Cardiac myocyte-specific AHR activation phenocopies TCDD-induced toxicity in zebrafish

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.

Protective Potential of a Botanical-Based Supplement Ingredient against the Impact of Environmental Pollution on Cutaneous and Cardiopulmonary Systems: Preclinical Study

Air pollution is a growing threat to human health. Airborne pollution effects on respiratory, cardiovascular and skin health are well-established. The main mechanisms of air-pollution-induced health effects involve oxidative stress and inflammation. The present study evaluates the potential of a polyphenol-enriched food supplement ingredient comprising Lippia citriodora, Olea europaea, Rosmarinus officinalis, and Sophora japonica extracts in mitigating the adverse effects of environmental pollution on skin and cardiopulmonary systems. Both in vitro and ex vivo studies were used to assess the blend's effects against pollution-induced damage. In these studies, the botanical blend was found to reduce lipid peroxidation, inflammation (by reducing IL-1α), and metabolic alterations (by regulating MT-1H, AhR, and Nrf2 expression) in human skin explants exposed to a mixture of pollutants. Similar results were also observed in keratinocytes exposed to urban dust. Moreover, the ingredient significantly reduced pollutant-induced ROS production in human endothelial cells and lung fibroblasts, while downregulating the expression of apoptotic genes (bcl-2 and bax) in lung fibroblasts. Additionally, the blend counteracted the effect of urban dust on the heart rate in zebrafish embryos. These results support the potential use of this supplement as an adjuvant method to reduce the impact of environmental pollution on the skin, lungs, and cardiovascular tissues.

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.

Proper modulation of AHR signaling is necessary for establishing neural connectivity and oligodendrocyte precursor cell development in the embryonic zebrafish brain

2,3,7,8-tetrachlorodibenzo-[p]-dioxin (TCDD) is a persistent global pollutant that exhibits a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. Epidemiological studies have associated AHR agonist exposure with multiple human neuropathologies. Consistent with the human data, research studies using laboratory models have linked pollutant-induced AHR activation to disruptions in learning and memory as well as motor impairments. Our understanding of endogenous AHR functions in brain development is limited and, correspondingly, scientists are still determining which cell types and brain regions are sensitive to AHR modulation. To identify novel phenotypes resulting from pollutant-induced AHR activation and ahr2 loss of function, we utilized the optically transparent zebrafish model. Early embryonic TCDD exposure impaired embryonic brain morphogenesis, resulted in ventriculomegaly, and disrupted neural connectivity in the optic tectum, habenula, cerebellum, and olfactory bulb. Altered neural network formation was accompanied by reduced expression of synaptic vesicle 2. Loss of ahr2 function also impaired nascent network development, but did not affect gross brain or ventricular morphology. To determine whether neural AHR activation was sufficient to disrupt connectivity, we used the Gal4/UAS system to express a constitutively active AHR specifically in differentiated neurons and observed disruptions only in the cerebellum; thus, suggesting that the phenotypes resulting from global AHR activation likely involve multiple cell types. Consistent with this hypothesis, we found that TCDD exposure reduced the number of oligodendrocyte precursor cells and their derivatives. Together, our findings indicate that proper modulation of AHR signaling is necessary for the growth and maturation of the embryonic zebrafish brain.

Synergistic protective effects of folic acid and resveratrol against fine particulate matter-induced heart malformations in zebrafish embryos

Ambient fine particulate matter (PM2.5) is a major environmental health problem worldwide, and recent studies indicate that maternal PM2.5 exposure is closely associated with congenital heart diseases (CHDs) in offspring. We previously found that supplementation with folic acid (FA) or Resveratrol (RSV) could protect against heart defects in zebrafish embryos exposed to extractable organic matter (EOM) from PM2.5 by targeting aryl hydrocarbon receptor (AHR) signaling and reactive oxygen species (ROS) production respectively. Thus, we hypothesized that FA combined with RSV may have a synergistic protective effect against PM2.5-induced heart defects. To test our hypothesis, we treated zebrafish embryos with EOM in the presence or absence of FA, RSV or a combination of both. We found that RSV and FA showed a clear synergistic protection against EOM-induced heart defects in zebrafish embryos. Further studies showed that FA and RSV suppressed EOM-induced AHR activity and ROS generation respectively. Although only RSV inhibited EOM-induced apoptosis, FA enhanced the inhibitory effect of RSV. Moreover, vitamin C (VC), a typical antioxidant, also exhibits a synergistic inhibitory effect with FA on EOM-induced apoptosis and heart defects. In conclusion, supplementation with FA and RSV have a synergistic protective effect against PM2.5-induced heart defects in zebrafish embryos by targeting AHR activity and ROS production respectively. Our results indicate that, in the presence of antioxidants, FA even at a low concentration level could protect against the high risk of CHDs caused by air pollution.

Perfluorooctane Sulfonamide (PFOSA) Induces Cardiotoxicity via Aryl Hydrocarbon Receptor Activation in Zebrafish

Perfluorooctane sulfonamide (PFOSA), a precursor of perfluorooctanesulfonate (PFOS), is widely used during industrial processes, though little is known about its toxicity, particularly to early life stage organisms that are generally sensitive to xenobiotic exposure. Here, following exposure to concentrations of 0.01, 0.1, 1, 10, and 100 μg/L PFOSA, transcriptional, morphological, physiological, and biochemical assays were used to evaluate the potential effects on aquatic organisms. The top Tox functions in exposed zebrafish were related to cardiac diseases predicted by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and Ingenuity Pathway Analysis (IPA) analysis. Consistent with impacts predicted by transcriptional changes, abnormal cardiac morphology, disordered heartbeat signals, as well as reduced heart rate and cardiac output were observed following the exposure of 0.1, 1, 10, or 100 μg/L PFOSA. Furthermore, these PFOSA-induced cardiac effects were either prevented or alleviated by supplementation with an aryl hydrocarbon receptor (AHR) antagonist or ahr2-morpholino knock-down, uncovering a seminal role of AHR in PFOSA-induced cardiotoxicity. Our results provide the first evidence in fish that PFOSA can impair proper heart development and function and raises concern for PFOSA analogues in the natural environment.

