Akt phosphorylation and kinase activity are down-regulated during hibernation in the 13-lined ground squirrel

Phosphorylation state of Akt in the heart during artificial deep hypothermia in Syrian hamsters

Hibernating animals show a remarkable decrease in body temperature without accompanying serious organ damage. Active hypometabolism may be involved in the protective mechanisms. Therefore, in the present study, the phosphorylation status of Akt was used to examine whether metabolism is actively reduced during artificial hypothermia in hamsters. Hypothermia induced by activation of the central adenosine A1 receptor (A1AR) resulted in dephosphorylation of Akt in the heart, as in that of hibernating hamsters. Since phosphorylated Akt almost totally disappeared, it is probable that cellular metabolism is actively reduced in A1AR-mediated hypothermia. Surprisingly, phosphorylation of Akt was significantly, but not totally, reduced even when hypothermia was induced by a combination of inhalation anesthesia and cooling. It can thus be considered that active regulation for the reduction of metabolic activity occurs in A1AR-mediated hypothermia as well as in anesthesia-induced hypothermia, though to a lesser extent. In both hypothermic conditions, serious arrhythmias were rarely observed. In anesthesia-induced hypothermia, during which dephosphorylation of Akt occurred only partially, there was no obvious organ damage in histopathological examination. Taken together, our findings suggest that artificial hypothermia causes active hypometabolism and has therapeutically applicable properties.

Mitochondrial microRNA profiles are altered in thirteen-lined ground squirrels (Ictidomys tridecemlineatus) during hibernation

Thirteen-lined ground squirrels (TLGSs) are obligate hibernators that cycle between torpor (low metabolic rate and body temperature) and interbout euthermia (IBE; typical euthermic body temperature and metabolism) from late autumn to spring. Many physiological changes occur throughout hibernation, including a reduction in liver mitochondrial metabolism during torpor, which is reversed during arousal to interbout euthermia. Nuclear-encoded microRNA (miRNA, small posttranscriptional regulator molecules) differ in abundance throughout TLGS hibernation and have been shown to regulate mitochondrial gene expression in mammalian cell culture (where they are referred to as mitomiRs). This study characterized differences in mitomiR profiles from TLGS liver mitochondria isolated during summer, torpor, and IBE, and predicted their mitochondrial targets. Using small RNA sequencing, differentially abundant mitomiRs were identified between hibernation states, and using quantitative PCR analysis, we quantified the expression of predicted mitochondrial mRNA targets. Most differences in mitomiR abundances were seasonal (i.e., between summer and winter) with only one mitomiR differentially abundant between IBE and torpor. Multiple factor analysis (MFA) revealed three clusters divided by hibernation states, where clustering was predominantly driven by mitomiR abundances. Nine of these differentially abundant mitomiRs had predicted mitochondrial RNA targets, including subunits of electron transfer system complexes I and IV, 12S rRNA, and two tRNAs. Overall, mitomiRs were predicted to suppress the expression of their mitochondrial targets and may have some involvement in regulating protein translation in mitochondria. This study found differences in mitomiR abundances between seasons and hibernation states of TLGS and suggests potential mechanisms for regulating the mitochondrial electron transfer system.NEW & NOTEWORTHY During the hibernation season, thirteen-lined ground squirrels periodically increase metabolism remarkably between torpor and interbout euthermia (IBE). This process involves rapid reactivation of mitochondrial respiration. We predicted that mitochondrial microRNA (mitomiRs) might be altered during this response. We found that the abundance of 38 liver mitomiRs differs based on hibernation state (summer, IBE, and torpor). Small RNA sequencing identified mitomiR profiles, including some mitomiRs that are predicted to bind to mitochondrial RNAs.

Torpor-responsive microRNAs in the heart of the Monito del monte, Dromiciops gliroides

The marsupial Monito del monte (Dromiciops gliroides) utilizes both daily and seasonal bouts of torpor to preserve energy and prolong survival during periods of cold and unpredictable food availability. Torpor involves changes in cellular metabolism, including specific changes to gene expression that is coordinated in part, by the posttranscriptional gene silencing activity of microRNAs (miRNA). Previously, differential miRNA expression has been identified in D. gliroides liver and skeletal muscle; however, miRNAs in the heart of Monito del monte remained unstudied. In this study, the expression of 82 miRNAs was assessed in the hearts of active and torpid D. gliroides, finding that 14 were significantly differentially expressed during torpor. These 14 miRNAs were then used in bioinformatic analyses to identify Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways that were predicted to be most affected by these differentially expressed miRNAs. Overexpressed miRNAs were predicted to primarily regulate glycosaminoglycan biosynthesis, along with various signaling pathways such as Phosphoinositide-3-kinase/protein kinase B and transforming growth factor-β. Similarly, signaling pathways including phosphatidylinositol and Hippo were predicted to be regulated by the underexpression of miRNAs during torpor. Together, these results suggest potential molecular adaptations that protect against irreversible tissue damage and enable continued cardiac and vascular function despite hypothermia and limited organ perfusion during torpor.

Differential AMPK-mediated metabolic regulation observed in hibernation-style polymorphisms in Siberian chipmunks

Hibernation is a unique physiological phenomenon allowing extreme hypothermia in endothermic mammals. Hypometabolism and hypothermia tolerance in hibernating animals have been investigated with particular interest; recently, studies of cultured cells and manipulation of the nervous system have made it possible to reproduce physiological states related to hypothermia induction. However, much remains unknown about the periodic regulation of hibernation. In particular, the physiological mechanisms facilitating the switch from an active state to a hibernation period, including behavioral changes and the acquisition of hypothermia tolerance remain to be elucidated. AMPK is a protein known to play a central role not only in feeding behavior but also in metabolic regulation in response to starvation. Our previous research has revealed that chipmunks activate AMPK in the brain during hibernation. However, whether AMPK is activated during winter in non-hibernating animals is unknown. Previous comparative studies between hibernating and non-hibernating animals have often been conducted between different species, consequently it has been impossible to account for the effects of phylogenetic differences. Our long-term monitoring of siberian chipmunks, has revealed intraspecific variation between those individuals that hibernate annually and those that never become hypothermic. Apparent differences were found between hibernating and non-hibernating types with seasonal changes in lifespan and blood HP levels. By comparing seasonal changes in AMPK activity between these polymorphisms, we clarified the relationship between hibernation and AMPK regulation. In hibernating types, phosphorylation of p-AMPK and p-ACC was enhanced throughout the brain during hibernation, indicating that AMPK-mediated metabolic regulation is activated. In non-hibernating types, AMPK and ACC were not seasonally activated. In addition, AMPK activation in the hypothalamus had already begun during high Tb before hibernation. Changes in AMPK activity in the brain during hibernation may be driven by circannual rhythms, suggesting a hibernation-regulatory mechanism involving AMPK activation independent of Tb. The differences in brain AMPK regulation between hibernators and non-hibernators revealed in this study were based on a single species thus did not involve phylogenetic differences, thereby supporting the importance of brain temperature-independent AMPK activation in regulating seasonal metabolism in hibernating animals.

