Mechanisms for increased levels of phosphorylation of elongation factor-2 during hibernation in ground squirrels

Hibernation slows epigenetic ageing in yellow-bellied marmots

Species that hibernate generally live longer than would be expected based solely on their body size. Hibernation is characterized by long periods of metabolic suppression (torpor) interspersed by short periods of increased metabolism (arousal). The torpor–arousal cycles occur multiple times during hibernation, and it has been suggested that processes controlling the transition between torpor and arousal states cause ageing suppression. Metabolic rate is also a known correlate of longevity; we thus proposed the ‘hibernation–ageing hypothesis’ whereby ageing is suspended during hibernation. We tested this hypothesis in a well-studied population of yellow-bellied marmots (Marmota flaviventer), which spend 7–8 months per year hibernating. We used two approaches to estimate epigenetic age: the epigenetic clock and the epigenetic pacemaker. Variation in epigenetic age of 149 samples collected throughout the life of 73 females was modelled using generalized additive mixed models (GAMM), where season (cyclic cubic spline) and chronological age (cubic spline) were fixed effects. As expected, the GAMM using epigenetic ages calculated from the epigenetic pacemaker was better able to detect nonlinear patterns in epigenetic ageing over time. We observed a logarithmic curve of epigenetic age with time, where the epigenetic age increased at a higher rate until females reached sexual maturity (two years old). With respect to circannual patterns, the epigenetic age increased during the active season and essentially stalled during the hibernation period. Taken together, our results are consistent with the hibernation–ageing hypothesis and may explain the enhanced longevity in hibernators.

Species that hibernate generally have longer lifespans than expected based on their body size. The authors show epigenetic ageing patterns from a natural population of hibernating yellow-bellied marmots consistent with the hypothesis that ageing is suspended during hibernation.

Suspended in time: Molecular responses to hibernation also promote longevity

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

AMPK activation, eEF2 inactivation, and reduced protein synthesis in the cerebral cortex of hibernating chipmunks

During hibernation, mammalian cells are exposed to severe environmental stressors such as low temperature, lowered O₂ supply, and glucose deficiency. The cellular metabolic rate is markedly reduced for adapting to these conditions. AMP-activated protein kinase (AMPK) senses the cellular energy status and regulates metabolism. Therefore, we examined AMPK signaling in several brain regions and peripheral tissues in hibernating chipmunk. Eukaryotic elongation factor 2 (eEF2) is a downstream target of AMPK. Phosphorylation of eEF2, indicating its inactivation, is enhanced in the cerebral cortex of hibernating chipmunks. The study indicated that the sequential regulation of AMPK-mammalian target of rapamycin complex 1-eEF2 signaling was altered and protein synthesis ability was reduced in the cerebral cortex of hibernating chipmunks.

Calcium-Binding Proteins in the Nervous System during Hibernation: Neuroprotective Strategies in Hypometabolic Conditions?

Calcium-binding proteins (CBPs) can influence and react to Ca2+ transients and modulate the activity of proteins involved in both maintaining homeostatic conditions and protecting cells in harsh environmental conditions. Hibernation is a strategy that evolved in vertebrate and invertebrate species to survive in cold environments; it relies on molecular, cellular, and behavioral adaptations guided by the neuroendocrine system that together ensure unmatched tolerance to hypothermia, hypometabolism, and hypoxia. Therefore, hibernation is a useful model to study molecular neuroprotective adaptations to extreme conditions, and can reveal useful applications to human pathological conditions. In this review, we describe the known changes in Ca2+-signaling and the detection and activity of CBPs in the nervous system of vertebrate and invertebrate models during hibernation, focusing on cytosolic Ca2+ buffers and calmodulin. Then, we discuss these findings in the context of the neuroprotective and neural plasticity mechanisms in the central nervous system: in particular, those associated with cytoskeletal proteins. Finally, we compare the expression of CBPs in the hibernating nervous system with two different conditions of neurodegeneration, i.e., platinum-induced neurotoxicity and Alzheimer's disease, to highlight the similarities and differences and demonstrate the potential of hibernation to shed light into part of the molecular mechanisms behind neurodegenerative diseases.

Regulation of protein phosphatase 2A during embryonic diapause process in the silkworm, Bombyx mori

Regulation of protein phosphorylation requires coordinated interactions between protein kinases and protein phosphatases. In the present study, we investigated regulation of protein phosphatase 2A (PP2A) during the embryonic diapause process of B. mori. An immunoblotting analysis showed that Bombyx eggs contained a catalytic C subunit, a major regulatory B subunit (B55/PR55 subunit), and a structural A subunit, with the A and B subunits undergoing differential changes between diapause and non-diapause eggs during embryonic process. In non-diapause eggs, eggs whose diapause initiation was prevented by HCl, and eggs in which diapause had been terminated by chilling of diapausing eggs at 5°C for 70days and then were transferred to 25°C, protein levels of the A and B subunits of PP2A gradually increased toward embryonic development. However, protein levels of the A and B subunits in diapause eggs remained at low levels during the first 8days after oviposition. The direct determination of PP2A enzymatic activity showed that the activity remained at low levels in diapause eggs during the first 8days after oviposition. However, in non-diapause eggs, eggs whose diapause initiation was prevented by HCl, and eggs in which diapause had been terminated by chilling, PP2A enzymatic activity sharply increased during the first several days, reached a peak during the middle embryonic development, and then greatly decreased 3 or 4days before hatching. Examination of temporal changes in mRNA expression levels of the catalytic β subunit and regulatory subunit of PP2A showed high levels in eggs whose diapause initiation was prevented by HCl compared to those in diapause eggs. These results demonstrate that the higher PP2A gene expression and PP2A A and B subunit protein levels and increased enzymatic activity are related to embryonic development of B. mori.

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.

