Pluripotent Stem Cell Assays: Modalities and Applications For Predictive Developmental Toxicity

Engineering a computable epiblast for in silico modeling of developmental toxicity

Developmental hazard evaluation is an important part of assessing chemical risks during pregnancy. Toxicological outcomes from prenatal testing in pregnant animals result from complex chemical-biological interactions, and while New Approach Methods (NAMs) based on in vitro bioactivity profiles of human cells offer promising alternatives to animal testing, most of these assays lack cellular positional information, physical constraints, and regional organization of the intact embryo. Here, we engineered a fully computable model of the embryonic disc in the CompuCell3D.org modeling environment to simulate epithelial-mesenchymal transition (EMT) of epiblast cells and self-organization of mesodermal domains (chordamesoderm, paraxial, lateral plate, posterior/extraembryonic). Mesodermal fate is modeled by synthetic activity of the BMP4-NODAL-WNT signaling axis. Cell position in the epiblast determines timing with respect to EMT for 988 computational cells in the computer model. An autonomous homeobox (Hox) clock hidden in the epiblast is driven by WNT-FGF4-CDX signaling. Executing the model renders a quantitative cell-level computation of mesodermal fate and consequences of perturbation based on known biology. For example, synthetic perturbation of the control network rendered altered phenotypes (cybermorphs) mirroring some aspects of experimental mouse embryology, with electronic knockouts, under-activation (hypermorphs) or over-activation (hypermorphs) particularly affecting the size and specification of the posterior mesoderm. This foundational model is trained on embryology but capable of performing a wide variety of toxicological tasks conversing through anatomical simulation to integrate in vitro chemical bioactivity data with known embryology. It is amenable to quantitative simulation for probabilistic prediction of early developmental toxicity.

Investigating the applicability domain of the hiPSC-based PluriLum assay: an embryotoxicity assessment of chemicals and drugs

To meet the growing demand for developmental toxicity assessment of chemicals, New Approach Methodologies (NAMs) are needed. Previously, we developed two 3D in vitro assays based on human-induced pluripotent stem cells (hiPSC) and cardiomyocyte differentiation: the PluriBeat assay, based on assessment of beating differentiated embryoid bodies, and the PluriLum assay, a reporter gene assay based on the expression of the early cardiac marker NKX2.5; both promising assays for predicting embryotoxic effects of chemicals and drugs. In this work, we aimed to further describe the predictive power of the PluriLum assay and compare its sensitivity with PluriBeat and similar human stem cell-based assays developed by others. For this purpose, we assessed the toxicity of a panel of ten chemicals from different chemical classes, consisting of the known developmental toxicants 5-fluorouracil, all-trans retinoic acid and valproic acid, as well as the negative control compounds ascorbic acid and folic acid. In addition, the fungicides epoxiconazole and prochloraz, and three perfluoroalkyl substances (PFAS), PFOS, PFOA and GenX were tested. Generally, the PluriLum assay displayed higher sensitivity when compared to the PluriBeat assay. For several compounds the luminescence readout of the PluriLum assay showed effects not detected by the PluriBeat assay, including two PFAS compounds and the two fungicides. Overall, we find that the PluriLum assay has the potential to provide a fast and objective detection of developmental toxicants and has a level of sensitivity that is comparable to or higher than other in vitro assays also based on human stem cells and cardiomyocyte differentiation for assessment of developmental toxicity.

