Epicardial Progenitor Cells Research Articles

Heartbeat-Driven Pericardiac Fluid Forces Contribute to Epicardium Morphogenesis

Hydrodynamic forces play a central role in organ morphogenesis. The role of blood flow in shaping the developing heart is well established, but the role of fluid forces generated in the pericardial cavity surrounding the heart is unknown. Mesothelial cells lining the pericardium generate the proepicardium (PE), the precursor cell population of the epicardium, the outer layer covering the myocardium, which is essential for its maturation and the formation of the heart valves and coronary vasculature. However, there is no evidence from in vivo studies showing how epicardial precursor cells reach and attach to the heart. Using optical tools for real-time analysis in the zebrafish, including high-speed imaging and optical tweezing, we show that the heartbeat generates pericardiac fluid advections that drive epicardium formation. These flow forces trigger PE formation and epicardial progenitor cell release and motion. The pericardial flow also influences the site of PE cell adhesion to the myocardium. We additionally identify novel mesothelial sources of epicardial precursors and show that precursor release and adhesion occur both through pericardiac fluid advections and through direct contact with the myocardium. Two hydrodynamic forces couple cardiac development and function: first, blood flow inside the heart, and second, the pericardial fluid advections outside the heart identified here. This pericardiac fluid flow is essential for epicardium formation and represents the first example of hydrodynamic flow forces controlling organogenesis through an action on mesothelial cells.

Abstract 011: Transcriptional Basis Of Epicardial Progenitor Cell Activation During Heart Development, Regeneration And Repair

The epicardium encapsulates the heart and functions as a source of multipotent progenitor cells and paracrine factors essential for cardiac development and repair. Injury of the adult heart results in reactivation of epicardial progenitor cells, which reengages a developmental gene program, but the underlying transcriptional basis has not been delineated. We established a mouse embryonic heart organ culture and gene expression system that facilitated the identification of epicardial enhancers activated during heart development and injury. Epicardial activation of these enhancers depends on a combinatorial transcriptional code centered on C/EBP, HOX, MEIS, and GRAINYHEAD families of transcription factors. Furthermore, disruption of C/EBP signaling in the adult epicardium reduced injury-induced neutrophil infiltration and improved cardiac function. These findings reveal a transcriptional basis for epicardial progenitor cell activation during heart development and injury, providing a platform for enhancing cardioprotection and regeneration.

Epicardial Lineages

The epicardium is the mono-layered epithelium that covers the outer surface of the myocardium from early in cardiac development. Long thought to act merely passively to protect the myocardium from frictional forces in the pericardial cavity during the enduring contraction and expansion cycles of the heart, it is now considered to be a crucial source of cells and signals that direct myocardial growth and formation of the coronary vasculature during development and regeneration. Lineage tracing efforts in the chick, the mouse and the zebrafish unambiguously identified fibroblasts in interstitial and perivascular locations as well as coronary smooth muscle cells as the two major lineages that derive from epithelial-mesenchymal transition and subsequent differentiation from individual epicardial cells. However, controversies exist about an additional endothelial and myocardial fate of epicardial progenitor cells. Here, we review epicardial fate mapping efforts in three vertebrate model systems, describe their conceptual differences and discuss their methodological limitations to reach a consensus of the potential of (pro-)epicardial cells in vitro and in vivo.

Role of Prokineticin Receptor-1 in Epicardial Progenitor Cells

G protein-coupled receptors (GPCRs) form a large class of seven transmembrane (TM) domain receptors. The use of endogenous GPCR ligands to activate the stem cell maintenance or to direct cell differentiation would overcome many of the problems currently encountered in the use of stem cells, such as rapid in vitro differentiation and expansion or rejection in clinical applications. This review focuses on the definition of a new GPCR signaling pathway activated by peptide hormones, called "prokineticins", in epicardium-derived cells (EPDCs). Signaling via prokineticin-2 and its receptor, PKR1, is required for cardiomyocyte survival during hypoxic stress. The binding of prokineticin-2 to PKR1 induces proliferation, migration and angiogenesis in endothelial cells. The expression of prokineticin and PKR1 increases during cardiac remodeling after myocardial infarction. Gain of function of PKR1 in the adult mouse heart revealed that cardiomyocyte-PKR1 signaling activates EPDCs in a paracrine fashion, thereby promoting de novo vasculogenesis. Transient PKR1 gene therapy after myocardial infarction in mice decreases mortality and improves heart function by promoting neovascularization, protecting cardiomyocytes and mobilizing WT1+ cells. Furthermore, PKR1 signaling promotes adult EPDC proliferation and differentiation to adopt endothelial and smooth muscle cell fate, for the induction of de novo vasculogenesis. PKR1 is expressed in the proepicardium and epicardial cells derived from mice kidneys. Loss of PKR1 causes deficits in EPDCs in the neonatal mice hearts and kidneys and impairs vascularization and heart and kidney function. Taken together, these data indicate a novel role for PKR1 in heart-kidney complex via EPDCs.

