Coronary Vessel Development

Epicardial EMT and cardiac repair: an update

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

Extracellular Matrix Structure and Composition in the Early Four-Chambered Embryonic Heart

During embryonic development, the heart undergoes complex morphogenesis from a liner tube into the four chambers consisting of ventricles, atria and valves. At the same time, the cardiomyocytes compact into a dense, aligned, and highly vascularized myocardium. The extracellular matrix (ECM) is known to play an important role in this process but understanding of the expression and organization remains incomplete. Here, we performed 3D confocal imaging of ECM in the left ventricle and whole heart of embryonic chick from stages Hamburger-Hamilton 28-35 (days 5-9) as an accessible model of heart formation. First, we observed the formation of a fibronectin-rich, capillary-like networks in the myocardium between day 5 and day 9 of development. Then, we focused on day 5 prior to vascularization to determine the relative expression of fibronectin, laminin, and collagen type IV. Cardiomyocytes were found to uniaxially align prior to vascularization and, while the epicardium contained all ECM components, laminin was reduced, and collagen type IV was largely absent. Quantification of fibronectin revealed highly aligned fibers with a mean diameter of ~500 nm and interfiber spacing of ~3 µm. These structural parameters (volume, spacing, fiber diameter, length, and orientation) provide a quantitative framework to describe the organization of the embryonic ECM.

Intriguing Roles for Endothelial ADAM10/Notch Signaling in the Development of Organ-Specific Vascular Beds

The vasculature is a remarkably interesting, complex, and interconnected organ. It provides a conduit for oxygen and nutrients, filtration of waste products, and rapid communication between organs. Much remains to be learned about the specialized vascular beds that fulfill these diverse, yet vital functions. This review was prompted by the discovery that Notch signaling in mouse endothelial cells is crucial for the development of specialized vascular beds found in the heart, kidneys, liver, intestines, and bone. We will address the intriguing questions raised by the role of Notch signaling and that of its regulator, the metalloprotease ADAM10, in the development of specialized vascular beds. We will cover fundamentals of ADAM10/Notch signaling, the concept of Notch-dependent cell fate decisions, and how these might govern the development of organ-specific vascular beds through angiogenesis or vasculogenesis. We will also consider common features of the affected vessels, including the presence of fenestra or sinusoids and their occurrence in portal systems with two consecutive capillary beds. We hope to stimulate further discussion and study of the role of ADAM10/Notch signaling in the development of specialized vascular structures, which might help uncover new targets for the repair of vascular beds damaged in conditions like coronary artery disease and glomerulonephritis.

Coronary Artery Development: Origin, Malformations, and Translational Vascular Reparative Therapy

After thickening of the cardiac chamber walls during embryogenesis, oxygen and nutrients can no longer be adequately supplied to cardiac cells via passive diffusion; therefore, a primitive vascular network develops to supply these vital structures. This plexus further matures into coronary arteries and veins, which ensures continued development of the heart. Various models have been proposed to account for the growth of the coronary arteries. However, lineage-tracing studies in the last decade have identified 3 major sources, namely, the proepicardium, the sinus venosus, and endocardium. Although the exact contribution of each source remains unknown, the emerging model depicts alternative pathways and progenitor cells, which ensure successful coronary angiogenesis. We aim to explore the current trends in coronary artery development, the cellular and molecular signals regulating heart vascularization, and its implications for heart disease and vascular regeneration.

Wdpcp promotes epicardial EMT and epicardium-derived cell migration to facilitate coronary artery remodeling

During coronary vasculature development, endothelial cells enclose the embryonic heart to form the primitive coronary plexus. This structure is remodeled upon recruitment of epicardial cells that may undergo epithelial-mesenchymal transition (EMT) to enable migration and that give rise to smooth muscle cells. In mice expressing a loss-of-function mutant form of Wdpcp, a gene involved in ciliogenesis, the enclosure of the surface of the heart by the subepicardial coronary plexus was accelerated because of enhanced chemotactic responses to Shh. Coronary arteries, but not coronary veins in Wdpcp mutant mice, showed reduced smooth muscle cell coverage. In addition, Wdpcp mutant hearts had reduced expression of EMT and mesenchymal markers and had fewer epicardium-derived cells (EPDCs) that showed impaired migration. Epicardium-specific deletion of Wdpcp recapitulated the coronary artery defect of the Wdpcp mutant. Thus, Wdpcp promotes epithelial EMT and EPDC migration, processes that are required for remodeling of the coronary primitive plexus. The Wdpcp mutant mice will be a useful tool to dissect the molecular mechanisms that govern the remodeling of the primitive plexus during coronary development.

Cellular and molecular mechanisms of coronary vessel development

Development of coronary vessels is a complex process in developmental biology and it may have clinical implications. Although coronary vessels develop as a form of vasculogenesis followed by angiogenesis, the cells of the entire coronary system do not arise from the developing heart. The key events of the coronary system formation include the generation of primordium and proepicardial organ; formation of epicardium; generation of subepicardial mesenchymal cells, and the formation, remodeling and maturation of the final vascular plexus. These events represent a complex regulation of the cell fate determination, cellular migration, epicardial/mesenchymal transformation, and patterning of vasculatures. Recent studies suggest that several transcription factors, adhesion molecules, growth factors and signaling molecules play essential roles in these events. This article reviews the literature on the development of coronary vessels, and discusses current advances and controversies of molecular and cellular mechanisms, thereby directing future investigations.

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.

Study of Third Coronary Artery in Adult Human Cadaveric Hearts

INTRODUCTION

Third coronary artery (TCA) is a direct branch arising from the anterior aortic sinus (right aortic sinus) which supplies right ventricular outflow tract. It is found frequently and may be an important source for collateral coronary blood flow through a vascular anastomotic bridge (circle of Vieussens) between the right and left coronary systems.

AIM

To evaluate the gross anatomy of third coronary artery in terms of their number, origin, extent and distribution.

MATERIALS AND METHODS

After an ethical approval, 150 formalin fixed adult human cadaveric hearts were collected from Department of Anatomy, BVDU Medical College and Hospital, Sangli and Pune over the period of six years. The careful dissection was carried out to note details about third coronary artery and data was analysed using SPSS computer software.

