Extracellular matrix from embryonic myocardium elicits an early morphogenetic event in cardiac endothelial differentiation

Decellularized heart extracellular matrix alleviates activation of hiPSC-derived cardiac fibroblasts

Human induced pluripotent stem cell derived cardiac fibroblasts (hiPSC-CFs) play a critical role in modeling human cardiovascular diseases in vitro. However, current culture substrates used for hiPSC-CF differentiation and expansion, such as Matrigel and tissue culture plastic (TCPs), are tissue mismatched and may provide pathogenic cues. Here, we report that hiPSC-CFs differentiated on Matrigel and expanded on tissue culture plastic (M-TCP-iCFs) exhibit transcriptomic hallmarks of activated fibroblasts limiting their translational potential. To alleviate pathogenic activation of hiPSC-CFs, we utilized decellularized extracellular matrix derived from porcine heart extracellular matrix (HEM) to provide a biomimetic substrate for improving hiPSC-CF phenotypes. We show that hiPSC-CFs differentiated and expanded on HEM (HEM-iCFs) exhibited reduced expression of hallmark activated fibroblast markers versus M-TCP-iCFs while retaining their cardiac fibroblast phenotype. HEM-iCFs also maintained a reduction in expression of hallmark genes associated with pathogenic fibroblasts when seeded onto TCPs. Further, HEM-iCFs more homogenously integrated into an hiPSC-derived cardiac organoid model, resulting in improved cardiomyocyte sarcomere development. In conclusion, HEM provides an improved substrate for the differentiation and propagation of hiPSC-CFs for disease modeling.

Molecular Pathways and Animal Models of Tricuspid Atresia and Univentricular Heart

The process of valve formation is a complex process that involves intricate interplay between various pathways at precise times. Although we have not completely elucidated the molecular pathways that lead to normal valve formation, we have identified a few major players in this process. We are now able to implicate TGF-ß, BMP, and NOTCH as suspects in tricuspid atresia (TA), as well as their downstream targets: NKX2-5, TBX5, NFATC1, GATA4, and SOX9. We know that the TGF-ß and the BMP pathways converge on the SMAD4 molecule, and we believe that this molecule plays a very important role to tie both pathways to TA. Similarly, we look at the NOTCH pathway and identify the HEY2 as a potential link between this pathway and TA. Another transcription factor that has been implicated in TA is NFATC1. While several mouse models exist that include part of the TA abnormality as their phenotype, no true mouse model can be said to represent TA. Bridging this gap will surely shed light on this complex molecular pathway and allow for better understanding of the disease process.

The Mesenchymal Cap of the Atrial Septum and Atrial and Atrioventricular Septation

In this publication, dedicated to Professor Robert H. Anderson and his contributions to the field of cardiac development, anatomy, and congenital heart disease, we will review some of our earlier collaborative studies. The focus of this paper is on our work on the development of the atrioventricular mesenchymal complex, studies in which Professor Anderson has played a significant role. We will revisit a number of events relevant to atrial and atrioventricular septation and present new data on the development of the mesenchymal cap of the atrial septum, a component of the atrioventricular mesenchymal complex which, thus far, has received only moderate attention.

The hLAMP-1-positive particulate matrix involved in cardiac mesenchyme formation in the chick does not include BMP-2

Early heart development involves the transformation of endocardial cells in the atrioventricular canal and outflow tract regions into mesenchymal cells, a process called endocardial mesenchymal transformation (EMT). This process is initiated by factors, termed the particulate matrix, that are secreted by the myocardium. The particulate matrix causes a subset of endocardial cells to hypertrophy, lose their cell-cell contacts, form migratory processes, transform into mesenchymal cells, and migrate into the underlying endocardial cushions. The particulate matrix can be extracted using EDTA and the EDTA extract can initiate the EMT process. Earlier reports from our laboratory have shown that the particulate matrix can be detected with the hLAMP-1 antibody in immunostaining and Western blot analysis. In addition, similar proteins have been isolated from the growth media of stage 15-16 chick embryo myocardial cultures (MyoCM). Since other investigators have identified a possible role for bone morphogenetic protein (BMP)-2 during the EMT process in the heart, we asked whether BMP-2 is a part of the chick hLAMP-1-positive particulate matrix. To answer this question, we double stained stage 15-16 chick embryo sections with hLAMP-1 and BMP-2 antibodies. We found that BMP-2 signals do not colocalize with hLAMP-1-stained particles. In addition, using immunoprecipitation-Western blot analysis, we demonstrated no association of BMP-2 with the hLAMP-1-bound fraction of the EDTA extract or MyoCM. Our results indicate that BMP-2 is not a component of the hLAMP-1-positive particulate matrix in the chick.

Symptomatic cardiac metastases of breast cancer 27 years after mastectomy: a case report with literature review--pathophysiology of molecular mechanisms and metastatic pathways, clinical aspects, diagnostic procedures and treatment modalities

Metastases to the heart and pericardium are rare but more common than primary cardiac tumours and are generally associated with a rather poor prognosis. Most cases are clinically silent and are undiagnosed in vivo until the autopsy. We present a female patient with a 27-year-old history of an operated primary breast cancer who was presented with dyspnoea, paroxysmal nocturnal dyspnoea and orthopnoea. The clinical signs and symptoms aroused suspicion of congestive heart failure. However, the cardiac metastases were detected during a routine cardiologic evaluation and confirmed with computed tomography imaging. Additionally, this paper outlines the pathophysiology of molecular and clinical mechanisms involved in the metastatic spreading, clinical presentation, diagnostic procedures and treatment of heart metastases. The present case demonstrates that a complete surgical resection and systemic chemotherapy may result in a favourable outcome for many years. However, a lifelong medical follow-up, with the purpose of a detection of metastases, is highly recommended. We strongly call the attention of clinicians to the fact that during the follow-up of all cancer patients, such heart failure may be a harbinger of the secondary heart involvement.

Embryological origin of the endocardium and derived valve progenitor cells: from developmental biology to stem cell-based valve repair

The cardiac valves are targets of both congenital and acquired diseases. The formation of valves during embryogenesis (i.e., valvulogenesis) originates from endocardial cells lining the myocardium. These cells undergo an endothelial-mesenchymal transition, proliferate and migrate within an extracellular matrix. This leads to the formation of bilateral cardiac cushions in both the atrioventricular canal and the outflow tract. The embryonic origin of both the endocardium and prospective valve cells is still elusive. Endocardial and myocardial lineages are segregated early during embryogenesis and such a cell fate decision can be recapitulated in vitro by embryonic stem cells (ESC). Besides genetically modified mice and ex vivo heart explants, ESCs provide a cellular model to study the early steps of valve development and might constitute a human therapeutic cell source for decellularized tissue-engineered valves. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Loss of muscleblind-like 1 promotes invasive mesenchyme formation in endocardial cushions by stimulating autocrine TGFβ3

BACKGROUND

Valvulogenesis and septation in the developing heart depend on the formation and remodeling of endocardial cushions in the atrioventricular canal (AVC) and outflow tract (OFT). These cushions are invaded by a subpopulation of endocardial cells that undergo an epithelial-mesenchymal transition in response to paracrine and autocrine transforming growth factor β (TGFβ) signals. We previously demonstrated that the RNA binding protein muscleblind-like 1 (MBNL1) is expressed specifically in the cushion endocardium, and knockdown of MBNL1 in stage 14 embryonic chicken AVC explants enhances TGFβ-dependent endocardial cell invasion.

