Initiation of apoptosis in the developing avian outflow tract myocardium

Hypoxia - as a Possible Regulator of the Activity of Epicardial Mesothelial Cells After Myocardial Infarction

Aim      To study the effect of hypoxia on the activity of epithelial-mesenchymal transition (EMT) in epicardial cells, which provides formation of a specialized microenvironment.Material and methods   This study used a model of experimental myocardial infarction created by ligation of the anterior descendent coronary artery. The activity of epicardial cells after a hypoxic exposure was studied with the hypoxia marker, pimonidazole, bromodeoxyuridine, immunofluorescent staining of heart cryosections, and in vitro mesothelial cell culture.Results The undamaged heart maintained the quiescent condition of mesothelial cells and low levels of their proliferation, extracellular matrix protein production, and of the EMT activity. Acute ischemic injury induced moderate hypoxia in the epicardial/subepicardial region. This caused a global rearrangement of this region due to the initiation of EMT in cells, changes in the cell composition, and accumulation of extracellular matrix proteins. We found that the initiation of EMT in mesothelial cells may result in the formation of smooth muscle cell precursors, fibroblasts, and a population of Sca-1+ cardiac progenitor cells, which may both participate in construction of new blood vessels and serve as a mesenchymal link for the paracrine support of microenvironmental cells. In in vitro experiments, we showed that 72‑h hypoxia facilitated activation of EMT regulatory genes, induced dissembling of intercellular contacts, cell uncoupling, and increased cell plasticity.Conclusion      The epicardium of an adult heart serves as a "reparative reserve" that can be reactivated by a hypoxic exposure. This creates a basis for an approach to influence the epicardium to modulate its activity for regulating reparative processes.

Transforming Growth Factor Beta (TGF-β1) Induces Pro-Reparative Phenotypic Changes in Epicardial Cells in Mice

We evaluated the content of active form of TGF-β1 in the intact and post-infarction heart and the effect of this factor on the properties of epicardial cells. During the acute stage after myocardial infarction, the production of TGF-β1 in the heart increased, which closely correlated with activation of epicardial cells (appearance of a pool of Wt1+ epicardial cells entering the epithelial-mesenchymal transition). The role of TGF-β1 as the factor of epicardial activation was confirmed by the results of in vitro experiments: addition of recombinant TGF-β1 to cultured epicardial cells led to enhanced expression of genes of epithelial-mesenchymal transition and phenotypic transformation of these cells leading to the appearance of cells with markers of smooth muscle cells and fibroblasts. Our findings suggest that the regulatory axis "TGF-β1-epicardium cells" can be considered as an important link of the post-infarction reparative process and adaptive response during heart remodeling after myocardial infarction and as the target for therapeutic interventions.

Oxygen and lack of oxygen in fetal and placental development, feto-placental coupling, and congenital heart defects

Low oxygen concentration (hypoxia) is part of normal embryonic development, yet the situation is complex. Oxygen (O₂ ) is a janus gas with low levels signaling through hypoxia-inducible transcription factor (HIF) that are required for development of fetal and placental vasculature and fetal red blood cells. This results in coupling of fetus and mother around midgestation as a functional feto-placental unit (FPU) for O₂ transport, which is required for continued growth and development of the fetus. Defects in these processes may leave the developing fetus vulnerable to O₂ deprivation or other stressors during this critical midgestational transition when common septal and conotruncal heart defects (CHDs) are likely to arise. Recent human epidemiological and case-control studies support an association between placental dysfunction, manifest as early onset pre-eclampsia (PE) and increased serum bio-markers, and CHD. Animal studies support this association, in particular those using gene inactivation in the mouse. Sophisticated methods for gene inactivation, cell fate mapping, and a quantitative bio-reporter of O₂ concentration support the premise that hypoxic stress at critical stages of development leads to CHD. The secondary heart field contributing to the cardiac outlet is a key target, with activation of the un-folded protein response and abrogation of FGF signaling or precocious activation of a cardiomyocyte transcriptional program for differentiation, suggested as mechanisms. These studies provide a strong foundation for further study of feto-placental coupling and hypoxic stress in the genesis of human CHD.

