Complete pulpodentin complex regeneration by modulating the stiffness of biomimetic matrix

Dental caries is one of the most prevalent chronic diseases in all populations. The regeneration of dentin-pulp tissues (pulpodentin) using a scaffold-based tissue engineering strategy is a promising approach to replacing damaged dental structures and restoring their biological functions. However, the current scaffolding design for pulpodentin regeneration does not take into account the distinct difference between pulp and dentin, therefore, is incapable of regenerating a complete tooth-like pulpodentin complex. In this study, we determined that scaffolding stiffness is a crucial biophysical cue to modulate dental pulp stem cell (DPSC) differentiation. The DPSCs on a high-stiffness three-dimensional (3D) nanofibrous gelatin (NF-gelatin) scaffold had more organized cytoskeletons and a larger spreading area than on a low-stiffness NF-gelatin scaffold. In the same differentiation medium, a high-stiffness NF-gelatin facilitated DPSC differentiation to form a mineralized tissue, while a low-stiffness NF-gelatin promoted a soft pulp-like tissue formation from the DPSCs. A facile method was then developed to integrate the low- and high-stiffness gelatin matrices into a single scaffold (S-scaffold) for pulpodentin complex regeneration. A 4-week in vitro experiment showed that biomineralization took place only in the high-stiffness peripheral area and formed a ring-like structure surrounding the non-mineralized central area of the DPSC/S-scaffold construct. A complete pulpodentin complex similar to natural pulpodentin was successfully regenerated after subcutaneous implantation of the DPSC/S-scaffold in nude mice for 4weeks. Histological staining showed a significant amount of extracellular matrix (ECM) formation in the newly formed pulpodentin complex, and a number of blood vessels were observed in the pulp tissue. Taken together, this work shows that modulating the stiffness of the NF-gelatin scaffold is a successful approach to regenerating a complete tooth-like pulpodentin complex.

Removal of SOST or blocking its product sclerostin rescues defects in the periodontitis mouse model

Understanding periodontal ligament (PDL) biology and developing an effective treatment for bone and PDL damage due to periodontitis have been long-standing aims in dental medicine. Here, we first demonstrated by cell lineage tracing and mineral double-labeling approaches that murine PDL progenitor cells display a 2- and 3-fold higher mineral deposition rate than the periosteum and endosteum at the age of 4 weeks, respectively. We next proved that the pathologic changes in osteocytes (Ocys; changes from a spindle shape to round shape with a >50% reduction in the dendrite number/length, and an increase in SOST) are the key pathologic factors responsible for bone and PDL damage in periostin-null mice (a periodontitis animal model) using a newly developed 3-dimensional FITC-Imaris technique. Importantly, we proved that deleting the Sost gene (a potent inhibitor of WNT signaling) or blocking sclerostin function by using the mAb in this periodontitis model significantly restores bone and PDL defects (n = 4-5; P < 0.05). Together, identification of the key contribution of the PDL in normal alveolar bone formation, the pathologic changes of the Ocys in periodontitis bone loss, and the novel link between sclerostin and Wnt signaling in the PDL will aid future drug development in the treatment of patients with periodontitis.

Osterix controls cementoblast differentiation through downregulation of Wnt-signaling via enhancing DKK1 expression

Osterix (Osx), a transcriptional factor essential for osteogenesis, is also critical for in vivo cellular cementum formation. However, the molecular mechanism by which Osx regulates cementoblasts is largely unknown. In this study, we initially demonstrated that overexpression of Osx in a cementoblast cell line upregulated the expression of markers vital to cementogenesis such as osteopontin (OPN), osteocalcin (OCN), and bone sialoprotein (BSP) at both mRNA and protein levels, and enhanced alkaline phosphatase (ALP) activity. Unexpectedly, we demonstrated a sharp increase in the expression of DKK1 (a potent canonical Wnt antagonist), and a great reduction in protein levels of β-catenin and its nuclear translocation by overexpression of Osx. Further, transient transfection of Osx reduced protein levels of TCF1 (a target transcription factor of β-catenin), which were partially reversed by an addition of DKK1. We also demonstrated that activation of canonical Wnt signaling by LiCl or Wnt3a significantly enhanced levels of TCF1 and suppressed the expression of OPN, OCN, and BSP, as well as ALP activity and formation of extracellular mineralized nodules. Importantly, we confirmed that there were a sharp reduction in DKK1 and a concurrent increase in β-catenin in Osx cKO mice (crossing between the Osx loxP and 2.3 Col 1-Cre lines), in agreement with the in vitro data. Thus, we conclude that the key role of Osx in control of cementoblast proliferation and differentiation is to maintain a low level of Wnt-β-catenin via direct up-regulation of DKK1.

