cGMP-stimulated protein kinase phosphorylates pyruvate kinase in an anoxia-tolerant marine mollusc

The regulation of AMPK signaling in a natural state of profound metabolic rate depression

In response to energy stress (and elevated AMP), the AMP-activated protein kinase (AMPK) coordinates the restoration of energy homeostasis. We determined that AMPK is activated in a model system (desert snail Otala lactea) during a physiological state of profound metabolic rate depression (estivation) in the absence of a rise in AMP. Kinetic characterization indicated a strong increase in AMPK activity and phosphorylation in estivation, consistent with an increase in P-Ser428 LKB, an established regulator of AMPK. Accordingly, approximately 2-fold increases in AMPKalpha1 protein and activity were observed with LKB1 immunoprecipitates from estivating snails. In vitro studies determined that AMPK in crude extracts was activated in the presence of cGMP and deactivated in conditions that permitted protein phosphatase type-2A (PP2A) activity. Furthermore, AMPKalpha1 protein and activity increased in PKG immunoprecipitates from estivating tissues, suggesting a novel role for PKG in the regulation of AMPK in vivo. We evaluated several downstream targets of AMPK. Acetyl-CoA carboxylase (ACC) activity was strongly inhibited in estivation, consistent with increased P-Ser79 content, and in vitro stimulation of AMPK negated citrate's ability to stimulate ACC aggregation. Analysis of other targets revealed a strong decrease in PPARgamma-coactivator 1alpha expression in both tissues, which was related to decreased gluconeogenic protein expression in hepatic tissue, but no changes in mitochondrial biogenesis markers in muscle. We concluded that AMPK activation in O. lactea aids in facilitating the suppression of anabolic pathways, without necessarily activating ATP-generating catabolism.

Tribute to P. L. Lutz: putting life on 'pause'--molecular regulation of hypometabolism

Entry into a hypometabolic state is an important survival strategy for many organisms when challenged by environmental stress, including low oxygen, cold temperatures and lack of food or water. The molecular mechanisms that regulate transitions to and from hypometabolic states, and stabilize long-term viability during dormancy, are proving to be highly conserved across phylogenic lines. A number of these mechanisms were identified and explored using anoxia-tolerant turtles as the model system, particularly from the research contributions made by Dr Peter L. Lutz in his explorations of the mechanisms of neuronal suppression in anoxic brain. Here we review some recent advances in understanding the biochemical mechanisms of metabolic arrest with a focus on ideas such as the strategies used to reorganize metabolic priorities for ATP expenditure, molecular controls that suppress cell functions (e.g. ion pumping, transcription, translation, cell cycle arrest), changes in gene expression that support hypometabolism, and enhancement of defense mechanisms (e.g. antioxidants, chaperone proteins, protease inhibitors) that stabilize macromolecules and promote long-term viability in the hypometabolic state.

Akt and its downstream targets play key roles in mediating dormancy in land snails

Estivation, a state of aerobic dormancy, facilitates survival during adverse environmental conditions and is characterized at the molecular level by regulatory protein phosphorylation. The Akt (protein kinase B) signaling pathway regulates diverse responses in cells and the present study analyzes its role in the estivating desert snail Otala lactea. Kinetic analysis (maximal velocity, substrate affinities) determined that Akt was activated in tissues of estivating snails and Western blotting and in vitro incubations promoting changes to Akt phosphorylation state both confirmed that higher amounts of active (phosphorylated Ser473) Akt were present during estivation. Akt protein stability was also enhanced during estivation as assessed from urea denaturation studies. Multiple downstream targets of Akt were differentially regulated during estivation. Estivating animals showed elevated levels of phosphorylated FOXO3a (Ser253) and BAD (Ser136), no change in mTOR (Ser2481 and Ser2448), and reduced amounts of phosphorylated glycogen synthase kinase-3 (GSK-3) beta subunit (Ser9). Kinetic analysis of GSK-3 showed 1.5-1.7 fold higher activities in estivating snails coupled with increased GSK-3 substrate affinities in hepatopancreas. The data suggest an active role for Akt signaling during estivation emphasizing anti-apoptotic actions but uncoupling growth/proliferation actions to help achieve life extension on a limited energy budget.

