Cellular origin and developmental program of coronary angiogenesis

Avian embryonic coronary arterio-venous patterning involves the contribution of different endothelial and endocardial cell populations

BACKGROUND

Coronary vasculature irrigates the myocardium and is crucial to late embryonic and adult heart function. Despite the developmental significance and clinical relevance of these blood vessels, the embryonic origin and the cellular and molecular mechanisms that regulate coronary arterio-venous patterning are not known in detail. In this study, we have used the avian embryo to dissect the ontogenetic origin and morphogenesis of coronary vasculature.

RESULTS

We show that sinus venosus endocardial sprouts and proepicardial angioblasts pioneer coronary vascular formation, invading the developing heart simultaneously. We also report that avian ventricular endocardium has the potential to contribute to coronary vessels, and describe the incorporation of cardiac distal outflow tract endothelial cells to the peritruncal endothelial plexus to participate in coronary vascular formation. Finally, our findings indicate that large sinus venosus-independent sections of the forming coronary vasculature develop without connection to the systemic circulation and that coronary arterio-venous shunts form a few hours before peritruncal arterial endothelium connects to the aortic root.

CONCLUSIONS

Embryonic coronary vasculature is a developmental mosaic, formed by the integration of vascular cells from, at least, four different embryological origins, which assemble in a coordinated manner to complete coronary vascular development. Developmental Dynamics 247:686-698, 2018. © 2017 Wiley Periodicals, Inc.

Enhancing the precision of genetic lineage tracing using dual recombinases

The Cre-loxP recombination system is the most widely used technology for in vivo tracing of stem or progenitor cell lineages. The precision of this genetic system largely depends on the specificity of Cre recombinase expression in targeted stem or progenitor cells. However, Cre expression in nontargeted cell types can complicate the interpretation of lineage-tracing studies and has caused controversy in many previous studies. Here we describe a new genetic lineage tracing system that incorporates the Dre-rox recombination system to enhance the precision of conventional Cre-loxP-mediated lineage tracing. The Dre-rox system permits rigorous control of Cre-loxP recombination in lineage tracing, effectively circumventing potential uncertainty of the cell-type specificity of Cre expression. Using this new system we investigated two topics of recent debates-the contribution of c-Kit⁺ cardiac stem cells to cardiomyocytes in the heart and the contribution of Sox9⁺ hepatic progenitor cells to hepatocytes in the liver. By overcoming the technical hurdle of nonspecific Cre-loxP-mediated recombination, this new technology provides more precise analysis of cell lineage and fate decisions and facilitates the in vivo study of stem and progenitor cell plasticity in disease and regeneration.

Non-muscle myosin IIB (Myh10) is required for epicardial function and coronary vessel formation during mammalian development

The coronary vasculature is an essential vessel network providing the blood supply to the heart. Disruptions in coronary blood flow contribute to cardiac disease, a major cause of premature death worldwide. The generation of treatments for cardiovascular disease will be aided by a deeper understanding of the developmental processes that underpin coronary vessel formation. From an ENU mutagenesis screen, we have isolated a mouse mutant displaying embryonic hydrocephalus and cardiac defects (EHC). Positional cloning and candidate gene analysis revealed that the EHC phenotype results from a point mutation in a splice donor site of the Myh10 gene, which encodes NMHC IIB. Complementation testing confirmed that the Myh10 mutation causes the EHC phenotype. Characterisation of the EHC cardiac defects revealed abnormalities in myocardial development, consistent with observations from previously generated NMHC IIB null mouse lines. Analysis of the EHC mutant hearts also identified defects in the formation of the coronary vasculature. We attribute the coronary vessel abnormalities to defective epicardial cell function, as the EHC epicardium displays an abnormal cell morphology, reduced capacity to undergo epithelial-mesenchymal transition (EMT), and impaired migration of epicardial-derived cells (EPDCs) into the myocardium. Our studies on the EHC mutant demonstrate a requirement for NMHC IIB in epicardial function and coronary vessel formation, highlighting the importance of this protein in cardiac development and ultimately, embryonic survival.

Endothelial-to-Mesenchymal Transition: A Potential Mechanism for Atherosclerosis Plaque Progression and Destabilization

Endothelial-to-mesenchymal transition (EndMT) is a cellular reprogramming mechanism by which endothelial cells acquire a mesenchymal phenotype. EndMT is associated with fibroproliferative diseases, such as cancer progression and metastasis and cardiac and kidney fibrosis, and this condition has been extensively investigated over the past decade. Recently, studies showed that EndMT contributes to the initiation and progression of atherosclerotic lesion and plaque destabilization. Unstable atherosclerotic plaque rupture and subsequent thrombosis are the main pathological causes of acute cardiovascular events. EndMT is plastic and reversible. Therefore, our enhanced understanding on the mechanisms controlling EndMT and its roles in the atherosclerosis plaque progression and instability may provide a basis for the development of novel therapeutic strategies to stabilize and reverse atherosclerotic plaques.