RESULTS

In this study, we demonstrate that the effect of MBNL1 knockdown on invasion remains dependent on TGFβ3 after it is no longer required to induce basal levels of invasion. TGFβ3, but not TGFβ2, levels are elevated in medium conditioned by MBNL1-depleted AVC explants. TGFβ3 is elevated even when the myocardium is removed, indicating that MBNL1 modulates autocrine TGFβ3 production in the endocardium. More TGFβ3-positive cells are observed in the endocardial monolayer following MBNL1 knockdown. Addition of exogenous TGFβ3 to AVC explants recapitulates the effects of MBNL1 knockdown. Time course experiments demonstrate that knockdown of MBNL1 induces precocious TGFβ3 secretion, and early exposure to excess TGFβ3 induces precocious invasion. MBNL1 expression precedes TGFβ3 in the AVC endocardium, consistent with a role in preventing precocious autocrine TGFβ3 signaling. The stimulatory effects of MBNL1 knockdown on invasion are lost in stage 16 AVC explants. Knockdown of MBNL1 in OFT explants similarly enhances cell invasion, but not activation. TGFβ is necessary and sufficient to mediate this effect.

CONCLUSIONS

Taken together, these data support a model in which MBNL1 negatively regulates cell invasion in the endocardial cushions by restricting the magnitude and timing of endocardial-derived TGFβ3 production.

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.

The role of nuclear pore complex in tumor microenvironment and metastasis

One of the main reasons for cancer mortality is caused by the highly invasive behavior of cancer cells, which often due to aggressive metastasis. Metastasis is mediated by various growth factors and cytokines, operating through numerous signaling pathways. Remarkably, all these metastatic signaling pathways must enter the nucleus through a single gatekeeper, the nuclear pore complex (NPC). NPCs are the only gateway between the cytoplasm and the nucleus. NPCs are among the largest proteinaceous assemblies in the cell and are composed of multiple copies of around 30 different proteins called nucleoporins. Here, we review what is currently known about the NPC, and its role in the mechanisms of tumor progression. We will also explore potential strategies to target metastatic pathways by manipulating the karyopherins (importins/exportins) of nucleocytoplasmic traffic through NPCs.

Cancer metastasis as disrupted developmental phenotype

Cancer metastasis is a complex processes, associated with the invasion to tissues with extensive degradation of the surrounding normal components, penetration into vessels, circulation, and then invasion to normal tissues in body. It would be not surprising that tumor cells usurp pathways critical to the developing embryo during metastasis. For the better understanding of tumor metastasis, this review will highlight the recent progress and significance of the signal transduction pathways, relevant to developmental biology.

Nfatc1 coordinates valve endocardial cell lineage development required for heart valve formation

RATIONALE

Formation of heart valves requires early endocardial to mesenchymal transformation (EMT) to generate valve mesenchyme and subsequent endocardial cell proliferation to elongate valve leaflets. Nfatc1 (nuclear factor of activated T cells, cytoplasmic 1) is highly expressed in valve endocardial cells and is required for normal valve formation, but its role in the fate of valve endocardial cells during valve development is unknown.

OBJECTIVE

Our aim was to investigate the function of Nfatc1 in cell-fate decision making by valve endocardial cells during EMT and early valve elongation.

METHODS AND RESULTS

Nfatc1 transcription enhancer was used to generate a novel valve endocardial cell-specific Cre mouse line for fate-mapping analyses of valve endocardial cells. The results demonstrate that a subpopulation of valve endocardial cells marked by the Nfatc1 enhancer do not undergo EMT. Instead, these cells remain within the endocardium as a proliferative population to support valve leaflet extension. In contrast, loss of Nfatc1 function leads to enhanced EMT and decreased proliferation of valve endocardium and mesenchyme. The results of blastocyst complementation assays show that Nfatc1 inhibits EMT in a cell-autonomous manner. We further reveal by gene expression studies that Nfatc1 suppresses transcription of Snail1 and Snail2, the key transcriptional factors for initiation of EMT.

CONCLUSIONS

These results show that Nfatc1 regulates the cell-fate decision making of valve endocardial cells during valve development and coordinates EMT and valve elongation by allocating endocardial cells to the 2 morphological events essential for valve development.

What chick and mouse models have taught us about the role of the endocardium in congenital heart disease

Specific cell and tissue interactions drive the formation and function of the vertebrate cardiovascular system. Although much attention has been focused on the muscular components of the developing heart, the endocardium plays a key role in the formation of a functioning heart. Endocardial cells exhibit heterogeneity that allows them to participate in events such as the formation of the valves, septation of the outflow tract, and trabeculation. Here we review, the contributions of the endocardium to cardiovascular development and outline useful approaches developed in the chick and mouse that have revealed endocardial cell heterogeneity, the signaling molecules that direct endocardial cell behavior, and how these insights have contributed to our understanding of cardiovascular development and disease.

Hypoxia, inflammation, and the tumor microenvironment in metastatic disease

Metastasis, the leading cause of cancer deaths, is an intricate process involving many important tumor and stromal proteins that have yet to be fully defined. This review discusses critical components necessary for the metastatic cascade, including hypoxia, inflammation, and the tumor microenvironment. More specifically, this review focuses on tumor cell and stroma interactions, which allow cell detachment from a primary tumor, intravasation to the blood stream, and extravasation at a distant site where cells can seed and tumor metastases can form. Central players involved in this process and discussed in this review include integrins, matrix metalloproteinases, and soluble growth factors/matrix proteins, including the connective tissue growth factor and lysyl oxidase.

Arsenic exposure perturbs epithelial-mesenchymal cell transition and gene expression in a collagen gel assay

Arsenic is a naturally occurring metalloid and environmental contaminant. Arsenic exposure in drinking water is reported to cause cancer of the liver, kidneys, lung, bladder, and skin as well as birth defects, including neural tube, facial, and vasculogenic defects. The early embryonic period most sensitive to arsenic includes a variety of cellular processes. One key cellular process is epithelial-mesenchymal transition (EMT) where epithelial sheets develop into three-dimensional structures. An embryonic prototype of EMT is found in the atrioventricular (AV) canal of the developing heart, where endothelia differentiate to form heart valves. Effects of arsenic on this cellular process were examined by collagen gel invasion assay (EMT assay) using explanted AV canals from chicken embryo hearts. AV canals treated with 12.5-500 ppb arsenic showed a loss of mesenchyme at 12.5 ppb, and mesenchyme formation was completely inhibited at 500 ppb. Altered gene expression in arsenic-treated explants was investigated by microarray analysis. Genes whose expression was altered consistently at exposure levels of 10, 25, and 100 ppb were identified, and results showed that 25 ppb in vitro was particularly effective. Three hundred and eighty two genes were significantly altered at this exposure level. Cytoscape analysis of the microarray data using the chicken interactome identified four clusters of altered genes based on published relationships and pathways. This analysis identified cytoskeleton and cell adhesion-related genes whose disruption is consistent with an altered ability to undergo EMT. These studies show that EMT is sensitive to arsenic and that an interactome-based approach can be useful in identifying targets.

