Heart field origin of great vessel precursors relies on nkx2.5-mediated vasculogenesis

Buffering Mechanism in Aortic Arch Artery Formation and Congenital Heart Disease

BACKGROUND

The resiliency of embryonic development to genetic and environmental perturbations has been long appreciated; however, little is known about the mechanisms underlying the robustness of developmental processes. Aberrations resulting in neonatal lethality are exemplified by congenital heart disease arising from defective morphogenesis of pharyngeal arch arteries (PAAs) and their derivatives.

METHODS

Mouse genetics, lineage tracing, confocal microscopy, and quantitative image analyses were used to investigate mechanisms of PAA formation and repair.

RESULTS

The second heart field (SHF) gives rise to the PAA endothelium. Here, we show that the number of SHF-derived endothelial cells (ECs) is regulated by VEGFR2 (vascular endothelial growth factor receptor 2) and Tbx1. Remarkably, when the SHF-derived EC number is decreased, PAA development can be rescued by the compensatory endothelium. Blocking such compensatory response leads to embryonic demise. To determine the source of compensating ECs and mechanisms regulating their recruitment, we investigated 3-dimensional EC connectivity, EC fate, and gene expression. Our studies demonstrate that the expression of VEGFR2 by the SHF is required for the differentiation of SHF-derived cells into PAA ECs. The deletion of 1 VEGFR2 allele (VEGFR2SHF-HET) reduces SHF contribution to the PAA endothelium, while the deletion of both alleles (VEGFR2SHF-KO) abolishes it. The decrease in SHF-derived ECs in VEGFR2SHF-HET and VEGFR2SHF-KO embryos is complemented by the recruitment of ECs from the nearby veins. Compensatory ECs contribute to PAA derivatives, giving rise to the endothelium of the aortic arch and the ductus in VEGFR2SHF-KO mutants. Blocking the compensatory response in VEGFR2SHF-KO mutants results in embryonic lethality shortly after mid-gestation. The compensatory ECs are absent in Tbx1+/- embryos, a model for 22q11 deletion syndrome, leading to unpredictable arch artery morphogenesis and congenital heart disease. Tbx1 regulates the recruitment of the compensatory endothelium in an SHF-non-cell-autonomous manner.

CONCLUSIONS

Our studies uncover a novel buffering mechanism underlying the resiliency of PAA development and remodeling.

NKX2-5 regulates vessel remodeling in scleroderma-associated pulmonary arterial hypertension

NKX2-5 is a member of the homeobox-containing transcription factors critical in regulating tissue differentiation in development. Here, we report a role for NKX2-5 in vascular smooth muscle cell phenotypic modulation in vitro and in vascular remodeling in vivo. NKX2-5 is upregulated in scleroderma patients with pulmonary arterial hypertension. Suppression of NKX2-5 expression in smooth muscle cells halted vascular smooth muscle proliferation and migration, enhanced contractility, and blocked the expression of extracellular matrix genes. Conversely, overexpression of NKX2-5 suppressed the expression of contractile genes (ACTA2, TAGLN, CNN1) and enhanced the expression of matrix genes (COL1) in vascular smooth muscle cells. In vivo, conditional deletion of NKX2-5 attenuated blood vessel remodeling and halted the progression to hypertension in a mouse chronic hypoxia model. This study revealed that signals related to injury such as serum and low confluence, which induce NKX2-5 expression in cultured cells, is potentiated by TGF-β and further enhanced by hypoxia. The effect of TGF-β was sensitive to ERK5 and PI3K inhibition. Our data suggest a pivotal role for NKX2-5 in the phenotypic modulation of smooth muscle cells during pathological vascular remodeling and provide proof of concept for therapeutic targeting of NKX2-5 in vasculopathies.

Identification of novel buffering mechanisms in aortic arch artery development and congenital heart disease

Rationale

The resiliency of embryonic development to genetic and environmental perturbations has been long appreciated; however, little is known about the mechanisms underlying the robustness of developmental processes. Aberrations resulting in neonatal lethality are exemplified by congenital heart disease (CHD) arising from defective morphogenesis of pharyngeal arch arteries (PAA) and their derivatives.

Objective

To uncover mechanisms underlying the robustness of PAA morphogenesis.

Methods and Results

The second heart field (SHF) gives rise to the PAA endothelium. Here, we show that the number of SHF-derived ECs is regulated by VEGFR2 and Tbx1 . Remarkably, when SHF-derived EC number is decreased, PAA development can be rescued by the compensatory endothelium. Blocking such compensatory response leads to embryonic demise. To determine the source of compensating ECs and mechanisms regulating their recruitment, we investigated three-dimensional EC connectivity, EC fate, and gene expression. Our studies demonstrate that the expression of VEGFR2 by the SHF is required for the differentiation of SHF-derived cells into PAA ECs. The deletion of one VEGFR2 allele (VEGFR2 SHF-HET ) reduces SHF contribution to the PAA endothelium, while the deletion of both alleles (VEGFR2 SHF-KO ) abolishes it. The decrease in SHF-derived ECs in VEGFR2 SHF-HET and VEGFR2 SHF-KO embryos is complemented by the recruitment of ECs from the nearby veins. Compensatory ECs contribute to PAA derivatives, giving rise to the endothelium of the aortic arch and the ductus in VEGFR2 SHF-KO mutants. Blocking the compensatory response in VEGFR2 SHF-KO mutants results in embryonic lethality shortly after mid-gestation. The compensatory ECs are absent in Tbx1 +/- embryos, a model for 22q11 deletion syndrome, leading to unpredictable arch artery morphogenesis and CHD. Tbx1 regulates the recruitment of the compensatory endothelium in an SHF-non-cell-autonomous manner.

Conclusions

Our studies uncover a novel buffering mechanism underlying the resiliency of PAA development and remodeling.

Nonstandard Abbreviations and Acronyms in Alphabetical Order

CHD - congenital heart disease; ECs - endothelial cells; IAA-B - interrupted aortic arch type B; PAA - pharyngeal arch arteries; RERSA - retro-esophageal right subclavian artery; SHF - second heart field; VEGFR2 - Vascular endothelial growth factor receptor 2.

Cardiac competence of the paraxial head mesoderm fades concomitant with a shift towards the head skeletal muscle programme

The vertebrate head mesoderm provides the heart, the great vessels, some smooth and most head skeletal muscle, in addition to parts of the skull. It has been speculated that the ability to generate cardiac and smooth muscle is the evolutionary ground-state of the tissue. However, whether indeed the entire head mesoderm has generic cardiac competence, how long this may last, and what happens as cardiac competence fades, is not clear. Bone morphogenetic proteins (Bmps) are known to promote cardiogenesis. Using 41 different marker genes in the chicken embryo, we show that the paraxial head mesoderm that normally does not engage in cardiogenesis has the ability to respond to Bmp for a long time. However, Bmp signals are interpreted differently at different time points. Up to early head fold stages, the paraxial head mesoderm is able to read Bmps as signal to engage in the cardiac programme; the ability to upregulate smooth muscle markers is retained slightly longer. Notably, as cardiac competence fades, Bmp promotes the head skeletal muscle programme instead. The switch from cardiac to skeletal muscle competence is Wnt-independent as Wnt caudalises the head mesoderm and also suppresses Msc-inducing Bmp provided by the prechordal plate, thus suppressing both the cardiac and the head skeletal muscle programmes. Our study for the first time suggests a specific transition state in the embryo when cardiac competence is replaced by skeletal muscle competence. It sets the stage to unravel the cardiac-skeletal muscle antagonism that is known to partially collapse in heart failure.

