Relative embryotoxicity of two classes of chemicals in a modified zebrafish embryotoxicity test and comparison with their in vivo potencies

Determination of the zebrafish embryo developmental toxicity assessment (ZEDTA) as an alternative non-mammalian approach for the safety assessment of agrochemicals

With the US Environment Protection Agency reducing requests for (and funding of) mammalian studies alongside the proposed elimination of requests by 2035, there is an urgent need for fully validated New Approach Methods (NAMs) to fill the resultant gap for safety assessment of agrochemicals. One promising NAM for assessing the potential for human prenatal developmental toxicity potential is the Zebrafish Embryo Developmental Toxicity Assessment, a bioassay that has been used by the pharmaceutical industry for more than a decade in early-stage drug safety assessment. Despite its promise, little data has been generated to assess the validity of ZEDTA for assessing Developmental and Reproductive Toxicity of new agrochemical products. Addressing this knowledge gap, we tested 67 compounds (insecticides, herbicides and fungicides) spanning multiple different chemical groupings and mechanisms of action. ZEDTA assay results were compared with the European Chemicals Agency (ECHA) Classification and Labelling (C&L) for mammalian hazard classification and with publicly available data to determine the ZEDTA's translation power. Overall, the ZEDTA assay had an effective detection capability of 65 % for sensitivity and 64 % for specificity as compared with the ECHA-C&L classification and publicly available data. Comparing the ZEDTA data there were both strengths and weaknesses in alignments for across the different chemical classes and chemical mechanisms of action. Overall, the data generated, show the performance of the ZEDTA assay was comparable with other bioassays highlighted as alternatives for mammalian assessment and holds good promise as a NAM for screening agrochemical prenatal developmental toxicity during new product human safety assessment.

Effects of effluents from the Villa Victoria Reservoir (Mexico) on the development of Danio rerio at early life stages through apoptotic response and oxidative-induced state

As one of Mexico's most crucial water storage facilities, the Villa Victoria Reservoir (VVR) supplies water to over six million people residing in the Mexico City Metropolitan Area. In recent years, this water resource has been subjected to significant risks due to several factors, including human population growth, alterations in global climate patterns, excessive resource utilization, and insufficient protective regulations, thereby endangering not only the biocenosis itself, but also the water supply for numerous inhabitants. This study aimed to evaluate the current state of the reservoir through the determination of conventional and emerging pollutants present in the sampling points, as well as embryotoxicity and oxidative damage in Danio rerio embryos exposed to effluents from the VVR. Embryotoxicity was quantified using the General Morphology Score (GMS) and teratogenic index, whereas oxidative damage was assessed based on lipid peroxidation, hydroperoxide content, oxidized proteins, antioxidant enzyme activity, and gene expression. These results revealed the presence of heavy metals, diverse pharmaceutical compounds, and pesticides. In addition, elevated lipid, hydroperoxide, and protein oxidation accompanied by alterations in superoxide dismutase (SOD) and catalase (CAT) enzymatic activity were observed during exposure. GMS resulted in impaired embryo development and teratogenic effects, including pericardial, axial, and skeletal edema. Furthermore, the upregulation of genes associated with apoptotic processes and antioxidant defense reflects a comprehensive response to oxidative stress. The study concluded that pollutants in VVR water induced oxidative damage, modified antioxidant activity, elicited embryotoxicity, and upregulated oxidative damage-related genes. The findings underscore the necessity of undertaking restoration efforts for water sources, as pollution can potentially endanger aquatic organisms and human well-being.

An adverse outcome pathway approach linking retinoid signaling disruption to teratogenicity and population-level outcomes

Recent research efforts in endocrine disruption have focused on evaluating non-EATS (estrogen, androgen, thyroid, and steroidogenesis) pathways. Retinoid signaling disruption is noteworthy because of its teratogenic effects and environmental relevance. However, current environmental risk assessments are limited in their ability to evaluate impacts on individuals and populations. This study characterizes an Adverse Outcome Pathway (AOP) network linking retinoid signaling disruption to teratogenicity and survival in zebrafish. We identified Retinoic Acid Receptor (RAR) overactivation as the molecular initiating event leading to key events including craniofacial (CFM) and tail (TM) malformations, posterior swim bladder (SB) non-inflation, impaired swimming performance, and reduced feeding, ultimately resulting in decreased survival. Our study (1) determines critical sensitivity windows for CFM, posterior SB non-inflation, and TM, (2) provides quantitative measurements for CFM and TM, and (3) defines impacts on higher biological levels including food ingestion, swimming, and survival. Results show that all-trans retinoic acid (ATRA) induces strong teratogenic effects with sensitivity windows between 4 and 48 h post fertilization (hpf) for CFM, TM, and posterior SB non-inflation. TM is the most sensitive indicator, with EC50 of 0.2 - 0.26 µg/L across exposure windows 4-48, 4-72, 4-96, and 4-120 hpf. Besides inducing known malformations, ATRA impaired posterior SB inflation with EC50 of 1 - 1.21 µg/L across the same exposure windows. ATRA exposure (1 µg/L) resulted in 50 % food ingestion inhibition at 7 days post fertilization (dpf) and 10 % survival at 14 dpf. This study provides a regulatory-relevant framework linking developmental effects to population outcomes, highlighting ecological risks and needs for improved risk assessments.

Molecular insights: zebrafish embryo damage linked to hospital effluent

This study addresses the pressing issue of pollutants, particularly heavy metals and pharmaceuticals, infiltrating aquatic ecosystems due to untreated hospital effluents. These contaminants, known for their toxicity and bioaccumulative potential, adversely affect water quality and ecosystem health. The research focuses on the intricate relationship between oxidative stress and embryonic damage in Danio rerio exposed to hospital effluent, offering a detailed understanding of the underlying mechanisms. Concentrations of pharmaceutical residues (ng L-1) such as NSAIDs, corticosteroids, proton pump inhibitors, H2-receptor antagonists, and heavy metals (mg L-1) like Cd, As, Cu, Cr, Hg, Ni, Pb, and Zn were meticulously quantified. The effluent exhibited a significant embryolethal potential with an LC50 of 2.328% and an EC50 for malformation at 2.607%. Notable embryonic malformations included yolk sac edema, tail abnormalities, pericardial edema, scoliosis, craniofacial deformities, eye hypopigmentation, developmental delays, and body malformations. Zebrafish embryos were exposed to varying concentrations of the effluent (0.5% to 4.0%) and assessed for lethality and malformations at specific intervals (12, 24, 48, 72, and 96 h post-fertilization). The study also scrutinized oxidative damage and monitored the expression of genes central to antioxidant processes, detoxification, and apoptosis (sod, cat, nrf2, cyp1a1, bax, casp3, casp6, casp7, and casp9) at 48-, 72-, and 96-h post-fertilization across all concentrations. Findings consistently revealed lipid and protein damage, heightened antioxidant activity, and altered gene expression at all time points and effluent concentrations. These results highlight the environmental threat posed by untreated hospital effluent, emphasizing the need for comprehensive effluent treatment measures to protect aquatic ecosystems from the detrimental impacts of pharmaceuticals and heavy metals. The study underscores the critical role of oxidative stress in embryonic damage and advocates for improved environmental stewardship and regulatory measures.

The zebrafish embryo as a model for chemically-induced steatosis: A case study with three pesticides

There is an increasing incidence and prevalence of fatty liver disease in the western world, with steatosis as the most prevalent variant. Known causes of steatosis include exposure to food-borne chemicals, and overconsumption of alcohol, carbohydrates and fat, and it is a well-known side effect of certain pharmaceuticals such as tetracycline, amiodarone and tamoxifen (drug-induced hepatic steatosis). Mechanistic knowledge on chemical-induced steatosis has greatly evolved and has been organized into adverse outcome pathways (AOPs) describing the chain of events from first molecular interaction of a substance with a biological system to the adverse outcome, intrahepatic lipid accumulation. In this study, three known steatosis-inducing pesticides (imazalil, clothianidin, and thiacloprid) were tested for their ability to induce hepatic triglyceride accumulation in the zebrafish (Danio rerio) embryo (ZFE) at 5 days post fertilization, both as single compounds and equipotent binary mixtures. The results indicate that the ZFE is very well applicable as a higher tier testing model to confirm effects in downstream key events in AOPs, that is, chemically-induced triglyceride accumulation in the whole organism and production of visible steatosis. Moreover, dose addition could be concluded for binary mixtures of substances with similar and with dissimilar modes of action.

Evaluation of temperature- and ethanol-related developmental degree variations by a new scoring system (FETAX-score) applicable to Frog Embryo Teratogenicity Assay: Xenopus

The aim of the present work is to propose a new quantitative assessment method (FETAX-score) for determining the degree of Xenopus laevis embryo development intended for use in embryotoxicity studies. Inspired by a similar scoring system used to evaluate developmental delays (young-for-age phenotypes) in rat embryos cultured in vitro, the FETAX-score was established by considering seven morphological features (head, naris, mouth, lower jaw, tentacles, intestine, anus) that are easily evaluable in tadpoles during the late stages of development at the conclusion of the test. Given that X. laevis development is temperature-dependent and that temperatures below 14°C and above 26°C are teratogenic, the FETAX-score was tested in embryos maintained at 17, 20, 23 and 26°C. No abnormalities were observed in any group, while the total score was temperature-related, suggesting that the FETAX-score is sensitive to moderate distress that does not influence general morphology. Intestine and anus were the least sensitive structures to temperature variations. To assess the applicability of the FETAX-score in developmental toxicological studies, we evaluated FETAX-score in tadpoles exposed during the morphogenetic period to Ethanol (Eth) at concentrations of 0, 0.25, 0.5, 1, 1.5, and 2 % v/v. Gross malformations were observed only in tadpoles from the Eth 2 % group. By contrast, data analysis of the other Eth groups showed dose-related reductions in the FETAX-score. Tentacles were the most sensitive structures to Eth-related delays. These results support the use of the FETAX-score to quantitatively assess developmental deviations in FETAX embryotoxicity studies.

