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Authentically Phosphorylated α-Synuclein at Ser129 Accelerates Neurodegeneration in a Rat Model of Familial Parkinson's Disease

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra (SN) and the appearance of fibrillar aggregates of insoluble α-synuclein (α-syn) called Lewy bodies (LBs). Approximately 90% of α-syn deposited in LBs is phosphorylated at serine 129 (Ser129). In contrast, only 4% of total α-syn is phosphorylated in normal brain, suggesting that accumulation of Ser129-phosphorylated α-syn is involved in the pathogenesis of PD. However, the role of Ser129 phosphorylation in α-syn neurotoxicity remains unclear. In this study, we coexpressed familial PD-linked A53T α-syn and G-protein-coupled receptor kinase 6 (GRK6) in the rat SN pars compacta using recombinant adeno-associated virus 2. Coexpression of these proteins yielded abundant Ser129-phosphorylated α-syn and significantly exacerbated degeneration of dopaminergic neurons when compared with coexpression of A53T α-syn and GFP. Immunohistochemical analysis revealed that Ser129-phosphorylated α-syn was preferentially distributed to swollen neurites. However, biochemical analysis showed that the increased expression of Ser129-phosphorylated α-syn did not promote accumulation of detergent-insoluble α-syn. Coexpression of catalytically inactive K215R mutant GRK6 failed to accelerate A53T α-syn-induced degeneration. Furthermore, introducing a phosphorylation-incompetent mutation, S129A, into A53T α-syn did not alter the pace of degeneration, even when GRK6 was coexpressed. Our study demonstrates that authentically Ser129-phosphorylated α-syn accelerates A53T α-syn neurotoxicity without the formation of detergent-insoluble α-syn, and suggests that the degenerative process could be constrained by inhibiting the kinase that phosphorylates α-syn at Ser129.