The role of cyp1a and heme oxygenase 1 gene expression for the toxicity of 3,4-dichloroaniline in zebrafish (Danio rerio) embryos

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.

Toxicity and ecological risk assessment for two AhR agonistic pesticides mepanipyrim and cyprodinil and their metabolites

Mepanipyrim and cyprodinil are widely used to control and/or prevent fungal diseases in fruit culture. They are frequently detected in the aquatic environment and some food commodities. Different from TCDD, mepanipyrim and cyprodinil are more easily metabolised in the environments. However, the risk of their metabolites to the ecological environment is unclear and needs to be further confirmed. In this study, we investigated the temporal pattern of mepanipyrim- and cyprodinil-induced CYP1A and AhR2 expression and EROD enzyme activity at different time frames during zebrafish embryonic and larval development. Then, we assessed the ecological risk of mepanipyrim, cyprodinil, and their metabolites to aquatic organisms. Our results showed that mepanipyrim and cyprodinil exposure could increase the expression level of cyp1a and ahr2 genes and EROD activity by a dynamic pattern in different developmental stages of zebrafish. Besides, their several metabolites showed strong AhR agonistic activity. Importantly, these metabolites could cause potential ecological risks to aquatic organisms and should be paid more attention to. Our results would provide an important reference value for environmental pollution control and the use management of mepanipyrim and cyprodinil.

Cylindrospermopsin impairs zebrafish (Danio rerio) embryo development

Cyanotoxins are among common contaminants that can impair human, animal, and environmental health. Cylindrospermopsin (CYN) is an abundant form of cyanotoxins elevated following algal bloom in the water worldwide. Previous studies have described CYN effects on several organs in mammals. However, little is known about its toxicity mechanisms in other vertebrates. This study aims to characterize the developmental effects of CYN using zebrafish larvae as an aquatic model organism. A wide range of CYN concentrations (0-2000 μg/L) was tested using a morphometric approach for survival, hatching, various growth and developmental abnormalities. We also investigated the expression of genes related to oxidative stress, osmoregulation, and thyroid function. Exposure to CYN resulted in decreased growth, increased developmental abnormalities such as pericardial and yolk sac edema as well as swim bladder absence. In addition, CYN increased tr1a, and decreased dio1 and dio3 transcript levels which are involved in thyroid-mediated function. It also increased transcript levels related to oxidative stress, including hsp70, ahr1a, cyp1a, gpx and cat. Lastly, CYN exposure increased aqp3a and decreased dab2, which are involved in osmoregulation with a threshold of 10 μg/L. The present study demonstrates multiple effects of exposure to environmentally relevant CYN concentrations in zebrafish embryos.

Physiochemical characterization and toxicity assessment of colloidal mercuric formulation-'Sivanar amirtham'

The study aims to characterize and understand the toxicological effects of colloidal mercuric formulation. The physiochemical characterization was carried out using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray microanalysis system (EDS), Inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), Zeta potential, Brunauer-Emmett-Teller (BET) and electron microscopy. Based on the physiochemical characterizations, the pairwise relationship between the parameters such as size, surface area, surface charge, reactivity and band gap energy were described. The biological effects of the sample were studied by both in vitro and in vivo assays. The in vitro cytotoxicity assay confirmed that the colloidal mercuric formulation was effective against cancer cells (MCF-7) and less toxic to normal cells (Hek 293). The formulation was effective against MCF-7 with more than 85% of apoptotic and necrotic cells, positive for PI staining when treated with 100 μg/mL. The inflammatory response on the macrophage cell lines was studied. The colloidal mercuric formulation upregulated the expression of TGF-β, IL-6 and TNF-α, due to the presence of arsenic and other organic compounds such as piperine. The in vivo developmental toxicity was observed in Zebrafish hampered growth and survival in a dose and time dependent manner. The formulation was safe at lower concentration and exhibit a dose and time dependent toxicity. Based on the results obtained, it is confirmed that the selective toxicity towards MCF-7 cells is promising to develop an effective formulation for the treatment of cancer, provided more such proofs obtained from in vivo experiments.