Characterization of the zebrafish gabra1sa43718/sa43718 germline loss of function allele confirms a function for Gabra1 in motility and nervous system development

Mutation of the GABRA1 gene is associated with neurodevelopmental defects and epilepsy. GABRA1 encodes for the α1 subunit of the γ-aminobutyric acid type A receptor (GABAAR), which regulates the fast inhibitory impulses of the nervous system. Multiple model systems have been developed to understand the function of GABRA1, but these models have produced complex and, at times, incongruent data. Thus, additional model systems are required to validate and substantiate previous results. We sought to provide initial phenotypic analysis of a novel germline mutant allele. Our analysis provides a solid foundation for the future use of this allele to characterize gabra1 functionally and pharmacologically using zebrafish. We investigated the behavioral swim patterns associated with a nonsense mutation of the zebrafish gabra1 (sa43718 allele) gene. The sa43718 allele causes a decrease in gabra1 mRNA expression, which is associated with light induced hypermotility, one phenotype previously associated with seizure like behavior in zebrafish. Mutation of gabra1 was accompanied by decreased mRNA expression of gabra2, gabra3, and gabra5, indicating a reduction in the expression of additional α sub-units of the GABAAR. Although multiple sub-units were decreased, larvae continued to respond to pentylenetetrazole (PTZ), indicating that a residual GABAAR exists in the sa43718 allele. Proteomics analysis demonstrated that mutation of gabra1 is associated with abnormal expression of proteins that regulate synaptic vesicle fusion, vesicle transport, synapse development, and mitochondrial protein complexes. These data support previous studies performed in a zebrafish nonsense allele created by CRISPR/Cas9 and validate that loss of function mutations in the gabra1 gene result in seizure-like phenotypes with abnormal development of the GABA synapse. Our results add to the existing body of knowledge as to the function of GABRA1 during development and validate that zebrafish can be used to provide complete functional characterization of the gene.

Myclobutanil induces neurotoxicity by activating autophagy and apoptosis in zebrafish larvae (Danio rerio)

Myclobutanil (MYC), a typical broad-spectrum triazole fungicide, is often detected in surface water. This study aimed to explore the neurotoxicity of MYC and the underlying mechanisms in zebrafish and in PC12 cells. In this study, zebrafish embryos were exposed to 0, 0.5 and 1 mg/L of MYC from 4 to 96 h post fertilization (hpf) and neurobehavior was evaluated. Our data showed that MYC decreased the survival rate, hatching rate and heart rate, but increased the malformation rate and spontaneous movement. MYC caused abnormal neurobehaviors characterized by decreased swimming distance and movement time. MYC impaired cerebral histopathological morphology and inhibited neurogenesis in HuC:egfp transgenic zebrafish. MYC also reduced the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), and downregulated neurodevelopment related genes (gfap, syn2a, gap43 and mbp) in zebrafish and PC12 cells. Besides, MYC activated autophagy through enhanced expression of the LC3-II protein and suppressed expression of the p62 protein and autophagosome formation, subsequently triggering apoptosis by upregulating apoptotic genes (p53, bax, bcl-2 and caspase 3) and the cleaved caspase-3 protein in zebrafish and PC12 cells. These processes were restored by the autophagy inhibitor 3-methyladenine (3-MA) both in vivo and in vitro, indicating that MYC induces neurotoxicity by activating autophagy and apoptosis. Overall, this study revealed the potential autophagy and apoptosis mechanisms of MYC-induced neurotoxicity and provided novel strategies to counteract its toxicity.

Polluted water from a storage dam (Villa Victoria, méxico) induces oxidative damage, AChE activity, embryotoxicity, and behavioral changes in Cyprinus carpio larvae

The Villa Victoria dam is one of the most important storage reservoirs in Mexico since it distributes water to more than 20 million inhabitants in the Metropolitan Zone of Mexico City. In this dam, the common carp (Cyprinus carpio) is an important food resource for the inhabitants, so the aim of this work was to evaluate the oxidative damage (lipoperoxidation, oxidized proteins, antioxidant enzymes activity and gene expression), AChE, embryotoxicity and behavioral changes in C. carpio embryos and larvae exposed to water from Villa Victoria dam for 24, 48, 72 and 96 h. The embryotoxicity was evaluated trough the General Morphology Score (GMS) and the teratogenic index. Behavioral changes in basal locomotor activity and thigmotaxis were evaluated in a DanioVision, Noldus ™. An increase in lipid and protein oxidation as well as modification of CAT, SOD and GPx enzymatic activity was observed during the exposure times. The GMS indicated a low development in the embryos, the teratogenic index was less than 1, however teratogenic effects as yolk edema, fin malformation, head malformation and scoliosis were observed. In parallel, an increase in AChE activity and gene expression was observed reflecting changes in distance traveled of the basal locomotor activity and thigmotaxis at the sampling points. In conclusion, pollutants in water from Villa Victoria dam caused oxidative damage, changes in SOD, CAT, GPx and AChE activity as well as embryotoxicity and modifications in the behavior of C. carpio larvae. This study demonstrates the need to implement restoration programs for this reservoir since, contamination in the Villa Victoria dam could eventually endanger aquatic life and human health.

Myclobutanil induces cardiotoxicity in developing zebrafish larvae by initiating oxidative stress and apoptosis: The protective role of curcumin

Myclobutanil (MYC) is a common triazole fungicide widely applied in agriculture. MYC extensively exists in the natural environment and can be detected in organisms. However, little is known about MYC-induced embryonic developmental damage. This study aimed to unravel the cardiotoxicity of MYC and the underlying mechanisms, as well as the cardioprotective effect of curcumin (CUR, an antioxidant polyphenol) using the zebrafish model. Here, zebrafish embryos were exposed to MYC at concentrations of 0, 0.5, 1 and 2 mg/L from 4 to 96 h post fertilization (hpf) and cardiac development was assessed. As results, MYC reduced the survival and hatching rate, body length and heart rate, but increased the malformation rate and spontaneous movement. MYC caused abnormal cardiac morphology and function in myl7:egfp transgenic zebrafish, and downregulated cardiac developmental genes. MYC promoted oxidative stress through excessive reactive oxygen species (ROS) accumulation and suppressed the activities of antioxidant enzymes, triggering cardiomyocytic apoptosis via upregulated expression of apoptosis-related genes. These adverse toxicities could be significantly ameliorated by the antioxidant properties of CUR, indicating that CUR rescued MYC-induced cardiotoxicity by inhibiting oxidative stress and apoptosis. Overall, our study revealed the potential mechanisms of oxidative stress and apoptosis in MYC-induced cardiotoxicity in zebrafish and identified the cardioprotection of CUR in this pathological process.

Hexaconazole induces developmental toxicities via apoptosis, inflammation, and alterations of Akt and MAPK signaling cascades

Hexaconazole is a highly effective triazole fungicide that is frequently applied in various countries to elevate crop productivity. Given its long half-life and high water solubility, this fungicide is frequently detected in the environment, including water sources. Moreover, hexaconazole exerts hazardous effects on nontarget organisms. However, little is known about the toxic effects of hexaconazole on animal development. Thus, this study aimed to investigate the developmental toxicity of hexaconazole to zebrafish, a valuable animal model for toxicological studies, and elucidate the underlying mechanisms. Results showed that hexaconazole affected the viability and hatching rate of zebrafish at 96 h postfertilization. Hexaconazole-treated zebrafish showed phenotypic defects, such as reduced size of head and eyes and enlarged pericardiac edema. Moreover, hexaconazole induced apoptosis, DNA fragmentation, and inflammation in developing zebrafish. Various organ defects, including neurotoxicity, cardiovascular toxicity, and hepatotoxicity, were observed in transgenic zebrafish models olig2:dsRed, fli1:eGFP, and l-fabp:dsRed. Furthermore, hexaconazole treatment altered the Akt and MAPK signaling pathways, which possibly triggered the organ defects and other toxic mechanisms. This study demonstrated the developmental toxicity of hexaconazole to zebrafish and elucidated the underlying mechanisms.

