Metabolic consequences of accelerated urea synthesis during seasonal dormancy of spadefoot toads,Scaphiopus couchi andScaphiopus multiplicatus

Regulation of the unfolded protein response during dehydration stress in African clawed frogs, Xenopus laevis

The unfolded protein response (UPR) is a wide-ranging cellular response to accumulation of malfolded proteins in the endoplasmic reticulum (ER) and acts as a quality control mechanism to halt protein processing and repair/destroy malfolded proteins under stress conditions of many kinds. Among vertebrate species, amphibians experience the greatest challenges in maintaining water and osmotic balance, the high permeability of their skin making them very susceptible to dehydration and challenging their ability to maintain cellular homeostasis. The present study evaluates the involvement of the UPR in dealing with dehydration-mediated disruption of protein processing in the tissues of African clawed frogs, Xenopus laevis. This primarily aquatic frog must deal with seasonal drought conditions in its native southern Africa environment. Key markers of cellular stress that impact protein processing were identified in six tissues of frogs that had lost 28% of total body water, as compared with fully hydrated controls. This included upregulation of glucose-regulated proteins (GRPs) that are resident chaperones in the ER, particularly 2-ninefold increases in GRP58, GRP75, and/or GRP94 in the lung and skin. Activating transcription factors (ATF3, ATF4, ATF6) that mediate UPR responses also responded to dehydration stress, particularly in skeletal muscle where both ATF3 and ATF4 rose strongly in the nucleus. Other protein markers of the UPR including GADD34, GADD153, EDEM, and XBP-1 also showed selective upregulation in frog tissues in response to dehydration and nuclear levels of the transcription factors XBP-1 and P-CREB rose indicating up-regulation of genes under their control.

Cardiac microRNA expression profile in response to estivation

Couch's spadefoot toad (Scaphiopus couchii) spends most of the year underground in a hypometabolic state known as estivation. During this time, they overcome significant dehydration and lack of food through many mechanisms including employing metabolic rate depression (MRD), increasing urea concentration, switching to lipid oxidation as the primary energy source, and decreasing their breathing and heart rate. MicroRNA (miRNA) are known to regulate translation by targeting messenger RNA (mRNA) for degradation or temporary storage, with several studies having reported that miRNA is differentially expressed during MRD, including estivation. Thus, we hypothesized that miRNA would be involved in gene regulation during estivation in S. couchii heart. Next-generation sequencing and bioinformatic analyses were used to assess changes in miRNA expression in response to two-month estivation and to predict the downstream effects of this expression. KEGG and GO analyses indicated that ribosome and cardiac muscle contraction are among the pathways predicted to be upregulated, whereas cell signaling and fatty acid metabolism were predicted to be downregulated. Together these results suggest that miRNAs contribute to the regulation of gene expression related to cardiac muscle physiology and energy metabolism during estivation.

Effect of salinity on locomotor performance and thermal extremes of metamorphic Andean Toads (Rhinella spinulosa) from Monte Desert, Argentina

Rhinella spinulosa is distributed from Peru to Argentina (from 1200 to 5000 m elevation), inhabiting arid mountain valleys of the Andes, characterized by salty soils. The variations in soil salinity, caused by high evapotranspiration of water, can create an osmotic constraint and high thermal oscillations for metamorphsed Andean toad (R. spinulosa), affecting their thermoregulation and extreme thermal tolerances. We investigated the changes in thermal tolerance parameters (critical thermal maximum and crystallization temperature) of a population of metamorphosed R. spinulosa from the Monte Desert of San Juan, Argentina, under different substrate salinity conditions. Our results suggest that the locomotor performance of metamorphs of R. spinulosa is affected by increasing salinity concentrations in the environment where they develop. On the other hand, the thermal extremes of metamorphs of R. spinulosa also showed changes associated with different salinity conditions. According to other studies on different organisms, the increase of the osmolarity of the internal medium may increase the thermal tolerance of this species. More studies are needed to understand the thermo-osmolar adjustments of the metamorphs of toads to environmental variability.

