Chronic Stress Aggravates Cognitive Impairment and Suppresses Insulin Associated Signaling Pathway in APP/PS1 Mice

Differences in brain function are a central determinant of individual variability in the stress response. Brain dysfunction, resulting from aging, illness, or genetic mutations, could reduce the tolerance of glucocorticoid stress hormones. When glucocorticoids exceed tolerable limits in the brain, especially in the hippocampus, this state can cause or aggravate structural or functional damage. However, the underlying mechanisms are not well understood. This study investigated the effects of chronic unpredictable mild stress (CUMS) in APP/PS1 and control mice. We showed that 4 weeks of CUMS exposure increased the levels of glucocorticoids, reduced glucocorticoids receptor expression, and promoted senile plaque deposition, neuronal injury, and cognitive impairment in APP/PS1 mice compared to controls. The phosphorylation of insulin receptor, insulin receptor substrate 1 and associated signaling pathways (Akt, mTOR, p70S6K, ERK1/2, and PTEN) were decreased in hippocampus in APP/PS1 mice compared to control mice, while no changes were found in GSK3 and TSC2 phosphorylation. Furthermore, insulin and Akt/mTOR signaling pathways were further decreased in APP/PS1 mice after CUMS, which may be related to the activation of the stress-activated protein kinase JNK, while no alterations in the levels of phosphorylated ERK1/2, GSK3, PTEN, or TSC2 were observed. These results suggest that chronic stress may affect the insulin and Akt/mTOR pathway, accelerating the progression of Alzheimer's disease in vulnerable individuals.