Reproductive and developmental toxicity of dioxin in fish

The influence of AHR on immune and tissue biology

The aryl hydrocarbon receptor is a ligand dependent transcription factor which functions as an environmental sensor. Originally discovered as the sensor for man made pollutants such as 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) it has recently gained prominence as an important mediator for environmental triggers via the diet or microbiota which influences many physiological functions in different cell types and tissues across the body. Notably AHR activity contributes to prevent excessive inflammation following tissue damage in barrier organs such as skin, lung or gut which has received wide attention in the past decade. In this review we will focus on emerging common AHR functions across cell types and tissues and discuss ongoing issues that confound the understanding of AHR physiology. Furthermore, we will discuss the need for deeper molecular understanding of the functional activity of AHR in different contexts with respect to development of potential therapeutic applications.

Characterization of medaka (Oryzias latipes) AHRs and the comparison of two model fishes-Medaka vs. zebrafish: The subform-specific sensitivity to dioxin

Dioxins and the emerging dioxin-like compounds (DLCs) have recruited increasing concerns about their environmental contamination, toxicity, health impacts, and mechanisms. Based on the structural similarity of dioxins and many DLCs, their toxicity was predominantly mediated by the dioxin receptor (aryl hydrocarbon receptor, AHR) in animals (including human), which can be different in expression and function among species and then possibly produce the species-specific risk or toxicity. To date, characterizing the AHR of additional species other than human and rodents can increase the accuracy of toxicity/risk evaluation and increase knowledge about AHR biology. As a key model, the medaka AHR has not been clearly characterized. Through genome survey and phylogenetic analysis, we identified four AHRs (olaAHR1a, olaAHR1b, olaAHR2a, and olaAHR2b) and two ARNTs (olaARNT1 and olaARNT2). The medaka AHR pathway was conserved in expression in nine tested tissues, of which olaAHR2a represented the predominant subform with greater abundance. Medaka AHRs and ARNTs were functional and could be efficiently transactivated by the classical dioxin congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), although olaAHR1a did not seem to cooperate with olaARNT2. In terms of function/sensitivity, the EC50 values of medaka olaAHR1a (9.01 ± 1.43 nM), olaAHR1b (4.00 ± 1.10 nM), olaAHR2a (8.75 ± 3.34 nM), and olaAHR2b (3.06 ± 0.81 nM) showed slight differences; however, they were all at the nM level. The sensitivity of four medaka AHRs to TCDD was similar to that of zebrafish dreAHR2 (the dominant form, EC50 = 3.14 ± 4.19 nM), but these medaka AHRs were more sensitive than zebrafish dreAHR1b (EC50 = 27.05 ± 18.51 nM). The additional comparison also indicated that the EC50 values in various species were usually within the nM range, but AHRs of certain subforms/species can vary by one or two orders of magnitude. In summary, the present study will enhance the understanding of AHR and help improve research on the ecotoxicity of dioxins/DLCs.

Ecotoxicological risk of asphalt pavements to aquatic animals associated with pollutant leaching

Contaminants such as heavy metals and polycyclic aromatic hydrocarbons (PAHs) can be released from asphalt pavement and transported through stormwater runoff to nearby water bodies, leading to water pollution and potential harm to living aquatic animals. This study characterizes the heavy metal and PAH leaching from various asphalt paving materials and their potential ecotoxicological effects on zebrafish Danio rerio. Artificial runoffs were prepared in the laboratory concerning the effects of water, temperature, and traffic. The concentrations of heavy metals and PAHs in the leachates were quantified, while the toxicity assessment encompassed mortality, metal stress, PAH toxicity, inflammation, carcinogenicity, and oxidative damage. Gene expressions of related proteins or transcription factors were assessed, including metallothionines, aryl hydrocarbon receptors, interleukin-1β, interleukin-10, nuclear factor-κB, tumor necrosis factor-α, tumor suppressor p53, heat shock protein 70, and reactive oxygen species (ROS). The findings demonstrate that leachates from asphalt pavements containing waste bottom ash, crumb rubber, or specific chemicals could induce notable stress and inflammation responses in zebrafish. In addition, potential carcinogenic effects and the elevation of ROS were identified within certain treatment groups. This study represents the first attempt to assess the ecotoxicity of pavement leachates employing a live fish model, thereby improving the current understanding of the environmental impact of asphalt pavements.

Transcriptomics reveals that NAD(P)H affects the development of the Zig-zag eel (Mastacembelus armatus ♀) × Spiny eel (Sinobdella sinensis ♂) hybrid offspring leading to low hatching rates

Zig-zag eel (Mastacembelus armatus (2 n = 48)) and Spiny eel (Sinobdella sinensis (2 n = 48)) are two species of the Mastacembelidae family commonly found in southern China. Hybridization between the two has a very high deformity rate and a very low hatching rate. In order to investigate the reasons for this, the first hybridization between M. armatus and S. sinensis was carried out using artificial insemination, and the embryonic development of the hybrid offspring was examined using microphotography, and the malformations of the hybrid offspring were investigated by transcriptomics. The experiments showed that the average egg production was 4265.7 ± 322.94 (Mean ± SD), the average fertilization rate of hybrid offspring was 98.67 ± 0.58 % (Mean ± SD), the hatching rate was 12.06 ± 3.44 % (Mean ± SD), the deformity rate was 98.15 ± 3.21 % (Mean ± SD), and the embryonic development successively went through the five main stages of fertilized egg, egg cleavage, embryo formation, organogenesis, and exertion of membranes. Transcriptomics showed that the expression of NAD(P)H-related enzyme activity DEGs was increased, and many DEGs related to cell signaling molecule transmission and metabolic regulation are enriched in KEGG pathways, such as IL-17 signaling pathway, Osteoclast differentiation, TNF signaling pathway and MAPK signaling pathway. etc. The major types of DEGs corresponded to those coding for proteins. This study suggests that the high malformation rate in hybrid offspring may be caused by impaired synthesis of proteins during embryonic development.

Dioxins vs. PFAS: Science and Policy Challenges

BACKGROUND

Dioxin-like chemicals are a group of ubiquitous environmental toxicants that received intense attention in the last two decades of the 20th century. Through extensive mechanistic research and validation, the global community has agreed upon a regulatory strategy for these chemicals that centers on their common additive activation of a single receptor. Applying these regulations has led to decreased exposure in most populations studied. As dioxin-like chemicals moved out of the limelight, research and media attention has turned to other concerning contaminants, including per- and polyfluoroalkyl substances (PFAS). During the 20th century, PFAS were also being quietly emitted into the environment, but only in the last 20 years have we realized the serious threat they pose to health. There is active debate about how to appropriately classify and regulate the thousands of known PFAS and finding a solution for these "forever chemicals" is of the utmost urgency.

OBJECTIVES

Here, we compare important features of dioxin-like chemicals and PFAS, including the history, mechanism of action, and effective upstream regulatory strategies, with the objective of gleaning insight from the past to improve strategies for addressing PFAS.

DISCUSSION

The differences between these two chemical classes means that regulatory strategies for dioxin-like chemicals will not be appropriate for PFAS. PFAS exert toxicity by both receptor-based and nonreceptor-based mechanisms, which complicates mixtures evaluation and stymies efforts to develop inexpensive assays that accurately capture toxicity. Furthermore, dioxin-like chemicals were unwanted byproducts, but PFAS are useful and valuable, which has led to intense resistance against efforts to restrict their production. Nonetheless, useful lessons can be drawn from dioxin-like chemicals and applied to PFAS, including eliminating nonessential production of new PFAS and proactive investment in environmental remediation to address their extraordinarily long environmental persistence. https://doi.org/10.1289/EHP14449.

Long-term exposure to aryl hydrocarbon receptor agonist neburon induces reproductive toxicity in male zebrafish (Danio rerio)

Neburon is a phenylurea herbicide that is widely used worldwide, but its toxicity is poorly studied. In our previous study, we found that neburon has strong aryl hydrocarbon receptor (AhR) agonist activity, but whether it causes reproductive toxicity is not clear. In the present study, zebrafish were conducted as a model organism to evaluate whether environmental concentrations of neburon (0.1, 1 and 10 µg/L) induce reproductive disorder in males. After exposure to neburon for 150 days from embryo to adult, that the average spawning egg number in high concentration group was 106.40, which was significantly lower than 193.00 in control group. This result was mainly due to the abnormal male reproductive behavior caused by abnormal transcription of genes associated with reproductive behavior in the brain, such as secretogranin-2a. The proportions of spermatozoa in the medium and high concentration groups were 82.40% and 83.84%, respectively, which were significantly lower than 89.45% in control group. This result was mainly caused by hormonal disturbances and an increased proportion of apoptotic cells. The hormonal disruption was due to the significant changes in the transcription levels of key genes in the hypothalamus-pituitary-gonadal axis following neburon treatment. Neburon treatment also significantly activated the AhR signaling pathway, causing oxidative stress damage and eventually leading to a significant increase in apoptosis in the exposed group. Together, these data filled the currently more vacant profile of neburon toxicity and might provide information to assess the ecotoxicity of neburon on male reproduction at environmentally relevant concentrations.

