The origin, formation and developmental significance of the epicardium: a review

Extracardiac septum transversum/proepicardial endothelial cells pattern embryonic coronary arterio-venous connections

Recent reports suggest that mammalian embryonic coronary endothelium (CoE) originates from the sinus venosus and ventricular endocardium. However, the contribution of extracardiac cells to CoE is thought to be minor and nonsignificant for coronary formation. Using classic (Wt1(Cre)) and previously undescribed (G2-Gata4(Cre)) transgenic mouse models for the study of coronary vascular development, we show that extracardiac septum transversum/proepicardium (ST/PE)-derived endothelial cells are required for the formation of ventricular coronary arterio-venous vascular connections. Our results indicate that at least 20% of embryonic coronary arterial and capillary endothelial cells derive from the ST/PE compartment. Moreover, we show that conditional deletion of the ST/PE lineage-specific Wilms' tumor suppressor gene (Wt1) in the ST/PE of G2-Gata4(Cre) mice and in the endothelium of Tie2(Cre) mice disrupts embryonic coronary transmural patterning, leading to embryonic death. Taken together, our results demonstrate that ST/PE-derived endothelial cells contribute significantly to and are required for proper coronary vascular morphogenesis.

Characterisation of the human embryonic and foetal epicardium during heart development

The epicardium is essential for mammalian heart development. At present, our understanding of the timing and morphogenetic events leading to the formation of the human epicardium has essentially been extrapolated from model organisms. Here, we studied primary tissue samples to characterise human epicardium development. We reveal that the epicardium begins to envelop the myocardial surface at Carnegie stage (CS) 11 and this process is completed by CS15, earlier than previously inferred from avian studies. Contrary to prevailing dogma, the formed human epicardium is not a simple squamous epithelium and we reveal evidence of more complex structure, including novel spatial differences aligned to the developing chambers. Specifically, the ventricular, but not atrial, epicardium exhibited areas of expanded epithelium, preferential cell alignment and spindle-like morphology. Likewise, we reveal distinct properties ex vivo, such that ventricular cells spontaneously differentiate and lose epicardial identity, whereas atrial-derived cells remained 'epithelial-like'. These data provide insight into the developing human epicardium that may contribute to our understanding of congenital heart disease and have implications for the development of strategies for endogenous cell-based cardiac repair.

Human fetal cardiac progenitors: The role of stem cells and progenitors in the fetal and adult heart

The human fetal heart is formed early during embryogenesis as a result of cell migrations, differentiation, and formative blood flow. It begins to beat around gestation day 22. Progenitor cells are derived from mesoderm (endocardium and myocardium), proepicardium (epicardium and coronary vessels), and neural crest (heart valves, outflow tract septation, and parasympathetic innervation). A variety of molecular disturbances in the factors regulating the specification and differentiation of these cells can cause congenital heart disease. This review explores the contribution of different cardiac progenitors to the embryonic heart development; the pathways and transcription factors guiding their expansion, migration, and functional differentiation; and the endogenous regenerative capacity of the adult heart including the plasticity of cardiomyocytes. Unfolding these mechanisms will become the basis for understanding the dynamics of specific congenital heart disease as well as a means to develop therapy for fetal as well as postnatal cardiac defects and heart failure.

Histology and Immunohistochemistry of the Cardiac Ventricular Structure in the Green Turtle (Chelonia mydas)

This study describes the implications of cardiac ventricular microscopy in Chelonia mydas relating to its ability to dive. For this work, 11 specimens of the marine turtle species C. mydas found dead on the coast of Rio Grande do Norte (Northeast Brazil) were used. After necropsy, fragments of the cardiac ventricular wall were fixed in 10% buffered formaldehyde solution for 24 h and then subjected to routine processing for light and scanning electron microscopy (SEM). The ventricle in this species is formed by the epicardium, myocardium and endocardium. The subepicardial layer consists of highly vascularised connective tissue that emits septa to reinforce the myocardium surface. There is an abundant and diffuse subepicardial nerve plexus shown by immunostaining technique. The thickness of the spongy myocardium and the nature of its trabeculae varied between the heart chambers. The endocardium shows no characteristic elements of the heart conduction system. The valves have a hyaline cartilage skeleton, coated by dense irregular connective tissues characterised by elastic fibres. These findings in the green turtle ventricular microscopy are related to hypoxia resistance during diving.

Developmental origin of age-related coronary artery disease

AIM

Age and injury cause structural and functional changes in coronary artery smooth muscle cells (caSMCs) that influence the pathogenesis of coronary artery disease. Although paracrine signalling is widely believed to drive phenotypic changes in caSMCs, here we show that developmental origin within the fetal epicardium can have a profound effect as well.

