Exposure to low-dose trichloroethylene alters shear stress gene expression and function in the developing chick heart

How studies in developmental epithelial-mesenchymal transition and mesenchymal-epithelial transition inspired new research paradigms in biomedicine

Epithelial-mesenchymal transition (EMT) and its reverse mechanism, mesenchymal-epithelial transition (MET), are evolutionarily conserved mechanisms initially identified in studies of early metazoan development. EMT may even have been established in choanoflagellates, the closest unicellular relative of Metazoa. These crucial morphological transitions operate during body plan formation and subsequently in organogenesis. These findings have prompted an increasing number of investigators in biomedicine to assess the importance of such mechanisms that drive epithelial cell plasticity in multiple diseases associated with congenital disabilities and fibrosis, and, most importantly, in the progression of carcinoma. EMT and MET also play crucial roles in regenerative medicine, notably by contributing epigenetic changes in somatic cells to initiate reprogramming into stem cells and their subsequent differentiation into distinct lineages.

Soft-Tissue Material Properties and Mechanogenetics during Cardiovascular Development

During embryonic development, changes in the cardiovascular microstructure and material properties are essential for an integrated biomechanical understanding. This knowledge also enables realistic predictive computational tools, specifically targeting the formation of congenital heart defects. Material characterization of cardiovascular embryonic tissue at consequent embryonic stages is critical to understand growth, remodeling, and hemodynamic functions. Two biomechanical loading modes, which are wall shear stress and blood pressure, are associated with distinct molecular pathways and govern vascular morphology through microstructural remodeling. Dynamic embryonic tissues have complex signaling networks integrated with mechanical factors such as stress, strain, and stiffness. While the multiscale interplay between the mechanical loading modes and microstructural changes has been studied in animal models, mechanical characterization of early embryonic cardiovascular tissue is challenging due to the miniature sample sizes and active/passive vascular components. Accordingly, this comparative review focuses on the embryonic material characterization of developing cardiovascular systems and attempts to classify it for different species and embryonic timepoints. Key cardiovascular components including the great vessels, ventricles, heart valves, and the umbilical cord arteries are covered. A state-of-the-art review of experimental techniques for embryonic material characterization is provided along with the two novel methods developed to measure the residual and von Mises stress distributions in avian embryonic vessels noninvasively, for the first time in the literature. As attempted in this review, the compilation of embryonic mechanical properties will also contribute to our understanding of the mature cardiovascular system and possibly lead to new microstructural and genetic interventions to correct abnormal development.

Cx43 overexpression is involved in the hyper-proliferation effect of trichloroethylene on human embryonic stem cells

Trichloroethylene (TCE) is a major environmental contaminant. Maternal exposure of TCE is linked to developmental defects, but the mechanisms remain to be elucidated. Along with a strategy of 3Rs principle, human embryonic stem cells (hESCs) are regarded as most promising in vitro models for developmental toxicity studies. TCE interfered with hESCs differentiation, but no report was available for TCE effects on hESCs proliferation. Here, we aimed to explore the toxic effects and mechanisms of TCE on hESCs proliferation. Treatment with TCE, did not affect the pluripotency genes expression. However, TCE enhanced hESCs proliferation, manifested by increased cell number, PCNA expression and EdU incorporation. Moreover, TCE exposure upregulated the protein expression levels of Cx43 and cyclin-dependent kinases. Knockdown of Cx43 attenuated the TCE-induced cell hyper-proliferation and CDK2 upregulation. Furthermore, TCE increased Akt phosphorylation, and the inhibition of Akt blocked the TCE-induced Cx43 overexpression and cell proliferation. In conclusion, TCE exposure resulted in upregulation of Cx43 via Akt phosphorylation, consequently stimulated CDK2 expression, contributing to hyper-proliferation in hESCs. Our study brings to light that TCE stimulated the proliferation of hESCs via Cx43, providing a new research avenue for the causes of TCE-induced developmental toxicity.

Mechanosensitive Pathways in Heart Development: Findings from Chick Embryo Studies

The heart is the first organ that starts to function in a developing embryo. It continues to undergo dramatic morphological changes while pumping blood to the rest of the body. Genetic regulation of heart development is partly governed by hemodynamics. Chick embryo is a major animal model that has been used extensively in cardiogenesis research. To reveal mechanosensitive pathways, a variety of surgical interferences and chemical treatments can be applied to the chick embryo to manipulate the blood flow. Such manipulations alter expressions of mechanosensitive genes which may anticipate induction of morphological changes in the developing heart. This paper aims to present different approaches for generating clinically relevant disturbed hemodynamics conditions using this embryonic chick model and to summarize identified mechanosensitive genes using the model, providing insights into embryonic origins of congenital heart defects.

