Epicardium-derived cells (EPDCs) in development, cardiac disease and repair of ischemia

miR-194-3p regulates epithelial-mesenchymal transition in embryonic epicardial cells via p120/β-catenin signaling

The epicardium is integral to cardiac development and facilitates endogenous heart regeneration and repair. While miR-194-3p is associated with cellular migration and invasion, its impact on epicardial cells remains uncharted. In this work we use gain-of-function and loss-of-function methodologies to investigate the function of miR-194-3p in cardiac development. We culture embryonic epicardial cells in vitro and subject them to transforming growth factor β (TGF-β) treatment to induce epithelial-mesenchymal transition (EMT) and monitor miR-194-3p expression. In addition, the effects of miR-194-3p mimics and inhibitors on epicardial cell development and changes in EMT are investigated. To validate the binding targets of miR-194-3p and its ability to recover the target gene-phenotype, we produce a mutant vector p120-catenin-3'UTR-MUT. In epicardial cells, TGF-β-induced EMT results in a notable overexpression of miR-194-3p. The administration of miR-194-3p mimics promotes EMT, which is correlated with elevated levels of mesenchymal markers. Conversely, miR-194-3p inhibitor attenuates EMT. Further investigations reveal a negative correlation between miR-194-3p and p120-catenin, which influences β-catenin level in the cell adhesion pathway. The suppression of EMT caused by the miR-194-3p inhibitor is balanced by silencing of p120-catenin. In conclusion, miR-194-3p directly targets p120-catenin and modulates its expression, which in turn alters β-catenin expression, critically influencing the EMT process in the embryonic epicardial cells via the cell adhesion mechanism.

Upregulation of Wilms' Tumor 1 in epicardial cells increases cardiac fibrosis in dystrophic mice

Cardiomyopathy is a primary cause of mortality in Duchenne muscular dystrophy (DMD) patients. Mechanistic understanding of cardiac fibrosis holds the key to effective DMD cardiomyopathy treatments. Here we demonstrate that upregulation of Wilms' tumor 1 (Wt1) gene in epicardial cells increased cardiac fibrosis and impaired cardiac function in 8-month old mdx mice lacking the RNA component of telomerase (mdx/mTR-/-). Levels of phosphorylated IƙBα and p65 significantly rose in mdx/mTR-/- dystrophic hearts and Wt1 expression declined in the epicardium of mdx/mTR-/- mice when nuclear factor κB (NF-κB) and inflammation were inhibited by metformin. This demonstrates that Wt1 expression in epicardial cells is dependent on inflammation-triggered NF-κB activation. Metformin effectively prevented cardiac fibrosis and improved cardiac function in mdx/mTR-/- mice. Our study demonstrates that upregulation of Wt1 in epicardial cells contributes to fibrosis in dystrophic hearts and metformin-mediated inhibition of NF-κB can ameliorate the pathology, and thus showing clinical potential for dystrophic cardiomyopathy. Translational Perspective: Cardiomyopathy is a major cause of mortality in Duchenne muscular dystrophy (DMD) patients. Promising exon-skipping treatments are moving to the clinic, but getting sufficient dystrophin expression in the heart has proven challenging. The present study shows that Wilms' Tumor 1 (Wt1) upregulation in epicardial cells is primarily responsible for cardiac fibrosis and dysfunction of dystrophic mice and likely of DMD patients. Metformin effectively prevents cardiac fibrosis and improves cardiac function in dystrophic mice, thus representing a treatment option for DMD patients on top of existing therapies.

A Systematic Review of Ebstein’s Anomaly with Left Ventricular Noncompaction

Traditional definitions of Ebstein’s anomaly (EA) and left ventricular noncompaction (LVNC), two rare congenital heart defects (CHDs), confine disease to either the right or left heart, respectively. Around 15–29% of patients with EA, which has a prevalence of 1 in 20,000 live births, commonly manifest with LVNC. While individual EA or LVNC literature is extensive, relatively little discussion is devoted to the joint appearance of EA and LVNC (EA/LVNC), which poses a higher risk of poor clinical outcomes. We queried PubMed, Medline, and Web of Science for all peer-reviewed publications from inception to February 2022 that discuss EA/LVNC and found 58 unique articles written in English. Here, we summarize and extrapolate commonalities in clinical and genetic understanding of EA/LVNC to date. We additionally postulate involvement of shared developmental pathways that may lead to this combined disease. Anatomical variation in EA/LVNC encompasses characteristics of both CHDs, including tricuspid valve displacement, right heart dilatation, and left ventricular trabeculation, and dictates clinical presentation in both age and severity. Disease treatment is non-specific, ranging from symptomatic management to invasive surgery. Apart from a few variant associations, mainly in sarcomeric genes MYH7 and TPM1, the genetic etiology and pathogenesis of EA/LVNC remain largely unknown.

Fibro-fatty remodelling in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is an inherited disorder characterized by lethal arrhythmias and a risk to sudden cardiac death. A hallmark feature of AC is the progressive replacement of the ventricular myocardium with fibro-fatty tissue, which can act as an arrhythmogenic substrate further exacerbating cardiac dysfunction. Therefore, identifying the processes underlying this pathological remodelling would help understand AC pathogenesis and support the development of novel therapies. In this review, we summarize our knowledge on the different models designed to identify the cellular origin and molecular pathways underlying cardiac fibroblast and adipocyte cell differentiation in AC patients. We further outline future perspectives and how targeting the fibro-fatty remodelling process can contribute to novel AC therapeutics.

Single-cell transcriptomics defines heterogeneity of epicardial cells and fibroblasts within the infarcted murine heart

In the adult heart, the epicardium becomes activated after injury, contributing to cardiac healing by secretion of paracrine factors. Here, we analyzed by single-cell RNA sequencing combined with RNA in situ hybridization and lineage tracing of Wilms tumor protein 1-positive (WT1+) cells, the cellular composition, location, and hierarchy of epicardial stromal cells (EpiSC) in comparison to activated myocardial fibroblasts/stromal cells in infarcted mouse hearts. We identified 11 transcriptionally distinct EpiSC populations, which can be classified into three groups, each containing a cluster of proliferating cells. Two groups expressed cardiac specification markers and sarcomeric proteins suggestive of cardiomyogenic potential. Transcripts of hypoxia-inducible factor (HIF)-1α and HIF-responsive genes were enriched in EpiSC consistent with an epicardial hypoxic niche. Expression of paracrine factors was not limited to WT1+ cells but was a general feature of activated cardiac stromal cells. Our findings provide the cellular framework by which myocardial ischemia may trigger in EpiSC the formation of cardioprotective/regenerative responses.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Cardiac telocytes inhibit cardiac microvascular endothelial cell apoptosis through exosomal miRNA-21-5p-targeted cdip1 silencing to improve angiogenesis following myocardial infarction

Promotion of cardiac angiogenesis in ischemic myocardium is a critical strategy for repairing and regenerating the myocardium after myocardial infarction (MI). Currently, effective methods to aid in the survival of endothelial cells, to avoid apoptosis in ischemic myocardium and to achieve long-term cardiac angiogenesis are still being pursued. Here, we investigated whether cardiac telocyte (CT)-endothelial cell communication suppresses apoptosis and promotes the survival of endothelial cells to facilitate cardiac angiogenesis during MI.