Cellular origin and developmental program of coronary angiogenesis

Coronary artery disease causes acute myocardial infarction and heart failure. Identifying coronary vascular progenitors and their developmental program could inspire novel regenerative treatments for cardiac diseases. The developmental origins of the coronary vessels have been shrouded in mystery and debated for several decades. Recent identification of progenitors for coronary vessels within the endocardium, epicardium, and sinus venosus provides new insights into this question. In addition, significant progress has been achieved in elucidating the cellular and molecular programs that orchestrate coronary artery development. Establishing adequate vascular supply will be an essential component of cardiac regenerative strategies, and these findings raise exciting new strategies for therapeutic cardiac revascularization.

Formal genesis of the outflow tracts of the heart revisited: previous works in the light of recent observations

The formal genesis of the great arteries continues to be controversial due to the lack of consensus of septation of the developing outflow tract. In order to make it clear how the great arteries are generated, we have re-examined our previous papers which emphasized the formation of the aorta and pulmonary trunk, concept of the aorticopulmonary septum, formation of the leaflets of semilunar valves, morphogenesis of the crista supraventricularis, programmed cell death and rotation of the outflow tract. In the present paper, we compare outcomes gained from the re-examination of our previous papers with prevalent interpretations of the arterial trunk. We obtained conclusions as follows: (i) The elongation of the fourth and sixth aortic arch arteries, which sprout from the wall of the aortic sac at the expense of the distal truncus, contributes to the formation of the aorta and pulmonary trunk; (ii) Smooth muscle cells of the tunica media of the arterial trunks do not arise from the transformation of the myocardial cells of the truncus wall (not 'arterialization'); (iii) Truncus swellings are divided into two parts: distal and proximal. The former contributes to the separation of the orifices of arterial trunks ('aorticopulmonary septum'). The latter contributes to the formation of the leaflets of the semilunar valves of the aorta and pulmonary trunk; (iv) The origin of the myocardial cells of the crista supraventricularis is a wall of the conus originated from secondary/anterior heart fields; and (v) There has been no acceptable proof that rotation and counterclockwise rotation are involved.

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.

Role of hypoxia in the evolution and development of the cardiovascular system

How multicellular organisms obtain and use oxygen and other substrates has evolved over hundreds of millions of years in parallel with the evolution of oxygen-delivery systems. A steady supply of oxygen is critical to the existence of organisms that depend on oxygen as a primary source of fuel (i.e., those that live by aerobic metabolism). Not surprisingly, a number of mechanisms have evolved to defend against oxygen deprivation. This review highlights evolutionary and developmental aspects of O2 delivery to allow understanding of adaptive responses to O2 deprivation (hypoxia). First, we consider how the drive for more efficient oxygen delivery from the heart to the periphery may have shaped the evolution of the cardiovascular system, with particular attention to the routing of oxygenated and deoxygenated blood in the cardiac outlet. Then we consider the role of O2 in the morphogenesis of the cardiovascular system of animals of increasing size and complexity. We conclude by suggesting areas for future research regarding the role of oxygen deprivation and oxidative stress in the normal development of the heart and vasculature or in the pathogenesis of congenital heart defects.

Epicardium-derived cells are important for correct development of the Purkinje fibers in the avian heart

During embryonic development, the proepicardial organ (PEO) grows out over the heart surface to form the epicardium. Following epithelial-mesenchymal transformation, epicardium-derived cells (EPDCs) migrate into the heart and contribute to the developing coronary arteries, to the valves, and to the myocardium. The peripheral Purkinje fiber network develops from differentiating cardiomyocytes in the ventricular myocardium. Intrigued by the close spatial relationship between the final destinations of migrating EPDCs and Purkinje fiber differentiation in the avian heart, that is, surrounding the coronary arteries and at subendocardial sites, we investigated whether inhibition of epicardial outgrowth would disturb cardiomyocyte differentiation into Purkinje fibers. To this end, epicardial development was inhibited mechanically with a membrane, or genetically, by suppressing epicardial epithelial-to-mesenchymal transformation with antisense retroviral vectors affecting Ets transcription factor levels (n=4, HH39-41). In both epicardial inhibition models, we evaluated Purkinje fiber development by EAP-300 immunohistochemistry and found that restraints on EPDC development resulted in morphologically aberrant differentiation of Purkinje fibers. Purkinje fiber hypoplasia was observed both periarterially and at subendocardial positions. Furthermore, the cells were morphologically abnormal and not aligned in orderly Purkinje fibers. We conclude that EPDCs are instrumental in Purkinje fiber differentiation, and we hypothesize that they cooperate directly with endothelial and endocardial cells in the development of the peripheral conduction system.

