Investigation of action of shear stress, which is produced by Mechanical loading applied to the skeleton, on osteodast formation and bone-resorbing activity

背景：施加于骨骼的机械载荷对骨骼完整性和结构的发育和维持至关重要。由于长时间的固定或太空失重，机械负荷的减少会降低骨形成率，导致骨质流失。另一方面，机械负荷的增加导致骨密度的增加。因此，骨组织对机械刺激很敏感。骨的机械载荷产生细胞外基质变形和流体流动，机械刺激分别转化为机械应变和流体剪切应力等机械信号。从体外研究中获得的证据表明，嵌入腔隙/小管系统的骨细胞和衬在骨表面的成骨细胞和骨细胞是机械传感器，可以检测负载引起的机械刺激。通过这些骨形成细胞，机械刺激被转化为细胞信号因子。然而，正常的骨代谢更多的是通过骨重塑来实现的，在骨形成细胞和骨吸收细胞之间保持平衡。迄今为止，流体剪切应力对成骨细胞的影响已被广泛研究；然而，对负责骨吸收的破骨细胞的作用仍有待阐明。因此，本研究的目的是阐明流体剪切应力直接或间接通过成骨细胞对破骨细胞发生和破骨细胞功能的影响。方法：我们从小鼠颅骨中分离成骨细胞，用单一的流动系统对这些细胞施加流体剪切应力。在破骨细胞形成的研究中，我们采用M-CSF依赖性骨髓细胞衍生的破骨细胞祖细胞体外培养系统，研究依赖RANKL的破骨细胞形成。结果和讨论：分离的小鼠颅骨来源的成骨细胞在长期培养中能够形成结节和钙化。当成骨细胞暴露于流体剪切应力时，Cox-2和RANKL mrna的表达早在暴露开始后2h就增加了。此外，RANKL的可溶性诱饵受体OPG的基因表达也在流体剪切应力作用下增加；但RANKL表达的增加幅度小于RANKL表达的增加幅度。低浓度RANKL诱导的低浓度剪切应力加载成骨细胞的条件培养基增加了破骨细胞的形成，而无应力加载成骨细胞的条件培养基对破骨细胞的形成没有刺激作用。短期剪应力加载足以诱导RANKL基因的表达，即使在解除加载后，这种诱导也能维持。剪应力诱导的RANKL表达依赖于Erk的激活，而不依赖于剪应力诱导的前列腺素。最后，流体剪切应力直接刺激M-CSF和低剂量RANKL处理的破骨细胞前体形成破骨细胞。综上所述，流体剪切应力通过诱导成骨细胞中RANKL的表达直接或间接地刺激破骨细胞的形成。少

Background : Mechanical loading applied to the skeleton is crucial to the development and maintenance of bone integrity and architecture. A decrease in the mechanical loading due to prolonged immobilization or weightlessness in space reduces the bone formation rate, resulting in bone loss. On the other hand, an increase in mechanical loading causes a gain in bone density. Thus, bone tissue is sensitive to mechanical stimulation. Mechanical loading on bone generates extracellular matrix deformation and fluid flow, and the mechanical stimuli are translated to mechanical signals such as mechanical strain and fluid shear stress, respectively. Evidence obtained from in vitro studies indicates that osteocytes embedded in the lacunae/canaliculi system and osteoblasts and bone cells lining the bone surface are mechanosensors that detect load-derived mechanical stimuli. By these bone-forming cells, the mechanical stimuli are translated into cellular signaling factors. However, normal bone metab … More olism is achieved by bone remodeling on balance between bone-forming cells and bone-resorbing cells. To date, the effect of fluid shear stress on osteoblasts has been extensively investigated ; however, the action on osteoclasts responsible for bone resorption remains to be elucidated. Therefore, the aim of this study is to elucidate the effect of fluid shear stress on osteoclastogenesis and osteoclast function directly or indirectly via osteoblasts.Methods : We isolated osteoblastic cells from mouse calvaria, and loaded fluid shear stress on these cells with a single flow-through system. In the case of study on osteoclast formation, we employed in vitro culture system of M-CSF dependent bone marrow cells-derived osteoclast progenitors for osteoclast formation dependent on RANKL. Results and Discussion : Isolated mouse calvalia-derived osteoblasts were capable of forming nodules and calcifing in a long-term culture. When the osteoblasts were exposed to fluid shear stress, expression of Cox-2 and RANKL mRNAs was increased as early as 2h after the start of exposure. In addition, gene expression of OPG, a soluble decoy receptor of RANKL, was also increased by the fluid shear stress in the cells ; however, the increase was less than that of RANKL expression. Conditioned medium obtained from the culture of shear stress-loading osteoblasts increased osteoclast formation induced by low concentration of RANKL, whereas the conditioned medium from the stress-unloading osteoblasts did not stimulated the formation. Short-term shear stress loading was sufficient for induction of RANKL gene expression, and the induction maintained even after freeing the loading. The shear stress-induced RANKL expression was dependent on the activation of Erk, but independent of prostaglangin induced by the shear stress. Finally, fluid shear stress directly stimulated the osteoclast formation from osteoclast precursors that were treated with M-CSF and low doses of RANKL. Taken together, fluid shear stress stimulates osteoclastogenesis directly or indirectly via inducing RANKL expression in osteoblasts. Less