METHODS AND RESULTS

Fluorescent dye and transgene pulse-labelling techniques in mice revealed that the majority of caSMCs are derived from Wt1(+), Gata5-Cre(+) cells that migrate before E12.5, whereas a minority of cells are derived from a later-emigrating, Wt1(+), Gata5-Cre(-) population. We functionally evaluated the influence of early emigrating cells on coronary artery development and disease by Gata5-Cre excision of Rbpj, which prevents their contribution to coronary artery smooth muscle cells. Ablation of the Gata5-Cre(+) population resulted in coronary arteries consisting solely of Gata5-Cre(-) caSMCs. These coronary arteries appeared normal into early adulthood; however, by 5-8 months of age, they became progressively fibrotic, lost the adventitial outer elastin layer, were dysfunctional and leaky, and animals showed early mortality.

CONCLUSION

Taken together, these data reveal heterogeneity in the fetal epicardium that is linked to coronary artery integrity, and that distortion of the coronaries epicardial origin predisposes to adult onset disease.

Casz1 is required for cardiomyocyte G1-to-S phase progression during mammalian cardiac development

Organ growth occurs through the integration of external growth signals during the G1 phase of the cell cycle to initiate DNA replication. Although numerous growth factor signals have been shown to be required for the proliferation of cardiomyocytes, genetic studies have only identified a very limited number of transcription factors that act to regulate the entry of cardiomyocytes into S phase. Here, we report that the cardiac para-zinc-finger protein CASZ1 is expressed in murine cardiomyocytes. Genetic fate mapping with an inducible Casz1 allele demonstrates that CASZ1-expressing cells give rise to cardiomyocytes in the first and second heart fields. We show through the generation of a cardiac conditional null mutation that Casz1 is essential for the proliferation of cardiomyocytes in both heart fields and that loss of Casz1 leads to a decrease in cardiomyocyte cell number. We further report that the loss of Casz1 leads to a prolonged or arrested S phase, a decrease in DNA synthesis, an increase in phospho-RB and a concomitant decrease in the cardiac mitotic index. Taken together, these studies establish a role for CASZ1 in mammalian cardiomyocyte cell cycle progression in both the first and second heart fields.

Generation of cardiac progenitor cells through epicardial to mesenchymal transition

The epithelial to mesenchymal transition (EMT) is a biological process that drives the formation of cells involved both in tissue repair and in pathological conditions, including tissue fibrosis and tumor metastasis by providing cancer cells with stem cell properties. Recent findings suggest that EMT is reactivated in the heart following ischemic injury. Specifically, epicardial EMT might be involved in the formation of cardiac progenitor cells (CPCs) that can differentiate into endothelial cells, smooth muscle cells, and, possibly, cardiomyocytes. The identification of mechanisms and signaling pathways governing EMT-derived CPC generation and differentiation may contribute to the development of a more efficient regenerative approach for adult heart repair. Here, we summarize key literature in the field.

Wilms' tumor 1 (WT1) protein expression in human developing tissues

Several genes playing crucial roles in human development often reproduce a key role also during the onset and progression of malignant tumors. WT1, a transcription factor expressed with a dynamic pattern during human development, has either oncogenic or suppressor tumor properties. A detailed analysis of the immunohistochemical profile of WT1 protein in human developmental tissues could be exploitable as the rational for better understanding its role in cancerogenesis and planning innovative WT1-based therapeutic approaches. This review focuses on the dynamic immunohistochemical expression and distribution of WT1 protein during human ontogenesis, providing illustrations and discussion on the most relevant findings. The possibility that WT1 nuclear/cytoplasmic expression in some tumors mirrors its normal developmental regulation will be emphasized.

Robust derivation of epicardium and its differentiated smooth muscle cell progeny from human pluripotent stem cells

The epicardium has emerged as a multipotent cardiovascular progenitor source with therapeutic potential for coronary smooth muscle cell, cardiac fibroblast (CF) and cardiomyocyte regeneration, owing to its fundamental role in heart development and its potential ability to initiate myocardial repair in injured adult tissues. Here, we describe a chemically defined method for generating epicardium and epicardium-derived smooth muscle cells (EPI-SMCs) and CFs from human pluripotent stem cells (HPSCs) through an intermediate lateral plate mesoderm (LM) stage. HPSCs were initially differentiated to LM in the presence of FGF2 and high levels of BMP4. The LM was robustly differentiated to an epicardial lineage by activation of WNT, BMP and retinoic acid signalling pathways. HPSC-derived epicardium displayed enhanced expression of epithelial- and epicardium-specific markers, exhibited morphological features comparable with human foetal epicardial explants and engrafted in the subepicardial space in vivo. The in vitro-derived epicardial cells underwent an epithelial-to-mesenchymal transition when treated with PDGF-BB and TGFβ1, resulting in vascular SMCs that displayed contractile ability in response to vasoconstrictors. Furthermore, the EPI-SMCs displayed low density lipoprotein uptake and effective lowering of lipoprotein levels upon treatment with statins, similar to primary human coronary artery SMCs. Cumulatively, these findings suggest that HPSC-derived epicardium and EPI-SMCs could serve as important tools for studying human cardiogenesis, and as a platform for vascular disease modelling and drug screening.