Apoptosis in the developing mouse heart

Apoptosis occurs at high frequency in the myocardium of the developing avian cardiac outflow tract (OFT). Up- or down-regulating apoptosis results in defects resembling human conotruncal heart anomalies. This finding suggested that regulated levels of apoptosis are critical for normal morphogenesis of the four-chambered heart. Recent evidence supports an important role for hypoxia of the OFT myocardium in regulating cell death and vasculogenesis. The purpose of this study was to determine whether apoptosis in the outflow tract myocardium occurs in the mouse heart during developmental stages comparable to the avian heart and to determine whether differential hypoxia is also present at this site in the murine heart. Apoptosis was detected using a fluorescent vital dye, Lysotracker Red (LTR), in the OFT myocardium of the mouse starting at embryonic day (E) 12.5, peaking at E13.5-14.5, and declining thereafter to low or background levels by E18.5. In addition, high levels of apoptosis were detected in other cardiac regions, including the apices of the ventricles and along the interventricular sulcus. Apoptosis in the myocardium was detected by double-labeling with LTR and cardiomyocyte markers. Terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) and immunostaining for cleaved Caspase-3 were used to confirm the LTR results. At the peak of OFT apoptosis in the mouse, the OFT myocardium was relatively hypoxic, as indicated by specific and intense EF5 staining and HIF1alpha nuclear localization, and was surrounded by the developing vasculature as in the chicken embryo. These findings suggest that cardiomyocyte apoptosis is an evolutionarily conserved mechanism for normal morphogenesis of the outflow tract myocardium in avian and mammalian species.

Extracardiac tissues and the epigenetic control of myocardial development in vertebrate embryos

During the past few years, research on the developing cardiovascular system has given new insights into the origin and development of the myocardium in vertebrate embryos. In the present paper, a review is given on our current knowledge about two aspects of myocardial development that have been found to depend on signals from extracardiac tissues. These two aspects are, firstly, the development of the so-called heart-forming fields and, secondly, the morphogenesis of the outer compact layer of the myocardial wall.

Experimental analyses of the function of the proepicardium using a new microsurgical procedure to induce loss-of-proepicardial-function in chick embryos

The proepicardium (PE) is a primarily extracardiac progenitor cell population that colonizes the embryonic heart and delivers the epicardium, the subepicardial and intramyocardial fibroblasts, and the coronary vessels. Recent data show that PE-derived cells additionally play important regulatory roles in myocardial development and possibly in the normal morphogenesis of the heart. Developmental Dynamics 233, 2005. Research on the latter topics profits from the fact that loss-of-PE-function can be experimentally induced in chick embryos. So far, two microsurgical techniques were used to produce such embryos: (1) blocking of PE cell transfer with pieces of the eggshell membrane, and (2) mechanical excision of PE. Both of these techniques, however, have their shortcomings. We have searched, therefore, for new techniques to eliminate the PE. Here, we show that loss-of-PE-function can be induced by photoablation of the PE. Chick embryos were treated in ovo by means of a window in the eggshell at Hamburger and Hamilton (HH) stage 16 (iday 3). The pericardial coelom was opened, and the PE was externally stained with a 1% solution of Rose Bengal by means of a micropipette. Photoactivation of the dye was accomplished by illumination of the operation field with visible light. Examination on postoperative day 1 (iday 4, HH stages 19/20) disclosed complete removal of PE in every experimental embryo. On iday 9 (HH stages 33/34), the survival rate of experimental embryos was 35.7% (15 of 42). Development of the PE-derivatives was compromised in the heart of every survivor. The abnormalities encompassed hydro- or hemopericardium, epicardium-free areas with aneurysmatic outward bulging of the ventricular wall, thin myocardium, defects of the coronary vasculature, and abnormal tissue bridges between the ventricles and the pericardial wall. Our results show that photoablation of the PE is a powerful technique to induce long-lasting loss-of-PE-function in chick embryos. We have additionally obtained new data that suggest that the embryonic epicardium may make important contributions to the passive mechanics of the developing heart.

