PCB126 exposure disrupts zebrafish ventricular and branchial but not early neural crest development

Exposure to the Polychlorinated biphenyl mixture Aroclor 1254 elicits neurological and cardiac developmental effects in early life stage zebrafish (Danio rerio)

The goal of this study was to compare the bioaccumulation of the PCB mixture Aroclor 1254 in zebrafish to cardiac and neurologic outcomes. The establishment of effect concentrations (ECs) for cardiac and neurotoxic effects of PCBs in early life stage fish is challenging due to a lack of measured PCB concentrations in test media (e.g., fish tissue), the lack of standard exposure methods, and the propensity of PCBs to adsorb to test glassware and materials resulting in discrepancies in ECs from different studies with similar endpoints. Reporting tissue concentrations in test organisms will allow for standardization across different tests and thus may improve estimations of effect thresholds. Early life stage zebrafish (Danio rerio) are a common environmental toxicological model well represented within the literature, making them ideal for comparisons across multiple studies. Embryos were exposed at 6 h post fertilization (hpf) to aqueous Aroclor 1254 for 96 h with or without renewal in addition to a PCB 126 positive control for cardiotoxicity. PCB concentrations were measured in both exposure solutions and tissue samples. Measured concentrations of Aroclor 1254 in test solutions ranged from 8.7% to 870% of nominal concentrations. Heart rate, pericardial edema, and neurological endpoints (eye tremors) were measured in 102 hpf larvae. Pericardial edema was not present in Aroclor 1254-treated zebrafish but was observed in those exposed to PCB-126. Concentration-dependent bradycardia was observed in zebrafish exposed to Aroclor 1254 and PCB-126. Similarly, a concentration-dependent increase in eye tremor behavior was observed in embryos exposed to Aroclor 1254. Data produced by this study demonstrate novel toxicological effects of Aroclor 1254 and highlight the importance of measuring PCBs in both exposure and receptor media.

Induction of oxidative stress and cardiac developmental toxicity in zebrafish embryos by arsenate at environmentally relevant concentrations

The contamination of water by arsenic (As) has emerged as a significant environmental concern due to its well-documented toxicity. Environmentally relevant concentrations of As have been reported to pose a considerable threat to fish. However, previous studies mainly focused on the impacts of As at environmentally relevant concentrations on adult fish, and limited information is available regarding its impacts on fish at early life stage. In this study, zebrafish embryos were employed to evaluate the environmental risks following exposure to different concentrations (0, 25, 50, 75 and 150 μg/L) of pentavalent arsenate (AsV) for 120 hours post fertilization. Our findings indicated that concentrations ≤ 150 μg/L AsV did not exert significant effects on survival or aberration; however, it conspicuously inhibited heart rate of zebrafish larvae. Furthermore, exposure to AsV significantly disrupted mRNA transcription of genes associated with cardiac development, and elongated the distance between the sinus venosus and bulbus arteriosus at 75 μg/L and 150 μg/L treatments. Additionally, AsV exposure enhanced superoxide dismutase (SOD) activity at 50, 75 and 150 μg/L treatments, and increased mRNA transcriptional levels of Cu/ZnSOD and MnSOD at 75 and 150 μg/L treatments. Concurrently, AsV suppressed metallothionein1 (MT1) and MT2 mRNA transcriptions while elevating heat shock protein70 mRNA transcription levels in zebrafish larvae resulting in elevated malondialdehyde (MDA) levels. These findings provide novel insights into the toxic effects exerted by low concentrations of AsV on fish at early life stage, thereby contributing to an exploration into the environmental risks associated with environmentally relevant concentrations.

Dihalogenated nitrophenols in drinking water: Prevalence, resistance to household treatment, and cardiotoxic impact on zebrafish embryo

Dihalogenated nitrophenols (2,6-DHNPs), an emerging group of aromatic disinfection byproducts (DBPs) detected in drinking water, have limited available information regarding their persistence and toxicological risks. The present study found that 2,6-DHNPs are resistant to major drinking water treatment processes (sedimentation and filtration) and households methods (boiling, filtration, microwave irradiation, and ultrasonic cleaning). To further assess their health risks, we conducted a series of toxicology studies using zebrafish embryos as the model organism. Our findings reveal that these emerging 2,6-DHNPs showed lethal toxicity 248 times greater than that of the regulated DBP, dichloroacetic acid. Specifically, at sublethal concentrations, exposure to 2,6-DHNPs generated reactive oxygen species (ROS), caused apoptosis, inhibited cardiac looping, and induced cardiac failure in zebrafish. Remarkably, the use of a ROS scavenger, N-acetyl-l-cysteine, considerably mitigated these adverse effects, emphasizing the essential role of ROS in 2,6-DHNP-induced cardiotoxicity. Our findings highlight the cardiotoxic potential of 2,6-DHNPs in drinking water even at low concentrations of 19 μg/L and the beneficial effect of N-acetyl-l-cysteine in alleviating the 2,6-DHNP-induced cardiotoxicity. This study underscores the urgent need for increased scrutiny of these emerging compounds in public health discussions.

Emerging disinfection byproducts 3-bromine carbazole induces cardiac developmental toxicity via aryl hydrocarbon receptor activation in zebrafish larvae

3-bromine carbazole (3-BCZ) represents a group of emerging aromatic disinfection byproducts (DBP) detected in drinking water; however, limited information is available regarding its potential cardiotoxicity. To assess its impacts, zebrafish embryos were exposed to 0, 0.06, 0.14, 0.29, 0.58, 1.44 or 2.88 mg/L of 3-BCZ for 120 h post fertilization (hpf). Our results revealed that ≥1.44 mg/L 3-BCZ exposure induced a higher incidence of heart malformation and an elevated pericardial area in zebrafish larvae; it also decreased the number of cardiac muscle cells and thins the walls of the ventricle and atrium while increasing cardiac output and impeding cardiac looping. Furthermore, 3-BCZ exposure also exhibited significant effects on the transcriptional levels of genes related to both cardiac development (nkx2.5, vmhc, gata4, tbx5, tbx2b, bmp4, bmp10, and bmp2b) and cardiac function (cacna1ab, cacna1da, atp2a1l, atp1b2b, atp1a3b, and tnnc1a). Notably, N-acetyl-L-cysteine, a reactive oxygen species scavenger, may alleviate the failure of cardiac looping induced by 3-BCZ but not the associated cardiac dysfunction or malformation; conversely, the aryl hydrocarbon receptor agonist CH131229 can completely eliminate the cardiotoxicity caused by 3-BCZ. This study provides new evidence for potential risks associated with ingesting 3-BCZ as well as revealing underlying mechanisms responsible for its cardiotoxic effects on zebrafish embryos.

