Regulation of UCP1 and UCP3 in arctic ground squirrels and relation with mitochondrial proton leak

Mitochondria Need Their Sleep: Redox, Bioenergetics, and Temperature Regulation of Circadian Rhythms and the Role of Cysteine-Mediated Redox Signaling, Uncoupling Proteins, and Substrate Cycles

Although circadian biorhythms of mitochondria and cells are highly conserved and crucial for the well-being of complex animals, there is a paucity of studies on the reciprocal interactions between oxidative stress, redox modifications, metabolism, thermoregulation, and other major oscillatory physiological processes. To address this limitation, we hypothesize that circadian/ultradian interaction of the redoxome, bioenergetics, and temperature signaling strongly determine the differential activities of the sleep–wake cycling of mammalians and birds. Posttranslational modifications of proteins by reversible cysteine oxoforms, S-glutathionylation and S-nitrosylation are shown to play a major role in regulating mitochondrial reactive oxygen species production, protein activity, respiration, and metabolomics. Nuclear DNA repair and cellular protein synthesis are maximized during the wake phase, whereas the redoxome is restored and mitochondrial remodeling is maximized during sleep. Hence, our analysis reveals that wakefulness is more protective and restorative to the nucleus (nucleorestorative), whereas sleep is more protective and restorative to mitochondria (mitorestorative). The “redox–bioenergetics–temperature and differential mitochondrial–nuclear regulatory hypothesis” adds to the understanding of mitochondrial respiratory uncoupling, substrate cycling control and hibernation. Similarly, this hypothesis explains how the oscillatory redox–bioenergetics–temperature–regulated sleep–wake states, when perturbed by mitochondrial interactome disturbances, influence the pathogenesis of aging, cancer, spaceflight health effects, sudden infant death syndrome, and diseases of the metabolism and nervous system.

Overexpression of UCP3 decreases mitochondrial efficiency in mouse skeletal muscle in vivo

Uncoupling protein-3 (UCP3) is a mitochondrial transmembrane protein highly expressed in the muscle that has been implicated in regulating the efficiency of mitochondrial oxidative phosphorylation. Increasing UCP3 expression in skeletal muscle enhances proton leak across the inner mitochondrial membrane and increases oxygen consumption in isolated mitochondria, but its precise function in vivo has yet to be fully elucidated. To examine whether muscle-specific overexpression of UCP3 modulates muscle mitochondrial oxidation in vivo, rates of ATP synthesis were assessed by ³¹ P magnetic resonance spectroscopy (MRS), and rates of mitochondrial oxidative metabolism were measured by assessing the rate of [2-¹³ C]acetate incorporation into muscle [4-¹³ C]-, [3-¹³ C]-glutamate, and [4-¹³ C]-glutamine by high-resolution ¹³ C/¹ H MRS. Using this approach, we found that the overexpression of UCP3 in skeletal muscle was accompanied by increased muscle mitochondrial inefficiency in vivo as reflected by a 42% reduction in the ratio of ATP synthesis to mitochondrial oxidation.

WITHDRAWN: Mitochondria need their sleep: Sleep-wake cycling and the role of redox, bioenergetics, and temperature regulation, involving cysteine-mediated redox signaling, uncoupling proteins, and substrate cycles

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Browning Epicardial Adipose Tissue: Friend or Foe?

The epicardial adipose tissue (EAT) is the visceral fat depot of the heart which is highly plastic and in direct contact with myocardium and coronary arteries. Because of its singular proximity with the myocardium, the adipokines and pro-inflammatory molecules secreted by this tissue may directly affect the metabolism of the heart and coronary arteries. Its accumulation, measured by recent new non-invasive imaging modalities, has been prospectively associated with the onset and progression of coronary artery disease (CAD) and atrial fibrillation in humans. Recent studies have shown that EAT exhibits beige fat-like features, and express uncoupling protein 1 (UCP-1) at both mRNA and protein levels. However, this thermogenic potential could be lost with age, obesity and CAD. Here we provide an overview of the physiological and pathophysiological relevance of EAT and further discuss whether its thermogenic properties may serve as a target for obesity therapeutic management with a specific focus on the role of immune cells in this beiging phenomenon.

