Normal fate and altered function of the cardiac neural crest cell lineage in retinoic acid receptor mutant embryos

In-Depth Genomic Analysis: The New Challenge in Congenital Heart Disease

The use of next-generation sequencing has provided new insights into the causes and mechanisms of congenital heart disease (CHD). Examinations of the whole exome sequence have detected detrimental gene variations modifying single or contiguous nucleotides, which are characterised as pathogenic based on statistical assessments of families and correlations with congenital heart disease, elevated expression during heart development, and reductions in harmful protein-coding mutations in the general population. Patients with CHD and extracardiac abnormalities are enriched for gene classes meeting these criteria, supporting a common set of pathways in the organogenesis of CHDs. Single-cell transcriptomics data have revealed the expression of genes associated with CHD in specific cell types, and emerging evidence suggests that genetic mutations disrupt multicellular genes essential for cardiogenesis. Metrics and units are being tracked in whole-genome sequencing studies.

Timeline of Developmental Defects Generated upon Genetic Inhibition of the Retinoic Acid Receptor Signaling Pathway

It has been established for almost 30 years that the retinoic acid receptor (RAR) signalling pathway plays essential roles in the morphogenesis of a large variety of organs and systems. Here, we used a temporally controlled genetic ablation procedure to precisely determine the time windows requiring RAR functions. Our results indicate that from E8.5 to E9.5, RAR functions are critical for the axial rotation of the embryo, the appearance of the sinus venosus, the modelling of blood vessels, and the formation of forelimb buds, lung buds, dorsal pancreatic bud, lens, and otocyst. They also reveal that E9.5 to E10.5 spans a critical developmental period during which the RARs are required for trachea formation, lung branching morphogenesis, patterning of great arteries derived from aortic arches, closure of the optic fissure, and growth of inner ear structures and of facial processes. Comparing the phenotypes of mutants lacking the 3 RARs with that of mutants deprived of all-trans retinoic acid (ATRA) synthesising enzymes establishes that cardiac looping is the earliest known morphogenetic event requiring a functional ATRA-activated RAR signalling pathway.

Lipoprotein(a), a Lethal Player in Calcific Aortic Valve Disease

Calcified aortic valve disease (CAVD) is the most common valvular cardiovascular disease with increasing incidence and mortality. The primary treatment for CAVD is surgical or transcatheter aortic valve replacement and there remains a lack of effective drug treatment. Recently, lipoprotein (a) (Lp(a)) has been considered to play a crucial role in CAVD pathophysiology. Multiple studies have shown that Lp(a) represents an independent risk factor for CAVD. Moreover, Lp(a) mediates the occurrence and development of CAVD by affecting aortic valve endothelial dysfunction, indirectly promoting foam cell formation through oxidized phospholipids (OxPL), inflammation, oxidative stress, and directly promotes valve calcification. However, there is a lack of clinical trials with Lp(a) reduction as a primary endpoint. This review aims to explore the relationship and mechanism between Lp(a) and CAVD, and focuses on the current drugs that can be used as potential therapeutic targets for CAVD.

Embryonic requirements for Tcf12 in the development of the mouse coronal suture

A major feature of Saethre-Chotzen syndrome is coronal craniosynostosis, the fusion of the frontal and parietal bones at the coronal suture. It is caused by heterozygous loss-of-function mutations in either of the bHLH transcription factors TWIST1 and TCF12. Although compound heterozygous Tcf12; Twist1 mice display severe coronal synostosis, the individual role of Tcf12 had remained unexplored. Here, we show that Tcf12 controls several key processes in calvarial development, including the rate of frontal and parietal bone growth, and the boundary between sutural and osteogenic cells. Genetic analysis supports an embryonic requirement for Tcf12 in suture formation, as combined deletion of Tcf12 in embryonic neural crest and mesoderm, but not in postnatal suture mesenchyme, disrupts the coronal suture. We also detected asymmetric distribution of mesenchymal cells on opposing sides of the wild-type frontal and parietal bones, which prefigures later bone overlap at the sutures. In Tcf12 mutants, reduced asymmetry is associated with bones meeting end-on-end, possibly contributing to synostosis. Our results support embryonic requirements of Tcf12 in proper formation of the overlapping coronal suture.

Genomic frontiers in congenital heart disease

The application of next-generation sequencing to study congenital heart disease (CHD) is increasingly providing new insights into the causes and mechanisms of this prevalent birth anomaly. Whole-exome sequencing analysis identifies damaging gene variants altering single or contiguous nucleotides that are assigned pathogenicity based on statistical analyses of families and cohorts with CHD, high expression in the developing heart and depletion of damaging protein-coding variants in the general population. Gene classes fulfilling these criteria are enriched in patients with CHD and extracardiac abnormalities, evidencing shared pathways in organogenesis. Developmental single-cell transcriptomic data demonstrate the expression of CHD-associated genes in particular cell lineages, and emerging insights indicate that genetic variants perturb multicellular interactions that are crucial for cardiogenesis. Whole-genome sequencing analyses extend these observations, identifying non-coding variants that influence the expression of genes associated with CHD and contribute to the estimated ~55% of unexplained cases of CHD. These approaches combined with the assessment of common and mosaic genetic variants have provided a more complete knowledge of the causes and mechanisms of CHD. Such advances provide knowledge to inform the clinical care of patients with CHD or other birth defects and deepen our understanding of the complexity of human development. In this Review, we highlight known and candidate CHD-associated human genes and discuss how the integration of advances in developmental biology research can provide new insights into the genetic contributions to CHD.

