No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
DNA Methylation Contributes to the Regulation of Sclerostin Expression in Human Osteocytes
Abstract
Sclerostin, encoded by the SOST gene, is a potent inhibitor of bone formation, produced by osteocytes, not by osteoblasts, but little is known about the molecular mechanisms controlling its expression. We aimed to test the hypothesis that epigenetic mechanisms, specifically DNA methylation, modulate SOST expression. We found two CpG-rich regions in SOST: region 1, located in the proximal promoter, and region 2, around exon 1. qMSP and pyrosequencing analysis of DNA methylation showed that region 2 was largely methylated in all samples analyzed. In contrast, marked differences were observed in region 1. Whereas the CpG-rich region 1 was hypermethylated in osteoblasts, this region was largely hypomethylated in microdissected human osteocytes. Bone lining cells showed a methylation profile between primary osteoblasts and osteocytes. Whereas SOST expression was detected at very low level or not at all by RT-qPCR in several human osteoblastic and nonosteoblastic cell lines, and human primary osteoblasts under basal conditions, it was dramatically upregulated (up to 1300-fold) by the demethylating agent AzadC. Experiments using reporter vectors demonstrated the functional importance of the region -581/+30 of the SOST gene, which contains the CpG-rich region 1. In vitro methylation of this CpG-island impaired nuclear protein binding and led to a 75 ± 12% inhibition of promoter activity. In addition, BMP-2-induced expression of SOST was markedly enhanced in cells demethylated by AzadC. Overall, these results strongly suggest that DNA methylation is involved in the regulation of SOST expression during osteoblast-osteocyte transition, presumably by preventing the binding of transcription factors to the proximal promoter. To our knowledge, our data provide first ever evidence of the involvement of DNA methylation in the regulation of SOST expression and may help to establish convenient experimental models for further studies of human sclerostin.
... For AzadC treatment Saos-2 cells were seeded in six-well plates at an initial density of 40,000 cells/cm 2 in complete medium and allowed to settle overnight. Cells were then treated once with AzadC at a final concentration of 1 μM as previously reported to increase SOST expression by demethylating its promoter (Delgado-Calle et al., 2012) and harvested three days later. Actinomycin D (ActD) was also from Sigma-Aldrich (Dorset, UK) dissolved in ethanol and used at a final concentration of 2 μM 1 hr prior to strain. ...
... RUNX2 may modulate SOST expression in Saos-2 cells by either activating (Sevetson et al., 2004) or suppressing (Byon et al., 2011) its promoter activity. Under our conditions, when SOST expression was increased with the DNA methylation inhibitor AzadC as previously reported (Delgado-Calle et al., 2012), cells lacking RUNX2 up-regulated SOST to a significantly lower extent than vector control cells (Fig 3A). As neither RUNX2 knockdown nor AzadC had any significant effect on RANKL expression (Fig 3B), we investigated the effect of the coupling agent PGE2 which is known to up-regulate RANKL. ...
... Epigenetic regulation of SOST expression through DNA methylation has previously been reported (Delgado-Calle et al., 2012;Reppe et al., 2015;Lhaneche et al., 2016;Stegen et al., 2018). In the present study, the up-regulation of SOST expression induced by demethylation was sub-maximal in cells lacking maximal RUNX2 expression. ...
Mechanical loading-related strains trigger bone formation by osteoblasts while suppressing resorption by osteoclasts, uncoupling the processes of formation and resorption. Osteocytes may orchestrate this process in part by secreting sclerostin (SOST), which inhibits osteoblasts, and expressing receptor activator of nuclear factor-κB ligand (RANKL/TNFSF11) which recruits osteoclasts. Both SOST and RANKL are targets of the master osteoblastic transcription factor RUNX2. Subjecting human osteoblastic Saos-2 cells to strain by four point bending down-regulates their expression of SOST and RANKL without altering RUNX2 expression. RUNX2 knockdown increases basal SOST expression, but does not alter SOST down-regulation following strain. Conversely, RUNX2 knockdown does not alter basal RANKL expression, but prevents its down-regulation by strain. Chromatin immunoprecipitation revealed RUNX2 occupies a region of the RANKL promoter containing a consensus RUNX2 binding site and its occupancy of this site decreases following strain. The expression of epigenetic acetyl and methyl writers and readers was quantified by RT-qPCR to investigate potential epigenetic bases for this change. Strain and RUNX2 knockdown both down-regulate expression of the bromodomain acetyl reader BRD2. BRD2 and RUNX2 co-immunoprecipitate, suggesting interaction within regulatory complexes, and BRD2 was confirmed to interact with the RUNX2 promoter. BRD2 also occupies the RANKL promoter and its occupancy was reduced following exposure to strain. Thus, RUNX2 may contribute to bone remodeling by suppressing basal SOST expression, while facilitating the acute strain-induced down-regulation of RANKL through a mechanosensitive epigenetic loop involving BRD2.
... [2,15]. The second site in the SOST gene is the upstream promoter region, where runt-related transcription factor 2 (Runx2) binds [16] and suppresses sclerostin expression [17]. Similarly, sirtuin 1, a histone deacetylase (HDAC), which is increased in hypoxia, suppresses the SOST gene in osteocytes by deacetylation of histone 3 at the SOST promoter [18,19] (Figure 2). the canonical Wnt-signaling pathway exists in an OPG-independent manner and probably acts on osteoclast precursors [14]. ...
... [2,15]. The second site in the SOST gene is the upstream promoter region, where runt-related transcription factor 2 (Runx2) binds [16] and suppresses sclerostin expression [17]. Similarly, sirtuin 1, a histone deacetylase (HDAC), which is increased in hypoxia, suppresses the SOST gene in osteocytes by deacetylation of histone 3 at the SOST promoter [18,19] (Figure 2). ...
Sclerostin has been identified as an important regulator of bone homeostasis through inhibition of the canonical Wnt-signaling pathway, and it is involved in the pathogenesis of many different skeletal diseases. Many studies have been published in the last few years regarding sclerostin’s origin, regulation, and mechanism of action. The ongoing research emphasizes the potential therapeutic implications of sclerostin in many pathological conditions with or without skeletal involvement. Antisclerostin antibodies have recently been approved for the treatment of osteoporosis, and several animal studies and clinical trials are currently under way to evaluate the effectiveness of antisclerostin antibodies in the treatment of other than osteoporosis skeletal disorders and cancer with promising results. Understanding the exact role of sclerostin may lead to new therapeutic approaches for the treatment of skeletal disorders.
... Extensive studies on the epigenetic state of gene coding sclerostin (SOST) have been performed. Delgado-Calle et al. [49] reported 2 CpG-rich regions, namely region 1 located in the proximal promoter and region 2 located in the exon 1 of SOST. While the CpG-rich promoter region 1 was hypomethylated in human osteocytes, it is hypermethylated in osteoblasts and extraskeletal cells. ...
... While the CpG-rich promoter region 1 was hypomethylated in human osteocytes, it is hypermethylated in osteoblasts and extraskeletal cells. Treatment of osteoblastic cells with the demethylating agent 5-aza-2′-deoxycytidine (AzadC) caused a significant increase in gene expression of SOST [49]. Thus, these authors suggested that methylation status of CpG dinucleotides influences the regulation of SOST gene expression in osteocytes and also during osteoblast-osteocyte transition. ...
Cellular senescence is a hallmark of aging that is characterized by irreversible cell cycle arrest in response to various stress stimuli. It is a normal physiological event, which plays essential role in development, prevention of cancer, wound healing process, etc. Nevertheless, accumulation of senescent cells is the trigger and forms the basis of aging of a tissue/organ and, thus, the organismal aging. During aging, cells display a senescence-associated secretory phenotype (SASP) that transforms normal physiology into pathophysiology of a tissue/organ, leading to its dysfunction. Similarly, the accumulation of senescent bone cells and their dysfunction leads to skeletal morbidity. Therefore, factors regulating the process of cellular senescence during aging can serve as potential therapeutic targets to contain morbidity and improve the bone health. Extensive investigations are currently focused on deciphering age-related bone loss and diseases. In this chapter, authors systematically review the research findings and advancements available on cellular senescence and aging, to understand their contributions to the pathophysiology of bone and the therapeutic options to treat the aging bone.
... As far as the regulation of bone remodeling process is concerned, it has recently been shown that sclerostin has a relevant role, displaying both autocrine and paracrine effects [66]. Sclerostin (a 22-kDa glycoprotein), due to SOST promoter hypomethylation [72,73], is currently considered the major mediator of the molecular osteocyte mechanisms involved in the process of adaptive bone responses. In the mature skeleton, sclerostin is mainly synthesized by differentiated mature osteocytes, while preosteocytes, bone lining cells, and osteoblasts express very low levels of sclerostin. ...
... Such opposite up-or down- As far as the regulation of bone remodeling process is concerned, it has recently been shown that sclerostin has a relevant role, displaying both autocrine and paracrine effects [66]. Sclerostin (a 22-kDa glycoprotein), due to SOST promoter hypomethylation [72,73], is currently considered the major mediator of the molecular osteocyte mechanisms involved in the process of adaptive bone responses. In the mature skeleton, sclerostin is mainly synthesized by differentiated mature osteocytes, while preosteocytes, bone lining cells, and osteoblasts express very low levels of sclerostin. ...
Osteocytes are the most abundant bone cells, entrapped inside the mineralized bone matrix. They derive from osteoblasts through a complex series of morpho-functional modifications; such modifications not only concern the cell shape (from prismatic to dendritic) and location (along the vascular bone surfaces or enclosed inside the lacuno-canalicular cavities, respectively) but also their role in bone processes (secretion/mineralization of preosseous matrix and/or regulation of bone remodeling). Osteocytes are connected with each other by means of different types of junctions, among which the gap junctions enable osteocytes inside the matrix to act in a neuronal-like manner, as a functional syncytium together with the cells placed on the vascular bone surfaces (osteoblasts or bone lining cells), the stromal cells and the endothelial cells, i.e., the bone basic cellular system (BBCS). Within the BBCS, osteocytes can communicate in two ways: by means of volume transmission and wiring transmission, depending on the type of signals (metabolic or mechanical, respectively) received and/or to be forwarded. The capability of osteocytes in maintaining skeletal and mineral homeostasis is due to the fact that it acts as a mechano-sensor, able to transduce mechanical strains into biological signals and to trigger/modulate the bone remodeling, also because of the relevant role of sclerostin secreted by osteocytes, thus regulating different bone cell signaling pathways. The authors want to emphasize that the present review is centered on the morphological aspects of the osteocytes that clearly explain their functional implications and their role as bone orchestrators.
... In vitro work showed that treatment with a demethylating agent removed methyl marks from the Sost proximal promoter and stimulated sclerostin expression in human osteoblastic cell lines. (62) To determine whether this could be a mechanism regulating Sost expression in human bone, we used ex vivo human bone organ cultures, and treated them with the same DNA methylation inhibitor. Similar to our in vitro studies, the expression of Sost in ex vivo bone organ cultures was increased. ...
... Similar to our in vitro studies, the expression of Sost in ex vivo bone organ cultures was increased. (62) Clinical studies inspired by these initial findings demonstrated that Sost/sclerostin expression negatively correlates with DNA methylation at the Sost proximal promoter in several patient populations. (63,64) In a different set of experiments, we employed ex vivo human bone organ cultures to determine the translatability of findings found in mouse models. ...
The integrity of the skeleton is maintained by the coordinated and balanced activities of the bone cells. Osteoclasts resorb bone, osteoblasts form bone, and osteocytes orchestrate the activities of osteoclasts and osteoblasts. A variety of in vitro approaches has been used in an attempt to reproduce the complex in vivo interactions among bone cells under physiological as well as pathological conditions and to test new therapies. Most cell culture systems lack the proper extracellular matrix, cellular diversity, and native spatial distribution of the components of the bone microenvironment. In contrast, ex vivo cultures of fragments of intact bone preserve key cell–cell and cell–matrix interactions and allow the study of bone cells in their natural 3D environment. Further, bone organ cultures predict the in vivo responses to genetic and pharmacologic interventions saving precious time and resources. Moreover, organ cultures using human bone reproduce human conditions and are a useful tool to test patient responses to therapeutic agents. Thus, these ex vivo approaches provide a platform to perform research in bone physiology and pathophysiology. In this review, we describe protocols optimized in our laboratories to establish ex vivo bone organ cultures and provide technical hints and suggestions. In addition, we present examples on how this technical approach can be employed to study osteocyte biology, drug responses in bone, cancer‐induced bone disease, and cross‐talk between bone and other organs © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.
... DNA methylation plays a critical role during development and differentiation by regulating the function of osteoblasts, osteocalcin and sclerostin. Methylation of alkaline phosphatase (ALP) gene is required for the regulation of osteoblasts transition to osteocytes [69]. Furthermore, changes in cytosine methylation have been implicated in the regulation of amelogenesis mediated by methyltransferase DNMT1 and the ten-eleven translocation (TET) protein family of demethylases [70]. ...
... Contributed to elevated expression of pro-inflammatory cytokines IL-2, IL-8, IL-1, IL-12 [67] miR-146a, miR-155 /Upregulation Regulation of TLR signaling to bacteria and inflammatory response [63] Dental carries ALP methylation Regulation of osteoblasts transition to osteocytes [69] Cytosine methylation by methyltransferase DNMT1 and demethylase TET Regulation of amelogenesis [70] Congenital orofacial malformations Clf2 methylation Implicated in cleft etiology [72] DNA methylation Regulation of tooth agenesis [75] prolonged survival in mice. Butyric acid derivatives have also been used as HDACis. ...
Background
Epigenetic mechanisms alter gene expression and regulate vital cellular processes that contribute to the onset and the progression of major dental diseases. Their reversible character may prove beneficial for therapeutic targeting. This review aims to provide an update on the main epigenetic changes that contribute to the pathogenesis of Oral Squamous Cell Carcinoma (OSCC), pulpitis and periodontitis as well as in dental caries and congenital orofacial malformations, in an effort to identify potential therapeutic targets.
Methods
We undertook a structured search of bibliographic databases (PubMed and MEDLINE) for peer-reviewed epigenetic research studies focused in oral diseases the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.
Results
Several epigenetic modifications have been associated with OSCC pathogenesis including promoter methylation of genes involved in DNA repair, cell cycle regulation and proliferation leading to malignant transformation. Additionally, epigenetic inactivation of tumor suppressor genes, overexpression of histone chaperones and several microRNAs are implicated in OSCC aggressiveness. Changes in the methylation patterns of IFN-γ and trimethylation of histone Η3Κ27 have been detected in pulpitis, along with aberrant expression of several microRNAs, mainly affecting cytokine production. Chronic periodontal disease has been associated with modifications in the methylation patterns of Toll-Like Receptor 2, Prostaglandin synthase 2, E-cadherin and some inflammatory cytokines, along with overexpression of miR-146a and miR155. Furthermore, DNA methylation was found to regulate amelogenesis and has been implicated in the pathogenesis of dental caries as well as in several congenital orofacial malformations.
Conclusion
Strong evidence indicates that epigenetic changes participate in the pathogenesis of oral diseases and epigenetic targeting may be considered as a complementary therapeutic scheme to current management of oral health.
... The upstream promoter region has binding sites for runt-related transcription factor 2 (Runx2), a master transcription factor of osteoblasts [82], and osterix (OSX) thereby promoting the transcription of SOST. This region also contains a methylation site and demethylation of this region in osteoblast differentiation results in increased SOST expression [83]. Endosteal hyperostosis develops in the absence of the downstream enhancer ECR5 region, which contains a binding sequence for the myocyte enhancer factor (Mef) 2c transcription factor. ...
... Osteoblast-specific Mef2c KO mice exhibited high bone mass [29,84] with decreased expression of sclerostin, suggesting that the binding of Mef2c to the ECR5 region of the SOST gene is important for SOST expression ( Figure 2). site and demethylation of this region in osteoblast differentiation results in increased SOST expression [83]. Endosteal hyperostosis develops in the absence of the downstream enhancer ECR5 region, which contains a binding sequence for the myocyte enhancer factor (Mef) 2c transcription factor. ...
