On-demand chlorine dioxide solution enhances odontoblast differentiation through desulfation of cell surface heparan sulfate proteoglycan and subsequent activation of canonical Wnt signaling

Heparan sulfate proteoglycans (HSPGs) surround the surface of odontoblasts, and their modification affects their affinity for Wnt ligands. This study proposes applying Matching Transformation System® (MA-T), a novel chlorinated oxidant, to enhance dentinogenesis. MA-T treatment in odontoblasts decreased sulfation of HSPG and upregulated the expression of dentin sialophosphoprotein (Dspp) and Dentin Matrix Protein 1 (Dmp1) via activation of canonical Wnt signaling in vitro. Ex vivo application of MA-T also enhanced dentin matrix formation in developing tooth explants. Reanalysis of a public single-cell RNA-seq dataset revealed significant Wnt activity in the odontoblast population, with enrichment for Wnt10a and Wnt6. Silencing assays showed that Wnt10a and Wnt6 were redundant in inducing Dspp and Dmp1 mRNA expression. These Wnt ligands' expression was upregulated by MA-T treatment, and TCF/LEF binding sites are present in their promoters. Furthermore, the Wnt inhibitors Notum and Dkk1 were enriched in odontoblasts, and their expression was also upregulated by MA-T treatment, together suggesting autonomous maintenance of Wnt signaling in odontoblasts. This study provides evidence that MA-T activates dentinogenesis by modifying HSPG and through subsequent activation of Wnt signaling.

First report in Argentina of a pathogenic DMP1 variant associated with autosomal recessive hypophosphatemic rickets

Hereditary hypophosphatemic rickets is a genetic condition associated with impaired bone mineralization caused by phosphate deficiency. It results in skeletal deformity and growth retardation in early childhood. Different inheritance patterns have been described according to the locus involved. Given the phenotypic overlapping and the difficulty in analyzing reduced genealogies, molecular studies are important to establish the genetic cause and implement a family-centered approach. The autosomal recessive form of hypophosphatemic rickets (ARHR, OMIM 241520) is an extremely rare condition reported in families of European and Middle Eastern descent. Loss-of-function mutations in the DMP1 (dentin matrix acidic phosphoprotein 1) gene are associated with hereditary hypophosphatemic rickets type 1. In this article, we describe the first report of an Argentine family with hereditary hypophosphatemic rickets due to a mutation in the DMP1 gene.

DMP1 and IFITM5 Regulate Osteogenic Differentiation of MC3T3-E1 on PEO-Treated Ti-6Al-4V-Ca2+/Pi surface

Despite numerous studies on various surface modifications on titanium and its alloys, it remains unclear what kind of titanium-based surface modifications are capable of controlling cell activity. This study aimed to understand the mechanism at the cellular and molecular levels and investigate the in vitro response of osteoblastic MC3T3-E1 cultured on the Ti-6Al-4V surface modified by plasma electrolytic oxidation (PEO) treatment. A Ti-6Al-4V surface was prepared by PEO at 180, 280, and 380 V for 3 or 10 min in an electrolyte containing Ca²⁺/P_i ions. Our results showed that PEO-treated Ti-6Al-4V-Ca²⁺/P_i surfaces enhanced the cell attachment and differentiation of MC3T3-E1 compared to the untreated Ti-6Al-4V control but did not affect cytotoxicity as shown by cell proliferation and cell death. Interestingly, on the Ti-6Al-4V-Ca²⁺/P_i surface treated by PEO at 280 V for 3 or 10 min, MC3T3-E1 showed a higher initial adhesion and mineralization. In addition, the alkaline phosphatase (ALP) activity significantly increased in MC3T3-E1 on the PEO-treated Ti-6Al-4V-Ca²⁺/P_i (280 V for 3 or 10 min). In RNA-seq analysis, the expression of dentin matrix protein 1 (DMP1), sortilin 1 (Sort1), signal-induced proliferation-associated 1 like 2 (SIPA1L2), and interferon-induced transmembrane protein 5 (IFITM5) was induced during the osteogenic differentiation of MC3T3-E1 on the PEO-treated Ti-6Al-4V-Ca²⁺/P_i. DMP1 and IFITM5 silencing decreased the expression of bone differentiation-related mRNAs and proteins and ALP activity in MC3T3-E1. These results suggest that the PEO-treated Ti-6Al-4V-Ca²⁺/P_i surface induces osteoblast differentiation by regulating the expression of DMP1 and IFITM5. Therefore, surface microstructure modification through PEO coatings with Ca²⁺/P_i ions could be used as a valuable method to improve biocompatibility properties of titanium alloys.

Dentin Matrix Protein 1 Regulates Mineralization of MC3T3-E1 Cells via the TNAP-ANK-ENPP1 Axis

BACKGROUND

Dentin matrix protein 1 (DMP1) is central to matrix mineralization. Clarification of the function of DMP1 is crucial to understanding normal bone formation and pathological calcification. The tissue-nonspecific alkaline phosphatase (TNAP) -progressive ankylosing enzyme (ANK) -extracellular nucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) axis induces deposition of hydroxyapatite (HA) and pyrophosphate dehydrate (CPPD) by regulating pyrophosphate (PPi). Here, we investigated the mechanism by which DMP1 and the TNAP-ANK-ENPP1 axis participate in mineralization.

METHODS

Expression of DMP1, TNAP, NPP1, and ANK genes in MC3T3-E1 cells was detected by RT-qPCR before and after treatment with DMP1 siRNA. An enzyme-linked immunosorbent assay was used to determine expression of DMP1 protein, TNAP activity was detected by SIGMAFAST p-nitrophenyl phosphate tablets, and mineralization of osteoblasts was determined by alizarin red staining. PPi levels were determined radiometrically and equalized for cell DNA. Levels of calcium, inorganic phosphate, zinc, and magnesium were assessed by standard laboratory techniques.

RESULTS

After DMP1 gene silencing, expressions of TNAP, ENPP1, and ANK were correspondingly reduced. DMP1 altered extravesicular and intravesicular ion levels through the TNAP-ENPP1-ANK axis in MC3T3-E1 cells.

CONCLUSIONS

DMP1 regulated mineralization of MC3T3-E1 cells via the TNAP-ANK-ENPP1 axis and affected TNAP activity by two processes-rapid regulation of the Zn²⁺ transporter (ZnT) and transcriptional regulation of hysteresis. However, DMP1 may affect expression of ENPP1 and ANK only via hysteresis transcriptional regulation. DMP1, as a calcium trap or catalytic enzyme, appears to have a role in collagen mineralization.

Whole exome sequencing identifies two novel variants in PHEX and DMP1 in Malaysian children with hypophosphatemic rickets

BACKGROUND

Hypophosphatemic rickets (HR) is a genetic disease of phosphate wasting that is characterized by defective bone mineralization. The most common cause of the disease is mutations in the phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX) gene. The aims of this study were to identify the gene variants responsible for HR in three cases of Malaysian origin from three independent families and to describe their clinical, biochemical, and radiological features.

METHODS

Whole exome sequencing (WES) was performed on all patients and their parents, followed by Sanger sequencing validation. Bioinformatics tools were used to provide supporting evidence for pathogenicity of variants. To confirm that a mutation is de novo, paternity test was carried out. High resolution melting curve analysis was performed to assess the allele frequency in normal controls for mutations that were found in the patients.

RESULTS

The patients showed typical characteristics of HR including lower limb deformity, hypophosphatemia, and elevated alkaline phosphatase. WES revealed two variants in the PHEX gene and one variant in the dentin matrix protein 1 (DMP1) gene. Two of the three variants were novel, including c.1946_1954del (p.Gly649_Arg651del) in PHEX and c.54 + 1G > A in DMP1. Our data suggests that the novel p.Gly649_Arg651del variant is likely pathogenic for HR disease.

CONCLUSIONS

This study extends the variant spectrum of the PHEX and DMP1 genes. Our findings indicate that WES is an advantageous approach for diagnosis of genetic diseases which are heterogeneous.

