Expression of a chimeric retroviral-lipase mRNA confers enhanced lipolysis in a hibernating mammal

Mass spectrometry of the white adipose metabolome in a hibernating mammal reveals seasonal changes in alternate fuels and carnitine derivatives

Mammalian hibernators undergo substantial changes in metabolic function throughout the seasonal hibernation cycle. We report here the polar metabolomic profile of white adipose tissue isolated from active and hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus). Polar compounds in white adipose tissue were extracted from five groups representing different timepoints throughout the seasonal activity-torpor cycle and analyzed using hydrophilic interaction liquid chromatography-mass spectrometry in both the positive and negative ion modes. A total of 224 compounds out of 660 features detected after curation were annotated. Unsupervised clustering using principal component analysis revealed discrete clusters representing the different seasonal timepoints throughout hibernation. One-way analysis of variance and feature intensity heatmaps revealed metabolites that varied in abundance between active and torpid timepoints. Pathway analysis compared against the KEGG database demonstrated enrichment of amino acid metabolism, purine metabolism, glycerophospholipid metabolism, and coenzyme A biosynthetic pathways among our identified compounds. Numerous carnitine derivatives and a ketone that serves as an alternate fuel source, beta-hydroxybutyrate (BHB), were among molecules found to be elevated during torpor. Elevated levels of the BHB-carnitine conjugate during torpor suggests the synthesis of beta-hydroxybutyrate in white adipose mitochondria, which may contribute directly to elevated levels of circulating BHB during hibernation.

Phenotypic changes in the metabolic profile and adiponectin activity during seasonal fattening and hibernation in female Daurian ground squirrels (Spermophilus dauricus)

Seasonal hibernation has provided an opportunity to study animals' phenotypic plasticity in adaptation to changing environment. In the present study focusing on the female Daurian ground squirrel (Spermophilus dauricus)-a well demonstrated seasonal hibernator-we examined their behavioral, morphological, and metabolic changes during fattening, hibernation, and emergence. Our data indicated high levels of food intake, fat deposition, and body mass increases during fattening compared to hibernation. The levels of serum glucose and triglycerides were also higher during fattening than during hibernation and emergence. Interestingly, although squirrels showed signs of obesity and elevated triglycerides in serum during fattening, triglyceride levels in the liver and skeletal muscles remained unchanged. Our data also indicated that adiponectin levels in serum and cerebrospinal fluid were different between fattening and hibernation. Levels of adiponectin receptor 1 in the skeletal muscle remained low during fattening but peaked in late hibernation. In contrast, adiponectin receptor 2 in the liver showed a steady increase during fattening, which was followed by a significant decrease at early hibernation. Our data indicate that adiponectin may play an important role in preventing heterotopic fat accumulation in a receptor- and organ-specific manner, as well as in facilitating the switch from glucose metabolism to lipid metabolism during fattening and hibernation in female Daurian ground squirrels.

Life in the fat lane: seasonal regulation of insulin sensitivity, food intake, and adipose biology in brown bears

Grizzly bears (Ursus arctos horribilis) have evolved remarkable metabolic adaptations including enormous fat accumulation during the active season followed by fasting during hibernation. However, these fluctuations in body mass do not cause the same harmful effects associated with obesity in humans. To better understand these seasonal transitions, we performed insulin and glucose tolerance tests in captive grizzly bears, characterized the annual profiles of circulating adipokines, and tested the anorectic effects of centrally administered leptin at different times of the year. We also used bear gluteal adipocyte cultures to test insulin and beta-adrenergic sensitivity in vitro. Bears were insulin resistant during hibernation but were sensitive during the spring and fall active periods. Hibernating bears remained euglycemic, possibly due to hyperinsulinemia and hyperglucagonemia. Adipokine concentrations were relatively low throughout the active season but peaked in mid-October prior to hibernation when fat content was greatest. Serum glycerol was highest during hibernation, indicating ongoing lipolysis. Centrally administered leptin reduced food intake in October, but not in August, revealing seasonal variation in the brain's sensitivity to its anorectic effects. This was supported by strong phosphorylated signal transducer and activator of transcription 3 labeling within the hypothalamus of hibernating bears; labeling virtually disappeared in active bears. Adipocytes collected during hibernation were insulin resistant when cultured with hibernation serum but became sensitive when cultured with active season serum. Heat treatment of active serum blocked much of this action. Clarifying the cellular mechanisms responsible for the physiology of hibernating bears may inform new treatments for metabolic disorders.

