Functional defects in cementoblasts with disrupted bone sialoprotein functional domains, in vitro

Bone sialoprotein (BSP) is a multifunctional extracellular matrix (ECM) protein present in bone and cementum. Global in vivo ablation of BSP leads to bone mineralization defects, lack of acellular cementum, and periodontal breakdown. BSP harbors three main functional domains: N-terminal collagen-binding domain, hydroxyapatite-nucleating domain, and C-terminal RGD integrin-binding signaling domain. How each of these domains contributes to BSP function(s) is not understood. We hypothesized that collagen-binding and RGD domains play distinct roles in cementoblast functions. Three CRISPR/Cas9 gene-edited cell lines were derived from control wild-type (WT) OCCM.30 murine immortalized cementoblasts: 1) deletion of the N-terminus of BSP after signal peptide, including entire collagen binding domain (Ibsp∆N-Term); 2) deletion of exon 4 (majority of collagen-binding domain; Ibsp∆Ex4); and 3) deletion of C-terminus of BSP including the integrin binding RGD domain (Ibsp∆C-Term). Compared to WT, Ibsp∆Ex4 and Ibsp∆C-Term cell lines showed reduced BSP secretion, in vitro. Abnormal cell morphology was observed in all mutant cell lines, with Ibsp∆C-Term showing highly disorganized cytoskeleton. All mutant cell lines showed significantly lower cell proliferation compared to WT at all timepoints. Ibsp∆N-Term cells showed reduced cell migration by 24 h. All mutants exhibited over 50 % significant reduced mineralization at days 6 and 10. While WT cells were largely unaffected by seeding density, mutant cells failed to mineralize at lower cell density. Mutant cell lines diverged from WT and from each other by dysregulated expression in 23 genes involved in mineralization, ECM, and cell signaling. In summary, disabling BSP functional domains led to profound and distinct changes in cementoblast cell functions, especially dysregulated gene expression and reduced mineralization, in a way did not align with a straightforward narrative where each functional domain caused specific, expected differences. Instead, the study uncovered a significant level of complexity in how different mutant forms of BSP affected cell functions, in vitro.

Optineurin regulates osteoblast function in an age-dependent fashion in a mouse model of Paget's disease of bone

Paget's disease of bone (PDB) is a degenerative disorder affecting the skull and bones. Hyperactive osteoclasts (OCs) initiate bone degradation in the early stage, followed by increased bone formation by osteoblasts (OBs) in trabecular bones during the advanced stage. This OB-OC uncoupling results in bone deformations and irregular trabecular bone patterns. Current mouse models poorly replicate the advanced-stage characteristics of PDB. Optineurin (Gene: OPTN in humans, Optn in mice, protein: OPTN) has been implicated in PDB by genetic analyses. We identified PDB-like cortical lesions associated with OC hyperactivation in an Optn knockout (Optn-/-) mouse model. However, the effects of OPTN dysfunction on OBs and trabecular bone in advanced PDB remain unclear. In this study, we used the Optn-/- mouse model to investigate trabecular bone abnormalities and OB activity in PDB. Micro-computed tomography analysis revealed severe pagetic alterations in craniofacial bones and femurs of aged Optn-/- mice, resembling clinical manifestations of PDB. Altered OB activity was observed in aged Optn-/- mice, implicating compensatory OB response in trabecular bone anomalies. To elucidate the role of OC-OB interactions in PDB, we conducted in vitro experiments using OC conditioned media (CM) to examine the effects on OB osteogenic potential. We found OC CM restored compromised osteogenic induction of Optn-/- bone marrow stromal cells (BMSCs) from young mice, suggesting OCs maintain OB activity through secreted factors. Strikingly, OC CM from aged Optn-/- mice significantly enhanced osteogenic capability of Optn-/- BMSCs, providing evidence for increased OB activity in advanced stages of PDB. We further identified TGF-β/BMP signaling pathway in mediating the effects of OC CM on OBs. Our findings provide insights into Optn's role in trabecular bone abnormalities and OB activity in PDB. This enhances understanding of PDB pathogenesis and may contribute to potential therapeutic strategies for PDB and related skeletal disorders.

Contributions of increased osteopontin and hypophosphatemia to dentoalveolar defects in osteomalacic Hyp mice

X-linked hypophosphatemia (XLH) is an inherited disorder caused by inactivating mutations in the PHEX gene leading to renal phosphate wasting, rickets and osteomalacia. XLH is also associated with dentoalveolar mineralization defects in tooth enamel, dentin and cementum, and in alveolar bone, which lead to an increased prevalence of dental abscesses, periodontal disease and tooth loss. Genetic mouse experiments, and deficiencies in XLH patient therapies where treatments do not fully ameliorate mineralization defects, suggest that other pathogenic mechanisms may exist in XLH. The mineralization-inhibiting, secreted extracellular matrix phosphoprotein osteopontin (OPN, gene Spp1) is a substrate for the PHEX enzyme whereby extensive and inactivating degradation of inhibitory OPN by PHEX facilitates mineralization. Conversely, excess OPN accumulation in skeletal and dental tissues - for example in XLH where inactivating mutations in the PHEX gene limit degradation of inhibitory OPN, or as occurs in Fgf23-null mice - contributes to mineralization defects. We hypothesized that Spp1/OPN ablation in Hyp mice (a mouse model for XLH) would reduce dentoalveolar mineralization defects. Immunostaining revealed increased OPN in Hyp vs. wild-type (WT) alveolar bone, particularly in osteocyte lacunocanalicular networks where Hyp mice have characteristic hypomineralized peri-osteocytic lesions (POLs). Micro-computed tomography and histology showed that ablation of Spp1 in Hyp mice (Hyp;Spp1-/-) on a normal diet did not ameliorate bulk defects in enamel, dentin, or alveolar bone. On a high-phosphate diet, both Hyp and Hyp;Spp1-/- mice showed improved mineralization of enamel, dentin, and alveolar bone. Silver staining indicated Spp1 ablation did not improve alveolar or mandibular bone osteocyte POLs in Hyp mice; however, they were normalized by a high-phosphate diet in both Hyp and Hyp;Spp1-/- mice, although inducing increased OPN. Collectively, these data indicate that despite changes in OPN content in the dentoalveolar mineralized tissues, there exist other compensatory mineralization mechanisms that arise from knockout of Spp1/OPN in the Hyp background.

Dentoalveolar Alterations in an Adenine-Induced Chronic Kidney Disease Mouse Model

Chronic kidney disease (CKD) is characterized by kidney damage and loss of renal function. CKD mineral and bone disorder (CKD-MBD) describes the dysregulation of mineral homeostasis, including hyperphosphatemia and elevated parathyroid hormone (PTH) secretion, skeletal abnormalities, and vascular calcification. CKD-MBD impacts the oral cavity, with effects including salivary gland dysfunction, enamel hypoplasia and damage, increased dentin formation, decreased pulp volume, pulp calcifications, and altered jaw bones, contributing to clinical manifestations of periodontal disease and tooth loss. Underlying mechanisms are not fully understood, and CKD mouse models commonly require invasive procedures with high rates of infection and mortality. We aimed to characterize the dentoalveolar effects of an adenine diet (AD)-induced CKD (AD-CKD) mouse model. Eight-week-old C57BL/6J mice were provided either a normal phosphorus diet control (CTR) or adenine and high-phosphorus diet CKD to induce kidney failure. Mice were euthanized at 15 weeks old, and mandibles were collected for micro-computed tomography and histology. CKD mice exhibited kidney failure, hyperphosphatemia, and hyperparathyroidism in association with porous cortical bone in femurs. CKD mice showed a 30% decrease in molar enamel volume compared to CTR mice. Enamel wear was associated with reduced ductal components, ectopic calcifications, and altered osteopontin (OPN) deposition in submandibular salivary glands of CKD mice. Molar cusps in CKD mice were flattened, exposing dentin. Molar dentin/cementum volume increased 7% in CKD mice and pulp volume decreased. Histology revealed excessive reactionary dentin and altered pulp-dentin extracellular matrix proteins, including increased OPN. Mandibular bone volume fraction decreased 12% and bone mineral density decreased 9% in CKD versus CTR mice. Alveolar bone in CKD mice exhibited increased tissue-nonspecific alkaline phosphatase localization, OPN deposition, and greater osteoclast numbers. AD-CKD recapitulated key aspects reported in CKD patients and revealed new insights into CKD-associated oral defects. This model has potential for studying mechanisms of dentoalveolar defects or therapeutic interventions. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Optineurin regulates NRF2-mediated antioxidant response in a mouse model of Paget's disease of bone

Degenerative diseases affecting the nervous and skeletal systems affect the health of millions of elderly people. Optineurin (OPTN) has been associated with numerous neurodegenerative diseases and Paget's disease of bone (PDB), a degenerative bone disease initiated by hyperactive osteoclastogenesis. In this study, we found age-related increase in OPTN and nuclear factor E2-related factor 2 (NRF2) in vivo. At the molecular level, OPTN could directly interact with both NRF2 and its negative regulator Kelch-like ECH-associated protein 1 (KEAP1) for up-regulating antioxidant response. At the cellular level, deletion of OPTN resulted in increased intracellular reactive oxygen species and increased osteoclastogenic potential. At the tissue level, deletion of OPTN resulted in substantially increased oxidative stress derived from leukocytes that further stimulate osteoclastogenesis. Last, curcumin attenuated hyperactive osteoclastogenesis induced by OPTN deficiency in aged mice. Collectively, our findings reveal an OPTN-NRF2 axis maintaining bone homeostasis and suggest that antioxidants have therapeutic potential for PDB.

