Non-Esterified Fatty Acids Generate Distinct Low-Molecular Weight Amyloid-β (Aβ42) Oligomers along Pathway Different from Fibril Formation

Amyloid-beta (A beta) peptide aggregation is known to play a central role in the etiology of Alzheimer's disease (AD). Among various aggregates, low-molecular weight soluble oligomers of A beta are increasingly believed to be the primary neurotoxic agents responsible for memory impairment. Anionic interfaces are known to influence the A beta aggregation process significantly. Here, we report the effects of interfaces formed by medium-chain (C9-C12), saturated non-esterified fatty acids (NEFAs) on A beta 42 aggregation. NEFAs uniquely affected A beta 42 aggregation rates that depended on both the ratio of A beta:NEFA as well the critical micelle concentration (CMC) of the NEFAs. More importantly, irrespective of the kind of NEFA used, we observed that two distinct oligomers, 12-18 mers and 4-5 mers were formed via different pathway of aggregation under specific experimental conditions: (i) 12-18 mers were generated near the CMC in which NEFAs augment the rate of A beta 42 aggregation towards fibril formation, and, (ii) 4-5 mers were formed above the CMC, where NEFAs inhibit fibril formation. The data indicated that both 12-18 mers and 4-5 mers are formed along an alternate pathway called 'off-pathway' that did not result in fibril formation and yet have subtle structural and morphological differences that distinguish their bulk molecular behavior. These observations, (i) reflect the possible mechanism of A beta aggregation in physiological lipid-rich environments, and (ii) reiterate the fact that all oligomeric forms of A beta need not be obligatory intermediates of the fibril formation pathway.