RESULTS

The TCA was present in 32% of the heart specimens. In 42 hearts (28%) single TCA and in 6 hearts (4%) double TCA were noted. It was found to be variably distributed to conus arteriosus, anterior wall of the right ventricle, interventricular septum and the apex of the heart. TCA was larger than right coronary artery in 8 hearts and later ended at inferior border of heart. Myocardial bridge was noted over large third coronary artery in one specimen.

CONCLUSION

TCA is present frequently. It anastomoses with branches of left anterior descending artery (LADA) and contributes to apical and septal perfusion. Hence role of TCA should always be considered during diagnostic and therapeutic interventions.

An overview of cardiac morphogenesis

Accurate knowledge of normal cardiac development is essential for properly understanding the morphogenesis of congenital cardiac malformations that represent the most common congenital anomaly in newborns. The heart is the first organ to function during embryonic development and is fully formed at 8 weeks of gestation. Recent studies stemming from molecular genetics have allowed specification of the role of cellular precursors in the field of heart development. In this article we review the different steps of heart development, focusing on the processes of alignment and septation. We also show, as often as possible, the links between abnormalities of cardiac development and the main congenital heart defects. The development of animal models has permitted the unraveling of many mechanisms that potentially lead to cardiac malformations. A next step towards a better knowledge of cardiac development could be multiscale cardiac modelling.

Epicardial calcineurin-NFAT signals through Smad2 to direct coronary smooth muscle cell and arterial wall development

AIMS

Congenital coronary artery anomalies produce serious events that include syncope, arrhythmias, myocardial infarction, or sudden death. Studying the mechanism of coronary development will contribute to the understanding of the disease and help design new diagnostic or therapeutic strategies. Here, we characterized a new calcineurin-NFAT signalling which specifically functions in the epicardium to regulate the development of smooth muscle wall of the coronary arteries.

METHODS AND RESULTS

Using tissue-specific gene deletion, we found that calcineurin-NFAT signals in the embryonic epicardium to direct coronary smooth muscle cell development. The smooth muscle wall of coronary arteries fails to mature in mice with epicardial deletion of calcineurin B1 (Cnb1), and accordingly these mutant mice develop cardiac dysfunction with reduced exercise capacity. Inhibition of calcineurin at various developmental windows shows that calcineurin-NFAT signals within a narrow time window at embryonic Day 12.5-13.5 to regulate coronary smooth muscle cell development. Within the epicardium, NFAT transcriptionally activates the expression of Smad2, whose gene product is critical for transducing transforming growth factor β (TGFβ)-Alk5 signalling to control coronary development.

CONCLUSION

Our findings demonstrate new spatiotemporal and molecular actions of calcineurin-NFAT that dictate coronary arterial wall development and a new mechanism by which calcineurin-NFAT integrates with TGFβ signalling during embryonic development.

Accelerated coronary angiogenesis by vegfr1-knockout endocardial cells

During mouse heart development, ventricular endocardial cells give rise to the coronary arteries by angiogenesis. Myocardially-derived vascular endothelial growth factor-a (Vegfa) regulates embryonic coronary angiogenesis through vascular endothelial growth factor-receptor 2 (Vegfr2) expressed in the endocardium. In this study, we investigated the role of endocardially-produced soluble Vegfr1 (sVegfr1) in the coronary angiogenesis. We deleted sVegfr1 in the endocardium of the developing mouse heart and found that this deletion resulted in a precocious formation of coronary plexuses. Using an ex vivo coronary angiogenesis assay, we showed that the Vegfr1-null ventricular endocardial cells underwent excessive angiogenesis and generated extensive endothelial tubular networks. We also revealed by qPCR analysis that expression of genes involved in the Vegf-Notch pathway was augmented in the Vegfr1-null hearts. We further showed that inhibition of Notch signaling blocked the formation of coronary plexuses by the ventricular endocardial cells. These results establish that Vegfr1 produced in the endocardium negatively regulates embryonic coronary angiogenesis, possibly by limiting the Vegf-Notch signaling.

Endocardial cells form the coronary arteries by angiogenesis through myocardial-endocardial VEGF signaling

The origins and developmental mechanisms of coronary arteries are incompletely understood. We show here by fate mapping, clonal analysis, and immunohistochemistry that endocardial cells generate the endothelium of coronary arteries. Dye tracking, live imaging, and tissue transplantation also revealed that ventricular endocardial cells are not terminally differentiated; instead, they are angiogenic and form coronary endothelial networks. Myocardial Vegf-a or endocardial Vegfr-2 deletion inhibited coronary angiogenesis and arterial formation by ventricular endocardial cells. In contrast, lineage and knockout studies showed that endocardial cells make a small contribution to the coronary veins, the formation of which is independent of myocardial-to-endocardial Vegf signaling. Thus, contrary to the current view of a common source for the coronary vessels, our findings indicate that the coronary arteries and veins have distinct origins and are formed by different mechanisms. This information may help develop better cell therapies for coronary artery disease.

Towards the therapeutic use of vascular smooth muscle progenitor cells

Recent advances in the development of alternative proangiogenic and revascularization processes, including recombinant protein delivery, gene therapy, and cell therapy, hold the promise of greater efficacy in the management of cardiovascular disease in the coming years. In particular, vascular progenitor cell-based strategies have emerged as an efficient treatment approach to promote vessel formation and repair and to improve tissue perfusion. During the past decade, considerable progress has been achieved in understanding therapeutic properties of endothelial progenitor cells, while the therapeutic potential of vascular smooth muscle progenitor cells (SMPC) has only recently been explored; the number of the circulating SMPC being correlated with cardiovascular health. Several endogenous SMPC populations with varying phenotypes have been identified and characterized in the peripheral blood, bone marrow, and vascular wall. While the phenotypic entity of vascular SMPC is not fully defined and remains an evolving area of research, SMPC are increasingly recognized to play a special role in cardiovascular biology. In this review, we describe the current approaches used to define vascular SMPC. We further summarize the data on phenotype and functional properties of SMPC from various sources in adults. Finally, we discuss the role of SMPC in cardiovascular disease, including the contribution of SMPC to intimal proliferation, angiogenesis, and atherosclerotic plaque instability as well as the benefits resulting from the therapeutic use of SMPC.