Photo-enhanced toxicity of crude oil on early developmental stages of Atlantic cod (Gadus morhua)

Photo-enhanced toxicity of crude oil is produced by exposure to ultraviolet (UV) radiation. Atlantic cod (Gadus morhua) embryos were exposed to crude oil with and without UV radiation (290-400 nm) from 3 days post fertilization (dpf) until 6 dpf. Embryos from the co-exposure experiment were continually exposed to UV radiation until hatching at 11 dpf. Differences in body burden levels and cyp1a expression in cod embryos were observed between the exposure regimes. High doses of crude oil produced increased mortality in cod co-exposed embryos, as well as craniofacial malformations and heart deformities in larvae from both experiments. A higher number of differentially expressed genes (DEGs) and pathways were revealed in the co-exposure experiment, indicating a photo-enhanced effect of crude oil toxicity. Our results provide mechanistic insights into crude oil and photo-enhanced crude oil toxicity, suggesting that UV radiation increases the toxicity of crude oil in early life stages of Atlantic cod.

Developmental toxicity testing of unsubstituted and methylated 4- and 5-ring polycyclic aromatic hydrocarbons using the zebrafish embryotoxicity test

The present study evaluates the in vitro developmental toxicity of 4- and 5-ring PAHs including benz[a]anthracene and benzo[a]pyrene and six of their monomethylated congeners, and dibenz[a,h]anthracene using the zebrafish embryotoxicity test (ZET). In general, the tested PAHs induced various developmental effects in the zebrafish embryos including unhatched embryos, no movement and circulation, yolk sac and pericardial edemas, deformed body shape, and cumulative mortality at 96 h post fertilization (hpf). The alkyl substituent on different positions of the aromatic ring of the PAHs appeared to change their in vitro developmental toxicity. Comparison to a previously reported molecular docking study showed that the methyl substituents may affect the interaction of the PAHs with the aryl hydrocarbon receptor (AhR) which is known to play a role in the developmental toxicity of some PAHs. Taken together, our results show that methylation can either increase or decrease the developmental toxicity of PAHs and suggest this may relate to effects on the molecular dimensions and resulting consequences for interactions with the AhR.

AHR/ROS-mediated mitochondria apoptosis contributes to benzo[a]pyrene-induced heart defects and the protective effects of resveratrol

Benzo[a]pyrene (BaP), a prototypical polycyclic aromatic hydrocarbon, is widely present in the environment. BaP-induced heart defects have been frequently reported, but the underlying molecular mechanisms remain elusive. Here, we found that BaP increased heart malformations in zebrafish embryos in a concentration-dependent manner, which were attenuated by supplementation with either CH223191 (CH), an aryl hydrocarbon receptor (AHR) inhibitor, or N-acetyl-l-cysteine (NAC), a reactive oxygen species (ROS) scavenger. While CH and NAC both inhibited BaP-induced ROS generation, NAC had no effect on BaP-induced AHR activation. We further demonstrated that BaP increased mitochondrial ROS, decreased mitochondrial membrane potential, and caused endogenous apoptosis, with all these effects being counteracted by supplementation with either CH or NAC. Resveratrol (RSV), a natural AHR antagonist and ROS scavenger, also counteracted the heart malformations caused by BaP. Further experiments showed that RSV attenuated BaP-induced oxidative stress, mitochondrial damage and apoptosis, but had no significant effect on AHR activation. In conclusion, our findings show that BaP induces oxidative stress via AHR activation, which causes mitochondria-mediated intrinsic apoptosis, resulting in heart malformations in zebrafish embryos, and that RSV had a protective effect against BaP-induced heart defects mainly by inhibiting oxidative stress rather than through antagonism of AHR activity.

Toxic Effects of Polychlorinated Biphenyl Congeners and Aroclors on Embryonic Growth and Development

Polychlorinated biphenyls (PCBs) cause significant health and reproductive problems in many vertebrates. Exposure during embryogenesis likely leads to defects in organ development, compromising survival and growth through adulthood. The present study identifies the impact of PCBs on the embryonic development of key organs and resulting consequences on survival and growth. Zebrafish embryos were treated with individual PCB congeners (126 or 104) or one of 4 Aroclor mixtures (1016, 1242, 1254, or 1260) and analyzed for changes in gross embryonic morphology. Specific organs were assessed for defects during embryonic development, using a variety of transgenic zebrafish to improve organ visualization. Resulting larvae were grown to adulthood while survival and growth were assayed. Embryonic gross development on PCB treatment was abnormal, with defects presenting in a concentration-dependent manner in the liver, pancreas, heart, and blood vessel organization. Polychlorinated biphenyl 126 treatment resulted in the most consistently severe and fatal phenotypes, whereas treatments with PCB 104 and Aroclors resulted in a range of more subtle organ defects. Survival of fish was highly variable although the growth rates of surviving fish were relatively normal, suggesting that maturing PCB-treated fish that survive develop compensatory strategies needed to reach adulthood. Life span analyses of fish from embryogenesis through adulthood, as in the present study, are scarce but important for the field because they help identify foci for further studies. Environ Toxicol Chem 2021;40:187-201. © 2020 SETAC.

The effects of exposure to crude oil or PAHs on fish swim bladder development and function

The failure of the swim bladder to inflate during fish development is a common and sensitive response to exposure to petrochemicals. Here, we review potential mechanisms by which petrochemicals or their toxic components (polycyclic aromatic hydrocarbons; PAHs) may affect swim bladder inflation, particularly during early life stages. Surface films formed by oil can cause a physical barrier to primary inflation by air gulping, and are likely important during oil spills. The act of swimming to the surface for primary inflation can be arduous for some species, and may prevent inflation if this behavior is limited by toxic effects on vision or musculature. Some studies have noted altered gene expression in the swim bladder in response to PAHs, and Cytochrome P450 1A (CYP1A) can be induced in swim bladder or rete mirabile tissue, suggesting that PAHs can have direct effects on swim bladder development. Swim bladder inflation failure can also occur secondarily to the failure of other systems; cardiovascular impairment is the best elucidated of these mechanisms, but other mechanisms might include non-inflation as a sequela of disruption to thyroid signaling or cholesterol metabolism. Failed swim bladder inflation has the potential to lead to chronic sublethal effects that are as yet unstudied.

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.

Developmental toxicity and transcriptome analysis of 4-epianhydrotetracycline to zebrafish (Danio rerio) embryos

As a primary degradation by-product of tetracycline (TC), 4-Epianhydrotetracycline (4-EATC) has been detected frequently in the aquatic environment, which may pose a potential environmental risk to aquatic organisms. Up to now, however, the toxicology study on 4-EATC to aquatic organisms is limited. In the present study, in order to better understand the toxic mechanism of 4-EATC, developmental toxicity including lethal and sublethal effects of 4-EATC and TC were investigated. The results showed that the developmental toxicity of 4-EATC to zebrafish embryos was stronger than that of TC. The 96 h LC₅₀ value of 4-EATC to zebrafish embryos was 29.13 mg/L. Malformations seemed to be the most sensitive sublethal endpoint of 4-EATC exposure, and the 96 h EC₅₀ value was 8.57 mg/L. Transcriptome response of 4-EATC to zebrafish embryos was determined. The results showed that 430 different expression genes (DEGs) caused by 4-EATC, and most enriched in tryptophan (TRP) metabolism pathway. Annotation of DEGs in the TRP metabolism demonstrated that expression of 4 gene products in tryptophan metabolized along the kynurenine (KYN) pathway were changed. Disorder of TRP catabolism in KYN pathway was a potential mechanism of 4-EATC toxicity to zebrafish embryos.

