The squirrel with the lagging eIF2: Global suppression of protein synthesis during torpor

The gp130/STAT3-endoplasmic reticulum stress axis regulates hepatocyte necroptosis in acute liver injury

AIM

To investigate the effect of the gp130/STAT3-endoplasmic reticulum (ER) stress axis on hepatocyte necroptosis during acute liver injury.

METHODS

ER stress and liver injury in LO2 cells were induced with thapsigargin, and in BALB/c mice with tunicamycin and carbon tetrachloride (CCl4). Glycoprotein 130 (gp130) expression, the degrees of ER stress, and hepatocyte necroptosis were assessed.

RESULTS

ER stress significantly upregulated gp130 expression in LO2 cells and mouse livers. The silencing of activating transcription factor 6 (ATF6), but not of ATF4, increased hepatocyte necroptosis and mitigated gp130 expression in LO2 cells and mice. Gp130 silencing reduced the phosphorylation of CCl4-induced signal transducer and activator of transcription 3 (STAT3), and aggravated ER stress, necroptosis, and liver injury in mice.

CONCLUSION

ATF6/gp130/STAT3 signaling attenuates necroptosis in hepatocytes through the negative regulation of ER stress during liver injury. Hepatocyte ATF6/gp130/STAT3 signaling may be used as a therapeutic target in acute liver injury.

Role of unfolded protein response and ER-associated degradation under freezing, anoxia, and dehydration stresses in the freeze-tolerant wood frogs

The North American amphibian, wood frogs, Rana sylvatica are the most studied anuran to comprehend vertebrate freeze tolerance. Multiple adaptations support their survival in frigid temperatures during winters, particularly their ability to produce glucose as natural cryoprotectant. Freezing and its component consequences (anoxia and dehydration) induce multiple stresses on cells. Among these is endoplasmic reticulum (ER) stress, a condition spawned by buildup of unfolded or misfolded proteins in the ER. The ER stress causes the unfolded protein response (UPR) and the ER-associated degradation (ERAD) pathway that potentially could lead to apoptosis. Immunoblotting was used to assess the responses of major proteins of the UPR and ERAD under freezing, anoxia, and dehydration stresses in the liver and skeletal muscle of the wood frogs. Targets analyzed included activating transcription factors (ATF3, ATF4, ATF6), the growth arrest and DNA damage proteins (GADD34, GADD153), and EDEM (ERAD enhancing α-mannosidase-like proteins) and XBP1 (X-box binding protein 1) proteins. UPR signaling was triggered under all three stresses (freezing, anoxia, dehydration) in liver and skeletal muscle of wood frogs with most tissue/stress responses consistent with an upregulation of the primary targets of all three UPR pathways (ATF4, ATF6, and XBP-1) to enhance the protein folding/refolding capacity under these stress conditions. Only frozen muscle showed preference for proteasomal degradation of misfolded proteins via upregulation of EDEM (ERAD). The ERAD response of liver was downregulated across three stresses suggesting preference for more refolding of misfolded/unfolded proteins. Overall, we conclude that wood frog organs activate the UPR as a means of stabilizing and repairing cellular proteins to best survive freezing exposures.

Hypothalamic remodeling of thyroid hormone signaling during hibernation in the arctic ground squirrel

Hibernation involves prolonged intervals of profound metabolic suppression periodically interrupted by brief arousals to euthermy, the function of which is unknown. Annual cycles in mammals are timed by a photoperiodically-regulated thyroid-hormone-dependent mechanism in hypothalamic tanycytes, driven by thyrotropin (TSH) in the pars tuberalis (PT), which regulates local TH-converting deiodinases and triggers remodeling of neuroendocrine pathways. We demonstrate that over the course of hibernation in continuous darkness, arctic ground squirrels (Urocitellus parryii) up-regulate the retrograde TSH/Deiodinase/TH pathway, remodel hypothalamic tanycytes, and activate the reproductive axis. Forcing the premature termination of hibernation by warming animals induced hypothalamic deiodinase expression and the accumulation of secretory granules in PT thyrotrophs and pituitary gonadotrophs, but did not further activate the reproductive axis. We suggest that periodic arousals may allow for the transient activation of hypothalamic thyroid hormone signaling, cellular remodeling, and re-programming of brain circuits in preparation for the short Arctic summer.

