Role of Rab27b in synucleinopathies

University Of Alabama At Birmingham, Birmingham AL

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Abstract

Recent evidence points to ?syn misfolding and cell-to-cell transmission as critical to neurodegeneration in Parkinson?s Disease (PD) and Dementia with Lewy Bodies (DLB). Key steps implicated in ?syn spread include release, uptake, misfolding, and impaired protein degradation, yet the key molecular mechanisms that regulate ?syn spread are poorly understood. Rab proteins are small GTPase proteins that control protein trafficking and degradation and have been implicated in ?syn pathogenesis. Among the Rab proteins, Rab27b is highly expressed at synaptic terminals in neurons in key brain areas affected in PD and DLB. Rab27b regulates synaptic vesicle (SV) exocytosis and recycling. In non-neuronal cells, Rab27b regulates the distal transport of lysosomes. We recently showed elevated Rab27b levels in ?syn models and in human PD and DLB. Furthermore, we observed that Rab27b KD increases ?syn toxicity by disrupting autophagic flux - pointing to a protective role for Rab27b in cells with high intracellular ?syn burden. Surprisingly, the effects of Rab27b were different in the ?syn fibril model, in which neurons were exposed to extracellular ?syn fibrils: Rab27 KO reduced fibrillary ?syn uptake to prevent ?syn inclusions and neuronal loss. Based on these data, we hypothesize that Rab27b plays two critical yet potentially opposing roles in ?syn handling: 1) Rab27b promotes ?syn autophagic-lysosomal degradation, yet 2) Rab27b can facilitate endocytosis of pathologic, extracellular ?syn as part of its role in SV recycling. Early in disease, we propose that Rab27b upregulation is neuroprotective: Rab27b aids intracellular ?syn clearance. Yet, as disease progresses and extracellular ?syn levels increase, Rab27b?s role in SV endocytosis becomes maladaptive, overcoming any protective function in protein degradation, and aids ?syn cell-to-cell transmission. Our proposed studies will examine the impact of Rab27b and its interactors on two critical biological processes implicated in ?syn pathogenesis. In Aim 1, we will test the mechanisms by which Rab27b promotes clearance of intracellular ?syn, and examine its interactions with other proteins involved in autophagic-lysosomal function. In Aim 2, we will elucidate the mechanisms by which Rab27b aids ?syn entry into neurons and determine its interaction with other synaptic trafficking proteins to mediate this effect. In Aim 3, we will test the overall consequences of Rab27b on ?syn pathology over time in two in vivo ?syn models. A greater understanding of the network of proteins that shape ?syn transmission will have significant impact on the development of future therapies for PD and DLB.