Reference guide to the stages of chick heart embryology

Cardiac progenitors of the splanchnic mesoderm (primary and secondary heart field), cardiac neural crest, and the proepicardium are the major embryonic contributors to chick heart development. Their contribution to cardiac development occurs with precise timing and regulation during such processes as primary heart tube fusion, cardiac looping and accretion, cardiac septation, and the development of the coronary vasculature. Heart development is even more complex if one follows the development of the cardiac innervation, cardiac pacemaking and conduction system, endocardial cushions, valves, and even the importance of apoptosis for proper cardiac formation. This review is meant to provide a reference guide (Table 1) on the developmental timing according to the staging of Hamburger and Hamilton (1951) (HH) of these important topics in heart development for those individuals new to a chick heart research laboratory. Even individuals outside of the heart field, who are working on a gene that is also expressed in the heart, will gain information on what to look for during chick heart development. This reference guide provides complete and easy reference to the stages involved in heart development, as well as a global perspective of how these cardiac developmental events overlap temporally and spatially, making it a good bench top companion to the many recently written in-depth cardiac reviews of the molecular aspects of cardiac development.

Apoptosis as an instrument in cardiovascular development

Cell death as a phenomenon in embryonic development was first described over 100 years ago. Approximately 30 years ago the process was named apoptosis, and its involvement is now recognized in many life processes, in virtually every animal species, and from fertilization to the death of an organism. In cardiovascular development, it coincides with major developmental processes in specific time windows. Both intrinsic (controlled by mitochondrial activity) and extrinsic (starting with death receptors) apoptotic pathways co-regulate developmental mechanisms. During cardiac development, many cell populations are recruited to the heart, where they differentiate into cardiomyocytes, fibroblasts, smooth muscle cells, endocardial and endothelial cells lining the inner surfaces, and epicardial cells lining the outer contours. In particular, neural crest-derived cell populations, which migrate to specific locations in the heart, are prone to apoptosis. During the complex geometric changes that occur in the primary heart tube and connected vessel segments, proper interaction of the respective cell populations guarantees the ensuing steps of differentiation. Growth factors, including endothelin, VEGF, and TGF-beta, as well as other factors, such as FasL, play dominant roles in these phases. Transgenic and knockout studies have provided strong evidence for aberrant patterns of apoptosis resulting in congenital malformations and syndromic malformations, including septation anomalies, interrupted aortic arch segments, coronary anomalies, and DiGeorge syndrome. Embryonic remodeling of the arterial system, including the coronary arteries, is accompanied by apoptosis patterns, the disruption of which results in severe malformations. It is interesting to note that hemodynamic factors, such as flow-driven shear stress, regulate the expression of genes that are important for signaling molecules such as endothelin and NO-synthase. In general, high shear stress protects against apoptosis, thus preventing the onset of disease processes in the fully-grown vasculature, and regulating the remodeling of the vascular system in the embryo.

Coronary Artery and Orifice Development Is Associated With Proper Timing of Epicardial Outgrowth and Correlated Fas Ligand Associated Apoptosis Patterns

The proepicardial organ provides differentiated cell types to the myocardial wall and facilitates coronary development. Ingrowth of the coronary arteries into the aorta has recently been linked to apoptosis. This study was set up to examine the effect of an inhibition of epicardial outgrowth on apoptotic patterning and coronary development. Epicardial outgrowth was blocked at HH15-17 in quail embryos, which survived until HH25-35 (n=33). Embryos with complete inhibition of outgrowth did not survive after HH29. These embryos presented with thin compact myocardium, devoid of vessels. In embryos with delayed epicardial outgrowth the phenotype was less severe, and surviving embryos were studied up to HH35. In these embryos, myocardial vascularization was poor and apoptosis in the peritruncal region at HH30 was diminished. Embryos at HH35 displayed an abnormal coronary network and absent coronary orifices. In a further set of experiments (n=10), outgrowth was inhibited in chicken embryos at HH15, followed by transplantation of a quail proepicardial organ into the pericardial cavity to rescue cardiac phenotype. These chimeras were studied at HH29 and HH35. Myocardial development was restored; however, in 3 of 4 embryos (HH35), the coronary orifices were absent. Examination of double stainings of quail-chicken chimeras revealed that EPDCs produce Fas ligand as an apoptotic inductor at sites of coronary ingrowth. In the absence of proper timing of epicardial outgrowth, myocardial development and vascularization are disturbed. Also apoptosis in the peritruncal region is diminished. During later development, this leads to defective or absent connections of the coronary system to the systemic circulation.

