Frs2alpha-deficiency in cardiac progenitors disrupts a subset of FGF signals required for outflow tract morphogenesis

The ERBB2 c.1795C>T, p.Arg599Cys variant is associated with left ventricular outflow tract obstruction defects in humans

Non-syndromic congenital heart defects (CHDs) are occasionally familial and left ventricular outflow tract obstruction (LVOTO) defects are among the subtypes with the highest hereditability. The aim of this study was to evaluate the pathogenicity of a heterozygous ERBB2 variant c.1795C>T, p.Arg599Cys identified in three families with LVOTO defects. Variant detection was done with exome sequencing. Western blotting, digital PCR, mass spectrometry (MS), MS microscopy, and flow cytometry were used to study the function of the ERBB2 variant c.1795C>T. Cardiac structure and function were studied in zebrafish embryos expressing human ERBB2 wild type or c.1795C>T. Proband-derived human induced pluripotent stem cell cardiomyocytes (hiPS-CMs) and endothelial cells (hiPS-ECs) were used for transcriptomic analyses. While phosphorylation of the ERBB2 p.Arg599Cys receptor was not altered, the variant affected dramatically the binding partners of the protein, indicating mislocalization of the mutant ERBB2 from plasma membrane to endoplasmic reticulum. Expression of human ERBB2 p.Arg599Cys in zebrafish embryos resulted in cardiomyocyte hypertrophy, increased cardiac wall thickness, and impaired fractional shortening. Transcriptomic analyses of hiPS-ECs and hiPS-CMs from an individual with the c.1795C>T variant showed aberrant expression of genes related to cardiovascular system development and abnormal response to oxidative stress in both cell types. In conclusion, the heterozygous variant ERBB2 c.1795C>T, p.Arg599Cys leads to abnormal cellular localization of the ERBB2 receptor and induces structural changes and dysfunction in the zebrafish embryo heart. This evidence expands previous findings from animal studies to humans and suggests variants in ERBB2 may be associated with CHD.

RNA-binding protein SAMD4A targets FGF2 to regulate cardiomyocyte lineage specification from human embryonic stem cells

BACKGROUND

RNA-binding proteins (RBPs) are essential in cardiac development. However, a large of them have not been characterized during the process.

METHODS

We applied the human embryonic stem cells (hESCs) differentiated into cardiomyocytes model and constructed SAMD4A-knockdown/overexpression hESCs to investigate the role of SAMD4A in cardiomyocyte lineage specification.

RESULTS

SAMD4A, an RBP, exhibits increased expression during early heart development. Suppression of SAMD4A inhibits the proliferation of hESCs, impedes cardiac mesoderm differentiation, and impairs the function of hESC-derived cardiomyocytes. Correspondingly, forced expression of SAMD4A enhances proliferation and promotes cardiomyogenesis. Mechanistically, SAMD4A specifically binds to FGF2 via a specific CNGG/CNGGN motif, stabilizing its mRNA and enhancing translation, thereby upregulating FGF2 expression, which subsequently modulates the AKT signaling pathway and regulates cardiomyocyte lineage differentiation. Additionally, supplementation of FGF2 can rescue the proliferation defect of hESCs in the absence of SAMD4A.

CONCLUSIONS

Our study demonstrates that SAMD4A orchestrates cardiomyocyte lineage commitment through the post-transcriptional regulation of FGF2 and modulation of AKT signaling. These findings not only underscore the essential role of SAMD4A in cardiac organogenesis, but also provide critical insights into the molecular mechanisms underlying heart development, thereby informing potential therapeutic strategies for congenital heart disease.

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.

Molecular convergence of risk variants for congenital heart defects leveraging a regulatory map of the human fetal heart

Congenital heart defects (CHD) arise in part due to inherited genetic variants that alter genes and noncoding regulatory elements in the human genome. These variants are thought to act during fetal development to influence the formation of different heart structures. However, identifying the genes, pathways, and cell types that mediate these effects has been challenging due to the immense diversity of cell types involved in heart development as well as the superimposed complexities of interpreting noncoding sequences. As such, understanding the molecular functions of both noncoding and coding variants remains paramount to our fundamental understanding of cardiac development and CHD. Here, we created a gene regulation map of the healthy human fetal heart across developmental time, and applied it to interpret the functions of variants associated with CHD and quantitative cardiac traits. We collected single-cell multiomic data from 734,000 single cells sampled from 41 fetal hearts spanning post-conception weeks 6 to 22, enabling the construction of gene regulation maps in 90 cardiac cell types and states, including rare populations of cardiac conduction cells. Through an unbiased analysis of all 90 cell types, we find that both rare coding variants associated with CHD and common noncoding variants associated with valve traits converge to affect valvular interstitial cells (VICs). VICs are enriched for high expression of known CHD genes previously identified through mapping of rare coding variants. Eight CHD genes, as well as other genes in similar molecular pathways, are linked to common noncoding variants associated with other valve diseases or traits via enhancers in VICs. In addition, certain common noncoding variants impact enhancers with activities highly specific to particular subanatomic structures in the heart, illuminating how such variants can impact specific aspects of heart structure and function. Together, these results implicate new enhancers, genes, and cell types in the genetic etiology of CHD, identify molecular convergence of common noncoding and rare coding variants on VICs, and suggest a more expansive view of the cell types instrumental in genetic risk for CHD, beyond the working cardiomyocyte. This regulatory map of the human fetal heart will provide a foundational resource for understanding cardiac development, interpreting genetic variants associated with heart disease, and discovering targets for cell-type specific therapies.

