Arrestment of carbohydrate metabolism during anaerobic dormancy and aerobic acidosis inArtemia embryos: determination of pH-sensitive control points

OXPHOS capacity is diminished and the phosphorylation system inhibited during diapause in an extremophile, embryos of Artemia franciscana

Diapause exhibited by embryos of Artemia franciscana is accompanied by severe arrest of respiration. A large fraction of this depression is attributable to downregulation of trehalose catabolism that ultimately restricts fuel to mitochondria. This study now extends knowledge on the mechanism by revealing metabolic depression is heightened by inhibitions within mitochondria. Compared with that in embryo lysates during post-diapause, oxidative phosphorylation (OXPHOS) capacity P is depressed during diapause when either NADH-linked substrates (pyruvate and malate) for electron transfer (electron transfer capacity, E) through respiratory Complex I or the Complex II substrate succinate are used. When pyruvate, malate and succinate were combined, respiratory inhibition by the phosphorylation system in diapause lysates was discovered as judged by P/E flux control ratios (two-way ANOVA; F1,24=38.78; P<0.0001). Inhibition was eliminated as the diapause extract was diluted (significant interaction term; F2,24=9.866; P=0.0007), consistent with the presence of a diffusible inhibitor. One candidate is long-chain acyl-CoA esters known to inhibit the adenine nucleotide translocator. Addition of oleoyl-CoA to post-diapause lysates markedly decreased the P/E ratio to 0.40±0.07 (mean±s.d.; P=0.002) compared with 0.79±0.11 without oleoyl-CoA. Oleoyl-CoA inhibits the phosphorylation system and may be responsible for the depressed P/E in lysates from diapause embryos. With isolated mitochondria, depression of P/E by oleoyl-CoA was fully reversed by addition of l-carnitine (control versus recovery with l-carnitine, P=0.338), which facilitates oleoyl-CoA transport into the matrix and elimination by β-oxidation. In conclusion, severe metabolic arrest during diapause promoted by restricting glycolytic carbon to mitochondria is reinforced by depression of OXPHOS capacity and the phosphorylation system.

Physiological strategies during animal diapause: lessons from brine shrimp and annual killifish

Diapause is a programmed state of developmental arrest that typically occurs as part of the natural developmental progression of organisms that inhabit seasonal environments. The brine shrimp Artemia franciscana and annual killifish Austrofundulus limnaeus share strikingly similar life histories that include embryonic diapause as a means to synchronize the growth and reproduction phases of their life history to favorable environmental conditions. In both species, respiration rate is severely depressed during diapause and thus alterations in mitochondrial physiology are a key component of the suite of characters associated with cessation of development. Here, we use these two species to illustrate the basic principles of metabolic depression at the physiological and biochemical levels. It is clear that these two species use divergent molecular mechanisms to achieve the same physiological and ecological outcomes. This pattern of convergent physiological strategies supports the importance of biochemical and physiological adaptations to cope with extreme environmental stress and suggests that inferring mechanism from transcriptomics or proteomics or metabolomics alone, without rigorous follow-up at the biochemical and physiological levels, could lead to erroneous conclusions.

Mechanisms of animal diapause: recent developments from nematodes, crustaceans, insects, and fish

Life cycle delays are beneficial for opportunistic species encountering suboptimal environments. Many animals display a programmed arrest of development (diapause) at some stage(s) of their development, and the diapause state may or may not be associated with some degree of metabolic depression. In this review, we will evaluate current advancements in our understanding of the mechanisms responsible for the remarkable phenotype, as well as environmental cues that signal entry and termination of the state. The developmental stage at which diapause occurs dictates and constrains the mechanisms governing diapause. Considerable progress has been made in clarifying proximal mechanisms of metabolic arrest and the signaling pathways like insulin/Foxo that control gene expression patterns. Overlapping themes are also seen in mechanisms that control cell cycle arrest. Evidence is emerging for epigenetic contributions to diapause regulation via small RNAs in nematodes, crustaceans, insects, and fish. Knockdown of circadian clock genes in selected insect species supports the importance of clock genes in the photoperiodic response that cues diapause. A large suite of chaperone-like proteins, expressed during diapause, protects biological structures during long periods of energy-limited stasis. More information is needed to paint a complete picture of how environmental cues are coupled to the signal transduction that initiates the complex diapause phenotype, as well as molecular explanations for how the state is terminated. Excellent examples of molecular memory in post-dauer animals have been documented in Caenorhabditis elegans It is clear that a single suite of mechanisms does not regulate diapause across all species and developmental stages.