Methods: CT exosomes were isolated from CT conditioned medium, and their miRNA profile was characterized by small RNA sequencing. A rat model of left anterior descending coronary artery ligation (LAD)-mediated MI was assessed with histology for infarct size and fibrosis, immunostaining for angiogenesis and cell apoptosis and echocardiography to evaluate the therapeutic effects. Cardiac microvascular endothelial cells (CMECs) and the LAD-MI model treated with CT exosomes or CT exosomal miRNA-21-5p in vitro and in vivo were assessed with cellular and molecular techniques to demonstrate the underlying mechanism.

Results: CTs exert therapeutic effects on MI via the potent paracrine effects of CT exosomes to facilitate the inhibition of apoptosis and survival of CMECs and promote cardiac angiogenesis. A novel mechanism of CTs is revealed, in which CT-endothelial cell communication suppresses apoptosis and promotes the survival of endothelial cells in the pathophysiological myocardium. CT exosomal miRNA-21-5p targeted and silenced the cell death inducing p53 target 1 (Cdip1) gene and thus down-regulated the activated caspase-3, which then inhibited the apoptosis of recipient endothelial cells under ischemic and hypoxic conditions, facilitating angiogenesis and regeneration following MI.

Conclusions: The present study is the first to show that CTs inhibit cardiac microvascular endothelial cell apoptosis through exosomal miRNA-21-5p-targeted Cdip1 silencing to improve angiogenesis in myocardial infarction. It is believed that these novel findings and the discovery of cellular and molecular mechanisms will provide new opportunities to tailor novel cardiac cell therapies and cell-free therapies for the functional and structural regeneration of the injured myocardium.

Cell Sheet Comprised of Mesenchymal Stromal Cells Overexpressing Stem Cell Factor Promotes Epicardium Activation and Heart Function Improvement in a Rat Model of Myocardium Infarction

Cell therapy of the post-infarcted myocardium is still far from clinical use. Poor survival of transplanted cells, insufficient regeneration, and replacement of the damaged tissue limit the potential of currently available cell-based techniques. In this study, we generated a multilayered construct from adipose-derived mesenchymal stromal cells (MSCs) modified to secrete stem cell factor, SCF. In a rat model of myocardium infarction, we show that transplantation of SCF producing cell sheet induced activation of the epicardium and promoted the accumulation of c-kit positive cells in ischemic muscle. Morphometry showed the reduction of infarct size (16%) and a left ventricle expansion index (0.12) in the treatment group compared to controls (24-28%; 0.17-0.32). The ratio of viable myocardium was more than 1.5-fold higher, reaching 49% compared to the control (28%) or unmodified cell sheet group (30%). Finally, by day 30 after myocardium infarction, SCF-producing cell sheet transplantation increased left ventricle ejection fraction from 37% in the control sham-operated group to 53%. Our results suggest that, combining the genetic modification of MSCs and their assembly into a multilayered construct, we can provide prolonged pleiotropic effects to the damaged heart, induce endogenous regenerative processes, and improve cardiac function.

HAND1 loss-of-function within the embryonic myocardium reveals survivable congenital cardiac defects and adult heart failure

AIMS

To examine the role of the basic Helix-loop-Helix (bHLH) transcription factor HAND1 in embryonic and adult myocardium.

METHODS AND RESULTS

Hand1 is expressed within the cardiomyocytes of the left ventricle (LV) and myocardial cuff between embryonic days (E) 9.5-13.5. Hand gene dosage plays an important role in ventricular morphology and the contribution of Hand1 to congenital heart defects requires further interrogation. Conditional ablation of Hand1 was carried out using either Nkx2.5 knockin Cre (Nkx2.5Cre) or α-myosin heavy chain Cre (αMhc-Cre) driver. Interrogation of transcriptome data via ingenuity pathway analysis reveals several gene regulatory pathways disrupted including translation and cardiac hypertrophy-related pathways. Embryo and adult hearts were subjected to histological, functional, and molecular analyses. Myocardial deletion of Hand1 results in morphological defects that include cardiac conduction system defects, survivable interventricular septal defects, and abnormal LV papillary muscles (PMs). Resulting Hand1 conditional mutants are born at Mendelian frequencies; but the morphological alterations acquired during cardiac development result in, the mice developing diastolic heart failure.

CONCLUSION

Collectively, these data reveal that HAND1 contributes to the morphogenic patterning and maturation of cardiomyocytes during embryogenesis and although survivable, indicates a role for Hand1 within the developing conduction system and PM development.

The Role of Transcription Factor 21 in Epicardial Cell Differentiation and the Development of Coronary Heart Disease

Transcription factor 21 (TCF21) is specific for mesoderm and is expressed in the embryos' mesenchymal derived tissues, such as the epicardium. It plays a vital role in regulating cell differentiation and cell fate specificity through epithelial-mesenchymal transformation during cardiac development. For instance, TCF21 could promote cardiac fibroblast development and inhibit vascular smooth muscle cells (VSMCs) differentiation of epicardial cells. Recent large-scale genome-wide association studies have identified a mass of loci associated with coronary heart disease (CHD). There is mounting evidence that TCF21 polymorphism might confer genetic susceptibility to CHD. However, the molecular mechanisms of TCF21 in heart development and CHD remain fundamentally problematic. In this review, we are committed to providing a detailed introduction of the biological roles of TCF21 in epicardial fate determination and the development of CHD.

MiR-195 enhances cardiomyogenic differentiation of the proepicardium/septum transversum by Smurf1 and Foxp1 modulation

Cardiovascular development is a complex developmental process in which multiple cell lineages are involved, namely the deployment of first and second heart fields. Beside the contribution of these cardiogenic fields, extracardiac inputs to the developing heart are provided by the migrating cardiac neural crest cells and the proepicardial derived cells. The proepicardium (PE) is a transitory cauliflower-like structure located between the cardiac and hepatic primordia. The PE is constituted by an internal mesenchymal component surrounded by an external epithelial lining. With development, cells derived from the proepicardium migrate to the neighboring embryonic heart and progressive cover the most external surface, leading to the formation of the embryonic epicardium. Experimental evidence in chicken have nicely demonstrated that epicardial derived cells can distinctly contribute to fibroblasts, endothelial and smooth muscle cells. Surprisingly, isolation of the developing PE anlage and ex vivo culturing spontaneously lead to differentiation into beating cardiomyocytes, a process that is enhanced by Bmp but halted by Fgf administration. In this study we provide a comprehensive characterization of the developmental expression profile of multiple microRNAs during epicardial development in chicken. Subsequently, we identified that miR-125, miR-146, miR-195 and miR-223 selectively enhance cardiomyogenesis both in the PE/ST explants as well as in the embryonic epicardium, a Smurf1- and Foxp1-driven process. In addition we identified three novel long non-coding RNAs with enhanced expression in the PE/ST, that are complementary regulated by Bmp and Fgf administration and well as by microRNAs that selectively promote cardiomyogenesis, supporting a pivotal role of these long non coding RNAs in microRNA-mediated cardiomyogenesis of the PE/ST cells.