Zebrafish Congenital Heart Disease Models: Opportunities and Challenges

Congenital heart defects (CHDs) are common human birth defects. Genetic mutations potentially cause the exhibition of various pathological phenotypes associated with CHDs, occurring alone or as part of certain syndromes. Zebrafish, a model organism with a strong molecular conservation similar to humans, is commonly used in studies on cardiovascular diseases owing to its advantageous features, such as a similarity to human electrophysiology, transparent embryos and larvae for observation, and suitability for forward and reverse genetics technology, to create various economical and easily controlled zebrafish CHD models. In this review, we outline the pros and cons of zebrafish CHD models created by genetic mutations associated with single defects and syndromes and the underlying pathogenic mechanism of CHDs discovered in these models. The challenges of zebrafish CHD models generated through gene editing are also discussed, since the cardiac phenotypes resulting from a single-candidate pathological gene mutation in zebrafish might not mirror the corresponding human phenotypes. The comprehensive review of these zebrafish CHD models will facilitate the understanding of the pathogenic mechanisms of CHDs and offer new opportunities for their treatments and intervention strategies.

Relevance of flounder caging and proteomics to explore the impact of a major industrial accident caused by fire on the Seine estuarine water quality

On September 26th 2019, a major fire occurred in the Lubrizol factory located near the Seine estuary, in Rouen-France. Juvenile flounders were captured in the Canche estuary (a reference system) and caged one month in the Canche and in the Seine downstream the accident site. No significant increases of PAHs, PCBs and PFAS was detected in Seine vs Canche sediments after the accident, but a significant increase of dioxins and furans was observed in water and sewage sludge in the Rouen wastewater treatment plant. The proteomics approach highlighted a dysregulation of proteins associated with cholesterol synthesis and lipid metabolism, in fish caged in the Seine. The overall results suggested that the fire produced air borne dioxins and furans that got deposited on soil and subsequently entered in the Seine estuarine waters via runoff; thus contaminating fish preys and caged flounders in the Seine estuary.

Comprehensive characterization and prioritization of halogenated organic compounds in fish and their implications for exposure

Fish are an important pollution indicator for biomonitoring of halogenated organic compounds (HOCs) in aquatic environments, and HOCs in fish may pose health threats to consumers. This study performed nontarget and comprehensive analyses of HOCs in fish from an e-waste recycling zone by gas chromatography-high-resolution mass spectrometry, and further prioritized their human exposure risks. A total of 1652 formulas of HOCs were found in the fish, of which 1222, 117, and 313 were organochlorines, organobromines, and organochlorine-bromines, respectively. The total concentrations of HOCs were 15.4-18.7 μg/g (wet weight), and organobromines were the predominant (14.1-16.8 μg/g). Of the HOCs, 41 % were elucidated with tentative structures and divided into 13 groups. The estimated total daily exposures of HOCs via dietary consumption of the fish for local adult residents were 3082-3744 ng/kg bw/day. The total exposures were dominated by several groups of HOCs with the following contribution order: polyhalogenated biphenyls and their derivatives > polyhalogenated diphenyl ethers > halo- (H-)alkanes/olefines > H-benzenes > H-dioxins > H-polycyclic aromatic hydrocarbons > H-phenols. The comprehensive characterization and prioritization results provide an overview of the species and distributions of HOCs in edible fish, and propose an inventory of crucial HOCs associated with high exposure risks.

Independently evolved pollution resistance in four killifish populations is largely explained by few variants of large effect

The genetic architecture of phenotypic traits can affect the mode and tempo of trait evolution. Human-altered environments can impose strong natural selection, where successful evolutionary adaptation requires swift and large phenotypic shifts. In these scenarios, theory predicts that adaptation is due to a few adaptive variants of large effect, but empirical studies that have revealed the genetic architecture of rapidly evolved phenotypes are rare, especially for populations inhabiting polluted environments. Fundulus killifish have repeatedly evolved adaptive resistance to extreme pollution in urban estuaries. Prior studies, including genome scans for signatures of natural selection, have revealed some of the genes and pathways important for evolved pollution resistance, and provide context for the genotype-phenotype association studies reported here. We created multiple quantitative trait locus (QTL) mapping families using progenitors from four different resistant populations, and using RAD-seq genetically mapped variation in sensitivity (developmental perturbations) following embryonic exposure to a model toxicant PCB-126. We found that one to two large-effect QTL loci accounted for resistance to PCB-mediated developmental toxicity. QTLs harbored candidate genes that govern the regulation of aryl hydrocarbon receptor (AHR) signaling. One QTL locus was shared across all populations and another was shared across three populations. One QTL locus showed strong signatures of recent natural selection in the corresponding wild population but another QTL locus did not. Some candidate genes for PCB resistance inferred from genome scans in wild populations were identified as QTL, but some key candidate genes were not. We conclude that rapidly evolved resistance to the developmental defects normally caused by PCB-126 is governed by few genes of large effect. However, other aspects of resistance beyond developmental phenotypes may be governed by additional loci, such that comprehensive resistance to PCB-126, and to the mixtures of chemicals that distinguish urban estuaries more broadly, may be more genetically complex.

Immunotoxicity and transcriptome analysis of zebrafish embryos exposure to Nitazoxanide

Nitazoxanide (NTZ) is a broad-spectrum immunomodulatory drug, and little information is about the immunotoxicity of aquatic organisms induced by NTZ. In the present study, reduced body length and decreased yolk sac absorption in the NTZ-treated group were observed. Meanwhile, the number of innate immune cells and adaptive immune cells was substantially reduced upon NTZ exposure, and the migration and retention of macrophages and neutrophils in the injured area were inhibited. Following NTZ stimulation, oxidative stress levels in the zebrafish increased obviously. Mechanistically, RNA-seq, a high-throughput method, was performed to analyze the global expression of differentially expressed genes (DEGs) in zebrafish embryos treated with NTZ. 531 DEGs were identified by comparative transcriptome analysis, including 121 up-regulated and 420 down-regulated genes in zebrafish embryos after NTZ exposure. The transcriptome sequences were further subjected to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and gene ontology (GO) and analysis, showing phototransduction and metabolic pathway, respectively, and were most enriched. In addition, some immune-related genes were inhibited after NTZ exposure. RNA-seq results confirmed by qRT-PCR were used to verify the expression of the 6 selected genes. The other immune-related genes such as two pro-inflammatory cytokines (IL-1β, tnfα) and two chemokines (CXCL8b.3, CXCL-c1c) were further confirmed and were differentially regulated after NTZ exposure. In summary, NTZ exposure could lead to immunotoxicity and increased ROS in zebrafish embryos, this study provides valuable information for future elucidating the molecular mechanism of exogenous stimuli-induced immunotoxicity in aquatic ecosystems.