Three-dimensional cardiac microtissues composed of cardiomyocytes and endothelial cells co-differentiated from human pluripotent stem cells

Cardiomyocytes and endothelial cells in the heart are in close proximity and in constant dialogue. Endothelium regulates the size of the heart, supplies oxygen to the myocardium and secretes factors that support cardiomyocyte function. Robust and predictive cardiac disease models that faithfully recapitulate native human physiology in vitro would therefore ideally incorporate this cardiomyocyte-endothelium crosstalk. Here, we have generated and characterized human cardiac microtissues in vitro that integrate both cell types in complex 3D structures. We established conditions for simultaneous differentiation of cardiomyocytes and endothelial cells from human pluripotent stem cells following initial cardiac mesoderm induction. The endothelial cells expressed cardiac markers that were also present in primary cardiac microvasculature, suggesting cardiac endothelium identity. These cell populations were further enriched based on surface markers expression, then recombined allowing development of beating 3D structures termed cardiac microtissues. This in vitro model was robustly reproducible in both embryonic and induced pluripotent stem cells. It thus represents an advanced human stem cell-based platform for cardiovascular disease modelling and testing of relevant drugs.

Summary: Co-differentiation of endothelial cells and cardiomyocytes from human pluripotent stem cells provides a cardiac microtissue model with potential applications for disease modelling and drug discovery.

Danshensu accelerates angiogenesis after myocardial infarction in rats and promotes the functions of endothelial progenitor cells through SDF-1α/CXCR4 axis

The present study was performed to investigate the potential role of Danshensu in therapeutic angiogenesis in ischemic myocardium and endothelial progenitor cells (EPCs) function. The rat model of myocardial infarction (MI) injury was induced by left anterior descending coronary artery ligation for 14 days. Danshensu significantly alleviated myocardial ischemia injury by ameliorating left ventricular function and reducing infarct size. Furthermore, Danshensu potentiated post-ischemia neovascularization as evidenced by increased microvessel density in infarction boundary zone, as well as the expression of marker proteins vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). Moreover, Danshensu notably promoted stromal cell-derived factor-1α (SDF-1α) level in plasma and C-X-C chemokine receptor type 4 (CXCR4) expression in peri-infarction myocardium, and AMD3100 (CXCR4 antagonist) could reverse the angiogenic and cardioprotective effects of Danshensu. For in vitro study, EPCs were isolated from bone marrow of rats. On the one hand, Danshensu provided significant cytoprotection against hypoxia insult by boosting EPCs viability and inhibiting apoptosis, and upregulated Akt phosphorylation. On the other hand, Danshensu enhanced proangiogenic functions of EPCs on cell migration and tube formation, and increased SDF-1α and CXCR4 expression. Likewise, the cytoprotection and proangiogenic functions of Danshensu on EPCs were partly negated by LY294002 (PI3K antagonist) and CXCR4 siRNA, respectively. Taken together, our results suggested that the cardioprotection of Danshensu in MI rats may be related to promoting myocardial neovascularization. The possible mechanisms may involve improving EPCs survival in hypoxia condition through Akt phosphorylation, and accelerating EPCs proangiogenic functions through SDF-1α/CXCR4 axis.

Fibroblasts in an endocardial fibroelastosis disease model mainly originate from mesenchymal derivatives of epicardium

Endocardial fibroelastosis (EFE) refers to the thickening of the ventricular endocardium as a result of de novo deposition of subendocardial fibrous tissue layers during neonatal heart development. The origin of EFE fibroblasts is proposed to be postnatal endocardial cells that undergo an aberrant endothelial-to-mesenchymal transition (EndMT). Genetic lineage tracing of endocardial cells with the inducible endocardial Cre line Npr3-CreER and the endothelial cell tracing line Cdh5-CreER on an EFE-like model did not reveal any contribution of neonatal endocardial cells to fibroblasts in the EFE-like tissues. Instead, lineage tracing of embryonic epicardium by Wt1-CreER suggested that epicardium-derived mesenchymal cells (MCs) served as the major source of EFE fibroblasts. By labeling MCs using Sox9-CreER, we confirmed that MCs of the embryonic heart expand and contribute to the majority of neonatal EFE fibroblasts. During this pathological process, TGFβ signaling, the key mediator of fibroblasts activation, was highly upregulated in the EFE-like tissues. Targeting TGFβ signaling by administration of its antagonist bone morphogenetic protein 7 effectively reduced fibroblast accumulation and tissue fibrosis in the EFE-like model. Our study provides genetic evidence that excessive fibroblasts in the EFE-like tissues mainly originate from the epicardium-derived MCs through epicardial to mesenchymal transition (EpiMT). These EpiMT-derived fibroblasts within the EFE-like tissues could serve as a potential therapeutic target.