Wnt, a secreted glycoprotein, has an approximate molecular weight of 40 kDa, and it is a cytokine involved in various biological phenomena including ontogeny, morphogenesis, carcinogenesis, and maintenance of stem cells. The Wnt signaling pathway can be classified into two main pathways: canonical and non-canonical. Of these, the canonical Wnt signaling pathway promotes osteogenesis. Sclerostin produced by osteocytes is an inhibitor of this pathway, thereby inhibiting osteogenesis. Recently, osteoporosis treatment using an anti-sclerostin therapy has been introduced. In this review, the basics of Wnt signaling, its role in bone metabolism and its involvement in skeletal disorders have been covered. Furthermore, the clinical significance and future scopes of Wnt signaling in osteoporosis, osteoarthritis, rheumatoid arthritis and neoplasia are discussed.
... Additionally, in Sost knockout mice, increased bone mass, bone formation, and bone strength have been reported. 37 The epigenetic status of SOST has also been widely studied; for example, Calle et al 38 noted that the proximal SOST promoter is hypomethylated in osteoblasts but hypermethylated in osteocytes, and that the in vitro treatment of osteoblasts with DNA methylation inhibitor 5-azacytidine significantly increases SOST synthesis. These findings suggest that DNA methylation affects protein binding to the proximal SOST promoter core, which in turn affects SOST transcription and regulation of the Wnt pathway. ...
Alterations to the human organism that are brought about by aging are comprehensive and detrimental. Of these, an imbalance in bone homeostasis is a major outward manifestation of aging. In older adults, the decreased osteogenic activity of bone marrow mesenchymal stem cells and the inhibition of bone marrow mesenchymal stem cell differentiation lead to decreased bone mass, increased risk of fracture, and impaired bone injury healing. In the past decades, numerous studies have reported the epigenetic alterations that occur during aging, such as decreased core histones, altered DNA methylation patterns, and abnormalities in noncoding RNAs, which ultimately lead to genomic abnormalities and affect the expression of downstream signaling osteoporosis treatment and promoter of fracture healing in older adults. The current review summarizes the impact of epigenetic regulation mechanisms on age‐related bone homeostasis imbalance.
... This probably because vascular SMCs are the main vascular cells that express SOST [39][40][41][42]. These findings indicate a specific role for colchicine in modulating vascular SMC homeostasis and new insights into SOST expression regulation, although we cannot rule out other possibilities of epigenetic or post-transcriptional modifications that may also involve in colchicinemediated SOST expression, such as histone acetylation or DNA methylation [67,68]. ...
Development of non-surgical treatment of human abdominal aortic aneurysm (AAA) has clinical significance. Colchicine emerges as an effective therapeutic regimen in cardiovascular diseases. Yet, whether colchicine slows AAA growth remain controversy. Here, we demonstrated that daily intragastric administration of low-dose colchicine blocked AAA formation, prevented vascular smooth muscle cell (SMC) phenotype switching and apoptosis, and vascular inflammation in both peri-aortic CaPO4 injury and subcutaneous angiotensin-II infusion induced experimental AAA mice models. Mechanistically, colchicine increased global mRNA stability by inhibiting the METTL14/YTHDC1-mediated m6A modification, resulting in increased sclerostin (SOST) expression and consequent inactivation of the WNT/β-catenin signaling pathway in vascular SMCs from mouse AAA lesions and in cultured human aortic SMCs. Moreover, human and mouse AAA lesions all showed increased m6A methylation, decreased SOST expression, and skewed synthetic SMC de-differentiation phenotype, compared to those without AAA. This study uncovers a novel mechanism of colchicine in slowing AAA development by using the METTL14/SOST/WNT/β-catenin axis to control vascular SMC homeostasis in mouse aortic vessels and in human aortic SMCs. Therefore, use of colchicine may benefit AAA patients in clinical practice.
... DNA methylation is also an important epigenetic mechanism that plays a role in the regulation of bone formation. This mechanism is characterized by the introduction of a heritable epigenetic mark involving a reversible modification of cytosines in the context of CpG dinucleotides by DNA methyltransferases [126]. It was shown that DNA methylation modulates SOST gene expression, a potent inhibitor of the Wnt pathway. ...
Bone formation is a complex process that occurs throughout life, and is normally limited to the skeletal system. In bone formation, osteoprogenitor cells follow several developmental stages, including differentiation in osteoblasts, proliferation, matrix maturation, and mineralization. The mechanisms involved in the mineralization process of bone, such as in the new bone formation, are extremely complex and have been under intense investigation for many years. Bone formation follows two distinct processes, intramembranous and endochondral ossification; both are regulated by signaling pathways involving numerous genes. Disturbance of these signaling pathways may cause a large spectrum of skeletal diseases characterized by new bone formation and bone growth anomalies. This review will only focus on the key genetic pathways involved in heterotopic bone formation. Wingless/integrated (Wnt), hedgehog (HH), and transforming growth factor beta (TGFβ)/bone morphogenetic protein (BMP) signaling pathways are described and illustrated; their relation with new bone formation is demonstrated through their involvement in bone formation disorders.
... These data, together with the findings of the present study, indicate that cell senescence, regulation of Sost expression, and possibly SASP, play important roles in aging-related osteoporosis. Epigenetic regulation of Sost controls sclerostin levels, which correlates with bone density and fracture rates 47 . The promoter region of Sost has a CpG-rich domain, which contributes to the transcriptional regulation of Sost 48 . ...
The mechanistic regulation of bone mass in aged animals is poorly understood. In this study, we examined the role of SIRT6, a longevity-associated factor, in osteocytes, using mice lacking Sirt6 in Dmp-1-expressing cells (cKO mice) and the MLO-Y4 osteocyte-like cell line. cKO mice exhibited increased osteocytic expression of Sost, Fgf23 and senescence inducing gene Pai-1 and the senescence markers p16 and Il-6, decreased serum phosphate levels, and low-turnover osteopenia. The cKO phenotype was reversed in mice that were a cross of PAI-1-null mice with cKO mice. Furthermore, senescence induction in MLO-Y4 cells increased the Fgf23 and Sost mRNA expression. Sirt6 knockout and senescence induction increased HIF-1α binding to the Fgf23 enhancer sequence. Bone mass and serum phosphate levels were higher in PAI-1-null aged mice than in wild-type mice. Therefore, SIRT6 agonists or PAI-1 inhibitors may be promising therapeutic options for aging-related bone metabolism disruptions.
... Reppe et al. revealed an increasing level of CpG methylation in SOST gene promoter region in 27 postmenopausal women with osteoporosis compared to 36 healthy controls [219]. The inhibitory role in bone formation of the glycoprotein sclerostin, encoded by SOST gene, was confirmed by Delgado-Calle et al., who in previous study highlighted a hypomethylation in the SOST promotor region in human osteocytes, suggesting, as an attracted possibility, targeting sclerostin for the treatment of osteoporosis [220]". ...
Aging is an inevitable outcome of life, characterized by a progressive decline in tissue and organ function. At a molecular level, it is marked by the gradual alterations of biomolecules. Indeed, important changes are observed on the DNA, as well as at a protein level, that are influenced by both genetic and environmental parameters. These molecular changes directly contribute to the development or progression of several human pathologies, including cancer, diabetes, osteoporosis, neurodegenerative disorders and others aging-related diseases. Additionally, they increase the risk of mortality. Therefore, deciphering the hallmarks of aging represents a possibility for identifying potential druggable targets to attenuate the aging process, and then the age-related comorbidities. Given the link between aging, genetic, and epigenetic alterations, and given the reversible nature of epigenetic mechanisms, the precisely understanding of these factors may provide a potential therapeutic approach for age-related decline and disease. In this review, we center on epigenetic regulatory mechanisms and their aging-associated changes, highlighting their inferences in age-associated diseases.
... The regulation of established pathways such as Wnt/b-catenin by SOST [81], as well as transcription factor regulation of SOST by Osterix and Runx2 [79], have been well documented; however, information on the epigenetic mechanisms of gene regulation remains incomplete. It has been reported that altered methylation of the SOST promoter regulated the osteoblast-osteocyte transition [82], as well as a correlation between an increase in methylation and increased fracture risk in post-menopausal women. More recently, SOST promoter methylation was found to disrupt the transactivation of key transcription factors Osterix, ERa and RUNX2 via the impairment of transcription factor binding [83]. ...
Many crucial epigenetic changes occur during early skeletal development and throughout life due to aging, disease and are heavily influenced by an individual’s lifestyle. Epigenetics is the study of heritable changes in gene expression as the result of changes in the environment without any mutation in the underlying DNA sequence. The epigenetic profiles of cells are dynamic and mediated by different mechanisms, including histone modifications, non-coding RNA-associated gene silencing and DNA methylation. Given the underlining role of dysfunctional mesenchymal tissues in common age-related skeletal diseases such as osteoporosis and osteoarthritis, investigations into skeletal stem cells or mesenchymal stem cells (MSC) and their functional deregulation during aging has been of great interest and how this is mediated by an evolving epigenetic landscape. The present review describes the recent findings in epigenetic changes of MSCs that effect growth and cell fate determination in the context of aging, diet, exercise and bone-related diseases.
... These data, together with the ndings of the present study, indicate that cell senescence, regulation of Sost expression, and possibly SASP, play important roles in aging-related osteoporosis. Epigenetic regulation of Sost controls sclerostin levels, which correlates with bone density and fracture rates 37 . The promoter region of Sost has a CpGrich domain, which contributes to the transcriptional regulation of Sost 38 . ...
The mechanistic regulation of bone mass in aged animals is poorly understood. In this study, we examined the role of SIRT6, a longevity-associated factor, in osteocytes, using mice lacking Sirt6 in Dmp-1 -expressing cells (cKO mice) and the MLO-Y4 osteocyte-like cell line. cKO mice exhibited increased osteocytic expression of Sost, Fgf23 and senescence inducing gene Pai-1 and the senescence markers p16 and Il-6 , decreased serum phosphate levels, and low-turnover osteopenia. The cKO phenotype was reversed in mice that were a cross of PAI-1-null mice with cKO mice. Furthermore, senescence induction in MLO-Y4 cells increased the Fgf23 and Sost mRNA expression. Sirt6 knockout and senescence induction increased HIF-1α binding to the Fgf23 enhancer sequence. Bone mass and serum phosphate levels were higher in PAI-1-null aged mice than in wild-type mice. Therefore, SIRT6 agonists or PAI-1 inhibitors may be promising therapeutic options for aging-related bone metabolism disruptions.
... In addition, the proximal promoter contains three estrogen response elements (ERES) that negatively control the expression of SOST through estrogen receptor B [407]. Furthermore, it has been determined that this region is subject to epigenetic modifications that regulate its expression, such as methylation (hypermethylated in osteoblasts to hypomethylated in osteocytes) [408,409] or acetylation (deacetylation of H3K9 by Sirt1) [410]. In addition to the proximal promoter, the ECR5 enhancer region regulates the correct expression of SOST in bone cells. ...
The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.
... 9 Promoter regions of osteoblast-specific genes such as Sost, Dlx5, and Sp7 appear to be hypomethylated in osteogenic cells but not in other cell types. [13][14][15][16] Exactly how methylation patterns are dynamically regulated as progenitor cells differentiate into osteoblasts remains unclear. Initial studies in embryonic stem cells revealed a crucial role for TET enzymes in the regulation of methylation and gene inactivation at stemnessspecific genes during differentiation. ...
DNA methylation is an epigenetic modification critical for the regulation of chromatin structure and gene expression during development and disease. The ten‐eleven translocation (TET) enzyme family catalyzes the hydroxymethylation and subsequent demethylation of DNA by oxidizing 5‐methylcytosine (5mC) to 5‐hydroxymethylcytosine (5hmC). Little is known about TET protein function due to a lack of pharmacological tools to manipulate DNA hydroxymethylation levels. In this study, we examined the role of TET‐mediated DNA hydroxymethylation during BMP‐induced C2C12 osteoblast differentiation using a novel cytosine‐based selective TET enzyme inhibitor, Bobcat339 (BC339). Treatment of C2C12 cells with BC339 increased global 5mC and decreased global 5hmC without adversely affecting cell viability, proliferation, or apoptosis. Furthermore, BC339 treatment inhibited osteoblast marker gene expression and decreased alkaline phosphatase activity during differentiation. Methylated DNA immunoprecipitation and bisulfite sequencing showed that inhibition of TET with BC339 led to increased 5mC at specific CpG‐rich regions at the promoter of Sp7, a key osteoblast transcription factor. Consistent with promoter 5mC marks being associated with transcriptional repression, luciferase activity of an Sp7‐promoter‐reporter construct was repressed by in vitro DNA methylation or BC339. Chromatin immunoprecipitation analysis confirmed that TET2 does indeed occupy the promoter region of Sp7. Accordingly, forced overexpression of SP7 rescued the inhibition of osteogenic differentiation by BC339. In conclusion, our data suggest that TET‐mediated DNA demethylation of genomic regions, including the Sp7 promoter, plays a role in the initiation of osteoblast differentiation. Furthermore, BC339 is a novel pharmacological tool for the modulation of DNA methylation dynamics for research and therapeutic applications.
... The upstream promoter region has binding sequences for osteoblast transcription factor runt-related transcription factor 2 (Runx2) [29] and also contains a methylation site. Demethylation of this region during osteoblast-osteocyte transition results in increased SOST expression [30]. ...
Idiopathic scoliosis is a disorder of unknown etiology. Bone biopsies from idiopathic scoliosis patients revealed changes at cellular and molecular level. Osteocytic sclerostin is downregulated, and serum level of sclerostin is decreased. Osteocytes in idiopathic scoliosis appear to be less active with abnormal canaliculi network. Differentiation of osteoblasts to osteocytes is decelerated, while Wnt/β-catenin signaling pathway is overactivated and affects normal bone mineralization that leads to inferior mechanical properties of the bone, which becomes susceptible to asymmetrical forces and causes deformity of the spinal column. Targeting bone metabolism during growth by stimulating sclerostin secretion from osteocytes and restoring normal function of Wnt/β-catenin signaling pathway could, in theory, increase bone strength and prevent deterioration of the scoliotic deformity.
... Estrogen prevents osteocyte and osteoblast apoptosis, induces osteoblast differentiation, increases osteoclast apoptosis, and reduces osteoclast differentiation (Khosla et al., 2012;Syed and Khosla, 2005;Shevde et al., 2000;Bord et al., 2003;Khalid and Krum, 2016). Estrogen also downregulates osteocyte expression of sclerostin (Drake and Khosla, 2017;Delgado-Calle et al., 2012;Dirkes et al., 2020a), which acts locally to inhibit bone growth. In older men, estrogen treatment lowered circulating sclerostin (Il Mödder et al., 2011), implying there is an inverse relationship between estrogen and sclerostin in males. ...
Bisphenol-A (BPA) and bisphenol-S (BPS) are endocrine disrupting chemicals (EDCs) found primarily in plastics. Estrogen is a primary hormonal regulator of skeletal growth and development; however, the impact of gestational BPA or BPS exposure on skeletal health of offspring remains relatively unknown. Here, adult female mice were randomized into three treatment groups: 200 μg BPA/kg BW (BPA), 200 μg BPS/kg BW (BPS) or control (CON). Animals were then further randomized to exercising (EX) or sedentary (SED) groups. Treatment continued through mating, gestation, and lactation. One male offspring from each dam (n = 6–8/group) was assessed at 16 weeks of age to evaluate effects of EDC exposure on the adult skeleton. Cortical geometry of the mid-diaphysis and trabecular microarchitecture of the distal femur were assessed via micro-CT. Biomechanical strength and mineral apposition rate of the femoral diaphysis were assessed via three-point bending and dynamic histomorphometry, respectively. Two-factor ANOVA or ANCOVA were used to determine the effects of maternal EX and BPA or BPS on trabecular and cortical bone outcomes. Maternal EX led to a significant decrease in body fat percentage and bone stiffness, independent of EDC exposure. Offspring exposed to BPA had significantly lower trabecular bone volume, trabecular number, connectivity density, cortical thickness, and greater trabecular spacing compared to BPS or CON animals. In conclusion, gestational BPA, but not BPS, exposure negatively impacted trabecular microarchitecture and cortical geometry in adult male offspring. If these findings translate to humans, this could have significant public health impacts on expecting women or those seeking to become pregnant.
