The translation state of differentially expressed mRNAs in the hibernating 13-lined ground squirrel (Spermophilus tridecemlineatus)

Tissue-specific response of the RB-E2F1 complex during mammalian hibernation

Metabolic rate depression during prolonged bouts of torpor is characteristic of mammalian hibernation, reducing energy expenditures over the winter. Cell cycle arrest is observed in quiescent cells during dormancy, partly due to the retinoblastoma (Rb) protein at G₁ /S, given cell division and proliferation are metabolic-costly processes. Rb binds to E2F transcription factors and recruits corepressors (e.g., SUV39H1) to E2F target genes, blocking their transcription and cell cycle passage. Phosphorylation by cyclin-CDK complexes at S780 or S795 abolishes Rb-mediated repression, allowing transition into S phase. The present study compares Rb-E2F1 responses between euthermic and torpid states in five organs (brain, heart, kidney, liver, skeletal muscle) of 13-lined ground squirrels (Ictidomys tridecemlineatus). Immunoblotting assessed the expression of Rb, pRb (S780, S795), E2F1, and SUV39H1. Our findings demonstrate multi-tissue upregulation of Rb and SUV39H1 during torpor, with tissue-specific changes to E2F1 and pRb (S780), suggesting Rb-E2F1 contributes to cell cycle control in hibernation.

Hypothalamic orexinergic neuron changes during the hibernation of the Syrian hamster

Hibernation in small mammals is a highly regulated process with periods of torpor involving drops in body temperature and metabolic rate, as well as a general decrease in neural activity, all of which proceed alongside complex brain adaptive changes that appear to protect the brain from extreme hypoxia and low temperatures. All these changes are rapidly reversed, with no apparent brain damage occurring, during the short periods of arousal, interspersed during torpor—characterized by transitory and partial rewarming and activity, including sleep activation, and feeding in some species. The orexins are neuropeptides synthesized in hypothalamic neurons that project to multiple brain regions and are known to participate in the regulation of a variety of processes including feeding behavior, the sleep-wake cycle, and autonomic functions such as brown adipose tissue thermogenesis. Using multiple immunohistochemical techniques and quantitative analysis, we have characterized the orexinergic system in the brain of the Syrian hamster—a facultative hibernator. Our results revealed that orexinergic neurons in this species consisted of a neuronal population restricted to the lateral hypothalamic area, whereas orexinergic fibers distribute throughout the rostrocaudal extent of the brain, particularly innervating catecholaminergic and serotonergic neuronal populations. We characterized the changes of orexinergic cells in the different phases of hibernation based on the intensity of immunostaining for the neuronal activity marker C-Fos and orexin A (OXA). During torpor, we found an increase in C-Fos immunostaining intensity in orexinergic neurons, accompanied by a decrease in OXA immunostaining. These changes were accompanied by a volume reduction and a fragmentation of the Golgi apparatus (GA) as well as a decrease in the colocalization of OXA and the GA marker GM-130. Importantly, during arousal, C-Fos and OXA expression in orexinergic neurons was highest and the structural appearance and the volume of the GA along with the colocalization of OXA/GM-130 reverted to euthermic levels. We discuss the involvement of orexinergic cells in the regulation of mammalian hibernation and, in particular, the possibility that the high activation of orexinergic cells during the arousal stage guides the rewarming as well as the feeding and sleep behaviors characteristic of this phase.

Comparison of Digestive Enzyme Activities and Expression of Small Intestinal Transporter Genes in Jinhua and Landrace Pigs

Digestive enzyme activity is involved in the regulation of growth performance because digestive enzymes function to improve the feed efficiency by digestion and in turn to modulate the process of nutrient metabolism. The objective of this study was to investigate the differences of the digestive enzyme activities and expression of nutrient transporters in the intestinal tract between Jinhua and Landrace pigs and to explore the potential breed-specificity in digestion and absorption. The pancreas segments and the digesta and mucosa of the duodenum, jejunum, and ileum were collected from 10 Jinhua pigs and Landrace pigs, respectively. The activities of trypsin, chymotrypsin, amylase, maltase, sucrase, and lipase were measured and the expression levels of PepT1, GLUT2, SGLT1, FABP1, FABP2, and FABP4 were examined. Results showed that the trypsin activity in the pancreas of Jinhua pigs was higher than that in Landrace pigs, but was lower in the small intestine, except for in the jejunal mucosa. The chymotrypsin activity in the small intestine of Jinhua pigs was higher than that in Landrace pigs, except for in jejunal mucosa and contents. Compared with Landrace pigs, the amylase and maltase activity in the small intestine of Jinhua pigs was lower, except for in ileal mucosa. The sucrase activity in the small intestine of Jinhua pigs was also lower than Landrace pigs, except for in jejunal mucosa. Furthermore, the lipase activity in the small intestine of Jinhua pigs was higher than that in Landrace pigs. The mRNA levels of PepT1 and GLUT2 in duodenal, jejunal and ileal mucosa showed no difference between Jinhua and Landrace pigs, whereas SGLT1 in ileal mucosa was lower in Jinhua pigs. The mRNA levels of FABP1, FABP2 and FABP4 in the small intestinal mucosa of Jinhua pigs were higher than in Landrace pigs. These findings indicate that there is a certain difference in the digestibility and absorption of nutrients in small intestine of Jinhua and Landrace pigs, partially resulting in their differences in growth development and fat deposition.

