Insight into Familial and Sporadic Parkinson’s Disease: -Synuclein Mutant Analysis in a Fission Yeast Model

Parkinson’s disease (PD) is the second most common neurodegenerative disease, affecting six million people worldwide. It results from the specific loss of substantia nigra dopaminergic neurons, which accumulate large filamentous structures called Lewy bodies composed mostly of one misfolded and aggregated protein called -synuclein. The aggregation and membrane phospholipid binding ability of -synuclein are both linked to cellular toxicity in familial and sporadic forms of PD, but their relative contributions are not resolved. This thesis utilized a Schizosaccharomyces pombe (fission yeast) model to get further insight into the nature of -synuclein toxicity in these two forms of PD and the data was comparatively evaluated with previous studies done in budding yeast. Three studies were conducted. First, for insight into familial PD, we tested the hypothesis that the newest mutant E46K -synuclein is toxic in fission yeast, but instead found it to be surprisingly slightly toxic. This lack of major toxicity correlated with extensive -synuclein aggregation and the lack of plasma membrane localization. In contrast, in budding yeast, E46K is known to be toxic, membrane localized, and not aggregated. Next, for insight into sporadic Parkinson’s disease, we first tested the hypothesis that alanine-76 in -synuclein as a major contributor to a-synuclein’s aggregation and found that an A76E mutant was indeed less aggregated in fission yeast. This finding supports past budding yeast work where A76E -synuclein is less plasma membrane localized. Lastly, we tested the hypothesis that posttranslational modifications in -synuclein contribute to its aggregation and to its known higher migration when run on protein gels. Surprisingly, phosphorylationdeficient (S87A and S129A) and nitrosylation-deficient mutants (Y39F and Y125F) of -synuclein remained aggregated and their migration size on gels was unchanged. In contrast, these mutants are significantly toxic to budding yeast and maintain membrane localization. We suggest that fission yeast is more resistant than budding yeast to -synuclein toxicity possibly because membrane localization contributes toxicity, while aggregation protects. It is also possible that fission yeast more efficiently suppresses synuclein toxicity by thus far unknown mechanisms. Importantly, the mutants studied in both models demonstrate that -synuclein intrinsically regulates its ability to aggregate and bind membrane phospholipids, providing new insight into the toxicity seen in both sporadic and familial PD.