Development of the cardiac venous system in prenatal human life

The human coronary sinus is an evolutionary modification of the terminal part of the left sinus horn. Anatomically, the coronary sinus is a short, broad vessel that runs along the coronary groove situated on the diaphragmatic surface of the heart. This structure, which opens into the right atrium, collects blood from the great cardiac vein and from other veins of the heart as well. In this study, we assessed the growth and dimensions of the coronary sinus at the fourth and eighth months of fetal development from whole material received from the Nicolaus Copernicus University, Collegium Medicum, Department of Histology and Embryology in Bydgoszcz. A group of 219 specimens, 105 male and 114 female fetuses, presented no visible malformations or developmental abnormalities. The results of this study determined that the dimension of the coronary sinus during prenatal development is not sexually dimorphic. Furthermore, following a monthly period of rapid growth in length of this structure, there are no further increases in length after the six months gestation. Finally, we concluded that the dimensions of the coronary sinus obtained during autopsy are similar to those determined through intravital ultrasound examination. The diameter of the coronary sinus is the best parameter to monitoring the fetal age and the growing of the fetus. Accordingly, we suggest that the best way of estimate for proper blood drainage from heart veins is study of coronary sinus volume.

Hand2 loss-of-function in Hand1-expressing cells reveals distinct roles in epicardial and coronary vessel development

RATIONALE

The basic helix-loop-helix (bHLH) transcription factors Hand1 and Hand2 are essential for embryonic development. Given their requirement for cardiogenesis, it is imperative to determine their impact on cardiovascular function.

OBJECTIVE

To deduce the role of Hand2 within the epicardium.

METHOD AND RESULTS

We engineered a Hand1 allele expressing Cre recombinase. Cardiac Hand1 expression is largely limited to cells of the primary heart field, overlapping little with Hand2 expression. Hand1 is expressed within the septum transversum, and the Hand1 lineage marks the proepicardial organ and epicardium. To examine Hand factor functional overlap, we conditionally deleted Hand2 from Hand1-expressing cells. Hand2 mutants display defective epicardialization and fail to form coronary arteries, coincident with altered extracellular matrix deposition and Pdgfr expression.

CONCLUSIONS

These data demonstrate a hierarchal relationship whereby transient Hand1 septum transversum expression defines epicardial precursors that are subsequently dependent on Hand2 function.

Perivascular cells as mesenchymal stem cells

IMPORTANCE OF THE FIELD

Mesenchymal stem cells are multipotent adult stem cell populations that have broad differentiation plasticity and immunosuppressive potential that render them of great importance in cell-based therapies. They are identified by in vitro characteristics based on their differentiation potential for clinical approaches while their biological properties and in vivo identities are often less understood.

AREAS COVERED IN THIS REVIEW

Recent research carried out in the last decade on mesenchymal stem cell biology suggests that mesenchymal stem cells from various tissues reside in a perivascular location and these can be identified as pericytes that function as mural cells in microvessels.

WHAT THE READER WILL GAIN

This review covers recent progress on understanding the link between pericytes and mesenchymal stem cells discussing specific points such as response to injury and tissue-specific functions.

TAKE HOME MESSAGE

Despite a long and controversial history, there is a growing acceptance that perivascular cells are connected with mesenchymal stem cells, all that is really lacking is genetic evidence to show differentiation of pericytes into different cells types.

The influence of extracellular matrix composition on the pathogenesis of coronary atherosclerosis

The modern concept of the development of atherosclerosis implies that the underlying pathogenesis of this disease is vascular remodeling as a response of the vessel wall to hypertension associated with hyperlipidemia and subsequent inflammation. However, even though this disease has been investigated for decades, both from a basic and clinical research aspect, there are still many doubts as to what the initial phase of the disease is. In contemporary literature there are an increasing number of papers that stress the importance of the extracellular matrix (ECM) of the blood vessels connective tissue, particularly proteoglycans, in the formation of early atherosclerotic lesions of human coronary arteries.

Resident vascular progenitor cells--diverse origins, phenotype, and function

The fundamental contributions that blood vessels make toward organogenesis and tissue homeostasis are reflected by the considerable ramifications that loss of vascular wall integrity has on pre- and postnatal health. During both neovascularization and vessel wall remodeling after insult, the dynamic nature of vascular cell growth and replacement vitiates traditional impressions that blood vessels contain predominantly mature, terminally differentiated cell populations. Recent discoveries have verified the presence of diverse stem/progenitor cells for both vascular and non-vascular progeny within the mural layers of the vasculature. During embryogenesis, this encompasses the emergence of definitive hematopoietic stem cells and multipotent mesoangioblasts from the developing dorsal aorta. Ancestral cells have also been identified and isolated from mature, adult blood vessels, showing variable capacity for endothelial, smooth muscle, and mesenchymal differentiation. At present, the characterization of these different vascular wall progenitors remains somewhat rudimentary, but there is evidence for their constitutive residence within organized compartments in the vessel wall, most compellingly in the tunica adventitia. This review overviews the spectrum of resident stem/progenitor cells that have been documented in macro- and micro-vessels during developmental and adult life and considers the implications for a local, vascular wall stem cell niche(s) in the pathogenesis and treatment of cardiovascular and other diseases.

Anomalous origin of the left coronary artery from the right pulmonary artery presenting following relief of left heart obstruction: a distinct and predictable clinico-pathological syndrome

INTRODUCTION

Pre-operative recognition of significant abnormalities of the coronary arteries is important in a variety of congenital cardiac conditions. Failure to diagnose anomalous origin of the coronary artery from the pulmonary artery during repair of other anomalies is important because reduction in pulmonary artery pressure will reduce myocardial perfusion pressure.

PATIENTS

We report two cases of the rare association of anomalous origin of the left coronary artery from the right pulmonary artery, aortic coarctation, and mitral stenosis.

