Metabolic rate depression and biochemical adaptation in anaerobiosis, hibernation and estivation

Metabolic mechanisms for anoxia tolerance and freezing survival in the intertidal gastropod, Littorina littorea

The gastropod mollusk, Littorina littorea L., is a common inhabitant of the intertidal zone along rocky coastlines of the north Atlantic. This species has well-developed anoxia tolerance and freeze tolerance and is extensively used as a model for exploring the biochemical adaptations that support these tolerances as well as for toxicological studies aimed at identifying effective biomarkers of aquatic pollution. This article highlights our current understanding of the molecular mechanisms involved in anaerobiosis and freezing survival of periwinkles, particularly with respect to anoxia-induced metabolic rate depression. Analysis of foot muscle and hepatopancreas metabolism includes anoxia-responsive changes in enzyme regulation, signal transduction, gene expression, post-transcriptional regulation of mRNA, control of translation, and cytoprotective strategies including chaperones and antioxidant defenses. New studies describe the regulation of glucose-6-phosphate dehydrogenase by reversible protein phosphorylation, the role of microRNAs in suppressing mRNA translation in the hypometabolic state, modulation of glutathione S-transferase isozyme patterns, and the regulation of the unfolded protein response.

Seasonal changes in the behaviour and respiration physiology of the freshwater duck mussel Anodonta anatina

For low-energy organisms like bivalves, the costs of thermal compensation of biological rates (synonymous with acclimation and acclimatization) may be higher than the benefits. In the first experiment, we examined the effects of seasonal temperature changes on the bivalve Anodonta anatina, making measurements each month for a year at the corresponding temperature for that time of year. Only burrowing rate was partially compensated. There was no evidence for compensation of valve closure duration, frequency or locomotory speed. In a second experiment, we compared A. anatina at summer and winter temperatures (24 and 4°C respectively) and found no evidence for compensation of the burrowing rate, valve closure duration, frequency, or oxygen consumption rates during burrowing, immediately after valve closure and at rest. Within the experimental error of this study, the evidence suggests that thermal compensation of biological rates is not a strategy employed by A. anatina. We argue that this is due to a lack of evolutionary pressure to acclimatize, or evolutionary pressure to not acclimatize. Firstly, there is little incentive to increase metabolic rate to enhance predatory ability given that these are filter feeders. Secondly, maintained low energetic demand, enhanced at winter temperatures, is essential for predator avoidance, i.e. valve closure. Thus, we suggest that the costs of acclimatization outweigh the benefits in A. anatina.

Decreased hydrogen peroxide production and mitochondrial respiration in skeletal muscle but not cardiac muscle of the green-striped burrowing frog, a natural model of muscle disuse

Suppression of disuse-induced muscle atrophy has been associated with altered mitochondrial reactive oxygen species (ROS) production in mammals. However, despite extended hindlimb immobility aestivating animals exhibit little skeletal muscle atrophy compared with artificially-immobilised mammalian models. Therefore, we studied mitochondrial respiration and ROS (H2O2) production in permeabilised muscle fibres of the green-striped burrowing frog, Cyclorana alboguttata. Mitochondrial respiration within saponin-permeabilised skeletal and cardiac muscle fibres was measured concurrently with ROS production using high-resolution respirometry coupled to custom-made fluorometers. After four months of aestivation, C. alboguttata had significantly depressed whole body metabolism by approximately 70% relative to control (active) frogs, and mitochondrial respiration in saponin-permeabilised skeletal muscle fibres decreased by almost 50% both in the absence of ADP and during oxidative phosphorylation. Mitochondrial ROS production showed up to an 88% depression in aestivating skeletal muscle when malate, succinate and pyruvate were present at concentrations likely reflecting those in vivo. The percentage ROS released per O2 molecule consumed was also approximately 94 % less at these concentrations indicating an intrinsic difference in ROS production capacities during aestivation. We also examined mitochondrial respiration and ROS production in permeabilised cardiac muscle fibres and found that aestivating frogs maintained respiratory flux and ROS production at control levels. These results show that aestivating C. alboguttata has the capacity to independently regulate mitochondrial function in skeletal and cardiac muscles. Furthermore, this work indicates that ROS production can be suppressed in the disused skeletal muscle of aestivating frogs, which may in turn protect against potential oxidative damage and preserve skeletal muscle structure during aestivation and following arousal.

Long-Term survival of anoxia despite rapid ATP decline in embryos of the annual killifish Austrofundulus limnaeus

Embryos of the annual killifish Austrofundulus limnaeus can survive for months in the complete absence of oxygen. Survival of anoxia is associated with entry into a state of metabolic dormancy known as diapause. However, extreme tolerance of anoxia is retained for several days of post-diapause development. Rates of heat dissipation in diapause II and 4 days post-diapause II embryos were measured under aerobic conditions and during the transition into anoxia. Phosphorylated adenylate compounds were quantified in embryos during entry into anoxia and after 12 hr of aerobic recovery. Rates of heat dissipation were not affected by exposure to anoxia in diapause II embryos, while post-diapause II embryos experienced a profound decrease in heat dissipation. ATP decreased substantially in both developmental stages upon exposure to anoxia, and all indicators of cellular energetic status indicated energetic stress, at least based on the mammalian paradigm. The rate of decline in ATP is the most acute reported for any vertebrate. The mechanisms responsible for cellular survival despite a clear dysregulation between energy production and energy consumption remain to be identified. Necrotic and apoptotic cell death in response to hypoxia contribute to poor survival during many diseases and pathological conditions in mammals. Understanding the mechanisms that are in place to prevent maladaptive cell death in embryos of A. limnaeus may greatly improve treatment strategies in diseases that involve hypoxia and reperfusion injuries.

