Onset of expression and regional deposition of alpha-smooth and sarcomeric actin during avian heart development

Correlation between in vitro expansion-related cell stiffening and differentiation potential of human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are an attractive cell source for tissue regeneration, given their self-renewal and multilineage potential. However, they are present in only small percentages in human bone marrow, and are generally propagated in vitro prior to downstream use. Previous work has shown that hMSC propagation can lead to alterations in cell behavior and differentiation potency, yet optimization of differentiation based on starting cell elastic modulus is an area still under investigation. To further advance the knowledge in this field, hMSCs were cultured and routinely passaged on tissue-culture polystyrene to investigate the correlation between cell stiffening and differentiation potency during in vitro aging. Local cell elastic modulus was measured at every passage using atomic force microscopy indentation. At each passage, cells were induced to differentiate down myogenic and osteogenic paths. Cells induced to differentiate, as well as undifferentiated cells were assessed for gene and protein expression using quantitative polymerase chain reaction and immunofluorescent staining, respectively, for osteogenic and myogenic markers. Myogenic and osteogenic cell potential are highly reliant on the elastic modulus of the starting cell population (of undifferentiated cells), and this potential appears to peak when the innate cell elastic modulus is close to that of differentiated tissue. However, the latent expression of the same markers in undifferentiated cells also appears to undergo a correlative relationship with cell elastic modulus, indicating some endogenous effects of cell elastic modulus and gene/protein expression. Overall, this study correlates age-related changes with regards to innate cell stiffening and gene/protein expression in commercial hMSCs, providing some guidance as to maintenance and future use of hMSCs in future tissue engineering applications.

Extracellular matrix promotes highly efficient cardiac differentiation of human pluripotent stem cells: the matrix sandwich method

RATIONALE

Cardiomyocytes (CMs) differentiated from human pluripotent stem cells (PSCs) are increasingly being used for cardiovascular research, including disease modeling, and hold promise for clinical applications. Current cardiac differentiation protocols exhibit variable success across different PSC lines and are primarily based on the application of growth factors. However, extracellular matrix is also fundamentally involved in cardiac development from the earliest morphogenetic events, such as gastrulation.

OBJECTIVE

We sought to develop a more effective protocol for cardiac differentiation of human PSCs by using extracellular matrix in combination with growth factors known to promote cardiogenesis.

METHODS AND RESULTS

PSCs were cultured as monolayers on Matrigel, an extracellular matrix preparation, and subsequently overlayed with Matrigel. The matrix sandwich promoted an epithelial-to-mesenchymal transition as in gastrulation with the generation of N-cadherin-positive mesenchymal cells. Combining the matrix sandwich with sequential application of growth factors (Activin A, bone morphogenetic protein 4, and basic fibroblast growth factor) generated CMs with high purity (up to 98%) and yield (up to 11 CMs/input PSC) from multiple PSC lines. The resulting CMs progressively matured over 30 days in culture based on myofilament expression pattern and mitotic activity. Action potentials typical of embryonic nodal, atrial, and ventricular CMs were observed, and monolayers of electrically coupled CMs modeled cardiac tissue and basic arrhythmia mechanisms.

CONCLUSIONS

Dynamic extracellular matrix application promoted epithelial-mesenchymal transition of human PSCs and complemented growth factor signaling to enable robust cardiac differentiation.

Stage-specific optimization of activin/nodal and BMP signaling promotes cardiac differentiation of mouse and human pluripotent stem cell lines

Efficient differentiation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) to a variety of lineages requires step-wise approaches replicating the key commitment stages found during embryonic development. Here we show that expression of PdgfR-α segregates mouse ESC-derived Flk-1 mesoderm into Flk-1(+)PdgfR-α(+) cardiac and Flk-1(+)PdgfR-α(-) hematopoietic subpopulations. By monitoring Flk-1 and PdgfR-α expression, we found that specification of cardiac mesoderm and cardiomyocytes is determined by remarkably small changes in levels of Activin/Nodal and BMP signaling. Translation to human ESCs and iPSCs revealed that the emergence of cardiac mesoderm could also be monitored by coexpression of KDR and PDGFR-α and that this process was similarly dependent on optimal levels of Activin/Nodal and BMP signaling. Importantly, we found that individual mouse and human pluripotent stem cell lines require optimization of these signaling pathways for efficient cardiac differentiation, illustrating a principle that may well apply in other contexts.

Phylogeny informs ontogeny: a proposed common theme in the arterial pole of the vertebrate heart

In chick and mouse embryogenesis, a population of cells described as the secondary heart field (SHF) adds both myocardium and smooth muscle to the developing cardiac outflow tract (OFT). Following this addition, at approximately HH stage 22 in chick embryos, for example, the SHF can be identified architecturally by an overlapping seam at the arterial pole, where beating myocardium forms a junction with the smooth muscle of the arterial system. Previously, using either immunohistochemistry or nitric oxide indicators such as diaminofluorescein 2-diacetate, we have shown that a similar overlapping architecture also exists in the arterial pole of zebrafish and some shark species. However, although recent work suggests that development of the zebrafish OFT may also proceed by addition of a SHF-like population of cells, the presence of a true SHF in zebrafish and in many other developmental biological models remains an open question. We performed a comprehensive morphological study of the OFT of a wide range of vertebrates. Our data suggest that all vertebrates possess three fundamental OFT components: a proximal myocardial component, a distal smooth muscle component, and a middle component that contains overlapping myocardium and smooth muscle surrounding and supporting the outflow valves. Because the middle OFT component of avians and mammals is derived from the SHF, our observations suggest that a SHF may be an evolutionarily conserved theme in vertebrate embryogenesis.