Identification of Novel Single-Nucleotide Variants With Potential of Mediating Malfunction of MicroRNA in Congenital Heart Disease

Congenital heart defects (CHDs) represent the most common human birth defects. Our previous study indicates that the malfunction of microRNAs (miRNAs) in cardiac neural crest cells (NCCs), which contribute to the development of the heart and the connected great vessels, is likely linked to the pathogenesis of human CHDs. In this study, we attempt to further search for causative single-nucleotide variants (SNVs) from CHD patients that mediate the mis-regulating of miRNAs on their downstream target genes in the pathogenesis of CHDs. As a result, a total of 2,925 3'UTR SNVs were detected from a CHD cohort. In parallel, we profiled the expression of miRNAs in cardiac NCCs and found 201 expressed miRNAs. A combined analysis with these data further identified three 3'UTR SNVs, including NFATC1 c.^*654C>T, FGFRL1 c.^*414C>T, and CTNNB1 c.^*729_^*730insT, which result in the malfunction of miRNA-mediated gene regulation. The dysregulations were further validated experimentally. Therefore, our study indicates that miRNA-mediated gene dysregulation in cardiac NCCs could be an important etiology of congenital heart disease, which could lead to a new direction of diagnostic and therapeutic investigation on congenital heart disease.

Excess Provisional Extracellular Matrix: A Common Factor in Bicuspid Aortic Valve Formation

A bicuspid aortic valve (BAV) is the most common cardiac malformation, found in 0.5% to 2% of the population. BAVs are present in approximately 50% of patients with severe aortic stenosis and are an independent risk factor for aortic aneurysms. Currently, there are no therapeutics to treat BAV, and the human mutations identified to date represent a relatively small number of BAV patients. However, the discovery of BAV in an increasing number of genetically modified mice is advancing our understanding of molecular pathways that contribute to BAV formation. In this study, we utilized the comparison of BAV phenotypic characteristics between murine models as a tool to advance our understanding of BAV formation. The collation of murine BAV data indicated that excess versican within the provisional extracellular matrix (P-ECM) is a common factor in BAV development. While the percentage of BAVs is low in many of the murine BAV models, the remaining mutant mice exhibit larger and more amorphous tricuspid AoVs, also with excess P-ECM compared to littermates. The identification of common molecular characteristics among murine BAV models may lead to BAV therapeutic targets and biomarkers of disease progression for this highly prevalent and heterogeneous cardiovascular malformation.

The role of glucose in physiological and pathological heart formation

Cells display distinct metabolic characteristics depending on its differentiation stage. The fuel type of the cells serves not only as a source of energy but also as a driver of differentiation. Glucose, the primary nutrient to the cells, is a critical regulator of rapidly growing embryos. This metabolic change is a consequence as well as a cause of changes in genetic program. Disturbance of fetal glucose metabolism such as in diabetic pregnancy is associated with congenital heart disease. In utero hyperglycemia impacts the left-right axis establishment, migration of cardiac neural crest cells, conotruncal formation and mesenchymal formation of the cardiac cushion during early embryogenesis and causes cardiac hypertrophy in late fetal stages. In this review, we focus on the role of glucose in cardiogenesis and the molecular mechanisms underlying heart diseases associated with hyperglycemia.

Genetic and Developmental Contributors to Aortic Stenosis

Aortic stenosis (AS) remains one of the most common forms of valve disease, with significant impact on patient survival. The disease is characterized by left ventricular outflow obstruction and encompasses a series of stenotic lesions starting from the left ventricular outflow tract to the descending aorta. Obstructions may be subvalvar, valvar, or supravalvar and can be present at birth (congenital) or acquired later in life. Bicuspid aortic valve, whereby the aortic valve forms with two instead of three cusps, is the most common cause of AS in younger patients due to primary anatomic narrowing of the valve. In addition, the secondary onset of premature calcification, likely induced by altered hemodynamics, further obstructs left ventricular outflow in bicuspid aortic valve patients. In adults, degenerative AS involves progressive calcification of an anatomically normal, tricuspid aortic valve and is attributed to lifelong exposure to multifactoral risk factors and physiological wear-and-tear that negatively impacts valve structure-function relationships. AS continues to be the most frequent valvular disease that requires intervention, and aortic valve replacement is the standard treatment for patients with severe or symptomatic AS. While the positive impacts of surgical interventions are well documented, the financial burden, the potential need for repeated procedures, and operative risks are substantial. In addition, the clinical management of asymptomatic patients remains controversial. Therefore, there is a critical need to develop alternative approaches to prevent the progression of left ventricular outflow obstruction, especially in valvar lesions. This review summarizes our current understandings of AS cause; beginning with developmental origins of congenital valve disease, and leading into the multifactorial nature of AS in the adult population.

Outflow Tract Formation-Embryonic Origins of Conotruncal Congenital Heart Disease

Anomalies in the cardiac outflow tract (OFT) are among the most frequent congenital heart defects (CHDs). During embryogenesis, the cardiac OFT is a dynamic structure at the arterial pole of the heart. Heart tube elongation occurs by addition of cells from pharyngeal, splanchnic mesoderm to both ends. These progenitor cells, termed the second heart field (SHF), were first identified twenty years ago as essential to the growth of the forming heart tube and major contributors to the OFT. Perturbation of SHF development results in common forms of CHDs, including anomalies of the great arteries. OFT development also depends on paracrine interactions between multiple cell types, including myocardial, endocardial and neural crest lineages. In this publication, dedicated to Professor Andriana Gittenberger-De Groot and her contributions to the field of cardiac development and CHDs, we review some of her pioneering studies of OFT development with particular interest in the diverse origins of the many cell types that contribute to the OFT. We also discuss the clinical implications of selected key findings for our understanding of the etiology of CHDs and particularly OFT malformations.

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.

Cardiac Neural Crest

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

The 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.