Systems Toxicology Approach for Testing Chemical Cardiotoxicity in Larval Zebrafish

Transcriptomic approaches can give insight into molecular mechanisms underlying chemical toxicity and are increasingly being used as part of toxicological assessments. To aid the interpretation of transcriptomic data, we have developed a systems toxicology method that relies on a computable biological network model. We created the first network model describing cardiotoxicity in zebrafish larvae-a valuable emerging model species in testing cardiotoxicity associated with drugs and chemicals. The network is based on scientific literature and represents hierarchical molecular pathways that lead from receptor activation to cardiac pathologies. To test the ability of our approach to detect cardiotoxic outcomes from transcriptomic data, we have selected three publicly available data sets that reported chemically induced heart pathologies in zebrafish larvae for five different chemicals. Network-based analysis detected cardiac perturbations for four out of five chemicals tested, for two of them using transcriptomic data collected up to 3 days before the onset of a visible phenotype. Additionally, we identified distinct molecular pathways that were activated by the different chemicals. The results demonstrate that the proposed integrational method can be used for evaluating the effects of chemicals on the zebrafish cardiac function and, together with observed cardiac apical end points, can provide a comprehensive method for connecting molecular events to organ toxicity. The computable network model is freely available and may be used to generate mechanistic hypotheses and quantifiable perturbation values from any zebrafish transcriptomic data.

Downregulation of miR-133a contributes to the cardiac developmental toxicity of trichloroethylene in zebrafish

Trichloroethylene (TCE), a widely used organic solvent, is a common environmental pollutant. Increasing evidence indicates that maternal TCE exposure is associated with congenital cardiac defects, but the underlining mechanisms remain largely unknown. In this study, we revealed that TCE exposure significantly induced heart defects and dysfunctions in zebrafish embryos. Heart tissues were dissected and subjected to high throughput sequencing and qPCR to identify differentially expressed miRNAs and mRNAs. The effects of miRNA were further verified by microinjection of antagomir or agomir. Reactive Oxygen Species (ROS) and cell proliferation were measured by using dichlorodihydrofluorescein diacetate (DCFH-DA) and EdU staining, respectively. Our results showed that 19 miRNAs were downregulated whereas 48 miRNAs were upregulated in the heart of zebrafish embryos. The downregulation of miR-133a and the upregulation of miR-182 were further validated. Moreover, we found that miR-133a agomir significantly alleviated the TCE-induced heart defects while miR-133a antagomir mimicked the toxic effect of TCE on heart development. Furthermore, miR-133a agomir significantly counteracted TCE-induced ROS production and excessive cell proliferation in the heart of zebrafish embryos. In conclusion, our results indicate that miR-133a mediates TCE-induced ROS generation, leading to excessive cell proliferation and heart defects.

Follow Me! A Tale of Avian Heart Development with Comparisons to Mammal Heart Development

Avian embryos have been used for centuries to study development due to the ease of access. Because the embryos are sheltered inside the eggshell, a small window in the shell is ideal for visualizing the embryos and performing different interventions. The window can then be covered, and the embryo returned to the incubator for the desired amount of time, and observed during further development. Up to about 4 days of chicken development (out of 21 days of incubation), when the egg is opened the embryo is on top of the yolk, and its heart is on top of its body. This allows easy imaging of heart formation and heart development using non-invasive techniques, including regular optical microscopy. After day 4, the embryo starts sinking into the yolk, but still imaging technologies, such as ultrasound, can tomographically image the embryo and its heart in vivo. Importantly, because like the human heart the avian heart develops into a four-chambered heart with valves, heart malformations and pathologies that human babies suffer can be replicated in avian embryos, allowing a unique developmental window into human congenital heart disease. Here, we review avian heart formation and provide comparisons to the mammalian heart.

