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

Smad4-dependent pathways control basement membrane deposition and endodermal cell migration at early stages of mouse development

Background

Smad4 mutant embryos arrest shortly after implantation and display a characteristic shortened proximodistal axis, a significantly reduced epiblast, as well as a thickened visceral endoderm layer. Conditional rescue experiments demonstrate that bypassing the primary requirement for Smad4 in the extra-embryonic endoderm allows the epiblast to gastrulate. Smad4-independent TGF-β signals are thus sufficient to promote mesoderm formation and patterning. To further analyse essential Smad4 activities contributed by the extra-embryonic tissues, and characterise Smad4 dependent pathways in the early embryo, here we performed transcriptional profiling of Smad4 null embryonic stem (ES) cells and day 4 embryoid bodies (EBs).

Results

Transcripts from wild-type versus Smad4 null ES cells and day 4 EBs were analysed using Illumina arrays. In addition to several known TGF-β/BMP target genes, we identified numerous Smad4-dependent transcripts that are mis-expressed in the mutants. As expected, mesodermal cell markers were dramatically down-regulated. We also observed an increase in non-canonical potency markers (Pramel7, Tbx3, Zscan4), germ cell markers (Aire, Tuba3a, Dnmt3l) as well as early endoderm markers (Dpp4, H19, Dcn). Additionally, expression of the extracellular matrix (ECM) remodelling enzymes Mmp14 and Mmp9 was decreased in Smad4 mutant ES and EB populations. These changes, in combination with increased levels of laminin alpha1, cause excessive basement membrane deposition. Similarly, in the context of the Smad4 null E6.5 embryos we observed an expanded basement membrane (BM) associated with the thickened endoderm layer.

Conclusion

Smad4 functional loss results in a dramatic shift in gene expression patterns and in the endodermal cell lineage causes an excess deposition of, or an inability to breakdown and remodel, the underlying BM layer. These structural abnormalities probably disrupt reciprocal signalling between the epiblast and overlying visceral endoderm required for gastrulation.

BMP signaling regulates sympathetic nervous system development through Smad4-dependent and -independent pathways

Induction of the sympathetic nervous system (SNS) from its neural crest (NC) precursors is dependent on BMP signaling from the dorsal aorta. To determine the roles of BMP signaling and the pathways involved in SNS development, we conditionally knocked out components of the BMP pathways. To determine if BMP signaling is a cell-autonomous requirement of SNS development, the Alk3 (BMP receptor IA) was deleted in the NC lineage. The loss of Alk3 does not prevent NC cell migration, but the cells die immediately after reaching the dorsal aorta. The paired homeodomain factor Phox2b, known to be essential for survival of SNS precursors, is downregulated, suggesting that Phox2b is a target of BMP signaling. To determine if Alk3 signals through the canonical BMP pathway, Smad4 was deleted in the NC lineage. Loss of Smad4 does not affect neurogenesis and ganglia formation; however, proliferation and noradrenergic differentiation are reduced. Analysis of transcription factors regulating SNS development shows that the basic helix-loop-helix factor Ascl1 is downregulated by loss of Smad4 and that Ascl1 regulates SNS proliferation but not noradrenergic differentiation. To determine if the BMP-activated Tak1 (Map3k7) pathway plays a role in SNS development, Tak1 was deleted in the NC lineage. We show that Tak1 is not involved in SNS development. Taken together, our results suggest multiple roles for BMP signaling during SNS development. The Smad4-independent pathway acts through the activation of Phox2b to regulate survival of SNS precursors, whereas the Smad4-dependent pathway controls noradrenergic differentiation and regulates proliferation by maintaining Ascl1 expression.

Signals controlling neural crest contributions to the heart

Cardiac neural crest cells represent a unique subpopulation of cranial neural crest cells that are specified, delaminate and migrate from the developing neural tube to the caudal pharynx where they support aortic arch artery development. From the caudal pharynx, a subset of these cells migrates into the cardiac outflow tract where they are needed for outflow septation. Many signaling factors are known to be involved in specifying and triggering the migration of neural crest cells. These factors have not been specifically studied in cardiac crest but are assumed to be the same as for the other regions of crest. Signaling factors like Ephs and Semaphorins guide the cells into the caudal pharynx. Support of the cells in the pharynx is from endothelin, PDGF and the TGFbeta/BMP signaling pathways. Mutants in the TGFbeta/BMP pathway show abnormal migration or survival in the pharynx, whereas the migration of the neural crest cells into the outflow tract is orchestrated by Semaphorin/Plexin signaling. Although TGFbeta family members have been well studied and show defective neural crest function in outflow septation, their mechanism of action remains unclear.

