Generation of Tg(cyp1a:gfp) Transgenic Zebrafish for Development of a Convenient and Sensitive In Vivo Assay for Aryl Hydrocarbon Receptor Activity

Exposure to the aryl hydrocarbon receptor agonist dioxin disrupts formation of the muscle, nerves, and vasculature in the developing jaw

Human exposure to environmental pollutants can disrupt embryonic development and impact juvenile and adult health outcomes by adversely affecting cell and organ function. Notwithstanding, environmental contamination continues to increase due to industrial development, insufficient regulations, and the mobilization of pollutants as a result of extreme weather events. Dioxins are a class of structurally related persistent organic pollutants that are highly toxic, carcinogenic, and teratogenic. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent dioxin compound and has been shown to induce toxic effects in developing organisms by activating the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor targeted by multiple persistent organic pollutants. Contaminant-induced AHR activation results in malformations of the craniofacial cartilages and neurocranium; however, the mechanisms mediating these phenotypes are not well understood. In this study, we utilized the optically transparent zebrafish model to elucidate novel cellular targets and potential transcriptional targets underlying TCDD-induced craniofacial malformations. To this end, we exposed zebrafish embryos at 4 h post fertilization to TCDD and employed a mixed-methods approach utilizing immunohistochemistry staining, transgenic reporter lines, fixed and in vivo confocal imaging, and timelapse microscopy to determine the targets mediating TCDD-induced craniofacial phenotypes. Our data indicate that embryonic TCDD exposure reduced jaw and pharyngeal arch Sox10+ chondrocytes and Tcf21+ pharyngeal mesoderm progenitors. Exposure to TCDD correspondingly led to a reduction in collagen type II deposition in Sox10+ domains. Embryonic TCDD exposure impaired development of tissues derived from or guided by Tcf21+ progenitors, namely: nerves, muscle, and vasculature. Specifically, TCDD exposure disrupted development of the hyoid and mandibular arch muscles, decreased neural innervation of the jaw, resulted in compression of cranial nerves V and VII, and led to jaw vasculature malformations. Collectively, these findings reveal novel structural targets and potential transcriptional targets of TCDD-induced toxicity, showcasing how contaminant exposures lead to congenital craniofacial malformations.

Generation and application of a novel transgenic zebrafish line Tg(GAcyp1a:eGFP/Luc) as an in vivo assay to sensitive and specific monitoring of DLCs in the environment

CYP1A is the most commonly used biomarker and transgenic fish which carrying a cyp1a promoter to drive a reporter gene can be used as reliable way to monitor dioxin/dioxin-like compounds (DLCs) in the environment. Here, we cloned the cyp1a promoter of Gambusia affinis and this promoter showed stronger transcriptional activity than that of zebrafish. Then, a Tg(GAcyp1a:eGFP/Luc) transgenic zebrafish line was first constructed with the G. affinis cyp1a promoter driving eGFP expression using meganuclease I-SceI mediated transgenesis technology. The Tg(GAcyp1a:eGFP/Luc) larvae at 72 h post-fertilization (hpf) were tested by exposing to TCDD for 72 h, and induced GFP was mainly expressed in the liver with low background. The Tg(GAcyp1a:eGFP/Luc) zebrafish showed high sensitivity (limit of detection of 0.322 ng/L TCDD and 0.7 TEQ-ng/L PCDD/Fs) and specificity (insensitive to responses to PAHs and PCBs). In addition, the transgenic line showed a low detection concentration of the DLCs contaminated environmental samples (as low as 1.8 TEQ-ng/L), and the eGFP fluorescence intensity and the chemical-TEQ values were closely correlated. In conclusion, a sensitively and specifically transgenic zebrafish line was established to convenient and effective to detect DLCs in the environment.

Comparative analysis of two new zebrafish models: The cyp1a low-expression line and cyp1a knockout line under PAHs exposure

Cytochrome P450 1 (CYP1) is an important enzyme family involved in the metabolism of pollutants, and used as a biomarker to monitor environmental pollution. In this study, a fluorescence-labeled cyp1a zebrafish line, named as KI (cyp1a^+/+-T2A-mCherry) (KICM), was originally constructed to monitor dioxin-like compounds in the environment. However, the cyp1a gene expression in the KICM line was inhibited by the fluorescence labeling, thus leading to a significantly increased sensitivity of KICM zebrafish line to PAHs. Then, a cyp1a knockout zebrafish line, named KOC, were constructed for comparative analysis with the cyp1a low-expression line. Interestingly, knockout of the cyp1a gene did not increase the sensitivity of zebrafish to PAHs as significantly as the cyp1a low-expression line. So, the expression levels of related genes in the aryl hydrocarbon receptor pathway were analyzed and the results showed that the expression level of cyp1b in KOC group was significantly higher than that of wild type and KICM under the same PAH exposure. This indicated that the effect of losing cyp1a was compensated by inducing expression of cyp1b. In conclusion, two new zebrafish models including cyp1a low-expression line and cyp1a knockout line were constructed in this study, which may provide a convenient model for subsequent studies on the toxicity mechanism of PAHs and the role of cyp1a in detoxification.

