Death-associated protein kinase 1 is associated with cognitive dysfunction in major depressive disorder

Abstract

We previously showed that death-associated protein kinase 1 (DAPK1) expression is increased in hippocampal tissue in a mouse model of major depressive disorde and is related to cognitive dysfunction in Alzheimer's disease. In addition, depression is a risk factor for developing Alzheimer's disease, as well as an early clinical manifestation of Alzheimer's disease. Meanwhile, cognitive dysfunction is a distinctive feature of major depressive disorder. Therefore, DAPK1 may be related to cognitive dysfunction in major depressive disorder. In this study, we established a mouse model of major depressive disorder by housing mice individually and exposing them to chronic, mild, unpredictable stressors. We found that DAPK1 and tau protein levels were increased in the hippocampal CA3 area, and tau was hyperphosphorylated at Thr231, Ser262, and Ser396 in these mice. Furthermore, DAPK1 shifted from axonal expression to overexpression on the cell membrane. Exercise and treatment with the antidepressant drug citalopram decreased DAPK1 expression and tau protein phosphorylation in hippocampal tissue and improved both depressive symptoms and cognitive dysfunction. These results indicate that DAPK1 may be a potential reason and therapeutic target of cognitive dysfunction in major depressive disorder.

Keywords: Alzheimer’s disease; antidepressant drug; behavioral tests; cognitive dysfunction; death-associated protein kinase 1; exercise; hippocampus; major depressive disorder; phosphorylation; tau protein.

Figure 1

MDD mice exhibit cognitive dysfunction. (A) Sucrose preference test (SPT): Total consumption of drinking water (sugar water intake + pure water intake) in the two experimental groups (sucrose preference value = sugar water intake/total water intake). (B) Forced swimming test (FST): Immobility time. (C) Tail suspension test (TST): Immobility time. (D–H) Morris water maze test. (D) Total distance traveled the first 5 days. (E) Escape latency during the first 5 days. (F) Percentage of time spent in the target quadrant for both groups. (G) Number of times the platform in the target quadrant was crossed. (H) Representative swim paths in the test session on day 6. Data are expressed as mean ± SEM (n = 9–12/group). **P < 0.01, ****P < 0.0001, vs. control group. Except for total distance (two-way analysis of variance with repeated measures followed by Tukey’s post hoc test) and average escape latency (two-way analysis of variance with repeated measures followed by Tukey’s post hoc test) in Morris water maze test, all behavioral results were analyzed by unpaired t-test. MDD: Major depressive disorder.

Figure 2

Expression of death-associated protein kinase-1 and tau in the hippocampus. (A) DAPK1 expression in the hippocampus. (B) Expression of Tau5, Tau1, pThr231- Tau, pSer262-Tau, and pSer396-Tau in the hippocampus. Relative protein expression was normalized to β-actin. Data are expressed as mean ± SEM (n = 3/group). *P < 0.05, **P < 0.01, ***P < 0.001, vs. control group (unpaired t-test). (C) DAPK1 and Tau5 expression in hippocampal sections. Cytoplasmic DAPK1 expression was higher in the MDD group than in the control group, and tau expression was increased in both neurites and cell membranes. (D) DAPK1 (red, Cy3) and Tau5 (green, Alexa Fluor® 488) DAPK1 expression and co-localization in the hippocampus. Greater co-localization of DAPK1 and tau was observed in neuronal membrane and neurites in MDD mice than in the control group. Scale bars: 100 μm in C, 50 μm in D. DAPK1: Death-associated protein kinase-1; MDD: major depressive disorder; p: phosphorylated.