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

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

Developmental signaling in myocardial progenitor cells: a comprehensive view of Bmp- and Wnt/beta-catenin signaling

The tight regulation of different signaling systems and the transcriptional and translational networks during embryonic development have been the focus of embryologists in recent decades. Defective developmental signaling due to genetic mutation or temporal and region-specific alteration of gene expression causes embryonic lethality or accounts for birth defects (e.g., congenital heart disease). The formation of the heart requires the coordinated integration of multiple cardiac progenitor cell populations derived from the first and second heart fields and from cardiac neural crest cells. This article summarizes what has been learned from conditional mutagenesis of Bmp pathway components and the Wnt effector, beta-catenin, in the developing heart of mice. Although Bmp signaling is required for cardiac progenitor cell specification, proliferation, and differentiation, recent studies have demonstrated distinct functions of Wnt/beta-catenin signaling at various stages of heart development.

Signaling pathways controlling second heart field development

Insight into the mechanisms underlying congenital heart defects and the use of stem cells for cardiac repair are major research goals in cardiovascular biology. In the early embryo, progenitor cells in pharyngeal mesoderm contribute to the rapid growth of the heart tube during looping morphogenesis. These progenitor cells constitute the second heart field (SHF) and were first identified in 2001. Direct or indirect perturbation of SHF addition to the heart results in congenital heart defects, including arterial pole alignment defects. Over the last 3 years, a number of studies have identified key intercellular signaling pathways that control the proliferation and deployment of SHF progenitor cells. Here, we review data concerning Wnt, fibroblast growth factor, bone morphogenetic protein, Hedgehog, and retinoic acid signaling that have begun to identify the ligand sources and responding cell types controlling SHF development. These studies have revealed the importance of signals from pharyngeal mesoderm itself, as well as critical inputs from adjacent pharyngeal epithelia and neural crest cells. Proliferation is emerging as a central checkpoint in the regulation of SHF development. Together, these studies contribute to defining the niche of cardiac progenitor cells in the early embryo, and we discuss the implications of these findings for the regulation of resident stem cell populations in the fetal and postnatal heart. Characterization of signals that maintain, expand, and regulate the differentiation of cardiac progenitor cells is essential for understanding both the etiology of congenital heart defects and the biomedical application of stem cell populations for cardiac repair.

Multiple lineage-specific roles of Smad4 during neural crest development

During vertebrate development, neural crest cells are exposed to multiple extracellular cues that drive their differentiation into neural and non-neural cell lineages. Insights into the signals potentially involved in neural crest cell fate decisions in vivo have been gained by cell culture experiments that have allowed the identification of instructive growth factors promoting either proliferation of multipotent neural crest cells or acquisition of specific fates. For instance, members of the TGFbeta factor family induce neurogenesis and smooth muscle cell formation at the expense of other fates in culture. In vivo, conditional ablation of various TGFbeta signaling components resulted in malformations of non-neural derivatives of the neural crest, but it is unclear whether these phenotypes involved aberrant fate decisions. Moreover, it remains to be shown whether neuronal determination indeed requires TGFbeta factor activity in vivo. To address these issues, we conditionally deleted Smad4 in the neural crest, thus inactivating all canonical TGFbeta factor signaling. Surprisingly, neural crest cell fates were not affected in these mutants, with the exception of sensory neurogenesis in trigeminal ganglia. Rather, Smad4 regulates survival of smooth muscle and proliferation of autonomic and ENS neuronal progenitor cells. Thus, Smad signaling plays multiple, lineage-specific roles in vivo, many of which are elicited only after neural crest cell fate decision.