Message in a Bottle: Upgrading Cardiac Repair into Rejuvenation

Ischaemic cardiac disease is associated with a loss of cardiomyocytes and an intrinsic lack of myocardial renewal. Recent work has shown that the heart retains limited cardiomyocyte proliferation, which remains inefficient when facing pathological conditions. While broadly active in the neonatal mammalian heart, this mechanism becomes quiescent soon after birth, suggesting loss of regenerative potential with maturation into adulthood. A key question is whether this temporary regenerative window can be enhanced via appropriate stimulation and further extended. Recently the search for novel therapeutic approaches for heart disease has centred on stem cell biology. The “paracrine effect” has been proposed as a promising strategy to boost endogenous reparative and regenerative mechanisms from within the cardiac tissue by exploiting the modulatory potential of soluble stem cell-secreted factors. As such, growing interest has been specifically addressed towards stem/progenitor cell-secreted extracellular vesicles (EVs), which can be easily isolated in vitro from cell-conditioned medium. This review will provide a comprehensive overview of the current paradigm on cardiac repair and regeneration, with a specific focus on the role and mechanism(s) of paracrine action of EVs from cardiac stromal progenitors as compared to exogenous stem cells in order to discuss the optimal choice for future therapy. In addition, the challenges to overcoming translational EV biology from bench to bedside for future cardiac regenerative medicine will be discussed.

Epicardial TGFβ and BMP Signaling in Cardiac Regeneration: What Lesson Can We Learn from the Developing Heart?

The epicardium, the outer layer of the heart, has been of interest in cardiac research due to its vital role in the developing and diseased heart. During development, epicardial cells are active and supply cells and paracrine cues to the myocardium. In the injured adult heart, the epicardium is re-activated and recapitulates embryonic behavior that is essential for a proper repair response. Two indispensable processes for epicardial contribution to heart tissue formation are epithelial to mesenchymal transition (EMT), and tissue invasion. One of the key groups of cytokines regulating both EMT and invasion is the transforming growth factor β (TGFβ) family, including TGFβ and Bone Morphogenetic Protein (BMP). Abundant research has been performed to understand the role of TGFβ family signaling in the developing epicardium. However, less is known about signaling in the adult epicardium. This review provides an overview of the current knowledge on the role of TGFβ in epicardial behavior both in the development and in the repair of the heart. We aim to describe the presence of involved ligands and receptors to establish if and when signaling can occur. Finally, we discuss potential targets to improve the epicardial contribution to cardiac repair as a starting point for future investigation.

Myocardial regeneration: role of epicardium and implicated genes

Lower invertebrates' hearts such as those of zebrafish have the capacity for scarless myocardial regeneration which is lost by mammalian hearts as they form a fibrotic scar tissue instead of regenerating the injured area. However, neonatal mammalian hearts have a remarkable capacity for regeneration highlighting conserved evolutionary mechanisms underlying such a process. Studies investigated the underlying mechanism of myocardial regeneration in species capable to do so, to see its applicability on mammals. The epicardium, the mesothelial outer layer of the vertebrate heart, has proven to play an important role in the process of repair and regeneration. It serves as an important source of smooth muscle cells, cardiac fibroblasts, endothelial cells, stem cells, and signaling molecules that are involved in this process. Here we review the role of the epicardium in myocardial regeneration focusing on the different involved; Activation, epithelial to mesenchymal transition, and differentiation. In addition, we will discuss its contributory role to different aspects that support myocardial regeneration. Of these we will discuss angiogenesis and the formation of a regenerate extracellular matrix. Moreover, we will discuss several factors that act on the epicardium to affect regeneration. Finally, we will highlight the utility of the epicardium as a mode of cell therapy in the treatment of myocardial injury.

SDF-1α promotes repair of myocardial ischemic necrosis zones in rats

OBJECTIVE

To explore the repair effect of stromal cell-derived factor-1α (SDF-1α) on myocardial ischemic necrosis zones.

METHODS

Lentivirus (LV-SDF-1α-GFP) containing SDF-1α target gene was established, the separated and cultured neonatal rat cardiac fibroblasts were transfected, and caudal intravenous injection of isoproterenol was conducted to prepare a rat model of myocardial ischemia. Small animal ultrasound was used to evaluate the effect on cardiac functions. Morphology and immunofluorescence were used to observe the change of ischemic necrosis zones and expressions of stem cellular markers c-kit, CD34, nkx2.5, and nanog, and a quantitative analysis was performed.

RESULTS

The established LV-SDF-1α-GFP was used to transfect myocardial fibroblasts which presented GFP green fluorescent expression and could secrete SDF-1α. The small animal ultrasound system showed that rat cardiac functions of the lentivirus group and cell group were improved to different degrees, myocardial ischemic necrosis zones of lentivirus group and cell group were reduced, and differences had statistical significances (P<0.05). Immunofluorescence showed that expressions of stem cellular markers c-kit, CD34, nkx2.5 and nanog in myocardial tissue ischemic zones in both the lentivirus group and cell group increased, and differences through inter-group comparison had statistical significances (P<0.05).

CONCLUSION

SDF-1α can promote migration and proliferation of stem cells into the myocardial ischemic necrosis zone, participate in repair of the myocardial necrosis zone, and improve cardiac function.

The Telocytes in the Subepicardial Niche

A great interest has developed over the last several years in research on interstitial Cajal-like cells (ICLCs), later renamed to telocytes (TCs). Such studies are restricted by diverse limitations. We aimed to critically review (sub)epicardial ICLCs/TCs and to bring forward supplemental immunohistochemical evidence on (sub)epicardial stromal niche inhabitants. We tested the epicardial expressions of CD117/c-kit, CD34, Cytokeratin 7 (CK7), Ki67, Platelet-Derived Growth Factor Receptor (PDGFR)-α and D2-40 in adult human cardiac samples. The mesothelial epicardial cells expressed D2-40, CK7, CD117/c-kit and PDGFR-α. Subepicardial D2-40-positive lymphatic vessels and isolated D2-40-positive and CK7-positive subepicardial cells were also found. Immediate submesothelial spindle-shaped cells expressed Ki-67. Submesothelial stromal cells and endothelial tubes were PDGFR-α-positive and CD34-positive. The expression of CD34 was pan-stromal, so a particular stromal cell type could not be distinguished. The stromal expression of CD117/c-kit was also noted. It seems that epicardial TCs could not be regarded as belonging to a unique cell type until (pre)lymphatic endothelial cells are inadequately excluded. Markers such as CD117/c-kit or CD34 seem to be improper for identifying TCs as a distinctive cell type. Care should be taken when using the immunohistochemical method and histological interpretations, as they may not produce accurate results.

Modulation of TNFα Activity by the microRNA Let-7 Coordinates Zebrafish Heart Regeneration

The adult zebrafish is capable of regenerating heart muscle, resolving collagen tissue, and fully restoring heart function throughout its life. In this study, we show that the highly upregulated, epicardium-enriched microRNA let-7i functions in wound closure and cardiomyocyte proliferation. RNA sequencing experiments identified upregulated expression of members of the tumor necrosis factor (TNF) signaling pathway in the absence of let-7. Importantly, co-suppression of TNF and let-7 activity rescued epicardium migration and cardiomyocyte proliferation defects induced by depletion of let-7 alone. Sensitizing animals to low levels of TNF activity before injury culminated in repressed cardiomyocyte proliferation and wound closure defects, suggesting that levels of inflammation at the onset of injury are critical for heart regeneration. Our studies indicate that injury-induced reduction in TNF signaling by let-7 in the epicardium creates a pro-regenerative environment for cardiomyocyte proliferation during adult heart regeneration.

