Hypoxia Suppressed Copper Toxicity during Early Development in Zebrafish Embryos in a Process Mediated by the Activation of the HIF Signaling Pathway

Effects of structural remodelling on gill physiology

The complex relationships between the structure and function of fish gills have been of interest to comparative physiologists for many years. Morphological plasticity of the gill provides a dynamic mechanism to reversibly alter its structure in response to changes in the conditions experienced by the fish. The best known example of gill remodelling is the growth or retraction of cell masses between the lamellae, a rapid process that alters the lamellar surface area that is exposed to the water (i.e. the functional lamellar surface area). Decreases in environmental O2 availability and/or increases in metabolic O2 demand stimulate uncovering of the lamellae, presumably to increase the capacity for O2 uptake. This review addresses four questions about gill remodelling: (1) what types of reversible morphological changes occur; (2) how do these changes affect physiological function from the gill to the whole animal; (3) what factors regulate reversible gill plasticity; and (4) is remodelling phylogenetically widespread among fishes? We address these questions by surveying the current state of knowledge of gill remodelling in fishes, with a focus on identifying gaps in our understanding that future research should consider.

Hypoxic and temporal variation in the endocrine disrupting toxicity of perfluorobutanesulfonate in marine medaka (Oryzias melastigma)

Perfluorobutanesulfonate (PFBS) is an emerging pollutant capable of potently disrupting the sex and thyroid endocrine systems of teleosts. However, the hypoxic and temporal variation in PFBS endocrine disrupting toxicity remain largely unknown. In the present study, adult marine medaka were exposed to environmentally realistic concentrations of PFBS (0 and 10 µg/L) under normoxia or hypoxia conditions for 7 days, aiming to explore the interactive behavior between PFBS and hypoxia. In addition, PFBS singular exposure was extended till 21 days under normoxia to elucidate the time-course progression in PFBS toxicity. The results showed that hypoxia inhibited the growth and caused the suspension of egg spawn regardless of PFBS exposure. With regard to the sex endocrine system, 7-day PFBS exposure led to an acute stimulation of transcriptional profiles in females, which, subsequently, recovered after the 21-day exposure. The potency of hypoxia to disturb the sex hormones was much stronger than PFBS. A remarkable increase in estradiol concentration was noted in medaka blood after hypoxia exposure. Changes in sex endocrinology of coexposed fish were largely determined by hypoxia, which drove the formation of an estrogenic environment. PFBS further enhanced the endocrine disrupting effects of hypoxia. However, the hepatic synthesis of vitellogenin and choriogenin, two commonly used sensitive biomarkers of estrogenic activity, failed to initiate in response to the estrogen stimulus. Compared to sex endocrine system, disturbances in thyroidal axis by PFBS or hypoxia were relatively mild. Overall, the present findings will advance our toxicological understanding about PFBS pollutant under the interference of hypoxia.

Role of HIF in fish inflammation

The hypoxia-inducing factor (HIF) is a central transcription factor in cellular oxygen sensing and regulation. It is common that the inflammation always appears in many diseases, like infectious diseases in fishes, and the inflammation is often accompanied by hypoxia, as a hallmark of inflammation. Besides coordinating cellular responses to low oxygen, HIF-mediated hypoxia signaling pathway is also crucial for immune responses such as the regulations of innate immune cell phenotype and function, as well as metabolic reprogramming under the inflammation. However, the understanding of the molecular mechanisms by which HIFs regulate the inflammatory response in fish is still very limited. Here, we review the characteristics of HIF as well as its roles in innate immune cells and the infections caused by bacteria and viruses. The regulatory effects of HIF on the metabolic reprogramming of innate immune cells are also discussed and the future research directions are outlooked. This paper will serve as a reference for elucidating the molecular mechanism of HIF regulating inflammation and identifying treatment strategies to target HIF for fish disease.

Toxicity of perfluorobutanesulfonate on gill functions of marine medaka (Oryzias melastigma): A time course and hypoxia co-exposure study

