FoxO3a-mediated activation of stress responsive genes during early torpor in a mammalian hibernator

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.

Genomic Signatures of Selection along a Climatic Gradient in the Northern Range Margin of the White-Footed Mouse (Peromyscus leucopus)

Identifying genetic variation involved in thermal adaptation is likely to yield insights into how species adapt to different climates. Physiological and behavioral responses associated with overwintering (e.g., torpor) are thought to serve important functions in climate adaptation. In this study, we use 2 isolated Peromyscus leucopus lineages on the northern margin of the species range to identify single nucleotide polymorphisms (SNPs) showing a strong environmental association and test for evidence of parallel evolution. We found signatures of clinal selection in each lineage, but evidence of parallelism was limited, with only 2 SNPs showing parallel allele frequencies across transects. These parallel SNPs map to a gene involved in protection against iron-dependent oxidative stress (Fxn) and to a gene with unknown function but containing a forkhead-associated domain (Fhad1). Furthermore, within transects, we find significant clinal patterns in genes enriched for functions associated with glycogen homeostasis, synaptic function, intracellular Ca2+ balance, H3 histone modification, as well as the G2/M transition of cell division. Our results are consistent with recent literature on the cellular and molecular basis of climate adaptation in small mammals and provide candidate genomic regions for further study.

Transcriptional changes in muscle of hibernating arctic ground squirrels (Urocitellus parryii): implications for attenuation of disuse muscle atrophy

Physical inactivity generates muscle atrophy in most mammalian species. In contrast, hibernating mammals demonstrate limited muscle loss over the prolonged intervals of immobility during winter, which suggests that they have adaptive mechanisms to reduce disuse muscle atrophy. To identify transcriptional programs that underlie molecular mechanisms attenuating muscle loss, we conducted a large-scale gene expression profiling in quadriceps muscle of arctic ground squirrels, comparing hibernating (late in a torpor and during torpor re-entry after arousal) and summer active animals using next generation sequencing of the transcriptome. Gene set enrichment analysis showed a coordinated up-regulation of genes involved in all stages of protein biosynthesis and ribosome biogenesis during both stages of hibernation that suggests induction of translation during interbout arousals. Elevated proportion of down-regulated genes involved in apoptosis, NFKB signaling as well as significant under expression of atrogenes, upstream regulators (FOXO1, FOXO3, NFKB1A), key components of the ubiquitin proteasome pathway (FBXO32, TRIM63, CBLB), and overexpression of PPARGC1B inhibiting proteolysis imply suppression of protein degradation in muscle during arousals. The induction of protein biosynthesis and decrease in protein catabolism likely contribute to the attenuation of disuse muscle atrophy through prolonged periods of immobility of hibernation.

Remarkable Protective Effects of Nrf2-Mediated Antioxidant Enzymes and Tissue Specificity in Different Skeletal Muscles of Daurian Ground Squirrels Over the Torpor-Arousal Cycle