Metabolic downregulation and inhibition of carbohydrate catabolism during diapause in embryos of Artemia franciscana

Diapause embryos were collected from ovigerous females of Artemia franciscana at the Great Salt Lake, Utah, and were synchronized to within 4 h of release. Respiration rate for these freshly released embryos across a subsequent 26-d time course showed a rapid decrease during the first several days followed thereafter by a much slower decline. The overall metabolic depression was estimated to be greater than 99%. However, proton conductance of mitochondria isolated from diapause and postdiapause embryos was identical. Because proton leak is apparently not downregulated during diapause, mitochondrial membrane potential is likely compromised because of the very low metabolic rate observed for diapause embryos. Given that trehalose is the primary fuel used by these embryos, we measured metabolic intermediates along the catabolic pathway from trehalose to acetyl-CoA for both diapause and postdiapause (active) embryos in order to identify sites of metabolic inhibition. Comparison of product-to-substrate ratios for sequential enzymatic steps revealed inhibition during diapause at trehalase, hexokinase, pyruvate kinase, and pyruvate dehydrogenase. Measurements of ATP, ADP, and AMP allowed calculations of substantial decreases in ATP:ADP ratio and in adenylate energy charge during diapause. The phosphorylation of site 1 for pyruvate dehydrogenase (PDH) subunit E1α was higher in diapause embryos than in postdiapause embryos, which is consistent with PDH inhibition during diapause. Taken together, our findings indicate that restricted substrate availability to mitochondria for oxidative phosphorylation contributes to downregulating metabolic rate during diapause.

pH-Induced Metabolic Transitions in Artemia Embryos Mediated by a Novel Hysteretic Trehalase

Gastrula-stage embryos of the brine shrimp Artemia undergo reversible transitions between metabolically active and dormant states that are promoted by changes in intracellular pH. A macromolecular mechanism for this suppression of energy metabolism that involves regulation of the enzyme trehalase is reported here. Isolated trehalase from these embryos existed in two active forms that interconverted when exposed to physiological transitions in pH. This hysteretic interconversion was reversible, required minutes for completion, and involved a change in enzyme polymerization. The two states differed twofold in molecular size and were distinguishable electrophoretically. Compared to the smaller species, the polymerized form was strongly inhibited by acidic pH, adenosine 5'-triphosphate, and the substrate trehalose. Thus, the shift in assembly equilibrium toward the aggregated enzyme caused by pH values less than or equal to 7.4 may mediate the arrest of trehalose-fueled metabolism and respiration during dormancy in this cryptobiotic organism.

Anaerobic dormancy quantified in artemia embryos: a calorimetric test of the control mechanism

Continuous measurement of heat dissipation from brine shrimp embryos during reversible transitions from aerobic development to anaerobic dormancy demonstrates a primary role for intracellular pH(pH(i))in this metabolic switching. Artificially elevating the depressed pH(i) during anoxia by adding ammonia markedly reactivates metabolism, as judged by increases in heat dissipation, trehalose catabolism, and the ratio of adenosine triphosphate to adenosine diphosphate. Energy flow during anaerobic dormancy is suppressed to 2.4 percent of aerobic values, which is the lowest percentage thus far reported for euryoxic animals. Use of diguanosine tetraphosphate stores cannot account for this observed heat dissipation. Thus, mobilizing trace amounts of trehalose may explain the energy metabolism during quiescence.

Mechanisms of apoptosis in Crustacea: What conditions induce versus suppress cell death?

Arthropoda is the largest of all animal phyla and includes about 90% of extant species. Our knowledge about regulation of apoptosis in this phylum is largely based on findings for the fruit fly Drosophila melanogaster. Recent work with crustaceans shows that apoptotic proteins, and presumably mechanisms of cell death regulation, are more diverse in arthropods than appreciated based solely on the excellent work with fruit flies. Crustacean homologs exist for many major proteins in the apoptotic networks of mammals and D. melanogaster, but integration of these proteins into the physiology and pathophysiology of crustaceans is far from complete. Whether apoptosis in crustaceans is mainly transcriptionally regulated as in D. melanogaster (e.g., RHG 'killer' proteins), or rather is controlled by pro- and anti-apoptotic Bcl-2 family proteins as in vertebrates needs to be clarified. Some phenomena like the calcium-induced opening of the mitochondrial permeability transition pore (MPTP) are apparently lacking in crustaceans and may represent a vertebrate invention. We speculate that differences in regulation of the intrinsic pathway of crustacean apoptosis might represent a prerequisite for some species to survive harsh environmental insults. Pro-apoptotic stimuli described for crustaceans include UV radiation, environmental toxins, and a diatom-produced chemical that promotes apoptosis in offspring of a copepod. Mechanisms that serve to depress apoptosis include the inhibition of caspase activity by high potassium in energetically healthy cells, alterations in nucleotide abundance during energy-limited states like diapause and anoxia, resistance to opening of the calcium-induced MPTP, and viral accommodation during persistent viral infection. Characterization of the players, pathways, and their significance in the core machinery of crustacean apoptosis is revealing new insights for the field of cell death.

