Article

Central Role of Pyrophosphate in Acellular Cementum Formation

Laboratory of Oral Connective Tissue Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland, United States of America.
PLoS ONE (Impact Factor: 3.53). 06/2012; 7(6):e38393. DOI: 10.1371/journal.pone.0038393
Source: PubMed

ABSTRACT Inorganic pyrophosphate (PP(i)) is a physiologic inhibitor of hydroxyapatite mineral precipitation involved in regulating mineralized tissue development and pathologic calcification. Local levels of PP(i) are controlled by antagonistic functions of factors that decrease PP(i) and promote mineralization (tissue-nonspecific alkaline phosphatase, Alpl/TNAP), and those that increase local PP(i) and restrict mineralization (progressive ankylosis protein, ANK; ectonucleotide pyrophosphatase phosphodiesterase-1, NPP1). The cementum enveloping the tooth root is essential for tooth function by providing attachment to the surrounding bone via the nonmineralized periodontal ligament. At present, the developmental regulation of cementum remains poorly understood, hampering efforts for regeneration. To elucidate the role of PP(i) in cementum formation, we analyzed root development in knock-out ((-/-)) mice featuring PP(i) dysregulation.
Excess PP(i) in the Alpl(-/-) mouse inhibited cementum formation, causing root detachment consistent with premature tooth loss in the human condition hypophosphatasia, though cementoblast phenotype was unperturbed. Deficient PP(i) in both Ank and Enpp1(-/-) mice significantly increased cementum apposition and overall thickness more than 12-fold vs. controls, while dentin and cellular cementum were unaltered. Though PP(i) regulators are widely expressed, cementoblasts selectively expressed greater ANK and NPP1 along the root surface, and dramatically increased ANK or NPP1 in models of reduced PP(i) output, in compensatory fashion. In vitro mechanistic studies confirmed that under low PP(i) mineralizing conditions, cementoblasts increased Ank (5-fold) and Enpp1 (20-fold), while increasing PP(i) inhibited mineralization and associated increases in Ank and Enpp1 mRNA.
Results from these studies demonstrate a novel developmental regulation of acellular cementum, wherein cementoblasts tune cementogenesis by modulating local levels of PP(i), directing and regulating mineral apposition. These findings underscore developmental differences in acellular versus cellular cementum, and suggest new approaches for cementum regeneration.