The epicardium as a source of multipotent adult cardiac progenitor cells: Their origin, role and fate

Since the regenerative capacity of the adult mammalian heart is limited, cardiac injury leads to the formation of scar tissue and thereby increases the risk of developing compensatory heart failure. Stem cell therapy is a promising therapeutic approach but is facing problems with engraftment and clinical feasibility. Targeting an endogenous stem cell population could circumvent these limitations. The epicardium, a membranous layer covering the outside of the myocardium, is an accessible cell population which plays a key role in the developing heart. Epicardial cells undergo epithelial to mesenchymal transition (EMT), thus providing epicardial derived cells (EPDCs) that migrate into the myocardium and cooperate in myocardial vascularisation and compaction. In the adult heart, injury activates the epicardium, and an embryonic-like response is observed which includes EMT and differentiation of the EPDCs into cardiac cell types. Furthermore, paracrine communication between the epicardium and myocardium improves the regenerative response. The significant role of the epicardium has been shown in both the developing and the regenerating heart. Interestingly, the epicardial contribution to cardiac repair can be improved in several ways. In this review, an overview of the epicardial origin and fate will be given and potential therapeutic approaches will be discussed.

Identification of a hybrid myocardial zone in the mammalian heart after birth

Noncompaction cardiomyopathy is characterized by the presence of extensive trabeculations, which could lead to heart failure and malignant arrhythmias. How trabeculations resolve to form compact myocardium is poorly understood. Elucidation of this process is critical to understanding the pathophysiology of noncompaction disease. Here we use genetic lineage tracing to mark the Nppa⁺ or Hey2⁺ cardiomyocytes as trabecular and compact components of the ventricular wall. We find that Nppa⁺ and Hey2⁺ cardiomyocytes, respectively, from the endocardial and epicardial zones of the ventricular wall postnatally. Interposed between these two postnatal layers is a hybrid zone, which is composed of cells derived from both the Nppa⁺ and Hey2⁺ populations. Inhibition of the fetal Hey2⁺ cell contribution to the hybrid zone results in persistence of excessive trabeculations in postnatal heart. Our findings indicate that the expansion of Hey2⁺ fetal compact component, and its contribution to the hybrid myocardial zone, are essential for normal formation of the ventricular walls.

Fetal trabecular muscles in the heart undergo a poorly described morphogenetic process that results into a solidified compact myocardium after birth. Tian et al. show that cardiomyocytes in the fetal compact layer also contribute to this process, forming a hybrid myocardial zone that is composed of cells derived from both trabecular and compact layers.

Preexisting endothelial cells mediate cardiac neovascularization after injury

The mechanisms that promote the generation of new coronary vasculature during cardiac homeostasis and after injury remain a fundamental and clinically important area of study in the cardiovascular field. Recently, it was reported that mesenchymal-to-endothelial transition (MEndoT) contributes to substantial numbers of coronary endothelial cells after myocardial infarction. Therefore, the MEndoT has been proposed as a paradigm mediating neovascularization and is considered a promising therapeutic target in cardiac regeneration. Here, we show that preexisting endothelial cells mainly beget new coronary vessels in the adult mouse heart, with essentially no contribution from other cell sources through cell-lineage transdifferentiation. Genetic-lineage tracing revealed that cardiac fibroblasts expand substantially after injury, but do not contribute to the formation of new coronary blood vessels, indicating no contribution of MEndoT to neovascularization. Moreover, genetic-lineage tracing with a pulse-chase labeling strategy also showed that essentially all new coronary vessels in the injured heart are derived from preexisting endothelial cells, but not from other cell lineages. These data indicate that therapeutic strategies for inducing neovascularization should not be based on targeting presumptive lineage transdifferentiation such as MEndoT. Instead, preexisting endothelial cells appear more likely to be the therapeutic target for promoting neovascularization and driving heart regeneration after injury.

