Temporal and distinct TGFbeta ligand requirements during mouse and avian endocardial cushion morphogenesis

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.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.

Smad3 signaling is required for epithelial-mesenchymal transition of lens epithelium after injury

Lens epithelial cells undergo epithelial-mesenchymal transition (EMT) after injury as in cataract extraction, leading to fibrosis of the lens capsule. Fibrosis of the anterior capsule can be modeled in the mouse by capsular injury in the lens, which results in EMT of the lens epithelium and subsequent deposition of extracellular matrix without contamination of other cell types from outside the lens. We have previously shown that signaling via Smad3, a key signal-transducing element downstream of transforming growth factor (TGF)-beta and activin receptors, is activated in lens epithelial cells by 12 hours after injury and that this Smad3 activation is blocked by administration of a TGF-beta 2-neutralizing antibody in mice. We now show that EMT of primary lens epithelial cells in vitro depends on TGF-beta expression and that injury-induced EMT in vivo depends, more specifically, on signaling via Smad3. Loss of Smad3 in mice blocks both morphological changes of lens epithelium to a mesenchymal phenotype and expression of the EMT markers snail, alpha-smooth muscle actin, lumican, and type I collagen in response to injury in vivo or to exposure to exogenous TGF-beta in organ culture. The results suggest that blocking the Smad3 pathway might be beneficial in inhibiting capsular fibrosis after injury and/or surgery.

Notch activation results in phenotypic and functional changes consistent with endothelial-to-mesenchymal transformation

Various studies have identified a critical role for Notch signaling in cardiovascular development. In this and other systems, Notch receptors and ligands are expressed in regions that undergo epithelial-to-mesenchymal transformation. However, there is no direct evidence that Notch activation can induce mesenchymal transdifferentiation. In this study we show that Notch activation in endothelial cells results in morphological, phenotypic, and functional changes consistent with mesenchymal transformation. These changes include downregulation of endothelial markers (vascular endothelial [VE]-cadherin, Tie1, Tie2, platelet-endothelial cell adhesion molecule-1, and endothelial NO synthase), upregulation of mesenchymal markers (alpha-smooth muscle actin, fibronectin, and platelet-derived growth factor receptors), and migration toward platelet-derived growth factor-BB. Notch-induced endothelial-to-mesenchymal transformation does not seem to require external regulation and is restricted to cells expressing activated Notch. Jagged1 stimulation of endothelial cells induces a similar mesenchymal transformation, and Jagged1, Notch1, and Notch4 are expressed in the ventricular outflow tract during stages of endocardial cushion formation. This is the first evidence that Jagged1-Notch interactions induce endothelial-to-mesenchymal transformation, and our findings suggest that Notch signaling may be required for proper endocardial cushion differentiation and/or vascular smooth muscle cell development.

Integration of TGF-beta/Smad and Jagged1/Notch signalling in epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transitions (EMTs) underlie cell plasticity required in embryonic development and frequently observed in advanced carcinogenesis. Transforming growth factor-beta (TGF-beta) induces EMT phenotypes in epithelial cells in vitro and has been associated with EMT in vivo. Here we report that expression of the hairy/enhancer-of-split-related transcriptional repressor Hey1, and the Notch-ligand Jagged1 (Jag1), was induced by TGF-beta at the onset of EMT in epithelial cells from mammary gland, kidney tubules, and epidermis. The HEY1 expression profile was biphasic, consisting of immediate-early Smad3-dependent, Jagged1/Notch-independent activation, followed by delayed, indirect Jagged1/Notch-dependent activation. TGF-beta-induced EMT was blocked by RNA silencing of HEY1 or JAG1, and by chemical inactivation of Notch. The EMT phenotype, biphasic activation of Hey1, and delayed expression of Jag1 were induced by TGF-beta in wild-type, but not in Smad3-deficient, primary mouse kidney tubular epithelial cells. Our findings identify a new mechanism for functional integration of Jagged1/Notch signalling and coordinated activation of the Hey1 transcriptional repressor controlled by TGF-beta/Smad3, and demonstrate functional roles for Smad3, Hey1, and Jagged1/Notch in mediating TGF-beta-induced EMT.