Drosophila as a Model to Understand Second Heart Field Development

The genetic model system Drosophila has contributed fundamentally to our understanding of mammalian heart specification, development, and congenital heart disease. The relatively simple Drosophila heart is a linear muscular tube that is specified and develops in the embryo and persists throughout the life of the animal. It functions at all stages to circulate hemolymph within the open circulatory system of the body. During Drosophila metamorphosis, the cardiac tube is remodeled, and a new layer of muscle fibers spreads over the ventral surface of the heart to form the ventral longitudinal muscles. The formation of these fibers depends critically upon genes known to be necessary for mammalian second heart field (SHF) formation. Here, we review the prior contributions of the Drosophila system to the understanding of heart development and disease, discuss the importance of the SHF to mammalian heart development and disease, and then discuss how the ventral longitudinal adult cardiac muscles can serve as a novel model for understanding SHF development and disease.

A Multi-Omics Approach Using a Mouse Model of Cardiac Malformations for Prioritization of Human Congenital Heart Disease Contributing Genes

Congenital heart disease (CHD) is the most common type of birth defect, affecting ~1% of all live births. Malformations of the cardiac outflow tract (OFT) account for ~30% of all CHD and include a range of CHDs from bicuspid aortic valve (BAV) to tetralogy of Fallot (TOF). We hypothesized that transcriptomic profiling of a mouse model of CHD would highlight disease-contributing genes implicated in congenital cardiac malformations in humans. To test this hypothesis, we utilized global transcriptional profiling differences from a mouse model of OFT malformations to prioritize damaging, de novo variants identified from exome sequencing datasets from published cohorts of CHD patients. Notch1 +/- ; Nos3 -/- mice display a spectrum of cardiac OFT malformations ranging from BAV, semilunar valve (SLV) stenosis to TOF. Global transcriptional profiling of the E13.5 Notch1 +/- ; Nos3 -/- mutant mouse OFTs and wildtype controls was performed by RNA sequencing (RNA-Seq). Analysis of the RNA-Seq dataset demonstrated genes belonging to the Hif1α, Tgf-β, Hippo, and Wnt signaling pathways were differentially expressed in the mutant OFT. Mouse to human comparative analysis was then performed to determine if patients with TOF and SLV stenosis display an increased burden of damaging, genetic variants in gene homologs that were dysregulated in Notch1 +/- ; Nos3 -/- OFT. We found an enrichment of de novo variants in the TOF population among the 1,352 significantly differentially expressed genes in Notch1 +/- ; Nos3 -/- mouse OFT but not the SLV population. This association was not significant when comparing only highly expressed genes in the murine OFT to de novo variants in the TOF population. These results suggest that transcriptomic datasets generated from the appropriate temporal, anatomic and cellular tissues from murine models of CHD may provide a novel approach for the prioritization of disease-contributing genes in patients with CHD.

Maternal iron deficiency perturbs embryonic cardiovascular development in mice

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

The Multifunctional Contribution of FGF Signaling to Cardiac Development, Homeostasis, Disease and Repair

The current focus on cardiovascular research reflects society’s concerns regarding the alarming incidence of cardiac-related diseases and mortality in the industrialized world and, notably, an urgent need to combat them by more efficient therapies. To pursue these therapeutic approaches, a comprehensive understanding of the mechanism of action for multifunctional fibroblast growth factor (FGF) signaling in the biology of the heart is a matter of high importance. The roles of FGFs in heart development range from outflow tract formation to the proliferation of cardiomyocytes and the formation of heart chambers. In the context of cardiac regeneration, FGFs 1, 2, 9, 16, 19, and 21 mediate adaptive responses including restoration of cardiac contracting rate after myocardial infarction and reduction of myocardial infarct size. However, cardiac complications in human diseases are correlated with pathogenic effects of FGF ligands and/or FGF signaling impairment. FGFs 2 and 23 are involved in maladaptive responses such as cardiac hypertrophic, fibrotic responses and heart failure. Among FGFs with known causative (FGFs 2, 21, and 23) or protective (FGFs 2, 15/19, 16, and 21) roles in cardiac diseases, FGFs 15/19, 21, and 23 display diagnostic potential. The effective role of FGFs on the induction of progenitor stem cells to cardiac cells during development has been employed to boost the limited capacity of postnatal cardiac repair. To renew or replenish damaged cardiomyocytes, FGFs 1, 2, 10, and 16 were tested in (induced-) pluripotent stem cell-based approaches and for stimulation of cell cycle re-entry in adult cardiomyocytes. This review will shed light on the wide range of beneficiary and detrimental actions mediated by FGF ligands and their receptors in the heart, which may open new therapeutic avenues for ameliorating cardiac complications.

Endothelial FGFR1 (Fibroblast Growth Factor Receptor 1) Deficiency Contributes Differential Fibrogenic Effects in Kidney and Heart of Diabetic Mice

Endothelial-to-mesenchymal transition (EndMT) has been shown to contribute to organ fibrogenesis. We have reported that N-acetyl-seryl-aspartyl- lysyl-proline (AcSDKP) restored levels of diabetes mellitus-suppressed FGFR1 (fibroblast growth factor receptor 1), the endothelial receptor essential for combating EndMT. However, the molecular regulation and biological/pathological significance of the AcSDKP-FGFR1 relationship has not been elucidated yet. Here, we demonstrated that endothelial FGFR1 deficiency led to AcSDKP-resistant EndMT and severe fibrosis associated with EndMT-stimulated fibrogenic programming in neighboring cells. Diabetes mellitus induced severe kidney fibrosis in endothelial FGFR1-deficient mice (FGFR1^fl/fl; VE-cadherin-Cre: FGFR1^EKO) but not in control mice (FGFR1^fl/fl); AcSDKP completely or partially suppressed kidney fibrosis in control or FGFR1^EKO mice. Severe fibrosis was also induced in hearts of diabetic FGFR1^EKO mice; however, AcSDKP had no effect on heart fibrosis in FGFR1^EKO mice. AcSDKP also had no effect on EndMT in either kidney or heart but partially suppressed epithelial-to-mesenchymal transition in kidneys of diabetic FGFR1^EKO mice. The medium from FGFR1-deficient endothelial cells stimulated TGFβ (transforming growth factor β)/Smad-dependent epithelial-to-mesenchymal transition in cultured human proximal tubule epithelial cell line, AcSDKP inhibited such epithelial-to-mesenchymal transition. These data demonstrated that endothelial FGFR1 is essential as an antifibrotic core molecule as the target of AcSDKP.

