Hironori Hojo

The University of Tokyo, Tōkyō, Japan

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Publications (29)107.2 Total impact

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    Hironori Hojo, Shinsuke Ohba, Ung-il Chung
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    ABSTRACT: Tissue engineering is an approach to the regeneration of tissues that uses a combination of cell sources, signaling factors and scaffolds. Among these three components, signaling factors for bone regeneration have not yet been established, and it is necessary to better understand osteoblast progenitors as a target cells. Several lines of evidence have revealed that, during bone formation, mesenchymal cells are specified and differentiate into osteoblasts through several stages of precursors. The osteoblast lineage is defined by the expression of stage-specific transcription factors. The specification and differentiation are organized by a variety of signaling pathways including hedgehog (Hh), Wnt, Notch, bone morphogenetic protein (BMP) and transforming growth factor-beta (TGFβ). In this review we integrate the known functions of these signaling pathways and discuss future tasks to gain a better understanding of the signaling network in osteogenesis for tissue engineering.
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    ABSTRACT: Hedgehog (Hh) signaling plays important roles in various development processes. This signaling is necessary for osteoblast formation during endochondral ossification. In contrast to the established roles of Hh signaling in embryonic bone formation, evidence of its roles in adult bone homeostasis is not complete. Here we report the involvement of Gli1, a transcriptional activator induced by Hh signaling activation, in postnatal bone homeostasis under physiological and pathological conditions. Skeletal analyses of Gli1+/- adult mice revealed that Gli1 haploinsufficiency caused decreased bone mass with reduced bone formation and accelerated bone resorption, suggesting an uncoupling of bone metabolism. Hh-mediated osteoblast differentiation was largely impaired in cultures of Gli1+/- precursors, and the impairment was rescued by Gli1 expression via adenoviral transduction. In addition, Gli1+/- precursors showed premature differentiation into osteocytes and increased ability to support osteoclastogenesis. When we compared fracture healing between wild-type and Gli1+/- adult mice, we found that the Gli1+/- mice exhibited impaired fracture healing with insufficient soft callus formation. These data suggest that Gli1, acting downstream of Hh signaling, contributes to adult bone metabolism, in which this molecule not only promotes osteoblast differentiation but also represses osteoblast maturation toward osteocytes to maintain normal bone homeostasis.
    PLoS ONE 10/2014; 9(10):e109597. DOI:10.1371/journal.pone.0109597 · 3.53 Impact Factor
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    ABSTRACT: Pluripotent stem cells are a promising tool for mechanistic studies of tissue development, drug screening, and cell-based therapies. Here, we report an effective and mass-producing strategy for the stepwise differentiation of mouse embryonic stem cells (mESCs) and mouse and human induced pluripotent stem cells (miPSCs and hiPSCs, respectively) into osteoblasts using four small molecules (CHIR99021 [CHIR], cyclopamine [Cyc], smoothened agonist [SAG], and a helioxanthin-derivative 4-(4-methoxyphenyl)pyrido[4',3':4,5]thieno[2,3-b]pyridine-2-carboxamide [TH]) under serum-free and feeder-free conditions. The strategy, which consists of mesoderm induction, osteoblast induction, and osteoblast maturation phases, significantly induced expressions of osteoblast-related genes and proteins in mESCs, miPSCs, and hiPSCs. In addition, when mESCs defective in runt-related transcription factor 2 (Runx2), a master regulator of osteogenesis, were cultured by the strategy, they molecularly recapitulated osteoblast phenotypes of Runx2 null mice. The present strategy will be a platform for biological and pathological studies of osteoblast development, screening of bone-augmentation drugs, and skeletal regeneration.
    06/2014; 2(6):751-60. DOI:10.1016/j.stemcr.2014.04.016
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    ABSTRACT: The effect of tetrapod-shaped alpha tricalcium phosphate granules (Tetrabones(®) [TB]) in combination with basic fibroblast growth factor (bFGF)-binding ion complex gel (f-IC gel) on bone defect repair was examined. Bilateral segmental defects 20-mm long were created in the radius of 5 dogs, stabilized with a plate and screws, and implanted with 1 of the following: TB (TB group), TB and bFGF solution (TB/f group), and TB and f-IC gel (TB/f-IC group). Dogs were euthanized 4 weeks after surgery. Radiographs showed well-placed TB granules in the defects and equal osseous callus formation in all the groups. Histomorphometry revealed that the number of vessels and volume of new bone in the TB/f-IC group were significantly higher than those in the other groups. However, no significant differences in neovascularization and new bone formation were observed between the TB/f and TB groups. Furthermore, no significant difference in the lamellar bone volume or rate of mineral apposition was observed among groups. These results suggest that increased bone formation might have been because of the promotion of neovascularization by the f-IC gel. Therefore, the combinatorial method may provide a suitable scaffold for bone regeneration in large segmental long bone defects.