Molecular and cellular mechanisms of thymosin β4–mediated cardioprotection

Coronary heart disease is still the leading cause of death in industrialized nations. Reduction of infarct size after acute myocardial infarction and, in addition, improvement of myocardial function and perfusion in acute and chronic myocardial ischemia would enhance cardiac survival. Thymosin β4, a 43-amino acid water-soluble peptide with pleiotropic abilities seems to be a promising candidate for the treatment of ischemic heart disease. During cardiac development, thymosin β4 is essential for vascularization of the myocardium, by targeting all three parts of vessel development, that is, vasculogenesis, angiogenesis, and arteriogenesis. In the adult, thymosin β4 is capable of inducing angiogenesis via activation of survival kinases in an actin-dependent and -independent manner. In addition, thymosin β4 has anti-inflammatory properties by reducing NF-κB p65 activation. These protective effects are further enhanced through increased myocyte and endothelial cell survival accompanied by differentiation of epicardial progenitor 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.

Mab21l2 Is Essential for Embryonic Heart and Liver Development

During mouse embryogenesis, proper formation of the heart and liver is especially important and is crucial for embryonic viability. In this study, we showed that Mab21l2 was expressed in the trabecular and compact myocardium, and that deletion of Mab21l2 resulted in a reduction of the trabecular myocardium and thinning of the compact myocardium. Mab21l2-deficient embryonic hearts had decreased expression of genes that regulate cell proliferation and apoptosis of cardiomyocytes. These results show that Mab21l2 functions during heart development by regulating the expression of such genes. Mab21l2 was also expressed in the septum transversum mesenchyme (STM). Epicardial progenitor cells are localized to the anterior surface of the STM (proepicardium), and proepicardial cells migrate onto the surface of the heart and form the epicardium, which plays an important role in heart development. The rest of the STM is essential for the growth and survival of hepatoblasts, which are bipotential progenitors for hepatocytes and cholangiocytes. Proepicardial cells in Mab21l2-deficient embryos had defects in cell proliferation, which led to a small proepicardium, in which α4 integrin expression, which is essential for the migration of proepicardial cells, was down-regulated, suggesting that defects occurred in its migration. In Mab21l2-deficient embryos, epicardial formation was defective, suggesting that Mab21l2 plays important roles in epicardial formation through the regulation of the cell proliferation of proepicardial cells and the migratory process of proepicardial cells. Mab21l2-deficient embryos also exhibited hypoplasia of the STM surrounding hepatoblasts and decreased hepatoblast proliferation with a resultant loss of defective morphogenesis of the liver. These findings demonstrate that Mab21l2 plays a crucial role in both heart and liver development through STM formation.

Repairing the heart from within

U.K. researchers have shown that priming epicardial progenitor cells with RegeneRx's thymosin β4 peptide could promote the generation of mature cardiomyocytes after myocardial infarction. The findings point to an alternative to cell transplants to help decrease ischemic injury and increase cardiac repair after a heart attack.

Generation of cardiomyocytes from epicardial progenitor cells after myocardial infarction (MI)

Generation of cardiomyocytes from epicardial progenitor cells after myocardial infarction (MI)

Markers of Progenitor Cell Recruitment and Differentiation Rise Early During Ischemia and Continue During Resuscitation in a Porcine Acute Ischemia Model

Clinical administration of bone marrow-derived stem cells in the setting of acute myocardial infarction (AMI) leads to improved left ventricular ejection fraction. Thymosin beta-4 (TB4) and vascular endothelial growth factor (VEGF) are linked to adult epicardial progenitor cell mobilization and neovascularization and is cardioprotective after myocardial ischemia. This study investigated the time course of TB4 and VEGF during AMI, cardiac arrest, and resuscitation. Fifteen anesthetized and instrumented domestic swine underwent balloon occlusion of the proximal left anterior descending coronary artery. During occlusion, venous blood samples were collected from the right atrium at 5-min intervals until 15 min after the onset of cardiopulmonary resuscitation (CPR). Plasma levels of TB4, VEGF, and matrix metalloproteinase-9 (MMP-9, selected as a marker for remodeling and repair) were measured by ELISA. Generalized linear mixed models were employed to model the time-dependent change in plasma concentration. All variables were natural log transformed, except TB4 values, to normalize distributions. Fifteen animals successfully underwent balloon occlusion of the left anterior descending coronary artery and samples were collected from these subjects. The average onset of spontaneous ventricular fibrillation was 28 min. TB4, VEGF, and MMP-9 demonstrated a statistically significant, time-dependent increase in concentration during ischemia. Following arrest and throughout the first 15 min of resuscitation, MMP-9 had an unchanged rate of rise when compared with the prearrest, ischemic period, with VEGF showing a deceleration in its time-dependent concentration trajectory and TB4 demonstrating an acceleration. Endogenous TB4 and VEGF increase shortly after the onset of AMI and increase through cardiac arrest and resuscitation in parallel to remodeling proteases. These markers continue to rise during successful resuscitation and may represent an endogenous mechanism to recruit undifferentiated stem cells to areas of myocardial injury.

