Role of covalent modification in the control of glycolytic enzymes in response to environmental anoxia in goldfish

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 elevated seawater pCO(2) on gene expression patterns in the gills of the green crab, Carcinus maenas

Background

The green crab Carcinus maenas is known for its high acclimation potential to varying environmental abiotic conditions. A high ability for ion and acid-base regulation is mainly based on an efficient regulation apparatus located in gill epithelia. However, at present it is neither known which ion transport proteins play a key role in the acid-base compensation response nor how gill epithelia respond to elevated seawater pCO₂ as predicted for the future. In order to promote our understanding of the responses of green crab acid-base regulatory epithelia to high pCO₂, Baltic Sea green crabs were exposed to a pCO₂ of 400 Pa. Gills were screened for differentially expressed gene transcripts using a 4,462-feature microarray and quantitative real-time PCR.

Results

Crabs responded mainly through fine scale adjustment of gene expression to elevated pCO₂. However, 2% of all investigated transcripts were significantly regulated 1.3 to 2.2-fold upon one-week exposure to CO₂ stress. Most of the genes known to code for proteins involved in osmo- and acid-base regulation, as well as cellular stress response, were were not impacted by elevated pCO₂. However, after one week of exposure, significant changes were detected in a calcium-activated chloride channel, a hyperpolarization activated nucleotide-gated potassium channel, a tetraspanin, and an integrin. Furthermore, a putative syntaxin-binding protein, a protein of the transmembrane 9 superfamily, and a Cl^-/HCO₃^- exchanger of the SLC 4 family were differentially regulated. These genes were also affected in a previously published hypoosmotic acclimation response study.

Conclusions

The moderate, but specific response of C. maenas gill gene expression indicates that (1) seawater acidification does not act as a strong stressor on the cellular level in gill epithelia; (2) the response to hypercapnia is to some degree comparable to a hypoosmotic acclimation response; (3) the specialization of each of the posterior gill arches might go beyond what has been demonstrated up to date; and (4) a re-configuration of gill epithelia might occur in response to hypercapnia.

Not only students can express alcohol dehydrogenase: goldfish can too!

This article describes a novel approach to study the metabolic regulation of the respiratory system in vertebrates that suits physiology lessons for undergraduate students. It consists of an experimental demonstration of the goldfish's (Carassius auratus) adaptations to anoxia. The goldfish is one of the few vertebrates showing strong enzymatic plasticity for the expression of alcohol dehydrogenase (ADH), which allows it to survive long periods of severe anoxia. Therefore, we propose two simple laboratory exercises in which students are first asked to characterize the distribution of ADH isozymes in the goldfish by performing cellulose acetate electrophoresis. The second part of this laboratory lesson is the determination of liver glycogen. To further student comprehension, an interspecies comparative component is integrated, in which the same subjects are studied in an anoxia-sensitive species, the brook charr (Salvelinus fontinalis). ADH in goldfish is restricted to skeletal muscles, where it catalyzes alcoholic fermentation, permitting ethanol excretion through the gills and therefore preventing lactate acidosis caused by sustained glycolysis during anoxia. Electrophoresis also reveals the occurrence of a liver isozyme in the brook charr, which ADH catalyzes in the opposite pathway, allowing the usual ethanol degradation. As for the liver glycogen assay, it shows largely superior content in the goldfish liver compared with the brook charr, providing goldfish with a sustained energy supply during anoxia. The results of this laboratory exercise clearly demonstrate several physiological strategies developed by goldfish to cope with such a crucial environmental challenge as oxygen depletion.

Purification and kinetic properties of 6-phosphofructo-1-kinase from gilthead sea bream muscle

The kinetic properties of 6-phosphofructo-1-kinase (PFK) from skeletal muscle (PFKM) of gilthead sea bream (Sparus aurata) were studied, after 10,900-fold purification to homogeneity. The native enzyme had an apparent molecular mass of 662 kDa and is composed of 81 kDa subunits, suggesting a homooctameric structure. At physiological pH, S. aurata PFKM exhibited sigmoidal kinetics for the substrates, fructose-6-phosphate (fru-6-P) and ATP. Fructose-2,6-bisphosphate (fru-2,6-P(2)) converted the saturation curves for fru-6-P to hyperbolic, activated PFKM synergistically with other positive effectors of the enzyme such as AMP and ADP, and counteracted ATP and citrate inhibition. The fish enzyme showed differences regarding other animal PFKs: it is active as a homooctamer, and fru-2,6-P(2) and pH affected affinity for ATP. By monitoring incorporation of (32)P from ATP, we show that fish PFKM is a substrate for the cAMP-dependent protein kinase. The mechanism involved in PFKM activation by phosphorylation contrasts with previous observations in other species: it increased V(max) and did not affect affinity for fru-6-P. Unlike the mammalian muscle enzyme, our findings support that phosphorylation of PFKM may exert a major role during starvation in fish muscle.