Gene Therapy Using Recombinant AAV Type 8 Vector Encoding TNAP-D10 Improves the Skeletal Phenotypes in Murine Models of Osteomalacia

Hypophosphatasia (HPP), caused by loss-of-function mutations in the ALPL gene encoding tissue-nonspecific alkaline phosphatase (TNAP), is characterized by skeletal and dental hypomineralization that can vary in severity from life-threatening to milder manifestations only in adulthood. PHOSPHO1 deficiency leads to early-onset scoliosis, osteomalacia, and fractures that mimic pseudo-HPP. Asfotase alfa, a life-saving enzyme replacement therapy approved for pediatric-onset HPP, requires subcutaneous injections 3 to 6 times per week. We recently showed that a single injection of an adeno-associated virus vector serotype 8 harboring TNAP-D10 (AAV8-TNAP-D10) effectively prevented skeletal disease and prolonged life in Alpl -/- mice phenocopying infantile HPP. Here, we aimed to determine the efficacy of AAV8-TNAP-D10 in improving the skeletal and dental phenotype in the Alpl Prx1/Prx1 and Phospho1 -/- mouse models of late-onset (adult) HPP and pseudo-HPP, respectively. A single dose of 3 × 1011 vector genomes per body (vg/b) was injected intramuscularly into 8-week-old Alpl Prx1/Prx1 and wild-type (WT) littermates, or into 3-day-old Phospho1 -/- and WT mice, and treatment efficacy was evaluated after 60 days for late-onset HPP mice and after 90 days for Phospho1 -/- mice. Biochemical analysis showed sustained serum alkaline phosphatase activity and reduced plasma PPi levels, and radiographic images, micro-computed tomography (micro-CT) analysis, and hematoxylin and eosin (H&E) staining showed improvements in the long bones in the late-onset HPP mice and corrected scoliosis in the Phospho1 -/- mice. Micro-CT analysis of the dentoalveolar complex did not reveal significant changes in the phenotype of late-onset HPP and pseudo-HPP models. Moreover, alizarin red staining analysis showed that AAV8-TNAP-D10 treatment did not promote ectopic calcification of soft organs in adult HPP mice after 60 days of treatment, even after inducing chronic kidney disease. Overall, the AAV8-TNAP-D10 treatment improved the skeletal phenotype in both the adult HPP and pseudo-HPP mouse models. This preclinical study will contribute to the advancement of gene therapy for the improvement of skeletal disease in patients with heritable forms of osteomalacia. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Dentoalveolar Defects of Hypophosphatasia are Recapitulated in a Sheep Knock-In Model

Hypophosphatasia (HPP) is the inherited error-of-metabolism caused by mutations in ALPL, reducing the function of tissue-nonspecific alkaline phosphatase (TNAP/TNALP/TNSALP). HPP is characterized by defective skeletal and dental mineralization and is categorized into several clinical subtypes based on age of onset and severity of manifestations, though premature tooth loss from acellular cementum defects is common across most HPP subtypes. Genotype-phenotype associations and mechanisms underlying musculoskeletal, dental, and other defects remain poorly characterized. Murine models that have provided significant insights into HPP pathophysiology also carry limitations including monophyodont dentition, lack of osteonal remodeling of cortical bone, and differing patterns of skeletal growth. To address this, we generated the first gene-edited large-animal model of HPP in sheep via CRISPR/Cas9-mediated knock-in of a missense mutation (c.1077C>G; p.I359M) associated with skeletal and dental manifestations in humans. We hypothesized that this HPP sheep model would recapitulate the human dentoalveolar manifestations of HPP. Compared to wild-type (WT), compound heterozygous (cHet) sheep with one null allele and the other with the targeted mutant allele exhibited the most severe alveolar bone, acellular cementum, and dentin hypomineralization defects. Sheep homozygous for the mutant allele (Hom) showed alveolar bone and hypomineralization effects and trends in dentin and cementum, whereas sheep heterozygous (Het) for the mutation did not exhibit significant effects. Important insights gained include existence of early alveolar bone defects that may contribute to tooth loss in HPP, observation of severe mantle dentin hypomineralization in an HPP animal model, association of cementum hypoplasia with genotype, and correlation of dentoalveolar defects with alkaline phosphatase (ALP) levels. The sheep model of HPP faithfully recapitulated dentoalveolar defects reported in individuals with HPP, providing a new translational model for studies into etiopathology and novel therapies of this disorder, as well as proof-of-principle that genetically engineered large sheep models can replicate human dentoalveolar disorders. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Perspective on Dentoalveolar Manifestations Resulting From PHOSPHO1 Loss-of-Function: A Form of Pseudohypophosphatasia?

Mineralization of the skeleton occurs by several physicochemical and biochemical processes and mechanisms that facilitate the deposition of hydroxyapatite (HA) in specific areas of the extracellular matrix (ECM). Two key phosphatases, phosphatase, orphan 1 (PHOSPHO1) and tissue-non-specific alkaline phosphatase (TNAP), play complementary roles in the mineralization process. The actions of PHOSPHO1 on phosphocholine and phosphoethanolamine in matrix vesicles (MVs) produce inorganic phosphate (P_i) for the initiation of HA mineral formation within MVs. TNAP hydrolyzes adenosine triphosphate (ATP) and the mineralization inhibitor, inorganic pyrophosphate (PP_i), to generate P_i that is incorporated into MVs. Genetic mutations in the ALPL gene-encoding TNAP lead to hypophosphatasia (HPP), characterized by low circulating TNAP levels (ALP), rickets in children and/or osteomalacia in adults, and a spectrum of dentoalveolar defects, the most prevalent being lack of acellular cementum leading to premature tooth loss. Given that the skeletal manifestations of genetic ablation of the Phospho1 gene in mice resemble many of the manifestations of HPP, we propose that Phospho1 gene mutations may underlie some cases of "pseudo-HPP" where ALP may be normal to subnormal, but ALPL mutation(s) have not been identified. The goal of this perspective article is to compare and contrast the loss-of-function effects of TNAP and PHOSPHO1 on the dentoalveolar complex to predict the likely dental phenotype in humans that may result from PHOSPHO1 mutations. Potential cases of pseudo-HPP associated with PHOSPHO1 mutations may resist diagnosis, and the dental manifestations could be a key criterion for consideration.

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.

Gene Therapy Using Adeno-Associated Virus Serotype 8 Encoding TNAP-D10 Improves the Skeletal and Dentoalveolar Phenotypes in Alpl-/- Mice

Hypophosphatasia (HPP) is caused by loss-of-function mutations in the ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP), whose deficiency results in the accumulation of extracellular inorganic pyrophosphate (PP_i ), a potent mineralization inhibitor. Skeletal and dental hypomineralization characterizes HPP, with disease severity varying from life-threatening perinatal or infantile forms to milder forms that manifest in adulthood or only affect the dentition. Enzyme replacement therapy (ERT) using mineral-targeted recombinant TNAP (Strensiq/asfotase alfa) markedly improves the life span, skeletal phenotype, motor function, and quality of life of patients with HPP, though limitations of ERT include frequent injections due to a short elimination half-life of 2.28 days and injection site reactions. We tested the efficacy of a single intramuscular administration of adeno-associated virus 8 (AAV8) encoding TNAP-D₁₀ to increase the life span and improve the skeletal and dentoalveolar phenotypes in TNAP knockout (Alpl^-/- ) mice, a murine model for severe infantile HPP. Alpl^-/- mice received 3 × 10¹¹ vector genomes/body of AAV8-TNAP-D₁₀ within 5 days postnatal (dpn). AAV8-TNAP-D₁₀ elevated serum ALP activity and suppressed plasma PP_i . Treatment extended life span of Alpl^-/- mice, and no ectopic calcifications were observed in the kidneys, aorta, coronary arteries, or brain in the 70 dpn observational window. Treated Alpl^-/- mice did not show signs of rickets, including bowing of long bones, enlargement of epiphyses, or fractures. Bone microstructure of treated Alpl^-/- mice was similar to wild type, with a few persistent small cortical and trabecular defects. Histology showed no measurable osteoid accumulation but reduced bone volume fraction in treated Alpl^-/- mice versus controls. Treated Alpl^-/- mice featured normal molar and incisor dentoalveolar tissues, with the exceptions of slightly reduced molar enamel and alveolar bone density. Histology showed the presence of cementum and normal periodontal ligament attachment. These results support gene therapy as a promising alternative to ERT for the treatment of HPP. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Effects of Active Vitamin D or FGF23 Antibody on Hyp Mice Dentoalveolar Tissues