Correction of Vascular Calcification and Hyperphosphatemia in CKD Rats Treated with ASARM Peptide

Background

Abnormalities in calcium, phosphorus, PTH, vitamin D metabolism, bone, and vascular calcification occur in chronic kidney disease mineral bone disorder (CKD-MBD). Calciphylaxis, involving painful, ulcerative skin lesions, is also a major problem associated with CKD-MBD. There are no quality medical interventions to address these clinical issues. Bone ASARM peptides are strong inhibitors of mineralization and induce hypophosphatemia by inhibiting phosphate uptake from the gut. We hypothesize treatment of CKD-MBD rats with ASARM peptides will reverse hyperphosphatemia, reduce soft-tissue calcification, and prevent calciphylaxis.

Methods

To test our hypothesis, we assessed the effects of synthetic ASARM peptide in rats that had undergone a subtotal 5/6th nephrectomy (56NEPHREX), a rodent model of CKD-MBD. All rats were fed a high phosphate diet (2% Pi) to worsen mineral metabolism defects. Changes in serum potassium, phosphate, BUN, creatinine, PTH, FGF23, and calcium were assessed in response to 28 days of ASARM peptide infusion. Also, changes in bone quality, soft-tissue calcification, and expression of gut Npt2b (Slc34a2) were studied following ASARM peptide treatment.

Results

Rats that had undergone 56NEPHREX treated with ASARM peptide showed major improvements in hyperphosphatemia, blood urea nitrogen (BUN), and bone quality compared with vehicle controls. Also, ASARM-infused 56NEPHREX rats displayed improved renal, brain, and cardiovascular calcification. Notably, ASARM peptide infusion prevented the genesis of subdermal medial blood vessel calcification and calciphylaxis-like lesions in 56NEPHREX rats compared with vehicle controls.

Conclusions

ASARM peptide infusion corrects hyperphosphatemia and improves vascular calcification, renal calcification, brain calcification, bone quality, renal function, and skin mineralization abnormalities in 56NEPHREX rats. These findings confirm our hypothesis and support the utility of ASARM peptide treatment in patients with CKD-MBD.

Improved cognitive impairments by silencing DMP1 via enhancing the proliferation of neural progenitor cell in Alzheimer-like mice

Alzheimer's disease (AD) is age-related progressive neurological dysfunction. Limited clinical benefits for current treatments indicate an urgent need for novel therapeutic strategies. Previous transcriptomic analysis showed that DMP1 expression level was increased in AD model animals whereas it can induce cell-cycle arrest in several cell lines. However, whether the cell-cycle arrest of neural progenitor cell induced by DMP1 affects cognitive function in Alzheimer-like mice still remains unknown. The objective of our study is to explore the issue. We found that DMP1 is correlated with cognitive function based on the clinical genomic analysis of ADNI database. The negative role of DMP1 on neural progenitor cell (NPC) proliferation was revealed by silencing and overexpressing DMP1 in vitro. Furthermore, silencing DMP1 could increase the number of NPCs and improve cognitive function in Alzheimer-like mice, through decreasing P53 and P21 levels, which suggested that DMP1-induced cell-cycle arrest could influence cognitive function.

Dentin Matrix Protein 1 on Titanium Surface Facilitates Osteogenic Differentiation of Stem Cells

Dentin matrix protein 1 (DMP1) contains a large number of acidic domains, multiple phosphorylation sites, a functional arginine-glycine-aspartate (RGD) motif, and a DNA binding domain, and has been shown to play essential regulatory function in dentin and bone mineralization. DMP1 could also orchestrate bone matrix formation, but the ability of DMP1 on Ti to human mesenchymal stem cell (hMSC) conversion to osteoblasts has not been studied. There is importance to test if the DMP1 coated Ti surface would promote cell migration and attachment to the metal surface and promote the differentiation of the attached stem cells to an osteogenic lineage. This study aimed to study the human mesenchymal stem cells (hMSCs) attachment and proliferation on DMP1 coated titanium (Ti) disks compared to non-coated disks, and to assess possible osteoblastic differentiation of attached hMSCs. Sixty-eight Ti disks were divided into two groups. Group 1 disks were coated with dentin matrix protein 1 and group 2 disks served as control. Assessment with light microscopy was used to verify hMSC attachment and proliferation. Cell viability was confirmed through fluorescence microscopy and mitochondrial dehydrogenase activity. Real-time polymerase chain reaction analysis was done to study the gene expression. The proliferation assay showed significantly greater cell proliferation with DMP1 coated disks compared to the control group (p-value < 0.001). Cell vitality analysis showed a greater density of live cells on DMP1 coated disks compared to the control group. Alkaline phosphatase staining revealed higher enzyme activity on DMP1 coated disks and showed itself to be significantly higher than the control group (p-value < 0.001). von Kossa staining revealed higher positive areas for mineralized deposits on DMP1 coated disks than the control group (p-value < 0.05). Gene expression analysis confirmed upregulation of runt-related transcription factor 2, osteoprotegerin, osteocalcin, osteopontin, and alkaline phosphatase on DMP1 coated disks (p-value < 0.001). The dentin matrix protein promoted the adhesion, proliferation, facilitation differentiation of hMSC, and mineralized matrix formation.

The Role of DMP1 in CKD-MBD

PURPOSE OF REVIEW

Chronic kidney disease-mineral and bone disorder (CKD-MBD) has become a global health crisis with very limited therapeutic options. Dentin matrix protein 1 (DMP1) is a matrix extracellular protein secreted by osteocytes that has generated recent interest for its possible involvement in CKD-MBD pathogenesis. This is a review of DMP1 established regulation and function, and early studies implicating DMP1 in CKD-MBD.

RECENT FINDINGS

Patients and mice with CKD show perturbations of DMP1 expression in bone, associated with impaired osteocyte maturation, mineralization, and increased fibroblast growth factor 23 (FGF23) production. In humans with CKD, low circulating DMP1 levels are independently associated with increased cardiovascular events. We recently showed that DMP1 supplementation lowers circulating FGF23 levels and improves bone mineralization and cardiac outcomes in mice with CKD. Mortality rates are extremely high among patients with CKD and have only marginally improved over decades. Bone disease and FGF23 excess contribute to mortality in CKD by increasing the risk of bone fractures and cardiovascular disease, respectively. Previous studies focused on DMP1 loss-of-function mutations have established its role in the regulation of FGF23 and bone mineralization. Recent studies show that DMP1 supplementation may fill a crucial therapeutic gap by improving bone and cardiac health in CKD.

The colocalizations of pulp neural stem cells markers with dentin matrix protein-1, dentin sialoprotein and dentin phosphoprotein in human denticle (pulp stone) lining cells

BACKGROUND

The primary dentin, secondary dentin, and reactive tertiary dentin are formed by terminal differentiated odontoblasts, whereas atubular reparative tertiary dentin is formed by odontoblast-like cells. Odontoblast-like cells differentiate from pulpal stem cells, which express the neural stem cell markers nestin, S100β, Sox10, and P0. The denticle (pulp stone) is an unique mineralized extracellular matrix that frequently occurs in association with the neurovascular structures in the dental pulp. However, to date, the cellular origin of denticles in human dental pulp is unclear. In addition, the non-collagenous extracellular dentin matrix proteins dentin matrix protein 1 (DMP1), dentin sialoprotein (DSP), and dentin phosphoprotein (DPP) have been well characterized in the dentin matrix, whereas their role in the formation and mineralization of the denticle matrix remains to be clarified.

METHODS

To characterize the formation of denticle, healthy human third molars (n = 59) were completely sectioned and evaluated by HE staining in different layers at 720 µm intervals. From these samples, molars with (n = 5) and without denticles (n = 8) were selected. Using consecutive cryo-sections from a layer containing denticles of different sizes, we examined DMP1, DSP, and DPP in denticle lining cells and tested their co-localizations with the glial stem cell markers nestin, S100β, Sox10, and P0 by quantitative and double staining methods.

RESULTS

DMP1, DSP and DPP were found in odontoblasts, whereas denticle lining cells were positive only for DMP1 and DSP but not for DPP. Nestin was detected in both odontoblasts and denticle lining cells. S100β, Sox10, and P0 were co-localized with DMP1 and DSP in different subpopulations of denticle lining cells.