Cellular origin and developmental program of coronary angiogenesis

Coronary artery disease causes acute myocardial infarction and heart failure. Identifying coronary vascular progenitors and their developmental program could inspire novel regenerative treatments for cardiac diseases. The developmental origins of the coronary vessels have been shrouded in mystery and debated for several decades. Recent identification of progenitors for coronary vessels within the endocardium, epicardium, and sinus venosus provides new insights into this question. In addition, significant progress has been achieved in elucidating the cellular and molecular programs that orchestrate coronary artery development. Establishing adequate vascular supply will be an essential component of cardiac regenerative strategies, and these findings raise exciting new strategies for therapeutic cardiac revascularization.

Induced pluripotent stem cell-derived vascular smooth muscle cells: methods and application

Vascular smooth muscle cells (VSMCs) play a major role in the pathophysiology of cardiovascular diseases. The advent of induced pluripotent stem cell (iPSC) technology and the capability of differentiating into virtually every cell type in the human body make this field a ray of hope for vascular regenerative therapy and understanding of the disease mechanism. In the present review, we first discuss the recent iPSC technology and vascular smooth muscle development from an embryo and then examine different methodologies to derive VSMCs from iPSCs, and their applications in regenerative therapy and disease modelling.

Programming and reprogramming a human heart cell

The latest discoveries and advanced knowledge in the fields of stem cell biology and developmental cardiology hold great promise for cardiac regenerative medicine, enabling researchers to design novel therapeutic tools and approaches to regenerate cardiac muscle for diseased hearts. However, progress in this arena has been hampered by a lack of reproducible and convincing evidence, which at best has yielded modest outcomes and is still far from clinical practice. To address current controversies and move cardiac regenerative therapeutics forward, it is crucial to gain a deeper understanding of the key cellular and molecular programs involved in human cardiogenesis and cardiac regeneration. In this review, we consider the fundamental principles that govern the "programming" and "reprogramming" of a human heart cell and discuss updated therapeutic strategies to regenerate a damaged heart.

Semaphorin signaling in cardiovascular development

Semaphorins were originally identified as neuronal guidance molecules mediating their attractive or repulsive signals by forming complexes with plexin and neuropilin receptors. Subsequent research has identified functions for semaphorin signaling in many organs and tissues outside of the nervous system. Vital roles for semaphorin signaling in vascular patterning and cardiac morphogenesis have been demonstrated, and impaired semaphorin signaling has been associated with various human cardiovascular disorders, including persistent truncus arteriosus, sinus bradycardia and anomalous pulmonary venous connections. Here, we review the functions of semaphorins and their receptors in cardiovascular development and disease and highlight important recent discoveries in the field.

Extracardiac control of embryonic cardiomyocyte proliferation and ventricular wall expansion

AIMS

The strategies that control formation of the ventricular wall during heart development are not well understood. In previous studies, we documented IGF2 as a major mitogenic signal that controls ventricular cardiomyocyte proliferation and chamber wall expansion. Our objective in this study was to define the tissue source of IGF2 in heart development and the upstream pathways that control its expression.

METHODS AND RESULTS

Using a number of mouse genetic tools, we confirm that the critical source of IGF2 is the epicardium. We find that epicardial Igf2 expression is controlled in a biphasic manner, first induced by erythropoietin and then regulated by oxygen and glucose with onset of placental function. Both processes are independently controlled by retinoic acid signalling.

CONCLUSIONS

Our results demonstrate that ventricular wall cardiomyocyte proliferation is subdivided into distinct regulatory phases. Each involves instructive cues that originate outside the heart and thereby act on the epicardium in an endocrine manner, a mode of regulation that is mostly unknown in embryogenesis.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

Cardiac-Specific Overexpression of Human Stem Cell Factor Promotes Epicardial Activation and Arteriogenesis After Myocardial Infarction

Background—

The adult epicardium is a potential source of cardiac progenitors after myocardial infarction (MI). We tested the hypothesis that cardiomyocyte-specific overexpression of membrane-associated human stem cell factor (hSCF) enhances epicardial activation, epicardium-derived cells (EPDCs) production, and myocardial arteriogenesis post MI.

Methods and Results—

Wild-type and the inducible cardiac-specific hSCF transgenic (hSCF/tetracycline transactivator) mice were subjected to MI. Wilms tumor-1 (Wt1)–positive epicardial cells were higher in hSCF/tetracycline transactivator compared with wild-type mice 3 days post MI. Arteriole density was significantly higher in the peri-infarct area of hSCF/tetracycline transactivator mice compared with wild-type mice 5 days post MI. In cultured EPDCs, adenoviral hSCF treatment significantly increased cell proliferation and growth factor expression. Furthermore, adenoviral hSCF treatment in wild-type cardiomyocytes significantly increased EPDC migration. These effects of hSCF overexpression on EPDC proliferation and growth factor expression were all abrogated by ACK2, a neutralizing antibody against c-kit . Finally, lineage tracing using ROSA mTmG ;Wt1 CreER mice showed that adenoviral hSCF treatment increased Wt1 + lineage–derived EPDC migration into the infarcted myocardium 5 days post MI, which was inhibited by ACK2.