The specific role of FAM20C in dentinogenesis

FAM20C is an evolutionarily reserved molecule highly expressed in mineralized tissues. Previously we demonstrated that Sox2-Cre;Fam20C(fl/fl) mice, in which Fam20C was ubiquitously inactivated, had dentin and enamel defects as well as hypophosphatemic rickets. We also showed that K14-Cre;Fam20C(fl/fl) mice, in which Fam20C was specifically inactivated in the epithelium, had enamel defects but lacked hypophosphatemia and defects in the bone and dentin. These results indicated that the enamel defects in the Sox2-Cre;Fam20C(fl/fl) mice were independent of dentin defects and hypophosphatemia. To determine if the dentin defects in the Sox2-Cre;Fam20C(fl/fl) mice were associated with the enamel defects and hypophosphatemia, we crossed Fam20C(fl/fl) mice with Wnt1-Cre and Osr2-Cre transgenic mice to inactivate Fam20C in the craniofacial mesenchymal cells that form dentin and alveolar bone. The resulting Wnt1-Cre;Fam20C(fl/fl) and Osr2-Cre;Fam20C(fl/fl) mice showed remarkable dentin and alveolar bone defects, while their enamel did not show apparent defects. The serum FGF23 levels in these mice were higher than normal but lower than those in the Sox2-Cre;Fam20C(fl/fl) mice; they developed a mild type of hypophosphatemia that did not cause major defects in long bones. These results indicate that the dentin defects in the Sox2-Cre;Fam20C(fl/fl) mice were independent of the enamel defects.

Osterix couples chondrogenesis and osteogenesis in post-natal condylar growth

Osterix (Osx) is a transcription factor essential for osteoblast differentiation and bone mineralization. Although there are indications that Osx also plays a regulatory role in cartilage, this has not been well-studied. The goal of this study was to define the function of Osx in the post-natal growth of the secondary cartilage at the mandibular condyle. Conditional Osx knockout (cKO) mice that were missing Osx only in cartilage were generated by crossing Osx-loxP mice to Aggrecan-Cre mice. Cre activity was induced by tamoxifen injection twice a week from day 12 to 1 mo of age, and specimens were collected at 1 and 5 mo of age. At 1 mo of age, the condylar hypertrophic chondrocyte zone in the cKO-mice was > three-fold thicker than that in the age-matched control, with little sign of endochondral bone formation. Immunohistochemistry and analysis of histological data revealed a defect in the coupling of chondrogenesis and osteogenesis in the cKO mice. In five-month-old mice examined to address whether late-stage removal of the Cre-deletion event would alleviate the phenotype, the hypertrophic chondrocyte zone in the cKO condyles was considerably larger than in wild-type mice. There were large discrete areas of calcified cartilage in the hypertrophic zone, few signs of endochondral bone formation, and large regions of disorganized intramembranous bone. Analysis of these data further strengthens the notion that Osterix is essential for the coupling of terminal cartilage differentiation and endochondral ossification in mandibular condylar cartilage.

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.

Constitutive nuclear expression of dentin matrix protein 1 fails to rescue the Dmp1-null phenotype

Dentin matrix protein 1 (DMP1) plays multiple roles in bone, tooth, phosphate homeostasis, kidney, salivary gland, reproductive cycles, and the development of cancer. In vitro studies have indicated two different biological mechanisms: 1) as a matrix protein, DMP1 interacts with αvβ3 integrin and activates MAP kinase signaling; and 2) DMP1 serves as a transcription co-factor. In vivo studies have demonstrated its key role in osteocytes. This study attempted to determine whether DMP1 functions as a transcription co-factor and regulates osteoblast functions. For gene expression comparisons using adenovirus constructs, we targeted the expression of DMP1 either to the nucleus only by replacing the endogenous signal peptide with a nuclear localization signal (NLS) sequence (referred to as (NLS)DMP1) or to the extracellular matrix as the WT type (referred to as (SP)DMP1) in MC3T3 osteoblasts. High levels of DMP1 in either form greatly increased osteogenic gene expression in an identical manner. However, the targeted (NLS)DMP1 transgene driven by a 3.6-kb rat Col 1α1 promoter in the nucleus of osteoblasts and osteocytes failed to rescue the phenotyope of Dmp1-null mice, whereas the (SP)DMP1 transgene rescued the rickets defect. These studies support the notion that DMP1 functions as an extracellular matrix protein, rather than as a transcription co-factor in vivo. We also show that DMP1 continues its expression in osteoblasts during postnatal development and that the deletion of Dmp1 leads to an increase in osteoblast proliferation. However, poor mineralization in the metaphysis indicates a critical role for DMP1 in both osteoblasts and osteocytes.

