钙敏感受体（CaSR）在力学负荷减少致废用性骨质疏松中的作用和分子机制研究

As the aging population problem is deepened in China, osteoporosis, especially the mechanical unloading induced immobilized osteoporosis is attracting the attention of researchers and the society. Immobilized osteoporosis, also called disuse osteoporosis, is caused by the less activity, long term in bed, which are common in aged people. However, the molecular mechanism of the mechanical unloading induced immobilized osteoporosis is unclear. Calcium-sensing receptor (CaSR) plays an important role in regulating calcium metabolism and maintaining the calcium homeostasis. Our previous study proved that activating mutation of CaSR mice model exhibited high bone mineral density, promoted osteoblast differentiation and enhanced bone mineralization. MAPK (mitogen-activated protein kinase)-ERK (extracellular regulated protein kinases) signaling pathway is reported to regulate osteogenic differentiation and osteoblast maturation. Phosphorylated ERK could bind to the promoter regions of Osteocalcin, which is essential gene for osteoblast differentiation. Thus, we hypothesize that activated CaSR in osteoblast may directly enhance the osteoblast and mineralization via MAPK-ERK signaling pathway, and regulate the mechanical loading associated bone formation. In this study, we plan to use the tail suspension mice model to mimic the mechanical unloading induced immobilized osteoporosis. We focus on the role of CaSR in the process of mechanical unloading induced bone loss, as well as its regulatory mechanism. In addition, we study the effect of CaSR on osteoblast differentiation, function, and mineralization through MAPK-ERK signaling pathway on molecular level, in order to prove innovative theory and new approach to therapy of immobilized osteoporosis.

随着社会老龄化加剧，骨质疏松，尤其是老年人因活动减少、长期卧床引起骨组织力学负荷减少相关的废用性骨质疏松已成为焦点问题，然而其发生的分子机制不清。钙敏感受体（CaSR）对调节和维持体内钙平衡有重要作用。我们的前期研究发现，CaSR激活突变小鼠表现为骨密度增高，成骨细胞分化及钙化能力增强。已经证实MAPK-ERK通路能调节骨形成基因Osteocalcin表达，促进成骨细胞分化。据此我们推测，激活成骨细胞表面的CaSR可能通过MAPK-ERK通路直接促进成骨细胞分化和钙化，调节力学负荷相关的骨形成。本研究拟使用鼠尾悬吊模型，模拟骨组织机械应力减少致骨量减低引起废用性骨质疏松，研究此过程中CaSR所扮演的角色，阐明CaSR通过MAPK-ERK通路直接调控成骨细胞分化、功能的分子机制，并探讨其受体拮抗剂、抑制剂对成骨细胞分化的作用，为废用性骨质疏松的发生提供新的理论依据，并找寻针对性治疗的新方向。

随着社会老龄化的加剧，骨质疏松症的发病率逐年攀升，成为威胁人们健康和生活质量的公共卫生问题。钙敏感受体（CaSR）在钙磷代谢中扮演重要角色，并参与骨代谢的调节。本课题探讨了CaSR在骨量维持和骨强度等方面起到的重要作用，研究钙敏感受体拮抗剂calcilytic对增加小鼠骨密度的作用及调节机制，并探讨calcilytic对骨质疏松小鼠模型的保护作用。实验结果证实，calcilytic可明显增加小鼠骨密度，增加骨形成标志物、骨形成转录因子Bglap2、Runx2的表达，较高钙高维生素D补充治疗组对小鼠骨密度有显著提升，而对骨吸收标志物Trap无明显影响。骨组织形态学分析结果显示，calcilytic除提升骨密度外，还可以改善骨转换水平，促进骨形成速度，改善骨组织的重建。较高浓度calcilytic可与外源性PTH（1-34）达到相近的增加骨密度效果，且不增加皮质骨多孔化的发生。此外，calcilytic对血钙的波动作用较小，并可减少尿钙排泄，减少肾结石的风险。我们应用CaSR激活突变小鼠进行验证，探讨基因突变小鼠的骨代谢异常表现，以及骨强度和皮质骨多孔化形成程度，并针对性研究高龄小鼠动物模型的骨代谢特点。CaSR激活突变小鼠表现出显著降低的骨强度，并随年龄增加，老龄小鼠的骨强度下降更为明显。CaSR拮抗剂calcilytic长期治疗可以显著改善骨强度、骨韧性。骨强度和骨骼微细结构表现的微小骨折与骨折发生风险显著相关。CaSR激活突变小鼠经calcilytic治疗后表现为微小骨折发生的减少。本研究探讨钙敏感受体在骨代谢和废用性骨质疏松发生中的调节作用，揭示了钙敏感受体在小鼠骨密度维持、骨转换过程、骨强度和骨韧性的调节过程中起到的重要角色，并研究了CaSR拮抗剂calcilytic对废用性骨质疏松的保护作用，证实Calcilytic对骨密度的增加、骨转换的改善和骨形成的促进方面的重要作用，丰富了CaSR对骨代谢调节的理论基础。本研究还证实calcilytic可以增加骨强度，对抗微小骨折的发生。通过以上结论可以推测，calcilytic可能作为抗骨质疏松的治疗药物，同时不增加小鼠的尿钙水平，不引起血钙波动，具有较好的安全性，为临床骨质疏松症的治疗提供新的思路和启示。