Neurotrophin mechanisms in neural development and disease

描述（由申请人提供）：发育神经生物学中的一个基本问题是，在神经系统发育期间，有限数量的生长因子如何协调精确神经元回路的建立。神经营养因子提供了一个最好的例子，目标衍生的发育线索调节神经元的生存，轴突和树突的生长和突触。神经营养因子研究中一个有趣的问题是，如此有限的一组神经营养因子及其受体如何触发大量的生物事件。我们以前报道过，两种不同的神经营养因子，神经生长因子（NGF）和神经营养因子-3（NT-3），采用相同的TrkA受体，以促进交感神经系统发育过程中轴突生长的顺序阶段。NT-3从血管系统沿着投射交感神经轴突的轨迹分泌，促进早期轴突生长。然而，终末器官的最终神经支配需要来自外周靶的NGF。一个共同的TrkA受体如何响应两种不同的神经营养因子，以促进轴突生长的不同阶段？我们确定了一个钙调神经磷酸酶依赖的TrkA内吞途径，这是关键的神经生长因子，但不是NT-3依赖的营养功能。钙调神经磷酸酶是一种钙响应性磷酸酶，通过将细胞内钙水平的微小变化转化为形态和转录变化来影响神经元发育的各个方面。因此，本提案的总体目标是检验两种神经营养因子NGF和NT-3差异调节TrkA信号传导和运输以促进交感神经轴突生长的不同阶段的假设。为此，我们将采用生物化学测定来鉴定TrkA信号通路，其允许靶源性NGF和NT-3差异激活交感神经元中的钙调磷酸酶。使用缺乏钙调磷酸酶或其下游靶点，内吞GT3，dynamin 1的突变小鼠，我们将测试的假设，TrkA受体的内吞作用是专门需要的神经生长因子，但不是NT-3介导的交感神经轴突生长在体内。我们还将确定TrkA内吞促进轴突生长的机制。总之，这些研究将提供新的见解，两个目标衍生的神经营养因子信号如何通过一个共同的TrkA受体在神经元发育过程中协同调节轴突生长，以及提供解决这些轴突生长程序的作用的基础在成年神经元损伤或疾病后介导再生生长。

DESCRIPTION (provided by applicant): A fundamental question in developmental neurobiology is how a limited number of growth factors coordinate the establishment of precise neuronal circuits during nervous system development. Neurotrophins provide one of the best examples of target-derived developmental cues regulating neuronal survival, axonal and dendritic growth and synaptogenesis. An intriguing question in neurotrophin research is how a very large number of biological events are triggered by such a limited set of neurotrophins and their receptors. We previously reported that two different neurotrophins, Nerve Growth Factor (NGF) and Neurotrophin-3 (NT-3), employ the same TrkA receptor to promote sequential stages of axonal growth during sympathetic nervous system development. NT-3, secreted from the vasculature along the trajectory of projecting sympathetic axons, promotes early axon outgrowth. However, NGF derived from peripheral targets is required for final innervation of end- organs. How does a common TrkA receptor respond to two different neurotrophins to facilitate distinct phases of axon growth? We identified a calcineurin-dependent TrkA endocytic pathway that is critical for NGF-, but not NT-3-dependent trophic functions. Calcineurin is a calcium-responsive phosphatase that influences diverse aspects of neuronal development by translating small changes in intracellular calcium levels to morphological and transcriptional changes. Thus, the overall goal of this proposal is to test the hypothesis that the two neurotrophins, NGF and NT-3, differentially regulate TrkA signaling and trafficking to promote distinct stages of sympathetic axon growth. To this end, we will employ biochemical assays to identify TrkA signaling pathways that allow target-derived NGF and NT-3 to differentially activate calcineurin in sympathetic neurons. Using mutant mice lacking calcineurin or its downstream target, the endocytic GTPase, dynamin1, we will test the hypothesis that endocytosis of TrkA receptors is specifically required for NGF, but not NT-3-mediated sympathetic axonal growth in vivo. We will also define the mechanisms by which TrkA endocytosis promotes axonal growth. Together, these studies will provide novel insights into how two target-derived neurotrophins signal via a common TrkA receptor to cooperatively regulate axonal growth during neuronal development, as well as provide the foundation for addressing the role of these axonal growth programs in mediating regenerative growth of adult neurons following injury or disease.