Reevaluation of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Equivalency Factors for Dioxin-Like Polychlorinated Dibenzo-p-Dioxins, Polychlorinated Dibenzofurans, and Polychlorinated Biphenyls for Fishes

An expert meeting was organized by the World Health Organization (WHO) in 1997 to streamline assessments of risk posed by mixtures of dioxin-like chemicals (DLCs) through development of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) equivalency factors (TEFs) for mammals, birds, and fishes. No reevaluation has been performed for fish TEFs. Therefore, the objective of the present study was to reevaluate the TEFs for fishes based on an updated database of relative potencies (RePs) for DLCs. Selection criteria consistent with the WHO meeting resulted in 53 RePs across 14 species of fish ultimately being considered. Of these RePs, 70% were not available at the time of the WHO meeting. These RePs were used to develop updated TEFs for fishes based on a similar decision process as used at the WHO meeting. The updated TEF for 16 DLCs was greater than the WHO TEF, but only four differed by more than an order of magnitude. Measured concentrations of DLCs in four environmental samples were used to compare 2,3,7,8-TCDD equivalents (TEQs) calculated using the WHO TEFs relative to the updated TEFs. The TEQs for none of these environmental samples differed by more than an order of magnitude. Therefore, present knowledge supports that the WHO TEFs are suitable potency estimates for fishes. However, the updated TEFs pull from a larger database with a greater breadth of data and as a result offer greater confidence relative to the WHO TEFs. Risk assessors will have different criteria in the selection of TEFs, and the updated TEFs are not meant to immediately replace the formal WHO TEFs; but those who value a larger database and increased confidence in TEQs could consider using the updated TEFs. Environ Toxicol Chem 2023;42:2215-2228. © 2023 Wiley Periodicals LLC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Lingering Effects of Legacy Industrial Pollution on Yellow Perch of the Detroit River

We used yellow perch (Perca flavescens) captured at four sites differing in legacy industrial pollution in the Lake St. Clair-Detroit River system to evaluate the lingering sublethal effects of industrial pollution. We emphasized bioindicators of direct (toxicity) and indirect (chronic stress, impoverished food web) effects on somatic and organ-specific growth (brain, gut, liver, heart ventricle, gonad). Our results show that higher sediment levels of industrial contaminants at the most downstream Detroit River site (Trenton Channel) are associated with increased perch liver detoxification activity and liver size, reduced brain size, and reduced scale cortisol content. Trenton Channel also displayed food web disruption, where adult perch occupied lower trophic positions than forage fish. Somatic growth and relative gut size were lower in perch sampled at the reference site in Lake St. Clair (Mitchell's Bay), possibly because of increased competition for resources. Models used to determine the factors contributing to site differences in organ growth suggest that the lingering effects of industrial pollution are best explained by trophic disruption. Thus, bioindicators of fish trophic ecology may prove advantageous to assess the health of aquatic ecosystems. Environ Toxicol Chem 2023;42:2158-2170. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

AOP Report: Aryl Hydrocarbon Receptor Activation Leads to Early-Life Stage Mortality via Sox9 Repression-Induced Craniofacial and Cardiac Malformations

The aryl hydrocarbon receptors (Ahrs) are evolutionarily conserved ligand-dependent transcription factors that are activated by structurally diverse endogenous compounds as well as environmental chemicals such as polycyclic aromatic hydrocarbons and halogenated aromatic hydrocarbons. Activation of the Ahr leads to several transcriptional changes that can cause developmental toxicity resulting in mortality. Evidence was assembled and evaluated for two novel adverse outcome pathways (AOPs) which describe how Ahr activation (molecular initiating event) can lead to early-life stage mortality (adverse outcome), via either SOX9-mediated craniofacial malformations (AOP 455) or cardiovascular toxicity (AOP 456). Using a key event relationship (KER)-by-KER approach, we collected evidence using both a narrative search and a systematic review based on detailed search terms. Weight of evidence for each KER was assessed to inform overall confidence of the AOPs. The AOPs link to previous descriptions of Ahr activation and connect them to two novel key events (KEs), increase in slincR expression, a newly characterized long noncoding RNA with regulatory functions, and suppression of SOX9, a critical transcription factor implicated in chondrogenesis and cardiac development. In general, confidence levels for KERs ranged between medium and strong, with few inconsistencies, as well as several opportunities for future research identified. While the majority of KEs have only been demonstrated in zebrafish with 2,3,7,8-tetrachlorodibenzo-p-dioxin as an Ahr activator, evidence suggests that the two AOPs likely apply to most vertebrates and many Ahr-activating chemicals. Addition of the AOPs into the AOP-Wiki (https://aopwiki.org/) helps expand the growing Ahr-related AOP network to 19 individual AOPs, of which six are endorsed or in progress and the remaining 13 relatively underdeveloped. Environ Toxicol Chem 2023;42:2063-2077. © 2023 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Tritium: Its relevance, sources and impacts on non-human biota

Tritium (³H) is a radioactive isotope of hydrogen that is abundantly released from nuclear industries. It is extremely mobile in the environment and in all biological systems, representing an increasing concern for the health of both humans and non-human biota (NHB). The present review examines the sources and characteristics of tritium in the environment, and evaluates available information pertaining to its biological effects at different levels of biological organisation in NHB. Despite an increasing number of publications in the tritium radiobiology field, there exists a significant disparity between data available for the different taxonomic groups and species, and observations are heavily biased towards marine bivalves, fish and mammals (rodents). Further limitations relate to the scarcity of information in the field relative to the laboratory, and lack of studies that employ forms of tritium other than tritiated water (HTO). Within these constraints, different responses to HTO exposure, from molecular to behavioural, have been reported during early life stages, but the potential transgenerational effects are unclear. The application of rapidly developing "omics" techniques could help to fill these knowledge gaps and further elucidate the relationships between molecular and organismal level responses through the development of radiation specific adverse outcome pathways (AOPs). The use of a greater diversity of keystone species and exposures to multiple stressors, elucidating other novel effects (e.g., by-stander, germ-line, transgenerational and epigenetic effects) offers opportunities to improve environmental risk assessments for the radionuclide. These could be combined with artificial intelligence (AI) including machine learning (ML) and ecosystem-based approaches.

Independently evolved pollution resistance in four killifish populations is largely explained by few variants of large effect

The genetic architecture of phenotypic traits can affect the mode and tempo of trait evolution. Human-altered environments can impose strong natural selection, where successful evolutionary adaptation requires swift and large phenotypic shifts. In these scenarios, theory predicts the influence of few adaptive variants of large effect, but empirical studies that have revealed the genetic architecture of rapidly evolved phenotypes are rare, especially for populations inhabiting polluted environments. Fundulus killifish have repeatedly evolved adaptive resistance to extreme pollution in urban estuaries. Prior studies, including genome scans for signatures of natural selection, have revealed some of the genes and pathways important for evolved pollution resistance, and provide context for the genotype-phenotype association studies reported here. We created multiple quantitative trait locus (QTL) mapping families using progenitors from four different resistant populations, and genetically mapped variation in sensitivity (developmental perturbations) following embryonic exposure to a model toxicant PCB-126. We found that a few large-effect QTL loci accounted for resistance to PCB-mediated developmental toxicity. QTLs harbored candidate genes that govern the regulation of aryl hydrocarbon receptor (AHR) signaling, where some (but not all) of these QTL loci were shared across all populations, and some (but not all) of these loci showed signatures of recent natural selection in the corresponding wild population. Some strong candidate genes for PCB resistance inferred from genome scans in wild populations were identified as QTL, but some key candidate genes were not. We conclude that rapidly evolved resistance to the developmental defects normally caused by PCB-126 is governed by few genes of large effect. However, other aspects of resistance beyond developmental phenotypes may be governed by additional loci, such that comprehensive resistance to PCB-126, and to the mixtures of chemicals that distinguish urban estuaries more broadly, may be more genetically complex.

Effects of Hudson River Stressors on Atlantic Tomcod: Contaminants and a Warming Environment

The Hudson River (HR) Estuary has a long history of pollution with a variety of contaminants including PCBs, and dioxins. In fact, 200 miles of the mainstem HR is designated a U.S. federal Superfund site, the largest in the nation, because of PCB contamination. The tidal HR hosts the southernmost spawning population of Atlantic tomcod, and studies revealed a correlation between exposure of juveniles to warm water temperature during summer to abundance of spawning adults of the same cohort in the following winter. Further, a battery of mechanistically linked biomarkers, ranging from the molecular to the population levels, were significantly impacted from contaminant exposures of the HR tomcod population. In response to xenobiotic insult, the HR tomcod population developed resistance to PCB sand TCDD toxicity resulting from a deletion in the aryl hydrocarbon receptor2 (AHR2) gene. Furthermore, RNA-Seq analysis of global gene expression demonstrated that effects of the AHR2 polymorphism were far more pervasive than anticipated. The most highly PCB-contaminated sediments in the upper HR were dredged between 2009 and 2015 with the objective of lowering PCB concentrations in fishes in the lower HR. Success of the remediation project has been controversial. These observations suggest that tomcod provides an informative model to evaluate the efficacy of HR PCB remediation efforts on downriver fish populations and possible interactive effects between contaminant exposure and a warming environment.

