No abstract

Osteoclasts are multinucleated cells that resorb bone. Although osteoclasts originate from the monocyte/macrophage lineage, osteoclast precursors are not well characterized in vivo. The relationship between proliferation and differentiation of osteoclast precursors is examined in this study using murine macrophage cultures treated with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB (RANK) ligand (RANKL). Cell cycle–arrested quiescent osteoclast precursors (QuOPs) were identified as the committed osteoclast precursors in vitro. In vivo experiments show that QuOPs survive for several weeks and differentiate into osteoclasts in response to M-CSF and RANKL. Administration of 5-fluorouracil to mice induces myelosuppression, but QuOPs survive and differentiate into osteoclasts in response to an active vitamin D3 analogue given to those mice. Mononuclear cells expressing c-Fms and RANK but not Ki67 are detected along bone surfaces in the vicinity of osteoblasts in RANKL-deficient mice. These results suggest that QuOPs preexist at the site of osteoclastogenesis and that osteoblasts are important for maintenance of QuOPs.

Myeloid differentiation factor 88 (MyD88) plays essential roles in the signaling of the Toll/interleukin (IL)-1 receptor family. Toll–IL-1 receptor domain-containing adaptor inducing interferon-β (TRIF)-mediated signals are involved in lipopolysaccharide (LPS)-induced MyD88-independent pathways. Using MyD88-deficient (MyD88−/−) mice and TRIF-deficient (TRIF−/−) mice, we examined roles of MyD88 and TRIF in osteoclast differentiation and function. LPS, diacyl lipopeptide, and IL-1α stimulated osteoclastogenesis in cocultures of osteoblasts and hemopoietic cells obtained from TRIF−/− mice, but not MyD88−/− mice. These factors stimulated receptor activator of nuclear factor-κB ligand mRNA expression in TRIF−/− osteoblasts, but not MyD88−/− osteoblasts. LPS stimulated IL-6 production in TRIF−/− osteoblasts, but not TRIF−/− macrophages. LPS and IL-1α enhanced the survival of TRIF−/− osteoclasts, but not MyD88−/− osteoclasts. Diacyl lipopeptide did not support the survival of osteoclasts because of the lack of Toll-like receptor (TLR)6 in osteoclasts. Macrophages expressed both TRIF and TRIF-related adaptor molecule (TRAM) mRNA, whereas osteoblasts and osteoclasts expressed only TRIF mRNA. Bone histomorphometry showed that MyD88−/− mice exhibited osteopenia with reduced bone resorption and formation. These results suggest that the MyD88-mediated signal is essential for the osteoclastogenesis and function induced by IL-1 and TLR ligands, and that MyD88 is physiologically involved in bone turnover.

Osteoporosis remains a major public health problem through its associated fragility fractures. Several animal models for the study of osteoporotic bone loss, such as ovariectomy (OVX) and denervation, require surgical skills and several weeks to establish. Osteoclast differentiation and activation is mediated by RANKL. Here we report the establishment of a novel and rapid bone loss model by the administration of soluble RANKL (sRANKL) to mice. Mice were injected intraperitoneally with sRANKL and used to evaluate existing anti-osteoporosis drugs. sRANKL decreased BMD within 50 h in a dose-dependent manner. The marked decrease in femoral trabecular BMD shown by pQCT and the 3D images obtained by mCT were indistinguishable from those observed in the OVX model. Histomorphometry showed that osteoclastic activity was significantly increased in the sRANKL-injected mice. In addition, serum biochemical markers of bone turnover such as Ca, C-telopeptide of type 1 collagen (CTX), and TRACP5b were also significantly increased in the sRANKL-injected mice in a dose-dependent manner. Bisphosphonates (BPs), selective estrogen receptor modulators (SERMs), and PTH are commonly used for the treatment of osteoporosis. We successfully evaluated the effects of anti-bone-resorbing agents such as BPs, a SERM, and anti-RANKL-neutralizing antibody on bone resorption in a couple of weeks. We also evaluated the effects of PTH on bone formation in 2 wk. A combination of sRANKL injections and OVX made it possible to evaluate a SERM. The sRANKL model is the simplest, fastest, and easiest of all osteoporosis models and could be useful in the evaluation of drug candidates for osteoporosis.

Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L‐glutamate. Osteoclasts secrete L‐glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+‐dependent manner. KCl‐ and ATP‐dependent secretion of L‐glutamate was absent in osteoclasts prepared from VGLUT1−/− knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L‐glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1−/− mice develop osteoporosis. Thus, in bone‐resorbing osteoclasts, L‐glutamate and bone degradation products are secreted through transcytosis and the released L‐glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

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.

No abstract

This study aimed to investigate the behavior and ultrastructure of osteoblastic cells after intermittent PTH treatment and attempted to elucidate the role of osteoclasts on the mediation of PTHdriven bone anabolism. After administering PTH intermittently to wildtype and c-fos 2/2 mice, immunohistochemical, histomorphometrical, ultrastructural, and statistical examinations were performed. Structural and kinetic parameters related to bone formation were increased in PTH-treated wildtype mice, whereas in the osteoclast-deficient c-fos 2/2 mice, there were no significant differences between groups. In wildtype and knockout mice, PTH administration led to significant increases in the number of cells double-positive for alkaline phosphatase and BrdU, suggesting active pre-osteoblastic proliferation. Ultrastructural examinations showed two major pre-osteoblastic subtypes: one rich in endoplasmic reticulum (ER), the hypER cell, and other with fewer and dispersed ER, the misER cell. The latter constituted the most abundant preosteoblastic phenotype after PTH administration in the wildtype mice. In c-fos 2/2 mice, misER cells were present on the bone surfaces but did not seem to be actively producing bone matrix. Several misER cells were shown to be positive for EphB4 and were eventually seen rather close to osteoclasts in the PTH-administered wildtype mice. We concluded that the absence of osteoclasts in c-fos 2/2 mice might hinder PTH-driven bone anabolism and that osteoclastic presence may be necessary for full osteoblastic differentiation and enhanced bone formation seen after intermittent PTH administration.