Perfluorobutanesulfonate (PFBS) is found in hypoxia regions. Results of previous studies have shown that hypoxia was capable of altering the inherent toxicity of PFBS. However, regarding gill functions, hypoxic influences and time course progression of toxic effects of PFBS remain unclear. In this study, with the aim to reveal the interaction behavior between PFBS and hypoxia, adult marine medaka Oryzias melastigma were exposed for 7 days to 0 or 10 μg PFBS/L under normoxic or hypoxic conditions. Subsequently, to explore the time-course transition in gill toxicity, medaka were exposed to PFBS for 21 days. The results showed that hypoxia dramatically increased the respiratory rate of medaka gill, which was further enhanced by exposure to PFBS; although exposure to PFBS under normoxic conditions for 7 days did not alter respiration, exposure to PFBS for 21 days significantly accelerated the respiration rate of female medaka. Concurrently, both hypoxia and PFBS were potent to interrupt the gene transcriptions and Na⁺, K⁺-ATPase enzymatic activity that play pivotal roles in the osmoregulation in gills of marine medaka, consequently disrupting homeostasis of major ions in blood, such as Na⁺, Cl^-, and Ca²⁺. In addition, composition and diversity of the microbiome residing on surfaces of the gill were profiled by using amplicon sequencing. Acute exposure to hypoxia for only 7 days caused a significant decrease in diversity of the bacterial community of gill whatever the presence of PFBS, while PFBS exposure for 21 days increased the diversity of gill microbial community. Principal component analysis revealed that, compared with PFBS, hypoxia was the predominant driver of gill microbiome dysbiosis. Depending on duration of exposure, a divergence was caused in the microbial community of gill. Overall, the current findings underline the interaction between hypoxia and PFBS on gill function and demonstrate the temporal variation in PFBS toxicity.

Hypoxia aggravates the burden of yellowstripe goby (Mugilogobius chulae) under atorvastatin exposure

In the present study, an estuarine benthic fish, Mugilogobius chulae (M. chulae), was exposed to hypoxia, atorvastatin (ATV), a highly used and widely detected lipid-lowering drug in aquatic environment, and the combination of hypoxia and ATV for 7 days, respectively, so as to address and compare the effects of the combination of hypoxia and ATV exposure on M. chulae. The results showed that lipid metabolism in M. chulae was greatly affected: lipid synthesis was blocked and catabolism was enhanced, exhibiting that lipids content were heavily depleted. The combined exposure of hypoxia and ATV caused oxidative stress and induced massive inflammatory response in the liver of M. chulae. Signaling pathways involving in energy metabolism and redox responses regulated by key factors such as HIF, PPAR, p53 and sirt1 play important regulatory roles in hypoxia-ATV stress. Critically, we found that the response of M. chulae to ATV was more sensitive under hypoxia than normoxia. ATV exposure to aquatic non-target organisms under hypoxic conditions may make a great impact on the detoxification and energy metabolism, especially lipid metabolism, and aggravate the oxidative pressure of the exposed organisms.

Copper exposure improves the upper thermal tolerance in a sex-specific manner, irrespective of fish thermal history

Ectotherms can respond to climate change via evolutionary adaptation, usually resulting in an increase of their upper thermal tolerance. But whether such adaptation influences the phenotypic plasticity of thermal tolerance when encountering further environmental stressors is not clear yet. This is crucial to understand because organisms experience multiple stressors, besides warming climate, in their natural environment and pollution is one of those. Here, we studied the phenotypic plasticity of thermal tolerance in three-spined stickleback populations inhabiting spatially replicated thermally polluted and pristine areas before and after exposing them to a sublethal concentration of copper for one week. We found that the upper thermal tolerance and its phenotypic plasticity after copper exposure did not depend on the thermal history of fish, suggesting that five decades of thermal pollution did not result in evolutionary adaptation to thermal tolerance. The upper thermal tolerance of fish was, on the other hand, increased by ∼ 1.5 °C after 1-week copper exposure in a sex-specific manner, with males having higher plasticity. To our knowledge this is the first study that shows an improvement of the upper thermal tolerance as a result of metal exposure. The results suggest that three-spined sticklebacks are having high plasticity and they are capable of surviving in a multiple-stressor scenario in the wild and that male sticklebacks seem more resilient to fluctuating environmental conditions than female.

Kinetically Orthogonal Probe for Simultaneous Measurement of H2S and Nitroreductase: A Refined Method to Predict the Invasiveness of Tumor Cells

The concentrations of nitroreductase and H₂S have been widely used to predict the invasiveness of tumors. However, the above two substrates always interfere with the measurement of each other as both substrates react with the typical nitroaromatic probe with the same process. Moreover, the above interferences may lead to the misjudgment of the tumor invasiveness. We used a strategy combining kinetical distinguishing and signal amplification to construct a kinetically orthogonal probe labeled KOP. The above strategy expanded the gap between the reactivity of KOP to H₂S and nitroreductase with an acceptable reactivity and could determine the concentration of coexisting nitroreductase and H₂S on a kinetic curve with a breakpoint. KOP could also indicate the correct invasiveness tendency in the cellular model with a complex H₂S generation pathway, while the traditional kinetically nonorthogonal probe could not indicate invasiveness correctly.

