The molecule mechanism of the bone mass regulation by a bone cell network

骨细胞通过骨突和骨小管形成网络。骨细胞有刺激或抑制骨形成和抑制骨吸收的报道。由于这些功能是通过骨细胞死亡后的骨变化来估计的，因此必须考虑坏死后微环境中的炎症反应。骨细胞网络由于其理想的解剖特性被认为是一种机械传感器和机械转导系统。我们建立了两系成骨细胞特异性BCL2转基因小鼠。出乎意料的是，BCL2在成骨细胞中的过度表达最终导致骨细胞凋亡。骨细胞突数减少，随着凋亡结构改变逐渐死亡，4月龄时死亡骨细胞达到75%。我们发现了一种新的机械应力负责分子，丙酮酸脱氢酶激酶4(Pdk4)，在卸载条件下，在骨细胞功能被破坏的BCL2转基因小鼠中，其表达上调。Pdk4^<-/->小鼠骨发育正常并得到维持。然而，在卸载时，野生型小鼠由于破骨细胞生成和Rankl表达增强而骨量减少，而Pdk4^<-/->小鼠则没有。在M-CSF和RANKL存在下，Pdk4^<-/->骨髓源性单核/巨噬细胞谱系细胞（BMMs）的破骨细胞分化受到抑制，野生型BMMs和Pdk4^<-/->成骨细胞共培养时，破骨细胞发生受损，其中RANKL的表达和启动子活性降低。此外，将Pdk4引入Pdk4^<-/-> BMMs和成骨细胞可增强破骨细胞生成和Rankl表达，并激活Rankl启动子。这些发现表明，Pdk4通过促进破骨细胞的发生，在卸载时骨丢失中起重要作用。

Osteocytes form a network through processes and canaliculi throughout bone. Osteocytes have been reported to stimulate or inhibit bone formation and to inhibit bone resorption. As these functions have been estimated from the bone changes after osteocyte death, however, an inflammatory reaction in the microenvironment after necrosis has to be considered. The osteocyte network is thought to be a mechanosensor and mechanotransduction system due to its ideal anatomical feature. We established two lines of osteoblast-specific BCL2 transgenic mice. Unexpectedly, overexpression of BCL2 in osteoblasts eventually caused osteocyte apoptosis. Osteocytes, which had a reduced number of processes, gradually died with apoptotic structural alterations, and dead osteocytes reached 75% at 4 months of age. We identified a novel mechanical stress-responsible molecule, pyruvate dehydrogenase kinase 4(Pdk4), whose expression was upregulated in osteoblasts at the unloaded condition, using BCL2 transgenic mice with the disrupted osteocyte function. Bone in Pdk4^<-/-> mice developed normally and was maintained. At unloading, however, bone mass was reduced due to enhanced osteoclastogenesis and Rankl expression in wild-type mice but not in Pdk4^<-/-> mice. Osteoclast differentiation of Pdk4^<-/-> bone marrow-derived monocyte/ macrophage lineage cells(BMMs) in the presence of M-CSF and RANKL was suppressed, and osteoclastogenesis was impaired in the coculture of wild-type BMMs and Pdk4^<-/-> osteoblasts, in which Rankl expression and promoter activity were reduced. Further, introduction of Pdk4 into Pdk4^<-/-> BMMs and osteoblasts enhanced osteoclastogenesis and Rankl expression and activated Rankl promoter. These findings indicate that Pdk4 plays an important role in bone loss at unloading by promoting osteoclastogenesis.