Cancer stem cells and chemoradiation resistance

Cancer is a disease of genetic and epigenetic alterations, which are emphasized as the central mechanisms of tumor progression in the multistepwise model. Discovery of rare subpopulations of cancer stem cells (CSCs) has created a new focus in cancer research. The heterogeneity of tumors can be explained with the help of CSCs supported by antiapoptotic signaling. CSCs mimic normal adult stem cells by demonstrating resistance to toxic injuries and chemoradiation therapy. Moreover, they might be responsible for tumor relapse following apparent beneficial treatments. Compared with hematopoietic malignancies, conventional therapy regimes in solid tumors have improved the overall survival marginally, illustrating the profound impact of treatment resistance. This implies that the present therapies, which follow total elimination of rapidly dividing and differentiated tumor cells, need to be modified to target CSCs that repopulate the tumor. In this review article, we report on recent findings regarding the involvement of CSCs in chemoradiation resistance and provide new insights into their therapeutic implications in cancer.

ROCK1 expression is regulated by TGFbeta3 and ALK2 during valvuloseptal endocardial cushion formation

During early heart development at the looped heart stage, endothelial cells in the outflow tract and atrioventricular (AV) regions transform into mesenchyme to generate endocardial cushion tissue. This endocardial epithelial-mesenchymal transition (EMT) is regulated by several regulatory pathways, including the transforming growth factor-beta (TGFbeta), bone morphogenetic protein (BMP), and Rho-ROCK pathways. Here, we investigated the spatiotemporal expression pattern of ROCK1 mRNA during EMT in chick and examined whether TGFbeta or BMP could induce the expression of ROCK1. At the onset of EMT, ROCK1 expression was up-regulated in endothelial/mesenchymal cells. A three-dimensional collagen gel assay was used to examine the mechanisms regulating the expression of ROCK1. In AV endocardium co-cultured with associated myocardium, ROCK1 expression was inhibited by either anti-TGFbeta3 antibody, anti-ALK2 antibody or noggin, but not SB431542 (ALK5 inhibitor). In cultured preactivated AV endocardium, TGFbeta3 protein induced the expression of ROCK1, but BMP did not. AV endothelial cells that were cultured in medium supplemented with TGFbeta3 plus anti-ALK2 antibody failed to express ROCK1. These results suggest that the expression of ROCK1 is up-regulated at the onset of EMT and that signaling mediated by TGFbeta3/ALK2 together with BMP is involved in the expression of ROCK1.

Epithelial--mesenchymal and mesenchymal--epithelial transitions in carcinoma progression

Like a set of bookends, cellular, molecular, and genetic changes of the beginnings of life mirror those of one of the most common cause of death--metastatic cancer. Epithelial to mesenchymal transition (EMT) is an important change in cell phenotype which allows the escape of epithelial cells from the structural constraints imposed by tissue architecture, and was first recognized by Elizabeth Hay in the early to mid 1980's to be a central process in early embryonic morphogenesis. Reversals of these changes, termed mesenchymal to epithelial transitions (METs), also occur and are important in tissue construction in normal development. Over the last decade, evidence has mounted for EMT as the means through which solid tissue epithelial cancers invade and metastasize. However, demonstrating this potentially rapid and transient process in vivo has proven difficult and data connecting the relevance of this process to tumor progression is still somewhat limited and controversial. Evidence for an important role of MET in the development of clinically overt metastases is starting to accumulate, and model systems have been developed. This review details recent advances in the knowledge of EMT as it occurs in breast development and carcinoma and prostate cancer progression, and highlights the role that MET plays in cancer metastasis. Finally, perspectives from a clinical and translational viewpoint are discussed.

TGF beta-mediated RhoA expression is necessary for epithelial-mesenchymal transition in the embryonic chick heart

Endothelia in the atrioventricular canal (AVC) of the embryonic heart undergo an epithelial-mesenchymal transition (EMT) and migrate into the underlying extracellular matrix. We explore here whether RhoA mediates this EMT. RhoA was detected in all cells of the chick heart during the stages studied. Expression was elevated when EMT was actively occurring. Explants treated with C3 exoenzyme in collagen gel cultures showed a significant decrease in mesenchymal cell numbers. siRNA was used to inhibit RhoA mRNA, and both activated endothelial and mesenchymal cells decreased significantly with treatment. Loss of RhoA produced a reduction of RhoB, cyclin-b2, and beta-catenin messages showing that these genes are regulated downstream of RhoA. In contrast, runx-2 was not reduced. Inhibition of TGFbeta3 or TGFbeta2 activity caused a large reduction of RhoA message. These data place RhoA in TGFbeta regulated pathways for both endothelial activation and mesenchymal invasion and demonstrate a functional requirement during EMT.

Multiple transforming growth factor-beta isoforms and receptors function during epithelial-mesenchymal cell transformation in the embryonic heart

Epithelial-mesenchymal cell transformation (EMT) is a critical process during development of the heart valves. Transition of endothelial cells into mesenchymal cells in the atrioventricular (AV) canal and the outflow tract regions of the heart form the cardiac cushions that eventually form the heart valves. Collagen gel invasion assay has aided in the identification of molecules that regulate EMT. Among those, transforming growth factor-beta (TGF-beta) ligands and receptors demonstrate a critical role during EMT. In the chick, TGF-beta ligands and some receptors have specific functions during EMT. TGF-beta2 mediates endothelial cell-cell activation and separation, and TGF-beta3 mediates cell invasion into the extracellular matrix. Receptors involved in the EMT process include TGF-beta receptor type II (TBRII), TBRIII, endoglin and the TBRI receptors, ALK2 and ALK5. In contrast, in the mouse model, TGF-beta2 is the only ligand involved in EMT. The TGF-beta2 null mouse has either increased EMT or a mesenchymal cell proliferation after EMT. However, functional studies of TGF-beta1 in vivo and in vitro showed that TGF-beta1 functions in the EMT of the mouse AV canal. Latent TGF-beta-binding protein (LTBP-1) and endoglin have a role in the EMT process. Therefore, TGF-betas mediate cardiac EMT in both embryonic species. Further studies will reveal the identification of ligand and receptor-specific activities.

Chick embryos exposed to trichloroethylene in an ex ovo culture model show selective defects in early endocardial cushion tissue formation

BACKGROUND

Formation of the primitive heart is a critical step for establishing a competent circulatory system necessary for continued morphogenesis, and as such has significant potential as a target for environmental insult. The goal of this study was to identify the initial cellular events that precede more superficially observable abnormalities resulting from exposing early chick embryos to trichloroethylene (TCE).

METHODS

A whole embryo culture method was used to assess the susceptibility of endocardial epithelial-mesenchymal transformation in the early chick heart to TCE. This method has the benefits of maintaining the anatomical relationships of developing tissues and organs, instantaneously exposing precisely staged embryos to quantifiable levels of TCE in a protein-free medium, and the ability to directly monitor developmental morphology.

