Polarized release of enveloped viruses in the embryonic chick heart: demonstration of epithelial polarity in the presumptive myocardium

The epicardium and epicardially derived cells (EPDCs) as cardiac stem cells

After its initial formation the epicardium forms the outermost cell layer of the heart. As a result of an epithelial-to-mesenchymal transformation (EMT) individual cells delaminate from this primitive epicardial epithelium and migrate into the subepicardial space (Pérez-Pomares et al., Dev Dyn 1997; 210:96-105; Histochem J 1998a;30:627-634). Several studies have demonstrated that these epicardially derived cells (EPDCs) subsequently invade myocardial and valvuloseptal tissues (Mikawa and Fischman, Proc Natl Acad Sci USA 1992;89:9504-9508; Mikawa and Gourdie, Dev Biol 1996;174:221-232; Dettman et al., Dev Biol 1998;193:169-181; Gittenberger de Groot et al., Circ Res 1998;82:1043-1052; Manner, Anat Rec 1999;255:212-226; Pérez-Pomares et al., Dev. Biol. 2002b;247:307-326). A subset of EPDCs continue to differentiate in a variety of different cell types (including coronary endothelium, coronary smooth muscle cells (CoSMCs), interstitial fibroblasts, and atrioventricular cushion mesenchymal cells), whereas other EPDCs remain in a more or less undifferentiated state. Based on its specific characteristics, we consider the EPDC as the ultimate 'cardiac stem cell'. In this review we briefly summarize what is known about events that relate to EPDC development and differentiation while at the same time identifying some of the directions where EPDC-related research might lead us in the near future.

Coronary Vessel Development

Development of the coronary vascular system is an interesting model in developmental biology with major implications for the clinical setting. Although coronary vessel development is a form of vasculogenesis followed by angiogenesis, this system uses several unique developmental processes not observed in the formation of other blood vessels. This review summarizes the literature that describes the development of the coronary system, highlighting the unique aspects of coronary vessel development. It should be noted that many of the basic mechanisms that govern vasculogenesis in other systems have not been analyzed in coronary vessel development. In addition, we present recent advances in the field that uncover the basic mechanisms regulating the generation of these blood vessels and identify areas in need of additional studies.

Epithelial-mesenchymal transitions: a mesodermal cell strategy for evolutive innovation in Metazoans

Epithelial-mesenchymal transitions (EMTs) are well known processes in which new mesenchyme is locally generated from epithelia. During the development of the vertebrate embryo, EMTs are a source of mesenchyme in diverse places and stages through embryonic morphogenesis, especially in mesodermal domains. In the present work we consider the embryo as a two-state system in which epithelium and mesenchyme represent the stable and unstable states, respectively. We think that a pattern of recurrent oscillations between the plasticity and exploratory behaviour of the mesenchyme and the stability of the epithelia can be recognized in the embryogenesis of vertebrates and, probably, in most tripoblastic Metazoans. Mesoderm, in particular, might be regarded as a cell layer able to oscillate between epithelial and mesenchymal states. The cellular and molecular mechanisms that enable these recurrent oscillations between stable (epithelial) and unstable (mesenchymal) states during embryogenesis provide the mesoderm with a large plasticity, an extended potential for innovation, and a better control of the three-dimensional (3D) body organization. In this scenario, it is conceivable that the origin of the mesoderm itself might be related to ancestral mechanisms regulating cell adhesion and detachment. We conclude that EMTs played a key role in the evolution of Metazoans, and are involved in the pathological and reparative processes of adult organisms.

