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

本研究的目的是阐明1)骨中矿物质调节的骨细胞功能，2)骨细胞微管系统对骨质量的影响，3)甲状旁腺素对骨矿物质运输的作用。(1)建立了表达骨细胞特异性白喉毒素受体的转基因小鼠。注射DT后，70%-80%的骨细胞死亡，导致骨矿物质丢失、皮质内孔洞和微骨折(骨矿物质调节)。这只转基因小鼠对卸载诱导的骨丢失也具有抵抗力，表明这是一种骨细胞机械转导。骨矿物质的调节和骨细胞的机械转导似乎都会影响骨质量。(2)用Schoen‘s银染法观察了骨细胞陷窝管系统(OLCS)在小鼠骨骼中的分布。骨转换率高的松质骨可见无明显排列的骨细胞胞浆突起。此外，许多肥大的骨细胞带有…更多的是以无序的方式嵌入矿化的骨中。然而，重塑良好的骨(骨骺小梁和皮质骨)显示骨细胞呈纺锤形，排列成平行于小梁骨纵轴的形式。根据收集的数据，随着正常骨重建的进行，OCs似乎呈现出有组织的、可能与功能相关的空间分布。OLCS似乎是一个影响骨质量的细胞因素。(3)观察PTH给药后是否会发生骨细胞性骨溶解。注射甲状旁腺素后，野生型小鼠骨细胞陷窝扩大，硬化素表达减少，而完全抑制破骨细胞发生的c-fos-/-小鼠则无此变化。如上所述，C-fos-/-小鼠表现出OCL被破坏，而野生型小鼠则有组织良好、功能正常的OCL。因此，PTH信号似乎通过功能性的OLCS从PTH受体的成骨细胞转导到骨细胞，导致骨细胞性骨质疏松。较少

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