Exposure to the persistent organic pollutant 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) disrupts development of the zebrafish inner ear

Dioxins are a class of highly toxic and persistent environmental pollutants that have been shown through epidemiological and laboratory-based studies to act as developmental teratogens. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent dioxin congener, has a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. TCDD-induced AHR activation during development impairs nervous system, cardiac, and craniofacial development. Despite the robust phenotypes previously reported, the characterization of developmental malformations and our understanding of the molecular targets mediating TCDD-induced developmental toxicity remains limited. In zebrafish, TCDD-induced craniofacial malformations are produced, in part, by the downregulation of SRY-box transcription factor 9b ( sox9b ), a member of the SoxE gene family. sox9b , along with fellow SoxE gene family members sox9a and sox10 , have important functions in the development of the otic placode, the otic vesicle, and, ultimately, the inner ear. Given that sox9b in a known target of TCDD and that transcriptional interactions exist among SoxE genes, we asked whether TCDD exposure impaired the development of the zebrafish auditory system, specifically the otic vesicle, which gives rise to the sensory components of the inner ear. Using immunohistochemistry, in vivo confocal imaging, and time-lapse microscopy, we assessed the impact of TCDD exposure on zebrafish otic vesicle development. We found exposure resulted in structural deficits, including incomplete pillar fusion and altered pillar topography, leading to defective semicircular canal development. The observed structural deficits were accompanied by reduced collagen type II expression in the ear. Together, our findings reveal the otic vesicle as a novel target of TCDD-induced toxicity, suggest that the function of multiple SoxE genes may be affected by TCDD exposure, and provide insight into how environmental contaminants contribute to congenital malformations.

Highlights

The zebrafish ear is necessary to detect changes in motion, sound, and gravity.Embryos exposed to TCDD lack structural components of the developing ear.TCDD exposure impairs formation of the fusion plate and alters pillar topography.The semicircular canals of the ear are required to detect changes in movement.Following TCDD exposure embryos fail to establish semicircular canals.

West African e-waste-soil assessed with a battery of cell-based bioassays

Soil samples randomly taken from major e-waste sites in West Africa (Nigeria, Benin and Ghana) were examined for an extensive range of organic contaminants. Cytotoxicity measurements and assessment of activation of xeno-sensing receptors from fish (Atlantic cod) were employed as a battery of in vitro biological assays to explore the quality and toxicity profile of West African e-waste soil. The concentrations of the measured contaminants of emerging concerns (CECs) and persistent organic pollutants (POPs) in the e-waste soil differs significantly from the reference soil with chemical profiles typically dominated by legacy polybrominated diphenyl ethers (PBDEs) (405.8 μgkg^-1) and emerging organophosphate ester flame retardant tris (1-chloro-2-propyl) phosphate (TCPP) (404 μgkg^-1), in addition to the short chain perfluorobutane sulfonate (PFBS) (275.3 μgkg^-1) and perfluorobutanoate (PFBA) (16 μgkg^-1). The study revealed that perfluorooctanoic acid (PFOA) occurred only in e-waste soil from Ghana and ranged from 2.6 to 5.0 μgkg^-1. Overall, non-polar e-waste soil-derived extracts had a stronger effect on COS-7 cell viability than the polar extracts and elutriates. The highest receptor activation was observed with single polar and non-polar extracts from the Nigeria and Benin sites, indicating hotspots with Er-, PPARa- and Ahr-agonist activities. Thus, the results obtained with our battery of in vitro biological assays underscored these e-waste sites as remarkably polluted spots with complex toxicity profiles of great concern for human and environmental health.

Transgenic zebrafish (Danio rerio) as an emerging model system in ecotoxicology and toxicology: Historical review, recent advances, and trends

Zebrafish (Danio rerio) is an alternative model system for drug screening, developing new products, and assessing ecotoxic effects of pollutants and biomonitor species in environmental risk assessment. However, the history and current use of transgenic zebrafish lines in ecotoxicology and toxicology studies remain poorly explored. Thus, the present study aimed to summarize and discuss the existing data in the literature about the applications of transgenic zebrafish lines in ecotoxicology and toxicology. The articles were analyzed according to publication year, journal, geographic distribution, and collaborations. Also, the bioassays were evaluated according to the tested chemical, transgenic lines, development stage, biomarkers, and exposure conditions (i.e., concentration, time, type, and route of exposure). Revised data showed that constitutive transgenic lines are the main type of transgenic used in the studies, besides most of uses embryos and larvae under static conditions. Tg(fli1: EGFP) was the main transgenic line, while the GFP and EGFP were the main reporter proteins. Transgenic zebrafish stands out in assessing vasotoxicity, neurotoxicity, systemic toxicity, hepatoxicity, endocrine disruption, cardiotoxicity, immunotoxicity, hematotoxicity, ototoxicity, and pancreotoxicity. This review showed that transgenic zebrafish lines are emerging as a suitable in vivo model system for assessing the mechanism of action and toxicity of chemicals and new biotechnology products, and the effects of traditional and emerging pollutants.