Metabolic Flexibility: Hibernation, Torpor, and Estivation

Many environmental conditions can constrain the ability of animals to obtain sufficient food energy, or transform that food energy into useful chemical forms. To survive extended periods under such conditions animals must suppress metabolic rate to conserve energy, water, or oxygen. Amongst small endotherms, this metabolic suppression is accompanied by and, in some cases, facilitated by a decrease in core body temperature-hibernation or daily torpor-though significant metabolic suppression can be achieved even with only modest cooling. Within some ectotherms, winter metabolic suppression exceeds the passive effects of cooling. During dry seasons, estivating ectotherms can reduce metabolism without changes in body temperature, conserving energy reserves, and reducing gas exchange and its inevitable loss of water vapor. This overview explores the similarities and differences of metabolic suppression among these states within adult animals (excluding developmental diapause), and integrates levels of organization from the whole animal to the genome, where possible. Several similarities among these states are highlighted, including patterns and regulation of metabolic balance, fuel use, and mitochondrial metabolism. Differences among models are also apparent, particularly in whether the metabolic suppression is intrinsic to the tissue or depends on the whole-animal response. While in these hypometabolic states, tissues from many animals are tolerant of hypoxia/anoxia, ischemia/reperfusion, and disuse. These natural models may, therefore, serve as valuable and instructive models for biomedical research.

Control of translation in the cold: implications for therapeutic hypothermia

Controlled whole-body cooling has been used since the 1950s to protect the brain from injury where cerebral blood flow is reduced. Therapeutic hypothermia has been used successfully during heart surgery, following cardiac arrest and with varied success in other instances of reduced blood flow to the brain. However, why reduced temperature is beneficial is largely unknown. Here we review the use of therapeutic hypothermia with a view to understanding the underlying biology contributing to the phenomenon. Interestingly, the benefits of cooling have recently been extended to treatment of chronic neurodegenerative diseases in two mouse models. Concurrently studies have demonstrated the importance of the regulation of protein synthesis, translation, to the cooling response, which is also emerging as a targetable process in neurodegeneration. Through these studies the potential importance of the rewarming process following cooling is also beginning to emerge. Altogether, these lines of research present new opportunities to manipulate cooling pathways for therapeutic gain.

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.

Brain hypometabolism triggers PHF-like phosphorylation of tau, a major hallmark of Alzheimer's disease pathology

Sporadic Alzheimer's disease (AD) is a chronic progressive neurodegenerative disorder of unknown cause characterized by fibrillar accumulation of the Aß-peptide and aggregates of the microtubule-associated protein tau in a hyperphosphorylated form. Already at preclinical stages, AD is characterized by hypometabolic states which are a good predictor of cognitive decline. Here, we summarize recent evidence derived from the study of hibernating animals that brain hypometabolism can trigger PHF-like hyperphosphorylation of tau. We put forward the concept that particular types of neurons respond to a hypometabolic state with an elevated phosphorylation of tau protein which represents a physiological mechanism involved in regulating synaptic gain. If, in contrast to hibernation, the hypometabolic state is not terminated after a definite time but rather persists and progresses, the elevated phosphorylation of tau protein endures and the protective reaction associated with it might turn into a pathological cascade leading to neurodegeneration.

To be or not to be: the regulation of mRNA fate as a survival strategy during mammalian hibernation

Mammalian hibernators undergo profound behavioral, physiological, and biochemical changes in order to cope with hypothermia, ischemia-reperfusion, and finite fuel reserves over days or weeks of continuous torpor. Against a backdrop of global reductions in energy-expensive processes such as transcription and translation, a subset of genes/proteins are strategically upregulated in order to meet challenges associated with hibernation. Consequently, hibernation involves substantial transcriptional and posttranscriptional regulatory mechanisms and provides a phenomenon with which to understand how a set of common genes/proteins can be differentially regulated in order to enhance stress tolerance beyond that which is possible for nonhibernators. The present review focuses on the involvement of messenger RNA (mRNA) interacting factors that play a role in the regulation of gene/protein expression programs that define the hibernating phenotype. These include proteins involved in mRNA processing (i.e., capping, splicing, and polyadenylation) and the possible role of alternative splicing as a means of enhancing protein diversity. Since the total pool of mRNA remains constant throughout torpor, mechanisms which enhance mRNA stability are discussed in the context of RNA binding proteins and mRNA decay pathways. Furthermore, mechanisms which control the global reduction of cap-dependent translation and the involvement of internal ribosome entry sites in mRNAs encoding stress response proteins are also discussed. Finally, the concept of regulating each of these factors in discrete subcellular compartments for enhanced efficiency is addressed. The analysis draws on recent research from several well-studied mammalian hibernators including ground squirrels, bats, and bears.

Inhibition of elongation factor-2 kinase augments the antitumor activity of Temozolomide against glioma

BACKGROUND

Glioblastoma multiforme (GBM), the most common form of brain cancer with an average survival of less than 12 months, is a highly aggressive and fatal disease characterized by survival of glioma cells following initial treatment, invasion through the brain parenchyma and destruction of normal brain tissues, and ultimately resistance to current treatments. Temozolomide (TMZ) is commonly used chemotherapy for treatment of primary and recurrent high-grade gliomas. Nevertheless, the therapeutic outcome of TMZ is often unsatisfactory. In this study, we sought to determine whether eEF-2 kinase affected the sensitivity of glioma cells to treatment with TMZ.

METHODOLOGY/PRINCIPAL FINDINGS

Using RNA interference approach, a small molecule inhibitor of eEF-2 kinase, and in vitro and in vivo glioma models, we observed that inhibition of eEF-2 kinase could enhance sensitivity of glioma cells to TMZ, and that this sensitizing effect was associated with blockade of autophagy and augmentation of apoptosis caused by TMZ.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrated that targeting eEF-2 kinase can enhance the anti-glioma activity of TMZ, and inhibitors of this kinase may be exploited as chemo-sensitizers for TMZ in treatment of malignant glioma.

