Global arrest of translation during invertebrate quiescence

Transcriptome analysis of FOXO-dependent hypoxia gene expression identifies Hipk as a regulator of low oxygen tolerance in Drosophila

When exposed to low oxygen or hypoxia, animals must alter their metabolism and physiology to ensure proper cell-, tissue-, and whole-body level adaptations to their hypoxic environment. These alterations often involve changes in gene expression. While extensive work has emphasized the importance of the HIF-1 alpha transcription factor on controlling hypoxia gene expression, less is known about other transcriptional mechanisms. We previously identified the transcription factor FOXO as a regulator of hypoxia tolerance in Drosophila larvae and adults. Here, we use an RNA-sequencing approach to identify FOXO-dependent changes in gene expression that are associated with these tolerance effects. We found that hypoxia altered the expression of over 2,000 genes and that ∼40% of these gene expression changes required FOXO. We discovered that hypoxia exposure led to a FOXO-dependent increase in genes involved in cell signaling, such as kinases, GTPase regulators, and regulators of the Hippo/Yorkie pathway. Among these, we identified homeodomain-interacting protein kinase as being required for hypoxia survival. We also found that hypoxia suppresses the expression of genes involved in ribosome synthesis and egg production, and we showed that hypoxia suppresses tRNA synthesis and mRNA translation and reduces female fecundity. Among the downregulated genes, we discovered that FOXO was required for the suppression of many ribosomal protein genes and genes involved in oxidative phosphorylation, pointing to a role for FOXO in limiting energetically costly processes such as protein synthesis and mitochondrial activity upon hypoxic stress. This work uncovers a widespread role for FOXO in mediating hypoxia changes in gene expression.

Effect of hypercapnic hypoxia and bacterial infection (Vibrio campbellii) on protein synthesis rates in the Pacific whiteleg shrimp,Litopenaeus vannamei

Estuarine species frequently encounter areas of simultaneously low dissolved O2(hypoxia) and high CO2(hypercapnia). Organisms exposed to hypoxia experience a metabolic depression that serves to decrease ATP utilization and O2demand during stress. This downregulation is typically facilitated by a reduction in protein synthesis, a process that can be responsible for up to 60% of basal metabolism. The added effects of hypercapnia, however, are unclear. Certain decapods also exhibit a metabolic depression in response to bacterial challenges, leading us to hypothesize that protein synthesis may also be reduced during infection. In the present study, we examined the effects of hypoxia (H), hypercapnic hypoxia (HH), and bacterial infection ( Vibrio campbellii) on tissue-specific (muscle and hepatopancreas) fractional protein synthesis rates ( ks) in Litopenaeus vannamei. We observed a significant decrease in ksin muscle after 24 h exposure to both H and HH, and in hepatopancreas after 24 h exposure to HH. Thus ksis responsive to changes in O2, and the combined effect of hypercapnic hypoxia on ksis more severe than hypoxia alone. These reductions in ksappear to be driven by changes in RNA translational efficiency ( kRNA), and not RNA capacity ( Cs). Bacterial infection, however, had no significant effect on ksin either tissue. These results suggest that crustaceans reduce metabolic demand during environmental hypoxia by reducing global protein synthesis, and that this effect is magnified when hypercapnia is concomitantly present. Conversely, an immune-mediated metabolic depression is not associated with a decrease in overall protein production.

Long-term survival of hydrated resting eggs from Brachionus plicatilis

Background

Several organisms display dormancy and developmental arrest at embryonic stages. Long-term survival in the dormant form is usually associated with desiccation, orthodox plant seeds and Artemia cysts being well documented examples. Several aquatic invertebrates display dormancy during embryonic development and survive for tens or even hundreds of years in a hydrated form, raising the question of whether survival in the non-desiccated form of embryonic development depends on pathways similar to those occurring in desiccation tolerant forms.

Methodology/Principal Findings

To address this question, Illumina short read sequencing was used to generate transcription profiles from the resting and amictic eggs of an aquatic invertebrate, the rotifer, Brachionus plicatilis. These two types of egg have very different life histories, with the dormant or diapausing resting eggs, the result of the sexual cycle and amictic eggs, the non-dormant products of the asexual cycle. Significant transcriptional differences were found between the two types of egg, with amictic eggs rich in genes involved in the morphological development into a juvenile rotifer. In contrast, representatives of classical “stress” proteins: a small heat shock protein, ferritin and Late Embryogenesis Abundant (LEA) proteins were identified in resting eggs. More importantly however, was the identification of transcripts for messenger ribonucleoprotein particles which stabilise RNA. These inhibit translation and provide a valuable source of useful RNAs which can be rapidly activated on the exit from dormancy. Apoptotic genes were also present. Although apoptosis is inconsistent with maintenance of prolonged dormancy, an altered apoptotic pathway has been proposed for Artemia, and this may be the case with the rotifer.

Conclusions

These data represent the first transcriptional profiling of molecular processes associated with dormancy in a non-desiccated form and indicate important similarities in the molecular pathways activated in resting eggs compared with desiccated dormant forms, specifically plant seeds and Artemia.

Cytochrome c oxidase is regulated by modulations in protein expression and mitochondrial membrane phospholipid composition in estivating African lungfish

We examined some of the potential mechanisms lungfish ( Protopterus dolloi ) use to regulate cytochrome c oxidase (CCO), during metabolic depression. CCO activity was reduced by 67% in isolated liver mitochondria of estivating fish. This was likely accomplished, in part, by the 46% reduction in CCO subunit I protein expression in the liver. No change in the mRNA expression levels of CCO subunits I, II, III, and IV were found in the liver, suggesting CCO is under translational regulation; however, in the kidney, messenger limitation may be a factor as the expression of subunits I and II were depressed (∼10-fold) during estivation, suggesting tissue-specific mechanisms of regulation. CCO is influenced by mitochondrial membrane phospholipids, particularly cardiolipin (CL). In P. dolloi , the phospholipid composition of the liver mitochondrial membrane changed during estivation, with a ∼2.3-fold reduction in the amount of CL. Significant positive correlations were found between CCO activity and the amount of CL and phosphatidylethanolamine within the mitochondrial membrane. It appears CCO activity is regulated through multiple mechanisms in P. dolloi , and individual subunits of CCO are regulated independently, and in a tissue-specific manner. It is proposed that altering the amount of CL within the mitochondrial membrane may be a means of regulating CCO activity during metabolical depression in the African lungfish, P. dolloi .

