Hypercementosis Associated with ENPP1 Mutations and GACI

[The tooth: A marker of developmental abnormalities]

Tooth formation results from specific epithelial-mesenchymal interactions, which summarize a number of developmental processes. Tooth anomalies may thus reflect subclinical diseases of the kidney, bone and more broadly of the mineral metabolism, skin or nervous system. Odontogenesis starts from the 3rd week of intrauterine life by the odontogenic orientation of epithelial cells by a first PITX2 signal. The second phase is the acquisition of the number, shape, and position of teeth. It depends on multiple transcription and growth factors (BMP, FGF, SHH, WNT). These ecto-mesenchymal interactions guide cell migration, proliferation, apoptosis and differentiation ending in the formation of the specific dental mineralized tissues. Thus, any alteration will have consequences on the tooth structure or shape. Resulting manifestations will have to be considered in the patient phenotype and the multidisciplinary care, but also may contribute to identify the altered genetic circuity.

Hypercementosis in Late Pleistocene Homo sapiens fossils from Klasies River Main Site, South Africa

OBJECTIVE

To examine early Homo sapiens fossils from the Late Pleistocene site of Klasies River Main Site, South Africa for evidence of hypercementosis. The specimens represent seven adult individuals dated to between 119,000 and 58,000 years ago. These observations are contextualized in relation to the incidences of hypercementosis among recent human populations and fossil human samples and the potential etiologies of hypercementosis.

DESIGN

The fossils were investigated utilizing micro-CT and nano-CT scanning to visualize and measure cementum apposition on permanent incisor, premolar and molar roots. Cementum thickness was measured at mid-root level, and the volume of the cementum sleeve was calculated for the two fossil specimens that display marked hypercementosis.

RESULTS

Two of the fossils display no evidence of cementum hypertrophy. Three exhibit moderate cementum thickening, barely attaining the quantitative threshold for hypercementosis. Two evince marked hypercementosis. One of the Klasies specimens with marked hypercementosis is judged to be an older individual with periapical abscessing. The second specimen is a younger adult, and seemingly similar in age to other Klasies fossils that exhibit only minimal cementum apposition. However, this second specimen exhibits dento-alveolar ankylosis of the premolar and molars.

CONCLUSIONS

These two fossils from Klasies River Main Site provide the earliest manifestation of hypercementosis in Homo sapiens.

Beyond dirty teeth: Integrating dental calculus studies with osteoarchaeological parameters

The study of ancient human dental calculus (mineralized dental plaque, also known as tartar) is becoming increasingly important in osteoarchaeology, human palaeoecology and environmental archaeology. Microremains of different origin (e.g. starch granules, pollen, phytoliths, feather barbules) as well as biomolecules and chemical compounds retrieved from its mineral matrix may represent an important link between past humans and their physical, biological and social environment, but they are rarely fully linked to the evidence from skeletal remains. This paper critically reviews the lines of evidence retrieved from dental calculus in relation to osteoarchaeological parameters, employing macroscopic, microscopic and biomolecular approaches, assessing synergy potential and limitations. The scope of this paper is also to contribute to the building of a much needed theoretical framework in this emerging subfield.

Npp1 prevents external tooth root resorption by regulation of cervical cementum integrity

Tooth roots embedded in the alveolar bone do not typically undergo resorption while the bone continues remodeling in its physiological state. In this study, we analyzed genetically modified mice with the functional inactivation of nucleotide pyrophosphatase 1 (Npp1), encoded by ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1). This mutation leads to the formation of ectopic cervical cementum vulnerable to external tooth root resorption. Cementoblasts with the inactivation of Enpp1 extensively expressed non-collagenous matrix proteins enriched with bone sialoprotein (Bsp), dentin matrix protein 1 (Dmp1), and osteopontin (Opn), which have roles in mineralization through nucleation and in cell adhesion through the Arg-Gly-Asp (RGD) motif. In cementoblasts with the inactivation of Enpp1, β-catenin was significantly activated and induced the expression of these non-collagenous matrix proteins. In addition, adenosine triphosphate (ATP), which is the most preferred substrate of Npp1, accumulated extracellularly and autocrinally induced the expression of the receptor activator of nuclear factor κB ligand (Rankl) in cementoblasts with inactivated Npp1. Consequently, these results strongly suggest that functional Npp1 preserves cervical cementum integrity and supports the anti-resorptive properties of tooth roots through ATP homeostasis in the physiological state of cervical cementum.

ENPP1 deficiency: A clinical update on the relevance of individual variants using a locus-specific patient database

Loss-of-function variants in the ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) cause ENPP1 Deficiency, a rare disorder characterized by pathological calcification, neointimal proliferation, and impaired bone mineralization. The consequence of ENPP1 Deficiency is a broad range of age dependent symptoms and morbidities including cardiovascular complications and 50% mortality in infants, autosomal recessive hypophosphatemic rickets type 2 (ARHR2) in children, and joint pain, osteomalacia and enthesopathies in adults. Recent research continues to add to the growing clinical presentation profile as well as expanding the role of ENPP1 itself. Here we review the current knowledge on the spectrum of clinical and genetic findings of ENPP1 Deficiency reported in patients diagnosed with GACI or ARHR2 phenotypes using a comprehensive database of known ENPP1 variants with associated clinical data. A total of 108 genotypes were identified from 154 patients. Of the 109 ENPP1 variants reviewed, 72.5% were demonstrably disease-causing, a threefold increase in pathogenic/likely pathogenic variants over other databases. There is substantial heterogeneity in disease severity, even among patients with the same variant. The approach to creating a continuously curated database of ENPP1 variants accessible to clinicians is necessary to increase the diagnostic yield of clinical genetic testing and accelerate diagnosis of ENPP1 Deficiency.

