The goal of this project is to examine the role of Rab27 in synucleinopathies, including Parkinson's disease and Dementia with Lewy Bodies. Misfolding and cell-to-cell propagation of ?syn are hypothesized to play a major role in the pathogenesis of Parkinson's Disease and Dementia with Lewy Bodies. Key steps implicated in ?-syn spread include active release, uptake, and misfolding, yet the key mechanisms that regulate the spread of ?-syn are poorly understood. Rab proteins are small GTPase proteins that interact with Rab effector proteins to control protein trafficking and degradation and have been implicated in ?syn pathogenesis. We recently identified a potential role for Rab27b in synucleinopathies. Rab27b, which is highly enriched in neurons, plays a role in protein secretion in multiple cell types including neurons, but has also been linked to autophagy and endocytosis. Alterations in Rab27b are associated with Alzheimer's disease, Dementia with Lewy Bodies, and X-linked dystonia parkinsonism syndrome. We recently found a dramatic increase in Rab27b in human PD postmortem brains and in an inducible ?syn cell line (isyn). Rab 27b knockdown (KD) in isyn cells reduces ?syn release and impairs autophagic clearance, consequently promoting ?syn toxicity. In contrast, Rab27a/b double knockout (DKO) leads to a dramatic reduction in ?syn aggregation in neurons exposed to extracellular ?syn protofibrils (PFFs) in vitro and in vivo, most likely due to reduced uptake. Based on these data, we hypothesize that Rab27b plays two independent roles that affect ?syn handling: 1) it regulates entry of extracellular ?syn via endocytosis; and 2) it promotes clearance of intracellular ?syn via secretion and autophagy. In disease, fibrillar ?syn co-opts cell entry pathways via Rab27b and thus indirectly disrupts Rab27b's clearance of intracellular ?syn. We propose that differential effects of Rab27b on autophagy, secretion, and uptake are mediated by different Rab27 effector pathways.
In Aim 1, we will test the ability of Rab27b to mediate ?syn entry into neurons and the relevant Rab27 effectors that mediate uptake.
In Aim 2, we will focus on the impact of Rab27b on clearance of intracellular ?-syn by active secretion and autophagy, determine the relevant Rab27 effectors that mediate these effects on autophagy and secretion, and test whether fibrillar ?syn induces a redistribution of Rab27b away from endolysosomal structures to interfere with clearance activities.
In Aim 3, we will test the consequences of Rab27b and relevant effectors on cognitive function, ?syn inclusion formation, and neuronal loss in the in vivo ?-syn PFF model. Further understanding of the mechanisms by which Rab27b affects ?syn trafficking is critical to understand its role in disease and its therapeutic potential.
Parkinson's disease (PD) and Dementia with Lewy Bodies (DLB) are progressive neurodegenerative disorders that cause significant motor and cognitive disability with no current therapies that slow disease progression. In this proposal, we will investigate the role of Rab27b in the pathophysiology of these disorders. Our long-term goal is to establish whether Rab27b pathways may serve as therapeutic targets for PD and DLB.
