Morphological and molecular left-right asymmetries in the development of the proepicardium: a comparative analysis on mouse and chick embryos

Epicardial EMT and cardiac repair: an update

Epicardial epithelial-to-mesenchymal transition (EMT) plays a pivotal role in both heart development and injury response and involves dynamic cellular changes that are essential for cardiogenesis and myocardial repair. Specifically, epicardial EMT is a crucial process in which epicardial cells lose polarity, migrate into the myocardium, and differentiate into various cardiac cell types during development and repair. Importantly, following EMT, the epicardium becomes a source of paracrine factors that support cardiac growth at the last stages of cardiogenesis and contribute to cardiac remodeling after injury. As such, EMT seems to represent a fundamental step in cardiac repair. Nevertheless, endogenous EMT alone is insufficient to stimulate adequate repair. Redirecting and amplifying epicardial EMT pathways offers promising avenues for the development of innovative therapeutic strategies and treatment approaches for heart disease. In this review, we present a synthesis of recent literature highlighting the significance of epicardial EMT reactivation in adult heart disease patients.

Exploring the Function of Epicardial Cells Beyond the Surface

The epicardium, previously viewed as a passive outer layer around the heart, is now recognized as an essential component in development, regeneration, and repair. In this review, we explore the cellular and molecular makeup of the epicardium, highlighting its roles in heart regeneration and repair in zebrafish and salamanders, as well as its activation in young and adult postnatal mammals. We also examine the latest technologies used to study the function of epicardial cells for therapeutic interventions. Analysis of highly regenerative animal models shows that the epicardium is essential in regulating cardiomyocyte proliferation, transient fibrosis, and neovascularization. However, despite the epicardium's unique cellular programs to resolve cardiac damage, it remains unclear how to replicate these processes in nonregenerative mammalian organisms. During myocardial infarction, epicardial cells secrete signaling factors that modulate fibrotic, vascular, and inflammatory remodeling, which differentially enhance or inhibit cardiac repair. Recent transcriptomic studies have validated the cellular and molecular heterogeneity of the epicardium across various species and developmental stages, shedding further light on its function under pathological conditions. These studies have also provided insights into the function of regulatory epicardial-derived signaling molecules in various diseases, which could lead to new therapies and advances in reparative cardiovascular medicine. Moreover, insights gained from investigating epicardial cell function have initiated the development of novel techniques, including using human pluripotent stem cells and cardiac organoids to model reparative processes within the cardiovascular system. This growing understanding of epicardial function holds the potential for developing innovative therapeutic strategies aimed at addressing developmental heart disorders, enhancing regenerative therapies, and mitigating cardiovascular disease progression.

Novel Insights into the Molecular Mechanisms Governing Embryonic Epicardium Formation

The embryonic epicardium originates from the proepicardium, an extracardiac primordium constituted by a cluster of mesothelial cells. In early embryos, the embryonic epicardium is characterized by a squamous cell epithelium resting on the myocardium surface. Subsequently, it invades the subepicardial space and thereafter the embryonic myocardium by means of an epithelial-mesenchymal transition. Within the myocardium, epicardial-derived cells present multilineage potential, later differentiating into smooth muscle cells and contributing both to coronary vasculature and cardiac fibroblasts in the mature heart. Over the last decades, we have progressively increased our understanding of those cellular and molecular mechanisms driving proepicardial/embryonic epicardium formation. This study provides a state-of-the-art review of the transcriptional and emerging post-transcriptional mechanisms involved in the formation and differentiation of the embryonic epicardium.

Fetal liver development and implications for liver disease pathogenesis

The metabolic, digestive and homeostatic roles of the liver are dependent on proper crosstalk and organization of hepatic cell lineages. These hepatic cell lineages are derived from their respective progenitors early in organogenesis in a spatiotemporally controlled manner, contributing to the liver's specialized and diverse microarchitecture. Advances in genomics, lineage tracing and microscopy have led to seminal discoveries in the past decade that have elucidated liver cell lineage hierarchies. In particular, single-cell genomics has enabled researchers to explore diversity within the liver, especially early in development when the application of bulk genomics was previously constrained due to the organ's small scale, resulting in low cell numbers. These discoveries have substantially advanced our understanding of cell differentiation trajectories, cell fate decisions, cell lineage plasticity and the signalling microenvironment underlying the formation of the liver. In addition, they have provided insights into the pathogenesis of liver disease and cancer, in which developmental processes participate in disease emergence and regeneration. Future work will focus on the translation of this knowledge to optimize in vitro models of liver development and fine-tune regenerative medicine strategies to treat liver disease. In this Review, we discuss the emergence of hepatic parenchymal and non-parenchymal cells, advances that have been made in in vitro modelling of liver development and draw parallels between developmental and pathological processes.

The Role of Bmp- and Fgf Signaling Modulating Mouse Proepicardium Cell Fate

Bmp and Fgf signaling are widely involved in multiple aspects of embryonic development. More recently non coding RNAs, such as microRNAs have also been reported to play essential roles during embryonic development. We have previously demonstrated that microRNAs, i.e., miR-130, play an essential role modulating Bmp and Fgf signaling during early stages of cardiomyogenesis. More recently, we have also demonstrated that microRNAs are capable of modulating cell fate decision during proepicardial/septum transversum (PE/ST) development, since over-expression of miR-23 blocked while miR-125, miR-146, miR-223 and miR-195 enhanced PE/ST-derived cardiomyogenesis, respectively. Importantly, regulation of these microRNAs is distinct modulated by Bmp2 and Fgf2 administration in chicken. In this study, we aim to dissect the functional role of Bmp and Fgf signaling during mouse PE/ST development, their implication regulating post-transcriptional modulators such as microRNAs and their impact on lineage determination. Mouse PE/ST explants and epicardial/endocardial cell cultures were distinctly administrated Bmp and Fgf family members. qPCR analyses of distinct microRNAs, cardiomyogenic, fibrogenic differentiation markers as well as key elements directly epithelial to mesenchymal transition were evaluated. Our data demonstrate that neither Bmp2/Bmp4 nor Fgf2/Fgf8 signaling is capable of inducing cardiomyogenesis, fibrogenesis or inducing EMT in mouse PE/ST explants, yet deregulation of several microRNAs is observed, in contrast to previous findings in chicken PE/ST. RNAseq analyses in mouse PE/ST and embryonic epicardium identified novel Bmp and Fgf family members that might be involved in such cell fate differences, however, their implication on EMT induction and cardiomyogenic and/or fibrogenic differentiation is limited. Thus our data support the notion of species-specific differences regulating PE/ST cardiomyogenic lineage commitment.