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.

The aryl hydrocarbon receptor as a target of environmental stressors - Implications for pollution mediated stress and inflammatory responses

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor regulating the expression of genes, for instance encoding the monooxygenases cytochrome P450 (CYP) 1A1 and CYP1A2, which are important enzymes in metabolism of xenobiotics. The AHR is activated upon binding of polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs), and related ubiquitous environmental chemicals, to mediate their biological and toxic effects. In addition, several endogenous and natural compounds can bind to AHR, thereby modulating a variety of physiological processes. In recent years, ambient particulate matter (PM) associated with traffic related air pollution (TRAP) has been found to contain significant amounts of PAHs. PM containing PAHs are of increasing concern as a class of agonists, which can activate the AHR. Several reports show that PM and AHR-mediated induction of CYP1A1 results in excessive generation of reactive oxygen species (ROS), causing oxidative stress. Furthermore, exposure to PM and PAHs induce inflammatory responses and may lead to chronic inflammatory diseases, including asthma, cardiovascular diseases, and increased cancer risk. In this review, we summarize findings showing the critical role that the AHR plays in mediating effects of environmental pollutants and stressors, which pose a risk of impacting the environment and human health.

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PAHs present on ambient air pollution particles are ligands of the cellular AHR.

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AHR-dependent induction of CYP1, AKR, NOX and COX-2 genes can be a source of ROS generation.

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AHR signaling and NRF2 signaling interact to regulate the expression of antioxidant genes.

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Air pollution and ROS can affect inflammation, which is partially triggered by AHR and associated immune responses.

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Skin, lung, and the cardiovascular system are major target sites for air pollution-induced inflammation.

AHR-mediated oxidative stress contributes to the cardiac developmental toxicity of trichloroethylene in zebrafish embryos

Trichloroethylene (TCE), a widely used chlorinated solvent, is a common environmental pollutant. Current evidence shows that TCE could induce heart defects during embryonic development, but the underlining mechanism(s) remain unclear. Since activation of the aryl hydrocarbon receptor (AHR) could induce oxidative stress, we hypothesized that AHR-mediated oxidative stress may play a role in the cardiac developmental toxicity of TCE. In this study, we found that the reactive oxygen species (ROS) scavenger, N-Acetyl-L-cysteine (NAC), and AHR inhibitors, CH223191 (CH) and StemRegenin 1, significantly counteracted the TCE-induced heart malformations in zebrafish embryos. Moreover, both CH and NAC suppressed TCE-induced ROS and 8-OHdG (8-hydroxy-2' -deoxyguanosine). TCE did not affect ahr2 and cyp1a expression, but increased cyp1b1 expression, which was restored by CH supplementation. CH also attenuated the TCE-induced mRNA expression changes of Nrf2 signalling genes (nrf2b, gstp2, sod2, ho1, nqo1) and cardiac differentiation genes (gata4, hand2, c-fos, sox9b). In addition, the TCE enhanced SOD activity was attenuated by CH. Morpholino knockdown confirmed that AHR mediated the TCE-induced ROS and 8-OHdG generation in the heart of zebrafish embryos. In conclusion, our results suggest that AHR mediates TCE-induced oxidative stress, leading to DNA damage and heart malformations in zebrafish embryos.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

Temporal and tissue-specific activation of aryl hydrocarbon receptor in discrete mouse models of kidney disease

Emerging evidence in animal models of chronic kidney disease (CKD) implicates Aryl Hydrocarbon Receptor (AHR) signaling as a mediator of uremic toxicity. However, details about its tissue-specific and time-dependent activation in response to various renal pathologies remain poorly defined. Here, a comprehensive analysis of AHR induction was conducted in response to discrete models of kidney diseases using a transgenic mouse line expressing the AHR responsive-promoter tethered to a β-galactosidase reporter gene. Following validation using a canonical AHR ligand (a dioxin derivative), the transgenic mice were subjected to adenine-induced and ischemia/reperfusion-induced injury models representing CKD and acute kidney injury (AKI), respectively, in humans. Indoxyl sulfate was artificially increased in mice through the drinking water and by inhibiting its excretion into the urine. Adenine-fed mice showed a distinct and significant increase in β-galactosidase in the proximal and distal renal tubules, cardiac myocytes, hepatocytes, and microvasculature in the cerebral cortex. The pattern of β-galactosidase increase coincided with the changes in serum indoxyl sulfate levels. Machine-learning-based image quantification revealed positive correlations between indoxyl sulfate levels and β-galactosidase expression in various tissues. This pattern of β-galactosidase expression was recapitulated in the indoxyl sulfate-specific model. The ischemia/reperfusion injury model showed increase in β-galactosidase in renal tubules that persisted despite reduction in serum indoxyl sulfate and blood urea nitrogen levels. Thus, our results demonstrate a relationship between AHR activation in various tissues of mice with CKD or AKI and the levels of indoxyl sulfate. This study demonstrates the use of a reporter gene mouse to probe tissue-specific manifestations of uremia in translationally relevant animal models and provide hypothesis-generating insights into the mechanism of uremic toxicity that warrant further investigation.

Effects of corexit 9500A and Corexit-crude oil mixtures on transcriptomic pathways and developmental toxicity in early life stage mahi-mahi (Coryphaena hippurus)

Crude oil and polycyclic aromatic hydrocarbon (PAH) exposure in early life stage fish has been well-characterized to induce phenotypic malformations such as altered heart development and other morphological impacts. The effects of chemical oil dispersants on toxicity are more controversial. To better understand how chemical dispersion of oil can impact toxicity in pelagic fish, embryos of mahi-mahi (Coryphaena hippurus) were exposed to three concentrations of the chemical dispersant Corexit 9500A, or Corexit 9500A-oil mixtures (chemically enhanced water accommodated fractions: CEWAF) of Deepwater Horizon crude oil for 48 h. RNA sequencing, gene ontology enrichment, and phenotypic measurements were conducted to assess toxicity. Exposure to Corexit 9500A altered expression of less than 50 genes at all concentrations (2.5, 5, and 10 mg/L nominal concentration) and did not induce acute mortality or phenotypic malformations, corroborating other studies showing minimal effects of Corexit 9500A on developing mahi-mahi embryos. CEWAF preparations contained environmentally relevant ∑PAH concentrations ranging from 1.4 to 3.1 μg/L and similarly did not alter larval morphology. Differentially expressed genes and significantly altered pathways related to cardiotoxicity, visual impairments, and Ca²⁺ homeostasis reinforced previous work that expression of genes associated with the heart and eye are highly sensitive molecular endpoints in oil-exposed early life stage fish. Differential expression and gene ontology pathways were similar across the three CEWAF treatments, indicating that increased chemical dispersion did not alter molecular outcomes within the range tested here. In addition, significant sublethal molecular responses occurred in the absence of observable phenotypic changes to the heart, indicating that effects of oil on early life stage fish may not be completely dependent on cardiac function.

