Cardiac arterial pole alignment is sensitive to FGF8 signaling in the pharynx

Mesodermal fibronectin controls cell shape, polarity, and mechanotransduction in the second heart field during cardiac outflow tract development

Failure in the elongation of the cardiac outflow tract (OFT) results in congenital heart disease due to the misalignment of the great arteries with the left and right ventricles. The OFT lengthens via the accretion of progenitors from the second heart field (SHF). SHF cells are exquisitely regionalized and organized into an epithelial-like layer, forming the dorsal pericardial wall (DPW). Tissue tension, cell polarity, and proliferation within the DPW are important for the addition of SHF-derived cells to the heart and OFT elongation. However, the genes controlling these processes are not completely characterized. Using conditional mutagenesis in the mouse, we show that fibronectin (FN1) synthesized by the mesoderm coordinates multiple cellular behaviors in the anterior DPW. FN1 is enriched in the anterior DPW and plays a role in OFT elongation by maintaining a balance between pro- and anti-adhesive cell-extracellular matrix (ECM) interactions and controlling DPW cell shape, polarity, cohesion, proliferation, and mechanotransduction.

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

Molecular Pathways and Animal Models of Tetralogy of Fallot and Double Outlet Right Ventricle

Tetralogy of Fallot and double-outlet right ventricle are outflow tract (OFT) alignment defects situated on a continuous disease spectrum. A myriad of upstream causes can impact on ventriculoarterial alignment that can be summarized as defects in either i) OFT elongation during looping morphogenesis or ii) OFT remodeling during cardiac septation. Embryological processes underlying these two developmental steps include deployment of second heart field cardiac progenitor cells, establishment and transmission of embryonic left/right information driving OFT rotation and OFT cushion and valve morphogenesis. The formation and remodeling of pulmonary trunk infundibular myocardium is a critical component of both steps. Defects in myocardial, endocardial, or neural crest cell lineages can result in alignment defects, reflecting the complex intercellular signaling events that coordinate arterial pole development. Importantly, however, OFT alignment is mechanistically distinct from neural crest-driven OFT septation, although neural crest cells impact indirectly on alignment through their role in modulating signaling during SHF development. As yet poorly understood nongenetic causes of alignment defects that impact the above processes include hemodynamic changes, maternal exposure to environmental teratogens, and stochastic events. The heterogeneity of causes converging on alignment defects characterizes the OFT as a hotspot of congenital heart defects.

Molecular Pathways and Animal Models of d-Transposition of the Great Arteries

During normal cardiovascular development, the outflow tract becomes septated and rotates so that the separate aorta and pulmonary trunk are correctly aligned with the left and right ventricles, respectively. However, when this process goes wrong, the aorta and pulmonary trunk are incorrectly positioned, resulting in oxygenated blood being directly returned to the lungs, with deoxygenated blood being delivered to the systemic circulation. This is termed transposition of the great arteries (TGA). The precise etiology of TGA is not known, but the use of animal models has elucidated that genes involved in determination of the left- embryonic body axis play key roles. Other factors such as retinoic acid levels are also crucial. This chapter reviews the animal models presenting with TGA that have been generated by genetic manipulation or with exogenous agents.

Novel rare genetic variants of familial and sporadic pulmonary atresia identified by whole-exome sequencing

Pulmonary atresia (PA) is a severe cyanotic congenital heart disease. Although some genetic mutations have been described to be associated with PA, the knowledge of pathogenesis is insufficient. The aim of this research was to use whole-exome sequencing (WES) to determine novel rare genetic variants in PA patients. We performed WES in 33 patients (27 patient-parent trios and 6 single probands) and 300 healthy control individuals. By applying an enhanced analytical framework to incorporate de novo and case-control rare variation, we identified 176 risk genes (100 de novo variants and 87 rare variants). Protein‒protein interaction (PPI) analysis and Genotype-Tissue Expression analysis revealed that 35 putative candidate genes had PPIs with known PA genes with high expression in the human heart. Expression quantitative trait loci analysis revealed that 27 genes that were identified as novel PA genes that could be affected by the surrounding single nucleotide polymorphism were screened. Furthermore, we screened rare damaging variants with a threshold of minor allele frequency at 0.5% in the ExAC_EAS and GnomAD_exome_EAS databases, and the deleteriousness was predicted by bioinformatics tools. For the first time, 18 rare variants in 11 new candidate genes have been identified that may play a role in the pathogenesis of PA. Our research provides new insights into the pathogenesis of PA and helps to identify the critical genes for PA.

Over-expression of Fgf8 in cardiac neural crest cells leads to persistent truncus arteriosus

During cardiogenesis, the outflow tract undergoes a complicated morphogenesis, including the re-alignment of the great blood vessels, and the separation of aorta and pulmonary trunk. The deficiency of FGF8 in the morphogenesis of outflow tract has been well studied, however, the effect of over-dosed FGF8 on the development of outflow tract remains unknown. In this study, Rosa26R-Fgf8 knock-in allele was constitutively activated by Wnt1-cre transgene in the mouse neural crest cells presumptive for the endocardial cushion of outflow tract. Surprisingly, Wnt1-cre; Rosa26R-Fgf8 mouse embryos exhibited persistent truncus arteriosus and died prior to E15.5. The cardiac neural crest cells in Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus did not degenerate as in WT controls, but proliferated into a thickened endocardial cushion and then, blocked the blood outflow from cardiac chambers into the lungs, which resulted in the embryonic lethality. Although the spiral aorticopulmonary septum failed to form, the differentiaion of the endothelium and smooth muscle in the Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus were impacted little. However, lineage tracing assay showed that the neural crest derived cells aggregated in the cushion layer, but failed to differentiate into the endothelium of Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus. Further investigation displayed the reduced p-Akt and p-Erk immunostaining, and the decreased Bmp2 and Bmp4 transcription in the endothelium of Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus. Our findings suggested that Fgf8 over-expression in cardiac neural crest impaired the formation of aorticopulmonary septum by suppressing the endothelial differentiation and stimulating the proliferation of endocardial cushion cells, which implicated a novel etiology of persistent truncus arteriosus.