Anoxia-reoxygenation alters H2O2 efflux and sensitivity of redox centers to copper in heart mitochondria

Mitochondrial reactive oxygen species (ROS) have been implicated in organ damage caused by environmental stressors, prompting studies on the effect of oxygen deprivation and metal exposure on ROS metabolism. However, how anoxia and copper (Cu) jointly influence heart mitochondrial ROS metabolism is not understood. We used rainbow trout heart mitochondria to probe the effects of anoxia-reoxygenation and Cu on hydrogen peroxide (H₂O₂) emission during oxidation of palmitoylcarnitine (PC), succinate, or glutamate-malate. In addition, we examined the influence of anoxia-reoxygenation and Cu on site-specific H₂O₂ emission capacities and key antioxidant enzymes, glutathione peroxidase (GPx) and thioredoxin reductase (TrxR). Results showed that anoxia-reoxygenation suppressed H₂O₂ emission regardless of substrate type or duration of anoxia. Anoxia-reoxygenation reduced mitochondrial sensitivity to Cu during oxidation of succinate or glutamate-malate whereas high Cu concentration additively stimulated H₂O₂ emission in mitochondria oxidizing PC. Prolonged anoxia-reoxygenation stimulated H₂O₂ emission from sites O_F and I_F, inhibited emission from sites I_Q, II_F and III_Qo, and disparately altered the sensitivity of the sites to Cu. Interestingly, anoxia-reoxygenation increased GPx and TrxR activities, more prominently when reoxygenation followed a short duration of anoxia. Cu did not alter GPx but reduced TrxR activity in normoxic and anoxic-reoxygenated mitochondria. Overall, our study revealed potential mechanisms that may reduce oxidative damage associated with anoxia-reoxygenation and Cu exposure in heart mitochondria. The increased and decreased H₂O₂ emission from NADH/NAD⁺ and QH₂/Q isopotential sites, respectively, may represent a balance between H₂O₂ required for oxygen deprivation-induced signaling and prevention of ROS burst associated with anoxia-reoxygenation.

Enhanced anti-angiogenic activity of novel melatonin-like agents

Hypoxia-inducible factor-1 (HIF-1) plays an important role in cellular responses to hypoxia, including the transcriptional activation of several genes involved in tumor angiogenesis. Melatonin, also known as N-acetyl-5-methopxytryptamine, is produced naturally by the pineal gland and has anti-angiogenic effects in cancer through its ability to modulate HIF-1α activity. However, the use of melatonin as a therapeutic is limited by its low oral bioavailability and short half-life. Here, we synthesized melatonin-like molecules with enhanced HIF-1α targeting activity and less toxicity and investigated their effects on tumor growth and angiogenesis, as well as the underlying molecular mechanisms. Among melatonin derivatives, N-butyryl-5-methoxytryptamine (NB-5-MT) showed the most potent HIF-1α targeting activity. This molecule was able to (a) reduce the expression of HIF-1α at the protein level, (b) reduce the transcription of HIF-1α target genes, (c) reduce reactive oxygen species (ROS) generation, (d) decrease angiogenesis in vitro and in vivo, and (e) suppress tumor size and metastasis. In addition, NB-5-MT showed improved anti-angiogenic activity compared with melatonin due to its enhanced cellular uptake. NB-5-MT is thus a promising lead for the future development of anticancer compounds with HIF-1α targeting activity. Given that HIF-1α is overexpressed in the majority of human cancers, the melatonin derivative NB-5-MT could represent a novel potent therapeutic agent for cancer.

Acclimation to cyclic hypoxia improves thermal tolerance and copper survival in the caridean shrimp Palaemon varians

In response to the continuous variation of environmental parameters, species must be able to adjust their physiology to overcome stressful conditions, a process known as acclimatization. Numerous laboratory studies have been conducted to understand and describe the mechanisms of acclimation to one environmental stressor (e.g. cyclic hypoxia), but currently our understanding of how acclimation to one stressor can change tolerance to a subsequent stressor is limited. Here, in two different experiments, we used the shrimp Palaemon varians to test how, following 28-days acclimation to cyclic hypoxia (mimicking a cyclic hypoxic regime currently found in its natural habitat), critical thermal maximum (CT_max) and sensitivity to copper (Cu²⁺) exposure (30 mgL^-1) changed in comparison to shrimp acclimated to normoxic conditions and then exposed to thermal stress or Cu²⁺. Acclimation to cyclic hypoxia improved both CT_max (~1 °C higher than controls) and survival to acute Cu²⁺ exposure (~30% higher than controls) and induced significant gene expression changes (i.e. up-regulation of heat shock protein 70 - HSP70, hypoxia inducible factor - HIF, phosphoenolpyruvate carboxykinase - PEPCK, glucose 6-P transporter - G6Pt, metallothionein - Mt, and down-regulation of hemocyanin - Hem) in animals acclimated to cyclic hypoxia. Our results demonstrate how acclimation to cyclic hypoxia improved tolerance to subsequent stressors, highlighting the complexity of predicting organismal performance in variable (i.e. where multiple parameters can simultaneously change during the day) environments.