    Journal of Veterinary Medical Science 03/2014; 76(7). DOI:10.1292/jvms.14-0027 · 0.88 Impact Factor
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    ABSTRACT: Induced pluripotent stem cells (iPSC) are a promising cell source for cartilage regenerative medicine; however, the methods for chondrocyte induction from iPSC are currently developing and not yet sufficient for clinical application. Here, we report the establishment of a fluorescent indicator system for monitoring chondrogenic differentiation from iPSC to simplify screening for effective factors that induce chondrocytes from iPSC. We generated iPSC from embryonic fibroblasts of Col2a1-EGFP transgenic mice by retroviral transduction of Oct4, Sox2, Klf4, and c-Myc. Among the 30 clones of Col2a1-EGFP iPSC we established, two clones showed high expression levels of embryonic stem cell (ESC) marker genes, similar to control ESC. A teratoma formation assay showed that the two clones were pluripotent and differentiated into cell types from all three germ layers. The fluorescent signal was observed during chondrogenic differentiation of the two clones concomitant with the increase in chondrocyte marker expression. In conclusion, Col2a1-EGFP iPSC are useful for monitoring chondrogenic differentiation and will contribute to research in cartilage regenerative medicine.
    PLoS ONE 09/2013; 8(9):e74137. DOI:10.1371/journal.pone.0074137 · 3.53 Impact Factor
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    ABSTRACT: The effect of tetrapod-shaped alpha tricalcium phosphate granules (TB) as a scaffold combined with basic fibroblast growth factor (bFGF)-binding ion complex gel (f-IC gel) on neovascularization and bone regeneration was evaluated in segmental femoral defects of rabbits. The defects were stabilized using a plate with a polypropylene mesh cage (PMC) containing one of the following: PMC alone (PMC group), TB (TB group), TB and bFGF (TB/f group), TB and IC gel (TB/IC group), or TB and f-IC gel (TB/f-IC group). Four rabbits from each group were euthanized at 2 and 4 weeks after surgery. Histomorphometry showed that the number of vessels and the volume of new bone in the TB/f-IC group were significantly higher than those in the other groups at all time points. There were no differences in the extent of neovascularization and new bone formation between the TB and TB/f groups. These findings suggest that the combination of TB and f-IC gel facilitated both neovascularization and new bone formation in segmental femoral defects of rabbits. This combination may be of considerable use for treating segmental long bone defects.
    Biomaterials 08/2013; DOI:10.1016/j.biomaterials.2013.08.014 · 8.31 Impact Factor
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    ABSTRACT: Nanoscaled drug-loaded carriers are of particular interest for efficient tumor therapy as numerous studies have shown improved targeting and efficacy. Nevertheless, most of these studies have been performed against allograft and xenograft tumor models, which have altered microenvironment features affecting the accumulation and penetration of nanocarriers. Conversely, the evaluation of nanocarriers on genetically engineered mice, which can gradually develop clinically relevant tumors, permits the validation of their design under normal processes of immunity, angiogenesis, and inflammation. Therefore, considering the poor prognosis of pancreatic cancer, we used the elastase 1-promoted luciferase and Simian virus 40 T and t antigens transgenic mice, which develop spontaneous bioluminescent pancreatic carcinoma, and showed that long circulating micellar nanocarriers, incorporating the parent complex of oxaliplatin, inhibited the tumor growth as a result of their efficient accumulation and penetration in the tumors. The reduction of the photon flux from the endogenous tumor by the micelles correlated with the decrease of serum carbohydrate-associated antigen 19-9 marker. Micelles also reduced the incidence of metastasis and ascites, extending the survival of the transgenic mice.
