Extension of Endocardium-Derived Vessels Generate Coronary Arteries in Neonates

Background: Unraveling how new coronary arteries develop may provide critical information for establishing novel therapeutic approaches to treating ischemic cardiac diseases. There are two distinct coronary vascular populations derived from different origins in the developing heart. Understanding the formation of coronary arteries may provide insights into new ways of promoting coronary artery formation after myocardial infarction. Methods: To understand how intramyocardial coronary arteries are generated to connect these two coronary vascular populations, we combined genetic lineage tracing, light-sheet microscopy, fluorescence micro-optical sectioning tomography, and tissue-specific gene knockout approaches to understand their cellular and molecular mechanisms. Results: We show that a subset of intramyocardial coronary arteries form by angiogenic extension of endocardium-derived vascular tunnels in the neonatal heart. Three-dimensional whole-mount fluorescence imaging showed that these endocardium-derived vascular tunnels or tubes adopt an arterial fate in neonates. Mechanistically, we implicate Mettl3 and Notch signaling in regulating endocardium-derived intramyocardial coronary artery formation. Functionally, these intramyocardial arteries persist into adulthood and play a protective role after myocardial infarction. Conclusions: A subset of intramyocardial coronary arteries form by extension of endocardium-derived vascular tunnels in the neonatal heart.

Dach1 Extends Artery Networks and Protects Against Cardiac Injury

[Figure: see text].

Maternal iron deficiency perturbs embryonic cardiovascular development in mice

Congenital heart disease (CHD) is the most common class of human birth defects, with a prevalence of 0.9% of births. However, two-thirds of cases have an unknown cause, and many of these are thought to be caused by in utero exposure to environmental teratogens. Here we identify a potential teratogen causing CHD in mice: maternal iron deficiency (ID). We show that maternal ID in mice causes severe cardiovascular defects in the offspring. These defects likely arise from increased retinoic acid signalling in ID embryos. The defects can be prevented by iron administration in early pregnancy. It has also been proposed that teratogen exposure may potentiate the effects of genetic predisposition to CHD through gene-environment interaction. Here we show that maternal ID increases the severity of heart and craniofacial defects in a mouse model of Down syndrome. It will be important to understand if the effects of maternal ID seen here in mice may have clinical implications for women.

Sox17 Controls Emergence and Remodeling of Nestin-Expressing Coronary Vessels

RATIONALE

The molecular mechanisms underlying the formation of coronary arteries during development and during cardiac neovascularization after injury are poorly understood. However, a detailed description of the relevant signaling pathways and functional TFs (transcription factors) regulating these processes is still incomplete.

OBJECTIVE

The goal of this study is to identify novel cardiac transcriptional mechanisms of coronary angiogenesis and vessel remodeling by defining the molecular signatures of coronary vascular endothelial cells during these complex processes.

METHODS AND RESULTS

We demonstrate that Nes-gfp and Nes-CreERT2 transgenic mouse lines are novel tools for studying the emergence of coronary endothelium and targeting sprouting coronary vessels (but not ventricular endocardium) during development. Furthermore, we identify Sox17 as a critical TF upregulated during the sprouting and remodeling of coronary vessels, visualized by a specific neural enhancer from the Nestin gene that is strongly induced in developing arterioles. Functionally, genetic-inducible endothelial deletion of Sox17 causes deficient cardiac remodeling of coronary vessels, resulting in improper coronary artery formation.

CONCLUSIONS

We demonstrated that Sox17 TF regulates the transcriptional activation of Nestin's enhancer in developing coronary vessels while its genetic deletion leads to inadequate coronary artery formation. These findings identify Sox17 as a critical regulator for the remodeling of coronary vessels in the developing heart.

Structural coronary artery remodelling in the rabbit fetus as a result of intrauterine growth restriction

Intrauterine growth restriction (IUGR) is a fetal condition that affects up to 10% of all pregnancies and is associated with cardiovascular structural and functional remodelling that persists postnatally. Some studies have reported an increase in myocardial coronary blood flow in severe IUGR fetuses which has been directly associated to the dilatation of the coronary arteries. However, a direct measurement of the coronaries’ lumen diameter in IUGR has not been reported before. The aim of this paper is to perform, for the first time, a quantitative analysis of the effects of IUGR in cardiac geometry and coronary vessel size in a well-known rabbit model of IUGR using synchrotron-based X-ray Phase Contrast Tomography Imaging (X-PCI). Eight rabbit fetal hearts were imaged non-destructively with X-PCI. 3D reconstructions of the coronary arterial tree were obtained after semi-automatic image segmentation. Different morphometric features including vessel lumen diameter of the three main coronaries were automatically quantified. IUGR fetuses had more globular hearts and dilated coronary arteries as compared to controls. We have quantitatively shown that IUGR leads to structural coronary vascular tree remodelling and enlargement as an adaptation mechanism in response to an adverse environment of restricted oxygen and nutrients and increased perfusion pressure.

Expression and localisation of thymosin beta-4 in the developing human early fetal heart

BACKGROUND

The objective of this study was to investigate the expression and localisation of thymosin β4 (Tβ4) in the developing human heart. Tβ4 is a cardioprotective protein which may have therapeutic potential. While Tβ4 is an endogenously produced protein with known importance during development, its role within the developing human heart is not fully understood. Elucidating the localisation of Tβ4 within the developing heart will help in understanding its role during cardiac development and is crucial for understanding its potential for cardioprotection and repair in the adult heart.

METHODS

Expression of Tβ4 mRNA in the early fetal human heart was assessed by PCR using both ventricular and atrial tissue. Fluorescence immunohistochemistry was used to assess the localisation of Tβ4 in sections of early fetal human heart. Co-staining with CD31, an endothelial cell marker, and with myosin heavy chain, a cardiomyocyte marker, was used to determine whether Tβ4 is localised to these cell types within the early fetal human heart.

RESULTS

Tβ4 mRNA was found to be expressed in both the atria and the ventricles of the early fetal human heart. Tβ4 protein was found to be primarily localised to CD31-expressing endothelial cells and the endocardium as well as being present in the epicardium. Tβ4-associated fluorescence was greater in the compact layer of the myocardial wall and the interventricular septum than in the trabecular layer of the myocardium.

CONCLUSIONS

The data presented illustrates expression of Tβ4 in the developing human heart and demonstrates for the first time that Tβ4 in the human heart is primarily localised to endothelial cells of the cardiac microvasculature and coronary vessels as-well as to the endothelial-like cells of the endocardium and to the epicardium.

