Smad signaling in the neural crest regulates cardiac outflow tract remodeling through cell autonomous and non-cell autonomous effects

Self-organizing human heart assembloids with autologous and developmentally relevant cardiac neural crest-derived tissues

Neural crest cells (NCCs) are a multipotent embryonic cell population of ectodermal origin that extensively migrate during early development and contribute to the formation of multiple tissues. Cardiac NCCs play a critical role in heart development by orchestrating outflow tract septation, valve formation, aortic arch artery patterning, parasympathetic innervation, and maturation of the cardiac conduction system. Abnormal migration, proliferation, or differentiation of cardiac NCCs can lead to severe congenital cardiovascular malformations. However, the complexity and timing of early embryonic heart development pose significant challenges to studying the molecular mechanisms underlying NCC-related cardiac pathologies. Here, we present a sophisticated functional model of human heart assembloids derived from induced pluripotent stem cells, which, for the first time, recapitulates cardiac NCC integration into the human embryonic heart in vitro . NCCs successfully integrated at developmentally relevant stages into heart organoids, and followed developmental trajectories known to occur in the human heart. They demonstrated extensive migration, differentiated into cholinergic neurons capable of generating nerve impulses, and formed mature glial cells. Additionally, they contributed to the mesenchymal populations of the developing outflow tract. Through transcriptomic analysis, we revealed that NCCs acquire molecular features of their cardiac derivatives as heart assembloids develop. NCC-derived parasympathetic neurons formed functional connections with cardiomyocytes, promoting the maturation of the cardiac conduction system. Leveraging this model's cellular complexity and functional maturity, we uncovered that early exposure of NCCs to antidepressants harms the development of NCC derivatives in the context of the developing heart. The commonly prescribed antidepressant Paroxetine disrupted the expression of a critical early neuronal transcription factor, resulting in impaired parasympathetic innervation and functional deficits in cardiac tissue. This advanced heart assembloid model holds great promise for high-throughput drug screening and unraveling the molecular mechanisms underlying NCC-related cardiac formation and congenital heart defects.

IN BRIEF

Human neural crest heart assembloids resembling the major directions of neural crest differentiation in the human embryonic heart, including parasympathetic innervation and the mesenchymal component of the outflow tract, provide a human-relevant embryonic platform for studying congenital heart defects and drug safety.

The chromatin regulator Ankrd11 controls cardiac neural crest cell-mediated outflow tract remodeling and heart function

ANKRD11 (Ankyrin Repeat Domain 11) is a chromatin regulator and a causative gene for KBG syndrome, a rare developmental disorder characterized by multiple organ abnormalities, including cardiac defects. However, the role of ANKRD11 in heart development is unknown. The neural crest plays a leading role in embryonic heart development, and its dysfunction is implicated in congenital heart defects. We demonstrate that conditional knockout of Ankrd11 in the murine embryonic neural crest results in persistent truncus arteriosus, ventricular dilation, and impaired ventricular contractility. We further show these defects occur due to aberrant cardiac neural crest cell organization leading to outflow tract septation failure. Lastly, knockout of Ankrd11 in the neural crest leads to impaired expression of various transcription factors, chromatin remodelers and signaling pathways, including mTOR, BMP and TGF-β in the cardiac neural crest cells. In this work, we identify Ankrd11 as a regulator of neural crest-mediated heart development and function.

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.

SMAD4: A Critical Regulator of Cardiac Neural Crest Cell Fate and Vascular Smooth Muscle Differentiation

Background

The pharyngeal arch arteries (PAAs) are precursor vessels which remodel into the aortic arch arteries (AAAs) during embryonic cardiovascular development. Cardiac neural crest cells (NCs) populate the PAAs and differentiate into vascular smooth muscle cells (vSMCs), which is critical for successful PAA-to-AAA remodeling. SMAD4, the central mediator of canonical TGFβ signaling, has been implicated in NC-to-vSMC differentiation; however, its distinct roles in vSMC differentiation and NC survival are unclear.

Results

Here, we investigated the role of SMAD4 in cardiac NC differentiation to vSMCs using lineage-specific inducible mouse strains in an attempt to avoid early embryonic lethality and NC cell death. We found that with global SMAD4 loss, its role in smooth muscle differentiation could be uncoupled from its role in the survival of the cardiac NC in vivo . Moreover, we found that SMAD4 may regulate the induction of fibronectin, a known mediator of NC-to-vSMC differentiation. Finally, we found that SMAD4 is required in NCs cell-autonomously for NC-to-vSMC differentiation and for NC contribution to and persistence in the pharyngeal arch mesenchyme.

Conclusions

Overall, this study demonstrates the critical role of SMAD4 in the survival of cardiac NCs, their differentiation to vSMCs, and their contribution to the developing pharyngeal arches.

Vanadium Modulates Proteolytic Activities and MMP-14-Like Levels during Paracentrotus lividus Embryogenesis

The increasing industrial use of vanadium (V), as well as its recent medical use in various pathologies has intensified its environmental release, making it an emerging pollutant. The sea urchin embryo has long been used to study the effects induced by metals, including V. In this study we used an integrated approach that correlates the biological effects on embryo development with proteolytic activities of gelatinases that could better reflect any metal-induced imbalances. V-exposure caused morphological/morphometric aberrations, mainly concerning the correct distribution of embryonic cells, the development of the skeleton, and the embryo volume. Moreover, V induced a concentration change in all the gelatinases expressed during embryo development and a reduction in their total proteolytic activity. The presence of three MMP-like gelatinases (MMP-2, -9, and -14) was also demonstrated and their levels depended on V-concentration. In particular, the MMP-14-like protein modified its expression level during embryo development in a time- and dose-dependent manner. This enzyme also showed a specific localization on filopodia, suggesting that primary mesenchyme cells (PMCs) could be responsible for its synthesis. In conclusion, these results indicate that an integrated study among morphology/morphometry, proteolytic activity, and MMP-14 expression constitutes an important response profile to V-action.