Human Pluripotent Stem Cell-Derived Multipotent Vascular Progenitors of the Mesothelium Lineage Have Utility in Tissue Engineering and Repair

In this report we describe a human pluripotent stem cell-derived vascular progenitor (MesoT) cell of the mesothelium lineage. MesoT cells are multipotent and generate smooth muscle cells, endothelial cells, and pericytes and self-assemble into vessel-like networks in vitro. MesoT cells transplanted into mechanically damaged neonatal mouse heart migrate into the injured tissue and contribute to nascent coronary vessels in the repair zone. When seeded onto decellularized vascular scaffolds, MesoT cells differentiate into the major vascular lineages and self-assemble into vasculature capable of supporting peripheral blood flow following transplantation. These findings demonstrate in vivo functionality and the potential utility of MesoT cells in vascular engineering applications.

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.

Recapitulation of developmental mechanisms to revascularize the ischemic heart

Restoring blood flow after myocardial infarction (MI) is essential for survival of existing and newly regenerated tissue. Endogenous vascular repair processes are deployed following injury but are poorly understood. We sought to determine whether developmental mechanisms of coronary vessel formation are intrinsically reactivated in the adult mouse after MI. Using pulse-chase genetic lineage tracing, we establish that de novo vessel formation constitutes a substantial component of the neovascular response, with apparent cellular contributions from the endocardium and coronary sinus. The adult heart reverts to its former hypertrabeculated state and repeats the process of compaction, which may facilitate endocardium-derived neovascularization. The capacity for angiogenic sprouting of the coronary sinus vein, the adult derivative of the sinus venosus, may also reflect its embryonic origin. The quiescent epicardium is reactivated and, while direct cellular contribution to new vessels is minimal, it supports the directional expansion of the neovessel network toward the infarcted myocardium. Thymosin β4, a peptide with roles in vascular development, was required for endocardial compaction, epicardial vessel expansion, and smooth muscle cell recruitment. Insight into pathways that regulate endogenous vascular repair, drawing on comparisons with development, may reveal novel targets for therapeutically enhancing neovascularization.

Embryonic mechanisms are redeployed to revascularize the ischemic heart, with contributions primarily from the endocardium and coronary sinus and processes that require thymosin β4.

Arrhythmogenic Cardiomyopathy: the Guilty Party in Adipogenesis

Arrhythmogenic cardiomyopathy (ACM) is a genetic cardiac condition characterized by the replacement of the ventricular myocardium with fibro-fatty tissue, by arrhythmias and sudden death. Adipogenesis in ACM is considered an aberrant remodeling following myocardial loss. Which cell type(s) is (are) responsible for the adipose replacement is still matter of debate. A systematic overview of the different cells that have been, over time, considered as main players in adipose replacement is provided. The comprehension of the cellular component giving rise to arrhythmogenic cardiomyopathy substrate defects may represent both an essential tool for mechanistic studies of disease pathogenesis and a novel possible therapeutic target.

Cardiac Progenitor Cells and the Interplay with Their Microenvironment

The microenvironment plays a crucial role in the behavior of stem and progenitor cells. In the heart, cardiac progenitor cells (CPCs) reside in specific niches, characterized by key components that are altered in response to a myocardial infarction. To date, there is a lack of knowledge on these niches and on the CPC interplay with the niche components. Insight into these complex interactions and into the influence of microenvironmental factors on CPCs can be used to promote the regenerative potential of these cells. In this review, we discuss cardiac resident progenitor cells and their regenerative potential and provide an overview of the interactions of CPCs with the key elements of their niche. We focus on the interaction between CPCs and supporting cells, extracellular matrix, mechanical stimuli, and soluble factors. Finally, we describe novel approaches to modulate the CPC niche that can represent the next step in recreating an optimal CPC microenvironment and thereby improve their regeneration capacity.

Harnessing Epicardial Progenitor Cells and Their Derivatives for Rescue and Repair of Cardiac Tissue After Myocardial Infarction

PURPOSE OF REVIEW

Ischemic heart disease and stroke lead to the greatest number of deaths worldwide. Despite decreased time to intervention and improvements in the standard of care, 1 out of 5 patients that survive a myocardial infarction (MI) still face long-term chronic heart failure and a 5-year mortality rate of about 50%. Based on their multi-potency for differentiation and paracrine activity, epicardial cells and their derivatives have potential to rescue jeopardized tissue and/or promote cardiac regeneration. Here we review the diagnosis and treatment of MI, basic epicardial cell biology, and potential treatment strategies designed to harness the reparative properties of epicardial cells.

RECENT FINDINGS

During cardiac development, epicardial cells covering the surface of the heart generate migratory progenitor cells that contribute to the coronary vasculature and the interstitial fibroblasts. Epicardial cells also produce paracrine signals required for myocardial expansion and cardiac growth. In adults with myocardial infarction, epicardial cells and their derivatives provide paracrine factors that affect myocardial remodeling and repair. At present, the intrinsic mechanisms and extrinsic signals that regulate epicardial cell fate and paracrine activity in adults remain poorly understood.

SUMMARY

Human diseases that result in heart failure due to negative remodeling or extensive loss of viable cardiac tissue require new, effective treatments. Improved understanding of epicardial cell function(s) and epicardial-mediated secretion of growth factors, cytokines and hormones during cardiac growth, homeostasis and injury may lead to new ways to treat patients with myocardial infarction.

[Role of telocytes in the heart in health and diseases]

This review summarizes the data available in the literature on the development, structure, and function of telocytes (TCs) and their role in the heart in health and diseases. At the present time, TCs have been found in many organs of mammals and humans. TC is a small oval cell that contains a nucleus surrounded by small amounts of cytoplasm, with extremely long and thin processes named telopodias. TCs have unique ultrastructural and immunohistochemical features; double positive labeling for CD34/PDGFR-β and CD34/vimentin is suitable for their identification. The role of TCs in the heart at different study stages is the subject of debate. There are currently available data on a decline in the number of cardiac TCs in patients with various heart diseases. Relying on a number of investigations showing that TCs are present in the subepicardial stem cell niches, the authors consider a hypothesis for the key role of cardiac TCs in the regeneration and reparation of the heart.

Phenotypic Screen for Cardiac Regeneration Identifies Molecules with Differential Activity in Human Epicardium-Derived Cells versus Cardiac Fibroblasts

Activation and proliferation of resident cardiac progenitor cells has therapeutic potential to repair the heart after injury. However, research has been impeded by a lack of well-defined and characterized cell sources and difficulties in translation to screening platforms. Here, we describe the development, validation, and use of a 384-well phenotypic assay in primary human epicardium-derived cells (EPDCs) to identify compounds that induce proliferation while maintaining the progenitor phenotype. Using this assay, we screened 7400 structurally diverse compounds where greater than 90% are biologically annotated and known to modulate a broad range of biological targets. From the primary screen, we identified and validated hits and expanded upon the lead molecules of interest. A counterscreen was developed in human cardiac fibroblasts to filter out compounds with a general proliferative effect, after which the activity of selected molecules was confirmed across multiple EPDC donors. To further examine the mechanism of action of compounds with annotated targets, we performed knockdown experiments to understand whether a single known target was responsible for the proliferative effect, confirming results with protein expression and activity assays. Here, we were able to show that the annotated targets of compounds of interest were not responsible for the proliferative effect, which highlights potential differences in cell types and signaling pathways and possible polypharmacology. These studies demonstrate the feasibility of using relevant human primary cells in a phenotypic screen to identify compounds as novel biological tools and starting points for drug discovery projects, and we disclose the first small molecules to proliferate human primary EPDCs.