Connecting Suborganismal Data to Bioenergetic Processes: Killifish Embryos Exposed to a Dioxin-Like Compound

A core challenge for ecological risk assessment is to integrate molecular responses into a chain of causality to organismal or population-level outcomes. Bioenergetic theory may be a useful approach for integrating suborganismal responses to predict organismal responses that influence population dynamics. We describe a novel application of dynamic energy budget (DEB) theory in the context of a toxicity framework (adverse outcome pathways [AOPs]) to make quantitative predictions of chemical exposures to individuals, starting from suborganismal data. We use early-life stage exposure of Fundulus heteroclitus to dioxin-like chemicals (DLCs) and connect AOP key events to DEB processes through "damage" that is produced at a rate proportional to the internal toxicant concentration. We use transcriptomic data of fish embryos exposed to DLCs to translate molecular indicators of damage into changes in DEB parameters (damage increases somatic maintenance costs) and DEB models to predict sublethal and lethal effects on young fish. By changing a small subset of model parameters, we predict the evolved tolerance to DLCs in some wild F. heteroclitus populations, a data set not used in model parameterization. The differences in model parameters point to reduced sensitivity and altered damage repair dynamics as contributing to this evolved resistance. Our methodology has potential extrapolation to untested chemicals of ecological concern. Environ Toxicol Chem 2023;42:2040-2053. © 2023 Oak Ridge National Laboratory and The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Sin3a associated protein 130 kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130 kDa (Sap130), part of the chromatin modifying SIN3A/HDAC complex, as a gene contributing to the etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cardiac function were dysregulated in sap130a, but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a, in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

Induced Pluripotent Stem Cell-Based Modeling of Single-Ventricle Congenital Heart Diseases

PURPOSE OF REVIEW

Congenital heart disease includes a wide variety of structural cardiac defects, the most severe of which are single ventricle defects (SVD). These patients suffer from significant morbidity and mortality; however, our understanding of the developmental etiology of these conditions is limited. Model organisms offer a window into normal and abnormal cardiogenesis yet often fail to recapitulate complex congenital heart defects seen in patients. The use of induced pluripotent stem cells (iPSCs) derived from patients with single-ventricle defects opens the door to studying SVD in patient-derived cardiomyocytes (iPSC-CMs) in a variety of different contexts, including organoids and chamber-specific cardiomyocytes. As the genetic and cellular causes of SVD are not well defined, patient-derived iPSC-CMs hold promise for uncovering mechanisms of disease development and serve as a platform for testing therapies. The purpose of this review is to highlight recent advances in iPSC-based models of SVD.

RECENT FINDINGS

Recent advances in patient-derived iPSC-CM differentiation, as well as the development of both chamber-specific and non-myocyte cardiac cell types, make it possible to model the complex genetic and molecular architecture involved in SVD development. Moreover, iPSC models have become increasingly complex with the generation of 3D organoids and engineered cardiac tissues which open the door to new mechanistic insight into SVD development. Finally, iPSC-CMs have been used in proof-of-concept studies that the molecular underpinnings of SVD may be targetable for future therapies. While each platform has its advantages and disadvantages, the use of patient-derived iPSC-CMs offers a window into patient-specific cardiogenesis and SVD development. Advancement in stem-cell based modeling of SVD promises to revolutionize our understanding of the developmental etiology of SVD and provides a tool for developing and testing new therapies.

Sin3a Associated Protein 130kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130kDa ( Sap130 ), a protein in the chromatin modifying SIN3A/HDAC1 complex, as a gene contributing to the digenic etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cell communication were dysregulated in sap130a , but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a , in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

Anisodamine affects the pigmentation, mineral density, craniofacial area, and eye development in zebrafish embryos

Anisodamine is one of the major components of the tropine alkaloid family and is widely used in the treatment of pain, motion sickness, pupil dilatation, and detoxification of organophosphorus poisoning. As a muscarinic receptor antagonist, the low toxicity and moderate drug effect of anisodamine often result in high doses for clinical use, making it important to fully investigate its toxicity. In this study, zebrafish embryos were exposed to 1.3-, 2.6-, and 5.2-mM anisodamine for 7 days to study the toxic effects of drug exposure on pigmentation, mineral density, craniofacial area, and eye development. The results showed that exposure to anisodamine at 1.3 mM resulted in cranial malformations and abnormal pigmentation in zebrafish embryos; 2.6- and 5.2-mM anisodamine resulted in significant eye development defects and reduced bone density in zebrafish embryos. The associated toxicities were correlated with functional development of neural crest cells through gene expression (col1a2, ddb1, dicer1, mab21l1, mab21l2, sox10, tyrp1b, and mitfa) in the dose of 5.2-mM exposed group. In conclusion, this study provides new evidence of the developmental toxicity of high doses of anisodamine in aqueous solutions to organisms and provides a warning for the safe use of this drug.

Impact of environmental chemicals on craniofacial skeletal development: Insights from investigations using zebrafish embryos

Craniofacial skeletal anomalies are among the most common structural birth defects around the world. Various studies using human populations and experimental animals have shown that genetic and environmental factors play significant roles in the causation and progression of these anomalies. Environmental factors, such as teratogens and toxin mixtures, induce craniofacial anomalies are gaining heightened attention. Among experimental investigations, the use of the zebrafish (Danio rerio) has been increasing. A major reason for the increased use is that the zebrafish boast a simple craniofacial structure, and facial morphogenesis is readily observed due to external fertilization and transparent embryo, making it a valuable platform to screen and identify environmental factors involved in the etiology of craniofacial skeletal malformation. This review provides an update on harmful effects from exposure to environmental chemicals, involving metallic elements, nanoparticles, persistent organic pollutants, pesticides and pharmaceutical formulations on craniofacial skeletal development in zebrafish embryos. The collected data provide a better understanding for induction of craniofacial skeletal anomalies and for development of better prevention strategies.

PCB and TCDD derived embryonic cardiac defects result from a novel AhR pathway

Polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are environmental contaminants known to impact cardiac development, a key step in the embryonic development of most animals. To date, little is understood of the molecular mechanism driving the observed cardiac defects in exposed fishes. The literature shows PCB & TCDD derived cardiac defects are concurrent with, but not caused by, expression of cyp1A, due to activation of the aryl hydrocarbon receptor (AhR) gene activation pathway. However, in this study, detailed visualization of fish hearts exposed to PCBs and TCDD show that, in addition to a failure of cardiac looping in early heart development, the inner endocardial lining of the heart fails to maintain proper cell adhesion and tissue integrity. The resulting gap between the endocardium and myocardium in both zebrafish and Atlantic sturgeon suggested functional faults in endothelial adherens junction formation. Thus, we explored the molecular mechanism triggering cardiac defects using immunohistochemistry to identify the location and phosphorylation state of key regulatory and adhesion molecules. We hypothesized that PCB and TCDD activates AhR, phosphorylating Src, which then phosphorylates the endothelial adherens junction protein, VEcadherin. When phosphorylated, VEcadherin dimers, found in the endocardium and vasculature, separate, reducing tissue integrity. In zebrafish, treatment with PCB and TCDD contaminants leads to higher phosphorylation of VEcadherin in cardiac tissue suggesting that these cells have reduced connectivity. Small molecule inhibition of Src phosphorylation prevents contaminant stimulated phosphorylation of VEcadherin and rescues both cardiac function and gross morphology. Atlantic sturgeon hearts show parallels to contaminant exposed zebrafish cardiac phenotype at the tissue level. These data suggest that the mechanism for PCB and TCDD action in the heart is, in part, distinct from the canonical mechanism described in the literature and that cardiac defects are impacted by this nongenomic mechanism.

Identification of molecular toxicity pathways across early life-stages of zebrafish exposed to PCB126 using a whole transcriptomics approach

Although withdrawn from the market in the 1980s, polychlorinated biphenyls (PCBs) are still found ubiquitously in the aquatic environment and pose a serious risk to biota due to their teratogenic potential. In fish, early life-stages are often considered most sensitive with regard to their exposure to PCBs and other dioxin-like compounds. However, little is known about the molecular drivers of the frequently observed teratogenic effects. Therefore, the aims of our study were to: (1) characterize the baseline transcriptome profiles at different embryonic life-stages in zebrafish (Danio rerio); and (2) to identify the molecular response to PCB exposure and life-stage specific-effects of the chemical on associated processes. For both objectives, embryos were sampled at 12, 48, and 96 h post-fertilization (hpf) and subjected to Illumina sequence-by-synthesis and RNAseq analysis. Results revealed that with increasing age more genes and related pathways were upregulated both in terms of number and magnitude. Yet, other transcripts followed an opposite pattern with greater transcript abundance at the earlier time points. Additionally, embryos were exposed to PCB126, a potent agonist of the aryl hydrocarbon receptor (AHR). ClueGO network analysis revealed significant enrichment of genes associated with basic cell metabolism, communication, and homeostasis as well as eye development, muscle formation, and skeletal formation. We selected eight genes involved in the affected pathways for an in-depth characterization of their regulation throughout normal embryogenesis and after exposure to PCB126 by quantification of transcript abundances every 12 h until 118 hpf. Among these, fgf7 and c9 stood out because of their strong upregulation by PCB126 exposure at 48 and 96 hpf, respectively. Cyp2aa12 was upregulated from 84 hpf on. Fabp10ab, myhz1.1, col8a1a, sulf1, and opn1sw1 displayed specific regulation depending on the developmental stage. Overall, we demonstrate that (1) the developmental transcriptome of zebrafish is highly dynamic, and (2) dysregulation of gene expression by exposure to PCB126 was significant and in several cases not directly connected to AHR-signaling. Hence, this study improves the understanding of linkages between molecular events and apical outcomes that are of regulatory relevance.

