Oxidative modification of methionine residue in α-synuclein by dopamine induces cellular vulnerability, oligomerization and secretion of α-synuclein.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the selective loss of dopamine (DA) neurons and presence of Lewy bodies, with prion-like propagation of α-synuclein (α-syn) also attracting attention recently. However, the specific causes for PD-related pathogenesis, including cell vulnerability and α-syn propagation, occurring only in selective neurons remain unclear. Therefore, we aimed to investigate the interactions between DA and α-syn protein to clarify its effects on α-syn degradation, secretion, and toxicity. We generated PC12 cells expressing human α-syn and M127A mutant in a tetracycline-inducible manner. In these cells, intracellular α-syn levels were controlled via autophagic/lysosomal degradation and secretion to extracellular space. Notably, M127A mutation decreased the intracellular degradation and secretion of α-syn. Using the generated cells, we investigated the association between cell viability and oxidized methionine [Met(O)] in α-syn. We also investigated the effects of Met(O) on α-syn toxicity and stability upon DA induction. Wildtype α-syn overexpression decreased the cell viability, and inhibition of methionine sulfoxide reductase, a methionine sulfoxide-reducing enzyme, further amplified this effect, suggesting that α-syn cytotoxicity is associated with methionine oxidation. Notably, vulnerabilities of M127A mutant cells were lower than those of wildtype α-syn-expressing cells. Overall, our results suggest M127 as the major target for oxidative modification by DA and that this modification is associated with both cell vulnerability and α-syn intracellular stability and secretion in PD pathogenesis.