Aestivation Induces Changes in the mRNA Expression Levels and Protein Abundance of Two Isoforms of Urea Transporters in the Gills of the African Lungfish, Protopterus annectens

The African lungfish, Protopterus annectens, is ammonotelic in water despite being ureogenic. When it aestivates in mucus cocoon on land, ammonia is detoxified to urea. During the maintenance phase of aestivation, urea accumulates in the body, which is subsequently excreted upon arousal. Urea excretion involves urea transporters (UT/Ut). This study aimed to clone and sequence the ut isoforms from the gills of P. annectens, and to test the hypothesis that the mRNA and/or protein expression levels of ut/Ut isoforms could vary in the gills of P. annectens during the induction, maintenance, and arousal phases of aestivation. Two isoforms of ut, ut-a2a and ut-a2b, were obtained from the gills of P. annectens. ut-a2a consisted of 1227 bp and coded for 408 amino acids with an estimated molecular mass of 44.7 kDa, while ut-a2b consisted of 1392 bp and coded for 464 amino acids with an estimated molecular mass of 51.2 kDa. Ut-a2a and Ut-a2b of P. annectens had a closer phylogenetic relationship with Ut/UT of tetrapods than Ut of fishes. While the mRNA expression pattern of ut-a2a and ut-a2b across various tissues of P. annectens differed, the transcript levels of ut-a2a and ut-a2b in the gills were comparable, indicating that they might be equally important for branchial urea excretion during the initial arousal phase of aestivation. During the maintenance phase of aestivation, the transcript level of ut-a2a increased significantly, but the protein abundance of Ut-a2a remained unchanged in the gills of P. annectens. This could be an adaptive feature to prepare for an increase in the production of Ut-a2a upon arousal. Indeed, arousal led to a significant increase in the branchial Ut-a2a protein abundance. Although the transcript level of ut-a2b remained unchanged, there were significant increases in the protein abundance of Ut-a2b in the gills of P. annectens throughout the three phases of aestivation. The increase in the protein abundance of Ut-a2b during the maintenance phase could also be an adaptive feature to prepare for efficient urea excretion when water becomes available.

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.

Cryoprotectants and extreme freeze tolerance in a subarctic population of the wood frog

Wood frogs (Rana sylvatica) exhibit marked geographic variation in freeze tolerance, with subarctic populations tolerating experimental freezing to temperatures at least 10-13 degrees Celsius below the lethal limits for conspecifics from more temperate locales. We determined how seasonal responses enhance the cryoprotectant system in these northern frogs, and also investigated their physiological responses to somatic freezing at extreme temperatures. Alaskan frogs collected in late summer had plasma urea levels near 10 μmol ml-1, but this level rose during preparation for winter to 85.5 ± 2.9 μmol ml-1 (mean ± SEM) in frogs that remained fully hydrated, and to 186.9 ± 12.4 μmol ml-1 in frogs held under a restricted moisture regime. An osmolality gap indicated that the plasma of winter-conditioned frogs contained an as yet unidentified osmolyte(s) that contributed about 75 mOsmol kg-1 to total osmotic pressure. Experimental freezing to -8°C, either directly or following three cycles of freezing/thawing between -4 and 0°C, or -16°C increased the liver's synthesis of glucose and, to a lesser extent, urea. Concomitantly, organs shed up to one-half (skeletal muscle) or two-thirds (liver) of their water, with cryoprotectant in the remaining fluid reaching concentrations as high as 0.2 and 2.1 M, respectively. Freeze/thaw cycling, which was readily survived by winter-conditioned frogs, greatly increased hepatic glycogenolysis and delivery of glucose (but not urea) to skeletal muscle. We conclude that cryoprotectant accrual in anticipation of and in response to freezing have been greatly enhanced and contribute to extreme freeze tolerance in northern R. sylvatica.

Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog

We investigated hibernation physiology and freeze tolerance in a population of the wood frog, Rana sylvatica, indigenous to Interior Alaska, USA, near the northernmost limit of the species' range. Winter acclimatization responses included a 233% increase in the hepatic glycogen depot that was subsidized by fat body and skeletal muscle catabolism, and a rise in plasma osmolality that reflected accrual of urea (to 106±10 μmol ml−1) and an unidentified solute (to ~73 μmol ml−1). In contrast, frogs from a cool-temperate population (southern Ohio, USA) amassed much less glycogen, had a lower uremia (28±5 μmol ml−1) and apparently lacked the unidentified solute. Alaskan frogs survived freezing at temperatures as low as −16°C, some 10–13°C below those tolerated by southern conspecifics, and endured a 2-month bout of freezing at −4°C. The profound freeze tolerance is presumably due to their high levels of organic osmolytes and bound water, which limits ice formation. Adaptive responses to freezing (−2.5°C for 48 h) and subsequent thawing (4°C) included synthesis of the cryoprotectants urea and glucose, and dehydration of certain tissues. Alaskan frogs differed from Ohioan frogs in retaining a substantial reserve capacity for glucose synthesis, accumulating high levels of cryoprotectants in brain tissue, and remaining hyperglycemic long after thawing. The northern phenotype also incurred less stress during freezing/thawing, as indicated by limited cryohemolysis and lactate accumulation. Post-glacial colonization of high latitudes by R. sylvatica required a substantial increase in freeze tolerance that was at least partly achieved by enhancing their cryoprotectant system.

