Systems bioengineering approaches for developmental toxicology

Developmental toxicology is the field of study that examines the effects of chemical and physical agents on developing organisms. By using principles of systems biology and bioengineering, a systems bioengineering approach could be applied to study the complex interactions between developing organisms, the environment, and toxic agents. This approach would result in a holistic understanding of the effects of toxic agents on organisms, by considering the interactions between different biological systems and the impacts of toxicants on those interactions. It would be useful in identifying key biological pathways and mechanisms affected by toxic agents, as well as in the development of predictive models to assess potential risks of exposure to toxicants during development. In this review, we discuss the relevance of systems bioengineering to the field of developmental toxicity and provide up-to-date examples that illustrate the use of engineering principles for this application.

Integrating in vitro chemical transplacental passage into a generic PBK model: A QIVIVE approach

With the increasing application of cell culture models as primary tools for predicting chemical safety, the quantitative extrapolation of the effective dose from in vitro to in vivo (QIVIVE) is of increasing importance. For developmental toxicity this requires scaling the in vitro observed dose-response characteristics to in vivo fetal exposure, while integrating maternal in vivo kinetics during pregnancy, in particular transplacental transfer. Here the transfer of substances across the placental barrier, has been studied using the in vitro BeWo cell assay and six embryotoxic compounds of different kinetic complexity. The BeWo assay results were incorporated in an existing generic Physiologically Based Kinetic (PBK) model which for this purpose was extended with rat pregnancy. Finally, as a "proof of principle", the BeWo PBK model was used to perform a QIVIVE based on developmental toxicity as observed in various different in vitro toxicity assays. The BeWo results illustrated different transport profiles of the chemicals across the BeWo monolayer, allocating the substances into two distinct groups: the 'quickly-transported' and the 'slowly-transported'. BeWo PBK exposure simulations during gestation were compared to experimentally measured maternal blood and fetal concentrations and a reverse dosimetry approach was applied to translate in vitro observed embryotoxicity into equivalent in vivo dose-response curves. This approach allowed for a direct comparison of the in vitro dose-response characteristics as observed in the Whole Embryo Culture (WEC), and the Embryonic Stem Cell test (cardiac:ESTc and neural:ESTn) with in vivo rat developmental toxicity data. Overall, the in vitro to in vivo comparisons suggest a promising future for the application of such QIVIVE methodologies for screening and prioritization purposes of developmental toxicants. Nevertheless, the clear need for further improvements is acknowledged for a wider application of the approach in chemical safety assessment.

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.

In vitro culture systems as an alternative for female reproductive toxicology studies

Studies have shown that daily exposure to different products, whether chemical or natural, can cause irreversible damage to women’s reproductive health. Therefore it is necessary to use tests that evaluate the safety and efficacy of these products. Most reproductive toxicology tests are performedin vivo. However, in recent years, various cell culture methods, including embryonic stem cells and tissues have been developed with the aim of reducing the use of animals in toxicological tests. This is a major advance in the area of toxicology, as these systems have the potential to become a widely used tool compared within vivotests routinely used in reproductive biology and toxicology. The present review describes and highlights data onin vitroculture processes used to evaluate reproductive toxicity as an alternative to traditional methods usingin vivotests.

Perfluorononanoic acid (PFNA) alters lipid accumulation in bovine blastocysts after oocyte exposure during in vitro maturation

Perfluorononanoic acid (PFNA) is one of the perfluoroalkyl acids present in human tissues. In this study, effects on early embryo development after PFNA exposure were investigated using the bovine in vitro production system. Oocytes were exposed to PFNA during maturation in vitro (10 μg mL^-1 and 0.1 μg mL^-1), and then fertilized and cultured in parallel with control groups. Developmental parameters (cleavage, blastocyst formation) were followed and embryo quality evaluated (stage, grade). Embryos developed after exposure to 0.1 μg mL^-1 were stained to distinguish nuclei, active mitochondria and neutral lipids. 10 μg mL^-1 of PFNA had a severe negative effect on blastocyst formation (OR: 0.27 p < 0.05), an effect not observed at 0.1 μg mL^-1. However, lipid droplet distribution was significantly altered in embryos exposed to 0.1 μg mL^-1, suggesting a disturbance of lipid metabolism after exposure to sublethal levels of PFNA during oocyte maturation in vitro.