RESULTS

A minority of embryos (Hamburger and Hamilton [HH] stage 13-14) exposed to TCE (10-80 ppm) were not viable after 24 hr in culture and exhibited a variety of gross malformations in a dose-dependent fashion. However, the majority of treated embryos remained viable and developed into HH stage 17 embryos that were superficially indistinguishable from vehicle-treated controls. Further analysis of the hearts of these superficially normal embryos by whole-mount confocal microscopy revealed selective reduction in the number of atrioventricular canal mesenchymal cells. Additionally, those mesenchymal cells that did develop migrated abnormally as long thin cords of adherent cells.

CONCLUSIONS

The regional selectivity of these effects in the chick heart suggests a critical window of susceptibility to TCE in the epithelial-mesenchymal transformation of atrioventricular canal endocardium.

Understanding heart development and congenital heart defects through developmental biology: a segmental approach

ABSTRACT The heart is the first organ to form and function during development. In the pregastrula chick embryo, cells contributing to the heart are found in the postero-lateral epiblast. During the pregastrula stages, interaction between the posterior epiblast and hypoblast is required for the anterior lateral plate mesoderm (ALM) to form, from which the heart will later develop. This tissue interaction is replaced by an Activin-like signal in culture. During gastrulation, the ALM is committed to the heart lineage by endoderm-secreted BMP and subsequently differentiates into cardiomyocyte. The right and left precardiac mesoderms migrate toward the ventral midline to form the beating primitive heart tube. Then, the heart tube generates a right-side bend, and the d-loop and presumptive heart segments begin to appear segmentally: outflow tract (OT), right ventricle, left ventricle, atrioventricular (AV) canal, atrium and sinus venosus. T-box transcription factors are involved in the formation of the heart segments: Tbx5 identifies the left ventricle and Tbx20 the right ventricle. After the formation of the heart segments, endothelial cells in the OT and AV regions transform into mesenchyme and generate valvuloseptal endocardial cushion tissue. This phenomenon is called endocardial EMT (epithelial-mesenchymal transformation) and is regulated mainly by BMP and TGFbeta. Finally, heart septa that have developed in the OT, ventricle, AV canal and atrium come into alignment and fuse, resulting in the completion of the four-chambered heart. Altered development seen in the cardiogenetic process is involved in the pathogenesis of congenital heart defects. Therefore, understanding the molecular nature regulating the 'nodal point' during heart development is important in order to understand the etiology of congenital heart defects, as well as normal heart development.

Genetic and cellular analyses of zebrafish atrioventricular cushion and valve development

Defects in cardiac valve morphogenesis and septation of the heart chambers constitute some of the most common human congenital abnormalities. Some of these defects originate from errors in atrioventricular (AV) endocardial cushion development. Although this process is being extensively studied in mouse and chick, the zebrafish system presents several advantages over these models, including the ability to carry out forward genetic screens and study vertebrate gene function at the single cell level. In this paper, we analyze the cellular and subcellular architecture of the zebrafish heart during stages of AV cushion and valve development and gain an unprecedented level of resolution into this process. We find that endocardial cells in the AV canal differentiate morphologically before the onset of epithelial to mesenchymal transformation, thereby defining a previously unappreciated step during AV valve formation. We use a combination of novel transgenic lines and fluorescent immunohistochemistry to analyze further the role of various genetic (Notch and Calcineurin signaling) and epigenetic (heart function) pathways in this process. In addition, from a large-scale forward genetic screen we identified 55 mutants, defining 48 different genes, that exhibit defects in discrete stages of AV cushion development. This collection of mutants provides a unique set of tools to further our understanding of the genetic basis of cell behavior and differentiation during AV valve development.

Heart valve development: endothelial cell signaling and differentiation

During the past decade, single gene disruption in mice and large-scale mutagenesis screens in zebrafish have elucidated many fundamental genetic pathways that govern early heart patterning and differentiation. Specifically, a number of genes have been revealed serendipitously to play important and selective roles in cardiac valve development. These initially surprising results have now converged on a finite number of signaling pathways that regulate endothelial proliferation and differentiation in developing and postnatal heart valves. This review highlights the roles of the most well-established ligands and signaling pathways, including VEGF, NFATc1, Notch, Wnt/beta-catenin, BMP/TGF-beta, ErbB, and NF1/Ras. Based on the interactions among and relative timing of these pathways, a signaling network model for heart valve development is proposed.

Frzb modulates Wnt-9a-mediated beta-catenin signaling during avian atrioventricular cardiac cushion development

Normal development of the cardiac atrioventricular (AV) endocardial cushions is essential for proper ventricular septation and morphogenesis of the mature mitral and tricuspid valves. In this study, we demonstrate spatially restricted expression of both Wnt-9a (formerly Wnt-14) and the secreted Wnt antagonist Frzb in AV endocardial cushions of the developing chicken heart. Wnt-9a expression is detected only in AV canal endocardial cells, while Frzb expression is detected in both endocardial and transformed mesenchymal cells of the developing AV cardiac cushions. We present evidence that Wnt-9a promotes cell proliferation in the AV canal and overexpression of Wnt-9a in ovo results in enlarged endocardial cushions and AV inlet obstruction. Wnt-9a stimulates beta-catenin-responsive transcription in AV canal cells, duplicates the embryonic axis upon ventral injections in Xenopus embryos and appears to regulate cell proliferation by activating a Wnt/beta-catenin signaling pathway. Additional functional studies reveal that Frzb inhibits Wnt-9a-mediated cell proliferation in cardiac cushions. Together, these data argue that Wnt-9a and Frzb regulate mesenchymal cell proliferation leading to proper AV canal cushion outgrowth and remodeling in the developing avian heart.

Cell Biology of Cardiac Cushion Development

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Rho kinases regulate endothelial invasion and migration during valvuloseptal endocardial cushion tissue formation

Rho-associated kinase (ROCK) is a downstream effector of small Rho-GTPases, and phosphorylates several substrates to regulate cell functions, including actin cytoskeletal reorganization and cellular motility. Endothelial-mesenchymal transformation (EMT) is a critical event in the formation of valves and septa during cardiogenesis. It has been reported that ROCK plays an important role in the regulation of endocardial cell differentiation and migration during mouse cardiogenesis (Zhao and Rivkees [2004] Dev. Biol. 275:183-191). Immunohistochemistry showed that, during chick cardiogenesis, ROCK1 and -2 were expressed in the transforming and migrating endothelial/mesenchymal cells in the outflow tract (OT) and atrioventricular (AV) canal regions from which valvuloseptal endocardial cushion tissue would later develop. Treatment with Y27632, a specific ROCK inhibitor, of cultured AV explants or AV endothelial monolayers of stage 14-minus heart (preactivated stage for EMT) on three-dimensional collagen gel perturbed the seeding of mesenchymal cells into the gel lattice. In these experiments, Y27632 did not suppress the expression of an early transformation marker, smooth muscle alpha-actin. Moreover, Y27632 inhibited the mesenchymal invasion in stage 14-18 AV explants, in which endothelial cells had committed to undergo EMT. ML-9, a myosin light chain kinase inhibitor, also inhibited the mesenchymal invasion in cultured AV explants. These results suggest that ROCKs have a critical role in the mesenchymal cell invasion/migration that occurs at the late onset of EMT.