Calcified Aortic Valve Disease in Patients With Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is a rare autosomal gene deficiency disease with increased low-density lipoprotein cholesterol, xanthoma, and premature coronary heart disease. Calcified aortic valve disease (CAVD) is prevalent in FH patients, resulting in adverse events and heavy health care burden. Aortic valve calcification is currently considered an active biological process, which shares several common risk factors with atherosclerosis, including aging, hypertension, dyslipidemia, and so on. Unfortunately, the pathogenesis and therapy of CAVD in FH are still controversial. There is no pharmacological intervention recommended to delay the development of CAVD in FH, and the only effective treatment for severe CAVD is aortic valve replacement. In this review, we summarize the detailed description of the pathophysiology, molecular mechanism, risk factors, and treatment of CAVD in FH patients.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Reduced maternal vitamin A status increases the incidence of normal aortic arch variants

While common in the general population, the developmental origins of "normal" anatomic variants of the aortic arch remain unknown. Aortic arch development begins with the establishment of the second heart field (SHF) that contributes to the pharyngeal arch arteries (PAAs). The PAAs remodel during subsequent development to form the mature aortic arch and arch vessels. Retinoic acid signaling involving the biologically active metabolite of vitamin A, plays a key role in multiple steps of this process. Recent work from our laboratory indicates that the E3 ubiquitin ligase Hectd1 is required for full activation of retinoic acid signaling during cardiac development. Furthermore, our study suggested that mild alterations in retinoic acid signaling combined with reduced gene dosage of Hectd1, results in a benign aortic arch variant where the transverse aortic arch is shortened between the brachiocephalic and left common carotid arteries. These abnormalities are preceded by hypoplasia of the fourth PAA. To further explore this interaction, we investigate whether reduced maternal dietary vitamin A intake can similarly influence aortic arch development. Our findings indicate that the incidence of hypoplastic fourth PAAs, as well as the incidence of shortened transverse arch are increased with reduced maternal vitamin A intake during pregnancy. These studies provide new insights as to the developmental origins of these benign aortic arch variants.

Retinoic acid signaling in heart development

Retinoic acid (RA) is a vitamin A metabolite that acts as a morphogen and teratogen. Excess or defective RA signaling causes developmental defects including in the heart. The heart develops from the anterior lateral plate mesoderm. Cardiogenesis involves successive steps, including formation of the primitive heart tube, cardiac looping, septation, chamber development, coronary vascularization, and completion of the four-chambered heart. RA is dispensable for primitive heart tube formation. Before looping, RA is required to define the anterior/posterior boundaries of the heart-forming mesoderm as well as to form the atrium and sinus venosus. In outflow tract elongation and septation, RA signaling is required to maintain/differentiate cardiogenic progenitors in the second heart field at the posterior pharyngeal arches level. Epicardium-secreted insulin-like growth factor, the expression of which is regulated by hepatic mesoderm-derived erythropoietin under the control of RA, promotes myocardial proliferation of the ventricular wall. Epicardium-derived RA induces the expression of angiogenic factors in the myocardium to form the coronary vasculature. In cardiogenic events at different stages, properly controlled RA signaling is required to establish the functional heart.

New imaging techniques project the cellular and molecular alterations underlying bicuspid aortic valve development

Bicuspid aortic valve (BAV) disease is the most common congenital cardiac malformation associated with an increased lifetime risk and a high rate of surgically-relevant valve deterioration and aortic dilatation. Genomic data revealed that different genes are associated with BAV. A dominant genetic factor for the recent past was the basis to the recommendation for a more extensive aortic intervention. However very recent evidence that hemodynamic stressors and alterations of wall shear stress play an important role independent from the genetic trait led to more conservative treatment recommendations. Therefore, there is a current need to improve the ability to risk stratify BAV patients in order to obtain an early detection of valvulopathy and aortopathy while also to predict valve dysfunction and/or aortic disease development. Imaging studies based on new cutting-edge technologies, such us 4-dimensional (4D) flow magnetic resonance imaging (MRI), two-dimensional (2D) or three-dimensional (3D) speckle-tracking imaging (STI) and computation fluid dynamics, combined with studies demonstrating new gene mutations, specific signal pathways alterations, hemodynamic influences, circulating biomarkers modifications, endothelial progenitor cell impairment and immune/inflammatory response, all detected BAV valvulopathy progression and aortic wall abnormality. Overall, the main purpose of this review article is to merge the evidences of imaging and basic science studies in a coherent hypothesis that underlies and thus projects the development of both BAV during embryogenesis and BAV-associated aortopathy and its complications in the adult life, with the final goal to identifying aneurysm formation/rupture susceptibility to improve diagnosis and management of patients with BAV-related aortopathy.

The ubiquitin ligase HECTD1 promotes retinoic acid signaling required for development of the aortic arch

The development of the aortic arch is a complex process that involves remodeling of the bilaterally symmetrical pharyngeal arch arteries (PAAs) into the mature asymmetric aortic arch. Retinoic acid signaling is a key regulator of this process by directing patterning of the second heart field (SHF), formation of the caudal PAAs and subsequent remodeling of the PAAs to form the aortic arch. Here, we identify the HECTD1 ubiquitin ligase as a novel modulator of retinoic acid signaling during this process. Hectd1^opm/opm homozygous mutant embryos show a spectrum of aortic arch abnormalities that occur following loss of 4th PAAs and increased SHF marker expression. This sequence of defects is similar to phenotypes observed in mutant mouse models with reduced retinoic acid signaling. Importantly, HECTD1 binds to and influences ubiquitination of the retinoic acid receptor, alpha (RARA). Furthermore, reduced activation of a retinoic acid response element (RARE) reporter is detected in Hectd1 mutant cells and embryos. Interestingly, Hectd1^opm/+ heterozygous embryos exhibit reduced retinoic acid signaling, along with intermediate increased expression of SHF markers; however, heterozygotes show normal development of the aortic arch. Decreasing retinoic acid synthesis by reducing Raldh2 (also known as Aldh1a2) gene dosage in Hectd1^opm/+ heterozygous embryos reveals a genetic interaction. Double heterozygous embryos show hypoplasia of the 4th PAA and increased incidence of a benign aortic arch variant, in which the transverse arch between the brachiocephalic and left common carotid arteries is shortened. Together, our data establish that HECTD1 is a novel regulator of retinoic acid signaling required for proper aortic arch development.

