LRP1-deficient leptin receptor-positive cells in periodontal ligament tissue reduce alveolar bone mass by inhibiting bone formation.

OBJECTIVE: Leptin receptor-positive (LepR+) periodontal ligament (PDL) cells play a crucial role in osteogenesis during tooth socket healing and orthodontic tooth movement; however, the factors regulating osteoblast differentiation remain unclear. This study aimed to demonstrate the function of low-density lipoprotein receptor-related protein 1 (LRP1) in alveolar bone formation by examining conditional knockout (cKO) mice lacking LRP1 in LepR+ cells. DESIGN: Bone mass and formation were examined via bone morphometric analysis. Bone formation and resorption activities were determined via histochemical staining. Additionally, PDL cells collected from molars were induced to differentiate into osteoblasts with the addition of BMP2 and to mineralize with the addition of osteogenic medium. Osteoblast differentiation of PDL cells was examined by measuring the expression of osteoblast markers. RESULTS: Bone morphometry analysis revealed decreased mineral apposition rate and alveolar bone mass in cKO mice. Additionally, cKO mice showed a decreased number of osterix-positive cells in the PDL. cKO mice had a large number of osteoclasts around the alveolar bone near the root apex and mesial surface of the tooth. In the PDL cells from cKO mice, inhibition of mineralized matrix formation and decreased expression of alkaline phosphatase, osterix, bone sialoprotein, and osteocalcin were observed even when BMP2 was added to the medium. BMP2, BMP4, and osteoprotegerin expression also decreased, but RANKL expression increased dominantly. CONCLUSION: LRP1 in LepR+ cells promotes bone formation by stimulating osteoblast differentiation. Our findings can contribute to clinical research on bone diseases and help elucidate bone metabolism in the periodontal tissue.

p53 deficiency promotes bone regeneration by functional regulation of mesenchymal stromal cells and osteoblasts.

INTRODUCTION: The detailed mechanism of the process during bone healing of drill-hole injury has been elucidated, but a crucial factor in regulating drill-hole healing has not been identified. The transcription factor p53 suppresses osteoblast differentiation through inhibition of osterix expression. In present study, we demonstrate the effects of p53 deficiency on the capacity of MSCs and osteoblasts during drill-hole healing. MATERIALS AND METHODS: Mesenchymal stromal cells (MSCs) and osteoblasts were collected from bone marrow and calvaria of p53 knockout (KO) mice, respectively. The activities of cell mobility, cell proliferation, osteoblast differentiation, and wound healing of MSCs and/or osteoblasts were determined by in vitro experiments. In addition, bone healing of drill-hole injury in KO mice was examined by micro-CT and immunohistological analysis using anti-osterix, Runx2, and sclerostin antibodies. RESULTS: KO MSCs stimulated cell mobility, cell proliferation, and osteoblast differentiation. Likewise, KO osteoblasts enhanced cell proliferation and wound healing. KO MSCs and osteoblasts showed high potency in the inflammation and callus formation phases compared to those from wild-type (WT) mice. In addition, increased expression of osterix and Runx2 was observed in KO MSCs and osteoblasts that migrated in the drill-hole. Conversely, sclerostin expression was inhibited in KO mice. Eventually, KO mice exhibited high repairability of drill-hole injury, suggesting a novel role of p53 in MSCs and osteoblasts in improving bone healing. CONCLUSION: p53 Deficiency promotes bone healing of drill-hole injury by enhancing the bone-regenerative ability of MSCs and osteoblasts.

Chemokine ligand 28 (CCL28) negatively regulates trabecular bone mass by suppressing osteoblast and osteoclast activities.