Neuronal plasticity in hibernation and the proposed role of the microtubule-associated protein tau as a "master switch" regulating synaptic gain in neuronal networks

The present paper provides an overview of adaptive changes in brain structure and learning abilities during hibernation as a behavioral strategy used by several mammalian species to minimize energy expenditure under current or anticipated inhospitable environmental conditions. One cellular mechanism that contributes to the regulated suppression of metabolism and thermogenesis during hibernation is reversible phosphorylation of enzymes and proteins, which limits rates of flux through metabolic pathways. Reversible phosphorylation during hibernation also affects synaptic membrane proteins, a process known to be involved in synaptic plasticity. This mechanism of reversible protein phosphorylation also affects the microtubule-associated protein tau, thereby generating a condition that in the adult human brain is associated with aggregation of tau protein to paired helical filaments (PHFs), as observed in Alzheimer's disease. Here, we put forward the concept that phosphorylation of tau is a neuroprotective mechanism to escape NMDA-mediated hyperexcitability of neurons that would otherwise occur during slow gradual cooling of the brain. Phosphorylation of tau and its subsequent targeting to subsynaptic sites might, thus, work as a kind of "master switch," regulating NMDA receptor-mediated synaptic gain in a wide array of neuronal networks, thereby enabling entry into torpor. If this condition lasts too long, however, it may eventually turn into a pathological trigger, driving a cascade of events leading to neurodegeneration, as in Alzheimer's disease or other "tauopathies".

SUMO and ischemic tolerance

Hibernating squirrels slow blood flow to a crawl, but sustain no damage to brain or other tissues. This phenomenon provides an excellent model of natural tolerance to ischemia. Small ubiquitin-like modifier (SUMO) is a 100-residue peptide that modifies other proteins by being attached to the epsilon amino group of specific lysine residues. The discovery of massive SUMOylation (by both SUMO-1 and SUMO-2/3) occurring in the brains of 13-lined ground squirrels (Ictidomys tridecemlineatus) during hibernation torpor had opened the door to the studies on SUMO and ischemic tolerance reviewed here. Ischemic stress was shown to increase the levels of SUMO conjugation, especially SUMO-2/3, mostly during reperfusion in animal models and during restoration of oxygen and glucose in cell culture systems. Over-expression or depletion of SUMOs and/or Ubc9 (the SUMO E2 conjugating enzyme) increases or decreases (respectively) the levels of SUMO conjugates. Elevated global SUMO conjugations were shown to cytoprotect from ischemic insults; conversely, depressed SUMOylation sensitized cells. Global protein conjugation not only by SUMOs, but also by other ubiquitin-like modifiers (ULMs) including NEDD8, ISG15, UFM1 and FUB1 was shown to be significantly increased in the brains of hibernating ground squirrels during torpor. These increases in multiple ULM conjugations may orchestrate the cellular events in hibernating ground squirrels that induce a state of natural tolerance through their multipronged effects. Certain miRNAs such as the miR-200 family and the miR-182 family were shown, at least partly, to control the levels of these ULM conjugations. Lowering the levels of these miRNAs leads to an increase in global SUMOylation/ULM conjugation, thereby providing the tolerance to ischemia. This suggests that these miRNAs may be good targets for therapeutic intervention in stroke.

Phosphorylation of elongation factor-2 kinase differentially regulates the enzyme's stability under stress conditions

Eukaryotic elongation factor-2 kinase (eEF-2K) is a Ca(2+)/calmodulin-dependent enzyme that negatively regulates protein synthesis. eEF-2K has been shown to be up-regulated in cancer, and to play an important role in cell survival through inhibition of protein synthesis. Post-translational modification of protein synthesis machinery is important for its regulation and could be critical for survival of cancer cells encountering stress. The purpose of our study was to examine the regulation of eEF-2K during stress with a focus on the roles of phosphorylation in determining the stability of eEF-2K. We found that stress conditions (nutrient deprivation and hypoxia) increase eEF-2K protein. mRNA levels are only transiently increased and shortly return to normal, while eEF-2K protein levels continue to increase after further exposure to stress. A seemingly paradoxical decrease in eEF-2K stability was found when glioma cells were subjected to stress despite increased protein expression. We further demonstrated that phosphorylation of eEF-2K differentially affects the enzyme's turnover under both normal and stress conditions, as evidenced by the different half-lives of phosphorylation-defective mutants of eEF-2K. We further found that the eEF-2K site (Ser398) phosphorylated by AMPK is pivotal to the protein's stability, as the half-life of S398A mutant increases to greater than 24h under both normal and stress conditions. These data indicate that eEF-2K is regulated at multiple levels with phosphorylation playing a critical role in the enzyme's turnover under stressful conditions. The complexity of eEF-2K phosphorylation highlights the intricacies of protein synthesis control during cellular stress.

Expression of elongation factor-2 kinase contributes to anoikis resistance and invasion of human glioma cells

AIM

To determine whether elongation factor-2 kinase (eEF-2 kinase) contributes to the malignant phenotype of glioblastoma multiforme by promoting the migration and invasion of glioma cells. The mechanism involved was also explored.

METHODS

Human glioma cell lines T98G and LN-229 were used. The expression of eEF-2 kinase was silenced using siRNA, and the invasive potential of tumor cells was assessed using a wound-healing assay and a Matrigel invasion assay. Apoptosis was determined using propidium iodide (PI) staining and Western blot analysis of cleaved caspase-3.

RESULTS

Silencing the expression of eEF-2 kinase by siRNA significantly suppressed both the migration and invasion of human glioma cells. Silencing eEF-2 kinase expression also sensitized glioma cells to anoikis, thereby decreasing tumor cell viability in the absence of attachment. Treatment of tumor cells with the caspase inhibitor z-VAD-fmk down-regulated Bim accumulation and abolished glioma cell sensitivity to anoikis.