Ribosomal analysis of rapid rates of protein synthesis in the Antarctic sea urchin Sterechinus neumayeri

Previous research has shown that developing stages of the Antarctic sea urchin Sterechinus neumayeri have high rates of protein synthesis that are comparable to those of similar species living in much warmer waters. Direct measurements of the biosynthetic capacities of isolated ribosomes have not been reported for marine organisms living in the extreme-cold environment of Antarctica. Such measurements are required for a mechanistic understanding of how the critical and highly complex processes involved in protein synthesis are regulated in animals living in the coldest marine environment on Earth (< -1 degrees C). We tested the hypothesis that high rates of protein synthesis in the cold are a direct result of high biosynthetic capacities of ribosomes engaged in protein synthesis. Our results show that the rate at which ribosomes manufacture proteins (i.e., the peptide elongation rate) at -1 degrees C is surprisingly similar to rates measured in other sea urchin species at temperatures that are over 15 degrees C warmer. Average peptide elongation rates for a range of developmental stages of the Antarctic sea urchin were 0.36 codons s(-1) (+/- 0.05, SE). On the basis of subcellular rate determinations of ribosomal activity, we calculated stage-specific rates of protein synthesis for blastulae and gastrulae to be 3.7 and 6.5 ng protein h(-1), respectively. These findings support the conclusion that the high rates of biosynthesis previously reported for the Antarctic sea urchin are an outcome of high ribosomal activities.

Mitochondria in energy-limited states: mechanisms that blunt the signaling of cell death

Cellular conditions experienced during energy-limited states--elevated calcium, shifts in cellular adenylate status, compromised mitochondrial membrane potential--are precisely those that trigger, at least in mammals, the mitochondrion to initiate opening of the permeability transition pore, to assemble additional protein release channels, and to release pro-apoptotic factors. These pro-apototic factors in turn activate initiator and executer caspases. How is activation of mitochondria-based pathways for the signaling of apoptotic and necrotic cell death avoided under conditions of hypoxia, anoxia, diapause, estivation and anhydrobiosis? Functional trade-offs in environmental tolerance may have occurred in parallel with the evolution of diversified pathways for the signaling of cell death in eukaryotic organisms. Embryos of the brine shrimp, Artemia franciscana, survive extended periods of anoxia and diapause, and evidence indicates that opening of the mitochondrial permeability transition pore and release of cytochrome c (cyt-c) do not occur. Further, caspase activation in this crustacean is not dependent on cyt-c. Its caspases display regulation by nucleotides that is consistent with ;applying the brakes' to cell death during energy limitation. Unraveling the mechanisms by which organisms in extreme environments avoid cell death may suggest possible interventions during disease states and biostabilization of mammalian cells.

Stress-induced activation of the AMP-activated protein kinase in the freeze-tolerant frog Rana sylvatica

Survival in the frozen state depends on biochemical adaptations that deal with multiple stresses on cells including long-term ischaemia and tissue dehydration. We investigated whether the AMP-activated protein kinase (AMPK) could play a regulatory role in the metabolic re-sculpting that occurs during freezing. AMPK activity and the phosphorylation state of translation factors were measured in liver and skeletal muscle of wood frogs (Rana sylvatica) subjected to anoxia, dehydration, freezing, and thawing after freezing. AMPK activity was increased 2-fold in livers of frozen frogs compared with the controls whereas in skeletal muscle, AMPK activity increased 2.5-, 4.5- and 3-fold in dehydrated, frozen and frozen/thawed animals, respectively. Immunoblotting with phospho-specific antibodies revealed an increase in the phosphorylation state of eukaryotic elongation factor-2 at the inactivating Thr56 site in livers from frozen frogs and in skeletal muscles of anoxic frogs. No change in phosphorylation state of eukaryotic initiation factor-2alpha at the inactivating Ser51 site was seen in the tissues under any of the stress conditions. Surprisingly, ribosomal protein S6 phosphorylation was increased 2-fold in livers from frozen frogs and 10-fold in skeletal muscle from frozen/thawed animals. However, no change in translation capacity was detected in cell-free translation assays with skeletal muscle extracts under any of the experimental conditions. The changes in phosphorylation state of translation factors are discussed in relation to the control of protein synthesis and stress-induced AMPK activation.

Effects of environmental hypercapnia on animal physiology: A 13C NMR study of protein synthesis rates in the marine invertebrate Sipunculus nudus

Global climate change is associated with a progressive rise in ocean CO(2) concentrations (hypercapnia) and, consequently, a drop in seawater pH. However, a comprehensive picture of the physiological mechanisms affected by chronic CO(2) stress in marine biota is still lacking. Here we present an analysis of protein biosynthesis rates in isolated muscle of the marine invertebrate Sipunculus nudus, a sediment dwelling worm living at various water depths. We followed the incorporation of (13)C-labelled phenylalanine into muscular protein via high-resolution NMR spectroscopy. Protein synthesis decreased by about 60% at a medium pH of 6.70 and a consequently lowered intracellular pH (pHi). The decrease in protein synthesis rates is much stronger than the concomitant suppression of protein degradation (60% versus 10-15%) possibly posing a threat to the cellular homeostasis of structural as well as functional proteins. Considering the progressive rise in ocean CO(2) concentrations, permanent disturbances of cellular protein turnover might seriously affect growth and reproductive performance in many marine organisms with as yet unexplored impacts on species density and composition in marine ecosystems.

