Anti-apoptotic response during anoxia and recovery in a freeze-tolerant wood frog (Rana sylvatica)

mRNA m6 A methylation in wood frog brain is maintained during freezing and anoxia

Freeze tolerance is an adaptive strategy that wood frogs (Rana sylvatica) use to survive the subzero temperatures of winter. It is characterized by a variety of metabolic and physiological changes that facilitate successful freezing and anoxia. As both mRNA regulation and posttranslation protein modification have been implicated in freeze tolerance, we hypothesized that posttranslational RNA regulation is also involved in coordinating freeze-thaw cycles and metabolic rate depression. As such, we investigated the most abundant RNA modification, adenosine methylation (N⁶ -methyladenosine; m⁶ A) in wood frog brains during 24 h periods of freezing and anoxia. This was followed by an examination of levels of RNA methyltransferases, demethyltransferases, and the readers of RNA methylation. Despite relative levels of methylation on mRNA remaining constant throughout freezing and anoxia, a significant increase in relative abundance of m⁶ A methyltransferases METTL3 and METTL14 was observed. In addition, we investigated the effect of m⁶ A RNA methylation on mRNA triaging to stress granules and report a significant increase in stress granule markers TIAR and TIA-1 in both freezing and anoxia. Our findings are the first report of RNA posttranslational regulation during metabolic rate depression in the wood frog brain and suggest that the dynamic RNA methylation observed is not directly linked to mRNA regulation during periods of extreme metabolic reorganization, warranting future investigations.

MicroRNA transcriptomics in liver of the freeze-tolerant gray tree frog (Dryophytes versicolor) indicates suppression of energy-expensive pathways

The rapid and reversible nature of microRNA (miRNA) transcriptional regulation is ideal for implementing global changes to cellular processes and metabolism, a necessary asset for the freeze-tolerant gray tree frog (Dryophytes versicolor). D. versicolor can freeze up to 42% of its total body water during the winter and then thaw completely upon more favorable conditions of spring. Herein, we examined the freeze-specific miRNA responses in the gray tree frog using RBiomirGS, a bioinformatic tool designed for the analysis of miRNA-seq transcriptomics in non-genome sequenced organisms. We identified 11 miRNAs differentially regulated during freezing (miR-140-3p, miR-181a-5p, miR-206-3p, miR-451a, miR-19a-3p, miR-101-3p, miR-30e-5p, miR-142-3p and -5p, miR-21-5p, and miR-34a-5p). Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis suggests these miRNAs play roles in downregulating signaling pathways, apoptosis, and nuclear processes while enhancing ribosomal biogenesis. Overall, these findings point towards miRNA inducing a state of energy conservation by downregulating energy-expensive pathways, while ribosomal biogenesis may lead to prioritization of critical processes for freeze-tolerance survival.

Role of unfolded protein response and ER-associated degradation under freezing, anoxia, and dehydration stresses in the freeze-tolerant wood frogs

The North American amphibian, wood frogs, Rana sylvatica are the most studied anuran to comprehend vertebrate freeze tolerance. Multiple adaptations support their survival in frigid temperatures during winters, particularly their ability to produce glucose as natural cryoprotectant. Freezing and its component consequences (anoxia and dehydration) induce multiple stresses on cells. Among these is endoplasmic reticulum (ER) stress, a condition spawned by buildup of unfolded or misfolded proteins in the ER. The ER stress causes the unfolded protein response (UPR) and the ER-associated degradation (ERAD) pathway that potentially could lead to apoptosis. Immunoblotting was used to assess the responses of major proteins of the UPR and ERAD under freezing, anoxia, and dehydration stresses in the liver and skeletal muscle of the wood frogs. Targets analyzed included activating transcription factors (ATF3, ATF4, ATF6), the growth arrest and DNA damage proteins (GADD34, GADD153), and EDEM (ERAD enhancing α-mannosidase-like proteins) and XBP1 (X-box binding protein 1) proteins. UPR signaling was triggered under all three stresses (freezing, anoxia, dehydration) in liver and skeletal muscle of wood frogs with most tissue/stress responses consistent with an upregulation of the primary targets of all three UPR pathways (ATF4, ATF6, and XBP-1) to enhance the protein folding/refolding capacity under these stress conditions. Only frozen muscle showed preference for proteasomal degradation of misfolded proteins via upregulation of EDEM (ERAD). The ERAD response of liver was downregulated across three stresses suggesting preference for more refolding of misfolded/unfolded proteins. Overall, we conclude that wood frog organs activate the UPR as a means of stabilizing and repairing cellular proteins to best survive freezing exposures.