Skeletal muscles of hibernating black bears show minimal atrophy and phenotype shifting despite prolonged physical inactivity and starvation

Hibernating mammals experience prolonged periods of torpor and starvation during winter for up to 5–7 months. Though physical inactivity and malnutrition generally lead to profound loss of muscle mass and metabolic dysfunction in humans, hibernating bears show limited muscle atrophy and can successfully maintain locomotive function. These physiological features in bears allow us to hypothesize that hibernating bears uniquely alter the regulation of protein and energy metabolism in skeletal muscle which then contributes to “muscle atrophy resistance” against continued physical inactivity. In this study, alteration of signaling pathways governing protein and energy metabolisms was examined in skeletal muscle of the Japanese black bear (Ursus thibetanus japonicus). Sartorius muscle samples were collected from bear legs during late November (pre-hibernation) and early April (post-hibernation). Protein degradation pathways, through a ubiquitin-proteasome system (as assessed by increased expression of murf1 mRNA) and an autophagy-dependent system (as assessed by increased expression of atg7, beclin1, and map1lc3 mRNAs), were significantly activated in skeletal muscle following hibernation. In contrast, as indicated by a significant increase in S6K1 phosphorylation, an activation state of mTOR (mammalian/mechanistic target of rapamycin), which functions as a central regulator of protein synthesis, increased in post-hibernation samples. Gene expression of myostatin, a negative regulator of skeletal muscle mass, was significantly decreased post-hibernation. We also confirmed that the phenotype shifted toward slow-oxidative muscle and mitochondrial biogenesis. These observations suggest that protein and energy metabolism may be altered in skeletal muscle of hibernating bears, which then may contribute to limited loss of muscle mass and efficient energy utilization.

Identification of a lipid-rich depot in the orbital cavity of the thirteen-lined ground squirrel

We discovered a previously undescribed orbital lipid depot in the thirteen-lined ground squirrel during the first ever magnetic resonance image (MRI) of this common experimental model of mammalian hibernation. In animals housed at constant ambient temperatures (5°C or 25°C, 12 h:12 h light:dark photoperiod), the volume of this depot increased in the autumn and decreased in the spring, suggesting an endogenous circannual pattern. Water-fat MRI revealed that throughout the year this depot is composed of ∼40% lipid, similar to brown adipose tissue (BAT). During arousal from torpor, thermal images showed higher surface temperatures near this depot before the rest of the head warmed, suggesting a thermoregulatory function. This depot, however, does not contain uncoupling protein 1, a BAT biomarker, or uncoupling protein 3. Histology shows blood vessels in close proximity to each other, suggesting it may serve as a vascular rete, perhaps to preferentially warm the eye and brain during arousals.

Transcription Regulators and Hormones Involved in the Development of Brown Fat and White Fat Browning: Transcriptional and Hormonal Control of Brown/Beige Fat Development

The high prevalence of obesity and related metabolic complications has inspired research on adipose tissues. Three kinds of adipose tissues are identified in mammals: brown adipose tissue (BAT), beige or brite adipose tissue and white adipose tissue (WAT). Beige adipocytes share some characteristics with brown adipocytes such as the expression of UCP1. Beige adipocytes can be activated by environmental stimuli or pharmacological treatment, and this change is accompanied by an increase in energy consumption. This process is called white browning, and it facilitates the maintenance of a lean and healthy phenotype. Thus, promoting beige adipocyte development in WAT shows promise as a new strategy in treating obesity and related metabolic consequences. In this review, we summarized the current understanding of the regulators and hormones that participate in the development of brown fat and white fat browning.

TRPs et al.: a molecular toolkit for thermosensory adaptations

The ability to sense temperature is crucial for the survival of an organism. Temperature influences all biological operations, from rates of metabolic reactions to protein folding, and broad behavioral functions, from feeding to breeding, and other seasonal activities. The evolution of specialized thermosensory adaptations has enabled animals to inhabit extreme temperature niches and to perform specific temperature-dependent behaviors. The function of sensory neurons depends on the participation of various types of ion channels. Each of the channels involved in neuronal excitability, whether through the generation of receptor potential, action potential, or the maintenance of the resting potential have temperature-dependent properties that can tune the neuron's response to temperature stimuli. Since the function of all proteins is affected by temperature, animals need adaptations not only for detecting different temperatures, but also for maintaining sensory ability at different temperatures. A full understanding of the molecular mechanism of thermosensation requires an investigation of all channel types at each step of thermosensory transduction. A fruitful avenue of investigation into how different molecules can contribute to the fine-tuning of temperature sensitivity is to study the specialized adaptations of various species. Given the diversity of molecular participants at each stage of sensory transduction, animals have a toolkit of channels at their disposal to adapt their thermosensitivity to their particular habitats or behavioral circumstances.

