Temporal-spatial ablation of neural crest in the mouse results in cardiovascular defects

SMAD4: A critical regulator of cardiac neural crest cell fate and vascular smooth muscle development

BACKGROUND

During embryogenesis, cardiac neural crest-derived cells (NCs) migrate into the pharyngeal arches and give rise to the vascular smooth muscle cells (vSMCs) of the pharyngeal arch arteries (PAAs). vSMCs are critical for the remodeling of the PAAs into their final adult configuration, giving rise to the aortic arch and its arteries (AAAs).

RESULTS

We investigated the role of SMAD4 in NC-to-vSMC differentiation using lineage-specific inducible mouse strains. We found that the expression of SMAD4 in the NC is indelible for regulating the survival of cardiac NCs. Although the ablation of SMAD4 at E9.5 in the NC lineage led to a near-complete absence of NCs in the pharyngeal arches, PAAs became invested with vSMCs derived from a compensatory source. Analysis of AAA development at E16.5 showed that the alternative vSMC source compensated for the lack of NC-derived vSMCs and rescued AAA morphogenesis.

CONCLUSIONS

Our studies uncovered the requisite role of SMAD4 in the contribution of the NC to the pharyngeal arch mesenchyme. We found that in the absence of SMAD4+ NCs, vSMCs around the PAAs arose from a different progenitor source, rescuing AAA morphogenesis. These findings shed light on the remarkable plasticity of developmental mechanisms governing AAA development.

Molecular Pathways and Animal Models of Truncus Arteriosus

In normal cardiovascular development in birds and mammals, the outflow tract of the heart is divided into two distinct channels to separate the oxygenated systemic blood flow from the deoxygenated pulmonary circulation. When the process of outflow tract septation fails, a single common outflow vessel persists resulting in a serious clinical condition known as persistent truncus arteriosus or common arterial trunk. In this chapter, we will review molecular pathways and the cells that are known to play a role in the formation and development of the outflow tract and how genetic manipulation of these pathways in animal models can result in common arterial trunk.

Nanoplastics causes extensive congenital malformations during embryonic development by passively targeting neural crest cells

Nanomaterials are widespread in the human environment as pollutants, and are being actively developed for use in human medicine. We have investigated how the size and dose of polystyrene nanoparticles affects malformations in chicken embryos, and have characterized the mechanisms by which they interfere with normal development. We find that nanoplastics can cross the embryonic gut wall. When injected into the vitelline vein, nanoplastics become distributed in the circulation to multiple organs. We find that the exposure of embryos to polystyrene nanoparticles produces malformations that are far more serious and extensive than has been previously reported. These malformations include major congenital heart defects that impair cardiac function. We show that the mechanism of toxicity is the selective binding of polystyrene nanoplastics nanoparticles to neural crest cells, leading to the death and impaired migration of those cells. Consistent with our new model, most of the malformations seen in this study are in organs that depend for their normal development on neural crest cells. These results are a matter of concern given the large and growing burden of nanoplastics in the environment. Our findings suggest that nanoplastics may pose a health risk to the developing embryo.

PAX Genes in Cardiovascular Development

The mammalian heart is a four-chambered organ with systemic and pulmonary circulations to deliver oxygenated blood to the body, and a tightly regulated genetic network exists to shape normal development of the heart and its associated major arteries. A key process during cardiovascular morphogenesis is the septation of the outflow tract which initially forms as a single vessel before separating into the aorta and pulmonary trunk. The outflow tract connects to the aortic arch arteries which are derived from the pharyngeal arch arteries. Congenital heart defects are a major cause of death and morbidity and are frequently associated with a failure to deliver oxygenated blood to the body. The Pax transcription factor family is characterised through their highly conserved paired box and DNA binding domains and are crucial in organogenesis, regulating the development of a wide range of cells, organs and tissues including the cardiovascular system. Studies altering the expression of these genes in murine models, notably Pax3 and Pax9, have found a range of cardiovascular patterning abnormalities such as interruption of the aortic arch and common arterial trunk. This suggests that these Pax genes play a crucial role in the regulatory networks governing cardiovascular development.