Embryonic toxicity of 3,4-dichloroaniline (3,4-DCA) on Javanese medaka (Oryzias javanicus Bleeker, 1854)

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The 96 h LC₅₀ of 3,4-dichloroaniline in Javanese medaka embryo is 32.87 mg.L⁻¹.

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3,4-DCA lowers heart rate of developing Javanese medaka embryos.

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The sublethal concentration of 3,4-DCA delays hatching in Javanese medaka embryo.

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The LOEC for deformities in embryos of Javanese medaka was 0.5 mg.L⁻¹.

Early-life exposure to toxic chemicals causes irreversible morphological and physiological abnormalities that may last for a lifetime. The present study aimed to determine the toxicity effect of 3,4-Dichloroaniline (3,4-DCA) on Javanese medaka (Oryzias javanicus) embryos. Healthy embryos were exposed to various 3,4-DCA concentrations for acute toxicity (5, 10, 25, 50, and 100 mg.L⁻¹) and sublethal toxicity (0.10, 0.50, 1.25, 2.50, and 5.00 mg.L⁻¹) for 96 h and 20 days respectively. Acute toxicity test revealed that the median lethal concentration (96h-LC₅₀) was 32.87 mg.L⁻¹ (95 % CI = 27.90–38.74, R² = 0.95). Sublethal exposure revealed that 1.25 mg.L^-1 at 3 days post-exposure (3 dpe) has a significant lower heartrate (120 ± 12.3 beats/min., p < 0.01), while at 7 dpe those exposed to 5 mg.L⁻¹ (141.8 ± 8.3 beats/min) had significantly (p < 0.01) lower heart rate compared to other treatments. Likewise, at 13 dpe, 5.00 mg.L⁻¹ (110.4 ± 17.3 beats/min) and 2.5 mg.L^-1 (130.4 ± 8.3 beats/min) were significantly lower (p < 0.001) compared to control. None of the embryos in 5.00 mg.L⁻¹ and 2.50 mg.L^-1 treatment groups survived at the end of the experiment. The results indicated a concentration-dependent response. The lowest observed effect concentration (LOEC) that exerted developmental deformities was 0.5 mg.L⁻¹. Javanese medaka embryo have low sensitivity to acute toxicity of 3,4-DCA, but developmental abnormalities at sublethal concentrations were observed.

Pxr- and Nrf2- mediated induction of ABC transporters by heavy metal ions in zebrafish embryos

Transcription factors including pregnane X receptor (Pxr) and nuclear factor-erythroid 2-related factor-2 (Nrf2) are important modulators of Adenosine triphosphate-binding cassette (ABC) transporters in mammalian cells. However, whether such modulation is conserved in zebrafish embryos remains largely unknown. In this manuscript, pxr- and nrf2-deficient models were constructed with CRISPR/Cas9 system, to evaluate the individual function of Pxr and Nrf2 in the regulation of ABC transporters and detoxification of heavy metal ions like Cd²⁺ and Ag⁺. As a result, both Cd²⁺ and Ag⁺ conferred extensive interactions with ABC transporters in wild type (WT) embryos: their accumulation and toxicity were affected by the activity of ABC transporters, and they significantly induced the mRNA expressions of ABC transporters. These induction effects were reduced by the mutation of pxr and nrf2, but elevations in the basal expression of ABC transporters compensated for the loss of their inducibility. This could be an explanation for remaining transporter function in both mutant models as well as the unaltered toxicity of metal ions in pxr-deficient embryos. However, mutation of nrf2 disrupted the production of glutathione (GSH), resulting in the enhanced toxicity of Cd²⁺/Ag⁺ in zebrafish embryos. In addition, elevated expressions of other transcription factors like aryl hydrocarbon receptor (ahr) 1b, peroxisome proliferator-activated receptor (ppar)-β, and nrf2 were found in pxr-deficient models without any treatment, while enhanced induction of ahr1b, ppar-β and pxr could only be seen in nrf2-deficient embryos after the treatment of metal ions, indicating different compensation phenomena for the absence of transcription factors. After all, pxr-deficient and nrf2-deficient zebrafish embryos are useful tools in the functional investigation of Pxr and Nrf2 in the early life stages of aquatic organisms. However, the compensatory mechanisms should be taken into consideration when interpreting the results and need in-depth investigations.