Expanded cardiovascular phenotype of Myhre syndrome includes tetralogy of Fallot suggesting a role for SMAD4 in human neural crest defects

Tetralogy of Fallot (ToF) can be associated with a wide range of extracardiac anomalies, with an underlying etiology identified in approximately 10% of cases. Individuals affected with Myhre syndrome due to recurrent SMAD4 mutations frequently have cardiovascular anomalies, including congenital heart defects. In addition to two patients in the literature with ToF, we describe five additional individuals with Myhre syndrome and classic ToF, ToF with pulmonary atresia and multiple aorto-pulmonary collaterals, and ToF with absent pulmonary valve. Aorta hypoplasia was documented in one patient and suspected in another two. In half of these individuals, postoperative cardiac dysfunction was thought to be more severe than classic postoperative ToF repair. There may be an increase in right ventricular pressure, and right ventricular dysfunction due to free pulmonic regurgitation. Noncardiac developmental abnormalities in our series and the literature, including corectopia, heterochromia iridis, and congenital miosis suggest an underlying defect of neural crest cell migration in Myhre syndrome. We advise clinicians that Myhre syndrome should be considered in the genetic evaluation of a child with ToF, short stature, unusual facial features, and developmental delay, as these children may be at risk for increased postoperative morbidity. Additional research is needed to investigate the hypothesis that postoperative hemodynamics in these patients may be consistent with restrictive myocardial physiology.

Dynamic Expression of Membrane Type 1-Matrix Metalloproteinase (Mt1-mmp/Mmp14) in the Mouse Embryo

MT1-MMP/MMP14 belongs to a subgroup of the matrix metalloproteinases family that presents a transmembrane domain, with a cytosolic tail and the catalytic site exposed to the extracellular space. Deficient mice for this enzyme result in early postnatal death and display severe defects in skeletal, muscle and lung development. By using a transgenic line expressing the LacZ reporter under the control of the endogenous Mt1-mmp promoter, we reported a dynamic spatiotemporal expression pattern for Mt1-mmp from early embryonic to perinatal stages during cardiovascular development and brain formation. Thus, Mt1-mmp shows expression in the endocardium of the heart and the truncus arteriosus by E8.5, and is also strongly detected during vascular system development as well as in endothelial cells. In the brain, LacZ reporter expression was detected in the olfactory bulb, the rostral cerebral cortex and the caudal mesencephalic tectum. LacZ-positive cells were observed in neural progenitors of the spinal cord, neural crest cells and the intersomitic region. In the limb, Mt1-mmp expression was restricted to blood vessels, cartilage primordium and muscles. Detection of the enzyme was confirmed by Western blot and immunohistochemical analysis. We suggest novel functions for this metalloproteinase in angiogenesis, endocardial formation and vascularization during organogenesis. Moreover, Mt1-mmp expression revealed that the enzyme may contribute to heart, muscle and brain throughout development.

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

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

The Cardiac Neural Crest Cells in Heart Development and Congenital Heart Defects

The neural crest (NC) is a multipotent and temporarily migratory cell population stemming from the dorsal neural tube during vertebrate embryogenesis. Cardiac neural crest cells (NCCs), a specified subpopulation of the NC, are vital for normal cardiovascular development, as they significantly contribute to the pharyngeal arch arteries, the developing cardiac outflow tract (OFT), cardiac valves, and interventricular septum. Various signaling pathways are shown to orchestrate the proper migration, compaction, and differentiation of cardiac NCCs during cardiovascular development. Any loss or dysregulation of signaling pathways in cardiac NCCs can lead to abnormal cardiovascular development during embryogenesis, resulting in abnormalities categorized as congenital heart defects (CHDs). This review focuses on the contributions of cardiac NCCs to cardiovascular formation, discusses cardiac defects caused by a disruption of various regulatory factors, and summarizes the role of multiple signaling pathways during embryonic development. A better understanding of the cardiac NC and its vast regulatory network will provide a deeper insight into the mechanisms of the associated abnormalities, leading to potential therapeutic advancements.

Single-Cell Multiomic Approaches Reveal Diverse Labeling of the Nervous System by Common Cre-Drivers

Neural crest development involves a series of dynamic, carefully coordinated events that result in human disease when not properly orchestrated. Cranial neural crest cells acquire unique multipotent developmental potential upon specification to generate a broad variety of cell types. Studies of early mammalian neural crest and nervous system development often use the Cre-loxP system to lineage trace and mark cells for further investigation. Here, we carefully profile the activity of two common neural crest Cre-drivers at the end of neurulation in mice. RNA sequencing of labeled cells at E9.5 reveals that Wnt1-Cre2 marks cells with neuronal characteristics consistent with neuroepithelial expression, whereas Sox10-Cre predominantly labels the migratory neural crest. We used single-cell mRNA and single-cell ATAC sequencing to profile the expression of Wnt1 and Sox10 and identify transcription factors that may regulate the expression of Wnt1-Cre2 in the neuroepithelium and Sox10-Cre in the migratory neural crest. Our data identify cellular heterogeneity during cranial neural crest development and identify specific populations labeled by two Cre-drivers in the developing nervous system.