... DNA methylation plays a role in the final stage of modification in the case of SOST gene expression. The upregulated SOST further promotes two osteoblasts genes RUNX2 and OSX (Delgado-Calle et al. 2012;Sebastian and Loots 2017). An increment in DNMT3B gene expression was noted with significant upregulation in DNA methylation (Oh et al. 2007). ...
Arterial and aortic valve calcifications are the most prevalent pathophysiological conditions among all the reported cases of cardiovascular calcifications. It increases with several risk factors like age, hypertension, external stimuli, mechanical forces, lipid deposition, malfunction of genes and signaling pathways, enhancement of naturally occurring calcium inhibitors, and many others. Modern-day lifestyle is affected by numerous environmental factors and harmful toxins that impair our health rather than providing benefits. Applying the combinatorial approach or targeting the exact mechanism could be a new strategy for drug designing or attenuating the severity of calcification. Most of the non-communicable diseases are life-threatening; thus, altering the phenotype and not the genotype may reveal the gateway for fighting with upcoming hurdles. Overall, this review summarizes the reason behind the generation of arterial and aortic valve calcification and its related signaling pathways and also the detrimental effects of calcification. In addition, the individual process of epigenetics and how the implementation of this process becomes a novel approach for diminishing the harmful effect of calcification are discussed. Noteworthy, as epigenetics is linked with genetics and environmental factors necessitates further clinical trials for complete and in-depth understanding and application of this strategy in a more specific and prudent manner.
... Estrogen can block osteocyte and osteoblast apoptosis, induce osteoblast differentiation, increases osteoclast apoptosis, and reduce osteoclast differentiation (Khosla et al., 2012;Syed and Khosla, 2005;Shevde et al., 2000;Bord et al., 2003;Khalid and Krum, 2016). More recently, it has also been suggested that estrogen can downregulate the expression of sclerostin (Drake and Khosla, 2017;Delgado-Calle et al., 2012;Dirkes et al., 2020a). Sclerostin is a protein produced in the osteocyte which acts locally within the bone to inhibit bone growth. ...
Bisphenol-A (BPA) and bisphenol-S (BPS) are estrogen disrupting chemicals (EDCs) found in the environment and common household items. Estrogen is a primary hormonal regulator of bone growth and development; however, the impact of gestational BPA or BPS exposure on skeletal health of offspring remains relatively unknown. In this longitudinal study, adult female mice were randomized into three groups: 200 μg BPA/kg BW (BPA), 200 μg BPS/kg BW (BPS) or control (CON). Animals in each group were further randomized to exercise treatment (EX) or sedentary (SED) control, resulting in six overall groups. BPA/BPS/CON and EX/SED treatment were initiated prior to mating and continued through mating, gestation, and lactation. One female offspring from each dam (n = 6/group) was assessed at 17 weeks of age to evaluate effects of EDC exposure on the adult skeleton. Cortical geometry of the mid-diaphysis and trabecular microarchitecture of the distal femur were assessed via micro-computed tomography. Biomechanical strength and mineral apposition rate of the femoral diaphysis were assessed via three-point bending and dynamic histomorphometry, respectively. Sclerostin expression was measured using immunohistochemistry. Two-factor ANOVA or ANCOVA were used to determine the effects of maternal exercise and BPA or BPS exposure on trabecular and cortical bone outcomes, respectively. Consistent with prior studies, there were no significant differences in body weight, femoral length, cortical geometry, trabecular microarchitecture, or biomechanical strength between groups in female offspring. In conclusion, gestational BPA exposure and maternal exercise have minimal impact on skeletal outcomes in female adult offspring.
... Binding of myocyte enhancer factor-2 (MEF2C) to evolutionarily conserved region 5 (ECR5) plays a critical role in the regulation of SOST expression (54)(55)(56). In addition, the loss of epigenetic DNA methyl marks at the SOST proximal promoter enables osteocytes to express SOST (57). Chromatin marks regulated by Sirtuin 1, a histone deacetylase, and HDAC5, a class IIa histone deacetylase, also directly regulate SOST expression in osteocytes (58)(59)(60)(61)(62). ...
Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone as well as in distant tissues. Osteocytes are a significant source of molecules that regulate bone homeostasis by integrating mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of various bone therapeutics used in the clinic. Herein, we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematological and metastatic cancers in the skeleton.
... Binding of myocyte enhancer factor-2 (MEF2C) to evolutionarily conserved region 5 (ECR5) plays a critical role in the regulation of SOST expression (54)(55)(56). In addition, the loss of epigenetic DNA methyl marks at the SOST proximal promoter enables osteocytes to express SOST (57). Chromatin marks regulated by Sirtuin 1, a histone deacetylase, and HDAC5, a class IIa histone deacetylase, also directly regulate SOST expression in osteocytes (58)(59)(60)(61)(62). ...
Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone as well as in distant tissues. Osteocytes are a significant source of molecules that regulate bone homeostasis by integrating mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of various bone therapeutics used in the clinic. Herein, we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematological and metastatic cancers in the skeleton.
... It was also observed that the increased methylation level of SOST in patients with low Bone Mineral Density (BMD) was functionally associated with the reduction of bone SOST mRNA and SOST serum levels [21]. The inhibitory role of SOST in bone formation was also confirmed by the Delgado-Calle team, who in a previous study showed hypomethylation in the SOST promoter region in human osteocytes [31]. These data suggest that the increased methylation of SOST in OP patients may be a compensatory counteracting mechanism leading to lower serum sclerostin levels, which reduces inhibition of Wnt signaling and promotes bone formation. ...
DNA methylation is one of the most studied epigenetic mechanisms that play a pivotal role in regulating gene expression. The epigenetic component is strongly involved in aging-bone diseases, such as osteoporosis and osteoarthritis. Both are complex multi-factorial late-onset disorders that represent a globally widespread health problem, highlighting a crucial point of investigations in many scientific studies. In recent years, new findings on the role of DNA methylation in the pathogenesis of aging-bone diseases have emerged. The aim of this systematic review is to update knowledge in the field of DNA methylation associated with osteoporosis and osteoarthritis, focusing on the specific tissues involved in both pathological conditions.
... In older men, estrogen treatment lowered circulating levels of sclerostin [32], implying that the inverse relationship between estrogen and sclerostin is also present in males. However, because circulating levels of sclerostin may not reflect bone sclerostin expression [33], further studies looking at the effect of estrogen on bone sclerostin expression in males are warranted. ...
Estrogen receptor-α knockout (ERKO) in female, but not male, mice results in an impaired osteogenic response to exercise, but the mechanisms behind this ability in males are unknown. We explored the main and interactive effects of ERKO and exercise on cortical geometry, trabecular microarchitecture, biomechanical strength, and sclerostin expression in male mice. At 12 weeks of age, male C57BL/6J ERKO and WT animals were randomized into two groups: exercise treatment (EX) and sedentary (SED) controls, until 22 weeks of age. Cortical geometry and trabecular microarchitecture were measured via μCT; biomechanical strength was assessed via three-point bending; sclerostin expression was measured via immunohistochemistry. Two-way ANOVA was used to assess sclerostin expression and trabecular microarchitecture; two-way ANCOVA with body weight was used to assess cortical geometry and biomechanical strength. ERKO positively impacted trabecular microarchitecture, and exercise had little effect on these outcomes. ERKO significantly impaired cortical geometry, but exercise was able to partially reverse these negative alterations. EX increased cortical thickness regardless of genotype. There were no effects of genotype or exercise on sclerostin expression. In conclusion, male ERKO mice retain the ability to build bone in response to exercise, but altering sclerostin expression is not one of the mechanisms involved.
... In particular, an agonist of SIRT1 stimulated osteoclasts bone formation and inhibited bone resorption by deacetylating Forkhead box O (FoxO) family proteins, making SIRT1 ligands promising therapeutic agents for osteoporosis in ovariectomized rats [51]. Natural compounds as resveratrol is an agonist of SIRT1 and downregulate specific miRNAs microRNA-338-3p associated with low BMD and high-risk fractures favoring osteogenesis [51,56]. ...
The study of epigenetics reaches its 50th anniversary, however, its clinical application is gradually coming into the clinical setting. Osteoporosis is one of the major and widely diffused bone diseases. Pathogenic mechanisms at the epigenetic level may interfere with bone remodeling occurring during osteoporosis. Preclinical models were used to understand whether such events may interfere with the disease. Besides, observational clinical trials investigated epigenetic-related biomarkers. This effort leads to some epigenetic-related therapies in clinical trials for the treatment of osteoporosis. Bisphosphonates (BPs), target therapy blocking RANK/RANKL pathway, and anti-sclerostin antibody (SOST) are the main therapeutic approaches. However, future large trials will reveal whether epigenetic therapies of osteoporosis will remain a work in progress or data will become more robust in the real-world management of these frailty patients.
... Several studies have suggested that methylome changes play an important role in osteoblast differentiation and activity. [27][28][29][30] Hypomethylation of the promoters of the runt-related transcription factor-2 (Runx-2), bone gamma-carboxyglutamate protein (Bglap, the coding gene for osteocalcin), and osterix (Osx) genes is involved in osteogenic differentiation of adipose-derived mesenchymal stromal cells (MSCs). 31 Other regulatory mechanisms involve microRNAs (miRs) that can regulate posttranscriptional gene expression. ...
Lactation‐induced bone loss occurs due to high calcium requirements for fetal growth but skeletal recovery is normally achieved promptly postweaning. Dietary protein is vital for fetus and mother but the effects of protein undernutrition on the maternal skeleton and skeletal muscles are largely unknown. We used mouse dams fed with normal (N, 20%) or low (L, 8%) protein diet during gestation and lactation and maintained on the same diets (NN, LL) or switched from low to normal (LN) during a 28 d skeletal restoration period post lactation. Skeletal muscle morphology and neuromuscular junction integrity was not different between any of the groups. However, dams fed the low protein diet showed extensive bone loss by the end of lactation, followed by full skeletal recovery in NN dams, partial recovery in LN and poor bone recovery in LL dams. Primary osteoblasts from low protein diet fed mice showed decreased in vitro bone formation and decreased osteogenic marker gene expression; promoter methylation analysis by pyrosequencing showed no differences in Bmpr1a, Ptch1, Sirt1, Osx, and Igf1r osteoregulators, while miR‐26a, ‐34a, and ‐125b expression was found altered in low protein fed mice. Therefore, normal protein diet is indispensable for maternal musculoskeletal health during the reproductive period.
... 75 The damages in the epigenomic level are linked to the cell senescence and aging by chromatin structure and telomere length. 76 In bone modeling and remodeling, epigenetic mechanisms can affect the osteoclastogenesis by RANKL, 22 OPG, 77 and osteocalcin, 78 and the osteoblastogenesis, by BMP-2, 79 sclerostin, 80 and C/EBPa 81 through DNA methylation. It is also reported that the RANKinduced NFATc1, which regulates the osteoclast differentiation, can be controlled by the histone demethylases. ...
The bone repair and substitute have been developed for decades with the bone defect repair applied successfully in clinical. However, implant complications may occur in more challenging situations, bone cell senescence and osteoporosis. Due to certain microenvironment conditions, including hormonal, nutritional, and aging factors, the bone cell responses are regulated to alteration and end into cell senescence and osteoporosis. Thus, the bone tissue regeneration is limited with site morbidity increased, leading to bone grafts failure. In such pathological state, bone grafts and substitutes are companied with osteoporosis therapies to improve bone tissue engineering and to enhance bone graft healing. Those substitutes with osteoporosis therapeutic-based applications have been becoming a growing field of interest in bone repair. Bone grafts, such as scaffolds with anti-osteoporotic drugs releasing, surface therapeutic modified implants, are studied in aim to significantly increase low bone density as well as improve impaired bone regeneration. In this review, we discuss a throughout understanding in bone remodeling process, bone cell senescence and osteoporosis. Based on the understanding, the review concentrates on the treatments to the osteoporosis and the bone grafts applied in the bone senescence model and provide a future direction in clinical trial.
... Studies showed that methylation of the SOST gene is important for regulating osteoblastosteocyte transition. Accordingly, the proximal promoter of the SOST gene is hypermethylated in OBLs and hypomethylated in osteocytes (Delgado-Calle et al., 2012). Ex vivo study on iliac bone biopsies showed that increase of CpGs methylation of the SOST proximal promoter region in osteoporotic women in comparison with healthy controls was associated with the lower serum SOST level, reduced inhibition of Wnt signaling, and promotion of bone formation (Reppe et al., 2015). ...
Wnt is a major signaling pathway involved in multifaceted roles of various biological processes. Bones are dynamic tissues which are able to remodel and maintain the tissue homeostasis. Wnt signaling cascade leads to the promotion of bone formation and suppression of bone resorption, leading to a balance in bone remodeling. Recent evidence has reinforced the inevitable role of Wnt signaling in osteoporosis. The complex genetic and epigenetic regulations of Wnt signaling factors and their interaction with other master signaling pathways such as TGF‐β, BMP, PI3K/AKT, and Hedgehog outline their importance in diagnosis and treatment of osteoporosis. In this review, we highlighted the recent advances in function of Wnt signaling‐related epigenetic regulation, different signaling pathways interacting with Wnt, and their roles in osteoporosis. Finally, we discussed novel promises in molecular targeted therapy of osteoporosis.
... DNA methylation can lead to variations in gene expression without changing its DNA sequence. It has been demonstrated that demethylation of the SOST promoter by 5-aza-2 ′ -deoxycytidine (AzadC) induces a strong increase in SOST expression in MG63 osteosarcoma cell line, presumably by facilitating the binding of transcription factors to the proximal promoter [16]. Reppe et al. have also found there is correlation between sclerostin expression and DNA methylation in promoter of the SOST gene [17]. ...
Purpose
SOST gene is one of the key factors in regulating bone absorption. Although there are reports showing diverse transcription factors, epigenetic modification could be responsible for regulating SOST gene expression. There is still little exploration on promoter methylation status of SOST gene in osteoporotic bone tissues. The aim of this study is to investigate the involvement of CpG methylation in regulation of SOST expression in patients with primary osteoporosis.
Methods
The diagnosis of osteoporosis was established on the basis of dual energy X-ray absorptiometry to measure BMD. All femoral bone tissues were separated in surgeries. After extracting total RNA and protein, we checked the relative expression levels of SOST by quantitative real-time PCR and western blot. Also, immunohistochemical staining was performed to observe the expression of SOST protein in the bone samples. The genomic DNA of non-OPF (non-osteoporotic fracture bone tissues) and OPF (osteoporotic fracture bone tissues) were treated by bisulfite modification, and methylation status of CpG sites in the CpG island of SOST gene promoter was determined by DNA sequencing.
Results
SOST gene expression in the non-OPF group was lower than that in OPF group. Bisulfite sequencing result showed that SOST gene promoter was slightly demethylated in the OPF group, as compared with non-OPF group.
Conclusion
Our study demonstrated that DNA methylation influenced the transcriptional expression of SOST gene, which probably may play an important role in the pathogenesis of primary osteoporosis.
Osteocyte, a cell type living within the mineralized bone matrix and connected to each other by means of numerous dendrites, appears to play a major role in body homeostasis. Benefiting from the maturation of osteocyte extraction and culture technique, many cross-sectional studies have been conducted as a subject of intense research in recent years, illustrating the osteocyte–organ/tissue communication not only mechanically but also biochemically. The present review comprehensively evaluates the new research work on the possible crosstalk between osteocyte and closely situated or remote vital organs/tissues. We aim to bring together recent key advances and discuss the mutual effect of osteocyte and brain, kidney, vascular calcification, muscle, liver, adipose tissue, and tumor metastasis and elucidate the therapeutic potential of osteocyte.