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

NADP-dependent isocitrate dehydrogenase (NADP-IDH, EC 1.1.1.42) catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate with the concomitant production of NADPH. NADPH plays important roles in many biosynthesis pathways, maintenance of proper oxidation-reduction balance, and protection against oxidative damage. This present study investigated the dynamic nature of NADP-IDH during hibernation by purifying it from the skeletal muscle of Richardson's ground squirrel (Urocitellus richardsonii) and analyzing its structural and functional changes in response to hibernation. Kinetic parameters of purified NADP-IDH from euthermic and hibernating ground squirrel skeletal muscle were characterized at 22 °C and 5 °C. Relative to euthermic muscle, -NADP-IDH in hibernating muscle had a higher affinity for its substrate, isocitrate at 22 °C, whereas at 5 °C, there was a significant decrease in isocitrate affinity. Western blot analysis revealed greater serine and threonine phosphorylation in hibernator NADP-IDH as compared to euthermic NADP-IDH. In addition, Bioinformatic analysis predicted the presence of 18 threonine and 21 serine phosphorylation sites on squirrel NADP-IDH. The structural and functional changes in NADP-IDH indicate the ability of the organism to reduce energy consumption during hibernation, while emphasizing increased NADPH production, and thus antioxidant activity, during torpor arousal cycles.

Integrative transcription start site analysis and physiological phenotyping reveal torpor-specific expression program in mouse skeletal muscle

Mice enter an active hypometabolic state, called daily torpor when they experience a lowered caloric intake under cold ambient temperature. During torpor, the oxygen consumption rate in some animals drops to less than 30% of the normal rate without harming the body. This safe but severe reduction in metabolism is attractive for various clinical applications; however, the mechanism and molecules involved are unclear. Therefore, here we systematically analyzed the gene expression landscape on the level of the RNA transcription start sites in mouse skeletal muscles under various metabolic states to identify torpor-specific transcribed regulatory patterns. We analyzed the soleus muscles from 38 mice in torpid and non-torpid conditions and identified 287 torpor-specific promoters out of 12,862 detected promoters. Furthermore, we found that the transcription factor ATF3 is highly expressed during torpor deprivation and its binding motif is enriched in torpor-specific promoters. Atf3 was also highly expressed in the heart and brown adipose tissue during torpor and systemically knocking out Atf3 affected the torpor phenotype. Our results demonstrate that mouse torpor combined with powerful genetic tools is useful for studying active hypometabolism.

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.

Differential protein metabolism and regeneration in hypertrophic diaphragm and atrophic gastrocnemius muscles in hibernating Daurian ground squirrels

NEW FINDINGS

What is the central question of this study? The aim was to investigate whether diaphragm hypertrophy and gastrocnemius atrophy during hibernation of Daurian ground squirrels involve differential regulation of protein metabolism and regeneration. What is the main finding and its importance? We clarified the differences in protein metabolism and muscle regenerative potential in the diaphragm and gastrocnemius of hibernating ground squirrels, reflecting the different adaptability of muscles.

ABSTRACT

Are differences in the regulation of protein metabolism and regeneration involved in the different phenotypic adaptation mechanisms of muscle hypertrophy and atrophy in hibernators? Two fast-type muscles (diaphragm and gastrocnemius) in summer active and hibernating Daurian ground squirrels were selected to detect changes in cross-sectional area (CSA) and protein expression indicative of protein synthesis metabolism (protein expression of P-Akt, P-mTORC1, P-S6K1 and P-4E-BP1), protein degradation metabolism (MuRF1, atrogin-1, calpain-1, calpain-2, calpastatin, desmin, troponin T, Beclin1 and LC3-II) and muscle regeneration (MyoD, myogenin and myostatin). In the hibernation group compared with the summer active group, the CSA of the diaphragm muscle increased significantly by 26.1%, whereas the CSA of the gastrocnemius muscle decreased significantly by 20.4%. Our study also indicated that increased protein synthesis, decreased protein degradation and increased muscle regenerative potential contributed to diaphragm muscle hypertrophy, whereas decreased protein synthesis, increased protein degradation and decreased muscle regenerative potential contributed to gastrocnemius muscle atrophy. In conclusion, the differences in muscle regeneration and regulatory pattern of protein metabolism might contribute to the different adaptive changes observed in the diaphragm and gastrocnemius muscles of ground squirrels.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

Renal Tubular Cells from Hibernating Squirrels are Protected against Cisplatin Induced Apoptosis

Hibernating 13-lined ground squirrels are characterized by tolerance of severe hypothermia and hypoperfusion during torpor, followed by periodic warm reperfusion during IBA, conditions which are lethal to nonhibernating mammals. The aim of the present study was to determine whether protection from apoptosis was specific to torpor arousal cycles during hibernation or will also apply to cisplatin treatment on squirrel renal tubular cells (RTECs) that were procured during hibernation. Squirrel and mouse RTECs were treated with cisplatin, a potent inducer of RTEC apoptosis. Squirrel RTECs subjected to cisplatin had significantly less apoptosis, no cleaved caspase-3, and increased XIAP, pAkt, and pBAD versus mouse RTECs. To determine whether XIAP and Akt1 are necessary for RTEC protection against cisplatin induced apoptotic cell death, gene expression of Akt1 or XIAP was silenced in squirrel RTECs. Squirrel RTECs deficient in Akt1 and XIAP had increased apoptosis and cleaved caspase-3 when treated with cisplatin. Our results thus demonstrates that 13-lined ground squirrel RTECs possess intrinsic intracellular mechanisms by which they protect themselves from apoptotic cell death. Cisplatin induced acute kidney injury (AKI) may be avoided in cancer patients by employing mechanisms used by squirrel RTECs to protect against cisplatin induced tubular cell apoptosis.

