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.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Exploratory/Developmental Grants (R21)
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Special Emphasis Panel (ZRG1-MDCN-T (02))
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Talley, Edmund M
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Stanford University
Schools of Medicine
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Chung, Won-Suk; Verghese, Philip B; Chakraborty, Chandrani et al. (2016) Novel allele-dependent role for APOE in controlling the rate of synapse pruning by astrocytes. Proc Natl Acad Sci U S A 113:10186-91
Chung, Won-Suk; Clarke, Laura E; Wang, Gordon X et al. (2013) Astrocytes mediate synapse elimination through MEGF10 and MERTK pathways. Nature 504:394-400
Chung, Won-Suk; Barres, Ben A (2012) The role of glial cells in synapse elimination. Curr Opin Neurobiol 22:438-45