Galphaq and Galpha11 proteins mediate endothelin-1 signaling in neural crest-derived pharyngeal arch mesenchyme

Endothelin signaling in development

Since the discovery of endothelin 1 (EDN1) in 1988, the role of endothelin ligands and their receptors in the regulation of blood pressure in normal and disease states has been extensively studied. However, endothelin signaling also plays crucial roles in the development of neural crest cell-derived tissues. Mechanisms of endothelin action during neural crest cell maturation have been deciphered using a variety of in vivo and in vitro approaches, with these studies elucidating the basis of human syndromes involving developmental differences resulting from altered endothelin signaling. In this Review, we describe the endothelin pathway and its functions during the development of neural crest-derived tissues. We also summarize how dysregulated endothelin signaling causes developmental differences and how this knowledge may lead to potential treatments for individuals with gene variants in the endothelin pathway.

Blockade of CD47 function attenuates restenosis by promoting smooth muscle cell efferocytosis and inhibiting their migration and proliferation

Cluster of differentiation 47 (CD47) plays an important role in the pathophysiology of various diseases including atherosclerosis, but its role in neointimal hyperplasia which contributes to restenosis, has not been studied. Using molecular approaches in combination with a mouse vascular endothelial denudation model, we studied the role of CD47 in injury-induced neointimal hyperplasia. We determined that thrombin induced CD47 expression both in human and mouse aortic smooth muscle cells (HASMCs and MASMCs). In exploring the mechanisms, we found that the protease-activated receptor 1 (PAR1)-Gα protein q/11 (Gαq/11)-phospholipase Cβ3 (PLCβ3)-nuclear factor of activated T cells c1 (NFATc1) signaling axis regulates thrombin-induced CD47 expression in HASMCs. Depletion of CD47 levels using its siRNA or interference of its function by its blocking antibody (bAb) blunted thrombin-induced migration and proliferation of HASMCs and MASMCs. In addition, we found that thrombin-induced HASMC migration requires CD47 interaction with integrin β3. On the other hand, thrombin-induced HASMC proliferation was dependent on CD47's role in nuclear export and degradation of CDK-interacting protein 1 (p21Cip1). In addition, suppression of CD47 function by its bAb rescued HASMC efferocytosis from inhibition by thrombin. We also found that vascular injury induces CD47 expression in intimal SMCs and that inhibition of CD47 function by its bAb, while alleviating injury-induced inhibition of SMC efferocytosis, attenuated SMC migration and proliferation resulting in reduced neointima formation. Thus, these findings reveal a pathological role for CD47 in neointimal hyperplasia.

Auriculocondylar syndrome 2 results from the dominant-negative action of PLCB4 variants

Auriculocondylar syndrome 2 (ARCND2) is a rare autosomal dominant craniofacial malformation syndrome linked to multiple genetic variants in the coding sequence of phospholipase C β4 (PLCB4). PLCB4 is a direct signaling effector of the endothelin receptor type A (EDNRA)-Gq/11 pathway, which establishes the identity of neural crest cells (NCCs) that form lower jaw and middle ear structures. However, the functional consequences of PLCB4 variants on EDNRA signaling is not known. Here, we show, using multiple signaling reporter assays, that known PLCB4 variants resulting from missense mutations exert a dominant-negative interference over EDNRA signaling. In addition, using CRISPR/Cas9, we find that F0 mouse embryos modeling one PLCB4 variant have facial defects recapitulating those observed in hypomorphic Ednra mouse models, including a bone that we identify as an atavistic change in the posterior palate/oral cavity. Remarkably, we have identified a similar osseous phenotype in a child with ARCND2. Our results identify the disease mechanism of ARCND2, demonstrate that the PLCB4 variants cause craniofacial differences and illustrate how minor changes in signaling within NCCs may have driven evolutionary changes in jaw structure and function. This article has an associated First Person interview with the first author of the paper.

ITPR1 Mutation Contributes to Hemifacial Microsomia Spectrum

Hemifacial microsomia (HM) is a craniofacial congenital defect involving the first and second branchial arch, mainly characterized by ocular, ear, maxilla-zygoma complex, mandible, and facial nerve malformation. HM follows autosomal dominant inheritance. Whole-exome sequencing of a family revealed a missense mutation in a highly conserved domain of ITPR1. ITPR1 is a calcium ion channel. By studying ITPR1's expression pattern, we found that ITPR1 participated in craniofacial development, especially the organs that corresponded to the phenotype of HM. In zebrafish, itpr1b, which is homologous to human ITPR1, is closely related to craniofacial bone formation. The knocking down of itpr1b in zebrafish could lead to a remarkable decrease in craniofacial skeleton formation. qRT-PCR suggested that knockdown of itpr1b could increase the expression of plcb4 while decreasing the mRNA level of Dlx5/6. Our findings highlighted ITPR1's role in craniofacial formation for the first time and suggested that ITPR1 mutation contributes to human HM.

Neratinib and entinostat combine to rapidly reduce the expression of K-RAS, N-RAS, Gαq and Gα11 and kill uveal melanoma cells

There is no efficacious standard of care therapy for uveal melanoma. Unlike cutaneous disease, uveal melanoma does not exhibit RAS mutations but instead contains mutations with ~90% penetrance in either Gα_q or Gα₁₁. Previously we demonstrated that neratinib caused ERBB1/2/4 and RAS internalization into autolysosomes which resulted in their proteolytic degradation. In PDX isolates of uveal melanoma, neratinib caused the internalization and degradation of Gα_q and Gα₁₁ in parallel with ERBB1 breakdown. These effects were enhanced by the HDAC inhibitor entinostat. Similar data were obtained using GFP/RFP tagged forms of K-RAS V12. Down regulation of Gα_q and Gα₁₁ expression and RAS-GFP/RFP fluorescence required Beclin1 and ATG5. The [neratinib + entinostat] combination engaged multiple pathways to mediate killing. One was from ROS-dependent activation of ATM via AMPK-ULK1-ATG13-Beclin1/ATG5. Another pathway was from CD95 via caspase 8-RIP1/RIP3. A third was from reduced expression of HSP70, HSP90, HDAC6 and phosphorylation of eIF2α. Downstream of the mitochondrion both caspase 9 and AIF played roles in tumor cell execution. Knock down of ATM/AMPK/ULK-1 prevented ATG13 phosphorylation and degradation of RAS and Gα proteins. Over-expression of activated mTOR prevented ATG13 phosphorylation and suppressed killing. Knock down of eIF2α maintained BCL-XL and MCL-1 expression. Within 6h, [neratinib + entinostat] reduced the expression of the immunology biomarkers PD-L1, ODC, IDO-1 and enhanced MHCA levels. Our data demonstrate that [neratinib + entinostat] down-regulates oncogenic RAS and the two key oncogenic drivers present in most uveal melanoma patients and causes a multifactorial form of killing via mitochondrial dysfunction and toxic autophagy. Abbreviations: ERK: extracellular regulated kinase; PI3K: phosphatidyl inositol 3 kinase; ca: constitutively active; dn: dominant negative; ER: endoplasmic reticulum; AIF: apoptosis inducing factor; AMPK: AMP-dependent protein kinase; mTOR: mammalian target of rapamycin; JAK: Janus Kinase; STAT: Signal Transducers and Activators of Transcription; MAPK: mitogen activated protein kinase; PTEN: phosphatase and tensin homologue on chromosome ten; ROS: reactive oxygen species; CMV: empty vector plasmid or virus; si: small interfering; SCR: scrambled; IP: immunoprecipitation; VEH: vehicle; HDAC: histone deacetylase.

