Neuronal Regulation of Skeletal Development and Repair

 描述（由申请人提供）：在这个新项目中，我们将探索感觉神经在骨骼中的作用。与骨发育中血管生成作用的研究日益增多相反，尽管有大量间接证据表明神经支配在骨骼发育和修复中的重要性，但很少有研究调查神经在骨骼中的作用。这种知识差距部分是由于识别钙化组织中的神经纤维的困难以及缺乏用于研究骨骼神经支配的易处理的模型系统。为了克服这些限制，我们的实验室已经验证了小鼠模型，用于可视化和破坏支配骨骼的感觉神经的功能信号。使用这些模型，我们表明，表达原肌球蛋白受体激酶A（TrkA）的感觉神经元发出的背根神经节项目轴突神经生长因子（NGF）表达软骨膜细胞的发展中的肢体与早期矿化的相邻软骨相一致。在胚胎发育过程中消除这些感觉神经中的TrkA激酶活性会延缓轴突长入并减弱新生骨矿化。在胚胎发生过程中TrkA信号的中断减少了出生后的骨获取，并延缓了成年骨对实验性骨折的再生。这些观察结果以及下文描述的其他研究为骨发育、获取和修复中需要TrkA感觉神经提供了直接证据。在这个建议中概述的研究将探索通过感觉神经功能促进成骨的机制。我们的方法将测试的假设，即神经生长因子（NGF）产生的发育和受伤的肢体间充质激活TrkA感觉神经元，以促进其生存，并引导其轴突的初级骨化中心，以启动成骨细胞分化。该假设将在骨发育（目标1）的环境中进行测试，并响应于已建立的实验性骨修复模型（目标2）。我们相信，我们的项目将对感觉神经在健康人体中的作用产生新的见解，并最终为与某些骨骼疾病相关的神经病理学表现提供信息。

 DESCRIPTION (provided by applicant): In this new project we will explore the role of sensory nerves in bone. In contrast to increasing body of work on to the role of angiogenesis in bone development, few studies have investigated the role of nerves in bone despite a wealth of circumstantial evidence implicating the importance of innervation in skeletal development and repair. This knowledge gap is in part due the difficulty of identifying nerve fibers in calcified tissue and the lack of tractable model systems for study skeletal innervation. To overcome these limitations, our laboratory has validated mouse models for visualizing and disrupting functional signaling of sensory nerves that innervate the skeleton. Using these models we demonstrate that tropomyosin receptor kinase A (TrkA) expressing sensory neurons emanating from the dorsal root ganglion project axons to nerve growth factor (NGF) expressing perichondral cells of developing limbs coincident with incipient mineralization of adjacent cartilage. Elimination of TrkA kinase activity in these sensory nerves during embryogenesis retards axonal ingrowth and attenuates nascent bone mineralization. Disruption of TrkA signaling during embryogenesis reduces postnatal bone acquisition and retards the regeneration of adult bone in response to experimental fracture. These observations, and other studies described below, provide direct evidence for the requirement of TrkA sensory nerves in bone development, acquisition and repair. The studies outlined in this proposal will explore the mechanisms through which sensory nerves function to promote osteogenesis. Our approach will test the hypothesis that nerve growth factor (NGF) produced in the developing and injured limb mesenchyme activates TrkA sensory neurons to promote their survival and guide their axons to primary ossification centers to initiate osteoblast differentiation. This hypothesis will be tested in a setting of bone development (Aim 1) and in response to an established model of experimental bone repair (Aim 2). We believe that our project will yield new insights into the role of sensory nerves in healthy humans and will ultimately inform on the neuropathological manifestations associated with certain bone disorders.