Cardiac Neural Crest Cells: Their Rhombomeric Specification, Migration, and Association with Heart and Great Vessel Anomalies

Outflow tract abnormalities are the most frequent congenital heart defects. These are due to the absence or dysfunction of the two main cell types, i.e., neural crest cells and secondary heart field cells that migrate in opposite directions at the same stage of development. These cells directly govern aortic arch patterning and development, ascending aorta dilatation, semi-valvular and coronary artery development, aortopulmonary septation abnormalities, persistence of the ductus arteriosus, trunk and proximal pulmonary arteries, sub-valvular conal ventricular septal/rotational defects, and non-compaction of the left ventricle. In some cases, depending on the functional defects of these cells, additional malformations are found in the expected spatial migratory area of the cells, namely in the pharyngeal arch derivatives and cervico-facial structures. Associated non-cardiovascular anomalies are often underestimated, since the multipotency and functional alteration of these cells can result in the modification of multiple neural, epidermal, and cervical structures at different levels. In most cases, patients do not display the full phenotype of abnormalities, but congenital cardiac defects involving the ventricular outflow tract, ascending aorta, aortic arch and supra-aortic trunks should be considered as markers for possible impaired function of these cells. Neural crest cells should not be considered as a unique cell population but on the basis of their cervical rhombomere origins R3-R5 or R6-R7-R8 and specific migration patterns: R3-R4 towards arch II, R5-R6 arch III and R7-R8 arch IV and VI. A better understanding of their development may lead to the discovery of unknown associated abnormalities, thereby enabling potential improvements to be made to the therapeutic approach.

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

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

Bearing My Heart: The Role of Extracellular Matrix on Cardiac Development, Homeostasis, and Injury Response

The extracellular matrix (ECM) is an essential component of the heart that imparts fundamental cellular processes during organ development and homeostasis. Most cardiovascular diseases involve severe remodeling of the ECM, culminating in the formation of fibrotic tissue that is deleterious to organ function. Treatment schemes effective at managing fibrosis and promoting physiological ECM repair are not yet in reach. Of note, the composition of the cardiac ECM changes significantly in a short period after birth, concurrent with the loss of the regenerative capacity of the heart. This highlights the importance of understanding ECM composition and function headed for the development of more efficient therapies. In this review, we explore the impact of ECM alterations, throughout heart ontogeny and disease, on cardiac cells and debate available approaches to deeper insights on cell–ECM interactions, toward the design of new regenerative therapies.

Dullard-mediated Smad1/5/8 inhibition controls mouse cardiac neural crest cells condensation and outflow tract septation

The establishment of separated pulmonary and systemic circulation in vertebrates, via cardiac outflow tract (OFT) septation, is a sensitive developmental process accounting for 10% of all congenital anomalies. Neural Crest Cells (NCC) colonising the heart condensate along the primitive endocardial tube and force its scission into two tubes. Here, we show that NCC aggregation progressively decreases along the OFT distal-proximal axis following a BMP signalling gradient. Dullard, a nuclear phosphatase, tunes the BMP gradient amplitude and prevents NCC premature condensation. Dullard maintains transcriptional programs providing NCC with mesenchymal traits. It attenuates the expression of the aggregation factor Sema3c and conversely promotes that of the epithelial-mesenchymal transition driver Twist1. Altogether, Dullard-mediated fine-tuning of BMP signalling ensures the timed and progressive zipper-like closure of the OFT by the NCC and prevents the formation of a heart carrying the congenital abnormalities defining the tetralogy of Fallot.

Specification and formation of the neural crest: Perspectives on lineage segregation

The neural crest is a fascinating embryonic population unique to vertebrates that is endowed with remarkable differentiation capacity. Thought to originate from ectodermal tissue, neural crest cells generate neurons and glia of the peripheral nervous system, and melanocytes throughout the body. However, the neural crest also generates many ectomesenchymal derivatives in the cranial region, including cell types considered to be of mesodermal origin such as cartilage, bone, and adipose tissue. These ectomesenchymal derivatives play a critical role in the formation of the vertebrate head, and are thought to be a key attribute at the center of vertebrate evolution and diversity. Further, aberrant neural crest cell development and differentiation is the root cause of many human pathologies, including cancers, rare syndromes, and birth malformations. In this review, we discuss the current findings of neural crest cell ontogeny, and consider tissue, cell, and molecular contributions toward neural crest formation. We further provide current perspectives into the molecular network involved during the segregation of the neural crest lineage.

Neural crest delamination and migration: Looking forward to the next 150 years

Neural crest (NC) cells were described for the first time in 1868 by Wilhelm His. Since then, this amazing population of migratory stem cells has been intensively studied. It took a century to fully unravel their incredible abilities to contribute to nearly every organ of the body. Yet, our understanding of the cell and molecular mechanisms controlling their migration is far from complete. In this review, we summarize the current knowledge on epithelial-mesenchymal transition and collective behavior of NC cells and propose further stops at which the NC train might be calling in the near future.