Prdm6 controls heart development by regulating neural crest cell differentiation and migration

The molecular mechanisms that drive the acquisition of distinct neural crest cell (NCC) fates is still poorly understood. Here, we identified Prdm6 as an epigenetic modifier that temporally and spatially regulates the expression of NCC specifiers and determines the fate of a subset of migrating cardiac NCCs (CNCCs). Using transcriptomic analysis and genetic and fate mapping approaches in transgenic mice, we showed that disruption of Prdm6 was associated with impaired CNCC differentiation, delamination, and migration and led to patent ductus arteriosus (DA) and ventricular noncompaction. Bulk and single-cell RNA-Seq analyses of the DA and CNCCs identified Prdm6 as a regulator of a network of CNCC specification genes, including Wnt1, Tfap2b, and Sox9. Loss of Prdm6 in CNCCs diminished its expression in the pre-epithelial–mesenchymal transition (pre-EMT) cluster, resulting in the retention of NCCs in the dorsal neural tube. This defect was associated with diminished H4K20 monomethylation and G1-S progression and augmented Wnt1 transcript levels in pre-EMT and neural tube clusters, which we showed was the major driver of the impaired CNCC migration. Altogether, these findings revealed Prdm6 as a key regulator of CNCC differentiation and migration and identified Prdm6 and its regulated network as potential targets for the treatment of congenital heart diseases.

Identification of astroglia-like cardiac nexus glia that are critical regulators of cardiac development and function

Glial cells are essential for functionality of the nervous system. Growing evidence underscores the importance of astrocytes; however, analogous astroglia in peripheral organs are poorly understood. Using confocal time-lapse imaging, fate mapping, and mutant genesis in a zebrafish model, we identify a neural crest-derived glial cell, termed nexus glia, which utilizes Meteorin signaling via Jak/Stat3 to drive differentiation and regulate heart rate and rhythm. Nexus glia are labeled with gfap, glast, and glutamine synthetase, markers that typically denote astroglia cells. Further, analysis of single-cell sequencing datasets of human and murine hearts across ages reveals astrocyte-like cells, which we confirm through a multispecies approach. We show that cardiac nexus glia at the outflow tract are critical regulators of both the sympathetic and parasympathetic system. These data establish the crucial role of glia on cardiac homeostasis and provide a description of nexus glia in the PNS.

Structural heart defects associated with ETB mutation, a cause of Hirschsprung disease

BACKGROUND

HSCR, a colonic neurocristopathy affecting 1/5000 births, is suggested to associate with cardiac septal defects and conotruncal malformations. However, we question subtle cardiac changes maybe more commonly present due to multi-regulations by HSCR candidate genes, in this instance, ETB. To investigate, we compared the cardiac morphology and quantitative measurements of sl/sl rat to those of the control group.

METHODS

Eleven neonatal rats were generated from heterozygote (ETB+/-) crossbreeding. Age and bodyweight were recorded at time of sacrifice. Diffusion-staining protocols with 1.5% iodine solution was completed prior to micro-CT scanning. All rats were scanned using an in vivo micro-CT scanner, Caliper Quantum FX, followed by two quality-control scans using a custom-built ex vivo micro-CT system. All scans were reviewed for gross cardiac dysmorphology. Micro-CT data were segmented semi-automatically post-NLM filtering for: whole-heart, LV, RV, LA, RA, and aortic arch. Measurements were taken with Drishti. Following image analysis, PCR genotyping of rats was performed: five sl/sl rats, three wildtype, and three heterozygotes. Statistical comparisons on organ volume, growth rate, and organ volume/bodyweight ratios were made between sl/sl and the control group.

RESULTS

Cardiac morphology and constituents were preserved. However, significant volumetric reductions were recorded in sl/sl rats with respect to the control: whole heart (38.70%, p value = 0.02); LV (41.22%, p value = 0.01), RV (46.15%, p value = 0.02), LA (44.93%, p value = 0.06), and RA (39.49%, p value = 0.02). Consistent trend was observed in growth rate (~ 20%) and organ-volume/bodyweight ratios (~ 25%). On the contrary, measurements on aortic arch demonstrated no significant difference among the two groups.

CONCLUSION

Despite the presence of normal morphology, significant cardiac growth retardation was detected in sl/sl rat, supporting the likely association of cardiac anomalies with HSCR, at least in ETB-/- subtype. Structural reduction was likely due to a combination of failure to thrive from enteric dysfunction, alterations to CaNCC colonization, and importantly coronary hypoperfusion from elevated ET-1/ETA-mediated hypervasoconstriction. Little correlation was detected between aortic arch development and sl/sl rat, supporting minor ETB role in large vessels. Although further clinical study is warranted, HSCR patients may likely require cardiac assessment in view of potential congenital cardiac defects.