The biology of uveal melanoma

Uveal melanoma (UM), a rare cancer of the eye, is distinct from cutaneous melanoma by its etiology, the mutation frequency and profile, and its clinical behavior including resistance to targeted therapy and immune checkpoint blockers. Primary disease is efficiently controlled by surgery or radiation therapy, but about half of UMs develop distant metastasis mostly to the liver. Survival of patients with metastasis is below 1 year and has not improved in decades. Recent years have brought a deep understanding of UM biology characterized by initiating mutations in the G proteins GNAQ and GNA11. Cytogenetic alterations, in particular monosomy of chromosome 3 and amplification of the long arm of chromosome 8, and mutation of the BRCA1-associated protein 1, BAP1, a tumor suppressor gene, or the splicing factor SF3B1 determine UM metastasis. Cytogenetic and molecular profiling allow for a very precise prognostication that is still not matched by efficacious adjuvant therapies. G protein signaling has been shown to activate the YAP/TAZ pathway independent of HIPPO, and conventional signaling via the mitogen-activated kinase pathway probably also contributes to UM development and progression. Several lines of evidence indicate that inflammation and macrophages play a pro-tumor role in UM and in its hepatic metastases. UM cells benefit from the immune privilege in the eye and may adopt several mechanisms involved in this privilege for tumor escape that act even after leaving the niche. Here, we review the current knowledge of the biology of UM and discuss recent approaches to UM treatment.

Regulator of G protein signaling 2 (Rgs2) regulates neural crest development through Pparδ-Sox10 cascade

Neural crest cells are multipotent progenitors that migrate extensively and differentiate into numerous derivatives. The developmental plasticity and migratory ability of neural crest cells render them an attractive model for studying numerous aspects of cell progression. We observed that zebrafish rgs2 was expressed in neural crest cells. Disrupting Rgs2 expression by using a dominant negative rgs2 construct or rgs2 morpholinos reduced GTPase-activating protein activity, induced the formation of neural crest progenitors, increased the proliferation of nonectomesenchymal neural crest cells, and inhibited the formation of ectomesenchymal neural crest derivatives. The transcription of pparda (which encodes Pparδ, a Wnt-activated transcription factor) was upregulated in Rgs2-deficient embryos, and Pparδ inhibition using a selective antagonist in the Rgs2-deficient embryos repaired neural crest defects. Our results clarify the mechanism through which the Rgs2-Pparδ cascade regulates neural crest development; specifically, Pparδ directly binds to the promoter and upregulates the transcription of the neural crest specifier sox10. This study reveals a unique regulatory mechanism, the Rgs2-Pparδ-Sox10 signaling cascade, and defines a key molecular regulator, Rgs2, in neural crest development.

Expression profiles of the Gα subunits during Xenopus tropicalis embryonic development

Heterotrimeric G protein signaling plays major roles during different cellular events. However, there is a limited understanding of the molecular mechanisms underlying G protein control during embryogenesis. G proteins are highly conserved and can be grouped into four subfamilies according to sequence homology and function. To further studies on G protein function during embryogenesis, the present analysis identified four Gα subunits representative of the different subfamilies and determined their spatiotemporal expression patterns during Xenopus tropicalis embryogenesis. Each of the Gα subunit transcripts was maternally and zygotically expressed, and, as development progressed, dynamic expression patterns were observed. In the early developmental stages, the Gα subunits were expressed in the animal hemisphere and dorsal marginal zone. While expression was observed at the somite boundaries, in vascular structures, in the eye, and in the otic vesicle during the later stages, expression was mainly found in neural tissues, such as the neural tube and, especially, in the cephalic vesicles, neural crest region, and neural crest-derived structures. Together, these results support the pleiotropism and complexity of G protein subfamily functions in different cellular events. The present study constitutes the most comprehensive description to date of the spatiotemporal expression patterns of Gα subunits during vertebrate development.

G-Protein α-Subunit Gsα Is Required for Craniofacial Morphogenesis

The heterotrimeric G protein subunit Gsα couples receptors to activate adenylyl cyclase and is required for the intracellular cAMP response and protein kinase A (PKA) activation. Gsα is ubiquitously expressed in many cell types; however, the role of Gsα in neural crest cells (NCCs) remains unclear. Here we report that NCCs-specific Gsα knockout mice die within hours after birth and exhibit dramatic craniofacial malformations, including hypoplastic maxilla and mandible, cleft palate and craniofacial skeleton defects. Histological and anatomical analysis reveal that the cleft palate in Gsα knockout mice is a secondary defect resulting from craniofacial skeleton deficiencies. In Gsα knockout mice, the morphologies of NCCs-derived cranial nerves are normal, but the development of dorsal root and sympathetic ganglia are impaired. Furthermore, loss of Gsα in NCCs does not affect cranial NCCs migration or cell proliferation, but significantly accelerate osteochondrogenic differentiation. Taken together, our study suggests that Gsα is required for neural crest cells-derived craniofacial development.

Expression and functionality of transient receptor potential melastatin 4 (TRPM4)-like channels during development of the zebrafish

Calcium signaling, from localized spikes to coordinated waves, are linked to cleavage, patterning, differentiation, and growth during embryonic development. The basis for control of these Ca(2+) signals is poorly defined. In this study, the expression and functionality of the transient receptor potential melastatin 4 protein (TRPM4), an ion channel that controls Ca(2+) entry into cells, was examined in the zebrafish embryo and adult. Originating with the human TRPM4 gene, Ensembl ortholog, NCBI BLAST, and Homologene searches identified a zebrafish TRPM4 "like" gene encoding a predicted protein of 1199 amino acids and sharing a 42-43% sequence identity with the mouse, rat, and human. Custom-designed zebrafish primers identified TRPM4 transcripts throughout the 0-123h period of embryonic development with greatest and lowest relative expression at 12 and 123h post-fertilization, respectively. Perforated patch clamp recordings in 27h embryonic cells revealed Ca(2+)-activated currents with the characteristics of those described for mammalian TRPM4. Similarly, TRPM4-like expression and functionality was observed in brain and liver cells from adult fish. These findings suggest that a TRPM4-like channel is available for Ca(2+) regulation during early development of the zebrafish.