Climate change impacts on pollutants mobilization and interactive effects of climate change and pollutants on toxicity and bioaccumulation of pollutants in estuarine and marine biota and linkage to seafood security

This article provides an overview of the impacts of climate change stressors (temperature, ocean acidification, sea-level rise, and hypoxia) on estuarine and marine biota (algae, crustaceans, molluscs, corals, and fish). It also assessed possible/likely interactive impacts (combined impacts of climate change stressors and pollutants) on pollutants mobilization, pollutants toxicity (effects on growth, reproduction, mortality) and pollutants bioaccumulation in estuarine and marine biota. An increase in temperature and extreme events may enhance the release, degradation, transportation, and mobilization of both hydrophobic and hydrophilic pollutants in the estuarine and marine environments. Based on the available pollutants' toxicity trend data and information it reveals that the toxicity of several high-risk pollutants may increase with increasing levels of climate change stressors. It is likely that the interactive effects of climate change and pollutants may enhance the bioaccumulation of pollutants in seafood organisms. There is a paucity of literature relating to realistic interactive effects of climate change and pollutants. Therefore, future research should be directed towards the combined effects of climate change stressors and pollutants on estuarine and marine bota. A sustainable solution for pollution control caused by both greenhouse gas emissions (that cause climate change) and chemical pollutants would be required to safeguard the estuarine and marine biota.

Ecotoxicological assessment of palm oil mill effluent final discharge by zebrafish (Danio rerio) embryonic assay

Most palm oil mills adopted conventional ponding system, including anaerobic, aerobic, facultative and algae ponds, for the treatment of palm oil mill effluent (POME). Only a few mills installed a bio-polishing plant to treat POME further before its final discharge. The present study aims to determine the quality and toxicity levels of POME final discharge from three different mills by using conventional chemical analyses and fish (Danio rerio) embryo toxicity (FET) test. The effluent derived from mill A which installed with a bio-polishing plant had lower values of BOD, COD and TSS at 45 mg/L, 104 mg/L, and 27 mg/L, respectively. Only mill A nearly met the industrial effluent discharge standard for BOD. In FET test, effluent from mill A recorded low lethality and most of the embryos were malformed after hatching (half-maximal effective concentration (EC50) = 20%). The highest toxicity was observed from the effluent of mill B and all embryos were coagulated after 24 h in samples greater than 75% of effluent (38% of half-maximal lethal concentration (LC50) at 96 h). The embryos in the effluent from mill C recorded high mortality after hatching, and the survivors were malformed after 96 h exposure (LC50 = 26%). Elemental analysis of POME final discharge samples showed Cu, Zn, and Fe concentrations were in the range of 0.10-0.32 mg/L, 0.01-0.99 mg/L, and 0.94-4.54 mg/L, respectively and all values were below the effluent permissible discharge limits. However, the present study found these metals inhibited D. rerio embryonic development at 0.12 mg/L of Cu, and 4.9 mg/L of Fe for 96 h-EC50. The present study found that bio-polishing plant installed in mill A effectively removing pollutants especially BOD and the FET test was a useful method to monitor quality and toxicity of the POME final discharge samples.

Global variation in freshwater physico-chemistry and its influence on chemical toxicity in aquatic wildlife

Chemical pollution is one of the major threats to global freshwater biodiversity and will be exacerbated through changes in temperature and rainfall patterns, acid-base chemistry, and reduced freshwater availability due to climate change. In this review we show how physico-chemical features of natural fresh waters, including pH, temperature, oxygen, carbon dioxide, divalent cations, anions, carbonate alkalinity, salinity and dissolved organic matter, can affect the environmental risk to aquatic wildlife of pollutant chemicals. We evidence how these features of freshwater physico-chemistry directly and/or indirectly affect the solubility, speciation, bioavailability and uptake of chemicals [including via alterations in the trans-epithelial electric potential (TEP) across the gills or skin] as well as the internal physiology/biochemistry of the organisms, and hence ultimately toxicity. We also show how toxicity can vary with species and ontogeny. We use a new database of global freshwater chemistry (GLORICH) to demonstrate the huge variability (often >1000-fold) for these physico-chemical variables in natural fresh waters, and hence their importance to ecotoxicology. We emphasise that a better understanding of chemical toxicity and more accurate environmental risk assessment requires greater consideration of the natural water physico-chemistry in which the organisms we seek to protect live.