Mutations in the PHEX gene lead to X-linked hypophosphatemia (XLH), a form of inherited rickets featuring elevated fibroblast growth factor 23 (FGF23), reduced 1,25-dihydroxyvitamin D (1,25D), and hypophosphatemia. Hyp mutant mice replicate the XLH phenotype, including dentin, alveolar bone, and cementum defects. We aimed to compare effects of 1,25D versus FGF23-neutralizing antibody (FGF23Ab) monotherapies on Hyp mouse dentoalveolar mineralization. Male Hyp mice, either injected subcutaneously with daily 1,25D or thrice weekly with FGF23 blocking antibody from 2 to 35 d postnatal, were compared to wild-type (WT) controls and untreated Hyp mice. Mandibles were analyzed by high-resolution micro-computed tomography (micro-CT), histology, and immunohistochemistry. Both interventions maintained normocalcemia, increased serum phosphate levels, and improved dentoalveolar mineralization in treated versus untreated Hyp mice. 1,25D increased crown dentin volume and thickness and root dentin/cementum volume, whereas FGF23Ab effects were limited to crown dentin volume. 1,25D increased bone volume fraction, bone mineral density, and tissue mineral density in Hyp mice, whereas FGF23Ab failed to significantly affect these alveolar bone parameters. Neither treatment fully attenuated dentin and bone defects to WT levels, and pulp volumes remained elevated regardless of treatment. Both treatments reduced predentin thickness and improved periodontal ligament organization, while 1,25D promoted a more profound improvement in acellular cementum thickness. Altered cell densities and lacunocanalicular properties of alveolar and mandibular bone osteocytes and cementocytes in Hyp mice were partially corrected by either treatment. Neither treatment normalized the altered distributions of bone sialoprotein and osteopontin in Hyp mouse alveolar bone. Moderate improvements from both 1,25D and FGF23Ab treatment regimens support further studies and collection of oral health data from subjects receiving a newly approved anti-FGF23 therapy. The inability of either treatment to fully correct Hyp mouse dentin and bone prompts further experiments into underlying pathological mechanisms to identify new therapeutic approaches.

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.

Multiple Idiopathic Cervical Root Resorption: A Challenge for a Transdisciplinary Medical-Dental Team

The goal of this perspective article is to use multiple idiopathic cervical root resorption (MICRR) as a model to demonstrate the need for transdisciplinary collaborations, from basic science to treatment planning, to improve the quality of health care for all. This is not a review of the literature on the current state of MICRR. Tooth root resorption is a normal physiological process required for resorption and exfoliation of primary teeth; however, root resorption of adult teeth is largely pathological. MICRR is an aggressive form of external root resorption, which occurs near the cemento-enamel junction (CEJ). The cause of MICRR remains elusive, however, it is mediated primarily by osteoclasts/odontoclasts. Accumulating case studies and experiments in animal models have provided insights into defining the etiologies and pathophysiological mechanisms for MICRR, which include: systemic conditions and syndromes, inherited genetic variants affecting osteoclast/odontoclast activity, altered periodontal structures, drug-induced root resorption and rebound effects after cessation of anti-resorptive treatment, chemotherapy, exposure to pets or viral infections, and other factors such as inflammatory conditions or trauma. To determine the causative factors for MICRR, as well as other oral-dental conditions, at minimum, a comprehensive health history should be collected for all patients by dental care providers, discussed with other health care providers and appropriate collaborations established. The examples highlighted in this perspective emphasize the need for transdisciplinary research collaborations coupled with integrated management strategies between medicine and dentistry in order to identify cause(s) early and improve clinical outcomes.

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.

Monocyte Subsets With High Osteoclastogenic Potential and Their Epigenetic Regulation Orchestrated by IRF8

Osteoclasts (OCs) are bone-resorbing cells formed by the serial fusion of monocytes. In mice and humans, three distinct subsets of monocytes exist; however, it is unclear if all of them exhibit osteoclastogenic potential. Here we show that in wild-type (WT) mice, Ly6C^hi and Ly6C^int monocytes are the primary source of OC formation when compared to Ly6C^- monocytes. Their osteoclastogenic potential is dictated by increased expression of signaling receptors and activation of preestablished transcripts, as well as de novo gain in enhancer activity and promoter changes. In the absence of interferon regulatory factor 8 (IRF8), a transcription factor important for myelopoiesis and osteoclastogenesis, all three monocyte subsets are programmed to display higher osteoclastogenic potential. Enhanced NFATc1 nuclear translocation and amplified transcriptomic and epigenetic changes initiated at early developmental stages direct the increased osteoclastogenesis in Irf8-deficient mice. Collectively, our study provides novel insights into the transcription factors and active cis-regulatory elements that regulate OC differentiation. © 2020 American Society for Bone and Mineral Research (ASBMR).

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.

Comparative proteomic analysis of dental cementum from deciduous and permanent teeth

BACKGROUND AND OBJECTIVES

Dental cementum (DC) is a mineralized tissue covering tooth roots that plays a critical role in dental attachment. Differences in deciduous vs. permanent tooth DC have not been explored. We hypothesized that proteomic analysis of DC matrix would identify compositional differences in deciduous (DecDC) vs. permanent (PermDC) cementum that might reflect physiological or pathological differences, such as root resorption that is physiological in deciduous teeth but can be pathological in the permanent dentition.

METHODS

Protein extracts from deciduous (n = 25) and permanent (n = 12) teeth were pooled (five pools of DecDC, five teeth each; four pools of PermDC, three teeth each). Samples were denatured, and proteins were extracted, reduced, alkylated, digested, and analyzed by liquid chromatography-mass spectrometry (LC-MS/MS). The beta-binomial statistical test was applied to normalized spectrum counts with 5% significance level to determine differentially expressed proteins. Immunohistochemistry was used to validate selected proteins.

RESULTS

A total of 510 proteins were identified: 123 (24.1%) exclusive to DecDC; 128 (25.1%) exclusive to PermDC; 259 (50.8%) commonly expressed in both DecDC and PermDC. Out of 60 differentially expressed proteins, 17 (28.3%) were detected in DecDC, including myeloperoxidase (MPO), whereas 43 (71.7%) were detected in PermDC, including decorin (DCN) and osteocalcin (BGLAP). Overall, Gene Ontology (GO) analysis indicated that all expressed proteins were related to GO biological processes that included localization and response to stress, and the GO molecular function of differentially expressed proteins was enriched in cell adhesion, molecular binding, cytoskeletal protein binding, structural molecular activity, and macromolecular complex binding. Immunohistochemistry confirmed the trends for selected differentially expressed proteins in human teeth.

CONCLUSIONS

Clear differences were found between the proteomes of DecDC and PermDC. These findings may lead to new insights into developmental differences between DecDC and PermDC, as well as to a better understanding of physiological/pathological events such as root resorption.

A novel de novo heterozygous ALPL nonsense mutation associated with adult hypophosphatasia

Using genetic, clinical, biochemical, and radiographic assessment and bioinformatic approaches, we present an unusual case of adult HPP caused by a novel de novo heterozygous nonsense mutation in the alkaline phosphatase (ALPL).

INTRODUCTION

Hypophosphatasia (HPP) is caused by genetic alterations of the ALPL gene, encoding the tissue-nonspecific isozyme of alkaline phosphatase (TNSALP). Here, the purpose was to perform clinical and molecular investigation in a 36-year-old Caucasian woman suspected to present adult HPP.

METHODS

Medical and dental histories were obtained for the proposita and family members, including biochemical, radiographic, and dental assessments. ALPL mutational analysis was performed by the Sanger sequencing method, and the functional impact prediction of the identified mutations was assessed by bioinformatic methods.

