Regulation of 5'-adenosine monophosphate deaminase in the freeze tolerant wood frog, Rana sylvatica

Adenosine monophosphate deaminase modulates BIN2 activity through hydrogen peroxide-induced oligomerization

The Arabidopsis thaliana GSK3-like kinase, BRASSINOSTEROID-INSENSITIVE2 (BIN2) is a key negative regulator of brassinosteroid (BR) signaling and a hub for crosstalk with other signaling pathways. However, the mechanisms controlling BIN2 activity are not well understood. Here we performed a forward genetic screen for resistance to the plant-specific GSK3 inhibitor bikinin and discovered that a mutation in the ADENOSINE MONOPHOSPHATE DEAMINASE (AMPD)/EMBRYONIC FACTOR1 (FAC1) gene reduces the sensitivity of Arabidopsis seedlings to both bikinin and BRs. Further analyses revealed that AMPD modulates BIN2 activity by regulating its oligomerization in a hydrogen peroxide (H2O2)-dependent manner. Exogenous H2O2 induced the formation of BIN2 oligomers with a decreased kinase activity and an increased sensitivity to bikinin. By contrast, AMPD activity inhibition reduced the cytosolic reactive oxygen species (ROS) levels and the amount of BIN2 oligomers, correlating with the decreased sensitivity of Arabidopsis plants to bikinin and BRs. Furthermore, we showed that BIN2 phosphorylates AMPD to possibly alter its function. Our results uncover the existence of an H2O2 homeostasis-mediated regulation loop between AMPD and BIN2 that fine-tunes the BIN2 kinase activity to control plant growth and development.

Short Time-Series Expression Transcriptome Data Reveal the Gene Expression Patterns of Dairy Cow Mammary Gland as Milk Yield Decreased Process

The existing research on dairy cow mammary gland genes is extensive, but there have been few reports about dynamic changes in dairy cow mammary gland genes as milk yield decrease. For the first time, transcriptome analysis based on short time-series expression miner (STEM) and histological observations were performed using the Holstein dairy cow mammary gland to explore gene expression patterns in this process of decrease (at peak, mid-, and late lactation). Histological observations suggested that the number of mammary acinous cells at peak/mid-lactation was significantly higher than that at mid-/late lactation, and the lipid droplets area secreted by dairy cows was almost unaltered across the three stages of lactation (p > 0.05). Totals of 882 and 1439 genes were differentially expressed at mid- and late lactation, respectively, compared to peak lactation. Function analysis showed that differentially expressed genes (DEGs) were mainly related to apoptosis and energy metabolism (fold change ≥ 2 or fold change ≤ 0.5, p-value ≤ 0.05). Transcriptome analysis based on STEM identified 16 profiles of differential gene expression patterns, including 5 significant profiles (false discovery rate, FDR ≤ 0.05). Function analysis revealed DEGs involved in milk fat synthesis were downregulated in Profile 0 and DEGs in Profile 12 associated with protein synthesis. These findings provide a foundation for future studies on the molecular mechanisms underlying mammary gland development in dairy cows.

5'-Adenosine monophosphate deaminase regulation in ground squirrels during hibernation

Hibernation is an important winter survival strategy for many small mammals. By sinking into a deep torpor where metabolic rate can be as low as 1-5% of the resting rate in euthermia, animals accrue huge energy savings that allow survival, typically without eating, for many months. Hibernating ground squirrels show a net reduction in the total adenylate pool of skeletal muscle during torpor, but the ATP/ADP ratio and adenylate energy charge remain stable. A key enzyme involved in managing adenylate pool size is 5'-adenosine monophosphate deaminase (AMPD). Assessing skeletal muscle AMPD from both Richardson's ground squirrels (Urocitellus richardsonii) (RGS) and 13-lined ground squirrels (Ictidomys tridecemlineatus) (TLGS), the present study shows that muscle AMPD of euthermic versus hibernating animals displays markedly different kinetic properties, differential responses to temperature and to effectors, and is regulated by reversible protein phosphorylation. AMPD activity decreased during hibernation in both TLGS and RGS skeletal muscle, by 70 and 84%, respectively. Stimulation of total protein phosphatases, total serine/threonine protein phosphatases, PP1, PP2B or PP2C, all reduced AMPD activity between 54 and 92% in extracts of euthermic RGS muscle. The same incubation did not change the activity of AMPD from muscle of hibernating animals. Oppositely, both euthermic and hibernating AMPD showed a strong increase in activity when incubated under conditions that promoted the enzyme phosphorylation by PKA, PKC or PKG. Overall, the data indicate that both low activity of AMPD and low affinity of the enzyme for AMP during torpor reduce the rate of adenylate degradation, the primary driver of these changes being covalent phosphorylation of AMPD.

