Misfolding and aggregation of alpha-synuclein (a-syn) are associated with a range of neurological disorders, including Parkinson's disease (PD). Fibrillar, insoluble aggregates of a-syn, known as Lewy bodies (LBs) are deposited in the substantia nigra and are a pathological hallmark of PD. a-syn is a natively unstructured protein, co-populating extended and more compact conformational forms under equilibrium. The fine balance of this equilibrium can be shifted due to changes in its environment such as alterations in metal content, ionic strength, free dopamine or others, promoting the assembly of a-syn into toxic conformations. Small, soluble oligomers preceding LB formation are thought to be causative, in vitro, different a-syn oligomers have been produced with alternate biochemical properties. Here the primary objective was to uncover the link between conformation and toxic gain of function by the use of functional assays in combination with ESI-IMS-MS. Epitope mapping procedures indicated that different a-syn oligomers have unique epitope features. Dye binding assays such as ThT and ANS fluorescence inferred that the various oligomer types differ in their amyloidogenicity and hydrophobicity. Furthermore, intracellular aggregation assays, MTT cell proliferation and Ca(ll) influx analysis in SH-SY5Y neuroblastoma cells showed that cellular effects correlated with structural features. ESI-IMS-MS spectra of the different oligomers have been acquired and allowed the conformations of the oligomer subsets to be determined. The oligomers assembled up to a hexameric form with a closed ring-like conformation. These results demonstrated that unique structural features are required for toxicity and that a subset of oligomers with characteristic structures may be pivotal in PD