INTRODUCTION: Bone metabolism imbalances cause bone metabolism diseases, like osteoporosis, through aging. Although some chemokines are known to be involved in bone mass regulation, many have not been investigated. Thus, the present study aimed to investigate the role of chemokine ligand 28 (CCL28) on bone metabolism. MATERIALS AND METHODS: To investigate the role of CCL28 on bone metabolism, 10-week-old male wild-type and Ccl28 knockout (Ccl28 KO) mice were analyzed. Microcomputed tomography analysis and bone tissue morphometry were used to investigate the effect of Ccl28 deficiency on the bone. CCL28 localization in bone tissue was assumed by immunohistochemistry. Osteoblast and osteoclast markers were evaluated by enzyme-linked immunosorbent assay and quantitative reverse transcription-polymerase chain reaction. Finally, in vitro experiments using MC3T3-E1 and bone marrow macrophages revealed the direct effect of CCL28 on osteoblast and osteoclast. RESULTS: This study showed that Ccl28 deficiency significantly increased bone mass and the number of mature osteoblasts. Immunoreactivity for CCL28 was observed in osteoblasts and osteoclasts on bone tissue. Additionally, Ccl28 deficiency promoted osteoblast and osteoclast maturation. Moreover, CCL28 treatment decreased osteoblast and osteoclast activities but did not affect differentiation. CONCLUSION: In summary, this study indicated that CCL28 is one of the negative regulators of bone mass by suppressing osteoblast and osteoclast activities. These results provide important insights into bone immunology and the selection of new osteoporosis treatments.

Intermittent parathyroid hormone 1-34 induces oxidation and deterioration of mineral and collagen quality in newly formed mandibular bone.

Intermittent parathyroid hormone (PTH) administration is known to promote bone healing after surgical procedures. However, the mechanism and influence of PTH on the mineral and collagen quality of the jaw are not well understood. Most studies have focused on analyzing the bone density and microstructure of the mandible, and have insufficiently investigated its mineral and collagen quality. Oxidative stress activates osteoclasts, produces advanced glycation end products, and worsens mineral and collagen quality. We hypothesized that PTH induces oxidation and affects the mineral and collagen quality of newly formed mandibular bone. To test this, we examined the mineral and collagen quality of newly formed mandibular bone in rats administered PTH, and analyzed serum after intermittent PTH administration to examine the degree of oxidation. PTH administration reduced mineralization and worsened mineral and collagen quality in newly formed bone. In addition, total anti-oxidant capacity in serum was significantly decreased and the oxidative-INDEX was increased among PTH-treated compared to vehicle-treated rats, indicating serum oxidation. In conclusion, intermittent administration of PTH reduced mineral and collagen quality in newly formed mandibular bone. This effect may have been induced by oxidation.

Establishment and characterization of a C57BL/6 mouse model of bone metastasis of breast cancer.

Bone is one of the most common sites of metastasis in patients with advanced breast cancer; however, the mechanisms of bone metastasis remain to be fully elucidated. Animal models are essential research tools for investigating the mechanisms of diseases and drug actions. To date, there have only been a few reports in which C57BL/6 mice were used for the study of bone metastases of breast cancer. In the current study, we found that intracardiac inoculation of C57BL/6 mouse-derived parental E0771 breast cancer cells (E0771/Pa) frequently lead to bone metastases in C57BL/6 mice within 2 weeks. The bone-metastatic clone of E0771 (E0771/Bone) established by sequential in vivo selection demonstrated a higher bone-metastatic potential. Although there were no apparent differences in cell morphology or proliferation in monolayer cultures, E0771/Bone showed increased tumorsphere formation in suspension cultures and tumor formation in the orthotopic mammary fat pad in C57BL/6 mice compared with E0771/Pa. Furthermore, E0771/Bone expressed breast cancer stem-like cell surface markers CD24-/CD44+. These findings suggest that E0771/Bone possesses cancer stem-like properties. Quantitative PCR analysis revealed that mRNA expression of parathyroid hormone-related protein (PTHrP), the most common mediator of osteolytic bone metastases of breast cancer, was significantly upregulated in E0771/Bone. Thus, cancer stem-like properties and elevated PTHrP expression likely contribute to the enhanced metastatic potential of E0771/Bone. We believe that this new mouse model is a useful tool for in vivo studies of bone metastases of breast cancer, especially for those using genetically engineered mice with a C57BL/6 background.