ETV2 Upregulation Marks the Specification of Early Cardiomyocytes and Endothelial Cells During Co-differentiation

The ability to differentiate human-induced pluripotent stem cells (hiPSCs) efficiently into defined cardiac lineages, such as cardiomyocytes and cardiac endothelial cells, is crucial to study human heart development and model cardiovascular diseases in vitro. The mechanisms underlying the specification of these cell types during human development are not well understood which limits fine-tuning and broader application of cardiac model systems. Here, we used the expression of ETV2, a master regulator of hematoendothelial specification in mice, to identify functionally distinct subpopulations during the co-differentiation of endothelial cells and cardiomyocytes from hiPSCs. Targeted analysis of single-cell RNA-sequencing data revealed differential ETV2 dynamics in the 2 lineages. A newly created fluorescent reporter line allowed us to identify early lineage-predisposed states and show that a transient ETV2-high-state initiates the specification of endothelial cells. We further demonstrated, unexpectedly, that functional cardiomyocytes can originate from progenitors expressing ETV2 at a low level. Our study thus sheds light on the in vitro differentiation dynamics of 2 important cardiac lineages.

RBPMS2 Is a Myocardial-Enriched Splicing Regulator Required for Cardiac Function

BACKGROUND

RBPs (RNA-binding proteins) perform indispensable functions in the post-transcriptional regulation of gene expression. Numerous RBPs have been implicated in cardiac development or physiology based on gene knockout studies and the identification of pathogenic RBP gene mutations in monogenic heart disorders. The discovery and characterization of additional RBPs performing indispensable functions in the heart will advance basic and translational cardiovascular research.

METHODS

We performed a differential expression screen in zebrafish embryos to identify genes enriched in nkx2.5-positive cardiomyocytes or cardiopharyngeal progenitors compared to nkx2.5-negative cells from the same embryos. We investigated the myocardial-enriched gene RNA-binding protein with multiple splicing (variants) 2 [RBPMS2)] by generating and characterizing rbpms2 knockout zebrafish and human cardiomyocytes derived from RBPMS2-deficient induced pluripotent stem cells.

RESULTS

We identified 1848 genes enriched in the nkx2.5-positive population. Among the most highly enriched genes, most with well-established functions in the heart, we discovered the ohnologs rbpms2a and rbpms2b, which encode an evolutionarily conserved RBP. Rbpms2 localizes selectively to cardiomyocytes during zebrafish heart development and strong cardiomyocyte expression persists into adulthood. Rbpms2-deficient embryos suffer from early cardiac dysfunction characterized by reduced ejection fraction. The functional deficit is accompanied by myofibril disarray, altered calcium handling, and differential alternative splicing events in mutant cardiomyocytes. These phenotypes are also observed in RBPMS2-deficient human cardiomyocytes, indicative of conserved molecular and cellular function. RNA-sequencing and comparative analysis of genes mis-spliced in RBPMS2-deficient zebrafish and human cardiomyocytes uncovered a conserved network of 29 ortholog pairs that require RBPMS2 for alternative splicing regulation, including RBFOX2, SLC8A1, and MYBPC3.

CONCLUSIONS

Our study identifies RBPMS2 as a conserved regulator of alternative splicing, myofibrillar organization, and calcium handling in zebrafish and human cardiomyocytes.

Chemokine signaling synchronizes angioblast proliferation and differentiation during pharyngeal arch artery vasculogenesis

Developmentally, the great vessels of the heart originate from the pharyngeal arch arteries (PAAs). During PAA vasculogenesis, PAA precursors undergo sequential cell fate decisions that are accompanied by proliferative expansion. However, how these two processes are synchronized remains poorly understood. Here, we find that the zebrafish chemokine receptor Cxcr4a is expressed in PAA precursors, and genetic ablation of either cxcr4a or the ligand gene cxcl12b causes PAA stenosis. Cxcr4a is required for the activation of the downstream PI3K/AKT cascade, which promotes not only PAA angioblast proliferation, but also differentiation. AKT has a well-known role in accelerating cell-cycle progression through the activation of cyclin-dependent kinases. Despite this, we demonstrate that AKT phosphorylates Etv2 and Scl, the key regulators of angioblast commitment, on conserved serine residues, thereby protecting them from ubiquitin-mediated proteasomal degradation. Altogether, our study reveals a central role for chemokine signaling in PAA vasculogenesis through orchestrating angioblast proliferation and differentiation.

Intrinsic myocardial defects underlie an Rbfox-deficient zebrafish model of hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is characterized by underdevelopment of left sided structures including the ventricle, valves, and aorta. Prevailing paradigm suggests that HLHS is a multigenic disease of co-occurring phenotypes. Here, we report that zebrafish lacking two orthologs of the RNA binding protein RBFOX2, a gene linked to HLHS in humans, display cardiovascular defects overlapping those in HLHS patients including ventricular, valve, and aortic deficiencies. In contrast to current models, we demonstrate that these structural deficits arise secondary to impaired pump function as these phenotypes are rescued when Rbfox is specifically expressed in the myocardium. Mechanistically, we find diminished expression and alternative splicing of sarcomere and mitochondrial components that compromise sarcomere assembly and mitochondrial respiration, respectively. Injection of human RBFOX2 mRNA restores cardiovascular development in rbfox mutant zebrafish, while HLHS-linked RBFOX2 variants fail to rescue. This work supports an emerging paradigm for HLHS pathogenesis that centers on myocardial intrinsic defects.

GATA4/5/6 family transcription factors are conserved determinants of cardiac versus pharyngeal mesoderm fate

GATA4/5/6 transcription factors play essential, conserved roles in heart development. To understand how GATA4/5/6 modulates the mesoderm-to-cardiac fate transition, we labeled, isolated, and performed single-cell gene expression analysis on cells that express gata5 at precardiac time points spanning zebrafish gastrulation to somitogenesis. We found that most mesendoderm-derived lineages had dynamic gata5/6 expression. In the absence of Gata5/6, the population structure of mesendoderm-derived cells was substantially altered. In addition to the expected absence of cardiac mesoderm, we confirmed a concomitant expansion of cranial-pharyngeal mesoderm. Moreover, Gata5/6 loss led to extensive changes in chromatin accessibility near cardiac and pharyngeal genes. Functional analyses in zebrafish and the tunicate Ciona, which has a single GATA4/5/6 homolog, revealed that GATA4/5/6 acts upstream of tbx1 to exert essential and cell-autonomous roles in promoting cardiac and inhibiting pharyngeal mesoderm identity. Overall, cardiac and pharyngeal mesoderm fate choices are achieved through an evolutionarily conserved GATA4/5/6 regulatory network.

Latent TGFβ binding proteins 1 and 3 protect the larval zebrafish outflow tract from aneurysmal dilatation

Aortic root aneurysm is a common cause of morbidity and mortality in Loeys-Dietz and Marfan Syndromes, where perturbations in TGFβ signaling play a causal or contributory role, respectively. Despite the advantages of cross-species disease modeling, animal models of aortic root aneurysm are largely restricted to genetically engineered mice. Here, we report that zebrafish devoid of latent TGFβ binding protein (ltbp) 1 and 3 develop rapid and severe aneurysm of the outflow tract (OFT), the aortic root equivalent. Similar to syndromic aneurysm tissue, the distended OFTs display evidence for paradoxical hyperactivated TGFβ signaling. RNA-sequencing revealed significant overlap between the molecular signatures of disease tissue from mutant zebrafish and Marfan mice. Lastly, chemical inhibition of TGFβ signaling in wild-type animals phenocopied mutants but chemical activation did not, demonstrating that TGFβ signaling is protective against aneurysm. Human relevance is supported by recent studies implicating genetic lesions in LTBP3 and potentially LTBP1 as heritable causes of aortic root aneurysm. Ultimately, our data demonstrate that zebrafish can now be leveraged to interrogate thoracic aneurysmal disease and identify novel lead compounds through small molecule suppressor screens.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

Retinoic acid signaling restricts the size of the first heart field within the anterior lateral plate mesoderm