AhR-mediated lipid peroxidation contributes to TCDD-induced cardiac defects in zebrafish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a persistent environmental contaminant that activates the aryl hydrocarbon receptor (AhR) pathway, has been reported to cause cardiac damage. However, the mechanism underlying AhR-induced cardiac defects in response to TCDD exposure remains unclear. In this study, we characterized the impacts of TCDD exposure on heart morphology and cardiac function in zebrafish. TCDD exposure in the early developmental stage of zebrafish embryos led to morphological heart malformation and pericardial edema, concomitant with reduced cardiac function. These cardiac defects were attenuated by inhibiting AhR activity with CH223191. Transcriptome profiling showed that, along with an upregulation of the AhR signaling pathway by TCDD treatment, the expression of pro-ferroptotic genes was upregulated, while that of genes implicated in glutathione metabolism were downregulated. Moreover, lipid peroxidation, as indicated by malonaldehyde (MDA) production, was increased in TCDD-exposed cardiac tissue. Accordingly, inhibiting lipid peroxidation with liproxstatin-1 reversed the adverse cardiac effects induced by TCDD treatment. Taken together, our findings demonstrate that AhR-mediated lipid peroxidation contributes to cardiac defects in the early developmental stage in zebrafish embryos exposed to TCDD.

Review of ecologically relevant in vitro bioassays to supplement current in vivo tests for whole effluent toxicity testing - Part 1: Apical endpoints

Whole effluent toxicity (WET) testing is commonly used to ensure that wastewater discharges do not pose an unacceptable risk to receiving environments. Traditional WET testing involves exposing animals to (waste)water samples to assess four major ecologically relevant apical endpoints: mortality, growth, development, and reproduction. Recently, with the widespread implementation of the 3Rs to replace, reduce and refine the use of animals in research and testing, there has been a global shift away from in vivo testing towards in vitro alternatives. However, prior to the inclusion of in vitro bioassays in regulatory frameworks, it is critical to establish their ecological relevance and technical suitability. This is part 1 of a two-part review that aims to identify in vitro bioassays that can be used in WET testing and relate them to ecologically relevant endpoints through toxicity pathways, providing the reader with a high-level overview of current capabilities. Part 1 of this review focuses on four apical endpoints currently included in WET testing: mortality, growth, development, and reproduction. For each endpoint, the link between responses at the molecular or cellular level, that can be measured in vitro, and the adverse outcome at the organism level were established through simplified toxicity pathways. Additionally, literature from 2015 to 2020 on the use of in vitro bioassays for water quality assessments was reviewed to identify a list of suitable bioassays for each endpoint. This review will enable the prioritization of relevant endpoints and bioassays for incorporation into WET testing.

Using food waste to cultivate safe, good-quality Sabah (giant hybrid) grouper: Dioxins and dioxin-like polychlorinated biphenyls

Dioxins and dioxin-like polychlorinated biphenyls (dl-PCBs) in fish fillet counteract the health benefits of fish products. In this study, food waste was used as a protein alternative to replacing fishmeal commonly used in the commercial fish feed, aiming to cultivate Sabah grouper with acceptable levels of dioxins and dl-PCBs. The concentrations of dioxins and dl-PCBs, as well as the fish growth performance, were compared between the fish groups fed with food waste-based feed (FWBF) and commercial feed (Nanyu®, control). The results showed that the concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) (1.22 pg/g dry weight (d.w.)) and non-ortho-dl-PCBs (13.0 pg/g d.w.) were significantly lower (p < 0.05) in the FWBF than in the control feed (commercial feed) (PCDDs: 2.35 pg/g d.w.; non-ortho-dl-PCBs: 27.2 pg/g d.w.). The growth performance of the fish group fed with the FWBF was comparable to that fed with the control feed. There were no significant differences between the WHO₂₀₀₅-TEQ values of different fish fillets (1.00, 1.11, and 1.10 pg WHO₂₀₀₅-TEQ/g d.w. for FWBF group, control feed group, and local market fish, respectively). Based on the guidelines provided by European Food Safety Authority (ESFA) and U.S. Environmental Protection Agency (USEPA), the fish fed with the FWBF were safe for human consumption (hazard index values: 0.284-0.522; cancer risk range: 2.59-2.97 × 10^-5). The findings of this study suggest that food waste could serve as an alternative protein source for cultivating Sabah grouper with acceptable levels of dioxins and dl-PCBs.

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

Aryl hydrocarbon receptor (AhR)-mediated signaling as a critical regulator of skeletal cell biology

The aryl hydrocarbon receptor (AhR) has been implicated in regulating skeletal progenitor cells and the activity of bone-forming osteoblasts and bone-resorbing osteoclasts, thereby impacting bone mass and the risk of skeletal fractures. The AhR also plays an important role in the immune system within the skeletal niche and in the differentiation of mesenchymal stem cells into other cell lineages including chondrocytes and adipocytes. This transcription factor responds to environmental pollutants which can act as AhR ligands, initiating or interfering with various signaling cascades to mediate downstream effects, and also responds to endogenous ligands including tryptophan metabolites. This review comprehensively describes the reported roles of the AhR in skeletal cell biology, focusing on mesenchymal stem cells, osteoblasts, and osteoclasts, and discusses how AhR exhibits sexually dimorphic effects in bone. The molecular mechanisms mediating AhR's downstream effects are highlighted to emphasize the potential importance of targeting this signaling cascade in skeletal disorders.

Aryl hydrocarbon receptor agonist diuron and its metabolites cause reproductive disorders in male marine medaka (Oryzias melastigma)

Diuron, a widely used phenylurea herbicide, has been frequently detected in marine organism and seawater all over the world. But the understanding of potential damage of diuron on reproduction in marine fish is currently not enough. Herein, marine medaka (Oryzias melastigma) were continuously exposed to 0, 5, 50, 500, and 5000 ng/L diuron from embryo (0 dpf) to adult (180 dpf) stage. The results suggested that diuron had an adverse influence on male reproduction for marine medaka, including decreased gonado somatic index, histological changes of testes, decreased mobility of sperm, and reduced fecundity through disrupting the balance of sex hormone and genes expression related to hypothalamus-pituitary-gonadal-liver (HPGL) axis. The reduced fecundity was reflected in abnormal sexual behaviors, further inhibited growth and development of F1 embryo and larvae. Moreover, the proportion of diuron metabolites (DCPMU and DCPU) was increased in fish, but the proportion of diuron was decreased with the increasing of exposure concentration. Diuron, DCPMU, and DCPU was identified as aryl hydrocarbon receptor agonist (AhR) agonist using in silico and in vivo models. DCPMU and DCPU induced the gene expression of AhR signaling and metabolizing enzymes (such as cyp1a1) in the livers. A great deal of major metabolites affected various organs related to HPGL axis of male marine medaka and led to serious reproductive disorders. Consequently, it reveals that long-term exposure to environmentally relevant concentrations of diuron and even AhR agonist pesticides pose a potential ecological risk for marine fish.

Embryonic cardio-respiratory impairments in rainbow trout (Oncorhynchus mykiss) following exposure to hydraulic fracturing flowback and produced water

During hydraulic fracturing, wastewaters - termed flowback and produced water (FPW) - are created as a by-product during hydrocarbon extraction. Given the large volumes of FPW that a single well can produce, and the history of FPW release to surface water bodies, it is imperative to understand the hazards that hydraulic fracturing and FPW pose to aquatic biota. Using rainbow trout embryos as model organisms, we investigated impacts to cardio-respiratory system development and function following acute (48 h) and sub-chronic (28-day) FPW exposure by examining occurrences of developmental deformities, rates of embryonic respiration (MO₂), and changes in expression of critical cardiac-specific genes. FPW-exposed embryos had significantly increased rates of pericardial edema, yolk-sac edema, and tail/trunk curvatures at hatch. Furthermore, when exposed at three days post-fertilization (dpf), acute 5% FPW exposures significantly increased embryonic MO₂ through development until 15 dpf, where a switch to significantly reduced MO₂ rates was subsequently recorded. A similar trend was observed during sub-chronic 1% FPW exposures. Interestingly, at certain specific developmental timepoints, previous salinity exposure seemed to affect embryonic MO₂; a result not previously observed. Following acute FPW exposures, embryonic genes for cardiac development and function were significantly altered, although at termination of sub-chronic exposures, significant changes to these same genes were not found. Together, our evidence of induced developmental deformities, modified embryonic MO₂, and altered cardiac transcript expression suggest that cardio-respiratory tissues are toxicologically targeted following FPW exposure in developing rainbow trout. These results may be helpful to regulatory bodies when developing hazard identification and risk management protocols concerning hydraulic fracturing activities.