Phosphofructokinase from muscle of the marine giant barnacle Austromegabalanus psittacus: kinetic characterization and effect of in vitro phosphorylation

The kinetic properties of phosphofructokinase from muscle of the giant cirripede Austromegabalanus psittacus were characterized, after partial purification by ion exchange chromatography on DEAE-cellulose. This enzyme showed differences regarding PFKs from other marine invertebrates: the affinity for fructose 6-phosphate (Fru 6-P) was very low, with an S(0.5) of 22.6+/-1.4 mM (mean+/-S.D., n=3), and a high cooperativity (n(H) of 2.90+/-0.21; mean+/-S.D., n=3). The barnacle PFK showed hyperbolic saturation kinetics for ATP (apparent K(m ATP)=70 microM, at 5 mM Fru 6-P, in the presence of 2 mM ammonium sulfate). ATP concentrations higher than 1 mM inhibited the enzyme. Ammonium sulfate activated the PFK several folds, increasing the affinity of the enzyme for Fru 6-P and V(max). 5'-AMP (0.2 mM) increased the affinity for Fru 6-P (S(0.5) of 6.2 mM). Fructose 2,6-bisphosphate activated the PFK, with a maximal activation at concentrations higher than 2 microM. Citrate reverted the activation of PFK produced by 0.2 mM 5'-AMP (IC(50 citrate)=2.0 mM), producing a higher inhibition than that exerted on other invertebrate PFKs. Barnacle muscular PFK was activated in vitro after exposure to exogenous cyclic-AMP (0.1 mM) as well as by phosphatidylserine (50 microg/ml), indicating a possible control by protein kinase A and a phospholipid dependent protein kinase (PKC). The results suggest a highly regulated enzyme in vivo, by allosteric mechanisms and also by protein phosphorylation.

Key metabolic enzymes and muscle structure in triplefin fishes (Tripterygiidae): a phylogenetic comparison

Metabolic potential and muscle development were investigated relative to habitat and phylogeny in seven species of New Zealand triplefin fishes. Activity was measured in three principal glycolytic enzymes (lactate dehydrogenase, pyruvate kinase and phosphofructokinase) and two oxidative enzymes (citrate synthase and L3-hydroxyacyl CoA:NAD(+) oxidoreductase). The non-bicarbonate buffering capacity of caudal muscle was also estimated. Phylogenetic independent contrast analyses were used to reduce the effects of phylogenetic history in analyses. A positive relationship between metabolic potential and the effective water velocity at respective habitat depths was found only after the exclusion from analyses of the semi-pelagic species Obliquichthys maryannae. O. maryannae showed high glycolytic enzyme activities, and displayed double the activity of both oxidative enzymes relative to the six benthic species. Histochemically stained sections taken immediately posterior to the vent showed that adult O. maryannae and larval Forsterygion lapillum had significantly more red muscle, and smaller cross-sectional areas of white and red muscle fibres, than adults of benthic species. The distribution of red muscle in adult O. maryannae resembled that of larval F. lapillum, and differed from the typical teleost pattern seen in adults of the six benthic species. Both adult O. maryannae and larval F. lapillum have an expansive lateralis superficialis muscle, typical of larval fish, which encompasses much of the caudal trunk. Results suggest that anaerobic potential in New Zealand triplefins: (a) increases with the locomotory requirements of different habitats, and (b) displays a negative relationship with depth-dependent water velocities in benthic species. O. maryannae appears to have increased aerobic potential for sustained swimming by paedomorphic retention of larval muscle architecture.

