Heat Shock Prevents Alpha-synuclein-induced Apoptosis in a Yeast Model of Parkinsonʹs Disease

We show that human wild-type alpha synuclein (WT α-syn), and the inherited mutants A53T or A30P, when expressed in the yeast Saccharomyces cerevisiae triggers events that are diagnostic of apoptosis: loss of membrane asymmetry due to the externalization of phosphatidylserine, accumulation of reactive oxygen species (ROS), and the release of cytochrome c from mitochondria. A brief heat shock was strikingly protective in that α-syn-expressing cells receiving a heat shock exhibited none of these apoptotic markers. Because the heat shock did not decrease the expression level of α-syn, a protective protein or proteins, induced by the heat shock, must be responsible for inhibition of α-syn-induced apoptosis. Using ROS accumulation as a marker of apoptosis, the role of various genes and various drugs in controlling α-syn-induced apoptosis was investigated. Treatment with geldanamycin or glutathione, overexpression of Ssa3 (Hsp70), or deletion of the yeast metacaspase gene YCA1 abolishes the ability of α-syn to induce ROS accumulation. Deletion of YCA1 also promotes vigorous growth of α-syn-expressing cells compared to cells that contain a functional copy of YCA1. These findings indicate that α-syn-induced ROS generation is mediated by the caspase, according to α-syn→caspase→ROS→apoptosis. It is shown by co-immunoprecipitation that Ssa3 binds to α-syn in a nucleotide-dependent manner. Thus, we propose that Hsp70 chaperones inhibit this sequence of events by binding and sequestering α-syn.