Dicer is required for survival of differentiating neural crest cells

The neural crest (NC) lineage gives rise to a wide array of cell types ranging from neurons and glia of the peripheral nervous system to skeletal elements of the head. The mechanisms regulating NC differentiation into such a large number of cell types remain largely unknown. MicroRNAs (miRNAs) play key roles in regulating developmental events suggesting they may also play a role during NC differentiation. To determine what roles miRNAs play in differentiation of NC-derived tissues, we deleted the miRNA processing gene Dicer in NC cells using the Wnt1-Cre deleter line. We show that deletion of Dicer soon after NC cells have formed does not affect their migration and colonization of their targets in the embryo. However, the post-migratory NC is dependent on Dicer for survival. In the head, loss of Dicer leads to a loss of NC-derived craniofacial bones while in the trunk, cells of the enteric, sensory and sympathetic nervous systems are lost during development. We found that loss of Dicer does not prevent the initial differentiation of NC but as development progresses, NC derivatives are lost due to apoptotic cell death. When Dicer is deleted, both Caspase-dependent and -independent apoptotic pathways are activated in the sensory ganglia but only the Caspase-dependent apoptotic program was activated in the sympathetic nervous system showing that the specific endogenous apoptotic programs are turned on by loss of Dicer. Our results show that Dicer and miRNAs, are required for survival of NC-derived tissues by preventing apoptosis during differentiation.     

SOX3 activity during pharyngeal segmentation is required for craniofacial morphogenesis

Craniofacial development is a complex multi-step process leading to the morphogenesis of the face and sense organs, and to that of the neck, including the anteriormost part of the respiratory and digestive apparatus and associated endocrine glands. In vertebrates, the process is initiated by the formation of the pharyngeal arches from ectoderm, endoderm and mesoderm. These arches are then populated by neural crest cells, which originate from the central nervous system. We show here that, in mouse, there is a requirement for the HMG box factor SOX3 during the earliest stage of pharyngeal development: the formation of the pharyngeal pouches that segment the pharyngeal region by individualising each arch. In Sox3-null mutants, these pouches are expanded at the detriment of the second pharyngeal arch. As a consequence, neural crest cell migration and ectoderm-derived epibranchial placode development are affected, leading to craniofacial defects. We also show that Sox3 genetically interacts both with FgfR1 and with Sox2, another member of the Soxb1 family, to fulfil its function in the pharyngeal region. Although the importance of the neural crest has long been recognised, our studies highlight the equally crucial role of the pharyngeal region in craniofacial morphogenesis. They also give insight into the formation of pharyngeal pouches, of which little is known in vertebrates. Finally, this work introduces two new players in craniofacial development - SOX3 and SOX2.     

Extracellular metalloproteinases in neural crest development and craniofacial morphogenesis

Abstract

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.     

Combined intrinsic and extrinsic influences pattern cranial neural crest migration and pharyngeal arch morphogenesis in axolotl

Cranial neural crest cells migrate in a precisely segmented manner to form cranial ganglia, facial skeleton and other derivatives. Here, we investigate the mechanisms underlying this patterning in the axolotl embryo using a combination of tissue culture, molecular markers, scanning electron microscopy and vital dye analysis. In vitro experiments reveal an intrinsic component to segmental migration; neural crest cells from the hindbrain segregate into distinct streams even in the absence of neighboring tissue. In vivo, separation between neural crest streams is further reinforced by tight juxtapositions that arise during early migration between epidermis and neural tube, mesoderm and endoderm. The neural crest streams are dense and compact, with the cells migrating under the epidermis and outside the paraxial and branchial arch mesoderm with which they do not mix. After entering the branchial arches, neural crest cells conduct an "outside-in" movement, which subsequently brings them medially around the arch core such that they gradually ensheath the arch mesoderm in a manner that has been hypothesized but not proven in zebrafish. This study, which represents the most comprehensive analysis of cranial neural crest migratory pathways in any vertebrate, suggests a dual process for patterning the cranial neural crest. Together with an intrinsic tendency to form separate streams, neural crest cells are further constrained into channels by close tissue apposition and sorting out from neighboring tissues.     