[Histological and Immunocytochemical Investigation of Human Coronary Vessel Development with ANTI-CD34 Antibodies]

Information about embryonic development of coronary endothelium is the main clue for the creation of new methods in tissue engineering for treatment of ischemic heart diseases. The purpose of the research was to describe human coronary vessels development on early stages of the prenatal ontogenesis. The first step in human coronary vessels development is the formation of endothelium de novo by transformation of some epicardial and, possibly, endocardial cells. The next step is the ingrowth of sinus venosus endothelium in subepicardium over ventricles and atria, which gives rise to the coronary vessels. Only after 7 days does the primitive coronary plexus of the heart communicate with aorta (third step). During this period, some subepicardial vessels invade myocardium and some intramyocardial vessels contact with the heart cavity. Such intercommunications could help in regulation of blood circulation in primitive coronary plexus before establishment of effective contacts between arterial and venous vessels—excess of blood could be discharged directly into the heart cavity. Additional population of CD34+ cells were revealed inside condensed mesenchyme of the conotruncus; it participates in the formation of vasa vasorum in the aorta. Epicardium and sinus venosus generate endothelium of coronary vessels by neovasculo- and angiogenesis, respectively. During a week after ingrowth of vessels from SV and before their ingrowth to the aorta, ventriculo-coronary communications could be found in the heart.

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.

Length to width ratio of the ductus venosus in simple screening for fetal congenital heart diseases in the second trimester

Antenatal diagnosis of congenital heart disease (CHD) is still low even though screening was first introduced over 25 years ago. The purpose of our study was to determine the efficacy of a second-trimester prenatal ultrasonographic method of screening for CHD.From September 2012 to September 2013, the length and width of the fetal ductus venosus were measured sonographically in 1006 singleton fetuses, and the ratio of length to width was calculated. The accuracy of each fetal measurement and Doppler ultrasonography were determined. The standard fetal echocardiographic evaluations including 2-dimensional gray-scale imaging, color, and Doppler color flow mapping were performed. The transducer was aligned to the long axis of the fetal trunk to view the ductus venosus in its full length, including the inlet (isthmus) and outlet portions of the vessel. The diameters of the vessel inner wall and mid-point of the ductus venosus were measured using calipers. All scans and fetal measurements were conducted by a registered sonographer with more than 20 years of perinatal ultrasound screening experience.Of the 1006 singleton fetuses between 19 and 28 weeks' gestation, 36 had CHD. The ductus venosus length/width ratio (DVR) for the first CHD screening was extremely sensitive at 88.90%, with a specificity of 99.10% for the cardiac abnormalities included in this study. Chromosomal anomalies accompanied CHD in 0.4% (4/1006) of all cases and 11.11% (4/36) of the CHD cases.The DVR differed significantly between fetuses with CHD and normal fetuses during the second trimester. Careful assessment of the ratio should be a part of the sonographic examination of every fetus. In the case of a small DVR, advanced echocardiography and karyotype analysis should be performed. The ratio is a helpful tool for screening CHD abnormalities prenatally in the Chinese population.

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.

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.

Origin, Specification, and Plasticity of the Great Vessels of the Heart

The pharyngeal arch arteries (PAAs) are a series of paired embryonic blood vessels that give rise to several major arteries that connect directly to the heart. During development, the PAAs emerge from nkx2.5-expressing mesodermal cells and connect the dorsal head vasculature to the outflow tract of the heart. Despite their central role in establishing the circulatory system, the embryonic origins of the PAA progenitors are only coarsely defined, and the factors that specify them and their regenerative potential are unclear. Using fate mapping and mutant analysis, we find that PAA progenitors are derived from the tcf21 and nkx2.5 double-positive head mesoderm and require these two transcription factors for their specification and survival. Unexpectedly, cell ablation shows that the tcf21+; nkx2.5+ PAA progenitors are not required for PAA formation. We find that this compensation is due to the replacement of ablated tcf21+; nkx2.5+ PAA cells by endothelial cells from the dorsal head vasculature. Together, these studies assign the embryonic origin of the great vessel progenitors to the interface between the pharyngeal and cardiac mesoderm, identify the transcription factor code required for their specification, and reveal an unexpected plasticity in the formation of the great vessels.

VEGF-C and aortic cardiomyocytes guide coronary artery stem development

Coronary arteries (CAs) stem from the aorta at 2 highly stereotyped locations, deviations from which can cause myocardial ischemia and death. CA stems form during embryogenesis when peritruncal blood vessels encircle the cardiac outflow tract and invade the aorta, but the underlying patterning mechanisms are poorly understood. Here, using murine models, we demonstrated that VEGF-C-deficient hearts have severely hypoplastic peritruncal vessels, resulting in delayed and abnormally positioned CA stems. We observed that VEGF-C is widely expressed in the outflow tract, while cardiomyocytes develop specifically within the aorta at stem sites where they surround maturing CAs in both mouse and human hearts. Mice heterozygous for islet 1 (Isl1) exhibited decreased aortic cardiomyocytes and abnormally low CA stems. In hearts with outflow tract rotation defects, misplaced stems were associated with shifted aortic cardiomyocytes, and myocardium induced ectopic connections with the pulmonary artery in culture. These data support a model in which CA stem development first requires VEGF-C to stimulate vessel growth around the outflow tract. Then, aortic cardiomyocytes facilitate interactions between peritruncal vessels and the aorta. Derangement of either step can lead to mispatterned CA stems. Studying this niche for cardiomyocyte development, and its relationship with CAs, has the potential to identify methods for stimulating vascular regrowth as a treatment for cardiovascular disease.