Bone morphogenetic protein-2 can mediate myocardial regulation of atrioventricular cushion mesenchymal cell formation in mice

Transformation of endocardial endothelial cells into invasive mesenchyme is a critical antecedent of cardiac cushion tissue formation. The message for bone morphogenetic protein (BMP)-2 is known to be expressed in myocardial cells in a manner consistent with the segmental pattern of cushion formation [Development 109(1990) 833]. In the present work, we localized BMP-2 protein in atrioventricular (AV) myocardium in mice at embryonic day (ED) 8.5 (12 somite stage) before the onset of AV mesenchymal cell formation at ED 9.5. BMP-2 protein expression was absent from ventricular myocardium throughout the stages examined. After cellularization of the AV cushion at ED 10.5, myocardial BMP-2 protein expression was diminished in AV myocardium, whereas cushion mesenchymal cells started expressing BMP protein. Expression of BMP-2 in cushion mesenchyme persisted during later stages of development, ED 13.5-16, during valuvulogenesis. Intense expression of BMP-2 persisted in the valve tissue in adult mice. Based on the expression pattern, we performed a series of experiments to test the hypothesis that BMP-2 mediates myocardial regulation of cardiac cushion tissue formation in mice. When BMP-2 protein was added to the 16-18 somite stage (ED 9.25) AV endocardial endothelium in culture, cushion mesenchymal cells were formed in the absence of AV myocardium, which invaded into collagen gels and expressed the mesenchymal marker, smooth muscle (SM) alpha-actin; whereas the endothelial marker, PECAM-1, was lost from the invaded cells. In contrast, when noggin, a specific antagonist to BMPs, was applied together with BMP-2 to the culture medium, AV endothelial cells remained as an epithelial monolayer with little expression of SM alpha-actin, and expression of PECAM-1 was retained in the endocardial cells. When noggin was added to AV endothelial cells cocultured with associated myocardium, it blocked endothelial transformation to mesenchyme. AV endothelium treated with BMP-2 expressed elevated levels of TGFbeta-2 in the absence of myocardium, as observed in the endothelium cocultured with myocardium. BMP-2-supported elevation of TGFbeta-2 expression in endocardial cells was abolished by noggin treatment. These data indicated that BMP signaling is required in and BMP-2 is sufficient for myocardial segmental regulation of AV endocardial cushion mesenchymal cell formation in mice.

Elevated glucose inhibits VEGF-A-mediated endocardial cushion formation: modulation by PECAM-1 and MMP-2

Atrioventricular (AV) septal defects resulting from aberrant endocardial cushion (EC) formation are observed at increased rates in infants of diabetic mothers. EC formation occurs via an epithelial-mesenchymal transformation (EMT), involving transformation of endocardial cells into mesenchymal cells, migration, and invasion into extracellular matrix. Here, we report that elevated glucose inhibits EMT by reducing myocardial vascular endothelial growth factor A (VEGF-A). This effect is reversed with exogenous recombinant mouse VEGF-A165, whereas addition of soluble VEGF receptor-1 blocks EMT. We show that disruption of EMT is associated with persistence of platelet endothelial cell adhesion molecule-1 (PECAM-1) and decreased matrix metalloproteinase-2 (MMP-2) expression. These findings correlate with retention of a nontransformed endocardial sheet and lack of invasion. The MMP inhibitor GM6001 blocks invasion, whereas explants from PECAM-1 deficient mice exhibit MMP-2 induction and normal EMT in high glucose. PECAM-1-negative endothelial cells are highly motile and express more MMP-2 than do PECAM-1-positive endothelial cells. During EMT, loss of PECAM-1 similarly promotes single cell motility and MMP-2 expression. Our findings suggest that high glucose-induced inhibition of AV cushion morphogenesis results from decreased myocardial VEGF-A expression and is, in part, mediated by persistent endocardial cell PECAM-1 expression and failure to up-regulate MMP-2 expression.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.

Retinoic acid administration is associated with changes in the extracellular matrix and cardiac mesenchyme within the endocardial cushion

Retinoic acid has been associated with a number of cardiac defects, some of which seem to be related to changes in the endocardial cushions. Studies in mice and older chick embryos have suggested that these defects may be associated with a decrease in mesenchymal cell formation within the cushion. In a previous report we showed that retinoic acid lowered the number of mesenchymal cells in a culture bioassay of mesenchyme formation and that this response was due to retinoic acid modifying the production of particulate matrix from the myocardium. In this study, we have extended these observations to the embryo by implanting a retinoic acid coated bead into the embryo and examined the effect on cardiac mesenchyme formation and in the production of the particulate matrix. In all cases the addition of retinoic acid resulted in a decrease in the number of mesenchymal cells invading the endocardial cushions. In addition retinoic acid increased the production of hLAMP-1 and fibronectin but not transferrin, confirming our earlier report. Finally, we measured the volume of the cushion and calculated the cell density of both the inferior and superior cushions. The results suggest that the superior cushion is more sensitive to retinoic acid treatment than the inferior cushion. Collectively, these results support our earlier work that suggests that the mechanism of retinoic acid cardiac abnormalities involves a disruption in the production of particulate matrix from the myocardium and a subsequent decrease in cardiac mesenchyme cells that results in a malformed cardiac cushions.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

Expression of chick Tbx-2, Tbx-3, and Tbx-5 genes during early heart development: evidence for BMP2 induction of Tbx2

Expression patterns of Tbx2, -3, and -5 genes were analyzed during chick embryonic heart development. Transcripts of these three cTbx genes were detected in overlapping patterns in the early cardiac crescent. cTbx2 and cTbx3 expression patterns closely overlapped with that of bmp2. cTbx5 expression diverged from cTbx2 and bmp2 during the elaboration and folding of the heart tube. In comparison, cTbx2 expression overlapped significantly with that of bmp2 and bmp4 during all stages of heart development and during later embryonic stages, suggestive of a specialized role for Tbx2 in septation. Coexpression of cTbx 2 and cTbx3 genes with bmp2 transcripts raised the possibility that these cTbx genes might be downstream bmp2 targets. Application of bmp2 selectively induced cTbx2 and cTbx3 expression in noncardiogenic embryonic tissue, and the bmp antagonist Noggin down-regulated cTbx2 gene activity. Moreover, the appearance of murine mTbx2 was blocked in bmp2 null mouse embryos. cTbx2 and to a lesser extent cTbx3 gene activity appears to be directed by BMPs during early cardiogenesis.

