Structural insight into the differential effects of omega-3 and omega-6 fatty acids on the production of Abeta peptides and amyloid plaques

Abstract

Several studies have shown the protective effects of dietary enrichment of various lipids in several late-onset animal models of Alzheimer Disease (AD); however, none of the studies has determined which structure within a lipid determines its detrimental or beneficial effects on AD. High-sensitivity enzyme-linked immunosorbent assay (ELISA) shows that saturated fatty acids (SFAs), upstream omega-3 FAs, and arachidonic acid (AA) resulted in significantly higher secretion of both Aβ 40 and 42 peptides compared with long chain downstream omega-3 and monounsaturated FAs (MUFA). Their distinct detrimental action is believed to be due to a structural template found in their fatty acyl chains that lack SFAs, upstream omega-3 FAs, and AA. Immunoblotting experiments and use of APP-C99-transfected COS-7 cells suggest that FA-driven altered production of Aβ is mediated through γ-secretase cleavage of APP. An early-onset AD transgenic mouse model expressing the double-mutant form of human amyloid precursor protein (APP); Swedish (K670N/M671L) and Indiana (V717F), corroborated in vitro findings by showing lower levels of Aβ and amyloid plaques in the brain, when they were fed a low fat diet enriched in DHA. Our work contributes to the clarification of aspects of structure-activity relationships.

FIGURE 1.

Effects of fatty acids with no or single double bond on in vitro Aβ levels. The BSA control normalized Aβ levels in the conditioned medium after 12 h of incubation with 25 μm (stated otherwise in results) of: A, saturated & monounsaturated FAs: PA, PalA, SA, OA, cis- and trans-conjugated double bonds containing FAs: CLA and cis-PA. B, upstream omega-3 FAs: αLNA, StA, EPA, TXB3, and omega-6 FA: AA. The mean ± S.E. are shown for all plots (n = 3, except where noted). ***, p < 0.001; **, p < 0.01; *, p < 0.05.

FIGURE 2.

Effects of downstream omega-3 fatty acids on in vitro Aβ levels. The BSA control normalized Aβ levels in the conditioned medium after 12 h of incubation with 25 μm: A, downstream omega-3 FAs: DPA, ethyl-DHA, DTA, and B, DHA. C, Western blot of APP-C99 and total Aβ levels in the extracts of APP-C99-transfected COS-7 cells after 12 h of incubation with DHA. Respective molecular masses (kDa) are shown on the left. Plot shows densitometric analysis of the Western blot. D, plot shows in vitro Aβ40/42 ratios versus DHA concentration, symbols show individual replicates at each concentration of DHA used. The mean ± S.E. are shown, (n = 3, except where noted). ***, p < 0.001; **, p < 0.01; *, p < 0.05.

FIGURE 3.

Effects of low fat/+DHA diet on plaque pathology and in vivo Aβ levels. A, photomicrographs of Aβ stained mice brains in the neocortex (NC), hippocampus (HC), and amygdala (AG) of chow, low fat/−DHA and low fat/+DHA mice. Arrows indicate Aβ-stained plaques. Plots show quantitative analyses of plaque density (B) and Aβ40 and Aβ42 levels in the hippocampi of chow, low fat/−DHA and low fat/+DHA mice (C). The mean ± S.E. are shown, (n = 5 for each group). ***, p < 0.001; **, p < 0.01; *, p < 0.05.