Use of 6-diazo-5-oxo-L-norleucine to study interaction between myocardial glycoconjugate secretion and endothelial activation in the early embryonic chick heart

Atrioventricular valve development: new perspectives on an old theme

Atrioventricular valve development commences with an EMT event whereby endocardial cells transform into mesenchyme. The molecular events that induce this phenotypic change are well understood and include many growth factors, signaling components, and transcription factors. Besides their clear importance in valve development, the role of these transformed mesenchyme and the function they serve in the developing prevalve leaflets is less understood. Indeed, we know that these cells migrate, but how and why do they migrate? We also know that they undergo a transition to a mature, committed cell, largely defined as an interstitial fibroblast due to their ability to secrete various matrix components including collagen type I. However, we have yet to uncover mechanisms by which the matrix is synthesized, how it is secreted, and how it is organized. As valve disease is largely characterized by altered cell number, cell activation, and matrix disorganization, answering questions of how the valves are built will likely provide us with information of real clinical relevance. Although expression profiling and descriptive or correlative analyses are insightful, to advance the field, we must now move past the simplicity of these assays and ask fundamental, mechanistic based questions aimed at understanding how valves are "built". Herein we review current understandings of atrioventricular valve development and present what is known and what isn't known. In most cases, basic, biological questions and hypotheses that were presented decades ago on valve development still are yet to be answered but likely hold keys to uncovering new discoveries with relevance to both embryonic development and the developmental basis of adult heart valve diseases. Thus, the goal of this review is to remind us of these questions and provide new perspectives on an old theme of valve development.

Early developmental expression pattern of retinoblastoma tumor suppressor mRNA indicates a role in the epithelial-to-mesenchyme transformation of endocardial cushion cells

The earliest stages of embryonic development are characterized by the generation of precursor cell populations that differentiate and coalesce into tissue and organ primordia. To provide sufficient numbers of differentiated cells for tissue and organ formation, the differentiative as well as the proliferative processes of cells must be controlled and coordinated. Potential regulators of the proliferative process include molecules that control the cell cycle, in particular, the tumor suppressor proteins. To begin to understand the role such molecules can play in development, we have studied the expression of the retinoblastoma tumor suppressor (Rb) gene in early chicken development. Our studies in early chicken embryos show that Rb is encoded by a single gene that gives rise to several Rb mRNA isoforms through alternative splicing of a primary transcript. These mRNA isoforms potentially encode Rb proteins that differ with respect to the number of sequence motifs known to target cyclin-dependent kinases to Rb, suggesting dynamic control of Rb phosphorylation and function during development. This complex expression pattern of Rb mRNA begins as early as the blastoderm stage of chicken development (stage 3) and continues through stage 18, the latest stage examined. Despite this early embryonic expression of Rb mRNA as detected by reverse transcription polymerase chain reaction, Rb mRNA levels sufficient to be detected by in situ hybridization were not expressed until after stage 14 of development. Rb mRNA was found to be localized to cells of the endocardial cushions of the early heart tube, cells of the epicardium, and myogenic cells of the somitic myotome. Interestingly, each of these cell types undergoes an epithelial-to-mesenchyme transformation to form a migratory and/or invasive population of mesenchymal cells. We have focused our studies on the expression of Rb mRNA in endocardial cells of the early heart tube, because the transition of these cells to mesenchyme initiates the important process of septation, an early step in the formation of heart valves.