Burn pit-related smoke causes developmental and behavioral toxicity in zebrafish: Influence of material type and emissions chemistry

Combustion of mixed materials during open air burning of refuse or structural fires in the wildland urban interface produces emissions that worsen air quality, contaminate rivers and streams, and cause poor health outcomes including developmental effects. The zebrafish, a freshwater fish, is a useful model for quickly screening the toxicological and developmental effects of agents in such species and elicits biological responses that are often analogous and predictive of responses in mammals. The purpose of this study was to compare the developmental toxicity of smoke derived from the burning of 5 different burn pit-related material types (plywood, cardboard, plastic, a mixture of the three, and the mixture plus diesel fuel as an accelerant) in zebrafish larvae. Larvae were exposed to organic extracts of increasing concentrations of each smoke 6-to-8-hr post fertilization and assessed for morphological and behavioral toxicity at 5 days post fertilization. To examine chemical and biological determinants of toxicity, responses were related to emissions concentrations of polycyclic hydrocarbons (PAH). Emissions from plastic and the mixture containing plastic caused the most pronounced developmental effects, including mortality, impaired swim bladder inflation, pericardial edema, spinal curvature, tail kinks, and/or craniofacial deformities, although all extracts caused concentration-dependent effects. Plywood, by contrast, altered locomotor responsiveness to light changes to the greatest extent. Some morphological and behavioral responses correlated strongly with smoke extract levels of PAHs including 9-fluorenone. Overall, the findings suggest that material type and emissions chemistry impact the severity of zebrafish developmental toxicity responses to burn pit-related smoke.

Ecological assessment of stream water polluted by phosphorus chemical plant: Physiological, biochemical, and molecular effects on zebrafish (Danio rerio) embryos

The rapid development of the phosphorus chemical industry has caused serious pollution problems in the regional eco-environment. However, understanding of their ecotoxic effects remains limited. This study aimed to investigate the developmental toxicity of a stream polluted by a phosphorus chemical plant (PCP) on zebrafish embryos. For this, zebrafish embryos were exposed to stream water (0, 25, 50, and 100% v/v) for 96 h, and developmental toxicity, oxidative stress, apoptosis, and DNA damage were assessed. Stream water-treated embryos exhibited decreased hatching rates, heart rates, and body lengths, as well as increased mortality and malformation rates. The general morphology score system indicated that the swim bladder and pigmentation were the main abnormal morphological endpoints. Stream water promoted antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and glutathione peroxidase (GPx)), lipid peroxidation, and DNA damage. It also triggered apoptosis in the embryos' heads, hearts, and spines by activating apoptotic enzymes (Caspase-3 and Caspase-9). Additionally, stream water influenced growth, oxidative stress, and apoptosis-related 19 gene expression. Notably, tyr, sod (Mn), and caspase9 were the most sensitive indicators of growth, oxidative stress, and apoptosis, respectively. The current trial concluded that PCP-polluted stream water exhibited significant developmental toxicity to zebrafish embryos, which was regulated by the oxidative stress-mediated activation of endogenous apoptotic signaling pathways.

Sargassum spp. Ethanolic Extract Elicits Toxic Responses and Malformations in Zebrafish (Danio rerio) Embryos

The amount of Sargassum spp. arriving in the Caribbean Sea has increased steadily in the last few years, producing a profound environmental impact on the ecological dynamics of the coasts of the Yucatan Peninsula. We characterized the toxicological effects of an ethanolic extract of Sargassum spp. on zebrafish (Danio rerio) embryos (ZFEs) in a 96-h static bioassay using T1 (0.01 mg/L), T2 (0.1 mg/L), T3 (1 mg/L), T4 (10 mg/L), T5 (25 mg/L), T6 (50 mg/L), T7 (75 mg/L), T8 (100 mg/L), T9 (200 mg/L), and T10 (400 mg/L). In this extract, we detected 74 compounds by gas chromatography-mass spectrometry (GC-MS), of which hexadecanoic acid methyl ester, and 2-pentanone 4-hydroxy-4-methyl, were the most abundant. In ZFEs, a median lethal concentration of 251 mg/L was estimated. Exposed embryos exhibited extensive morphological changes, including edema in the yolk sac, scoliosis, and loss of pigmentation, as well as malformations of the head, tail, and eyes. By integrating these abnormalities using the Integrated Biological Response (IBRv2) and General Morphological Score (GMS) indices, we were able to determine that ZFEs exposed to 200 mg/L (T9) exhibited the most pronounced biological response in comparison with the other groups. In the comparative transcriptomic analysis, 66 genes were upregulated, and 246 genes were downregulated in the group exposed to 200 mg/L compared with the control group. In the upregulated genes, we identified several gene ontology-enriched terms, such as response to xenobiotic stimuli, cellular response to chemical stimulus, transcriptional regulation, pigment metabolic process, erythrocyte differentiation and embryonic hemopoiesis, extracellular matrix organization, and chondrocyte differentiation involved in endochondral bone morphogenesis, among others. In the down-regulated genes, we found many genes associated with nervous system processes, sensory and visual perception, response to abiotic stimulus, and the nucleoside phosphate biosynthetic process. The probable connections among the morphological changes observed in the transcriptome are thoroughly discussed. Our findings suggest that Sargassum spp. exposure can induce a wide negative impact on zebrafish embryos. Environ Toxicol Chem 2024;00:1-15. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Impact of exposure to glyphosate-based herbicide on morphological and physiological parameters in embryonic and larval development of zebrafish

Glyphosate-based herbicides (GBH) have been commonly used in agriculture to inhibit weed growth and increase yields. However, due to the high solubility of these herbicides in water, they can reach aquatic environments, by infiltration, erosion, and/or lixiviation, affecting non target organisms. Thus, this study aimed to characterize the toxicity of GBH Roundup WG® (RWG®) during the embryonic and larval development of Danio rerio. Embryos (3 hours post fertilization, hpf-until hatching) and larvae (3 days post fertilization, dpf to 6 dpf) were exposed to concentrations of 0.065 and 6.5 mg L-1 . They were evaluated for survival, hatching, spontaneous movements, heartbeat, morphology, and morphometry by in vivo photographs in microscope, cell proliferation and apoptosis by immunohistochemistry, and exploratory behavior and phototropism by video recording. Our results showed an increase in embryo and larvae mortality in those exposed to 0.065 mg L-1 , as well as a reduction in spontaneous embryo movements. The larval heartbeats showed a decrease at 4 dpf in the group exposed to 0.065 mg L-1 and an increase at 5 and 6 dpf in both exposed groups. Cell proliferation was reduced in both groups exposed in embryos and only in the 0.065 mg L-1 group in larvae, while cell death increased in embryos exposed to 6.5 mg L-1 . These results demonstrated the toxic effect of low concentrations of the herbicide RWG® during embryonic and larval development of non target organisms, as well as the importance of constantly reviewing acceptable limits for exposure in natural environments.

Dose addition in mixtures of compounds with dissimilar endocrine modes of action in in vitro receptor activation assays and the zebrafish sexual development test

BACKGROUND

Human exposure to pesticides is being associated with feminisation for which a decrease of the anogenital distance (AGD) is a sensitive endpoint. Dose addition for the cumulative risk assessment of pesticides in food is considered sufficiently conservative for combinations of compounds with both similar and dissimilar modes of action (MoA).

OBJECTIVE

The present study was designed to test the dose addition hypothesis in a binary mixture of endocrine active compounds with a dissimilar mode of action for the endpoint feminisation.

METHODS

Compounds were selected from a list of chemicals of which exposure is related to a decrease of the AGD in rats and completed with reference compounds. These chemicals were characterised using specific in vitro transcriptional activation (TA) assays for estrogenic and androgenic properties, leading to a final selection of dienestrol as an ER-agonist and flutamide, linuron, and deltamethrin as AR-antagonists. These compounds were then tested in an in vivo model, i.e. in zebrafish (Danio rerio), using sex ratio in the population as an endpoint in order to confirm their feminising effect and MoA. Ultimately, the fish model was used to test a binary mixture of flutamide and dienestrol.

RESULTS

Statistical analysis of the binary mixture of flutamide and dienestrol in the fish sexual development tests (FSDT) with zebrafish supported dose addition.

Toxicity of Titanium Dioxide-Cerium Oxide Nanocomposites to Zebrafish Embryos: A Preliminary Evaluation

The widespread use of metal nanoparticles in different fields has raised many doubts regarding their possible toxicity to living organisms and the accumulation and discharge of metals in fish species. Among these nanoparticles, titanium dioxide (TiO2) and cerium oxide (CeO2) nanoparticles have mainly been employed in photocatalysis and water depuration. The aim of this research was to evaluate the potential toxic effects, after a co-exposure of TiO2-3%CeO2 nanoparticles, on zebrafish development, using an acute toxicity test. Increasing concentrations of TiO2-3%CeO2 nanoparticles were used (0.1-1-10-20 mg/L). The heartbeat rate was assessed using DanioscopeTM software (version 1.2) (Noldus, Leesburg, VA, USA), and the responses to two biomarkers of exposure (Heat shock proteins-70 and Metallothioneins) were evaluated through immunofluorescence. Our results showed that the co-exposure to TiO2-3%CeO2 nanoparticles did not affect the embryos' development compared to the control group; a significant difference (p < 0.05) at 48 hpf heartbeat for the 1, 10, and 20 mg/L groups was found compared to the unexposed group. A statistically significant response (p < 0.05) to Heat shock proteins-70 (Hsp70) was shown for the 0.1 and 1 mg/L groups, while no positivity was observed in all the exposed groups for Metallothioneins (MTs). These results suggest that TiO2-3%CeO2 nanocomposites do not induce developmental toxicity; instead, when considered separately, TiO2 and CeO2 NPs are harmful to zebrafish embryos, as previously shown.