Cardiac Neural Crest

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

Early Embryonic Expression of AP-2α Is Critical for Cardiovascular Development

Congenital cardiovascular malformation is a common birth defect incorporating abnormalities of the outflow tract and aortic arch arteries, and mice deficient in the transcription factor AP-2α (Tcfap2a) present with complex defects affecting these structures. AP-2α is expressed in the pharyngeal surface ectoderm and neural crest at mid-embryogenesis in the mouse, but the precise tissue compartment in which AP-2α is required for cardiovascular development has not been identified. In this study we describe the fully penetrant AP-2α deficient cardiovascular phenotype on a C57Bl/6J genetic background and show that this is associated with increased apoptosis in the pharyngeal ectoderm. Neural crest cell migration into the pharyngeal arches was not affected. Cre-expressing transgenic mice were used in conjunction with an AP-2α conditional allele to examine the effect of deleting AP-2α from the pharyngeal surface ectoderm and the neural crest, either individually or in combination, as well as the second heart field. This, surprisingly, was unable to fully recapitulate the global AP-2α deficient cardiovascular phenotype. The outflow tract and arch artery phenotype was, however, recapitulated through early embryonic Cre-mediated recombination. These findings indicate that AP-2α has a complex influence on cardiovascular development either being required very early in embryogenesis and/or having a redundant function in many tissue layers.

Epicardial cells derived from human embryonic stem cells augment cardiomyocyte-driven heart regeneration

The epicardium and its derivatives provide trophic and structural support for the developing and adult heart. Here we tested the ability of human embryonic stem cell (hESC)-derived epicardium to augment the structure and function of engineered heart tissue in vitro and to improve efficacy of hESC-cardiomyocyte grafts in infarcted athymic rat hearts. Epicardial cells markedly enhanced the contractility, myofibril structure and calcium handling of human engineered heart tissues, while reducing passive stiffness compared with mesenchymal stromal cells. Transplanted epicardial cells formed persistent fibroblast grafts in infarcted hearts. Cotransplantation of hESC-derived epicardial cells and cardiomyocytes doubled graft cardiomyocyte proliferation rates in vivo, resulting in 2.6-fold greater cardiac graft size and simultaneously augmenting graft and host vascularization. Notably, cotransplantation improved systolic function compared with hearts receiving either cardiomyocytes alone, epicardial cells alone or vehicle. The ability of epicardial cells to enhance cardiac graft size and function makes them a promising adjuvant therapeutic for cardiac repair.

Integrating gene regulatory pathways into differential network analysis of gene expression data

The advent of next-generation sequencing has introduced new opportunities in analyzing gene expression data. Research in systems biology has taken advantage of these opportunities by gleaning insights into gene regulatory networks through the analysis of gene association networks. Contrasting networks from different populations can reveal the many different roles genes fill, which can lead to new discoveries in gene function. Pathologies can also arise from aberrations in these gene-gene interactions. Exposing these network irregularities provides a new avenue for understanding and treating diseases. A general framework for integrating known gene regulatory pathways into a differential network analysis between two populations is proposed. The framework importantly allows for any gene-gene association measure to be used, and inference is carried out through permutation testing. A simulation study investigates the performance in identifying differentially connected genes when incorporating known pathways, even if the pathway knowledge is partially inaccurate. Another simulation study compares the general framework with four state-of-the-art methods. Two RNA-seq datasets are analyzed to illustrate the use of this framework in practice. In both examples, the analysis reveals genes and pathways that are known to be biologically significant along with potentially novel findings that may be used to motivate future research.

Loss of embryonic neural crest derived cardiomyocytes causes adult onset hypertrophic cardiomyopathy in zebrafish

Neural crest cells migrate to the embryonic heart and transform into a small number of cardiomyocytes, but their functions in the developing and adult heart are unknown. Here, we show that neural crest derived cardiomyocytes (NC-Cms) in the zebrafish ventricle express Notch ligand jag2b, are adjacent to Notch responding cells, and persist throughout life. Genetic ablation of NC-Cms during embryogenesis results in diminished jag2b, altered Notch signaling and aberrant trabeculation patterns, but is not detrimental to early heart function or survival to adulthood. However, embryonic NC-Cm ablation results in adult fish that show severe hypertrophic cardiomyopathy (HCM), altered cardiomyocyte size, diminished adult heart capacity and heart failure in cardiac stress tests. Adult jag2b mutants have similar cardiomyopathy. Thus, we identify a cardiomyocyte population and genetic pathway that are required to prevent adult onset HCM and provide a zebrafish model of adult-onset HCM and heart failure.

