Knockout of Rab27b exacerbates neuropathology in alpha-synuclein mouse models

Parkinson's Disease (PD) and other synucleinopathies are characterized by the formation of inclusions comprised of alpha-synuclein (αsyn) among other proteins, but the mechanisms by which these inclusions form and cause toxicity are not well understood. We have previously reported that the small GTPase Rab27b modulates autophagic-lysosomal function in neurons and supports lysosomal degradation of αsyn across multiple αsyn cellular models. Knockout (KO) and knockdown (KD) of Rab27b damage lysosomal degradative capacity and exacerbate αsyn pathology, while Rab27b overexpression is conversely protective in cellular αsyn models. Elevations of Rab27b seen in human synucleinopathies suggest a compensatory role for Rab27b in these disorders. Here, we examined the role Rab27b plays in vivo in the context of both A53T genetic αsyn overexpression and viral AAV αsyn overexpression mouse models. Rab27b knockout in A53T⁺ mice did not alter motor behavior or survival. However, Rab27b knockout increased proteinase-K resistant αsyn in the cortex, striatum, and substantia nigra of A53T mice starting as early as six months of age. Additionally, Rab27b KO increased phosphorylated S129 αsyn in the cortex and nigra. Astrocyte and microglial activation were also observed upon Rab27b KO in the A53T model. In the AAV αsyn model, Rab27b KO resulted in dopaminergic cell loss in the nigra, which was not observed in WT mice. Collectively, we report that loss of Rab27b results in elevated neuropathology in PD-relevant brain regions, validating its role as a therapeutic target in synucleinopathies.