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.

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.

Generation and Application of the Zebrafish heg1 Mutant as a Cardiovascular Disease Model

Cardiovascular disease (CVD) is the leading cause of global mortality, which has caused a huge burden on the quality of human life. Therefore, experimental animal models of CVD have become essential tools for analyzing the pathogenesis, developing drug screening, and testing potential therapeutic strategies. In recent decades, zebrafish has entered the field of CVD as an important model organism. HEG1, a heart development protein with EGF like domains 1, plays important roles in the development of vertebrate cardiovascular system. Loss of HEG1 will affect the stabilization of vascular endothelial cell connection and eventually lead to dilated cardiomyopathy (DCM). Here, we generated a heg1-specific knockout zebrafish line using CRISPR/Cas9 technology. Zebrafish heg1 mutant demonstrated severe cardiovascular malformations, including atrial ventricular enlargement, heart rate slowing, venous thrombosis and slow blood flow, which were similar to human heart failure and thrombosis phenotype. In addition, the expression of zebrafish cardiac and vascular markers was abnormal in heg1 mutants. In order to apply zebrafish heg1 mutant in cardiovascular drug screening, four Traditional Chinese Medicine (TCM) herbs and three Chinese herbal monomers were used to treat heg1 mutant. The pericardial area, the distance between sinus venosus and bulbus arteriosus (SV-BA), heart rate, red blood cells (RBCs) accumulation in posterior cardinal vein (PCV), and blood circulation in the tail vein were measured to evaluate the therapeutic effects of those drugs on DCM and thrombosis. Here, a new zebrafish model of DCM and thrombosis was established, which was verified to be suitable for drug screening of cardiovascular diseases. It provided an alternative method for traditional in vitro screening, and produced potential clinical related drugs in a rapid and cost-effective way.

Finite element modelling of discontinuous action potential propagation in larval zebrafish and human cardiac tissue

Recently the larval zebrafish has emerged as a model organism which is used to assist in the studies of human cardiac electrophysiology. Although they share many similar electrophysiological characteristics, it has been found that the conduction velocity (CV) of action potential (AP) propagation in larval zebrafish heart is up to two orders of magnitude smaller than in the adult mammalian heart. To address this difference, we have developed three dimensional discrete models of larval zebrafish ventricular fibres (LZVF) in order to simulate AP propagation, taking into account the cellular nature of the tissues and intercellular conduction via gap junctions. Since our ultimate goal is to simulate a whole larval zebrafish heart, we have used the phenomenological Fitzhugh Nagumo (FHN) equations to describe transmembrane currents, and manually adjusted the FHN parameters, to fit published AP shapes for larval zebrafish ventricular cells. This has the benefit of reduced computational load compared to approaches based on biophysical ion current models. We have created models for 48 and 72 h post fertilisation LZVF tissue using published AP and cell size data for zebrafish embryos and used mammalian values for passive electrical parameters. Using the gap junction resistivity per myocyte as an adjustable parameter, we were able to obtain CVs in both of our LZVF models which agree with experimental observations. In order to validate our approach, we have applied it to a human ventricular fibre (HVF) model similar in structure and parameters to other models of the mammalian heart, but adjusting the FHN parameters to fit published AP shapes for human ventricular cells. We find good agreement with the human models. The gap junction resistivities used in the LZVF models are significantly higher than in the HVF case and are consistent with a lower density of gap junctions connecting cells.

Long-term effects of an early-life exposure of fathead minnows to sediments containing bitumen. Part I: Survival, deformities, and growth

The aim of this study was to investigate the long-term effects of a short exposure to natural sediments within the Athabasca oil sand formation to critical stages of embryo-larval development in fathead minnows (Pimephales promelas). Three different sediments were used: Ref sediment from the upper Steepbank River tested at 3 g/L (containing 12.2 ng/g ∑PAHs), and two bitumen-rich sediments tested at 1 and 3 g/L; one from the Ells River (Ells downstream, 6480 ng/g ∑PAHs) and one from the Steepbank River (Stp downstream, 4660 ng/g ∑PAHs). Eggs and larvae were exposed to sediments for 21 days, then transferred to clean water for a 5-month grow-out and recovery period. Larval fish had significantly decreased survival after exposure to 3 g/L sediment from Stp downstream, and decreased growth (length and weight at 16 days post hatch) in Ells and Stp downstream sediments at both 1 and 3 g/L. Decreased tail length was a sensitive endpoint in larval fish exposed to Ells and Stp downstream sediments for 21 days compared to Ref sediment. After the grow-out in clean water, all growth effects from the bitumen-containing sediments recovered, but adult fish from Stp downstream 3 g/L sediment had significant increases in jaw deformities. The study shows the potential for fish to recover from the decreased growth effects caused by sediments containing oil sands-related compounds, but that some effects of the early-life sediment exposure occur later on in adult fish.

Sex-specific alterations of lipid metabolism in zebrafish exposed to polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic pollutants (POPs) mixtures exerting environmental health risk. In mammals, PCBs have been shown to disrupt metabolic state, especially lipid metabolism, and energy balance, but their effects on lipid metabolism in fish are largely unknown. The zebrafish were selected as model and both male and female adult zebrafish were exposed to different concentrations of PCBs at gradient concentrations of 0.2, 2.0 and 20.0 μg/L for 6 weeks. PCB exposure did not affect survival, but a significant inhibition of growth was observed in the males after exposure to 20.0 μg/L. The lower concentrations of 0.2 and 2.0 μg/L increased hepatic lipid accumulation to a greater extent in male fish, but the higher concentration of 20.0 μg/L did not cause significant fat accumulation in either male or female fish. In males, the expression of genes related to lipogenesis and lipid catabolism was upregulated in a concentration-dependent manner in the liver and visceral mass without liver and gonad; the effects of exposure on lipid metabolism-related genes in female fish were less pronounced. PCB exposure did not induce significant oxidative stress, but did upregulate the expression of stress- and apoptosis-related genes, mostly in male fish. The low concentrations of PCBs (0.2 μg/L and 2.0 μg/L) exerted sex-specific effects on zebrafish lipid metabolism, and male fish were more sensitive to the exposure. This study provides new mechanistic insights into the complex interactions between PCBs, lipid metabolism, and sex in zebrafish, and may contribute to a future systematic assessment of the effects of PCBs on aquatic ecosystems.