Conclusions—

Cardiomyocyte-specific overexpression of hSCF promotes epicardial activation and myocardial arteriogenesis post MI.

Epicardium is required for sarcomeric maturation and cardiomyocyte growth in the ventricular compact layer mediated by transforming growth factor β and fibroblast growth factor before the onset of coronary circulation

The epicardium, which is derived from the proepicardial organ (PE) as the third epithelial layer of the developing heart, is crucial for ventricular morphogenesis. An epicardial deficiency leads to a thin compact layer for the developing ventricle; however, the mechanisms leading to the impaired development of the compact layer are not well understood. Using chick embryonic hearts, we produced epicardium-deficient hearts by surgical ablation or blockade of the migration of PE and examined the mechanisms underlying a thin compact myocardium. Sarcomeric maturation (distance between Z-lines) and cardiomyocyte growth (size) were affected in the thin compact myocardium of epicardium-deficient ventricles, in which the amounts of phospho-smad2 and phospho-ERK as well as expression of transforming growth factor (TGF)β2 and fibroblast growth factor (FGF)2 were reduced. TGFβ and FGF were required for the maturation of sarcomeres and growth of cardiomyocytes in cultured ventricles. In ovo co-transfection of dominant negative (dN)-Alk5 (dN-TGFβ receptor I) and dN-FGF receptor 1 to ventricles caused a thin compact myocardium. Our results suggest that immature sarcomeres and small cardiomyocytes are the causative architectures of an epicardium-deficient thin compact layer and also that epicardium-dependent signaling mediated by TGFβ and FGF plays a role in the development of the ventricular compact layer before the onset of coronary circulation.

Developmental heterogeneity of cardiac fibroblasts does not predict pathological proliferation and activation

RATIONALE

Fibrosis is mediated partly by extracellular matrix-depositing fibroblasts in the heart. Although these mesenchymal cells are reported to have multiple embryonic origins, the functional consequence of this heterogeneity is unknown.

OBJECTIVE

We sought to validate a panel of surface markers to prospectively identify cardiac fibroblasts. We elucidated the developmental origins of cardiac fibroblasts and characterized their corresponding phenotypes. We also determined proliferation rates of each developmental subset of fibroblasts after pressure overload injury.

METHODS AND RESULTS

We showed that Thy1(+)CD45(-)CD31(-)CD11b(-)Ter119(-) cells constitute the majority of cardiac fibroblasts. We characterized these cells using flow cytometry, epifluorescence and confocal microscopy, and transcriptional profiling (using reverse transcription polymerase chain reaction and RNA-seq). We used lineage tracing, transplantation studies, and parabiosis to show that most adult cardiac fibroblasts derive from the epicardium, a minority arises from endothelial cells, and a small fraction from Pax3-expressing cells. We did not detect generation of cardiac fibroblasts by bone marrow or circulating cells. Interestingly, proliferation rates of fibroblast subsets on injury were identical, and the relative abundance of each lineage remained the same after injury. The anatomic distribution of fibroblast lineages also remained unchanged after pressure overload. Furthermore, RNA-seq analysis demonstrated that Tie2-derived and Tbx18-derived fibroblasts within each operation group exhibit similar gene expression profiles.

CONCLUSIONS

The cellular expansion of cardiac fibroblasts after transaortic constriction surgery was not restricted to any single developmental subset. The parallel proliferation and activation of a heterogeneous population of fibroblasts on pressure overload could suggest that common signaling mechanisms stimulate their pathological response.

Cardiac fibroblast in development and wound healing

Cardiac fibroblasts are the most abundant cell type in the mammalian heart and comprise approximately two-thirds of the total number of cardiac cell types. During development, epicardial cells undergo epithelial-mesenchymal-transition to generate cardiac fibroblasts that subsequently migrate into the developing myocardium to become resident cardiac fibroblasts. Fibroblasts form a structural scaffold for the attachment of cardiac cell types during development, express growth factors and cytokines and regulate proliferation of embryonic cardiomyocytes. In post natal life, cardiac fibroblasts play a critical role in orchestrating an injury response. Fibroblast activation and proliferation early after cardiac injury are critical for maintaining cardiac integrity and function, while the persistence of fibroblasts long after injury leads to chronic scarring and adverse ventricular remodeling. In this review, we discuss the physiologic function of the fibroblast during cardiac development and wound healing, molecular mediators of activation that could be possible targets for drug development for fibrosis and finally the use of reprogramming technologies for reversing scar. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Cell-cell interaction in the heart via Wnt/β-catenin pathway after cardiac injury

