Hypoxic survival strategies in two fishes: extreme anoxia tolerance in the North European crucian carp and natural hypoxic preconditioning in a coral-reef shark

Neural Processing without O2 and Glucose Delivery: Lessons from the Pond to the Clinic

Neuronal activity requires a large amount of ATP, leading to a rapid collapse of brain function when aerobic respiration fails. Here, we summarize how rhythmic motor circuits in the brain stem of adult frogs, which normally have high metabolic demands, transform to produce proper output during severe hypoxia associated with emergence from hibernation. We suggest that general principles underlying plasticity in brain bioenergetics may be uncovered by studying nonmammalian models that face extreme environments, yielding new insights to combat neurological disorders involving dysfunctional energy metabolism.

Differential production of mitochondrial reactive oxygen species between mouse (Mus musculus) and crucian carp (Carassius carassius)

AIM

In most vertebrates, oxygen deprivation and subsequent re-oxygenation are associated with mitochondrial impairment and excess production of reactive oxygen species (ROS) like hydrogen peroxide (H2O2). This in turn triggers a cascade of cell-damaging events in a temperature-dependent manner. The crucian carp (Carassius carassius) is one of few vertebrates that survives months without oxygen at cold temperatures and overcomes oxidative damage during re-oxygenation periods. Mitochondria of this anoxia-tolerant species therefore serve as an excellent model in translational research to study adaptation and resilience to low oxygen conditions and thermal variability.

METHODS

Here, we used high-resolution respirometry on isolated mitochondria from hearts of crucian carp and the anoxia-intolerant mouse (Mus musculus), at 37 and 8°C; two temperatures relevant for transplantation medicine (i.e., graft preservation and subsequent rewarming).

RESULTS

We find: (1) a striking difference in H2O2 release between the two species at 37°C despite comparable mitochondrial efficiency and capacity, (2) a massive H2O2 release after inhibition of complex V in mouse at 37°C that is absent in crucian carp, and prevented in mouse by incubation at 8°C or uncoupling with a protonophore at 37°C, and (3) indications that differences in mitochondrial complex I and II capacity and thermal sensitivity influence the release of mitochondrial H2O2 relative to respiration.

CONCLUSION

Our findings provide comparative insights into a spectrum of mitochondrial adaptations in vertebrates and the importance of thermal variability. Furthermore, the species- and temperature-related changes associated with mitochondria highlighted in this study may help identify mitochondria-based targets for translational medicine.

The vulnerability of sharks, skates, and rays to ocean deoxygenation: Physiological mechanisms, behavioral responses, and ecological impacts

Levels of dissolved oxygen in open ocean and coastal waters are decreasing (ocean deoxygenation), with poorly understood effects on marine megafauna. All of the more than 1000 species of elasmobranchs (sharks, skates, and rays) are obligate water breathers, with a variety of life-history strategies and oxygen requirements. This review demonstrates that although many elasmobranchs typically avoid hypoxic water, they also appear capable of withstanding mild to moderate hypoxia with changes in activity, ventilatory responses, alterations to circulatory and hematological parameters, and morphological alterations to gill structures. However, such strategies may be insufficient to withstand severe, progressive, or prolonged hypoxia or anoxia where anaerobic metabolic pathways may be used for limited periods. As water temperatures increase with climate warming, ectothermic elasmobranchs will exhibit elevated metabolic rates and are likely to be less able to tolerate the effects of even mild hypoxia associated with deoxygenation. As a result, sustained hypoxic conditions in warmer coastal or surface-pelagic waters are likely to lead to shifts in elasmobranch distributions. Mass mortalities of elasmobranchs linked directly to deoxygenation have only rarely been observed but are likely underreported. One key concern is how reductions in habitat volume as a result of expanding hypoxia resulting from deoxygenation will influence interactions between elasmobranchs and industrial fisheries. Catch per unit of effort of threatened pelagic sharks by longline fisheries, for instance, has been shown to be higher above oxygen minimum zones compared to adjacent, normoxic regions, and attributed to vertical habitat compression of sharks overlapping with increased fishing effort. How a compound stressor such as marine heatwaves alters vulnerability to deoxygenation remains an open question. With over a third of elasmobranch species listed as endangered, a priority for conservation and management now lies in understanding and mitigating ocean deoxygenation effects in addition to population declines already occurring from overfishing.

Phosphoproteomic changes in response to anoxia are tissue-specific in the anoxia-tolerant crucian carp (Carassius carassius)

Crucian carp (Carassius carassius), a freshwater fish, can survive chronic anoxia for several months at low temperatures. Consequently, anoxia-related physiological and biochemical adaptations in this species have been studied for more than half a century. Still, despite for the well-known role of protein phosphorylation in regulating cellular processes, no studies have comprehensively characterized the phosphoproteome in crucian carp. In this study, we report the global phosphoproteome in crucian carp brain and liver during anoxia and reoxygenation. By applying a bottom-up proteomic approach on enriched phosphopeptides we found that the brain phosphoproteome shows surprisingly few changes during anoxia-reoxygenation exposure with only 109 out of 4200 phosphopeptides being differentially changed compared to normoxic controls. By contrast, in the liver 395 out of 1287 phosphopeptides changed. Although most changes occurred in the liver phosphoproteome, the pattern of changes indicated metabolic depression and decreased translation in both brain and liver. We also found changes in phosphoproteins involved in apoptotic regulation and reactive oxygen species handling in both tissues. In the brain, some of the most changed phosphopeptides belonged to proteins involved in central nervous system development and neuronal activity at the synaptic cleft. Changed phosphoproteins specific for liver tissue were related to glucose metabolism, such as glycolytic flux and glycogenolysis. In conclusion, protein phosphorylation in response to anoxia and reoxygenation showed both common and tissue-specific changes related to the functional differences between brain and liver.

Gulf toadfish (Opsanus beta) gill neuroepithelial cells in response to hypoxia exposure

Neuroepithelial cells (NECs) within the fish gill contain the monoamine neurochemical serotonin (5-HT), sense changes in the partial pressure of oxygen (PO2) in the surrounding water and blood, and initiate the cardiovascular and ventilatory responses to hypoxia. The distribution of neuroepithelial cells (NECs) within the gill is known for some fish species but not for the Gulf toadfish, Opsanus beta, a fish that has always been considered hypoxia tolerant. Furthermore, whether NEC size, number, or distribution changes after chronic exposure to hypoxia, has never been tested. We hypothesize that toadfish NECs will respond to hypoxia with an increase in NEC size, number, and a change in distribution. Juvenile toadfish (N = 24) were exposed to either normoxia (21.4 ± 0.0 kPa), mild hypoxia (10.2 ± 0.3 kPa), or severe hypoxia (3.1 ± 0.2 kPa) for 7 days and NEC size, number, and distribution for each O2 regime were measured. Under normoxic conditions, juvenile toadfish have similar NEC size, number, and distribution as other fish species with NECs along their filaments but not throughout the lamellae. The distribution of NECs did not change with hypoxia exposure. Mild hypoxia exposure had no effect on NEC size or number, but fish exposed to severe hypoxia had a higher NEC density (# per mm filament) compared to mild hypoxia-exposed fish. Fish exposed to severe hypoxia also had longer gill filament lengths that could not be explained by body weight. These results point to signs of phenotypic plasticity in these juvenile, lab-bred fish with no previous exposure to hypoxia and a strategy to deal with hypoxia exposure that differs in toadfish compared to other fish.

Evolution of a novel regulatory mechanism of hypoxia inducible factor in hypoxia-tolerant electric fishes

Hypoxia is a significant source of metabolic stress that activates many cellular pathways involved in cellular differentiation, proliferation, and cell death. Hypoxia is also a major component in many human diseases and a known driver of many cancers. Despite the challenges posed by hypoxia, there are animals that display impressive capacity to withstand lethal levels of hypoxia for prolonged periods of time and thus offer a gateway to a more comprehensive understanding of the hypoxic response in vertebrates. The weakly electric fish genus Brachyhypopomus inhabits some of the most challenging aquatic ecosystems in the world, with some species experiencing seasonal anoxia, thus providing a unique system to study the cellular and molecular mechanisms of hypoxia tolerance. In this study, we use closely related species of Brachyhypopomus that display a range of hypoxia tolerances to probe for the underlying molecular mechanisms via hypoxia inducible factors (HIFs)-transcription factors known to coordinate the cellular response to hypoxia in vertebrates. We find that HIF1⍺ from hypoxia tolerant Brachyhypopomus species displays higher transactivation in response to hypoxia than that of intolerant species, when overexpressed in live cells. Moreover, we identified two SUMO-interacting motifs near the oxygen-dependent degradation and transactivation domains of the HIF1⍺ protein that appear to boost transactivation of HIF1, regardless of the genetic background. Together with computational analyses of selection, this shows that evolution of HIF1⍺ are likely to underlie adaptations to hypoxia tolerance in Brachyhypopomus electric fishes, with changes in two SUMO-interacting motifs facilitating the mechanism of this tolerance.