Differential levels of tissue hypoxia in the developing chicken heart

Tissue hypoxia plays a critical role in normal development, including cardiogenesis. Previously, we showed that oxygen concentration, as assessed by the hypoxia indicator EF5, is lowest in the outflow tract (OFT) myocardium of the developing chicken heart and may be regulating events in OFT morphogenesis. In this study, we identified additional areas of the embryonic chicken heart that were intensely positive for EF5 within the myocardium in discrete regions of the atrial wall and the interventricular septum (IVS). The region of the IVS that is EF5-positive includes a portion of the developing central conduction system identified by HNK-1 co-immunostaining. The EF5 positive tissues were also specifically positive for nuclear-localized hypoxia inducible factor 1alpha (HIF-1alpha), the oxygen-sensitive component of the hypoxia inducible factor 1 (HIF-1) heterodimer. The pattern of the most intensely EF5-stained myocardial regions of the atria and IVS resemble the pattern of the major coronary vessels that form in later stages within or immediately adjacent to these particular regions. These vessels include the sinoatrial nodal artery that is a branch of the right coronary artery within the atrial wall and the anterior/posterior interventricular vessels of the IVS. These findings indicate that a portion of the developing central conduction system and the patterning of coronary vessels may be subject to a level of regulation that is dependent on differential oxygen concentration within cardiac tissues and subsequent HIF-1 regulation of gene expression.

Spatiotemporal analysis of programmed cell death during mouse cardiac septation

Cell death is thought to play an important role in mammalian cardiogenesis, although a precise map of its distribution during the crucial period of cardiac septation has so far been lacking. In this study, the spatiotemporal distribution of programmed cell death (PCD) during mouse cardiac septation is described between embryonic days 10.5 and 13.5. Two types of foci of cell death can be demonstrated in the developing heart. Those with high-intensity, with a PCD index greater than 1%, are clearly visible on individual TUNEL-assayed sections. Low-intensity foci, with a PCD index of less than 1%, become visible only following summation of data. High-intensity foci occur exclusively within the endocardial cushions of the outflow tract and atrioventricular region, appearing at the 52-54 somite stage (late E11.5), concomitant with the formation of the central mesenchymal mass. Low-intensity foci are present throughout the period of cardiac development from E10.5 to E13.5 and are frequently localized to regions of septation, such as the muscular ventricular septum and the mesenchymal cap of the primary atrial septum. Expression of Fas and FasL corresponds to these low-intensity foci, but not those with high-intensity, suggesting that activation of this death receptor may be specifically involved in molecular control of the low-intensity foci.

Teratogenic effects of bis-diamine on the developing myocardium

BACKGROUND

Bis-diamine induces conotruncal anomalies and disproportional ventricular development in rat embryos when administered to the mother. To evaluate the mechanisms of disproportional ventricular development in the anomalous heart, we analyzed the morphology of the embryonic heart and investigated cardiomyocytic DNA synthesis and apoptosis.

METHODS

A single dose of 200 mg of bis-diamine was administered to pregnant rats Wistar on day 9.5 of pregnancy. The embryos were removed on each embryonic day from 10.5 to 18.5. Expression of cardiotrophin-1 and hepatocyte growth factor was investigated on the sections, and cardiotrophin-1, hepatocyte growth factor and myocyte enhancer factor 2 mRNA expression was examined by reverse transcriptase-polymerase chain reaction. Myocardial DNA synthesis was investigated using 5-bromo-2'-deoxyuridine and the labeling index was calculated for each heart. Apoptosis was also analyzed using TUNEL reaction and electrophoresis of DNA fragmentation.

RESULTS

The embryos treated with bis-diamine had conotruncal anomalies associated with thin left ventricular wall in the later stage. The labeling index on embryonic day 15.5 and 16.5 was significantly lower than those in the controls. Hepatocyte growth factor and cardiotrophin-1 mRNA expression was upregulated on embryonic day 12.5 and 15.5 in bis-diamine-treated hearts. Fewer apoptotic cells were detected in the hearts of bis-diamine-treated embryos than in control hearts from embryonic day 14.5 to 16.5.