Editor's choice: The HECTD1 ubiquitin ligase is a novel modulator of retinoic acid signaling during aortic arch development and provides a model for complex interactions underlying variations in aortic arch development.

Meis2 represses the osteoblastic transdifferentiation of aortic valve interstitial cells through the Notch1/Twist1 pathway

AIM

Calcific aortic valve disease (CAVD) is the most common valvular disease worldwide. The osteoblastic transdifferentiation of aortic valve interstitial cells (VICs) is the essential process of CAVD, but the underlying mechanisms are poorly understood. Aortic VICs are generated from epithelial-to-mesenchymal transition (EMT) and migration of neural crest cells (NCCs).Meis2 has been associated with EMT and NCCs migration during development, but its role in CAVD is unknown. This study aims to elucidate the specific functions of Meis2 and its downstream targets in aortic valve calcification.

MATERIAL AND METHODS

Levels of Meis2 were examined in calcified (n = 30) and normal (n = 30) human aortic valve tissues, respectively. Meis2 was inhibited in porcine aortic VICs in vitro, and the effect on osteoblastic transdifferentiation and its downstream pathway were studied.

RESULTS

Meis2 gene and protein expression decreased significantly in calcified human aortic valve tissue compared with the normal ones. Inhibiting Meis2 by siRNAs reduced the gene and protein expression of Notch1 and Twist1, and induced the osteoblastic transdifferentiation of the porcine aortic VICs in vitro.

CONCLUSIONS

The present study indicated that Meis2 repress the osteoblastic transdifferentiation of aortic VICs through the Notch1/Twist1 signaling pathway. The Results identify Meis2 as a potential intervention target for the prevention of CAVD.

The Genetic Regulation of Aortic Valve Development and Calcific Disease

Heart valves are dynamic, highly organized structures required for unidirectional blood flow through the heart. Over an average lifetime, the valve leaflets or cusps open and close over a billion times, however in over 5 million Americans, leaflet function fails due to biomechanical insufficiency in response to wear-and-tear or pathological stimulus. Calcific aortic valve disease (CAVD) is the most common valve pathology and leads to stiffening of the cusp and narrowing of the aortic orifice leading to stenosis and insufficiency. At the cellular level, CAVD is characterized by valve endothelial cell dysfunction and osteoblast-like differentiation of valve interstitial cells. These processes are associated with dysregulation of several molecular pathways important for valve development including Notch, Sox9, Tgfβ, Bmp, Wnt, as well as additional epigenetic regulators. In this review, we discuss the multifactorial mechanisms that contribute to CAVD pathogenesis and the potential of targeting these for the development of novel, alternative therapeutics beyond surgical intervention.

Neuropilins guide preganglionic sympathetic axons and chromaffin cell precursors to establish the adrenal medulla

The adrenal medulla is composed of neuroendocrine chromaffin cells that secrete adrenaline into the systemic circulation to maintain physiological homeostasis and enable the autonomic stress response. How chromaffin cell precursors colonise the adrenal medulla and how they become connected to central nervous system-derived preganglionic sympathetic neurons remain largely unknown. By combining lineage tracing, gene expression studies, genetic ablation and the analysis of mouse mutants, we demonstrate that preganglionic axons direct chromaffin cell precursors into the adrenal primordia. We further show that preganglionic axons and chromaffin cell precursors require class 3 semaphorin (SEMA3) signalling through neuropilins (NRP) to target the adrenal medulla. Thus, SEMA3 proteins serve as guidance cues to control formation of the adrenal neuroendocrine system by establishing appropriate connections between preganglionic neurons and adrenal chromaffin cells that regulate the autonomic stress response.

Summary: A new role is revealed for semaphorin/neuropilin signalling in guiding preganglionic sympathetic axons and chromaffin cell precursors into the adrenal primordia, ensuring correct regulation of the autonomic stress response.

Probing the origin of matching functional jaws: roles of Dlx5/6 in cranial neural crest cells

Gnathostome jaws derive from the first pharyngeal arch (PA1), a complex structure constituted by Neural Crest Cells (NCCs), mesodermal, ectodermal and endodermal cells. Here, to determine the regionalized morphogenetic impact of Dlx5/6 expression, we specifically target their inactivation or overexpression to NCCs. NCC-specific Dlx5/6 inactivation (NCC^∆Dlx5/6) generates severely hypomorphic lower jaws that present typical maxillary traits. Therefore, differently from Dlx5/6 null-embryos, the upper and the lower jaws of NCC^∆Dlx5/6 mice present a different size. Reciprocally, forced Dlx5 expression in maxillary NCCs provokes the appearance of distinct mandibular characters in the upper jaw. We conclude that: (1) Dlx5/6 activation in NCCs invariably determines lower jaw identity; (2) the morphogenetic processes that generate functional matching jaws depend on the harmonization of Dlx5/6 expression in NCCs and in distinct ectodermal territories. The co-evolution of synergistic opposing jaws requires the coordination of distinct regulatory pathways involving the same transcription factors in distant embryonic territories.

Development of the Myocardial Interstitium

The space between cardiac myocytes is commonly referred-to as the cardiac interstitium (CI). The CI is a unique, complex and dynamic microenvironment in which multiple cell types, extracellular matrix molecules, and instructive signals interact to crucially support heart homeostasis and promote cardiac responses to normal and pathologic stimuli. Despite the biomedical and clinical relevance of the CI, its detailed cellular structure remains to be elucidated. In this review, we will dissect the organization of the cardiac interstitium by following its changing cellular and molecular composition from embryonic developmental stages to adulthood, providing a systematic analysis of the biological components of the CI. The main goal of this review is to contribute to our understanding of the CI roles in health and disease. Anat Rec, 302:58-68, 2019. © 2018 Wiley Periodicals, Inc.