Retinoic acid (RA) signaling is required to restrict heart size through limiting the posterior boundary of the vertebrate cardiac progenitor field within the anterior lateral plate mesoderm (ALPM). However, we still do not fully understand how different cardiac progenitor populations that contribute to the developing heart, including earlier-differentiating first heart field (FHF), later-differentiating second heart field (SHF), and neural crest-derived progenitors, are each affected in RA-deficient embryos. Here, we quantified the number of cardiac progenitors and differentiating cardiomyocytes (CMs) in RA-deficient zebrafish embryos. While Nkx2.5+ cells were increased overall in the nascent hearts of RA-deficient embryos, unexpectedly, we found that the major effect within this population was a significant expansion in the number of differentiating FHF CMs. In contrast to the expansion of the FHF, there was a progressive decrease in SHF progenitors at the arterial pole as the heart tube elongated. Temporal differentiation assays and immunostaining in RA-deficient embryos showed that the outflow tracts (OFTs) of the hearts were significantly smaller, containing fewer differentiated SHF-derived ventricular CMs and a complete absence of SHF-derived smooth muscle at later stages. At the venous pole of the heart, pacemaker cells of the sinoatrial node also failed to differentiate in RA-deficient embryos. Interestingly, genetic lineage tracing showed that the number of neural-crest derived CMs was not altered within the enlarged hearts of RA-deficient zebrafish embryos. Altogether, our data show that the enlarged hearts in RA-deficient zebrafish embryos are comprised of an expansion in earlier differentiating FHF-derived CMs coupled with a progressive depletion of the SHF, suggesting RA signaling determines the relative ratios of earlier- and later-differentiation cardiac progenitors within an expanded cardiac progenitor pool.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Cell-Extracellular Matrix Interactions Play Multiple Essential Roles in Aortic Arch Development

RATIONALE

Defects in the morphogenesis of the fourth pharyngeal arch arteries (PAAs) give rise to lethal birth defects. Understanding genes and mechanisms regulating PAA formation will provide important insights into the etiology and treatments for congenital heart disease.

OBJECTIVE

Cell-ECM (extracellular matrix) interactions play essential roles in the morphogenesis of PAAs and their derivatives, the aortic arch artery and its major branches; however, their specific functions are not well-understood. Previously, we demonstrated that integrin α5β1 and Fn1 (fibronectin) expressed in the Isl1 lineages regulate PAA formation. The objective of the current studies was to investigate cellular mechanisms by which integrin α5β1 and Fn1 regulate aortic arch artery morphogenesis.

METHODS AND RESULTS

Using temporal lineage tracing, whole-mount confocal imaging, and quantitative analysis of the second heart field (SHF) and endothelial cell (EC) dynamics, we show that the majority of PAA EC progenitors arise by E7.5 in the SHF and contribute to pharyngeal arch endothelium between E7.5 and E9.5. Consequently, SHF-derived ECs in the pharyngeal arches form a plexus of small blood vessels, which remodels into the PAAs by 35 somites. The remodeling of the vascular plexus is orchestrated by signals dependent on the pharyngeal ECM microenvironment, extrinsic to the endothelium. Conditional ablation of integrin α5β1 or Fn1 in the Isl1 lineages showed that signaling by the ECM regulates aortic arch artery morphogenesis at multiple steps: (1) accumulation of SHF-derived ECs in the pharyngeal arches, (2) remodeling of the EC plexus in the fourth arches into the PAAs, and (3) differentiation of neural crest-derived cells adjacent to the PAA endothelium into vascular smooth muscle cells.

CONCLUSIONS

PAA formation is a multistep process entailing dynamic contribution of SHF-derived ECs to pharyngeal arches, the remodeling of endothelial plexus into the PAAs, and the remodeling of the PAAs into the aortic arch artery and its major branches. Cell-ECM interactions regulated by integrin α5β1 and Fn1 play essential roles at each of these developmental stages.

Pharyngeal pouches provide a niche microenvironment for arch artery progenitor specification

The paired pharyngeal arch arteries (PAAs) are transient blood vessels connecting the heart with the dorsal aorta during embryogenesis. Although PAA malformations often occur along with pharyngeal pouch defects, the functional interaction between these adjacent tissues remains largely unclear. Here, we report that pharyngeal pouches are essential for PAA progenitor specification in zebrafish embryos. We reveal that the segmentation of pharyngeal pouches coincides spatiotemporally with the emergence of PAA progenitor clusters. These pouches physically associate with pharyngeal mesoderm in discrete regions and provide a niche microenvironment for PAA progenitor commitment by expressing BMP proteins. Specifically, pouch-derived BMP2a and BMP5 are the primary niche cues responsible for activating the BMP/Smad pathway in pharyngeal mesoderm, thereby promoting progenitor specification. In addition, BMP2a and BMP5 play an inductive function in the expression of the cloche gene npas4l in PAA progenitors. cloche mutants exhibit a striking failure to specify PAA progenitors and display ectopic expression of head muscle markers in the pharyngeal mesoderm. Therefore, our results support a crucial role for pharyngeal pouches in establishing a progenitor niche for PAA morphogenesis via BMP2a/5 expression.

The Endocardium and Heart Valves

Endocardial cells are specialized endothelial cells that, during embryogenesis, form a lining on the inside of the developing heart, which is maintained throughout life. Endocardial cells are an essential source for several lineages of the cardiovascular system including coronary endothelium, endocardial cushion mesenchyme, cardiomyocytes, mural cells, fibroblasts, liver vasculature, adipocytes, and hematopoietic cells. Alterations in the differentiation programs that give rise to these lineages has detrimental effects, including premature lethality or significant structural malformations present at birth. Here, we will review the literature pertaining to the contribution of endocardial cells to valvular, and nonvalvular lineages and highlight critical pathways required for these processes. The lineage differentiation potential of embryonic, and possibly adult, endocardial cells has therapeutic potential in the regeneration of damaged cardiac tissue or treatment of cardiovascular diseases.

SNPs in Sites for DNA Methylation, Transcription Factor Binding, and miRNA Targets Leading to Allele-Specific Gene Expression and Contributing to Complex Disease Risk: A Systematic Review

INTRODUCTION

The complex genetic diversity among human populations results from an assortment of factors acting at various sequential levels, including mutations, population migrations, genetic drift, and selection. Although there are a plethora of DNA sequence variations identified through genome-wide association studies (GWAS), the challenge remains to explain the mechanisms underlying interindividual phenotypic disparity accounting for disease susceptibility. Single nucleotide polymorphisms (SNPs) present in the sites for DNA methylation, transcription factor (TF) binding, or miRNA targets can alter the gene expression. The systematic review aimed to evaluate the complex crosstalk among SNPs, miRNAs, DNA methylation, and TFs for complex multifactorial disease risk.

METHODS

PubMed and Scopus databases were used from inception until May 15, 2019. Initially, screening of articles involved studies assessing the interaction of SNPs with TFs, DNA methylation, or miRNAs resulting in allele-specific gene expression in complex multifactorial diseases. We also included the studies which provided experimental validation of the interaction of SNPs with each of these factors. The results from various studies on multifactorial diseases were assessed.

RESULTS

A total of 11 articles for SNPs interacting with DNA methylation, 30 articles for SNPs interacting with TFs, and 11 articles for SNPs in miRNA binding sites were selected. The interactions of SNPs with epigenetic factors were found to be implicated in different types of cancers, autoimmune diseases, cardiovascular diseases, diabetes, and asthma.

CONCLUSION

The systematic review provides evidence for the interplay between genetic and epigenetic risk factors through allele-specific gene expression in various complex multifactorial diseases.

Endothelial cells on the move: dynamics in vascular morphogenesis and disease

The vascular system is a hierarchically organized network of blood vessels that play crucial roles in embryogenesis, homeostasis and disease. Blood vessels are built by endothelial cells – the cells lining the interior of blood vessels – through a process named vascular morphogenesis. Endothelial cells react to different biomechanical signals in their environment by adjusting their behavior to: (1) invade, proliferate and fuse to form new vessels (angiogenesis); (2) remodel, regress and establish a hierarchy in the network (patterning); and (3) maintain network stability (quiescence). Each step involves the coordination of endothelial cell differentiation, proliferation, polarity, migration, rearrangements and shape changes to ensure network integrity and an efficient barrier between blood and tissues. In this review, we highlighted the relevance and the mechanisms involving endothelial cell migration during different steps of vascular morphogenesis. We further present evidence on how impaired endothelial cell dynamics can contribute to pathology.

