ApoE4 Age-dependent Increase Susceptibility To Stress-induced Depression-like Behavior And Cognitive Dysfunction

Object Depression is one of mood disorders that is the most common prevalence in the world, Clinical studies found that apolipoprotein E4(Apo E4) may increase the risk of the elderly population suffering from depression, and many studies confirm that Apo E4 has a close relationship with cognitive dysfunction, but the relationship among the three still unclear. Our project investigated the possible mechanisms of susceptibility and aging dependent changes in the Apo E4-TR mice to stress-induced depression-like behavioral and cognitive dysfunction.Methods 1. The first part of the study: we apply 3-month-old Apo E3 / E4-TR mice through a period of six weeks chronic unpredictable mild stress(CUMS) successfully established depression model, then stop CUMS intervention, some mice were sacrificed for collecting specimens, part of them was normal feeding to the age of 12 months. We apply the sugar preferences and depressive behavioral tests(elevated plus maze, open field test, tail suspension test) to detect depressive behavior in mice, and cognitive behavioral test(Morris water maze) to evaluate the visual-spatial learning and memory in 3-month-old and 12-month-old Apo E-TR mice. Then we detect the expression of GABAergic neurons and secreted Reelin protein of the Apo E-TR mice in the prefrontal cortex and hippocampus through immunohistochemistry and RT-PCR. And by Western blot method, we detect the Reelin-Fyn-NMDAR signaling pathway in the prefrontal cortex and hippocampus. This part of the work in order to investigate the possible mechanisms of early-life stress leads to impaired spatial learning and memory in middle-aged Apo E4-TR mice.2. The second part of the study: we apply chronic unpredictable mild stress(CUMS) successfully established depression model in the 3-month-old and 8-month-old Apo E3 / E4-TR mice. Then we detect the depression-like behavior and cognitive function of 3-month-old and 8-month-old Apo E-TR mice through depression and cognitive behavioral tests, then observed GABAergic neurons and the expression of Reelin protein in the prefrontal cortex and hippocampus of the Apo E-TR mice by immunohistochemical staining, and by Western blot method, we detect the expression of Reelin-Fyn-NMDAR signaling pathway, synaptic terminal PKMζ and PSD95, PDK1 protein in the prefrontal cortex and hippocampus.This part of the work was to investigate the mechanism of Apo E4 age-dependent increase susceptibility to stress-induced depression-like behavior and cognitive dysfunction.Results 1. The first part of the study: By a period of six weeks CUMS procedure successfully induces depression-like behaviors in 3-month-old Apo E-TR mice.It was shown as: compared with the control groups, the weight and sugar consumption preferences in CUMS intervention group was significantly reduced; and from the results of depressive behavioral test, in the CUMS intervention group, in the elevated plus maze test, the percentage of time and entries into the open arms were significantly reduced; and in the open-field test, its horizontal movement, vertical movement, and the total number of movements that were less than the control groups; in the tail suspension test, the resting time in the CUMS intervention groups were significantly prolonged. But there was no significant difference between CUMS-E4 group and CUMS-E3 group. Then stop CUMS intervention, naturally rearing to 12 months old, all Apo E-TR mice had recover from the depressive state, but the visual-spatial learning and memory capacity in 12-month-old CUMS-E4 mice was significantly impaired, in the Morris maze test, compared with the other groups, it performed as significantly reduced escape latency to find the target platform, and the target quadrant time and the number of crossing platform position were significantly decreased, however, the learning and memory capacity in 12-month-old CUMS-E3 were not significantly different with the control-E3 group. And the numbers of GABAergic neurons in prefrontal cortex and hippocampus in the middle-aged CUMS-E4 mice were significantly reduced, and the expression of Reelin, Fyn, P-NMDAR2 B and P-CREB were significantly decreased compared with the age-matched control group and CUMS-E3 group.2. The second part of the study: we successfully induced 8-month-old mice Apo E-TR depression-like behavior through a six-week CUMS intervention. Interestingly, the weight and sucrose consumption percentage in the CUMS-E4 groups were significant less than that of in the CUMS-E3 group, and as the results of the depressive behavior test, in the CUMS-E4 group,in the elevated plus maze test, the percentage of time and entries into the open arms were significantly reduced relative to the CUMS-E3 group; and in the open-field test, its horizontal movement, vertical movement, and the total number of movements that were less than the CUMS-E3 group; and in the tail suspension test, the resting time in the CUMS-E4 group were significantly longer than that of the CUMS-E3 group. In the Morris maze test, the escape latency in the CUMS-E4 group significantly longer than that of CUMS-E3 group, and the target quadrant time and the frequency of crossing platform positions were less than that of the CUMS-E3 group. And the numbers of GABAergic neuron in the prefrontal cortex and hippocampus in the middle-aged CUMS-E4 group were significant reduced, and the expression of Reelin, Apo ER2, Fyn, NMDAR were significantly decreased compared with the age-matched the control group and CUMS-E3 group. The levels of PDK1, PSD95 and p-PKM ζ in the hippocampus and hippocampal synaptosomes in the CUMS-E4 group were significantly decreased compared with the control group and CUMS-E3 group.Conclusion 1. Early-life stress leads to impaired spatial learning and memory in middle-aged Apo E4-TR mice, and the possible mechanism of the chronic stress may affect the numbers of GABAergic neurons in the prefrontal cortex and hippocampus of the middle-aged Apo E4-TR mice, and reduce the expression of secreted Reelin, resulting in down-regulated the signaling pathways of Reelin- Fyn-NMDAR-CREB which closely related with cognitive function.2. The chronic stress age-dependent increase the susceptibility to depression-like behavior and cognitive impairment in the Apo E4-TR mice, the possible mechanism through reducing the numbers of GABAergic neurons in the prefrontal cortex and hippocampus in the middle-aged Apo E4-TR mice, and decreasing the mount of secreted Reelin, then down-regulating the signaling pathways of Reelin-Apo ER2-Fyn-NMDAR, followed by reducing the expression of p-PKMζand PSD95.