Trkb signaling in pericytes is required for cardiac microvessel stabilization

Pericyte and vascular smooth muscle cell (SMC) recruitment to the developing vasculature is an important step in blood vessel maturation. Brain-derived neurotrophic factor (BDNF), expressed by endothelial cells, activates the receptor tyrosine kinase TrkB to stabilize the cardiac microvasculature in the perinatal period. However, the effects of the BDNF/TrkB signaling on pericytes/SMCs and the mechanisms downstream of TrkB that promote vessel maturation are unknown. To confirm the involvement of TrkB in vessel maturation, we evaluated TrkB deficient (trkb^−/−) embryos and observed severe cardiac vascular abnormalities leading to lethality in late gestation to early prenatal life. Ultrastructural analysis demonstrates that trkb^−/− embryos exhibit defects in endothelial cell integrity and perivascular edema. As TrkB is selectively expressed by pericytes and SMCs in the developing cardiac vasculature, we generated mice deficient in TrkB in these cells. Mice with TrkB deficiency in perivascular cells exhibit reduced pericyte/SMC coverage of the cardiac microvasculature, abnormal endothelial cell ultrastructure, and increased vascular permeability. To dissect biological actions and the signaling pathways downstream of TrkB in pericytes/SMCs, human umbilical SMCs were treated with BDNF. This induced membranous protrusions and cell migration, events dependent on myosin light chain phosphorylation. Moreover, inhibition of Rho GTPase and the Rho-associated protein kinase (ROCK) prevented membrane protrusion and myosin light chain phosphorylation in response to BDNF. These results suggest an important role for BDNF in regulating migration of TrkB-expressing pericytes/SMCs to promote cardiac blood vessel ensheathment and functional integrity during development.

Altering HIF-1α through 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure affects coronary vessel development

Differential tissue hypoxia drives normal cardiogenic events including coronary vessel development. This requirement renders cardiogenic processes potentially susceptible to teratogens that activate a transcriptional pathway that intersects with the hypoxia-inducible factor (HIF-1) pathway. The potent toxin 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to cause cardiovascular defects by way of reduced myocardial hypoxia, inhibition of angiogenic stimuli, and alterations in responsiveness of endothelial cells to those stimuli. Our working hypothesis is that HIF-1 levels and thus HIF-1 signaling in the developing myocardium will be reduced by TCDD treatment in vivo during a critical stage and in particularly sensitive sites during heart morphogenesis. This inadequate HIF-1 signaling will subsequently result in outflow tract (OFT) and coronary vasculature defects. Our current data using the chicken embryo model showed a marked decrease in the intensity of immunostaining for HIF-1α nuclear expression in the OFT myocardium of TCDD-treated embryos. This area at the base of the OFT is particularly hypoxic during normal development; where endothelial cells initially form a concentrated anastomosing network known as the peritruncal ring; and where the left and right coronary arteries eventually connect to the aortic lumen. Consistent with this finding, anomalies of the proximal coronaries were detected after TCDD treatment and HIF-1α protein levels decreased in a TCDD dose-dependent manner.

The fetal three-vessel and tracheal view revisited

The routine use of four-chamber screening of the fetal heart was pioneered in the early 1980s and has been shown to detect reliably mainly univentricular hearts in the fetus. Many conotruncal anomalies and ductal-dependent lesions may, however, not be detected with the four-chamber view alone and additional planes are needed. The three-vessel and tracheal (3VT) view is a transverse plane in the upper mediastinum demonstrating simultaneously the course and the connection of both the aortic and ductal arches, their relationship to the trachea and the visualization of the superior vena cava. The purpose of the article is to review the two-dimensional anatomy of this plane and the contribution of colour Doppler and to present a checklist to be achieved on screening ultrasound. Typical suspicions include the detection of abnormal vessel number, abnormal vessel size, abnormal course and alignment and abnormal colour Doppler pattern. Anomalies such as pulmonary and aortic stenosis and atresia, aortic coarctation, interrupted arch, tetralogy of Fallot, common arterial trunk, transposition of the great arteries, right aortic arch, double aortic arch, aberrant right subclavian artery, left superior vena cava are some of the anomalies showing an abnormal 3VT image. Recent studies on the comprehensive evaluation of the 3VT view and adjacent planes have shown the potential of visualizing the thymus and the left brachiocephalic vein during fetal echocardiography and in detecting additional rare conditions. National and international societies are increasingly recommending the use of this plane during routine ultrasound in order to improve prenatal detection rates of critical cardiac defects.

A cell-autonomous role of Cited2 in controlling myocardial and coronary vascular development

AIMS

Myocardial development is dependent on concomitant growth of cardiomyocytes and a supporting vascular network. The coupling of myocardial and coronary vascular development is partly mediated by vascular endothelial growth factor (VEGFA) signalling and additional unknown mechanisms. We examined the cardiomyocyte specific role of the transcriptional co-activator Cited2 on myocardial microstructure and vessel growth, in relation to Vegfa expression.

METHODS AND RESULTS

A cardiomyocyte-specific knockout of mouse Cited2 (Cited2(Nkx)) was analysed using magnetic resonance imaging and histology. Ventricular septal defects and significant compact layer thinning (P < 0.02 at right ventricular apex, P < 0.009 at the left ventricular apex in Cited2(Nkx) vs. controls, n = 11 vs. n = 7, respectively) were found. This was associated with a significant decrease in the number of capillaries to larger vessels (ratio 1.56 ± 0.56 vs. 3.25 ± 1.63, P = 2.7 × 10(-6) Cited2(Nkx) vs. controls, n = 11 vs. n = 7, respectively) concomitant with a 1.5-fold reduction in Vegfa expression (P < 0.02, Cited2(Nkx) vs. controls, n = 12 vs. n = 12, respectively). CITED2 was subsequently found at the Vegfa promoter in mouse embryonic hearts using chromatin immunoprecipitation, and moreover found to stimulate human VEGFA promoter activity in cooperation with TFAP2 transcription factors in transient transfection assays. There was no change in the myocardial expression of the left-right patterning gene Pitx2c, a previously known target of CITED2.

CONCLUSIONS

This study delineates a novel cell-autonomous role of Cited2 in regulating VEGFA transcription and the development of myocardium and coronary vasculature in the mouse. We suggest that coupling of myocardial and coronary growth in the developing heart may occur in part through a Cited2→Vegfa pathway.

Coronary veins determine the pattern of sympathetic innervation in the developing heart

Anatomical congruence of peripheral nerves and blood vessels is well recognized in a variety of tissues. Their physical proximity and similar branching patterns suggest that the development of these networks might be a coordinated process. Here we show that large diameter coronary veins serve as an intermediate template for distal sympathetic axon extension in the subepicardial layer of the dorsal ventricular wall of the developing mouse heart. Vascular smooth muscle cells (VSMCs) associate with large diameter veins during angiogenesis. In vivo and in vitro experiments demonstrate that these cells mediate extension of sympathetic axons via nerve growth factor (NGF). This association enables topological targeting of axons to final targets such as large diameter coronary arteries in the deeper myocardial layer. As axons extend along veins, arterial VSMCs begin to secrete NGF, which allows axons to reach target cells. We propose a sequential mechanism in which initial axon extension in the subepicardium is governed by transient NGF expression by VSMCs as they are recruited to coronary veins; subsequently, VSMCs in the myocardium begin to express NGF as they are recruited by remodeling arteries, attracting axons toward their final targets. The proposed mechanism underlies a distinct, stereotypical pattern of autonomic innervation that is adapted to the complex tissue structure and physiology of the heart.