Cardiovascular Effects of PCB 126 (3,3',4,4',5-Pentachlorobiphenyl) in Zebrafish Embryos and Impact of Co-Exposure to Redox Modulating Chemicals

The developing cardiovascular system of zebrafish is a sensitive target for many environmental pollutants, including dioxin-like compounds and pesticides. Some polychlorinated biphenyls (PCBs) can compromise the cardiovascular endothelial function by activating oxidative stress-sensitive signaling pathways. Therefore, we exposed zebrafish embryos to PCB126 or to several redox-modulating chemicals to study their ability to modulate the dysmorphogenesis produced by PCB126. PCB126 produced a concentration-dependent induction of pericardial edema and circulatory failure, and a concentration-dependent reduction of cardiac output and body length at 80 hours post fertilization (hpf). Among several modulators tested, the effects of PCB126 could be both positively and negatively modulated by different compounds; co-treatment with α-tocopherol (vitamin E liposoluble) prevented the adverse effects of PCB126 in pericardial edema, whereas co-treatment with sodium nitroprusside (a vasodilator compound) significantly worsened PCB126 effects. Gene expression analysis showed an up-regulation of cyp1a, hsp70, and gstp1, indicative of PCB126 interaction with the aryl hydrocarbon receptor (AhR), while the transcription of antioxidant genes (sod1, sod2; cat and gpx1a) was not affected. Further studies are necessary to understand the role of oxidative stress in the developmental toxicity of low concentrations of PCB126 (25 nM). Our results give insights into the use of zebrafish embryos for exploring mechanisms underlying the oxidative potential of environmental pollutants.

Prenatal exposure to PCBs in Cyp1a2 knock-out mice interferes with F1 fertility, impairs long-term potentiation, reduces acoustic startle and impairs conditioned freezing contextual memory with minimal transgenerational effects

Polychlorinated biphenyls (PCBs) are toxic environmental pollutants. Humans are exposed to PCB mixtures via contaminated food or water. PCB exposure causes adverse effects in adults and after exposure in utero. PCB toxicity depends on the congener mixture and CYP1A2 gene activity. For coplanar PCBs, toxicity depends on ligand affinity for the aryl hydrocarbon receptor (AHR). Previously, we found that perinatal exposure of mice to a three-coplanar/five-noncoplanar PCB mixture induced deficits in novel object recognition and trial failures in the Morris water maze in Cyp1a2^-/- ::Ahr^b1 C57BL6/J mice compared with wild-type mice (Ahr^b1  = high AHR affinity). Here we exposed gravid Cyp1a2^-/- ::Ahr^b1 mice to a PCB mixture on embryonic day 10.5 by gavage and examined the F₁ and F₃ offspring (not F₂ ). PCB-exposed F₁ mice exhibited increased open-field central time, reduced acoustic startle, greater conditioned contextual freezing and reduced CA1 hippocampal long-term potentiation with no change in spatial learning or memory. F₁ mice also had inhibited growth, decreased heart rate and cardiac output, and impaired fertility. F₃ mice showed few effects. Gene expression changes were primarily in F₁ PCB males compared with wild-type males. There were minimal RNA and DNA methylation changes in the hippocampus from F₁ to F₃ with no clear relevance to the functional effects. F₀ PCB exposure during a period of rapid DNA de-/remethylation in a susceptible genotype produced clear F₁ effects with little evidence of transgenerational effects in the F₃ generation. While PCBs show clear developmental neurotoxicity, their effects do not persist across generations for effects assessed herein.

Fish life-history traits are affected after chronic dietary exposure to an environmentally realistic marine mixture of PCBs and PBDEs

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants that have been shown to affect fish life-history traits such as reproductive success, growth and survival. At the individual level, their toxicity and underlying mechanisms of action have been studied through experimental exposure. However, the number of experimental studies approaching marine environmental situations is scarce, i.e., in most cases, individuals are exposed to either single congeners, or single types of molecules, or high concentrations, so that results can hardly be transposed to natural populations. In the present study, we evaluated the effect of chronic dietary exposure to an environmentally realistic marine mixture of PCB and PBDE congeners on zebrafish life-history traits from larval to adult stage. Exposure was conducted through diet from the first meal and throughout the life cycle of the fish. The mixture was composed so as to approach environmentally relevant marine conditions in terms of both congener composition and concentrations. Life-history traits of exposed fish were compared to those of control individuals using several replicate populations in each treatment. We found evidence of slower body growth, but to a larger asymptotic length, and delayed spawning probability in exposed fish. In addition, offspring issued from early spawning events of exposed fish exhibited a lower larval survival under starvation condition. Given their strong dependency on life-history traits, marine fish population dynamics and associated fisheries productivity for commercial species could be affected by such individual-level effects of PCBs and PBDEs on somatic growth, spawning probability and larval survival.

Early Life Exposure to Low Levels of AHR Agonist PCB126 (3,3',4,4',5-Pentachlorobiphenyl) Reprograms Gene Expression in Adult Brain

Early life exposure to environmental chemicals can have long-term consequences that are not always apparent until later in life. We recently demonstrated that developmental exposure of zebrafish to low, nonembryotoxic levels of 3,3',4,4',5-pentachlorobiphenyl (PCB126) did not affect larval behavior, but caused changes in adult behavior. The objective of this study was to investigate the underlying molecular basis for adult behavioral phenotypes resulting from early life exposure to PCB126. We exposed zebrafish embryos to PCB126 during early development and measured transcriptional profiles in whole embryos, larvae and adult male brains using RNA-sequencing. Early life exposure to 0.3 nM PCB126 induced cyp1a transcript levels in 2-dpf embryos, but not in 5-dpf larvae, suggesting transient activation of aryl hydrocarbon receptor with this treatment. No significant induction of cyp1a was observed in the brains of adults exposed as embryos to PCB126. However, a total of 2209 and 1628 genes were differentially expressed in 0.3 and 1.2 nM PCB126-exposed groups, respectively. KEGG pathway analyses of upregulated genes in the brain suggest enrichment of calcium signaling, MAPK and notch signaling, and lysine degradation pathways. Calcium is an important signaling molecule in the brain and altered calcium homeostasis could affect neurobehavior. The downregulated genes in the brain were enriched with oxidative phosphorylation and various metabolic pathways, suggesting that the metabolic capacity of the brain is impaired. Overall, our results suggest that PCB exposure during sensitive periods of early development alters normal development of the brain by reprogramming gene expression patterns, which may result in alterations in adult behavior.

Mixture-specific gene expression in zebrafish (Danio rerio) embryos exposed to perfluorooctane sulfonic acid (PFOS), perfluorohexanoic acid (PFHxA) and 3,3',4,4',5-pentachlorobiphenyl (PCB126)

Perfluorooctane sulfonic acid (PFOS) and 3,3',4,4',5-pentachlorobiphenyl (PCB126) are persistent organic pollutants of high concern because of their environmental persistence, bioaccumulation and toxic properties. Besides, the amphiphilic properties of fluorinated compounds such as PFOS and perfluorohexanoic acid (PFHxA) suggest a role in increasing cell membrane permeability and solubilizing chemicals. The present study aimed at investigating whether PFOS and PFHxA are capable of modifying the activation of PCB126 toxicity-related pathways. For this purpose, zebrafish embryos were exposed in semi-static conditions to 7.5μg/L of PCB126 alone, in the presence of 25mg/L of PFOS, 15.7mg/L of PFHxA or in the presence of both PFOS and PFHxA. Quantitative PCR was performed on embryos aged from 24h post fertilization (hpf) to 96 hpf to investigate expression changes of genes involved in metabolism of xenobiotics (ahr2, cyp1a), oxidative stress (gpx1a, tp53), lipids metabolism (acaa2, osbpl1a), and epigenetic mechanisms (dnmt1, dnmt3ba). Cyp1a and ahr2 expression were significantly induced by the presence of PCB126. However, after 72 and 78h of exposure, induction of cyp1a expression was significantly lower when embryos were co-exposed to PCB126+PFOS+PFHxA when compared to PCB126-exposed embryos. Significant upregulation of gpx1a occurred after exposure to PCB126+PFHxA and to PCB126+PFOS+PFHxA at 30 and 48 hpf. Besides, embryos appeared more sensitive to PCB126+PFOS+PFHxA at 78 hpf: acaa2 and osbpl1a were significantly downregulated; dnmt1 was significantly upregulated. While presented as environmentally safe, PFHxA demonstrated that it could affect gene expression patterns in zebrafish embryos when combined to PFOS and PCB126, suggesting that such mixture may increase PCB126 toxicity. This is of particular relevance since PFHxA is persistent and still being ejected into the environment. Moreover, it provides additional information as to the importance to integrate mixture effects of chemicals in risk assessment and biomonitoring frameworks.