Frogs and estivation: transcriptional insights into metabolism and cell survival in a natural model of extended muscle disuse

Green-striped burrowing frogs (Cyclorana alboguttata) survive in arid environments by burrowing underground and entering into a deep, prolonged metabolic depression known as estivation. Throughout estivation, C. alboguttata is immobilized within a cast-like cocoon of shed skin and ceases feeding and moving. Remarkably, these frogs exhibit very little muscle atrophy despite extended disuse and fasting. Little is known about the transcriptional regulation of estivation or associated mechanisms that may minimize degradative pathways of atrophy. To investigate transcriptional pathways associated with metabolic depression and maintenance of muscle function in estivating burrowing frogs, we assembled a skeletal muscle transcriptome using next-generation short read sequencing and compared gene expression patterns between active and 4 mo estivating C. alboguttata. This identified a complex suite of gene expression changes that occur in muscle during estivation and provides evidence that estivation in burrowing frogs involves transcriptional regulation of genes associated with cytoskeletal remodeling, avoidance of oxidative stress, energy metabolism, the cell stress response, and apoptotic signaling. In particular, the expression levels of genes encoding cell cycle and prosurvival proteins, such as serine/threonine-protein kinase Chk1, cell division protein kinase 2, survivin, and vesicular overexpressed in cancer prosurvival protein 1, were upregulated during estivation. These data suggest that estivating C. alboguttata are able to regulate the expression of genes in several major cellular pathways critical to the survival and viability of cells, thus preserving muscle function while avoiding the deleterious consequences often seen in laboratory models of muscle disuse.

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.

Skeletal muscle atrophy occurs slowly and selectively during prolonged aestivation in Cyclorana alboguttata (Günther 1867)

We investigated the effect of prolonged immobilisation of six and nine months duration on the morphology and antioxidant biochemistry of skeletal muscles in the amphibian aestivator Cyclorana alboguttata. We hypothesised that, in the event of atrophy occurring during aestivation,larger jumping muscles were more likely to be preserved over smaller non-jumping muscles. Whole muscle mass (g), muscle cross-sectional area (CSA)(μm2), water content (%) and myofibre number (per mm2) remained unchanged in the cruralis muscle after six to nine months of aestivation; however, myofibre area (μm2) was significantly reduced. Whole muscle mass, water content, myofibre number and myofibre CSA remained unchanged in the gastrocnemius muscle after six to nine months of aestivation. However, iliofibularis dry muscle mass, whole muscle CSA and myofibre CSA was significantly reduced during aestivation. Similarly,sartorius dry muscle mass, water content and whole muscle CSA was significantly reduced during aestivation. Endogenous antioxidants were maintained at control levels throughout aestivation in all four muscles. The results suggest changes to muscle morphology during aestivation may occur when lipid reserves have been depleted and protein becomes the primary fuel substrate for preserving basal metabolic processes. Muscle atrophy as a result of this protein catabolism may be correlated with locomotor function, with smaller non-jumping muscles preferentially used as a protein source during fasting over larger jumping muscles. Higher levels of endogenous antioxidants in the jumping muscles may confer a protective advantage against oxidative damage during aestivation; however, it is not clear whether they play a role during aestivation or upon resumption of normal metabolic activity.

Water balance and arginine vasotocin in the cocooning frog Cyclorana platycephala (hylidae)

It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.

