Abstract
<jats:p>Different conformations, or strains, of α-synuclein (α-syn) aggregates are believed to be responsible for the distinct seeding propensities, propagation profiles, and clinical presentations in Lewy body diseases (LBD) and multiple system atrophy (MSA). While biochemical properties and strain differences of insoluble deposits have been extensively characterized, the understanding of what influence soluble α-syn species may have on these processes is limited to a small number of studies focusing on complex mixtures of soluble species or on a single α-synucleinopathy. Given that soluble oligomers are considered highly pathologically relevant, we isolated and characterized the biochemical, seeding, and toxicity properties of size-fractionated soluble α-syn species from MSA and LBD brains, comparing them to species from control brains without known neurological disease (Ctrl). We observed that levels of differently sized oligomers phosphorylated at Ser129, as well as soluble large oligomers (>450 kDa), were increased in LBD compared to both MSA and Ctrl brains. Nevertheless, species derived from MSA brain exhibited seeding activity across the spectrum of α-syn species (oligomers, monomers, and truncated forms) in the seed amplification assay, whereas only oligomeric species (>150 kDa) from LBD cases were seeding-prone. In the HEK293 α-syn (A53T)-YFP biosensor line, as well as in murine primary neurons, only large oligomers (>450 kDa) from MSA cases induced seeding and aggregation of α-syn. Taken together, our study suggests that soluble α-syn species derived from MSA and LBD brains show different biochemical, aggregation and seeding patterns, presumably due to strain variations of the respective oligomers. Our findings provide novel insight into the pathogenesis of different α-synucleinopathies, which may guide us in the development of targeted therapeutics.</jats:p>