Validation of the Embryonic Stem Cell Test in the International ECVAM Validation Study on Three In Vitro Embryotoxicity Tests

[Embryonic stem cells in pharmacology]

Because of their self-renewal and pluripotency properties, human embryonic stem cells (hES) receive a marked attention from scientists and clinicians for regenerative medicine. The most recent application of hES cells may however reside in their use as a tool in drug development. The currently available cellular models for preclinical testing consist in primary and immortalized cells that display limitations in terms of available amount and likeliness to their in vivo counterparts, respectively. hES cells have the potential to revolutionize drug discovery by providing a physiological model for any human cell type in the desired amount for the earliest steps of drug development, notably for pharmacological, metabolic and toxicity evaluation. This new generation of model may contribute to reduce, refine or replace animal testing and decrease drug attrition.

P19 neuronal differentiation and retinoic acid metabolism as criteria to investigate atrazine, nitrite, and nitrate developmental toxicity

Atrazine and nitrogenous fertilizers are agrochemical contaminants frequently detected in water systems in North America. Several studies reported their ability to affect amphibian and mammalian development. Retinoids, supplied in the diet or synthesized by cells, are essential to embryogenesis. Disturbance of their homeostasis may lead to teratogenic effects. Retinoic acid (RA) is a major retinoid regulator of cell proliferation and differentiation. Previous studies reported alterations of retinoid stores in bullfrogs of Yamaska River subwatersheds (Québec, Canada), a region of intensive agricultural activities associated with atrazine, nitrate, and nitrite contaminants. These contaminants could affect RA metabolism and RA-mediated processes. Mouse P19 embryonic stem cells, which can differentiate to neurons in response to RA, were used to test this hypothesis. Cells were cultured in the absence or presence of contaminants during neuroinduction with RA and assayed by flow cytometry for expression of stage-specific embryonic antigen-1 (SSEA1) (embryonic marker) and betaIII-tubulin (neuronal marker). Cell cultures were also analyzed for RA metabolism by high performance liquid chromotagraphy (HPLC). Downregulation of SSEA1 paralleled betaIII-tubulin upregulation in an RA concentration-dependent manner. Atrazine, nitrate, and nitrite did not affect differentiation at environmentally encountered micromolar concentrations. However, low molar nitrite prevented RA-induced SSEA1 downregulation and decreased betaIII-tubulin appearance. Decreased cell viability/proliferation accompanied these differentiation effects. P19 cells metabolized RA to polar retinoids. RA metabolism was not affected at any concentration of atrazine, nitrate, or nitrite. Environmentally relevant levels of these contaminants, thus, had no gross effect on neurodifferentiation and RA catabolism of embryonic stem cells. P19 cell-based bioassays may provide valuable tools in monitoring developmental toxicity.

In vitro developmental toxicity test detects inhibition of stem cell differentiation by silica nanoparticles

While research into the potential toxic properties of nanomaterials is now increasing, the area of developmental toxicity has remained relatively uninvestigated. The embryonic stem cell test is an in vitro screening assay used to investigate the embryotoxic potential of chemicals by determining their ability to inhibit differentiation of embryonic stem cells into spontaneously contracting cardiomyocytes. Four well characterized silica nanoparticles of various sizes were used to investigate whether nanomaterials are capable of inhibition of differentiation in the embryonic stem cell test. Nanoparticle size distributions and dispersion characteristics were determined before and during incubation in the stem cell culture medium by means of transmission electron microscopy (TEM) and dynamic light scattering. Mouse embryonic stem cells were exposed to silica nanoparticles at concentrations ranging from 1 to 100 microg/ml. The embryonic stem cell test detected a concentration dependent inhibition of differentiation of stem cells into contracting cardiomyocytes by two silica nanoparticles of primary size 10 (TEM 11) and 30 (TEM 34) nm while two other particles of primary size 80 (TEM 34) and 400 (TEM 248) nm had no effect up to the highest concentration tested. Inhibition of differentiation of stem cells occurred below cytotoxic concentrations, indicating a specific effect of the particles on the differentiation of the embryonic stem cells. The impaired differentiation of stem cells by such widely used particles warrants further investigation into the potential of these nanoparticles to migrate into the uterus, placenta and embryo and their possible effects on embryogenesis.

Embryonic stem cell-derived cardiomyocytes as a model system to study cardioprotective effects of dexamethasone in doxorubicin cardiotoxicity

Embryonic stem cell (ESC)-derived beating cardiomyocytes may be considered as a suitable model for in vitro assessment of pharmacological and toxicological studies. In this model, laboratory animals are not required. In addition, physiological functions, such as heart beat, are assessed rather than single parameters such as cell viability. Here we report that doxorubicin (DOX) cardiotoxicity on mouse ESC-derived beating cardiomyocytes can be ameliorated by treatment with dexamethasone (DEX) when DEX is administrated only before DOX and not in combination with DOX. DEX effect appears to be mediated via glucocorticoid receptor and increases cardiomyocyte-specific gene expression. Cardiotoxicity of DOX can be augmented by calcium channel blocker, verapamil (VER) which also decreases the expression of cardiac gene markers. This model provides us with a clinical suggestion which proposes that the beneficial effect of DEX is obtained when DEX was added before DOX administration.