Pbx4 limits heart size and fosters arch artery formation by partitioning second heart field progenitors and restricting proliferation

Vertebrate heart development requires the integration of temporally distinct differentiating progenitors. However, few signals are understood that restrict the size of the later-differentiating outflow tract (OFT). We show that improper specification and proliferation of second heart field (SHF) progenitors in zebrafish lazarus (lzr) mutants, which lack the transcription factor Pbx4, produces enlarged hearts owing to an increase in ventricular and smooth muscle cells. Specifically, Pbx4 initially promotes the partitioning of the SHF into anterior progenitors, which contribute to the OFT, and adjacent endothelial cell progenitors, which contribute to posterior pharyngeal arches. Subsequently, Pbx4 limits SHF progenitor (SHFP) proliferation. Single cell RNA sequencing of nkx2.5+ cells revealed previously unappreciated distinct differentiation states and progenitor subpopulations that normally reside within the SHF and arterial pole of the heart. Specifically, the transcriptional profiles of Pbx4-deficient nkx2.5+ SHFPs are less distinct and display characteristics of normally discrete proliferative progenitor and anterior, differentiated cardiomyocyte populations. Therefore, our data indicate that the generation of proper OFT size and arch arteries requires Pbx-dependent stratification of unique differentiation states to facilitate both homeotic-like transformations and limit progenitor production within the SHF.

Ibuprofen and diclofenac impair the cardiovascular development of zebrafish (Danio rerio) at low concentrations

The non-steroidal anti-inflammatory drugs (NSAIDs) ibuprofen and diclofenac are highly prescribed worldwide and their presence in aquatic system may pose a potential risk to aquatic organisms. Here, we systematically assessed their cardiovascular disruptive effects in zebrafish (Danio rerio) at environmentally relevant concentrations between 0.04 and 25.0 μg/L. Ibuprofen significantly increased the cardiac outputs of zebrafish embryos at actual concentrations of 0.91, 4.3 and 21.9 μg/L. It up-regulated the blood cell velocity, total blood flow and down-regulated the blood cell density at concentrations of 4.3 μg/L and higher. In comparison, diclofenac led to inhibition of spontaneous muscle contractions and decreased hatching rate of zebrafish embryos at the highest concentration (24.1 μg/L), while it had negligible effects on the cardiac physiology and hemodynamics. Transcriptional analysis of biomarker genes involved in cardiovascular physiology, such as the significantly up-regulated nppa and nkx2.5 expressions response to ibuprofen but not to diclofenac, is consistent with these observations. In addition, both ibuprofen and diclofenac altered the morphology of intersegmental vessels at high concentrations. Our results revealed unexpected cardiovascular functional alterations of NSAIDs to fish at environmental or slightly higher than surface water concentrations and thus provided novel insights into the understanding of their potential environmental risks.

Failed Progenitor Specification Underlies the Cardiopharyngeal Phenotypes in a Zebrafish Model of 22q11.2 Deletion Syndrome

Microdeletions involving TBX1 result in variable congenital malformations known collectively as 22q11.2 deletion syndrome (22q11.2DS). Tbx1-deficient mice and zebrafish recapitulate several disease phenotypes, including pharyngeal arch artery (PAA), head muscle (HM), and cardiac outflow tract (OFT) deficiencies. In zebrafish, these structures arise from nkx2.5⁺ progenitors in pharyngeal arches 2-6. Because pharyngeal arch morphogenesis is compromised in Tbx1-deficient animals, the malformations were considered secondary. Here, we report that the PAA, HM, and OFT phenotypes in tbx1 mutant zebrafish are primary and arise prior to pharyngeal arch morphogenesis from failed specification of the nkx2.5⁺ pharyngeal lineage. Through in situ analysis and lineage tracing, we reveal that nkx2.5 and tbx1 are co-expressed in this progenitor population. Furthermore, we present evidence suggesting that gdf3-ALK4 signaling is a downstream mediator of nkx2.5⁺ pharyngeal lineage specification. Collectively, these studies support a cellular mechanism potentially underlying the cardiovascular and craniofacial defects observed in the 22q11.2DS population.

Exploring the Activities of RBPMS Proteins in Myocardial Biology

Numerous RNA-binding proteins (RBPs) are expressed in the heart, and mutations in several RBPs have been implicated in cardiovascular disease through genetic associations, animal modeling, and mechanistic studies. However, the functions of many more cardiac RBPs, and their relevance to disease states, remain to be elucidated. Recently, we have initiated studies to characterize the functions of the RBPs RBPMS and RBPMS2 in regulating myocardial biology in zebrafish and higher vertebrate species. These studies began when we learned, using an unbiased gene discovery approach, that rbpms2a and rbpms2b in zebrafish are robust markers of embryonic myocardium. This observation, which is consistent with published data, suggests that the encoded proteins are likely to be performing critical functions in regulating one or more aspects of cardiomyocyte differentiation, proliferation, survival, and/or contractility. This notion is supported by recent reports demonstrating that zebrafish embryos with disrupted Rbpms2 function exhibit gross signs of cardiac distress. Interestingly, a 20-year-old study determined that myocardial tissue from the frog, chick, and mouse also express high levels of Rbpms and/or Rbpms2, which is suggestive of evolutionary conservation of function. In this review, we will provide a historical account of how RBPMS and RBPMS2 genes were discovered, attempt to clarify some potentially confusing nomenclature, and summarize published observations that inform our ongoing studies.

Endocardially Derived Macrophages Are Essential for Valvular Remodeling

During mammalian embryogenesis, de novo hematopoiesis occurs transiently in multiple anatomical sites including the yolk sac, dorsal aorta, and heart tube. A long-unanswered question is whether these local transient hematopoietic mechanisms are essential for embryonic growth. Here, we show that endocardial hematopoiesis is critical for cardiac valve remodeling as a source of tissue macrophages. Colony formation assay from explanted heart tubes and genetic lineage tracing with the endocardial specific Nfatc1-Cre mouse revealed that hemogenic endocardium is a de novo source of tissue macrophages in the endocardial cushion, the primordium of the cardiac valves. Surface marker characterization, gene expression profiling, and ex vivo phagocytosis assay revealed that the endocardially derived cardiac tissue macrophages play a phagocytic and antigen presenting role. Indeed, genetic ablation of endocardially derived macrophages caused severe valve malformation. Together, these data suggest that transient hemogenic activity in the endocardium is indispensable for the valvular tissue remodeling in the heart.

BMP signaling is required for nkx2.3-positive pharyngeal pouch progenitor specification in zebrafish

Pharyngeal pouches, a series of outpocketings that bud from the foregut endoderm, are essential to the formation of craniofacial skeleton as well as several important structures like parathyroid and thymus. However, whether pharyngeal pouch progenitors exist in the developing gut tube remains unknown. Here, taking advantage of cell lineage tracing and transgenic ablation technologies, we identified a population of nkx2.3⁺ pouch progenitors in zebrafish embryos and demonstrated an essential requirement of ectodermal BMP2b for their specification. At early somite stages, nkx2.3⁺ cells located at lateral region of pharyngeal endoderm give rise to the pouch epithelium except a subpopulation expressing pdgfαa rather than nkx2.3. A small-scale screen of chemical inhibitors reveals that BMP signaling is necessary to specify these progenitors. Loss-of-function analyses show that BMP2b, expressed in the pharyngeal ectoderm, actives Smad effectors in endodermal cells to induce nkx2.3⁺ progenitors. Collectively, our study provides in vivo evidence for the existence of pouch progenitors and highlights the importance of BMP2b signaling in progenitor specification.

