Urea and glucose modulation during freezing exposure in three temperate frogs reveals specific targets in relation to climate

Amphibian diversity is most prominent in the warm and humid tropical and subtropical regions across the globe. Nonetheless, amphibians also inhabit high-altitude tropical mountains and regions at medium and high latitudes, exposing them to subzero temperatures and requiring behavioural or physiological adaptations to endure freezing events. While freeze tolerance has been predominantly reported in high-latitude zones where species endure prolonged freezing (several weeks or months), less is known about mid-latitudes amphibians exposed to occasional subzero temperatures. In this study, we employed a controlled ecological protocol, subjecting three frog species from the Iberian Peninsula (Rana parvipalmata, Epidalea calamita, and Pelobates cultripes) to a 2-h exposure to temperatures of -2 °C to investigate the accumulation of urea and glucose as physiological mechanisms associated with survival at freezing temperatures. Our results revealed a moderate response in the production of cryoprotectant metabolites under experimental freezing conditions, particularly urea, with notable findings in R. parvipalmata and E. calamita and no response in P. cultripes. However, no significant alterations in glucose concentrations were observed in any of the studied frog species. This relatively weak freezing tolerance response differs from the strong response exhibited by amphibians inhabiting high latitudes and enduring prolonged freezing conditions, suggesting potential reliance on behavioural adaptations to cope with occasional freezing episodes.

A large-effect fitness trade-off across environments is explained by a single mutation affecting cold acclimation

Identifying the genetic basis of local adaptation and fitness trade-offs across environments is a central goal of evolutionary biology. Cold acclimation is an adaptive plastic response for surviving seasonal freezing, and costs of acclimation may be a general mechanism for fitness trade-offs across environments in temperate zone species. Starting with locally adapted ecotypes of Arabidopsis thaliana from Italy and Sweden, we examined the fitness consequences of a naturally occurring functional polymorphism in CBF2. This gene encodes a transcription factor that is a major regulator of cold-acclimated freezing tolerance and resides within a locus responsible for a genetic trade-off for long-term mean fitness. We estimated the consequences of alternate genotypes of CBF2 on 5-y mean fitness and fitness components at the native field sites by comparing near-isogenic lines with alternate genotypes of CBF2 to their genetic background ecotypes. The effects of CBF2 were validated at the nucleotide level using gene-edited lines in the native genetic backgrounds grown in simulated parental environments. The foreign CBF2 genotype in the local genetic background reduced long-term mean fitness in Sweden by more than 10%, primarily via effects on survival. In Italy, fitness was reduced by more than 20%, primarily via effects on fecundity. At both sites, the effects were temporally variable and much stronger in some years. The gene-edited lines confirmed that CBF2 encodes the causal variant underlying this genetic trade-off. Additionally, we demonstrated a substantial fitness cost of cold acclimation, which has broad implications for potential maladaptive responses to climate change.

Snow flies self-amputate freezing limbs to sustain behavior at sub-zero temperatures

Temperature profoundly impacts all living creatures. In spite of the thermodynamic constraints on biology, some animals have evolved to live and move in extremely cold environments. Here, we investigate behavioral mechanisms of cold tolerance in the snow fly (Chionea spp.), a flightless crane fly that is active throughout the winter in boreal and alpine environments of the northern hemisphere. Using thermal imaging, we show that adult snow flies maintain the ability to walk down to an average body temperature of -7°C. At this supercooling limit, ice crystallization occurs within the snow fly's hemolymph and rapidly spreads throughout the body, resulting in death. However, we discovered that snow flies frequently survive freezing by rapidly amputating legs before ice crystallization can spread to their vital organs. Self-amputation of freezing limbs is a last-ditch tactic to prolong survival in frigid conditions that few animals can endure. Understanding the extreme physiology and behavior of snow insects holds particular significance at this moment when their alpine habitats are rapidly changing due to anthropogenic climate change. VIDEO ABSTRACT.

Biochemical response of two earthworm taxa exposed to freezing

Several earthworm species are known to be able to withstand freezing. At the biochemical level, this ability is based on cryoprotectant accumulation as well as several other mechanisms. In this study, we used ¹H NMR to investigate metabolomic changes in two freeze-tolerant earthworm taxa, Dendrobaena octaedra and one of the genetic lineages of Eisenia sp. aff. nordenskioldi f. pallida. A total of 45 metabolites were quantified. High concentrations of glucose were present in frozen tissues of both taxa. No other putative cryoprotectants were found. We detected high levels of glycolysis end products and succinate in frozen animals, indicating the activation of glycolysis. Concentrations of many other substances also significantly increased. On the whole, metabolic change in response to freezing was much more pronounced in the specimens of Eisenia sp. aff. nordenskioldi f. pallida, including signs of nucleotide degradation.

Hepatic citrate synthase suppression in the freeze-tolerant wood frog (Rana sylvatica)

The wood frog, Rana sylvatica endures whole body freezing for weeks/months while overwintering at subzero temperatures. Survival of long-term freezing requires not only cryoprotectants but also strong metabolic rate depression (MRD) and reorganization of essential processes in order to maintain a balance between ATP-producing and ATP-consuming processes. Citrate synthase (CS) (E.C. 2.3.3.1) is an important irreversible enzyme of the tricarboxylic acid (TCA) cycle and forms a crucial checkpoint for many metabolic processes. Present study investigated the regulation of CS from wood frog liver during freezing. CS was purified to homogeneity by a two-step chromatographic process. Kinetic and regulatory parameters of the enzyme were investigated and, notably, demonstrated a significant decrease in the V_max of the purified form of CS from frozen frogs as compared to controls when assayed at both 22 °C and 5 °C. This was further supported by a decrease in the maximum activity of CS from liver of frozen frogs. Immunoblotting also showed changes in posttranslational modifications with a significant decrease in threonine phosphorylation (by 49 %) for CS from frozen frogs. Taken together, these results suggest that CS is suppressed and TCA flux is inhibited during freezing, likely to support MRD survival of harsh winters.

Lethal and sublethal implications of low temperature exposure for three intertidal predators

Benthic invertebrate predators play a key role in top-down trophic regulation in intertidal ecosystems. While the physiological and ecological consequences of predator exposure to high temperatures during summer low tides are increasingly well-studied, the effects of cold exposure during winter low tides remain poorly understood. To address this knowledge gap, we measured the supercooling points, survival, and feeding rates of three intertidal predator species in British Columbia, Canada - the sea stars Pisaster ochraceus and Evasterias troschelii and the dogwhelk Nucella lamellosa - in response to exposure to sub-zero air temperatures. Overall, we found that all three predators exhibited evidence of internal freezing at relatively mild sub-zero temperatures, with sea stars exhibiting an average supercooling point of -2.50 °C, and the dogwhelk averaging approximately -3.99 °C. None of the tested species are strongly freeze tolerant, as evidenced by moderate-to-low survival rates after exposure to -8 °C air. All three predators exhibited significantly reduced feeding rates over a two-week period following a single 3-h sublethal (-0.5 °C) exposure event. We also quantified variation in predator body temperature among thermal microhabitats during winter low tides. Predators that were found at the base of large boulders, on the sediment, and within crevices had higher body temperatures during winter low tides, as compared to those situated in other microhabitats. However, we did not find evidence of behavioural thermoregulation via selective microhabitat use during cold weather. Since these intertidal predators are less freeze tolerant than their preferred prey, winter low temperature exposures can have important implications for organism survival and predator-prey dynamics across thermal gradients at both local (habitat-driven) and geographic (climate-driven) scales.