Unraveling the Big Sleep: Molecular Aspects of Stem Cell Dormancy and Hibernation

Tissue-resident stem cells may enter a dormant state, also known as quiescence, which allows them to withstand metabolic stress and unfavorable conditions. Similarly, hibernating mammals can also enter a state of dormancy used to evade hostile circumstances, such as food shortage and low ambient temperatures. In hibernation, the dormant state of the individual and its cells is commonly known as torpor, and is characterized by metabolic suppression in individual cells. Given that both conditions represent cell survival strategies, we here compare the molecular aspects of cellular quiescence, particularly of well-studied hematopoietic stem cells, and torpor at the cellular level. Critical processes of dormancy are reviewed, including the suppression of the cell cycle, changes in metabolic characteristics, and cellular mechanisms of dealing with damage. Key factors shared by hematopoietic stem cell quiescence and torpor include a reversible activation of factors inhibiting the cell cycle, a shift in metabolism from glucose to fatty acid oxidation, downregulation of mitochondrial activity, key changes in hypoxia-inducible factor one alpha (HIF-1α), mTOR, reversible protein phosphorylation and autophagy, and increased radiation resistance. This similarity is remarkable in view of the difference in cell populations, as stem cell quiescence regards proliferating cells, while torpor mainly involves terminally differentiated cells. A future perspective is provided how to advance our understanding of the crucial pathways that allow stem cells and hibernating animals to engage in their 'great slumbers.'

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.

Effects of cold exposure revealed by global transcriptomic analysis in ferret peripheral blood mononuclear cells

Animal studies, mostly performed in rodents, show the beneficial anti-obesity effects of cold studies. This is due to thermogenic activation of brown adipose tissue (BAT), a tissue also recently discovered in adult humans. Studies in humans, however, are hampered by the accessibility of most tissues. In contrast, peripheral blood mononuclear cells (PBMC) are accessible and share the expression profile of different sets of genes with other tissues, including those that reflect metabolic responses. Ferrets are an animal model physiologically closer to humans than rodents. Here, we investigated the effects on ferrets of one-week acclimation to 4 °C by analysing the PBMC transcriptome. Cold exposure deeply affected PBMC gene expression, producing a widespread down-regulation of genes involved in different biological pathways (cell cycle, gene expression regulation/protein synthesis, immune response, signal transduction, and genes related to extracellular matrix/cytoskeleton), while thermogenic and glycogenolysis-related processes were increased. Results obtained in PBMC reflected those of adipose tissue, but hardly those of the liver. Our study, using ferret as a model, reinforce PBMC usefulness as sentinel biological material for cold-exposure studies in order to deepen our understanding of the general and specific pathways affected by cold acclimation. This is relevant for future development of therapies to be used clinically.

Functional impact of microRNA regulation in models of extreme stress adaptation

When confronted with severe environmental stress, some animals are able to undergo a substantial reorganization of their cellular environment that enables long-term survival. One molecular mechanism of adaptation that has received considerable attention in recent years has been the action of reversible transcriptome regulation by microRNA. The implementation of new computational and high-throughput experimental approaches has started to uncover the vital contributions of microRNA towards stress adaptation. Indeed, recent studies have suggested that microRNA may have a major regulatory influence over a number of cellular processes that are essential to prolonged environmental stress survival. To date, a number of studies have highlighted the role of microRNA in the regulation of a metabolically depressed state, documenting stress-responsive microRNA expression during mammalian hibernation, frog and insect freeze tolerance, and turtle and marine snail anoxia tolerance. These studies collectively indicate a conserved principle of microRNA stress response across phylogeny. As we are on the verge of dissecting the role of microRNA in environmental stress adaptation, this review summarizes recent research advances and the hallmark expression patterns that facilitate stress survival.

Identification of a prosurvival neuroprotective mitochondrial peptide in a mammalian hibernator

Hibernation requires the intricate regulation of physiological and biochemical adaptations to facilitate the decrease in metabolic rate and activation of prosurvival factors needed for winter survival. Mitochondria play important roles in eliciting these responses and in coordinating the required energy shifts. Herein, we report the presence of a novel mitochondrial peptide, s-humanin, in the hibernating 13-lined ground squirrel, Ictidomys tridecemlineatus. S-humanin was shown to have strong structural and sequence similarities to its human analogue, humanin-a powerful neuroprotective mitochondrial peptide. An assessment of the protein and gene expression levels of this peptide in ground squirrels revealed stark tissue-specific regulatory responses whereby transcript levels increased in brain cortex, skeletal muscle, and adipose tissues during hibernation, suggesting a protective torpor-induced activation. Accompanying peptide measurements found that s-humanin levels were suppressed in liver of torpid squirrels but enhanced in brain cortex. The enhanced transcript and protein levels of s-humanin in brain cortex suggest that it is actively involved in protecting delicate brain tissues and neuronal connections from hibernation-associated stresses. We propose that this squirrel-specific peptide is involved in modulating tissue-specific cytoprotective functions, expanding its role from human-specific neuroprotection to environmental stress protection. SIGNIFICANCE OF THE STUDY: Understanding the molecular mechanisms, which protect against oxidative stress in a model hibernator such as the ground squirrel, could be pivotal to the regulation of cytoprotection. This study expands on our knowledge of metabolic rate depression and could suggest a potential role for humanin therapy in neurodegenerative diseases.

RNA-binding proteins: The next step in translating skeletal muscle adaptations?

The decline of skeletal muscle mass during illness, injury, disuse, and aging is associated with poor health outcomes. Therefore, it is important to pursue a greater understanding of the mechanisms that dictate skeletal muscle adaptation. In this review, we propose that RNA-binding proteins (RBPs) comprise a critical regulatory node in the orchestration of adaptive responses in skeletal muscle. While RBPs have broadly pleiotropic molecular functions, our discussion is constrained at the outset by observations from hibernating animals, which suggest that RBP regulation of RNA stability and its impact on translational reprogramming is a key component of skeletal muscle response to anabolic and catabolic stimuli. We discuss the limited data available on the expression and functions of RBPs in adult skeletal muscle in response to disuse, aging, and exercise. A model is proposed in which dynamic changes in RBPs play a central role in muscle adaptive processes through their differential effects on mRNA stability. While limited, the currently available data suggest that understanding how adaptive (and maladaptive) changes in the expression of RBPs regulate mRNA stability in skeletal muscle could be an informative and productive research area for finding new strategies to limit atrophy and promote hypertrophy.

