Alpha-synuclein inhibits the secretion of extracellular vesicles through disruptions in YKT6 lipidation

Parkinson's disease is characterized by the presence of α-synuclein (α-syn) primarily containing Lewy bodies in neurons. Despite decades of extensive research on α-syn accumulation, its molecular mechanisms have remained largely unexplored. Recent studies by us and others have suggested that extracellular vesicles (EVs), especially exosomes, can mediate the release of α-syn from cells, and inhibiting this pathway could result in increased intracellular α-syn levels. In this study, we have discovered that elevated levels of α-syn themselves lead to reduced α-syn -containing EVs in α-syn inducible H4 cells and induced pluripotent stem cell-derived dopaminergic (DA) neurons from both sexes. Our investigations have revealed that the impairment in EV secretion is not due to their generation but rather a consequence of changes in a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein, YKT6. Specifically, as α-syn levels increase, membrane-associated YKT6 is reduced. Pharmacological inhibition of farnesylation using FTI has led to decreased EV secretion and subsequent elevated levels of α-syn. In summary, our findings suggest that increased levels of α-syn impair YKT6-mediated EV secretion, establishing a detrimental cycle of intracellular α-syn accumulation in human DA neurons.Significance Statement Neurodegenerative disorders, including Parkinson's disease (PD), are characterized by the pathological accumulation of insoluble proteins, primarily in neurons. Regulating intracellular levels of these proteins is critical. Despite extensive research for decades, the precise mechanism of these protein deposits remains unexplained. In this study, we discovered that extracellular vesicles (EVs) play a pivotal role in regulating alpha-synuclein (a-syn) levels through release from neurons. Furthermore, increased levels of a-syn impede its EV secretion, creating a pathological loop. Elevated levels of a-syn interfere with the localization of YKT6, a SNARE protein, to the vesicle membrane by inhibiting YKT6 palmitoylation. These findings illustrate a novel mechanism of a-syn accumulation in neurons and provide a target to potentially mitigate PD pathology.