Comparative proteomic analysis of hypoxia-treated and untreated human leukemic U937 cells

We reported recently that moderate hypoxia and hypoxia-mimetic agents could induce growth arrest and differentiation of leukemic cells via the mediation of hypoxia-inducible factor 1 alpha (HIF-1alpha), but the exact molecular mechanisms remain largely unknown. In this study, human acute promonocytic leukemic U937 cells were incubated under 2% O2 or in 50 microM of the hypoxia mimetic agent cobalt chloride (CoCl2) and normal oxygen for 24 h, and their protein expression profiles were compared by 2-DE coupled with MALDI-TOF/TOF MS/MS. We identified 62 and 16 proteins that were significantly deregulated by hypoxia and CoCl2 treatment, respectively. These proteins were mainly involved in metabolism, gene expression regulation, signal transduction, cell proliferation, differentiation and apoptosis. As an example, N-myc downstream regulated gene 1 (NDRG1), a putative differentiation-related gene, was up-regulated in both 2% O2- and CoCl2-treated U937 cells. Moreover, enforced HIF-1alpha expression also elevated NDRG1 mRNA and protein in U937 cells. These data will provide some clues for understanding mechanisms by which leukemic cells response to hypoxia.

Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Defining the physiological mechanisms that set metabolic rates and the`cost of living' is important for understanding the energy costs of development. Embryos and larvae of the sea urchin Lytechinus pictus(Verrill) were used to test hypotheses regarding differential costs of protein synthesis in animals differing in size, rates of protein synthesis, and physiological feeding states. For embryos, the rate of protein synthesis was 0.22±0.014 ng protein embryo-1 h-1 (mean ±s.e.m.) and decreased in unfed larvae to an average rate of 0.05±0.001 ng protein larva-1 h-1. Fed larvae had rates of synthesis that were up to 194 times faster than unfed larvae (9.7±0.81 ng protein larva-1 h-1). There was no significant difference, however, in the cost of protein synthesis between these larvae with very different physiological states. Furthermore, the cost of synthesis in the larval stages was also similar to costs measured for blastula and gastrula embryos of 8.4±0.99 J mg-1 protein synthesized. The cost of protein synthesis was obtained using both direct (`inhibitor') and indirect (`correlative') measurements; both methods gave essentially identical results. Protein synthesis accounted for up to 54±8% of metabolic rate in embryos. Percent of metabolism accounted for by protein synthesis in larvae was dependent on their physiological feeding state, with protein synthesis accounting for 16±4% in unfed larvae and 75±11% in fed larvae. This regulation of metabolic rate was due to differential rates of synthesis for a fixed energy cost per unit mass of protein synthesized. The cost of synthesizing a unit of protein did not change with increasing rates of protein synthesis. We conclude that the cost of protein synthesis is independent of the rate of synthesis, developmental stage, size and physiological feeding state during sea urchin development.

Temperature-dependent protein synthesis capacities in Antarctic and temperate (North Sea) fish (Zoarcidae)

For an evaluation of effects of seasonal cold acclimation and evolutionary cold adaptation on protein synthesis capacity, the protein synthesis apparatus was isolated from the gills and white muscle of Antarctic eelpout Pachycara brachycephalum and North Sea eelpout Zoarces viviparus. Both species had been acclimated to 0 degrees C (control) and 5 degrees C (Antarctic) and 5 degrees C and 10 degrees C (North Sea control). The translational capacities of the protein synthesis machineries were determined in an optimised cell-free in vitro system. The results demonstrate that tissues from the polar zoarcid possess cold-adapted protein synthesis machineries, indicated by low activation energies and, especially, high RNA translational capacities at similar RNA:protein ratios when compared to temperate zoarcids at 10 degrees C. When both species were brought to 5 degrees C, the temperate species displayed cold compensated protein synthesis capacities caused by elevated RNA:protein ratios. Warm exposure (from 0 to 5 degrees C) of the Antarctic zoarcid revealed a capacity for thermal acclimation indicated by a reduction in protein synthesis capacities associated with lower RNA:protein ratios.

Mitochondrial permeability transition in the crustaceanArtemia franciscana: absence of a calcium-regulated pore in the face of profound calcium storage

When mammalian mitochondria are exposed to high calcium and phosphate, a massive swelling, uncoupling of respiration, and release of cytochrome c occur. These changes are mediated by opening of the mitochondrial permeability transition pore (MPTP). Activation of the MPTP in vivo in response to hypoxic and oxidative stress leads to necrotic and apoptotic cell death. Considering that embryos of the brine shrimp Artemia franciscana tolerate anoxia for years, we investigated the MPTP in this crustacean to reveal whether pore opening occurs. Minimum molecular constituents of the regulated MPTP in mammals are believed to be the voltage-dependent anion channel, the adenine nucleotide translocators, and cyclophilin D. Western blot analysis revealed that mitochondria from A. franciscana possess all three required components. When measured with a calcium-sensitive fluorescent probe, rat liver mitochondria are shown to release matrix calcium after addition of ≥100 μM extramitochondrial calcium (MPTP opening), whereas brine shrimp mitochondria continue to take up extramitochondrial calcium and do not release internal stores even up to 1.0 mM exogenously added calcium (no MPTP opening). Furthermore, no swelling of A. franciscana mitochondria in response to added calcium was observed, and no release of cytochrome c could be detected. HgCl2-dependent swelling and cytochrome c release were readily confirmed, which is consistent with the presence of an “unregulated pore.” Although the absence of a regulated MPTP in A. franciscana mitochondria could contribute to the extreme hypoxia tolerance in this species, we speculate that absence of the regulated MPTP may be a general feature of invertebrates.

Internal translation initiation mediated by the angiogenic factor Tie2

Tie2 is an endothelium-specific receptor tyrosine kinase required for normal blood vessel maturation. We report that Tie2 mRNA translation is maintained under hypoxic conditions. To identify the mechanism responsible for this, we undertook structure/function analysis of the Tie2 5'-untranslated region (UTR). Transcription start site mapping indicates the existence of a several mRNA isoforms containing unusually long 5'-UTRs (>350 nucleotides) with five upstream open reading frames. We find internal ribosome binding activity that allows the Tie2 mRNA to initiate in a cap-independent fashion. Our data provide a framework for understanding how Tie2 mRNA is translated despite a cumbersome structured 5'-UTR and how its production is secured under unfavorable environmental conditions.