Gene Expression Profiling in the Hibernating Primate, Cheirogaleus Medius

Hibernation is a complex physiological response that some mammalian species employ to evade energetic demands. Previous work in mammalian hibernators suggests that hibernation is activated not by a set of genes unique to hibernators, but by differential expression of genes that are present in all mammals. This question of universal genetic mechanisms requires further investigation and can only be tested through additional investigations of phylogenetically dispersed species. To explore this question, we use RNA-Seq to investigate gene expression dynamics as they relate to the varying physiological states experienced throughout the year in a group of primate hibernators-Madagascar's dwarf lemurs (genus Cheirogaleus). In a novel experimental approach, we use longitudinal sampling of biological tissues as a method for capturing gene expression profiles from the same individuals throughout their annual hibernation cycle. We identify 90 candidate genes that have variable expression patterns when comparing two active states (Active 1 and Active 2) with a torpor state. These include genes that are involved in metabolic pathways, feeding behavior, and circadian rhythms, as might be expected to correlate with seasonal physiological state changes. The identified genes appear to be critical for maintaining the health of an animal that undergoes prolonged periods of metabolic depression concurrent with the hibernation phenotype. By focusing on these differentially expressed genes in dwarf lemurs, we compare gene expression patterns in previously studied mammalian hibernators. Additionally, by employing evolutionary rate analysis, we find that hibernation-related genes do not evolve under positive selection in hibernating species relative to nonhibernators.

Comparative genomics of mammalian hibernators using gene networks

In recent years, the study of the molecular processes involved in mammalian hibernation has shifted from investigating a few carefully selected candidate genes to large-scale analysis of differential gene expression. The availability of high-throughput data provides an unprecedented opportunity to ask whether phylogenetically distant species show similar mechanisms of genetic control, and how these relate to particular genes and pathways involved in the hibernation phenotype. In order to address these questions, we compare 11 datasets of differentially expressed (DE) genes from two ground squirrel species, one bat species, and the American black bear, as well as a list of genes extracted from the literature that previously have been correlated with the drastic physiological changes associated with hibernation. We identify several genes that are DE in different species, indicating either ancestral adaptations or evolutionary convergence. When we use a network approach to expand the original datasets of DE genes to large gene networks using available interactome data, a higher agreement between datasets is achieved. This indicates that the same key pathways are important for activating and maintaining the hibernation phenotype. Functional-term-enrichment analysis identifies several important metabolic and mitochondrial processes that are critical for hibernation, such as fatty acid beta-oxidation and mitochondrial transport. We do not detect any enrichment of positive selection signatures in the coding sequences of genes from the networks of hibernation-associated genes, supporting the hypothesis that the genetic processes shaping the hibernation phenotype are driven primarily by changes in gene regulation.

Transcriptomic analysis of brown adipose tissue across the physiological extremes of natural hibernation

We used RNAseq to generate a comprehensive transcriptome of Brown Adipose Tissue (BAT) over the course of a year in the naturally hibernating thirteen-lined ground squirrel, Ictidomys tridecemlineatus. During hibernation ground squirrels do not feed and use fat stored in White Adipose Tissue (WAT) as their primary source of fuel. Stored lipid is consumed at high rates by BAT to generate heat at specific points during the hibernation season. The highest rate of BAT activity occurs during periodic arousals from hypothermic torpor bouts, referred to as Interbout Arousals (IBAs). IBAs are characterized by whole body re-warming (from 5 to 37 °C) in 2-3 hours, and provide a unique opportunity to determine the genes responsible for the highly efficient lipid oxidation and heat generation that drives the arousal process. Illumina HighSeq sequencing identified 14,573 distinct BAT mRNAs and quantified their levels at four points: active ground squirrels in April and October, and hibernating animals during both torpor and IBA. Based on significant changes in mRNA levels across the four collection points, 2,083 genes were shown to be differentially expressed. In addition to providing detail on the expression of nuclear genes encoding mitochondrial proteins, and genes involved in beta-adrenergic and lipolytic pathways, we identified differentially expressed genes encoding various transcription factors and other regulatory proteins which may play critical roles in high efficiency fat catabolism, non-shivering thermogenesis, and transitions into and out of the torpid state.