CONCLUSION

The results suggest that the expression of eEF-2 kinase contributes to migration and invasion of human glioma cells by protecting them from anoikis. eEF-2 kinase expression may serve as a prognostic marker and a novel target for cancer therapy.

The alpha-kinase family: an exceptional branch on the protein kinase tree

The alpha-kinase family represents a class of atypical protein kinases that display little sequence similarity to conventional protein kinases. Early studies on myosin heavy chain kinases in Dictyostelium discoideum revealed their unusual propensity to phosphorylate serine and threonine residues in the context of an alpha-helix. Although recent studies show that some members of this family can also phosphorylate residues in non-helical regions, the name alpha-kinase has remained. During evolution, the alpha-kinase domains combined with many different functional subdomains such as von Willebrand factor-like motifs (vWKa) and even cation channels (TRPM6 and TRPM7). As a result, these kinases are implicated in a large variety of cellular processes such as protein translation, Mg(2+) homeostasis, intracellular transport, cell migration, adhesion, and proliferation. Here, we review the current state of knowledge on different members of this kinase family and discuss the potential use of alpha-kinases as drug targets in diseases such as cancer.

Seasonal protein changes support rapid energy production in hibernator brainstem

During the torpor phase of mammalian hibernation when core body temperature is near 4 degrees C, the autonomic system continues to maintain respiration, blood pressure and heartbeat despite drastic reductions in brain activity. In addition, the hibernator's neuronal tissues enter into a protected state in which the potential for ischemia-reperfusion injury is markedly minimized. Evolutionary adaptations for continued function and neuroprotection throughout cycles of torpor and euthermia in winter are predicted to manifest themselves partly in changes in the brainstem proteome. Here, we compare the soluble brainstem protein complement from six summer active ground squirrels and six in the early torpor (ET) phase of hibernation. Thirteen percent of the approximately 1,500 quantifiable 2D gel spots alter significantly from summer to ET; the proteins identified in these differing spots are known to play roles in energy homeostasis via the tricarboxylic acid cycle (8 proteins), cytoarchitecture and cell motility (14 proteins), anabolic protein processes (13 proteins), redox control (11 proteins) and numerous other categories including protein catabolism, oxidative phosphorylation, signal transduction, glycolysis, intracellular protein trafficking and antiapoptotic function. These protein changes represent, at least in part, the molecular bases for restructuring of cells in the brainstem, a shift away from glucose as the primary fuel source for brain in the winter, and the generation of a streamlined mechanism capable of efficient and rapid energy production and utilization during the torpor and arousal cycles of hibernation.

Seasonal proteomic changes reveal molecular adaptations to preserve and replenish liver proteins during ground squirrel hibernation

Hibernators are unique among mammals in their ability to survive extended periods of time with core body temperatures near freezing and with dramatically reduced heart, respiratory, and metabolic rates in a state known as torpor. To gain insight into the molecular events underlying this remarkable physiological phenotype, we applied a proteomic screening approach to identify liver proteins that differ between the summer active (SA) and the entrance (Ent) phase of winter hibernation in 13-lined ground squirrels. The relative abundance of 1,600 protein spots separated on two-dimensional gels was quantitatively determined using fluorescence difference gel electrophoresis, and 74 unique proteins exhibiting significant differences between the two states were identified using liquid chromatography followed by tandem mass spectrometry (LC-MS/MS). Proteins elevated in Ent hibernators included liver fatty acid-binding protein, fatty acid transporter, and 3-hydroxy-3-methylglutaryl-CoA synthase, which support the known metabolic fuel switch to lipid and ketone body utilization in winter. Several proteins involved in protein stability and protein folding were also elevated in the Ent phase, consistent with previous findings. In contrast to transcript screening results, there was a surprising increase in the abundance of proteins involved in protein synthesis during Ent hibernation, including several initiation and elongation factors. This finding, coupled with decreased abundance of numerous proteins involved in amino acid and nitrogen metabolism, supports the intriguing hypothesis that the mechanism of protein preservation and resynthesis is used by hibernating ground squirrels to help avoid nitrogen toxicity and ensure preservation of essential amino acids throughout the long winter fast.

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.

Matching cellular metabolic supply and demand in energy-stressed animals

Certain environmental stressors can impair cellular ATP production to the point of harming or even killing an animal. Some exceptional animals employ strategies that maintain the balance between ATP production and consumption, allowing them to tolerate prolonged exposure to stressors such as hypoxia and anoxia. Anoxia- and hypoxia-tolerant animals reduce ATP consumption by ion-motive ATPases while concomitant reductions in passive ion flux reduce the demand for ion pumping and maintain transmembrane ion gradients. Reductions in gene transcription and protein turnover decrease ATP demand in hibernating and hypoxia-tolerant animals. Proton leak uncouples mitochondrial substrate oxidation from ATP synthesis and accounts for a considerable proportion of cellular energy demand, but there is little evidence that the proton permeability of inner mitochondrial membranes decreases in animals that tolerate energy stress. Indeed in some cases proton leak increases, possibly reducing reactive oxygen species production. Because substrate oxidation is important to the control of cellular metabolism, the downregulation of ATP supply pathways contributes significantly to metabolic suppression under energy stress. Mechanisms that coordinate the downregulation of both ATP supply and demand pathways include AMP kinase and ATP-sensitive ion channels. Strategies employed by animals tolerant to one energy stress often convey "cross-tolerance" to completely different stresses.

Mammalian hibernation: a naturally reversible model for insulin resistance in man?

