Early Hedgehog signaling from neural to oral epithelium organizes anterior craniofacial development

Selective vulnerability of the primitive meningeal layer to prenatal Smo activation for skull base meningothelial meningioma formation

Somatic activating mutations of smoothened (SMO), a component of the embryonic sonic hedgehog (SHH) signaling pathway, are found in 3-5% of grade I meningiomas, most of them corresponding to meningothelial meningiomas located at the anterior skull base. By generating different developmental stage-specific conditional activations in mice, we define a restricted developmental window during which conditional activation of Smo in Prostaglandin D2-synthase-positive mesoderm-derived meningeal layer of the skull base results in meningothelial meningioma formation. We show a selective vulnerability of the arachnoid from the skull base to Smo activation to initiate tumor development. This prenatal period and specific topography are correlated to the timing and location of SHH signaling involvement in the formation of craniofacial and meninges patterning, strongly corroborating the hypothesis of a developmental origin for Smo-activated meningiomas. Finally, we provide preclinical in vitro evidence of the efficacy of the SMO-inhibitor Sonidegib, supporting further preclinical and clinical evaluation of targeted treatment for refractory SMO-mutant meningiomas.

Essential Role of Nr2f Nuclear Receptors in Patterning the Vertebrate Upper Jaw

The jaw is central to the extensive variety of feeding and predatory behaviors across vertebrates. The bones of the lower but not upper jaw form around an early-developing cartilage template. Whereas Endothelin1 patterns the lower jaw, the factors that specify upper-jaw morphology remain elusive. Here, we identify Nuclear Receptor 2f genes (Nr2fs) as enriched in and required for upper-jaw formation in zebrafish. Combinatorial loss of Nr2fs transforms maxillary components of the upper jaw into lower-jaw-like structures. Conversely, nr2f5 misexpression disrupts lower-jaw development. Genome-wide analyses reveal that Nr2fs repress mandibular gene expression and early chondrogenesis in maxillary precursors. Rescue of lower-jaw defects in endothelin1 mutants by reducing Nr2f dosage further demonstrates that Nr2f expression must be suppressed for normal lower-jaw development. We propose that Nr2fs shape the upper jaw by protecting maxillary progenitors from early chondrogenesis, thus preserving cells for later osteogenesis.

The developmental effects of low-level procymidone towards zebrafish embryos and involved mechanism

Procymidone (PCM), a dicarboximide fungicide, is widely used in agriculture to control plant diseases. In the present study, zebrafish embryos were exposed to PCM at 0, 10, 100 and 1000 ng/L for 72 h, the development and cardiac functioning of larvae were observed and determined. The results showed that hatching rate was significantly decreased in the 1000 ng/L treatment, and pericardial edema rate and spine curvature rate were significantly increased in the 100 and 1000 ng/L groups. The PCM-treated larvae exhibited an increased heart rate as well as arrhythmia, and shortened low jaw. The transcription levels of cardiac development-related genes tbx5, nkx2.5, tnnt2, gata4, myh6, myl7, cdh2, ryr2 were altered, which might be responsible for the cardiac developmental and functioning defects in the larvae. The deformation in bone development might be related with the impaired transcription levels of ihh, shh, bmp2b, bmp4, gh, igf1, sox9, gli2. The activities of Na⁺/K⁺-ATPase and Ca²⁺-ATPase were significantly inhibited by 100 ng/L and 1000 ng/L PCM exposure, which might be a cause for the occurrence of pericardial edema and skeletal deformation. The results of this study will be helpful in evaluating the potential threat of PCM to fish population in the aquatic environment.

