FETAX Assay for Evaluation of Developmental Toxicity

In chemico methodology for engineered nanomaterial categorization according to number, nature and oxidative potential of reactive surface sites

Methanol probe chemisorption quantifies the number of reactive sites at the surface of engineered nanomaterials, enabling normalization per reactive site in reactivity and toxicity tests, rather than per mass or physical surface area. Subsequent temperature-programmed surface reaction (TPSR) of chemisorbed methanol identifies the reactive nature of surface sites (acidic, basic, redox or combination thereof) and their reactivity. Complementary to the methanol assay, a dithiothreitol (DTT) probe oxidation reaction is used to evaluate the oxidation capacity. These acellular approaches to quantify the number, nature, and reactivity of surface sites constitute a new approach methodology (NAM) for site-specific classification of nanomaterials. As a proof of concept, CuO, CeO2, ZnO, Fe3O4, CuFe2O4, Co3O4 and two TiO2 nanomaterials were probed. A harmonized reactive descriptor for ENMs was obtained: the DTT oxidation rate per reactive surface site, or oxidative turnover frequency (OxTOF). CuO and CuFe2O4 ENMs exhibit the largest reactive site surface density and possess the highest oxidizing ability in the series, as estimated by the DTT probe reaction, followed by CeO2 NM-211 and then titania nanomaterials (DT-51 and NM-101) and Fe3O4. DTT depletion for ZnO NM-110 was associated with dissolved zinc ions rather than the ZnO particles; however, the basic characteristics of the ZnO NM-110 particles were evidenced by methanol TPSR. These acellular assays allow ranking the eight nanomaterials into three categories with statistically different oxidative potentials: CuO, CuFe2O4 and Co3O4 are the most reactive; ceria exhibits a moderate reactivity; and iron oxide and the titanias possess a low oxidative potential.

Effects of combined exposure to two bisphenol plasticizers (BPA and BPB) on Xenopus laevis development

Due to its endocrine disruptive activity, the plastic additive Bisphenol A (BPA) is classified as substance of very high concern (EU ECHA 2017). A correlation between environmental exposure to BPA and congenital defects has been described in humans and in experimental species including the amphibian Xenopus laevis, where severe branchial defects were associated to lethality. The exposure of X. laevis embryos to the BPA analogue bisphenol B (BPB) was recently linked to similar teratogenic effects, with BPB having relative potency about 3 times higher than BPA. The combined BPA-BPB exposure is realistic as both BPA and BPB are detected in human samples and environment. Limited experimental data are available on the combined developmental toxicity of BPA and BPB. The aim of the present work is to evaluate the effects of BPA and BPB mixture in the X. laevis development model, using R-FETAX procedure. The exposure was limited to the first day of development (corresponding to the phylotypic developmental period, common to all vertebrates). Samples were monitored for lethal effects during the full six-day test period and the external morphology was evaluated at the end of the test. Mixture effects were described by modelling, using the PROAST software package. Overall data modelling showed that dose-addiction could not be rejected, suggesting a health concern for co-exposure.

Ibuprofen-induced multiorgan malformation during embryogenesis in Xenopus laevis (FETAX)

Ibuprofen, one of the most commonly prescribed nonsteroidal anti-inflammatory drugs, has not been fully assessed for embryonic toxicity in vertebrates. Here, we systematically assessed the embryotoxicity of ibuprofen in Xenopus laevis at various concentrations during embryogenesis. Embryos were treated with different concentrations of ibuprofen, ranging from 8 to 64 mg/L, at 23 °C for 96 h, and examined daily and evaluated at 72 hpf. Lethal or teratogenic effects were documented. For histological analysis, paraffin embedded embryos were transversely sectioned at a thickness of 10-μm and stained with hematoxylin and eosin. Total RNA was isolated from embryos at stages 6, 12, 22 and 36, and real-time quantitative PCR was performed. Ibuprofen-treated embryos showed delayed or failed dorsal lip formation and its closure at the beginning of gastrulation. This resulted in herniation of the endodermal mass after gastrulation under high concentrations of ibuprofen-treated embryos. Underdeveloped intestines with stage and/or intestinal malrotation, distorted microcephaly, and hypoplastic heart, lungs, and pronephric tubules were observed in ibuprofen-treated embryos. Cephalic, cardiac, and truncal edema were also observed in them. The severity of the deformities was observed in a concentration-dependent manner. The teratogenic index was 2.28. These gross and histological disruptions correlated well with the altered expression of each organ marker gene. In conclusion, ibuprofen induced delayed and disrupted gastrulation in the early developmental stage and multiorgan malformation later in the organogenesis stage of Xenopus laevis embryos.