Water balance of field-excavated aestivating Australian desert frogs, the cocoon-formingNeobatrachus aquiloniusand the non-cocooningNotaden nichollsi(Amphibia: Myobatrachidae)

Burrowed aestivating frogs of the cocoon-forming species Neobatrachus aquilonius and the non-cocooning species Notaden nichollsi were excavated in the Gibson Desert of central Australia. Their hydration state(osmotic pressure of the plasma and urine) was compared to the moisture content and water potential of the surrounding soil. The non-cocooning N. nichollsi was consistently found in sand dunes. While this sand had favourable water potential properties for buried frogs, the considerable spatial and temporal variation in sand moisture meant that frogs were not always in positive water balance with respect to the surrounding soil. The cocoon-forming N. aquilonius was excavated from two distinct habitat types, a claypan in which frogs had a well-formed cocoon and a dune swale where frogs did not have a cocoon. Cocoons of excavated frogs ranged in thickness from 19.4 μm to 55.61 μm and consisted of 81-229 layers. Cocooned claypan N. aquilonius were nearing exhaustion of their bladder water reserves and had a urine osmolality approaching that of the plasma. By contrast, non-cocooned N. aquilonius from the dune swale were fully hydrated, although soil moisture levels were not as high as calculated to be necessary to maintain water balance. Both species had similar plasma arginine vasotocin (AVT) concentrations ranging from 9.4 to 164 pg ml-1, except for one cocooned N. aquilonius with a higher concentration of 394 pg ml-1. For both species, AVT showed no relationship with plasma osmolality over the lower range of plasma osmolalities but was appreciably increased at the highest osmolality recorded. This study provides the first evidence that cocoon formation following burrowing is not obligatory in species that are capable of doing so, but that cocoon formation occurs when soil water conditions are more desiccating than for non-cocooned frogs.

Effect of soil type on burrowing behavior and cocoon formation in the green-striped burrowing frog, Cyclorana alboguttata

This study examined the effect of soil type on burrowing behaviour and cocoon formation during aestivation in the green-striped burrowing frog, Cyclorana alboguttata (Günther, 1867). Given a choice, frogs always chose to burrow in wet sand in preference to wet clay. Frogs buried themselves faster and dug deeper burrows in sandy soil. However, under my laboratory conditions, there was little difference in the pattern of soil drying between the two soil types. Frogs in both sand and clay soil experienced hydrating conditions for the first 3 months and dehydrating conditions for the last 3 months of the 6-month aestivation period, and cocoons were not formed until after 3 months of aestivation. After 6 months, there were more layers in the cocoons of frogs aestivating in sand than those aestivating in clay. Frogs were able to absorb water from sandy soil with water potentials greater than –400 kPa, but lost water when placed on sand with a water potential of –1000 kPa.

Life in the slow lane: molecular mechanisms of estivation

Estivation is a state of aerobic hypometabolism used by organisms to endure seasonally arid conditions, often in desert environments. Estivating species are often active for only a few weeks each year to feed and breed and then retreat to estivate in sheltered sites, often underground. In general, estivation includes a strong reduction in metabolic rate, a primary reliance on lipid oxidation to fuel metabolism, and methods of water retention, both physical (e.g. cocoons) and metabolic (e.g. urea accumulation). The present review focuses on several aspects of metabolic adaptation during estivation including changes in the activities of enzymes of intermediary metabolism and antioxidant defenses, the effects of urea on estivator enzymes, enzyme regulation by reversible protein phosphorylation, protein kinases and phosphatases involved in signal transduction mechanisms, and the role of gene expression in estivation. The focus is on two species: the spadefoot toad, Scaphiopus couchii, from the Arizona desert; and the land snail, Otala lactea, a native of the Mediterranean region. The mechanisms of metabolic depression in estivators are similar to those seen in hibernation and anaerobiosis, and contribute to the development of a unified set of biochemical principles for the control of metabolic arrest in nature.

The muscle fatty acid binding protein of spadefoot toad (Scaphiopus couchii)

Fatty acid binding protein was purified from skeletal muscle of the spadefoot toad (Scaphiopus couchii), an estivating species. While estivating, this animal relies on the fatty acid oxidation for energy. Hence we were interested in the behaviour of fatty acid binding protein under conditions of elevated urea (up to 200 mM) and potassium chloride such as exist during estivation. Also we examined whether there were interactions between glycolytic intermediates and the binding ability of the protein. The amount of bound fatty acid (a fluorescence assay using cis-parinarate) was not affected (P < 0.05) by glucose, fructose 6-phosphate or phosphoenolpyruvate at physiological concentrations. By contrast, glucose 6-phosphate increased the amount of bound cis-parinarate but the apparent dissociation constant was not different from the control. Fructose 1,6-bisphosphate but not fructose 2,6-phosphate decreased cis-parinarate binding by 40%, commensurate with doubling the apparent dissociation constant (1.15-2.62 microM). Urea, guanidinium and trimethylamine N-oxide at 200 mM increased cis-parinarate binding 60% over controls. Urea (1 M) and KCl (200 mM) did not affect cis-parinarate binding compared to controls. The interaction of this fatty acid transporter with fructose 1,6-bisphosphate is discussed in terms of reciprocal interaction with phosphofructokinase since fatty acid is also an inhibitor of phosphofructokinase.