Evolvability of the vertebrate craniofacial skeleton

The skull is a vertebrate novelty. Morphological adaptations of the skull are associated with major evolutionary transitions, including the shift to a predatory lifestyle and the ability to masticate while breathing. These adaptations include the chondrocranium, dermatocranium, articulated jaws, primary and secondary palates, internal choanae, the middle ear, and temporomandibular joint. The incredible adaptive diversity of the vertebrate skull indicates an underlying bauplan that promotes evolvability. Comparative studies in craniofacial development suggest that the craniofacial bauplan includes three secondary organizers, two that are bilaterally placed at the Hinge of the developing jaw, and one situated in the midline of the developing face (the FEZ). These organizers regulate tissue interactions between the cranial neural crest, the neuroepithelium, and facial and pharyngeal epithelia that regulate the development and evolvability of the craniofacial skeleton.

In vivo zebrafish morphogenesis shows Cyp26b1 promotes tendon condensation and musculoskeletal patterning in the embryonic jaw

Integrated development of diverse tissues gives rise to a functional, mobile vertebrate musculoskeletal system. However, the genetics and cellular interactions that drive the integration of muscle, tendon, and skeleton are poorly understood. In the vertebrate head, neural crest cells, from which cranial tendons derive, pattern developing muscles just as tendons have been shown to in limb and trunk tissue, yet the mechanisms of this patterning are unknown. From a forward genetic screen, we determined that cyp26b1 is critical for musculoskeletal integration in the ventral pharyngeal arches, particularly in the mandibulohyoid junction where first and second arch muscles interconnect. Using time-lapse confocal analyses, we detail musculoskeletal integration in wild-type and cyp26b1 mutant zebrafish. In wild-type fish, tenoblasts are present in apposition to elongating muscles and condense in discrete muscle attachment sites. In the absence of cyp26b1, tenoblasts are generated in normal numbers but fail to condense into nascent tendons within the ventral arches and, subsequently, muscles project into ectopic locales. These ectopic muscle fibers eventually associate with ectopic tendon marker expression. Genetic mosaic analysis demonstrates that neural crest cells require Cyp26b1 function for proper musculoskeletal development. Using an inhibitor, we find that Cyp26 function is required in a short time window that overlaps the dynamic window of tenoblast condensation. However, cyp26b1 expression is largely restricted to regions between tenoblast condensations during this time. Our results suggest that degradation of RA by this previously undescribed population of neural crest cells is critical to promote condensation of adjacent scxa-expressing tenoblasts and that these condensations are subsequently required for proper musculoskeletal integration.

Zebrafish models of orofacial clefts

Zebrafish is a model organism that affords experimental advantages toward investigating the normal function of genes associated with congenital birth defects. Here we summarize zebrafish studies of genes implicated in orofacial cleft (OFC). The most common use of zebrafish in this context has been to explore the normal function an OFC-associated gene product in craniofacial morphogenesis by inhibiting expression of its zebrafish ortholog. The most frequently deployed method has been to inject embryos with antisense morpholino oligonucleotides targeting the desired transcript. However, improvements in targeted mutagenesis strategies have led to widespread adoption of CRISPR/Cas9 technology. A second application of zebrafish has been for functional assays of gene variants found in OFC patients; such in vivo assays are valuable because the success of in silico methods for testing allele severity has been mixed. Finally, zebrafish have been used to test the tissue specificity of enhancers that harbor single nucleotide polymorphisms associated with risk for OFC. We review examples of each of these approaches in the context of genes that are implicated in syndromic and non-syndromic OFC. Developmental Dynamics 246:897-914, 2017. © 2017 Wiley Periodicals, Inc.

Bardet-Biedl syndrome 3 regulates the development of cranial base midline structures

