Islet 1 is expressed in distinct cardiovascular lineages, including pacemaker and coronary vascular cells

Genetic dissection of Down syndrome-associated congenital heart defects using a new mouse mapping panel

Down syndrome (DS), caused by trisomy of human chromosome 21 (Hsa21), is the most common cause of congenital heart defects (CHD), yet the genetic and mechanistic causes of these defects remain unknown. To identify dosage-sensitive genes that cause DS phenotypes, including CHD, we used chromosome engineering to generate a mapping panel of 7 mouse strains with partial trisomies of regions of mouse chromosome 16 orthologous to Hsa21. Using high-resolution episcopic microscopy and three-dimensional modeling we show that these strains accurately model DS CHD. Systematic analysis of the 7 strains identified a minimal critical region sufficient to cause CHD when present in 3 copies, and showed that it contained at least two dosage-sensitive loci. Furthermore, two of these new strains model a specific subtype of atrio-ventricular septal defects with exclusive ventricular shunting and demonstrate that, contrary to current hypotheses, these CHD are not due to failure in formation of the dorsal mesenchymal protrusion.

Down syndrome is a condition caused by having an extra copy of one of the 46 chromosomes found inside human cells. Specifically, instead of two copies, people with Down syndrome are born with three copies of chromosome 21. This results in many different effects, including learning and memory problems, heart defects and Alzheimer’s disease. Each of these different effects is caused by having a third copy of one or more of the approximately 230 genes found on chromosome 21. However, it is not known which of these genes cause any of these effects, and how an extra copy of the genes results in such changes.

Now, Lana-Elola et al. have investigated which genes on chromosome 21 cause the heart defects seen in Down syndrome, and how those heart defects come about. This involved engineering a new strain of mouse that has an extra copy of 148 mouse genes that are very similar to 148 genes found on chromosome 21 in humans. Like people with Down syndrome, this mouse strain developed heart defects when it was an embryo.

Using a series of six further mouse strains, Lana-Elola et al. then narrowed down the potential genes that, when in three copies, are needed to cause the heart defects, to a list of just 39 genes. Further experiments then showed that at least two genes within these 39 genes were required in three copies to cause the heart defects.

The next step will be to identify the specific genes that actually cause the heart defects, and then work out how a third copy of these genes causes the developmental problems.

cKit+ cardiac progenitors of neural crest origin

The degree to which cKit-expressing progenitors generate cardiomyocytes in the heart is controversial. Genetic fate-mapping studies suggest minimal contribution; however, whether or not minimal contribution reflects minimal cardiomyogenic capacity is unclear because the embryonic origin and role in cardiogenesis of these progenitors remain elusive. Using high-resolution genetic fate-mapping approaches with cKit(CreERT2/+) and Wnt1::Flpe mouse lines, we show that cKit delineates cardiac neural crest progenitors (CNC(kit)). CNC(kit) possess full cardiomyogenic capacity and contribute to all CNC derivatives, including cardiac conduction system cells. Furthermore, by modeling cardiogenesis in cKit(CreERT2)-induced pluripotent stem cells, we show that, paradoxically, the cardiogenic fate of CNC(kit) is regulated by bone morphogenetic protein antagonism, a signaling pathway activated transiently during establishment of the cardiac crescent, and extinguished from the heart before CNC invasion. Together, these findings elucidate the origin of cKit(+) cardiac progenitors and suggest that a nonpermissive cardiac milieu, rather than minimal cardiomyogenic capacity, controls the degree of CNC(kit) contribution to myocardium.

Histone Deacetylase 3 Coordinates Deacetylase-independent Epigenetic Silencing of Transforming Growth Factor-β1 (TGF-β1) to Orchestrate Second Heart Field Development

About two-thirds of human congenital heart disease involves second heart field-derived structures. Histone-modifying enzymes, histone deacetylases (HDACs), regulate the epigenome; however, their functions within the second heart field remain elusive. Here we demonstrate that histone deacetylase 3 (HDAC3) orchestrates epigenetic silencing of Tgf-β1, a causative factor in congenital heart disease pathogenesis, in a deacetylase-independent manner to regulate development of second heart field-derived structures. In murine embryos lacking HDAC3 in the second heart field, increased TGF-β1 bioavailability is associated with ascending aortic dilatation, outflow tract malrotation, overriding aorta, double outlet right ventricle, aberrant semilunar valve development, bicuspid aortic valve, ventricular septal defects, and embryonic lethality. Activation of TGF-β signaling causes aberrant endothelial-to-mesenchymal transition and altered extracellular matrix homeostasis in HDAC3-null outflow tracts and semilunar valves, and pharmacological inhibition of TGF-β rescues these defects. HDAC3 recruits components of the PRC2 complex, methyltransferase EZH2, EED, and SUZ12, to the NCOR complex to enrich trimethylation of Lys-27 on histone H3 at the Tgf-β1 regulatory region and thereby maintains epigenetic silencing of Tgf-β1 specifically within the second heart field-derived mesenchyme. Wild-type HDAC3 or catalytically inactive HDAC3 expression rescues aberrant endothelial-to-mesenchymal transition and epigenetic silencing of Tgf-β1 in HDAC3-null outflow tracts and semilunar valves. These findings reveal that epigenetic dysregulation within the second heart field is a predisposing factor for congenital heart disease.

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.

Hoxb1 regulates proliferation and differentiation of second heart field progenitors in pharyngeal mesoderm and genetically interacts with Hoxa1 during cardiac outflow tract development

Outflow tract (OFT) anomalies are among the most common congenital heart defects found at birth. The embryonic OFT grows by the progressive addition of cardiac progenitors, termed the second heart field (SHF), which originate from splanchnic pharyngeal mesoderm. Development of the SHF is controlled by multiple intercellular signals and transcription factors; however the relationship between different SHF regulators remains unclear. We have recently shown that Hoxa1 and Hoxb1 are expressed in a sub-population of the SHF contributing to the OFT. Here, we report that Hoxb1 deficiency results in a shorter OFT and ventricular septal defects (VSD). Mechanistically, we show that both FGF/ERK and BMP/SMAD signaling, which regulate proliferation and differentiation of cardiac progenitor cells and OFT morphogenesis, are enhanced in the pharyngeal region in Hoxb1 mutants. Absence of Hoxb1 also perturbed SHF development through premature myocardial differentiation. Hence, the positioning and remodeling of the mutant OFT is disrupted. Hoxa1(-/-) embryos, in contrast, have low percentage of VSD and normal SHF development. However, compound Hoxa1(-/-); Hoxb1(+/-) embryos display OFT defects associated with premature SHF differentiation, demonstrating redundant roles of these factors during OFT development. Our findings provide new insights into the gene regulatory network controlling SHF and OFT formation.