Novel fluorescent and secreted transcriptional reporters for quantifying activity of the xenobiotic sensor aryl hydrocarbon receptor (AHR)

Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that plays a critical role in diverse biological processes, including xenobiotic metabolism, carcinogenesis, and physiological functions such as regulation of the immune system and cell differentiation. To improve studies of AHR activity, we constructed two new reporter genes: a fluorescent GFP-tagged histone 2B (XRE-H2B-eGFP) and a secreted nanoluciferase (XRE-pNL1.3[secNluc]). Here, we demonstrate how these reporters can be used to monitor AHR activity in different types of cells, including human primary trophoblasts and cell lines, following incubation with a strong AHR ligand, benzo[a]pyrene (B[a]P), or an AHR inhibitor (CH223191). Compared to vehicle control cells, a significant increase in AHR activity was observed in cells treated with 0.5 and/or 2 µM B[a]P and a significant decrease was detected in response to treatment with 3 µM CH223191. These new plasmids have great potential for use in a variety of applications, such as screening for endogenous or exogenous ligands of AHR.

Altered Larval Yellow Perch Swimming Behavior Due to Methylmercury and PCB126 Detected Using Hidden Markov Chain Models

Fish swimming behavior is a commonly measured response in aquatic ecotoxicology because behavior is considered a whole organism-level effect that integrates many sensory systems. Recent advancements in animal behavior models, such as hidden Markov chain models (HMM), suggest an improved analytical approach for toxicology. Using both new and traditional approaches, we examined the sublethal effects of PCB126 and methylmercury on yellow perch (YP) larvae (Perca flavescens) using three doses. Both approaches indicate larvae increase activity after exposure to either chemical. The middle methylmercury-dosed larvae showed multiple altered behavior patterns. First, larvae had a general increase in activity, typically performing more behavior states, more time swimming, and more swimming bouts per second. Second, when larvae were in a slow or medium swimming state, these larvae tended to switch between these states more often. Third, larvae swam slower during the swimming bouts. The upper PCB126-dosed larvae exhibited a higher proportion and a fast swimming state, but the total time spent swimming fast decreased. The middle PCB126-dosed larvae transitioned from fast to slow swimming states less often than the control larvae. These results indicate that developmental exposure to very low doses of these neurotoxicants alters YP larvae overall swimming behaviors, suggesting neurodevelopment alteration.

Zebrafish, an In Vivo Platform to Screen Drugs and Proteins for Biomedical Use

The nearly simultaneous convergence of human genetics and advanced molecular technologies has led to an improved understanding of human diseases. At the same time, the demand for drug screening and gene function identification has also increased, albeit time- and labor-intensive. However, bridging the gap between in vitro evidence from cell lines and in vivo evidence, the lower vertebrate zebrafish possesses many advantages over higher vertebrates, such as low maintenance, high fecundity, light-induced spawning, transparent embryos, short generation interval, rapid embryonic development, fully sequenced genome, and some phenotypes similar to human diseases. Such merits have popularized the zebrafish as a model system for biomedical and pharmaceutical studies, including drug screening. Here, we reviewed the various ways in which zebrafish serve as an in vivo platform to perform drug and protein screening in the fields of rare human diseases, social behavior and cancer studies. Since zebrafish mutations faithfully phenocopy many human disorders, many compounds identified from zebrafish screening systems have advanced to early clinical trials, such as those for Adenoid cystic carcinoma, Dravet syndrome and Diamond-Blackfan anemia. We also reviewed and described how zebrafish are used to carry out environmental pollutant detection and assessment of nanoparticle biosafety and QT prolongation.