Proteostasis in ice: the role of heat shock proteins and ubiquitin in the freeze tolerance of the intertidal mussel, Mytilus trossulus

The bay mussel, Mytilus trossulus, is an animal that can survive extracellular ice formation. Depending on air and ocean temperatures, freeze tolerant intertidal organisms, like M. trossulus, may freeze and thaw many times during the winter. Freezing can cause protein denaturation, leading to an induction of the heat shock response with expression of chaperone proteins like the 70 kDa heat shock protein (HSP70), and an increase in ubiquitin-conjugated proteins. There has been little work on the mechanisms of freeze tolerance in intertidal species, limiting our understanding of this survival strategy. Additionally, this limited research has focused solely on the effects of single freezing events, but the act of repeatedly crossing the freezing threshold may present novel physiological or biochemical stressors that have yet to be discovered. Mytilus are important ecosystem engineers and provide habitat for other intertidal species, thus understanding their physiology under thermal extremes is important for preserving shoreline health. We predicted that repeated freeze exposures would increase mortality, upregulate HSP70 expression, and increase ubiquitin conjugates in mussels, relative to single, prolonged freeze exposures. Mytilus trossulus from Vancouver, Canada were repeatedly frozen for a combination of 1 × 8 h, 2 × 4 h, or 4 × 2 h. We then compared mortality, HSP70 expression, and the quantity of ubiquitin-conjugated proteins across experimental groups. We found a single 8-h freeze caused significantly more mortality than repeated freeze-thaw cycles. We also found that HSP70 and ubiquitinated protein was upregulated exclusively after freeze-thaw cycles, suggesting that freeze-thaw cycles offer a period of damage repair between freezes. This indicates that freeze-thaw cycles, which happen naturally in the intertidal, are crucial for M. trossulus survival in sub-zero temperatures.

Variations of Supercooling Capacity in Intertidal Gastropods

Winter low-temperature confines species distribution. Intertidal gastropods are distributed from tropical to polar zones, facing variable intensities and durations of low temperatures. They usually set their supercooling points (SCPs) at high subzero temperatures to stimulate freezing. However, the variations in SCP in intertidal gastropods at intraspecific and interspecific levels remain poorly understood. Here, we measured the body size, cooling rate, and SCP of nine intertidal gastropod species in China. These species were distributed in high or middle intertidal zone with different geographic distributions. The average SCPs (-4.27~-7.10 °C) and the coefficients of variation of SCP (22.6%~45.9%) were high in all species. At the intraspecific level, the supercooling capacity was positively correlated with the cooling rate. Interspecifically, the supercooling capacity was closely related to the cooling rate, and also to the species' geographical distribution. Northern high-shore species showed lower SCPs, while southern high-shore species had higher SCPs. There was no difference in SCP between widespread high- and mid-shore species. Our results indicated that the supercooling capability is potentially an adaptative response to the local winter temperatures, and the cooling rate is a factor in determining the difference in SCP at the intraspecific and interspecific levels.

Microhabitat-specific diurnal metabolomic responses of the intertidal limpet Cellana toreuma to winter low temperature

High-throughput determination of circadian rhythms in metabolic response and their divergent patterns in various microhabitats are crucial for understanding how organisms respond to environmental stresses. A mid-intertidal limpet Cellana toreuma was collected at various time points across both daytime and nighttime in winter during low tide for investigating the diurnal metabolomic responses to cold stress and elucidating the divergent metabolic responses to temperature variations across microhabitats. Temperatures of emergent rock microhabitats were lower than the tidal pool and even aggravated at night. A series of metabolomic responses exhibited coordinated diurnal changes in winter. Metabolic responses which were associated with cellular stress responses and energy metabolism of emergent rock microhabitat individuals were highly induced compared to the tidal pool ones. This study shed light on the diurnal patterns of metabolomic responses of intertidal molluscs in the field and emphasized the variations in metabolic responses between microhabitats.

Lessons from nature: Leveraging the freeze-tolerant wood frog as a model to improve organ cryopreservation and biobanking

The freeze-tolerant wood frog, Rana sylvatica, is one of the very few vertebrate species known to endure full body freezing in winter and thaw in early spring without any significant sign of damage. Once frozen, wood frogs show no cardiac or lung activity, brain function, or physical movement yet resume full physiological and biochemical functions within hours after thawing. The miraculous ability to tolerate such extreme stresses makes wood frogs an attractive model for identifying the molecular mechanisms that can promote freeze/thaw endurance. Recapitulating these pro-survival strategies in transplantable human cells and organs could improve viability post-thaw leading to better post-transplant outcomes, in addition to providing more time for adequate distribution of these transplantable materials across larger geographical areas. Indeed, several laboratories are beginning to mimic the pro-survival responses observed in wood frogs to preservation of human cells, tissues and organs and, to date, a few trials have been successful in extending preservation time prior to transplantation. In this review, we discuss the biology of the freeze-tolerant wood frog, current advances in biobanking based on these animals, and extend our discussion to future prospects for cryopreservation as an aid to regenerative medicine.

Alteration of Gut Microbiota of a Food-Storing Hibernator, Siberian Chipmunk Tamias sibiricus

Hibernation represents a state of fasting because hibernators cease eating in the torpid periods. Therefore, food deprivation during hibernation is expected to modify the gut microbiota of host. However, there are few reports of gut microbiota in food-storing hibernators that feed during the interbout arousals. Here we collected fecal samples of Siberian chipmunk T. sibiricus to character and examine changes in the gut microbiota at various stages relative to hibernation: pre-hibernation, early-hibernation, mid-hibernation, late-hibernation, and post-hibernation. Compared to the pre-hibernation state, alpha-diversity of gut microbiota was significantly increased during the interbout arousal periods. In addition, beta-diversity of the fecal communities from pre-hibernation and interbout arousal periods grouped together, and post-hibernation gut microbiota resembled the counterpart at late-hibernation. Hibernation significantly decreased the relative abundance of Firmicutes but increased Bacteroidetes, reflecting a shift of microbiota toward taxa in favor of host-derived substrates. The increased abundance of Ruminococcaceae_UCG-014, Lactobacillus, and Christensenellaceae_R-7_group in gut microbiota may help the chipmunks reduce intestinal inflammation and then maintain healthy bowel during hibernation. KEGG pathway indicated that hibernation altered the metabolic function of gut microflora of T. sibiricus. Our study provides evidence that the gut microbiota of food-storing hibernators, despite feeding during the interbout arousals, shows similar response to hibernation that has well documented in fat-storing counterparts, suggesting the potential for a core gut microbiota during hibernation of mammals. Importantly, these results will broaden our understanding of the effects of hibernation on gut microbiota of mammal hibernators.

