Does phagocytosis by astrocytes mediate synapse elimination?

描述（由申请人提供）：如何消除突触？为了实现精确的神经连接并形成成熟的神经回路，神经系统需要进行广泛的重塑。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，消除过程在发育期间塑造神经回路以及调节突触可塑性以响应成年期的经验和记忆方面至关重要。此外，某些神经退行性疾病，如阿尔茨海默病（AD），与疾病过程早期的突触严重丧失有关，这强调了理解控制突触丧失的机制的重要性。对哺乳动物、苍蝇和蠕虫的研究已经证明，神经胶质细胞通过吞噬作用在清除凋亡神经元和变性轴突中发挥重要作用。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，我们发现几种吞噬途径--MEGF 10/德雷珀/ced 1途径介导果蝇的轴突修剪和MERTK途径介导视网膜色素上皮细胞的外节清除--在发育和成年CNS中的小鼠星形胶质细胞中特异性和高度表达。虽然已经假设哺乳动物CNS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消除多余的轴突、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具有清除轴突碎片的高吞噬活性。在本申请中，我们将检验星形胶质细胞通过体外和体内星形胶质细胞表达的MEGF 10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，我们将测试MEGF 10和MERTK在星形胶质细胞介导的神经碎片和突触的吞噬作用，通过使用体外吞噬试验的作用。在第二个目标中，我们将使用星形胶质细胞特异性条件性基因敲除小鼠来确定星形胶质细胞是否介导发育中和成年CNS内突触的消除。这些研究有可能揭示突触通常是如何消除的，在学习和记忆过程中突触消除和转换在正常成人CNS中发生的程度，以及如何预防神经系统疾病中的突触丢失。 公共卫生相关性：为了实现精确的神经连接并形成成熟的神经回路，需要对预先形成的轴突和突触进行广泛的重塑，但对负责的机制知之甚少。在这个建议中，我们专注于星形胶质细胞通过MEGF 10和Mertk通路积极吞噬发育中和成年大脑中的突触的假设。更好地理解突触雕刻可能会导致更好地理解电路是如何形成的，成人突触可塑性是学习和记忆的基础，以及为什么突触在神经退行性疾病（如阿尔茨海默病）中丢失。

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.