The potential mechanism of mitochondrial dysfunction in septic cardiomyopathy

Septic cardiomyopathy is one of the most serious complications of sepsis or septic shock. Basic and clinical research has studied the mechanism of cardiac dysfunction for more than five decades. It has become clear that myocardial depression is not related to hypoperfusion. As the heart is highly dependent on abundant adenosine triphosphate (ATP) levels to maintain its contraction and diastolic function, impaired mitochondrial function is lethally detrimental to the heart. Research has shown that mitochondria play an important role in organ damage during sepsis. The mitochondria-related mechanisms in septic cardiomyopathy have been discussed in terms of restoring mitochondrial function. Mitochondrial uncoupling proteins located in the mitochondrial inner membrane can promote proton leakage across the mitochondrial inner membrane. Recent studies have demonstrated that proton leakage is the essential regulator of mitochondrial membrane potential and the generation of reactive oxygen species (ROS) and ATP. Other mechanisms involved in septic cardiomyopathy include mitochondrial ROS production and oxidative stress, mitochondria Ca²⁺ handling, mitochondrial DNA in sepsis, mitochondrial fission and fusion, mitochondrial biogenesis, mitochondrial gene regulation and mitochondria autophagy. This review will provide an overview of recent insights into the factors contributing to septic cardiomyopathy.

Nature's fat-burning machine: brown adipose tissue in a hibernating mammal

Brown adipose tissue (BAT) is a unique thermogenic tissue in mammals that rapidly produces heat via nonshivering thermogenesis. Small mammalian hibernators have evolved the greatest capacity for BAT because they use it to rewarm from hypothermic torpor numerous times throughout the hibernation season. Although hibernator BAT physiology has been investigated for decades, recent efforts have been directed toward understanding the molecular underpinnings of BAT regulation and function using a variety of methods, from mitochondrial functional assays to 'omics' approaches. As a result, the inner-workings of hibernator BAT are now being illuminated. In this Review, we discuss recent research progress that has identified players and pathways involved in brown adipocyte differentiation and maturation, as well as those involved in metabolic regulation. The unique phenotype of hibernation, and its reliance on BAT to generate heat to arouse mammals from torpor, has uncovered new molecular mechanisms and potential strategies for biomedical applications.

The single IGF-1 partial deficiency is responsible for mitochondrial dysfunction and is restored by IGF-1 replacement therapy

BACKGROUND & AIMS

We previously described in cirrhosis and aging, both conditions of IGF-1 deficiency, a clear hepatic mitochondrial dysfunction with increased oxidative damage. In both conditions, the hepatic mitochondrial function was improved with low doses of IGF-1. The aim of this work was to explore if the only mere IGF-1 partial deficiency, without any exogenous insult, is responsible for hepatic mitochondrial dysfunction.

METHODS

Heterozygous (igf1^+/-) mice were divided into two groups: untreated and treated mice with low doses of IGF-1. WT group was used as controls. Parameters of hepatic mitochondrial function were determined by flow cytometry, antioxidant enzyme activities were determined by spectrophotometry, and electron chain transport enzyme levels were determined by immunohistochemistry and immunofluorescence analyses. Liver expression of genes coding for proteins involved in mitochondrial protection and apoptosis was studied by microarray analysis and RT-qPCR.

RESULTS

Hz mice showed a significant reduction in hepatic mitochondrial membrane potential (MMP) and ATPase activity, and an increase in intramitochondrial free radical production and proton leak rates, compared to controls. These parameters were normalized by IGF-1 replacement therapy. No significant differences were found between groups in oxygen consumption and antioxidant enzyme activities, except for catalase, whose activity was increased in both Hz groups. Relevant genes coding for proteins involved in mitochondrial protection and survival were altered in Hz group and were reverted to normal in Hz+IGF-1 group.

CONCLUSIONS

The mere IGF-1 partial deficiency is per se associated with hepatic mitochondrial dysfunction sensitive to IGF-1 replacement therapy. Results in this work prove that IGF-1 is involved in hepatic mitochondrial protection, because it is able to reduce free radical production, oxidative damage and apoptosis. All these IGF-1 actions are mediated by the modulation of the expression of genes encoding citoprotective and antiapoptotic proteins.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Evolutionary adaptation to thermosensation

Organisms continuously evolve to adapt to changing environmental conditions. Chief among these are daily and seasonal temperature fluctuations. Relatively small in terms of real physical values, temperature fluctuations of just a few degrees can profoundly affect organismal functions. In vertebrates, temperature is detected by primary afferents of somatosensory neurons, which express thermo-gated ion channels. Most of our knowledge about temperature receptors comes from seminal studies in mice and rats. Recent work uncovered thermosensory mechanisms in other vertebrates, shedding light onto the diversity of thermosensory adaptations. Here, we summarize molecular mechanisms of thermosensation in different species and discuss the need to use the standard laboratory rodents and non-standard species side-by-side in order to understand fundamental principles of somatosensation.

