Antibodies directed against the chicken type II TGFbeta receptor identify endothelial cells in the developing chicken and quail

Endocardial cell epithelial-mesenchymal transformation requires Type III TGFβ receptor interaction with GIPC

An early event in heart valve formation is the epithelial-mesenchymal transformation (EMT) of a subpopulation of endothelial cells in specific regions of the heart tube, the endocardial cushions. The Type III TGFβ receptor (TGFβR3) is required for TGFβ2- or BMP-2-stimulated EMT in atrioventricular endocardial cushion (AVC) explants in vitro but the mediators downstream of TGFβR3 are not well described. Using AVC and ventricular explants as an in vitro assay, we found an absolute requirement for specific TGFβR3 cytoplasmic residues, GAIP-interacting protein, C terminus (GIPC), and specific Activin Receptor-Like Kinases (ALK)s for TGFβR3-mediated EMT when stimulated by TGFβ2 or BMP-2. The introduction of TGFβR3 into nontransforming ventricular endocardial cells, followed by the addition of either TGFβ2 or BMP-2, results in EMT. TGFβR3 lacking the entire cytoplasmic domain, or only the 3C-terminal amino acids that are required to bind GIPC, fails to support EMT in response to TGFβ2 or BMP-2. Overexpression of GIPC in AVC endocardial cells enhanced EMT while siRNA-mediated silencing of GIPC in ventricular cells overexpressing TGFβR3 significantly inhibited EMT. Targeting of specific ALKs by siRNA revealed that TGFβR3-mediated EMT requires ALK2 and ALK3, in addition to ALK5, but not ALK4 or ALK6. Taken together, these data identify GIPC, ALK2, ALK3, and ALK5 as signaling components required for TGFβR3-mediated endothelial cell EMT.

Transforming growth factor beta2 is negatively regulated by endogenous retinoic acid during early heart morphogenesis

Vitamin A-deficient (VAD) quail embryos lack the vitamin A-active form, retinoic acid (RA) and are characterized by a phenotype that includes a grossly abnormal cardiovascular system that can be rescued by RA. Here we report that the transforming growth factor, TGFbeta2 is involved in RA-regulated cardiovascular development. In VAD embryos TGFbeta2 mRNA and protein expression are greatly elevated. The expression of TGFbeta receptor II is also elevated in VAD embryos but is normalized by treatment with TGFbeta2-specific antisense oligonucleotides (AS). Administration of this AS or an antibody specific for TGFbeta2 to VAD embryos normalizes posterior heart development and vascularization, while the administration of exogenous active TGFbeta2 protein to normal quail embryos mimics the excessive TGFbeta2 status of VAD embryos and induces VAD cardiovascular phenotype. In VAD embryos pSmad2/3 and pErk1 are not activated, while pErk2 and pcRaf are elevated and pSmad1/5/8 is diminished. We conclude that in the early avian embryo TGFbeta2 has a major role in the retinoic acid-regulated posterior heart morphogenesis for which it does not use Smad2/3 pathways, but may use other signaling pathways. Importantly, we conclude that retinoic acid is a critical negative physiological regulator of the magnitude of TGFbeta2 signals during vertebrate heart formation.

Embryonic cardiomyocyte expression of endothelial genes

Vertebrate precardiac mesoderm contains cells destined to become cardiomyocyte or endothelial cells. To determine the stability of these phenotypes freshly isolated embryonic day (E) 2.5-E6 chicken hearts were immunostained for myosin heavy chain (MyHC) to identify cardiomyocytes, and von Willebrand factor (vWF) and Flk-1 to identify endothelial cells. At E2.5-E3, 90% of cells express only MyHC and 6% express only vWF/Flk-1. However, 2% MyHC+ cells in E2.5-E3 hearts and 0.3% in E4-E6 hearts, also express vWF/Flk-1; and when cultured 3 days, >40% of the MyHC+ cells express vWF/Flk-1, but they do not express Vezf1, vascular endothelial cadherin, or Tie2. Thus, only a subset of endothelial genes are induced in cultured cardiomyocytes. While the subsequent developmental fate of embryonic heart cells exhibiting a vWF+/MyHC+ phenotype is unknown, analysis of this phenotype may provide information pertinent to mechanisms of cell phenotype stability, cellular transdifferentiation, and induction of stable cell types from embryonic stem cells.