Transcription factor ISL1 is essential for pacemaker development and function

The sinoatrial node (SAN) maintains a rhythmic heartbeat; therefore, a better understanding of factors that drive SAN development and function is crucial to generation of potential therapies, such as biological pacemakers, for sinus arrhythmias. Here, we determined that the LIM homeodomain transcription factor ISL1 plays a key role in survival, proliferation, and function of pacemaker cells throughout development. Analysis of several Isl1 mutant mouse lines, including animals harboring an SAN-specific Isl1 deletion, revealed that ISL1 within SAN is a requirement for early embryonic viability. RNA-sequencing (RNA-seq) analyses of FACS-purified cells from ISL1-deficient SANs revealed that a number of genes critical for SAN function, including those encoding transcription factors and ion channels, were downstream of ISL1. Chromatin immunoprecipitation assays performed with anti-ISL1 antibodies and chromatin extracts from FACS-purified SAN cells demonstrated that ISL1 directly binds genomic regions within several genes required for normal pacemaker function, including subunits of the L-type calcium channel, Ank2, and Tbx3. Other genes implicated in abnormal heart rhythm in humans were also direct ISL1 targets. Together, our results demonstrate that ISL1 regulates approximately one-third of SAN-specific genes, indicate that a combination of ISL1 and other SAN transcription factors could be utilized to generate pacemaker cells, and suggest ISL1 mutations may underlie sick sinus syndrome.

"String theory" of c-kit(pos) cardiac cells: a new paradigm regarding the nature of these cells that may reconcile apparently discrepant results

Although numerous preclinical investigations have consistently demonstrated salubrious effects of c-kit(pos) cardiac cells administered after myocardial infarction, the mechanism of action remains highly controversial. We and others have found little or no evidence that these cells differentiate into mature functional cardiomyocytes, suggesting paracrine effects. In this review, we propose a new paradigm predicated on a comprehensive analysis of the literature, including studies of cardiac development; we have (facetiously) dubbed this conceptual construct "string theory" of c-kit(pos) cardiac cells because it reconciles multifarious and sometimes apparently discrepant results. There is strong evidence that, during development, the c-kit receptor is expressed in different pools of cardiac progenitors (some capable of robust cardiomyogenesis and others with little or no contribution to myocytes). Accordingly, c-kit positivity, in itself, does not define the embryonic origins, lineage capabilities, or differentiation capacities of specific cardiac progenitors. C-kit(pos) cells derived from the first heart field exhibit cardiomyogenic potential during development, but these cells are likely depleted shortly before or after birth. The residual c-kit(pos) cells found in the adult heart are probably of proepicardial origin, possess a mesenchymal phenotype (resembling bone marrow mesenchymal stem/stromal cells), and are capable of contributing significantly only to nonmyocytic lineages (fibroblasts, smooth muscle cells, and endothelial cells). If these 2 populations (first heart field and proepicardium) express different levels of c-kit, the cardiomyogenic potential of first heart field progenitors might be reconciled with recent results of c-kit(pos) cell lineage tracing studies. The concept that c-kit expression in the adult heart identifies epicardium-derived, noncardiomyogenic precursors with a mesenchymal phenotype helps to explain the beneficial effects of c-kit(pos) cell administration to ischemically damaged hearts despite the observed paucity of cardiomyogenic differentiation of these cells.

Casz1 is required for cardiomyocyte G1-to-S phase progression during mammalian cardiac development

Organ growth occurs through the integration of external growth signals during the G1 phase of the cell cycle to initiate DNA replication. Although numerous growth factor signals have been shown to be required for the proliferation of cardiomyocytes, genetic studies have only identified a very limited number of transcription factors that act to regulate the entry of cardiomyocytes into S phase. Here, we report that the cardiac para-zinc-finger protein CASZ1 is expressed in murine cardiomyocytes. Genetic fate mapping with an inducible Casz1 allele demonstrates that CASZ1-expressing cells give rise to cardiomyocytes in the first and second heart fields. We show through the generation of a cardiac conditional null mutation that Casz1 is essential for the proliferation of cardiomyocytes in both heart fields and that loss of Casz1 leads to a decrease in cardiomyocyte cell number. We further report that the loss of Casz1 leads to a prolonged or arrested S phase, a decrease in DNA synthesis, an increase in phospho-RB and a concomitant decrease in the cardiac mitotic index. Taken together, these studies establish a role for CASZ1 in mammalian cardiomyocyte cell cycle progression in both the first and second heart fields.

Head muscle development

The developmental paths that lead to the formation of skeletal muscles in the head are distinct from those operating in the trunk. Craniofacial muscles are associated with head and neck structures. In the embryo, these structures derive from distinct mesoderm populations. Distinct genetic programs regulate different groups of muscles within the head to generate diverse muscle specifications. Developmental and lineage studies in vertebrates and invertebrates demonstrated an overlap in progenitor populations derived from the pharyngeal mesoderm that contribute to certain head muscles and the heart. These studies reveal that the genetic program controlling pharyngeal muscles overlaps with that of the heart. Indeed cardiac and craniofacial birth defects are often linked. Recent studies suggest that early chordates, the last common ancestor of tunicates and vertebrates, had an ancestral pharyngeal mesoderm lineage that later during evolution gave rise to both heart and craniofacial structures. This chapter summarizes studies related to the origins, signaling, genetics, and evolution of the head musculature, highlighting its heterogeneous characteristics in all these aspects.

The sinus venosus myocardium contributes to the atrioventricular canal: potential role during atrioventricular node development?

The presence of distinct electrophysiological pathways within the atrioventricular node (AVN) is a prerequisite for atrioventricular nodal reentrant tachycardia to occur. In this study, the different cell contributions that may account for the anatomical and functional heterogeneity of the AVN were investigated. To study the temporal development of the AVN, the expression pattern of ISL1, expressed in cardiac progenitor cells, was studied in sequential stages performing co-staining with myocardial markers (TNNI2 and NKX2-5) and HCN4 (cardiac conduction system marker). An ISL1+/TNNI2+/HCN4+ continuity between the myocardium of the sinus venosus and atrioventricular canal was identified in the region of the putative AVN, which showed a pacemaker-like phenotype based on single cell patch-clamp experiments. Furthermore, qPCR analysis showed that even during early development, different cell populations can be identified in the region of the putative AVN. Fate mapping was performed by in ovo vital dye microinjection. Embryos were harvested and analysed 24 and 48 hrs post-injection. These experiments showed incorporation of sinus venosus myocardium in the posterior region of the atrioventricular canal. The myocardium of the sinus venosus contributes to the atrioventricular canal. It is postulated that the myocardium of the sinus venosus contributes to nodal extensions or transitional cells of the AVN since these cells are located in the posterior region of the AVN. This finding may help to understand the origin of atrioventricular nodal reentrant tachycardia.

Programming and reprogramming a human heart cell

The latest discoveries and advanced knowledge in the fields of stem cell biology and developmental cardiology hold great promise for cardiac regenerative medicine, enabling researchers to design novel therapeutic tools and approaches to regenerate cardiac muscle for diseased hearts. However, progress in this arena has been hampered by a lack of reproducible and convincing evidence, which at best has yielded modest outcomes and is still far from clinical practice. To address current controversies and move cardiac regenerative therapeutics forward, it is crucial to gain a deeper understanding of the key cellular and molecular programs involved in human cardiogenesis and cardiac regeneration. In this review, we consider the fundamental principles that govern the "programming" and "reprogramming" of a human heart cell and discuss updated therapeutic strategies to regenerate a damaged heart.