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.

Embryonic stem cell test remastered: comparison between the validated EST and the new molecular FACS-EST for assessing developmental toxicity in vitro

The embryonic stem cell test (EST) represents a reliable, scientifically validated in vitro system for the detection and classification of compounds according to their teratogenic potency. However, some serious issues were frequently raised against the widespread implementation and practicability of the EST in its original version. Most importantly, the evaluation of the morphological endpoint of beating cell agglomerates requires extensive experimental experience and is prone to misjudgment. Also, the testing period of 10 days is too long and costly to be attractive for industries interested in high-throughput screening of potential drug candidates. These drawbacks prompted us to work out a new molecular approach based on analysis of the expression of certain marker proteins specific for developing heart tissue. We have previously reported that quantitative flow cytometry of marker proteins (i.e., sarcomeric myosin heavy chain and alpha-actinin) can be performed at day 7 in embryonic stem cells from mice and combined with concurrent cell viability analysis. In the present study, extensive investigations were performed in order to explore the predictive power and validity of the newly established EST, subsequently referred to as molecular fluorescence activated cell sorting (FACS)-EST, by applying and comparing a set of 10 well-known embryotoxicants that encompasses the full range of chemical inherent embryotoxic potencies possible. While the molecular FACS-EST offered the same sensitivity compared to the validated EST protocol, the test duration could be significantly reduced. Due to significant improvements, this new molecular method holds promise as a sensitive, more rapid and reproducible screen highly suited to predict developmental toxicity in vivo from in vitro data.

[Stem cell-based in vitro models as alternative methods for toxicity and efficacy tests in animals]

Regarding toxicity and efficacy tests of pharmacological and chemical substances (REACH legislation in Europe), there is a strong need to develop alternative methods for animal in vivo studies, in particular for human in vitro models. Here we present results from early phases of projects exploring the potential of embryonic stem cell models, with a special emphasis on embryo toxicity and neuronal stress.We have been able to demonstrate key functional read-outs of neural hESC models, in addition to representing mechanistic aspects which are characteristic for ischemia or excitotoxicity. There is agreement that these mechanisms underlie a variety of human neurodegenerative diseases. We discuss the possibilities to develop more precise endpoints on the molecular level and present an example of a protein biomarker signature emerging from a European FP6 project about embryo toxicity (www.reprotect.eu), employing murine and human embryonic stem cell models.

Evaluation of the embryotoxic potency of compounds in a newly revised high throughput embryonic stem cell test

The ability of murine-derived embryonic stem cells (D3) to differentiate into cardiomyocytes is the basis of the embryonic stem cell test (EST). With the EST, chemicals and pharmaceuticals can be assessed for their embryotoxic potency early on in the development process. In order to come to a higher throughput EST, a 96-well based method was developed based on low attachment well plates that allow for the formation of embryonic bodies from which the stem cells can differentiate. Twelve test compounds were selected based on their reported in vitro and in vivo embryotoxic potency. In the 96-well based EST, reportedly strong embryotoxic compounds 5-fluorouracil, 6-aminonicotinamide (6AN), methylmercury chloride, and hydroxyurea were correctly ranked with corresponding Relative Embryotoxic Potency values (REP, based on the EC(50) (microM) value of 6AN) of 2.6 +/- 2.9, 1, 2.0 +/- 3.1, and 0.07 +/- 0.05, respectively. Moderately embryotoxic compounds valproic acid, boric acid, methoxyacetic acid, and lithium chloride resulted in a correct ranking with REP values of 0.01 +/- 0.003, 0.001 +/- 0.001, 0.0007 +/- 0.001, and 0.0006 +/- 0.0004, respectively. The included nonembryotoxic compounds Penicillin G, acrylamide, and saccharin did not result in an inhibition of D3 cells to differentiate into cardiomyocytes, other than related to cytotoxicity (REP value of 0.00001). However, diphenhydramine resulted in an inhibitory effect similarly to the strong embryotoxic compound hydroxyurea, with a REP value of 0.40 +/- 0.36. However, further evaluation suggested this was due to direct inhibition of the contractile capacity of the D3 cardiomyocytes, rather than an embryotoxic mechanism. The 96-well based EST is a promising addition to the screening process of newly developed chemicals and pharmaceuticals.