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.

Histopathological changes in zebrafish embryos exposed to DLPCBs extract from Zhanjiang coastal sediment

Dioxin-like polychlorinated biphenyls (DLPCBs) are ubiquitous persistent pollutants that cause adverse effects in many environmental organisms. DLPCBs in marine sediments can be absorbed by benthic organisms, bioaccumulate, and biomagnify through the food chain and threaten animal and human health. There are no reports of DLPCBs concentrations in the Zhanjiang Gulf seabed. This study was designed to investigate the concentration of DLPCBs in the Zhanjiang coastal sediment and histopathological changes in zebrafish (Diano rerio) embryos exposed to environmentally relevant concentrations of DLPCBs. Of the five sites selected, two sites TS and JSW contained DLPCBs at concentrations of 0.08 and 22.54 ng/g dry sediment, respectively. Two groups of zebrafish embryos were used. One group was exposed to 3.75, 7.5, 15, 30, and 60 mg/ml of DLPCBs extracted from the sediments sampled from the TS site and the second group to 4.375, 8.75, 17.5, 35, and 70 mg/ml of DLPCBs from JSW site from 0.75 h post-fertilization (hpf) to 96 hpf. The zebrafish exposed to 60 and 70 mg/ml of DLPCBs at 96 hpf displayed gross histopathological changes with cardiac lesions including pericardial edema being the most deleterious. Other changes observed were hydropic degeneration of gill filaments and hepatocytes, loss of intestinal folds, and uninflated swim bladder. It appears that only a few sites of the Zhanjiang gulf are contaminated with DLPCBs. This is the first report of histopathological changes in the gills, hepatocytes, intestines, heart, and the swim bladder in zebrafish embryos exposed to DLPCBs from a coastal sediment. Further studies with sampling at different stages of development are required to identify which organ/tissue is most sensitive to DLPCBs.

Learning to Fish with Genetics: A Primer on the Vertebrate Model Danio rerio

In the last 30 years, the zebrafish has become a widely used model organism for research on vertebrate development and disease. Through a powerful combination of genetics and experimental embryology, significant inroads have been made into the regulation of embryonic axis formation, organogenesis, and the development of neural networks. Research with this model has also expanded into other areas, including the genetic regulation of aging, regeneration, and animal behavior. Zebrafish are a popular model because of the ease with which they can be maintained, their small size and low cost, the ability to obtain hundreds of embryos on a daily basis, and the accessibility, translucency, and rapidity of early developmental stages. This primer describes the swift progress of genetic approaches in zebrafish and highlights recent advances that have led to new insights into vertebrate biology.

A Novel Cardiotoxic Mechanism for a Pervasive Global Pollutant

The Deepwater Horizon disaster drew global attention to the toxicity of crude oil and the potential for adverse health effects amongst marine life and spill responders in the northern Gulf of Mexico. The blowout released complex mixtures of polycyclic aromatic hydrocarbons (PAHs) into critical pelagic spawning habitats for tunas, billfishes, and other ecologically important top predators. Crude oil disrupts cardiac function and has been associated with heart malformations in developing fish. However, the precise identity of cardiotoxic PAHs, and the mechanisms underlying contractile dysfunction are not known. Here we show that phenanthrene, a PAH with a benzene 3-ring structure, is the key moiety disrupting the physiology of heart muscle cells. Phenanthrene is a ubiquitous pollutant in water and air, and the cellular targets for this compound are highly conserved across vertebrates. Our findings therefore suggest that phenanthrene may be a major worldwide cause of vertebrate cardiac dysfunction.

3D Finite Element Electrical Model of Larval Zebrafish ECG Signals

Assessment of heart function in zebrafish larvae using electrocardiography (ECG) is a potentially useful tool in developing cardiac treatments and the assessment of drug therapies. In order to better understand how a measured ECG waveform is related to the structure of the heart, its position within the larva and the position of the electrodes, a 3D model of a 3 days post fertilisation (dpf) larval zebrafish was developed to simulate cardiac electrical activity and investigate the voltage distribution throughout the body. The geometry consisted of two main components; the zebrafish body was modelled as a homogeneous volume, while the heart was split into five distinct regions (sinoatrial region, atrial wall, atrioventricular band, ventricular wall and heart chambers). Similarly, the electrical model consisted of two parts with the body described by Laplace's equation and the heart using a bidomain ionic model based upon the Fitzhugh-Nagumo equations. Each region of the heart was differentiated by action potential (AP) parameters and activation wave conduction velocities, which were fitted and scaled based on previously published experimental results. ECG measurements in vivo at different electrode recording positions were then compared to the model results. The model was able to simulate action potentials, wave propagation and all the major features (P wave, R wave, T wave) of the ECG, as well as polarity of the peaks observed at each position. This model was based upon our current understanding of the structure of the normal zebrafish larval heart. Further development would enable us to incorporate features associated with the diseased heart and hence assist in the interpretation of larval zebrafish ECGs in these conditions.

The metabolite 3,4,3',4'-tetrachloroazobenzene (TCAB) exerts a higher ecotoxicity than the parent compounds 3,4-dichloroaniline (3,4-DCA) and propanil

3,4,3',4'-tetrachloroazobenzene (TCAB) is not commercially manufactured but formed as an unwanted by-product in the manufacturing of 3,4-dichloroaniline (3,4-DCA) or metabolized from the degradation of chloranilide herbicides, like propanil. While a considerable amount of research has been done concerning the toxicological and ecotoxicological effects of propanil and 3,4-DCA, limited information is available on TCAB. Our study examined the toxicity of TCAB in comparison to its parent compounds propanil and 3,4-DCA, using a battery of bioassays including in vitro with aryl hydrocarbon receptor (AhR) mediated activity by the 7-ethoxyresorufin-O-deethylase (EROD) assay and micro-EROD, endocrine-disrupting activity with chemically activated luciferase gene expression (CALUX) as well as in vivo with fish embryo toxicity (FET) assays with Danio rerio. Moreover, the quantitative structure activity response (QSAR) concepts were applied to simulate the binding affinity of TCAB to certain human receptors. It was shown that TCAB has a strong binding affinity to the AhR in EROD and micro-EROD induction assay, with the toxic equivalency factor (TEF) of 8.7×10(-4) and 1.2×10(-5), respectively. TCAB presented to be a weak endocrine disrupting compound with a value of estradiol equivalence factor (EEF) of 6.4×10(-9) and dihydrotestosterone equivalency factor (DEF) of 1.1×10(-10). No acute lethal effects of TCAB were discovered in FET test after 96h of exposure. Major sub-lethal effects detected were heart oedema, yolk malformation, as well as absence of blood flow and tail deformation. QSAR modelling suggested an elevated risk to environment, particularly with respect to binding to the AhR. An adverse effect potentially triggering ERβ, mineralocorticoid, glucocorticoid and progesterone receptor activities might be expected. Altogether, the results obtained suggest that TCAB exerts a higher toxicity than both propanil and 3,4-DCA. This should be considered when assessing the impact of these compounds for the environment and also for regulatory decisions.