Systematic evaluation of mechanistic data in assessing in utero exposures to trichloroethylene and development of congenital heart defects

The hypothesis that in utero exposures to low levels of trichloroethylene (TCE) may increase the risk of congenital heart defects (CHDs) in offspring remains a subject of substantial controversy within the scientific community due primarily to the reliance on an inconsistent and unreproducible experimental study in rats. To build on previous assessments that have primarily focused on epidemiological and experimental animal studies in developing conclusions, the objective of the current study is to conduct a systematic evaluation of mechanistic data related to in utero exposures to TCE and the development of CHDs. The evidence base was heterogeneous; 79 mechanistic datasets were identified, characterizing endpoints which ranged from molecular to organismal responses in seven species, involving both in vivo and in vitro study designs in mammalian and non-mammalian models. Of these, 24 datasets were considered reliable following critical appraisal using a study quality tool that employs metrics specific to the study type. Subsequent synthesis and integration demonstrated that the available mechanistic data: 1) did not support the potential for CHD hazard in humans, 2) did not support the biological plausibility of a response in humans based on organization via a putative adverse outcome pathway for valvulo-septal cardiac defects, and 3) were not suitable for serving as candidate studies in risk assessment. Findings supportive of an association were generally limited to in ovo chicken studies, in which TCE was administered in high concentration solutions via direct injection. Results of these in ovo studies were difficult to interpret for human health risk assessment given the lack of generalizability of the study models (including dose relevance, species-specific biological differences, variations in the construct of the study design, etc.). When the mechanistic data are integrated with findings from previous evaluations of human and animal evidence streams, the totality of evidence does not support CHDs as a critical effect in TCE human health risk assessment.

Developmental toxicity of trichloroethylene in zebrafish (Danio rerio)

Trichloroethylene (TCE), an industrial solvent and degreaser, is an environmental toxicant that contaminates over half of Superfund sites, is a known carcinogen, and is linked to congenital defects and neurodegenerative disease. The developmental toxicity of TCE near ecologically relevant levels needs further characterization in order to better assess health risks of exposure. In this study, the toxicodynamics of TCE in the zebrafish (Danio rerio) model was investigated through the establishment of a LC50 concentration and by monitoring the acute developmental toxicity of ecologically relevant concentrations (0, 5, 50, and 500 parts per billion; ppb) of TCE during two different exposure lengths (1-72 hours post fertilization (hpf) and 1-120 hpf). Acute developmental toxicity was assessed by monitoring survival and hatching, larval morphology, larval heart rate, and behavioral responses during an embryonic photomotor response test and a larval visual motor response test. Embryonic exposure to TCE was associated with decreased percent hatch at 48 hpf, altered larval morphology, increased heart rate, and altered behavioral responses during the photomotor response test and visual motor response test. Larval morphology and behavioral alterations were more pronounced in the 1-120 hpf exposure length trials. The observed alterations suggest developmental TCE toxicity is still a concern at regulatory concentrations and that timing of exposure influences developmental toxicity.

HNF4a transcription is a target of trichloroethylene toxicity in the embryonic mouse heart

In exploration of congenital heart defects produced by TCE, Hepatocyte Nuclear Factor 4 alpha (HNF4a) transcriptional activity was identified as a central component. TCE exposure altered gene transcription in the chick heart in a non-monotonic pattern where only low dose exposure inhibited transcription by HNF4a. As the chick embryo is non-placental, we examine here HNF4a as a target of TCE in developing mouse embryos. Benfluorex and Bi6015, published agonist and antagonist, respectively, of HNF4a were compared to low dose TCE exposure. Pregnant mice were exposed to 10 ppb (76 nM) TCE, 5 μM Benfluorex, 5 μM Bi6015, or a combination of Bi6015 and TCE in drinking water. Litters (E12) were collected during a sensitive window in heart development. Embryonic hearts were collected, pooled for extraction of RNA and marker expression was examined by quantitative PCR. Multiple markers, previously identified as sensitive to TCE exposure in chicks or as published targets of HNF4a transcription were significantly affected by Benfluorex, Bi6015 and TCE. Activity of TCE and both HNF4a-specific reagents on transcription argues that HNF4a is a component of TCE cardiotoxicity and likely a proximal target of low dose exposure during development. The effectiveness of these reagents after delivery in maternal drinking water suggests that neither maternal metabolism, nor placental transport is protective of exposure.