DNA methylation and regulation of DNA methyltransferases in a freeze-tolerant vertebrate

The wood frog is one of the few freeze-tolerance vertebrates. This is accomplished in part by the accumulation of cryoprotectant glucose, metabolic rate depression, and stress response activation. These may be achieved by mechanisms such as DNA methylation, which is typically associated with transcriptional repression. Hyperglycemia is also associated with modifications to epigenetic profiles, indicating an additional role that the high levels of glucose play in freeze tolerance. We sought to determine whether DNA methylation is affected during freezing exposure, and whether this is due to the wood frog's response to hyperglycemia. We examined global DNA methylation and DNA methyltransferases (DNMTs) in the liver and muscle of frozen and glucose-loaded wood frogs. The results showed that levels of 5-methylcytosine (5mC) increased in the muscle, suggesting elevated DNA methylation during freezing. DNMT activities also decreased in muscle during thawing, glucose loading, and in vitro glucose experiments. Liver DNMT activities were similar to muscle; however, a varied response to DNMT levels and a decrease in 5mC highlight the metabolic role the liver plays during freezing. Glucose was also shown to decrease DNMT activity levels in the wood frog, in vitro, elucidating a potentially novel regulatory mechanism. Together these results suggest an interplay between freeze tolerance and hyperglycemic regulation of DNA methylation.

Glucose and urea metabolic enzymes are differentially phosphorylated during freezing, anoxia, and dehydration exposures in a freeze tolerant frog

Vertebrate freeze tolerance requires multiple adaptations underpinned by specialized biochemistry. Freezing of extracellular water leads to intracellular dehydration as pure water is incorporated into growing ice crystals and also results in the cessation of blood supply to tissues, creating an anoxic cellular environment. Hence, the freeze tolerant wood frog, Rana sylvatica, must endure both dehydration and anoxia stresses in addition to freezing. The metabolic responses to freezing, dehydration and anoxia involve both protein/enzyme adaptations and the production of metabolites with metabolic or osmotic functions, particularly glucose and urea. The present study uses a phosphoproteome analysis to examine the differential phosphorylation of metabolic enzymes involved in the production of these two metabolites in liver in response to freezing, anoxia, or dehydration exposures. Our results show stress-specific responses in the abundance of phosphopeptides retrieved from nine glycolytic enzymes and three urea cycle enzymes in liver of wood frogs exposed to 24 h freezing, 24 h anoxia, or dehydration to 40% of total body water loss, as compared with 5 °C acclimated controls. Data show changes in the abundance of phosphopeptides belonging to glycogen phosphorylase (GP) and phosphofructokinase 2 (PFK2) that were consistent with differential phosphorylation control of glycogenolysis and a metabolic block at PFK1 that can facilitate glucose synthesis as the cryoprotectant during freezing. Anoxia-exposed animals showed similar changes in GP phosphorylation but no changes to PFK2; changes that would facilitate mobilization of glycogen as a fermentative fuel for anaerobic glycolysis. Urea is commonly produced as a compatible osmolyte in response to amphibian dehydration. Selected urea cycle enzymes showed small changes in phosphopeptide abundance in response to dehydration, but during freezing differential phosphorylation occurred that may facilitate this ATP expensive process when energy resources are sparse. These results add to the growing body of literature demonstrating the importance and efficiency of reversible protein phosphorylation as a regulatory mechanism allowing animals to rapidly respond to environmental stress.

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.

Passive regeneration of glutathione: Glutathione reductase regulation from the freeze-tolerant North American wood frog, Rana sylvatica