CONCLUSIONS

The ventricular disproportion in the bis-diamine-treated heart may be caused by the early myocardial differentiation delay and poor proliferation and reduced apoptosis associated with anomalous circulatory condition in the later stage.

Hypoxia-responsive signaling regulates the apoptosis-dependent remodeling of the embryonic avian cardiac outflow tract

We proposed a model in which myocardial hypoxia triggers the apoptosis-dependent remodeling of the avian outflow tract (OFT) in the transition of the embryo to a dual circulation. In this study, we examined hypoxia-dependent signaling in cardiomyocyte apoptosis and outflow tract remodeling. The hypoxia-inducible transcription factor HIF-1alpha was specifically present in the nuclei of OFT cardiomyocytes from stages 25-32, the period of hypoxia-dependent OFT remodeling. HIF-1alpha expression was sensitive to changes in ambient oxygen concentrations, while its dimerization partner HIF-1beta was constitutively expressed. There was not a simple relationship between HIF-1alpha expression and apoptosis. Apoptotic cardiomyocytes were detected in HIF-1alpha-positive and -negative regions, and a hypoxic stimulus sufficient to induce nuclear accumulation of HIF-1alpha did not induce cardiomyocyte apoptosis. The hypoxia-dependent expression of the vascular endothelial growth factor receptor (VEGFR2) in the distal OFT myocardium may be protective as cardiomyocyte apoptosis in the early stages (25-30) of OFT remodeling was absent from this region. Furthermore, recombinant adenoviral-mediated expression of dominant negative Akt, an inhibitor of tyrosine kinase receptor signaling, augmented cardiomyocyte apoptosis in the OFT and constitutively active Akt suppressed it. Adenovirus-mediated forced expression of VEGF165 induced conotruncal malformation such as double outlet right ventricle (DORV) and ventricular septal defect (VSD), similar to defects observed when apoptosis-dependent remodeling of the OFT was specifically targeted. We conclude that normal developmental remodeling of the embryonic avian cardiac OFT involves hypoxia/HIF-1-dependent signaling and cardiomyocyte apoptosis. Autocrine signaling through VEGF/VEGFR2 and Akt provides survival signals for the hypoxic OFT cardiomyocytes, and regulated VEGF signaling is required for the normal development of the OFT.

The Development of the Embryonic Outflow Tract Provides Novel Insights into Cardiac Differentiation and Remodeling

The embryonic cardiac outflow tract (OFT) connects the developing ventricles with the aortic sac. In birds and mammals, OFT cardiomyocytes are generated from a "secondary (anterior)," heart-forming field well after the formation of the primitive heart tube. The OFT cardiomyocytes have unique properties and developmental fates as compared with the myocytes of the atrial and ventricular chambers. Many of the OFT cardiomyocytes of the avian embryo are eliminated by programmed cell death (PCD) during OFT remodeling in the transition from a single- to a dual-series circulation. Targeted PCD gain and loss-of-function studies indicate that PCD drives the shortening and rotation of the OFT required for the aorta and pulmonary artery to connect with the left and right ventricles, respectively. Defects in this process model aspects of the relatively common and often life-threatening congenital human conotruncal heart defects. Using indicators of tissue hypoxia, we suggest that OFT myocardial hypoxia may be the trigger for the PCD-dependent remodeling of the OFT. This review discusses these aspects of the formation and remodeling of the embryonic OFT in the context of the broader questions of cardiac muscle biology.

Role of myocardial hypoxia in the remodeling of the embryonic avian cardiac outflow tract

