Metformin Ameliorates A β Pathology by Insulin-Degrading Enzyme in a Transgenic Mouse Model of Alzheimer's Disease

Abstract

Alzheimer's disease (AD) is the most common neurodegenerative disease. The accumulation of amyloid beta (Aβ) is the main pathology of AD. Metformin, a well-known antidiabetic drug, has been reported to have AD-protective effect. However, the mechanism is still unclear. In this study, we tried to figure out whether metformin could activate insulin-degrading enzyme (IDE) to ameliorate Aβ-induced pathology. Morris water maze and Y-maze results indicated that metformin could improve the learning and memory ability in APP^swe/PS1^dE9 (APP/PS1) transgenic mice. ¹⁸F-FDG PET-CT result showed that metformin could ameliorate the neural dysfunction in APP/PS1 transgenic mice. PCR analysis showed that metformin could effectively improve the mRNA expression level of nerve and synapse-related genes (Syp, Ngf, and Bdnf) in the brain. Metformin decreased oxidative stress (malondialdehyde and superoxide dismutase) and neuroinflammation (IL-1β and IL-6) in APP/PS1 mice. In addition, metformin obviously reduced the Aβ level in the brain of APP/PS1 mice. Metformin did not affect the enzyme activities and mRNA expression levels of Aβ-related secretases (ADAM10, BACE1, and PS1). Meanwhile, metformin also did not affect the mRNA expression levels of Aβ-related transporters (LRP1 and RAGE). Metformin increased the protein levels of p-AMPK and IDE in the brain of APP/PS1 mice, which might be the key mechanism of metformin on AD. In conclusion, the well-known antidiabetic drug, metformin, could be a promising drug for AD treatment.

Figure 1

Metformin improves learning and memory impairment in APP/PS1 mice. (a) Escape latency of the five-day Morris water maze. (b) Time spent in the target quadrant in the Morris water maze. (c) Crossing times of the target platform in the Morris water maze. (d) Swimming speed in the Morris water maze. (e) Percentage of spontaneous alternation of Y-maze. Data represent the mean ± SEM (n = 15 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.

Figure 2

Metformin improves glucose metabolism in APP/PS1 mice. (a) PET-CT images. (b) ¹⁸F-FDG uptake of mice brains. Data represent the mean ± SEM (n = 3 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.

Figure 3

Metformin improves neurotrophic factors in APP/PS1 mice. The mRNA levels of (a) Syp, (b) Bdnf, and (c) Ngf in the APP/PS1 mice. Data represent the mean ± SEM (n = 6 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.

Figure 4

Metformin ameliorates oxidative stress and neuroinflammation in APP/PS1 mice. The level of (a) MDA and the activity of (b) SOD in the brain of APP/PS1 mice. The levels of (a) IL-1β and (b) IL-6 in the brain of APP/PS1 mice. Experimental values were expressed as the mean ± SEM (n = 6 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.

Figure 5

Metformin decreases Aβ levels in APP/PS1 mice. The levels of (a) Aβ1-40 and (b) Aβ1-42 in the brain of APP/PS1 mice. ThT staining of the brain slides in APP/PS1 mice. Experimental values were expressed as the mean ± SEM (n = 6 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1. Bar: 100 μm.

Figure 6

Metformin has no effect on Aβ production and transportation-related genes in APP/PS1 mice. The activities of (a) α-, (b) β-, and (c) γ-secretases. The mRNA expressions of (d) ADAM10, (e) BACE1, (f) PS1, (g) LRP1, and (h) RAGE. Experimental values were expressed as the mean ± SEM (n = 6 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.

Figure 7

Metformin activates AMPK and increases IDE in the brain of APP/PS1 mice. (a) The representative bands of p-AMPK, AMPK, IDE, NEP, and ACTB. Western blot analysis: (b) p-AMPK/AMPK, (c) IDE/ACTB, and (d) NEP/ACTB. Experimental values were expressed as the mean ± SEM (n = 3 per group). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 vs. WT; ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 vs. APP/PS1.