Microplastics and TiO2 nanoparticles mixture as an emerging threat to amphibians: A case study on bullfrog embryos

Emerging contaminants can act as contributing factors to the decline of amphibian populations worldwide. Recently, scientists have drawn attention to the potential ecotoxicity of microplastics and nanomaterials in amphibians, however, their possible effects on embryonic developmental stages are still absent. Thus, the present study analyzed the developmental toxicity of environmentally relevant concentrations of polyethylene microplastics (PE MPs; 60 mg/L) and titanium dioxide nanoparticles (TiO2 NPs; 10 μg/L), isolated or in combination (Mix group) on bullfrog embryos, Aquarana catesbeiana, adapting the Frog Embryo Teratogenesis Assay (FETAX, 96h). Allied to the FETAX protocol, we also analyzed the heart rate and morphometric data. The exposure reduced the survival and hatching rates in groups exposed to TiO2 NPs, and to a lesser extent, also affected the Mix group. TiO2 NPs possibly interacted with the hatching enzymes of the embryos, preventing hatching, and reducing their survival. The reduced effects in the Mix group are due to the agglomeration of both toxicants, making the NPs less available for the embryos. PE MPs got attached to the gelatinous capsule of the chorion (confirmed by fluorescence microscopy), which protected the embryos from eventual direct effects of the microplastics on the hatching and survival rates. Although there were no cardiotoxic effects nor morphometric alterations, there was a significant increase in abdominal edemas in the hatched embryos of the PE MPs group, which indicates that osmoregulation might have been affected by the attachment of the microplastics on the embryos' gelatinous capsule. This study presents the first evidence of developmental toxicity of environmental mixtures of microplastics and nanoparticles on amphibians and reinforces the need for more studies with other amphibian species, especially neotropical specimens that could present bigger sensibility. Our study also highlighted several features of the FETAX protocol as useful tools to evaluate the embryotoxicity of several pollutants on amphibians.

Biochemical Studies to Understand Teratogenicity and Lethality Outcomes in Modified-FETAX

The frog embryo teratogenesis assay-Xenopus (FETAX) is a standardized test used to assess the toxic and teratogenic effects of xenobiotics. With this test, toxic and/or teratogenic concentrations of xenobiotic substances can be determined using morphological parameters such as lethality, length, and malformations in stage 8-11 Xenopus laevis embryos after 96 h exposure. These parameters enable the determination of the median lethal and effective concentrations (LC50 and EC50), minimum concentration to inhibit growth (MCIG), and teratogenic index of the tested chemical to reveal the short-term effects of relatively high concentrations. On the other hand, although FETAX provides quantitative and qualitative data on teratogenicity and toxicity, the biochemical and molecular mechanisms of these effects cannot be explained. Recent studies have tried to elucidate the mechanisms causing malformations and to explain the underlying causes of toxicity and teratogenicity by biochemical marker analysis. This chapter describes methods to analyze modified-FETAX and some detoxification and oxidative stress-related biomarkers during the early embryonic development of X. laevis.