Hibernating mammals experience conditions of extreme oxidative stress, such as fasting, muscle disuse, and repeated hypoxic ischemia-reperfusion, during the torpor-arousal cycle. Despite this, they experience little oxidative injury and are thus an interesting model of anti-oxidative damage. Thus, in the current study, we explored the levels and underlying mechanism of oxidative stress and antioxidant capacity in three skeletal muscles [slow-twitch soleus (SOL), fast-twitch extensor digitorum longus (EDL), and mixed gastrocnemius (GAS)] of Daurian ground squirrels (Spermophilus dauricus) during hibernation. Results showed that hydrogen peroxide content in the EDL and GAS decreased significantly during pre-hibernation (PRE) and late torpor (LT) compared to levels in the summer active (SA) group. Furthermore, relative to SA levels, malondialdehyde content decreased significantly during interbout arousal (IBA) and early torpor (ET) in all three skeletal muscles and decreased in the EDL and GAS during LT. Compared with the SA group, glutathione peroxidase 1 (GPx1) and catalase (CAT) protein expression in the SOL and superoxide dismutase 1 (SOD1) and SOD2 expression in the GAS increased significantly during the entire hibernation season. Furthermore, SOD1 in the IBA group and CAT and GPx1 in the ET and LT groups increased significantly in the EDL. The activities of most tested antioxidant enzymes were higher in the IBA group than in the LT group, whereas CAT remained highly active throughout the hibernation season in all three muscles. Nrf2 and p-Nrf2 protein levels were significantly elevated in the SOL and EDL during hibernation, and increased during the PRE, IBA, and ET states in the GAS. Thus, activation of the Nrf2/Keap1 antioxidant pathway resulted in the elimination of excess reactive oxygen species (ROS). Specifically, ROS levels were maintained at physiological levels by the up-regulation of antioxidant enzyme expression in skeletal muscles under oxidative stress during hibernation, thus preventing oxidative injury over the torpor-arousal cycle. Different antioxidant patterns and oxidative stress levels were also observed among the different skeletal muscles of hibernating Daurian ground squirrels.

Stress-induced antioxidant defense and protein chaperone response in the freeze-tolerant wood frog Rana sylvatica

Freeze tolerance is an adaptive response utilized by the wood frog Rana sylvatica to endure the sub-zero temperatures of winter. Survival of whole body freezing requires wood frogs to trigger cryoprotective mechanisms to deal with potential injuries associated with conversion of 65-70% of total body water into ice, including multiple consequences of ice formation such as cessation of blood flow and cell dehydration caused by water loss into ice masses. To understand how wood frogs defend against these stressors, we measured the expression of proteins known to be involved in the antioxidant defense and protein chaperone stress responses in brain and heart of wood frogs comparing freezing, anoxia, and dehydration stress. Our results showed that most stress proteins were regulated in a tissue- and stress-specific manner. Notably, protein levels of the cytosolic superoxide dismutase (SOD1) were upregulated by 1.37 ± 0.11-fold in frozen brain, whereas the mitochondrial SOD2 isoform rose by 1.38 ± 0.37-fold in the heart during freezing. Catalase protein levels were upregulated by 3.01 ± 0.47-fold in the brain under anoxia stress, but remained unchanged in the heart. Similar context-specific regulatory patterns were also observed for the heat shock protein (Hsp) molecular chaperones. Hsp27 protein was down-regulated in the brain across the three stress conditions, whereas the mitochondrial Hsp60 was upregulated in anoxic brain by 1.73 ± 0.38-fold and by 2.13 ± 0.57-fold in the frozen heart. Overall, our study provides a snapshot of the regulatory expression of stress proteins in wood frogs under harsh environment conditions and shows that they are controlled in a tissue- and stress-specific manner.

Investigating Nrf2-associated non-coding RNAs in the hibernating ground squirrel, Ictidomys tridecemlineatus

Small mammals hibernate to deal with environmental conditions associated with the winter season. Numerous physiological changes occur during a typical torpor-arousal cycle including variations in heart rate and blood flow. Such cycle possesses characteristics of ischemia-reperfusion cycles that can lead to oxidative stress in non-hibernating models. Interestingly, hibernators can cope with these conditions and the complete molecular picture underlying this adaptation is not fully understood. Non-coding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), can impact expression and activity of various targets and have been associated with oxidative stress response. This work was aimed at assessing expression of oxidative stress-associated non-coding RNAs and their targets during hibernation. Measurement of miRNAs miR-93, miR-141, miR-144 and miR-200a, lncRNAs Mhrt and ODRUL, as well as of several targets associated with the Nrf2 signaling cascade including Keap1 was conducted using qRT-PCR in hibernating hearts of the thirteen-lined ground squirrel, Ictidomys tridecemlineatus. Elevated Nrf2 levels and reduced miR-200a levels were notably observed in hibernating versus euthermic samples. Functional analysis of targets predicted to be regulated by the investigated miRNAs was performed and revealed transcriptional regulation and phosphorylation as relevant processes. These results highlight a potential interplay between non-coding RNAs and targets associated with oxidative stress response during hibernation and further strengthen the underlying importance of non-coding RNAs in cold torpor.

