Linking Oxidative Stress and Magnitude of Compensatory Responses with Life-Stage Specific Differences in Sensitivity of White Sturgeon (Acipenser transmontanus) to Copper or Cadmium

Cadmium impacts on calcium mineralization of zebrafish skeletal development and behavioral impairment

Cadmium (Cd) poses significant risks to aquatic organisms due to its toxicity and ability to disrupt the cellular processes. Given the similar atomic radius of Cd and calcium (Ca), Cd may potentially affect the Ca homeostasis, which can lead to impaired mineralization of skeletal structures and behavioral abnormalities. The formation of the spinal skeleton involves Ca transport and mineralization. In this study, we conducted an in-depth investigation on the effects of Cd at environmental concentrations on zebrafish (Danio rerio) skeletal development and the underlying molecular mechanisms. As the concentration of Cd increased, the accumulation of Cd in zebrafish larvae also rose, while the Ca content decreased significantly by 3.0 %-57.3 %, and vertebral deformities were observed. Transcriptomics analysis revealed that sixteen genes involved in metal absorption were affected. Exposure to 2 µg/L Cd significantly upregulated the expression of these genes, whereas exposure to 10 µg/L resulted in their downregulation. Consequently, exposure of zebrafish larvae to 10 µg/L of Cd inhibited the body segmentation growth and skeletal mineralization development by 29.1 %-56.7 %. This inhibition was evidenced by the downregulation of mineral absorption genes and decreased Ca accumulation. The findings of this study suggested that the inhibition of skeletal mineralization was likely attributed to the disruption of mineral absorption, thus providing novel insights into the mechanisms by which metal pollutants inhibit the skeletal development of fish.

Predicting the Metal Mixture Toxicity with a Toxicokinetic-Toxicodynamic Model Considering the Time-Dependent Adverse Outcome Pathways

Chemicals mainly exist in ecosystems as mixtures, and understanding and predicting their effects are major challenges in ecotoxicology. While the adverse outcome pathway (AOP) and toxicokinetic-toxicodynamic (TK-TD) models show promise as mechanistic approaches in chemical risk assessment, there is still a lack of methodology to incorporate the AOP into a TK-TD model. Here, we describe a novel approach that integrates the AOP and TK-TD models to predict mixture toxicity using metal mixtures (specifically Cd-Cu) as a case study. We preliminarily constructed an AOP of the metal mixture through temporal transcriptome analysis together with confirmatory bioassays. The AOP revealed that prolonged exposure time activated more key events and adverse outcomes, indicating different modes of action over time. We selected a potential key event as a proxy for damage and used it as a measurable parameter to replace the theoretical parameter (scaled damage) in the TK-TD model. This refined model, which connects molecular responses to organism outcomes, effectively predicts Cd-Cu mixture toxicity over time and can be extended to other metal mixtures and even multicomponent mixtures. Overall, our results contribute to a better understanding of metal mixture toxicity and provide insights for integrating the AOP and TK-TD models to improve risk assessment for chemical mixtures.

Melatonin as an Ameliorative Agent Against Cadmium- and Lead-Induced Toxicity in Fish: an Overview

Diverse anthropogenic activities and lack of knowledge on its consequences have promoted serious heavy metal contaminations in different aquatic systems throughout the globe. The non-biodegradable nature of most of these toxic heavy metals has increased the concern on their possible bioaccumulation in aquatic organisms as well as in other vertebrates. Among these aquatic species, fish are most sensitive to such contaminated water that not only decreases their chance of survivability in the nature but also increases the probability of biomagnifications of these heavy metals in higher order food chain. After entering the fish body, heavy metals induce detrimental changes in different vital organs by impairing multiple physiological and biochemical pathways that are essential for the species. Such alterations may include tissue damage, induction of oxidative stress, immune-suppression, endocrine disorders, uncontrolled cell proliferation, DNA damage, and even apoptosis. Although uncountable reports have explored the toxic effects of different heavy metals in diverse fish species, but surprisingly, only a few attempts have been made to ameliorate such toxic effects. Since, oxidative stress seems to be the underlying common factor in such heavy metal-induced toxicity, therefore, a potent and endogenous antioxidant with no side effect may be an appropriate therapeutic solution. Apart from summarizing the toxic effects of two important toxicants, i.e., cadmium and lead in fish, the novelty of the present treatise lies in its arguments in favor of using melatonin, an endogenous free radical scavenger and indirect antioxidant, in ameliorating the toxic effects of heavy metals in any fish species.

Elucidating the Differences in Metal Toxicity by Quantitative Adverse Outcome Pathways

Numerous studies have reported that the toxicity differences among metals are widespread; however, little is known about the mechanism of differences in metal toxicity to aquatic organisms due to the lack of quantitative understanding of their adverse outcome pathway. Here, we investigated the effects of Cd and Cu on bioaccumulation, gene expression, physiological responses, and apical effects in zebrafish larvae. RNA sequencing was conducted to provide supplementary mechanistic information for the effects of Cd and Cu exposure. On this basis, we proposed a quantitative adverse outcome pathway (qAOP) suitable for metal risk assessment of aquatic organisms. Our work provides a mechanistic explanation for the differences in metal toxicity where the strong bioaccumulation of Cu enables the newly accumulated Cu to reach the threshold that causes different adverse effects faster than Cd in zebrafish larvae, resulting in a higher toxicity of Cu than that of Cd. Furthermore, we proposed a parameter C_IT/BCF (the ratio of internal threshold concentration and bioaccumulation factor) that helps to understand the toxicity differences by combining the information of bioaccumulation and internal threshold of adverse effects. This work demonstrated that qAOP is an effective quantitative tool for understanding the toxicity mechanism and highlight the importance of toxicokinetics and toxicodynamics at different biological levels in determining the metal toxicity.