Identification of cardiac hemo-vascular precursors and their requirement of sphingosine-1-phosphate receptor 1 for heart development

The cardiac endothelium plays a crucial role in the development of a functional heart. However, the precise identification of the endocardial precursors and the mechanisms they require for their role in heart morphogenesis are not well understood. Using in vivo and in vitro cell fate tracing concomitant with specific cell ablation and embryonic heart transplantation studies, we identified a unique set of precursors which possess hemogenic functions and express the stem cell leukemia (SCL) gene driven by its 5' enhancer. These hemo-vascular precursors give rise to the endocardium, atrioventricular cushions and coronary vascular endothelium. Furthermore, deletion of the sphingosine-1-phosphate receptor 1 (S1P1) in these precursors leads to ventricular non-compaction cardiomyopathy, a poorly understood condition leading to heart failure and early mortality. Thus, we identified a distinctive population of hemo-vascular precursors which require S1P1 to exert their functions and are essential for cardiac morphogenesis.

Coronary Artery Development: Progenitor Cells and Differentiation Pathways

Coronary artery disease (CAD) is the number one cause of death worldwide and involves the accumulation of plaques within the artery wall that can occlude blood flow to the heart and cause myocardial infarction. The high mortality associated with CAD makes the development of medical interventions that repair and replace diseased arteries a high priority for the cardiovascular research community. Advancements in arterial regenerative medicine could benefit from a detailed understanding of coronary artery development during embryogenesis and of how these pathways might be reignited during disease. Recent research has advanced our knowledge on how the coronary vasculature is built and revealed unexpected features of progenitor cell deployment that may have implications for organogenesis in general. Here, we highlight these recent findings and discuss how they set the stage to interrogate developmental pathways during injury and disease.

Analysis of Coronary Vessels in Cleared Embryonic Hearts

Whole mount visualization of the embryonic coronary plexus from which the capillary and arterial networks will form is rendered problematic using standard microscopy techniques, due to the scattering of imaging light by the thick heart tissue, as these vessels are localized deep within the walls of the developing heart. As optical clearing of tissues using organic solvents such as BABB (1 part benzyl alcohol to 2 parts benzyl benzoate) has been shown to greatly improve the optical penetration depth that can be achieved, we combined clearance of whole, PECAM1-immunostained hearts, with laser-scanning confocal microscopy, in order to obtain high-resolution images of vessels throughout the entire heart. BABB clearance of embryonic hearts takes place rapidly and also acts to preserve the fluorescent signal for several weeks; in addition, samples can be imaged multiple times without loss of signal. This straightforward method is also applicable to imaging other types of blood vessels in whole embryos.

Insulin-Like Growth Factor 1 Receptor-Dependent Pathway Drives Epicardial Adipose Tissue Formation After Myocardial Injury

BACKGROUND

Epicardial adipose tissue volume and coronary artery disease are strongly associated, even after accounting for overall body mass. Despite its pathophysiological significance, the origin and paracrine signaling pathways that regulate epicardial adipose tissue's formation and expansion are unclear.

METHODS

We used a novel modified mRNA-based screening approach to probe the effect of individual paracrine factors on epicardial progenitors in the adult heart.

RESULTS

Using 2 independent lineage-tracing strategies in murine models, we show that cells originating from the Wt1⁺ mesothelial lineage, which includes epicardial cells, differentiate into epicardial adipose tissue after myocardial infarction. This differentiation process required Wt1 expression in this lineage and was stimulated by insulin-like growth factor 1 receptor (IGF1R) activation. IGF1R inhibition within this lineage significantly reduced its adipogenic differentiation in the context of exogenous, IGF1-modified mRNA stimulation. Moreover, IGF1R inhibition significantly reduced Wt1 lineage cell differentiation into adipocytes after myocardial infarction.

CONCLUSIONS

Our results establish IGF1R signaling as a key pathway that governs epicardial adipose tissue formation in the context of myocardial injury by redirecting the fate of Wt1⁺ lineage cells. Our study also demonstrates the power of modified mRNA -based paracrine factor library screening to dissect signaling pathways that govern progenitor cell activity in homeostasis and disease.

Vascular ontogeny within selected thoracoabdominal organs and the limbs

The cardiovascular system is fundamental to life. Its vessels are the conduits for delivery of nutrients and oxygen to organs and the removal of wastes. During embryonic development, the vascular system is instrumental in the formation of organs. It contributes to the form and pattern of organs as diverse as the limbs and the gonads. Recent advances in molecular biology and genomics have afforded great insight to the control of vascular development at subcellular levels of organization. Nevertheless, there is little assembled information concerning the vascular development of the organ systems of the body. This paper begins by reviewing the modes of formation of embryonic blood vessels. This is followed by summaries of the ontogeny of the vasculature that supplies selected major thoracic and abdominal organs (heart, gut, liver, gonads, and kidney). The paper concludes with a description of the arterial development of the upper and lower extremities.

