Mechanisms of alpha-synuclein neurotoxicity in Parkinson's disease

Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. α-synuclein, a small protein localizing to the nucleus and the synapse, plays a central role in the pathogenesis of both rare autosomal dominant and prevalent sporadic forms of the disease. The mechanism by which α-synuclein induces loss of dopaminergic neurons is unknown. In the first part of my dissertation, I examined the role of nuclear α-synuclein in promoting neurotoxicity. Targeting α-synuclein to the nucleus promoted toxicity, while cytoplasmic sequestration was protective in both neuroblastoma cells and transgenic Drosophila. Since α-synuclein has been shown to physically bind histones (Goers et al., 2003), we examined whether over-expression of α-synuclein affected histone acetylation levels. We created stable cell lines of syn NLS and syn NES , and found that histone H3 was significantly hypoacetylated in stable syn NLS cells, relative to untransfected cells and stably transfected syn NES cells. Toxicity of α-synuclein was rescued by administration of histone deacetylase inhibitors in both cell culture and transgenic flies. α-synuclein associated with histones, reduced the level of acetylated histone H3 in cultured cells, and inhibited acetylation in histone acetyltransferase assays. These results suggest that α-synuclein may mediate toxicity in the nucleus by influencing histone acetylation states. In the second part of my dissertation, I identified calmodulin as a genetic mediator of α-synuclein dependent toxicity. In the Drosophila brain, reducing calmodulin expression suppressed α-synuclein-dependent toxicity, whereas overexpressing wild-type calmodulin enhanced toxicity. Administration of calmodulin antagonists also rescued α-synuclein toxicity. These exciting findings potentially implicate the calmodulin signaling network in Parkinson's disease pathogenesis, and raise a number of interesting questions regarding the specific mechanisms by which calmodulin may influence α-synuclein neurotoxicity. In conclusion, I have described two novel mechanisms influencing α-synuclein toxicity. First, I showed that α-synuclein acts in the nucleus to inhibit histone acetylation and promote neurotoxicity. Second, I identified calmodulin as a genetic modifier of α-synuclein toxicity. Taken together, this dissertation provides a major contribution to our understanding of mechanisms underlying neurotoxicity in Parkinson's disease, and carries implications for future studies investigating these mechanisms at the cellular and organismal levels.