Oxygen consumption rate during recovery from loss of equilibrium induced by warming, hypoxia, or exhaustive exercise in rainbow darter (Etheostoma caeruleum)

Animals routinely encounter environmental (e.g., high temperatures and hypoxia) as well as physiological perturbations (e.g., exercise and digestion) that may threaten homeostasis. However, comparing the relative threat or "disruptiveness" imposed by different stressors is difficult, as stressors vary in their mechanisms, effects, and timescales. We exploited the fact that several acute stressors can induce the loss of equilibrium (LOE) in fish to (i) compare the metabolic recovery profiles of three environmentally relevant stressors and (ii) test the concept that LOE could be used as a physiological calibration for the intensity of different stressors. We focused on Etheostoma caeruleum, a species that routinely copes with environmental fluctuations in temperature and oxygen and that relies on burst swimming to relocate and avoid predators, as our model. Using stop-flow (intermittent) respirometry, we tracked the oxygen consumption rate (MO2) as E. caeruleum recovered from LOE induced by hypoxia (PO2 at LOE), warming (critical thermal maximum, CTmax), or exhaustive exercise. Regardless of the stressor used, E. caeruleum recovered rapidly, returning to routine MO2 within ~3 h. Fish recovering from hypoxia and warming had similar maximum MO2, aerobic scopes, recovery time, and total excess post-hypoxia or post-warming oxygen consumption. Though exhaustive exercise induced a greater maximum MO2 and corresponding higher aerobic scope than warming or hypoxia, its recovery profile was otherwise similar to the other stressors, suggesting that "calibration" to a physiological state such as LOE may be a viable conceptual approach for investigators interested in questions related to multiple stressors, cross tolerance, and how animals cope with challenges to homeostasis.

Does hypometabolism constrain innate immune defense?

Many animals routinely make energetic trade-offs to adjust to environmental demands and these trade-offs often have significant implications for survival. For example, environmental hypoxia is commonly experienced by many organisms and is an energetically challenging condition because reduced oxygen availability constrains aerobic energy production, which can be lethal. Many hypoxia-tolerant species downregulate metabolic demands when oxygen is limited; however, certain physiological functions are obligatory and must be maintained despite the need to conserve energy in hypoxia. Of particular interest is immunity (including both constitutive and induced immune functions) because mounting an immune response is among the most energetically expensive physiological processes but maintaining immune function is critical for survival in most environments. Intriguingly, physiological responses to hypoxia and pathogens share key molecular regulators such as hypoxia-inducible factor-1α, through which hypoxia can directly activate an immune response. This raises an interesting question: do hypoxia-tolerant species mount an immune response during periods of hypoxia-induced hypometabolism? Unfortunately, surprisingly few studies have examined interactions between immunity and hypometabolism in such species. Therefore, in this review, we consider mechanistic interactions between metabolism and immunity, as well as energetic trade-offs between these two systems, in hypoxia-tolerant animals but also in other models of hypometabolism, including neonates and hibernators. Specifically, we explore the hypothesis that such species have blunted immune responses in hypometabolic conditions and/or use alternative immune pathways when in a hypometabolic state. Evidence to date suggests that hypoxia-tolerant animals do maintain immunity in low oxygen conditions, but that the sensitivity of immune responses may be blunted.

Plasticity in the Functional Properties of NMDA Receptors Improves Network Stability during Severe Energy Stress

Brain energy stress leads to neuronal hyperexcitability followed by a rapid loss of function and cell death. In contrast, the frog brainstem switches into a state of extreme metabolic resilience that allows them to maintain motor function during hypoxia as they emerge from hibernation. NMDA receptors (NMDARs) are Ca2+-permeable glutamate receptors that contribute to the loss of homeostasis during hypoxia. Therefore, we hypothesized that hibernation leads to plasticity that reduces the role of NMDARs within neural networks to improve function during hypoxia. To test this, we assessed a circuit with a large involvement of NMDAR synapses, the brainstem respiratory network of female bullfrogs, Lithobates catesbeianus Contrary to our expectations, hibernation did not alter the role of NMDARs in generating network output, nor did it affect the amplitude, kinetics, and hypoxia sensitivity of NMDAR currents. Instead, hibernation strongly reduced NMDAR Ca2+ permeability and enhanced desensitization during repetitive stimulation. Under severe hypoxia, the normal NMDAR profile caused network hyperexcitability within minutes, which was mitigated by blocking NMDARs. After hibernation, the modified complement of NMDARs protected against hyperexcitability, as disordered output did not occur for at least one hour in hypoxia. These findings uncover state-dependence in the plasticity of NMDARs, whereby multiple changes to receptor function improve neural performance during metabolic stress without interfering with their normal role during healthy conditions.

Geographical and taxonomic patterns in aerobic traits of marine ectotherms

The metabolism and hypoxia tolerance of marine ectotherms play key roles in limiting species geographical ranges, but underlying traits have only been directly measured for a small fraction of biodiversity. Here we diagnose and analyse spatial and phylogenetic patterns in hypoxia tolerance and its temperature sensitivity at ecologically active metabolic rates, by combining a model of organismal oxygen (O2) balance with global climate and biogeographic data for approximately 25 000 animal species from 13 phyla. Large-scale spatial trait patterns reveal that active hypoxia tolerance is greater and less temperature sensitive among tropical species compared to polar ones, consistent with sparse experimental data. Species energetic demands for activity vary less with temperature than resting costs, an inference confirmed by available rate measurements. Across the tree of life, closely related species share similar hypoxia traits, indicating that evolutionary history shapes physiological tolerances to O2 and temperature. Trait frequencies are highly conserved across phyla, suggesting the breadth of global aerobic conditions selects for convergent trait diversity. Our results support aerobic limitation as a constraint on marine habitat distributions and their responses to climate change and highlight the under-sampling of aerobic traits among species living in the ocean's tropical and polar oxythermal extremes. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Characterization of OsPIN2 Mutants Reveal Novel Roles for Reactive Oxygen Species in Modulating Not Only Root Gravitropism but Also Hypoxia Tolerance in Rice Seedlings

Tolerance to submergence-induced hypoxia is an important agronomic trait especially for crops in lowland and flooding-affected areas. Although rice (Oryza sativa) is considered a flood-tolerant crop, only limited cultivars display strong tolerance to prolonged submergence and/or hypoxic stress. Therefore, characterization of hypoxic resistant genes and/or germplasms have important theoretical and practical significance for rice breeding and sustained improvements. Previous investigations have demonstrated that loss-of-function of OsPIN2, a gene encoding an auxin efflux transporter, results in the loss of root gravitropism due to disrupted auxin transport in the root tip. In this study, we revealed a novel connection between OsPIN2 and reactive oxygen species (ROS) in modulating root gravitropism and hypoxia tolerance in rice. It is shown that the OsPIN2 mutant had decreased accumulation of ROS in root tip, due to the downregulation of glycolate oxidase encoding gene OsGOX6, one of the main H2O2 sources. The morphological defects of root including waved rooting and agravitropism in OsPIN2 mutant may be rescued partly by exogenous application of H2O2. The OsPIN2 mutant exhibited increased resistance to ROS toxicity in roots due to treatment with H2O2. Furthermore, it is shown that the OsPIN2 mutant had increased tolerance to hypoxic stress accompanied by lower ROS accumulation in roots, because the hypoxia stress led to over production of ROS in the roots of the wild type but not in that of OsPIN2 mutant. Accordingly, the anoxic resistance-related gene SUB1B showed differential expression in the root of the WT and OsPIN2 mutant in response to hypoxic conditions. Notably, compared with the wild type, the OsPIN2 mutant displayed a different pattern of auxin distribution in the root under hypoxia stress. It was shown that hypoxia stress caused a significant increase in auxin distribution in the root tip of the WT but not in that of the war1 mutant. In summary, these results suggested that OsPIN2 may play a role in regulating ROS accumulation probably via mediating auxin transport and distribution in the root tip, affecting root gravitropism and hypoxic tolerance in rice seedlings. These findings may contribute to the genetic improvement and identification of potential hypoxic tolerant lines in rice.

