Membrane binding, internalization, and sorting of alpha-synuclein in the cell

Abstract

Alpha-synuclein (aSyn) plays a crucial role in Parkinson’s disease (PD) and other synucleinopathies, since it misfolds and accumulates in typical proteinaceous inclusions. While the function of aSyn is thought to be related to vesicle binding and trafficking, the precise molecular mechanisms linking aSyn with synucleinopathies are still obscure. aSyn can spread in a prion-like manner between interconnected neurons, contributing to the propagation of the pathology and to the progressive nature of synucleinopathies. Here, we investigated the interaction of aSyn with membranes and trafficking machinery pathways using cellular models of PD that are amenable to detailed molecular analyses. We found that different species of aSyn can enter cells and form high molecular weight species, and that membrane binding properties are important for the internalization of aSyn. Once internalized, aSyn accumulates in intracellular inclusions. Interestingly, we found that internalization is blocked in the presence of dynamin inhibitors (blocked membrane scission), suggesting the involvement of the endocytic pathway in the internalization of aSyn. By screening a pool of small Rab-GTPase proteins (Rabs) which regulate membrane trafficking, we found that internalized aSyn partially colocalized with Rab5A and Rab7. Initially, aSyn accumulated in Rab4A-labelled vesicles and, at later stages, it reached the autophagy-lysosomal pathway (ALP) where it gets degraded. In total, our study emphasizes the importance of membrane binding, not only as part of the normal function but also as an important step in the internalization and subsequent accumulation of aSyn. Importantly, we identified a fundamental role for Rab proteins in the modulation of aSyn processing, clearance and spreading, suggesting that targeting Rab proteins may hold important therapeutic value in PD and other synucleinopathies.Electronic supplementary materialThe online version of this article (10.1186/s40478-018-0578-1) contains supplementary material, which is available to authorized users.

The uptake of tau amyloid fibrils is facilitated by the cellular prion protein and hampers prion propagation in cultured cells

“…A similar hypothesis has been proved true in the case of intercellular transfer of tau amyloids through tunneling nanotubes, where extracellular tau species (monomers and fibrils) activated the formation of new nanotubes and therefore facilitated fibrillar tau transport between neurons (15). On the other hand, protein aggregates (tau and synuclein) are trafficked in early endosomes and lysosomes after the internalization (20,40,41). The need of the cell to deal quickly with a large amount of exogenous and potentially dangerous material might clog the degradation system, with subsequent impact on the normal turnover of the endogenous proteins.…”

Physiological C-terminal truncation of α-synuclein potentiates the prion-like formation of pathological inclusions

“…Cumulatively, the in vitro and cell culture experiments revealed that C-truncated ␣syn is primed to aggregate, which may be crucial in initial disease pathogenesis. Also, ␣syn fibrils are known to be trafficked to lysosomes in neurons (21,98), where these truncations are likely formed (22, 30 -34), and may play a role in initial fibril-induced propagation of pathology, as the C-truncated forms of ␣syn are much more readily induced to aggregate compared with FL ␣syn.…”

In vitro models of synucleinopathies: informing on molecular mechanisms and protective strategies

“…Monomers readily passed through the membrane via diffusion while oligomers and fibrils can be taken up via dynamindependent endocytosis or macropinocytosis (Holmes et al 2013). Recent evidence suggests that membrane binding is a crucial step for a-Syn internalization, which is mediated by Rab proteins (Masaracchia et al 2018). A recent study characterizing the internalization mechanisms of a-Syn containing exosomes found that the process was an active as opposed to a passive process, yet resistant to conventional blockers of both clathrin and caveolin-mediated endocytosis as well as micropinocytosis (Delenclos et al 2017).…”