Comparison of hypoxia- and hyperoxia-induced alteration of epigene expression pattern in lungs of Pleurodeles waltl and Mus musculus

Epigenetics is an emerging field of research because of its involvement in susceptibility to diseases and aging. Hypoxia and hyperoxia are known to be involved widely in various pathophysiologies. Here, we compared the differential epigene expression pattern between Pleurodeles waltl and Mus musculus (commonly known as Iberian ribbed newt and mouse, respectively) exposed to hypoxia and hyperoxia. Adult healthy newts and mice were exposed to normobaric hypoxia (8% O2) and hyperoxia (80% O2) for 2 hours. We collected the lungs and analyzed the expression of hypoxia-inducible factor 1 alpha (Hif1α) and several key epigenes from DNA methyltransferase (DNMT) family, histone deacetylase (HDAC) family, and methyl-CpG binding domain (MBD) family. The exposure to hypoxia significantly increased the mRNA levels of DNA methyltransferase 3 alpha (Dnmt3α), methyl-CpG binding domain protein 2 (Mbd2), Mbd3, and histone deacetylase 2 (Hdac2) in lungs of newts, but decreased the mRNA levels of DNA methyltransferase 1 (Dnmt1) and Dnmt3α in lungs of mice. The exposure to hyperoxia did not significantly change the expression of any gene in either newts or mice. The differential epigene expression pattern in response to hypoxia between newts and mice may provide novel insights into the prevention and treatment of disorders developed due to hypoxia exposure.

Resilience and phenotypic plasticity of Arctic char (Salvelinus alpinus) facing cyclic hypoxia: insights into growth, energy stores and hepatic metabolism

Arctic char (Salvelinus alpinus) is facing the decline of its southernmost populations due to several factors including rising temperatures and eutrophication. These conditions are also conducive to episodes of cyclic hypoxia, another possible threat to this species. In fact, lack of oxygen and reoxygenation can both have serious consequences on fish as a result of altered ATP balance and an elevated risk of oxidative burst. Thus, fish must adjust their phenotype to survive and equilibrate their energetic budget. However, their energy allocation strategy could imply a reduction in growth which could be deleterious for their fitness. Although the impact of cyclic hypoxia is a major issue for ecosystems and fisheries worldwide, our knowledge on how salmonid deal with high oxygen fluctuations remains limited. Our objective was to characterize the effects of cyclic hypoxia on growth and metabolism in Arctic char. We monitored growth parameters (specific growth rate, condition factor), hepatosomatic and visceral indexes, relative heart mass and hematocrit of Arctic char exposed to 30 days of cyclic hypoxia. We also measured the hepatic protein synthesis rate, hepatic triglycerides as well as muscle glucose, glycogen and lactate and quantified hepatic metabolites during this treatment. The first days of cyclic hypoxia slightly reduce growth performance with a downward trend in specific growth rate in mass and condition factor variation compared to the control group. This acute exposure also induced a profound metabolome reorganization in the liver with an alteration of amino acid, carbohydrate and lipid metabolisms. However, fish rebalanced their metabolic activities and successfully maintained their growth and energetic reserves after 1 month of cyclic hypoxia. These results demonstrate the impressive ability of Arctic char to cope with its changing environment but also highlight a certain vulnerability of this species during the first days of a cyclic hypoxia event.

Thermal sensitivity of metabolic rate mirrors biogeographic differences between teleosts and elasmobranchs

Environmental temperature affects physiological functions, representing a barrier for the range expansions of ectothermic species. To understand the link between thermal physiology and biogeography, a key question is whether among-species thermal sensitivity of metabolic rates is mechanistically constrained or buffered through physiological remodeling over evolutionary time. The former conception, the Universal Temperature Dependence hypothesis, predicts similar among- and within-species thermal sensitivity. The latter conception, the Metabolic Cold Adaptation hypothesis, predicts lower among-species thermal sensitivity than within-species sensitivity. Previous studies that tested these hypotheses for fishes overwhelmingly investigated teleosts with elasmobranchs understudied. Here, we show that among-species thermal sensitivity of resting metabolic rates is lower than within-species sensitivity in teleosts but not in elasmobranchs. Further, species richness declines with latitude more rapidly in elasmobranchs than in teleosts. Metabolic Cold Adaptation exhibited by teleosts might underpin their high diversity at high latitudes, whereas the inflexible thermal sensitivity approximated by Universal Temperature Dependence of elasmobranchs might explain their low diversity at high latitudes.

Metabolites, gene expression, and gut microbiota profiles suggest the putative mechanisms via which dietary creatine increases the serum taurine and g-ABA contents in Megalobrama amblycephala

A 90-day experiment was conducted to explore the effects of creatine on growth performance, liver health status, metabolites, and gut microbiota in Megalobrama amblycephala. There were 6 treatments as follows: control (CD, 29.41% carbohydrates), high carbohydrate (HCD, 38.14% carbohydrates), betaine (BET, 1.2% betaine + 39.76% carbohydrates), creatine 1 (CRE1, 0.5% creatine + 1.2% betaine + 39.29% carbohydrates), creatine 2 (CRE2, 1% creatine + 1.2% betaine + 39.50% carbohydrates), and creatine 3 (CRE3, 2% creatine + 1.2% betaine + 39.44% carbohydrates). The results showed that supplementing creatine and betaine together reduced the feed conversion ratio significantly (P < 0.05, compared to CD and HCD) and improved liver health (compared to HCD). Compared with the BET group, dietary creatine significantly increased the abundances of Firmicutes, Bacteroidota, ZOR0006, and Bacteroides and decreased the abundances of Proteobacteria, Fusobacteriota, Vibrio, Crenobacter, and Shewanella in the CRE1 group. Dietary creatine increased the content of taurine, arginine, ornithine, γ-aminobutyric acid (g-ABA), and creatine (CRE1 vs. BET group) and the expression of creatine kinase (ck), sulfinoalanine decarboxylase (csad), guanidinoacetate N-methyltransferase (gamt), glycine amidinotransferase (gatm), agmatinase (agmat), diamine oxidase1 (aoc1), and glutamate decarboxylase (gad) in the CRE1 group. Overall, these results suggested that dietary supplementation of creatine (0.5-2%) did not affect the growth performance, but it altered the gut microbial composition at the phylum and genus levels, which might be beneficial to the gut health of M. amblycephala; dietary creatine also increased the serum content of taurine by enhancing the expressions of ck and csad and increased the serum content of g-ABA by enhancing the arginine content and the expressions of gatm, agmat, gad, and aoc1.

Comprehensive transcriptional and metabolomic analysis reveals the neuroprotective mechanism of dietary gamma-aminobutyric acid response to hypoxic stress in the Chinese mitten crab (Eriocheir sinensis)

Hypoxia is one of the serious stress challenges that aquatic animals face throughout their life. Our previous study found that hypoxia stress could induce neural excitotoxicity and neuronal apoptosis in Eriocheir sinensis, and observed that gamma-aminobutyric acid (GABA) has a positive neuroprotective effect on juvenile crabs under hypoxia. To reveal the neuroprotective pathway and metabolic regulatory mechanism of GABA in E. sinensis exposed to hypoxia stress, an 8-week feeding trial and acute hypoxia challenge were performed. Subsequently, we performed a comprehensive transcriptomic and metabolomic analysis of the thoracic ganglia of juvenile crabs. Differential genes and differential metabolites were co-annotated to 11 KEGG pathways, and further significant analysis showed that only the sphingolipid signaling pathway and the arachidonic acid metabolism pathway were significantly enriched. In the sphingolipid signaling pathway, GABA treatment significantly increased long-chain ceramide content in thoracic ganglia, which exerted neuroprotective effects by activating downstream signals to inhibit hypoxia-induced apoptosis. Moreover, in the arachidonic acid metabolism pathway, GABA could increase the content of neuroprotective active substances and reduce the content of harmful metabolites by regulating the metabolism of arachidonic acid for inflammatory regulation and neuroprotection. Furthermore, the decrease of glucose and lactate levels in the hemolymph suggests the positive role of GABA in metabolic regulation. This study reveals the neuroprotective pathways and possible mechanisms of GABA in juvenile E. sinensis exposed to hypoxia stress and inspires the discovery of new targets for improving hypoxia tolerance in aquatic animals.

Effects of hypoxia and reoxygenation on oxidative stress, histological structure, and apoptosis in a new hypoxia-tolerant variety of blunt snout bream (Megalobrama amblycephala)

A new hypoxia-tolerant variety of blunt snout bream was obtained by successive breeding of the wild population, which markedly improved hypoxia tolerance. In this study, the hypoxia-tolerant variety was exposed to hypoxia (2.0 mg O₂·L^-1) for 4, 7 days. The contents of blood biochemical indicators including the number of red blood cells (RBC), total cholesterol (T-CHO), total protein (TP), triglyceride (TG), glucose (GLU), and lactic acid (LD) increased significantly (P < 0.05) under hypoxia. The glycogen content in the liver and muscle decreased significantly (P < 0.05) and the LD content in the brain, muscle and liver increased significantly (P < 0.05) under hypoxia. The levels of oxidative stress-related indicators i.e., superoxide dismutase (SOD), malondialdehyde (MDA), glutathione (GSH), catalase (CAT), and total antioxidant capacity (T-AOC) also changed significantly (P < 0.05) in the heart, liver, and intestine of the new variety under hypoxia. Additionally, hypoxia has caused injuries to the heart, liver, and intestine, but it shows amazing repair ability during reoxygenation. The apoptotic cells and apoptosis rate in the heart, liver, and intestine increased under hypoxia. Under hypoxia, the expression of the B-cell lymphomas 2 (Bcl-2) gene in the heart, liver, and intestine was significantly (P < 0.05) down-regulated, while the expression of the BCL2-associated agonist of cell death (Bad) gene was significantly (P < 0.05) up-regulated. These results are of great significance for enriching the basic data of blunt snout bream new variety in response to hypoxia and promoting the healthy development of its culture industry.