Developmental origin and lineage plasticity of endogenous cardiac stem cells

Over the past two decades, several populations of cardiac stem cells have been described in the adult mammalian heart. For the most part, however, their lineage origins and in vivo functions remain largely unexplored. This Review summarizes what is known about different populations of embryonic and adult cardiac stem cells, including KIT(+), PDGFRα(+), ISL1(+)and SCA1(+)cells, side population cells, cardiospheres and epicardial cells. We discuss their developmental origins and defining characteristics, and consider their possible contribution to heart organogenesis and regeneration. We also summarize the origin and plasticity of cardiac fibroblasts and circulating endothelial progenitor cells, and consider what role these cells have in contributing to cardiac repair.

Epicardial Epithelial-to-Mesenchymal Transition in Heart Development and Disease

The epicardium is an epithelial monolayer that plays a central role in heart development and the myocardial response to injury. Recent developments in our understanding of epicardial cell biology have revealed this layer to be a dynamic participant in fundamental processes underlying the development of the embryonic ventricles, the coronary vasculature, and the cardiac valves. Likewise, recent data have identified the epicardium as an important contributor to reparative and regenerative processes in the injured myocardium. These essential functions of the epicardium rely on both non-cell autonomous and cell-autonomous mechanisms, with the latter featuring the process of epicardial Epithelial-to-Mesenchymal Transition (EMT). This review will focus on the induction and regulation of epicardial EMT, as it pertains to both cardiogenesis and the response of the myocardium to injury.

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.

Beyond the Mammalian Heart: Fish and Amphibians as a Model for Cardiac Repair and Regeneration

The epidemic of heart disease, the leading cause of death worldwide, is made worse by the fact that the adult mammalian heart is especially poor at repair. Damage to the mammal heart-such as that caused by myocardial infarction-leads to scarring, resulting in cardiac dysfunction and heart failure. In contrast, the hearts of fish and urodele amphibians are capable of complete regeneration of cardiac tissue from multiple types of damage, with full restoration of functionality. In the last decades, research has revealed a wealth of information on how these animals are able to perform this remarkable feat, and non-mammalian models of heart repair have become a burgeoning new source of data on the morphological, cellular, and molecular processes necessary to heal cardiac damage. In this review we present the major findings from recent research on the underlying mechanisms of fish and amphibian heart regeneration. We also discuss the tools and techniques that have been developed to answer these important questions.

Transition of mesothelial cell to fibroblast in peritoneal dialysis: EMT, stem cell or bystander?

Long-term peritoneal dialysis (PD) can lead to fibrotic changes in the peritoneum, characterized by loss of mesothelial cells (MCs) and thickening of the submesothelial area with an accumulation of collagen and myofibroblasts. The origin of myofibroblasts is a central question in peritoneal fibrosis that remains unanswered at present. Numerous clinical and experimental studies have suggested that MCs, through epithelial-mesenchymal transition (EMT), contribute to the pool of peritoneal myofibroblasts. However, recent work has placed significant doubts on the paradigm of EMT in organ fibrogenesis (in the kidney particularly), highlighting the need to reconsider the role of EMT in the generation of myofibroblasts in peritoneal fibrosis. In particular, selective cell isolation and lineage-tracing experiments have suggested the existence of progenitor cells in the peritoneum, which are able to switch to fibroblast-like cells when stimulated by the local environment. These findings highlight the plastic nature of MCs and its contribution to peritoneal fibrogenesis. In this review, we summarize the key findings and caveats of EMT in organ fibrogenesis, with a focus on PD-related peritoneal fibrosis, and discuss the potential of peritoneal MCs as a source of myofibroblasts.

Regional differences in WT-1 and Tcf21 expression during ventricular development: implications for myocardial compaction

BACKGROUND

Morphological and functional differences of the right and left ventricle are apparent in the adult human heart. A differential contribution of cardiac fibroblasts and smooth muscle cells (populations of epicardium-derived cells) to each ventricle may account for part of the morphological-functional disparity. Here we studied the relation between epicardial derivatives and the development of compact ventricular myocardium.

RESULTS

Wildtype and Wt1CreERT2/+ reporter mice were used to study WT-1 expressing cells, and Tcf21lacZ/+ reporter mice and PDGFRα-/-;Tcf21LacZ/+ mice to study the formation of the cardiac fibroblast population. After covering the heart, intramyocardial WT-1+ cells were first observed at the inner curvature, the right ventricular postero-lateral wall and left ventricular apical wall. Later, WT-1+ cells were present in the walls of both ventricles, but significantly more pronounced in the left ventricle. Tcf21-LacZ + cells followed the same distribution pattern as WT-1+ cells but at later stages, indicating a timing difference between these cell populations. Within the right ventricle, WT-1+ and Tcf21-lacZ+ cell distribution was more pronounced in the posterior inlet part. A gradual increase in myocardial wall thickness was observed early in the left ventricle and at later stages in the right ventricle. PDGFRα-/-;Tcf21LacZ/+ mice showed deficient epicardium, diminished number of Tcf21-LacZ + cells and reduced ventricular compaction.

CONCLUSIONS

During normal heart development, spatio-temporal differences in contribution of WT-1 and Tcf21-LacZ + cells to right versus left ventricular myocardium occur parallel to myocardial thickening. These findings may relate to lateralized differences in ventricular (patho)morphology in humans.

Myogenic differentiation and reparative activity of stromal cells derived from pericardial adipose in comparison to subcutaneous origin

Introduction

Adipose tissue-derived stromal cells (ADSCs) are abundant and easy to obtain, but the diversity of differentiation potential from different locations may vary with the developmental origin of their mesenchymal compartment. We therefore aim to compare the myogenic differentiation and reparative activity of ADSCs derived from the pericardial tissue to ADSCs of subcutaneous origin.

Methods

Pericardial and inguinal adipose tissues from Wistar rats were surgically obtained, and the stromal fraction was isolated after enzymatic digestion. The phenotypic epitopes of the resultant two types of ADSCs were analyzed with flow cytometry, and the expression of transcriptional factors was analyzed with immunostaining. Furthermore, their potential toward adipogenic, osteogenic, and myogenic differentiation also was compared. Finally, the reparative activity and the resultant functional benefits were examined by allograft transplantation into an infarcted model in rats.

Results

ADSCs from two adipose sources showed identical morphology and growth curve at the initial stage, but inguinal ADSCs (ingADSCs) sustained significantly vigorous growth after 25 days of cultivation. Although both ADSCs shared similar immunophenotypes, the pericardial ADSCs (periADSC) intrinsically exhibited partial expression of transcription factors for cardiogenesis (such as GATA-4, Isl-1, Nkx 2.5, and MEF-2c) and more-efficient myogenic differentiation, but less competent for adipogenic and osteogenic differentiation. After in vivo transplantation, periADSCs exhibited significantly vigorous reparative activity evidenced by thickening of ventricular wall and pronounced vasculogenesis and myogenesis, although the majority of prelabeled cells disappeared 28 days after transplantation. The structural repair also translated into functional benefits of hearts after infarction.