CONCLUSIONS

The co-localization of S100β, Sox10, and P0 with DMP1 and DSP in denticle lining cells suggest that denticle lining cells are originated from glial and/or endoneurial mesenchymal stem cells which are involved in biomineralization of denticle matrix by secretion of DMP1 and DSP. Since denticles are atubular compared to primary, secondary, reactionary tertiary dentin and denticle formed by odontoblasts, our results suggest that DPP could be one of the proteins involved in the complex regulation of dentinal tubule formation.

Odontoblast markers and dentine reactions in carious primary molars with and without hypomineralised enamel defects

BACKGROUND

Wnt/β-Catenin signalling and DMP1 have key roles in tertiary dentinogenesis.

AIM

To compare the relationship between remaining dentine thickness (RDT), tertiary dentine thickness (TDT), β-catenin and dentine matrix protein 1 (DMP1) in carious second primary molar teeth with normal (SPM) and hypomineralised enamel (HSPM).

DESIGN

Extracted carious SPM and HSPM were fixed, sectioned (5 μm) and stained with haematoxylin and eosin or with indirect immunofluorescence for β-catenin and DMP1. Image analysis was performed to determine RDT, TDT, β-catenin and DMP1 intensity in the odontoblast layer and dentine-pulp complex.

RESULTS

Carious SPM (n = 11; mean RDT = 1536.1 μm) and HSPM (n = 12; mean RDT = 1179.9 μm) had mean TDT 248.6 μm and 518.1 μm, respectively (P = .02). There were no significant differences in intensity values in the odontoblast layer and dentine-pulp complex for β-catenin and DMP1 for both groups.

CONCLUSION

There was no observable variation in Wnt/β-catenin and DMP1 expression between HSPM and SPM despite a statistically significant twofold increased TDT in HSPM compared with SPM that had similar RDT. Thus, the observed increased TDT in HSPM is more likely due to an earlier onset of repair processes rather than an amplified response to caries.

Functions of Matricellular Proteins in Dental Tissues and Their Emerging Roles in Orofacial Tissue Development, Maintenance, and Disease

Matricellular proteins (MCPs) are defined as extracellular matrix (ECM) associated proteins that are important regulators and integrators of microenvironmental signals, contributing to the dynamic nature of ECM signalling. There is a growing understanding of the role of matricellular proteins in cellular processes governing tissue development as well as in disease pathogenesis. In this review, the expression and functions of different MP family members (periostin, CCNs, TSPs, SIBLINGs and others) are presented, specifically in relation to craniofacial development and the maintenance of orofacial tissues, including bone, gingiva, oral mucosa, palate and the dental pulp. As will be discussed, each MP family member has been shown to have non-redundant roles in development, tissue homeostasis, wound healing, pathology and tumorigenesis of orofacial and dental tissues.

Osteocytes but not osteoblasts directly build mineralized bone structures

Bone-forming osteoblasts have been a cornerstone of bone biology for more than a century. Most research toward bone biology and bone diseases center on osteoblasts. Overlooked are the 90% of bone cells, called osteocytes. This study aims to test the hypothesis that osteocytes but not osteoblasts directly build mineralized bone structures, and that defects in osteocytes lead to the onset of hypophosphatemia rickets. The hypothesis was tested by developing and modifying multiple imaging techniques, including both in vivo and in vitro models plus two types of hypophosphatemia rickets models (Dmp1-null and Hyp, Phex mutation mice), and Dmp1-Cre induced high level of β-catenin models. Our key findings were that osteocytes (not osteoblasts) build bone similar to the construction of a high-rise building, with a wire mesh frame (i.e., osteocyte dendrites) and cement (mineral matrices secreted from osteocytes), which is a lengthy and slow process whose mineralization direction is from the inside toward the outside. When osteoblasts fail to differentiate into osteocytes but remain highly active in Dmp-1-null or Hyp mice, aberrant and poor bone mineralization occurs, caused by a sharp increase in Wnt-β-catenin signaling. Further, the constitutive expression of β-catenin in osteocytes recaptures a similar osteomalacia phenotype as shown in Dmp1 null or Hyp mice. Thus, we conclude that osteocytes directly build bone, and osteoblasts with a short life span serve as a precursor to osteocytes, which challenges the existing dogma.

Experience with the targeted next-generation sequencing in the diagnosis of hereditary hypophosphatemic rickets

OBJECTIVES

Hereditary Hypophosphatemic Rickets (HHR) is a heterogeneous group of disorders characterized by hypophosphatemia. Although the X-linked dominant HHR is the most common form, the genetic etiology of HHR is variable. Recently, developed next-generation sequencing techniques may provide opportunities for making HHR diagnosis in a timely and efficient way.

METHODS

We investigated clinical and genetic features for 18 consecutive probands and their 17 affected family members with HHR. All patient's clinical and biochemical data were collected. We first analyzed a single gene with Next-generation sequencing if the patients have a strong clue for an individual gene. For the remaining cases, a Hypophosphatemic Rickets gene panel, including all known HHR genes by Next-generation sequencing, was employed.

RESULTS

We were able to diagnosis all of the consecutive 35 patients in our tertiary care center. We detected nine novel and 10 previously described variants in PHEX (9; 50%), SLC34A3 (3; 17%), ENPP1 (3; 17%), SLC34A1 (1; 5%), CLCN5 (1; 5%), and DMP1 (1; 5%).

CONCLUSIONS

To delineate the etiology of HHR cases in a cost and time-efficient manner, we propose single gene analysis by next-generation sequencing if findings of patients indicate a strong clue for an individual gene. If that analysis is negative or for all other cases, a Next-generation Sequence gene panel, which includes all known HHR genes, should be employed.

Krüppel-like factor 6 promotes odontoblastic differentiation through regulating the expression of dentine sialophosphoprotein and dentine matrix protein 1 genes

AIM

To investigate the potential role of Krüppel-like factor 6 (KLF6) in the odontoblastic differentiation of immortalized dental papilla mesenchymal cells (iMDP-3) cells.

METHODOLOGY

Alizarin Red S (ARS) and Alkaline phosphatase (ALP) staining was used to examine the mineralization effect of iMDP-3 cells after odontoblastic induction. Real-time PCR and Western blotting were employed to analyse dentine sialophosphoprotein (DSPP), dentine matrix protein 1 (DMP1), RUNX family transcription factor 2 (RUNX2), ALP and KLF6 expression during this process. Co-expression of the KLF6 with DMP1, DSPP and RUNX2 was detected by double immunofluorescence staining to explore their local relationship in the cell. To further investigate KLF6 functions, Klf6 gain- and loss-of-function assays followed by ARS and ALP stainings, real-time PCR and Western blotting were performed using Klf6-overexpression plasmids and Klf6 siRNA to investigate whether changes in Klf6 expression affect the odontoblastic differentiation of iMDP-3 cells. Dual-luciferase reporter assays were used to elucidate the mechanistic regulation of Dspp and Dmp1 expression by Klf6. Means were compared using the unpaired t-test and Kruskal-Wallis one-way anova with P < 0.05 and P < 0.01 defined as statistical significance levels.

RESULTS

The expression levels of Klf6 (P < 0.01), Dspp (P < 0.05), Dmp1 (P < 0.01), Runx2 (P < 0.01) and Alp (P < 0.01) were significantly elevated during odontoblastic differentiation of iMDP-3 cells. KLF6 was co-localized with DSPP, DMP1 and RUNX2 in the cytoplasm and nucleus of iMDP-3 cells. Overexpression of Klf6 promoted the odontoblastic differentiation of iMDP-3, whereas the inhibition of Klf6 prevented this procession. Dual-luciferase assays revealed that Klf6 upregulates Dspp and Dmp1 transcription in iMDP-3 cells during odontoblastic differentiation.

CONCLUSION

Klf6 promoted odontoblastic differentiation by targeting the transcription promoter of Dmp1 and Dspp. This study may offer novel insights into strategies for treating injuries to dental pulp tissue.

