Heart field: from mesoderm to heart tube

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.

Genetic and functional genomics approaches targeting the Notch pathway in cardiac development and congenital heart disease

The Notch signalling pathway plays crucial roles in cardiac development and postnatal cardiac homoeostasis. Gain- and loss-of-function approaches indicate that Notch promotes or inhibits cardiogenesis in a stage-dependent manner. However, the molecular mechanisms are poorly defined because many downstream effectors remain to be identified. Genome-scale analyses are shedding light on the genes that are regulated by Notch signalling and the mechanisms underlying this regulation. We review the functional data that implicates Notch in cardiac morphogenetic processes and expression profiling studies that enlighten the regulatory networks behind them. A recurring theme is that Notch cross-talks reiteratively with other key signalling pathways including Wnt and Bmp to coordinate cell and tissue interactions during cardiogenesis.

Retinoid signaling in control of progenitor cell differentiation during mouse development

The vitamin A metabolite retinoic acid (RA) serves as a ligand for nuclear RA receptors that control differentiation of progenitor cells important for vertebrate development. Genetic studies in mouse embryos deficient for RA-generating enzymes have been invaluable for deciphering RA function. RA first begins to act during early organogenesis when RA generated in trunk mesoderm begins to function as a diffusible signal controlling progenitor cell differentiation. In neuroectoderm, RA functions as an instructive signal to stimulate neuronal differentiation of progenitor cells in the hindbrain and spinal cord. RA is not required for early neuronal differentiation of the forebrain, but at later stages RA stimulates neuronal differentiation in forebrain basal ganglia. RA also acts as a permissive signal for differentiation by repressing fibroblast growth factor (FGF) signaling in differentiated cells as they emerge from progenitor populations in the caudal progenitor zone and second heart field. In addition, RA signaling stimulates differentiation of spermatogonial germ cells and induces meiosis in male but not female gonads. A more complete understanding of the normal functions of RA signaling during development will guide efforts to use RA as a differentiation agent for therapeutic purposes.

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

Congenital heart defects affect approximately 1-5 % of human newborns each year, and of these cardiac defects 20-30 % are due to heart valve abnormalities. Recent literature indicates that the key factors and pathways that regulate valve development are also implicated in congenital heart defects and valve disease. Currently, there are limited options for treatment of valve disease, and therefore having a better understanding of valve development can contribute critical insight into congenital valve defects and disease. There are three major signaling pathways required for early specification and initiation of endothelial-to-mesenchymal transformation (EMT) in the cardiac cushions: BMP, TGF-β, and Notch signaling. BMPs secreted from the myocardium set up the environment for the overlying endocardium to become activated; Notch signaling initiates EMT; and both BMP and TGF-β signaling synergize with Notch to promote the transition of endothelia to mesenchyme and the mesenchymal cell invasiveness. Together, these three essential signaling pathways help form the cardiac cushions and populate them with mesenchyme and, consequently, set off the cascade of events required to develop mature heart valves. Furthermore, integration and cross-talk between these pathways generate highly stratified and delicate valve leaflets and septa of the heart. Here, we discuss BMP, TGF-β, and Notch signaling pathways during mouse cardiac cushion formation and how they together produce a coordinated EMT response in the developing mouse valves.

miR-34b regulates multiciliogenesis during organ formation in zebrafish

Multiciliated cells (MCCs) possess multiple motile cilia and are distributed throughout the vertebrate body, performing important physiological functions by regulating fluid movement in the intercellular space. Neither their function during organ development nor the molecular mechanisms underlying multiciliogenesis are well understood. Although dysregulation of members of the miR-34 family plays a key role in the progression of various cancers, the physiological function of miR-34b, especially in regulating organ formation, is largely unknown. Here, we demonstrate that miR-34b expression is enriched in kidney MCCs and the olfactory placode in zebrafish. Inhibiting miR-34b function using morpholino antisense oligonucleotides disrupted kidney proximal tubule convolution and the proper distribution of distal transporting cells and MCCs. Microarray analysis of gene expression, cilia immunostaining and a fluid flow assay revealed that miR-34b is functionally required for the multiciliogenesis of MCCs in the kidney and olfactory placode. We hypothesize that miR-34b regulates kidney morphogenesis by controlling the movement and distribution of kidney MCCs and fluid flow. We found that cmyb was genetically downstream of miR-34b and acted as a key regulator of multiciliogenesis. Elevated expression of cmyb blocked membrane docking of centrioles, whereas loss of cmyb impaired centriole multiplication, both of which resulted in defects in the formation of ciliary bundles. Thus, miR-34b serves as a guardian to maintain the proper level of cmyb expression. In summary, our studies have uncovered an essential role for miR-34b-Cmyb signaling during multiciliogenesis and kidney morphogenesis.