Zebrafish as the toxicant screening model: Transgenic and omics approaches

The production of large amounts of synthetic industrial and biomedical compounds, together with environmental pollutants, poses a risk to our ecosystem and induces negative effects on the health of wildlife and human beings. With the emergence of the global problem of chemical contamination, the adverse biological effects of these chemicals are gaining attention among the scientific communities, industry, governments, and the public. Among these chemicals, endocrine disrupting chemicals (EDCs) are regarded as one of the major global issues that potentially affecting our health. There is an urgent need of understanding the potential hazards of such chemicals. Zebrafish have been widely used in the aquatic toxicology. In this review, we first discuss the strategy of transgenic lines that used in the toxicological studies, followed by summarizing the current omics approaches (transcriptomics, proteomics, metabolomics, and epigenomics) on toxicities of EDCs in this model. We will also discuss the possible transgenerational effects in zebrafish and future prospective of the integrated omics approaches with customized transgenic organism. To conclude, we summarize the current findings in the field, and provide our opinions on future environmental toxicity research in the zebrafish model.

Zebrafish (Danio rerio) using as model for genotoxicity and DNA repair assessments: Historical review, current status and trends

Genotoxic pollutants lead to both DNA damage and changes in cell repair mechanisms. Selecting suitable biomonitors is a fundamental step in genotoxicity studies. Thus, zebrafish have become a popular model used to assess the genotoxicity of different pollutants in recent years. They have orthologous genes with humans and hold almost all genes involved in different repair pathways. Therefore, the aim of the current study is to summarize the existing literature on zebrafish using as model system to assess the genotoxicity of different pollutants. Revised data have shown that comet assay is the main technique adopted in these studies. However, it is necessary standardizing the technique applied to zebrafish in order to enable better result interpretation and comparisons. Overall, pollutants lead to single-strand breaks (SSB), double-strand breaks (DSB), adduct formation, as well as to changes in the expression of genes involved in repair mechanisms. Although analyzing repair mechanisms is essential to better understand the genotoxic effects caused by pollutants, few studies have analyzed repair capacity. The current review reinforces the need of conducting further studies on the role played by repair pathways in zebrafish subjected to DNA damage. Revised data have shown that zebrafish are a suitable model to assess pollutant-induced genotoxicity.

Development and Evaluation of Olive Flounder cyp1a1-Luciferase Assay for Effective Detection of CYP1A-Inducing Contaminants in Coastal Sediments

Flounders have been widely used as indicator species for monitoring the benthic environment of marine coastal regions owing to their habitat and feeding preferences in or on sandy sediments. Here, a single-step, sensitive, specific, and simple luciferase assay was developed, using the olive flounder cyp1a1 gene, for effective detection of CYP1A-inducing contaminants in coastal sediments. The developed cyp1a1-luciferase assay was highly sensitive to the widely used CYP1A inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), benzo[a]pyrene (B[a]P), and 3,3',4,4',5-pentachlorobiphenyl (PCB 126). In the case of TCDD, significant dose-dependent increases in luciferase activity (0.3-300 ng/L) were detected. The assay was more sensitive to PCB 126 than to B[a]P. The assay also involved the highly sensitive expression of luciferase to extracted mixtures of PCBs and polycyclic aromatic hydrocarbons (PAHs) collected from coastal sediments. PCBs were more capable of cyp1a1 induction in the assay system at small doses than PAHs in environmental samples. Using the cyp1a1-luciferase assay along with water or sediment chemistry will certainly aid in diagnosing CYP1A-inducing contaminants in coastal environments.

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

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

Functional cytochrome P450 1A enzymes are induced in mouse and human islets following pollutant exposure

AIMS/HYPOTHESIS

Exposure to environmental pollution has been consistently linked to diabetes incidence in humans, but the potential causative mechanisms remain unclear. Given the critical role of regulated insulin secretion in maintaining glucose homeostasis, environmental chemicals that reach the endocrine pancreas and cause beta cell injury are of particular concern. We propose that cytochrome P450 (CYP) enzymes, which are involved in metabolising xenobiotics, could serve as a useful biomarker for direct exposure of islets to pollutants. Moreover, functional CYP enzymes in islets could also impact beta cell physiology. The aim of this study was to determine whether CYP1A enzymes are activated in islets following direct or systemic exposure to environmental pollutants.

METHODS

Immortalised liver (HepG2) and rodent pancreatic endocrine cell lines (MIN6, βTC-6, INS1, α-TC1, α-TC3), as well as human islets, were treated in vitro with known CYP1A inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene (3-MC). In addition, mice were injected with either a single high dose of TCDD or multiple low doses of TCDD in vivo, and islets were isolated 1, 7 or 14 days later.

RESULTS

CYP1A enzymes were not activated in any of the immortalised beta or alpha cell lines tested. However, both 3-MC and TCDD potently induced CYP1A1 gene expression and modestly increased CYP1A1 enzyme activity in human islets after 48 h. The induction of CYP1A1 in human islets by TCDD was prevented by cotreatment with a cytokine mixture. After a systemic single high-dose TCDD injection, CYP1A1 enzyme activity was induced in mouse islets ~2-fold, ~40-fold and ~80-fold compared with controls after 1, 7 and 14 days, respectively, in vivo. Multiple low-dose TCDD exposure in vivo also caused significant upregulation of Cyp1a1 in mouse islets. Direct TCDD exposure to human and mouse islets in vitro resulted in suppressed glucose-induced insulin secretion. A single high-dose TCDD injection resulted in lower plasma insulin levels, as well as a pronounced increase in beta cell death.