Natural hypometabolism during hibernation and daily torpor in mammals

Daily torpor and hibernation are the most powerful measures of endotherms to reduce their energy expenditure. During entrance into these torpid states metabolic rate is suppressed to a fraction of euthermic metabolism, paralleled by reductions in ventilation and heart rate. Body temperature gradually decreases towards the level of ambient temperature. In deep torpor body temperature as well as metabolic rate are controlled at a hypothermic and hypometabolic level. Torpid states are terminated by an arousal where metabolic rate spontaneously returns to normal levels again and euthermic body temperature is established by a burst of heat production. In recent years some of the cellular mechanisms which contribute to hypometabolism have been disclosed. Transcription, translation, as well as protein synthesis are largely suppressed. Cell proliferation in highly proliferating epithelia like the intestine is suspended. ATP production from glucose is reduced and lipids serve as the major substrate for remaining energy requirements. All these changes are rapidly reverted to normometabolism during arousal. Hibernation and daily torpor are found in small mammals inhabiting temperate as well as tropical climates. It indicates that this behaviour is not primarily aimed for cold defense, instead points to a general role of hypometabolism, as a measure to cope with a timely limited or seasonal bottleneck of energy supply.

Metabolic rate depression in animals: transcriptional and translational controls

Metabolic rate depression is an important survival strategy for many animal species and a common element of hibernation, torpor, aestivation, anaerobiosis, diapause, and anhydrobiosis. Studies of the biochemical mechanisms that regulate reversible transitions to and from hypometabolic states are identifying principles of regulatory control that are conserved across phylogenetic lines and that are broadly applied to the control of multiple cell functions. One such mechanism is reversible protein phosphorylation which is now known to contribute to the regulation of fuel metabolism, to ion channel arrest, and to the suppression of protein synthesis during hypometabolism. The present review focuses on two new areas of research in hypometabolism: (1) the role of differential gene expression in supplying protein products that adjust metabolism or protect cell functions for long-term survival, and (2) the mechanisms of protein life extension in hypometabolism involving inhibitory controls of transcription, translation and protein degradation. Control of translation examines reversible phosphorylation regulation of ribosomal initiation and elongation factors, the dissociation of polysomes and storage of mRNA transcripts during hypometabolism, and control over the translation of different mRNA types by differential sequestering of mRNA into polysome versus monosome fractions. The analysis draws primarily from current research on two animal models, hibernating mammals and anoxia-tolerant molluscs, with selected examples from multiple other sources.

Energy budget of hepatocytes from Antarctic fish (Pachycara brachycephalumandLepidonotothen kempi) as a function of ambient CO2: pH-dependent limitations of cellular protein biosynthesis?

Scenarios of rising CO2 concentration in surface waters due to atmospheric accumulation of anthropogenic CO2, or in the deep sea due to anticipated industrial dumping of CO2, suggest that hypercapnia (elevated partial pressure of CO2) will become a general stress factor in aquatic environments, with largely unknown effects on species survival and well being, especially in cold and deep waters. For an analysis of CO2 effects at the cellular level, isolated hepatocytes were prepared from two representatives of the Antarctic fish fauna, Pachycara brachycephalum and Lepidonotothen kempi. Correlated changes in energy and protein metabolism were investigated by determining the rates of oxygen consumption at various levels of PCO2, of intra- and extracellular pH, and after inhibition of protein synthesis by cycloheximide. A decrease in extracellular pH (pHe) from control levels (pHe 7.90) to pHe 6.50 caused a reduction in aerobic metabolic rate of 34-37% under both normocapnic and hypercapnic conditions. Concomitantly, protein biosynthesis was inhibited by about 80%under conditions of severe acidosis in hepatocytes from both species. A parallel drop in intracellular pH probably mediates this effect. In conclusion, the present data indicate that elevated PCO2 may limit the functional integrity of the liver due to a pronounced depression in protein anabolism. This process may contribute to the limits of whole-animal tolerance to raised CO2levels.

Mitochondrial mRNA stability and polyadenylation during anoxia-induced quiescence in the brine shrimp Artemia franciscana

Polyadenylation of messenger RNA is known to be an important mechanism for regulating mRNA stability in a variety of systems, including bacteria, chloroplasts and plant mitochondria. By comparison, little is known about the role played by polyadenylation in animal mitochondrial gene expression. We have used embryos of the brine shrimp Artemia franciscana to test hypotheses regarding message stability and polyadenylation under conditions simulating anoxia-induced quiescence. In response to anoxia, these embryos undergo a profound and acute metabolic downregulation, characterized by a steep drop in intracellular pH (pH(i)) and ATP levels. Using dot blots of total mitochondrial RNA, we show that during in organello incubations both O(2) deprivation and acidic pH (pH 6.4) elicit increases in half-lives of selected mitochondrial transcripts on the order of five- to tenfold or more, relative to normoxic controls at pH 7.8. Polyadenylation of these transcripts was measured under the same incubation conditions using a reverse transcriptase-polymerase chain reaction (RT-PCR)-based assay. The results demonstrate that low pH and anoxia promote significant deadenylation of the stabilized transcripts in several cases, measured either as change over time in the amount of polyadenylation within a given size class of poly(A)(+) tail, or as the total amount of polyadenylation at the endpoint of the incubation. This study is the first direct demonstration that for a metazoan mitochondrion, polyadenylation is associated with destabilized mRNA. This pattern has also been demonstrated in bacteria, chloroplasts and plant mitochondria and may indicate a conserved mechanism for regulating message half-life that differs from the paradigm for eukaryotic cytoplasm, where increased mRNA stability is associated with polyadenylation.

