Evidence for phosphorylation/dephosphorylation control of phosphofructokinase from organs of the Anoxia-Tolerant sea musselMytilus edulis

Energy homeostasis as an integrative tool for assessing limits of environmental stress tolerance in aquatic invertebrates

Energy balance is a fundamental requirement of stress adaptation and tolerance. We explore the links between metabolism, energy balance and stress tolerance using aquatic invertebrates as an example and demonstrate that using key parameters of energy balance (aerobic scope for growth, reproduction and activity; tissue energy status; metabolic rate depression; and compensatory onset of anaerobiosis) can assist in integrating the effects of multiple stressors and their interactions and in predicting the whole-organism and population-level consequences of environmental stress. We argue that limitations of both the amount of available energy and the rates of its acquisition and metabolic conversions result in trade-offs between basal maintenance of a stressed organism and energy costs of fitness-related functions such as reproduction, development and growth and can set limit to the tolerance of a broad range of environmental stressors. The degree of stress-induced disturbance of energy balance delineates transition from moderate stress compatible with population persistence (pejus range) to extreme stress where only time-limited existence is possible (pessimum range). It also determines the predominant adaptive strategy of metabolic responses (energy compensation vs. conservation) that allows an organism to survive the disturbance. We propose that energy-related biomarkers can be used to determine the conditions when these metabolic transitions occur and thus predict ecological consequences of stress exposures. Bioenergetic considerations can also provide common denominator for integrating stress responses and predicting tolerance limits under the environmentally realistic scenarios when multiple and often variable stressors act simultaneously on an organism. Determination of bioenergetic sustainability at the organism's level (or lack thereof) has practical implications. It can help identify the habitats and/or conditions where a population can survive (even if at the cost of reduced reproduction and growth) and those that are incapable of supporting viable populations. Such an approach will assist in explaining and predicting the species' distribution limits in the face of the environmental change and informing the conservation efforts and resource management practices.

High-resolution analysis of metabolic cycles in the intertidal mussel Mytilus californianus

Inhabitants of the marine rocky intertidal live in an environment that alternates between aquatic and terrestrial due to the rise and fall of the tide. The tide creates a cyclical availability of oxygen with animals having access to oxygenated water during episodes of submergence, while access to oxygen is restricted during aerial emergence. Here we performed liquid chromatography and gas chromatography-mass spectrometry enabled metabolomic profiling of gill samples isolated from the California ribbed mussel, Mytilus californianus, to investigate how metabolism is orchestrated in this variable environment. We created a simulated intertidal environment in which mussels were acclimated to alternating high and low tides of 6 h duration, and samples were taken every 2 h for 72 h to capture reproducible changes in metabolite levels over six high and six low tides. We quantified 169 named metabolites of which 24 metabolites cycled significantly with a 12-h period that was linked to the tidal cycle. These data confirmed the presence of alternating phases of fermentation and aerobic metabolism and highlight a role for carnitine-conjugated metabolites during the anaerobic phase of this cycle. Mussels at low tide accumulated eight carnitine-conjugated metabolites, arising from the degradation of fatty acids, branched-chain amino acids, and mitochondrial β-oxidation end products. The data also implicate sphingosine as a potential signaling molecule during aerial emergence. These findings identify new levels of metabolic control whose role in intertidal adaptation remains to be elucidated.

Intermediary metabolism in mudskippers,Periophthalmodon schlosseriandBoleophthalmus boddarti, during immersion or emersion

This study aimed to examine effects of 48 h emersion on intermediary metabolism in the mudskippers Periophthalmodon schlosseri (Pallas, 1770) and Boleophthalmus boddarti (Pallas, 1770). The glycogen content increased significantly, while the lactate content remained unchanged, in the muscle of P. schlosseri during 48 h of emersion. Thus, emersion led to either a decrease in glycogen utilization or an increase in glycogen synthesis. Because there was no change in energy charge, P. schlosseri probably used energy stores other than glycogen during emersion. The phosphofructose kinase-1 (PFK) from the muscle of P. schlosseri became more sensitive to ATP inhibition after 6 h of emersion, indicating that a transient decrease in the glycolytic flux indeed occurred at this locus that led to an increase in glycogen content. In contrast, such phenomena were not observed in the muscle of B. boddarti during 48 h of emersion. Emersion also led to a significant increase in the muscle fructose-2,6-bisphosphate (F26P2) content in P. schlosseri, but not in B. boddarti. Based on results obtained from P. schlosseri undergoing forced exercise, we propose that a correlation may exist between the F26P2content and the utilization of amino acids as an energy source in this mudskipper during emersion.

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.