Status of the Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathway in liver and skin of the freeze tolerant wood frog

The wood frog (Rana sylvatica) has adapted full-body freezing and thawing as a means of sub-zero winter survival and early-breeding in ephemeral pools. One such protective process implicated recently in freeze-thaw tolerance is that of anti-apoptotic signaling, which has been proposed to play a cytoprotective role by modulating stress-induced death signals. This study employed the use of immunoblotting to examine response of a potent cell cycle and apoptosis regulator, known as the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway, to freezing and thawing in the liver and skin of the wood frog. This pathway demonstrably exhibits factor- and tissue-specific changes between non-frozen, 24 h-frozen, and 8 h-thawed conditions. There were few changes in JAK-STAT proteins in frozen frogs, but protective changes were observed upon thaw: Elevated levels of pJAK3 and nuclear localization of pSTAT3 and pSTAT5 suggested an increase in anti-apoptotic signaling after thaw. By contrast, both STAT1 and STAT3 signaling appeared to increase in frozen skin, suggesting frogs use homeostatic regulation of apoptotic- and anti-apoptotic signals, in an antagonistic and compensatory manner. As such, these findings support that JAK-STAT pathway signaling modulation is a plausible adaptation that contributes to fast and reversible manipulation of anti-apoptotic signals, thus assisting in freeze survival of the wood frog.

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.

RAGE management: ETS1- EGR1 mediated transcriptional networks regulate angiogenic factors in wood frogs

Freeze-tolerant species, such as wood frogs (Rana sylvatica), are susceptible to multiple co-occurring stresses that they must overcome to survive. Freezing is accompanied by mechanical stress and dehydration due to ice crystal formation in the extracellular space, ischemia/anoxia due to interruption in blood flood, and hyperglycemia due to cryoprotective measures. Wood frogs can survive dehydration, anoxia, and high glucose stress independently of freezing, thereby creating a multifactorial model for studying freeze-tolerance. Oxidative stress and high glucose levels favors the production of pro-oxidant molecules and advanced glycation end product (AGE) adducts that could cause substantial cellular damage. In this study, the involvement of the high mobility group box 1 (HMGB1)-AGE/RAGE (receptor for AGE) axis and the regulation of ETS1 and EGR1-mediated angiogenic responses were investigated in liver of wood frogs expose to freeze/thaw, anoxia/reoxygenation and dehydration/rehydration treatments. HMGB1 and not AGE-adducts are likely to induce the activation of ETS1 and EGR1 via the RAGE pathway. The increase in nuclear localization of both ETS1 and EGR1, but not DNA binding activity in response to stress hints to a potential spatial and temporal regulation in inducing angiogenic factors. Freeze/thaw and dehydration/rehydration treatments increase the levels of both pro- and anti-angiogenic factors, perhaps to prepare for the distribution of cryoprotectants or enable the repair of damaged capillaries and wounds when needed. Overall, wood frogs appear to anticipate the need for angiogenesis in response to freezing and dehydration but not anoxic treatments, probably due to mechanical stress associated with the two former conditions.

DNA Hypomethylation May Contribute to Metabolic Recovery of Frozen Wood Frog Brains

Transcriptional suppression is characteristic of extreme stress responses, speculated to preserve energetic resources in the maintenance of hypometabolism. In recent years, epigenetic regulation has become heavily implicated in stress adaptation of many animals, including supporting freeze tolerance of the wood frog (Rana sylvatica). However, nervous tissues are frequently lacking in these multi-tissue analyses which warrants investigation. The present study examines the role of DNA methylation, a core epigenetic mechanism, in the response of wood frog brains to freezing. We use immunoblot analysis to track the relative expression of DNA methyltransferases (DNMT), methyl-CpG-binding domain (MBD) proteins and ten-eleven-translocation (TET) demethylases across the freeze-thaw cycle in R. sylvatica brain, including selected comparisons to freeze-associated sub-stresses (anoxia and dehydration). Global methyltransferase activities and 5-hmC content were also assessed. The data show coordinated evidence for DNA hypomethylation in wood frog brains during freeze-recovery through the combined roles of depressed DNMT3A/3L expression driving lowered DNMT activity and increased TET2/3 levels leading to elevated 5-hmC genomic content (p < 0.05). Raised levels of DNMT1 during high dehydration were also noteworthy. The above suggest that alleviation of transcriptionally repressive 5-mC DNA methylation is a necessary component of the wood frog freeze-thaw cycle, potentially facilitating the resumption of a normoxic transcriptional state as frogs thaw and resume normal metabolic activities.

Epigenetics of the frozen brain: roles for lysine methylation in hypometabolism

Wood frog (Rana sylvatica) freeze tolerance necessitates metabolic rate depression, where costly processes such as gene transcription are commonly suppressed. Epigenetic mechanisms, such as histone lysine methylation, have recently been implicated in hypometabolic states of various animals, although they are underreported in nervous tissues. In the present study, we track the expression of eight lysine methyltransferases, as well as the activity on, and abundance of putative histone products across the freeze-thaw cycle and freeze-associated sub-stresses (anoxia, dehydration) of wood frog brains. Our results suggest that hypomethylation of transcriptionally repressive H3K9 may be a key facet of metabolic recovery during the thawing of nervous tissue, which we speculate may have a positive effect on global gene transcription. Some non-histone roles for lysine methylation are also proposed.

