Lack of additive role of ageing in nigrostriatal neurodegeneration triggered by alpha-synuclein overexpression

Introduction: Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons as well as the presence of proteinaceous inclusions named Lewy bodies. alpha synuclein (alpha-syn) is a major constituent of Lewy bodies, and the first disease-causing protein characterized in PD. Several alpha-syn-based animal models of PD have been developed to investigate the pathophysiology of PD, but none of them recapitulate the full picture of the disease. Ageing is the most compelling and major risk factor for developing PD but its impact on alpha-syn toxicity remains however unexplored. In this study, we developed and exploited a recombinant adeno-associated viral (AAV) vector of serotype 9 overexpressing mutated alpha-syn to elucidate the influence of ageing on the dynamics of PD-related neurodegeneration associated with alpha-syn pathology in different mammalian species. Results: Identical AAV pseudotype 2/9 vectors carrying the DNA for human mutant p.A53T alpha-syn were injected into the substantia nigra to induce neurodegeneration and synucleinopathy in mice, rats and monkeys. Rats were used first to validate the ability of this serotype to replicate alpha-syn pathology and second to investigate the relationship between the kinetics of alpha-syn-induced nigrostriatal degeneration and the progressive onset of motor dysfunctions, strikingly reminiscent of the impairments observed in PD patients. In mice, AAV2/9-h alpha-syn injection into the substantia nigra was associated with accumulation of alpha-syn and phosphorylated h alpha-syn, regardless of mouse strain. However, phenotypic mutants with either accelerated senescence or resistance to senescence did not display differential susceptibility to h alpha-syn overexpression. Of note, p-alpha-syn levels correlated with nigrostriatal degeneration in mice. In monkeys, h alpha-syn-induced degeneration of the nigrostriatal pathway was not affected by the age of the animals. Unlike mice, m onkeys did not exhibit correlations between levels of phosphorylated alpha-syn and neurodegeneration. Conclusions: In conclusion, AAV2/9-mediated h alpha-syn induces robust nigrostriatal neurodegeneration in mice, rats and monkeys, allowing translational comparisons among species. Ageing, however, neither exacerbated nigrostriatal neurodegeneration nor alpha-syn pathology per se. Our unprecedented multi-species investigation thus favours the multiple-hit hypothesis for PD wherein ageing would merely be an aggravating, additive, factor superimposed upon an independent disease process.