Backgrounding
Stayability, which may be defined as the probability of a cow remaining in the herd until a reference age or at a specific number of calvings, is usually measured late in the animal's life. Thus, if used as selection criteria, it will increase the generation interval and consequently might decrease the annual genetic gain. Measuring stay-ability at an earlier age could be a reasonable strategy to avoid this problem. In this sense, a better understanding of the genetic architecture of this trait at different ages and/or at different calvings is important. This study was conducted to identify possible regions with major effects on stayability measured considering different numbers of calv-ings in Nellore cattle as well as pathways that can be involved in its expression throughout the female's productive life.
Results
The top 10 most important SNP windows explained, on average, 17.60% of the genetic additive variance for stayability, varying between 13.70% (at the eighth calving) and 21% (at the fifth calving). These SNP windows were located on 17 they harbored a total of 176 annotated genes. The functional analyses of these genes, in general, indicate that the expression of stayabil-ity from the second to the sixth calving is mainly affected by genetic factors related to reproductive performance, and nervous and immune systems. At the seventh and eighth calvings, genes and pathways related to animal health, such as density bone and cancer, might be more relevant.
Conclusion
Our results indicate that part of the target genomic regions in selecting for stayability at earlier ages (from the 2th to the 6th calving) would be different than selecting for this trait at later ages (7th and 8th calvings). While the expression of stayability at earlier ages appeared to be more influenced by genetic factors linked to reproductive performance together with an overall health/immunity, at later ages genetic factors related to an overall animal health gain relevance. These results support that selecting for stayability at earlier ages (perhaps at the second calving) could be applied, having practical implications in breeding programs since it could drastically reduce the generation interval, accelerating the genetic progress.
This is a reprint of articles from the Special Issue published online in the open access journal Journal of Clinical Medicine (ISSN 2077-0383) (available at: https://www.mdpi.com/journal/jcm/ special issues/scoliosis spinal disorders).
Preface to ”Advances on Scoliogeny, Diagnosis and Management of Scoliosis and Spinal Disorders” Scoliosis, a 3-D deformity of the spine and the thorax, mainly affects children, who are the future of any society. The medical societies that specialize in this ailment have recently focused intensely on the study of the epidemiology, etiology, pathobiomechanics and laboratory work, in addition to
clinical and imaging documentation and treatment, either non-operative or operative. The advent of new technologies is key in the study and advancement of our insight into these diseases, with the aim of improving the quality of life of those who live with the condition. Our ultimate goal is to diminish or even eliminate the disease.
It is interesting to note the impressive developments in the implementation of growth
modulation for the surgical treatment of early-onset scoliosis. These developments have led to better patient quality of life compared to what was experienced in the past. However, this topic is still under development and new instrumentation systems are being introduced. When proper management is not implemented, spinal disorders may lead to signicant social problems and enormous economic burdens. Therefore, treatment decisions based on the recent evidence-based literature will result in the optimum outcomes. Proper management, including prevention and treament, whether operative or not, must be tailored and implemented. Therefore, it is very important to increase awareness and advocacy for a social mission regarding
the early detection of scoliosis and prevention of progressive spinal deformity. It is imperative to raise awareness about scoliosis and to inform the public, as well as the healthcare and policy-making communities, about the individual, familial and societal burdens of spinal deformity, as well as the benets of proper detection, diagnosis and optimal care for all patients. This Special Issue and its papers aim to serve the above-mentioned objectives.
Theodoros B. Grivas
Editor
1. Valgus-varus deformity (VVD) is a common leg bone problem in broilers that causes serious economic losses to the breeding industry. The genetic etiology of VVD is not clear, which restricts the genetic control of VVD.
2. In this study, knee cartilage of 35-day-old VVD and normal broilers was sequenced by whole-genome bisulphite sequencing (WGBS). The unique whole-genome DNA methylation profile of VVD broilers was described, and the methylation data and transcription data were used for joint analysis.
3. The mean methylation level of the VVD group was greater than that in the normal group. A total of 4315 differentially methylated regions (DMRs) were detected from methylation data, with the highest DMR density on chromosomes 25, 27, 31 and 33. DMRs were mainly located in introns, which accounted for more than 60%, followed by promoter and exon regions.
4. A total of 2326 differentially methylated genes (DMGs) were identified from DMRs, including 1159 genes with upregulated DMRs, 936 genes with downregulated DMRs, and 231 genes with two types of DMRs.
5. The ESPL1 gene may be an important epigenetic gene of VVD. The methylation of particular CpG17, CpG18 and CpG19 sites in the promoter region of the ESPL1 gene may hinder the binding of transcription factors and promoters and increase the expression of ESPL1.
This article has been withdrawn at the request of the Author. The Author apologizes for any inconvenience this may cause.
The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.
Osteoporosis, characterised by low bone mass, poor bone structure, and an increased risk of fracture, is a major public health problem. There is increasing evidence that the influence of the environment on gene expression, through epigenetic processes, contributes to variation in BMD and fracture risk across the lifecourse. Such epigenetic processes include DNA methylation, histone and chromatin modifications and non-coding RNAs. Examples of associations with phenotype include DNA methylation in utero linked to maternal vitamin D status, and to methylation of target genes such as OPG and RANKL being associated with osteoporosis in later life. Epigenome-wide association studies and multi-omics technologies have further revealed susceptibility loci, and histone acetyltransferases, deacetylases and methylases are being considered as therapeutic targets. This review encompasses recent advances in our understanding of epigenetic mechanisms in the regulation of bone mass and osteoporosis development, and outlines possible diagnostic and prognostic biomarker applications.
Objective: The aim of this study was to investigate the effect of low-level erbium-doped yttrium aluminum garnet (Er:YAG) laser irradiation on gene expression in osteogenic cells from rat calvariae. Background: Previous studies showed beneficial effects of laser irradiation on bone-related cells. However, few studies have examined the gene expression alteration by laser irradiation on osteogenic cells in a calcified condition. Materials and methods: Osteogenic cells were prepared by culturing rat calvarial osteoblast-like cells in osteoinductive medium for 21 days. The cells at the bottom of the culture dish were irradiated with Er:YAG laser (wavelength: 2.94 μm, energy density: 3.1 and 8.2 J/cm2) positioned at distance of 25 cm. Lactate dehydrogenase (LDH) assay of the irradiated cells was performed. After screening for genes related to bone formation, mechanotransduction, and thermal effect by quantitative polymerase chain reaction (qPCR), gene expression at 3 h after 3.1 J/cm2 irradiation was comprehensively analyzed using microarray. Results: No dramatical increase in surface temperature and LDH activities after laser irradiation were observed. Sost expression was significantly reduced at 3 h after 3.1 J/cm2 irradiation. Bcar1 and Hspa1a expression was significantly increased following 8.2 J/cm2 irradiation. Microarray analysis identified 116 differentially expressed genes. Gene set enrichment analysis showed enrichment of histone H3-K9 methylation and modification gene sets. Conclusions: Er:YAG laser irradiation, especially at 3.1 J/cm2, showed positive effect on the expression of genes related to bone formation in osteogenic cells, without inducing significant cell damage. These findings may represent critical mechanisms of early bone formation after Er:YAG laser irradiation.
BACKGROUND
Periodontitis is a highly prevalent inflammatory disease affecting the periodontium that results from an imbalance between periodontopathogens and host mechanisms. Continuous progression of the disease may lead to tissue and bone destruction, eventually resulting in tooth loss. The extent of bone loss depends on the dysregulated host immune response. Various host-elicited molecules play a major role in disease progression. The discovery of the glycoprotein sclerostin and its role as a regulator of bone mass has led to a better understanding of bone metabolism.
HIGHLIGHT
Sclerostin, which is primarily expressed by osteocytes, is a negative regulator of bone formation. It is a potent antagonist of the canonical Wingless-related integration site (Wnt) pathway, which is actively involved in bone homeostasis. Sclerostin is known to stimulate bone resorption by altering the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa- β ligand (RANKL) balance. Additionally, in periodontitis, activation of the inflammatory cascade also increases the synthesis of sclerostin.
CONCLUSION
The recently discovered sclerostin antibody has emerged as a positive therapeutic tool for the treatment of metabolic bone diseases. It has been reported to improve bone strength, bone formation, osseointegration around implants and lower the risk of bone fractures in various animal and human models. This review describes the properties and action of sclerostin, its role in periodontal diseases, and the advent and efficacy of sclerostin antibodies.
Introduction:
Cell-free DNA (cfDNA) methylation is an important molecular biomarker, which provides information about the regulation of gene expression in the tissue of origin. There is an inverse correlation between SOST gene methylation and expression levels.
Methods:
We analyzed SOST promoter methylation in cfDNA from serum, and compared it with DNA from blood and bone cells from patients undergoing hip replacement surgery. We also measured cfDNA methylation in 28 osteoporotic patients at baseline and after 6 months of antiosteoporotic therapy (alendronate, teriparatide, or denosumab).
Results:
SOST gene promoter methylation levels in serum cfDNA were very similar to those of bone-derived DNA (79% ± 12% and 82% ± 7%, respectively), but lower than methylation levels in blood cell DNA (87% ± 10%). Furthermore, there was a positive correlation between an individual's SOST DNA methylation values in serum and bone. No differences in either serum sclerostin levels or SOST methylation were found after 6-months of therapy with antiosteoporotic drugs.
Conclusions:
Our results suggest that serum cfDNA does not originate from blood cells, but rather from bone. However, since we did not confirm changes in this marker after therapy with bone-active drugs, further studies examining the correlation between bone changes of SOST expression and SOST methylation in cfDNA are needed to confirm its potential role as a bone biomarker.
Periprosthetic osteolysis (PIO) caused by wear particles is the main cause of implant failure, which is regulated by nuclear factor κ B receptor activator ligand (RANKL)/osteoprotegerin (OPG) system. At present, there is a lack of effective drugs to prevent or treat PIO. Previous studies have confirmed that DNA methylation is closely related to postmenopausal osteoporosis and can affect the expression of OPG and RANKL. However, the relationship between DNA methylation and PIO is not clear. In this study, we investigated the inhibitory effect of 5-Aza-2-deoxycytidine (AzadC) on osteolysis induced by titanium particles in a mouse model. This inhibition mechanism is achieved by changing the ratio of RANKL/OPG in the osteolysis model. In conclusion, there is a relationship between DNA methylation and PIO. AzadC has a certain inhibitory effect on osteolysis induced by titanium particles. Regulating DNA methylation may be a new way to treat PIO. Our findings lay a foundation for epigenetic understanding and intervention of osteolysis.
Altered DNA methylation upon ageing may result in many age‐related diseases such as osteoporosis. However, the changes in DNA methylation that occur in cortical bones, the major osteocytic areas, remain unknown. In our study, we extracted total DNA and RNA from the cortical bones of 6‐month‐old and 24‐month‐old mice and systematically analysed the differentially methylated regions (DMRs), differentially methylated promoters (DMPs) and differentially expressed genes (DEGs) between the mouse groups. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the DMR‐related genes revealed that they were mainly associated with metabolic signalling pathways, including glycolysis, fatty acid and amino acid metabolism. Other genes with DMRs were related to signalling pathways that regulate the growth and development of cells, including the PI3K‐AKT, Ras and Rap1 signalling pathways. The gene expression profiles indicated that the DEGs were mainly involved in metabolic pathways and the PI3K‐AKT signalling pathway, and the profiles were verified through real‐time quantitative PCR ( RT‐qPCR). Due to the pivotal roles of the affected genes in maintaining bone homeostasis, we suspect that these changes may be key factors in age‐related bone loss, either together or individually. Our study may provide a novel perspective for understanding the osteocyte and its relationship with osteoporosis during ageing.
Significance of the study
Our study identified age‐related changes in gene expressions in osteocytic areas through whole‐genome bisulfite sequencing (WGBS) and RNA‐seq, providing new theoretical foundations for the targeted treatment of senile osteoporosis.
Adequate and continuous expression of genes lies beneath all cellular activities. Due to that, the complex and dynamic network of gene regulation is a critical component of the cellular homeostasis, often impaired in multiple pathologies. Epigenetics is a growing field of biomedical research that investigates mechanisms underlying the ability of cells to inherit certain means of gene regulation independently of the genetic sequences. A growing body of evidence accumulated over the last years has connected the suboptimal activity of these molecular mechanisms to the process of aging, which characterizes all living cells and organisms. This chapter discusses the historical and current nuances of epigenetic research and focuses on the major aspects of epigenetic changes in aging. It further reviews the emerging involvement of impaired epigenetic regulation in age-related pathologies of muscles and bones. Particular emphasis is put on the latest discoveries in the field in order to acquaint the readers with cutting-edge research related to these topics.
Introducción
Varios estudios de barrido genómico (GWAS) y otros focalizados en el gen de la esclerostina (SOST) han encontrado que algunos polimorfismos de SOST se asocian con la masa ósea y el riesgo de fracturas. El objetivo de este estudio fue analizar la relevancia funcional de ciertos polimorfismos de la región promotora de SOST, en relación con la expresión y la metilación de dicho gen
Material y método
Para ello, se determinaron los alelos de los polimorfismos rs851054, rs851056, rs10534024, rs1234612 y se analizó la metilación de ADN de 33 muestras de suero y de hueso, procedentes de pacientes intervenidos para colocar una prótesis de cadera, mediante pirosecuenciación tras conversión con bisulfito. Además, en el hueso se estudió la expresión de SOST. Por último, se clonaron diferentes alelos del promotor de SOST en vectores reporteros dobles con el gen de la luciferasa bajo dicho promotor y el gen de la fosfatasa alcalina bajo un promotor constitutivo.
Resultados
El análisis de metilación de la región promotora de SOST en ADN libre en suero y en ADN de hueso no reveló diferencias estadísticamente significativas en relación con los alelos de los polimorfismos analizados (p>0,05). Sin embargo, las transfecciones con los vectores reporteros mostraron una elevada actividad transcripcional, independiente mente del vector utilizado.
Conclusión
No hemos encontrado una asociación clara entre los distintos alelos y la metilación de ADN de la región promotora del gen SOST. Son necesarios más estudios para determinar los efectos funcionales de los polimorfismos sobre la metilación y expresión del gen de SOST y los efectos sobre la masa ósea.
Exciting discoveries over the last years have propelled osteocytes, originally considered passive and metabolically inactive bone cells, to the category of master regulators of bone homeostasis. Osteocytes differentiate from osteoblasts when they become surrounded by matrix during the process of bone formation. Osteoblast to osteocyte differentiation is complex and involves profound modifications in gene expression that result in morphological changes and transform osteocytes in dynamic and multifunctional cells. In addition to the traditional role of osteocytes in the integration of mechanical signals, new osteocytic functions are emerging. Osteocytes are now considered a major source of molecules that coordinate the activity of osteoclasts and osteoblasts in response to both physical and hormonal cues. In addition, accumulating evidence supports the notion that dysregulation of osteocyte function underlies the pathophysiology of several skeletal disorders, ranging from rare to common diseases such as osteoporosis. Further, the increased understanding of osteocyte biology has led to the development of therapeutic approaches targeting osteocytes and their derived factors. In this chapter, we summarize the current knowledge on osteocyte biology and its different functions and discuss novel observations that support the role of osteocytes as endocrine regulators of body composition and energy metabolism and as key players in the deleterious effects of cancer and diabetes on bone.
Background:
Our study aimed to explore more mechanistic insights into the epigenetic regulation of osteoarthritis (OA).
Methods:
The expression profiles (accession number: GSE64393 and GSE64394) were downloaded from the Gene Expression Omnibus database. The differentially hydroxymethylated regions (DhMRs) and differentially expressed genes (DEGs) between OA and control groups were identified. The distribution of DhMRs in the whole genome and the correlation between DhMRs and DEGs were analyzed. Functional module mining for the DEGs and DhMRs was conducted, followed by protein-protein interaction (PPI) analysis. The transcriptional factor (TF) was predicted.