Forced expression of mouse progerin attenuates the osteoblast differentiation interrupting ß-catenin signal pathway in vitro.

Nuclear protein, lamin A, which is a component of inner membrane on nucleoplasm, plays a role in nuclear formation and cell differentiation. The expression of mutated lamin A, termed progerin, causes a rare genetic aging disorder, Hutchinson-Gilford progeria syndrome, which shows abnormal bone formation with the decrease in a number of osteoblasts and osteocytes. However, exact molecular mechanism how progerin exerts depressive effects on osteogenesis has not been fully understood. Here, we created mouse lamin A dC50 cDNA encoding progerin that lacks 50 amino acid residues at C-terminus, transfected it in mouse preosteoblast-like MC3T3-E1 cells, and examined the changes in osteoblast phenotype. When lamin A dC50-expressed cells were cultured with differentiation-inductive medium, alkaline phosphatase (ALP) activity and mRNA levels of major osteoblast markers, type I collagen (Col1), bone sialoprotein (BSP), dentine matrix protein 1 (DMP1), and Runx2 were significantly decreased, and no mineralized nodules were detected as seen in control cells expressing empty vector. In the culture with mineralization-inductive medium, mRNA levels of BSP, osteocalcin, DMP1, Runx2, and osterix were strongly decreased parallel with loss of mineralization in lamin A dC50-expressed cells, while mineralized nodules appear at 21 days in control cells. Furthermore, lamin A dC50 expression was depressed nuclear localization of ß-catenin with the decrease of GSK-3ß phosphorylation level. These results suggest that lamin A dC50 depresses osteoblast differentiation in both early and late stages, and it negatively regulates ß-catenin activity interacting with GSK-3ß in cytoplasm.

Treatment of OPG-deficient mice with WP9QY, a RANKL-binding peptide, recovers alveolar bone loss by suppressing osteoclastogenesis and enhancing osteoblastogenesis.

Osteoblasts express two key molecules for osteoclast differentiation, receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), a soluble decoy receptor for RANKL. RANKL induces osteoclastogenesis, while OPG inhibits it by blocking the binding of RANKL to RANK, a cellular receptor of RANKL. OPG-deficient (OPG-/-) mice exhibit severe alveolar bone loss with enhanced bone resorption. WP9QY (W9) peptide binds to RANKL and blocks RANKL-induced osteoclastogenesis. W9 is also reported to stimulate bone formation in vivo. Here, we show that treatment with W9 restores alveolar bone loss in OPG-/-mice by suppressing osteoclastogenesis and enhancing osteoblastogenesis. Administration of W9 or risedronate, a bisphosphonate, to OPG-/-mice significantly decreased the osteoclast number in the alveolar bone. Interestingly, treatment with W9, but not risedronate, enhanced Wnt/ß-catenin signaling and induced alveolar bone formation in OPG-/-mice. Expression of sclerostin, an inhibitor of Wnt/ß-catenin signaling, was significantly lower in tibiae of OPG-/-mice than in wild-type mice. Treatment with risedronate recovered sclerostin expression in OPG-/-mice, while W9 treatment further suppressed sclerostin expression. Histomorphometric analysis confirmed that bone formation-related parameters in OPG-/-mice, such as osteoblast number, osteoblast surface and osteoid surface, were increased by W9 administration but not by risedronate administration. These results suggest that treatment of OPG-/-mice with W9 suppressed osteoclastogenesis by inhibiting RANKL signaling and enhanced osteoblastogenesis by attenuating sclerostin expression in the alveolar bone. Taken together, W9 may be a useful drug to prevent alveolar bone loss in periodontitis.

Enhanced bone-forming activity of side population cells in the periodontal ligament.