Transgenerational effects of zinc in zebrafish following early life stage exposure

Although toxic effects of zinc (Zn) have been well established in the different developmental stages in fish, long-lasting effects of Zn exposure during embryonic development have not been explored. Exposure to an environmentally relevant Zn concentration of 10 μM (650 μg/L) during the first five days after fertilization did not affect survival, body weight, malformations or overall hatching success of F0 and F1 larvae. Zn exposure did, however, result in delayed hatching in both the F0 and F1 generations and caused significant changes in homeostasis of Zn and selenium (Se) in F0 and F1 fish. This was especially pronounced when F1 embryos from Zn-exposed parents were treated with 30 μM (2000 μg/L) Zn. In the F0 generation, skewed sex ratio towards males and changes in homeostasis of Zn, Se and manganese (Mn) in the brain, gill, liver and gonad of adult fish were also observed. These changes were associated with altered expression of Zn- and Mn-regulatory genes and sex differentiation genes in F0 and F1 fish. The present study suggests that fish may carry memory from embryo-larval Zn exposure into adulthood and further to the next generation. The present study shows that ecotoxicological risk of an exposure to Zn during embryo-larval development may persist long after recovery and may also manifest in the F1 generation.

Cadmium induced oxidative stress, endoplasmic reticulum (ER) stress and apoptosis with compensative responses towards the up-regulation of ribosome, protein processing in the ER, and protein export pathways in the liver of zebrafish

The present study identified that exposure to 5, 10, and 20 µg/L Cd for 48 days reduced growth, increased Cd accumulation and levels of reactive oxygen species (ROS) and lipid peroxidation, and induced ER stress and cellular apoptosis in the liver in a dose-dependent manner. However, the survival rate was not affected by Cd. The increased production of ROS might result from reduced catalase (CAT) and copper/zinc-superoxide dismutase (Cu/Zn-SOD) activities, which might trigger ER stress pathways and subsequently induce apoptotic responses, ultimately leading to growth inhibition. Transcriptomic analyses indicated that the differentially expressed genes (DEGs) involved in metabolic pathways were significantly enriched and dysregulated by Cd, suggesting that metabolic disturbances may contribute to Cd toxicity. However, there were increases in glutathione peroxidase (GPX) activity, protein levels of metallothioneins (MTs) and heat shock protein 70 (HSP70), and mRNA levels of sod1, cat, gpx, mt2, and hsp70. Furthermore, DEGs related to ribosome, protein processing in the ER, and protein export pathways were significantly enriched and up-regulated by Cd. These increases may be compensatory responses following oxidative stress, ER stress, and apoptosis to resist negative effects. Taken together, we demonstrated that environmentally relevant levels of Cd induced adaptive responses with compensatory mechanisms in fish, which may help to maintain fish survival at the cost of growth.

Effects of continuous and intermittent cadmium exposure on HPGL axis, GH/IGF axis and circadian rhythm signaling and their consequences on reproduction in female zebrafish: Biomarkers independent of exposure regimes

Typical biomarkers of cadmium (Cd) pollution have well been confirmed in fish from continuous exposure pattern. However, in a natural environment, fish may be exposed to Cd intermittently. In this study, juvenile female zebrafish were exposed for 48 days to 10 μg/L Cd continuously, 20 μg/L for 1 day in every 2 days or 30 μg/L for 1 day in every 3 days. The toxic effects were evaluated using 8 various physiological and biochemical endpoints like specific growth rate (SGR), 17β-estradiol (E2) and vitellogenin (VTG) concentrations in plasma, reproductive parameters (gonadosomatic index (GSI), egg-laying amount, spawning percentage, and hatching and mortality rate of embryos). Transcription of 59 genes related to hypothalamic-pituitary-gonadal-liver (HPGL) axis, circadian rhythm signaling and insulin-like growth factor (IGF) system was examined. SGR, spawning percentage, E2 and VTG levels declined in fish exposed to 10 and 20 μg/L Cd but remained relatively stable in fish exposed to 30 μg/L Cd. Exposure to 10, 20 and 30 μg/L Cd significantly reduced GSI, hatching rate and mortality rate. Similarly, mRNA expression of 27 genes were sensitive to both continuous and intermittent Cd exposure. Among these genes, expression levels of 10 genes had more than 5-fold increase or decrease, including mRNA levels of vtg1, vtg2, vtg3, esr1, igf2a, igf2b, igfbp5b, nr1d1, gnrh3 and gnrhr4. The most sensitive molecular biomarker was vtg3 expression with 1500-3100 fold increase in the liver. The present study, for the first time, provides effective candidate biomarkers for Cd, which are independent of exposure regimes.