Autophagy and Akt-mTOR signaling display periodic oscillations during torpor-arousal cycles in oxidative skeletal muscle of Daurian ground squirrels (Spermophilus dauricus)

Whether hibernation accelerates or suppresses autophagy is still unknown. In the current study, we examined changes in autophagy in oxidative soleus (SOL) muscle in summer active (SA), pre-hibernation (PRE), torpor (TOR), interbout arousal (IBA), and post-hibernation groups of Daurian ground squirrels (Spermophilus dauricus). Here, the SOL muscle showed no significant atrophy during hibernation in regard to muscle wet weight, fiber cross-sectional area, or MuRF1 protein level. Autophagy-related proteins beclin1 and Atg7 increased significantly, whereas LC3-II decreased significantly in the PRE group compared with the SA group. However, neither the expression nor activity of cathepsin L showed any differences between the SA and PRE groups. In addition, beclin1, LC3-II, and the LC3-II/LC3-I ratio increased, p62 decreased, LC3 puncta increased, p62 puncta decreased, and cathepsin L activity increased in the TOR group compared with the PRE group. In contrast, beclin1, LC3-II, and the LC3-II/LC3-I ratio decreased, p62 increased, LC3 puncta decreased, p62 puncta increased, and cathepsin L activity declined in the IBA group compared with the TOR group. Moreover, the phosphorylation of Akt (Ser473) and mTOR (Ser2448) changed significantly during hibernation and showed an inverse relationship with autophagy changes. In conclusion, autophagy proteins displayed periodic oscillation in the torpor-arousal cycle, which may be advantageous in maintaining SOL muscle mass during the entire hibernation period. Furthermore, the Akt-mTOR signaling was decreased in TOR and increased in IBA group in the SOL muscle of Daurian ground squirrels during hibernation.

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

During winter hibernation in mammals, body temperature falls to near-ambient levels, metabolism shifts to favor lipid oxidation, and metabolic rate is strongly suppressed by inhibiting many ATP-expensive processes (e.g., transcription, translation) for animals in order to survive for many months on limited reserves of body fuels. Regulation of such profound changes (i.e., metabolic rate depression) requires rapid and reversible controls provided by protein posttranslational modifications. Protein lysine methylation provides one mechanism by which the functionality, activity, and stability of cellular proteins and enzymes can be modified for the needs of the hibernator. The present study reports the responses of seven lysine methyltransferases (SMYD2, SUV39H1, SET8, SET7/9, G9a, ASH2L, and RBBP5) in skeletal muscle and liver over seven stages of the torpor/arousal cycle in 13-lined ground squirrels (Ictidomys tridecemlineatus). A tissue-specific and stage-specific analysis revealed significant changes in the protein levels of lysine methyltransferases, methylation patterns on histone H3, histone methyltransferase activity, and methylation of the p53 transcription factor. Enzymes typically increased in protein amount in either torpor, arousal, or the transitory periods. Methylation of histone H3 and p53 typically followed the patterns of the methyltransferase enzymes. Overall, these data show that protein lysine methylation is an important regulator of the mammalian hibernation phenotype.

Molecular control of protein synthesis, glucose metabolism, and apoptosis in the brain of hibernating thirteen-lined ground squirrels

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) are excellent models for studying acute brain ischemia because they show high resistance to reductions in blood flow and oxygen delivery without evidence of neurological damage. In this study, we analyzed the insulin signaling pathway and regulation of mitochondrial substrate oxidation in three regions of ground squirrel brain (forebrain, cerebellum, and brainstem), comparing summer, late torpor, and interbout arousal conditions. We found select decreases in phospho-Akt in the cerebellum during torpor compared with summer animals, as well as select increases in the forebrain during interbout arousal, suggesting that Akt may influence either metabolism or cytoprotective pathways. The phosphoprotein abundance of glycogen synthase kinase 3 beta (GSK3β) showed the most consistent trend across all three brain regions, with peak increases observed during deep torpor, suggesting a crucial role for this protein during hibernation. Furthermore, all three regions of the brain showed increased phospho-protein abundance of pyruvate dehydrogenase at serine 232 during both deep torpor and interbout arousal, and serine 300 during interbout arousal only, whereas other phosphorylation sites showed a region-specific expression pattern. Information collected from these studies sheds light on the molecular controls governing insulin signaling and fuel utilization in the brain of hibernating ground squirrels.

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

Hibernating mammals use strong metabolic rate depression and a reduction in body temperature to near-ambient to survive the cold winter months. During torpor, protein synthesis is suppressed but can resume during interbout arousals. The current study aimed to identify molecular targets responsible for the global suppression of protein synthesis during torpor as well as possible mechanisms that could allow for selective protein translation to continue over this time. Relative changes in protein expression and/or phosphorylation levels of key translation factors (ribosomal protein S6, eIF4E, eIF2α, eEF2) and their upstream regulators (mTOR, TSC2, p70 S6K, 4EBP) were analyzed in liver and kidney of 13-lined ground squirrels (Ictidomys tridecemlineatus) sampled from six points over the torpor-arousal cycle. The results indicate that both organs reduce protein synthesis during torpor by decreasing mTOR and TSC2 phosphorylation between 30 and 70% of control levels. Translation resumes during interbout arousal when p-p70 S6K, p-rpS6, and p-4EBP levels returned to control values or above. Only liver translation factors were activated or disinhibited during periods of torpor itself, with >3-fold increases in total eIF2α and eEF2 protein levels, and a decrease in p-eEF2 (T56) to as low as 16% of the euthermic control value. These data shed light on a possible molecular mechanism involving eIF2α that could enable the translation of key transcripts during times of cell stress.

Akt Protein Kinase, miR-200/miR-182 Expression and Epithelial-Mesenchymal Transition Proteins in Hibernating Ground Squirrels

Hibernating 13-lined ground squirrels (Ictidomys tridecemlineatus; TLGS) rank among the most brain hypoperfusion-tolerant mammals known. Herein we provide some evidence of cycling between an epithelial phenotype and a hybrid epithelial/mesenchymal (E/M) phenotype (partial EMT) within the brains of TLGS during each bout of hibernation torpor. During hibernation torpor, expression of the epithelial marker E-cadherin (E-CDH) was reduced, while expression of the well-known mesenchymal markers vimentin and Sox2 were increased. P-cadherin (P-CDH), which has recently been proposed as a marker of intermediate/partial EMT, also increased during torpor, suggesting that a partial EMT may be taking place during hibernation torpor. Members of the miR-200 family and miR-182 cluster and Akt isoforms (Akt1, Akt2), well-known EMT regulators, were also differentially regulated in the TLGS brain during hibernation bouts. Using SHSY5Y cells, we also demonstrate that the Akt1/Akt2 ratio determined the expression levels of miR-200/miR-182 miRNA family members, and that these miRNAs controlled the expression of EMT-related proteins. Accordingly, we propose that such cell state transitions (EMT/MET) may be one of the mechanisms underlying the extraordinary ischemic tolerance of the TLGS brain during hibernation bouts; hibernator brain cells appear to enter reversible states that confer the stress survival characteristics of cancer cells without the risk of neoplastic transformation.