    Proceedings of the National Academy of Sciences 06/2013; DOI:10.1073/pnas.1301348110 · 9.81 Impact Factor
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    ABSTRACT: Articular cartilage is a permanent tissue, with poor self-regenerative capacity. Consequently, a tissue engineering approach to cartilage regenerative therapy could greatly advance the current treatment options for patients with cartilage degeneration and/or defects. A successful tissue engineering approach would require not only induction of chondrogenic differentiation, but also suppression of subsequent endochondral ossification and chondrocyte dedifferentiation. We previously reported that direct injection of the thienoindazole derivative, TD-198946, into the knee joints of mice halted the progression of osteoarthritis; the compound induced chondrogenic differentiation without promoting endochondral ossification. In the present study, we applied TD-198946 to a cell-based cartilage reconstruction model, taking advantage of the cell-sheet technology. Cartilaginous cell-sheets were generated by culturing mouse and canine costal chondrocytes and human mesenchymal stem cells with TD-198946 on temperature-responsive dishes. The transplanted cell-sheets were then successfully used to promote the reconstruction of permanent cartilage, with no evidence of chondrocyte hypertrophy in the knee articular cartilage defects created in mice and canines. Thus, TD-198946 is a promising candidate for cell-based cartilage reconstruction therapies, enabling us to avoid any concern surrounding the use of scaffolds or cytokines to stimulate regeneration.
    Biomaterials 04/2013; DOI:10.1016/j.biomaterials.2013.04.008 · 8.31 Impact Factor
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    ABSTRACT: Although bone grafts and prosthetic implants have shown some clinical success in the treatment of bone defects, the graft availability, biocompatibility, function, and longevity still remain to be improved. One possible solution to these problems is to develop bone implants acting on host cells to induce rapid bone regeneration. Here, we demonstrate bone healing by means of a sterilizable and osteogenic molecule-eluting implant system in which two small molecules, a smoothened agonist (SAG) and a helioxanthin derivative (TH), are loaded onto tetrapod-shaped calcium phosphate granules (Tetrabone). We succeeded in directing progenitor cells toward mature osteoblasts with the combined application of the two small molecules acting on different stages of osteogenesis. Tetrabone released SAG and TH for prolonged periods when loaded with these molecules. EOG sterilization did not affect the osteogenic activity of the SAG- and TH-loaded Tetrabones. The combinatorial use of SAG- and TH-loaded Tetrabones achieved bone healing without cell transplantation in a rat femur bone defect model within two weeks. This system will allow us to vary the combination rate of implants loaded with different osteogenic factors depending on the types and sizes of defects, potentially allowing full temporal and spatial control of the bone regeneration.
    Biomaterials 04/2013; DOI:10.1016/j.biomaterials.2013.03.089 · 8.31 Impact Factor
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    ABSTRACT: Tenomodulin (Tnmd) is a type II transmembrane protein characteristically expressed in dense connective tissues such as tendons and ligaments. Its expression in the periodontal ligament (PDL) has also been demonstrated, though the timing and function remain unclear. We investigated the expression of Tnmd during murine tooth eruption and explored its biological functions in vitro. Tnmd expression was related to the time of eruption when occlusal force was transferred to the teeth and surrounding tissues. Tnmd overexpression enhanced cell adhesion in NIH3T3 and human PDL cells. In addition, Tnmd-knockout fibroblasts showed decreased cell adhesion. In the extracellular portions of Tnmd, the BRICHOS domain or CS region was found to be responsible for Tnmd-mediated enhancement of cell adhesion. These results suggest that Tnmd acts on the maturation or maintenance of the PDL by positively regulating cell adhesion via its BRICHOS domain.