Insights into cardiac conduction system formation provided by HCN4 expression

Specialized myocytes of the cardiac conduction system (CCS) are essential to coordinate sequential contraction of cardiac atria and ventricles. Anomalies of the CCS can result in lethal cardiac arrhythmias, including sick sinus syndrome and atrial or ventricular fibrillation. To develop future therapies and regenerative medicine aimed at cardiac arrhythmias, it is important to understand formation and function of distinct components of the CCS. Essential to this understanding is the development of CCS-specific markers. In this review, we briefly summarize available mouse models of CCS markers and focus on those involving the hyperpolarization cation-selective nucleotide-gated cation channel, HCN4, which selectively marks all components of the specialized CCS in adult heart. Recent studies have revealed, however, that HCN4 expression during development is highly dynamic in cardiac precursors. These studies have offered insights into the contributions of the first and second heart field to myocyte and conduction system lineages and suggested the timing of allocation of specific conduction system precursors during development. Altogether, they have highlighted the utility of HCN4 as a cell surface marker for distinct components of the CCS at distinct stages of development, which can be utilized to facilitate purification and characterization of CCS precursors in mouse and human model systems and pave the way for regenerative therapies.

Vangl2-regulated polarisation of second heart field-derived cells is required for outflow tract lengthening during cardiac development

Planar cell polarity (PCP) is the mechanism by which cells orient themselves in the plane of an epithelium or during directed cell migration, and is regulated by a highly conserved signalling pathway. Mutations in the PCP gene Vangl2, as well as in other key components of the pathway, cause a spectrum of cardiac outflow tract defects. However, it is unclear why cells within the mesodermal heart tissue require PCP signalling. Using a new conditionally floxed allele we show that Vangl2 is required solely within the second heart field (SHF) to direct normal outflow tract lengthening, a process that is required for septation and normal alignment of the aorta and pulmonary trunk with the ventricular chambers. Analysis of a range of markers of polarised epithelial tissues showed that in the normal heart, undifferentiated SHF cells move from the dorsal pericardial wall into the distal outflow tract where they acquire an epithelial phenotype, before moving proximally where they differentiate into cardiomyocytes. Thus there is a transition zone in the distal outflow tract where SHF cells become more polarised, turn off progenitor markers and start to differentiate to cardiomyocytes. Membrane-bound Vangl2 marks the proximal extent of this transition zone and in the absence of Vangl2, the SHF-derived cells are abnormally polarised and disorganised. The consequent thickening, rather than lengthening, of the outflow wall leads to a shortened outflow tract. Premature down regulation of the SHF-progenitor marker Isl1 in the mutants, and accompanied premature differentiation to cardiomyocytes, suggests that the organisation of the cells within the transition zone is important for maintaining the undifferentiated phenotype. Thus, Vangl2-regulated polarisation and subsequent acquisition of an epithelial phenotype is essential to lengthen the tubular outflow vessel, a process that is essential for on-going cardiac morphogenesis.

Congenital heart defects are common, affecting almost 1% of all live births. Many of these affect the outflow region, where the aorta and pulmonary trunk connect with the main ventricular chambers. Congenital heart defects arise from disruption of normal developmental processes and can be modelled in mice. Thus, studying normal development, together with mouse mutants that develop heart malformations, should shed light on why these common anomalies arise. We have studied cardiac development in a mouse mutant for the Vangl2 gene, a key component of the planar cell polarity (PCP) pathway. This pathway controls the orientations of cells in epithelia and during directional cell migration. Here, we show that PCP signalling is required by cells derived from the second heart field, which forms the outflow tract walls. We show that in the absence of Vangl2, the cells within the distal outflow tract walls are non-polarised and disorganised. As a consequence the outflow tract is shortened and does not align properly with the ventricles. Thus, we show why disruption of a key PCP gene leads to outflow tract malformations. This is important for understanding heart development, but also more generally for understanding how PCP signalling regulates growth of tubular structures.

Cardiac stem cells: translation to human studies

The discovery of multiple classes of cardiac progenitor cells in the adult mammalian heart has generated hope for their use as a therapeutic in heart failure. However, successful results from animal models have not always yielded similar findings in human studies. Recent Phase I/II trials of c-Kit (SCIPIO) and cardiosphere-based (CADUCEUS) cardiac progenitor cells have demonstrated safety and some therapeutic efficacy. Gaps remain in our understanding of the origins, function and relationships between the different progenitor cell families, many of which are heterogeneous populations with overlapping definitions. Another challenge lies in the limitations of small animal models in replicating the human heart. Cryopreserved human cardiac tissue provides a readily available source of cardiac progenitor cells and may help address these questions. We review important findings and relative unknowns of the main classes of cardiac progenitor cells, highlighting differences between animal and human studies.

Assessment of DNA synthesis in Islet-1⁺ cells in the adult murine heart

RATIONALE

Islet-1 positive (Islet-1(+)) cardiac progenitor cells give rise to the right ventricle, atria and outflow tract during murine cardiac development. In the adult heart Islet-1 expression is limited to parasympathetic neurons, few cardiomyocytes, smooth muscle cells, within the proximal aorta and pulmonary artery and sinoatrial node cells. Its role in these cells is unknown. Here we tested the hypothesis that Islet-1(+) cells retain proliferative activity and may therefore play a role in regenerating specialized regions in the heart.

METHODS AND RESULTS

DNA synthesis was analyzed by the incorporation of tritiated thymidine ((3)H-thymidine) in Isl-1-nLacZ mice, a transgenic model with an insertion of a nuclear beta-galactosidase in the Islet-1 locus. Mice received daily injections of (3)H-thymidine for 5 days. DNA synthesis was visualized throughout the heart by dipping autoradiography of cryosections. Colocalization of an nLacZ-signal and silver grains would indicate DNA synthesis in Islet-1(+) cells. Whereas Islet(-) non-myocyte nuclei were regularly marked by accumulation of silver grains, colocalization with nLacZ-signals was not detected in >25,000 cells analyzed.

CONCLUSIONS

Islet-1(+) cells are quiescent in the adult heart, suggesting that, under normal conditions, even pacemaking cells do not proliferate at higher rates than normal cardiac myocytes.

How to make a heart valve: from embryonic development to bioengineering of living valve substitutes

Cardiac valve disease is a significant cause of ill health and death worldwide, and valve replacement remains one of the most common cardiac interventions in high-income economies. Despite major advances in surgical treatment, long-term therapy remains inadequate because none of the current valve substitutes have the potential for remodeling, regeneration, and growth of native structures. Valve development is coordinated by a complex interplay of signaling pathways and environmental cues that cause disease when perturbed. Cardiac valves develop from endocardial cushions that become populated by valve precursor mesenchyme formed by an epithelial-mesenchymal transition (EMT). The mesenchymal precursors, subsequently, undergo directed growth, characterized by cellular compartmentalization and layering of a structured extracellular matrix (ECM). Knowledge gained from research into the development of cardiac valves is driving exploration into valve biomechanics and tissue engineering directed at creating novel valve substitutes endowed with native form and function.

Irx4 identifies a chamber-specific cell population that contributes to ventricular myocardium development

BACKGROUND

The ventricular myocardium is the most prominent layer of the heart, and the most important for mediating cardiac physiology. Although the ventricular myocardium is critical for heart function, the cellular hierarchy responsible for ventricle-specific myocardium development remains unresolved.

RESULTS

To determine the pattern and time course of ventricular myocardium development, we investigated IRX4 protein expression, which has not been previously reported. We identified IRX4+ cells in the cardiac crescent, and these cells were positive for markers of the first or second heart fields. From the onset of chamber formation, IRX4+ cells were restricted to the ventricular myocardium. This expression pattern persisted into adulthood. Of interest, we observed that IRX4 exhibits developmentally regulated dynamic intracellular localization. Throughout prenatal cardiogenesis, and up to postnatal day 4, IRX4 was detected in the cytoplasm of ventricular myocytes. However, between postnatal days 5–6, IRX4 translocated to the nucleus of ventricular myocytes.