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.

Energizing an Invertebrate Embryo: Bafilomycin‐Dependent Respiration and the Metabolic Cost of Proton Pumping by the V‐ATPase

We examine herein the contribution of V-ATPase activity to the energy budget of aerobically developing embryos of Artemia franciscana and discuss the results in the context of quiescence under anoxia. (31)P-NMR analysis indicates that intracellular pH and NTP levels are unaffected by acute incubation of dechorionated embryos with the V-ATPase inhibitor, bafilomycin A(1). Bafilomycin A(1) also has no significant effect on oxygen consumption by isolated mitochondria. Taken together, these data indicate that bafilomycin does not affect energy-producing pathways in the developing embryo. However, the V-ATPase inhibitor exhibits a concentration-dependent inhibition of oxygen consumption in aerobic embryos. A conservative analysis of respirometric data indicates that proton pumping by the V-ATPase, and processes immediately dependent on this activity, constitutes approximately 31% of the aerobic energy budget of the preemergent embryo. Given the complete absence of detectable Na(+)K(+)-ATPase activity during the first hours of aerobic development, it is plausible that the V-ATPase is performing a role in both the acidification of intracellular compartments and the energization of plasma membranes. Importantly, the high metabolic cost associated with maintaining these diverse proton gradients requires that V-ATPase activity be downregulated under anoxia in order to attain the almost complete metabolic depression observed in the quiescent embryo.

Caspase activity during cell stasis: avoidance of apoptosis in an invertebrate extremophile, Artemia franciscana

Evaluation of apoptotic processes downstream of the mitochondrion reveals caspase-9- and low levels of caspase-3-like activities in partly purified extracts of Artemia franciscana embryos. However, in contrast to experiments with extracts of human hepatoma cells, cytochrome c fails to activate caspase-3 or -9 in extracts from A. franciscana. Furthermore, caspase-9 activity is sensitive to exogenous calcium. The addition of 5 mM calcium leads to a 4.86 +/- 0.19 fold (SD) (n = 3) increase in activity, which is fully prevented with 150 mM KCl. As with mammalian systems, high ATP (>1.25 mM) suppresses caspase activity in A. franciscana extracts. A strong inhibition of caspase-9 activity was also found by GTP. Comparison of GTP-induced inhibition of caspase-9 at 0 and 2.5 mM MgCl(2) indicates that free (nonchelated) GTP is likely to be the inhibitory form. The strongest inhibition among all nucleotides tested was with ADP. Inhibition by ADP in the presence of Mg(2+) is 60-fold greater in diapause embryos than in postdiapause embryos. Because ADP does not change appreciably in concentration between the two physiological states, it is likely that this differential sensitivity to Mg(2+)-ADP is important in avoiding caspase activation during diapause. Finally, mixtures of nucleotides that mimic physiological concentrations in postdiapause and diapause states underscore the depressive action of these regulators on caspase-9 during diapause. Our biochemical characterization of caspase-like activity in A. franciscana extracts reveals that multiple mechanisms are in place to reduce the probability of apoptosis under conditions of energy limitation in this embryo.

V-ATPase expression during development of Artemia franciscana embryos: potential role for proton gradients in anoxia signaling

Under anoxia, Artemia franciscana embryos downregulate metabolic processes and approach an ametabolic state. Entrance into this quiescent state is accompanied by a profound acidification of the intracellular space, and more than two decades of research now clearly demonstrates that this acidification is critical to metabolic downregulation in anoxic embryos. However, the proximal mechanisms responsible for the pH shift remain largely unidentified. Here, we report evidence demonstrating expression of the V-ATPase in encysted embryos and present an argument for its involvement in the intracellular acidification induced by anoxia. We identified a single B-subunit cDNA sharing the greatest degree of sequence similarity with ;generalist-type' homologues from mammals (brain-type) and invertebrates. Quantitative analysis of B-subunit mRNA demonstrates differential expression throughout early development, and western blot analyses confirm the expression of at least six V-ATPase subunits in both heavy membranes and microsomal vesicles. The critical need for proton pumping during the anoxia-tolerant stage of development is demonstrated by incubation with the V-ATPase inhibitor bafilomycin A1, which halts embryonic development. Importantly, net proton flux from V-ATPase-acidified compartments to the surrounding cytoplasm is likely under anoxia and may significantly contribute to the enigmatic acidification critical to quiescence.

The biochemistry of metabolic depression: a history of perceptions

Our interest in animals that 'turn off' dates back at least 300 years. This phenomenon has been reported in most of the major invertebrate phyla and in all vertebrate classes, and has implications for our understanding of a wide range of homeostasis and metabolic control issues. Surprisingly however, it took 20 years of biochemical research before the realization dawned that metabolic depression is the frontline strategy utilized by these animals to survive environmental stress. In this essay, the history of this research is treated in five stages, defined in terms of how the phenomenon now known as metabolic depression was perceived at the time. The two initial stages clearly show that the researchers involved were refractory to the concept of metabolic depression until about 1982 (stage 3). The two stages after 1982 reflect the impact of the acknowledgement of metabolic depression per se and show how research is now being directed towards both the mechanisms involved in, and the cellular targets of metabolic depression.