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.

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.

Seasonal loss and resumption of circadian rhythms in hibernating arctic ground squirrels

Circadian clocks are near universal among organisms and play a key role in coordinating physiological and metabolic functions to anticipate or coincide with predictable daily changes in the physical and social environment. However, whether circadian rhythms persist and are functionally important during hibernation in all mammals is currently unclear. We examined whether circadian rhythms of body temperature (T _b) persist during multi-day, steady-state torpor and investigated the association between timing of animal emergence, exposure to light, and resumption of activity and T _b rhythms in free-living and captive male arctic ground squirrels. High-resolution (0.02 °C) temperature loggers revealed that circadian rhythms of T _b were not present during deep torpor in free-living arctic ground squirrels. Significant circadian rhythms of T _b resumed, however, following the resumption of euthermia, but prior to emergence, though rhythms became much more robust coincident with aboveground emergence. Additionally, squirrels maintained in captivity under conditions of constant darkness spontaneously developed significant circadian rhythms of T _b and activity soon after ending torpor. Exposing animals to a 5-s pulse of light within a week when they ended torpor increased the strength of rhythms. Our results are consistent with the hypothesis that circadian clock function is inhibited during hibernation in arctic ground squirrels, and we postulate that exposure to external stimuli, such as light in free-living animals, and meals or acute disturbance for captive squirrels, may enhance T _b rhythmicity by synchronizing loosely coupled circadian oscillators within the suprachiasmatic nucleus.

Transcriptome Analysis of Hypothalamic Gene Expression during Daily Torpor in Djungarian Hamsters (Phodopus sungorus)

Animals living at high or temperate latitudes are challenged by extensive changes in environmental conditions over seasons. Djungarian hamsters (Phodopus sungorus) are able to cope with extremely cold ambient temperatures and food scarcity in winter by expressing spontaneous daily torpor. Daily torpor is a circadian controlled voluntary reduction of metabolism that can reduce energy expenditure by up to 65% when used frequently. In the past decades it has become more and more apparent, that the hypothalamus is likely to play a key role in regulating induction and maintenance of daily torpor, but the molecular signals, which lead to the initiation of daily torpor, are still unknown. Here we present the first transcriptomic study of hypothalamic gene expression patterns in Djungarian hamsters during torpor entrance. Based on Illumina sequencing we were able to identify a total number of 284 differentially expressed genes, whereby 181 genes were up- and 103 genes down regulated during torpor entrance. The 20 most up regulated group contained eight genes coding for structure proteins, including five collagen genes, dnha2 and myo15a, as well as the procoagulation factor vwf. In a proximate approach we investigated these genes by quantitative real-time PCR (qPCR) analysis over the circadian cycle in torpid and normothermic animals at times of torpor entrance, mid torpor, arousal and post-torpor. These qPCR data confirmed up regulation of dnah2, myo15a, and vwf during torpor entrance, but a decreased mRNA level for all other investigated time points. This suggests that gene expression of structure genes as well as the procoagulation factor are specifically initiated during the early state of torpor and provides evidence for protective molecular adaptions in the hypothalamus of Djungarian hamsters including changes in structure, transport of biomolecules and coagulation.

Tissue-specific response of carbohydrate-responsive element binding protein (ChREBP) to mammalian hibernation in 13-lined ground squirrels

Mammalian hibernation is characterized by a general suppression of energy expensive processes and a switch to lipid oxidation as the primary fuel source. Glucose-responsive carbohydrate responsive element binding protein (ChREBP) has yet to be studied in hibernating organisms, which prepare for the cold winter months by feeding until they exhibit an obesity-like state that is accompanied by naturally-induced and completely reversible insulin resistance. Studying ChREBP expression and activity in the hibernating 13-lined ground squirrel is important to better understand the molecular mechanisms that regulate energy metabolism under cellular stress. Immunoblotting was used to determine the relative expression level and subcellular localization of ChREBP, as well as serine phosphorylation at 95 kDa, comparing euthermic and late torpid ground squirrel liver, kidney, heart and muscle. DNA-binding ELISAs and RT-PCR were used to explore ChREBP transcriptional activity during cold stress. ChREBP activity seemed generally suppressed in liver and kidney. During torpor, ChREBP total protein levels decreased to 44% of EC in liver, phosphoserine levels increased 2.1-fold of EC in kidney, and downstream Fasn/Pkl transcript levels decreased to <60% of EC in liver. By contrast, ChREBP activity generally increased during torpor in cardiac and skeletal muscle, where ChREBP total protein levels increased over 1.5-fold and 5-fold of EC in muscle and heart, respectively; where DNA-binding increased by ∼2-fold of EC in muscle; and where Fasn transcript levels increased over 3-fold and 7-fold in both muscle and heart, respectively. In summary, ChREBP has a tissue-specific role in regulating energy metabolism during hibernation.

Regulation of Torpor in the Gray Mouse Lemur: Transcriptional and Translational Controls and Role of AMPK Signaling

