Mechanisms of natural phosphatidylserine exposure in the nervous system

项目概要/摘要 功能性神经系统需要支持细胞主动维持神经元结构。一些 支持细胞是吞噬细胞，通过检测"吃我"来吞噬不必要的或受损的神经元部分。 暴露在神经元膜上的信号磷脂酰丝氨酸（PS）已被证明是一种有效的神经元调节剂， Eat-me信号，在许多神经退行性事件中介导神经元的吞噬作用。通常隐藏在 在质膜的内小叶上，这种磷脂特异性地暴露在细胞膜的退化部分上。 神经元以诱导吞噬作用。然而，尚不清楚健康的神经元是否在其表面暴露PS 与体内吞噬作用无关。使用体内系统可视化外周血PS暴露 在感觉神经元中，发现果蝇中的某些神经元在它们的树突上自然地暴露PS， 空间上限定的模式而不诱导吞噬作用。这一发现提出了两个具体和重要的 问题：（1）PS暴露如何在空间上维持在这些树突上？(2)是什么阻止了这些PS- 暴露树突以免被吞噬细胞吞噬本提案的总体目标是 研究这种意外的PS暴露在健康神经元上的机制原因，并阐明如何 这些暴露于PS的神经元被保护免于吞噬作用诱导的变性。回答这些 问题是朝着我们阐明神经元-吞噬细胞相互作用的长期目标迈出的重要一步。 神经系统的发育、维持和退化。为了实现这些目标， 本研究的主要目的有两个：（1）阐明树突上天然PS暴露的机制。 具体地，内源性PS翻转酶ATP8A将在特定神经元中被荧光标记以研究 翻转酶沿沿着树突的分布是否定义了天然PS暴露的模式。此外该 树突激发和膜电位在自然PS暴露中的作用将被检查。(2)定义 调节树突对PS诱导的变性的易感性的途径。一个试验性的RNAi屏幕显示， 隔膜连接组分和Deg/ENaC钠通道参与调节阻力 暴露于PS的树突被吞噬。因此，将测试隔膜连接蛋白是否保护PS-1。 暴露树突被吞噬以及神经元活动是否促进PS相关的变性。 总之，这些目标将定义天然神经元PS暴露的分子基础，并揭示新的 神经保护机制。在这个项目中获得的知识将是重要的理解如何 不同的神经元类型利用相同的吞噬信号以不同的方式与吞噬细胞相互作用。

PROJECT SUMMARY/ABSTRACT A functional nervous system requires active maintenance of neuronal structures by supporting cells. Some supporting cells are phagocytes that engulf unnecessary or damaged neuronal parts by detecting “eat-me” signals exposed on neuronal membranes. Phosphatidylserine (PS) has been shown to be a potent neuronal eat-me signal that mediates phagocytosis of neurons in many neurodegenerative events. Normally hidden in the inner leaflet of the plasma membrane, this phospholipid is exposed specifically on the degenerative parts of neurons to induce phagocytosis. However, it is unknown whether heathy neurons expose PS on their surface in ways unrelated to phagocytosis in vivo. Using an in vivo system to visualize PS exposure on peripheral sensory neurons, it was discovered that certain neurons in Drosophila naturally expose PS on their dendrites in spatially defined patterns without inducing phagocytosis. This finding raises two specific and important questions: (1) How is PS exposure spatially maintained on these dendrites? (2) What prevents these PS- exposing dendrites from being engulfed by phagocytes? The overall objectives of this proposal are to investigate the mechanistic causes of this unexpected PS exposure on healthy neurons and to elucidate how these PS-exposing neurons are protected against phagocytosis-induced degeneration. Answering these questions is an important step toward our long-term goal of elucidating neuron-phagocyte interactions in the development, maintenance, and degeneration of the nervous system. To achieve these objectives, the following two aims are proposed: (1) Elucidate the mechanisms underlying natural PS exposure on dendrites. Specifically, the endogenous PS flippase ATP8A will be fluorescently tagged in specific neurons to investigate whether the flippase distribution along dendrites defines the patterns of natural PS exposure. In addition, the roles of dendrite excitation and membrane potential in natural PS exposure will be examined. (2) Define the pathways that regulate susceptibility of dendrites to PS-induced degeneration. A pilot RNAi screen revealed the involvement of a septate junction component and a Deg/ENaC sodium channel in regulating the resistance of PS-exposing dendrites to phagocytosis. Thus, it will be tested whether septate junction proteins protect PS- exposing dendrites from being engulfed and whether neuronal activity promotes PS-related degeneration. Together, these aims will define the molecular basis of natural neuronal PS exposure and reveal new mechanisms of neuroprotection. The knowledge gained in this project will be important for understanding how different neuronal types utilize the same phagocytic signal to interact with phagocytes in diverse ways.