Trabecular myocytes of the embryonic heart require N-cadherin for migratory unit identity

The myocardial wall of the vertebrate heart changes from a simple epithelium to a trabeculated structure during embryogenesis. This process occurs when epithelioid cardiomyocytes migrate toward the endocardium, which we show is coincident with up-regulation of the cell adhesion molecule, N-cadherin. To study the role of N-cadherin expressed at the trabeculation stage, a replication-defective retrovirus expressing a dominant negative mutant of N-cadherin (delta N-cadherin) was engineered. Control viruses were designed to express beta-galactosidase or a full-length N-cadherin. Viruses were introduced into epithelioid presumptive myocytes at the time they initiate the epithelial-mesenchymal transformation. Individual cells infected with control viruses generated daughter myocytes which migrated toward endocardium as a tight cluster, thereby generating a clone that forms a single or at most two trabeculae. In contrast, myocytes expressing delta N-cadherin were sparsely distributed within the myocardium and failed to form the ridge-shaped clone. Thus, in addition to its known roles in myocyte epithelialization and intercalated disc formation, N-cadherin appears to play a role in homotypic interactions between nonepithelial migratory myocytes during trabecular formation of the embryonic heart.

The neural cell adhesion molecule expresses a tyrosine-independent basolateral sorting signal

Transmembrane isoforms of the neural cell adhesion molecule, N-CAM (N-CAM-140 and N-CAM-180), are vectorially targeted from the trans-Golgi network to the basolateral domain upon expression in transfected Madin-Darby canine kidney cells (Powell, S. K., Cunningham, B. A., Edelman, G. M., and Rodriguez-Boulan, E. (1991) Nature 353, 76-77). To localize basolateral targeting information, mutant forms of N-CAM-140 were constructed and their surface distribution analyzed in Madin-Darby canine kidney cells. N-CAM-140 deleted of its cytoplasmic domain shows a non-polar steady state distribution, resulting from delivery from the trans-Golgi network to both the apical and basolateral surfaces. This result suggests that entrance into the basolateral pathway may occur without cytoplasmic signals, implying that apical targeting from the trans-Golgi network is not a default mechanism but, rather, requires positive sorting information. Subsequent construction and analysis of a nested set of C-terminal deletion mutants identified a region of 40 amino acids (amino acids 749-788) lacking tyrosine residues required for basolateral targeting. Addition of these 40 amino acids is sufficient to restore basolateral targeting to both the non-polar cytoplasmic deletion mutant of N-CAM as well as to the apically expressed cytoplasmic deletion mutant of the p75 low affinity neurotrophin receptor (p75(NTR)), indicating that this tyrosine-free sequence is capable of functioning independently as a basolateral sorting signal. Deletion of both cytoplasmic and transmembrane domains resulted in apical secretion of N-CAM, demonstrating that the ectodomain of this molecule carries recessive apical sorting information.

Establishment of the mesodermal cell line QCE-6. A model system for cardiac cell differentiation

The QCE-6 cell line was derived from precardiac mesoderm of the Japanese quail. As previously reported, these cells are able to differentiate into two distinct cardiac cell types with myocardial or endocardial endothelial cell properties. This present communication describes in detail the derivation of this cell line and further characterizes the nontreated and induced myocardial and endothelial phenotypes of these cells. The QCE-6 cells exhibit an epithelial morphology, as well as the pattern of protein expression, that is characteristic of precardiac mesoderm. Treatment with retinoic acid, basic fibroblast growth factor (bFGF), transforming growth factor (TGF)-beta 2, and TGF-beta 3 induces these cells to differentiate and produce mixed cultures of epithelial and mesenchymal cells. The epithelial cells express myosin, desmin, and cardiac troponin I in a punctate pattern throughout the cytoplasm. These sarcomeric proteins become organized in a premyofibrillar pattern when TGF-beta 1, platelet-derived growth factor (PDGF)-BB, and insulin-like growth factor (IGF) II are added in combination along with retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3. Also, these treatments induce Na+,K(+)-ATPase expression. When the QCE-6 cells are cultured on collagen type I, the mesenchymal cells that are promoted by retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3 will invade the gel. These mesenchymal cells are positive for QH1 and JB3, which are both markers for presumptive endocardial cells within the early cardiogenic mesoderm. The addition of both PDGF-BB and IGF II to QCE-6 cell cultures will inhibit the ability of retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3 to induce both the mesenchymal morphology and QH1 and JB3 expression. Collectively, these results suggest that the proces of cardiac cell differentiation is regulated by multiple signals and that early cardiogenic mesoderm contains a bipotential stem cell that can give rise to both the myocardial and endocardial lineages. More important, since the QCE-6 cells are representative of early cardiogenic cells, this cell line offers a unique model system to study cardiac cell differentiation.