Regeneration of alveolar bone is critical for the successful treatment of periodontal diseases. The periodontal ligament (PDL) has been widely investigated as a source of cells for the regeneration of periodontal tissues. In the present study where we attempted to develop an effective strategy for alveolar bone regeneration, we examined the osteogenic potential of side population (SP) cells, a stem cell-containing population that has been shown to be highly abundant in several kinds of tissues, in PDL cells. Isolated SP cells from the rat PDL exhibited a superior ability to differentiate into osteoblastic cells compared with non-SP (NSP) and unsorted PDL cells in vitro. The mRNA expressions of osteoblast markers and bone morphogenetic protein (BMP) 2 were significantly upregulated in SP cells and were further increased by osteogenic induction. To examine the bone-forming activity of SP cells in vivo, PDL SP cells isolated from green fluorescent protein (GFP)-transgenic rats were transplanted with hydroxyapatite (HA) disks into wild-type animals. SP cells exhibited a high ability to induce the mineralized matrix compared with NSP and unsorted PDL cells. At 12 weeks after the implantation, some of the pores in the HA disks with SP cells were filled with mineralized matrices, which were positive for bone matrix proteins, such as osteopontin, bone sialoprotein, and osteocalcin. Furthermore, osteoblast- and osteocyte-like cells on and in the bone-like mineralized matrices were GFP positive, suggesting that the matrices were directly formed by the transplanted cells. These results suggest that PDL SP cells possess enhanced osteogenic potential and could be a potential source for cell-based regenerative therapy for alveolar bone.

Osteoprotegerin-deficient male mice as a model for severe alveolar bone loss: comparison with RANKL-overexpressing transgenic male mice.

Periodontitis, an inflammatory disease of periodontal tissues, is characterized by excessive alveolar bone resorption. An increase in the receptor activator of nuclear factor-κB ligand (RANKL) to osteoprotegerin (OPG) ratio is thought to reflect the severity of periodontitis. Here, we examined alveolar bone loss in OPG-deficient (OPG(-/-)) mice and RANKL-overexpressing transgenic (RANKL-Tg) mice. Alveolar bone loss in OPG(-/-) mice at 12 weeks was significantly higher than that in RANKL-Tg mice. OPG(-/-) but not RANKL-Tg mice exhibited severe bone resorption especially in cortical areas of the alveolar bone. An increased number of osteoclasts was observed in the cortical areas in OPG(-/-) but not in RANKL-Tg mice. Immunohistochemical analyses showed many OPG-positive signals in osteocytes but not osteoblasts. OPG-positive osteocytes in the cortical area of alveolar bones and long bones were abundant in both wild-type and RANKL-Tg mice. This suggests the resorption in cortical bone areas to be prevented by OPG produced locally. To test the usefulness of OPG(-/-) mice as an animal model for screening drugs to prevent alveolar bone loss, we administered an antimouse RANKL antibody or risedronate, a bisphosphonate, to OPG(-/-) mice. They suppressed alveolar bone resorption effectively. OPG(-/-) mice are useful for screening therapeutic agents against alveolar bone loss.

Localization of SUMOylation factors and Osterix in odontoblast lineage cells during dentin formation and regeneration.

Small ubiquitin-related modifier (SUMO) conjugation (SUMOylation) is a post-translational modification involved in various cellular processes including the regulation of transcription factors. In this study, to analyze the involvement of SUMOylation in odontoblast differentiation, we examined the immunohistochemical localization of SUMO-1, SUMO-2/3, and Osterix during rat tooth development. At the bud and cap stages, localization of SUMOs and Osterix was hardly detected in the dental mesenchyme. At the bell stage, odontoblasts just beginning dentin matrix secretion and preodontoblasts near these odontoblasts showed intense immunoreactivity for these molecules. However, after the root-formation stage, these immunoreactivities in the odontoblasts decreased in intensity. Next, to examine whether the SUMOylation participates in dentin regeneration, we evaluated the distribution of SUMOs and Osterix in the dental pulp after cavity preparation. In the coronal pulp chamber of an untreated rat molar, odontoblasts and pulp cells showed no immunoreactivity. At 4 days after cavity preparation, positive cells for SUMOs and Osterix appeared on the surface of the dentin beneath the cavity. Odontoblast-like cells forming reparative dentin were immunopositive for SUMOs and Osterix at 1 week, whereas these immunoreactivities disappeared after 8 weeks. Additionally, we further analyzed the capacity of SUMO-1 to bind Osterix by performing an immunoprecipitation assay using C2C12 cells, and showed that Osterix could undergo SUMOylation. These results suggest that SUMOylation might regulate the transcriptional activity of Osterix in odontoblast lineage cells, and thus play important roles in odontoblast differentiation and regeneration.