Cardiac neural crest expression of Hand2 regulates outflow and second heart field development

The cardiac neural crest (cNC) lineage plays key roles in heart development by directly contributing to heart structures and regulating development of other heart lineages. The basic helix-loop-helix factor Hand2 regulates development of cardiovascular structures and NC-derived tissues including those that contribute to face and peripheral nervous system. Although Hand2 is expressed in cNC, its role has not been examined because of an early embryonic lethality when Hand2 is deleted in the NC lineage. We find that the lethality is attributable to loss of norepinephrine synthesis that can be overcome by activating adrenergic receptors. In rescued embryos, loss of Hand2 in the NC lineage leads to the misalignment of the outflow tract and aortic arch arteries. Defects include pulmonary stenosis, interrupted aortic artery, retroesophageal right subclavian artery, and ventricular septum defect, which resemble congenital heart defects attributed to defects in the NC. Hand2 functions in part by regulating signaling from the cNC to other cardiac lineages but not by regulating migration or survival of the cNC. Loss of Hand2 in NC also uncovered a novel role for the cNC in regulating proliferation and differentiation of the second heart field-derived myocardium that persists late into development. These results show that the cNC functions as a major signaling center for heart development and Hand2 plays a pivotal role in regulating both cell-autonomous and -nonautonomous functions of the cNC.

Ablation of developing podocytes disrupts cellular interactions and nephrogenesis both inside and outside the glomerulus

Podocyte loss in adults leads to glomerulosclerosis. However, the impact of podocyte loss on glomerulogenesis and the development of the kidney as a whole has not been directly studied. Here, we used a podocyte-specific Cre transgene to direct the production of diphtheria toxin (DTA) inside podocytes during nephrogenesis. Affected podocytes underwent translational arrest and apoptosis, leading to oliguria, proteinuria, hematuria, interstitial hemorrhage, and perinatal death. Glomerular cell-cell interactions were disrupted, even before overt podocyte apoptosis. VEGF production by podocytes was greatly decreased, and this was associated with reduced endothelial fenestration and altered glomerular vascular architecture. In addition to these glomerular anomalies, embryonic podocyte ablation also led to structural changes and increased apoptosis in proximal tubules. The collecting ducts, however, only showed molecular changes that are likely an indirect effect of the greatly reduced urine flow. Although podocyte loss significantly impacted the development and maintenance of the vasculature both inside and outside the glomerulus, our results suggest that there is a lack of long-range signaling from deep-seated, mature glomeruli to the differentiating cells in the outer nephrogenic zone. This study illustrates the tight integration of various cell types in the developing kidney and shows that the impact of podocyte loss during development is much greater than that in adults. This study also shows the specificity and effectiveness of a genetically controlled podocyte ablation system in mice where the additional readily available tools can further expand its applications.

Disruption of Smad4 in neural crest cells leads to mid-gestation death with pharyngeal arch, craniofacial and cardiac defects

TGFbeta/BMP signaling pathways are essential for normal development of neural crest cells (NCCs). Smad4 encodes the only common Smad protein in mammals, which is a critical nuclear mediator of TGFbeta/BMP signaling. In this work, we sought to investigate the roles of Smad4 for development of NCCs. To overcome the early embryonic lethality of Smad4 null mice, we specifically disrupted Smad4 in NCCs using a Cre/loxP system. The mutant mice died at mid-gestation with defects in facial primordia, pharyngeal arches, outflow tract and cardiac ventricles. Further examination revealed that mutant embryos displayed severe molecular defects starting from E9.5. Expression of multiple genes, including Msx1, 2, Ap-2 alpha, Pax3, and Sox9, which play critical roles for NCC development, was downregulated by NCC disruption of Smad4. Moreover, increased cell death was observed in pharyngeal arches from E10.5. However, the cell proliferation rate in these areas was not substantially altered. Taken together, these findings provide compelling genetic evidence that Smad4-mediated activities of TGFbeta/BMP signals are essential for appropriate NCC development.

Mouse models of congenital cardiovascular disease

Congenital heart defects occur in nearly 1% of human live births and many are lethal if not surgically repaired. In addition, the genetic contribution to congenital or acquired cardiovascular diseases that are silent at birth, but progress to cause significant disease in later life is being increasingly appreciated. Heart development and structure are highly conserved between mouse and human. The discoveries that are being made in this model system are highly relevant to understanding the pathogenesis of human heart defects whether they occus in isolation, or in the context of a syndrome. Many of the genes required for cardiovascular development were discovered fortuitously when early lethality or structural defects were observed in mouse mutants generated for other purposes, and relevant genes continue to be defined in this manner. Candidate genes for this process are being identified by their roles other species, or by their expression in pertinent tissues in mice. In this review, I will briefly summarize heart development as currently understood in the mouse, and then discuss how complementary studies in mouse and human have identified genes and pathways that are critical for normal cardiovascular development, and for maintaining the structure and function of this organ system throughout life.