Distinguishing the Effect on the Rate and Yield of Aβ42 Aggregation by Green Tea Polyphenol EGCG

Deposition of A beta 42 aggregates in the form of amyloid plaques is a pathological hallmark of Alzheimer's disease. A desired avenue of intervention is the inhibition of A beta 42 aggregation. Epigallocatechin gallate (EGCG), the main polyphenol in green tea, has been generally considered an inhibitor of A beta aggregation. However, previous experiments focused on the reduction of the amount of A beta 42 aggregates, while the effect of EGCG on the rate of A beta 42 aggregation was not critically analyzed. Here we performed an experimental evaluation of A beta 42 aggregation kinetics in the absence and presence of EGCG at a wide range of concentrations. We found that EGCG reduced thioflavin T fluorescence in an EGCG concentration-dependent manner, suggesting that EGCG reduced the amount of A beta 42 fibrils. The effect of EGCG on the rate of A beta 42 aggregation appears to be bimodal. We found that higher EGCG-to-A beta 42 ratios promoted the rate of A beta 42 aggregation, while lower EGCG-toA beta 42 ratios inhibited the aggregation rate. To confirm that the reduction of thioflavin T fluorescence is due to the lowered aggregate quantity, but not due to perturbation of thioflavin T binding to A/342 fibrils, we probed the effect of EGCG on A beta 42 aggregation using site-directed spin labeling. Electron paramagnetic resonance of spin-labeled A beta 42 aggregates suggests that high EGCG-to-A beta 42 ratios led to a greatly reduced amount of A beta 42 fibrils, and these aggregates adopt similar structures as the fibrils in the no-EGCG sample. Potential implications of this work in designing prevention or therapeutic strategies using EGCG are discussed.