Regulation of Apoptosis and Autophagy During Anoxia in the Freshwater Crayfish, Faxonius virilis

The ability of an animal to survive prolonged periods of oxygen deprivation is a critical area of study, both in terms of its importance to better understanding the physiology of these incredible animals and to its potential applicability to medical fields. The freshwater crayfish, Faxonius virilis, is one such animal capable of resisting anoxia, but it remains understudied and much of the metabolic mechanisms underlying this anoxia tolerance remain largely unprofiled. This study examines the activity and regulation of apoptosis and autophagy in F. virilis in response to 20-h anoxia. Apoptosis signaling was assessed through pro- and anti-apoptosis targets, whereas autophagy was assessed via expression response of multiple autophagy proteins. An anoxia-triggered, tissue-specific result arose, potentially based on the importance of individual organ integrity through hypometabolism. Tail muscle, which showed increased expression profiles of all three target groups, contrasted with hepatopancreas, which appeared to not be susceptible to either apoptotic or autophagic signaling during anoxia. This is likely due to the importance of the hepatopancreas, given that apoptosis or autophagy of this organ at any significant level could be fatal to the organism. The data provides a comprehensive overview of the responses and integration of multiple stress-responsive signaling pathways in F. virilis that provide a novel contribution to our understanding of pro-survival mechanisms supporting invertebrate anoxia resistance.

Mitochondrial DNA methyltransferases and their regulation under freezing and dehydration stresses in the freeze tolerant wood frog, Rana sylvatica

Wood frogs are one of a few vertebrate species that can survive whole-body freezing. Multiple adaptations support this including cryoprotectant production (glucose), metabolic rate depression and selective changes in gene/protein expression to activate pro-survival pathways. The role of DNA methylation machinery (DNA methyltransferases, DNMTs) in regulating nuclear gene expression supporting freezing survival has already been established. However, a comparable role for DNMTs in mitochondria has not been explored in wood frogs. We examined the mitochondrial protein levels of DNMT-1, DNMT-3A, DNMT-3B and DNMT-3L as well as mitochondrial DNMT activity in the liver and heart to assess DNMT involvement in the survival of freezing and dehydration stresses (cellular dehydration being one component of freezing). Our results showed stress and tissue-specific response by mitochondrial DNMT-1 protein in liver and heart respectively. During 24h freezing and whole-body dehydration, we saw an overall downregulation of mitochondrial DNMT-1, a major protein involved in maintaining methylation levels relating to its role in selective transcription of mitochondrial genes as well as antioxidant response. Tissue-specific response of protein levels of DNMT-3A, DNMT-3B and DNMT-3L and DNMT activity in the liver suggested a preference for higher methylation state in the liver under both freezing and dehydration stresses but not in the heart.

Role of MicroRNAs in Extreme Animal Survival Strategies

The critical role microRNAs play in modulating global functions is emerging, both in the maintenance of homeostatic mechanisms and in the adaptation to diverse environmental stresses. When stressed, cells must divert metabolic requirements toward immediate survival and eventual recovery and the unique features of miRNAs, such as their relatively ATP-inexpensive biogenesis costs, and the quick and reversible nature of their action, renders them excellent "master controllers" for rapid responses. Many animal survival strategies for dealing with extreme environmental pressures involve prolonged retreats into states of suspended animation to extend the time that they can survive on their limited internal fuel reserves until conditions improve. The ability to retreat into such hypometabolic states is only possible by coupling the global suppression of nonessential energy-expensive functions with an activation of prosurvival networks, a process in which miRNAs are now known to play a major role. In this chapter, we discuss the activation, expression, biogenesis, and unique attributes of miRNA regulation required to facilitate profound metabolic rate depression and implement stress-specific metabolic adaptations. We examine the role of miRNA in strategies of biochemical adaptation including mammalian hibernation, freeze tolerance, freeze avoidance, anoxia and hypoxia survival, estivation, and dehydration tolerance. By comparing these seemingly different adaptive programs in traditional and exotic animal models, we highlight both unique and conserved miRNA-meditated mechanisms for survival. Additional topics discussed include transcription factor networks, temperature dependent miRNA-targeting, and novel species-specific and stress-specific miRNAs.