Developmental exposure window influences silver toxicity but does not affect the susceptibility to subsequent exposures in zebrafish embryos

Silver is a non-essential, toxic metal widespread in freshwaters and capable of causing adverse effects to wildlife. Its toxic effects have been studied in detail but less is known about how sensitivity varies during development and whether pre-exposures affect tolerance upon re-exposure. We address these knowledge gaps using the zebrafish embryo (Danio rerio) model to investigate whether exposures encompassing stages of development prior to mid-blastula transition, when chorion hardening and epigenetic reprogramming occur, result in greater toxicity compared to those initiated after this period. We conducted exposures to silver initiated at 0.5 h post fertilisation (hpf) and 4 hpf to determine if toxicity differed. In parallel, we exposed embryos to the methylation inhibitor 5-azacytidine as a positive control. Toxicity increased when exposures started from 0.5 hpf compared to 4 hpf and LC50 were significantly lower by 1.2 and 7.6 times for silver and 5-azacyitidine, respectively. We then investigated whether pre-exposure to silver during early development (from 0.5 or 4 hpf) affected the outcome of subsequent exposures during the larvae stage, and found no alterations in toxicity compared to naïve larvae. Together, these data demonstrate that during early development zebrafish embryos are more sensitive to silver when experiments are initiated at the one-cell stage, but that pre-exposures do not influence the outcome of subsequent exposures, suggesting that no long-lasting memory capable of influencing future susceptibility was maintained under our experimental conditions. The finding that toxicity is greater for exposures initiated at the one-cell stage has implications for designing testing systems to assess chemical toxicity.

Modulation of copper-induced antioxidant defense, Cu transport, and mitophagy by hypoxia in the large yellow croaker (Larimichthys crocea)

This study aimed to investigate the effects of hypoxia on Cu-induced antioxidant defense, Cu transport, and mitophagy in the liver of the large yellow croaker. Fish were exposed to hypoxia (3.0 mg L^-1), Cu (120 μg L^-1), and hypoxia (3.0 mg L^-1) plus Cu (120 μg L^-1) for 48 h. Hypoxia exposure increased antioxidant abilities to maintain cellular redox balance. Although Cu exposure alone improved antioxidant defense, Cu transport, and mitophagy, these stress responses could not completely neutralize Cu toxicity, as reflected by the elevated reactive oxygen species (ROS) and lipid peroxidation (LPO) and hepatic vacuoles. When compared with Cu stress alone, hypoxia increased Cu toxicity by inhibiting antioxidant defense, Cu transport, and mitophagy, leading to the increment of mortality, ROS, and LPO, and the deterioration of histological structure. The adverse effects of hypoxia on Cu-induced metal transport and mitophagy might be involved in metal-responsive element-binding transcription factor-1 (MTF-1) and Forkhead box O-3 (FoxO3) signaling pathways, respectively. Overall, hypoxia reduced antioxidant response, Cu transport, and mitophagy in fish exposed to Cu, which contributes to understanding the molecular mechanisms underlying negative effects of hypoxia on Cu toxicity in fish.

Activation of HIF-1 signaling ameliorates liver steatosis in zebrafish atp7b deficiency (Wilson's disease) models

Wilson's disease is an autosomal recessive disease characterized by excess copper accumulated in the liver and brain. It is caused by mutations in the copper transporter gene ATP7B. However, based on the poor understanding of the transcriptional program involved in the pathogenesis of Wilson's disease and the lack of more safe and efficient therapies, the identification of novel pathways and the establishment of complementary model systems of Wilson's disease are urgently needed. Herein, we generated two zebrafish atp7b-mutant lines using the CRISPR/Cas9 editing system, and the mutants developed hepatic and behavioral deficits similar to those observed in humans with Wilson's disease. Interestingly, we found that atp7b-deficient zebrafish embryos developed liver steatosis under low-dose Cu exposure, and behavioral deficits appeared under high-dose Cu exposure. Analyses of publicly available transcriptomic data from ATP7B-knockout HepG2 cells demonstrated that the HIF-1 signaling pathway is downregulated in ATP7B-knockout HepG2 cells compared with wildtype cells following Cu exposure. The HIF-1 signaling pathway was also downregulated in our atp7b-deficient zebrafish mutants following Cu exposure. Furthermore, we demonstrate that activation of the HIF-1 signaling pathway with the chemical compound FG-4592 or DMOG ameliorates liver steatosis and reduces accumulated Cu levels in zebrafish atp7b deficiency models. These findings introduce a novel prospect that modulation of the HIF-1 signaling pathway should be explored as a novel strategy to reduce copper toxicity in Wilson's disease patients.