2,3,7,8-Tetrachlorodibenzo-p-dioxin promotes injury-induced vascular neointima formation in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental pollutant that causes cardiovascular toxicity. The phenotypic transformation of vascular smooth muscle cells (VSMCs) from the contractile to the synthetic phenotype is a hallmark of vascular response to injury. However, the precise role and molecular mechanism of TCDD in vascular remodeling remains unknown. In the present study, we found that TCDD treatment promoted VSMC phenotypic transition from contractile to synthetic phenotype and exaggerated vascular neointimal hyperplasia after wire injury in mice. TCDD treatment enhanced VSMC entry into cell cycle from G0/G1 phase to S and G2/M phase. The expression of cyclin D1, cyclin-dependent kinase 4 (CDK4), and its phosphorylation were coordinately increased in response to TCDD treatment. Knocking down of aryl hydrocarbon receptor (AHR) inhibited VSMC phenotypic transition induced by TCDD and promoted S/G2 phase cell cycle arrest. TCDD treatment markedly increased oncogenic c-Jun gene expression in VSMCs. ChIP assay revealed the direct binding of AHR on the promoter of c-Jun to up-regulate the mRNA expression of c-Jun. Silencing of c-Jun gene enhanced the expression of p53 and p21, whereas attenuated the expression of CDK4 and cyclin D1 leading to the decrease in the TCDD-stimulated VSMC proliferation and synthetic phenotype transition in vitro. In vivo study showed that genetic ablation of c-Jun in VSMCs restricted injury-induced neointimal hyperplasia in TCDD-treated mice. Thus, TCDD exposure exaggerated injury-induced vascular remodeling by the activation of AHR and up-regulation of the expression of its target gene c-Jun, indicating that inhibition of AHR may be a promising prevention strategy for TCDD-associated cardiovascular diseases.-Guo, S., Zhang, R., Liu, Q., Wan, Q., Wang, Y., Yu, Y., Liu, G., Shen, Y., Yu, Y., Zhang, J. 2,3,7,8-Tetrachlorodibenzo-p-dioxin promotes injury-induced vascular neointima formation in mice.

Use of biological detection methods to assess dioxin-like compounds in sediments of Bohai Bay, China

Bohai Bay, in the western region of northeastern China's Bohai Sea, receives water from large rivers containing various pollutants including dioxin-like compounds (DLCs). This study used the established zebrafish (Danio rerio) model, its known developmental toxicity endpoints and sensitive molecular analyses to evaluate sediments near and around an industrial effluent site in Bohai Bay. The primary objective was to assess the efficacy of rapid biological detection methods as an addition to chemical analyses. Embryos were exposed to various concentrations of sediment extracts as well as a 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) positive control. Exposure to sediment extract nearest the discharge site (P1) resulted in the most severe- and highest rates of change in embryos and larvae, suggesting that DLC contaminated sediment probably did not occur much beyond it. P1 extract resulted in concentration dependent increases in mortality and pericardial edema. Its highest concentration caused up-regulation of P-450 (CYP)-1A1(CYP1A) mRNA expression at 72 h post fertilization (hpf), an increase in its expression in gill arches as observed by whole mount in situ hybridization, and an increased signal in the Tg(cyp1a: mCherry) transgenic line. The pattern and magnitude of response was very similar to that of TCDD and supported the presence of DLCs in these sediment samples. Follow-up chemical analysis confirmed this presence and identified H7CDF, O8CDF and O8CDD as the main components in P1 extract. This study validates the use of biological assays as a rapid, sensitive, and cost-effective method to evaluate DLCs and their effects in sediment samples. Additionally, it provides support for the conclusion that DLCs have limited remobilization capacity in marine sediments.

Evaluation of the developmental toxicity of 2,7-dibromocarbazole to zebrafish based on transcriptomics assay

Polyhalogenated carbazoles (PHCZs), which have the similar structure of dioxin, have been reported ubiquitous in the environments and drawn wide concerns. However, their potential ecological and health risks are still poorly understood. Here, wildtype zebrafish embryos were used to evaluate the environmental risks of 2,7-dibromocarbazole (2,7-DBCZ), 3,6-dibromocarbazole (3,6-DBCZ), and 3,6-dichlorocarbazole (3,6-DCCZ). 2,7-DBCZ was the most toxic compound with the 96-h LC₅₀ value of 581.8 ± 29.3 μg·L^-1 and the EC₅₀ value of 201.5 ± 6.5 μg·L^-1 for pericardial edema. The teratogenic effects of 2,7-DBCZ were tested using transgenic zebrafish larvae. The transcriptomic analysis revealed that 90 genes in zebrafish expressed differently after exposure to 2,7-DBCZ, and many pathways were related to aryl hydrocarbon receptor (AhR) activation. The qRT-PCR also showed that expression levels of AhR1 and CYP1 A in zebrafish were significantly up-regulated after exposure to 2,7-DBCZ. In conclusion, 2,7-DBCZ exhibited more potent toxicity and cardiac teratogenic effects, and presented developmental toxicity partially consistent with AhR activation. Our results will be of great help to the risk assessment and regulation-making of PHCZs. Meanwhile, further studies should be promoted to illustrate the potential mechanism between PHCZs and AhR in the near future.