Aestivation: signaling and hypometabolism

Aestivation is a survival strategy used by many vertebrates and invertebrates to endure arid environmental conditions. Key features of aestivation include strong metabolic rate suppression, strategies to retain body water, conservation of energy and body fuel reserves, altered nitrogen metabolism, and mechanisms to preserve and stabilize organs, cells and macromolecules over many weeks or months of dormancy. Cell signaling is crucial to achieving both a hypometabolic state and reorganizing multiple metabolic pathways to optimize long-term viability during aestivation. This commentary examines the current knowledge about cell signaling pathways that participate in regulating aestivation, including signaling cascades mediated by the AMP-activated kinase, Akt, ERK, and FoxO1.

Metabolic responses of the South American ornate horned frog (Ceratophrys ornata) to estivation

We examined the metabolic responses of the South American frog, Ceratophrys ornata, to laboratory-induced estivation. Whole-animal and mass-specific oxygen consumption rates (VO(2)) did not change during fasting or 56days of estivation, despite observing significant decreases in body mass. The maintenance of mass-specific metabolic rate at routine levels during estivation suggests that metabolic rate suppression is not a major response to estivation in this species. There was a significant decline in liver glycogen and a loss of adipose tissue mass during estivation, suggesting that both carbohydrate and lipid pathways are used to fuel metabolism during estivation. The activity of pyruvate dehydrogenase, an important regulator of carbohydrate oxidation, and carnitine palmitoyltransferase and 3-hydroxyacyl-CoA dehydrogenase, regulators of lipid oxidation, showed no significant change in activity in liver, heart, and muscle between estivating and active frogs. There was an increase in plasma osmolality, which is characteristic of estivating animals. Overall, our metabolic analysis of estivation in C. ornata indicates that this species does not employ a dramatic suppression metabolic rate to survive dehydration stress and that both endogenous carbohydrates and lipids are used as metabolic fuels.

Suppression of MAPKAPK2 during mammalian hibernation

Metabolic signaling coordinates the transition by hibernating mammals from euthermia into profound torpor. Organ-specific responses by activated p38 mitogen activated protein kinase (MAPK) are known to contribute to this transition. Therefore, we hypothesized that the MAPK-activated protein kinase-2 (MAPKAPK2), a downstream target of p38 MAPK, would also be active in establishing the torpid state. Kinetic parameters of MAPKAPK2 from skeletal muscle of Richardson's ground squirrels, Spermophilus richardsonii, were analyzed using a fluorescence assay. MAPKAPK2 activity was 27.4±1.27 pmol/min/mg in muscle from euthermic squirrels and decreased by ∼63% during cold torpor, while total protein levels were unchanged (as assessed by immunoblotting). In vitro treatment of MAPKAPK2 via stimulation of endogenous phosphatases and addition of commercial alkaline phosphatase decreased enzyme activity to only ∼3-5% of its original value in muscle extracts from both euthermic and hibernating squirrels suggesting that posttranslational modification suppresses MAPKAPK2 during the transition from euthermic to torpid states. Enzyme S₀.₅ and n(H) values for ATP and peptide substrates changed significantly between euthermia and torpor, and also between assays at 22 versus 10 °C but, kinetic parameters were actually closely conserved when values for the euthermic enzyme at 22 °C were directly compared with the hibernator enzyme at 10 °C. Arrhenius plots showed significantly different activation energies of 40.8±0.7 and 54.3±2.7 kJ/mol for the muscle enzyme from euthermic versus torpid animals, respectively but MAPKAPK2 from the two physiological states showed no difference in sensitivity to urea denaturation. Overall, the results show that total activity of MAPKAPK2 is in fact reduced, despite previous findings of p38 MAPK activation, and kinetic parameters are altered when ground squirrels enter torpor but protein stability is not apparently changed. The data suggest that MAPKAPK2 suppression may have a significant role in the differential regulation of muscle target proteins when ground squirrels enter torpor.

Cell cycle regulation during development and dormancy in embryos of the annual killifish Austrofundulus limnaeus

Embryos of the annual killifish Austrofundulus limnaeus can enter into a state of metabolic dormancy, termed diapause, as a normal part of their development. In addition, these embryos can also survive for prolonged sojourns in the complete absence of oxygen. Dormant embryos support their metabolism using anaerobic metabolic pathways, regardless of oxygen availability. Dormancy in diapause is associated with high ATP and a positive cellular energy status, while anoxia causes a severe reduction in ATP content and large reductions in adenylate energy charge and ATP/ADP ratios. Most cells are arrested in the G 1/G 0 phase of the cell cycle during diapause and in response to oxygen deprivation. In this paper, we review what is known about the physiological and biochemical mechanisms that support metabolic dormancy in this species. We also highlight the great potential that this model holds for identifying novel therapies for human diseases such as heart attack, stroke and cancer.