Catalysis-Independent ENPP1 Protein Signaling Regulates Mammalian Bone Mass

Biallelic ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) deficiency induces vascular/soft tissue calcifications in generalized arterial calcification of infancy (GACI), and low bone mass with phosphate-wasting rickets in GACI survivors (autosomal hypophosphatemic rickets type-2). ENPP1 haploinsufficiency induces early-onset osteoporosis and mild phosphate wasting in adults. Both conditions demonstrate the unusual combination of reduced accrual of skeletal mineral, yet excess and progressive heterotopic mineralization. ENPP1 is the only enzyme that generates extracellular pyrophosphate (PPi), a potent inhibitor of both bone and heterotopic mineralization. Life-threatening vascular calcification in ENPP1 deficiency is due to decreased plasma PPi; however, the mechanism by which osteopenia results is not apparent from an understanding of the enzyme's catalytic activity. To probe for catalysis-independent ENPP1 pathways regulating bone, we developed a murine model uncoupling ENPP1 protein signaling from ENPP1 catalysis, Enpp1T238A mice. In contrast to Enpp1asj mice, which lack ENPP1, Enpp1T238A mice have normal trabecular bone microarchitecture and favorable biomechanical properties. However, both models demonstrate low plasma Pi and PPi, increased fibroblast growth factor 23 (FGF23), and by 23 weeks, osteomalacia demonstrating equivalent phosphate wasting in both models. Reflecting findings in whole bone, calvarial cell cultures from Enpp1asj mice demonstrated markedly decreased calcification, elevated transcription of Sfrp1, and decreased nuclear β-catenin signaling compared to wild-type (WT) and Enpp1T238A cultures. Finally, the decreased calcification and nuclear β-catenin signaling observed in Enpp1asj cultures was restored to WT levels by knockout of Sfrp1. Collectively, our findings demonstrate that catalysis-independent ENPP1 signaling pathways regulate bone mass via the expression of soluble Wnt inhibitors such as secreted frizzled-related protein 1 (SFRP1), whereas catalysis dependent pathways regulate phosphate homeostasis through the regulation of plasma FGF23. © 2022 American Society for Bone and Mineral Research (ASBMR).

Mutation update: Variants of the ENPP1 gene in pathologic calcification, hypophosphatemic rickets, and cutaneous hypopigmentation with punctate keratoderma

ENPP1 encodes ENPP1, an ectonucleotidase catalyzing hydrolysis of ATP to AMP and inorganic pyrophosphate (PPi), and an endogenous plasma protein physiologically preventing ectopic calcification of connective tissues. Mutations in ENPP1 have been reported in association with a range of human genetic diseases. In this mutation update, we provide a comprehensive review of all the pathogenic variants, likely pathogenic variants, and variants of unknown significance in ENPP1 associated with three autosomal recessive disorders-generalized arterial calcification of infancy (GACI), autosomal recessive hypophosphatemic rickets type 2 (ARHR2), and pseudoxanthoma elasticum (PXE), as well as with a predominantly autosomal dominant disorder-Cole disease. The classification of all variants is determined using the latest ACMG guidelines. A total of 140 ENPP1 variants were curated consisting of 133 previously reported variants and seven novel variants, with missense variants being the most prevalent (70.0%, 98/140). While the pathogenic variants are widely distributed in the ENPP1 gene of patientsgen without apparent genotype-phenotype correlation, eight out of nine variants associated with Cole disease are confined to the somatomedin-B-like (SMB) domains critical for homo-dimerization of the ENPP1 protein.

Between a rock and a hard place: Regulation of mineralization in the periodontium

The periodontium supports and attaches teeth via mineralized and nonmineralized tissues. It consists of two, unique mineralized tissues, cementum and alveolar bone. In between these tissues, lies an unmineralized, fibrous periodontal ligament (PDL), which distributes occlusal forces, nourishes and invests teeth, and harbors progenitor cells for dentoalveolar repair. Many unanswered questions remain regarding periodontal biology. This review will focus on recent research providing insights into one enduring mystery: the precise regulation of the hard-soft tissue borders in the periodontium which define the interfaces of the cementum-PDL-alveolar bone structure. We will focus on advances in understanding the molecular mechanisms that maintain the unmineralized PDL "between a rock and a hard place" by regulating the mineralization of cementum and alveolar bone.

Prevalence and Manifestations of Dental Ankylosis in Primary Molars Using Panoramic X-rays: A Cross-Sectional Study

Dental ankylosis is a serious condition defined as the process that causes the fusion between the dentin or the cementum of the root and the alveolar bone, with the obliteration of the periodontal ligament becoming progressively replaced by bone tissue. The aim of the study was to determine the prevalence, location, severity, and association of dental ankylosis in primary molars with other dental anomalies such as the agenesis of permanent buds. For this study 150 panoramic x-rays were selected from patients with temporary or mixed dentition, aged six to twelve years old, from a private dentistry office and from the Pediatric Dentistry Department of UMFST in Targu-Mures, Romania. In order to identify the cases with dental ankylosis, the presence and severity of the infraocclusion, displacements of the neighboring teeth, the appearance of the root area, and the relationship with the bone tissue were examined. For evaluation of the categorical data we used Fischer’s exact test and the Chi-squared test and the chosen significance level was set at 0.05. The results showed that the highest percentage of cases with ankylosis was found in the first group (six to nine years old), respectively, with 72% of cases compared with the second group (ten to twelve years old) with 28% of cases. Findings showed that there was no positive association between dental ankylosis and gender, but a strong correlation was found regarding the location on the dental arches. Most cases were identified on the lower arch with a higher percentage in quadrant three. Of the two primary molars, the most affected by ankylosis was the first molar in quadrant three, followed by the second molar, and finally the first molar in quadrant four. Most cases diagnosed with ankylosis had a mild to moderate degree of infraocclusion; therefore, changes in the functional balance of the dental arch and on neighboring teeth were insignificant. There were some differences obtained between our results and studies from the literature, especially regarding the localization in the lower left dental arch, but these differences can be attributed to the number of the subjects selected and from the methodology of dental ankylosis diagnosis. Based on the data obtained, it was concluded that ankylosis is a dental condition which occurs in children in early mixed dentition, especially in the lower arch, with the first primary molar being the most affected tooth. The presence of infraocclusion and the absence of dental mobility, especially during the stage of primary molars’ root resorption, are the main signs which must be followed to make an early diagnosis and prevent further complications.

Matrix Vesicle-Mediated Mineralization and Potential Applications

Hard tissues, including the bones and teeth, are a fundamental part of the body, and their formation and homeostasis are critically regulated by matrix vesicle-mediated mineralization. Matrix vesicles have been studied for 50 y since they were first observed using electron microscopy. However, research progress has been hampered by various technical barriers. Recently, there have been great advancements in our understanding of the intracellular biosynthesis of matrix vesicles. Mitochondria and lysosomes are now considered key players in matrix vesicle formation. The involvement of mitophagy, mitochondrial-derived vesicles, and mitochondria-lysosome interaction have been suggested as potential detailed mechanisms of the intracellular pathway of matrix vesicles. Their main secretion pathway may be exocytosis, in addition to the traditionally understood mechanism of budding from the outer plasma membrane. This basic knowledge of matrix vesicles should be strengthened by novel nano-level microscopic technologies, together with basic cell biologies, such as autophagy and interorganelle interactions. In the field of tissue regeneration, extracellular vesicles such as exosomes are gaining interest as promising tools in cell-free bone and periodontal regenerative therapy. Matrix vesicles, which are recognized as a special type of extracellular vesicles, could be another potential alternative. In this review, we outline the recent significant progress in the process of matrix vesicle-mediated mineralization and the potential clinical applications of matrix vesicles for tissue regeneration.