Figure 3

Effect of swim training on depressive symptoms and cognitive ability in MDD mice. (A) Sucrose preference test (SPT): Total drinking water consumption and sucrose preference. (B) Forced swimming test (FST): Immobility time. (C) Tail suspension test (TST): Immobility time. (D) Y maze: Total time spent in the arms and the spontaneous alternation accuracy rate. (E–H) Morris water maze test. (E) Average escape latency during the first 5 days. (F) Number of times the platform in the target quadrant was crossed. (G) Trajectories of mice in the spatial exploration experiment. The number of target crossings and amount of time spent in the target quadrant were lower in the MDD group than in the control group, and both parameters improved after swim training. (H) Percentage of time spent in the target quadrant. Data are expressed as mean ± SEM (n = 9–10/group). *P < 0.05, **P < 0.01, ***P < 0.001, vs. control group; #P < 0.05, ##P < 0.01, ###P < 0.001, vs. MDD + no further treatment (MDD only) group. Except for average escape latency (two-way analysis of variance with repeated measures followed by Tukey’s post hoc test) in the Morris water maze test, all behavioral results were analyzed by using one-way analysis of variance followed by Tukey’s post hoc test. MDD: Major depressive disorder.

Figure 4

Effect of swim training on DAPK1 expression and activity in the hippocampal CA3 area of MDD mice. (A, B) Effect of swim training on DAPK1 and p-DAPK1 expression in the hippocampal CA3 area of MDD mice. (C, D) Effect of swim training on Tau1, Tau5, pThr231-Tau, and pSer396-Tau expression in the hippocampus of MDD mice. Data (normalized by β-actin) are expressed as mean ± SEM (n = 3). **P < 0.01, ***P < 0.001, vs. control group; #P < 0.05, ##P < 0.01, vs. MDD group (one-way analysis of variance followed by Tukey’s post hoc test). DAPK1: Death-associated protein kinase-1; MDD: major depressive disorder; p: phosphorylated.

Figure 5

Effect of citalopram on depressive behaviors and cognitive ability in MDD mice. (A) Forced swimming test (FST): Immobility time. (B) Tail suspension test (TST): Immobility time. (C) Elevated plus-maze test (EPM): Percentage of time spent in the open arm. (D) Open field test (OFT): Time spent in the central area. (E–H) Morris water maze test. (E) Average escape latency during the first 5 days. (F) Number of times that the platform was crossed. (G) Percentage of stay spent in the target quadrant. Data are expressed as mean ± SEM (n = 7–10/group). *P < 0.05, ***P < 0.001, ****P < 0.0001, vs. control group; #P < 0.05, ##P < 0.01, vs. MDD + NS group. Except for average escape latency (two-way analysis of variance with repeated measures followed by Tukey’s post hoc test) in the Morris water maze test, all behavioral results were analyzed by using one-way analysis of variance followed by Tukey’s post hoc test. (H) Representative trajectory maps of spatial exploration. In the MDD + NS group, the number of target crossings and the amount of time spent in the target quadrant were lower than in the control group, and both parameters improved after antidepressant treatment. MDD: Major depressive disorder; MDD + CA: MDD + citalopram; MDD + NS: MDD + normal saline.

Figure 6

Effect of citalopram on the expression of death-associated protein kinase-1, Tau5, pSer262-Tau, and pSer396-Tau in the hippocampi of MDD mice. (A, B) Expression of DAPK1, Tau5, pSer262-Tau, and pSer396-Tau in the hippocampus. Data are expressed as mean ± SEM (n = 3). *P < 0.05, **P < 0.01, ***P < 0.001, vs. control group; #P < 0.05, ##P < 0.01, ###P < 0.001, vs. MDD + NS group (one-way analysis of variance followed by Tukey’s post hoc test). (C) Expression and cellular distribution of tau phosphorylated at Ser396 in the hippocampus. In MDD mice, pSer396-Tau was concentrated on the cell membrane, and the hippocampal neuron neurites were shorter; both of these effects were reversed by antidepressant treatment. (D) Immunofluorescence staining showing the expression and co-localization of DAPK1 (red, Cy3) and Tau5 (green, Alexa Fluor® 488) in the hippocampus. The co-localization of neuronal membrane and neurites in the hippocampus was increased in MDD mice compared with control mice and was significantly reduced after antidepressant treatment. Scale bars: 100 μm. DAPK1: Death-associated protein kinase-1; MDD: major depressive disorder; MDD + CA: MDD + citalopram; MDD + NS: MDD + normal saline.