Rab27b promotes lysosomal function and alpha-synuclein clearance in neurons

Alpha-synuclein (αsyn) is the key pathogenic protein implicated in synucleinopathies including Parkinson’s Disease (PD) and Dementia with Lewy Bodies (DLB). In these diseases, αsyn is thought to spread between cells where it accumulates and induces pathology; however, mechanisms that drive its propagation or aggregation are poorly understood. We have previously reported that the small GTPase Rab27b is elevated in human PD and DLB and that it can mediate the autophagic clearance and toxicity of αsyn in a paracrine αsyn cell culture model. Here, we expanded our previous work and characterized a role for Rab27b in neuronal lysosomal processing and αsyn clearance. We found that Rab27b KD in this αsyn inducible neuronal model resulted in lysosomal dysfunction and increased αsyn levels in lysosomes. Similar lysosomal proteolytic defects and enzymatic dysfunction were observed in both primary neuronal cultures and brain lysates from male and female Rab27b knockout (KO) mice. αSyn aggregation was exacerbated in Rab27b KO neurons upon treatment with αsyn preformed fibrils. We found no changes in lysosomal counts or lysosomal pH in either model, but we did identify changes in acidic vesicle trafficking and in lysosomal enzyme maturation and localization, which may drive lysosomal dysfunction and promote αsyn aggregation. Rab27b OE enhanced lysosomal activity and reduced insoluble αsyn accumulation. Finally we found elevated Rab27b levels in human postmortem incidental Lewy Body Disease (iLBD) subjects relative to healthy controls. These data suggest a role for Rab27b in neuronal lysosomal activity and identify it as a potential therapeutic target in synucleinopathies. Significance statement Alpha-synuclein aggregation in Parkinson’s disease is associated with autophagic-lysosomal dysfunction, yet the molecular mechanisms underlying alpha-synuclein clearance are not well understood. We identified the small GTPase Rab27b as a novel regulator of the lysosomal clearance of alpha-synuclein. Using several alpha-synuclein models, we found that Rab27b knockdown or knockout impairs lysosomal function, increases alpha-synuclein lysosomal accumulation, and increases alpha-synuclein aggregation. Conversely, Rab27b overexpression promotes lysosomal function and reduces alpha-synuclein aggregation. We also identified defects in lysosomal enzyme maturation and localization and acidic vesicle trafficking upon Rab27b loss, which may drive lysosomal dysfunction. These findings suggest that targeting Rab27b could boost lysosomal clearance of alpha-synuclein in synucleinopathies.