Insect cold hardiness: metabolic, gene, and protein adaptation1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Winter survival for thousands of species of insects relies on adaptive strategies for cold hardiness. Two basic mechanisms are widely used (freeze avoidance by deep supercooling and freeze tolerance where insects endure ice formation in extracellular fluid spaces), whereas additional strategies (cryoprotective dehydration, vitrification) are also used by some polar species in extreme environments. This review assesses recent research on the biochemical adaptations that support insect cold hardiness. We examine new information about the regulation of cryoprotectant biosynthesis, mechanisms of metabolic rate depression, role of aquaporins in water and glycerol movement, and cell preservation strategies (chaperones, antioxidant defenses and metal binding proteins, mitochondrial suppression) for survival over the winter. We also review the new information coming from the use of genomic and proteomic screening methods that are greatly widening the scope for discovery of genes and proteins that support winter survival.

Cold tolerance and freeze-induced glucose accumulation in three terrestrial slugs

Cold tolerance and metabolic responses to freezing of three slug species common in Scandinavia (Arion ater, Arion rufus and Arion lusitanicus) are reported. Autumn collected slugs were cold acclimated in the laboratory and subjected to freezing conditions simulating likely winter temperatures in their habitat. Slugs spontaneously froze at about -4 °C when cooled under dry conditions, but freezing of body fluids was readily induced at -1 °C when in contact with external ice crystals. All three species survived freezing for 2 days at -1 °C, and some A. rufus and A. lusitanicus also survived freezing at -2 °C. (1)H NMR spectroscopy revealed that freezing of body fluids resulted in accumulation of lactate, succinate and glucose. Accumulation of lactate and succinate indicates that ATP production occurred via fermentative pathways, which is likely a result of oxygen depletion in frozen tissues. Glucose increased from about 6 to 22 μg/mg dry tissue upon freezing in A. rufus, but less so in A. ater and A. lusitanicus. Glucose may thus act as a cryoprotectant in these slugs, although the concentrations are not as high as reported for other freeze tolerant invertebrates.

Each to their own: skeletal muscles of different function use different biochemical strategies during aestivation at high temperature

Preservation of muscle morphology depends on a continuing regulatory balance between molecules that protect, and molecules that damage, muscle structural integrity. Excessive disruption of the biochemical balance that favours reactive oxygen species (ROS) in disused muscles may lead to oxidative stress; which in turn is associated with increased atrophic or apoptotic signalling and/or oxidative damage to the muscle and thus muscle disuse atrophy. Increases in rate of oxygen consumption likely increase the overall generation of ROS in vivo. Temperature-induced increases in muscle oxygen consumption rate occur in some muscles of ectotherms undergoing prolonged muscular disuse during aestivation. In the green-striped burrowing frog, Cyclorana alboguttata, both large jumping muscles and small non-jumping muscles undergo atrophy seemingly commensurate with their rate of oxygen consumption during aestivation. However, since the extent of atrophy in these muscles is not enhanced at higher temperatures despite a temperature sensitive rate of oxygen consumption in the jumping muscle, we proposed that muscles are protected by biochemical means that when mobilised at higher temperatures inhibit atrophy. We proposed the biochemical response to temperature would be muscle-specific. We examined the effect of temperature on the antioxidant and heat shock protein systems and evidence of oxidative damage to lipids and proteins in two functionally different skeletal muscles, gastrocnemius (jumping muscle) and iliofibularis (non-jumping muscle), by aestivating frogs at 24 and 30oC for six months. We assayed small molecule antioxidant capacity, mitochondrial and cytosolic SOD and Hsp70 to show that protective mechanisms in disused muscles are differentially regulated both with respect to temperature and aestivation. High aestivation temperature results in an antioxidant response in the metabolically temperature-sensitive jumping muscle. We assayed lipid peroxidation and protein oxidation to show that oxidative damage is apparent during aestivation and its pattern is muscle-specific, but unaffected by temperature. Consideration is given to how the complex responses of muscle biochemistry inform of the different strategies muscles may use in regulating their oxidative environment during extended disuse and disuse at high temperature.

Adaptive evolution of Leptin in heterothermic bats

Heterothermy (hibernation and daily torpor) is a key strategy that animals use to survive in harsh conditions and is widely employed by bats, which are found in diverse habitats and climates. Bats comprise more than 20% of all mammals and although heterothermy occurs in divergent lineages of bats, suggesting it might be an ancestral condition, its evolutionary history is complicated by complex phylogeographic patterns. Here, we use Leptin, which regulates lipid metabolism and is crucial for thermogenesis of hibernators, as molecular marker and combine physiological, molecular and biochemical analyses to explore the possible evolutionary history of heterothermy in bat. The two tropical fruit bats examined here were homeothermic; in contrast, the two tropical insectivorous bats were clearly heterothermic. Molecular evolutionary analyses of the Leptin gene revealed positive selection in the ancestors of all bats, which was maintained or further enhanced the lineages comprising mostly heterothermic species. In contrast, we found evidence of relaxed selection in homeothermic species. Biochemical assays of bat Leptin on the activity on adipocyte degradation revealed that Leptin in heterothermic bats was more lipolytic than in homeothermic bats. This shows that evolutionary sequence changes in this protein are indeed functional and support the interpretation of our physiological results and the molecular evolutionary analyses. Our combined data strongly support the hypothesis that heterothermy is the ancestral state of bats and that this involved adaptive changes in Leptin. Subsequent loss of heterothermy in some tropical lineages of bats likely was associated with range and dietary shifts.