Flow Cytometry to Evaluate Potential Developmental Toxicants in the Embryonic Stem Cell

Embryonic stem cells (ESC) are widely used due to their unlimited capacity of differentiation into different cell lineages, which makes ESC a viable choice as a toxicology test model. Toxicological analysis using embryonic stem cells (ESC) has become an important tool in toxicology procedures. Regarding toxicological analysis methods, flow cytometry (FC) is one technique designed to detect and evaluate cells in suspension, for example, ESC suspension, thus making possible to study different biological, physical, and/or chemical characteristics of cells. Thus, FC can be very useful for cell toxicology and tumorigenic analyses.

Systems Toxicology of Male Reproductive Development: Profiling 774 Chemicals for Molecular Targets and Adverse Outcomes

BACKGROUND

Trends in male reproductive health have been reported for increased rates of testicular germ cell tumors, low semen quality, cryptorchidism, and hypospadias, which have been associated with prenatal environmental chemical exposure based on human and animal studies.

OBJECTIVE

In the present study we aimed to identify significant correlations between environmental chemicals, molecular targets, and adverse outcomes across a broad chemical landscape with emphasis on developmental toxicity of the male reproductive system.

METHODS

We used U.S. EPA's animal study database (ToxRefDB) and a comprehensive literature analysis to identify 774 chemicals that have been evaluated for adverse effects on male reproductive parameters, and then used U.S. EPA's in vitro high-throughput screening (HTS) database (ToxCastDB) to profile their bioactivity across approximately 800 molecular and cellular features.

RESULTS

A phenotypic hierarchy of testicular atrophy, sperm effects, tumors, and malformations, a composite resembling the human testicular dysgenesis syndrome (TDS) hypothesis, was observed in 281 chemicals. A subset of 54 chemicals with male developmental consequences had in vitro bioactivity on molecular targets that could be condensed into 156 gene annotations in a bipartite network.

CONCLUSION

Computational modeling of available in vivo and in vitro data for chemicals that produce adverse effects on male reproductive end points revealed a phenotypic hierarchy across animal studies consistent with the human TDS hypothesis. We confirmed the known role of estrogen and androgen signaling pathways in rodent TDS, and importantly, broadened the list of molecular targets to include retinoic acid signaling, vascular remodeling proteins, G-protein coupled receptors (GPCRs), and cytochrome P450s.

CITATION

Leung MC, Phuong J, Baker NC, Sipes NS, Klinefelter GR, Martin MT, McLaurin KW, Setzer RW, Darney SP, Judson RS, Knudsen TB. 2016. Systems toxicology of male reproductive development: profiling 774 chemicals for molecular targets and adverse outcomes. Environ Health Perspect 124:1050-1061; http://dx.doi.org/10.1289/ehp.1510385.

Distinct gene expression responses of two anticonvulsant drugs in a novel human embryonic stem cell based neural differentiation assay protocol

Hazard assessment of chemicals and pharmaceuticals is increasingly gaining from knowledge about molecular mechanisms of toxic action acquired in dedicated in vitro assays. We have developed an efficient human embryonic stem cell neural differentiation test (hESTn) that allows the study of the molecular interaction of compounds with the neural differentiation process. Within the 11-day differentiation protocol of the assay, embryonic stem cells lost their pluripotency, evidenced by the reduced expression of stem cell markers Pou5F1 and Nanog. Moreover, stem cells differentiated into neural cells, with morphologically visible neural structures together with increased expression of neural differentiation-related genes such as βIII-tubulin, Map2, Neurogin1, Mapt and Reelin. Valproic acid (VPA) and carbamazepine (CBZ) exposure during hESTn differentiation led to concentration-dependent reduced expression of βIII-tubulin, Neurogin1 and Reelin. In parallel VPA caused an increased gene expression of Map2 and Mapt which is possibly related to the neural protective effect of VPA. These findings illustrate the added value of gene expression analysis for detecting compound specific effects in hESTn. Our findings were in line with and could explain effects observed in animal studies. This study demonstrates the potential of this assay protocol for mechanistic analysis of specific compound-induced inhibition of human neural cell differentiation.

Usefulness of bovine and porcine IVM/IVF models for reproductive toxicology

Women presenting fertility problems are often helped by Assisted Reproductive Techniques (ART), such as in vitro fertilization (IVF) programs. However, in many cases the etiology of the in/subfertility remains unknown even after treatment. Although several aspects should be considered when assisting a woman with problems to conceive, a survey on the patients' exposure to contaminants would help to understand the cause of the fertility problem, as well as to follow the patient properly during IVF. Daily exposure to toxic compounds, mainly environmental and dietary ones, may result in reproductive impairment. For instance, because affects oocyte developmental competence. Many of these compounds, natural or synthetic, are endocrine disruptors or endocrine active substances that may impair reproduction. To understand the risks and the mechanism of action of such chemicals in human cells, the use of proper in vitro models is essential. The present review proposes the bovine and porcine models to evaluate toxic compounds on oocyte maturation, fertilization and embryo production in vitro. Moreover, we discuss here the species-specific differences when mice, bovine and porcine are used as models for human.