Identification of ENPP1 Haploinsufficiency in Patients With Diffuse Idiopathic Skeletal Hyperostosis and Early-Onset Osteoporosis

Homozygous ENPP1 mutations are associated with autosomal recessive hypophosphatemic rickets type 2 (ARHR2), severe ossification of the spinal ligaments, and generalized arterial calcification of infancy type 1. There are a limited number of reports on phenotypes associated with heterozygous ENPP1 mutations. Here, we report a series of three probands and their families with heterozygous and compound heterozygous ENPP1 mutations. The first case (case 1) was a 47-year-old male, diagnosed with early-onset osteoporosis and low-normal serum phosphate levels, which invoked suspicion for hypophosphatemic rickets. The second and third cases were 77- and 54-year-old females who both presented with severe spinal ligament ossification and the presumptive diagnosis of diffuse idiopathic skeletal hyperostosis (DISH). Upon workup, fibroblast growth factor 23 (FGF23) was noted to be relatively high in case 2 and serum phosphorous was low-normal in case 3, and the diagnoses of X-linked hypophosphatemic rickets (XLH) and ARHR2 were considered. Genetic testing for genes related to congenital hypophosphatemic rickets was therefore performed, revealing heterozygous ENPP1 variants in cases 1 and 2 (case 1, c.536A>G, p.Asn179Ser; case 2, c.1352A>G, p.Tyr451Cys) and compound heterozygous ENPP1 variants in case 3 constituting the same variants present in cases 1 and 2 (c.536A>G, p.Asn179Ser and c.1352A>G, p.Tyr451Cys). Several in silico tools predicted the two variants to be pathogeneic, a finding confirmed by in vitro biochemical analysis demonstrating that the p.Asn179Ser and p.Tyr451Cys ENPP1 variants possessed a catalytic velocity of 45% and 30% compared with that of wild-type ENPP1, respectively. Both variants were therefore categorized as pathogenic loss-of-function mutations. Our findings suggest that ENPP1 mutational status should be evaluated in patients presenting with the diagnosis of idiopathic DISH, ossification of the posterior longitudinal ligament (OPLL), and early-onset osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Short histological kaleidoscope - recent findings in histology. Part I

This article is a review of new advances in histology, concerning either classification or structure of different tissular elements (basement membrane, hemidesmosomes, urothelium, glandular epithelia, adipose tissue, astrocytes), and various organs' constituents (blood-brain barrier, human dental cementum, tubarial salivary glands, hepatic stellate cells, pineal gland, fibroblasts of renal interstitium, Leydig testicular cells, ovarian hilar cells), as well as novel biotechnological techniques (tissue engineering in angiogenesis), recently introduced.

Orthodontic tooth movement alters cementocyte ultrastructure and cellular cementum proteome signature

Cementum is a mineralized tissue that covers tooth roots and functions in the periodontal attachment complex. Cementocytes, resident cells of cellular cementum, share many characteristics with osteocytes, are mechanoresponsive cells that direct bone remodeling based on changes in loading. We hypothesized that cementocytes play a key role during orthodontic tooth movement (OTM). To test this hypothesis, we used 8-week-old male Wistar rats in a model of OTM for 2, 7, or 14 days (0.5 N), whereas unloaded contralateral teeth served as controls. Tissue and cell responses were analyzed by high-resolution micro-computed tomography, histology, tartrate-resistant acid phosphatase staining for odontoclasts/osteoclasts, and transmission electron microscopy. In addition, laser capture microdissection was used to collect cellular cementum, and extracted proteins were identified by liquid chromatography coupled to tandem mass spectrometry. The OTM model successfully moved first molars mesially more than 250 μm by 14 days introducing apoptosis in a small number of cementocytes and areas of root resorption on mesial and distal aspects. Cementocytes showed increased nuclear size and proportion of euchromatin suggesting cellular activity. Proteomic analysis identified 168 proteins in cellular cementum with 21 proteins found only in OTM sites and 54 proteins only present in control samples. OTM-down-regulated several extracellular matrix proteins, including decorin, biglycan, asporin, and periostin, localized to cementum and PDL by immunostaining. Furthermore, type IV collagen (COL14A1) was the protein most down-regulated (-45-fold) by OTM and immunolocalized to cells at the cementum-dentin junction. Eleven keratins were significantly increased by OTM, and a pan-keratin antibody indicated keratin localization primarily in epithelial remnants of Hertwig's epithelial root sheath. These experiments provide new insights into biological responses of cementocytes and cellular cementum to OTM.

A Perspective: Integrating Dental and Medical Research Improves Overall Health

The past decade has seen marked increases in research findings identifying oral-systemic links. Yet, much of dental research remains poorly integrated with mainstream biomedical research. The historic separation of dentistry from medicine has led to siloed approaches in education, research and practice, ultimately depriving patients, providers, and policy makers of findings that could benefit overall health and well-being. These omissions amount to lost opportunities for risk assessment, diagnosis, early intervention and prevention of disease, increasing cost and contributing to a fragmented and inefficient healthcare delivery system. This perspective provides examples where fostering interprofessional research collaborations has advanced scientific understanding and yielded clinical benefits. In contrast are examples where failure to include dental research findings has limited progress and led to adverse health outcomes. The impetus to overcome the dental-medical research divide gains further urgency today in light of the coronavirus pandemic where contributions that dental research can make to understanding the pathophysiology of the SARS-CoV-2 virus and in diagnosing and preventing infection are described. Eliminating the research divide will require collaborative and trans-disciplinary research to ensure incorporation of dental research findings in broad areas of biomedical research. Enhanced communication, including interoperable dental/medical electronic health records and educational efforts will be needed so that the public, health care providers, researchers, professional schools, organizations, and policymakers can fully utilize oral health scientific information to meet the overall health needs of the public.

Generalized Arterial Calcification of Infancy Type 1 (GACI1): Identification of a Novel Pathogenic Variant (c.1715T>C (p.Leu572Ser))

Generalized Arterial Calcification of Infancy (GACI) is a rare disease inherited in a recessive manner, with severe and diffuse early onset of calcifications along the internal elastic lamina in large and medium size arteries. The diagnosis results are from clinical manifestations, imaging, histopathologic exams, and genetic tests. GACI is predominantly caused by biallelic pathogenic variant in the ENPP1 gene (GACI1, OMIM#208000) and, to a lesser extent, by pathogenic variants in the ABCC6 gene (GACI2, OMIM#614473). We present a novel variation in the ENPP1 gene identified in a patient clinically diagnosed with GACI and confirmed by genetic investigation and autopsy as GACI type 1. The sequence analysis of the patient's ENPP1 gene detected two heterozygous variants c.1412A>G (p.Tyr471Cys) and c.1715T>C (p.Leu572Ser). The variant c.1715T>C (p.Leu572Ser) has not been described yet in the literature and in mutation databases. A genetic analysis was also carried out for the parents of the newborn; the heterozygous pathogenic variant c.1412A>G (p.Tyr471Cys) was detected in the mother's ENPP1 gene, and a sequence analysis of the father's ENPP1 gene revealed the novel heterozygous variant c.1715T>C (p.Leu572Ser). Our results showed that the variant c.1715T>C (p.Leu572Ser) may have a pathogenic role in the development of GACI type1 (GACI1, OMIM#208000), at least when associated with the pathogenic c.1412A>G (p.Tyr471Cys) variant. The identification of novel mutations potentially enabled genotype/phenotype associations that will ultimately have an impact on clinical management and prognosis for the disease.