Thy-1-positive cells in the subodontoblastic layer possess high potential to differentiate into hard tissue-forming cells.

The cells of the subodontoblastic cell-rich layer in dental pulp are speculated to contain odontoblast progenitor cells because of their positional relationship with odontoblasts as well as their high alkaline phosphatase (ALP) activity. However, it has yet to be determined whether these cells have the ability to differentiate into odontoblastic cells. In the present study, we firstly found that the majority of cells in the subodontoblastic layer expressed Thy-1, a cell-surface marker of stem and progenitor cells. Then, we evaluated the capacity of Thy-1 high- and low-expressing (Thy-1(high) and Thy-1(low)) cells separated from rat dental pulp cells by use of a fluorescence-activated cell sorter to differentiate into hard tissue-forming cells in vitro and in vivo. Following stimulation with bone morphogenetic protein-2, Thy-1(high) cells in vitro showed accelerated induction of ALP activity and formation of alizarin red-positive mineralized matrix compared with Thy-1(low) cells. Furthermore, subcutaneous implantation of Thy-1(high) cells efficiently induced the formation of bone-like matrix. These results collectively suggest that Thy-1-positive dental pulp cells localized in the subodontoblastic layer had the ability to differentiate into hard tissue-forming cells, and thus these cells may serve as a source of odontoblastic cells.

Tetracyclines convert the osteoclastic-differentiation pathway of progenitor cells to produce dendritic cell-like cells.

Tetracyclines, such as doxycycline and minocycline, are used to suppress the growth of bacteria in patients with inflammatory diseases. Tetracyclines have been shown to prevent bone loss, but the mechanism involved is unknown. Osteoclasts and dendritic cells (DCs) are derived from common progenitors, such as bone marrow-derived macrophages (BMMs). In this article, we show that tetracyclines convert the differentiation pathway, resulting in DC-like cells not osteoclasts. Doxycycline and minocycline inhibited the receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis of BMMs, but they had no effects on cell growth and phagocytic activity. They influenced neither the proliferation nor the differentiation of bone-forming osteoblasts. Surprisingly, doxycycline and minocycline induced the expression of DC markers, CD11c and CD86, in BMMs in the presence of RANKL. STAT5 is involved in DC differentiation induced by GM-CSF. Midostaurin, a STAT5-signaling inhibitor, and an anti-GM-CSF-neutralizing Ab suppressed the differentiation induced by GM-CSF but not by tetracyclines. In vivo, the injection of tetracyclines into RANKL-injected mice and RANKL-transgenic mice suppressed RANKL-induced osteoclastogenesis and promoted the concomitant appearance of CD11c(+) cells. These results suggested that tetracyclines prevent bone loss induced by local inflammation, including rheumatoid arthritis and periodontitis, through osteoclast-DC-like cell conversion.

1α,25-Dihydroxyvitamin D3 inhibits osteoblastic differentiation of mouse periodontal fibroblasts.