Transcriptional Profiling of Cultured, Embryonic Epicardial Cells Identifies Novel Genes and Signaling Pathways Regulated by TGFβR3 In Vitro

The epicardium plays an important role in coronary vessel formation and Tgfbr3^-/- mice exhibit failed coronary vessel development associated with decreased epicardial cell invasion. Immortalized Tgfbr3^-/- epicardial cells display the same defects. Tgfbr3^+/+ and Tgfbr3^-/- cells incubated for 72 hours with VEH or ligands known to promote invasion via TGFβR3 (TGFβ1, TGFβ2, BMP2), for 72 hours were harvested for RNA-seq analysis. We selected for genes >2-fold differentially expressed between Tgfbr3^+/+ and Tgfbr3^-/- cells when incubated with VEH (604), TGFβ1 (515), TGFβ2 (553), or BMP2 (632). Gene Ontology (GO) analysis of these genes identified dysregulated biological processes consistent with the defects observed in Tgfbr3^-/- cells, including those associated with extracellular matrix interaction. GO and Gene Regulatory Network (GRN) analysis identified distinct expression profiles between TGFβ1-TGFβ2 and VEH-BMP2 incubated cells, consistent with the differential response of epicardial cells to these ligands in vitro. Despite the differences observed between Tgfbr3^+/+ and Tgfbr3^-/- cells after TGFβ and BMP ligand addition, GRNs constructed from these gene lists identified NF-ĸB as a key nodal point for all ligands examined. Tgfbr3^-/- cells exhibited decreased expression of genes known to be activated by NF-ĸB signaling. NF-ĸB activity was stimulated in Tgfbr3^+/+ epicardial cells after TGFβ2 or BMP2 incubation, while Tgfbr3^-/- cells failed to activate NF-ĸB in response to these ligands. Tgfbr3^+/+ epicardial cells incubated with an inhibitor of NF-ĸB signaling no longer invaded into a collagen gel in response to TGFβ2 or BMP2. These data suggest that NF-ĸB signaling is dysregulated in Tgfbr3^-/- epicardial cells and that NF-ĸB signaling is required for epicardial cell invasion in vitro. Our approach successfully identified a signaling pathway important in epicardial cell behavior downstream of TGFβR3. Overall, the genes and signaling pathways identified through our analysis yield the first comprehensive list of candidate genes whose expression is dependent on TGFβR3 signaling.

Wars2 is a determinant of angiogenesis

Coronary flow (CF) measured ex vivo is largely determined by capillary density that reflects angiogenic vessel formation in the heart in vivo. Here we exploit this relationship and show that CF in the rat is influenced by a locus on rat chromosome 2 that is also associated with cardiac capillary density. Mitochondrial tryptophanyl-tRNA synthetase (Wars2), encoding an L53F protein variant within the ATP-binding motif, is prioritized as the candidate at the locus by integrating genomic data sets. WARS2(L53F) has low enzyme activity and inhibition of WARS2 in endothelial cells reduces angiogenesis. In the zebrafish, inhibition of wars2 results in trunk vessel deficiencies, disordered endocardial-myocardial contact and impaired heart function. Inhibition of Wars2 in the rat causes cardiac angiogenesis defects and diminished cardiac capillary density. Our data demonstrate a pro-angiogenic function for Wars2 both within and outside the heart that may have translational relevance given the association of WARS2 with common human diseases.

Blood supply to the heart is crucial for cardiac function. Here, the authors show that the mitochondrial tryptophanyl-tRNA synthetase, WARS2, drives blood vessel generation in zebrafish and rats and that inhibition of Wars2 diminishes blood vessel growth both within and outside in the heart, suggesting a new target for manipulating angiogenesis.

Endocardium Minimally Contributes to Coronary Endothelium in the Embryonic Ventricular Free Walls

RATIONALE

There is persistent uncertainty regarding the developmental origins of coronary vessels, with 2 principal sources suggested as ventricular endocardium or sinus venosus (SV). These 2 proposed origins implicate fundamentally distinct mechanisms of vessel formation. Resolution of this controversy is critical for deciphering the programs that result in the formation of coronary vessels and has implications for research on therapeutic angiogenesis.

OBJECTIVE

To resolve the controversy over the developmental origin of coronary vessels.

