Does phagocytosis by astrocytes mediate synapse elimination?

描述（由申请人提供）：突触是如何消除的？为了实现精确的神经连接和形成成熟的神经回路，需要对神经系统进行广泛的改造。这些重塑过程包括消除多余的轴突、树突、突触及其碎片。越来越多的证据表明，消除过程在发育过程中形成神经回路以及在成年期对经验和记忆的反应中调节突触可塑性方面是必不可少的。此外，某些神经退行性疾病，如阿尔茨海默病（AD），在疾病过程的早期与突触的严重丧失有关，这强调了理解控制突触丧失机制的重要性。对哺乳动物、苍蝇和蠕虫的研究表明，神经胶质细胞通过吞噬作用在清除凋亡神经元和退化轴突中发挥核心作用。然而，在哺乳动物中驱动这些现象的细胞和分子机制仍然知之甚少。最近，通过基因分析，我们发现了几种吞噬通路——介导果蝇轴突修剪的MEGF10/draper/ced1通路和介导视网膜色素上皮细胞外段清除的MERTK通路——在发育和成年中枢神经系统中都被小鼠星形胶质细胞特异性和高度表达。尽管人们一直认为哺乳动物中枢神经系统中的小胶质细胞主要负责清除神经碎片，但我们的研究结果表明，哺乳动物星形胶质细胞可能积极参与清除多余的轴突、树突、突触及其碎片。在我们的初步研究中，我们发现星形胶质细胞在体外清除轴突碎片方面具有很高的吞噬活性。在本应用中，我们将在体外和体内验证星形胶质细胞通过表达的MEGF10和/或MERTK吞噬受体途径吞噬神经碎片和突触的假设。在第一个目标中，我们将通过体外吞噬实验来测试星形胶质细胞中MEGF10和MERTK在介导神经碎片和突触吞噬中的作用。在第二个目标中，我们将使用星形胶质细胞特异性条件敲除小鼠来确定星形胶质细胞是否介导发育和成年中枢神经系统内突触的消除。这些研究有可能揭示突触是如何正常消除的，正常成人中枢神经系统在学习和记忆过程中突触消除和更新的程度，以及如何预防神经系统疾病中的突触丧失。

DESCRIPTION (provided by applicant): How are synapses eliminated? To achieve precise neural connectivity and form mature neural circuits, the nervous system needs to be remodeled extensively. These remodeling processes include elimination of excess axons, dendrites, synapses and their debris. Growing evidence suggests that elimination processes are essential in shaping neural circuits during development as well as in regulating synaptic plasticity in response to experience and memory in adulthood. Moreover, certain neurodegenerative diseases, such as Alzheimer's disease (AD), are associated with a profound loss of synapses early in the disease process, underscoring the importance of understanding the mechanisms controlling synapse loss. Studies in mammals, flies and worms have previously demonstrated that glial cells play central roles in clearing apoptotic neurons and degenerating axons through an engulfment process called phagocytosis. However, the cellular and molecular mechanisms that drive these phenomena in mammals are still poorly understood. Recently by gene profiling, we have found that several phagocytic pathways--the MEGF10/draper/ced1 pathway that mediates axon pruning in flies and the MERTK pathway that mediates outer segment clearance by retinal pigment epithelial cells--are specifically and highly expressed by mouse astrocytes in both the developing and adult CNS. Although it has been assumed that microglia in the mammalian CNS are largely responsible for clearing neural debris, our findings suggest that mammalian astrocytes may actively participate in eliminating excess axons, dendrites, synapses and their debris. In our preliminary studies, we have found that astrocytes have high phagocytic activity in clearing axonal debris in vitro. In this application, we will test the hypothesis that astrocytes phagocytose neural debris and synapses through the MEGF10 and/or MERTK phagocytic receptor pathways expressed by astrocytes in vitro and in vivo. In the first aim, we will test the roles of MEGF10 and MERTK in astrocytes in mediating phagocytosis of neural debris and synapses by using an in vitro engulfment assay. In the second aim, we will use astrocyte-specific conditional knockout mice to determine whether astrocytes mediate the elimination of synapses within the developing and adult CNS. These studies have the potential to shed new light onto how synapses are normally eliminated, the extent to which synapse elimination and turnover occurs in normal adult CNS during learning and memory, and on how synapse loss in neurological diseases can be prevented. PUBLIC HEALTH RELEVANCE: To achieve precise neural connectivity and form mature neural circuits, pre-formed axons and synapses need to be remodeled extensively but the mechanisms responsible are poorly understood. In this proposal we focus on the hypothesis that astrocytes actively phagocytose synapses in developing and adult brains by the MEGF10 and Mertk pathways. A better understanding of synapse sculpting may lead to improved understanding of how circuits are formed, the adult synaptic plasticity that underlies learning and memory, and why synapses are lost in neurodegenerative diseases such as Alzheimer's disease.