Adaptive strategies of aquatic mammals: Exploring the role of the HIF pathway and hypoxia tolerance

Aquatic mammals (marine and freshwater species) share significant and similar adaptations, enabling them to tolerate hypoxia during regular breath-hold diving. Despite the established importance of HIF1A, a master regulator in the molecular mechanism of hypoxia response, and other associated genes, their role in the evolutionary adaptation of aquatic mammals is not fully understood. In this study, we investigated this topic by employing a candidate gene approach to analyze 11 critical genes involved in the HIF1A signaling pathway in aquatic mammals. Our gene analyses included evaluating positive and negative selection, relaxation or constriction of selection, and molecular convergence compared to other terrestrial mammals, including subterranean mammals. Evidence of selection suggested a significant role of negative selection, as well as relaxation of the selective regime in cetaceans for most of these genes. We found that the glutamine 68 variant in the HIF3α protein is unique to cetaceans and initial evaluations indicated a destabilizing effect on protein structure. However, further analyses are necessary to evaluate its functional impact and adaptive relevance in this taxon.

Back to the Womb: A Perinatal Perspective on Mammalian Hibernation

AbstractThe idea of putting astronauts into a hibernation-like state during interplanetary spaceflights has sparked new interest in the evolutionary roots of hibernation and torpor. In this context, it should be noted that mammalian fetuses and neonates respond to the environmental challenges in the perinatal period with a number of physiological mechanisms that bear striking similarity to hibernation and torpor. These include three main points: first, prenatal deviation from the overall metabolic size relationship, which adapts the fetus to the low-oxygen conditions in the womb and corresponds to the metabolic reduction during hibernation and estivation; second, intranatal diving bradycardia in response to shortened O₂ supply during birth, comparable to the decrease in heart rate preceding the drop in body temperature upon entry into torpor; and third, postnatal onset of nonshivering thermogenesis in the brown adipose tissue, along with the increase in basal metabolic rate up to the level expected from body size, such as during arousal from hibernation. The appearance of hibernation-like adaptations in the perinatal period suggests that, conversely, hibernation and torpor may be composed of mechanisms shared by all mammals around birth. This hypothesis sheds new light on the origins of hibernation and supports its potential accessibility to nonhibernating species, including humans.

Metabolic Responses and Resilience to Environmental Challenges in the Sedentary Batrachoid Halobatrachus didactylus (Bloch & Schneider, 1801)

In the context of climate change, warming of the seas and expansion of hypoxic zones are challenges that most species of fish are, or will be subjected to. Understanding how different species cope with these changes in their environment at the individual level can shed light on how populations and ecosystems will be affected. We provide first-time estimates on the metabolic rates, thermal, and oxygen-related limits for Halobatrachus didactylus, a coastal sedentary fish that lives in intertidal environments of the Northeast Atlantic. Using respirometry in different experimental designs, we found that this species is highly resistant to acute thermal stress (CT_max: 34.82 ± 0.66 °C) and acute hypoxia (P_crit: 0.59-1.97 mg O₂ L^-1). We found size-specific differences in this stress response, with smaller individuals being more sensitive. We also quantified its aerobic scope and daily activity patterns, finding this fish to be extremely sedentary, with one of the lowest standard metabolic rates found in temperate fish (SMR: 14.96 mg O₂ kg^-1h^-1). H. didactylus activity increases at night, when its metabolic rate increases drastically (RMR: 36.01 mg O₂ kg^-1h^-1). The maximum metabolic rate of H. didactylus was estimated to be 67.31 mg O₂ kg^-1h^-1, producing an aerobic scope of 52.35 mg O₂ kg^-1h^-1 (77.8% increase). The metrics obtained in this study prove that H. didactylus is remarkably resilient to acute environmental variations in temperature and oxygen content, which might enable it to adapt to the extreme abiotic conditions forecasted for the world's oceans in the near future.

Gypenosides (GPs) alleviates hypoxia-induced injury in PC12 cells and enhances tolerance to anoxia in C57BL/6 mice

Oxygen is a necessary substance for life activities, but reduced oxygen utilization due to high altitude exposure and respiratory dysfunction diseases could lead to pathological changes in the organisms. Herein gypenosides, the active ingredients in the food and medicine resource plant Gynostemma pentaphyllum (Thunb.) Makino were found to alleviate hypoxia-induced injury in PC12 cells. Moreover, hypoxia induced an increase in Ca²⁺ and reactive oxygen species content, and such patterns were both significantly reduced by gypenosides treatment. At the same time, gypenosides significantly blocked the decrease of both NO content and mitochondrial membrane potential caused by hypoxia. Furthermore, gypenosides gavage treatment significantly prolonged the survival time of C57BL/6 mice in confinement up to 24.3% and enhanced the locomotor ability of mice. Therefore, gypenosides have good neuroprotective effects and hypoxia tolerance activity and have the prospect of being developed as a preventive and therapeutic drug for hypoxia-related diseases. PRACTICAL APPLICATIONS: Gypenosides can enhance tolerance of cells and mice to hypoxia and have the potential to be developed into hypoxia-resistant health food and drugs.

Methyltransferase SMYD3 impairs hypoxia tolerance by augmenting hypoxia signaling independent of its enzymatic activity

Hypoxia-inducible factor (HIF)1α, a main transcriptional regulator of the cellular response to hypoxia, also plays important roles in oxygen homeostasis of aerobic organisms, which is regulated by multiple mechanisms. However, the full cellular response to hypoxia has not been elucidated. In this study, we found that expression of SMYD3, a methyltransferase, augments hypoxia signaling independent of its enzymatic activity. We demonstrated SMYD3 binds to and stabilizes HIF1α via co-immunoprecipitation and Western blot assays, leading to the enhancement of HIF1α transcriptional activity under hypoxia conditions. In addition, the stabilization of HIF1α by SMYD3 is independent of HIF1α hydroxylation by prolyl hydroxylases and the intactness of the von Hippel-Lindau ubiquitin ligase complex. Furthermore, we showed SMYD3 induces reactive oxygen species accumulation and promotes hypoxia-induced cell apoptosis. Consistent with these results, we found smyd3-null zebrafish exhibit higher hypoxia tolerance compared to their wildtype siblings. Together, these findings define a novel role of SMYD3 in affecting hypoxia signaling and demonstrate that SMYD3-mediated HIF1α stabilization augments hypoxia signaling, leading to the impairment of hypoxia tolerance.

The Role of Hypoxia-Inducible Factor in the Mechanisms of Aging

Aging is accompanied by a reduction in the oxygen delivery to all organs and tissues and decrease in the oxygen partial pressure in them, resulting in the development of hypoxia. The lack of oxygen activates cell signaling pathway mediated by the hypoxia-inducible transcription factor (HIF), which exists in three isoforms - HIF-1, HIF-2, and HIF-3. HIF regulates expression of several thousand genes and is a potential target for the development of new drugs for the treatment of many diseases, including those associated with age. Human organism and organisms of laboratory animals differ in their tolerance to hypoxia and expression of HIF and HIF-dependent genes, which may contribute to the development of inflammatory, tumor, and cardiovascular diseases. Currently, the data on changes in the HIF expression with age are contradictory, which is mostly due to the fact that such studies are conducted in different age groups, cell types, and model organisms, as well as under different hypoxic conditions and mainly in vitro. Furthermore, the observed discrepancies can be due to the individual tolerance of the studied organisms to hypoxia, which is typically not taken into account. Therefore, the purpose of this review was to analyze the published data on the connection between the mechanisms of aging, basal tolerance to hypoxia, and changes in the level of HIF expression with age. Here, we summarized the data on the age-related changes in the hypoxia tolerance, HIF expression and the role of HIF in aging, which is associated with its involvement in the molecular pathways mediated by insulin and IGF-1 (IIS), sirtuins (SIRTs), and mTOR. HIF-1 interacts with many components of the IIS pathway, in particular with FOXO, the activation of which reduces production of reactive oxygen species (ROS) and increases hypoxia tolerance. Under hypoxic conditions, FOXO is activated via both HIF-dependent and HIF-independent pathways, which contributes to a decrease in the ROS levels. The activity of HIF-1 is regulated by all members of the sirtuin family, except SIRT5, while the mechanisms of SIRT interaction with HIF-2 and HIF-3 are poorly understood. The connection between HIF and mTOR and its inhibitor, AMPK, has been identified, but its exact mechanism has yet to be studied. Understanding the role of HIF and hypoxia in aging and pathogenesis of age-associated diseases is essential for the development of new approaches to the personalized therapy of these diseases, and requires further research.