Critical role of Bmpr1a in mandibular condyle growth

The importance of Bone Morphogenetic Proteins (BMPs) in the regulation of cell fate, differentiation and proliferation in the growth plate is well-known. However, in secondary cartilages (such as that in the temporomandibular joint) that grow by proliferation of prechondrocytes and differ in their pattern of growth, the role of BMPs is largely unexplored. To examine this question, we ablated Bmpr1a in the condylar cartilage of neonatal mice and assessed the consequences for mandibular condyle growth and organization at intervals over the ensuing 4 weeks. Bmpr1a deficiency caused significant chondrodysplasia and almost eliminated the chondrocytic phenotype in the TMJ. Expression of Sox9, collagen II, proteoglycan were all greatly reduced, and cell proliferation as detected by BrdU was almost non-existent in the knockout mice. Primary bone spongiosa formation was also disturbed and was accompanied by reduced Osterix expression. These findings strongly suggest that Bmpr1a is critical for the development and growth of the mandibular condyle via its effect on proliferation of prechondroblasts and chondrocyte differentiation.

Twist1- and Twist2-haploinsufficiency results in reduced bone formation

Background

Twist1 and Twist2 are highly homologous bHLH transcription factors that exhibit extensive highly overlapping expression profiles during development. While both proteins have been shown to inhibit osteogenesis, only Twist1 haploinsufficiency is associated with the premature synostosis of cranial sutures in mice and humans. On the other hand, biallelic Twist2 deficiency causes only a focal facial dermal dysplasia syndrome or additional cachexia and perinatal lethality in certain mouse strains. It is unclear how these proteins cooperate to synergistically regulate bone formation.

Methods

Twist1 floxed mice (Twist1^f/f) were bred with Twist2-Cre knock-in mice (Twist2^Cre/+) to generate Twist1 and Twist2 haploinsufficient mice (Twist1^f/+; Twist2^Cre/+). X-radiography, micro-CT scans, alcian blue/alizarin red staining, trap staining, BrdU labeling, immunohistochemistry, in situ hybridizations, real-time PCR and dual luciferase assay were employed to investigate the overall skeletal defects and the bone-associated molecular and cellular changes of Twist1^f/+;Twist2^Cre/+ mice.

Results

Twist1 and Twist2 haploinsufficient mice did not present with premature ossification and craniosynostosis; instead they displayed reduced bone formation, impaired proliferation and differentiation of osteoprogenitors. These mice exhibited decreased expressions of Fgf2 and Fgfr1–4 in bone, resulting in a down-regulation of FGF signaling. Furthermore, in vitro studies indicated that both Twist1 and Twist2 stimulated 4.9 kb Fgfr2 promoter activity in the presence of E12, a Twist binding partner.

Conclusion

These data demonstrated that Twist1- and Twist2-haploinsufficiency caused reduced bone formation due to compromised FGF signaling.

Magnesium-containing nanostructured hybrid scaffolds for enhanced dentin regeneration

Dental caries is one of the most prevalent chronic diseases in the United States, affecting 92% of adults aged 20-64 years. Scaffold-based tissue engineering represents a promising strategy to replace damaged dental structures and restore their biological functions. Current single-component scaffolding materials used for dental tissue regeneration, however, cannot provide the proper microenvironment for dental stem/progenitor cell adhesion, proliferation, and differentiation; new biomimetic hybrid scaffolds are needed to promote better dental tissue formation. In this work, we developed a biomimetic approach to prepare three-dimensional (3D) nanofibrous gelatin/magnesium phosphate (NF-gelatin/MgP) hybrid scaffolds. These scaffolds not only mimic the nanostructured architecture and the chemical composition of natural dentin matrices but also constantly present favorable chemical signals (Mg ions) to dental pulp stem cells (DPSCs), thus providing a desirable microenvironment to facilitate DPSC proliferation, differentiation, and biomineralization. Synthesized hybrid NF-gelatin/MgP possesses natural extracellular matrix (ECM)-like architecture, high porosity, high pore interconnectivity, well-defined pore size, and controlled Mg ion release from the scaffold. Adding MgP into NF-gelatin also increased the mechanical strength of the hybrid scaffold. The sustained release of Mg ions from the NF-gelatin/MgP (MgP=10% wt/wt) scaffold significantly enhanced the proliferation, differentiation, and biomineralization of human DPSCs in vitro. The alkaline phosphatase (ALP) activity and the gene expressions for odontogenic differentiation (collagen I [Col I], ALP, osteocalcin [OCN], dentin sialophosphoprotein [DSPP], and dentin matrix protein 1 [DMP1]) were all significantly higher (p<0.05) in the NF-gelatin/MgP group than in the NF-gelatin group. Those results were further confirmed by hematoxylin and eosin (H&E) and von Kossa staining, as shown by greater ECM secretion and mineral deposition in the hybrid scaffold. Consistent with the in vitro study, the DPSCs/NF-gelatin/MgP constructs produced greater ECM deposition, hard tissue formation, and expression of marker proteins (DSPP, DMP1, Col I) for odontogenic differentiation than did the DPSCs/NF-gelatin after 5 weeks of ectopic implantation in rude mice. The controlled release of metallic ions from biomimetic nanostructured hybrid scaffolds, therefore, is a promising approach to enhancing the biological capability of the scaffolds for dental tissue regeneration.

