The Aggregation Conditions Define Whether EGCG is an Inhibitor or Enhancer of α-Synuclein Amyloid Fibril Formation

Abstract

The amyloid fibril formation by α -synuclein is a hallmark of various neurodegenerative disorders, most notably Parkinson’s disease. Epigallocatechin gallate (EGCG) has been reported to be an efficient inhibitor of amyloid formation by numerous proteins, among them α -synuclein. Here, we show that this applies only to a small region of the relevant parameter space, in particular to solution conditions where EGCG readily oxidizes, and we find that the oxidation product is a much more potent inhibitor compared to the unmodified EGCG. In addition to its inhibitory effects, EGCG and its oxidation products can under some conditions even accelerate α -synuclein amyloid fibril formation through facilitating its heterogeneous primary nucleation. Furthermore, we show through quantitative seeding experiments that, contrary to previous reports, EGCG is not able to re-model α -synuclein amyloid fibrils into seeding-incompetent structures. Taken together, our results paint a complex picture of EGCG as a compound that can under some conditions inhibit the amyloid fibril formation of α -synuclein, but the inhibitory action is not robust against various physiologically relevant changes in experimental conditions. Our results are important for the development of strategies to identify and characterize promising amyloid inhibitors.

Tannic Acid Inhibits α-Synuclein Amyloid Fibril Formation via Binding to the Monomer N-terminal Domain

“…However, the conditions under which these compounds are able to successfully prevent amyloid aggregation are varied. EGCG was found to inhibit insulin fibril formation at pH 7.0 but not at pH 6.0 [32], while it completely inhibits αS aggregation at pH 7.0 and actually accelerates aggregation at pH 6.0 [33]. The results of the current work show that TA is an efficient inhibitor of αS at pH 7.4 but not at pH 6.0, which is consistent with previous work in the literature utilizing similar polyphenols and IDPs [32,33].…”

“…S2a, orange). This behavior is similar to previous observations of the interaction between the polyphenol EGCG and αS [32,33]. To ensure that TA does not interfere with the ability of ThT to fluoresce upon binding to amyloid fibrils and to gain a detailed picture of the aggregates formed in the presence of TA, we acquired AFM images of the endpoints of the αS control (Fig.…”

“…EGCG was found to inhibit insulin fibril formation at pH 7.0 but not at pH 6.0 [32], while it completely inhibits αS aggregation at pH 7.0 and actually accelerates aggregation at pH 6.0 [33]. The results of the current work show that TA is an efficient inhibitor of αS at pH 7.4 but not at pH 6.0, which is consistent with previous work in the literature utilizing similar polyphenols and IDPs [32,33]. The pH dependent inhibitory behavior of these polyphenols is thought to be a result of autooxidation at neutral pH [42,43].…”

“…Slc) of TA at pH 7.4 relative to pH 6.0. Because of these changes, it is likely that TA is undergoing pH induced autooxidation at pH 7.4 where the vicinal hydroxyl groups form an assortment of branched ortho-quinone (oquinone) like compounds [32,33,35].…”

Green Tea Epigallocatechin-3-gallate (EGCG) Targeting Protein Misfolding in Drug Discovery for Neurodegenerative Diseases

“…In a recent study, researchers confirmed that EGCG can accelerate α-syn amyloid fibril formation by facilitating its heterogeneous primary nucleation. This work showed that the aggregation conditions define whether EGCG is an inhibitor or enhancer of α-syn amyloid fibril formation, since the inhibitory action is not robust against various physiologically relevant changes in experimental conditions [241].…”