Hypoxia: from molecular responses to ecosystem responses

Hypoxia affects thousands of km2 of marine waters all over the world, and has caused mass mortality of marine animals, benthic defaunation and decline in fisheries production in many places. The severity, frequency occurrence and spatial scale of hypoxia have increased in the last few decades. Due to rapid human population growth and global warming, the problem of hypoxia is likely to become worse in the coming years. Molecular responses of marine animals to hypoxia are poorly known. In many animals, a haem protein probably serves as the cellular sensor for oxygen, and reactive oxygen species are generated as signaling molecules. In mammal and fish, a heterodimeric transcription factor, hypoxia-inducible factor 1 (HIF-1) has been identified. HIF-1 receives signals from the molecular oxygen senor through redox reactions and/or phosphorylation, and in turn, regulates the transcription of a number of hypoxia-inducible genes, including genes involved in erythropoiesis, angiogenesis and glycolysis. These molecular responses then cascade into a series of biochemical and physiological adjustments, enabling the animal to survive better under hypoxic conditions. Marine animals respond to hypoxia by first attempting to maintain oxygen delivery (e.g. increases in respiration rate, number of red blood cells, or oxygen binding capacity of hemoglobin), then by conserving energy (e.g. metabolic depression, down regulation of protein synthesis and down regulation/modification of certain regulatory enzymes). Upon exposure to prolonged hypoxia, animals must eventually resort to anaerobic respiration. Hypoxia reduces growth and feeding, which may eventually affect individual fitness. Effects of hypoxia on reproduction and development of marine animals, albeit important in affecting species survival, remain almost unknown. Many fish and marine organisms can detect, and actively avoid hypoxia. Some benthos may leave their burrows and move to sediment surface during hypoxia. These behaviorial changes may render the animals more vulnerable to predation. Hypoxia may eliminate sensitive species, thereby causing major changes in species composition of benthic, fish and phytoplankton communities. Decreases in species diversity and species richness are well documented, and changes in trophodynamics and functional groups have also been reported. Under hypoxic conditions, there is a general tendency for suspended feeders to be replaced by deposit feeders; demersal fish by pelagic fish; and macrobenthos by meiobenthos. Microflagellates and nanoplankton also tend to dominate in the phytoplankton community in hypoxic environments. Existing evidence suggest that recovery of benthic communities in temperate region take two to several years. Recovery however, appears to be much quicker in subtropical environments. In natural conditions, hypoxia is often associated with increases in ammonia, hydrogen sulphide and particulate organic materials. The inability to isolate effects of hypoxia from interactions of these compounding factors makes it difficult to attribute many of the observed ecological effects to hypoxia.

Glycolytic controls in estivation and anoxia: a comparison of metabolic arrest in land and marine molluscs

Facultative metabolic rate depression is the common adaptive strategy underlying various animal mechanisms for surviving harsh environmental conditions. This strategy is common among molluscs, enabling animals to survive over days or even months in the absence of oxygen or under extremely dry conditions. The large reductions in metabolic rate during estivation and anoxia can translate into considerable energy savings when dormant animals are compared to active animals. A complex metabolic coordination is required during the transition into the dormant state to maintain cellular homeostasis and involves both energy-consuming and energy-producing pathways. With regard to energy-producing pathways, several different mechanisms have been identified that participate in controlling flux. One such mechanism, enzyme phosphorylation, can have a wide-ranging effect. For example, phosphorylated enzymes exhibit altered substrate, activator, and inhibitor affinities. This effect may be magnified by changes in the concentrations of allosteric effectors, such as fructose 2,6-bisphosphate, that occur during hypometabolic states. Changes in fructose 2,6-bisphosphate are related to changes in enzyme phosphorylation through changes in the relative activity of phosphofructokinase-2. Alterations in glycolytic enzyme binding can also be brought about through changes in enzyme phosphorylation. The present review focuses on identifying hypometabolism-related changes in enzyme phosphorylation as well as characterizing the mechanisms involved in mediating these phosphorylation events.

Metabolic adaptations supporting anoxia tolerance in reptiles: recent advances

Animal survival during severe hypoxia and/or anoxia is enhanced by a variety of biochemical adaptations including adaptations of fermentative pathways of energy production and, most importantly, the ability to sharply reduce metabolic rate by 5-20 fold and enter a hypometabolic state. The biochemical regulation of metabolic arrest is proving to have common molecular principles that extend across phylogenetic lines and that are conserved in different types of arrested states (not only anaerobiosis but also estivation, hibernation, etc.). Our new studies with anoxia-tolerant vertebrates have identified a variety of regulatory mechanisms involved in both metabolic rate depression and in the aerobic recovery process using as models the freshwater turtle Trachemys scripta elegans and garter snakes Thamnophis sirtalis parietalis. Mechanisms include: 1) post-translational modification of cellular and functional proteins by reversible phosphorylation and changes in protein kinase (PKA, PKC) and/or phosphatase activities to regulate this, 2) reversible enzyme binding associations with subcellular structural elements, 3) differential gene expression and/or mRNA translation producing new mRNA variants and new protein products, 4) changes in protease activity, particularly the multicatalytic proteinase complex, and 5) both constitutive and anoxia-induced modifications to cellular antioxidant systems to deal with oxidative stress during the anoxic-aerobic transition of recovery.