Essential role for ADAM19 in cardiovascular morphogenesis

Congenital heart disease is the most common form of human birth defects, yet much remains to be learned about its underlying causes. Here we report that mice lacking functional ADAM19 (mnemonic for a disintegrin and metalloprotease 19) exhibit severe defects in cardiac morphogenesis, including a ventricular septal defect (VSD), abnormal formation of the aortic and pulmonic valves, leading to valvular stenosis, and abnormalities of the cardiac vasculature. During mouse development, ADAM19 is highly expressed in the conotruncus and the endocardial cushion, structures that give rise to the affected heart valves and the membranous ventricular septum. ADAM19 is also highly expressed in osteoblast-like cells in the bone, yet it does not appear to be essential for bone growth and skeletal development. Most adam19(-/-) animals die perinatally, likely as a result of their cardiac defects. These findings raise the possibility that mutations in ADAM19 may contribute to human congenital heart valve and septal defects.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Making more heart muscle

Postnatally, heart muscle cells almost completely lose their ability to divide, which makes their loss after trauma irreversible. Potential repair by cell grafting or mobilizing endogenous cells is of particular interest for possible treatments for heart disease, where the poor capacity for cardiomyocyte proliferation probably contributes to the irreversibility of heart failure. Knowledge of the molecular mechanisms that underly formation of heart muscle cells might provide opportunities to repair the diseased heart by induction of (trans) differentiation of endogenous or exogenous cells into heart muscle cells. We briefly review the molecular mechanisms involved in early development of the linear heart tube by differentiation of mesodermal cells into heart muscle cells. Because the initial heart tube does not comprise all the cardiac compartments present in the adult heart, heart muscle cells are added to the distal borders of the tube and within the tube. At both distal borders, mesodermal cell are recruited into the cardiac lineage and, within the heart tube, muscular septa are formed. In this review, the relative late additions of heart muscle cells to the linear heart tube are described and the potential underlying molecular mechanisms are discussed.

Annexin II-mediated plasmin generation activates TGF-β3 during epithelial–mesenchymal transformation in the developing avian heart

Epithelial-mesenchymal transformation (EMT), the process by which epithelial cells are converted into motile, invasive mesenchymal cells, is critical to valvulogenesis. Transforming growth factor-beta3 (TGF-beta3), an established mediator of avian atrioventricular (AV) canal EMT, is secreted as a latent complex. In vitro, plasmin-mediated proteolysis has been shown to release active TGF-betas from the latent complex. Annexin II, a co-receptor for tissue plasminogen activator (tPA) and plasminogen, promotes cell-surface generation of the serine protease plasmin. Here, we show that annexin II-mediated plasmin activity regulates release of active TGF-beta3 during chick AV canal EMT. Primary embryonic endocardial-derived cells express annexin II which promotes plasminogen activation in vitro. Incubation of heart explant cultures with either alpha(2)antiplasmin (alpha(2)AP), a major physiological plasmin inhibitor, or anti-annexin II IgG, blocked EMT by approximately 80%, and 50%, respectively. Anti-annexin II IgG-mediated inhibition of EMT was overcome by the addition of recombinant TGF-beta3. Upon treatment with anti-annexin II IgG or alpha(2)AP, conditioned medium from heart explant cultures showed absence of the active fragment of TGF-beta3 by Western blot analysis and a approximately 50% decrease in TGF-beta specific bioactivity. Our results suggest that annexin II-mediated plasmin activity regulates the release of active TGF-beta during cardiac valve development in the avian heart.