The Classification of VACTERL Association into 3 Groups According to the Limb Defect

The VACTERL association (VA) is defined as the nonrandom co-occurrence of 6 anomalies: vertebral anomalies (V), Anal atresia (A), Cardiac defects (C), Tracheo-esophageal fistula (TE), Renal defects (R), and Limb anomalies (L). The current communication presents an argument that patients with VA should be classified into three district groups based on their limb defects: VACTERL1: patients with normal limbs; VACTERL2: patients with limb anomalies other than radial ray defects of the upper limbs; and VACTERL3: patients with radial ray defects of the upper limbs. The author will demonstrate that the rationale behind the L1-3 classification in patients in VA is based on the embryogenesis of the 6 affected anatomical areas in VA. The pathogenesis of VACTERL1 is secondary to perturbations of Sonic Hedgehog (SHH) interactions. SHH signaling is known to have a major role in the normal development of the vertebrae, ano-rectal area, heart, tracheo-esophageal area, and kidney. However, SHH is not involved in the development of the radial ray; hence, patients present with no limb defects. The pathogenesis of VACTERL2 is variable depending on the type of gene mutation. The pathogenesis of VACTERL3 is related to errors in a group of proteins (namely, the proteins of the TBX5-SALL4-SALL1 loop and the FGF8-FGF10 loop/ pathway). These proteins are essential for the normal development of the radial ray and they interact in the development of the other anatomical areas of VA including the heart and kidney. Hence, VACTERL3 patients present with radial ray deficiency.

Cardiac Neural Crest

Cardiac neural crest (CNC) cells are pluripotent cells derived from the dorsal neural tube that migrate and contribute to the remodeling of pharyngeal arch arteries and septation of the cardiac outflow tract (OFT). Numerous molecular cascades regulate the induction, specification, delamination, and migration of the CNC. Extensive analyses of the CNC ranging from chick ablation models to molecular biology studies have explored the mechanisms of heart development and disease, particularly involving the OFT and aortic arch (AA) system. Recent studies focus more on reciprocal signaling between the CNC and cells originated from the second heart field (SHF), which are essential for the development of the OFT myocardium, providing new insights into the molecular mechanisms underlying congenital heart diseases (CHDs) and some human syndromes.

The heart of the neural crest: cardiac neural crest cells in development and regeneration

Cardiac neural crest cells (cNCCs) are a migratory cell population that stem from the cranial portion of the neural tube. They undergo epithelial-to-mesenchymal transition and migrate through the developing embryo to give rise to portions of the outflow tract, the valves and the arteries of the heart. Recent lineage-tracing experiments in chick and zebrafish embryos have shown that cNCCs can also give rise to mature cardiomyocytes. These cNCC-derived cardiomyocytes appear to be required for the successful repair and regeneration of injured zebrafish hearts. In addition, recent work examining the response to cardiac injury in the mammalian heart has suggested that cNCC-derived cardiomyocytes are involved in the repair/regeneration mechanism. However, the molecular signature of the adult cardiomyocytes involved in this repair is unclear. In this Review, we examine the origin, migration and fates of cNCCs. We also review the contribution of cNCCs to mature cardiomyocytes in fish, chick and mice, as well as their role in the regeneration of the adult heart.

Nkx2-5 Second Heart Field Target Gene Ccdc117 Regulates DNA Metabolism and Proliferation

The cardiac transcription factor Nkx2-5 is essential for normal outflow tract (OFT) and right ventricle (RV) development. Nkx2-5-/- null mouse embryos display severe OFT and RV hypoplasia and a single ventricle phenotype due to decreased proliferation of Second Heart Field (SHF) cells, a pool of cardiac progenitors present in anterior pharyngeal arch mesoderm at mid-gestation. However, definition of the precise role of Nkx2-5 in facilitating SHF expansion is incomplete. We have found that Nkx2-5 positively and directly regulates a novel target gene, Ccdc117, in cells of the SHF at these stages. The nuclear/mitotic spindle associated protein Ccdc117 interacts with the MIP18/MMS19 cytoplasmic iron-sulfur (FeS) cluster assembly (CIA) complex, which transfers critical FeS clusters to several key enzymes with functions in DNA repair and replication. Loss of cellular Ccdc117 expression results in reduced proliferation rates associated with a delay at the G1-S transition, decreased rates of DNA synthesis, and unresolved DNA damage. These results implicate a novel role for Nkx2-5 in the regulation of cell cycle events in the developing heart, through Ccdc117's interaction with elements of the CIA pathway and the facilitation of DNA replication during SHF expansion.

Copy number variants in Ebstein anomaly

Background

Ebstein anomaly (EA) is a rare congenital defect characterized by apical displacement of the septal tricuspid leaflets and atrialization of the right ventricle. The etiology of EA is unclear; however, recurrence in families and the association of EA with genetic syndromes and copy number variants (CNVs) suggest a genetic component.

Objective

We performed a population-based study to search for recurrent and novel CNVs in a previously unreported set of EA cases.

Methods

We genotyped 60 EA cases identified from all live births (2,891,076) from selected California counties (1991–2010) using the Illumina HumanOmni2.5–8 array. We identified 38 candidate CNVs in 28 (46%) cases and prioritized and validated 11 CNVs based on the genes included.