Functional exploration of SNP mutations in HIF2αb gene correlated with hypoxia tolerance in blunt snout bream (Megalobrama amblycephala)

Blunt snout bream (Megalobrama amblycephala) is sensitive to hypoxia environment. Hypoxia-inducible factor (HIF) is the most critical factor in the HIF pathway, which strictly regulates the hypoxia stress process of fish. In this study, we found six hifα genes in blunt snout bream that demonstrated different expressions under hypoxia conditions. In HEK293T cells, all six hifαs were detected to activate the HRE region by luciferase reporter assay. More importantly, we identified two linkage-disequilibrium SNP sites at exon 203 and 752 of the hif2αb gene in blunt snout bream. Haplotype II (A²⁰³A⁷⁵²) and its homozygous diplotype II (A²⁰³A²⁰³A⁷⁵²A⁷⁵²) appeared frequently in a selected strain of blunt snout bream with hypoxia tolerance. Diplotype II has a lower oxygen tension threshold for loss of equilibrium (LOE_crit) over a similar range of temperatures. Moreover, its erythrocyte number increased significantly (p < 0.05) than those in diplotype I and diplotype III strains at 48 h of hypoxia. The enzymes related with hypoxia tolerant traits, i.e., reduced glutathione, superoxide dismutase, and catalase, were also significantly (p < 0.05) induced in diplotype II than in diplotype I or III. In addition, the expression of epo in the liver of diplotype II was significantly (p < 0.01) higher than that in the diplotype I or III strains at 48 h of hypoxia. Taken together, our results found that the hypoxia-tolerant-related diplotype II of hif2αb has the potential to be used as a molecular marker in future genetic breeding of hypoxia-tolerant strain.

Surviving without oxygen involves major tissue specific changes in the proteome of crucian carp (Carassius carassius)

The crucian carp (Carassius carassius) can survive complete oxygen depletion (anoxia) for several months at low temperatures, making it an excellent model for studying molecular adaptations to anoxia. Still, little is known about how its global proteome responds to anoxia and reoxygenation. By applying mass spectrometry-based proteome analyses on brain, heart and liver tissue from crucian carp exposed to normoxia, five days anoxia, and reoxygenation, we found major changes in particularly cardiac and hepatic protein levels in response to anoxia and reoxygenation. These included tissue-specific differences in mitochondrial proteins involved in aerobic respiration and mitochondrial membrane integrity. Enzymes in the electron transport system (ETS) decreased in heart and increased massively in liver during anoxia and reoxygenation but did not change in the brain. Importantly, the data support a special role for the liver in succinate handling upon reoxygenation, as suggested by a drastic increase of components of the ETS and uncoupling protein 2, which could allow for succinate metabolism without excessive formation of reactive oxygen species (ROS). Also during reoxygenation, the levels of proteins involved in the cristae junction organization of the mitochondria changed in the heart, possibly functioning to suppress ROS formation. Furthermore, proteins involved in immune (complement) system activation changed in the anoxic heart compared to normoxic controls. The results emphasize that responses to anoxia are highly tissue-specific and related to organ function.

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.

Gamma-aminobutyric acid enhances hypoxia tolerance of juvenile Chinese mitten crab (Eriocheir sinensis) by regulating respiratory metabolism and alleviating neural excitotoxicity

With climate change and intensive aquaculture development, environmental hypoxia in aquaculture water has become a common challenge for many aquatic species. Therefore, it is crucial to improve the hypoxic tolerance of animals through nutritional strategies. This study explored the positive role of dietary gamma-aminobutyric acid (GABA) supplementation in enhancing hypoxia tolerance of juvenile Eriocheir sinensis through respiratory regulation and alleviation of hypoxia-induced neural excitotoxicity. Acute hypoxia stress significantly up-regulated the mRNA expression level of hypoxia-inducible factor 1α, oxygen consumption rate and anaerobic respiratory metabolism-related enzyme activities. On the other hand, aerobic respiratory metabolism-related enzyme activities were significantly decreased. However, dietary GABA supplementation remodeled the respiratory metabolism pattern of juvenile crabs exposed to hypoxia stress. In addition, acute hypoxic stress significantly increased the contents of free glutamate and GABA in the nervous tissue. The expression levels of N-Methyl-d-aspartate-related receptor genes and calcium-dependent degradation enzyme-related genes were significantly up-regulated. Similarly, neuronal apoptosis rates, expression levels of apoptosis-related genes, and vesicular glutamate transporter genes were also significantly increased. The high-affinity neuronal glutamate transporter decreased significantly in the crabs exposed to hypoxia stress. However, dietary GABA supplementation could effectively prevent acute hypoxia stress-induced neural excitotoxicity. Furthermore, dietary GABA could significantly improve the redox status in vivo exposed to hypoxia stress. In conclusion, acute hypoxia stress can affect respiratory metabolism and redox state and induce neural excitotoxicity in juvenile E. sinensis. GABA supplementation could improve hypoxia tolerance through multiple physiological regulation pathways.

Thresholds for Reduction in Fish Growth and Consumption Due to Hypoxia: Implications for Water Quality Guidelines to Protect Aquatic Life

Control of hypoxia is a key element of water quality management, and guidelines are usually based on qualitative reviews of hypoxia impacts. In this study we use segmented regression to identify both thresholds for growth reduction and rate of decline of fish growth and food consumption under hypoxia; and then evaluate whether current freshwater guidelines for dissolved oxygen based on qualitative reviews are consistent with the quantitative analysis of hypoxia thresholds. Segmented regressions were fit to data from published growth-hypoxia studies for freshwater (N = 17) and marine fishes (N = 13). To understand potential drivers of hypoxia tolerance, we also modelled thresholds as simple functions of environmental and ecological covariates for each species including trophic level, marine vs. freshwater environment, maximum fish length, fish weight, and maximum temperature tolerance. The average threshold for growth reduction (G_crit; 5.1 mg·l^-1 DO) and decreased food consumption (C_crit = 5.6 mg·l^-1 DO) were not significantly different, and did not differ between marine and freshwater taxa. However, salmonids showed a significantly steeper decline in growth with increasing hypoxia relative to other taxa. Growth declined by 22% for every mg·l^-1 reduction in DO below average G_crit, and significant regressions indicate that warmwater (R² = 0.25) and smaller-bodied (R² = 0.44) species are more likely to be hypoxia tolerant. Observed mean G_crit and C_crit in the range of 5-6 mg·l^-1 broadly match minimum water quality guidelines for the protection of aquatic life in freshwater in representative industrialized countries. However, this is much higher than the definition of hypoxia typically used in marine systems (2-2.5 mg·l^-1), indicating a need to reconcile definition of hypoxia in the marine environment with empirical data. The principal challenge in freshwater hypoxia management is now translating discretionary guidelines into effective regulatory frameworks to reduce the incidence and severity of hypoxia.

Effect of acute hypoxia on the brain energy metabolism of the scorpionfish Scorpaena porcus Linnaeus, 1758: the pattern of oxidoreductase activity and adenylate system

The activity of oxidoreductases, malate dehydrogenase and lactate dehydrogenase (MDH, 1.1.1.37; LDH, 1.1.1.27), as well as parameters of adenylate system-[ATP], [ADP], [AMP], total adenylate pool (AP), and adenylate energy charge (AEC) in medulla oblongata (MB) and forebrain, midbrain, and diencephalon (FDMB)-were studied in the scorpionfish under acute hypoxia (0.9-1.2 mg O₂·L^-1, 90 min). A higher MDH activity level was observed in MB and FDMB, as compared to LDH (p < 0.05). At the same time, MB showed a higher adenylate content and increased AP (p < 0.05). AEC did not exceed ~ 0.7 (vs. the maximum of this index ~ 0.9-1.0) in the brain of the scorpionfish indicating adaptation of the tissue energy status to hypoxia. A rapid decrease in MDH activity (p < 0.05) was observed in MB under acute hypoxia. These changes were accompanied by insignificant LDH activation. A pronounced LDH activation (p < 0.05), a decrease in MDH activity, and the highest AP raise (p < 0.05) were observed in FDMB, suggesting activation of glycolysis and simultaneous decrease in the rate of ATP consumption. MB and FDMB demonstrated the ability to a relative retention of AEC during hypoxia. The unidirectional metabolic adaptation was based on the intensification of glycolysis, a decrease of ATP consumption, and a subsequent increase in adenylate concentration that allowed the scorpionfish brain structures to maintain the energy status under acute hypoxia.

