Age-Related Decline in Nrf2/ARE Signaling Is Associated with the Mitochondrial DNA Damage and Cognitive Impairments

Abstract

In this research, we compared the cognitive parameters of 2-, 7-, and 15-month-old mice, changes in mitochondrial DNA (mtDNA) integrity and expression of genes involved in the nuclear erythroid 2-related factor 2/antioxidant response element (Nrf2/ARE) signaling pathway. We showed an age-related decrease in the Nfe2l2 expression in the cerebral cortex, not in the hippocampus. At the same time, we find an increase in the mtDNA copy number in the cerebral cortex, despite the lack of an increase in gene expression, which is involved in the mitochondrial biogenesis regulation. We suppose that increase in mtDNA content is associated with mitophagy downregulation. We supposed that mitophagy downregulation may be associated with an age-related increase in the mtDNA damage. In the hippocampus, we found a decrease in the Bdnf expression, which is involved in the pathways, which play an essential role in regulating long-term memory formation. We showed a deficit of working and reference memory in 15-month-old-mice in the water Morris maze, and a decrease in the exploratory behavior in the open field test. Cognitive impairments in 15-month-old mice correlated with a decrease in Bdnf expression in the hippocampus, Nfe2l2 expression, and an increase in the number of mtDNA damage in the cerebral cortex. Thus, these signaling pathways may be perspective targets for pharmacological intervention to maintain mitochondrial quality control, neuronal plasticity, and prevent the development of age-related cognitive impairment.

Keywords: Morris water maze; aging; brain-derived neurotrophic factor; cognitive deficit; mammalian target of rapamycin complex 1; mitochondrial DNA damage; nuclear erythroid 2-related factor 2.

Figure 1

The age-related changes in mtDNA in the cerebral cortex in the three age groups of mice. (A) The mean number of damages through all studied fragments of mtDNA. (B) Normalized mtDNA level in the frontal cortex * p < 0.05, ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 2

The normalized gene expression in the cerebral cortex: (A) Bdnf; (B). Il1b; (C) Nrf1; (D) Nfe2l2; (E) Cox1; (F) Tfam; (G) Sod2; (H) p62. ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 3

The normalized gene expression in the hippocampus: (A) Akt1; (B) Mtor; (C) Nfe2l2; (D) Pten; (E) Bdnf; (F) Il1b. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9). * p < 0.05; ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test.

Figure 4

The results of the open field test for the three age groups. (A) Locomotor activity (s); (B) rearing acts (quantity); (C) entering to the center (quantity); (D) time in the center area (s); (E) grooming acts (quantity); (F) grooming time (s); (G) hole-poking acts (quantity); (H) defecation acts (quantity). * p < 0.05; ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 5

The results of the dark-light box test. (A) Time in the dark compartment of the box (s); (B) Number of transitions between compartments of the box. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 6

The results of the Morris water maze for reference memory. (A) Distance points on the sixth trial day; (B) distance points on the twelfth trial day; (C) latency time points on the sixth trial day; (D) latency time points on the twelfth trial day; (E) time in a quadrant with the platform(s) on the sixth trial day; (F) time in a quadrant with the platform (s) on the twelfth trial day. * p < 0.05; ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 7

The results of the Morris water maze for reference memory. The most representative pathways for mice from 3 groups are shown on the sixth trial day (left column) and the twelfth trial day (right column).

Figure 8

The results of the Morris water maze for working memory (points). * p < 0.05 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 9

The results of the string test (scores). * p < 0.05; ** p < 0.01 differences between age groups were statistically significant according to the Mann–Whitney test. The 2-month-old mice (n = 9), 7-month-old mice (n = 9), 15-month-old mice (n = 9).

Figure 10

Timeline of the experiment. The open field was performed at the 1st day. The string test was performed at the 2nd day. The dark-light box (DLB) test was performed at the 3rd day. Morris water maze (MWM) for assessment of reference memory was performed from the 4th to the 15th day. MWM for assessment of working memory was performed from the 16th to the 36th day. Mice were sacrificed at the 37th day of experiment.