From discovery of the CHOP axis and targeting ClpP to the identification of additional axes of the UPRmt driven by the estrogen receptor and SIRT3

The mitochondrial UPR (UPR^mt) is rapidly gaining attention. While most studies on the UPR^mt have focused on its role in aging, emerging studies suggest an important role of the UPR^mt in cancer. Further, several of the players of the UPR^mt in mammalian cells have well reported roles in the maintenance of the organelle. The goal of this review is to emphasize aspects of the UPR^mt that have been overlooked in the current literature, describe the role of specific players of the UPR^mt in the biology of the mitochondria and highlight the intriguing possibility that targeting the UPR^mt in cancer may be already within reach.

Regulation of Smad mediated microRNA transcriptional response in ground squirrels during hibernation

Mammalian hibernation is a state of dormancy that is used by some animals to survive through the unfavorable conditions of winter, and is characterized by coordinated suppression of basal metabolism that is supported by global inhibition of energy/ATP-consuming processes. In this study, we examine the regulation of the anti-proliferatory TGF-β/Smad transcription factor signaling pathway in the liver tissue of the hibernating 13-lined ground squirrel Ictidomys tridecemlineatus. The TGF-β/Smad signaling pathway is known to mediate cell cycle arrest through induction of cell cycle dependent kinase inhibitors, and more recently, has been shown to regulate a wide range of cellular processes via its control of microRNA biosynthesis. We show that phosphorylation levels of the Smad3 protein at its activation residue is increased by ~1.5-fold during torpor, and this is associated with an increase in nuclear localization and DNA binding activity of Smad3. Expression levels of several TGF-β induced microRNAs previously described in human cells were also activated in ground squirrel during torpor. Among these were miR-21, miR-23a, and miR-107, which contain either the conserved R-SBE or R-SBE related motif found in microRNAs that are post-transcriptionally processed by Smad proteins. We show that levels of miR-21 were highly elevated at multiple stages of torpor, and predicted gene targets of miR-21 were enriched to multiple pro-growth cellular processes. Overall, we provide evidence that show the Smad3 transcription factor is activated in ground squirrels during torpor, and suggest a role for this signaling pathway in mediating anti-proliferatory signals via its transcriptional control of cell cycle inhibitors and downstream microRNAs.

A signature of renal stress resistance induced by short-term dietary restriction, fasting, and protein restriction

During kidney transplantation, ischemia-reperfusion injury (IRI) induces oxidative stress. Short-term preoperative 30% dietary restriction (DR) and 3-day fasting protect against renal IRI. We investigated the contribution of macronutrients to this protection on both phenotypical and transcriptional levels. Male C57BL/6 mice were fed control food ad libitum, underwent two weeks of 30%DR, 3-day fasting, or received a protein-, carbohydrate- or fat-free diet for various periods of time. After completion of each diet, renal gene expression was investigated using microarrays. After induction of renal IRI by clamping the renal pedicles, animals were monitored seven days postoperatively for signs of IRI. In addition to 3-day fasting and two weeks 30%DR, three days of a protein-free diet protected against renal IRI as well, whereas the other diets did not. Gene expression patterns significantly overlapped between all diets except the fat-free diet. Detailed meta-analysis showed involvement of nuclear receptor signaling via transcription factors, including FOXO3, HNF4A and HMGA1. In conclusion, three days of a protein-free diet is sufficient to induce protection against renal IRI similar to 3-day fasting and two weeks of 30%DR. The elucidated network of common protective pathways and transcription factors further improves our mechanistic insight into the increased stress resistance induced by short-term DR.