Anoxia-induced changes in reactive oxygen species and cyclic nucleotides in the painted turtle

The Western painted turtle survives months without oxygen. A key adaptation is a coordinated reduction of cellular ATP production and utilization that may be signaled by changes in the concentrations of reactive oxygen species (ROS) and cyclic nucleotides (cAMP and cGMP). Little is known about the involvement of cyclic nucleotides in the turtle's metabolic arrest and ROS have not been previously measured in any facultative anaerobes. The present study was designed to measure changes in these second messengers in the anoxic turtle. ROS were measured in isolated turtle brain sheets during a 40-min normoxic to anoxic transition. Changes in cAMP and cGMP were determined in turtle brain, pectoralis muscle, heart and liver throughout 4 h of forced submergence at 20-22 degrees C. Turtle brain ROS production decreased 25% within 10 min of cyanide or N(2)-induced anoxia and returned to control levels upon reoxygenation. Inhibition of electron transfer from ubiquinol to complex III caused a smaller decrease in [ROS]. Conversely, inhibition of complex I increased [ROS] 15% above controls. In brain [cAMP] decreased 63%. In liver [cAMP] doubled after 2 h of anoxia before returning to control levels with prolonged anoxia. Conversely, skeletal muscle and heart [cAMP] remained unchanged; however, skeletal muscle [cGMP] became elevated sixfold after 4 h of submergence. In liver and heart [cGMP] rose 41 and 127%, respectively, after 2 h of anoxia. Brain [cGMP] did not change significantly during 4 h of submergence. We conclude that turtle brain ROS production occurs primarily between mitochondrial complexes I and III and decreases during anoxia. Also, cyclic nucleotide concentrations change in a manner suggestive of a role in metabolic suppression in the brain and a role in increasing liver glycogenolysis.

Identification of a 115kDa MAP-kinase activated by freezing and anoxic stresses in the marine periwinkle, Littorina littorea

The mitogen-activated protein kinase (MAPK) cascade regulates changes in gene transcription by transmitting extracellular stimuli from the plasma membrane to the cell nucleus and has an important role to play in organismal responses to environmental stresses. The activities of MAPKs were investigated in the marine gastropod mollusk, Littorina littorea, a species that tolerates both extracellular freezing and long term oxygen deprivation. In-gel kinase assays revealed the presence of two MAPKs in foot muscle and hepatopancreas, a 42 and a 115kDa protein. Immunoblot analysis showed that both were MAPK proteins and that one was the periwinkle homologue of p42(ERK2). Size exclusion chromatography confirmed the 115kDa size of the novel snail MAPK and its role as the dominant MAPK activity in foot muscle. In-gel kinase assays, immunoblotting with phospho-specific ERK antibody, as well as kinase activity profiles from hydroxyapatite chromatography demonstrated that p115 MAPK kinase activity was increased in foot muscle in response to in vivo freezing or anoxia exposures. The results suggest a role for this novel kinase in environmental stress response.

Suppression of Na+/K+-ATPase activity during estivation in the land snail Otala lactea

Entry into the hypometabolic state of estivation requires a coordinated suppression of the rate of cellular ATP turnover, including both ATP-generating and ATP-consuming reactions. As one of the largest consumers of cellular ATP, the plasma membrane Na+/K+-ATPase is a potentially key target for regulation during estivation. Na+/K+-ATPase was investigated in foot muscle and hepatopancreas of the land snail Otala lactea, comparing active and estivating states. In both tissues enzyme properties changed significantly during estivation: maximal activity was reduced by about one-third, affinity for Mg.ATP was reduced (Km was 40% higher), and activation energy (derived from Arrhenius plots) was increased by approximately 45%. Foot muscle Na+/K+-ATPase from estivated snails also showed an 80% increase in Km Na+ and a 60% increase in Ka Mg2+ as compared with active snails, whereas hepatopancreas Na+/K+-ATPase showed a 70% increase in I50 K+ during estivation. Western blotting with antibodies recognizing the alpha subunit of Na+/K+-ATPase showed no change in the amount of enzyme protein during estivation. Instead, the estivation-responsive change in Na+/K+-ATPase activity was linked to posttranslational modification. In vitro incubations manipulating endogenous kinase and phosphatase activities indicated that Na+/K+-ATPase from estivating snails was a high phosphate, low activity form, whereas dephosphorylation returned the enzyme to a high activity state characteristic of active snails. Treatment with protein kinases A, C or G could all mediate changes in enzyme properties in vitro that mimicked the effect of estivation, whereas treatments with protein phosphatase 1 or 2A had the opposite effect. Reversible phosphorylation control of Na+/K+-ATPase can provide the means of coordinating ATP use by this ion pump with the rates of ATP generation by catabolic pathways in estivating snails.

Glucose-6-phosphate dehydrogenase regulation during hypometabolism

Glucose-6-phosphate dehydrogenase (G6PDH) from hepatopancreas of the land snail, Otala lactea, shows distinct changes in properties between active and estivating (dormant) states, providing the first evidence of pentose phosphate cycle regulation during hypometabolism. Compared with active snails, G6PDH Vmax increased by 50%, Km for glucose-6-phosphate decreased by 50%, Ka Mg x citrate decreased by 35%, and activation energy (from Arrhenius plots) decreased by 35% during estivation. DEAE-Sephadex chromatography separated two peaks of activity and in vitro incubations stimulating protein kinases or phosphatases showed that peak I (low phosphate) G6PDH was higher in active snails (57% of activity) whereas peak II (high phosphate) G6PDH dominated during estivation (71% of total). Kinetic properties of peaks I and II forms mirrored the enzyme from active and estivated states, respectively. Peak II G6PDH also showed reduced sensitivity to urea inhibition of activity and greater stability to thermolysin protease treatment. The interconversion of G6PDH between active and estivating forms was linked to protein kinase G and protein phosphatase 1. Estivation-induced phosphorylation of G6PDH may enhance relative carbon flow through the pentose phosphate cycle, compared with glycolysis, to help maintain NADPH production for use in antioxidant defense.