Biology of advanced uveal melanoma and next steps for clinical therapeutics

Uveal melanoma is the most common intraocular malignancy although it is a rare subset of all melanomas. Uveal melanoma has distinct biology relative to cutaneous melanoma, with widely divergent patient outcomes. Patients diagnosed with a primary uveal melanoma can be stratified for risk of metastasis by cytogenetics or gene expression profiling, with approximately half of patients developing metastatic disease, predominately hepatic in location, over a 15-yr period. Historically, no systemic therapy has been associated with a clear clinical benefit for patients with advanced disease, and median survival remains poor. Here, as a joint effort between the Melanoma Research Foundation's ocular melanoma initiative, CURE OM and the National Cancer Institute, the current understanding of the molecular and immunobiology of uveal melanoma is reviewed, and on-going laboratory research into the disease is highlighted. Finally, recent investigations relevant to clinical management via targeted and immunotherapies are reviewed, and next steps in the development of clinical therapeutics are discussed.

Endothelin regulates neural crest deployment and fate to form great vessels through Dlx5/Dlx6-independent mechanisms

Endothelin-1 (Edn1), originally identified as a vasoconstrictor peptide, is involved in the development of cranial/cardiac neural crest-derived tissues and organs. In craniofacial development, Edn1 binds to Endothelin type-A receptor (Ednra) to induce homeobox genes Dlx5/Dlx6 and determines the mandibular identity in the first pharyngeal arch. However, it remains unsolved whether this pathway is also critical for pharyngeal arch artery development to form thoracic arteries. Here, we show that the Edn1/Ednra signaling is involved in pharyngeal artery development by controlling the fate of neural crest cells through a Dlx5/Dlx6-independent mechanism. Edn1 and Ednra knock-out mice demonstrate abnormalities in pharyngeal arch artery patterning, which include persistent first and second pharyngeal arteries, resulting in additional branches from common carotid arteries. Neural crest cell labeling with Wnt1-Cre transgene and immunostaining for smooth muscle cell markers revealed that neural crest cells abnormally differentiate into smooth muscle cells at the first and second pharyngeal arteries of Ednra knock-out embryos. By contrast, Dlx5/Dlx6 knockout little affect the development of pharyngeal arch arteries and coronary arteries, the latter of which is also contributed by neural crest cells through an Edn-dependent mechanism. These findings indicate that the Edn1/Ednra signaling regulates neural crest differentiation to ensure the proper patterning of pharyngeal arch arteries, which is independent of the regional identification of the pharyngeal arches along the dorsoventral axis mediated by Dlx5/Dlx6.

Genetic interactions between neurofibromin and endothelin receptor B in mice

When mutations in two different genes produce the same mutant phenotype, it suggests that the encoded proteins either interact with each other, or act in parallel to fulfill a similar purpose. Haploinsufficiency of Neurofibromin and over-expression of Endothelin 3 both cause increased numbers of melanocytes to populate the dermis during mouse development, and thus we are interested in how these two signaling pathways might intersect. Neurofibromin is mutated in the human genetic disease, neurofibromatosis type 1, which is characterized by the development of Schwann cell based tumors and skin hyper-pigmentation. Neurofibromin is a GTPase activating protein, while the Endothelin 3 ligand activates Endothelin receptor B, a G protein coupled receptor. In order to study the genetic interactions between endothelin and neurofibromin, we defined the deletion breakpoints of the classical Ednrb piebald lethal allele (Ednrb(s-l) ) and crossed these mice to mice with a loss-of-function mutation in neurofibromin, Dark skin 9 (Dsk9). We found that Neurofibromin haploinsufficiency requires Endothelin receptor B to darken the tail dermis. In contrast, Neurofibromin haploinsufficiency increases the area of the coat that is pigmented in Endothelin receptor B null mice. We also found an oncogenic mutation in the G protein alpha subunit, GNAQ, which couples to Endothelin receptor B, in a uveal melanoma from a patient with neurofibromatosis type 1. Thus, this data suggests that there is a complex relationship between Neurofibromin and Endothelin receptor B.

A human homeotic transformation resulting from mutations in PLCB4 and GNAI3 causes auriculocondylar syndrome

Auriculocondylar syndrome (ACS) is a rare, autosomal-dominant craniofacial malformation syndrome characterized by variable micrognathia, temporomandibular joint ankylosis, cleft palate, and a characteristic "question-mark" ear malformation. Careful phenotypic characterization of severely affected probands in our cohort suggested the presence of a mandibular patterning defect resulting in a maxillary phenotype (i.e., homeotic transformation). We used exome sequencing of five probands and identified two novel (exclusive to the patient and/or family studied) missense mutations in PLCB4 and a shared mutation in GNAI3 in two unrelated probands. In confirmatory studies, three additional novel PLCB4 mutations were found in multigenerational ACS pedigrees. All mutations were confirmed by Sanger sequencing, were not present in more than 10,000 control chromosomes, and resulted in amino-acid substitutions located in highly conserved protein domains. Additionally, protein-structure modeling demonstrated that all ACS substitutions disrupt the catalytic sites of PLCB4 and GNAI3. We suggest that PLCB4 and GNAI3 are core signaling molecules of the endothelin-1-distal-less homeobox 5 and 6 (EDN1-DLX5/DLX6) pathway. Functional studies demonstrated a significant reduction in downstream DLX5 and DLX6 expression in ACS cases in assays using cultured osteoblasts from probands and controls. These results support the role of the previously implicated EDN1-DLX5/6 pathway in regulating mandibular specification in other species, which, when disrupted, results in a maxillary phenotype. This work defines the molecular basis of ACS as a homeotic transformation (mandible to maxilla) in humans.

The canonical Wnt signaling activator, R-spondin2, regulates craniofacial patterning and morphogenesis within the branchial arch through ectodermal-mesenchymal interaction

R-spondins are a recently characterized family of secreted proteins that activate Wnt/β-catenin signaling. Herein, we determine R-spondin2 (Rspo2) function in craniofacial development in mice. Mice lacking a functional Rspo2 gene exhibit craniofacial abnormalities such as mandibular hypoplasia, maxillary and mandibular skeletal deformation, and cleft palate. We found that loss of the mouse Rspo2 gene significantly disrupted Wnt/β-catenin signaling and gene expression within the first branchial arch (BA1). Rspo2, which is normally expressed in BA1 mesenchymal cells, regulates gene expression through a unique ectoderm-mesenchyme interaction loop. The Rspo2 protein, potentially in combination with ectoderm-derived Wnt ligands, up-regulates Msx1 and Msx2 expression within mesenchymal cells. In contrast, Rspo2 regulates expression of the Dlx5, Dlx6, and Hand2 genes in mesenchymal cells via inducing expression of their upstream activator, Endothelin1 (Edn1), within ectodermal cells. Loss of Rspo2 also causes increased cell apoptosis, especially within the aboral (or caudal) domain of the BA1, resulting in hypoplasia of the BA1. Severely reduced expression of Fgf8, a survival factor for mesenchymal cells, in the ectoderm of Rspo2(-/-) embryos is likely responsible for increased cell apoptosis. Additionally, we found that the cleft palate in Rspo2(-/-) mice is not associated with defects intrinsic to the palatal shelves. A possible cause of cleft palate is a delay of proper palatal shelf elevation that may result from the small mandible and a failure of lowering the tongue. Thus, our study identifies Rspo2 as a mesenchyme-derived factor that plays critical roles in regulating BA1 patterning and morphogenesis through ectodermal-mesenchymal interaction and a novel genetic factor for cleft palate.

