Screening for angiogenic inhibitors in zebrafish to evaluate a predictive model for developmental vascular toxicity

Dynamics in zebrafish development define transcriptomic specificity after angiogenesis inhibitor exposure

Testing for developmental toxicity is an integral part of chemical regulations. The applied tests are laborious and costly and require a large number of vertebrate test animals. To reduce animal numbers and associated costs, the zebrafish embryo was proposed as an alternative model. In this study, we investigated the potential of transcriptome analysis in the zebrafish embryo model to support the identification of potential biomarkers for key events in developmental toxicity, using the inhibition of angiogenesis as a proof of principle. Therefore, the effects on the zebrafish transcriptome after exposure to the tyrosine kinase inhibitors, sorafenib (1.3 µM and 2.4 µM) and SU4312 (1 µM, 2 µM, and 5 µM), and the putative vascular disruptor compound rotenone (25 nM and 50 nM) were analyzed. An early (2 hpf-hours post fertilization) and a late (24 hpf) exposure start with a time resolved transcriptome analysis was performed to compare the specificity and sensitivity of the responses with respect to anti-angiogenesis. We also showed that toxicodynamic responses were related to the course of the internal concentrations. To identify differentially expressed genes (DEGs) the time series data were compared by applying generalized additive models (GAMs). We observed mainly unspecific developmental toxicity in the early exposure scenario, while a specific repression of vascular related genes was only partially observed. In contrast, differential expression of vascular-related genes could be identified clearly in the late exposure scenario. Rotenone did not show angiogenesis-specific response on a transcriptomic level, indicating that the observed mild phenotype of angiogenesis inhibition may represent a secondary effect.

Developmental toxicity assay of xanthatin in zebrafish embryos

Xanthatin (XAN), a xanthanolide sesquiterpene lactone, isolated from Chinese herb, Xanthium strumarium L, has various pharmacological activities, such as antitumor activity and anti-inflammatory. However, little is known about its potential toxicity and the mechanism. Here, zebrafish model was used to study the developmental toxicity in vivo. Our results indicated that xanthatin increased the mortality and led to the morphological abnormalities including pericardial edema, yolk sac edema, curved body shape and hatching delay. Furthermore, xanthatin damaged the normal structure and/or function of heart, liver, immune and nervous system. ROS elevation and much more apoptosis cells were observed after xanthatin exposure. Gene expression results showed that oxidative stress-related genes nrf2 was inhibited, while oxidative stress-related genes (keap1 and nqo1) and apoptotic genes (caspase3, caspase9 and p53) were increased after xanthatin exposure. Mitophagy related genes pink1 and parkin, and wnt pathway (β-catenin, wnt8a and wnt11) were significantly increased after xanthatin exposure. Taken together, our finding indicated that xanthatin induced developmental toxicity, and the ROS elevation, apoptosis activation, dysregulation of mitophagy and wnt pathways were involved in the toxicity caused by xanthatin.

Investigation of Peroxisome Proliferator-Activated Receptor Genes as Requirements for Visual Startle Response Hyperactivity in Larval Zebrafish Exposed to Structurally Similar Per- and Polyfluoroalkyl Substances (PFAS)

BACKGROUND

Per- and polyfluoroalkyl Substances (PFAS) are synthetic chemicals widely detected in humans and the environment. Exposure to perfluorooctanesulfonic acid (PFOS) or perfluorohexanesulfonic acid (PFHxS) was previously shown to cause dark-phase hyperactivity in larval zebrafish.

OBJECTIVES

The objective of this study was to elucidate the mechanism by which PFOS or PFHxS exposure caused hyperactivity in larval zebrafish.

METHODS

Swimming behavior was assessed in 5-d postfertilization (dpf) larvae following developmental (1-4 dpf) or acute (5 dpf) exposure to 0.43 - 7.86 μ M PFOS, 7.87 - 120 μ M PFHxS, or 0.4% dimethyl sulfoxide (DMSO). After developmental exposure and chemical washout at 4 dpf, behavior was also assessed at 5-8 dpf. RNA sequencing was used to identify differences in global gene expression to perform transcriptomic benchmark concentration-response (BMC T ) modeling, and predict upstream regulators in PFOS- or PFHxS-exposed larvae. CRISPR/Cas9-based gene editing was used to knockdown peroxisome proliferator-activated receptors (ppars) pparaa/ab, pparda/db, or pparg at day 0. Knockdown crispants were exposed to 7.86 μ M PFOS or 0.4% DMSO from 1-4 dpf and behavior was assessed at 5 dpf. Coexposure with the ppard antagonist GSK3787 and PFOS was also performed.

RESULTS

Transient dark-phase hyperactivity occurred following developmental or acute exposure to PFOS or PFHxS, relative to the DMSO control. In contrast, visual startle response (VSR) hyperactivity only occurred following developmental exposure and was irreversible up to 8 dpf. Similar global transcriptomic profiles, BMC T estimates, and enriched functions were observed in PFOS- and PFHxS-exposed larvae, and ppars were identified as putative upstream regulators. Knockdown of pparda/db, but not pparaa/ab or pparg, blunted PFOS-dependent VSR hyperactivity to control levels. This finding was confirmed via antagonism of ppard in PFOS-exposed larvae.

DISCUSSION

This work identifies a novel adverse outcome pathway for VSR hyperactivity in larval zebrafish. We demonstrate that developmental, but not acute, exposure to PFOS triggered persistent VSR hyperactivity that required ppard function. https://doi.org/10.1289/EHP13667.