Role of Toxicokinetic and Toxicodynamic Parameters in Explaining the Sensitivity of Zebrafish Larvae to Four Metals

Given the persistence and toxic potencies of metal contaminants in ecosystems, animals, and human beings, they are considered to be hazardous global pollutants. While the lethality of metal toxicities (e.g., LC₅₀) can significantly vary, even within the same species, the underlying mechanisms are less well-understood. In this study, we developed a subcellular two-compartment toxicokinetic-toxicodynamic (TK-TD) model for zebrafish larvae when exposed to four metals (cadmium, lead, copper, and zinc) to reveal whether differences in metal toxicity (LC₅₀ values) were dominated by the TK or TD processes. Results showed that the subcellular TK and TD parameters of the four metals were significantly different, and the bioconcentration factor (BCF) value of copper was higher than those of the other metals. We also found that the TD parameter internal threshold concentration (C_IT) was significantly positively correlated to the LC₅₀ values (R² = 0.7), suggesting a dominant role of TD processes in metal toxicity. Furthermore, the combined parameter C_IT/BCF for a metal-sensitive fraction (BCF_MSF), which linked exposure to effects through the TK-TD approach, explained up to 89% of the variation in toxicity to the four metals. The present study suggests that the observed variation in toxicity of these four metals was mainly determined by TD processes but that TK processes should not be ignored, especially for copper.

Zn Induces Lipophagy via the Deacetylation of Beclin1 and Alleviates Cu-Induced Lipotoxicity at Their Environmentally Relevant Concentrations

In this study, the mechanisms of environmentally relevant doses of Cu and Zn mixtures influencing lipid deposition and metabolism were investigated in freshwater teleost yellow catfish Pelteobagrus fulvidraco (2 months old, 4.95 (t0.01 g, mean ± SEM). Our study indicated that waterborne Cu exposure increased lipid content, while Zn activated lipophagic flux and alleviated Cu-induced lipid accumulation. Yellow catfish hepatocytes treated with Zn or Zn + Cu activated autophagy-specific lipophagy, decreased lipid storage, and increased nonesterified fatty acid (NEFA) release, suggesting a causal relationship between lipophagy and lipid droplet (LD) breakdown under Zn and Zn + Cu conditions. Our further investigation found that Beclin1 deacetylation by sirtuin 1 (SIRT1) was required for Zn- and Zn + Cu-induced lipophagy and lipolysis, and lysine residues 427 and 434 were key sites for Beclin1 deacetylation. Taken together, these findings show that the Zn-induced deacetylation of Beclin1 promotes lipophagy as an important pathway to alleviate Cu-induced lipid accumulation in fish, which reveals a previously unidentified mechanism for understanding the antagonistic effects of Cu and Zn on metabolism at their environmentally relevant concentrations. Our results highlight the importance of combined exposure when the biological effects of heavy metals are evaluated during environmental risk assessments.

Differences in the subcellular distribution of cadmium and copper in the gills and liver of white sturgeon (Acipenser transmontanus) and rainbow trout (Oncorhynchus mykiss)

Recent studies have shown that white sturgeon (Acipenser transmontanus) are more resistant to cadmium (Cd) compared to rainbow trout (Oncorhynchus mykiss), whereas they are more sensitive than rainbow trout when exposed to copper (Cu). Differences in the subcellular distribution of metals among species could be one of the factors responsible for the differences in the sensitivity to metals. Although, subcellular distribution has been studied extensively in many species with many metals, its direct role in species-specific differences in the sensitivity has not been well studied. The objective of this study was to evaluate the role of subcellular distribution of metals in species-specific differences in the sensitivity to metals between sturgeon and trout. We compared the subcellular distribution of metals Cd and Cu in the cellular debris, heat-stable proteins, heat-denatured fraction, metal-rich granules, and organelles fractions from the gills and liver after exposure of juveniles of both species to 1.25 and 20 μg/L Cd and Cu for 8 days, respectively. Sturgeon diverted a higher amount of Cd towards biologically inactive metal pool (BIM) and a lower amount towards the biologically active metal pool (BAM) compared to trout in both tissues. This explained why sturgeon are able to tolerate a relatively higher exposure level to Cd compared to trout. For Cu, there was no statistically significant species-specific differences in the amounts diverted towards either BAM or BIM; hence, white sturgeon's greater sensitivity to Cu was not explained by its subcellular distribution strategies.