CONCLUSIONS/INTERPRETATION

Transient exposure to TCDD results in long-term upregulation of CYP1A1 enzyme activity in islets. This provides evidence for direct exposure of islets to lipophilic pollutants in vivo and may have implications for islet physiology.

Generation of a Tg(cyp1a-12DRE:EGFP) transgenic zebrafish line as a rapid in vivo model for detecting dioxin-like compounds

Dioxin-like compounds (DLCs) are extremely stable toxic organic compounds and can cause serious health risks. To develop a convenient biomonitoring tool for the detection of DLCs in the environment, we generated a transgenic line-Tg(cyp1a-12DRE:EGFP)-with a zebrafish cyp1a promoter recombined with multiple dioxin-responsive elements (DREs) that drive EGFP expression. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced EGFP expression was observed in the head cartilage (most sensitive), gut, otic vesicle, pectoral fin bud and eye of larvae. The lowest observed effect concentration of TCDD was estimated to be approximately 1 ng/L. Compared with existing zebrafish lines, our transgenic fish displayed comparable or even higher detection sensitivity to DLCs and could serve as an improved and rapid assay in an in vivo context. The Tg(cyp1a-12DRE:EGFP) transgenic zebrafish line also had higher stability for inducing EGFP expression (nearly 100% of our zebrafish induced EGFP at approximately 1 ng/L TCDD) than other lines. In addition, Tg(cyp1a-12DRE:EGFP) zebrafish could serve as a convenient and straightforward tool to assess potential cranial malformations and related health effects.

Mechanistic toxicity of DEHP at environmentally relevant concentrations (ERCs) and ecological risk assessment in the Three Gorges Reservoir Area, China

Di-(2-ethylhexyl) phthalate (DEHP) associated in vitro/vivo toxicity at current environmentally relevant concentration (ERC) with attendant ecological risks in the Three Gorges Reservoir Area (TGRA) is still elusive. Responding to this challenge, a novel integrated study based on analytical and biological assays was designed to elucidate the underlying mechanisms for toxicity of DEHP and its ecological risks at ERC. In this study, GC-MS analysis showed that the highest environmental concentration of DEHP in the TGRA surface water was nearly double that of WHO and USEPA standards. Both distribution and ecological risk decreased from the upper to middle and lower reaches of the TGRA. In vitro toxicity was assessed by cell viability and DNA damage assays: DEHP exposure at ERCs (100-800 μg/L) caused significant reduction in cell viability and elevated DNA damage. Further, DEHP exposure above 400 μg/L resulted in enhanced migration behavior of cancer cells. For in vivo toxicity assessment, short term acute exposure (7 d, 400 μg/L) apparently activated the PI3K-AKT-mTOR pathway, and chronic low-level exposure (3 months, 10-33 μg/L) suppressed the hypothalamus pituitary thyroid (HPT) axis pathway in zebrafish. In addition, acute low-level exposure (5 d, 33-400 μg/L) to DEHP increased aryl hydrocarbon receptor (AhR) activity in Tg(cyp1a:gfp) zebrafish in a concentration-dependent manner. In short, DEHP at ERC has extended potential to induce diverse in vitro and in vivo toxicity at concentrations that also cause impairment of biochemical function in aquatic species of the TGRA.

Immune response induced by major environmental pollutants through altering neutrophils in zebrafish larvae

Environmental pollutants may cause adverse effects on the immune system of aquatic organisms. However, the cellular effects of pollutants on fish immune system are largely unknown. Here, we exploited the transgenic zebrafish Tg(lysC:DsRed2) larva as a preliminary screening system to evaluate the potential inflammatory effects of environmental pollutants. Tg(lysC:DsRED2) larvae aged 7-day-postfertilization (7 dpf) were treated with selected environmental chemicals for 24 h (24 h) and the number of neutrophils were quantified using both image analysis and fluorescence activated cell sorting (FACS). We found that the numbers of neutrophils in the Tg(lysC:DsRED2) larvae were significantly increased by most of the organic chemicals tested, including E2 (17β-estradiol), BPA (Bisphenol-A), NDEA (N-nitrosodiethylamine), 4-NP (4-Nitrophenol) and Lindane (γ-hexachlorocyclohexane). Neutrophil numbers were also increased by all the metals tested (Na2HAsO4· 7H2O, Pb(NO3)2, HgCl₂, CdCl2, CuSO₄·5H₂O, ZnSO4, and K2Cr2O7). The only exception was TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), which significantly reduced the number of neutrophils after exposure. Additionally, the transcription of genes (lyz, mpo, tnfα and il8) related to fish immune system were significantly modulated upon exposure to some of the selected chemicals such as E2, TCDD, Cu and Cd. This study revealed that representatives of major categories of environmental pollutants could cause an acute inflammatory response in zebrafish larvae as shown by alterations in the neutrophils, which may imply a common immunotoxicity mechanism for most environmental pollutants. This study has also demonstrated that Tg(lyz:DsRed2) transgenic zebrafish is an excellent tool for screening environmental chemicals with potential inflammatory effects through FACS-facilitated neutrophil counting.