CONCLUSIONS

Given the ventricle-specific expression of Irx4 in later stages of heart development, we hypothesize that IRX4+ cells in the cardiac crescent represent the earliest cell population in the cellular hierarchy underlying ventricular myocardium development.

Early patterning and specification of cardiac progenitors in gastrulating mesoderm

Mammalian heart development requires precise allocation of cardiac progenitors. The existence of a multipotent progenitor for all anatomic and cellular components of the heart has been predicted but its identity and contribution to the two cardiac progenitor 'fields' has remained undefined. Here we show, using clonal genetic fate mapping, that Mesp1+ cells in gastrulating mesoderm are rapidly specified into committed cardiac precursors fated for distinct anatomic regions of the heart. We identify Smarcd3 as a marker of early specified cardiac precursors and identify within these precursors a compartment boundary at the future junction of the left and right ventricles that arises prior to morphogenesis. Our studies define the timing and hierarchy of cardiac progenitor specification and demonstrate that the cellular and anatomical fate of mesoderm-derived cardiac cells is specified very early. These findings will be important to understand the basis of congenital heart defects and to derive cardiac regeneration strategies.

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.

Nkx2-5 regulates cardiac growth through modulation of Wnt signaling by R-spondin3

A complex regulatory network of morphogens and transcription factors is essential for normal cardiac development. Nkx2-5 is among the earliest known markers of cardiac mesoderm that is central to the regulatory pathways mediating second heart field (SHF) development. Here, we have examined the specific requirements for Nkx2-5 in the SHF progenitors. We show that Nkx2-5 potentiates Wnt signaling by regulating the expression of the R-spondin3 (Rspo3) gene during cardiogenesis. R-spondins are secreted factors and potent Wnt agonists that in part regulate stem cell proliferation. Our data show that Rspo3 is markedly downregulated in Nkx2-5 mutants and that Rspo3 expression is regulated by Nkx2-5. Conditional inactivation of Rspo3 in the Isl1 lineage resulted in embryonic lethality secondary to impaired development of SHF. More importantly, we find that Wnt signaling is significantly attenuated in Nkx2-5 mutants and that enhancing Wnt/β-catenin signaling by pharmacological treatment or by transgenic expression of Rspo3 rescues the SHF defects in the conditional Nkx2-5(+/-) mutants. We have identified a previously unrecognized genetic link between Nkx2-5 and Wnt signaling that supports continued cardiac growth and proliferation during development. Identification of Rspo3 in cardiac development provides a new paradigm in temporal regulation of Wnt signaling by cardiac-specific transcription factors.

Cardiac cell lineages that form the heart

Myocardial cells ensure the contractility of the heart, which also depends on other mesodermal cell types for its function. Embryological experiments had identified the sources of cardiac precursor cells. With the advent of genetic engineering, novel tools have been used to reconstruct the lineage tree of cardiac cells that contribute to different parts of the heart, map the development of cardiac regions, and characterize their genetic signature. Such knowledge is of fundamental importance for our understanding of cardiogenesis and also for the diagnosis and treatment of heart malformations.

Regional innervation of the heart in the goldfish, Carassius auratus: a confocal microscopy study

The intracardiac nervous system represents the final common pathway for autonomic control of the vertebrate heart in maintaining cardiovascular homeostasis. In teleost fishes, details of the organization of this system are not well understood. Here we investigated innervation patterns in the heart of the goldfish, a species representative of a large group of cyprinids. We used antibodies against the neuronal markers zn-12, acetylated tubulin, and human neuronal protein C/D, as well as choline acetyltransferase, tyrosine hydroxylase, nitric oxide synthetase, and vasoactive intestinal polypeptide (VIP) to detect neural elements and their transmitter contents in wholemounts and sections of cardiac tissue. All chambers of the heart were innervated by choline acetyltransferase-positive axons, implying cholinergic regulation; and by tyrosine hydroxylase-containing axons, implying adrenergic regulation. The mean total number of intracardiac neurons was 713 ± 78 (SE), nearly half of which were cholinergic. Neuronal somata were mainly located in a ganglionated plexus around the sinoatrial valves. Somata were contacted by cholinergic, adrenergic, nitrergic, and VIP-positive terminals. Putative pacemaker cells, identified by immunoreactivity for hyperpolarization activated, cyclic nucleotide-gated channel 4, were located in the base of the sinoatrial valves, and this region was densely innervated by cholinergic and adrenergic terminals. We have shown that the goldfish heart possesses the necessary neuroanatomical substrate for fine, region-by-region autonomic control of the myocardial effectors that are involved in determining cardiac output.

Pulmonary endoderm, second heart field and the morphogenesis of distal outflow tract in mouse embryonic heart

The second heart field (SHF), foregut endoderm and sonic hedgehog (SHH) signaling pathway are all reported to associate with normal morphogenesis and septation of outflow tract (OFT). However, the morphological relationships of the development of foregut endoderm and expression of SHH signaling pathway members with the development of surrounding SHF and OFT are seldom described. In this study, serial sections of mouse embryos from ED9 to ED13 (midgestation) were stained with a series of marker antibodies for specifically highlighting SHF (Isl-1), endoderm (Foxa2), basement membrane (Laminin), myocardium (MHC) and smooth muscle (α-SMA) respectively, or SHH receptors antibodies including patched1 (Ptc1), patched2 (Ptc2) and smoothened, to observe the spatiotemporal relationship between them and their contributions to OFT morphogenesis. Our results demonstrated that the development of an Isl-1 positive field in the splanchnic mesoderm ventral to foregut, a subset of SHF, is closely coupled with pulmonary endoderm or tracheal groove, the Isl-1 positive cells surrounding pulmonary endoderm are distributed in a special cone-shaped pattern and take part in the formation of the lateral walls of the intrapericardial aorta and pulmonary trunk and the transient aortic-pulmonary septum, and Ptc1 and Ptc2 are exclusively expressed in pulmonary endoderm during this Isl-l positive field development, suggesting special roles played in inducing the Isl-l positive field formation by pulmonary endoderm. It is indicated that pulmonary endoderm plays a role in the development and specification of SHF in midgestation, and that pulmonary endoderm-associated Isl-l positive field is involved in patterning the morphogenesis and septation of the intrapericardial arterial trunks.

LIM homeodomain transcription factor Isl1 directs normal pyloric development by targeting Gata3

Background

Abnormalities in pyloric development or in contractile function of the pylorus cause reflux of duodenal contents into the stomach and increase the risk of gastric metaplasia and cancer. Abnormalities of the pyloric region are also linked to congenital defects such as the relatively common neonatal hypertrophic pyloric stenosis, and primary duodenogastric reflux. Therefore, understanding pyloric development is of great clinical relevance. Here, we investigated the role of the LIM homeodomain transcription factor Isl1 in pyloric development.