Gill remodeling increases the respiratory surface area of grass carp (Ctenopharyngodon idella) under hypoxic stress

Seasonal changes, diurnal variations, and eutrophication result in periodic hypoxia in fish habitats, thus affecting the success of commercial aquaculture. In this study, the grass carp (Ctenopharyngodon idella) presented moderate hypoxia tolerance; they showed a medium critical oxygen tension during the loss of equilibrium. In response to 7 d of hypoxic exposure, the erythrocyte count and hemoglobin (Hb) concentration significantly increased (p < 0.01). To cope with the hypoxic environment, the grass carp underwent gill remodeling marked by reduction in the interlamellar cell mass (ILCM) and an increase in respiratory surface area. The gill remodeling under hypoxia was enabled by apoptosis induction. Although apoptotic signals were not found on ILCM cells, transferase dUTP nick end labeling (TUNEL) assay results indicated that after 1 d of hypoxic exposure, the number of TUNEL-positive cells per lamella increased until 4 d and then began to decrease. Consistent with the results of the TUNEL assay, the mRNA expression of apoptosis-related genes, caspase-3, Bax, and Bcl-2, increased at 1, 4, and 7 d of the hypoxia treatment. In addition, gill remodeling significantly (p < 0.01) decreased the concentration of sodium and chloride ions in the fish serum. These findings provide evidence that grass carps increase their respiratory surface area through gill remodeling by apoptosis in the gill filaments to acclimate to a hypoxic environment. This study expands our understanding of the morphological and physiological changes in grass carp in response to a hypoxic environment; therefore, it could be useful for maintaining grass carp production.

Aerobic scope in fishes with different lifestyles and across habitats: Trade-offs among hypoxia tolerance, swimming performance and digestion

Exercise and aerobic scope in fishes have attracted scientists' attention for several decades. While it has been suggested that aerobic scope may limit behavioral expression and tolerance to environmental stressors in fishes, the exact importance of aerobic scope in an ecological context remains poorly understood. In this review, we examine the ecological relevance of aerobic scope by reconsidering and reanalyzing the existing literature on Chinese freshwater fishes across a wide-range of habitats and lifestyles. The available evidence suggests that natural selection in fast-flowing aquatic habitats may favor species with a high aerobic scope and anaerobic capacity for locomotion, whereas in relatively slow-flowing habitats, hypoxia tolerance may be favored at the cost of reduced locomotor capacity. In addition, while physical activity can usually cause fishes from fast-flowing habitats to reach their aerobic metabolic ceiling (i.e., maximum metabolic rate), possibly due to selection pressure on locomotion, most species from slow-flowing habitats can only reach their metabolic ceiling during digestion, either alone or in combination with physical activity. Overall, we suggest that fish exhibit a continuum of metabolic types, from a 'visceral metabolic type' with a higher digestive performance to a 'locomotion metabolic type' which appears to have reduced capacity for digestion but enhanced locomotor performance. Generally, locomotor-type species can either satisfy the demands of their high swimming capacity with a high oxygen uptake capacity or sacrifice digestion while swimming. In contrast, most visceral-type species show a pronounced decrease in swimming performance while digesting, probably owing to conflicts within their aerobic scope. In conclusion, the ecological relevance of aerobic scope and the consequent effects on other physiological functions are closely related to habitat and the lifestyle of a given species. These results suggest that swimming performance, digestion and hypoxia tolerance might coevolve due to dependence on metabolic traits such as aerobic scope.

What to do with low O2: Redox adaptations in vertebrates native to hypoxic environments

Reactive oxygen species (ROS) are important cellular signalling molecules but sudden changes in redox balance can be deleterious to cells and lethal to the whole organism. ROS production is inherently linked to environmental oxygen availability and many species live in variable oxygen environments that can range in both severity and duration of hypoxic exposure. Given the importance of redox homeostasis to cell and animal viability, it is not surprising that early studies in species adapted to various hypoxic niches have revealed diverse strategies to limit or mitigate deleterious ROS changes. Although research in this area is in its infancy, patterns are beginning to emerge in the suites of adaptations to different hypoxic environments. This review focuses on redox adaptations (i.e., modifications of ROS production and scavenging, and mitigation of oxidative damage) in hypoxia-tolerant vertebrates across a range of hypoxic environments. In general, evidence suggests that animals adapted to chronic lifelong hypoxia are in homeostasis, and do not encounter major oxidative challenges in their homeostatic environment, whereas animals exposed to seasonal chronic anoxia or hypoxia rapidly downregulate redox balance to match a hypometabolic state and employ robust scavenging pathways during seasonal reoxygenation. Conversely, animals adapted to intermittent hypoxia exposure face the greatest degree of ROS imbalance and likely exhibit enhanced ROS-mitigation strategies. Although some progress has been made, research in this field is patchy and further elucidation of mechanisms that are protective against environmental redox challenges is imperative for a more holistic understanding of how animals survive hypoxic environments.

Hypoxia-mediated immunotoxicity in the blood clam Tegillarca granosa

In marine ecosystems, dissolved oxygen (DO) is essential for maintaining intracellular energy balance during aerobic metabolism. Bivalve mollusks are frequently exposed to hypoxia environments due to tides, temperature changes, and anthropogenic activities. The blood clam, Tegillarca granosa, mainly inhabits intertidal mudflats and is more susceptible to low oxygen events. In this study, we investigated the effect of hypoxia on immune responses in clams, and showed that hypoxia exposure reduced total hemocyte counts (THC), hemoglobin concentrations, and intracellular reactive oxygen species (ROS) levels. Also, phagocytic and cell activities of hemocyte were significantly inhibited. Furthermore, immune-related gene expression was also down-regulated. In conclusion, hypoxia greatly affected immune functions in blood clams, and our research provided the foundation for further mechanistic studies on hypoxia tolerance in clams.

Hematogenesis Adaptation to Long-Term Hypoxia Acclimation in Zebrafish (Danio rerio)

When fish live in the wild or are cultured artificially, they will inevitably suffer from hypoxia. At the same time, blood physiological indexes represent the physiological state of fish. In order to study the effect of long-term hypoxia acclimation on fish hematogenesis, we cultured zebrafish embryos into adulthood in a hypoxia incubator (1.5 ± 0.2 mg/L). Then we compared the hematological parameters of zebrafish cultured in normoxia and hypoxia conditions. Transcriptome sequencing analysis of the main hematopoietic tissue, the head kidney, was also compared between the two groups. Results showed that the number of erythrocytes increased significantly in the long-term hypoxia acclimated group, while the size of several cell types, such as red blood cells, eosinophils, basophils, small lymphocytes and thrombocytes, decreased significantly. The transcriptomic comparisons revealed that there were 6475 differentially expressed genes (DEGs) between the two groups. A Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that hematopoiesis and cell proliferation signaling were the most significantly enriched pathways in the head kidney of hypoxia acclimated zebrafish. In addition, many genes involved in the hematopoietic process showed significantly higher levels of expression in the hypoxia acclimated zebrafish, when compared to the normoxia zebrafish. When considered together, these data allowed us to conclude that long-term hypoxia can promote the hematopoiesis process and cell proliferation signaling in the zebrafish head kidney, which resulted in higher red blood cell production. Higher numbers of red blood cells allow for better adaptation to the hypoxic environment. In conclusion, this study provides a basis for the in-depth understanding of the effects of hypoxia on hematogenesis in fish species.

Epigenetic and post-transcriptional repression support metabolic suppression in chronically hypoxic goldfish

Goldfish enter a hypometabolic state to survive chronic hypoxia. We recently described tissue-specific contributions of membrane lipid composition remodeling and mitochondrial function to metabolic suppression across different goldfish tissues. However, the molecular and especially epigenetic foundations of hypoxia tolerance in goldfish under metabolic suppression are not well understood. Here we show that components of the molecular oxygen-sensing machinery are robustly activated across tissues irrespective of hypoxia duration. Induction of gene expression of enzymes involved in DNA methylation turnover and microRNA biogenesis suggest a role for epigenetic transcriptional and post-transcriptional suppression of gene expression in the hypoxia-acclimated brain. Conversely, mechanistic target of rapamycin-dependent translational machinery activity is not reduced in liver and white muscle, suggesting this pathway does not contribute to lowering cellular energy expenditure. Finally, molecular evidence supports previously reported chronic hypoxia-dependent changes in membrane cholesterol, lipid metabolism and mitochondrial function via changes in transcripts involved in cholesterol biosynthesis, β-oxidation, and mitochondrial fusion in multiple tissues. Overall, this study shows that chronic hypoxia robustly induces expression of oxygen-sensing machinery across tissues, induces repressive transcriptional and post-transcriptional epigenetic marks especially in the chronic hypoxia-acclimated brain and supports a role for membrane remodeling and mitochondrial function and dynamics in promoting metabolic suppression.