Synthesis of extracellular matrix and adhesion through beta(1) integrins are critical for fetal ventricular myocyte proliferation

Extracellular matrix (ECM) regulates vascular smooth muscle cell proliferation. The role of ECM in myocardial growth is unexplored. We sought to determine whether human fetal ventricular myocytes (HFVMs) produce ECM and whether synthesis and attachment to ECM are necessary for their epidermal growth factor (EGF)-dependent and -independent proliferation. Cultured HFVMs proliferate in the presence but not absence of serum and EGF, as determined by increase in cell number and [(3)H]thymidine and [(14)C]leucine incorporation (measures of DNA and protein synthesis, respectively). Using a cyanogen bromide digestion technique to measure collagen and elastin and using affinity chromatography for fibronectin, we found that HFVMs synthesized collagen and fibronectin but not elastin. HFVMs grown on exogenous ECM (including fibronectin and type I collagen and laminin) demonstrated no change in proliferation or DNA and protein synthesis with or without EGF. However, inhibition of collagen synthesis using cis-4-hydroxyproline resulted in a decrease in EGF-related HFVM proliferation and DNA and protein synthesis, which was reversed by exposure to L-proline but not by growth on type I collagen. Use of beta(1) but not beta(3) integrin antibody to inhibit cell interaction with ECM resulted in a decrease in HFVM proliferation and DNA and protein synthesis in response to EGF. Furthermore, EGF-dependent proliferation was enhanced by alpha(1)beta(1) and alpha(5)beta(1) antibodies that act as functional ligands, but not alpha(3)beta(1), the only beta(1) subtype expressed in adult myocytes. In conclusion, proliferating HFVMs synthesize collagen and fibronectin. The proliferative response of HFVMs to EGF requires the synthesis of collagen as well as attachment to specific alpha/beta(1) integrin heterodimers.

Slug is an essential target of TGFbeta2 signaling in the developing chicken heart

An epithelial-mesenchymal cell transformation (EMT) occurs during the development of endocardial cushions in the atrioventricular (AV) canal of the heart. This is a complex developmental process regulated by multiple extracellular signals and signal transduction pathways. It was recently shown that the transcription factor Slug is expressed in the AV canal and is required for initial steps of EMT. Treatment of AV canal explants with either antisense oligodeoxynucleotides toward Slug or anti-TGFbeta2 antibody inhibited initial steps of EMT. Others have identified roles for HGF and BMP during EMT in the heart. Both HGF and BMP are known to regulate Slug in other cell types. To determine whether TGFbeta2 or other signaling factors regulate Slug expression during EMT in the heart, we cultured AV canal explants in the presence of anti-TGFbeta2 antibody, anti-TGFbeta3 antibody, pertussis toxin, retinoic acid, noggin, or anti-HGF antibody. Only treatment with anti-TGFbeta2 antibody or retinoic acid inhibited Slug expression in AV canal explants. Consistent with these data, we found that retinoic acid disrupted initial steps of EMT, while antagonists of BMP and HGF signaling disrupted later steps of EMT. Transfection of AV canal explants with Slug rescued the inhibitory effect of anti-TGFbeta2 antibody but not retinoic acid on EMT. Slug is thus an essential target of TGFbeta2 signaling during EMT in the developing chicken heart.

Mechanisms involved in valvuloseptal endocardial cushion formation in early cardiogenesis: roles of transforming growth factor (TGF)-beta and bone morphogenetic protein (BMP)

Endothelial-mesenchymal transformation (EMT) is a critical event in the generation of the endocardial cushion, the primordia of the valves and septa of the adult heart. This embryonic phenomenon occurs in the outflow tract (OT) and atrioventricular (AV) canal of the embryonic heart in a spatiotemporally restricted manner, and is initiated by putative myocardially derived inductive signals (adherons) which are transferred to the endocardium across the cardiac jelly. Abnormal development of endocardial cushion tissue is linked to many congenital heart diseases. At the onset of EMT in chick cardiogenesis, transforming growth factor (TGFbeta)-3 is expressed in transforming endothelial and invading mesenchymal cells, while bone morphogenetic protein (BMP)-2 is expressed in the subjacent myocardium. Three-dimensional collagen gel culture experiments of the AV endocardium show that 1) myocardially derived inductive signals upregulate the expression of AV endothelial TGFbeta3 at the onset of EMT, 2) TGFbeta3 needs to be expressed by these endothelial cells to trigger the initial phenotypic changes of EMT, and 3) myocardial BMP2 acts synergistically with TGFbeta3 in the initiation of EMT.

Retinoic acid inhibition of cardiac mesenchyme formation in vitro correlates with changes in the secretion of particulate matrix from the myocardium

Retinoic acid has been associated with a variety of cardiac defects. A percentage of these defects are related to changes in the endocardial cushions. Studies in mice and older chick embryos have shown a decrease in mesenchymal cell formation attributable to retinoic acid and have suggested that retinoic acid was affecting the extracellular matrix. In this study we have tested the effect of retinoic acid on cardiac mesenchyme formation in vitro and then tested retinoic acid treated myocyte cultures for changes in the expression of hLAMP-1, fibronectin and transferrin members of the particulate matrix that is required for mesenchyme formation. Initial experiments tested the effect of retinoic acid on mesenchymal cell formation first in atrioventricular canal and outflow tract explant cultures and then in AV endothelial monolayer cultures using myocyte conditioned media or the particulate matrix fraction from retinoic acid treated myocyte cultures. In all cases, mesenchymal cell formation was suppressed while no suppression was observed when MyoCM was included with retinoic acid. Protein analysis showed that retinoic acid had a stimulatory effect on protein synthesis. ELISA assays revealed that retinoic acid treated myocyte cultures contained significantly more hLAMP-1 and fibronectin than either normal or DMSO controls. However, transferrin was not affected by retinoic acid treatment in these experiments. Our results suggest that retinoic acid affects the expression of the particulate matrix and that these changes may be responsible for the observed decrease in mesenchymal cell formation.

Antisense oligodeoxynucleotide complementary to smooth muscle alpha-actin inhibits endothelial-mesenchymal transformation during chick cardiogenesis

alpha-Smooth-muscle actin (SMA) is the major isoform of adult vascular tissues. During early development, SMA is expressed in various mesodermally derived tissues in a spatiotemporally restricted manner; however, its exact role remains unknown. We examined its role in the formation of chicken atrioventricular (AV) endocardial cushion tissue. This developmental process possesses the characteristics of endothelial-mesenchymal transformation and is partly TGF beta-dependent. Immunohistochemistry showed that SMA was (1) expressed homogeneously in the newly formed appendages of transforming endothelial/mesenchymal cells, and (2) distributed in a punctate manner in the lamellipodia/filopodia of invading mesenchymal cells. Antisense oligodeoxynucleotide (ODNs) specific for SMA reduced both SMA expression and mesenchymal formation in AV endothelial cells cultured with myocardium on a collagen gel lattice. Perturbation of SMA by antisense ODN also inhibited TGF beta-inducible migratory appendage formation in a cultured AV endothelial monolayer. However, it did not inhibit cell:cell separation or cellular hypertrophy. These results suggest that the expression of SMA is necessary for migratory appendage formation during the TGF beta-dependent initial phenotypic changes that occur in endothelial-mesenchymal transformation.