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.

Metabolic hormone FGF21 is induced in ground squirrels during hibernation but its overexpression is not sufficient to cause torpor

Hibernation is a natural adaptation that allows certain mammals to survive physiological extremes that are lethal to humans. Near freezing body temperatures, heart rates of 3-10 beats per minute, absence of food consumption, and depressed metabolism are characteristic of hibernation torpor bouts that are periodically interrupted by brief interbout arousals (IBAs). The molecular basis of torpor induction is unknown, however starved mice overexpressing the metabolic hormone fibroblast growth factor 21 (FGF21) promote fat utilization, reduce body temperature, and readily enter torpor-all hallmarks of mammalian hibernation. In this study we cloned FGF21 from the naturally hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and found that levels of FGF21 mRNA in liver and FGF21 protein in serum are elevated during hibernation torpor bouts and significantly elevated during IBAs compared to summer active animals. The effects of artificially elevating circulating FGF21 concentrations 50 to 100-fold via adenoviral-mediated overexpression were examined at three different times of the year. This is the first time that a transgenic approach has been used in a natural hibernator to examine mechanistic aspects of hibernation. Surgically implanted transmitters measured various metrics of the hibernation phenotype over a 7-day period including changes in motor activity, heart rate and core body temperature. In April fed-state animals, FGF21 overexpression decreased blood insulin and free fatty acid concentrations, effects similar to those seen in obese mice. However, elevated FGF21 concentrations did not cause torpor in these fed-state animals nor did they cause torpor or affect metabolic parameters in fasted-state animals in March/April, August or October. We conclude that FGF21 is strongly regulated during torpor and IBA but that its overexpression is not sufficient to cause torpor in naturally hibernating ground squirrels.

Deep sequencing the transcriptome reveals seasonal adaptive mechanisms in a hibernating mammal

Mammalian hibernation is a complex phenotype involving metabolic rate reduction, bradycardia, profound hypothermia, and a reliance on stored fat that allows the animal to survive for months without food in a state of suspended animation. To determine the genes responsible for this phenotype in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) we used the Roche 454 platform to sequence mRNA isolated at six points throughout the year from three key tissues: heart, skeletal muscle, and white adipose tissue (WAT). Deep sequencing generated approximately 3.7 million cDNA reads from 18 samples (6 time points ×3 tissues) with a mean read length of 335 bases. Of these, 3,125,337 reads were assembled into 140,703 contigs. Approximately 90% of all sequences were matched to proteins in the human UniProt database. The total number of distinct human proteins matched by ground squirrel transcripts was 13,637 for heart, 12,496 for skeletal muscle, and 14,351 for WAT. Extensive mitochondrial RNA sequences enabled a novel approach of using the transcriptome to construct the complete mitochondrial genome for I. tridecemlineatus. Seasonal and activity-specific changes in mRNA levels that met our stringent false discovery rate cutoff (1.0 × 10(-11)) were used to identify patterns of gene expression involving various aspects of the hibernation phenotype. Among these patterns are differentially expressed genes encoding heart proteins AT1A1, NAC1 and RYR2 controlling ion transport required for contraction and relaxation at low body temperatures. Abundant RNAs in skeletal muscle coding ubiquitin pathway proteins ASB2, UBC and DDB1 peak in October, suggesting an increase in muscle proteolysis. Finally, genes in WAT that encode proteins involved in lipogenesis (ACOD, FABP4) are highly expressed in August, but gradually decline in expression during the seasonal transition to lipolysis.

Torpor induction in mammals: recent discoveries fueling new ideas

When faced with a harsh climate or inadequate food, some mammals enter a state of suspended animation known as torpor. A major goal of torpor research is to determine mechanisms that integrate environmental cues, gene expression and metabolism to produce periods of torpor lasting from hours to weeks. Recent discoveries spanning the Metazoa suggest that sirtuins, the mammalian circadian clock, fibroblast growth factor 21 (FGF21) and lipids are involved in torpor induction. For example, sirtuins link cellular energy status to the mammalian circadian clock, oxidative stress and metabolic fuel selection. In this review, we discuss how these recent discoveries form a new hypothesis linking changes in the physical environment with changes in the expression of genes that regulate torpor induction.