Epicardial Contribution to the Developing and Injured Heart: Exploring the Cellular Composition of the Epicardium

The epicardium is an essential cell population during cardiac development. It contributes different cell types to the developing heart through epithelial-to-mesenchymal transition (EMT) and it secretes paracrine factors that support cardiac tissue formation. In the adult heart the epicardium is a quiescent layer of cells which can be reactivated upon ischemic injury, initiating an embryonic-like response in the epicardium that contributes to post-injury repair processes. Therefore, the epicardial layer is considered an interesting target population to stimulate endogenous repair mechanisms. To date it is still not clear whether there are distinct cell populations in the epicardium that contribute to specific lineages or aid in cardiac repair, or that the epicardium functions as a whole. To address this putative heterogeneity, novel techniques such as single cell RNA sequencing (scRNA seq) are being applied. In this review, we summarize the role of the epicardium during development and after injury and provide an overview of the most recent insights into the cellular composition and diversity of the epicardium.

Notch and Bmp signaling pathways act coordinately during the formation of the proepicardium

BACKGROUND

The epicardium is the outer mesothelial layer of the heart. It encloses the myocardium and plays key roles in heart development and regeneration. It derives from the proepicardium (PE), cell clusters that appear in the dorsal pericardium (DP) close to the atrioventricular canal and the venous pole of the heart, and are released into the pericardial cavity. PE cells are advected around the beating heart until they attach to the myocardium. Bmp and Notch signaling influence PE formation, but it is unclear how both signaling pathways interact during this process in the zebrafish.

RESULTS

Here, we show that the developing PE is influenced by Notch signaling derived from the endothelium. Overexpression of the intracellular receptor of notch in the endothelium enhances bmp expression, increases the number of pSmad1/5 positive cells in the DP and PE, and enhances PE formation. On the contrary, pharmacological inhibition of Notch1 impairs PE formation. bmp2b overexpression can rescue loss of PE formation in the presence of a Notch1 inhibitor, but Notch gain-of-function could not recover PE formation in the absence of Bmp signaling.

CONCLUSIONS

Endothelial Notch signaling activates bmp expression in the heart tube, which in turn induces PE cluster formation from the DP layer.

Analysis of wt1a reporter line expression levels during proepicardium formation in the zebrafish

The epicardium is the outer mesothelial layer of the heart. It covers the myocardium and plays important roles in both heart development and regeneration. It is derived from the proepicardium (PE), groups of cells that emerges at early developmental stages from the dorsal pericardial layer (DP) close to the atrio-ventricular canal and the venous pole of the heart-tube. In zebrafish, PE cells extrude apically into the pericardial cavity as a consequence of DP tissue constriction, a process that is dependent on Bmp pathway signaling. Expression of the transcription factor Wilms tumor-1, Wt1, which is a leader of important morphogenetic events such as apoptosis regulation or epithelial-mesenchymal cell transition, is also necessary during PE formation. In this study, we used the zebrafish model to compare intensity level of the wt1a reporter line epi:GFP in PE and its original tissue, the DP. We found that GFP is present at higher intensity level in the PE tissue, and differentially wt1 expression at pericardial tissues could be involved in the PE formation process. Our results reveal that bmp2b overexpression leads to enhanced GFP level both in DP and in PE tissues.

Spontaneous Left Cardiac Isomerism in Chick Embryos: Case Report, Review of the Literature, and Possible Significance for the Understanding of Ventricular Non-Compaction Cardiomyopathy in the Setting of Human Heterotaxy Syndromes

The outer shape of most vertebrates is normally characterized by bilateral symmetry. The inner organs, on the other hand, are normally arranged in bilaterally asymmetric patterns. Congenital deviations from the normal organ asymmetry can occur in the form of mirror imagery of the normal arrangement (situs inversus), or in the form of arrangements that have the tendency for the development of bilateral symmetry, either in a pattern of bilateral left-sidedness (left isomerism) or bilateral right-sidedness (right isomerism). The latter two forms of visceral situs anomalies are called “heterotaxy syndromes”. During the past 30 years, remarkable progress has been made in uncovering the genetic etiology of heterotaxy syndromes. However, the pathogenetic mechanisms causing the spectrum of cardiovascular defects found in these syndromes remain poorly understood. In the present report, a spontaneous case of left cardiac isomerism found in an HH-stage 23 chick embryo is described. The observations made in this case confirmed the existence of molecular isomerism in the ventricular chambers previously noted in mouse models. They, furthermore, suggest that hearts with left cardiac isomerism may have the tendency for the development of non-compaction cardiomyopathy caused by defective development of the proepicardium.

The deployment of cell lineages that form the mammalian heart

The function of the mammalian heart depends on the interplay between different cardiac cell types. The deployment of these cells, with precise spatiotemporal regulation, is also important during development to establish the heart structure. In this Review, we discuss the diverse origins of cardiac cell types and the lineage relationships between cells of a given type that contribute to different parts of the heart. The emerging lineage tree shows the progression of cell fate diversification, with patterning cues preceding cell type segregation, as well as points of convergence, with overlapping lineages contributing to a given tissue. Several cell lineage markers have been identified. However, caution is required with genetic-tracing experiments in comparison with clonal analyses. Genetic studies on cell populations provided insights into the mechanisms for lineage decisions. In the past 3 years, results of single-cell transcriptomics are beginning to reveal cell heterogeneity and early developmental trajectories. Equating this information with the in vivo location of cells and their lineage history is a current challenge. Characterization of the progenitor cells that form the heart and of the gene regulatory networks that control their deployment is of major importance for understanding the origin of congenital heart malformations and for producing cardiac tissue for use in regenerative medicine.

Actin dynamics and the Bmp pathway drive apical extrusion of proepicardial cells

The epicardium, the outer mesothelial layer enclosing the myocardium, plays key roles in heart development and regeneration. During embryogenesis, the epicardium arises from the proepicardium (PE), a cell cluster that appears in the dorsal pericardium (DP) close to the venous pole of the heart. Little is known about how the PE emerges from the pericardial mesothelium. Using a zebrafish model and a combination of genetic tools, pharmacological agents and quantitative in vivo imaging, we reveal that a coordinated collective movement of DP cells drives PE formation. We found that Bmp signaling and the actomyosin cytoskeleton promote constriction of the DP, which enables PE cells to extrude apically. We provide evidence that cell extrusion, which has been described in the elimination of unfit cells from epithelia and the emergence of hematopoietic stem cells, is also a mechanism for PE cells to exit an organized mesothelium and fulfil their developmental fate to form a new tissue layer, the epicardium.