Results:
Total 52,282 DhMRs were obtained, among which 31,452 ones were annotated to 9726 genes. Additionally, 1806 DEGs were selected. Hydroxymethylation mainly occurred in gene body region. Correlation analysis revealed that more than 70% of DhMRs were uncorrelated with DEGs expression. Functional module mining for the DEGs and DhMRs identified 2 functional modules, which were involved in pathways of regulation of actin cytoskeleton, and TGF-β signaling pathway. A PPI network was constructed, and ITGB3 had the highest degree. Furthermore, 7 TFs were predicted, which regulated 12 candidate genes, such as HES1-PTEN.
Conclusions:
The onset and progression of OA may be associated with the upregulated hydroxymethylation in gene body region of PTEN. HES1 may be important TF in the pathogenesis of OA. Additionally, pathways of regulation of actin cytoskeleton, and TGF-beta signaling pathway may also play important roles in OA progression.
El tejido óseo difiere de otros tejidos conjuntivos en su capacidad de mineralizarse a sí mismo formando cristales de hidroxiapatita. Esto le confiere una propiedad de protección y resistencia al estrés mecánico, pero también se convierte en un reservorio de calcio y fosfato para el organismo. Los intercambios entre el tejido óseo y el organismo están controlados por hormonas calciótropas como la parathormona, la vitamina D y la calcitonina, pero también por el factor de crecimiento de los fibroblastos 23 (FGF23) para el fosfato. El tejido óseo también incluye la hematopoyesis, que proporciona las diversas células necesarias para la defensa del organismo, así como las células precursoras implicadas en la remodelación ósea, como los osteoclastos que reabsorben el hueso viejo y los osteoblastos que producen nuevo tejido óseo. Estas células se activan movilizando factores de transcripción que guían la diferenciación de las células madre en células óseas, como Runx2 para los osteoblastos. La activación de la vía del receptor activador de NF-κB/ligando/osteoprotegerina (RANK/RANKL/OPG) moviliza a los osteoclastos y regula el acoplamiento con la acción de los osteoblastos. La vía Wnt/LRP5 controla el nivel de formación ósea de los osteoblastos. También hay muchas interacciones entre el tejido óseo y el resto del organismo, como la regulación del apetito y la función sexual con la leptina o la participación en la regulación de las células pancreáticas con la osteocalcina. El osteocito desempeña un papel importante tanto en la regulación de la adaptación de la remodelación ósea a las fuerzas mecánicas como en la homeostasis de los fosfatos mediante la secreción de FGF23, una hormona fosfatúrica. La remodelación ósea también está bajo el control del sistema neurovascular, cuyo papel principal está en constante crecimiento. Una mejor comprensión de estos mecanismos que regulan la remodelación ósea y sus interacciones con otros órganos ofrece dianas terapéuticas prometedoras en perspectiva.
Environmental influences, including social-related factors, determine, along genetic factors and random events, the risk of many diseases, such as cancer, neurological disorders, and skeletal disorders. Environmental factors may influence the phenotype and disease risk by modifying the epigenome, both in utero and after birth. In particular, several lines of evidence suggest that factors related to the socioeconomic status impact the epigenome. The factors leading that influence are unclear but may include nutrition, life habits, working conditions, and other ambient exposures, as well as psychological factors. Social deprivation is associated with low-grade inflammation and activation of the stress response. Changes in gene promoter methylation and other epigenetic marks may be involved in this phenomenon. Those changes likely have a negative impact in bone homeostasis and may be involved in the increased risk of osteoporosis observed in population groups of lower social status. Although the transgenerational transmission of epigenetic marks in mammals is unclear, the capability of environmental influences to determine the epigenome of the exposed individual and his/her offspring underscores the need of building the healthiest possible physical and psychological environment.
El tejido óseo se diferencia de los demás tejidos conjuntivos por su capacidad de mineralizarse formando cristales de hidroxiapatita. Esto le confiere una propiedad de protección y resistencia a las presiones mecánicas, pero también es un reservorio de calcio y de fosfato para el organismo. Los intercambios entre el tejido óseo y el organismo están controlados por hormonas calciótropas como la parathormona, la vitamina D y la calcitonina, pero también por el FGF23 para el fosfato. El tejido óseo también alberga la hematopoyesis, que proporciona las diferentes células necesarias para la defensa del organismo, así como las células precursoras implicadas en la remodelación ósea, como los osteoclastos, que reabsorben el hueso viejo, y los osteoblastos, que fabrican tejido óseo nuevo. Estas células entran en acción gracias a la movilización de factores de transcripción que orientan la diferenciación de células madre hacia las células óseas, como Runx2 para los osteoblastos. La activación de la vía RANK/RANKL/osteoprotegerina moviliza los osteoclastos y regula el acoplamiento con la acción de los osteoblastos. La vía Wnt/LRP5 controla el grado de formación ósea de los osteoblastos. Además, existen numerosas interacciones entre el tejido óseo y el resto del organismo, como la regulación del apetito y de la función sexual con la leptina o la participación en la regulación de las células pancreáticas con la osteocalcina. El osteocito desempeña un papel esencial a la vez en la regulación de la adaptación de la remodelación ósea a las presiones mecánicas y en la homeostasis del fosfato, a través de la secreción de FGF23, hormona fosfatúrica. La remodelación ósea también está controlada por el sistema neurovascular, cuyo importante papel no deja de aumentar. La mejor comprensión de estos mecanismos que regulan la remodelación ósea y sus interacciones con los demás órganos ofrece buenas dianas terapéuticas en perspectiva.
Unlabelled:
Expression of the osteocyte-derived bone formation inhibitor sclerostin in adult bone requires a distant enhancer. We show that MEF2 transcription factors control this enhancer and mediate inhibition of sclerostin expression by PTH.
Introduction:
Sclerostin encoded by the SOST gene is a key regulator of bone formation. Lack of SOST expression is the cause for the progressive bone overgrowth disorders sclerosteosis and Van Buchem disease. We have previously identified a distant enhancer within the 52-kb Van Buchem disease deletion downstream of the SOST gene that is essential for its expression in adult bone. Furthermore, we and others have reported that SOST expression is suppressed by PTH. The aim of this study was to identify transcription factors involved in SOST bone enhancer activity and mediating PTH responsiveness.
Materials and methods:
Regulation of the SOST enhancer and promoter was studied by luciferase reporter gene assays. Transcription factor binding sites were mapped by footprint analysis and functional mutation analyses using transient transfections of osteoblast-like UMR-106 cells that exhibit endogenous SOST expression. Specific transcription factor binding was predicted by sequence analysis and shown by gel retardation assays and antibody-induced supershifts. Expression of myocyte enhancer factors 2 (MEF2) was detected by in situ hybridization, quantitative RT-PCR (qPCR), and immunohistochemistry. The role of MEF2s in SOST expression was assessed by reporter gene assays and siRNA-mediated RNA knockdown.
Results:
PTH completely suppressed the transcriptional activity of the SOST bone enhancer but did not affect the SOST promoter. A MEF2 response element was identified in the bone enhancer. It was essential for transcriptional activation, bound MEF2 transcription factors, and mediated PTH responsiveness. Expression of MEF2s in bone was shown by qPCR, in situ hybridization, and immunohistochemistry. MEF2s and sclerostin co-localized in osteocytes. Enhancer activity was stimulated by MEF2C overexpression and inhibited by co-expression of a dominant negative MEF2C mutant. Finally, siRNA-mediated knockdown of MEF2A, C, and D suppressed endogenous SOST expression in UMR-106 osteoblast-like cells.
Conclusions:
These data strongly suggest that SOST expression in osteocytes of adult bone and its inhibition by PTH is mediated by MEF2A, C, and D transcription factors controlling the SOST bone enhancer. Hence, MEF2s are implicated in the regulation of adult bone mass.
The last decade has provided a virtual explosion of data on the molecular biology and function of osteocytes. Far from being the passive placeholder in bone, this cell has been found to have numerous functions, such as acting as an orchestrator of bone remodeling through regulation of both osteoclast and osteoblast activity and also functioning as an endocrine cell. The osteocyte is a source of soluble factors not only to target cells on the bone surface but also to target distant organs, such as kidney, muscle, and other tissues. This cell plays a role in both phosphate metabolism and calcium availability and can remodel its perilacunar matrix. Osteocytes compose 90% to 95% of all bone cells in adult bone and are the longest lived bone cell, up to decades within their mineralized environment. As we age, these cells die, leaving behind empty lacunae that frequently micropetrose. In aged bone such as osteonecrotic bone, empty lacunae are associated with reduced remodeling. Inflammatory factors such as
Sclerostin is a product of mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation. While evidence suggests that sclerostin has an anti-anabolic role, the possibility also exists that sclerostin has catabolic activity. To test this we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin, or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated the expression of receptor activator of nuclear factor kappa B (RANKL) mRNA and down-regulated that of osteoprotegerin (OPG) mRNA, causing an increase in the RANK:OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold) compared to untreated co-cultures. The increased resorption was abolished by co-addition of recombinant OPG. In co-cultures of MLO-Y4 cells with PBMC, SCL also increased the number and size of the TRAP-positive multinucleated cells formed. Importantly, rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Further, rhSCL did not induce apoptosis of MLO-Y4 cells, as determined by caspase activity assays, demonstrating that the osteoclastic response was not driven by dying osteocytes. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes, in a RANKL-dependent manner.
Osteocytes play a central role in the regulation of bone remodeling. The aim of this study was to explore osteocyte function, and particularly the expression of SOST, a Wnt inhibitor, in patients with hip fractures. Serum sclerostin levels were measured by ELISA. The expression of several osteocytic genes was studied by quantitative PCR in trabecular samples of the femoral head of patients with hip fractures, hip osteoarthritis and control subjects. The presence of sclerostin protein and activated caspase 3 was revealed by immunostaining. There were no significant differences in serum sclerostin between the three groups. Patients with fractures have fewer lacunae occupied by osteocytes (60 ± 5% vs. 64 ± 6% in control subjects, P = 0.014) and higher numbers of osteocytes expressing activated caspase 3, a marker of apoptosis. The proportion of sclerostin-positive lacunae was lower in patients with fractures than in control subjects (34 ± 11% vs. 69 ± 10%, P = 2 × 10(-8)). The proportion of sclerostin-positive osteocytes was also lower in patients. RNA transcripts of SOST, FGF23 and PHEX were also less abundant in fractures than in control bones (P = 0.002, 5 × 10(-6), and 0.04, respectively). On the contrary, in patients with osteoarthritis, there was a decreased expression of SOST and FGF23, without differences in PHEX transcripts or osteocyte numbers. Osteocyte activity is altered in patients with hip fractures, with increased osteocyte apoptosis and reduced osteocyte numbers, as well as decreased transcription of osteocytic genes. Therefore, these results suggest that an osteocyte deficiency may play a role in the propensity to hip fractures.
Humans lacking sclerostin display progressive bone overgrowth due to increased bone formation. Although it is well established that sclerostin is an osteocyte-secreted bone formation inhibitor, the underlying molecular mechanisms are not fully elucidated. We identified in tandem affinity purification proteomics screens LRP4 (low density lipoprotein-related protein 4) as a sclerostin interaction partner. Biochemical assays with recombinant proteins confirmed that sclerostin LRP4 interaction is direct. Interestingly, in vitro overexpression and RNAi-mediated knockdown experiments revealed that LRP4 specifically facilitates the previously described inhibitory action of sclerostin on Wnt1/β-catenin signaling. We found the extracellular β-propeller structured domain of LRP4 to be required for this sclerostin facilitator activity. Immunohistochemistry demonstrated that LRP4 protein is present in human and rodent osteoblasts and osteocytes, both presumed target cells of sclerostin action. Silencing of LRP4 by lentivirus-mediated shRNA delivery blocked sclerostin inhibitory action on in vitro bone mineralization. Notably, we identified two mutations in LRP4 (R1170W and W1186S) in patients suffering from bone overgrowth. We found that these mutations impair LRP4 interaction with sclerostin and its concomitant sclerostin facilitator effect. Together these data indicate that the interaction of sclerostin with LRP4 is required to mediate the inhibitory function of sclerostin on bone formation, thus identifying a novel role for LRP4 in bone.
The Wnt signaling pathway is a robust regulator of skeletal homeostasis. Gain-of-function mutations promote high bone mass, whereas loss of Lrp5 or Lrp6 co-receptors decrease bone mass. Similarly, mutations in antagonists of Wnt signaling influence skeletal integrity, in an inverse relation to Lrp receptor mutations. Loss of the Wnt antagonist Sclerostin (Sost) produces the generalized skeletal hyperostotic condition of sclerosteosis, which is characterized by increased bone mass and density due to hyperactive osteoblast function. Here we demonstrate that prostaglandin E(2) (PGE(2)), a paracrine factor with pleiotropic effects on osteoblasts and osteoclasts, decreases Sclerostin expression in osteoblastic UMR106.01 cells. Decreased Sost expression correlates with increased expression of Wnt/TCF target genes Axin2 and Tcf3. We also show that the suppressive effect of PGE(2) is mediated through a cyclic AMP/PKA pathway. Furthermore, selective agonists for the PGE(2) receptor EP2 mimic the effect of PGE(2) upon Sost, and siRNA reduction in Ptger2 prevents PGE(2)-induced Sost repression. These results indicate a functional relationship between prostaglandins and the Wnt/β-catenin signaling pathway in bone.
Understanding the mechanisms regulating expression of retinal ganglion cell (RGC) specific and axon-guidance genes during development and in retinal stem cells will be critical for successful optic nerve regeneration. Müller glia have some characteristics of retinal stem cells but in mammals have demonstrated limited potential to differentiate into RGCs. Chromatin remodeling through histone deacetylation and DNA methylation are a potential mechanism for silencing genes necessary for neuronal differentiation of glial cells. We investigated DNA methylation as a mechanism for regulating expression of mouse EphA5, one member of a large family of ephrin receptor genes that regulate patterning of the topographic connections of RGCs during visual system development. We analyzed spatial and age-related patterns of EphA5 promoter methylation by bisulfite sequencing and mRNA expression by quantitative RT-PCR in the mouse retina. The CpG island in the EphA5 promoter was hypomethylated in the retina and showed no change in overall methylation with age, despite a decline in EphA5 mRNA expression levels in the adult retina. In the nasal retina of post-natal day 0 mice, there was a modest, but statistically significant increase in methylation. Increased methylation corresponded with lower levels of receptor mRNA expression in the nasal retina. We cloned the EphA5 promoter and found that site-specific differences in methylation could preferentially activate or repress promoter activity in transient transfections of rat retinal progenitor cells (R28) using luciferase assays. In sphere cultures generated by EGF/FGF2 stimulation of conditionally immortalized mouse Müller glia (ImM10), EphA5 promoter was hypermethylated and EphA5 mRNA was not detected. Demethylation using 5-azadeoxycytidine (AzadC) resulted in a significant decrease of methylation of the EphA5 promoter and re-expression of the EphA5 mRNA. The inverse relationship between EphA5 promoter methylation and mRNA expression is consistent with a role for DNA methylation in modulating the spatial patterns of EphA5 gene expression in the retina and in silencing EphA5 expression in ImM10 cells. The robust up-regulation of EphA5 in ImM10 cells following demethylation suggests that modulation of chromatin structure may be a useful approach for promoting expression of silenced developmental genes and increasing the neurogenic potential of Müller glia.
Epigenetic regulation of gene expression occurs due to alterations in chromatin proteins that do not change DNA sequence, but alter the chromatin architecture and the accessibility of genes, resulting in changes to gene expression that are preserved during cell division. Through this process genes are switched on or off in a more durable fashion than other transient mechanisms of gene regulation, such as transcription factors. Thus, epigenetics is central to cellular differentiation and stem cell linage commitment. One such mechanism is DNA methylation, which is associated with gene silencing and is involved in a cell's progression towards a specific fate. Mechanical signals are a crucial regulator of stem cell behavior and important in tissue differentiation; however, there has been no demonstration of a mechanism whereby mechanics can affect gene regulation at the epigenetic level. In this study, we identified candidate DNA methylation sites in the promoter regions of three osteogenic genes from bone marrow derived mesenchymal stem cells (MSCs). We demonstrate that mechanical stimulation alters their epigenetic state by reducing DNA methylation and show an associated increase in expression. We contrast these results with biochemically induced differentiation and distinguish expression changes associated with durable epigenetic regulation from those likely to be due to transient changes in regulation. This is an important advance in stem cell mechanobiology as it is the first demonstration of a mechanism by which the mechanical micro-environment is able to induce epigenetic changes that control osteogenic cell fate, and that can be passed to daughter cells. This is a first step to understanding that will be vital to successful bone tissue engineering and regenerative medicine, where continued expression of a desired long-term phenotype is crucial.