Molecular chaperones, stress resistance and development in Artemia franciscana

Embryos of the brine shrimp, Artemia franciscana, either develop directly into swimming larvae or are released from females as encysted gastrulae (cysts) which enter diapause, a reversible state of dormancy. Metabolic activity in diapause cysts is very low and these embryos are remarkably resistant to physiological stresses. Encysting embryos, but not those undergoing uninterrupted development, synthesize large amounts of two proteins, namely p26 and artemin. Cloning and sequencing demonstrated p26 is a small heat shock/alpha-crystallin protein while artemin has structural similarity to ferritin. p26 exhibits molecular chaperone activity in vitro, moves reversibly into nuclei during stress and confers thermotolerance on transformed organisms, suggesting critical roles in cyst development. The function of artemin is unknown. Encysted Artemia also contain an abundance of trehalose, a disaccharide capable of protecting embryos. Artemia represent a novel experimental system where the developmental functions of small heat shock/alpha-crystallin proteins and other stress response elements can be explored.

The role of vitrification in anhydrobiosis

Numerous organisms are capable of surviving more or less complete dehydration. A common feature in their biochemistry is that they accumulate large amounts of disaccharides, the most common of which are sucrose and trehalose. Over the past 20 years, we have provided evidence that these sugars stabilize membranes and proteins in the dry state, most likely by hydrogen bonding to polar residues in the dry macromolecular assemblages. This direct interaction results in maintenance of dry proteins and membranes in a physical state similar to that seen in the presence of excess water. An alternative viewpoint has been proposed, based on the fact that both sucrose and trehalose form glasses in the dry state. It has been suggested that glass formation (vitrification) is in itself sufficient to stabilize dry biomaterials. In this review we present evidence that, although vitrification is indeed required, it is not in itself sufficient. Instead, both direct interaction and vitrification are required. Special properties have often been claimed for trehalose in this regard. In fact, trehalose has been shown by many workers to be remarkably (and sometimes uniquely) effective in stabilizing dry or frozen biomolecules, cells, and tissues. Others have not observed any such special properties. We review evidence here showing that trehalose has a remarkably high glass-transition temperature (Tg). It is not anomalous in this regard because it lies at the end of a continuum of sugars with increasing Tg. However, it is unusual in that addition of small amounts of water does not depress Tg, as in other sugars. Instead, a dihydrate crystal of trehalose forms, thereby shielding the remaining glassy trehalose from effects of the added water. Thus under less than ideal conditions such as high humidity and temperature, trehalose does indeed have special properties, which may explain the stability and longevity of anhydrobiotes that contain it. Further, it makes this sugar useful in stabilization of biomolecules of use in human welfare.

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%.

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.

Heat dissipation during long-term anoxia in Artemia franciscana embryos: identification and fate of metabolic fuels

Microcalorimetric measurements of brine shrimp embryos during 6 days of anoxia indicated that heat dissipation was rapidly suppressed to 2.7% of control (aerobic) values over the first 9 h. Energy flow continued to decline slowly to 31 microW.g dry mass-1 (0.4% of control) during the subsequent 5.5 days. Within 2 h after returning anoxic embryos to aerobic conditions, heat dissipation rose to 77% of control rates. The calorimetric/respirometric (CR) ratio across this 2-h recovery period increased steadily from -226 to -346 kJ.mol O2-1). Prior to the anoxic exposures, hydrated embryos were incubated aerobically for 10 h to insure full initiation of carbohydrate metabolism (CR ratio = -484 kJ.mol O2-1). During the 6-day asymptotic approach to a nearly ametabolic state, trehalose and glycogen levels declined 18% and 13%, respectively. The majority of this utilization occurred within the first three days. Thermochemical calculations showed that carbohydrate catabolism accounted for 84% of the total heat dissipation measured over the 6-day anoxic bout; only 3% of the heat could be explained by the catabolism of diguanosine tetraphosphate (Gp4G). Analyses of embryo extracts by high performance liquid chromatography indicated that multiple acid end products were accumulated. Lactate and propionate reached 4.5 mM and 1.0 mM, respectively, but these compounds did not account quantitatively for the amount of carbohydrate utilized. However, the largest chromatographic peak that accumulated under anoxia has not been successfully identified. Fumarate and pyruvate levels decreased as anoxia proceeded. Thus, a perceptible energy flow in Artemia franciscana embryos still remained after 6 days of anoxia.(ABSTRACT TRUNCATED AT 250 WORDS)