Regulation of facial morphogenesis by endothelin signaling: insights from mice and fish

Craniofacial morphogenesis is accomplished through a complex set of developmental events, most of which are initiated in neural crest cells within the pharyngeal arches. Local patterning cues from the surrounding environment induce gene expression within neural crest cells, leading to formation of a diverse set of skeletal elements. Endothelin-1 (Edn1) is one of the primary signals that establishes the identity of neural crest cells within the mandibular portion of the first pharyngeal arch. Signaling through its cognate receptor, the endothelin-A receptor, is critical for patterning the ventral/distal portion of the arch (lower jaw) and also participates with Hox genes in patterning more posterior arches. Edn1/Ednra signaling is highly conserved between mouse and zebrafish, and genetic analyses in these two species have provided complementary insights into the patterning cues responsible for establishing the craniofacial complex as well as the genetic basis of facial birth defect syndromes.

Molecular mechanisms of cranial neural crest cell migration and patterning in craniofacial development

During vertebrate craniofacial development, neural crest cells (NCCs) contribute much of the cartilage, bone and connective tissue that make up the developing head. Although the initial patterns of NCC segmentation and migration are conserved between species, the variety of vertebrate facial morphologies that exist indicates that a complex interplay occurs between intrinsic genetic NCC programs and extrinsic environmental signals during morphogenesis. Here, we review recent work that has begun to shed light on the molecular mechanisms that govern the spatiotemporal patterning of NCC-derived skeletal structures – advances that are central to understanding craniofacial development and its evolution.

Serotonin 2B receptor signaling is required for craniofacial morphogenesis and jaw joint formation in Xenopus

Serotonin (5-HT) is a neuromodulator that plays many different roles in adult and embryonic life. Among the 5-HT receptors, 5-HT2B is one of the key mediators of 5-HT functions during development. We used Xenopus laevis as a model system to further investigate the role of 5-HT2B in embryogenesis, focusing on craniofacial development. By means of gene gain- and loss-of-function approaches and tissue transplantation assays, we demonstrated that 5-HT2B modulates, in a cell-autonomous manner, postmigratory skeletogenic cranial neural crest cell (NCC) behavior without altering early steps of cranial NCC development and migration. 5-HT2B overexpression induced the formation of an ectopic visceral skeletal element and altered the dorsoventral patterning of the branchial arches. Loss-of-function experiments revealed that 5-HT2B signaling is necessary for jaw joint formation and for shaping the mandibular arch skeletal elements. In particular, 5-HT2B signaling is required to define and sustain the Xbap expression necessary for jaw joint formation. To shed light on the molecular identity of the transduction pathway acting downstream of 5-HT2B, we analyzed the function of phospholipase C beta 3 (PLC) in Xenopus development and showed that PLC is the effector of 5-HT2B during craniofacial development. Our results unveiled an unsuspected role of 5-HT2B in craniofacial development and contribute to our understanding of the interactive network of patterning signals that is involved in the development and evolution of the vertebrate mandibular arch.

The zebrafish dyrk1b gene is important for endoderm formation

Nodal-signaling is required for specification of mesoderm, endoderm, establishing left-right asymmetry, and craniofacial development. Wdr68 is a WD40-repeat domain-containing protein recently shown to be required for endothelin-1 (edn1) expression and subsequent lower jaw development. Previous reports detected the Wdr68 protein in multiprotein complexes containing mammalian members of the dual-specificity tyrosine-regulated kinase (dyrk) family. Here we describe the characterization of the zebrafish dyrk1b homolog. We report the detection of a physical interaction between Dyrk1b and Wdr68. We also found perturbations of nodal signaling in dyrk1b antisense morpholino knockdown (dyrk1b-MO) animals. Specifically, we found reduced expression of lft1 and lft2 (lft1/2) during gastrulation and a near complete loss of the later asymmetric lft1/2 expression domains. Although wdr68-MO animals did not display lft1/2 expression defects during gastrulation, they displayed a near complete loss of the later asymmetric lft1/2 expression domains. While expression of ndr1 was not substantially effected during gastrulation, ndr2 expression was moderately reduced in dyrk1b-MO animals. Analysis of additional downstream components of the nodal signaling pathway in dyrk1b-MO animals revealed modestly expanded expression of the dorsal axial mesoderm marker gsc while the pan-mesodermal marker bik was largely unaffected. The endodermal markers cas and sox17 were also moderately reduced in dyrk1b-MO animals. Notably, and similar to defects previously reported for wdr68 mutant animals, we also found reduced expression of the pharyngeal pouch marker edn1 in dyrk1b-MO animals. Taken together, these data reveal a role for dyrk1b in endoderm formation and craniofacial patterning in the zebrafish.

Independent regulation of hair and skin color by two G protein-coupled pathways

Hair color and skin color are frequently coordinated in mammalian species. To explore this, we have studied mutations in two different G protein coupled pathways, each of which affects the darkness of both hair and skin color. In each mouse mutant (Gnaq(Dsk1), Gna11(Dsk7), and Mc1r(e)), we analyzed the melanocyte density and the concentrations of eumelanin (black pigment) and pheomelanin (yellow pigment) in the hair or skin to determine the mechanisms regulating pigmentation. Surprisingly, we discovered that each mutation affects hair and skin color differently. Furthermore, we have found that in the epidermis, the melanocortin signaling pathway does not couple the synthesis of eumelanin with pheomelanin, as it does in hair follicles. Even by shared signaling pathways, hair and skin melanocytes are regulated quite independently.

Biological mechanisms in palatogenesis and cleft palate

Clefts of the palate are common birth defects requiring extensive treatment. They appear to be caused by multiple genetic and environmental factors during palatogenesis. This may result in local changes in growth factors, extracellular matrix (ECM), and cell adhesion molecules. Several clefting factors have been implicated by studies in mouse models, while some of these have also been confirmed by genetic screening in humans. Here, we discuss several knockout mouse models to examine the role of specific genes in cleft formation. The cleft is ultimately caused by interference with shelf elevation, attachment, or fusion. Shelf elevation is brought about by mesenchymal proliferation and changes in the ECM induced by growth factors such as TGF-betas. Crucial ECM molecules are collagens, proteoglycans, and glycosaminoglycans. Shelf attachment depends on specific differentiation of the epithelium involving TGF-beta3, sonic hedgehog, and WNT signaling, and correct expression of epithelial adhesion molecules such as E-cadherin. The final fusion requires epithelial apoptosis and epithelium-to-mesenchyme transformation regulated by TGF-beta and WNT proteins. Other factors may interact with these signaling pathways and contribute to clefting. Normalization of the biological mechanisms regulating palatogenesis in susceptible fetuses is expected to contribute to cleft prevention.

