A role for fibronectin in the migration of avian precardiac cells. I. Dose-dependent effects of fibronectin antibody

Quantifying endodermal strains during heart tube formation in the developing chicken embryo

In the early avian embryo, the developing heart forms when bilateral fields of cardiac progenitor cells, which reside in the lateral plate mesoderm, move toward the embryonic midline, and fuse above the anterior intestinal portal (AIP) to form a straight, muscle-wrapped tube. During this process, the precardiac mesoderm remains in close contact with the underlying endoderm. Previous work has shown that the endoderm around the AIP actively contracts to pull the cardiac progenitors toward the midline. The morphogenetic deformations associated with this endodermal convergence, however, remain unclear, as do the signaling pathways that might regulate this process. Here, we fluorescently labeled populations of endodermal cells in early chicken embryos and tracked their motion during heart tube formation to compute time-varying strains along the anterior endoderm. We then determined how the computed endodermal strain distributions are affected by the pharmacological inhibition of either myosin II or fibroblast growth factor (FGF) signaling. Our data indicate that a mediolateral gradient in endodermal shortening is present around the AIP, as well as substantial convergence and extension movements both anterior and lateral to the AIP. These active endodermal deformations are disrupted if either actomyosin contractility or FGF signaling are inhibited pharmacologically. Taken together, these results demonstrate how active deformations along the anterior endoderm contribute to heart tube formation within the developing embryo.

Cardiac differentiation of human pluripotent stem cells using defined extracellular matrix proteins reveals essential role of fibronectin

Research and therapeutic applications using human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) require robust differentiation strategies. Efforts to improve hPSC-CM differentiation have largely overlooked the role of extracellular matrix (ECM). The present study investigates the ability of defined ECM proteins to promote hPSC cardiac differentiation. Fibronectin (FN), laminin-111, and laminin-521 enabled hPSCs to attach and expand. However, only addition of FN promoted cardiac differentiation in response to growth factors Activin A, BMP4, and bFGF in contrast to the inhibition produced by laminin-111 or laminin-521. hPSCs in culture produced endogenous FN which accumulated in the ECM to a critical level necessary for effective cardiac differentiation. Inducible shRNA knockdown of FN prevented Brachyury+ mesoderm formation and subsequent hPSC-CM generation. Antibodies blocking FN binding integrins α4β1 or αVβ1, but not α5β1, inhibited cardiac differentiation. Furthermore, inhibition of integrin-linked kinase led to a decrease in phosphorylated AKT, which was associated with increased apoptosis and inhibition of cardiac differentiation. These results provide new insights into defined matrices for culture of hPSCs that enable production of FN-enriched ECM which is essential for mesoderm formation and efficient cardiac differentiation.

Bioengineering approaches to mature induced pluripotent stem cell-derived atrial cardiomyocytes to model atrial fibrillation

Induced pluripotent stem cells (iPSCs) serve as a robust platform to model several human arrhythmia syndromes including atrial fibrillation (AF). However, the structural, molecular, functional, and electrophysiological parameters of patient-specific iPSC-derived atrial cardiomyocytes (iPSC-aCMs) do not fully recapitulate the mature phenotype of their human adult counterparts. The use of physiologically inspired microenvironmental cues, such as postnatal factors, metabolic conditioning, extracellular matrix (ECM) modulation, electrical and mechanical stimulation, co-culture with non-parenchymal cells, and 3D culture techniques can help mimic natural atrial development and induce a more mature adult phenotype in iPSC-aCMs. Such advances will not only elucidate the underlying pathophysiological mechanisms of AF, but also identify and assess novel mechanism-based therapies towards supporting a more 'personalized' (i.e. patient-specific) approach to pharmacologic therapy of AF.

Bearing My Heart: The Role of Extracellular Matrix on Cardiac Development, Homeostasis, and Injury Response

The extracellular matrix (ECM) is an essential component of the heart that imparts fundamental cellular processes during organ development and homeostasis. Most cardiovascular diseases involve severe remodeling of the ECM, culminating in the formation of fibrotic tissue that is deleterious to organ function. Treatment schemes effective at managing fibrosis and promoting physiological ECM repair are not yet in reach. Of note, the composition of the cardiac ECM changes significantly in a short period after birth, concurrent with the loss of the regenerative capacity of the heart. This highlights the importance of understanding ECM composition and function headed for the development of more efficient therapies. In this review, we explore the impact of ECM alterations, throughout heart ontogeny and disease, on cardiac cells and debate available approaches to deeper insights on cell–ECM interactions, toward the design of new regenerative therapies.

Basic Biology of Extracellular Matrix in the Cardiovascular System, Part 1/4: JACC Focus Seminar

The extracellular matrix (ECM) is the noncellular component of tissues in the cardiovascular system and other organs throughout the body. It is formed of filamentous proteins, proteoglycans, and glycosaminoglycans, which extensively interact and whose structure and dynamics are modified by cross-linking, bridging proteins, and cleavage by matrix degrading enzymes. The ECM serves important structural and regulatory roles in establishing tissue architecture and cellular function. The ECM of the developing heart has unique properties created by its emerging contractile nature; similarly, ECM lining blood vessels is highly elastic in order to sustain the basal and pulsatile forces imposed on their walls throughout life. In this part 1 of a 4-part JACC Focus Seminar, we focus on the role, function, and basic biology of the ECM in both heart development and in the adult.

Matrix metalloproteinases regulate ECM accumulation but not larval heart growth in Drosophila melanogaster

The Drosophila heart provides a simple model to examine the remodelling of muscle insertions with growth, extracellular matrix (ECM) turnover, and fibrosis. Between hatching and pupation, the Drosophila heart increases in length five-fold. If major cardiac ECM components are secreted remotely, how is ECM "self assembly" regulated? We explored whether ECM proteases were required to maintain the morphology of a growing heart while the cardiac ECM expanded. An increase in expression of Drosophila's single tissue inhibitor of metalloproteinase (TIMP), or reduced function of metalloproteinase MMP2, resulted in fibrosis and ectopic deposition of two ECM Collagens; type-IV and fibrillar Pericardin. Significant accumulations of Collagen-IV (Viking) developed on the pericardium and in the lumen of the heart. Congenital defects in Pericardin deposition misdirected further assembly in the larva. Reduced metalloproteinase activity during growth also increased Pericardin fibre accumulation in ECM suspending the heart. Although MMP2 expression was required to remodel and position cardiomyocyte cell junctions, reduced MMP function did not impair expansion of the heart. A previous study revealed that MMP2 negatively regulates the size of the luminal cell surface in the embryonic heart. Cardiomyocytes align at the midline, but do not adhere to enclose a heart lumen in MMP2 mutant embryos. Nevertheless, these embryos hatch and produce viable larvae with bifurcated hearts, indicating a secondary pathway to lumen formation between ipsilateral cardiomyocytes. MMP-mediated remodelling of the ECM is required for organogenesis, and to prevent assembly of excess or ectopic ECM protein during growth. MMPs are not essential for normal growth of the Drosophila heart.