Development of an Automated Morphometric Approach to Assess Vascular Outcomes following Exposure to Environmental Chemicals in Zebrafish

BACKGROUND

Disruptions in vascular formation attributable to chemical insults is a pivotal risk factor or potential etiology of developmental defects and various disease settings. Among the thousands of chemicals threatening human health, the highly concerning groups prevalent in the environment and detected in biological monitoring in the general population ought to be prioritized because of their high exposure risks. However, the impacts of a large number of environmental chemicals on vasculature are far from understood. The angioarchitecture complexity and technical limitations make it challenging to analyze the entire vasculature efficiently and identify subtle changes through a high-throughput in vivo assay.

OBJECTIVES

We aimed to develop an automated morphometric approach for the vascular profile and assess the vascular morphology of health-concerning environmental chemicals.

METHODS

High-resolution images of the entire vasculature in Tg(fli1a:eGFP) zebrafish were collected using a high-content imaging platform. We established a deep learning-based quantitative framework, ECA-ResXUnet, combined with MATLAB to segment the vascular networks and extract features. Vessel scores based on the rates of morphological changes were calculated to rank vascular toxicity. Potential biomarkers were identified by vessel-endothelium-gene-disease integrative analysis.

RESULTS

Whole-trunk blood vessels and the cerebral vasculature in larvae exposed to 150 representative chemicals were automatically segmented as comparable to human-level accuracy, with sensitivity and specificity of 95.56% and 95.81%, respectively. Chemical treatments led to heterogeneous vascular patterns manifested by 31 architecture indexes, and the common cardinal vein (CCV) was the most affected vessel. The antipsychotic medicine haloperidol, flame retardant 2,2-bis(chloromethyl)trimethylenebis[bis(2-chloroethyl) phosphate], and tert-butylphenyl diphenyl phosphate ranked as the top three in vessel scores. Pesticides accounted for the largest group, with a vessel score of ≥ 1 , characterized by a remarkable inhibition of subintestinal venous plexus and delayed development of CCV. Multiple-concentration evaluation of nine per- and polyfluoroalkyl substances (PFAS) indicated a low-concentration effect on vascular impairment and a positive association between carbon chain length and benchmark concentration. Target vessel-directed single-cell RNA sequencing of f l i 1 a + cells from larvae treated with λ -cyhalothrin , perfluorohexanesulfonic acid, or benzylbutyl phthalate, along with vessel-endothelium-gene-disease integrative analysis, uncovered potential associations with vascular disorders and identified biomarker candidates.

DISCUSSION

This study provides a novel paradigm for phenotype-driven screenings of vascular-disrupting chemicals by converging morphological and transcriptomic profiles at a high-resolution level, serving as a powerful tool for large-scale toxicity tests. Our approach and the high-quality morphometric data facilitate the precise evaluation of vascular effects caused by environmental chemicals. https://doi.org/10.1289/EHP13214.

Analysis of vascular disruption in zebrafish embryos as an endpoint to predict developmental toxicity

Inhibition of angiogenesis is an important mode of action for the teratogenic effect of chemicals and drugs. There is a gap in the availability of simple, experimental screening models for the detection of angiogenesis inhibition. The zebrafish embryo represents an alternative test system which offers the complexity of developmental differentiation of an entire organism while allowing for small-scale and high-throughput screening. Here we present a novel automated imaging-based method to detect the inhibition of angiogenesis in early life stage zebrafish. Video subtraction was used to identify the location and number of functional intersegmental vessels according to the detection of moving blood cells. By exposing embryos to multiple tyrosine kinase inhibitors including SU4312, SU5416, Sorafenib, or PTK787, we confirmed that this method can detect concentration-dependent inhibition of angiogenesis. Parallel assessment of arterial and venal aorta ruled out a potential bias by impaired heart or blood cell development. In contrast, the histone deacetylase inhibitor valproic acid did not affect ISV formation supporting the specificity of the angiogenic effects. The new test method showed higher sensitivity, i.e. lower effect concentrations, relative to a fluorescent reporter gene strain (Tg(KDR:EGFP)) exposed to the same tyrosine kinase inhibitors indicating that functional effects due to altered tubulogenesis or blood transport can be detected before structural changes of the endothelium are visible by fluorescence imaging. Comparison of exposure windows indicated higher specificity for angiogenesis when exposure started at later embryonic stages (24 h post-fertilization). One of the test compounds was showing particularly high specificity for angiogenesis effects (SU4312) and was, therefore, suggested as a model compound for the identification of molecular markers of angiogenic disruption. Our findings establish video imaging in wild-type strains as viable, non-invasive, high-throughput method for the detection of chemical-induced angiogenic disruption in zebrafish embryos.

Embryonic exposure to chloroxylenol induces developmental defects and cardiovascular toxicity via oxidative stress, inflammation, and apoptosis in zebrafish

Chloroxylenol is an extensively consumed anti-microbial compound. Since its usage is on the rise due to the coronavirus pandemic and ban on other antimicrobial ingredients, recent studies have suggested the necessity of estimating its potential for ecotoxicity. The detrimental effect of chloroxylenol on zebrafish (Danio rerio) viability has been reported; however, research on the mechanisms underlying its toxicity is quite limited. Therefore, we applied the zebrafish model for elucidating responses against chloroxylenol to predict its toxicity toward human health and ecology. Zebrafish exposed to chloroxylenol (0, 0.5, 1, 2.5, 5, and 10 mg/L) at the embryonic stage (from 6 h post-fertilization (hpf) to 96 hpf) showed impaired viability and hatchability, and pathological phenotypes. To address these abnormalities, cellular responses such as oxidative stress, inflammation, and apoptosis were confirmed via in vivo imaging of a fluorescent dye or measurement of the transcriptional changes related to each response. In particular, developmental defects in the cardiovascular system of zebrafish exposed to 0, 0.5, 1, and 2.5 mg/L of chloroxylenol from 6 to 96 hpf were identified by structural analyses of the system in the flk1:eGFP transgenic line. Additional experiments were conducted using human umbilical vein endothelial cells (HUVECs) to predict the adverse impacts of chloroxylenol on the human vascular system. Chloroxylenol impairs the viability and tube formation ability of HUVECs by modulating ERK signaling. The findings obtained using the zebrafish model provide evidence of the possible risks of chloroxylenol exposure and suggest the importance of more in-depth ecotoxicological studies.