Elucidating timing and function of endothelin-A receptor signaling during craniofacial development using neural crest cell-specific gene deletion and receptor antagonism

Cranial neural crest cells (NCCs) play an intimate role in craniofacial development. Multiple signaling cascades participate in patterning cranial NCCs, some of which are regulated by endothelin-A receptor (Ednra) signaling. Ednra(-/-) embryos die at birth from severe craniofacial defects resulting from disruption of neural crest cell patterning and differentiation. These defects include homeotic transformation of lower jaw structures into upper jaw-like structures, suggesting that some cephalic NCCs alter their "identity" in the absence of Ednra signaling. To elucidate the temporal necessity for Ednra signaling in vivo, we undertook two strategies. We first used a conditional knockout strategy in which mice containing a conditionally targeted Ednra allele (Ednra(fl)) were bred with mice from the Hand2-Cre and Wnt1-Cre transgenic mouse strains, two strains in which Cre expression occurs at different time periods within cranial NCCs. In our second approach, we used an Ednra-specific antagonist to treat wild type pregnant mice between embryonic days E8.0 and E10.0, a time frame encompassing the early migration and proliferation of cranial NCCs. The combined results suggest that Ednra function is crucial for NCC development between E8.25 and E9.0, a time period encompassing the arrival of NCCs in the arches and/or early post-migratory patterning. After this time period, Ednra signaling is dispensable. Interestingly, middle ear structures are enlarged and malformed in a majority of Ednra(fl/fl);Wnt1-Cre embryos, instead resembling structures found in extinct predecessors of mammals. These observations suggest that the advent of Ednra signaling in cranial NCCs may have been a crucial event in the evolution of the mammalian middle ear ossicles.

A twist of insight - the role of Twist-family bHLH factors in development

Members of the Twist-family of bHLH proteins play a pivotal role in a number of essential developmental programs. Twist-family bHLH proteins function by dimerizing with other bHLH members and binding to cis- regulatory elements, called E-boxes. While Twist-family members may simply exhibit a preference in terms of high-affinity binding partners, a complex, multilevel cascade of regulation creates a dynamic role for these bHLH proteins. We summarize in this review information on each Twist-family member concerning expression pattern, function, regulation, downstream targets, and interactions with other bHLH proteins. Additionally, we focus on the phospho-regulatory mechanisms that tightly control posttranslational modification of Twist-family member bHLH proteins.

Transcriptional regulation during development of the ductus arteriosus

The ductus arteriosus is a specialized blood vessel containing highly differentiated and contractile vascular smooth muscle, derived largely from neural crest cells, that is essential for fetal life but typically closes after birth. Impaired development of the ductus arteriosus or disruption of signaling pathways that initiate postnatal closure can result in persistent patency of the ductus arteriosus, the third most common congenital heart defect. We found that Tfap2beta, a transcription factor associated with patent ductus arteriosus in humans, was uniquely expressed in mouse ductal smooth muscle. Endothelin-1 and the hypoxia-induced transcription factor, Hif2alpha were also highly enriched in ductal smooth muscle at embryonic day 13.5 and were dependent on Tfap2beta for their expression in this domain. Hif2alpha functioned as a negative regulator of Tfap2beta-induced transcription by disrupting protein-DNA interactions, suggesting a negative feedback loop regulating Tfap2beta activity. Our data indicate that Tfap2beta, Et-1, and Hif2alpha act in a transcriptional network during ductal smooth muscle development and that disruption of this pathway may contribute to patent ductus arteriosus by affecting the development of smooth muscle within the ductus arteriosus.

Recombinase-mediated cassette exchange reveals the selective use of Gq/G11-dependent and -independent endothelin 1/endothelin type A receptor signaling in pharyngeal arch development

The endothelin (Edn) system comprises three ligands (Edn1, Edn2 and Edn3) and their G-protein-coupled type A (Ednra) and type B (Ednrb) receptors. During embryogenesis, the Edn1/Ednra signaling is thought to regulate the dorsoventral axis patterning of pharyngeal arches via Dlx5/Dlx6 upregulation. To further clarify the underlying mechanism, we have established mice in which gene cassettes can be efficiently knocked-in into the Ednra locus using recombinase-mediated cassette exchange (RMCE) based on the Cre-lox system. The first homologous recombination introducing mutant lox-flanked Neo resulted in homeotic transformation of the lower jaw to an upper jaw, as expected. Subsequent RMCE-mediated knock-in of lacZ targeted its expression to the cranial/cardiac neural crest derivatives as well as in mesoderm-derived head mesenchyme. Knock-in of Ednra cDNA resulted in a complete rescue of craniofacial defects of Ednra-null mutants. By contrast, Ednrb cDNA could not rescue them except for the most distal pharyngeal structures. At early stages, the expression of Dlx5, Dlx6 and their downstream genes was downregulated and apoptotic cells distributed distally in the mandible of Ednrb-knock-in embryos. These results, together with similarity in craniofacial defects between Ednrb-knock-in mice and neural-crest-specific Galpha(q)/Galpha(11)-deficient mice, indicate that the dorsoventral axis patterning of pharyngeal arches is regulated by the Ednra-selective, G(q)/G(11)-dependent signaling, while the formation of the distal pharyngeal region is under the control of a G(q)/G(11)-independent signaling, which can be substituted by Ednrb. This RMCE-mediated knock-in system can serve as a useful tool for studies on gene functions in craniofacial development.

mef2ca is required in cranial neural crest to effect Endothelin1 signaling in zebrafish

Mef2 genes encode highly conserved transcription factors involved in somitic and cardiac mesoderm development in diverse bilaterians. Vertebrates have multiple mef2 genes. In mice, mef2c is required for heart and vascular development. We show that a zebrafish mef2c gene (mef2ca) is required in cranial neural crest (CNC) for proper head skeletal patterning. mef2ca mutants have head skeletal phenotypes resembling those seen upon partial loss-of-function of endothelin1 (edn1). Furthermore, mef2ca interacts genetically with edn1, arguing that mef2ca functions within the edn1 pathway. mef2ca is expressed in CNC and this expression does not require edn1 signaling. Mosaic analyses reveal that mef2ca is required in CNC for pharyngeal skeletal morphogenesis. Proper expression of many edn1-dependent target genes including hand2, bapx1, and gsc, depends upon mef2ca function. mef2ca plays a critical role in establishing the proper nested expression patterns of dlx genes. dlx5a and dlx6a, known Edn1 targets, are downregulated in mef2ca mutant pharyngeal arch CNC. Surprisingly, dlx4b and dlx3b are oppositely affected in mef2ca mutants. dlx4b expression is abolished while the edn1-dependent dlx3b is ectopically expressed in more dorsal CNC. Together our results support a model in which CNC cells require mef2ca downstream of edn1 signaling for proper craniofacial development.