Influence of elevated temperature on metabolism during aestivation: implications for muscle disuse atrophy

Reactive oxygen species (ROS), produced commensurate with aerobic metabolic rate, contribute to muscle disuse atrophy (MDA) in immobilised animals by damaging myoskeletal protein and lipids. Aestivating frogs appear to avoid MDA in part by substantially suppressing metabolic rate. However, as ectotherms, metabolic rate is sensitive to environmental temperature, and the high ambient temperatures that may be experienced by frogs during aestivation could in fact promote MDA. In this study, we investigated the effect of temperature on the metabolic rate of the aestivating frog Cyclorana alboguttata and its skeletal muscles in order to determine their likely susceptibility to MDA. Compared with non-aestivating frogs, a significant decrease in metabolic rate was recorded for aestivating frogs at 20, 24 and 30°C. At 30°C, however, the metabolic rate of aestivating frogs was significantly higher, approximately double that of frogs aestivating at 20 or 24°C, and the magnitude of the metabolic depression was significantly reduced at 30°C compared with that at 20°C. Temperature effects were also observed at the tissue level. At 24 and 30°C the metabolic rate of all muscles from aestivating frogs was significantly depressed compared with that of muscles from non-aestivating frogs. However, during aestivation at 30°C the metabolic rates of gastrocnemius, sartorius and cruralis were significantly elevated compared with those from frogs aestivating at 24°C. Our data show that the metabolism of C. alboguttata and its skeletal muscles is elevated at higher temperatures during aestivation and that the capacity of the whole animal to actively depress metabolism is impaired at 30°C.

The regulation of food intake in mammalian hibernators: a review

One of the most profound hallmarks of mammalian hibernation is the dramatic reduction in food intake during the winter months. Several species of hibernator completely cease food intake (aphagia) for nearly 7 months regardless of ambient temperature and in many cases, whether or not food is available to them. Food intake regulation has been studied in mammals that hibernate for over 50 years and still little is known about the physiological mechanisms that control this important behavior in hibernators. It is well known from lesion experiments in non-hibernators that the hypothalamus is the main brain region controlling food intake and therefore body mass. In hibernators, the regulation of food intake and body mass is presumably governed by a circannual rhythm since there is a clear seasonal rhythm to food intake: animals increase food intake in the summer and early autumn, food intake declines in autumn and actually ceases in winter in many species, and resumes again in spring as food becomes available in the environment. Changes in circulating hormones (e.g., leptin, insulin, and ghrelin), nutrients (glucose, and free fatty acids), and cellular enzymes such as AMP-activated protein kinase (AMPK) have been shown to determine the activity of neurons involved in the food intake pathway. Thus, it appears likely that the food intake pathway is controlled by a variety of inputs, but is also acted upon by upstream regulators that are presumably rhythmic in nature. Current research examining the molecular mechanisms and integration of environmental signals (e.g., temperature and light) with these molecular mechanisms will hopefully shed light on how animals can turn off food intake and survive without eating for months on end.

Glucose-6-Phosphate Dehydrogenase Regulation in Anoxia Tolerance of the Freshwater Crayfish Orconectes virilis

Glucose-6-phosphate dehydrogenase (G6PDH), the enzyme which catalyzes the rate determining step of the pentose phosphate pathway (PPP), controls the production of nucleotide precursor molecules (R5P) and powerful reducing molecules (NADPH) that support multiple biosynthetic functions, including antioxidant defense. G6PDH from hepatopancreas of the freshwater crayfish (Orconectes virilis) showed distinct kinetic changes in response to 20 h anoxic exposure. K(m) values for both substrates decreased significantly in anoxic crayfish; K(m) NADP(+) dropped from 0.015 ± 0.008 mM to 0.012 ± 0.008 mM, and K(m) G6P decreased from 0.13 ± 0.02 mM to 0.08 ± 0.007 mM. Two lines of evidence indicate that the mechanism involved is reversible phosphorylation. In vitro incubations that stimulated protein kinase or protein phosphatase action mimicked the effects on anoxia on K(m) values, whereas DEAE-Sephadex chromatography showed the presence of two enzyme forms (low- and high-phosphate) whose proportions changed during anoxia. Incubation studies implicated protein kinase A and G in mediating the anoxia-responsive changes in G6PDH kinetic properties. In addition, the amount of G6PDH protein (measured by immunoblotting) increased by ∼60% in anoxic hepatopancreas. Anoxia-induced phosphorylation of G6PDH could contribute to modifying carbon flow through the PPP under anoxic conditions, potentially maintaining NADPH supply for antioxidant defense during prolonged anoxia-induced hypometabolism.