Transcriptomic characterization of C57BL/6 mouse embryonic stem cell differentiation and its modulation by developmental toxicants

The Tox21 program calls for transforming toxicology testing from traditional in vivo tests to less expensive and higher throughput in vitro methods. In developmental toxicology, a spectrum of alternative methods including cell line based tests has been developed. In particular, embryonic stem cells (ESCs) have received widespread attention as a promising alternative model for developmental toxicity assessment. Here, we characterized gene expression changes during mouse ESC differentiation and their modulation by developmental toxicants. C57BL/6 ESCs were allowed to differentiate spontaneously and RNA of vehicle controls was collected at 0, 24, 48, 72, 96, 120 and 168 h after embryoid body (EB) formation; RNA of compound-exposed EBs were collected at 24 h. Samples were hybridized to Affymetrix Mouse Gene 2.0 ST Array; using stringent cut-off criteria of Bonferroni-adjusted p<0.05 and fold change >2.0, a total of 1996 genes were found differentially expressed among the vehicle controls at different time points. Gene ontology (GO) analysis showed these regulated genes were mostly involved in differentiation-related processes such as development, morphogenesis, metabolism, cell differentiation, cell organization and biogenesis, embryonic development, and reproduction. Biomarkers of all three germ layers or of their derivative early cell types were identified in the gene list. Principal component analysis (PCA) based on these genes showed that the unexposed vehicle controls appeared in chronological order in the PCA plot, and formed a differentiation track when connected. Cultures exposed to thalidomide, monobutyl phthalate, or valproic acid deviated significantly from the differentiation track, manifesting the capacity of the differentiation track to identify the modulating effects of diverse developmental toxicants. The differentiation track defined in this study may be further exploited as a baseline for developmental toxicity testing, with compounds causing significant deviation from the differentiation track being predicted as potential developmental toxicants.

Stem cells and stem cell-derived tissues and their use in safety assessment

Toxicology has long relied on animal models in a tedious approach to understanding risk of exposure to an uncharacterized molecule. Stem cell-derived tissues can be made in high purity, quality, and quantity to enable a new approach to this problem. Currently, stem cell-derived tissues are primarily "generic" genetic backgrounds; the future will see the integration of various genetic backgrounds and complex three-dimensional models to create truly unique in vitro organoids. This minireview focuses on the state of the art of a number of stem cell-derived tissues and details their application in toxicology.

Predictive models and computational toxicology

Understanding the potential health risks posed by environmental chemicals is a significant challenge elevated by the large number of diverse chemicals with generally uncharacterized exposures, mechanisms, and toxicities. The ToxCast computational toxicology research program was launched by EPA in 2007 and is part of the federal Tox21 consortium to develop a cost-effective approach for efficiently prioritizing the toxicity testing of thousands of chemicals and the application of this information to assessing human toxicology. ToxCast addresses this problem through an integrated workflow using high-throughput screening (HTS) of chemical libraries across more than 650 in vitro assays including biochemical assays, human cells and cell lines, and alternative models such as mouse embryonic stem cells and zebrafish embryo development. The initial phase of ToxCast profiled a library of 309 environmental chemicals, mostly pesticidal actives having rich in vivo data from guideline studies that include chronic/cancer bioassays in mice and rats, multigenerational reproductive studies in rats, and prenatal developmental toxicity endpoints in rats and rabbits. The first phase of ToxCast was used to build models that aim to determine how well in vivo animal effects can be predicted solely from the in vitro data. Phase I is now complete and both the in vitro data (ToxCast) and anchoring in vivo database (ToxRefDB) have been made available to the public (http://actor.epa.gov/). As Phase II of ToxCast is now underway, the purpose of this chapter is to review progress to date with ToxCast predictive modeling, using specific examples on developmental and reproductive effects in rats and rabbits with lessons learned during Phase I.