Hypersensitive assessment of aryl hydrocarbon receptor transcriptional activity using a novel truncated cyp1a promoter in zebrafish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent organic pollutant (POP), an unintentional byproduct of various industrial processes, and a human carcinogen. The expression of the cytochrome P450 1A (cyp1a) gene is upregulated in the presence of TCDD through activating the aryl hydrocarbon receptor pathway in a dose-dependent manner. Several essential response elements, including the 8 potential xenobiotic response elements in the cyp1a promoter region, have been identified to be the main functional parts for the response to TCDD. Thus, we aimed to develop a convenient and sensitive biomonitoring tool to examine the level of POPs in the environment and evaluate its potential human health risks by TCDD. Here, we established a transgenic zebrafish model with a red fluorescent reporter gene ( mCherry) using the truncated cyp1a promoter. Under exposure to TCDD, the expression pattern of mCherry in the reporter zebrafish mirrored that of endogenous cyp1a mRNA, and the primary target tissues for TCDD were the brain vessels, liver, gut, cloaca, and skin. Our results indicated that exposure of the embryos to TCDD at concentrations as low as 0.005 nM for 48 h, which did not elicit morphologic abnormalities in the embryos, markedly increased mCherry expression. In addition, the reporter embryos responded to other POPs, and primary liver cell culture of zebrafish revealed that Cyp1a protein was mainly expressed in the cytoplasm of liver cells. Furthermore, our transgenic fish embryos demonstrated that TCDD exposure can regulate the expression levels of several tumor-related factors, including epidermal growth factor, TNF-α, C-myc, proliferating cell nuclear antigen, TGF-β, serine/threonine kinase (Akt), and phosphorylated Akt, suggesting that our transgenic fish can be used as a sensitive model to evaluate the carcinogenicity induced by TCDD exposure.-Luo, J.-J., Su, D.-S., Xie, S.-L., Liu, Y., Liu, P., Yang, X.-J., Pei D.-S. Hypersensitive assessment of aryl hydrocarbon receptor transcriptional activity using a novel truncated cyp1a promoter in zebrafish.

Development of a high-throughput in vivo screening platform for particulate matter exposures

Particulate matter (PM) exposure is a public health burden with poorly understood health effect mechanisms and lacking an efficient model to compare the vast diversity of PM exposures. Zebrafish (Danio rerio) are amenable to high-throughput screening (HTS), but few studies have investigated PM toxicity in zebrafish, despite the multitude of advantages. To develop standardized exposure procedures, the urban PM standard reference material (SRM) 1649b was used to systematically determine sample preparation methods, design experimental controls, determine concentration ranges and evaluation procedures. Embryos (n = 32/treatment) were dechorionated and placed into 96-well plates containing SRM1649b (0-200 μg/mL) at 6 h post fertilization (hpf). Developmental toxicity was assessed at 24 and 120 hpf by evaluating morphological changes, embryonic/larval photomotor behavior, and mortality. Differences from blank medium and particle controls were observed for all biological responses measured. Differences due to SRM1649b concentration and preparation method were also observed. Exposure to SRM1649b from DMSO extraction was associated with changes in morphology and mortality and hypoactivity in photomotor responses compared to the DMSO control for the whole particle suspension (76, 68%) and soluble fraction (59, 54%) during the embryonic and larval stages, respectively. Changes in behavioral responses were not observed following exposure to the insoluble fraction of SRM1649b from DMSO extraction. The toxicity bias from PM preparation provided further impetus to select a single HTS exposure method. Based on the biological activity results, the soluble fraction of SRM1649b from DMSO extraction was selected and shown to have concentration dependent cyp1a/GFP expression. This rapid, sensitive and consistently scalable model is a potentially cost-effective vertebrate approach to study the toxicology of PM from diverse locations, and provides a path to identifying the toxic material(s) in these samples, and discover the mechanisms of toxicity.