Differential Sensitivity to In Vitro Inhibition of Cytochrome P450 Aromatase (CYP19) Activity Among 18 Freshwater Fishes

There is significant concern regarding potential impairment of fish reproduction associated with endocrine disrupting chemicals. Aromatase (CYP19) is a steroidogenic enzyme involved in the conversion of androgens to estrogens. Inhibition of aromatase by chemicals can result in reduced concentrations of estrogens leading to adverse reproductive effects. These effects have been extensively investigated in a small number of laboratory model fishes, such as fathead minnow (Pimephales promelas), Japanese medaka (Oryzias latipes), and zebrafish (Danio rerio). But, differences in sensitivity among species are largely unknown. Therefore, this study took a first step toward understanding potential differences in sensitivity to aromatase inhibitors among fishes. Specifically, a standard in vitro aromatase inhibition assay using subcellular fractions of whole tissue homogenates was used to evaluate the potential sensitivity of 18 phylogenetically diverse species of freshwater fish to the nonsteroidal aromatase inhibitor fadrozole. Sensitivity to fadrozole ranged by more than 52-fold among these species. Five species were further investigated for sensitivity to up to 4 additional nonsteroidal aromatase inhibitors, letrozole, imazalil, prochloraz, and propiconazole. Potencies of each of these chemicals relative to fadrozole ranged by up to 2 orders of magnitude among the 5 species. Fathead minnow, Japanese medaka, and zebrafish were among the least sensitive to all the investigated chemicals; therefore, ecological risks of aromatase inhibitors derived from these species might not be adequately protective of more sensitive native fishes. This information could guide more objective ecological risk assessments of native fishes to chemicals that inhibit aromatase.

Comparative analyses of oxidative stress response and metallothionein induction in white sturgeon and rainbow trout during acute waterborne copper exposure

Early life-stages of the endangered white sturgeon (Acipenser transmontanus) have been shown to be among the most sensitive fishes to aqueous copper (Cu) exposure. In a recent analogous study, we examined the role of whole-body Cu accumulation and Na homeostasis in species-specific differences between the sensitivity of white sturgeon and a common laboratory fish model, rainbow trout, to Cu. However, the potential roles of important mechanisms such as Cu-induced oxidative stress and/or metallothionein (MT) induction as potential drivers of sensitivity of white sturgeon to Cu have not been investigated to date. Here, rainbow trout and white sturgeon from three different early life-stages were exposed to waterborne Cu for 96 h, following which major antioxidant parameters, lipid peroxidation and MT gene expression were evaluated. Results indicated that during larval and swim-up life-stages, Cu induced oxidative damage in white sturgeon was greater than in rainbow trout. Moreover, baseline glutathione (GSH) was significantly greater in rainbow trout than white sturgeon. Observations also suggested that trout exceedingly relied on GSH to combat Cu-induced oxidative stress as they grew older. In contrast, sturgeon recruited an increasing level of MT to neutralize Cu-induced oxidative stress and/or Cu loading. In our recent study, we demonstrated that Na homeostasis is more susceptible to Cu in white sturgeon than in rainbow trout. Collectively, these findings indicate that the greater degree of oxidative damage in early life-stages, in addition to the higher magnitude of the disruption of Na homeostasis, contributes to the higher sensitivity of white sturgeon to Cu exposure.

Copper Concentrations in the Upper Columbia River as a Limiting Factor in White Sturgeon Recruitment and Recovery

Currently there is little natural recruitment of white sturgeon (Acipenser transmontanus) in the Upper Columbia River located in British Columbia, Canada and Washington, USA. This review of life history, physiology, and behavior of white sturgeon, along with data from recent toxicological studies, suggest that trace metals, especially Cu, affect survival and behavior of early life stage fish. Sturgeon free embryos, first feeding embryos, and mixed feeding embryos utilize interstitial spaces between gravel. Although concentrations of Cu in the water column of the Upper Columbia River are typically less than US water quality criteria defined to protect aquatic life, samples at the sediment-water interface were as large as 24 µg/L and exceed the criteria. Toxicological studies reviewed here demonstrate mortality, loss of equilibrium, and immobility at Cu concentrations of 1.5 to <16 µg/L and reduced swimming activity was documented at 0.88 to 7 μg/L. Contaminated invertebrates and slag particles provide other routes of exposure. These additional routes of exposure can cause indirect effects from starvation due to potential lack of prey items and ingestion of contaminated prey or slag particles. The lack of food in stomachs during these critical early life stages may coincide with a threshold "point of no return" at which sturgeon will be unable to survive even if food becomes available following that early time frame. These findings become especially important as work progresses to enhance white sturgeon recruitment in the Upper Columbia River. To date, decisions against including trace metals as a factor in sturgeon recovery have focused on surface-water concentrations and measurements of lethality (LC50) to establish threshold concentrations for sturgeon sensitivity. However, information provided here suggests that measurements from the sediment-water interface and effect concentrations (EC50) be considered with white sturgeon life history characteristics. These data support minimizing Cu exposure risk to enhance a successful white sturgeon recovery effort. Integr Environ Assess Manag 2020;16:378-391. Published 2020. This article is a US Government work and is in the public domain inthe USA.

Influence of Hardness and Dissolved Organic Carbon on the Acute Toxicity of Copper to Zebrafish (Danio rerio) at Different Life Stages

Copper (Cu) bioavailability varies under water conditions. In the present study, the whole life of zebrafish was divided into three different life stages (larvae, juvenile and adult) based on the growth curve, then the influences of water hardness and dissolved organic carbon (DOC) concentration on the acute toxicity of zebrafish were respectively investigated. The results indicated that the life stages had significant effects on Cu toxicity. The larvae stage was less sensitive to Cu than both the juvenile and adult stages. With the increase of water hardness, the toxicity of Cu on zebrafish was decreased, a linear relationship was observed between water hardness and Cu toxicity, and the same was true for DOC concentration. The results showed that taking the 24 days juvenile zebrafish to study the water quality criteria of Cu was stable, sensitive and economical.