Zebrafish second heart field development relies on progenitor specification in anterior lateral plate mesoderm and nkx2.5 function

Second heart field (SHF) progenitors perform essential functions during mammalian cardiogenesis. We recently identified a population of cardiac progenitor cells (CPCs) in zebrafish expressing latent TGFβ-binding protein 3 (ltbp3) that exhibits several defining characteristics of the anterior SHF in mammals. However, ltbp3 transcripts are conspicuously absent in anterior lateral plate mesoderm (ALPM), where SHF progenitors are specified in higher vertebrates. Instead, ltbp3 expression initiates at the arterial pole of the developing heart tube. Because the mechanisms of cardiac development are conserved evolutionarily, we hypothesized that zebrafish SHF specification also occurs in the ALPM. To test this hypothesis, we Cre/loxP lineage traced gata4(+) and nkx2.5(+) ALPM populations predicted to contain SHF progenitors, based on evolutionary conservation of ALPM patterning. Traced cells were identified in SHF-derived distal ventricular myocardium and in three lineages in the outflow tract (OFT). We confirmed the extent of contributions made by ALPM nkx2.5(+) cells using Kaede photoconversion. Taken together, these data demonstrate that, as in higher vertebrates, zebrafish SHF progenitors are specified within the ALPM and express nkx2.5. Furthermore, we tested the hypothesis that Nkx2.5 plays a conserved and essential role during zebrafish SHF development. Embryos injected with an nkx2.5 morpholino exhibited SHF phenotypes caused by compromised progenitor cell proliferation. Co-injecting low doses of nkx2.5 and ltbp3 morpholinos revealed a genetic interaction between these factors. Taken together, our data highlight two conserved features of zebrafish SHF development, reveal a novel genetic relationship between nkx2.5 and ltbp3, and underscore the utility of this model organism for deciphering SHF biology.

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.

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.

Epithelial machines of morphogenesis and their potential application in organ assembly and tissue engineering

Sheets of embryonic epithelial cells coordinate their efforts to create diverse tissue structures such as pits, grooves, tubes, and capsules that lead to organ formation. Such cells can use a number of cell behaviors including contractility, proliferation, and directed movement to create these structures. By contrast, tissue engineers and researchers in regenerative medicine seeking to produce organs for repair or replacement therapy can combine cells with synthetic polymeric scaffolds. Tissue engineers try to achieve these goals by shaping scaffold geometry in such a way that cells embedded within these scaffold self-assemble to form a tissue, for instance aligning to synthetic fibers, and assembling native extracellular matrix to form the desired tissue-like structure. Although self-assembly is a dominant process that guides tissue assembly both within the embryo and within artificial tissue constructs, we know little about these critical processes. Here, we compare and contrast strategies of tissue assembly used by embryos to those used by engineers during epithelial morphogenesis and highlight opportunities for future applications of developmental biology in the field of tissue engineering.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Differentiation of human embryonic stem cells and induced pluripotent stem cells to cardiomyocytes: a methods overview

Since human embryonic stem cells were first differentiated to beating cardiomyocytes a decade ago, interest in their potential applications has increased exponentially. This has been further enhanced over recent years by the discovery of methods to induce pluripotency in somatic cells, including those derived from patients with hereditary cardiac diseases. Human pluripotent stem cells have been among the most challenging cell types to grow stably in culture, but advances in reagent development now mean that most laboratories can expand both embryonic and induced pluripotent stem cells robustly using commercially available products. However, differentiation protocols have lagged behind and in many cases only produce the cell types required with low efficiency. Cardiomyocyte differentiation techniques were also initially inefficient and not readily transferable across cell lines, but there are now a number of more robust protocols available. Here, we review the basic biology underlying the differentiation of pluripotent cells to cardiac lineages and describe current state-of-the-art protocols, as well as ongoing refinements. This should provide a useful entry for laboratories new to this area to start their research. Ultimately, efficient and reliable differentiation methodologies are essential to generate desired cardiac lineages to realize the full promise of human pluripotent stem cells for biomedical research, drug development, and clinical applications.

Asymmetric fate of the posterior part of the second heart field results in unexpected left/right contributions to both poles of the heart

RATIONALE

The second heart field (SHF) contains progenitors of all heart chambers, excluding the left ventricle. The SHF is patterned, and the anterior region is known to be destined to form the outflow tract and right ventricle.

OBJECTIVE

The aim of this study was to map the fate of the posterior SHF (pSHF).

METHODS AND RESULTS

We examined the contribution of pSHF cells, labeled by lipophilic dye at the 4- to 6-somite stage, to regions of the heart at 20 to 25 somites, using mouse embryo culture. Cells more cranial in the pSHF contribute to the atrioventricular canal (AVC) and atria, whereas those more caudal generate the sinus venosus, but there is intermixing of fate throughout the pSHF. Caudal pSHF contributes symmetrically to the sinus venosus, but the fate of cranial pSHF is left/right asymmetrical. Left pSHF moves to dorsal left atrium and superior AVC, whereas right pSHF contributes to right atrium, ventral left atrium, and inferior AVC. Retrospective clonal analysis shows the relationships between AVC and atria to be clonal and that right and left progenitors diverge before first and second heart lineage separation. Cranial pSHF cells also contribute to the outflow tract: proximal and distal at 4 somites, and distal only at 6 somites. All outflow tract-destined cells are intermingled with those that will contribute to inflow and AVC.

CONCLUSIONS

These observations show asymmetric fate of the pSHF, resulting in unexpected left/right contributions to both poles of the heart and can be integrated into a model of the morphogenetic movement of cells during cardiac looping.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Genetic insights into bicuspid aortic valve formation

Bicuspid aortic valve (BAV) is the most common congenital heart defect, affecting 1-2% of the population. It is generally diagnosed late in adulthood when deterioration of the abnormal leaflet becomes clinically evident. BAV patients have an increased risk of developing serious complications, including stenosis, regurgitation, endocarditis, dilation of the aorta, aortic dissection, and aneurysm. BAV is a heritable trait, but the genetic basis underlying this cardiac malformation remains poorly understood. In the last decade, thanks to studies in animal models as well as genetic and biochemical approaches, a large number of genes that play important roles in heart development have been identified. These discoveries provided valuable insight into cardiac morphogenesis and uncovered congenital-heart-disease-causing genes. This paper will summarize the current knowledge of valve morphogenesis as well as our current understanding of the genetic pathways involved in BAV formation. The impact of these advances on human health including diagnosis of BAV and prevention of cardiovascular complications in individuals with BAV or with a family history of BAV is also discussed.