Expression of the atrial-specific myosin heavy chain AMHC1 and the establishment of anteroposterior polarity in the developing chicken heart

A unique myosin heavy chain cDNA (AMHC1), which is expressed exclusively in the atria of the developing chicken heart, was isolated and used to study the generation of diversified cardiac myocyte cell lineages. The pattern of AMHC1 gene expression during heart formation was determined by whole-mount in situ hybridization. AMHC1 is first activated in the posterior segment of the heart when these myocytes initially differentiate (Hamburger and Hamilton stage 9+). The anterior segment of the heart at this stage does not express AMHC1 although the ventricular myosin heavy chain isoform is strongly expressed beginning at stage 8+. Throughout chicken development, AMHC1 continues to be expressed in the posterior heart tube as it develops into the diversified atria. The early activation of AMHC1 expression in the posterior cardiac myocytes suggests that the heart cells are diversified when they differentiate initially and that the anterior heart progenitors differ from the posterior heart progenitors in their myosin isoform gene expression. The expression domain of AMHC1 can be expanded anteriorly within the heart tube by treating embryos with retinoic acid as the heart primordia fuse. Embryos treated with retinoic acid prior to the initiation of fusion of the heart primordia express AMHC1 throughout the entire heart-forming region and fusion of the heart primordia is inhibited. These data indicate that retinoic acid treatment produces an expansion of the posterior (atrial) domain of the heart and suggests that diversified fates of cardiomyogenic progenitors can be altered.

Behavior of embryonic chick heart cells in culture. 2. Cellular responses to epidermal growth factor and other growth signals

Muscle cell-enriched primary cell cultures were prepared from 8-day embryonic chick heart ventricles (74% of these cells showed positive staining with anti-cardiac myosin antibody). To determine if Epidermal Growth Factor (EGF) affects cardiac muscle cells, immunostaining and autoradiography were performed to find the Muscle Cell Labeling Index (MLI). MLI represents the proportion of cardiac myosin-positive cells that specifically incorporated [3H]thymidine. The MLI for EGF-treated cells was 51%. Controls in Serum-free Nutrient Medium (SFNM) had a MLI of 34.5%. Combinations of growth signals also were tested. EGF, IGF-I (Insulin-like Growth Factor-I), or PDGF (Platelet-derived Growth Factor) alone increased [3H]thymidine incorporation in the cells. Adding IGF-I or PDGF simultaneously with EGF enhanced the response of the cells to EGF by increasing [3H]thymidine incorporation. TGF-beta (Transforming Growth Factor-beta) alone was shown to have an inhibitory effect on [3H]thymidine incorporation, and when TGF-beta was added together with EGF, it attenuated the stimulatory effect of EGF on [3H]thymidine incorporation. Phorbol 12-Myristate 13-Acetate (PMA), a tumor promoter, alone had no effect on [3H]thymidine incorporation, but its addition suppressed the stimulatory effect of EGF when they were added simultaneously in the presence of 5% FBS. Developmental response of the heart cells to growth signals also was tested. Heart cells from 18-day embryos were used to test the effect of insulin and EGF. Although both insulin and EGF increased [3H]thymidine incorporation in heart cells from 8-day embryos, different responses to insulin and EGF occurred with heart cells from 18-day embryos. Whereas the heart cells from 18-day embryos still responded to EGF by increasing [3H]thymidine incorporation, they did not show a response to insulin as measured by [3H]thymidine incorporation, suggesting that the loss of response of the heart cells to growth signals may occur at the receptor level. Further studies show that EGF, TGF-alpha, aFGF, and PDGF increased the total numbers of heart cells, and that aFGF and PDGF also increased the percentages of heart muscle cells.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus. III: Polyclonal origin of adjacent ventricular myocytes