Alkylation of Benz[a]anthracene Affects Toxicity to Early-Life Stage Zebrafish and In Vitro Aryl Hydrocarbon Receptor 2 Transactivation in a Position-Dependent Manner

Polycyclic aromatic hydrocarbons (PAHs) are structurally diverse organic chemicals that can have adverse effects on the health of fishes through activation of aryl hydrocarbon receptor 2 (AhR2). They are ubiquitous in the environment, but alkyl PAHs are more abundant in some environmental matrices. However, relatively little is known regarding the effects of alkylation on the toxicity of PAHs to fishes in vivo and how this relates to potency for activation of AhR2 in vitro. Therefore, the objectives of the present study were to determine the toxicity of benz[a]anthracene and three alkylated homologs representing various alkylation positions to early life stages of zebrafish (Danio rerio) and to assess the potency of each for activation of the zebrafish AhR2 in a standardized in vitro AhR transactivation assay. Exposure of embryos to each of the PAHs caused a dose-dependent increase in mortality and malformations characteristic of AhR2 activation. Each alkyl homolog had in vivo toxicities and in vitro AhR2 activation potencies different from those of the parent PAH in a position-dependent manner. However, there was no statistically significant linear relationship between responses measured in these assays. The results suggest a need for further investigation into the effect of alkylation on the toxicity of PAHs to fishes and greater consideration of the contribution of alkylated homologs in ecological risk assessments. Environ Toxicol Chem 2022;41:1993-2002. © 2022 SETAC.

Effects of dietary 2-(2H-benzotriazol-2-yl)-4-methylphenol (UV-P) exposure on Japanese medaka (Oryzias latipes) in a short-term reproduction assay

Benzotriazole ultraviolet stabilizers (BZT-UVs) are added to various products to prevent damage caused by UV light and have emerged as contaminants of concern. Although BZT-UVs are detected in aquatic biota globally, few studies have assessed their potential toxic effects. The objective of the present study was to assess effects of 2-(2H-Benzotriazol-2-yl)-4-methylphenol (UV-P) on reproductive success of Japanese medaka (Oryzias latipes) in a standard 21-day reproduction assay. Japanese medaka were exposed to dietary UV-P at concentrations of 0, 36, 158, and 634 ng UV-P/g food, for a total of 28 days which included 7 days of exposure prior to the start of the 21-day reproduction assay. No significant effect on egg production or fertilization success was observed. Abundances of transcripts of erα, vtgI, cyp1a, or cyp3a4 were not significantly different in livers from male or female fish exposed to UV-P. However, abundances of transcripts of cyp11a and cyp19a were significantly lower in gonads from female fish. There was a trend of increasing concentrations of E2 and a non-significant increase of T in the 634 ng/g treatment in plasma from female fish exposed to UV-P. Concentrations of 11-KT were unchanged in plasma from males exposed to UV-P. These responses suggest weak perturbation of steroidogenesis, consistent with an antiandrogenic mode of action. However, this perturbation was insufficient to impair reproductive performance. Metabolomics analysis of female livers suggests altered concentrations of various metabolites and biological pathways, including glutathione metabolism, suggesting that UV-P might cause responses related to oxidative stress or phase II metabolism. However, metabolomics revealed no obvious mechanism of toxicity. Overall, results of this study indicate that dietary exposure to UV-P up to 634 ng/g food does not significantly impact reproductive performance of Japanese medaka but impacts on steroidogenesis could indicate a potential mechanism of toxicity which might lead to reproductive impairment in more sensitive species.

Epidermal cell cultures from white and green sturgeon (Acipenser transmontanus and medirostris): Expression of TGM1-like transglutaminases and CYP4501A

Using a system optimized for propagating human keratinocytes, culture of skin samples from white and green sturgeons generated epithelial cells capable of making cross-linked protein envelopes. Two distinct forms of TGM1-like mRNA were molecularly cloned from the cells of white sturgeon and detected in green sturgeon cells, accounting for their cellular envelope forming ability. The protein translated from each displayed a cluster of cysteine residues resembling the membrane anchorage region expressed in epidermal cells of teleosts and tetrapods. One of the two mRNA forms (called A) was present at considerably higher levels than the other (called B) in both species. Continuous lines of white sturgeon epidermal cells were established and characterized. Size measurements indicated that a substantial fraction of the cells became enlarged, appearing similar to squames in human epidermal keratinocyte cultures. The cultures also expressed CYP1A, a cytochrome P450 enzyme inducible by activation of aryl hydrocarbon receptor 2 in fish. The cells gradually improved in growth rate over a dozen passages while retaining envelope forming ability, TGM1 expression and CYP1A inducibility. These cell lines are thus potential models for studying evolution of fish epidermis leading to terrestrial adaptation and for testing sturgeon sensitivity to environmental stresses such as pollution.

Comparative toxicity study of waterborne two booster biocides (CuPT and ZnPT) on embryonic flounder (Paralichthys olivaceus)

A new generation of booster biocides that include metal pyrithiones (PTs) such as copper pyrithione (CuPT) and zinc pyrithione (ZnPT) are being used as tributyltin alternatives. In the marine environment, ZnPT can easily transchelate Cu to form CuPT, and the environmental fate and persistence of these two metal pyrithiones are closely related. Although some data on the toxicity of biocides on marine fish are available, little is known about their toxicity and toxic pathway. We thus compared the toxic effects of CuPT and ZnPT on embryonic olive flounder (Paralichthys olivaceus) by investigating their adverse effects based on developmental morphogenesis and transcriptional variation. In our study, the toxic potency of CuPT was greater with respect to developmental malformation and mortality than ZnPT. Consistent with the developmental effects, the expression of genes related to tail fin malformation (including plod2, furin, and wnt3a) was higher in embryonic flounder exposed to CuPT than in those exposed to ZnPT. Genes related to muscle and nervous system development exhibited significant changes on differential gene expression profiles using RNA sequencing (cutoff value P < 0.05). Gene ontology analysis of embryos exposed to CuPT revealed affected cellular respiration and kidney development, whereas genes associated with cell development, nervous system development and heart development showed significant variation in embryonic flounder exposed to ZnPT. Overall, our study clarifies the common and unique developmental toxic effects of CuPT and ZnPT through transcriptomic analyses in embryonic flounder.

The antidepressant venlafaxine perturbs cardiac development and function in larval zebrafish

Venlafaxine, a selective serotonin and norepinephrine reuptake inhibitor, is a highly prescribed antidepressant and is detected at µg/L concentrations in waterways receiving municipal wastewater effluents. We previously showed that early-life venlafaxine exposure disrupted the normal development of the nervous system and reduces larval activity in zebrafish (Danio rerio). However, it is unclear whether the reduced swimming activity may be associated with impaired cardiac function. Here we tested the hypothesis that zygotic exposure to venlafaxine impacts the development and function of the larval zebrafish heart. Venlafaxine (0, 1 or 10 ng) was administered by microinjection into freshly fertilized zebrafish embryos (1-4 cell stage) to assess heart development and function during early-life stages. Venlafaxine deposition in the zygote led to precocious development of the embryo heart, including the timing of the first heartbeat, increased heart size, and a higher heart rate at 24- and 48-hours post-fertilization (hpf). Also, waterborne exposure to environmental levels of this antidepressant during early development increased the heart rate at 48 hpf of zebrafish larvae mimicking the zygotic deposition. The venlafaxine-induced higher heart rate in the embryos was abolished in the presence of NAN-190, an antagonist of the 5HT_1A receptor. Also, heart rate dropped below control levels in the 10 ng, but not 1 ng venlafaxine group at 72 and 96 hpf. An acute stressor reduced the venlafaxine-induced heart rate at 48 hpf but did not affect the already reduced heart rate at 72 and 96 hpf in the 10 ng venlafaxine group. Our results suggest that the higher heart rate in the venlafaxine group may be due to an enhanced serotonin stimulation of the 5HT_1A receptor. Taken together, early-life venlafaxine exposure disrupts cardiac development and has the potential to compromise the cardiovascular performance of larval zebrafish.