Arctic ground squirrels hibernating in darkness activate the pars tuberalis - hypothalamus thyroid hormone signaling pathway, remodel hypothalamic tanycytes, and activate the reproductive axis.

Muscles in Winter: The Epigenetics of Metabolic Arrest

The winter months are challenging for many animal species, which often enter a state of dormancy or hypometabolism to “wait out” the cold weather, food scarcity, reduced daylight, and restricted mobility that can characterize the season. To survive, many species use metabolic rate depression (MRD) to suppress nonessential metabolic processes, conserving energy and limiting tissue atrophy particularly of skeletal and cardiac muscles. Mammalian hibernation is the best recognized example of winter MRD, but some turtle species spend the winter unable to breathe air and use MRD to survive with little or no oxygen (hypoxia/anoxia), and various frogs endure the freezing of about two-thirds of their total body water as extracellular ice. These winter survival strategies are highly effective, but create physiological and metabolic challenges that require specific biochemical adaptive strategies. Gene-related processes as well as epigenetic processes can lower the risk of atrophy during prolonged inactivity and limited nutrient stores, and DNA modifications, mRNA storage, and microRNA action are enacted to maintain and preserve muscle. This review article focuses on epigenetic controls on muscle metabolism that regulate MRD to avoid muscle atrophy and support winter survival in model species of hibernating mammals, anoxia-tolerant turtles and freeze-tolerant frogs. Such research may lead to human applications including muscle-wasting disorders such as sarcopenia, or other conditions of limited mobility.

mTOR Signaling in Metabolic Stress Adaptation

The mechanistic target of rapamycin (mTOR) is a central regulator of cellular homeostasis that integrates environmental and nutrient signals to control cell growth and survival. Over the past two decades, extensive studies of mTOR have implicated the importance of this protein complex in regulating a broad range of metabolic functions, as well as its role in the progression of various human diseases. Recently, mTOR has emerged as a key signaling molecule in regulating animal entry into a hypometabolic state as a survival strategy in response to environmental stress. Here, we review current knowledge of the role that mTOR plays in contributing to natural hypometabolic states such as hibernation, estivation, hypoxia/anoxia tolerance, and dauer diapause. Studies across a diverse range of animal species reveal that mTOR exhibits unique regulatory patterns in an environmental stressor-dependent manner. We discuss how key signaling proteins within the mTOR signaling pathways are regulated in different animal models of stress, and describe how each of these regulations uniquely contribute to promoting animal survival in a hypometabolic state.

Markers of tissue remodeling and inflammation in the white and brown adipose tissues of a model hibernator

The thirteen-lined ground squirrel is a model fat-storing hibernator that nearly doubles its weight in the fall to fuel metabolism with triglycerides throughout the winter months. Hibernator brown and white adipose tissue (BAT, WAT) are important to study in terms of their inflammatory profile and tissue remodeling mechanisms since controlled and natural regulation of these processes could inform new pharmacological interventions that limit oxidative stress and inflammation in the adipose tissues of humans suffering from obesity, promote non-shivering thermogenesis-mediated weight loss, or prevent tissue damage in transplantable organs emerging from cold-storage. Thus, markers of inflammation like cytokines and soluble receptors and tissue remodeling proteins such as matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) were investigated in normothermic, torpid, and arousing ground squirrels. Multiplex protein assays and western blotting revealed fewer changes in WAT compared to BAT. Pro-inflammatory IL-1α levels increased during torpor and soluble epidermal growth factor receptor protein levels increased during arousal in BAT. Given their known roles in other model systems, these proteins could regulate processes like adipogenesis, lipid catabolism, or cell motility. Decreased TIMP2 levels combined with maintained MMP2 or MMP3 protein levels suggested that BAT may avoid tissue remodeling until arousal. No changes in WAT inflammatory cytokines or soluble receptors as well as decreased MMP2 levels during torpor and arousal suggested inflammation and modification to the extracellular matrix is likely suppressed in WAT. This study emphasizes the fat-but-fit nature of the hibernating ground squirrel and the ability of its fat stores to suppress inflammation.