Results

Five CNVs (41%) overlapped or were close to genes involved in early myocardial development, including NODAL, PDLIM5, SIX1, ASF1A and FGF12. We also replicated a previous association of EA with CNVs at 1p34.1 and AKAP12. Finally, we identified four CNVs overlapping or in close proximity to the transcription factors HES3, TRIM71, CUX1 and EIF4EBP2.

Conclusions

This study supports the relationship of genetic factors to EA and demonstrates that defects in cardiomyocytes and myocardium differentiation may play a role. Abnormal differentiation of cardiomyocytes and how genetic factors contribute should be examined for their association with EA.

Temperature-activated ion channels in neural crest cells confer maternal fever-associated birth defects

Birth defects of the heart and face are common, and most have no known genetic cause, suggesting a role for environmental factors. Maternal fever during the first trimester is an environmental risk factor linked to these defects. Neural crest cells are precursor populations essential to the development of both at-risk tissues. We report that two heat-activated transient receptor potential (TRP) ion channels, TRPV1 and TRPV4, were present in neural crest cells during critical windows of heart and face development. TRPV1 antagonists protected against the development of hyperthermia-induced defects in chick embryos. Treatment with chemical agonists of TRPV1 or TRPV4 replicated hyperthermia-induced birth defects in chick and zebrafish embryos. To test whether transient TRPV channel permeability in neural crest cells was sufficient to induce these defects, we engineered iron-binding modifications to TRPV1 and TRPV4 that enabled remote and noninvasive activation of these channels in specific cellular locations and at specific developmental times in chick embryos with radio-frequency electromagnetic fields. Transient stimulation of radio frequency-controlled TRP channels in neural crest cells replicated fever-associated defects in developing chick embryos. Our data provide a previously undescribed mechanism for congenital defects, whereby hyperthermia activates ion channels that negatively affect fetal development.

Gestational stress induces the unfolded protein response, resulting in heart defects

Congenital heart disease (CHD) is an enigma. It is the most common human birth defect and yet, even with the application of modern genetic and genomic technologies, only a minority of cases can be explained genetically. This is because environmental stressors also cause CHD. Here we propose a plausible non-genetic mechanism for induction of CHD by environmental stressors. We show that exposure of mouse embryos to short-term gestational hypoxia induces the most common types of heart defect. This is mediated by the rapid induction of the unfolded protein response (UPR), which profoundly reduces FGF signaling in cardiac progenitor cells of the second heart field. Thus, UPR activation during human pregnancy might be a common cause of CHD. Our findings have far-reaching consequences because the UPR is activated by a myriad of environmental or pathophysiological conditions. Ultimately, our discovery could lead to preventative strategies to reduce the incidence of human CHD.

Craniofacial Muscle Development

The developmental mechanisms that control head muscle formation are distinct from those that operate in the trunk. Head and neck muscles derive from various mesoderm populations in the embryo and are regulated by distinct transcription factors and signaling molecules. Throughout the last decade, developmental, and lineage studies in vertebrates and invertebrates have revealed the peculiar nature of the pharyngeal mesoderm that forms certain head muscles and parts of the heart. Studies in chordates, the ancestors of vertebrates, revealed an evolutionarily conserved cardiopharyngeal field that progressively facilitates the development of both heart and craniofacial structures during vertebrate evolution. This ancient regulatory circuitry preceded and facilitated the emergence of myogenic cell types and hierarchies that exist in vertebrates. This chapter summarizes studies related to the origins, signaling circuits, genetics, and evolution of the head musculature, highlighting its heterogeneous characteristics in all these aspects, with a special focus on the FGF-ERK pathway. Additionally, we address the processes of head muscle regeneration, and the development of stem cell-based therapies for treatment of muscle disorders.

Heparan Sulfate Biosynthesis Enzyme, Ext1, Contributes to Outflow Tract Development of Mouse Heart via Modulation of FGF Signaling

Glycosaminoglycans are important regulators of multiple signaling pathways. As a major constituent of the heart extracellular matrix, glycosaminoglycans are implicated in cardiac morphogenesis through interactions with different signaling morphogens. Ext1 is a glycosyltransferase responsible for heparan sulfate synthesis. Here, we evaluate the function of Ext1 in heart development by analyzing Ext1 hypomorphic mutant and conditional knockout mice. Outflow tract alignment is sensitive to the dosage of Ext1. Deletion of Ext1 in the mesoderm induces a cardiac phenotype similar to that of a mutant with conditional deletion of UDP-glucose dehydrogenase, a key enzyme responsible for synthesis of all glycosaminoglycans. The outflow tract defect in conditional Ext1 knockout(Ext1^f/f:Mesp1Cre) mice is attributable to the reduced contribution of second heart field and neural crest cells. Ext1 deletion leads to downregulation of FGF signaling in the pharyngeal mesoderm. Exogenous FGF8 ameliorates the defects in the outflow tract and pharyngeal explants. In addition, Ext1 expression in second heart field and neural crest cells is required for outflow tract remodeling. Our results collectively indicate that Ext1 is crucial for outflow tract formation in distinct progenitor cells, and heparan sulfate modulates FGF signaling during early heart development.

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.

