Free Cholesterol Induces Higher b-Sheet Content in Ab Peptide Oligomers by Aromatic Interaction with Phe19

Accumulating experimental evidence support an enhancing effect of free cholesterol on amyloid-beta (Ab) aggregation. To probe the mechanisms of cholesterol-mediated Ab aggregation, we applied all-atom molecular dynamic simulations on Ab42 peptides in presence of free cholesterol. Several control systems were also designed to examine the specificity of cholesterol-residue interactions, including mutation on aromatic residue, substitution of cholesterol with sphingomyelin (SM) and DPPC bilayer, and a mixing SM and cholesterol. Each system was performed 4 independent simulations, with a total time of 560 ns. It was found that cholesterol increased b-sheet formation by 4 folds, but the Phe19RSer mutation on Ab42 peptide totally eliminated cholesterol’s effect. A stable contact was recognized between the steroid group of cholesterol and the Benzyl group of Phe19. Interestingly, our simulation revealed a regular 1 ns time interval between the establishment of cholesterol-phenylalanine contact and consequent b-sheet formation, suggesting an important role of steroid-benzyl interaction in cholesterol-mediated aggregation. The presence of SM slightly increased b-sheet formation, but the mixture of cholesterol and SM had a strong induction effect. Also, the measurement of Phe19-lipid distance indicates that aromatic side chains of peptides prone to bind to cholesterol on the surface of the mixed micelle. In the DPPC system, polar chains were attracted to the surface of membrane, yielding moderate increase of b-sheet formation. These results shed light on the mechanism of cholesterol-mediated fibrillogenesis, and help to differentiate the effects o