Impact of Fibronectin Knockout on Proliferation and Differentiation of Human Infrapatellar Fat Pad-Derived Stem Cells

Fibronectin plays an essential role in tissue development and regeneration. However, the effects of fibronectin knockout (FN1-KO) on stem cells' proliferation and differentiation remain unknown. In this study, CRISPR/Cas9 generated FN1-KO in human infrapatellar fat pad-derived stem cells (IPFSCs) was evaluated for proliferation ability including cell cycle and surface markers as well as stemness gene expression and for differentiation capacity including chondrogenic and adipogenic differentiation. High passage IPFSCs were also evaluated for proliferation and differentiation capacity after expansion on decellularized ECM (dECM) deposited by FN1-KO cells. Successful FN1-KO in IPFSCs was confirmed by Sanger sequencing and Inference of CRISPR Edits analysis (ICE) as well as immunostaining for fibronectin expression. Compared to the GFP control, FN1-KO cells showed an increase in cell growth, percentage of cells in the S and G2 phases, and CD105 and CD146 expression but a decrease in expression of stemness markers CD73, CD90, SSEA4, and mesenchymal condensation marker CDH2 gene. FN1-KO decreased both chondrogenic and adipogenic differentiation capacity. Interestingly, IPFSCs grown on dECMs deposited by FN1-KO cells exhibited a decrease in cell proliferation along with a decline in CDH2 expression. After induction, IPFSCs plated on dECMs deposited by FN1-KO cells also displayed decreased expression of both chondrogenic and adipogenic capacity. We concluded that FN1-KO increased human IPFSCs' proliferation capacity; however, this capacity was reversed after expansion on dECM deposited by FN1-KO cells. Significance of fibronectin in chondrogenic and adipogenic differentiation was demonstrated in both FN1-KO IPFSCs and FN(-) matrix microenvironment.

Foraging ecology influences the number of vertebrae in hydrophiine sea snakes

The number of vertebrae in snakes is highly variable both within and among species. Across ophidian taxa, the number of vertebrae has been linked to many aspects of ecology and performance. Herein, I test the hypothesis that variation in the number of vertebrae and the length of the anterior region of sea snakes are associated with foraging ecology. I predicted that sea snakes that invade burrows and crevices for prey would have relatively longer anterior regions as a result of a greater number of vertebrae. Using radiographs, I counted the number of vertebrae between the head and atria and between the atria and cloaca for 22 species of hydrophiine sea snakes. The length between the cranium and atria was positively associated with the frequency of burrowing prey consumed. The number of vertebrae in the pre-atrial region showed a positive association with diet, although the analysis only approached statistical significance. No association was observed between diet and the number of vertebrae between the atria and cloaca, indicating that heart position is constrained with respect to the cloaca. These data indicate that sea snakes specializing on burrowing prey have adapted elongated, anterior regions of the body through an increased number of vertebrae.

Interspecific variation in organ position in hydrophiine snakes is explained by modifications to the vertebral column

Interspecific disparities in the position of the internal organs of snakes have been associated with evolutionary history and cardiovascular performance, as influenced by habitat use. For snakes, the positions of internal organs are typically determined as a linear measurement relative to body length. Therefore, interspecific variation in organ position could be explained either as heterotopic shifts in organ position or by modifications to the vertebral column. Using vertebral counts from radiographs, I determined the positions of the atria and pyloric sphincter relative to the cloaca in hydrophiine sea snakes. I found interspecific variation in the number of pre-atrial vertebrae to be labile, whereas the number of vertebrae in the atria to pyloric sphincter region and in the pyloric sphincter to cloaca region was relatively constrained. Furthermore, the number of pre-atrial vertebrae was dissociated from the number of vertebrae between the atria and cloaca, indicating that these two regions of the vertebral column can evolve independently. I conclude that variation in organ position among hydrophiine sea snake species is attributable, in part, to differences in the number of vertebrae among regions of the vertebral column rather than to heterotopic shifts in the positions of the internal organs.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.

Cell migration during heart regeneration in zebrafish

Zebrafish possess the remarkable ability to regenerate injured hearts as adults, which contrasts the very limited ability in mammals. Although very limited, mammalian hearts do in fact have measurable levels of cardiomyocyte regeneration. Therefore, elucidating mechanisms of zebrafish heart regeneration would provide information of naturally occurring regeneration to potentially apply to mammalian studies, in addition to addressing this biologically interesting phenomenon in itself. Studies over the past 13 years have identified processes and mechanisms of heart regeneration in zebrafish. After heart injury, pre-existing cardiomyocytes dedifferentiate, enter the cell cycle, and repair the injured myocardium. This process requires interaction with epicardial cells, endocardial cells, and vascular endothelial cells. Epicardial cells envelope the heart, while endocardial cells make up the inner lining of the heart. They provide paracrine signals to cardiomyocytes to regenerate the injured myocardium, which is vascularized during heart regeneration. In addition, accumulating results suggest that local migration of these major cardiac cell types have roles in heart regeneration. In this review, we summarize the characteristics of various heart injury methods used in the research community and regeneration of the major cardiac cell types. Then, we discuss local migration of these cardiac cell types and immune cells during heart regeneration. Developmental Dynamics 245:774-787, 2016. © 2016 Wiley Periodicals, Inc.

Expression and function of Ccbe1 in the chick early cardiogenic regions are required for correct heart development

During the course of a differential screen to identify transcripts specific for chick heart/hemangioblast precursor cells, we have identified Ccbe1 (Collagen and calcium-binding EGF-like domain 1). While the importance of Ccbe1 for the development of the lymphatic system is now well demonstrated, its role in cardiac formation remained unknown. Here we show by whole-mount in situ hybridization analysis that cCcbe1 mRNA is initially detected in early cardiac progenitors of the two bilateral cardiogenic fields (HH4), and at later stages on the second heart field (HH9-18). Furthermore, cCcbe1 is expressed in multipotent and highly proliferative cardiac progenitors. We characterized the role of cCcbe1 during early cardiogenesis by performing functional studies. Upon morpholino-induced cCcbe1 knockdown, the chick embryos displayed heart malformations, which include aberrant fusion of the heart fields, leading to incomplete terminal differentiation of the cardiomyocytes. cCcbe1 overexpression also resulted in severe heart defects, including cardia bifida. Altogether, our data demonstrate that although cardiac progenitors cells are specified in cCcbe1 morphants, the migration and proliferation of cardiac precursors cells are impaired, suggesting that cCcbe1 is a key gene during early heart development.

Improved fibroblast functionalities by microporous pattern fabricated by microelectromechanical systems

Fibroblasts, which play an important role in biological seal formation and maintenance, determine the long-term success of percutaneous implants. In this study, well-defined microporous structures with micropore diameters of 10-60 µm were fabricated by microelectromechanical systems and their influence on the fibroblast functionalities was observed. The results show that the microporous structures with micropore diameters of 10-60 µm did not influence the initial adherent fibroblast number; however, those with diameters of 40 and 50 µm improved the spread, actin stress fiber organization, proliferation and fibronectin secretion of the fibroblasts. The microporous structures with micropore diameters of 40-50 µm may be promising for application in the percutaneous part of an implant.

Fibronectin is deposited by injury-activated epicardial cells and is necessary for zebrafish heart regeneration

Unlike adult mammals, adult zebrafish vigorously regenerate lost heart muscle in response to injury. The epicardium, a mesothelial cell layer enveloping the myocardium, is activated to proliferate after cardiac injury and can contribute vascular support cells or provide mitogens to regenerating muscle. Here, we applied proteomics to identify secreted proteins that are associated with heart regeneration. We found that Fibronectin, a main component of the extracellular matrix, is induced and deposited after cardiac damage. In situ hybridization and transgenic reporter analyses indicated that expression of two fibronectin paralogues, fn1 and fn1b, are induced by injury in epicardial cells, while the itgb3 receptor is induced in cardiomyocytes near the injury site. fn1, the more dynamic of these paralogs, is induced chamber-wide within one day of injury before localizing epicardial Fn1 synthesis to the injury site. fn1 loss-of-function mutations disrupted zebrafish heart regeneration, as did induced expression of a dominant-negative Fibronectin cassette, defects that were not attributable to direct inhibition of cardiomyocyte proliferation. These findings reveal a new role for the epicardium in establishing an extracellular environment that supports heart regeneration.