Induced ablation of Bmp1 and Tll1 produces osteogenesis imperfecta in mice

Osteogenesis imperfecta (OI), or brittle bone disease, is most often caused by dominant mutations in the collagen I genes COL1A1/COL1A2, whereas rarer recessive OI is often caused by mutations in genes encoding collagen I-interacting proteins. Recently, mutations in the gene for the proteinase bone morphogenetic 1 (BMP1) were reported in two recessive OI families. BMP1 and the closely related proteinase mammalian tolloid-like 1 (mTLL1) are co-expressed in various tissues, including bone, and have overlapping activities that include biosynthetic processing of procollagen precursors into mature collagen monomers. However, early lethality of Bmp1- and Tll1-null mice has precluded use of such models for careful study of in vivo roles of their protein products. Here we employ novel mouse strains with floxed Bmp1 and Tll1 alleles to induce postnatal, simultaneous ablation of the two genes, thus avoiding barriers of Bmp1(-/-) and Tll1(-/-) lethality and issues of functional redundancy. Bones of the conditionally null mice are dramatically weakened and brittle, with spontaneous fractures-defining features of OI. Additional skeletal features include osteomalacia, thinned/porous cortical bone, reduced processing of procollagen and dentin matrix protein 1, remarkably high bone turnover and defective osteocyte maturation that is accompanied by decreased expression of the osteocyte marker and Wnt-signaling inhibitor sclerostin, and by marked induction of canonical Wnt signaling. The novel animal model presented here provides new opportunities for in-depth analyses of in vivo roles of BMP1-like proteinases in bone and other tissues, and for their roles, and for possible therapeutic interventions, in OI.

The plastic nature of the human bone-periodontal ligament-tooth fibrous joint

This study investigates bony protrusions within a narrowed periodontal ligament space (PDL-space) of a human bone-PDL-tooth fibrous joint by mapping structural, biochemical, and mechanical heterogeneity. Higher resolution structural characterization was achieved via complementary atomic force microscopy (AFM), nano-transmission X-ray microscopy (nano-TXM), and microtomography (MicroXCT™). Structural heterogeneity was correlated to biochemical and elemental composition, illustrated via histochemistry and microprobe X-ray fluorescence analysis (μ-XRF), and mechanical heterogeneity evaluated by AFM-based nanoindentation. Results demonstrated that the narrowed PDL-space was due to invasion of bundle bone (BB) into PDL-space. Protruded BB had a wider range with higher elastic modulus values (2-8GPa) compared to lamellar bone (0.8-6GPa), and increased quantities of Ca, P and Zn as revealed by μ-XRF. Interestingly, the hygroscopic 10-30μm interface between protruded BB and lamellar bone exhibited higher X-ray attenuation similar to cement lines and lamellae within bone. Localization of the small leucine rich proteoglycan biglycan (BGN) responsible for mineralization was observed at the PDL-bone interface and around the osteocyte lacunae. Based on these results, it can be argued that the LB-BB interface was the original site of PDL attachment, and that the genesis of protruded BB identified as protrusions occurred as a result of shift in strain. We emphasize the importance of bony protrusions within the context of organ function and that additional study is warranted.