Activation of the Hippo Pathway in Rana sylvatica: Yapping Stops in Response to Anoxia

Wood frogs (Rana sylvatica) display well-developed anoxia tolerance as one component of their capacity to endure prolonged whole-body freezing during the winter months. Under anoxic conditions, multiple cellular responses are triggered to efficiently cope with stress by suppressing gene transcription and promoting activation of mechanisms that support cell survival. Activation of the Hippo signaling pathway initiates a cascade of protein kinase reactions that end with phosphorylation of YAP protein. Multiple pathway components of the Hippo pathway were analyzed via immunoblotting, qPCR or DNA-binding ELISAs to assess the effects of 24 h anoxia and 4 h aerobic recovery, compared with controls, on liver and heart metabolism of wood frogs. Immunoblot results showed significant increases in the relative levels of multiple proteins of the Hippo pathway representing an overall activation of the pathway in both organs under anoxia stress. Upregulation of transcript levels further confirmed this. A decrease in YAP and TEAD protein levels in the nuclear fraction also indicated reduced translocation of these proteins. Decreased DNA-binding activity of TEAD at the promoter region also suggested repression of gene transcription of its downstream targets such as SOX2 and OCT4. Furthermore, changes in the protein levels of two downstream targets of TEAD, OCT4 and SOX2, established regulated transcriptional activity and could possibly be associated with the activation of the Hippo pathway. Increased levels of TAZ in anoxic hearts also suggested its involvement in the repair mechanism for damage caused to cardiac muscles during anoxia. In summary, this study provides the first insights into the role of the Hippo pathway in maintaining cellular homeostasis in response to anoxia in amphibians.

Seasonal cellular stress phenomena and phenotypic plasticity in land snail Helix lucorum populations from different altitudes

Temperature, a major abiotic environmental factor, regulates various physiological functions in land snails and therefore determines their biogeographical distribution. Thus, species with different distributions may present different thermal tolerance limits. Additionally, the intense reactivation of snail metabolic rate upon arousal from hibernation or estivation may provoke stress. Land snails, Helix lucorum, display a wide altitudinal distribution resulting in populations being exposed to different seasonal temperature variations. The aim of the present study was to investigate the expression of heat shock proteins (Hsps), mitogen activated protein kinases (MAPKs) and proteins that are related to apoptosis (Bcl-2, ubiquitin), that have 'cytoprotective' roles and are also considered to be reliable indicators of stress because of their crucial role in maintaining cellular homeostasis. These proteins were assessed in H. lucorum individuals from two different populations, one at Axios (sea level, 0 m) and the other at Kokkinopilos (Olympus, 1250 m), as well as after mutual population exchanges, in order to find out whether the different responses of these stress-related proteins depend solely on the environmental temperature. The results showed seasonally altered levels in all studied proteins in the hepatopancreas and foot of snails, both among different populations and between the same populations exposed to varying altitudes. However, individuals of the same population in their native habitat or acclimatized to a different habitat showed a relatively similar pattern of expression, supporting the induction of the specific proteins according to the life history of each species.

Hypoxic Jumbo Squid Activate Neuronal Apoptosis but Not MAPK or Antioxidant Enzymes during Oxidative Stress

AbstractThe limitations that hypoxia imparts on mitochondrial oxygen supply are circumvented by the activation of anaerobic metabolism and prosurvival mechanisms in hypoxia-tolerant animals. To deal with the hypoxia that jumbo squid (Dosidicus gigas) experience in the ocean's depth, they depress their metabolic rate by up to 52% relative to normoxic conditions. This is coupled with molecular reorganization to facilitate their daily descents into the ocean's oxygen minimum zone, where they face not only low oxygen levels but also higher pressures and colder frigid waters. Our current study explores the tissue-specific hypoxia responses of three central processes: (1) antioxidant enzymes responsible for defending against oxidative stress, (2) early apoptotic machinery that signals the activation of cell death, and (3) mitogen-activated protein kinases (MAPKs) that act as central regulators of numerous cellular processes. Luminex xMAP technology was used to assess protein levels and phosphorylation states under normoxic and hypoxic conditions in brains, branchial hearts, and mantle muscles. Hypoxic brains were found to activate apoptosis via upregulation of phospho-p38, phospho-p53, activated caspase 8, and activated caspase 9, whereas branchial hearts were the only tissue to show an increase in antioxidant enzyme levels. Hypoxic muscles seemed the least affected by hypoxia. Our results suggest that hypoxic squid do not undergo large dynamic changes in the phosphorylation states of key apoptotic and central MAPK factors, except for brains, suggesting that these mechanisms are involved in squid hypometabolic responses.