The effect of warming on pesticide toxicity is reversed between developmental stages in the mosquito Culex pipiens

A better understanding of interactions between pesticides and warming is important to improve ecological risk assessment in a warming world. Current insights are almost exclusively based on studies that exposed animals simultaneously to both warming and a pesticide and focused on effects during the pesticide exposure period and within a single developmental stage. We studied two ignored aspects of the interplay between warming and pesticide exposure: (i) the role of delayed effects after the pesticide exposure period, and (ii) the dependence on the developmental stage. We carried out a longitudinal experiment from the egg stage to the adult stage in the mosquito Culex pipiens where we crossed a warming treatment (20 °C vs 24 °C) with 48 h exposures to the pesticide chlorpyrifos in three developmental stages (early L1 larvae, late L4 larvae and adults). Chlorpyrifos induced mild to moderate mortality in all developmental stages (10-30%). A key finding was that warming shaped the chlorpyrifos-induced mortality but in opposite directions between stages. Chlorpyrifos was 7% less toxic under warming in L1 larvae, yet more toxic under warming in L4 larvae (22%) and in adult males (33%), while toxicity did not change under warming in adult females. We hypothesize that the general, stage-specific differences in the effects of warming on body size (increased size in early larvae, decreased size in later stages) caused the reversal of the effects of warming on toxicity between stages. Previous larval exposure to chlorpyrifos caused delayed effects that strongly reduced survival to the adult stage (̰25% at 24 °C). Notably, warming also modulated these delayed mortality effects in opposite ways between developmental stages, matching the patterns of mortality during the pesticide exposure periods. Integrating the general stage-specific patterns of how warming shapes body size is important to advance our mechanistic understanding of the interactions between pesticides and warming.

Pre-hypoxia exposure inhibited copper toxicity by improving energy metabolism, antioxidant defence and mitophagy in the liver of the large yellow croaker Larimichthys crocea

This study investigated the effects of moderate hypoxia pre-exposure on energy metabolism, antioxidant defence and mitophagy in the liver of the large yellow croaker Larimichthys crocea exposed to Cu. Fish were pre-exposed to either normoxia or hypoxia (~3.0 mg L^-1, 42% O₂ saturation) for 48 h, and subsequently were subjected to either control (without Cu addition) or Cu (168 μg L^-1) under normoxic conditions for another 48 h. Copper exposure under normoxia induced Cu toxicity that increased mortality, the production of reactive oxygen species (ROS) and malondialdehyde, and aberrant hepatic mitochondrial ultrastructure. Interestingly, hypoxia pre-exposure improved energy metabolism, antioxidant ability and mitophagy response, and reduced the Cu content to inhibit Cu toxicity, reflecting the enhanced survival rate and reduced oxidative damage. In these processes, hypoxia-inducible factor-1α (HIF-1α), transcription factors NFE2-related nuclear factor 2 (Nrf2), and forkhead box O-3 (FoxO3) mRNA levels were correlated with expression of genes related to energy metabolism, antioxidant defence and mitophagy, respectively, indicating HIF-1α, Nrf2, and FoxO3 are required for the induction of their respective target genes. Overall, moderate hypoxia pre-exposure was able to generate adaptive responses to mitigate Cu-induced toxicological effects, underlining a central role of hormesis.

Sublethal exposure to copper supresses the ability to acclimate to hypoxia in a model fish species

Hypoxia is one of the major threats to biodiversity in aquatic systems. The association of hypoxia with nutrient-rich effluent input into aquatic systems results in scenarios where hypoxic waters could be contaminated with a wide range of chemicals, including metals. Despite this, little is known about the ability of fish to respond to hypoxia when exposures occur in the presence of environmental toxicants. We address this knowledge gap by investigating the effects of exposures to different levels of oxygen in the presence or absence of copper using the three-spined sticklebacks (Gasterosteus aculeatus) model. Fish were exposed to different air saturations (AS; 100%, 75% and 50%) in combination with copper (20 μg/L) over a 4 day period. The critical oxygen level (P_crit), an indicator of acute hypoxia tolerance, was 54.64 ± 2.51% AS under control conditions, and 36.21 ± 2.14% when fish were chronically exposed to hypoxia (50% AS) for 4 days, revealing the ability of fish to acclimate to low oxygen conditions. Importantly, the additional exposure to copper (20 μg/L) prevented this improvement in P_crit, impairing hypoxia acclimation. In addition, an increase in ventilation rate was observed for combined copper and hypoxia exposure, compared to the single stressors or the controls. Interestingly, in the groups exposed to copper, a large increase in variation in the measured P_crit was observed between individuals, both under normoxic and hypoxic conditions. This variation, if observed in wild populations, may lead to selection for a tolerant phenotype and alterations in the gene pool of the populations, with consequences for their sustainability. Our findings provide strong evidence that copper reduces the capacity of fish to respond to hypoxia by preventing acclimation and will inform predictions of the consequences of global increases of hypoxia in water systems affected by other pollutants worldwide.