Disruption of FGF Signaling Ameliorates Inflammatory Response in Hepatic Stellate Cells

It is a well-documented event that fibroblast growth factors (FGFs) regulate liver development and homeostasis in autocrine, paracrine, and endocrine manners via binding and activating FGF receptors (FGFRs) tyrosine kinase in hepatocytes. Recent research reveals that hepatic stellate cells (HSCs) play a fundamental role in liver immunology. However, how FGF signaling in HSCs regulates liver inflammation remains unclear. Here, we report that FGF promoted NF-κB signaling, an inflammatory pathway, in human HSCs, which was associated with FGFR1 expression. Both FGF and NF-κB signaling in HSCs were compromised by FGFR1 tyrosine kinase inhibitor. After stimulating HSCs with proinflammatory cytokines, expression of multiple FGF ligands was significantly increased. However, disruption of FGF signaling with FGFR inhibitors prominently reduced the apoptosis, inflammatory response, NF-κB nuclear translocation, and expression of matrix metalloproteinase-9 (MMP-9) induced by TNFα in HSCs. Interestingly, FGF21 significantly alleviated the inflammation responses in the concanavalin A (Con A)-induced acutely injured liver. Unlike canonic FGFs that elicit signals through activating the FGFR–heparan sulfate complex, FGF21 activates the FGFR–KLB complex and elicits a different set of signals. Therefore, the finding here indicates the urgency of developing pathway-specific inhibitors that only suppress canonical FGF, but not non-canonical FGF21, signaling for alleviating inflammation in the liver, which is presented in all stages of diseased liver.

N-acetyl-seryl-aspartyl-lysyl-proline is a valuable endogenous antifibrotic peptide for kidney fibrosis in diabetes: An update and translational aspects

N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP) is an endogenous peptide that has been confirmed to show excellent organ-protective effects. Even though originally discovered as a modulator of hemotopoietic stem cells, during the recent two decades, AcSDKP has been recognized as valuable antifibrotic peptide. The antifibrotic mechanism of AcSDKP is not yet clear; we have established that AcSDKP could target endothelial-mesenchymal transition program through the induction of the endothelial fibroblast growth factor receptor signaling pathway. Also, recent reports suggested the clinical significance of AcSDKP. The aim of this review was to update recent advances of the mechanistic action of AcSDKP and discuss translational research aspects.

Alternative splicing of jnk1a in zebrafish determines first heart field ventricular cardiomyocyte numbers through modulation of hand2 expression

The planar cell polarity pathway is required for heart development and whilst the functions of most pathway members are known, the roles of the jnk genes in cardiac morphogenesis remain unknown as mouse mutants exhibit functional redundancy, with early embryonic lethality of compound mutants. In this study zebrafish were used to overcome early embryonic lethality in mouse models and establish the requirement for Jnk in heart development. Whole mount in-situ hybridisation and RT-PCR demonstrated that evolutionarily conserved alternative spliced jnk1a and jnk1b transcripts were expressed in the early developing heart. Maternal zygotic null mutant zebrafish lines for jnk1a and jnk1b, generated using CRISPR-Cas9, revealed a requirement for jnk1a in formation of the proximal, first heart field (FHF)-derived portion of the cardiac ventricular chamber. Rescue of the jnk1a mutant cardiac phenotype was only possible by injection of the jnk1a EX7 Lg alternatively spliced transcript. Analysis of mutants indicated that there was a reduction in the size of the hand2 expression field in jnk1a mutants which led to a specific reduction in FHF ventricular cardiomyocytes within the anterior lateral plate mesoderm. Moreover, the jnk1a mutant ventricular defect could be rescued by injection of hand2 mRNA. This study reveals a novel and critical requirement for Jnk1 in heart development and highlights the importance of alternative splicing in vertebrate cardiac morphogenesis. Genetic pathways functioning through jnk1 may be important in human heart malformations with left ventricular hypoplasia.

Intratumor δ-catenin heterogeneity driven by genomic rearrangement dictates growth factor dependent prostate cancer progression

Only a small number of genes are bona fide oncogenes and tumor suppressors such as Ras, Myc, β-catenin, p53, and APC. However, targeting these cancer drivers frequently fail to demonstrate sustained cancer remission. Tumor heterogeneity and evolution contribute to cancer resistance and pose challenges for cancer therapy due to differential genomic rearrangement and expression driving distinct tumor responses to treatments. Here we report that intratumor heterogeneity of Wnt/β-catenin modulator δ-catenin controls individual cell behavior to promote cancer. The differential intratumor subcellular localization of δ-catenin mirrors its compartmentalization in prostate cancer xenograft cultures as result of mutation-rendered δ-catenin truncations. Wild-type and δ-catenin mutants displayed distinct protein interactomes that highlight rewiring of signal networks. Localization specific δ-catenin mutants influenced p120ctn-dependent Rho GTPase phosphorylation and shifted cells towards differential bFGF-responsive growth and motility, a known signal to bypass androgen receptor dependence. Mutant δ-catenin promoted Myc-induced prostate tumorigenesis while increasing bFGF-p38 MAP kinase signaling, β-catenin-HIF-1α expression, and the nuclear size. Therefore, intratumor δ-catenin heterogeneity originated from genetic remodeling promotes prostate cancer expansion towards androgen independent signaling, supporting a neomorphism model paradigm for targeting tumor progression.