RESULTS

We identified a novel heterozygous nonsense mutation in the ALPL gene (NM_000478.6:c.768G>A; W[TGG]>*[TGA]) associated with spontaneous vertebral fracture, severe back pain, musculoskeletal pain, low bone density, and short-rooted permanent teeth loss. Functional prediction analysis revealed that the Trp256Ter mutation led to a complete loss of TNSALP crown domain and extensive loss of other functional domains (calcium-binding domain, active site vicinity, and zinc-binding site) and over 60% loss of homodimer interface residues, suggesting that the mutant TNSALP molecules are nonfunctional and form unstable homodimers. Genotyping of the ALPL in the proposita's parents, sister, and niece revealed that in this case, HPP occurred due to a de novo mutation.

CONCLUSION

The present study describes a novel genotype-phenotype and structure-function relationship for HPP, contributing to a better molecular comprehension of HPP etiology and pathophysiology.

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.

Dental malformations associated with biallelic MMP20 mutations

Background

Matrix metallopeptidase 20 (MMP20) is an evolutionarily conserved protease that is essential for processing enamel matrix proteins during dental enamel formation. MMP20 mutations cause human autosomal recessive pigmented hypomaturation‐type amelogenesis imperfecta (AI2A2; OMIM #612529). MMP20 is expressed in both odontoblasts and ameloblasts, but its function during dentinogenesis is unclear.

Methods

We characterized 10 AI kindreds with MMP20 defects, characterized human third molars and/or Mmp20^−/− mice by histology, Backscattered Scanning Electron Microscopy (bSEM), µCT, and nanohardness testing.

Results

We identified six novel MMP20 disease‐causing mutations. Four pathogenic variants were associated with exons encoding the MMP20 hemopexin‐like (PEX) domain, suggesting a necessary regulatory function. Mutant human enamel hardness was softest (13% of normal) midway between the dentinoenamel junction (DEJ) and the enamel surface. bSEM and µCT analyses of the third molars revealed reduced mineral density in both enamel and dentin. Dentin close to the DEJ showed an average hardness number 62%–69% of control. Characterization of Mmp20^−/− mouse dentin revealed a significant reduction in dentin thickness and mineral density and a transient increase in predentin thickness, indicating disturbances in dentin matrix secretion and mineralization.

Conclusion

These results expand the spectrum of MMP20 disease‐causing mutations and provide the first evidence for MMP20 function during dentin formation.

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.

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.

Loss of Discoidin Domain Receptor 1 Predisposes Mice to Periodontal Breakdown

The discoidin domain receptors, DDR1 and DDR2, are nonintegrin collagen receptors and tyrosine kinases. DDRs regulate cell functions, and their extracellular domains affect collagen fibrillogenesis and mineralization. Based on the collagenous nature of dentoalveolar tissues, we hypothesized that DDR1 plays an important role in dentoalveolar development and function. Radiography, micro-computed tomography (micro-CT), histology, histomorphometry, in situ hybridization (ISH), immunohistochemistry (IHC), and transmission electron microscopy (TEM) were used to analyze Ddr1 knockout (Ddr1^-/-) mice and wild-type (WT) controls at 1, 2, and 9 mo, and ISH and quantitative polymerase chain reaction (qPCR) were employed to assess Ddr1/DDR1 messenger RNA expression in mouse and human tissues. Radiographic images showed normal molars but abnormal mandibular condyles, as well as alveolar bone loss in Ddr1^-/- mice versus WT controls at 9 mo. Histological, histomorphometric, micro-CT, and TEM analyses indicated no differences in enamel or dentin Ddr1^-/- versus WT molars. Total volumes (TVs) and bone volumes (BVs) of subchondral and ramus bone of Ddr1^-/- versus WT condyles were increased and bone volume fraction (BV/TV) was reduced at 1 and 9 mo. There were no differences in alveolar bone volume at 1 mo, but at 9 mo, severe periodontal defects and significant alveolar bone loss (14%; P < 0.0001) were evident in Ddr1^-/- versus WT mandibles. Histology, ISH, and IHC revealed disrupted junctional epithelium, connective tissue destruction, bacterial invasion, increased neutrophil infiltration, upregulation of cytokines including macrophage colony-stimulating factor, and 3-fold increased osteoclast numbers (P < 0.05) in Ddr1^-/- versus WT periodontia at 9 mo. In normal mouse tissues, ISH and qPCR revealed Ddr1 expression in basal cell layers of the oral epithelia and in immune cells. We confirmed a similar expression pattern in human oral epithelium by ISH and qPCR. We propose that DDR1 plays an important role in periodontal homeostasis and that absence of DDR1 predisposes mice to periodontal breakdown.

A novel combination of biallelic ALPL mutations associated with adult hypophosphatasia: A phenotype-genotype association and computational analysis study

Hypophosphatasia (HPP) is an inherited metabolic disorder that causes defective skeletal and dental mineralization. HPP exhibits a markedly heterogeneous range of clinical manifestations caused by dysfunction of the tissue-nonspecific isozyme of alkaline phosphatase (TNSALP), resulting from loss-of-function mutations in the ALPL gene. HPP has been associated with predominantly missense mutations in ALPL, and a number of compound heterozygous genotypes have been identified. Here, we describe a case of a subject with adult-onset HPP caused by a novel combination of missense mutations p.Gly473Ser and p.Ala487Val, resulting in chronic musculoskeletal pain, myopathy, persistent fatigue, vomiting, and an uncommon dental phenotype of short-rooted permanent teeth. Pedigree and biochemical analysis indicated that severity of symptoms was correlated with levels of residual ALP activity, and co-segregated with the p.Gly473Ser missense mutation. Bioinformatic analysis to predict the structural and functional impact of each of the point mutations in the TNSALP molecule, and its potential contribution to the clinical symptoms, revealed that the affected Gly473 residue is localized in the homodimer interface and predicted to have a dominant negative effect. The affected Ala487 residue was predicted to bind to Tyr479, which is closely located the N-terminal α-helix of TNSALP monomer 2, suggesting that both changes may impair dimer stability and catalytic functions. In conclusion, these findings assist in defining genotype-phenotype associations for HPP, and further define specific sites within the TNSALP molecule potentially related to neuromuscular manifestations in adult HPP, allowing for a better understanding of HPP pathophysiology.

Inactivating Mutation in IRF8 Promotes Osteoclast Transcriptional Programs and Increases Susceptibility to Tooth Root Resorption

This is the first study to our knowledge to report a novel mutation in the interferon regulatory factor 8 gene (IRF8G388S ) associated with multiple idiopathic tooth root resorption, a form of periodontal disease. The IRF8G388S variant in the highly conserved C-terminal motif is predicted to alter the protein structure, likely impairing IRF8 function. Functional assays demonstrated that the IRF8G388S mutant promoted osteoclastogenesis and failed to inhibit NFATc1-dependent transcriptional activation when compared with IRF8WT control. Further, similar to subjects with heterozygous IRF8G388S mutation, Irf8+/- mice exhibited increased osteoclast activity in the mandibular alveolar bone surrounding molar teeth. Immunohistochemistry illustrated increased NFATc1 expression in the dentoalveolar region of Irf8-/- and Irf8+/- mice when compared with Irf8+/+ controls. Genomewide analyses revealed that IRF8 constitutively bound to regulatory regions of several thousand genes in osteoclast precursors, and genetic aberration of IRF8 significantly enhanced many osteoclast-specific transcripts. Collectively, this study delineates the critical role of IRF8 in defining osteoclast lineage and osteoclast transcriptional program, which may help in better understanding of various osteoclast-mediated disorders, including periodontal disease. © 2019 American Society for Bone and Mineral Research.

Alkaline Phosphatase Replacement Therapy for Hypophosphatasia in Development and Practice

Hypophosphatasia (HPP) is an inherited disorder that affects bone and tooth mineralization characterized by low serum alkaline phosphatase. HPP is caused by loss-of-function mutations in the ALPL gene encoding the protein, tissue-nonspecific alkaline phosphatase (TNSALP). TNSALP is expressed by mineralizing cells of the skeleton and dentition and is associated with the mineralization process. Generalized reduction of activity of the TNSALP leads to accumulation of its substrates, including inorganic pyrophosphate (PP_i) that inhibits physiological mineralization. This leads to defective skeletal mineralization, with manifestations including rickets, osteomalacia, fractures, and bone pain, all of which can result in multi-systemic complications with significant morbidity, as well as mortality in severe cases. Dental manifestations are nearly universal among affected individuals and feature most prominently premature loss of deciduous teeth. Management of HPP has been limited to supportive care until the introduction of a TNSALP enzyme replacement therapy (ERT), asfotase alfa (AA). AA ERT has proven to be transformative, improving survival in severely affected infants and increasing overall quality of life in children and adults with HPP. This chapter provides an overview of TNSALP expression and functions, summarizes HPP clinical types and pathologies, discusses early attempts at therapies for HPP, summarizes development of HPP mouse models, reviews design and validation of AA ERT, and provides up-to-date accounts of AA ERT efficacy in clinical trials and case reports, including therapeutic response, adverse effects, limitations, and potential future directions in therapy.