Heat shock protein expression in relation to reproductive cycle in land snails: Implications for survival

Land snails are subject to daily and seasonal variations in temperature and in water availability and use heat shock proteins (HSPs) as part of their survival strategy. We tested whether the reproductive cycle of land snails affects the endogenous levels of HSPs, and their involvement in the reproductive process. We examined HSP levels in the foot tissue of two Sphincterochila species, S. cariosa and S. zonata, before and after laying eggs, and analyzed the albumen gland (reproductive organ) of both species and eggs of S. cariosa for the presence and quantity of various HSPs. Our study shows reduction in the expression level of Hsp70 isoforms and Hsp90 in S. zonata foot and of Hsp74 in S. cariosa foot during the period preceding egg laying compared to the post-reproductive stage. Hsp70 isoforms and Hsp25 were highly expressed in both large albumen glands and in freshly laid eggs of S. cariosa, whereas large albumen glands of S. zonata expressed mainly Hsp70 isoforms. We conclude that a trade-off between survival and fertility is responsible for the expression level of HSPs in the foot tissue of Sphincterochila snails. Our study shows that HSPs are involved in the reproductive process. We propose that parental provision of HSPs may be part of a "be prepared" strategy of Sphincterochila snails, and that HSPs may play important roles in the survival strategy of land snails during the early life stages. Our observations also highlight the importance of the reproductive status in study of whole organisms, especially when assessing the HSP response to stress.

Do season and distribution affect thermal energetics of a hibernating bat endemic to the tropics and subtropics?

Although many tropical and subtropical areas experience pronounced seasonal changes in weather and food availability, few studies have examined and none have compared the thermal physiology and energetics of a hibernating mammal that is restricted to these regions. We quantified thermal energetics of northern long-eared bats (Nyctophilus bifax; body mass ∼10 g) during summer, winter, and spring from a subtropical habitat, and also during winter from a tropical habitat, to determine how N. bifax cope with climate and seasonal changes in weather. We captured bats in the wild and measured metabolic rates via open-flow respirometry. The basal metabolic rate of subtropical bats at an ambient temperature (T(a)) of 32.6 ± 0.7°C was 1.28 ± 0.06 ml O(2)·g(-1)·h(-1) during both summer and winter, similar to other species of Nyctophilus. Resting metabolic rates below the thermoneutral zone increased similarly with decreasing T(a) during all seasons and in both regions. All individuals showed a high proclivity to enter torpor at T(a) values below the thermoneutral zone. Metabolic rates in torpid thermoconforming bats fell with T(a) and body temperature, and mean minimum metabolic rates during torpor were similar during all seasons and in both regions and as predicted from body mass in temperate zone hibernators. At very low T(a), torpid N. bifax thermoregulated, and this threshold T(a) differed significantly between subtropical (T(a) = 3.5 ± 0.3°C) and tropical (T(a) = 6.7 ± 0.7°C) individuals, but not between seasons. Our data show that thermal energetics of N. bifax do not vary seasonally and in many aspects are similar in tropical and subtropical bats; however, torpid individuals from the subtropics allow body temperature to fall to significantly lower values than those from the tropics.

Natural annual cycle of heat shock protein expression in land snails: desert versus Mediterranean species of Sphincterochila

Land snails are subject to daily and seasonal variations in temperature and in water availability, and have evolved annual cycles of activity and aestivation as part of their survival strategy. We tested in the field whether adaptation to different habitats affects the endogenous levels of heat shock proteins (HSPs) in two closely related Sphincterochila snail species, a desiccation-resistant desert species, Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species, S. cariosa. We examined HSP levels in various tissues of snails during aestivation and after resumption of activity. Our study shows that, during aestivation, S. cariosa had higher standing stocks of Hsp70 in the foot and the hepatopancreas, and of small HSPs (sHSPs) in all the examined tissues, whereas S. zonata had higher stocks of Hsp70 in the kidney and of Hsp90 in the kidney and in the hepatopancreas. Arousal induced a general upregulation of HSPs, except for Hsp90, the expression of which in the foot was higher during aestivation. We suggest that the stress protein machinery is upregulated during arousal in anticipation of possible oxidative stress ensuing from the accelerating metabolic rate and the exit from the deep hypometabolic state. Our findings support the concept that, in land snails, aestivation and activity represent two distinct physiological states, and suggest that land snails use HSPs as important components of the aestivation mechanism, and as part of their survival strategy during and after arousal. Our study also indicates that adaptation to different habitats results in the development of distinct strategies of HSP expression with likely consequences for the ecology and distribution of land snails.

Energetics of insect diapause

Managing metabolic resources is critical for insects during diapause when food sources are limited or unavailable. Insects accumulate reserves prior to diapause, and metabolic depression during diapause promotes reserve conservation. Sufficient reserves must be sequestered to both survive the diapause period and enable postdiapause development that may involve metabolically expensive functions such as metamorphosis or long-distance flight. Nutrient utilization during diapause is a dynamic process, and insects appear capable of sensing their energy reserves and using this information to regulate whether to enter diapause and how long to remain in diapause. Overwintering insects on a tight energy budget are likely to be especially vulnerable to increased temperatures associated with climate change. Molecular mechanisms involved in diapause nutrient regulation remain poorly known, but insulin signaling is likely a major player. We also discuss other possible candidates for diapause-associated nutrient regulation including adipokinetic hormone, neuropeptide F, the cGMP-kinase For, and AMPK.