A tissue-specific, Gata6-driven transcriptional program instructs remodeling of the mature arterial tree

Connection of the heart to the systemic circulation is a critical developmental event that requires selective preservation of embryonic vessels (aortic arches). However, why some aortic arches regress while others are incorporated into the mature aortic tree remains unclear. By microdissection and deep sequencing in mouse, we find that neural crest (NC) only differentiates into vascular smooth muscle cells (SMCs) around those aortic arches destined for survival and reorganization, and identify the transcription factor Gata6 as a crucial regulator of this process. Gata6 is expressed in SMCs and its target genes activation control SMC differentiation. Furthermore, Gata6 is sufficient to promote SMCs differentiation in vivo, and drive preservation of aortic arches that ought to regress. These findings identify Gata6-directed differentiation of NC to SMCs as an essential mechanism that specifies the aortic tree, and provide a new framework for how mutations in GATA6 lead to congenital heart disorders in humans.

Hoxa1 and Hoxb1 are required for pharyngeal arch artery development

Hox transcription factors play critical roles during early vertebrate development. Previous studies have revealed an overlapping function of Hoxa1 and Hoxb1 during specification of the rhombomeres from which neural crest cells emerge. A recent study on Hoxa1 mutant mice documented its function during cardiovascular development, however, the role of Hoxb1 is still unclear. Here we show using single and compound Hoxa1;Hoxb1 mutant embryos that reduction of Hoxa1 gene dosage in Hoxb1-null genetic background is sufficient to result in abnormal pharyngeal aortic arch (PAA) development and subsequently in great artery defects. Endothelial cells in the 4th PAAs of compound mutant differentiate normally whereas vascular smooth muscle cells of the vessels are absent in the defective PAAs. The importance of Hoxa1 and Hoxb1, and their interaction during specification of cardiac NCCs is demonstrated. Together, our data reveal a critical role for anterior Hox genes during PAA development, providing new mechanistic insights into the etiology of congenital heart defects.

HIRA Gene is Lower Expressed in the Myocardium of Patients with Tetralogy of Fallot

Background:

The most typical cardiac abnormality is conotruncal defects (CTDs) in patients with 22q11 deletion syndrome (22q11DS). HIRA (histone cell cycle regulator) gene, as one of the candidate genes located at the critical region of 22q11DS, was reported as possibly relevant to CTD in animal models. This study aimed to analyze the level of expression of the HIRA gene in tetralogy of Fallot (TOF) patients and the potential DNA sequence variations in the promoter region.

Methods:

The messenger RNA (mRNA) expression was examined with quantitative real-time polymerase chain reaction in 39 myocardial tissues of the right ventricular outflow tract (RVOT) from TOF patients and 4 myocardial tissues of RVOT from noncardiac death children. The protein expression was detected using immunohistochemistry in 12 TOF patients and 4 controls. A total of 100 TOF cases and 200 healthy controls were recruited for DNA sequencing.

Results:

The mRNA and protein expressions of the HIRA gene in the myocardium of the TOF patients were both significantly lower as compared to the controls (P < 0.05). Five single nucleotide polymorphisms (SNPs), including g.4111A>G (rs1128399), g.4265C>A (rs4585115), g.4369T>G (rs2277837), g.4371C>A (rs148516780), and g.4543T>C (rs111802956), were found in the promoter region of the HIRA gene. There were no significant differences of frequencies in these SNPs between the TOF patients and the controls (P > 0.05).

Conclusion:

The abnormal lower expression of the HIRA gene in the myocardium may participate in the pathogenesis of TOF.

Neural crest requires Impdh2 for development of the enteric nervous system, great vessels, and craniofacial skeleton

Mutations that impair the proliferation of enteric neural crest-derived cells (ENCDC) cause Hirschsprung disease, a potentially lethal birth defect where the enteric nervous system (ENS) is absent from distal bowel. Inosine 5' monophosphate dehydrogenase (IMPDH) activity is essential for de novo GMP synthesis, and chemical inhibition of IMPDH induces Hirschsprung disease-like pathology in mouse models by reducing ENCDC proliferation. Two IMPDH isoforms are ubiquitously expressed in the embryo, but only IMPDH2 is required for life. To further understand the role of IMPDH2 in ENS and neural crest development, we characterized a conditional Impdh2 mutant mouse. Deletion of Impdh2 in the early neural crest using the Wnt1-Cre transgene produced defects in multiple neural crest derivatives including highly penetrant intestinal aganglionosis, agenesis of the craniofacial skeleton, and cardiac outflow tract and great vessel malformations. Analysis using a Rosa26 reporter mouse suggested that some or all of the remaining ENS in Impdh2 conditional-knockout animals was derived from cells that escaped Wnt1-Cre mediated DNA recombination. These data suggest that IMPDH2 mediated guanine nucleotide synthesis is essential for normal development of the ENS and other neural crest derivatives.