Clinical Characteristics and Bone Features of Autosomal Recessive Hypophosphatemic Rickets Type 1 in Three Chinese Families: Report of Five Chinese Cases and Review of the Literature

Autosomal recessive hypophosphatemic rickets type 1 (ARHR1) was reported to be caused by homozygous mutation of dentin matrix protein 1 (DMP1). To date, very few cases have been reported. Here, we summarized clinical, laboratory and imaging findings of ARHR1 patients in our hospital. Literature review was performed to analyze genotype-phenotype correlation. Five Chinese patients from three unrelated pedigrees presented with lower extremity deformity and short stature. Hypophosphatemia, elevated alkaline phosphatase, high intact fibroblast growth factor 23 and sclerostin were found. X-ray uncovered coexistence of osteomalacia and osteosclerosis. Although areal bone mineral density (aBMD) of axial bone measured by dual-energy X-ray absorptiometry was relatively high in all patients, volumetric BMD (vBMD) and microstructure of one adult patient's peripheral bone detected by HR-pQCT were damaged. Mutation analyses of DMP1 revealed three homozygous mutations including two novel mutations, c.54 + 1G > C and c.94C > A (p.E32X), and a reported mutation c.184-1G > A. Genotype-phenotype correlation analysis including 30 cases (25 from literature review and 5 from our study) revealed that patients harboring mutations affecting C-terminal fragment of DMP1 presented with shorter stature (Z score of height = - 3.4 ± 1.6 vs - 1.0 ± 1.6, p = 0.001) and lower serum phosphate level (0.70 ± 0.15 vs 0.84 ± 0.16, p = 0.03) than those harboring mutations only affecting N-terminal fragment. In summary, we reported five Chinese ARHR1 patients and identified two novel DMP1 mutations. High aBMD and local osteosclerosis in axial bone with low vBMD and damaged microstructure in peripheral bone were featured. Genotype-phenotype correlation analysis confirmed the important role of C-terminal fragment of DMP1.

Distinguished properties of cells isolated from the dentin-pulp interface

BACKGROUND

Dental odontoblasts produce dentin mineralized matrix, trigger immune responses and act as sensory cells. The understanding of the mechanisms of these functions has been particularly restricted due to the lack of odontoblasts being cultivable in vitro. Because of the lack of specific markers to identify cells of the odontoblastic lineage, properties of the cells isolated from the dentin-pulp interface were compared to dental pulp cells, periodontal ligament cells, osteoblasts, skin fibroblasts, epithelial cells (A549) and HeLa in the present study.

METHODS

After surgical procedures, the pulp tissue was removed from the tooth crown, and cells were scrapped off the dentin-pulp interface. Explants from teeth of three patients were routinely cultivated, and cells were harvested after several weeks. Cell morphology and ultrastructure was studied by light microscopy (LM), scanning (SEM) or transmission electron microscopy (TEM). Expression of dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), TRPV4, and S100 calcium binding protein A4 (S100A4) were analyzed at the protein level by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting using specific antibodies. The differential expression of S100A4 in the various cell lines was further investigated at the gene level by semiquantitative real-time PCR. Mineralization in the various cell types was observed after alizarin red staining after a 28 days incubation period. The immunophenotype of the cells was examined by flow cytometry using monoclonal anti-human antibodies CD90-FITC, CD73-PE, CD105-PE, CD29-PE, CD140a-FITC, CD144-PE, CD45-FITC or CD34-FITC. Differences between median values were statistically analyzed (Mann-Whitney U-test).

RESULTS

Cells from the dentin-pulp interface retain the polarity of odontoblast morphology in culture with an elongated, rounded cell body, and an extended cellular process. Ultrastructural analysis of the cells indicates high secretory activity including the extracellular deposition of fibrillar collagen. An extended rough endoplasmic reticulum is lined by a large number of ribosomes, and a vast number of secretory granules merges with the cell membrane. Protein expression of DSPP, DMP1, and TRPV4 as a transient receptor potential cation was detected in all cell lines. S100A4 was found differentially expressed in cultures of cells from tooth tissues. High expression of S100A4 was observed at the protein and gene level in two fractions of cells isolated from the dentin-pulp interface, but was absent or only weakly expressed in pulp cells. S100A4 expression in cells from the dentin-pulp interface and pulp cells is consistent with the intensity of the formation of mineralized nodules detected by alizarin red staining. Immunophenotyping revealed that a high percentage of CD73 (ecto-5-nucleotidase), an enzyme active on the surface of immune-competent cells, was expressed in cells of the dentin-pulp interface. While 72%-78% of positive cells were detected in dentin-pulp interface fractions, only 28-64% of the cells in pulp cell cultures were stained.

CONCLUSIONS

The present findings obtained with a variety of cells of different origin provide experimental evidence that cells isolated from the dentin-pulp interface express unique properties different from dental pulp cells in particular. The differential expression of S100A4 is a relevant marker candidate for differentiating between dental pulp cells and cells of the odontoblast lineage.

Hypophosphatemic rickets accelerate chondrogenesis and cell trans-differentiation from TMJ chondrocytes into bone cells via a sharp increase in β-catenin

Dentin matrix protein 1 (DMP1) is primarily expressed in osteocytes, although a low level of DMP1 is also detected in chondrocytes. Removing Dmp1 in mice or a mutation in humans leads to hypophosphatemic rickets (identical to X-linked hypophosphatemia). The deformed skeletons were currently thought to be a consequence of an inhibition of chondrogenesis (leading to an accumulation of hypertrophic chondrocytes and a failure in the replacement of cartilage by bone). To precisely study the mechanisms by which DMP1 and phosphorus control temporomandibular condyle formation, we first showed severe malformed condylar phenotypes in Dmp1-null mice (great expansions of deformed cartilage layers and subchondral bone), which worst as aging. Next, we excluded the direct role of DMP1 in condylar hypertrophic-chondrogenesis by conditionally deleting Dmp1 in hypertrophic chondrocytes using Col10a1-Cre and Dmp1 loxP mice (displaying no apparent phosphorous changes and condylar phenotype). To address the mechanism by which the onset of endochondral phenotypes takes place, we generated two sets of tracing lines in the Dmp1 KO background: AggrecanCreERT2-ROSA-tdTomato and Col 10a1-Cre-ROSA-tdTomato, respectively. Both tracing lines displayed an acceleration of chondrogenesis and cell trans-differentiation from chondrocytes into bone cells in the Dmp1 KO. Next, we showed that administrations of neutralizing fibroblast growth factor 23 (FGF23) antibodies in Dmp1-null mice restored hypophosphatemic condylar cartilage phenotypes. In further addressing the rescue mechanism, we generated compound mice containing Col10a1-Cre with ROSA-tdTomato and Dmp1 KO lines with and without a high Pi diet starting at day 10 for 39 days. We demonstrated that hypophosphatemia leads to an acceleration of chondrogenesis and trans-differentiation of chondrocytes to bone cells, which were largely restored under a high Pi diet. Finally, we identified the causative molecule (β-catenin). Together, this study demonstrates that the Dmp1-null caused hypophosphatemia, leading to acceleration (instead of inhibition) of chondrogenesis and bone trans-differentiation from chondrocytes but inhibition of bone cell maturation due to a sharp increase in β-catenin. These findings will aid in the future treatment of hypophosphatemic rickets with FGF23 neutralizing antibodies.

DMP1 Ablation in the Rabbit Results in Mineralization Defects and Abnormalities in Haversian Canal/Osteon Microarchitecture

DMP1 (dentin matrix protein 1) is an extracellular matrix protein highly expressed in bones. Studies of Dmp1 knockout (KO) mice led to the discovery of a rare autosomal recessive form of hypophosphatemic rickets (ARHR) caused by DMP1 mutations. However, there are limitations for using this mouse model to study ARHR, including a lack of Haversian canals and osteons (that occurs only in large mammalian bones), high levels of fibroblast growth factor 23 (FGF23), and PTH, in comparison with a moderate elevation of FGF23 and unchanged PTH in human ARHR patients. To better understand this rare disease, we deleted the DMP1 gene in rabbit using CRISPR/Cas9. This rabbit model recapitulated many features of human ARHR, such as the rachitic rosary (expansion of the anterior rib ends at the costochondral junctions), moderately increased FGF23, and normal PTH levels, as well as severe defects in bone mineralization. Unexpectedly, all DMP1 KO rabbits died by postnatal week 8. They developed a severe bone microarchitecture defect: a major increase in the central canal areas of osteons, concurrent with massive accumulation of osteoid throughout all bone matrix (a defect in mineralization), suggesting a new paradigm, where rickets is caused by a combination of a defect in bone microarchitecture and a failure in mineralization. Furthermore, a study of DMP1 KO bones found accelerated chondrogenesis, whereas ARHR has commonly been thought to be involved in reduced chondrogenesis. Our findings with newly developed DMP1 KO rabbits suggest a revised understanding of the mechanism underlying ARHR. © 2019 American Society for Bone and Mineral Research.