Myocardialization of the cardiac outflow tract

During development, the single-circuited cardiac tube transforms into a double-circuited four-chambered heart by a complex process of remodeling, differential growth, and septation. In this process the endocardial cushion tissues of the atrioventricular junction and outflow tract (OFT) play a crucial role as they contribute to the mesenchymal components of the developing septa and valves in the developing heart. After fusion, the endocardial ridges in the proximal portion of the OFT initially form a mesenchymal outlet septum. In the adult heart, however, this outlet septum is basically a muscular structure. Hence, the mesenchyme of the proximal outlet septum has to be replaced by cardiomyocytes. We have dubbed this process "myocardialization." Our immunohistochemical analysis of staged chicken hearts demonstrates that myocardialization takes place by ingrowth of existing myocardium into the mesenchymal outlet septum. Compared to other events in cardiac septation, it is a relatively late process, being initialized around stage H/H28 and being basically completed around stage H/H38. To unravel the molecular mechanisms that are responsible for the induction and regulation of myocardialization, an in vitro culture system in which myocardialization could be mimicked and manipulated was developed. Using this in vitro myocardialization assay it was observed that under the standard culture conditions (i) whole OFT explants from stage H/H20 and younger did not spontaneously myocardialize the collagen matrix, (ii) explants from stage H/H21 and older spontaneously formed extensive myocardial networks, (iii) the myocardium of the OFT could be induced to myocardialize and was therefore "myocardialization-competent" at all stages tested (H/H16-30), (iv) myocardialization was induced by factors produced by, most likely, the nonmyocardial component of the outflow tract, (v) at none of the embryonic stages analyzed was ventricular myocardium myocardialization-competent, and finally, (vi) ventricular myocardium did not produce factors capable of supporting myocardialization.

Induction of endocardial cushion tissue in the avian heart is regulated, in part, by TGFbeta-3-mediated autocrine signaling

Valvuloseptal morphogenesis of the primitive heart tube into a four-chambered organ requires the formation of endocardial cushion tissue. The latter is the outcome of an inductive interaction in which endocardial (endothelial) cells are induced to transform into mesenchyme by paracrine signals secreted by the adjacent myocardium. In this study, we propose that transforming endothelial/mesenchymal cells themselves secrete a factor-TGFbeta-3-that functions in an autocrine mode to promote/sustain mesenchyme formation and possibly in a paracrine manner to amplify the original (myocardial) inductive event. Cushion mesenchyme-conditioned medium, previously demonstrated to be an endogenous source of autocrine, migration-promoting factors, was found in the present study to contain TGFbeta-3, as detected by immunoblot analysis. Immunoneutralization of TGFbeta-3 in preparations of cushion mesenchyme-conditioned medium resulted in a failure of treated target endocardial cells to migrate as mesenchyme, whereas inclusion of a control antibody did not inhibit the migration-promoting activity of the conditioned medium. Similar to treatment with the conditioned medium, direct addition of TGFbeta-3 to target endocardial cells also elicited invasive migration but only in cultures which had been activated in vivo by inductive interaction with the myocardium prior to treatment. Selective inhibition of TGFbeta-3-mediated autocrine signaling in continuous cocultures of endocardium plus myocardium resulted in endocardial cells which did not migrate, even though they had expressed early markers associated with endocardial cell activation (e.g., alpha-smooth muscle actin, ES/130, and TGFbeta-3). Collectively, these results suggest that (i) two signaling pathways, myocardial and endocardial, are required to start and complete epithelial-mesenchymal transformation in cushion-forming regions of the heart and (ii) the endocardial pathway signals through iteration of TGFbeta-3 and is not functionally redundant to the myocardial pathway.

Early development of quail heart epicardium and associated vascular and glandular structures

As in the other vertebrates the epicardium of the quail embryo develops from proepicardial tissue located between the sinus horns and the liver primordium. The cuboidal cells of the coelomic lining above the proepicardium are transformed into mesothelial cells which in cooperation with the underlying mesenchymal cells elaborate a large quantity of extracellular matrix, so producing the villous outgrowths of the proepicardium. The mesenchymal cells of this area are attached to each other with typical desmosomes and have anti-alpha cytokeratin-stained tonofilament bundles. These cells resemble keratinocytes and are designated as proepicardial matrix keratinocytes. The proepicardium proliferates first in the sulci of the U-shaped tubular heart, and within 2 days (between stages 15-25) establishes the visceral layer of the epicardium. The proliferating proepicardium consists of gland-like tubular strands, formed by the invaginations of the surface mesothelial cells, mesenchymal cells, fibroblasts, angioblasts, blood cells and capillaries. Because of its heterogeneous structure and multiple functions, the proepicardium is considered a transitory organ of the developing heart. In the quail embryo the forerunners of the coronary vessels grow from the perihepatic area into the proepicardial organ, and when the epicardial covering is completed, but before the coronary artery orifices open, these primordial vessels form a subepicardial and intramural vascular network in the ventricular myocardium. After the completion of the epicardial covering the proepicardium involutes and is not seem from stage 26 onward.