Isoforms of cAMP-dependent protein kinase in the bivalve mollusk Mytilus galloprovincialis: activation by cyclic nucleotides and effect of temperature

Two different isoforms of cAMP-dependent protein kinase (PKA) have been partially purified from the posterior adductor muscle and the mantle tissue of the sea mussel Mytilus galloprovincialis. The holoenzymes contain as regulatory subunit (R) the previously identified isoforms Rmyt1 and Rmyt2, and were named PKAmyt1 and PKAmyt2, respectively. Both cAMP and cGMP can activate these PKA isoforms completely, although they exhibit a sensitivity approximately 100-fold higher for cAMP than for cGMP. When compared to PKAmyt2, the affinity of PKAmyt1 for cAMP and cGMP is 2- and 3.5-fold higher, respectively. The effect of temperature on the protein kinase activity of both PKA isoforms was examined. Temperature changes did not affect significantly the apparent activation constants (Ka) for cAMP. However, the protein kinase activity was clearly modified and a remarkable difference was observed between both PKA isoforms. PKAmyt1 showed a linear Arrhenius plot over the full range of temperature tested, with an activation energy of 15.3+/-1.5 kJ/mol. By contrast, PKAmyt2 showed a distinct break in the Arrhenius plot at 15 degrees C; the activation energy when temperature was above 15 degrees C was 7-fold higher than that of lower temperatures (70.9+/-8.1 kJ/mol vs 10.6+/-6.5 kJ/mol). These data indicate that, above 15 degrees C, PKAmyt2 activity is much more temperature-dependent than that of PKAmyt1. This different behavior would be related to the different role that these isoforms may play in the tissues where they are located.

Characterization of a novel gene up-regulated during anoxia exposure in the marine snail, Littorina littorea

Gene expression was investigated during anoxia exposure in the marine snail, Littorina littorea. Differential screening of a cDNA library made from hepatopancreas of anoxic L. littorea yielded a 525 bp clone coding for the novel gene kvn. The deduced amino acid sequence of the KVN protein contained 99 amino acid residues with a predicted molecular weight of 12 kDa and showed an N-terminal secretory signal. Analysis of hepatopancreas samples over a time course of anoxia exposure showed a maximum increase in transcript levels of 5.8-fold after 48 h relative to normoxic animals, with a subsequent decrease in transcript levels during normoxic recovery. Nuclear run-off assays confirmed the observed transcriptional up-regulation of kvn during anoxia. Organ culture experiments were performed to determine a possible pathway of up-regulation of kvn, with data indicating a putative role for cGMP in signal transduction. Profiles of ribosome distribution in polysomes versus monosomes revealed a reduction in the polysome peak during anoxia and a shift in the position of kvn transcripts to association with the lower density polysome/higher density monosome region. The data suggest that the kvn transcript is both transcribed and translated during anoxia, indicating a possible significant role for the KVN protein in the survival of anoxia by L. littorea.

Transcription pattern of ribosomal protein L26 during anoxia exposure in Littorina littorea

Differential screening of a hepatopancreas cDNA library derived from the marine snail Littorina littorea yielded a 421-bp clone coding for ribosomal protein L26 that was up-regulated during anoxia exposure. The deduced amino acid sequence, containing 144 residues with a predicted molecular weight of 17 kDa, showed 80% amino acid sequence identity to the mammalian ribosomal protein L26. Analysis of hepatopancreas and foot muscle samples from a time course of anoxia exposure showed a maximal transcript increase of 4- and 3-fold after 96 hr and 48 hr, respectively, relative to normoxic animals, with a subsequent decrease in transcript levels during normoxic recovery. Nuclear run-off assays confirmed the observed transcriptional up-regulation of L26 during anoxia. Organ culture experiments were performed to determine a possible pathway of up-regulation of L26, with data indicating a putative role for cGMP in signal transduction. The transcriptional up-regulation of L26 during anoxia may stabilize the existing mRNA pool, via a possible cGMP-mediated signaling cascade, until oxygen reappears and protein synthesis resumes.

Effects of seasonal change and prolonged anoxia on metabolic enzymes of Littorina littorea

The effects of seasonal change (July versus November) and prolonged anoxia (N2 atmosphere at 5 or 10°C for 6 days) exposure in vivo on the activities of 18 enzymes, as well as the kinetic properties of phosphofructokinase (PFK) and pyruvate kinase (PK), were investigated in foot muscle and digestive gland of the marine periwinkle Littorina littorea L. Seasonal differences in enzyme maximal activities were tissue-specific, with generally increased activities during the summer and changes in a greater number of enzymes in digestive gland than in foot muscle. Seasonal differences in the kinetic properties of PFK and PK were observed in both tissues. PK from digestive gland of winter animals showed a much higher S0.5 for phosphoenolpyruvate and stronger changes in enzyme kinetic properties in response to anoxia than did the enzyme in summer animals; this may suggest the presence of seasonal isozymes. The effects of anoxia were tissue- and season-specific. Anoxia exposure during the winter induced a greater number of changes in enzyme maximal activities in foot muscle than in digestive gland. Anoxia-induced changes in the kinetic properties of both PFK and PK were also seen in both organs. For PK, these changes were consistent with less active enzyme forms in the anoxic state. Hence, both seasonal and environmental (anoxia) factors influence enzyme maximal activities and kinetic properties in L. littorea.