[The effects of different herbal compound and extracts from different extraction methods on hypoxia tolerance in mice]

Objective: To investigate the effects of different prescription compositions of traditional Chinese medicine and its different extraction methods of compound formula extracts on hypoxia tolerance in mice, in order to preferably select their prescription compositions and preparation extraction methods. Methods: Male BALB/c mice were randomly divided into 6 groups: blank control group, compound danshen group, compound Rhodiola Rosea alcohol-water extract group (Rhodiola rosea, Astragali Radix, Polygonati Rhizoma, Lycii Fructus), compound Rhodiola Rosea water extract group, compound Astragalus alcohol-water extract group (Astragali Radix, Polygonati Rhizoma, Lycii Fructus) and compound Astragalus water extract group, 30 mice in each group. Each group was administered continuously by gavage for 10 d. The blank group was gavaged with sterilized injection water. The mice in the other groups were treated with 0.15 g/kg of compound danshen, 3 g/kg of compound Rhodiola Rosea alcohol-water extract or water extract, and 1.7 g/kg of compound Astragalus alcohol-water extract or water extract, respectively. Each group was subjected to normobaric hypoxia tolerance test, sodium nitrite toxicity survival test and acute cerebral ischemia-hypoxia test 1 h after the last gavage, and the mice brain tissues were used to determine the activity of antioxidant enzymes and metabolites related to oxidative stress. Results: Compared with the blank control group, in normobaric hypoxia tolerance test, the survival time of mice in the compound danshen group and the compound Astragalus alcohol-water extract group and water extraction group was prolonged significantly (P＜0.01), and the number of open-mouth gasping after cerebral ischemia and hypoxia was increased significantly (P＜0.05). There was no statistical difference in survival time after sodium nitrite injection in each group. Compared with the blank control group, the activities of T-AOC, SOD, GSH and CAT were increased significantly (P＜0.05, P＜0.01) and the content of MDA was decreased significantly (P＜0.01) in the compound Astragalus water extract group. Compared with the compound danshen group, the activities of SOD, CAT and GSH were increased significantly (P＜0.01, P＜0.05) and the content of MDA was decreased significantly (P＜0.05). Conclusion: Compound Astragalus water extraction has the best effect of hypoxia tolerance, compound Rhodiola Rosea can eliminate Rhodiola rosea and consists of Astragali Radix, Polygonati Rhizoma, Lycii Fructus and its extraction method is water extraction.

Transformation to ischaemia tolerance of frog brain function corresponds to dynamic changes in mRNA co-expression across metabolic pathways

Neural activity is costly and requires continuous ATP from aerobic metabolism. Brainstem motor function of American bullfrogs normally collapses after minutes of ischaemia, but following hibernation, it becomes ischaemia-tolerant, generating output for up to 2 h without oxygen or glucose delivery. Transforming the brainstem to function during ischaemia involves a switch to anaerobic glycolysis and brain glycogen. We hypothesized that improving neural performance during ischaemia involves a transcriptional program for glycogen and glucose metabolism. Here we measured mRNA copy number of genes along the path from glycogen metabolism to lactate production using real-time quantitative PCR. The expression of individual genes did not reflect enhanced glucose metabolism. However, the number of co-expressed gene pairs increased early into hibernation, and by the end, most genes involved in glycogen metabolism, glucose transport and glycolysis exhibited striking linear co-expression. By contrast, co-expression of genes in the Krebs cycle and electron transport chain decreased throughout hibernation. Our results uncover reorganization of the metabolic transcriptional network associated with a shift to ischaemia tolerance in brain function. We conclude that modifying gene co-expression may be a critical step in synchronizing storage and use of glucose to achieve ischaemia tolerance in active neural circuits.

Indicators of Hypoxia Tolerance as Determined by Cellular Elements of Rat Blood

Although hypoxia tolerance is mainly determined genetically, it is important to study individual variability of animal organisms in order to identify the factors that underlie their tolerance to hypoxic exposure. We investigated blood cell counts and coagulograms in Wistar rats as predictors allowing the animal population to be split into hypoxia-tolerant and hypoxia-intolerant individuals. The validity of the specific predictors’ choice was proved by a coincidence between the population split in accordance with the detected individual parameters and the results of testing animals in a decompression chamber at a rarefaction corresponding to the “rise to an altitude” of 11500 m above sea level. Circulating blood cells were quantitatively assessed by eighteen indicators before and after hypoxic exposure. The differences between animals low-tolerant (LT), high-tolerant (HT), and medium-tolerant (MT) to hypoxia were determined by five indicators: white blood cell count (WBC), granulocyte count (Gran#), red blood cell count (RBC), reticulocyte count/percent (RTC), and mean corpuscular hemoglobin (MCH). The RBC, RTC, and MCH values in HT rats were significantly higher than in LT animals (by 1.4, 1.9, and 1.1 times, respectively). The WBC and Gran# values in HT rats were lower than in LT individuals. The hypoxia tolerance indices (HTI) were calculated using the original formula. It was established that in LT rats, the HTI ≤ 0.203, in HT rats ≥ 0.335, and in MT rats < 0.335 but > 0.203. After testing in a decompression chamber, the activated partial thromboplastin time (APTT), thrombin time (TT), and prothrombin time (PT) decreased, but the fibrinogen level increased. LT rats were characterized by the lowest APTT, TT, and PT values and the highest values of the fibrinogen level. Our results indicate that one of the most important mechanisms underlying a high hypoxia tolerance in rats consists in sustaining reciprocal relationships between the complex of RBC indicators, which tend to increase under hypoxia, and Gran# indicators, which tend to decrease after hypoxic exposure.

Acute measures of upper thermal and hypoxia tolerance are not reliable predictors of mortality following environmental challenges in rainbow trout (Oncorhynchus mykiss)

Anthropogenic climate change threatens freshwater biodiversity and poses a challenge for fisheries management, as fish will increasingly be exposed to episodes of high temperature and low oxygen (hypoxia). Here, we examine the extent of variation in tolerance of acute exposure to these stressors within and among five strains of rainbow trout (Oncorhynchus mykiss) currently being used or under consideration for use in stocking programmes in British Columbia, Canada. We used incipient lethal oxygen saturation (ILOS) as an index of acute hypoxia tolerance, critical thermal maximum (CT_max) as an index of acute upper thermal tolerance and mortality following these two acute exposure trials to assess the relative resilience of individuals and strains to climate change-relevant stressors. We measured tolerance across two brood years and two life stages (fry and yearling), using a highly replicated design with hundreds of individuals per strain and life stage. There was substantial within-strain variation in CT_max and ILOS, but differences among strains, although statistically significant, were small. In contrast, there were large differences in post-trial mortality among strains, ranging from less than 2% mortality in the most resilient strain to 55% mortality in the least resilient. There was a statistically significant, but weak, correlation between CT_max and ILOS at both life stages for some strains, with thermally tolerant individuals tending to be hypoxia tolerant. These data indicate that alternative metrics of tolerance may result in different conclusions regarding resilience to climate change stressors, which has important implications for stocking and management decisions for fish conservation in a changing climate.