Antioxidant Rescue of Selenomethionine-Induced Teratogenesis in Zebrafish Embryos

Selenium (Se) is an essential micronutrient that can be found at toxic concentrations in surface waters contaminated by runoff from agriculture and coal mining. Zebrafish (Danio rerio) embryos were exposed to aqueous Se in the form of selenate, selenite, and l-selenomethionine (SeMet) in an attempt to determine if oxidative stress plays a role in selenium embryo toxicity. Selenate and selenite exposure did not induce embryo deformities (lordosis and craniofacial malformation). l-selenomethionine, however, induced significantly higher deformity rates at 100 µg/L compared with controls. SeMet exposure induced a dose-dependent increase in the catalytic subunit of glutamate-cysteine ligase (gclc) and reached an 11.7-fold increase at 100 µg/L. SeMet exposure also reduced concentrations of TGSH, RGSH, and the TGSH:GSSG ratio. Pretreatment with 100 µM N-acetylcysteine significantly reduced deformities in the zebrafish embryos secondarily treated with 400 µg/L SeMet from approximately 50–10 % as well as rescued all three of the significant glutathione level differences seen with SeMet alone. Selenite exposure induced a 6.6-fold increase in expression of the glutathione-S-transferase pi class 2 (gstp2) gene, which is involved in xenobiotic transformation and possibly oxidative stress. These results suggest that aqueous exposure to SeMet can induce significant embryonic teratogenesis in zebrafish that are at least partially attributed to oxidative stress.

Developmental toxicity, EROD, and CYP1A mRNA expression in zebrafish embryos exposed to dioxin-like PCB126

Dioxin-like PCB126 is a persistent organic pollutant that causes a range of syndromes including developmental toxicity. Dioxins have a high affinity for aryl hydrocarbon receptor (AhR) and induce cytochrome P4501A (CYP1A). However, the role of CYP1A activity in developmental toxicity is less clear. To better understand dioxin induced developmental toxicity, we exposed zebrafish (Danio rerio) embryos to PCB126 at concentrations of 0, 16, 32, 64, and 128 μg L(-1) from 3-h post-fertilization (hpf) to 168 hpf. The embryonic survival rate decreased at 144 and 168 hpf. The fry at 96 hpf displayed gross developmental malformations, including pericardial and yolk sac edema, spinal curvature, abnormal lower jaw growth, and non-inflated swim bladder. The pericardial and yolk sac edema rate significantly increased and the heart rate declined from 96 hpf compared with the controls. PCB126 did not alter the hatching rate. To elucidate the mechanism of PCB126-induced developmental toxicity, we conducted ethoxyresorufin-O-deethylase (EROD) in vivo assay to determine CYP1A enzyme activity, and real-time PCR to study the induction of CYP1A mRNA gene expression in embryo/larval zebrafish at 24, 72, 96, and 132 hpf. In vivo EROD activity was induced by PCB126 at 16 μg L(-1) concentration as early as 72 hpf but significant increases were observed only in zebrafish exposed to 64 and 128 μg L(-1) doses (p < 0.005) at 72, 96, and 132 hpf. Induction of CYP1A mRNA expression was significantly upregulated in zebrafish exposed to 32 and 64 μg L(-1) at 24, 72, 96, and 132 hpf. Overall, the severe pericardial and yolk sac edema and reduced heart rate suggest that heart defects are a sensitive endpoint, and the general trend of dose-dependent increase in EROD activity and induction of CYP1A mRNA gene expression provide evidence that the developmental toxicity of PCB126 to zebrafish embryos is mediated by activation of AhR.

Retene causes multifunctional transcriptomic changes in the heart of rainbow trout (Oncorhynchus mykiss) embryos

Fish are particularly sensitive to aryl hydrocarbon receptor (AhR)-mediated developmental toxicity. The molecular mechanisms behind these adverse effects have remained largely unresolved in salmonids, and for AhR-agonistic polycyclic aromatic hydrocarbons (PAHs). This study explored the cardiac transcriptome of rainbow trout (Oncorhynchus mykiss) eleuteroembryos exposed to retene, an AhR-agonistic PAH. The embryos were exposed to retene (nominal concentration 32 μg/L) and control, their hearts were collected before, at and after the onset of the visible signs of developmental toxicity, and transcriptomic changes were studied by microarray analysis. Retene up- or down-regulated 122 genes. The largest Gene Ontology groups were signal transduction, transcription, apoptosis, cell growth, cytoskeleton, cell adhesion/mobility, cardiovascular development, xenobiotic metabolism, protein metabolism, lipid metabolism and transport, and amino acid metabolism. Together these findings suggest that retene affects multiple signaling cascades in the heart of rainbow trout embryos, and potentially disturbs processes related to cardiovascular development and function.

Effects of embryonic exposure to polychlorinated biphenyls (PCBs) on anxiety-related behaviors in larval zebrafish

The zebrafish (Danio rerio) is an excellent model system for assessing the effects of toxicant exposure on behavior and neurodevelopment. In the present study, we examined the effects of sub-chronic embryonic exposure to polychlorinated biphenyls (PCBs), a ubiquitous anthropogenic pollutant, on anxiety-related behaviors. We found that exposure to the PCB mixture, Aroclor (A) 1254, from 2 to 26h post-fertilization (hpf) induced two statistically significant behavioral defects in larvae at 7 days post-fertilization (dpf). First, during 135min of free swimming, larvae that had been exposed to 2ppm, 5ppm or 10ppm A1254 exhibited enhanced thigmotaxis (edge preference) relative to control larvae. Second, during the immediately ensuing 15-min visual startle assay, the 5ppm and 10ppm PCB-exposed larvae reacted differently to a visual threat, a red 'bouncing' disk, relative to control larvae. These results are consistent with the anxiogenic and attention-disrupting effects of PCB exposure documented in children, monkeys and rodents and merit further investigation.

Early life exposure to PCB126 results in delayed mortality and growth impairment in the zebrafish larvae

The occurrence of chronic or delayed toxicity resulting from the exposure to sublethal chemical concentrations is an increasing concern in environmental risk assessment. The Fish Embryo Toxicity (FET) test with zebrafish provides a reliable prediction of acute toxicity in adult fish, but it cannot yet be applied to predict the occurrence of chronic or delayed toxicity. Identification of sublethal FET endpoints that can assist in predicting the occurrence of chronic or delayed toxicity would be advantageous. The present study characterized the occurrence of delayed toxicity in zebrafish larvae following early exposure to PCB126, previously described to cause delayed effects in the common sole. The first aim was to investigate the occurrence and temporal profiles of delayed toxicity during zebrafish larval development and compare them to those previously described for sole to evaluate the suitability of zebrafish as a model fish species for delayed toxicity assessment. The second aim was to examine the correlation between the sublethal endpoints assessed during embryonal and early larval development and the delayed effects observed during later larval development. After exposure to PCB126 (3-3000ng/L) until 5 days post fertilization (dpf), larvae were reared in clean water until 14 or 28 dpf. Mortality and sublethal morphological and behavioural endpoints were recorded daily, and growth was assessed at 28 dpf. Early life exposure to PCB126 caused delayed mortality (300 ng/L and 3000 ng/L) as well as growth impairment and delayed development (100 ng/L) during the clean water period. Effects on swim bladder inflation and cartilaginous tissues within 5 dpf were the most promising for predicting delayed mortality and sublethal effects, such as decreased standard length, delayed metamorphosis, reduced inflation of swim bladder and column malformations. The EC50 value for swim bladder inflation at 5 dpf (169 ng/L) was similar to the LC50 value at 8 dpf (188 and 202 ng/L in two experiments). Interestingly, the patterns of delayed mortality and delayed effects on growth and development were similar between sole and zebrafish. This indicates the comparability of critical developmental stages across divergent fish species such as a cold water marine flatfish and a tropical freshwater cyprinid. Additionally, sublethal effects in early embryo-larval stages were found promising for predicting delayed lethal and sublethal effects of PCB126. Therefore, the proposed method with zebrafish is expected to provide valuable information on delayed mortality and delayed sublethal effects of chemicals and environmental samples that may be extrapolated to other species.