Regulation of AKT activity prevents autonomic nervous system imbalance

Autonomic nervous system (ANS) imbalances are involved in the etiology of cancer, allergy, and collagen diseases. Previously, we hypothesized that FoxO and HSF-1 limit autonomic stress responses via negative feedback on the ANS. Here, we evaluated the role of AKT, a negative regulator of FoxO, during activation of the ANS by loneliness stress in mice. Spontaneous motility was increased during loneliness stress and decreased after release from stress. The AKT activator SC79 attenuated stress-induced spontaneous motility, whereas the AKT inhibitor API-2 prevented decreases in motility after stress release. Our results show that AKT activity regulates ANS responses to loneliness stress.

Skeletal muscle mass and composition during mammalian hibernation

Hibernation is characterized by prolonged periods of inactivity with concomitantly low nutrient intake, conditions that would typically result in muscle atrophy combined with a loss of oxidative fibers. Yet, hibernators consistently emerge from winter with very little atrophy, frequently accompanied by a slight shift in fiber ratios to more oxidative fiber types. Preservation of muscle morphology is combined with down-regulation of glycolytic pathways and increased reliance on lipid metabolism instead. Furthermore, while rates of protein synthesis are reduced during hibernation, balance is maintained by correspondingly low rates of protein degradation. Proposed mechanisms include a number of signaling pathways and transcription factors that lead to increased oxidative fiber expression, enhanced protein synthesis and reduced protein degradation, ultimately resulting in minimal loss of skeletal muscle protein and oxidative capacity. The functional significance of these outcomes is maintenance of skeletal muscle strength and fatigue resistance, which enables hibernating animals to resume active behaviors such as predator avoidance, foraging and mating immediately following terminal arousal in the spring.

Regulation of gene expression by NFAT transcription factors in hibernating ground squirrels is dependent on the cellular environment

Calcineurin is a calmodulin-stimulated phosphatase that regulates the nuclear translocation of nuclear factor of activated T cell (NFAT) c1-4 through dephosphorylation. We believe that this mechanism plays various roles in the remodeling and maintenance of Ictidomys tridecemlineatus skeletal muscle. During hibernation, bouts of torpor and arousal take place, and squirrels do not lose muscle mass despite being inactive. Protein expression of Ca(2+) signaling proteins were studied using immunoblotting. A DNA-protein interaction ELISA technique was created to test the binding of NFATs in the nucleus to DNA probes containing the NFAT response element under environmental conditions reflective of those during hibernation. Calcineurin protein levels increased by 3.08-fold during torpor (compared to euthermic control), whereas calpain1 levels also rose by 3.66-fold during torpor. Calmodulin levels were elevated upon entering torpor. NFATc4 binding to DNA showed a 1.4-fold increase during torpor, and we found that this binding was further enhanced when 600 nM of Ca(2+) was supplemented. We also found that decreasing the temperature of ELISAs resulted in progressive decreases in the binding of NFATs c1, c3, and c4 to DNA. In summary, calmodulin and calpain1 appear to activate calcineurin and NFATc4 during torpor. NFAT binding to target promoters is affected by intranuclear [Ca(2+)] and environmental temperatures. Therefore, Ca(2+) signaling and temperature changes play key roles in regulation of the NFAT-calcineurin pathway in skeletal muscle of hibernating 13-lined ground squirrels over the torpor-arousal cycle, and they may contribute to the avoidance of disuse-induced muscle atrophy that occurs naturally in these animals.