A role for nuclear receptors in mammalian hibernation

Hibernation is one of the most dramatic examples of phenotypic plasticity in mammals. During periods of food shortage and/or reduced ambient temperatures hibernating mammals become heterothermic, allowing their body temperature to decrease while entering an energy-conserving torpid state. In order to survive the multi-month hibernation season many species engage in hyperphagy, dramatically increasing adipose stores prior to the onset of hibernation. Nuclear receptors are a superfamily of transcription factors many of which bind lipophilic molecules as ligands. They regulate a variety of processes including energy homeostasis, carbohydrate and lipid metabolism, inflammation and circadian rhythm. Given that lipids are integral in the hibernation phenotype they may play important regulatory roles through their interactions with nuclear receptors. Here we review current knowledge and suggest possible roles in mammalian hibernation for peroxisome proliferator-activated receptors (PPARs), farnesoid X receptors (FXRs), liver X receptors (LXRs), retinoid-related orphan receptors (RORs) and Rev-ERBs.

Endocrine regulation of the fasting response by PPARalpha-mediated induction of fibroblast growth factor 21

Peroxisome proliferator-activated receptor alpha (PPARalpha) regulates the utilization of fat as an energy source during starvation and is the molecular target for the fibrate dyslipidemia drugs. Here, we identify the endocrine hormone fibroblast growth factor 21 (FGF21) as a mediator of the pleiotropic actions of PPARalpha. FGF21 is induced directly by PPARalpha in liver in response to fasting and PPARalpha agonists. FGF21 in turn stimulates lipolysis in white adipose tissue and ketogenesis in liver. FGF21 also reduces physical activity and promotes torpor, a short-term hibernation-like state of regulated hypothermia that conserves energy. These findings demonstrate an unexpected role for the PPARalpha-FGF21 endocrine signaling pathway in regulating diverse metabolic and behavioral aspects of the adaptive response to starvation.

Brain energy metabolism and neurotransmission at near-freezing temperatures: in vivo (1)H MRS study of a hibernating mammal

The brain of a hibernating mammal withstands physiological extremes that would result in cerebral damage and death in a non-hibernating species such as humans. To examine the possibility that this neuroprotection results from alterations in cerebral metabolism, we used in vivo(1)H NMR spectroscopy at high field (9.4 T) to measure the concentration of 18 metabolites (neurochemical profile) in the brain of 13-lined ground squirrels (Spermophilus tridecemlineatus) before, during, and after hibernation. Resolved in vivo(1)H NMR spectra were obtained even at low temperature in torpid hibernators ( approximately 7 degrees C). The phosphocreatine-to-creatine ratio was increased during torpor (+143%) indicating energy storage, and remained increased to a lesser extent during interbout arousal (IBA) (+83%). The total gamma-aminobutyric acid concentration was increased during torpor (+135%) and quickly returned to baseline during IBA. Glutamine (Gln) was decreased (-54%) during torpor but quickly returned to normal levels during IBA and after terminal arousal in the spring. Glutamate (Glu) was also decreased during torpor (-17%), but remained decreased during IBA (-20% compared with fall), and returned to normal level in the spring. Our observation that Glu and Gln levels are depressed in the brain of hibernators suggests that the balance between anaplerosis and loss of Glu and Gln (because of glutamatergic neurotransmission or other mechanisms) is altered in hibernation.

Advances in molecular biology of hibernation in mammals

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

Detection of differential gene expression in brown adipose tissue of hibernating arctic ground squirrels with mouse microarrays