Neural excitotoxicity and the toxic mechanism induced by acute hypoxia in Chinese mitten crab (Eriocheir sinensis)

Hypoxia can induce neural excitotoxicity in mammals, but this adverse effect has not been investigated in aquatic animals to date, especially in crustaceans. This study explored the induction effect and toxic mechanism of acute hypoxia stress (1.0 ± 0.1 mg dissolved oxygen /L) for 24 h on neural excitotoxicity in juvenile Chinese mitten crab, Eriocheir sinensis. The results showed that hemolymph glucose and serum lactic acid content were significantly increased, and the mRNA expression of crustacean hyperglycemic hormone and hypoxia-inducible factor 1α were significantly up-regulated in the hypoxia group compared with control. RNA-Seq results confirmed that acute hypoxia stress had a more significant impact on carbohydrate metabolism than lipid and protein metabolism. In addition, the TUNEL assay showed that the apoptosis rate of nerve cells was significantly higher in the hypoxia group than in the control, and similar trends were observed in the expression of apoptosis-related genes. RNA-Seq results also showed that acute hypoxia stress-induced neuronal apoptosis by regulating multiple apoptosis-related pathways. Moreover, free glutamate and GABA contents in the nerve tissue of thoracic ganglia were significantly higher in the hypoxia group than in the control group. Furthermore, the mRNA expression of NMDA related receptors was significantly up-regulated in the hypoxia group compared with the control. Similar trends were observed in the expression of calcium-dependent degrading enzymes and endogenous antioxidant-related proteins or enzymes. Meanwhile, the mRNA expression level of high-affinity neuronal glutamate transporter in the hypoxia group was significantly up-regulated compared with the control, whereas the vesicular glutamate transporter was significantly down-regulated. Furthermore, NMDA-R antagonists (MK-801 and Ro25-6981) injection showed that NMDA-R served as the bridge and core position of glutamate-induced neural neurotoxicity. This study provides a new perspective and theoretical guidance for exploring the regulation of hypoxic tolerance in E. sinensis.

Comparative transcriptome analysis provides novel insights into the molecular mechanism of the silver carp (Hypophthalmichthys molitrix) brain in response to hypoxia stress

The brain of fish plays an important role in regulating growth and adapting to environmental changes. However, few studies have been performed to address the changes in gene expression profiles in fish brains under hypoxic stress. In the present study, silver carp (Hypophthalmichthys molitrix) were kept under hypoxic experimental conditions by using the method of natural oxygen consumption, which resulted in a significant decrease in malondialdehyde (MDA) and glutathione (GSH) content and superoxide dismutase (SOD) activity in the brain. In addition, RNA sequencing (RNA-Seq) was performed to analyze transcriptional regulation in the brains of silver carp under normoxia (control group), hypoxia, semi-asphyxia, and asphyxia conditions. The results of KEGG enrichment pathway analysis showed that the immune system, such as antigen processing and presentation, natural killer cell-mediated cytotoxicity, was enriched in the hypoxia group; the nervous system (e.g., "glutamatergic synapse"), signal transduction (e.g., "calcium signaling pathway"; "foxo signaling pathway"), and signaling molecules and interactions (e.g., "neuroactive ligand-receptor interaction") were enriched in the semi-asphyxia group; and signaling molecules and interactions (e.g., "neuroactive ligand-receptor interaction") were enriched in the asphyxia group. These results provide novel insights into the molecular regulatory mechanism of the fish brain coping with hypoxia stress.

Adaptive strategies of sponges to deoxygenated oceans

Ocean deoxygenation is one of the major consequences of climate change. In coastal waters, this process can be exacerbated by eutrophication, which is contributing to an alarming increase in the so-called 'dead zones' globally. Despite its severity, the effect of reduced dissolved oxygen has only been studied for a very limited number of organisms, compared to other climate change impacts such as ocean acidification and warming. Here, we experimentally assessed the response of sponges to moderate and severe simulated hypoxic events. We ran three laboratory experiments on four species from two different temperate oceans (NE Atlantic and SW Pacific). Sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O₂  L^-1 , over 7-12-days). We found that sponges are generally very tolerant of hypoxia. All the sponges survived in the experimental conditions, except Polymastia crocea, which showed significant mortality at the lowest oxygen concentration (0.13 mg O₂  L^-1 , lethal median time: 286 h). In all species except Suberites carnosus, hypoxic conditions do not significantly affect respiration rate down to 0.4 mg O₂  L^-1 , showing that sponges can uptake oxygen at very low concentrations in the surrounding environment. Importantly, sponges displayed species-specific phenotypic modifications in response to the hypoxic treatments, including physiological, morphological and behavioural changes. This phenotypic plasticity likely represents an adaptive strategy to live in reduced or low oxygen water. Our results also show that a single sponge species (i.e., Suberites australiensis) can display different strategies at different oxygen concentrations. Compared to other sessile organisms, sponges generally showed higher tolerance to hypoxia, suggesting that sponges could be favoured and survive in future deoxygenated oceans.

The transcriptomic responses of blunt snout bream (Megalobrama amblycephala) to acute hypoxia stress alone, and in combination with bortezomib

BACKGROUND

Blunt snout bream (Megalobrama amblycephala) is sensitive to hypoxia. A new blunt snout bream strain, "Pujiang No.2", was developed to overcome this shortcoming. As a proteasome inhibitor, bortezomib (PS-341) has been shown to affect the adaptation of cells to a hypoxic environment. In the present study, bortezomib was used to explore the hypoxia adaptation mechanism of "Pujiang No.2". We examined how acute hypoxia alone (hypoxia-treated, HN: 1.0 mg·L^- 1), and in combination with bortezomib (hypoxia-bortezomib-treated, HB: Use 1 mg bortezomib for 1 kg fish), impacted the hepatic ultrastructure and transcriptome expression compared to control fish (normoxia-treated, NN).

RESULTS

Hypoxia tolerance was significantly decreased in the bortezomib-treated group (LOE_crit, loss of equilibrium, 1.11 mg·L^- 1 and 1.32 mg·L^- 1) compared to the control group (LOE_crit, 0.73 mg·L^- 1 and 0.85 mg·L^- 1). The HB group had more severe liver injury than the HN group. Specifically, the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the HB group (52.16 U/gprot, 32 U/gprot) were significantly (p < 0.01) higher than those in the HN group (32.85 U/gprot, 21. 68 U/gprot). In addition, more severe liver damage such as vacuoles, nuclear atrophy, and nuclear lysis were observed in the HB group. RNA-seq was performed on livers from the HN, HB and NN groups. KEGG pathway analysis disclosed that many DEGs (differently expressed genes) were enriched in the HIF-1, FOXO, MAPK, PI3K-Akt and AMPK signaling pathway and their downstream.

CONCLUSION

We explored the adaptation mechanism of "Pujiang No.2" to hypoxia stress by using bortezomib, and combined with transcriptome analysis, accurately captured the genes related to hypoxia tolerance advantage.

Adaptations to a hypoxic lifestyle in naked mole-rats

Hypoxia is one of the strongest environmental drivers of cellular and physiological adaptation. Although most mammals are largely intolerant of hypoxia, some specialized species have evolved mitigative strategies to tolerate hypoxic niches. Among the most hypoxia-tolerant mammals are naked mole-rats (Heterocephalus glaber), a eusocial species of subterranean rodent native to eastern Africa. In hypoxia, naked mole-rats maintain consciousness and remain active despite a robust and rapid suppression of metabolic rate, which is mediated by numerous behavioural, physiological and cellular strategies. Conversely, hypoxia-intolerant mammals and most other hypoxia-tolerant mammals cannot achieve the same degree of metabolic savings while staying active in hypoxia and must also increase oxygen supply to tissues, and/or enter torpor. Intriguingly, recent studies suggest that naked mole-rats share many cellular strategies with non-mammalian vertebrate champions of anoxia tolerance, including the use of alternative metabolic end-products and potent pH buffering mechanisms to mitigate cellular acidification due to upregulation of anaerobic metabolic pathways, rapid mitochondrial remodelling to favour increased respiratory efficiency, and systemic shifts in energy prioritization to maintain brain function over that of other tissues. Herein, I discuss what is known regarding adaptations of naked mole-rats to a hypoxic lifestyle, and contrast strategies employed by this species to those of hypoxia-intolerant mammals, closely related African mole-rats, other well-studied hypoxia-tolerant mammals, and non-mammalian vertebrate champions of anoxia tolerance. I also discuss the neotenic theory of hypoxia tolerance – a leading theory that may explain the evolutionary origins of hypoxia tolerance in mammals – and highlight promising but underexplored avenues of hypoxia-related research in this fascinating model organism.