FRS2α-dependent cell fate transition during endocardial cushion morphogenesis

Atrioventricular valve development requires endothelial-to-mesenchymal transition (EndMT) that induces cushion endocardial cells to give rise to mesenchymal cells crucial to valve formation. In the adult endothelium, deletion of the docking protein FRS2α induces EndMT by activating TGFβ signaling in a miRNA let-7-dependent manner. To study the role of endothelial FRS2α during embryonic development, we generated mice with an inducible endothelial-specific deletion of Frs2α (FRS2α^iECKO). Analysis of the FRS2α^iECKO embryos uncovered a combination of impaired EndMT in AV cushions and defective maturation of AV valves leading to development of thickened, abnormal valves when Frs2α was deleted early (E7.5) in development. At the same time, no AV valve developmental abnormalities were observed after late (E10.5) deletion. These observations identify FRS2α as a pivotal controller of cell fate transition during both EndMT and post-EndMT valvulogenesis.

A single-cell transcriptional roadmap for cardiopharyngeal fate diversification

In vertebrates, multipotent progenitors located in the pharyngeal mesoderm form cardiomyocytes and branchiomeric head muscles, but the dynamic gene expression programmes and mechanisms underlying cardiopharyngeal multipotency and heart versus head muscle fate choices remain elusive. Here, we used single-cell genomics in the simple chordate model Ciona to reconstruct developmental trajectories forming first and second heart lineages and pharyngeal muscle precursors and characterize the molecular underpinnings of cardiopharyngeal fate choices. We show that FGF-MAPK signalling maintains multipotency and promotes the pharyngeal muscle fate, whereas signal termination permits the deployment of a pan-cardiac programme, shared by the first and second heart lineages, to define heart identity. In the second heart lineage, a Tbx1/10-Dach pathway actively suppresses the first heart lineage programme, conditioning later cell diversity in the beating heart. Finally, cross-species comparisons between Ciona and the mouse evoke the deep evolutionary origins of cardiopharyngeal networks in chordates.

HDAC1-mediated repression of the retinoic acid-responsive gene ripply3 promotes second heart field development

Coordinated transcriptional and epigenetic mechanisms that direct development of the later differentiating second heart field (SHF) progenitors remain largely unknown. Here, we show that a novel zebrafish histone deacetylase 1 (hdac1) mutant allele cardiac really gone (crg) has a deficit of ventricular cardiomyocytes (VCs) and smooth muscle within the outflow tract (OFT) due to both cell and non-cell autonomous loss in SHF progenitor proliferation. Cyp26-deficient embryos, which have increased retinoic acid (RA) levels, have similar defects in SHF-derived OFT development. We found that nkx2.5⁺ progenitors from Hdac1 and Cyp26-deficient embryos have ectopic expression of ripply3, a transcriptional co-repressor of T-box transcription factors that is normally restricted to the posterior pharyngeal endoderm. Furthermore, the ripply3 expression domain is expanded anteriorly into the posterior nkx2.5⁺ progenitor domain in crg mutants. Importantly, excess ripply3 is sufficient to repress VC development, while genetic depletion of Ripply3 and Tbx1 in crg mutants can partially restore VC number. We find that the epigenetic signature at RA response elements (RAREs) that can associate with Hdac1 and RA receptors (RARs) becomes indicative of transcriptional activation in crg mutants. Our study highlights that transcriptional repression via the epigenetic regulator Hdac1 facilitates OFT development through directly preventing expression of the RA-responsive gene ripply3 within SHF progenitors.

Congenital heart defects are the most common malformations found in newborns, with many of these defects disrupting development of the outflow tract, the structure where blood is expelled from the heart. Despite their frequency, we do not have a grasp of the molecular and genetic mechanisms that underlie most congenital heart defects. Here, we show that zebrafish embryos containing a mutation in a gene called histone deacetylase 1 (hdac1) have smaller hearts with a reduction in the size of the ventricle and outflow tract. Hdac1 proteins limit accessibility to DNA and repress gene expression. We find that loss of Hdac1 in zebrafish embryos leads to increased expression of genes that are also induced by excess retinoic acid, a teratogen that induces similar outflow tract defects. Genetic loss-of-function studies support that ectopic expression of ripply3, a common target of both Hdac1 and retinoic acid signaling that is normally restricted to a subset of posterior pharyngeal cells, contributes to the smaller hearts found in zebrafish hdac1 mutants. Our study establishes a mechanism whereby the coordinated repression of genes downstream of Hdac1 and retinoic acid signaling is necessary for normal vertebrate outflow tract development.

FGF10 Signaling in Heart Development, Homeostasis, Disease and Repair

Essential muscular organ that provides the whole body with oxygen and nutrients, the heart is the first organ to function during embryonic development. Cardiovascular diseases, including acquired and congenital heart defects, are the leading cause of mortality in industrialized countries. Fibroblast Growth Factors (FGFs) are involved in a variety of cellular responses including proliferation, differentiation, and migration. Among the 22 human/mouse FGFs, the secreted FGF10 ligand through the binding of its specific receptors (FGFR1b and FGFR2b) and subsequent activation of downstream signaling is known to play essential role in cardiac development, homeostasis and disease. FGF10 is one of the major marker of the early cardiac progenitor cells and a crucial regulator of differentiated cardiomyocyte proliferation in the developing embryo. Increasing evidence support the hypothesis that a detailed understanding of developmental processes is essential to identify targets for cardiac repair and regeneration. Indeed the activation of resident cardiomyocyte proliferation together with the injection of cardiac progenitors represent the most promising therapeutical strategies for cardiac regenerative medicine. The recent findings showing that FGF10 promotes adult cardiomyocyte cell cycle reentry and directs stem cell differentiation and cell reprogramming toward the cardiogenic lineage provide new insights into therapeutical strategies for cardiac regeneration and repair.