Two developmentally distinct populations of neural crest cells contribute to the zebrafish heart

Cardiac neural crest cells are essential for outflow tract remodeling in animals with divided systemic and pulmonary circulatory systems, but their contributions to cardiac development in animals with a single-loop circulatory system are less clear. Here we genetically labeled neural crest cells and examined their contribution to the developing zebrafish heart. We identified two populations of neural crest cells that contribute to distinct compartments of zebrafish cardiovascular system at different developmental stages. A stream of neural crest cells migrating through pharyngeal arches 1 and 2 integrates into the myocardium of the primitive heart tube between 24 and 30 h post fertilization and gives rise to cardiomyocytes. A second wave of neural crest cells migrating along aortic arch 6 envelops the endothelium of the ventral aorta and invades the bulbus arteriosus after three days of development. Interestingly, while inhibition of FGF signaling has no effect on the integration of neural crest cells to the primitive heart tube, it prevents these cells from contributing to the outflow tract, demonstrating disparate responses of neural crest cells to FGF signaling. Furthermore, neural crest ablation in zebrafish leads to multiple cardiac defects, including reduced heart rate, defective myocardial maturation and a failure to recruit progenitor cells from the second heart field. These findings add to our understanding of the contribution of neural crest cells to the developing heart and provide insights into the requirement for these cells in cardiac maturation.

Signaling Pathways and Gene Regulatory Networks in Cardiomyocyte Differentiation

Strategies for harnessing stem cells as a source to treat cell loss in heart disease are the subject of intense research. Human pluripotent stem cells (hPSCs) can be expanded extensively in vitro and therefore can potentially provide sufficient quantities of patient-specific differentiated cardiomyocytes. Although multiple stimuli direct heart development, the differentiation process is driven in large part by signaling activity. The engineering of hPSCs to heart cell progeny has extensively relied on establishing proper combinations of soluble signals, which target genetic programs thereby inducing cardiomyocyte specification. Pertinent differentiation strategies have relied as a template on the development of embryonic heart in multiple model organisms. Here, information on the regulation of cardiomyocyte development from in vivo genetic and embryological studies is critically reviewed. A fresh interpretation is provided of in vivo and in vitro data on signaling pathways and gene regulatory networks (GRNs) underlying cardiopoiesis. The state-of-the-art understanding of signaling pathways and GRNs presented here can inform the design and optimization of methods for the engineering of tissues for heart therapies.

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.

Heterogeneity in the Segmental Development of the Aortic Tree: Impact on Management of Genetically Triggered Aortic Aneurysms

An extensive search of the medical literature examining the development of the thoracic aortic tree reveals that the thoracic aorta does not develop as one unit or in one stage: the oldest part of the thoracic aorta is the descending aorta with the aortic arch being the second oldest, developing under influence from the neural crest cell. Following in chronological order are the proximal ascending aorta and aortic root, which develop from a conotruncal origin. Different areas of the thoracic aorta develop under the influence of different gene sets. These parts develop from different cell lineages: the aortic root (the conotruncus), developing from the mesoderm; the ascending aorta and aortic arch, developing from the neural crest cells; and the descending aorta from the mesoderm. Findings illustrate that the thoracic aorta is not a single entity, in developmental terms. It develops from three or four distinct areas, at different stages of embryonic life, and under different sets of genes and signaling pathways. Genetically triggered thoracic aortic aneurysms are not a monolithic group but rather share a multi-genetic origin. Identification of therapeutic targets should be based on the predilection of certain genes to cause aneurysmal disease in specific aortic segments.

Mesodermal expression of integrin α5β1 regulates neural crest development and cardiovascular morphogenesis

Integrin α5-null embryos die in mid-gestation from severe defects in cardiovascular morphogenesis, which stem from defective development of the neural crest, heart and vasculature. To investigate the role of integrin α5β1 in cardiovascular development, we used the Mesp1(Cre) knock-in strain of mice to ablate integrin α5 in the anterior mesoderm, which gives rise to all of the cardiac and many of the vascular and muscle lineages in the anterior portion of the embryo. Surprisingly, we found that mutant embryos displayed numerous defects related to the abnormal development of the neural crest such as cleft palate, ventricular septal defect, abnormal development of hypoglossal nerves, and defective remodeling of the aortic arch arteries. We found that defects in arch artery remodeling stem from the role of mesodermal integrin α5β1 in neural crest proliferation and differentiation into vascular smooth muscle cells, while proliferation of pharyngeal mesoderm and differentiation of mesodermal derivatives into vascular smooth muscle cells was not defective. Taken together our studies demonstrate a requisite role for mesodermal integrin α5β1 in signaling between the mesoderm and the neural crest, thereby regulating neural crest-dependent morphogenesis of essential embryonic structures.

Evolutionary and developmental origins of the cardiac neural crest: building a divided outflow tract

The cardiac neural crest cells (CNCCs) have played an important role in the evolution and development of the vertebrate cardiovascular system: from reinforcement of the developing aortic arch arteries early in vertebrate evolution, to later orchestration of aortic arch artery remodeling into the great arteries of the heart, and finally outflow tract septation in amniotes. A critical element necessary for the evolutionary advent of outflow tract septation was the co-evolution of the cardiac neural crest cells with the second heart field. This review highlights the major transitions in vertebrate circulatory evolution, explores the evolutionary developmental origins of the CNCCs from the third stream cranial neural crest, and explores candidate signaling pathways in CNCC and outflow tract evolution drawn from our knowledge of DiGeorge Syndrome.