Inhibition of skeletal muscle atrophy during torpor in ground squirrels occurs through downregulation of MyoG and inactivation of Foxo4

Foxo4 and MyoG proteins regulate the transcription of numerous genes, including the E3 ubiquitin ligases MAFbx and MuRF1, which are activated in skeletal muscle under atrophy-inducing conditions. In the thirteen-lined ground squirrel, there is little muscle wasting that occurs during hibernation, a process characterized by bouts of torpor and arousal, despite virtual inactivity. Consequently, we were interested in studying the regulatory role of Foxo4 and MyoG on ubiquitin ligases throughout torpor-arousal cycles. Findings indicate that MAFbx and MuRF1 decreased during early torpor (ET) by 42% and 40%, respectively, relative to euthermic control (EC), although MuRF1 expression subsequently increased at late torpor (LT). The expression pattern of MyoG most closely resembled that of MAFbx, with levels decreasing during LT. In addition, the phosphorylation of Foxo4 at Thr-451 showed an initial increase during EN, followed by a decline throughout the remainder of the torpor-arousal cycle, suggesting Foxo4 inhibition. This trend was mirrored by inhibition of the Ras-Ral pathway, as the Ras and Ral proteins were decreased by 77% and 41% respectively, at ET. Foxo4 phosphorylation at S197 was depressed during entrance and torpor, suggesting Foxo4 nuclear localization, and possibly regulating the increase in MuRF1 levels at LT. These findings indicate that signaling pathways involved in regulating muscle atrophy, such as MyoG and Foxo4 through the Ras-Ral pathway, contribute to important muscle-specific changes during hibernation. Therefore, this data provides novel insight into the molecular mechanisms regulating muscle remodeling in a hibernator model.

Transcriptional activation of muscle atrophy promotes cardiac muscle remodeling during mammalian hibernation

Background. Mammalian hibernation in thirteen-lined ground squirrels (Ictidomys tridecemlineatus) is characterized by dramatic changes on a physiological and molecular level. During hibernation, mammalian hearts show a propensity to hypertrophy due to the need for increasing contractility to pump colder and more viscous blood. While cardiac hypertrophy is quite often a process characterized by decompensation, the ground squirrel studied is an excellent model of cardiac plasticity and cardioprotection under conditions of hypothermia and ischemia. The forkhead box O (Foxo) family of proteins and myogenin (MyoG) are transcription factors that control protein degradation and muscle atrophy by regulating the expression of the E3 ubiquitin ligases, MAFbx and MuRF1. These ligases are part of the ubiquitin proteasome system by transferring ubiquitin to proteins and targeting these proteins for degradation. Regulation of Foxo1 and 3a occurs through phosphorylation at different residues. The threonine-24 (Thr-24) and serine-319 (Ser-319) residues on Foxo1, and the Thr-32 residue on Foxo3a are phosphorylated by Akt, leading to cytoplasmic localization of Foxo. We propose that the described mechanism contributes to the changes taking place in cardiac muscle throughout hibernation.

Methods. Total and phosphorylated protein levels of Foxo1 and Foxo3a, as well as total protein levels of MyoG, MAFbx, and MuRF1, were studied using immunoblotting.

Results. Immunoblotting results demonstrated upregulations in Foxo1 and Foxo3a total protein levels (1.3- and 4.5-fold increases relative to euthermic control, for Foxo1 and 3a respectively) during late torpor, and protein levels remained elevated throughout the rest of torpor and at interbout arousal. We also observed decreases in inactive, phosphorylated Foxo1 and 3a proteins during throughout torpor, where levels of p-Foxo1 Ser³¹⁹ and Thr²⁴, as well as p-Foxo3a Thr³² decreased by at least 45% throughout torpor. MyoG was upregulated only during late torpor by 2.4-fold. Protein levels of MAFbx and MuRF1 increased in late torpor as well as during early arousal by as much as 2.8-fold, and MAFbx levels remained elevated during interbout arousal, whereas MuRF1 levels returned to control levels.

Discussion. The present results indicate that upregulation and activation of Foxo1 and 3a, in addition to the increase in MyoG levels at late torpor, may be upregulating the expression of MAFbx and MuRF1. These findings suggest that there is activation of the ubiquitin proteasome system (UPS) as ground squirrels arouse from torpor. Therefore, the signalling pathway involving MyoG, and the E3 ligases MAFbx and MuRF1, plays a significant role in cardiac muscle remodelling during hibernation. These findings provide insights into the regulation of protein degradation and turnover in the cardiac muscle of a hibernator model.