Pharyngeal pouches are essential to the formation of craniofacial skeleton as well as several important structures like parathyroid and thymus, but whether their progenitors exist in the developing gut tube remains unknown. Our study provide in vivo evidence that, in the early somite stages, nkx2.3⁺ cells are present in the lateral pharyngeal endoderm and give rise to the pouch epithelium. We further reveal that ectodermal BMP2b is essential for the activation of Smad effectors in endodermal cells, thereby facilitating pouch progenitor specification. Collectively, our discoveries shed new light on the cellular and molecular mechanisms of pharyngeal pouch development.

Nr2f-dependent allocation of ventricular cardiomyocyte and pharyngeal muscle progenitors

Multiple syndromes share congenital heart and craniofacial muscle defects, indicating there is an intimate relationship between the adjacent cardiac and pharyngeal muscle (PM) progenitor fields. However, mechanisms that direct antagonistic lineage decisions of the cardiac and PM progenitors within the anterior mesoderm of vertebrates are not understood. Here, we identify that retinoic acid (RA) signaling directly promotes the expression of the transcription factor Nr2f1a within the anterior lateral plate mesoderm. Using zebrafish nr2f1a and nr2f2 mutants, we find that Nr2f1a and Nr2f2 have redundant requirements restricting ventricular cardiomyocyte (CM) number and promoting development of the posterior PMs. Cre-mediated genetic lineage tracing in nr2f1a; nr2f2 double mutants reveals that tcf21⁺ progenitor cells, which can give rise to ventricular CMs and PM, more frequently become ventricular CMs potentially at the expense of posterior PMs in nr2f1a; nr2f2 mutants. Our studies reveal insights into the molecular etiology that may underlie developmental syndromes that share heart, neck and facial defects as well as the phenotypic variability of congenital heart defects associated with NR2F mutations in humans.

Many developmental syndromes include both congenital heart and craniofacial defects, necessitating a better understanding of the mechanisms underlying the correlation of these defects. During early vertebrate development, cardiac and pharyngeal muscle cells originate from adjacent, partially overlapping progenitor fields within the anterior mesoderm. However, signals that allocate the cells from the adjacent cardiac and pharyngeal muscle progenitor fields are not understood. Mutations in the gene NR2F2 are associated with variable types of congenital heart defects in humans. Our recent work demonstrates that zebrafish Nr2f1a is the functional equivalent to Nr2f2 in mammals and promotes atrial development. Here, we identify that zebrafish nr2f1a and nr2f2 have redundant requirements at earlier stages of development than nr2f1a alone to restrict the number of ventricular CMs in the heart and promote posterior pharyngeal muscle development. Therefore, we have identified an antagonistic mechanism that is necessary to generate the proper number of cardiac and pharyngeal muscle progenitors in vertebrates. These studies provide evidence to help explain the variability of congenital heart defects from NR2F2 mutations in humans and a novel molecular framework for understanding developmental syndromes with heart and craniofacial defects.

Endothelial to Mesenchymal Transition in Cardiovascular Disease: JACC State-of-the-Art Review

Endothelial to mesenchymal transition (EndMT) is a process whereby an endothelial cell undergoes a series of molecular events that lead to a change in phenotype toward a mesenchymal cell (e.g., myofibroblast, smooth muscle cell). EndMT plays a fundamental role during development, and mounting evidence indicates that EndMT is involved in adult cardiovascular diseases (CVDs), including atherosclerosis, pulmonary hypertension, valvular disease, and fibroelastosis. Therefore, the targeting of EndMT may hold therapeutic promise for treating CVD. However, the field faces a number of challenges, including the lack of a precise functional and molecular definition, a lack of understanding of the causative pathological role of EndMT in CVDs (versus being a "bystander-phenomenon"), and a lack of robust human data corroborating the extent and causality of EndMT in adult CVDs. Here, we review this emerging but exciting field, and propose a framework for its systematic advancement at the molecular and translational levels.

Left-right asymmetry in heart development and disease: forming the right loop

Extensive studies have shown how bilateral symmetry of the vertebrate embryo is broken during early development, resulting in a molecular left-right bias in the mesoderm. However, how this early asymmetry drives the asymmetric morphogenesis of visceral organs remains poorly understood. The heart provides a striking model of left-right asymmetric morphogenesis, undergoing rightward looping to shape an initially linear heart tube and align cardiac chambers. Importantly, abnormal left-right patterning is associated with severe congenital heart defects, as exemplified in heterotaxy syndrome. Here, we compare the mechanisms underlying the rightward looping of the heart tube in fish, chick and mouse embryos. We propose that heart looping is not only a question of direction, but also one of fine-tuning shape. This is discussed in the context of evolutionary and clinical perspectives.

Hey2 regulates the size of the cardiac progenitor pool during vertebrate heart development

A key event in heart development is the timely addition of cardiac progenitor cells, defects in which can lead to congenital heart defects. However, how the balance and proportion of progenitor proliferation versus addition to the heart is regulated remains poorly understood. Here, we demonstrate that Hey2 functions to regulate the dynamics of cardiac progenitor addition to the zebrafish heart. We found that the previously noted increase in myocardial cell number found in the absence of Hey2 function was due to a pronounced expansion in the size of the cardiac progenitor pool. Expression analysis and lineage tracing of hey2-expressing cells showed that hey2 is active in cardiac progenitors. Hey2 acted to limit proliferation of cardiac progenitors, prior to heart tube formation. Use of a transplantation approach demonstrated a likely cell-autonomous (in cardiac progenitors) function for Hey2. Taken together, our data suggest a previously unappreciated role for Hey2 in controlling the proliferative capacity of cardiac progenitors, affecting the subsequent contribution of late-differentiating cardiac progenitors to the developing vertebrate heart.

LncRNA-uc.40 silence promotes P19 embryonic cells differentiation to cardiomyocyte via the PBX1 gene

Uc.40 is a long noncoding RNA that is highly conserved among different species, although its function is unknown. It is highly expressed in abnormal human embryonic heart. We previously reported that overexpression of uc.40 promoted apoptosis and inhibited proliferation of P19 cells, and downregulated PBX1, which was identified as a potential target gene of uc.40. The current study evaluated the effects of uc40-siRNA-44 (siRNA against uc.40) on the differentiation, proliferation, apoptosis, and mitochondrial function in P19 cells, and investigated the relationship between uc.40 and PBX1 in cardiomyocytes. The uc.40 silencing expression was confirmed by quantitative real-time polymerase chain reaction (RT-PCR). Observation of morphological changes in transfected P19 cells during different stages of differentiation revealed that uc40-siRNA-44 increased the number of cardiomyocyes. There was no significant difference in the morphology or time of differentiation between the uc40-siRNA-44 group and the control group. uc40-siRNA-44 significantly promoted proliferation of P19 cells and inhibited serum starvation-induced apoptosis. There was no significant difference in mitochondrial DNA copy number or cellular ATP level between the two groups, and ROS levels were significantly decreased in uc40-siRNA-44-transfected cells. The levels of PBX1 and myocardial markers of differentiation were examined in transfected P19 cells; uc40-siRNA-44 downregulated myocardial markers and upregulated PBX1 expression. These results suggest that uc.40 may play an important role during the differentiation of P19 cells by regulation of PBX1 to promote proliferation and inhibit apoptosis. These studies provide a foundation for further study of uc.40/PBX1 in cardiac development.

TGF-β Signaling Is Necessary and Sufficient for Pharyngeal Arch Artery Angioblast Formation

The pharyngeal arch arteries (PAAs) are transient embryonic blood vessels that mature into critical segments of the aortic arch and its branches. Although defects in PAA development cause life-threating congenital cardiovascular defects, the molecular mechanisms that orchestrate PAA morphogenesis remain unclear. Through small-molecule screening in zebrafish, we identified TGF-β signaling as indispensable for PAA development. Specifically, chemical inhibition of the TGF-β type I receptor ALK5 impairs PAA development because nkx2.5⁺ PAA progenitor cells fail to differentiate into tie1⁺ angioblasts. Consistent with this observation, we documented a burst of ALK5-mediated Smad3 phosphorylation within PAA progenitors that foreshadows angioblast emergence. Remarkably, premature induction of TGF-β receptor activity stimulates precocious angioblast differentiation, thereby demonstrating the sufficiency of this pathway for initiating the PAA progenitor to angioblast transition. More broadly, these data uncover TGF-β as a rare signaling pathway that is necessary and sufficient for angioblast lineage commitment.