Low temperature mitigates cardia bifida in zebrafish embryos

The coordinated migration of bilateral cardiomyocytes and the formation of the cardiac cone are essential for heart tube formation. We investigated gene regulatory mechanisms involved in myocardial migration, and regulation of the timing of cardiac cone formation in zebrafish embryos. Through screening of zebrafish treated with ethylnitrosourea, we isolated a mutant with a hypomorphic allele of mil (s1pr2)/edg5, called s1pr2^as10 (as10). Mutant embryos with this allele expressed less mil/edg5 mRNA and exhibited cardia bifida prior to 28 hours post-fertilization. Although the bilateral hearts of the mutants gradually fused together, the resulting formation of two atria and one tightly-packed ventricle failed to support normal blood circulation. Interestingly, cardia bifida of s1pr2^as10 embryos could be rescued and normal circulation could be restored by incubating the embryos at low temperature (22.5°C). Rescue was also observed in gata5 and bon cardia bifida morphants raised at 22.5°C. The use of DNA microarrays, digital gene expression analyses, loss-of-function, as well as mRNA and protein rescue experiments, revealed that low temperature mitigates cardia bifida by regulating the expression of genes encoding components of the extracellular matrix (fibronectin 1, tenascin-c, tenascin-w). Furthermore, the addition of N-acetyl cysteine (NAC), a reactive oxygen species (ROS) scavenger, significantly decreased the effect of low temperature on mitigating cardia bifida in s1pr2^as10 embryos. Our study reveals that temperature coordinates the development of the heart tube and somitogenesis, and that extracellular matrix genes (fibronectin 1, tenascin-c and tenascin-w) are involved.

In vitro cellular models for cardiac development and pharmacotoxicology

Permanent cultures of cardiac cells described so far have limited value for studying cell biology and pharmacology of the developing heart because of the loss of proliferative capacity and cardiac-specific properties of cardiomyocytes during long-term cultivation. Pluripotent embryonic carcinoma (EC) and embryonic stem (ES) cells cultivated as permanent lines offer a new approach for studying cardiogenic differentiation in vitro. We describe cardiogenesis in vitro by differentiating EC and ES cells by way of embryo-like aggregates (embryoid bodies) into spontaneously beating cardiomyocytes. During cardiomyocyte differentiation three distinct developmental stages were defined by expression of specific action potentials and ionic currents measured by the whole-cell patch-clamp technique. Whereas early differentiated cardiomyocytes are characterized by action potentials and ionic currents typical for early pacemaker cells, terminally differentiated cardiomyocytes show action potentials and ionic currents inherent to ventricular-, atrial- or sinus nodal-like cells. These functional characteristics are in accordance with the expression of alpha- and beta-cardiac myosin heavy chain at early differentiation stages and the additional expression of ventricular-specific MLC-2V and atrial-specific ANF genes at terminal stages demonstrated by reverse transcription polymerase chain reaction (RT-PCR) analysis. Pharmacological studies performed by measuring chronotropic responses and by analysing the Ca(2+) channel activity correspond to data obtained with cardiac cells from living organisms. For testing the influence of exogenous compounds on cardiac differentiation the teratogenic compound retinoic acid (RA) was applied during distinct stages of embryoid body development. A temporally controlled influence of RA on cardiac differentiation and expression of cardiac-specific genes was found. We conclude that ES cell-derived cardiomyocytes provide an excellent cellular model to study early cardiac development and to perform pharmacological and embryotoxicological investigations.

Not just inductive: a crucial mechanical role for the endoderm during heart tube assembly

The heart is the first functioning organ to form during development. During gastrulation, the cardiac progenitors reside in the lateral plate mesoderm but maintain close contact with the underlying endoderm. In amniotes, these bilateral heart fields are initially organized as a pair of flat epithelia that move towards the embryonic midline and fuse above the anterior intestinal portal (AIP) to form the heart tube. This medial motion is typically attributed to active mesodermal migration over the underlying endoderm. In this model, the role of the endoderm is twofold: to serve as a mechanically passive substrate for the crawling mesoderm and to secrete various growth factors necessary for cardiac specification and differentiation. Here, using computational modeling and experiments on chick embryos, we present evidence supporting an active mechanical role for the endoderm during heart tube assembly. Label-tracking experiments suggest that active endodermal shortening around the AIP accounts for most of the heart field motion towards the midline. Results indicate that this shortening is driven by cytoskeletal contraction, as exposure to the myosin-II inhibitor blebbistatin arrested any shortening and also decreased both tissue stiffness (measured by microindentation) and mechanical tension (measured by cutting experiments). In addition, blebbistatin treatment often resulted in cardia bifida and abnormal foregut morphogenesis. Moreover, finite element simulations of our cutting experiments suggest that the endoderm (not the mesoderm) is the primary contractile tissue layer during this process. Taken together, these results indicate that contraction of the endoderm actively pulls the heart fields towards the embryonic midline, where they fuse to form the heart tube.

N-cadherin cell-cell adhesion complexes are regulated by fibronectin matrix assembly

Fibronectin is a principal component of the extracellular matrix. Soluble fibronectin molecules are assembled into the extracellular matrix as insoluble, fibrillar strands via a cell-dependent process. In turn, the interaction of cells with the extracellular matrix form of fibronectin stimulates cell functions critical for tissue repair. Cross-talk between cell-cell and cell-extracellular matrix adhesion complexes is essential for the organization of cells into complex, functional tissue during embryonic development and tissue remodeling. Here, we demonstrate that fibronectin matrix assembly affects the organization, composition, and function of N-cadherin-based adherens junctions. Using fibronectin-null mouse embryonic myofibroblasts, we identified a novel quaternary complex composed of N-cadherin, β-catenin, tensin, and actin that exists in the absence of a fibronectin matrix. In the absence of fibronectin, homophilic N-cadherin ligation recruited both tensin and α5β1 integrins into nascent cell-cell adhesions. Initiation of fibronectin matrix assembly disrupted the association of tensin and actin with N-cadherin, released α5β1 integrins and tensin from cell-cell contacts, stimulated N-cadherin reorganization into thin cellular protrusions, and decreased N-cadherin adhesion. Fibronectin matrix assembly has been shown to recruit α5β1 integrins and tensin into fibrillar adhesions. Taken together, these studies suggest that tensin serves as a common cytoskeletal link for integrin- and cadherin-based adhesions and that the translocation of α5β1 integrins from cell-cell contacts into fibrillar adhesions during fibronectin matrix assembly is a novel mechanism by which cell-cell and cell-matrix adhesions are coordinated.

Calcium channel blockade in embryonic cardiac progenitor cells disrupts normal cardiac cell differentiation

We suggest that characterization of processes involved in differentiation of the pluripotential cardiac precursor cells in their embryonic environment will permit identifying pathways important for induction of diverse stem cells toward the cardiac phenotype. Phenotypic characteristics of cardiac cells are their contractile and electrical properties. The objective of the present study was to define whether calcium (Ca(++)) has a regulatory role in the pluripotential precursor cell population during commitment into cardiomyocytes. We used the chick embryo model because of ease of staging the embryos and visibility of heart development. Using the Ca(++) indicator Fluo-3/acetoxymethyl and confocal microscopy, we demonstrated the existence of higher free Ca(++) levels in the cardiogenic precursor cells than in neighboring cell populations outside of the heart fields. Subsequently, gastrulation stage 4/5 chick embryos were set up in modified New cultures in the medium containing either the L-type Ca channel blocker, diltiazem, or the N-type Ca channel inhibitor, ω-conotoxin. The embryos were incubated for 22-24 h during which time the control embryos developed, beating looping hearts. At the end of incubation, exposure to the L-type channel blockade with diltiazem resulted in an inhibition of cardiomyogenesis in the most posterior, uncommitted, part of the heart fields. N-type channel blockade with ω-conotoxin was less intense. Cells in the most anterior cardiogenic regions that were already committed at time of exposure continued to differentiate. Thus, regulation and maintenance of normal cytosolic Ca levels are necessary for the early steps of cardiomyocyte specification and commitment leading to differentiation.