Summary: Proepicardial cells emerge from the pericardial mesothelium through apical extrusion, a process that depends on BMP signaling and actomyosin rearrangements.

Nitric Oxide Reverses the Position of the Heart during Embryonic Development

Nitric oxide (NO) produced by endothelial nitric oxide synthase (eNOS) plays crucial roles in cardiac homeostasis. Adult cardiomyocyte specific overexpression of eNOS confers protection against myocardial-reperfusion injury. However, the global effects of NO overexpression in developing cardiovascular system is still unclear. We hypothesized that nitric oxide overexpression affects the early migration of cardiac progenitor cells, vasculogenesis and function in a chick embryo. Vehicle or nitric oxide donor DEAN (500 mM) were loaded exogenously through a small window on the broad side of freshly laid egg and embryonic development tracked by live video-microscopy. At Hamburg Hamilton (HH) stage 8, the cardiac progenitor cells (CPC) were isolated and cell migration analysed by Boyden Chamber. The vascular bed structure and heart beats were compared between vehicle and DEAN treated embryos. Finally, expression of developmental markers such as BMP4, Shh, Pitx2, Noggin were measured using reverse transcriptase PCR and in-situ hybridization. The results unexpectedly showed that exogenous addition of pharmacological NO between HH stage 7⁻8 resulted in embryos with situs inversus in 28 out of 100 embryos tested. Embryos treated with NO inhibitor cPTIO did not have situs inversus, however 10 embryos treated with L-arginine showed a situs inversus phenotype. N-acetyl cysteine addition in the presence of NO failed to rescue situs inversus phenotype. The heart beat is normal (120 beats/min) although the vascular bed pattern is altered. Migration of CPCs in DEAN treated embryos is reduced by 60% compared to vehicle. BMP4 protein expression increases on the left side of the embryo compared to vehicle control. The data suggests that the NO levels in the yolk are important in turning of the heart during embryonic development. High levels of NO may lead to situs inversus condition in avian embryo by impairing cardiac progenitor cell migration through the NO-BMP4-cGMP axis.

Coronary Vasculature in Cardiac Development and Regeneration

Functional coronary circulation is essential for a healthy heart in warm-blooded vertebrates, and coronary diseases can have a fatal consequence. Despite the growing interest, the knowledge about the coronary vessel development and the roles of new coronary vessel formation during heart regeneration is still limited. It is demonstrated that early revascularization is required for efficient heart regeneration. In this comprehensive review, we first describe the coronary vessel formation from an evolutionary perspective. We further discuss the cell origins of coronary endothelial cells and perivascular cells and summarize the critical signaling pathways regulating coronary vessel development. Lastly, we focus on the current knowledge about the molecular mechanisms regulating heart regeneration in zebrafish, a genetically tractable vertebrate model with a regenerative adult heart and well-developed coronary system.

Proepicardium: Current Understanding of its Structure, Induction, and Fate

The proepicardium (PE) is a transitory extracardiac embryonic structure which plays a crucial role in cardiac morphogenesis and delivers various cell lineages to the developing heart. The PE arises from the lateral plate mesoderm (LPM) and is present in all vertebrate species. During development, mesothelial cells of the PE reach the naked myocardium either as free-floating aggregates in the form of vesicles or via a tissue bridge; subsequently, they attach to the myocardium and, finally, form the third layer of a mature heart-the epicardium. After undergoing epithelial-to-mesenchymal transition (EMT) some of the epicardial cells migrate into the myocardial wall and differentiate into fibroblasts, smooth muscle cells, and possibly other cell types. Despite many recent findings, the molecular pathways that control not only proepicardial induction and differentiation but also epicardial formation and epicardial cell fate are poorly understood. Knowledge about these events is essential because molecular mechanisms that occur during embryonic development have been shown to be reactivated in pathological conditions, for example, after myocardial infarction, during hypertensive heart disease or other cardiovascular diseases. Therefore, in this review we intended to summarize the current knowledge about PE formation and structure, as well as proepicardial cell fate in animals commonly used as models for studies on heart development. Anat Rec, 302:893-903, 2019. © 2018 Wiley Periodicals, Inc.

Does cardiac development provide heart research with novel therapeutic approaches?

Embryonic heart progenitors arise at specific spatiotemporal periods that contribute to the formation of distinct cardiac structures. In mammals, the embryonic and fetal heart is hypoxic by comparison to the adult heart. In parallel, the cellular metabolism of the cardiac tissue, including progenitors, undergoes a glycolytic to oxidative switch that contributes to cardiac maturation. While oxidative metabolism is energy efficient, the glycolytic-hypoxic state may serve to maintain cardiac progenitor potential. Consistent with this proposal, the adult epicardium has been shown to contain a reservoir of quiescent cardiac progenitors that are activated in response to heart injury and are hypoxic by comparison to adjacent cardiac tissues. In this review, we discuss the development and potential of the adult epicardium and how this knowledge may provide future therapeutic approaches for cardiac repair.

Cells with hematopoietic potential reside within mouse proepicardium

During embryonic development, hematopoietic cells are present in areas of blood-vessel differentiation. These hematopoietic cells emerge from a specific subpopulation of endothelial cells called the hemogenic endothelium. We have previously found that mouse proepicardium contained its own population of endothelial cells forming a network of vascular tubules. We hypothesize that this EC population contains cells of hematopoietic potential. Therefore, we investigated an in vitro hematopoietic potential of proepicardial cell populations. The CD31+/CD45-/CD71- cell population cultured for 10 days in MethocultTM gave numerous colonies of CFU-GEMM, CFU-GM, and CFU-E type. These colonies consisted of various cell types. Flk-1+/CD31-/CD45-/CD71-, and CD45+ and/or CD71+ cell populations produced CFU-GEMM and CFU-GM, or CFU-GM and CFU-E colonies, respectively. Immunohistochemical evaluations of smears prepared from colonies revealed the presence of cells of different hematopoietic lineages. These cells were characterized by labeling with various combinations of antibodies directed against CD31, CD41, CD71, c-kit, Mpl, Fli1, Gata-2, and Zeb1 markers. Furthermore, we found that proepicardium-specific marker WT1 co-localized with Runx1 and Zeb1 and that single endothelial cells bearing CD31 molecule expressed Runx1 in the proepicardial area of embryonic tissue sections. We have shown that cells of endothelial and/or hematopoietic phenotypes isolated from mouse proepicardium possess hematopoietic potential in vitro and in situ. These results are supported by RT-PCR analyses of proepicardial extract, which revealed the expression of mRNA for crucial regulatory factors for hemogenic endothelium specification, i.e., Runx1, Notch1, Gata2, and Sox17. Our data are in line with previous observation on hemangioblast derivation from the quail PE.

