Axon Degeneration and Apoptosis: Molecular Intersection of Two Distinct Pathways

描述(由申请人提供)：神经元具有激活通路的能力，这些通路可导致整个细胞因细胞凋亡而退化，或仅引起轴突退化。轴突特异性变性在生理上很重要，因为它允许神经元移除过多的或被错误引导的轴突分支，并允许神经元连接的可塑性。然而，神经元如何激活和分割这条退化的路径，以在不危及细胞其余部分的情况下摧毁其轴突，目前尚不清楚。这一点特别有趣，因为最近的研究发现，Bax是细胞凋亡途径中的关键蛋白，也可以调节轴突特异性退化。我们的目标是关注细胞凋亡和轴突特异性退变途径之间的分子交集。首先，我们建立了一个基于微流控小室的交感神经元模型，在该模型中，从轴突间室剥夺神经生长因子(NGF)只会诱导轴突特异性退化，而从轴突和胞体间室剥夺NGF会诱导细胞凋亡。其次，我们发现细胞凋亡和轴突特异性退变之间有很大的重叠，但也发现了明显和意想不到的差异。例如，虽然细胞凋亡需要APAF-1和Caspase-9(CASP9)，但我们发现轴突退化需要CASP9，但令人惊讶的是，APAF-1不需要。我们的结果还表明，神经元能够选择性地进行凋亡或轴突特异性变性，因为成熟的神经元完全限制了凋亡，但对轴突变性仍然是允许的。在这项提议中，我们将研究神经元调节轴突特异性变性的几种新机制。我们将特别关注确定Bax在这一途径中是如何激活的(目标1)，研究CASP9独立于APAF-1激活的机制(目标2)，并探索成熟神经元选择性地限制凋亡但允许轴突变性的机制(目标3)。这些研究无疑将揭示神经元如何利用许多相同的成分进行细胞凋亡和轴突选择性退化的关键方面，但使用不同的机制来允许对这些通路的激活进行精确的时空控制。

DESCRIPTION (provided by applicant): Neurons have the capability to activate pathways that cause degeneration of either the entire cell by apoptosis or only the axons. Axon-specific degeneration is physiologically important as it allows neurons to remove excessive or misguided axon branches and permit plasticity in neuronal connections. However, exactly how a neuron can activate and compartmentalize this degenerative pathway to destroy its axon without putting the rest of the cell at risk is unclear. This is particularly interesting since recent studies idenified Bax, a key protein in the apoptosis pathway, to also regulate axon-specific degeneration. Our goal is to focus on the molecular intersection between apoptosis and axon-specific degeneration pathways. First, we established a microfluidic chamber-based model of sympathetic neurons where deprivation of nerve growth factor (NGF) from the axon compartment only induces axon-specific degeneration whereas NGF deprivation from the axon and soma compartments induces apoptosis. Second, we found substantial overlap between apoptosis and axon-specific degeneration but also identified distinct and unexpected differences. For example, while apoptosis required both Apaf-1 and Caspase-9 (Casp9), we found axon degeneration to require Casp9 but, surprisingly, not Apaf-1. Our results also show that neurons are exquisitely capable of selectively engaging apoptosis or axon-specific degeneration as mature neurons completely restrict apoptosis but remain permissive for axon degeneration. In this proposal, we will investigate several novel mechanisms by which neurons regulate axon-specific degeneration. We will focus specifically on determining how Bax is activated in this pathway (Aim 1), examining the mechanism by which Casp9 is activated independently of Apaf-1 (Aim 2), and probing the mechanism by which mature neurons selectively restrict apoptosis but remain permissive for axon degeneration (Aim 3). These studies will undoubtedly uncover critical aspects of how neurons utilize many of the same components for apoptosis and axon-selective degeneration but engage distinct mechanisms to allow precise spatial and temporal control over the activation of these pathways.