Extreme freeze-tolerance in cryophilic tardigrades relies on controlled ice formation but does not involve significant change in transcription

Subzero temperatures are among the most significant factors defining the distribution of organisms, yet, certain taxa have evolved to overcome this barrier. The microscopic tardigrades are among the most freeze-tolerant animals, with selected species reported to survive milli-Kelvin temperatures. Here, we estimate survival of fully hydrated eutardigrades of the species Ramazzottius varieornatus following exposures to -20 °C and  -80 °C as well as -196 °C with or without initial cooling to -80 °C. The tardigrades easily survive these temperatures, yet with a significant decrease in viability following rapid cooling by direct exposure to -196 °C. Hence, post-freeze recovery of R. varieornatus seems to rely on cooling rate and thus controlled ice formation. Cryophilic organisms are renowned for having cold-active enzymes that secure appropriate reaction rates at low temperatures. Hence, extreme freeze-tolerance in R. varieornatus could potentially involve syntheses of cryoprotectants and de novo transcription. We therefore generated a reference transcriptome for this cryophilic R. varieornatus population and explored for differential gene expression patterns following cooling to -80 °C as compared to active 5 °C controls. Specifically, we tested for fast transcription potentially occurring within 25 min of cooling from room temperature to a supercooling point of ca. -20 °C, at which the tardigrades presumably freeze and enter into the ametabolic state of cryobiosis. Our analyses revealed no evidence for differential gene expression. We, therefore, conclude that extreme freeze-tolerance in R. varieornatus relies on controlled extracellular freezing with any freeze-tolerance related genes being constitutively expressed.

One-step purification and regulation of fructose 1,6-bisphosphatase from the liver of the freeze-tolerant wood frog, Rana sylvatica

The wood frog (Rana sylvatica) undergoes numerous changes to its physiology and metabolic processes to survive the winter months, including adaptations that let them endure whole-body freezing. The regulation of key enzymes of central carbohydrate metabolism in the liver plays a crucial role in mediating the synthesis and maintenance of high concentrations of glucose as a cryoprotectant during freezing as well as glucose reconversion to glycogen after thawing. The present study characterized the regulation of fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) from wood frog liver during freezing, FBPase being a crucial enzyme regulating gluconeogenesis. Liver FBPase was purified to homogeneity from control and frozen wood frogs by a one-step chromatographic process. Kinetic and regulatory parameters of the enzyme were investigated and demonstrated a significant decrease in sensitivity to its substrate fructose-1,6-bisphosphate in the liver of frozen frogs, as compared with controls. Immunoblotting also revealed freeze-responsive changes in posttranslational modifications with a significant decrease in serine phosphorylation (by 53%) for FBPase from frozen frogs. Taken together, these results suggest that FBPase is suppressed, and gluconeogenesis is inhibited during freezing. This response acts as an important component of the metabolic survival strategy of the wood frog.

Metabolomic Analysis Reveals That the Moor Frog Rana arvalis Uses Both Glucose and Glycerol as Cryoprotectants

Simple Summary

The moor frog Rana arvalis can tolerate freezing to low temperatures, up to −16 °C. We performed metabolomic analysis of the liver and hindlimb muscles of frozen and control R. arvalis. We found that the moor frog synthesizes glucose and glycerol in similar concentrations as low molecular weight cryoprotectants. This is the first such case reported for the genus Rana, which was believed to use glucose only. We found that freezing upregulates glycolysis, with the accumulation of several end products: lactate, alanine, ethanol, and, possibly, 2,3-butanediol. To our knowledge, this is also the first report of ethanol as an end product of glycolysis in terrestrial vertebrates. We observed highly increased concentrations of nucleotide degradation products, implying high level of stress. We found almost no signs of adaptations to reoxygenation stress, with overall low levels of antioxidants. We also performed metabolomics analysis of subcutaneous ice that was found to contain glucose, glycerol, and several other substances.

Abstract

The moor frog Rana arvalis is one of a few amphibians that can tolerate freezing to low temperatures, up to −16 °C. In this study, we performed metabolomic analysis of the liver and hindlimb muscles of frozen and control R. arvalis. We found that the moor frog synthesizes glucose and glycerol in similar concentrations as low molecular weight cryoprotectants. This is the first such case reported for the genus Rana, which was believed to use glucose only. We found that freezing upregulates glycolysis, with the accumulation of several end products: lactate, alanine, ethanol, and, possibly, 2,3-butanediol. To our knowledge, this is also the first report of ethanol as an end product of glycolysis in terrestrial vertebrates. We observed highly increased concentrations of nucleotide degradation products, implying high level of stress. The Krebs cycle arrest resulted in high concentrations of succinate, which is common for animals. However, we found almost no signs of adaptations to reoxygenation stress, with overall low levels of antioxidants. We also performed metabolomics analysis of subcutaneous ice that was found to contain glucose, glycerol, and several other substances.

1H NMR Profiling of Honey Bee Bodies Revealed Metabolic Differences between Summer and Winter Bees

Simple Summary

The European honey bee, Apis mellifera, is well-known to have two distinct populations in temperate climate zone: short-living summer bees and long-living winter bees. Several biological factors related to the different lifespans of the two populations have been studied. However, the link between the metabolic changes and basic physiological features in the bodies of summer bees and winter bees is limited. This study aimed to identify the metabolic fingerprints that characterize summer and winter bees using proton nuclear magnetic resonance (¹H NMR) spectroscopy. In total, we found 28 significantly changed metabolites between the two populations. The results suggest that the metabolites detected in honey bee bodies can distinguish the summer and winter bees. Changes in carbohydrates, amino acids, choline-containing compounds, and an unknown compound were noticeable during the transition from summer bees to winter bees. The results from this study give us a broad perspective on honey bee metabolism that will support future research related to honey bee lifespan and overwintering management.

Abstract

In temperate climates, honey bee workers of the species Apis mellifera have different lifespans depending on the seasonal phenotype: summer bees (short lifespan) and winter bees (long lifespan). Many studies have revealed the biochemical parameters involved in the lifespan differentiation of summer and winter bees. However, comprehensive information regarding the metabolic changes occurring in their bodies between the two is limited. This study used proton nuclear magnetic resonance (¹H NMR) spectroscopy to analyze the metabolic differences between summer and winter bees of the same age. The multivariate analysis showed that summer and winter bees could be distinguished based on their metabolic profiles. Among the 36 metabolites found, 28 metabolites have displayed significant changes from summer to winter bees. Compared to summer bees, trehalose in winter bees showed 1.9 times higher concentration, and all amino acids except for proline and alanine showed decreased patterns. We have also detected an unknown compound, with a CH₃ singlet at 2.83 ppm, which is a potential biomarker that is about 13 times higher in summer bees. Our results show that the metabolites in summer and winter bees have distinctive characteristics; this information could provide new insights and support further studies on honey bee longevity and overwintering.