MicroRNA expression patterns in the brown fat of hibernating 13-lined ground squirrels

The sequence diversity of microRNAs (miRNAs) allows these potent regulators of mRNA fate to bind multiple transcripts, giving them the power to inhibit diverse cellular processes. Therefore, miRNAs may regulate metabolic rate suppression (also termed torpor), an adaptation used by capable species to reduce energy expenditure, minimize tissue damage, and prolong life. Small RNA-sequencing of brown fat from control (37 °C) and torpid (5-8 °C) ground squirrels revealed a central role for miRNAs in torpor. Unsupervised clustering analysis of all 319 conserved miRNAs showed separation of control and torpor samples, which was supported by PCA analysis. Of the 76 miRNAs that were differentially expressed, 45 were upregulated during torpor. KEGG and GO analyses suggested these miRNAs inhibit genes within the ribosome, oxidative phosphorylation, and glycolysis/gluconeogenesis pathways. Some of the most downregulated miRNAs (miR-1-3p, miR-206 and miR-133a/b) had significant Pearson correlation coefficients, suggesting these myomiRs may be co-expressed in control animals. Only 3 of the 16 enriched KEGG pathways were less targeted by miRNAs during torpor, including cytokine-cytokine receptor interactions and the coagulation and complement cascades, suggesting epigenetic or post-translation modifications may inhibit these potentially damaging processes. Alternatively, their activation could promote damage sensing, wound repair, and improve tissue homeostasis. Overall, miRNA-seq analysis of brown fat revealed a strong role for miRNAs in the downregulation of central metabolic processes necessary for MRS, and highlighted miRNAs that could be inhibited by antagomiRs to promote brown fat activity in potential obesity treatments, or that could be used to replicate torpor in non-hibernating mammals.

The Torpid State: Recent Advances in Metabolic Adaptations and Protective Mechanisms†

Torpor and hibernation are powerful strategies enabling animals to survive periods of low resource availability. The state of torpor results from an active and drastic reduction of an individual's metabolic rate (MR) associated with a relatively pronounced decrease in body temperature. To date, several forms of torpor have been described in all three mammalian subclasses, i.e., monotremes, marsupials, and placentals, as well as in a few avian orders. This review highlights some of the characteristics, from the whole organism down to cellular and molecular aspects, associated with the torpor phenotype. The first part of this review focuses on the specific metabolic adaptations of torpor, as it is used by many species from temperate zones. This notably includes the endocrine changes involved in fat- and food-storing hibernating species, explaining biomedical implications of MR depression. We further compare adaptive mechanisms occurring in opportunistic vs. seasonal heterotherms, such as tropical and sub-tropical species. Such comparisons bring new insights into the metabolic origins of hibernation among tropical species, including resistance mechanisms to oxidative stress. The second section of this review emphasizes the mechanisms enabling heterotherms to protect their key organs against potential threats, such as reactive oxygen species, associated with the torpid state. We notably address the mechanisms of cellular rehabilitation and protection during torpor and hibernation, with an emphasis on the brain, a central organ requiring protection during torpor and recovery. Also, a special focus is given to the role of an ubiquitous and readily-diffusing molecule, hydrogen sulfide (H2S), in protecting against ischemia-reperfusion damage in various organs over the torpor-arousal cycle and during the torpid state. We conclude that (i) the flexibility of torpor use as an adaptive strategy enables different heterothermic species to substantially suppress their energy needs during periods of severely reduced food availability, (ii) the torpor phenotype implies marked metabolic adaptations from the whole organism down to cellular and molecular levels, and (iii) the torpid state is associated with highly efficient rehabilitation and protective mechanisms ensuring the continuity of proper bodily functions. Comparison of mechanisms in monotremes and marsupials is warranted for understanding the origin and evolution of mammalian torpor.