Notch promotes epithelial-mesenchymal transition during cardiac development and oncogenic transformation

Epithelial-to-mesenchymal transition (EMT) is fundamental to both embryogenesis and tumor metastasis. The Notch intercellular signaling pathway regulates cell fate determination throughout metazoan evolution, and overexpression of activating alleles is oncogenic in mammals. Here we demonstrate that Notch activity promotes EMT during both cardiac development and oncogenic transformation via transcriptional induction of the Snail repressor, a potent and evolutionarily conserved mediator of EMT in many tissues and tumor types. In the embryonic heart, Notch functions via lateral induction to promote a selective transforming growth factor-beta (TGFbeta)-mediated EMT that leads to cellularization of developing cardiac valvular primordia. Embryos that lack Notch signaling elements exhibit severely attenuated cardiac snail expression, abnormal maintenance of intercellular endocardial adhesion complexes, and abortive endocardial EMT in vivo and in vitro. Accordingly, transient ectopic expression of activated Notch1 (N1IC) in zebrafish embryos leads to hypercellular cardiac valves, whereas Notch inhibition prevents valve development. Overexpression of N1IC in immortalized endothelial cells in vitro induces EMT accompanied by oncogenic transformation, with corresponding induction of snail and repression of VE-cadherin expression. Notch is expressed in embryonic regions where EMT occurs, suggesting an intimate and fundamental role for Notch, which may be reactivated during tumor metastasis.

Transforming growth factor beta in cardiovascular development and function

Transforming growth factor betas (TGFbetas) are pleiotropic cytokines involved in many biological processes. Genetic engineering and tissue explanation studies have revealed specific non-overlapping roles for TGFbeta ligands and their signaling molecules in development and in normal function of the cardiovascular system in the adult. In the embryo, TGFbetas appear to be involved in epithelial-mesenchymal transformations (EMT) during endocardial cushion formation, and in epicardial epithelial-mesenchymal transformations essential for coronary vasculature, ventricular myocardial development and compaction. In the adult, TGFbetas are involved in cardiac hypertrophy, vascular remodeling and regulation of the renal renin-angiotensin system. The evidence for TGFbeta activities during cardiovascular development and physiologic function will be given and areas which need further investigation will be discussed.

An essential role of Bmp4 in the atrioventricular septation of the mouse heart

Proper septation and valvulogenesis during cardiogenesis depend on interactions between the myocardium and the endocardium. By combining use of a hypomorphic Bone morphogenetic protein 4 (Bmp4) allele with conditional gene inactivation, we here identify Bmp4 as a signal from the myocardium directly mediating atrioventricular septation. Defects in this process cause one of the most common human congenital heart abnormalities, atrioventricular canal defect (AVCD). The spectrum of defects obtained through altering Bmp4 expression in the myocardium recapitulates the range of AVCDs diagnosed in patients, thus providing a useful genetic model with AVCD as the primary defect.

CD40 and CD40L expression in the chicken embryo aorta: possible role in the endothelial-mesenchymal transdifferentiation process

Endothelial-mesenchymal transdifferentiation (EMT) is believed to play a crucial role in embryonic vascular development and intimal thickening, which contributes to the pathogenesis of atherosclerotic lesions. However, the mechanisms by which it occurs, as well as the signals that control it, have not yet been elucidated. Given the important role played by the CD40-CD40 ligand (CD40L) system during the initiation and progress of atherosclerosis, we investigated whether both CD40 and CD40L were present in the aortic wall during EMT and the advanced stages of chicken embryo development. CD40-CD40L expression was found on endothelial cells (ECs), mesenchymal cells, and smooth muscle cells (SMCs) at all stages examined, and appeared to be distributed across the aortic wall. However, some notable differences between the expression patterns were observed. CD40 had a more restricted distribution compared to CD40L, and did not stain every cell type of the aortic wall. According to immunoblotting and enzyme-linked immunosorbent assay (ELISA) analyses, the CD40L content was highest at day 7 of development. An important and novel finding was the expression of CD40L in areas where ECs transdifferentiate into mesenchymal cells. Specifically, CD40L was associated to the surface of cells that were detaching and migrating from the monolayer of ECs, whereas for CD40 a very diffuse subcellular localization was seen at the monolayer and the detaching and migrating cells. These data suggest a possible role for CD40-CD40L interactions during EMT and the remodeling of the aorta.