Hibernation is an energy-saving strategy adopted by a wide range of mammals to survive highly seasonal or unpredictable environments. Arctic ground squirrels living in Alaska provide an extreme example, with 6- to 9-mo-long hibernation seasons when body temperature alternates between levels near 0 degrees C during torpor and 37 degrees C during arousal episodes. Heat production during hibernation is provided, in part, by nonshivering thermogenesis that occurs in large deposits of brown adipose tissue (BAT). BAT is active at tissue temperatures from 0 to 37 degrees C during rewarming and continuously at near 0 degrees C during torpor in subfreezing conditions. Despite its crucial role in hibernation, the global gene expression patterns in BAT during hibernation compared with the nonhibernation season remain largely unknown. We report a large-scale study of differential gene expression in BAT between winter hibernating and summer active arctic ground squirrels using mouse microarrays. Selected differentially expressed genes identified on the arrays were validated by quantitative real-time PCR using ground squirrel specific primers. Our results show that the mRNA levels of the genes involved in nearly every step of the biochemical pathway leading to nonshivering thermogenesis are significantly increased in BAT during hibernation, whereas those of genes involved in protein biosynthesis are significantly decreased compared with summer active animals in August. Surprisingly, the differentially expressed genes also include adipocyte differentiation-related protein or adipophilin (Adfp), gap junction protein 1 (Gja1), and secreted protein acidic and cysteine-rich (Sparc), which may play a role in enhancing thermogenesis at low tissue temperatures in BAT.

Characterization of the human CD5 endogenous retrovirus-E in B lymphocytes

All T lymphocytes and some B lymphocytes express CD5. This coreceptor is encoded by one gene that consists of 11 exons. We have previously described a B cell-specific alternative exon 1, leading to the synthesis of a protein, devoid of leader peptide, and, therefore, retained in the cytoplasm. The novel exon 1 originates from a human endogenous retrovirus (HERV) at a time interval between the divergence of New World monkeys from Old World monkeys, and prior to the divergence of humans from Old World monkeys. Based on sequence similarity to gamma-retroviruses, it was categorized as class I: based on the specificity of its primer binding site, it was allotted to the subclass E, and based on its location within the cd5 gene, named HERV-E.CD5. Alternative transcripts were detected in lymphoid organs including fetal liver (not adult liver), more particularly in CD5-negative cell surface B-1b than in CD5-positive cell surface B-1a, and not at all in B-2 cells. By alignment of 5' long terminal repeats, HERV-E.CD5 was distinguished from similar proviruses. This could be central to the regulation of membrane expression of CD5 in human B lymphocytes, and, thereby, to the strength of the B-cell antigen receptor signaling.

Digital transcriptome analysis indicates adaptive mechanisms in the heart of a hibernating mammal

Survival of near-freezing body temperatures and reduced blood flow during hibernation is likely the result of changes in the expression of specific genes. In this study, we described a comprehensive survey of mRNAs in the heart of the thirteen-lined ground squirrel ( Spermophilus tridecemlineatus) before and during hibernation. The heart was chosen for this study because it is a contractile organ that must continue to work despite body temperatures of 5°C and the lack of food for periods of 5–6 mo. We used a digital gene expression assay involving high-throughput sequencing of directional cDNA libraries from hearts of active and hibernating ground squirrels to determine the identity and frequency of 3,532 expressed sequence tags (ESTs). Statistical analysis of the active and hibernating heart expression profile indicated the differential regulation of 48 genes based on a P ≤ 0.03 threshold. Several of the differentially expressed genes identified in this screen encode proteins that likely account for uninterrupted cardiac function during hibernation, including those involved in metabolism, contractility, Ca2+handling, and low-temperature catalysis. A sampling of genes showing higher expression during hibernation includes phosphofructokinase, pancreatic triacylglycerol lipase, pyruvate dehydrogenase kinase 4 (PDK4), aldolase A, sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a), titin, and four-and-a-half LIM domains protein 2 (FHL2). Genes showing reduced levels of expression during hibernation include cyclin-dependent kinase 2-associated protein 1 (CDK2AP1), troponin C, phospholamban, Ca2+/calmodulin-dependent protein kinase II (CaMKII), calmodulin, and four subunits of cytochrome c oxidase.