Prioritization of chemicals in food for risk assessment by integrating exposure estimates and new approach methodologies: A next generation risk assessment case study

Next generation risk assessment is defined as a knowledge-driven system that allows for cost-efficient assessment of human health risk related to chemical exposure, without animal experimentation. One of the key features of next generation risk assessment is to facilitate prioritization of chemical substances that need a more extensive toxicological evaluation, in order to address the need to assess an increasing number of substances. In this case study focusing on chemicals in food, we explored how exposure data combined with the Threshold of Toxicological Concern (TTC) concept could be used to prioritize chemicals, both for existing substances and new substances entering the market. Using a database of existing chemicals relevant for dietary exposure we calculated exposure estimates, followed by application of the TTC concept to identify substances of higher concern. Subsequently, a selected set of these priority substances was screened for toxicological potential using high-throughput screening (HTS) approaches. Remarkably, this approach resulted in alerts for a selection of substances that are already on the market and represent relevant exposure in consumers. Taken together, the case study provides proof-of-principle for the approach taken to identify substances of concern, and this approach can therefore be considered a supportive element to a next generation risk assessment strategy.

Advances in computational methods along the exposure to toxicological response paradigm

Human health risk assessment is a function of chemical toxicity, bioavailability to reach target biological tissues, and potential environmental exposure. These factors are complicated by many physiological, biochemical, physical and lifestyle factors. Furthermore, chemical health risk assessment is challenging in view of the large, and continually increasing, number of chemicals found in the environment. These challenges highlight the need to prioritize resources for the efficient and timely assessment of those environmental chemicals that pose greatest health risks. Computational methods, either predictive or investigative, are designed to assist in this prioritization in view of the lack of cost prohibitive in vivo experimental data. Computational methods provide specific and focused toxicity information using in vitro high throughput screening (HTS) assays. Information from the HTS assays can be converted to in vivo estimates of chemical levels in blood or target tissue, which in turn are converted to in vivo dose estimates that can be compared to exposure levels of the screened chemicals. This manuscript provides a review for the landscape of computational methods developed and used at the U.S. Environmental Protection Agency (EPA) highlighting their potentials and challenges.

Predicting Prenatal Developmental Toxicity Based On the Combination of Chemical Structures and Biological Data

For hazard identification, classification, and labeling purposes, animal testing guidelines are required by law to evaluate the developmental toxicity potential of new and existing chemical products. However, guideline developmental toxicity studies are costly, time-consuming, and require many laboratory animals. Computational modeling has emerged as a promising, animal-sparing, and cost-effective method for evaluating the developmental toxicity potential of chemicals, such as endocrine disruptors, without the use of animals. We aimed to develop a predictive and explainable computational model for developmental toxicants. To this end, a comprehensive dataset of 1244 chemicals with developmental toxicity classifications was curated from public repositories and literature sources. Data from 2140 toxicological high-throughput screening assays were extracted from PubChem and the ToxCast program for this dataset and combined with information about 834 chemical fragments to group assays based on their chemical-mechanistic relationships. This effort revealed two assay clusters containing 83 and 76 assays, respectively, with high positive predictive rates for developmental toxicants identified with animal testing guidelines (PPV = 72.4 and 77.3% during cross-validation). These two assay clusters can be used as developmental toxicity models and were applied to predict new chemicals for external validation. This study provides a new strategy for constructing alternative chemical developmental toxicity evaluations that can be replicated for other toxicity modeling studies.

A systematic comparison of the developmental vascular toxicity of bisphenol A and its alternatives in vivo and in vitro

Due to the potential toxicity of bisphenol A (BPA), several bisphenols (BPs), including bisphenol F (BPF), bisphenol S (BPS) and bisphenol AF (BPAF), have been gradually used as its main substitutes, and the levels of these alternatives in different environmental media have been constantly increasing. Although some previous studies have shown that bisphenol substitutes have similar or greater acute toxicity and estrogenic effects than BPA, comparative studies on the cardiovascular toxicity of BPs have not been evaluated. In this study, the developmental vascular toxicity of BPA and three predominant substitutes (BPF, BPS and BPAF) were evaluated using zebrafish embryos and human vascular endothelial cells (HUVECs). BP exposure at a sublethal concentration of 1/10 96 h median lethal concentration (96 h-LC₅₀) significantly hindered intersegmental vessel (ISV) growth, delayed common cardinal vein (CCV) remodeling and decreased subintestinal vessels (SIVs) in Tg (fli1:EGFP) zebrafish embryos. Meanwhile, the results of the endothelial tube formation assay showed that in vitro angiogenesis was inhibited by BP exposure. Mechanistically, BP exposure increased oxidative stress characterized by a significant decrease in superoxide dismutase (SOD) and catalase (CAT) activity, accompanied by increased levels of malondialdehyde (MDA) and reactive oxygen species (ROS) in both zebrafish and HUVECs. Therefore, the vascular toxicity and oxidative stress potency of the BPs were compared and evaluated, ranking as follows: BPAF > BPF > BPA > BPS. To the best of our knowledge, the present work, for the first time, systematically provides direct evidence for BPA and its alternatives on developmental vascular toxicity in vitro and in vivo. Therefore, these findings will provide insight into the rational and safe application of BPA substitutes.