Vitamin a requirement for early cardiovascular morphogenesis specification in the vertebrate embryo: insights from the avian embryo

Vitamin A is required throughout the life cycle, including crucial stages of embryonic and fetal development. With the identification of retinoic acid-specific nuclear transcription factors, the retinoid receptors, considerable advances have been made in understanding the molecular function of vitamin A. The requirement for vitamin A during early embryogenesis has successfully been examined in the vitamin A-deficient avian embryo during neurulation, when in the vertebrates crucial developmental decisions take place. These studies revealed that retinoic acid is essential during these early stages of embryogenesis for the initiation of organogenesis (i.e., formation of the heart). If retinoic acid is not present at this time, abnormal development ensues, leading to early embryonic death. Though the initial insult of the absence of vitamin A appears to be on the specification of cardiovascular tissues, subsequently all development is adversely affected and the embryo dies. Molecular and functional studies revealed that retinoic acid regulates the expression of the cardiogenic transcription factor GATA-4 and several heart asymmetry genes, which explains why the heart position is random in vitamin A-deficient quail embryos. During the crucial retinoic acid-requiring developmental window, retinoic acid transduces its signals to genes for heart morphogenesis via the receptors RARalpha2, RARgamma, and RXRalpha. Elucidation of the function of vitamin A during early embryonic development may lead to a better understanding of the cardiovascular birth defects prevalent in the Western world.

Hepatoma-derived growth factor is a pulmonary endothelial cell-expressed angiogenic factor

Hepatoma-derived growth factor (HDGF) was previously identified as a developmentally regulated cardiovascular and renal gene that is mitogenic for vascular smooth muscle and aortic endothelial cells. As reciprocal interactions of smooth muscle and endothelial cells are necessary for vascular formation, we examined whether HDGF plays a role in angiogenesis. According to immunohistochemistry, HDGF was highly expressed in endothelial cells of nonmuscularized, forming blood vessels of the fetal lung. HDGF was also expressed in endothelial cells of small (20 microm) mature arteries and veins. By Western immunoblotting, HDGF was highly expressed by human pulmonary microvascular endothelial cells in vitro. Adenoviral overexpression of HDGF was mitogenic for human pulmonary microvascular endothelial cells in serum-free medium, stimulating a 1.75-fold increase in bromodeoxyuridine (BrdU) uptake and a twofold increase in cell migration. With the chick chorioallantoic membrane (CAM), a biologic assay for angiogenesis, exogenous recombinant HDGF significantly stimulated blood vessel formation and a dose-dependent reorganization of cells within the CAM into a more compact, linear alignment reminiscent of tube formation. According to double immunostaining for endothelial cells with a transforming growth factor-betaII receptor antibody and BrdU as a marker of cell proliferation, exogenous HDGF selectively stimulated endothelial cell BrdU uptake. HDGF also activated specific ERK1/2 signaling and did not overlap with VEGF SAPK/JNK, Akt-mediated pathways. We conclude that HDGF is a highly expressed vascular endothelial cell protein in vivo and is a potent endothelial mitogen and regulator of endothelial cell migration by mechanisms distinct from VEGF.