Effects of embryonic exposure to polychlorinated biphenyls (PCBs) on larval zebrafish behavior

Developmental disorders such as anxiety, autism, and attention deficit hyperactivity disorders have been linked to exposure to polychlorinated biphenyls (PCBs), a ubiquitous anthropogenic pollutant. The zebrafish is widely recognized as an excellent model system for assessing the effects of toxicant exposure on behavior and neurodevelopment. In the present study, we examined the effect of sub-chronic embryonic exposure to the PCB mixture, Aroclor (A) 1254 on anxiety-related behaviors in zebrafish larvae at 7 days post-fertilization (dpf). We found that exposure to low concentrations of A1254, from 2 to 26 h post-fertilization (hpf) induced specific behavioral defects in two assays. In one assay with intermittent presentations of a moving visual stimulus, 5 ppm and 10 ppm PCB-exposed larvae displayed decreased avoidance behavior but no significant differences in thigmotaxis or freezing relative to controls. In the other assay with intermittent presentations of a moving visual stimulus and a stationary visual stimulus, 5 ppm and 10 ppm PCB-exposed larvae had elevated baseline levels of thigmotaxis but no significant differences in avoidance behavior relative to controls. The 5 ppm larvae also displayed higher terminal levels of freezing relative to controls. Collectively, our results show that exposure to ecologically valid PCB concentrations during embryonic development can induce functional deficits and alter behavioral responses to a visual threat.

Grouping of histone deacetylase inhibitors and other toxicants disturbing neural crest migration by transcriptional profiling

Functional assays, such as the "migration inhibition of neural crest cells" (MINC) developmental toxicity test, can identify toxicants without requiring knowledge on their mode of action (MoA). Here, we were interested, whether (i) inhibition of migration by structurally diverse toxicants resulted in a unified signature of transcriptional changes; (ii) whether statistically-identified transcript patterns would inform on compound grouping even though individual genes were little regulated, and (iii) whether analysis of a small group of biologically-relevant transcripts would allow the grouping of compounds according to their MoA. We analyzed transcripts of 35 'migration genes' after treatment with 16 migration-inhibiting toxicants. Clustering, principal component analysis and correlation analyses of the data showed that mechanistically related compounds (e.g. histone deacetylase inhibitors (HDACi), PCBs) triggered similar transcriptional changes, but groups of structurally diverse toxicants largely differed in their transcriptional effects. Linear discriminant analysis (LDA) confirmed the specific clustering of HDACi across multiple separate experiments. Similarity of the signatures of the HDACi trichostatin A and suberoylanilide hydroxamic acid to the one of valproic acid (VPA), suggested that the latter compound acts as HDACi when impairing neural crest migration. In conclusion, the data suggest that (i) a given functional effect (e.g. inhibition of migration) can be associated with highly diverse signatures of transcript changes; (ii) statistically significant grouping of mechanistically-related compounds can be achieved on the basis of few genes with small regulations. Thus, incorporation of mechanistic markers in functional in vitro tests may support read-across procedures, also for structurally un-related compounds.

Temporal cohesion of the structural, functional and molecular characteristics of the developing zebrafish heart

Heart formation is a complex, dynamic and highly coordinated process of molecular, morphogenetic and functional factors with each interacting and contributing to formation of the mature organ. Cardiac abnormalities in early life can be lethal in mammals but not in the zebrafish embryo which has been widely used to study the developing heart. While early cardiac development in the zebrafish has been well characterized, functional changes during development and how these relate to architectural, cellular and molecular aspects of development have not been well described previously. To address this we have carefully characterised cardiac structure, function, cardiomyocyte proliferation and cardiac-specific gene expression between 48 and 120 hpf in the zebrafish. We show that the zebrafish heart increases in volume and changes shape significantly between 48 and 72 hpf accompanied by a 40% increase in cardiomyocyte number. Between 96 and 120 hpf, while external heart expansion slows, there is rapid formation of a mature and extensive trabecular network within the ventricle chamber. While ejection fraction does not change during the course of development other determinants of contractile function increase significantly particularly between 72 and 96 hpf leading to an increase in cardinal vein blood flow. This study has revealed a number of novel aspects of cardiac developmental dynamics with striking temporal orchestration of structure and function within the first few days of development. These changes are associated with changes in expression of developmental and maturational genes. This study provides important insights into the complex temporal relationship between structure and function of the developing zebrafish heart.

Ecotoxicology of polychlorinated biphenyls in fish--a critical review

Polychlorinated biphenyls (PCBs) are widespread persistent anthropogenic contaminants that can accumulate in tissues of fish. The toxicity of PCBs and their transformation products has been investigated for nearly 50 years, but there is a lack of consensus regarding the effects of these environmental contaminants on wild fish populations. The objective of this review is to critically examine these investigations and evaluate publicly available databases for evidence of effects of PCBs in wild fish. Biological activity of PCBs is limited to a small proportion of PCB congeners [e.g., dioxin-like PCBs (DL-PCBs)] and occurs at concentrations that are typically orders of magnitude higher than PCB levels detected in wild fish. Induction of biomarkers consistent with PCB exposure (e.g., induction of cytochrome P450 monooxygenase system) has been evaluated frequently and shown to be induced in fish from some environments, but there does not appear to be consistent reports of damage (i.e., biomarkers of effect) to biomolecules (i.e., oxidative injury) in these fish. Numerous investigations of endocrine system dysfunction or effects on other organ systems have been conducted in wild fish, but collectively there is no consistent evidence of PCB effects on these systems in wild fish. Early life stage toxicity of DL-PCBs does not appear to occur at concentrations reported in wild fish embryos, and results do not support an association between PCBs and decreased survival of early life stages of wild fish. Overall, there appears to be little evidence that PCBs have had any widespread effect on the health or survival of wild fish.

Calcium signaling regulates ventricular hypertrophy during development independent of contraction or blood flow

In utero interventions aimed at restoring left ventricular hemodynamic forces in fetuses with prenatally diagnosed hypoplastic left heart syndrome failed to stimulate ventricular myocardial growth during gestation, suggesting chamber growth during development may not rely upon fluid forces. We therefore hypothesized that ventricular hypertrophy during development may depend upon fundamental Ca(2+)-dependent growth pathways that function independent of hemodynamic forces. To test this hypothesis, zebrafish embryos were treated with inhibitors or activators of Ca(2+) signaling in the presence or absence of contraction during the period of chamber development. Abolishment of contractile function alone in the setting of preserved Ca(2+) signaling did not impair ventricular hypertrophy. In contrast, inhibition of L-type voltage-gated Ca(2+) influx abolished contraction and led to reduced ventricular hypertrophy, whereas increasing L-type voltage-gated Ca(2+) influx led to enhanced ventricular hypertrophy in either the presence or absence of contraction. Similarly, inhibition of the downstream Ca(2+)-sensitive phosphatase calcineurin, a known regulator of adult cardiac hypertrophy, led to reduced ventricular hypertrophy in the presence or absence of contraction, whereas hypertrophy was rescued in the absence of L-type voltage-gated Ca(2+) influx and contraction by expression of a constitutively active calcineurin. These data suggest that ventricular cardiomyocyte hypertrophy during chamber formation is dependent upon Ca(2+) signaling pathways that are unaffected by heart function or hemodynamic forces. Disruption of Ca(2+)-dependent hypertrophy during heart development may therefore represent one mechanism for impaired chamber formation that is not related to impaired blood flow.

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

Exposure of zebrafish embryos to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activates the zebrafish aryl hydrocarbon receptor 2 (AHR) to produce developmental and cardiovascular toxicity. AHR is found in the heart; however, AHR activation by TCDD is not confined to the heart and occurs throughout the organism. In order to understand the cause of cardiotoxicity, we constructed a constitutively active AHR (caAHR) based on the zebrafish AHR2 and expressed it specifically in cardiomyocytes. We show that AHR activation within the cardiomyocytes can account for the heart failure induced by TCDD. Expression of the caAHR within the heart produced cardiac malformations, loss of circulation, and pericardial edema. The heart-specific activation of AHR reproduced several other well-characterized endpoints of TCDD toxicity outside of the cardiovascular system, including defects in swim bladder and craniofacial development. This work identifies a single cellular site of TCDD action, the myocardial cell, that can account for the severe cardiovascular collapse observed following early life stage exposure to TCDD, and contributes to other forms of toxicity.