Calcific Aortic Valve Disease: a Developmental Biology Perspective

PURPOSE OF REVIEW

This review aims to highlight the past and more current literature related to the multifaceted pathogenic programs that contribute to calcific aortic valve disease (CAVD) with a focus on the contribution of developmental programs.

RECENT FINDINGS

Calcification of the aortic valve is an active process characterized by calcific nodule formation on the aortic surface leading to a less supple and more stiffened cusp, thereby limiting movement and causing clinical stenosis. The mechanisms underlying these pathogenic changes are largely unknown, but emerging studies have suggested that signaling pathways common to valvulogenesis and bone development play significant roles and include Transforming Growth Factor-β (TGF-β), bone morphogenetic protein (BMP), Wnt, Notch, and Sox9. This comprehensive review of the literature highlights the complex nature of CAVD but concurrently identifies key regulators that can be targeted in the development of mechanistic-based therapies beyond surgical intervention to improve patient outcome.

Transcriptome profiling reveals expression signatures of cranial neural crest cells arising from different axial levels

Background

Cranial neural crest cells (NCCs) are a unique embryonic cell type which give rise to a diverse array of derivatives extending from neurons and glia through to bone and cartilage. Depending on their point of origin along the antero-posterior axis cranial NCCs are rapidly sorted into distinct migratory streams that give rise to axial specific structures. These migratory streams mirror the underlying segmentation of the brain with NCCs exiting the diencephalon and midbrain following distinct paths compared to those exiting the hindbrain rhombomeres (r). The genetic landscape of cranial NCCs arising at different axial levels remains unknown.

Results

Here we have used RNA sequencing to uncover the transcriptional profiles of mouse cranial NCCs arising at different axial levels. Whole transcriptome analysis identified over 120 transcripts differentially expressed between NCCs arising anterior to r3 (referred to as r1-r2 migratory stream for simplicity) and the r4 migratory stream. Eight of the genes differentially expressed between these populations were validated by RT-PCR with 2 being further validated by in situ hybridisation. We also explored the expression of the Neuropilins (Nrp1 and Nrp2) and their co-receptors and show that the A-type Plexins are differentially expressed in different cranial NCC streams.

Conclusions

Our analyses identify a large number of genes differentially regulated between cranial NCCs arising at different axial levels. This data provides a comprehensive description of the genetic landscape driving diversity of distinct cranial NCC streams and provides novel insight into the regulatory networks controlling the formation of specific skeletal elements and the mechanisms promoting migration along different paths.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0147-z) contains supplementary material, which is available to authorized users.

Facial shape differences between rats selected for tame and aggressive behaviors

Domestication has been consistently accompanied by a suite of traits called the domestication syndrome. These include increased docility, changes in coat coloration, prolonged juvenile behaviors, modified function of adrenal glands and reduced craniofacial dimensions. Wilkins et al recently proposed that the mechanistic factor underlying traits that encompass the domestication syndrome was altered neural crest cell (NCC) development. NCC form the precursors to a large number of tissue types including pigment cells, adrenal glands, teeth and the bones of the face. The hypothesis that deficits in NCC development can account for the domestication syndrome was partly based on the outcomes of Dmitri Belyaev's domestication experiments initially conducted on silver foxes. After generations of selecting for tameness, the foxes displayed phenotypes observed in domesticated species. Belyaev also had a colony of rats selected over 64 generations for either tameness or defensive aggression towards humans. Here we focus on the facial morphology of Belyaev's tame, 'domesticated' rats to test whether: 1) tameness in rats causes craniofacial changes similar to those observed in the foxes; 2) facial shape, i.e. NCC-derived region, is distinct in the tame and aggressive rats. We used computed-tomography scans of rat skulls and landmark-based geometric morphometrics to quantify and analyze the facial skeleton. We found facial shape differences between the tame and aggressive rats that were independent of size and which mirrored changes seen in domesticated animals compared to their wild counterparts. However, there was no evidence of reduced sexual dimorphism in the face of the tame rats. This indicates that not all morphological changes in NCC-derived regions in the rats follow the pattern of shape change reported in domesticated animals or the silver foxes. Thus, certain phenotypic trends that are part of the domestication syndrome might not be consistently present in all experimental animal models.

Unique spatiotemporal requirements for intraflagellar transport genes during forebrain development

Primary cilia are organelles extended from virtually all cells and are required for the proper regulation of a number of canonical developmental pathways. The role in cortical development of proteins important for ciliary form and function is a relatively understudied area. Here we have taken a genetic approach to define the role in forebrain development of three intraflagellar transport proteins known to be important for primary cilia function. We have genetically ablated Kif3a, Ift88, and Ttc21b in a series of specific spatiotemporal domains. The resulting phenotypes allow us to draw several conclusions. First, we conclude that the Ttc21b cortical phenotype is not due to the activity of Ttc21b within the brain itself. Secondly, some of the most striking phenotypes are from ablations in the neural crest cells and the adjacent surface ectoderm indicating that cilia transduce critical tissue—tissue interactions in the developing embryonic head. Finally, we note striking differences in phenotypes from ablations only one embryonic day apart, indicating very discrete spatiotemporal requirements for these three genes in cortical development.

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.