Loss of distinct arterial and venous boundaries in mice lacking endoglin, a vascular-specific TGFbeta coreceptor

Several characteristic morphological and functional differences distinguish arteries from veins. It was thought that hemodynamic forces shaped these differences; however, increasing evidence suggests that morphogenetic programs play a central role in blood vessel differentiation. Hereditary hemorrhagic telangiectasia (HHT) is a vascular dysplasia characterized by the inappropriate fusion of arterioles with venules. The genes implicated in this disease, ALK1 and endoglin, may be involved in defining the fundamental boundaries between arteries and veins. We previously showed that mice lacking Alk1 lost structural, molecular, and functional distinctions between arteries and veins. Here, we report that mice lacking endoglin develop arterial-venous malformations and fail to confine intraembryonic hematopoiesis to arteries. In contrast to Alk1 mutants, endoglin mutants do not show profound vessel dilation or downregulation of arterial ephrinB2. Finally, our data indicate that a failure in cardiac cushion formation observed in both strains may be secondary to the peripheral vasculature defect. The phenotypic similarities, yet reduced severity, implicates endoglin as an accessory coreceptor that specifically modulates Alk1 signaling. We propose that endoglin and Alk1 are necessary for the maintenance of distinct arterial-venous vascular beds and that attenuation of the Alk1 signaling pathway is the precipitating event in the etiology of HHT.

Control of endocardial cushion and cardiac valve maturation by BMP signaling pathways

Congenital heart defects, the leading cause of deaths from birth defects, are estimated to occur in close to 1% of live newborns. Among these, abnormal septation of the heart and valve anomalies are the most frequent forms. Despite progress defining several genes involved in normal heart development, we still have a limited understanding of the signaling pathways involved in morphogenesis of the outflow tract (OFT) and, to date, very few genes have been identified that are responsible for defects in humans. Bone Morphogenetic Protein (BMP) signaling pathways are emerging as vital regulators of multiple aspects of cardiogenesis, including the septation of the OFT and valve maturation. Genetic and other in vivo evidence is now supporting the role for BMPs as inducers of endocardial cushion epithelial-to-mesenchymal transformation that was suggested by in vitro explant studies as well as by their patterns of expression in the developing heart. Here, we review briefly the in vitro data, and detail the novel mouse models where perturbed BMP signaling pathways result in impaired OFT septation and semilunar valvulogenesis. We propose that growth of the OFT valve cushions is regulated by the level of BMP signaling, under the control of other signaling pathways.

gp38k (CHI3L1) is a novel adhesion and migration factor for vascular cells

gp38k (CHI3L1) is a secreted heparin-binding glycoprotein whose expression, in vitro, is associated with vascular smooth muscle cell (VSMC) migration and invasion into the underlying gelatinous matrix. gp38k is expressed at high levels in postconfluent "nodular" VSMC cultures and at low levels in subconfluent proliferating cultures. In vivo, expression of gp38k homologs is high in regions of tissue remodeling and now has been detected in atherosclerotic plaques and in the developing heart. We tested the hypothesis that gp38k functions to modulate VSMC adhesion and migration. By use of modified Boyden chambers, gp38k at a concentration as low as 1 ng/ml has profound effects on VSMC migration but little or no effect on fibroblast migration. In addition, gp38k adsorbed to polystyrene surfaces directly promotes VSMC attachment and spreading. Attachment is inhibited in the presence of affinity-purified anti-gp38k or 10 mM EDTA. These results establish that gp38k is a new vascular cell adhesion and migration factor that may have a role in processes leading to vascular occlusion and heart development. gp38k may interact with VSMC via an EDTA-sensitive mechanism consistent with integrin mediated cell-matrix interaction.

