Transforming growth factor-beta 1 localized within the heart of the chick embryo

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.

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.

An autocrine function for transforming growth factor (TGF)-beta3 in the transformation of atrioventricular canal endocardium into mesenchyme during chick heart development

Transformation of atrioventricular canal endocardium into invasive mesenchyme is a critical antecedent of cardiac septation and valvulogenesis. Previous studies by Potts et al. (Proc. Natl. Acad. Sci. USA 88, 1510-1520, 1991) showed that treatment of atrioventricular canal endocardial and myocardial cocultures with TGFbeta3 antisense oligodeoxynucleotides blocked mesenchyme formation. Based on this observation, we sought to: (i) identify the target tissue of TGFbeta3 antisense oligos in this transformation bioassay, and (ii) more clearly define the mechanism of TGFbeta3 function in atrioventricular canal mesenchyme formation. In situ hybridization and immunohistochemistry showed little or no TGFbeta3 mRNA or protein in the atrioventricular canal myocardium or endocardium prior to mesenchyme formation (stage 14; paraformaldehyde fixation). However, by stage 18 transforming atrioventricular canal endocardial cells and mesenchyme as well as myocardium were positive for both TGFbeta3 mRNA and protein. In culture bioassays, atrioventricular canal endocardial monolayers pretreated with antisense phosphorothioate oligodeoxynucleotides to TGFbeta3 did not transform into invasive mesenchyme in response to cardiocyte conditioned medium: the subsequent addition of exogenous TGFbeta3 protein relieved this inhibition. Control cultures without pretreatment or those receiving missense oligos generated similar numbers of invasive mesenchyme in response to cardiocyte conditioned medium. Direct addition of TGFbeta3 protein to atrioventricular canal endocardial monolayers in the absence of cardiocyte conditioned medium resulted in loss of cell:cell associations and stimulated cellular hypertrophy, but did not engender invasive mesenchyme formation or alter endocardial proliferation after 24 h of culture. Similar results were obtained with TGFbeta2 protein, either alone or in combination with TGFbeta3. The results of this study indicate that: (i) atrioventricular canal endocardium expresses TGFbeta3 in response to a myocardially derived signal other than TGFbeta3, (ii) atrioventricular canal endocardial TGFbeta3 functions in an autocrine fashion to elicit selected characteristics necessary for cushion tissue formation, and (iii) TGFbeta3 alone or in combination with TGFbeta2 is insufficient to transform atrioventricular canal endocardium into invasive mesenchyme in culture.

Contribution of the primitive epicardium to the subepicardial mesenchyme in hamster and chick embryos

A study about the hypothetical contribution of the epicardial cells to the subepicardial mesenchyme was carried out in Syrian hamster embryos of 9-12 days post coitum (dpc) and chick embryos of 3-5 days of incubation. In the epicardium and subepicardium of these embryos we have immunolocated the proteins cytokeratin (CK), vimentin (VIM), fibronectin (FN), and two antigens related to the transformation of endocardial cells into valvuloseptal mesenchyme, ES/130 and JB3. In the hamster embryos, CK+ subepicardial mesenchymal cells (SEMC) were apparently migrating from the primitive epicardium from 9.5 dpc at the atrioventricular (AV) groove and proximal outflow tract (OFT). The morphological signs of delamination extended by 11 dpc to the epicardium of the interventricular groove and the dorsal part of the ventricle. The relative abundance of the CK+ SEMC decreased in embryos of 12 dpc. VIM colocalized with CK in most SEMC, and in some epicardial mesothelial cells, mainly at the areas of delamination. CK immunoreactivity was also found in some early subepicardial capillaries. Similar observations were made in the chick embryos studied. The immunoreactive patterns obtained at the subepicardium with anti-FN, ES/130, and JB3 antibodies were similar to those reported in the areas of endothelial transformation of the endocardial cushions. We suggest that these observations are compatible with an epithelial-mesenchymal transformation involving the epicardial mesothelium and originating at least a part of the SEMC.

Expression of smooth muscle alpha-actin in mesenchymal cells during formation of avian endocardial cushion tissue: a role for transforming growth factor beta3

During early cardiac morphogenesis, outflow tract (OT) and atrio-ventricular (AV) endothelial cells differentiate into mesenchymal cells, which have characteristics of smooth muscle-like myofibroblasts, and which form endocardial cushion tissue, the primordia of valves, and septa in the adult heart. During this embryonic event, transforming growth factor beta3 (TGF beta3) is an essential element in the progression of endothelial-transformation into mesenchyme. TGF beta(s) are known to be a potent inducer for mesodermal differentiation and a promoter for differentiation of endothelial cells into smooth muscle-like cells. Using a monoclonal antibody against smooth muscle-specific alpha-actin (SMA), we examined the immunohistochemical staining of this form of actin in avian endocardial cushion tissue formation. To determine whether TGF beta3 initiates the expression of SMA, the pre-migratory AV endothelial monolayer was cultured with or without chicken recombinant TGF beta3 and the expression of SMA was examined immunochemically. Migrating mesenchymal cells expressed SMA beneath the cell surface membrane. These cells showed a reduction of endothelial specific marker antigen, QH1. Stationary endothelial cells did not express SMA. The deposition of SMA in the mesenchymal tissue persisted until the end of the fetal period. Pre-migratory endothelial cells cultured in complete medium (CM199) that contained TGF beta3 expressed SMA, whereas cells cultured in CM199 alone did not. At the onset of the endothelial-mesenchymal transformation, migrating mesenchymal cells express SMA and the expression of this form of actin is upregulated by TGF beta3. The induction of the expression of SMA by TGF beta3 is one of the initial events in the cytoskeletal reorganization in endothelial cells which separate from one another during the initial phenotypic change associated with the endothelial-mesenchymal transformation.