Identification of Candidate Genes Associated With Hypoxia Tolerance in Trachinotus blochii Using Bulked Segregant Analysis and RNA-Seq

Golden Pompano (Trachinotus blochii) has rapidly developed into the one of the main valuable fish species in Chinese marine aquaculture. Due to its rapid growth, active metabolism, and high oxygen consumption, hypoxia will increase its mortality and cause serious economic losses. We constructed two experimental groups of fish with different degrees of tolerance to hypoxia, used BSR-Seq analysis based on genome and genetic linkage groups to locate SNPs and genes that were related to the differences in hypoxia tolerance. The results showed that hypoxia tolerance SNPs of golden pompano may be jointly determined by multiple linkage groups, especially linkage groups 18 and 22. There were 768 and 348 candidate genes located in the candidate regions of the brain and liver, respectively. These genes were mainly involved in anaerobic energy metabolism, stress response, immune response, waste discharge, and cell death. The prostaglandin-endoperoxide synthase 2 (PTGS2) on LG8, which is involved in the metabolism of arachidonic acid, has a G/A nonsynonymous mutation at position 20641628, and the encoded amino acid was changed from hydrophobic aspartic acid to asparaginate. The specific pathway of the RIG-I-like receptor signaling pathway in the liver may mediate the metabolic system and the immune system, linking glucose metabolism with immune regulation. The death of the hypoxia-intolerant group may be due to the accumulation of lactic acid caused by the activation of anaerobic glycolysis during the early stage of hypoxia stress, and the activation of type I interferon was inhibited, which resulted in decreased immunity. Among the genes involved in the RIG-I-like receptor signaling pathway, the CYLD Lysine 63 Deubiquitinase (CYLD) located on LG16 had a G/T nonsynonymous mutation at position 13629651, and the encoded amino acid was changed from alanine acid to valine. The interferon induced with helicase C domain 1 (Ifih1) located on LG18 has a G/C nonsynonymous mutation at position 16153700, and the encoded hydrophilic glycine was changed to hydrophobic alanine. Our findings suggest these SNPs may assist in the molecular breeding of hypoxia-tolerant golden pompano, and speculate that the balance of glucose and lipid metabolism plays a key role in Trachinotus blochii under acute hypoxia.

Bioinspiration as a method of problem-based STEM education: A case study with a class structured around the COVID-19 crisis

Bioinspiration is a promising lens for biology instruction as it allows the instructor to focus on current issues, such as the COVID-19 pandemic. From social distancing to oxygen stress, organisms have been tackling pandemic-related problems for millions of years. What can we learn from such diverse adaptations in our own applications? This review uses a seminar course on the COVID-19 crisis to illustrate bioinspiration as an approach to teaching biology content. At the start of the class, students mind-mapped the entire problem; this range of subproblems was used to structure the biology content throughout the entire class. Students came to individual classes with a brainstormed list of biological systems that could serve as inspiration for a particular problem (e.g., absorptive leaves in response to the problem of toilet paper shortages). After exploration of relevant biology content, discussion returned to the focal problem. Students dug deeper into the literature in a group project on mask design and biological systems relevant to filtration and transparency. This class structure was an engaging way for students to learn principles from ecology, evolution, behavior, and physiology. Challenges with this course design revolved around the interdisciplinary and creative nature of the structure; for instance, the knowledge of the participants was often stretched by engineering details. While the present class was focused on the COVID-19 crisis, a course structured through a bioinspired approach can be applied to other focal problems, or subject areas, giving instructors a powerful method to deliver interdisciplinary content in an integrated and inquiry-driven way.

Comparative transcriptomic analysis of the brain in Takifugu rubripes shows its tolerance to acute hypoxia

Hypoxia in water that caused by reduced levels of oxygen occurred frequently, due to the complex aquatic environment. Hypoxia tolerance for fish depends on a complete set of coping mechanisms such as oxygen perception and gene-protein interaction regulation. The present study examined the short-term effects of hypoxia on the brain in Takifugu rubripes. We sequenced the transcriptomes of the brain in T. rubripes to study their response mechanism to acute hypoxia. A total of 167 genes were differentially expressed in the brain of T. rubripes after exposed to acute hypoxia. Gene ontology and KEGG enrichment analysis indicated that hypoxia could cause metabolic and neurological changes, showing the clues of their adaptation to acute hypoxia. As the most complex and important organ, the brain of T. rubripes might be able to create a self-protection mechanism to resist or reduce damage caused by acute hypoxia stress.

Burrowing star-nosed moles (Condylura cristata) are not hypoxia tolerant

Star-nosed moles (Condylura cristata) have an impressive diving performance and burrowing lifestyle, yet no ventilatory data are available for this or any other talpid mole species. We predicted that, like many other semi-aquatic and fossorial small mammals, star-nosed moles would exhibit: (i) a blunted (i.e. delayed or reduced) hypoxic ventilatory response, (ii) a reduced metabolic rate and (iii) a lowered body temperature (Tb) in hypoxia. We thus non-invasively measured these variables from wild-caught star-nosed moles exposed to normoxia (21% O2) or acute graded hypoxia (21-6% O2). Surprisingly, star-nosed moles did not exhibit a blunted HVR or decreased Tb in hypoxia, and only manifested a significant, albeit small (<8%), depression of metabolic rate at 6% O2 relative to normoxic controls. Unlike small rodents inhabiting similar niches, star-nosed moles are thus intolerant to hypoxia, which may reflect an evolutionary trade-off favouring the extreme sensory biology of this unusual insectivore.

Increased polyamine levels and maintenance of γ-aminobutyric acid (Gaba) homeostasis in the gills is indicative of osmotic plasticity in killifish

The Fundulus genus of killifish includes species that inhabit marshes along the U.S. Atlantic coast and the Gulf of Mexico, but differ in their ability to adjust rapidly to fluctuations in salinity. Previous work suggests that euryhaline killifish stimulate polyamine biosynthesis and accumulate putrescine in the gills during acute hypoosmotic challenge. Despite evidence that polyamines have an osmoregulatory role in euryhaline killifish species, their function in marine species is unknown. Furthermore, the consequences of hypoosmotic-induced changes in polyamine synthesis on downstream pathways, such as ƴ-aminobutyric acid (Gaba) production, have yet to be explored. Here, we examined the effects of acute hypoosmotic exposure on polyamine, glutamate, and Gaba levels in the gills of a marine (F. majalis) and two euryhaline killifish species (F. heteroclitus and F. grandis). Fish acclimated to 32 ppt or 12 ppt water were transferred to fresh water, and concentrations of glutamate (Glu), Gaba, and the polyamines putrescine (Put), spermidine (Spd), and spermine (Spm) were measured in the gills using high-performance liquid chromatography. F. heteroclitus and F. grandis exhibited an increase in gill Put concentration, but showed no change in Glu or Gaba levels following freshwater transfer. F. heteroclitus also accumulated Spd in the gills, whereas F. grandis showed transient increases in Spd and Spm levels. In contrast, gill Put, Spm, Glu, and Gaba levels decreased in F. majalis following freshwater transfer. Together, these findings suggest that increasing polyamine levels and maintaining Glu and Gaba levels in the gills may enable euryhaline teleosts to acclimate to shifts in environmental salinity.

Hypoxia tolerance and metabolic coping strategies in Oreochromis niloticus

The Nile tilapia (Oreochromis niloticus) is widely farmed in tropical and subtropical pond culture. O. niloticus is recognized as a species that is tolerant of hypoxic conditions, a trait that may largely be responsible for the success of this species in aquaculture. Until now, neither coping mechanisms nor a comparison of various indices of hypoxia tolerance to characterize the response to hypoxia, have been described. In the present study, Nile tilapia were subjected to hypoxia of increasing severity and duration to examine effects on metabolic rate (MO₂) and post hypoxic oxygen debt. MO₂ was measured during periods of severe hypoxia at 2.1 kPa O₂ (10% oxygen saturation) lasting between 2 and 24 h at 27 °C. Hypoxia tolerance was assessed by determining the critical oxygen tension (P_crit) and the pO₂ at which loss of equilibrium (LOE) occurred. We show that the tolerance of Nile tilapia to severe hypoxia is largely achieved through a capacity for metabolic depression. Despite prolonged exposure to dissolved oxygen levels below P_crit, the fish showed little excess post-hypoxic oxygen consumption (EPHOC) upon return to normoxic conditions. LOE did not occur until conditions became near-anoxic. Blood pH was not affected by severe hypoxia (2.1 kPa O₂), but a significant acidosis occurred during LOE, accompanied by a significant elevation in lactate and glucose levels. The results from the present study indicate that Nile tilapia do not switch to anaerobic metabolism during hypoxia until pO₂ falls below 2.1 kPa.

Naked mole-rat skeletal muscle mitochondria exhibit minimal functional plasticity in acute or chronic hypoxia

Oxidative phosphorylation is compromised in hypoxia, but many organisms live and exercise in low oxygen environments. Hypoxia-driven adaptations at the mitochondrial level are common and may enhance energetic efficiency or minimize deleterious reactive oxygen species (ROS) generation. Mitochondria from various hypoxia-tolerant animals exhibit robust functional changes following in vivo hypoxia and we hypothesized that similar plasticity would occur in naked mole-rat skeletal muscle. To test this, we exposed adult subordinate naked mole-rats to normoxia (21% O₂) or acute (4 h, 7% O₂) or chronic hypoxia (4-6 weeks, 11% O₂) and then isolated skeletal muscle mitochondria. Using high-resolution respirometry and a fluorescent indicator of ROS production, we then probed for changes in: i) lipid- (palmitoylcarnitine-malate), ii) carbohydrate- (pyruvate-malate), and iii) succinate-fueled metabolism, and also iv) complex IV electron transfer capacity, and v) H₂O₂ production. Compared to normoxic values, a) lipid-fueled uncoupled respiration was reduced ~15% during acute and chronic hypoxia, b) complex I-II capacity and the rate of ROS efflux were both unaffected, and c) complex II and IV uncoupled respiration were supressed ~16% following acute hypoxia. Notably, complex II-linked H₂O₂ efflux was 33% lower after acute hypoxia, which may reduce deleterious ROS bursts during reoxygenation. These mild changes in lipid- and carbohydrate-fueled respiratory capacity may reflect the need for this animal to exercise regularly in highly variable and intermittently hypoxic environments in which more robust plasticity may be energetically expensive.