Conclusions

Although two sources of ADSCs are phenotypically identical, pericADSCs constituted intrinsic properties toward myogenesis and vasculogenesis, and thus provided more potent reparative effects after transplantation; therefore, they represent an attractive candidate cell donor for cardiac therapy.

Electrophysiology of human cardiac atrial and ventricular telocytes

Telocytes (TCs) with exceptionally long cellular processes of telopodes have been described in human epicardium to act as structural supporting cells in the heart. We examined myocardial chamber-specific TCs identified in atrial and ventricular fibroblast culture using immunocytochemistry and studied their electrophysiological property by whole-cell patch clamp. Atrial and ventricular TCs with extended telopodes and alternating podoms and podomers that expressed CD34, c-Kit and PDGFR-β were identified. These cells expressed large conductance Ca²⁺-activated K⁺ current (BK_Ca) and inwardly rectifying K⁺ current (IK_ir), but not transient outward K⁺ current (I_to) and ATP-sensitive potassium current (K_ATP). The active channels were functionally competent with demonstrated modulatory response to H₂S and transforming growth factor (TGF)-β1 whereby H₂S significantly inhibited the stimulatory effect of TGF-β1 on current density of both BK_Ca and IK_ir. Furthermore, H₂S attenuated TGF-β1-stimulated KCa1.1/Kv1.1 (encode BK_Ca) and Kir2.1 (encode IK_ir) expression in TCs. Our results show that functionally competent K⁺ channels are present in human atrial and ventricular TCs and their modulation may have significant implications in myocardial physiopathology.

Epicardium-Derived Heart Repair

In the last decade, cell replacement therapy has emerged as a potential approach to treat patients suffering from myocardial infarction (MI). The transplantation or local stimulation of progenitor cells with the ability to form new cardiac tissue provides a novel strategy to overcome the massive loss of myocardium after MI. In this regard the epicardium, the outer layer of the heart, is a tractable local progenitor cell population for therapeutic pursuit. The epicardium has a crucial role in formation of the embryonic heart. After activation and migration into the developing myocardium, epicardial cells differentiate into several cardiac cells types. Additionally, the epicardium provides instructive signals for the growth of the myocardium and coronary angiogenesis. In the adult heart, the epicardium is quiescent, but recent evidence suggests that it becomes reactivated upon damage and recapitulates at least part of its embryonic functions. In this review we provide an update on the current knowledge regarding the contribution of epicardial cells to the adult mammalian heart during the injury response.

Molecular evidence of stress-induced acute heart injury in a mouse model simulating posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is a common condition induced by life-threatening stress, such as that experienced by soldiers under battlefield conditions. Other than the commonly recognized behavioral and psychological dysfunction, epidemiological studies have also revealed that PTSD patients have a higher risk of other diseases, such as cardiovascular disorders. Using a PTSD mouse model, we investigated the longitudinal transcriptomic changes in heart tissues after the exposure to stress through intimidation. Our results revealed acute heart injury associated with the traumatic experience, reflecting the underlying biological injury processes of the immune response, extracellular matrix remodeling, epithelial-to-mesenchymal cell transitions, and cell proliferation. Whether this type of injury has any long-term effects on heart function is yet to be determined. The differing responses to stress leading to acute heart injury in different inbred strains of mice also suggest that this response has a genetic as well as an environmental component. Accordingly, the results from this study suggest a molecular basis for the observed higher risk of cardiovascular disorders in PTSD patients, which raises the likelihood of cardiac dysfunction induced by long-term stress exposures.

The Structural Basis of Functional Improvement in Response to Human Umbilical Cord Blood Stem Cell Transplantation in Hearts With Postinfarct LV Remodeling

Cellular therapy for myocardial repair has been one of the most intensely investigated interventional strategies for acute myocardial infarction. Although the therapeutic potential of stem cells has been demonstrated in various studies, the underlying mechanisms for such improvements are poorly understood. In the present study, we investigated the long-term effects of stem cell therapy on both myocardial fiber organization and regional contractile function using a rat model of postinfarct remodeling. Human nonhematopoietic umbilical cord blood stem cells (nh-UCBSCs) were administered via tail vein to rats 2 days after infarct surgery. Animals were maintained without immunosuppressive therapy. In vivo and ex vivo MR imaging was performed on infarct hearts 10 months after cell transplantation. Compared to the age-matched rats exposed to the identical surgery, both global and regional cardiac functions of the nh-UCBSC-treated hearts, such as ejection fraction, ventricular strain, and torsion, were significantly improved. More importantly, the treated hearts exhibited preserved fiber orientation and water diffusivities that were similar to those in sham-operated control hearts. These data provide the first evidence that nh-UCBSC treatment may prevent/delay untoward structural remodeling in postinfarct hearts, which supports the improved LV function observed in vivo in the absence of immunosuppression, suggesting a beneficial paracrine effect occurred with the cellular therapy.

The cardiac hypoxic niche: emerging role of hypoxic microenvironment in cardiac progenitors

Resident stem cells persist throughout the entire lifetime of an organism where they replenishing damaged cells. Numerous types of resident stem cells are housed in a low-oxygen tension (hypoxic) microenvironment, or niches, which seem to be critical for survival and maintenance of stem cells. Recently our group has identified the adult mammalian epicardium and subepicardium as a hypoxic niche for cardiac progenitor cells. Similar to hematopoietic stem cells (LT-HSCs), progenitor cells in the hypoxic epicardial niche utilize cytoplasmic glycolysis instead of mitochondrial oxidative phosphorylation, where hypoxia inducible factor 1α (Hif-1α) maintains them in glycolytic undifferentiated state. In this review we summarize the relationship between hypoxic signaling and stem cell function, and discuss potential roles of several cardiac stem/progenitor cells in cardiac homeostasis and regeneration.

Differential expression of embryonic epicardial progenitor markers and localization of cardiac fibrosis in adult ischemic injury and hypertensive heart disease

During embryonic heart development, the transcription factors Tcf21, Wt1, and Tbx18 regulate activation and differentiation of epicardium-derived cells, including fibroblast lineages. Expression of these epicardial progenitor factors and localization of cardiac fibrosis were examined in mouse models of cardiovascular disease and in human diseased hearts. Following ischemic injury in mice, epicardial fibrosis is apparent in the thickened layer of subepicardial cells that express Wt1, Tbx18, and Tcf21. Perivascular fibrosis with predominant expression of Tcf21, but not Wt1 or Tbx18, occurs in mouse models of pressure overload or hypertensive heart disease, but not following ischemic injury. Areas of interstitial fibrosis in ischemic and hypertensive hearts actively express Tcf21, Wt1, and Tbx18. In all areas of fibrosis, cells that express epicardial progenitor factors are distinct from CD45-positive immune cells. In human diseased hearts, differential expression of Tcf21, Wt1, and Tbx18 also is detected with epicardial, perivascular, and interstitial fibrosis, indicating conservation of reactivated developmental mechanisms in cardiac fibrosis in mice and humans. Together, these data provide evidence for distinct fibrogenic mechanisms that include Tcf21, separate from Wt1 and Tbx18, in different fibroblast populations in response to specific types of cardiac injury.