The role of biomineralization in disorders of skeletal development and tooth formation

The major mineralized tissues are bone and teeth, which share several mechanisms governing their development and mineralization. This crossover includes the hormones that regulate circulating calcium and phosphate concentrations, and the genes that regulate the differentiation and transdifferentiation of cells. In developing endochondral bone and in developing teeth, parathyroid hormone-related protein (PTHrP) acts in chondrocytes to delay terminal differentiation, thereby increasing the pool of precursor cells. Chondrocytes and (in specific circumstances) pre-odontoblasts can also transdifferentiate into osteoblasts. Moreover, bone and teeth share outcomes when affected by systemic disorders of mineral homeostasis or of the extracellular matrix, and by adverse effects of treatments such as bisphosphonates and fluoride. Unlike bone, teeth have more permanent effects from systemic disorders because they are not remodelled after they are formed. This Review discusses the normal processes of bone and tooth development, followed by disorders that have effects on both bone and teeth, versus disorders that have effects in one without affecting the other. The takeaway message is that bone specialists should know when to screen for dental disorders, just as dental specialists should recognize when a tooth disorder should raise suspicions about a possible underlying bone disorder.

Cementocyte alterations associated with experimentally induced cellular cementum apposition in hyp mice

BACKGROUND

Cellular cementum, a mineralized tissue covering apical tooth roots, grows by apposition to maintain the tooth in its occlusal position. We hypothesized that resident cementocytes would show morphological changes in response to cementum apposition, possibly implicating a role in cementum biology.

METHODS

Mandibular first molars were induced to super-erupt (EIA) by extraction of maxillary molars, promoting rapid new cementum formation. Tissue and cell responses were analyzed at 6 and/or 21 days post-procedure (dpp).

RESULTS

High-resolution micro-computed tomography (micro-CT) and confocal laser scanning microscopy showed increased cellular cementum by 21 dpp. Transmission electron microscopy (TEM) revealed that cementocytes under EIA were 50% larger than control cells, supported by larger pore sizes detected by micro-CT. Cementocytes under EIA displayed ultrastructural changes consistent with increased activity, including increased cytoplasm and nuclear size. We applied EIA to Hyp mutant mice, where cementocytes have perilacunar hypomineralization defects, to test cell and tissue responses in an altered mechanoresponsive milieu. Hyp and WT molars displayed similar super-eruption, with Hyp molars exhibiting 28% increased cellular cementum area versus 22% in WT mice at 21 dpp. Compared to control, Hyp cementocytes featured well-defined, disperse euchromatin and a thick layer of peripherally condensed heterochromatin in nuclei, indicating cellular activity. Immunohistochemistry (IHC) for cementum markers revealed intense dentin matrix protein-1 expression and abnormal osteopontin deposition in Hyp mice. Both WT and Hyp cementocytes expressed gap junction protein, connexin 43.

CONCLUSION

This study provides new insights into the EIA model and cementocyte activity in association with new cementum formation.

Delivery of Alkaline Phosphatase Promotes Periodontal Regeneration in Mice

Factors regulating the ratio of pyrophosphate (PP_i) to phosphate (P_i) modulate biomineralization. Tissue-nonspecific alkaline phosphatase (TNAP) is a key promineralization enzyme that hydrolyzes the potent mineralization inhibitor PP_i. The goal of this study was to determine whether TNAP could promote periodontal regeneration in bone sialoprotein knockout mice (Ibsp^-/- mice), which are known to have a periodontal disease phenotype. Delivery of TNAP was accomplished either systemically (through a lentiviral construct expressing a mineral-targeted TNAP-D₁₀ protein) or locally (through addition of recombinant human TNAP to a fenestration defect model). Systemic TNAP-D₁₀ delivered by intramuscular injection at 5 d postnatal (dpn) increased circulating alkaline phosphatase (ALP) levels in Ibsp^-/- mice by 5-fold at 30 dpn, with levels returning to normal by 60 dpn when tissues were evaluated by micro-computed tomography and histology. Local delivery of recombinant human TNAP to fenestration defects in 5-wk-old wild type (WT) and Ibsp^-/- mice did not alter long-term circulating ALP levels, and tissues were evaluated by micro-computed tomography and histology at postoperative day 45. Systemic and local delivery of TNAP significantly increased alveolar bone volume (20% and 37%, respectively) and cementum thickness (3- and 42-fold) in Ibsp^-/- mice, with evidence for periodontal ligament attachment and bone/cementum marker localization. Local delivery significantly increased regenerated cementum and bone in WT mice. Addition of 100-μg/mL bovine intestinal ALP to culture media to increase ALP in vitro increased media P_i concentration, mineralization, and Spp1 and Dmp1 marker gene expression in WT and Ibsp^-/- OCCM.30 cementoblasts. Use of phosphonoformic acid, a nonspecific inhibitor of sodium P_i cotransport, indicated that effects of bovine intestinal ALP on mineralization and marker gene expression were in part through P_i transport. These findings show for the first time through multiple in vivo and in vitro approaches that pharmacologic modulation of P_i/PP_i metabolism can overcome periodontal breakdown and accomplish regeneration.