Genetic engineering a large animal model of human hypophosphatasia in sheep

The availability of tools to accurately replicate the clinical phenotype of rare human diseases is a key step toward improved understanding of disease progression and the development of more effective therapeutics. We successfully generated the first large animal model of a rare human bone disease, hypophosphatasia (HPP) using CRISPR/Cas9 to introduce a single point mutation in the tissue nonspecific alkaline phosphatase (TNSALP) gene (ALPL) (1077 C > G) in sheep. HPP is a rare inherited disorder of mineral metabolism that affects bone and tooth development, and is associated with muscle weakness. Compared to wild-type (WT) controls, HPP sheep have reduced serum alkaline phosphatase activity, decreased tail vertebral bone size, and metaphyseal flaring, consistent with the mineralization deficits observed in human HPP patients. Computed tomography revealed short roots and thin dentin in incisors, and reduced mandibular bone in HPP vs. WT sheep, accurately replicating odonto-HPP. Skeletal muscle biopsies revealed aberrant fiber size and disorganized mitochondrial cristae structure in HPP vs. WT sheep. These genetically engineered sheep accurately phenocopy human HPP and provide a novel large animal platform for the longitudinal study of HPP progression, as well as other rare human bone diseases.

Profile of asfotase alfa in the treatment of hypophosphatasia: design, development, and place in therapy

Hypophosphatasia (HPP) is a multi-systemic metabolic disorder caused by loss-of-function mutations in the ALPL gene that encodes the mineralization-associated enzyme, tissue-nonspecific alkaline phosphatase (TNSALP). HPP is characterized by defective bone and dental mineralization, leading to skeletal abnormalities with complications resulting in significant morbidity and mortality. Management of HPP has been limited to supportive care until the introduction of a recently approved enzyme replacement therapy employing bone-targeted recombinant human TNSALP, asfotase alfa (AA). This new therapy has been transformative as it improves survival in severely affected infants, and overall quality of life in children and adults with HPP. This review provides an overview of HPP, focusing on important steps in the development of AA enzyme replacement therapy, including the drug design, preclinical studies in the HPP mouse model, and outcomes from clinical trials and case report publications to date, with special attention given to response to therapy of skeletal manifestations, biochemical features, and other clinical manifestations. The limitations, adverse effects, and outcomes of AA are outlined and the place in therapy for individuals with HPP is discussed.

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.

MMP20 Overexpression Disrupts Molar Ameloblast Polarity and Migration

Ameloblasts responsible for enamel formation express matrix metalloproteinase 20 (MMP20), an enzyme that cleaves enamel matrix proteins, including amelogenin (AMELX) and ameloblastin (AMBN). Previously, we showed that continuously erupting incisors from transgenic mice overexpressing active MMP20 had a massive cell infiltrate present within their enamel space, leading to enamel mineralization defects. However, effects of MMP20 overexpression on mouse molars were not analyzed, although these teeth more accurately represent human odontogenesis. Therefore, MMP20-overexpressing mice ( Mmp20^+/+Tg⁺) were assessed by multiscale analyses, combining several approaches from high-resolution micro-computed tomography to enamel organ immunoblots. During the secretory stage at postnatal day 6 (P6), Mmp20^+/+Tg⁺ mice had a discontinuous ameloblast layer and, unlike incisors, molar P12 maturation stage ameloblasts abnormally migrated away from the enamel layer into the stratum intermedium/stellate reticulum. TOPflash assays performed in vitro demonstrated that MMP20 expression promoted β-catenin nuclear localization and that MMP20 expression promoted invasion through Matrigel-coated filters. However, for both assays, significant differences were eliminated in the presence of the β-catenin inhibitor ICG-001. This suggests that MMP20 activity promotes cell migration via the Wnt pathway. In vivo, the unique molar migration of amelogenin-expressing ameloblasts was associated with abnormal deposition of ectopic calcified nodules surrounding the adherent enamel layer. Enamel content was assessed just prior to eruption at P15. Compared to wild-type, Mmp20^+/+Tg⁺ molars exhibited significant reductions in enamel thickness (70%), volume (60%), and mineral density (40%), and MMP20 overexpression resulted in premature cleavage of AMBN, which likely contributed to the severe defects in enamel mineralization. In addition, Mmp20^+/+Tg⁺ mouse molar enamel organs had increased levels of inactive p-cofilin, a protein that regulates cell polarity. These data demonstrate that increased MMP20 activity in molars causes premature degradation of ameloblastin and inactivation of cofilin, which may contribute to pathological Wnt-mediated cell migration away from the enamel layer.

Osteopontin regulates dentin and alveolar bone development and mineralization

The periodontal complex is essential for tooth attachment and function and includes the mineralized tissues, cementum and alveolar bone, separated by the unmineralized periodontal ligament (PDL). To gain insights into factors regulating cementum-PDL and bone-PDL borders and protecting against ectopic calcification within the PDL, we employed a proteomic approach to analyze PDL tissue from progressive ankylosis knock-out (Ank^-/-) mice, featuring reduced PP_i, rapid cementogenesis, and excessive acellular cementum. Using this approach, we identified the matrix protein osteopontin (Spp1/OPN) as an elevated factor of interest in Ank^-/- mouse molar PDL. We studied the role of OPN in dental and periodontal development and function. During tooth development in wild-type (WT) mice, Spp1 mRNA was transiently expressed by cementoblasts and strongly by alveolar bone osteoblasts. Developmental analysis from 14 to 240days postnatal (dpn) indicated normal histological structures in Spp1^-/- comparable to WT control mice. Microcomputed tomography (micro-CT) analysis at 30 and 90dpn revealed significantly increased volumes and tissue mineral densities of Spp1^-/- mouse dentin and alveolar bone, while pulp and PDL volumes were decreased and tissue densities were increased. However, acellular cementum growth was unaltered in Spp1^-/- mice. Quantitative PCR of periodontal-derived mRNA failed to identify potential local compensators influencing cementum in Spp1^-/- vs. WT mice at 26dpn. We genetically deleted Spp1 on the Ank^-/- mouse background to determine whether increased Spp1/OPN was regulating periodontal tissues when the PDL space is challenged by hypercementosis in Ank^-/- mice. Ank^-/-; Spp1^-/- double deficient mice did not exhibit greater hypercementosis than that in Ank^-/- mice. Based on these data, we conclude that OPN has a non-redundant role regulating formation and mineralization of dentin and bone, influences tissue properties of PDL and pulp, but does not control acellular cementum apposition. These findings may inform therapies targeted at controlling soft tissue calcification.

Hypercementosis Associated with ENPP1 Mutations and GACI

Mineralization of bones and teeth is tightly regulated by levels of extracellular inorganic phosphate (P_i) and pyrophosphate (PP_i). Three regulators that control pericellular concentrations of P_i and PP_i include tissue-nonspecific alkaline phosphatase (TNAP), progressive ankylosis protein (ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Inactivation of these factors results in mineralization disorders affecting teeth and their supporting structures. This study for the first time analyzed the effect of decreased PP_i on dental development in individuals with generalized arterial calcification of infancy (GACI) due to loss-of-function mutations in the ENPP1 gene. Four of the 5 subjects reported a history of infraocclusion, overretained primary teeth, ankylosis, and/or slow orthodontic tooth movement, suggesting altered mineral metabolism contributing to disrupted tooth movement and exfoliation. All subjects had radiographic evidence of unusually protruding cervical root morphology in primary and/or secondary dentitions. High-resolution micro-computed tomography (micro-CT) analyses of extracted primary teeth from 3 GACI subjects revealed 4-fold increased cervical cementum thickness ( P = 0.00007) and a 23% increase in cementum density ( P = 0.009) compared to age-matched healthy control teeth. There were no differences in enamel and dentin densities between GACI and control teeth. Histology revealed dramatically expanded cervical cementum in GACI teeth, including cementocyte-like cells and unusual patterns of cementum resorption and repair. Micro-CT analysis of Enpp1 mutant mouse molars revealed 4-fold increased acellular cementum thickness ( P = 0.002) and 5-fold increased cementum volume ( P = 0.002), with no changes in enamel or dentin. Immunohistochemistry identified elevated ENPP1 expression in cementoblasts of human and mouse control teeth. Collectively, these findings reveal a novel dental phenotype in GACI and identify ENPP1 genetic mutations associated with hypercementosis. The sensitivity of cementum to reduced PP_i levels in both human and mouse teeth establishes this as a well-conserved and fundamental biological process directing cementogenesis across species (ClinicalTrials.gov NCT00369421).

