PROJECT SUMMARY Niemann-Pick type C (NPC) is a lysosomal storage disease that is inherited in an autosomal recessive manner. Mutations in Npc1, a protein that exports cholesterol out of the lysosomes, cause the abnormal accumulation of cholesterol in lysosomes; a hallmark of NPC. Neurological symptoms of NPC include ataxia, cognitive impairment, and dementia that lead to premature death. Purkinje cells (PCs) are among the most susceptible neurons to Npc1 deficiency leading to their early degeneration and the development of early neurological symptoms such as clumsiness, gait defects, and ataxia. The pathological changes in the cerebellum that precede and possibly precipitate Purkinje cells degeneration in NPC are not completely known. Studies in our laboratory and by others have shown that microglia reactivity and increased phagocytic responses occur early before the degeneration of Purkinje cells and contribute to the loss of these cells in the cerebellum. Given that the manifestation of neurological symptoms is the result of severe neurodegeneration, a major goal of the proposed study is to understand how NPC1 deficiency impacts microglia and Purkinje cells during postnatal development, an asymptomatic stage in childhood NPC that precedes the degeneration of Purkinje cells. A great portion of the anatomical and functional development of the cerebellum occurs postnatally, and we have found that the development of both, microglia and Purkinje cells, is significantly affected by NPC1 deficiency. Developmental defects in microglia include delayed migration, impaired differentiation, and increased proliferation and phagocytic activity. These early developmental changes in microglia were accompanied by dendritic and synaptic defects in Purkinje cells, including impaired synaptic remodeling of climbing fibers (CF) and decreased synaptic inputs. For this proposed study we will be elucidating the mechanism by which the lack of NPC1 alters the development, function, and phenotype of the cerebellar microglia using the Npc1nmf164 mouse model. We will be investigating the following two aims: In Aim 1 we will elucidate the mechanism by which lack of Npc1 alters microglia metabolism, identity, and activation during postnatal development. Also, it will be determined if by reverting these early changes in microglia the developmental defects in Purkinje cells are prevented. In Aim 2, we will specifically test whether deficiency of the protein CLEC7A prevents microglia developmental deficits, phagocytic phenotype, and the degeneration of Purkinje cells in Npc1nmf164 mice. Understanding the pathological developmental events that precede Purkinje cell degeneration and disease onset in NPC is critical for the designing and testing of effective therapeutic interventions that prevent or delay the progression of NPC.