Antioxidant Defenses in the Brains of Bats during Hibernation

Hibernation is a strategy used by some mammals to survive a cold winter. Small hibernating mammals, such as squirrels and hamsters, use species- and tissue-specific antioxidant defenses to cope with oxidative insults during hibernation. Little is known about antioxidant responses and their regulatory mechanisms in hibernating bats. We found that the total level of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the brain of each of the two distantly related hibernating bats M. ricketti and R. ferrumequinum at arousal was lower than that at torpid or active state. We also found that the levels of malondialdehyde (product of lipid peroxidation) of the two hibernating species of bats were significantly lower than those of non-hibernating bats R. leschenaultia and C. sphinx. This observation suggests that bats maintain a basal level of ROS/RNS that does no harm to the brain during hibernation. Results of Western blotting showed that hibernating bats expressed higher amounts of antioxidant proteins than non-hibernating bats and that M. ricketti bats upregulated the expression of some enzymes to overcome oxidative stresses, such as superoxide dismutase, glutathione reductase, and catalase. In contrast, R. ferrumequinum bats maintained a relatively high level of superoxide dismutase 2, glutathione reductase, and thioredoxin-2 throughout the three different states of hibernation cycles. The levels of glutathione (GSH) were higher in M. ricketti bats than in R. ferrumequinum bats and were significantly elevated in R. ferrumequinum bats after torpor. These data suggest that M. ricketti bats use mainly antioxidant enzymes and R. ferrumequinum bats rely on both enzymes and low molecular weight antioxidants (e.g., glutathione) to avoid oxidative stresses during arousal. Furthermore, Nrf2 and FOXOs play major roles in the regulation of antioxidant defenses in the brains of bats during hibernation. Our study revealed strategies used by bats against oxidative insults during hibernation.

Response of the JAK-STAT pathway to mammalian hibernation in 13-lined ground squirrel striated muscle

Over the course of the torpor-arousal cycle, hibernators must make behavioral, physiological, and molecular rearrangements in order to keep a very low metabolic rate and retain organ viability. 13-lined ground squirrels (Ictidomys tridecemlineatus) remain immobile during hibernation, and although the mechanisms of skeletal muscle survival are largely unknown, studies have shown minimal muscle loss in hibernating organisms. Additionally, the ground squirrel heart undergoes cold-stress, reversible cardiac hypertrophy, and ischemia-reperfusion without experiencing fatal impairment. This study examines the role of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway in the regulation of cell stress in cardiac and skeletal muscles, comparing euthermic and hibernating ground squirrels. Immunoblots showed a fivefold decrease in JAK3 expression during torpor in skeletal muscle, along with increases in STAT3 and 5 phosphorylation and suppressors of cytokine signaling-1 (SOCS1) protein levels. Immunoblots also showed coordinated increases in STAT1, 3 and 5 phosphorylation and STAT1 inhibitor protein expression in cardiac muscle during torpor. PCR analysis revealed that the activation of these pro-survival signaling cascades did not result in coordinate changes in downstream genes such as anti-apoptotic B-cell lymphoma-2 (Bcl-2) family gene expression. Overall, these results indicate activation of the JAK-STAT pathway in both cardiac and skeletal muscles, suggesting a response to cellular stress during hibernation.