Alpha actin isoforms expression in human and rat adult cardiac conduction system

In the adult heart, cardiac muscle comprises the working myocardium and the conduction system (CS). The latter includes the sinoatrial node (SAN), the internodal tract or bundle (IB), the atrioventricular node (AVN), the atrioventricular bundle (AVB), the bundle branches (BB) and the peripheral Purkinje fibers (PF). Most of the information concerning the phenotypic features of CS tissue derives from the characterization of avian and rodent developing hearts; data concerning the expression of actin isoforms in adult CS cardiomyocytes are scarce. Using specific antibodies, we investigated the distribution of alpha-skeletal (alpha-SKA), alpha-cardiac (alpha-CA), alpha-smooth muscle (alpha-SMA) actin isoforms and other muscle-typical proteins in the CS of human and rat hearts at different ages. SAN and IB cardiomyocytes were characterized by the presence of alpha-SMA, alpha-CA, calponin and caldesmon, whereas alpha-SKA and vimentin were absent. Double immunofluorescence demonstrated the co-localisation of alpha-SMA and alpha-CA in I-bands of SAN cardiomyocytes. AVN, AVB, BB and PF cardiomyocytes were alpha-SMA, calponin, caldesmon and vimentin negative, and alpha-CA and alpha-SKA positive. No substantial differences in actin isoform distribution were observed in human and rat hearts, except for the presence of isolated subendocardial alpha-SMA positive cardiomyocytes co-expressing alpha-CA in the ventricular septum of the rat. Aging did not influence CS cardiomyocyte actin isoform expression profile. These findings support the concept that cardiomyocytes of SAN retain the phenotype of a developing myogenic cell throughout the entire life span.

Fibroblast growth factors and Hedgehogs: at the heart of the epicardial signaling center

Over the past several years, increasing attention has been focused on understanding signaling pathways that control key events during midgestational heart development. During this period of development, the heart tube transforms into a functioning organ that must maintain its own blood supply and grow and respond to the physiologic needs of the organism. A critical event that occurs during midgestational heart development is the formation of the epicardium, which functions as a source of cells and as a signaling center that regulates myocardial growth and coronary vascular development. This review will describe our understanding of the role and the mechanism by which the epicardium governs these developmental events, primarily as a result of studies in the mouse. We focus on two key growth factor pathways: fibroblast growth factor and Hedgehog signaling.

LIM-only protein, CRP2, switched on smooth muscle gene activity in adult cardiac myocytes

Smooth muscle alpha-actin gene activity appears in promyocardial cells well before cardiac myocyte differentiation and is down-regulated during the onset of rhythmic contractility and cardiac morphogenesis. The levels of LIM-only CRP2 correlated well with smooth muscle gene activity. Cardiomyocyte-specific expression of CRP2 in transgenic mice showed robust expression of smooth muscle cell-specific transcripts and protein filaments in the adult heart. Protein transduction of a recombinant CRP2 protein, fused to the protein transduction domain of HIV, into neonatal heart cells induced de novo synthesis of smooth muscle cell-specific transcripts and proteins. The LIM zinc fingers in CRP2 were found to collaborate with Brg1 of the SNF/SWI complexes, recruited serum response factor, and remodeled smooth muscle target gene chromatin through histone acetylation. CRP2 may have a cytoskeletal role, but as a nuclear protein, CRP2 acted as a potent transcription coadaptor that remodeled silent cardiac myocyte chromatin and directed serum response factor-dependent smooth muscle gene activity.

Heart myofibrillogenesis occurs in isolated chick posterior blastoderm: a culture model

Early cardiogenesis including myofibrillogenesis is a critical event during development. ­Recently we showed that prospective cardiomyocytes reside in the posterior lateral blastoderm in the chick embryo. Here we cultured the posterior region of the chick blastoderm in serum-free medium and observed the process of myofibrillogenesis by immunohistochem­istry. After 48 hours, explants expressed sarcomeric proteins (sarcomeric α-actinin, 61%; smooth muscle α-actin, 95%; Z-line titin, 56%; sarcomeric myosin, 48%); however, they did not yet show a mature striation. After 72 hours, more than 92% of explants expressed I-Z-I proteins, which were incorporated into the striation in 75% of explants or more (sarcomeric α-actinin, 75%; smooth muscle α-actin, 81%; Z-line titin, 83%). Sarcomeric myosin was ­expressed in 63% of explants and incorporated into A-bands in 37%. The percentage incidence of expression or striation of I-Z-I proteins was significantly higher than that of sarcomeric myosin. Results suggested that the nascent I-Z-I components appeared to be gener­ated independently of A-bands in the cultured posterior blastoderm, and that the process of myofibrillogenesis observed in our culture model faithfully reflected that in vivo. Our blastoderm culture model appeared to be useful to investigate the mechanisms regulating the early cardiogenesis.

Cardiac myxoma cells exhibit embryonic endocardial stem cell features

The origin of myxoma, the most frequent tumour of the heart, remains uncertain. Previous phenotypic characterizations have shown heterogeneous results and the most recent hypothesis suggests that cardiac myxoma originates from a primitive pluripotential cardiogenic cell. We investigated the expression of actin isoforms in 30 left atrial myxomas by immunohistochemistry and in eight consecutive tumours by RT-PCR. alpha-Smooth muscle actin (alpha-SMA) protein and/or transcripts were detected in all cases, whereas alpha-cardiac actin was observed in few cases and alpha-skeletal actin was always absent. Besides classical features, vessel-like structures were characterized by cells expressing CD34 and, less frequently, alpha-SMA. Confocal microscopy showed focal co-expression of CD34 and alpha-SMA in myxoma cells, suggesting a gradual loss of stem endothelial markers and the acquisition of myocytic antigens. In order to confirm this hypothesis, early cardiac differentiation markers were also investigated. RT-PCR documented the presence of transcripts for Sox9 (100%), Notch1 (87.5%), NFATc1 (37.5%), Smad6, metalloproteinases 1 and 2 alone or in variable combinations and the absence of ErbB3 and WT1. Myxoma cells maintained phenotypic heterogeneity in vitro, including the expression of alpha-SMA and the presence of stress fibres. These findings document in cardiac myxoma cells phenotypic markers of the embryonic endothelial-to-mesenchymal transformation that precedes terminal differentiation of endocardial cushions, supporting the hypothesis that cardiac myxoma cells may derive from adult developmental remnants.

Endocardial and epicardial derived FGF signals regulate myocardial proliferation and differentiation in vivo

The epicardium regulates growth and survival of the underlying myocardium. This activity depends on intrinsic retinoic acid (RA) and erythropoietin signals. However, these signals do not act directly on the myocardium and instead are proposed to regulate the production of an unidentified soluble epicardial derived mitogen. Here, we show that Fgf9, Fgf16, and Fgf20 are expressed in the endocardium and epicardium and that RA can induce epicardial expression of Fgf9. Using knockout mice and an embryonic heart organ culture system, we show that endocardial and epicardial derived FGF signals regulate myocardial proliferation during midgestation heart development. We further show that this FGF signal is received by both FGF receptors 1 and 2 acting redundantly in the cardiomyoblast. In the absence of this signal, premature differentiation results in cellular hypertrophy and newborn mice develop a dilated cardiomyopathy. FGFs thus constitute all or part of the epicardial signal regulating myocardial growth and differentiation.

