Abstract This project will exploit the power of zebrafish genetics to discover new genes that are essential for the development, migration, and activation of microglia and neural macrophages. Microglia are highly motile, phagocytic glial cells within the central nervous system (CNS) that destroy pathogens and clear debris such as apoptotic cells and damaged axons. Macrophages perform similar functions in peripheral nerves. In disease or after injury, inappropriate activation of microglia and macrophages can cause damage to the nervous system. For example, in multiple sclerosis (MS) and other diseases in the CNS, activated microglia release cytotoxic factors that harm myelinated axons. Similarly, activated macrophages damage myelinated axons in diabetic peripheral neuropathy. Despite the importance of microglia and macrophages in the healthy and diseased nervous system, there are fundamental gaps in the understanding of the development, migration, and activation of these cells. Thus investigation of the mechanisms that regulate the function of microglia and neural macrophages will provide important insights into the pathophysiology of diseases of the nervous system, including MS, peripheral neuropathies, and many others. In addition, these studies will suggest new avenues toward therapies to prevent and repair damage to the nervous system. To discover new genes essential for the development and function of microglia and neural macrophages, we will conduct a genetic screen for mutations in which these cells are disrupted. Using a rapid, robust marker assay for microglia and macrophages, we have found that microglia and macrophages are increased and strongly activated in a mutant recovered in our previous screens for myelination mutants, nsf. Interestingly, a drug that blocks activation of microglia and macrophages ameliorates the phenotype of nsf mutants, suggesting that activated phagocytes contribute to pathology in these mutants. These results demonstrate the feasibility of finding mutations that affect microglia and neural macrophages, and also underscore the importance of the relationship between myelinated axons and the associated phagocytes. Analysis of additional mutants with abnormal microglia and macrophages will test the hypothesis that activation of these cells contributes to diverse pathologies of the nervous system, such that they act to exacerbate and ameliorate symptoms in different contexts. This project will isolate more mutations in genes with essential functions in microglia and macrophages and characterize the functions of these genes at the cellular and biochemical level. These experiments will elucidate fundamental aspects of vertebrate neurobiology, establish zebrafish models of important human diseases, add to the understanding of the processes that are disrupted in diseased and injured axons, and provide a basis to pursue the therapeutic repair of damage to the nervous system.