Exosomal cell-to-cell transmission of alpha synuclein oligomers

Background: Aggregation of alpha-synuclein (asyn) and resulting cytotoxicity is a hallmark of sporadic and familial Parkinson's disease (PD) as well as dementia with Lewy bodies, with recent evidence implicating oligomeric and pre-fibrillar forms of asyn as the pathogenic species. Recent in vitro studies support the idea of transcellular spread of extracellular, secreted asyn across membranes. The aim of this study is to characterize the transcellular spread of asyn oligomers and determine their extracellular location. Results: Using a novel protein fragment complementation assay where asyn is fused to non-bioluminescent amino-or carboxy-terminus fragments of humanized Gaussia Luciferase we demonstrate here that asyn oligomers can be found in at least two extracellular fractions: either associated with exosomes or free. Exosome-associated asyn oligomers are more likely to be taken up by recipient cells and can induce more toxicity compared to free asyn oligomers. Specifically, we determine that asyn oligomers are present on both the outside as well as inside of exosomes. Notably, the pathway of secretion of asyn oligomers is strongly influenced by autophagic activity. Conclusions: Our data suggest that asyn may be secreted via different secretory pathways. We hypothesize that exosome-mediated release of asyn oligomers is a mechanism whereby cells clear toxic asyn oligomers when autophagic mechanisms fail to be sufficient. Preventing the early events in asyn exosomal release and uptake by inducing autophagy may be a novel approach to halt disease spreading in PD and other synucleinopathies.