Steroid-associated hip joint collapse in bipedal emus

In this study we established a bipedal animal model of steroid-associated hip joint collapse in emus for testing potential treatment protocols to be developed for prevention of steroid-associated joint collapse in preclinical settings. Five adult male emus were treated with a steroid-associated osteonecrosis (SAON) induction protocol using combination of pulsed lipopolysaccharide (LPS) and methylprednisolone (MPS). Additional three emus were used as normal control. Post-induction, emu gait was observed, magnetic resonance imaging (MRI) was performed, and blood was collected for routine examination, including testing blood coagulation and lipid metabolism. Emus were sacrificed at week 24 post-induction, bilateral femora were collected for micro-computed tomography (micro-CT) and histological analysis. Asymmetric limping gait and abnormal MRI signals were found in steroid-treated emus. SAON was found in all emus with a joint collapse incidence of 70%. The percentage of neutrophils (Neut %) and parameters on lipid metabolism significantly increased after induction. Micro-CT revealed structure deterioration of subchondral trabecular bone. Histomorphometry showed larger fat cell fraction and size, thinning of subchondral plate and cartilage layer, smaller osteoblast perimeter percentage and less blood vessels distributed at collapsed region in SAON group as compared with the normal controls. Scanning electron microscope (SEM) showed poor mineral matrix and more osteo-lacunae outline in the collapsed region in SAON group. The combination of pulsed LPS and MPS developed in the current study was safe and effective to induce SAON and deterioration of subchondral bone in bipedal emus with subsequent femoral head collapse, a typical clinical feature observed in patients under pulsed steroid treatment. In conclusion, bipedal emus could be used as an effective preclinical experimental model to evaluate potential treatment protocols to be developed for prevention of ON-induced hip joint collapse in patients.

Impact of extracellular matrix derived from osteoarthritis subchondral bone osteoblasts on osteocytes: role of integrinβ1 and focal adhesion kinase signaling cues

INTRODUCTION

Our recent study indicated that subchondral bone pathogenesis in osteoarthritis (OA) is associated with osteocyte morphology and phenotypic abnormalities. However, the mechanism underlying this abnormality needs to be identified. In this study we investigated the effect of extracellular matrix (ECM) produced from normal and OA bone on osteocytic cells function.

METHODS

De-cellularized matrices, resembling the bone provisional ECM secreted from primary human subchondral bone osteoblasts (SBOs) of normal and OA patients were used as a model to study the effect on osteocytic cells. Osteocytic cells (MLOY4 osteocyte cell line) cultured on normal and OA derived ECMs were analyzed by confocal microscopy, scanning electron microscopy (SEM), cell attachment assays, zymography, apoptosis assays, qRT-PCR and western blotting. The role of integrinβ1 and focal adhesion kinase (FAK) signaling pathways during these interactions were monitored using appropriate blocking antibodies.

RESULTS

The ECM produced by OA SBOs contained less mineral content, showed altered organization of matrix proteins and matrix structure compared with the matrices produced by normal SBOs. Culture of osteocytic cells on these defective OA ECM resulted in a decrease of integrinβ1 expression and the de-activation of FAK cell signaling pathway, which subsequently affected the initial osteocytic cell's attachment and functions including morphological abnormalities of cytoskeletal structures, focal adhesions, increased apoptosis, altered osteocyte specific gene expression and increased Matrix metalloproteinases (MMP-2) and -9 expression.

CONCLUSION

This study provides new insights in understanding how altered OA bone matrix can lead to the abnormal osteocyte phenotypic changes, which is typical in OA pathogenesis.

Failure to process dentin sialophosphoprotein into fragments leads to periodontal defects in mice

Dentin sialophosphoprotein (DSPP) plays a vital role in dentinogenesis. Previously, we showed that, in addition to dentin, DSPP is also highly expressed in alveolar bone and cellular cementum, and plays a crucial role in maintaining periodontal integrity; Dspp-deficient mice demonstrate severe periodontal defects, including alveolar bone loss, decreased cementum deposition, abnormal osteocyte morphology in the alveolar bone, and apical migration of periodontal ligament. Dentin sialophosphoprotein in dentin and bone is cleaved into NH₂ -terminal and COOH-terminal fragments. Whilst our previous study showed that the proteolytic processing of DSPP is critical for dentinogenesis, it is unclear whether the post-translational cleavage of DSPP also plays an essential role in maintaining a healthy periodontium. In this study, we analyzed the periodontal tissues from transgenic mice expressing the uncleavable full-length DSPP in the Dspp knockout (Dspp-KO) background (named 'Dspp-KO/D452A-Tg mice'), in comparison with those from wild-type mice, Dspp-KO mice, and mice expressing the normal Dspp transgene in the Dspp-KO background (designated 'Dspp-KO/normal-Tg mice'). We found that transgenic expression of the normal DSPP fully rescued the periodontal defects of the Dspp-KO mice, whereas this was not the case in Dspp-KO/D452A-Tg mice. These results indicate that proteolytic processing of DSPP is essential to periodontal integrity.