The ontogeny, activation and function of the epicardium during heart development and regeneration

The epicardium plays a key role during cardiac development, homeostasis and repair, and has thus emerged as a potential target in the treatment of cardiovascular disease. However, therapeutically manipulating the epicardium and epicardium-derived cells (EPDCs) requires insights into their developmental origin and the mechanisms driving their activation, recruitment and contribution to both the embryonic and adult injured heart. In recent years, studies of various model systems have provided us with a deeper understanding of the microenvironment in which EPDCs reside and emerge into, of the crosstalk between the multitude of cardiovascular cell types that influence the epicardium, and of the genetic programmes that orchestrate epicardial cell behaviour. Here, we review these discoveries and discuss how technological advances could further enhance our knowledge of epicardium-based repair mechanisms and ultimately influence potential therapeutic outcomes in cardiovascular regenerative medicine.

CDC42 is required for epicardial and pro-epicardial development by mediating FGF receptor trafficking to the plasma membrane

The epicardium contributes to multiple cardiac lineages and is essential for cardiac development and regeneration. However, the mechanism of epicardium formation is unclear. This study aimed to establish the cellular and molecular mechanisms underlying the dissociation of pro-epicardial cells (PECs) from the pro-epicardium (PE) and their subsequent translocation to the heart to form the epicardium. We used lineage tracing, conditional deletion, mosaic analysis and ligand stimulation in mice to determine that both villous protrusions and floating cysts contribute to PEC translocation to myocardium in a CDC42-dependent manner. We resolved a controversy by demonstrating that physical contact of the PE with the myocardium constitutes a third mechanism for PEC translocation to myocardium, and observed a fourth mechanism in which PECs migrate along the surface of the inflow tract to reach the ventricles. Epicardial-specific Cdc42 deletion disrupted epicardium formation, and Cdc42 null PECs proliferated less, lost polarity and failed to form villous protrusions and floating cysts. FGF signaling promotes epicardium formation in vivo, and biochemical studies demonstrated that CDC42 is involved in the trafficking of FGF receptors to the cell membrane to regulate epicardium formation.

Highlighted article: During epicardial formation in mice, four different mechanisms of pro-epicardial cell translocation to the myocardium can be identified, with CDC42 playing a key role.

More than Just a Simple Cardiac Envelope; Cellular Contributions of the Epicardium

The adult pumping heart is formed by distinct tissue layers. From inside to outside, the heart is composed by an internal endothelial layer, dubbed the endocardium, a thick myocardial component which supports the pumping capacity of the heart and exteriorly covered by a thin mesothelial layer named the epicardium. Cardiac insults such as coronary artery obstruction lead to ischemia and thus to an irreversible damage of the myocardial layer, provoking in many cases heart failure and death. Thus, searching for new pathways to regenerate the myocardium is an urgent biomedical need. Interestingly, the capacity of heart regeneration is present in other species, ranging from fishes to neonatal mammals. In this context, several lines of evidences demonstrated a key regulatory role for the epicardial layer. In this manuscript, we provide a state-of-the-art review on the developmental process leading to the formation of the epicardium, the distinct pathways controlling epicardial precursor cell specification and determination and current evidences on the regenerative potential of the epicardium to heal the injured heart.

Foxa2 identifies a cardiac progenitor population with ventricular differentiation potential

The recent identification of progenitor populations that contribute to the developing heart in a distinct spatial and temporal manner has fundamentally improved our understanding of cardiac development. However, the mechanisms that direct atrial versus ventricular specification remain largely unknown. Here we report the identification of a progenitor population that gives rise primarily to cardiovascular cells of the ventricles and only to few atrial cells (<5%) of the differentiated heart. These progenitors are specified during gastrulation, when they transiently express Foxa2, a gene not previously implicated in cardiac development. Importantly, Foxa2+ cells contribute to previously identified progenitor populations in a defined pattern and ratio. Lastly, we describe an analogous Foxa2+ population during differentiation of embryonic stem cells. Together, these findings provide insight into the developmental origin of ventricular and atrial cells, and may lead to the establishment of new strategies for generating chamber-specific cell types from pluripotent stem cells.

The Role of Mesothelial Cells in Liver Development, Injury, and Regeneration

Mesothelial cells (MCs) cover the surface of visceral organs and the parietal walls of cavities, and they synthesize lubricating fluids to create a slippery surface that facilitates movement between organs without friction. Recent studies have indicated that MCs play active roles in liver development, fibrosis, and regeneration. During liver development, the mesoderm produces MCs that form a single epithelial layer of the mesothelium. MCs exhibit an intermediate phenotype between epithelial cells and mesenchymal cells. Lineage tracing studies have indicated that during liver development, MCs act as mesenchymal progenitor cells that produce hepatic stellate cells, fibroblasts around blood vessels, and smooth muscle cells. Upon liver injury, MCs migrate inward from the liver surface and produce hepatic stellate cells or myofibroblast depending on the etiology, suggesting that MCs are the source of myofibroblasts in capsular fibrosis. Similar to the activation of hepatic stellate cells, transforming growth factor β induces the conversion of MCs into myofibroblasts. Further elucidation of the biological and molecular changes involved in MC activation and fibrogenesis will contribute to the development of novel approaches for the prevention and therapy of liver fibrosis.

Current Perspectives in Cardiac Laterality

The heart is the first organ to break symmetry in the developing embryo and onset of dextral looping is the first indication of this event. Looping is a complex process that progresses concomitantly to cardiac chamber differentiation and ultimately leads to the alignment of the cardiac regions in their final topology. Generation of cardiac asymmetry is crucial to ensuring proper form and consequent functionality of the heart, and therefore it is a highly regulated process. It has long been known that molecular left/right signals originate far before morphological asymmetry and therefore can direct it. The use of several animal models has led to the characterization of a complex regulatory network, which invariably converges on the Tgf-β signaling molecule Nodal and its downstream target, the homeobox transcription factor Pitx2. Here, we review current data on the cellular and molecular bases of cardiac looping and laterality, and discuss the contribution of Nodal and Pitx2 to these processes. A special emphasis will be given to the morphogenetic role of Pitx2 and to its modulation of transcriptional and functional properties, which have also linked laterality to atrial fibrillation.