Induction of initial cardiomyocyte α-actin—smooth muscle α-actin—in cultured avian pregastrula epiblast: A role for nodal and BMP antagonist

During early cardiogenesis, endoderm-derived bone morphogenetic protein (BMP) induces the expression of both heart-specific transcription factors and sarcomeric proteins. However, BMP antagonists do not inhibit the expression of the "initial heart alpha-actin"--smooth muscle alpha-actin (SMA)--which is first expressed in the anterior lateral mesoderm and then recruited into the initial myofibrils (Nakajima et al. [2002] Dev. Biol. 245:291-303). Therefore, mechanisms that regulate the expression of SMA in the heart-forming mesoderm are not well-understood. Regional explantation experiments using chick blastoderm showed that the posterolateral region of the epiblast differentiated into cardiomyocytes. Posterior epiblast cultured with or without the associated hypoblast showed that interaction between the tissues of these two germ layers at the early pregastrula stage (stages X-XI) was a prerequisite for the expression of SMA. Posterior epiblast that is cultured without hypoblast could also be induced to express SMA if TGF-beta or activin was added to the culture medium. However, neither neutralizing antibodies against TGF-betas nor follistatin perturbed the expression of SMA in cultured blastoderm. Adding BMP to the cultured blastoderm inhibited the expression of SMA, whereas BMP antagonists, such as chordin, were able to induce the expression of SMA in cultured posterior epiblast. Furthermore, adding lefty-1, a nodal antagonist, to the blastoderm inhibited the expression of SMA, and nodal plus BMP antagonist up-regulated the expression of SMA in cultured posterior epiblast. Results indicate that the interaction between the tissues of the posterior epiblast and hypoblast is necessary to initiate the expression of SMA during early cardiogenesis and that nodal and BMP antagonist may play an important role in the regulation of SMA expression.

An In Vitro Analysis of Myocardial Potential Indicates That Phenotypic Plasticity Is an Innate Property of Early Embryonic Tissue

Explants from gastrula-stage avian embryos have provided an important culture model for examining the formation of the vertebrate heart. Explants harvested from anterior regions containing the precardiac mesoderm faithfully recapitulate cardiogenesis and generate contractile tissue in culture. Posterior regions of the early embryo do not supply cellular material to the developing heart in situ, and thus have been commonly employed as negative control tissues for studying cardiogenic induction. To begin to understand the cellular mechanisms that account for the distinct cell fates of precardiac and posterior tissue within the embryo, we undertook a comprehensive investigation on the myocardial potential of presumptive noncardiac tissue. Myocardial differentiation was assayed by expression of the myocardium-associated transcription factor gene Nkx2.5 and positive immunostaining for sarcomeric myosin, muscle alpha-actinin, and smooth muscle alpha-actin. Our results demonstrate that regions of the early embryo that do not provide a cellular contribution to the myocardium in situ are capable of generating myocardial tissue when removed from their normal embryonic environment and placed in culture under nontreated conditions. Although treatment with the presumptive cardiac inducer Dickkopf-1 increased the frequency that cardiac tissue appeared within cultures of posterior tissue, no difference was observed in either the size or morphology of the myocardium-positive areas among treated and nontreated explants. These findings suggest that progenitor cells within the early embryo possess an innate phenotypic plasticity and that presumptive cardiac inducing signals do not induce cardiac differentiation but instead augment a pre-existing cardiac potential of embryonic tissue.

Neural crest stem cell and cardiac endothelium defects in the TrkC null mouse

TrkC null mice have multiple cardiac malformations. Since neural crest cells participate in cardiac outflow tract septation, the aim of this study was to determine at the cellular level the putative neural crest defect. We have identified three types of progenitor cells: stem cells that undergo self-renewal and can generate many cell types, cells that are restricted in their developmental potentials, and cells that are committed to the smooth muscle cell lineage. In TrkC null mice, there is a greater than 50% decrease in stem cell numbers and an equivalent increase in fate-restricted cells. The outflow tract wall is thickened and the endothelial tube is disorganized. We conclude that deletion of the TrkC gene causes precocious fate restrictions of the neural crest stem cell and a defect of the outflow tract endothelium, both of which may contribute to the outflow tract malformations that occur in TrkC null mice.

Myofibrillogenesis in the developing chicken heart: Role of actin isoforms and of the pointed end actin capping protein tropomodulin during thin filament assembly

Recently, important differences between myofibrillogenesis in cultured cardiomyocytes vs. the three-dimensional setting in situ could be determined. We investigated thin filament assembly in situ by confocal microscopy of whole-mount preparations of immunostained embryonic chicken hearts. Of interest, a distinct localisation of different actin isoforms was observed in immature thin filaments. Cardiac alpha-actin is restricted to filaments with a length comparable to mature thin filaments as soon as the first contractions occur, while vascular alpha-actin makes up filaments that extend toward the M-band. The pointed-end actin filament capping protein tropomodulin can be found initially in close association with the plasma membrane, but attains its mature localisation pattern at the ends of the thin filaments only comparatively late during myofibrillogenesis. Thus tropomodulin acts as a length stabilising element of actin filaments also in developing cardiomyocytes in situ, but plays an additional role together with membrane-associated actin filaments in the earliest steps of myofibril assembly.

Inhibition of the cardiac alpha-actin gene in embryonic cardiac myocytes by dominant-negative serum response factor

Serum response factor (SRF), a transcription factor ubiquitously involved in the processes of cellular proliferation and differentiation, has been implicated in cardiac and skeletal muscle development because of its strong expression in embryonic muscle lineages, and its necessity for the transcription of transiently transfected muscle genes that contain SRF binding sites. This study was designed to ascertain whether SRF is required for the expression of an endogenous SRF-dependent gene during differentiation of early embryonic cardiac myocytes by introducing a dominant-negative SRF construct via retroviral delivery. Although no effect on overt cellular differentiation was detected, semi-quantitative RT-PCR revealed that expression of the SRF-dependent gene cardiac alpha-actin was inhibited, whereas expression of the non-SRF-dependent genes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cardiac troponin-C was unaffected. No effect on myocyte proliferation was detected. Curiously, immunohistochemical localization of SRF protein suggested that whereas endogenous SRF was homogeneously dispersed throughout the cytoplasm and nucleus, the dominant-negative SRF protein was concentrated in the nucleus. These results extend previous findings using transiently transfected genes to the endogenous level, indicating that SRF is required for the full expression of muscle genes that contain SRF binding sites during cardiac myocyte differentiation.