The gray mouse lemur (Microcebus murinus) is one of few primate species that is able to enter daily torpor or prolonged hibernation in response to environmental stresses. With an emerging significance to human health research, lemurs present an optimal model for exploring molecular adaptations that regulate primate hypometabolism. A fundamental challenge is how to effectively regulate energy expensive cellular processes (e.g., transcription and translation) during transitions to/from torpor without disrupting cellular homeostasis. One such regulatory mechanism is reversible posttranslational modification of selected protein targets that offers fine cellular control without the energetic burden. This study investigates the role of phosphorylation and/or acetylation in regulating key factors involved in energy homeostasis (AMP-activated protein kinase, or AMPK, signaling pathway), mRNA translation (eukaryotic initiation factor 2α or eIF2α, eukaryotic initiation factor 4E or eIF4E, and initiation factor 4E binding protein or 4EBP), and gene transcription (histone H3) in six tissues of torpid and aroused gray mouse lemurs. Our results indicated selective tissue-specific changes of these regulatory proteins. The relative level of Thr172-phosphorylated AMPKα was significantly elevated in the heart but reduced in brown adipose tissue during daily torpor, as compared to the aroused lemurs, implicating the regulation of AMPK activity during daily torpor in these tissues. Interestingly, the levels of the phosphorylated eIFs were largely unaltered between aroused and torpid animals. Phosphorylation and acetylation of histone H3 were examined as a marker for transcriptional regulation. Compared to the aroused lemurs, level of Ser10-phosphorylated histone H3 decreased significantly in white adipose tissue during torpor, suggesting global suppression of gene transcription. However, a significant increase in acetyl-histone H3 in the heart of torpid lemurs indicated a possible stimulation of transcriptional activity of this tissue. Overall, our study demonstrates that AMPK signaling and posttranslational regulation of selected proteins may play crucial roles in the control of transcription/translation during daily torpor in mouse lemurs.

Insight into post-transcriptional gene regulation: stress-responsive microRNAs and their role in the environmental stress survival of tolerant animals

Living animals are constantly faced with various environmental stresses that challenge normal life, including: oxygen limitation, very low or high temperature, as well as restriction of water and food. It has been well established that in response to these stresses, tolerant organisms regularly respond with a distinct suite of cellular modifications that involve transcriptional, translational and post-translational modification. In recent years, a new mechanism of rapid and reversible transcriptome regulation, via the action of non-coding RNA molecules, has emerged into post-transcriptional regulation and has since been shown to be part of the survival response. However, these RNA-based mechanisms by which tolerant organisms respond to stressed conditions are not well understood. Recent studies have begun to show that non-coding RNAs control gene expression and translation of mRNA to protein, and can also have regulatory influence over major cellular processes. For example, select microRNAs have been shown to have regulatory influence over the cell cycle, apoptosis, signal transduction, muscle atrophy and fatty acid metabolism during periods of environmental stress. As we are on the verge of dissecting the roles of non-coding RNA in environmental stress adaptation, this Commentary summarizes the hallmark alterations in microRNA expression that facilitate stress survival.

Regulation of hypometabolism: insights into epigenetic controls

For many animals, survival of severe environmental stress (e.g. to extremes of heat or cold, drought, oxygen limitation, food deprivation) is aided by entry into a hypometabolic state. Strong depression of metabolic rate, often to only 1–20% of normal resting rate, is a core survival strategy of multiple forms of hypometabolism across the animal kingdom, including hibernation, anaerobiosis, aestivation and freeze tolerance. Global biochemical controls are needed to suppress and reprioritize energy use; one such well-studied control is reversible protein phosphorylation. Recently, we turned our attention to the idea that mechanisms previously associated mainly with epigenetic regulation can also contribute to reversible suppression of gene expression in hypometabolic states. Indeed, situations as diverse as mammalian hibernation and turtle anoxia tolerance show coordinated changes in histone post-translational modifications (acetylation, phosphorylation) and activities of histone deacetylases, consistent with their use as mechanisms for suppressing gene expression during hypometabolism. Other potential mechanisms of gene silencing in hypometabolic states include altered expression of miRNAs that can provide post-transcriptional suppression of mRNA translation and the formation of ribonuclear protein bodies in the nucleus and cytoplasm to allow storage of mRNA transcripts until animals rouse themselves again. Furthermore, mechanisms first identified in epigenetic regulation (e.g. protein acetylation) are now proving to apply to many central metabolic enzymes (e.g. lactate dehydrogenase), suggesting a new layer of regulatory control that can contribute to coordinating the depression of metabolic rate.

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.

The involvement of mRNA processing factors TIA-1, TIAR, and PABP-1 during mammalian hibernation

Mammalian hibernators survive low body temperatures, ischemia-reperfusion, and restricted nutritional resources via global reductions in energy-expensive cellular processes and selective increases in stress pathways. Consequently, studies that analyze hibernation uncover mechanisms which balance metabolism and support survival by enhancing stress tolerance. We hypothesized processing factors that influence messenger ribonucleic acid (mRNA) maturation and translation may play significant roles in hibernation. We characterized the amino acid sequences of three RNA processing proteins (T cell intracellular antigen 1 (TIA-1), TIA1-related (TIAR), and poly(A)-binding proteins (PABP-1)) from thirteen-lined ground squirrels (Ictidomys tridecemlineatus), which all displayed a high degree of sequence identity with other mammals. Alternate Tia-1 and TiaR gene variants were found in the liver with higher expression of isoform b versus a in both cases. The localization of RNA-binding proteins to subnuclear structures was assessed by immunohistochemistry and confirmed by subcellular fractionation; TIA-1 was identified as a major component of subnuclear structures with up to a sevenfold increase in relative protein levels in the nucleus during hibernation. By contrast, there was no significant difference in the relative protein levels of TIARa/TIARb in the nucleus, and a decrease was observed for TIAR isoforms in cytoplasmic fractions of torpid animals. Finally, we used solubility tests to analyze the formation of reversible aggregates that are associated with TIA-1/R function during stress; a shift towards the soluble fraction (TIA-1a, TIA-1b) was observed during hibernation suggesting enhanced protein aggregation was not present during torpor. The present study identifies novel posttranscriptional regulatory mechanisms that may play a role in reducing translational rates and/or mRNA processing under unfavorable environmental conditions.