Results

Examination of Isl1 expression in developing mouse stomach by immunohistochemistry, whole mount in situ hybridization and real-time quantitative PCR demonstrated that Isl1 is highly expressed in developing mouse stomach, principally in the smooth muscle layer of the pylorus. Isl1 expression was also examined by immunofluorescence in human hypertrophic pyloric stenosis where the majority of smooth muscle cells were found to express Isl1. Isl1 function in embryonic stomach development was investigated utilizing a tamoxifen-inducible Isl1 knockout mouse model. Isl1 deficiency led to nearly complete absence of the pyloric outer longitudinal muscle layer at embryonic day 18.5, which is consistent with Gata3 null mouse phenotype. Chromatin immunoprecipitation, luciferase assays, and electrophoretic mobility shift assays revealed that Isl1 ensures normal pyloric development by directly targeting Gata3.

Conclusions

This study demonstrates that the Isl1-Gata3 transcription regulatory axis is essential for normal pyloric development. These findings are highly clinically relevant and may help to better understand pathways leading to pyloric disease.

Tissue engineering approaches to heart repair

The heart is a large organ containing many cell types, each of which is necessary for normal function. Because of this, cardiac regenerative medicine presents many unique challenges. Because each of the many types of cells within the heart has unique physiological and electrophysiological characteristics, donor cells must be well matched to the area of the heart into which they are grafted to avoid mechanical dysfunction or arrhythmia. In addition, grafted cells must be functionally integrated into host tissue to effectively repair cardiac function. Because of its size and physiological function, the metabolic needs of the heart are considerable. Therefore grafts must contain not only cardiomyocytes but also a functional vascular network to meet their needs for oxygen and nutrition. In this article we review progress in the use of pluripotent stem cells as a source of donor cardiomyocytes and highlight current unmet needs in the field. We also examine recent tissue engineering approaches integrating cells with various engineered materials that should address some of these unmet needs.

Heart fields: spatial polarity and temporal dynamics

In chick and mouse, heart fields undergo dynamic morphological spatiotemporal changes during heart tube formation. Here, the dynamic change in spatial polarity of such fields is discussed and a new perspective on the heart fields is proposed. The heart progenitor cells delaminate through the primitive streak and migrate in a semicircular trajectory craniolaterally forming the bilateral heart fields as part of the splanchnic mesoderm. They switch their polarity from anteroposterior to mediolateral. The anterior intestinal portal posterior descent inverts the newly formed heart field mediolateral polarity into lateromedial by 125° bending. The heart fields revert back to their original anteroposterior polarity and fuse at the midline forming a semi heart tube by completing their half circle movement. Several names and roles were assigned to different portions of the heart fields: posterior versus anterior, first versus second, and primary versus secondary heart field. The posterior and anterior heart fields define basically physical fields that form the inflow-outflow axis of the heart tube. The first and second heart fields are, in contrast, temporal fields of differentiating cardiomyocytes expressing myosin light chain 2a and undifferentiated and proliferating precardiac mesoderm expressing Isl1 gene, respectively. The two markers present a complementary pattern and are expressed transiently in all myocardial lineages. Thus, Isl1 is not restricted to a portion of the heart field or one of the two heart lineages as has been often assumed.

Islet-1 may function as an assistant factor for histone acetylation and regulation of cardiac development-related transcription factor Mef2c expression

Objective

Islet-1 is an important transcription factor for cardiac development through mediating extensive interactions between DNA and proteins. The present study was to investigate the role of Islet-1 in regulating the expression of cardiac development-related transcription factors and mechanism.

Methods and Results

The expression of Islet-1 and histone acetylases (HATs) subtype p300 was determined in newborn mouse hearts and mouse embryonic hearts at different development stages using Western blot. The expression of Islet-1 and cardiac development-related transcription factors Mef2c, GATA4 and Tbx5 as well as histone H3 acetylation level were determined in cardiac progenitor cells with and without transfection of Islet-1 interference RNA (RNAi) in lentivirus using PCR and Western blot. Islet-1 peak expression occurred on day E14.5 in mouse embryonic heart, and was present in the promoter regions of Mef2c, GATA4 and Tbx5 that were precipitated with p300 antibody. When Islet-1 was inhibited with specific RNAi in cardiac progenitor cells, the expression of Mef2c and Tbx5, but not GATA4, was significantly suppressed along with selective reduction in histone H3 acetylation in the promoter region of Mef2c, but not GATA4 and Tbx5. The level of Mef2c DNA, not GATA4 and Tbx5, in the complex associated with p300 was significantly decreased in the cells with Islet-1 knockdown.

Conclusions

These data suggested that Islet-1 might function as an assistant factor that was involved in the regulation of histone acetylation and Mef2c expression via assisting p300 on specifically targeting the promoter of Mef2c.

Common Expression of Stemness Molecular Markers and Early Cardiac Transcription Factors in Human Wharton's Jelly-Derived Mesenchymal Stem Cells and Embryonic Stem Cells

At present, there are still significant barriers that impede the clinical use of hESCs and iPS cells, including ethics, immunorejection, tumorigenesis from hESCs, and teratoma formation from iPS cells. It is therefore necessary to search for alternative sources of stem cells. WJ-MSCs originate from embryonic epiblasts and possess properties intermediate between hESCs and adult stem cells. However, the stemness properties of molecules in WJ-MSCs remain unclear compared to those of hESCs. In the present study, we isolated WJ-MSCs by a nonenzymatic method. Further, using microarray analysis by Affymetrix GeneChip and functional network analyses, we determined the degree of expression of stemness genes exhibited by the Human Stem Cell Pluripotency array. We also defined a wide range of stem cell gene expression in the WJ-MSCs in comparison with hESCs. At the same time, the definitive markers of early cardiac precursor cells and more committed progenitors were further characterized in WJ-MSCs. Our results demonstrated for the first time that WJ-MSCs had significant expression of undifferentiated human embryonic stem cell core markers, such as SOX2, NANOG, LIN28, SSEA1, SSEA3, SSEA4, KLF4, c-MYC, CRIPTO, and REX1, with a relatively lower level of expression than in hESCs. We also found WJ-MSCs have high expression of early cardiac transcription factors, such as Flk-1, Isl-1, and Nkx2.5. Functional analysis revealed signature genes of WJ-MSCs with specific roles involved in immune, cytoskeletal, and chemokine regulation, cell adhesion, and cell signaling. Our study indicated that there is a significant overlap between the stemness genes expressed by hESCs and WJ-MSCs. WJ-MSCs harbor a true stem cell population and are promising cells for stem cell-based therapies.

Driving vascular endothelial cell fate of human multipotent Isl1+ heart progenitors with VEGF modified mRNA

Distinct families of multipotent heart progenitors play a central role in the generation of diverse cardiac, smooth muscle and endothelial cell lineages during mammalian cardiogenesis. The identification of precise paracrine signals that drive the cell-fate decision of these multipotent progenitors, and the development of novel approaches to deliver these signals in vivo, are critical steps towards unlocking their regenerative therapeutic potential. Herein, we have identified a family of human cardiac endothelial intermediates located in outflow tract of the early human fetal hearts (OFT-ECs), characterized by coexpression of Isl1 and CD144/vWF. By comparing angiocrine factors expressed by the human OFT-ECs and non-cardiac ECs, vascular endothelial growth factor (VEGF)-A was identified as the most abundantly expressed factor, and clonal assays documented its ability to drive endothelial specification of human embryonic stem cell (ESC)-derived Isl1⁺ progenitors in a VEGF receptor-dependent manner. Human Isl1-ECs (endothelial cells differentiated from hESC-derived ISL1⁺ progenitors) resemble OFT-ECs in terms of expression of the cardiac endothelial progenitor- and endocardial cell-specific genes, confirming their organ specificity. To determine whether VEGF-A might serve as an in vivo cell-fate switch for human ESC-derived Isl1-ECs, we established a novel approach using chemically modified mRNA as a platform for transient, yet highly efficient expression of paracrine factors in cardiovascular progenitors. Overexpression of VEGF-A promotes not only the endothelial specification but also engraftment, proliferation and survival (reduced apoptosis) of the human Isl1⁺ progenitors in vivo. The large-scale derivation of cardiac-specific human Isl1-ECs from human pluripotent stem cells, coupled with the ability to drive endothelial specification, engraftment, and survival following transplantation, suggest a novel strategy for vascular regeneration in the heart.