Short-term exposure to dexamethasone at environmentally relevant concentrations impairs embryonic development in Cyprinus carpio: Bioconcentration and alteration of oxidative stress-related gene expression patterns

In recent years and as a result of the Covid-19 pandemic, the consumption of dexamethasone (DXE) has increased. This favors that this corticosteroid is highly released in aquatic environments, generating deleterious effects in aquatic organisms. The information on the toxic effects of DXE in the environment is still limited. Thus, the objective of this work was to determine whether DXE at short-term exposure can cause alterations to embryonic development and alteration of oxidative stress-related gene expression patterns in Cyprinus carpio. For this purpose, common carp embryos (2 hpf) were exposed to realistic concentrations of DXE until 96 hpf. Alterations to embryonic development were evaluated at 12, 24, 48, 72 and 96 hpf. In addition, oxidative stress in carp embryos at 72 and 96 hpf was evaluated by cellular oxidation biomarkers (lipoperoxidation level, hydroperoxide and carbonyl protein content) and antioxidant enzymes activities (superoxide dismutase and catalase). Oxidative stress-related gene expression (sod, cat and gpx1) was also evaluated. Our results showed that DXE concentrations above 35 ng/L are capable of producing alterations to embryonic development in 50 % of the embryo population. Furthermore, DXE was able to induce alterations such as scoliosis, hypopigmentation, craniofacial malformations, pericardial edema and growth retardation, leading to the death of half of the population at 50 ng/L of DXE. Concerning oxidative stress, the results demonstrated that DXE induce oxidative damage on the embryos of C. carpio. In conclusion, DXE is capable of altering embryonic development and generating oxidative stress in common carp C. carpio.

Modeling Sarcoglycanopathy in Danio rerio

Sarcoglycanopathies, also known as limb girdle muscular dystrophy 3-6, are rare muscular dystrophies characterized, although heterogeneous, by high disability, with patients often wheelchair-bound by late adolescence and frequently developing respiratory and cardiac problems. These diseases are currently incurable, emphasizing the importance of effective treatment strategies and the necessity of animal models for drug screening and therapeutic verification. Using the CRISPR/Cas9 genome editing technique, we generated and characterized δ-sarcoglycan and β-sarcoglycan knockout zebrafish lines, which presented a progressive disease phenotype that worsened from a mild larval stage to distinct myopathic features in adulthood. By subjecting the knockout larvae to a viscous swimming medium, we were able to anticipate disease onset. The δ-SG knockout line was further exploited to demonstrate that a δ-SG missense mutant is a substrate for endoplasmic reticulum-associated degradation (ERAD), indicating premature degradation due to protein folding defects. In conclusion, our study underscores the utility of zebrafish in modeling sarcoglycanopathies through either gene knockout or future knock-in techniques. These novel zebrafish lines will not only enhance our understanding of the disease's pathogenic mechanisms, but will also serve as powerful tools for phenotype-based drug screening, ultimately contributing to the development of a cure for sarcoglycanopathies.

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

Nervous system development related gene expression regulation in the zebrafish embryo after exposure to valproic acid and retinoic acid: A genome wide approach

The evaluation of chemical and pharmaceutical safety for humans is moving from animal studies to New Approach Methodologies (NAM), reducing animal use and focusing on mechanism of action, whilst enhancing human relevance. In developmental toxicology, the mechanistic approach is facilitated by the assessment of predictive biomarkers, which allow mechanistic pathways perturbation monitoring at the basis of human hazard assessment. In our search for biomarkers of maldevelopment, we focused on chemically-induced perturbation of the retinoic acid signaling pathway (RA-SP), a major pathway implicated in a plethora of developmental processes. A genome-wide expression screening was performed on zebrafish embryos treated with two teratogens, all-trans retinoic acid (ATRA) and valproic acid (VPA), and a non-teratogen reference compound, folic acid (FA). Each compound was found to have a specific mRNA expression profile with 248 genes commonly dysregulated by both teratogenic compounds but not by FA. These genes were implicated in several developmental processes (e.g., the circulatory and nervous system). Given the prominent response of neurodevelopmental gene sets, and the crucial need to better understand developmental neurotoxicity, our study then focused on nervous system development. We found 62 genes that are potential early neurodevelopmental toxicity biomarker candidates. These results advance NAM-based safety assessment evaluation by highlighting the usefulness of the RA-SP in providing early toxicity biomarker candidates.

Developmental defects and potential mechanisms in F1 generation of parents exposed to difenoconazole at different life stages of zebrafish (Danio rerio)

As a typical triazole fungicide, difenoconazole is extensively used to control plant diseases; however, its residue in environmental waters poses a risk to aquatic organisms. In this study, we investigated the acute toxicity of different life stages and sub-lethal toxicity in embryonic yolk sac stage of difenoconazole to zebrafish, and the developmental toxicity in F1 generation of parents exposed to difenoconazole at different life stages of zebrafish. Furthermore, we used transcriptomics to explore the potential mechanisms of difenoconazole on the F1 larvae of parents exposed to the chemical at the embryonic stage. The results of this study showed that developmental defects were observed in the F1 embryo/larvae of parents exposed to 3, 30, and 300 μg/L of difenoconazole at different (embryo, larval, juvenile, and adult) life stages, and exposure to difenoconazole at the embryonic stage caused more severe developmental toxicity than those at other life stages. Developmental defects (malformation, inhibition of heartbeat and body length) were observed in the F1 embryos and larvae of parents exposed to difenoconazole at the embryonic stage. In addition, the total cholesterol and triglyceride contents were significantly reduced in the F1 larvae, and RNA-seq analysis revealed significant alterations in the expression of nine genes (msmo1, hsd17b7, sc5d, tm7sf2, ebp, cyp2r1, lss, cyp51, and cyp27b1) in the steroid synthesis pathway. This is suggested that F1 larvae of parents exposed to difenoconazole at the embryonic stage show abnormalities in the steroid biosynthetic pathway. These results reveal the differences in toxicity of difenoconazole to zebrafish at different life stages, improve studies on difenoconazole toxicity to zebrafish, and provide a new perspective for assessing the risk of contaminants to aquatic organisms.

Chronic intake of an enriched diet with spirulina (Arthrospira maxima) alleviates the embryotoxic effects produced by realistic concentrations of tetracycline in Danio rerio

Tetracycline (TC) is one of the most consumed antibiotics worldwide. Due to its high consumption, recent studies have reported its presence in aquatic environments and have assessed its effects on fish, algae, and daphniids. However, in most of those works, authors have tested TC toxicity at concentrations higher than the ones reported in the water matrix. Herein, we aimed to assess the likely embryotoxic and oxidative damage induced by environmentally relevant concentrations of TC in embryos of Danio rerio. Moreover, we seek to determine whether or not an enriched diet with spirulina can alleviate the embryotoxic damage produced by TC. Our findings indicated that TC at concentrations of 50 to 500 ng/L induced pericardial edema, tail deformities, and absence of head and fin in embryos after 96 h of exposure. Moreover, this antibiotic prompted the death of embryos in a concentration-dependent manner. According to our integrated biomarker response index, TC induced oxidative damage on Danio rerio embryos, as star plots showed a tendency to lipoperoxidation, hydroperoxides, and protein carbonyl content. Spirulina reduced the toxicity of TC by diminishing the levels of oxidative damage biomarkers, which resulted in a decrease in the rate of death and malformed embryos. Overall, TC at concentrations of ng/L prompted oxidative stress and embryotoxicity in the early life stages of Danio rerio; nonetheless, the algae spirulina was able to reduce the severity of those effects.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is dependent on the ionic strength of exposure media

Pericardial edema is commonly observed in zebrafish embryo-based chemical toxicity screens, and a mechanism underlying edema may be disruption of embryonic osmoregulation. Therefore, the objective of this study was to identify whether triphenyl phosphate (TPHP) - a widely used aryl phosphate ester-based flame retardant - induces pericardial edema via impacts on osmoregulation within embryonic zebrafish. In addition to an increase in TPHP-induced microridges in the embryonic yolk sac epithelium, an increase in ionic strength of exposure media exacerbated TPHP-induced pericardial edema when embryos were exposed from 24 to 72 h post-fertilization (hpf). However, there was no difference in embryonic sodium concentrations in situ within TPHP-exposed embryos relative to embryos exposed to vehicle (0.1% DMSO) from 24 to 72 hpf. Interestingly, increasing the osmolarity of exposure media with mannitol (an osmotic diuretic which mitigates TPHP-induced pericardial edema) and increasing the ionic strength of the exposure media (which exacerbates TPHP-induced pericardial edema) did not affect embryonic doses of TPHP, suggesting that TPHP uptake was not altered under these varying experimental conditions. Overall, our findings suggest that TPHP-induced pericardial edema within zebrafish embryos is dependent on the ionic strength of exposure media, underscoring the importance of further standardization of exposure media and embryo rearing protocols in zebrafish-based chemical toxicity screening assays.

