Dietary docosahexaenoic acid and docosapentaenoic acid ameliorate amyloid-beta and tau pathology via a mechanism involving presenilin 1 levels

Abstract

The underlying cause of sporadic Alzheimer disease (AD) is unknown, but a number of environmental and genetic factors are likely to be involved. One environmental factor that is increasingly being recognized as contributing to brain aging is diet, which has evolved markedly over modern history. Here we show that dietary supplementation with docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, in the 3xTg-AD mouse model of AD reduced the intraneuronal accumulation of both amyloid-beta (Abeta) and tau. In contrast, combining DHA with n-6 fatty acids, either arachidonic acid or docosapentaenoic acid (DPAn-6), diminished the efficacy of DHA over a 12 month period. Here we report the novel finding that the mechanism accounting for the reduction in soluble Abeta was attributable to a decrease in steady-state levels of presenilin 1, and not to altered processing of the amyloid precursor protein by either the alpha- or beta-secretase. Furthermore, the presence of DPAn-6 in the diet reduced levels of early-stage phospho-tau epitopes, which correlated with a reduction in phosphorylated c-Jun N-terminal kinase, a putative tau kinase. Collectively, these results suggest that DHA and DPAn-6 supplementations could be a beneficial natural therapy for AD.

Figure 1.

Whole-brain and RBC fatty acid profiles in 3xTg-AD mice after PUFA dietary supplementation. a, Whole-brain homogenates and RBC fatty acid profiles of the four groups after 3 months (n = 6) of supplementation with various PUFAs. Values are expressed as a weight percentage of total brain (a) or RBC (b) fatty acids. The four diets are shown as control, DHA, DHA–DPA, and DHA–ARA. Error bars indicate SEM. DMA, Dimethylacetals; LA, linoleic acid (n-6); ADRENIC, adrenic acid (n-6).

Figure 2.

Whole-brain phospholipid fatty acid profiles in 3xTg-AD mice after dietary supplementation. Whole-brain phospholipid fatty acid profiles of the four groups after 3 months of dietary supplementation (n = 6) are shown. Values are expressed as a weight percentage of total brain fatty acids. The four diets are shown as control, DHA, DHA–DPA, and DHA–ARA. Error bars indicate SEM. DMA, Dimethylacetals; LA, linoleic acid (n-6); ADRENIC, adrenic acid (n-6).

Figure 3.

Aβ levels in 3xTg-AD mice after PUFA dietary supplementation. a–f, Soluble and insoluble Aβ₄₀ and Aβ₄₂ levels were measured from 3xTg-AD whole-brain homogenates after 3 months (a, b; n = 6), 6 months (c, d; n = 6), and 9 months (e, f; n = 6) of dietary supplementation. Soluble Aβ₄₀ and Aβ₄₂ levels were significantly decreased in the treatment diets after 3 months of supplementation (p < 0.05), in the DHA and DHA–DPA diets after 6 months of treatment (p < 0.05), and only Aβ₄₀ in the DHA group after 9 months of supplementation (p < 0.05). Insoluble Aβ₄₀ and Aβ₄₂ levels were unaffected but demonstrated a gradual increase in levels as time progressed. g, DAB staining with 6E10 shows Aβ-like immunoreactivity in 40 μm sections from 3 month diet-treated mice. The hippocampus region is shown (original magnification, 5×). DAB Aβ-like immunoreactivity was elevated in cell bodies of the hippocampus and amygdale, corresponding to the decreased soluble Aβ₄₀ and Aβ₄₂ as measured by ELISA. h, Quantification of g. Error bars indicate SEM.

Figure 4.

Aβ oligomer, APP fragments, and IDE, TTR, and ApoE steady-state levels in 3xTg-AD mice after DHA dietary supplementation. Western blot analyses of protein extracts from whole-brain homogenates of 3 month diet-treated 3xTg-AD mice (n = 6) are shown as alternating lanes: C, control; D, DHA; DP, DHA–DPA; DA, DHA–ARA. a, Dot blot analyses for oligomer levels in brain homogenates from 6-, 9-, and 12-month-old 3xTg-AD mice treated with diets for 3, 6, and 9 months, respectively (n = 6 per dietary group). b, Quantification of a. c, Steady-state levels of APP and APP CTFs C83 and C99. A longer exposure of APP is also shown to expose a band at 56 kDa corresponding to the Aβ 12mer oligomer (APP56). d, Quantification of protein blots from c shown normalized to β-actin levels as a loading control. e, Steady-state levels of IDE, TTR, and ApoE. f, Quantification of protein blots from e shown normalized to β-actin levels as a loading control. Error bars indicate SEM.

Figure 5.

Secretase steady-state levels in 3xTg-AD mice after DHA dietary treatment. Western blot analyses of protein extracts from whole-brain homogenates of 3 month diet-treated 3xTg-AD mice (n = 6) are shown as alternating lanes: C, control; D, DHA; DP, DHA–DPA; DA, DHA–ARA. a, Steady-state levels of ADAM10 and BACE. b, Quantification of protein blots from a shown normalized to β-actin levels as a loading control. c, Steady-state levels of PS1 and nicastrin. d, Quantification of protein blots from c shown normalized to β-actin levels as a loading control. e, Steady-state levels of PS1 from 6 and 9 months of dietary treatment brain homogenates. f, Quantification of protein blots from e shown normalized to β-actin levels as a loading control. g, SHSY5Y cells treated for 48 h with either 0.3 μg/ml DHA complexed 3:1 to BSA (n = 3) or with the equivalent BSA alone (n = 3). mRNA was extracted, and real-time PCR was used to quantify PS1 mRNA levels. The graph shows PS1 mRNA levels normalized to β-actin mRNA as a loading control. DHA significantly reduced PS1 mRNA. The asterisk indicates significance (p < 0.05). Error bars indicate SEM.

Figure 6.

Tau steady-state levels in 3xTg-AD mice after DHA dietary enhancement. Western blot analyses of protein extracts from whole-brain homogenates of 3 month dietary-treated 3xTg-AD mice (n = 6) are shown as alternating lanes: C, control; D, DHA; DP, DHA–DPA; DA, DHA–ARA. a, Steady-state levels of total human tau from 3 month PUFA-treated 3xTg-AD mice. b, Quantification of protein blots from a shown normalized to β-actin levels as a loading control. c, d, Representative HT7 DAB staining of hippocampus from control and DHA–DPA-treated animals. e, Steady-state levels of total human tau from 6 month PUFA-treated 3xTg-AD mice. f, Quantification of protein blots from e shown normalized to β-actin levels as a loading control. g, Steady-state levels of total human tau from 9 month PUFA-treated 3xTg-AD mice. h, Quantification of protein blots from g shown normalized to β-actin levels as a loading control. The asterisk denotes significance (p < 0.05). Error bars indicate SEM.

Figure 7.

DPA reduces conformationally altered and phosphorylated tau in 3xTg-AD mice after dietary supplementation. a, DAB staining with MC1 shows conformationally altered tau immunoreactivity in 40 μm sections from 3 month diet-treated mice (control diet, DHA diet, and DHA–DPA diet). The hippocampus region is shown (magnification, 5×). DAB tau immunoreactivity was elevated in cell bodies of the hippocampus. b, Quantification of a. c, Steady-state levels of phospho-tau epitopes from 9 month PUFA-treated 3xTg-AD mice. d, Steady-state levels of putative tau kinases from 9 month PUFA-treated 3xTg-AD mice. e, Quantification of protein blots from c and d shown normalized to β-actin levels as a loading control. The asterisk denotes significance (p < 0.05). Error bars indicate SEM.