Neural crest cell-autonomous roles of fibronectin in cardiovascular development

The chemical and mechanical properties of extracellular matrices (ECMs) modulate diverse aspects of cellular fates; however, how regional heterogeneity in ECM composition regulates developmental programs is not well understood. We discovered that fibronectin 1 (Fn1) is expressed in strikingly non-uniform patterns during mouse development, suggesting that regionalized synthesis of the ECM plays cell-specific regulatory roles during embryogenesis. To test this hypothesis, we ablated Fn1 in the neural crest (NC), a population of multi-potent progenitors expressing high levels of Fn1. We found that Fn1 synthesized by the NC mediated morphogenesis of the aortic arch artery and differentiation of NC cells into vascular smooth muscle cells (VSMCs) by regulating Notch signaling. We show that NC Fn1 signals in an NC cell-autonomous manner through integrin α5β1 expressed by the NC, leading to activation of Notch and differentiation of VSMCs. Our data demonstrate an essential role of the localized synthesis of Fn1 in cardiovascular development and spatial regulation of Notch signaling.

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

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

Influence of intracellular zinc on cultures of rat cardiac neural crest cells

BACKGROUND

Developmental zinc (Zn) deficiency increases the incidence of heart anomalies in rat fetuses, in regions and structures derived from the outflow tract. Given that the development of the outflow tract requires the presence of cardiac neural crest cells (cNCC), we speculated that Zn deficiency selectively kills cNCC and could lead to heart malformations.

METHODS

Cardiac NCC were isolated from E10.5 rat embryos and cultured in control media (CTRL), media containing 3 μM of the cell permeable metal chelator N, N, N', N'-tetrakis (2-pyridylmethyl) ethylene diamine (TPEN), or in TPEN-treated media supplemented with 3 μM Zn (TPEN + Zn). Cardiac NCC were collected after 6, 8, and 24 h of treatment to assess cell viability, proliferation, and apoptosis.

RESULTS

The addition of TPEN to the culture media reduced free intracellular Zn pools and cell viability as assessed by low ATP production, compared to cells grown in control or Zn-supplemented media. There was an accumulation of reactive oxygen species, a release of mitochondrial cytochrome c into the cytoplasm, and an increased cellular expression of active caspase-3 in TPEN-treated cNCC compared to cNCC cultured in CTRL or TPEN + Zn media.

CONCLUSION

Zn deficiency can result in oxidative stress in cNCC, and subsequent decreases in their population and metabolic activity. These data support the concept that Zn deficiency associated developmental heart defects may arise in part as a consequence of altered cNCC metabolism.

Neural crest cells in cardiovascular development

Cardiac neural crest cells (NCCs) are a transient, migratory cell population exclusive to vertebrate embryos. Ablation, transplantation, and lineage-tracing experiments in chick and mouse have demonstrated their essential role in the remodeling of the initially bilateral and symmetric pharyngeal artery pairs into an aortic arch and for the septation of the cardiac outflow tract into the base of the pulmonary artery and aorta. Accordingly, defective cardiac NCC function is a common cause of congenital birth defects. Here, we review our current understanding of cardiac NCC-mediated vascular remodeling and signaling pathways important for this process. We additionally discuss their contribution to the cardiac valves as well as the still contentious role of cardiac NCCs in the development of the myocardium and conductive system of the heart.

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

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

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

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

Connecting teratogen-induced congenital heart defects to neural crest cells and their effect on cardiac function

Neural crest cells play many key roles in embryonic development, as demonstrated by the abnormalities that result from their specific absence or dysfunction. Unfortunately, these key cells are particularly sensitive to abnormalities in various intrinsic and extrinsic factors, such as genetic deletions or ethanol-exposure that lead to morbidity and mortality for organisms. This review discusses the role identified for a segment of neural crest in regulating the morphogenesis of the heart and associated great vessels. The paradox is that their derivatives constitute a small proportion of cells to the cardiovascular system. Findings supporting that these cells impact early cardiac function raises the interesting possibility that they indirectly control cardiovascular development at least partially through regulating function. Making connections between insults to the neural crest, cardiac function, and morphogenesis is more approachable with technological advances. Expanding our understanding of early functional consequences could be useful in improving diagnosis and testing therapies.

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.