Hand2 ensures an appropriate environment for cardiac fusion by limiting Fibronectin function

Heart formation requires the fusion of bilateral cardiomyocyte populations as they move towards the embryonic midline. The bHLH transcription factor Hand2 is essential for cardiac fusion; however, the effector genes that execute this function of Hand2 are unknown. Here, we provide in zebrafish the first evidence for a downstream component of the Hand2 pathway that mediates cardiac morphogenesis. Although hand2 is expressed in cardiomyocytes, mosaic analysis demonstrates that it plays a non-autonomous role in regulating cardiomyocyte movement. Gene expression profiles reveal heightened expression of fibronectin 1 (fn1) in hand2 mutant embryos. Reciprocally, overexpression of hand2 leads to decreased Fibronectin levels. Furthermore, reduction of fn1 function enables rescue of cardiac fusion in hand2 mutants: bilateral cardiomyocyte populations merge and exhibit improved tissue architecture, albeit without major changes in apicobasal polarity. Together, our data provide a novel example of a tissue creating a favorable environment for its morphogenesis: the Hand2 pathway establishes an appropriate environment for cardiac fusion through negative modulation of Fn1 levels.

The extracellular matrix in development and morphogenesis: a dynamic view

The extracellular matrix (ECM) is synthesized and secreted by embryonic cells beginning at the earliest stages of development. Our understanding of ECM composition, structure and function has grown considerably in the last several decades and this knowledge has revealed that the extracellular microenvironment is critically important for cell growth, survival, differentiation and morphogenesis. ECM and the cellular receptors that interact with it mediate both physical linkages with the cytoskeleton and the bidirectional flow of information between the extracellular and intracellular compartments. This review considers the range of cell and tissue functions attributed to ECM molecules and summarizes recent findings specific to key developmental processes. The importance of ECM as a dynamic repository for growth factors is highlighted along with more recent studies implicating the 3-dimensional organization and physical properties of the ECM as it relates to cell signaling and the regulation of morphogenetic cell behaviors. Embryonic cell and tissue generated forces and mechanical signals arising from ECM adhesion represent emerging areas of interest in this field.

The role of mechanical forces in the torsional component of cardiac looping

During early development, the initially straight heart tube (HT) bends and twists (loops) into a curved tube to lay out the basic plan of the mature heart. The physical mechanisms that drive and regulate looping are not yet completely understood. This paper reviews our recent studies of the mechanics of cardiac torsion during the first phase of looping (c-looping). Experiments and computational modeling show that torsion is primarily caused by forces exerted on the HT by the primitive atria and the splanchnopleure, a membrane that presses against the ventral surface of the heart. Experimental and numerical results are described and integrated to propose a hypothesis for cardiac torsion, and key aspects of our hypothesis are tested using experiments that perturb normal looping. For each perturbation, the models predict the correct qualitative response. These studies provide new insight into the mechanisms that drive and regulate cardiac looping.

Extra-embryonic syndecan 2 regulates organ primordia migration and fibrillogenesis throughout the zebrafish embryo

One of the first steps in zebrafish heart and gut organogenesis is the migration of bilateral primordia to the midline to form cardiac and gut tubes. The mechanisms that regulate this process are poorly understood. Here we show that the proteoglycan syndecan 2 (Sdc2) expressed in the extra-embryonic yolk syncytial layer (YSL) acts locally at the YSL-embryo interface to direct organ primordia migration, and is required for fibronectin and laminin matrix assembly throughout the embryo. Surprisingly, neither endogenous nor exogenous sdc2 expressed in embryonic cells can compensate for knockdown of sdc2 in the YSL, indicating that Sdc2 expressed in extra-embryonic tissues is functionally distinct from Sdc2 in embryonic cells. The effects of sdc2 knockdown in the YSL can be rescued by extra-embryonic Sdc2 lacking an extracellular proteolytic cleavage (shedding) site, but not by extra-embryonic Sdc2 lacking extracellular glycosaminoglycan (GAG) addition sites, suggesting that distinct GAG chains on extra-embryonic Sdc2 regulate extracellular matrix assembly, cell migration and epithelial morphogenesis of multiple organ systems throughout the embryo.

Cellular nonmuscle myosins NMHC-IIA and NMHC-IIB and vertebrate heart looping

Flectin, a protein previously described to be expressed in a left-dominant manner in the embryonic chick heart during looping, is a member of the nonmuscle myosin II (NMHC-II) protein class. During looping, both NMHC-IIA and NMHC-IIB are expressed in the mouse heart on embryonic day 9.5. The patterns of localization of NMHC-IIB, rather than NMHC-IIA in the mouse looping heart and in neural crest cells, are equivalent to what we reported previously for flectin. Expression of full-length human NMHC-IIA and -IIB in 10 T1/2 cells demonstrated that flectin antibody recognizes both isoforms. Electron microscopy revealed that flectin antibody localizes in short cardiomyocyte cell processes extending from the basal layer of the cardiomyocytes into the cardiac jelly. Flectin antibody also recognizes stress fibrils in the cardiac jelly in the mouse and chick heart; while NMHC-IIB antibody does not. Abnormally looping hearts of the Nodal(Delta 600) homozygous mouse embryos show decreased NMHC-IIB expression on both the mRNA and protein levels. These results document the characterization of flectin and extend the importance of NMHC-II and the cytoskeletal actomyosin complex to the mammalian heart and cardiac looping.

Asymmetric involution of the myocardial field drives heart tube formation in zebrafish

Many vertebrate organs are derived from monolayered epithelia that undergo morphogenesis to acquire their shape. Whereas asymmetric left/right gene expression within the zebrafish heart field has been well documented, little is known about the tissue movements and cellular changes underlying early cardiac morphogenesis. Here, we demonstrate that asymmetric involution of the myocardium of the right-posterior heart field generates the ventral floor, whereas the noninvoluting left heart field gives rise to the dorsal roof of the primary heart tube. During heart tube formation, asymmetric left/right gene expression within the myocardium correlates with asymmetric tissue morphogenesis. Disruption of left/right gene expression causes randomized myocardial tissue involution. Time-lapse analysis combined with genetic analyses reveals that motility of the myocardial epithelium is a tissue migration process. Our results demonstrate that asymmetric morphogenetic movements of the 2 bilateral myocardial cell populations generate different dorsoventral regions of the zebrafish heart tube. Failure to generate a heart tube does not affect the acquisition of atrial versus ventricular cardiac cell shapes. Therefore, establishment of basic cardiac cell shapes precedes cardiac function. Together, these results provide the framework for the integration of single cell behaviors during the formation of the vertebrate primary heart tube.

miles-apart-Mediated regulation of cell-fibronectin interaction and myocardial migration in zebrafish

The migration of myocardial precursor cells towards the embryonic midline underlies the formation of the heart tube and is a key process of heart organogenesis. The zebrafish mutation miles-apart (mil), which affects the gene encoding a sphingosine-1-phosphate receptor, is characterized by defective migration of myocardial precursor cells and results in the formation of two laterally positioned hearts, a condition known as cardia bifida. The mechanism that disrupts myocardial migration in mil mutants remains largely unclear. To investigate how mil regulates this process, here we analyze the interactions between mil and other mediators of myocardial migration. We show that mil function is associated with the other known cardia bifida locus, natter/fibronectin (nat/fn), which encodes fibronectin, a major component of the extracellular matrix, in the control of myocardial migration. By using a primary culture system of embryonic zebrafish cells, we also show that signaling from the sphingosine-1-phosphate receptor regulates cell-fibronectin interactions in zebrafish. In addition, localized inhibition and activation of cell-fibronectin interactions during the stages of myocardial migration reveal that the temporal regulation of cell-fibronectin interaction by mil is required for proper myocardial migration. Our study reveals novel functional links between sphingosine-1-phosphate receptor signaling and cell-fibronectin interaction in the control of myocardial migration during zebrafish heart organogenesis.

