Monitoring of teratogenic effects in vitro by analysing a selected gene expression pattern

The Human Induced Pluripotent Stem Cell Test as an Alternative Method for Embryotoxicity Testing

The evaluation of substances for their potency to induce embryotoxicity is controlled by safety regulations. Test guidelines for reproductive and developmental toxicity rely mainly on animal studies, which make up the majority of animal usage in regulatory toxicology. Therefore, there is an urgent need for alternative in vitro methods to follow the 3R principles. To improve human safety, cell models based on human cells are of great interest to overcome species differences. Here, human induced pluripotent stem cells (hiPSCs) are an ideal cell source as they largely recapitulate embryonic stem cells without bearing ethical concerns and they are able to differentiate into most cell types of the human body. Here, we set up and characterized a fetal bovine serum (FBS)-free hiPSC-based in vitro test method, called the human induced pluripotent stem cell test (hiPS Test), to evaluate the embryotoxic potential of substances. After 10 days in culture, hiPSCs develop into beating cardiomyocytes. As terminal endpoint evaluations, cell viability, qPCR analyses as well as beating frequency and area of beating cardiomyocytes by video analyses are measured. The embryotoxic positive and non-embryotoxic negative controls, 5-Fluorouracil (5-FU) and Penicillin G (PenG), respectively, were correctly assessed in the hiPS Test. More compounds need to be screened in the future for defining the assay’s applicability domain, which will inform us of the suitability of the hiPS Test for detecting adverse effects of substances on embryonic development.

Stem cell toxicology: a powerful tool to assess pollution effects on human health

Environmental pollution is a global problem; the lack of comprehensive toxicological assessments may lead to increased health risks. To fully understand the health effects of pollution, it is paramount to implement fast, efficient and specific toxicity screening that relies on human models rather than on time-consuming, expensive and often inaccurate tests involving live animals. Human stem cell toxicology represents a valid alternative to traditional toxicity assays because it takes advantage of the ability of stem cells to differentiate into multiple cell types and tissues of the human body. Thus, this branch of toxicology provides a possibility to assess cellular, embryonic, developmental, reproductive and functional toxicity in vitro within a single system highly relevant to human physiology. In this review, we describe the development, performance and future perspectives of stem cell toxicology, with an emphasis on how it can meet the increasing challenges posed by environmental pollution in the modern world.

Alternative models in developmental toxicology

In light of various pressures, toxicologists have been searching for alternative methods for safety testing of chemicals. According to a recent policy in the European Union (Regulation, Evaluation Authorisation and Restriction of Chemicals, REACH), it has been estimated that over the next twelve to fifteen years, approximately 30,000 chemicals may need to be tested for safety, and under current guidelines such testing would require the use of approximately 7.2 million laboratory animals [ Hofer et al. 2004 ]. It has also been estimated that over 80% of all animals used for safety testing under REACH legislation would be used for examining reproductive and developmental toxicity [Hofer et al., 2004]. In addition to REACH initiatives, it has been estimated that out of 5,000 to 10,000 new drug entities that a pharmaceutical company may start with, only one is finally approved by the Food and Drug Administration at a cost of over one billion dollars [ Garg et al. 2011 ]. A large portion of this cost is due to animal testing. Therefore, both the pharmaceutical and chemical industries are interested in using alternative models and in vitro tests for safety testing. This review will examine the current state of three alternative models - whole embryo culture (WEC), the mouse embryonic stem cell test (mEST), and zebrafish. Each of these alternatives will be reviewed, and advantages and disadvantages of each model will be discussed. These models were chosen because they are the models most commonly used and would appear to have the greatest potential for future applications in developmental toxicity screening and testing.

Analysis of altered gene expression specific to embryotoxic chemical treatment during embryonic stem cell differentiation into myocardiac and neural cells