Global DNA modifications suppress transcription in brown adipose tissue during hibernation

Hibernation is crucial to winter survival for many small mammals and is characterized by prolonged periods of torpor during which strong global controls are applied to suppress energy-expensive cellular processes. We hypothesized that one strategy of energy conservation is a global reduction in gene transcription imparted by reversible modifications to DNA and to proteins involved in chromatin packing. Transcriptional regulation during hibernation was examined over euthermic control groups and five stages of the torpor/arousal cycle in brown adipose tissue of thirteen-lined ground squirrels (Ictidomys tridecemlineatus). Brown adipose is crucial to hibernation success because it is responsible for the non-shivering thermogenesis that rewarms animals during arousal. A direct modification of DNA during torpor was revealed by a 1.7-fold increase in global DNA methylation during long term torpor as compared with euthermic controls. Acetylation of histone H3 (on Lys23) was reduced by about 50% when squirrels entered torpor, which would result in increased chromatin packing (and transcriptional repression). This was accompanied by strong increases in histone deacetylase protein levels during torpor; e.g. HDAC1 and HDAC4 levels rose by 1.5- and 6-fold, respectively. Protein levels of two co-repressors of transcription, MBD1 and HP1, also increased by 1.9- and 1.5-fold, respectively, in long-term torpor and remained high during early arousal. MBD1, HP1 and HDACs all returned to near control values during interbout indicating a reversal of their inhibitory actions. Overall, the data presents strong evidence for a global suppression of transcription during torpor via the action of epigenetic regulatory mechanisms in brown adipose tissue of hibernating thirteen-lined ground squirrels.

Intrinsic circannual regulation of brown adipose tissue form and function in tune with hibernation

Winter hibernators repeatedly cycle between cold torpor and rewarming supported by nonshivering thermogenesis in brown adipose tissue (BAT). In contrast, summer animals are homeotherms, undergoing reproduction, growth, and fattening. This life history confers variability to BAT recruitment and activity. To address the components underlying prewinter enhancement and winter activation, we interrogated the BAT proteome in 13-lined ground squirrels among three summer and five winter states. We also examined mixed physiology in fall and spring individuals to test for ambient temperature and seasonal effects, as well as the timing of seasonal transitions. BAT form and function differ circannually in these animals, as evidenced by morphology and proteome dynamics. This intrinsic pattern distinguished homeothermic groups and early vs. late winter hibernators. Homeothermic variation derived from postemergence delay in growth and substrate biosynthesis. The heterothermic proteome varied less despite extreme winter physiological shifts and was optimized to exploit lipids by enhanced fatty acid binding, β-oxidation, and mitochondrial protein translocation. Surprisingly, ambient temperature did not affect the BAT proteome during transition seasons; rather, the pronounced summer-winter shift preceded environmental changes and phenotypic progression. During fall transition, differential regulation of two fatty acid binding proteins provides further evidence of recruitment and separates proteomic preparation from successful hibernation. Abundance of FABP4 correlates with torpor bout length throughout the year, clarifying its potential function in hibernation. Metabolically active BAT is a target for treating human obesity and metabolic disorders. Understanding the hibernator's extreme and seasonally distinct recruitment and activation control strategies offers untapped potential to identify novel, therapeutically relevant regulatory pathways.

Metabolic suppression during protracted exposure to hypoxia in the jumbo squid, Dosidicus gigas, living in an oxygen minimum zone

The jumbo squid, Dosidicus gigas, can survive extended forays into the oxygen minimum zone (OMZ) of the Eastern Pacific Ocean. Previous studies have demonstrated reduced oxygen consumption and a limited anaerobic contribution to ATP production, suggesting the capacity for substantial metabolic suppression during hypoxic exposure. Here we provide a more complete description of energy metabolism and explore the expression of proteins indicative of transcriptional and translational arrest that may contribute to metabolic suppression. We demonstrate a suppression of total ATP demand under hypoxic conditions (1% oxygen, PO2 = 0.8 kPa) in both juveniles (52%) and adults (35%) of the jumbo squid. Oxygen consumption rates are reduced to 20% under hypoxia relative to air-saturated controls. Concentrations of Arg-P and ATP declined initially, reaching a new steady state (~30% of controls) after the first hour of hypoxic exposure. Octopine began accumulating after the first hour of hypoxic exposure, once Arg-P breakdown resulted in sufficient free arginine for substrate. Octopine reached levels near 30 mmoles g-1 after 3.4 h of hypoxic exposure. Succinate did increase through hypoxia but contributed minimally to total ATP production. Glycogenolysis in mantle muscle presumably serves to maintain muscle functionality and balance energetics during hypoxia. We provide evidence that post-translational modifications on histone proteins and translation factors serve as a primary means of energy conservation and that select components of the stress response are altered in hypoxic squids. Reduced ATP consumption under hypoxia serves to maintain ATP levels, prolong fuel store use and minimize the accumulation of acidic intermediates of anaerobic ATP generating pathways during prolonged diel forays into the OMZ. Metabolic suppression likely limits active, daytime foraging at depth in the core of the OMZ, but confers an energetic advantage over competitors that must remain in warm, oxygenated surface waters. Moreover, capacity for metabolic suppression provides habitat flexibility as oxygen minimum zones expand due to climate change.

The impact of cold acclimation and hibernation on antioxidant defenses in the ground squirrel (Spermophilus citellus): an update

Any alteration in oxidative metabolism is coupled with a corresponding response by an antioxidant defense (AD) in appropriate subcellular compartments. Seasonal hibernators pass through circannual metabolic adaptations that allow them to either maintain euthermy (cold acclimation) or enter winter torpor with body temperature falling to low values. The present study aimed to investigate the corresponding pattern of AD enzyme protein expressions associated with these strategies in the main tissues involved in whole animal energy homeostasis: brown and white adipose tissues (BAT and WAT, respectively), liver, and skeletal muscle. European ground squirrels (Spermophilus citellus) were exposed to low temperature (4 ± 1 °C) and then divided into two groups: (1) animals fell into torpor (hibernating group) and (2) animals stayed active and euthermic for 1, 3, 7, 12, or 21 days (cold-exposed group). We examined the effects of cold acclimation and hibernation on the tissue-dependent protein expression of four enzymes which catalyze the two-step detoxification of superoxide to water: superoxide dismutase 1 and 2 (SOD 1 and 2), catalase (CAT), and glutathione peroxidase (GSH-Px). The results showed that hibernation induced an increase of AD enzyme protein expressions in BAT and skeletal muscle. However, AD enzyme contents in liver were largely unaffected during torpor. Under these conditions, different WAT depots responded by elevating the amounts of specific enzymes, as follows: SOD 1 in retroperitoneal WAT, GSH-Px in gonadal WAT, and CAT in subcutaneous WAT. Similar perturbations of AD enzymes contents were seen in all tissues during cold acclimation, often in a time-dependent manner. It can be concluded that BAT and muscle AD capacity undergo the most dramatic changes during both cold acclimation and hibernation, while liver is relatively unaffected by either condition. Additionally, this study provides a basis for further metabolic study that will illuminate the causes of these tissue-specific AD responses, particularly the novel finding of distinct responses by different WAT depots in hibernators.