Membrane stabilizer Poloxamer 188 improves yield of primary isolated rat cardiomyocytes without impairing function

Intact cardiomyocytes are used to investigate cardiac contractility and evaluate the efficacy of new therapeutic compounds. Primary enzymatic isolation of adult rodent cardiomyocytes has limitations, including low cardiomyocyte survival, which is likely due to ischemic conditions and/or membrane damage. The addition of Poloxamer 188 (P188) has been used to reduce ischemia‐ and membrane‐related damage in ischemia–reperfusion and muscular dystrophy studies. P188 stabilizes membranes, reducing cell death. Cardiomyocytes were isolated from rats, under three conditions: (1) using standard isolation solutions, (2) with P188 added during cannulation (ischemic event), and (3) with P188 added during cannulation, enzymatic digestion, and trituration. Cell survival was assessed by quantifying the number of rod‐shaped versus contracted cells on the day of isolation and up to 3 days post‐isolation. Adding P188 only during cannulation yielded improved survival on the day of isolation. Little difference in survival was seen among the three conditions in the days post‐isolation. Cardiomyocyte function was assessed by measuring calcium transients and unloaded sarcomere lengths for up to 2 days post‐isolation. P188 did not consistently alter calcium handling or sarcomere shortening in the isolated cardiomyocytes. We conclude that the addition of P188 to the cannulation (e.g., wash) of the isolated heart may improve initial survival of cardiomyocytes upon primary enzymatic isolation.

Trichloroethylene perturbs HNF4a expression and activity in the developing chick heart

Exposure to trichloroethylene (TCE) is linked to formation of congenital heart defects in humans and animals. Prior interactome analysis identified the transcription factor, Hepatocyte Nuclear Factor 4 alpha (HNF4a), as a potential target of TCE exposure. As a role for HNF4a is unknown in the heart, we examined developing avian hearts for HNF4a expression and for sensitivity to TCE and the HNF4a agonist, Benfluorex. In vitro analysis using a HNF4a reporter construct showed both TCE and HFN4a to be antagonists of HNF4a-mediated transcription at the concentrations tested. HNF4a mRNA is expressed transiently in the embryonic heart during valve formation and cardiac development. Embryos were examined for altered gene expression in the presence of TCE or Benfluorex. TCE altered expression of selected mRNAs including HNF4a, TRAF6 and CYP2C45. There was a transition between inhibition and induction of marker gene expression in embryos as TCE concentration increased. Benfluorex was largely inhibitory to selected markers. Echocardiography of exposed embryos showed reduced cardiac function with both TCE and Benfluorex. Cardiac contraction was reduced by 29% and 23%, respectively at 10 ppb. The effects of TCE and Benfluorex on autocrine regulation of HNF4a, selected markers and cardiac function argue for a functional interaction of TCE and HNF4a. Further, the dose-sensitive shift between inhibition and induction of marker expression may explain the nonmonotonic-like dose response observed with TCE exposure in the heart.

Human aortic endothelial cells respond to shear flow in well-plate microfluidic devices

Although chronic progressive cardiovascular diseases such as atherosclerosis are often challenging to fully model in vitro, it has been shown that certain in vitro methods can effectively evaluate some aspects of disease progression. This has been demonstrated in in vitro and in vivo studies of endothelial cells that have illustrated the effects of nitric oxide (NO) production, filamentous actin (F-actin) formation, and cell and actin angle alignment on vascular function and homeostasis. Systems utilising shear flow have been established, in order to create a physiologically relevant environment for cells that require shear flow for homeostasis. Here, we investigated the use of a well-plate microfluidic system and associated devices (0-20dyn/cm²) to demonstrate applied shear effects on primary Human Aortic Endothelial Cells (HAECs). Changes in cell and actin alignment in the direction of flow, real-time production of NO and gross cell membrane shape changes in response to physiological shear flow were observed. These commercial systems have a range of potential applications, including within the consumer and pharmaceutical industries, thereby reducing the dependency on animal testing for regulatory safety assessments.

RNA-seq analysis of the kidneys of broiler chickens fed diets containing different concentrations of calcium

Calcium (Ca) is required for normal growth and is involved in cellular physiology, signal transduction, and bone mineralization. In humans, inadequate Ca intake causes hypocalcaemia, and excessive Ca intake causes hypercalcemia. In chicken, Ca is also required for body weight gain and eggshell formation. However, transcriptomic responses to low/high Ca intake, and mechanisms affecting body weight have not been explored. In this study, we performed comparative RNA sequencing (RNA-seq) using the kidney of broiler chickens fed diets containing 0.8, 1.0, and 1.2% Ca. Annotation of RNA-seq data revealed a significant number of differentially expressed genes (DEGs) in the kidney via pairwise comparison using Cufflinks and edgeR. Using edgeR, we identified 12 DEGs; seven overlapped with those found by cufflinks. Seven DEGs were validated by real-time quantitative-PCR (qRT-PCR) in Ca-supplemented kidneys, and the results correlated with the RNA-seq data. DEGs identified by cufflinks/edgeR were subjected to pathway enrichment, protein/protein interaction, and co-occurrence analyses to determine their involvement in disease. The National Research Council (NRC) recommended Ca intake for 21-day post-hatch broilers is about 1.0%. Our findings suggest that higher-than-recommended Ca intake (1.2%) could reduce body weight gain in broilers, and that affected DEGs are related to stress-induced diseases, such as hypertension.

