Natural silibinin modulates amyloid precursor protein processing and amyloid-β protein clearance in APP/PS1 mice

Abstract

Silibinin has been shown to attenuate cognitive dysfunction and inhibit amyloid-beta (Aβ) aggregation in Alzheimer's disease (AD) models. However, the underlying mechanism by which silibinin improves cognition remains poorly understood. In this study, we investigated the effect of silibinin on β-secretase levels, Aβ enzymatic degradation, and oxidative stress in the brains of APP/PS1 mice with cognitive impairments. Oral administration of silibinin for 2 months significantly attenuated the cognitive deficits of APP/PS1 mice in the Y-maze test, novel object recognition test, and Morris water maze test. Biochemical analyses revealed that silibinin decreased Aβ deposition and the levels of soluble Aβ1-40/1-42 in the hippocampus by downregulating APP and BACE1 and upregulating NEP in APP/PS1 mice. In addition, silibinin decreased the MDA content and increased the activities of the antioxidant enzymes CAT, SOD, and NO. Based on our findings, silibinin is a potentially promising agent for preventing AD-associated Aβ pathology.

Keywords: Alzheimer’s disease; Antioxidant; Aβ degradation; Aβ generation; Silibinin.

Fig. 1

Silibinin increases spontaneous alternation behavior in the Y-maze effects of silibinin on learning and memory deficits of APP/PS1 mice in the Y-maze test. The 8-week silibinin treatment significantly increased spontaneous alternation behaviors in APP/PS1 mice. a Alternations (%) in the Y-maze test. b The total number of arm entries in the Y-maze test. The data are presented as means ± SEM and were analyzed with a one-way ANOVA, N = 10 animals per group, ^###p < 0.001 compared with the WT mice; **p < 0.01 compared with the placebo group

Fig. 2

Silibinin improves visual recognition in the novel object recognition test. Effects of silibinin on learning and memory deficits of APP/PS1 mice in the novel object recognition test. Silibinin groups showed significant improvements in the DI and PI. a The discrimination index recorded in the 1-h novel object recognition test. b Preferential index recorded in the retention session of the 1-h novel object recognition test. c Total time spent exploring objects A1 and A2. The data are presented as means ± SEM and were analyzed with a one-way ANOVA, N = 10 animals per group, ^###p < 0.001 compared with the WT mice; **p < 0.01 and ***p < 0.001 compared with the placebo group

Fig. 3

Silibinin improvement of spatial memory in Morris water maze test. Effects of silibinin on learning and memory deficits in the Morris water maze. Silibinin groups exhibited significant increases in time spent in the target quadrant and number of platform crossings. a Swimming speed in the training period. b Escape latency in the training period. c Mean time spent in the target quadrant in probe trial. d Mean number of platform crossings during the probe trial. e Representative traces of each group. The data are presented as means ± SEM and were analyzed with a one-way ANOVA, from ten mice per group, ^##p < 0.01 and ^###p < 0.001 compared with the WT mice; *p < 0.05, **p < 0.01, and ***p < 0.001 compared with the placebo group

Fig. 4

Silibinin reduces Aβ plaque deposits and soluble Aβ1-42, Aβ1-40 in APP/PS1 mice. Effects of silibinin on attenuating Aβ deposition in the hippocampus of APP/PS1 transgenic mice. The number of Aβ plaques and levels of soluble Aβ1-42 and Aβ1-40 were significantly reduced in the silibinin-treated groups. Images of the hippocampus from the a WT mice, b placebo group, c 100 mg/kg silibinin group, and d 200 mg/kg silibinin groups (images were captured using a high power 40× objective). e Quantification of Aβ deposits in each group. Results were calculated as the percentage of the Aβ area. f Soluble Aβ1-40 levels in the hippocampus of APP/PS1 mice. g Soluble Aβ1-42 levels in the hippocampus of APP/PS1 mice. Data are presented as the means ± SEM from five animals per group and four sections from each animal. ^###p < 0.001 compared with the WT mice and **p < 0.01, ***p < 0.001 compared with the placebo group

Fig. 5

Effects of silibinin on cortical oxidative damage (lipid peroxidation, nitrite, superoxide dismutase, and catalase levels. Silibinin exerted antioxidant effects on the cortex of APP/PS1 mice. Treatment with silibinin for 8 weeks reduced the MDA content and increased SOD, NO, and CAT activity in APP/PS1 mice. Data are presented as the means ± SEM from five animals per group. ^###p < 0.001 compared with the WT mice; ***P < 0.001 compared with the placebo group

Fig. 6

Silibinin regulates Aβ processing in APP/PS1 mice. Representative Western blots showing the levels of APP, BACE, IDE, and DEP in the hippocampus of APP/PS1 mice. After 8 weeks of treatment, silibinin significantly decreased the levels of APP and BACE in the hippocampus of APP/PS1 mice. Meanwhile, silibinin 200 mg/kg group significantly increased levels of the NEP protein in the hippocampus. Data are presented as the means ± SEM from four animals per group. ^###p < 0.001 compared with the WT mice; *p < 0.05 and **p < 0.01 compared with the placebo group