Nuclear to cytoplasmic shuttling of ERK promotes differentiation of muscle stem/progenitor cells

The transition between the proliferation and differentiation of progenitor cells is a key step in organogenesis, and alterations in this process can lead to developmental disorders. The extracellular signal-regulated kinase 1/2 (ERK) signaling pathway is one of the most intensively studied signaling mechanisms that regulates both proliferation and differentiation. How a single molecule (e.g. ERK) can regulate two opposing cellular outcomes is still a mystery. Using both chick and mouse models, we shed light on the mechanism responsible for the switch from proliferation to differentiation of head muscle progenitors and implicate ERK subcellular localization. Manipulation of the fibroblast growth factor (FGF)-ERK signaling pathway in chick embryos in vitro and in vivo demonstrated that blockage of this pathway accelerated myogenic differentiation, whereas its activation diminished it. We next examined whether the spatial subcellular localization of ERK could act as a switch between proliferation (nuclear ERK) and differentiation (cytoplasmic ERK) of muscle progenitors. A myristoylated peptide that blocks importin 7-mediated ERK nuclear translocation induced robust myogenic differentiation of muscle progenitor/stem cells in both head and trunk. In the mouse, analysis of Sprouty mutant embryos revealed that increased ERK signaling suppressed both head and trunk myogenesis. Our findings, corroborated by mathematical modeling, suggest that ERK shuttling between the nucleus and the cytoplasm provides a switch-like transition between proliferation and differentiation of muscle progenitors.

Embryonic heart progenitors and cardiogenesis

The mammalian heart is a highly specialized organ, comprised of many different cell types arising from distinct embryonic progenitor populations during cardiogenesis. Three precursor populations have been identified to contribute to different myocytic and nonmyocytic cell lineages of the heart: cardiogenic mesoderm cells (CMC), the proepicardium (PE), and cardiac neural crest cells (CNCCs). This review will focus on molecular cues necessary for proper induction, expansion, and lineage-specific differentiation of these progenitor populations during cardiac development in vivo. Moreover, we will briefly discuss how the knowledge gained on embryonic heart progenitor biology can be used to develop novel therapeutic strategies for the management of congenital heart disease as well as for improvement of cardiac function in ischemic heart disease.

Misregulation of SDF1-CXCR4 signaling impairs early cardiac neural crest cell migration leading to conotruncal defects

RATIONALE

Cardiac neural crest cells (NCs) contribute to heart morphogenesis by giving rise to a variety of cell types from mesenchyme of the outflow tract, ventricular septum, and semilunar valves to neurons of the cardiac ganglia and smooth muscles of the great arteries. Failure in cardiac NC development results in outflow and ventricular septation defects commonly observed in congenital heart diseases. Cardiac NCs derive from the vagal neural tube, which also gives rise to enteric NCs that colonize the gut; however, so far, molecular mechanisms segregating these 2 populations and driving cardiac NC migration toward the heart have remained elusive.

OBJECTIVE

Stromal-derived factor-1 (SDF1) is a chemokine that mediates oriented migration of multiple embryonic cells and mice deficient for Sdf1 or its receptors, Cxcr4 and Cxcr7, exhibit ventricular septum defects, raising the possibility that SDF1 might selectively drive cardiac NC migration toward the heart via a chemotactic mechanism.

METHODS AND RESULTS

We show in the chick embryo that Sdf1 expression is tightly coordinated with the progression of cardiac NCs expressing Cxcr4. Cxcr4 loss-of-function causes delayed migration and enhanced death of cardiac NCs, whereas Sdf1 misexpression results in their diversion from their normal pathway, indicating that SDF1 acts as a chemoattractant for cardiac NCs. These alterations of SDF1 signaling result in severe cardiovascular defects.

CONCLUSIONS

These data identify Sdf1 and its receptor Cxcr4 as candidate genes responsible for cardiac congenital pathologies in human.

Fibronectin and integrin alpha 5 play requisite roles in cardiac morphogenesis

Fibronectin and its major receptor, integrin α5β1 are required for embryogenesis. These mutants have similar phenotypes, although, defects in integrin α5-deficient mice are milder. In this paper, we examined heart development in those mutants, in which the heart is formed, and discovered that both fibronectin and integrin α5 were required for cardiac morphogenesis, and in particular, for the formation of the cardiac outflow tract. We found that Isl1+ precursors are specified and migrate into the heart in fibronectin- or integrin α5-mutant embryos, however, the hearts in these mutants are of aberrant shape, and the cardiac outflow tracts are short and malformed. We show that these defects are likely due to the requirement for cell adhesion to fibronectin for proliferation of myocardial progenitors and for Fgf8 signaling in the pharyngeal region.

Human embryonic stem cell-derived cardiomyocytes migrate in response to gradients of fibronectin and Wnt5a

An improved understanding of the factors that regulate the migration of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) would provide new insights into human heart development and suggest novel strategies to improve their electromechanical integration after intracardiac transplantation. Since nothing has been reported as to the factors controlling hESC-CM migration, we hypothesized that hESC-CMs would migrate in response to the extracellular matrix and soluble signaling molecules previously implicated in heart morphogenesis. To test this, we screened candidate factors by transwell assay for effects on hESC-CM motility, followed by validation via live-cell imaging and/or gap-closure assays. Fibronectin (FN) elicited a haptotactic response from hESC-CMs, with cells seeded on a steep FN gradient showing nearly a fivefold greater migratory activity than cells on uniform FN. Studies with neutralizing antibodies indicated that adhesion and migration on FN are mediated by integrins α-5 and α-V. Next, we screened 10 soluble candidate factors by transwell assay and found that the noncanonical Wnt, Wnt5a, elicited an approximately twofold increase in migration over controls. This effect was confirmed using the gap-closure assay, in which Wnt5a-treated hESC-CMs showed approximately twofold greater closure than untreated cells. Studies with microfluidic-generated Wnt5a gradients showed that this factor was chemoattractive as well as chemokinetic, and Wnt5a-mediated responses were inhibited by the Frizzled-1/2 receptor antagonist, UM206. In summary, hESC-CMs show robust promigratory responses to FN and Wnt5a, findings that have implications on both cardiac development and cell-based therapies.