OBJECTIVE: Periodontal ligament (PDL) is a non-mineralized tissue connecting cementum and alveolar bone. Recent studies have suggested that periodontal fibroblasts can differentiate into mineralized tissue-forming cells such as osteoblasts and cementoblasts. We elucidated the immunolocalization of vitamin D receptor (VDR) and the effects of vitamin D(3) (VD(3)) on mouse periodontal fibroblasts to clarify the role of VDR and VD(3) in the differentiation of periodontal fibroblasts. DESIGN: Immunohistochemical analysis using anti-VDR antibody was performed in paraffin sections of mouse mandibles at E13, E14, E17, P10, and P35. The roles of VD(3) in osteoblastic differentiation and mineralization were estimated by alkaline phosphatase (ALP) and alizarin red (AR) staining. In addition, the mRNA expression of osteoblast markers and mineralization inhibitors was examined by real-time PCR. RESULTS: At the bud, cap and early bell stages, immunoreactivity for VDR was scarcely seen in the cells of dental follicles. Labelling was detected in the nuclei of cemetoblasts, periodontal fibroblasts and osteoblasts during root formation. VD(3) inhibited ALP activity and AR-positive mineralized nodule formation of periodontal fibroblasts in osteogenic culture medium. Real-time PCR revealed that VD(3) down-regulated the levels of osteoblast markers. In contrast, VD(3) significantly up-regulated the level of periodontal ligament associated protein (PLAP)-1, a negative regulator of osteoblastic differentiation. CONCLUSION: These results suggest that VD(3) negatively regulates the osteoblastic differentiation of mouse periodontal fibroblasts and prevents the periodontal ligament from mineralization via increase of PLAP-1.

Increased bone mass in mice after single injection of anti-receptor activator of nuclear factor-kappaB ligand-neutralizing antibody: evidence for bone anabolic effect of parathyroid hormone in mice with few osteoclasts.

Receptor activator of nuclear factor-κB ligand (RANKL) is a pivotal osteoclast differentiation factor. To investigate the effect of RANKL inhibition in normal mice, we prepared an anti-mouse RANKL-neutralizing monoclonal antibody (Mab, clone OYC1) and established a new mouse model with high bone mass induced by administration of OYC1. A single subcutaneous injection of 5 mg/kg OYC1 in normal mice significantly augmented the bone mineral density in the distal femoral metaphysis from day 2 to day 28. The OYC1 treatment markedly reduced the serum level of tartrate-resistant acid phosphatase-5b (TRAP-5b, a marker for osteoclasts) on day 1, and this level was undetectable from day 3 to day 28. The serum level of alkaline phosphatase (a marker for osteoblasts) declined significantly following the reduction of TRAP-5b. Histological analysis revealed few osteoclasts in femurs of the treated mice on day 4, and both osteoclasts and osteoblasts were markedly diminished on day 14. Daily injection of parathyroid hormone for 2 weeks increased the bone mineral density in trabecular and cortical bone by stimulating bone formation in the OYC1-treated mice. These results suggest that parathyroid hormone exerted its bone anabolic activity in mice with few osteoclasts. The mouse anti-RANKL neutralizing antibody OYC1 may be a useful tool to investigate unknown functions of RANKL in vivo.

Rab27a and Rab27b are involved in stimulation-dependent RANKL release from secretory lysosomes in osteoblastic cells.

The quantity of the receptor activator of NF-κB ligand (RANKL) expressed at the cell surface of osteoblastic cells is an important factor regulating osteoclast activation. Previously, RANKL was found to be localized to secretory lysosomes in osteoblastic cells and to translocate to the cell surface in response to stimulation with RANK-Fc-conjugated beads. However, the in vivo significance of stimulation-dependent RANKL release has not been elucidated. In this study we show that small GTPases Rab27a and Rab27b are involved in the stimulation-dependent RANKL release pathway in osteoblastic cells. Suppression of either Rab27a or Rab27b resulted in a marked reduction in RANKL release after stimulation. Slp4-a, Slp5, and Munc13-4 acted as effector molecules that coordinated Rab27a/b activity in this pathway. Suppression of Rab27a/b or these effector molecules did not inhibit accumulation of RANKL in lysosomal vesicles around the stimulated sites but did inhibit the fusion of these vesicles to the plasma membrane. In osteoblastic cells, suppression of the effector molecules resulted in reduced osteoclastogenic ability. Furthermore, Jinx mice, which lack a functional Munc13-4 gene, exhibited a phenotype characterized by increased bone volume near the tibial metaphysis caused by low bone resorptive activity. In conclusion, stimulation-dependent RANKL release is mediated by Rab27a/b and their effector molecules, and this mechanism may be important for osteoclast activation in vivo.