METHODS AND RESULTS

We first generated nuclear factor of activated T cells (Nfatc1)-Cre and Nfatc1-Dre lineage tracers for endocardium labeling. We found that Nfatc1 recombinases also label a significant portion of SV endothelial cells in addition to endocardium. Therefore, restricted endocardial lineage tracing requires a specific marker that distinguishes endocardium from SV. By single-cell gene expression analysis, we identified a novel endocardial gene natriuretic peptide receptor 3 (Npr3). Npr3 is expressed in the entirety of the endocardium but not in the SV. Genetic lineage tracing based on Npr3-CreER showed that endocardium contributes to a minority of coronary vessels in the free walls of embryonic heart. Intersectional genetic lineage tracing experiments demonstrated that endocardium minimally contributes to coronary endothelium in the embryonic ventricular free walls.

CONCLUSIONS

Our study suggested that SV, but not endocardium, is the major origin for coronary endothelium in the embryonic ventricular free walls. This work thus resolves the recent controversy over the developmental origin of coronary endothelium, providing the basis for studying coronary vessel formation and regeneration after injury.

Genetic lineage tracing identifies endocardial origin of liver vasculature

The hepatic vasculature is essential for liver development, homeostasis and regeneration, yet the developmental program of hepatic vessel formation and the embryonic origin of the liver vasculature remain unknown. Here we show in mouse that endocardial cells form a primitive vascular plexus surrounding the liver bud and subsequently contribute to a substantial portion of the liver vasculature. Using intersectional genetics, we demonstrate that the endocardium of the sinus venosus is a source for the hepatic plexus. Inhibition of endocardial angiogenesis results in reduced endocardial contribution to the liver vasculature and defects in liver organogenesis. We conclude that a substantial portion of liver vessels derives from the endocardium and shares a common developmental origin with coronary arteries.

Smooth muscle origin of postnatal 2nd CVP is pre-determined in early embryo

Recent identification of the neonatal 2nd coronary vascular population (2nd CVP) suggests that a subset of these vessels form de novo and mature in the inner myocardial wall of the postnatal heart. However, the origin of smooth muscle cells (SMCs) in the postnatal 2nd CVP remains undetermined. Using a tamoxifen-inducible Wt1-CreER driver and a Rosa26-RFP reporter line, we traced the lineage of epicardial cells to determine if they contribute to SMCs of the 2nd CVP. Late embryonic and postnatal induction of Wt1-CreER activity demonstrated that at these stages Wt1-labeled epicardium does not significantly migrate into the myocardium to form SMCs. However, following tamoxifen treatment at an early embryonic stage (E10.5), we detected Wt1 descendants (epicardium-derived cells, or EPDCs) in the outer myocardial wall at E17.5. When the 2nd CVP forms and remodels at postnatal stage, these early labeled EDPCs re-migrate deep into the inner myocardial wall and contribute to 2nd CVP-SMCs in the adult heart. Our findings reveal that SMCs in the postnatal 2nd CVP are pre-specified as EPDCs from the earliest wave of epicardial cell migration. Rather than the re-activation and migration of epicardial cells at later stages, these resident EPDCs mobilize and contribute to smooth muscle of the 2nd CVP during postnatal development.

Epicardium is required for cardiac seeding by yolk sac macrophages, precursors of resident macrophages of the adult heart

A subset of macrophages that reside in adult tissues originate from the fetal yolk sac, while others derive from circulating monocytes. These ontologically different macrophage subsets have distinct roles in tissue injury responses, with the embryonic population overall having beneficial activity in cardiac repair. Here we show that fetal yolk macrophages are recruited to a niche within and just below the epicardium, the mesothelial covering of the heart. The epicardium was required for establishment of yolk sac macrophages in this region of the fetal heart, and this function of epicardium depended on its expression of the transcription factor WT1. Thus, tissue-specific cues and transcriptional programs recruit or retain embryonic macrophages in their final abodes, where they help to shape organ homeostasis and injury responses.

The developmental origins and lineage contributions of endocardial endothelium

Endocardial development involves a complex orchestration of cell fate decisions that coordinate with endoderm formation and other mesodermal cell lineages. Historically, investigations into the contribution of endocardium in the developing embryo was constrained to the heart where these cells give rise to the inner lining of the myocardium and are a major contributor to valve formation. In recent years, studies have continued to elucidate the complexities of endocardial fate commitment revealing a much broader scope of lineage potential from developing endocardium. These studies cover a wide range of species and model systems and show direct contribution or fate potential of endocardium giving rise to cardiac vasculature, blood, fibroblast, and cardiomyocyte lineages. This review focuses on the marked expansion of knowledge in the area of endocardial fate potential. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Coronary stem development in wild-type and Tbx1 null mouse hearts

BACKGROUND

Coronary artery (CA) stems connect the ventricular coronary tree with the aorta. Defects in proximal CA patterning are a cause of sudden cardiac death. In mice lacking Tbx1, common arterial trunk is associated with an abnormal trajectory of the proximal left CA. Here we investigate CA stem development in wild-type and Tbx1 null embryos.