New insights into organ-specific oxidative stress mechanisms using a novel biosensor zebrafish

BACKGROUND

Reactive oxygen species (ROS) arise as a result from, and are essential in, numerous cellular processes. ROS, however, are highly reactive and if left unneutralised by endogenous antioxidant systems, can result in extensive cellular damage and/or pathogenesis. In addition, exposure to a wide range of environmental stressors can also result in surplus ROS production leading to oxidative stress (OS) and downstream tissue toxicity.

OBJECTIVES

Our aim was to produce a stable transgenic zebrafish line, unrestricted by tissue-specific gene regulation, which was capable of providing a whole organismal, real-time read-out of tissue-specific OS following exposure to a wide range of OS-inducing environmental contaminants and conditions. This model could, therefore, serve as a sensitive and specific mechanistic in vivo biomarker for all environmental conditions that result in OS.

METHODS

To achieve this aim, we exploited the pivotal role of the electrophile response element (EpRE) as a globally-acting master regulator of the cellular response to OS. To test tissue specificity and quantitative capacity, we selected a range of chemical contaminants known to induce OS in specific organs or tissues, and assessed dose-responsiveness in each using microscopic measures of mCherry fluorescence intensity.

RESULTS

We produced the first stable transgenic zebrafish line Tg (3EpRE:hsp70:mCherry) with high sensitivity for the detection of cellular RedOx imbalances, in vivo in near-real time. We applied this new model to quantify OS after exposure to a range of environmental conditions with high resolution and provided quantification both of compound- and tissue-specific ROS-induced toxicity.

DISCUSSION

Our model has an extremely diverse range of potential applications not only for biomonitoring of toxicants in aqueous environments, but also in biomedicine for identifying ROS-mediated mechanisms involved in the progression of a number of important human diseases, including cancer.

Cu pre-exposure alters antioxidant defense and energy metabolism in large yellow croaker Larimichthys crocea in response to severe hypoxia

The aim of the present study was to evaluate the effects of Cu pre-exposure on antioxidant defense and energy metabolism in the liver of the large yellow croaker exposed to severe hypoxia. Fish were pre-acclimated to 0 and 30 μg Cu L^-1 for 96 h, and subsequently exposed to 7.0 and 1.5 mg DO L^-1 for another 24 h. Hypoxic stress alone increased reactive oxygen species and hepatic vacuoles. When compared to hypoxic stress alone, hypoxic stress plus Cu pre-exposure increased mortality and ROS production, and worsened histological structure by inhibiting antioxidant defense and aerobic metabolism, and enhancing anaerobic metabolism, suggesting Cu pre-acclimation aggravated hypoxia-induced oxidative damage. NFE2-related nuclear factor 2 and hypoxia-inducible factor-1α might participate in the transcriptional regulation of genes related to antioxidant response and energy metabolism, respectively. In conclusion, Cu pre-acclimation had a synergistic effect on antioxidant response and energy metabolism in fish under severe hypoxia, which contributes to understanding the molecular mechanisms underlying negative effects of Cu pre-acclimation against hypoxic damage in fish.

Influence of Diltiazem on Fathead Minnows Across Dissolved Oxygen Gradients

Water resources in many arid to semi-arid regions are stressed by population growth and drought. Growing populations and climatic changes are influencing contaminant and water chemistry dynamics in urban inland waters, where flows can be dominated by, or even dependent on, wastewater effluent discharge. In these watersheds, interacting stressors such as dissolved oxygen and environmental contaminants (e.g., pharmaceuticals) have the potential to affect fish physiology and populations. Recent field observations from our group identified the calcium channel blocker (CCB) diltiazem in fish plasma exceeding human therapeutic doses (e.g., C_min ) in aquatic systems impaired because of nonattainment of dissolved oxygen water quality standards. Therefore our study objectives examined: 1) standard acute and chronic effects of dissolved oxygen and diltiazem to fish, 2) influences of dissolved oxygen at criteria levels deemed protective of aquatic life on diltiazem toxicity to fish, and 3) whether sublethal effects occur at diltiazem water concentrations predicted to cause a human therapeutic level (therapeutic hazard value [THV]) in fish plasma. Dissolved oxygen × diltiazem co-exposures significantly decreased survival at typical stream, lake, and reservoir water quality standards of 5.0 and 3.0 mg dissolved oxygen/L. Dissolved oxygen and diltiazem growth effects were observed at 2 times and 10 times lower than median lethal concentration (LC50) values (1.7 and 28.2 mg/L, respectively). Larval fathead minnow (Pimephales promelas) swimming behavior following low dissolved oxygen and diltiazem exposure generally decreased and was significantly reduced in light-to-dark bursting distance traveled, number of movements, and duration at concentrations as low as the THV. Individual and population level consequences of such responses are not yet understood, particularly in older organisms or other species; however, these findings suggest that assessments with pharmaceuticals and other cardioactive contaminants may underestimate adverse outcomes in fish across dissolved oxygen levels considered protective of aquatic life. Environ Toxicol Chem 2018;37:2835-2850. © 2018 SETAC.