Cre-driver lines used for genetic fate mapping of neural crest cells in the mouse: An overview

The neural crest is one of the embryonic structures with the broadest developmental potential in vertebrates. Morphologically, neural crest cells emerge during neurulation in the dorsal folds of the neural tube before undergoing an epithelial‐to‐mesenchymal transition (EMT), delaminating from the neural tube, and migrating to multiple sites in the growing embryo. Neural crest cells generate cell types as diverse as peripheral neurons and glia, melanocytes, and so‐called mesectodermal derivatives that include craniofacial bone and cartilage and smooth muscle cells in cardiovascular structures. In mice, the fate of neural crest cells has been determined mainly by means of transgenesis and genome editing technologies. The most frequently used method relies on the Cre‐loxP system, in which expression of Cre‐recombinase in neural crest cells or their derivatives genetically enables the expression of a Cre‐reporter allele, thus permanently marking neural crest‐derived cells. Here, we provide an overview of the Cre‐driver lines used in the field and discuss to what extent these lines allow precise neural crest stage and lineage‐specific fate mapping.

microRNA expression profiling and functional annotation analysis of their targets modulated by oxidative stress during embryonic heart development in diabetic mice

Maternal pregestational diabetes mellitus (PGDM) induces congenital heart defects (CHDs). The molecular mechanism underlying PGDM-induced CHDs is unknown. microRNAs (miRNAs), small non-coding RNAs, repress gene expression at the posttranscriptional level and play important roles in heart development. We performed a global miRNA profiling study to assist in revealing potential miRNAs modulated by PGDM and possible developmental pathways regulated by miRNAs during heart development. A total of 149 mapped miRNAs in the developing heart were significantly altered by PGDM. Bioinformatics analysis showed that the majority of the 2111 potential miRNA target genes were associated with cardiac development-related pathways including STAT3 and IGF-1 and transcription factors (Cited2, Zeb2, Mef2c, Smad4 and Ets1). Overexpression of the antioxidant enzyme, superoxide dismutase 1, reversed PGDM-altered miRNAs, suggesting that oxidative stress is responsible for dysregulation of miRNAs. Thus, our study provides the foundation for further investigation of a miRNA-dependent mechanism underlying PGDM-induced CHDs.

Rac1 Signaling Is Required for Anterior Second Heart Field Cellular Organization and Cardiac Outflow Tract Development

BACKGROUND

The small GTPase Rac1 regulates diverse cellular functions, including both apicobasal and planar cell polarity pathways; however, its role in cardiac outflow tract (OFT) development remains unknown. In the present study, we aimed to examine the role of Rac1 in the anterior second heart field (SHF) splanchnic mesoderm and subsequent OFT development during heart morphogenesis.

METHODS AND RESULTS

Using the Cre/loxP system, mice with an anterior SHF-specific deletion of Rac1 (Rac1(SHF)) were generated. Embryos were collected at various developmental time points for immunostaining and histological analysis. Intrauterine echocardiography was also performed to assess aortic valve blood flow in embryos at embryonic day 18.5. The Rac1(SHF) splanchnic mesoderm exhibited disruptions in SHF progenitor cellular organization and proliferation. Consequently, this led to a spectrum of OFT defects along with aortic valve defects in Rac1(SHF) embryos. Mechanistically, it was found that the ability of the Rac1(SHF) OFT myocardial cells to migrate into the proximal OFT cushion was severely reduced. In addition, expression of the neural crest chemoattractant semaphorin 3c was decreased. Lineage tracing showed that anterior SHF contribution to the OFT myocardium and aortic valves was deficient in Rac1(SHF) hearts. Furthermore, functional analysis with intrauterine echocardiography at embryonic day 18.5 showed aortic valve regurgitation in Rac1(SHF) hearts, which was not seen in control hearts.

CONCLUSIONS

Disruptions of Rac1 signaling in the anterior SHF results in aberrant progenitor cellular organization and defects in OFT development. Our data show Rac1 signaling to be a critical regulator of cardiac OFT formation during embryonic heart development.

Embryonic Development of the Bicuspid Aortic Valve

Bicuspid aortic valve (BAV) is the most common congenital valvular heart defect with an overall frequency of 0.5%–1.2%. BAVs result from abnormal aortic cusp formation during valvulogenesis, whereby adjacent cusps fuse into a single large cusp resulting in two, instead of the normal three, aortic cusps. Individuals with BAV are at increased risk for ascending aortic disease, aortic stenosis and coarctation of the aorta. The frequent occurrence of BAV and its anatomically discrete but frequent co-existing diseases leads us to suspect a common cellular origin. Although autosomal-dominant transmission of BAV has been observed in a few pedigrees, notably involving the gene NOTCH1, no single-gene model clearly explains BAV inheritance, implying a complex genetic model involving interacting genes. Several sequencing studies in patients with BAV have identified rare and uncommon mutations in genes of cardiac embryogenesis. But the extensive cell-cell signaling and multiple cellular origins involved in cardiac embryogenesis preclude simplistic explanations of this disease. In this review, we examine the series of events from cellular and transcriptional embryogenesis of the heart, to development of the aortic valve.

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.

AcvR1-mediated BMP signaling in second heart field is required for arterial pole development: implications for myocardial differentiation and regional identity

BMP signaling plays an essential role in second heart field-derived heart and arterial trunk development, including myocardial differentiation, right ventricular growth, and interventricular, outflow tract and aortico-pulmonary septation. It is mediated by a number of different BMP ligands, and receptors, many of which are present simultaneously. The mechanisms by which they regulate morphogenetic events and degree of redundancy amongst them have still to be elucidated. We therefore assessed the role of BMP Type I receptor AcvR1 in anterior second heart field-derived cell development, and compared it with that of BmpR1a. By removing Acvr1 using the driver Mef2c[AHF]-Cre, we show that AcvR1 plays an essential role in arterial pole morphogenesis, identifying defects in outflow tract wall and cushion morphology that preceded a spectrum of septation defects from double outlet right ventricle to common arterial trunk in mutants. Its absence caused dysregulation in gene expression important for myocardial differentiation (Isl1, Fgf8) and regional identity (Tbx2, Tbx3, Tbx20, Tgfb2). Although these defects resemble to some degree those in the equivalent Bmpr1a mutant, a novel gene knock-in model in which Bmpr1a was expressed in the Acvr1 locus only partially restored septation in Acvr1 mutants. These data show that both BmpR1a and AcvR1 are needed for normal heart development, in which they play some non-redundant roles, and refine our understanding of the genetic and morphogenetic processes underlying Bmp-mediated heart development important in human congenital heart disease.