Pyruvate kinase: current status of regulatory and functional properties

Pyruvate kinase (PK) is a key enzyme for the glycolytic pathway and carbon metabolism in general. On the basis of the relevance and enormous diverse properties of this enzyme, this paper describes the results of a current and extensive review that determines the sites of conservation and/or difference in PK sequences, and the differences in the functional and regulatory properties of the enzymes. An alignment and analysis of 50 PK sequences from different sources and a phylogenetic tree are presented. This analysis was performed with reference to crystallographically characterized PK principally from E. coli, cat and rabbit muscle. A number of attributes of the enzyme that make it of particular interest in biomedicine and industry are also discussed.

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.

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.

Protein kinase involvement in land snail aestivation and anoxia: protein kinase A kinetic properties and changes in second messenger compounds during depressed metabolism

In response to environmental stress (low water, low oxygen) snails sharply suppress their metabolic rate, a process that is coordinated at the molecular level by reversible protein phosphorylation of key enzymes and functional proteins. Factors affecting protein kinase activity are, therefore, critical to metabolic suppression. Changes in the concentration of protein kinase second messenger compounds were followed over the first 24 h of aestivation and anoxia exposure in the terrestrial snail Otala lactea (Muller) (Pulmonata, Helicidae). The results showed declining concentrations of cyclic AMP over the first 24 h of anoxia exposure and aestivation in foot. Cyclic AMP concentrations in hepatopancreas transiently decreased with the lowest concentration observed at 4 h in both anoxic and aestivating animals. A transient increase in foot muscle cyclic GMP concentrations was apparent 4 h after the start of aestivation whereas a slow, steady increase was seen in anoxic foot muscle. Foot muscle 1,4,5-inositol triphosphate (IP3) concentrations decreased transiently during anoxia exposure and aestivation. Hepatopancreas IP3 concentrations were significantly lower in 24 h anoxic snails and foot IP3 concentrations were significantly lower in 24 h aestivating snails. Kinetic characterization of purified PKA catalytic subunit was also performed. Snail PKA catalytic subunit had an absolute requirement for Mg2+ ion but was inhibited at Mg2+ concentrations above 0.5 mM. Increasing concentrations of neutral salts and phosphate also inhibited activity although the inhibition by phosphate appeared to be specific since the inhibition constant (I50 = 39 mM) was much lower than that of the neutral salts (I50 approximately 240 mM). The enzyme exhibited a broad pH optimum between pH 6.5-8.5. Arrhenius plots gave an activation energy of 13.3 kcal/mol corresponding to a Q10 value of 2.3. The relationship between these results and temporal control of enzyme phosphorylation is discussed.

The role of protein kinases in anoxia tolerance in facultative anaerobes: purification and characterization of a protein kinase that phosphorylates pyruvate kinase

A protein kinase which phosphorylates pyruvate kinase (PK) in vitro was purified and characterized from the foot muscle of the anoxia-tolerant gastropod mollusc Busycon canaliculatum. Purification involved four steps: poly(ethylene glycol) fractionation, affinity chromatography on Blue agarose, ion-exchange chromatography on phosphocellulose and preparative isoelectric focusing (pI = 5.5). The activity was monitored by following changes in pyruvate kinase I50 values for L-alanine which have previously been linked to changes in the degree of enzyme phosphorylation. The correlation between enzyme phosphorylation and changes in the L-alanine inhibition constant was also directly demonstrated in the present paper by radioactively labelling PK with [tau-32P]ATP. The final purified protein kinase solution gave a single band on SDS-gel electrophoresis with a molecular weight of 37,000 +/- 2000. Kinetic analysis of the purified protein kinase (PK-kinase) showed a pH optimum of 7.0, an absolute requirement for magnesium ions (Km = 1.29 mM), a relatively high affinity for MgATP (Km = 57 microM), and inhibition by increasing salt concentrations (I50 = 55 mM KCl). The protein kinase activity was not affected by either spermine, heparin, cAMP, cGMP or concentrations of CaCl2 less than 10 mM. The enzyme did not phosphorylate either phosphofructokinase or glycogen phosphorylase, two enzymes that are also phosphorylated during anoxia in whelks. The purified enzyme is different from the catalytic subunit of cAMP-dependent protein kinase as shown by the inability of cAMP to stimulate the protein kinase at all stages of the preparation; cAMP did not activate either crude enzyme, the 7% poly(ethylene glycol) supernatant, or any of the column eluant peak fractions when measured by changes in pyruvate kinase kinetic parameters.