Overlapping functions of bone sialoprotein and pyrophosphate regulators in directing cementogenesis

Although acellular cementum is essential for tooth attachment, factors directing its development and regeneration remain poorly understood. Inorganic pyrophosphate (PP_i), a mineralization inhibitor, is a key regulator of cementum formation: tissue-nonspecific alkaline phosphatase (Alpl/TNAP) null mice (increased PP_i) feature deficient cementum, while progressive ankylosis protein (Ank/ANK) null mice (decreased PP_i) feature increased cementum. Bone sialoprotein (Bsp/BSP) and osteopontin (Spp1/OPN) are multifunctional extracellular matrix components of cementum proposed to have direct and indirect effects on cell activities and mineralization. Studies on dentoalveolar development of Bsp knockout (Bsp^-/-) mice revealed severely reduced acellular cementum, however underlying mechanisms remain unclear. The similarity in defective cementum phenotypes between Bsp^-/- mice and Alpl^-/- mice (the latter featuring elevated PP_i and OPN), prompted us to examine whether BSP is operating by modulating PP_i-associated genes. Genetic ablation of Bsp caused a 2-fold increase in circulating PP_i, altered mRNA expression of Alpl, Spp1, and Ank, and increased OPN protein in the periodontia. Generation of a Bsp knock-out (KO) cementoblast cell line revealed significantly decreased mineralization capacity, 50% increased PP_i in culture media, and increased Spp1 and Ank mRNA expression. While addition of 2μg/ml recombinant BSP altered Spp1, Ank, and Enpp1 expression in cementoblasts, changes resulting from this dose were not dependent on the integrin-binding RGD motif or MAPK/ERK signaling pathway. Decreasing PP_i by genetic ablation of Ank on the Bsp^-/- mouse background reestablished cementum formation, allowing >3-fold increased acellular cementum volume compared to wild-type (WT). However, deleting Ank did not fully compensate for the absence of BSP. Bsp^-/-; Ank^-/- double-deficient mice exhibited mean 20-27% reduced cementum thickness and volume compared to Ank^-/- mice. From these data, we conclude that the perturbations in PP_i metabolism are not solely driving the cementum pathology in Bsp^-/- mice, and that PP_i is more potent than BSP as a cementum regulator, as shown by the ability to override loss of BSP by lowering PP_i. We propose that BSP and PP_i work in concert to direct mineralization in cementum and likely other mineralized tissues.

On the discovery of cementum

Though cementum of the tooth root is critical for periodontal structure and tooth attachment and function, this tissue was not discovered and characterized on human teeth until a full century later than enamel and dentin. Early observations from the seventeenth to the nineteenth centuries by Marcello Malpighi, Antonie van Leeuwenhoek, Robert Blake, Jacques Tenon and Georges Cuvier founded a confusing and conflicting nomenclature that obscured the nature of cementum, often conflating it with bone. Advances in microscopy and histological procedures yielded the first detailed descriptions of human cementum in the 1830s by Jan Purkinje and Anders Retzius, who identified for the first time acellular and cellular types of cementum, and the resident cementocytes embedded in the latter. Comparative anatomy studies by Richard Owen and others over the latter half of the nineteenth century identified coronal and radicular cementum varieties across the Reptilia and Mammalia. The functional importance of cementum was not appreciated until detailed anatomical studies of the periodontium were performed by G.V. Black and others in the late nineteenth and early twentieth centuries. These early studies on cementum laid the foundation for more advanced understanding of cementum ultrastructure, composition, development, physiology, disease, genetics, repair and regeneration throughout the twentieth and into the twenty-first century.

Microproteome of dentoalveolar tissues

Proteomic analysis of extracellular matrices (ECM) of dentoalveolar tissues can provide insights into developmental, pathological, and reparative processes. However, targeted dissection of mineralized tissues, dental cementum (DC), alveolar bone (AB), and dentin (DE), presents technical difficulties. We demonstrate an approach combining EDTA decalcification and laser capture microdissection (LCM), followed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), to analyze proteome profiles of these tissues. Using the LCM-LC-MS/MS approach, a total of 243 proteins was identified from all tissues, 193 proteins in DC, 147 in AB, and 135 proteins DE. Ninety proteins (37% of total) were common to all tissues, whereas 52 proteins (21%) were overlapping in only two. Also, 101 (42%) proteins were exclusively detected in DC (60), AB (15), or DE (26). Identification in all tissues of expected ECM proteins including collagen alpha-1(I) chain (COL1A1), collagen alpha-1(XII) chain (COL12A1), biglycan (BGN), asporin (ASPN), lumican (LUM), and fibromodulin (FMOD), served to validate the approach. Principal component analysis (PCA) and hierarchical clustering identified a high degree of similarity in DC and AB proteomes, whereas DE presented a distinct dataset. Exclusively and differentially identified proteins were detected from all three tissues. The protein-protein interaction network (interactome) of DC was notable for its inclusion of several indicators of metabolic function (e.g. mitochondrial proteins, protein synthesis, and calcium transport), possibly reflecting cementocyte activity. The DE proteome included known and novel mineralization regulators, including matrix metalloproteinase 20 (MMP-20), 5' nucleotidase (NT5E), and secreted phosphoprotein 24 (SPP-24 or SPP-2). Application of the LCM-LC-MS/MS approach to dentoalveolar tissues would be of value in many experimental designs, including developmental studies of transgenic animals, investigation of treatment effects, and identification of novel regenerative factors.

Cementoblast Gene Expression is Regulated by Porphyromonas gingivalis Lipopolysaccharide Partially via Toll-like Receptor-4/MD-2

Lipopolysaccharides are potent inflammatory mediators considered to contribute to destruction of periodontal tissues. Here, we hypothesized that Porphyromonas gingivalis lipopolysaccharide (P-LPS) treatment would regulate gene expression in murine cementoblasts through Toll-like receptor 4. Real-time (RT)-PCR and Northern blot analysis indicated that P-LPS decreased expression of transcripts for osteocalcin (OCN) and receptor activator of nuclear factor κB ligand (RANKL). In contrast, a dose-dependent up-regulation in mRNA levels for osteopontin (OPN) and osteoprotegerin (OPG) was observed. Similarly, ELISA demonstrated decreased RANKL and increased OPG levels. A monoclonal antibody specific for mouse TLR-4/MD-2 partially neutralized the P-LPS effect on cementoblasts. These results indicate that exposure of cementoblasts to P-LPS can alter cell function by regulating markers of osteoclastic activity ( e.g., RANKL/OPG), thereby potentially affecting the inflammation-associated resorption of mineralized tissues.

Conditional Alpl Ablation Phenocopies Dental Defects of Hypophosphatasia

Loss-of-function mutations in ALPL result in hypophosphatasia (HPP), an inborn error of metabolism that causes defective skeletal and dental mineralization. ALPL encodes tissue-nonspecific alkaline phosphatase, an enzyme expressed in bone, teeth, liver, and kidney that hydrolyzes the mineralization inhibitor inorganic pyrophosphate. As Alpl-null mice die before weaning, we aimed to generate mouse models of late-onset HPP with extended life spans by engineering a floxed Alpl allele, allowing for conditional gene ablation (conditional knockout [cKO]) when crossed with Cre recombinase transgenic mice. The authors hypothesized that targeted deletion of Alpl in osteoblasts and selected dental cells ( Col1a1-cKO) or deletion in chondrocytes, osteoblasts, and craniofacial mesenchyme ( Prx1-cKO) would phenocopy skeletal and dental manifestations of late-onset HPP. Col1a1-cKO and Prx1-cKO mice were viable and fertile, and they did not manifest the epileptic seizures characteristic of the Alpl^-/- model of severe infantile HPP. Both cKO models featured normal postnatal body weight but significant reduction as compared with wild type mice by 8 to 12 wk. Plasma alkaline phosphatase for both cKO models at 24 wk was reduced by approximately 75% as compared with controls. Radiography revealed profound skeletal defects in cKO mice, including rachitic changes, hypomineralized long bones, deformations, and signs of fractures. Microcomputed tomography confirmed quantitative differences in cortical and trabecular bone, including decreased cortical thickness and mineral density. Col1a1-cKO mice exhibited classic signs of HPP dentoalveolar disease, including short molar roots with thin dentin, lack of acellular cementum, and osteoid accumulation in alveolar bone. Prx1-cKO mice exhibited the same array of periodontal defects but featured less affected molar dentin. Both cKO models exhibited reduced alveolar bone height and 4-fold increased numbers of osteoclast-like cells versus wild type at 24 wk, consistent with HPP-associated periodontal disease. These novel models of late-onset HPP can inform on long-term skeletal and dental manifestations and will provide essential tools to further studies of etiopathologies and therapeutic interventions.