Inactivation of Cdc42 in neural crest cells causes craniofacial and cardiovascular morphogenesis defects

Neural crest cells (NCCs) are physically responsible for craniofacial skeleton formation, pharyngeal arch artery remodeling and cardiac outflow tract septation during vertebrate development. Cdc42 (cell division cycle 42) is a Rho family small GTP-binding protein that works as a molecular switch to regulate cytoskeleton remodeling and the establishment of cell polarity. To investigate the role of Cdc42 in NCCs during embryonic development, we deleted Cdc42 in NCCs by crossing Cdc42 flox mice with Wnt1-cre mice. We found that the inactivation of Cdc42 in NCCs caused embryonic lethality with craniofacial deformities and cardiovascular developmental defects. Specifically, Cdc42 NCC knockout embryos showed fully penetrant cleft lips and short snouts. Alcian Blue and Alizarin Red staining of the cranium exhibited an unfused nasal capsule and palatine in the mutant embryos. India ink intracardiac injection analysis displayed a spectrum of cardiovascular developmental defects, including persistent truncus arteriosus, hypomorphic pulmonary arteries, interrupted aortic arches, and right-sided aortic arches. To explore the underlying mechanisms of Cdc42 in the formation of the great blood vessels, we generated Wnt1Cre-Cdc42-Rosa26 reporter mice. By beta-galactosidase staining, a subpopulation of Cdc42-null NCCs was observed halting in their migration midway from the pharyngeal arches to the conotruncal cushions. Phalloidin staining revealed dispersed, shorter and disoriented stress fibers in Cdc42-null NCCs. Finally, we demonstrated that the inactivation of Cdc42 in NCCs impaired bone morphogenetic protein 2 (BMP2)-induced NCC cytoskeleton remodeling and migration. In summary, our results demonstrate that Cdc42 plays an essential role in NCC migration, and inactivation of Cdc42 in NCCs impairs craniofacial and cardiovascular development in mice.

Neural crest cells are required for correct positioning of the developing outflow cushions and pattern the arterial valve leaflets

Aims

Anomalies of the arterial valves, principally bicuspid aortic valve (BAV), are the most common congenital anomalies. The cellular mechanisms that underlie arterial valve development are poorly understood. While it is known that the valve leaflets derive from the outflow cushions, which are populated by cells derived from the endothelium and neural crest cells (NCCs), the mechanism by which these cushions are sculpted to form the leaflets of the arterial valves remains unresolved. We set out to investigate how NCCs participate in arterial valve formation, reasoning that disrupting NCC within the developing outflow cushions would result in arterial valve anomalies, in the process elucidating the normal mechanism of arterial valve leaflet formation.

Methods and results

By disrupting Rho kinase signalling specifically in NCC using transgenic mice and primary cultures, we show that NCC condensation within the cardiac jelly is required for correct positioning of the outflow cushions. Moreover, we show that this process is essential for normal patterning of the arterial valve leaflets with disruption leading to a spectrum of valve leaflet patterning anomalies, abnormal positioning of the orifices of the coronary arteries, and abnormalities of the arterial wall.

Conclusion

NCCs are required at earlier stages of arterial valve development than previously recognized, playing essential roles in positioning the cushions, and patterning the valve leaflets. Abnormalities in the process of NCC condensation at early stages of outflow cushion formation may provide a common mechanism underlying BAV, and also explain the link with arterial wall anomalies and outflow malalignment defects.

How insights from cardiovascular developmental biology have impacted the care of infants and children with congenital heart disease

To illustrate the impact developmental biology and genetics have already had on the clinical management of the million infants born worldwide each year with CHD, we have chosen three stories which have had particular relevance for pediatric cardiologists, cardiothoracic surgeons, cardiac anesthesiologists, and cardiac nurses. First, we show how Margaret Kirby's finding of the unexpected contribution of an ectodermal cell population - the cranial neural crest - to the aortic arch arteries and arterial pole of the embryonic avian heart provided a key impetus to the field of cardiovascular patterning. Recognition that a majority of patients affected by the neurocristopathy DiGeorge syndrome have a chromosome 22q11 deletion, have also spurred tremendous efforts to characterize the molecular mechanisms contributing to this pathology, assigning a major role to the transcription factor Tbx1. Second, synthesizing the work of the last two decades by many laboratories on a wide gamut of metazoans (invertebrates, tunicates, agnathans, teleosts, lungfish, amphibians, and amniotes), we review the >20 major modifications and additions to the ancient circulatory arrangement composed solely of a unicameral (one-chambered), contractile myocardial tube and a short proximal aorta. Two changes will be discussed in detail - the interposition of a second cardiac chamber in the circulation and the septation of the cardiac ventricle. By comparing the developmental genetic data of several model organisms, we can better understand the origin of the various components of the multicameral (multi-chambered) heart seen in humans. Third, Martina Brueckner's discovery that a faulty axonemal dynein was responsible for the phenotype of the iv/iv mouse (the first mammalian model of human heterotaxy) focused attention on the biology of cilia. We discuss how even the care of the complex cardiac and non-cardiac anomalies seen in heterotaxy syndrome, which have long seemed impervious to advancements in surgical and medical intensive care, may yet yield to strategies grounded in a better understanding of the cilium. The fact that all cardiac defects seen in patients with full-blown heterotaxy can also be seen in patients without obvious laterality defects hints at important roles for ciliary function not only in left-right axis specification but also in cardiovascular morphogenesis. These three developmental biology stories illustrate how the remaining unexplained mortality and morbidity of congenital heart disease can be solved.