Natural cardiogenesis-based template predicts cardiogenic potential of induced pluripotent stem cell lines

BACKGROUND

Cardiac development is a complex process resulting in an integrated, multilineage tissue with developmental corruption in early embryogenesis leading to congenital heart disease. Interrogation of individual genes has provided the backbone for cardiac developmental biology, yet a comprehensive transcriptome derived from natural cardiogenesis is required to gauge innate developmental milestones.

METHODS AND RESULTS

Stage-specific cardiac structures were dissected from 8 distinctive mouse embryonic time points to produce genome-wide expressome analysis across cardiogenesis. With reference to this native cardiogenic expression roadmap, divergent induced pluripotent stem cell-derived cardiac expression profiles were mapped from procardiogenic 3-factor (SOX2, OCT4, KLF4) and less-cardiogenic 4-factor (plus c-MYC) reprogrammed cells. Expression of cardiac-related genes from 3-factor-induced pluripotent stem cell differentiated in vitro at days 5 and 11 and recapitulated expression profiles of natural embryos at days E7.5-E8.5 and E14.5-E18.5, respectively. By contrast, 4-factor-induced pluripotent stem cells demonstrated incomplete cardiogenic gene expression profiles beginning at day 5 of differentiation. Differential gene expression within the pluripotent state revealed 23 distinguishing candidate genes among pluripotent cell lines with divergent cardiogenic potentials. A confirmed panel of 12 genes, differentially expressed between high and low cardiogenic lines, was transformed into a predictive score sufficient to discriminate individual induced pluripotent stem cell lines according to relative cardiogenic potential.

CONCLUSIONS

Transcriptome analysis attuned to natural embryonic cardiogenesis provides a robust platform to probe coordinated cardiac specification and maturation from bioengineered stem cell-based model systems. A panel of developmental-related genes allowed differential prognosis of cardiogenic competency, thus prioritizing cell lines according to natural blueprint to streamline functional applications.

Transcriptional Control of Cell Lineage Development in Epicardium-Derived Cells

Epicardial derivatives, including vascular smooth muscle cells and cardiac fibroblasts, are crucial for proper development of the coronary vasculature and cardiac fibrous matrix, both of which support myocardial integrity and function in the normal heart. Epicardial formation, epithelial-to-mesenchymal transition (EMT), and epicardium-derived cell (EPDC) differentiation are precisely regulated by complex interactions among signaling molecules and transcription factors. Here we review the roles of critical transcription factors that are required for specific aspects of epicardial development, EMT, and EPDC lineage specification in development and disease. Epicardial cells and subepicardial EPDCs express transcription factors including Wt1, Tcf21, Tbx18, and Nfatc1. As EPDCs invade the myocardium, epicardial progenitor transcription factors such as Wt1 are downregulated. EPDC differentiation into SMC and fibroblast lineages is precisely regulated by a complex network of transcription factors, including Tcf21 and Tbx18. These and other transcription factors also regulate epicardial EMT, EPDC invasion, and lineage maturation. In addition, there is increasing evidence that epicardial transcription factors are reactivated with adult cardiac ischemic injury. Determining the function of reactivated epicardial cells in myocardial infarction and fibrosis may improve our understanding of the pathogenesis of heart disease.

Tissue-resident Sca1+ PDGFRα+ mesenchymal progenitors are the cellular source of fibrofatty infiltration in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is a disease of the heart involving myocardial dystrophy leading to fibrofatty scarring of the myocardium and is associated with an increased risk of both ventricular arrhythmias and sudden cardiac death. It often affects the right ventricle but may also involve the left. Although there has been significant progress in understanding the role of underlying desmosomal genetic defects in AC, there is still a lack of data regarding the cellular processes involved in its progression. The development of cardiac fibrofatty scarring is known to be a principal pathological process associated with ventricular arrhythmias, and it is vital that we elucidate the role of various cell populations involved in the disease if targeted therapeutics are to be developed. The known role of mesenchymal progenitor cells in the reparative process of both the heart and skeletal muscle has provided inspiration for the identification of the cellular basis of fibrofatty infiltration in AC. Here we hypothesize that reparative processes triggered by myocardial degeneration lead to the differentiation of tissue-resident Sca1+ PDGFRα+ mesenchymal progenitors into adipocytes and fibroblasts, which compose the fibrofatty lesions characteristic of AC.

Endocardial and epicardial epithelial to mesenchymal transitions in heart development and disease

Epithelial to mesenchymal transition (EMT) converts epithelial cells to mobile and developmentally plastic mesenchymal cells. All cells in the heart arise from one or more EMTs. Endocardial and epicardial EMTs produce most of the noncardiomyocyte lineages of the mature heart. Endocardial EMT generates valve progenitor cells and is necessary for formation of the cardiac valves and for complete cardiac septation. Epicardial EMT is required for myocardial growth and coronary vessel formation, and it generates cardiac fibroblasts, vascular smooth muscle cells, a subset of coronary endothelial cells, and possibly a subset of cardiomyocytes. Emerging studies suggest that these developmental mechanisms are redeployed in adult heart valve disease, in cardiac fibrosis, and in myocardial responses to ischemic injury. Redirection and amplification of disease-related EMTs offer potential new therapeutic strategies and approaches for treatment of heart disease. Here, we review the role and molecular regulation of endocardial and epicardial EMT in fetal heart development, and we summarize key literature implicating reactivation of endocardial and epicardial EMT in adult heart disease.

Cardiac-specific inducible and conditional gene targeting in mice

Mouse genetic engineering has revolutionized our understanding of the molecular and genetic basis of heart development and disease. This technology involves conditional tissue-specific and temporal transgenic and gene targeting approaches, as well as introduction of polymorphisms into the mouse genome. These approaches are increasingly used to elucidate the genetic pathways underlying tissue homeostasis, physiology, and pathophysiology of adult heart. They have also led to the development of clinically relevant models of human cardiac diseases. Here, we review the technologies and their limitations in general and the cardiovascular research community in particular.

Heart repair and regeneration: recent insights from zebrafish studies

Cardiovascular disease is the leading cause of death in the U.S. and worldwide. Failure to properly repair or regenerate damaged cardiac tissues after myocardial infarction is a major cause of heart failure. In contrast to humans and other mammals, zebrafish hearts regenerate after substantial injury or tissue damage. Here, we review recent progress in studying zebrafish heart regeneration, addressing the molecular and cellular responses in the three tissue layers of the heart: myocardium, epicardium, and endocardium. We also compare different injury models utilized to study zebrafish heart regeneration and discuss the differences in responses to injury between mammalian and zebrafish hearts. By learning how zebrafish hearts regenerate naturally, we can better design therapeutic strategies for repairing human hearts after myocardial infarction.