GABA metabolism is crucial for long-term survival of anoxia in annual killifish embryos

In most vertebrates, a lack of oxygen quickly leads to irreparable damages to vital organs, such as the brain and heart. However, there are some vertebrates that have evolved mechanisms to survive periods of no oxygen (anoxia). The annual killifish (Austrofundulus limnaeus) survives in ephemeral ponds in the coastal deserts of Venezuela and their embryos have the remarkable ability to tolerate anoxia for months. When exposed to anoxia, embryos of A. limnaeus respond by producing significant amounts of γ-aminobutyric acid (GABA). This study aims to understand the role of GABA in supporting the metabolic response to anoxia. To explore this, we investigated four developmentally distinct stages of A. limnaeus embryos that vary in their anoxia tolerance. We measured GABA and lactate concentrations across development in response to anoxia and aerobic recovery. We then inhibited enzymes responsible for the production and degradation of GABA and observed GABA and lactate concentrations, as well as embryo mortality. Here, we show for the first time that GABA metabolism affects anoxia tolerance in A. limnaeus embryos. Inhibition of enzymes responsible for GABA production (glutamate decarboxylase) and degradation (GABA-transaminase and succinic acid semialdehyde dehydrogenase) led to increased mortality, supporting a role for GABA as an intermediate product and not a metabolic end-product. We propose multiple roles for GABA during anoxia and aerobic recovery in A. limnaeus embryos, serving as a neurotransmitter, an energy source, and an anti-oxidant.

De Novo Transcriptomic and Metabolomic Analyses Reveal the Ecological Adaptation of High-Altitude Bombus pyrosoma

Bombus pyrosoma is one of the most abundant bumblebee species in China, with a distribution range of very varied geomorphology and vegetation, which makes it an ideal pollinator species for research into high-altitude adaptation. Here, we sequenced and assembled transcriptomes of B. pyrosoma from the low-altitude North China Plain and the high-altitude Tibet Plateau. Subsequent comparative analysis of de novo transcriptomes from the high- and low-altitude groups identified 675 common upregulated genes (DEGs) in the high-altitude B. pyrosoma. These genes were enriched in metabolic pathways and corresponded to enzyme activities involved in energy metabolism. Furthermore, according to joint analysis with comparative metabolomics, we suggest that the metabolism of coenzyme A (CoA) and the metabolism and transport of energy resources contribute to the adaptation of high-altitude B. pyrosoma. Meanwhile, we found many common upregulated genes enriched in the Toll and immune deficiency (Imd)signaling pathways that act as important immune defenses in insects, and hypoxia and cold temperatures could induce the upregulation of immune genes in insects. Therefore, we suppose that the Toll and Imd signaling pathways also participated in the high-altitude adaptation of B. pyrosoma. Like other organisms, we suggest that the high-altitude adaptation of B. pyrosoma is controlled by diverse mechanisms.

Meta-Transcriptomic Analysis of RNAseq Data Reveals Pacu and Loach Fish with Unusually High Levels of Myoglobin Expression in Skeletal Muscles

Simple Summary

Oxygen-binding proteins that mediate oxygen-binding for storage and consumption, to reduce energy, are very diverse in fish, depending on their habitats. In the present study, oxygen-binding protein gene expression in the skeletal muscle of 25 diverse fish species was examined by a meta-transcriptomic approach. By using RNAseq data, this is the first study to examine the high level of myoglobin, one of the oxygen-binding proteins, transcripts in pacu and loach fish that might be related to their high tolerance for the oxygen-deficient environment. In addition, this study presents the power of the current method to compare the fish oxygen-binding protein expression and its putative gene expansion event.

Abstract

Oxygen-binding proteins, such as myoglobin, hemoglobin, neuroglobin, and cytoglobin, play a role in oxygen binding and delivery to tissues. In icefish, the loss of myoglobin and hemoglobin genes has been reported to be an adaptive evolution event. This interesting finding prompted us to exam oxygen-binding protein expression in diverse fish species. Taking advantage of substantial RNAseq data deposited in the NCBI (National Center for Biotechnology Information) database, we adopted a meta-transcriptomic approach to explore and compare four oxygen-binding protein gene expression levels in the skeletal muscle of 25 diverse fish species for the first time. RNAseq data were downloaded from the NCBI Sequence Read Archive (SRA) database, and de novo assembly was performed to generate transcript contigs. The genes encoding oxygen-binding proteins were then identified by the BLAST search, and the relative expression level of oxygen-binding protein genes was normalized by the RPKM (Reads per Kilobase Million) method. By performing expression profiling, hierarchy clustering, and principal component analysis, pacu and loach fish were noticed by their high myoglobin expression levels in skeletal muscle tissues among 25 diverse fish species. In conclusion, we demonstrated that meta-transcriptomic analysis of RNAseq data is an informative approach to compare the oxygen-binding protein expression and putative gene expansion event in fish.

Role of adenosine in functional recovery following anoxic coma in Locusta migratoria

When exposed to prolonged anoxia insects enter a reversible coma during which neural and muscular systems temporarily shut down. Nervous system shut down is a result of spreading depolarization throughout neurons and glial cells. Upon return to normoxia, recovery occurs following the restoration of ion gradients. However, there is a delay in the functional recovery of synaptic transmission following membrane repolarization. In mammals, the build-up of extracellular adenosine following spreading depolarization contributes to this delay. Adenosine accumulation is a marker of metabolic stress and it has many downstream effects through the activation of adenosine receptors, including the inhibition of cAMP production. Here we demonstrate that adenosine lengthens the time to functional recovery following anoxic coma in locusts. Caffeine, used as an adenosine receptor antagonist, decreased the time to recovery in intact animals and lengthened the time to recovery in semi-intact animals. A cAMP inhibitor, NKH 477, delayed recovery time in male animals. Our results show that the rate of recovery in insect systems is affected by the presence of adenosine.

Effects of hypoxia on the behavior and physiology of kelp forest fishes

Forecasts from climate models and oceanographic observations indicate increasing deoxygenation in the global oceans and an elevated frequency and intensity of hypoxic events in the coastal zone, which have the potential to affect marine biodiversity and fisheries. Exposure to low dissolved oxygen (DO) conditions may have deleterious effects on early life stages in fishes. This study aims to identify thresholds to hypoxia while testing behavioral and physiological responses of two congeneric species of kelp forest fish to four DO levels, ranging from normoxic to hypoxic (8.7, 6.0, 4.1, and 2.2 mg O2 /L). Behavioral tests identified changes in exploratory behavior and turning bias (lateralization), whereas physiological tests focused on determining changes in hypoxia tolerance (pCrit), ventilation rates, and metabolic rates, with impacts on the resulting capacity for aerobic activity. Our findings indicated that copper rockfish (Sebastes caurinus) and blue rockfish (Sebastes mystinus) express sensitivity to hypoxia; however, the strength of the response differed between species. Copper rockfish exhibited reduced absolute lateralization and increased escape time at the lowest DO levels, whereas behavioral metrics for blue rockfish did not vary with oxygen level. Both species exhibited decreases in aerobic scope (as a function of reduced maximum metabolic rate) and increases in ventilation rates to compensate for decreasing oxygen levels. Blue rockfish had a lower pCrit and stronger acclimation response compared to copper rockfish. The differences expressed by each species suggest that acclimatization to changing ocean conditions may vary, even among related species that recruit to the same kelp forest habitat, leading to winners and losers under future ocean conditions. Exposure to hypoxia can decrease individual physiological fitness through metabolic and aerobic depression and changes to anti-predator behavior, with implications for the outcome of ecological interactions and the management of fish stocks in the face of climate change.

The OxymiR response to oxygen limitation: a comparative microRNA perspective

From squid at the bottom of the ocean to humans at the top of mountains, animals have adapted to diverse oxygen-limited environments. Surviving these challenging conditions requires global metabolic reorganization that is orchestrated, in part, by microRNAs that can rapidly and reversibly target all biological functions. Herein, we review the involvement of microRNAs in natural models of anoxia and hypoxia tolerance, with a focus on the involvement of oxygen-responsive microRNAs (OxymiRs) in coordinating the metabolic rate depression that allows animals to tolerate reduced oxygen levels. We begin by discussing animals that experience acute or chronic periods of oxygen deprivation at the ocean's oxygen minimum zone and go on to consider more elevated environments, up to mountain plateaus over 3500 m above sea level. We highlight the commonalities and differences between OxymiR responses of over 20 diverse animal species, including invertebrates and vertebrates. This is followed by a discussion of the OxymiR adaptations, and maladaptations, present in hypoxic high-altitude environments where animals, including humans, do not enter hypometabolic states in response to hypoxia. Comparing the OxymiR responses of evolutionarily disparate animals from diverse environments allows us to identify species-specific and convergent microRNA responses, such as miR-210 regulation. However, it also sheds light on the lack of a single unified response to oxygen limitation. Characterizing OxymiRs will help us to understand their protective roles and raises the question of whether they can be exploited to alleviate the pathogenesis of ischemic insults and boost recovery. This Review takes a comparative approach to addressing such possibilities.

