The phosphorylation state of Ser-129 in human α-synuclein determines neurodegeneration in a rat model of Parkinson disease

Studies have shown that alpha-synuclein (alpha-syn) deposited in Lewy bodies in brain tissue from patients with Parkinson disease (PD) is extensively phosphorylated at Ser-129. We used recombinant Adeno-associated virus (rAAV) to overexpress human wild-type (wt) alpha-syn and two human alpha-syn mutants with site-directed replacement of Ser-129 to alanine (S129A) or to aspartate (S129D) in the nigrostriatal tract of the rat to investigate the effect of Ser-129 phosphorylation state on dopaminergic neuron pathology. Rats were injected with rAAV2/5 vectors in the substantial nigra pars compacta (SNc) on one side of the brain; the other side remained as a nontransduced control. The level of human wt or mutant alpha-syn expressed on the injected side was about four times the endogenous rat alpha-syn. There was a significant reduction of dopaminergic neurons in the SNc and dopamine (DA) and tyrosine hydroxylase (TH) levels in the striatum of all S129A-treated rats as early as 4 wk postinjection. Nigral DA pathology occurred more slowly in the wt-injected animals, but by 26 wk the wt alpha-syn group lost nigral TH neurons equivalent to the mutated S129A group at 8 wk. In stark contrast, we did not observe any pathological changes in S129D-treated animals. Therefore, the nonphosphorylated form of S129 exacerbates alpha-syn-induced nigral pathology, whereas Ser-129 phosphorylation eliminates alpha-syn-induced nigrostriatal degeneration. This suggests possible new therapeutic targets for Parkinson Disease.