Mitochondria as a target and central hub of energy division during cold stress in insects

Temperature stress is one of the crucial factors determining geographical distribution of insect species. Most of them are active in moderate temperatures, however some are capable of surviving in extremely high as well as low temperatures, including freezing. The tolerance of cold stress is a result of various adaptation strategies, among others the mitochondria are an important player. They supply cells with the most prominent energy carrier—ATP, needed for their life processes, but also take part in many other processes like growth, aging, protection against stress injuries or cell death. Under cold stress, the mitochondria activity changes in various manner, partially to minimize the damages caused by the cold stress, partially because of the decline in mitochondrial homeostasis by chill injuries. In the response to low temperature, modifications in mitochondrial gene expression, mtDNA amount or phosphorylation efficiency can be observed. So far study also showed an increase or decrease in mitochondria number, their shape and mitochondrial membrane permeability. Some of the changes are a trigger for apoptosis induced via mitochondrial pathway, that protects the whole organism against chill injuries occurring on the cellular level. In many cases, the observed modifications are not unequivocal and depend strongly on many factors including cold acclimation, duration and severity of cold stress or environmental conditions. In the presented article, we summarize the current knowledge about insect response to cold stress focusing on the role of mitochondria in that process considering differences in results obtained in different experimental conditions, as well as depending on insect species. These differentiated observations clearly indicate that it is still much to explore.

Variations in oxidative stress and antioxidant defense level during different phases of hibernation in common Asian toad, Duttaphrynus melanostictus

To assess redox status during hibernation with metabolic depression, oxidative stress parameters and antioxidant defense were assessed during different phases of hibernation including active period, hibernation, arousal, and post-arousal period, in the liver and brain tissues of Duttaphrynus melanostictus. We hypothesized low levels of oxidative stress and antioxidant defense during the hibernation period in comparison to the summer active period, due to hypometabolism and their subsequent increase during the arousal period following an increase in body temperature and metabolism. Contrary to our hypothesis, increased oxidative stress with significantly higher lipid peroxidation, protein carbonylation, oxidized glutathione (GSSG): glutathione (GSH) ratio, and elevated antioxidants defense consisting of higher catalase activity and high ascorbic acid content to control oxidative stress were found during hibernation. However, GSH and uric acid levels were found low with super oxide dismutase (SOD) activities at a steady level during hibernation. Supporting our hypothesis, increased oxidative stress with high lipid peroxidation and GSSG:GSH ratio were found during arousal from hibernation owing to increased oxygen consumption and rewarming. Augmented catalase and SOD activities and nonenzymatic antioxidants (GSH, ascorbic acid, and uric acid) level were found to counteract oxidative stress during arousal periods as it was expected. A steady level of protein carbonylation, indicating no oxidative damage during arousal from hibernation due to elevated antioxidant defense, shows the significance of hibernation to overcome food and water scarcity and cold climatic condition. Decrease in antioxidants levels accompanying coming down of lipid peroxidation, protein carbonylation, and GSSG:GSH ratio to their lower levels during the post-arousal period showing normalcy in redox status as it was during active period indicates controllability of oxidative stress in hibernating toads.

Freeze tolerance in neotropical frogs: an intrageneric comparison using Pristimantis species of high elevation and medium elevation

Paramos are high-elevation tropical Andean ecosystems above the tree line that display variable temperature and frequent freezing spells. Because a significant anuran community lives in this environment, physiological protection against freezing must characterise individuals in this community. Antifreeze protection has been studied in amphibians from other communities, and it is likely that Paramo anurans rely on the same underlying molecules that convey such protection to Nearctic species. However, given the pervasive presence of freezing spells in the Paramos year-round, the processes of activating protection mechanisms may differ from that of seasonal counterparts. Accordingly, this study investigated cryoprotection strategies in high-elevation tropical frogs, using as a model the terrestrial and nocturnal genus Pristimantis, specifically P. bogotensis, P. elegans and P. nervicus from Paramos, and the warm ecosystem counterparts P. insignitus, P. megalops and P. sanctaemartae. We focused on freeze tolerance and its relationship with glucose accumulation and ice formation. Under field conditions, the highest elevation P. nervicus exhibited higher glucose concentration at dawn compared to noon (1.7 ± 0.6 mmol/L versus 3.5 ± 1.32 mmol/L). Under experimental thermal freeze exposure for 2 hours between −2 and −4 ºC, the glucose concentration of the three Paramo species increased but physiological diversity was evident (P. nervicus 126%; P. bogotensis 100%; and P. elegans 55%). During this test, body ice formation was assessed calorimetrically. The species with the highest body ice formation was P. bogotensis (17% ± 5.37; maximum value: 63%; n = 8), followed by P. nervicus (5% ± 3.27; maximum value: 11%; n = 5) and P. elegans (0.34% ± 0.09; maximum value: 1%; n = 4). The study shows physiological diversity both within a genus and across the amphibian community around the freezing contour. Overall, Paramo species differ in freezing physiology from their low-elevation counterparts. Thus, climate shifts increasing freezing spells may affect the structure of communities in this zone.

Surviving winter: Physiological regulation of energy balance in a temperate ectotherm entering and exiting brumation

Characterizing the physiological response to prolonged cold exposure is essential for understanding the maintenance of long-term energy balance. As part of their natural life cycle, temperate ectotherms are often exposed to seasonal variation in temperatures, including extended periods of cold well below their activity range. Relatively little is known about variation in physiological responses as vertebrate ectotherms enter and exit brumation in response to sustained cold temperatures. We tested the influence of temperature on physiology before, during, and after a simulated brumation in the checkered garter snake (Thamnophis marcianus), a widespread ectothermic vertebrate. We tested for the relative effect of immediate temperature and physiological context (entering or exiting brumation) on hormones regulating energy balance, indicators of energy availability, and resting metabolic rate (V̇_O2). Plasma corticosterone, glucose, and insulin, as well as immune cell heterophil: lymphocyte ratios responded to temperature, though they did so with different thermal response curves. Thermal sensitivity varied both among and within physiological measures depending on whether animals were going into or coming out of brumation. Additionally, V̇_O2 was regulated beyond simple temperature-dependence, whereby post-brumation measures were depressed relative to pre-brumation measures at the same temperature. This pattern was characterized by a change in the temperature coefficient (Q₁₀), with a larger pre-brumation Q_10, suggesting reduced thermal sensitivity of metabolic rate following a period of extended cold exposure. The integrated physiological response presented here demonstrates not only temperature dependence across physiological axes, but seasonal variation in thermal responsiveness. Our results suggest that energy allocation decisions and hormonal regulation of underlying processes promote differing levels of thermal sensitivity when entering or exiting brumation.