Recruitment of intra- and extracardiac cells into the myocardial lineage during mouse development

The tubular heart differentiates from the bilateral cardiac fields in the splanchnic mesoderm. The expression of smooth muscle proteins has been shown to accompany the early phases of cardiac muscle formation. In this study we show that during elongation of the arterial pole of the mouse linear heart tube, alpha-smooth muscle actin (alpha-Sma) expression extends in the area that has been shown to become recruited into the myocardial lineage, but does not yet express myocardial markers. These data suggest that alpha-Sma identifies mesodermal cells that during subsequent development will be recruited into the myocardial lineage. Myocardium formation is not only observed at the arterial pole, but also at the venous pole and in the intracardiac mesenchyme. This results in the formation of the caval and pulmonary myocardium, the smooth-walled atrial myocardium, the myocardial atrioventricular septum, and the myocardial outlet septum. To determine whether recruitment into the myocardial lineage also takes place in these regions, the spatiotemporal pattern of expression of alpha-Sma and of the myocardial markers sarcoplasmatic reticulum calcium ATPase (Serca2a), alpha-myosin heavy chain (Mhc), and beta-Mhc were examined. We show that prior to the expression of myocardial markers, alpha-Sma is expressed in these regions, which suggests that these mesodermal cells become recruited into the cardiac lineage after formation of the linear heart tube.

Alpha-actin isoform distribution in normal and failing human heart: a morphological, morphometric, and biochemical study

We investigated the distribution of alpha-skeletal, alpha-cardiac, and alpha-smooth muscle actin isoforms in human heart during development, hypertrophy, and failure. At 20 weeks of fetal life, alpha-skeletal actin was localized in a small proportion of subendocardial and papillary muscle cardiomyocytes. At this gestation time, diffuse alpha-cardiac actin staining was observed, associated with focal expression of alpha-smooth muscle actin. In normal adult subjects, alpha-skeletal actin positive cardiomyocytes were distributed in a transmural gradient with the highest proportion located subendocardially. In myocardial hypertrophy and cardiomyopathies, the amount of alpha-skeletal actin was increased and diffuse staining was seen in all layers of ventricular myocardium, with the exception of idiopathic dilated cardiomyopathies. Cardiomyocytes were negative for alpha-smooth muscle actin in all pathological situations studied. As expected, fibroblasts in post-infarct scars expressed alpha-smooth muscle actin and transforming growth factor-beta1 but, surprisingly, were negative for these proteins in interstitial fibrosis. Our results demonstrate that increased expression of alpha-skeletal actin in the diseased human heart is associated with increased myocyte stretch, increased wall stress, and pressure overload, but not with idiopathic dilated cardiomyopathies. They also suggest that fibrotic changes develop with different mechanisms in scars versus interstitial fibrosis.

Significance of bone morphogenetic protein-4 function in the initial myofibrillogenesis of chick cardiogenesis

The heart is the first organ to form and function during vertebrate embryogenesis. Using a secreted protein, noggin, which specifically antagonizes bone morphogenetic protein (BMP)-2 and -4, we examined the role played by BMP during the initial myofibrillogenesis in chick cultured precardiac mesoendoderm (mesoderm + endoderm; ME). Conditioned medium from COS7 cells transfected with Xenopus noggin cDNA inhibited the expression of sarcomeric proteins (such as sarcomeric alpha-actinin, Z-line titin, and sarcomeric myosin), and so myofibrillogenesis was perturbed in cultured stage 4 precardiac ME; however, it did not inhibit the expression of smooth muscle alpha-actin (the first isoform of alpha-actin expressed during cardiogenesis). In cultured stage 5 precardiac ME, noggin did not inhibit either the formation of I-Z-I components or the expression of sarcomeric myosin, but it did inhibit the formation of A-bands. Although BMP4 was required to induce expressions of sarcomeric alpha-actinin, titin, and sarcomeric myosin in cultured stage 6 posterolateral mesoderm (noncardiogenic mesoderm), smooth muscle alpha-actin was expressed without the addition of BMP4. Interestingly, in cultured stage 6 posterolateral mesoderm, BMP2 induced the expressions of sarcomeric alpha-actinin and titin, but not of sarcomeric myosin. These results suggest that (1) BMP4 function lies upstream of the initial formation of I-Z-I components and A-bands separately in a stage-dependent manner, and (2) at least two signaling pathways are involved in the initial cardiac myofibrillogenesis: one is an unknown pathway responsible for the expression of smooth muscle alpha-actin; the other is BMP signaling, which is involved in the expression of sarcomeric alpha-actinin, titin, and sarcomeric myosin.

Spatiotemporal and tissue specific distribution of apoptosis in the developing chick heart

To investigate spatial and temporal distributions of apoptosis in the embryonic chick heart and its relation to different tissue types, we examined apoptosis in the embryonic chick heart from Hamburger and Hamilton stage 17 through 3 days after hatching. MF20 antibody, alpha-smooth muscle actin (SMA) antibody and EAP-300 antibody were applied to delineate specific cell types. During early development of the embryonic chick heart, very few apoptotic cells were detected. The first distinctive zone of apoptosis was observed in the outflow tract at stage 25. This focus was most prominent during septation of the pulmonary artery from the aorta (i.e., between stages 28 and 29), and diminished to virtually background level by stage 32, except in the subconal regions. Subsequently, remarkable apoptosis appeared in the atrioventricular cushions by stage 26, peaked at stages 29-31, and dropped significantly thereafter. Characteristic distribution patterns of apoptotic cells were also detected in the cardiac conduction tissues, including the His bundle, the bundle branches, and the ventricular trabeculae. After stage 36, cell death dropped to background level, except in developing coronary vessels. MF20 and TUNEL double staining revealed that apoptosis in cardiomyocytes was limited to a few specific regions, much less than in cushion tissues. SMA and TUNEL double staining demonstrated that vascular structures were the major foci of apoptosis from stage 40 to 44, whereas adjacent perivascular Purkinje cells displayed significantly less cell death at these stages. The characteristic spatiotemporal locations of apoptosis parallel the morphologic changes and tissue differentiation during heart development, suggesting that apoptosis is crucial to the transformation of the heart from a simple tube to a complex multichambered pump.