Teratogenic and neuro-behavioural toxic effects of bisphenol A (BPA) and B (BPB) on Xenopus laevis development

Bisphenol A (BPA) is a plastic additive with endocrine disruptive activity, classified in 2017 by EU ECHA as substance of very high concern. A correlation between environmental exposure to BPA and congenital defects has been described in humans and in experimental species, including the amphibian Xenopus laevis. Among BPA analogues, bisphenol B (BPB) is used as alternative in different not-EU countries, including US, but seems to share with BPA its endocrine disruptor properties. Aim of the present work is the evaluation of the effects of BPB versus BPA exposure in a X. laevis developmental model. A windowed exposure (R-FETAX method) was applied covering the developmental phylotypic period (teratogenicity window), or the late tailbud stages (neuro-behavioural toxicity window, corresponding to the spontaneous swimming acquisition period). Samples were monitored for lethal effects during the full test period. External morphology evaluation and deglutition functional test were applied in any group. Abnormal tadpoles were also processed for cartilage staining. In groups exposed during neuro-behavioural toxicity window the swimming test was also applied. Lethality and malformations were obtained only in samples exposed during the teratogenicity window; these data were modelled using PROAST software and BPB relative potency resulted about 3 times higher than BPA. The day-by-day evaluation revealed that lethality was correlated to embryonic abnormal development of gills and apoptosis in gill primordia. Teratogenicity was never detected in groups exposed during the neuro-behavioural toxicity window, where some significant neuro-behavioural deficits were detected in tadpoles exposed to the highest tested concentrations of BPA and BPB.

Environmental endocrine disruptors and amphibian immunity: A bridge between the thyroid hormone axis and T cell development

Immunity is susceptible to reprogramming by environmental chemical and endocrine signals. Notably, numerous thyroid disrupting chemicals (TDCs) have the potential to perturb immune endpoints, but data are lacking on the mechanisms by which TDCs can influence the development of the immune system. T cell immunity is particularly vulnerable to modulation by TDCs during thymic education, differentiation, and selection. The following review discusses the ways in which thyroid hormones may influence T cell development, as well as emerging TDCs with potential to impact both thyroid hormone physiology and immune outcomes. To overcome the challenges of studying TDC impacts on immune toxicological endpoints, a comparative approach using the amphibian Xenopus laevis is recommended. X. laevis are ideally suited to studying TDC impacts on immunity due to the importance of thyroid hormones for metamorphosis, and the wealth of immunological models to measure immune endpoints in both tadpoles and adult frogs.

Normal Table of Xenopus development: a new graphical resource

Normal tables of development are essential for studies of embryogenesis, serving as an important resource for model organisms, including the frog Xenopus laevis. Xenopus has long been used to study developmental and cell biology, and is an increasingly important model for human birth defects and disease, genomics, proteomics and toxicology. Scientists utilize Nieuwkoop and Faber's classic 'Normal Table of Xenopus laevis (Daudin)' and accompanying illustrations to enable experimental reproducibility and reuse the illustrations in new publications and teaching. However, it is no longer possible to obtain permission for these copyrighted illustrations. We present 133 new, high-quality illustrations of X. laevis development from fertilization to metamorphosis, with additional views that were not available in the original collection. All the images are available on Xenbase, the Xenopus knowledgebase (http://www.xenbase.org/entry/zahn.do), for download and reuse under an attributable, non-commercial creative commons license. Additionally, we have compiled a 'Landmarks Table' of key morphological features and marker gene expression that can be used to distinguish stages quickly and reliably (https://www.xenbase.org/entry/landmarks-table.do). This new open-access resource will facilitate Xenopus research and teaching in the decades to come.

The Xenopus phenotype ontology: bridging model organism phenotype data to human health and development

Background

Ontologies of precisely defined, controlled vocabularies are essential to curate the results of biological experiments such that the data are machine searchable, can be computationally analyzed, and are interoperable across the biomedical research continuum. There is also an increasing need for methods to interrelate phenotypic data easily and accurately from experiments in animal models with human development and disease.