0 Followers
 · 
306 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Hypophosphatasia (HPP) results from ALPL gene mutations, which lead to a deficiency of tissue-nonspecific alkaline phosphatase (TNAP), and accumulation of inorganic pyrophosphate, a potent inhibitor of mineralization that is also a natural substrate of TNAP, in the extracellular space. HPP causes mineralization disorders including soft bones (rickets or osteomalacia) and defects in teeth and periodontal tissues. Enzyme replacement therapy using mineral-targeting recombinant TNAP has proven effective in preventing skeletal and dental defects in TNAP knockout (Alpl(-/-)) mice, a model for life-threatening HPP. Here, we show that the administration of a soluble, intestinal-like chimeric alkaline phosphatase (ChimAP) improves the manifestations of HPP in Alpl(-/-) mice. Mice received daily subcutaneous injections of ChimAP at doses of 1, 8 or 16mg/kg, from birth for up to 53days. Lifespan and body weight of Alpl(-/-) mice were normalized, and vitamin B6-associated seizures were absent with 16mg/kg/day of ChimAP. Radiographs, μCT and histological analyses documented improved mineralization in cortical and trabecular bone and secondary ossification centers in long bones of ChimAP16-treated mice. There was no evidence of craniosynostosis in the ChimAP16-treated mice and we did not detect ectopic calcification by radiography and histology in the aortas, stomachs, kidneys or lungs in any of the treatment groups. Molar tooth development and function improved with the highest ChimAP dose, including enamel, dentin, and tooth morphology. Cementum remained deficient and alveolar bone mineralization was reduced compared to controls, though ChimAP-treated Alpl(-/-) mice featured periodontal attachment and retained teeth. This study provides the first evidence for the pharmacological efficacy of ChimAP for use in the treatment of skeletal and dental manifestations of HPP. Copyright © 2014. Published by Elsevier Inc.
    Bone 11/2014; DOI:10.1016/j.bone.2014.11.017 · 4.46 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cementum is critical for anchoring the insertion of periodontal ligament fibers to the tooth root. Several aspects of cementogenesis remain unclear, including differences between acellular cementum and cellular cementum, and between cementum and bone. Biomineralization is regulated by the ratio of inorganic phosphate (Pi) to mineral inhibitor pyrophosphate (PPi), where local Pi and PPi concentrations are controlled by phosphatases including tissue-nonspecific alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). The focus of this study was to define the roles of these phosphatases in cementogenesis. TNAP was associated with earliest cementoblasts near forming acellular and cellular cementum. With loss of TNAP in the Alpl null mouse, acellular cementum was inhibited, while cellular cementum production increased, albeit as hypomineralized cementoid. In contrast, NPP1 was detected in cementoblasts after acellular cementum formation, and at low levels around cellular cementum. Loss of NPP1 in the Enpp1 null mouse increased acellular cementum, with little effect on cellular cementum. Developmental patterns were recapitulated in a mouse model for acellular cementum regeneration, with early TNAP expression and later NPP1 expression. In vitro, cementoblasts expressed Alpl gene/protein early, whereas Enpp1 gene/protein expression was significantly induced only under mineralization conditions. These patterns were confirmed in human teeth, including widespread TNAP, and NPP1 restricted to cementoblasts lining acellular cementum. These studies suggest that early TNAP expression creates a low PPi environment promoting acellular cementum initiation, while later NPP1 expression increases PPi, restricting acellular cementum apposition. Alterations in PPi have little effect on cellular cementum formation, though matrix mineralization is affected.
    International Journal of Oral Science 12/2014; 7(1). DOI:10.1038/ijos.2014.62 · 2.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Background: Historically, periodontal regeneration has focused predominantly on bone substitutes and/or barrier membrane application to provide for defect fill and/or selected cell repopulation of the lesion. More recently, a number of technologies have evolved which can be viewed as emerging therapeutic approaches for periodontal regeneration and these technologies were considered in the review paper and by the consensus group. The goal of this Consensus Report on emerging regenerative approaches for periodontal hard and soft tissue reconstruction was to develop a consensus document based on the accompanying review paper and on additional materials submitted prior to and at the consensus group session. Methods: The review paper was sent to all the consensus group participants in advance of the consensus conference. In addition and also prior to the conference, individual consensus group members submitted additional material for consideration by the group. At the conference, each consensus group participant introduced themselves and provided disclosure of any potential conflicts of interest. The review paper was briefly presented by two of the authors and discussed by the consensus group. A discussion of each of the following topics then occurred first based on the content of the review: a general summary of the topic, implications for patient reported outcomes and, suggested research priorities for the future. As each topic was discussed based upon the review article, supplemental information was then added that the consensus group agreed upon. Lastly, an updated reference list was created. Results: The application of protein and peptide therapy, cell-based therapy, genetic therapy, application of scaffolds, bone anabolics and lasers were found to be emerging technologies for periodontal regeneration. Other approaches included: therapies directed at the resolution of inflammation, therapies that took into account the influence of the microbiome, therapies involving the local regulation of phosphate and pyrophosphate metabolism, as well as approaches directed at harnessing current therapies used for other purposes. The results indicate that with most emerging technologies, the specific mechanisms of action are not well understood nor are the specific target cells identified. Patient related outcomes were typically not addressed in the literature. Numerous recommendations can be made for future research priorities for both basic science and clinical application of emerging therapies. The need to emphasize the importance of regeneration of a functional periodontal organ system was noted. The predictability and efficacy of outcomes as well as safety concerns and the cost/benefit ratio were also identified as key factors for emerging technologies. Conclusion: A number of technologies appear viable as emerging regenerative approaches for periodontal hard and soft tissue regeneration and are expanding the potential of reconstructing the entire periodontal organ system. The cost/benefit ratio as well as safety issues are important considerations for any new emerging therapies. Clinical Recommendation: At this time, there is insufficient evidence on emerging periodontal regenerative technologies to warrant definitive clinical recommendations.
    Journal of Periodontology 10/2014; 86(2-s):1-5. DOI:10.1902/jop.2015.140381 · 2.57 Impact Factor