In vitro protein synthesis capacities in a cold stenothermal and a temperate eurythermal pectinid

The translational system was isolated from the gills of the Antarctic scallop Adamussium colbecki (Smith) and the European scallop Aequipecten opercularis (Linnaeus) for in vitro protein synthesis capacities microg protein mg FW(-1) day(-1)) and the translational capacities of RNA (k(RNA in vitro) mg protein mg RNA(-1) day(-1)). In vitro protein synthesis capacity in the cold-adapted pectinid at 0 degrees C was similar to the one found in the temperate scallop at 25 degrees C. These findings might reflect cold compensated rates in Adamussium colbecki, partly explainable by high tissue levels of RNA. Cold-compensated in vitro protein synthesis capacities may further result from increments in the translational capacity of RNA. The thermal sensitivity of the translation machinery was slightly different in the two species, with significantly lower levels of Arrhenius activation energies E(a) and Q(10) in Adamussium colbecki in the temperature range 0-15 degrees C. Reduced protein synthesis and translational capacities were found in vitro in gills of long-term aquarium-maintained Adamussium colbecki and were accounted for by a loss of protein synthesis machinery, i.e. a reduction in RNA levels, as well as a decrease in the amount of protein synthesized per milligram of RNA (RNA translational capacity, k(RNA in vitro)). Such changes may involve food uptake or mirror metabolic depression strategies, like those occurring during winter. Consequences of high in vitro RNA translational capacities found in the permanently cold-adapted species are discussed in the context of seasonal food availability and growth rates at high latitudes.

The role of eukaryotic initiation factor 2alpha during the metabolic depression associated with estivation

We have investigated the role of eukaryotic initiation factor 2alpha (eIF2alpha) in two estivating organisms previously shown to downregulate protein synthesis during metabolic depression, the land snail Helix aspersa Müller and the desert frog Neobatrachus sutor Main 1957. We have developed a method using a single antibody (which binds specifically to the phosphorylated, conserved phosphorylation region) by which the total levels of eIF2alpha and the ratio of phosphorylated eIF2alpha [eIF2alpha(P)] to total (phosphorylated and unphosphorylated) eIF2alpha can be determined. In H. aspersa, we have shown that the level of eIF2alpha mRNA expression is unchanged between the awake and estivating states. The amount of total eIF2alpha is the same in the estivating and awake states, and eIF2alpha(P) is undetectable and must represent < or =10% of total eIF2alpha in both states. Conversely, in N. sutor during estivation, the level of total eIF2alpha increases approximately 1.6-fold and the ratio of eIF2alpha(P)/eIF2alpha increases from 0.22+/-0.11 to 0.52+/-0.08, implicating eIF2alpha phosphorylation in the downregulation of protein synthesis during estivation in this animal. The differences in the amounts of eIF2alpha and the level of its phosphorylation between these two species also suggest possible differences either in the mechanism by which protein synthesis is downregulated during estivation or in the sensitivity of the initiation of translation to eIF2alpha(P) levels.

Transcriptional initiation under conditions of anoxia-induced quiescence in mitochondria from Artemia franciscana embryos

In response to anoxia, embryos of the brine shrimp Artemia franciscana are able coordinately to downregulate metabolism to levels low enough to permit survival for several years at room temperature. In addition to dramatic decreases in free ATP levels and heat production, intracellular pH drops from 7.8 to 6.3 overnight. Use of isolated mitochondria to study transcriptional responses to anoxia offers several advantages: (1). the localized nature of transcript initiation, processing and degradation, all of which may be followed in organello; (2). the relatively simple cis- and trans-machinery involved and (3). the ability to provide relevant physiological treatments in vitro. In response to anoxic incubation of embryos in vivo for 4 h followed by anoxic mitochondrial isolation and anoxic transcription assay at pH 6.4, a significant decrease in overall UTP incorporation (77%) was seen after 30 min relative to normoxic, pH 7.9 controls. A less severe inhibition of transcription under anoxia (52%) was observed compared with controls when pH was raised to 7.9. Similarly, under normoxia, the incubation at low pH (6.4) reduced transcription by 59%. Ribonuclease protection assays showed that the contribution of in vitro initiation during the assay fell from 78% at pH 7.9 to approximately 32% at pH 6.4 under either normoxic or anoxic conditions. DNA footprinting of putative transcriptional promoters revealed proteins at regular intervals upstream of the 12S rRNA in the control region, which previously had been indirectly inferred to contain promoters for H-strand transcription. The area between 1230 and 12065 contains a sequence in the tRNA(leu) gene believed to bind the transcription termination factor mTERF or TERM, and we provide the first evidence that this sequence is protein-bound in A. franciscana. However, our hypothesis that initiation is reduced at low pH because of a change in DNA binding by mitochondrial transcription factors was not confirmed. We propose that regulation of initiation may be mediated by covalent modification or by protein-protein interactions not detected by footprinting.

The protein synthesis machinery operates at the same expense in eurythermal and cold stenothermal pectinids

Translationally active cell-free systems from gills of the Antarctic scallop Adamussium colbecki and the European scallop Aequipecten opercularis were developed, characterised, and optimised for an analysis of translational capacity. The aim was to determine the energetic cost of protein synthesis in the in vitro cell-free system by directly measuring the required energy equivalents in the lysates. Protein synthesis rate in assays conducted with lysates of A. colbecki (1.029+/-0.061 micromol Phe min(-1) at 15 degrees C; Phe=phenylalanine) were higher compared with lysates of A. opercularis (0.087+/-0.013 micromol Phe min(-1) at 15 degrees C and 0.156+/-0.023 micromol Phe min(-1) at 25 degrees C). This can in part be attributed to the naturally occurring higher RNA content in lysates of A. colbecki (0.883+/-0.037 mg RNA mL(-1) lysate) compared with A. opercularis (0.468+/-0.013 mg RNA mL(-1) lysate). There was no significant difference in the energetic costs of protein synthesis in cell-free systems of gill lysates of the cold stenothermal A. colbecki with 4.3+/-0.7 energy equivalents per peptide bond formed and the eurythermal A. opercularis with 5.6+/-0.6 energy equivalents, indicating that there are no differences in the efficiency of the translation machinery. The energetic costs specified for protein synthesis correspond with the generally accepted theoretical value of four energy equivalents per peptide bond formed, especially in gill lysates of A. colbecki, whereas the value for gill lysates of A. opercularis was slightly higher.