The adult mammalian heart predominantly comprises myocytes, fibroblasts, endothelial cells, smooth muscle cells, and epicardial cells arranged in a precise three-dimensional framework. Following cardiac injury, the spatial arrangement of cells is disrupted as different populations of cells are recruited to the heart in a temporally regulated manner. The alteration of the cellular composition of the heart after cardiac injury thus enables different phenotypes of cells to interact with each other in a spatio-temporal-dependent manner. It can be argued that the integrated study of such cellular interactions rather than the examination of single populations of cells can provide more insights into the biology of cardiac repair especially at an organ-wide level. Many signalling systems undoubtedly mediate such cross talk between cells after cardiac injury. The Wnt/β-catenin system plays an important role during cardiac development and disease. Here, we describe how cell populations in the heart after cardiac injury mediate their interactions via the Wnt/β-catenin pathway, determine how such interactions can affect a cardiac repair response and finally suggest an integrated approach to study cardiac cellular interactions.

Embryonic origins of human vascular smooth muscle cells: implications for in vitro modeling and clinical application

Vascular smooth muscle cells (SMCs) arise from multiple origins during development, raising the possibility that differences in embryological origins between SMCs could contribute to site-specific localization of vascular diseases. In this review, we first examine the developmental pathways and embryological origins of vascular SMCs and then discuss in vitro strategies for deriving SMCs from human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). We then review in detail the potential for vascular disease modeling using iPSC-derived SMCs and consider the pathological implications of heterogeneous embryonic origins. Finally, we touch upon the role of human ESC-derived SMCs in therapeutic revascularization and the challenges remaining before regenerative medicine using ESC- or iPSC-derived cells comes of age.

The chick embryo as an expanding experimental model for cancer and cardiovascular research

A long and productive history in biomedical research defines the chick as a model for human biology. Fundamental discoveries, including the description of directional circulation propelled by the heart and the link between oncogenes and the formation of cancer, indicate its utility in cardiac biology and cancer. Despite the more recent arrival of several vertebrate and invertebrate animal models during the last century, the chick embryo remains a commonly used model for vertebrate biology and provides a tractable biological template. With new molecular and genetic tools applied to the avian genome, the chick embryo is accelerating the discovery of normal development and elusive disease processes. Moreover, progress in imaging and chick culture technologies is advancing real-time visualization of dynamic biological events, such as tissue morphogenesis, angiogenesis, and cancer metastasis. A rich background of information, coupled with new technologies and relative ease of maintenance, suggest an expanding utility for the chick embryo in cardiac biology and cancer research.

Embryonic heart progenitors and cardiogenesis

The mammalian heart is a highly specialized organ, comprised of many different cell types arising from distinct embryonic progenitor populations during cardiogenesis. Three precursor populations have been identified to contribute to different myocytic and nonmyocytic cell lineages of the heart: cardiogenic mesoderm cells (CMC), the proepicardium (PE), and cardiac neural crest cells (CNCCs). This review will focus on molecular cues necessary for proper induction, expansion, and lineage-specific differentiation of these progenitor populations during cardiac development in vivo. Moreover, we will briefly discuss how the knowledge gained on embryonic heart progenitor biology can be used to develop novel therapeutic strategies for the management of congenital heart disease as well as for improvement of cardiac function in ischemic heart disease.

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.

Epicardial function of canonical Wnt-, Hedgehog-, Fgfr1/2-, and Pdgfra-signalling

AIMS

The embryonic epicardium is a source of smooth muscle cells and fibroblasts of the coronary vasculature and of the myocardium, but the signalling pathways that control mobilization and differentiation of epicardial cells are only partly known. We aimed to (re-)evaluate the relevance of canonical Wnt-, Hedgehog (Hh)-, Fibroblast growth factor receptor (Fgfr)1/2-, and platelet-derived growth factor receptor alpha (Pdgfra)-signalling in murine epicardial development.

METHODS AND RESULTS

We used a T-box 18 (Tbx18)(cre)-mediated conditional approach to delete and to stabilize, respectively, the downstream mediator of canonical Wnt-signalling, beta-catenin (Ctnnb1), to delete and activate the mediator of Hh-signalling, smoothened (Smo), and to delete Fgfr1/Fgfr2 and Pdgfra in murine epicardial development. We show that epicardial loss of Ctnnb1, Smo, or Fgfr1/Fgfr2 does not affect cardiac development, whereas the loss of Pdgfra prevents the differentiation of epicardium-derived cells into mature fibroblasts. Epicardial expression of a stabilized version of Ctnnb1 results in the formation of hyperproliferative epicardial cell clusters; epicardial expression of a constitutively active version of Smo leads to epicardial thickening and loss of epicardial mobilization.

