Microglial exosomes facilitate α-synuclein transmission in Parkinson's disease

Accumulation of neuronal alpha-synuclein is a prominent feature in Parkinson's disease. More recently, such abnormal protein aggregation has been reported to spread from cell to cell and exosomes are considered as important mediators. The focus of such research, however, has been primarily in neurons. Given the increasing recognition of the importance of non-cell autonomous-mediated neurotoxicity, it is critical to investigate the contribution of glia to alpha-synuclein aggregation and spread. Microglia are the primary phagocytes in the brain and have been well-documented as inducers of neuroinflammation. How and to what extent microglia and their exosomes impact alpha-synuclein pathology has not been well delineated. We report here that when treated with human alpha-synuclein preformed fibrils, exosomes containing alpha-synuclein released by microglia are fully capable of inducing protein aggregation in the recipient neurons. Additionally, when combined with microglial proinflammatory cytokines, these exosomes further increased protein aggregation in neurons. Inhibition of exosome synthesis in microglia reduced alpha-synuclein transmission. The in vivo significance of these exosomes was demonstrated by stereotaxic injection of exosomes isolated from alpha-synuclein preformed fibrils treated microglia into the mouse striatum. Phosphorylated alpha-synuclein was observed in multiple brain regions consistent with their neuronal connectivity. These animals also exhibited neurodegeneration in the nigrostriatal pathway in a time-dependent manner. Depleting microglia in vivo dramatically suppressed the transmission of alpha-synuclein after stereotaxic injection of preformed fibrils. Mechanistically, we report here that alpha-synuclein preformed fibrils impaired autophagy flux by upregulating PELI1, which in turn, resulted in degradation of LAMP2 in activated microglia. More importantly, by purifying microglia/macrophage derived exosomes in the CSF of Parkinson's disease patients, we confirmed the presence of alpha-synuclein oligomer in CD11b + exosomes, which were able to induce alpha-synuclein aggregation in neurons, further supporting the translational aspect of this study. Taken together, our study supports the view that microglial exosomes contribute to the progression of alpha-synuclein pathology and therefore, they may serve as a promising therapeutic target for Parkinson's disease.