Coordinated expression of Jumonji and AHCY under OCT transcription factor control to regulate gene methylation in wood frogs during anoxia

Wood frogs (Rana sylvatica) can survive extended periods of whole body freezing. Freezing imparts multiple stresses on cells that include anoxia and dehydration, but these can also be experienced as independent stresses. Under anoxia stress, energy metabolism is suppressed, and pro-survival pathways are prioritized to differentially regulate some transcription factors including OCT1 and OCT4. Jumonji C domain proteins (JMJD1A and JMJD2C) are hypoxia responsive demethylases whose expression is accelerated by OCT1 and OCT4 which act to demethylate genes related to the methionine cycle. The responses by these factors to 24 h anoxia exposure and 4 h aerobic recovery was analyzed in liver and skeletal muscle of wood frogs to assess their involvement in metabolic adaptation to oxygen limitation. Immunoblot results showed a decrease in JMJD1A levels under anoxia in liver and muscle, but an increase was observed in JMJD2C demethylase protein in anoxic skeletal muscle. Protein levels of adenosylhomocysteinase (AHCY) and methionine adenosyl transferase (MAT), enzymes of the methionine cycle, also showed an increase in the reoxygenated liver, whereas the levels decreased in muscle. A transcription factor ELISA showed a decrease in DNA binding by OCT1 in the reoxygenated liver and anoxic skeletal muscle, and transcript levels also showed tissue specific gene expression. The present study provides the first analysis of the role of the OCT1 transcription factor, associated proteins, and lysine demethylases in mediating responses to anoxia by wood frog tissues.

RAGE against the stress: Mitochondrial suppression in hypometabolic hearts

The wood frog (Rana sylvatica) can tolerate full body freezing in winter. As a protective response, wood frogs dehydrate their cells and accumulate large quantities of glucose as an intracellular cryoprotectant. Freezing causes ischemia since blood delivery to organs is interrupted. Fascinatingly, wood frogs can tolerate dehydration, extreme hyperglycemia, and anoxia independently of freezing. In response to low oxygen levels, wood frogs strategically reduce their metabolic rates and allocate the finite amount of intracellular fuel available to pro-survival processes while reducing or interrupting all others. In this study, the involvement of advanced glycation end products (AGEs) and the high mobility group box 1 (HMGB1) protein in activating RAGE (AGE receptor) were investigated. The results show that freezing, anoxia and dehydration induced the expression of total HMGB1 and its acetylation in the heart. RAGE levels were induced in response to all stress conditions, which resulted in differential regulation of the ETS1 transcription factor. While the nuclear localization of total ETS1 was not affected, the DNA binding activity of total and its active form increased in response to freezing and dehydration but not in response to anoxia. Current results indicate that ETS1 acts as a transcriptional activator for peroxiredoxin 1 in response to freezing but acts as a transcriptional repressor of several nuclear-encoded mitochondrial genes in response to all stresses. Altogether, current results show that the HMGB1/RAGE axis may activate ETS1 and that this activation could result in both transcriptional activation and/or repression in a stress-dependent manner.

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.

The Ratio of Linoleic and Linolenic Acid in the Pre-hibernation Diet Influences NFκB Signaling in Garden Dormice During Torpor

The fatty acid composition of a pre-hibernation diet can influence the depth and duration of metabolic suppression achieved by hibernators. More specifically, a diet high in n-6 polyunsaturated fatty acids (PUFAs) relative to n-3 PUFAs is essential to maximize torpor expression. However, few studies have investigated how diets with different n-6/n-3 PUFA ratios change stress-inducible cell signaling. Garden dormice (Eliomys quercinus) were fed one of three diets designed with different ratios of n-6 PUFA linoleic acid (LA) and n-3 PUFA linolenic acid (ALA). Then, NFκB signaling was assessed in the white adipose, brown adipose, and liver tissues of euthermic and hibernating dormice via multiplex and RT-qPCR analyses of relative protein and transcript levels, respectively. Dormice fed a high LA diet regulated NFκB signaling in a protective manner in all tissues. NFκB signaling was generally decreased in the high LA group, with significant decreases in the protein levels of NFκB mediators IKKα/β, IκBα, and downstream pro-apoptotic protein FADD. Liver and white adipose from torpid dormice fed a high LA diet increased sod2 expression relative to the other diets or relative to euthermic controls, indicating protection against ROS generated from potentially increased β-oxidation of n-6 PUFAs. The low LA diet increased biomarkers for apoptosis relative to other diets and relative to euthermia, suggesting low LA diets may be detrimental to hibernator health. Overall, this study suggests that changes in the ratio of n-6/ n-3 PUFAs in the diet influences apoptotic and antioxidant responses in white adipose, brown adipose, and liver of hibernating garden dormice.

Regulation of antioxidant systems in response to anoxia and reoxygenation in Rana sylvatica