Generation and application of a novel transgenic zebrafish line Tg(cyp1a:mCherry) as an in vivo assay to sensitively monitor PAHs and TCDD in the environment

The polycyclic aromatic hydrocarbons (PAHs) and 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) are classified as human carcinogens, and can also cause serious health problems. To develop a convenient bio-monitoring tool for the detection of PAHs and TCDD in the environment, we generated a transgenic zebrafish line Tg(cyp1a:mCherry) with cyp1a promoter driving mCherry expression. Here, Tg(cyp1a:mCherry) embryos were treated with different concentrations of TCDD and five US EPA priority PAHs congeners. The results showed that the expressions of mCherry and endogenous cyp1a were consistent with the PAHs exposure concentrations and were largely induced by TCDD and ≥4-ring PAHs. Moreover, the sensitivity of Tg(cyp1a:mCherry) embryos was also evaluated through monitoring of the PAHs contamination in the water and soil samples. The elevated red fluorescent signals and cyp1a expression levels were observed in Tg(cyp1a:mCherry) zebrafish after exposure to water samples and soil organic extracts with higher concentrations of ≥4-ring PAHs. These results further strengthen our findings of concentration- and congener-dependent response of the newly established zebrafish. Taken together, the newly established zebrafish line will prove as a sensitive, efficient and convenient tool for monitoring PAHs and TCDD contamination in the environment.

Differential sensitivities to dioxin-like compounds PCB 126 and PeCDF between Tg(cyp1a:gfp) transgenic medaka and zebrafish larvae

It has been intensively documented that there are species-differences in the sensitivity to dioxin-like compounds (DLCs) in mammalian and avian. However, this issue is still unclear in fish. This study aimed at evaluating the differential sensitivities to DLCs in fish larvae. Here, larvae of Tg(cyp1a:gfp) medaka and Tg(cyp1a:gfp) zebrafish were tested with 2,3,7,8-Tetrachlorodibenzodioxin (TCDD), polychlorinated biphenyl 126 (PCB 126) and 2,3,4,7,8,-Pentachlorodibenzofuran (PeCDF). Comparative analyses were performed on induction of GFP fluorescence, expression of endogenous cyp1a mRNAs and EROD activity between the two species after exposure to these chemicals. We found that PCB 126 and PeCDF exposure at high concentrations induced strong GFP expression in multiple organs (liver, head kidney and gut) in both medaka and zebrafish larvae. Moreover, the expression of endogenous cyp1a mRNA was significantly elevated in the zebrafish larvae exposed to TCDD, PCB 126 and PeCDF at different concentrations. Likewise, almost all the exposure conditions could cause prominent elevation of EROD activity in the zebrafish larvae, while the EROD activities were just slightly elevated in the medaka larvae exposed to 1 nM and 0.5 nM of TCDD as well as to 1.5 nM and 15 nM of PeCDF, but not in the medaka larvae exposed to PCB 126. Taken together, zebrafish was proved to be more sensitive than medaka to PCB 126 and to PeCDF in this study. The findings suggested species-specific sensitivity to DLCs in fish and will facilitate choosing a sensitive and reliable fish model or tool to evaluate the risk of dioxins and DLCs exposure.

Diverse toxicological risks of PAHs in surface water with an impounding level of 175m in the Three Gorges Reservoir Area, China

The impounding level of 175m for the Three Gorges Reservoir (TGR) is of vital importance for efficient flood control, power generation and convenient navigation in China. However, little is known about the spatial distribution and toxicological risks of major pollutants in the Three Gorges Reservoir Area (TGRA) at that stage. The aim of this study is to probe the ubiquitous polycyclic aromatic hydrocarbons (PAHs) contamination and toxicological impacts in the surface water of the TGRA at the highest water impoundment level of 175m. Our results showed that the ƩPAHs levels ranged from 83 to 1631ng/L in the upper reaches, 354 to 1159ng/L in the middle reaches, and 23 to 747ng/L in the lower reaches of the TGRA. Source apportionment of PAHs indicated that coal combustion, industrial emissions, heavy traffic, agriculture and shipping activities were the primary sources. Compositional pattern highlighted >85% dominancy of low molecular weight (LMW) PAHs in the reservoir. Risk assessment based on risk quotients (RQs) implied moderate to high ecological risks: the upper reaches>the middle reaches>the lower reaches. However, gene expression profiles portrayed contrary scenario because of the presence of relatively higher footprints of high molecular weight (HMW) PAHs in the middle and the lower reaches, which was confirmed by Cox hazard proportional model. Moreover, the transgenic zebrafish Tg(cyp1a:gfp) induced by PAHs also expressed stronger fluorescent signals in the middle and lower reaches. Taken together, different approaches were employed to firstly reveal the real status of ecological toxicity of PAHs and explore the underlying mechanisms at the highest impounding level of 175m in the TGRA.