Modulation of Gene Expression in Key Survival Pathways During Daily Torpor in the Gray Mouse Lemur, Microcebus murinus

A variety of mammals employ torpor as an energy-saving strategy in environments of marginal or severe stress either on a daily basis during their inactive period or on a seasonal basis during prolonged multi-day hibernation. Recently, a few Madagascar lemur species have been identified as the only primates that exhibit torpor; one of these is the gray mouse lemur (Microcebus murinus). To explore the regulatory mechanisms that underlie daily torpor in a primate, we analyzed the expression of 28 selected genes that represent crucial survival pathways known to be involved in squirrel and bat hibernation. Array-based real-time PCR was used to compare gene expression in control (aroused) versus torpid lemurs in five tissues including the liver, kidney, skeletal muscle, heart, and brown adipose tissue. Significant differences in gene expression during torpor were revealed among genes involved in glycolysis, fatty acid metabolism, antioxidant defense, apoptosis, hypoxia signaling, and protein protection. The results showed upregulation of select genes primarily in liver and brown adipose tissue. For instance, both tissues showed elevated gene expression of peroxisome proliferator activated receptor gamma (ppargc), ferritin (fth1), and protein chaperones during torpor. Overall, the data show that the expression of only a few genes changed during lemur daily torpor, as compared with the broader expression changes reported for hibernation in ground squirrels. These results provide an indication that the alterations in gene expression required for torpor in lemurs are not as extensive as those needed for winter hibernation in squirrel models. However, identification of crucial genes with altered expression that support lemur torpor provides key targets to be explored and manipulated toward a goal of translational applications of inducible torpor as a treatment option in human biomedicine.

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

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

Neuronal UCP1 expression suggests a mechanism for local thermogenesis during hibernation

Hibernating mammals possess a unique ability to reduce their body temperature to ambient levels, which can be as low as -2.9 °C, by active down-regulation of metabolism. Despite such a depressed physiologic phenotype, hibernators still maintain activity in their nervous systems, as evidenced by their continued sensitivity to auditory, tactile, and thermal stimulation. The molecular mechanisms that underlie this adaptation remain unknown. We report, using differential transcriptomics alongside immunohistologic and biochemical analyses, that neurons from thirteen-lined ground squirrels (Ictidomys tridecemlineatus) express mitochondrial uncoupling protein 1 (UCP1). The expression changes seasonally, with higher expression during hibernation compared with the summer active state. Functional and pharmacologic analyses show that squirrel UCP1 acts as the typical thermogenic protein in vitro. Accordingly, we found that mitochondria isolated from torpid squirrel brain show a high level of palmitate-induced uncoupling. Furthermore, torpid squirrels during the hibernation season keep their brain temperature significantly elevated above ambient temperature and that of the rest of the body, including brown adipose tissue. Together, our findings suggest that UCP1 contributes to local thermogenesis in the squirrel brain, and thus supports nervous tissue function at low body temperature during hibernation.

To each its own: Thermoregulatory strategy varies among neonatal polar phocids

Cold environmental conditions and small body size promote heat loss and may create thermoregulatory challenges for marine mammals born in polar regions. However, among polar-born phocid seal species there are variations in physical attributes and environmental conditions at birth, allowing for an interesting contrast in thermoregulatory strategy. We compared thermoregulatory strategies through morphometrics, sculp attributes (conductivity and resistance), nonshivering thermogenesis (NST via uncoupling protein 1; UCP1), and muscle thermogenesis (via enzyme activity) in neonatal harp (Pagophilus groenlandicus), hooded (Cystophora cristata), and Weddell seals (Leptonychotes weddellii). Harp seals are the smallest at birth (9.8±0.7 kg), rely on lanugo (82.49±3.70% of thermal resistance), and are capable of NST through expression of UCP1 in brown adipose tissue (BAT). In contrast, hooded seal neonates (26.8±1.3 kg) have 2.06±0.23 cm of blubber, accounting for 38.19±6.07% of their thermal resistance. They are not capable of NST, as UCP1 is not expressed. The large Weddell seal neonates (31.5±4.9 kg) rely on lanugo (89.85±1.25% of thermal resistance) like harp seals, but no evidence of BAT was found. Muscle enzyme activity was highest in Weddell seal neonates, suggesting that they rely primarily on muscle thermogenesis. Similar total thermal resistance, combined with marked differences in thermogenic capacity of NST and ST among species, strongly supports that thermoregulatory strategy in neonatal phocids is more closely tied to pups' surface area to volume ratio (SA:V) and potential for early water immersion rather than mass and ambient environmental conditions.

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.