The neural crest in cardiac congenital anomalies

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

Notch pathway regulation of neural crest cell development in vivo

BACKGROUND

The function of Notch signaling in murine neural crest-derived cell lineages in vivo was examined.

RESULTS

Conditional gain (Wnt1Cre;Rosa(Notch)) or loss (Wnt1Cre;RBP-J(f/f)) of Notch signaling in neural crest cells (NCCs) in vivo results in craniofacial, cardiac, and trunk abnormalities. Severe craniofacial malformations are apparent in Wnt1Cre;Rosa(Notch) embryos, while less severe skull abnormalities are evident in Wnt1Cre;RBP-J(f/f) mice. Deficient cardiac neural crest migration, resulting in cardiac outflow tract malformations, occurs with increased or decreased Notch signaling in NCCs. Smooth muscle cell differentiation also is impaired in pharyngeal NCC derivatives in both Wnt1Cre;Rosa(Notch) and Wnt1Cre;RBP-J(f/f) embryos. Neurogenesis is absent and gliogenesis is increased in the dorsal root ganglia of Wnt1Cre;Rosa(Notch) embryos, while neurogenesis is increased and gliogenesis is decreased in Wnt1Cre;RBP-J(f/f) embryos.

CONCLUSIONS

Together, these studies demonstrate essential cell-autonomous roles for appropriate levels of Notch signaling during NCC migration, proliferation, and differentiation with critical implications in craniofacial, cardiac, and neurogenic development and disease.

Pax3 is essential for normal cardiac neural crest morphogenesis but is not required during migration nor outflow tract septation

Systemic loss-of-function studies have demonstrated that Pax3 transcription factor expression is essential for dorsal neural tube, early neural crest and muscle cell lineage morphogenesis. Cardiac neural crest cells participate in both remodeling of the pharyngeal arch arteries and outflow tract septation during heart development, but the lineage specific role of Pax3 in neural crest function has not yet been determined. To gain insight into the requirement of Pax3 within the neural crest, we conditionally deleted Pax3 in both the premigratory and migratory neural crest populations via Wnt1-Cre and Ap2α-Cre and via P0-Cre in only the migratory neural crest, and compared these phenotypes to the pulmonary atresia phenotype observed following the systemic loss of Pax3. Surprisingly, using Wnt1-Cre deletion there are no resultant heart defects despite the loss of Pax3 from the premigratory and migratory neural crest. In contrast, earlier premigratory and migratory Ap2α-Cre mediated deletion resulted in double outlet right ventricle alignment heart defects. In order to assess the tissue-specific contribution of neural crest to heart development, genetic ablation of neural crest lineage using a Wnt1-Cre-activated diphtheria toxin fragment-A cell-killing system was employed. Significantly, ablation of Wnt1-Cre-expressing neural crest cells resulted in fully penetrant persistent truncus arteriosus malformations. Combined, the data show that Pax3 is essential for early neural crest progenitor formation, but is not required for subsequent cardiac neural crest progeny morphogenesis involving their migration to the heart or septation of the outflow tract.

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

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

What chick and mouse models have taught us about the role of the endocardium in congenital heart disease

Specific cell and tissue interactions drive the formation and function of the vertebrate cardiovascular system. Although much attention has been focused on the muscular components of the developing heart, the endocardium plays a key role in the formation of a functioning heart. Endocardial cells exhibit heterogeneity that allows them to participate in events such as the formation of the valves, septation of the outflow tract, and trabeculation. Here we review, the contributions of the endocardium to cardiovascular development and outline useful approaches developed in the chick and mouse that have revealed endocardial cell heterogeneity, the signaling molecules that direct endocardial cell behavior, and how these insights have contributed to our understanding of cardiovascular development and disease.

