Extracellular α-synuclein drives sphingosine 1-phosphate receptor subtype 1 out of lipid rafts, leading to impaired inhibitory G-protein signaling

α-Synuclein (α-Syn)-positive intracytoplasmic inclusions, known as Lewy bodies, are thought to be involved in the pathogenesis of Lewy body diseases, such as Parkinson's disease (PD). Although growing evidence suggests that cell-to-cell transmission of α-Syn is associated with the progression of PD and that extracellular α-Syn promotes formation of inclusion bodies, its precise mechanism of action in the extracellular space remains unclear. Here, as indicated by both conventional fractionation techniques and FRET-based protein-protein interaction analysis, we demonstrate that extracellular α-Syn causes expulsion of sphingosine 1-phosphate receptor subtype 1 (S1P₁R) from the lipid raft fractions. S1P₁R regulates vesicular trafficking, and its expulsion involved α-Syn binding to membrane-surface gangliosides. Consequently, the S1P₁R became refractory to S1P stimulation required for activating inhibitory G-protein (G_i) in the plasma membranes. Moreover, the extracellular α-Syn also induced uncoupling of the S1P₁R on internal vesicles, resulting in the reduced amount of CD63 molecule (CD63) in the lumen of multivesicular endosomes, together with a decrease in CD63 in the released exosomes from α-Syn-treated cells. Furthermore, cholesterol-depleting agent-induced S1P₁R expulsion from the rafts also resulted in S1P₁R uncoupling. Taken together, these results suggest that extracellular α-Syn-induced expulsion of S1P₁R from lipid rafts promotes the uncoupling of S1P₁R from G_i, thereby blocking subsequent G_i signals, such as inhibition of cargo sorting into exosomal vesicles in multivesicular endosomes. These findings help shed additional light on PD pathogenesis.

Involvement of Gβγ subunits of Gi protein coupled with S1P receptor on multivesicular endosomes in F-actin formation and cargo sorting into exosomes

Exosomes play a critical role in cell-to-cell communication by delivering cargo molecules to recipient cells. However, the mechanism underlying the generation of the exosomal multivesicular endosome (MVE) is one of the mysteries in the field of endosome research. Although sphingolipid metabolites such as ceramide and sphingosine 1-phosphate (S1P) are known to play important roles in MVE formation and maturation, the detailed molecular mechanisms are still unclear. Here, we show that Rho family GTPases, including Cdc42 and Rac1, are constitutively activated on exosomal MVEs and are regulated by S1P signaling as measured by fluorescence resonance energy transfer (FRET)-based conformational changes. Moreover, we detected S1P signaling-induced filamentous actin (F-actin) formation. A selective inhibitor of Gβγ subunits, M119, strongly inhibited both F-actin formation on MVEs and cargo sorting into exosomal intralumenal vesicles of MVEs, both of which were fully rescued by the simultaneous expression of constitutively active Cdc42 and Rac1. Our results shed light on the mechanism underlying exosomal MVE maturation and inform the understanding of the physiological relevance of continuous activation of the S1P receptor and subsequent downstream G protein signaling to Gβγ subunits/Rho family GTPases-regulated F-actin formation on MVEs for cargo sorting into exosomal intralumenal vesicles.