Urea and amphibian water economy

Accumulation of urea in the body fluids enables some amphibians to tolerate high ambient salinities (Bufo viridis, Xenopus laevis, Rana cancrivora, Ambystoma tigrinum, Batrachoseps spp.) or to estivate in soil with low water potentials (Scaphiopus spp.). These species are assumed not only to accumulate urea produced in the normal metabolism, but to synthesize urea in response to water shortage. Re-examination of the data did not support the view of an osmoregulatory urea synthesis. Increased urea synthesis on exposure to high salinities in X. laevis, R. cancrivora and Batrachoseps spp. seemed to reflect reactions to an adverse environment. It is suggested that in amphibians, solute concentration in the plasma and rate of excretion of urea are coordinated so that at a certain plasma concentration, urea is excreted at the same rate at which it is produced. The higher the level of urea in the body fluids at balance between production and excretion, the higher the tolerance of the species of low external water potentials. The mechanisms that integrate the relationship between plasma solute concentration and handling of urea by the kidneys are not known.

Reproductive and lipid cycles in the male frog Rana ridibunda in northern Greece

1. Reproductive and lipid cycles in the male frog Rana ridibunda were studied. 2. The spermatogenesis of Rana ridibunda is of the potentially continuous type. 3. During prehibernating season (September-November) a part of lipid is mobilized from fat bodies to other body sites or is transformed to other metabolites. 4. During wintering this frog consumes mainly glycogen. 5. In February the lipid is accumulated in the fat bodies and the liver mass shows a second peak, probably as a result of glycogen accumulation. 6. The greatest decrease of metabolites was observed during the breeding season and this is the result of the intensive activities related to the reproduction and maintenance.

A comparison of energy substrates and reproductive patterns of two anurans. Acris crepitans and Bufo woodhousei

The seasonal pattern of carcass, liver and ovary lipid and liver non-lipid mass was examined in the cricket frog, Acris crepitans, and Woodhouse's toad, Bufo woodhousei. Reproductive patterns were also studied. The over-winter reduction of body lipid and liver non-lipid material was attributed to metabolism in Acris crepitans. Male, but not female, Bufo woodhousei exhibited seasonal variation in lipid stores that was attributable to metabolism. Females, but not males, showed seasonal variation in liver non-lipid mass and quantified liver glycogen. Females of both species mobilized body lipid during the period of ovarian development; however, an inverse relationship between fatbody mass and ovary mass was evident for Acris, only. Female Acris crepitans depleted all ovarian follicles prior to brumation; however, Bufo woodhousei retained enlarged follicles throughout the year. Variation in metabolic substrate use between species was related to the differences in reproductive patterns exhibited by the two anurans. These variations in reproductive strategy represent adaptations that enhance the survivability of different species in dissimilar habitats.

Urea tolerance and osmoregulation in Bufo viridis and Rana ridibunda

1. The tolerance and adaptation to urea solutions by terrestrial green toads (Bufo viridis) and semi-aquatic frogs (Rana Ridibunda) were studied. 2. the green toad showed tolerance to urea solution of 800 mM and the frogs showed tolerance only to about 400 mM urea solution. 3. The plasma concentrations of both species was hyperosmotic to the external medium in all the different urea solutions. 4. Blood osmolality, urea, Na+ and Cl- concentrations of B. viridis were always higher than in R. ridibunda. 5. The urea concentration in muscle of R. ridibunda was higher than the urea concentration in muscle of B. viridis. 6. The muscle tissue weight loss of B. viridis was significantly lower than R. ridibunda.

The role of diet in glycogen storage by juvenile bullfrogs prior to overwintering

Juvenile bullfrogs from natural populations doubled their glycogen stores during their predormancy period, while their blood glucose and lipid stores declined. Glycogen stores were replenished in natural populations as food consumption decreased. Some liver glycogen was stored during fasting if the bullfrogs were collected in the storage phase of their energy reserve cycle, but the amount of glycogen stored was increased by feeding.