Phosphofructokinase from white muscle of the rainbow trout, Oncorhynchus mykiss: purification and properties

Phosphofructokinase was purified and characterized from the white skeletal muscle of rainbow trout Oncorhynchus mykiss. Purification involved three steps: ion-exchange chromatography on hydroxyapatite and affinity chromatography on phosphocellulose and ATP-agarose. A final specific activity of 75 units per mg of protein at 22 degrees C and pH 7.2 with 40% recovery was obtained. The purified enzyme gave a single band on SDS-PAGE with a subunit molecular mass of 76.5 +/- 0.6 kDa. Based on gel filtration analysis, the active form of the enzyme was found to be composed of six identical subunits. A high isoelectric point (7.1) was found for this enzyme. Arrhenius plots of the enzyme activity showed a sharp transition at 15-16 degrees C. The pH optimum of the enzyme was 8.0-8.5 at physiological level of ATP and positive modulators shifted the optimum to lower pH values. Amino-acid analysis revealed a lower content of the aromatic residues Phe, Tyr and Trp and higher level of Ser residue than in the rabbit muscle enzyme.

Phosphofructokinase from a vertebrate facultative anaerobe: effects of temperature and anoxia on the kinetic parameters of the purified enzyme from turtle white muscle

The effects of low temperature and anoxia were determined on phosphofructokinase (PFK) purified from white skeletal muscle of the freshwater turtle, Pseudemys scripta. These effects were assayed by comparing PFK kinetic constants measured at a high (20 degrees C) and low (6 degrees C) temperature using enzyme obtained from animals held under normoxic and anoxic conditions. When assayed at 20 degrees C, PFK from anoxic animals had a lower Ka for phosphate, a lower Ka for AMP and showed no inhibition with increasing concentrations of ATP (up to 10 mM) when compared to enzyme from normoxic animals. At 6 degrees C, anoxic enzyme had a higher Km for fructose 6-phosphate and a higher I50 value for citrate with respect to normoxic enzyme. Decreasing temperature also had a differential effect on PFK kinetic parameters depending on the source of the enzyme. When normoxic enzymes were compared at 20 and 6 degrees C, the enzyme measured at 6 degrees C showed a lower Km for ATP and a lower Ka for AMP. Comparison of anoxic enzymes at these two temperatures showed that anoxic PFK at 6 degrees C had a higher Ka for phosphate, a higher Ka for AMP, and a larger Hill coefficient. A comparison of maximal velocities at varying temperature showed that normoxic enzyme (Q10 = 2.22) was more temperature sensitive than the anoxic enzyme (Q10 = 1.80). It is possible to interconvert the normoxic and anoxic forms of PFK by incubating normoxic enzyme with the active subunit of protein kinase, suggesting that the kinetic changes observed during anoxia resulted from enzyme phosphorylation. These data are discussed with respect to the mechanisms underlying white muscle function during diving and hibernation in red-eared turtles.

Purification of phosphofructokinase using transition-state analogue affinity chromatography

A novel purification of phosphofructokinase has been achieved in a two step process using ion-exchange affinity chromatography and a transition-state analogue affinity column matrix. The procedure can be performed in one day, and gives a 25% yield of the starting material. The transition-state analogue chromatography is carried out using an ADP-agarose column in the presence of fructose 6-phosphate, magnesium ions and nitrate ions. In the presence of nitrate ion plus substrate, phosphofructokinase binds immobilized ADP while other proteins pass through the column. Previous studies with creatine kinase have shown that the nitrate ion mimics the planar phosphate in the transition state resulting in a complex which is stable under the relatively high ionic strength of the column buffer. This permits the elution of phosphofructokinase in a single peak of high specific activity. This column typically results in a 20-30 fold increase in specific activity with only a small loss of activity.