The GTPase Rab27b regulates the release, autophagic clearance, and toxicity of ?-synuclein

?-Synuclein (?syn) is the primary component of proteinaceous aggregates termed Lewy bodies that pathologically define synucleinopathies including Parkinson's 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 cell-to-cell release and uptake of ?syn are considered important processes for its prion-like spread. Rab27b is one of several GTPases essential to the endosomal-lysosomal pathway and is implicated in protein secretion and clearance, but its role in ?syn spread has yet to be characterized. In this study, we used a paracrine ?syn in vitro neuronal model to test the impact of Rab27b on ?syn release, clearance, and toxicity. shRNA-mediated knockdown (KD) of Rab27b increased ?syn-mediated paracrine toxicity. Rab27b reduced ?syn release primarily through nonexosomal pathways, but the ?syn released after Rab27b KD was of higher-molecular-weight species, as determined by size-exclusion chromatography. Rab27b KD increased intracellular levels of insoluble ?syn and led to an accumulation of endogenous light chain 3 (LC3)-positive puncta. Rab27b KD also decreased LC3 turnover after treatment with an autophagosome-lysosome fusion inhibitor, chloroquine, indicating that Rab27b KD induces a defect in autophagic flux. Rab27b protein levels were increased in brain lysates obtained from postmortem tissues of individuals with PD and DLB compared with healthy controls. These data indicate a role for Rab27b in the release, clearance, and toxicity of ?syn and, ultimately, in the pathogenesis of synucleinopathies.