Gene loss, thermogenesis, and the origin of birds

Compared to related taxa, birds have exceptionally enlarged and diversified skeletal muscles, features that are closely associated with skeletal diversification and are commonly explained by a diversity of avian ecological niches and locomotion types. The thermogenic muscle hypothesis (TMH) for the origin of birds proposes that such muscle hyperplasia and the associated skeletal innovations are instead the consequence of the avian clade originating from an ancestral population that underwent several successive episodes of loss of genes associated with thermogenesis, myogenesis, and skeletogenesis. Direct bird ancestors met this challenge with a combination of behavioral strategies (e.g., brooding of nestlings) and acquisition of a variety of adaptations for enhanced nonshivering thermogenesis in skeletal muscle. The latter include specific biochemical alterations promoting muscle heat generation and dramatic expansion of thigh and breast muscle mass. The TMH proposes that such muscle hyperplasia facilitated bipedality, freeing upper limbs for new functions (e.g., flight, swimming), and, by altering the mechanical environment of embryonic development, generated skeletal novelties, sometimes abruptly, that became distinctive features of the avian body plan.

Changes in glucose tolerance and leptin responsiveness of rats offered a choice of lard, sucrose, and chow

Rats offered chow, lard, and 30% sucrose solution (choice) rapidly become obese. We tested metabolic disturbances in rats offered choice, chow+lard, or chow+30% sucrose solution [chow+liquid sucrose (LS)] and compared them with rats fed a composite 60% kcal fat, 7% sucrose diet [high-fat diet (HFD)], or a 10% kcal fat, 35% sucrose diet [low-fat diet (LFD)]. Choice rats had the highest energy intake, but HFD rats gained the most weight. After 23 days carcass fat was the same for choice, HFD, chow+lard, and chow+LS groups. Glucose clearance was the same for all groups during an intraperitoneal glucose tolerance test (GTT) on day 12, but fasting insulin was increased in choice, LFD fed, and chow+LS rats. By contrast, only choice and chow+LS rats were resistant to an intraperitoneal injection of 2 mg leptin/kg on day 17. In experiment 2 choice rats were insulin insensitive during an intraperitoneal GTT, but this was corrected in an oral GTT due to GLP-1 release. UCP-1 protein was increased in brown fat and inguinal white fat in choice rats, and this was associated with a significant increase in energy expenditure of choice rats during the dark period whether expenditure was expressed on a per animal or a metabolic body size basis. The increase in expenditure obviously was not great enough to prevent development of obesity. Further studies are required to determine the mechanistic basis of the rapid onset of leptin resistance in choice rats and how consumption of sucrose solution drives this process.

Cholesterol and lipoprotein dynamics in a hibernating mammal

Hibernating mammals cease feeding during the winter and rely primarily on stored lipids to fuel alternating periods of torpor and arousal. How hibernators manage large fluxes of lipids and sterols over the annual hibernation cycle is poorly understood. The aim of this study was to investigate lipid and cholesterol transport and storage in ground squirrels studied in spring, summer, and several hibernation states. Cholesterol levels in total plasma, HDL and LDL particles were elevated in hibernators compared with spring or summer squirrels. Hibernation increased plasma apolipoprotein A-I expression and HDL particle size. Expression of cholesterol 7 alpha-hydroxylase was 13-fold lower in hibernators than in active season squirrels. Plasma triglycerides were reduced by fasting in spring but not summer squirrels. In hibernators plasma β-hydroxybutyrate was elevated during torpor whereas triglycerides were low relative to normothermic states. We conclude that the switch to a lipid-based metabolism during winter, coupled with reduced capacity to excrete cholesterol creates a closed system in which efficient use of lipoproteins is essential for survival.

Increased lipolysis and energy expenditure in a mouse model with severely impaired glucagon secretion

Background

Secretion of insulin and glucagon is triggered by elevated intracellular calcium levels. Although the precise mechanism by which the calcium signal is coupled to insulin and glucagon granule exocytosis is unclear, synaptotagmin-7 has been shown to be a positive regulator of calcium-dependent insulin and glucagon secretion, and may function as a calcium sensor for insulin and glucagon granule exocytosis. Deletion of synaptotagmin-7 leads to impaired glucose-stimulated insulin secretion and nearly abolished Ca²⁺-dependent glucagon secretion in mice. Under non-stressed resting state, however, synaptotagmin-7 KO mice exhibit normal insulin level but severely reduced glucagon level.

Methodology/Principal Findings

We studied energy expenditure and metabolism in synaptotagmin-7 KO and control mice using indirect calorimetry and biochemical techniques. Synaptotagmin-7 KO mice had lower body weight and body fat content, and exhibited higher oxygen consumption and basal metabolic rate. Respiratory exchange ratio (RER) was lower in synaptotagmin-7 KO mice, suggesting an increased use of lipid in their energy production. Consistent with lower RER, gene expression profiles suggest enhanced lipolysis and increased capacity for fatty acid transport and oxidation in synaptotagmin-7 KO mice. Furthermore, expression of uncoupling protein 3 (UCP3) in skeletal muscle was approximately doubled in the KO mice compared with control mice.