Evidence for a role of Smad6 in chick cardiac development

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-beta (TGF-beta) superfamily, are obligatory growth factors for early embryogenesis and heart formation. SMAD proteins transduce signals of the TGF-beta superfamily. We isolated chicken Smad6 (cSmad6), a member of inhibitory SMADs, and found its expression to be remarkably restricted to the developing heart, eyes, and limbs. cSmad6 expression was detected in the cardiogenic region of stage 5 embryos and overlapped Nkx2-5 and bmp-2, -4, and -7 expression. Throughout development, cSmad6 was expressed strongly in the heart, primarily in the myocardium, endocardium, and endocardial cushion tissue. Myocardial expression of cSmad6 was stronger in the forming septum, where highly localized expression of bmp-2 and -4 was also observed. Ectopically applied BMP-2 protein induced the expression of cSmad6, a putative negative regulator of BMP-signaling pathway, in anterior medial mesoendoderm of stage 4-5 embryos. In addition, blocking of BMP signaling using Noggin downregulated cSmad6 in cardiogenic tissue. cSmad1, one of the positive mediators of BMP signaling, was also expressed in cardiogenic region, but was not BMP-2 inducible. Our data suggest that cSmad6 has a role in orchestrating BMP-mediated cardiac development. We propose the possible mechanism of action of cSmad6 as modulating BMP signal by keeping a balance between constitutively expressed pathway-specific cSmad1 and ligand-induced inhibitory cSmad6 in the developing heart.

Urokinase regulates embryonic cardiac cushion cell migration without converting plasminogen

Urokinase-type plasminogen activator (uPA) activation of plasminogen is an important mediator of cell migration in many cell types. In the developing avian heart, uPA has been implicated as a mediator of atrioventricular (AV) cushion cell migration; however, the role of the plasminogen/plasmin system has not been examined. The purpose of this study was to test the hypothesis that uPA conversion of plasminogen to plasmin mediates AV cushion cell migration in vitro. Stage 17/18 chicken atrioventricular tissue lysates converted plasminogen into plasmin through uPA activity but no tissue-type plasminogen activator activity was detected. Zymograms on living cultured AV explants also activated plasminogen producing plasmin that degraded extracellular protein. The migratory capacity of cushion cells was assessed in the presence or absence of various test reagents known to alter the plasminogen/plasmin system. Addition of either human or chicken plasminogen or aprotinin (an inhibitor of plasmin) had no effect on cell migration. However, an anti-catalytic uPA antibody that blocked AV uPA activity, significantly decreased cell migration at all concentrations tested. These results showed that uPA mediated a portion of cushion cell migration in vitro. Although AV segments activated plasminogen and degraded extracellular proteins, uPA's functional role in cushion cell migration did not involve the plasminogen/plasmin system.

Neuregulin stimulates DNA synthesis in embryonic chick heart cells

Neuregulins are a family of growth factors that have been shown to promote the growth or differentiation of various cell types. Recently, targeted mutations of the genes for neuregulins or their putative receptors by homologous recombination resulted in embryonic lethality characterized by cardiac malformation. Here we investigate a role for neuregulin in the growth of cultured chick heart cells. Neuregulin induced the tyrosine phosphorylation of a 185-kDa protein in cultured heart cells, and it also stimulated an increase in [(3)H]thymidine incorporation and BrDU labeling in the cell cultures. Immunocytochemistry revealed that the increased DNA synthesis was primarily in mesenchymal cells and not detected in myocytes or endocardial cells. These data suggest that neuregulin may function as a paracrine signal in mesenchymal-endothelial interactions during cardiac development.

Slug is a mediator of epithelial-mesenchymal cell transformation in the developing chicken heart

An epithelial-mesenchymal cell transformation occurs during the development of the endocardial cushions in the atrioventricular (AV) canal of the heart. We hypothesized that the transcription factor Slug is required for this epithelial-mesenchymal cell transformation since Slug is required for similar transformations during gastrulation and neural crest differentiation in chicken embryos. We found by RT-PCR and immunostaining that the temporal and spatial localization of Slug in the embryonic chicken heart is consistent with a role for Slug in endocardial cushion formation. Moreover, we found that Slug expression by AV canal endothelial cells is induced by a signal provided by AV canal myocardium. Slug appears to be required for epithelial-mesenchymal cell transformation in the chicken heart since treatment of AV canal explants with antisense Slug oligodeoxynucleotides inhibited mesenchymal cell formation in vitro. Antisense Slug oligodeoxynucleotides prevented endothelial cell-cell separation, suggesting that Slug acts early in the transformation pathway.

Bone morphogenetic protein-2 acts synergistically with transforming growth factor-beta3 during endothelial-mesenchymal transformation in the developing chick heart

In the early embryonic heart, endothelial cells in atrioventricular (AV) and outflow tract (OT) regions are transformed into the invasive mesenchymal cells that form endocardial cushion tissue (endothelial-mesenchymal transformation). It has been reported that bone morphogenetic proteins (BMPs) are transcribed in the AV and OT regions of the embryonic mouse heart. We previously reported that transforming growth factor beta 3 (TGFbeta3) triggers the initial phenotypic changes seen in endothelial-mesenchymal transformation. We cloned BMP2 from embryonic chick hearts and examined its functional role during endocardial cushion tissue formation. In situ hybridization showed BMP2 transcripts in the myocardium of the AV and OT regions, but not in endothelial/mesenchymal cells. Antisense oligodeoxynucleotides to BMP2 inhibited mesenchyme formation in AV endocardium cocultured with associated myocardium. This inhibitory effect was reversed by the addition of recombinant BMP2. In cultured AV endothelial monolayers, recombinant BMP2 did not induce any cellular phenotypic changes characteristic of endothelial-mesenchymal transformation. However, BMP2 enhanced the TGFbeta-induced initial phenotypic changes associated with endothelial-mesenchymal transformation. These results suggest that BMP2 1) plays an important role in the formation of endocardial cushion tissue and 2) acts synergistically with TGFbeta3 in the regulation of this developmental event.

TGFbeta2 and TGFbeta3 have separate and sequential activities during epithelial-mesenchymal cell transformation in the embryonic heart

Heart valve formation is initiated by an epithelial-mesenchymal cell transformation (EMT) of endothelial cells in the atrioventricular (AV) canal. Mesenchymal cells formed from cardiac EMTs are the initial cellular components of the cardiac cushions and progenitors of valvular and septal fibroblasts. It has been shown that transforming growth factor beta (TGFbeta) mediates EMT in the AV canal, and TGFbeta1 and 2 isoforms are expressed in the mouse heart while TGFbeta 2 and 3 are expressed in the avian heart. Depletion of TGFbeta3 in avian or TGFbeta2 in mouse leads to developmental defects of heart tissue. These observations raise questions as to whether multiple TGFbeta isoforms participate in valve formation. In this study, we examined the localization and function of TGFbeta2 and TGFbeta3 in the chick heart during EMT. TGFbeta2 was present in both endothelium and myocardium before and after EMT. TGFbeta2 antibody inhibited endothelial cell-cell separation. In contrast, TGFbeta3 was present only in the myocardium before EMT and was in the endothelium at the initiation of EMT. TGFbeta3 antibodies inhibited mesenchymal cell formation and migration into the underlying matrix. Both TGFbeta2 and 3 increased fibrillin 2 expression. However, only TGFbeta2 treatment increased cell surface beta-1,4-galactosyltransferase expression. These data suggest that TGFbeta2 and TGFbeta3 are sequentially and separately involved in the process of EMT. TGFbeta2 mediates initial endothelial cell-cell separation while TGFbeta3 is required for the cell morphological change that enables the migration of cells into the underlying ECM.