Replication-incompetent variants of the avian spleen necrosis virus (SNV) encoding cytoplasmic or nuclear-directed beta-galactosidase (beta-gal) have been used to trace the clonal growth of myocytes during left ventricular free-wall formation. Tubular-stage hearts were infected with a mixed suspension of both retroviruses and, after hatching, the progeny of marked cells in the ventricular wall were examined by X-gal histochemistry. When a small number of virions was introduced individual blue patches contained myocytes with only one label type (cytoplasmic or nuclear). These results confirmed our previous conclusion that each cluster or patch represents a single clone (Mikawa et al., 1992, Dev. Dynamics, 193:11-23). Each of these clones formed a clone-shaped patch which often extended through the entire thickness of the ventricular myocardium, but typically each patch was heterogeneous, containing a mixture of labeled and unlabeled cells. We then asked whether the two populations of myocytes in each patch were clonally related or generated from more than one progenitor. When hearts were infected with high titer viral suspensions many patches were observed in which cytoplasmic-tagged myocytes were intermingled with nuclear-tagged myocytes. Thus, the cone-shaped myocyte patches in the ventricular wall are polyclones derived from separate progenitors in the precardiac mesoderm. This finding led us to examine the separation of clonally related ventricular myocytes in the developing hearts. Embryos were infected with retroviral suspensions at varying stages of development and the resulting colonies examined after hatching.(ABSTRACT TRUNCATED AT 250 WORDS)

N-cadherin localization in early heart development and polar expression of Na+,K(+)-ATPase, and integrin during pericardial coelom formation and epithelialization of the differentiating myocardium

N-cadherin, a Ca(2+)-dependent cell adhesion molecule, has been localized previously to the mesoderm during chick gastrulation and to adherens junctions in beating avian hearts. However, a systematic study of the dynamic nature of N-cadherin localization in the critical early stages of heart development is lacking. The presented work defines the changes in the spatial and temporal expression of N-cadherin during early stages of chick heart development, principally between Hamburger and Hamilton stages 5-8, 18-29 hr of development. During gastrulation N-cadherin appears evenly distributed in the heart forming region. As development proceeds to form the pericardial coelom (stages 6, 7, and 8, i.e., between 22 and 26 hr of development) N-cadherin localization becomes restricted to the more central areas of the mesoderm. The localization also shows a periodicity that correlates closely with the distance between foci of cavities that eventually coalesce to form the coelom. This distribution suggests that N-cadherin may have a function in the sorting out of somatic and splanchnic mesoderm cells to form the coelom. This separation of the mesoderm in the embryo for the first time physically delineates the precardiac mesoderm population. Concomitant with cell sorting during coelom formation, the precardiac cells change shape and show a distinct polarity as conveyed by (1) the apical expression of N-cadherin on precardiac cell surfaces lining the pericardial coelom, (2) the primarily lateral expression of Na+,K(+)-ATPase, and (3) an enrichment of integrin (beta 1 subunit) on basal cell surfaces. The somatic mesoderm cells apparently down-regulate N-cadherin expression. N-cadherin is also absent from the precardiac cells close to the endoderm. The latter cells eventually form the endocardium, i.e., the endothelial lining of the heart. By contrast, in the tubular, beating heart N-cadherin is found throughout the myocardium. In summary, immunolocalization patterns of N-cadherin during early cardiogenesis suggest that this cell adhesion molecule has a major role in the dynamics of pericardial coelom formation. Subsequently, its continued expression during cell differentiation of the cardiomyocyte to form the myocardium, but not endocardium, suggests N-cadherin is an essential morphoregulatory molecule in heart organogenesis.