Hox Genes in Cardiovascular Development and Diseases

Congenital heart defects (CHD) are the leading cause of death in the first year of life. Over the past 20 years, much effort has been focused on unraveling the genetic bases of CHD. In particular, studies in human genetics coupled with those of model organisms have provided valuable insights into the gene regulatory networks underlying CHD pathogenesis. Hox genes encode transcription factors that are required for the patterning of the anterior–posterior axis in the embryo. In this review, we focus on the emerging role of anteriorly expressed Hox genes (Hoxa1, Hoxb1, and Hoxa3) in cardiac development, specifically their contribution to patterning of cardiac progenitor cells and formation of the great arteries. Recent evidence regarding the cooperative regulation of heart development by Hox proteins with members of the TALE-class of homeodomain proteins such as Pbx and Meis transcription factors is also discussed. These findings are highly relevant to human pathologies as they pinpoint new genes that increase susceptibility to cardiac anomalies and provide novel mechanistic insights into CHD.

The Hippo pathway member YAP enhances human neural crest cell fate and migration

The Hippo/YAP pathway serves as a major integrator of cell surface-mediated signals and regulates key processes during development and tumorigenesis. The neural crest is an embryonic tissue known to respond to multiple environmental cues in order to acquire appropriate cell fate and migration properties. Using multiple in vitro models of human neural development (pluripotent stem cell-derived neural stem cells; LUHMES, NTERA2 and SH-SY5Y cell lines), we investigated the role of Hippo/YAP signaling in neural differentiation and neural crest development. We report that the activity of YAP promotes an early neural crest phenotype and migration, and provide the first evidence for an interaction between Hippo/YAP and retinoic acid signaling in this system.

Common arterial trunk and ventricular non-compaction in Lrp2 knockout mice indicate a crucial role of LRP2 in cardiac development

Lipoprotein-related receptor protein 2 (LRP2) is important for development of the embryonic neural crest and brain in both mice and humans. Although a role in cardiovascular development can be expected, the hearts of Lrp2 knockout (KO) mice have not yet been investigated. We studied the cardiovascular development of Lrp2 KO mice between embryonic day 10.5 (E10.5) and E15.5, applying morphometry and immunohistochemistry, using antibodies against Tfap2α (neural crest cells), Nkx2.5 (second heart field), WT1 (epicardium derived cells), tropomyosin (myocardium) and LRP2. The Lrp2 KO mice display a range of severe cardiovascular abnormalities, including aortic arch anomalies, common arterial trunk (persistent truncus arteriosus) with coronary artery anomalies, ventricular septal defects, overriding of the tricuspid valve and marked thinning of the ventricular myocardium. Both the neural crest cells and second heart field, which are essential for the lengthening and growth of the right ventricular outflow tract, are abnormally positioned in the Lrp2 KO. This explains the absence of the aorto-pulmonary septum, which leads to common arterial trunk and ventricular septal defects. Severe blebbing of the epicardial cells covering the ventricles is seen. Epithelial-mesenchymal transition does occur; however, there are fewer WT1-positive epicardium-derived cells in the ventricular wall as compared to normal, coinciding with the myocardial thinning and deep intertrabecular spaces. LRP2 plays a crucial role in cardiovascular development in mice. This corroborates findings of cardiac anomalies in humans with LRP2 mutations. Future studies should reveal the underlying signaling mechanisms in which LRP2 is involved during cardiogenesis.

Summary: This paper sheds a new light on the role of the second heart field and neural crest cells in outflow tract formation in the mouse embryo. Depletion of the LPR2 results in a disturbed contribution pattern and subsequent common arterial trunk.

Dysregulated endocardial TGFβ signaling and mesenchymal transformation result in heart outflow tract septation failure

Heart outflow tract septation in mouse embryos carrying mutations in retinoic acid receptor genes fails with complete penetrance. In this mutant background, ectopic TGFβ signaling in the distal outflow tract is responsible for septation failure, but it was uncertain what tissue was responsive to ectopic TGFβ and why this response interfered with septation. By combining RAR gene mutation with tissue-specific Cre drivers and a conditional type II TGFβ receptor (Tgfbr2) allele, we determined that ectopic activation of TGFβ signaling in the endocardium is responsible for septation defects. Ectopic TGFβ signaling results in ectopic mesenchymal transformation of the endocardium and thereby in improperly constituted distal OFT cushions. Our analysis highlights the interactions between myocardium, endocardium, and neural crest cells in outflow tract morphogenesis, and demonstrates the requirement for proper TGFβ signaling in outflow tract cushion organization and septation.

Cardiac outflow morphogenesis depends on effects of retinoic acid signaling on multiple cell lineages

BACKGROUND

Retinoic acid (RA), the bioactive derivative of vitamin A, is essential for vertebrate heart development. Both excess and reduced RA signaling lead to cardiovascular malformations affecting the outflow tract (OFT). To address the cellular mechanisms underlying the effects of RA signaling during OFT morphogenesis, we used transient maternal RA supplementation to rescue the early lethality resulting from inactivation of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene.

RESULTS

By embryonic day 13.5, all rescued Raldh2(-/-) hearts exhibit severe, reproducible OFT septation defects, although wild-type and Raldh2(+/-) littermates have normal hearts. Cardiac neural crest cells (cNCC) were present in OFT cushions of Raldh2(-/-) mutant embryos but ectopically located in the periphery of the endocardial cushions, rather than immediately underlying the endocardium. Excess mesenchyme was generated by Raldh2(-/-) mutant endocardium, which displaced cNCC derivatives from their subendocardial, medial position.

CONCLUSIONS

RA signaling affects not only cNCC numbers but also their position relative to endocardial mesenchyme during the septation process. Our study shows that inappropriate coordination between the different cell types of the OFT perturbs its morphogenesis and leads to a severe congenital heart defect, persistent truncus arteriosus.