In vivo downregulation of protein synthesis in the snail Helix apersa during estivation

Protein synthesis is downregulated during metabolic depression in a number of systems where the metabolic depression is effected by obvious extrinsic cues. The metabolic depression of the estivating land snail Helix apersa occurs in the absence of any obvious physiological stress and has an intrinsic component independent of temperature, pH, O(2) status, or osmolality. We show that this metabolic depression is accompanied by a downregulation of protein synthesis in vivo. The rate of protein synthesis decreases in two major tissues during estivation: to 23% and 53% of the awake rate in hepatopancreas and foot muscle, respectively. We show from calculations of the theoretical contribution of protein synthesis to total O(2) consumption that the depression of protein synthesis must be a significant, obligate, in vivo component of metabolic depression in H. aspersa.

Changes in metabolic rate and N excretion in the marine invertebrateSipunculus nudusunder conditions of environmental hypercapnia

Increased CO2 partial pressures (hypercapnia) as well as hypoxia are natural features of marine environments like the intertidal zone. Nevertheless little is known about the specific effects of CO2 on metabolism, except for the well-described effects on acid—base variables and regulation. Accordingly, the sediment-dwelling worm Sipunculus nudus was used as an experimental model to investigate the correlation of acid—base-induced metabolic depression and protein/amino acid catabolism, by determining the rates of oxygen consumption, ammonia excretion and O/N ratios in non-perfused preparations of body wall musculature at various levels of extra- and intracellular pH, PCO2 and [HCO3-]. A decrease in extracellular pH from control level (7.9) to 6.7 caused a reduction in aerobic metabolic rate of both normocapnic and hypercapnic tissues by 40-45 %. O/N ratios of 4.0-4.5 under control conditions indicate that amino acid catabolism meets the largest fraction of aerobic energy demand. A significant 10-15 % drop in ammonia excretion, a simultaneous reduction of O/N ratios and a transient accumulation of intracellular bicarbonate during transition to extreme acidosis suggest a reduction in net amino acid catabolism and a shift in the selection of amino acids used,favouring monoamino dicarboxylic acids and their amines (asparagine,glutamine, aspartic and glutamic acids). A drop in intracellular pH was identified as mediating this effect. In conclusion, the present data provide evidence for a regulatory role of intracellular pH in the selection of amino acids used by catabolism.

Intrinsic metabolic depression in cells isolated from the hepatopancreas of estivating snails

Many animals across the phylogenetic scale are routinely capable of depressing their metabolic rate to 5-15% of that at rest, remaining in this state sometimes for years. However, despite its widespread occurrence, the biochemical processes associated with metabolic depression remain obscure. We demonstrate here the development of an isolated cell model for the study of metabolic depression. The isolated cells from the hepatopancreas (digestive gland) of the land snail (Helix aspersa) are oxygen conformers; i.e., their rate of respiration depends on pO(2). Cells isolated from estivating snails show a stable metabolic depression to 30% of control (despite the long and invasive process of cell isolation) when metabolic rate at the physiological pH and pO(2) of the hemolymph of estivating snails is compared with metabolic rate at the physiological pH and pO(2) of the hemolymph of control snails. When the extrinsic effects of pH and pO(2) are excluded, the intrinsic metabolic depression of the cells from estivating snails is still to below 50% of control snails. The in vitro effect of pO(2) on metabolic rate is independent of pH and state (awake or estivating), but the effects of pH and state significantly interact. This suggests that pH and state change affect metabolic depression by similar mechanisms but that the metabolic depression by hypoxia involves a separate mechanism.

Depression of nuclear transcription and extension of mRNA half-life under anoxia in Artemia franciscana embryos

Transcriptional activity, as assessed by nuclear run-on assays, was constant during 10 h of normoxic development for embryos of the brine shrimp Artemia franciscana. Exposure of embryos to only 4 h of anoxia resulted in a 79.3+/−1 % decrease in levels of in-vivo-initiated transcripts, and transcription was depressed by 88. 2+/−0.7 % compared with normoxic controls after 24 h of anoxia (means +/− s.e.m., N=3). Initiation of transcription was fully restored after 1 h of normoxic recovery. Artificially lowering the intracellular pH of aerobic embryos to the value reflective of anoxia (pH 6.7) showed that acidification alone explained over half the transcriptional arrest. Initiation of transcription was not rescued by application of 80 % carbon monoxide under anoxia, which suggests that heme-based oxygen sensing is not involved in this global arrest. When these transcriptional data are combined with the finding that mRNA levels are unchanged for at least 6 h of anoxia, it is clear that the half-life of mRNA is extended at least 8.5-fold compared with that in aerobic embryos. In contrast to the activation of compensatory mechanisms to cope with anoxia that occurs in mammalian cells, A. franciscana embryos enter a metabolically depressed state in which gene expression and mRNA turnover are cellular costs apparently not compatible with survival and in which extended tolerance supercedes the requirement for continued metabolic function.

Regulatory features of transcription in isolated mitochondria from Artemia franciscana embryos

Optimal conditions were developed for an in organello transcriptional run-on assay using mitochondria isolated from Artemia franciscana embryos to investigate potential regulatory features of RNA synthesis under conditions of anoxia-induced quiescence. Transcription is not dependent on oxidative phosphorylation for maximal activity when exogenous ATP is available. Bona fide transcription products, as assessed by hybridization with specific mitochondrial cDNAs from A. franciscana, are produced in an inhibitor-sensitive manner. Transcription rate measured at pH 7.9 is reduced 80% when pH is lowered to 6.3, a pH range that mimics the in vivo change seen on exposure of embryos to anoxia. The proton sensitivity of mitochondrial RNA synthesis may provide a mechanism to depress this significant energy expenditure during quiescence. The influence of nucleotide concentration on kinetics is complicated by an interdependence among nucleotide species. ATP inhibition observed at subsaturating UTP concentrations is relieved when UTP is at saturating, physiologically relevant levels. Taken together, these data suggest that local (versus nuclear mediated) control is important in dictating mitochondrial transcription during rapid modulations in gene expression, such as those observed under anoxia-induced quiescence.