Transcriptional Activation of p53 during Cold Induced Torpor in the 13-Lined Ground Squirrel Ictidomys tridecemlineatus

The transcription factor p53 is located at the centre of multiple pathways relating the cellular response to stress. Commonly known as a tumor suppressor, it is responsible for initiating diverse actions to protect the integrity of the genome, ranging from cell cycle arrest to apoptosis. This study investigated the regulation of p53 protein in hibernating 13-lined ground squirrel Ictidomys tridecemlineatus during multiple stages of the torpor-arousal cycle. Transcript and protein levels of p53 were both elevated in the skeletal muscle during early and late torpor stages of the hibernation cycle. Nuclear localization of p53 was also increased during late torpor, and this is associated with an increase in its DNA binding activity and expression of p53 transcriptional targets p21CIP, gadd45α, and 14-3-3σ. The increase in p53 transcriptional activity appears to be independent of its phosphorylation at Ser-15, Ser-46, and Ser-392, consistent with an absence of checkpoint kinase activation during torpor. Sequence analysis revealed unique amino acid substitutions in the ground squirrel p53 protein, which may contribute to an increase in protein stability compared to nonhibernators. Overall, the study results provided evidences for a potential role of p53 in the protection of the skeletal muscle during torpor.

Expression of nuclear factor of activated T cells (NFAT) and downstream muscle-specific proteins in ground squirrel skeletal and heart muscle during hibernation

The thirteen-lined ground squirrel (Ictidomys tridecemlineatus) undergoes remarkable adaptive changes during hibernation. Interestingly, skeletal muscle remodelling occurs during the torpor-arousal cycle of hibernation to prevent net muscle loss despite inactivity. Reversible cardiomyocyte hypertrophy occurs in cardiac muscle, allowing the heart to preserve cardiac output during hibernation, while avoiding chronic maladaptive hypertrophy post-hibernation. We propose that calcium signalling proteins [calcineurin (Cn), calmodulin (CaM), and calpain], the nuclear factor of activated T cell (NFAT) family of transcription factors, and the NFAT targets myoferlin and myomaker contribute significantly to adaptations taking place in skeletal and cardiac muscle during hibernation. Protein-level analyses were performed over several conditions: euthermic room temperature (ER), euthermic cold room (EC), entrance into (EN), early (ET), and late torpor (LT) time points, in addition to early (EA), interbout (IA), and late arousal (LA) time points using immunoblotting and DNA-protein interaction (DPI) enzyme-linked immunosorbent assay (ELISAs). In skeletal and cardiac muscle, NFATc2 protein levels were elevated during torpor. NFATc4 increased throughout the torpor-arousal cycle in both tissues, and NFATc1 showed this trend in cardiac muscle only. NFATc3 showed an elevation in DNA-binding activity but not expression during torpor. Myoferlin protein levels dramatically increased during torpor in both skeletal and cardiac muscle. Myomaker levels also increased significantly in cardiac muscle during torpor. Cardiac Cn levels remained stable, whereas CaM and calpain decreased throughout the torpor-arousal cycle. Activation and/or upregulation of NFATc2, c3, myoferlin, and myomaker at torpor could be part of a stress-response mechanism to preserve skeletal muscle mass, whereas CaM and calpain appear to initiate the rapid reversal of cardiac hypertrophy during arousal through downregulation of the NFAT-Cn pathway.

Post-translational regulation of PTEN catalytic function and protein stability in the hibernating 13-lined ground squirrel

BACKGROUND

The insulin signaling pathway functions as a major regulator of many metabolic and cellular functions, and has been shown to be reversibly suppressed in many species during hibernation. This study characterized the regulation of PTEN phosphatase, a negative regulator of the insulin receptor network, over the torpor-arousal cycle of hibernation in the skeletal muscle of Ictidomys tridecemlineatus.

METHODS

Western blotting and RT-PCR were used to analyze post-translational and transcriptional regulations of PTEN respectively. Enzymatic activities were determined by the malachite green assay, while protein stability was assessed the using pulse-proteolysis method.