Regulation of the futile cycle of fructose phosphate in sea mussel

Carbohydrate metabolism in mussels shows two phases separated seasonally. During summer and linked to food supply, carbohydrates, mainly glycogen, are accumulated in the mantle tissue. During winter, mantle glycogen decreases concomitantly with an increase in triglyceride synthesis. In spring, after spawning, the animals go in to metabolic rest until the beginning of a new cycle. This cycle is regulated by the futile cycle of fructose phosphate that implicates PFK-1 and FBPase-1 activities. These enzymes and the bifunctional PFK-2/FBPase-2 that regulates the Fru-2,6-P2 levels, are seasonally modulated by covalent phosphorylation/dephosphorylation mechanisms, as a response to unknown factors. The futile cycle of the fructose phosphates also controls the transition from physiological aerobiosis to hypoxia. The process is independent of the phosphorylation state. In this sense, a pH decrease triggers a small Pasteur effect during the first 24 h of aerial exposure. Variations in the concentration of Fru-2,6-P2 and AMP are the sole factor responsible for this effect. Longer periods of hypoxia induce a metabolic depression characterized by a decrease in Fru-2,6-P2 which is hydrolyzed by drop in the pH. In this review, the authors speculate on the two regulation processes.

Purification of a novel isoform of the regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Cytosolic extracts from the posterior adductor muscle of the bivalve mollusk Mytilus galloprovincialis contain significant amounts of both cGMP-binding and cGMP-stimulated protein kinase activities. However, photoaffinity labeling with 8-azido-[32P]cGMP revealed only a major cGMP-binding protein with an apparent molecular mass of 54 kDa (p54), lacking protein kinase activity itself. Instead, the purified and cGMP-free p54 protein has the ability to inhibit a mussel protein kinase homologous to the mammalian cAMP-dependent protein kinase (cAPK) catalytic subunit, the inhibition being relieved by cAMP or cGMP, which suggests that it can act as a regulatory subunit of cAPK. However, p54 failed to be recognized by a specific antibody against the regulatory subunit (type RII) previously isolated from mussel. Therefore, p54 must be a novel isoform of cAPK regulatory subunit that seems to have high affinity for both cGMP and cAMP.

In vivo phosphorylation of phosphofructokinase from the bivalve mollusk Mytilus galloprovincialis

The phosphorylation state of phosphofructokinase from the mantle tissue of the facultative anaerobe mollusk Mytilus galloprovincialis was determined by a back-phosphorylation technique. The incubation of intact mantle tissue with 8-bromoadenosine 3':5'-cyclic monophosphate increased significantly the phosphate content of phosphofructokinase, which indicates that the enzyme can be phosphorylated in vivo by endogenous cAMP-dependent protein kinase. The phosphate content of mussel phosphofructokinase changes significantly during the year, in agreement with the kinetic data that show a more active enzyme form in earlier autumn. These results suggest that cAMP-dependent phosphorylation of phosphofructokinase can be partially responsible for the observed glycolytic changes associated with the annual gametogenic cycle that takes place in the mantle tissue of the mollusk. On the contrary, no differences were observed between aerobic and 24-h hypoxic mussels with regard to the phosphorylation state and the kinetic constants of phosphofructokinase. This result is inconsistent with the hypothesis that phosphorylation of phosphofructokinase is involved in the glycolytic depression that takes place during the long-term environmental hypoxia that the mollusk can undergo.

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.

The Pasteur effect in facultative anaerobic metazoa

The existence and the regulatory mechanisms of the Pasteur effect in facultative anaerobic metazoa are discussed. There are three reasons for the controversy surrounding this phenomenon. 1) The different definitions of the Pasteur effect, 2) the antagonistic effect of metabolic depression and its species specific response to hypoxia, as well as 3) the laboratory-specific differences in the experimental procedures for analyzing the Pasteur effect and its regulation. This review aims to clarify the confusion about the existence of the Pasteur effect in facultative anaerobic metazoa and to offer possible molecular mechanisms.

Phosphorylation-activated 6-phosphofructo-2-kinase from mantle tissue of marine mussels

PKF-2 from mussel mantle was phosphorylated by cAMP-dependent protein kinase. The phosphorylation does not change the enzyme activity at neutral pH values, but at acid pH the activity of the phosphorylated form is higher than the native PFK-2. With respect to the native enzyme, the activation consisted of a reduction in the Km for Fru-6-P and a decrease in the inhibitory effect of PEP. These results are in keeping with the stabilized concentration of Fru-2,6-P2 found in the mussel mantle during the physiological hypoxia caused by the closure of the valves.