Guidelines for Micro-Computed Tomography Analysis of Rodent Dentoalveolar Tissues

Micro–computed tomography (μCT) has become essential for analysis of mineralized as well as nonmineralized tissues and is therefore widely applicable in the life sciences. However, lack of standardized approaches and protocols for scanning, analyzing, and reporting data often makes it difficult to understand exactly how analyses were performed, how to interpret results, and if findings can be broadly compared with other models and studies. This problem is compounded in analysis of the dentoalveolar complex by the presence of four distinct mineralized tissues: enamel, dentin, cementum, and alveolar bone. Furthermore, these hard tissues interface with adjacent soft tissues, the dental pulp and periodontal ligament (PDL), making for a complex organ. Drawing on others' and our own experience analyzing rodent dentoalveolar tissues by μCT, we introduce techniques to successfully analyze dentoalveolar tissues with similar or disparate compositions, densities, and morphological characteristics. Our goal is to provide practical guidelines for μCT analysis of rodent dentoalveolar tissues, including approaches to optimize scan parameters (filters, voltage, voxel size, and integration time), reproducibly orient samples, define regions and volumes of interest, segment and subdivide tissues, interpret findings, and report methods and results. We include illustrative examples of analyses performed on genetically engineered mouse models with phenotypes in enamel, dentin, cementum, and alveolar bone. The recommendations are designed to increase transparency and reproducibility, promote best practices, and provide a basic framework to apply μCT analysis to the dentoalveolar complex that can also be extrapolated to a variety of other tissues of the body. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Dental defects in the primary dentition associated with hypophosphatasia from biallelic ALPL mutations

ALPL encodes tissue-nonspecific alkaline phosphatase (TNAP), an enzyme expressed in bone, teeth, liver, and kidney. ALPL loss-of-function mutations cause hypophosphatasia (HPP), an inborn error-of-metabolism that produces skeletal and dental mineralization defects. Case reports describe widely varying dental phenotypes, making it unclear how HPP comparatively affects the three unique dental mineralized tissues: enamel, dentin, and cementum. We hypothesized that HPP affected all dental mineralized tissues and aimed to establish quantitative measurements of dental tissues in a subject with HPP. The female proband was diagnosed with HPP during childhood based on reduced alkaline phosphatase activity (ALP), mild rachitic skeletal effects, and premature primary tooth loss. The diagnosis was subsequently confirmed genetically by the presence of compound heterozygous ALPL mutations (exon 5: c.346G>A, p.A116T; exon 10: c.1077C>G, p.I359M). Dental defects in 8 prematurely exfoliated primary teeth were analyzed by high resolution micro-computed tomography (micro-CT) and histology. Similarities to the Alpl^-/- mouse model of HPP were identified by additional analyses of murine dentoalveolar tissues. Primary teeth from the proband exhibited substantial remaining root structure compared to healthy control teeth. Enamel and dentin densities were not adversely affected in HPP vs. control teeth. However, analysis of discrete dentin regions revealed an approximate 10% reduction in the density of outer mantle dentin of HPP vs. control teeth. All 4 incisors and the molar lacked acellular cementum by micro-CT and histology, but surprisingly, 2 of 3 prematurely exfoliated canines exhibited apparently normal acellular cementum. Based on dentin findings in the proband's teeth, we examined dentoalveolar tissues in a mouse model of HPP, revealing that the delayed initiation of mineralization in the incisor mantle dentin was associated with a broader lack of circumpulpal dentin mineralization. This study describes a quantitative approach to measure effects of HPP on dental tissues. This approach has uncovered a previously unrecognized novel mantle dentin defect in HPP, as well as a surprising and variable cementum phenotype within the teeth from the same HPP subject.

Congenital Conditions of Hypophosphatemia in Children

Great strides over the past few decades have increased our understanding of the pathophysiology of hypophosphatemic disorders. Phosphate is critically important to a variety of physiologic processes, including skeletal growth, development and mineralization, as well as DNA, RNA, phospholipids, and signaling pathways. Consequently, hypophosphatemic disorders have effects on multiple systems, and may cause a variety of nonspecific signs and symptoms. The acute effects of hypophosphatemia include neuromuscular symptoms and compromise. However, the dominant effects of chronic hypophosphatemia are the effects on musculoskeletal function including rickets, osteomalacia and impaired growth during childhood. While the most common causes of chronic hypophosphatemia in children are congenital, some acquired conditions also result in hypophosphatemia during childhood through a variety of mechanisms. Improved understanding of the pathophysiology of these congenital conditions has led to novel therapeutic approaches. This article will review the pathophysiologic causes of congenital hypophosphatemia, their clinical consequences and medical therapy.

Ablation of Pyrophosphate Regulators Promotes Periodontal Regeneration

Biomineralization is regulated by inorganic pyrophosphate (PP_i), a potent physiological inhibitor of hydroxyapatite crystal growth. Progressive ankylosis protein (ANK) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) act to increase local extracellular levels of PP_i, inhibiting mineralization. The periodontal complex includes 2 mineralized tissues, cementum and alveolar bone (AB), both essential for tooth attachment. Previous studies demonstrated that loss of function of ANK or ENPP1 (reducing PP_i) resulted in increased cementum formation, suggesting PP_i metabolism may be a target for periodontal regenerative therapies. To compare the effects of genetic ablation of Ank, Enpp1, and both factors concurrently on cementum and AB regeneration, mandibular fenestration defects were created in Ank knockout (Ank KO), Enpp1 mutant (Enpp1^asj/asj), and double KO (dKO) mice. Genetic ablation of Ank, Enpp1, or both factors increased cementum regeneration compared to controls at postoperative days (PODs) 15 and 30 (Ank KO: 8-fold, 3-fold; Enpp1^asj/asj: 7-fold, 3-fold; dKO: 11-fold, 4-fold, respectively) associated with increased fluorochrome labeling and expression of mineralized tissue markers, dentin matrix protein 1 (Dmp1/DMP1), osteopontin (Spp1/OPN), and bone sialoprotein (Ibsp/BSP). Furthermore, dKO mice featured increased cementum thickness compared to single KOs at POD15 and Ank KO at POD30. No differences were noted in AB volume between genotypes, but osteoblast/osteocyte markers were increased in all KOs, partially mineralized osteoid volume was increased in dKO versus controls at POD15 (3-fold), and mineral density was decreased in Enpp1^asj/asj and dKOs at POD30 (6% and 9%, respectively). Increased numbers of osteoclasts were present in regenerated AB of all KOs versus controls. These preclinical studies suggest PP_i modulation as a potential and novel approach for cementum regeneration, particularly targeting ENPP1 and/or ANK. Differences in cementum and AB regeneration in response to reduced PP_i conditions highlight the need to consider tissue-specific responses in strategies targeting regeneration of the entire periodontal complex.

Tissue-Nonspecific Alkaline Phosphatase-A Gatekeeper of Physiological Conditions in Health and a Modulator of Biological Environments in Disease

Tissue-nonspecific alkaline phosphatase (TNAP) is a ubiquitously expressed enzyme that is best known for its role during mineralization processes in bones and skeleton. The enzyme metabolizes phosphate compounds like inorganic pyrophosphate and pyridoxal-5′-phosphate to provide, among others, inorganic phosphate for the mineralization and transportable vitamin B6 molecules. Patients with inherited loss of function mutations in the ALPL gene and consequently altered TNAP activity are suffering from the rare metabolic disease hypophosphatasia (HPP). This systemic disease is mainly characterized by impaired bone and dental mineralization but may also be accompanied by neurological symptoms, like anxiety disorders, seizures, and depression. HPP characteristically affects all ages and shows a wide range of clinical symptoms and disease severity, which results in the classification into different clinical subtypes. This review describes the molecular function of TNAP during the mineralization of bones and teeth, further discusses the current knowledge on the enzyme’s role in the nervous system and in sensory perception. An additional focus is set on the molecular role of TNAP in health and on functional observations reported in common laboratory vertebrate disease models, like rodents and zebrafish.