Toxic effects of triclocarban on larval zebrafish: A focus on visual dysfunction

Triclocarban (TCC) is considered an endocrine disruptor and shows antagonist activity on thyroid receptors. In view of the report that thyroid hormone signaling mediates retinal cone photoreceptor specification, we hypothesize that TCC could impair visual function, which is vital to wildlife. In order to verify our hypothesis, we assessed alteration in the retinal structure (retinal layer thickness and cell density), visually-mediated behavior, cone and rod opsin gene expression, and photoreceptor immunostaining in zebrafish larvae exposed to TCC at environmentally realistic concentrations (0.16 ± 0.005 µg/L, L-group) and one-fifth of the median lethal concentrations (25.4 ± 1.02 µg/L, H-group). Significant decrease in eye size, ganglion cell density, optokinetic response, and phototactic response can be observed in the L-group, while the thickness of outer nuclear layer, where the cell bodies of cone and rod cells are located, was significantly reduced with the down-regulation of critical opsin gene (opn1sw2, opn1mw1, opn1mw3, opn1lw1, opn1lw2, and rho) expression and rhodopsin immunofluorescence in the H-group. It should be noted that TCC could affect the sensitivity of zebrafish larvae to red and green light according to the results of behavioral and opsin gene expression analysis. These findings provide the first evidence to support our hypothesis that the visual system, a novel toxicological target, is affected by TCC. Consequently, we urgently call for a more in-depth exploration of TCC-induced ocular toxicity to aquatic organisms and even to humans.

Liver toxicity of macrolide antibiotics in zebrafish

Macrolide antibiotics (macrolides) are among the most commonly prescribed antibiotics worldwide and are used for a wide range of infections, but macrolides also expose people to the risk of adverse events include hepatotoxicity. Here, we report the liver toxicity of macrolides with different structures in zebrafish. The absorption, distribution, metabolism, excretion and toxicology (ADMET) parameters of macrolide compounds were predicted and contrasted by utilizing in silico analysis. Fluorescence imaging and Oil Red O stain assays showed all the tested macrolide drugs induced liver degeneration, changed liver size and liver steatosis in larval zebrafish. Through RNA-seq analysis, we found seven co-regulated differentially expressed genes (co-DEGs) associated with metabolism, apoptosis and immune system biological processes, and two co-regulated significant pathways including amino sugar and nucleotide sugar metabolism and apoptosis signaling pathway. We found that only fosab of seven co-DEGs was in the two co-regulated significant pathways. fosab encoded proto-oncogene c-Fos, which was closely associated with liver diseases. The whole-mount in situ hybridization showed high transcription of c-Fos induced by macrolide compounds mainly in the liver region of zebrafish larvae. Cell Counting Kit-8 (CCK-8) and lactate dehydrogenase (LDH) leakage assays revealed that macrolides exerts significant cytotoxic effects on L02 cells. qRT-PCR and western blot analysis demonstrated macrolides also promoted human c-Fos expression in L02 cells. The c-Fos overexpression significantly reduced cell viability by using CCK-8 assay. These data indicate that hepatotoxicity induced by macrolides may be correlated with c-Fos expression activated by these compounds. This study may provide a biomarker for the further investigations on the mechanism of hepatotoxicity induced by macrolide drugs with different structures, and extend our understanding for improving rational clinical application of macrolides.

Evaluation of Developmental Toxicity, Developmental Neurotoxicity, and Tissue Dose in Zebrafish Exposed to GenX and Other PFAS

BACKGROUND

Per- and polyfluoroalkyl substances (PFAS) are a diverse class of industrial chemicals with widespread environmental occurrence. Exposure to long-chain PFAS is associated with developmental toxicity, prompting their replacement with short-chain and fluoroether compounds. There is growing public concern over the safety of replacement PFAS.

OBJECTIVE

We aimed to group PFAS based on shared toxicity phenotypes.

METHODS

Zebrafish were developmentally exposed to 4,8-dioxa-3H-perfluorononanoate (ADONA), perfluoro-2-propoxypropanoic acid (GenX Free Acid), perfluoro-3,6-dioxa-4-methyl-7-octene-1-sulfonic acid (PFESA1), perfluorohexanesulfonic acid (PFHxS), perfluorohexanoic acid (PFHxA), perfluoro-n-octanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), or 0.4% dimethyl sulfoxide (DMSO) daily from 0-5 d post fertilization (dpf). At 6 dpf, developmental toxicity and developmental neurotoxicity assays were performed, and targeted analytical chemistry was used to measure media and tissue doses. To test whether aliphatic sulfonic acid PFAS cause the same toxicity phenotypes, perfluorobutanesulfonic acid (PFBS; 4-carbon), perfluoropentanesulfonic acid (PFPeS; 5-carbon), PFHxS (6-carbon), perfluoroheptanesulfonic acid (PFHpS; 7-carbon), and PFOS (8-carbon) were evaluated.

RESULTS

PFHxS or PFOS exposure caused failed swim bladder inflation, abnormal ventroflexion of the tail, and hyperactivity at nonteratogenic concentrations. Exposure to PFHxA resulted in a unique hyperactivity signature. ADONA, PFESA1, or PFOA exposure resulted in detectable levels of parent compound in larval tissue but yielded negative toxicity results. GenX was unstable in DMSO, but stable and negative for toxicity when diluted in deionized water. Exposure to PFPeS, PFHxS, PFHpS, or PFOS resulted in a shared toxicity phenotype characterized by body axis and swim bladder defects and hyperactivity.

CONCLUSIONS

All emerging fluoroether PFAS tested were negative for evaluated outcomes. Two unique toxicity signatures were identified arising from structurally dissimilar PFAS. Among sulfonic acid aliphatic PFAS, chemical potencies were correlated with increasing carbon chain length for developmental neurotoxicity, but not developmental toxicity. This study identified relationships between chemical structures and in vivo phenotypes that may arise from shared mechanisms of PFAS toxicity. These data suggest that developmental neurotoxicity is an important end point to consider for this class of widely occurring environmental chemicals. https://doi.org/10.1289/EHP5843.