Endothelial-to-mesenchymal transition in cardiovascular diseases: Developmental signaling pathways gone awry

The process named endothelial-to-mesenchymal transition (EndMT) was observed for the first time during the development of the chicken embryo several decades ago. Of interest, accumulating evidence suggests that EndMT plays a critical role in the onset and progression of multiple postnatal cardiovascular diseases. EndMT is controlled by a set of developmental signaling pathways, very similar to the process of epithelial-to-mesenchymal transition, which determine the activity of several EndMT transcriptional effectors. Once activated, these EndMT effectors regulate the expression of endothelial- and mesenchymal-specific genes, in part by interacting with specific motifs in promoter regions, eventually leading to the down-regulation of endothelial-specific features and acquisition of a fibroblast-like phenotype. Important technical advances in lineage tracing methods combined with experimental mouse models demonstrated the pathophysiological importance of EndMT for human diseases. In this review, we discuss the major signal transduction pathways involved in the activation and regulation of the EndMT program. Furthermore, we will review the latest discoveries on EndMT, focusing on cardiovascular diseases, and in particular on its role in vascular calcification, pulmonary arterial hypertension, and organ fibrosis. Developmental Dynamics 247:492-508, 2018. © 2017 Wiley Periodicals, Inc.

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.

Conserved signaling mechanisms in Drosophila heart development

Signal transduction through multiple distinct pathways regulates and orchestrates the numerous biological processes comprising heart development. This review outlines the roles of the FGFR, EGFR, Wnt, BMP, Notch, Hedgehog, Slit/Robo, and other signaling pathways during four sequential phases of Drosophila cardiogenesis-mesoderm migration, cardiac mesoderm establishment, differentiation of the cardiac mesoderm into distinct cardiac cell types, and morphogenesis of the heart and its lumen based on the proper positioning and cell shape changes of these differentiated cardiac cells-and illustrates how these same cardiogenic roles are conserved in vertebrates. Mechanisms bringing about the regulation and combinatorial integration of these diverse signaling pathways in Drosophila are also described. This synopsis of our present state of knowledge of conserved signaling pathways in Drosophila cardiogenesis and the means by which it was acquired should facilitate our understanding of and investigations into related processes in vertebrates. Developmental Dynamics 246:641-656, 2017. © 2017 Wiley Periodicals, Inc.

Pdgfrα functions in endothelial-derived cells to regulate neural crest cells and the development of the great arteries

Originating as a single vessel emerging from the embryonic heart, the truncus arteriosus must septate and remodel into the aorta and pulmonary artery to support postnatal life. Defective remodeling or septation leads to abnormalities collectively known as conotruncal defects, which are associated with significant mortality and morbidity. Multiple populations of cells must interact to coordinate outflow tract remodeling, and the cardiac neural crest has emerged as particularly important during this process. Abnormalities in the cardiac neural crest have been implicated in the pathogenesis of multiple conotruncal defects, including persistent truncus arteriosus, double outlet right ventricle and tetralogy of Fallot. However, the role of the neural crest in the pathogenesis of another conotruncal abnormality, transposition of the great arteries, is less well understood. In this report, we demonstrate an unexpected role of Pdgfra in endothelial cells and their derivatives during outflow tract development. Loss of Pdgfra in endothelium and endothelial-derived cells results in double outlet right ventricle and transposition of the great arteries. Our data suggest that loss of Pdgfra in endothelial-derived mesenchyme in the outflow tract endocardial cushions leads to a secondary defect in neural crest migration during development.

Summary: Loss of Pdgfrα in endothelial-derived mesenchyme results in defective neural crest behavior and is associated with conotruncal defects including, surprisingly, transposition of the great arteries.

Association Between Single Nucleotide Polymorphisms in NFATC1 Signaling Pathway Genes and Susceptibility to Congenital Heart Disease in the Chinese Population

The nuclear factor of activated T lymphocytes (NFATC1) signaling has been demonstrated to play important roles in cardiac valve and septal development. Genetic variants in genes involved in NFATC1 signaling may affect their expression and promote the formation of congenital heart disease (CHD). The goal of this study was to investigate the associations of single nucleotide polymorphism (SNP) in seven genes (NFATC1, VEGFR, VEGF, RANKL, FGFR1, BCL-6 and ZNRD1) with the risk of CHD. Twenty-nine polymorphisms were genotyped by using MassARRAY RS1000 platform in 277 CHD child patients and 293 controls from the Henan Province in China. Fours SNPs were excluded for the association analysis because of deviation from the Hardy-Weinberg equilibrium. Of the 25 SNPs, only two were found to be significantly associated with increased CHD risk after Bonferroni correction (RANKL, rs4531631: homozygous, AA vs. GG; OR 2.38, 95 % CI 1.40-4.07, p = 0.001; recessive, AA vs. AG + GG; OR 2.54, 95 % CI 1.53-4.22, p = 0.0003; FGFR1, rs13317: recessive, CC vs. CT + TT; OR 2.06, 95 % CI 1.30-3.25, p = 0.00196). Our findings suggest rs4531631 and rs13317 may be potential biomarkers for genetic diagnosis and treatment of CHD.

Wnt/β-catenin signaling enables developmental transitions during valvulogenesis

Heart valve development proceeds through coordinated steps by which endocardial cushions (ECs) form thin, elongated and stratified valves. Wnt signaling and its canonical effector β-catenin are proposed to contribute to endocardial-to-mesenchymal transformation (EMT) through postnatal steps of valvulogenesis. However, genetic redundancy and lethality have made it challenging to define specific roles of the canonical Wnt pathway at different stages of valve formation. We developed a transgenic mouse system that provides spatiotemporal inhibition of Wnt/β-catenin signaling by chemically inducible overexpression of Dkk1. Unexpectedly, this approach indicates canonical Wnt signaling is required for EMT in the proximal outflow tract (pOFT) but not atrioventricular canal (AVC) cushions. Furthermore, Wnt indirectly promotes pOFT EMT through its earlier activity in neighboring myocardial cells or their progenitors. Subsequently, Wnt/β-catenin signaling is activated in cushion mesenchymal cells where it supports FGF-driven expansion of ECs and then AVC valve extracellular matrix patterning. Mice lacking Axin2, a negative Wnt regulator, have larger valves, suggesting that accumulating Axin2 in maturing valves represents negative feedback that restrains tissue overgrowth rather than simply reporting Wnt activity. Disruption of these Wnt/β-catenin signaling roles that enable developmental transitions during valvulogenesis could account for common congenital valve defects.