Multiple glycoproteins localize to a particulate form of extracellular matrix in regions of the embryonic heart where endothelial cells transform into mesenchyme

Cells derived from an epithelial-mesenchymal transformation within the atrioventricular canal and outflow tract are involved in the partitioning of the early embryonic heart into a four-chambered organ. This transformation process has been shown to proceed from an inductive interaction between the myocardium and competent, target endothelial cells within these regions of the heart. Interestingly, immunohistochemistry revealed the presence of fibronectin-positive particulates within the matrix of mesenchyme-forming regions (Mjaatvedt et al., 1987). This particulate matrix is extractable by EDTA and can elicit the epithelial-mesenchymal transformation in culture (Mjaatvedt and Markwald, 1989). Analysis of EDTA extracts of embryonic heart tissue revealed the presence of fibronectin and about 40 unidentified proteins, 6 of which appeared to be enriched in the biologically active 100,000g pellet fraction (Mjaatvedt and Markwald, 1989). Based on these and other data we have proposed that the particulate matrix is composed of a multicomponent complex of fibronectin and one or more of the low-molecular-weight proteins in this pellet. The purpose of the present study was to begin a biochemical characterization of the nonfibronectin proteins thought to be present in the matrix particulates. Given that many matrix constituents are glycoproteins, lectins were used to initially characterize the particulate constituents. Of the lectins tested, soybean agglutinin (SBA) was found to be specific only for matrix particulates. Histochemical analyses showed that SBA and antibodies against fibronectin colocalized regionally and temporally to the same matrix particulates in embryonic heart tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Epithelial-mesenchymal transformation of embryonic cardiac endothelial cells is inhibited by a modified antisense oligodeoxynucleotide to transforming growth factor beta 3

During early cardiac development, the progenitor cells of the heart valves and membranous septa undergo an epithelial-mesenchymal transformation. Previous studies have shown that this transformation depends on the activity of a transforming growth factor beta (TGF beta) molecule produced by the heart. In the present study, we have used modified antisense oligodeoxynucleotides generated to nonconserved regions of TGF beta 1, -2, -3, and -4 to examine the possible roles of these members in this transformation. A phosphoramidate-modified oligonucleotide complementary to TGF beta 3 mRNA was capable of inhibiting normal epithelial-mesenchymal transformation by 80%. Unmodified oligonucleotides to TGF beta 3, modified oligonucleotides to TGF beta 1, -2, and -4, and two modified control oligonucleotides were unable to inhibit the transformation. These data demonstrate that a specific member of the TGF beta family, TGF beta 3, is essential for the epithelial-mesenchymal cell transformation.

A comparison of fibronectin, laminin, and galactosyltransferase adhesion mechanisms during embryonic cardiac mesenchymal cell migration in vitro