Wood frogs inhabit a broad range across North America, extending from the southern tip of the Appalachian Mountains to the northern boreal forest. Remarkably they can survive the winter in a frozen state, where as much as 70% of their body water is converted into ice. During the frozen state, their hearts cease to pump blood, causing their cells to experience ischemia which can dramatically increase the production of reactive oxygen species produced within the cell. To overcome this, wood frogs have elevated levels of glutathione, a primary antioxidant. We examined the regulation of glutathione reductase, the enzyme involved in recycling glutathione, in both the frozen and unfrozen state (control). Glutathione reductase activity from both the control and frozen state showed dramatic reduction in substrate specificity (Km) for oxidized glutathione (50%) when measured in the presence of glucose (300mM) and a increase (157%) when measured in the presence of levels of urea (75mM) encountered in the frozen state. However, when we tested the synergistic effect of urea and glucose simultaneously, we observed a substantial reduction in the Km for oxidized glutathione (43%) to a value similar to that of glucose alone. In fact, we found no observable differences in the kinetic and structural properties of glutathione reductase between the two states. Therefore, a significant increase in the affinity for oxidized glutathione in the presence of endogenous levels of glucose, suggests that increased glutathione recycling may result due to passive regulation of glutathione reductase by rising levels of glucose during freezing.

Free-radical first responders: the characterization of CuZnSOD and MnSOD regulation during freezing of the freeze-tolerant North American wood frog, Rana sylvatica

BACKGROUND

The North American wood frog, Rana sylvatica, is able to overcome subzero conditions through overwintering in a frozen state. Freezing imposes ischemic and oxidative stress on cells as a result of cessation of blood flow. Superoxide dismutases (SODs) catalyze the redox reaction involving the dismutation of superoxide (O(2)(-)) to molecular oxygen and hydrogen peroxide.

METHODS

The present study investigated the regulation of CuZnSOD and MnSOD kinetics as well as the transcript, protein and phosphorylation levels of purified enzyme from the muscle of control and frozen R. sylvatica.

RESULTS

CuZnSOD from frozen muscle showed a significantly higher V(max) (1.52 fold) in comparison to CuZnSOD from the muscle of control frogs. MnSOD from frozen muscle showed a significantly lower Km for O(2)(-) (0.66 fold) in comparison to CuZnSOD from control frogs. MnSOD from frozen frogs showed higher phosphorylation of serine (2.36 fold) and tyrosine (1.27 fold) residues in comparison to MnSOD from control animals. Susceptibility to digestion via thermolysin after incubation with increasing amount of urea (C(m)) was tested, resulting in no significant changes for CuZnSOD, whereas a significant change in MnSOD stability was observed between control (2.53 M urea) and frozen (2.92 M urea) frogs. Expressions of CuZnSOD and MnSOD were quantified at both mRNA and protein levels in frog muscle, but were not significantly different.

CONCLUSION

The physiological consequence of freeze-induced SOD modification appears to adjust SOD function in freezing frogs.

GENERAL SIGNIFICANCE

Augmented SOD activity may increase the ability of R. sylvatica to overcome oxidative stress associated with ischemia.

Glycogen synthase kinase-3: cryoprotection and glycogen metabolism in the freeze-tolerant wood frog

The terrestrial anuran Rana sylvatica tolerates extended periods of whole-body freezing during the winter. Freezing survival is facilitated by extensive glycogen hydrolysis and distribution of high concentrations of the cryoprotectant glucose into blood and all tissues. As glycogenesis is both an energy-expensive process and counter-productive to maintaining sustained high cryoprotectant levels, we proposed that glycogen synthase kinase-3 (GSK-3) would be activated when wood frogs froze and would phosphorylate its downstream substrates to inactivate glycogen synthesis. Western blot analysis determined that the amount of phosphorylated (inactive) GSK-3 decreased in all five tissues tested in 24 h frozen frogs compared with unfrozen controls. Total GSK-3 protein levels did not change, with the exception of heart GSK-3, indicating that post-translational modification was the primary regulatory mechanism for this kinase. Kinetic properties of skeletal muscle GSK-3 from control and frozen frogs displayed differential responses to a temperature change (22 versus 4°C) and high glucose. For example, when assayed at 4°C, the K(m) for the GSK-3 substrate peptide was ∼44% lower for frozen frogs than the corresponding value in control frogs, indicating greater GSK-3 affinity for its substrates in the frozen state. This indicates that at temperatures similar to the environment encountered by frogs, GSK-3 in frozen frogs will phosphorylate its downstream targets more readily than in unfrozen controls. GSK-3 from skeletal muscle of control frogs was also allosterically regulated. AMP and phosphoenolpyruvate activated GSK-3 whereas inhibitors included glucose, glucose 6-phosphate, pyruvate, ATP, glutamate, glutamine, glycerol, NH(4)Cl, NaCl and KCl. The combination of phosphorylation and allosteric control argues for a regulatory role of GSK-3 in inactivating glycogenesis to preserve high glucose cryoprotectant levels throughout each freezing bout.