1Molecular Pharmacology Research Center, School of Pharmaceutical Science, Wenzhou Medical University, Wenzhou, Zhejiang, China 2The Institute of Life Sciences, Wenzhou University, Wenzhou, Zhejiang, China 3Department of Anesthesia, The First Affiliated Hospital, Gannan Medical University, Jiangxi, China
Autophagy is a lysosome-dependent cellular catabolic mechanism that mediates the turnover of dysfunctional organelles and aggregated proteins. It has a neuroprotective role on neurodegenerative diseases. Here, we hypothesized that autophagy may also have a neuroprotective role in diabetes-associated cognitive decline (DACD). In current study, we found that db/db mice display cognitive decline with inferior learning and memory function. The accumulation of β-amyloid1-42 (Aβ1-42), which is a characteristic of Alzheimer’s disease (AD), was markedly higher in the prefrontal cortex (PFC), cornu ammon1 (CA1), and dentate gyrus (DG) areas of the hippocampus in db/db mice. Moreover, BDNF and microtubule associated protein 2 (MAP2) levels were lower in the hippocampus of db/db mice. However, there was no noticeable differences in the level of apoptosis in the hippocampus between control (CON) mice and db/db mice. Markers of autophagy in the hippocampus were elevated in db/db mice. The expression levels of ATG5, ATG7, and LC3B were higher, and the level of P62 was lower. An autophagy inhibitor, 3-MA, and ATG7 siRNA significantly reversed the activation of autophagy in vitro, which was accompanied with a higher level of apoptosis. Taken together, our current study suggests that diabetes is associated with cognitive decline, and activation of autophagy has a neuroprotective role in DACD.
Figure 1. Diabetes resulted in cognitive decline with inferior learning and memory function. (A) The learning curve of training during six blocks in the Morris water maze test of CON mice and db/db mice. (B) Number of crossings over the original platform location in CON mice and db/db mice at 1 hr and 24 hr after training. (C) Representative swimming track of CON mice and db/db mice at 1 hr and 24 hr after training. (D) Swimming speed of CON mice and db/db mice. CON: control. **p < 0.01 vs. CON, ***p < 0.001 vs. CON, and N = 10.
Figure 2. Diabetes induced Aβ1-42 accumulation and downregulated the expression of BDNF and MAP2. (A-B) The immunohistochemical staining of Aβ1-42 and BDNF in CA1, CA3, and PFC of the hippocampus. (C) The immunofluorescence staining of MAP2 in CA1, CA3, and PFC. CON: control, PFC: prefrontal cortex, CA1: cornu ammon1, and CA3: cornu ammon3Scale bar = 50 μm, and N = 4.
Figure 3. The morphological structure and level of apoptosis in the hippocampus in diabetes. (A) H&E staining of CA1 and DG. (B) TUNEL staining of the hippocampus in CON mice and db/db mice. CON: control. Scale bar = 50 μm, and N = 4.
Figure 4. The expression levels of LC3B and P62 in the hippocampus. (A) Western blot and quantitative analysis of LC3B and P62 expression. (B) Immunofluorescence staining of LC3B in CA1. CON: control. Scale bar = 50 µm and 10 µm. **p < 0.01 vs. CON, ***p <0.001 vs. CON, and N = 6.
Figure 5. The expression levels of ATG5 and ATG7 in the hippocampus. (A) Western blot and quantitative analysis of ATG5 and ATG7. (B) Immunohistochemical staining of ATG5 and ATG7 in CA1 and CA3. CON: control. Scale bar = 50 µm. **p < 0.01 vs. CON, and N = 6.
Figure 6. HG treatment increased the expression of autophagy markers in PC12 cells. PC12 cells were cultured in HG (30 mM) conditions for 12, 24, 36, and 48 hr. (A) Western blot analysis of ATG7, ATG5, P62, and LC3B expression. (B) Quantitative analysis of LC3B and P62. (C) Quantitative analysis of ATG5 and ATG7. (D) Immunofluorescence of LC3B in PC12 cells. CON: control, HG: high glucose. Scale bar = 50 μm, *P < 0.05 vs.CON, **P < 0.01vs. CON, ***P < 0.001 vs. CON, and N = 3.
Figure 7. 3-MA treatment inhibited HG-mediated autophagic activation, resulting in the induction of apoptosis. (A) Western blot analysis of ATG7, ATG5, P62, and LC3B expression. (B) Quantitative analysis of ATG5 and ATG7. (C) Quantitative analysis of LC3B and P62. (D) Immunofluorescence staining of LC3B in PC12 cells. (E) Cells were collected and stained with annexin V-FITC/propidium iodide and detected by flow cytometry. The lower right panel indicates the apoptotic cells. (F) The level of apoptosis in PC12 cells. CON: control, HG: high glucose. Scale bar = 50 μm, **P < 0.01 vs. CON, ***P < 0.001 vs. CON, #P < 0.05 vs. HG group, ##P < 0.01 vs. HG group, ###P < 0.001 vs. HG group, and N = 3.
Figure 8. ATG7 siRNA treatment abolished the neuroprotective role of autophagy under HG conditions. (A) Western blot analysis of ATG7, ATG5, P62, and LC3B expression. (B) Quantitative analysis of ATG5 and ATG7. (C) Quantitative analysis of LC3B and P62. (D) Immunofluorescence staining of LC3B in PC12 cells. (E) The results of flow cytometry under different conditions. (F) The level of apoptosis in PC12 cells. CON: control, HG: high glucose. Scale bar = 50 μm. *P < 0.05 vs. CON, **P < 0.01 vs. CON, ***P < 0.001 vs. CON, #P < 0.05 vs. HG group, ##P < 0.01 vs. HG group, ###P < 0.001vs. HG group, and N = 3.
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