Hepatocyte FRS2α is essential for the endocrine fibroblast growth factor to limit the amplitude of bile acid production induced by prandial activity

In addition to being positively regulated by prandial activity, bile acid production is also negatively controlled by the endocrine fibroblast growth factor 19 (FGF19) or the mouse ortholog FGF15 from the ileum that represses hepatic cholesterol 7 α-hydroxylase (Cyp7a1) expression through activating FGF receptor four (FGFR4). However, how these two regulatory mechanisms interplay to control bile acid homeostasis in the body and the downstream pathways by which FGFR4 regulates Cyp7a1 expression are not fully understood. Here we report that hepatocyte FGFR substrate 2α (FRS2α), a scaffold protein essential for canonical FGFRs to activate the ERK and AKT pathways, was required for the regulation of bile acid production by the FGF15/19-FGFR4 signaling axis. This occurred through limiting the extent of increases in Cyp7a1 expression induced by prandial activity. Excess FGFR4 kinase activity reduced the amplitude of the increase whereas a lack of FGFR4 augmented the increase of Cyp7a1 expression in the liver. Ablation of Frs2α alleles in hepatocytes abrogated the regulation of Cyp7a1 expression by FGFR4. Together, the results demonstrate that FRS2α-mediated pathways are essential for the FGF15/FGF19-FGFR4 signaling axis to control bile acid homeostasis.

Loss of Wnt5a disrupts second heart field cell deployment and may contribute to OFT malformations in DiGeorge syndrome

Outflow tract (OFT) malformation accounts for ∼30% of human congenital heart defects and manifests frequently in TBX1 haplo-insufficiency associated DiGeorge (22q11.2 deletion) syndrome. OFT myocardium originates from second heart field (SHF) progenitors in the pharyngeal and splanchnic mesoderm (SpM), but how these progenitors are deployed to the OFT is unclear. We find that SHF progenitors in the SpM gradually gain epithelial character and are deployed to the OFT as a cohesive sheet. Wnt5a, a non-canonical Wnt, is expressed specifically in the caudal SpM and may regulate oriented cell intercalation to incorporate SHF progenitors into an epithelial-like sheet, thereby generating the pushing force to deploy SHF cells rostrally into the OFT. Using enhancer trap and Cre transgenes, our lineage tracing experiments show that in Wnt5a null mice, SHF progenitors are trapped in the SpM and fail to be deployed to the OFT efficiently, resulting in a reduction in the inferior OFT myocardial wall and its derivative, subpulmonary myocardium. Concomitantly, the superior OFT and subaortic myocardium are expanded. Finally, in chick embryos, blocking the Wnt5a function in the caudal SpM perturbs polarized elongation of SHF progenitors, and compromises their deployment to the OFT. Collectively, our results highlight a critical role for Wnt5a in deploying SHF progenitors from the SpM to the OFT. Given that Wnt5a is a putative transcriptional target of Tbx1, and the similar reduction of subpulmonary myocardium in Tbx1 mutant mice, our results suggest that perturbing Wnt5a-mediated SHF deployment may be an important pathogenic mechanism contributing to OFT malformations in DiGeorge syndrome.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

A roadmap to investigate the genetic basis of bicuspid aortic valve and its complications: insights from the International BAVCon (Bicuspid Aortic Valve Consortium)

Bicuspid aortic valve (BAV) is the most common adult congenital heart defect and is found in 0.5% to 2.0% of the general population. The term "BAV" refers to a heterogeneous group of disorders characterized by diverse aortic valve malformations with associated aortopathy, congenital heart defects, and genetic syndromes. Even after decades of investigation, the genetic determinants of BAV and its complications remain largely undefined. Just as BAV phenotypes are highly variable, the genetic etiologies of BAV are equally diverse and vary from complex inheritance in families to sporadic cases without any evidence of inheritance. In this paper, the authors discuss current concepts in BAV genetics and propose a roadmap for unraveling unanswered questions about BAV through the integrated analysis of genetic and clinical data.

The functional diversity of essential genes required for mammalian cardiac development

Genes required for an organism to develop to maturity (for which no other gene can compensate) are considered essential. The continuing functional annotation of the mouse genome has enabled the identification of many essential genes required for specific developmental processes including cardiac development. Patterns are now emerging regarding the functional nature of genes required at specific points throughout gestation. Essential genes required for development beyond cardiac progenitor cell migration and induction include a small and functionally homogenous group encoding transcription factors, ligands and receptors. Actions of core cardiogenic transcription factors from the Gata, Nkx, Mef, Hand, and Tbx families trigger a marked expansion in the functional diversity of essential genes from midgestation onwards. As the embryo grows in size and complexity, genes required to maintain a functional heartbeat and to provide muscular strength and regulate blood flow are well represented. These essential genes regulate further specialization and polarization of cell types along with proliferative, migratory, adhesive, contractile, and structural processes. The identification of patterns regarding the functional nature of essential genes across numerous developmental systems may aid prediction of further essential genes and those important to development and/or progression of disease. genesis 52:713–737, 2014.