Results

Here we present the Xenopus phenotype ontology (XPO) to annotate phenotypic data from experiments in Xenopus, one of the major vertebrate model organisms used to study gene function in development and disease. The XPO implements design patterns from the Unified Phenotype Ontology (uPheno), and the principles outlined by the Open Biological and Biomedical Ontologies (OBO Foundry) to maximize interoperability with other species and facilitate ongoing ontology management. Constructed in Web Ontology Language (OWL) the XPO combines the existing uPheno library of ontology design patterns with additional terms from the Xenopus Anatomy Ontology (XAO), the Phenotype and Trait Ontology (PATO) and the Gene Ontology (GO). The integration of these different ontologies into the XPO enables rich phenotypic curation, whilst the uPheno bridging axioms allows phenotypic data from Xenopus experiments to be related to phenotype data from other model organisms and human disease. Moreover, the simple post-composed uPheno design patterns facilitate ongoing XPO development as the generation of new terms and classes of terms can be substantially automated.

Conclusions

The XPO serves as an example of current best practices to help overcome many of the inherent challenges in harmonizing phenotype data between different species. The XPO currently consists of approximately 22,000 terms and is being used to curate phenotypes by Xenbase, the Xenopus Model Organism Knowledgebase, forming a standardized corpus of genotype–phenotype data that can be directly related to other uPheno compliant resources.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-022-04636-8.

Toxic effects of SiO2NPs in early embryogenesis of Xenopuslaevis

The exposure of organisms to the nanoparticulate is potentially hazardous, particularly when it occurs during embryogenesis. The effects of commercial SiO₂NPs in early development were studied, using Xenopus laevis as a model to investigate their possible future employment by means of the Frog Embryo Teratogenesis Assay-Xenopus test (FETAX). The SiO₂NPs did not change the survival but produced several abnormalities in developing embryos, in particular, the dorsal pigmentation, the cartilages of the head and branchial arches were modified; the encephalon, spinal cord and nerves are anomalous and the intestinal brush border show signs of suffering; these embryos are also bradycardic. In addition, the expression of genes involved in the early pathways of embryo development was modified. Treated embryos showed an increase of reactive oxygen species. This study suggests that SiO₂NPs are toxic but non-lethal and showed potential teratogenic effects in Xenopus. The latter may be due to their cellular accumulation and/or to the effect caused by the interaction of SiO₂NPs with cytoplasmic and/or nuclear components. ROS production could contribute to the observed effects. In conclusion, the data indicates that the use of SiO₂NPs requires close attention and further studies to better clarify their activity in animals, including humans.

E-liquids and vanillin flavoring disrupts retinoic acid signaling and causes craniofacial defects in Xenopus embryos

Environmental teratogens such as smoking are known risk factors for developmental disorders such as cleft palate. While smoking rates have declined, a new type of smoking, called vaping is on the rise. Vaping is the use of e-cigarettes to vaporize and inhale an e-liquid containing nicotine and food-like flavors. There is the potential that, like smoking, vaping could also pose a danger to the developing human. Rather than waiting for epidemiological and mammalian studies, we have turned to an aquatic developmental model, Xenopus laevis, to more quickly assess whether e-liquids contain teratogens that could lead to craniofacial malformations. Xenopus, like zebrafish, has the benefit of being a well-established developmental model and has also been effective in predicting whether a chemical could be a teratogen. We have determined that embryonic exposure to dessert flavored e-liquids can cause craniofacial abnormalities, including an orofacial cleft in Xenopus. To better understand the underlying mechanisms contributing to these defects, transcriptomic analysis of the facial tissues of embryos exposed to a representative dessert flavored e-liquid vapor extract was performed. Analysis of differentially expressed genes in these embryos revealed several genes associated with retinoic acid metabolism or the signaling pathway. Consistently, retinoic acid receptor inhibition phenocopied the craniofacial defects as those embryos exposed to the vapor extract of the e-liquid. Such malformations also correlated with a group of common differentially expressed genes, two of which are associated with midface birth defects in humans. Further, e-liquid exposure sensitized embryos to forming craniofacial malformations when they already had depressed retinoic acid signaling. Moreover, 13-cis-retinoic acid treatment could significantly reduce the e-liquid induced malformation in the midface. Such results suggest the possibility of an interaction between retinoic acid signaling and e-liquid exposure. One of the most popular and concentrated flavoring chemicals in dessert flavored e-liquids is vanillin. Xenopus embryos exposed to this chemical closely resembled embryos exposed to dessert-like e-liquids and a retinoic acid receptor antagonist. In summary, we determined that e-liquid chemicals, in particular vanillin, can cause craniofacial defects potentially by dysregulating retinoic acid signaling. This work warrants the evaluation of vanillin and other such flavoring additives in e-liquids on mammalian development.

