Neurotrophin mechanisms in neural development and disease

描述(申请人提供)：发育神经生物学的一个基本问题是，在神经系统发育过程中，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调控神经元存活、轴突和树突生长以及突触发生的靶向发育线索的最佳例子之一。神经营养素研究中一个有趣的问题是，如此有限的一组神经营养素及其受体是如何触发大量生物学事件的。我们先前报道了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)，在交感神经系统发育过程中使用相同的TrkA受体来促进轴突生长的顺序阶段。NT-3由血管系统沿投射交感神经轴突的轨迹分泌，促进早期轴突的生长。然而，来自外周靶点的NGF是终末器官神经支配所必需的。一种常见的TrkA受体如何对两种不同的神经营养因子做出反应，以促进轴突生长的不同阶段？我们确定了一条钙调神经磷酸酶依赖的TrkA内吞途径，它对NGF-而不是NT-3依赖的营养功能是关键的。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的变化来影响神经元发育的各个方面。因此，这项提议的总体目标是检验这两种神经营养因子NGF和NT-3以不同方式调节TrkA信号和运输以促进交感神经轴突生长的不同阶段的假设。为此，我们将使用生化分析来确定TrkA信号通路，该通路允许靶源NGF和NT-3以不同方式激活交感神经元中的钙调神经磷酸酶。使用缺乏钙调神经磷酸酶或其下游靶点内细胞性GTP酶Dynamin1的突变小鼠，我们将检验这一假设，即在体内，TrkA受体的内吞是NGF特定需要的，而不是NT-3介导的交感轴突生长所必需的。我们还将确定TrkA内吞作用促进轴突生长的机制。总之，这些研究将为两个靶源性神经营养因子如何通过共同的TrkA受体发出信号以协同调节神经元发育过程中的轴突生长提供新的见解，并为研究这些轴突生长计划在介导成年神经元损伤或疾病后再生生长中的作用提供基础。

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.