Molecular Basis for Dysregulated Activation of NKX2-5 in the Vascular Remodeling of Systemic Sclerosis

OBJECTIVE

NKX2-5 is a homeobox transcription factor that is required for the formation of the heart and vessels during development, with significant postnatal down-regulation and reactivation in disease states, characterized by vascular remodeling. The purpose of this study was to investigate mechanisms that activate NKX2-5 expression in diseased vessels, such as systemic sclerosis (scleroderma; SSc)-associated pulmonary hypertension (PH), and to identify genetic variability that potentially underlies susceptibility to specific vascular complications.

METHODS

We explored NKX2-5 expression in biopsy samples from patients with SSc-associated PH and in pulmonary artery smooth muscle cells (PASMCs) from patients with scleroderma. Disease-associated putative functional single-nucleotide polymorphisms (SNPs) at the NKX2-5 locus were cloned and studied in reporter gene assays. SNP function was further examined through protein-DNA binding assays, chromatin immunoprecipitation assays, and RNA silencing analyses.

RESULTS

Increased NKX2-5 expression in biopsy samples from patients with SSc-associated PH was localized to remodeled vessels and PASMCs. Meta-analysis of 2 independent scleroderma cohorts revealed an association of rs3131917 with scleroderma (P = 0.029). We demonstrated that disease-associated SNPs are located in a novel functional enhancer, which increases NKX2-5 transcriptional activity through the binding of GATA-6, c-Jun, and myocyte-specific enhancer factor 2C. We also characterized an activator/coactivator transcription-enhancer factor domain 1 (TEAD1)/Yes-associated protein 1 (YAP1) complex, which was bound at rs3095870, another functional SNP, with TEAD1 binding the risk allele and activating the transcription of NKX2-5.

CONCLUSION

NKX2-5 is genetically associated with scleroderma, pulmonary hypertension, and fibrosis. Functional evidence revealed a regulatory mechanism that results in NKX2-5 transcriptional activation in PASMCs through the interaction of an upstream promoter and a novel downstream enhancer. This mechanism can act as a model for NKX2-5 activation in cardiovascular disease characterized by vascular remodeling.

Unique developmental trajectories and genetic regulation of ventricular and outflow tract progenitors in the zebrafish second heart field

During mammalian embryogenesis, cardiac progenitor cells constituting the second heart field (SHF) give rise to the right ventricle and primitive outflow tract (OFT). In zebrafish, previous lineage-tracing and mutant analyses suggested that SHF ventricular and OFT progenitors co-migrate to the arterial pole of the zebrafish heart tube soon after their specification in the nkx2.5⁺ field of anterior lateral plate mesoderm (ALPM). Using additional prospective lineage tracing, we demonstrate that while SHF ventricular progenitors migrate directly to the arterial pole, OFT progenitors become temporarily sequestered in the mesodermal cores of pharyngeal arch 2 (PA2), where they downregulate nkx2.5 expression. While there, they intermingle with precursors for PA2-derived head muscles (HMs) and hypobranchial artery endothelium, which we demonstrate are co-specified with SHF progenitors in the nkx2.5⁺ ALPM. Soon after their sequestration in PA2, OFT progenitors migrate to the arterial pole of the heart and differentiate into OFT lineages. Lastly, we demonstrate that SHF ventricular and OFT progenitors exhibit unique sensitivities to a mutation in fgf8a Our data highlight novel aspects of SHF, OFT and HM development in zebrafish that will inform mechanistic interpretations of cardiopharyngeal phenotypes in zebrafish models of human congenital disorders.

Vascular heterogeneity and specialization in development and disease

Blood and lymphatic vessels pervade almost all body tissues and have numerous essential roles in physiology and disease. The inner lining of these networks is formed by a single layer of endothelial cells, which is specialized according to the needs of the tissue that it supplies. Whereas the general mechanisms of blood and lymphatic vessel development are being defined with increasing molecular precision, studies of the processes of endothelial specialization remain mostly descriptive. Recent insights from genetic animal models illuminate how endothelial cells interact with each other and with their tissue environment, providing paradigms for vessel type- and organ-specific endothelial differentiation. Delineating these governing principles will be crucial for understanding how tissues develop and maintain, and how their function becomes abnormal in disease.

Point mutations in murine Nkx2-5 phenocopy human congenital heart disease and induce pathogenic Wnt signaling

Mutations in the Nkx2-5 gene are a main cause of congenital heart disease. Several studies have addressed the phenotypic consequences of disrupting the Nkx2-5 gene locus, although animal models to date failed to recapitulate the full spectrum of the human disease. Here, we describe a new Nkx2-5 point mutation murine model, akin to its human counterpart disease-generating mutation. Our model fully reproduces the morphological and physiological clinical presentations of the disease and reveals an understudied aspect of Nkx2-5-driven pathology, a primary right ventricular dysfunction. We further describe the molecular consequences of disrupting the transcriptional network regulated by Nkx2-5 in the heart and show that Nkx2-5-dependent perturbation of the Wnt signaling pathway promotes heart dysfunction through alteration of cardiomyocyte metabolism. Our data provide mechanistic insights on how Nkx2-5 regulates heart function and metabolism, a link in the study of congenital heart disease, and confirms that our models are the first murine genetic models to our knowledge to present all spectra of clinically relevant adult congenital heart disease phenotypes generated by NKX2-5 mutations in patients.

Nkx2.5 marks angioblasts that contribute to hemogenic endothelium of the endocardium and dorsal aorta

Novel regenerative therapies may stem from deeper understanding of the mechanisms governing cardiovascular lineage diversification. Using enhancer mapping and live imaging in avian embryos, and genetic lineage tracing in mice, we investigated the spatio-temporal dynamics of cardiovascular progenitor populations. We show that expression of the cardiac transcription factor Nkx2.5 marks a mesodermal population outside of the cardiac crescent in the extraembryonic and lateral plate mesoderm, with characteristics of hemogenic angioblasts. Extra-cardiac Nkx2.5 lineage progenitors migrate into the embryo and contribute to clusters of CD41⁺/CD45⁺ and RUNX1⁺ cells in the endocardium, the aorta-gonad-mesonephros region of the dorsal aorta and liver. We also demonstrated that ectopic expression of Nkx2.5 in chick embryos activates the hemoangiogenic gene expression program. Taken together, we identified a hemogenic angioblast cell lineage characterized by transient Nkx2.5 expression that contributes to hemogenic endothelium and endocardium, suggesting a novel role for Nkx2.5 in hemoangiogenic lineage specification and diversification.

DOI:http://dx.doi.org/10.7554/eLife.20994.001

As an animal embryo develops, it establishes a circulatory system that includes the heart, vessels and blood. Vessels and blood initially form in the yolk sac, a membrane that surrounds the embryo. These yolk sac vessels act as a rudimentary circulatory system, connecting to the heart and blood vessels within the embryo itself. In older embryos, cells in the inner layer of the largest blood vessel (known as the dorsal aorta) generate blood stem cells that give rise to the different types of blood cells.

A gene called Nkx2.5 encodes a protein that controls the activity of a number of complex genetic programs and has been long studied as a key player in the development of the heart. Nkx2.5 is essential for forming normal heart muscle cells and for shaping the primitive heart and its surrounding vessels into a working organ. Interfering with the normal activity of the Nkx2.5 gene results in severe defects in blood vessels and the heart. However, many details are missing on the role played by Nkx2.5 in specifying the different cellular components of the circulatory system and heart.