Development of cardiovascular and neurodevelopmental metrics as sublethal endpoints for the Fish embryo toxicity test

The fathead minnow fish embryo toxicity (FET) test has been proposed as a more humane alternative to current toxicity testing methods as younger organisms are thought to experience less distress during toxicant exposure. However, the FET test protocol does not include endpoints that allow for the prediction of sublethal adverse outcomes, limiting its utility relative to other test types. Researchers have proposed the development of sublethal endpoints for the FET test to increase its utility. The present study 1) developed methods for previously unmeasured sublethal metrics in fathead minnows (i.e., spontaneous contraction frequency and heart rate) and 2) investigated the responsiveness of several sublethal endpoints related to growth (wet wt, length, and growth-related gene expression), neurodevelopment (spontaneous contraction frequency, eye size, and neurodevelopmental gene expression), and cardiovascular function and development (pericardial area, heart rate, and cardiovascular system-related gene expression) as additional FET test metrics using the model toxicant 3,4-dichloroaniline. Of the growth, neurological, and cardiovascular endpoints measured, length, eye size, and pericardial area were found to be more responsive than the other endpoints evaluated. Future studies linking alterations in these endpoints to longer-term adverse impacts are needed to fully evaluate the predictive power of these metrics in chemical and whole-effluent toxicity testing. Environ Toxicol Chem 2018;37:2530-2541. © 2018 SETAC.

Liver Transcriptome and miRNA Analysis of Silver Carp (Hypophthalmichthys molitrix) Intraperitoneally Injected With Microcystin-LR

Next-generation sequencing was used to analyze the effects of toxic microcystin-LR (MC-LR) on silver carp (Hypophthalmichthys molitrix). Silver carps were intraperitoneally injected with MC-LR, and RNA-seq and miRNA-seq in the liver were analyzed at 0.25, 0.5, and 1 h. The expression of glutathione S-transferase (GST), which acts as a marker gene for MC-LR, was tested to determine the earliest time point at which GST transcription was initiated in the liver tissues of the MC-LR-treated silver carps. Hepatic RNA-seq/miRNA-seq analysis and data integration analysis were conducted with reference to the identified time point. Quantitative PCR (qPCR) was performed to detect the expression of the following genes at the three time points: heme oxygenase 1 (HO-1), interleukin-10 receptor 1 (IL-10R1), apolipoprotein A-I (apoA-I), and heme binding protein 2 (HBP2). Results showed that the liver GST expression was remarkably decreased at 0.25 h (P < 0.05). RNA-seq at this time point revealed that the liver tissue contained 97,505 unigenes, including 184 significantly different unigenes and 75 unknown genes. Gene Ontology (GO) term enrichment analysis suggested that 35 of the 145 enriched GO terms were significantly enriched and mainly related to the immune system regulation network. KEGG pathway enrichment analysis showed that 18 of the 189 pathways were significantly enriched, and the most significant was a ribosome pathway containing 77 differentially expressed genes. miRNA-seq analysis indicated that the longest miRNA had 22 nucleotides (nt), followed by 21 and 23 nt. A total of 286 known miRNAs, 332 known miRNA precursor sequences, and 438 new miRNAs were predicted. A total of 1,048,575 mRNA–miRNA interaction sites were obtained, and 21,252 and 21,241 target genes were respectively predicted in known and new miRNAs. qPCR revealed that HO-1, IL-10R1, apoA-I, and HBP2 were significantly differentially expressed and might play important roles in the toxicity and liver detoxification of MC-LR in fish. These results were consistent with those of high-throughput sequencing, thereby verifying the accuracy of our sequencing data. RNA-seq and miRNA-seq analyses of silver carp liver injected with MC-LR provided valuable and new insights into the toxic effects of MC-LR and the antitoxic mechanisms of MC-LR in fish.