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 cardiac progenitor cells in the early mouse embryo

At the end of the first week of mouse gestation, cardiomyocyte differentiation initiates in the cardiac crescent to give rise to the linear heart tube. The heart tube subsequently elongates by addition of cardiac progenitor cells from adjacent pharyngeal mesoderm to the growing arterial and venous poles. These progenitor cells, termed the second heart field, originate in splanchnic mesoderm medial to cells of the cardiac crescent and are patterned into anterior and posterior domains adjacent to the arterial and venous poles of the heart, respectively. Perturbation of second heart field cell deployment results in a spectrum of congenital heart anomalies including conotruncal and atrial septal defects seen in human patients. Here, we briefly review current knowledge of how the properties of second heart field cells are controlled by a network of transcriptional regulators and intercellular signaling pathways. Focus will be on 1) the regulation of cardiac progenitor cell proliferation in pharyngeal mesoderm, 2) the control of progressive progenitor cell differentiation and 3) the patterning of cardiac progenitor cells in the dorsal pericardial wall. Coordination of these three processes in the early embryo drives progressive heart tube elongation during cardiac morphogenesis. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Calcium dependent CAMTA1 in adult stem cell commitment to a myocardial lineage

The phenotype of somatic cells has recently been found to be reversible. Direct reprogramming of one cell type into another has been achieved with transduction and over expression of exogenous defined transcription factors emphasizing their role in specifying cell fate. To discover early and novel endogenous transcription factors that may have a role in adult-derived stem cell acquisition of a cardiomyocyte phenotype, mesenchymal stem cells from human and mouse bone marrow and rat liver were co-cultured with neonatal cardiomyocytes as an in vitro cardiogenic microenvironment. Cell-cell communications develop between the two cell types as early as 24 hrs in co-culture and are required for elaboration of a myocardial phenotype in the stem cells 8–16 days later. These intercellular communications are associated with novel Ca²⁺ oscillations in the stem cells that are synchronous with the Ca²⁺ transients in adjacent cardiomyocytes and are detected in the stem cells as early as 24–48 hrs in co-culture. Early and significant up-regulation of Ca²⁺-dependent effectors, CAMTA1 and RCAN1 ensues before a myocardial program is activated. CAMTA1 loss-of-function minimizes the activation of the cardiac gene program in the stem cells. While the expression of RCAN1 suggests involvement of the well-characterized calcineurin-NFAT pathway as a response to a Ca²⁺ signal, the CAMTA1 up-regulated expression as a response to such a signal in the stem cells was unknown. Cell-cell communications between the stem cells and adjacent cardiomyocytes induce Ca²⁺ signals that activate a myocardial gene program in the stem cells via a novel and early Ca²⁺-dependent intermediate, up-regulation of CAMTA1.

The neural crest in cardiac congenital anomalies

This review discusses the function of neural crest as they relate to cardiovascular defects. The cardiac neural crest cells are a subpopulation of cranial neural crest discovered nearly 30 years ago by ablation of premigratory neural crest. The cardiac neural crest cells are necessary for normal cardiovascular development. We begin with a description of the crest cells in normal development, including their function in remodeling the pharyngeal arch arteries, outflow tract septation, valvulogenesis, and development of the cardiac conduction system. The cells are also responsible for modulating signaling in the caudal pharynx, including the second heart field. Many of the molecular pathways that are known to influence specification, migration, patterning and final targeting of the cardiac neural crest cells are reviewed. The cardiac neural crest cells play a critical role in the pathogenesis of various human cardiocraniofacial syndromes such as DiGeorge, Velocardiofacial, CHARGE, Fetal Alcohol, Alagille, LEOPARD, and Noonan syndromes, as well as Retinoic Acid Embryopathy. The loss of neural crest cells or their dysfunction may not always directly cause abnormal cardiovascular development, but are involved secondarily because crest cells represent a major component in the complex tissue interactions in the head, pharynx and outflow tract. Thus many of the human syndromes linking defects in the heart, face and brain can be better understood when considered within the context of a single cardiocraniofacial developmental module with the neural crest being a key cell type that interconnects the regions.

The second heart field

Ten years ago, a population of cardiac progenitor cells was identified in pharyngeal mesoderm that gives rise to a major part of the amniote heart. These multipotent progenitor cells, termed the second heart field (SHF), contribute progressively to the poles of the elongating heart tube during looping morphogenesis, giving rise to myocardium, smooth muscle, and endothelial cells. Research into the mechanisms of SHF development has contributed significantly to our understanding of the properties of cardiac progenitor cells and the origins of congenital heart defects. Here recent data concerning the regulation, clinically relevant subpopulations, evolution and lineage relationships of the SHF are reviewed. Proliferation and differentiation of SHF cells are controlled by multiple intercellular signaling pathways and a transcriptional regulatory network that is beginning to be elucidated. Perturbation of SHF development results in common forms of congenital heart defects and particular progenitor cell subpopulations are highly relevant clinically, including cells giving rise to myocardium at the base of the pulmonary trunk and the interatrial septum. A SHF has recently been identified in amphibian, fish, and agnathan embryos, highlighting the important contribution of these cells to the evolution of the vertebrate heart. Finally, SHF-derived parts of the heart share a lineage relationship with craniofacial skeletal muscles revealing that these progenitor cells belong to a broad cardiocraniofacial field of pharyngeal mesoderm. Investigation of the mechanisms underlying the dynamic process of SHF deployment is likely to yield further insights into cardiac development and pathology.

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.