Conclusions

These results show that the lean phenotype in synaptotagmin-7 KO mice was mostly attributed to increased lipolysis and energy expenditure, and suggest that reduced glucagon level may have broad influence on the overall metabolism in the mouse model.

Uncoupled respiration, ROS production, acute lipotoxicity and oxidative damage in isolated skeletal muscle mitochondria from UCP3-ablated mice

The function of uncoupling protein 3 (UCP3) is still not established. Mitochondrial uncoupling, control of ROS production, protection against lipotoxicity and protection against oxidative stress are functions classically discussed. To establish a role for UCP3 in these functions, we have here used UCP3 (-/-) mice, backcrossed for 10 generations on a C57Bl/6 background. In isolated skeletal muscle mitochondria, we examined uncoupled respiration, both unstimulated and in the presence of fatty acids. We did not observe any difference between mitochondria from wildtype and UCP3 (-/-) mice. We measured H(2)O(2) production rate and respiration rate under reactive oxygen species-generating conditions (succinate without rotenone) but found no effect of UCP3. We tested two models of acute lipotoxicity-fatty acid-induced oxidative inhibition and fatty acid-induced swelling-but did not observe any protective effect of UCP3. We examined oxidative stress by quantifying 4-hydroxynonenal protein adducts and protein carbonyls in the mitochondria-but did not observe any protective effect of UCP3. We conclude that under the experimental conditions tested here, we find no evidence for the function of UCP3 being basal or induced uncoupling, regulation of ROS production, protection against acute lipotoxicity or protection against oxidative damage.

Expression of uncoupling protein 3 in mitochondria protects against stress-induced myocardial injury: a proteomic study

It has been confirmed that stress plays an important role in the induction and development of cardiovascular diseases, but its mechanism and molecular basis remain unknown. In the present study, a myocardial injury model induced by restraint stress was established in rat. To screen for the related proteins involved in stress-induced myocardial injury, proteomic techniques based on 2-DE and mass spectrometry were used. In our results, ten proteins were found to be altered. The expression of eight of these proteins was increased after restraint stress, including cardiac myosin heavy chain, dihydrolipoamide succinyltransferase component of 2-oxoglutarate dehydrogenase complex, mitochondrial aldehyde dehydrogenase, H(+)-transporting ATP synthase, albumin, and apolipoprotein A-I precursor. The expression of uncoupling protein 3 (UCP3) and mitochondrial aconitase was decreased. Most of the proteins were related to energy metabolism. Further research indicated that UCP3 may mediate the myocardial cell response induced by restraint stress.

Global analysis of circulating metabolites in hibernating ground squirrels

Hibernation in mammals involves major alterations in nutrition and metabolism that would be expected to affect levels of circulating molecules. To gain insight into these changes we conducted a non-targeted LC-MS based metabolomic analysis of plasma using hibernating ground squirrels in late torpor (LT, T(b)~5 °C) or during an interbout arousal period (IBA, T(b)~5 °C) and non-hibernating squirrels in spring (T(b)~37 °C). Several metabolites varied and allowed differentiation between hibernators and spring squirrels, and between torpid and euthermic squirrels. Methionine and the short-chain carnitine esters of propionate and butyryate/isobutyrate were reduced in LT compared with the euthermic groups. Pantothenic acid and several lysophosphatidylcholines were elevated in LT relative to the euthermic groups, whereas lysophosphatidylethanolamines were elevated during IBA compared to LT and spring animals. Two regulatory lipids varied among the groups: sphingosine 1-phosphate was lower in LT vs. euthermic groups, whereas cholesterol sulfate was elevated in IBA compared to spring squirrels. Levels of long-chain fatty acids (LCFA) and total NEFA tended to be elevated in hibernators relative to spring squirrels. Three long-chain acylcarnitines were reduced in LT relative to IBA; free carnitine was also lower in LT vs. IBA. Our results identified several biochemical changes not previously observed in the seasonal hibernation cycle, including some that may provide insight into the metabolic limitations of mammalian torpor.