CONCLUSION

Canonical Wnt-, Hh-, and Fgfr1/Fgfr2-signalling are dispensable for epicardial development, but Pdgfra-signalling is crucial for the differentiation of cardiac fibroblasts from epicardium-derived cells.

Epicardium Formation as a Sensor in Toxicology

Zebrafish (Danio rerio) are an excellent vertebrate model for studying heart development, regeneration and cardiotoxicity. Zebrafish embryos exposed during the temporal window of epicardium development to the aryl hydrocarbon receptor (AHR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibit severe heart malformations. TCDD exposure prevents both proepicardial organ (PE) and epicardium development. Exposure later in development, after the epicardium has formed, does not produce cardiac toxicity. It is not until the adult zebrafish heart is stimulated to regenerate does TCDD again cause detrimental effects. TCDD exposure prior to ventricular resection prevents cardiac regeneration. It is likely that TCDD-induced inhibition of epicardium development and cardiac regeneration occur via a common mechanism. Here, we describe experiments that focus on the epicardium as a target and sensor of zebrafish heart toxicity.

Induction of the Proepicardium

The proepicardium is a transient extracardiac embryonic tissue that gives rise to the epicardium and a number of coronary vascular cell lineages. This important extracardiac tissue develops through multiple steps of inductive events, from specification of multiple cell lineages to morphogenesis. This article will review our current understanding of inductive events involved in patterning of the proepicardium precursor field, specification of cell types within the proepicardium, and their extension and attachment to the heart.

Development of the human heart

Molecular and genetic studies around the turn of this century have revolutionized the field of cardiac development. We now know that the primary heart tube, as seen in the early embryo contains little more than the precursors for the left ventricle, whereas the precursor cells for the remainder of the cardiac components are continuously added, to both the venous and arterial pole of the heart tube, from a single center of growth outside the heart. While the primary heart tube is growing by addition of cells, it does not show significant cell proliferation, until chamber differentiation and expansion starts locally in the tube, by which the chambers balloon from the primary heart tube. The transcriptional repressors Tbx2 and Tbx3 locally repress the chamber-specific program of gene expression, by which these regions are allowed to differentiate into the distinct components of the conduction system. Molecular genetic lineage analyses have been extremely valuable to assess the distinct developmental origin of the various component parts of the heart, which currently can be unambiguously identified by their unique molecular phenotype. Despite the enormous advances in our knowledge on cardiac development, even the most common congenital cardiac malformations are only poorly understood. The challenge of the newly developed molecular genetic techniques is to unveil the basic gene regulatory networks underlying cardiac morphogenesis.

Comparative Analyses of MicroRNA Microarrays during Cardiogenesis: Functional Perspectives

Cardiovascular development is a complex process in which several transcriptional pathways are operative, providing instructions to the developing cardiomyocytes, while coping with contraction and morphogenetic movements to shape the mature heart. The discovery of microRNAs has added a new layer of complexity to the molecular mechanisms governing the formation of the heart. Discrete genetic ablation of the microRNAs processing enzymes, such as Dicer and Drosha, has highlighted the functional roles of microRNAs during heart development. Importantly, selective deletion of a single microRNA, miR-1-2, results in an embryonic lethal phenotype in which both morphogenetic, as well as impaired conduction, phenotypes can be observed. In an effort to grasp the variability of microRNA expression during cardiac morphogenesis, we recently reported the dynamic expression profile during ventricular development, highlighting the importance of miR-27 on the regulation of a key cardiac transcription factor, Mef2c. In this review, we compare the microRNA expression profile in distinct models of cardiogenesis, such as ventricular chamber development, induced pluripotent stem cell (iPS)-derived cardiomyocytes and the aging heart. Importantly, out of 486 microRNAs assessed in the developing heart, 11% (55) displayed increased expression, many of which are also differentially expressed in distinct cardiogenetic experimental models, including iPS-derived cardiomyocytes. A review on the functional analyses of these differentially expressed microRNAs will be provided in the context of cardiac development, highlighting the resolution and power of microarrays analyses on the quest to decipher the most relevant microRNAs in the developing, aging and diseased heart.

The transcription factors Tbx18 and Wt1 control the epicardial epithelial-mesenchymal transition through bi-directional regulation of Slug in murine primary epicardial cells