Control of carbohydrate metabolism in an anoxia-tolerant nervous system

Anoxia-tolerant animal models are crucial to understand protective mechanisms during low oxygen excursions. As glycogen is the main fermentable fuel supporting energy production during oxygen tension reduction, understanding glycogen metabolism can provide important insights about processes involved in anoxia survival. In this report we studied carbohydrate metabolism regulation in the central nervous system (CNS) of an anoxia-tolerant land snail during experimental anoxia exposure and subsequent reoxygenation. Glucose uptake, glycogen synthesis from glucose, and the key enzymes of glycogen metabolism, glycogen synthase (GS) and glycogen phosphorylase (GP), were analyzed. When exposed to anoxia, the nervous ganglia of the snail achieved a sustained glucose uptake and glycogen synthesis levels, which seems important to maintain neural homeostasis. However, the activities of GS and GP were reduced, indicating a possible metabolic depression in the CNS. During the aerobic recovery period, the enzyme activities returned to basal values. The possible strategies used by Megalobulimus abbreviatus CNS to survive anoxia are discussed.

Acute in vitro hypoxia and high-altitude (4,559 m) exposure decreases leukocyte oxygen consumption

Hypoxia impairs metabolic functions by decreasing activity and expression of ATP-consuming processes. To separate hypoxia from systemic effects, we tested whether hypoxia at high altitude affects basal and PMA-stimulated leukocyte metabolism and how this compares to acute (15 min) and 24 h of in vitro hypoxia. Leukocytes were prepared at low altitude and ∼24 h after arrival at 4559 m. Mitochondrial oxygen consumption (JO₂) was measured by respirometry, oxygen radicals by electron spin resonance spectroscopy, both at a Po₂ = 100 mmHg (JO₂,₁₀₀) and 20 mmHg (JO₂,₂₀). Acute hypoxia of leukocytes decreased JO₂ at low altitude. Exposure to high altitude decreased JO₂,₁₀₀, whereas JO₂,₂₀ was not affected. Acute hypoxia of low-altitude samples decreased the activity of complexes I, II, and III. At high altitude, activity of complexes I and III were decreased when measured in normoxia. Stimulation of leukocytes with PMA increased JO₂,₁₀₀ at low (twofold) and high altitude (five-fold). At both locations, PMA-stimulated JO₂ was decreased by acute hypoxia. Basal and PMA-stimulated reactive oxygen species (ROS) production were unchanged at high altitude. Separate in vitro experiments performed at low altitude show that ∼75% of PMA-induced increase in JO₂ was due to increased extra-mitochondrial JO₂ (JO₂(,res); in the presence of rotenone and antimycin A). JO₂(,res) was doubled by PMA. Acute hypoxia decreased basal JO₂(,res) by ∼70% and PMA-stimulated JO₂(,res) by about 50% in cells cultured in normoxia and hypoxia (1.5% O₂; 24 h). Conversely, 24 h in vitro hypoxia decreased mitochondrial JO₂,₁₀₀ and JO₂,₂₀, extra-mitochondrial, basal, and PMA-stimulated JO₂ were not affected. These results show that 24 h of high altitude but not 24 h in vitro hypoxia decreased basal leukocyte metabolism, whereas PMA-induced JO₂ and ROS formation were not affected, indicating that prolonged high-altitude hypoxia impairs mitochondrial metabolism but does not impair respiratory burst. In contrast, acute hypoxia impairs respiratory burst at either altitude.

Cyclic AMP signaling in bivalve molluscs: an overview

The cyclic AMP (cAMP)-dependent signaling accounts for the control of cellular cascades involved in many physiological functions, and a wealth of information is available on the cAMP system that operates in mammalian cells. Nevertheless, cAMP has a central role also in nonmammalian vertebrates and invertebrates. The present review aims at examining the information available on bivalve molluscs, from the first studies carried out in the early 1980s to the last progresses made in the present days. The major focus is on the structural and operational characteristics of the main actors of the signaling pathway, i.e., adenylyl cyclase, G proteins, and protein kinase A, and on the role played by the cyclic nucleotide on smooth muscle, heart, gills, gonads, and metabolism regulation. Moreover, recent evidence regarding the cAMP system as a target of environmental stress factors are discussed. It will become clear that cAMP does play a wide and important role in bivalve physiology. Several issues have been sufficiently clarified, although investigated only in a few model species. However, further fundamental aspects remain unknown, mainly regarding molecular features and interactions with other signaling pathways, thus requiring further elucidation.

Heat shock proteins and resistance to desiccation in congeneric land snails

Land snails are subject to daily and seasonal variations in temperature and in water availability and depend on a range of behavioral and physiological adaptations for coping with problems of maintaining water, ionic, and thermal balance. Heat shock proteins (HSPs) are a multigene family of proteins whose expression is induced by a variety of stress agents. We used experimental desiccation to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desiccation-resistant, desert species Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species Sphincterochila cariosa. We examined the HSP response in the foot, hepatopancreas, and kidney tissues of snails exposed to normothermic desiccation. Our findings show variations in the HSP response in both timing and magnitude between the two species. The levels of endogenous Hsp72 in S. cariosa were higher in all the examined tissues, and the induction of Hsp72, Hsp74, and Hsp90 developed earlier than in S. zonata. In contrary, the induction of sHSPs (Hsp25 and Hsp30) was more pronounced in S. zonata compared to S. cariosa. Our results suggest that land snails use HSPs as part of their survival strategy during desiccation and as important components of the aestivation mechanism in the transition from activity to dormancy. Our study underscores the distinct strategy of HSP expression in response to desiccation, namely the delayed induction of Hsp70 and Hsp90 together with enhanced induction of sHSPs in the desert-dwelling species, and suggests that evolution in harsh environments will result in selection for reduced Hsp70 expression.