Ontogeny of Carbon Monoxide-Related Gene Expression in a Deep-Diving Marine Mammal

Marine mammals such as northern elephant seals (NES) routinely experience hypoxemia and ischemia-reperfusion events to many tissues during deep dives with no apparent adverse effects. Adaptations to diving include increased antioxidants and elevated oxygen storage capacity associated with high hemoprotein content in blood and muscle. The natural turnover of heme by heme oxygenase enzymes (encoded by HMOX1 and HMOX2) produces endogenous carbon monoxide (CO), which is present at high levels in NES blood and has been shown to have cytoprotective effects in laboratory systems exposed to hypoxia. To understand how pathways associated with endogenous CO production and signaling change across ontogeny in diving mammals, we measured muscle CO and baseline expression of 17 CO-related genes in skeletal muscle and whole blood of three age classes of NES. Muscle CO levels approached those of animals exposed to high exogenous CO, increased with age, and were significantly correlated with gene expression levels. Muscle expression of genes associated with CO production and antioxidant defenses (HMOX1, BVR, GPX3, PRDX1) increased with age and was highest in adult females, while that of genes associated with protection from lipid peroxidation (GPX4, PRDX6, PRDX1, SIRT1) was highest in adult males. In contrast, muscle expression of mitochondrial biogenesis regulators (PGC1A, ESRRA, ESRRG) was highest in pups, while genes associated with inflammation (HMOX2, NRF2, IL1B) did not vary with age or sex. Blood expression of genes involved in regulation of inflammation (IL1B, NRF2, BVR, IL10) was highest in pups, while HMOX1, HMOX2 and pro-inflammatory markers (TLR4, CCL4, PRDX1, TNFA) did not vary with age. We propose that ontogenetic upregulation of baseline HMOX1 expression in skeletal muscle of NES may, in part, underlie increases in CO levels and expression of genes encoding antioxidant enzymes. HMOX2, in turn, may play a role in regulating inflammation related to ischemia and reperfusion in muscle and circulating immune cells. Our data suggest putative ontogenetic mechanisms that may enable phocid pups to transition to a deep-diving lifestyle, including high baseline expression of genes associated with mitochondrial biogenesis and immune system activation during postnatal development and increased expression of genes associated with protection from lipid peroxidation in adulthood.

Hypoxia Tolerant Species: The Wisdom of Nature Translated into Targets for Stroke Therapy

Human neurons rapidly die after ischemia and current therapies for stroke management are limited to restoration of blood flow to prevent further brain damage. Thrombolytics and mechanical thrombectomy are the available reperfusion treatments, but most of the patients remain untreated. Neuroprotective therapies focused on treating the pathogenic cascade of the disease have widely failed. However, many animal species demonstrate that neurons can survive the lack of oxygen for extended periods of time. Here, we reviewed the physiological and molecular pathways inherent to tolerant species that have been described to contribute to hypoxia tolerance. Among them, Foxo3 and Eif5A were reported to mediate anoxic survival in Drosophila and Caenorhabditis elegans, respectively, and those results were confirmed in experimental models of stroke. In humans however, the multiple mechanisms involved in brain cell death after a stroke causes translation difficulties to arise making necessary a timely and coordinated control of the pathological changes. We propose here that, if we were able to plagiarize such natural hypoxia tolerance through drugs combined in a pharmacological cocktail it would open new therapeutic opportunities for stroke and likely, for other hypoxic conditions.

HIF-Dependent Mechanisms of Relationship between Hypoxia Tolerance and Tumor Development

Oxygen deficiency is one of the key pathogenetic factors determining development and severity of many diseases, including inflammatory, infectious diseases, and cancer. Lack of oxygen activates the signaling pathway of the hypoxia-inducible transcription factor HIF in cells that has three isoforms, HIF-1, HIF-2, HIF-3, regulating expression of several thousand genes. Throughout tumor progression, HIF activation stimulates angiogenesis, promotes changes in cell metabolism, adhesion, invasiveness, and ability to metastasize. HIF isoforms can play opposite roles in the development of inflammatory and neoplastic processes. Humans and laboratory animals differ both in tolerance to hypoxia and in the levels of expression of HIF and HIF-dependent genes, which may lead to predisposition to the development of certain oncological disorders. In particular, the ratio of different histogenetic types of tumors may vary among people living in the mountains and at the sea level. However, despite the key role of hypoxia at almost all stages of tumor development, basal tolerance to oxygen deficiency is not considered as a factor of predisposition to the tumor growth initiation. In literature, there are many works characterizing the level of local hypoxia in various tumors, and suggesting fundamental approaches to its mitigation by HIF inhibition. HIF inhibitors, as a rule, have a systemic effect on the organism, however, basal tolerance of an organism to hypoxia as well as the level of HIF expression are not taken into account in the process of their use. The review summarizes the literature data on different HIF isoforms and their role in tumor progression, with extrapolation to organisms with high and low tolerance to hypoxia, as well as on the prevalence of various types of tumors in the populations living at high altitudes.

Hydrogen Sulfide Enhances Plant Tolerance to Waterlogging Stress

Hydrogen sulfide (H₂S) is considered the third gas signal molecule in recent years. A large number of studies have shown that H₂S not only played an important role in animals but also participated in the regulation of plant growth and development and responses to various environmental stresses. Waterlogging, as a kind of abiotic stress, poses a serious threat to land-based waterlogging-sensitive plants, and which H₂S plays an indispensable role in response to. In this review, we summarized that H₂S improves resistance to waterlogging stress by affecting lateral root development, photosynthetic efficiency, and cell fates. Here, we reviewed the roles of H₂S in plant resistance to waterlogging stress, focusing on the mechanism of its promotion to gained hypoxia tolerance. Finally, we raised relevant issues that needed to be addressed.

Hypometabolic Responses to Chronic Hypoxia: A Potential Role for Membrane Lipids

Metabolic suppression is an essential strategy to cope with chronic hypoxia. This review examines the physiological processes used to survive in low oxygen environments. It proposes a novel mechanism-the remodeling of membrane lipids-to suppress ATP use and production. Temperature (homeoviscous adaptation), diet (natural doping in migrant birds) and body mass (membrane pacemaker of metabolism) have an impact on the lipid composition of membranes, which, in turn, modulates metabolic capacity. Vertebrate champions of hypoxia tolerance show extensive changes in membrane lipids upon in vivo exposure to low oxygen. These changes and those observed in hibernating mammals can promote the downregulation of ion pumps (major ATP consumers), ion channels, mitochondrial respiration capacity (state 3, proton leak, cytochrome c oxidase), and energy metabolism (β-oxidation and glycolysis). A common membrane signal regulating the joint inhibition of ion pumps and channels could be an exquisite way to preserve the balance between ATP supply and demand in hypometabolic states. Membrane remodeling together with more traditional mechanisms could work in concert to cause metabolic suppression.

Spleen Volume and Contraction During Apnea in Mt. Everest Climbers and Everest Base Camp Trekkers

The human spleen can contract and transiently boost the blood with stored erythrocytes. We measured spleen volume and contraction during apneas in two groups, each containing 12 Caucasian participants (each 3 women): one group planning to summit Mt. Everest (8848 m; "Climbers") and another trekking to Everest Base Camp (5300 m; "Trekkers"). Tests were done in Kathmandu (1370 m) 1-3 days after arrival, before the Climb/Trek. Age, height, weight, vital capacity, resting heart rate, and arterial oxygen saturation were similar between groups (not significant). After 15 minutes of sitting rest, all participants performed a 1-minute apnea and, after 2 minutes of rest, 1 maximal duration apnea was performed. Six of the climbers did a third apnea and hemoglobin concentration (Hb) was measured. Three axial spleen diameters were measured by ultrasonic imaging before and after the apneas for spleen volume calculation. Mean (standard deviation) baseline spleen volume was larger in Climbers [367 (181) mL] than in Trekkers [228 (70) mL; p = 0.022]. Spleen contraction occurred during apneas in both groups, with about twice the magnitude in Climbers. Three apneas in six of the Climbers resulted in a spleen volume reduction from 348 (145) to 202 (91) mL (p = 0.005) and an Hb elevation from 147.9 (13.1) to 153.3 (11.3) g/L (p = 0.024). Maximal apneic duration was longer in Climbers [88 (23) seconds vs. 67 (18) seconds in Trekkers; p = 0.023]. We concluded that a large spleen characterizes Climbers, suggesting that spleen function may be important for high-altitude climbing performance.