The RNA/miRNA data are available from the NCBI database Registration No. : SRP075165.

The use of mrp1-deficient (Danio rerio) zebrafish embryos to investigate the role of Mrp1 in the toxicity of cadmium chloride and benzo[a]pyrene

Previous studies in our lab have revealed that both P-glycoprotein (Pgp) and multi-resistance associated protein (Mrp) 1 played important roles in the detoxification of heavy metals and polycyclic aromatic hydrocarbon (PAH) in zebrafish embryos. This paper aims to extend this research by using mrp1-deficient model to illustrate the individual function of Mrp1. In this respect, CRISPR/Cas9 system was employed to generate a frameshift mutation in zebrafish mrp1 causing premature translational stops in Mrp1. Significant reduction on the efflux function of Mrps was found in mutant zebrafish embryos, which correlated well with the significantly enhanced accumulation and toxicity of cadmium chloride (CdCl₂) and benzo[a]pyrene (BαP), indicating the protective role of the corresponding protein. The different alteration on the accumulation and toxicity of Cd²⁺ and BαP could be attributed to the fact that Cd²⁺ and its metabolites were mainly excreted by Mrp1, while BαP was primarily pumped out by Pgp. More importantly, the compensation mechanism for the absence of Mrp1, including elevated glutathione (GSH) level and up-regulated expression of pgp and mrp2 were also found. Thus, mrp1-deficient zebrafish embryo could be a useful tool in the investigation of Mrp1 functions in the early life stages of aquatic organisms. However, compensation mechanism should be taken into consideration in the interpretation of results obtained with mrp1-deficient fish.

Developmental toxicity from exposure to various forms of mercury compounds in medaka fish (Oryzias latipes) embryos

This study examined developmental toxicity of different mercury compounds, including some used in traditional medicines. Medaka (Oryzias latipes) embryos were exposed to 0.001–10 µM concentrations of MeHg, HgCl2, α-HgS (Zhu Sha), and β-HgS (Zuotai) from stage 10 (6–7 hpf) to 10 days post fertilization (dpf). Of the forms of mercury in this study, the organic form (MeHg) proved the most toxic followed by inorganic mercury (HgCl₂), both producing embryo developmental toxicity. Altered phenotypes included pericardial edema with elongated or tube heart, reduction of eye pigmentation, and failure of swim bladder inflation. Both α-HgS and β-HgS were less toxic than MeHg and HgCl₂. Total RNA was extracted from survivors three days after exposure to MeHg (0.1 µM), HgCl₂ (1 µM), α-HgS (10 µM), or β-HgS (10 µM) to examine toxicity-related gene expression. MeHg and HgCl₂ markedly induced metallothionein (MT) and heme oxygenase-1 (Ho-1), while α-HgS and β-HgS failed to induce either gene. Chemical forms of mercury compounds proved to be a major determinant in their developmental toxicity.

Evidence for a role of heme oxygenase-1 in the control of cardiac function in zebrafish (Danio rerio) larvae exposed to hypoxia

Carbon monoxide (CO) is a gaseous neurotransmitter produced from the breakdown of heme via heme oxygenase-1 (HO-1; hypoxia-inducible isoform) and heme oxygenase-2 (HO-2; constitutively expressed isoform). In mammals, CO is involved in modulating cardiac function. The role of the HO-1/CO system in the control of heart function in fish, however, is unknown and investigating its physiological function in lower vertebrates will provide a better understanding of the evolution of this regulatory mechanism. We explored the role of the HO-1/CO system in larval zebrafish (Danio rerio) in vivo by investigating the impact of translational gene knockdown of HO-1 on cardiac function. Immunohistochemistry revealed the presence of HO-1 in the pacemaker cells of the heart at 4 days post-fertilization and thus the potential for CO production at these sites. Sham-treated zebrafish larvae (experiencing normal levels of HO-1) significantly increased heart rate (fH) when exposed to hypoxia (PwO2 =30 mmHg). Zebrafish larvae lacking HO-1 expression after morpholino knockdown (morphants) exhibited significantly higher fH under normoxic (but not hypoxic) conditions when compared with sham-treaded fish. The increased fH in HO-1 morphants was rescued (fH was restored to control levels) after treatment of larvae with a CO-releasing molecule (40 µmol l(-1) CORM). The HO-1-deficient larvae developed significantly larger ventricles and when exposed to hypoxia they displayed higher cardiac output ([Formula: see text]) and stroke volume (SV). These results suggest that under hypoxic conditions, HO-1 regulates [Formula: see text] and SV presumably via the production of CO. Overall, this study provides a better understanding of the role of the HO-1/CO system in controlling heart function in lower vertebrates. We demonstrate for the first time the ability for CO to be produced in presumptive pacemaker cells of the heart where it plays an inhibitory role in setting the resting cardiac frequency.