Strategies of biochemical adaptation for hibernation in a South American marsupial, Dromiciops gliroides: 2. Control of the Akt pathway and protein translation machinery

When faced with harsh environmental conditions, the South American marsupial, monito del monte (Dromiciops gliroides), reduces its body temperature and uses either daily torpor or multiday hibernation to survive. This study used ELISA and multiplex assays to characterize the responses to hibernation by three regulatory components of protein translation machinery [p-eIF2α(S51), p-eIF4E(S209), p-4EBP(Thr37/46)] and eight targets involved in upstream signaling control of translation [p-IGF-1R(Tyr1135/1136), PTEN(S380), p-Akt(S473), p-GSK-3α(S21), p-GSK-3β(S9), p-TSC2(S939), p-mTOR(S2448), and p70S6K(T412)]. Liver, brain and kidney were analyzed comparing control and hibernation (4days continuous torpor) conditions. In the liver, increased phosphorylation of IGF-1R, Akt, GSK-3β, TSC2, mTOR, eIF2α, and 4EBP (1.60-1.98 fold compared to control) occurred during torpor suggesting that the regulatory phosphorylation cascade and protein synthesis remained active during torpor. However, responses by brain and kidney differed; torpor resulted in increased phosphorylation of GSK-3β (2.15-4.17 fold) and TSC2 (2.03-3.65 fold), but phosphorylated Akt decreased (to 34-62% of control levels). Torpor also led to an increase in phosphorylated eIF2α (1.4 fold) content in the brain. These patterns of differential protein phosphorylation in brain and kidney were indicative of suppression of protein translation but also could suggest an increase in antioxidant and anti-apoptotic signaling during torpor. Previous studies of liver metabolism in hibernating eutherian mammals have shown that Akt kinase and its downstream signaling components play roles in facilitating hypometabolism by suppressing energy expensive anabolic processes during torpor. However, the results in this study reveal differences between eutherian and marsupial hibernators, suggesting alternative actions of liver Akt during torpor.

Regulation of the insulin-Akt signaling pathway and glycolysis during dehydration stress in the African clawed frog Xenopus laevis

Estivation is an adaptive stress response utilized by some amphibians during periods of drought in the summer season. In this study, we examine the regulation of the insulin signaling cascade and glycolysis pathway in the African clawed frog Xenopus laevis during the dehydration stress induced state of estivation. We show that in the brain and heart of X. laevis, dehydration reduces the phosphorylation of the insulin growth factor-1 receptor (IGF-1R), and this is followed by similar reductions in the phosphorylation of the Akt and mechanistic target of rapamycin (mTOR) kinase. Interestingly, phosphorylation levels of IGF-1R and mTOR were not affected in the kidney, and phosphorylation levels of P70S6K and the ribosomal S6 protein were elevated during dehydration stress. Animals under estivation are also susceptible to periods of hypoxia, suggesting that glycolysis may also be affected. We observed that protein levels of many glycolytic enzymes remained unchanged during dehydration; however, the hypoxia response factor-1 alpha (HIF-1α) protein was elevated by greater than twofold in the heart during dehydration. Overall, we provide evidence that shows that the insulin signaling pathway in X. laevis is regulated in a tissue-specific manner during dehydration stress and suggests an important role for this signaling cascade in mediating the estivation response.

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

Mammalian hibernation includes re-programing of metabolic capacities, partially, encouraged by microRNAs (miRNAs). Albeit much is known about the functions of miRNAs, we need learning on low temperature miRNAs target determination. As hibernators can withstand low body temperatures (TB) for a long time without anguish tissue damage, understanding the means and mechanisms that empower them to do as such are of restorative intrigue. Nonetheless, these mechanisms by which miRNAs and the hibernators react to stressful conditions are not much clear. It is evident from recent data that the gene expression and the translation of mRNA to protein are controlled by miRNAs. The miRNAs also influence regulation of major cellular processes. As the significance of miRNAs in stress conditions adaptation are getting clearer, this audit article abridges the key alterations in miRNA expression and the mechanism that facilitates stress survival.

Protection From Apoptotic Cell Death During Cold Storage Followed by Rewarming in 13-Lined Ground Squirrel Tubular Cells: The Role of Prosurvival Factors X-Linked Inhibitor of Apoptosis and PhosphoAkt

BACKGROUND

Hibernators, such as the 13-lined ground squirrel, endure severe hypothermia during torpor followed by periodic rewarming (REW) during interbout arousal (IBA), proapoptotic conditions that are lethal to nonhibernating mammals. We have previously shown that 13-lined ground squirrel tubular cells are protected from apoptotic cell death during IBA. To understand the mechanism of protection, we developed an in vitro model of prolonged cold storage (CS) followed by REW, which is akin to the in vivo changes of hypothermia followed by REW observed during IBA. We hypothesized that renal tubular epithelial cells (RTECs) isolated from hibernating ground squirrels would be protected against apoptosis during CS/REW versus nonhibernating mouse RTECs.

METHODS

Isolated hibernating ground squirrel and mouse RTECs were subjected to CS at 4°C for 24 hours followed by REW to 37°C for 24 hours (CS/REW).

RESULTS

Ground squirrel RTECs had significantly less apoptosis compared to mouse RTECs when subjected to CS/REW. Next, we hypothesized that the mechanism of protection was related to the antiapoptotic proteins X-linked inhibitor of apoptosis (XIAP), phospho-Akt (pAkt), and phospho-BAD. There was a significantly increased pAkt and pBAD expression in ground squirrel versus mouse RTECs subjected to CS/REW. The XIAP expression was maintained in ground squirrel RTECs but was significantly decreased in mouse RTECs after CS/REW. Ground squirrel RTECs in which gene expression of Akt1 and XIAP was silenced lost their protection and demonstrated increased apoptosis and cleaved caspase-3 expression after CS/REW.

CONCLUSIONS

Our findings suggest that ground squirrel RTECs are protected against apoptosis during prolonged CS/REW by the "prosurvival" factors XIAP and pAkt.