Connecting teratogen-induced congenital heart defects to neural crest cells and their effect on cardiac function

Neural crest cells play many key roles in embryonic development, as demonstrated by the abnormalities that result from their specific absence or dysfunction. Unfortunately, these key cells are particularly sensitive to abnormalities in various intrinsic and extrinsic factors, such as genetic deletions or ethanol-exposure that lead to morbidity and mortality for organisms. This review discusses the role identified for a segment of neural crest in regulating the morphogenesis of the heart and associated great vessels. The paradox is that their derivatives constitute a small proportion of cells to the cardiovascular system. Findings supporting that these cells impact early cardiac function raises the interesting possibility that they indirectly control cardiovascular development at least partially through regulating function. Making connections between insults to the neural crest, cardiac function, and morphogenesis is more approachable with technological advances. Expanding our understanding of early functional consequences could be useful in improving diagnosis and testing therapies.

Toxic effects of polychlorinated biphenyls on cardiac development in zebrafish

Polychlorinated biphenyls (PCBs) are ubiquitous environmental pollutants that may pose significant health-risks to various organisms including humans. Although the mixed PCB Aroclor 1254 is widespread in the environment, its potential toxic effect on heart development and the mechanism underlying its developmental toxicity have not been previously studied. Here, we used the zebrafish as a toxicogenomic model to examine the effects of Aroclor 1254 on heart development. We found that PCB exposure during zebrafish development induced heart abnormalities including pericardial edema and cardiac looping defects. Further malformations of the zebrafish embryo were observed and death of the larvae occurred in a time- and dose-dependent manner. Our mechanistic studies revealed that abnormalities in the arylhydrocarbon receptor, Wnt and retinoic acid signaling pathways may underlie the effects of PCBs on zebrafish heart development. Interestingly, co-administration of Aroclor 1254 and diethylaminobenzaldehyde, an inhibitor of retinaldehyde dehydrogenase, partially rescued the toxic effects of PCBs on zebrafish heart development. In conclusion, PCBs can induce developmental defects in the zebrafish heart, which may be mediated by abnormal RA signaling.

Functional analysis of candidate genes in 2q13 deletion syndrome implicates FBLN7 and TMEM87B deficiency in congenital heart defects and FBLN7 in craniofacial malformations

Recurrent 2q13 deletion syndrome is associated with incompletely penetrant severe cardiac defects and craniofacial anomalies. We used an atypical, overlapping 1.34 Mb 2q13 deletion in a patient with pathogenically similar congenital heart defects (CHD) to narrow the putative critical region for CHD to 474 kb containing six genes. To determine which of these genes is responsible for severe cardiac and craniofacial defects noted in the patients with the deletions, we used zebrafish morpholino knockdown to test the function of each orthologue during zebrafish development. Morpholino-antisense-mediated depletion of fibulin-7B, a zebrafish orthologue of fibulin-7 (FBLN7), resulted in cardiac hypoplasia, deficient craniofacial cartilage deposition and impaired branchial arch development. TMEM87B depletion likewise resulted in cardiac hypoplasia but with preserved branchial arch development. Depletion of both fibulin-7B and TMEM87B resulted in more severe defects of cardiac development, suggesting that their concurrent loss may enhance the risk of a severe cardiac defect. We postulate that heterozygous loss of FBLN7 and TMEM87B account for some of the clinical features, including cardiac defects and craniofacial abnormalities associated with 2q13 deletion syndrome.

Long-term dietary-exposure to non-coplanar PCBs induces behavioral disruptions in adult zebrafish and their offspring

The use of polychlorinated biphenyls (PCBs) has been banned for several decades. PCBs have a long biological half-life and high liposolubility which leads to their bioaccumulation and biomagnification through food chains over a wide range of trophic levels. Exposure can lead to changes in animal physiology and behavior and has been demonstrated in both experimental and field analyses. There are also potential risks to high trophic level predators, including humans. A maternal transfer has been demonstrated in fish as PCBs bind to lipids in eggs. In this study, behavioral traits (exploration and free swimming, with or without challenges) of contaminated zebrafish (Danio rerio) adults and their offspring (both as five-day-old larvae and as two-month-old fish reared under standard conditions) were measured using video-tracking. Long-term dietary exposure to a mixture of non-coplanar PCBs was used to mimic known environmental contamination levels and congener composition. Eight-week-old fish were exposed for eight months at 26-28 °C. Those exposed to an intermediate dose (equivalent to that found in the Loire Estuary, ∑(CB)=515 ng g⁻¹ dry weight in food) displayed behavioral disruption in exploration capacities. Fish exposed to the highest dose (equivalent to that found in the Seine Estuary, ∑(CB)=2302 ng g⁻¹ dry weight in food) displayed an increased swimming activity at the end of the night. In offspring, larval activity was increased and two-month-old fish occupied the bottom section of the tank less often. These findings call for more long-term experiments using the zebrafish model; the mechanisms underlying behavioral disruptions need to be understood due to their implications for both human health and their ecological relevance in terms of individual fitness and survival.

Using placenta to evaluate the polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) exposure of fetus in a region with high prevalence of neural tube defects

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants suspected to have various toxic effects, including reproductive toxicity. The aim of this study was to determine the concentrations of PCBs and PBDEs in human placentas and to examine the potential association between in utero exposure to these pollutants and the risk of neural tube defects. Subjects were recruited from a birth defects surveillance program in a rural area of Shanxi Province, China, from 2005 to 2007. 80 placental samples from fetuses/neonates with neural tube defects and 50 samples from healthy newborn infants were analyzed for PCBs and PBDEs using electron-capture negative-ionization gas chromatographic mass spectrometry. The median concentrations were 0.89 and 0.54ng/g lipid for the eight PCB congeners and six PBDE congeners detected, respectively. The median concentration of total PCBs was slightly higher in the case samples than in the controls (0.91 vs. 0.89ng/g lipid), but the difference was not significant (P=0.46), as also found for the median concentration of total PBDEs (0.55 vs. 0.54ng/g lipid, P=0.61). For both PCBs and PBDEs, when their placental concentration was above the median of all samples, it was associated with a non-significantly higher or equal risk of neural tube defects. Low levels of PCBs and PBDEs are not likely risk factors for neural tube defects in this population.

Dioxin inhibits zebrafish epicardium and proepicardium development

Embryonic exposure to the environmental contaminant and aryl hydrocarbon receptor agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin), disrupts cardiac development and function in fish, birds, and mammals. In zebrafish, the temporal window of sensitivity to the cardiotoxic effects of TCDD coincides with epicardium formation. We hypothesized that this TCDD-induced heart failure results from disruption of epicardial development. To determine whether embryonic TCDD exposure inhibits epicardium and proepicardium (PE) development in zebrafish, we used histology and fluorescence immunocytochemistry to examine the epicardium formation in fish exposed to TCDD. TCDD exposure prevented epicardium formation. Using live imaging and in situ hybridization, we found that TCDD exposure blocked the formation of the PE cluster. In situ hybridization experiments showed that TCDD exposure also prevented the expression of the PE marker tcf21 at the site where the PE normally forms. TCDD also inhibited expansion of the epicardial layer across the developing heart: Exposure after PE formation was completed prevented further expansion of the epicardium. However, TCDD exposure did not affect epicardial cells already present. Because TCDD blocks epicardium formation, but is not directly toxic to the epicardium once complete, we propose that inhibition of epicardium formation can account for the window of sensitivity to TCDD cardiotoxicity in developing zebrafish. Epicardium development is crucial to heart development. Loss of this layer during development may account for most if not all of the TCDD-induced cardiotoxicity in zebrafish.