Not just inductive: a crucial mechanical role for the endoderm during heart tube assembly

The heart is the first functioning organ to form during development. During gastrulation, the cardiac progenitors reside in the lateral plate mesoderm but maintain close contact with the underlying endoderm. In amniotes, these bilateral heart fields are initially organized as a pair of flat epithelia that move towards the embryonic midline and fuse above the anterior intestinal portal (AIP) to form the heart tube. This medial motion is typically attributed to active mesodermal migration over the underlying endoderm. In this model, the role of the endoderm is twofold: to serve as a mechanically passive substrate for the crawling mesoderm and to secrete various growth factors necessary for cardiac specification and differentiation. Here, using computational modeling and experiments on chick embryos, we present evidence supporting an active mechanical role for the endoderm during heart tube assembly. Label-tracking experiments suggest that active endodermal shortening around the AIP accounts for most of the heart field motion towards the midline. Results indicate that this shortening is driven by cytoskeletal contraction, as exposure to the myosin-II inhibitor blebbistatin arrested any shortening and also decreased both tissue stiffness (measured by microindentation) and mechanical tension (measured by cutting experiments). In addition, blebbistatin treatment often resulted in cardia bifida and abnormal foregut morphogenesis. Moreover, finite element simulations of our cutting experiments suggest that the endoderm (not the mesoderm) is the primary contractile tissue layer during this process. Taken together, these results indicate that contraction of the endoderm actively pulls the heart fields towards the embryonic midline, where they fuse to form the heart tube.

Islet1-expressing cardiac progenitor cells: a comparison across species

Adult mammalian cardiac stem cells express the LIM-homeodomain transcription factor Islet1 (Isl1). They are considered remnants of Isl1-positive embryonic cardiac progenitor cells. During amniote heart development, Isl1-positive progenitor cells give rise mainly to the outflow tract, the right ventricle, and parts of the atria. This led to the hypothesis that the development of the right ventricle of the amniote heart depends on the recruitment of additional cells to the primary heart tube. The region from which these additional, Isl1-positive cells originate is called second heart field, as opposed to the first heart field whose cells form the primary heart tube. Here, we review the available data about Isl1 in different species, demonstrating that Isl1 is an important component of the core transcription factor network driving early cardiogenesis in animals of the two clades, deuterostomes, and protostomes. The data support the view of a single cardiac progenitor cell population that includes Isl1-expressing cells and which differentiates into the various cardiac lineages during embryonic development in vertebrates but not in other phyla of the animal kingdom.

Tbx5 overexpression favors a first heart field lineage in murine embryonic stem cells and in Xenopus laevis embryos

The T-box transcription factor Tbx5 is involved in several developmental processes including cardiogenesis. Early steps of cardiac development are characterised by the formation of two cardiogenic lineages, the first (FHF) and the second heart field (SHF) lineage, which arise from a common cardiac progenitor cell population. To further investigate the function of Tbx5 during cardiogenesis, we generated a murine embryonic stem cell line constitutively overexpressing Tbx5. Differentiation of these cells is characterised by an earlier and increased appearance of contracting cardiomyocytes that beat with a higher frequency than control cells. In semi-quantitative and quantitative RT-PCR analyses, we observed an up-regulation of cardiac marker genes such as Troponin T, endogenous Tbx5, and Nkx2.5 and a down-regulation of others like BMP4 and Hand2. Similar data were gained in Xenopus laevis arguing for a conserved function of Tbx5. Furthermore, markers of the conduction system and atrial cardiomyocytes were increased.

CIP, a cardiac Isl1-interacting protein, represses cardiomyocyte hypertrophy

RATIONALE

Mammalian heart has minimal regenerative capacity. In response to mechanical or pathological stress, the heart undergoes cardiac remodeling. Pressure and volume overload in the heart cause increased size (hypertrophic growth) of cardiomyocytes. Whereas the regulatory pathways that activate cardiac hypertrophy have been well-established, the molecular events that inhibit or repress cardiac hypertrophy are less known.

OBJECTIVE

To identify and investigate novel regulators that modulate cardiac hypertrophy.

METHODS AND RESULTS

Here, we report the identification, characterization, and functional examination of a novel cardiac Isl1-interacting protein (CIP). CIP was identified from a bioinformatic search for novel cardiac-expressed genes in mouse embryonic hearts. CIP encodes a nuclear protein without recognizable motifs. Northern blotting, in situ hybridization, and reporter gene tracing demonstrated that CIP is highly expressed in cardiomyocytes of developing and adult hearts. Yeast two-hybrid screening identified Isl1, a LIM/homeodomain transcription factor essential for the specification of cardiac progenitor cells in the second heart field, as a cofactor of CIP. CIP directly interacted with Isl1, and we mapped the domains of these two proteins, which mediate their interaction. We show that CIP represses the transcriptional activity of Isl1 in the activation of the myocyte enhancer factor 2C. The expression of CIP was dramatically reduced in hypertrophic cardiomyocytes. Most importantly, overexpression of CIP repressed agonist-induced cardiomyocyte hypertrophy.

CONCLUSIONS

Our studies therefore identify CIP as a novel regulator of cardiac hypertrophy.