Early temporal-specific responses and differential sensitivity to lithium and Wnt-3A exposure during heart development

Members of both Wnt and bone morphogenetic protein (BMP) families of signaling molecules are important in heart development. We previously demonstrated that beta-catenin, a key downstream intermediary of the canonical Wnt signaling pathway, delineates the dorsal boundary of the cardiac compartments in an anteroposterior progression. We hypothesized the progression involves canonical Wnt signaling and reflects development of the primary body axis of the embryo. A similar anteroposterior signaling wave leading to cardiac cell specification involves inductive signaling by BMP-2 synthesized by the underlying endoderm in anterior bilateral regions. Any molecule that disrupts the normal balance of Wnt and BMP concentrations within the heart field may be expected to affect early heart development. The canonical Wnt signaling step mimicked by lithium involves inactivation of glycogen synthase kinase-3beta (GSK-3beta; Klein and Melton [1996] Proc. Natl. Acad. Sci. U. S. A. 93:8455-8459). We show that lithium, Wnt-3A, and an inhibitor of GSK-3beta, SB415286, affect early heart development at the cardiac specification stages. We demonstrate that normal expression patterns of key signaling molecules as Notch-1 and Dkk-1 are altered in the anterior mesoderm within the heart fields by a one-time exposure to lithium, or by noggin inhibition of BMP, at Hamburger and Hamilton (HH) stage 3 during chick embryonic development. The severity of developmental defects is greatest with exposure to lithium or Wnt-3A at HH stage 3 and decreases at HH stage 4. Taken together, our results demonstrate that there are temporal-specific responses and differential sensitivities to lithium/Wnt-3A exposure during early heart development.

Cardiac morphogenesis: matrix metalloproteinase coordination of cellular mechanisms underlying heart tube formation and directionality of looping

During heart organogenesis, the spatiotemporal organization of the extracellular matrix (ECM) undergoes significant remodeling. Because matrix metalloproteinases (MMPs) are known to be key regulators of cell-matrix interactions, we analyzed the role(s) of MMPs, and specifically MMP-2, in early heart development. Both MMP-2 neutralizing antibody and the broad-spectrum MMP inhibitor Ilomastat in a temporal manner, when applied between chick embryonic stages 5 (primitive streak stage) to stage 12 ( approximately 16-somites), produced severe heart tube defects. Exposure to the MMP inhibitor at stage 5 produced various degrees of cardia bifida. At the seven-somite stage, MMP-2/Ilomastat inhibition caused a shift in normal left-right patterning of cell proliferation within the dorsal mesocardium and mesoderm of the anterior heart field that correlated with a change in looping direction. MMP inhibition at the 10- to 12-somite stage resulted in an arrest of heart tube bending by inhibiting the breakdown of the dorsal mesocardial ECM. The experimental observations suggest that MMP activity regulates the coordination of early heart organogenesis by affecting ventral closure of the heart and gut tubes, asymmetric cell proliferation in the dorsal mesocardium to drive looping direction, and ECM degradation within the dorsal mesocardium allowing looping to proceed toward completion.

Cross talk between cell-cell and cell-matrix adhesion signaling pathways during heart organogenesis: implications for cardiac birth defects

The anterior-posterior and dorsal-ventral progression of heart organogenesis is well illustrated by the patterning and activity of two members of different families of cell adhesion molecules: the calcium-dependent cadherins, specifically N-cadherin, and the extracellular matrix glycoproteins, fibronectin. N-cadherin by its binding to the intracellular molecule beta-catenin and fibronectin by its binding to integrins at focal adhesion sites, are involved in regulation of gene expression by their association with the cytoskeleton and through signal transduction pathways. The ventral precardiac mesoderm cells epithelialize and become stably committed by the activation of these cell-matrix and intracellular signaling transduction pathways. Cross talk between the adhesion signaling pathways initiates the characteristic phenotypic changes associated with cardiomyocyte differentiation: electrical activity and organization of myofibrils. The development of both organ form and function occurs within a short interval thereafter. Mutations in any of the interacting molecules, or environmental insults affecting either of these signaling pathways, can result in embryonic lethality or fetuses born with severe heart defects. As an example, we have defined that exposure of the embryo temporally to lithium during an early sensitive developmental period affects a canonical Wnt pathway leading to beta-catenin stabilization. Lithium exposure results in an anterior-posterior progression of severe cardiac defects.

The role of mechanical forces in dextral rotation during cardiac looping in the chick embryo

Cardiac looping is a vital morphogenetic process that transforms the initially straight heart tube into a curved tube normally directed toward the right side of the embryo. While recent work has brought major advances in our understanding of the genetic and molecular pathways involved in looping, the biophysical mechanisms that drive this process have remained poorly understood. This paper examines the role of biomechanical forces in cardiac rotation during the initial stages of looping, when the heart bends and rotates into a c-shaped tube (c-looping). Embryonic chick hearts were subjected to mechanical and chemical perturbations, and tissue stress and strain were studied using dissection and fluorescent labeling, respectively. The results suggest that (1) the heart contains little or no intrinsic ability to rotate, as external forces exerted by the splanchnopleure (SPL) and the omphalomesenteric veins (OVs) drive rotation; (2) unbalanced forces in the omphalomesenteric veins play a role in left-right looping directionality; and (3) in addition to ventral bending and rightward rotation, the heart tube also bends slightly toward the right. The results of this study may help investigators searching for the link between gene expression and the mechanical processes that drive looping.

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

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

Trophoblast differentiation in vitro: establishment and characterisation of a serum-free culture model for murine secondary trophoblast giant cells

Differentiation of trophoblast giant cells is an early event during the process of murine embryo implantation. However, differentiation of secondary trophoblast giant cells in the rodent is still only partially understood, probably because of the lack of suitablein vitromodels and cell markers. In order to advance our understanding of trophoblast differentiation, suitablein vitromodels and markers are required to study their development. The objectives of this study were to establish and characterise a serum-freein vitromodel for murine secondary trophoblast cells. Secondary trophoblast giant cells growingin vitroand paraffin sections of day 8.5 postcoitum mouse embryos were processed for immunostaining to establish the expression of potential markers using antibodies to blood group antigens, E-cadherin, α7integrins and activator protein-γ, as well as placental lactogen-II. Within 3 days in serum-free culture, ectoplacental cone-derived secondary trophoblast cells underwent simultaneous induction of both morphological and functional differentiation. Secondary trophoblasts grewin vitroas a monolayer of cells with giant nuclei and expressed B and Le-b/Le-y blood group antigens, α7integrins and placental lactogen-II, as well as activator protein-γ. Transcripts for activator protein-γ and placental lactogen-II were detected in cultures by RT-PCR and for placental lactogen-II byin situhybridisation. At later time-points apoptosis increased. A fibronectin substrate significantly increased secondary trophoblast cell numbers and surface area of outgrowth. The increase in cells with giant nuclei coincided with induction of placental lactogen-II expression. A relationship was found between the nuclear area of secondary trophoblast cells and expression of placental lactogen-II.