In Silico Site-Directed Mutagenesis Informs Species-Specific Predictions of Chemical Susceptibility Derived From the Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) Tool

Chemical hazard assessment requires extrapolation of information from model organisms to all species of concern. The Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) tool was developed as a rapid, cost-effective method to aid cross-species extrapolation of susceptibility to chemicals acting on specific protein targets through evaluation of protein structural similarities and differences. The greatest resolution for extrapolation of chemical susceptibility across species involves comparisons of individual amino acid residues at key positions involved in protein-chemical interactions. However, a lack of understanding of whether specific amino acid substitutions among species at key positions in proteins affect interaction with chemicals made manual interpretation of alignments time consuming and potentially inconsistent. Therefore, this study used in silico site-directed mutagenesis coupled with docking simulations of computational models for acetylcholinesterase (AChE) and ecdysone receptor (EcR) to investigate how specific amino acid substitutions impact protein-chemical interaction. This study found that computationally derived substitutions in identities of key amino acids caused no change in protein-chemical interaction if residues share the same side chain functional properties and have comparable molecular dimensions, while differences in these characteristics can change protein-chemical interaction. These findings were considered in the development of capabilities for automatically generated species-specific predictions of chemical susceptibility in SeqAPASS. These predictions for AChE and EcR were shown to agree with SeqAPASS predictions comparing the primary sequence and functional domain sequence of proteins for more than 90% of the investigated species, but also identified dramatic species-specific differences in chemical susceptibility that align with results from standard toxicity tests. These results provide a compelling line of evidence for use of SeqAPASS in deriving screening level, species-specific, susceptibility predictions across broad taxonomic groups for application to human and ecological hazard assessment.

Sensitivity of white sturgeon and rainbow trout to waterborne copper exposure: A comparative study of copper-induced disruption of sodium homeostasis

Recent studies have demonstrated that white sturgeon are more sensitive to acute exposure to Cu than rainbow trout (Oncorhynchus mykiss), especially during early life-stages. However, the physiological mechanisms underlying this difference in sensitivity to Cu is not known. In the present study, we first confirmed the higher sensitivity (lower 96 h LC₅₀ values) of white sturgeon to Cu at three different life stages (larva, swim-up, and juvenile) relative to their counterparts in rainbow trout. We also demonstrated that acute exposure to Cu (50 μg/L for 4.5 h) caused a significantly greater reduction in the rate of waterborne Na uptake in white sturgeon relative to that in rainbow trout across all three life-stages. In agreement with this observation, we also found that acute exposure to Cu (20 μg/L for 48 h) elicits a significantly greater decrease in whole body Na level in all life stages of white sturgeon compared to rainbow trout. In contrast, white sturgeon demonstrated a higher or similar level of Cu body burden relative to rainbow trout during acute Cu exposure (20 μg/L for 24 h), thereby indicating that Cu bioaccumulation is not a good indicator of its toxicity in these species. Overall, our study demonstrated that the differences in sensitivity to acute Cu exposure between white sturgeon and rainbow trout can be explained on the basis of differential effects of Cu on Na homeostasis.

Combined effects of polystyrene microplastics and natural organic matter on the accumulation and toxicity of copper in zebrafish

As emerging contaminants, microplastics (MPs) are predicted to act as vectors for other contaminants and their combined effects are largely unknown. In this study, the combined effects of MPs and natural organic matter (NOM) on the accumulation and toxicity of copper (Cu) in zebrafish (Danio rerio) were investigated. As a result, small-size MPs could absorb more Cu than large-size MPs. The presence of NOM promoted Cu adsorption on MPs in the pH range of 6-8. Our results demonstrate that the combination of MPs and NOM increased Cu accumulation in the livers and guts in a size-depended manner. Correspondingly, the results of biochemical test showed that MPs and NOM could aggravate Cu-toxicity in the livers and guts, which is manifested in the increased levels of malonaldehyde (MDA) and metallothionein (MT) and decreased levels of superoxide dismutase (SOD). Furthermore, the results of transcriptomic analysis suggested that such aggravation of toxicity was mainly attributed to the inhibition of Cu-ion transport and the enhanced oxidative stress. Since the co-existence of MPs and NOM in the environment is inevitable, their enhancement effects on the bioaccumulation and toxicity of other pollutants such as heavy metals deserve more attention.

