Interaction of alpha-synuclein and Parkin in iron toxicity on SH-SY5Y cells: implications in the pathogenesis of Parkinson's disease

The toxicity of accumulated alpha-synuclein plays a key role in the neurodegeneration of Parkinson's disease (PD). This study has demonstrated that iron in varying concentrations (up to 400 mu M) causes an increase in alpha-synuclein content in SH-SY5Y cells associated with mitochondrial depolarization, decreased cellular ATP content and loss of cell viability during incubation up to 96 h. Knocking-down alpha-synuclein expression prevents cytotoxic actions of iron, which can also be prevented by cyclosporine A (a blocker of mitochondrial permeability transition pore). These results indicate that iron cytotoxicity is mediated by alpha-synuclein acting on mitochondria. Likewise siRNA mediated knock-down of Parkin causes an accumulation of alpha-synuclein accompanied by mitochondrial dysfunction and cell death during 48 h incubation under basal conditions, but these changes are not further aggravated by co-incubation with iron (400 mu M). We have also analyzed mitochondrial dysfunction and cell viability in SH-SY5Y cells under double knock-down (alpha-synuclein and Parkin concurrently) conditions during incubation for 48 h with or without iron. Our results tend to suggest that iron inactivates Parkin in SH-SY5Y cells and thereby inhibits the proteasomal degradation of alpha-synuclein, and the accumulated alpha-synuclein causes mitochondrial dysfunction and cell death. These results have implications in the pathogenesis of sporadic PD and also familial type with Parkin mutations.