Assembly of thick, thin, and titin filaments in chick precardiac explants

De novo cardiac myofibril assembly has been difficult to study due to the lack of available cell culture models that clearly and accurately reflect heart muscle development in vivo. However, within precardiac chick embryo explants, premyocardial cells differentiate and commence beating in a temporal pattern that corresponds closely with myocyte differentiation in the embryo. Immunofluorescence staining of explants followed by confocal microscopy revealed that distinct stages of cardiac myofibril assembly, ranging from the earliest detection of sarcomeric proteins to the late appearance of mature myofibrils, were consistently recognized in precardiac cultures. Assembly events involved in the early formation of sarcomeres were clearly visualized and accurately reflected observations described by others during chick heart muscle development. Specifically, the early colocalization of alpha-actinin and titin dots was observed near the cell periphery representing I-Z-I-like complex formation. Myosin-containing thick filaments assembled independently of actin-containing thin filaments and appeared centered within sarcomeres when titin was also linearly aligned at or near cell borders. An N-terminal epitope of titin was detected earlier than a C-terminal epitope; however, both epitopes were observed to alternate near the cell periphery concomitant with the earliest formation of myofibrils. Although vascular actin was detected within cells during early assembly stages, cardiac actin predominated as the major actin isoform in mature thin filaments. Well-aligned thin filaments were also observed in the absence of organized staining for tropomodulin at thin filament pointed ends, suggesting that tropomodulin is not required to define thin filament lengths. Based on these findings, we conclude that the use of the avian precardiac explant system accurately allows for direct investigation of the mechanisms regulating de novo cardiac myofibrillogenesis.

Impaired vascular contractility and blood pressure homeostasis in the smooth muscle alpha-actin null mouse

The smooth muscle (SM) alpha-actin gene activated during the early stages of embryonic cardiovascular development is switched off in late stage heart tissue and replaced by cardiac and skeletal alpha-actins. SM alpha-actin also appears during vascular development, but becomes the single most abundant protein in adult vascular smooth muscle cells. Tissue-specific expression of SM alpha-actin is thought to be required for the principal force-generating capacity of the vascular smooth muscle cell. We wanted to determine whether SM alpha-actin gene expression actually relates to an actin isoform's function. Analysis of SM alpha-actin null mice indicated that SM alpha-actin is not required for the formation of the cardiovascular system. Also, SM alpha-actin null mice appeared to have no difficulty feeding or reproducing. Survival in the absence of SM alpha-actin may result from other actin isoforms partially substituting for this isoform. In fact, skeletal alpha-actin gene, an actin isoform not usually expressed in vascular smooth muscle, was activated in the aortas of these SM alpha-actin null mice. However, even with a modest increase in skeletal alpha-actin activity, highly compromised vascular contractility, tone, and blood flow were detected in SM alpha-actin-defective mice. This study supports the concept that SM alpha-actin has a central role in regulating vascular contractility and blood pressure homeostasis, but is not required for the formation of the cardiovascular system.

N-cadherin is required for the differentiation and initial myofibrillogenesis of chick cardiomyocytes

To investigate initial stages of cardiac myofibrillogenesis, heart-forming mesoderm was excised from stage 6 chick embryos and explanted on fibronectin-coated coverglasses. The explants were fixed at various times and immunofluorescently stained with antibodies to N-cadherin, alpha-catenin, beta-catenin, sarcomeric myosin, pan and sarcomeric alpha-actinins, or rhodamine phalloidin. After 7 hours in culture the cells appeared epithelial. N-cadherin, alpha- and beta-catenin, pan alpha-actinin, and F-actin showed circumferential localization at cell borders. No cells in the explant were positive for sarcomeric alpha-actinin or sarcomeric myosin at this stage. Sarcomeric alpha-actinin and sarcomeric myosin were detected around 10 hours after plating. Sarcomeric alpha-actinin initially appeared as small beads along thin actin filaments. Mature Z-lines began to be organized at 20 hours, at the same time the cells started to contract. When the rat monoclonal antibody NCD-2, which inhibits N-cadherin function, was added to the culture at early time-points, cells lost cell-cell contacts, became spherical in shape, and contained tangled actin fibers. The expression of sarcomeric alpha-actinin and sarcomeric myosin was suppressed. These results indicate that 1) the precardiac mesoderm explant cells differentiate and form well-organized myofibrils in culture, 2) N-cadherin-mediated cell-cell interactions are necessary for early differentiation of cardiomyocytes and organization of myofibrils.

Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha-actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] Dev. Biol. 159:706-719; Schoenwolf and Garcia-Martinez [1995] Cell Mol. Biol. Res. 41:233-240; Garcia-Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so-called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent-shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight-heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post-transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events.

Antisense oligodeoxynucleotide complementary to smooth muscle alpha-actin inhibits endothelial-mesenchymal transformation during chick cardiogenesis

alpha-Smooth-muscle actin (SMA) is the major isoform of adult vascular tissues. During early development, SMA is expressed in various mesodermally derived tissues in a spatiotemporally restricted manner; however, its exact role remains unknown. We examined its role in the formation of chicken atrioventricular (AV) endocardial cushion tissue. This developmental process possesses the characteristics of endothelial-mesenchymal transformation and is partly TGF beta-dependent. Immunohistochemistry showed that SMA was (1) expressed homogeneously in the newly formed appendages of transforming endothelial/mesenchymal cells, and (2) distributed in a punctate manner in the lamellipodia/filopodia of invading mesenchymal cells. Antisense oligodeoxynucleotide (ODNs) specific for SMA reduced both SMA expression and mesenchymal formation in AV endothelial cells cultured with myocardium on a collagen gel lattice. Perturbation of SMA by antisense ODN also inhibited TGF beta-inducible migratory appendage formation in a cultured AV endothelial monolayer. However, it did not inhibit cell:cell separation or cellular hypertrophy. These results suggest that the expression of SMA is necessary for migratory appendage formation during the TGF beta-dependent initial phenotypic changes that occur in endothelial-mesenchymal transformation.