The challenge of reproductive and developmental toxicology under REACH

The European Union's REACH regulation has explicit requirements for reproductive and developmental toxicity data on all substances manufactured in or imported into the European Union at > or = 10 metric tons/year. Meeting the data requirements with whole-animal testing could result in the use of almost 22 million vertebrate animals for the registration of existing chemicals and cost up to several hundred thousand dollars per registered substance. The requirement for financial and animal resources can be reduced by the use of in vitro testing, quantitative structure-activity relationship models, and grouping of related substances. Although REACH strongly encourages these methods of avoiding vertebrate animal testing, it does not appear that in vitro testing or quantitative structure-activity relationship analysis will be able to replace whole-animal reproductive and developmental toxicity testing. Grouping of related compounds offers the possibility, perhaps in conjunction with in vitro testing and structure-activity analysis, of reducing vertebrate animal testing provided there is sufficient information on the related compounds and sufficient reason to believe that the related compounds will have similar toxicological properties. The designation of a substance as a reproductive or developmental toxicant follows criteria that do not consider the dose level of the substance at which reproductive or developmental effects occur, as long as excessive generalized toxicity does not occur. This method of labeling substances without consideration of effective dose level does not provide information on the actual risk of the chemical. Designation of a substance as a reproductive or developmental toxicant may have important implications under REACH and can be expected to result in the need to obtain authorization for marketing of the substance in the European Union.

Paraquat toxicity in a mouse embryonic stem cell model

The objective of this in vitro study was to use a mouse embryonic stem (mES) cell model to better understand pesticide injury that may adversely affect early pregnancy and to evaluate an antioxidant intervention. Undifferentiated D3 mES cells were incubated 24h with control, reference dose (RfD), no observed effect level (NOEL), or lowest observed effect level (LOEL) of paraquat, a commonly used, toxic agricultural herbicide. Pesticide effects were evaluated at 0 and 24h using assays for cell proliferation, total reactive oxygen species (ROS), viability, and alkaline phosphatase activity. Compared to 0 h, cell proliferation increased significantly in the 24h control treatment and was stalled in all paraquat dilutions tested. ROS production and percent necrotic and apoptotic cells were significantly increased at all paraquat concentrations examined. Alkaline phosphatase activity suggested that cells remained undifferentiated during the study period. Experiments with ascorbic acid suggested that pesticide effects on cell viability and ROS production were minimized by the recommended daily allowance (RDA) of vitamin C. Data suggest pesticide-induced injury can occur very early in development and at concentrations predicted without health consequences. Mouse ES cells may provide a useful in vitro model for rapidly screening developmental toxicants and protective interventions.

First steps in establishing a developmental toxicity test method based on human embryonic stem cells

The use of embryonic stem cells is currently the most promising approach to assess developmental toxicity in vitro. In addition, the possibility of using human embryonic stem (hES) cells will increase safety of consumers and patients as false classification of substances due to inter-species variations can be avoided. One validated test based on murine embryonic stem cells, the embryonic stem cell test (EST), consists of following endpoints: IC(50) values of fibroblasts and embryonic stem cells as well as the inhibition of differentiation of mES cells into cardiomyocytes. As a follow up of its successful validation study we established a cytotoxicity assay based on hES cells and human fibroblasts employing two developmental toxicants: 5-fluorouracil (5-FU) and all-trans retinoic acid (RA). The results were compared to historical data from the EST. For 5-FU, no significant differences were obtained between the different cell lines. However, for RA, both test systems produced higher IC(50) values for the fibroblasts than for the stem cells, which is a well-known effect of developmental toxicants. Moreover, the reliability and relevance of several marker genes as possible toxicological endpoints were tested. During early differentiation Oct-4, hTert and Dusp6 showed the most reliable results. Brachyury and GATA-4 were found to be best suited to monitor cardiac differentiation. The late cardiac marker gene TNNT2 demonstrated significant results until day 18. Therefore, these marker genes have the highest potential to serve as endpoints for a developmental toxicity test.

Human embryonic stem cells: current technologies and emerging industrial applications

The efficiency and accuracy of the drug development process is severely restricted by the lack of functional human cell systems. However, the successful derivation of pluripotent human embryonic stem (hES) cell lines in the late 1990s is expected to revolutionize biomedical research in many areas. Due to their growth capacity and unique developmental potential to differentiate into almost any cell type of the human body, hES cells have opened novel avenues both in basic and applied research as well as for therapeutic applications. In this review we describe, from an industrial perspective, the basic science that underlies the hES cell technology and discuss the current and future prospects for hES cells in novel and improved stem cell based applications for drug discovery, toxicity testing as well as regenerative medicine.