During cardiac development, a subpopulation of epicardial cells migrates into the heart as part of the epicardial epithelial-mesenchymal transition (EMT) and differentiates into smooth muscle cells and fibroblasts. However, the roles of transcription factors in the epicardial EMT are poorly understood. Here, we show that two transcription factors expressed in the developing epicardium, T-box18 (Tbx18) and Wilms’ tumor 1 homolog (Wt1), bi-directionally control the epicardial EMT through their effects on Slug expression in murine primary epicardial cells. Knockdown of Wt1 induced the epicardial EMT, which was accompanied by an increase in the migration and expression of N-cadherin and a decrease in the expression of ZO-1 as an epithelial marker. By contrast, knockdown of Tbx18 inhibited the mesenchymal transition induced by TGFβ1 treatment and Wt1 knockdown. The expression of Slug but not Snail decreased as a result of Tbx18 knockdown, but Slug expression increased following knockdown of Wt1. Knockdown of Slug also attenuated the epicardial EMT induced by TGFβ1 treatment and Wt1 knockdown. Furthermore, in normal murine mammary gland-C7 (NMuMG-C7) cells, Tbx18 acted to increase Slug expression, while Wt1 acted to decrease Slug expression. Chromatin immunoprecipitation and promoter assay revealed that Tbx18 and Wt1 directly bound to the Slug promoter region and regulated Slug expression. These results provide new insights into the regulatory mechanisms that control the epicardial EMT.

Characterization of epicardial-derived cardiac interstitial cells: differentiation and mobilization of heart fibroblast progenitors

The non-muscular cells that populate the space found between cardiomyocyte fibers are known as ‘cardiac interstitial cells’ (CICs). CICs are heterogeneous in nature and include different cardiac progenitor/stem cells, cardiac fibroblasts and other cell types. Upon heart damage CICs soon respond by initiating a reparative response that transforms with time into extensive fibrosis and heart failure. Despite the biomedical relevance of CICs, controversy remains on the ontogenetic relationship existing between the different cell kinds homing at the cardiac interstitium, as well as on the molecular signals that regulate their differentiation, maturation, mutual interaction and role in adult cardiac homeostasis and disease. Our work focuses on the analysis of epicardial-derived cells, the first cell type that colonizes the cardiac interstitium. We present here a characterization and an experimental analysis of the differentiation potential and mobilization properties of a new cell line derived from mouse embryonic epicardium (EPIC). Our results indicate that these cells express some markers associated with cardiovascular stemness and retain part of the multipotent properties of embryonic epicardial derivatives, spontaneously differentiating into smooth muscle, and fibroblast/myofibroblast-like cells. Epicardium-derived cells are also shown to initiate a characteristic response to different growth factors, to display a characteristic proteolytic expression profile and to degrade biological matrices in 3D in vitro assays. Taken together, these data indicate that EPICs are relevant to the analysis of epicardial-derived CICs, and are a god model for the research on cardiac fibroblasts and the role these cells play in ventricular remodeling in both ischemic or non/ischemic myocardial disease.

Thymosin β4 mobilizes mesothelial cells for blood vessel repair

Mesothelium is the simple squamous epithelium covering all abdominal organs and the coeloms in which those organs reside. While the structural characteristics of this cell type were documented a century ago, its potential in development, disease, and wound healing is only now becoming apparent. In the embryo, mesothelia provide vasculogenic cells for the developing heart, lungs, and gut. Furthermore, adult mesothelial cells can be reactivated using thymosin β4 and mobilized to aid in tissue repair. Despite their positive role in development and repair, mesothelia are also susceptible to adhesion and tumor formation. With knowledge that the mesothelium is an important mediator of tissue repair as well as disease, it will be important to identify other factors like thymosin β4 that have the ability to potentiate these cells. Future use of chemical and genetic agents in conjunction with mesothelial cells will lead to enhanced therapeutic potential and mitigation of deleterious outcomes.

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.

Epithelial-to-mesenchymal transition in epicardium is independent of Snail1

The epicardium is the outer epithelial covering the heart. This tissue undergoes an epithelial-to-mesenchymal transition (EMT) to generate mesenchymal epicardial-derived cells (EPDCs) that populate the extracellular matrix of the subepicardium and contribute to the development of the coronary vessels and cardiac interstitial cells. Although epicardial EMT plays a crucial role in heart development, the molecular regulation of this process is incompletely understood. Here we examined the possible role of the EMT regulator Snail1 in this process. Snail1 is expressed in the epicardium and EPDCs during mouse cardiac development. To determine the function of Snail1 in epicardial EMT, we deleted Snail1 in the epicardium using Wt1- and Tbx18-Cre drivers. Unexpectedly, epicardial-specific Snail1 mutants are viable and fertile and do not display any obvious morphological or functional cardiac abnormalities. Molecular analysis of these mice reveals that epicardial EMT occurs normally, and epicardial derivatives are established in these mutants. We conclude that Snail1 is not required for the initiation and progression of embryonic epicardial EMT.