Inhibition of heart formation by lithium is an indirect result of the disruption of tissue organization within the embryo

Lithium is a commonly used drug for the treatment of bipolar disorder. At high doses, lithium becomes teratogenic, which is a property that has allowed this agent to serve as a useful tool for dissecting molecular pathways that regulate embryogenesis. This study was designed to examine the impact of lithium on heart formation in the developing frog for insights into the molecular regulation of cardiac specification. Embryos were exposed to lithium at the beginning of gastrulation, which produced severe malformations of the anterior end of the embryo. Although previous reports characterized this deformity as a posteriorized phenotype, histological analysis revealed that the defects were more comprehensive, with disfigurement and disorganization of all interior tissues along the anterior-posterior axis. Emerging tissues were poorly segregated and cavity formation was decreased within the embryo. Lithium exposure also completely ablated formation of the heart and prevented myocardial cell differentiation. Despite the complete absence of cardiac tissue in lithium treated embryos, exposure to lithium did not prevent myocardial differentiation of precardiac dorsal marginal zone explants. Moreover, precardiac tissue freed from the embryo subsequent to lithium treatment at gastrulation gave rise to cardiac tissue, as demonstrated by upregulation of cardiac gene expression, display of sarcomeric proteins, and formation of a contractile phenotype. Together these data indicate that lithium's effect on the developing heart was not due to direct regulation of cardiac differentiation, but an indirect consequence of disrupted tissue organization within the embryo.

Nodal mutant eXtraembryonic ENdoderm (XEN) stem cells upregulate markers for the anterior visceral endoderm and impact the timing of cardiac differentiation in mouse embryoid bodies

Interactions between the endoderm and mesoderm that mediate myocardial induction are difficult to study in vivo because of the small size of mammalian embryos at relevant stages. However, we and others have demonstrated that signals from endodermal cell lines can influence myocardial differentiation from both mouse and human embryoid bodies (EBs), and because of this, assays that utilize embryonic stem (ES) cells and endodermal cell lines provide excellent in vitro models to study early cardiac differentiation. Extraembryonic endoderm (XEN) stem cells have a particular advantage over other heart-inducing cell lines in that they can easily be derived from both wild type and mutant mouse blastocysts. Here we describe the first isolation of a Nodal mutant XEN stem cell line. Nodal^−/− XEN cell lines were not isolated at expected Mendelian ratios, and those that were successfully established, showed an increase in markers for the anterior visceral endoderm (AVE). Since AVE represents the heart-inducing endoderm in the mouse, cardiac differentiation was compared in EBs treated with conditioned medium (CM) collected from wild type or Nodal^−/− XEN cells. EBs treated with CM from Nodal^−/− cells began beating earlier and showed early activation of myocardial genes, but this early cardiac differentiation did not cause an overall increase in cardiomyocyte yield. By comparison, CM from wild type XEN cells both delayed cardiac differentiation and caused a concomitant increase in overall cardiomyocyte formation. Detailed marker analysis suggested that early activation of cardiac differentiation by Nodal^−/− XEN CM caused premature differentiation and subsequent depletion of cardiac progenitors.

Localized and temporal gene regulation in heart development

The heart is a structurally complex and functionally heterogeneous organ. The repertoire of genes active in a given cardiac cell defines its shapes and function. This process of localized or heterogeneous gene expression is regulated to a large extent at the level of transcription, dictating the degree particular genes in a cell are active. Therefore, errors in the regulation of localized gene expression are at the basis of misregulation of the delicate process of heart development and function. In this review, we provide an overview of the origin of the different components of the vertebrate heart, and discuss our current understanding of the regulation of localized gene expression in the developing heart. We will also discuss where future research may lead to gain more insight into this process, which should provide much needed insight into the dysregulation of heart development and function, and the etiology of congenital defects.

FRS2α-mediated FGF signals suppress premature differentiation of cardiac stem cells through regulating autophagy activity

RATIONALE

Although the fibroblast growth factor (FGF) signaling axis plays important roles in heart development, the molecular mechanism by which the FGF regulates cardiogenesis is not fully understood.

OBJECTIVE

To investigate the mechanism by which FGF signaling regulates cardiac progenitor cell differentiation.

METHODS AND RESULTS

Using mice with tissue-specific ablation of FGF receptors and FGF receptor substrate 2α (Frs2α) in heart progenitor cells, we demonstrate that disruption of FGF signaling leads to premature differentiation of cardiac progenitor cells in mice. Using embryoid body cultures of mouse embryonic stem cells, we reveal that FGF signaling promotes mesoderm differentiation in embryonic stem cells but inhibits cardiomyocyte differentiation of the mesoderm cells at later stages. Furthermore, we also report that inhibiting FRS2α-mediated signals increases autophagy and that activating autophagy promotes myocardial differentiation and vice versa.

CONCLUSIONS

The results indicate that the FGF/FRS2α-mediated signals prevent premature differentiation of heart progenitor cells through suppressing autophagy. The findings provide the first evidence that autophagy plays a role in heart progenitor differentiation.

Expression of the Tgfβ2 gene during chick embryogenesis

We performed a comprehensive analysis of the expression of transforming growth factor (TGF) β2 during chick embryogenesis from stage 6 to 30 (Hamburger and Hamilton, J Morphol 1951;88:49-92) using in situ hybridization. During cardiogenesis, Tgfβ2 was expressed in the endothelial/mesenchymal cells of the valvulo-septal endocardial cushion tissue and in the epicardium until the end of embryogenesis. During the formation of major arteries, Tgfβ2 was localized in smooth muscle progenitors but not in the vascular endothelium. During limb development, Tgfβ2 was expressed in the mesenchymal cells in the presumptive limb regions at stage 16, and thereafter it was localized in the skeletal muscle progenitors. In addition, strong Tgfβ2 expression was seen in the mesenchymal cells in the pharyngeal arches. Tgfβ2 mRNA was also detected in other mesoderm-derived tissues, such as the developing bone and pleura. During ectoderm development, Tgfβ2 was expressed in the floor plate of the neural tube, lens, optic nerve, and otic vesicle. In addition, Tgfβ2 was expressed in the developing gut epithelium. Our results suggest that TGFβ2 plays an important role not only in epithelial-mesenchymal interactions but also in cell differentiation and migration and cell death during chick embryogenesis. We also found that chick and mouse Tgfβ2 RNA show very similar patterns of expression during embryogenesis. Chick embryos can serve as a useful model to increase our understanding in the roles of TGFβ2 in cell-cell interactions, cell differentiation, and proliferation during organogenesis.

