Role of IGF-I/integrin signaling in the periosteal response to load

 描述（由申请方提供）：骨基质卸载导致骨形成停止和骨吸收初始增加。这些变化伴随着对胰岛素样生长因子1（IGF1）对骨的合成代谢作用的抵抗。另一方面，骨骼负荷增加IGF1的产生，并增强IGF1对IGF1R的激活，作为其增加骨形成的机制的一部分。在寻找IGF 1信号传导的负荷诱导调节的机制时，我们发现骨骼卸载与整合素表达的减少有关，特别是具有b1（IGTB 1）和b3（ITGB 3）亚基的整合素。我们发现，在成骨细胞中，IGF 1增加了ITGB 3与IGF 1R的结合，如果ITGB 3下调，IGF 1就不能再激活IGF 1R。粘着斑激酶（FAK）和/或其相关家族成员蛋白酪氨酸激酶2 β（PTK2B）在整联蛋白和生长因子受体途径（包括ITGB 3和IGF1R之间）之间提供联系。IGF1激活IGF1R导致FAK的磷酸化（假定激活），而FAK的抑制则通过IGF1或负荷阻断IGF1R的激活。为了直接在体内测试IGF1信号传导的作用，我们开发了成熟成骨细胞中IGF1R缺失的小鼠，并检查了这种小鼠是否会对骨骼卸载或重新加载做出反应。缺乏IGF1R的小鼠在卸载过程中表现出与对照组相当的骨形成减少，但在重新加载过程中未能增加骨形成。惊人的结果是， 这种对再负荷的反应失败仅见于骨膜骨形成;骨内膜和小梁骨形成的反应与对照组相当。这些结果将我们的注意力集中在骨膜上，其中骨祖细胞不仅暴露于从骨细胞和肌肉产生的负荷诱导的IGF 1，而且暴露于整联蛋白配体骨膜蛋白，我们假设这种组合将使它们的增殖、分化和新骨的形成响应于机械负荷而最大化。在这个项目中，我们将测试的假设，骨骼负荷刺激IGF1的生产在骨细胞和肌肉和形成的IGF1R/整合素复合物在骨膜骨祖细胞（pOP）所需的IGF1介导的合成代谢反应，这些细胞的负荷。目的1--确定体外骨膜细胞中响应于负荷和/或IGF 1而形成的IGF 1R复合物的组分，并评估它们在促成该响应中的作用;目的2--确定体内从pOP中删除Igf 1r和Itgb 3对它们介导骨骼对负荷的响应的能力的影响;目的3-确定IGF 1的来源，促进负荷诱导的骨膜骨形成，特别注意骨细胞和肌肉。我们将利用新的动物模型和最先进的技术来实现这些目标。结果将提供 IGF1/整合素信号相互作用如何调节骨骼对机械负荷的反应的新认识，为进一步研究开辟了新途径，并为预防固定和衰老引起的骨丢失提供了潜在的靶点。

 DESCRIPTION (provided by applicant): Skeletal unloading results in a cessation in bone formation and an initial increase in bone resorption. These changes are accompanied by resistance to the anabolic actions of insulin like growth factor 1 (IGF1) on bone. On the other hand skeletal loading increases IGF1 production, and enhances IGF1 activation of IGF1R as part of the mechanism by which it increases bone formation. In seeking a mechanism for the load induced regulation of IGF1 signaling we discovered that skeletal unloading was associated with a decrease in the expression of integrins, in particular integrins with b1 (IGTB1) and b3 (ITGB3) subunits. We discovered that in osteoblasts, IGF1 increased the binding of ITGB3 to IGF1R, and that if ITGB3 were downregulated, IGF1 could no longer activate IGF1R. Focal adhesion kinase (FAK) and/or its related family member protein tyrosine kinase 2 beta (PTK2B) provide a link between the integrin and growth factor receptor pathways including between ITGB3 and IGF1R. IGF1 activation of IGF1R results in phosphorylation (presumed activation) of FAK, whereas inhibition of FAK blocks activation of IGF1R either by IGF1 or by load. To test the role of IGF1 signaling directly in vivo we developed mice in which the IGF1R was deleted in mature osteoblasts and examined whether this mouse would respond to skeletal unloading or reloading. Mice lacking IGF1R showed an equivalent decrease in bone formation during unloading to controls, but failed to increase bone formation during reloading. The striking result, however, was that this failure to respond to reloading was found only in periosteal bone formation; endosteal and trabecular bone formation responded comparable to controls. These results focused our attention on the periosteum where the osteoprogenitors are exposed not only to load induced IGF1 emanating from both osteocytes and muscle, but to the integrin ligand periostin, a combination we hypothesize will maximize their proliferation, differentiation, and formation of new bone in response to mechanical load. In this project we will test the hypothesis that skeletal loading stimulates IGF1 production in osteocytes and muscle and formation of the IGF1R/integrin complex in periosteal osteoprogenitors (pOP) required for the IGF1 mediated anabolic response of these cells to load. This will be achieved in the following three aims: Aim1--Determine the components of the IGF1R complex that forms in response to load and/or IGF1 in periosteal cells in vitro, and assess their role in contributing to that response; Aim 2-- Determin the impact of deleting Igf1r and Itgb3 from pOP in vivo with respect to their ability to mediate th skeletal response to load; Aim 3--Determine the source(s) of IGF1 facilitating load induced periosteal bone formation with particular attention to osteocytes and muscle. We will be utilizing novel animal models and state of the art techniques to fulfill these aims. The results will provide new understanding of how IGF1/integrin signaling interactions regulate the skeletal response to mechanical load, open up new avenues for further investigation, and provide potential targets for preventing the bone loss of immobilization and aging.