Dioxin inhibits zebrafish epicardium and proepicardium development

Embryonic exposure to the environmental contaminant and aryl hydrocarbon receptor agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin), disrupts cardiac development and function in fish, birds, and mammals. In zebrafish, the temporal window of sensitivity to the cardiotoxic effects of TCDD coincides with epicardium formation. We hypothesized that this TCDD-induced heart failure results from disruption of epicardial development. To determine whether embryonic TCDD exposure inhibits epicardium and proepicardium (PE) development in zebrafish, we used histology and fluorescence immunocytochemistry to examine the epicardium formation in fish exposed to TCDD. TCDD exposure prevented epicardium formation. Using live imaging and in situ hybridization, we found that TCDD exposure blocked the formation of the PE cluster. In situ hybridization experiments showed that TCDD exposure also prevented the expression of the PE marker tcf21 at the site where the PE normally forms. TCDD also inhibited expansion of the epicardial layer across the developing heart: Exposure after PE formation was completed prevented further expansion of the epicardium. However, TCDD exposure did not affect epicardial cells already present. Because TCDD blocks epicardium formation, but is not directly toxic to the epicardium once complete, we propose that inhibition of epicardium formation can account for the window of sensitivity to TCDD cardiotoxicity in developing zebrafish. Epicardium development is crucial to heart development. Loss of this layer during development may account for most if not all of the TCDD-induced cardiotoxicity in zebrafish.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

The expanding role of the epicardium and epicardial-derived cells in cardiac development and disease

PURPOSE OF REVIEW

In this review, we aim at presenting and discussing the cellular and molecular mechanisms of embryonic epicardial development that may underlie the origin of congenital heart disease (CHD).

RECENT FINDINGS

New discoveries on the multiple cell lineages that form part of the original pool of epicardial progenitors and the roles played by epicardial transcription factors and morphogens in the regulation of epicardial epithelial-to-mesenchymal transition, epicardial-derived cell (EPDCs) differentiation, coronary blood vessel morphogenesis and cardiac interstitium formation are presented in a comprehensive manner.

SUMMARY

We have provided evidence on the critical participation of epicardial cells and EPDCs in normal and abnormal cardiac development, suggesting the implication of defective epicardial development in various forms of CHD.

Endocardial cushion morphogenesis and coronary vessel development require chicken ovalbumin upstream promoter-transcription factor II

OBJECTIVE

Septal defects and coronary vessel anomalies are common congenital heart defects, yet their ontogeny and the underlying genetic mechanisms are not well understood. Here, we investigated the role of chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII, NR2F2) in cardiac organogenesis.

METHODS AND RESULTS

We analyzed embryos deficient in COUP-TFII and observed a spectrum of cardiac defects, including atrioventricular septal defect, thin-walled myocardium, and abnormal coronary morphogenesis. We show by expression analysis that COUP-TFII is expressed in the endocardium and the epicardium but not in the myocardium of the ventricle. Using endothelial-specific COUP-TFII mutants and molecular approaches, we show that COUP-TFII deficiency resulted in endocardial cushion hypoplasia. This was attributed to the reduced growth and survival of atrioventricular cushion mesenchymal cells and defective epithelial-mesenchymal transformation (EMT) in the underlying endocardium. In addition, the endocardial EMT defect was accompanied by downregulation of Snai1, one of the master regulators of EMT, and upregulation of vascular endothelial-cadherin. Furthermore, we show that although COUP-TFII does not play a major role in the formation of epicardial cell cysts, it is critically important for the formation of epicardium. Ablation of COUP-TFII impairs epicardial EMT and coronary plexus formation.

CONCLUSIONS

Our results reveal that COUP-TFII plays cell-autonomous roles in the endocardium and the epicardium for endocardial and epicardial EMT, which are required for proper valve and coronary vessel formation during heart development.

Wt1 and epicardial fate mapping

RATIONALE

The embryonic epicardium is a crucial cell source of the cardiac fibrous skeleton as well as of the coronary system. Genetic lineage tracing systems based on Wt1 regulatory sequences provided evidence that epicardium-derived cells also adopt a myocardial fate in the mouse.

OBJECTIVE

To define the adequacy of Wt1-based lineage tracing systems for epicardial fate mapping.

METHODS AND RESULTS

Using in situ hybridization analysis and immunofluorescence on tissue sections, we detected endogenous expression of Wt1 mRNA and Wt1 protein in the proepicardium and epicardium and also in endothelial cells throughout cardiogenesis. Expression analysis of a sensitive GFP reporter showed that recombination mediated by cre recombinase in the Wt1(creEGFP) line occurs randomly and sporadically in all cells of the embryo. Recombination in cardiomyocytes was found in the linear heart tube before establishment of a (pro)epicardium. In contrast, the tamoxifen-inducible Wt1(creERT2) mouse line mediated poor and variable recombination in the epicardium. Recombination in cardiomyocytes was not detected in this case.

CONCLUSIONS

Frequently used Wt1 based cre-mediated lineage tracing systems are not suitable for epicardial fate mapping because of endogenous endothelial expression of Wt1, ectopic recombination (Wt1(creEGFP)), and poor recombination efficiency (Wt1(creERT2)) in the developing heart. We conclude that claims of a cardiomyocyte fate of epicardial cells in the mouse are not substantiated.

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.