Expression of retinol binding protein and transthyretin during early embryogenesis

Previous studies have shown that anterior lateral plate endoderm from stage 6 chicken embryos is necessary and sufficient to enable precardiac mesoderm to complete its cardiogenic program in vitro, culminating in a rhythmically contractile multicellular vesicle (Sugi and Lough [1994] Dev. Dyn. 200:155-162). To identify cardiogenic factors, we have begun to characterize proteins that are secreted by endoderm cell explants. Fluorography of proteins from endoderm-conditioned medium revealed 1-2 dozen bands, the most prominent of which migrated at approximately 17 and 25 kD. The bulk of the 17-kD band, which migrates near FGFs and subunits of the transforming growth factor-beta family, was identified by N-terminal sequencing as transthyretin (TTR). A component of the 25-kD band was identified by Western blotting as retinol binding protein (RBP). RT/PCR analysis revealed that mRNAs for both proteins are in the embryo as early as stage 3. In situ hybridization localized these mRNAs to the extraembryonic endoderm at stage 6, after which they were detected in endoderm overlying the embryo proper, including the developing heart. Later, RBP and TTR mRNA and protein were detected in cells associated with the developing heart. Western blotting of whole embryo proteins revealed the presence of RBP by stage 7, followed by sequential increases to stage 25; by contrast, content of RBP in isolated hearts peaked at stage 14, then declined. Immunohistochemistry revealed the presence of RBP protein in the extracellular matrix subjacent to lateral plate endoderm beginning at stage 8; upon formation of the definitive heart, intense staining was observed in the cardiac "jelly." By contrast TTR was intracellular, first detected as subtle deposits in stage 6 embryonic endoderm, which by stage 8 were prominent in the dorsally invaginated endoderm subjacent to the precardiac splanchnic mesoderm. At stages 11-14, TTR was detected only in myocardial cells. Such localization of RBP and TTR may indicate a role in the transport and distribution of retinol and thyroid hormone, respectively, from yolk to embryo prior to establishment of the circulatory system, and is suggestive of a subsequent role in heart development.

BMP-2 induces ectopic expression of cardiac lineage markers and interferes with somite formation in chicken embryos

In Drosophila induction of the homeobox gene tinman and subsequent heart formation are dependent on dpp signaling from overlying ectoderm. In order to define vertebrate heart-inducing signals we screened for dpp-homologues expressed in HH stage 4 chicken embryos. The majority of transcripts were found to be BMP-2 among several other members of the BMP family. From embryonic HH stage 4 onwards cardiogenic mesoderm appeared to be in close contact to BMP-2 expressing cells which initially were present in lateral mesoderm and subsequently after headfold formation in the pharyngeal endoderm. In order to assess the role of BMP-2 for heart formation, gastrulating chick embryos in New culture were implanted with BMP-2 producing cells. BMP-2 implantation resulted in ectopic cardiac mesoderm specification. BMP-2 was able to induce Nkx2-5 expression ectopically within the anterior head domain, while GATA-4 was also induced more caudally. Cardiogenic induction by BMP-2, however remained incomplete, since neither Nkx2-8 nor the cardiac-restricted structural gene VMHC-1 became ectopically induced. BMP-2 expressing cells implanted adjacent to paraxial mesoderm resulted in impaired somite formation and blocked the expression of marker genes, such as paraxis, Pax-3, and the forkhead gene cFKH-1. These results suggest that BMP-2 is part of the complex of cardiogenic signals and is involved in the patterning of early mesoderm similar to the role of dpp in Drosophila.

Expression of the smooth-muscle proteins alpha-smooth-muscle actin and calponin, and of the intermediate filament protein desmin are parameters of cardiomyocyte maturation in the prenatal rat heart

BACKGROUND

Coexpression of alpha- and beta-myosin heavy chain (MHC) is a characteristic of the primary myocardial tube. To establish if the smooth-muscle proteins alpha-smooth-muscle actin (alpha-SMA) and calponin, and the intermediate filament protein, desmin, contribute to the specific functional properties of these early cardiomyocytes, we studied their spatiotemporal expression pattern.

METHODS

Sections of prenatal and neonatal Wistar rats were stained with antibodies against alpha- and beta-MHC, alpha-SMA, calponin, and desmin.

RESULTS

The expression of alpha-SMA and calponin in embryonic cardiomyocytes increases to reach its highest level at ED14. Subsequently, these proteins gradually disappear, beginning in the interventricular septum (IVS) and followed successively by the compact myocardium of the left ventricle, the right ventricle, and the central atrium. Expression of alpha-SMA persists longer in the ventricular conduction system, making it a convenient marker for the ventricular conduction system of the fetal rat. Desmin becomes expressed one day later than alpha-SMA, but also reaches its peak at ED14, suggesting that a relatively high concentration is required to form mature sarcomeres.

CONCLUSIONS

The results indicate that alpha-SMA, calponin, and desmin are involved in the myofibrillar development in rat heart. The presence of spatiotemporal differences in the expression of these proteins reveals regional differences in the developmental timing of cardiomyocyte maturation. The maturation process extends from the compact myocardium in the IVS to the left and right ventricular free walls, whereas the atrioventricular junction, the ventricular trabeculae, and developing ventricular conduction system show a relatively slow maturation. Smooth-muscle proteins may contribute to the slow shortening speed that is characteristic of the embryonic myocardium.

Proliferation and differentiation of smooth muscle cell precursors occurs simultaneously during the development of the vessel wall

Formation of the blood vessel wall depends on the recruitment, proliferation, and differentiation of smooth muscle cell (SMC) precursors. The temporal events associated with the onset of expression of several SMC proteins have been well characterized in mouse and avian species. However, the timing of cell proliferation during this process has not been explored. More importantly, it has not been clear whether commitment to the smooth muscle pathway precludes proliferation during development. In the present study, we have determined the kinetics of replication in developing chick aortae between days 2.5 and 19 and have correlated these data with the expression of various SMC differentiation markers. We found that proliferation of aortic SMC precursors occurs in two waves; an early phase of rapid proliferation (15-17%; between days 4 and 12), and a second phase, when replication was reduced to less than 5% (days 16 to hatching). Proliferation of SMC during the first wave occurred concomitantly with the progressive accumulation of SMC contractile proteins, such as SM alpha-actin, calponin, myosin heavy chain, and the 1E12 antigen. We also found that the relative proliferation capacity within each compartment of the vessel wall, ie., intima, media, and adventitia varies throughout development. Approximately, 55-63% of all replicating cells were found in the tunica adventitia from days 6 to 12, whereas 35% were found in the tunica media (tunica media:adventitia = 1:2). This ratio was inverted after day 12, when most of the replicating cells were located in the tunica media (tunica media:adventitia = 2:1). In addition, we observed a ventral-to-dorsal gradient in the proliferation of SMC precursors between days 2.5 and 5. The ventral-to-dorsal proliferation gradient was similar to the previously described differential expression of two early SMC markers: alpha-actin and the 1E12 antigen. These data support the concept that a polarity exists either in the pool of SMC precursors or, in expression of factors that regulate recruitment of presumptive SMC.