Oncogenic deregulation of NKL homeobox gene MSX1 in mantle cell lymphoma

NKL homeobox gene MSX1 is physiologically expressed during embryonic hematopoiesis. Here, we detected MSX1 overexpression in three examples of mantle cell lymphoma (MCL) and one of acute myeloid leukemia (AML) by screening 96 leukemia/lymphoma cell lines via microarray profiling. Moreover, in silico analysis identified significant overexpression of MSX1 in 3% each of patients with MCL and AML, confirming aberrant activity in subsets of both types of malignancies. Comparative expression profiling analysis and subsequent functional studies demonstrated overexpression of histone acetyltransferase PHF16 together with transcription factors FOXC1 and HLXB9 as activators of MSX1 transcription. Additionally, we identified regulation of cyclin D1/CCND1 by MSX1 and its repressive cofactor histone H1C. Fluorescence in situ hybridization in MCL cells showed that t(11;14)(q13;q32) results in detachment of CCND1 from its corresponding repressive MSX1 binding site. Taken together, we uncovered regulators and targets of homeobox gene MSX1 in leukemia/lymphoma cells, supporting the view of a recurrent genetic network that is reactivated in malignant transformation.

Requirement for integrin-linked kinase in neural crest migration and differentiation and outflow tract morphogenesis

BACKGROUND

Neural crest defects lead to congenital heart disease involving outflow tract malformation. Integrin-linked-kinase (ILK) plays important roles in multiple cellular processes and embryogenesis. ILK is expressed in the neural crest, but its role in neural crest and outflow tract morphogenesis remains unknown.

RESULTS

We ablated ILK specifically in the neural crest using the Wnt1-Cre transgene. ILK ablation resulted in abnormal migration and overpopulation of neural crest cells in the pharyngeal arches and outflow tract and a significant reduction in the expression of neural cell adhesion molecule (NCAM) and extracellular matrix components. ILK mutant embryos exhibited an enlarged common arterial trunk and ventricular septal defect. Reduced smooth muscle differentiation, but increased ossification and neurogenesis/innervation were observed in ILK mutant outflow tract that may partly be due to reduced transforming growth factor β2 (TGFβ2) but increased bone morphogenetic protein (BMP) signaling. Consistent with these observations, microarray analysis of fluorescence-activated cell sorting (FACS)-sorted neural crest cells revealed reduced expression of genes associated with muscle differentiation, but increased expression of genes of neurogenesis and osteogenesis.

CONCLUSIONS

Our results demonstrate that ILK plays essential roles in neural crest and outflow tract development by mediating complex crosstalk between cell matrix and multiple signaling pathways. Changes in these pathways may collectively result in the unique neural crest and outflow tract phenotypes observed in ILK mutants.

Extracellular metalloproteinases in neural crest development and craniofacial morphogenesis

The neural crest (NC) is a population of migratory stem/progenitor cells that is found in early vertebrate embryos. NC cells are induced during gastrulation, and later migrate to multiple destinations and contribute to many types of cells and tissues, such as craniofacial structures, cardiac tissues, pigment cells and the peripheral nervous system. Recently, accumulating evidence suggests that many extracellular metalloproteinases, including matrix metalloproteinases (MMPs), a disintegrin and metalloproteinases (ADAMs), and ADAMs with thrombospondin motifs (ADAMTSs), play important roles in various stages of NC development. Interference with metalloproteinase functions often causes defects in craniofacial structures, as well as in other cells and tissues that are contributed by NC cells, in humans and other vertebrates. In this review, we summarize the current state of the field concerning the roles of these three families of metalloproteinases in NC development and related tissue morphogenesis, with a special emphasis on craniofacial morphogenesis.

Heterogeneous susceptibility of valve endothelial cells to mesenchymal transformation in response to TNFα

Lack of understanding of the early mechanisms of aortic valve stenosis and calcification hinders the development of diagnostic and therapeutic intervention strategies. Inflammation is a known component of early aortic valve disease and can induce mesenchymal transformation in a subset of aortic valve endothelial cells. Here we present a three-dimensional culture system that allows transforming and non-transforming cells to be independently isolated and analyzed. We have used the system to identify and characterize the dynamic invasion and phenotypic transition of two distinct subsets of endothelial cells: those that invade and transform under TNFα treatment, and those that resist mesenchymal transformation and remain endothelial. We determine that non-transformed cells maintain control levels of endothelial genes VE-cadherin and eNOS, while transformed cells lose these endothelial characteristics and upregulate α-smooth muscle actin. Both subsets of cells have an inflammatory phenotype marked by increased ICAM-1, but transformed cells have increased MMP-9, Notch1, TGF-β, and BMP-4, while non-transformed cells do not. Transformed cells also have distinct effects on alignment of collagen fibers as they invade the hydrogel system, which is not found in control endothelial or interstitial valve cells. Understanding the role of transforming and non-transforming endothelial cells in valve disease will provide an important pathological link between early inflammation and later stages of disease. Discovery of the molecular signature of transformation-resistant endothelial cells could inform development of treatment strategies that promote survival of the valve endothelium.