Aerobic and anaerobic starvation metabolism in methanotrophic bacteria

The capacity for anaerobic metabolism of endogenous and selected exogenous substrates in carbon- and energy-starved methanotrophic bacteria was examined. The methanotrophic isolate strain WP 12 survived extended starvation under anoxic conditions while metabolizing 10-fold less endogenous substrate than did parallel cultures starved under oxic conditions. During aerobic starvation, the cell biomass decreased by 25% and protein and lipids were the preferred endogenous substrates. Aerobic protein degradation (24% of total protein) took place almost exclusively during the initial 24 h of starvation. Metabolized carbon was recovered mainly as CO(inf2) during aerobic starvation. In contrast, cell biomass decreased by only 2.4% during anaerobic starvation, and metabolized carbon was recovered mainly as organic solutes in the starvation medium. During anaerobic starvation, only the concentration of intracellular low-molecular-weight compounds decreased, whereas no significant changes were measured for cellular protein, lipids, polysaccharides, and nucleic acids. Strain WP 12 was also capable of a limited anaerobic glucose metabolism in the absence of added electron acceptors. Small amounts of CO(inf2) and organic acids, including acetate, were produced from exogenous glucose under anoxic conditions. Addition of potential anaerobic electron acceptors (fumarate, nitrate, nitrite, or sulfate) to starved cultures of the methanotrophs Methylobacter albus BG8, Methylosinus trichosporium OB3b, and strain WP 12 did not stimulate anaerobic survival. However, anaerobic starvation of these bacteria generally resulted in better survival than did aerobic starvation. The results suggest that methanotrophic bacteria can enter a state of anaerobic dormancy accompanied by a severe attenuation of endogenous metabolism. In this state, maintenance requirements are presumably provided for by fermentation of certain endogenous substrates. In addition, low-level catabolism of exogenous substrates may support long-term anaerobic survival of some methanotrophic bacteria.

Regulation of Akt during torpor in the hibernating ground squirrel, Ictidomys tridecemlineatus

The 13-lined ground squirrel (Ictidomys tridecemlineatus) is capable of entering into extended periods of torpor during winter hibernation. The state of torpor represents a hypometabolic shift wherein the rate of oxygen consuming processes are strongly repressed in an effort to maintain cellular homeostasis as the availability of food energy becomes limited. We are interested in studying hibernation/torpor because of the robust state of tolerance to constrained oxygen delivery, oligemia, and hypothermia achieved by the tissues of hibernating mammals. The role of the serine/threonine kinase Akt (also known as PKB) has been examined in torpor in previous studies. However, this is the first study that examines the level of Akt phosphorylation in the liver during the two transition phases of the hibernation cycle: entrance into torpor, and the subsequent arousal from torpor. Our results indicate that Akt is activated in the squirrel liver by phosphorylation of two key residues (Thr(308) and Ser(473)) during entrance into torpor and arousal from torpor. Moreover, we observed increased phosphorylation of key substrates of Akt during the two transition stages of torpor. Finally, this study reports the novel finding that PRAS40, a component of the TORC1 multi-protein complex and a potentially important modulator of metabolism, is regulated during torpor.

Cytochrome c oxidase is regulated by modulations in protein expression and mitochondrial membrane phospholipid composition in estivating African lungfish

We examined some of the potential mechanisms lungfish ( Protopterus dolloi ) use to regulate cytochrome c oxidase (CCO), during metabolic depression. CCO activity was reduced by 67% in isolated liver mitochondria of estivating fish. This was likely accomplished, in part, by the 46% reduction in CCO subunit I protein expression in the liver. No change in the mRNA expression levels of CCO subunits I, II, III, and IV were found in the liver, suggesting CCO is under translational regulation; however, in the kidney, messenger limitation may be a factor as the expression of subunits I and II were depressed (∼10-fold) during estivation, suggesting tissue-specific mechanisms of regulation. CCO is influenced by mitochondrial membrane phospholipids, particularly cardiolipin (CL). In P. dolloi , the phospholipid composition of the liver mitochondrial membrane changed during estivation, with a ∼2.3-fold reduction in the amount of CL. Significant positive correlations were found between CCO activity and the amount of CL and phosphatidylethanolamine within the mitochondrial membrane. It appears CCO activity is regulated through multiple mechanisms in P. dolloi , and individual subunits of CCO are regulated independently, and in a tissue-specific manner. It is proposed that altering the amount of CL within the mitochondrial membrane may be a means of regulating CCO activity during metabolical depression in the African lungfish, P. dolloi .

An overview of stress response and hypometabolic strategies in Caenorhabditis elegans: conserved and contrasting signals with the mammalian system

Studies of the molecular mechanisms that are involved in stress responses (environmental or physiological) have long been used to make links to disease states in humans. The nematode model organism, Caenorhabditis elegans, undergoes a state of hypometabolism called the 'dauer' stage. This period of developmental arrest is characterized by a significant reduction in metabolic rate, triggered by ambient temperature increase and restricted oxygen/ nutrients. C. elegans employs a number of signal transduction cascades in order to adapt to these unfavourable conditions and survive for long times with severely reduced energy production. The suppression of cellular metabolism, providing energetic homeostasis, is critical to the survival of nematodes through the dauer period. This transition displays molecular mechanisms that are fundamental to control of hypometabolism across the animal kingdom. In general, mammalian systems are highly inelastic to environmental stresses (such as extreme temperatures and low oxygen), however, there is a great deal of conservation between the signal transduction pathways of nematodes and mammals. Along with conserving many of the protein targets in the stress response, many of the critical regulatory mechanisms are maintained, and often differ only in their level of expression. Hence, the C. elegans model outlines a framework of critical molecular mechanisms that may be employed in the future as therapeutic targets for addressing disease states.