phospholipase C, beta 3 is required for Endothelin1 regulation of pharyngeal arch patterning in zebrafish

Genetic and pharmacological studies demonstrate that Endothelin1 (Edn1) is a key signaling molecule for patterning the facial skeleton in fish, chicks, and mice. When Edn1 function is reduced early in development the ventral lower jaw and supporting structures are reduced in size and often fused to their dorsal upper jaw counterparts. We show that schmerle (she) encodes a zebrafish ortholog of Phospholipase C, beta 3 (Plcbeta3) required in cranial neural crest cells for Edn1 regulation of pharyngeal arch patterning. Sequencing and co-segregation demonstrates that two independent she (plcbeta3) alleles have missense mutations in conserved residues within the catalytic domains of Plcbeta3. Homozygous plcbeta3 mutants are phenotypically similar to edn1 mutants and exhibit a strong arch expression defect in Edn1-dependent Distalless (Dlx) genes as well as expression defects in several Edn1-dependent intermediate and ventral arch domain transcription factors. plcbeta3 also genetically interacts with edn1, supporting a model in which Edn1 signals through a G protein-coupled receptor to activate Plcbeta3. Mild skeletal defects occur in plcbeta3 heterozygotes, showing the plcbeta3 mutations are partially dominant. Through a morpholino-mediated deletion in the N-terminal PH domain of Plcbeta3, we observe a partial rescue of facial skeletal defects in homozygous plcbeta3 mutants, supporting a hypothesis that an intact PH domain is necessary for the partial dominance we observe. In addition, through mosaic analyses, we show that wild-type neural crest cells can efficiently rescue facial skeletal defects in homozygous plcbeta3 mutants, demonstrating that Plcbeta3 function is required in neural crest cells and not other cell types to pattern the facial skeleton.

Requirements for Endothelin type-A receptors and Endothelin-1 signaling in the facial ectoderm for the patterning of skeletogenic neural crest cells in zebrafish

Genetic studies in mice and zebrafish have revealed conserved requirements for Endothelin 1 (Edn1) signaling in craniofacial development. Edn1 acts through its cognate type-A receptor (Ednra) to promote ventral skeletal fates and lower-jaw formation. Here, we describe the isolation and characterization of two zebrafish ednra genes - ednra1 and ednra2 -both of which are expressed in skeletal progenitors in the embryonic neural crest. We show that they play partially redundant roles in lower-jaw formation and development of the jaw joint. Knockdown of Ednra1 leads to fusions between upper- and lower-jaw cartilages, whereas the combined loss of Ednra1 and Ednra2 eliminates the lower jaw, similar to edn1-/-mutants. edn1 is expressed in pharyngeal arch ectoderm, mesoderm and endoderm. Tissue-mosaic studies indicate that, among these tissues, a crucial source of Edn1 is the surface ectoderm. This ectoderm also expresses ednrA1 in an edn1-dependent manner, suggesting that edn1 autoregulates its own expression. Collectively, our results indicate that Edn1 from the pharyngeal ectoderm signals through Ednra proteins to direct early dorsoventral patterning of the skeletogenic neural crest.

Conditional mutagenesis of G-protein coupled receptors and G-proteins

The G-protein-coupled receptor signaling system, consisting of a huge variety of receptors as well as of many G-proteins and effectors, operates in every cell and is involved in many physiological and pathological processes. The versatility of this system and the involvement of specific components makes G-protein-coupled receptors and their signaling pathways ideal targets for pharmacological interventions. Classical mouse knockout models have often provided important preliminary insights into the biological roles of individual receptors and signaling pathways and they are routinely used in the process of target validation. The recent development of efficient conditional mutagenesis techniques now allows a much more detailed analysis of G-protein-mediated signaling transduction processes. This review summarizes some of the areas in which progress has recently been made by applying conditional mutagenesis of genes coding for G-proteins and G-protein-coupled receptors.

Regulation of dHAND protein expression by all-trans retinoic acid through ET-1/ETAR signaling in H9c2 cells

dHAND is thought to be a cardiac-restricted transcription factor during embryonic development. Vertebrate heart development involves many transcription factors such as Nkx2.5, GATA, and tbx5. All-trans retinoic acid (AtRA), the oxidative metabolite of vitamin A, can regulate the expression of these factors to affect embryonic heart development. However, the action of atRA on the expression of dHAND is rarely reported. To clarify whether atRA regulate the dHAND expression, we exposed cultured H9c2 cells (rat embryonic cardiomyocytes) to atRA and detected the protein expression of dHAND by Western blot analysis. We observed atRA can regulate the dHAND expression in a dose- and time-dependent manner. AtRA also inhibited endothelin-1 (ET-1) expression in a time-dependent manner. Further studies revealed that pretreatment with 10 microM BQ-123, a selective endothelin-1 receptor (ETAR) antagonist, for 2 h can significantly counteract the inhibition of 5 microM atRA treatment for 2 h of dHAND mRNA and protein expression. Taken together, these results suggest that atRA regulates dHAND expression by ET-1/ETAR signal transduction pathway in H9c2 cells. The mechanism of ET-1/ETAR signaling in controlling the level of dHAND protein is to reduce the levels of dHAND mRNA. It is possible for atRA to exert its cardiac teratogenesis during vertebrate embryonic development in this way.

A zebrafish screen for craniofacial mutants identifies wdr68 as a highly conserved gene required for endothelin-1 expression

BACKGROUND

Craniofacial birth defects result from defects in cranial neural crest (NC) patterning and morphogenesis. The vertebrate craniofacial skeleton is derived from cranial NC cells and the patterning of these cells occurs within the pharyngeal arches. Substantial efforts have led to the identification of several genes required for craniofacial skeletal development such as the endothelin-1 (edn1) signaling pathway that is required for lower jaw formation. However, many essential genes required for craniofacial development remain to be identified.

RESULTS

Through screening a collection of insertional zebrafish mutants containing approximately 25% of the genes essential for embryonic development, we present the identification of 15 essential genes that are required for craniofacial development. We identified 3 genes required for hyomandibular development. We also identified zebrafish models for Campomelic Dysplasia and Ehlers-Danlos syndrome. To further demonstrate the utility of this method, we include a characterization of the wdr68 gene. We show that wdr68 acts upstream of the edn1 pathway and is also required for formation of the upper jaw equivalent, the palatoquadrate. We also present evidence that the level of wdr68 activity required for edn1 pathway function differs between the 1st and 2nd arches. Wdr68 interacts with two minibrain-related kinases, Dyrk1a and Dyrk1b, required for embryonic growth and myotube differentiation, respectively. We show that a GFP-Wdr68 fusion protein localizes to the nucleus with Dyrk1a in contrast to an engineered loss of function mutation Wdr68-T284F that no longer accumulated in the cell nucleus and failed to rescue wdr68 mutant animals. Wdr68 homologs appear to exist in all eukaryotic genomes. Notably, we found that the Drosophila wdr68 homolog CG14614 could substitute for the vertebrate wdr68 gene even though insects lack the NC cell lineage.