Hibernating above the permafrost: effects of ambient temperature and season on expression of metabolic genes in liver and brown adipose tissue of arctic ground squirrels

Hibernating arctic ground squirrels (Urocitellus parryii), overwintering in frozen soils, maintain large gradients between ambient temperature (T(a)) and body temperature (T(b)) by substantially increasing metabolic rate during torpor while maintaining a subzero T(b). We used quantitative reverse-transcription PCR (qRT-PCR) to determine how the expression of 56 metabolic genes was affected by season (active in summer vs hibernating), metabolic load during torpor (imposed by differences in T(a): +2 vs -10°C) and hibernation state (torpid vs after arousal). Compared with active ground squirrels sampled in summer, liver from hibernators showed increased expression of genes associated with fatty acid catabolism (CPT1A, FABP1 and ACAT1), ketogenesis (HMGCS2) and gluconeogenesis (PCK1) and decreased expression of genes associated with fatty acid synthesis (ACACB, SCD and ELOVL6), amino acid metabolism, the urea cycle (PAH, BCKDHA and OTC), glycolysis (PDK1 and PFKM) and lipid metabolism (ACAT2). Stage of hibernation (torpid vs aroused) had a much smaller effect, with only one gene associated with glycogen synthesis (GSY1) in liver showing consistent differences in expression levels between temperature treatments. Despite the more than eightfold increase in energetic demand associated with defending T(b) during torpor at a T(a) of -10 vs +2°C, transcript levels in liver and brown adipose tissue differed little. Our results are inconsistent with a hypothesized switch to use of non-lipid fuels when ambient temperatures drop below freezing.

Heart-type fatty acid-binding protein is essential for efficient brown adipose tissue fatty acid oxidation and cold tolerance

Brown adipose tissue has a central role in thermogenesis to maintain body temperature through energy dissipation in small mammals and has recently been verified to function in adult humans as well. Here, we demonstrate that the heart-type fatty acid-binding protein, FABP3, is essential for cold tolerance and efficient fatty acid oxidation in mouse brown adipose tissue, despite the abundant expression of adipose-type fatty acid-binding protein, FABP4 (also known as aP2). Fabp3(-/-) mice exhibit extreme cold sensitivity despite induction of uncoupling and oxidative genes and hydrolysis of brown adipose tissue lipid stores. However, using FABP3 gain- and loss-of-function approaches in brown adipocytes, we detected a correlation between FABP3 levels and the utilization of exogenous fatty acids. Thus, Fabp3(-/-) brown adipocytes fail to oxidize exogenously supplied fatty acids, whereas enhanced Fabp3 expression promotes more efficient oxidation. These results suggest that FABP3 levels are a determinant of fatty acid oxidation efficiency by brown adipose tissue and that FABP3 represents a potential target for modulation of energy dissipation.

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.

Transcriptome survey of the anhydrobiotic tardigrade Milnesium tardigradum in comparison with Hypsibius dujardini and Richtersius coronifer

Background

The phenomenon of desiccation tolerance, also called anhydrobiosis, involves the ability of an organism to survive the loss of almost all cellular water without sustaining irreversible damage. Although there are several physiological, morphological and ecological studies on tardigrades, only limited DNA sequence information is available. Therefore, we explored the transcriptome in the active and anhydrobiotic state of the tardigrade Milnesium tardigradum which has extraordinary tolerance to desiccation and freezing. In this study, we present the first overview of the transcriptome of M. tardigradum and its response to desiccation and discuss potential parallels to stress responses in other organisms.

Results

We sequenced a total of 9984 expressed sequence tags (ESTs) from two cDNA libraries from the eutardigrade M. tardigradum in its active and inactive, anhydrobiotic (tun) stage. Assembly of these ESTs resulted in 3283 putative unique transcripts, whereof ~50% showed significant sequence similarity to known genes. The resulting unigenes were functionally annotated using the Gene Ontology (GO) vocabulary. A GO term enrichment analysis revealed several GOs that were significantly underrepresented in the inactive stage. Furthermore we compared the putative unigenes of M. tardigradum with ESTs from two other eutardigrade species that are available from public sequence databases, namely Richtersius coronifer and Hypsibius dujardini. The processed sequences of the three tardigrade species revealed similar functional content and the M. tardigradum dataset contained additional sequences from tardigrades not present in the other two.

Conclusions

This study describes novel sequence data from the tardigrade M. tardigradum, which significantly contributes to the available tardigrade sequence data and will help to establish this extraordinary tardigrade as a model for studying anhydrobiosis. Functional comparison of active and anhydrobiotic tardigrades revealed a differential distribution of Gene Ontology terms associated with chromatin structure and the translation machinery, which are underrepresented in the inactive animals. These findings imply a widespread metabolic response of the animals on dehydration. The collective tardigrade transcriptome data will serve as a reference for further studies and support the identification and characterization of genes involved in the anhydrobiotic response.

