(-)-Epigallocatechin-3-gallate ameliorates learning and memory deficits by adjusting the balance of TrkA/p75NTR signaling in APP/PS1 transgenic mice

Abstract

Alzheimer's disease (AD) is pathologically characterized by deposition of β-amyloid (Aβ) peptides, which closely correlates with the balance of nerve growth factor (NGF)-related TrkA/p75NTR signaling. (-)-Epigallocatechin-3-gallate (EGCG) is used for prevention and treatment of many neurodegenerative diseases, including AD. However, whether the neuroprotective effects of EGCG treatment were via modulating the balance of TrkA/p75NTR signaling was still unknown. In this study, we found that EGCG treatment (2 mg·kg(-1)·day(-1)) dramatically ameliorated the cognitive impairments, reduced the overexpressions of Aβ(1-40) and amyloid precursor protein (APP), and inhibited the neuronal apoptosis in the APP/PS1 mice. Interestingly, the EGCG treatment enhanced the relative expression level of NGF by increasing the NGF/proNGF ratio in the APP/PS1 mice. Moreover, after EGCG treatment, TrkA signaling was activated by increasing the phosphorylation of TrkA following the increased phosphorylation of c-Raf, ERK1/2, and cAMP response element-binding protein (CREB), simultaneously the p75NTR signaling was significantly inhibited by decreasing the p75ICD expression, JNK2 phosphorylation, and cleaved-caspase 3 expression, so that the Aβ deposits and neuronal apoptosis in the hippocampus were inhibited.

Fig. 1

EGCG treatment ameliorated the latency (a) and the frequencies of entering the dark compartment (b) of APP/PS1 mice in the passive avoidance test (n = 10) **P < 0.01, compared with WT group; ^## P < 0.01, compared with APP/PS1 group

Fig. 2

EGCG treatment improved the escape latency (a) and path length (b) of APP/PS1 mice in the navigation test of Morris water Maze (n = 10). *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 3

EGCG treatment improved the performance of the probe trial in Morris water maze of APP/PS1 mice (n = 10). a The representive locus plot after EGCG treatment in the probe trial. b EGCG treatment prolonged the time spent in target quadrant. c EGCG treatment increased the frequencies of passing through the goal. *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 4

EGCG treatment did not affect locomotivity (a) and the frequencies of stand-up (b) of APP/PS1 mice in locomotivity test (n = 10). P > 0.05 among all the groups

Fig. 5

EGCG treatment ameliorated the overexpression of Aβ(1–40) in the hippocampus of APP/PS1 mice by immunohistochemical staining (a) and Western blot (b) (n = 5). **P < 0.01, compared with WT group; ^## P < 0.01, compared with APP/PS1 group

Fig. 6

EGCG treatment decreased the APP expression levels in the hippocampus of APP/PS1 mice by immunohistochemical staining (a) and Western blot (b) (n = 5). **P < 0.01, compared with WT group; ^## P < 0.01, compared with APP/PS1 group

Fig. 7

EGCG treatment ameliorated the neural apoptosis in the hippocampus of APP/PS1 mice by TUNEL staining (n = 5). Red arrow indicated the TUNEL positive cell. *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 8

EGCG treatment decreased the expression level of caspase 3 in the hippocampus of APP/PS1 mice by immunohistochemical staining (n = 3). *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 9

EGCG treatment improved neurodegeneration in the hippocampus of APP/PS1 mice by Fluoro-Jade B staining (n = 3). *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 10

MEM treatment elevated the expression levels of NGF and its precursor neurotrophin proNGF (a), and increased the relative expression level of NGF versus proNGF (b) in the hippocampus of APP/PS1 mice (n = 3). *P < 0.05, **P < 0.01, compared with WT group; ^# P < 0.05, ^## P < 0.01, compared with APP/PS1 group

Fig. 11

MEM treatment activated NGF-TrkA signaling by increasing the phosphorylation levels of TrkA (a), c-Raf (b), and ERK1/2 (c), finally upregulated the phosphorylation level of the CREB (d) downstream, which was closely related with memory and learning in the hippocampus of APP/PS1 mice by Western blot (n = 3). **P < 0.01, compared with WT group; ^## P < 0.01, compared with APP/PS1 group

Fig. 12

MEM treatment inhibited the cleavage ability of p75^NTR and its related signaling by decreasing the cleavage product p75^ICD (a), reducing the phosphorylation of JNK2 (b), so as to decreasing the target substrates downstream p53 (c) and cleaved-caspase 3 (d) in the hippocampus of APPcxxPS1 mice by Western blot (n = 3). **P < 0.01, compared with WT group; ^## P < 0.01, compared with APP/PS1 group