Disruption of cardiogenesis in human embryonic stem cells exposed to trichloroethylene

Trichloroethylene (TCE) is ubiquitous in our living environment, and prenatal exposure to TCE is reported to cause congenital heart disease in humans. Although multiple studies have been performed using animal models, they have limited value in predicting effects on humans due to the unknown species-specific toxicological effects. To test whether exposure to low doses of TCE induces developmental toxicity in humans, we investigated the effect of TCE on human embryonic stem cells (hESCs) and cardiomyocytes (derived from the hESCs). In the current study, hESCs cardiac differentiation was achieved by using differentiation medium consisting of StemPro-34. We examined the effects of TCE on cell viability by cell growth assay and cardiac inhibition by analysis of spontaneously beating cluster. The expression levels of genes associated with cardiac differentiation and Ca²⁺ channel pathways were measured by immunofluorescence and qPCR. The overall data indicated the following: (1) significant cardiac inhibition, which was characterized by decreased beating clusters and beating rates, following treatment with low doses of TCE; (2) significant up-regulation of the Nkx2.5/Hand1 gene in cardiac progenitors and down regulation of the Mhc-7/cTnT gene in cardiac cells; and (3) significant interference with Ca²⁺ channel pathways in cardiomyocytes, which contributes to the adverse effect of TCE on cardiac differentiation during early embryo development. Our results confirmed the involvement of Ca²⁺ turnover network in TCE cardiotoxicity as reported in animal models, while the inhibition effect of TCE on the transition of cardiac progenitors to cardiomyocytes is unique to hESCs, indicating a species-specific effect of TCE on heart development. This study provides new insight into TCE biology in humans, which may help explain the development of congenital heart defects after TCE exposure. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1372-1380, 2016.

A systematic evaluation of the potential effects of trichloroethylene exposure on cardiac development

The 2011 EPA trichloroethylene (TCE) IRIS assessment, used developmental cardiac defects from a controversial drinking water study in rats (Johnson et al. [51]), along with several other studies/endpoints to derive reference values. An updated literature search of TCE-related developmental cardiac defects was conducted. Study quality, strengths, and limitations were assessed. A putative adverse outcome pathway (AOP) construct was developed to explore key events for the most commonly observed cardiac dysmorphologies, particularly those involved with epithelial-mesenchymal transition (EMT) of endothelial origin (EndMT); several candidate pathways were identified. A hypothesis-driven weight-of-evidence analysis of epidemiological, toxicological, in vitro, in ovo, and mechanistic/AOP data concluded that TCE has the potential to cause cardiac defects in humans when exposure occurs at sufficient doses during a sensitive window of fetal development. The study by Johnson et al. [51] was reaffirmed as suitable for hazard characterization and reference value derivation, though acknowledging study limitations and uncertainties.

Mitochondrial Dysfunction, Disruption of F-Actin Polymerization, and Transcriptomic Alterations in Zebrafish Larvae Exposed to Trichloroethylene

Trichloroethylene (TCE) is primarily used as an industrial degreasing agent and has been in use since the 1940s. TCE is released into the soil, surface, and groundwater. From an environmental and regulatory standpoint, more than half of Superfund hazardous waste sites on the National Priority List are contaminated with TCE. Occupational exposure to TCE occurs primarily via inhalation, while environmental TCE exposure also occurs through ingestion of contaminated drinking water. Current literature links TCE exposure to various adverse health effects including cardiovascular toxicity. Current studies aiming to address developmental cardiovascular toxicity utilized rodent and avian models, with the majority of studies using relatively higher parts per million (mg/L) doses. In this study, to further investigate developmental cardiotoxicity of TCE, zebrafish embryos were treated with 0, 10, 100, or 500 parts per billion (ppb; μg/L) TCE during embryogenesis and/or through early larval stages. After the appropriate exposure period, angiogenesis, F-actin, and mitochondrial function were assessed. A significant dose-response decrease in angiogenesis, F-actin, and mitochondrial function was observed. To further complement this data, a transcriptomic profile of zebrafish larvae was completed to identify gene alterations associated with the 10 ppb TCE exposure. Results from the transcriptomic data revealed that embryonic TCE exposure caused significant changes in genes associated with cardiovascular disease, cancer, and organismal injury and abnormalities with a number of targets in the FAK signaling pathway. Overall, results from our study support TCE as a developmental cardiovascular toxicant, provide molecular targets and pathways for investigation in future studies, and indicate a need for continued priority for environmental regulation.