Embryonic stem cells (ES cells), pluripotent cells derived from the inner cell mass of blastocysts, differentiate in vitro into a variety of cell types representing all three germ layers. They therefore constitute one of the most promising in vitro tools for developmental toxicology. To assess the developmental toxicity of chemicals using ES cells easily, identification of effective marker genes is a high priority. We report here altered gene expression during ES cell differentiation into myocardiac and neural cells on treatment with some embryotoxic and non-embryotoxic chemicals. Decreases in several undifferentiated markers such as Oct3/4 and Nanog, and elevated expression of genes associated with heart development or the central nervous system, respectively, were found on microarray analysis. Under differentiation of ES cells into myocardic cells, 107 genes were substantially up-regulated. Decrease in the expression of 13 genes of these (Hand1, Pim2, Tbx20, Myl4, Myl7, Hbb-bh1, Hba-a1, Col1a2, Hba-x, Cmya1, Pitx2, Smyd1 and Adam19) was observed specifically by embryotoxic chemicals. Of the 107 genes up-regulated under differentiation into neurons, 22 genes (Map2, Cpe, Marcks, Ptbp2, Sox11, Tubb2b, Vim, Arx, Emx2, Pax6, Basp1, Ddr1, Ndn, Sfrp, Ttc3, Ubqln2, Six3, Dcx, L1cam, Reln, Wnt1 and Nnat) showed reduced expression specifically by embryotoxic chemicals. Almost all gene sets identified in this study are known to be indispensable for differentiation and development of heart and brain tissues, and thus may serve in early detection or prediction of embryotoxicity of chemicals in vitro.

Human pluripotent stem cells for modeling toxicity

The development of xenobiotics, driven by the demand for therapeutic, domestic and industrial uses continues to grow. However, along with this increasing demand is the risk of xenobiotic-induced toxicity. Currently, safety screening of xenobiotics uses a plethora of animal and in vitro model systems which have over the decades proven useful during compound development and for application in mechanistic studies of xenobiotic-induced toxicity. However, these assessments have proven to be animal-intensive and costly. More importantly, the prevalence of xenobiotic-induced toxicity is still significantly high, causing patient morbidity and mortality, and a costly impediment during drug development. This suggests that the current models for drug safety screening are not reliable in toxicity prediction, and the results not easily translatable to the clinic due to insensitive assays that do not recapitulate fully the complex phenotype of a functional cell type in vivo. Recent advances in the field of stem cell research have potentially allowed for a readily available source of metabolically competent cells for toxicity studies, derived using human pluripotent stem cells harnessed from embryos or reprogrammed from mature somatic cells. Pluripotent stem cell-derived cell types also allow for potential disease modeling in vitro for the purposes of drug toxicology and safety pharmacology, making this model possibly more predictive of drug toxicity compared with existing models. This article will review the advances and challenges of using human pluripotent stem cells for modeling metabolism and toxicity, and offer some perspectives as to where its future may lie.

The embryonic stem cell test combined with toxicogenomics as an alternative testing model for the assessment of developmental toxicity

One of the most studied in vitro alternative testing methods for identification of developmental toxicity is the embryonic stem cell test (EST). Although the EST has been formally validated, the applicability domain as well as the predictability of the model needs further study to allow successful implementation of the EST as an alternative testing method in regulatory toxicity testing. Genomics technologies have already provided a proof of principle of their value in identification of toxicants such as carcinogenic compounds. Also within the EST, gene expression profiling has shown its value in the identification of developmental toxicity and in the evaluation of factors critical for risk assessment, such as dose and time responses. It is expected that the implementation of genomics into the EST will provide a more detailed end point evaluation as compared to the classical morphological scoring of differentiation cultures. Therefore, genomics may contribute to improvement of the EST, both in terms of definition of its applicability domain as well as its predictive capacity. In the present review, we present the progress that has been made with regard to the prediction of developmental toxicity using the EST combined with transcriptomics. Furthermore, we discuss the developments of additional aspects required for further optimization of the EST, including kinetics, the use of human embryonic stem cells (ESC) and computational toxicology. Finally, the current and future use of the EST model for prediction of developmental toxicity in testing strategies and in regulatory toxicity evaluations is discussed.

Human embryonic stem cells as a model for embryotoxicity screening

Reproductive toxicity encompasses harmful effects of various agents on all aspects and stages of the reproductive cycle, including infertility and the induction of adverse effects in the embryo/fetus. In developing a model for reproductive toxicity screening, it is important to define the stage of the human reproductive cycle that this specific model is going to recreate in vitro and to identify molecular targets that are critical for this stage of development. In this review, we focus our discussion on modeling pre-implantation embryotoxicity. The rationale for this is that despite advances on both clinical and biological levels, many unresolved infertility cases may be due to our lack of knowledge regarding environmental influences on this short, but critical stage of development. Data from in vitro fertilization practice suggest that the early-dividing embryo is very sensitive to numerous factors present in its microenvironment. In vivo, as the embryo travels down the oviduct, physical or chemical insults can directly damage the embryo and/or prevent implantation, and cause infertility. Multiple lines of evidence point to the differences between mouse and human pre-implantation development and between mouse and human embryonic stem cells (hESCs). In light of these data we present the case that hESCs and their derivatives are better suited as in vitro models for human pre-implantation development than their mouse counterparts. We then describe some of the most promising hESC-based systems that are used today to model certain aspects of development in the human pre-implantation embryo and that have the potential to be used for embryo toxicity screening tests in the near future. Described systems model two major events during differentiation of the human pre-implantation embryo: differentiation of the trophectoderm and segregation of the inner cell mass into epiblast and hypoblast. The first event is replicated in vitro by triggering either direct or indirect (through embryoid body stage) differentiation into trophectoderm. The second event can be modeled using the recently described system of high-throughput generation of embryoid bodies that recapitulate segregation of inner cell mass. We conclude by discussing the potential of these existing models in toxicology studies and the possibilities for their improvement in the future.