    PLoS ONE 04/2013; 8(4):e60203. DOI:10.1371/journal.pone.0060203 · 3.53 Impact Factor
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    ABSTRACT: Specification of progenitors into the osteoblast lineage is an essential event for skeletogenesis. During endochondral ossification, cells in the perichondrium give rise to osteoblast precursors. Hedgehog (Hh) and bone morphogenetic protein (BMP) are suggested to regulate the commitment of these cells. However, properties of perichondrial cells and regulatory mechanisms of the specification process are still poorly understood. Here, we investigated the machineries by combining a novel organ culture system and single-cell expression analysis with mouse genetics and biochemical analyses. In a metatarsal organ culture reproducing bone collar formation, activation of BMP signaling enhanced the bone collar formation cooperatively with Hh input, while the signaling induced ectopic chondrocyte formation in the perichondrium without Hh input. Similar phenotypes were also observed in compound mutant mice, where signaling activities of Hh and BMP were genetically manipulated. Single-cell RT-qPCR analyses showed heterogeneity of perichondrial cells in terms of natural characteristics and responsiveness to Hh input. In vitro analyses revealed that Hh signaling suppressed BMP-induced chondrogenic differentiation; Gli1 inhibited the expression of Sox5, Sox6, and Sox9 as well as transactivation by Sox9. Indeed, ectopic expression of chondrocyte maker genes were observed in the perichondrium of metatarsals in Gli1-/- fetuses, and the phenotype was more severe in Gli1-/-;Gli2-/- newborns. These data suggest that Hh-Gli activators alter the function of BMP to specify perichondrial cells into osteoblasts; the timing of Hh input and its target populations are critical for BMP function.
    Journal of Biological Chemistry 02/2013; 288(14). DOI:10.1074/jbc.M112.409342 · 4.60 Impact Factor
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    ABSTRACT: Heterotopic ossification or calcification follows any type of musculoskeletal trauma and is known to occur after arthroplasties of hip, knee, shoulder, or elbow; fractures; joint dislocations; or tendon ruptures. Histamine receptor H2 (Hrh2) has been shown to be effective for reducing pain and decreasing calcification in patients with calcifying tendinitis, which suggested that H2 blockers were effective for the treatment of tendon ossification or calcification. However, the detailed mechanisms of its action on tendon remain to be clarified. We investigated the mechanisms underlying H2 blocker-mediated suppression of tendon calcification, with a focus on the direct action of the drug on tendon cells. Famotidine treatment suppressed the mRNA expressions of Col10a1 and osteocalcin, ossification markers, in a tendon-derived cell line TT-D6, as well as a preosteoblastic one MC3T3-E1. Both of the cell lines expressed Hrh2; histamine treatment induced osteocalcin expression in these cells. Famotidine administration suppressed calcification in the Achilles tendon of ttw mice, a mouse model of ectopic ossification. These data suggest that famotidine inhibits osteogenic differentiation of tendon cells in vitro, and this inhibition may underlie the anti-calcification effects of the drug in vivo. This study points to the use of H2 blockers as a promising strategy for treating heterotopic ossification or calcification in tendon, and provides evidence in support of the clinical use of famotidine. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1958-1962, 2012.
    Journal of Orthopaedic Research 12/2012; 30(12):1958-62. DOI:10.1002/jor.22146 · 2.97 Impact Factor
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    ABSTRACT: OBJECTIVES: To identify a new disease-modifying osteoarthritis drug (DMOAD) candidate that can effectively repair cartilage by promoting chondrogenic differentiation and halt osteoarthritis (OA) progression by suppressing aberrant hypertrophy. METHODS: We screened 2500 natural and synthetic small compounds for chondrogenic agents via four steps using the Col2GFP-ATDC5 system and identified a small thienoindazole derivative compound, TD-198946, as a novel DMOAD candidate. We tested its efficacy as a DMOAD via intra-articular injections directly into the joint space in a surgically-induced mouse model of OA both at the onset (prevention model) and 4 weeks after (repair model) OA induction. The downstream molecules were screened by microarray analysis. We further investigated the mechanism of the drug action and its molecular target using in vitro and in vivo assays. RESULTS: TD-198946 strongly induced chondrogenic differentiation without promoting hypertrophy in cell and metatarsal organ cultures. When administered directly into the joint space, TD-198946 successfully prevented and repaired degeneration of the articular cartilage. TD-198946 exerted its effect through the regulation of Runx1 expression, which was downregulated in both mouse and human OA cartilage compared with normal tissue. CONCLUSIONS: Our data suggest that TD-198946 is a novel class of DMOAD candidate, and that targeting Runx1 will provide a promising new approach in the development of disease-modifying drugs against OA.