CONCLUSION

This work represents a systematic identification of approximately 25% of the essential genes required for craniofacial development. The identification of zebrafish models for two human disease syndromes indicates that homologs to the other genes are likely to also be relevant for human craniofacial development. The initial characterization of wdr68 suggests an important role in craniofacial development for the highly conserved Wdr68-Dyrk1 protein complexes.

Smad6 represses Dlx3 transcriptional activity through inhibition of DNA binding

Dlx3 (Distal-less 3) is a homeobox-containing transcription factor required for normal placental development in mice. Here we demonstrate that Dlx3 interacts with Smad6, a member of a larger family of transcriptional regulators generally thought to regulate transforming growth factor beta/bone morphogenetic protein signaling. Immunocytochemical and immunoprecipitation studies demonstrate overlapping nuclear localization and physical interaction between Dlx3 and Smad6 in human choriocarcinoma cells and in differentiated trophoblasts from human placenta. In vitro protein interaction studies mapped the Smad6 interaction domain within Dlx3 to residues 80-163, a region of Dlx3 that includes a portion of the homeodomain. Dlx3 and Dlx4 share homology within this region, and Dlx4 was also found to bind Smad6. Using the Esx1 gene promoter as a model for a Dlx3-responsive gene, studies demonstrate two near consensus Dlx3 binding sites within the proximal 2.3 kb of the transcription start site. Interestingly, binding of Dlx3 to one of these two sites was inhibited by interaction with Smad6. Consistent with this result, expression of an Esx1 promoter luciferase reporter was increased by overexpression of Dlx3; this effect was reversed with co-expression of Smad6. Further, small interference RNA-mediated knockdown of endogenous Smad6 increased Dlx3-dependent expression of the Esx1 gene promoter. Thus, Smad6 appears to functionally interact with Dlx3, altering the ability of Dlx3 to bind target gene promoters. Smad6 appears to play a modulatory role in the regulation of Dlx3-dependent gene transcription within placental trophoblasts.

Zebrafish furin mutants reveal intricacies in regulating Endothelin1 signaling in craniofacial patterning

Endothelin1 (Edn1) signaling promotes ventral character to the facial skeleton. In zebrafish edn1 mutants, the ventral jaw structures are severely reduced and fused to their dorsal counterparts, with a loss of joints that normally form at an intermediate dorsal-ventral position. Loss of function at another locus, sturgeon, also yields joint losses, but only mild reductions in the ventral jaw structures. We show that sturgeon encodes one of two orthologs of Furin present in zebrafish, and that both furin genes may function partially redundantly to activate Edn1 signaling. Supporting this hypothesis, early expression of edn1-dependent genes is downregulated in sturgeon (furinA) mutants. Later in development, expression of most of these genes recovers to near wild-type levels in furinA mutants but not in edn1 mutants. The recovery explains the less severe furinA mutant skeletal phenotype and suggests that late gene expression is dependent on a critical level of Edn1 signaling not present in the more severe edn1 mutants. However, expression defects in the intermediate joint-forming domains in both mutants persist, explaining the joint losses observed later in both mutants. We further show that in both mutants the arches fail to correctly undergo ventral elongation before skeletogenesis begins and propose a model in which this failure is largely responsible for the loss of an Edn1-dependent compartmentation of the arch into the intermediate and ventral domains.

Expression pattern of Dlx3 during cell differentiation in mineralized tissues

The present study was designed to compare the expression pattern of Dlx3 in four different mineralized tissues because of: 1-its role in skeleton patterning, 2-its expression in dental epithelium and mesenchyme during morphogenesis, 3-the membranous and endochondral bone and tooth phenotype of tricho-dento-osseous syndrome related to Dlx3 gene mutation and 4-recently emerging knowledge on Dlx family members in the bone field. Ameloblasts, odontoblasts, osteoblasts and chondrocytes were analyzed in vitro and in vivo. Dlx3 transcripts were detected by RT-PCR in established model systems (microdissected dental epithelium and mesenchyme; primary cultures of rat chondrocytes), as recently performed in osteoblasts in vitro. A human 414-bp Dlx3 probe was generated. A 4.5-kb human Dlx3 sense RNA was identified in maxillo-facial samples by Northern blotting. Immunolabeling and in situ hybridization were performed in mice from Theiler stage E 14.5 until birth. In teeth, although Dlx3 was still expressed in differentiated ameloblasts, it was down regulated during odontoblast polarization. During endochondral bone formation, Dlx3 protein was detected in chondrocytes and was most strongly expressed in the prehypertrophic cartilage zone and in differentiating and differentiated osteoblasts of metaphyseal periosteum. In vitro, real-time PCR studies supported this upregulation in prehypertrophic chondrocytes, closely correlated with Ihh variations. In membranous bone, Dlx3 was present in preosteoblasts, osteoblasts and osteoid-osteocytes. The present data on Dlx3 and recently published functional studies show that this transcription factor may be instrumental during growth in the control of matrix deposition and biomineralization in the entire skeleton.

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Characteristic defects in neural crest cell-specific Galphaq/Galpha11- and Galpha12/Galpha13-deficient mice

The endothelin/endothelin receptor system plays a critical role in the differentiation and terminal migration of particular neural crest cell subpopulations. Targeted deletion of the G-protein-coupled endothelin receptors ET(A) and ET(B) was shown to result in characteristic developmental defects of derivatives of cephalic and cardiac neural crest and of neural crest-derived melanocytes and enteric neurons, respectively. Since both endothelin receptors are coupled to G-proteins of the G(q)/G(11)- and G(12)/G(13)-families, we generated mouse lines lacking Galpha(q)/Galpha(11) or Galpha(12)/Galpha(13) in neural crest cells to study their roles in neural crest development. Mice lacking Galpha(q)/Galpha(11) in a neural crest cell-specific manner had craniofacial defects similar to those observed in mice lacking the ET(A) receptor or endothelin-1 (ET-1). However, in contrast to ET-1/ET(A) mutant animals, cardiac outflow tract morphology was intact. Surprisingly, neither Galpha(q)/Galpha(11)- nor Galpha(12)/Galpha(13)-deficient mice showed developmental defects seen in animals lacking either the ET(B) receptor or its ligand endothelin-3 (ET-3). Interestingly, Galpha(12)/Galpha(13) deficiency in neural crest cell-derived cardiac cells resulted in characteristic cardiac malformations. Our data show that G(q)/G(11)- but not G(12)/G(13)-mediated signaling processes mediate ET-1/ET(A)-dependent development of the cephalic neural crest. In contrast, ET-3/ET(B)-mediated development of neural crest-derived melanocytes and enteric neurons appears to involve G-proteins different from G(q)/G(11)/G(12)/G(13).