Functional interaction between Foxd3 and Pax3 in cardiac neural crest development

The transcription factors Foxd3 and Pax3 are important early regulators of neural crest (NC) progenitor cell properties. Homozygous mutations of Pax3 or a homozygous NC-specific deletion of Foxd3 cause marked defects in most NC derivatives, but neither loss of both Foxd3 alleles nor loss of one Pax3 allele alone greatly affects overall development of cardiac NC derivatives. In contrast, compound mutant embryos homozygous for a NC-specific Foxd3 mutation and heterozygous for Pax3 have fully penetrant persistent truncus arteriosus, severe thymus hypoplasia, and midgestation lethality. Foxd3; Pax3 compound mutant embryos have increased cell death in the neural folds and a drastic early reduction of NC cells, with an almost complete absence of NC caudal to the first pharyngeal arch. The genetic interaction between these genes implicates gene dosage-sensitive roles for Foxd3 and Pax3 in cardiac NC progenitors. Foxd3 and Pax3 act together to affect survival and maintenance of cardiac NC progenitors, and loss of these progenitors catastrophically affects key aspects of later cardiovascular development.

Early regulative ability of the neuroepithelium to form cardiac neural crest

The cardiac neural crest (arising from the level of hindbrain rhombomeres 6-8) contributes to the septation of the cardiac outflow tract and the formation of aortic arches. Removal of this population after neural tube closure results in severe septation defects in the chick, reminiscent of human birth defects. Because neural crest cells from other axial levels have regenerative capacity, we asked whether the cardiac neural crest might also regenerate at early stages in a manner that declines with time. Accordingly, we find that ablation of presumptive cardiac crest at stage 7, as the neural folds elevate, results in reformation of migrating cardiac neural crest by stage 13. Fate mapping reveals that the new population derives largely from the neuroepithelium ventral and rostral to the ablation. The stage of ablation dictates the competence of residual tissue to regulate and regenerate, as this capacity is lost by stage 9, consistent with previous reports. These findings suggest that there is a temporal window during which the presumptive cardiac neural crest has the capacity to regulate and regenerate, but this regenerative ability is lost earlier than in other neural crest populations.

Outflow tract cushions perform a critical valve-like function in the early embryonic heart requiring BMPRIA-mediated signaling in cardiac neural crest

Neural crest-specific ablation of BMP type IA receptor (BMPRIA) causes embryonic lethality by embryonic day (E) 12.5, and this was previously postulated to arise from a myocardial defect related to signaling by a small population of cardiac neural crest cells (cNCC) in the epicardium. However, as BMP signaling via cNCC is also required for proper development of the outflow tract cushions, precursors to the semilunar valves, a plausible alternate or additional hypothesis is that heart failure may result from an outflow tract cushion defect. To investigate whether the outflow tract cushions may serve as dynamic valves in regulating hemodynamic function in the early embryo, in this study we used noninvasive ultrasound biomicroscopy-Doppler imaging to quantitatively assess hemodynamic function in mouse embryos with P0-Cre transgene mediated neural crest ablation of Bmpr1a (P0 mutants). Similar to previous studies, the neural crest-deleted Bmpr1a P0 mutants died at approximately E12.5, exhibiting persistent truncus arteriosus, thinned myocardium, and congestive heart failure. Surprisingly, our ultrasound analyses showed normal contractile indices, heart rate, and atrioventricular conduction in the P0 mutants. However, reversed diastolic arterial blood flow was detected as early as E11.5, with cardiovascular insufficiency and death rapidly ensuing by E12.5. Quantitative computed tomography showed thinning of the outflow cushions, and this was associated with a marked reduction in cell proliferation. These results suggest BMP signaling to cNCC is required for growth of the outflow tract cushions. This study provides definitive evidence that the outflow cushions perform a valve-like function critical for survival of the early mouse embryo.

MAP3Ks as central regulators of cell fate during development

The cytoplasmic serine/threonine kinases transduce extracellular signals into regulatory events that impact cellular responses. The induction of one kinase triggers the activation of several downstream kinases, leading to the regulation of transcription factors to affect gene function. This arrangement allows for the kinase cascade to be amplified, and integrated according to the cellular context. An upstream mitogen or growth factor signal initiates a module of three kinases: a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK; e.g., Raf) that phosphorylates and activates a MAP kinase kinase (MAPKK; e.g., MEK) and finally activation of MAP kinase (MAPK; e.g., ERK). Thus, this MAP3K-MAP2K-MAPK module represents critical effectors that regulate extracellular stimuli into cellular responses, such as differentiation, proliferation, and apoptosis all of which function during development. There are 21 characterized MAP3Ks that activate known MAP2Ks, and they function in many aspects of developmental biology. This review summarizes known transduction routes linked to each MAP3K and highlights mouse models that provide clues to their physiological functions. This perspective reveals that some of these MAP3K effectors may have redundant functions, and also serve as unique nexus depending on the context of the signaling pathway.