Parkin cleaves intracellular α-synuclein inclusions via the activation of calpain

Mutations in the alpha-synuclein and parkin genes cause heritable forms of Parkinson's disease. In the present study, we examined the possible functional relationship between the parkin and alpha-synuclein genes in a conditionally immortalized embryonic hippocampal cell (H19-7) line. Whereas transient transfection of alpha-synuclein into neuronal H19-7 cells caused the formation of its intracytoplasmic inclusions and a significant cell death, the combined overexpression of parkin restored the alpha-synuclein-induced decrease in cell viability to control levels. In addition, the overexpression of parkin was found to generate selective cleavage of alpha-synuclein. Furthermore, the cytoprotective effect of parkin on alpha-synuclein-induced cell death was not inhibited in the presence of a proteasome inhibitor. Interestingly, the overexpression of parkin induced the activation of an intracellular cysteine protease, calpain, but not caspase, and the cytoprotective effect of parkin on alpha-synuclein cytotoxicity was significantly inhibited by the presence of calpain-specific inhibitors. In conclusion, our results suggest that parkin accelerates the degradation of alpha-synuclein via the activation of the nonproteasomal protease, calpain, leading to the prevention of alpha-synuclein-induced cell death in embryonic hippocampal progenitor cells.