The responses of cytochrome redox state and energy metabolism to dehydration support a role for cytoplasmic viscosity in desiccation tolerance

To characterize the depression of metabolism in anhydrobiotes, the redox state of cytochromes and energy metabolism were studied during dehydration of soaked cowpea (Vigna unguiculata) cotyledons and pollens of Typha latifolia and Impatiens glandulifera. Between water contents (WC) of 1.0 and 0.6 g H2O/g dry weight (g/g), viscosity as measured by electron spin resonance spectroscopy increased from 0.15 to 0.27 poise. This initial water loss was accompanied by a 50% decrease in respiration rates, whereas the adenylate energy charge remained constant at 0.8, and cytochrome c oxidase (COX) remained fully oxidized. From WC of 0.6 to 0.2 g/g, viscosity increased exponentially. The adenylate energy charge declined to 0.4 in seeds and 0.2 in pollen, whereas COX became progressively reduced. At WC of less than 0.2 g/g, COX remained fully reduced, whereas respiration ceased. When dried under N2, COX remained 63% reduced in cotyledons until WC was 0.7 g/g and was fully reduced at 0.2 g/g. During drying under pure O2, the pattern of COX reduction was similar to that of air-dried tissues, although the maximum reduction was 70% in dried tissues. Thus, at WC of less than 0.6 g/g, the reduction of COX probably originates from a decreased O2 availability as a result of the increased viscosity and impeded diffusion. We suggest that viscosity is a valuable parameter to characterize the relation between desiccation and decrease in metabolism. The implications for desiccation tolerance are discussed.

The role of protein synthesis during metabolic depression in the Australian desert frog Neobatrachus centralis

Little is known about the role of energy consuming processes during metabolic depression. We have shown that aestivation in the Australian desert frog Neobatrachus centralis is accompanied by an in vivo metabolic depression of 77%. Using an in vitro liver slice preparation, we have measured an in vitro metabolic depression in liver of 55%, with a concomitant 67% decrease in the rate of protein synthesis. The decrease in protein synthesis accounts for 52% of the metabolic depression of the tissue, but only 4.9% of the metabolic depression of the whole animal. No in vitro metabolic depression or decrease in protein synthesis during aestivation was measured in muscle, but a decrease in the low rate of protein synthesis in muscle in vivo could not, in any case, account for more than 3% of the metabolic depression of the whole animal. The liver, although not a quantitatively important tissue in terms of metabolic depression in vivo, offers the opportunity to characterise the regulation of protein synthesis in a system in which metabolic depression is not confounded by changes in ambient temperature and PO2.

Effectors of metabolic depression in an estivating pulmonate snail (Helix aspersa): whole animal and in vitro tissue studies

We have examined metabolic depression in the land snail (Helix aspersa) during estivation, and have developed a tissue model of metabolic depression using an in vitro mantle preparation. The metabolic rate of H. aspersa is depressed by 84% in vivo within 4 weeks of onset of estivation, and this metabolic depression is accompanied by a decrease in haemolymph PO2 and pH, and an increase in haemolymph PCO2. The in vitro mantle preparation has a stable O2 consumption and energy charge, and an energy charge similar to that of mantle in vivo. The in vitro mantle is an O2-conforming tissue, with VO2 varying curvilinearly with PO2. Consequently, we have developed a mathematical method of calculating tissue VO2 at any PO2. These calculations show that under appropriate incubation conditions of pH and PO2, the mantle from estivating animals shows a stable in vitro metabolic depression of 48% compared to mantle from control snails. The extrinsic effects of PO2 and pH account for 70% of the total in vitro metabolic depression of mantle tissue; intrinsic effectors contribute a further 30%.

Unifying theory of hypoxia tolerance: molecular/metabolic defense and rescue mechanisms for surviving oxygen lack

We develop a unifying theory of hypoxia tolerance based on information from two cell level models (brain cortical cells and isolated hepatocytes) from the highly anoxia tolerant aquatic turtle and from other more hypoxia sensitive systems. We propose that the response of hypoxia tolerant systems to oxygen lack occurs in two phases (defense and rescue). The first lines of defense against hypoxia include a balanced suppression of ATP-demand and ATP-supply pathways; this regulation stabilizes (adenylates) at new steady-state levels even while ATP turnover rates greatly decline. The ATP demands of ion pumping are down-regulated by generalized "channel" arrest in hepatocytes and by "spike" arrest in neurons. Hypoxic ATP demands of protein synthesis are down-regulated probably by translational arrest. In hypoxia sensitive cells this translational arrest seems irreversible, but hypoxia-tolerant systems activate "rescue" mechanisms if the period of oxygen lack is extended by preferentially regulating the expression of several proteins. In these cells, a cascade of processes underpinning hypoxia rescue and defense begins with an oxygen sensor (a heme protein) and a signal-transduction pathway, which leads to significant gene-based metabolic reprogramming-the rescue process-with maintained down-regulation of energy-demand and energy-supply pathways in metabolism throughout the hypoxic period. This recent work begins to clarify how normoxic maintenance ATP turnover rates can be drastically (10-fold) down-regulated to a new hypometabolic steady state, which is prerequisite for surviving prolonged hypoxia or anoxia. The implications of these developments are extensive in biology and medicine.