CONCLUSIONS

Definitive imaging of coronary artery anatomy by echocardiography or other modalities should form a routine part of diagnostic assessment in all congenital heart disease patients but particularly those with left heart obstruction.

A case of persistent left and absent right superior caval vein: An anatomical and embryological perspective

The relatively common persistent left superior caval vein (LSCV) is in most cases associated with doubling of the superior caval vein. A persistent LSCV with absent right superior caval vein (RSCV)-a rather rare event-was found during our course of gross anatomy. The LSCV drained into an enlarged coronary sinus, which was partly accompanied by an apparent "double" sinus of normal size draining into this enlarged coronary sinus. Histological and immunofluorescence studies using antibodies against smooth and cardiac muscle actins were performed. The terminal part of the LSCV near the opening into the right atrium contained cardiac actin as expected for a normal derivative of the left sinus horn. Previously only one case of doubled coronary sinus with LSCV has been reported and this abnormality was explained by splitting of the sinus. In our case, the partly doubled coronary sinus had the structure of coronary veins. Mechanical forces have been invoked for the obliteration of the LSCV. Therefore, we examined thirteen human embryos from 15 mm to 32 mm crown-rump length. In one embryo, we found a persistent LSCV together with an enormously enlarged left atrium. Contrary to previous suggestions our data indicate that during normal development a compression of the left anterior cardinal vein does not sufficiently explain the obliteration of the left and the persistence of the right vein. We therefore believe that beside a left dominated blood flow of head and arm, genes for left-right signaling may have to be taken into consideration.

Expression of lymphatic markers during avian and mouse cardiogenesis

The adult heart has been reported to have an extensive lymphatic system, yet the development of this important system during cardiogenesis is still largely unexplored. The nuclear-localized transcription factor Prox-1 identified a sheet of Prox-1-positive cells on the developing aorta and pulmonary trunk in avian and murine embryos just before septation of the four heart chambers. The cells coalesced into a branching lymphatic network that spread within the epicardium to cover the heart. These vessels eventually expressed the lymphatic markers LYVE-1, VEGFR-3, and podoplanin. Before the Prox-1-positive cells were detected in the mouse epicardium, LYVE-1, a homologue of the CD44 glycoprotein, was primarily expressed in individual epicardial cells. Similar staining patterns were observed for CD44 in avian embryos. The proximity of these LYVE-1/CD44-positive mesenchymal cells to Prox-1-positive vessels suggests that they may become incorporated into the lymphatics. Unexpectedly, we detected LYVE-1/PECAM/VEGFR-3-positive vessels within the embryonic and adult myocardium, which remained Prox-1/podoplanin-negative. Lymphatic markers were surprisingly found in adult rat and embryonic mouse epicardial cell lines, with Prox-1 also exhibiting nuclear-localized expression in primary cultures of embryonic avian epicardial cells. Our data identified three types of cells in the embryonic heart expressing lymphatic markers: (1) Prox-1-positive cells from an extracardiac source that migrate within the serosa of the outflow tract into the epicardium of the developing heart, (2) individual LYVE-1-positive cells in the epicardium that may be incorporated into the Prox-1-positive lymphatic vasculature, and (3) LYVE-1-positive cells/vessels in the myocardium that do not become Prox-1-positive even in the adult heart.

Loss of glypican-3 function causes growth factor-dependent defects in cardiac and coronary vascular development

Glypican-3 (Gpc3) is a heparan sulfate proteoglycan (HSPG) expressed widely during vertebrate development. Loss-of-function mutations cause Simpson-Golabi-Behmel syndrome (SGBS), a rare and complex congenital overgrowth syndrome with a number of associated developmental abnormalities including congenital heart disease. We found that Gpc3-deficient mice display a high incidence of congenital cardiac malformations like ventricular septal defects, common atrioventricular canal and double outlet right ventricle. In addition we observed coronary artery fistulas, which have not been previously reported in SGBS. Coronary artery fistulas are noteworthy because little is known about the molecular basis of this abnormality. Formation of the coronary vascular plexus in Gpc3-deficient embryos was delayed compared to wild-type, and consistent with GPC3 functioning as a co-receptor for fibroblast growth factor-9 (FGF9), we found a reduction in Sonic Hedgehog (Shh) mRNA expression and signaling in embryonic mutant hearts. Interestingly, we found an asymmetric reduction in SHH signaling in cardiac myocytes, as compared with perivascular cells, resulting in excessive coronary artery formation in the Gpc3-deficient animals. We hypothesize that the excessive development of coronary arteries over veins enables the formation of coronary artery fistulas. This work has broad significance to understanding the genetic basis of coronary development and potentially to molecular mechanisms relevant to revascularization following ischemic injury to the heart.

Spatial and temporal regulation of coronary vessel formation by calcineurin-NFAT signaling

Formation of the coronary vasculature requires reciprocal signaling between endothelial, epicardially derived smooth muscle and underlying myocardial cells. Our studies show that calcineurin-NFAT signaling functions in endothelial cells within specific time windows to regulate coronary vessel development. Mouse embryos exposed to cyclosporin A (CsA), which inhibits calcineurin phosphatase activity, failed to develop normal coronary vasculature. To determine the cellular site at which calcineurin functions for coronary angiogenesis, we deleted calcineurin in endothelial, epicardial and myocardial cells. Disruption of calcineurin-NFAT signaling in endothelial cells resulted in the failure of coronary angiogenesis, recapitulating the coronary phenotype observed in CsA-treated embryos. By contrast, deletion of calcineurin in either epicardial or myocardial cells had no effect on coronary vasculature during early embryogenesis. To define the temporal requirement for NFAT signaling, we treated developing embryos with CsA at overlapping windows from E9.5 to E12.5 and examined coronary development at E12.5. These experiments demonstrated that calcineurin-NFAT signaling functions between E10.5 and E11.5 to regulate coronary angiogenesis. Consistent with these in vivo observations, endothelial cells exposed to CsA within specific time windows in tissue culture were unable to form tubular structures and their cellular responses to VEGF-A were blunted. Thus, our studies demonstrate specific temporal and spatial requirements of NFAT signaling for coronary vessel angiogenesis. These requirements are distinct from the roles of NFAT signaling in the angiogenesis of peripheral somatic vessels, providing an example of the environmental influence of different vascular beds on the in vivo endothelial responses to angiogenic stimuli.