The embryonic cardiac outflow myocardium originates from a secondary heart-forming field to connect the developing ventricles with the aortic sac. The outflow tract (OFT) subsequently undergoes complex remodeling in the transition of the embryo to a dual circulation. In avians, elimination of OFT cardiomyocytes by apoptosis (stages 25-32) precedes coronary vasculogenesis and is necessary for the shortening of the OFT and the posterior rotation of the aorta. We hypothesized that regional myocardial hypoxia triggers OFT remodeling. We used immunohistochemical detection of the nitroimidazole EF5, administered by intravascular infusion in ovo, as an indicator of relative tissue oxygen concentrations. EF5 binding was increased in the OFT myocardium relative to other myocardium during these stages (25-32) of OFT remodeling. The intensity of EF5 binding paralleled the prevalence of apoptosis in the OFT myocardium, which are first detected at stage 25, maximal at stage 30, and diminished by stage 32. Evidence of coincident hypoxia-dependent responses included the expression of the vascular endothelial growth factor (VEGF) receptor 2 by the OFT myocardium, the predominant expression of VEGF122 (diffusible) isoform in the OFT, and the recruitment of QH1-positive pro-endothelial cells to the OFT and vasculogenesis. Exposure of embryos to hyperoxia (95% O(2)/5% CO(2)) during this developmental window reduced the prevalence of cardiomyocyte apoptosis and attenuated the shortening and rotation of the OFT, resulting in double-outlet right ventricle morphology, similar to that observed when apoptosis is directly inhibited. These results suggest that regional myocardial hypoxia triggers cardiomyocyte apoptosis and remodeling of the OFT in the transition to a dual circulation, and that VEGF autocrine/paracrine signaling may regulate these processes.

Fas ligand gene transfer to the embryonic heart induces programmed cell death and outflow tract defects

The remodeling of the embryonic avian cardiac outflow tract (OFT) involves the removal of cardiomyocytes by programmed cell death (PCD). In contrast, the prevalence of PCD is low in the atrial or ventricular myocytes during this period of development. To determine if this selective PCD is due to the unique ability of the OFT cardiomyocytes to execute PCD, we transduced the embryonic chicken heart in ovo with recombinant adenovirus expressing a death (FasL) ligand. This resulted in programmed cell death in atrial, ventricular, and OFT cardiomyocytes as evidenced by chromosomal fragmentation, accumulation of lysosomes, and Caspase enzymatic activity. Consistent with the widespread induction of PCD, transcripts for the Fas receptor were detected in all chambers of the heart throughout development. The precocious and widespread activation of PCD in the OFT myocardium resulted in a marked dimunition of the subpulmonic myocardial infundibulum, and transposition of the aorta side-by-side with the pulmonary artery and connecting to the right ventricle. Defects in other cardiac structures are also described. We conclude that the regulated removal of OFT cardiomyocytes by PCD is required for the great vessels to make their proper connections with the ventricles in the transition to a dual circulation. The malalignment of the great vessels described in this animal model are similar to those described in congenital human conotruncal heart defects, suggesting that PCD-dependent remodeling of the OFT myocardium could be a target of genetic mutations or teratogens that cause human conotruncal heart defects.

Neural crest stem cell and cardiac endothelium defects in the TrkC null mouse

TrkC null mice have multiple cardiac malformations. Since neural crest cells participate in cardiac outflow tract septation, the aim of this study was to determine at the cellular level the putative neural crest defect. We have identified three types of progenitor cells: stem cells that undergo self-renewal and can generate many cell types, cells that are restricted in their developmental potentials, and cells that are committed to the smooth muscle cell lineage. In TrkC null mice, there is a greater than 50% decrease in stem cell numbers and an equivalent increase in fate-restricted cells. The outflow tract wall is thickened and the endothelial tube is disorganized. We conclude that deletion of the TrkC gene causes precocious fate restrictions of the neural crest stem cell and a defect of the outflow tract endothelium, both of which may contribute to the outflow tract malformations that occur in TrkC null mice.

Making more heart muscle

Postnatally, heart muscle cells almost completely lose their ability to divide, which makes their loss after trauma irreversible. Potential repair by cell grafting or mobilizing endogenous cells is of particular interest for possible treatments for heart disease, where the poor capacity for cardiomyocyte proliferation probably contributes to the irreversibility of heart failure. Knowledge of the molecular mechanisms that underly formation of heart muscle cells might provide opportunities to repair the diseased heart by induction of (trans) differentiation of endogenous or exogenous cells into heart muscle cells. We briefly review the molecular mechanisms involved in early development of the linear heart tube by differentiation of mesodermal cells into heart muscle cells. Because the initial heart tube does not comprise all the cardiac compartments present in the adult heart, heart muscle cells are added to the distal borders of the tube and within the tube. At both distal borders, mesodermal cell are recruited into the cardiac lineage and, within the heart tube, muscular septa are formed. In this review, the relative late additions of heart muscle cells to the linear heart tube are described and the potential underlying molecular mechanisms are discussed.