Goldfish Response to Chronic Hypoxia: Mitochondrial Respiration, Fuel Preference and Energy Metabolism

Hypometabolism is a hallmark strategy of hypoxia tolerance. To identify potential mechanisms of metabolic suppression, we have used the goldfish to quantify the effects of chronically low oxygen (4 weeks; 10% air saturation) on mitochondrial respiration capacity and fuel preference. The responses of key enzymes from glycolysis, β-oxidation and the tricarboxylic acid (TCA) cycle, and Na+/K+-ATPase were also monitored in various tissues of this champion of hypoxia tolerance. Results show that mitochondrial respiration of individual tissues depends on oxygen availability as well as metabolic fuel oxidized. All the respiration parameters measured in this study (LEAK, OXPHOS, Respiratory Control Ratio, CCCP-uncoupled, and COX) are affected by hypoxia, at least for one of the metabolic fuels. However, no common pattern of changes in respiration states is observed across tissues, except for the general downregulation of COX that may help metabolic suppression. Hypoxia causes the brain to switch from carbohydrates to lipids, with no clear fuel preference in other tissues. It also downregulates brain Na+/K+-ATPase (40%) and causes widespread tissue-specific effects on glycolysis and beta-oxidation. This study shows that hypoxia-acclimated goldfish mainly promote metabolic suppression by adjusting the glycolytic supply of pyruvate, reducing brain Na+/K+-ATPase, and downregulating COX, most likely decreasing mitochondrial density.

Neuromodulation or energy failure? Metabolic limitations silence network output in the hypoxic amphibian brainstem

Hypoxia tolerance in the vertebrate brain often involves chemical modulators that arrest neuronal activity to conserve energy. However, in intact networks, it can be difficult to determine whether hypoxia triggers modulators to stop activity in a protective manner or whether activity stops because rates of ATP synthesis are insufficient to support network function. Here, we assessed the extent to which neuromodulation or metabolic limitations arrest activity in the respiratory network of bullfrogs-a circuit that survives moderate periods of oxygen deprivation, presumably, by activating an inhibitory noradrenergic pathway. We confirmed that hypoxia and norepinephrine (NE) reduce network output, consistent with the view that hypoxia may cause the release of NE to inhibit activity. However, these responses differed qualitatively; hypoxia, but not NE, elicited a large motor burst and silenced the network. The stereotyped response to hypoxia persisted in the presence of both NE and an adrenergic receptor blocker that eliminates sensitivity to NE, indicating that noradrenergic signaling does not cause the arrest. Pharmacological inhibition of glycolysis and mitochondrial respiration recapitulated all features of hypoxia on network activity, implying that reduced ATP synthesis underlies the effects of hypoxia. Finally, activating modulatory mechanisms that dampen neuronal excitability when ATP levels fall, KATP channels and AMP-dependent protein kinase, did not resemble the hypoxic response. These results suggest that energy failure-rather than inhibitory modulation-silences the respiratory network during hypoxia and emphasize the need to account for metabolic limitations before concluding that modulators arrest activity as an adaptation for energy conservation in the nervous system.

Mechanism and effects of fructose diphosphate on anti-hypoxia fatigue and learning memory ability

This study aims to investigate the mechanisms through which fructose diphosphate (FDP) causes anti-hypoxia and anti-fatigue effects and improves learning and memory. Mice were divided into three groups: low-dose FDP (FDP-L), high-dose FDP (FDP-H), and a control group. Acute toxic hypoxia induced by carbon monoxide, sodium nitrite, and potassium cyanide and acute cerebral ischemic hypoxia were used to investigate the anti-hypoxia ability of FDP. The tests of rod-rotating, mouse tail suspension, and swimming endurance were used to explore the anti-fatigue effects of FDP. The Morris water maze experiment was used to determine the impact of FDP on learning and memory ability. Poisoning-induced hypoxic tests showed that mouse survival time was significantly prolonged in the FDP-L and FDP-H groups compared with the control group (p < 0.05). In the exhaustive swimming test, FDP significantly shortened struggling time and prolonged the time of mass-loaded swimming; the rod-rotating test showed that endurance time was significantly prolonged by using FDP (p < 0.05). FDP significantly decreased lactate and urea nitrogen levels and increased hepatic and muscle glycogen and glucose transporter-4 and Na⁺-K⁺-ATPase (p < 0.05). To conclude, FDP enhances hypoxia tolerance and fatigue resistance and improves learning and memory ability through regulating glucose and energy metabolism.

Naked mole-rats suppress energy metabolism and modulate membrane cholesterol in chronic hypoxia

Naked mole-rats (NMRs) are mammalian champions of hypoxia tolerance that enter metabolic suppression to survive in low oxygen environments. Common physiological mechanisms used by animals to suppress metabolic rate include downregulating energy metabolism (ATP supply) as well as ion pumps (primary cellular ATP consumers). A recent goldfish study demonstrated that remodeling of membrane lipids may mediate these responses, but it is unknown if NMR employs the same strategies; therefore, we aimed to test the hypotheses that these fossorial mammals 1) downregulate the activity of key enzymes of glycolysis, tricarboxylic acid (TCA) cycle, and β-oxidation, 2) inhibit sodium-potassium-ATPase, and 3) alter membrane lipids in response to chronic hypoxia. We found that NMRs exposed to 11% oxygen for 4 wk had a lower metabolic rate by 34%. This suppression occurs concurrently with tissue-specific 25-99% decreases in metabolic enzymes activities, a 77% decrease in brain sodium/potassium-ATPase activity, and widespread changes in membrane cholesterol abundance. By reducing glycolytic and β-oxidation fluxes, NMRs decrease the supply of acetyl-CoA to the TCA cycle. By contrast, there is a 94% upregulation of citrate synthase in the heart, possibly to support circulation and thus oxygen supply to other organs. Taken together, these responses may reflect a coordinated physiological response to hypoxia, but a clear functional link between changes in membrane composition and enzyme activities could not be established. Nevertheless, this is the first demonstration that hypometabolic NMRs alter the lipid composition of their membranes in response to chronic in vivo exposure to hypoxia.

Loss of hypoxia-inducible factor 1α affects hypoxia tolerance in larval and adult zebrafish (Danio rerio)

The coordination of the hypoxic response is attributed, in part, to hypoxia-inducible factor 1α (Hif-1α), a regulator of hypoxia-induced transcription. After the teleost-specific genome duplication, most teleost fishes lost the duplicate copy of Hif-1α, except species in the cyprinid lineage that retained both paralogues of Hif-1α (Hif1aa and Hif1ab). Little is known about the contribution of Hif-1α, and specifically of each paralogue, to hypoxia tolerance. Here, we examined hypoxia tolerance in wild-type (Hif1aa^+/+ab^+/+) and Hif-1α knockout lines (Hif1aa^-/-; Hif1ab^-/-; Hif1aa^-/-ab^-/-) of zebrafish (Danio rerio). Critical O₂ tension (P_crit; the partial pressure of oxygen (PO₂) at which O₂ consumption can no longer be maintained) and time to loss of equilibrium (LOE), two indices of hypoxia tolerance, were assessed in larvae and adults. Knockout of both paralogues significantly increased P_crit (decreased hypoxia tolerance) in larval fish. Prior exposure of larvae to hypoxia decreased P_crit in wild-type fish, an effect mediated by the Hif1aa paralogue. In adults, individuals with a knockout of either paralogue exhibited significantly decreased time to LOE but no difference in P_crit. Together, these results demonstrate that in zebrafish, tolerance to hypoxia and improved hypoxia tolerance after pre-exposure to hypoxia (pre-conditioning) are mediated, at least in part, by Hif-1α.

Expression of SIRT5 protein in gastric cancer cells

The purpose of this paper was to investigate the dynamic trend of SIRT5 (Sirtuins 5) protein expression in gastric cancer cells, for which a hypoxic gastric cancer cell line was established. Afterward, the parental gastric cancer cell group and hypo group were designed, and the levels of related proteins and mRNAs were detected by using Western blot along with real-time quantitative fluorescence PCR (RT-PCR). The results showed that the expression of SIRT5 in hypoxic gastric cancer cells increased significantly, which could increase the phosphorylation level of c-MET (hepatocyte growth factor receptor) and promote the migration of gastric cancer cells. When the expression of SIRT5 protein was observed under hypoxic conditions, SIRT5 silencing significantly reduced the migration ability of MGC803/hypo cells. It could be predicted that SIRT5-mediated protein desuccinylation played an important role in promoting the migration of hypoxic gastric cancer cells. Therefore, the migration rate of gastric cancer cells could be affected by controlling the expression of SIRT5 protein, which provides a novel idea for the treatment of gastric cancer in the future.