RESULTS

During torpor, the ratio of non-phosphorylated PTEN (S380/T382/T383) was significantly elevated by 1.4-fold during late torpor compared with euthermic controls; this was coupled with an increase in substrate affinity for PIP3 (by 56%) in late torpor. Two proteolytic cleavage PEST motifs were identified in the C-terminus that overlapped with the phosphorylation sites of PTEN; pulse-proteolysis analysis of PTEN protein showed a decrease in protein stability during late torpor (Cm of urea decreased by 21%). Furthermore, the increase in PTEN activity observed was correlated with a decrease in PDK-1 phosphorylation by 32%, suggesting a downstream effect of PTEN activation during torpor. Transcriptional analysis showed that mRNA expression of pten and pdk-1 remain unchanged during hibernation, suggesting post-translation modification as the primary regulatory mechanism of PTEN function.

CONCLUSION

Phosphorylation plays an important role in the regulation of PTEN enzymatic activity and protein stability.

GENERAL SIGNIFICANCE

Activation of PTEN during torpor can regulate insulin signaling during periods of low energy state.

Induction of Antioxidant and Heat Shock Protein Responses During Torpor in the Gray Mouse Lemur, Microcebus murinus

A natural tolerance of various environmental stresses is typically supported by various cytoprotective mechanisms that protect macromolecules and promote extended viability. Among these are antioxidant defenses that help to limit damage from reactive oxygen species and chaperones that help to minimize protein misfolding or unfolding under stress conditions. To understand the molecular mechanisms that act to protect cells during primate torpor, the present study characterizes antioxidant and heat shock protein (HSP) responses in various organs of control (aroused) and torpid gray mouse lemurs, Microcebus murinus. Protein expression of HSP70 and HSP90α was elevated to 1.26 and 1.49 fold, respectively, in brown adipose tissue during torpor as compared with control animals, whereas HSP60 in liver of torpid animals was 1.15 fold of that in control (P<0.05). Among antioxidant enzymes, protein levels of thioredoxin 1 were elevated to 2.19 fold in white adipose tissue during torpor, whereas Cu-Zn superoxide dismutase 1 levels rose to 1.1 fold in skeletal muscle (P<0.05). Additionally, total antioxidant capacity was increased to 1.6 fold in liver during torpor (P<0.05), while remaining unchanged in the five other tissues. Overall, our data suggest that antioxidant and HSP responses are modified in a tissue-specific manner during daily torpor in gray mouse lemurs. Furthermore, our data also show that cytoprotective strategies employed during primate torpor are distinct from the strategies in rodent hibernation as reported in previous studies.

Forkhead factor FOXQ1 promotes TGF-β1 expression and induces epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) promotes tumor invasion and metastasis, but the coordination and integration mechanisms of these processes are still not fully understood. In this study, we used a cross-species expression profiling strategy of Hela cells to determine an important genetic program transfers. In particular, we have discovered a new transfer function, which is not previously known about transcription factor forkhead box Q1 (FOXQ1). The shRNA anti-FOXQ1 gene was synthesized and transfected into the Hela and EpRas cells. RT-PCR assay was performed to detect the mRNA levels in cells. Cell adhesion and separation assay were used to examine the cell-cell adhesion and separation among cells. Wound healing assay was utilized to examine cell migration and invasion ability. Chromatin immunoprecipitation assay was used to investigate the interaction between E-cadherin and N-cadherin and FOXQ1 promoter region. The results indicated that ectopic expression of FOXQ1 increased cell migration and invasion in vitro, enhanced mammary epithelial cells in vivo lung metastasis, and triggered significant EMT. In contrast, the opposite effects in vitro and in vivo of FOXQ1 knockdown phenotypes were caused by these mechanisms. Notably, FOXQ1 repressed core EMT regulation of the expression of TGF-β1. FOXQ1 protein directly interacts with E-cadherin and N-cadherin promoter region. And surveys show that FOXQ1 expression regulation by TGF-β1 and blockade induced EMT both morphological and molecular levels. Our findings emphasize the feasibility of cross-species expression profiles, as a strategy to identify metastasis-related genes. The induction of EMT by FOXQ1 defines a new transfer function in promoting cancer behind possible mechanisms.