Genetic and Cell Biological Dissection of Alpha-Synuclein Trafficking Defects in Yeast and Mammalian Cells

a-synuclein, a major player in idiopathic and familial forms of Parkinson's disease pathogenesis, is thought to impair vesicle trafficking as a toxic gain of function through elevated levels of the protein. Genetic screens for modifiers of a-synuclein toxicity in model organisms have overwhelmingly identified a significant subset of genes involved in vesicle trafficking. While recent studies in yeast have shown that at toxic levels a-synuclein disrupts Rab homeostasis, causing an initial ER-to-Golgi block that precedes ageneralized trafficking collapse, further studies in mammalian systems suggested that several Rab proteins downstream of ER-Golgi trafficking can also rescue a-synuclein toxicity. Here, we show that constitutive expression of a-synuclein in yeast causes an accumulation of endocytic vesicles and impairs late-exocytic, early-endocytic and/or recycling trafficking and, further, that this vesicle trafficking impairment can be attenuated by yeast casein kinase 1 and the small rab protein Ypt1p. Our work in mammalian cell culture demonstrates that human orthologs of vacuolar protein sortinggenes involved in endosomal transport modulate the toxicity of a-synuclein. Furthermore, our findings suggest that a-synuclein-induced toxicity is due to impairment in the endosomal trafficking pathway of neuronal cells. Taken together, our results provide evidence of the potential contribution of endosome anomalies to the pathogenesis of Parkinson's disease