Cold tolerance of UCP1-ablated mice: a skeletal muscle mitochondria switch toward lipid oxidation with marked UCP3 up-regulation not associated with increased basal, fatty acid- or ROS-induced uncoupling or enhanced GDP effects

Mice lacking the thermogenic mitochondrial membrane protein UCP1 (uncoupling protein 1)--and thus all heat production from brown adipose tissue--can still adapt to a cold environment (4 degrees C) if successively transferred to the cold. The mechanism behind this adaptation has not been clarified. To examine possible adaptive processes in the skeletal muscle, we isolated mitochondria from the hind limb muscles of cold-acclimated wild-type and UCP1(-/-) mice and examined their bioenergetic chracteristics. We observed a switch in metabolism, from carbohydrate towards lipid catabolism, and an increased total mitochondrial complement, with an increased total ATP production capacity. The UCP1(-/-) muscle mitochondria did not display a changed state-4 respiration rate (no uncoupling) and were less sensitive to the uncoupling effect of fatty acids than the wild-type mitochondria. The content of UCP3 was increased 3-4 fold, but despite this, endogenous superoxide could not invoke a higher proton leak, and the small inhibitory effect of GDP was unaltered, indicating that it was not mediated by UCP3. Double mutant mice (UCP1(-/-) plus superoxide dismutase 2-overexpression) were not more cold sensitive than UCP1(-/-), bringing into question an involvement of reactive oxygen species (ROS) in activation of any alternative thermogenic mechanism. We conclude that there is no evidence for an involvement of UCP3 in basal, fatty-acid- or superoxide-stimulated oxygen consumption or in GDP sensitivity. The adaptations observed did not imply any direct alternative process for nonshivering thermogenesis but the adaptations observed would be congruent with adaptation to chronically enhanced muscle activity caused by incessant shivering in these mice.

The regulation and turnover of mitochondrial uncoupling proteins

Uncoupling proteins (UCP1, UCP2 and UCP3) are important in regulating cellular fuel metabolism and as attenuators of reactive oxygen species production through strong or mild uncoupling. The generic function and broad tissue distribution of the uncoupling protein family means that they are increasingly implicated in a range of pathophysiological processes including obesity, insulin resistance and diabetes mellitus, neurodegeneration, cardiovascular disease, immunity and cancer. The significant recent progress describing the turnover of novel uncoupling proteins, as well as current views on the physiological roles and regulation of UCPs, is outlined.

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.

Matching cellular metabolic supply and demand in energy-stressed animals

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

Molecular evolution of UCP1 and the evolutionary history of mammalian non-shivering thermogenesis

Background

Uncoupling protein 1 (UCP1) is a mitochondrial anion carrier, expressed in brown adipose tissue (BAT) of Eutherians. UCP1 is responsible for uncoupling mitochondrial proton transport from the production of ATP, thereby dissipating heat; it is essential for non-shivering thermogenesis (NST) in mammalian BAT. UCP1 orthologs have been identified in non-Eutherian mammals, fish and amphibians. Yet, UCP1 has a unique function in Eutherians in that it is necessary in the production of heat (NST). As such, this study aims to determine the evolutionary mode of UCP1 in Eutherians, where there is clear evidence of UCP1-dependent NST in BAT.

Results

Models of adaptive evolution through phylogenetic analysis of amino acid sequences by maximum likelihood were implemented to determine the mode of UCP1 protein evolution in Eutherians. An increase in the rate of amino acid substitutions on the branch leading to Eutherians is observed, but is best explained by relaxed constraints, not positive selection. Further, evidence for branch and site heterogeneity in selection pressures, as well as divergent selection pressures between UCP1 and its paralogs (UCP2-3) is observed.

Conclusion

We propose that the unique thermogenic function of UCP1 in Eutherians may be best explained by neutral processes. Along with other evidence, this suggests that the primary biochemical properties of UCP1 may not differ between Eutherians and non-Eutherians.

Mitochondrial metabolism in hibernation and daily torpor: a review

Hibernation and daily torpor involve substantial decreases in body temperature and metabolic rate, allowing birds and mammals to cope with cold environments and/or limited food. Regulated suppression of mitochondrial metabolism probably contributes to energy savings: state 3 (phosphorylating) respiration is lower in liver mitochondria isolated from mammals in hibernation or daily torpor compared to normothermic controls, although data on state 4 (non-phosphorylating) respiration are equivocal. However, no suppression is seen in skeletal muscle, and there is little reliable data from other tissues. In both daily torpor and hibernation, liver state 3 substrate oxidation is suppressed, especially upstream of electron transport chain complex IV. In hibernation respiratory suppression is reversed quickly in arousal even when body temperature is very low, implying acute regulatory mechanisms, such as oxaloacetate inhibition of succinate dehydrogenase. Respiratory suppression depends on in vitro assay temperature (no suppression is evident below approximately 30 degrees C) and (at least in hibernation) dietary polyunsaturated fats, suggesting effects on inner mitochondrial membrane phospholipids. Proton leakiness of the inner mitochondrial membrane does not change in hibernation, but this also depends on dietary polyunsaturates. In contrast proton leak increases in daily torpor, perhaps limiting reactive oxygen species production.