Embryonic hearts contain a homogeneous population of mesenchymal cells which migrate through an extensive extracellular matrix (ECM) to become the earliest progenitors of the cardiac valves. Since these cells normally migrate through an ECM containing several adhesion substrates, this study was undertaken to examine and compare three ECM binding mechanisms for mesenchymal cell migration in an in vitro model. Receptor mechanisms for the ECM glycoproteins fibronectin (FN) and laminin (LM) and the cell surface receptor galactosyltransferase (GalTase), which binds an uncharacterized ECM substrate, were compared. Primary cardiac explants from stage 17 chick embryos were cultured on three-dimensional collagen gels. Mesenchymal cell outgrowth was recorded every 24 hr and is reported as a percentage of control. Migration was perturbed using specific inhibitors for each of the three receptor mechanisms. These included the hexapeptide GRGDSP (300-1000 micrograms/ml), which mimics a cell binding domain of FN, the pentapeptide YIGSR (300-1000 micrograms/ml), which mimics a binding domain of LM, and alpha-lactalbumin (1-10 mg/ml), a protein modifier of GalTase activity. The functional role of these adhesion mechanisms was further tested using antibodies to avian integrin (JG22) and avian GalTase. While the FN-related peptide had no significant effect on cell migration it did produce a rounded cellular morphology. The LN-related peptide inhibited mesenchymal migration 70% and alpha-lactalbumin inhibited cell migration 50%. Antibodies against integrin and GalTase inhibited mesenchymal cell migration by 80 and 50%, respectively. The substrate for GalTase was demonstrated to be a single high molecular weight substrate which was not LM or FN. Control peptides, proteins and antibodies demonstrated the specificity of these effects. These data demonstrate that multiple adhesion mechanisms, including cell surface GalTase, are potentially functional during cardiac mesenchymal cell migration. The sensitivity of cell migration to the various inhibitors suggests that occupancy of specific ECM receptors can modulate the activity of other, unrelated, ECM adhesion mechanisms utilized by these cells.

Spatial differences of endogenous lectin expression within the cellular organization of the human heart: a glycohistochemical, immunohistochemical, and glycobiochemical study

Protein-carbohydrate recognition may be involved in an array of molecular interactions on the cellular and subcellular levels. To gain insight into the role of proteins in this type of interaction, surgically removed specimens of human endomyocardial tissue were processed for histochemical and biochemical analysis. The inherent capacity of these sections to bind individual sugar moieties, which are constituents of the carbohydrate part of cellular glycoconjugates, was assessed using a panel of biotinylated neoglycoproteins according to a standardized procedure. Together with appropriate controls, it primarily allowed localization of endogenous lectins. Differences in lectin expression were observed between layers of endocardial tissue, myocardial cell constituents, connective-tissue elements, and vascular structures. The endocardium proved to be positive with beta-galactoside-bearing probes; with neoglycoproteins carrying beta-xylosides, alpha-fucosides, and galactose-6-phosphate moieties; and with probes containing a carboxyl group within the carbohydrate structure, namely sialic acid and glucuronic acid. In contrast, only fucose-and maltose-specific receptors were apparent in the elastic layers of the endocardium. Aside from ascertaining the specificity of the protein-carbohydrate interaction by controls, i.e., lack of binding of the probe in the presence of the unlabelled neoglycoprotein and lack of binding of the labelled sugar-free carrier protein, respective sugar receptors were isolated from heart extracts by using histochemically effective carbohydrates as immobilized affinity ligand. Moreover, affinity chromatography using immobilized lactose as affinity ligand as well as the use of polyclonal antibodies against the predominant beta-galactoside-specific lectin of heart demonstrated that the lactose-specific neoglycoprotein binding was due to this lectin. Remarkably, the labelled endogenous lectin, preferred to plant lectins for detecting ligands of the endogenous lectin, localized ligands in tissue parts where the lectin itself was detected glycohistochemically as well as immunohistologically. This demonstration of receptor-ligand presence in the same system is a further step toward functional assignment of the recorded protein-carbohydrate interaction. Overall, the observed patterns of lectin expression may serve as a guideline to elucidate the precise physiological relevance of lectins and to analyze pathological conditions comparatively.