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.

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.

Current perspectives of the signaling pathways directing neural crest induction

The neural crest is a migratory population of embryonic cells with a tremendous potential to differentiate and contribute to nearly every organ system in the adult body. Over the past two decades, an incredible amount of research has given us a reasonable understanding of how these cells are generated. Neural crest induction involves the combinatorial input of multiple signaling pathways and transcription factors, and is thought to occur in two phases from gastrulation to neurulation. In the first phase, FGF and Wnt signaling induce NC progenitors at the border of the neural plate, activating the expression of members of the Msx, Pax, and Zic families, among others. In the second phase, BMP, Wnt, and Notch signaling maintain these progenitors and bring about the expression of definitive NC markers including Snail2, FoxD3, and Sox9/10. In recent years, additional signaling molecules and modulators of these pathways have been uncovered, creating an increasingly complex regulatory network. In this work, we provide a comprehensive review of the major signaling pathways that participate in neural crest induction, with a focus on recent developments and current perspectives. We provide a simplified model of early neural crest development and stress similarities and differences between four major model organisms: Xenopus, chick, zebrafish, and mouse.

Analysis of the patterning of cardiac outflow tract and great arteries with angiography and vascular casting

Formation of the cardiac outflow tract and great arteries involves complex morphogenetic processes, whose abnormities result in several clinically important diseases. Studies of these developmental processes are therefore important for understanding congenital vascular defects. However, the three-dimensional structure of arteries makes it challenging to analyze the pattern of vasculature using conventional histological approaches. Here we describe a vascular casting method to visualize the branching and connections of great arteries in developing embryos as well as in adult mice. This technique can be used to study the development of cardiac outflow tract, semilunar valves, and great arteries as demonstrated previously (Circ Res, 2008; Development 135: 3577-3586, 2008).

The second heart field

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

FRS2α-mediated FGF signals suppress premature differentiation of cardiac stem cells through regulating autophagy activity

RATIONALE

Although the fibroblast growth factor (FGF) signaling axis plays important roles in heart development, the molecular mechanism by which the FGF regulates cardiogenesis is not fully understood.

OBJECTIVE

To investigate the mechanism by which FGF signaling regulates cardiac progenitor cell differentiation.

METHODS AND RESULTS

Using mice with tissue-specific ablation of FGF receptors and FGF receptor substrate 2α (Frs2α) in heart progenitor cells, we demonstrate that disruption of FGF signaling leads to premature differentiation of cardiac progenitor cells in mice. Using embryoid body cultures of mouse embryonic stem cells, we reveal that FGF signaling promotes mesoderm differentiation in embryonic stem cells but inhibits cardiomyocyte differentiation of the mesoderm cells at later stages. Furthermore, we also report that inhibiting FRS2α-mediated signals increases autophagy and that activating autophagy promotes myocardial differentiation and vice versa.

CONCLUSIONS

The results indicate that the FGF/FRS2α-mediated signals prevent premature differentiation of heart progenitor cells through suppressing autophagy. The findings provide the first evidence that autophagy plays a role in heart progenitor differentiation.

A decade of advances in the molecular embryology and genetics underlying congenital heart defects

Congenital heart defects (CHD) are the most common type of human birth defect and result in significant mortality worldwide. Despite numerous epidemiologic studies in the past decades, few genetic causes have been identified until recently. CHD result from abnormal morphogenesis of the systematic cardiovascular construction during development. Recent advances in molecular embryology, including the discovery of a new source of cardiac progenitor cells termed the second heart field (SHF), have revealed that the heart arises from multiple distinct embryonic origins. Cells derived from the SHF contribute to the development of the cardiac outflow tract, together with the other progenitor cell lineage called cardiac neural crest cells. Numerous cardiac transcription factors regulate these progenitor cells during heart development. Elucidation of the transcriptional network for these cardiac progenitor cells is essential for further understanding cardiac development and providing new insights into the morphogenesis of CHD. This review outlines the recent discoveries of the molecular embryology of the normal heart and the genetic basis of CHD.

Redundant and dosage sensitive requirements for Fgf3 and Fgf10 in cardiovascular development

Heart development requires contributions from, and coordinated signaling interactions between, several cell populations, including splanchnic and pharyngeal mesoderm, postotic neural crest and the proepicardium. Here we report that Fgf3 and Fgf10, which are expressed dynamically in and near these cardiovascular progenitors, have redundant and dosage sensitive requirements in multiple aspects of early murine cardiovascular development. Embryos with Fgf3(-/+);Fgf10(-/-), Fgf3(-/-);Fgf10(-/+) and Fgf3(-/-);Fgf10(-/-) genotypes formed an allelic series of increasing severity with respect to embryonic survival, with double mutants dead by E11.5. Morphologic analysis of embryos with three mutant alleles at E11.5-E13.5 and double mutants at E9.5-E11.0 revealed multiple cardiovascular defects affecting the outflow tract, ventricular septum, atrioventricular cushions, ventricular myocardium, dorsal mesenchymal protrusion, pulmonary arteries, epicardium and fourth pharyngeal arch artery. Assessment of molecular markers in E8.0-E10.5 double mutants revealed abnormalities in each progenitor population, and suggests that Fgf3 and Fgf10 are not required for specification of cardiovascular progenitors, but rather for their normal developmental coordination. These results imply that coding or regulatory mutations in FGF3 or FGF10 could contribute to human congenital heart defects.