Xenobiotic metabolism and its physiological consequences in high-Antarctic Notothenioid fishes

The Antarctic ecosystem is progressively exposed to anthropogenic contaminants, such as polycyclic aromatic hydrocarbons (PAHs). So far, it is largely unknown if PAHs leave a mark in the physiology of high-Antarctic fish. We approached this issue via two avenues: first, we examined the functional response of the aryl hydrocarbon receptor (Ahr), which is a molecular initiating event of many toxic effects of PAHs in biota. Chionodraco hamatus and Trematomus loennbergii served as representatives for high-Antarctic Notothenioids, and Atlantic cod, Gadus morhua as non-polar reference species. We sequenced and cloned the Ahr ligand binding domain (LBD) of the Notothenioids and deployed a GAL4-based luciferase reporter gene assay expressing the Ahr LBD. Benzo[a]pyrene (BaP), beta-naphthoflavone and chrysene were used as ligands for the reporter gene assay. Second, we investigated the energetic costs of Ahr activation in isolated liver cells of the Notothenioids during acute, non-cytotoxic BaP exposure. In the reporter assay, the Ahr LBD of Atlantic cod and the Antarctic Notothenioids were activated by the ligands tested herein. In the in vitro assays with isolated liver cells of high-Antarctic Notothenioids, BaP exposure had no effect on overall respiration, but caused shifts in the respiration dedicated to protein synthesis. Thus, our study demonstrated that high-Antarctic fish possess a functional Ahr that can be ligand-activated in a concentration-dependent manner by environmental contaminants. This is associated with altered cost for cellular protein synthesis. Future studies have to show if the toxicant-induced activation of the Ahr pathway may lead to altered organism performance of Antarctic fish.

Species-Specific Differences in Aryl Hydrocarbon Receptor Responses: How and Why?

The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates a wide range of biological and toxicological effects by binding to specific ligands. AhR ligands exist in various internal and external ecological systems, such as in a wide variety of hydrophobic environmental contaminants and naturally occurring chemicals. Most of these ligands have shown differential responses among different species. Understanding the differences and their mechanisms helps in designing better experimental animal models, improves our understanding of the environmental toxicants related to AhR, and helps to screen and develop new drugs. This review systematically discusses the species differences in AhR activation effects and their modes of action. We focus on the species differences following AhR activation from two aspects: (1) the molecular configuration and activation of AhR and (2) the contrast of cis-acting elements corresponding to AhR. The variations in the responses seen in humans and other species following the activation of the AhR signaling pathway can be attributed to both factors.

Using In Vitro and Machine Learning Approaches to Determine Species-Specific Dioxin-like Potency and Congener-Specific Relative Sensitivity among Birds for Brominated Dioxin Analogues

There is a paucity of experimental data regarding dioxin-like toxicity of polybrominated dibenzo-p-dioxins/dibenzofurans (PBDD/Fs) and non-ortho polybrominated biphenyls (PBBs). In this study, avian aryl hydrocarbon receptor 1 (AHR1)-luciferase reporter gene assays were used to determine their species-specific dioxin-like potencies (DLPs) and congener-specific interspecies relative sensitivities in birds. The results suggested that DLPs of the brominated congeners for chicken-like (Ile324_Ser380) species did not always follow World Health Organization toxicity equivalency factors of their chlorinated analogues. For ring-necked pheasant-like (Ile324_Ala380) and Japanese quail-like (Val324_Ala380) species, the difference in DLP for several congeners was 1 or even 2 orders of magnitude. Moreover, molecular docking and molecular dynamics simulation were performed to explore the interactions between the brominated congeners and AHR1-ligand-binding domain (LBD). The molecular mechanics energy (E_MM) between each congener and each individual amino acid (AA) residue in AHR1-LBD was calculated. These E_MM values could finely characterize the final conformation of species-specific AHR1-LBD for each brominated congener. Based on this, mechanism-driven generalized linear models were successfully built using machine learning algorithms and the spline approximation method, and these models could qualitatively predict the complex relationships between AHR1 conformations and DLPs or avian interspecies relative sensitivity to brominated dioxin-like compounds (DLCs). In addition, several AAs conserved among birds were found to potentially interact with species-specific AAs, thereby inducing species-specific interactions between AHR1 and brominated DLCs. The present study provides a novel strategy to facilitate the development of mechanism-driven computational prediction models for supporting safety assessment of DLCs, as well as a basis for the ecotoxicological risk assessment of brominated congeners in birds.

Reproductive toxicity due to herbicide exposure in freshwater organisms

Excessively used pesticides in agricultural areas are spilled into aquatic environments, wherein they are suspended or sedimented. Owing to climate change, herbicides are the fastest growing sector of the pesticide industry and are detected in surface water, groundwater, and sediments near agricultural areas. In freshwater, organisms, including mussels, snails, frogs, and fish, are exposed to various types and concentrations of herbicides. Invertebrates are sensitive to herbicide exposure because their defense systems are incomplete. At the top of the food chain in freshwater ecosystems, fish show high bioaccumulation of herbicides. Herbicide exposure causes reproductive toxicity and population declines in freshwater organisms and further contamination of fish used for consumption poses a risk to human health. In addition, it is important to understand how environmental factors are physiologically processed and assess their impacts on reproductive parameters, such as gonadosomatic index and steroid hormone levels. Zebrafish is a good model for examining the effects of herbicides such as atrazine and glyphosate on embryonic development in freshwater fish. This review describes the occurrence and role of herbicides in freshwater environments and their potential implications for the reproduction and embryonic development of freshwater organisms.

Immunotoxicity of Xenobiotics in Fish: A Role for the Aryl Hydrocarbon Receptor (AhR)?

The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.

Oxidative stress inducers potentiate 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated pre-cardiac edema in larval zebrafish

We reported the involvement of oxidative stress and prostaglandins including thromboxane and prostacyclin in pre-cardiac edema (early edema) caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). While the involvement of oxidative stress in TCDD-induced toxicity has been frequently reported, the mechanism of its action is still unclear. In the present study, oxidative stress inducers including paraquat, hydrogen peroxide (H₂O₂) and rotenone augmented early edema (edema) induced by a low concentration of TCDD (0.1 ppb) at 55 hr post fertilization (hpf), while each of them alone did not cause edema. Edema caused by TCDD plus oxidative stress inducers was almost abolished by antioxidants, an antagonist for thromboxane receptor (ICI-192,605) and an agonist for prostacyclin receptor (beraprost), suggesting that the site of action of these inducers was in the regular signaling pathway after activation of aryl hydrocarbon receptor type 2 (AHR2) by TCDD. Oxidative stress inducers also enhanced edema caused by an agonist for the thromboxane receptor (U46619), and the enhancement was also inhibited by antioxidants. Sulforaphane and auranofin, activators of Nrf2 that is a master regulator of anti-oxidative response, did not affect U46619-evoked edema but almost abolished TCDD-induced edema and potentiation by paraquat in both TCDD- and U46619-induced edema. Taken together, the results suggest that oxidative stress augments pre-cardiac edema caused by TCDD via activation of thromboxane receptor-mediated signaling in developing zebrafish. As paraquat and other oxidative stress inducers used also are environmental pollutants, interaction between dioxin-like compounds and exogenous source of oxidative stress should also be considered.

Dibenzyl trisulfide binds to and competitively inhibits the cytochrome P450 1A1 active site without impacting the expression of the aryl hydrocarbon receptor

The toxicological manifestation of many pollutants relies upon their binding to the aryl hydrocarbon receptor (AHR), and it follows a cascade of reactions culminating in an elevated expression of cytochrome P450 (CYP) 1 enzymes. CYP1A1 and CYP1B1 are associated with enhanced carcinogenesis when chronically exposed to certain polyaromatic hydrocarbons, and their inhibition may lead to chemoprevention. We evaluated dibenzyl trisulfide (DTS), expressed in the ethnomedical plant, Petiveria alliacea, for such potential chemoprevention. Using recombinant human CYP1A1 and CYP1B1 bactosomes on a fluorogenic assay, we first demonstrated that DTS moderately inhibited both enzymes with half maximal inhibitory concentration (IC50) values of 1.3 ± 0.3 and 1.7 ± 0.3 μM, respectively. Against CYP1A1, DTS was a reversible, competitive inhibitor with an apparent inhibitory constant (Ki) of 4.55 ± 0.37 μM. In silico molecular modeling showed that DTS binds with an affinity of -39.8 kJ·mol-1, situated inside the binding pocket, approximately 4.3 Å away from the heme group, exhibiting interactions with phenylalanine residue 123 (Phe-123), Phe-224, and Phe-258. Lastly, zebrafish (Danio rerio) embryos were exposed to 0.08-0.8 μM DTS from 24 to 96 h post fertilization (hpf) with the in vivo ethoxyresorufin-O-deethylase (EROD) assay, and, at 96 hpf, DTS significantly suppressed EROD CYP1A activity in a dose-dependent manner, with up to 60% suppression in the highest 0.8 μM exposure group. DTS had no impact on gene transcription levels for cyp1a and aryl hydrocarbon receptor 2 (ahr2). In co-exposure experiments, DTS suppressed CYP1A activity induced by both B[a]P and PCB-126, although these reductions were not significant. Taken together, these results demonstrate that DTS is a direct, reversible, competitive inhibitor of the carcinogen-activating CYP1A enzyme, binding in the active site pocket close to the heme site, and shows potential in chemoprevention.

