Mechanisms Neurotrophin Signaling Through P75 Receptor

描述（申请人提供）：神经元凋亡和轴突修剪是脊椎动物神经发育的正常部分；然而，这些过程的异常发生是许多神经病理的基础。神经元死亡和退化的一个关键调节因子是p75神经营养因子受体。P75是一种多功能信号蛋白，激活促进神经元存活、细胞凋亡、轴突变性和抑制轴突生长的通路。在哺乳动物发育过程中，它在调节许多神经群的凋亡中起着重要作用。此外，p75信号传导有助于各种神经损伤后的细胞死亡，并被认为在许多神经病变中看到的神经元丧失和变性中发挥作用。这项建议的总体目标是阐明p75介导神经退行性变的分子机制。不幸的是，受体如何转导其信号以及它们如何被差异调节以决定最终的细胞反应尚不清楚。我们最近证明，BDNF在交感神经元中激活p75导致c-Jun N末端激酶（JNK）的刺激，这导致受体蛋白水解，首先是金属蛋白酶TACE，然后是3-分泌酶。p75的裂解释放了其胞内结构域（ICD）和NRIF（一种DNA结合蛋白），从而促进了NRIF易位到细胞核，从而导致细胞凋亡。p75介导的神经元凋亡需要受体蛋白水解和NRIF核易位。有趣的是，p75和NRIF不仅存在于神经元胞体中，也存在于轴突中。此外，仅在交感神经元的远端轴突添加BDNF可触发体细胞死亡，这表明p75可以激活逆行凋亡信号。然而，如果神经元同时给予NGF，那么局部BDNF治疗导致轴突变性，而不是细胞凋亡。我们建议确定这种逆行死亡信号的性质及其产生的机制。具体来说，我们假设p75的凋亡信号涉及JNK的激活，JNK刺激受体的内吞作用和切割，导致涉及转录因子NRIF的逆行死亡信号。此外，我们提出阻止逆行信号导致局部轴突变性。我们将通过以下具体目的来解决这些假设：(1)确定NRIF是否在p75被凋亡配体激活时逆行转运。(2)确定p75内化和裂解在逆行凋亡信号传导中是否必要。(3)确定JNK在逆行信号传导中的作用。(4)判断逆行凋亡信号被阻断时是否发生轴突变性。这些研究的结果将阐明神经元死亡和轴突变性的一些不同机制，这两者都涉及许多神经病理学。

DESCRIPTION (provided by applicant): Neuronal apoptosis and axon pruning are a normal part of vertebrate neurodevelopment; however, abnormal occurrences of these processes are the basis for many neuropathologies. One key regulator of neuronal death and degeneration is the p75 neurotrophin receptor. P75 is a multifunctional signaling protein, activating pathways that promote neuronal survival as well as apoptosis, axonal degeneration and the inhibition of axon growth. It has an essential role in regulating apoptosis of many neural populations during mammalian development. In addition, p75 signaling contributes to cell death following a wide variety of neural insults and has been suggested to play a role in the neuronal loss and degeneration seen in a number of neuropathologies. The overall objective of this proposal is to elucidate the molecular mechanisms by which p75 mediates neurodegeneration. Unfortunately, how the receptor transduces its signals and how they are differentially regulated to determine the ultimate cellular response is not well understood. We recently demonstrated that activation of p75 by BDNF in sympathetic neurons results in the stimulation of c-Jun N- terminal kinase (JNK) and this causes receptor proteolysis, first by the metalloprotease TACE followed by 3- secretase. The cleavage of p75 releases its intracellular domain (ICD) along with NRIF, a DNA binding protein, thereby facilitating NRIF translocation to the nucleus, which leads to apoptosis. Both receptor proteolysis and NRIF nuclear translocation are required for p75-mediated apoptosis in neurons. Interestingly, p75 and NRIF are detected not only in the neuronal cell body, but also in axons. Furthermore, addition of BDNF exclusively to distal axons in sympathetic neurons triggered cell death back at the soma, indicating that p75 can activate a retrograde apoptotic signal. However, if the neurons were simultaneously given NGF, then local BDNF treatment resulted in axon degeneration, but not apoptosis. We propose to determine the nature of this retrograde death signal and the mechanisms by which it is generated. Specifically, we hypothesize that p75's apoptotic signal involves activation of JNK, which stimulates endocytosis and cleavage of the receptor, resulting in a retrograde death signal involving the transcription factor NRIF. Furthermore, we propose that preventing retrograde signaling results in local axon degeneration. We will address these hypotheses by the following specific aims: (1) Determine whether NRIF is retrogradely trafficked in response to p75 activation by apoptotic ligands. (2) Determine whether p75 internalization and cleavage are necessary for retrograde apoptotic signaling. (3) Determine the role of JNK in retrograde signaling. (4) Determine whether axonal degeneration occurs when the retrograde apoptotic signal is blocked. The results from these studies will elucidate some of the differential mechanisms underlying neuronal death and axonal degeneration, both of which are involved in many neuropathologies.