Coronary endothelial proliferation and morphogenesis are regulated by a VEGF-mediated pathway

Though development of the coronary vasculature is a critical event during embryogenesis, the molecular mechanisms that regulate its formation are not well characterized. Two unique approaches were used to investigate interactions between cardiac myocytes and proepicardial (PE) cells, which are the coronary anlagen. One of these experimental approaches used a 3-D collagen scaffold system on which specific cell-cell and cell-matrix interactions were studied. The other approach used a whole heart culture system that allowed for the analysis of epicardial to mesenchymal transformation (EMT). The VEGF signaling system has been implicated previously as an important regulator of coronary development. Our results demonstrated that a specific isoform of VEGF-A, VEGF(164), increased PE-derived endothelial cell proliferation and also increased EMT. However, VEGF-stimulated endothelial cells did not robustly coalesce into endothelial tubes as they did when cocultured with cardiac myocytes. Interestingly, blocking VEGF signaling via flk-1 inhibition reduced endothelial tube formation despite the presence of cardiac myocytes. These results indicate that VEGF signaling is complex during coronary development and that combinatorial signaling by other VEGF-A isoforms or other flk-1-binding VEGFs are likely to regulate endothelial tube formation.

Expression of active Notch1 in avian coronary development

Notch1 is an important regulator of intercellular interactions in cardiovascular development. We show that the nuclear-localized, cleaved and active form of Notch1, the Notch1 intracellular domain (N1ICD), appeared in mesothelial cells of the pro-epicardium during epicardial formation at looped heart stages. N1ICD was also present in mesothelial cells and mesenchymal cells specifically within the epicardium at sulcus regions. N1ICD-positive endothelial cells were detected within the nascent vessel plexus at the atrioventricular junction and within the compact myocardium (Hamburger and Hamilton stage [HH] 25-HH30). The endothelial cells expressing N1ICD were surrounded by N1ICD-positive smooth muscle cells after coronary orifice formation (HH32-HH35), while N1ICD expression was absent in the mesenchymal and mesothelial cells surrounding mature coronary vessels. We propose that differential activation of the hypoxia/HIF1-VEGF-Notch pathway may play a role in epicardial cell interactions that promote epicardial epithelial/mesenchymal transition and coronary progenitor cell differentiation during epicardial development and coronary vasculogenesis in particularly hypoxic sulcus regions.

Shared circuitry: developmental signaling cascades regulate both embryonic and adult coronary vasculature

Ischemic heart disease is the most common cause of heart failure and is among the leading causes of mortality worldwide. Therapies used for the treatment of this disease aim to restore blood flow to severely narrowed or occluded coronary arteries by either catheter-based or surgical means. Although these strategies prove efficacious for many patients, a substantial number of individuals fail to improve following these procedures. Recently, a noninvasive strategy has been proposed, focusing on the use of endogenous growth factors that trigger the growth of new coronary arteries. Using the developing heart as a model, several groups have identified some of the key pathways that not only govern the development of the coronary vascular system but also promote the growth of the adult coronary vasculature. Here, we review the major morphological events and signaling cascades that mediate the formation of the coronary vasculature in the embryo. We further describe the mechanism by which many of these same pathways also regulate the adult coronary vasculature and their potential use in the treatment of ischemic heart disease.

Vascular wall resident progenitor cells: a review

The vessel wall has usually been thought to be relatively quiescent. But the discovery of progenitor cells in many tissues and in the vasculature itself has led to a reconsideration of the vascular biology. The presence of circulating endothelial and smooth muscle progenitors able to home to the injured vascular wall is a firm acquisition; less known is the notion, coming from embryonic and adult tissue studies, that stem cells able to differentiate into endothelial cells and smooth muscle cells also reside in the arterial wall. Moreover, the existence of a vasculogenic zone has recently been identified in adult human arteries; this niche-like zone is believed to act as a source of progenitors for postnatal vasculogenesis. From the literature it is already apparent that a complex interplay between circulating and resident vascular wall progenitors takes place during embryonal and postnatal life; a structural/functional disarray of these intimate stem cell compartments could hamper appropriate vascular repair, the development of vascular wall disease being the direct clinical consequence in adult life. This review gives an overview of adult large vessel progenitors established in the vascular wall during embryogenesis and their role in the maintenance of wall homeostasis.

Brothers and sisters: molecular insights into arterial-venous heterogeneity

The molecular differences between arteries and veins are genetically predetermined and are evident even before the first embryonic heart beat. Although ephrinB2 and EphB4 are expressed in cells that will ultimately differentiate into arteries and veins, respectively, many other genes have been shown to play a significant role in cell fate determination. The expression patterns of ephrinB2 and EphB4 are restricted to arterial-venous boundaries, and Eph/ephrin signaling provides repulsive cues at arterial-venous boundaries that are thought to prevent intermixing of arterial- and venous-fated cells. However, the maintenance of arterial-venous fate is susceptible to some degree of plasticity. Thus, in response to signals from the ambient microenvironment and shear stress, there is flow-mediated intercalation of the arteries and veins that ultimately leads to the formation of a functional, closed-loop circulation. In addition, cells in the blood vessels of each organ undergo epigenetic, morphological, and functional adaptive changes that are specific to the proximate function of their cognate organ(s). These adaptive changes result in an interorgan and intraorgan vessel heterogeneity that manifest clinically in a disparate response of different organs to identical risk factors and injury in the same animal. In this review, we focus on the molecular and physiological factors influencing arterial-venous heterogeneity between and within different organ(s). We explore arterial-venous differences in selected organs, as well as their respective endothelial cell architectural organization that results in their inter- and intraorgan heterogeneity.