Lectin histochemistry of the embryonic heart: expression of terminal and penultimate galactose residues in developing rats and chicks

Rat embryos at days 10-18 of gestation and chicken embryos at days 3-6 of incubation were fixed and processed for lectin histochemistry. The distribution of binding sites for a lectin from the peanut Arachis hypogaea (PNA) conjugated to horseradish peroxidase (HRP) was determined on tissue sections both before and after enzymatic cleavage of sialic acid with neuraminidase (sialidase). Endocardial cushion tissue in the rat, but not in the chick, reacted with PNA-HRP prior to digestion with sialidase. Endocardium of both species (12 and 13 days in rat, 5 and 6 days in chick), particularly at the level of endocardial cushions, reacted strongly with the sialidase-PNA sequence; this staining decreased markedly after day 14 of gestation in the rat. PNA binding sites capped by sialic acid were most abundant in the developing rat heart during the critical period of endocardial cushion formation and decreased as development proceeded. The marked changes in the appearance and distribution of cardiac cell and tissue glycoconjugates during cardiogenesis support the concept that rapid changes occur in the structure of complex carbohydrates during embryonic and fetal development. The findings also suggest that such glycosylation-related events may be species specific.

Initial expression of type I procollagen in chick cardiac mesenchyme is dependent upon myocardial stimulation

Formation of the atrioventricular (AV) mesenchyme is a critical step in early heart development. Endothelial cells are activated and transformed into a mesenchymal population that invades the cell-free myocardial basement membrane. This process can be duplicated in collagen gel culture, where it has been established that myocardium or its secretory products activate the endothelium. The purpose of the present study was to determine when these activated endothelial and/or mesenchymal cells start producing type I collagen in situ. These results were compared to those obtained from a culture model of mesenchyme formation. The production of type I collagen was monitored using a monoclonal antibody (M38) that recognizes the carboxy-terminal propeptide of human type I procollagen. The initial expression of the latter within activated AV endothelial and mesenchymal cells in ovo was 48 hr following activation. Prior to this time, only the myocardium was reactive with M38. AV explants of early hearts on collagen gels revealed staining of activated endothelial and mesenchymal cells with M38 after 48 hr in coculture with myocardial tissue. Explants that were prevented from activating (myocardium removed) never expressed the M38 antigen. Similarly, AV endothelial monolayers grown in the presence of myocardial conditioned medium activated and expressed type I collagen after 48 hr in culture, whereas those grown in standard medium did not. These results establish the initial expression of type I collagen within activated AV endothelium and mesenchyme. In addition, the data suggest that the expression of type I collagen within the AV mesenchyme may be dependent on extrinsic influences that induce the AV endothelium to transform into mesenchyme.

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

A critical step in early cardiac morphogenesis can be faithfully duplicated in culture using a hydrated collagen substratum, and thereby serves as a useful model system for studying the molecular mechanisms of cell differentiation. Results from previous work suggested that the myocardium in the atrioventricular canal (AV) region of the developing chick heart secretes extracellular proteins into its associated basement membrane, which may function to promote an epithelial-mesenchymal transition of endothelium to form prevalvular fibroblasts (E. L. Krug, R. B. Runyan, and R. R. Markwald, 1985, Dev. Biol. 112, 414-426; C. H. Mjaatvedt, R. C. Lepera, and R. R. Markwald, 1987, Dev. Biol., in press). In the present study we show that an EDTA-soluble extract of embryonic chick hearts can substitute for the presence of myocardium, the presumptive stimulator tissue, in initiating mesenchyme formation from AV endothelium in culture. Ventricular endothelium was unresponsive to this material in keeping with observed in situ behavior. AV endothelial cells did not survive beyond 4-5 days when cultured in the absence of either the EDTA-soluble heart extract, myocardial conditioned medium, or the myocardium itself. Antibody prepared against a particulate fraction of the EDTA-solubilized heart extract immunohistochemically localized this material to the myocardial basement membrane. In addition, conditioned medium from embryonic myocardial cultures effectively induced mesenchyme formation. Neither a variety of growth factors nor a sarcoma basement membrane preparation were effective in promoting mesenchyme formation indicating a selectivity of the responding embryonic AV endothelial cells to myocardial basement membrane. These observations reflect a truly inductive phenomenon as there was an absolute dependence on the presence of the stimulating substance/tissue and retention, in culture, of both the temporal and regional characteristics observed in situ. This is in contrast to the results of others investigating the cytodifferentiation of committed cells whose phenotypic expression can be either accelerated or diminished but not obligatorily regulated by a specific agent, thus making the interpretation of data difficult, if not irrelevant, to the study of differentiation. The results of this study provide direct experimental support for the hypothesis that extracellular matrix can indeed serve as a direct stimulator or "secondary inducer" of cytodifferentiation.