Mitochondria and the Frozen Frog

The wood frog, Rana sylvatica, is the best-studied of a small group of amphibian species that survive whole body freezing during the winter months. These frogs endure the freezing of 65-70% of their total body water in extracellular ice masses. They have implemented multiple adaptations that manage ice formation, deal with freeze-induced ischemia/reperfusion stress, limit cell volume reduction with the production of small molecule cryoprotectants (glucose, urea) and adjust a wide variety of metabolic pathways for prolonged life in a frozen state. All organs, tissues, cells and intracellular organelles are affected by freeze/thaw and its consequences. This article explores mitochondria in the frozen frog with a focus on both the consequences of freezing (e.g., anoxia/ischemia, cell volume reduction) and mitigating defenses (e.g., antioxidants, chaperone proteins, upregulation of mitochondria-encoded genes, enzyme regulation, etc.) in order to identify adaptive strategies that defend and adapt mitochondria in animals that can be frozen for six months or more every year. A particular focus is placed on freeze-responsive genes in wood frogs that are encoded on the mitochondrial genome including ATP6/8, ND4 and 16S RNA. These were strongly up-regulated during whole body freezing (24 h at -2.5 °C) in the liver and brain but showed opposing responses to two component stresses: strong upregulation in response to anoxia but no response to dehydration stress. This indicates that freeze-responsive upregulation of mitochondria-encoded genes is triggered by declining oxygen and likely has an adaptive function in supporting cellular energetics under indeterminate lengths of whole body freezing.

Drivers of plasticity in freeze tolerance in the intertidal mussel Mytilus trossulus

Freezing is an extreme stress to living cells, and so freeze-tolerant animals often accumulate protective molecules (termed cryoprotectants) to prevent the cellular damage caused by freezing. The bay mussel, Mytilus trossulus, is an ecologically important intertidal invertebrate that can survive freezing. Although much is known about the biochemical correlates of freeze tolerance in insects and vertebrates, the cryoprotectants that are used by intertidal invertebrates are not well characterized. Previous work has proposed two possible groups of low-molecular weight cryoprotectants in intertidal invertebrates: osmolytes and anaerobic byproducts. In our study, we examined which group of candidate cryoprotectants correlate with plasticity in freeze tolerance in mussels using 1H NMR metabolomics. We found that the freeze tolerance of M. trossulus varies on a seasonal basis, along an intertidal shore-level gradient, and with changing salinity. Acclimation to increased salinity (30 ppt compared with 15 ppt) increased freeze tolerance, and mussels were significantly more freeze tolerant during the winter. Mussel freeze tolerance also increased with increasing shore level. There was limited evidence that anaerobic byproduct accumulation was associated with increased freeze tolerance. However, osmolyte accumulation was correlated with increased freeze tolerance after high salinity acclimation and in the winter. The concentration of most low molecular weight metabolites did not vary with shore level, indicating that another mechanism is likely responsible for this pattern of variation in freeze tolerance. By identifying osmolytes as a group of molecules that assist in freezing tolerance, we have expanded the known biochemical repertoire of the mechanisms of freeze tolerance.

Mercury (Hg2+) interferes with physiological adaptations to freezing in the arctic earthworm Enchytraeus albidus

Freezing temperatures is an important stressor in the arctic regions and has a significant influence on the population dynamics and geographic distribution of terrestrial invertebrates. Toxic metals in the environment can interfere with protective cold-acclimation responses of organisms. It is therefore important to evaluate the combined effects of cold stress and environmental contaminants. Here, we aimed to investigate the effects of Hg (HgCl₂) on various physiological aspects of freeze-tolerance in the earthworm (Enchytraeus albidus). We measured the levels of the cryoprotectant glucose, the glycogen content (source of glucose molecules for cryoprotection and fuel for metabolism), and changes in the composition of membrane phospholipid fatty acids (PLFA) as an indicator of lipid peroxidation. Freezing at -6 °C had no effect on survival in uncontaminated soil, however, survival of freezing in Hg contaminated soil was clearly reduced, especially at extended exposure times. Thus, the LC₅₀ value in frozen soil decreased from 8.3 mg Hg kg^-1 (when exposed for 17 days) to only 4.2 mg Hg kg^-1 after 36 days' exposure indicating that combined effects of Hg and freezing became larger at prolonged exposure times. Hg caused a depletion of glycogen reserves (almost 50% at 12 mg kg^-1 dry soil), but despite this effect worms were able to maintain a constant cryoprotectant level (about 0.12 mg glucose mg^-1 dry weight) at all Hg concentrations. Hg had clear negative effects on the proportion of unsaturated PLFAs, which could be an indication of lipid peroxidation. Since a high proportion of unsaturated fatty acids in the membrane is important for invertebrate freeze-tolerance, our results suggest that the negative effect of Hg on freeze-tolerance in E. albidus is related to degraded membrane functionality at low temperature.

Control of ice recrystallization in liver tissues using small molecule carbohydrate derivatives

Minimizing ice recrystallization injury in tissues and organs has historically been sought using biological antifreeze proteins. However, the size of these compounds can limit permeation and their potential immunogenicity disqualifies them from use in several cryopreservation applications. Novel small molecule carbohydrate-derived ice recrystallization inhibitors (IRIs) are not subject to these constraints, and thus we sought to evaluate the ability of a highly active IRI to permeate liver tissue and control recrystallization. Rat liver tissue blocks (0.5 mm²) were incubated with the IRI for 6 h at 22 °C and subsequently plunged in liquid nitrogen. Ice crystals within the tissue were fixed using a formal acetic alcohol fixative as it was rewarmed from -80 °C to 22 °C over the course of 48 h. The untreated control demonstrated a gradient of increasing crystal size from the exterior to the interior region of the tissue; however, the IRI-treated condition had no such gradient and exhibited small crystals throughout. Threshold segmentation confirmed a significant reduction in the ice crystal size within the interior region of the IRI-treated condition, suggesting the IRI permeated throughout and effectively controlled recrystallization within the tissue.

MondoA:MLX complex regulates glucose-dependent gene expression and links to circadian rhythm in liver and brain of the freeze-tolerant wood frog, Rana sylvatica

The wood frog, Rana sylvatica, is one of only a few vertebrate species that display natural freeze tolerance. Frogs survive the freezing of about two-thirds of their body water as extracellular ice over the winter months. Multiple adaptations support freeze tolerance including metabolic rate depression and the production of huge amounts of glucose (often 200 mM or more) as a cryoprotectant that protects cells from freeze damage. To understand how high glucose levels affect gene expression, we studied MondoA, a glucose sensing transcription factor, and its partner MLX (Max-like protein) to assess their ability to modulate the expression of genes involved in glucose metabolism and circadian rhythm. Wood frog liver and brain tissues were analyzed, assessing protein levels, nuclear distribution, and DNA binding activity of MondoA:MLX during freezing (24 h at - 2.5 °C) and subsequent thawing (8 h returned to 5 °C), as compared with 5 °C controls. Downstream targets of MondoA:MLX were also evaluated: TXNIP (thioredoxin interacting protein), ARRDC4 (arrestin domain containing 4), HK-2 (hexokinase-2), PFKFB-3 (6-phosphofructo-2-kinase isozyme 3) and KLF-10 (Kruppel-like factor-10). Both KLF-10 and PFKFB-3 are also involved in circadian dependant regulation which was also explored in the current study via analysis of BMAL-1 (aryl hydrocarbon receptor nuclear translocator-like protein 1) and CLOCK (circadian locomotor output cycles kaput) proteins. Our data establish the MondoA-MLX complex as active under the hyperglycemic conditions in liver to regulate glucose metabolism and may also link to circadian rhythm in liver via KLF-10 and PFKFB-3 but not in brain.