Annual lipid cycles in hibernators: integration of physiology and behavior

Mammalian hibernation is a temporary suspension of euthermia allowing endotherms to undergo reversible hypothermia and generate a marked savings in energy expenditure. In most fat-storing hibernator species, seasonal changes in food intake, triacylglycerol deposition, metabolism, and reproductive development are controlled by a circannual clock. In ground-dwelling sciurid rodents (ground squirrels and marmots), for example, energy intake increases during a summer body mass gain phase, and toward the end of this phase metabolic rate also begins to decrease, resulting in a profound increase in lipid deposition as fat. Increased activity of lipogenic hormones and enzymes correspond with this increase. The hibernation mass loss phase begins after the body mass peak in the fall and ends in spring. During this phase, stored lipids are slowly utilized in a programmed manner by undergoing deep torpor or hibernation during which the hypothalamic setpoint for body temperature is typically reduced to just above 0 degrees C. Throughout the hibernation season, bouts of deep torpor are punctuated by periodic arousals in which brown adipose tissue thermogenesis plays a critical role. Lipid oxidation nearly exclusively fuels deep torpor and most of the rewarming process. The fatty acid composition of stored lipids can affect the depth and duration of deep torpor, and saturated fatty acids may be preferentially used during hibernation, whereas polyunsaturated fatty acids may be preferentially retained. Female and underweight male hibernators terminate hibernation in spring when aboveground food becomes available; in contrast, heavier males with sufficient lipid reserves spontaneously terminate hibernation several weeks before females and independent of food availability. Mating occurs shortly after emergence from hibernation, and the lipid cycle begins again with the completion of reproduction. Lipid deposition and mobilization, temperature regulation, reproduction, and circannual timing are intimately interdependent. The unique manner in which they are controlled during the annual cycle, especially lipid reserves, makes hibernators valuable and promising models for research into the mechanisms underlying these processes in all mammals.

Genes controlling the metabolic switch in hibernating mammals

Hibernating mammals have the ability to decrease their metabolic rate and survive up to 6 months without food in an inactive state where body temperatures approach 0°C. In hibernating 13-lined ground squirrels (Spermophilus tridecemlineatus), oxygen consumption holds at 1/30 to 1/50 of the aroused condition and heart rates are as low as 3–10 beats/min, compared with 200–300 beats/min when the animal is active. This seasonal adaptation requires a metabolic shift away from the oxidation of carbohydrates and towards the combustion of stored fatty acids as the primary source of energy. A key element in this fuel switch is the differential expression of the gene encoding pyruvate dehydrogenase kinase isoenzyme 4. Pyruvate dehydrogenase kinase isoenzyme 4 inhibits pyruvate dehydrogenase and thus minimizes carbohydrate oxidation by preventing the flow of glycolytic products into the tricarboxylic acid cycle. Hibernators also exploit the low-temperature activity of PTL (pancreatic triacylglycerol lipase) in both heart and white adipose tissue. Lipolytic activity at body temperatures associated with hibernation was examined using recombinant ground squirrel and human PTL expressed in yeast. Enzymes from both humans and ground squirrel displayed high activity at temperatures as low as 0°C and showed Q10=1.2–1.5 over the temperature range 37–7°C. These studies indicate that low-temperature lipolysis is a general property of PTL and does not require protein modifications unique to mammalian cells and/or the hibernating state.

Pancreatic triacylglycerol lipase in a hibernating mammal. II. Cold-adapted function and differential expression

Thirteen-lined ground squirrels (Spermophilus tridecemlineatus) exploit the low-temperature activity of pancreatic triacylglycerol lipase (PTL) during hibernation. Lipolytic activity at body temperatures associated with hibernation was examined using recombinant ground squirrel and human PTLs expressed in yeast. Both the human and ground squirrel enzymes displayed high activity at temperatures as low as 0 degrees C and showed Q10 values of 1.2-1.5 over a range of 37-7 degrees C. These studies indicate that low-temperature lipolysis is a general property of PTL and does not require protein modifications unique to mammalian cells and/or the hibernating state. Western blots show elevated levels of PTL protein during hibernation in both heart and white adipose tissue (WAT). Significant increases in PTL gene expression are seen in heart, WAT, and testes; but not in pancreas, where PTL mRNA levels are highest. Upregulation of PTL in testes is also accompanied by expression of the PTL-specific cofactor, colipase. The multi-tissue expression of PTL during hibernation supports its role as a key enzyme that shows high activity at low temperatures.