Endodermal growth factors promote endocardial precursor cell formation from precardiac mesoderm

We previously demonstrated that the initial emergence of endocardial precursor cells (endocardial angioblasts) occurred within the precardiac mesoderm and that the endodermal secretory products promoted delamination of cells from the precardiac mesoderm and expression of endothelial lineage markers [Dev. Biol. 175 (1996), 66]. In this study, we sought to extend our original study to the identification of candidate molecules derived from the endoderm that might have induced endocardial precursor cell formation. We have detected expression of transforming growth factors beta (TGFbeta) 2, 3, and 4 in anterior endoderm at Hamburger and Hamilton (H-H) stage 5 by RT-PCR. To address the role of growth factors known to be present in the endoderm, precardiac mesodermal explants were isolated from H-H stage 5 quail embryos and cultured on the surface of collagen gels with serum-free defined medium 199. Similar to the effect of explants cocultured with anterior endoderm, when cultured with TGFbetas 1-3 (3 ng/ml each), explants formed QH-1 (anti-quail endothelial marker)-positive mesenchymal cells, which invaded the gel and expressed the extracellular marker, cytotactin (tenascin). Another member of the TGFbeta superfamily, bone morphogenetic protein-2 (BMP-2; 100 ng/ml), did not induce QH-1-positive mesenchymal cell formation but promoted formation of an epithelial monolayer on the surface of the collagen gel; this monolayer did not express QH-1. Explants treated with vascular endothelial growth factor (VEGF(165), 100 ng/ml) also did not invade the gel but formed an epithelial-like outgrowth on the surface of the gel. However, this monolayer did express the QH-1 marker. Fibroblast growth factor-2 (FGF-2; 250 ng/ml)-treated explants expressed QH-1 and exhibited separation of the cells on the surface of the gel. Finally, a combination of TGFbetas and VEGF enhanced formation of QH-1-positive cord-like structures within the gel from mesenchyme that had previously invaded the gel. Luminization of the cords, however, was not clearly evident. These findings suggest that TGFbetas, among the growth factors tested, mediate the initial step of endocardial formation, i.e., delamination of endothelial precursor cells from precardiac mesoderm, whereas VEGF may primarily effect early vasculogenesis (cord-like structure formation).

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.

Expression of the Flk1 receptor and its ligand VEGF in the developing chick central nervous system

The receptor tyrosine kinase Flk1 is known to mediate signals of vascular endothelial growth factor (VEGF) during vasculogenesis and hematopoiesis. We demonstrate by in situ hybridization that in addition to endothelial cells, chick Flk1 mRNA is also expressed in the notochord and in the neural epithelial cells of the ventral diencephalon, hindbrain, and spinal cord. During the development of the avascular chick retina, Flk1 mRNA is detected in the proliferative zone of the neural epithelium, whereas the VEGF ligand is expressed by differentiated retinal ganglion cells. Moreover, expression patterns of Flk1 in the retina are conserved among chick, quail and mouse, thus suggesting a distinct role of Flk1 and VEGF in the development of the vertebrate central nervous system.

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.

Activin receptor-like kinase 2 can mediate atrioventricular cushion transformation

Epithelial-mesenchymal transformation in the atrioventricular (AV) cushion of the tubular heart is a critical step in the formation of the valves and membranous septa. Transforming growth factor beta (TGFbeta) ligands are a primary signal of this transformation. To investigate the expression and function of specific Type I TGFbeta receptors during AV cushion transformation, we cloned and characterized the chicken homologues of two mammalian activin receptor-like kinases (ALK), ALK2 and ALK5, and generated specific, polyclonal antibodies against the extracellular binding domains of each. Both the chicken ALK2 (ChALK2) and the chicken ALK5 (ChALK5) cDNAs encode proteins that bind TGFbeta1 in the presence of the Type II TGFbeta receptor. However, as expected, only ChALK5 stimulated the TGFbeta-responsive PAI-1 promoter. These data establish that ChALK2 and ChALK5 are the chicken homologues of the mammalian receptors ALK2 and ALK5. Both ChALK2 and ChALK5 are expressed by AV endocardial cells. AV cushion explants harvested from stage 13-18 embryos were incubated with antisera to ChALK2 or ChALK5. Anti-ChALK2 antisera inhibited mesenchyme formation by 34-50% while neutralizing anti-ChALK5 antisera were without effect. These data demonstrate that ChALK2 can mediate transformation in the AV cushion.