BMP receptor 1A determines the cell fate of the postnatal growth plate

Bone morphogenic proteins (BMPs) are critical for both chondrogenesis and osteogenesis. Previous studies reported that embryos deficient in Bmp receptor (Bmpr)1a or Bmpr1b in cartilage display subtle skeletal defects; however, double mutant embryos develop severe skeletal defects, suggesting a functional redundancy that is essential for early chondrogenesis. In this study, we examined the postnatal role of Bmpr1a in cartilage. In the Bmpr1a conditional knockout (cKO, a cross between Bmpr1a flox and aggrecan-CreER^T2 induced by a one-time-tamoxifen injection at birth and harvested at ages of 2, 4, 8 and 20 weeks), there was essentially no long bone growth with little expression of cartilage markers such as SOX9, IHH and glycoproteins. Unexpectedly, the null growth plate was replaced by bone-like tissues, supporting the notions that the progenitor cells in the growth plate, which normally form cartilage, can form other tissues such as bone and fibrous; and that BMPR1A determines the cell fate. A working hypothesis is proposed to explain the vital role of BMPR1A in postnatal chondrogenesis.

The specific role of FAM20C in amelogenesis

Previously, we showed that Sox2-Cre;Fam20C(fl/fl) mice in which Fam20C was ubiquitously inactivated had severe defects in dentin, enamel, and bone, along with hypophosphatemia. It remains to be determined if the enamel defects in the mice with universal inactivation of Family with sequence similarity 20-C (FAM20C) were associated with the dentin defects and whether hypophosphatemia in the knockout mice contributed to the enamel defects. In this study, we crossed Fam20C(fl/fl) mice with keratin 14-Cre (K14-Cre) transgenic mice to specifically inactivate Fam20C in the epithelial cells, including the dental epithelial cells that are responsible for forming tooth enamel. X-ray, backscattered scanning electron microscopic, and histological analyses showed that the K14-Cre;Fam20C(fl/fl) mice had severe enamel and ameloblast defects, while their dentin and alveolar bone were not significantly affected. Accordingly, serum biochemistry of the K14-Cre;Fam20C(fl/fl) mice showed normal phosphate and FGF23 levels in the circulation. Analysis of these data indicates that, while FAM20C is a molecule essential to amelogenesis, its inactivation in the dental epithelium does not significantly affect dentinogenesis. Hypophosphatemia makes no significant contribution to the enamel defects in the mice with the ubiquitous deletion of Fam20C.

Osteocyte regulation of phosphate homeostasis and bone mineralization underlies the pathophysiology of the heritable disorders of rickets and osteomalacia

Although recent studies have established that osteocytes function as secretory cells that regulate phosphate metabolism, the biomolecular mechanism(s) underlying these effects remain incompletely defined. However, investigations focusing on the pathogenesis of X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR), heritable disorders characterized by abnormal renal phosphate wasting and bone mineralization, have clearly implicated FGF23 as a central factor in osteocytes underlying renal phosphate wasting, documented new molecular pathways regulating FGF23 production, and revealed complementary abnormalities in osteocytes that regulate bone mineralization. The seminal observations leading to these discoveries were the following: 1) mutations in FGF23 cause ADHR by limiting cleavage of the bioactive intact molecule, at a subtilisin-like protein convertase (SPC) site, resulting in increased circulating FGF23 levels and hypophosphatemia; 2) mutations in DMP1 cause ARHR, not only by increasing serum FGF23, albeit by enhanced production and not limited cleavage, but also by limiting production of the active DMP1 component, the C-terminal fragment, resulting in dysregulated production of DKK1 and β-catenin, which contributes to impaired bone mineralization; and 3) mutations in PHEX cause XLH both by altering FGF23 proteolysis and production and causing dysregulated production of DKK1 and β-catenin, similar to abnormalities in ADHR and ARHR, but secondary to different central pathophysiological events. These discoveries indicate that ADHR, XLH, and ARHR represent three related heritable hypophosphatemic diseases that arise from mutations in, or dysregulation of, a single common gene product, FGF23 and, in ARHR and XLH, complimentary DMP1 and PHEX directed events that contribute to abnormal bone mineralization.

