Image of yeast cells expressing α-synuclein tagged with green fluorescent protein courtesy of Aaron Gitler, Whitehead Institute for Biomedical Research, Cambridge, MA.
Researchers are brewing new approaches to the study of neurological disorders. Mutation or duplication of the 
-synuclein (Syn) gene is associated with Parkinson disease. However, the function of Syn and its role in Parkinson disease have been difficult to address in complex, multicellular organisms. Cooper et al. find that Syn is important in protein trafficking by examining its function in yeast in a recent article in Science.
Multiple copies of the Syn gene reduced yeast viability. Eight hours after Syn induction by a galactose-inducible promoter, colony formation declined by 60%. Endoplasmic reticulum (ER) stress results from a buildup of misfolded proteins in the ER that require transfer to the cytoplasm for proteasome degradation. Using an unfolded protein response reporter assay, the authors found increased ER stress in yeast overexpressing Syn and yeast expressing Syn-A53T, a mutation that is associated with early-onset Parkinson disease in humans. Yeast expressing excess or mutant Syn showed normal degradation of a proteasome substrate, but elevated Syn expression disrupted the degradation of CPY*, a misfolded protein that requires trafficking from the ER to the Golgi apparatus. The authors found that even correctly folded proteins that required ER–Golgi vesicular transfer were blocked from trafficking by excess or mutant Syn within 4 hours, suggesting that Syn toxicity specifically results from blocked vesicular transport between the ER and the Golgi apparatus.
The authors then screened a yeast library to identify gene products that modified Syn toxicity and found 34 genes that counteracted and 20 genes that worsened Syn toxicity. They ensured specificity by removing from their dataset genes that modulated the toxicity of a protein associated with Huntington disease. Many of the genes that specifically suppressed Syn toxicity showed direct involvement in ER–Golgi trafficking, including the Rab GTPase Ypt1p and SNARE. Conversely, Syn toxicity was enhanced by Gyp8, a Rab GTPase activating protein (GAP) that turns Ypt1 off. Ypt1 improved and Gyp8 worsened the trafficking of properly folded CPY to the Golgi apparatus.
Rab1, the mouse Ypt1 ortholog, rescued cells from several species from Syn-mediated cell death. Drosophila with either an Syn or an Syn-A53T transgene had fewer dopamine neurons relative to wild-type Drosophila. A Rab1 transgene rescued dopamine-expressing neurons from both excess and mutant Syn. In C. elegans, an Syn transgene reduced the number of dopamine neurons by 60%, but Rab1 rescued dopamine neuron loss. Similarly, in primary cultures of rat midbrain neurons with lentiviral Syn-A53T transgenes, the authors found a 50% reduction in dopamine cells that was rescued by coadministration of a Rab1 transgene.
Ypt1/Rab1 is involved in the docking of vesicles on the Golgi apparatus. Therefore, Rab1 and other agents that encourage the vesicular transport of proteins to the Golgi apparatus are exciting targets for therapeutic intervention in Parkinson disease.