Validation of a mouse 3D gastruloid-based embryotoxicity assay in reference to the ICH S5(R3) guideline chemical exposure list

There is growing interest in establishing alternative methods in place of conventional animal tests to assess the developmental and reproductive toxicity (DART) of chemicals. Gastruloids are 3D aggregates of pluripotent stem cells that spontaneously exhibit axial elongation morphogenesis similar to gastrulation. They have been explored as in vitro embryogenesis models for developmental and toxicological studies. Here, a mouse gastruloid-based assay was validated for DART assessment in accordance with the ICH S5(R3) guideline, which provides the plasma concentration data of various reference drugs in rodents, specifically Cmax and AUC for NOAEL and LOAEL. First, adverse effect concentrations of the reference drugs and their known metabolites on gastruloid development were determined based on morphological impact, namely reduced growth or aberrant elongation. Then, the NOAEL to LOAEL concentration range obtained from the gastruloid assay was compared with that in rodents to examine similarities in sensitivity between the in vitro and in vivo assays for each chemical. For 18 out of the 24 reference drugs that have both NOAEL and LOAEL information in rodents, the sensitivity of the gastruloid assay was comparable to the in vivo assay within an 8-fold concentration margin. For 7 out of the 8 additional reference drugs that have only NOAEL or LOAEL information in rodents, the gastruloid assay was in line with the in vivo data. Altogether, these results support the effectiveness of the gastruloid assay, which may be exploited as a non-animal alternative method for DART assessment.

Validation of a new protocol for a zebrafish MEFL (malformation or embryo-fetal lethality) test method that conforms to the ICH S5 (R3) guideline

Detecting the toxic effects of chemicals on reproduction and development without using mammalian animal models is crucial in the exploitation of pharmaceuticals for human use. Zebrafish are a promising animal model for investigating pharmacological effects and toxicity during vertebrate development. Several studies have suggested the use of zebrafish embryos for the assessment of malformations or embryo-fetal lethality (MEFL). However, a reproducible protocol as a standard for the zebrafish MEFL test method that fulfills global requests has not been established based on the International Council of Harmonisation (ICH) S5 (R3) guidelines. To establish such a toxicity test method, we developed a new and easy protocol to detect MEFL caused by chemicals, especially those with teratogenic potential, using fertilized zebrafish eggs (embryos) within 5 days of development. Our toxicity test trials using the same protocol in two to four different laboratories corroborated the high inter-laboratory reproducibility. Our test method enabled the detection of 18 out of 22 test compounds that induced rat MEFL. Thus, the prediction rate of our zebrafish test method for MEFL was almost 82% compared with that of rat MEFL. Collectively, our study proposes the establishment of an easy and reproducible protocol for the zebrafish MEFL test method for reproductive and developmental toxicity that meets ICH guideline S5 (R3), which can be further considered in combination with information from other sources for regulatory use.

Reproductive-Toxicity-Related Endpoints in C. elegans Are Consistent with Reduced Concern for Dimethylarsinic Acid Exposure Relative to Inorganic Arsenic

Exposures to arsenic and mercury are known to pose significant threats to human health; however, the effects specific to organic vs. inorganic forms are not fully understood. Caenorhabditis elegans' (C. elegans) transparent cuticle, along with the conservation of key genetic pathways regulating developmental and reproductive toxicology (DART)-related processes such as germ stem cell renewal and differentiation, meiosis, and embryonic tissue differentiation and growth, support this model's potential to address the need for quicker and more dependable testing methods for DART hazard identification. Organic and inorganic forms of mercury and arsenic had different effects on reproductive-related endpoints in C. elegans, with methylmercury (meHgCl) having effects at lower concentrations than mercury chloride (HgCl₂), and sodium arsenite (NaAsO₂) having effects at lower concentrations than dimethylarsinic acid (DMA). Progeny to adult ratio changes and germline apoptosis were seen at concentrations that also affected gravid adult gross morphology. For both forms of arsenic tested, germline histone regulation was altered at concentrations below those that affected progeny/adult ratios, while concentrations for these two endpoints were similar for the mercury compounds. These C. elegans findings are consistent with corresponding mammalian data, where available, suggesting that small animal model test systems may help to fill critical data gaps by contributing to weight of evidence assessments.