Insights into dental mineralization from three heritable mineralization disorders

Teeth are comprised of three unique mineralized tissues, enamel, dentin, and cementum, that are susceptible to developmental defects similar to those affecting bone. X-linked hypophosphatemia (XLH), caused by PHEX mutations, leads to increased fibroblast growth factor 23 (FGF23)-driven hypophosphatemia and local extracellular matrix disturbances. Hypophosphatasia (HPP), caused by ALPL mutations, results in increased levels of inorganic pyrophosphate (PP_i), a mineralization inhibitor. Generalized arterial calcification in infancy (GACI), caused by ENPP1 mutations, results in vascular calcification due to decreased PP_i, later compounded by FGF23-driven hypophosphatemia. In this perspective, we compare and contrast dental defects in primary teeth associated with XLH, HPP, and GACI, briefly reviewing genetic and biochemical features of these disorders and findings of clinical and preclinical studies to date, including some of our own recent observations. The distinct dental defects associated with the three heritable mineralization disorders reflect unique processes of the respective dental hard tissues, revealing insights into their development and clues about pathological mechanisms underlying such disorders.

Genetic and pharmacologic modulation of cementogenesis via pyrophosphate regulators

Pyrophosphate (PP_i) serves as a potent and physiologically important regulator of mineralization, with systemic and local concentrations determined by several key regulators, including: tissue-nonspecific alkaline phosphatase (ALPL gene; TNAP protein), the progressive ankylosis protein (ANKH; ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1; ENPP1). Results to date have indicated important roles for PP_i in cementum formation, and we addressed several gaps in knowledge by employing genetically edited mouse models where PP_i metabolism was disrupted and pharmacologically modulating PP_i in a PP_i-deficient mouse model. We demonstrate that acellular cementum growth is inversely proportional to PP_i levels, with reduced cementum in Alpl KO (increased PP_i levels) mice and excess cementum in Ank KO mice (decreased PP_i levels). Moreover, simultaneous ablation of Alpl and Ank results in reestablishment of functional cementum in dKO mice. Additional reduction of PP_i by dual deletion of Ank and Enpp1 does not further increase cementogenesis, and PDL space is maintained in part through bone modeling/remodeling by osteoclasts. Our results provide insights into cementum formation and expand our knowledge of how PP_i regulates cementum. We also demonstrate for the first time that pharmacologic manipulation of PP_i through an ENPP1-Fc fusion protein can regulate cementum growth, supporting therapeutic interventions targeting PP_i metabolism.

Generalized Arterial Calcification of Infancy: New Insights, Controversies, and Approach to Management

PURPOSE OF REVIEW

This review summarizes current understanding of generalized arterial calcification of infancy (GACI), emphasizing pathophysiology, clinical presentation, and approaches and controversies in management.

RECENT FINDINGS

Identification of causative ENPP1 mutations revealed that GACI arises from deficiencies in inorganic pyrophosphate (leading to calcifications) and adenosine monophosphate (leading to intimal proliferation). Identification of genotypic and phenotypic overlap with pseudoxanthoma elasticum and autosomal recessive hypophosphatemic rickets further advanced understanding of GACI as a complex, multisystemic disease. Clinical data is limited to small, retrospective samples; it is therefore unknown whether commonly used medications, such as bisphosphonates and hypophosphatemia treatment, are therapeutic or potentially harmful. ENPP1-Fc replacement represents a promising approach warranting further study. Knowledge gaps in natural history place clinicians at high risk of assigning causality to interventions that are correlated with changes in clinical status. There is thus a critical need for improved natural history studies to develop and test targeted therapies.

Clinical and Biochemical Phenotypes in a Family With ENPP1 Mutations

Inactivating mutations of the ENPP1 gene are associated with generalized arterial calcification of infancy (GACI) and less often autosomal-recessive hypophosphatemic rickets type 2 (ARHR2). We aimed to investigate the spectrum of phenotypes in a family with monoallelic and biallelic mutations of ENPP1 after identification through whole exome sequencing of a 54-year-old female with biallelic mutation of ENPP1, c.323G > T; p.Cys108Phe and c.1441C > T; p.Arg481Trp. Including the proband, 2 subjects had biallelic mutations, 5 had monoallelic mutations, and 2 had no mutation of ENPP1. The maternal mutation, a known pathogenic variant associated with GACI, was found in 3 subjects with monoallelic mutations, while the paternal mutation, which was not previously reported, was present in 2 subjects with monoallelic mutations. Both subjects with biallelic mutations had bowing of bilateral femurs, periarticular mineral deposition, normocalcemic primary hyperparathyroidism with multigland parathyroidectomy, increased carotid intima-media thickness, and enthesopathy was also noted in one subject. Intact FGF23 was elevated in both subjects with biallelic mutations, while C-terminal FGF23 was only elevated in one and PPi was reduced in one. Subjects with monoallelic mutations did not have periarticular calcifications or bone deformities. To conclude, patients with biallelic GACI causing mutations can survive well into adulthood, and despite the same biallelic ENPP1 pathogenic variants, clinical and biochemical manifestations can significantly differ, and include enthesopathy and primary hyperparathyroidism, which have not been previously described. Although carriers of monoallelic ENPP1 variants appear unaffected by classic disease manifestations, there may be subtle biochemical and clinical findings that warrant further investigation. © 2019 American Society for Bone and Mineral Research.

Dental and craniofacial defects in the Crtap-/- mouse model of osteogenesis imperfecta type VII

BACKGROUND

Inactivating mutations in the gene for cartilage-associated protein (CRTAP) cause osteogenesis imperfecta type VII in humans, with a phenotype that can include craniofacial defects. Dental and craniofacial manifestations have not been a focus of case reports to date. We analyzed the craniofacial and dental phenotype of Crtap^-/- mice by skull measurements, micro-computed tomography (micro-CT), histology, and immunohistochemistry.