Factors controlling cardiac neural crest cell migration

Cardiac neural crest cells originate as part of the postotic caudal rhombencephalic neural crest stream. Ectomesenchymal cells in this stream migrate to the circumpharyngeal ridge and then into the caudal pharyngeal arches where they condense to form first a sheath and then the smooth muscle tunics of the persisting pharyngeal arch arteries. A subset of the cells continue migrating into the cardiac outflow tract where they will condense to form the aorticopulmonary septum. Cell signaling, extracellular matrix and cell-cell contacts are all critical for the initial migration, pauses, continued migration, and condensation of these cells. This review elucidates what is currently known about these factors.

BMP signaling in congenital heart disease: new developments and future directions

Congenital heart malformations are the most common of all congenital human birth anomalies. During the past decade, research with zebrafish, chick, and mouse models have elucidated many fundamental genetic pathways that govern early cardiac patterning and differentiation. This review highlights the roles of the bone morphogenetic protein (BMP) signaling pathway in cardiogenesis and how defective BMP signals can disrupt the intricate steps of cardiac formation and cause congenital heart defects.

Cell death serves as a single etiological cause of a wide spectrum of congenital urinary tract defects

PURPOSE

We genetically disrupted the wolffian duct in mice to study the affected organogenesis processes and to test the hypothesis that cell loss can be the developmental basis for a wide spectrum of congenital anomalies in the kidney and urinary tract.

MATERIALS AND METHODS

We used Hoxb7-Cre transgenic lines (HC1 and HC2) to induce diphtheria toxin production from a ROSA(DTA) allele, disrupting the wolffian duct and derivatives.

RESULTS

The first set of mutants (HC1;ROSA(DTA/+)) exhibited agenesis of the kidney, ureter and reproductive tracts. The second set of mutants (HC2;ROSA(DTA/+)) exhibited diverse defects, including renal agenesis/hypoplasia, hydronephrosis, hydroureter, ureter-vas deferens fistulas in males and ureter-oviduct/uterus fistulas in females. The phenotypic differences correspond to the degree of apoptosis induced caudal truncation of the wolffian duct, which is less severe and more variable in HC2;ROSA(DTA/+) mice. Whenever the wolffian duct failed to reach the urogenital sinus, the ureter failed to separate from the wolffian duct, suggesting that ureteral migration along the common nephric duct to the cloaca and the subsequent common nephric duct degeneration constitute the only pathway for separating the ureter and wolffian duct derivatives.

CONCLUSIONS

The diverse and severe defects observed emphasize the central role of the wolffian duct in providing progenitors and signals for urogenital development. These results also indicate that the quantitative difference in cell death induced caudal truncation of the wolffian duct can lead to a wide range of qualitatively distinct defects, and that cell death can serve as a single etiological cause of a wide spectrum of congenital kidney and urinary tract defects.

Tbx1, subpulmonary myocardium and conotruncal congenital heart defects

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

TGFβ superfamily signaling in the neural crest lineage

The neural crest cell (NCC) lineage is often referred to as the fourth germ layer in embryos, as its wide range of migration and early colonization of multiple tissues and organ systems throughout the developing body is astounding. Many human birth defects are thought to have their origins within the NCC lineage. Exciting recent conditional mouse targeting and transgenic combinatorial suppression approaches have revealed that the Tgf-b superfamily is a key signaling pathway within the cardiac and cranial NCC subpopulations. Given the complexity of Tgf-b superfamily signaling and that multiple ligand and receptor combinations have already been shown to be expressed within the NCC subpopulations, and the difficulty in transgenically targeting entire signaling cascades, we review several up-to-date transgenic approaches that are revealing unexpected consequences.

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

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

Cell autonomous requirement of endocardial Smad4 during atrioventricular cushion development in mouse embryos

Atrioventricular (AV) cushions are the precursors of AV septum and valves. In this study, we examined roles of Smad4 during AV cushion development using a conditional gene inactivation approach. We found that endothelial/endocardial inactivation of Smad4 led to the hypocellular AV cushion defect and that both reduced cell proliferation and increased apoptosis contributed to the defect. Expression of multiple genes critical for cushion development was down-regulated in mutant embryos. In collagen gel assays, the number of mesenchymal cells formed is significantly reduced in mutant AV explants compared to that in control explants, suggesting that the reduction of cushion mesenchyme formation in mutants is unlikely secondary to their gross vasculature abnormalities. Using a previously developed immortal endocardial cell line, we showed that Smad4 is required for BMP signaling- stimulated upregulation of Tbx20 and Gata4. Therefore, our data collectively support the cell-autonomous requirement of endocardial Smad4 in regulating AV cushion development.

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

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

Trigenic neural crest-restricted Smad7 over-expression results in congenital craniofacial and cardiovascular defects

Smad7 is a negative regulator of TGFbeta superfamily signaling. Using a three-component triple transgenic system, expression of the inhibitory Smad7 was induced via doxycycline within the NCC lineages at pre- and post-migratory stages. Consistent with its role in negatively regulating both TGFbeta and BMP signaling in vitro, induction of Smad7 within the NCC significantly suppressed phosphorylation levels of both Smad1/5/8 and Smad2/3 in vivo, resulting in subsequent loss of NCC-derived craniofacial, pharyngeal and cardiac OFT cushion cells. At the cellular level, increased cell death was observed in pharyngeal arches. However, cell proliferation and NCC-derived smooth muscle differentiation were unaltered. NCC lineage mapping demonstrated that cardiac NCC emigration and initial migration were not affected, but subsequent colonization of the OFT was significantly reduced. Induction of Smad7 in post-migratory NCC resulted in interventricular septal chamber septation defects, suggesting that TGFbeta superfamily signaling is also essential for cardiac NCC at post-migratory stages to govern normal cardiac development. Taken together, the data illustrate that tightly regulated TGFbeta superfamily signaling plays an essential role during craniofacial and cardiac NCC colonization and cell survival in vivo.