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.

Identification of a novel marker for primordial smooth muscle and its differential expression pattern in contractile vs noncontractile cells

The assembly of the vessel wall from its cellular and extracellular matrix components is an essential event in embryogenesis. Recently, we used the descending aorta of the embryonic quail to define the morphological events that initiate the formation of a multilayered vessel wall from a nascent endothelial cell tube (Hungerford, J.E., G.K. Owens, W.S. Argraves, and C.D. Little. 1996. Dev. Biol. 178:375-392). We generated an mAb, 1E12, that specifically labels smooth muscle cells from the early stages of development to adulthood. The goal of our present study was to characterize further the 1E12 antigen using both cytological and biochemical methods. The 1E12 antigen colocalizes with the actin cytoskeleton in smooth muscle cells grown on planar substrates in vitro; in contrast, embryonic vascular smooth muscle cells in situ contain 1E12 antigen that is distributed in threadlike filaments and in cytoplasmic rosette-like patterns. Initial biochemical analysis shows that the 1E12 mAb recognizes a protein, Mr = 100,000, in lysates of adult avian gizzard. An additional polypeptide band, Mr = 40,000, is also recognized in preparations of lysate, when stronger extraction conditions are used. We have identified the 100-kD polypeptide as smooth muscle alpha-actinin by tandem mass spectroscopy analysis. The 1E12 antibody is an IgM isotype. To prepare a more convenient 1E12 immunoreagent, we constructed a single chain antibody (sFv) using recombinant protein technology. The sFv recognizes a single 100-kD protein in gizzard lysates. Additionally, the recombinant antibody recognizes purified smooth muscle alpha-actinin. Our results suggest that the 1E12 antigen is a member of the alpha-actinin family of cytoskeletal proteins; furthermore, the onset of its expression defines a primordial cell restricted to the smooth muscle lineage.

Rescue of cardiac alpha-actin-deficient mice by enteric smooth muscle gamma-actin

The muscle actins in higher vertebrates display highly conserved amino acid sequences, yet they show distinct expression patterns. Thus, cardiac alpha-actin, skeletal alpha-actin, vascular smooth muscle alpha-actin, and enteric smooth muscle gamma-actin comprise the major actins in their respective tissues. To assess the functional and developmental significance of cardiac alpha-actin, the murine (129/SvJ) cardiac alpha-actin gene was disrupted by homologous recombination. The majority ( approximately 56%) of the mice lacking cardiac alpha-actin do not survive to term, and the remainder generally die within 2 weeks of birth. Increased expression of vascular smooth muscle and skeletal alpha-actins is observed in the hearts of newborn homozygous mutants and also heterozygotes but apparently is insufficient to maintain myofibrillar integrity in the homozygous mutants. Mice lacking cardiac alpha-actin can be rescued to adulthood by the ectopic expression of enteric smooth muscle gamma-actin using the cardiac alpha-myosin heavy chain promoter. However, the hearts of such rescued cardiac alpha-actin-deficient mice are extremely hypodynamic, considerably enlarged, and hypertrophied. Furthermore, the transgenically expressed enteric smooth muscle gamma-actin reduces cardiac contractility in wild-type and heterozygous mice. These results demonstrate that alterations in actin composition in the fetal and adult heart are associated with severe structural and functional perturbations.

Differential expression of a novel isoform of alpha-tropomyosin in cardiac and skeletal muscle of the Mexican axolotl (Ambystoma mexicanum)

Alternative mRNA splicing is a fundamental process in eukaryotes that contributes to tissue-specific and developmentally regulated patterns of tropomyosin (TM) gene expression. Northern blot analyses suggest the presence of multiple transcripts of tropomyosin in skeletal and cardiac muscle of adult Mexican axolotls. We have cloned and sequenced two tropomyosin cDNAs designated ATmC-1 and ATmC-2 from axolotl heart tissue and one TM cDNA from skeletal muscle, designated ATmS-1. Nucleotide sequence analyses suggest that ATmC-1 and ATmC-2 are the products of the same alpha-TM gene produced via alternate splicing, whereas ATmC-1 and ATmS-1 are the identical isoforms generated from the alpha-gene. RT-PCR analysis using isoform-specific primer pairs and detector oligonucleotides suggests that ATmC-2 is expressed predominantly in adult axolotl hearts. ATmC-2 is a novel isoform, which unlike ATmC-1 and other known striated muscle isoforms expresses exon 2a instead of exon 2b.

Insulin-like growth factor-II/mannose-6-phosphate receptor expression during early heart development

Expression of the insulin-like growth factor-II/mannose-6-phosphate (ICF-II/ M6P) receptor was examined during the major stages of heart morphogenesis in the chicken embryo. By using an affinity-purified antibody, Western blot analysis of total embryonic proteins from stages 5-24 revealed little if any IGF-II/M6P receptor protein until stage 7, approximately 8 hours prior to the appearance of the rudimentary myocardial tubes. Thereafter, receptor accumulation increased until stage 14, after which receptor protein levels remained constant, up to 7 days in ovo. Immunohistochemical localization revealed that, among all embryonic tissues at stages 10-24, the predominant site of receptor expression was the developing myocardium. Receptor expression was also immunohistochemically evaluated in a defined in vitro model of cardiogenesis in which explanted precardiac mesoderm is induced to undergo differentiation by co-explanted endoderm. In this system, as in vivo, IGF-II/M6P receptors were only detected after precardiac mesoderm had differentiated into a synchronously contractile multilayer which expressed cardiac alpha-actin. These findings indicate that the IGF-II/M6P receptor has an important role during early heart development.