Ibuprofen and diclofenac impair the cardiovascular development of zebrafish (Danio rerio) at low concentrations

The non-steroidal anti-inflammatory drugs (NSAIDs) ibuprofen and diclofenac are highly prescribed worldwide and their presence in aquatic system may pose a potential risk to aquatic organisms. Here, we systematically assessed their cardiovascular disruptive effects in zebrafish (Danio rerio) at environmentally relevant concentrations between 0.04 and 25.0 μg/L. Ibuprofen significantly increased the cardiac outputs of zebrafish embryos at actual concentrations of 0.91, 4.3 and 21.9 μg/L. It up-regulated the blood cell velocity, total blood flow and down-regulated the blood cell density at concentrations of 4.3 μg/L and higher. In comparison, diclofenac led to inhibition of spontaneous muscle contractions and decreased hatching rate of zebrafish embryos at the highest concentration (24.1 μg/L), while it had negligible effects on the cardiac physiology and hemodynamics. Transcriptional analysis of biomarker genes involved in cardiovascular physiology, such as the significantly up-regulated nppa and nkx2.5 expressions response to ibuprofen but not to diclofenac, is consistent with these observations. In addition, both ibuprofen and diclofenac altered the morphology of intersegmental vessels at high concentrations. Our results revealed unexpected cardiovascular functional alterations of NSAIDs to fish at environmental or slightly higher than surface water concentrations and thus provided novel insights into the understanding of their potential environmental risks.

A Case Study with Triazole Fungicides to Explore Practical Application of Next-Generation Hazard Assessment Methods for Human Health

The ongoing developments in chemical risk assessment have led to new concepts building on integration of sophisticated nonanimal models for hazard characterization. Here we explore a pragmatic approach for implementing such concepts, using a case study of three triazole fungicides, namely, flusilazole, propiconazole, and cyproconazole. The strategy applied starts with evaluating the overall level of concern by comparing exposure estimates to toxicological potential, followed by a combination of in silico tools and literature-derived high-throughput screening assays and computational elaborations to obtain insight into potential toxicological mechanisms and targets in the organism. Additionally, some targeted in vitro tests were evaluated for their utility to confirm suspected mechanisms of toxicity and to generate points of departure. Toxicological mechanisms instead of the current "end point-by-end point" approach should guide the selection of methods and assays that constitute a toolbox for next-generation risk assessment. Comparison of the obtained in silico and in vitro results with data from traditional in vivo testing revealed that, overall, nonanimal methods for hazard identification can produce adequate qualitative hazard information for risk assessment. Follow-up studies are needed to further refine the proposed approach, including the composition of the toolbox, toxicokinetics models, and models for exposure assessment.

Evaluation of the Developmental Toxicity Induced by E804 in Zebrafish Embryos

E804, a derivative of indirubin, have multi-biological activities such as anticancer and anti-inflammatory activities, but little is known about its developmental toxicity. In this study, we investigated the toxicity of E804 on the developments of zebrafish embryos. Our results showed that E804 treatment caused a significant increase of the malformation rate compared with the control groups. Pericardial edema and curved body shape were the most morphological abnormalities observed in E804-treated group. The hatching rates and body length of the zebrafish larvae was significantly decreased in E804-treated groups. E804 also affect the development of heart, liver, phagocytes and vascular formation. Further studies showed that the level of reactive oxygen species was significantly increased. The activity of total superoxide dismutase decreased and the concentration of malondialdehyde were increased. Much more apoptotic cells were detected in E804-treated group, compared with the control. In addition, gene-expression results showed that the pathways of oxidative stress and apoptosis were provoked in E804 treated groups. Taken together, our findings will be helpful to understanding E804-induced developmental toxicity and the underlying mechanism.

PHMH, a diarylheptanoid from Alpinia officinarum attenuates VEGF-induced angiogenesis via inhibition of the VEGFR-2 signaling pathway

The rhizome of Alpinia officinarum Hance, a popular spice used as a condiment in China and Europe, has various reported bioactivities, including anticancer, anti-inflammatory and antioxidant effects. However, its anti-angiogenic activity has not previously been reported. In this study, a diarylheptanoid was isolated from Alpinia officinarum and identified as 1-phenyl-7-(4-hydroxy-3-methoxyphenyl)-4E-en-3-heptanone (PHMH). We demonstrated that PHMH exerts anti-angiogenic activity both in vitro and in vivo. PHMH inhibited vascular endothelial growth factor (VEGF)-induced viability, migration, invasion and tube formation in human umbilical vein endothelial cells (HUVECs) in vitro, and also suppressed VEGF-induced sprout formation of rat aorta ex vivo. Furthermore, PHMH was found to block VEGF-induced vessel formation in mice and suppress angiogenesis in both zebrafish and chorioallantoic membrane models. Mechanistic studies indicated that PHMH inhibited VEGF-induced VEGF receptor-2 (VEGFR-2) auto-phosphorylation and resulted in the blockage of VEGFR-2-mediated signaling cascades in HUVECs, including the Akt/mTOR, ERK1/2, and FAK pathways. Our findings provide new insights into the potential application of PHMH as a therapeutic agent for anti-angiogenesis.

Translational Toxicology in Zebrafish

A major goal of translational toxicology is to identify adverse chemical effects and determine whether they are conserved or divergent across experimental systems. Translational toxicology encompasses assessment of chemical toxicity across multiple life stages, determination of toxic mode-of-action, computational prediction modeling, and identification of interventions that protect or restore health following toxic chemical exposures. The zebrafish is increasingly used in translational toxicology because it combines the genetic and physiological advantages of mammalian models with the higher-throughput capabilities and genetic manipulability of invertebrate models. Here, we review recent literature demonstrating the power of the zebrafish as a model for addressing all four activities of translational toxicology. Important data gaps and challenges associated with using zebrafish for translational toxicology are also discussed.