Heparan sulfate expression in the neural crest is essential for mouse cardiogenesis

Impaired heparan sulfate (HS) synthesis in vertebrate development causes complex malformations due to the functional disruption of multiple HS-binding growth factors and morphogens. Here, we report developmental heart defects in mice bearing a targeted disruption of the HS-generating enzyme GlcNAc N-deacetylase/GlcN N-sulfotransferase 1 (NDST1), including ventricular septal defects (VSD), persistent truncus arteriosus (PTA), double outlet right ventricle (DORV), and retroesophageal right subclavian artery (RERSC). These defects closely resemble cardiac anomalies observed in mice made deficient in the cardiogenic regulator fibroblast growth factor 8 (FGF8). Consistent with this, we show that HS-dependent FGF8/FGF-receptor2C assembly and FGF8-dependent ERK-phosphorylation are strongly reduced in NDST1(-/-) embryonic cells and tissues. Moreover, WNT1-Cre/LoxP-mediated conditional targeting of NDST function in neural crest cells (NCCs) revealed that their impaired HS-dependent development contributes strongly to the observed cardiac defects. These findings raise the possibility that defects in HS biosynthesis may contribute to congenital heart defects in humans that represent the most common type of birth defect.

The Role of fibroblast growth factor receptor substrate 2 (FRS2) in the regulation of two activity levels of the components of the extracellular signal-regulated kinase (ERK) pathway in the mouse epididymis

The components of the extracellular signal-regulated kinase (ERK) pathway are involved in the regulation of epididymal cellular processes. Interestingly, our previous studies showed that there are two different activity levels of the ERK pathway components in the epididymal epithelium: a basal level in most regions and a higher level in the differentiated initial segment (IS). In this study we analyzed the role of fibroblast growth factor receptor substrate 2 (FRS2) in the regulation of these two levels. Two mouse models were generated. In the first model, Frs2 was deleted from epithelial cells of most epididymal regions except for the IS from the embryonic period onward. Loss of Frs2 dampened the basal activity level of the ERK pathway components, which resulted in an increase in apoptosis along the epididymal duct. This was observed during the period when FRS2 expression level was highest in wild-type epididymides. In the second model, Frs2 was deleted from the proximal epididymal epithelium from Postnatal Day 17 onward. Most of the epididymides in this model exhibited normal morphology. Loss of Frs2 in these epididymides did not affect the high activity level of the ERK pathway components in the IS. However, a subgroup of epididymides in the second model showed increased apoptosis which resulted in an abnormally shaped proximal region or development of granulomas. Therefore, data from these two models showed that FRS2 played different roles in the regulation of two activity levels of the ERK pathway components in the epididymis.

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.

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

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

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.

The role of endothelial-mesenchymal transition in development and pathological process

Epithelial-mesenchymal transition is an important developmental process, participates in tumor's formation, invasion, and metastasis and has been extensively studied. Recently, endothelial-mesenchymal transition (EndMT), a newly recognized type of cellular transdifferentiation, has been demonstrated to participate in a number of diseases by causing morphology changes and pathological processes. Previous studies showed that EndMT was a critical process of embryonic cardiac development. Not only that recent advances also suggested that EndMT occurred postnatally in cancer and cardiac fibrosis and emerged as a possible source of cancer-associated fibroblasts (CAFs). CAFs were found to acquire properties that promoted tumor development and metastasis formation. Resident endothelial cells undergoing EndMT lose their endothelial markers, acquire a mesenchymal or myofibroblastic phenotype, express mesenchymal cell products such as α-smooth muscle actin and type I collagen and develop invasive and migratory abilities. EndMT-derived cells are believed to function as fibroblasts in damaged tissue and may therefore have an important role in pathological process. However, little is known about the signaling mechanisms that cause endothelial cells to transform into mesenchymal cells. Transforming growth factor-β, Notch, or other signaling pathways could direct or interact to mediate EndMT. Therefore, to explore the signaling mechanisms of EndMT may provide novel therapeutic strategies for treating cancer.

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.

PI3K/mTOR signaling regulates prostatic branching morphogenesis

Prostatic branching morphogenesis is an intricate event requiring precise temporal and spatial integration of numerous hormonal and growth factor-regulated inputs, yet relatively little is known about the downstream signaling pathways that orchestrate this process. In this study, we use a novel mesenchyme-free embryonic prostate culture system, newly available mTOR inhibitors and a conditional PTEN loss-of-function model to investigate the role of the interconnected PI3K and mTOR signaling pathways in prostatic organogenesis. We demonstrate that PI3K levels and PI3K/mTOR activity are robustly induced by androgen during murine prostatic development and that PI3K/mTOR signaling is necessary for prostatic epithelial bud invasion of surrounding mesenchyme. To elucidate the cellular mechanism by which PI3K/mTOR signaling regulates prostatic branching, we show that PI3K/mTOR inhibition does not significantly alter epithelial proliferation or apoptosis, but rather decreases the efficiency and speed with which the developing prostatic epithelial cells migrate. Using mTOR kinase inhibitors to tease out the independent effects of mTOR signaling downstream of PI3K, we find that simultaneous inhibition of mTORC1 and mTORC2 activity attenuates prostatic branching and is sufficient to phenocopy combined PI3K/mTOR inhibition. Surprisingly, however, mTORC1 inhibition alone has the reverse effect, increasing the number and length of prostatic branches. Finally, simultaneous activation of PI3K and downstream mTORC1/C2 via epithelial PTEN loss-of-function also results in decreased budding reversible by mTORC1 inhibition, suggesting that the effect of mTORC1 on branching is not primarily mediated by negative feedback on PI3K/mTORC2 signaling. Taken together, our data point to an important role for PI3K/mTOR signaling in prostatic epithelial invasion and migration and implicates the balance of PI3K and downstream mTORC1/C2 activity as a critical regulator of prostatic epithelial morphogenesis.

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.

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.