Quantitative Response-Response Relationships Linking Aromatase Inhibition to Decreased Fecundity are Conserved Across Three Fishes with Asynchronous Oocyte Development

Quantitative adverse outcome pathways (qAOPs) describe quantitative response-response relationships that can predict the probability or severity of an adverse outcome for a given magnitude of chemical interaction with a molecular initiating event. However, the taxonomic domain of applicability for these predictions is largely untested. The present study began defining this applicability for a previously described qAOP for aromatase inhibition leading to decreased fecundity developed using data from fathead minnow (Pimephales promelas). This qAOP includes quantitative response-response relationships describing plasma 17β-estradiol (E2) as a function of plasma fadrozole, plasma vitellogenin (VTG) as a function of plasma E2, and fecundity as a function of plasma VTG. These quantitative response-response relationships simulated plasma E2, plasma VTG, and fecundity measured in female zebrafish (Danio rerio) exposed to fadrozole for 21 days but not these responses measured in female Japanese medaka (Oryzias latipes). However, Japanese medaka had different basal levels of plasma E2, plasma VTG, and fecundity. Normalizing basal levels of each measurement to equal those of female fathead minnow enabled the relationships to accurately simulate plasma E2, plasma VTG, and fecundity measured in female Japanese medaka. This suggests that these quantitative response-response relationships are conserved across these three fishes when considering relative change rather than absolute measurements. The present study represents an early step toward defining the appropriate taxonomic domain of applicability and extending the regulatory applications of this qAOP.

Species and life-stage specific differences in cadmium accumulation and cadmium induced oxidative stress, metallothionein and heat shock protein responses in white sturgeon and rainbow trout

Understanding the mechanistic basis of differences in the sensitivity of fishes to metals is important for developing informed ecological risk assessment approaches for metals. Whole body metal accumulation, metallothionein induction, oxidative stress and associated antioxidant response, as well as heat shock proteins (mainly HSP70) are known to play important roles in determining the toxicity of metals in fish. Hence, in this study we have cross-evaluated these parameters as a function of Cd exposure across different life-stages of two evolutionarily distinct fish species, namely rainbow (Oncorhynchus mykiss) trout and white sturgeon (Acipenser transmontanus). These two species have been shown to differ significantly in their physiological and apical responses to Cd exposure. The findings of the present study suggest that species-specific differences in the sensitivity to Cd could partially be explained by HSP70 gene response and oxidative damage biomarkers. However, not all the parameters studied in this study could explain the life-stage specific differences universally and were limited to only some life-stages. Based on the observations in the present study and other recent studies, it is apparent that species- and life-stage specific differences in the sensitivity to Cd and possibly other metals is a complex phenomenon and could be driven by multiple toxicokinetic and toxicodynamic factors.

Negative effects of acute cadmium on stress defense, immunity, and metal homeostasis in liver of zebrafish: The protective role of environmental zinc dpre-exposure

In the study, zebrafish were exposed to 0 and 200 μg/L Zn for 8 weeks, and then both groups were transferred to water including 0, 100, and 200 μg/L Cd for 4 days, respectively. Acute Cd exposure caused negative effects on stress defense, immune, and metal transport systems by increasing lipid peroxidation, iNOS activity and mRNA levels of il-6 and inos, and decreasing Cu/Zn-SOD and HSP70 levels, and mRNA levels of sod1, cat, hsp70, p65, mtf-1, znt5, zip7, atp7a, and atp7b. Lipid peroxidation was significantly reduced by Zn pre-exposure under Cd exposure, which may be explained by the enhanced stress defense capacity and the weaken inflammatory response. Firstly, Zn pre-exposure increased MTs and HSP70 levels and CAT activity in Cd-free water, which may facilitate fish quick response to Cd. Secondly, Zn pre-exposure reduced Cd accumulation at 100 and 200 μg/L Cd, down-regulated il-6 and il-1β at 100 μg/L Cd and p65 at 200 μg/L Cd, and increased Cu/Zn-SOD and CAT activities at 200 μg/L Cd. Thirdly, Zn pre-exposure alone up-regulated transcription factors (hsf1, hsf2, and mtf-1, and nrf2) and their target genes (sod1, cat, hsp70, and mt2) under Cd exposure in a dose-dependent manner. It should be noted that Zn pre-exposure down-regulated several metal transport genes dramatically at 0 and 100 μg/L Cd, which may be an important mechanism for reducing Cd import into livers. Overall, long-term and environmental Zn pre-exposure mitigated Cd toxicity by the enhanced stress defense capacity and the down-regulated metal transport and inflammatory responses.

Toxicokinetic and toxicodynamic (TK-TD) modeling to study oxidative stress-dependent toxicity of heavy metals in zebrafish

Adverse outcome pathways (AOP) have been proposed as a new method to improve the ecological risk assessment of pollutants, but it requires quantitation linkage between exposure, biomarker response and toxicity of pollutants. A toxicokinetic and toxicodynamic (TK-TD) model was used to quantify AOP of the toxicity of Cd and Pb to zebrafish, including the quantitative relationship between Cd and Pb accumulation in gill and oxidative damage level based on ROS or MDA, and LC₅₀ values at different times. Significant relationships were found between the oxidative damage level characterized by ROS and MDA content and Cd or Pb accumulation in gill (R² > 0.60), and the TK model could better simulate the Pb accumulation in the gills (R² > 0.60) than Cd. The increasing of Cd or Pb concentrations induced the generation of ROS and the formation of ROS initiated the fluctuation of MDA level in the cells as compared to controls (p < 0.05). For the individual level effect, the Damage Assessment Model (DAM) could successfully explain the change of LC_50-ROS and LC_50-MDA values at different times (R² > 0.99). Our findings suggested that the TK-TD model based on ROS and MDA could be used as a quantitative AOP to predict toxicity of metals to zebrafish.

