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

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.

Methylene blue elicits non-genotoxic H2O2 production and protects brain mitochondria from rotenone toxicity.

Methylene blue (MB) is a promising compound with a broad range of neuroprotective activity. One of therapeutic effects is the activation of mitochondrial biogenesis via Nrf2/ARE signaling cascade. Probably, mild oxidative stress caused by MB-depended H2O2 production is a trigger for activation of this signaling cascade. So mechanistically, MB can be regarded as prooxidant. We investigated the dose-dependent H2O2 production in intact brain mitochondria and showed the increase in the H2O2 production after adding as little as 50 nM MB. We have not found genotoxic effect of therapeutic concentration of MB to mitochondrial genome. 100 µM MB selectively damaged fragments of mitochondrial DNA, which correlated with the number of purine-T-G-purine (RTGR)-sequences in studied fragments. Furthermore, 20 µM MB combined with the red light caused the formation of singlet oxygen, which strongly damaged mitochondrial DNA in all studied fragments. We did not observe mitochondrial DNA lesions in brain after single intraperitoneal injection of MB in the concentration of 50 mg/kg. Furthermore, we showed the neuroprotective properties of MB pretreatments after rotenone injection. Therefore, we suggest that MB-induced mild oxidative stress does not have genotoxic effect on mitochondrial DNA.