PROJECT SUMMARY/ABSTRACT Neuroinflammation plays a critical role in injury and degeneration in the central nervous system (CNS). MG are specialized resident myeloid cells in the CNS that play essential roles the innate immune response. CNS ischemia and traumatic brain injury (TBI) cause recruitment of circulating immune cells and activation of resident MG. Activated MG perform dynamic functions that can be both supportive and destructive to neuronal health. MG also have essential roles in CNS development, plasticity and immune surveillance. However the ontogeny of MG in the adult CNS is still not fully understood. While MG progenitors that colonize the developing brain are born in the embryonic yolk sac, recent reports demonstrate that MG progenitor cells are also present in the adult CNS. When adult MG are depleted, these progenitor cells devide and differentiate into mature adult MG. These findings suggest that MG plasticity not only refers to the molecular and morphological changes of existing cells, but also the generation of a new MG populations. Currently, little is known regarding the potential role for MG progenitors in the normal, aging or injured brain. We are interested in understanding the regulation of MG behavior, including the generation and differentiation of newly born MG during acute and chronic CNS injury and neurodegeneration. We have developed methods to isolate and culture MG progenitor cells from adult mouse brain. This progenitor population is present in the uninjured adult CNS, can be isolated by cell sorting or positive selection and will differentiate into mature MG in vitro. The goals of this proposal are to determine how newly born MG contribute to the mature MG population in the setting of advanced age or disease and to identify molecular pathways that distinguish MG progenitors from mature MG. We propose to study the process of MG repopulation in aging mice and to determine whether age influences the rate of cellular proliferation in adult CNS resident erythromeyloid precursor cells, MG progenitor cells and mature MG. We will employ this approach to determine if the APP/PS1 mouse model of Alzheimer's disease (AD) influences the size of the progenitor and mature MG populations. In addition, to determine the specific molecular signature of MG progenitors cells, we will isolate progenitors and mature MG by ex-vivo flow cytometry and compare the global gene expression profiles of the three populations using RNA sequencing. In summary, the accomplishment of these aims will help to further understand the molecular signals that regulate MG and their progenitors within the CNS.