    Annals of the rheumatic diseases 10/2012; 72(5). DOI:10.1136/annrheumdis-2012-201745 · 9.27 Impact Factor
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    ABSTRACT: Objectives: This study aimed to investigate the expression of tenomodulin (Tnmd) during murine tooth eruption and explore its biological function in vitro. Methods: Specific antibodies against murine Tnmd were established. Expression of Tnmd during the three eruption phases (pre-eruptive, eruptive or post-eruptive phase) of murine molars was determined by immunohistochemistry in wild type (WT) or Tnmd knockout (KO) mice specimens. To determine roles of Tnmd in cell adhesion, cell adhesion assay was performed in NIH3T3 cells transfected with Tnmd or its extracellular domain deletion mutants. Results: The specificity of the Tnmd-antibody was confirmed by immunohistochemistry of WT or Tnmd KO mice tail tendons. This antibody revealed the expression of Tnmd in periodontal ligament; the Tnmd expression was increased at eruptive phase and sustained after eruption. Given that dental attrition was observed in eruptive and post-eruptive phases, teeth were likely exposed to occlusal forces at these time points. Functional analyses in vitro revealed that Tnmd overexpression resulted in enhancement of cell adhesion in NIH3T3 cells. The enhancement was confirmed in fibroblasts derived from WT or Tnmd KO mice. Deletion of cleavage site or BRICHOS domain diminished cell adhesion, whereas deletion of C-terminal domain (CTD) did not. The result suggests that BRICHOS domain, but not CTD, is responsible for the Tnmd-mediated enhancement of cell adhesion. Conclusions: Expression of Tnmd was correlated with the time of eruption when occlusal force was transferred to teeth and surrounding tissues. This observation suggests that the regulation of Tnmd expression may be controlled partially by mechanical forces in dense connective tissues. In addition, Tnmd is likely to have positive effects on cell adhesion. Further analyses will reveal the regulation of Tnmd expression and its function in more detail.
    PER/IADR Congress 2012; 09/2012
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    Shinsuke Ohba, Hironori Hojo, Ung-Il Chung
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    ABSTRACT: THERE ARE THREE COMPONENTS FOR THE CREATION OF NEW TISSUES: cell sources, scaffolds, and bioactive factors. Unlike conventional medical strategies, regenerative medicine requires not only analytical approaches but also integrative ones. Basic research has identified a number of bioactive factors that are necessary, but not sufficient, for organogenesis. In skeletal development, these factors include bone morphogenetic proteins (BMPs), transforming growth factor β TGF-β, Wnts, hedgehogs (Hh), fibroblast growth factors (FGFs), insulin-like growth factors (IGFs), SRY box-containing gene (Sox) 9, Sp7, and runt-related transcription factors (Runx). Clinical and preclinical studies have been extensively performed to apply the knowledge to bone and cartilage regeneration. Given the large number of findings obtained so far, it would be a good time for a multi-disciplinary, collaborative effort to optimize these known factors and develop appropriate drug delivery systems for delivering them.
    07/2012; 2(3):193-203.
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    ABSTRACT: With regard to Hedgehog signaling in mammalian development, the majority of research has focused on Gli2 and Gli3 rather than Gli1. This is because Gli1(-/-) mice do not show any gross abnormalities in adulthood, and no detailed analyses of fetal Gli1(-/-) mice are available. In this study, we investigated the physiological role of Gli1 in osteogenesis. Histological analyses revealed that bone formation was impaired in Gli1(-/-) fetuses compared with WT fetuses. Gli1(-/-) perichondrial cells expressed neither runt-related transcription factor 2 (Runx2) nor osterix, master regulators of osteogenesis, in contrast to WT cells. In vitro analyses showed that overexpression of Gli1 up-regulated early osteogenesis-related genes in both WT and Runx2(-/-) perichondrial cells, and Gli1 activated transcription of those genes via its association with their 5'-regulatory regions, underlying the function of Gli1 in the perichondrium. Moreover, Gli1(-/-);Gli2(-/-) mice showed more severe phenotypes of impaired bone formation than either Gli1(-/-) or Gli2(-/-) mice, and osteoblast differentiation was impaired in Gli1(-/-);Gli3(-/-) perichondrial cells compared with Gli3(-/-) cells in vitro. These data suggest that Gli1 itself can induce early osteoblast differentiation, at least to some extent, in a Runx2-independent manner. It also plays a redundant role with Gli2 and is involved in the repressor function of Gli3 in osteogenesis. On the basis of these findings, we propose that upon Hedgehog input, Gli1 functions collectively with Gli2 and Gli3 in osteogenesis.