Myc overexpression enhances of epicardial contribution to the developing heart and promotes extensive expansion of the cardiomyocyte population

Myc is an essential regulator of cell growth and proliferation. Myc overexpression promotes the homeostatic expansion of cardiomyocyte populations by cell competition, however whether this applies to other cardiac lineages remains unknown. The epicardium contributes signals and cells to the developing and adult injured heart and exploring strategies for modulating its activity is of great interest. Using inducible genetic mosaics, we overexpressed Myc in the epicardium and determined the differential expansion of Myc-overexpressing cells with respect to their wild type counterparts. Myc-overexpressing cells overcolonized all epicardial-derived lineages and showed increased ability to invade the myocardium and populate the vasculature. We also found massive colonization of the myocardium by Wt1Cre-derived Myc-overexpressing cells, with preservation of cardiac development. Detailed analyses showed that this contribution is unlikely to derive from Cre activity in early cardiomyocytes but does not either derive from established epicardial cells, suggesting that early precursors expressing Wt1Cre originate the recombined cardiomyocytes. Myc overexpression does not modify the initial distribution of Wt1Cre-recombined cardiomyocytes, indicating that it does not stimulate the incorporation of early expressing Wt1Cre lineages to the myocardium, but differentially expands this initial population. We propose that strategies using epicardial lineages for heart repair may benefit from promoting cell competitive ability.

Wtip is required for proepicardial organ specification and cardiac left/right asymmetry in zebrafish

Wilm's tumor 1 interacting protein (Wtip) was identified as an interacting partner of Wilm's tumor protein (WT1) in a yeast two-hybrid screen. WT1 is expressed in the proepicardial organ (PE) of the heart, and mouse and zebrafish wt1 knockout models appear to lack the PE. Wtip's role in the heart remains unexplored. In the present study, we demonstrate that wtip expression is identical in wt1a-, tcf21-, and tbx18-positive PE cells, and that Wtip protein localizes to the basal body of PE cells. We present the first genetic evidence that Wtip signaling in conjunction with WT1 is essential for PE specification in the zebrafish heart. By overexpressing wtip mRNA, we observed ectopic expression of PE markers in the cardiac and pharyngeal arch regions. Furthermore, wtip knockdown embryos showed perturbed cardiac looping and lacked the atrioventricular (AV) boundary. However, the chamber-specific markers amhc and vmhc were unaffected. Interestingly, knockdown of wtip disrupts early left-right (LR) asymmetry. Our studies uncover new roles for Wtip regulating PE cell specification and early LR asymmetry, and suggest that the PE may exert non-autonomous effects on heart looping and AV morphogenesis. The presence of cilia in the PE, and localization of Wtip in the basal body of ciliated cells, raises the possibility of cilia-mediated PE signaling in the embryonic heart.

The Early Stages of Heart Development: Insights from Chicken Embryos

The heart is the first functioning organ in the developing embryo and a detailed understanding of the molecular and cellular mechanisms involved in its formation provides insights into congenital malformations affecting its function and therefore the survival of the organism. Because many developmental mechanisms are highly conserved, it is possible to extrapolate from observations made in invertebrate and vertebrate model organisms to humans. This review will highlight the contributions made through studying heart development in avian embryos, particularly the chicken. The major advantage of chick embryos is their accessibility for surgical manipulation and functional interference approaches, both gain- and loss-of-function. In addition to experiments performed in ovo, the dissection of tissues for ex vivo culture, genomic, or biochemical approaches is straightforward. Furthermore, embryos can be cultured for time-lapse imaging, which enables tracking of fluorescently labeled cells and detailed analysis of tissue morphogenesis. Owing to these features, investigations in chick embryos have led to important discoveries, often complementing genetic studies in mice and zebrafish. As well as including some historical aspects, we cover here some of the crucial advances made in understanding early heart development using the chicken model.

Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel-shaped and contains small amounts of myocardium. The AV valve is formed by two pocket-like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well-defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen-muscle boundary marks the SV-atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853-865, 2016. © 2016 Wiley Periodicals, Inc.

Interplay between cardiac function and heart development

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease. This requires cardiomyocytes to be mechanically durable and able to mount coordinated responses to a variety of environmental signals on different time scales, including cardiac pressure loading and electrical and hemodynamic forces. During physiological growth, myocytes, endocardial and epicardial cells have to adaptively remodel to these mechanical forces. Here we review some of the recent advances in the understanding of how mechanical forces influence cardiac development, with a focus on fluid flow forces. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

The Lhx9-integrin pathway is essential for positioning of the proepicardial organ

The development of the vertebrate embryonic heart occurs by hyperplastic growth as well as the incorporation of cells from tissues outside of the initial heart field. Amongst these tissues is the epicardium, a cell structure that develops from the precursor proepicardial organ on the right side of the septum transversum caudal to the developing heart. During embryogenesis, cells of the proepicardial organ migrate, adhere and envelop the maturing heart, forming the epicardium. The cells of the epicardium then delaminate and incorporate into the heart giving rise to cardiac derivatives, including smooth muscle cells and cardiac fibroblasts. Here, we demonstrate that the LIM homeodomain protein Lhx9 is transiently expressed in Xenopus proepicardial cells and is essential for the position of the proepicardial organ on the septum transversum. Utilizing a small-molecule screen, we found that Lhx9 acts upstream of integrin-paxillin signaling and consistently demonstrate that either loss of Lhx9 or disruption of the integrin-paxillin pathway results in mis-positioning of the proepicardial organ and aberrant deposition of extracellular matrix proteins. This leads to a failure of proepicardial cell migration and adhesion to the heart, and eventual death of the embryo. Collectively, these studies establish a requirement for the Lhx9-integrin-paxillin pathway in proepicardial organ positioning and epicardial formation.