RESULTS

Crtap^-/- mice exhibited a brachycephalic skull shape with fusion of the nasofrontal suture and facial bones, resulting in mid-face retrusion and a class III dental malocclusion. Loss of CRTAP also resulted in decreased dentin volume and decreased cellular cementum volume, though acellular cementum thickness was increased. Periodontal dysfunction was revealed by decreased alveolar bone volume and mineral density, increased periodontal ligament (PDL) space, ectopic calcification within the PDL, bone-tooth ankylosis, altered immunostaining of extracellular matrix proteins in bone and PDL, increased pSMAD5, and more numerous osteoclasts on alveolar bone surfaces.

CONCLUSIONS

Crtap^-/- mice serve as a useful model of the dental and craniofacial abnormalities seen in individuals with osteogenesis imperfecta type VII.

Transcriptome analysis of ankylosed primary molars with infraocclusion

Primary molar ankylosis with infraocclusion can retard dental arch development and cause dental asymmetry. Despite its widespread prevalence, little is known about its molecular etiology and pathogenesis. To address this, RNA sequencing was used to generate transcriptomes of furcal bone from infraoccluded (n = 7) and non-infraoccluded (n = 9) primary second molars, all without succeeding biscuspids. Of the 18 529 expressed genes, 432 (2.3%) genes were differentially expressed between the two groups (false discovery rate < 0.05). Hierarchical clustering and principal component analysis showed clear separation in gene expression between infraoccluded and non-infraoccluded samples. Pathway analyses indicated that molar ankylosis is associated with the expression of genes consistent with the cellular inflammatory response and epithelial cell turnover. Independent validation using six expressed genes by immunohistochemical analysis demonstrated that the corresponding proteins are strongly expressed in the developing molar tooth germ, in particular the dental follicle and inner enamel epithelium. The descendants of these structures include the periodontal ligament, cementum, bone and epithelial rests of Malassez; tissues that are central to the ankylotic process. We therefore propose that ankylosis involves an increased inflammatory response associated with disruptions to the developmental remnants of the dental follicle and epithelial rests of Malassez.

Dentoalveolar Defects in the Hyp Mouse Model of X-linked Hypophosphatemia

Mutations in PHEX cause X-linked hypophosphatemia (XLH), a form of hypophosphatemic rickets. Hyp (Phex mutant) mice recapitulate the XLH phenotype. Dental disorders are prevalent in individuals with XLH; however, underlying dentoalveolar defects remain incompletely understood. We analyzed Hyp mouse dentoalveolar defects at 42 and 90 d postnatal to comparatively define effects of XLH on dental formation and function. Phex mRNA was expressed by odontoblasts (dentin), osteocytes (bone), and cementocytes (cellular cementum) in wild-type (WT) mice. Enamel density was unaffected, though enamel volume was significantly reduced in Hyp mice. Dentin defects in Hyp molars were indicated histologically by wide predentin, thin dentin, and extensive interglobular dentin, confirming micro-computed tomography (micro-CT) findings of reduced dentin volume and density. Acellular cementum was thin and showed periodontal ligament detachment. Mechanical testing indicated dramatically altered periodontal mechanical properties in Hyp versus WT mice. Hyp mandibles demonstrated expanded alveolar bone with accumulation of osteoid, and micro-CT confirmed decreased bone volume fraction and alveolar bone density. Cellular cementum area was significantly increased in Hyp versus WT molars owing to accumulation of hypomineralized cementoid. Histology, scanning electron microscopy, and nanoindentation revealed hypomineralized "halos" surrounding Hyp cementocyte and osteocyte lacunae. Three-dimensional micro-CT analyses confirmed larger cementocyte/osteocyte lacunae and significantly reduced perilacunar mineral density. While long bone and alveolar bone osteocytes in Hyp mice overexpressed fibroblast growth factor 23 (Fgf23), its expression in molars was much lower, with cementocyte Fgf23 expression particularly low. Expression and distribution of other selected markers were disturbed in Hyp versus WT long bone, alveolar bone, and cementum, including osteocyte/cementocyte marker dentin matrix protein 1 (Dmp1). This study reports for the first time a quantitative analysis of the Hyp mouse dentoalveolar phenotype, including all mineralized tissues. Novel insights into cellular cementum provide evidence for a role for cementocytes in perilacunar mineralization and cementum biology.

Differential Diagnosis of Periapical Radiopacities and Radiolucencies

Periapical pathoses represent changes noted at the apices of teeth within the alveolar process that are suspected on examination, visualized via imaging, and confirmed via histopathology. They can be bone forming or destructive. There are pathologic entities that show both types of changes at the apical regions. These lesions must be identified if they are odontogenic in origin because treatment modalities differ. This article discusses identification of radiopaque and radiolucent lesions noted on radiographs. The common opacities and lucencies are described. When required, advanced imaging is used to depict changes within the bone near the periapical regions of mandibular and maxillary teeth.

ENPP1, an Old Enzyme with New Functions, and Small Molecule Inhibitors-A STING in the Tale of ENPP1

Ectonucleotide pyrophosphatase/phosphodiesterase I (ENPP1) was identified several decades ago as a type II transmembrane glycoprotein with nucleotide pyrophosphatase and phosphodiesterase enzymatic activities, critical for purinergic signaling. Recently, ENPP1 has emerged as a critical phosphodiesterase that degrades the stimulator of interferon genes (STING) ligand, cyclic GMP-AMP (cGAMP). cGAMP or analogs thereof have emerged as potent immunostimulatory agents, which have potential applications in immunotherapy. This emerging role of ENPP1 has placed this "old" enzyme at the frontier of immunotherapy. This review highlights the roles played by ENPP1, the mechanism of cGAMP hydrolysis by ENPP1, and small molecule inhibitors of ENPP1 with potential applications in diverse disease states, including cancer.

Isolation and characterization of a human cementocyte-like cell line, HCY-23

Cementum is the mineralized tissue covering the tooth root that functions in tooth attachment and post-eruptive adjustment of tooth position. It has been reported to be highly similar to bone in several respects but remains poorly understood in terms of development and regeneration. Here, we investigate whether cementocytes, the residing cells in cellular cementum, have the potential to be protagonist in cementum homeostasis, responding to endocrine signals and directing local cementum metabolism. Cells from healthy erupted human teeth were isolated using sequential collagenase/EDTA digestions, and maintained in standard cell culture conditions. A cementocyte-like cell line was cloned (HCY-23, for human cementocyte clone 23), which presented a cementocyte compatible gene expression signature, including the expression of dentin matrix protein 1 ( DMP1 ), sclerostin ( SOST ), and E11/gp38/podoplanin ( E11 ). In contrast, these cells did not express the odontoblast/dentin marker dentin sialoprotein ( DSPP ). HCY-23 cells produced mineral-like nodules in vitro under differentiation conditions, and were highly responsive to inorganic phosphate (Pi). Within the limits of the present study, it can be concluded that cementocytes are phosphate-responsive cells, and have the potential do play a key role in periodontal homeostasis and regeneration.

