Scutellarin inhibits the uninduced and metal-induced aggregation of α-Synuclein and disaggregates preformed fibrils: implications for Parkinson's disease

The aggregation of the protein alpha synuclein (alpha-Syn), a known contributor in Parkinson's disease (PD) pathogenesis is triggered by transition metal ions through occupational exposure and disrupted metal ion homeostasis. Naturally occurring small molecules such as polyphenols have emerged as promising inhibitors of alpha-Syn fibrillation and toxicity and could be potential therapeutic agents against PD. Here, using an array of biophysical tools combined with cellular assays, we demonstrate that the novel polyphenolic compound scutellarin efficiently inhibits the uninduced and metal-induced fibrillation of alpha-Syn by acting at the nucleation stage and stabilizes a partially folded intermediate of alpha-Syn to form SDS-resistant, higher-order oligomers (similar to 680 kDa) and also disaggregates preformed fibrils of alpha-Syn into similar type of higher-order oligomers. ANS binding assay, fluorescence lifetime measurements and cell-toxicity experiments reveal scutellarin-generated oligomers as compact, low hydrophobicity structures with modulated surface properties and significantly reduced cytotoxicity than the fibrillation intermediates of alpha-Syn control. Fluorescence spectroscopy and isothermal titration calorimetry establish the binding between scutellarin and alpha-Syn to be non-covalent in nature and of moderate affinity (K-a similar to 10(5) M-1). Molecular docking approaches suggest binding of scutellarin to the residues present in the NAC region and C-terminus of monomeric alpha-Syn and the C-terminal residues of fibrillar alpha-Syn, demonstrating inhibition of fibrillation upon binding to these residues and possible stabilization of the autoinhibitory conformation of alpha-Syn. These findings reveal interesting insights into the mechanism of scutellarin action and establish it as an efficient modulator of uninduced as well as metal-induced alpha-Syn fibrillation and toxicity.