Maternal diabetes: effects on embryonic vascular development--a vascular endothelial growth factor-A-mediated process

Major congenital malformations, many of which result from abnormal cardiovascular patterning, remain the leading cause in infant mortality and morbidity. Targeted mutations of several genes (including VEGF and VEGF receptors) and certain teratogenic agents (including excess alpha-D-glucose) give rise to embryonic lethal phenotypes associated with failure in the formation of a functional vitelline circulation and aberrant organogenesis. Our work to date has demonstrated that yolk sac vasculopathy and failure of endocardial cushion epithelial-mesenchymal transformation occurs in hyperglycemic conditions in murine whole conceptus culture and in embryos from streptozotocin-induced diabetic mice. These cardiovascular abnormalities are associated with changes in expression and phosphorylation state of adhesion molecules such as platelet endothelial growth factor-1 and expression of growth factors such as vascular endothelial growth factor (VEGF-A). Further understanding of the effects of maternal diabetes on yolk sac and embryonic vasculogenesis/angiogenesis and organogenesis may lead to novel approaches in treating and preventing major birth defects.

Expression patterns of Tgfbeta1-3 associate with myocardialisation of the outflow tract and the development of the epicardium and the fibrous heart skeleton

Transforming growth factor-beta (Tgfbeta) is essential for normal embryogenesis. The cardiac phenotypes obtained after knockout of each of the three mammalian isoforms suggest different roles during morphogenesis. We studied cardiovascular expression of Tgfbeta1-3 in parallel tissue sections of normal mouse embryos from 9.5 to 15.5 days post coitum (dpc) by using radioactive in situ hybridisation. The Tgfbeta isoforms are differentially expressed in unique and in overlapping patterns during cardiovascular development. In the vessels, Tgfbeta1 is found in the intima, whereas Tgfbeta2 and -beta3 are mainly present in the media and adventitia of the great arteries. Tgfbeta1 is present in the endocardium at all stages examined. The Tgfbeta2 signal in the endocardium of the atrioventricular canal and outflow tract (9.5 dpc) shifts during epithelial-mesenchymal transformation toward the mesenchymal cushions (10.5-11.5 dpc) after which it exhibits a marked spatiotemporal expression pattern as the cushion differentiation progresses (11.5-15.5 dpc). The myocardium underlying the endocardial cushions and the atrial muscular septum are intensely positive for Tgfbeta2 at early stages (9.5-11.5 dpc) and expression decreases at 12.5 days. In contrast to earlier reports, we find marked overlap of Tgfbeta2 and -beta3 expression. Tgfbeta3 expression shows a characteristic distribution in the mesenchymal cushions, suggesting a role in cushion differentiation, possibly additional to Tgfbeta2. From 14.5 dpc onward, a strong Tgfbeta3 signal is found in the fibrous septum primum of the atrium and in the fibrous skeleton of the heart. Special attention was paid to the proepicardial organ and its derivatives. The proepicardial organ strongly expresses Tgfbeta2 as early as 9.5 days, and all isoforms are present in the epicardium from 12.5 dpc onward. The spatiotemporal cardiovascular expression of Tgfbeta1-3 supports both specific and complementary functions during cardiovascular development that might explain functional redundancy between the Tgfbeta-isoforms. The information provided favors novel roles of Tgfbeta1-3 in epicardial development, of Tgfbeta2 in myocardialisation, and of Tgfbeta3 in differentiation of the fibrous structures of the heart.

Cardiac muscle cell formation after development of the linear heart tube

After the development of the linear heart tube, additional myocardium is formed leading to the muscular mantle around the caval and pulmonary veins and the muscular septa in the embryonic heart. Here, we report the results of our in vivo and in vitro studies of this late myocardium-generating process in the mouse. By using an immunohistochemical approach, we determined that myocardium formation starts around embryonic day 12 in the dorsal mesocardium. In subsequent stages of development, the process extends downstream into the intracardiac mesenchymal tissues of the atrioventricular canal and outflow tract and upstream into the extracardiac mediastinal mesenchyme embedding the pulmonary and caval veins. Given the spatiotemporal pattern of myocardium formation, we applied a three-dimensional in vitro explant culture assay to investigate the myocardium-generating potential of the different cardiac compartments. We determined that this potential is stage- and mesenchyme-dependent. This latter finding suggests an important role for mesenchyme in myocardium formation after the development of the linear heart tube.