Mechanisms of Developmental Toxicity of Dioxins and Related Compounds

Dioxins and related compounds induce morphological abnormalities in developing animals in an aryl hydrocarbon receptor (AhR)-dependent manner. Here we review the studies in which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is used as a prototypical compound to elucidate the pathogenesis of morphological abnormalities. TCDD-induced cleft palate in fetal mice involves a delay in palatogenesis and dissociation of fused palate shelves. TCDD-induced hydronephrosis, once considered to be caused by the anatomical obstruction of the ureter, is now separated into TCDD-induced obstructive and non-obstructive hydronephrosis, which develops during fetal and neonatal periods, respectively. In the latter, a prostaglandin E₂ synthesis pathway and urine concentration system are involved. TCDD-induced abnormal development of prostate involves agenesis of the ventral lobe. A suggested mechanism is that AhR activation in the urogenital sinus mesenchyme by TCDD modulates the wingless-type MMTV integration site family (WNT)/β-catenin signaling cascade to interfere with budding from urogenital sinus epithelium. TCDD exposure to zebrafish embryos induces loss of epicardium progenitor cells and heart malformation. AHR2-dependent downregulation of Sox9b expression in cardiomyocytes is a suggested underlying mechanism. TCDD-induced craniofacial malformation in zebrafish is considered to result from the AHR2-dependent reduction in SRY-box 9b (SOX9b), probably partly via the noncoding RNA slincR, resulting in the underdevelopment of chondrocytes and cartilage.

Characterization of AHR1 and its functional activity in Atlantic sturgeon and shortnose sturgeon

Sturgeon species are imperiled world-wide by a variety of anthropogenic stressors including chemical contaminants. Atlantic sturgeon, Acipenser oxyrinchus, and shortnose sturgeon, Acipenser brevirostrum, are largely sympatric acipenserids whose young life-stages are often exposed to high levels of benthic-borne PCBs and PCDD/Fs in large estuaries along the Atlantic Coast of North America. In previous laboratory studies, we demonstrated that both sturgeon species are sensitive to early life-stage toxicities from exposure to environmentally relevant concentrations of coplanar PCBs and TCDD. The sensitivity of young life-stages of fishes to these contaminants varies among species by three orders of magnitude and often is due to variation in the structure and function of the aryl hydrocarbon receptor (AHR) pathway. Unlike mammals, fishes have two forms of AHR (AHR1 and AHR2) with AHR2 usually being more highly expressed across tissues and functional in mediating toxicities. Based on previous studies in white sturgeon, A. transmontanus, we hypothesized that sturgeon taxa are unusually sensitive to these contaminants because of higher levels of expression and functional activity of AHR1 than in other fish taxa. To address this possibility, we characterized AHR1 in both Atlantic Coast sturgeon species, evaluated its' in vivo expression in young life-stages and in multiple tissues of shortnose sturgeon, and tested its ability to drive reporter gene expression in AHR-deficient cells treated with graded doses of PCB126 and TCDD. Similar to white sturgeon and lake sturgeon, AHR1 amino acid sequences in Atlantic sturgeon and shortnose sturgeon were more similar to mammalian AHRs and avian AHR1s than to AHR1 in other fishes, suggesting their greater functionality in sturgeon species than in other fishes. Exposure to graded doses of coplanar PCBs and TCDD usually failed to significantly induce AHR1 expression in young life-stages or most tissues of shortnose sturgeon. However, in reporter gene assays, AHR1 drove higher levels of gene expression than AHR2 alone, but their binary combination failed to drive higher levels of expression than either AHR alone. In total, our results suggest that AHR1 may be more functional in sturgeon species than in other fishes, but probably does not explain their heightened sensitivity to these contaminants.

sox9b is required in cardiomyocytes for cardiac morphogenesis and function

The high mobility group transcription factor SOX9 is expressed in stem cells, progenitor cells, and differentiated cell-types in developing and mature organs. Exposure to a variety of toxicants including dioxin, di(2-ethylhexyl) phthalate, 6:2 chlorinated polyfluorinated ether sulfonate, and chlorpyrifos results in the downregulation of tetrapod Sox9 and/or zebrafish sox9b. Disruption of Sox9/sox9b function through environmental exposures or genetic mutations produce a wide range of phenotypes and adversely affect organ development and health. We generated a dominant-negative sox9b (dnsox9b) to inhibit sox9b target gene expression and used the Gal4/UAS system to drive dnsox9b specifically in cardiomyocytes. Cardiomyocyte-specific inhibition of sox9b function resulted in a decrease in ventricular cardiomyocytes, an increase in atrial cardiomyocytes, hypoplastic endothelial cushions, and impaired epicardial development, ultimately culminating in heart failure. Cardiomyocyte-specific dnsox9b expression significantly reduced end diastolic volume, which corresponded with a decrease in stroke volume, ejection fraction, and cardiac output. Further analysis of isolated cardiac tissue by RT-qPCR revealed cardiomyocyte-specific inhibition of sox9b function significantly decreased the expression of the critical cardiac development genes nkx2.5, nkx2.7, and myl7, as well as c-fos, an immediate early gene necessary for cardiomyocyte progenitor differentiation. Together our studies indicate sox9b transcriptional regulation is necessary for cardiomyocyte development and function.

Genome-wide scan reveals signatures of selection related to pollution adaptation in non-model estuarine Atlantic killifish (Fundulus heteroclitus)

In many human-altered ecosystems, organisms are increasingly faced with more diverse and complex environmental stressors and pollutant mixtures, to which the adaptations necessary to survive exposure are likely to be numerous and varied. Improving our understanding of the molecular mechanisms that underlie complex polygenic adaptations in natural settings requires significant toxicological, biochemical, physiological, and genomic data rarely available for non-model organisms. Here, we build upon two decades of study of adaptation to anthropogenic pollutants in a population of Atlantic killifish (Fundulus heteroclitus) that inhabits the creosote-contaminated Atlantic Wood Industries Superfund (AW) site on the Elizabeth River, Virginia in the United States. To better understand the genotypes that underlie previously characterized resistance to PCBs and PAHs, we performed Restriction site-Associated DNA sequencing (RADseq) on killifish from AW and two relatively clean reference sites (King's Creek-KC, and Mains Creek-MC). Across the genome, we analyzed over 83,000 loci and 12,000 single nucleotide polymorphisms (SNPs). Shared across both comparisons of killifish from polluted (AW) and relatively unpolluted (KC and MC) sites, we found eight genomic regions with smoothed FST values significantly (p < 0.001) elevated above background. Using the recently published F. heteroclitus reference genome, we identified candidate genes in these significant regions involved in the AHR pathway (e.g. AIP, ARNT1c), as well as genes relating to cardiac structure and function. These genes represent both previously characterized and potentially novel molecular adaptations involved with various aspects of resistance to these environmental toxins.