Epithelial-mesenchymal transformation in the embryonic heart is mediated through distinct pertussis toxin-sensitive and TGFbeta signal transduction mechanisms

During early development, progenitors of the heart valves and septa are formed by epithelial-mesenchymal transformation (EMT) of endothelial cells in the atrioventricular (AV) canal. Previously, we showed that pertussis toxin, a specific inhibitor of a subset of G proteins, inhibited EMT in chick AV canal cultures. This study examines in detail the effects of pertussis toxin on the process of EMT. One of the major mediators of EMT is Transforming Growth Factor beta 3 (TGFbeta3) which acts through the TGFbeta Type II receptor. To determine whether pertussis toxin affects EMT via the TGFbeta Type II receptor pathway, we compared AV cultures treated with pertussis toxin and TGFbeta Type II receptor blocking antibody. Pertussis toxin inhibited several elements of EMT. At all stages tested, pertussis toxin blocked endothelial cell-cell separation, cell hypertrophy, and the cellular polarization associated with endothelial activation. These activities were unaffected by TGFbeta Type II receptor antibodies. Pertussis toxin also reduced transformed mesenchymal cell migration by 61%. The expression patterns of several proteins (as markers of EMT) were analyzed in untreated, pertussis toxin-treated, and TGFbeta Type II receptor blocking antibody-treated cultures. These markers were alpha-smooth muscle actin, Mox-1, fibrillin 2, tenascin, cell surface beta 1,4 galactosyltransferase (GalTase), and integrin alpha6. Clear differences in marker expression were found between the two inhibitors. For example, in all cells, pertussis toxin inhibited expression of alpha-smooth muscle actin and GalTase while TGFbeta Type II receptor antibody treatment increased expression of these two proteins. These data suggest that G protein-mediated signaling is required for several elements of EMT. Furthermore, distinct G protein and TGFbeta signal transduction pathways mediate discrete components of EMT.

The Cspg2 gene, disrupted in the hdf mutant, is required for right cardiac chamber and endocardial cushion formation

The heart defect (hdf) mouse is a recessive lethal that arose from a transgene insertional mutation on chromosome 13. Embryos homozygous for the transgene die in utero by embryonic day 10.5 postcoitus and exhibit specific defects along the anterior-posterior cardiac axis. The future right ventricle and conus/truncus of the single heart tube fail to form and the endocardial cushions in the atrioventricular and conus/truncus regions are absent. Because the hdf mouse mutation provided the opportunity to identify a gene required for endocardial cushion formation and for specification or maintenance of the anterior most segments of the heart, we initiated studies to further characterize the phenotype, clone the insertion site, and identify the gene disrupted. Chromosome mapping studies first identified the gene, Cspg2 (versican), as a candidate hdf gene. In addition, an antibody recognizing a glycosaminoglycan epitope on versican was found to be positive by immunohistochemistry in the extracellular matrix of normal wild-type embryonic hearts, but absent in homozygous hearts. Expression analysis of the Cspg2 gene showed that the 6/8, 6/9, and 7/9 Cspg2 exon boundaries were present in mRNA of normal wild-type embryonic hearts but absent in the homozygous mutant embryos. DNA sequence flanking the transgene was used to isolate from a normal mouse library overlapping genomic DNA segments that span the transgene insertion site. The contiguous genomic DNA segment was found to contain exon 7 of the Cspg2 in a position 3' to the transgene insertion site. These four separate lines of evidence support the hypothesis that Cspg2 is the gene disrupted by the transgene insertion in the hdf mouse line. The findings of this study and our previous studies of the hdf insertional mutant mouse have shown that normal expression of the Cspg2 gene is required for the successful development of the endocardial cushion swellings and the embryonic heart segments that give rise to the right ventricle and conus/truncus in the outlet of the looped heart.

Versican expression is associated with chamber specification, septation, and valvulogenesis in the developing mouse heart

The versican (PG-M) gene encodes a chondroitin sulfate proteoglycan that is nonpermissive for cell migration and appears in association with slow cell proliferation and cytodifferentiation. Using the techniques of in situ hybridization and immunocytochemistry on sectioned mouse embryos, we found that the mRNA and protein for versican show similar distributions and are expressed in a dynamic pattern during development of the heart. Versican exhibits generalized expression in the tubular heart but becomes rapidly downregulated in the atrium and exhibits higher transcript levels on the right side of the ventricular chamber than the left, before the onset of ventricular septation. Versican is expressed strongly in the trabeculated ventricular myocardium, whereas the compact proliferative zone has lower transcript abundance. It is expressed in the outer layers and on the crest of the ventricular septum and is prominent on the mesenchymal cap of the primary atrial septum. Versican is particularly strongly expressed in the endocardial cushions of the atrioventricular and outflow tract regions and in the atrioventricular, semilunar, and venous valves. This study raises the possibility that versican may be involved in specification of the ventricular chambers, in growth and fusion of the atrial and ventricular septa, and in the transformation from epithelium to mesenchyme that characterizes development of the endocardial cushions. Versican may be a key participant in cardiogenesis, responding to the many diffusible signals that mediate interactions between the developing endocardium and myocardium.

Expression and function of activin beta A during mouse cardiac cushion tissue formation

The formation of cardiac cushion tissue, which ultimately contributes to formation of the valves and septa, is dependent on the regional activation of cardiac endothelial cells to undergo an epithelial-mesenchymal transition. This endothelial transition was correlated with activin betaA mRNA expression by Northern and in situ hybridization in both a temporal and spatial manner in developing mouse embryos. Activin betaA was the only subunit of the inhibin family detected during the initial phase of endothelial cell transition; activin betaB was detected at later stages, and inhibin alpha was not detectable in the heart. An in vitro assay that has been used to study mesenchymal cell formation in chick was modified for use with mammalian embryos. Conditioned media from embryonic mouse cardiocyte cultures was shown to substitute for the endogenous inductive signal in these assays. The presence of activin betaA was demonstrated by Western blot analysis of the cardiocyte conditioned media (CCM). Modified antisense oligonucleotides to activin betaA inhibited the endothelial-mesenchymal transition in the assay system, which was not affected by control oligonucleotides. Adapting the avian culture system for use with mice enabled the use of tissue from mice with a null allele for activin betaA. CCM produced from embryos homozygous for the mutant betaA allele did not contain activin betaA and was used in in vitro assays. CCM lacking activin betaA produced fewer mesenchymal cells from cardiac endothelial monolayers than CCM with activin betaA. Localized expression of activin betaA in the embryonic heart indicates a possible role in the endothelial-mesenchymal transition. Bioassays in which activin betaA expression is blocked or activin betaA is absent from the media indicate that activin betaA promotes the formation of mesenchymal cells in the endothelial cushions, which are required for normal septation.