Mef2cb regulates late myocardial cell addition from a second heart field-like population of progenitors in zebrafish

Two populations of cells, termed the first and second heart field, drive heart growth during chick and mouse development. The zebrafish has become a powerful model for vertebrate heart development, partly due to the evolutionary conservation of developmental pathways in this process. Here we provide evidence that the zebrafish possesses a conserved homolog to the murine second heart field. We developed a photoconversion assay to observe and quantify the dynamic late addition of myocardial cells to the zebrafish arterial pole. We define an extra-cardiac region immediately posterior to the arterial pole, which we term the late ventricular region. The late ventricular region has cardiogenic properties, expressing myocardial markers such as vmhc and nkx2.5, but does not express a full complement of differentiated cardiomyocyte markers, lacking myl7 expression. We show that mef2cb, a zebrafish homolog of the mouse second heart field marker Mef2c, is expressed in the late ventricular region, and is necessary for late myocardial addition to the arterial pole. FGF signaling after heart cone formation is necessary for mef2cb expression, the establishment of the late ventricular region, and late myocardial addition to the arterial pole. Our study demonstrates that zebrafish heart growth shows more similarities to murine heart growth than previously thought. Further, as congenital heart disease is often associated with defects in second heart field development, the embryological and genetic advantages of the zebrafish model can be applied to study the vertebrate second heart field.

The FGF-BMP signaling axis regulates outflow tract valve primordium formation by promoting cushion neural crest cell differentiation

RATIONALE

Heart valves develop from precursor structures called cardiac cushions, an endothelial-lined cardiac jelly that resides in the inner side of the heart tube. The cushions are then invaded by cells from different sources, undergo a series of complicated and poorly understood remodeling processes, and give rise to valves. Disruption of the fibroblast growth factor (FGF) signaling axis impairs morphogenesis of the outflow tract (OFT). Yet, whether FGF signaling regulates OFT valve formation is unknown.

OBJECTIVE

To study how OFT valve formation is regulated and how aberrant cell signaling causes valve defects.

METHODS AND RESULTS

By using mouse genetic manipulation, cell lineage tracing, ex vivo heart culture, and molecular biology approaches, we demonstrated that FGF signaling in the OFT myocardium upregulated Bmp4 expression, which then enhanced smooth muscle differentiation of neural crest cells (NCCs) in the cushion. FGF signaling also promoted OFT myocardial cell invasion to the cushion. Disrupting FGF signaling interrupted cushion remodeling with reduced NCCs differentiation into smooth muscle and less cardiomyocyte invasion and resulted in malformed OFT valves.

CONCLUSIONS

The results demonstrate a novel mechanism by which the FGF-BMP signaling axis regulates formation of OFT valve primordia by controlling smooth muscle differentiation of cushion NCCs.

Partial rescue of the Tbx1 mutant heart phenotype by Fgf8: genetic evidence of impaired tissue response to Fgf8

Tbx1 is the candidate gene of DiGeorge syndrome and is required in humans and mice for the development of the cardiac outflow tract (OFT) and aortic arch arteries. Loss of function mutants present with reduced cell proliferation and premature differentiation of cardiac progenitor cells of the second heart field (SHF). Tbx1 regulates Fgf8 expression hence the hypothesis that the proliferation impairment may contribute to the heart phenotype of mutants. Here we show that forced Fgf8 expression modifies and partially rescues the OFT septation defects of Tbx1 mutants but only if there is some residual expression of Tbx1. This genetic experiment suggests that Tbx1, directly or indirectly, affects tissue response to Fgf8. Indeed, Tbx1(-/-) mouse embryonic fibroblasts were unable to respond to Fgf8 added to the culture media and showed defective response of Erk1/2 and Rsk1. Our data suggest a coordinated pathway modulating Fgf8 ligand expression and tissue response to it in the SHF.

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.

The formation of the embryonic mouse heart: heart fields and myocardial cell lineages

During cardiogenesis in the mouse, the second heart field (SHF) is the source of the myocardium of the outflow tract and it contributes to other regions of the heart with the exception of the primitive left ventricle. This contribution corresponds with that of the second myocardial cell lineage, identified by retrospective clonal analysis. Gene regulatory networks, signaling pathways, and heterogeneity within the SHF are discussed, together with the question of regulation of myocardial progenitor cells within the first heart field. The extension of the SHF into the mesodermal core of the arches also gives rise to endothelial cells of the pharyngeal arch arteries. Knowledge about the origin and genetic regulation of cells that contribute to the heart and associated vasculature is important for the diagnosis and treatment of congenital heart malformations.

Notch and cardiac outflow tract development

Congenital heart disease represents the most common form of human birth defect, occurring in nearly 1 in 100 live births. An increasing number of patients with these defects are surviving infancy. Approximately one-third of congenital heart defects involve malformations of the outflow tract. Related defects are found in isolation and as part of common human syndromes. Our laboratory has investigated mechanisms of cardiac morphogenesis with particular attention to outflow tract formation. During cardiogenesis, neural crest cells interact with second heart field myocardium and endocardial cushion mesenchyme. Our recent work demonstrates that Jagged1/Notch signaling within the second heart field initiates a signaling cascade involving Fgf8, Bmp4, and downstream effectors that modulate outflow tract development and aortic arch artery patterning. Hence, complex tissue-tissue interactions and integration of multiple pathways converge to orchestrate proper patterning of the outflow region. The role of Notch signaling in adult cardiac homeostasis and disease is an area of active investigation.

Tissue-tissue interactions during morphogenesis of the outflow tract

The heart forms as a linear heart tube that loops and septates to produce a mature four-chambered structure. The single vessel emerging from the embryonic heart, the truncus arteriosus, divides into the aorta and the pulmonary artery as part of this septation process, and a series of additional morphogenetic events result in the proper alignment and orientation of the cardiac outflow tract. Recent evidence indicates that this process involves the complex interactions of multiple cell types including primary and secondary heart fields, neural crest, pharyngeal mesenchyme, endoderm, and endothelium. Among the many signals that mediate tissue-tissue interactions during the formation of the outflow tract, we have focused on the role of the Notch signaling pathway. Here, we focus on recent advances in our understanding of Notch-mediated regulation of cardiac development with specific attention to the formation of the cardiac outflow tract.

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.

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.