Human pluripotent stem cell-based approaches for myocardial repair: from the electrophysiological perspective

Heart diseases are a leading cause of mortality worldwide. Terminally differentiated adult cardiomyocytes (CMs) lack the innate ability to regenerate. Their malfunction or significant loss can lead to conditions from cardiac arrhythmias to heart failure. For myocardial repair, cell- and gene-based therapies offer promising alternatives to donor organ transplantation. Human embryonic stem cells (hESCs) can self-renew while maintaining their pluripotency. Direct reprogramming of adult somatic cells to become pluripotent hES-like cells (also known as induced pluripotent stem cells or iPSCs) has been achieved. Both hESCs and iPSCs have been successfully differentiated into genuine human CMs. In this review, we describe our current knowledge of the structure-function properties of hESC/iPSC-CMs, with an emphasis on their electrophysiology and Ca(2+) handling, along with the hurdles faced and potential solutions for translating into clinical and other applications (e.g., disease modeling, cardiotoxicity and drug screening).

Zebrafish cardiac development requires a conserved secondary heart field

The secondary heart field is a conserved developmental domain in avian and mammalian embryos that contributes myocardium and smooth muscle to the definitive cardiac arterial pole. This field is part of the overall heart field and its myocardial component has been fate mapped from the epiblast to the heart in both mammals and birds. In this study we show that the population that gives rise to the arterial pole of the zebrafish can be traced from the epiblast, is a discrete part of the mesodermal heart field, and contributes myocardium after initial heart tube formation, giving rise to both smooth muscle and myocardium. We also show that Isl1, a transcription factor associated with undifferentiated cells in the secondary heart field in other species, is active in this field. Furthermore, Bmp signaling promotes myocardial differentiation from the arterial pole progenitor population, whereas inhibiting Smad1/5/8 phosphorylation leads to reduced myocardial differentiation with subsequent increased smooth muscle differentiation. Molecular pathways required for secondary heart field development are conserved in teleosts, as we demonstrate that the transcription factor Tbx1 and the Sonic hedgehog pathway are necessary for normal development of the zebrafish arterial pole.

Identification of differentially expressed genes in the heart precursor cells of the chick embryo

Genetic evidence has implicated several genes as being critical for heart development. However, the inducers of these genes as well as their targets and pathways they are involved with, remain largely unknown. Previous studies in the avian embryo showed that at HH4 Cerberus (cCer) transcripts are detected in the anterior endomesoderm including the heart precursor cells and later in the left lateral plate mesoderm. We have identified a promoter element of chick cCer able to drive EGFP expression in a population of cells that consistently exit from the anterior primitive streak region, from as early as stage HH3+, and that later will populate the heart. Using this promoter element as a tool allowed us to identify novel genes previously not known to potentially play a role in heart development. In order to identify and study genes expressed and involved in the correct development and differentiation of the vertebrate heart precursor cell (HPC) lineages, a differential screening using Affymetrix GeneChip system technologies was performed. Remarkably, this screening led to the identification of more than 700 transcripts differentially expressed in the heart forming regions (HFR). Bioinformatic tools allowed us to filter the large amount of data generated from this approach and to select a few transcripts for in vivo validation. Whole-mount in situ hybridization and sectioning of selected genes showed heart and vascular expression patterns for these transcripts during early chick development. We have developed an effective strategy to specifically identify genes that are differentially expressed in the HPC lineages. Within this set we have identified several genes that are expressed in the heart, blood and vascular lineages, which are likely to play a role in their development. These genes are potential candidates for future functional studies on early embryonic patterning.

An eye on the head: the development and evolution of craniofacial muscles

Skeletal muscles exert diverse functions, enabling both crushing with great force and movement with exquisite precision. A remarkably distinct repertoire of genes and ontological features characterise this tissue, and recent evidence has shown that skeletal muscles of the head, the craniofacial muscles, are evolutionarily, morphologically and molecularly distinct from those of the trunk. Here, we review the molecular basis of craniofacial muscle development and discuss how this process is different to trunk and limb muscle development. Through evolutionary comparisons of primitive chordates (such as amphioxus) and jawless vertebrates (such as lampreys) with jawed vertebrates, we also provide some clues as to how this dichotomy arose.