Biomimetic engineering of nanofibrous gelatin scaffolds with noncollagenous proteins for enhanced bone regeneration

Biomimetic approaches are widely used in scaffolding designs to enhance tissue regeneration. In this study, we integrated noncollagenous proteins (NCPs) from bone extracellular matrix (ECM) with three-dimensional nanofibrous gelatin (NF-Gelatin) scaffolds to form an artificial matrix (NF-Gelatin-NCPs) mimicking both the nano-structured architecture and chemical composition of natural bone ECM. Through a chemical coupling process, the NCPs were evenly distributed over all the surfaces (inner and outer) of the NF-gelatin-NCPs. The in vitro study showed that the number of osteoblasts (MC3T3-E1) on the NF-Gelatin-NCPs was significantly higher than that on the NF-Gelatin after being cultured for 14 days. Both the alkaline phosphatase (ALP) activity and the expression of osteogenic genes (OPN, BSP, DMP1, CON, and Runx2) were significantly higher in the NF-Gelatin-NCPs than in the NF-Gelatin at 3 weeks. Von Kossa staining, backscattered scanning electron microscopy, and microcomputed tomography all revealed a higher amount of mineral deposition in the NF-Gelatin-NCPs than in the NF-Gelatin after in vitro culturing for 3 weeks. The in vivo calvarial defect study indicated that the NF-Gelatin-NCPs recruited more host cells to the defect and regenerated a higher amount of bone than the controls after implantation for 6 weeks. Immunohistochemical staining also showed high-level mineralization of the bone matrix in the NF-Gelatin-NCPs. Taken together, both the in vitro and in vivo results confirmed that the incorporation of NCPs onto the surfaces of the NF-Gelatin scaffold significantly enhanced osteogenesis and mineralization. Biomimetic engineering of the surfaces of the NF-Gelatin scaffold with NCPs, therefore, is a promising strategy to enhance bone regeneration.

Loss of dentin sialophosphoprotein leads to periodontal diseases in mice

BACKGROUND AND OBJECTIVE

Dentin sialophosphoprotein (DSPP) and its cleaved products, dentin phosphoprotein (DPP) and dentin sialoprotein (DSP), play important roles in biomineralization. Recently, we observed that DSPP is highly expressed in the alveolar bone and cementum, indicating that this molecule may play an important role in the formation and maintenance of a healthy periodontium, and its deletion may cause increased susceptibility to periodontal diseases. The objective of this investigation was to study the effects of Dspp ablation on periodontal tissues by analyzing Dspp null mice.

MATERIAL AND METHODS

Newborn to 6-mo-old Dspp null mice were examined, and the 3- and 6-mo-old Dspp null mice were characterized in detail using X-ray radiography, histology and scanning electron microscopy (backscattered as well as resin-infiltrating). Wild-type mice of the same age groups served as the normal controls.

RESULTS

The Dspp null mice showed significant loss of alveolar bone and cementum, particularly in the furcation and interproximal regions of the molars. The alveolar bone appeared porous while the quantity of cementum was reduced in the apical region. The canalicular systems and osteocytes in the alveolar bone were abnormal, with reduced numbers of canaliculi and altered osteocyte morphology. The loss of alveolar bone and cementum along with the detachment of the periodontal ligaments (PDL) led to the apical migration of the epithelial attachment and formation of periodontal pockets.

CONCLUSION

Inactivation of DSPP leads to the loss of alveolar bone and cementum and increased susceptibility to bacterial infections in PDL of Dspp null mice. The fact that the loss of DSPP results in periodontal diseases indicates that this molecule plays a vital role in maintaining the health of the periodontium.

The rescue of dentin matrix protein 1 (DMP1)-deficient tooth defects by the transgenic expression of dentin sialophosphoprotein (DSPP) indicates that DSPP is a downstream effector molecule of DMP1 in dentinogenesis

Dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein (DSPP) are essential for the formation of dentin. Previous in vitro studies have indicated that DMP1 might regulate the expression of DSPP during dentinogenesis. To examine whether DMP1 controls dentinogenesis through the regulation of DSPP in vivo, we cross-bred transgenic mice expressing normal DSPP driven by a 3.6-kb rat Col1a1 promoter with Dmp1 KO mice to generate mice expressing the DSPP transgene in the Dmp1 KO genetic background (referred to as "Dmp1 KO/DSPP Tg mice"). We used morphological, histological, and biochemical techniques to characterize the dentin and alveolar bone of Dmp1 KO/DSPP Tg mice compared with Dmp1 KO and wild-type mice. Our analyses showed that the expression of endogenous DSPP was remarkably reduced in the Dmp1 KO mice. Furthermore, the transgenic expression of DSPP rescued the tooth and alveolar bone defects of the Dmp1 KO mice. In addition, our in vitro analyses showed that DMP1 and its 57-kDa C-terminal fragment significantly up-regulated the Dspp promoter activities in a mesenchymal cell line. In contrast, the expression of DMP1 was not altered in the Dspp KO mice. These results provide strong evidence that DSPP is a downstream effector molecule that mediates the roles of DMP1 in dentinogenesis.