Fibronectin regulates epithelial organization during myocardial migration in zebrafish

Several genes have been implicated in heart tube formation, yet we know little about underlying cellular mechanisms. We analyzed the cellular architecture of the migrating myocardial precursors, and find that they form coherent epithelia that mature as they move medially. Mutant analyses indicate that the cardia bifida locus natter (nat) is required for the integrity of the myocardial epithelia. We positionally cloned nat and show that it encodes Fibronectin. During myocardial migration, Fibronectin is deposited at the midline between the endoderm and endocardial precursors, and laterally around the myocardial precursors. Further analyses show that Fibronectin deposition at the midline is required for the timely migration of myocardial precursors, but dispensable for the migration process itself. In the complete absence of Fibronectin, adherens junctions between myocardial precursors do not form properly, suggesting that cell-substratum interactions are required for epithelial organization. These data suggest that myocardial migration is dependent on epithelial integrity.

Coordinating morphogenesis: epithelial integrity during heart tube assembly

Formation of the embryonic heart tube requires the medial migration and merger of bilateral precursor populations. A new study of zebrafish cardiogenesis published in this issue of Developmental Cell indicates that precursor migration involves formation of a coherent epithelium and that fibronectin plays an important role in maintaining cardiac epithelial integrity.

Increased fibronectin deposition in embryonic hearts of retinol-binding protein-null mice

Precise regulation of retinoid levels is critical for normal heart development. Retinol-binding protein (RBP), an extracellular retinol transporter, is strongly secreted by cardiogenic endoderm. This study addresses whether RBP gene ablation affects heart development. Despite exhibiting an >85% decrease in serum retinol, adult RBP-null mice are viable, breed, and have normal vision when maintained on a vitamin A-sufficient diet. Comparison of RBP-null with wild-type (WT) hearts from embryos at day 9.0 (E9.0) through E12.5 revealed an RBP-null phenotype similar to that of other retinoid-deficient models. At an early stage, RBP-null hearts display retarded trabecular development, which recovers by E9.5. This is accompanied at E9.5 and E10.5 by precocious differentiation of subepicardial cardiac myocytes. Most remarkably, RBP-null hearts display augmented deposition of fibronectin protein in the cardiac jelly at E9.0 through E10.5 and in the outflow tract at E12.5. This phenomenon, which was detected by immunohistochemistry and Western blotting without increased fibronectin transcript levels, is accompanied by increased numbers of mesenchymal cells in the outflow tract but not in the atrioventricular canal. RBP-null cardiac myocytes, especially in the subepicardial layer, display increased cell proliferation. This phenotype may present a model of subclinical retinoid insufficiency characterized by alteration of an extracellular matrix component and altered cellular differentiation and proliferation, changes that may have functional consequences for adult cardiac function. This murine model may have relevance to fetal development in human populations with inadequate retinoid intake.

Regulation of heart morphology: current molecular and cellular perspectives on the coordinated emergence of cardiac form and function

BACKGROUND

During early heart development, in addition to cells being induced to differentiate into cardiomyocytes, pathways are activated that lead to cardiac morphogenesis or the development of form.

METHODS

Orchestration of organogenesis involves the incremental activation of regulatory pathways that lead to pivotal transition points, such as cardiac compartment delineation and looping. Each embryonic stage sets up the correct patterning of morphoregulatory molecules that will regulate the next process, until an organ is formed from the mesoderm layer after gastrulation. The current review provides an understanding of the morphoregulatory, cell adhesion and extracellular matrix-mediated, processes that coordinate development of heart form with that of function. The period reviewed encompasses the formation of a definitive cardiac compartment from the lateral plate mesoderm to the time-point in which the single, beating heart tube loops directionally to the right. Looping results in the correct spatial orientation for subsequent modeling of the four-chambered heart. Even subtle alterations in looping can form the basis upon which malformations of the inlet or the outlet regions of the heart, or both, are superimposed.

RESULTS

In the future, DNA microarray data sets may allow modeling the specific sequence of gene regulatory dynamics leading to these transition points to discover the regulatory "modes" that the cells adopt during heart organogenesis. The regulatory genes, however, can only specify the proteins that will be present.

CONCLUSIONS

To fully understand the timing and mechanisms underlying heart development, it is necessary to define the sequential synthesis, patterning, and interaction of the proteins, and of still other receptors, which eventually drive cells to organize into functioning organs.

Directionality of heart looping: effects of Pitx2c misexpression on flectin asymmetry and midline structures

A critical regulatory laterality gene expressed in the left side of the straight heart tube during development is Pitx2, which when mutated in humans underlies Rieger's Syndrome. Previously reported results have indicated that, when using gain-of-function and loss-of function approaches of the chick cPitx2c isoform, this results in randomization of heart looping. To determine whether Pitx2c misexpression affects downstream morphogenesis by altering the expression of specific proteins in the myocardium during looping, after experimental manipulations, we analyzed immunohistochemically for the extracellular matrix molecule flectin that normally is expressed predominantly in the left lateral plate mesoderm (LPM) and left side of the straight chick heart tube before and during looping. We show here that the left-side predominance of flectin is due to a delay in the timing of expression in one heart field vs the other. Experimental results indicate that misexpression of Pitx2c in the heart fields using antisense or retroviral delivery perturbs the normal temporal pattern of flectin expression in the left LPM relative to the right: abnormally leftward looping hearts show predominate right-sided flectin expression in the dorsal mesocardial regions around the foregut ventral midline. Additionally, Pitx2c misexpression affects the positioning of the developing foregut to more lateral areas, either on the right or left side of the embryonic midline. The position of the heart with respect to the embryo midline is defined by the position of the foregut. Incubating embryos in the presence of flectin antibody caused randomization of heart looping or no looping.

Cellular interactions in vascular growth and differentiation

In nature, mammalian cells do not exist in isolation, but rather are involved in interactions with other cells and matrix. In this review, several aspects of cellular interactions that are important in vascular growth and development will be highlighted. The cardiovascular system is the earliest to develop in the embryo. A number of growth factors and their receptors mediate the complex stages of migration, assembly, organization, and stabilization of developing vessels. In the adult organism, normal angiogenesis is restricted primarily to tissue growth (such as muscle and fat), the wound healing process and the female reproductive system. However, pathological angiogenesis, such as with tumor growth, diabetic retinopathy, and arthritis, is of great concern. The identification and/or development of exogenous and endogenous angiogenesis inhibitors has added to the understanding of these pathological processes. In addition to cellular interactions via ligands and receptors, cells also interact directly through physical contacts. These interactions facilitate anchorage, communication, and permeability. Since vessels serve as non-leaky conduits for blood flow as well as interfaces for molecular diffusion, the physical interactions between the cells that make up vessels must be specific for the function at hand. Permeability is a specialized function of vessels and is mediated by intracellular mechanisms and intercellular interactions. Cells also interact with the surrounding extracellular matrix. Integrin-matrix interaction is a two-way exchange critical for angiogenesis. Matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases play major roles in embryonic remodeling, adult injury, and pathological conditions. Several experimental model systems have been useful in our understanding of cellular interactions. These in vitro models incorporate heterotypic cell-cell interactions and/or allow cell-matrix interactions to occur.