Bardet-Biedl Syndrome (BBS) is an autosomal recessive disorder and is classified as one of the ciliopathy. The patients manifest a characteristic craniofacial dysmorphology but the effects of Bbs3 deficiency in the developmental process during the craniofacial pathogenesis are still incompletely understood. Here, we analyzed a cranial development of a BBS model Bbs3^-/- mouse. It was previously reported that these mutant mice exhibit a dome-shape cranium. We show that Bbs3^-/- mouse embryos present mid-facial hypoplasia and solitary central upper incisor. Morphologically, these mutant mice show synchondrosis of the cranial base midline due to the failure to fuse in association with loss of intrasphenoidal synchondrosis. The cranial base was laterally expanded and longitudinally shortened. In the developing cartilaginous primordium of cranial base, cells present in the midline were less in Bbs3^-/- embryos. Expression of BBS3 was observed specifically in a cell population lying between condensed ectomesenchyme in the midline and the ventral midbrain at this stage. Finally, siRNA-based knockdown of Bbs3 in ATDC5 cells impaired migration in culture. Our data suggest that BBS3 is required for the development of cranial base via regulation of cell migration toward the midline where they promote the condensation of ectomesenchyme and form the future cartilaginous templates of cranial base.

The Cyprinodon variegatus genome reveals gene expression changes underlying differences in skull morphology among closely related species

BACKGROUND

Understanding the genetic and developmental origins of phenotypic novelty is central to the study of biological diversity. In this study we identify modifications to the expression of genes at four developmental stages that may underlie jaw morphological differences among three closely related species of pupfish (genus Cyprinodon) from San Salvador Island, Bahamas. Pupfishes on San Salvador Island are trophically differentiated and include two endemic species that have evolved jaw morphologies unlike that of any other species in the genus Cyprinodon.

RESULTS

We find that gene expression differs significantly across recently diverged species of pupfish. Genes such as Bmp4 and calmodulin, previously implicated in jaw diversification in African cichlid fishes and Galapagos finches, were not found to be differentially expressed among species of pupfish. Instead we find multiple growth factors and cytokine/chemokine genes to be differentially expressed among these pupfish taxa. These include both genes and pathways known to affect craniofacial development, such as Wnt signaling, as well as novel genes and pathways not previously implicated in craniofacial development. These data highlight both shared and potentially unique sources of jaw diversity in pupfish and those identified in other evolutionary model systems such as Galapagos finches and African cichlids.

CONCLUSIONS

We identify modifications to the expression of genes involved in Wnt signaling, Igf signaling, and the inflammation response as promising avenues for future research. Our project provides insight into the magnitude of gene expression changes contributing to the evolution of morphological novelties, such as jaw structure, in recently diverged pupfish species.

Molecular control of gut formation in the spider Parasteatoda tepidariorum

The development of a digestive system is an essential feature of bilaterians. Studies of the molecular control of gut formation in arthropods have been studied in detail in the fruit fly Drosophila melanogaster. However, little is known in other arthropods, especially in noninsect arthropods. To better understand the evolution of arthropod alimentary system, we investigate the molecular control of gut development in the spider Parasteatoda tepidariorum (Pt), the primary chelicerate model species for developmental studies. Orthologs of the ectodermal genes Pt-wingless (Pt-wg) and Pt-hedgehog (Pt-hh), of the endodermal genes, Pt-serpent (Pt-srp) and Pt-hepatocyte-nuclear factor-4 (Pt-hnf4) and of the mesodermal gene Pt-twist (Pt-twi) are expressed in the same germ layers during spider gut development as in D. melanogaster. Thus, our expression data suggest that the downstream molecular components involved in gut development in arthropods are conserved. However, Pt-forkhead (Pt-fkh) expression and function in spiders is considerably different from its D. melanogaster ortholog. Pt-fkh is expressed before gastrulation in a cell population that gives rise to endodermal and mesodermal precursors, suggesting a possible role for this factor in specification of both germ layers. To test this hypothesis, we knocked down Pt-fkh via RNA interference. Pt-fkh RNAi embryos not only fail to develop a proper gut, but also lack the mesodermal Pt-twi expressing cells. Thus, in spiders Pt-fkh specifies endodermal and mesodermal germ layers. We discuss the implications of these findings for the evolution and development of gut formation in Ecdysozoans.