Mammalian hibernators such as ground squirrels store massive amounts of fat each autumn. These fat depots serve as the main source of metabolic fuel throughout the winter, gradually decreasing over a period of months until the animals emerge from hibernation each spring. Fat deposition occurs on an approximately annual, i.e. on a circannual, basis. Although this rhythm occurs in the absence of environmental temperature and light cues, it is entrained by the length of daylight, with peak fat deposition occurring as days shorten in the autumn. Here we examine the circ-annual cycle of hibernation, and then explore the similarities and differences between the obligatory, yet reversible, natural obesity and accompanying insulin resistance of natural hibernation, and the pandemic of human obesity and metabolic syndrome.

Amino acids Thr56 and Thr58 are not essential for elongation factor 2 function in yeast

Yeast elongation factor 2 is an essential protein that contains two highly conserved threonine residues, T56 and T58, that could potentially be phosphorylated by the Rck2 kinase in response to environmental stress. The importance of residues T56 and T58 for elongation factor 2 function in yeast was studied using site directed mutagenesis and functional complementation. Mutations T56D, T56G, T56K, T56N and T56V resulted in nonfunctional elongation factor 2 whereas mutated factor carrying point mutations T56M, T56C, T56S, T58S and T58V was functional. Expression of mutants T56C, T56S and T58S was associated with reduced growth rate. The double mutants T56M/T58W and T56M/T58V were also functional but the latter mutant caused increased cell death and considerably reduced growth rate. The results suggest that the physiological role of T56 and T58 as phosphorylation targets is of little importance in yeast under standard growth conditions. Yeast cells expressing mutants T56C and T56S were less able to cope with environmental stress induced by increased growth temperatures. Similarly, cells expressing mutants T56M and T56M/T58W were less capable of adapting to increased osmolarity whereas cells expressing mutant T58V behaved normally. All mutants tested were retained their ability to bind to ribosomes in vivo. However, mutants T56D, T56G and T56K were under-represented on the ribosome, suggesting that these nonfunctional forms of elongation factor 2 were less capable of competing with wild-type elongation factor 2 in ribosome binding. The presence of nonfunctional but ribosome binding forms of elongation factor 2 did not affect the growth rate of yeast cells also expressing wild-type elongation factor 2.

Protein SUMOylation is massively increased in hibernation torpor and is critical for the cytoprotection provided by ischemic preconditioning and hypothermia in SHSY5Y cells

Hibernation torpor provides an excellent natural model of tolerance to profound reductions in blood flow to the brain and other organs. Here, we report that during torpor of 13-lined ground squirrels, massive SUMOylation occurs in the brain, liver, and kidney. The level of small ubiquitin-related modifier (SUMO) conjugation coincides with the expression level of Ubc9, the SUMO specific E2-conjugating enzyme. Hypothermia alone also increased SUMO conjugation, but not as markedly as hibernation torpor. Increased SUMO conjugation (induced by Ubc9 overexpression, ischemic preconditioning (PC)+/-hypothermia) was necessary and sufficient for tolerance of SHSY5Y neuroblastoma cells to oxygen/glucose deprivation (OGD) ('in vitro ischemia'); decreased SUMO conjugation (induced by a dominant-negative Ubc9) severely reduced tolerance to OGD in these cells. These data indicate that post-translational modification of proteins by SUMOylation is a prominent feature of hibernation torpor and is critical for cytoprotection by ischemic PC+/-hypothermia in SHSY5Y cells subjected to OGD.

Evidence for a reduced transcriptional state during hibernation in ground squirrels

During mammalian hibernation, metabolic rate can be reduced to <5% of the euthermic rate as a result of coordinated suppression of multiple energy expensive metabolic processes. Gene transcription is one of these and the present study examines mechanisms of transcriptional control that could contribute to lowering the rate of gene expression in torpor. Histone deacetylases (HDAC) have been linked to gene silencing and measured HDAC activity was 1.82-fold higher in skeletal muscle of hibernating thirteen-lined ground squirrels, Spermophilus tridecemlineatus, compared with euthermic controls. Western blotting also showed that HDAC1 and HDAC4 protein levels were 1.21-and 1.48-fold higher, respectively, in muscle from torpid animals. Histone H3 was also evaluated by Western blotting. Total histone H3 was unchanged but two forms of covalently modified histone H3 that are associated with active transcription (phosphorylated Ser 10 and acetylated Lys 23) were significantly reduced by 38-39% in muscle during hibernation. Finally, RNA polymerase II activity was measured using a PCR-based approach; activity in muscle from hibernating squirrels was only 57% of the euthermic value. These data support an overall decrease in transcriptional activity in skeletal muscle of hibernating animals that is accomplished by multiple molecular mechanisms.

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.

Up-regulation of the endoplasmic reticulum molecular chaperone GRP78 during hibernation in thirteen-lined ground squirrels

Hibernating mammals endure conditions of low body temperature and oxidative stress that would be highly injurious to humans and most other mammals. Stress conditions frequently trigger the production of molecular chaperones; in the endoplasmic reticulum the glucose-regulated protein-78 (GRP78) helps to minimize protein misfolding under stress. The present study evaluated the GRP78 response in seven organs of hibernating thirteen-lined ground squirrels, Spermophilus tridecemlineatus. Transcript levels of grp78, assessed by RT-PCR, were significantly higher (3.5- to 4.1-fold) in brown adipose tissue and brain of hibernating squirrels compared with euthermic control animals but remained low or stable in all other tissues. GRP78 protein content, assessed by Western blotting, was also elevated in brown adipose and brain during hibernation by 1.4-1.6 fold. A 2490 bp cDNA sequence was retrieved that contained the full length open reading frame of ground squirrel grp78 and the translated protein sequence of 654 amino acids shared 98-99% identity with GRP78 from other mammalian sources. Selected specific amino acid substitutions were found in the ground squirrel sequence that may aid GRP78 function under the near 0 degrees C body temperatures of the hibernating state. Electrophoretic mobility shift and supershift assays showed that the activating transcription factor, ATF4, binds to the promoter region of the grp78 gene in ground squirrel brain and may be responsible for grp78 up-regulation during hibernation. Changes in grp78 gene and protein expression appear to aid stress tolerance in two highly oxygen-dependent organs that are critical to whole animal survival during hibernation.