Distinct compartments of the proepicardial organ give rise to coronary vascular endothelial cells

The proepicardial organ is an important transient structure that contributes cells to various cardiac lineages. However, its contribution to the coronary endothelium has been disputed, with conflicting data arising in chick and mouse. Here we resolve this conflict by identifying two proepicardial markers, Scleraxis (Scx) and Semaphorin3D (Sema3D), that genetically delineate heretofore uncharacterized proepicardial subcompartments. In contrast to previously fate-mapped Tbx18/WT-1-expressing cells that give rise to vascular smooth muscle, Scx- and Sema3D-expressing proepicardial cells give rise to coronary vascular endothelium both in vivo and in vitro. Furthermore, Sema3D(+) and Scx(+) proepicardial cells contribute to the early sinus venosus and cardiac endocardium, respectively, two tissues linked to vascular endothelial formation at later stages. Taken together, our studies demonstrate that the proepicardial organ is a molecularly compartmentalized structure, reconciling prior chick and mouse data and providing a more complete understanding of the progenitor populations that establish the coronary vascular endothelium.

The podocyte protein nephrin is required for cardiac vessel formation

Nephrin (NPHS1) has been described as an important structural protein of kidney podocytes. Mutations in this gene lead to the Finnish-type congenital nephrotic syndrome. More recently, a role of nephrin as a signalling molecule in kidney podocytes has been identified. Here, we show that nephrin not only has a function in kidney podocytes, but is also required for cardiovascular development. Nephrin is expressed in the epicardium and coronary vessels during human and mouse embryonic development. Nephrin knockout embryos showed abnormal epicardial cell morphology and, at later stages of development, a reduced number of coronary vessels due to increased apoptosis, and in addition, cardiac fibrosis. Connexin 43, which is required for coronary vessel formation, was downregulated in nephrin knockout embryos. Expression of the p75NTR neurotrophin receptor, a known mediator of apoptosis, was increased in mutants. Furthermore, co-immunoprecipitation studies demonstrated a direct interaction of nephrin with p75NTR. Primary nephrin-deficient cardiac cells showed a 5-fold higher rate of apoptosis in response to progenitor of nerve growth factor compared with wild-type cells, which could be rescued by RNAi against p75NTR. Taken together, our data demonstrate that nephrin directly interacts with p75NTR and reveal an important role for nephrin in murine cardiac development by permitting survival of cardiovascular progenitor cells.

The relationship between the dimensions of the right coronary artery and the type of coronary vasculature in human foetuses

BACKGROUND

The area of vascular supply of particular coronary arteries is directly linked to the varying typology of the coronary vasculature. This factor may have a significant influence on the coronary vessel diameters. To date there has been no published research that analyses the relationship between the type of coronary vasculature and the dimensions of the epicardial arteries in the human foetus. There are only a few papers that deal with this issue in the postnatal period of human life.

MATERIAL AND METHODS

The study was carried out on a group of 187 human foetuses aged five to seven months of intrauterine life. Prior to examination all foetuses had been conserved in a 9% formaldehyde solution for a minimum of three months. All foetuses had been aborted naturally. None of them had any external signs of malformations or developmental abnormalities. The number of foetuses in the particular age groups was variable. Adachi/Bianchi classification was used to categorize the particular vasculature types: type I -- classic, neither artery is dominating; type II -- dominant right coronary artery; type III -- dominant left coronary artery.

RESULTS AND CONCLUSIONS

The analysis of differences between the artery dimensions in particular types of coronary vasculature revealed that such differences existed between types I and II and also between types II and III.

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.

Coronary vessel development and insight towards neovascular therapy

Formation of the coronary arteries consists of a precisely orchestrated series of morphogenetic and molecular events which can be divided into three distinct processes: vasculogenesis, angiogenesis and arteriogenesis (Risau 1997; Carmeliet 2000). Even subtle perturbations in this process may lead to congenital coronary artery anomalies, as occur in 0.2-1.2% of the general population (von Kodolitsch et al. 2004). Contrary to the previously held dogma, the process of vasculogenesis is not limited to prenatal development. Both vasculogenesis and angiogenesis are now known to actively occur within the adult heart. When the need for regeneration arises, for example in the setting of coronary artery disease, a reactivation of embryonic processes ensues, redeploying many of the same molecular regulators. Thus, an understanding of the mechanisms of embryonic coronary vasculogenesis and angiogenesis may prove invaluable in developing novel strategies for cardiovascular regeneration and therapeutic coronary angiogenesis.

FGFR-1 is required by epicardium-derived cells for myocardial invasion and correct coronary vascular lineage differentiation

Critical steps in coronary vascular formation include the epithelial-mesenchyme transition (EMT) that epicardial cells undergo to become sub-epicardial; the invasion of the myocardium; and the differentiation of coronary lineages. However, the factors controlling these processes are not completely understood. Epicardial and coronary vascular precursors migrate to the avascular heart tube during embryogenesis via the proepicardium (PE). Here, we show that in the quail embryo fibroblast growth factor receptor (FGFR)-1 is expressed in a spatially and temporally restricted manner in the PE and epicardium-derived cells, including vascular endothelial precursors, and is up-regulated in epicardial cells after EMT. We used replication-defective retroviral vectors to over-express or knock-down FGFR-1 in the PE. FGFR-1 over-expression resulted in increased epicardial EMT. Knock-down of FGFR-1, however, did not inhibit epicardial EMT but greatly compromised the ability of PE progeny to invade the myocardium. The latter could, however, contribute to endothelia and smooth muscle of sub-epicardial vessels. Correct FGFR-1 levels were also important for correct coronary lineage differentiation with, at E12, an increase in the proportion of endothelial cells amongst FGFR-1 over-expressing PE progeny and a decrease in the proportion of smooth muscle cells in antisense FGFR-1 virus-infected PE progeny. Finally, in a heart explant system, constitutive activation of FGFR-1 signaling in epicardial cells resulted in increased delamination from the epicardium, invasion of the sub-epicardium, and invasion of the myocardium. These data reveal novel roles for FGFR-1 signaling in epicardial biology and coronary vascular lineage differentiation, and point to potential new therapeutic avenues.