Identification of qRT-PCR reference genes for analysis of opioid gene expression in a hibernator

Previous work has suggested that central and peripheral opioid signaling are involved in regulating torpor behavior and tissue protection associated with the hibernation phenotype. We used quantitative real-time PCR (qRT-PCR) to measure mRNA levels of opioid peptide precursors and receptors in the brain and heart of summer ground squirrels (Ictidomys tridecemlineatus) and winter hibernating squirrels in the torpid or interbout arousal states. The use of appropriate reference genes for normalization of qRT-PCR gene expression data can have profound effects on the analysis and interpretation of results. This may be particularly important when experimental subjects, such as hibernating animals, undergo significant morphological and/or functional changes during the study. Therefore, an additional goal of this study was to identify stable reference genes for use in qRT-PCR studies of the 13-lined ground squirrel. Expression levels of 10 potential reference genes were measured in the small intestine, liver, brain, and heart, and the optimal combinations of the most stable reference genes were identified by the GeNorm Excel applet. Based on this analysis, we provide recommendations for reference genes to use in each tissue that would be suitable for comparative studies among different activity states. When appropriate normalization of mRNA levels was used, there were no changes in opioid-related genes in heart among the three activity states; in brain, DOR expression was highest during torpor, lowest in interbout arousal and intermediate in summer. The results support the idea that changes in DOR expression may regulate the level of neuronal activity in brain during the annual hibernation cycle and may contribute to hibernation-associated tissue protection.

Cryopreservation: an optimizing factor for therapeutic potential of fetoplacental complex products

The therapeutic potential of cryopreserved fetoplacental complex cells (FPCP) has been explored for the treatment of autoimmune diseases, including autoimmune hemolytical anemia (AIHA). In this study, a murine AIHA model was utilized to investigate the application of cryopreserved fetal liver cells (cFLC), from varying gestation intervals (14 and 19 postcoital days) to correct the functional state of the lymphohemopoietic complex (LHPC). The results demonstrate that cryopreservation had little negative effect on FLC-14 corrective ability from that of nonfrozen FLC-14. Cryopreservation of FLC-19 (later gestation period) resulted in an increase in functional corrective potential over that of matched controls. Further, the increase in corrective effect of the cFLC-19 was found to be similar to that of both FLC-14 populations. In summary, these results demonstrate the potential for the use of cryopreserved FLCs as a therapeutic option for the treatment of AIHA.

Coping with the stress: expression of ATF4, ATF6, and downstream targets in organs of hibernating ground squirrels

Perturbation of the endoplasmic reticulum (ER) protein folding apparatus via any one of several environmental or metabolic stresses rapidly triggers a complex program of cellular responses that is termed the unfolded protein response (UPR). Stresses that trigger this response in mammals can include low temperature, hypoxia, ischemia, and oxidative stress. All of these can be natural features of mammalian hibernation, and hence the UPR might be integral to long term survival in a state of cold torpor. The present study analyzes changes in gene and/or protein expression of multiple markers of the UPR in tissues of euthermic (control) versus hibernating ground squirrels, Spermophilus tridecemlineatus. Immunoblot analysis of ATF4 protein expression revealed strong increases of 1.9- to 2.5-fold in brown adipose tissue, skeletal muscle, and brain during hibernation. However, transcript levels of atf4 were unchanged or lowered which suggests that ATF4 protein levels were regulated at the translational level. Subcellular localization studies showed that ATF4 translocated into the nucleus during hibernation, as did its cofactor, the phosphorylated form of CREB-1, which rose by 25- to 39-fold in nuclear extracts of brain and skeletal muscle of torpid animals. The responses of other proteins involved in the UPR including p-PERK, ATF6, GADD153, and GADD34 were also evaluated. The data suggest that ATF4 up-regulation may play an important role in coordinating gene expression responses that support the hibernating phenotype.

Tribute to P. L. Lutz: putting life on 'pause'--molecular regulation of hypometabolism

Entry into a hypometabolic state is an important survival strategy for many organisms when challenged by environmental stress, including low oxygen, cold temperatures and lack of food or water. The molecular mechanisms that regulate transitions to and from hypometabolic states, and stabilize long-term viability during dormancy, are proving to be highly conserved across phylogenic lines. A number of these mechanisms were identified and explored using anoxia-tolerant turtles as the model system, particularly from the research contributions made by Dr Peter L. Lutz in his explorations of the mechanisms of neuronal suppression in anoxic brain. Here we review some recent advances in understanding the biochemical mechanisms of metabolic arrest with a focus on ideas such as the strategies used to reorganize metabolic priorities for ATP expenditure, molecular controls that suppress cell functions (e.g. ion pumping, transcription, translation, cell cycle arrest), changes in gene expression that support hypometabolism, and enhancement of defense mechanisms (e.g. antioxidants, chaperone proteins, protease inhibitors) that stabilize macromolecules and promote long-term viability in the hypometabolic state.

Perichromatin fibrils as early markers of transcriptional alterations

Perichromatin fibrils represent the morphological expression of transcription and co-transcriptional processing of pre-mRNA. They can be considered, hence, an example of work in progress. High resolution techniques such as electron microscopy demonstrate that perichromatin fibrils play a role as early markers of transcriptional alterations. In this paper, we review some experimental and physiological conditions impairing or modulating transcription as well as their effects on perichromatin fibrils. In all the situations reported, perichromatin fibrils show modifications in their amount and/or their associated proteins. Their movements are also affected, as well as their export or their intra-nuclear storage forms. Perichromatin fibrils therefore represent highly sensitive markers not only for monitoring transcriptional and processing rate but also for identifying the maturation level of pre-mRNA/mRNA occurring in the cell nucleus and the functional correlation with the cellular metabolic state.

