Comparison of α-Synuclein Fibril Inhibition by Four Different Amyloid Inhibitors

Aggregation of alpha-synuclein (alpha-Syn) into toxic oligomers and fibrils leads to Parkinson's disease (PD) pathogenesis. Molecules that can inhibit the fibrillization and oligomerization of alpha-Syn have potential therapeutic value. Here, we studied four selective amyloid inhibitors: dopamine (Dopa), amphotericin-B (Amph), epigallocatechingallate (EGCG), and quinacrinedihydrochloride (Quin) for their effect on oligomerization, fibrillization, and preformed fibrils of alpha-Syn. The aggregation kinetics of alpha-Syn using ThT fluorescence and conformational transition by circular dichroism (CD) in the presence and absence of these four compounds suggest that, except Quin, the remaining three molecules inhibit alpha-Syn aggregation in a concentration dependent manner. Consistent with the aggregation kinetics data, the morphological study of aggregates formed in the presence of these compounds showed corresponding decrease in fibrillar size. The analysis of cell viability using MTT assay showed reduction in toxicity of alpha-Syn aggregates formed in the presence of these compounds, which also correlates with reduction of exposed hydrophobic surface as studied by ANS binding. Additionally, these inhibitors, except Quin, demonstrated reduction in size as well as the toxicity of oligomeric/fibrillar aggregates of alpha-Syn. The residue specific interaction to low molecular weight (LMW) species of alpha-Syn by 2D NMR study revealed that, the region and extent of binding are different for all these molecules. Furthermore, fibril-binding data using SPR suggested that there is no direct relationship between the binding affinity and fibril inhibition by these compounds. The present study suggests that sequence based interaction of small molecules with soluble a-Syn might dictate their inhibition or modulation capacity, which might be helpful in designing modulators of alpha-Syn aggregation.