In vitro tests to evaluate embryotoxicity and irritation of Chinese herbal medicine (Pentaherbs formulation) for atopic dermatitis

ETHNOPHARMACOLOGICAL RELEVANCE

Atopic dermatitis (AD) is a common chronic inflammatory skin disorder and its prevalence is increasing in the last few decades. No treatment can cure the condition. Pregnancy often worsens the clinical manifestation. There are considerable interests in Chinese Herbal Medicine (CHM) as an alternative treatment for AD. A well tolerated CHM formula (Pentaherbs formulation, PHF) has been proven efficacious in improving life quality and reducing topical corticosteroid use in children with moderate-to-severe AD. However, safety data of PHF are not available.

AIM OF THE STUDY

Our study aimed to evaluate the safety of PHF and its 5 individual herbal extracts, including embryotoxicity by Embryonic Stem Cell Test (EST) and irritation by Skin Irritation Test (SIT).

MATERIALS AND METHODS

Quality of 5 herbal extracts of PHF was confirmed by chromatography. In EST, mouse embryonic stem cell line (D3) and mouse fibroblast cell line (3T3) were used to study potential embryotoxicity. Three endpoints were assessed by concentration-response curves after 10 days' culture: 50% inhibition of D3 differentiation into beating cardiomyocytes (ID₅₀D3), 50% cytotoxic effects on D3 (IC₅₀D3) and on fibroblasts (IC₅₀3T3). A biostatistically based prediction model (PM) was applied to predict the embryotoxic potentials of each CHM. In SIT, epidermis equivalent commercially available kits (EpiDerm™) were used, and concentration-viability curves were obtained by MTT assay to detect skin irritations of each CHM.

RESULTS

Chemical authentication confirmed that 5 test herbal extracts contained their main active compounds. EST results indicated that the formula PHF and its individual CHMs were non-embryotoxic, except one CHM, Amur Corktree Bark (Huang Bai, Phellodendron chinense C.K.Schneid), was weakly embryotoxic. SIT results showed that cell viability was above 50% after treatment with different concentrations of all tested CHMs.

CONCLUSIONS

Our in vitro tests provided preliminary evidence for safety of the formula PHF in embryonic stem cell test and skin irritation model, but PHF shall be cautiously used in pregnant women with AD. Further studies are needed to support its clinical application as an alternative treatment for AD, especially to the patients who plan for pregnancy or at lactation stages.

Developmental toxicity of remdesivir, an anti-COVID-19 drug, is implicated by in vitro assays using morphogenetic embryoid bodies of mouse and human pluripotent stem cells

BACKGROUND

Remdesivir is an antiviral drug approved for the treatment of COVID-19, whose developmental toxicity remains unclear. More information about the safety of remdesivir is urgently needed for people of childbearing potential, who are affected by the ongoing pandemic. Morphogenetic embryoid bodies (MEBs) are three-dimensional (3D) aggregates of pluripotent stem cells that recapitulate embryonic body patterning in vitro, and have been used as effective embryo models to detect the developmental toxicity of chemical exposures specifically and sensitively.

METHODS

MEBs were generated from mouse P19C5 and human H9 pluripotent stem cells, and used to examine the effects of remdesivir. The morphological effects were assessed by analyzing the morphometric parameters of MEBs after exposure to varying concentrations of remdesivir. The molecular impact of remdesivir was evaluated by measuring the transcript levels of developmental regulator genes.

RESULTS

The mouse MEB morphogenesis was impaired by remdesivir at 1-8 μM. Remdesivir affected MEBs in a manner dependent on metabolic conversion, and its potency was higher than GS-441524 and GS-621763, presumptive anti-COVID-19 drugs that act similarly to remdesivir. The expressions of developmental regulator genes, particularly those involved in axial and somite patterning, were dysregulated by remdesivir. The early stage of MEB development was more vulnerable to remdesivir exposure than the later stage. The morphogenesis and gene expression profiles of human MEBs were also impaired by remdesivir at 1-8 μM.

CONCLUSIONS

Remdesivir impaired mouse and human MEBs at concentrations that are comparable to the therapeutic plasma levels in humans, urging further investigation into the potential impact of remdesivir on developing embryos.