Resolvin D2 Induces Resolution of Periapical Inflammation and Promotes Healing of Periapical Lesions in Rat Periapical Periodontitis

Periapical periodontitis results from pulpal infection leading to pulpal necrosis and resorption of periapical bone. The current treatment is root canal therapy, which attempts to eliminate infection and necrotic tissue. But, in some cases periapical inflammation doesn't resolve even after treatment. Resolvins belongs to a large family of specialized pro-resolving lipid mediators that actively resolves inflammation signaling via specific receptors. Resolvin D2 (RvD2), a metabolite of docosahexaenoic acid (DHA), was tested as an intracanal medicament in rats in vivo. Mechanism was evaluated in rat primary dental pulp cells (DPCs) in vitro. The results demonstrate that RvD2 reduces inflammatory cell infiltrate, periapical lesion size, and fosters pulp like tissue regeneration and healing of periapical lesion. RvD2 enhanced expression of its receptor, GPR18, dentin matrix acidic phosphoprotein 1 (DMP1) and mineralization in vivo and in vitro. Moreover, RvD2 induces phosphorylation of Stat3 transcription factor in dental pulp cells. We conclude that intracanal treatment with RvD2 resolves inflammation and promoting calcification around root apex and healing of periapical bone lesions. The data suggest that RvD2 induces active resolution of inflammation with pulp-like tissue regeneration after root canal infection and thus maybe suitable for treating periapical lesions.

Dental pulp stem cells senescence and regenerative potential relationship

Uncomplicated treatments for pulpitis and periodontitis continues to be challenging and regenerative approaches could meet this contingency. Dental pulp stem cells (DPSCs) represent a good candidate for oral recovering therapies. Here, we investigated changes in morphology, proliferation, and in vitro differentiation toward mesenchymal and neuronal phenotypes of human DPSCs harvested from differently aged donors. Aging is a physiologic phenomenon occurring with time that hamper body's capability to maintain homeostasis also affecting the functional reserve. Cytofluorimetric, immunohistochemical, quantitative reverse-transcription polymerase chain reaction (qRT-PCR), and western blot analyses were performed to gain insight for successful regenerative strategies in elderly. We observed a decline in DPSCs proliferation and differentiation potential with age. Interestingly, these cells behaved differently under osteogenic or odontogenic stimuli, showing different age-related mineralization capabilities. Similarly, neurogenic differentiation decreased with age. In conclusion, our observations represent a valid tool for the development of tailored regenerative strategies in an aging society.

Notch expressed by osteocytes plays a critical role in mineralisation

Notch is actively involved in various life processes including osteogenesis; however, the role of Notch signalling in the terminal mineralisation of bone is largely unknown. In this study, it was noted that Hey1, a downstream target of Notch signalling was highly expressed in mature osteocytes compared to osteoblasts, indicating a potential role of Notch in osteocytes. Using a recently developed thermosensitive cell line (IDG-SW3), we demonstrated that dentin matrix acidic phosphoprotein 1 (DMP1) expression was inhibited and mineralisation process was significantly altered when Notch pathway was inactivated via administration of N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), an inhibitor of Notch. Dysregulation of Notch in osteocyte differentiation can result in spontaneous deposition of calcium phosphate on collagen fibrils, disturbed transportation of intracellular mineral vesicles, alteration of mineral crystal structure, decreased bonding force between minerals and organic matrix, and suppression of dendrite development coupled with decreased expression of E11. In conclusion, the evidence presented here suggests that Notch plays a critical role in osteocyte differentiation and biomineralisation process.

KEY MESSAGES

Notch plays a regulatory role in osteocyte phenotype. Notch modulates the mineralisation mediated by osteocytes. Notch activity influences the ultrastructural properties of bone mineralisation.

Overexpression of ID1 reverses the repression of human dental pulp stem cells differentiation induced by TWIST1 silencing

Multiple studies showed that the cessation of TWIST1 expression is the prerequisite for osteoblasts' maturation. However, recent reports revealed that the function of TWIST1 is different in the dental pulp stem cells (DPSCs), where a high level of TWIST1 expression promoted DPSCs' differentiation. The aim of the study was to investigate the impact of TWIST1 and ID1 on the differentiation process in the human DPSCs.

METHODS

TWIST1 and ID1 expression in the DSPCs was modulated by lentivirus transduction. Genes expression was assessed with qRT-PCR. The proteins level was evaluated by Western blot. The DPSCs differentiation was assessed with the proliferation, alkaline phosphatase (ALP) activity, and calcium concentration assays.

RESULTS

TWIST1 silencing suppressed the expression of ID1 and both the early and late markers of odontoblasts' differentiation detected at the transcript and protein level. The forced overexpression of ID1 increased the expression of the late markers of odontoblasts differentiation but diminished the expression of the early markers. DPCSs with the silenced TWIST1 and subsequent ID1 overexpression displayed an increase in the expression of the late markers of odontoblasts differentiation. Cells with silenced TWIST1 and overexpressing ID1 had increased activity of ALP, higher calcium concentration and decreased proliferation rate. The high level of ID1 expression might be a critical factor stimulating DPSCs differentiation and it might compensate the repressed differentiation of DPSCs caused by TWIST1 silencing.

CONCLUSION

The mutual correlation between the expression level of TWIST1 and ID1 might be a critical factor driving the process of the human odontoblasts' differentiation.

A novel auditory ossicles membrane and the development of conductive hearing loss in Dmp1-null mice

Genetic mouse models are widely used for understanding human diseases but we know much less about the anatomical structure of the auditory ossicles in the mouse than we do about human ossicles. Furthermore, current studies have mainly focused on disease conditions such as osteomalacia and rickets in patients with hypophosphatemia rickets, although the reason that these patients develop late-onset hearing loss is unknown. In this study, we first analyzed Dmp1 lac Z knock-in auditory ossicles (in which the blue reporter is used to trace DMP1 expression in osteocytes) using X-gal staining and discovered a novel bony membrane surrounding the mouse malleus. This finding was further confirmed by 3-D micro-CT, X-ray, and alizarin red stained images. We speculate that this unique structure amplifies and facilitates sound wave transmissions in two ways: increasing the contact surface between the eardrum and malleus and accelerating the sound transmission due to its mineral content. Next, we documented a progressive deterioration in the Dmp1-null auditory ossicle structures using multiple imaging techniques. The auditory brainstem response test demonstrated a conductive hearing loss in the adult Dmp1-null mice. This finding may help to explain in part why patients with DMP1 mutations develop late-onset hearing loss, and supports the critical role of DMP1 in maintaining the integrity of the auditory ossicles and its bony membrane.

Skeletal Muscle, but not Cardiovascular Function, Is Altered in a Mouse Model of Autosomal Recessive Hypophosphatemic Rickets

Autosomal recessive hypophosphatemic rickets (ARHR) is a heritable disorder characterized by hypophosphatemia, osteomalacia, and poor bone development. ARHR results from inactivating mutations in the DMP1 gene with the human phenotype being recapitulated in the Dmp1 null mouse model which displays elevated plasma fibroblast growth factor 23. While the bone phenotype has been well-characterized, it is not known what effects ARHR may also have on skeletal, cardiac, or vascular smooth muscle function, which is critical to understand in order to treat patients suffering from this condition. In this study, the extensor digitorum longus (EDL-fast-twitch muscle), soleus (SOL–slow-twitch muscle), heart, and aorta were removed from Dmp1 null mice and ex-vivo functional tests were simultaneously performed in collaboration by three different laboratories. Dmp1 null EDL and SOL muscles produced less force than wildtype muscles after normalization for physiological cross sectional area of the muscles. Both EDL and SOL muscles from Dmp1 null mice also produced less force after the addition of caffeine (which releases calcium from the sarcoplasmic reticulum) which may indicate problems in excitation contraction coupling in these mice. While the body weights of the Dmp1 null were smaller than wildtype, the heart weight to body weight ratio was higher. However, there were no differences in pathological hypertrophic gene expression compared to wildtype and maximal force of contraction was not different indicating that there may not be cardiac pathology under the tested conditions. We did observe a decrease in the rate of force development generated by cardiac muscle in the Dmp1 null which may be related to some of the deficits observed in skeletal muscle. There were no differences observed in aortic contractions induced by PGF_2α or 5-HT or in endothelium-mediated acetylcholine-induced relaxations or endothelium-independent sodium nitroprusside-induced relaxations. In summary, these results indicate that there are deficiencies in both fast twitch and slow twitch muscle fiber type contractions in this model of ARHR, while there was less of a phenotype observed in cardiac muscle, and no differences observed in aortic function. These results may help explain skeletal muscle weakness reported by some patients with osteomalacia and need to be further investigated.