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.

Live imaging of blood flow in mammalian embryos using Doppler swept-source optical coherence tomography

Studying hemodynamic changes during early mammalian embryonic development is critical for further advances in prevention, diagnostics, and treatment of congenital cardiovascular (CV) birth defects and diseases. Doppler optical coherence tomography (OCT) has been shown to provide sensitive measurements of blood flow in avian and amphibian embryos. We combined Doppler swept-source optical coherence tomography (DSS-OCT) and live mouse embryo culture to analyze blood flow dynamics in early embryos. SS-OCT structural imaging was used for the reconstruction of embryo morphology and the orientation of blood vessels, which is required for calculating flow velocity from the Doppler measurements. Spatially and temporally resolved blood flow profiles are presented for the dorsal aorta and a yolk sac vessel in a 9.5-day embryo. We demonstrate that DSS-OCT can be successfully used for structural analysis and spatially and temporally resolved hemodynamic measurements in developing early mammalian embryos.

'Big-eyed frog' sign on spatiotemporal image correlation (STIC) in the antenatal diagnosis of transposition of the great arteries

OBJECTIVE

To determine the value of simultaneous visualization of the cross-sectional view of both atrioventricular (AV) valves, the pulmonary artery and the aorta (en-face view of the AV valves and great vessels) in the identification of fetuses with transposition of the great arteries (TGA).

METHODS

This was a retrospective analysis of volume datasets obtained with the spatiotemporal image correlation (STIC) technique from 56 fetuses with and 30 fetuses without congenital heart defects. Volume datasets were reviewed offline to compare the en-face view of the AV valves and great vessels between fetuses with normal echocardiography and those with TGA.

RESULTS

The en-face view of both AV valves and great vessels in fetuses with TGA displayed the main pulmonary artery situated side-by-side with the aorta ('big-eyed frog' sign). In contrast, fetuses with normal hearts did not have this characteristic sonographic sign. This novel sonographic sign also helped to identify additional cases of TGA in 17 fetuses with complex heart defects.

CONCLUSION

The big-eyed frog sign may prove helpful in the prenatal diagnosis of TGA.

Transposition of the great arteries in the fetus: assessment of the spatial relationships of the arterial trunks by four-dimensional echocardiography

OBJECTIVE

Coronary arterial abnormalities can be one of the few negative prognostic indicators in transposition of the great arteries (TGA), and their occurrence is related to the type of spatial relationship of the great arteries. The main objective of this study was to assess whether the use of the reconstructed en-face view with color Doppler imaging of the four cardiac valves can demonstrate the different types of spatial relationship of the arterial trunks in fetuses with TGA, in order to derive the risk of coronary abnormalities. A secondary end-point was the evaluation of the type of coronary arterial branching pattern.

METHODS

Twenty-three fetuses with a confirmed diagnosis of TGA underwent four-dimensional (4D) echocardiography at 19-33 gestational weeks. The en-face view of the four cardiac valves and color Doppler with high persistence were employed to assess the spatial relationships of the great arteries. In all cases, confirmation of the vessels' arrangement and coronary arterial distribution was obtained at neonatal echocardiography and/or surgery.

RESULTS

The spatial relationships of the great vessels was identified correctly in 20/23 (87%) cases. The aorta was found to be located anterior to and to the right of the pulmonary trunk in 13/23 (56.5%) cases and just anterior to the pulmonary artery in 6/23 (26.1%) cases; in the remaining four (17.4%) cases, the two vessels were side by side. With respect to the association between the spatial relationship of the great arteries and the occurrence of an unusual pattern of coronary arterial branching, five of the TGA fetuses had abnormal coronary arterial distribution.

CONCLUSIONS

Using 4D echocardiography with color Doppler, it is possible to define the spatial relationships of the great arteries in fetuses with TGA with a high degree of accuracy. This information can be used during counseling to predict the likelihood of abnormal coronary arterial distribution.

Primary and immortalized mouse epicardial cells undergo differentiation in response to TGFbeta

Cells derived from the epicardium are required for coronary vessel development. Transforming growth factor beta (TGFbeta) induces loss of epithelial character and smooth muscle differentiation in chick epicardial cells. Here, we show that epicardial explants from embryonic day (E) 11.5 mouse embryos incubated with TGFbeta1 or TGFbeta2 lose epithelial character and undergo smooth muscle differentiation. To further study TGFbeta Signaling, we generated immortalized mouse epicardial cells. Cells from E10.5, 11.5, and 13.5 formed tightly packed epithelium and expressed the epicardial marker Wilm's tumor 1 (WT1). TGFbeta induced the loss of zonula occludens-1 (ZO-1) and the appearance of SM22alpha and calponin consistent with smooth muscle differentiation. Inhibition of activin receptor-like kinase (ALK) 5 or p160 rho kinase activity prevented the effects of TGFbeta while inhibition of p38 mitogen activated protein (MAP) kinase did not. These data demonstrate that TGFbeta induces epicardial cell differentiation and that immortalized epicardial cells provide a suitable model for differentiation.

[Transposition of the great arteries--difficulties in prenatal diagnosis]

Transposition of the great arteries is a rare congenital heart malformation associated with significant perinatal morbidity and mortality. Prenatal diagnosis is possible after detailed examination of the fetal heart and knowledge of the ultrasound appearance of the two types of transposition--complete and corrected form. We present a case report of antenatally diagnosed complete transposition of the great arteries in 22 weeks of gestation and discuss the difficulties in prenatal diagnosis.