Embryonic retinoic acid synthesis is essential for heart morphogenesis in the mouse

Retinoic acid (RA), the active derivative of vitamin A, has been implicated in various steps of cardiovascular development, but its contribution to early heart morphogenesis has not been clearly established in a mammalian system. To block endogenous RA synthesis, we have disrupted the gene encoding RALDH2, the first retinaldehyde dehydrogenase whose expression has been detected during early mouse post-implantation development. We describe here the heart abnormalities of the RA-deficient Raldh2 mutants that die in utero at gestational day 10.5. The embryonic heart tube forms properly, but fails to undergo rightward looping and, instead, forms a medial distended cavity. Expression of early heart determination factors is not altered in mutants, and the defect in heart looping does not appear to involve the Nodal/Lefty/Pitx2 pathway. Histological and molecular analysis reveal distinct anteroposterior components in the mutant heart tube, although posterior chamber (atria and sinus venosus) development is severely impaired. Instead of forming trabeculae, the developing ventricular myocardium consists of a thick layer of loosely attached cells. Ultrastructural analysis shows that most of the ventricular wall consists of prematurely differentiated cardiomyocytes, whereas undifferentiated cells remain clustered rostrally. We conclude that embryonic RA synthesis is required for realization of heart looping, development of posterior chambers and proper differentiation of ventricular cardiomyocytes. Nevertheless, the precise location of this synthesis may not be crucial, as these defects can mostly be rescued by systemic (maternal) RA administration. However, cardiac neural crest cells cannot be properly rescued in Raldh2(−/−)embryos, leading to outflow tract septation defects.

Expression and cellular distribution of alpha(v)integrins in beta(1)integrin-deficient embryonic stem cell-derived cardiac cells

beta(1)integrin-deficient (beta(1)-/-) ES cells showed increased differentiation of cardiac cells characterized by reduced adhesion and high beating frequency. Whereas in whole embryoid body outgrowths of beta(1)-/- cells maximum levels of alpha(v), beta(3)and beta(5)integrin mRNA were delayed and transiently upregulated, in cardiac clusters isolated from beta(1)-/- cells, only beta(3)integrin mRNA levels were enhanced in comparison to wild-type (wt) cells. To answer the question, whether alpha(v)and beta(3)integrins may compensate, at least partially, the loss of beta(1)integrin function during cardiac differentiation, the distribution of alpha(v)and beta(3)integrins in beta(1)-/- and wt pacemaker-like cardiac cells was analyzed. A different distribution of alpha(v)and beta(3)integrins in beta(1)-/- v wt cardiac cells was found. In wt cardiac cells, beta(1)integrin was localized in specialized subsarcolemmal regions, in particular, at focal contacts and costameres, but alpha(v)integrin was diffusely distributed. In contrast, in beta(1)-/- cardiac cells, alpha(v)integrin was preponderantly localized at cell membranes, focal contacts and costameres. beta(3)integrin displayed a diffuse pattern both in wt and in beta(1)-/- pacemaker-like cells at early differentiation stages, whereas at terminal stages, beta(3)was colocalized with sarcomeres in wt, but not in beta(1)-/- pacemaker-like cells. Quantitative immunofluorescence analysis revealed increased alpha(v)and beta(3)integrin levels in beta(1)-/- pacemaker-like cardiac cells. Our results led us to conclude that altered cellular distribution of alpha(v)integrin and upregulation of beta(3)integrin correlate with growth and survival of beta(1)-/- cardiac pacemaker-like cells at an early developmental state. However, alpha(v)and beta(3)integrins cannot functionally compensate the loss of beta(1)integrin during terminal differentiation of cardiac cells implicating that cardiomyocytes require specific beta(1)integrin functions for cardiac specialization.

Wnt/frizzled-2 signaling induces aggregation and adhesion among cardiac myocytes by increased cadherin-beta-catenin complex

Wingless is known to be required for induction of cardiac mesoderm in Drosophila, but the function of Wnt family proteins, vertebrate homologues of wingless, in cardiac myocytes remains unknown. When medium conditioned by HEK293 cells overexpressing Wnt-3a or -5a was applied to cultured neonatal cardiac myocytes, Wnt proteins induced myocyte aggregation in the presence of fibroblasts, concomitant with increases in beta-catenin and N-cadherin in the myocytes and with E- and M-cadherins in the fibroblasts. The aggregation was inhibited by anti-N-cadherin antibody and induced by constitutively active beta-catenin, but was unaffected by dominant negative and dominant positive T cell factor (TCF) mutants. Thus, increased stabilization of complexed cadherin-beta-catenin in both cell types appears crucial for the morphological effect of Wnt on cardiac myocytes. Furthermore, myocytes overexpressing a dominant negative frizzled-2, but not a dominant negative frizzled-4, failed to aggregate in response to Wnt, indicating frizzled-2 to be the predominant receptor mediating aggregation. By contrast, analysis of bromodeoxyuridine incorporation and transcription of various cardiogenetic markers showed Wnt to have little or no impact on cell proliferation or differentiation. These findings suggest that a Wnt-frizzled-2 signaling pathway is centrally involved in the morphological arrangement of cardiac myocytes in neonatal heart through stabilization of complexed cadherin- beta-catenin.

Common role for each of the cGATA-4/5/6 genes in the regulation of cardiac morphogenesis

The GATA-4/5/6 genes encode transcription factors implicated previously in the regulation of cardiac-specific differentiation programs. However, recent analyses of mouse GATA-4 null mutations found evidence for function in endoderm development (in vitro) and embryonic morphogenesis (in vivo). Whether each of the three cardiac-associated GATA factors function within distinct or common developmental programs was previously untested; past studies defined specific and distinct roles for each of the GATA-1/2/3 genes in embryonic hematopoiesis. In this study, we compare the transcript patterns of cGATA-4/5/6 during chick embryogenesis. Each of the three GATA factors is expressed in a similar pattern within gastrulating cells of the primitive streak, prior to determination of the cardiomyocyte progenitors, and later within the lateral plate mesoderm and associated endoderm layer. The patterns overlap but extend beyond the presumptive cardiomyocyte population expressing cNkx-2.5. Later in development, cGATA-4/5/6 are all transcribed throughout the differentiating heart, in similar but not identical patterns, within the endocardium, myocardium, and great vessels. In order to test the function of GATA factors during chick cardiogenesis, embryos were cultured in vitro in the presence of antisense oligomers designed to deplete specifically transcripts encoding cGATA-4/5/6, beginning around stage 7. When oligomers are used to target transcripts for all three genes, a high percentage of the embryos develop abnormal hearts related to the failure to form a normal primitive heart tube. In the most severe phenotype, cardiac bifida results in two bilateral beating hearts. In some embryos, the paired heart primordia undergo partial fusion but fail to form a single looping heart tube. In all cases, cellular differentiation is not obviously affected, as the abnormal hearts form beating tissue. Depletion of transcripts encoding any single GATA factor, or any combination of two GATA factors, does not affect development. The partial depletion of all three genes in chick results in a remarkably similar phenotype compared to the null GATA-4 mutation in mouse. Therefore, in the chick, each of the GATA-4/5/6 genes functions in a common pathway, at the time of cardiac crescent formation, for regulating early embryonic cardiac morphogenesis, apparently associated with embryonic folding or the migration of primordia to form a primitive tube.