Loss of Gq/11 family G proteins in the nervous system causes pituitary somatotroph hypoplasia and dwarfism in mice

Heterotrimeric G proteins of the Gq/11 family transduce signals from a variety of neurotransmitter and hormone receptors and have therefore been implicated in various functions of the nervous system. Using the Cre/loxP system, we generated mice which lack the genes coding for the alpha subunits of the two main members of the Gq/11 family, gnaq and gna11, selectively in neuronal and glial precursor cells. Mice with defective gnaq and gna11 genes were morphologically normal, but they died shortly after birth. Mice carrying a single gna11 allele survived the early postnatal period but died within 3 to 6 weeks as anorectic dwarfs. In these mice, postnatal proliferation of pituitary somatotroph cells was strongly impaired, and plasma growth hormone (GH) levels were reduced to 15%. Hypothalamic levels of GH-releasing hormone (GHRH), an important stimulator of somatotroph proliferation, were strongly decreased, and exogenous administration of GHRH restored normal proliferation. The hypothalamic effects of ghrelin, a regulator of GHRH production and food intake, were reduced in these mice, suggesting that an impairment of ghrelin receptor signaling might contribute to GHRH deficiency and abnormal eating behavior. Taken together, our findings show that Gq/11 signaling is required for normal hypothalamic function and that impairment of this signaling pathway causes somatotroph hypoplasia, dwarfism, and anorexia.

Understanding endothelin-1 function during craniofacial development in the mouse and zebrafish

Morphogenesis of the face and neck is driven by an intricate relay of signaling molecules and transcription factors organized into hierarchical pathways. The coordinated action of these pathways regulates the development of neural crest cells within the pharyngeal arches, resulting in proper spatiotemporal formation of bone, cartilage, and connective tissue. While the functions of many genes involved in these processes were initially elucidated through the use of knockout technology in the mouse, increasing numbers of zebrafish craniofacial mutants have led to a rapid expansion in the identification of genes involved in craniofacial development. A comparative analysis of signaling pathways involved in these processes between mouse and zebrafish holds the potential not only to pinpoint conserved and therefore crucial gene functions in craniofacial development, but also to rapidly identify and study downstream effectors. These complementary approaches will also allow rapid identification of candidate genes and gene functions disrupted in human craniofacial dysmorphologies. In this brief review, we present a comparative analysis of one molecule involved in craniofacial development, endothelin-1, a small, secreted protein that is crucial for patterning the neural crest cells that give rise to lower jaw and throat structures.

Deletion of the endothelin-A receptor gene within the developing mandible

Signaling from the endothelin-A (Ednra) receptor is responsible for initiating multiple signaling pathways within neural crest cells (NCCs). Loss of this initiation is presumably the basis for the craniofacial defects observed in Ednra-/- embryos. However, it is not known whether continued Ednra signaling in NCC derivatives is required for subsequent development of the lower jaw. To address this question, mice containing loxP recombination sequences flanking a portion of the Ednra gene were bred with transgenic mice that express Cre recombinase under control of a Dlx5/6 enhancer element. We find that while Ednra gene inactivation within the mandibular arch of these Ednra conditional knockout embryos is detectable by embryonic day (E) 10.5, mandibular arch-specific gene expression is normal, as is overall mandible development. These results suggest that while Ednra receptor signaling is crucial for early NCC patterning, subsequent Ednra signaling is not essential for mandible bone development.

Endothelin-1 regulates the dorsoventral branchial arch patterning in mice

Endothelin-1 (ET-1), a 21-amino acid peptide secreted by the epithelium and core mesenchyme in the branchial arches as well as vascular endothelium, is involved in craniofacial and cardiovascular development through endothelin receptor type-A (EdnrA) expressed in the neural crest-derived ectomesenchyme. Here we show that ET-1(-/-) mutant mice exhibit a homeotic-like transformation of the lower jaw to an upper jaw. Most of the maxillary arch-derived components are duplicated and replaced mandibular arch-derived structures, resulting in a mirror image of the upper and lower jaws in the ET-1(-/-) mutant. As for hyoid arch-derivatives, the ventral structures are severely affected in comparison to the dorsal ones in the ET-1(-/-) mutant. Correspondingly, the expression of Dlx5 and Dlx6, Distalless-related homeobox genes determining the ventral identity of the anterior branchial arches, and of the mandibular marker gene Pitx1 is significantly downregulated in the ET-1(-/-) mutant, whereas the expression of Dlx2 and the maxillary marker gene Prx2 is unaffected or rather upregulated. These findings indicate that the ET-1/EdnrA signaling may contribute to the dorsoventral axis patterning of the branchial arch system as a mediator of the regional intercellular interactions.

Effects of G-protein mutations on skin color

A new class of dominant dark skin (Dsk) mutations discovered in a screen of approximately 30,000 mice is caused by increased dermal melanin. We identified three of four such mutations as hypermorphic alleles of Gnaq and Gna11, which encode widely expressed Galphaq subunits, act in an additive and quantitative manner, and require Ednrb. Interactions between Gq and Kit receptor tyrosine kinase signaling can mediate coordinate or independent control of skin and hair color. Our results provide a mechanism that can explain several aspects of human pigmentary variation and show how polymorphism of essential proteins and signaling pathways can affect a single physiologic system.

Endothelin-A receptor-dependent and -independent signaling pathways in establishing mandibular identity

The lower jaw skeleton is derived from cephalic neural crest (CNC) cells that reside in the mandibular region of the first pharyngeal arch. Endothelin-A receptor (Ednra) signaling in crest cells is crucial for their development, as Ednra(-/-) mice are born with severe craniofacial defects resulting in neonatal lethality. In this study, we undertook a more detailed analysis of mandibular arch development in Ednra(-/-) embryos to better understand the cellular and molecular basis for these defects. We show that most lower jaw structures in Ednra(-/-) embryos undergo a homeotic transformation into maxillary-like structures similar to those observed in Dlx5/Dlx6(-/-) embryos, though lower incisors are still present in both mutant embryos. These structural changes are preceded by aberrant expansion of proximal first arch gene expression into the distal arch, in addition to the previously described loss of a Dlx6/Hand2 expression network. However, a small distal Hand2 expression domain remains. Although this distal expression is not dependent on either Ednra or Dlx5/Dlx6 function, it may require one or more GATA factors. Using fate analysis, we show that these distal Hand2-positive cells probably contribute to lower incisor formation. Together, our results suggest that the establishment of a 'mandibular identity' during lower jaw development requires both Ednra-dependent and -independent signaling pathways.

Mouse models to study G-protein-mediated signaling

The G-protein-mediated signaling system has evolved as one of the most widely used transmembrane signaling mechanisms in eukaryotic organisms. Mammalian cells express many G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. This review focuses on recent data from studies in mutant mice, which have elucidated some of the roles of G-protein-mediated signaling in physiology and pathophysiology.