The Role of WT1 in Embryonic Development and Normal Organ Homeostasis

The Wilms' tumor suppressor gene 1 (Wt1) is critically involved in a number of developmental processes in vertebrates, including cell differentiation, control of the epithelial/mesenchymal phenotype, proliferation, and apoptosis. Wt1 proteins act as transcriptional and post-transcriptional regulators, in mRNA splicing and in protein-protein interactions. Furthermore, Wt1 is involved in adult tissue homeostasis, kidney function, and cancer. For these reasons, Wt1 function has been extensively studied in a number of animal models to establish its spatiotemporal expression pattern and the developmental fate of the cells expressing this gene. In this chapter, we review the developmental anatomy of Wt1, collecting information about its dynamic expression in mesothelium, kidney, gonads, cardiovascular system, spleen, nervous system, lung, and liver. We also describe the adult expression of Wt1 in kidney podocytes, gonads, mesothelia, visceral adipose tissue, and a small fraction of bone marrow cells. We have reviewed the available animal models for Wt1-expressing cell lineage analysis, including direct Wt1 expression reporters and systems for permanent Wt1 lineage tracing, based on constitutive or inducible Cre recombinase expression under control of a Wt1 promoter. Finally we provide a number of laboratory protocols to be used with these animal models in order to assess reporter expression.

3-D reconstruction and multiple marker analysis of mouse proepicardial endothelial cell population

BACKGROUND

The proepicardium (PE), a transient embryonic structure crucial for the development of the epicardium and heart, contains its own population of endothelial cells (ECs). The aim of our study was to determine the pattern, anatomical orientation and phenotypic marker expression of the endothelial cell network within the PE.

RESULTS

Immunohistochemical findings revealed that proepicardial ECs express both early and late EC-specific markers such as CD31, Flk-1, Lyve-1 and Tie-2 but not SCL/Tal1, vWF, Dll4 or Notch1. Proepicardial ECs are present in the vicinity of the sinus venosus (SV) and form a continuous network of vascular sprouts/tubules connected with the SV endothelium, with Ter-119-positive erythroblasts in the vascular lumina.

CONCLUSIONS

On the basis of our results, we postulate the existence of a continuous network of ECs in the PE, exhibiting connection and/or patency with the SV and forming vessels/tubules/strands. Marker expression suggests that ECs are immature and undifferentiated, which was also confirmed with a transmission electron microscopy (TEM) analysis. Our results deliver new data for a better understanding of the nature of proepicardial ECs.

Cardiac cell lineages that form the heart

Myocardial cells ensure the contractility of the heart, which also depends on other mesodermal cell types for its function. Embryological experiments had identified the sources of cardiac precursor cells. With the advent of genetic engineering, novel tools have been used to reconstruct the lineage tree of cardiac cells that contribute to different parts of the heart, map the development of cardiac regions, and characterize their genetic signature. Such knowledge is of fundamental importance for our understanding of cardiogenesis and also for the diagnosis and treatment of heart malformations.

Multiple modes of proepicardial cell migration require heartbeat

Background

The outermost layer of the vertebrate heart, the epicardium, forms from a cluster of progenitor cells termed the proepicardium (PE). PE cells migrate onto the myocardium to give rise to the epicardium. Impaired epicardial development has been associated with defects in valve development, cardiomyocyte proliferation and alignment, cardiac conduction system maturation and adult heart regeneration. Zebrafish are an excellent model for studying cardiac development and regeneration; however, little is known about how the zebrafish epicardium forms.

Results

We report that PE migration occurs through multiple mechanisms and that the zebrafish epicardium is composed of a heterogeneous population of cells. Heterogeneity is first observed within the PE and persists through epicardium formation. Using in vivo imaging, histology and confocal microscopy, we show that PE cells migrate through a cellular bridge that forms between the pericardial mesothelium and the heart. We also observed the formation of PE aggregates on the pericardial surface, which were released into the pericardial cavity. It was previously reported that heartbeat-induced pericardiac fluid advections are necessary for PE cluster formation and subsequent epicardium development. We manipulated heartbeat genetically and pharmacologically and found that PE clusters clearly form in the absence of heartbeat. However, when heartbeat was inhibited the PE failed to migrate to the myocardium and the epicardium did not form. We isolated and cultured hearts with only a few epicardial progenitor cells and found a complete epicardial layer formed. However, pharmacologically inhibiting contraction in culture prevented epicardium formation. Furthermore, we isolated control and silent heart (sih) morpholino (MO) injected hearts prior to epicardium formation (60 hpf) and co-cultured these hearts with “donor” hearts that had an epicardium forming (108 hpf). Epicardial cells from donor hearts migrated on to control but not sih MO injected hearts.

Conclusions

Epicardial cells stem from a heterogeneous population of progenitors, suggesting that the progenitors in the PE have distinct identities. PE cells attach to the heart via a cellular bridge and free-floating cell clusters. Pericardiac fluid advections are not necessary for the development of the PE cluster, however heartbeat is required for epicardium formation. Epicardium formation can occur in culture without normal hydrodynamic and hemodynamic forces, but not without contraction.

Homeobox transcription factor Pitx2: The rise of an asymmetry gene in cardiogenesis and arrhythmogenesis

The homeobox transcription factor Pitx2 displays a highly specific expression pattern during embryogenesis. Gain and loss of function experiments have unraveled its pivotal role in left-right signaling. Conditional deletion in mice has demonstrated a complex and intricate role for Pitx2 in distinct aspects of cardiac development and more recently a link to atrial fibrillation has been proposed based on genome-wide association studies. In this review we will revise the role of Pitx2 in the developing heart, starting from the early events of left-right determination followed by its role in cardiac morphogenesis and ending with its role in cardiac arrhythmogenesis.

Subpopulation of proepicardial cells is derived from the somatic mesoderm in the chick embryo

RATIONALE

The proepicardium (PE) is a transient structure forming at the venous pole of the heart and gives rise to the epicardium, fibroblasts, and smooth muscle cells. The embryological origin of the PE is presently unclear. Asymmetrical formation of the PE on the right inflow tract is a conserved feature of many vertebrate embryos, and in the chicken is under the control of fibroblast growth factor 8 and snail homolog 1.

OBJECTIVE

To gain further insight into the process of asymmetrical PE formation, we studied the role of TWIST1 during PE formation in the chick embryo.