In vivo differences between endothelial transcriptional profiles of coronary and iliac arteries revealed by microarray analysis

Endothelial cells (ECs) from different vascular beds display a remarkable heterogeneity in both structure and function. Phenotypic heterogeneity among arterial ECs is particularly relevant to atherosclerosis since the disease occurs predominantly in major arteries, which vary in their atherosusceptibility. To explore EC heterogeneity between typical atheroprone and atheroresistant arteries, we used DNA microarrays to compare gene expression profiles of freshly harvested porcine coronary (CECs) and iliac artery (IECs) ECs. Statistical analysis revealed 51 genes that were differentially expressed in CECs relative to IECs at a false discovery rate of 5%. Seventeen of these genes are known to be involved in atherogenesis. Consistent with coronary arteries being more atherosusceptible, almost all putative atherogenic genes were overexpressed in CECs, whereas all atheroprotective genes were downregulated, relative to IECs. A subset of the identified genes was validated by quantitative polymerase chain reaction (PCR). PCR results suggest that the differences in expression levels between CECs and IECs for the HOXA10 and HOXA9 genes were >100-fold. Gene ontology (GO) and biological pathway analysis revealed a global expression difference between CECs and IECs. Genes in twelve GO categories, including complement immune activation, immunoglobulin-mediated response, and system development, were significantly upregulated in CECs. CECs also overexpressed genes involved in several inflammatory pathways, including the classical pathway of complement activation and the IGF-1-mediated pathway. The in vivo transcriptional differences between CECs and IECs found in this study may provide new insights into the factors responsible for coronary artery atherosusceptibility.

Signaling via the Tgf-beta type I receptor Alk5 in heart development

Trophic factors secreted both from the endocardium and epicardium regulate appropriate growth of the myocardium during cardiac development. Epicardially-derived cells play also a key role in development of the coronary vasculature. This process involves transformation of epithelial (epicardial) cells to mesenchymal cells (EMT). Similarly, a subset of endocardial cells undergoes EMT to form the mesenchyme of endocardial cushions, which function as primordia for developing valves and septa. While it has been suggested that transforming growth factor-betas (Tgf-beta) play an important role in induction of EMT in the avian epi- and endocardium, the function of Tgf-betas in corresponding mammalian tissues is still poorly understood. In this study, we have ablated the Tgf-beta type I receptor Alk5 in endo-, myo- and epicardial lineages using the Tie2-Cre, Nkx2.5-Cre, and Gata5-Cre driver lines, respectively. We show that while Alk5-mediated signaling does not play a major role in the myocardium during mouse cardiac development, it is critically important in the endocardium for induction of EMT both in vitro and in vivo. Moreover, loss of epicardial Alk5-mediated signaling leads to disruption of cell-cell interactions between the epicardium and myocardium resulting in a thinned myocardium. Furthermore, epicardial cells lacking Alk5 fail to undergo Tgf-beta-induced EMT in vitro. Late term mutant embryos lacking epicardial Alk5 display defective formation of a smooth muscle cell layer around coronary arteries, and aberrant formation of capillary vessels in the myocardium suggesting that Alk5 is controlling vascular homeostasis during cardiogenesis. To conclude, Tgf-beta signaling via Alk5 is not required in myocardial cells during mammalian cardiac development, but plays an irreplaceable cell-autonomous role regulating cellular communication, differentiation and proliferation in endocardial and epicardial cells.

Differential effects of cyclic and static stretch on coronary microvascular endothelial cell receptors and vasculogenic/angiogenic responses

Mechanical stretch, an important growth stimulus, results not only from pulsatile blood flow and diastolic stretch of the ventricles [cyclic stretch (CS)] but also from tissue expansion during growth [constant static stretch (SS)]. We compared growth factor receptor expression and vasculogenic/angiogenic responses of rat coronary microvascular endothelial cells (ECs) by exposing cells to CS (10% elongation at 30 cycles/min) and SS (constant 10% elongation). Both CS and SS increased VEGF receptor (VEGF-R)2 protein levels and the extent of tube formation and branching. Moreover, both CS and SS enhanced VEGF-induced cell proliferation and tube formation, indicating that both types of stretch increase the sensitivity of ECs to VEGF. Blockade of VEGF-R2 prevented the increases in EC proliferation and aggregate tube length. However, CS but not SS enhanced EC Tie-2 protein and migration. CS affected a greater increase in tube length and branch formation than did SS. A unique finding was that SS but not CS increased VEGFR-1 in ECs. Our study is the first to distinguish between the effects of CS and SS on growth factor receptor expression and rat coronary microvascular EC proliferation, migration, and tube formation. In conclusion, EC angiogenic responses to these two types of stretch display both differences and similarities, but both CS and SS are dependent on VEGF-R2 signaling for their vasculogenic/angiogenic effects.

Mutation of the HEXIM1 gene results in defects during heart and vascular development partly through downregulation of vascular endothelial growth factor

Our previous studies and those of others indicated that the transcription factor Hexamethylene-bis-acetamide-inducible protein 1 (HEXIM1) is a tumor suppressor and cyclin-dependent kinase inhibitor, and that these HEXIM1 functions are mainly dependent on its C-terminal region. We provide evidence here that the HEXIM1 C-terminal region is critical for cardiovascular development. HEXIM1 protein was detected in the heart during critical time periods in cardiac growth and chamber maturation. We created mice carrying an insertional mutation in the HEXIM1 gene that disrupted its C-terminal region and found that this resulted in prenatal lethality. Heart defects in HEXIM1(1 to 312) mice included abnormal coronary patterning and thin ventricular walls. The thin myocardium can be partly attributed to increased apoptosis. Platelet endothelial cell adhesion molecular precursor-1 staining of HEXIM1(1 to 312) heart sections revealed decreased vascularization of the myocardium despite the presence of coronary vasculature in the epicardium. The expression of vascular endothelial growth factor (VEGF), known to affect angioblast invasion and myocardial proliferation and survival, was decreased in HEXIM1(1 to 312) mice compared with control littermates. We also observed decreased fibroblast growth factor 9 (FGF9) expression, suggesting that effects of HEXIM1 in the myocardium are partly mediated through epicardial FGF9 signaling. Together our results suggest that HEXIM1 plays critical roles in coronary vessel development and myocardial growth. The basis for this role of HEXIM1 is that VEGF is a direct transcriptional target of HEXIM1, and involves attenuation a repressive effects of C/EBPalpha on VEGF gene transcription.