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

Bromuconazole exposure induces cardiotoxicity and lipid transport disorder in larval zebrafish

Bromuconazole (BRO), as one of the typical triazole fungicides, has not been reported on its effects on aquatic organisms. In this study, zebrafish embryos were used as experimental objects to evaluate the toxicity of BRO. In the acute embryo toxicity test, it was observed that the heart rate and growing development were affected by BRO in a concentration-dependent manner, and the half-lethal concentration (LC50) of BRO at 96 h post-fertilization (hpf) was about 11.83 mg/L. Then, low concentrations of BRO (50 ng/L, 0.075 mg/L, 0.3 mg/L, 1.2 mg/L), which were set according to the LC50 and environmental related concentrations, were used to analyze the toxic effects on the different endpoints in larval zebrafish. Interestingly, the transcriptomic analysis found that most different expressed genes (DEGs) could be focused on the pathways of lipid metabolism, myocardial function, glycometabolism, indicating that heart function and lipid metabolism in larval zebrafish were disrupted by BRO. For supporting this idea, we re-exposed the transgenic zebrafish and WT zebrafish embryos, proved that BRO caused damage to heart development and lipid transport on morphological and genetic level, which was consistent with transcriptomic results. In addition, BRO exposure caused oxidative damage in the larvae. Taken together, BRO exposure could affect the myocardial contraction function and lipid transport in larval zebrafish, accompanied by disturbances in the level of oxidative stress, which was of great significance for improving the biotoxicological information of BRO.

Triflumizole Induces Developmental Toxicity, Liver Damage, Oxidative Stress, Heat Shock Response, Inflammation, and Lipid Synthesis in Zebrafish

Triflumizole (TFZ) toxicity must be investigated in the aquatic environment to understand the potential risks to aquatic species. Accordingly, the adverse effects of TFZ exposure in zebrafish were investigated. Results demonstrate that, after TFZ exposure, the lethal concentration 50 (LC50) in 3 d post-fertilization (dpf) embryos and 6 dpf larvae were 4.872 and 2.580 mg/L, respectively. The development (including pericardium edema, yolk sac retention, and liver degeneration) was apparently affected in 3 dpf embryos. Furthermore, the alanine aminotransferase (ALT) activity, superoxide dismutase (SOD) activity, catalase (CAT) activity, and malondialdehyde (MDA) content in 6 dpf larvae were significantly increased. Additionally, the expression of heat shock response genes (including hsp70, grp78, hsp90, and grp94), inflammatory genes (including p65-nfκb, il-1β, and cox2a), and lipid synthetic genes (including srebp1, fas, acc, and ppar-γ) in 3 dpf embryos was significantly increased, which was also partially observed in the intestinal cell line form Pampus argenteus. Taken together, TFZ could affect the development of zebrafish, accompanied by disturbances of oxidative stress, heat shock response, inflammation, and lipid synthesis. Our findings provide an original insight into the potential risks of TFZ to the aquatic ecosystem.

Protection against Paraquat-Induced Oxidative Stress by Curcuma longa Extract-Loaded Polymeric Nanoparticles in Zebrafish Embryos

The link between oxidative stress and environmental factors plays an important role in chronic degenerative diseases; therefore, exogenous antioxidants could be an effective alternative to combat disease progression and/or most significant symptoms. Curcuma longa L. (CL), commonly known as turmeric, is mostly composed of curcumin, a multivalent molecule described as having antioxidant, anti-inflammatory and neuroprotective properties. Poor chemical stability and low oral bioavailability and, consequently, poor absorption, rapid metabolism, and limited tissue distribution are major restrictions to its applicability. The advent of nanotechnology, by combining nanosacale with multi-functionality and bioavailability improvement, offers an opportunity to overcome these limitations. Therefore, in this work, poly-Ɛ-caprolactone (PCL) nanoparticles were developed to incorporate the methanolic extract of CL, and their bioactivity was assessed in comparison to free or encapsulated curcumin. Their toxicity was evaluated using zebrafish embryos by applying the Fish Embryo Acute Toxicity test, following recommended OECD guidelines. The protective effect against paraquat-induced oxidative damage of CL extract, free or encapsulated in PCL nanoparticles, was evaluated. This herbicide is known to cause oxidative damage and greatly affect neuromotor functions. The overall results indicate that CL-loaded PCL nanoparticles have an interesting protective capacity against paraquat-induced damage, particularly in neuromuscular development that goes well beyond that of CL extract itself and other known antioxidants.

Metallated phthalocyanines and their hydrophilic derivatives for multi-targeted oncological photodynamic therapy

BACKGROUND AND AIM

A photosensitizer (PS) delivery and comprehensive tumor targeting platform was developed that is centered on the photosensitization of key pharmacological targets in solid tumors (cancer cells, tumor vascular endothelium, and cellular and non-cellular components of the tumor microenvironment) before photodynamic therapy (PDT). Interstitially targeted liposomes (ITLs) encapsulating zinc phthalocyanine (ZnPC) and aluminum phthalocyanine (AlPC) were formulated for passive targeting of the tumor microenvironment. In previous work it was established that the PEGylated ITLs were taken up by cultured cholangiocarcinoma cells. The aim of this study was to verify previous results in cancer cells and to determine whether the ITLs can also be used to photosensitize cells in the tumor microenvironment and vasculature. Following positive results, rudimentary in vitro and in vivo experiments were performed with ZnPC-ITLs and AlPC-ITLs as well as their water-soluble tetrasulfonated derivatives (ZnPCS4 and AlPCS4) to assemble a research dossier and bring this platform closer to clinical transition.

METHODS

Flow cytometry and confocal microscopy were employed to determine ITL uptake and PS distribution in cholangiocarcinoma (SK-ChA-1) cells, endothelial cells (HUVECs), fibroblasts (NIH-3T3), and macrophages (RAW 264.7). Uptake of ITLs by endothelial cells was verified under flow conditions in a flow chamber. Dark toxicity and PDT efficacy were determined by cell viability assays, while the mode of cell death and cell cycle arrest were assayed by flow cytometry. In vivo systemic toxicity was assessed in zebrafish and chicken embryos, whereas skin phototoxicity was determined in BALB/c nude mice. A PDT efficacy pilot was conducted in BALB/c nude mice bearing human triple-negative breast cancer (MDA-MB-231) xenografts.

RESULTS

The key findings were that (1) photodynamically active PSs (i.e., all except ZnPCS4) were able to effectively photosensitize cancer cells and non-cancerous cells; (2) following PDT, photodynamically active PSs were highly toxic-to-potent as per anti-cancer compound classification; (3) the photodynamically active PSs did not elicit notable systemic toxicity in zebrafish and chicken embryos; (4) ITL-delivered ZnPC and ZnPCS4 were associated with skin phototoxicity, while the aluminum-containing PSs did not exert detectable skin phototoxicity; and (5) ITL-delivered ZnPC and AlPC were equally effective in their tumor-killing capacity in human tumor breast cancer xenografts and superior to other non-phthalocyanine PSs when appraised on a per mole administered dose basis.

CONCLUSIONS

AlPC(S4) are the safest and most effective PSs to integrate into the comprehensive tumor targeting and PS delivery platform. Pending further in vivo validation, these third-generation PSs may be used for multi-compartmental tumor photosensitization.

Multi-biomarker approach and IBR index to evaluate the effects of bisphenol A on embryonic stages of zebrafish (Danio rerio)

This study assessed the effects of Bisphenol A in embryonic stages of zebrafish, applying an IBR multi-biomarker approach that included alterations in growth and oxidative status and relates it with the expression of Nrf1, Nrf2, Wnt3a, Wnt8a, COX-2, Qdpra, and DKK1 genes. For this purpose, we exposed zebrafish embryos to eight environmentally relevant concentrations of BPA (220, 380, 540, 700, 860, 1180, 1340, and 1500 ng L^-1) until 96 h post-fertilization. Our results show that BPA induces several malformations in embryos (developmental delay, hypopigmentation, tail malformations, and on), leading to their death. The LC₅₀, EC₅₀ of malformations, and teratogenic index (TI) were 1234.60 ng L^-1, 987.77 ng L^-1, and 1.25, respectively; thus, this emerging contaminant is teratogenic. Regarding oxidative stress and gene expression, we demonstrated BPA altered oxidative status and the gene expression in embryos of Danio rerio.