FGF8 signaling is chemotactic for cardiac neural crest cells

Cardiac neural crest cells migrate into the pharyngeal arches where they support development of the pharyngeal arch arteries. The pharyngeal endoderm and ectoderm both express high levels of FGF8. We hypothesized that FGF8 is chemotactic for cardiac crest cells. To begin testing this hypothesis, cardiac crest was explanted for migration assays under various conditions. Cardiac neural crest cells migrated more in response to FGF8. Single cell tracing indicated that this was not due to proliferation and subsequent transwell assays showed that the cells migrate toward an FGF8 source. The migratory response was mediated by FGF receptors (FGFR) 1 and 3 and MAPK/ERK intracellular signaling. To test whether FGF8 is chemokinetic and/or chemotactic in vivo, dominant negative FGFR1 was electroporated into the premigratory cardiac neural crest. Cells expressing the dominant negative receptor migrated slower than normal cardiac neural crest cells and were prone to remain in the vicinity of the neural tube and die. Treating with the FGFR1 inhibitor, SU5402 or an FGFR3 function-blocking antibody also slowed neural crest migration. FGF8 over-signaling enhanced neural crest migration. Neural crest cells migrated to an FGF8-soaked bead placed dorsal to the pharynx. Finally, an FGF8 producing plasmid was electroporated into an ectopic site in the ventral pharyngeal endoderm. The FGF8 producing cells attracted a thick layer of mesenchymal cells. DiI labeling of the neural crest as well as quail-to-chick neural crest chimeras showed that neural crest cells migrated to and around the ectopic site of FGF8 expression. These results showing that FGF8 is chemotactic and chemokinetic for cardiac neural crest adds another dimension to understanding the relationship of FGF8 and cardiac neural crest in cardiovascular defects.

Tbx1, subpulmonary myocardium and conotruncal congenital heart defects

Conotruncal congenital heart defects, including defects in septation and alignment of the ventricular outlets, account for approximately a third of all congenital heart defects. Failure of the left ventricle to obtain an independent outlet results in incomplete separation of systemic and pulmonary circulation at birth. The embryonic outflow tract, a transient cylinder of myocardium connecting the embryonic ventricles to the aortic sac, plays a critical role in this process during normal development. The outflow tract (OFT) is derived from a population of cardiac progenitor cells called the second heart field that contributes to the arterial pole of the heart tube during cardiac looping. During septation, the OFT is remodeled to form the base of the ascending aorta and pulmonary trunk. Tbx1, the major candidate gene for DiGeorge syndrome, is a critical transcriptional regulator of second heart field development. DiGeorge syndrome patients are haploinsufficient for Tbx1 and present a spectrum of conotruncal anomalies including tetralogy of Fallot, pulmonary atresia, and common arterial trunk. In this review, we focus on the role of Tbx1 in the regulation of second heart field deployment and, in particular, in the development of a specific population of myocardial cells at the base of the pulmonary trunk. Recent data characterizing additional properties and regulators of development of this region of the heart, including the retinoic acid, hedgehog, and semaphorin signaling pathways, are discussed. These findings identify future subpulmonary myocardium as the clinically relevant component of the second heart field and provide new mechanistic insight into a spectrum of common conotruncal congenital heart defects.

Notochordal and foregut abnormalities correlate with elevated neural crest apoptosis in Patch embryos

Although Patch mutants show severe abnormalities in many neural crest-derived structures including the face and the heart, there is a paucity of information characterizing the mechanisms underlying these congenital defects. Via manipulating the genetic background to circumvent early embryonic lethality, our results revealed that Patch phenotypes are most likely due to a significant decrease in migratory neural crest lineage due to diminished neural crest survival and elevated apoptosis. Homozygous mutant neural crest precursors can undergo typical expansion within the neural tube, epithelial-to-mesenchymal transformation, and initiate normal neural crest emigration. Moreover, in vitro explant culture demonstrated that when isolated from the surrounding mesenchyme, Patch mutant neural crest cells (NCCs) can migrate appropriately. Additionally, Patch foregut, notochord and somitic morphogenesis, and Sonic hedgehog expression profiles were all perturbed. Significantly, the timing of lethality and extent of apoptosis correlated with the degree of severity of Patch mutant foregut, notochord, and somite dysfunction. Finally, analysis of Balb/c-enriched surviving Patch mutants revealed that not all the neural crest subpopulations are affected and that Patch mutant neural crest-derived sympathetic ganglia and dorsal root ganglia were unaffected. We hypothesize that loss of normal coordinated signaling from the notochord, foregut, and somites underlies the diminished survival of the neural crest lineage within Patch mutants resulting in subsequent neural crest-deficient phenotypes.

Pharyngeal mesoderm development during embryogenesis: implications for both heart and head myogenesis

The pharyngeal mesoderm (PM), located in the head region of the developing embryo, recently triggered renewed interest as the major source of cells contributing to broad regions of the heart as well as to the head musculature. What exactly is PM? In this review, we describe the anatomical and molecular characteristics of this mesodermal population and its relationship to the first and second heart fields in chick and mouse embryos. The regulatory network of transcription factors and signalling molecules that regulate PM development is also discussed. In addition, we summarize recent studies into the evolutionary origins of this tissue and its multipotential contributions to both cardiac and pharyngeal muscle progenitors.