Growth factor signaling alters the morphology of the zebrafish ethmoid plate

Craniofacial development relies on coordinated tissue interactions that allow for patterning and growth of the face. We know a priori that the Wingless, fibroblast growth factor, Hedgehog and transforming growth factor-beta growth factor signaling pathways are required for the development of the face, but how they contribute to the shape of the face is largely untested. Here, we test how each signaling pathway contributes to the overall morphology of the zebrafish anterior neurocranium. We tested the contribution of each signaling pathway to the development of the ethmoid plate during three distinct time periods: the time of neural crest migration [10 hour post fertilization (hpf)]; once the neural crest is resident in the face (20 hpf); and finally at the time at which the cartilaginous condensations are being initiated (48 hpf). Using geometric morphometric analysis, we conclude that each signaling pathway contributes to the shape, size and morphology of the ethmoid plate in a dose-, and time-dependent fashion.

Developmental toxicity of the common UV filter, benophenone-2, in zebrafish embryos

Benozophenone (BP) type UV filters are extensively used in the personal care products to provide protection against the harmful effects of UV radiation. BPs are one of the primary components in the UV filter family, in which benophenone-2 (BP2) is widely used as a UV filter reagent in the sunscreen. Humans used these personal care products directly on skin and the chemicals will be washed away to the water system. BP2 has been identified as one of the endocrine disruptor chemicals, which can inference the synthesis, metabolism, and action of endogenous hormones. Environmentally, it has been found to contaminate water worldwide. In this study, we aimed to unfold the possible developmental toxicology of this chemical. Zebrafish are used as the screening model to perform in situ hybridization staining to investigate the effects of BP2 on segmentation, brain regionalization, and facial formation at four developmental stages (10-12 somite, prim-5, 2 and 5 days post-fertilization). Results showed 40 μM (9.85 mg L^-1) or above BP2 exposure in zebrafish embryos for 5 days resulted in lipid accumulation in the yolk sac and facial malformation via affecting the lipid processing and the expression of cranial neural crest cells respectively. To conclude, the study alarmed its potential developmental toxicities at high dosage exposure.

Signalling pathways in trophic skeletal development and morphogenesis: Insights from studies on teleost fish

During the development of the vertebrate feeding apparatus, a variety of complicated cellular and molecular processes participate in the formation and integration of individual skeletal elements. The molecular mechanisms regulating the formation of skeletal primordia and their development into specific morphological structures are tightly controlled by a set of interconnected signalling pathways. Some of these pathways, such as Bmp, Hedgehog, Notch and Wnt, are long known for their pivotal roles in craniofacial skeletogenesis. Studies addressing the functional details of their components and downstream targets, the mechanisms of their interactions with other signals as well as their potential roles in adaptive morphological divergence, are currently attracting considerable attention. An increasing number of signalling pathways that had previously been described in different biological contexts have been shown to be important in the regulation of jaw skeletal development and morphogenesis. In this review, I provide an overview of signalling pathways involved in trophic skeletogenesis emphasizing studies of the most species-rich group of vertebrates, the teleost fish, which through their evolutionary history have undergone repeated episodes of spectacular trophic diversification.

Inhibition of the 3-hydroxy-3-methyl-glutaryl-CoA reductase induces orofacial defects in zebrafish

BACKGROUND

Orofacial clefts (OFCs) are common birth defects, which include a range of disorders with a complex etiology affecting formation of craniofacial structures. Some forms of syndromic OFCs are produced by defects in the cholesterol pathway. The principal enzyme of the cholesterol pathway is the 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCR). Our aim is to study whether defects of HMGCR function would produce orofacial malformation similar to those found in disorders of cholesterol synthesis.

METHODS

We used zebrafish hmgcrb mutants and HMGCR inhibition assay using atorvastatin during early and late stages of orofacial morphogenesis in zebrafish. To describe craniofacial phenotypes, we stained cartilage and bone and performed in situ hybridization using known craniofacial markers. Also, we visualized neural crest cell migration in a transgenic fish.