Pod1/Tcf21 is regulated by retinoic acid signaling and inhibits differentiation of epicardium-derived cells into smooth muscle in the developing heart

Epicardium-derived cells (EPDCs) invade the myocardium and differentiate into fibroblasts and vascular smooth muscle (SM) cells, which support the coronary vessels. The transcription factor Pod1 (Tcf21) is expressed in subpopulations of the epicardium and EPDCs in chicken and mouse embryonic hearts, and the transcription factors WT1, NFATC1, and Tbx18 are expressed in overlapping and distinct subsets of Pod1-expressing cells. Expression of Pod1 and WT1, but not Tbx18 or NFATC1, is activated with all-trans-retinoic acid (RA) treatment of isolated chick EPDCs in culture. In intact chicken hearts, RA inhibition leads to decreased Pod1 expression while RA treatment inhibits SM differentiation. The requirements for Pod1 in differentiation of EPDCs in the developing heart were examined in mice lacking Pod1. Loss of Pod1 in mice leads to epicardial blistering, increased SM differentiation on the surface of the heart, and a paucity of interstitial fibroblasts, with neonatal lethality. Epicardial epithelial-to-mesenchymal transition (EMT) and endothelial differentiation of coronary vessels are relatively unaffected. On the surface of the myocardium, expression of multiple SM markers is increased in Pod1-deficient EPDCs, demonstrating premature SM differentiation. Increased SM differentiation also is observed in Pod1-deficient lung mesenchyme. Together, these data demonstrate a critical role for Pod1 in controlling mesenchymal progenitor cell differentiation into SM and fibroblast lineages during cardiac development.

Cardiac explant-derived cells are regulated by Notch-modulated mesenchymal transition

Background

Progenitor cell therapy is emerging as a novel treatment for heart failure. However the molecular mechanisms regulating the generation of cardiac progenitor cells is not fully understood. We hypothesized that cardiac progenitor cells are generated from cardiac explant via a process similar to epithelial to mesenchymal transition (EMT).

Methods/Findings

Explant-derived cells were generated from partially digested atrial tissue. After 21 days in culture, c-Kit+ cells were isolated from cell outgrowth. The majority of explant-originated c-Kit+ cells expressed the epicardial marker Wt1. Cardiac cell outgrowth exhibits a temporal up-regulation of EMT-markers. Notch stimulation augmented, while Notch inhibition suppressed, mesenchymal transition in both c-Kit+ and c-Kit- cells. In c-Kit+ cells, Notch stimulation reduced, while Notch inhibition up-regulated pluripotency marker expressions such as Nanog and Sox2. Notch induction was associated with degradation of β-catenin in c-Kit- cells. In contrast, Notch inhibition resulted in β-catenin accumulation, acquisition of epitheloid morphology, and up-regulation of Wnt target genes in c-Kit- cells.

Conclusion

Our study suggests that Notch-mediated reversible EMT process is a mechanism that regulates explant-derived c-Kit+ and c-Kit- cells.

Epigenetic regulation of cardiac development and function by polycomb group and trithorax group proteins

Heart disease is a leading cause of death and disability in developed countries. Heart disease includes a broad range of diseases that affect the development and/or function of the cardiovascular system. Some of these diseases, such as congenital heart defects, are present at birth. Others develop over time and may be influenced by both genetic and environmental factors. Many of the known heart diseases are associated with abnormal expression of genes. Understanding the factors and mechanisms that regulate gene expression in the heart is essential for the detection, treatment, and prevention of heart diseases. Polycomb Group (PcG) and Trithorax Group (TrxG) proteins are special families of chromatin factors that regulate developmental gene expression in many tissues and organs. Accumulating evidence suggests that these proteins are important regulators of development and function of the heart as well. A better understanding of their roles and functional mechanisms will translate into new opportunities for combating heart disease.

Twist factor regulation of non-cardiomyocyte cell lineages in the developing heart

The heart is a complex organ that is composed of numerous cell types, which must integrate their programs for proper specification, differentiation and cardiac morphogenesis. During cardiogenesis members of the Twist-family of basic helix-loop-helix (bHLH) transcription factors play distinct roles within cardiac lineages such as the endocardium and extra-cardiac lineages such as the cardiac neural crest (cNCC) and epicardium. While the study of these cell populations is often eclipsed by that of cardiomyocytes, the contributions of non-cardiomyocytes to development and disease are increasingly being appreciated as both dynamic and essential. This review summarizes what is known regarding Twist-family bHLH function in extra-cardiac cell populations and the endocardium, with a focus on regulatory mechanisms, downstream targets, and expression profiles. Improving our understanding of the molecular pathways that Twist-family bHLH factors mediate in these lineages will be necessary to ascertain how their dysfunction leads to congenital disease and adult pathologies such as myocardial infarctions and cardiac fibroblast induced fibrosis. Indeed, this knowledge will prove to be critical to clinicians seeking to improve current treatments.

Experimental acute myocardial infarction: telocytes involvement in neo-angiogenesis

We used rat experimental myocardial infarction to study the ultrastructural recovery, especially neo-angiogenesis in the infarction border zone. We were interested in the possible role(s) of telocytes (TCs), a novel type of interstitial cell very recently discovered in myocardim (see http://www.telocytes.com). Electron microscopy, immunocytochemistry and analysis of several proangiogenic microRNAs provided evidence for TC involvement in neo-angiogenesis after myocardial infarction. Electron microscopy showed the close spatial association of TCs with neoangiogenetic elements. Higher resolution images provided the following information: (a) the intercellular space between the abluminal face of endothelium and its surrounding TCs is frequently less than 50 nm; (b) TCs establish multiple direct nanocontacts with endothelial cells, where the extracellular space seems obliterated; such nanocontacts have a length of 0.4–1.5 μm; (c) the absence of basal membrane on the abluminal face of endothelial cell. Besides the physical contacts (either nanoscopic or microscopic) TCs presumably contribute to neo-angiognesis via paracrine secretion (as shown by immunocytochemistry for VEGF or NOS2). Last but not least, TCs contain measurable quantities of angiogenic microRNAs (e.g. let-7e, 10a, 21, 27b, 100, 126-3p, 130a, 143, 155, 503). Taken together, the direct (physical) contact of TCs with endothelial tubes, as well as the indirect (chemical) positive influence within the ‘angiogenic zones’, suggests an important participation of TCs in neo-angiogenesis during the late stage of myocardial infarction.

Epithelial-to-mesenchymal transformation alters electrical conductivity of human epicardial cells

The myocardium of the developing heart tube is covered by epicardium. These epicardial cells undergo a process of epithelial-to-mesenchymal transformation (EMT) and develop into epicardium-derived cells (EPDCs). The ingrowing EPDCs differentiate into several celltypes of which the cardiac fibroblasts form the main group. Disturbance of EMT of the epicardium leads to serious hypoplasia of the myocardium, abnormal coronary artery differentiation and Purkinje fibre paucity. Interestingly, the electrophysiological properties of epicardial cells and whether EMT influences electrical conductivity of epicardial cells is not yet known. We studied the electrophysiological aspects of epicardial cells before and after EMT in a dedicated in vitro model, using micro-electrode arrays to investigate electrical conduction across epicardial cells. Therefore, human adult epicardial cells were placed between two neonatal rat cardiomyocyte populations. Before EMT the epicardial cells have a cobblestone (epithelium-like) phenotype that was confirmed by staining for the cell-adhesion molecule β-catenin. After spontaneous EMT in vitro the EPDCs acquired a spindle-shaped morphology confirmed by vimentin staining. When comparing both types we observed that the electrical conduction is influenced by EMT, resulting in significantly reduced conductivity of spindle-shaped EPDCs, associated with a conduction block. Furthermore, the expression of both gap junction (connexins 40, Cx43 and Cx45) and ion channel proteins (SCN5a, CACNA1C and Kir2.1) was down-regulated after EMT. This study shows for the first time the conduction differences between epicardial cells before and after EMT. These differences may be of relevance for the role of EPDCs in cardiac development, and in EMT-related cardiac dysfunction.