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

TGFβ/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 TGFβ/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-2α, 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 TGFβ/BMP signals are essential for appropriate NCC development.     

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

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

The actin depolymerizing factor n-cofilin is essential for neural tube morphogenesis and neural crest cell migration

Cofilin/ADF proteins are a ubiquitously expressed family of F-actin depolymerizing factors found in eukaryotic cells including plants. In vitro, cofilin/ADF activity has been shown to be essential for actin driven motility, by accelerating actin filament turnover. Three actin depolymerizing factors (n-cofilin, m-cofilin, ADF) can be found in mouse and human. Here we show that in mouse the non-muscle-specific gene-n-cofilin-is essential for migration of neural crest cells as well as other cell types in the paraxial mesoderm. The main defects observed in n-cofilin mutant embryos are an impaired delamination and migration of neural crest cells, affecting the development of neural crest derived tissues. Neural crest cells lacking n-cofilin do not polarize, and F-actin bundles or fibers are not detectable. In addition, n-cofilin is required for neuronal precursor cell proliferation and scattering. These defects result in a complete lack of neural tube closure in n-cofilin mutant embryos. Although ADF is overexpressed in mutant embryos, this cannot compensate the lack of n-cofilin, suggesting that they might have a different function in embryonic development. Our data suggest that in mammalian development, regulation of the actin cytoskeleton by the F-actin depolymerizing factor n-cofilin is critical for epithelial-mesenchymal type of cell shape changes as well as cell proliferation.     

Retinoic acid-dependent eye morphogenesis is orchestrated by neural crest cells

Using genetic approaches in the mouse, we show that the primary target tissue of retinoic acid (RA) action during eye morphogenesis is not the retina nor the corneal ectoderm, which both express RA-synthesizing retinaldehyde dehydrogenases (RALDH1 and RALDH3), but the neural crest cell-derived periocular mesenchyme (POM), which is devoid of RALDH. In POM, the effects of the paracrine RA signal are mediated by the nuclear RA receptors heterodimers RXRalpha/RARbeta and RXRalpha/RARgamma. These heterodimers appear to control: (1) the remodeling of the POM through activation of Eya2-related apoptosis; (2) the expression of Foxc1 and Pitx2, which play crucial roles in anterior eye segment development; and (3) the growth of the ventral retina. We additionally show that RALDH1 and RALDH3 are the only enzymes that are required for RA synthesis in the eye region from E10.5 to E13.5, and that patterning of the dorsoventral axis of the retina does not require RA.     

PDGF signaling from pharyngeal pouches promotes arch artery morphogenesis

The great vessels of the heart originate from the pharyngeal arch arteries(PAAs).Anomalies of the PAAs often occur together with pharyngeal pouch malfo rmations,but the reasons for this phenomenon are not fully understood.In the current study,we show that platelet-derived growth factor(PDGF) signaling derived from the pharyngeal pouches plays an important function in PAA vasculogenesis,During PAA development in zebrafish embryos,pdgfaa and pdgfab are expressed in the developing pharyngeal pouches.Results from loss-of-function experiments revealed a critical role of these genes in PAA formation.We found that nitroreductase(NTR)-mediated pouch ablation distinctly decreased PDGF receptor tyrosine phosphorylation,yielding a severe loss of PAAs.Importantly,pouch-specific overexpression of pdgfaa in pdgfaa~(-/-);pdgfab~(-/-)mutants significantly relieved the PAA defects,which indicated a primary role of pharyngeal pouch-expressed PDGF ligands in signal activation and PAA morphogenesis.Our findings further showed that PDGF signaling was indispensable for the proliferation of PAA angioblasts.Together,these results established a role for PDGFaa-and PDGFab-mediated tissuetissue interaction during PAA development.     