Histological and Transcriptomic Changes in Male Zebrafish Testes Due to Early Life Exposure to Low Level 2,3,7,8-Tetrachlorodibenzo-p-Dioxin

We have shown that zebrafish (Danio rerio) are an excellent model for evaluating the link between early life stage exposure to environmental chemicals and disease in adulthood and subsequent unexposed generations. Previously, we used this model to identify transgenerational effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) on skeletal development, sex ratio, and reproductive capacity. Transgenerational inheritance of TCDD toxicity, notably decreased reproductive capacity, appears to be mediated through the male germ line. Thus, we examine testicular tissue for structural and gene expression changes using histology, microarray, and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Histological analysis revealed decreased spermatozoa with concurrent increase in spermatogonia, and decreased germinal epithelium thickness in TCDD-exposed males compared with controls. We also identified altered expression of genes associated with testis development, steroidogenesis, spermatogenesis, hormone metabolism, and xenobiotic response. Altered genes are in pathways involving lipid metabolism, molecular transport, small molecule biochemistry, cell morphology, and metabolism of vitamins and minerals. These data will inform future investigations to elucidate the mechanism of adult-onset and transgenerational infertility due to TCDD exposure in zebrafish.

Molecular Mechanisms of Crude Oil Developmental Toxicity in Fish

With major oil spills in Korea, the United States, and China in the past decade, there has been a dramatic increase in the number of studies characterizing the developmental toxicity of crude oil and its associated polycyclic aromatic compounds (PACs). The use of model fish species with associated tools for genetic manipulation, combined with high throughput genomics techniques in nonmodel fish species, has led to significant advances in understanding the cellular and molecular bases of functional and morphological defects arising from embryonic exposure to crude oil. Following from the identification of the developing heart as the primary target of crude oil developmental toxicity, studies on individual PACs have revealed a diversity of cardiotoxic mechanisms. For some PACs that are strong agonists of the aryl hydrocarbon receptor (AHR), defects in heart development arise in an AHR-dependent manner, which has been shown for potent organochlorine agonists, such as dioxins. However, crude oil contains a much larger fraction of compounds that have been found to interfere directly with cardiomyocyte physiology in an AHR-independent manner. By comparing the cellular and molecular responses to AHR-independent and AHR-dependent toxicity, this review focuses on new insights into heart-specific pathways underlying both acute and secondary adverse outcomes to crude oil exposure during fish development.

Sensitivity of lake sturgeon (Acipenser fulvescens) early life stages to 2,3,7,8-tetrachlorodibenzo-P-dioxin and 3,3',4,4',5-pentachlorobiphenyl

The aquatic food web of the Great Lakes has been contaminated with polychlorinated biphenyls (PCBs) since the mid-20th century. Threats of PCB exposures to long-lived species of fish, such as lake sturgeon (Acipenser fulvescens), have been uncertain because of a lack of information on the relative sensitivity of the species. The objective of the present study was to evaluate the sensitivity of early-life stage lake sturgeon to 3,3',4,4',5-pentachlorobiphenyl (PCB-126) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. Mortality, growth, morphological and tissue pathologies, swimming performance, and activity levels were used as assessment endpoints. Pericardial and yolk sac edema, tubular heart, yolk sac hemorrhaging, and small size were the most commonly observed pathologies in both TCDD and PCB-126 exposures, beginning as early as 4 d postfertilization, with many of these pathologies occurring in a dose-dependent manner. Median lethal doses for PCB-126 and TCDD in lake sturgeon were 5.4 ng/g egg (95% confidence interval, 3.9-7.4 ng/g egg) and 0.61 ng/g egg (0.47-0.82 ng/g egg), respectively. The resulting relative potency factor for PCB-126 (0.11) was greater than the World Health Organization estimate for fish (toxic equivalency factor = 0.005), suggesting that current risk assessments may underestimate PCB toxicity toward lake sturgeon. Swimming activity and endurance were reduced in lake sturgeon survivors from the median lethal doses at 60 d postfertilization. Threshold and median toxicity values indicate that lake sturgeon, like other Acipenser species, are more sensitive to PCB and TCDD than the other genus of sturgeon, Scaphirhynchus, found in North America. Indeed, lake sturgeon populations in the Great Lakes and elsewhere are susceptible to PCB/TCDD-induced developmental toxicity in embryos and reductions in swimming performance. Environ Toxicol Chem 2017;36:988-998. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Novel adverse outcome pathways revealed by chemical genetics in a developing marine fish

Crude oil spills are a worldwide ocean conservation threat. Fish are particularly vulnerable to the oiling of spawning habitats, and crude oil causes severe abnormalities in embryos and larvae. However, the underlying mechanisms for these developmental defects are not well understood. Here, we explore the transcriptional basis for four discrete crude oil injury phenotypes in the early life stages of the commercially important Atlantic haddock (Melanogrammus aeglefinus). These include defects in (1) cardiac form and function, (2) craniofacial development, (3) ionoregulation and fluid balance, and (4) cholesterol synthesis and homeostasis. Our findings suggest a key role for intracellular calcium cycling and excitation-transcription coupling in the dysregulation of heart and jaw morphogenesis. Moreover, the disruption of ionoregulatory pathways sheds new light on buoyancy control in marine fish embryos. Overall, our chemical-genetic approach identifies initiating events for distinct adverse outcome pathways and novel roles for individual genes in fundamental developmental processes.

Crude oil exposures reveal roles for intracellular calcium cycling in haddock craniofacial and cardiac development

Recent studies have shown that crude oil exposure affects cardiac development in fish by disrupting excitation-contraction (EC) coupling. We previously found that eggs of Atlantic haddock (Melanogrammus aeglefinus) bind dispersed oil droplets, potentially leading to more profound toxic effects from uptake of polycyclic aromatic hydrocarbons (PAHs). Using lower concentrations of dispersed crude oil (0.7-7 μg/L ∑PAH), here we exposed a broader range of developmental stages over both short and prolonged durations. We quantified effects on cardiac function and morphogenesis, characterized novel craniofacial defects, and examined the expression of genes encoding potential targets underlying cardiac and craniofacial defects. Because of oil droplet binding, a 24-hr exposure was sufficient to create severe cardiac and craniofacial abnormalities. The specific nature of the craniofacial abnormalities suggests that crude oil may target common craniofacial and cardiac precursor cells either directly or indirectly by affecting ion channels and intracellular calcium in particular. Furthermore, down-regulation of genes encoding specific components of the EC coupling machinery suggests that crude oil disrupts excitation-transcription coupling or normal feedback regulation of ion channels blocked by PAHs. These data support a unifying hypothesis whereby depletion of intracellular calcium pools by crude oil-derived PAHs disrupts several pathways critical for organogenesis in fish.