Endo-MitoEGFP mice: a novel transgenic mouse with fluorescently marked mitochondria in microvascular endothelial cells

Blood vessel-specific fluorescent transgenic mice are excellent tools to study the development of the vasculature and angiogenic processes. There is growing interest in the biological processes relevant to endothelial cells but limited tools exist to selectively evaluate subcellular functions of this cell type in vivo. Here, we report a novel transgenic animal model that expresses mitochondrially targeted enhanced green fluorescent protein (EGFP) via the Hb9 promoter, a homeobox transcription factor with limited known involvement in the vasculature. Random integration of the transgene, containing the entire mouse Hb9 promoter, was found to be expressed in a variety of vascularised tissues. Further inspection revealed that Mito-EGFP localizes to the endothelial cells (ECs) of a subset of microvascular blood vessels, especially in the central nervous system (CNS), heart, spleen, thymus, lymph nodes and skin. We demonstrate the utility of this novel transgenic mouse, named Endo-MitoEGFP, in the detection, imaging, and isolation of microvascular ECs and evaluation of EC mitochondrial function isolated from adult animals. These transgenic mice will be useful to studies of ECs in development, physiology, and pathology.

Expression of Isl1 during mouse development

The LIM-homeodomain transcription factor Isl1 plays essential roles in cell proliferation, differentiation and survival during embryogenesis. To better visualize Isl1 expression and provide insight into the role of Isl1 during development, we generated an Isl1 nuclear LacZ (nLacZ) knockin mouse line. We have analyzed Isl1nlacZ expression during development by Xgal staining, and compared expression of Isl1nlacZ with endogenous Isl1 by coimmunostaining with antibodies to Isl1 and β-galactosidase. Results demonstrated that during development, Isl1 nLacZ is expressed in a pattern that recapitulates endogenous Isl1 protein expression. Consistent with previous in situ and immunohistochemistry data, we observed Isl1nlacZ expression in multiple tissues and cell types, including the central and peripheral nervous system, neural retina, inner ear, pharyngeal mesoderm and endoderm and their derivatives (craniofacial structures, thymus, thyroid gland and trachea), cardiovascular system (cardiac outflow tract, carotid arteries, umbilical vessels, sinoatrial node and atrial septum), gastrointestinal system (oral epithelium, stomach, pancreas, mesentery) and hindlimb.

Coordination of heart and lung co-development by a multipotent cardiopulmonary progenitor

Co-development of the cardiovascular and pulmonary systems is a recent evolutionary adaption to terrestrial life that couples cardiac output with the gas exchange function of the lung ¹. In this report, we show that the pulmonary vasculature develops even in the absence of lung development. We have identified a population of multi-potent cardiopulmonary mesoderm progenitors (CPPs) within the posterior pole of the heart that are marked by the expression of Wnt2/Gli1/Isl1. We show that CPPs arise from cardiac progenitors prior to lung development. Lineage tracing and clonal analysis demonstrates that CPPs generate the mesoderm lineages within the cardiac inflow tract and lung including cardiomyocytes, pulmonary vascular and airway smooth muscle, proximal vascular endothelium, and pericyte-like cells. CPPs are regulated by hedgehog expression from the foregut endoderm, which is required for connection of the pulmonary vasculature to the heart. Together, these studies identify a novel population of multipotent cardiopulmonary progenitors that coordinates heart and lung co-development that is required for adaptation to terrestrial existence.

Nkx2-5(+)islet1(+) mesenchymal precursors generate distinct spleen stromal cell subsets and participate in restoring stromal network integrity

Secondary lymphoid organ stromal cells comprise different subsets whose origins remain unknown. Herein, we exploit a genetic lineage-tracing approach to show that splenic fibroblastic reticular cells (FRCs), follicular dendritic cells (FDCs), marginal reticular cells (MRCs), and mural cells, but not endothelial cells, originate from embryonic mesenchymal progenitors of the Nkx2-5(+)Islet1(+) lineage. This lineage include embryonic mesenchymal cells with lymphoid tissue organizer (LTo) activity capable also of supporting ectopic lymphoid-like structures and a subset of resident spleen stromal cells that proliferate and regenerate the splenic stromal microenvironment following resolution of a viral infection. These findings identify progenitor cells that generate stromal diversity in spleen development and repair and suggest the existence of multipotent stromal progenitors in the adult spleen with regenerative capacity.

Preotic neural crest cells contribute to coronary artery smooth muscle involving endothelin signalling

Neural crest cells constitute a multipotent cell population that gives rise to diverse cell lineages. The neural crest arising from the postotic hindbrain is known as the 'cardiac' neural crest, and contributes to the great vessels and outflow tract endocardial cushions, but the neural crest contribution to structures within the heart remains largely controversial. Here we demonstrate that neural crest cells from the preotic region migrate into the heart and differentiate into coronary artery smooth muscle cells. Preotic neural crest cells preferentially distribute to the conotruncal region and interventricular septum. Ablation of the preotic neural crest causes abnormalities in coronary septal branch and orifice formation. Mice and chicks lacking endothelin signalling show similar abnormalities in the coronary artery, indicating its involvement in neural crest-dependent coronary artery formation. This is the first report that reveals the preotic neural crest contribution to heart development and smooth muscle heterogeneity within a coronary artery.

HCN4 dynamically marks the first heart field and conduction system precursors

RATIONALE

To date, there has been no specific marker of the first heart field to facilitate understanding of contributions of the first heart field to cardiac lineages. Cardiac arrhythmia is a leading cause of death, often resulting from abnormalities in the cardiac conduction system (CCS). Understanding origins and identifying markers of CCS lineages are essential steps toward modeling diseases of the CCS and for development of biological pacemakers.

OBJECTIVE

To investigate HCN4 as a marker for the first heart field and for precursors of distinct components of the CCS, and to gain insight into contributions of first and second heart lineages to the CCS.

METHODS AND RESULTS

HCN4CreERT2, -nuclear LacZ, and -H2BGFP mouse lines were generated. HCN4 expression was examined by means of immunostaining with HCN4 antibody and reporter gene expression. Lineage studies were performed using HCN4CreERT2, Isl1Cre, Nkx2.5Cre, and Tbx18Cre, coupled to coimmunostaining with CCS markers. Results demonstrated that, at cardiac crescent stages, HCN4 marks the first heart field, with HCN4CreERT2 allowing assessment of cell fates adopted by first heart field myocytes. Throughout embryonic development, HCN4 expression marked distinct CCS precursors at distinct stages, marking the entire CCS by late fetal stages. We also noted expression of HCN4 in distinct subsets of endothelium at specific developmental stages.