Coronary Vessel Development Is Dependent on the Type III Transforming Growth Factor β Receptor

Transforming growth factor (TGF)β receptor III (TGFβR3), or β-glycan, binds all 3 TGFβ ligands and inhibin with high affinity but lacks the serine/threonine kinase domain found in the type I and type II receptors (TGFβR1, TGFβR2). TGFβR3 facilitates signaling via TGFβR1/TGFβR2 but also has been suggested to play a unique and nonredundant role in TGFβ signaling. Targeted deletion of Tgfbr3 revealed a requirement for Tgfbr3 during development of the coronary vessels. Coronary vasculogenesis is significantly impaired in null mice, with few vessels evident and numerous, persistent blood islands found throughout the epicardium. Tgfbr3 -null mice die at embryonic day 14.5, the time when functional coronary vasculature is required for embryo viability. However, in null mice nascent coronary vessels attach to the aorta, form 2 coronary ostia, and initiate smooth muscle recruitment by embryonic day 14. Analysis of earlier developmental stages revealed defects in the epicardium. At embryonic day 13.5, these defects include an irregular and hypercellular epicardium with abundant subepicardial mesenchyme and a thin compact zone myocardium. Tgfbr3 -null mice also displayed other defects in coronary development, including dysmorphic and distended vessels along the atrioventricular groove and subepicardial hemorrhage. In null mice, vessels throughout the yolk sac and embryo form and recruit smooth muscle in a pattern indistinguishable from heterozygous or wild-type littermates. These data demonstrate a requirement for Tgfbr3 during coronary vessel development that is essential for embryonic viability.

Epicardium-derived cells in cardiogenesis and cardiac regeneration

During cardiogenesis, the epicardium grows from the proepicardial organ to form the outermost layer of the early heart. Part of the epicardium undergoes epithelial-mesenchymal transformation, and migrates into the myocardium. These epicardium- derived cells differentiate into interstitial fibroblasts, coronary smooth muscle cells, and perivascular fibroblasts. Moreover, epicardium-derived cells are important regulators of formation of the compact myocardium, the coronary vasculature, and the Purkinje fiber network, thus being essential for proper cardiac development. The fibrous structures of the heart such as the fibrous heart skeleton and the semilunar and atrioventricular valves also depend on a contribution of these cells during development. We hypothesise that the essential properties of epicardium-derived cells can be recapitulated in adult diseased myocardium. These cells can therefore be considered as a novel source of adult stem cells useful in clinical cardiac regeneration therapy.

A 3-D model of coronary vessel development

Coronary vascular disease is one of the leading causes of mortality and morbidity in the United States. Therefore, a mechanistic understanding of coronary vessel morphogenesis would aid in the innovation of new therapies targeting vascular disorders. Moreover, a functionally equivalent in vitro model system allows for the delineation of the molecular mechanisms that regulate coronary vessel development. In this study, we present a novel in vitro model system. This three-dimensional (3-D) model system consists of a tubular scaffold, which is engineered from type-I collagen and has been optimized to support the growth of embryonic cardiac tissues. In this report, proepicardial (PE) cells, the developmental precursors of coronary vessels, have been isolated from several model species and cultured on this scaffold. In this model system, the PE cells were able to recapitulate several aspects of coronary vessel morphogenesis including epicardial formation, the epicardial to mesenchymal transformation, and de novo coronary vessel development or vasculogenesis. The differentiation of PE cells was characterized using a variety of specific protein markers. The potential uses of this novel coronary developmental model are discussed.

Early morphogenesis of the sinuatrial region of the chick heart: a contribution to the understanding of the pathogenesis of direct pulmonary venous connections to the right atrium and atrial septal defects in hearts with right isomerism of the atrial appendages

The morphogenesis of the sinuatrial region of embryonic hearts is still not well understood. Current matters of dispute are the topogenesis of the future pulmonary vein orifice and the topogenesis of the primary atrial septum. We analyzed the development of the sinuatrial region in chick embryos ranging from Hamburger and Hamilton (HH) stage 14 to 25. Our study disclosed three features of sinuatrial development. First, the primitive atrium of the HH stage 16 chick embryo heart has a separate inflow component. This inflow component takes up the mouth of the confluence of the systemic veins (sinus venosus) as well as the future mouth of the common pulmonary vein (pulmonary pit). The left portion of the atrial inflow component becomes incorporated into the left atrium and its right portion becomes incorporated into the right atrium. Rightward growth of the sinuatrial fold separates the sinus venosus from the left atrium. Second, the pulmonary pit originally forms as a bilaterally paired structure. Its left and right portions are connected to the left and right portions of the atrial inflow component, respectively. Normally, only the left portion of the pulmonary pit deepens to form the common pulmonary vein orifice, whereas the right portion disappears. Third, the primary atrial septum of the chick heart is not formed at the original midline of the embryonic heart, but is formed to the left of the original midline. This finding is in accord with molecular data suggesting that the primary atrial septum derives from the left heart-forming field. Our findings shed new light on the pathogenesis of direct pulmonary venous connections to the right atrium and atrial septal defects in hearts with right isomerism of the atrial appendages.

The proepicardium delivers hemangioblasts but not lymphangioblasts to the developing heart

The mass of the myocardium and endocardium of the vertebrate heart derive from the heart-forming fields of the lateral plate mesoderm. Further components of the mature heart such as the epicardium, cardiac interstitium and coronary blood vessels originate from a primarily extracardiac progenitor cell population: the proepicardium (PE). The coronary blood vessels are accompanied by lymph vessels, suggesting a common origin of the two vessel types. However, the origin of cardiac lymphatics has not been studied yet. We have grafted PE of HH-stage 17 (day 3) quail embryos hetero- and homotopically into chick embryos, which were re-incubated until day 15. Double staining with the quail endothelial cell (EC) marker QH1 and the lymphendothelial marker Prox1 shows that the PE of avian embryos delivers hemangioblasts but not lymphangioblasts. We have never observed quail ECs in lymphatics of the chick host. However, one exception was a large lymphatic trunk at the base of the chick heart, indicating a lympho-venous anastomosis and a 'homing' mechanism of venous ECs into the lymphatic trunk. Cardiac lymphatics grow from the base toward the apex of the heart. In murine embryos, we observed a basal to apical gradient of scattered Lyve-1+/CD31+/CD45+ cells in the subepicardium at embryonic day 12.5, indicating a contribution of immigrating lymphangioblasts to the cardiac lymphatic system. Our studies show that coronary blood and lymph vessels are derived from different sources, but grow in close association with each other.

Transcriptional pathways in second heart field development

The heart is the first organ to form and function during vertebrate development and is absolutely essential for life. The left ventricle is derived from the classical primary or first heart field (FHF), while the right ventricle and outflow tract are derived from a distinct second heart field (SHF). The recent discovery of the SHF has raised several fundamental and important questions about how the two heart fields are integrated into a single organ and whether unique molecular programs control the development of the two heart fields. This review briefly highlights the contributions of the SHF to the developing and mature heart and then focuses primarily on our current understanding of the transcriptional pathways that function in the development of the SHF and its derivatives.