Hypophosphorylation of ribosomal protein S6 is a molecular mechanism underlying ischemic tolerance induced by either hibernation or preconditioning

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) have an extraordinary capacity to withstand prolonged and profound reductions in blood flow and oxygen delivery to the brain without incurring any cellular damage. As such, the hibernation torpor of I. tridecemlineatus provides a valuable model of tolerance to ischemic stress. Herein, we report that during hibernation torpor, a marked reduction in the phosphorylation of the ribosomal protein S6 (rpS6) occurs within the brains of I. tridecemlineatus. Of note, rpS6 phosphorylation was shown to increase in the brains of rats that underwent an occlusion of the middle cerebral artery. However, such an increase was attenuated after the implementation of an ischemic preconditioning paradigm. In addition, cultured cortical neurons treated with the rpS6 kinase (S6K) inhibitors, D-glucosamine or PF4708671, displayed a decrease in rpS6 phosphorylation and a subsequent increase in tolerance to oxygen/glucose deprivation, an in vitro model of ischemic stroke. Collectively, such evidence suggests that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning. Further identification and characterization of the mechanisms used by hibernating species to increase ischemic tolerance may eventually clarify how the loss of homeostatic control that occurs during and after cerebral ischemia in the clinic can ultimately be minimized and/or prevented. Mammalian hibernation provides a valuable model of tolerance to ischemic stress. Herein, we demonstrate that marked reductions in the phosphorylation of ribosomal protein S6 (rpS6), extracellular signal-regulated kinase family of mitogen-activated protein (MAP) kinase p44/42 (p44/42MAPK) and ribosomal protein S6 kinase (S6K) occur within the brains of both hibernating squirrels and rats, which have undergone an ischemic preconditioning paradigm. We therefore propose that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning, via a suppression of protein synthesis and/or energy consumption.

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.

Bone marrow Schwann cells induce hematopoietic stem cell hibernation

Hematopoietic stem cells (HSCs) are clonogenic cells capable of both self-renewal and multilineage differentiation. In adult mouse bone marrow (BM), most HSCs remain in the non-dividing G0-phase of cell cycle, in close contact with supporting cells known as the HSC "niche". In the present study, we focused on signaling mechanisms that regulate stem cell dormancy in the BM niche. We show that TGF-β type II receptor deficiency causes reduced phosphorylation of Smad2/3 and impairs long-term repopulating activity in HSCs, suggesting a significant role for TGF-β/Smad signaling in hematopoiesis. Furthermore, we aimed at defining the candidate BM niche responsible for homeostasis of hematopoiesis, and revealed that non-myelinating Schwann cells sustain HSC hibernation by converting TGF-β from its latent to its active form.

Suppression of MAPKAPK2 during mammalian hibernation

Metabolic signaling coordinates the transition by hibernating mammals from euthermia into profound torpor. Organ-specific responses by activated p38 mitogen activated protein kinase (MAPK) are known to contribute to this transition. Therefore, we hypothesized that the MAPK-activated protein kinase-2 (MAPKAPK2), a downstream target of p38 MAPK, would also be active in establishing the torpid state. Kinetic parameters of MAPKAPK2 from skeletal muscle of Richardson's ground squirrels, Spermophilus richardsonii, were analyzed using a fluorescence assay. MAPKAPK2 activity was 27.4±1.27 pmol/min/mg in muscle from euthermic squirrels and decreased by ∼63% during cold torpor, while total protein levels were unchanged (as assessed by immunoblotting). In vitro treatment of MAPKAPK2 via stimulation of endogenous phosphatases and addition of commercial alkaline phosphatase decreased enzyme activity to only ∼3-5% of its original value in muscle extracts from both euthermic and hibernating squirrels suggesting that posttranslational modification suppresses MAPKAPK2 during the transition from euthermic to torpid states. Enzyme S₀.₅ and n(H) values for ATP and peptide substrates changed significantly between euthermia and torpor, and also between assays at 22 versus 10 °C but, kinetic parameters were actually closely conserved when values for the euthermic enzyme at 22 °C were directly compared with the hibernator enzyme at 10 °C. Arrhenius plots showed significantly different activation energies of 40.8±0.7 and 54.3±2.7 kJ/mol for the muscle enzyme from euthermic versus torpid animals, respectively but MAPKAPK2 from the two physiological states showed no difference in sensitivity to urea denaturation. Overall, the results show that total activity of MAPKAPK2 is in fact reduced, despite previous findings of p38 MAPK activation, and kinetic parameters are altered when ground squirrels enter torpor but protein stability is not apparently changed. The data suggest that MAPKAPK2 suppression may have a significant role in the differential regulation of muscle target proteins when ground squirrels enter torpor.

Regulation of the mTOR signaling network in hibernating thirteen-lined ground squirrels

For many small mammals, survival over the winter months is a serious challenge because of low environmental temperatures and limited food availability. The solution for many species, such as thirteen-lined ground squirrels (Ictidomys tridecemlineatus), is hibernation, an altered physiological state characterized by seasonal heterothermy and entry into long periods of torpor that are interspersed with short arousals back to euthermia. During torpor, metabolic rate is strongly reduced to achieve major energy savings, and a coordinated depression of non-essential ATP-expensive functions such as protein synthesis takes place. This study examines the mammalian target of rapamycin (mTOR) signaling pathway, a crucial component of the insulin receptor network, over six stages of the torpor–arousal cycle of hibernation. Immunoblots showed that the phosphorylation state of mTORSer2448 was strongly reduced in skeletal muscle (by 55%) during late torpor but increased by 200% during early arousal compared with euthermia. However, the phosphorylation state of this residue remained relatively constant in cardiac muscle during torpor but was enhanced during entrance into torpor and early arousal from torpor stages (by 2.9- and 3.2-fold, respectively). Phosphorylation states of upstream regulators of mTOR, p-AktThr473 and p-TSC2Thr1462, were also suppressed in skeletal muscle by 55 and 51%, respectively, during late torpor, as were selected downstream substrates – p-4E-BP1Thr46 and p-S6Ser235 contents dropped by 74 and 41%, respectively. Overall, the results indicate suppressed mTOR signaling in skeletal muscle, but not cardiac muscle, during torpor. By contrast, activation of mTOR and other components of the mTORC1 complex (p-PRAS40Thr246 and GβL) occurred during early arousal in both skeletal and cardiac muscle.

Neuroprotection: lessons from hibernators

Mammals that hibernate experience extreme metabolic states and body temperatures as they transition between euthermia, a state resembling typical warm blooded mammals, and prolonged torpor, a state of suspended animation where the brain receives as low as 10% of normal cerebral blood flow. Transitions into and out of torpor are more physiologically challenging than the extreme metabolic suppression and cold body temperatures of torpor per se. Mammals that hibernate show unprecedented capacities to tolerate cerebral ischemia, a decrease in blood flow to the brain caused by stroke, cardiac arrest or brain trauma. While cerebral ischemia often leads to death or disability in humans and most other mammals, hibernating mammals suffer no ill effects when blood flow to the brain is dramatically decreased during torpor or experimentally induced during euthermia. These animals, as adults, also display rapid and pronounced synaptic flexibility where synapses retract during torpor and rapidly re-emerge upon arousal. A variety of coordinated adaptations contribute to tolerance of cerebral ischemia in these animals. In this review we discuss adaptations in heterothermic mammals that may suggest novel therapeutic targets and strategies to protect the human brain against cerebral ischemic damage and neurodegenerative disease.

