Inducible expression of mutant alpha-synuclein decreases proteasome activity and increases sensitivity to mitochondria-dependent apoptosis

Parkinson's disease (PD) is a common progressive neurodegenerative disorder caused by the loss of dopaminergic neurons in the substantia nigra, Althou gh mutations in alpha -synuclein have been identified in autosomal dominant PD, the mechanism by which dopaminergic neural cell death occurs remains u nknown. Proteins encoded by two other genes in which mutations cause famili al PD, parkin and UCH-L1, are involved in regulation of the ubiquitin-prote asome pathway, suggesting that dysregulation of the ubiquitin-proteasome pa thway is involved in the mechanism by which these mutations cause PD, We es tablished inducible PC12 cell lines in which wild-type or mutant alpha -syn uclein can be de-repressed by removing doxycycline, Differentiated PC12 cel l lines expressing mutant alpha -synuclein showed decreased activity of pro teasomes without direct toxicity, Cells expressing mutant alpha -synuclein showed increased sensitivity to apoptotic cell death when treated with sub- toxic concentrations of an exogenous proteasome inhibitor. Apoptosis was ac companied by mitochondrial depolarization and elevation of caspase-3 and -9 , and was blocked by cyclosporin A, These data suggest that expression of m utant alpha -synuclein results in sensitivity to impairment of proteasome a ctivity, leading to mitochondrial abnormalities and neuronal cell death.