Expression of sarcomeric myosin in the presumptive myocardium of chicken embryos occurs within six hours of myocyte commitment

The distribution of sarcomeric myosin heavy chain (MyHC) has been examined immunocytochemically in the presumptive myocardial cells of chicken embryos (stages 6-10) prior to the onset of the heart beat. Embryos were stained with monoclonal antibody MF20, a reagent which recognizes all chicken sarcomeric MyHCs (Bader et al., 1982), and then examined both in whole mount by immunofluorescence and in semithin, plastic-embedded sections following immunoperoxidase labeling. We observed that myosin could be detected as early as stage 7 (0-2 pairs of somites) in 29% of the 31 embryos examined, and by stage 8 (4 pairs of somites) more than 80% of the embryos were MF20+. Every embryo with 5 pairs of somites (stage 8+) labeled strongly with MF20. Labeling was first detected at stage 7 to 7+ as a diffuse fluorescent signal within pleomorphic cells of the splanchnic mesoderm located in two crescent-shaped regions bordering each side of the anterior intestinal portal (AIP). With progressive development, the two crescent-shaped regions merged at the apex of the AIP, and as the two heart tubes began fusion at stage 9, the MyHC+ regions extended cranially and medially. By somite stages 9-10, the myosin-positive cells completely encircled the heart tube. From stages 7 to 9 the myosin signal had no sarcomeric distribution; i.e., there were no MyHC striations nor periodic repeats evident in the presumptive myocytes until late stage 9 and stage 10. Semithin sections revealed that myosin was first distributed in apical regions of the myocytes, adjacent to the pericardial coelom. The implications of these findings for myocyte determination, differentiation and morphogenesis are discussed.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus: I. Formation of the ventricular myocardium

Cells of the precardiac mesoderm (stages 4-6) and dividing myocytes of early hearts (stages 10-15) were tagged with a replication-incompetent retrovirus (CXL) (Mikawa et al., 1991b) encoding bacterial beta-galactosidase (beta-gal). Two protocols were used to infect the cardiogenic cells. (1) Small blocks (approximately 50 micron 2) of anterolateral mesoderm were dissected from gastrula-stage embryos (stages 4-6) and incubated in liquid medium containing the retrovirus. After removal of CXL, the tissues were dispersed into single-cell suspensions and pressure injected into the precardiac areas of recipient embryos (stages 4-6). Such embryos were then incubated in vitro at 37 degrees C for 2 days (New, 1968), and those embryos with beating hearts were fixed for X-gal histochemistry and paraffin serial sectioning. (2) CXL was pressure injected in ovo (embryonic stages 4-15) into cardiogenic tissues and the eggs subsequently returned to an incubator. At selected stages of development embryos or whole hearts were fixed, stained with X-gal, and serially sectioned after paraffin embedding. The first method showed that (1) cells of the precardiac mesoderm could be infected with the retrovirus, (2) the transplanted cells would differentiate into beating myocytes, and (3) beta-gal expression was sufficiently high to be detected histochemically. With the second procedure we could show that (1) beta-gal-tagged cells formed colonies in the myocardium, (2) the labeled cells were exclusively myocytes, (3) the number of cells per colony increased with increasing age of embryonic development, (4) the size of colonies was larger in the left than the right ventricle, (5) many of the colonies were transmural, i.e., they extended from epicardial to endocardial layers of the myocardium and generally exhibited a cone or funnel-shape with the base of the cone nearest the epicardium, (6) the orientation of myocytes within each colony changed at different layers of the myocardium, and (7) the cones contained both beta-gal+ and beta-gal- myocytes. DNA labeling studies with [3H]thymidine indicated that cardiogenic cells divided every 16-18 hr during the first week of development and that the CXL-labeled cells divided indistinguishably from unlabeled myocytes. Based on these observations a model for the growth of the myocardium is presented.