Repeatability of Hypoxia Tolerance of Individual Juvenile Striped Bass Morone saxatilis and Effects of Social Status

Chesapeake Bay is the primary nursery for striped bass (Morone saxatilis), which are increasingly being exposed to hypoxic waters. Tolerance to hypoxia in fish is generally determined by a single exposure of an isolated individual or by exposing large groups of conspecifics to hypoxia without regard to social status. The importance of social context in determining physiological responses to stressors is being increasingly recognized. To determine whether social interactions influence hypoxia tolerance (HT) in striped bass, loss of equilibrium HT was assessed in the same fish while manipulating the social environment around it. Small group settings were used to be more representative of the normal sociality experienced by this species than the paired encounters typically used. After establishing the dominance hierarchy within a group of fish, HT was determined collectively for the individuals in that group, and then new groups were constructed from the same pool of fish. Individuals could then be followed across multiple settings for both repeatability of HT and hierarchy position ( X ¯ = 4.2 ± 0.91 SD groups per individual). HT increased with repeated exposures to hypoxia ( P < 0.001 ), with a significant increase by a third exposure ( P = 0.004 ). Despite this changing HT, rank order of HT was significantly repeatable across trials for 6 mo ( P = 0.012 ). Social status was significantly repeatable across trials of different group composition ( P = 0.02 ) and unrelated to growth rate but affected HT weakly in a complex interaction with size. Final HT was significantly correlated with blood [hemoglobin] and hematocrit. The repeatability and large intraspecific variance of HT in juvenile striped bass suggest that HT is potentially an important determinant of Darwinian fitness in an increasingly hypoxic Chesapeake Bay.

Acidosis Maintains the Function of Brain Mitochondria in Hypoxia-Tolerant Triplefin Fish: A Strategy to Survive Acute Hypoxic Exposure?

The vertebrate brain is generally very sensitive to acidosis, so a hypoxia-induced decrease in pH is likely to have an effect on brain mitochondria (mt). Mitochondrial respiration (JO₂) is required to generate an electrical gradient (ΔΨm) and a pH gradient to power ATP synthesis, yet the impact of pH modulation on brain mt function remains largely unexplored. As intertidal fishes within rock pools routinely experience hypoxia and reoxygenation, they would most likely experience changes in cellular pH. We hence compared four New Zealand triplefin fish species ranging from intertidal hypoxia-tolerant species (HTS) to subtidal hypoxia-sensitive species (HSS). We predicted that HTS would tolerate acidosis better than HSS in terms of sustaining mt structure and function. Using respirometers coupled to fluorimeters and pH electrodes, we titrated lactic-acid to decrease the pH of the media, and simultaneously recorded JO₂, ΔΨm, and H⁺ buffering capacities within permeabilized brain and swelling of mt isolated from non-permeabilized brains. We then measured ATP synthesis rates in the most HTS (Bellapiscus medius) and the HSS (Forsterygion varium) at pH 7.25 and 6.65. Mitochondria from HTS brain did have greater H⁺ buffering capacities than HSS mt (∼10 mU pH.mg_protein ^-1). HTS mt swelled by 40% when exposed to a decrease of 1.5 pH units, and JO₂ was depressed by up to 15% in HTS. However, HTS were able to maintain ΔΨm near -120 mV. Estimates of work, in terms of charges moved across the mt inner-membrane, suggested that with acidosis, HTS mt may in part harness extra-mt H⁺ to maintain ΔΨm, and could therefore support ATP production. This was confirmed with elevated ATP synthesis rates and enhanced P:O ratios at pH 6.65 relative to pH 7.25. In contrast, mt volumes and ΔΨm decreased downward pH 6.9 in HSS mt and paradoxically, JO₂ increased (∼25%) but ATP synthesis and P:O ratios were depressed at pH 6.65. This indicates a loss of coupling in the HSS with acidosis. Overall, the mt of these intertidal fish have adaptations that enhance ATP synthesis efficiency under acidic conditions such as those that occur in hypoxic or reoxygenated brain.

Tolerance traits related to climate change resilience are independent and polygenic

The resilience of organisms to climate change through adaptive evolution is dependent on the extent of genetically based variation in key phenotypic traits and the nature of genetic associations between them. For aquatic animals, upper thermal tolerance and hypoxia tolerance are likely to be a important determinants of sensitivity to climate change. To determine the genetic basis of these traits and to detect associations between them, we compared naturally occurring populations of two subspecies of Atlantic killifish, Fundulus heteroclitus, that differ in both thermal and hypoxia tolerance. Multilocus association mapping demonstrated that 47 and 35 single nucleotide polymorphisms (SNPs) explained 43.4% and 51.9% of variation in thermal and hypoxia tolerance, respectively, suggesting that genetic mechanisms underlie a substantial proportion of variation in each trait. However, no explanatory SNPs were shared between traits, and upper thermal tolerance varied approximately linearly with latitude, whereas hypoxia tolerance exhibited a steep phenotypic break across the contact zone between the subspecies. These results suggest that upper thermal tolerance and hypoxia tolerance are neither phenotypically correlated nor genetically associated, and thus that rates of adaptive change in these traits can be independently fine-tuned by natural selection. This modularity of important traits can underpin the evolvability of organisms to complex future environmental change.

Metabolic depression and the evolution of hypoxia tolerance in threespine stickleback, Gasterosteus aculeatus

Anthropogenic increases in global temperature and agricultural runoff are increasing the prevalence of aquatic hypoxia throughout the world. We investigated the potential for a relatively rapid evolution of hypoxia tolerance using two isolated (for less than 11 000 years) populations of threespine stickleback: one from a lake that experiences long-term hypoxia (Alta Lake, British Columbia) and one from a lake that does not (Trout Lake, British Columbia). Loss-of-equilibrium (LOE) experiments revealed that the Alta Lake stickleback were significantly more tolerant of hypoxia than the Trout Lake stickleback, and calorimetry experiments revealed that the enhanced tolerance of Alta Lake stickleback may be associated with their ability to depress metabolic rate (as indicated by metabolic heat production) by 33% in hypoxia. The two populations showed little variation in their capacities for O₂ extraction and anaerobic metabolism. These results reveal that intraspecific variation in hypoxia tolerance can develop over relatively short geological timescales, as can metabolic rate depression, a complex biochemical response that may be favoured in long-term hypoxic environments.

A rainbow trout Oncorhynchus mykiss strain with higher aerobic scope in normoxia also has superior tolerance of hypoxia

This study compared parr from three strains of rainbow trout Oncorhynchus mykiss to examine intraspecific variation in metabolic traits, hypoxia tolerance and upper thermal tolerance in this species. At the strain level, variation in absolute aerobic scope (AAS), critical oxygen level (O_2crit ), incipient lethal oxygen saturation (ILOS) and critical thermal maximum (CT_max ) generally exhibited consistent differences among the strains, suggesting the possibility of functional associations among these traits. This possibility was further supported at the individual level by a positive correlation between ILOS and O_2crit and a negative correlation between O_2crit and AAS. These results indicate that intraspecific differences in hypoxia tolerance among strains of O. mykiss may be primarily determined by differences in the ability to maintain oxygen uptake in hypoxia and that variation in aerobic scope in normoxia probably plays a role in determining the ability of these fish to sustain metabolism aerobically as water oxygen saturation is reduced.

Evolution of Cytochrome c Oxidase in Hypoxia Tolerant Sculpins (Cottidae, Actinopterygii)

Vertebrate hypoxia tolerance can emerge from modifications to the oxygen (O2) transport cascade, but whether there is adaptive variation to O2 binding at the terminus of this cascade, mitochondrial cytochrome c oxidase (COX), is not known. In order to address the hypothesis that hypoxia tolerance is associated with enhanced O2 binding by mitochondria we undertook a comparative analysis of COX O2 kinetics across species of intertidal sculpins (Cottidae, Actinopterygii) that vary in hypoxia tolerance. Our analysis revealed a significant relationship between hypoxia tolerance (critical O2 tension of O2 consumption rate; Pcrit), mitochondrial O2 binding affinity (O2 tension at which mitochondrial respiration was half maximal; P50), and COX O2-binding affinity (apparent Michaelis-Menten constant for O2 binding to COX; Km,app O2). The more hypoxia tolerant species had both a lower mitochondrial P50 and lower COX Km,app O2, facilitating the maintenance of mitochondrial function to a lower O2 tension than in hypoxia intolerant species. Additionally, hypoxia tolerant species had a lower overall COX Vmax but higher mitochondrial COX respiration rate when expressed relative to maximal electron transport system respiration rate. In silico analyses of the COX3 subunit postulated as the entry point for O2 into the COX protein catalytic core, points to variation in COX3 protein stability (estimated as free energy of unfolding) contributing to the variation in COX Km,app O2. We propose that interactions between COX3 and cardiolipin at four amino acid positions along the same alpha-helix forming the COX3 v-cleft represent likely determinants of interspecific differences in COX Km,app O2.