Investigating the Role of Ionoregulatory Processes in the Species- and Life-Stage-Specific Differences in Sensitivity of Rainbow Trout and White Sturgeon to Cadmium

There are huge variations in life-stage- and species-specific sensitivities among the fishes to the exposure with metals; however, the physiological mechanisms underlying these differences are not well understood to date. This study revealed significant life-stage-specific (larval, swim-up, and juvenile) and species-specific differences between two evolutionary distant species of fishes, rainbow trout ( Oncorhynchus mykiss) and white sturgeon ( Acipenser transmontanus), following acute exposures to Cd. Although the 96 h LC50 of Cd was similar in both species at the larval stage, trout demonstrated an increased sensitivity to Cd at later life stages as compared to sturgeon. Moreover, exposure to Cd disrupted calcium (Ca) uptake and whole body Ca levels in trout by a greater degree relative to that in sturgeon regardless of life stage. Finally, white sturgeon demonstrated a lower affinity for Cd uptake relative to the more sensitive rainbow trout. This infers a differential nature of the interaction between Cd and Ca transport pathways in the two species and partially explains the differences in Cd sensitivity between rainbow trout and white sturgeon described previously. Overall, our results suggest that species- and life-stage-specific differences in sensitivity to waterborne Cd in fish are likely a function of the interplay between Cd uptake and Cd-induced disruption of Ca homeostasis.

Life-cycle exposure to cadmium induced compensatory responses towards oxidative stress in the liver of female zebrafish

The current study examined effects of waterborne cadmium (Cd) at environmental relevant concentrations (0, 2.5 and 5 μg/L) on growth, survival, histology, ultrastructure, metal homeostasis, and antioxidant responses in female zebrafish from embryos to sexually maturity for 15 weeks. Growth and survival rate were not significantly affected by Cd exposure. There were no significant changes in ultrastructure of cellular organelles, reactive oxygen species (ROS) levels, lipid peroxidation (LPO) in liver. However, Cd exposure increased Cd and lipid accumulation, reduced contents of zinc, copper and reduced glutathione (GSH), and down-regulated activity of copper/zinc-superoxide dismutase (Cu/Zn-SOD) in liver of zebrafish. Contrarily, the mRNA and activity levels of catalase (CAT), the mRNA levels of Cu/Zn-SOD, and the mRNA and protein levels of metallothioneins (MTs) were up-regulated. The transcriptional regulation of Cu and Zn transporters might be a vital mechanism by which fish slow the Zn and Cu uptake. Taken together, our data demonstrated that long-term and low-dose Cd induced adaptive responses with interlinked compensatory mechanism, which may protect fish against oxidative stress.

Influence of microplastics on the accumulation and chronic toxic effects of cadmium in zebrafish (Danio rerio)

As the accumulation of microplastics (MPs) in the environment continues to rise, more concerns focus on the health risk of combined exposure to MPs and other contaminants. The aim of this study is to investigate the influences of MPs on the tissue-accumulation of cadmium (Cd) in zebrafish and explore the related chronic toxic effects induced by combined exposure of Cd and MPs. After co-exposure to MPs and Cd for 3 weeks, 20 and 200 μg/L MPs increased the accumulation of Cd in zebrafish livers (46% and 184%), guts (10% and 25%) and gills (9% and 46%). The Cd accumulation was gill > gut > liver. Comprehensive analyzes of biochemical biomarkers, histopathological observation and functional gene expression firstly demonstrated that the presence of MPs enhanced the toxicity of Cd on zebrafish and the combined exposure caused oxidative damage and inflammation in zebrafish tissues. Collectively, our results highlight the chronic effects of combined exposure to MPs and heavy metals.

Expression stability and selection of optimal reference genes for gene expression normalization in early life stage rainbow trout exposed to cadmium and copper

Gene expression analysis represents a powerful approach to characterize the specific mechanisms by which contaminants interact with organisms. One of the key considerations when conducting gene expression analyses using quantitative real-time reverse transcription-polymerase chain reaction (qPCR) is the selection of appropriate reference genes, which is often overlooked. Specifically, to reach meaningful conclusions when using relative quantification approaches, expression levels of reference genes must be highly stable and cannot vary as a function of experimental conditions. However, to date, information on the stability of commonly used reference genes across developmental stages, tissues and after exposure to contaminants such as metals is lacking for many vertebrate species including teleost fish. Therefore, in this study, we assessed the stability of expression of 8 reference gene candidates in the gills and skin of three different early life-stages of rainbow trout after acute exposure (24h) to two metals, cadmium (Cd) and copper (Cu) using qPCR. Candidate housekeeping genes were: beta actin (b-actin), DNA directed RNA polymerase II subunit I (DRP2), elongation factor-1 alpha (EF1a), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), glucose-6-phosphate dehydrogenase (G6PD), hypoxanthine phosphoribosyltransferase (HPRT), ribosomal protein L8 (RPL8), and 18S ribosomal RNA (18S). Four algorithms, geNorm, NormFinder, BestKeeper, and the comparative ΔCt method were employed to systematically evaluate the expression stability of these candidate genes under control and exposed conditions as well as across three different life-stages. Finally, stability of genes was ranked by taking geometric means of the ranks established by the different methods. Stability of reference genes was ranked in the following order (from lower to higher stability): HPRT<GAPDH<EF1a<G6PD<RPL8<DRP2<b-actin in gills of fish exposed to Cd; b-actin<GAPDH<G6PD<DRP2<RPL8<HPRT<EF1a in gills of fish exposed to Cu; RPL8<HPRT<GAPDH<G6PD<EF1a<DRP2<b-actin in the skin of fish exposed to Cd; and EF1a<GAPDH<RPL8<HPRT<G6PD<b-actin<DRP2 in the skin of fish exposed to Cu. The results demonstrated that the stability of reference genes depended on the metal, life-stage and/or organ in question. Thus, attention should be paid to these factors before selection of reference gene for relative quantification of the gene expressions.