Dynamic patterns of apoptosis in the developing chicken heart

The outflow tract (OFT) is abnormal in many congenital heart defects. One critical mechanism for morphogenesis of this complex structure is apoptosis. Chicken embryos (stages 19-38; ED4-10) stained with a fluorescent supravital lysosomal dye (LysoTracker Red; LTR) revealed the three-dimensional relationship between structural changes and apoptosis. The LTR staining peaked in the OFT myocardium at stages 27-32, consistent with our previous analyses using other apoptosis assays. While LTR stained under both the pulmonary artery and the aorta, it was most prevalent in the subaortic myocardium before its elimination. Furthermore, LTR staining was most abundant in the myocardium under intensely cytokeratin-positive, thick epicardium. These data support the hypothesis that temporally and spatially restricted apoptosis in the OFT myocardium allows the aorta and pulmonary artery to dock at the appropriate angle and level with the proper ventricle. These data also support a relationship between the differentiating epicardium and cardiomyocyte apoptosis.

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.

Epicardial-like cells on the distal arterial end of the cardiac outflow tract do not derive from the proepicardium but are derivatives of the cephalic pericardium

A series of recent studies strongly suggests that the myocardium of the cardiac distal outflow tract (d-OFT) does not derive from the original precardiac mesoderm but, instead, differentiates from a so-called anterior heart field. Similar findings were also reported for the endocardium of the d-OFT. However, very little information is available on the origin of the epicardium of the OFT. To address this issue, we have performed a study in which we have combined experimental in vivo and in vitro techniques (construction of proepicardial chimeras, proepicardial ablation, OFT insertion of eggshell membrane pieces, and culture on collagen gels) with molecular characterization techniques to determine this origin and define the properties of d-OFT epicardium compared with proepicardially derived epicardium. Our results demonstrate that the coelomic/pericardial epithelium in the vicinity of the aortic sac (and not the proepicardium) is the origin of d-OFT epicardium. This "pericardially" derived epicardium and the proepicardially derived epicardial tissues differ in their morphologic appearance, gene-expression profile, and in their ability to undergo epithelial-to-mesenchymal transformation. We conclude that the heterogeneity in the epicardial cell population of the OFT could be a factor in the complex developmental remodeling events at the arterial pole of the heart.

Septation and separation within the outflow tract of the developing heart

The developmental anatomy of the ventricular outlets and intrapericardial arterial trunks is a source of considerable confusion. First, major problems exist because of the multiple names and definitions used to describe this region of the heart as it develops. Second, there is no agreement on the boundaries of the described components, nor on the number of ridges or cushions to be found dividing the outflow tract, and the pattern of their fusion. Evidence is also lacking concerning the role of the fused cushions relative to that of the so-called aortopulmonary septum in separating the intrapericardial components of the great arterial trunks. In this review, we discuss the existing problems, as we see them, in the context of developmental and postnatal morphology. We concentrate, in particular, on the changes in the nature of the wall of the outflow tract, which is initially myocardial throughout its length. Key features that, thus far, do not seem to have received appropriate attention are the origin, and mode of separation, of the intrapericardial portions of the arterial trunks, and the formation of the walls of the aortic and pulmonary valvar sinuses. Also as yet undetermined is the formation of the free-standing muscular subpulmonary infundibulum, the mechanism of its separation from the aortic valvar sinuses, and its differentiation, if any, from the muscular ventricular outlet septum.

Sculpting the cardiac outflow tract

The cardiac outflow tract is the site of anomalies that affect a substantial proportion of individuals with congenital heart defects. The morphogenesis of this site is complex, and requires coordinated development of many cell types and tissues. It is therefore not surprising that developmental mistakes arise here, and that the steps and mechanisms of morphogenesis are still controversial and poorly understood, despite advances in molecular techniques. Recent findings have provided new insight into mechanisms of outflow tract morphogenesis, including clarification of its origins and the fate of cardiomyocytes, as well as invading cell populations. Application of new and old techniques and a wide range of approaches to tackle the unanswered questions about the outflow tract calls for collaboration among investigators from different disciplines including anatomists, physiologists, and molecular biologists.