Developmental Hypoxia Has Negligible Effects on Long-Term Hypoxia Tolerance and Aerobic Metabolism of Atlantic Salmon (Salmo salar)

Exposure to developmental hypoxia can have long-term impacts on the physiological performance of fish because of irreversible plasticity. Wild and captive-reared Atlantic salmon (Salmo salar) can be exposed to hypoxic conditions during development and continue to experience fluctuating oxygen levels as juveniles and adults. Here, we examine whether developmental hypoxia impacts subsequent hypoxia tolerance and aerobic performance of Atlantic salmon. Individuals at 8°C were exposed to 50% (hypoxia) or 100% (normoxia) dissolved oxygen (DO) saturation (as percent of air saturation) from fertilization for ∼100 d (800 degree days) and then raised in normoxic conditions for a further 15 mo. At 18 mo after fertilization, aerobic scope was calculated in normoxia (100% DO) and acute (18 h) hypoxia (50% DO) from the difference between the minimum and maximum oxygen consumption rates ([Formula: see text] and [Formula: see text], respectively) at 10°C. Hypoxia tolerance was determined as the DO at which loss of equilibrium (LOE) occurred in a constantly decreasing DO environment. There was no difference in [Formula: see text], [Formula: see text], or aerobic scope between fish raised in hypoxia or normoxia. There was some evidence that hypoxia tolerance was lower (higher DO at LOE) in hypoxia-raised fish compared with those raised in normoxia, but the magnitude of the effect was small (12.52% DO vs. 11.73% DO at LOE). Acute hypoxia significantly reduced aerobic scope by reducing [Formula: see text], while [Formula: see text] remained unchanged. Interestingly, acute hypoxia uncovered individual-level relationships between DO at LOE and [Formula: see text], [Formula: see text], and aerobic scope. We discuss our findings in the context of developmental trajectories and the role of aerobic performance in hypoxia tolerance.

Tropical fish in a warming world: thermal tolerance of Nile perch Lates niloticus (L.) in Lake Nabugabo, Uganda

Key to predicting the response of fishes to climate change is quantifying how close fish are to their critical thermal limits in nature and their ability to adjust their thermal sensitivity to maintain performance. Here, we evaluated the effects of body size and habitat on aerobic scope (AS) and thermal tolerance of Nile perch Lates niloticus (L.), a fish of great economic and food security importance in East Africa, using respirometry and critical thermal maximum (CTmax) trials. Juvenile Nile perch from distinct habitats (high or low dissolved oxygen concentrations) of Lake Nabugabo, Uganda were exposed for 4.6 ± 0.55 days to a temperature treatment (25.5, 27.5, 29.5 or 31.5°C) prior to experimentation, with the lowest temperature corresponding to the mean annual daytime temperature in Lake Nabugabo and the highest temperature being 3°C higher than the maximal monthly average. As expected, metabolic rates increased with body mass. Although resting metabolic rate increased with temperature, maximal metabolic rate showed no change. Likewise, AS did not vary across treatments. The CTmax increased with acclimation temperature. There was no effect of habitat on maximal metabolic rate, AS or CTmax; however, there was a trend towards a lower resting metabolic rate for Nile perch captured in the low-dissolved oxygen habitat than in well-oxygenated waters. This study shows that juvenile Nile perch maintain a large AS at temperatures near the upper limit of their natural thermal range and provides evidence that Nile perch have physiological mechanisms to deal with acute exposure to thermal stress.

Evolutionary Genetics of Hypoxia Tolerance in Cetaceans during Diving

Hypoxia was a major challenge faced by cetaceans during the course of secondary aquatic adaptation. Although physiological traits of hypoxia tolerance in cetaceans have been well characterized, the underlying molecular mechanisms remain unknown. We investigated the sequences of 17 hypoxia-tolerance-related genes in representative cetaceans to provide a comprehensive insight into the genetic basis of hypoxia tolerance in these animals. Genes involved in carrying and transporting oxygen in the blood and muscle (hemoglobin-α and β, myoglobin), and genes involved in the regulation of vasoconstriction (endothelin-1, -2, and -3; endothelin receptor type A and B; adrenergic receptor α-1D; and arginine vasopressin) appear to have undergone adaptive evolution, evidence for positive selection on their particular sites, and radical physiochemical property changes of selected condons. Interestingly, “long-diving” cetaceans had relatively higher ω (d_N/d_S) values than “short-diving” cetaceans for the hemoglobin β gene, indicating divergent selective pressure presented in cetacean lineages with different diving abilities. Additionally, parallel positive selection or amino acid changes (ADRA1D: P50A, A53G, AVPR1B: I/V270T) among animals exposed to different hypoxia habitats reflect functional convergence or similar genetic mechanisms of hypoxia tolerance. In summary, positive selection, divergent selective pressures, and parallel evolution at the molecular level provided some new insights into the genetic adaptation of hypoxia tolerance.

Hypoxia-inducible factor-1 mediates adaptive developmental plasticity of hypoxia tolerance in zebrafish, Danio rerio

In recent years, natural and anthropogenic factors have increased aquatic hypoxia the world over. In most organisms, the cellular response to hypoxia is mediated by the master regulator hypoxia-inducible factor-1 (HIF-1). HIF-1 also plays a critical role in the normal development of the cardiovascular system of vertebrates. We tested the hypothesis that hypoxia exposures which resulted in HIF-1 induction during embryogenesis would be associated with enhanced hypoxia tolerance in subsequent developmental stages. We exposed zebrafish (Danio rerio) embryos to just 4 h of severe hypoxia or total anoxia at 18, 24 and 36 h post-fertilization (hpf). Of these, exposure to hypoxia at 24 and 36 hpf as well as anoxia at 36 hpf activated the HIF-1 cellular pathway. Zebrafish embryos that acutely upregulated the HIF-1 pathway had an increased hypoxia tolerance as larvae. The critical window for hypoxia sensitivity and HIF-1 signalling was 24 hpf. Adult male fish had a lower critical oxygen tension (Pcrit) compared with females. Early induction of HIF-1 correlated directly with an increased proportion of males in the population. We conclude that mounting a HIF-1 response during embryogenesis is associated with long-term impacts on the phenotype of later stages which could influence both individual hypoxia tolerance and population dynamics.

A product of its environment: the epaulette shark (Hemiscyllium ocellatum) exhibits physiological tolerance to elevated environmental CO2

Ocean acidification, resulting from increasing anthropogenic CO2 emissions, is predicted to affect the physiological performance of many marine species. Recent studies have shown substantial reductions in aerobic performance in some teleost fish species, but no change or even enhanced performance in others. Notably lacking, however, are studies on the effects of near-future CO2 conditions on larger meso and apex predators, such as elasmobranchs. The epaulette shark (Hemiscyllium ocellatum) lives on shallow coral reef flats and in lagoons, where it may frequently encounter short-term periods of environmental hypoxia and elevated CO2, especially during nocturnal low tides. Indeed, H. ocellatum is remarkably tolerant to short periods (hours) of hypoxia, and possibly hypercapnia, but nothing is known about its response to prolonged exposure. We exposed H. ocellatum individuals to control (390 µatm) or one of two near-future CO2 treatments (600 or 880 µatm) for a minimum of 60 days and then measured key aspects of their respiratory physiology, namely the resting oxygen consumption rate, which is used to estimate resting metabolic rate, and critical oxygen tension, a proxy for hypoxia sensitivity. Neither of these respiratory attributes was affected by the long-term exposure to elevated CO2. Furthermore, there was no change in citrate synthase activity, a cellular indicator of aerobic energy production. Plasma bicarbonate concentrations were significantly elevated in sharks exposed to 600 and 880 µatm CO2 treatments, indicating that acidosis was probably prevented by regulatory changes in acid-base relevant ions. Epaulette sharks may therefore possess adaptations that confer tolerance to CO2 levels projected to occur in the ocean by the end of this century. It remains uncertain whether other elasmobranchs, especially pelagic species that do not experience such diurnal fluctuations in their environment, will be equally tolerant.