Full Spectrum of Postnatal Tooth Phenotypes in a Novel Irf6 Cleft Lip Model

Clefting of the lip, with or without palatal involvement (CLP), is associated with a higher incidence of developmental tooth abnormalities, including hypodontia and supernumerary teeth, aberrant crown and root morphologies, and enamel defects, although the underlying mechanistic link is poorly understood. As most CLP genes are expressed throughout the oral epithelium, the authors hypothesized that the expression of CLP genes may persist in the dental epithelium and thus, in addition to their earlier role in labiopalatine development, may play an important functional role in subsequent tooth patterning and amelogenesis. To address this, the authors generated a unique conditional knockout model involving the major CLP gene, Irf6, that overcomes the previously reported perinatal lethality to enable assessment of any posteruption dental phenotypes. A dental epithelium-specific Irf6 conditional knockout (Irf6-cKO) mouse was generated via a Pitx2-Cre driver line. Dental development was analyzed by microcomputed tomography, scanning electron microscopy, histology, immunohistochemistry, and quantitative polymerase chain reaction. Irf6-cKO mice displayed variable hypodontia, occasional supernumerary incisors and molars, as well as crown and root patterning anomalies, including peg-shaped first molars and taurodontic and C-shaped mandibular second molars. Enamel density was reduced in preeruption Irf6-cKO mice, and some shearing of enamel rods was noted in posteruption incisors. There was also rapid attrition of Irf6-cKO molars following eruption. Histologically, Irf6-cKO ameloblasts exhibited disturbances in adhesion and polarity, and delayed enamel formation was confirmed immunohistochemically. Altered structure of Hertwig's epithelial root sheath was also observed. These data support a role for IRF6 in tooth number, crown and root morphology and amelogenesis that is likely due to a functional role of Irf6 in organization and polarity of epithelial cell types. This data reinforce the notion that various isolated tooth defects could be considered part of the CLP spectrum in relatives of an affected individual.

Global proteome profiling of dental cementum under experimentally-induced apposition

Dental cementum (DC) covers the tooth root and has important functions in tooth attachment and position. DC can be lost to disease, and regeneration is currently unpredictable due to limited understanding of DC formation. This study used a model of experimentally-induced apposition (EIA) in mice to identify proteins associated with new DC formation. Mandibular first molars were induced to super-erupt for 6 and 21days after extracting opposing maxillary molars. Decalcified and formalin-fixed paraffin-embedded mandible sections were prepared for laser capture microdissection. Microdissected protein extracts were analyzed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and the data submitted to repeated measure ANOVA test (RM-ANOVA, alpha=5%). A total of 519 proteins were identified, with 97 (18.6%) proteins found exclusively in EIA sites and 50 (9.6%) proteins exclusively expressed in control sites. Fifty six (10.7%) proteins were differentially regulated by RM-ANOVA (p<0.05), with 24 regulated by the exclusive effect of EIA (12 proteins) or the interaction between EIA and time (12 proteins), including serpin 1a, procollagen C-endopeptidase enhancer, tenascin X (TNX), and asporin (ASPN). In conclusion, proteomic analysis demonstrated significantly altered protein profile in DC under EIA, providing new insights on DC biology and potential candidates for tissue engineering applications.

SIGNIFICANCE

Dental cementum (DC) is a mineralized tissue that covers the tooth root surface and has important functions in tooth attachment and position. DC and other periodontal tissues can be lost to disease, and regeneration is currently unpredictable due to lack of understanding of DC formation. This study used a model of experimentally-induced apposition (EIA) in mice to promote new cementum formation, followed by laser capture microdissection (LCM) and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) proteomic analysis. This approach identified proteins associated with new cementum formation that may be targets for promoting cementum regeneration.

A Familial Pattern of Multiple Idiopathic Cervical Root Resorption With a 30-Year Follow-Up

BACKGROUND

Multiple idiopathic cervical root resorption (MICRR) is a rare entity distinct from pathologic root resorption that occurs as a result of several local and systemic factors.

METHODS

This report describes a familial pattern of MICRR, including a recently identified case and a 30-year follow-up on previously described cases.

RESULTS

The previously reported father (aged 95 years) and son (aged 64 years), and the recently affected daughter (aged 61 years) recounted non-contributory medical history. The resorptive lesions were asymptomatic, unassociated with any predisposing factors, and first identified during the fourth to sixth decades of life. All tooth types were affected, with posterior teeth being affected earlier and with greater frequency; however, distal root surfaces were never affected. The resorptive lesions were progressive in nature, with additional teeth becoming involved as the condition was followed over time. In many instances, surrounding alveolar bone extended into the existing resorptive defects, but without clinical evidence of ankylosis. Gingival tissues, periodontal probing, and tooth mobility were within normal limits. Microcomputed tomography of extracted teeth demonstrated that the lesions were more extensive than clinically evident and rarely invaded the pulp chamber. Histologically, many resorptive lesions were noted along the cementum surface, with evidence of isolated cemental repair. Management of MICRR focused on restoring damaged root surfaces and extracting teeth with extensive root resorption.

CONCLUSIONS

MICRR is a challenging entity with unknown etiology and a lack of well-established preventive and management strategies. The familial pattern noted in this report necessitates future studies to investigate the role of genetic components in MICRR development.

The Cementocyte-An Osteocyte Relative?

Cementum is a mineralized tissue covering the tooth root that functions in tooth attachment and posteruptive adjustment of tooth position. During formation of the apically located cellular cementum, some cementoblasts become embedded in the cementoid matrix and become cementocytes. As apparently terminally differentiated cells embedded in a mineralized extracellular matrix, cementocytes are part of a select group of specialized cells, also including osteocytes, hypertrophic chondrocytes, and odontoblasts. The differentiation and potential function(s) of cementocytes are virtually unknown, and the question may be posed whether the cementocyte is a dynamic actor in cementum in comparable fashion with the osteocyte in the skeleton, responding to changing tooth functions and endocrine signals and actively directing local cementum metabolism. This review summarizes the literature with regard to cementocytes, comparing them to their closest "cousins," the osteocytes, where insights gained from osteocyte studies serve to inform the critical examination of cementocytes. The review identifies important unanswered questions about these cells regarding their origins, differentiation, morphology and lacuno-canalicular system, selective markers, and potential functions.

Role of PHOSPHO1 in Periodontal Development and Function

The tooth root and periodontal apparatus, including the acellular and cellular cementum, periodontal ligament (PDL), and alveolar bone, are critical for tooth function. Cementum and bone mineralization is regulated by factors including enzymes and extracellular matrix proteins that promote or inhibit hydroxyapatite crystal growth. Orphan Phosphatase 1 (Phospho1, PHOSPHO1) is a phosphatase expressed by chondrocytes, osteoblasts, and odontoblasts that functions in skeletal and dentin mineralization by initiating deposition of hydroxyapatite inside membrane-limited matrix vesicles. The role of PHOSPHO1 in periodontal formation remains unknown and we aimed to determine its functional importance in these tissues. We hypothesized that the enzyme would regulate proper mineralization of the periodontal apparatus. Spatiotemporal expression of PHOSPHO1 was mapped during periodontal development, and Phospho1(-/-) mice were analyzed using histology, immunohistochemistry, in situ hybridization, radiography, and micro-computed tomography. The Phospho1 gene and PHOSPHO1 protein were expressed by active alveolar bone osteoblasts and cementoblasts during cellular cementum formation. In Phospho1(-/-) mice, acellular cementum formation and mineralization were unaffected, whereas cellular cementum deposition increased although it displayed delayed mineralization and cementoid. Phospho1(-/-) mice featured disturbances in alveolar bone mineralization, shown by accumulation of unmineralized osteoid matrix and interglobular patterns of protein deposition. Parallel to other skeletal sites, deposition of mineral-regulating protein osteopontin (OPN) was increased in alveolar bone in Phospho1(-/-) mice. In contrast to the skeleton, genetic ablation of Spp1, the gene encoding OPN, did not ameliorate dentoalveolar defects in Phospho1(-/-) mice. Despite alveolar bone mineralization defects, periodontal attachment and function appeared undisturbed in Phospho1(-/-) mice, with normal PDL architecture and no evidence of bone loss over time. This study highlights the role of PHOSPHO1 in mineralization of alveolar bone and cellular cementum, further revealing that acellular cementum formation is not substantially regulated by PHOSPHO1 and likely does not rely on matrix vesicle-mediated initiation of mineralization.