Redundant and dosage sensitive requirements for Fgf3 and Fgf10 in cardiovascular development

Heart development requires contributions from, and coordinated signaling interactions between, several cell populations, including splanchnic and pharyngeal mesoderm, postotic neural crest and the proepicardium. Here we report that Fgf3 and Fgf10, which are expressed dynamically in and near these cardiovascular progenitors, have redundant and dosage sensitive requirements in multiple aspects of early murine cardiovascular development. Embryos with Fgf3(-/+);Fgf10(-/-), Fgf3(-/-);Fgf10(-/+) and Fgf3(-/-);Fgf10(-/-) genotypes formed an allelic series of increasing severity with respect to embryonic survival, with double mutants dead by E11.5. Morphologic analysis of embryos with three mutant alleles at E11.5-E13.5 and double mutants at E9.5-E11.0 revealed multiple cardiovascular defects affecting the outflow tract, ventricular septum, atrioventricular cushions, ventricular myocardium, dorsal mesenchymal protrusion, pulmonary arteries, epicardium and fourth pharyngeal arch artery. Assessment of molecular markers in E8.0-E10.5 double mutants revealed abnormalities in each progenitor population, and suggests that Fgf3 and Fgf10 are not required for specification of cardiovascular progenitors, but rather for their normal developmental coordination. These results imply that coding or regulatory mutations in FGF3 or FGF10 could contribute to human congenital heart defects.

Extracellular matrix and heart development

The extracellular matrix (ECM) of the developing heart contains numerous molecules that form a dynamic environment that plays an active and crucial role in the regulation of cellular events. ECM molecules found in the heart include hyaluronan, fibronectin, fibrillin, proteoglycans, and collagens. Tight regulation of the spatiotemporal expression, and the proteolytic processing of ECM components by proteases including members of the ADAMTS family, is essential for normal cardiac development. Perturbation of the expression of genes involved in matrix composition and remodeling can interfere with a myriad of events involved in the formation of the four-chambered heart and result in prenatal lethality or cardiac malformations as seen in humans with congenital heart disease. In this review, we summarize what is known about the specific importance of some of the components of the ECM in relation to the cardiovascular development.

Epigenetic factors and cardiac development

Congenital heart malformations remain the leading cause of death related to birth defects. Recent advances in developmental and regenerative cardiology have shed light on a mechanistic understanding of heart development that is controlled by a transcriptional network of genetic and epigenetic factors. This article reviews the roles of chromatin remodelling factors important for cardiac development with the current knowledge of cardiac morphogenesis, regeneration, and direct cardiac differentiation. In the last 5 years, critical roles of epigenetic factors have been revealed in the cardiac research field.

Bioenergetics, mitochondria, and cardiac myocyte differentiation

Cardiac metabolism is finely tuned, and disruption of myocardial bioenergetics can be clinically devastating. Many cardiomyopathies that present early in life are due to disruption of the maturation of these metabolic pathways. However, this bioenergetic maturation begins well before birth, when the embryonic heart is first beginning to beat, and continues into the mature animal. Thus, the changes in energy production seen after birth are actually part of a continuum that coincides with the structural and functional changes that occur as the cardiac myocyte differentiates and the heart undergoes morphogenesis. Therefore, although bioenergetics and mitochondrial biology have not been studied in great detail in the developing heart, bioenergetic maturation should be considered an important component of normal myocyte differentiation.Although events occurring after birth will be discussed, this review will focus on the changes in bioenergetics and mitochondrial biology that coincide with myocyte differentiation and cardiac morphogenesis. The relationship of these changes to the etiology and presentation of cardiomyopathies will be used as a starting point for this discussion. Then, after reviewing cardiac development and mitochondrial biology, the published data on bioenergetics and mitochondrial structure and function in the developing heart will be presented. Finally, the case will be made that mitochondria may be critical regulators of cardiac myocyte differentiation and cardiac development.

The representation of heart development in the gene ontology

An understanding of heart development is critical in any systems biology approach to cardiovascular disease. The interpretation of data generated from high-throughput technologies (such as microarray and proteomics) is also essential to this approach. However, characterizing the role of genes in the processes underlying heart development and cardiovascular disease involves the non-trivial task of data analysis and integration of previous knowledge. The Gene Ontology (GO) Consortium provides structured controlled biological vocabularies that are used to summarize previous functional knowledge for gene products across all species. One aspect of GO describes biological processes, such as development and signaling. In order to support high-throughput cardiovascular research, we have initiated an effort to fully describe heart development in GO; expanding the number of GO terms describing heart development from 12 to over 280. This new ontology describes heart morphogenesis, the differentiation of specific cardiac cell types, and the involvement of signaling pathways in heart development. This work also aligns GO with the current views of the heart development research community and its representation in the literature. This extension of GO allows gene product annotators to comprehensively capture the genetic program leading to the developmental progression of the heart. This will enable users to integrate heart development data across species, resulting in the comprehensive retrieval of information about this subject. The revised GO structure, combined with gene product annotations, should improve the interpretation of data from high-throughput methods in a variety of cardiovascular research areas, including heart development, congenital cardiac disease, and cardiac stem cell research. Additionally, we invite the heart development community to contribute to the expansion of this important dataset for the benefit of future research in this area.

Phylogeny informs ontogeny: a proposed common theme in the arterial pole of the vertebrate heart

In chick and mouse embryogenesis, a population of cells described as the secondary heart field (SHF) adds both myocardium and smooth muscle to the developing cardiac outflow tract (OFT). Following this addition, at approximately HH stage 22 in chick embryos, for example, the SHF can be identified architecturally by an overlapping seam at the arterial pole, where beating myocardium forms a junction with the smooth muscle of the arterial system. Previously, using either immunohistochemistry or nitric oxide indicators such as diaminofluorescein 2-diacetate, we have shown that a similar overlapping architecture also exists in the arterial pole of zebrafish and some shark species. However, although recent work suggests that development of the zebrafish OFT may also proceed by addition of a SHF-like population of cells, the presence of a true SHF in zebrafish and in many other developmental biological models remains an open question. We performed a comprehensive morphological study of the OFT of a wide range of vertebrates. Our data suggest that all vertebrates possess three fundamental OFT components: a proximal myocardial component, a distal smooth muscle component, and a middle component that contains overlapping myocardium and smooth muscle surrounding and supporting the outflow valves. Because the middle OFT component of avians and mammals is derived from the SHF, our observations suggest that a SHF may be an evolutionarily conserved theme in vertebrate embryogenesis.