2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure disrupts development of the visceral and ocular vasculature

The aryl hydrocarbon receptor (AHR) has endogenous functions in mammalian vascular development and is necessary for mediating the toxic effects of a number of environmental contaminants. Studies in mice have demonstrated that AHR is necessary for the formation of the renal, retinal, and hepatic vasculature. In fish, exposure to the prototypic AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces expression of the AHR biomarker cyp1a throughout the developing vasculature and produces vascular malformations in the head and heart. However, it is not known whether the vascular structures that are sensitive to loss of AHR function are also disrupted by aberrant AHR activation. Here, we report that TCDD-exposure in zebrafish disrupts development of 1) the subintestinal venous plexus (SIVP), which vascularizes the developing liver, kidney, gut, and pancreas, and 2) the superficial annular vessel (SAV), an essential component of the retinal vasculature. Furthermore, we determined that TCDD exposure increased the expression of bmp4, a key molecular mediator of SIVP morphogenesis. We hypothesize that the observed SIVP phenotypes contribute to one of the hallmarks of TCDD exposure in fish - the failure of the yolk sac to absorb. Together, our data describe novel TCDD-induced vascular phenotypes and provide molecular insight into critical factors producing the observed vascular malformations.

Maternal transfer of polychlorinated biphenyls in Pacific sand lance (Ammodytes personatus), Puget Sound, Washington

We measured polychlorinated biphenyls (PCBs) in multiple age and size classes of Pacific sand lance (Ammodytes personatus), including eggs, young-of-the year, and adults to evaluate maternal transfer as a pathway for contaminant uptake and to add to the limited information on the occurrence of PCBs in sand lance in Puget Sound. Sampling was replicated at an urban embayment (Eagle Harbor) and a state park along an open shoreline (Clayton Beach), during spring and fall. Lipid-normalized concentrations of PCBs in sand lance at Eagle Harbor were 5-11 times higher than PCB concentrations in comparable samples at Clayton Beach. This was true for every life stage and size class of sand lance, including eggs removed from females. The same trend was observed in environmental samples. In Eagle Harbor, PCB concentrations in unfiltered water (0.19 ng/L), sieved (<63 μm) nearshore bed sediments (0.78 ng/g dw) and suspended particulate matter (1.69 ng/g dw) were 2-3 times higher than equivalent samples from near Clayton Beach. Sand lance collected in the fall (buried in sediment during presumed winter dormancy) had lower lipid content and up to four times higher PCB concentrations than comparably sized fish collected in the spring (by beach seine). Lipid content was 5-8% in spring fish and was reduced in fall fish (1-3%). Male sand lance had higher PCB concentrations than comparable females. All egg samples contained PCBs, and the lipid normalized egg/female concentration ratios were close to 1 (0.87-0.96), confirming that maternal transfer of PCBs occurred, resulting in sand lance eggs and early life stages being contaminated with PCBs even before they are exposed to exogenous sources. These life stages are prey for an even wider range of species than consume adult sand lance, creating additional exposure pathways for biota and increasing the challenges for mitigation of PCBs in the food web.

Zebrafish (Danio rerio) as an excellent vertebrate model for the development, reproductive, cardiovascular, and neural and ocular development toxicity study of hazardous chemicals

In the past decades, the type of chemicals has gradually increased all over the world, and many of these chemicals may have a potentially toxic effect on human health. The zebrafish, as an excellent vertebrate model, is increasingly used for assessing chemical toxicity and safety. This review summarizes the efficacy of zebrafish as a model for the study of developmental toxicity, reproductive toxicity, cardiovascular toxicity, neurodevelopmental toxicity, and ocular developmental toxicity of hazardous chemicals, and the transgenic zebrafish as biosensors are used to detect the environmental pollutants.

The sulfate metabolite of 3,3'-dichlorobiphenyl (PCB-11) impairs Cyp1a activity and increases hepatic neutral lipids in zebrafish larvae (Danio rerio)

The environmental contaminant 3,3'-dichlorobiphenyl (PCB-11) is widely detected in environmental samples, and this parent compound along with its metabolites 4-OH-PCB-11 and 4-PCB-11-Sulfate are detected in human serum. Our previous research in zebrafish (Danio rerio) embryos shows exposure to 20 μM PCB-11 inhibits Cyp1a enzyme activity and perturbs lipid metabolism pathways. In this study, wildtype AB embryos underwent acute exposures from 1 to 4 days post fertilization (dpf) to 0.002-20 μM 4-OH-PCB-11 or 0.2-20 μM 4-PCB-11-Sulfate, with and without co-exposures to 100 μg/L benzo[a]pyrene (B[a]P) or 5 nM 3,3',4,4',5-pentachlorobiphenyl (PCB-126), and were assessed for in vivo EROD activity and morphometrics. Chronic exposures from 1 to 15 dpf to assess lipid accumulation using Oil-Red-O staining were also conducted with 0.2 μM parent or metabolite compounds, alongside a co-exposure experiment of 0.002-0.2 μM 4-PCB-11-Sulfate and 10 μg/L B[a]P. For acute experiments, 2 and 20 μM 4-OH-PCB-11 was lethal but no Cyp1a or morphological effects were observed at lower concentrations; 20 μM 4-PCB-11-Sulfate significantly lowered the Cyp1a activity of B[a]P and PCB-126 but did not alter morphological development. For chronic experiments, 0.2 μM 4-PCB-11-Sulfate significantly increased lipid accumulation 30% in single exposures and 44% in co-exposures with B[a]P. Further long-term studies would better elucidate the effects of this contaminant, particularly in the context of environmentally-relevant mixtures.

A Review of the Functional Roles of the Zebrafish Aryl Hydrocarbon Receptors

Over the last 2 decades, the zebrafish (Danio rerio) has emerged as a stellar model for unraveling molecular signaling events mediated by the aryl hydrocarbon receptor (AHR), an important ligand-activated receptor found in all eumetazoan animals. Zebrafish have 3 AHRs-AHR1a, AHR1b, and AHR2, and studies have demonstrated the diversity of both the endogenous and toxicological functions of the zebrafish AHRs. In this contemporary review, we first highlight the evolution of the zebrafish ahr genes, and the characteristics of the receptors including developmental and adult expression, their endogenous and inducible roles, and the predicted ligands from homology modeling studies. We then review the toxicity of a broad spectrum of AHR ligands across multiple life stages (early stage, and adult), discuss their transcriptomic and epigenetic mechanisms of action, and report on any known interactions between the AHRs and other signaling pathways. Through this article, we summarize the promising research that furthers our understanding of the complex AHR pathway through the extensive use of zebrafish as a model, coupled with a large array of molecular techniques. As much of the research has focused on the functions of AHR2 during development and the mechanism of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) toxicity, we illustrate the need to address the considerable knowledge gap in our understanding of both the mechanistic roles of AHR1a and AHR1b, and the diverse modes of toxicity of the various AHR ligands.

Exposure to the AhR agonist cyprodinil impacts the cardiac development and function of zebrafish larvae

Cyprodinil is a broad-spectrum pyrimidine amine fungicide that has been reportedly used worldwide. However, toxicity studies of cyprodinil on aquatic organisms, specifically zebrafish (Danio rerio), are lacking. In our present study, we predicted cyprodinil binding to the aryl hydrocarbon receptor (AhR) by using molecular docking simulation. Then, we used recombinant HepG2 cells and Tg(cyp1a1-12DRE:egfp) transgenic zebrafish to further assess the AhR agonistic activity of cyprodinil. Besides, the significant upregulation of cyp1a1 further verified that statement. Moreover, we found that zebrafish exposure to cyprodinil induced developmental toxicity in the larvae, particularly during cardiac development. The expression levels of cardiac development-related genes, namely tbx5, nkx2.5, gata4, and tnnt2, were markedly altered, which might cause the adverse effects of cyprodinil on cardiac function and development. In summary, we found that cyprodinil, as an AhR agonist, induced development toxicity in zebrafish larvae, especially on cardiac. Data here can assess the potential effects on organisms in the aquatic environment and promote the regulation and safe use of cyprodinil.