The life and death of translation elongation factor 2

eEF2 (eukaryotic elongation factor 2) occupies an essential role in protein synthesis where it catalyses the translocation of the two tRNAs and the mRNA after peptidyl transfer on the 80 S ribosome. Recent crystal structures of eEF2 and the cryo-electron microscopy reconstruction of its 80 S complex now provide a substantial structural framework for dissecting the functional properties of this factor. The factor can be modified by either phosphorylation or ADP-ribosylation, which results in cessation of translation. We review the structural and functional properties of eEF2 with particular emphasis on the unique diphthamide residue, which is ADP-ribosylated by diphtheria toxin from Corynebacterium diphtheriae and exotoxin A from Pseudomonas aeruginosa.

Evaluation of the role of AMP-activated protein kinase and its downstream targets in mammalian hibernation

Mammalian hibernation requires an extensive reorganization of metabolism that typically includes a greater than 95% reduction in metabolic rate, selective inhibition of many ATP-consuming metabolic activities and a change in fuel use to a primary dependence on the oxidation of lipid reserves. We investigated whether the AMP-activated protein kinase (AMPK) could play a regulatory role in this reorganization. AMPK activity and the phosphorylation state of multiple downstream targets were assessed in five organs of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) comparing euthermic animals with squirrels in deep torpor. AMPK activity was increased 3-fold in white adipose tissue from hibernating ground squirrels compared with euthermic controls, but activation was not seen in liver, skeletal muscle, brown adipose tissue or brain. Immunoblotting with phospho-specific antibodies revealed an increase in phosphorylation of eukaryotic elongation factor-2 at the inactivating Thr56 site in white adipose tissue, liver and brain of hibernators, but not in other tissues. Acetyl-CoA carboxylase phosphorylation at the inactivating Ser79 site was markedly increased in brown adipose tissue from hibernators, but no change was seen in white adipose tissue. No change was seen in the level of phosphorylation of the Ser565 AMPK site of hormone-sensitive lipase in adipose tissues of hibernating animals. In conclusion, AMPK does not appear to participate in the metabolic re-organization and/or the metabolic rate depression that occurs during ground squirrel hibernation.

Effects of hibernation on multicatalytic proteinase complex in thirteen-lined ground squirrels, Spermophilus tridecemlineatus

Multicatalytic proteinase complex (MCP) was studied in skeletal muscle of the hibernating ground squirrel, Spermophilus tridecemlineatus. MCP was partially purified using a S-400 gel filtration column and Centricon concentrating devices and assayed fluorometrically using three AMC-labeled substrates. K(m) and V(max) values were determined for each substrate with no significant differences between the enzyme from euthermic versus hibernating animals when assayed at 23 degrees C. However, properties of MCP from euthermic and hibernating ground squirrels were differentially affected by low assay temperature (8-10 degrees C) and also differed from the mouse enzyme, the data indicating that ground squirrel MCP is better suited for low temperature function. MCP preferentially degrades oxidatively-damaged proteins and quantification of protein carbonyl content showed that the level of oxidatively-damaged protein in skeletal muscle decreased by > 75% during hibernation suggesting a continuing role for the MCP in the torpid state.

Proteolysis is depressed during torpor in hibernators at the level of the 20S core protease

Protein synthesis is depressed during mammalian hibernation in concordance with metabolic demands. In the absence of significant protein synthesis, continued proteolysis would rapidly deplete protein pools. Since ubiquitin-dependent proteolysis is implicated in the turnover of most regulatory proteins, we examined the fate of this system during hibernation. Ubiquitin-dependent proteolysis consists of two major steps: (1) the tagging of a protein substrate by ubiquitin and (2) the protein substrate's subsequent degradation by the 26S proteasome. An earlier study revealed a two to threefold elevation of ubiquitin conjugate concentrations during hibernation: an unexpected result that seemingly would suggest increased proteolytic activity. A more likely explanation for these data would be that proteolysis per se was depressed and that the increased levels of ubiquitylated proteins reflect an inability to degrade tagged proteins. We employed an assay based on the cleavage of fluorogenic substrates to address the well characterized proteolytic activities of the proteasome. All activities show little to no activity at temperatures associated with deep torpor. Coordinated depression of proteolytic activities by low temperature supports the hypothesis that the increased levels of ubiquitylated proteins during hibernation is explained by a net accumulation due to an inability to degrade the tagged proteins.

Cloning and expression of hypoxia-inducible factor 1α from the hibernating ground squirrel, Spermophilus tridecemlineatus

Mammalian hibernation is associated with apnoic breathing patterns and a hypoxia-hypothermia connection has been suggested as part of the mechanism by which body temperature is reduced as animals enter torpor. Hence, we hypothesized that changes in the expression of the hypoxia inducible factor (HIF-1) may potentially be involved in regulating hibernation-responsive gene targets. The expression of the alpha subunit of HIF-1 was quantified at both gene and protein levels in four organs of the thirteen-lined ground squirrel, Spermophilus tridecemlineatus. Reverse transcription-PCR showed no change in hif-1alpha transcript levels in the liver, lung, skeletal muscle or brown adipose tissue of euthermic versus hibernating animals but HIF-1alpha protein levels were elevated by 60-70% in the two organs responsive for thermogenesis (brown adipose and skeletal muscle). Furthermore, assessment of DNA binding by HIF-1 in nuclear extracts from brown adipose revealed 6-fold higher levels in hibernator tissue than in euthermic controls suggesting increased expression of HIF-1 responsive genes during hibernation. The complete nucleotide sequence of hif-1alpha from ground squirrels, the first hif-1alpha sequence amplified from a hibernating mammal, was obtained using PCR amplification and 3' and 5' RACE. Amino acid sequence analysis revealed 90-95% identity with the HIF-1alpha protein from other mammals. Several unique amino acid sequence substitutions were identified that may affect protein conformation and could possibly function to counteract low temperature effects on HIF-1alpha conformation at near 0 degrees C body temperatures during torpor.