Synectin/syndecan-4 regulate coronary arteriolar growth during development

Syndecan-4 and its cytoplasmic binding partner, synectin, are known to play a role in FGF-2 signaling and vascular growth. To determine their roles in coronary artery/arteriolar formation and growth, we compared syndecan-4 and synectin null mice with their wild-type counterparts. Image analysis of arterioles visualized by smooth muscle alpha-actin immunostaining revealed that synectin (-/-) mice had lower arteriolar length and volume densities than wild-type mice. As shown by electron microscopic analysis, arterioles from the two did not differ in morphology, including their endothelial cell junctions, and the organization and distribution of smooth muscle. Using micro-computer tomography, we found that the size and branching patterns of coronary arteries (diameters > 50 microm) were similar for the two groups, a finding that indicates that the growth of arteries is not influenced by a loss of synectin. Syndecan-4 null male mice also had lower arteriolar length densities than their gender wild-type controls. However, female syndecan-4 null mice were characterized by higher arteriolar length and volume densities than their gender-matched wild-type controls. Thus, we conclude that both synectin and syndecan-4 play a role in arteriolar development, a finding that is consistent with previous evidence that FGF-2 plays a role in coronary arterial growth. Moreover, our data reveal that gender influences the arteriolar growth response to syndecan-4 but not to synectin.

Intrauterine growth restriction delays cardiomyocyte maturation and alters coronary artery function in the fetal sheep

There is now extensive evidence suggesting that intrauterine perturbations are linked with an increased risk of developing cardiovascular disease. Human epidemiological studies, supported by animal models, have demonstrated an association between low birth weight, a marker of intrauterine growth restriction (IUGR), and adult cardiovascular disease. However, little is known of the early influence of IUGR on the fetal heart and vessels. The aim of this study was to determine the effects of late gestational IUGR on coronary artery function and cardiomyocyte maturation in the fetus. IUGR was induced by placental embolization in fetal sheep from 110 to 130 days of pregnancy (D110-130); term approximately D147; control fetuses received saline. At necropsy (D130), wire and pressure myography was used to test endothelial and smooth muscle function, and passive mechanical wall properties, respectively, in small branches of left descending coronary arteries. Myocardium was dissociated for histological analysis of cardiomyocytes. At D130, IUGR fetuses (2.7 +/- 0.1 kg) were 28% lighter than controls (3.7 +/- 0.3 kg; P = 0.02). Coronary arteries from IUGR fetuses had enhanced responsiveness to the vasoconstrictors, angiotensin II and the thromboxane analogue U46619, than controls (P < 0.01). Endothelium-dependent and -independent relaxations were not different between groups. Coronary arteries of IUGR fetuses were more compliant (P = 0.02) than those of controls. The incidence of cardiomyocyte binucleation was lower in the left ventricles of IUGR fetuses (P = 0.02), suggestive of retarded cardiomyocyte maturation. We conclude that late gestational IUGR alters the reactivity and mechanical wall properties of coronary arteries and cardiomyocyte maturation in fetal sheep, which could have lifelong implications for cardiovascular function.

Thymosin beta4 induces adult epicardial progenitor mobilization and neovascularization

Cardiac failure has a principal underlying aetiology of ischaemic damage arising from vascular insufficiency. Molecules that regulate collateral growth in the ischaemic heart also regulate coronary vasculature formation during embryogenesis. Here we identify thymosin beta4 (Tbeta4) as essential for all aspects of coronary vessel development in mice, and demonstrate that Tbeta4 stimulates significant outgrowth from quiescent adult epicardial explants, restoring pluripotency and triggering differentiation of fibroblasts, smooth muscle cells and endothelial cells. Tbeta4 knockdown in the heart is accompanied by significant reduction in the pro-angiogenic cleavage product N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP). Although injection of AcSDKP was unable to rescue Tbeta4 mutant hearts, it significantly enhanced endothelial cell differentiation from adult epicardially derived precursor cells. This study identifies Tbeta4 and AcSDKP as potent stimulators of coronary vasculogenesis and angiogenesis, and reveals Tbeta4-induced adult epicardial cells as a viable source of vascular progenitors for continued renewal of regressed vessels at low basal level or sustained neovascularization following cardiac injury.

Vasculogenesis of the embryonic heart: contribution of nucleated red blood cells to early vascular structures

During embryogenesis, coronary vessels develop via vasculogenesis and angiogenesis. Vasculogenesis is formation in situ of primary vessels from angioblasts - endothelial cell progenitors, and angiogenesis is formation of vessels from the existing ones. In the embryonic heart vasculogenesis precedes and overlaps angiogenesis and lasts till the end of the fetal life. What is unique about heart vasculogenesis is the fact that nucleated blood cells accompany early angioblasts in a spatiotemporal way. Morphologically these structures resemble yolk sac blood islands, thus, they have been called blood-island-like structures. In addition, these early vascular structures (blood-island-like) are found in the heart before coronary vessel system connects with the systemic circulation. We present the recent data regarding endothelial cell properties and derivation during coronary vessel formation and hypotheses concerning a source of blood cells in early vascular structures of the heart; the latter has received little attention in the literature. This review summarizes current knowledge on the endothelial cell origination from epicardial mesothelium or liver primordium. This review also focuses on blood cell contribution to coronary vessel vasculogenesis. The role of proepicardium in the epicardial cover formation and the epicardium as a source of cellular components of coronary vasculature and interstitial fibroblasts is presented. It seems that blood cells and angioblasts, which form the early vascular structures do not derive from the same hemangioblastic precursor.

Vascular patterning of the quail coronary system during development

Recent studies have provided insights into specific events that contribute to vasculogenesis and angiogenesis in the developing coronary vasculature. This study focused on the developmental progression of coronary vascularization beginning with tube formation and ending with the establishment of a coronary arterial tree. We used electron microscopy, histology of serial sections, and immunohistochemistry in order to provide a comprehensive view of coronary vessel formation during the embryonic and fetal periods of the quail heart, a species that has been used in a number of studies addressing myocardial vascularization. Our data reveal features of progenitor cells and blood islands, tubular formation, and the anatomical relationship of a transformed periarterial tubular network and sympathetic ganglia to the emergence and branching of the right and left coronary arteries. We have traced the pattern of coronary artery branching and documented its innervation. Finally, our data include the relationship of fibronectin, laminin, and apoptosis to coronary artery growth. Our findings bring together morphological events that occur over the embryonic and fetal periods and provide a baseline for studies into the mechanisms that regulate the various events that occur during these time periods.