Systems Modeling of Developmental Vascular Toxicity

The more than 80,000 chemicals in commerce present a challenge for hazard assessments that toxicity testing in the 21^st century strives to address through high-throughput screening (HTS) assays. Assessing chemical effects on human development adds an additional layer of complexity to the screening, with a need to capture complex and dynamic events essential for proper embryo-fetal development. HTS data from ToxCast/Tox21 informs systems toxicology models, which incorporate molecular targets and biological pathways into mechanistic models describing the effects of chemicals on human cells, 3D organotypic culture models, and small model organisms. Adverse Outcome Pathways (AOPs) provide a useful framework for integrating the evidence derived from these in silico and in vitro systems to inform chemical hazard characterization. To illustrate this formulation, we have built an AOP for developmental toxicity through a mode of action linked to embryonic vascular disruption (Aop43). Here, we review the model for quantitative prediction of developmental vascular toxicity from ToxCast HTS data and compare the HTS results to functional vascular development assays in complex cell systems, virtual tissues, and small model organisms. ToxCast HTS predictions from several published and unpublished assays covering different aspects of the angiogenic cycle were generated for a test set of 38 chemicals representing a range of putative vascular disrupting compounds (pVDCs). Results boost confidence in the capacity to predict adverse developmental outcomes from HTS in vitro data and model computational dynamics for in silico reconstruction of developmental systems biology. Finally, we demonstrate the integration of the AOP and developmental systems toxicology to investigate the unique modes of action of two angiogenesis inhibitors.

Orthogonal Drug Pooling Enhances Phenotype-Based Discovery of Ocular Antiangiogenic Drugs in Zebrafish Larvae

Unbiased screening of large randomized chemical libraries in vivo is a powerful tool to find new drugs and targets. However, forward chemical screens in zebrafish can be time consuming and usually >99% of test compounds have no significant effect on the desired phenotype. Here, we sought to find bioactive drugs more efficiently and to comply with the 3R principles of replacement, reduction, and refinement of animals in research. We investigated if pooling of drugs to simultaneously test 8–10 compounds in zebrafish larvae can increase the screening efficiency of an established assay that identifies drugs inhibiting developmental angiogenesis in the eye. In a phenotype-based screen, we tested 1,760 small molecule compounds from the ChemBridge DIVERSet™ chemical library for their ability to inhibit the formation of distinct primary hyaloid vessels in the eye. Applying orthogonal pooling of the chemical library, we treated zebrafish embryos from 3 to 5 days post fertilization with pools of 8 or 10 compounds at 10 μM each. This reduced the number of tests from 1,760 to 396. In 63% of cases, treatment showed sub-threshold effects of <40% reduction of primary hyaloid vessels. From 18 pool hits, we identified eight compounds that reduce hyaloid vessels in the larval zebrafish eye by at least 40%. Compound 4-[4-(1H-benzimidazol-2-yl)phenoxy]aniline ranked as the most promising candidate with reproducible and dose-dependent effects. To our knowledge, this is the first report of a self-deconvoluting matrix strategy applied to drug screening in zebrafish. We conclude that the orthogonal drug pooling strategy is a cost-effective, time-saving, and unbiased approach to discover novel inhibitors of developmental angiogenesis in the eye. Ultimately, this approach may identify new drugs or targets to mitigate disease caused by pathological angiogenesis in the eye, e.g., diabetic retinopathy or age-related macular degeneration, wherein blood vessel growth and leaky vessels lead to vision impairment or clinical blindness.

A multi-institutional study benchmarking the zebrafish developmental assay for prediction of embryotoxic plasma concentrations from rat embryo-fetal development studies

Predicting embryotoxicity of pharmaceutical compounds or industrial chemicals is crucial for public safety. Conventional studies which monitor embryo-fetal development in rats and rabbits are costly and time consuming. Alternative assays which are simpler and less costly are being pursued. The purpose of this research was to assess the capacity for the zebrafish development assay to predict mammalian plasma levels that are embryotoxic. Previously published data on rat plasma levels associated with embryotoxicity were used to guide concentration ranges for each of 25 chemicals dissolved in the media bathing developing zebrafish embryos. Embryotoxic media concentrations were compared to embryotoxic rat plasma concentrations. Assays were conducted in parallel at multiple sites as a consortium effort through the Health and Environmental Sciences Institute (HESI). Considering results from all sites, the zebrafish embryo development assay predicted (within 1-log) the rat maternal exposure levels associated with embryotoxicity 75% of the time.

Construction of a Quantitative Structure Activity Relationship (QSAR) Model to Predict the Absorption of Cephalosporins in Zebrafish for Toxicity Study

Cephalosporins are beta-lactam antibiotics that are widely used in China. Five generations of cephalosporins have been introduced in clinical practice to date; moreover, some new candidates are also undergoing clinical evaluations. To improve the success rates of new drug development, we need to have a comprehensive understanding about the relationship between the structure of cephalosporins and the toxicity that it induces at an early stage. In the cephalosporins toxicity study using zebrafish, the drug absorption is a key point. In this study, we determined the absorption of cephalosporins in zebrafish during toxicity test. The internal concentrations of 19 cephalosporins in zebrafish were determined using a developed liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. Furthermore, a quantitative structure-activity relationship (QSAR) model was established by multilinear regression; moreover, it was used to predict the absorption of cephalosporins in zebrafish. During leave-one-out cross-validation, a satisfactory performance was obtained with a predictive ability (q ²) of 0.839. The prediction ability of the model was further confirmed when the predictive ability (q ²) was 0.859 in external prediction. The best QSAR model, which was based on five molecular descriptors, exhibited a promising predictive performance and robustness. In experiments involving drug toxicity, the developed QSAR model was used to estimate internal concentrations of cephalosporins. Thus, the toxicity results were correlated with the internal concentration of the drug within the larvae. The developed model served as a new powerful tool in zebrafish toxicity tests.