Fibroblast growth factors and Hedgehogs: at the heart of the epicardial signaling center

Over the past several years, increasing attention has been focused on understanding signaling pathways that control key events during midgestational heart development. During this period of development, the heart tube transforms into a functioning organ that must maintain its own blood supply and grow and respond to the physiologic needs of the organism. A critical event that occurs during midgestational heart development is the formation of the epicardium, which functions as a source of cells and as a signaling center that regulates myocardial growth and coronary vascular development. This review will describe our understanding of the role and the mechanism by which the epicardium governs these developmental events, primarily as a result of studies in the mouse. We focus on two key growth factor pathways: fibroblast growth factor and Hedgehog signaling.

Endothelial Precursor Cell Migration During Vasculogenesis

Migration of endothelial precursor cells (so-called "angioblasts" in embryos and "endothelial progenitor cells" in adults) during vasculogenesis is a requirement for the formation of a primary vascular plexus. The migration is initiated by the change of endothelial precursors to their migratory phenotype. The endothelial precursor cells are then guided to the position where the primary vascular plexus is formed. Migration is stopped by the reversion of the cells to their nonmigratory phenotype. A combination of regulatory mechanisms and factors controls this process. These include gradients of soluble factors, extracellular matrix-cell interaction and cell-cell interaction. In this review, we give an overview of the regulation of angioblast migration during embryonic vasculogenesis and its relationship to the migration of endothelial progenitors during postnatal vascular development.

Rebuilding the coronary vasculature: hedgehog as a new candidate for pharmacologic revascularization

Myocardial infarction and ischemic heart disease are among the most common causes of morbidity and mortality in the industrial world. Surgical and percutaneous intravascular approaches are commonly used to treat these diseases. Regrettably, a significant number of patients are either ineligible or demonstrate suboptimal responses to these therapies. In an attempt to provide such patients improved therapeutic options, much effort has been spent developing noninvasive approaches to restore coronary vascular perfusion. One such strategy, termed therapeutic revascularization or angiogenesis, involves administration of proangiogenic factors, which improve coronary perfusion by promoting growth of the coronary vasculature. Thus far, two potential proangiogenic factors have been intensively examined, fibroblast growth factor and vascular endothelial growth factor. Unfortunately, despite their apparent efficacy in animal models, neither factor has performed adequately in the clinic to date. Within the past year a new factor, hedgehog, has been shown to effectively promote the growth of the coronary vasculature and thus has been proposed as a novel candidate for therapeutic revascularization. In this review, we discuss the discovery of the hedgehog pathway as an essential regulator of the development of the coronary vasculature, as an inducer of adult coronary vascular growth, and as a therapeutic in the treatment of ischemic heart disease.

Developmental basis of vascular smooth muscle diversity

The origins of vascular smooth muscle are far more diverse than previously thought. Lineage mapping studies show that the segmental organization of early vertebrate embryos leaves footprints on the adult vascular system in the form of a mosaic pattern of different smooth muscle types. Moreover, evolutionarily conserved tissue forming pathways produce vascular smooth muscle from a variety of unanticipated sources. A closer look at the diversity of smooth muscle origins in vascular development provides new perspectives about how blood vessels differ from one another and why they respond in disparate ways to common risk factors associated with vascular disease. The origins of vascular smooth muscle are far more diverse than previously thought. A closer look at the diversity of smooth muscle origins in vascular development provides new perspectives about how blood vessels differ from one another and why they respond in disparate ways to common risk factors associated with vascular disease.

Pancreatic islet production of vascular endothelial growth factor--a is essential for islet vascularization, revascularization, and function

To investigate molecular mechanisms controlling islet vascularization and revascularization after transplantation, we examined pancreatic expression of three families of angiogenic factors and their receptors in differentiating endocrine cells and adult islets. Using intravital lectin labeling, we demonstrated that development of islet microvasculature and establishment of islet blood flow occur concomitantly with islet morphogenesis. Our genetic data indicate that vascular endothelial growth factor (VEGF)-A is a major regulator of islet vascularization and revascularization of transplanted islets. In spite of normal pancreatic insulin content and beta-cell mass, mice with beta-cell-reduced VEGF-A expression had impaired glucose-stimulated insulin secretion. By vascular or diffusion delivery of beta-cell secretagogues to islets, we showed that reduced insulin output is not a result of beta-cell dysfunction but rather caused by vascular alterations in islets. Taken together, our data indicate that the microvasculature plays an integral role in islet function. Factors modulating VEGF-A expression may influence islet vascularity and, consequently, the amount of insulin delivered into the systemic circulation.

Epicardial development in the rat: a new perspective

Development of the epicardium is critical to proper heart formation. It provides all of the precursor cells that form the coronary system and supplies signals that stimulate cardiac myocyte proliferation. The epicardium forms from mesothelial cells associated with the septum transversum and is referred to as the proepicardium (PE). Two different methods by which these PE cells colonize the developing heart have been described. In avians, PE cells form a bridge to the heart over which PE cells migrate onto the heart. In fish and mammals, PE cells form vesicles of cells that detach from the mesothelium, float through the pericardial cavity, and attach to the heart. A previous study of rat PE development investigated this process at the histological level. Protein markers have been developed since this study. Thus, we investigated this important developmental process coupled with these new markers using other visualization techniques such as scanning electron microscopy (SEM) and confocal microscopy. Finally, a novel, three-dimensional (3-D) culture system was used to confirm the identity of the PE cells. In this study, we found convincing evidence that the rat PE cells directly attach to the heart in a manner similar to that observed in avians.