Toxicity evaluation of traditional and organic yerba mate (Ilex paraguariensis A. St.-Hil.) extracts

Yerba mate (Ilex paraguariensis A. St.-Hil.) is an important source of biologically active compounds with pharmacological potential. The aim of this study was to examine the toxicity of different extracts obtained from either traditional or organic cultivated yerba mate in vitro and in vivo. Aqueous, ethanolic and methanolic extracts were obtained from commercial samples of yerba mate and total phenolic content was determined employing Folin-Ciocalteau reagent. The aqueous extracts presented higher content of total phenols, compared to ethanolic and methanolic extracts, and also demonstrated lower cytotoxicity, which is the basis for testing were carried out only using aqueous extracts. The main phenolic acids found in traditional aqueous (TA) extract were chlorogenic, gallic and protocatechuic acids. Gallic and hydroxybenzoic acids were detected in aqueous cultivated organic (OA) extract. Pretreatment with OA extract (100 µg/ml, 1 hr) was cytoprotective against rotenone-induced toxicity (1 µM). For in vivo toxicity assay, zebrafish embryos were exposed to OA or TA extracts (10-160 µg/ml) at 4 hr post fertilization. TA extract decreased embryos survival in a concentration-dependent manner, reduced the hatching rate at 40 µg/ml, increased edema frequency at 80 µg/ml and altered body curvature at 120 µg/ml. Further, TA extract produced locomotor disorders at concentrations equal to or greater than 10 µg/ml. In contrast, OA extract exhibited no apparent toxic effect on organogenesis and behavior up to 100 µg/ml. In summary, the OA cultivated extract showed the lowest cytotoxicity in vitro, enhanced reduction in rotenone-induced toxicity, and produced less toxicity in zebrafish embryos compared to the TA extract.

miR-29b-3p Inhibitor Alleviates Hypomethylation-Related Aberrations Through a Feedback Loop Between miR-29b-3p and DNA Methylation in Cardiomyocytes

As a member of the miR-29 family, miR-29b regulates global DNA methylation through target DNA methyltransferases (DNMTs) and acts as both a target and a key effector in DNA methylation. In this study, we found that miR-29b-3p expression was inversely correlated with DNMT expression in the heart tissues of patients with congenital heart disease (CHD), but whether it interacts with DNMTs in cardiomyocytes remains unknown. Further results revealed a feedback loop between miR-29b-3p and DNMTs in cardiomyocytes. Moreover, miR-29b-3p inhibitor relieved the deformity of hypomethylated zebrafish and restored the DNA methylation patterns in cardiomyocytes, resulting in increased proliferation and renormalization of gene expression. These results suggest mutual regulation between miR-29b-3p and DNMTs in cardiomyocytes and support the epigenetic normalization of miRNA-based therapy in cardiomyocytes.

Dose Addition in the Induction of Craniofacial Malformations in Zebrafish Embryos Exposed to a Complex Mixture of Food-Relevant Chemicals with Dissimilar Modes of Action

BACKGROUND

Humans are exposed to combinations of chemicals. In cumulative risk assessment (CRA), regulatory bodies such as the European Food Safety Authority consider dose addition as a default and sufficiently conservative approach. The principle of dose addition was confirmed previously for inducing craniofacial malformations in zebrafish embryos in binary mixtures of chemicals with either similar or dissimilar modes of action (MOAs).

OBJECTIVES

In this study, we explored a workflow to select and experimentally test multiple compounds as a complex mixture with each of the compounds at or below its no observed adverse effect level (NOAEL), in the same zebrafish embryo model.

METHODS

Selection of candidate compounds that potentially induce craniofacial malformations was done using in silico methods-structural similarity, molecular docking, and quantitative structure-activity relationships-applied to a database of chemicals relevant for oral exposure in humans via food (EuroMix inventory, n = 1,598 ). A final subselection was made manually to represent different regulatory fields (e.g., food additives, industrial chemicals, plant protection products), different chemical families, and different MOAs.

RESULTS

A final selection of eight compounds was examined in the zebrafish embryo model, and craniofacial malformations were observed in embryos exposed to each of the compounds, thus confirming the developmental toxicity as predicted by the in silico methods. When exposed to a mixture of the eight compounds, each at its NOAEL, substantial craniofacial malformations were observed; according to a dose-response analysis, even embryos exposed to a 7-fold dilution of this mixture still exhibited a slight abnormal phenotype. The cumulative effect of the compounds in the mixture was in accordance with dose addition (added doses of the individual compounds after adjustment for relative potencies), despite different MOAs of the compounds involved.

DISCUSSION

This case study of a complex mixture inducing craniofacial malformations in zebrafish embryos shows that dose addition can adequately predicted the cumulative effect of a mixture of multiple substances at low doses, irrespective of the (expected) MOA. The applied workflow may be useful as an approach for CRA in general. https://doi.org/10.1289/EHP9888.

Exposure to Landfill Leachates Affects the Embryonic Development of Zebrafish, Danio rerio: A Case Study in Yucatan, Mexico

We report the chemical characterisation and toxic effects of municipal solid waste landfill leachates on the embryonic development of Danio rerio. The results of the Fourier transform infrared spectroscopy (FTIR) revealed the presence of nitrogen-containing groups and aromatic functional groups associated with highly toxic pollutants such as ammonia and heavy metal-humic complexes. Mortalities of up to 93 and 100% were observed in 1:64 and 1:32 landfill leachate dilutions, v/v, respectively. The hatching percentages of the fish were also severely affected, with very low percentages ranging from 0 to 33.3% for 1:32, 1:64 and 1:128 dilutions, v/v. Morphologically, a developmental arrest was evident for all treatments. This study reveals the high toxicity of landfill leachates that could contaminate the aquifer of the Yucatan Peninsula and threaten the health of living organisms.

Acute exposure to environmentally relevant concentrations of phenytoin damages early development and induces oxidative stress in zebrafish embryos

Phenytoin (PHE) is an antiepileptic drug that has been widely used in clinical practice for about 80 years. It is mainly used in the treatment of tonic-clonic and partial seizures. The widespread consumption of this drug around the world has led to PHE being introduced into water bodies through municipal, hospital, and industrial effluent discharges. Since the toxic effects of this drug on aquatic species has been scarcely explored, the aim of this work was to investigate the influence of low (25-400 ngL^-1) and high (500-1500 ngL^-1) environmentally relevant concentrations of PHE on the development and oxidative status of zebrafish (Danio rerio) embryos. The toxicity of PHE was evaluated from 12 to 96 h after fertilization in D. rerio at concentrations between 25 and 1500 ngL^-1. In both the control group and the 0.05% DMSO system, no malformations were observed, all embryos developed normally after 96 h. The severity and frequency of malformations increased with increasing PHE concentration compared to embryos in the control group. Malformations observed included developmental delay, hypopigmentation, miscellaneous (more than one malformation in the same embryo), modified chorda structure, tail malformation, and yolk deformation. Concerning the biomarkers of oxidative stress, an increase in the degree of lipid peroxidation, protein carbonylation, and hydroperoxide content was observed (p < 0.05) concerning the control. In addition, a significant increase (p < 0.05) in antioxidant enzymes (SOD, CAT, and GPx) was observed at low exposure concentrations (25-400 ngL^-1), with a decrease in enzyme activity at high concentrations (500-1500 ngL^-1). Our IBR analysis demonstrated that oxidative damage biomarkers got more influence at 500ngL^-1 of PHE. The results demonstrated that PHE may affect the embryonic development of zebrafish and that oxidative stress may be involved in the generation of this embryotoxic process.

Effects of oxidative stress induced by environmental relevant concentrations of fluoxetine on the embryonic development on Danio rerio

Fluoxetine (FLX) is a psychoactive drug that acts as an antidepressant. FLX is one of the world's best-selling prescription antidepressants. FLX is widely used for the treatment of various psychiatric disorders. For these reasons, this drug may eventually end up in the aquatic environment via municipal, industrial, and hospital discharges. Even though the occurrence of FLX in aquatic environments has been reported as ubiquitous, the toxic effects that this drug may induce, especially at environmentally relevant concentrations, on essential biological processes of aquatic organisms require more attention. In the light of this information, this work aimed to investigate the influence that fluoxetine oxidative stress-induced got over the embryonic development of Danio rerio. For this purpose, D. rerio embryos (4 h post fertilization) were exposed to environmentally relevant concentrations (5, 10, 15, 20, 25, 30, 35, and 40 ng L^-1) of fluoxetine, until 96 h post fecundation. Along the exposure, survival, alterations to embryonic development, and teratogenic effects were evaluated using a stereomicroscope. Furthermore, oxidative stress biomarkers (superoxide dismutase, catalase, glutathione peroxidase, lipid peroxidation, hydroperoxide, and carbonyl content) were evaluated at 72 and 96 h post fecundation. LC₅₀, EC_50m, and teratogenic index were 30 ng L^-1, 16 ng L^-1, and 1.9, respectively. The main teratogenic effects induced by fluoxetine were pericardial edema, hatching retardation, spine alterations and craniofacial malformations. Concerning oxidative stress, our integrated biomarkers (IBR) analysis demonstrated that as the concentration increased, oxidative damage biomarkers got more influence over the embryos than antioxidant enzymes. Thus, fluoxetine induces an important oxidative stress response on the embryos of D. rerio. Collectively, our results allow us to concluded that FLX is a dangerous drug in the early life stages of D. rerio due to its high teratogenic potential and that FLX-oxidative stress induced may be involved in this toxic process.

Functional Therapeutic Target Validation Using Pediatric Zebrafish Xenograft Models

Simple Summary

Despite the major progress of precision and personalized oncology, a significant therapeutic benefit is only achieved in cases with directly druggable genetic alterations. This highlights the need for additional methods that reliably predict each individual patient’s response in a clinically meaningful time, e.g., through ex vivo functional drug screen profiling. Moreover, patient-derived xenograft (PDX) models are more predictive than cell culture studies, as they reconstruct cell–cell and cell–extracellular matrix (ECM) interactions and consider the tumor microenvironment, drug metabolism and toxicities. Zebrafish PDXs (zPDX) are nowadays emerging as a fast model allowing for multiple drugs to be tested at the same time in a clinically relevant time window. Here, we show that functional drug response profiling of zPDX from primary material obtained through the INdividualized Therapy FOr Relapsed Malignancies in Childhood (INFORM) pediatric precision oncology pipeline provides additional key information for personalized precision oncology.