The influence of heart developmental anatomy on cardiotoxicity-based adverse outcome pathways in fish

The developing fish heart is vulnerable to a diverse array of toxic chemical contaminants in freshwater, estuarine, and marine habitats. Globally occurring examples of cardiotoxic agents include dioxins, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). The disruption of cardiac function during the process of heart morphogenesis can lead to adverse outcome pathways (AOPs) that can negatively affect fish survival at hatching as well as later life stages. Proximal impacts include cardiogenic fluid accumulation (edema) and defects of the body axis and jaw that preclude larval feeding. More subtle changes in heart development can produce permanent structural defects in the heart that reduce cardiac output and swimming performance in older fish. In recent decades, the presence of edema in fish embryos and larvae has been a very common bioindicator of cardiotoxicity. However, the different ways that edema forms in fish from different habitats (i.e., freshwater vs. marine, pelagic vs. demersal) has not been rigorously examined. Oil spills are an important source of PAHs in fish spawning areas worldwide, and research is revealing how patterns of cardiogenic edema are shaped by species-specific differences in developmental anatomy and ionoregulatory physiology. Here we review the visible evidence for circulatory disruption across nine freshwater and marine fish species, exposed to crude oils from different parts of the world. We focus on the close interconnectedness of the cardiovascular and osmoregulatory systems during early development, and corresponding implications for fish in hyperosmotic and hyposmotic habitats. Finally, we suggest there may be poorly understood adverse outcomes pathways related to osmotic gradients and water movement within embryos, the latter causing extreme shifts in tissue osmolality.

Corresponding morphological and molecular indicators of crude oil toxicity to the developing hearts of mahi mahi

Crude oils from distinct geological sources worldwide are toxic to developing fish hearts. When oil spills occur in fish spawning habitats, natural resource injury assessments often rely on conventional morphometric analyses of heart form and function. The extent to which visible indicators correspond to molecular markers for cardiovascular stress is unknown for pelagic predators from the Gulf of Mexico. Here we exposed mahi (Coryphaena hippurus) embryos to field-collected crude oil samples from the 2010 Deepwater Horizon disaster. We compared visible heart defects (edema, abnormal looping, reduced contractility) to changes in expression of cardiac-specific genes that are diagnostic of heart failure in humans or associated with loss-of-function zebrafish cardiac mutants. Mahi exposed to crude oil during embryogenesis displayed typical symptoms of cardiogenic syndrome as larvae. Contractility, looping, and circulatory defects were evident, but larval mahi did not exhibit downstream craniofacial and body axis abnormalities. A gradation of oil exposures yielded concentration-responsive changes in morphometric and molecular responses, with relative sensitivity being influenced by age. Our findings suggest that 1) morphometric analyses of cardiac function are more sensitive to proximal effects of crude oil-derived chemicals on the developing heart, and 2) molecular indicators reveal a longer-term adverse shift in cardiogenesis trajectory.

A high throughput passive dosing format for the Fish Embryo Acute Toxicity test

High throughput testing according to the Fish Embryo Acute Toxicity (FET) test (OECD Testing Guideline 236) is usually conducted in well plates. In the case of hydrophobic test substances, sorptive and evaporative losses often result in declining and poorly controlled exposure conditions. Therefore, our objective was to improve exposure conditions in FET tests by evaluating a passive dosing format using silicone O-rings in standard 24-well polystyrene plates. We exposed zebrafish embryos to a series of phenanthrene concentrations until 120h post fertilization (hpf), and obtained a linear dilution series. We report effect values for both mortality and sublethal morphological effects based on (1) measured exposure concentrations, (2) (lipid normalized) body residues and (3) chemical activity. The LC50 for 120hpf was 310μg/L, CBR50 (critical body residue) was 2.72mmol/kg fresh wt and La50 (lethal chemical activity) was 0.047. All values were within ranges expected for baseline toxicity. Impaired swim bladder inflation was the most pronounced morphological effect and swimming activity was reduced in all exposure concentrations. Further analysis showed that the effect on swimming activity was not attributed to impaired swim bladder inflation, but rather to baseline toxicity. We conclude that silicone O-rings (1) produce a linear dilution series of phenanthrene in the 120hpf FET test, (2) generate and maintain aqueous concentrations for reliable determination of effect concentrations, and allow for obtaining mechanistic toxicity information, and (3) cause no toxicity, demonstrating its potential as an extension of the FET test when testing hydrophobic chemicals.

Regulation of Ahr signaling by Nrf2 during development: Effects of Nrf2a deficiency on PCB126 embryotoxicity in zebrafish (Danio rerio)

The embryotoxicity of co-planar PCBs is regulated by the aryl hydrocarbon receptor (Ahr), and has been reported to involve oxidative stress. Ahr participates in crosstalk with another transcription factor, Nfe2l2, or Nrf2. Nrf2 binds to antioxidant response elements to regulate the adaptive response to oxidative stress. To explore aspects of the crosstalk between Nrf2 and Ahr and its impact on development, we used zebrafish (Danio rerio) with a mutated DNA binding domain in Nrf2a (nrf2a(fh318/fh318)), rendering these embryos more sensitive to oxidative stress. Embryos were exposed to 2 nM or 5 nM PCB126 at 24 h post fertilization (prim-5 stage of pharyngula) and examined for gene expression and morphology at 4 days post fertilization (dpf; protruding - mouth stage). Nrf2a mutant eleutheroembryos were more sensitive to PCB126 toxicity at 4 dpf, and in the absence of treatment also displayed some subtle developmental differences from wildtype embryos, including delayed inflation of the swim bladder and smaller yolk sacs. We used qPCR to measure changes in expression of the nrf gene family, keap1a, keap1b, the ahr gene family, and known target genes. cyp1a induction by PCB126 was enhanced in the Nrf2a mutants (156-fold in wildtypes vs. 228-fold in mutants exposed to 5 nM). Decreased expression of heme oxygenase (decycling) 1 (hmox1) in the Nrf2a mutants was accompanied by increased nrf2b expression. Target genes of Nrf2a and AhR2, NAD(P)H:quinone oxidoreductase 1 (nqo1) and glutathione S-transferase, alpha-like (gsta1), showed a 2-5-fold increase in expression in the Nrf2a mutants as compared to wildtype. This study elucidates the interaction between two important transcription factor pathways in the developmental toxicity of co-planar PCBs.