RESULTS

Genetic lineage tracing reveals that limited outgrowth of aortic endothelium contributes to proximal CA stems. Immunohistochemistry and fluorescent tracer injections identify a periarterial vascular plexus present at the onset of CA stem development. Transplantation experiments in avian embryos indicate that the periarterial plexus originates in mesenchyme distal to the outflow tract. Tbx1 is required for the patterning but not timing of CA stem development and a Tbx1 reporter allele is expressed in myocardium adjacent to the left but not right CA stem. This expression domain is maintained in Sema3c(-/-) hearts with a common arterial trunk and leftward positioned CA. Ectopic myocardial differentiation is observed on the left side of the Tbx1(-/-) common arterial trunk.

CONCLUSIONS

A periarterial plexus bridges limited outgrowth of the aortic endothelium with the ventricular plexus during CA stem development. Molecular differences associated with left and right CA stems provide new insights into the etiology of CA patterning defects.

Genetic lineage tracing discloses arteriogenesis as the main mechanism for collateral growth in the mouse heart

Aims

Capillary and arterial endothelial cells share many common molecular markers in both the neonatal and adult hearts. Herein, we aim to establish a genetic tool that distinguishes these two types of vessels in order to determine the cellular mechanism underlying collateral artery formation.

Methods and results

Using Apln-GFP and Apln-LacZ reporter mice, we demonstrate that APLN expression is enriched in coronary vascular endothelial cells. However, APLN expression is reduced in coronary arterial endothelial cells. Genetic lineage tracing, using an Apln-CreER mouse line, robustly labelled capillary endothelial cells, but not arterial endothelial cells. We leveraged this differential activity of Apln-CreER to study collateral artery formation following myocardial infarction (MI). In a neonatal heart MI model, we found that Apln-CreER-labelled capillary endothelial cells do not contribute to the large collateral arteries. Instead, these large collateral arteries mainly arise from pre-existing, infrequently labelled coronary arteries, indicative of arteriogenesis. Furthermore, in an adult heart MI model, Apln-CreER activity also distinguishes large and small diameter arteries from capillaries. Lineage tracing in this setting demonstrated that most large and small coronary arteries in the infarcted myocardium and border region are derived not from capillaries, but from pre-existing arteries.

Conclusion

Apln-CreER-mediated lineage tracing distinguishes capillaries from large arteries, in both the neonatal and adult hearts. Through genetic fate mapping, we demonstrate that pre-existing arteries, but not capillaries, extensively contribute to collateral artery formation following myocardial injury. These results suggest that arteriogenesis is the major mechanism underlying collateral vessel formation.

Congenital coronary artery anomalies: a bridge from embryology to anatomy and pathophysiology--a position statement of the development, anatomy, and pathology ESC Working Group

Congenital coronary artery anomalies are of major significance in clinical cardiology and cardiac surgery due to their association with myocardial ischaemia and sudden death. Such anomalies are detectable by imaging modalities and, according to various definitions, their prevalence ranges from 0.21 to 5.79%. This consensus document from the Development, Anatomy and Pathology Working Group of the European Society of Cardiology aims to provide: (i) a definition of normality that refers to essential anatomical and embryological features of coronary vessels, based on the integrated analysis of studies of normal and abnormal coronary embryogenesis and pathophysiology; (ii) an animal model-based systematic survey of the molecular and cellular mechanisms that regulate coronary blood vessel development; (iii) an organization of the wide spectrum of coronary artery anomalies, according to a comprehensive anatomical and embryological classification scheme; (iv) current knowledge of the pathophysiological mechanisms underlying symptoms and signs of coronary artery anomalies, with diagnostic and therapeutic implications. This document identifies the mosaic-like embryonic development of the coronary vascular system, as coronary cell types differentiate from multiple cell sources through an intricate network of molecular signals and haemodynamic cues, as the necessary framework for understanding the complex spectrum of coronary artery anomalies observed in human patients.

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

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

Endocardium Contributes to Cardiac Fat

RATIONALE

Unraveling the developmental origin of cardiac fat could offer important implications for the treatment of cardiovascular disease. The recent identification of the mesothelial source of epicardial fat tissues reveals a heterogeneous origin of adipocytes in the adult heart. However, the developmental origin of adipocytes inside the heart, namely intramyocardial adipocytes, remains largely unknown.

OBJECTIVE

To trace the developmental origin of intramyocardial adipocytes.