Fibroblast growth factor signals regulate a wave of Hedgehog activation that is essential for coronary vascular development

Myocardial infarction and ischemic heart disease are the leading cause of death in the industrial world. Therapies employed for treating these diseases are aimed at promoting increased blood flow to cardiac tissue. Pharmacological induction of new coronary growth has recently been explored, however, clinical trials with known proangiogenic factors have been disappointing. To identify novel therapeutic targets, we have explored signaling pathways that govern embryonic coronary development. Using a combination of genetically engineered mice and an organ culture system, we identified novel roles for fibroblast growth factor (FGF) and Hedgehog (HH) signaling in coronary vascular development. We show that FGF signals promote coronary growth indirectly by signaling to the cardiomyoblast through redundant function of Fgfr1 and Fgfr2. Myocardial FGF signaling triggers a wave of HH activation that is essential for vascular endothelial growth factor (Vegf)-A, Vegf-B, Vegf-C, and angiopoietin-2 (Ang2) expression. We demonstrate that HH is necessary for coronary vascular development and activation of HH signaling is sufficient to promote coronary growth and to rescue coronary defects due to loss of FGF signaling. These studies implicate HH signaling as an essential regulator of coronary vascular development and as a potential therapeutic target for coronary neovascularization. Consistent with this, activation of HH signaling in the adult heart leads to an increase in coronary vessel density.

In vitro model for mouse coronary vasculogenesis

To analyze the molecular mechanisms of coronary vessel formation, we performed in vitro experiments on explant cultures of proepicardial organs (PEOs) excised from embryos taken from 9.5-day pregnant mice. When plated on coverglasses coated with rat tail collagen I, fibronectin, or laminin, PEO cells spread and formed an epithelial sheet. When PEOs were cultured on collagen gel in the presence of fetal calf serum (FCS), small projections were seen around the explants 3 days after plating. Around day 6, cord-like structures began to grow from the explants, gradually elongating, increasing in number, and forming a branching network. Histological sections demonstrated that the cells migrated into the gel and formed tube-like structures similar to the vascular channels of the embryonic heart. The cells lining the lumen of the tube-like structures were positive for platelet endothelial cell adhesion molecule (PECAM). Reverse transcriptase-polymerase chain reaction analyses demonstrated that the expression of PECAM, basic fibroblast growth factor (bFGF), and smooth muscle 22-alpha (SM22alpha) was upregulated in association with the tube formation, whereas the expression of Flk-1, Flt-1, and hepatocyte growth factor (HGF) was gradually downregulated. Vascular endothelial growth factor (VEGF) was continuously expressed during the culture. These changes were not observed when PEOs were explanted without FCS. Furthermore, addition of any one or combinational addition of the growth factors, including bFGF, VEGF, or HGF, did not induce tube formation. These results suggest that PEOs contain precursor cells of coronary vasculature and that vasculogenesis may be simultaneously regulated by multiple factors.

Forkhead transcription factors, Foxc1 and Foxc2, are required for the morphogenesis of the cardiac outflow tract

Previous studies have shown that Foxc1 and Foxc2, closely related Fox transcription factors, have interactive roles in cardiovascular development. However, little is known about their functional overlap during early heart morphogenesis. Here, we show that Foxc genes are coexpressed in a novel heart field, the second heart field, as well as the cardiac neural crest cells (NCCs), endocardium, and proepicardium. Notably, compound Foxc1; Foxc2 mutants have a wide spectrum of cardiac abnormalities, including hypoplasia or lack of the outflow tract (OFT) and right ventricle as well as the inflow tract, dysplasia of the OFT and atrioventricular cushions, and abnormal formation of the epicardium, in a dose-dependent manner. Most importantly, in the second heart field, compound mutants exhibit significant downregulation of Tbx1 and Fgf8/10 and a reduction in cell proliferation. Moreover, NCCs in compound mutants show extensive apoptosis during migration, leading to a failure of the OFT septation. Taken together, our results demonstrate that Foxc1 and Foxc2 play pivotal roles in the early processes of heart development, especially acting upstream of the Tbx1-FGF cascade during the morphogenesis of the OFT.

Pulmonary atresia, intact ventricular septum, right ventricle-dependent coronary artery circulation, and systemic collateral arteries supplying the left coronary artery

We present a 14-day-old with pulmonary artery atresia with intact ventricular septum, right ventricle-dependent coronary circulation, a single aortic root coronary ostia (left), congenital collateral arterial supply to the left coronary artery from the left internal mammary artery, and bilateral paravertebral arteries, with obstructive coronary artery lesions.

Increased vascular endothelial growth factor expression but impaired vascular endothelial growth factor receptor signaling in the myocardium of type 2 diabetic patients with chronic coronary heart disease

OBJECTIVES

The aim of the present study was to evaluate the expression and the activity of vascular endothelial growth factor (VEGF) in the hearts of diabetic patients with chronic coronary heart disease (CHD).

BACKGROUND

Diabetes is characterized by a decreased collateral vessel formation in response to coronary ischemic events, although the role of VEGF in human diabetic macroangiopathy has not been fully investigated.

METHODS

Biopsies of left ventricular (LV) myocardium were obtained from 10 patients with type 2 diabetes and 10 non-diabetic patients with chronic CHD, all undergoing surgical coronary revascularization. Right ventricle myocardial samples taken from normal hearts were used as control specimens. Vascular endothelial growth factor and VEGF-receptors (flt-1 and flk-1) were evaluated by Western blot, reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time RT-PCR. Akt and endothelial nitric oxide synthase (eNOS) protein expression and their phosphorylated forms were also evaluated by Western blot.

RESULTS

Vascular endothelial growth factor, flt-1, and flk-1 messenger ribonucleic acid (mRNA) and protein expressions were increased in non-diabetic patients with CHD compared with control subjects. Remarkably, in diabetic patients, VEGF mRNA and protein levels were significantly higher, whereas flt-1, flk-1 mRNA, and protein were lower when compared with non-diabetic patients. Interestingly, phospho-flk-1 was reduced in diabetic patients compared with non-diabetic patients. As a consequence, Akt phosphorylation, eNOS protein and its phosphorylated form were significantly higher in the samples from non-diabetic patients compared with diabetic patients.

CONCLUSIONS

Chronic CHD in diabetic patients is characterized by an increased VEGF myocardial expression and a decreased expression of its receptors along with a down-regulation of its signal transduction. The latter could be partially responsible for the reduced neoangiogenesis in diabetic patients with ischemic cardiomyopathy.