A novel forward osmosis system in landfill leachate treatment for removing polycyclic aromatic hydrocarbons and for direct fertigation

Landfill leachate (LL) is harmful to aquatic environment because it contains high concentrations of dissolved organic matter, inorganic components, heavy metals, and other xenobiotics. Thus, the remediation of LL is crucial for environmental conservation. Here, a potential application of the forward osmosis (FO) filtration process with ammonium bicarbonate (NH₄HCO₃) as a draw solution (DS) was investigated to remediate membrane bioreactor-treated LL (M-LL). After the leachate treatment, the toxicity and removal efficiencies of polycyclic aromatic hydrocarbons (PAHs) were evaluated using zebrafish and cultured human cells. The water recovery rate was improved using the current protocol up to 86.6% and 91.6% by both the pressure retarded osmosis (PRO) mode and the forward osmosis (FO) mode. Water flux increased with the increasing DS concentrations, but solution velocities decreased with the operation time. Toxicity tests revealed that the M-LL treated by NH₄HCO₃ had no toxic effect on zebrafish and human cells. Moreover, green fluorescent protein (GFP) expression in the transgenic zebrafish Tg(cyp1a:gfp) induced by PAHs was very weak compared to the effects induced by untreated M-LL. Since the diluted DS met local safety requirements of liquid fertilizer, it could be directly applied as the liquid fertilizer for fertigation. In conclusion, this novel FO system using NH₄HCO₃ as the DS provides a cheap and efficient protocol to effectively remove PAHs and other pollutants in LL, and the diluted DS can be directly applied to crops as a liquid fertilizer, indicating that this technique is effective and eco-friendly for the treatment of different types of LL.

Zebrafish in Toxicology and Environmental Health

As manufacturing processes and development of new synthetic compounds increase to keep pace with the expanding global demand, environmental health, and the effects of toxicant exposure are emerging as critical public health concerns. Additionally, chemicals that naturally occur in the environment, such as metals, have profound effects on human and animal health. Many of these compounds are in the news: lead, arsenic, and endocrine disruptors such as bisphenol A have all been widely publicized as causing disease or damage to humans and wildlife in recent years. Despite the widespread appreciation that environmental toxins can be harmful, there is limited understanding of how many toxins cause disease. Zebrafish are at the forefront of toxicology research; this system has been widely used as a tool to detect toxins in water samples and to investigate the mechanisms of action of environmental toxins and their related diseases. The benefits of zebrafish for studying vertebrate development are equally useful for studying teratogens. Here, we review how zebrafish are being used both to detect the presence of some toxins as well as to identify how environmental exposures affect human health and disease. We focus on areas where zebrafish have been most effectively used in ecotoxicology and in environmental health, including investigation of exposures to endocrine disruptors, industrial waste byproducts, and arsenic.

Optimizing multi-dimensional high throughput screening using zebrafish

The use of zebrafish for high throughput screening (HTS) for chemical bioactivity assessments is becoming routine in the fields of drug discovery and toxicology. Here we report current recommendations from our experiences in zebrafish HTS. We compared the effects of different high throughput chemical delivery methods on nominal water concentration, chemical sorption to multi-well polystyrene plates, transcription responses, and resulting whole animal responses. We demonstrate that digital dispensing consistently yields higher data quality and reproducibility compared to standard plastic tip-based liquid handling. Additionally, we illustrate the challenges in using this sensitive model for chemical assessment when test chemicals have trace impurities. Adaptation of these better practices for zebrafish HTS should increase reproducibility across laboratories.

Zebrafish and Medaka: new model organisms for modern biomedical research

Although they are primitive vertebrates, zebrafish (Danio rerio) and medaka (Oryzias latipes) have surpassed other animals as the most used model organisms based on their many advantages. Studies on gene expression patterns, regulatory cis-elements identification, and gene functions can be facilitated by using zebrafish embryos via a number of techniques, including transgenesis, in vivo transient assay, overexpression by injection of mRNAs, knockdown by injection of morpholino oligonucleotides, knockout and gene editing by CRISPR/Cas9 system and mutagenesis. In addition, transgenic lines of model fish harboring a tissue-specific reporter have become a powerful tool for the study of biological sciences, since it is possible to visualize the dynamic expression of a specific gene in the transparent embryos. In particular, some transgenic fish lines and mutants display defective phenotypes similar to those of human diseases. Therefore, a wide variety of fish model not only sheds light on the molecular mechanisms underlying disease pathogenesis in vivo but also provides a living platform for high-throughput screening of drug candidates. Interestingly, transgenic model fish lines can also be applied as biosensors to detect environmental pollutants, and even as pet fish to display beautiful fluorescent colors. Therefore, transgenic model fish possess a broad spectrum of applications in modern biomedical research, as exampled in the following review.