Spatio-temporal development of CYP1 activity in early life-stages of zebrafish (Danio rerio)

Endpoints of planar halogenated aromatic hydrocarbon (pHAH) and polycyclic aromatic hydrocarbon (PAH) toxicity are mediated via activation of the aryl hydrocarbon receptor (AhR) followed by activation of the so called "AhR-battery" of genes including the cytochrome P450 1 (CYP1) isoforms. The aim of this study was to develop a method to identify CYP1 activity in early life-stages of zebrafish (Danio rerio) in order to elucidate the spatio-temporal pattern of basal and induced CYP1 activities. Preliminary experiments with the fish embryo toxicity test (FET) were carried out to determine toxic effect thresholds of the AhR agonist β-naphthoflavone. To assess basal and β-naphthoflavone-induced CYP1 activity during early life-stages of zebrafish, the commonly used 7-ethoxyresorufin-O-deethylase (EROD) assay was developed further for use in confocal laser scanning microscopy (CLSM) and spectrometry. Following exposure to selected cytochrome P450 inducers, zebrafish embryos were dechorionated, anaesthetized and inspected in vivo under the CLSM. Alternatively, embryos were homogenized, and EROD activity was measured using classical spectrometry in vitro. CLSM of CYP-induced fluorescence allowed for the in vivo detection of CYP1 enzyme activity down to the cellular level as early as in the gastrulation stage. Basal and induced CYP1 activity was detected at all time points examined from 8h post-fertilization to early adulthood and showed a highly dynamic spatio-temporal pattern throughout zebrafish development. Basal and induced EROD activity was prominent in tissues of the cardiovascular system, the urinary tract, the digestive system, and parts of the brain as well as in the central portion of the eye and the otic vesicle during distinct stages of development. The differentiation between constitutive and induced spatio-temporal patterns of CYP1 activity even as early as the gastrula stage provide further insights into the endogenous role of CYP1 activity.

The role of Nrf2 and MAPK pathways in PFOS-induced oxidative stress in zebrafish embryos

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant and causes oxidative stress, apoptosis, and developmental toxicity in zebrafish embryos. In the present study, we examined nuclear factor erythroid 2-related factor 2 (Nrf2)- and mitogen-activated protein kinases (MAPKs)-mediated oxidative stress pathways in zebrafish embryos upon exposure to PFOS. Four-hour postfertilization (hpf) zebrafish embryos were exposed to 0.2, 0.4, and 1.0 mg/l PFOS until 96 hpf. PFOS enhanced production of reactive oxygen species (ROS) in a concentration-dependent manner. Activity of antioxidative enzymes, including superoxide dismutase, catalase, and glutathione peroxidase, was significantly induced in zebrafish larvae in all PFOS-treated groups relative to the control. Exposure to 1.0 mg/l PFOS significantly increased malondialdehyde production in zebrafish larvae. The Nrf2 and heme oxygenase-1 (HO-1) gene expressions were both significantly upregulated compared with the control group. For MAPKs, we investigated gene expression profiles of extracellular signal-regulated protein kinase (ERK), c-Jun NH (2)-terminal kinase (JNK), and p38. The ERK gene expression levels were unchanged, whereas JNK and p38 gene expressions were significantly upregulated, which could be linked to PFOS-induced cell apoptosis in zebrafish larvae. In addition, we found that coexposure with sulforaphane, an Nrf2 activator, could significantly protect against PFOS-induced ROS generation, whereas inhibition of MAPKs did not exhibit significant effects on PFOS-induced HO-1 gene expression and ROS production. Furthermore, we showed that morpholino-mediated knockdown of Nrf2 reduced PFOS-induced HO-1 gene expression. These findings demonstrate that Nrf2 is protective against PFOS-induced oxidative stress in zebrafish larvae.