Simvastatin modulates mouse embryonic stem cell-derived chondrogenesis in vitro

It has been studied in detail that cellular differentiation during chondrogenesis can be recapitulated in vitro by differentiation of embryonic stem (ES) cells as embryoid bodies (EBs). We here used this model system of cartilage development to analyze the effect of simvastatin, a potentially embryotoxic substance. Statins are a group of drugs used to treat hypercholesterolaemia. We found that simvastatin activated cartilage nodule formation during EB differentiation. Extended application of simvastatin resulted in enhanced expression of cartilage marker molecules and prolonged persistence of cartilage nodules. Expression of collagen type II was upregulated during simvastatin-induced chondrogenic ES cell differentiation as demonstrated by quantitative real time PCR. However, immunostaining for cartilage marker molecules revealed that cartilage nodules within simvastatin-treated EBs were defective, bearing cavities of cell loss. Furthermore, caspase activity was reduced in comparison to untreated controls indicating reduced apoptosis. Taken together, we may speculate that simvastatin prolongs survival of chondrocytes and disrupts cellular integrity of cartilage nodules during EB development by affecting apoptotic mechanisms. The study underlines that ES cell-derived EBs are a useful in vitro model to screen substances for their embryotoxic and teratogenic potential.

The bovine oocyte in vitro maturation model: a potential tool for reproductive toxicology screening

Reproductive toxicity testing according to the present guidelines requires a high number of animals. Therefore, the development of alternative in vitro methods is urgently required. The aim of the present study was to investigate the applicability domain of the bovine oocyte in vitro maturation assay (bIVM) to study female reproductive toxicology. Therefore, bovine oocytes were exposed to a broad set of chemicals of two distinct biological function groups: (a) affecting female fertility and (b) affecting embryonic development and having a broad range of physical and chemical properties. The endpoints evaluated were the oocyte nuclear maturation (progression of meiosis) and general cytotoxicity. The oocyte nuclear maturation was negatively affected by all compounds tested and the effect was observed at concentrations lower than the cytotoxic ones. The bIVM assay correctly predicted the classification of compounds between those predefined groups. Additionally, the bIVM model contributes significantly for the 3R principle, since no test animals are used in this assay. In conclusion, the bIVM is a sensitive and valuable alternative assay to identify potential chemical hazard on female fertility.

The embryonic stem cell test as tool to assess structure-dependent teratogenicity: the case of valproic acid

Teratogenicity can be predicted in vitro using the embryonic stem cell test (EST). The EST, which is based on the morphometric measurement of cardiomyocyte differentiation and cytotoxicity parameters, represents a scientifically validated method for the detection and classification of chemicals according to their teratogenic potency. Furthermore, an abbreviated protocol applying flow cytometry of intracellular marker proteins to determine differentiation into the cardiomyocyte lineage is available. Although valproic acid (VPA) is in worldwide clinical use as antiepileptic drug, it exhibits two severe side effects, i.e., teratogenicity and hepatotoxicity. These limitations have led to extensive research into derivatives of VPA. Here we chose VPA as model compound to test the applicability domain and to further evaluate the reliability of the EST. To this end, we study six closely related congeners of VPA and demonstrate that both the standard and the molecular flow cytometry-based EST are well suited to indicate differences in the teratogenic potency among VPA analogs that differ only in chirality or side chain length. Our data show that identical results can be obtained by using the standard EST or a shortened protocol based on flow cytometry of intracellular marker proteins. Both in vitro protocols enable to reliably determine differentiation of murine stem cells toward the cardiomyocyte lineage and to assess its chemical-mediated inhibition.

Present state and future perspectives of using pluripotent stem cells in toxicology research

The use of novel drugs and chemicals requires reliable data on their potential toxic effects on humans. Current test systems are mainly based on animals or in vitro–cultured animal-derived cells and do not or not sufficiently mirror the situation in humans. Therefore, in vitro models based on human pluripotent stem cells (hPSCs) have become an attractive alternative. The article summarizes the characteristics of pluripotent stem cells, including embryonic carcinoma and embryonic germ cells, and discusses the potential of pluripotent stem cells for safety pharmacology and toxicology. Special attention is directed to the potential application of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) for the assessment of developmental toxicology as well as cardio- and hepatotoxicology. With respect to embryotoxicology, recent achievements of the embryonic stem cell test (EST) are described and current limitations as well as prospects of embryotoxicity studies using pluripotent stem cells are discussed. Furthermore, recent efforts to establish hPSC-based cell models for testing cardio- and hepatotoxicity are presented. In this context, methods for differentiation and selection of cardiac and hepatic cells from hPSCs are summarized, requirements and implications with respect to the use of these cells in safety pharmacology and toxicology are presented, and future challenges and perspectives of using hPSCs are discussed.