Differential expression of flectin in the extracellular matrix and left-right asymmetry in mouse embryonic heart during looping stages

A novel extracellular matrix protein flectin (250 kD M(r)) shows specific left-right asymmetric expression before and throughout the looping process during heart development in avian embryos [Tsuda et al., 1996]. Flectin is a candidate molecule to provide directionality to the looping process in the avian model. In this study on mouse embryonic heart development, flectin is shown to be developmentally regulated and to be expressed in a specific asymmetric fashion, but in a different pattern from that observed in avian hearts. The molecules involved in development tend to be the same, but timing of expression, modulation, and asymmetry are different. In the mouse embryo, flectin is expressed symmetrically when the cardiogenic plate is formed. As looping progresses, flectin expression becomes asymmetric. There is right side predominance at the outflow tract and left side predominance at the ventricular portion of the tubular heart. The left side predominance of flectin develops in an anteroposterior direction, while right side predominance of the outflow tract remains relatively unchanged. These differential expression patterns of flectin decrease once the looping process is completed. After looping, flectin becomes restricted to the epicardium and subepicardial extracellular regions. In inv/inv mice, a known mouse model for human situs inversus, in which the directionality of heart looping is inverted, flectin expression pattern is mirror image of that of normal mouse embryos during looping stages. Our study indicates that, in the mouse, flectin shows a specific asymmetric expression pattern after initiation of heart looping and that this asymmetric expression pattern is related to the directionality of looping. The remodeling of the extracellular matrix (ECM) including specific flectin expression begins with the looping process. This morphogenetic change of the ECM coincides with the differentiation of each region of the tubular heart.

MMP-2 expression during early avian cardiac and neural crest morphogenesis

Matrix metalloproteinase-type 2 (MMP-2) degrades extracellular matrix, mediates cell migration and tissue remodeling, and is implicated in mediating neural crest (NC) and cardiac development. However, there is little information regarding the expression and distribution of MMP-2 during cardiogenesis and NC morphogenesis. To elucidate the role of MMP-2, we performed a comprehensive study on the temporal and spatial distribution of MMP-2 mRNA and protein during critical stages of early avian NC and cardiac development. We found that ectodermally derived NC cells did not express MMP-2 mRNA during their initial formation and early emigration but encountered MMP-2 protein in basement membranes deposited by mesodermal cells. While NC cells did not synthesize MMP-2 mRNA early in migration, MMP-2 expression was seen in NC cells within the cranial paraxial and pharyngeal arch mesenchyme at later stages but was never detected in NC-derived neural structures. This suggested NC MMP-2 expression was temporally and spatially dependent on tissue interactions or differed within the various NC subpopulations. MMP-2 was first expressed within cardiogenic splanchnic mesoderm before and during the formation of the early heart tube, at sites of active pharyngeal arch and cardiac remodeling, and during cardiac cushion cell migration. Collectively, these results support the postulate that MMP-2 has an important functional role in early cardiogenesis, NC cell and cardiac cushion migration, and remodeling of the pharyngeal arches and cardiac heart tube.

Knowing left from right: the molecular basis of laterality defects

The apparent symmetry of the vertebrate body conceals profound asymmetries in the development and placement of internal organs. Asymmetric organ development is controlled in part by genes expressed asymmetrically in the early embryo, and alterations in the activities of these genes can result in severe defects during organogenesis. Recently, data from different vertebrates have allowed researchers to put forward a model of genetic interactions that explains how asymmetric patterns of gene expression in the early embryo are translated into spatial patterns of asymmetric organ development. This model helps us to understand the molecular basis of a number of congenital malformations in humans.

N-cadherin/catenin-mediated morphoregulation of somite formation

Somitogenesis during early stages in the chick and mouse embryo was examined in relation to N-cadherin-mediated adhesion. Previous studies indicated that N-cadherin localizes to the somite regions during their formation. Those observations were extended to include a spatiotemporal immunohistochemical analyses of beta-catenin and alpha-catenin, as well as a more detailed study of N-cadherin, during segmentation, compaction, and compartmentalization of the somite. N-cadherin and the catenins appear early within the segmental plate and are expressed as small patch-like foci throughout this tissue. The small foci of immunostaining coalesce into larger clusters of N-cadherin/catenin-expressing regions. The clusters subsequently coalesce into a region of centrally localized cells that express N-cadherin/catenins at their apical surfaces. The multiple clusters are spaced wide apart in the anterior segmental plates that form the first 6 somite pairs, as contrasted to segmental plates that form somites 7 and beyond. To examine the functional significance of N-cadherin, segmental plates were exposed to antibodies that perturb N-cadherin-mediated adhesion in the chick embryo. The multiple, anomalous somites that result in these experiments indicate that each N-cadherin/catenin-expressing cluster can give rise to a somitic structure. beta-Catenin involvement in somitogenesis suggests a role for Wnt-mediated signaling. Embryos treated with LiCl also show induction of similar anomalous somites indicating further the possibility that Wnt-mediated signaling may be involved in the clustering event. It is suggested that beta-catenin serves to initiate the adhesion process which is spread then by N-cadherin. Later during compartmentalization, N-cadherin/catenins remain expressed by the myotome compartment. Taken together, these results suggest that the Ca2+-dependent cell adhesion molecule N-cadherin and the intracellular catenins are important in segmentation and formation of the somite and myotome compartment. It is proposed that the N-cadherin-mediated adhesion process may serve as a common, evolutionarily conserved, link in the differentiation pathways of skeletal and cardiac muscle.

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.

N-cadherin-catenin interaction: necessary component of cardiac cell compartmentalization during early vertebrate heart development

During early heart development the expression pattern of N-cadherin, a calcium-dependent cell adhesion molecule, suggests its involvement in morphoregulation and the stabilization of cardiomyocyte differentiation. N-cadherin's adhesive activity is dependent upon its interaction with the intracellular catenins. An association with alpha-catenin and beta-catenin also is believed to be involved in cell signaling. This study details the expression patterns of alpha-catenin, beta-catenin, and gamma-catenin, during definition of the cardiac cell population as distinct compartments in the anterior regions of the chick embryo between stages 5 and 9. The restriction of N-cadherin/catenin localization at stage 5+ from a uniform pattern in vivo, to specific cell clusters that demarcate areas where mesoderm separation is initiated, suggests that the N-cadherin/catenin complex is involved in boundary formation and in the subsequent cell sorting. The latter two processes lead to the specification and formation of the somatic and cardiac splanchnic mesoderm. N-cadherin colocalized with alpha- and beta-catenin at the cell membrane before and during the time that its expression becomes restricted to the lateral mesoderm and continues cephalocaudad into stage 8. These proteins continue to colocalize in the myocardium of the tubular heart. Plakoglobin is not expressed in this region during stages 6-8, but is detected in the myocardium later at stage 13. The observed in vivo expression patterns of alpha-catenin, beta-catenin, and plakoglobin suggest that these proteins are directly linked with the developmental regulation of cell junctions, as cardiac cells become stably committed and phenotypically differentiated to eventually form a mature myocardium. The localization of N-CAM also was analyzed during these stages to determine whether the N-cadherin-catenin localization was unique or whether other cell adhesion molecules were expressed similarly. The results indicate that the unique pattern of N-cadherin expression is not shared with N-CAM. We also show that perturbation of N-cadherin using a function perturbing N-cadherin antibody (NCD-2) inhibits normal early heart development and myogenesis in a cephalocaudad, stage-dependent manner. We propose a model whereby myocardial cell compartmentalization also defines the endocardial population. The presence of beta-catenin suggests that a similar signaling pathway involving Wnt (wingless)-mediated events may function in myocardial cell compartmentalization during early vertebrate heart development, as in Drosophila contractile vessel development.