Tbx1 expression in pharyngeal epithelia is necessary for pharyngeal arch artery development

During embryonic life, the initially paired pharyngeal arch arteries (PAAs) follow a precisely orchestrated program of persistence and regression that leads to the formation of the mature aortic arch and great vessels. When this program fails, specific cardiovascular defects arise that may be life threatening or mild, according to the identity of the affected artery. Fourth PAA-derived cardiovascular defects occur commonly in DiGeorge syndrome and velocardiofacial syndrome (22q11DS), and in Tbx1(+/-) mice that model the 22q11DS cardiovascular phenotype. Tbx1 is expressed in pharyngeal mesoderm, endoderm and ectoderm, and, in addition, we show that it is expressed in precursors of the endothelial cells that line the PAAs, thus expanding the number of tissues in which Tbx1 is potentially required for fourth PAA development. In this study, we have used cell fate mapping and tissue-specific gene deletion, driven by six different Cre lines, to explore Tbx1 gene-dosage requirements in the embryonic pharynx for fourth PAA development. Through this approach, we have resolved the spatial requirements for Tbx1 in this process, and we show pharyngeal epithelia to be a critical tissue. We also thereby demonstrate conclusively that the role of Tbx1 in fourth PAA development is cell non-autonomous.     

Loss of Gbx2 results in neural crest cell patterning and pharyngeal arch artery defects in the mouse embryo

Several syndromes characterized by defects in cardiovascular and craniofacial development are associated with a hemizygous deletion of chromosome 22q11 in humans and involve defects in pharyngeal arch and neural crest cell development. Recent efforts have focused on identifying 22q11 deletion syndrome modifying loci. In this study, we show that mouse embryos deficient for Gbx2 display aberrant neural crest cell patterning and defects in pharyngeal arch-derived structures. Gbx2(-/-) embryos exhibit cardiovascular defects associated with aberrant development of the fourth pharyngeal arch arteries including interrupted aortic arch type B, right aortic arch, and retroesophageal right subclavian artery. Other developmental abnormalities include overriding aorta, ventricular septal defects, cranial nerve, and craniofacial skeletal patterning defects. Recently, Fgf8 has been proposed as a candidate modifier for 22q11 deletion syndromes. Here, we demonstrate that Fgf8 and Gbx2 expression overlaps in regions of the developing pharyngeal arches and that they interact genetically during pharyngeal arch and cardiovascular development.     

Cytoplasmic protein methylation is essential for neural crest migration

As they initiate migration in vertebrate embryos, neural crest cells are enriched for methylation cycle enzymes, including S-adenosylhomocysteine hydrolase (SAHH), the only known enzyme to hydrolyze the feedback inhibitor of trans-methylation reactions. The importance of methylation in neural crest migration is unknown. Here, we show that SAHH is required for emigration of polarized neural crest cells, indicating that methylation is essential for neural crest migration. Although nuclear histone methylation regulates neural crest gene expression, SAHH and lysine-methylated proteins are abundant in the cytoplasm of migratory neural crest cells. Proteomic profiling of cytoplasmic, lysine-methylated proteins from migratory neural crest cells identified 182 proteins, several of which are cytoskeleton related. A methylation-resistant form of one of these proteins, the actin-binding protein elongation factor 1 alpha 1 (EF1α1), blocks neural crest migration. Altogether, these data reveal a novel and essential role for post-translational nonhistone protein methylation during neural crest migration and define a previously unknown requirement for EF1α1 methylation in migration.     