Antagonistic interactions between osterix and pyrophosphate during cementum formation

During cementum formation, the key roles of osterix (Osx) and inorganic pyrophosphate (PPi), mainly controlled by nucleotide pyrophosphatase 1 (Npp1; encoded by the Enpp1 gene) and progressive ankylosis protein (Ank), have been demonstrated by animal models displaying altered cementum formation. In this study, we analyzed the relationship of Osx and local PPi during cementum formation using compound mutant mice with their wildtype and corresponding single gene mutants. Importantly, functional defects in PPi regulation led to the induction of Osx expression at the cervical cementum as demonstrated by Enpp1 mutant mice and cementoblasts with the retroviral transduction of small hairpin RNA for Enpp1 or Ank. Conversely, cementoblasts exposed to inorganic PPi or with the enforced expression of Enpp1 or Ank reduced Osx expression in a concentration-dependent manner. Furthermore, the loss of Osx induced the higher expression of Npp1 and Ank at the apical region of the developing tooth root as observed in Osx-deficient mice. The activity of PPi-generating ectoenzymes (nucleoside triphosphate pyrophosphohydrolase, NTPPPHase) and the level of extracellular PPi were significantly increased in Osx-knockdown cementoblasts. However, the formation of ectopic cervical cementum was not completely diminished by inactivation of Osx in Enpp1 mutant mice. In addition, fibroblast growth factor (FGF) receptor 1 (Fgfr1) was strongly localized in cementoblasts lining the acellular cementum and involved in the inhibitory regulation of matrix accumulation and further mineralization by supporting PPi production. Taken together, these results suggest that local PPi suppresses matrix accumulation and further mineralization through an antagonistic interaction with Osx under the synergistic influence of FGF signaling during cementum formation.

Basic fibroblast growth factor regulates phosphate/pyrophosphate regulatory genes in stem cells isolated from human exfoliated deciduous teeth

Background

Basic fibroblast growth factor (bFGF) regulates maintenance of stemness and modulation of osteo/odontogenic differentiation and mineralization in stem cells from human exfoliated deciduous teeth (SHEDs). Mineralization in the bones and teeth is in part controlled by pericellular levels of inorganic phosphate (P_i), a component of hydroxyapatite, and inorganic pyrophosphate (PP_i), an inhibitor of mineralization. The progressive ankylosis protein (gene ANKH; protein ANKH) and ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1/ENPP1) increase PP_i and inhibit mineralization, while tissue-nonspecific alkaline phosphatase (ALPL; TNAP) is a critical pro-mineralization enzyme that hydrolyzes PP_i. We hypothesized that regulation by bFGF of mineralization in SHEDs occurs by modulation of P_i/PP_i-associated genes.

Methods

Cells were isolated from human exfoliated deciduous teeth and characterized for mesenchymal stem cell characteristics. Cells were treated with bFGF, and the osteogenic differentiation ability was determined. The mRNA expression was evaluated using real-time polymerase chain reaction. The mineralization was examined using alizarin red S staining.

Results

Cells isolated from primary teeth expressed mesenchymal stem cell markers, CD44, CD90, and CD105, and were able to differentiate into osteo/odontogenic and adipogenic lineages. Addition of 10 ng/ml bFGF to SHEDs during in vitro osteo/odontogenic differentiation decreased ALPL mRNA expression and ALP enzyme activity, increased ANKH mRNA, and decreased both P_i/PP_i ratio and mineral deposition. Effects of bFGF on ALPL and ANKH expression were detected within 24 h. Addition of 20 mM fibroblast growth factor receptor (FGFR) inhibitor SU5402 revealed the necessity of FGFR-mediated signaling, and inclusion of 1 μg/ml cyclohexamide (CHX) implicated the necessity of protein synthesis for effects on ALPL and ANKH. Addition of exogenous 10 μm PP_i inhibited mineralization and increased ANKH, collagen type 1a1 (COL1A1), and osteopontin (SPP1) mRNA, while addition of exogenous P_i increased mineralization and osterix (OSX), ANKH, SPP1, and dentin matrix protein 1 (DMP1) mRNA. The effects of PP_i and P_i on mineralization could be replicated by short-term 3- and 7-day treatments, suggesting signaling effects in addition to physicochemical regulation of mineral deposition.

Conclusion

This study reveals for the first time the effects of bFGF on P_i/PP_i regulators in SHEDs and implicates these factors in how bFGF directs osteo/odontogenic differentiation and mineralization by these cells.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1093-9) contains supplementary material, which is available to authorized users.

Reduced Orthodontic Tooth Movement in Enpp1 Mutant Mice with Hypercementosis

Previous studies revealed that cementum formation is tightly regulated by inorganic pyrophosphate (PP_i), a mineralization inhibitor. Local PP_i concentrations are determined by regulators, including ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), which increases PP_i concentrations by adenosine triphosphate hydrolysis. Orthodontic forces stimulate alveolar bone remodelling, leading to orthodontic tooth movement (OTM). To better understand how disturbed mineral metabolism and the resulting altered periodontal structures affect OTM, we employed Enpp1 mutant mice that feature reduced PP_i and increased cervical cementum in a model of OTM induced by a stretched closed-coil spring ligated between the maxillary left first molar and maxillary incisors. We analyzed tooth movement, osteoclast/odontoclast response, and tooth root resorption by micro-computed tomography, histology, histomorphometry, and immunohistochemistry. Preoperatively, we noted an altered periodontium in Enpp1 mutant mice, with significantly increased periodontal ligament (PDL) volume and thickness, as well as increased PDL-bone/tooth root surface area, compared to wild-type (WT) controls. After 11 d of orthodontic treatment, Enpp1 mutant mice displayed 38% reduced tooth movement versus WT mice. Molar roots in Enpp1 mutant mice exhibited less change in PDL width in compression and tension zones compared to WT mice. Root resorption was noted in both groups with no difference in average depths, but resorption lacunae in Enpp1 mutant mice were almost entirely limited to cementum, with 150% increased cementum resorption and 92% decreased dentin resorption. Osteoclast/odontoclast cells were reduced by 64% in Enpp1 mutant mice, with a predominance of tartrate-resistant acid phosphatase (TRAP)-positive cells on root surfaces, compared to WT mice. Increased numbers of TRAP-positive cells on root surfaces were associated with robust immunolocalization of osteopontin (OPN) and receptor-activator of NF-κB ligand (RANKL). Collectively, reduced response to orthodontic forces, decreased tooth movement, and altered osteoclast/odontoclast distribution suggests Enpp1 loss of function has direct effects on clastic function/recruitment and/or indirect effects on periodontal remodeling via altered periodontal structure or tissue mineralization.