Metabolic and cardiac signaling effects of inhaled hydrogen sulfide and low oxygen in male rats

Low concentrations of inhaled hydrogen sulfide (H(2)S) induce hypometabolism in mice. Biological effects of H(2)S in in vitro systems are augmented by lowering O(2) tension. Based on this, we hypothesized that reduced O(2) tension would increase H(2)S-mediated hypometabolism in vivo. To test this, male Sprague-Dawley rats were exposed to 80 ppm H(2)S at 21% O(2) or 10.5% O(2) for 6 h followed by 1 h recovery at room air. Rats exposed to H(2)S in 10.5% O(2) had significantly decreased body temperature and respiration compared with preexposure levels. Heart rate was decreased by H(2)S administered under both O(2) levels and did not return to preexposure levels after 1 h recovery. Inhaled H(2)S caused epithelial exfoliation in the lungs and increased plasma creatine kinase-MB activity. The effect of inhaled H(2)S on prosurvival signaling was also measured in heart and liver. H(2)S in 21% O(2) increased Akt-P(Ser473) and GSK-3β-P(Ser9) in the heart whereas phosphorylation was decreased by H(2)S in 10.5% O(2), indicating O(2) dependence in regulating cardiac signaling pathways. Inhaled H(2)S and low O(2) had no effect on liver Akt. In summary, we found that lower O(2) was needed for H(2)S-dependent hypometabolism in rats compared with previous findings in mice. This highlights the possibility of species differences in physiological responses to H(2)S. Inhaled H(2)S exposure also caused tissue injury to the lung and heart, which raises concerns about the therapeutic safety of inhaled H(2)S. In conclusion, these findings demonstrate the importance of O(2) in influencing physiological and signaling effects of H(2)S in mammalian systems.

Sustained ligand-activated preconditioning via δ-opioid receptors

We have previously described novel cardioprotection in response to sustained morphine exposure, efficacious in young to aged myocardium and mechanistically distinct from conventional opioid or preconditioning (PC) responses. We further investigate opioid-dependent sustained ligand-activated preconditioning (SLP), assessing duration of protection, opioid receptor involvement, additivity with conventional responses, and signaling underlying preischemic induction of the phenotype. Male C57BL/6 mice were treated with morphine (75-mg subcutaneous pellet) for 5 days followed by morphine-free periods (0, 3, 5, or 7 days) before ex vivo assessment of myocardial tolerance to 25-min ischemia/45-min reperfusion. SLP substantially reduced infarction (by ∼50%) and postischemic contractile dysfunction (eliminating contracture, doubling force development). Cardioprotection persisted for 5 to 7 days after treatment. SLP was induced specifically by δ-receptor and not κ- or μ-opioid receptor agonism, was eliminated by δ-receptor and nonselective antagonism, and was additive with adenosinergic but not acute morphine- or PC-triggered protection. Cotreatment during preischemic morphine exposure with the phosphoinositide-3 kinase (PI3K) inhibitor wortmannin, but not the protein kinase A (PKA) inhibitor myristoylated PKI-(14-22)-amide, prevented induction of SLP. This was consistent with shifts in total and phospho-Akt during the induction period. In summary, data reveal that SLP triggers sustained protection from ischemia for up to 7 days after stimulus, is δ-opioid receptor mediated, is induced in a PI3K-dependent/PKA-independent manner, and augments adenosinergic protection. Mechanisms underlying SLP may be useful targets for manipulation of ischemic tolerance in young or aged myocardium.

Endocannabinoid signalling: has it got rhythm?

Endogenous cannabinoid signalling is widespread throughout the body, and considerable evidence supports its modulatory role in many fundamental physiological processes. The daily and seasonal cycles of the relationship of the earth and sun profoundly affect the terrestrial environment. Terrestrial species have adapted to these cycles in many ways, most well studied are circadian rhythms and hibernation. The purpose of this review was to examine literature support for three hypotheses: (i) endocannabinoid signalling exhibits brain region-specific circadian rhythms; (ii) endocannabinoid signalling modulates the rhythm of circadian processes in mammals; and (iii) changes in endocannabinoid signalling contribute to the state of hibernation. The results of two novel studies are presented. First, we report the results of a study of healthy humans demonstrating that plasma concentrations of the endocannabinoid, N-arachidonylethanolamine (anandamide), exhibit a circadian rhythm. Concentrations of anandamide are threefold higher at wakening than immediately before sleep, a relationship that is dysregulated by sleep deprivation. Second, we investigated differences in endocannabinoids and congeners in plasma from Marmota monax obtained in the summer and during the torpor state of hibernation. We report that 2-arachidonoylglycerol is below detection in M. monax plasma and that concentrations of anandamide are not different. However, plasma concentrations of the anorexigenic lipid oleoylethanolamide were significantly lower in hibernation, while the concentrations of palmitoylethanolamide and 2-oleoylglycerol were significantly greater in hibernation. We conclude that available data support a bidirectional relationship between endocannabinoid signalling and circadian processes, and investigation of the contribution of endocannabinoid signalling to the dramatic physiological changes that occur during hibernation is warranted.

Regulation of Akt during torpor in the hibernating ground squirrel, Ictidomys tridecemlineatus

The 13-lined ground squirrel (Ictidomys tridecemlineatus) is capable of entering into extended periods of torpor during winter hibernation. The state of torpor represents a hypometabolic shift wherein the rate of oxygen consuming processes are strongly repressed in an effort to maintain cellular homeostasis as the availability of food energy becomes limited. We are interested in studying hibernation/torpor because of the robust state of tolerance to constrained oxygen delivery, oligemia, and hypothermia achieved by the tissues of hibernating mammals. The role of the serine/threonine kinase Akt (also known as PKB) has been examined in torpor in previous studies. However, this is the first study that examines the level of Akt phosphorylation in the liver during the two transition phases of the hibernation cycle: entrance into torpor, and the subsequent arousal from torpor. Our results indicate that Akt is activated in the squirrel liver by phosphorylation of two key residues (Thr(308) and Ser(473)) during entrance into torpor and arousal from torpor. Moreover, we observed increased phosphorylation of key substrates of Akt during the two transition stages of torpor. Finally, this study reports the novel finding that PRAS40, a component of the TORC1 multi-protein complex and a potentially important modulator of metabolism, is regulated during torpor.

