Acetyl-CoA carboxylase control of fatty acid oxidation in hearts from hibernating Richardson's ground squirrels

Cardiac adaptation and cardioprotection against arrhythmias and ischemia-reperfusion injury in mammalian hibernators

Hibernation allows animals to enter an energy conserving state to survive severe drops in external temperatures and a shortage of food. It has been observed that the hearts of mammalian hibernators exhibit intrinsic protection against ischemia-reperfusion (I/R) injury and cardiac arrhythmias in the winter whether they are hibernating or not. However, the molecular and ionic mechanisms for cardioprotection in mammalian hibernators remain elusive. Recent studies in woodchucks (Marmota monax) have suggested that cardiac adaptation occurs at different levels and mediates an intrinsic cardioprotection prior to/in the winter. The molecular/cellular remodeling in the winter (with or without hibernation) includes (1) an upregulation of transcriptional factor, anti-apoptotic factor, nitric oxide synthase, protein kinase C-ε, and phosphatidylinositol-4,5-bisphosphate 3-kinase; (2) an upregulation of antioxidant enzymes (e.g. superoxide dismutase and catalase); (3) a reduction in the oxidation level of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII); and (4) alterations in the expression and activity of multiple ion channels/transporters. Therefore, the cardioprotection against I/R injury in the winter is most likely mediated by enhancement in signaling pathways that are shared by preconditioning, reduced cell apoptosis, and increased detoxification of reactive oxygen species (ROS). The resistance to cardiac arrhythmias and sudden cardiac death in the winter is closely associated with an upregulation of the antioxidant catalase and a downregulation of CaMKII activation. This remodeling of the heart is associated with a reduction in the incidence of afterdepolarizations and triggered activities. In this short review article, we will discuss the seasonal changes in gene and protein expression profiles as well as alterations in the function of key proteins that are associated with the occurrence of cardioprotection against myocardial damage from ischemic events and fatal arrhythmias in a mammalian hibernator. Understanding the intrinsic cardiac adaptive mechanisms that confer cardioprotection in hibernators may offer new strategies to protect non-hibernating animals, especially humans, from I/R injury and ischemia-induced fatal cardiac arrhythmias.

Tissue-specific seasonal changes in mitochondrial function of a mammalian hibernator

Mammalian hibernators, such as golden-mantled ground squirrels ( Callospermophilus lateralis; GMGS), cease to feed while reducing metabolic rate and body temperature during winter months, surviving exclusively on endogenous fuels stored before hibernation. We hypothesized that mitochondria, the cellular sites of oxidative metabolism, undergo tissue-specific seasonal adjustments in carbohydrate and fatty acid utilization to facilitate or complement this remarkable phenotype. To address this, we performed high-resolution respirometry of mitochondria isolated from GMGS liver, heart, skeletal muscle, and brown adipose tissue (BAT) sampled during summer (active), fall (prehibernation), and winter (hibernation) seasons using multisubstrate titration protocols. Mitochondrial phospholipid composition was examined as a postulated intrinsic modulator of respiratory function across tissues and seasons. Respirometry revealed seasonal variations in mitochondrial oxidative phosphorylation capacity, substrate utilization, and coupling efficiency that reflected the distinct functions and metabolic demands of the tissues they support. A consistent finding across tissues was a greater influence of fatty acids (palmitoylcarnitine) on respiratory parameters during the prehibernation and hibernation seasons. In particular, fatty acids had a greater suppressive effect on pyruvate-supported oxidative phosphorylation in heart, muscle, and liver mitochondria and enhanced uncoupled respiration in BAT and muscle mitochondria in the colder seasons. Seasonal variations in the mitochondrial membrane composition reflected changes in the supply and utilization of polyunsaturated fatty acids but were generally mild and inconsistent with functional variations. In conclusion, mitochondria respond to seasonal variations in physical activity, temperature, and nutrient availability in a tissue-specific manner that complements circannual shifts in the bioenergetic and thermoregulatory demands of mammalian hibernators.

Mammalian hibernation and regulation of lipid metabolism: a focus on non-coding RNAs

Numerous species will confront severe environmental conditions by undergoing significant metabolic rate reduction. Mammalian hibernation is one such natural model of hypometabolism. Hibernators experience considerable physiological, metabolic, and molecular changes to survive the harsh challenges associated with winter. Whether as fuel source or as key signaling molecules, lipids are of primary importance for a successful bout of hibernation and their careful regulation throughout this process is essential. In recent years, a plethora of non-coding RNAs has emerged as potential regulators of targets implicated in lipid metabolism in diverse models. In this review, we introduce the general characteristics associated with mammalian hibernation, present the importance of lipid metabolism prior to and during hibernation, as well as discuss the potential relevance of non-coding RNAs such as miRNAs and lncRNAs during this process.

Substrate-specific changes in mitochondrial respiration in skeletal and cardiac muscle of hibernating thirteen-lined ground squirrels

During torpor, the metabolic rate (MR) of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) is considerably lower relative to euthermia, resulting in part from temperature-independent mitochondrial metabolic suppression in liver and skeletal muscle, which together account for ~40% of basal MR. Although heart accounts for very little (<0.5%) of basal MR, in the present study, we showed that respiration rates were decreased up to 60% during torpor in both subsarcolemmal (SS) and intermyofibrillar (IM) mitochondria from cardiac muscle. We further demonstrated pronounced seasonal (summer vs. winter [i.e., interbout] euthermia) changes in respiration rates in both mitochondrial subpopulations in this tissue, consistent with a shift in fuel use away from carbohydrates and proteins and towards fatty acids and ketones. By contrast, these seasonal changes in respiration rates were not observed in either SS or IM mitochondria isolated from hind limb skeletal muscle. Both populations of skeletal muscle mitochondria, however, did exhibit metabolic suppression during torpor, and this suppression was 2- to 3-fold greater in IM mitochondria, which provide ATP for Ca(2+)- and myosin ATPases, the activities of which are likely quite low in skeletal muscle during torpor because animals are immobile. Finally, these changes in mitochondrial respiration rates were still evident when standardized to citrate synthase activity rather than to total mitochondrial protein.