VEGF modulates early heart valve formation

Although hypoxic and/or nutritional insults during gestation are believed to contribute to congenital heart defects, the mechanisms responsible for these anomalies are not understood. Given the role vascular endothelial growth factor (VEGF) plays in response to hypoxia, it is a likely candidate for mediating deleterious effects of embryonic hypoxia. The ectopic or overproduction of endogenous factors such as VEGF may contribute to specific heart defects. Here we compared hypoxia-induced precocious production of VEGF during early heart valve development to normal VEGF production. Mouse prevalvular cardiac endocardial cushions were explanted onto hydrated type I collagen gels under normoxic or hypoxic conditions. The extent of transformation of cardiac endothelium into mesenchyme was inversely correlated with the levels of VEGF during the various culture conditions. A soluble VEGF antagonist confirmed that endogenous production of VEGF was specific for blocking normal cushion mesenchyme formation. We further demonstrated that E10.5 endocardium retains the ability to transform into cardiac mesenchyme in the absence of endogenous VEGF.

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.

Tempting fate: BMP signals for cardiac morphogenesis

Heart muscle cell specification (cardiac myogenesis) and creating the four-chambered heart (cardiac morphogenesis) are subject to regulation, in certain model organisms, by bone morphogenetic proteins and their receptors. Extrapolation to mammals from organisms that develop outside the mother (flies, fish, frogs, and avians) has been confounded by very early lethality-at gastrulation-of many null alleles needed to prove cause-effect relations in this pathway. Here, we describe the use of lineage- or compartment-restricted null alleles as well as hypomorphic alleles, which circumvent these limitations and pinpoint novel essential functions for the bone morphogenetic protein cascade in mammalian cardiac development.

Early events in valvulogenesis: a signaling perspective

The proper formation and function of the vertebrate heart requires a multitude of specific cell and tissue interactions. These interactions drive the early specification and assembly of components of the cardiovascular system that lead to a functioning system before the attainment of the definitive cardiac and vascular structures seen in the adult. Many of these adult structures are hypothesized to require both proper molecular and physical cues to form correctly. Unlike any other organ system in the embryo, the cardiovascular system requires concurrent function and formation for the embryo to survive. An example of this complex interaction between molecular and physical cues is the formation of the valves of the heart. Both molecular cues that regulate cell transformation, migration, and extracellular matrix deposition, and physical cues emanating from the beating heart, as well as hemodynamic forces, are required for valvulogenesis. This review will focus on molecules and emerging pathways that guide early events in valvulogenesis.

Hyaluronan: genetic insights into the complex biology of a simple polysaccharide

It is appropriate that this review should appear in a volume dedicated to Mert Bernfield. Much of my interest in the cell biology of the extracellular matrix, particularly during development, echoes Mert's pioneering studies. His kind but provocative questioning during meetings is especially missed. The glycosaminoglycan hyaluronan is ubiquitous, and is especially abundant during embryogenesis. Hydrated matrices rich in hyaluronan expand the extracellular space, facilitating cell migration. The viscoelastic properties of hyaluronan are also essential for proper function of cartilage and joints. Recent understanding of hyaluronan biology has benefited from the identification of genes encoding hyaluronan synthases and hyaluronidases, genetic analysis of the roles of hyaluronan during development, elucidation of the biochemical mechanisms of hyaluronan synthesis, and by studies of human genetics and tumors. This review focuses on recent studies utilizing hyaluronan-deficient, gene targeted mice with null alleles for the principal source of hyaluronan during mid-gestation, hyaluronan synthase-2 (has-2).