Cold-inducible RNA-binding protein Cirp, but not Rbm3, may regulate transcript processing and protection in tissues of the hibernating ground squirrel

RNA-binding proteins (RBPs) have important roles in transcription, pre-mRNA processing/transport, mRNA degradation, translation, and non-coding RNA processing, among others. RBPs that are expressed in response to cold stress, such as Cirp and Rbm3, could regulate RNA stability and translation in hibernating mammals that reduce their body temperatures from 37 °C to as low as 0-5 °C during torpor bouts. RBPs including Cirp, Rbm3, and stress-inducible HuR translocate from the nucleus to stabilize mRNAs in the cytoplasm, and thereby could regulate which mRNA transcripts are protected from degradation and are translated, versus stored, for future protein synthesis or degraded by nucleases during cell stress associated with metabolic rate depression. This is the first study to explore the transcriptional/translational regulation, and subcellular localization of cold-inducible RBPs in a model hibernator, the 13-lined ground squirrel (Ictidomys tridecemlineatus). Cirp protein levels were upregulated in liver, skeletal muscle, and brown adipose tissue throughout the torpor-arousal cycle whereas Rbm3 protein levels stayed constant or decreased, suggesting an important role for Cirp, but likely not Rbm3, in the hibernator stress response. Increased cytoplasmic localization of Cirp in liver and muscle and HuR in liver during torpor, but no changes in the relative levels of Rbm3 in the cytoplasm, emphasizes a role for Cirp and possibly HuR in regulating mRNA processing during torpor. This study informs our understanding of the natural adaptations that extreme animals use in the face of stress, and highlight natural stress response mediators that could be used to bolster cryoprotection of human organs donated for transplant.

Endoplasmic Reticulum Stress Increases Multidrug-resistance Protein 2 Expression and Mitigates Acute Liver Injury

BACKGROUND

Multidrug-resistance protein (MRP) 2 is a key membrane transporter that is expressed on hepatocytes and regulated by nuclear factor kappa B (NF-κB). Interestingly, endoplasmic reticulum (ER) stress is closely associated with liver injury and the activation of NF-κB signaling.

OBJECTIVE

Here, we investigated the impact of ER stress on MRP2 expression and the functional involvement of MRP2 in acute liver injury.

METHODS

ER stress, MRP2 expression, and hepatocyte injury were analyzed in a carbon tetrachloride (CCl4)-induced mouse model of acute liver injury and in a thapsigargin (TG)-induced model of ER stress.

RESULTS

CCl4 and TG induced significant ER stress, MRP2 protein expression and NF- κB activation in mice and LO2 cells (P < 0.05). Pretreatment with ER stress inhibitor 4- phenyl butyric acid (PBA) significantly mitigated CCl4 and TG-induced ER stress and MRP2 protein expression (P < 0.05). Moreover, pretreatment with pyrrolidine dithiocarbamic acid (PDTC; NF-κB inhibitor) significantly inhibited CCl4-induced NF-κB activation and reduced MRP2 protein expression (1±0.097 vs. 0.623±0.054; P < 0.05). Furthermore, hepatic downregulation of MRP2 expression significantly increased CCl4- induced ER stress, apoptosis, and liver injury.

CONCLUSION

ER stress enhances intrahepatic MRP2 protein expression by activating NF-κB. This increase in MRP2 expression mitigates ER stress and acute liver injury.

Suspended in time: Molecular responses to hibernation also promote longevity

Aging in most animals is an inevitable process that causes or is a result of physiological, biochemical, and molecular changes in the body, and has a strong influence on an organism's lifespan. Although advancement in medicine has allowed humans to live longer, the prevalence of age-associated medical complications is continuously burdening older adults worldwide. Current animal models used in research to study aging have provided novel information that has helped investigators understand the aging process; however, these models are limiting. Aging is a complex process that is regulated at multiple biological levels, and while a single manipulation in these models can provide information on a process, it is not enough to understand the global regulation of aging. Some mammalian hibernators live up to 9.8-times higher than their expected average lifespan, and new research attributes this increase to their ability to hibernate. A common theme amongst these mammalian hibernators is their ability to greatly reduce their metabolic rate to a fraction of their normal rate and initiate cytoprotective responses that enable their survival. Metabolic rate depression is strictly regulated at different biological levels in order to enable the animal to not only survive, but to also do so by relying mainly on their limited internal fuels. As such, understanding both the global and specific regulatory mechanisms used to promote survival during hibernation could, in theory, allow investigators to have a better understanding of the aging process. This can also allow pharmaceutical industries to find therapeutics that could delay or reverse age-associated medical complications and promote healthy aging and longevity in humans.