α-Synucleinopathies are a subset of neurodegenerative diseases, including Parkinson's disease (PD), dementia with Lewy Bodies (DLB) and multiple system atrophy (MSA) (1), which are characterized by abnormal accumulation of misfolded α-synuclein (α-syn) protein in neurons or glial cells. Emerging evidence suggests that the cell-to-cell transmission of pathological α-syn substantially cause neurodegeneration. However, the molecular mechanism of α-syn transmission is poorly understood. We have identified three transmembrane proteins that strongly bind with α-syn PFF: (i) lymphocyte activation gene-3 (LAG3), (ii) amyloid β precursor-like protein 1 (APLP1), and (iii) neurexin 1-β. LAG3 is an essential receptor, mediating internalization of α-syn PFF; however, substantial α-syn PFF binds to LAG3-/- (knockout) neurons, suggesting that a unidentified candidate(s) (e.g. APLP1) binds with pathologic α-syn, and facilitate the internalization. In this proposal, we hypothesize: (i) APLP1 is an essential receptor, that mediates α-syn transmission; (ii) APLP1-LAG3 complex synergistically mediates α-syn transmission; (iii) depletion of APLP1, LAG3, or APLP1- LAG3 complex, can reduce α-syn-induced neurodegeneration of α-syn transgenic mice. Accordingly, experiments are proposed to characterize the roles of APLP1, LAG3 and the AL complex in mediating the pathogenesis of α-synucleinopathies, in cell-to-cell transmission models and the α-syn transgenic mouse model. Experiments in two mice models with α-synucleinopathies, will be complemented by studies in cells and cell-free experiments, thus allowing the deciphering of each spreading step in the interaction of APLP1, LAG3, or the AL complex with pathologic α-syn. The successful completion of these studies will greatly enhance our understanding of the molecular mechanisms of α-syn cell-to-cell transmission via receptors, by: (i) identifying APLP1 as a novel receptor that mediates α-syn spreading, (ii) understanding the synergistic effect of the AL complex on mediating binding and internalization of α-syn PFF, (iii) providing essential preliminary evaluation of anti-LAG3 antibody as a potential therapeutic agent against PD and related α-synucleinopathies, and (iv) understanding the roles of APLP1, LAG3, and the AL complex in mediating neurodegeneration of α-syn transgenic mice.