High-acceleration whole body vibration stimulates cortical bone accrual and increases bone mineral content in growing mice

Whole body vibration (WBV) is a promising tool for counteracting bone loss. Most WBV studies on animals have been performed at acceleration <1g and frequency between 30 and 90Hz. Such WBV conditions trigger bone growth in osteopenia models, but not in healthy animals. In order to test the ability of WBV to promote osteogenesis in young animals, we exposed seven-week-old male mice to vibration at 90Hz and 2g peak acceleration for 15min/day, 5 days/week. We examined the effects on skeletal tissues with micro-computed tomography and histology. We also quantified bone vascularization and mechanosensitive osteocyte proteins, sclerostin and DMP1. Three weeks of WBV resulted in an increase of femur cortical thickness (+5%) and area (+6%), associated with a 25% decrease of sclerostin expression, and 35% increase of DMP1 expression in cortical osteocytes. Mass-structural parameters of trabecular bone were unaltered in femur or vertebra, while osteoclastic parameters and bone formation rate were increased at both sites. Three weeks of WBV resulted in higher blood vessel numbers (+23%) in the distal femoral metaphysis. After 9-week WBV, we have not observed the difference in structural cortical or trabecular parameters. However, the tissue mineral density of cortical bone was increased by 2.5%. Three or nine weeks of 2g/90Hz WBV treatment did not affect longitudinal growth rate or body weight increase under our experimental conditions, indicating that these are safe to use. These results validate a potential of 2g/90Hz WBV to stimulate trabecular bone cellular activity, accelerate cortical bone growth, and increase bone mineral density.

Deletion of Mbtps1 (Pcsk8, S1p, Ski-1) Gene in Osteocytes Stimulates Soleus Muscle Regeneration and Increased Size and Contractile Force with Age

Conditional deletion of Mbtps1 (cKO) protease in bone osteocytes leads to an age-related increase in mass (12%) and in contractile force (30%) in adult slow twitch soleus muscles (SOL) with no effect on fast twitch extensor digitorum longus muscles. Surprisingly, bone from 10-12-month-old cKO animals was indistinguishable from controls in size, density, and morphology except for a 25% increase in stiffness. cKO SOL exhibited increased expression of Pax7, Myog, Myod1, Notch, and Myh3 and 6-fold more centralized nuclei, characteristics of postnatal regenerating muscle, but only in type I myosin heavy chain-expressing cells. Increased expression of gene pathways mediating EGF receptor signaling, circadian exercise, striated muscle contraction, and lipid and carbohydrate oxidative metabolism were also observed in cKO SOL. This muscle phenotype was not observed in 3-month-old mice. Although Mbtps1 mRNA and protein expression was reduced in cKO bone osteocytes, no differences in Mbtps1 or cre recombinase expression were observed in cKO SOL, explaining this age-related phenotype. Understanding bone-muscle cross-talk may provide a fresh and novel approach to prevention and treatment of age-related muscle loss.

Sclerostin antibody (Scl-Ab) improves osteomalacia phenotype in dentin matrix protein 1(Dmp1) knockout mice with little impact on serum levels of phosphorus and FGF23

Unlike treatments for most rickets, the treatment using 1,25-(OH)2 vitamin D3 has little efficacy on patients with hypophosphatemic rickets, a set of rare genetic diseases. Thus, understanding the local cause for osteomalacia in hypophosphatemic rickets and developing an effective treatment to restore mineralization in this rare disease has been a longstanding goal in medicine. Here, we used Dmp1 knockout (KO) mice (whose mutations led to the same type of autosomal recessive hypophosphatemic rickets in humans) as the model in which the monoclonal antibody of sclerostin (Scl-Ab) was tested in two age groups for 8weeks: the prevention group (starting at age 4weeks) and the treatment group (starting at age 12weeks). Applications of Scl-Ab greatly improved the osteomalacia phenotype (>15%) and the biomechanical properties (3-point bending, ~60%) in the treated long-bone group. Our studies not only showed improvement of the osteomalacia in the alveolar bone, which has the highest bone metabolism rate, as well as the long bone phenotypes in treated mice. All these improvements attributed to the use of Scl-Ab are independent of the change in serum levels of phosphorus and FGF23, since Scl-Ab had little efficacy on those parameters. Finally, we propose a model to explain how Scl-Ab can improve the Dmp1 KO osteomalacia phenotype, in which the sclerostin level is already low.

Do ASARM peptides play a role in nephrogenic systemic fibrosis?

Nephrogenic systemic fibrosis (NSF) is a devastating condition associated with gadolinium (Gd3+)-based contrast agents (GBCAs) in patients with kidney disease. The release of toxic Gd3+ from GBCAs likely plays a major role in NSF pathophysiology. The cause and etiology of Gd3+ release from GBCAs is unknown. Increased Acidic Serine Aspartate Rich MEPE-associated peptides (ASARM peptides) induce bone mineralization abnormalities and contribute to renal phosphate-handling defects in inherited hypophosphatemic rickets and tumor-induced osteomalacia. The proteolytic cleavage of related bone matrix proteins with ASARM motifs results in release of ASARM peptide into bone and circulation. ASARM peptides are acidic, reactive, phosphorylated inhibitors of mineralization that bind Ca2+ and hydroxyapatite. Since the ionic radius of Gd3+ is close to that of Ca2+, we hypothesized that ASARM peptides increase the risk of NSF by inducing release of Gd3+ from GBCAs. Here, we show 1) ASARM peptides bind and induce release of Gd3+ from GBCAs in vitro and in vivo; 2) A bioengineered peptide (SPR4) stabilizes the Gd3+-GBCA complex by specifically binding to ASARM peptide in vitro and in vivo; and 3) SPR4 peptide infusion prevents GBCA-induced NSF-like pathology in a murine model with increased ASARM peptide (Hyp mouse). We conclude ASARM peptides may play a role in NSF and SPR4 peptide is a candidate adjuvant for preventing or reducing risk of disease.

Effects of chronic hypergravity: from adaptive to deleterious responses in growing mouse skeleton

One of the most important but least studied environmental factors playing a major role in bone physiology is gravity. While the knowledge of deleterious effects of microgravity on the skeleton is expanding, little is known about hypergravity and its osteogenic potential. Centrifugation was used to assess effects of 21-day continuous 2- or 3-g acceleration on femur and L2-vertebra of 7-wk-old male C57BL/6 mice. Under 3 g, body mass growth slowed down, and deleterious skeletal effects were found (P < 0.05 compared with control): cortical thinning, osteoclasts surface increase (+41% in femur, +20% in vertebra), and bone formation rate decrease (-34% in femur, -38% in vertebra). A 2-g centrifugation did not reduce body mass and improved trabecular volume (+18% in femur, +13% in vertebra) and microarchitecture (+32% connectivity density in femur, +9% trabecular thickness in vertebra, P < 0.05 compared with control). Centrifugation at 2 g also decreased osteoclast surfaces (-36% in femur, -16% in vertebra) and increased the extent of mineralized surfaces (+31% in femur, +48% in vertebra, P < 0.05 compare to control). Quantitative immunohistochemistry revealed an increase of dentin matrix acidic phosphoprotein 1 (DMP1) and decrease of sclerostin (+60% and -35% respectively, P < 0.001 compared with control) in the femur cortex of 2-g mice. In the distal femur metaphysis, the number and volume of blood vessels increased by 22 and 44%, respectively (P < 0.05 compared with control). In conclusion, the effects of continuous hypergravity were bone compartment-specific and depended on the gravity level, with a threshold between beneficial 2-g and deleterious 3-g effects.