Immunosuppressive effects and associated compensatory responses in zebrafish after full life-cycle exposure to environmentally relevant concentrations of cadmium

In natural environments, fish survive in polluted water by cadmium (Cd) throughout their whole life cycle. However, little information is available on Cd toxicity considering a life cycle assessment. The present study investigated effects of environmental levels of cadmium (0, 2.5, and 5μg/L) on immune responses in liver and spleen of zebrafish for 15 weeks, from embryos to sexually maturity. Nitric oxide (NO) levels and iNOS activity declined in liver and spleen of zebrafish exposed to 5μg/L Cd, suggesting an immunosuppressive effect. The result was further supported by the decreased transcriptional levels of proinflammatory cytokines by Cd, such as interleukin-6 (IL-6), interleukin-10 (IL-10), interleukin-1β (IL-1β), and tumour necrosis factor-α (TNF-α) in liver. However, a sharp increase in the mRNA levels of these cytokines was observed in spleen of zebrafish exposed to Cd. The increased mRNA expression of these proinflammatory cytokines may be the secondary effect following immunosuppression and just reflect a compensatory mechanism for coping with the decreased immunity, which may explain an increase in mRNA levels and a decrease in iNOS activity in spleen of zebrafish exposed to Cd. In liver, the down-regulated mRNA levels of iNOS paralleled with the decreased iNOS activity, suggesting a synchronous response from a molecular level to a biochemical level. Positive correlations between mRNA expression levels of nuclear transcription factor κB (NF-κB) and proinflammatory cytokines were also observed, suggesting that NF-κB might be required for the protracted induction of inflammatory genes. The corresponding changes in the mRNA levels of the inhibitor of κBα (IκBαa and IκBαb) may form a feedback loop to restore transcriptional activity of NF-κB. Furthermore, splenic ROS levels were increased by 5μg/L Cd, possibly activating NF-κB pathway. Taken together, immunosuppressive effects and tissue-dependent compensatory responses were demonstrated in zebrafish after full life-cycle exposure to environmental levels of Cd, indicating a compromise between survival and immunity.

Is hepatic oxidative stress a main driver of dietary selenium toxicity in white sturgeon (Acipenser transmontanus)?

Most species of sturgeon have experienced significant population declines and poor recruitment over the past decades, leading many, including white sturgeon (Acipenser transmontanus), to be listed as endangered. Reasons for these declines are not yet fully understood but benthic lifestyle, longevity, and delayed sexual maturation likely render sturgeon particularly susceptible to factors such as habitat alteration and contaminant exposures. One contaminant of particular concern to white sturgeon is selenium (Se), especially in its more bioavailable form selenomethionine (SeMet), as it is known to efficiently bioaccumulate in prey items of this species. Studies have shown white sturgeon to be among the most sensitive species of fish to dietary SeMet as well as other pollutants such as metals, dioxin-like chemicals and endocrine disrupters. One of the primary hypothesized mechanisms of toxicity of SeMet in fish is oxidative stress; however, little is know about the specific mode by which SeMet affects the health of white sturgeon. Therefore, the aim of this study was to characterize oxidative stress and associated antioxidant responses as a molecular event of toxicity, and to link it with the pathological effects observed previously. Specifically, three-year-old white sturgeon were exposed for 72 days via their diet to 1.4, 5.6, 22.4 or 104.4µg Se per g feed (dm). Doses were chosen to range over a necessary Se intake level, current environmentally relevant intakes and an intake representing predicted increases of Se release. Lipid hydroperoxides, which are end products of lipid oxidation, were quantified as a marker of oxidative stress. Changes in gene expression of glutathione peroxidase (GPx), superoxide dismutase, catalase, glutathione S-transferase, apoptosis inducing factor and caspase 3 were quantified as markers of the response to oxidative stress. Concentrations of lipid hydroperoxides were highly variable within dose groups and no dose response was observed. GPx expression was significantly increased in the low dose group indicating an induced antioxidant response. Expression of other genes were not significantly induced or suppressed. Overall, there was very little evidence of oxidative stress, and therefore, in contrast to previous reports on other species of teleost fishes, oxidative stress is not believed to be a main driver of toxicity in white sturgeon exposed to SeMet.