AMPK and ACCchange with fasting and physiological condition in euthermic and hibernating golden-mantled ground squirrels (Callospermophilus lateralis)

AMP-activated protein kinase (AMPK) is a cellular energy sensor that responds to low endogenous energy by stimulating fatty acid oxidation (through inactivation of acetyl-CoA carboxylase (ACC)) and food intake. Fasting generally stimulates phosphorylation of AMPK (pAMPK) and ACC (pACC), but it is unclear how AMPK and ACC react to a long-term fast (i.e. hibernation). We performed Western blots for total and pAMPK and pACC on tissues from a species of hibernator (Callospermophilus lateralis) after short-term summer fasting (1-5 days) and long-term winter fasting (3 months). Winter animals were sacrificed during hibernation at low body temperature (torpid, T(b)~5°C) or at normal high T(b)(euthermic, T(b)~37°C). We found a general increase in pAMPK in most tissues (liver, muscle, and white adipose tissue (WAT), but not hypothalamus) and pACC in all tissues after a short-term summer fast. Response of AMPK and ACC to a long-term winter fast differed by tissue-in liver, there was no difference in total or pAMPK or pACC between groups, but in muscle, WAT and BAT, euthermic GMGS had lower relative abundance of pAMPK and pACC than torpid animals. Therefore, AMPK may be an important energy sensor at all points in hibernator's circannual cycles of food intake and T(b).

An APPL1-AMPK signaling axis mediates beneficial metabolic effects of adiponectin in the heart

Adiponectin promotes cardioprotection by various mechanisms, and this study used primary cardiomyocytes and the isolated working perfused heart to investigate cardiometabolic effects. We show in adult cardiomyocytes that adiponectin increased CD36 translocation and fatty acid uptake as well as insulin-stimulated glucose transport and Akt phosphorylation. Coimmunoprecipitation showed that adiponectin enhanced association of AdipoR1 with APPL1, subsequent binding of APPL1 with AMPKα2, which led to phosphorylation and inhibition of ACC and increased fatty acid oxidation. Using siRNA to effectively knockdown APPL1 in neonatal cardiomyocytes, we demonstrated an essential role for APPL1 in mediating increased fatty acid uptake and oxidation by adiponectin. Importantly, enhanced fatty acid oxidation in conjunction with AMPK and ACC phosphorylation was also observed in the isolated working heart. Despite increasing fatty acid oxidation and myocardial oxygen consumption, adiponectin increased hydraulic work and maintained cardiac efficiency. In summary, the present study documents several beneficial metabolic effects mediated by adiponectin in the heart and provides novel insight into the mechanisms behind these effects, in particular the importance of APPL1.

Malonyl-CoA decarboxylase is a major regulator of myocardial fatty acid oxidation

The energy demands of the heart are normally met by oxidation of both glucose and fatty acids. Fatty acid oxidation is limited by the uptake of fatty acyl coenzyme A (CoA) into the mitochondria, a process regulated by carnitine palmitoyltransferase (CPT)1. Malonyl CoA is a potent endogenous inhibitor of CPT1, and therefore plays an integral role in the control of myocardial fatty acid oxidation. Malonyl-CoA decarboxylase (MCD) is responsible for the removal of malonyl CoA and may control myocardial fatty acid oxidation. Indeed, strategies using MCD inhibitors and MCD knockout mice have provided the first evidence for a direct role of MCD in the control of myocardial fatty acid oxidation. Based on these studies, pharmacologic inhibition of MCD has been proposed to be a viable approach for the treatment of ischemic heart disease resulting from a variety of pathologic conditions, including coronary artery diseases, pathologic hypertrophy, and hypertension.

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

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

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.

Changes of peroxisomal fatty acid metabolism during cold acclimatization in hibernating jerboa (Jaculus orientalis)

Jerboa (Jaculus orientalis) is a deep hibernator originating from sub-desert highlands and represents an excellent model to help to understand the incidence of seasonal variations of food intake and of body as well as environmental temperatures on lipid metabolism. In jerboa, hibernation processes are characterized by changes in the size of mitochondria, the number of peroxisomes in liver and in the expression of enzymes linked to fatty acid metabolism. In liver and kidney, cold acclimatization shows an opposite effect on the activities of the mitochondrial acyl-CoA dehydrogenase (-50%) and the peroxisomal acyl-CoA oxidase (AOX) (+50%), while in brown and white adipose tissues, both activities are decreased down to 85%. These enzymes activities are subject to a strong induction in brown and in white adipose tissue (3.4- to 7.5-fold, respectively) during the hibernation period which is characterized by a low body temperature (around 10 degrees C) and by starvation. Expression level of AOX mRNA and protein are increased during both pre-hibernation and hibernation periods. Unexpectedly, treatment with ciprofibrate, a hypolipemic agent, deeply affects lipolysis in brown adipose tissue by increasing acyl-CoA dehydrogenase activity (3.4-fold), both AOX activity and mRNA levels (2.8- and 3.8-fold, respectively) during pre-hibernation. Therefore, during pre-hibernation acclimatization, there is a negative regulation of fatty acid degradation allowing to accumulate a lipid stock which is later degraded during the hibernation period (starvation) due to a positive regulation of enzymes providing the required energy for animal survival.

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.