Isolation and Functional Analysis of an Immortalized Murine Cementocyte Cell Line, IDG-CM6

The dental cementum covering the tooth root is similar to bone in several respects but remains poorly understood in terms of development and differentiation of cementoblasts, as well as the potential function(s) of cementocytes residing in the cellular cementum. It is not known if the cementocyte is a dynamic actor in cementum metabolism, comparable to the osteocyte in the bone. Cementocytes exhibit irregular spacing and lacunar shape, with fewer canalicular connections compared with osteocytes. Immunohistochemistry and quantitative PCR (qPCR) revealed that the in vivo expression profile of cementocytes paralleled that of osteocytes, including expression of dentin matrix protein 1 (Dmp1/DMP1), Sost/sclerostin, E11/gp38/podoplanin, Tnfrsf11b (osteoprotegerin [OPG]), and Tnfsf11 (receptor activator of NF-κB ligand [RANKL]). We used the Immortomouse(+/-); Dmp1-GFP(+/-) mice to isolate cementocytes as Dmp1-expressing cells followed by immortalization using the interferon (IFN)-γ-inducible promoter driving expression of a thermolabile large T antigen to create the first immortalized line of cementocytes, IDG-CM6. This cell line reproduced the expression profile of cementocytes observed in vivo, including alkaline phosphatase activity and mineralization. IDG-CM6 cells expressed higher levels of Tnfrsf11b and lower levels of Tnfsf11 compared with IDG-SW3 osteocytes, and under fluid flow shear stress, IDG-CM6 cells significantly increased OPG while decreasing RANKL, leading to a significantly increased OPG/RANKL ratio, which would inhibit osteoclast activation. These studies indicate similarities yet potentially important differences in the function of cementocytes compared with osteocytes and support cementocytes as mechanically responsive cells.

Mineralization defects in cementum and craniofacial bone from loss of bone sialoprotein

Bone sialoprotein (BSP) is a multifunctional extracellular matrix protein found in mineralized tissues, including bone, cartilage, tooth root cementum (both acellular and cellular types), and dentin. In order to define the role BSP plays in the process of biomineralization of these tissues, we analyzed cementogenesis, dentinogenesis, and osteogenesis (intramembranous and endochondral) in craniofacial bone in Bsp null mice and wild-type (WT) controls over a developmental period (1-60 days post natal; dpn) by histology, immunohistochemistry, undecalcified histochemistry, microcomputed tomography (microCT), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative PCR (qPCR). Regions of intramembranous ossification in the alveolus, mandible, and calvaria presented delayed mineralization and osteoid accumulation, assessed by von Kossa and Goldner's trichrome stains at 1 and 14 dpn. Moreover, Bsp(-/-) mice featured increased cranial suture size at the early time point, 1 dpn. Immunostaining and PCR demonstrated that osteoblast markers, osterix, alkaline phosphatase, and osteopontin were unchanged in Bsp null mandibles compared to WT. Bsp(-/-) mouse molars featured a lack of functional acellular cementum formation by histology, SEM, and TEM, and subsequent loss of Sharpey's collagen fiber insertion into the tooth root structure. Bsp(-/-) mouse alveolar and mandibular bone featured equivalent or fewer osteoclasts at early ages (1 and 14 dpn), however, increased RANKL immunostaining and mRNA, and significantly increased number of osteoclast-like cells (2-5 fold) were found at later ages (26 and 60 dpn), corresponding to periodontal breakdown and severe alveolar bone resorption observed following molar teeth entering occlusion. Dentin formation was unperturbed in Bsp(-/-) mouse molars, with no delay in mineralization, no alteration in dentin dimensions, and no differences in odontoblast markers analyzed. No defects were identified in endochondral ossification in the cranial base, and craniofacial morphology was unaffected in Bsp(-/-) mice. These analyses confirm a critical role for BSP in processes of cementogenesis and intramembranous ossification of craniofacial bone, whereas endochondral ossification in the cranial base was minimally affected and dentinogenesis was normal in Bsp(-/-) molar teeth. Dissimilar effects of loss of BSP on mineralization of dental and craniofacial tissues suggest local differences in the role of BSP and/or yet to be defined interactions with site-specific factors.

PTH and Vitamin D Repress DMP1 in Cementoblasts

A complex feedback mechanism between parathyroid hormone (PTH), 1,25(OH)2D3 (1,25D), and fibroblast growth factor 23 (FGF-23) maintains mineral homeostasis, in part by regulating calcium and phosphate absorption/reabsorption. Previously, we showed that 1,25D regulates mineral homeostasis by repressing dentin matrix protein 1 (DMP1) via the vitamin D receptor pathway. Similar to 1,25D, PTH may modulate DMP1, but the underlying mechanism remains unknown. Immortalized murine cementoblasts (OCCM.30), similar to osteoblasts and known to express DMP1, were treated with PTH (1-34). Real-time quantitative polymerase chain reaction (PCR) and Western blot revealed that PTH decreased DMP1 gene transcription (85%) and protein expression (30%), respectively. PTH mediated the downregulation of DMP1 via the cAMP/protein kinase A (PKA) pathway. Immunohistochemistry confirmed the decreased localization of DMP1 in vivo in cellular cementum and alveolar bone of mice treated with a single dose (50 µg/kg) of PTH (1-34). RNA-seq was employed to further identify patterns of gene expression shared by PTH and 1,25D in regulating DMP1, as well as other factors involved in mineral homeostasis. PTH and 1,25D mutually upregulated 36 genes and mutually downregulated 27 genes by ≥2-fold expression (P ≤ 0.05). Many identified genes were linked with the regulation of bone/tooth homeostasis, cell growth and differentiation, calcium signaling, and DMP1 transcription. Validation of RNA-seq results via PCR array confirmed a similar gene expression pattern in response to PTH and 1,25D treatment. Collectively, these results suggest that PTH and 1,25D share complementary effects in maintaining mineral homeostasis by mutual regulation of genes/proteins associated with calcium and phosphate metabolism while also exerting distinct roles on factors modulating mineral metabolism. Furthermore, PTH may modulate phosphate homeostasis by downregulating DMP1 expression via the cAMP/PKA pathway. Targeting genes/proteins mutually governed by PTH and 1,25D may be a viable approach for designing new therapies for preserving mineralized tissue health.

Mechanical Forces Exacerbate Periodontal Defects in Bsp-null Mice

Bone sialoprotein (BSP) is an acidic phosphoprotein with collagen-binding, cell attachment, and hydroxyapatite-nucleating properties. BSP expression in mineralized tissues is upregulated at onset of mineralization. Bsp-null (Bsp(-/-)) mice exhibit reductions in bone mineral density, bone turnover, osteoclast activation, and impaired bone healing. Furthermore, Bsp(-/-) mice have marked periodontal tissue breakdown, with a lack of acellular cementum leading to periodontal ligament detachment, extensive alveolar bone and tooth root resorption, and incisor malocclusion. We hypothesized that altered mechanical stress from mastication contributes to periodontal destruction observed in Bsp(-/-) mice. This hypothesis was tested by comparing Bsp(-/-) and wild-type mice fed with standard hard pellet diet or soft powder diet. Dentoalveolar tissues were analyzed using histology and micro-computed tomography. By 8 wk of age, Bsp(-/-) mice exhibited molar and incisor malocclusion regardless of diet. Bsp(-/-) mice with hard pellet diet exhibited high incidence (30%) of severe incisor malocclusion, 10% lower body weight, 3% reduced femur length, and 30% elevated serum alkaline phosphatase activity compared to wild type. Soft powder diet reduced severe incisor malocclusion incidence to 3% in Bsp(-/-) mice, supporting the hypothesis that occlusal loading contributed to the malocclusion phenotype. Furthermore, Bsp(-/-) mice in the soft powder diet group featured normal body weight, long bone length, and serum alkaline phosphatase activity, suggesting that tooth dysfunction and malnutrition contribute to growth and skeletal defects reported in Bsp(-/-) mice. Bsp(-/-) incisors also erupt at a slower rate, which likely leads to the observed thickened dentin and enhanced mineralization of dentin and enamel toward the apical end. We propose that the decrease in eruption rate is due to a lack of acellular cementum and associated defective periodontal attachment. These data demonstrate the importance of BSP in maintaining proper periodontal function and alveolar bone remodeling and point to dental dysfunction as causative factor of skeletal defects observed in Bsp(-/-) mice.