Zero or not? Causes and consequences of zero-flow stream gage readings

Streamflow observations can be used to understand, predict, and contextualize hydrologic, ecological, and biogeochemical processes and conditions in streams. Stream gages are point measurements along rivers where streamflow is measured, and are often used to infer upstream watershed-scale processes. When stream gages read zero, this may indicate that the stream has fully dried; however, zero-flow readings can also be caused by a wide range of other factors. Our ability to identify whether or not a zero-flow gage reading indicates a dry fluvial system has far reaching environmental implications. Incorrect identification and interpretation by the data user can lead to hydrologic, ecological, and/or biogeochemical predictions from models and analyses. Here, we describe several causes of zero-flow gage readings: frozen surface water, flow reversals, instrument error, and natural or human-driven upstream source losses or bypass flow. For these examples, we discuss the implications of zero-flow interpretations. We also highlight additional methodss for determining flow presence, including direct observations, statistical methods, and hydrologic models, which can be applied to interpret causes of zero-flow gage readings and implications for reach- and watershed-scale dynamics. Such efforts are necessary to improve our ability to understand and predict surface flow activation, cessation, and connectivity across river networks. Developing this integrated understanding of the wide range of possible meanings of zero-flows will only attain greater importance in a more variable and changing hydrologic climate.

The lizard abides: cold hardiness and winter refuges of Liolaemus pictus argentinus in Patagonia, Argentina

In environments where the temperature periodically drops below zero, it is remarkable that some lizards can survive. Behaviorally, lizards can find microsites for overwintering where temperatures do not drop as much as the air temperature. Physiologically, they can alter their biochemical balance to tolerate freezing or avoid it by supercooling. We evaluated the cold hardiness of a population of Liolaemus pictus argentinus Müller and Hellmich, 1939 in the mountains of Esquel (Patagonia, Argentina) during autumn. Additionally, we assessed the thermal quality (in degree-days) of potential refuges in a mid-elevation forest (1100 m above sea level (asl)) and in the high Andean steppe (1400 m asl). We analyzed the role of urea, glucose, total proteins, and albumin as possible cryoprotectants, comparing a group of lizards gradually exposed to temperatures lower than 0 °C with a control group maintained at room temperature. However, we found no evidence to support the presence of freeze tolerance or supercooling mechanisms in this species as related to the analyzed metabolites. Instead, the low frequency of degree-days below 0 °C and temperatures never lower than −3 °C in potential refuges suggest that L. p. argentinus might avoid physiological investments (such as supercooling and freeze tolerance) by behaviorally selecting appropriate refuges to overcome cold environmental temperatures.

Combined effect of environmental temperature and density-dependent processes on the evolution of seasonal metabolic rate patterns

Species exhibit large diversity in their seasonal metabolic patterns which are traditionally explained by habitat specific seasonal temperature changes and various thermoregulatory adaptations. However, due to seasonal changes in resource abundances, density-dependent, ecological processes can also be important selective forces shaping the evolution of metabolic patterns. In the present theoretical study, the combined effect of environmental temperature and resource availability on the evolutionarily stable metabolic strategies is investigated in a consumer-resource model. Our results suggest that, under a broad range of circumstances, density-dependent mechanisms favor the selection of active metabolic regulation, where the metabolic rate differs from the thermally optimal value. This effect may be temporary, limited only to a brief period at seasonal changes, or permanent depending on the implied energetic cost and the relative timescale of environmental changes as compared to the generation time.

Metabolic reorganization in winter: Regulation of pyruvate dehydrogenase (PDH) during long-term freezing and anoxia

Wood frogs, Rana sylvatica, can undergo prolonged periods of whole body freezing during winter, locking as much as 65-70% of total body water into extracellular ice and imposing both anoxia and dehydration on their cells. Metabolic rate depression (MRD) is an adaptation used by R. sylvatica to survive these environmental stresses, where a finite amount of ATP generated through anaerobic metabolism is directed towards maintaining pro-survival functions, while most ATP-expensive cellular processes are temporarily reduced in function. Pyruvate dehydrogenase (PDH) is a vital metabolic enzyme that links anaerobic glycolysis to the aerobic TCA cycle and is an important regulatory site in MRD. PDH enzymatic activity is regulated via reversible protein phosphorylation in response to energetic demands of cells. This study explored the posttranslational regulation of PDH at three serine sites (S232, S293, S300) on the catalytic E1α subunit along with protein expression of four pyruvate dehydrogenase kinases (PDHK1-4) in response to 24 h Freezing, 8 h Thaw, 24 h Anoxia, and 4 h Recovery in the liver and skeletal muscle of R. sylvatica using Luminex multiplex technology and western immunoblotting. Overall, inhibitory regulation of PDH was evident during 24 h Freezing and 24 h Anoxia, which could indicate a notable reduction in glycoytic flux and carbon entry into the tricarboxylic acid cycle as part of MRD. Furthermore, the expression of PDHK1-4 and phosphorylation of PDH at S232, S293, and S300 were highly tissue and stress-specific, indicative of how different tissues respond differently to stress within the same organism.

n-Octyl (Thio)glycosides as Potential Cryoprotectants: Glass Transition Behaviour, Membrane Permeability, and Ice Recrystallization Inhibition Studies

A series of eight n-octyl (thio)glycosides (1α, β–4α, β) with d-glucose or d-galactose-configured head groups and varying anomeric configuration were synthesized and evaluated for glass transition behaviour, membrane permeability, and ice recrystallization inhibition (IRI) activity. Of these, n-octyl β-d-glucopyranoside (2β) exhibited a high glass transition temperatures (Tg), both as a neat sample and 20 wt-% aqueous solution. Membrane permeability studies of this compound revealed cellular uptake to concentrations relevant to the inhibition of intracellular ice formation, thus presenting a promising lead candidate for further biophysical and cryopreservation studies. Compounds were also evaluated as ice recrystallization inhibitors; however, no detectable activity was observed for the newly tested compounds.

