|dc.description.abstracteng||We hypothesized that cells release and transfer α-Syn associated with EVs, followed by highly efficient internalization and induction of α-Syn aggregation in previously healthy neurons. EVs of 40-100 nm diameter can either be derived from the multivesicular endosome (MVE) (exosomes) or shedded from the plasma membrane (microvesicles). Both types of vesicles are involved in the release of toxic cellular content and intercellular transfer of proteins, lipids and RNA and vesicular α-Syn may be internalized more efficiently by recipient cells than the free protein and induce greater toxicity (Danzer et al. 2012).
We could show that α-Syn is released with EVs and targeted to EVs by a completely novel mechanism based on SUMO modification. So far, ubiquitination had been regarded as an exclusive, necessary and sufficient signal for EV release of proteins. We could decipher the molecular mechanism of this novel sorting pathway, demonstrating that SUMO-dependent targeting to EVs depends on the endosomal sorting complex required for transport (ESCRT). Interestingly, the interaction of SUMO with ESCRT formation sites is mediated by SUMO binding to phosphoinositol containing membrane microdomains, most likely at the inner leaflet of the plasma membrane. By NMR spectrometry, we mapped the lipid interaction domain of SUMO to its hydrophobic cleft. Moreover, we could demonstrate that inhibition of SUMOylation by different genetic approaches strongly decreases the release of α-Syn with EVs. In contrast, enhancing SUMOylation by co-expression of SUMO or fusion of SUMO to α-Syn increased α-Syn sorting to EVs. Similar to SUMO, the release of α-Syn within EVs was inhibited by co expression of a dominant-negative VPS4 mutant, indicating that EV sorting by SUMO modification is ESCRT-dependent.
Our findings are thus of highest relevance for the understanding of Parkinson’s disease pathogenesis and progression at the molecular level. Moreover, we propose that SUMO-dependent sorting constitutes a mechanism with more general implications for cell biology.||de