Extracellular fibrillar structure of latent TGF beta binding protein-1: role in TGF beta-dependent endothelial-mesenchymal transformation during endocardial cushion tissue formation in mouse embryonic heart

Transforming growth factor-beta (TGF beta) is a dimeric peptide growth factor which regulates cellular differentiation and proliferation during development. Most cells secrete TGF beta as a large latent TGF beta complex containing mature TGF beta, latency associated peptide, and latent TGF beta-binding protein (LTBP)-1. The biological role of LTBP-1 in development remains unclear. Using a polyclonal antiserum specific for LTBP-1 (Ab39) and three-dimensional collagen gel culture assay of embryonic heart, we examined the tissue distribution of LTBP-1 and its functional role during the formation of endocardial cushion tissue in the mouse embryonic heart. Mature TGF beta protein was required at the onset of the endothelial-mesenchymal transformation to initiate endocardial cushion tissue formation. Double antibody staining showed that LTBP-1 colocalized with TGF beta 1 as an extracellular fibrillar structure surrounding the endocardial cushion mesenchymal cells. Immunogold electronmicroscopy showed that LTBP-1 localized to 40-100 nm extracellular fibrillar structure and 5-10-nm microfibrils. The anti-LTBP-1 antiserum (Ab39) inhibited the endothelial-mesenchymal transformation in atrio-ventricular endocardial cells cocultured with associated myocardium on a three-dimensional collagen gel lattice. This inhibitory effect was reversed by administration of mature TGF beta proteins in culture. These results suggest that LTBP-1 exists as an extracellular fibrillar structure and plays a role in the storage of TGF beta as a large latent TGF beta complex.

Roles for growth and differentiation factors in avian embryonic development

We review the evidence for a role for transforming growth factor-beta (TGF-beta) and for tumor necrosis factor-alpha (TNF-alpha) in the development of the avian embryo. Transforming growth factor-beta is expressed in a number of locations in the early embryo with a distribution consistent with a function in epithelial-mesenchymal transformation and modulation of the composition of the extracellular matrix. During gastrulation, this factor is found in the mesoderm cell layer as well as in the endoderm underlying the primitive streak. In vivo and in vitro investigations suggest that TGF-beta may be involved in the regulation of phenotypic transformation, matrix deposition, and cell proliferation. Tumor necrosis factor-alpha and its two receptors are also located with distributions that suggest important involvement for this pleiotropic factor in early morphogenetic processes. Tumor necrosis factor-alpha is found in several cell populations from the time of gastrulation onwards, including the lens. In vitro investigations, using tissue from the gastrulating embryo as well as from the lens, suggest that this factor may be associated with the extensive cell death that occurs throughout the first 6 d of development, and with nuclear degeneration in the lens. We hypothesize that TNF-alpha, acting in a paracrine or autocrine fashion, may be involved in the signalling pathways that effect the regulation of cell death in development.

Latent transforming growth factor-beta is present in the extracellular matrix of embryonic hearts in situ

Transforming growth factor-beta (TGF-beta) is an important regulator of development. In vitro, TGF-beta is secreted in a latent, inactive form and can be activated by pH extremes, chaotropic agents, or cell-surface proteases. However, there is little evidence for the existence of latent TGF-beta in vivo. In this study, we determined whether (1) cultured embryonic cardiac segments secrete latent or active TGF-beta, (2) binding of TGF-beta antibody to TGF-beta was conformation-dependent (i.e., active vs. latent), and (3) immunostaining of embryonic hearts changed after exposure to activating conditions. Only latent TGF-beta 3 (acid activatable) was detected in conditioned medium of stage 14-16 chick cardiac segments as measured by a growth inhibition bioassay. No growth-inhibitory activity was present in nonacidified control medium. When blotted onto a membrane, only transiently acidified conditioned medium bound TGF-beta antibody. These data showed that cardiac segments secrete latent TGF-beta which binds with antibody if activated. To determine if antibody binding to tissue sections required exposure to TGF-beta-activating conditions, stage 14-16 embryos were fixed and sectioned under conditions that maximally retained extracellular matrix (ECM). Under these conditions, immunostaining was found in the myocardium but not in the endocardium or cardiac ECM. Limited immunostaining was found in other areas of the embryo and was always cell-associated. In addition to the above staining, when tissue sections were exposed to TGF-beta activating conditions, immunopositive staining was present within most of the embryonic ECM including the cardiac ECM. All immunostaining was blocked by preabsorption with TGF-beta 3 protein. These data suggest that active TGF-beta has a very limited distribution while latent TGF-beta is more abundant in embryonic ECM. Therefore, in vivo activation of TGF-beta may play an important role in mediating the expression of TGF-beta function during development.