Comparative Toxicological Evaluation of Tattoo Inks on Two Model Organisms

Tattooing is a technique that introduces colored substances under the skin in order to color it permanently. Decomposition products of tattoo pigments produce numerous damages for the skin and other organs. We studied the effects of a commercial red ink tattoo, PR170, on Xenopus laevis embryos and Daphnia magna nauplii using concentrations of 10, 20, and 40 mg/L. For Xenopus, we applied the FETAX protocol analyzing survival, malformations, growth, heart rate, and the expression of genes involved in the development. In D. magna, we evaluated the toxicity with an immobilization test. Moreover, we investigated the production of ROS, antioxidant enzymes, and the expression of the ATP-binding cassette in both models. Our results indicate that PR170 pigment has nanoparticle dimensions, modifies the survival and the ATP-binding cassette activity, and induces oxidative stress that probably produces the observed effects in both models. Deformed embryos were observed in Xenopus, probably due to the modification of expression of genes involved in development. The expression of pro-inflammatory cytokines was also modified in this amphibian. We think that these effects are due to the accumulation of PR170 and, in particular, to the presence of the azoic group in the chemical structure of this pigment. Further studies needed to better understand the effects of commercial tattoo inks.

Bisphenol A exposure induces apoptosis and impairs early embryonic development in Xenopus laevis

Bisphenol A (BPA), an endocrine-disrupting chemical that is largely produced and used in the plastics industry, causes environmental pollution and is absorbed by humans through consumption of food and liquids in polycarbonate containers. BPA exerts developmental and genetic toxicities to embryos and offsprings, but the embryotoxicity mechanism of this chemical is unclear. This study aimed to explore the toxic effect of BPA on embryonic development and elucidate its toxicity mechanism. Embryos of Xenopus laevis as a model were treated with different concentrations (0.1, 1, 10, and 20 μM) of BPA at the two-cell stage to investigate the developmental toxicity of BPA. Embryonic development and behaviors were monitored 24 h-96 h of BPA exposure. BPA concentrations greater than 1 μM exerted significant teratogenic effects on the Xenopus embryos, which showed short tail axis, miscoiled guts, and bent notochord as the main malformations. The 20 μM BPA-treated embryos were seriously damaged in all aspects and exhibited deformity, impaired behavioral ability, and tissue damage. The DNA integrity and apoptosis of the Xenopus embryos were also investigated. Exposure to BPA concentrations higher than 0.1 μM significantly induced DNA damage (p < 0.05). The 10 and 20 μM BPA-treated embryos exhibited higher levels of cleaved caspase-3 protein than the control. The ratios of bax/bcl-2 mRNA were significantly higher in the 10 μM and 20 μM-treated embryos than the ratio in the control group. Overall, data indicated that BPA can delay the early development, induce DNA damage and apoptosis, and eventually cause multiple malformations in Xenopus embryos.

4-Octylphenol induces developmental abnormalities and interferes the differentiation of neural crest cells in Xenopus laevis embryos