Exposure of Daphnia magna to trichloroethylene (TCE) and vinyl chloride (VC): evaluation of gene transcription, cellular activity, and life-history parameters

Trichloroethylene (TCE) is a ubiquitous contaminant classified as a human carcinogen. Vinyl chloride (VC) is primarily used to manufacture polyvinyl chloride and can also be a degradation product of TCE. Very few data exist on the toxicity of TCE and VC in aquatic organisms particularly at environmentally relevant concentrations. The aim of this study was to evaluate the sub-lethal effects (10 day exposure; 0.1; 1; 10 µg/L) of TCE and VC in Daphnia magna at the gene, cellular, and life-history levels. Results indicated impacts of VC on the regulation of genes related to glutathione-S-transferase (GST), juvenile hormone esterase (JHE), and the vitelline outer layer membrane protein (VMO1). On the cellular level, exposure to 0.1, 1, and 10 µg/L of VC significantly increased the activity of JHE in D. magna and TCE increased the activity of chitinase (at 1 and 10 µg/L). Results for life-history parameters indicated a possible tendency of TCE to affect the number of molts at the individual level in D. magna (p=0.051). Measurement of VG-like proteins using the alkali-labile phosphates (ALP) assay did not show differences between TCE treated organisms and controls. However, semi-quantitative measurement using gradient gel electrophoresis (213-218 kDa) indicated significant decrease in VG-like protein levels following exposure to TCE at all three concentrations. Overall, results indicate effects of TCE and VC on genes and proteins related to metabolism, reproduction, and growth in D. magna.

Low-dose trichloroethylene alters cytochrome P450-2C subfamily expression in the developing chick heart

Trichloroethylene (TCE) is an organic solvent and common environmental contaminant. TCE exposure is associated with heart defects in humans and animal models. Primary metabolism of TCE in adult rodent models is by specific hepatic cytochrome P450 enzymes (Lash et al. in Environ Health Perspect 108:177-200, 2000). As association of TCE exposure with cardiac defects is in exposed embryos prior to normal liver development, we investigated metabolism of TCE in the early embryo. Developing chick embryos were dosed in ovo with environmentally relevant doses of TCE (8 and 800 ppb) and RNA was extracted from cardiac and extra-cardiac tissue (whole embryo without heart). Real-time PCR showed upregulation of CYP2H1 transcripts in response to TCE exposure in the heart. No detectable cytochrome expression was found in extra-cardiac tissue. As seen previously, the dose response was non-monotonic and 8 ppb elicited stronger upregulation than 800 ppb. Immunostaining for CYP2C subfamily expression confirmed protein expression and showed localization in both myocardium and endothelium. TCE exposure increased protein expression in both tissues. These data demonstrate that the earliest embryonic expression of phase I detoxification enzymes is in the developing heart. Expression of these CYPs is likely to be relevant to the susceptibility of the developing heart to environmental teratogens.

Collagen gel analysis of epithelial-mesenchymal transition in the embryo heart: an in vitro model system for the analysis of tissue interaction, signal transduction, and environmental effects

The cellular process of epithelial-mesenchymal cell transition (EMT) is a critical event in development that is reiterated in adult pathologies of metastasis and organ fibrosis. An initial understanding of the cellular and molecular events of this process emerged from an in vitro examination of heart valve development. Explants of the chick atrioventricular valve-forming region were placed on collagen gels and removed to show that EMT was regulated by a tissue interaction. Subsequent studies showed that specific TGFβ isoforms and receptors were required and steps of activation and invasion could be distinguished. The assay was modified for mouse hearts and has been used to explore signal transduction and gene expression in both species. The principle advantages of the system are a defined temporal window, when EMT takes place and the ability to isolate cells at various stages of the EMT process. These advantages are largely unavailable in other developmental or adult models. As the mesenchymal cells produced by EMT in the heart are involved in defects found in congenital heart disease, there is also a direct relevance of cardiac EMT to human birth defects. This relationship has been explored in relation to environmental exposures and in a number of genetic models. This review provides both an overview of the findings developed from the assay and protocols to enable the use of the assay by other laboratories. The assay provides a versatile platform to explore roles of specific gene products, drugs, and environmental agents on a critical cellular process.