Neural crest cell-derived VEGF promotes embryonic jaw extension

Craniofacial development is a complex morphogenic event that relies on highly orchestrated interactions between multiple cell types. Since the first description of Meckel’s cartilage in the lower jaw more than 180 years ago, we have come to realize that expansion of this specialized structure underpins correct mandible development. Here we demonstrate that an intricate association between neural crest cells and blood vessels plays an important role in promoting chondrocyte proliferation and expansion of Meckel’s cartilage as a prerequisite of correct mandibular morphogenesis. These findings provide direct insight into the origins and potential treatments of highly prevalent disorders affecting the mandible.

Jaw morphogenesis depends on the growth of Meckel’s cartilage during embryogenesis. However, the cell types and signals that promote chondrocyte proliferation for Meckel’s cartilage growth are poorly defined. Here we show that neural crest cells (NCCs) and their derivatives provide an essential source of the vascular endothelial growth factor (VEGF) to enhance jaw vascularization and stabilize the major mandibular artery. We further show in two independent mouse models that blood vessels promote Meckel’s cartilage extension. Coculture experiments of arterial tissue with NCCs or chondrocytes demonstrated that NCC-derived VEGF promotes blood vessel growth and that blood vessels secrete factors to instruct chondrocyte proliferation. Computed tomography and X-ray scans of patients with hemifacial microsomia also showed that jaw hypoplasia correlates with mandibular artery dysgenesis. We conclude that cranial NCCs and their derivatives provide an essential source of VEGF to support blood vessel growth in the developing jaw, which in turn is essential for normal chondrocyte proliferation, and therefore jaw extension.

Neuropilins define distinct populations of neural crest cells

Background

Neural crest cells (NCCs) are a transient embryonic cell type that give rise to a wide spectrum of derivatives, including neurons and glia of the sensory and autonomic nervous system, melanocytes and connective tissues in the head. Lineage-tracing and functional studies have shown that trunk NCCs migrate along two distinct paths that correlate with different developmental fates. Thus, NCCs migrating ventrally through the anterior somite form sympathetic and sensory ganglia, whereas NCCs migrating dorsolaterally form melanocytes. Although the mechanisms promoting migration along the dorsolateral path are well defined, the molecules providing positional identity to sympathetic and sensory-fated NCCs that migrate along the same ventral path are ill defined. Neuropilins (Nrp1 and Nrp2) are transmembrane glycoproteins that are essential for NCC migration. Nrp1 and Nrp2 knockout mice have disparate phenotypes, suggesting that these receptors may play a role in sorting NCCs biased towards sensory and sympathetic fates to appropriate locations.

Results

Here we have combined in situ hybridisation, immunohistochemistry and lineage-tracing analyses to demonstrate that neuropilins are expressed in a non-overlapping pattern within NCCs. Whereas Nrp1 is expressed in NCCs emigrating from hindbrain rhombomere 4 (r4) and within trunk NCCs giving rise to sympathetic and sensory ganglia, Nrp2 is preferentially expressed in NCCs emigrating from r2 and in trunk NCCs giving rise to sensory ganglia. By generating a tamoxifen-inducible lineage-tracing system, we further demonstrate that Nrp2-expressing NCCs specifically populate sensory ganglia including the trigeminal ganglia (V) in the head and the dorsal root ganglia in the trunk.

Conclusions

Taken together, our results demonstrate that Nrp1 and Nrp2 are expressed in different populations of NCCs, and that Nrp2-expressing NCCs are strongly biased towards a sensory fate. In the trunk, Nrp2-expressing NCCs specifically give rise to sensory ganglia, whereas Nrp1-expressing NCCs likely give rise to both sensory and sympathetic ganglia. Our findings therefore suggest that neuropilins play an essential role in coordinating NCC migration with fate specification.

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

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

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.

Twist1 controls a cell-specification switch governing cell fate decisions within the cardiac neural crest

Neural crest cells are multipotent progenitor cells that can generate both ectodermal cell types, such as neurons, and mesodermal cell types, such as smooth muscle. The mechanisms controlling this cell fate choice are not known. The basic Helix-loop-Helix (bHLH) transcription factor Twist1 is expressed throughout the migratory and post-migratory cardiac neural crest. Twist1 ablation or mutation of the Twist-box causes differentiation of ectopic neuronal cells, which molecularly resemble sympathetic ganglia, in the cardiac outflow tract. Twist1 interacts with the pro-neural factor Sox10 via its Twist-box domain and binds to the Phox2b promoter to repress transcriptional activity. Mesodermal cardiac neural crest trans-differentiation into ectodermal sympathetic ganglia-like neurons is dependent upon Phox2b function. Ectopic Twist1 expression in neural crest precursors disrupts sympathetic neurogenesis. These data demonstrate that Twist1 functions in post-migratory neural crest cells to repress pro-neural factors and thereby regulate cell fate determination between ectodermal and mesodermal lineages.

During vertebrate development, a unique population of cells, termed neural crest cells, migrates throughout the developing embryo, generating various cell types, for example, the smooth muscle that divides the aorta and pulmonary artery where they connect to the heart, and the autonomic neurons, which coordinate organ function. The distinctions between neural crest cells that will form smooth muscle and those that will become neurons are thought to occur prior to migration. Here, we show that, in mice with mutations of the transcription factor Twist1, a subpopulation of presumptive smooth muscle cells, following migration to the heart, instead mis-specify to resemble autonomic neurons. Twist1 represses transcription of the pro-neural factor Phox2b both through antagonism of its upstream effector, Sox10, and through direct binding to its promoter. Phox2b is absolutely required for autonomic neuron development, and indeed, the aberrant neurons in Twist1 mutants disappear when Phox2b is also mutated. Ectopic Twist1 expression within all neural crest cells disrupts the specification of normal autonomic neurons. Collectively, these data reveal that neural crest cells can alter their cell fate from mesoderm to ectoderm after they have migrated and that Twist1 functions to maintain neural crest cell potency during embryonic development.