Zamir et al. genetically engineered chick and mouse embryos to produce fluorescent markers that could be used to trace the cells that become part of blood vessels and heart. The experiments found that some of the cells that form the blood and vessels in the yolk sac originate from within the membranes surrounding the embryo, outside of the areas previously reported to give rise to the heart. The Nkx2.5 gene is active in these cells for only a short period of time as they migrate toward the heart and dorsal aorta, where they give rise to blood stem cells

These findings suggest that Nkx2.5 plays an important role in triggering developmental processes that eventually give rise to blood vessels and blood cells. The next step following on from this work will be to find out what genes the protein encoded by Nkx2.5 regulates to drive these processes. Mapping the genes that control the early origins of blood and blood-forming vessels will help biologists understand this complex and vital tissue system, and develop new treatments for patients with conditions that affect their circulatory system. In the future, this knowledge may also help to engineer synthetic blood and blood products for use in trauma and genetic diseases.

DOI:http://dx.doi.org/10.7554/eLife.20994.002

The pro-inflammatory signalling regulator Stat4 promotes vasculogenesis of great vessels derived from endothelial precursors

Vasculogenic defects of great vessels (GVs) are a major cause of congenital cardiovascular diseases. However, genetic regulators of endothelial precursors in GV vasculogenesis remain largely unknown. Here we show that Stat4, a transcription factor known for its regulatory role of pro-inflammatory signalling, promotes GV vasculogenesis in zebrafish. We find stat4 transcripts highly enriched in nkx2.5⁺ endothelial precursors in the pharynx and demonstrate that genetic ablation of stat4 causes stenosis of pharyngeal arch arteries (PAAs) by suppressing PAAs 3–6 angioblast development. We further show that stat4 is a downstream target of nkx2.5 and that it autonomously promotes proliferation of endothelial precursors of the mesoderm. Mechanistically, stat4 regulates the emerging PAA angioblasts by inhibiting the expression of hdac3 and counteracting the effect of stat1a. Altogether, our study establishes a role for Stat4 in zebrafish great vessel development, and suggests that Stat4 may serve as a therapeutic target for GV defects.

Stat4 is a transcription factor known to regulate pro-inflammatory signalling. Here, Meng et al. show that Stat4 is not only regulating inflammation but it is also crucial for great vessels development and endothelial precursor proliferation in zebrafish, by inhibiting the expression of hdac3 and counteracting the effect of Stat1a.

Nkx2.5 regulates endothelin converting enzyme-1 during pharyngeal arch patterning

In gnathostomes, dorsoventral (D-V) patterning of neural crest cells (NCC) within the pharyngeal arches is crucial for the development of hinged jaws. One of the key signals that mediate this process is Endothelin-1 (EDN1). Loss of EDN1 binding to the Endothelin-A receptor (EDNRA) results in loss of EDNRA signaling and subsequent facial birth defects in humans, mice and zebrafish. A rate-limiting step in this crucial signaling pathway is the conversion of immature EDN1 into a mature active form by Endothelin converting enzyme-1 (ECE1). However, surprisingly little is known about how Ece1 transcription is induced or regulated. We show here that Nkx2.5 is required for proper craniofacial development in zebrafish and acts in part by upregulating ece1 expression. Disruption of nkx2.5 in zebrafish embryos results in defects in both ventral and dorsal pharyngeal arch-derived elements, with changes in ventral arch gene expression consistent with a disruption in Ednra signaling. ece1 mRNA rescues the nkx2.5 morphant phenotype, indicating that Nkx2.5 functions through modulating Ece1 expression or function. These studies illustrate a new function for Nkx2.5 in embryonic development and provide new avenues with which to pursue potential mechanisms underlying human facial disorders.

Zebrafish as a Vertebrate Model System to Evaluate Effects of Environmental Toxicants on Cardiac Development and Function

Environmental pollution is a serious problem of the modern world that possesses a major threat to public health. Exposure to environmental pollutants during embryonic development is particularly risky. Although many pollutants have been verified as potential toxicants, there are new chemicals in the environment that need assessment. Heart development is an extremely sensitive process, which can be affected by environmentally toxic molecule exposure during embryonic development. Congenital heart defects are the most common life-threatening global health problems, and the etiology is mostly unknown. The zebrafish has emerged as an invaluable model to examine substance toxicity on vertebrate development, particularly on cardiac development. The zebrafish offers numerous advantages for toxicology research not found in other model systems. Many laboratories have used the zebrafish to study the effects of widespread chemicals in the environment on heart development, including pesticides, nanoparticles, and various organic pollutants. Here, we review the uses of the zebrafish in examining effects of exposure to external molecules during embryonic development in causing cardiac defects, including chemicals ubiquitous in the environment and illicit drugs. Known or potential mechanisms of toxicity and how zebrafish research can be used to provide mechanistic understanding of cardiac defects are discussed.

Endothelium in the pharyngeal arches 3, 4 and 6 is derived from the second heart field

Oxygenated blood from the heart is directed into the systemic circulation through the aortic arch arteries (AAAs). The AAAs arise by remodeling of three symmetrical pairs of pharyngeal arch arteries (PAAs), which connect the heart with the paired dorsal aortae at mid-gestation. Aberrant PAA formation results in defects frequently observed in patients with lethal congenital heart disease. How the PAAs form in mammals is not understood. The work presented in this manuscript shows that the second heart field (SHF) is the major source of progenitors giving rise to the endothelium of the pharyngeal arches 3 - 6, while the endothelium in the pharyngeal arches 1 and 2 is derived from a different source. During the formation of the PAAs 3 - 6, endothelial progenitors in the SHF extend cellular processes toward the pharyngeal endoderm, migrate from the SHF and assemble into a uniform vascular plexus. This plexus then undergoes remodeling, whereby plexus endothelial cells coalesce into a large PAA in each pharyngeal arch. Taken together, our studies establish a platform for investigating cellular and molecular mechanisms regulating PAA formation and alterations that lead to disease.

Cyp26 Enzymes Facilitate Second Heart Field Progenitor Addition and Maintenance of Ventricular Integrity

Although retinoic acid (RA) teratogenicity has been investigated for decades, the mechanisms underlying RA-induced outflow tract (OFT) malformations are not understood. Here, we show zebrafish embryos deficient for Cyp26a1 and Cyp26c1 enzymes, which promote RA degradation, have OFT defects resulting from two mechanisms: first, a failure of second heart field (SHF) progenitors to join the OFT, instead contributing to the pharyngeal arch arteries (PAAs), and second, a loss of first heart field (FHF) ventricular cardiomyocytes due to disrupted cell polarity and extrusion from the heart tube. Molecularly, excess RA signaling negatively regulates fibroblast growth factor 8a (fgf8a) expression and positively regulates matrix metalloproteinase 9 (mmp9) expression. Although restoring Fibroblast growth factor (FGF) signaling can partially rescue SHF addition in Cyp26 deficient embryos, attenuating matrix metalloproteinase (MMP) function can rescue both ventricular SHF addition and FHF integrity. These novel findings indicate a primary effect of RA-induced OFT defects is disruption of the extracellular environment, which compromises both SHF recruitment and FHF ventricular integrity.

Retinoic acid (RA) is the most active metabolic product of vitamin A. The embryonic heart is particularly sensitive to inappropriate RA levels, with cardiac outflow tract (OFT) defects among the most common RA-induced malformations. However, the mechanisms underlying these RA-induced defects are not understood. Cyp26 enzymes facilitate degradation of RA and thus are required to limit RA levels in early development. Here, we present evidence that loss of Cyp26 enzymes induces cardiac OFT defects through two mechanisms. First, we find that Cyp26-deficient zebrafish embryos fail to add later-differentiating ventricular cardiac progenitors to the OFT, with some of these progenitors instead contributing to the nearby arch arteries. Second, Cyp26-deficient embryos cannot maintain the integrity of the nascent heart tube, with ventricular cells within the heart tube losing their polarity and being extruded. Our data indicate that excess expression of matrix metalloproteinase 9, an enzyme that degrades the extracellular matrix, underlies both the cardiac progenitor addition and heart tube integrity defects seen in Cyp26-deficient embryos. Our findings highlight perturbation of the extracellular matrix as a major cause of RA-induced cardiac OFT defects that specifically disrupt ventricular development at later stages than previously appreciated.