Differential regulation of Tbx5 protein expression and sub-cellular localization during heart development

The T-box transcription factor Tbx5 can interact with Nkx2.5 and Gata4 transcription factors to synergistically regulate heart-specific genes in the nucleus. While a nuclear role for Tbx5 is clearly defined, we have previously shown that Tbx5 shuttles from nuclear to cytoplasmic sites, forming a complex with the PDZ-LIM protein LMP4 on the actin cytoskeleton. In this study, using a developmental series of chicken hearts, we provide the first evidence for differential Tbx5 protein expression and sub-cellular localization during cardiogenesis. At the tissue level, we show temporally and spatially restricted Tbx5 co-expression with LMP4. In cells co-expressing LMP4 and Tbx5 we demonstrate dynamic Tbx5 re-localization from exclusively nuclear to nuclear and cytoplasmic expression in the atrio-ventricular cushion. Furthermore, in coronary vessel development we show exclusive cytoplasmic localization of Tbx5, indicating a function for Tbx5 in the cytoplasm. In addition, we discover unknown regulation of Tbx5 and LMP4 expression in epicardial tissue, suggesting a specific role for Tbx5 in epicardial formation. These studies provide in vivo significance of the LMP4/Tbx5 protein interaction, suggesting both nuclear and cytoplasmic roles for Tbx5. The shuttling between nuclear and cytoplasmic sites reveals a novel mechanism for Tbx transcription factor regulation in chicken heart development allowing new insights for a better understanding of the molecular basis of hand/heart birth defects associated with TBX5 mutations.

Cardiovascular expression of the mouse WNK1 gene during development and adulthood revealed by a BAC reporter assay

Large deletions in WNK1 are associated with inherited arterial hypertension. WNK1 encodes two types of protein: a kidney-specific isoform (KS-WNK1) lacking kinase activity and a ubiquitously expressed full-length isoform (L-WNK1) with serine threonine kinase activity. Disease is thought to result from hypermorphic mutations increasing the production of one or both isoforms. However, the pattern of L-WNK1 expression remains poorly characterized. We generated transgenic mice bearing a murine WNK1 BAC containing the nlacZ reporter gene for monitoring L-WNK1 expression during development and adulthood. We observed previously unsuspected early expression in the vessels and primitive heart during embryogenesis, consistent with the early death of WNK1(-/-) mice. The generalized cardiovascular expression observed in adulthood may also suggest a possible kidney-independent role in blood pressure regulation. The second unsuspected site of L-WNK1 expression was the granular layer and Purkinje cells of the cerebellum, suggesting a role in local ion balance or cell trafficking. In the kidney, discordance between endogenous L-WNK1 and transgene expression suggests that either cis-regulatory elements important for physiological renal expression lie outside the BAC sequence or that illegitimate interactions occur between promoters. Despite this limitation, this transgenic model is a potentially valuable tool for the analysis of spatial and temporal aspects of WNK1 expression and regulation.

Molecular mechanisms controlling the coupled development of myocardium and coronary vasculature

Cardiac failure affects 1.5% of the adult population and is predominantly caused by myocardial dysfunction secondary to coronary vascular insufficiency. Current therapeutic strategies improve prognosis only modestly, as the primary cause -- loss of normally functioning cardiac myocytes -- is not being corrected. Adult cardiac myocytes are unable to divide and regenerate to any significant extent following injury. New cardiac myocytes are, however, created during embryogenesis from progenitor cells and then by cell division from existing cardiac myocytes. This process is intimately linked to the development of coronary vasculature from progenitors originating in the endothelium, the proepicardial organ and neural crest. In this review, we systematically evaluate approx. 90 mouse mutations that impair heart muscle growth during development. These studies provide genetic evidence for interactions between myocytes, endothelium and cells derived from the proepicardial organ and the neural crest that co-ordinate myocardial and coronary vascular development. Conditional knockout and transgenic rescue experiments indicate that Vegfa, Bmpr1a (ALK3), Fgfr1/2, Mapk14 (p38), Hand1, Hand2, Gata4, Zfpm2 (FOG2), Srf and Txnrd2 in cardiac myocytes, Rxra and Wt1 in the proepicardial organ, EfnB2, Tek, Mapk7, Pten, Nf1 and Casp8 in the endothelium, and Bmpr1a and Pax3 in neural crest cells are key molecules controlling myocardial development. Coupling of myocardial and coronary development is mediated by BMP (bone morphogenetic protein), FGF (fibroblast growth factor) and VEGFA (vascular endothelial growth factor A) signalling, and also probably involves hypoxia. Pharmacological targeting of these molecules and pathways could, in principle, be used to recreate the embryonic state and achieve coupled myocardial and coronary vascular regeneration in failing hearts.

In vivo and in vitro analysis of the vasculogenic potential of avian proepicardial and epicardial cells

Coronary vessel formation is a special case in the context of embryonic vascular development. A major part of the coronary cellular precursors (endothelial, smooth muscle, and fibroblastic cells) derive from the proepicardium and the epicardium in what can be regarded as a late event of angioblastic and smooth muscle cell differentiation. Thus, coronary morphogenesis is dependent on the epithelial-mesenchymal transformation of the proepicardium and the epicardium. In this study, we present several novel observations about the process of coronary vasculogenesis in avian embryos, namely: (1) The proepicardium displays a high vasculogenic potential, both in vivo (as shown by heterotopic transplants) and in vitro, which is modulated by vascular endothelial growth factor (VEGF) and basic fibroblast growth factor signals; (2) Proepicardial and epicardial cells co-express receptors for platelet-derived growth factor-BB and VEGF; (3) Coronary angioblasts (found all through the epicardial, subepicardial, and compact myocardial layers) express the Wilms' tumor associated transcription factor and the retinoic acid-synthesizing enzyme retinaldehyde-dehydrogenase-2, two markers of the coelomic epithelium involved in coronary endothelium development. All these results contribute to the development of our knowledge on the vascular potential of proepicardial/epicardial cells, the existent interrelationships between the differentiating coronary cell lineages, and the molecular mechanisms involved in the regulation of coronary morphogenesis.