Amyloid Fibril Formation of α‐Synuclein Is Modulated via the Estrogen Receptor Ligand Binding Domain of Estrogen Receptor α Bound with Tamoxifen‐based Small Molecules

Alpha‐synuclein (αSyn) forms amyloid fibrils in the brain of patients suffering from Parkinson's disease. Aiming at the development of a new therapy via regulating amyloid fibril formation of αSyn, we studied estrogen receptor ligand‐binding domain (ERLBD) of estrogen receptor α (ΕRα). The ERLBD encoding gene was cloned, expressed by IPTG induction, and successfully purified by Ni 2+ ‐affinity chromatography followed by anion‐exchange chromatography. In the thioflavin T (ThT) fluorescence assay, we examined the kinetics of amyloid fibril formation of αSyn in the presence of ERLBD protein. Dose‐dependent inhibition of amyloid fibril formation of αSyn by ERLBD protein was observed, indicating that ERLBD protein binds to αSyn, thereby blocking amyloid fibril formation. The kinetics of amyloid fibril formation of αSyn were compared in the absence and presence of a small molecule, such as 17β‐estradiol (E2, agonist) or tamoxifen (TMX, antagonist) of ΕRα. Regardless of the presence of E2, we found that ERLBD protein inhibits αSyn from forming amyloid fibrils. On the other hand, TMX reversed the inhibition effect of ERLBD protein on amyloid fibril formation of αSyn. Interestingly, the photo‐reactive hybrid ligand 2‐phenylquinoline‐4‐hydroxytamoxifen (PQ‐OHT) was expected to exhibit the anti‐inhibitory effect of ERLBD on amyloid fibril formation of αSyn. Upon illuminating with a UV lamp, PQ‐OHT was found to degrade ERLBD protein as well. These observations suggest the examination of the possibility that a complex of ERLBD‐αSyn amyloid fibril can be eliminated via the ligand PQ‐OHT upon exposure to light for therapeutic purposes.