Molecular mechanism of osteocyte lacunar canalicular system on PTH and bone quality

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The aim of this research project is to clarify 1) osteocytic function of mineral regulation in bone, 2) effects of osteocytelacunar canalicular system onto bone quality, and 3) PTH action on the transport of bone mineral. (1) We generated a transgenic mouse expressing osteocytes-specific diphtheria toxin (DT) receptor. Following a injection of DT, 70%-80% of the osteocytes were dead, resulting in loss of bone minerals, intracortical porosity and microfractures (bone mineral regulation). This transgenic mouse was also resistant to unloading-induced bone loss, suggesting the osteocytic mechanotransduction. Both of bone mineral regulation and mechanotransduction by osteocytes appears to affect bone quality. (2) Using Schoen's silver staining, we have examined the distribution of the osteocytic lacunar canalicular system (OLCS) in mouse bones. Trabecular bones with high bone turnover displayed osteocytic cytoplasmic processes without any perceptible alignment. Also, many plump osteocytes w … More ere embedded in the mineralized bone in a disorderly manner. However, well-remodeled bone (epiphyseal trabecules and cortical bone), showed osteocytes with their spindle shape, organized so as to parallel the longitudinal axis of trabecular bone. Given the data gathered, the OLCS appears to assume an organized, probably function-related spatial distribution as normal bone remodeling goes on. OLCS seems to be a cellular factor to affect bone quality. (3) Finally, we have examined whether osteocytic osteolysis would occur after PTH administration. When injecting PTH, the size of osteocytic lacunae enlarged and the expression of sclerostin was reduced in the wild-type mice, but no such changes in the c-fos-/- mice in which osetoclastogenesis is totally inhibited. C-fos-/- mice showed disrupted OCLS, while wild-type mice had well-organized, functional OLCS, as described above. Thus, PTH signaling appears to be transduced from PTH receptor-bearing osteoblasts into osteocytes by means of functional OLCS, resulting in osteocytic ostelysis. Less