Ligand-Specific Transcriptional Mechanisms Underlie Aryl Hydrocarbon Receptor-Mediated Developmental Toxicity of Oxygenated PAHs

Polycyclic aromatic hydrocarbons (PAHs) are priority environmental contaminants that exhibit mutagenic, carcinogenic, proinflammatory, and teratogenic properties. Oxygen-substituted PAHs (OPAHs) are formed during combustion processes and via phototoxidation and biological degradation of parent (unsubstituted) PAHs. Despite their prevalence both in contaminated industrial sites and in urban air, OPAH mechanisms of action in biological systems are relatively understudied. Like parent PAHs, OPAHs exert structure-dependent mutagenic activities and activation of the aryl hydrocarbon receptor (AHR) and cytochrome p450 metabolic pathway. Four-ring OPAHs 1,9-benz-10-anthrone (BEZO) and benz(a)anthracene-7,12-dione (7,12-B[a]AQ) cause morphological aberrations and induce markers of oxidative stress in developing zebrafish with similar potency, but only 7,12-B[a]AQ induces robust Cyp1a protein expression. We investigated the role of the AHR in mediating the toxicity of BEZO and 7,12-B[a]AQ, and found that knockdown of AHR2 rescued developmental effects caused by both compounds. Using RNA-seq and molecular docking, we identified transcriptional responses that precede developmental toxicity induced via differential interaction with AHR2. Redox-homeostasis genes were affected similarly by these OPAHs, while 7,12-B[a]AQ preferentially activated phase 1 metabolism and BEZO uniquely decreased visual system genes. Analysis of biological functions and upstream regulators suggests that BEZO is a weak AHR agonist, but interacts with other transcriptional regulators to cause developmental toxicity in an AHR-dependent manner. Identifying ligand-dependent AHR interactions and signaling pathways is essential for understanding toxicity of this class of environmentally relevant compounds.

Dioxin inhibition of swim bladder development in zebrafish: is it secondary to heart failure?

The swim bladder is a gas-filled organ that is used for regulating buoyancy and is essential for survival in most teleost species. In zebrafish, swim bladder development begins during embryogenesis and inflation occurs within 5 days post fertilization (dpf). Embryos exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) before 96 h post fertilization (hpf) developed swim bladders normally until the growth/elongation phase, at which point growth was arrested. It is known that TCDD exposure causes heart malformations that lead to heart failure in zebrafish larvae, and that blood circulation is a key factor in normal development of the swim bladder. The adverse effects of TCDD exposure on the heart occur during the same period of time that swim bladder development and growth occurs. Based on this coincident timing, and the dependence of swim bladder development on proper circulatory development, we hypothesized that the adverse effects of TCDD on swim bladder development were secondary to heart failure. We compared swim bladder development in TCDD-exposed embryos to: (1) silent heart morphants, which lack cardiac contractility, and (2) transiently transgenic cmlc2:caAHR-2AtRFP embryos, which mimic TCDD-induced heart failure via heart-specific, constitutive activation of AHR signaling. Both of these treatment groups, which were not exposed to TCDD, developed hypoplastic swim bladders of comparable size and morphology to those found in TCDD-exposed embryos. Furthermore, in all treatment groups swim bladder development was arrested during the growth/elongation phase. Together, these findings support a potential role for heart failure in the inhibition of swim bladder development caused by TCDD.

Combination effects of AHR agonists and Wnt/β-catenin modulators in zebrafish embryos: Implications for physiological and toxicological AHR functions

Wnt/β-catenin signaling regulates essential biological functions and acts in developmental toxicity of some chemicals. The aryl hydrocarbon receptor (AHR) is well-known to mediate developmental toxicity of persistent dioxin-like compounds (DLCs). Recent studies indicate a crosstalk between β-catenin and the AHR in some tissues. However the nature of this crosstalk in embryos is poorly known. We observed that zebrafish embryos exposed to the β-catenin inhibitor XAV939 display effects phenocopying those of the dioxin-like 3,3',4,4',5-pentachlorobiphenyl (PCB126). This led us to investigate the AHR interaction with β-catenin during development and ask whether developmental toxicity of DLCs involves antagonism of β-catenin signaling. We examined phenotypes and transcriptional responses in zebrafish embryos exposed to XAV939 or to a β-catenin activator, 1-azakenpaullone, alone or with AHR agonists, either PCB126 or 6-formylindolo[3,2-b]carbazole (FICZ). Alone 1-azakenpaullone and XAV939 both were embryo-toxic, and we found that in the presence of FICZ, the toxicity of 1-azakenpaullone decreased while the toxicity of XAV939 increased. This rescue of 1-azakenpaullone effects occurred in the time window of Ahr2-mediated toxicity and was reversed by morpholino-oligonucleotide knockdown of Ahr2. Regarding PCB126, addition of either 1-azakenpaullone or XAV939 led to lower mortality than with PCB126 alone but surviving embryos showed severe edemas. 1-Azakenpaullone induced transcription of β-catenin-associated genes, while PCB126 and FICZ blocked this induction. The data indicate a stage-dependent antagonism of β-catenin by Ahr2 in zebrafish embryos. We propose that the AHR has a physiological role in regulating β-catenin during development, and that this is one point of intersection linking toxicological and physiological AHR-governed processes.

AHR2-Mediated transcriptomic responses underlying the synergistic cardiac developmental toxicity of PAHs

Polycyclic aromatic hydrocarbons (PAHs) induce developmental defects including cardiac deformities in fish. The aryl hydrocarbon receptor (AHR) mediates the toxicity of some PAHs. Exposure to a simple PAH mixture during embryo development consisting of an AHR agonist (benzo(a)pyrene-BaP) with fluoranthene (FL), an inhibitor of cytochrome p450 1(CYP1)--a gene induced by AHR activation--results in cardiac deformities. Exposure to BaP or FL alone at similar concentrations alters heart rates, but does not induce morphological deformities. Furthermore, AHR2 knockdown prevents the toxicity of BaP + FL mixture. Here, we used a zebrafish microarray analysis to identify heart-specific transcriptomic changes during early development that might underlie cardiotoxicity of BaP + FL. We used AHR2 morphant embryos to determine the role of this receptor in mediating toxicity. Control and knockdown embryos at 36 h post-fertilization were exposed to DMSO, 100 μg/l BaP, 500 μg/l FL, or 100 μg/l BaP + 500 μg/l FL, and heart tissues for RNA were extracted at 2, 6, 12, and 18 h-post-exposure (hpe), prior to the appearance of cardiac deformities. Data show AHR2-dependent BaP + FL effects on expression of genes involved in protein biosynthesis and neuronal development in addition to signaling molecules and their associated molecular pathways. Ca(2+)-cycling and muscle contraction genes were the most significantly differentially expressed category of transcripts when comparing BaP + FL-treated AHR2 morphant and control embryos. These differences were most prominent at 2 and 6 hpe. Therefore, we postulate that BaP + FL may affect cellular Ca(2+) levels and subsequently cardiac muscle function, potentially underlying BaP + FL cardiotoxicity.