The bone morphogenetic protein (BMP) and Wnt signaling pathways both contribute essential roles in regulating bone mass. However, the molecular interactions between these pathways in osteoblasts are poorly understood. We recently reported that osteoblast-targeted conditional knockout (cKO) of BMP receptor type IA (BMPRIA) resulted in increased bone mass during embryonic development, where diminished expression of Sost as a downstream effector of BMPRIA resulted in increased Wnt/beta-catenin signaling. Here, we report that Bmpr1a cKO mice exhibit increased bone mass during weanling stages, again with evidence of enhanced Wnt/beta-catenin signaling as assessed by Wnt reporter TOPGAL mice and TOPFLASH luciferase. Consistent with negative regulation of the Wnt pathway by BMPRIA signaling, treatment of osteoblasts with dorsomorphin, an inhibitor of Smad-dependent BMP signaling, enhanced Wnt signaling. In addition to Sost, Wnt inhibitor Dkk1 also was downregulated in cKO bone. Expression levels of Dkk1and Sost were upregulated by BMP2 treatment and downregulated by Noggin. Moreover, expression of a constitutively active Bmpr1a transgene in mice resulted in the upregulation of both Dkk1 and Sost and partially rescued the Bmpr1a cKO bone phenotype. These effectors are differentially regulated by mitogen-activated protein kinase (MAPK) p38 because pretreatment of osteoblasts with SB202190 blocked BMP2-induced Dkk1 expression but not Sost. These results demonstrate that BMPRIA in osteoblasts negatively regulates endogenous bone mass and Wnt/beta-catenin signaling and that this regulation may be mediated by the activities of Sost and Dkk1. This study highlights several interactions between BMP and Wnt signaling cascades in osteoblasts that may be amenable to therapeutic intervention for the modification of bone mass density.
Periostin (gene Postn) is a secreted extracellular matrix protein involved in cell recruitment and adhesion and plays an important role in odontogenesis. In bone, periostin is preferentially expressed in the periosteum, but its functional significance remains unclear. We investigated Postn(-/-) mice and their wild type littermates to elucidate the role of periostin in the skeletal response to moderate physical activity and direct axial compression of the tibia. Furthermore, we administered a sclerostin-blocking antibody to these mice in order to demonstrate the influence of sustained Sost expression in their altered bone phenotypes. Cancellous and cortical bone microarchitecture as well as bending strength were altered in Postn(-/-) compared with Postn(+/+) mice. Exercise and axial compression both significantly increased bone mineral density and trabecular and cortical microarchitecture as well as biomechanical properties of the long bones in Postn(+/+) mice by increasing the bone formation activity, particularly at the periosteum. These changes correlated with an increase of periostin expression and a consecutive decrease of Sost in the stimulated bones. In contrast, mechanical stimuli had no effect on the skeletal properties of Postn(-/-) mice, where base-line expression of Sost levels were higher than Postn(+/+) and remained unchanged following axial compression. In turn, the concomitant injection of sclerostin-blocking antibody rescued the bone biomechanical response in Postn(-/-) mice. Taken together, these results indicate that the matricellular periostin protein is required for Sost inhibition and thereby plays an important role in the determination of bone mass and microstructural in response to loading.
Epigenetic modifications at the histone level affect gene regulation in response to extracellular signals. However, regulated epigenetic modifications at the DNA level, especially active DNA demethylation, in gene activation are not well understood. Here we report that DNA methylation/demethylation is hormonally switched to control transcription of the cytochrome p450 27B1 (CYP27B1) gene. Reflecting vitamin-D-mediated transrepression of the CYP27B1 gene by the negative vitamin D response element (nVDRE), methylation of CpG sites ((5m)CpG) is induced by vitamin D in this gene promoter. Conversely, treatment with parathyroid hormone, a hormone known to activate the CYP27B1 gene, induces active demethylation of the (5m)CpG sites in this promoter. Biochemical purification of a complex associated with the nVDRE-binding protein (VDIR, also known as TCF3) identified two DNA methyltransferases, DNMT1 and DNMT3B, for methylation of CpG sites, as well as a DNA glycosylase, MBD4 (ref. 10). Protein-kinase-C-phosphorylated MBD4 by parathyroid hormone stimulation promotes incision of methylated DNA through glycosylase activity, and a base-excision repair process seems to complete DNA demethylation in the MBD4-bound promoter. Such parathyroid-hormone-induced DNA demethylation and subsequent transcriptional derepression are impaired in Mbd4(-/-) mice. Thus, the present findings suggest that methylation switching at the DNA level contributes to the hormonal control of transcription.
Cell-free circulating DNA isolated from the plasma of individuals with cancer has been shown to harbor cancer-associated changes in DNA methylation, and thus it represents an attractive target for biomarker discovery. However, the reliable detection of DNA methylation changes in body fluids has proven to be technically challenging. Here we describe a novel combination of methods that allows quantitative and sensitive detection of DNA methylation in minute amounts of DNA present in body fluids (quantitative Methylation Analysis of Minute DNA amounts after whole Bisulfitome Amplification, qMAMBA). This method involves genome-wide amplification of bisulphite-modified DNA template followed by quantitative methylation detection using pyrosequencing and allows analysis of multiple genes from a small amount of starting DNA. To validate our method we used qMAMBA assays for four genes and LINE1 repetitive sequences combined with plasma DNA samples as a model system. qMAMBA offered high efficacy in the analysis of methylation levels and patterns in plasma samples with extremely small amounts of DNA and low concentrations of methylated alleles. Therefore, qMAMBA will facilitate methylation studies aiming to discover epigenetic biomarkers, and should prove particularly valuable in profiling a large sample series of body fluids from molecular epidemiology studies as well as in tracking disease in early diagnostics.
Differentiation and malignant transformation of stem cells are regulated by epigenetic mechanisms. We analyzed promoter methylation and expression of the stem cell determining genes Brachyury, DPPA5, FGF4, FOXD3, LIN28, NESTIN and ZFP42 depending on the differentiation state in human mesenchymal stem cells (MSC), human embryonal carcinoma cells (ECC) and somatic tumor cells. Differentiation of MSC into osteoblasts and adipocytes was accompanied with a loss of expression of the Brachyury gene and downregulation of LIN28. Inactivation of Brachyury was associated with progressive methylation of its CpG island promoter. In ECC promoter methylation of stem cell markers was more frequent in the differentiated subgroup (71%) compared to undifferentiated ECC (29%) and this was associated with downregulation of Brachyury, DPPA5, FGF4, FOXD3, LIN28 and ZFP42. DPPA5 was methylated and NESTIN was unmethylated in most tumor cells. In somatic tumor cells, methylation of stem cell markers (Brachyury, DPPA5, FGF4, FOXD3, LIN28 and ZFP42) was frequently observed (85%). Treatment of cell lines with an inhibitor of DNA methyltransferase reactivated the expression of DPPA5, FGF4, FOXD3, LIN28 and ZFP42, indicating that aberrant promoter methylation is a crucial event that results in their silencing. Our results suggest that epigenetic inactivation of stem cell associated genes is mediated by promoter methylation and that this may represent a fundamental mechanism during normal differentiation processes.
Bone lining cells cover > 80% of endosteal surfaces of human cancellous bone. Current research assigns to them a dual role: (1) as a biological membrane regulating exchange of substrates between the bone fluid compartment and the extracellular fluid of bone marrow and (2) as a signaling link between the osteocytic network as mechanical receptor and the osteoclastic cell pool for local induction of bone resorption. Furthermore, a catabolic role has been considered. We therefore examined the presence of matrix-metalloproteinases (MMPs) and their physiological tissue inhibitors (TIMPs) as putative proteolytic elements. Firstly, human cancellous bone from 60 patients was examined by immunofluorescence with antibodies against MMPs and TIMPs. Secondly, we applied laser-assisted microdissection (LMD) to isolate bone lining cells from frozen sections of human trabecular bone. mRNA analysis was performed using a single-cell PCR protocol. Three laser microdissection systems were tested: the new generation of Leica LMD and P.A.L.M. laser pressure catapulting (LPC) were compared to P.A.L.M. laser microdissection and micromanipulation (LMM). In a few pooled cell profiles, mRNA of MMP13, MMP14, TIMP1, and CBFA-1 was clearly detected. By immunofluorescence MMP13 and -14 as well as TIMP1 and -2 were strongly present in lining cells, while MMP2, TIMP3, and TIMP4 showed weak or negative signals. Although the functional impact of these enzymatic components remains open, there is additional evidence for a catabolic function of lining cells. The new diode-laser microdissection with LMD and LPC proved to be especially suitable to gain new insights into the properties of bone lining cells.
The culture of osteoblast-like cells of human origin has become an important experimental model in bone biology. We report here a comparison and evaluation of three of the most widely used systems available today: bone marrow stroma cell cultures (BMSC), human osteoblast explant cultures (hOB) and osteoblast explant cultures from collagenase-treated bone (hOBcol). Cultures from 16 bone specimens obtained from various donors were established and their expression of the osteoblast phenotype were then compared in secondary cultures by use of biochemical markers. BMSC had the highest basal and 1,25-dihydroxyvitaminD3 (1,25(OH)2D3)-induced alkaline phosphatase activities in all cell isolations, with levels approximately twice those in explant cultures. Basal osteocalcin secretion was low-to-undetectable in all cell cultures but was detected in 1,25(OH)2D3-stimulated cultures. BMSC produced half of the amount of osteocalcin synthesized in explant cultures. The BMSC cultures also synthesized the lowest amounts of type I collagen, whereas collagen type III synthesis did not differ significantly among the various cultures. When secondary cultures were treated with 100 nM dexamethasone in the presence of ascorbic acid (50 microg/mL) and beta-glycerophosphate (10 mM), cultures deposited calcium mineral into the cell layer within 2-4 weeks. PTH-induced cAMP formation was detected in only 5 of 15 isolations and no consistent isolation-dependent response pattern was seen. We conclude that BMSC cultures differ significantly from explant cultures obtained from the same bone specimen. However, all cultures represent cells which can differentiate further and induce mineralization of the extracellular matrix in response to osteoinductive drugs.
Sclerosteosis is a progressive sclerosing bone dysplasia with an autosomal recessive mode of inheritance. Radiologically, it is characterized by a generalized hyperostosis and sclerosis leading to a markedly thickened and sclerotic skull, with mandible, ribs, clavicles and all long bones also being affected. Due to narrowing of the foramina of the cranial nerves, facial nerve palsy, hearing loss and atrophy of the optic nerves can occur. Sclerosteosis is clinically and radiologically very similar to van Buchem disease, mainly differentiated by hand malformations and a large stature in sclerosteosis patients. By linkage analysis in one extended van Buchem family and two consanguineous sclerosteosis families we previously mapped both disease genes to the same chromosomal 17q12-q21 region, supporting the hypothesis that both conditions are caused by mutations in the same gene. After reducing the disease critical region to approximately 1 Mb, we used the positional cloning strategy to identify the SOST gene, which is mutated in sclerosteosis patients. This new gene encodes a protein with a signal peptide for secretion and a cysteine-knot motif. Two nonsense mutations and one splice site mutation were identified in sclerosteosis patients, but no mutations were found in a fourth sclerosteosis patient nor in the patients from the van Buchem family. As the three disease-causing mutations lead to loss of function of the SOST protein resulting in the formation of massive amounts of normal bone throughout life, the physiological role of SOST is most likely the suppression of bone formation. Therefore, this gene might become an important tool in the development of therapeutic strategies for osteoporosis.
We have identified a mutation (−49C>T) in the low-density lipoprotein receptor (LDLR) gene in a Spanish familial hypercholesterolemia (FH) patient. The mutation maps within repeat 3 of the LDLR gene promoter. This region binds Sp1 and collaborates with repeat 2 in the regulation of LDLR gene by sterols. To evaluate whether the mutation influenced the activity of the promoter, luciferase reporter plasmids containing 296 bp of the proximal promoter region were constructed. In transient transfection assays in HepG2 cells, the mutation resulted in an 80% reduction of promoter activity. Also, gel-shift assays demonstrated that the mutation severely affects Sp1 binding. However, the mutated promoter still retains the ability to respond to low sterol concentrations. As the analysis of the LDLR gene did not reveal any other changes, we conclude that the −49C>T mutation is the cause of FH in the patient. The analysis of the proband's pedigree indicated that not all the members of the family having the mutation disclose a FH phenotype. These results support the view that factors other than the presence of the mutation are important in the determination of the clinical phenotype in FH.
—Mozas P., R. Galetto, M. Albajar, E. Ros, M. Pocoví, and J. C. Rodríguez-Rey. A mutation (−49C>T) in the promoter of the low density lipoprotein receptor gene associated with familial hypercholesterolemia. J. Lipid Res. 2002. 43: 13–18.
Loss-of-function mutations in the sclerosteosis gene (SOST) cause a rare sclerosing bone dysplasia characterized by skeletal overgrowth. Cbfa1/RUNX2 is a key transcriptional regulator
of osteoblast function. Here we link these two pathways by demonstrating, via gel shift and transient transfection analyses,
that Cbfa1 binding to the proximal SOST promoter contributes to differential SOST expression in two osteosarcoma cell lines. Additionally, an E-box binding motif
in the 1.8-kb proximal SOST promoter appears to be functional in SAOS-2 cells, but does not account for SAOS-specific expression of SOST. The regulation
of SOST expression by Cbfa1 suggests a potential role for the sclerosteosis gene in homeostatic regulation of osteoblast differentiation
and function. Furthermore, the juxtaposition of Cbfa1, E-box, and C/EBP binding sites in the SOST proximal promoter bears an intriguing resemblance to the promoter for osteocalcin, another osteoblast-specific gene with
a loss-of-function phenotype of bone overgrowth.
Sclerosteosis, a skeletal disorder characterized by high bone mass due to increased osteoblast activity, is caused by loss of the SOST gene product, sclerostin. The localization in bone and the mechanism of action of sclerostin are not yet known, but it has been hypothesized that it may act as a bone morphogenetic protein (BMP) antagonist. We show here that SOST/sclerostin is expressed exclusively by osteocytes in mouse and human bone and inhibits the differentiation and mineralization of murine preosteoblastic cells (KS483). Although sclerostin shares some of the actions of the BMP antagonist noggin, we show here that it also has actions distinctly different from it. In contrast to noggin, sclerostin did not inhibit basal alkaline phosphatase (ALP) activity in KS483 cells, nor did it antagonize BMP-stimulated ALP activity in mouse C2C12 cells. In addition, sclerostin had no effect on BMP-stimulated Smad phosphorylation and direct transcriptional activation of MSX-2 and BMP response element reporter constructs in KS483 cells. Its unique localization and action on osteoblasts suggest that sclerostin may be the previously proposed osteocyte-derived factor that is transported to osteoblasts at the bone surface and inhibits bone formation.