Up-regulation of a thioredoxin peroxidase-like protein, proliferation-associated gene, in hibernating bats

Two-dimensional gel electrophoresis was used to assess differential protein expression between euthermic and hibernating states in heart of Myotis lucifugus. A hibernation-induced protein was identified by mass spectrometry as a thioredoxin peroxidase-like protein known as PAG. Western blotting confirmed up-regulation (>2-fold) and RT-PCR also revealed up-regulation (>5-fold) of pag mRNA. Cloning revealed a highly conserved sequence suggesting a conserved function for PAG. Oxidative stress markers, p-IkappaB-alpha (Ser 32) and p-HSP27 (Ser 78/82), were also up-regulated in heart and skeletal muscle during hibernation. Although there are selected increases in gene/protein expression during hibernation, general translation inhibition occurs as part of metabolic rate depression. This was confirmed by elevated levels of the inactive forms of the eIF2alpha (Ser 51) in both heart and skeletal muscle (2- to 5-fold higher than in euthermia) and the eEF2 (Thr 51) in skeletal muscle (a 15-fold increase). This study suggests that hibernators may use up-regulation of specific proteins to counteract oxidative stress.

Neurodegeneration and plasticity

Neurofibrillary degeneration, associated with the formation of paired helical filaments (PHF), is one of the critical neuropathological hallmarks of Alzheimer's disease (AD). Although the microtubule-associated protein tau in a hyperphosphorylated form has been established as primary PHF constituent, the process of tau phosphorylation and its potential link to degeneration is not very well understood, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. PHF formation in AD follows a hierarchical pattern of development throughout different cortical areas, which closely matches the pattern of neuronal plasticity in the adult brain. Those brain areas are most early and most severely affected which are involved in the regulation of memory, learning, perception, self-awareness, consciousness, and higher brain functions that require a life-long re-fitting of connectivity, a process based on a particularly high degree of plasticity. Failures of synaptic plasticity are, thus, assumed to represent early events in the course of AD that eventually lead to alteration of tau phosphorylation. Recently, we have used the hibernation cycle, a physiological model of adaptation associated with an extraordinary high degree of structural neuronal plasticity, to analyze the potential link between synaptic plasticity, synaptic detachment and the regulation of tau phosphorylation. During torpor, a natural state of hypothermia, synaptic contacts between mossy fibers and hippocampal pyramidal neurons undergo dramatic regressive changes that are fully reversible very rapidly during euthermy. This rapid, reversible, and repeated regression of synaptic and dendritic components on CA3 neurons is associated with a reversible PHF-like phosphorylation of tau at a similar time course. The repeated formation and degradation of PHF-tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau, potentially involved in neurofibrillary degeneration.

Seasonal and state-dependent changes of eIF4E and 4E-BP1 during mammalian hibernation: implications for the control of translation during torpor

Mammalian hibernation involves cessation of energetically costly processes typical of homeostatic regulation including protein synthesis. To further elucidate the mechanisms employed in depressing translation, we surveyed key eukaryotic initiation factors [eIF2, eIF4B, eIF4E, eIF4GI and -II, and 4E-binding protein-1 (4E-BP1), -2, and -3] for their availability and phosphorylation status in the livers of golden-mantled ground squirrels (Spermophilus lateralis) across the hibernation cycle. Western blot analyses indicated only one significant locus for regulation of translational initiation in ground squirrel liver: control of eIF4E. We found seasonal variation in a potent regulator of eIF4E activity, 4E-BP1. Summer squirrels lack 4E-BP1 and apparently control eIF4E activity through direct phosphorylation. In winter, eIF4E is regulated through binding with 4E-BP1. During the euthermic periods that separate bouts of torpor (interbout arousal), 4E-BP1 is hyperphosphorylated to promote initiation. However, during torpor, 4E-BP1 is hypophosphorylated and cap-dependent initiation of translation is restricted. The regulation of cap-dependent initiation of translation may allow for the differential expression of proteins directed toward enhancing survivorship.

Metabolic rate depression in animals: transcriptional and translational controls

Metabolic rate depression is an important survival strategy for many animal species and a common element of hibernation, torpor, aestivation, anaerobiosis, diapause, and anhydrobiosis. Studies of the biochemical mechanisms that regulate reversible transitions to and from hypometabolic states are identifying principles of regulatory control that are conserved across phylogenetic lines and that are broadly applied to the control of multiple cell functions. One such mechanism is reversible protein phosphorylation which is now known to contribute to the regulation of fuel metabolism, to ion channel arrest, and to the suppression of protein synthesis during hypometabolism. The present review focuses on two new areas of research in hypometabolism: (1) the role of differential gene expression in supplying protein products that adjust metabolism or protect cell functions for long-term survival, and (2) the mechanisms of protein life extension in hypometabolism involving inhibitory controls of transcription, translation and protein degradation. Control of translation examines reversible phosphorylation regulation of ribosomal initiation and elongation factors, the dissociation of polysomes and storage of mRNA transcripts during hypometabolism, and control over the translation of different mRNA types by differential sequestering of mRNA into polysome versus monosome fractions. The analysis draws primarily from current research on two animal models, hibernating mammals and anoxia-tolerant molluscs, with selected examples from multiple other sources.

Does phosphorylation of eukaryotic elongation factor eEF2 regulate protein synthesis in ischemic preconditioning?