The wood frog (Rana sylvatica) is a remarkable species. These frogs can endure prolonged oxygen deprivation as well as dehydration to ~60% of total body water lost and, combining these two abilities, they survive whole body freezing for weeks at a time during the winter. Episodes of anoxia/reoxygenation or freeze/thaw can trigger elevated production of reactive oxygen species (ROS) causing cellular damage, especially when oxygen is reintroduced during reoxygenation or thawing. To mitigate ROS damage, stress-responsive transcription factors such as the Octamer Binding Transcription factor (OCT4) and Nuclear factor (erythroid-derived 2)-like 2 transcription factor (Nrf2) were postulated to be involved in enhancing pro-survival pathways and antioxidant defenses. The present study used immunoblotting to analyze OCT4 and Nrf2 responses (and downstream factors under their control) to 24 h anoxia and 4 h reoxygenation in liver and skeletal muscle of wood frogs, with an emphasis on antioxidant systems. Surprisingly, no change was observed in relative total protein expression of either of the two transcription factors in liver. Furthermore, a significant decrease in total protein levels of OCT4 and Nrf2 occurred in skeletal muscle after 4 h recovery. However, essential cofactors of OCT4 and Nrf2 were significantly upregulated during anoxia and/or recovery. Downstream targets of the Nrf2-ARE pathway were evaluated, including glutathione-S-transferases (GSTs) and aldo-keto reductases (AKRs). Significant increases in GSTT1 and GSTP1 were observed in liver and muscle whereas AKRs showed a tissue specific response to both anoxia and recovery from anoxia. This study demonstrates activation of antioxidants as a cell protective mechanism against generation of reactive oxygen species during anoxia in wood frogs.

Mitochondria, metabolic control and microRNA: Advances in understanding amphibian freeze tolerance

Winter survival for many animal species depends freeze tolerance, a capacity to endure the conversion of as much as 65-70% of total body water into extracellular ice while reorganizing metabolism to provide cells with cryoprotection against insults that include prolonged ischemia and hyperosmotic stress. Natural freeze tolerance involves not just de novo preservation mechanisms such as synthesis of high levels of cryoprotectants or novel proteins that manage ice formation, but also requires attention to and co-ordination of many cellular processes. The present review examines recent studies of the freeze-tolerant wood frog (Rana sylvatica) that probed previously unexplored areas of metabolic adaptation for freezing survival, with a particular emphasis on mitochondria. Post-translational controls on enzyme function play a prominent role in resculpting metabolic responses of the wood frog to freezing including reversible phosphorylation control over fuel processing at the pyruvate dehydrogenase locus and modulation of antioxidant defense enzymes (Mn-SOD, catalase). Enzymes involved in mitochondrial nitrogen metabolism (glutamate dehydrogenase, carbamoyl phosphate synthetase) are also differentially regulated during freezing but by different post-translational modifications including ADP-ribosylation, lysine acetylation or glutarylation. The action of microRNAs in mediating post-translational controls on gene expression aid the suppression of energy-expensive (cell cycle) or destructive (apoptosis) processes in the frozen state while also providing storage of transcripts that will be immediately available for repair or reactivation of metabolic processes after thawing. The effects of low temperature in strengthening mRNA-microRNA interactions can also provide a passive mechanism of metabolic suppression in the frozen state.

Advances and applications of environmental stress adaptation research

Evolution has produced animals that survive extreme fluctuations in environmental conditions including freezing temperatures, anoxia, desiccating conditions, and prolonged periods without food. For example, the wood frog survives whole-body freezing every winter, arresting all gross physiological functions, but recovers functions upon thawing in the spring. Likewise, many small mammals hibernate for months at a time with minimal metabolic activity, organ perfusion, and movement, yet do not suffer significant muscle atrophy upon arousal. These conditions and the biochemical adaptations employed to deal with them can be viewed as Nature's answer to problems that humans wish to answer, particularly in a biomedical context. This review focuses on recent advances in the field of animal environmental stress adaptation, starting with an emphasis on new areas of research such as epigenetics and microRNA. We then examine new and emerging technologies such as genome editing, novel sequencing applications, and single cell analysis and how these can push us closer to a deeper understanding of biochemical adaptation. Next, evaluate the potential contributions of new high-throughput technologies (e.g. next-generation sequencing, mass spectrometry proteomics) to better understanding the adaptations that support these extreme phenotypes. Concluding, we examine some of the human applications that can be gained from understanding the principles of biochemical adaptation including organ preservation and treatments for conditions such as ischemic stroke and muscle disuse atrophy.

Biomarker-based assessment of the muscle maintenance and energy status of anurans from an extremely seasonal semi-arid environment, the Brazilian Caatinga

Strongly seasonal environments pose challenges for performance and survival of animals, especially when resource abundance seasonally fluctuates. We investigated the seasonal variation of key metabolic biomarkers in the muscles of males from three species (Rhinella jimi, R. granulosa and Pleurodema diplolister) of anurans from the drastically seasonal Brazilian semi-arid area, Caatinga. We examined the expression of proteins regulating energy turnover (AMP-activated protein kinase [AMPK] and protein kinase B [AKT]), protein synthesis and homeostasis (total and phosphorylated eukaryotic initiation factor 2α [eIF2α and p-eIF2α] and chaperone proteins [HSP 60, 70, and 90]) in muscles predominantly related to reproduction and locomotion. Cytochrome c oxidase (COX) activity was also assessed as an index of the muscle aerobic capacity. The expression pattern of metabolic biomarkers indicates that the maintenance of muscular function is regulated in a species-specific manner during the drastic seasonal variation. Rhinella jimi and R. granulosa that remain active during the drought appear to maintain muscles through more energy expensive pathways including elevated protein synthesis, while the aestivating P. diplolister employs energy conservation strategy suppressing protein synthesis, decreasing chaperone expression and increasing expression of AMPK. Two (P. diplolister and R. granulosa) of the three studied species activate cell survival pathways during the drought likely to prevent muscle atrophy, and all three studied species maintain the muscle aerobic capacity throughout the year, despite the resource limitation. These strategies are important considering the unpredictability of the reproductive event and high demand on muscular activity during the reproductive season in these amphibians. SUMMARY STATEMENT: We studied seasonal variation of key metabolic biomarkers in the muscles of anurans that experience drastic variation in environmental conditions and differ in seasonal activity patterns.