Myocardial lineage development

The myocardium of the heart is composed of multiple highly specialized myocardial lineages, including those of the ventricular and atrial myocardium, and the specialized conduction system. Specification and maturation of each of these lineages during heart development is a highly ordered, ongoing process involving multiple signaling pathways and their intersection with transcriptional regulatory networks. Here, we attempt to summarize and compare much of what we know about specification and maturation of myocardial lineages from studies in several different vertebrate model systems. To date, most research has focused on early specification, and although there is still more to learn about early specification, less is known about factors that promote subsequent maturation of myocardial lineages required to build the functioning adult heart.

Probing human cardiovascular congenital disease using transgenic mouse models

Congenital heart defects (CHDs) impact in utero embryonic viability, children, and surviving adults. Since the first transfer of genes into mice, transgenic mouse models have enabled researchers to experimentally study and genetically test the roles of genes in development, physiology, and disease progression. Transgenic mice have become a bona fide human CHD pathology model and their use has dramatically increased within the past two decades. Now that the entire mouse and human genomes are known, it is possible to knock out, mutate, misexpress, and/or replace every gene. Not only have transgenic mouse models changed our understanding of normal development, CHD processes, and the complex interactions of genes and pathways required during heart development, but they are also being used to identify new avenues for medical therapy.

Human cardiomyogenesis and the need for systems biology analysis

Cardiovascular disease remains the leading cause of death in the Western world and myocardial infarction is one of the primary facets of this disease. The limited natural self-renewal of cardiac muscle following injury and restricted supply of heart transplants has encouraged researchers to investigate other means to stimulate regeneration of damaged myocardium. The plasticity of stem cells toward multiple lineages offers the potential to repair the heart following injury. Embryonic stem cells have been extensively studied for their ability to differentiate into early cardiomyocytes, however, the pathway has only been partially defined and inadequate efficiency limits their clinical applicability. Some studies have shown cardiomyogenesis from adult mesenchymal stem cells, from both bone marrow and adipose tissue, but their differentiation pathway remains poorly detailed and these results remain controversial. Despite promising results using stem cells in animal models of cardiac injury, the driving mechanisms behind their differentiation down a cardiomyogenic pathway have yet to be determined. Currently, there is a paucity of information regarding cardiomyogenesis on the systemic level. Stem cell differentiation results from multiple signaling parameters operating in a tightly regulated spatiotemporal pattern. Investigating this phenomenon from a systems biology perspective could unveil the abstruse mechanisms controlling cardiomyogenesis that would otherwise require extensive in vitro testing.

Endothelin receptor type A expression defines a distinct cardiac subdomain within the heart field and is later implicated in chamber myocardium formation

The avian and mammalian heart originates from two distinct embryonic regions: an early differentiating first heart field and a dorsomedially located second heart field. It remains largely unknown when and how these subdivisions of the heart field divide into regions with different fates. Here, we identify in the mouse a subpopulation of the first (crescent-forming) field marked by endothelin receptor type A (Ednra) gene expression, which contributes to chamber myocardium through a unique type of cell behavior. Ednra-lacZ/EGFP-expressing cells arise in the ventrocaudal inflow region of the early linear heart tube, converge to the midline, move anteriorly along the outer curvature and give rise to chamber myocardium mainly of the left ventricle and both atria. This movement was confirmed by fluorescent dye-labeling and transplantation experiments. The Ednra-lacZ/EGFP-expressing subpopulation is characterized by the presence of Tbx5-expressing cells. Ednra-null embryonic hearts often demonstrate hypoplasia of the ventricular wall, low mitotic activity and decreased Tbx5 expression with reciprocal expansion of Tbx2 expression. Conversely, endothelin 1 stimulates ERK phosphorylation and Tbx5 expression in the early embryonic heart. These results indicate that early Ednra expression defines a subdomain of the first heart field contributing to chamber formation, in which endothelin 1/Ednra signaling is involved. The present finding provides an insight into how subpopulations within the crescent-forming (first) heart field contribute to the coordination of heart morphogenesis through spatiotemporally defined cell movements.

The multiple phases and faces of wnt signaling during cardiac differentiation and development

Understanding heart development on a molecular level is a prerequisite for uncovering the causes of congenital heart diseases. Therapeutic approaches that try to enhance cardiac regeneration or that involve the differentiation of resident cardiac progenitor cells or patient-specific induced pluripotent stem cells will also benefit tremendously from this knowledge. Wnt proteins have been shown to play multiple roles during cardiac differentiation and development. They are extracellular growth factors that activate different intracellular signaling branches. Here, we summarize our current understanding of how these factors affect different aspects of cardiogenesis, starting from early specification of cardiac progenitors and continuing on to later developmental steps, such as morphogenetic processes, valve formation, and establishment of the conduction system.

Early chordate origins of the vertebrate second heart field

The vertebrate heart is formed from diverse embryonic territories, including the first and second heart fields. The second heart field (SHF) gives rise to the right ventricle and outflow tract, yet its evolutionary origins are unclear. We found that heart progenitor cells of the simple chordate Ciona intestinalis also generate precursors of the atrial siphon muscles (ASMs). These precursors express Islet and Tbx1/10, evocative of the splanchnic mesoderm that produces the lower jaw muscles and SHF of vertebrates. Evidence is presented that the transcription factor COE is a critical determinant of ASM fate. We propose that the last common ancestor of tunicates and vertebrates possessed multipotent cardiopharyngeal muscle precursors, and that their reallocation might have contributed to the emergence of the SHF.