Abstract

The survival rate among children with relapsed tumors remains poor, due to tumor heterogeneity, lack of directly actionable tumor drivers and multidrug resistance. Novel personalized medicine approaches tailored to each tumor are urgently needed to improve cancer treatment. Current pediatric precision oncology platforms, such as the INFORM (INdividualized Therapy FOr Relapsed Malignancies in Childhood) study, reveal that molecular profiling of tumor tissue identifies targets associated with clinical benefit in a subgroup of patients only and should be complemented with functional drug testing. In such an approach, patient-derived tumor cells are exposed to a library of approved oncological drugs in a physiological setting, e.g., in the form of animal avatars injected with patient tumor cells. We used molecularly fully characterized tumor samples from the INFORM study to compare drug screen results of individual patient-derived cell models in functional assays: (i) patient-derived spheroid cultures within a few days after tumor dissociation; (ii) tumor cells reisolated from the corresponding mouse PDX; (iii) corresponding long-term organoid-like cultures and (iv) drug evaluation with the corresponding zebrafish PDX (zPDX) model. Each model had its advantage and complemented the others for drug hit and drug combination selection. Our results provide evidence that in vivo zPDX drug screening is a promising add-on to current functional drug screening in precision medicine platforms.

Teratogenic effects induced by paracetamol, ciprofloxacin, and their mixture on Danio rerio embryos: Oxidative stress implications

Even though the toxic effects of paracetamol (PCM) and ciprofloxacin (CPX) have been deeply studied in the last decades, the impact of the PCM-CPX mixture may induce in aquatic organisms is poorly known. Thus, the objective of this work was to investigate the teratogenic effects and oxidative stress that PCM, CPX, and their mixture induce in Danio rerio embryos. Moreover, we aimed to determine whether the PCM-CPX mixture induces more severe effects on the embryos than the individual drugs. For this purpose, zebrafish embryos (4 hpf) were exposed to environmentally relevant concentrations of PCM, CPX, and their mixture until 96 hpf. In addition, at 72 hpf and 96 hpf, we also evaluated the oxidative stress biomarkers (superoxide dismutase, catalase, glutathione peroxidase, lipid peroxidation, and hydroperoxides and carbonyl content) in the embryos. Our results demonstrated that PCM, CPX, and their mixture reduced the survival rate of embryos by up to 75%. In addition, both drugs, induced morphological alterations in the embryos, causing their death. The most observed malformations were: scoliosis, craniofacial malformations, hypopigmentation, growth retardation, pericardial edema. Concerning oxidative stress, our integrated biomarkers response (IBR) analysis demonstrated that PCM, CPX, and their mixture induce oxidative damage on the embryos. In conclusion, PCM, CPX, and their mixture can alter zebrafish embryonic development via an oxidative stress response.

Acute exposure to 17-α-ethinylestradiol disrupt the embryonic development and oxidative status of Danio rerio

17-Alpha-ethinylestradiol (EE2) is an estrogen derived from estradiol (E2). This compound and is one of the most widely used drugs both in humans and animals. Numerous studies have reported the ability of EE2 to alter sex determination and delay sexual maturity, but there are toxic effects that need to be explored. In this work, we analyzed the effect of EE2 on embryonic development and oxidative stress biomarkers in Danio rerio. For this effect, zebrafish embryos in the blastula period (2.5 h post fecundation) were exposed to different concentrations of EE2 (36-106 ng L^-1) until 96 hpf. Survival, alterations to embryonic development, and teratogenic effects were evaluated using a stereomicroscope. Furthermore, oxidative stress biomarkers: superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX) activities, lipid peroxidation (LPX), hydroperoxide content (HPX), and protein carbonyl content (POX) were evaluated at 72 and 96 hpf using spectrophotometric methods. LC₅₀ and EC₅₀ of malformations got values of 82 ng L^-1 and 57.7 ng L^-1, respectively. The main teratogenic effects found were: chorda malformation, body malformation, and developmental delay. These alterations occurred at 86, 96, and 106 ng L^-1. Integrated biomarker index showed that the oxidative stress biomarkers that had the most influence on embryos were SOD, CAT, GPX, and LPX. Overall, our results allow us to conclude that low concentrations of EE2 may potentially alter the development and oxidative status in the early life stages of zebrafish. Therefore, this bio-active estrogen can be considered a hazardous substance for fish.

Ethylene oxide derived glycol ethers: A review of the alkyl glycol ethers potential to cause endocrine disruption

The 'ethylene glycol ethers' (EGE) are a broad family of solvents and hydraulic fluids produced through the reaction of ethylene oxide and a monoalcohol. Certain EGE derived from methanol and ethanol are well known to cause toxicity to the testes and fetotoxicity and that this is caused by the common metabolites methoxy and ethoxyacetic acid, respectively. There have been numerous published claims that EGE fall into the category of 'endocrine disruptors' often without substantiated evidence. This review systematically evaluates all of the available and relevant in vitro and in vivo data across this family of substances using an approach based around the EFSA/ECHA 2018 guidance for the identification of endocrine disruptors. The conclusion reached is that there is no significant evidence to show that EGE target any endocrine organs or perturb endocrine pathways and that any toxicity that is seen occurs by non-endocrine modes of action.

Developmental alterations, teratogenic effects, and oxidative disruption induced by ibuprofen, aluminum, and their binary mixture on Danio rerio

Several studies highlighted the ubiquitous presence of ibuprofen and aluminum in the aquatic environment around the world and demonstrated their potential to induce embryotoxic and teratogenic defects on aquatic species individually. Although studies that evaluate developmental alterations induced by mixtures of these pollutants are scarce; and, since environmental contamination presented in the form of a mixture of toxicants with different chemical properties and toxicity mechanisms capable of generating interactions; the objective of this study was to evaluate the developmental defects, teratogenic alterations, and oxidative stress induced by individual forms and the mixture of ibuprofen (IBU) and aluminum (Al) on zebrafish embryos. Oocytes exposed to environmentally relevant concentrations of IBU (0.1-20 μg L-1) and Al (0.01-8 mg L-1) and one binary mixture. The LC50 and EC50 were obtained to calculate the teratogenic index (TI). The IBU LC50, EC50, and TI were 8.06 μg L-1, 2.85 μg L-1 and 2.82. In contrast, Al LC50 was 5.0 mg L-1with an EC50 of 3.58 mg L-1 and TI of 1.39. The main alterations observed for individual compounds were hatching alterations, head malformation, skeletal deformities, hypopigmentation, pericardial edema, and heart rate impairment. The mixture also showed significant delays to embryonic development. Moreover, oxidative stress biomarkers of cellular oxidation and antioxidant defenses at 72 and 96 hpf significantly increased. Results show that environmentally relevant concentrations of ibuprofen (IBU), aluminum (Al), and their mixture promote a series of developmental defects, teratogenic effects, and oxidative disruption on D. rerio embryos, and the interaction of both substances altered the response. In conclusion, morphological and biochemical tests are suitable tools for assessing the health risk of aquatic wildlife by exposure to individual and mixed pollutants in freshwater bodies.

Contamination and Toxicity of Surface Waters Along Rural and Urban Regions of the Capibaribe River in Tropical Northeastern Brazil

The Capibaribe River provides water to a population of 1.7 million people in the Brazilian northeast, while receiving agricultural runoff and industrial and domestic effluents along its 280 km. The present study evaluated the ecotoxicity of surface waters along ten sites in rural and urban areas using zebrafish (Danio rerio) early-life stages and related it to water quality indices and chemical abiotic variables. Lethality rates, delays in embryo-larval development quantified by the general morphology score (GMS), and frequencies of developmental abnormalities were analyzed. A correlation was detected between zebrafish GMS and water quality index (WQI), sensitivity to domestic sewage contamination, and trophic state index, focused on eutrophication. These indices agreed in identifying a spatial pattern of smaller impact in terms of ecotoxicity, domestic sewage contamination, and eutrophication risk at three sites in rural areas (mean GMS 16.9), an intermediate impact at four sites with urban and agricultural influence (mean GMS 16.4), and greatest impacts at three more urbanized sites (mean GMS 14.9). Most frequent developmental abnormalities included noninflation of the swim bladder, delayed hatching, nonprotrusion of the mouth, blood stasis, and nondevelopment of pectoral fins. Toxic NH₃ concentrations varied spatially, with higher concentrations in urban sites; and blood stasis correlated positively with NH₃ , suggesting a causal relationship. Polycyclic aromatic hydrocarbons were detected in both rural and urbanized sites, contributing to detected toxicity. The present study demonstrates the potential of zebrafish early-life stages as an ecotoxicological model that may contribute to a better understanding of surface water quality and ecotoxicity in tropical river systems. Environ Toxicol Chem 2021;40:3063-3077. © 2021 SETAC.

