Rab27b regulates the release, autophagic clearance, and toxicity of alpha-synuclein

Alpha synuclein (syn) is the primary component of proteinaceous aggregates termed Lewy Bodies that pathologically define synucleinopathies including Parkinsons disease (PD) and Dementia with Lewy Bodies (DLB). Syn is hypothesized to spread through the brain in a prion-like fashion by misfolded protein forming a template for aggregation of endogenous syn. The release and uptake of syn from cell to cell are considered important processes for this prion-like spread. Rab27b is one of several GTPases essential to the endosomal-lysosomal pathway and is implicated in protein secretion and clearance but has yet to be characterized in its role in syn spread. In this study, we used a paracrine syn in vitro model to test the impact of Rab27b on syn release, clearance, and toxicity. shRNA-mediated knockdown (KD) of Rab27b increased syn-mediated paracrine toxicity. While Rab27b reduced syn release primarily through non-exosomal pathways, the syn released under KD conditions was of higher molecular weight species by size exclusion chromatography. Rab27b KD increased intracellular insoluble syn levels and led to an accumulation of endogenous LC3 positive puncta. Rab27b KD also decreased LC3 turnover with chloroquine treatment, indicating a defect in autophagic flux. Rab27b protein levels were increased in postmortem human brain lysates from PD and DLB subjects compared to healthy controls. These data indicate a role for Rab27b in the release, clearance, and toxicity of syn and ultimately in the pathogenesis of synucleinopathies.