Developmental toxicity of 4-octylphenol (OP), an estrogenic endocrine disruptor was verified using frog embryo teratogenesis assay Xenopus. LC₅₀, EC_{50Malformtion} and EC_{50Melanocyte-dysgenesis} of OP were 9.9, 10.5, and 2.4 μM, respectively. In tadpoles, despite the low teratogenic index, 2 μM OP significantly inhibited head cartilage development and tail malformation. The total length of tadpole was significantly increased at 5 μM and decreased at 10 μM OP. In OP-treated tadpoles, head cartilages were frequently missed and col2a1 mRNA was decreased at 2 μM, indicating a chondrogenic defect in developing head. In the head skin of 1 μM OP-treated tadpoles, number of melanocytes and melanogenic pathway genes expression were significantly decreased. In the head-neck junction of stage 22 embryos, OP increased foxd3 and sox10 mRNA and SOX10(+) neural crest cells (NCCs) in somite mesoderm and endoderm, indicating the inhibition of chondrogenic differentiation, ectopic migration to endoderm, and undifferentiation of NCCs by OP. Together, OP-induced head dysplasia and inhibition of melanogenesis may be attributable to deregulation of neural crest cells in embryos. In tadpoles, OP at 1 μM significantly increased lipid hydroperoxide and induced spliced xbp1 mRNA, an IRE1 pathway endoplasmic reticulum stress (ERS) marker and p-eIF2α protein, a PERK pathway ERS marker. OP at 10 μM induced CHOP mRNA, pro-apoptotic genes expression, DNA fragmentation, and cleaved caspase-3, suggesting that OP differentially induced ERS and apoptosis according to the concentration in embryos. In 5-10 μM OP-treated stage 22 embryos and stage 45 tadpole heads, Ki67 was significantly increased, suggesting the apoptosis-induced proliferation of embryonic cells in the OP-treated embryos. Together, OP should be managed as a developmental toxicant altering the behavior of NCCs in vertebrates.

Xenbase: Facilitating the Use of Xenopus to Model Human Disease

At a fundamental level most genes, signaling pathways, biological functions and organ systems are highly conserved between man and all vertebrate species. Leveraging this conservation, researchers are increasingly using the experimental advantages of the amphibian Xenopus to model human disease. The online Xenopus resource, Xenbase, enables human disease modeling by curating the Xenopus literature published in PubMed and integrating these Xenopus data with orthologous human genes, anatomy, and more recently with links to the Online Mendelian Inheritance in Man resource (OMIM) and the Human Disease Ontology (DO). Here we review how Xenbase supports disease modeling and report on a meta-analysis of the published Xenopus research providing an overview of the different types of diseases being modeled in Xenopus and the variety of experimental approaches being used. Text mining of over 50,000 Xenopus research articles imported into Xenbase from PubMed identified approximately 1,000 putative disease- modeling articles. These articles were manually assessed and annotated with disease ontologies, which were then used to classify papers based on disease type. We found that Xenopus is being used to study a diverse array of disease with three main experimental approaches: cell-free egg extracts to study fundamental aspects of cellular and molecular biology, oocytes to study ion transport and channel physiology and embryo experiments focused on congenital diseases. We integrated these data into Xenbase Disease Pages to allow easy navigation to disease information on external databases. Results of this analysis will equip Xenopus researchers with a suite of experimental approaches available to model or dissect a pathological process. Ideally clinicians and basic researchers will use this information to foster collaborations necessary to interrogate the development and treatment of human diseases.

Automated Morphological Feature Assessment for Zebrafish Embryo Developmental Toxicity Screens

Detection of developmental phenotypes in zebrafish embryos typically involves a visual assessment and scoring of morphological features by an individual researcher. Subjective scoring could impact results and be of particular concern when phenotypic effect patterns are also used as a diagnostic tool to classify compounds. Here we introduce a quantitative morphometric approach based on image analysis of zebrafish embryos. A software called FishInspector was developed to detect morphological features from images collected using an automated system to position zebrafish embryos. The analysis was verified and compared with visual assessments of 3 participating laboratories using 3 known developmental toxicants (methotrexate, dexamethasone, and topiramate) and 2 negative compounds (loratadine and glibenclamide). The quantitative approach exhibited higher sensitivity and made it possible to compare patterns of effects with the potential to establish a grouping and classification of developmental toxicants. Our approach improves the robustness of phenotype scoring and reliability of assay performance and, hence, is anticipated to improve the predictivity of developmental toxicity screening using the zebrafish embryo.