Profiles of nuclear and mitochondrial encoded mRNAs in developing and quiescent embryos of Artemia franciscana

Embryos of the brine shrimp Artemia franciscana are able to withstand long bouts of environmental anoxia by entering a quiescent state during which metabolism is greatly depressed. Recent evidence supports a global arrest of protein synthesis during quiescence. In this study we measured the amounts of mRNA for a mitochondrial-encoded subunit of cytochrome c oxidase (COX I) and for nuclear-encoded actin during aerobic development, anaerobiosis, and aerobic acidosis (artificial quiescence imposed by intracellular acidification under aerobic conditions). The levels of both COX I and actin transcripts increased significantly during aerobic development. COX I mRNA levels were tightly correlated with previous measures of COX catalytic activity, which suggests that COX synthesis could be regulated by message concentration during aerobic development. The ontogenetic increase for these mRNAs was blocked by anoxia and aerobic acidosis. Importantly, the levels of COX I and actin mRNA did not decline appreciably during the 6 h bouts of quiescence, even though protein synthesis is acutely arrested by these same treatments. Thus, the constancy of mRNA levels during quiescence indicate that reduced protein synthesis is not caused by message limitation, but rather, is likely controlled at the translational level. One advantage of this regulatory mechanism is the conservation of mRNA molecules during quiescence, which would potentially favor a quick resumption of translation as soon as oxygen is returned to the embryos. Finally, because anoxia and aerobic acidosis are both characterized by acidic intracellular pH, the reduction in pH may serve, directly or indirectly, as one signal regulating levels of mRNA in this embryo during quiescence.

Acute depression of mitochondrial protein synthesis during anoxia: contributions of oxygen sensing, matrix acidification, and redox state

Mitochondrial protein synthesis is acutely depressed during anoxia-induced quiescence in embryos of Artemia franciscana. Oxygen deprivation is accompanied in vivo by a dramatic drop in extramitochondrial pH, and both of these alterations strongly inhibit protein synthesis in isolated mitochondria. Here we show that the oxygen dependence is not explained simply by blockage of the electron transport chain or by the increased redox state. Whereas oxygen deprivation substantially depressed protein synthesis within 5 min and resulted in a 77% reduction after 1 h, aerobic incubations with saturating concentrations of cyanide or antimycin A had little effect during the first 20 min and only a modest effect after 1 h (36 and 20% reductions, respectively). Yet the mitochondrial NAD(P)H pools were fully reduced after 2-3 min with all three treatments. This cyanide- and antimycin-insensitive but hypoxia-sensitive pattern of protein synthesis depression suggests the presence of a molecular oxygen sensor within the mitochondrion. Second, we show for the first time that acidification of extramitochondrial pH exerts inhibition on protein synthesis specifically through changes in matrix pH. Matrix pH was 8.2 during protein synthesis assays performed at the extramitochondrial pH optimum of 7.5. When this proton gradient was abolished with nigericin, the extramitochondrial pH optimum for protein synthesis displayed an alkaline shift of approximately 0.7 pH unit. These data suggest the presence of proton-sensitive translational components within the mitochondrion.

Oxygen and pH regulation of protein synthesis in mitochondria from Artemia franciscana embryos

To identify factors responsible for the down-regulation of mitochondrial biosynthetic processes during anoxia in encysted Artemia franciscana embryos, the effects of oxygen limitation and pH on protein synthesis were investigated in isolated mitochondria. At the optimal pH of 7.5, exposure of mitochondria to anoxia decreases the protein synthesis rate by 79%. Rates were suppressed by a further 10% at pH 6.8, the intracellular pH (pHi) measured under anoxia in vivo. Matrix pH, measured under identical conditions, was 8.43 +/- 0.01 at an extra-mitochondrial pH of 7.9 (mean +/- S.E.M., n = 3), 8.05 +/- 0.01 at pH 7.5, and 7.10 +/- 0.01 at pH 6.8. The matrix pH did not vary (P > or = 0.20) as a function of oxygen availability during the 1 h assays. Intramitochondrial purine nucleotides varied little as a function of pH. In contrast, after 1 h of protein synthesis under anoxia, ATP levels decreased by up to 40%, whereas AMP, ADP and GDP concentrations increased, and GTP and GMP concentrations remained relatively constant. The addition of 1 mM ATP at the onset of anoxia maintained the ATP/ADP ratio at the aerobic value, but did not stabilized the GTP/GDP ratio or rescue rates of protein synthesis. Thus, at present, we cannot eliminate the possibility that the decrease in the GTP/GDP ratio during anoxia may contribute to the suppression of protein synthesis. The effect of anoxia was reversible; the rate of protein synthesis upon reoxygenation after a 30 min bout of anoxia was comparable (P = 0.14) with the pre-anoxic rate (193 +/- 17 and 174 +/- 6 pmol of leucine per mg of protein respectively, mean +/- S.E.M., n = 3). The array of mitochondrial translation products did not differ qualitatively as a function of either oxygen availability or pH. Finally, similar pH profiles for protein synthesis were obtained with either [3H]leucine or [3H]histidine (known to use different transporters). Consequently, it is improbable that the pH-sensitivity of protein synthesis can be explained by a specific protein effect on the import of the radiolabelled amino acid used. In summary, both oxygen limitation and acidic pH suppress rates of mitochondrial protein synthesis and are likely to contribute to the arrest of mitochondrial anabolic processes during anoxia-induced quiescence in A. franciscana embryos.

Translational arrest in hypoxic potato tubers is correlated with the aberrant association of elongation factor EF-1 alpha with polysomes

Translation elongation factor EF-1 alpha became stably associated with potato tuber polysomes at the onset of hypoxia, coincident with a sharp rise in lactate and decrease in tissue pH. This aberrant association of EF-1 alpha with polysomes also occurred when aerobic tuber extracts were acidified in vitro. Upon resumption of protein synthesis, an increase in the steady-state levels of EF-1 alpha, and expression of an EF-1 alpha/GUS transgene was observed. These results indicate that translational arrest results from to the failure of EF-1 alpha to dissociate from ribosomes during the elongation cycle, and that restoration of protein synthesis is coordinated with expression of EF-1 alpha.