Low-temperature immobilization of water in Antarctic Turgidosculum complicatulum and in Prasiola crispa. Part I. Turgidosculum complicatulum

The studies of low-temperature immobilization of bound water in Antarctic lichenized fungus Turgidosculum complicatulum were performed using ¹H NMR and DSC over a wide range of thallus hydration. ¹H NMR free induction decays were decomposed into a solid component well described by the Gaussian function and two exponentially decaying components coming from a tightly bound water and from a loosely bound water fraction. ¹H NMR spectra revealed one averaged mobile proton signal component. ¹H NMR measurements recorded in time and in frequency domain suggest the non-cooperative bound water immobilization in T. complicatulum thallus. The threshold of the hydration level estimated by ¹H NMR analysis at which the cooperative bound water freezing was detected was Δm/m₀ ≈ 0.39, whereas for DSC analysis was equal to Δm/m₀ = 0.375. Main ice melting estimated from DSC measurements for zero hydration level of the sample starts at t_m = -(19.29 ± 1.19)°C. However, DSC melting peak shows a composed form being a superposition of the main narrow peak (presumably melting of mycobiont areas) and a broad low-temperature shoulder (presumably melting of isolated photobiont cells). DSC traces recorded after two-hour incubation of T. complicatulum thallus at -20 °C suggest much lower threshold level of hydration at which the ice formation occurs (Δm/m₀ = 0.0842). Presumably it is a result of diffusion induced migration of separated water molecules to ice microcrystallites already present in thallus, but still beyond the calorimeter resolution.

Hibernation Leads to Altered Gut Communities in Bumblebee Queens (Bombus terrestris)

Many reptiles, amphibians, mammals, and insects practice some form of hibernation during which their metabolic rate is drastically reduced. This allows them to conserve energy and survive the harsh winter conditions with little or no food. While it can be expected that a reduction in host metabolism has a substantial influence on the gut microbial community, little is known about the effects of hibernation on the composition of the microbial gut community, especially for insects. In this study, we assessed and compared the bacterial gut community composition within the midgut and ileum of indoor-reared queens of Bombus terrestris before and after an artificial hibernation period of 16 weeks. Deep sequencing of 16S ribosomal RNA gene amplicons and clustering of sequence reads into operational taxonomic units (OTUs) at a similarity threshold of 97% revealed several bacterial taxa that are known to be strongly associated with corbiculate bees. Bacterial community composition after hibernation compared to before hibernation was characterized by higher OTU richness and evenness, with decreased levels of the core bacteria Gilliamella (Proteobacteria, Orbaceae) and Snodgrassella (Proteobacteria, Neisseriaceae), and increased relative abundance of non-core bacteria, including several psychrophilic and psychrotrophic taxa.

The influence of low temperature and diapause phase on sugar and polyol content in the European corn borer Ostrinia nubilalis (Hbn.)

The European corn borer (ECB, Ostrinia nubilalis Hbn.) is a major pest in temperate regions of Europe and North America. Fifth instar ECB larvae enter diapause before winter and gradually develop cold hardiness. Here we investigated the combined influence of diapause phase and low temperature on sugar and polyol content in ECB larvae. Larvae in mid-diapause or diapause termination were acclimated at 5 °C, -3°C or -16 °C, and sugar and polyol content was measured using GC-MS. Control GC-MS measurements were conducted on untreated non-diapausing larvae. We detected differences in polyol (glycerol, sorbitol, myo-inositol) and sugar (trehalose, fructose, glucose) levels in diapausing versus non-diapausing larvae. Glycerol and trehalose were the most abundant of all analyzed cryoprotective compounds in diapausing larvae. Exposure of diapausing larvae to decreasing temperatures induced changes in polyol and sugar levels that depended on the phase of diapause. In mid-diapause larvae, decreasing temperatures induced a significant increase in glycerol and a decrease in sorbitol and myo-inositol. In larvae at diapause termination, polyol content was lower and less influenced by decreasing temperatures. In contrast, sugar levels were lower in larvae at mid-diapause versus diapause termination. Exposure of larvae to -16 °C induced a significant increase in the levels of all detected sugars. In particular, glucose levels were significantly higher in larvae at diapause termination following exposure to -16 °C. We propose that this shift toward sugar synthesis following low temperature exposure in larvae at diapause termination is a consequence of NADPH dependent polyol synthesis, and may be a mechanism for preservation of carbon reserves needed for post-diapause development.

Urea and plasma ice-nucleating proteins promoted the modest freeze tolerance in Pleske's high altitude frog Nanorana pleskei

The frog Nanorana pleskei (Dicroglossidae) is indigenous to the Qinghai-Tibetan Plateau. To identify its strategies in coping with the cold climate, we measured the hibernacula microhabitat temperature during winter. We also examined the freezing-induced and seasonal variation of several putative cryoprotectants in the heart, liver, brain, kidney and muscle, as well as ice-nucleating protein in plasma. Our results showed that N. pleskei survived exposure to temperatures as low as - 2.5 ± 0.40 °C during hibernation, which was lower than the body fluid freezing point (- 0.43 ± 0.01 °C). Experimental freezing results indicated that four of six specimens could survive 12 h of freezing at - 2 °C with 27.5 ± 2.5% of total body water as ice. Concomitantly, the water contents of all examined organs decreased after being frozen for 24 h at - 2 °C. The levels of urea in heart significantly increased from 71.05 ± 7.19 to 104.59 ± 10.11 µmol g^-1, and in muscle increased from 72.23 ± 3.40 to 102.42 ± 6.24 µmol g^-1 when exposed to freezing; other cryoprotectants (glucose, glycerol, and lactate) showed no significant increase in all examined tissues. In addition, urea levels were significantly higher in fall-collected frogs than summer-collected frogs in the tissues of heart, brain, kidney, and muscle. The results of differential scanning calorimetry indicated that the ice-nucleating protein was present only in cold-acclimated and fall-collected frogs' plasma. We concluded that the urea serves as a primary cryoprotectant and accumulates in anticipation of freezing in N. pleskei, coupling with the seasonal production of plasma ice-nucleating protein.

The role of MEF2 transcription factors in dehydration and anoxia survival in Rana sylvatica skeletal muscle

The wood frog (Rana sylvatica) can endure freezing of up to 65% of total body water during winter. When frozen, wood frogs enter a dormant state characterized by a cessation of vital functions (i.e., no heartbeat, blood circulation, breathing, brain activity, or movement). Wood frogs utilize various behavioural and biochemical adaptations to survive extreme freezing and component anoxia and dehydration stresses, including a global suppression of metabolic functions and gene expression. The stress-responsive myocyte enhancer factor-2 (MEF2) transcription factor family regulates the selective expression of genes involved in glucose transport, protein quality control, and phosphagen homeostasis. This study examined the role of MEF2A and MEF2C proteins as well as select downstream targets (glucose transporter-4, calreticulin, and muscle and brain creatine kinase isozymes) in 40% dehydration and 24 h anoxia exposure at the transcriptional, translational, and post-translational levels using qRT-PCR, immunoblotting, and subcellular localization. Mef2a/c transcript levels remained constant during dehydration and anoxia. Total, cytoplasmic, and nuclear MEF2A/C and phospho-MEF2A/C protein levels remained constant during dehydration, whereas a decrease in total MEF2C levels was observed during rehydration. Total and phospho-MEF2A levels remained constant during anoxia, whereas total MEF2C levels decreased during 24 h anoxia and P-MEF2C levels increased during 4 h anoxia. In contrast, cytoplasmic MEF2A levels and nuclear phospho-MEF2A/C levels were upregulated during anoxia. MEF2 downstream targets remained constant during dehydration and anoxia, with the exception of glut4 which was upregulated during anoxia. These results suggest that the upregulated MEF2 response reported in wood frogs during freezing may in part stem from their cellular responses to surviving prolonged anoxia, rather than dehydration, leading to an increase in GLUT4 expression which may have an important role during anoxia survival.