An in vitro embryotoxicity assay based on the disturbance of the differentiation of murine embryonic stem cells into endothelial cells. II. Testing of compounds

The embryonic stem cell test (EST) developed by Spielmann et al. [Spielmann, H., Pohl, I., Doering, B., Liebsch, M., Moldenhauer, F., 1997. The embryonic stem cell test, an in vitro embryotoxicity test using two permanent mouse cell lines: 3T3 fibroblasts and embryonic stem cells. In Vitro. Toxicol. 10, 119-127] is currently the most promising in vitro assay to predict the embryotoxic potential of compounds. In this assay the disturbance of the differentiation of embryonic stem (ES) cells into contracting cardiomyocytes by test compounds as well as the direct cytotoxicity of the test compounds on ES cells and 3T3 fibroblasts is analyzed. On the basis of these results and by applying a biostatistical prediction model (PM) [Genschow, E., Scholz, G., Brown, N., Piersma, A., Brady, M., Clemann, N., Huuskonen, H., Paillard, F., Bremer, S., Becker, K., Spielmann, H., 2000. Development of prediction models for three in vitro embryotoxicity tests in an ECVAM validation study. In Vitr. Mol. Toxicol. 13, 51-66; Genschow, E., Spielmann, H., Scholz, G., Pohl, I., Seiler, A., Clemann, N., Bremer, S., Becker, K., 2004. Validation of the embryonic stem cell test in the international ECVAM validation study on three in vitro embryotoxicity tests. Altern. Lab. Anim. 32, 209-244; Genschow, E., Spielmann, H., Scholz, G., Seiler, A., Brown, N., Piersma, A., Brady, M., Clemann, N., Huuskonen, H., Paillard, F., Bremer, S., Becker, K., 2002. The ECVAM international validation study on in vitro embryotoxicity tests: results of the definitive phase and evaluation of prediction models. European Centre for the Validation of Alternative Methods. Altern. Lab. Anim. 30, 151-176] test compounds can be classified as non-embryotoxic, weakly or strongly embryotoxic. In order to introduce a further endpoint into the EST, the disturbance of vasculogenesis and/or angiogenesis, a protocol to differentiate ES cells into endothelial cells, was established in the accompanying paper. PECAM-1 and VE-Cadherin gene expressions, quantified by real-time TaqMan PCR, were shown to be appropriate molecular markers for the differentiation of ES cells into endothelial cells. In the present study, the disturbance of the differentiation of ES cells into endothelial cells (i.e. the reduction in the expression of PECAM-1 and VE-Cadherin) by six test compounds with known embryotoxic potential was investigated: all-trans-retinoic acid (RA) and 5-fluorouracil (5-FU) are strongly embryotoxic, diphenylhydantoin (DPH) and valproic acid (Val) are weakly embryotoxic and saccharin (Sacch) and penicillin G (Pen G) are non-embryotoxic. In a first step the concentration of the test compound resulting in a 50% inhibition of PECAM-1 and VE-Cadherin gene expression and the concentration leading to a 50% decrease in the viability of ES cells and 3T3 fibroblasts were determined. In a second step and in a first attempt to assess the predictive potential of the newly developed test system the concentration values obtained were applied in the PM of the established EST to classify the selected test compounds. All six test compounds were correctly classified (i.e. the data obtained in vitro correlated with their known embryotoxic potential in vivo). Taken together it can be concluded that the disturbance of the differentiation of murine ES cells into endothelial cells represents a very promising new endpoint in a broadened EST with PECAM-1 and VE-Cadherin as specific differentiation marker genes.

The human embryonic stem cell-derived cardiomyocyte as a pharmacological model

Embryonic stem (ES) cells are specialised cells derived from the early embryo, which are capable of both sustained propagation in the undifferentiated state as well as subsequent differentiation into the majority of cell lineages. Human ES cells are being developed for clinical tissue repair, but a number of problems must be addressed before this becomes a reality. However, they also have potential for translational benefit through its use as a test system for screening pharmaceutical compounds. In the cardiac field, present model systems are not ideal for either screening or basic pharmacological/physiological studies. Cardiomyocytes produced from human ES differentiation have advantages for these purposes over the primary isolated cells or the small number of cell lines available. This review describes the methodology for obtaining cardiomyocytes from human embryonic stem cell-derived cardiomyocyte (hESCM), for increasing the proportion of cardiomyocytes in the preparation and for isolating single embryonic stem cell-derived cardiomyocyte (ESCM) from clusters. Their morphological, contractile and electrophysiological characteristics are compared to mature and immature primary cardiomyocytes. The advantages and disadvantages of the hESCM preparation for long term culture and genetic manipulation are described. Basic pharmacological studies on adrenoceptors and muscarinic receptors in hESCM have been performed, and have given stable and reproducible responses. Prolongation of repolarisation can be detected using hESCM cultured on multielectrode arrays (MEA). Human ESCM have a clear potential to improve model systems available for both basic scientific studies and pharmaceutical screening of cardiac target compounds.