Modulating Nrf2 transcription factor activity: Revealing the regulatory mechanisms of antioxidant defenses during hibernation in 13‐lined ground squirrels

Abstract

Mammalian hibernators undergo major behavioural, physiological and biochemical changes to survive hypothermia, ischaemia‐reperfusion and finite fuel reserves during days or weeks of continuous torpor. During hibernation, the 13‐lined ground squirrel (Ictidomys tridecemlineatus) undergoes a global suppression of energetically expensive processes such as transcription and translation, while selectively upregulating certain genes/proteins to mitigate torpor‐related damage. Antioxidant defenses are critical for preventing damage caused by reactive oxygen species (ROS) during torpor and arousal, and Nrf2 is a critical regulator of these antioxidant genes. This study analysed the relative protein expression levels of Nrf2, KEAP1, small Mafs (MafF, MafK and MafG) and catalase and the regulation of Nrf2 transcription factors by post‐translational modifications (PTMs) and protein‐protein interactions with a negative regulator (KEAP1) during hibernation. It was found that a significant increase in MafK during late torpor predicated an increase in relative Nrf2 and catalase levels seen in arousal. Additionally, Nrf2‐KEAP1 protein‐protein interactions and Nrf2 PTMs, including serine phosphorylation and lysine acetylation, were responsive to cycles of torpor‐arousal with peak responses occurring during arousal. These peaks seen during arousal correspond to a surge in oxygen consumption, which causes increased ROS production. Thus, these regulatory mechanisms could be important during hibernation because they provide mechanisms for mitigating the deleterious effects of oxidative stress by modifying Nrf2 expression and function in an energetically inexpensive manner.

Purification and characterization of NADP-isocitrate dehydrogenase from skeletal muscle of Urocitellus richardsonii

“…These results seen during arousal correspond to a surge in oxygen consumption, which causes increased reactive oxygen species production [31]. Overall, this is also contributing to the idea that ground squirrels increase oxidative stress defense during hibernation to cope with increased oxygen uctuations during periodic arousals and to preserve cells during reduced repair periods in hypometabolism.…”

“…Richardson's ground squirrel is additionally faced with oxidative stress during hibernation, particularly during arousals in which oxygen consumption increases drastically [9,27]. In many instances of hypometabolism, an increase in antioxidant enzyme activity is observed to prepare for imminent oxidative stress upon arousal and preserve current cell integrity during periods of reduced repair [28][29][30][31]. NADP-IDH often contributes signi cantly to the NADPH pool required for reductive fatty acid biosynthesis.…”

Histone Arginine Methylation in the Freeze-tolerant Wood Frog, Rana sylvatica

“…This is because periods of freeze/thaw, ischemia/reperfusion or anoxia/reoxygenation increase the production of reactive oxygen species (ROS), and thus tissues face oxidative stress during recovery. The activation of antioxidant defenses, typically mediated by the transcription factors, Nrf2 and Nf-kB, is a common response in many species that undergo cycles of transition between low and high metabolic rates such as torpor-arousal cycles in mammals and anoxia/reoxygenation in many species including frogs and turtles (Breedon et al, 2021;Childers et al, 2019;Tessier et al, 2021a). Interestingly, methylation on H3R17 and H4R3 by PRMT4 and PRMT1, respectively, appears to enable Nrf2 to bind more efficiently to the antioxidant responsive element (ARE) of various genes, thus activating the antioxidant defense response (Huang et al, 2013).…”

“…During recovery, tissues can be exposed to oxidative damage due to an increase in reactive oxygen species generated when tissue reperfusion by blood is reinstated. Thus, species such as wood frogs that experience stress-recovery must employ antioxidant defenses as part of their metabolic response to avoid tissue damage (Breedon et al, 2021;Childers et al, 2019;Tessier et al, 2021a). Arginine methylation of H3R17m2a and H4R3m2a have previously been linked to enhancing the antioxidant response and allowing Nrf2 to efficiently bind to the antioxidant responsive element (ARE), thereby contributing to the activation of the defence response (Huang et al, 2013).…”

MicroRNA regulation and expression in the anoxia-tolerant red-eared slider turtle, Trachemys scripta elegans

“…Indeed, there was an observed difference in the miRNA silencing response between organs, supporting the idea that because miRNA expression is tissue-specific, miRNA biogenesis and the resulting mRNA fate is also tissue-specific. The expression pattern of biogenesis proteins in the liver provided two main theories: (1) overall downregulation of miRNA synthesis as a result of decreased nuclear processing, or…”