Does phagocytosis by astrocytes mediate synapse elimination?

描述(申请人提供)：突触是如何消除的？为了实现精确的神经连接并形成成熟的神经电路，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，消除过程在发育过程中塑造神经回路以及在成年后调节突触可塑性以回应经验和记忆是必不可少的。此外，某些神经退行性疾病，如阿尔茨海默病(AD)，与疾病早期突触的严重丢失有关，这突显了了解控制突触丢失的机制的重要性。此前对哺乳动物、苍蝇和蠕虫的研究表明，胶质细胞通过一种称为吞噬作用的吞噬过程，在清除凋亡神经元和退化轴突方面发挥核心作用。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，通过基因图谱，我们发现了几条吞噬细胞通路--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清除外节的MERTK通路--在发育中的和成年的中枢神经系统中由小鼠星形胶质细胞特异性地高表达。虽然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，但我们的发现表明，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具有高吞噬活性来清除轴突碎片。在这一应用中，我们将检验这一假说，星形胶质细胞吞噬神经碎片和突触是通过星形胶质细胞在体外和体内表达的MEGF10和/或MERTK吞噬细胞受体途径。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过程中的作用。在第二个目标中，我们将使用星形胶质细胞特异性条件性基因敲除小鼠来确定星形胶质细胞是否参与了发育中和成年中枢神经系统中突触的消除。这些研究有可能为突触正常消除的方式、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防止神经疾病中的突触丢失提供新的线索。 与公共健康相关：为了实现精确的神经连接并形成成熟的神经回路，预先形成的轴突和突触需要广泛重塑，但对相关机制了解甚少。在这项研究中，我们关注的假设是星形胶质细胞通过MEGF10和MerTK通路在发育中和成人脑中主动吞噬突触。对突触塑造的更好理解可能会有助于更好地理解电路是如何形成的，学习和记忆的基础是成人突触的可塑性，以及为什么突触在阿尔茨海默病等神经退行性疾病中丢失。

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.