The AP-1 transcription factor component Fosl2 potentiates the rate of myocardial differentiation from the zebrafish second heart field

The vertebrate heart forms through successive phases of cardiomyocyte differentiation. Initially, cardiomyocytes derived from first heart field (FHF) progenitors assemble the linear heart tube. Thereafter, second heart field (SHF) progenitors differentiate into cardiomyocytes that are accreted to the poles of the heart tube over a well-defined developmental window. Although heart tube elongation deficiencies lead to life-threatening congenital heart defects, the variables controlling the initiation, rate and duration of myocardial accretion remain obscure. Here, we demonstrate that the AP-1 transcription factor, Fos-like antigen 2 (Fosl2), potentiates the rate of myocardial accretion from the zebrafish SHF. fosl2 mutants initiate accretion appropriately, but cardiomyocyte production is sluggish, resulting in a ventricular deficit coupled with an accumulation of SHF progenitors. Surprisingly, mutant embryos eventually correct the myocardial deficit by extending the accretion window. Overexpression of Fosl2 also compromises production of SHF-derived ventricular cardiomyocytes, a phenotype that is consistent with precocious depletion of the progenitor pool. Our data implicate Fosl2 in promoting the progenitor to cardiomyocyte transition and uncover the existence of regulatory mechanisms to ensure appropriate SHF-mediated cardiomyocyte contribution irrespective of embryonic stage.

Vascular Development and Regeneration in the Mammalian Heart

Cardiovascular diseases including coronary artery disease are the leading cause of death worldwide. Unraveling the developmental origin of coronary vessels could offer important therapeutic implications for treatment of cardiovascular diseases. The recent identification of the endocardial source of coronary vessels reveals a heterogeneous origin of coronary arteries in the adult heart. In this review, we will highlight recent advances in finding the sources of coronary vessels in the mammalian heart from lineage-tracing models as well as differentiation studies using pluripotent stem cells. Moreover, we will also discuss how we induce neovascularization in the damaged heart through transient yet highly efficient expression of VEGF-modified mRNAs as a potentially therapeutic delivery platform.

Normal formation of a vertebrate body plan and loss of tissue maintenance in the absence of ezh2

Polycomb group (PcG) proteins are transcriptional repressors of numerous genes, many of which regulate cell cycle progression or developmental processes. We used zebrafish to study Enhancer of zeste homolog 2 (Ezh2), the PcG protein responsible for placing the transcriptional repressive H3K27me3 mark. We identified a nonsense mutant of ezh2 and generated maternal zygotic (MZ) ezh2 mutant embryos. In contrast to knockout mice for PcG proteins, MZezh2 mutant embryos gastrulate seemingly normal, but die around 2 days post fertilization displaying pleiotropic phenotypes. Expression analyses indicated that genes important for early development are not turned off properly, revealing a regulatory role for Ezh2 during zygotic gene expression. In addition, we suggest that Ezh2 regulates maternal mRNA loading of zygotes. Analyses of tissues arising later in development, such as heart, liver, and pancreas, indicated that Ezh2 is required for maintenance of differentiated cell fates. Our data imply that the primary role of Ezh2 is to maintain tissues after tissue specification. Furthermore, our work indicates that Ezh2 is essential to sustain tissue integrity and to set up proper maternal mRNA contribution, and presents a novel and powerful tool to study how PcG proteins contribute to early vertebrate development.

Isolation and Characterization of Single Cells from Zebrafish Embryos

The zebrafish (Danio rerio) is a powerful model organism to study vertebrate development. Though many aspects of zebrafish embryonic development have been described at the morphological level, little is known about the molecular basis of cellular changes that occur as the organism develops. With recent advancements in microfluidics and multiplexing technologies, it is now possible to characterize gene expression in single cells. This allows for investigation of heterogeneity between individual cells of specific cell populations to identify and classify cell subtypes, characterize intermediate states that occur during cell differentiation, and explore differential cellular responses to stimuli. This study describes a protocol to isolate viable, single cells from zebrafish embryos for high throughput multiplexing assays. This method may be rapidly applied to any zebrafish embryonic cell type with fluorescent markers. An extension of this method may also be used in combination with high throughput sequencing technologies to fully characterize the transcriptome of single cells. As proof of principle, the relative abundance of cardiac differentiation markers was assessed in isolated, single cells derived from nkx2.5 positive cardiac progenitors. By evaluation of gene expression at the single cell level and at a single time point, the data support a model in which cardiac progenitors coexist with differentiating progeny. The method and work flow described here is broadly applicable to the zebrafish research community, requiring only a labeled transgenic fish line and access to microfluidics technologies.

The developmental origins and lineage contributions of endocardial endothelium

Endocardial development involves a complex orchestration of cell fate decisions that coordinate with endoderm formation and other mesodermal cell lineages. Historically, investigations into the contribution of endocardium in the developing embryo was constrained to the heart where these cells give rise to the inner lining of the myocardium and are a major contributor to valve formation. In recent years, studies have continued to elucidate the complexities of endocardial fate commitment revealing a much broader scope of lineage potential from developing endocardium. These studies cover a wide range of species and model systems and show direct contribution or fate potential of endocardium giving rise to cardiac vasculature, blood, fibroblast, and cardiomyocyte lineages. This review focuses on the marked expansion of knowledge in the area of endocardial fate potential. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Origin, Specification, and Plasticity of the Great Vessels of the Heart

The pharyngeal arch arteries (PAAs) are a series of paired embryonic blood vessels that give rise to several major arteries that connect directly to the heart. During development, the PAAs emerge from nkx2.5-expressing mesodermal cells and connect the dorsal head vasculature to the outflow tract of the heart. Despite their central role in establishing the circulatory system, the embryonic origins of the PAA progenitors are only coarsely defined, and the factors that specify them and their regenerative potential are unclear. Using fate mapping and mutant analysis, we find that PAA progenitors are derived from the tcf21 and nkx2.5 double-positive head mesoderm and require these two transcription factors for their specification and survival. Unexpectedly, cell ablation shows that the tcf21+; nkx2.5+ PAA progenitors are not required for PAA formation. We find that this compensation is due to the replacement of ablated tcf21+; nkx2.5+ PAA cells by endothelial cells from the dorsal head vasculature. Together, these studies assign the embryonic origin of the great vessel progenitors to the interface between the pharyngeal and cardiac mesoderm, identify the transcription factor code required for their specification, and reveal an unexpected plasticity in the formation of the great vessels.

Prdm1 functions in the mesoderm of the second heart field, where it interacts genetically with Tbx1, during outflow tract morphogenesis in the mouse embryo

Congenital heart defects affect at least 0.8% of newborn children and are a major cause of lethality prior to birth. Malformations of the arterial pole are particularly frequent. The myocardium at the base of the pulmonary trunk and aorta and the arterial tree associated with these great arteries are derived from splanchnic mesoderm of the second heart field (SHF), an important source of cardiac progenitor cells. These cells are controlled by a gene regulatory network that includes Fgf8, Fgf10 and Tbx1. Prdm1 encodes a transcriptional repressor that we show is also expressed in the SHF. In mouse embryos, mutation of Prdm1 affects branchial arch development and leads to persistent truncus arteriosus (PTA), indicative of neural crest dysfunction. Using conditional mutants, we show that this is not due to a direct function of Prdm1 in neural crest cells. Mutation of Prdm1 in the SHF does not result in PTA, but leads to arterial pole defects, characterized by mis-alignment or reduction of the aorta and pulmonary trunk, and abnormalities in the arterial tree, defects that are preceded by a reduction in outflow tract size and loss of caudal pharyngeal arch arteries. These defects are associated with a reduction in proliferation of progenitor cells in the SHF. We have investigated genetic interactions with Fgf8 and Tbx1, and show that on a Tbx1 heterozygote background, conditional Prdm1 mutants have more pronounced arterial pole defects, now including PTA. Our results identify PRDM1 as a potential modifier of phenotypic severity in TBX1 haploinsufficient DiGeorge syndrome patients.