METHODS AND RESULTS

TWIST1 is asymmetrically expressed on the right side in the somatic mesoderm under the control of snail homolog 1. Fate mapping experiments revealed a contribution of the somatic mesoderm to the PE. After colonization of the heart, this cell lineage gives rise to the epicardium, smooth muscle cells, and potentially fibroblast. Suppression of TWIST1 function in the right coelomic cavity caused a severe disruption of the villous protrusions of the PE and Wilms tumor 1 and transcription factor 21 expression. Rescue with the corresponding mouse cDNA normalized gene expression and PE morphology. Forced expression of TWIST1 on the left side induced ectopic expression domains of Wilms tumor 1 and transcription factor 21.

CONCLUSIONS

A significant proportion of the PE has its origin outside of the currently proposed domain in the splanchnic layer of the lateral plate mesoderm. The phenotype in embryos subjected to TWIST1 loss- or gain-of-function suggests an important contribution of somatic mesoderm to the mesothelial cell layer of the PE.

Tcf21 regulates the specification and maturation of proepicardial cells

The epicardium is a mesothelial cell layer essential for vertebrate heart development and pertinent for cardiac repair post-injury in the adult. The epicardium initially forms from a dynamic precursor structure, the proepicardial organ, from which cells migrate onto the heart surface. During the initial stage of epicardial development crucial epicardial-derived cell lineages are thought to be determined. Here, we define an essential requirement for transcription factor Tcf21 during early stages of epicardial development in Xenopus, and show that depletion of Tcf21 results in a disruption in proepicardial cell specification and failure to form a mature epithelial epicardium. Using a mass spectrometry-based approach we defined Tcf21 interactions and established its association with proteins that function as transcriptional co-repressors. Furthermore, using an in vivo systems-based approach, we identified a panel of previously unreported proepicardial precursor genes that are persistently expressed in the epicardial layer upon Tcf21 depletion, thereby confirming a primary role for Tcf21 in the correct determination of the proepicardial lineage. Collectively, these studies lead us to propose that Tcf21 functions as a transcriptional repressor to regulate proepicardial cell specification and the correct formation of a mature epithelial epicardium.

On the form problem of embryonic heart loops, its geometrical solutions, and a new biophysical concept of cardiac looping

BACKGROUND

Cardiac looping is an essential process in the morphogenesis of embryonic hearts. Unfortunately, relatively little is known about the form and biophysics of embryonic heart loops. Thompson regarded the form of an object as "a 'diagram of forces' … from it we can … deduce the forces that are acting or have acted upon it." Therefore, the present study was conducted to uncover the best geometrical solution of the form problem of embryonic heart loops. This approach may help to identify the biophysics of cardiac looping.

RESULTS

Analysis of the tendrils of climbing plants disclosed striking resemblance between the configurations of embryonic heart loops and a form motif named helical perversion. Helical perversion occurs in helically wound objects where they connect two helical segments of opposite handedness (two-handed helix). Helical perversion evolves in living and non-living filamentary objects such as the tendrils of climbing plants and helical telephone cords.

CONCLUSIONS

Helical perversion may be the best geometrical solution of the form problem of embryonic heart loops. The dynamics and mechanics of the emergence of helical perversions are relatively well known. The behavior of looping embryonic hearts may be interpreted in light of this knowledge.

New insights into the developmental mechanisms of coronary vessels and epicardium

During heart development, the epicardium, which originates from the proepicardial organ (PE), is a source of coronary vessels. The PE develops from the posterior visceral mesoderm of the pericardial coelom after stimulation with a combination of weak bone morphogenetic protein and strong fibroblast growth factor (FGF) signaling. PE-derived cells migrate across the heart surface to form the epicardial sheet, which subsequently seeds multipotent subepicardial mesenchymal cells via epithelial-mesenchymal transition, which is regulated by several signaling pathways including retinoic acid, FGF, sonic hedgehog, Wnt, transforming growth factor-β, and platelet-derived growth factor. Subepicardial endothelial progenitors eventually generate the coronary vascular plexus, which acquires an arterial or venous phenotype, connects with the sinus venosus and aortic sinuses, and then matures through the recruitment of vascular smooth muscle cells under the regulation of complex growth factor signaling pathways. These developmental programs might be activated in the adult heart after injury and play a role in the regeneration/repair of the myocardium.

Identification of a novel developmental mechanism in the generation of mesothelia

Mesothelium is the surface layer of all coelomic organs and is crucial for the generation of their vasculature. Still, our understanding of the genesis of this essential cell type is restricted to the heart where a localized exogenous population of cells, the proepicardium, migrates to and envelops the myocardium supplying mesothelial, vascular and stromal cell lineages. Currently it is not known whether this pattern of development is specific to the heart or applies broadly to other coelomic organs. Using two independent long-term lineage-tracing studies, we demonstrate that mesothelial progenitors of the intestine are intrinsic to the gut tube anlage. Furthermore, a novel chick-quail chimera model of gut morphogenesis reveals these mesothelial progenitors are broadly distributed throughout the gut primordium and are not derived from a localized and exogenous proepicardium-like source of cells. These data demonstrate an intrinsic origin of mesothelial cells to a coelomic organ and provide a novel mechanism for the generation of mesothelial cells.

Epicardial progenitor cells in cardiac development and regeneration

The epicardium forms an epithelial layer on the surface of the heart. It is derived from a cluster of mesothelial cells, which is termed the proepicardium. The proepicardium gives rise not only to the epicardium but also to epicardium-derived cells. These cells populate the myocardial wall and differentiate into smooth muscle cells, fibroblast, and possibly endothelial cells. In this review, the formation of the proepicardium is discussed. Marker genes, suitable to identify these cells in the embryo and in the adult, are introduced. Recent evidence suggests that the PE is made up of distinct cell populations. These cell lineages can be distinguished on the basis of marker gene expression and differ in their differentiation potential. The role of the epicardium as a resource for cardiac stem cells and its importance in cardiac regeneration is also discussed.

