Acceleration of brain amyloidosis in an Alzheimer's disease mouse model by a folate, vitamin B6 and B12-deficient diet

Abstract

Epidemiological and clinical studies indicate that elevated circulating level of homocysteine (Hcy) is a risk factor for developing Alzheimer's disease (AD). Dietary deficiency of folate, vitamin B6 and B12 results in a significant increase of Hcy levels, a condition also known as hyperhomocysteinemia (HHcy). In the present study we tested the hypothesis that a diet deficient for these three important factors when administered to a mouse model of AD, i.e. Tg2576, will result in HHcy and in an acceleration of their amylodotic phenotype. Compared with Tg2576 mice on regular chow, the ones receiving the diet deficient for folate, B6 and B12 developed HHcy. This condition was associated with a significant increase in Abeta levels in the cortex and hippocampus, and an elevation of Abeta deposits in the same regions. No significant changes were observed for steady-state levels of total APP, BACE-1, ADAM-10, PS1 and nicastrin in the brains of mice with HHcy. No differences were observed for the main Abeta catabolic pathways, i.e. IDE and neprilysin proteins, or the Abeta chaperone apolipoprotein E. Our findings demonstrate that a dietary condition which leads to HHcy may also result in increased Abeta levels and deposition in a transgenic mouse model of AD-like amylodosis. They further support the concept that dietary factors can contribute to the development of AD neuropathology.

Figure 1

Dietary induction of HHcy in Tg2576 mice. Tg2576 mice fed for 7 months with a folate, vitamin B6 and vitamin B12 deficient diet (Diet group, n = 6) had higher level of plasma homocysteine than Tg2576 fed with vehicle diet (Ctrl group, n = 7). Values represent mean ± S.E.M. *P < 0.05.

Figure 2

Diet-induced HHcy in Tg2576 mice results in increased Aβ peptide levels. RIPA-soluble (RIPA) and formic acid (FA) extractable Aβ1-40 (A, B), and Aβ1-42 (C, D) levels in the cortex (Ctx) and hippocampus (Hippo) from Tg2576 on a folate deficient diet (Diet group, n = 6) or vehicle (Ctrl group, n = 7) were measured by sandwich ELISA. Values represent mean ± S.E.M. *P < 0.05.

Figure 3

HHcy in Tg2576 mice results in increased Aβ deposition. A) Representative sections of brains of Tg2576 receiving folate deficient diet (Diet group, n = 5), or vehicle (Ctrl group, n = 6) immunostained with 4G8 antibody. B) Quantification of the area occupied by Aβ immunoreactivity in hippocampus and somatosensory cortex (SSC) of Tg2576. Values represent mean ± S.E.M. *P < 0.05.

Figure 4

APP metabolism in Tg2576 mice with diet-induced HHcy A) Representative western blots of APP, sAPPα, sAPPβ, CTFs (C99 and C83), ADAM10, BACE1, PS1 and Nicastrin in brain homogenates from Diet group (n = 6) or Ctrl group (n = 7). B) Densitometric analyses of the immunoreactivities to the antibodies shown in panel A (white bars: Ctrl group; black bars: Diet group). Values represent mean ± S.E.M. *P < 0.05.

Figure 5

Protein levels of neprilysin (NEP), IDE and apoE in Tg2576 mice with diet-induced HHcy. A) Representative western blots of NEP, IDE and apoE in brain homogenates from Tg2576 on control diet (Ctrl group, n = 7) or folate deficient diet (Diet group, n = 6). B) Densitometric analyses of the immunoreactivities to the antibodies shown in panel A (white bars: Ctrl group; black bars: Diet group). Values represent mean ± S.E.M