Aortic arch and pharyngeal phenotype in the absence of BMP-dependent neural crest in the mouse

Neural crest cells are essential for proper development of a variety of tissues and structures, including peripheral and autonomic nervous systems, facial skeleton, aortic arches and pharyngeal glands like the thymus and parathyroids. Previous work has shown that bone morphogenic protein (BMP) signalling is required for the production of migratory neural crest cells that contribute to the neurogenic and skeletogenic lineages. We show here that BMP-dependent neural crest cells are also required for development of the embryonic aortic arches and pharynx-derived glands. Blocking formation or migration of this crest cell population from the caudal hindbrain resulted in strong phenotypes in the cardiac outflow tract and the thymus. Thymic aplasia or hypoplasia occurs despite uncompromised gene induction in the pharyngeal endoderm. In addition, when hypoplastic thymic tissue is found, it is ectopically located, but functional in thymopoiesis. Our data indicate that thymic phenotypes produced by neural crest deficits result from aberrant formation of pharyngeal pouches and impaired migration of thymic primordia because the mesenchymal content in the branchial arches is below a threshold level.     

Overlapping origins of pharyngeal arch crest cells on the postotic hind-brain

The developing hind-brain of vertebrates consists of segmental units called rhombomeres. Although crest cells emigrate from the hind-brain, they are subsequently subdivided into several cell populations that are attached to restricted regions of the hind-brain. At the preotic level, only even-numbered rhombomeres are accompanied by crest cells, while the odd-numbered ones are not. At the postotic level, such the birhombomeric repetition becomes obscure. In order to map the origins and distributions of postotic crest cells, focal injections of Dil were made into various axial levels of the postotic neural tube. Cephalic crest cells at the postotic level first form a single cell population deposited by cells along the dorsolateral pathway. They are called the circumpharyngeal crest cells (CP cells) and are secondarily subdivided into each pharyngeal arch ectomesenchyme. The neural tube extending from r5 to the somite 3/4 boundary gave rise to CP cells. The neuraxial origins of each pharyngeal ectomesenchyme extended for more than three somite lengths, most of which overlapped with the other. Unlike in the preotic region, there is no segmental registration between neuraxial levels and pharyngeal arches. Caudal portions of the CP cell population show a characteristic distribution pattern that circumscribes the postotic pharyngeal arches caudally. Heterotopic transplantation of the Dil-labeled neural crest into the somite 3 level resulted in a distribution of labeled cells similar to that of CP cells, suggesting that the pattern of distribution depends upon dynamic modification of the body wall associated with pharyngeal arch formation.     

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: (1) what establishes the pathways of migration? And (2), what controls the final destination and differentiation of various neural crest subpopulations? These questions will be addressed in this Review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk.The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: (1) the vagal-level neural crest cells exhibit modest developmental bias; (2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively and (3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but take the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.Key words: neural crest, morphogenesis, cell migration, chicken embryo, fate restriction, vagal neural crest, pathways     

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively, and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: 1) what establishes the pathways of migration and 2) what controls the final destination and differentiation of various neural crest subpopulations. These questions will be addressed in this review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube, and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted, or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk. The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: 1) the vagal-level neural crest cells exhibit modest developmental bias; 2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively; 3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.     

Heparan sulfate expression in the neural crest is essential for mouse cardiogenesis

Impaired heparan sulfate (HS) synthesis in vertebrate development causes complex malformations due to the functional disruption of multiple HS-binding growth factors and morphogens. Here, we report developmental heart defects in mice bearing a targeted disruption of the HS-generating enzyme GlcNAc N-deacetylase/GlcN N-sulfotransferase 1 (NDST1), including ventricular septal defects (VSD), persistent truncus arteriosus (PTA), double outlet right ventricle (DORV), and retroesophageal right subclavian artery (RERSC). These defects closely resemble cardiac anomalies observed in mice made deficient in the cardiogenic regulator fibroblast growth factor 8 (FGF8). Consistent with this, we show that HS-dependent FGF8/FGF-receptor2C assembly and FGF8-dependent ERK-phosphorylation are strongly reduced in NDST1^−/− embryonic cells and tissues. Moreover, WNT1-Cre/LoxP-mediated conditional targeting of NDST function in neural crest cells (NCCs) revealed that their impaired HS-dependent development contributes strongly to the observed cardiac defects. These findings raise the possibility that defects in HS biosynthesis may contribute to congenital heart defects in humans that represent the most common type of birth defect.