Estivation-responsive microRNAs in a hypometabolic terrestrial snail

When faced with extreme environmental conditions, the milk snail (Otala lactea) enters a state of dormancy known as estivation. This is characterized by a strong reduction in metabolic rate to <30% of normal resting rate that is facilitated by various behavioural, physiological, and molecular mechanisms. Herein, we investigated the regulation of microRNA in the induction of estivation. Changes in the expression levels of 75 highly conserved microRNAs were analysed in snail foot muscle, of which 26 were significantly upregulated during estivation compared with controls. These estivation-responsive microRNAs were linked to cell functions that are crucial for long-term survival in a hypometabolic state including anti-apoptosis, cell-cycle arrest, and maintenance of muscle functionality. Several of the microRNA responses by snail foot muscle also characterize hypometabolism in other species and support the existence of a conserved suite of miRNA responses that regulate environmental stress responsive metabolic rate depression across phylogeny.

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.

Effects of anoxic exposure on the nuclear factor of activated T cell (NFAT) transcription factors in the stress-tolerant wood frog

The wood frog, Lithobates sylvaticus (also known as Rana sylvatica), is used for studying natural freeze tolerance. These animals convert 65% to 70% of their total body water into extracellular ice and survive freezing for weeks in winter. Freezing interrupts oxygen delivery to organs; thus, wood frogs limit their ATP usage by depressing their metabolism and redirecting the available energy only to prosurvival processes. Here, we studied the nuclear factor of activated T cell (NFAT) transcription factor family in response to 24-hour anoxia, and 4-hour aerobic recovery in liver and skeletal muscle. Protein expression levels of NFATc1-c4, calcineurin A and glycogen synthase kinase 3β (NFAT regulators), osteopontin, and atrial natriuretic peptide (ANP) (targets of NFATc3 and NFATc4, respectively) were measured by immunoblotting, and the DNA-binding activities of NFATc1-c4 were measured by DNA-protein interaction ELISAs. Results show that NFATc4, calcineurin, and ANP protein expression as well as NFATc4 DNA binding increased during anoxia in liver where calcineurin and ANP protein levels and NFATc4 DNA binding remaining high after aerobic recovery. Anoxia caused a significant increase in NFATc3 protein expression but not DNA-binding activity in muscle. Our results show that anoxia can increase NFATc4 transcriptional activity in liver, leading to the increase in expression of cytoprotective genes in the wood frog. Understanding the molecular mechanisms involved in mediating survival under anoxia/reoxygenation conditions in a naturally stress-tolerant model, such as the wood frog, provides insightful information on the prosurvival regulatory mechanisms involved in combating stress. This information will also further our understanding of metabolic rate depression and answer the question of how frogs tolerate prolonged periods of oxygen deprivation and resume to full function upon recovery without facing any detrimental side effects as other animals would.

The Living Dead: Mitochondria and Metabolic Arrest

Mitochondria are not just the powerhouses of the cell; these 'end of function' organelles are crucial components of cellular physiology and influence many central metabolic and signaling pathways that support complex multicellular life. Not surprisingly, these organelles play vital roles in adaptations for extreme survival strategies including hibernation and freeze tolerance, both of which are united by requirements for a strong reduction and reprioritization of metabolic processes. To facilitate metabolic rate depression, adaptations of all aspects of mitochondrial function are required, including; energetics, physiology, abundance, gene regulation, and enzymatic controls. This review discusses these factors with a focus on the stress-specific nature of mitochondrial genes and transcriptional regulators, and processes including apoptosis and chaperone protein responses. We also analyze the regulation of glutamate dehydrogenase and pyruvate dehydrogenase, central mitochondrial enzymes involved in coordinating the shifts in metabolic fuel use associated with extreme survival strategies. Finally, an emphasis is given to the novel mitochondrial research areas of microRNAs, peptides, epigenetics, and gaseous mediators and their potential roles in facilitating hypometabolism. © 2018 IUBMB Life, 70(12):1260-1266, 2018.