The arterial and cardiac epicardium in development, disease and repair

The importance of the epicardium covering the heart and the intrapericardial part of the great arteries has reached a new summit. It has evolved as a major cellular component with impact both in development, disease and more recently also repair potential. The role of the epicardium in development, its differentiation from a proepicardial organ at the venous pole (vPEO) and the differentiation capacities of the vPEO initiating cardiac epicardium (cEP) into epicardium derived cells (EPDCs) have been extensively described in recent reviews on growth and transcription factor pathways. In short, the epicardium is the source of the interstitial, the annulus fibrosus and the adventitial fibroblasts, and differentiates into the coronary arterial smooth muscle cells. Furthermore, EPDCs induce growth of the compact myocardium and differentiation of the Purkinje fibers. This review includes an arterial pole located PEO (aPEO) that provides the epicardium covering the intrapericardial great vessels. In avian and mouse models disturbance of epicardial outgrowth and maturation leads to a broad spectrum of cardiac anomalies with main focus on non-compaction of the myocardium, deficient annulus fibrosis, valve malformations and coronary artery abnormalities. The discovery that in disease both arterial and cardiac epicardium can again differentiate into EPDCs and thus reactivate its embryonic program and potential has highly broadened the scope of research interest. This reactivation is seen after myocardial infarction and also in aneurysm formation of the ascending aorta. Use of EPDCs for cell therapy show their positive function in paracrine mediated repair processes which can be additive when combined with the cardiac progenitor stem cells that probably share the same embryonic origin with EPDCs. Research into the many cell-autonomous and cell-cell-based capacities of the adult epicardium will open up new realistic therapeutic avenues.

Cell lineages, growth and repair of the mouse heart

The formation of the heart involves diversification of lineages which differentiate into distinct cardiac cell types or contribute to different regions such as the four cardiac chambers. The heart is the first organ to form in the embryo. However, in parallel with the growth of the organism, before or after birth, the heart has to adapt its size to maintain pumping efficiency. The adult heart has only a mild regeneration potential; thus, strategies to repair the heart after injury are based on the mobilisation of resident cardiac stem cells or the transplantation of external sources of stem cells. We discuss current knowledge on these aspects and raise questions for future research.

Origin and fates of the proepicardium

The embryonic heart initially consists of only two cell layers, the endocardium and the myocardium. The epicardium, which forms an epithelial layer on the surface of the heart, is derived from a cluster of mesothelial cells developing at the base of the venous inflow tract of the early embryonic heart. This cell cluster is termed the proepicardium and gives rise not only to the epicardium but also to epicardium-derived cells. These cells populate the myocardial wall and differentiate into smooth muscle cells and fibroblasts, while the contribution to the vascular endothelial lineage is uncertain. In this review we will discuss the signaling molecules involved in recruiting mesodermal cells to undergo proepicardium formation and guide these cells to the myocardial surface. Marker genes which are suitable to follow these cells during proepicardium formation and cell migration will be introduced. We will address whether the proepicardium consists of a homogenous cell population or whether different cell lineages are present. Finally the role of the epicardium as a source for cardiac stem cells and its importance in cardiac regeneration, in particular in the zebrafish and mouse model systems is discussed.

Effects of endothelial growth media on proepicardial cell gene expression and morphogenesis in 3D collagen matrices

Proepicardial cells (PE) contribute to embryonic coronary vessel and epicardial development. Cells from the PE region can differentiate into coronary vascular smooth muscle cells and fibroblasts in vitro, but the endothelial specification capability of these cells is controversial. We sought to examine the effects of endothelial cell growth media on gene expression and the morphogenic properties of proepicardial cells in three-dimensional (3D) matrices. A primary culture of avian PE cells was subjected to molecular characterization with selected endothelial specific markers. Morphogenic properties of PE cells were assessed by in vitro assays of coronary vasculogenesis and invasion, which utilized highly defined, serum free, three-dimensional matrix conditions. PE cells maintained mixed cell population properties in the culture based on morphogenic features, immunohistochemistry, and the gene expression data. When suspended in a 3D vasculogenesis in vitro assay, PE cells formed intracellular vacuoles and assembled into multicellular tubes. Further, ultrastructural analysis revealed the presence of pinocytic vacuoles, intercellular junctions, and endothelial specific Weibel Palade bodies. In the invasion assay, PE cells spontaneously invaded control matrices. This invasion was markedly enhanced by lysophosphatidic acid (94±9.6 vs. 285.6±54.9, p<0.05) and was completely blocked with synthetic broad-spectrum metalloproteinase inhibitor GM6001. Isolated PE cells grown in endothelial cell media represent mixed-cell population, characterized by both smooth muscle and endothelial gene expression. When placed in 3D in vitro assays, PE cells manifest morphogenic properties, including multicellular tube assembly and invasion.

NFATC1 promotes epicardium-derived cell invasion into myocardium

Epicardium-derived cells (EPDCs) contribute to formation of coronary vessels and fibrous matrix of the mature heart. Nuclear factor of activated T-cells cytoplasmic 1 (NFATC1) is expressed in cells of the proepicardium (PE), epicardium and EPDCs in mouse and chick embryos. Conditional loss of NFATC1 expression in EPDCs in mice causes embryonic death by E18.5 with reduced coronary vessel and fibrous matrix penetration into myocardium. In osteoclasts, calcineurin-mediated activation of NFATC1 by receptor activator of NFκB ligand (RANKL) signaling induces cathepsin K (CTSK) expression for extracellular matrix degradation and cell invasion. RANKL/NFATC1 pathway components also are expressed in EPDCs, and loss of NFATC1 in EPDCs causes loss of CTSK expression in the myocardial interstitium in vivo. Likewise, RANKL treatment induces Ctsk expression in PE-derived cell cultures via a calcineurin-dependent mechanism. In chicken embryo hearts, RANKL treatment increases the distance of EPDC invasion into myocardium, and this response is calcineurin dependent. Together, these data demonstrate a crucial role for the RANKL/NFATC1 signaling pathway in promoting invasion of EPDCs into the myocardium by induction of extracellular matrix-degrading enzyme gene expression.

Role of fibroblast growth factor signaling during proepicardium formation in the chick embryo

The proepicardium forms at the venous pole of the embryonic heart and gives rise to several cell types of the mature heart. We investigated the role of fibroblast growth factors (FGFs) during proepicardium formation in the chick embryo. Several FGF ligands (Fgf2, Fgf10, and Fgf12) and receptors (Fgfr1, Fgfr2, and Fgfr4) are expressed in the proepicardium. Experimental modulation of FGF signaling in explant cultures affected cell proliferation and survival. In contrast, expression of Tbx18, Wt1, or Tbx5 were unaffected by FGF inhibition. In agreement with the explant data, villous outgrowth of the proepicardium was strongly impaired by FGF inhibition in vivo, however Tbx18 expression was maintained. These data suggest that during proepicardium formation, FGF ligands act as autocrine or paracrine growth factors to prevent apoptosis, maintain proliferation, and to promote villous outgrowth of the proepicardium. However, FGF is not involved in the induction or maintenance of proepicardium-specific marker gene expression.