A Tbx1-Six1/Eya1-Fgf8 genetic pathway controls mammalian cardiovascular and craniofacial morphogenesis

Shared molecular programs govern the formation of heart and head during mammalian embryogenesis. Development of both structures is disrupted in human chromosomal microdeletion of 22q11.2 (del22q11), which causes DiGeorge syndrome (DGS) and velo-cardio-facial syndrome (VCFS). Here, we have identified a genetic pathway involving the Six1/Eya1 transcription complex that regulates cardiovascular and craniofacial development. We demonstrate that murine mutation of both Six1 and Eya1 recapitulated most features of human del22q11 syndromes, including craniofacial, cardiac outflow tract, and aortic arch malformations. The mutant phenotypes were attributable in part to a reduction of fibroblast growth factor 8 (Fgf8), which was shown to be a direct downstream effector of Six1 and Eya1. Furthermore, we showed that Six1 and Eya1 genetically interacted with Fgf8 and the critical del22q11 gene T-box transcription factor 1 (Tbx1) in mice. Together, these findings reveal a Tbx1-Six1/Eya1-Fgf8 genetic pathway that is crucial for mammalian cardiocraniofacial morphogenesis and provide insights into the pathogenesis of human del22q11 syndromes.

BMP signaling modulates hedgehog-induced secondary heart field proliferation

Sonic hedgehog signaling in the secondary heart field has a clear role in cardiac arterial pole development. In the absence of hedgehog signaling, proliferation is reduced in secondary heart field progenitors, and embryos predominantly develop pulmonary atresia. While it is expected that proliferation in the secondary heart field would be increased with elevated hedgehog signaling, this idea has never been tested. We hypothesized that up-regulating hedgehog signaling would increase secondary heart field proliferation, which would lead to arterial pole defects. In culture, secondary heart field explants proliferated up to 6-fold more in response to the hedgehog signaling agonist SAG, while myocardial differentiation and migration were unaffected. Treatment of chick embryos with SAG at HH14, just before the peak in secondary heart field proliferation, resulted unexpectedly in stenosis of both the aortic and pulmonary outlets. We examined proliferation in the secondary heart field and found that SAG-treated embryos exhibited a much milder increase in proliferation than was indicated by the in vitro experiments. To determine the source of other signaling factors that could modulate increased hedgehog signaling, we co-cultured secondary heart field explants with isolated pharyngeal endoderm or outflow tract and found that outflow tract co-cultures prevented SAG-induced proliferation. BMP2 is made and secreted by the outflow tract myocardium. To determine whether BMP signaling could prevent SAG-induced proliferation, we treated explants with SAG and BMP2 and found that BMP2 inhibited SAG-induced proliferation. In vivo, SAG-treated embryos showed up-regulated BMP2 expression and signaling. Together, these results indicate that BMP signaling from the outflow tract modulates hedgehog-induced proliferation in the secondary heart field.

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.

BMP-mediated inhibition of FGF signaling promotes cardiomyocyte differentiation of anterior heart field progenitors

The anterior heart field (AHF) encompasses a niche in which mesoderm-derived cardiac progenitors maintain their multipotent and undifferentiated nature in response to signals from surrounding tissues. Here, we investigate the signaling mechanism that promotes the shift from proliferating cardiac progenitors to differentiating cardiomyocytes in chick embryos. Genomic and systems biology approaches, as well as perturbations of signaling molecules, in vitro and in vivo, reveal tight crosstalk between the bone morphogenetic protein (BMP) and fibroblast growth factor (FGF) signaling pathways within the AHF niche: BMP4 promotes myofibrillar gene expression and cardiomyocyte contraction by blocking FGF signaling. Furthermore, inhibition of the FGF-ERK pathway is both sufficient and necessary for these processes, suggesting that FGF signaling blocks premature differentiation of cardiac progenitors in the AHF. We further revealed that BMP4 induced a set of neural crest-related genes, including MSX1. Overexpression of Msx1 was sufficient to repress FGF gene expression and cell proliferation, thereby promoting cardiomyocyte differentiation. Finally, we show that BMP-induced cardiomyocyte differentiation is diminished following cranial neural crest ablation, underscoring the key roles of these cells in the regulation of AHF cell differentiation. Hence, BMP and FGF signaling pathways act via inter- and intra-regulatory loops in multiple tissues, to coordinate the balance between proliferation and differentiation of cardiac progenitors.

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.

Role of mesodermal FGF8 and FGF10 overlaps in the development of the arterial pole of the heart and pharyngeal arch arteries

RATIONALE

The genes encoding fibroblast growth factor (FGF) 8 and 10 are expressed in the anterior part of the second heart field that constitutes a population of cardiac progenitor cells contributing to the arterial pole of the heart. Previous studies of hypomorphic and conditional Fgf8 mutants show disrupted outflow tract (OFT) and right ventricle (RV) development, whereas Fgf10 mutants do not have detectable OFT defects.

OBJECTIVES

Our aim was to investigate functional overlap between Fgf8 and Fgf10 during formation of the arterial pole.

METHODS AND RESULTS

We generated mesodermal Fgf8; Fgf10 compound mutants with MesP1Cre. The OFT/RV morphology in these mutants was affected with variable penetrance; however, the incidence of embryos with severely affected OFT/RV morphology was significantly increased in response to decreasing Fgf8 and Fgf10 gene dosage. Fgf8 expression in the pharyngeal arch ectoderm is important for development of the pharyngeal arch arteries and their derivatives. We now show that Fgf8 deletion in the mesoderm alone leads to pharyngeal arch artery phenotypes and that these vascular phenotypes are exacerbated by loss of Fgf10 function in the mesodermal core of the arches.

CONCLUSIONS

These results show functional overlap of FGF8 and FGF10 signaling from second heart field mesoderm during development of the OFT/RV, and from pharyngeal arch mesoderm during pharyngeal arch artery formation, highlighting the sensitivity of these key aspects of cardiovascular development to FGF dosage.