Minimum reporting standards based on a comprehensive review of the zebrafish embryo teratogenicity assay

Reproductive toxicity chemical safety assessment involves extensive use of vertebrate animals for regulatory testing purposes. Although alternative methods such as the zebrafish embryo teratogenicity assay (identified in the present manuscript by the acronym ZETA) are promising for replacing tests with mammals, challenges to regulatory application involve lack of standardization and incomplete validation. To identify key protocol aspects and ultimately support improving this situation, a comprehensive review of the literature on the level of harmonization/standardization and validation status of the ZETA has been conducted. The gaps and needed advances of the available ZETA protocols were evaluated and discussed with respect to its applicability as an alternative approach for teratogenicity assessment. Based on the review outcomes, a set of minimum reporting standards for the experimental protocol is proposed. Together with other initiatives towards implementation of alternative approaches at the screening and regulatory levels, the application of minimum reporting requirements is anticipated to support future method standardization and validation, as well as identifying potential improvement aspects. Present findings are expected to ultimately support advancing the ongoing validation initiatives towards the regulatory acceptance of the ZETA.

Exploring Neurobehaviour in Zebrafish Embryos as a Screening Model for Addictiveness of Substances

Tobacco use is the leading cause of preventable death worldwide and is highly addictive. Nicotine is the main addictive compound in tobacco, but less is known about other components and additives that may contribute to tobacco addiction. The zebrafish embryo (ZFE) has been shown to be a good model to study the toxic effects of chemicals on the neurological system and thus may be a promising model to study behavioral markers of nicotine effects, which may be predictive for addictiveness. We aimed to develop a testing protocol to study nicotine tolerance in ZFE using a locomotion test with light-dark transitions as behavioral trigger. Behavioral experiments were conducted using three exposure paradigms: (1) Acute exposure to determine nicotine’s effect and potency. (2) Pre-treatment with nicotine dose range followed by a single dose of nicotine, to determine which pre-treatment dose is sufficient to affect the potency of acute nicotine. (3) Pre-treatment with a single dose combined with acute exposure to a dose range to confirm the hypothesized decreased potency of the acute nicotine exposure. These exposure paradigms showed that (1) acute nicotine exposure decreased ZFE activity in response to dark conditions in a dose-dependent fashion; (2) pre-treatment with increasing concentrations dose-dependently reversed the effect of acute nicotine exposure; and (3) a fixed pre-treatment dose of nicotine induced a decreased potency of the acute nicotine exposure. This effect supported the induction of tolerance to nicotine by the pre-treatment, likely through neuroadaptation. The interpretation of these effects, particularly in view of prediction of dependence and addictiveness, and suitability of the ZFE model to test for such effects of other compounds than nicotine, are discussed.

Preliminary Insight of Pyrrolylated-Chalcones as New Anti-Methicillin-Resistant Staphylococcus aureus (Anti-MRSA) Agents

Bacterial infections are regarded as one of the leading causes of fatal morbidity and death in patients infected with diseases. The ability of microorganisms, particularly methicillin-resistant Staphylococcus aureus (MRSA), to develop resistance to current drugs has evoked the need for a continuous search for new drugs with better efficacies. Hence, a series of non-PAINS associated pyrrolylated-chalcones (1–15) were synthesized and evaluated for their potency against MRSA. The hydroxyl-containing compounds (8, 9, and 10) showed the most significant anti-MRSA efficiency, with the MIC and MBC values ranging from 0.08 to 0.70 mg/mL and 0.16 to 1.88 mg/mL, respectively. The time-kill curve and SEM analyses exhibited bacterial cell death within four hours after exposure to 9, suggesting its bactericidal properties. Furthermore, the docking simulation between 9 and penicillin-binding protein 2a (PBP2a, PDB ID: 6Q9N) suggests a relatively similar bonding interaction to the standard drug with a binding affinity score of −7.0 kcal/mol. Moreover, the zebrafish model showed no toxic effects in the normal embryonic development, blood vessel formation, and apoptosis when exposed to up to 40 µM of compound 9. The overall results suggest that the pyrrolylated-chalcones may be considered as a potential inhibitor in the design of new anti-MRSA agents.

A Systematic Review to Compare Chemical Hazard Predictions of the Zebrafish Embryotoxicity Test With Mammalian Prenatal Developmental Toxicity

Originally developed to inform the acute toxicity of chemicals on fish, the zebrafish embryotoxicity test (ZET) has also been proposed for assessing the prenatal developmental toxicity of chemicals, potentially replacing mammalian studies. Although extensively evaluated in primary studies, a comprehensive review summarizing the available evidence for the ZET's capacity is lacking. Therefore, we conducted a systematic review of how well the presence or absence of exposure-related findings in the ZET predicts prenatal development toxicity in studies with rats and rabbits. A two-tiered systematic review of the developmental toxicity literature was performed, a review of the ZET literature was followed by one of the mammalian literature. Data were extracted using DistillerSR, and study validity was assessed with an amended SYRCLE's risk-of-bias tool. Extracted data were analyzed for each species and substance, which provided the basis for comparing the 2 test methods. Although limited by the number of 24 included chemicals, our results suggest that the ZET has potential to identify chemicals that are mammalian prenatal developmental toxicants, with a tendency for overprediction. Furthermore, our analysis confirmed the need for further standardization of the ZET. In addition, we identified contextual and methodological challenges in the application of systematic review approaches to toxicological questions. One key to overcoming these challenges is a transition to more comprehensive and transparent planning, conduct and reporting of toxicological studies. The first step toward bringing about this change is to create broad awareness in the toxicological community of the need for and benefits of more evidence-based approaches.

Effects of humic substances on the aqueous stability of cerium dioxide nanoparticles and their toxicity to aquatic organisms

The impacts of humic substances (HS) on the aquatic stability and toxicity of nano‑cerium dioxide (nCeO₂) to three organisms with different exposure characteristics were investigated. Addition of HS to suspensions of nCeO₂ lowered the surface zeta potential of the particles, reduced their hydrodynamic size, and increased the energy barrier as indicated by the total potential energy profile. This resulted in a more stable suspension compared to suspensions without HS added. Moreover, a higher concentration of HS further stabilized nCeO₂ in the suspension. Acute toxicity of the suspensions to the unicellular green alga Raphidocelis subcapitata and to the crustacean Chydorus sphaericus was lower as compared to exposure without HS added. The acute toxicity of nCeO₂ suspensions to the zebrafish (Danio rerio) eleutheroembryo was on the other hand significantly enhanced (additive and synergistic) upon increasing HS concentration. Our findings emphasize that HS is important to stabilize the nano-suspensions and that its impact on nCeO₂ toxicity differs across different aquatic organisms. Emphasizing the exposure characteristics of each of the organisms selected from the trophic levels can explain how particle stability impacts particle toxicity.

Embryonic toxicity of epoxiconazole exposure to the early life stage of zebrafish

Epoxiconazole (EPX), as a broad-spectrum triazole fungicide, is widely used in agriculture to resist pests and diseases, while it may have potential toxicity to non-target organisms. In the present study, early developmental stage zebrafish were used as the subject organisms to assess the toxicity of EPX, and the possible mechanism of toxicity was also discussed by biochemical and transcriptomic analysis. Through embryo toxicity test, we had made it clear that the 96 h LC₅₀ of embryo was 7.204 mg/L, and acute exposure to EPX effected hatching rate, heartbeats, body length and even morphological defects. Then, by being exposed to EPX for 7 days at concentrations of 175 (1/40 LC₅₀), 350 (1/20 LC₅₀) and 700 (1/10 LC₅₀), biochemical parameters were affected, mainly manifested as increase of the triglyceride (TG) level and decrease of glucose content. Correspondingly, the transcription of genes related of glucose metabolism, lipid metabolism and cholesterol metabolism were also affected significantly in larval zebrafish. Moreover, some pathways, including lipid metabolism, glucose metabolism and amino acid metabolism were affected through transcriptome sequencing analysis in the larval zebrafish. Further data analysis based on the sequencing, EPX exposure also affected the expression of genes related to cell apoptosis. We further conformed that the bright fluorescence on the liver and bright spots near the liver by acridine orange staining. In addition, the mRNA levels of apoptosis related genes were also significantly affected in the EPX exposed larval zebrafish. Taken together, the work could provide an insight into toxic effects of EPX on the zebrafish larvae at embryo toxicity and transcriptional levels, providing some evidences for the toxic effects of triazole fungicides on non-target organisms.

An adverse outcome pathway on the disruption of retinoic acid metabolism leading to developmental craniofacial defects

Adverse outcome pathway (AOP) is a conceptual framework that links a molecular initiating event (MIE) via intermediate key events (KEs) with adverse effects (adverse outcomes, AO) relevant for risk assessment, through defined KE relationships (KERs). The aim of the present work is to describe a linear AOP, supported by experimental data, for skeletal craniofacial defects as the AO. This AO was selected in view of its relative high incidence in humans and the suspected relation to chemical exposure. We focused on inhibition of CYP26, a retinoic acid (RA) metabolizing enzyme, as MIE, based on robust previously published data. Conazoles were selected as representative stressors. Intermediate KEs are RA disbalance, aberrant HOX gene expression, disrupted specification, migration, and differentiation of neural crest cells, and branchial arch dysmorphology. We described the biological basis of the postulated events and conducted weight of evidence (WoE) assessments. The biological plausibility and the overall empirical evidence were assessed as high and moderate, respectively, the latter taking into consideration the moderate evidence for concordance of dose-response and temporal relationships. Finally, the essentiality assessment of the KEs, considered as high, supported the robustness of the presented AOP. This AOP, which appears of relevance to humans, thus contributes to mechanistic underpinning of selected test methods, thereby supporting their application in integrated new approach test methodologies and strategies and application in a regulatory context.