METHODS AND RESULTS

In this study, we identified that the majority of intramyocardial adipocytes were restricted to myocardial regions in close proximity to the endocardium. Using a genetic lineage tracing model of endocardial cells, we found that Nfatc1(+) endocardial cells contributed to a substantial number of intramyocardial adipocytes. Despite the capability of the endocardium to generate coronary vascular endothelial cells surrounding the intramyocardial adipocytes, results from our lineage tracing analyses showed that intramyocardial adipocytes were not derived from coronary vessels. Nevertheless, the endocardium of the postnatal heart did not contribute to intramyocardial adipocytes during homeostasis or after myocardial infarction.

CONCLUSIONS

Our in vivo fate-mapping studies demonstrated that the developing endocardium, but not the vascular endothelial cells, gives rise to intramyocardial adipocytes in the adult heart.

Sequential Notch activation regulates ventricular chamber development

Ventricular chambers are essential for the rhythmic contraction and relaxation occurring in every heartbeat throughout life. Congenital abnormalities in ventricular chamber formation cause severe human heart defects. How the early trabecular meshwork of myocardial fibres forms and subsequently develops into mature chambers is poorly understood. We show that Notch signalling first connects chamber endocardium and myocardium to sustain trabeculation, and later coordinates ventricular patterning and compaction with coronary vessel development to generate the mature chamber, through a temporal sequence of ligand signalling determined by the glycosyltransferase manic fringe (MFng). Early endocardial expression of MFng promotes Dll4-Notch1 signalling, which induces trabeculation in the developing ventricle. Ventricular maturation and compaction require MFng and Dll4 downregulation in the endocardium, which allows myocardial Jag1 and Jag2 signalling to Notch1 in this tissue. Perturbation of this signalling equilibrium severely disrupts heart chamber formation. Our results open a new research avenue into the pathogenesis of cardiomyopathies.

The anatomy and development of normal and abnormal coronary arteries

At present, there is significant interest in the morphology of the coronary arteries, not least due to the increasingly well-recognised association between anomalous origin of the arteries and sudden cardiac death. Much has also been learnt over the last decade regarding the embryology of the arteries. In this review, therefore, we provide a brief introduction into the recent findings regarding their development. In particular, we emphasise that new evidence, derived using the developing murine heart, points to the arterial stems growing out from the adjacent sinuses of the aortic root, rather than the arteries growing in, as is currently assumed. As we show, the concept of outgrowth provides an excellent explanation for several of the abnormal arrangements encountered in the clinical setting. Before summarising these abnormal features, we draw attention to the need to describe the heart in an attitudinally appropriate manner, following the basic rule of human anatomy, rather than describing the cardiac components with the heart in the "Valentine" orientation. We then show how the major abnormalities involving the coronary arteries in humans can be summarised in terms of abnormal origin from the pulmonary circulation, abnormal aortic origin, or fistulous communications between the coronary arteries and the cardiac cavities. In the case of abnormal aortic origin, we highlight those malformations known to be associated with sudden cardiac death.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Endothelial plasticity drives arterial remodeling within the endocardium after myocardial infarction

RATIONALE

Revascularization of injured, ischemic, and regenerating organs is essential to restore organ function. In the postinfarct heart, however, the mechanisms underlying the formation of new coronary arteries are poorly understood.

OBJECTIVE

To study vascular remodeling of coronary arteries after infarction.

METHODS AND RESULTS

We performed permanent left coronary ligation on Connexin40-GFP mice expressing green fluorescent protein (GFP) in endothelial cells of coronary arteries but not veins, capillaries, or endocardium. GFP(+) endothelial foci were identified within the endocardium in the infarct zone. These previously undescribed structures, termed endocardial flowers, have a distinct endothelial phenotype (Cx40(+), VEGFR2(+), and endoglin(-)) to the surrounding endocardium (Cx40(-), VEGFR2(-), and endoglin(+)). Endocardial flowers are contiguous with coronary vessels and associated with subendocardial smooth muscle cell accumulation. Genetic lineage tracing reveals extensive endothelial plasticity in the postinfarct heart, showing that endocardial flowers develop by arteriogenesis of Cx40(-) cells and by outgrowth of pre-existing coronary arteries. Finally, endocardial flowers exhibit angiogenic features, including early VEGFR2 expression and active proliferation of adjacent endocardial and smooth muscle cells.

CONCLUSIONS

Arterial endothelial foci within the endocardium reveal extensive endothelial cell plasticity in the infarct zone and identify the endocardium as a site of endogenous arteriogenesis and source of endothelial cells to promote vascularization in regenerative strategies.

Generation of cardiac progenitor cells through epicardial to mesenchymal transition

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