Conditioning of native substrates by chondroitin sulfate proteoglycans during cardiac mesenchymal cell migration

It is generally proposed that embryonic mesenchymal cells use sulfated macromolecules during in situ migration. Attempts to resolve the molecular mechanisms for this hypothesis using planar substrates have been met with limited success. In the present study, we provide evidence that the functional significance of certain sulfated macromolecules during mesenchyme migration required the presence of the endogenous migratory template; i.e., native collagen fibrils. Using three-dimensional collagen gel lattices and whole embryo culture procedures to produce metabolically labeled sulfated macromolecules in embryonic chick cardiac tissue, we show that these molecules were primarily proteoglycan (PG) in nature and that their distribution was class specific; i.e., heparan sulfate PG, the minor labeled component (15%), remained pericellular while chondroitin sulfate (CS) PG, the predominately labeled PG (85%), was associated with collagen fibrils as "trails" of 50-60-nm particles when viewed by scanning electron microscopy. Progressive "conditioning" of collagen with CS-PG inhibited the capacity of the template to support subsequent cell migration. Lastly, metabolically labeled, PG-derived CS chains were compared with respect to degree of sulfation in either the C-6 or C-4 position by chromatographic separation of chondroitinase AC digestion products. Results from temporal and regional comparisons of in situ-labeled PGs indicated a positive correlation between the presence of mesenchyme and an enrichment of disaccharide-4S relative to that from regions lacking mesenchyme (i.e., principally myocardial tissue). The suggestion of a mesenchyme-specific CS-PG was substantiated by similarly examining the PGs synthesized solely by cardiac mesenchymal cells migrating within hydrated collagen lattice in culture. These data were incorporated into a model of "substratum conditioning" which provides a molecular mechanism by which secretion of mesenchyme-specific CS-PGs not only provides for directed and sustained cell movement, but ultimately inhibits migration of the cell population as a whole.

Protein extracts from early embryonic hearts initiate cardiac endothelial cytodifferentiation

Prior to the formation of multiple chambers, the embryonic heart consists of two epithelial tubes, one within the other. As development proceeds, portions of the inner epithelium, i.e., the endothelium, undergo a morphological transformation into a migrating mesenchymal cell population. Our results show that this transformation is affected by proteins secreted by the outer epithelium, i.e., the myocardium, into the extracellular matrix between these two tissues. This conclusion is based on tissue autoradiographic studies of whole embryo cultures with 3H-amino acids. Continuous labeling conditions generated an apparent gradient of proteins extending away from the myocardium and contacting the endothelium just prior to the formation of mesenchyme, i.e., activation of the transformation sequence. Pulse/chase studies confirmed this directional movement of matrix protein. By performing sequential extractions of preactivation staged embryonic hearts with EDTA and testicular hyaluronidase followed by ammonium sulfate precipitation we obtained an enriched preparation of cardiac extracellular matrix. This fraction was capable of eliciting several of the events characteristic of endothelial activation in vitro. These events included: (i) cell-cell separation, (ii) lateral cell mobility, and (iii) hypertrophy and polarization of intracellular PAS staining (Golgi apparati). The biological activity of the extract was sensitive to heat denaturation: a homogenate of the remaining extracted tissue would not substitute for the matrix extract. Morphologically the extracted hearts appeared intact, however, the extracellular matrix space was significantly diminished. No more than 6% of the total lactic dehydrogenase activity, a cytosolic enzyme, was found in the extract. Preliminary electrophoretic characterization of the extract (metabolically labeled with 14C-amino acids) indicated that it may contain as many as 35 proteins or subunits. The relationship of ECM to endothelial differentiation in cardiac morphogenesis is discussed as a model for other developmental systems.