Cardiac outflow tract anomalies

The mature outflow tract (OFT) is, in basic terms, a short conduit. It is a simple, although vital, connection situated between contracting muscular heart chambers and a vast embryonic vascular network. Unfortunately, it is also a focal point underlying many multifactorial congenital heart defects (CHDs). Through the use of various animal models combined with human genetic investigations, we are beginning to comprehend the molecular and cellular framework that controls OFT morphogenesis. Clear roles of neural crest cells (NCC) and second heart field (SHF) derivatives have been established during OFT formation and remodeling. The challenge now is to determine how the SHF and cardiac NCC interact, the complex reciprocal signaling that appears to be occurring at various stages of OFT morphogenesis, and finally how endocardial progenitors and primary heart field (PHF) communicate with both these colonizing extra-cardiac lineages. Although we are beginning to understand that this dance of progenitor populations is wonderfully intricate, the underlying pathogenesis and the spatiotemporal cell lineage interactions remain to be fully elucidated. What is now clear is that OFT alignment and septation are independent processes, invested via separate SHF and cardiac neural crest (CNC) lineages. This review will focus on our current understanding of the respective contributions of the SHF and CNC lineage during OFT development and pathogenesis.

Increased dietary intake of vitamin A promotes aortic valve calcification in vivo

OBJECTIVE

Calcific aortic valve disease (CAVD) is a major public health problem with no effective treatment available other than surgery. We previously showed that mature heart valves calcify in response to retinoic acid (RA) treatment through downregulation of the SRY transcription factor Sox9. In this study, we investigated the effects of excess vitamin A and its metabolite RA on heart valve structure and function in vivo and examined the molecular mechanisms of RA signaling during the calcification process in vitro.

METHODS AND RESULTS

Using a combination of approaches, we defined calcific aortic valve disease pathogenesis in mice fed 200 IU/g and 20 IU/g of retinyl palmitate for 12 months at molecular, cellular, and functional levels. We show that mice fed excess vitamin A develop aortic valve stenosis and leaflet calcification associated with increased expression of osteogenic genes and decreased expression of cartilaginous markers. Using a pharmacological approach, we show that RA-mediated Sox9 repression and calcification is regulated by classical RA signaling and requires both RA and retinoid X receptors.

CONCLUSIONS

Our studies demonstrate that excess vitamin A dietary intake promotes heart valve calcification in vivo. Therefore suggesting that hypervitaminosis A could serve as a new risk factor of calcific aortic valve disease in the human population.

Partitioning the heart: mechanisms of cardiac septation and valve development

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

Cellular decisions in cardiac outflow tract and coronary development: an act by VEGF and NOTCH

Congenital cardiac abnormalities are, due to their relatively high frequency and severe impact on quality of life, an important focus in cardiovascular research. Recently, various human studies have revealed a high coincidence of VEGF and NOTCH polymorphisms with cardiovascular outflow tract anomalies, such as bicuspid aortic valves and Tetralogy of Fallot, next to predisposition for cardiovascular pathologies, including atherosclerosis and aortic valve calcification. This genetic association between VEGF/NOTCH mutations and congenital cardiovascular defects in humans has been supported by substantial proof from animal models, revealing interaction of both pathways in cellular processes that are crucial for cardiac development. This review focuses on the role of VEGF and NOTCH signaling and their interplay in cardiogenesis with special interest to coronary and outflow tract development. An overview of the association between congenital malformations and VEGF/NOTCH polymorphisms in humans will be discussed along with their potential mechanisms and processes as revealed by transgenic mouse models. The molecular and cellular interaction of VEGF and subsequent Notch-signaling in these processes will be highlighted.

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.

A retinoic acid responsive Hoxa3 transgene expressed in embryonic pharyngeal endoderm, cardiac neural crest and a subdomain of the second heart field

A transgenic mouse line harbouring a β-galacdosidase reporter gene controlled by the proximal 2 kb promoter of Hoxa3 was previously generated to investigate the regulatory cues governing Hoxa3 expression in the mouse. Examination of transgenic embryos from embryonic day (E) 8.0 to E15.5 revealed regionally restricted reporter activity in the developing heart. Indeed, transgene expression specifically delineated cells from three distinct lineages: a subpopulation of the second heart field contributing to outflow tract myocardium, the cardiac neural crest cells and the pharyngeal endoderm. Manipulation of the Retinoic Acid (RA) signaling pathway showed that RA is required for correct expression of the transgene. Therefore, this transgenic line may serve as a cardiosensor line of particular interest for further analysis of outflow tract development.

Mesodermal retinoic acid signaling regulates endothelial cell coalescence in caudal pharyngeal arch artery vasculogenesis

Disruption of retinoic acid signaling causes a variety of pharyngeal arch artery and great vessel defects, as well as malformations in many other tissues, including those derived from the pharyngeal endoderm. Previous studies implied that arch artery defects in the context of defective RA signaling occur secondary to pharyngeal pouch segmentation defects, although this model has never been experimentally verified. In this study, we examined arch artery morphogenesis during mouse development, and the role of RA in this process. We show in normal embryos that the arch arteries form by vasculogenic differentiation of pharyngeal mesoderm. Using various genetic backgrounds and tissue-specific mutation approaches, we segregate pharyngeal arch artery and pharyngeal pouch defects in RA receptor mutants, and show that RA signal transduction only in pharyngeal mesoderm is required for arch artery formation. RA does not control pharyngeal mesodermal differentiation to endothelium, but instead promotes the aggregation of endothelial cells into nascent vessels. Expression of VE-cadherin was substantially reduced in RAR mutants, and this deficiency may underlie the arch artery defects. The consequences of disrupted mesodermal and endodermal RA signaling were restricted to the 4th and 6th arch arteries and to the 4th pharyngeal pouch, respectively, suggesting that different regulatory mechanisms control the formation of the more anterior arch arteries and pouches.