Harmine, a ß-carboline alkaloid, inhibits osteoclast differentiation and bone resorption in vitro and in vivo.

Bone homeostasis is controlled by the balance between osteoblastic bone formation and osteoclastic bone resorption. Excessive bone resorption is involved in the pathogenesis of bone-related disorders such as osteoporosis, arthritis and periodontitis. To obtain new antiresorptive agents, we searched for natural compounds that can inhibit osteoclast differentiation and function. We found that harmine, a ß-carboline alkaloid, inhibited multinucleated osteoclast formation induced by receptor activator of nuclear factor-κB ligand (RANKL) in RAW264.7 cells. Similar results were obtained in cultures of bone marrow macrophages supplemented with macrophage colony-stimulating factor and RANKL, as well as in cocultures of bone marrow cells and osteoblastic UAMS-32 cells in the presence of vitamin D(3) and prostaglandin E(2). Furthermore, harmine prevented RANKL-induced bone resorption in both cell and bone tissue cultures. Treatment with harmine (10 mg/kg/day) also prevented bone loss in ovariectomized osteoporosis model mice. Structure-activity relationship studies showed that the C3-C4 double bond and 7-methoxy group of harmine are important for its inhibitory activity on osteoclast differentiation. In mechanistic studies, we found that harmine inhibited the RANKL-induced expression of c-Fos and subsequent expression of nuclear factor of activated T cells (NFAT) c1, which is a master regulator of osteoclastogenesis. However, harmine did not affect early signaling molecules such as ERK, p38 MAPK and IκBα. These results indicate that harmine inhibits osteoclast formation via downregulation of c-Fos and NFATc1 induced by RANKL and represses bone resorption. These novel findings may be useful for the treatment of bone-destructive diseases.

Prostaglandin E(2) receptor EP(4)-selective agonist (ONO-4819) increases bone formation by modulating mesenchymal cell differentiation.

Prostaglandin E(2) (PGE(2)) positively regulates bone resorption and formation mainly mediated through the EP(4) receptor, a subtype of PGE(2) receptors. ONO-4819, an EP(4) receptor-selective agonist, has been shown to increase bone volume, density, and strength; however, the mechanism of these effects has yet to be fully elucidated. To explore this matter, ONO-4819 (10µg/kg) was injected into intact rats twice a day for 5weeks, and their bones were then analyzed by morphological techniques. The effects of ONO-4819 on the differentiation of bone cells were also examined in vitro. Bone morphometric analysis showed that osteoblast number, bone volume, mineral apposition rate, and bone formation rate were significantly increased by ONO-4819, whereas osteoclast number was not affected. Immunohistochemical examination demonstrated that ONO-4819 increased the number of Runx2- and Osterix-positive osteoblasts in rats. In vitro studies using the multipotent mesenchymal cell line C3H10T1/2 showed that ONO-4819 induced alkaline phosphatase (ALPase) activity and up-regulated the mRNA expression of ALPase and Osterix. In contrast, ONO-4819 reduced the mRNA expression of peroxisome proliferator-activated receptor γ (PPARγ) and inhibited adipocyte differentiation of C3H10T1/2 cells, which findings are consistent with the observation that the age-dependent increase in adipocyte number in the bone marrow was significantly suppressed in the ONO-4819-treated animals. ONO-4819 also dose-dependently increased osteoclast-like cell formation in vitro, but the required concentrations were much higher than those to induce osteoblast differentiation. These results collectively suggest that ONO-4819 increased bone formation by stimulating osteoblast differentiation and function, possibly through modulating mesenchymal cell differentiation in the bone.

Deficiency of vitamin A delays bone healing process in association with reduced BMP2 expression after drill-hole injury in mice.