Role of the NH2 -terminal fragment of dentin sialophosphoprotein in dentinogenesis

Dentin sialophosphoprotein (DSPP) is a large precursor protein that is proteolytically processed into a NH2 -terminal fragment [composed of dentin sialoprotein (DSP) and a proteoglycan form (DSP-PG)] and a COOH-terminal fragment [dentin phosphoprotein (DPP)]. In vitro studies indicate that DPP is a strong initiator and regulator of hydroxyapatite crystal formation and growth, but the role(s) of the NH2 -terminal fragment of DSPP (i.e., DSP and DSP-PG) in dentinogenesis remain unclear. This study focuses on the function of the NH2 -terminal fragment of DSPP in dentinogenesis. Here, transgenic (Tg) mouse lines expressing the NH2 -terminal fragment of DSPP driven by a 3.6-kb type I collagen promoter (Col 1a1) were generated and cross-bred with Dspp null mice to obtain mice that express the transgene but lack the endogenous Dspp (Dspp KO/DSP Tg). We found that dentin from the Dspp KO/DSP Tg mice was much thinner, more poorly mineralized, and remarkably disorganized compared with dentin from the Dspp KO mice. The fact that Dspp KO/DSP Tg mice exhibited more severe dentin defects than did the Dspp null mice indicates that the NH2 -terminal fragment of DSPP may inhibit dentin mineralization or may serve as an antagonist against the accelerating action of DPP and serve to prevent predentin from being mineralized too rapidly during dentinogenesis.

Osteoblast lineage-specific effects of notch activation in the skeleton

Transgenic overexpression of the Notch1 intracellular domain inhibits osteoblast differentiation and causes osteopenia, and inactivation of Notch1 and Notch2 increases bone volume transiently and induces osteoblastic differentiation. However, the biology of Notch is cell-context-dependent, and consequences of Notch activation in cells of the osteoblastic lineage at various stages of differentiation and in osteocytes have not been defined. For this purpose, Rosa(Notch) mice, where a loxP-flanked STOP cassette placed between the Rosa26 promoter and the NICD coding sequence, were crossed with transgenics expressing the Cre recombinase under the control of the Osterix (Osx), Osteocalcin (Oc), Collagen 1a1 (Col2.3), or Dentin matrix protein1 (Dmp1) promoters. At 1 month, Osx-Cre;Rosa(Notch) and Oc-Cre;Rosa(Notch) mice exhibited osteopenia due to impaired bone formation. In contrast, Col2.3-Cre;Rosa(Notch) and Dmp1-Cre;Rosa(Notch) exhibited increased femoral trabecular bone volume due to a decrease in osteoclast number and eroded surface. In the four lines studied, cortical bone was either not present, was porous, or had the appearance of trabecular bone. Oc-Cre;Rosa(Notch) and Col2.3-Cre;Rosa(Notch) mice exhibited early lethality so that their adult phenotype was not established. At 3 months, Osx-Cre;Rosa(Notch) and Dmp1-Cre;Rosa(Notch) mice displayed increased bone volume, and increased osteoblasts although calcein-demeclocycline labels were diffuse and fragmented, indicating abnormal bone formation. In conclusion, Notch effects in the skeleton are cell-context-dependent. When expressed in immature osteoblasts, Notch arrests their differentiation, causing osteopenia, and when expressed in osteocytes, it causes an initial suppression of bone resorption and increased bone volume, a phenotype that evolves as the mice mature.

Hexa-D-arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype

Inactivating mutations of the "phosphate regulating gene with homologies to endopeptidases on the X chromosome" (PHEX/Phex) underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum fibroblast growth factor 23 (FGF-23) underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild-type mice with the proprotein convertase (PC) inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (Dec), increases serum FGF-23 and produces the HYP phenotype. Because PC2 is uniquely colocalized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, whereas Signe1 (7B2) RNA interference (RNAi) transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of the precursor to bone morphogenetic protein 1 (proBMP1) to active BMP1, which resulted in limited cleavage of dentin matrix acidic phosphoprotein 1 (DMP1), and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest that decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.