Blood-brain barrier development: Systems modeling and predictive toxicology

The blood-brain barrier (BBB) serves as a gateway for passage of drugs, chemicals, nutrients, metabolites, and hormones between vascular and neural compartments in the brain. Here, we review BBB development with regard to the microphysiology of the neurovascular unit (NVU) and the impact of BBB disruption on brain development. Our focus is on modeling these complex systems. Extant in silico models are available as tools to predict the probability of drug/chemical passage across the BBB; in vitro platforms for high-throughput screening and high-content imaging provide novel data streams for profiling chemical-biological interactions; and engineered human cell-based microphysiological systems provide empirical models with which to investigate the dynamics of NVU function. Computational models are needed that bring together kinetic and dynamic aspects of NVU function across gestation and under various physiological and toxicological scenarios. This integration will inform adverse outcome pathways to reduce uncertainty in translating in vitro data and in silico models for use in risk assessments that aim to protect neurodevelopmental health.

Quantification of Estradiol Uptake in Zebrafish Embryos and Larvae

Zebrafish are a powerful model system to assess the molecular and cellular effects of exposure to toxic chemicals during embryonic development. To study the effects of environmental endocrine disruptors, embryos and larvae are commonly exposed to supraphysiologic concentrations of these compounds in the water, but their bioavailability in zebrafish is largely unknown. One hypothesis is that supraphysiologic concentrations of estrogens in the water are required to achieve physiologic levels in vivo; however, this has not been directly tested. To test this hypothesis, we developed an assay using radiolabeled estradiol ([3H]E2) to measure uptake from water at multiple concentrations and exposure durations in developing zebrafish from 0 to 5 days postfertilization (dpf). We found that [3H]E2 uptake increased with increasing concentration, duration, and developmental stage. Percent uptake from the total volume of treatment solution increased with increasing exposure duration and developmental stage, but remained constant with increasing concentration. We also found that the chorion, an acellular envelope surrounding embryos through 3 dpf, did not substantially affect [3H]E2 uptake. Finally, we found that at 1 dpf, E2 was preferentially taken up by the yolk at multiple exposure durations, while at 2 dpf E2 was preferentially taken up into the embryonic body. Our results support the hypothesis that exposing zebrafish embryos and larvae to supraphysiologic concentrations of estrogens is required to achieve physiologically relevant doses in vivo. The isotopic assay reported here will provide a foundation for determining the uptake of other compounds for teratogenicity, toxicology and drug discovery studies.

Assaying uptake of endocrine disruptor compounds in zebrafish embryos and larvae

To study the effects of environmental endocrine disruptor compounds (EDCs) on aquatic animals, embryos and larvae are typically incubated in water containing defined concentrations of EDCs. However, the amount of EDC uptake into the animal is often difficult to determine. Using radiolabeled estradiol ([³H]E2), we previously developed a rapid, straightforward assay to measure estradiol uptake from water into zebrafish embryos and larvae. Here, we extend this approach to measure the uptake of two additional EDCs, bisphenol A (BPA) and ethinyl estradiol (EE2). As with E2, the uptake of each compound by individual larvae was low (<6%), and increased with increasing concentration, duration, and developmental stage. We found that E2 and EE2 had similar uptake under equivalent exposure conditions, while BPA had comparatively lower uptake. One application of this assay is to test factors that influence EDC uptake or efflux. It has been suggested that persistent organic pollutants (POPs) inhibit ABC transporters that may normally efflux EDCs and their metabolites, inducing toxicity in aquatic organisms. We measured [³H]E2 levels in zebrafish in the presence or absence of the POP PDBE-100, and cyclosporine A, a known inhibitor of ABC transporters. Neither chemical significantly affected [³H]E2 levels in zebrafish, suggesting that zebrafish maintain estradiol efflux in the presence of PDBE-100, independently of cyclosporine A-responsive transporters. These uptake results will be a valuable reference for EDC exposure studies in developing zebrafish, and provide a rapid assay to screen for chemicals that influence estrogen-like EDC levels in vivo.

Cardiopulmonary consequences of gestational toxicant exposure: Symposium overview at the 56th annual SOT meeting, Baltimore, MD

Xenobiotic exposures affect the maternal and/or in utero environment resulting in impairments in fetal development. During the period of rapid fetal growth, developing cardiovascular systems are especially vulnerable to their environment. Furthermore, fetal exposures can evoke changes in epigenetic signatures that result in permanent modifications in gene expression. This symposium focused on the intersection between maternal and fetal exposure and the developing cardiovascular system. The impact of maternal exposures on prenatal development is of major concern for regulatory agencies given the unique vulnerability of the embryo/fetus to environmental factors, the importance of vascular biology to maternal-fetal interactions, and the adverse consequences of vascular disruption to children's health. Speakers provided data from diverse exposures: nanomaterials, particulate matter or air pollution (PM_2.5), nicotine, and environmental chemicals. The current findings related to susceptible gestational windows for cardiovascular development and epigenetic, transcriptomic, toxicokinetic, and toxicodynamic changes in vascular physiology and cardiac function. In response to these concerns, new concepts in predictive modeling and risk assessment associated with in utero exposures were presented as future avenues of research within developmental toxicology. Finally, current applications using an Adverse Outcome Pathway framework for developmental toxicity were presented to integrate data from in vitro profiling of chemical libraries (e.g. ToxCast™) with computational models for in silico toxicology. In summary, this symposium addressed the significant threats to cardiovascular health that are associated with fetal/perinatal exposures, and offered new insights into the predictive, mechanistic, and risk assessment strategies in developmental toxicology.