CONCLUSIONS

This study provides insight into contributions of first and second heart lineages to the CCS and highlights the potential use of HCN4 in conjunction with other markers for optimization of protocols for generation and isolation of specific conduction system precursors.

Resolving cell lineage contributions to the ventricular conduction system with a Cx40-GFP allele: a dual contribution of the first and second heart fields

BACKGROUND

The ventricular conduction system (VCS) coordinates the heartbeat and is composed of central components (the atrioventricular node, bundle, and right and left bundle branches) and a peripheral Purkinje fiber network. Conductive myocytes develop from common progenitor cells with working myocytes in a bimodal process of lineage restriction followed by limited outgrowth. The lineage relationship between progenitor cells giving rise to different components of the VCS is unclear.

RESULTS

Cell lineage contributions to different components of the VCS were analysed by a combination of retrospective clonal analysis, regionalized transgene expression studies, and genetic tracing experiments using Connexin40-GFP mice that precisely delineate the VCS. Analysis of a library of hearts containing rare large clusters of clonally related myocytes identifies two VCS lineages encompassing either the right Purkinje fiber network or left bundle branch. Both lineages contribute to the atrioventricular bundle and right bundle branch that segregate early from working myocytes. Right and left VCS lineages share the transcriptional program of the respective ventricular working myocytes and genetic tracing experiments discount alternate progenitor cell contributions to the VCS.

CONCLUSIONS

The mammalian VCS is comprised of cells derived from two lineages, supporting a dual contribution of first and second heart field progenitor cells.

Early mesodermal cues assign avian cardiac pacemaker fate potential in a tertiary heart field

Cardiac pacemaker cells autonomously generate electrical impulses that initiate and maintain the rhythmic contraction of the heart. Although the majority of heart cells are thought to originate from the primary and secondary heart fields, we found that chick pacemaker cells arise from a discrete region of mesoderm outside of these fields. Shortly after gastrulation, canonical Wnts promote the recruitment of mesodermal cells within this region into the pacemaker lineage. These findings suggest that cardiac pacemaker cells are physically segregated and molecularly programmed in a tertiary heart field prior to the onset of cardiac morphogenesis.

Antagonism between retinoic acid and fibroblast growth factor signaling during limb development

The vitamin A metabolite retinoic acid (RA) provides patterning information during vertebrate embryogenesis, but the mechanism through which RA influences limb development is unclear. During patterning of the limb proximodistal axis (upper limb to digits), avian studies suggest that a proximal RA signal generated in the trunk antagonizes a distal fibroblast growth factor (FGF) signal. However, mouse and zebrafish genetic studies suggest that loss of RA suppresses forelimb initiation. Here, using genetic and pharmacological approaches, we demonstrate that limb proximodistal patterning is not RA dependent, thus indicating that RA-FGF antagonism does not occur along the proximodistal axis of the limb. Instead, our studies show that RA-FGF antagonism acts prior to limb budding along the anteroposterior axis of the trunk lateral plate mesoderm to provide a patterning cue that guides formation of the forelimb field. These findings reconcile disparate ideas regarding RA-FGF antagonism and provide insight into how endogenous RA programs the early embryo.

A molecular and genetic outline of cardiac morphogenesis

Perturbations in cardiac development result in congenital heart disease, the leading cause of birth defect-related infant morbidity and mortality. Advances in cardiac developmental biology have significantly augmented our understanding of signalling pathways and transcriptional networks underlying heart formation. Cardiogenesis is initiated with the formation of mesodermal multipotent cardiac progenitor cells and is governed by cross-talk between developmental cues emanating from endodermal, mesodermal and ectodermal cells. The molecular and transcriptional machineries that direct the specification and differentiation of these cardiac precursors are part of an evolutionarily conserved programme that includes the Nkx-, Gata-, Hand-, T-box- and Mef2 family of transcription factors. Unravelling the hierarchical networks governing the fate and differentiation of cardiac precursors is crucial for our understanding of congenital heart disease and future stem cell-based and gene therapies. Recent molecular and genetic lineage analyses have revealed that subpopulations of cardiac progenitor cells follow distinctive specification and differentiation paths, which determine their final contribution to the heart. In the last decade, progenitor cells that contribute to the arterial pole and right ventricle have received much attention, as abnormal development of these cells frequently results in congenital defects of the aortic and pulmonary outlets, representing the most commonly occurring congenital cardiac defects. In this review, we provide an overview of the building plan of the vertebrate four-chambered heart, with a special focus on cardiac progenitor cell specification, differentiation and deployment during arterial pole development.

Second heart field and the development of the outflow tract in human embryonic heart

The second heart field (SHF) is indicated to contribute to the embryonic heart development. However, less knowledge is available about SHF development of human embryo due to the difficulty of collecting embryos. In this study, serial sections of human embryos from Carnegie stage 10 (CS10) to CS16 were stained with antibodies against Islet-1 (Isl-1), Nkx2.5, GATA4, myosin heavy chain (MHC) and α-smooth muscle actin (α-SMA) to observe spatiotemporal distribution of SHF and its contribution to the development of the arterial pole of cardiac tube. Our findings suggest that during CS10 to CS12, SHF of the human embryo is composed of the bilateral pharyngeal mesenchyme, the central mesenchyme of the branchial arch and splanchnic mesoderm of the pericardial cavity dorsal wall. With development, SHF translocates and consists of ventral pharyngeal mesenchyme and dorsal wall of the pericardial cavity. Hence, the SHF of human embryo shows a dynamic spatiotemporal distribution pattern. The formation of the Isl-1 positive condense cell prongs provides an explanation for the saddle structure formation at the distal pole of the outflow tract. In human embryo, the Isl-1 positive cells of SHF may contribute to the formation of myocardial outflow tract (OFT) and the septum during different development stages.

Islet1 is a direct transcriptional target of the homeodomain transcription factor Shox2 and rescues the Shox2-mediated bradycardia

The heart's rhythm is initiated and regulated by a group of specialized cells in the sinoatrial node (SAN), the primary pacemaker of the heart. Abnormalities in the development of the SAN can result in irregular heart rates (arrhythmias). Although several of the critical genes important for SAN formation have been identified, our understanding of the transcriptional network controlling SAN development remains at a relatively early stage. The homeodomain transcription factor Shox2 is involved in the specification and patterning of the SAN. While the Shox2 knockout in mice results in embryonic lethality due to severe cardiac defects including improper SAN development, Shox2 knockdown in zebrafish causes a reduced heart rate (bradycardia). In order to gain deeper insight into molecular pathways involving Shox2, we compared gene expression levels in right atria of wildtype and Shox2 (-/-) hearts using microarray experiments and identified the LIM homeodomain transcription factor Islet1 (Isl1) as one of its putative target genes. The downregulation of Isl1 expression in Shox2 (-/-) hearts was confirmed and the affected region narrowed down to the SAN by whole-mount in situ hybridization. Using luciferase reporter assays and EMSA studies, we identified two specific SHOX2 binding sites within intron 2 of the ISL1 locus. We also provide functional evidence for Isl1 as a transcriptional target of Shox2 by rescuing the Shox2-mediated bradycardia phenotype with Isl1 using zebrafish as a model system. Our findings demonstrate a novel epistatic relationship between Shox2 and Isl1 in the heart with important developmental consequences for SAN formation and heart beat.