Long-term fasting decreases mitochondrial avian UCP-mediated oxygen consumption in hypometabolic king penguins

In endotherms, regulation of the degree of mitochondrial coupling affects cell metabolic efficiency. Thus it may be a key contributor to minimizing metabolic rate during long periods of fasting. The aim of the present study was to investigate whether variation in mitochondrial avian uncoupling proteins (avUCP), as putative regulators of mitochondrial oxidative phosphorylation, may contribute to the ability of king penguins (Aptenodytes patagonicus) to withstand fasting for several weeks. After 20 days of fasting, king penguins showed a reduced rate of whole animal oxygen consumption (Vo2; -33%) at rest, together with a reduced abundance of avUCP and peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC1-alpha) mRNA in pectoralis muscle (-54%, -36%, respectively). These parameters were restored after the birds had been refed for 3 days. Furthermore, in recently fed, but not in fasted penguins, isolated muscle mitochondria showed a guanosine diphosphate-inhibited, fatty acid plus superoxide-activated respiration, indicating the presence of a functional UCP. It was calculated that variation in mitochondrial UCP-dependent respiration in vitro may contribute to nearly 20% of the difference in resting Vo2 between fed or refed penguins and fasted penguins measured in vivo. These results suggest that the lowering of avUCP activity during periods of long-term energetic restriction may contribute to the reduction in metabolic rate and hence the ability of king penguins to face prolonged periods of fasting.

Low doses of insulin-like growth factor-I induce mitochondrial protection in aging rats

Serum IGF-I levels decline with age. We have recently reported that in aging rats the exogenous administration of IGF-I restores IGF-I circulating levels and age related-changes, improving glucose and lipid metabolisms, increasing testosterone levels and serum total antioxidant capability, and reducing oxidative damage in the brain and liver associated with a normalization of antioxidant enzyme activities. Understanding that mitochondria are one of the most important cellular targets of IGF-I, the aims of this study were to characterize mitochondrial dysfunction and study the effect of IGF-I therapy on mitochondria, leading to cellular protection in the following experimental groups: young controls, untreated old rats, and aging rats treated with IGF-I. Compared with young controls, untreated aging rats showed an increase of oxidative damage in isolated mitochondria with a mitochondrial dysfunction characterized by: depletion of membrane potential with increased proton leak rates and intramitochondrial free radical production, and a significant reduction of ATPase and complex IV activities. In addition, mitochondrial respiration from untreated aging rats was atractyloside insensitive, suggesting that the adenine nucleotide translocator was uncoupled. The adenine nucleotide translocator has been shown to be one of the most sensitive locations for pore opening. Accordingly, untreated aging rats showed a significant overexpression of the active fragment of caspases 3 and 9. IGF-I therapy corrected these parameters of mitochondrial dysfunction and reduced caspase activation. In conclusion, these results show that the cytoprotective effect of IGF-I is closely related to a mitochondrial protection, leading to reduce free radical production, oxidative damage, and apoptosis, and to increased ATP production.

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.

Resistance to high-fat-diet-induced obesity and sexual dimorphism in the metabolic responses of transgenic mice with moderate uncoupling protein 3 overexpression in glycolytic skeletal muscles

AIMS/HYPOTHESIS

Uncoupling protein (UCP) 3 is a mitochondrial inner membrane protein expressed predominantly in glycolytic skeletal muscles. Its role in vivo remains poorly understood. The aim of the present work was to produce a mouse model with moderate overproduction and proper fibre-type distribution of UCP3.

METHODS

Transgenic mice were created with a 16 kb region encompassing the human UCP3 gene. Mitochondrial uncoupling was investigated on permeabilised muscle fibres. Changes in body weight, adiposity and glucose or insulin tolerance were assessed in mice fed chow and high-fat diets. Indirect calorimetry was used to determine whole-body energy expenditure and substrate utilisation.

RESULTS

Transgenic mice showed a twofold increase in UCP3 protein levels specifically in glycolytic muscles. Mitochondrial respiration revealed an increase of uncoupling in glycolytic but not in oxidative muscles. Transgenic mice gained less weight than wild-type littermates due to lower adipose tissue accretion when fed a high-fat diet. Animals showed a sexual dimorphism in metabolic responses. Female transgenic mice were more glucose-sensitive than wild-type animals, while male transgenic mice with high body weights had impaired glucose and insulin tolerance. Measurements of RQs in mice fed chow and high-fat diets suggested an impairment of metabolic flexibility in transgenic male mice.

CONCLUSIONS/INTERPRETATION

Our data show that physiological overproduction of UCP3 in glycolytic muscles results in mitochondrial uncoupling, resistance to high-fat diet-induced obesity and sex specificity regarding insulin sensitivity and whole-body substrate utilisation.