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: 3.23). 06/2012; 7(6):e38393. DOI: 10.1371/journal.pone.0038393
Source: PubMed


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.

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Available from: Brian L Foster, Mar 29, 2014
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    • "Then, cells migrate until they reach their correct position in the brain. TNAP is strongly expressed in these early stages in the neural tube, and in migrating primordial germ cells [19] [29] [33], a subpopulation of neuroectodermal cells. In mice, these cells are characterized by moving from the epithelium of the hind gut to the genital ridges around embryonic day 8 (E8) [34]. "
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    ABSTRACT: Tissue-nonspecific alkaline phosphatase (TNAP) is one of four isozymes in humans and mice that have the capacity to hydrolyze phosphate groups from a wide spectrum of physiological substrates. Among these, TNAP degrades substrates implicated in neurotransmission. Transgenic mice lacking TNAP activity display the characteristic skeletal and dental phenotype of infantile hypophosphatasia, as well as spontaneous epileptic seizures and die around 10 days after birth. This physiopathology, linked to the expression pattern of TNAP in the central nervous system (CNS) during embryonic stages, suggests an important role for TNAP in neuronal development and synaptic function, situating it as a good target to be explored for the treatment of neurological diseases. In this review, we will focus mainly on the role that TNAP plays as an ectonucleotidase in the central nervous system regulating the levels of extracellular ATP and consequently purinergic signaling.
    Computational and Structural Biotechnology Journal 12/2014; 2. DOI:10.1016/j.csbj.2014.12.004
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    • "It remains unclear how particular ALPL mutations cause more severe clinical forms of HPP, in contrast to mild forms, including odonto-HPP. Dental tissues have been reported to be highly sensitive to dysregulation of phosphate and pyrophosphate metabolism [42] [43] [44], perhaps indicating that less deleterious ALPL mutations may preferentially affect the dentition , but not the broader skeleton. "
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    ABSTRACT: Hypophosphatasia (HPP) is an inherited disorder of mineral metabolism caused by mutations in ALPL, encoding tissue non-specific alkaline phosphatase (TNAP). Here, we report the molecular findings from monozygotic twins, clinically diagnosed with tooth-specific odontohypophosphatasia (odonto-HPP). Sequencing of ALPL identified two genetic alterations in the probands, including a heterozygous missense mutation c.454C>T, leading to change of arginine 152 to cysteine (p.R152C), and a novel heterozygous gene deletion c.1318_1320delAAC, leading to the loss of an asparagine residue at codon 440 (p.N440del). Clinical identification of low serum TNAP activity, dental abnormalities, and pedigree data strongly suggest a genotype-phenotype correlation between p.N440del and odonto-HPP in this family. Computational analysis of the p.N440del protein structure revealed an alteration in tertiary structure affecting the collagen-binding site (loop 422-452), which could potentially impair the mineralization process. Nevertheless, the Probands (compound heterozygous: p.[N440del];[R152C]) feature early-onset and severe odonto-HPP phenotype, whereas the father (p.[N440del];[=]) has only moderate symptoms, suggesting p.R152C may contribute or predispose to a more severe dental phenotype in combination with the deletion. These results assist in defining the genotype-phenotype associations for odonto-HPP, and further identify the collagen-binding site as a region of potential structural importance for TNAP function in the biomineralization.
    Bone 06/2013; 56(2). DOI:10.1016/j.bone.2013.06.010 · 3.97 Impact Factor
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    • "Mice with background of CD1 or mixed background of 129S1/SvImJ or C57BL/6 were used for histological and immunohistochemical staining studies. Mice null for tissue nonspecific alkaline phosphatase (gene Alpl) were employed as a model featuring deficient cementum formation.9 Alpl−/− mice were prepared and genotyped as previously described.9,10 "
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    ABSTRACT: The tooth root cementum is a thin, mineralized tissue covering the root dentin that is present primarily as acellular cementum on the cervical root and cellular cementum covering the apical root. While cementum shares many properties in common with bone and dentin, it is a unique mineralized tissue and acellular cementum is critical for attachment of the tooth to the surrounding periodontal ligament (PDL). Resources for methodologies for hard tissues often overlook cementum and approaches that may be of value for studying this tissue. To address this issue, this report offers detailed methodology, as well as comparisons of several histological and immunohistochemical stains available for imaging the cementum-PDL complex by light microscopy. Notably, the infrequently used Alcian blue stain with nuclear fast red counterstain provided utility in imaging cementum in mouse, porcine and human teeth. While no truly unique extracellular matrix markers have been identified to differentiate cementum from the other hard tissues, immunohistochemistry for detection of bone sialoprotein (BSP), osteopontin (OPN), and dentin matrix protein 1 (DMP1) is a reliable approach for studying both acellular and cellular cementum and providing insight into developmental biology of these tissues. Histological and immunohistochemical approaches provide insight on developmental biology of cementum.
    International Journal of Oral Science 09/2012; 4(3):119-28. DOI:10.1038/ijos.2012.57 · 2.53 Impact Factor
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