Differential Notch Signaling in the Epicardium Is Required for Cardiac Inflow Development and Coronary Vessel Morphogenesis

The proepicardium is a transient structure comprising epicardial progenitor cells located at the posterior limit of the embryonic cardiac inflow. A network of signals regulates proepicardial cell fate and defines myocardial and nonmyocardial domains at the venous pole of the heart. During cardiac development, epicardial-derived cells also contribute to coronary vessel morphogenesis. To study Notch function during proepicardium development and coronary vessel formation in the mouse. Using in situ hybridization, RT-PCR, and immunohistochemistry, we find that Notch pathway elements are differentially activated throughout the proepicardial-epicardial-coronary transition. Analysis of RBPJk-targeted embryos indicates that Notch ablation causes ectopic procardiogenic signaling in the proepicardium that in turn promotes myocardial differentiation in adjacent mesodermal progenitors, resulting in a premature muscularization of the sinus venosus horns. Epicardium-specific Notch1 ablation using a Wt1-Cre driver line disrupts coronary artery differentiation, reduces myocardium wall thickness and myocyte proliferation, and reduces Raldh2 expression. Ectopic Notch1 activation disrupts epicardium development and causes thinning of ventricular walls. Epicardial Notch modulates cell differentiation in the proepicardium and adjacent pericardial mesoderm. Notch1 is later required for arterial endothelium commitment and differentiation and for vessel wall maturation during coronary vessel development and myocardium growth.

Thymosin β4 mediated PKC activation is essential to initiate the embryonic coronary developmental program and epicardial progenitor cell activation in adult mice in vivo

Hypoxic heart disease is a predominant cause of disability and death worldwide. Since adult mammalian hearts are incapable of regeneration after hypoxia, attempts to modify this deficiency are critical. As demonstrated in zebrafish, recall of the embryonic developmental program may be the key to success. Because thymosin β4 (TB4) is beneficial for myocardial cell survival and essential for coronary development in embryos, we hypothesized that it reactivates the embryonic developmental program and initiates epicardial progenitor mobilization in adult mammals. We found that TB4 stimulates capillary-like tube formation of adult coronary endothelial cells and increases embryonic endothelial cell migration and proliferation in vitro. The increase of blood vessel/epicardial substance (Bves) expressing cells accompanied by elevated VEGF, Flk-1, TGF-β, F gfr-2, F gfr-4, F gf-17 and β-Catenin expression and increase of Tbx-18 and Wt-1 positive myocardial progenitors suggested organ-wide recall of the embryonic program in the adult epicardium. TB4 also positively regulated the expression and phosphorylation of myristoylated alanine-rich C-kinase substrate (Marcks), a direct substrate and indicator of protein kinase C (PKC) activity in vitro and in vivo. PKC inhibition significantly reduced TB4 initiated epicardial thickening, capillary growth and the number of myocardial progenitors. Our results demonstrate that TB4 is the first known molecule capable of organ-wide activation of the embryonic coronary developmental program in the adult mammalian heart after systemic administration and that PKC plays a significant role in the process.

Abstract 473: Genetic Fate Mapping Identifies Second Heart Field Epicardial Progenitor Cells as a Source of Adipocytes in Arrhythmogenic Right Ventricular Cardiomyopathy