Arterial pole progenitors interpret opposing FGF/BMP signals to proliferate or differentiate

During heart development, a subpopulation of cells in the heart field maintains cardiac potential over several days of development and forms the myocardium and smooth muscle of the arterial pole. Using clonal and explant culture experiments, we show that these cells are a stem cell population that can differentiate into myocardium, smooth muscle and endothelial cells. The multipotent stem cells proliferate or differentiate into different cardiovascular cell fates through activation or inhibition of FGF and BMP signaling pathways. BMP promoted myocardial differentiation but not proliferation. FGF signaling promoted proliferation and induced smooth muscle differentiation, but inhibited myocardial differentiation. Blocking the Ras/Erk intracellular pathway promoted myocardial differentiation, while the PLCgamma and PI3K pathways regulated proliferation. In vivo, inhibition of both pathways resulted in predictable arterial pole defects. These studies suggest that myocardial differentiation of arterial pole progenitors requires BMP signaling combined with downregulation of the FGF/Ras/Erk pathway. The FGF pathway maintains the pool of proliferating stem cells and later promotes smooth muscle differentiation.

Geometry and force control of cell function

Tissue engineering is becoming increasingly ambitious in its efforts to create functional human tissues, and to provide stem cell scientists with culture systems of high biological fidelity. Novel engineering designs are being guided by biological principles, in an attempt to recapitulate more faithfully the complexities of native cellular milieu. Three-dimensional (3D) scaffolds are being designed to mimic native-like cell environments and thereby elicit native-like cell responses. Also, the traditional focus on molecular regulatory factors is shifting towards the combined application of molecular and physical factors. Finally, methods are becoming available for the coordinated presentation of molecular and physical factors in the form of controllable spatial and temporal gradients. Taken together, these recent developments enable the interrogation of cellular behavior within dynamic culture settings designed to mimic some aspects of native tissue development, disease, or regeneration. We discuss here these advanced cell culture environments, with emphasis on the derivation of design principles from the development (the biomimetic paradigm) and the geometry-force control of cell function (the biophysical regulation paradigm).

[Lineage tracing of epicardial cells during development and regeneration]

Tracing the history of individual cells during embryonic morphogenesis in a structure as complex as the cardiovascular system is one of the major challenges of developmental biology. It involves determining the relationships between the various lineages of cells forming an organ at different stages, describing the topological rearrangements tissues undergo during morphogenesis, and characterizing the interactions between cells in different structures. However, despite the great expectations raised in the field of regenerative medicine, only limited progress has been made in using regenerative therapy to repair the cardiovascular system. Recent research has highlighted the role of the epicardium during cardiac regeneration, but it is still unclear whether it is important for molecular signaling or acts as a source of progenitor cells during this process. Consequently, increasing knowledge about the origin, diversification and potential of epicardial cells during development and homeostasis and under pathological conditions is of fundamental importance both for basic research and for the development of effective cellular therapies. The aims of this article were to provide a general overview of the classical techniques used for tracing cell lineages, including their potential and limitations, and to describe novel techniques for studying the origin and differentiation of the epicardium and its role in cardiac regeneration.

Brn3a regulates the transition from neurogenesis to terminal differentiation and represses non-neural gene expression in the trigeminal ganglion

The POU-domain transcription factor Brn3a is expressed in developing sensory neurons at all levels of the neural axis, including the trigeminal ganglion, hindbrain sensory ganglia, and dorsal root ganglia. Changes in global gene expression in the trigeminal ganglion from E11.5 to E13.5 reflect the repression of early neurogenic genes, exit from the cell cycle, and initiation of the expression of definitive markers of sensory function. A majority of these developmental changes are perturbed in the trigeminal ganglia of Brn3a knockout mice. At E13.5, Brn3a(-/-) trigeminal neurons fail to repress a battery of developmental regulators that are highly expressed at E11.5 and are normally down-regulated as development progresses, and also fail to appropriately activate a set of definitive sensory genes. Remarkably, developing Brn3a(-/-) trigeminal neurons also ectopically express multiple regulatory genes associated with cardiac and/or cranial mesoderm development, although definitive myogenic programs are not activated. The majority of these genes are not ectopically expressed in the dorsal root ganglia of Brn3a null mice, perhaps due to redundant mechanisms of repression at spinal levels. These results underscore the importance of gene repression in regulating neuronal development, and the need for unbiased screens in the determination of developmental gene regulatory programs.

Second lineage of heart forming region provides new understanding of conotruncal heart defects

Abnormal heart development causes various congenital heart defects. Recent cardiovascular biology studies have elucidated the morphological mechanisms involved in normal and abnormal heart development. The primitive heart tube originates from the lateral-most part of the heart forming mesoderm and mainly gives rise to the left ventricle. Then, during the cardiac looping, the outflow tract is elongated by the addition of cardiogenic cells from the both pharyngeal and splanchnic mesoderm (corresponding to anterior and secondary heart field, respectively), which originate from the mediocaudal region of the heart forming mesoderm and are later located anteriorly (rostrally) to the dorsal region of the heart tube. Therefore, the heart progenitors that contribute to the outflow tract region are distinct from those that form the left ventricle. The knowledge that there are two different lineages of heart progenitors in the four-chambered heart provides new understanding of the morphological and molecular etiology of conotruncal heart defects.