A potential antiapoptotic regulation: The interaction of heat shock protein 70 and apoptosis-inducing factor mitochondrial 1 during heat stress and aestivation in sea cucumber

The sea cucumber (Apostichopus japonicus) has become a good model organism for studying environmentally induced aestivation in marine invertebrates. A characteristic feature of aestivation in this species is the degeneration of the intestine. In the current study, we hypothesized that energy conservation and cytoprotective strategies need to be coordinated in the intestine to ensure long-term survival during aestivation, and there was potential relationship between heat shock protein 70 (HSP70) and apoptosis-inducing factor mitochondrial 1 (AIFM1) during extreme environmental stress. AIFM1 is a bifunctional flavoprotein that is involved in the caspase-independent activation of apoptosis. The gene and protein expression profiles of AjAIFM1 and AjHSP70 in intestinal tissue during aestivation were analyzed and results showed an inverse correlation between them, AjAIFM1 being suppressed during aestivation whereas AjHSP70 was strongly upregulated. Comparable responses were also seen when intestinal cells were isolated and analyzed in vitro for responses to heat stress at 25°C (a water temperature typical during aestivation), compared with 15°C control cells. Combined with co-immunoprecipitation studies in vivo and in vitro, our results suggested that AjHSP70 protein may have potential interaction with AjAIFM1. To determine the influence of heat stress on apoptotic rate of intestinal cells, we also assessed the DNA fragmentation by terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay, and results also supported a potential antiapoptotic response in sea cucumber during heat stress. This type of cytoprotective mechanism could be used to preserve the existing cellular components during long-term aestivation in sea cucumber.

Effects of both cold and heat stresses on the liver of giant spiny frog Quasipaa spinosa: stress response and histological changes

Ambient temperature associated stress can affect the normal physiological functions in ectotherms. To assess the effects of cold or heat stress on amphibians, the giant spiny frogs, Quasipaa spinosa, were acclimated at 22 °C followed by being treated at 5 °C or 30 °C for 0, 3, 6, 12, 24 and 48 h, respectively. Histological alterations, apoptotic index, mitochondrial reactive oxygen species (ROS) generation, antioxidant activity indices and stress-response gene expressions in frog livers were subsequently determined. Results showed that many fat droplets appeared after 12 h of heat stress. Percentage of melanomacrophages centres significantly changed during 48 h at both stress conditions. Furthermore, the mitochondrial ROS levels were elevated in a time-dependent manner up to 6 h and 12 h in the cold and heat stress groups, respectively. The activities of superoxide dismutase, glutathione peroxidase and catalase were successively increased along the cold or heat exposure, and most of their gene expression levels showed similar changes at both stress conditions. Most tested HSP genes were sensitive to temperature exposure, and the expression profiles of most apoptosis-related genes was significantly up-regulated at 3 and 48 h under cold and heat stress, respectively. Apoptotic index at 48 h under cold stress was significantly higher than that under heat stress. Notably, lipid droplets, HSP30, HSP70 and HSP110 might be suitable bioindicators of heat stress. The results of these alterations at physiological, biochemical and molecular levels might contribute to a better understanding of the stress response of Q. spinosa and even amphibians under thermal stresses.

MicroRNAs regulate survival in oxygen-deprived environments

Some animals must endure prolonged periods of oxygen deprivation to survive. One such extreme model is the Northern Crayfish (Orconectes virilis), that regularly survives year-round hypoxic and anoxic stresses in its warm stagnant summer waters and in its cold, ice-locked winter waters. To elucidate the molecular underpinnings of anoxia-resistance in this natural model, we surveyed the expression profiles of 76 highly-conserved microRNAs in crayfish hepatopancreas and tail muscle from normoxic, acute 2hr anoxia, and chronic 20hr anoxia exposures. MicroRNAs are known to regulate a diverse array of cellular functions required for environmental stress adaptations, and here we explore their role in anoxia tolerance. The tissue-specific anoxia responses observed herein, with 22 anoxia-responsive microRNAs in hepatopancreas and only 4 changing microRNAs in muscle, suggest that microRNAs facilitate a reprioritization of resources to preserve crucial organ functions. Bioinformatic microRNA target enrichment analysis predicted that the anoxia-downregulated microRNAs in hepatopancreas targeted hippo-signalling, suggesting that cell proliferation and apoptotic signalling are highly regulated in this liver-like organ during anoxia. Compellingly, miR-125-5p, miR-33-5p, and miR-190-5p, all known to target the master regulator of oxygen deprivation responses HIF1 (Hypoxia Inducible Factor-1), were anoxia-downregulated in hepatopancreas. The anoxia-increased transcript levels of the oxygen dependent subunit HIF1α, highlight a potential critical role for miRNA-HIF targeting in facilitating a successful anoxia response. Studying the cytoprotective mechanisms in place to protect against the challenges associated with surviving in oxygen-poor environments is critical to elucidating microRNAs’ vast and substantial role in the regulation of metabolism and stress in aquatic invertebrates.

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.

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.