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.

p38MAPK regulation of transcription factor targets in muscle and heart of the hibernating bat,Myotis lucifugus

Mammalian hibernation combines a profound net metabolic rate suppression with the selective up-regulation of key genes whose protein products address specific metabolic needs of the hibernator. The signal transduction pathways and transcription factors involved in regulating hibernation-responsive gene expression are of great interest. The present study suggests an important role for the p38 mitogen-activated protein kinase (p38 MAPK) and selected downstream transcription factors under its control (CREB, ATF-2, Elk-1) in the metabolic response by skeletal muscle during hibernation of little brown bats, Myotis lucifugus. Western blotting was used to quantify both total protein and levels of the phosphorylated, active forms of p38 MAPK, CREB, ATF-2 and Elk-1 in both skeletal muscle and heart of euthermic and hibernating bats. The p38 MAPK pathway was not apparently activated in heart during torpor but skeletal muscle showed strong increases (2.2-11-fold) in the amounts of phosphorylated p38 T180/Y182, CREB S133, ATF-2T69/71 and Elk-1(S383) in the torpid versus aroused state. By contrast both total and phosphorylated levels of Elk-1 in heart were reduced during hibernation to just 30% of the euthermic values. These data implicate p38 MAPK and its transcription factor targets, CREB, ATF-2 and Elk-1 in skeletal muscle maintenance during hibernation.

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.

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.

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.

Maintenance of slow type I myosin protein and mRNA expression in overwintering prairie dogs (Cynomys leucurus and ludovicianus) and black bears (Ursus americanus)

Hibernating mammals have the remarkable ability to withstand long periods of fasting and reduced activity with dramatic maintenance of skeletal muscle function and protein composition. We investigated several hindlimb muscles of white-tailed prairie dogs (Cynomys leucurus) and black bears (Ursus americanus), two very different hibernators who are dormant and fasting during winter. The black-tailed prairie dog (C. ludovicianus) remains active during winter, but suffers minor skeletal muscle atrophy; nevertheless, they also demonstrate apparent skeletal muscle adaptations. Using SDS-PAGE, we measured myosin protein isoform profiles before and after the hibernation season. All species maintained or increased levels of slow myosin, despite the collective physiological challenges of hypophagia and reduced activity. This contrasts markedly with standard mammalian models of skeletal muscle inactivity and atrophy predicting significant loss of slow myosin. A mechanism for changes in myosin isoforms was investigated using reverse-transcription PCR, following partial sequencing of the adult MHC isoforms in C. leucurus and U. americanus. However, mRNA expression was not well correlated with changes in MHC protein isoforms, and other synthesis and degradation pathways may be involved besides transcriptional control. The muscles of hibernating mammals demonstrate surprising and varied physiological responses to inactivity and atrophy with respect to slow MHC expression.

Gene hunting in hypoxia and exercise

New technologies in genomics and proteomics are revolutionizing the study of adaptation to environmental stress. These approaches provide a comprehensive overview of the responses of thousands of genes/proteins to stress and enormously expand our view of the molecular and metabolic changes that underlie physiological responses. Several new technologies can help physiological labs to become gene hunters. DNA array screening is particularly effective for two purposes: (1) identifying coordinated responses by functional groups of gene/proteins such as multiple members of a signal transduction cascade or enzymes of a metabolic pathway, and (2) highlighting cell functions that have never before been linked with the stress under consideration. We have shown that heterologous screening of DNA arrays can be a highly effective method of gene hunting for the comparative biochemist provided that it is followed up by species-specific analyses including PCR to quantify transcript levels and Western blotting to analyze protein responses. Recent work in my lab has used cDNA array screening to evaluate responses to low oxygen by multiple hypoxia/anoxia tolerant systems, revealing common gene responses across phylogeny. Analysis of vertebrate facultative anaerobiosis in freshwater turtles reveals an interesting mixture of gene responses, including up-regulation of antioxidant enzymes, protease inhibitors, and proteins of iron metabolism; a few of these are coordinated by the hypoxia inducible factor in other systems but most are not. Array screening is also providing new insights into how exercise stimulates the growth of differentiated muscle cells and studies in our lab are identifying the gene responses associated with "anti-exercise"--gene up-regulation that aids hibernating mammals to maintain their muscle mass despite months of inactivity.

Partial gene deletion of heart-type fatty acid-binding protein limits the severity of dietary-induced insulin resistance

The aim of this study was to determine the contribution of heart-type fatty acid-binding protein (H-FABP) to glucose and long-chain fatty acid (LCFA) utilization in dietary-induced insulin resistance. We tested the hypothesis that H-FABP facilitates increases in LCFA flux present in glucose-intolerant states and that a partial reduction in the amount of this protein would compensate for all or part of the impairment. Transgenic H-FABP heterozygotes (HET) and wild-type (WT) littermates were studied following chow diet (CHD) or high-fat diet (HFD) for 12 weeks. Catheters were surgically implanted in the carotid artery and jugular vein for sampling and infusions, respectively. Following 5 days of recovery, mice received either a saline infusion or underwent a euglycemic insulin clamp (4 mU x kg(-1) x min(-1)) for 120 min. At 90 min, a bolus of 2-deoxyglucose and [125I]-15-(rho-iodophenyl)-3-R,S-methylpentadecanoic acid were administered to obtain indexes of glucose and LCFA utilization. At 120 min, skeletal muscles were excised for tracer determination. All HFD mice were obese and hyperinsulinemic; however, only HFD-WT mice were hyperglycemic. Glucose infusion rates during insulin clamps were 49 +/- 4, 59 +/- 4, 16 +/- 4, and 33 +/- 4 mg x kg(-1) x min(-1) for CHD-WT, CHD-HET, HFD-WT, and HFD-HET mice, respectively, showing that HET limited the severity of whole-body insulin resistance with HFD. Insulin-stimulated muscle glucose utilization was attenuated in HFD-WT but unaffected in HFD-HET mice. Conversely, rates of LCFA clearance were increased with HFD feeding in HFD-WT but not in HFD-HET mice. In conclusion, a partial reduction in H-FABP protein normalizes fasting glucose levels and improves whole-body insulin sensitivity in HFD-fed mice despite obesity.

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.

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.