Expression of TGF beta 1/beta 3 during early chick embryo development

We have used an antibody against a TGF beta peptide fragment to localize this growth factor in the early chick embryo from laying to the ten-somite stage of development. Western blotting showed that the antibody reacted with both mammalian TGF beta 1 and chicken TGF beta 3. By immunocytochemistry we find that at the earliest developmental stage (stage X of Eyal-Giladi and Kochav) immunoreactivity to this antibody is primarily located in the cells of the area opaca and marginal zone, as well as in the most peripheral edge cells of the blastoderm. The yolk is non-reactive, except in a highly localized region subjacent to the edge cells. This pattern persists at stage XII, and at both stages individual isolated cells in the epiblast and hypoblast are also reactive. By the time of gastrulation, reactivity in the epiblast is polarized to the ventral extremity of the cells, and again some isolated cells in this layer are intensely immunoreactive. At this stage also, the endoderm cells, particularly those underlying the primitive streak, are positive, as are the mesoderm cells lateral to the streak. At somite stages, the neuroepithelium is not reactive but the ectoderm lateral to it is strongly positive. At the caudal primitive streak levels of early somite embryos, the ectoderm and endoderm are immunoreactive while the mesoderm loses the reactivity it showed at the early gastrulation stages. The neuroepithelial cells later show reactivity at their apical poles, and, as at the earlier stages, individual cells show intense labelling. These results indicate that TGF beta 1 and/or TGF beta 3 immunoreactivity is developmentally regulated from very early stages of morphogenesis in the chick, and together with data from earlier functional studies, suggest that this factor has roles in embryonic axis formation and in blastoderm expansion.

Fibroblast behavior in the embryonic chick heart

Intracardiac fibroblasts (mesenchymal cells) of Hamburger and Hamilton stage 36 chick heart reside in the epicardium and atrioventricular valves. The characteristics of the epicardial fibroblasts include segregation from the myocytes of the heart wall myocardium, voluminous extracellular matrix production, and some cell proliferation activity. The atrioventricular fibroblasts intermingle with myocytes at the mutual border between these tissues, produce smaller amounts of extracellular matrix, and show very active cell proliferation. Is the behavior of each population of fibroblasts predetermined or is each responding in a reversible fashion to local environment? A cell aggregate culture system, which permits 3-dimensional cell-cell and cell-matrix interactions, is used to study the behavior of each isolated population of fibroblasts in vitro. In the presence of serum-free medium, each population produces very little extracellular matrix, has relatively low mitotic activity, and does not segregate from myocytes when the aggregate is composed of randomly intermixed myocytes and fibroblasts. In the presence of chicken serum, each population increases matrix production, increases cell proliferation, and sorts from myocytes. Thus, we suggest that the two populations of fibroblasts in the developing heart are responding to local environments and the differences observed in vivo are not the consequence of irreversible states of cellular differentiation.

Transforming growth factor-beta 1 in heart development

Defined biochemical stimuli regulating neonatal ventricular myocyte (cardiomyocyte) development have not been established. Since cardiomyocytes stop proliferating during the first 3-5 days of age in the rodent, locally generated 'anti-proliferative' and/or differentiation signals can be hypothesized. The transforming growth factor-beta (TGF-beta) family of peptides are multifunctional regulators of proliferation and differentiation of many different cell types. We have determined in neonatal and maturing rat hearts that TGF-beta 1 gene expression occurs in pups of both normotensive (Wistar Kyoto, WKY) and hypertrophy-prone rats (spontaneously hypertensive, SHR). TGF-beta 1 transcript levels were readily apparent in total ventricular RNA from SHR pups within 1 day of age and elevated in 3-7 day old WKY and SHR hearts when cardiomyocyte proliferation indices are diminished. TGF-beta 1 transcript levels remain at a 'relatively' high level throughout maturation and into adulthood in both strains. Further, TGF-beta 1 transcripts were localized to cardiomyocytes of neonatal rat ventricular tissue sections by in situ hybridization. Immunoreactive TGF-beta was co-localized to the intracellular compartment of neonatal cardiomyocytes at the light and electron microscopic level. In vitro analysis using primary cultures of fetal and neonatal cardiomyocytes indicated that TGF-beta s inhibit mitogen stimulated DNA synthesis and thymidine incorporation. From these data, we propose that locally generated TGF-beta s may act as autocrine and/or paracrine regulators of cardiomyocyte proliferation and differentiation as intrinsic components of a multifaceted biochemical regulatory process governing heart development.