Characterization of adipocyte stress response pathways during hibernation in thirteen-lined ground squirrels

Abstract

To avoid the harsh conditions of winter climates, hibernating mammals undergo a systematic depression of physiological function by reducing their metabolic rate. During this process, hibernators are exposed to significant stresses (e.g., low body temperature, ischemia-reperfusion) that must be dealt with appropriately to avoid irreversible tissue damage. Consequently, we investigated the contribution of stress-responsive antioxidant enzymes, heat shock proteins, signal transduction pathways (e.g., mitogen-activated protein kinases, MAPK), and transcription factors for their role in conferring tolerance to stress in the hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus). Using a combination of multiplex protein panels and traditional immunoblotting procedures, we have focused on these stress factors in brown adipose tissue (BAT) and white adipose tissue (WAT) over cycles of torpor-arousal since they provide the means for heat production as a result of non-shivering thermogenesis and the mobilization of critical energy reserves, respectively. We show the differential and tissue-specific regulation of stress factors including a unified upregulation of the antioxidant enzyme Thioredoxin 1 in both tissues, an upregulation of superoxide dismutase (SOD1 and SOD2) in WAT, and an increase in heat shock proteins during the transitory periods of the torpor-arousal cycle (HSP90α in BAT and HSP60 in WAT). Additionally, an upregulation of the active form of ERK1/2 and p38 in BAT and select transcription factors (e.g., CREB-1 and ELK-1) in both tissues were identified. These data provide us with greater insight into the molecular mechanisms responsible for this animal's natural stress tolerance and outline molecular signatures which define stress resistance.

Optical visualisation of thermogenesis in stimulated single-cell brown adipocytes

“…In UCP1-ablated mice, peak rewarming rates from induced torpor were reduced by 50% (UCP1 +/+ : 0.24 ± 0.08 °C min −1 ; UCP1 −/− : 0.12 ± 0.04 °C min −1 ) 63 , strengthening the hypothesis that UCP1-facilitated non-shivering thermogenesis in BAT provides an extraordinarily high heat output to allow fast rewarming of the animal. A number of protective mechanisms exist in BAs to prevent cellular damage during these fast and relatively high temperature changes and ensure that hibernating animals endure little to no damage to their tissues during torpor arousal, such as antioxidants and heat shock proteins 64 . Moreover, it was recently demonstrated that membrane lipids, such as cardiolipin, stabilise UCP1, increasing its thermal stability 65 and conferring a higher temperature tolerance for mitochondrial membrane proteins.…”

Hibernation impacts lysine methylation dynamics in the 13‐lined ground squirrel, Ictidomys tridecemlineatus

“…However, reductions in gene expression and protein synthesis are not global over the entire genome; the expression of certain genes necessary for regulating and maintaining the hibernation state is maintained or upregulated (Grabek, Martin, & Hindle, 2015;Wu & Storey, 2016). For example, some cytoprotective pathways are upregulated during torpor, such as to prevent muscle atrophy, or enhance antioxidant defenses (Rouble, Tessier, & Storey, 2014). Clearly, an intricate level of selection over biological pathways is required during hibernation.…”

Stress‐responsive microRNAs are involved in re‐programming of metabolic functions in hibernators

“…Additionally, differential-regulation of MAPK signaling could be controlled in an organ-specific approach in hibernating thirteen lined ground squirrels during torpor (Rouble, Tessier, et al, 2014b). Likewise Wu, Biggar, and Storey (2013) protein-responsible for recruiting the Dicer complex to Argonaute 2 for microRNA processing), is stimulated by the activation of ERK signaling.…”

“…The results of the study showed that pharmacologically upregulation of this pathway significantly increases the stability of skeletal muscle of Myotis lucifugus during torpor (Eddy & Storey, ) (Figure ). Additionally, differential‐regulation of MAPK signaling could be controlled in an organ‐specific approach in hibernating thirteen lined ground squirrels during torpor (Rouble, Tessier, et al, ). Likewise Wu, Biggar, and Storey () examined the expressions of other 10 miRNAs (miR‐1, ‐16a, ‐21, ‐29b, ‐34a, ‐125b, ‐133a, ‐181a, ‐203, ‐210) in ventral skin, kidney and liver tissues of Xenopus laevis during dehydration.…”