Struggles for equivalence: In vitro developmental toxicity model evolution in pharmaceuticals in 2006

Our group has been using the ECVAM Embryonic Stem Cell assay to predict developmental toxicity. In order to improve the separation of non-teratogens from weak teratogens, we have employed measures of gene expression, and different statistical methods from those originally used to develop the test. These approaches have fundamentally not improved the discrimination of 'weaks' from 'nons'. A realization that a very low value for cytotoxicity IC50 would drive a final result for the test in ways that were inappropriate for pharmaceuticals has led us to re-examine the cytotoxicity component. Our current efforts are focused on other, perhaps more sensitive, measures of cytotoxicity, combined with gene expression changes in mouse stem cells in an attempt to correctly identify weak teratogens and non-teratogens.

Regulation of dHAND protein expression by all-trans retinoic acid through ET-1/ETAR signaling in H9c2 cells

dHAND is thought to be a cardiac-restricted transcription factor during embryonic development. Vertebrate heart development involves many transcription factors such as Nkx2.5, GATA, and tbx5. All-trans retinoic acid (AtRA), the oxidative metabolite of vitamin A, can regulate the expression of these factors to affect embryonic heart development. However, the action of atRA on the expression of dHAND is rarely reported. To clarify whether atRA regulate the dHAND expression, we exposed cultured H9c2 cells (rat embryonic cardiomyocytes) to atRA and detected the protein expression of dHAND by Western blot analysis. We observed atRA can regulate the dHAND expression in a dose- and time-dependent manner. AtRA also inhibited endothelin-1 (ET-1) expression in a time-dependent manner. Further studies revealed that pretreatment with 10 microM BQ-123, a selective endothelin-1 receptor (ETAR) antagonist, for 2 h can significantly counteract the inhibition of 5 microM atRA treatment for 2 h of dHAND mRNA and protein expression. Taken together, these results suggest that atRA regulates dHAND expression by ET-1/ETAR signal transduction pathway in H9c2 cells. The mechanism of ET-1/ETAR signaling in controlling the level of dHAND protein is to reduce the levels of dHAND mRNA. It is possible for atRA to exert its cardiac teratogenesis during vertebrate embryonic development in this way.

Detection of tissue-specific effects by methotrexate on differentiating mouse embryonic stem cells

BACKGROUND

Pluripotent embryonic stem (ES) cells offer a unique possibility to monitor the differentiation of several cell types in vitro. This study attempts to identify marker genes during in vitro cell differentiation of murine ES cells and allow a prediction of chemical effects on cell differentiation of specific target tissues. The study focused on the expression pattern of key genes involved in cardiomyocyte and osteoblast differentiation: Oct-4, Brachyury, Nkx2.5, alpha myosin heavy chain, Cbfa1, and Osteocalcin.

METHODS

Methotrexate was selected due to its well-characterized teratogenic effects. Several in vivo studies have demonstrated the specific interactions of methotrexate with bone formation whereas the cardiovascular system is not specifically affected after exposure to low concentration. The capability of murine ES cells to differentiate in vitro into cardiomyocytes as well as into osteoblasts have been used to demonstrate the target cell specificity in vitro, at non-cytotoxic concentration.

RESULTS

Exposure of differentiating ES cells did not result in any gene profile modification of the selected cardiomyocyte specific genes, whereas the expression of osteoblast specific key genes, Cbfa1 and Osteocalcin, decreased. At the latter stages of skeletal differentiation we observed a 30% decrease in gene expression for Cbfa1 and a 60% decrease for Osteocalcin, with reference to the control. Early marker genes for undifferentiated cells and mesodermal cells were not modified after methotrexate treatment.

CONCLUSIONS

These results show the possibility to integrate specific in vitro tests for teratogenicity in a test strategy for developmental toxicity.