Pancreatic triacylglycerol lipase in a hibernating mammal. I. Novel genomic organization

Pancreatic triacylglycerol lipase ( PTL) is expressed in novel locations during hibernation in the thirteen-lined ground squirrel ( Spermophilus tridecemlineatus). PTL cDNAs isolated from two of these locations, heart and white adipose tissue (WAT), contain divergent 5′-untranslated regions (5′-UTRs) suggesting alternative promoter usage or the possibility of multiple PTL genes in the ground squirrel genome. In addition, cDNAs isolated from WAT contain tracts of retroviral sequence in their 5′-UTRs. Our examination of PTL genomic clones isolated from a thirteen-lined ground squirrel genomic DNA library, coupled with genomic Southern blot analysis, enabled us to conclude that PTL mRNAs expressed in heart and WAT are the products of the same single-copy gene. The 5′ portion of this gene spans 9.2 kb, is composed of 6 exons, and contains a full-length endogenous retroviral genome with conserved long terminal repeats (LTRs). Alignment of the ground squirrel PTL gene with the mouse, rat, and human PTL genes indicates that this retrovirus inserted into the ground squirrel genome ∼200 bases upstream of the original PTL transcriptional start site. The insertion is a relatively recent event based on largely intact open-reading frames containing minimal frame-shift and nonsense mutations. The high-percentage identity (99.2%) shared between the 5′- and 3′-LTRs of this endogenous retrovirus suggests that the insertion occurred as recently as 300,000 years ago.

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.

Quantitative assessment of ground squirrel mRNA levels in multiple stages of hibernation

Hibernators in torpor dramatically reduce their metabolic, respiratory, and heart rates and core body temperature. These extreme physiological conditions are frequently and rapidly reversed during the winter hibernation season via endogenous mechanisms. This phenotype must derive from regulated expression of the hibernator's genome; to identify its molecular components, a cDNA subtraction was used to enrich for seasonally upregulated mRNAs in liver of golden-mantled ground squirrels. The relative steady-state levels for seven mRNAs identified by this screen, plus five others, were measured and analyzed for seasonal and stage-specific differences using kinetic RT-PCR. Four mRNAs show seasonal upregulation in which all five winter stages differ significantly from and are higher than summer (alpha2-macroglobulin, apolipoprotein A1, cathepsin H, and thyroxine-binding globulin). One of these mRNAs, alpha2-macroglobulin, varies during the winter stages with significantly lower levels at late torpor. None of the 12 mRNAs increased during torpor. The implications for these newly recognized upregulated mRNAs for hibernation as well as more global issues of maintaining steady-state levels of mRNA during torpor are discussed.

Invited review: molecular adaptations in mammalian hibernators: unique adaptations or generalized responses?

Hibernators are unique among mammals in their ability to attain, withstand, and reverse low body temperatures. Hibernators repeatedly cycle between body temperatures near zero during torpor and 37 degrees C during euthermy. How do these mammals maintain cardiac function, cell integrity, blood fluidity, and energetic balance during their prolonged periods at low body temperature and avoid damage when they rewarm? Hibernation is often considered an example of a unique adaptation for low-temperature function in mammals. Although such adaptation is apparent at the level of whole animal physiology, it is surprisingly difficult to demonstrate clear examples of adaptations at the cellular and biochemical levels that improve function in the cold and are unique to hibernators. Instead of adaptation for improved function in the cold, the key molecular adaptations of hibernation may be to exploit the cold to depress most aspects of biochemical function and then rewarm without damage to restore optimal function of all systems. These capabilities are likely due to novel regulation of biochemical pathways shared by all mammals, including humans.

Coordinate expression of the PDK4 gene: a means of regulating fuel selection in a hibernating mammal

Hibernation in mammals requires a metabolic shift away from the oxidation of carbohydrates and toward the combustion of stored fatty acids as the primary source of energy during torpor. A key element involved in this fuel selection is pyruvate dehydrogenase kinase isoenzyme 4 (PDK4). PDK4 inhibits pyruvate dehydrogenase and thus minimizes carbohydrate oxidation by preventing the flow of glycolytic products into the tricarboxylic acid cycle. This paper examines expression of the PDK4 gene during hibernation in heart, skeletal muscle, and white adipose tissue (WAT) of the 13-lined ground squirrel, Spermophilus tridecemlineatus. During hibernation PDK4 mRNA levels increase 5-fold in skeletal muscle and 15-fold in WAT compared with summer-active levels. Similarly, PDK4 protein is increased threefold in heart, fivefold in skeletal muscle, and eightfold in WAT. High levels of serum insulin, likely to have an inhibitory effect on PDK4 gene expression, are seen during fall when PDK4 mRNA levels are low. Coordinate upregulation of PDK4 in three distinct tissues suggests a common signal that regulates PDK4 expression and fuel selection during hibernation.