Ischemia/reperfusion-associated translation inhibition in the hippocampus is attenuated significantly at reinitiation and elongation steps by ischemic preconditioning (Burda et al. [2003] Neurochem. Res. 28:1237-1243). To address potential regulation of the elongation step by changes in eukaryotic elongation factor 2 (eEF2) phosphorylation with and without acquired ischemic tolerance (IT), Wistar rats were preconditioned by 5-min sublethal ischemia and 2 days later, 30-min lethal ischemia was induced. Given the important role that oxidative stress plays in the ischemic process, eEF2 phosphorylation was also studied in a model of oxidative stress in vitro. Three blocks of our results support a lack of correlation between eEF2 phosphorylation status and protein synthesis rate. First, eEF2 was dephosphorylated significantly (activated) after transient cerebral ischemia in rats with and without IT or H2O2-treated cells; however, protein synthesis was significantly inhibited under these three conditions. Second, after 30-min reperfusion, the protein synthesis rate was maintained below control levels in cortex and hippocampus of rats without IT. Eukaryotic EF2 phosphorylated levels were notably low only in the cortex, whereas levels in the hippocampus were close to that of sham controls. In rats with IT, protein synthesis was virtually restored in both brain regions, but phosphorylated eEF2 levels were even higher than in rats without IT. Third, after 4-hr reperfusion, the protein synthesis rate in cortex and hippocampus was observed to be below sham control values in rats with and without IT. Conversely, phosphorylated eEF2 levels were below sham control in rats with IT and reached sham control values in rats without IT.

Determination In Vivo of Newly Synthesized Gene Expression in Hamsters During Phases of the Hibernation Cycle

This study measured in vivo synthesis of total RNA and protein from cortex, cerebellum and midbrain/brainstem and 6 major organs from Syrian hamsters (Mesocricetus auratus) during (a) 33 h of torpor (body temperature 5-6 degrees C); (b) 90 min of the early arousal; (c) 90 min of the middle arousal; (d) 90 min in cold adapted cenothermic (CEN) hamsters of the same circannual period. Appropriate physiological parameters were used to confirm the phase of the hibernation cycle during infusion and incorporation of [3H]-uridine and [14C]-leucine. In torpor, RNA synthesis was 5-25% of CEN levels depending upon tissue. In brain and heart mRNA was not preferentially synthesized. Protein was synthesized at low, tissue specific levels during torpor. Initiation of arousal and the warming of anterior organs via non-shivering thermogenesis during the early arousal occurred without measurable synthesis of RNA or proteins. Tissue specific levels of RNA and protein synthesis occurred later after shivering thermogenesis had been recruited and was strongly influenced by thermal gradients in the body. In the middle arousal phase, protein synthesis is most active in the brain despite modest synthesis of RNA and mRNA. The majority of molecular processing required for the induction and maintenance of torpor and the arousal from torpor up until the onset of shivering thermogenesis occurs during the cenothermic period before the hamster initiates the hibernation cycle.

Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature

Mammalian hibernators undergo a remarkable phenotypic switch that involves profound changes in physiology, morphology, and behavior in response to periods of unfavorable environmental conditions. The ability to hibernate is found throughout the class Mammalia and appears to involve differential expression of genes common to all mammals, rather than the induction of novel gene products unique to the hibernating state. The hibernation season is characterized by extended bouts of torpor, during which minimal body temperature (Tb) can fall as low as -2.9 degrees C and metabolism can be reduced to 1% of euthermic rates. Many global biochemical and physiological processes exploit low temperatures to lower reaction rates but retain the ability to resume full activity upon rewarming. Other critical functions must continue at physiologically relevant levels during torpor and be precisely regulated even at Tb values near 0 degrees C. Research using new tools of molecular and cellular biology is beginning to reveal how hibernators survive repeated cycles of torpor and arousal during the hibernation season. Comprehensive approaches that exploit advances in genomic and proteomic technologies are needed to further define the differentially expressed genes that distinguish the summer euthermic from winter hibernating states. Detailed understanding of hibernation from the molecular to organismal levels should enable the translation of this information to the development of a variety of hypothermic and hypometabolic strategies to improve outcomes for human and animal health.

Effect of ischaemic preconditioning on genomic response to cerebral ischaemia: similarity to neuroprotective strategies in hibernation and hypoxia-tolerant states

BACKGROUND

Molecular mechanisms of neuroprotection that lead to ischaemic tolerance are incompletely understood. Identification of genes involved in the process would provide insight into cell survival and therapeutic approaches for stroke. We developed a mouse model of neuroprotection in stroke and did gene expression profiling to identify potential neuroprotective genes and their associated pathways.

METHODS

Eight mice per condition were subjected to occlusion of the middle cerebral artery for 15 min (preconditioning), 60 min (injurious ischaemia), or preconditioning followed 72 h later by injurious ischaemia. RNA was extracted from the cortical regions of the ischaemic and non-ischaemic hemispheres. Three pools per condition were generated, and RNA was hybridised to oligonucleotide microarrays for comparison of ischaemic and non-ischaemic hemispheres. Real-time PCR and western blots were used to validate results. Follow-up experiments were done to address the biological relevance of findings.

FINDINGS

Microarray analysis revealed changes in gene expression with little overlap among the conditions of injurious ischaemia, ischaemic preconditioning, or both. Injurious ischaemia induced upregulation of gene expression; 49 (86%) of 57 genes regulated showed increased expression in the ischaemic hemisphere. By contrast, preconditioning followed by injurious ischaemia resulted in pronounced downregulation; 47 (77%) of 61 regulated genes showed lower expression. Preconditioning resulted in transcriptional changes involved in suppression of metabolic pathways and immune responses, reduction of ion-channel activity, and decreased blood coagulation.

INTERPRETATION

Preconditioning reprogrammes the response to ischaemic injury. Similar changes reported by others support an evolutionarily conserved endogenous response to decreased blood flow and oxygen limitation such as seen during hibernation.