SPR4-peptide alters bone metabolism of normal and HYP mice

CONTEXT

ASARM-peptides are substrates and ligands for PHEX, the gene responsible for X-linked hypophosphatemic rickets (HYP). PHEX binds to the DMP1-ASARM-motif to form a trimeric-complex with α5β3-integrin on the osteocyte surface and this suppresses FGF23 expression. ASARM-peptide disruption of this complex increases FGF23 expression. We used a 4.2kDa peptide (SPR4) that binds to ASARM-peptide and ASARM-motif to study DMP1-PHEX interactions and to assess SPR4 for treating inherited hypophosphatemic rickets.

DESIGN

Subcutaneously transplanted osmotic pumps were used to infuse SPR4-peptide or vehicle into wild-type mice (WT) and HYP-mice for 4 weeks.

RESULTS

Asymmetrically distributed mineralization defects occurred with WT-SPR4 femurs. Specifically, SPR4 induced negative effects on trabecular bone and increased bone volume and mineralization in cortical-bone. Markedly increased sclerostin and reduced active β-catenin occurred with HYP mice. SPR4-infusion suppressed sclerostin and increased active β-catenin in WT and HYP mice and improved HYP-mice trabecular mineralization defects but not cortical mineralization defects.

CONCLUSIONS

SPR4-peptide has bimodal activity and acts by: (1) preventing DMP1 binding to PHEX and (2) sequestering an inhibitor of DMP1-PHEX binding, ASARM-peptide. In PHEX defective HYP-mice the second pathway predominates. Although SPR4-peptide improved trabecular calcification defects, decreased sclerostin and increased active β-catenin it did not correct HYP-mice cortical mineralization defects on a normal phosphate diet. Thus, for inherited hypophosphatemic rickets patients on a normal phosphate diet, SPR4-peptide is not a useful therapeutic.

Osteocytes serve as a progenitor cell of osteosarcoma

Osteosarcoma (OSA) is the most common primary bone tumor in humans. However, the cell of origin of OSA is not clearly defined although there is evidence that osteoblasts may serve as OSA progenitors. The role of osteocytes, terminally differentiated osteoblasts, as OSA progenitors has yet to be described. Analysis of patient cDNA from publicly available microarray data revealed that patients with OSA have increased expression of dentin matrix phosphoprotein 1 (DMP1), a marker of osteocytes. Analysis of multiple murine, human, and canine OSA cell lines revealed DMP1 expression. To test the tumorigenic potential of osteocytes, MLO-Y4, a SV-40 immortalized murine osteocyte cell line, was injected into subcutaneous and orthotopic (intratibial) sites of mice. Tumor growth occurred in both locations. Orthotopic MLO-Y4 tumors produced mixed osteoblastic/osteolytic radiographic lesions; a hallmark of OSA. Together, these data demonstrate for the first time that osteocytes can serve as OSA progenitors.

Dentin Matrix Proteins in Bone Tissue Engineering

Dentin and bone are mineralized tissue matrices comprised of collagen fibrils and reinforced with oriented crystalline hydroxyapatite. Although both tissues perform different functionalities, they are assembled and orchestrated by mesenchymal cells that synthesize both collagenous and noncollagenous proteins albeit in different proportions. The dentin matrix proteins (DMPs) have been studied in great detail in recent years due to its inherent calcium binding properties in the extracellular matrix resulting in tissue calcification. Recent studies have shown that these proteins can serve both as intracellular signaling proteins leading to induction of stem cell differentiation and also function as nucleating proteins in the extracellular matrix. These properties make the DMPs attractive candidates for bone and dentin tissue regeneration. This chapter will provide an overview of the DMPs, their functionality and their proven and possible applications with respect to bone tissue engineering.

Exome sequencing reveals a mutation in DMP1 in a family with familial sclerosing bone dysplasia

INTRODUCTION

Hypophosphatemic rickets (HR) comprises a rare group of inherited diseases. Very recently, mutations in the dentin matrix protein 1 (DMP1) gene were identified in patients with an extremely rare autosomal recessive form of HR (ARHR). To date, very few cases of these mutations were reported.

MATERIALS AND METHODS

A Lebanese consanguineous family with 2 affected sisters was studied. Patients aged 45 and 47years old presented with short stature, severe genu varum, cranial hyperostosis and a very high bone density that led to a diagnosis of a familial sclerosing bone dysplasia. Molecular analysis of known genes involved in osteopetrosis showed normal results. A combination of genotyping and exome sequencing was performed in order to elucidate the genetic basis of this pathology.

RESULTS

Biochemical analysis was consistent with normal serum calcium and 1-25(OH)2D levels, low to normal serum phosphorus and elevated PTH values. Serum c-terminal FGF-23 was elevated in one of the two patients. A homozygous mutation disrupting the initiation codon of the DMP1 gene (OMIM 600980), NM_001079911.2: c.1A>G, p.Met1Val, was identified by exome sequencing and confirmed by Sanger sequencing.

CONCLUSION

We report here a family of ARHR secondary to a DMP1 mutation located in the first coding exon of the gene. Our cases show that some ARHR cases may develop with age an unaccountable increase in bone density and bone overgrowth.

Functional analysis of mutant FAM20C in Raine syndrome with FGF23-related hypophosphatemia

Raine syndrome is an autosomal recessive disorder characterized by generalized osteosclerosis with periosteal bone formation and a distinctive facial phenotype. Either homozygous or compound heterozygous mutations in family with sequence similarity 20, member C (FAM20C) have been reported to cause this syndrome. Recently, it was reported that fibroblast growth factor 23 (FGF23)-related hypophosphatemia was found in patients with non-lethal Raine syndrome, and Fam20c conditional knockout mice presented Fgf23-related hypophosphatemic rickets. To clarify the mechanism of how FAM20C regulates FGF23, we performed functional analysis of mutant FAM20C proteins reported in Raine syndrome. We analyzed 6 mutant FAM20C proteins (T268M, P328S, R408W, D451N, D478A, and R549W) for their distributions, kinase activities, and effects on dentin matrix protein (DMP1) promoter activity. We also analyzed the effect of Fam20c knockdown on Dmp1 and Fgf23 mRNA levels in UMR-106 cells. As a result, all the mutant FAM20C proteins showed decreased kinase activities compared to wild-type (WT) FAM20C, and most of them also showed impaired secretion. Overexpression of WT FAM20C increased DMP1 promoter activity in Saos-2 cells while mutant FAM20C did not. Fam20c knockdown decreased Dmp1 mRNA and increased Fgf23 mRNA in UMR-106 cells. In conclusion, our results suggest that FAM20C suppresses FGF23 production by enhancing DMP1 expression, and inactivating mutations in FAM20C cause FGF23-related hypophosphatemia by decreasing transcription of DMP1.

Overexpression of Dmp1 fails to rescue the bone and dentin defects in Fam20C knockout mice

FAM20C is a kinase phosphorylating the small-integrin-binding ligand, N-linked glycoproteins (SIBLINGs), a group of extracellular matrix proteins that are essential for bone and dentin formation. Previously, we showed that Sox2-Cre;Fam20Cfl/fl mice had bone and dentin defects, along with hypophosphatemia and significant downregulation of dentin matrix protein 1 (DMP1). While the assumed phosphorylation failure of the SIBLINGs is likely associated with the defects in the Fam20C-deficient mice, it remains unclear if the downregulation of Dmp1 contributes to these phenotypes. In this study, we crossed 3.6 kb Col1-Dmp1 transgenic mice with 3.6 kb Col1-Cre;Fam20Cfl/fl mice to overexpress Dmp1 in the mineralized tissues of Fam20C conditional knockout (cKO) mice. X-ray, micro-computed tomography, serum biochemistry and histology analyses showed that expressing the Dmp1 transgene failed to rescue the bone and dentin defects, as well as the serum levels of FGF23 and phosphate in the Fam20C-cKO mice. These results indicated that the downregulation of Dmp1 may not directly associate with, or significantly contribute to the bone and dentin defects in the Fam20C-cKO mice.