Molecular Physiology of Freeze Tolerance in Vertebrates

Freeze tolerance is an amazing winter survival strategy used by various amphibians and reptiles living in seasonally cold environments. These animals may spend weeks or months with up to ∼65% of their total body water frozen as extracellular ice and no physiological vital signs, and yet after thawing they return to normal life within a few hours. Two main principles of animal freeze tolerance have received much attention: the production of high concentrations of organic osmolytes (glucose, glycerol, urea among amphibians) that protect the intracellular environment, and the control of ice within the body (the first putative ice-binding protein in a frog was recently identified), but many other strategies of biochemical adaptation also contribute to freezing survival. Discussed herein are recent advances in our understanding of amphibian and reptile freeze tolerance with a focus on cell preservation strategies (chaperones, antioxidants, damage defense mechanisms), membrane transporters for water and cryoprotectants, energy metabolism, gene/protein adaptations, and the regulatory control of freeze-responsive hypometabolism at multiple levels (epigenetic regulation of DNA, microRNA action, cell signaling and transcription factor regulation, cell cycle control, and anti-apoptosis). All are providing a much more complete picture of life in the frozen state.

Osmolyte regulation by TonEBP/NFAT5 during anoxia-recovery and dehydration-rehydration stresses in the freeze-tolerant wood frog (Rana sylvatica)

BACKGROUND

The wood frog, Rana sylvatica, tolerates freezing as a means of winter survival. Freezing is considered to be an ischemic/anoxic event in which oxygen delivery is significantly impaired. In addition, cellular dehydration occurs during freezing because water is lost to extracellular compartments in order to promote freezing. In order to prevent severe cell shrinkage and cell death, it is important for the wood frog to have adaptive mechanisms for osmoregulation. One important mechanism of cellular osmoregulation occurs through the cellular uptake/production of organic osmolytes like sorbitol, betaine, and myo-inositol. Betaine and myo-inositol are transported by the proteins BGT-1 and SMIT, respectively. Sorbitol on the other hand, is synthesized inside the cell by the enzyme aldose reductase. These three proteins are regulated at the transcriptional level by the transcription factor, NFAT5/TonEBP. Therefore, the objective of this study was to elucidate the role of NFAT5/TonEBP in regulating BGT-1, SMIT, and aldose reductase, during dehydration and anoxia in the wood frog muscle, liver, and kidney tissues.

METHODS

Wood frogs were subjected to 24 h anoxia-4 h recovery and 40% dehydration-full rehydration experiments. Protein levels of NFAT5, BGT-1, SMIT, and aldose reductase were studied using immunoblotting in muscle, liver, and kidney tissues.

RESULTS

Immunoblotting results demonstrated downregulations in NFAT5 protein levels in both liver and kidney tissues during anoxia (decreases by 41% and 44% relative to control for liver and kidney, respectively). Aldose reductase protein levels also decreased in both muscle and kidney tissues during anoxia (by 37% and 30% for muscle and kidney, respectively). On the other hand, BGT-1 levels increased during anoxia in muscle (0.9-fold compared to control) and kidney (1.1-fold). Under 40% dehydration, NFAT5 levels decreased in liver by 53%. Aldose reductase levels also decreased by 42% in dehydrated muscle, and by 35% in dehydrated liver. In contrast, BGT-1 levels increased by 1.4-fold in dehydrated liver. SMIT levels also increased in both dehydrated muscle and liver (both by 0.8-fold).

DISCUSSION

Overall, we observed that osmoregulation through an NFAT5-mediated pathway is both tissue- and stress-specific. In both anoxia and dehydration, there appears to be a general reduction in NFAT5 levels resulting in decreased aldose reductase levels, however BGT-1 and SMIT levels still increase in certain tissues. Therefore, the regulation of osmoregulatory genes during dehydration and anoxia occurs beyond the transcriptional level, and it possibly involves RNA processing as well. These novel findings on the osmoregulatory mechanisms utilized by the wood frog advances our knowledge of osmoregulation during anoxia and dehydration. In addition, these findings highlight the importance of using this model to study molecular adaptations during stress.

Freezing of body fluids induces metallothionein gene expression in earthworms (Dendrobaena octaedra)

The molecular mechanisms activated by environmental contaminants and natural stressors such as freezing need to be investigated in order to better understand the mechanisms of interaction and potential effects that combined stressors may have on organisms. Using the freeze-tolerant earthworm Dendrobaena octaedra as model species, we exposed worms to freezing and exposure to sublethal copper in a factorial design and investigated the transcription of candidate genes for metal and cold stress. We hypothesised that both freezing and copper would induce transcription of genes coding for heat shock proteins (hsp10 and hsp70), metallothioneins (mt1 and mt2), and glutathione-S-transferase (gst), and that the combined effects of these two stressors would be additive. The gene transcripts hsp10, hsp70, and gst were significantly upregulated by freezing, but only hsp10 was upregulated by copper. We found that copper at the time of sampling had no effect on transcription of two metallothionein genes whereas transcription was strongly upregulated by freezing. Moreover, there was a significant interaction causing more than additive transcription rates of mt1 in the copper/freezing treatment suggesting that freeze-induced cellular dehydration increases the concentration of free copper ions in the cytosol. This metallothionein response to freezing is likely adaptive and possibly provides protection against freeze-induced elevated metal concentrations in the cytosol and excess ROS levels due to hypoxia during freezing.

Warmer winters reduce frog fecundity and shift breeding phenology, which consequently alters larval development and metamorphic timing

One widely documented phenological response to climate change is the earlier occurrence of spring-breeding events. While such climate change-driven shifts in phenology are common, their consequences for individuals and populations have rarely been investigated. I addressed this gap in our knowledge by using a multi-year observational study of six wood frog (Rana sylvatica) populations near the southern edge of their range. I tested first if winter temperature or precipitation affected the date of breeding and female fecundity, and second if timing of breeding affected subsequent larval development rate, mass at metamorphosis, date of metamorphosis, and survival. Warmer winters were associated with earlier breeding but reduced female fecundity. Winter precipitation did not affect breeding date, but was positively associated with female fecundity. There was no association between earlier breeding and larval survival or mass at metamorphosis, but earlier breeding was associated with delayed larval development. The delay in larval development was explained through a counterintuitive correlation between breeding date and temperature during larval development. Warmer winters led to earlier breeding, which in turn was associated with cooler post-breeding temperatures that slowed larval development. The delay in larval development did not fully compensate for the earlier breeding, such that for every 2 days earlier that breeding took place, the average date of metamorphosis was 1 day earlier. Other studies have found that earlier metamorphosis is associated with increased postmetamorphic growth and survival, suggesting that earlier breeding has beneficial effects on wood frog populations.