An acetyltransferase conferring tolerance to toxic aromatic amine chemicals: molecular and functional studies

Aromatic amines (AA) are a major class of environmental pollutants that have been shown to have genotoxic and cytotoxic potentials toward most living organisms. Fungi are able to tolerate a diverse range of chemical compounds including certain AA and have long been used as models to understand general biological processes. Deciphering the mechanisms underlying this tolerance may improve our understanding of the adaptation of organisms to stressful environments and pave the way for novel pharmaceutical and/or biotechnological applications. We have identified and characterized two arylamine N-acetyltransferase (NAT) enzymes (PaNAT1 and PaNAT2) from the model fungus Podospora anserina that acetylate a wide range of AA. Targeted gene disruption experiments revealed that PaNAT2 was required for the growth and survival of the fungus in the presence of toxic AA. Functional studies using the knock-out strains and chemically acetylated AA indicated that tolerance of P. anserina to toxic AA was due to the N-acetylation of these chemicals by PaNAT2. Moreover, we provide proof-of-concept remediation experiments where P. anserina, through its PaNAT2 enzyme, is able to detoxify the highly toxic pesticide residue 3,4-dichloroaniline in experimentally contaminated soil samples. Overall, our data show that a single xenobiotic-metabolizing enzyme can mediate tolerance to a major class of pollutants in a eukaryotic species. These findings expand the understanding of the role of xenobiotic-metabolizing enzyme and in particular of NATs in the adaptation of organisms to their chemical environment and provide a basis for new systems for the bioremediation of contaminated soils.

The zebrafish embryo model in environmental risk assessment--applications beyond acute toxicity testing

BACKGROUND, AIM, AND SCOPE

The use of fish embryos is not regulated by current legislations on animal welfare and is therefore considered as a refinement, if not replacement of animal experiments. Fish embryos represent an attractive model for environmental risk assessment of chemicals since they offer the possibility to perform small-scale, high-throughput analyses.

MAIN FEATURES

Beyond their application for determining the acute toxicity, fish embryos are also excellent models for studies aimed at the understanding of toxic mechanisms and the indication of possible adverse and long-term effects. Therefore, we have reviewed the scientific literature in order to indicate alternative applications of the fish embryo model with focus on embryos of the zebrafish.

RESULTS AND DISCUSSIONS

The analysis of the mode of action is important for the risk assessment of environmental chemicals and can assist in indicating adverse and long-term effects. Toxicogenomics present a promising approach to unravel the potential mechanisms. Therefore, we present examples of the use of zebrafish embryos to study the effect of chemicals on gene and protein patterns, and the potential implications of differential expression for toxicity. The possible application of other methods, such as kinase arrays or metabolomic profiling, is also highlighted. Furthermore, we show examples of toxicokinetic studies (bioconcentration, ABC transporters) and discuss limitations that might be caused by the potential barrier function of the chorion. Finally, we demonstrate that biomarkers of endocrine disruption, immune modulation, genotoxicity or chronic toxicity could be used as indicators or predictors of sub-acute and long-term effects.

CONCLUSIONS

The zebrafish embryo represents a model with an impressive range of possible applications in environmental sciences. Particularly, the adaptation of molecular, system-wide approaches from biomedical research is likely to extend its use in ecotoxicology.

RECOMMENDATIONS AND PERSPECTIVES

Challenges for future research are (1) the identification of further suitable molecular markers as indicators of the mode of action, (2) the establishment of strong links between (molecular) effects in short-term assays in embryos and long-term (toxic) effects on individuals, (3) the definition of limitations of the model and (4) the development of tests that can be used for regulatory purposes.