A null mutation in the SOST gene is associated with sclerosteosis, an inherited disorder characterized by a high bone mass phenotype. The protein product of the SOST gene, sclerostin, is a bone morphogenetic protein (BMP) antagonist that decreases osteoblast activity and reduces the differentiation of osteoprogenitors. We sought to delineate the mechanism by which sclerostin modulated osteoblastic function by examining the effects of the protein on differentiating cultures of human mesenchymal stem cells (hMSC). Sclerostin significantly decreased alkaline phosphatase (ALP) activity and the proliferation of hMSC cells. In addition, hMSC cells treated with sclerostin displayed a marked increase in caspase activity. Elevated levels of fragmented histone-associated DNA in these cells were detected by ELISA and by TUNEL staining. Other BMP antagonists including noggin, Chordin, Gremlin, and Twisted gastrulation did not affect caspase activity. The sclerostin-mediated increase in caspase activity was blocked by caspase-1 and caspase-3 inhibitors. Sclerostin-induced changes in ALP activity and the survival of hMSC cells were partially restored by BMP-6, suggesting the involvement of additional growth factors. These findings show that sclerostin selectively controls the apoptosis of bone cells. The ability of sclerostin to interact with important growth factors such as BMPs likely serves as the basis by which it modulates the survival of osteoblasts. By making these growth factors unavailable for cell function, sclerostin promotes the apoptosis of bone cells, providing a novel level of control in the regulation of bone formation.
The loss of the SOST gene product sclerostin leads to sclerosteosis characterized by high bone mass. In this report, we found that sclerostin could antagonize canonical Wnt signaling in human embryonic kidney A293T cells and mouse osteoblastic MC3T3 cells. This sclerostin-mediated antagonism could be reversed by overexpression of Wnt co-receptor low density lipoprotein receptor-related protein (LRP) 5. In addition, we found that sclerostin bound to LRP5 as well as LRP6 and identified the first two YWTD-EGF repeat domains of LRP5 as being responsible for the binding. Although these two repeat domains are required for transduction of canonical Wnt signals, canonical Wnt did not appear to compete with sclerostin for binding to LRP5. Examination of the expression of sclerostin and Wnt7b, an autocrine canonical Wnt, during primary calvarial osteoblast differentiation revealed that sclerostin is expressed at late stages of osteoblast differentiation coinciding with the expression of osteogenic marker osteocalcin and trailing after the expression of Wnt7b. Given the plethora of evidence indicating that canonical Wnt signaling stimulates osteogenesis, we believe that the high bone mass phenotype associated with the loss of sclerostin may be attributed, at least in part, to an increase in canonical Wnt signaling resulting from the reduction in sclerostin-mediated Wnt antagonism.
Intermittent parathyroid hormone (PTH) application is an established pharmacological principle to stimulate bone formation. Yet, the molecular mechanisms underlying this bone anabolic action are not fully understood. Recently, SOST (sclerostin) was identified as a potent osteocyte expressed negative regulator of bone formation in vitro, in murine models and in patients with the bone overgrowth disorders Sclerosteosis and Van Buchem disease. Therefore, we have studied the impact of PTH on SOST regulation. First, we analyzed SOST expression during PTH-induced bone formation in a classical model of local bone formation. 8-month-old mice received intermittently 100 nM hPTH(1-34) or vehicle onto the calvaria for 5 days. PTH stimulated bone formation as assessed by fluorochrome-marker-based histomorphometry. SOST expression was reduced in PTH-treated calvariae 4 h after the last administration as evaluated by real-time quantitative PCR. Next, we observed a decrease of SOST expression in femoral cortical bone of 6-month-old rats following single subcutaneous systemic administration of 80 microg/kg PTH(1-34). Finally, we studied SOST mRNA expression in bone of 11-month-old osteopenic estrogen-deprived (OVX) rats following 8-week systemic intermittent administration of 5 microg/kg PTH(1-34). PTH-treated animals displayed increases in bone mineral density as detected by pQCT, while SOST mRNA levels were decreased compared to vehicle-treated OVX and SHAM controls. PTH decreased SOST expression also in vitro. 100 nM PTH(1-34) inhibited expression in mouse calvaria organ cultures and in osteoblastic UMR-106 cells within 6 h by 95%. An IC50 of 1 nM was determined for PTH(1-34) in UMR-106 cells, whereas the PTH antagonist (d-Trp12,Tyr34)-bPTH(7-34) did not efficiently reduce SOST expression. Furthermore, SOST inhibition by PTH was not blocked by the protein synthesis inhibitor cycloheximide, indicating direct regulation, and PTH did not influence SOST mRNA degradation, implying transcriptional regulation. Finally, we observed full suppression of SOST by the cAMP inducer forskolin, partial inhibition by ionomycin, and no effect with PMA, indicating that PTH action is mainly mediated via the cAMP/PKA pathway. In summary, we have shown that PTH directly inhibits SOST transcription in vivo and in vitro, suggesting that SOST regulation may play a role in mediating PTH action in bone.
Mutations in distant regulatory elements can have a negative impact on human development and health, yet because of the difficulty of detecting these critical sequences, we predominantly focus on coding sequences for diagnostic purposes. We have undertaken a comparative sequence-based approach to characterize a large noncoding region deleted in patients affected by Van Buchem (VB) disease, a severe sclerosing bone dysplasia. Using BAC recombination and transgenesis, we characterized the expression of human sclerostin (SOST) from normal (SOST(wt)) or Van Buchem (SOST(vbDelta) alleles. Only the SOST(wt) allele faithfully expressed high levels of human SOST in the adult bone and had an impact on bone metabolism, consistent with the model that the VB noncoding deletion removes a SOST-specific regulatory element. By exploiting cross-species sequence comparisons with in vitro and in vivo enhancer assays, we were able to identify a candidate enhancer element that drives human SOST expression in osteoblast-like cell lines in vitro and in the skeletal anlage of the embryonic day 14.5 (E14.5) mouse embryo, and discovered a novel function for sclerostin during limb development. Our approach represents a framework for characterizing distant regulatory elements associated with abnormal human phenotypes.
The SOST gene product, sclerostin, is produced by mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation (1, 2). While it is clear that sclerostin has an anti-anabolic role, we hypothesised that sclerostin also has a direct catabolic activity. In support of this, both human (3) and animal (4) studies employing a neutralising anti-sclerostin antibody observed decreases in bone resorption parameters. To test this hypothesis we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin (2), or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated expression of RANKL mRNA and down-regulated that of OPG mRNA, causing an increase in the RANKL:OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood-derived mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold). The increased resorption was abolished by co-addition of recombinant OPG. Interestingly, the numbers of tartrate resistant acid phosphatase (TRAP)-positive multinucleated cells formed was not affected by rhSCL treatment, but remained basally high. rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes in a RANKL-dependent manner. These novel findings may help explain the dramatic increase in bone mass observed when SOST is mutated in humans or sclerostin is neutralized in animal models, in addition to its anti-anabolic actions.
(1) Paszty C, et al 2010 J Bone Miner Res 25:1897-1904
(2) Atkins GJ, et al 2011 Sclerostin is a locally acting regulator of late-osteoblast/pre-osteocyte differentiation and regulates mineralization through a MEPE-ASARM dependent mechanism. J Bone Miner Res (In Press)
(3) Padhi D, et al 2011 J Bone Miner Res 26:19-26
(4) Li X, et al 2009 J Bone Miner Res 24:578-588
Sources of Research Support: National Health and Medical Research Council of Australia Project Grant Scheme.
Nothing to Disclose: GJA, MK, DMF, ARW
Mutations in sclerostin function or expression cause sclerosing bone dysplasias, involving decreased antagonism of Wnt/Lrp5 signaling. Conversely, deletion of the VHL tumor suppressor in osteoblasts, which stabilize HIF-α isoforms and thereby enables HIF-α/β-driven gene transcription, increases bone mineral content and cross-sectional area compared to wild-type controls. We examined the influence of cellular hypoxia (1% oxygen) upon sclerostin expression and canonical Wnt signaling. Osteoblasts and osteocytes cultured under hypoxia revealed decreased sclerostin transcript and protein, and increased expression and nuclear localization of activated β-catenin. Similarly, both hypoxia and the hypoxia mimetic DFO increased β-catenin gene reporter activity. Hypoxia and its mimetics increased expression of the BMP antagonists gremlin and noggin and decreased Smad-1/5/8 phosphorylation. As a partial explanation for the mechanism of regulation of sclerostin by oxygen, MEF2 reporter assays revealed decreased activity. Modulation of VEGF signaling under normoxia or hypoxia revealed no influence upon Sost transcription. These data suggest that hypoxia inhibits sclerostin expression, through enhanced antagonism of BMP signaling independent of VEGF. J. Cell. Biochem. 110: 457–467, 2010. © 2010 Wiley-Liss, Inc.
Sirt1, the mammalian ortholog of the yeast Sir2 (silent information regulator 2), was shown to play an important role in metabolism and in age-associated diseases, but its role in skeletal homeostasis and osteoporosis has yet not been studied. Using 129/Sv mice with a germline mutation in the Sirt1 gene, we demonstrate that Sirt1 haplo-insufficient (Sirt1(+/-)) female mice exhibit a significant reduction in bone mass characterized by decreased bone formation and increased marrow adipogenesis. Importantly, we identify Sost, encoding for sclerostin, a critical inhibitor of bone formation, as a novel target of Sirt1. Using chromatin immunoprecipitation analysis, we reveal that Sirt1 directly and negatively regulates Sost gene expression by deacetylating histone 3 at lysine 9 at the Sost promoter. Sost down-regulation by small interfering RNA and the administration of a sclerostin-neutralizing antibody restore gene expression of osteocalcin and bone sialoprotein as well as mineralized nodule formation in Sirt1(+/-) marrow-derived mesenchymal stem cells induced to osteogenesis. These findings reveal a novel role for Sirt1 in bone as a regulator of bone mass and a repressor of sclerostin, and have potential implications suggesting that Sirt1 is a target for promoting bone formation as an anabolic approach for treatment of osteoporosis.
In this study, we investigated the relative expression of the Rous sarcoma virus (RSV) promoter-driven expression of enhanced green fluorescent protein (EGFP) in fibroblasts of transgenic quails. We analyzed the direct influence of CpG methylation of the RSV promoter on the transcriptional activity of delivered transgenes. Embryonic fibroblasts collected from homozygous transgenic quail (TQ2) were treated with 50 μM of DNA methyltransferase inhibitor followed by 5-aza-2'-deoxycytidine (5-azadC) for 48 h, and changes in expression were then analyzed by flow cytometry. The results show a significant increase of EGFP expression in TQ2 embryonic fibroblasts (QEFs) (2.64% to 79.84%). Subsequent methylation-specific amplification revealed that 5-azadC significantly reduced the CpG methylation status in the RSV promoters of the QEFs (86.42 to 48.41%); even after 5-azadC was withdrawn, CpG methylation remained decreased in expanded culture (16.28%). Further analysis showed that potential transcription factor binding sites existed in the CpG methylation site of the RSV promoter. These results may provide the basis for understanding the epigenetic mechanism responsible for transgenic animal production and genetic preservation.
The identity of the cell type responsive to sclerostin, a negative regulator of bone mass, is unknown. Since sclerostin is expressed in vivo by mineral-embedded osteocytes, we tested the hypothesis that sclerostin would regulate the behavior of cells actively involved in mineralization in adult bone, the preosteocyte. Differentiating cultures of human primary osteoblasts exposed to recombinant human sclerostin (rhSCL) for 35 days displayed dose- and time-dependent inhibition of in vitro mineralization, with late cultures being most responsive in terms of mineralization and gene expression. Treatment of advanced (day 35) cultures with rhSCL markedly increased the expression of the preosteocyte marker E11 and decreased the expression of mature markers DMP1 and SOST. Concomitantly, matrix extracellular phosphoglycoprotein (MEPE) expression was increased by rhSCL at both the mRNA and protein levels, whereas PHEX was decreased, implying regulation through the MEPE-ASARM axis. We confirmed that mineralization by human osteoblasts is exquisitely sensitive to the triphosphorylated ASARM-PO4 peptide. Immunostaining revealed that rhSCL increased the endogenous levels of MEPE-ASARM. Importantly, antibody-mediated neutralization of endogenous MEPE-ASARM antagonized the effect of rhSCL on mineralization, as did the PHEX synthetic peptide SPR4. Finally, we found elevated Sost mRNA expression in the long bones of HYP mice, suggesting that sclerostin may drive the increased MEPE-ASARM levels and mineralization defect in this genotype. Our results suggest that sclerostin acts through regulation of the PHEX/MEPE axis at the preosteocyte stage and serves as a master regulator of physiologic bone mineralization, consistent with its localization in vivo and its established role in the inhibition of bone formation.
Appropriate patterns of DNA methylation and histone modifications are required to assure cell identity, and their deregulation can contribute to human diseases, such as cancer. Our aim here is to provide an overview of how epigenetic factors, including genomic DNA methylation, histone modifications, and microRNA regulation, contribute to normal development, paying special attention to their role in regulating tissue-specific genes. In addition, we summarize how these epigenetic patterns go awry during human cancer development. The possibility of "resetting" the abnormal cancer epigenome by applying pharmacological or genetic strategies is also discussed.
Clinical imperatives for new bone to replace or restore the function of traumatized or bone lost as a consequence of age or disease has led to the need for therapies or procedures to generate bone for skeletal applications. However, current in vitro methods for the differentiation of human bone marrow stromal cells (HBMSCs) do not, to date, produce homogeneous cell populations of the osteogenic or chondrogenic lineages. As epigenetic modifiers are known to influence differentiation, we investigated the effects of the DNA demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) or the histone deacetylase inhibitor trichostatin A (TSA) on osteogenic and chondrogenic differentiation. Monolayer cultures of HBMSCs were treated for 3 days with the 5-aza-dC or TSA, followed by culture in the absence of modifiers. Cells were subsequently grown in pellet culture to determine matrix production. 5-aza-dC stimulated osteogenic differentiation as evidenced by enhanced alkaline phosphatase activity, increased Runx-2 expression in monolayer, and increased osteoid formation in 3D cell pellets. In pellets cultured in chondrogenic media, TSA enhanced cartilage matrix formation and chondrogenic structure. These findings indicate the potential of epigenetic modifiers, as agents, possibly in combination with other factors, to enhance the ability of HBMSCs to form functional bone or cartilage with significant therapeutic implications therein.
Sclerostin, an osteocyte-secreted protein, negatively regulates osteoblasts and inhibits bone formation. In this first-in-human study, a sclerostin monoclonal antibody (AMG 785) was administered to healthy men and postmenopausal women. In this phase I, randomized, double-blind, placebo-controlled, ascending, single-dose study, 72 healthy subjects received AMG 785 or placebo (3:1) subcutaneously (0.1, 0.3, 1, 3, 5, or 10 mg/kg) or intravenously (1 or 5 mg/kg). Depending on dose, subjects were followed for up to 85 days. The effects of AMG 785 on safety and tolerability (primary objectives) and pharmacokinetics, bone turnover markers, and bone mineral density (secondary objectives) were evaluated. AMG 785 generally was well tolerated. One treatment-related serious adverse event of nonspecific hepatitis was reported and was resolved. No deaths or study discontinuations occurred. AMG 785 pharmacokinetics were nonlinear with dose. Dose-related increases in the bone-formation markers procollagen type 1 N-propeptide (P1NP), bone-specific alkaline phosphatase (BAP), and osteocalcin were observed, along with a dose-related decrease in the bone-resorption marker serum C-telopeptide (sCTx), resulting in a large anabolic window. In addition, statistically significant increases in bone mineral density of up to 5.3% at the lumbar spine and 2.8% at the total hip compared with placebo were observed on day 85. Six subjects in the higher-dose groups developed anti-AMG 785 antibodies, 2 of which were neutralizing, with no discernible effect on the pharmacokinetics or pharmacodynamics. In summary, single doses of AMG 785 generally were well tolerated, and the data support further clinical investigation of sclerostin inhibition as a potential therapeutic strategy for conditions that could benefit from increased bone formation.