Starvation physiology: reviewing the different strategies animals use to survive a common challenge

All animals face the possibility of limitations in food resources that could ultimately lead to starvation-induced mortality. The primary goal of this review is to characterize the various physiological strategies that allow different animals to survive starvation. The ancillary goals of this work are to identify areas in which investigations of starvation can be improved and to discuss recent advances and emerging directions in starvation research. The ubiquity of food limitation among animals, inconsistent terminology associated with starvation and fasting, and rationale for scientific investigations into starvation are discussed. Similarities and differences with regard to carbohydrate, lipid, and protein metabolism during starvation are also examined in a comparative context. Examples from the literature are used to underscore areas in which reporting and statistical practices, particularly those involved with starvation-induced changes in body composition and starvation-induced hypometabolism can be improved. The review concludes by highlighting several recent advances and promising research directions in starvation physiology. Because the hundreds of studies reviewed here vary so widely in their experimental designs and treatments, formal comparisons of starvation responses among studies and taxa are generally precluded; nevertheless, it is my aim to provide a starting point from which we may develop novel approaches, tools, and hypotheses to facilitate meaningful investigations into the physiology of starvation in animals.

Antioxidant defenses and oxidative stress parameters in tissues of mud crab (Scylla serrata) with reference to changing salinity

The effects of salinity (10, 17 and 35 ppt) on O(2) consumption, CO(2) release and NH(3) excretion by crabs and oxidative stress parameters and antioxidant defenses of its tissues were reported. An increase in salinity caused a decrease in O(2) consumption and CO(2) release and an increase in ammonia excretion by crabs. Lipid peroxidation, protein carbonyl, H(2)O(2) levels and total antioxidant capacity of the tissues elevated significantly at 35 ppt salinity except in abdominal muscle where H(2)O(2) content was low. Ascorbic acid content of tissues was higher at 17 ppt salinity than at 10 and 35 ppt salinities. With increasing salinity, a gradual decrease in SOD, an increase in catalase, no change in GPx and a decrease followed by an increase in GR activities were recorded for abdominal muscle. While for hepatopancreas, an increase followed by a decrease in SOD and catalase, decrease in GPx and GR activities were noticed with increasing salinity. In the case of gills, a decrease followed by an increase in SOD, a decrease in catalase and GPx and an increase in GR activities were noted when the salinity increased from 10 ppt to 35 ppt. These results suggest that salinity modulation of oxidative stress and antioxidant defenses in Scylla serrata is tissue specific.

Aestivation in mammals and birds

Aestivation, which in the context of this paper refers to avian and mammalian torpor in summer/at high ambient temperatures (T (a)), does not appear to differ functionally from other forms of torpor, and to a large extent reflects the higher body temperatures (T (b)) caused by high T (a). However, from an ecological point of view, aestivation results in different challenges and requirements than does torpor use in winter, because heat can cause reduced food and water availability in many regions, but without the access to low T (a) for a substantial reduction of T (b). Aestivation is used by a diversity of adult mammals and birds both in the field and laboratory, as well as by growing young to reduce thermoregulatory energy expenditure. Torpor occurs at high T (a) including the thermo-neutral zone and even under these conditions the reduction in energy expenditure and water requirements or water loss is substantial. Although data from the laboratory and, especially, from the field are limited, they show that torpor at high T (a) is an effective survival strategy and suggest that it is employed by many mammals and birds in a diversity of habitats.

Bone strength is maintained after 8 months of inactivity in hibernating golden-mantled ground squirrels, Spermophilus lateralis

Prolonged inactivity leads to disuse atrophy, a loss of muscle and bone mass. Hibernating mammals are inactive for 6-9 months per year but must return to full activity immediately after completing hibernation. This necessity for immediate recovery presents an intriguing conundrum, as many mammals require two to three times the period of inactivity to recover full bone strength. Therefore, if hibernators experience typical levels of bone disuse atrophy during hibernation, there would be inadequate time available to recover during the summer active season. We examined whether there were mechanical consequences as a result of the extended inactivity of hibernation. We dissected femur and tibia bones from squirrels in various stages of the annual hibernation cycle and measured the amount of force required to fracture these bones. Three groups were investigated; summer active animals were captured during the summer and immediately killed, animals in the 1 month detraining group were captured in the summer and killed following a 1-month period of restricted mobility, hibernating animals were killed after 8 months of inactivity. A three-point bend test was employed to measure the force required to break the bones. Apparent flexural strength and apparent flexural modulus (material stiffness) were calculated for femurs. There were no differences between groups for femur fracture force, tibia fracture force, or femur flexural strength. Femur flexural modulus was significantly less for the 1 month detraining group than for the hibernation and summer active groups. Thus, hibernators seem resistant to the deleterious effects of prolonged inactivity during the winter. However, they may be susceptible to immobilization-induced bone loss during the summer.