H2S-producing enzymes in anoxia-tolerant vertebrates: Effects of cold acclimation, anoxia exposure and reoxygenation on gene and protein expression

To lend insight into the potential role of the gasotransmitter hydrogen sulfide (H₂S) in facilitating anoxia survival of anoxia-tolerant vertebrates, we quantified the gene expression of the primary H₂S-synthesizing enzymes, 3-mercaptopyruvate sulfurtransferase (3MST), cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), in ventricle and brain of normoxic, anoxic and reoxygenated 21 °C- and 5 °C-acclimated freshwater turtles (Trachemys scripta) and 10 °C-acclimated crucian carp (Carassius carassius). Semi-quantitative Western blotting analysis was also conducted to assess 3MST and CBS protein abundance in ventricle and brain of 5 °C turtles and 10 °C crucian carp subjected to normoxia, anoxia and reoxygenation. We hypothesized that if H₂S was advantageous for anoxia survival, expression levels would remain unchanged or be upregulated with anoxia and/or reoxygenation. Indeed, for both species, gene and protein expression were largely maintained with anoxia exposure (24 h, 21 °C; 5 d, 10 °C; 14 d, 5 °C). With reoxygenation, 3MST expression was increased in turtle and crucian carp brain at the protein and gene level, respectively. Additionally, the effect of cold acclimation on gene expression was assessed in several tissues of the turtle. Expression levels were maintained in most tissues, but decreased in others. The maintenance of gene and protein expression of the H₂S-producing enzymes with anoxia exposure and the up-regulation of 3MST with reoxygenation suggests that H₂S may facilitate anoxic survival of the two champions of vertebrate anoxia survival. The differential effects of cold acclimation on H₂S enzyme expression may influence blood flow to different tissues during winter anoxia.

Interspecific variation in hypoxia tolerance and hypoxia acclimation responses in killifish from the family Fundulidae

Hypoxia is a pervasive stressor in aquatic environments, and both phenotypic plasticity and evolutionary adaptation could shape the ability to cope with hypoxia. We investigated evolved variation in hypoxia tolerance and the hypoxia acclimation response across fundulid killifishes that naturally experience different patterns of hypoxia exposure. We compared resting O2 consumption rate (ṀO2 ), and various indices of hypoxia tolerance [critical O2 tension (Pcrit), regulation index (RI), O2 tension (PO2 ) at loss of equilibrium (PLOE) and time to LOE (tLOE) at 0.6 kPa O2] in Fundulus confluentus, Fundulus diaphanus, Fundulus heteroclitus, Fundulus rathbuni, Lucania goodei and Lucania parva We examined the effects of chronic (28 days) exposure to constant hypoxia (2 kPa) or nocturnal intermittent hypoxia (12 h normoxia:12 h hypoxia) in a subset of species. Some species exhibited a two-breakpoint model in ṀO2  caused by early, modest declines in ṀO2  in moderate hypoxia. We found that hypoxia tolerance varied appreciably across species: F. confluentus was the most tolerant (lowest PLOE and Pcrit, longest tLOE), whereas F. rathbuni and F. diaphanus were the least tolerant. However, there was not a consistent pattern of interspecific variation for different indices of hypoxia tolerance, with or without taking phylogenetic relatedness into account, probably because these different indices are underlain by partially distinct mechanisms. Hypoxia acclimation generally improved hypoxia tolerance, but the magnitude of plasticity and responsiveness to different hypoxia patterns varied interspecifically. Our results therefore suggest that hypoxia tolerance is a complex trait that is best appreciated by considering multiple indices of tolerance.

Dietary GABA enhances hypoxia tolerance of a bottom-dwelling carp, Cirrhinus mrigala by modulating HIF-1α, thyroid hormones and metabolic responses

The Indian major carp, mrigal (Cirrhinus mrigala), is a bottom-dwelling fish that can survive hypoxic episodes in its natural environment. We hypothesise that it can better survive hypoxic conditions by altering metabolic responses through GABA (Gamma-aminobutyric acid) supplementation. In the first experiment, the hypoxia tolerance time of the fishes was evaluated under extreme anoxic conditions after feeding with GABA, which showed that GABA had improved survival time under hypoxia. To study the response of dietary GABA in hypoxia-exposed fish, the branchial HIF-1α expression levels, serum thyroid hormone levels and hepatic metabolic responses were assessed in the subsequent experiment. The treatment groups were fed for 60 days with experimental diets containing 4 levels of GABA (0.00% G, 0.50% G, 0.75% G and 1.0%G) and were subjected to 72-h hypoxia exposure (0.5 ± 0.02 mg L^-1 dissolved oxygen (DO)) whereas a control group was maintained under normoxic conditions (6.0 ± 0.21 mg L^-1 DO). The five treatment groups with three replicates were C0 (0% G + normoxia), H0 (0% G + hypoxia), H0.5 (0.50% G + hypoxia), H0.75 (0.75% G + hypoxia) and H1.0 (1.00% G + hypoxia). The results indicated that GABA supplementation triggered downregulation of HIF 1 alpha expression. When compared with the control group, decreased thyroxine (T4) and triiodothyronine (T3) levels were observed in the GABA-fed hypoxic groups. However, TSH (thyroid stimulating hormone) level remained unchanged in all the treatments. The LDH (lactate dehydrogenase) level in hypoxia-exposed groups was decreased by GABA supplementation. Our study demonstrated that GABA supplementation restores acute hypoxia-induced HIF-1α expression, thyroid hormone levels and LDH activities. On the other hand, it enhanced the citrate synthase (CS) activities at 0.5-1.00%, which showed a sharp decline in hypoxia. Hypoxia caused increase in the serum metabolites such as glucose, lactate, cholesterol and triglycerides. However, GABA supplementation was partially effective in reducing glucose and lactate level while triglycerides and cholesterol values remained unchanged. Overall, our results suggested a potential role of GABA in suppressing metabolism during hypoxia exposure, which can increase the chances of survival of the species Cirrhinus mrigala during hypoxia.

Hypoxic acclimation negatively impacts the contractility of steelhead trout (Oncorhynchus mykiss) spongy myocardium

Cardiac stroke volume (SV) is compromised in Atlantic cod and rainbow trout following acclimation to hypoxia (i.e., 40% air saturation; ~8 kPa O2) at 10-12°C, and this is not due to changes in heart morphometrics or maximum achievable in vitro end-diastolic volume. To examine if this diminished SV may be related to compromised myocardial contractility, we used the work-loop method to measure work and power in spongy myocardial strips from normoxic- and hypoxic-acclimated steelhead trout when exposed to decreasing Po2 levels (21 to 1.5 kPa) at several frequencies (30-90 contractions/min) at 14°C (their acclimation temperature). Work required to lengthen the muscle, as during filling of the heart, was strongly frequency dependent (i.e., increased with contraction rate) but was not affected by hypoxic acclimation or test Po2. In contrast, although shortening work was less frequency dependent, this parameter and network (and power) 1) were consistently lower (by ~30-50 and ~15%, respectively) in strips from hypoxic-acclimated fish and 2) fell by ~40-50% in both groups from 20 to 1.5 kPa Po2, despite the already-reduced myocardial performance in the hypoxic-acclimated group. In addition, strips from hypoxic-acclimated trout showed a poorer recovery of net power (by ~15%) when returned to normoxia. These results strongly suggest that hypoxic acclimation reduces myocardial contractility, and in turn, may limit SV (possibly by increasing end-systolic volume), but that this diminished performance does not improve the capacity to maintain myocardial performance under oxygen limiting conditions.

The endemic and endangered Maugean Skate (Zearaja maugeana) exhibits short-term severe hypoxia tolerance

The endangered and range-restricted Maugean skate (Zearaja maugeana) is subjected to large environmental variability coupled with anthropogenic stressors in its endemic habitat, Macquarie Harbour, Tasmania. However, little is known about the basic biology/physiology of this skate, or how it may respond to future environmental challenges predicted from climate change and/or increases in human activities such as aquaculture. These skate live at a preferred depth of 5-15 m where the dissolved oxygen (DO) levels are moderate (~55% air saturation), but can be found in areas of the Harbour where DO can range from 100% saturation to anoxia. Given that the water at their preferred depth is already hypoxic, we sought to investigate their response to further decreases in DO that may arise from potential increases in anthropogenic stress. We measured oxygen consumption, haematological parameters, tissue-enzyme capacity and heat shock protein (HSP) levels in skate exposed to 55% dissolved O2 saturation (control) and 20% dissolved O2 saturation (hypoxic) for 48 h. We conclude that the Maugean skate appears to be an oxyconformer, with a decrease in the rate of O2 consumption with increasing hypoxia. Increases in blood glucose and lactate at 20% O2 suggest that skate are relying more on anaerobic metabolism to tolerate periods of very low oxygen. Despite these metabolic shifts, there was no difference in HSP70 levels between groups, suggesting this short-term exposure did not elicit a cellular stress response. The metabolic state of the skate suggests that low oxygen stress for longer periods of time (i.e. >48 h) may not be tolerable and could potentially result in loss of habitat or shifts in their preferred habitat. Given its endemic distribution and limited life-history information, it will be critical to understand its tolerance to environmental challenges to create robust conservation strategies.