Sclerostin is the product of the SOST gene. Loss-of-function mutations in the SOST gene result in a high-bone-mass phenotype, demonstrating that sclerostin is a negative regulator of bone mass. Primarily expressed by osteocytes in bone, sclerostin is reported to bind the LRP5/6 receptor, thereby antagonizing canonical Wnt signaling and negatively regulating bone formation. We therefore investigated whether systemic administration of a sclerostin-neutralizing antibody would increase the regeneration of traumatized metaphyseal bone in rats. Young male rats had a screw inserted in the proximal tibia and were divided into six groups given 25 mg/kg of sclerostin antibody or control twice a week subcutaneously for 2 or 4 weeks. In four groups, the screws were tested for pull-out strength. At the time of euthanasia, a similar screw also was inserted in the contralateral tibia and pull-out tested immediately. Sclerostin antibody significantly increased the pull-out force by almost 50% compared with controls after 2 and 4 weeks. Also, the screws inserted at the time of euthanasia showed increased pull-out force. Micro-computed tomography (µCT) of the remaining two groups showed that the antibody led to a 30% increase in bone volume fraction in a region surrounding the screw. There also was a general increase in trabecular thickness in cancellous bone. Thus, as measured by the amount of bone and its mechanical resistance, the sclerostin antibody increased bone formation during metaphyseal repair but also in untraumatized bone.
Osteocytes are derived from osteoblasts and make up over 90% of the cells in bone. However, the mechanisms that control the differentiation of osteoblasts into osteocytes embedded in bone matrix are not well understood. With the recent developments of transgenic models for manipulating gene expression in osteocytes and of transgenic mice carrying lineage reporters for osteoblasts and osteocytes, unprecedented new insights are becoming possible. In this article we review recent advances, such as comparative gene and protein expression studies, that are delineating the changes in gene and protein expression that accompany osteocyte differentiation. We also review recent studies in which time-lapse dynamic imaging approaches have been used to visualize osteoblast and osteocyte populations within bone. These approaches reveal the key role of cell motility in bone cell function and highlight the dynamic nature of mineralized tissues. Changes in motile properties of the cell may be key in the transition from osteoblast to osteocyte, as reflected in the altered expression of many molecules involved in cytoskeletal function.
The anabolic effect of dynamic mechanical loading on skeletal architecture has been repeatedly demonstrated, but the cellular and molecular events occurring between load and ultimate bone formation remain obscure. The discovery of sclerostin, an antagonist of Wnt/Lrp5 signaling, and the sclerosing bone dysplasias that result from its mutation suggest its pivotal role in modulating bone formation. We examined expression of Sost mRNA across a variety of clonal cell lines spanning the osteogenic phenotype from immature osteoblast to mature osteocyte. No sclerostin expression was detected in immature MC3T3-E1 osteoblasts and, surprisingly, mature MLO-Y4 osteocytes, whereas immature MLO-A5 osteocytic cells expressed very low levels of Sost. Highest expression was observed in mature UMR 106.01 osteoblasts. We examined the influence of bone morphogenetic proteins on Sost expression. Treatment with BMP-2, -4 or -6 was without effect on Sost in mature MLO-Y4 osteocytes but elicited a robust increase in Sost expression in immature MLO-A5 osteocytes. Oscillatory fluid flow applied to mature UMR 106.01 osteoblasts transiently decreased expression of sclerostin at both the mRNA and protein level. Overall, our results indicate that BMP treatment and in vitro mechanical loading demonstrate opposite effects upon sclerostin expression.
The identification of molecules that are down-regulated in malignant phenotype is important for understanding tumor biology and their role in tumor suppression. We compared the expression profile of four normal nasal mucosal (NNM) epithelia and a series of nasopharyngeal cancinoma (NPC) cell lines using cDNA microarray and confirmed the actual expression of the selected genes, and found osteoprotegerin (OPG) to be ubiquitously deficient in NPC cells. We also found OPG to be down-regulated in various cancer cell lines, including oral, cervical, ovarian, lung, breast, pancreas, colon, renal, prostate cancer, and hepatoma. Administration of recombinant OPG (rOPG) brought about a reduction in cancer cell growth through apoptotic mechanism. We generated eleven monoclonal antibodies (MAbs) against OPG to study OPG's expression and biological functions in cancer cells. OPG was detected in the tumor stromal regions, but not in the cancer cell per se in surgical specimens of liver cancer. Quantitative reverse transcription-polymerase chain reaction (Q-RT-PCR) revealed that OPG was down-regulated in NPC tissues compared with normal nasal polyp (NNP) tissues. In addition, we showed OPG silencing to be associated with promoter methylation as well as histone modifications. In OPG-silenced cancer cell lines, the OPG gene promoter CpG dinucleotides were highly methylated. Compared to normal cells, silenced OPG gene in cancer cells were found to have reduced histone 3 lysine 4 tri-methylation (H3K4me3) and increased histone 3 lysine 27 tri-methylation (H3K27me3). Taken together, these results suggest that OPG silencing in carcinoma cancer cells occurs through epigenetic repression.
Sclerosteosis is an autosomal recessive sclerosing bone dysplasia characterized by progressive skeletal overgrowth. The majority of affected individuals have been reported in the Afrikaner population of South Africa, where a high incidence of the disorder occurs as a result of a founder effect. Homozygosity mapping in Afrikaner families along with analysis of historical recombinants localized sclerosteosis to an interval of ∼2 cM between the loci D17S1787 and D17S930 on chromosome 17q12-q21. Here we report two independent mutations in a novel gene, termed “SOST.” Affected Afrikaners carry a nonsense mutation near the amino terminus of the encoded protein, whereas an unrelated affected person of Senegalese origin carries a splicing mutation within the single intron of the gene. The SOST gene encodes a protein that shares similarity with a class of cystine knot–containing factors including dan, cerberus, gremlin, prdc, and caronte. The specific and progressive effect on bone formation observed in individuals affected with sclerosteosis, along with the data presented in this study, together suggest that the SOST gene encodes an important new regulator of bone homeostasis.
Studies on primary osteocytes, which compose >90-95% of bone cells, embedded throughout the mineralized matrix, are a major challenge because of their difficult accessibility and the very rare models available in vitro. We engineered a 3D culture method of primary human osteoblast differentiation into osteocytes. These 3D-differentiated osteocytes were compared with 2D-cultured cells and with human microdissected cortical osteocytes obtained from bone cryosections. Human primary osteoblasts were seeded either within the interspace of calibrated biphasic calcium phosphate particles or on plastic culture dishes and cultured for 4 wk in the absence of differentiation factors. Osteocyte differentiation was assessed by histological and immunohistological analysis after paraffin embedding of culture after various times, as well as by quantitative RT-PCR analysis of a panel of osteoblast and osteocyte markers after nucleic acid extraction. Histological analysis showed, after only 1 wk, the presence of an osteoid matrix including many lacunae in which the cells were individually embedded, exhibiting characteristics of osteocyte-like cells. Real-time PCR expression of a set of bone-related genes confirmed their osteocyte phenotype. Comparison with plastic-cultured cells and mature osteocytes microdissected from human cortical bone allowed to assess their maturation stage as osteoid-osteocytes. This model of primary osteocyte differentiation is a new tool to gain insights into the biology of osteocytes. It should be a suitable method to study the osteoblast-osteocyte differentiation pathway, the osteocyte interaction with the other bone cells, and orchestration of bone remodeling transmitted by mechanical loading and shear stress. It should be used in important cancer research areas such as the cross-talk of osteocytes with tumor cells in bone metastasis, because it has been recently shown that gene expression in osteocytes is strongly affected by cancer cells of different origin. It could also be a very efficient tool for drug testing and bone tissue engineering applications.
We have recently shown that TNF-related weak inducer of apoptosis (TWEAK) is a mediator of inflammatory bone remodeling. The aim of this study was to investigate the role of TWEAK in modulating human osteoblast activity, and how TWEAK and TNFalpha might interact in this context. Recombinant TWEAK and TNF were both mitogenic for human primary osteoblasts (NHBC). TWEAK dose- and time-dependently regulated the expression of the osteoblast transcription factors RUNX2 and osterix. TWEAK inhibited in vitro mineralization and downregulated the expression of osteogenesis-associated genes. Significantly, TWEAK and TWEAK/TNF induced the expression of the osteoblast differentiation inhibitor and SOST gene product, sclerostin. Sclerostin induction was mitogen-activated protein kinase (MAPK) dependent. The SOST mRNA levels induced by TWEAK were equivalent to or exceeded those seen in steady-state human bone, and the TWEAK/TNF induction of SOST mRNA was recapitulated in fresh cancellous bone explants. TWEAK-induced sclerostin expression was observed in immature osteoblastic cells, both in cycling (Ki67(+)) primary NHBC and in the cell lines MC3T3-E1 and MG-63, as well as in human osteocyte-like cells and in the osteocyte cell line, MLO-Y4. Treatment of NHBC with recombinant human sclerostin mimicked the effects of TWEAK to suppress RUNX2 and osteocalcin (OCN). TWEAK, TNF, and sclerostin treatment of NHBC similarly altered levels of phosphorylated and total GSK3beta and active and total levels of beta-catenin, implying that the Wnt signaling pathway was affected by all three stimuli. Sclerostin also rapidly activated ERK-1/2 MAPK signaling, indicating the involvement of additional signaling pathways. Together, our findings suggest that TWEAK, alone and with TNF, can regulate osteoblast function, at least in part by inducing sclerostin expression. Our results also suggest new roles and modes of action for sclerostin.
Sox9 is a key transcription factor which plays an important role in chondrogenesis. Although Bone Morphogenetic Protein-2 (BMP-2) has been reported to induce Sox9 expression, the underlying molecular mechanism remains elusive. Here, we used in vivo approaches to characterize BMP-2-induced alterations in chromatin organization around the Sox9 core promoter. Nuclease hypersensitive site mapping following BMP-2 stimulation showed an inducible hypersensitive site in the Sox9 proximal promoter. Immunoprecipitation (IP) experiments demonstrated that BMP-2 increased the association of the transcription factor NF-Y with histone acetyltransferase p300/CBP. Chromatin immunoprecipitation (ChIP) analysis showed the binding of the NF-Y-p300 complex to the Sox9 gene proximal promoter along with PCAF and RNA polymerase II. We also found that BMP-2 stimulation caused histone hyperacetylation and methylation at the Sox9 gene. Collectively, these data suggest that the activation of Sox9 gene transcription by BMP-2 is associated with chromatin remodeling and histone modification.
The effect of strontium ranelate (SR) on human osteoblast differentiation was tested. SR induced osteoblastic proliferation, in vitro mineralization, and increased the expression of osteocyte markers. SR also elicited an osteoprotegerin (OPG) secretory response. We conclude that SR promotes the osteoblast maturation and osteocyte differentiation while promoting an additional antiresorptive effect.
SR is a new treatment for osteoporosis that reduces the risk of hip and vertebral fractures in postmenopausal women. This study sought to investigate the extent, to which SR modulates human osteoblast differentiation.
Adult human primary osteoblasts (NHBC) were exposed to SR under mineralizing conditions in long-term cultures. Osteoblast differentiation status was investigated by cell-surface phenotypic analysis. Expression of genes associated with osteoblast/osteocyte differentiation was examined using real-time RT-PCR. Secreted OPG was assayed by enzyme-linked immunosorbent assay.
SR significantly increased osteoblast replication. SR time- and dose-dependently induced an osteocyte-like phenotype, as determined by cell surface alkaline phosphatase and STRO-1 expression. SR at 5 mM or greater dramatically increased in vitro mineralization. In parallel, mRNA levels of dentin matrix protein (DMP)-1 and sclerostin were higher under SR treatment, strongly suggestive of the presence of osteocytes. SR also increased the OPG/RANKL ratio throughout the culture period, consistent with an effect to inhibit osteoblast-induced osteoclastogenesis.
This study suggests that SR can promote osteoblast maturation and an osteocyte-like phenotype. Coupled with its effect on the OPG/RANKL system, these findings are consistent with in vivo effects in patients receiving SR for the treatment of osteoporosis.
Methylation-specific PCR (MSP) is frequently used to distinguish methylated alleles in the genome. Sequences that have been incompletely converted during bisulfite treatment are frequently co-amplified during MSP. For accurate MSP, it is important to detect methylated sequences in a background of unconverted DNA with a high level of sensitivity. We report here sensitive techniques, bisulfite conversion-specific MSP (BS-MSP) to accurately evaluate CpG methylation. BS-MSP provides accurate results across a wide spectrum of bisulfite conversion levels. BS-MSP is also confirmed to be a useful technique for the routine analysis of clinical tumor specimens that were paraffin-embedded and microdissected. BS-MSP thus provides the powerful features of ease of use and compatibility with paraffin sections. We recommend that methylation analysis should include a step to eliminate unconverted DNA to avoid overestimation of the DNA methylation level in the samples.
A new trabecular bone explant model was used to examine osteocyte-osteoblast interactions under DHP loading. DHP loading enhanced osteocyte viability as well as osteoblast function measured by osteoid formation. However, live osteocytes were necessary for osteoblasts to form osteoids in response to DHP, which directly show osteoblast-osteocyte interactions in this in vitro culture.
A trabecular bone explant model was characterized and used to examine the effect of osteocyte and osteoblast interactions and dynamic hydrostatic pressure (DHP) loading on osteocyte viability and osteoblast function in long-term culture.
Trabecular bone cores obtained from metacarpals of calves were cleaned of bone marrow and trabecular surface cells and divided into six groups, (1) live cores + dynamic hydrostatic pressure (DHP), (2) live cores + sham, (3) live cores + osteoblast + DHP, (4) live cores + osteoblast + sham, (5) devitalized cores + osteoblast + DHP, and (6) devitalized cores + osteoblast + sham, with four culture durations (2, 8, 15, and 22 days; n = 4/group). Cores from groups 3-6 were seeded with osteoblasts, and cores from groups 5 and 6 were devitalized before seeding. Groups 1, 3, and 5 were subjected to daily DHP loading. Bone histomorphometry was performed to quantify osteocyte viability based on morphology and to assess osteoblast function based on osteoid surface per bone surface (Os/Bs). TUNEL staining was performed to evaluate the mode of osteocyte death under various conditions.
A portion of osteocytes remained viable for the duration of culture. DHP loading significantly enhanced osteocyte viability up to day 8, whereas the presence of seeded osteoblasts significantly decreased osteocyte viability. Cores with live osteocytes showed higher Os/Bs compared with devitalized cores, which reached significant levels over a greater range of time-points when combined with DHP loading. DHP loading did not increase Os/Bs in the absence of live osteocytes. The percentage of apoptotic cells remained the same regardless of treatment or culture duration.
Enhanced osteocyte viability with DHP suggests the necessity of mechanical stimulation for osteocyte survival in vitro. Furthermore, osteocytes play a critical role in the transmission of signals from DHP loading to modulate osteoblast function. This explant culture model may be used for mechanotransduction studies in long-term cultures.
Osteocytes are the most abundant cells in bone and are ideally located to influence bone turnover through their syncytial relationship with surface bone cells. Osteocyte-derived signals have remained largely enigmatic, but it was recently reported that human osteocytes secrete sclerostin, an inhibitor of bone formation. Absent sclerostin protein results in the high bone mass clinical disorder sclerosteosis. Here we report that within adult iliac bone, newly embedded osteocytes were negative for sclerostin staining but became positive at or after primary mineralization. The majority of mature osteocytes in mineralized cortical and cancellous bone was positive for sclerostin with diffuse staining along dendrites in the osteocyte canaliculi. These findings provide for the first time in vivo evidence to support the concept that osteocytes secrete sclerostin after they become embedded in a mineralized matrix to limit further bone formation by osteoblasts. Sclerostin did not appear to influence the formation of osteocytes. We propose that sclerostin production by osteocytes may regulate the linear extent of formation and the induction or maintenance of a lining cell phenotype on bone surfaces. In doing so, sclerostin may act as a key inhibitory signal governing skeletal microarchitecture.
Methylation of DNA at position five of the cytosine ring occurs at most CpG dinucleotides in the mammalian genome and is essential for embryonic viability. With several of the key proteins now known, it has become possible to approach the biological significance of this epigenetic system through both biochemistry and genetics. As a result, advances have been made in our understanding of the mechanisms by which DNA methylation is targeted to specific regions of the genome and interpreted by methyl-CpG-binding proteins. Recent studies have illuminated the role of DNA methylation in controlling gene expression and have strengthened its links with histone modification and chromatin remodelling.