Although it is predicted that vitamin A and its active form, retinoic acid, regulate osteoblast lineage, this has not been elucidated in growing mammalians. To clarify the direct effect of retinoic acid on bone, we observed the process of filling up newly generating bone into a drill-hole of the bone, which is understood as membranous ossification, in vitamin A-deficient mice. Mice were assigned to three groups: a vitamin A-deficient group (VAD), which was fed a diet without vitamin A from the 10th day of gestation to the end of the experiments; a vitamin A-deficient-sufficient group (VADS), which was fed a diet without vitamin A from the 10th day of gestation to 4 weeks of age; and a vitamin A-sufficient group (VAS), which was fed a standard diet to the end of the experiment. In mice at 10 weeks of age (day 0), a drill-hole injury was made with a diameter of 1mm at the anterior portion of the diaphysis of the bilateral femurs. In VAD, retardation in repairing the drill-hole was demonstrated by in vivo micro-CT and histomorphometry from day 7 and after surgery. During repair of the bone defect, increases of bmp2, dlx5, msx2, col1a1, and osteocalcin mRNA expression were suppressed, and runx2-p2 mRNA expression was accelerated in VAD. Implantation of BMP2 in the bone defect of VAD normalized the delayed bone healing and mRNA expressions. We concluded that vitamin A regulates bmp2 mRNA expression and plays a crucial role in osteoblastogenesis and bone formation.

Function of OPG as a traffic regulator for RANKL is crucial for controlled osteoclastogenesis.

The amount of the receptor activator of NF-κB ligand (RANKL) on the osteoblastic cell surface is considered to determine the magnitude of the signal input to osteoclast precursors and the degree of osteoclastogenesis. Previously, we have shown that RANKL is localized predominantly in lysosomal organelles, but little is found on the osteoblastic cell surface, and consequently, the regulated subcellular trafficking of RANKL in osteoblastic cells is important for controlled osteoclastogenesis. Here we have examined the involvement of osteoprotegerin (OPG), which is currently recognized as a decoy receptor for RANKL, in the regulation of RANKL behavior. It was suggested that OPG already makes a complex with RANKL in the Golgi apparatus and that the complex formation is necessary for RANKL sorting to the secretory lysosomes. It was also shown that each structural domain of OPG is indispensable for exerting OPG function as a traffic regulator. In particular, the latter domains of OPG, whose physiologic functions have been unclear, were indicated to sort RANKL molecules to lysosomes from the Golgi apparatus. In addition, the overexpression of RANK-OPG chimeric protein, which retained OPG function as a decoy receptor but lost the function as a traffic regulator, inhibited endogenous OPG function as a traffic regulator selectively in osteoblastic cells and resulted in the upregulation of osteoclastogenic ability despite the increased number of decoy receptor molecules. Conclusively, OPG function as a traffic regulator for RANKL is crucial for regulating osteoclastogenesis at least as well as that as a decoy receptor.

Administration of the bisphosphonate zoledronic acid during tooth development inhibits tooth eruption and formation and induces dental abnormalities in rats.

Bisphosphonates (BPs) are potent inhibitors of osteoclastic bone resorption and widely used for the treatment of osteoporosis and metastatic bone diseases. Recently, BPs have also been shown to benefit children with primary and secondary osteoporosis, including osteogenesis imperfecta; however, their long-term safety has not been established yet. Clinical and experimental studies have demonstrated that BPs delay or inhibit tooth eruption. The failure of tooth eruption causes several dental abnormalities. In this study, to determine the effects of BPs on tooth formation, the BP zoledronic acid (ZOL) was injected into 7- and 14-day-old rats, and the development of the mandibular teeth was examined. X-ray analysis demonstrated that ZOL inhibited the eruption of both incisors and molars and their formation, especially in the molar roots. Histological examination showed that, in ZOL-treated animals, alveolar bone remained unresorbed around tooth crowns, which injured ameloblasts and enamel matrix, leading to defects of the enamel. Furthermore, haphazard proliferation of odontogenic epithelium and mesenchyme associated with primitive tooth structures, which resembles human odontomas, was induced at the basal end of incisors but not around the molars. Tooth ankylosis to alveolar bone was occasionally observed in molars. These results suggest that administration of BPs during tooth development has the potential to inhibit tooth eruption and formation and to induce several types of dental abnormalities, which may be attributed to the altered osteoclastic activities.