Imaging of intracellular sulfane sulfur expression changes under hypoxic stress via a selenium-containing near-infrared fluorescent probe

Hypoxia is a significant global issue affecting the health of organisms. Oxygen homeostasis is critical for mammalian cell survival and cellular activities. Hypoxic stress can lead to cell injury and death, which contributes to many diseases. Sulfane sulfur is involved in crucial roles in physiological processes of maintaining intracellular redox state and ameliorating oxidative damage. Therefore, real-time imaging of changes in sulfane sulfur levels is important for understanding their biofunctions in cells. In this study, we develop a new near-infrared (NIR) fluorescent probe BD-diSeH for imaging of sulfane sulfur changes in cells and in vivo under hypoxic stress. The probe includes two moieties: an NIR azo-BODIPY fluorophore equipped with a strong nucleophilic phenylselenol group (-SeH). The probe is capable of tracing dynamic changes of endogenous sulfane sulfur based on a fast and spontaneous intramolecular cyclization reaction. The probe has been successfully used for imaging sulfane sulfur in 3D-multicellular spheroid and mouse hippocampus under hypoxic stress. The overall levels of sulfane sulfur are affected by the degree and length of hypoxic stress. The results reveal a close relationship between sulfane sulfur and hypoxia in living cells and in vivo, allowing better understanding of physiological and pathological processes involving sulfane sulfur. Moreover, to investigate the effects of environmental hypoxia on aquatic animals, this probe has been applied for sulfane sulfur detection in hypoxic zebrafish.

Copper alters hypoxia sensitivity and the behavioural emersion response in the amphibious fish Kryptolebias marmoratus

Elevated levels of metals have been reported in mangrove ecosystems worldwide. Mangrove fishes also routinely experience severe environmental stressors, such as hypoxia. In the amphibious fish Kryptolebias marmoratus (mangrove rivulus), a key behavioural response to avoid aquatic stress is to leave water (emersion). We hypothesized that copper (Cu) exposure would increase the sensitivity of this behavioural hypoxia avoidance response due to histopathological effects of Cu on gill structure and function. K. marmoratus were exposed to either control (no added Cu) or Cu (300μg/L) for 96h. Following this period, fish were exposed to an acute hypoxic challenge (decline in dissolved oxygen to ∼0% over 15min), and the emersion response was recorded. Gills were examined for histological changes. Fish exposed to Cu emersed at a higher dissolved oxygen level (7.5±0.6%), relative to the control treatment group (5.8±0.4%). Histological analysis showed that the gill surface area increased and the interlamellar cell mass (ILCM) was reduced following Cu exposure, contrary to our prediction. Overall, these data indicate that Cu induces hypoxia-like changes to gill morphology and increases the sensitivity of the hypoxia emersion response.

Contrasting effects of hypoxia on copper toxicity during development in the three-spined stickleback (Gasterosteus aculeatus)

Hypoxia is a global problem in aquatic systems and often co-occurs with pollutants. Despite this, little is known about the combined effects of these stressors on aquatic organisms. The objective of this study was to investigate the combined effects of hypoxia and copper, a toxic metal widespread in the aquatic environment. We used the three-spined stickleback (Gasterosteus aculeatus) as a model because of its environmental relevance and amenability for environmental toxicology studies. We focused on embryonic development as this is considered to be a sensitive life stage to environmental pollution. We first investigated the effects of hypoxia alone on stickleback development to generate the information required to design subsequent studies. Our data showed that exposure to low oxygen concentrations (24.7 ± 0.9% air saturation; AS) resulted in strong developmental delays and increased mortalities, whereas a small decrease in oxygen (75.0 ± 0.5%AS) resulted in premature hatching. Stickleback embryos were then exposed to a range of copper concentrations under hypoxia (56.1 ± 0.2%AS) or normoxia (97.6 ± 0.1%AS), continuously, from fertilisation to free swimming larvae. Hypoxia caused significant changes in copper toxicity throughout embryonic development. Prior to hatching, hypoxia suppressed the occurrence of mortalities, but after hatching hypoxia significantly increased copper toxicity. Interestingly, when exposures were conducted only after hatching, the onset of copper-induced mortalities was delayed under hypoxia compared to normoxia, but after 48 h, copper was more toxic to hatched embryos under hypoxia. This is the second species for which the protective effect of hypoxia on copper toxicity prior to hatching, followed by its exacerbating effect after hatching is demonstrated, suggesting the hypothesis that this pattern may be common for teleost species. Our research highlights the importance of considering the interactions between multiple stressors, as understanding these interactions is essential to facilitate the accurate prediction of the consequences of exposure to complex stressors in a rapidly changing environment.