P19 H-ras induces G1/S phase delay maintaining cells in a reversible quiescence state

Background

Three functional c-ras genes, known as c-H-ras, c-K-ras, and c-N-ras, have been largely studied in mammalian cells with important insights into normal and tumorigenic cellular signal transduction events. Two K-Ras mRNAs are obtained from the same pre-mRNA by alternative splicing. H-Ras pre-mRNA can also be alternatively spliced in the IDX and 4A terminal exons, yielding the p19 and p21 proteins, respectively. However, despite the Ras gene family's established role in tumorigenic cellular signal transduction events, little is known about p19 function. Previous results showed that p19 did not interact with two known p21 effectors, Raf1 and Rin1, but was shown to interact with RACK1, a scaffolding protein that promotes multi-protein complexes in different signaling pathways (Cancer Res 2003, 63 p5178). This observation suggests that p19 and p21 play differential and complementary roles in the cell.

Principal Findings

We found that p19 regulates telomerase activity through its interaction with p73α/β proteins. We also found that p19 overexpression induces G1/S phase delay; an observation that correlates with hypophosphorylation of both Akt and p70SK6. Similarly, we also observed that FOXO1 is upregulated when p19 is overexpressed. The three observations of (1) hypophosphorylation of Akt, (2) G1/S phase delay and (3) upregulation of FOXO1 lead us to conclude that p19 induces G1/S phase delay, thereby maintaining cells in a reversible quiescence state and preventing entry into apoptosis. We then assessed the effect of p19 RNAi on HeLa cell growth and found that p19 RNAi increases cell growth, thereby having the opposite effect of arrest of the G1/S phase or producing a cellular quiescence state.

Significance

Interestingly, p19 induces FOXO1 that in combination with the G1/S phase delay and hypophosphorylation of both Akt and p70SK6 leads to maintenance of a reversible cellular quiescence state, thereby preventing entry into apoptosis.

Stimulation of movement in a quiescent, hibernation-like form of Caenorhabditis elegans by dopamine signaling

One of the characteristics of animals in hibernation is reduced behavioral activity. The Caenorhabditis elegans dauer state is a hibernation-like state of diapause that displays a dramatic reduction in spontaneous locomotion. A similar dauer-like quiescent state is produced in adults by relatively strong mutations in the insulin/IGF-1 receptor homolog daf-2. In this study, we show that mutations affecting the neurotransmitter dopamine, which regulates voluntary movement in many organisms, can stimulate movement in dauers and dauer-like quiescent adults. Surprisingly, the movement of quiescent animals is stimulated by conditions that reduce dopamine signaling and also by conditions predicted to increase dopamine signaling. Reducing dopamine signaling is likely to stimulate movement by activating a foraging response also seen in nondauers after withdrawal of food. In contrast, the stimulation of movement by increased dopamine is much more pronounced in quiescent daf-2(-) dauer and dauer-like adult animals than in nondauaer animals. This altered response to dopamine is primarily attributable to activity of the FOXO (forkhead box O) transcription factor DAF-16 in neurons. We suggest that dauers and dauer-like quiescent adults may have underlying changes in the dopamine system that enable them to respond differently to environmental stimulation.

Differential regulation of peroxisome proliferator-activated receptor (PPAR)-alpha1 and truncated PPARalpha2 as an adaptive response to fasting in the control of hepatic peroxisomal fatty acid beta-oxidation in the hibernating mammal

Seasonal obesity and fasting-associated hibernation are the two major metabolic events governing hepatic lipid metabolism in hibernating mammals. In this process, however, the role of the nuclear receptor known as peroxisome proliferator-activated receptor (PPAR)-alpha has not been elucidated yet. Here we show, as in human, that jerboa (Jaculus orientalis) liver expresses both active wild-type PPARalpha (PPARalpha1wt) and truncated PPARalpha forms and that the PPARalpha1wt to truncated PPARalpha2 ratio, which indicates the availability of active PPARalpha1wt, is differentially regulated during fasting-associated hibernation. Functional activation of hepatic jerboa PPARalpha, during prehibernating and hibernating states, was demonstrated by the induction of its target genes, which encode peroxisomal proteins such as acyl-CoA oxidase 1, peroxisomal membrane protein 70, and catalase, accompanied by a concomitant induction of PPARalpha thermogenic coactivator PPARgamma coactivator-1alpha. Interestingly, sustained activation of PPARalpha by its hypolipidemic ligand, ciprofibrate, abrogates the adaptive fasting response of PPARalpha during prehibernation and overinduces its target genes, disrupting the prehibernation fattening process. In striking contrast, during fasting-associated hibernation, jerboas exhibit preferential up-regulation of hepatic peroxisomal fatty acid oxidation instead of the mitochondrial pathway, which is down-regulated. Taken together, our results strongly suggest that PPARalpha is subject to a hibernation-dependent splicing regulation in response to feeding-fasting conditions, which defines the activity of PPARalpha and the activation of its target genes during hibernation bouts of jerboas.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

Comparative molecular physiological genomics. Heterologous probing of cDNA arrays

The use of DNA microarrays has gained wider acceptance as a standard tool for molecular biology studies over the past decade. In particular, biomedical studies embraced this technology as soon as arrays were produced for the common laboratory species. Slower to develop, however, has been the use of microarray screening with non-standard animal models, even though these species present fascinating physiological phenomena for study. The very high cost and huge amount of work involved in developing and producing a DNA array or microarray for a new species is prohibitive for most researchers working in comparative biology. The alternative is to explore the use of heterologous array hybridization, screening for stress-induced gene expression in one species using an array developed for another species. This chapter provides a comprehensive review of the current literature on heterologous DNA array hybridization and explores the factors that must be taken into account when performing heterologous microarray analysis on nonstandard species. Changes in methodology (e.g. hybridization conditions, stringency of washing) to optimize the percent cross reaction, the potential for false positives and false negatives to occur, and techniques for downstream analysis and confirmation of array data are all discussed. Examples of cross-hybridization using human microarrays are discussed using phylogenetically diverse species ranging from ground squirrels to frogs to snails. As with any new technology, the willingness to grasp cross-species analysis has been slow but the future looks bright for heterologous DNA hybridization and microarray analysis now that the initial hurdles have been overcome.