Jun is required in Isl1-expressing progenitor cells for cardiovascular development

Jun is a highly conserved member of the multimeric activator protein 1 transcription factor complex and plays an important role in human cancer where it is known to be critical for proliferation, cell cycle regulation, differentiation, and cell death. All of these biological functions are also crucial for embryonic development. Although all Jun null mouse embryos die at mid-gestation with persistent truncus arteriosus, a severe cardiac outflow tract defect also seen in human congenital heart disease, the developmental mechanisms are poorly understood. Here we show that murine Jun is expressed in a restricted pattern in several cell populations important for cardiovascular development, including the second heart field, pharyngeal endoderm, outflow tract and atrioventricular endocardial cushions and post-migratory neural crest derivatives. Several genes, including Isl1, molecularly mark the second heart field. Isl1 lineages include myocardium, smooth muscle, neural crest, endocardium, and endothelium. We demonstrate that conditional knockout mouse embryos lacking Jun in Isl1-expressing progenitors display ventricular septal defects, double outlet right ventricle, semilunar valve hyperplasia and aortic arch artery patterning defects. In contrast, we show that conditional deletion of Jun in Tie2-expressing endothelial and endocardial precursors does not result in aortic arch artery patterning defects or embryonic death, but does result in ventricular septal defects and a low incidence of semilunar valve defects, atrioventricular valve defects and double outlet right ventricle. Our results demonstrate that Jun is required in Isl1-expressing progenitors and, to a lesser extent, in endothelial cells and endothelial-derived endocardium for cardiovascular development but is dispensable in both cell types for embryonic survival. These data provide a cellular framework for understanding the role of Jun in the pathogenesis of congenital heart disease.

Functional vascular smooth muscle-like cells derived from adult mouse uterine mesothelial cells

In mammalian visceral organs, vascular smooth muscle cells (VSMCs) originate from an epithelial-to-mesenchymal transition (EMT) of embryonic mesothelial cells (MCs). The ability of adult MCs to recapitulate EMT and to acquire smooth muscle (SM) markers upon provasculogenic culture suggested they might retain embryonic vasculogenic differentiation potential. However, it remains unknown whether adult MCs-derived SM-like cells may acquire specific vascular SM lineage markers and the functionality of differentiated contractile VSMCs. Here, we describe how a gentle trypsinization of adult mouse uterine cords could selectively detach their outermost uterine mesothelial layer cells. As other MCs; uterine MCs (UtMCs) uniformly expressed the epithelial markers β-catenin, ZO-1, E-cadherin, CD54, CD29, and CK18. When cultured in a modified SM differentiation media (SMDM) UtMCs initiated a loss of epithelial characteristics and gained markers expression of EMT (Twist, Snail, and Slug), stem and progenitor (Nanog, Sox2, C-kit, Gata-4, Isl-1, and nestin), SM (α-SMA, calponin, caldesmon, SM22α, desmin, SM-MHC, and smoothelin-B) and cardiac (BMP2, BMP4, ACTC1, sACTN, cTnI, cTnT, ANF, Cx43, and MLC2a). UtMCs repeatedly subcultured in SMDM acquired differentiated VSM-like characteristics and expressed smoothelin-B in the typical stress-fiber pattern expression of contractile VSMCs. Relevantly, UtMCs-derived VSM-like cells could generate "mechanical force" to compact collagen lattices and displayed in diverse degree voltage (K(+)) and receptor (endothelin-1, oxytocin, norepinephrine, carbachol and vasopressin)-induced [Ca(2+)](i) rises and contraction. Thus, we show for the first time that UtMCs could recapitulate in vitro differentiative events of early cardiovascular differentiation and transdifferentiate in cells exhibiting molecular and functional characteristics of VSMCs.

Haemogenic endocardium contributes to transient definitive haematopoiesis

Haematopoietic cells arise from spatiotemporally restricted domains in the developing embryo. Although studies of non-mammalian animal and in vitro embryonic stem cell models suggest a close relationship among cardiac, endocardial and haematopoietic lineages, it remains unknown whether the mammalian heart tube serves as a haemogenic organ akin to the dorsal aorta. Here we examine the haemogenic activity of the developing endocardium. Mouse heart explants generate myeloid and erythroid colonies in the absence of circulation. Haemogenic activity arises from a subset of endocardial cells in the outflow cushion and atria earlier than in the aorta-gonad-mesonephros region, and is transient and definitive in nature. Interestingly, key cardiac transcription factors, Nkx2-5 and Isl1, are expressed in and required for the haemogenic population of the endocardium. Together, these data suggest that a subset of endocardial/endothelial cells serve as a de novo source for transient definitive haematopoietic progenitors.

Fate mapping identifies the origin of SHF/AHF progenitors in the chick primitive streak

Heart development depends on the spatio-temporally regulated contribution of progenitor cells from the primary, secondary and anterior heart fields. Primary heart field (PHF) cells are first recruited to form a linear heart tube; later, they contribute to the inflow myocardium of the four-chambered heart. Subsequently cells from the secondary (SHF) and anterior heart fields (AHF) are added to the heart tube and contribute to both the inflow and outflow myocardium. In amniotes, progenitors of the linear heart tube have been mapped to the anterior-middle region of the early primitive streak. After ingression, these cells are located within bilateral heart fields in the lateral plate mesoderm. On the other hand SHF/AHF field progenitors are situated anterior to the linear heart tube, however, the origin and location of these progenitors prior to the development of the heart tube remains elusive. Thus, an unresolved question in the process of cardiac development is where SHF/AHF progenitors originate from during gastrulation and whether they come from a region in the primitive streak distinct from that which generates the PHF. To determine the origin and location of SHF/AHF progenitors we used vital dye injection and tissue grafting experiments to map the location and ingression site of outflow myocardium progenitors in early primitive streak stage chicken embryos. Cells giving rise to the AHF ingressed from a rostral region of the primitive streak, termed region 'A'. During development these cells were located in the cranial paraxial mesoderm and in the pharyngeal mesoderm. Furthermore we identified region 'B', located posterior to 'A', which gave rise to progenitors that contributed to the primary heart tube and the outflow tract. Our studies identify two regions in the early primitive streak, one which generates cells of the AHF and a second from which cardiac progenitors of the PHF and SHF emerge.

Lost in transgenesis: a user's guide for genetically manipulating the mouse in cardiac research

The advent of modern mouse genetics has benefited many fields of diseased-based research over the past 20 years, none perhaps more profoundly than cardiac biology. Indeed, the heart is now arguably one of the easiest tissues to genetically manipulate, given the availability of an ever-growing tool chest of molecular reagents/promoters and "facilitator" mouse lines. It is now possible to modify the expression of essentially any gene or partial gene product in the mouse heart at any time, either gain or loss of function. This review is designed as a handbook for the nonmouse geneticist and/or junior investigator to permit the successful manipulation of any gene or RNA product in the heart, while avoiding artifacts. In the present review, guidelines, pitfalls, and limitations are presented so that rigorous and appropriate examination of cardiac genotype-phenotype relationships can be performed. This review uses examples from the field to illustrate the vast spectrum of experimental and design details that must be considered when using genetically modified mouse models to study cardiac biology.