Rab7 induces clearance of α-synuclein aggregates

Parkinson's disease can be caused by mutations in the alpha-synuclein gene and is characterized by aggregates of alpha-synuclein protein. Aggregates are degraded by the autophago-lysosomal pathway. Since Rab7 has been shown to regulate trafficking of late endosomes and autophagosomes, we hypothesized that over-expressing Rab7 might be beneficial in Parkinson's disease. To test this hypothesis, we expressed the pathogenic A53T mutant of alpha-synuclein in HEK293 cells and Drosophila melanogaster. In HEK293 cells, EGFP-Rab7-decorated vesicles contain alpha-synuclein. Rab7 over-expression reduced the percentage of cells with alpha-synuclein particles and the amount of alpha-synuclein protein. Time-lapse microscopy confirmed that particles frequently disappeared with Rab7 over-expression. Clearance of alpha-synuclein is explained by the increased occurrence of acidified y-synuclein vesicles with Rab7 over-expression, presumably representing autolysosomes. Rab7 over-expression reduced apoptosis and the percentage of dead cells in trypan blue staining. In the fly model, Rab7 rescued the locomotor deficit induced by neuronal expression of A53T-alpha-synuclein. These beneficial effects were not produced by Rab7 missense mutations causing Charcot Marie Tooth neuropathy, or by the related GTPases Rab5, Rab9, or Rab23. Using mass spectrometry, we identified Rab7 in neuromelanin granules purified from human substantia nigra, indicating that Rab7 might be involved in the biogenesis of these possibly protective, autophagosome-like organelles in doparninergic neurons. Taken together, Rab7 increased the clearance of alpha-synuclein aggregates, reduced cell death, and rescued the phenotype in a fly model of Parkinson's disease. These findings indicate that Rab7 is rate-limiting for aggregate clearance, and that Rab7 activation may offer a therapeutic strategy for Parkinson's disease.