Akt and its downstream targets play key roles in mediating dormancy in land snails

Estivation, a state of aerobic dormancy, facilitates survival during adverse environmental conditions and is characterized at the molecular level by regulatory protein phosphorylation. The Akt (protein kinase B) signaling pathway regulates diverse responses in cells and the present study analyzes its role in the estivating desert snail Otala lactea. Kinetic analysis (maximal velocity, substrate affinities) determined that Akt was activated in tissues of estivating snails and Western blotting and in vitro incubations promoting changes to Akt phosphorylation state both confirmed that higher amounts of active (phosphorylated Ser473) Akt were present during estivation. Akt protein stability was also enhanced during estivation as assessed from urea denaturation studies. Multiple downstream targets of Akt were differentially regulated during estivation. Estivating animals showed elevated levels of phosphorylated FOXO3a (Ser253) and BAD (Ser136), no change in mTOR (Ser2481 and Ser2448), and reduced amounts of phosphorylated glycogen synthase kinase-3 (GSK-3) beta subunit (Ser9). Kinetic analysis of GSK-3 showed 1.5-1.7 fold higher activities in estivating snails coupled with increased GSK-3 substrate affinities in hepatopancreas. The data suggest an active role for Akt signaling during estivation emphasizing anti-apoptotic actions but uncoupling growth/proliferation actions to help achieve life extension on a limited energy budget.

Insights into cytoprotection from ground squirrel hibernation, a natural model of tolerance to profound brain oligaemia

Progression of acute ischaemic brain damage is complex and multifactorial. Also, evidence suggests that participating molecules and signal transduction pathways can function differently in different cellular contexts. Hibernation torpor, a model of natural tolerance to profoundly reduced blood flow and oxygen delivery to brain, along with models of induced ischaemic tolerance can guide efforts to identify cytoprotective mechanisms that are multifactorial and that target multiple mechanisms in multiple cellular contexts. Post-translational modification of proteins by conjugation with the SUMO (small ubiquitin-related modifier) is massively increased in hibernation and may be such a mechanism.

Advances in molecular biology of hibernation in mammals

Mammalian hibernation is characterized by profound reductions in metabolism, oxygen consumption and heart rate. As a result, the animal enters a state of suspended animation where core body temperatures can plummet as low as -2.9 degrees C. Not only can hibernating mammals survive these physiological extremes, but they also return to a normothermic state of activity without reperfusion injury or other ill effects. This review examines recent findings on the genes, proteins and small molecules that control the induction and maintenance of hibernation in mammals. The molecular events involved with remodeling metabolism, inducing hypothermia and maintaining organ function are discussed and considered with respect to analogous processes in non-hibernating mammals such as mice and humans. The advent of sequenced genomes from three distantly related hibernators, a bat, hedgehog and ground squirrel, provides additional opportunities for molecular biologists to explore the mechanistic aspects of this biological adaptation in greater detail.

Of bears, frogs, meat, mice and men: complexity of factors affecting skeletal muscle mass and fat

Extreme loss of skeletal muscle mass (atrophy) occurs in human muscles that are not used. In striking contrast, skeletal muscles do not rapidly waste away in hibernating mammals such as bears, or aestivating frogs, subjected to many months of inactivity and starvation. What factors regulate skeletal muscle mass and what mechanisms protect against muscle atrophy in some species? Severe atrophy also occurs with ageing and there is much clinical interest in reducing such loss of muscle mass and strength (sarcopenia). In the meat industry, a key aim is optimizing the control of skeletal muscle growth and meat quality. The impaired response of muscle to insulin resulting in diabetes, that is a consequence of the metabolic impact of increasing obesity and fat deposition in humans, is also of increasing clinical concern. Intensive research in these fields, combined with mouse models, is reviewed with respect to the molecular control of muscle growth (myogenesis) and atrophy/hypertrophy and fat deposition (adipogenesis) in skeletal muscle, with a focus on IGF-1/insulin signaling.

Potential for discovery of neuroprotective factors in serum and tissue from hibernating species

Hibernation is a unique phenotype displayed by a phylogenetically diverse group of organisms including several species of mammals and one species of primate. Here we review evidence for blood and tissue borne signaling molecules in hibernating animals, achievements in isolating and characterizing these molecules, and potential medicinal applications.

Differential expression of selected mitochondrial genes in hibernating little brown bats,Myotis lucifugus

High rates of non-shivering thermogenesis by brown adipose tissue accompanied by additional shivering thermogenesis in skeletal muscle provide the powerful reheating of body organs that allows hibernating mammals to return from their state of cold torpor back to euthermic function. Previous studies have suggested that changes to brown adipose mitochondria occur during hibernation and are partially responsible for its capacity for non-shivering thermogenesis. The current study shows that selected mitochondrial enzyme activities are elevated and selected genes and proteins are induced during torpor in brown adipose tissue of the little brown bat, Myotis lucifugus. Cytochrome oxidase activity in brown adipose tissue was more than 3-fold higher during torpor than in euthermic animals. Transcript levels of mitochondria-encoded genes, coxII and nad4, were also 3-4-fold higher during torpor, as evidenced by northern blotting. By contrast, transcripts of these genes were unchanged in skeletal muscle during torpor. Protein levels of carnitine palmitoyl transferase-1beta, an enzyme embedded in the outer membrane of the mitochondria that is the rate-limiting step enzyme in beta-oxidation, were also elevated by 2-fold during torpor in brown adipose but were unchanged in skeletal muscle. Cloning and sequencing of a 624 bp segment of cpt-1beta revealed a number of amino acid substitutions in the bat protein as compared to CPT-1beta from other mammals; these may be beneficial for enzyme function at low body temperatures during torpor. This study provides further evidence for a key role of mitochondria in hibernation.

Cytokine signals modulated via lipid rafts mimic niche signals and induce hibernation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) niche in a noncycling state and enter the cell cycle at long intervals. However, little is known about inter- and intracellular signaling mechanisms underlying this unique property of HSCs. Here, we show that lipid raft clustering is a key event in the regulation of HSC dormancy. Freshly isolated HSCs from the BM niche lack lipid raft clustering, exhibit repression of the AKT-FOXO signaling pathway, and express abundant p57(Kip2) cyclin-dependent kinase inhibitor. Lipid raft clustering induced by cytokines is essential for HSC re-entry into the cell cycle. Conversely, inhibition of lipid raft clustering caused sustained nuclear accumulation of FOXO transcription factors and induced HSC hibernation ex vivo. These data establish a critical role for lipid rafts in regulating the cell cycle, the survival, and the entry into apoptosis of HSCs and uncover a striking similarity in HSC hibernation and Caenorhabditis elegans dauer formation.