Generation and application of a Tg(cyp1a:egfp) transgenic marine medaka (Oryzias melastigma) line as an in vivo assay to sensitively detect dioxin-like compounds in the environment

Large-range environmental pollution by dioxin and dioxin-like compounds (DLCs) is becoming a serious problem. To establish an in vivo method for the detection of DLCs in seawater, a Tg(cyp1a-12DRE:egfp) transgenic marine medaka (Oryzias melastigma) line was first developed with the modified cyp1a-12DRE promoter driving enhanced green fluorescent protein (EGFP) expression using Tol2 transgenesis technology. With increasing concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), the EGFP fluorescence intensity increased significantly. The Tg(cyp1a-12DRE:egfp) medaka possessed high sensitivity (limit of detection of 1 ng/L TCDD) and specificity and low background. This transgenic line is capable of detecting DLCs in environmental seawater in which the concentration of DLCs is at least 0.12207 ng/L TCDD after sample enrichment. The fluorescence-toxic equivalency (TEQ) values from EGFP intensity were closely correlated with the chemical-TEQ values obtained from chemical analyses. Furthermore, the Tg(cyp1a-12DRE:egfp) medaka can directly detect DLCs in seawater samples after a serious pollution accident and screen unknown aryl hydrocarbon receptor (AhR) agonists for risk assessment. For the first time, a convenient method has been established that sensitively and specifically responds to DLCs using the Tg(cyp1a-12DRE:egfp) marine medaka, which could be a highly efficient tool for detecting seawater DLCs in the future.

Using Zebrafish to Analyze the Genetic and Environmental Etiologies of Congenital Heart Defects

Congenital heart defects (CHDs) are among the most common human birth defects. However, the etiology of a large proportion of CHDs remains undefined. Studies identifying the molecular and cellular mechanisms that underlie cardiac development have been critical to elucidating the origin of CHDs. Building upon this knowledge to understand the pathogenesis of CHDs requires examining how genetic or environmental stress changes normal cardiac development. Due to strong molecular conservation to humans and unique technical advantages, studies using zebrafish have elucidated both fundamental principles of cardiac development and have been used to create cardiac disease models. In this chapter we examine the unique toolset available to zebrafish researchers and how those tools are used to interrogate the genetic and environmental contributions to CHDs.

Casting a wide net: use of diverse model organisms to advance toxicology

Toxicology - the study of how chemicals interact with biological systems - has clear relevance to human health and disease. Persistent exposure to natural and synthetic chemicals is an unavoidable part of living on our planet; yet, we understand very little about the effects of exposure to the vast majority of chemicals. While epidemiological studies can provide strong statistical inference linking chemical exposure to disease, research in model systems is essential to elucidate the mechanisms of action and to predict outcomes. Most research in toxicology utilizes a handful of mammalian models that represent a few distinct branches of the evolutionary tree. This narrow focus constrains the understanding of chemical-induced disease processes and systems that have evolved in response to exposures. We advocate for casting a wider net in environmental toxicology research to utilize diverse model systems, including zebrafish, and perform more mechanistic studies of cellular responses to chemical exposures to shift the perception of toxicology as an applied science to that of a basic science. This more-inclusive perspective will enrich the field and should remain central to research on chemical-induced disease.

Host monitoring of quorum sensing during Pseudomonas aeruginosa infection

Pseudomonas aeruginosa rapidly adapts to altered conditions by quorum sensing (QS), a communication system that it uses to collectively modify its behavior through the production, release, and detection of signaling molecules. QS molecules can also be sensed by hosts, although the respective receptors and signaling pathways are poorly understood. We describe a pattern of regulation in the host by the aryl hydrocarbon receptor (AhR) that is critically dependent on qualitative and quantitative sensing of P. aeruginosa quorum. QS molecules bind to AhR and distinctly modulate its activity. This is mirrored upon infection with P. aeruginosa collected from diverse growth stages and with QS mutants. We propose that by spying on bacterial quorum, AhR acts as a major sensor of infection dynamics, capable of orchestrating host defense according to the status quo of infection.

Cardiotoxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure through lactation in mice

Dioxins are a group of structurally related chemicals that persist in the environment. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener, is a suspected risk factor for cardiac diseases in humans. TCDD induces signs of cardiotoxicity in various animals. Mouse models of TCDD exposure suggest cardiotoxicity phenotypes develop differently depending on the timing and time-course of exposure. In order to clarify and characterize the TCDD-induced cardiotoxicity in the developing period, we utilized mouse pups exposed to TCDD. One day after delivery, groups of nursing C57BL/6J dams were orally administered TCDD at a dose of 0 (Control), 20 (TCDD-20), or 80 μg/kg (TCDD-80) body weight (BW). On postnatal days (PNDs) 7 and 21, pups' hearts were examined by histological and gene expression analyses. The TCDD-80 group was found to have a left ventricular remodeling on PND 7, and to develop heart hypertrophy on PND 21. It was accompanied by fibrosis and increased expression of associated genes, such as those for atrial natriuretic peptide (ANP), β-myosin heavy chain (β-MHC), and endothelin-1 (ET-1). These results revealed that TCDD directly induces cardiotoxicity in the postnatal period represented by progressive hypertrophy in which ANP, β-MHC, and ET-1 have potentials to mediate the cardiac hypertrophy and heart failure.

The cardiovascular toxicity induced by high doses of gatifloxacin and ciprofloxacin in zebrafish

As a new type of pollutant, fluoroquinolones (FQs) antibiotics are ubiquitous in environment and have some threat to human health and ecological environment. Their ecological toxicity to the environment urgently need to be assessed. Therefore, we firstly explored the toxic effects and possible mechanism of cardiovascular toxicity induced by gatifloxacin (GTFX) and ciprofloxacin (CPFX) using zebrafish model. After 24 h exposure, the zebrafish treated with GTFX showed pericardial edema which was further investigated by histopathological examination, while CPFX exposure did not induce morphological abnormalities. However, both of them induced cardiac dysfunction, such as decreased heart rate and cardiac output which was showed a positive correlation with the concentration. To better understand the possible molecular mechanisms underlying cardiovascular toxicity in zebrafish, we investigated the transcriptional level of genes related to calcium signaling pathway and cardiac muscle contraction. The results indicated that the expression of ATPase (atp2a1l) and cardiac troponin C (tnnc1a) genes were significantly inhibited, the expression of calcium channel (cacna1ab) gene showed slight promoted trend after CPFX exposure. For zebrafish treated with GTFX, the expression of atp2a1l genes was also significantly inhibited, while the expression of tnnc1a genes was slightly inhibited and cacna1ab genes expression had no obvious effect. The present study firstly revealed that GTFX exposure can induce morphological and functional abnormalities on the cardiovascular system of zebrafish. Though CPFX exposure did not induce morphological abnormalities, the function of cardiovascular system was still damaged. Mechanistically, this toxicity might result from the pressure of down-regulation of genes associated with calcium signaling pathway and cardiac muscle contraction. The results of this study can provide a valuable theoretical basis for the establishment of FQs environmental quality standards in water environment, environmental drug regulation and risk management.

The emerging contaminant 3,3'-dichlorobiphenyl (PCB-11) impedes Ahr activation and Cyp1a activity to modify embryotoxicity of Ahr ligands in the zebrafish embryo model (Danio rerio)

3,3'-dichlorobiphenyl (PCB-11) is an emerging PCB congener widely detected in environmental samples and human serum, but its toxicity potential is poorly understood. We assessed the effects of three concentrations of PCB-11 on embryotoxicity and Aryl hydrocarbon receptor (Ahr) pathway interactions in zebrafish embryos (Danio rerio). Wildtype AB or transgenic Tg(gut:GFP) strain zebrafish embryos were exposed to static concentrations of PCB-11 (0, 0.2, 2, or 20 μM) from 24 to 96 h post fertilization (hpf), and gross morphology, Cytochrome P4501a (Cyp1a) activity, and liver development were assessed via microscopy. Ahr interactions were probed via co-exposures with PCB-126 or beta-naphthoflavone (BNF). Embryos exposed to 20 μM PCB-11 were also collected for PCB-11 body burden, qRT-PCR, RNAseq, and histology. Zebrafish exposed to 20 μM PCB-11 absorbed 0.18% PCB-11 per embryo at 28 hpf and 0.61% by 96 hpf, and their media retained 1.36% PCB-11 at 28 hpf and 0.84% at 96 hpf. This concentration did not affect gross morphology, but altered the transcription of xenobiotic metabolism and liver development genes, impeded liver development, and increased hepatocyte vacuole formation. In co-exposures, 20 μM PCB-11 prevented deformities caused by PCB-126 but exacerbated deformities in co-exposures with BNF. This study suggests that PCB-11 can affect liver development, act as a partial agonist/antagonist of the Ahr pathway, and act as an antagonist of Cyp1a activity to modify the toxicity of compounds that interact with the Ahr pathway.