Climate sensitivity across marine domains of life: limits to evolutionary adaptation shape species interactions

Organisms in all domains, Archaea, Bacteria, and Eukarya will respond to climate change with differential vulnerabilities resulting in shifts in species distribution, coexistence, and interactions. The identification of unifying principles of organism functioning across all domains would facilitate a cause and effect understanding of such changes and their implications for ecosystem shifts. For example, the functional specialization of all organisms in limited temperature ranges leads us to ask for unifying functional reasons. Organisms also specialize in either anoxic or various oxygen ranges, with animals and plants depending on high oxygen levels. Here, we identify thermal ranges, heat limits of growth, and critically low (hypoxic) oxygen concentrations as proxies of tolerance in a meta-analysis of data available for marine organisms, with special reference to domain-specific limits. For an explanation of the patterns and differences observed, we define and quantify a proxy for organismic complexity across species from all domains. Rising complexity causes heat (and hypoxia) tolerances to decrease from Archaea to Bacteria to uni- and then multicellular Eukarya. Within and across domains, taxon-specific tolerance limits likely reflect ultimate evolutionary limits of its species to acclimatization and adaptation. We hypothesize that rising taxon-specific complexities in structure and function constrain organisms to narrower environmental ranges. Low complexity as in Archaea and some Bacteria provide life options in extreme environments. In the warmest oceans, temperature maxima reach and will surpass the permanent limits to the existence of multicellular animals, plants and unicellular phytoplankter. Smaller, less complex unicellular Eukarya, Bacteria, and Archaea will thus benefit and predominate even more in a future, warmer, and hypoxic ocean.

Roles of air stored in burrows of the mudskipper Boleophthalmus pectinirostris for adult respiration and embryonic development

Air was stored in 90% of Boleophthalmus pectinirostris burrows in summer breeding months when fish were active on the mudflat surface during low tide but only in 50% of burrows in overwintering months when the fish confined themselves to burrows. The volume of gas recovered from the burrows ranged from 30 to > 400 ml. The partial pressure of oxygen (PO₂) of the gas varied from 5 to 20 kPa and was inversely related to the partial pressure of carbon dioxide (PCO₂) in all but the wintering periods. Sampling in July demonstrated that gas was stored in both male and female burrows with no difference in volume, PO₂ or PCO₂ between them. Adult fish were able to survive total submersion in hypoxic (PO₂ = 1.96 kPa) water for 8 h, but no embryos survived to hatch in the hypoxic aquatic environment. Thus, the deposited air appears to be a crucial source of oxygen for the embryos developing in the egg chamber of the burrow, but may play only a subsidiary role for adult respiration during presumed high-tide confinement.

anaerobic scope: sibling species of fish indicate that temperature dependence of hypoxia tolerance can predict future survival

The temperature dependence of aerobic scope has been suggested to be a major determinant of how marine animals will cope with future rises in environmental temperature. Here, we present data suggesting that in some animals, the temperature dependence of anaerobic scope (i.e., the capacity for surviving severe hypoxia) may determine present-day latitudinal distributions and potential for persistence in a warmer future. As a model for investigating the role of anaerobic scope, we studied two sibling species of coral-dwelling gobies, Gobiodon histrio, and G. erythrospilus, with different latitudinal distributions, but which overlap in equal abundance at Lizard Island (14°40'S) on the Great Barrier Reef. These species did not differ in the temperature dependence of resting oxygen consumption or critical oxygen concentration (the lowest oxygen level where resting oxygen consumption can be maintained). In contrast, the more equatorial species (G. histrio) had a better capacity to endure anaerobic conditions at oxygen levels below the critical oxygen concentration at the high temperatures (32-33 °C) more likely to occur near the equator, or in a warmer future. These results suggest that anaerobic scope, in addition to aerobic scope, could be important in determining the impacts of global warming on some marine animals.

Low oxygen tolerance of different life stages of temperate freshwater fish species

Data on low dissolved oxygen (DO₂) tolerance of freshwater fish species of north-western Europe were used to create species sensitivity distributions (SSD). Lowest observed effect concentrations (LOEC) and 100% lethal concentrations (LC₁₀₀) data were collected from the scientific literature. Comparisons were made among life stages as well as between native and exotic species. In addition, lethal DO₂ concentrations were compared to oxygen concentrations corresponding to maximum tolerable water temperatures of the same species. Fish eggs and embryos were the least tolerant. Juveniles had a significantly lower mean LOEC than adults, but there was no difference in mean LC₁₀₀ between the two groups. The difference in lethal oxygen concentrations between adults and juveniles was largest for three salmonids, although it remains uncertain if this was a result of smoltification. There were no significant differences between native and exotic species; however, data on exotics are limited. DO₂ concentrations converted from maximum tolerable water temperatures were 3·9 times higher than the measured lethal DO₂ concentrations, which may reflect changes in respiration rates (Q₁₀) and may also relate to the simplicity of the model used.

Genetic animal models of preconditioning

The preconditioning phenomena have been well established in heart as well as brain. In this review, we detail some of the original studies on preconditioning as well as studies from our lab using rodents and a genetic model system (fruit fly). We have used Drosophila in our lab to solve some of the questions related to tolerance or susceptibility to hypoxia. We believe that these pro-survival strategies and genetic pathways help us understand some of the preconditioning mechanisms that protect the brain from ischemia or ischemia.

Phenotypic plasticity and genetic adaptation to high-altitude hypoxia in vertebrates

High-altitude environments provide ideal testing grounds for investigations of mechanism and process in physiological adaptation. In vertebrates, much of our understanding of the acclimatization response to high-altitude hypoxia derives from studies of animal species that are native to lowland environments. Such studies can indicate whether phenotypic plasticity will generally facilitate or impede adaptation to high altitude. Here, we review general mechanisms of physiological acclimatization and genetic adaptation to high-altitude hypoxia in birds and mammals. We evaluate whether the acclimatization response to environmental hypoxia can be regarded generally as a mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to genetic adaptation. In cases in which the acclimatization response to hypoxia is maladaptive, selection will favor an attenuation of the induced phenotypic change. This can result in a form of cryptic adaptive evolution in which phenotypic similarity between high- and low-altitude populations is attributable to directional selection on genetically based trait variation that offsets environmentally induced changes. The blunted erythropoietic and pulmonary vasoconstriction responses to hypoxia in Tibetan humans and numerous high-altitude birds and mammals provide possible examples of this phenomenon. When lowland animals colonize high-altitude environments, adaptive phenotypic plasticity can mitigate the costs of selection, thereby enhancing prospects for population establishment and persistence. By contrast, maladaptive plasticity has the opposite effect. Thus, insights into the acclimatization response of lowland animals to high-altitude hypoxia can provide a basis for predicting how altitudinal range limits might shift in response to climate change.

From record performance to hypoxia tolerance: respiratory transition in damselfish larvae settling on a coral reef

The fastest swimming fishes in relation to size are found among coral reef fish larvae on their way to settle on reefs. By testing two damselfishes, Chromis atripectoralis and Pomacentrus amboinensis, we show that the high swimming speeds of the pre-settlement larvae are accompanied by the highest rates of oxygen uptake ever recorded in ectothermic vertebrates. As expected, these high rates of oxygen uptake occur at the cost of poor hypoxia tolerance. However, hypoxia tolerance is needed when coral reef fishes seek nocturnal shelter from predators within coral colonies, which can become severely hypoxic microhabitats at night. When the larvae settle on the reef, we found that they go through a striking respiratory transformation, i.e. the capacity for rapid oxygen uptake falls, while the ability for high-affinity oxygen uptake at low oxygen levels is increased. This transition to hypoxia tolerance is needed when they settle on the reef; this was strengthened by our finding that small resident larvae of Acanthochromis polyacanthus, a damselfish lacking a planktonic larval stage, do not display such a transition, being well adapted to hypoxia and showing relatively low maximum rates of oxygen uptake that change little with age.