3S - Systematic, systemic, and systems biology and toxicology

A biological system is more than the sum of its parts - it accomplishes many functions via synergy. Deconstructing the system down to the molecular mechanism level necessitates the complement of reconstructing functions on all levels, i.e., in our conceptualization of biology and its perturbations, our experimental models and computer modelling. Toxicology contains the somewhat arbitrary subclass "systemic toxicities"; however, there is no relevant toxic insult or general disease that is not systemic. At least inflammation and repair are involved that require coordinated signaling mechanisms across the organism. However, the more body components involved, the greater the challenge to reca-pitulate such toxicities using non-animal models. Here, the shortcomings of current systemic testing and the development of alternative approaches are summarized. We argue that we need a systematic approach to integrating existing knowledge as exemplified by systematic reviews and other evidence-based approaches. Such knowledge can guide us in modelling these systems using bioengineering and virtual computer models, i.e., via systems biology or systems toxicology approaches. Experimental multi-organ-on-chip and microphysiological systems (MPS) provide a more physiological view of the organism, facilitating more comprehensive coverage of systemic toxicities, i.e., the perturbation on organism level, without using substitute organisms (animals). The next challenge is to establish disease models, i.e., micropathophysiological systems (MPPS), to expand their utility to encompass biomedicine. Combining computational and experimental systems approaches and the chal-lenges of validating them are discussed. The suggested 3S approach promises to leverage 21st century technology and systematic thinking to achieve a paradigm change in studying systemic effects.

Versatile synthetic alternatives to Matrigel for vascular toxicity screening and stem cell expansion

The physiological relevance of Matrigel as a cell-culture substrate and in angiogenesis assays is often called into question. Here, we describe an array-based method for the identification of synthetic hydrogels that promote the formation of robust in vitro vascular networks for the detection of putative vascular disruptors, and that support human embryonic stem cell expansion and pluripotency. We identified hydrogel substrates that promoted endothelial-network formation by primary human umbilical vein endothelial cells and by endothelial cells derived from human induced pluripotent stem cells, and used the hydrogels with endothelial networks to identify angiogenesis inhibitors. The synthetic hydrogels show superior sensitivity and reproducibility over Matrigel when evaluating known inhibitors, as well as in a blinded screen of a subset of 38 chemicals, selected according to predicted vascular disruption potential, from the Toxicity ForeCaster library of the US Environmental Protection Agency. The identified synthetic hydrogels should be suitable alternatives to Matrigel for common cell-culture applications.

Beyond organoids: In vitro vasculogenesis and angiogenesis using cells from mammals and zebrafish

The ability to culture complex organs is currently an important goal in biomedical research. It is possible to grow organoids (3D organ-like structures) in vitro; however, a major limitation of organoids, and other 3D culture systems, is the lack of a vascular network. Protocols developed for establishing in vitro vascular networks typically use human or rodent cells. A major technical challenge is the culture of functional (perfused) networks. In this rapidly advancing field, some microfluidic devices are now getting close to the goal of an artificially perfused vascular network. Another development is the emergence of the zebrafish as a complementary model to mammals. In this review, we discuss the culture of endothelial cells and vascular networks from mammalian cells, and examine the prospects for using zebrafish cells for this objective. We also look into the future and consider how vascular networks in vitro might be successfully perfused using microfluidic technology.

Embryonic vascular disruption adverse outcomes: Linking high throughput signaling signatures with functional consequences

Embryonic vascular disruption is an important adverse outcome pathway (AOP) as chemical disruption of cardiovascular development induces broad prenatal defects. High throughput screening (HTS) assays aid AOP development although linking in vitro data to in vivo apical endpoints remains challenging. This study evaluated two anti-angiogenic agents, 5HPP-33 and TNP-470, across the ToxCastDB HTS assay platform and anchored the results to complex in vitro functional assays: the rat aortic explant assay (AEA), rat whole embryo culture (WEC), and the zebrafish embryotoxicity (ZET) assay. Both were identified as putative vascular disruptive compounds (pVDCs) in ToxCastDB and disrupted angiogenesis and embryogenesis in the functional assays. Differences were observed in potency and adverse effects: 5HPP-33 was embryolethal (WEC and ZET); TNP-470 produced caudal defects at lower concentrations. This study demonstrates how a tiered approach using HTS signatures and complex functional in vitro assays might be used to prioritize further in vivo developmental toxicity testing.

Applying evolutionary genetics to developmental toxicology and risk assessment

Evolutionary thinking continues to challenge our views on health and disease. Yet, there is a communication gap between evolutionary biologists and toxicologists in recognizing the connections among developmental pathways, high-throughput screening, and birth defects in humans. To increase our capability in identifying potential developmental toxicants in humans, we propose to apply evolutionary genetics to improve the experimental design and data interpretation with various in vitro and whole-organism models. We review five molecular systems of stress response and update 18 consensual cell-cell signaling pathways that are the hallmark for early development, organogenesis, and differentiation; and revisit the principles of teratology in light of recent advances in high-throughput screening, big data techniques, and systems toxicology. Multiscale systems modeling plays an integral role in the evolutionary approach to cross-species extrapolation. Phylogenetic analysis and comparative bioinformatics are both valuable tools in identifying and validating the molecular initiating events that account for adverse developmental outcomes in humans. The discordance of susceptibility between test species and humans (ontogeny) reflects their differences in evolutionary history (phylogeny). This synthesis not only can lead to novel applications in developmental toxicity and risk assessment, but also can pave the way for applying an evo-devo perspective to the study of developmental origins of health and disease.