Transforming growth factor β, bone morphogenetic protein, and vascular endothelial growth factor mediate phenotype maturation and tissue remodeling by embryonic valve progenitor cells: relevance for heart valve tissue engineering

Despite years of research, limited understanding of heart valve cell and tissue biology remains a key impediment to valvular tissue engineering progress. Heart valves rapidly evolve structural and cellular composition naturally during embryonic development, which suggests that mimicking these signaling events could advance engineered valve tissue research. Many inductive factors participate in the initial endocardial to mesenchymal transformation event necessary to form the prevalvular cushion, but far less is known about the regulation of cushion remodeling into fibrous leaflets and the associated maturation of valvular progenitors into fibroblasts. In this study, we combine in vitro three-dimensional tissue-engineered models of embryonic valvular remodeling with in vivo analysis to determine the roles of three prominent growth factors during avian mitral valvulogenesis. We show that transforming growth factor-β3 (TGFβ3), bone morphogenetic protein 2 (BMP2), and vascular endothelial growth factor A (VEGFA) are expressed in spatiotemporally distinct patterns and at significantly different levels within remodeling embryonic valves in vivo. We then establish dose-dependent functional roles for each growth factor in 3D cultured embryonic valve progenitor cells. TGFβ3 induced cell migration, invasion, and matrix condensation; BMP2 induced invasion. VEGFA inhibited invasion but increased migration. Finally, we determine that TGFβ3 induced myofibroblastic differentiation in a dose-dependent manner, whereas VEGFA and BMP2 did not. Collectively, these findings frame a naturally derived blueprint for controlling valvulogenic remodeling and phenotype maturation, which can be integrated into clinically needed regenerative strategies for heart valve disease and to accelerate the development of engineered tissue valves.

Targeted deletion of Hand2 in cardiac neural crest-derived cells influences cardiac gene expression and outflow tract development

The basic helix-loop-helix DNA binding protein Hand2 has critical functions in cardiac development both in neural crest-derived and mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest has allowed us to genetically dissect Hand2-dependent defects specifically in outflow tract and cardiac cushion independent of Hand2 functions in mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest results in misalignment of the aortic arch arteries and outflow tract, contributing to development of double outlet right ventricle (DORV) and ventricular septal defects (VSD). These neural crest-derived developmental anomalies are associated with altered expression of Hand2-target genes we have identified by gene profiling. A number of Hand2 direct target genes have been identified using ChIP and ChIP-on-chip analyses. We have identified and validated a number of genes related to cell migration, proliferation/cell cycle and intracellular signaling whose expression is affected by Hand2 deletion in the neural crest and which are associated with development of VSD and DORV. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting expression of target genes associated with a number of functional interactions in neural crest-derived cells required for proper patterning of the outflow tract, generation of the appropriate number of neural crest-derived cells for elongation of the conotruncus and cardiac cushion organization. Our genetic model has made it possible to investigate the molecular genetics of neural crest contributions to outflow tract morphogenesis and cell differentiation.

Midline signaling regulates kidney positioning but not nephrogenesis through Shh

The role of axial structures, especially the notochord, in metanephric kidney development has not been directly examined. Here, we showed that disruption of the notochord and floor plate by diphtheria toxin (DTA)-mediated cell ablation did not disrupt nephrogenesis, but resulted in kidney fusions, resembling horseshoe kidneys in humans. Axial disruptions led to more medially positioned metanephric mesenchyme (MM) in midgestation. However, neither axial disruption nor the ensuing positional shift of the MM affected the formation of nephrons and other structures within the kidney. Response to Shh signaling was greatly reduced in midline cell populations in the mutants. To further ascertain the molecular mechanism underlying these abnormalities, we specifically inactivated Shh in the notochord and floor plate. We found that depleting the axial source of Shh was sufficient to cause kidney fusion, even in the presence of the notochord. These results suggested that the notochord is dispensable for nephrogenesis but required for the correct positioning of the metanephric kidney. Axial Shh signal appears to be critical in conferring the effects of axial structures on kidney positioning along the mediolateral axis. These studies also provide insights into the pathogenesis of horseshoe kidneys and how congenital kidney defects can be caused by signals outside the renal primordia.

Tissue-tissue interactions during morphogenesis of the outflow tract

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

Notch and cardiac outflow tract development

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

miRNA-processing enzyme Dicer is necessary for cardiac outflow tract alignment and chamber septation

MicroRNAs (miRNAs) have previously been implicated in a number of developmental processes, including development of the ventricular myocardium of the heart. To determine what, if any, additional roles miRNAs play in cardiogenesis, we deleted the miRNA-processing enzyme Dicer specifically in the developing murine heart. Embryos lacking cardiac Dicer lived longer than reported in previous studies using different alleles to remove cardiac Dicer activity and displayed a highly penetrant phenotype of double outlet right ventricle with a concurrent ventricular septal defect. Before the defect's onset, Pitx2c and Sema3c, both required for outflow tract morphogenesis, were up-regulated in Dicer-deficient hearts. Interestingly, mesenchymal apoptosis in the outflow tract normally required for outflow tract alignment was greatly decreased in the mutants, likely contributing directly to the observed phenotype. In sum, we demonstrate here a specific developmental process, that of outflow tract morphogenesis, being hindered by the deletion of miRNAs during cardiogenesis.