The pathological hallmark of arrhythmogenic right ventricular cardiomyopathy (ARVC), a genetic disease of desmosomal proteins, is fibro-adipocytic replacement of cardiac myocytes. The cellular origin of adipocytes in ARVC is an enigma. In desmosomal ARVC the impetus has to instigate from cells that express the mutant protein. Cardiac myocytes are the only cells in the heart known to express the desmosomal proteins. Adult myocytes are terminally differentiated, and hence, an unlikely source. In contrast, cardiac progenitor cells can differentiate to different lineages. Thus, we conditionally deleted Dsp, encoding desmoplakin, and concomitantly expressed enhanced yellow fluorescent protein (EYFP) using the Cre-LoxP technique, regulated by Nkx2.5, an early cardiac lineage promoter. We also screened the human hearts with ARVC for co-expression of markers of cardiac cell lineages and adipocytes. We generated Nkx-2.5-Cre:Dsp W/F and Nkx-2.5-Cre:Dsp W/F :R26-EYFP F/F mice, which showed enlarged hearts, depressed cardiac systolic function and patchy areas of fibro-adiposis, particularly at the epicardium. We detected expression of EYFP by immunoblotting (IB) and immunofluorescence (IF) and expression of cardiac progenitor cells and adipocytes markers by IF. A subset of epicardial cells in the Nkx-2.5-Cre:Dsp W/F :R26-EYFP F/F mice co-expressed adipogenic transcription factors C/EBP-α and PPAR-γ along with EYFP and the second heart field markers IsI1 and Mef2c. Likewise, we detected co-expression of C/EBP-α and IsI1 or Mef2C in the fibro-adipocytic regions in human hearts with ARVC. To determine the specific cardiac cell lineage that differentiates to adipocytes, we co-stained sections of human and lineage trace mouse hearts and detected co-expression of SM actin, a marker of smooth muscle cells (SMCs) and C/EBP-α as well as co-expression of cardiac α-actin, a marker of cardiac myocytes, and C/EBP-α but not co-expression of PECAM1, a marker of endothelial cells and C/EBP-α. We conclude excess adipocytes in desmosomal ARVC originate from second heart field epicardial SMCs/myocyte progenitor cells.

PAR3 is essential for cyst-mediated epicardial development by establishing apical cortical domains

Epithelial cysts are one of the fundamental architectures for mammalian organogenesis. Although in vitro studies using cultured epithelial cells have revealed proteins required for cyst formation, the mechanisms that orchestrate the functions of these proteins in vivo remain to be clarified. We show that the targeted disruption of the mouse Par3 gene results in midgestational embryonic lethality with defective epicardial development. The epicardium is mainly derived from epicardial cysts and essential for cardiomyocyte proliferation during cardiac morphogenesis. PAR3-deficient epicardial progenitor (EPP) cells do not form cell cysts and show defects in the establishment of apical cortical domains, but not in basolateral domains. In PAR3-deficient EPP cells, the localizations of aPKC, PAR6beta and ezrin to the apical cortical domains are disturbed. By contrast, ZO1 and alpha4/beta1 integrins normally localize to cell-cell junctions and basal domains, respectively. Our observations indicate that EPP cell cyst formation requires PAR3 to interpret the polarity cues from cell-cell and cell-extracellular matrix interactions so that each EPP cell establishes apical cortical domains. These results also provide a clear example of the proper organization of epithelial tissues through the regulation of individual cell polarity.

Dual functions of [alpha]4[beta]1 integrin in epicardial development: initial migration and long-term attachment.

The epicardium of the mammalian heart arises from progenitor cells outside the developing heart. The epicardial progenitor (EPP) cells migrate onto the heart through a cyst-mediated mechanism in which the progenitors are released from the tissue of origin as cysts; the cysts float in the fluid of the pericardial cavity and attach to the naked myocardial surface of the heart, and cells in the cysts then migrate out to form an epithelial sheet. In this paper, we show that the gene encoding the α4 subunit of α4β1 integrin (α4β1) is essential for this migratory process. We have generated a knockin mutation in mice replacing the α4 integrin gene with the lacZ reporter gene, placing lacZ under the control of the α4 integrin promoter. We show that in homozygous mutant embryos, the migration of EPP progenitor cells is impaired due to inefficient budding of the cysts and a failure of the cells in the cysts to migrate on the heart. This study provides direct genetic evidence for essential roles for α4β1 integrin–mediated cell adhesion in the migration of progenitor cells to form the epicardium, in addition to a previous finding that α4β1 is essential for maintaining the epicardium (Yang, J.T., H. Rayburn, and R.O. Hynes. 1995. Development. 121:549–560).

Alpha4 integrin is expressed in a subset of cranial neural crest cells and in epicardial progenitor cells during early mouse development.

By RNA in situ hybridization and immunohistochemical analyses of early stage mouse embryos, we find that alpha 4 integrin gene is expressed in migratory cranial neural crest cells originating from the presumptive forebrain, midbrain, and rhombomeres 1 and 2 of the presumptive hindbrain. alpha 4 is also expressed in epicardial progenitor cells in the septum transversum that migrate to the heart.