    Journal of Biological Chemistry 04/2012; 287(21):17860-9. DOI:10.1074/jbc.M112.347716 · 4.60 Impact Factor
  • Shinsuke Ohba, Hironori Hojo, Ung-il Chung
    01/2012; 27(1):1. DOI:10.3803/EnM.2012.27.1.1
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    ABSTRACT: Bone mass is regulated by osteoblast-mediated bone formation and osteoclast-mediated bone resorption. We previously reported that harmine, a β-carboline alkaloid, inhibits osteoclast differentiation and bone resorption in vitro and in vivo. In this study, we investigated the effects of harmine on osteoblast proliferation, differentiation and mineralization. Harmine promoted alkaline phosphatase (ALP) activity in MC3T3-E1 cells without affecting their proliferation. Harmine also increased the mRNA expressions of the osteoblast marker genes ALP and Osteocalcin. Furthermore, the mineralization of MC3T3-E1 cells was enhanced by treatment with harmine. Harmine also induced osteoblast differentiation in primary calvarial osteoblasts and mesenchymal stem cell line C3H10T1/2 cells. Structure-activity relationship studies using harmine-related β-carboline alkaloids revealed that the C3-C4 double bond and 7-hydroxy or 7-methoxy group of harmine were important for its osteogenic activity. The bone morphogenetic protein (BMP) antagonist noggin and its receptor kinase inhibitors dorsomorphin and LDN-193189 attenuated harmine-promoted ALP activity. In addition, harmine increased the mRNA expressions of Bmp-2, Bmp-4, Bmp-6, Bmp-7 and its target gene Id1. Harmine also enhanced the mRNA expressions of Runx2 and Osterix, which are key transcription factors in osteoblast differentiation. Furthermore, BMP-responsive and Runx2-responsive reporters were activated by harmine treatment. Taken together, these results indicate that harmine enhances osteoblast differentiation probably by inducing the expressions of BMPs and activating BMP and Runx2 pathways. Our findings suggest that harmine has bone anabolic effects and may be useful for the treatment of bone-decreasing diseases and bone regeneration as a lead compound.
    Biochemical and Biophysical Research Communications 06/2011; 409(2):260-5. DOI:10.1016/j.bbrc.2011.05.001 · 2.28 Impact Factor
  • Fumiko Yano, Hironori Hojo, Ung il Chung
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    ABSTRACT: Aiming at regeneration of articular cartilage, we have established stable lines of mouse chondrogenic ATDC5 cells expressing green fluorescent protein under the control of type II collagen promoter fused with four repeats of a SOX9 enhancer (COL2A1-GFP) , as a monitoring system for chondrogenic differentiation. A screening of natural and synthetic compound libraries using the system identified some novel compounds. Combined with cell-sheet technology, a novel small compound was applied to the treatment of full-thickness knee cartilage defects in murine and canine models.
    Clinical calcium 06/2011; 21(6):871-7.
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    ABSTRACT: Mammalian bones have three distinct origins (paraxial mesoderm, lateral plate mesoderm, and neural crest) and undergo two different modes of formation (intramembranous and endochondral). Bones derived from the paraxial mesoderm and lateral plate mesoderm mainly form through the endochondral process. During this process, hypertrophic chondrocytes play a vital role in inducing osteogenesis. So far, a number of published papers have provided evidence that chondrocyte hypertrophy and osteoblast differentiation are controlled by a variety of signaling pathways and factors; however, little is known about their hierarchy (which are upstream? which are most potent?). In this review, we discuss the signaling pathways and transcriptional factors regulating chondrocyte hypertrophy and osteoblast differentiation based on the evidence that has been reported and confirmed by multiple independent groups. We then discuss which factor would provide the most coherent evidence for its role in endochondral ossification.
    Journal of Bone and Mineral Metabolism 09/2010; 28(5):489-502. DOI:10.1007/s00774-010-0199-7 · 2.11 Impact Factor

Publication Stats

265 Citations
107.20 Total Impact Points


  • 2009–2014
    • The University of Tokyo
      • Center for Disease Biology and Integrative Medicine
      Tōkyō, Japan
  • 2006–2009
    • Kanazawa University
      Kanazawa, Ishikawa, Japan
  • 2008
    • Japan Society for the Promotion of Science
      Edo, Tōkyō, Japan