Project Summary/Abstract Local degeneration of neuronal processes is an important mechanism in neural circuit remodeling and neuronal injury. The neuronal debris resulting from degeneration must be promptly cleared by phagocytes to prevent inflammation and to facilitate the subsequent neuronal regrowth. Although aberrant recognition and clearance of neuronal debris are implicated in neuroinflammation, autoimmunity, and neurodegenerative diseases, it is unknown how phagocytes distinguish degenerative neurites from surrounding healthy ones. In particular, three important questions remain unanswered: what is the signal on degenerating neurites that allows the recognition by phagocytes? What is the receptor for the recognition signal of degenerating neurites? How is the recognition signal specifically exposed on degenerating neurites? Our new in vivo data provided important clues that will help us to solve these puzzles. Using our new in vivo probes, we discovered that the ?eat-me? signal phosphatidylserine (PS) is absent on the surface of healthy dendrites but is exposed on degenerating dendrites in both developmental remodeling and physical injury. Building on these observations, this project aims to elucidate the in vivo mechanisms of PS exposure and recognition in dendrite degeneration using Drosophila sensory neurons as a model system. Our long term objective is to uncover autonomous and non-autonomous mechanisms of dendrite degeneration and repair. For this project, we propose the following three aims: 1) Determine the role of PS exposure in the recognition and engulfment of degenerating dendrites. The necessity of PS exposure in engulfment of dendrites after injury will be determined by (i) masking PS on the dendrite surface with PS-binding proteins, and (ii) blocking the biosynthesis of PS in specific neurons. The sufficiency of PS in triggering dendrite engulfment and degeneration will be tested by ectopically inducing PS exposure in neurons. 2) Investigate how the CED-1 family member Draper recognizes degenerating dendrites. Our results suggest that Draper recognizes degenerating dendrites. Two complementary in vivo competition assays will be performed to determine if Draper directly interacts with PS. 3) Determine how PS exposure is regulated in neurons and degenerating dendrites. By conducting loss-of-function studies of candidate genes, the identities of PS flippases and scramblases that regulate PS exposure during dendrite degeneration will be determined. The role of caspases in PS exposure will be investigated by examining caspase activity after dendrite injury and by disrupting the caspase pathway in neurons. Together, these aims will reveal in vivo mechanisms of neuronal debris sensing. As the clearance of neuronal debris in both mammals and insects requires the same CED-1 family of engulfment receptor, this study will reveal conserved mechanisms that may be relevant to neurodegenerative disorders.