Transcriptional regulation of dentin matrix protein 1 (DMP1) in odontoblasts and osteoblasts

Dentin matrix protein 1 (DMP1) is a noncollagenous protein important for the mineralization of bones and teeth. Examination of the transcription factor binding sites within the 6.24 kb upstream sequence of rat DMP1 promoter by Matinspector software revealed that TCF11 had the highest number (six) of binding sites with 100% matrix similarity. Four of these sites are conserved in the mouse DMP1 promoter. TCF11 is a member of the Cap-n-Collar (cnc) family of basic leucine zipper transcription factors. Results from this study showed that TCF11 can bind specifically to the DMP1 promoter and activate its transcription in odontoblasts and osteoblasts. This could be attributed to both direct and indirect effects of TCF11. Electrophoretic mobility shift (EMSA) assay showed differential interaction between TCF11 and its binding sites on the DMP1 promoter. 21 bp oligos spanning the TCF11 matrix were used as probes in EMSA, and the results showed that the binding was specific to the sequence of the TCF11 matrix as well as the flanking sequences and this is typical of a heterodimer binding site. Results also showed changes in the binding pattern when cells were differentiated in osteogenic medium for 2 d. Thus, TCF11 may play an important role in the transcriptional regulation of DMP1 gene.

A novel way to statistically analyze morphologic changes in Dmp1-null osteocytes

Recent studies have revealed multiple roles of osteocytes in bone metabolism. However, detailed analyses of the embedded osteocytes in bone structure are still limited because of the high mineral content around these cells. In this study, we developed an innovative technique, the "FITC-Imaris technique", which combines FITC ([2, 5]-Fluorescein isothiocyanate), confocal microscopy and Imaris software. With this method, we could not only visualize the 3-D morphology of embedded osteocytes, but more importantly, we were able to statistically quantitate the osteocyte structure in the cell surface area, total cell volume, and dendrite numbers. Furthermore, we made a side-by-side comparison of the new method with the acid-etched SEM imaging technique, a common imaging method for studies of osteocyte morphology with a much smaller cell depth (< 3 μm). Finally, we used the FITC-Imaris technique to show both the morphological and statistical differences in the osteocyte structure between the Dmp1-null mice (the osteomalacia model) and their age-matched control littermates. We expect that this newly developed technique will become a powerful tool to disclose more roles that osteocytes play in bone health and diseases.

Site specificity of DSP-PP cleavage by BMP1

Bone morphogenic protein 1 (BMP1), a metalloproteinase, is known to cleave a wide variety of extracellular matrix proteins, suggesting that a consensus substrate cleavage amino acid sequence might exist. However, while such a consensus sequence has been proposed based on P4 to P4' (i.e. the four amino acids flanking either side of the BMP1 cleavage site; P4P3P2P1|P1'P2'P3'P4') sequence homologies between two BMP1 substrates, dentin matrix protein 1 and dentin sialoprotein phosphophoryn (DSP-PP) (i.e. xMQx|DDP), no direct testing has so far been attempted. Using an Sf9 cell expression system, we have been able to produce large amounts of uncleaved DSP-PP. Point mutations introduced into this recombinant DSP-PP were then tested for their effects on DSP-PP cleavage by either Sf9 endogenous tolloid-related protein 1 (TLR-1) or by its human homolog, BMP1. Here, we have measured DSP-PP cleavage efficiencies after modifications based on P4-P4' sequence comparisons with dentin matrix protein 1, as well as for prolysyl oxidase and chordin, two other BMP1 substrates. Our results demonstrate that any mutations within or outside of the DSP-PP P4 to P4' cleavage site can block, impair or accelerate DSP-PP cleavage, and suggest that its BMP1 cleavage site is highly conserved in order to regulate its cleavage efficiency, possibly with additional assistance from its conserved exosites. Thus, BMP1 cleavage cannot be based on a consensus substrate cleavage site.

Transcriptional factor ATF6 is involved in odontoblastic differentiation

ATF6 is an endoplasmic reticulum (ER) membrane-bound transcription factor that regulates various cellular functions. The purpose of this study was to investigate the role of ATF6 in odontoblast differentiation. Rat tooth germs were isolated, changes in gene expression were evaluated over time, and localization of ATF6 was determined by immunohistochemistry. Human dental pulp cells (HDPCs) were cultured with 50 µg/mL ascorbic acid and 5 mmol/L β-glycerophosphate or 100 ng/mL bone morphogenetic protein 2 to induce differentiation. Translocation of ATF6 was observed by immunofluorescence and confocal microscopy. Overexpression of ATF6 was performed with an adenoviral vector. Matrix mineralization was evaluated by alizarin red staining. Immunoreactivity to anti-ATF6 was observed in the odontoblastic layer of the molar tooth germ, and expressions of ATF6, dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1) increased gradually during tooth germ development. When HDPCs were cultured in differentiation media, ATF6, DSPP, and DMP1 expression increased with the expression of unfolded protein response (UPR) markers, BiP and CHOP. Immunofluorescence results showed that ATF6 protein moved from cytoplasm to nucleus when cells were exposed to differentiation media. Notably, overexpression of ATF6 increased DSPP and DMP1 expression, alkaline phosphatase (ALP) activity, and matrix mineralization in HDPC cultures. Inhibition of ATF6 decreased ALP activity and mineralization. These results suggest that ER membrane-bound transcriptional factor ATF6 may be involved in odontoblastic differentiation.

Expression of odontogenic genes in human bone marrow mesenchymal stem cells

OBJECTIVE

Tooth loss is a common problem and since current tooth replacement methods cannot counter balance with biological tooth structures, regenerating natural tooth structures has become an ideal goal. A challenging problem in tooth regeneration is to find a proper clinically feasible cell to seed.This study was designed to investigate the odontogenic potential of human bone marrow mesenchymal stem cells (HBMSCs) for seeding in tooth regeneration.

MATERIALS AND METHODS

In this experimental study, three pregnant Sprague Dawley (SD) rats were used at the eleventh embryonic day and rat fetuses were removed surgically using semilunar flap under general anesthesia. The primary mandible was cut using a stereomicroscope. The epithelial and mesenchymal components were separated and the dissected oral epithelium was cultured for 3 days. We used flow cytometry analysis to confirm presence of mesenchymal stem cells and not hematopoietic cells and to demonstrate the presence of oral epithelium. Bone marrow mesenchymal stem cells (BMSCs) and cultured oral epithelium were then co-cultured for 14 days. BMSCs cultured alone were used as controls. Expression of two odontogenic genes Pax9 and DMP1 was assessed using quantitative reverse transcription- polymerase chain reaction (RT-PCR).

RESULTS

Expression of two odontogenic genes, Pax9 and DMP1, were detected in BMSCs co-cultured with oral epithelium but not in the control group.

CONCLUSION

Expression of Pax9 and DMP1 by human BMSCs in the proximity of odontogenic epithelium indicates odontogenic potential of these cells.

Circling behavior developed in Dmp1 null mice is due to bone defects in the vestibular apparatus

With age, there is a progressive loss of body balance function. Yet, the potential influence of osteoporosis on body balance is largely unknown. Dentin matrix protein 1 (DMP1) is highly expressed in bone and required for phosphate homeostasis and mineralization. Dmp1 null mice display striking defects in bone structure. In this study we reported circling behavior and hyper reaction to touching in Dmp1 null mice. Our histology, tartrate resistant acid phosphatase (TRAP) staining and µCT data showed dramatic changes, such as an expansion of poorly mineralized matrices, in the Dmp1 null porous bony structure in the vestibular apparatus. The targeted re-expression of DMP1 in the Dmp1 null bone fully rescued not only the bone phenotype, but also circling behavior and hyper reaction. Furthermore, X-gal stain and DMP1 immunohistochemistry assay showed that DMP1 was not expressed in neuron cells or balance related cells in the inner ear, suggesting that a defect in the bony labyrinth of the internal ear is indirectly responsible for the circling behavior and/or hyper reaction to touching. Finally, discovery of DMP1 lacZ signal in pericyte-like cells may suggest a new function of DMP1 in angiogenesis.