Expression of alpha SM actin in terrestrial ectothermic vertebrates

alpha-Smooth muscle (alpha SM) actin of endothermic vertebrates is selectively recognized by the monoclonal antibody anti-alpha SM-1. Immunoreactivity to this antibody has been shown to be localized in the NH2-terminal sequence Ac-EEED (Chaponnier et al. 1994). Among terrestrial ectothermic vertebrates, two amphibian (Triturus vulgaris, Rana esculenta) and three reptilian species (Pseudemys scripta elegans, Natrix natrix, Podarcis sicula) were screened to investigate if their vascular and visceral smooth muscles were stained by anti-alpha SM-1. In all the specimens tested, Western-blot analysis of tissue extracts immunodecorated with anti-alpha SM-1 revealed a single polypeptide chain having the same electrophoretic mobility as bovine alpha SM actin. The binding to amphibian and reptilian tissue extracts was inhibited by the synthetic peptide Ac-EEED, but not Ac-DEED, as occurs in mammals. alpha SM actin expression was found in vascular and visceral smooth muscle cells of the species tested. The media of small and large blood vessels was labelled by anti-alpha SM-1. In the stomach and intestine the outer longitudinal and inner circular layers of the muscularis and of the muscularis mucosae were stained. In addition, myofibroblasts of the subepithelial layer were labelled. A more restricted expression of this isoactin was detected in turtle (P. scripta elegans) visceral smooth muscle cells, which may be related to the involvement of the digestive system in respiratory activity. These data suggest that in vertebrate evolution alpha SM actin arose earlier than previously proposed.

Developmental expression of fibroblast growth factor receptor-1 (cek-1; flg) during heart development

Previous work in this laboratory has indicated that fibroblast growth factor-2 (FGF-2; bFGF) regulates the initial stages of avian heart development in paracrine and autocrine fashion (Parlow et al. [1991] Dev. Biol. 146:139-147; Sugi et al. [1993] Dev. Biol. 157:28-37). Because these findings inferred the presence of a functional receptor for fibroblast growth factor (FGFR), we have immunochemically assessed the appearance of FGFR-1 (cek-1; flg) during development. Using a peptide-generated antibody, Western blots of total embryonic proteins revealed that FGFR-1 was barely detectable at pre-heart stages, followed by sequential increases in relative abundance that peaked at stage 24, followed by a decline at days 7-14. Western blots of proteins from isolated embryonic hearts demonstrated a similar developmental pattern, except that FGFR-1 expression was not decreased at later stages. The presence of FGFR-1 mRNA was verified by reverse transcription/polymerase chain reaction (RT/PCR) amplification. Immunohistochemical examination revealed punctate deposits of FGFR-1 in the precardiac endoderm at stage 6, followed by detection in the endoderm, foregut, and pre-cardiac splanchnic mesoderm at stage 8 and in the newly formed myocardium at the heart tube stage (9/10). By stage 13, FGFR-1 staining was observed only in the myocardium, a pattern which persisted at least until stage 30 (day 7), after which only isolated hearts were examined. After stage 30, staining was diminished in the ventricle, but not in the atrium. Staining of cardiac endothelial cells was not observed at any stage. A functional role for FGFR-1 was indicated by experiments in which anti-FGFR-1, but not pre-absorbed antiserum, retarded proliferation and multilayering of cardiogenic cells in an in vitro model of cardiac morphogenesis.

Smooth muscle myosin heavy chain exclusively marks the smooth muscle lineage during mouse embryogenesis

We cloned a portion of the mouse smooth muscle myosin heavy chain (SM-MHC) cDNA and analyzed its mRNA expression in adult tissues, several cell lines, and developing mouse embryos to determine the suitability of the SM-MHC promoter as a tool for identifying smooth muscle-specific transcription factors and to define the spatial and temporal pattern of smooth muscle differentiation during mouse development. RNase protection assays showed SM-MHC mRNA in adult aorta, intestine, lung, stomach, and uterus, with little or no signal in brain, heart, kidney, liver, skeletal muscle, spleen, and testes. From an analysis of 14 different cell lines, including endothelial cells, fibroblasts, and rhabdomyosarcomas, we failed to detect any SM-MHC mRNA; all of the cell lines induced to differentiate also showed no detectable SM-MHC. In situ hybridization of staged mouse embryos first revealed SM-MHC transcripts in the early developing aorta at 10.5 days post coitum (dpc). No hybridization signal was demonstrated beyond the aorta and its arches until 12.5 to 13.5 dpc, when SM-MHC mRNA appeared in smooth muscle cells (SMCs) of the developing gut and lungs as well as peripheral blood vessels. By 17.5 dpc, SM-MHC transcripts had accumulated in esophagus, bladder, and ureters. Except for blood vessels, no SM-MHC transcripts were ever observed in developing brain, heart, or skeletal muscle. These results indicate that smooth muscle myogenesis begins by 10.5 days of embryonic development in the mouse and establish SM-MHC as a highly specific marker for the SMC lineage. The SM-MHC promoter should therefore serve as a useful model for defining the mechanisms that govern SMC transcription during development and disease.

Anterior endoderm is a specific effector of terminal cardiac myocyte differentiation of cells from the embryonic heart forming region

The ability of anterior lateral plate mesoderm cells in the heart-forming region (HFR) of stage 6 chicken embryos to respond to cardiogenic stimuli from cells in adjacent germ layers has been investigated using explants cultured under defined conditions. Two types of explantation were evaluated: those in which two germ layers were explanted in contiguity, and those in which germ layers were isolated and co-cultured. Two parameters--contractility and expression of sarcomeric alpha-actin--were monitored to evaluate the terminal differentiation of cardiac myocytes. Contiguously explanted anterior endoderm/mesoderm became multilayered and underwent terminal differentiation within 2 days. By contrast, although contiguous anterior ectoderm/mesoderm or posterior endoderm/mesoderm co-explants also became multilayered, these explants did not differentiate, up to 5 days. To ascertain the cardiogenic potential of cells from different regions of the embryo, individual germ layers were isolated and co-cultured by placing the explants in separate areas of the culture chamber. These determinations demonstrated that anterior, but not posterior, endoderm effected differentiation of anterior mesoderm. As before, mesoderm in both types of co-culture survived and became multilayered; by contrast, mesoderm did not survive when cultured in isolation. These experiments provide evidence that anterior endoderm regulates the terminal differentiation, as opposed to growth, of presumptive cardiac myocytes in mesoderm cells from the anterior lateral plate. Finally, anterior endoderm was co-cultured with mesoderm from the posterior half of the embryo, which does not contain an HFR. The failure of these co-cultured explants to differentiate infers that pre-cardiac myoblasts in stage 6 anterior mesoderm are previously specified to respond to the terminal cardiogenic effects of endoderm.