RESULTS

Our results showed that mutants displayed loss of cartilage and diminished orofacial outgrowth, and in some cases palatal cleft. Late treatments with statin show a similar phenotype. Affected-siblings displayed a moderate phenotype, whereas early-treated embryos had a minor cleft. We found reduced expression of the downstream component of Sonic Hedgehog-signaling gli1 in ventral brain, oral ectoderm, and pharyngeal endoderm in mutants and in late atorvastatin-treated embryos.

CONCLUSION

Our results suggest that HMGCR loss-of-function primarily affects postmigratory cranial neural crest cells through abnormal Sonic Hedgehog signaling, probably induced by reduction in metabolites of the cholesterol pathway. Malformation severity correlates with the grade of HMGCR inhibition, developmental stage of its disruption, and probably with availability of maternal lipids. Together, our results might help to understand the spectrum of orofacial phenotypes found in cholesterol synthesis disorders. Birth Defects Research (Part A) 106:814-830, 2016. © 2016 Wiley Periodicals, Inc.

Formation of a "Pre-mouth Array" from the Extreme Anterior Domain Is Directed by Neural Crest and Wnt/PCP Signaling

The mouth arises from the extreme anterior domain (EAD), a region where the ectoderm and endoderm are directly juxtaposed. Here, we identify a "pre-mouth array" in Xenopus that forms soon after the cranial neural crest has migrated to lie on either side of the EAD. Initially, EAD ectoderm comprises a wide and short epithelial mass that becomes narrow and tall with cells and nuclei changing shape, a characteristic of convergent extension. The resulting two rows of cells-the pre-mouth array-later split down the midline to surround the mouth opening. Neural crest is essential for convergent extension and likely signals to the EAD through the Wnt/planar cell polarity (PCP) pathway. Fzl7 receptor is locally required in EAD ectoderm, while Wnt11 ligand is required more globally. Indeed, heterologous cells expressing Wnt11 can elicit EAD convergent extension. The study reveals a precise cellular mechanism that positions and contributes to the future mouth.

Ihha induces hybrid cartilage-bone cells during zebrafish jawbone regeneration

The healing of bone often involves a cartilage intermediate, yet how such cartilage is induced and utilized during repair is not fully understood. By studying a model of large-scale bone regeneration in the lower jaw of adult zebrafish, we show that chondrocytes are crucial for generating thick bone during repair. During jawbone regeneration, we find that chondrocytes co-express genes associated with osteoblast differentiation and produce extensive mineralization, which is in marked contrast to the behavior of chondrocytes during facial skeletal development. We also identify the likely source of repair chondrocytes as a population of Runx2(+)/Sp7(-) cells that emanate from the periosteum, a tissue that normally contributes only osteoblasts during homeostasis. Analysis of Indian hedgehog homolog a (ihha) mutants shows that the ability of periosteal cells to generate cartilage in response to injury depends on a repair-specific role of Ihha in the induction as opposed to the proliferation of chondrocytes. The large-scale regeneration of the zebrafish jawbone thus employs a cartilage differentiation program distinct from that seen during development, with the bone-forming potential of repair chondrocytes potentially due to their derivation from osteogenic cells in the periosteum.

An Fgf-Shh signaling hierarchy regulates early specification of the zebrafish skull

The neurocranium generates most of the craniofacial skeleton and consists of prechordal and postchordal regions. Although development of the prechordal is well studied, little is known of the postchordal region. Here we characterize a signaling hierarchy necessary for postchordal neurocranial development involving Fibroblast growth factor (Fgf) signaling for early specification of mesodermally-derived progenitor cells. The expression of hyaluron synthetase 2 (has2) in the cephalic mesoderm requires Fgf signaling and Has2 function, in turn, is required for postchordal neurocranial development. While Hedgehog (Hh)-deficient embryos also lack a postchordal neurocranium, this appears primarily due to a later defect in chondrocyte differentiation. Inhibitor studies demonstrate that postchordal neurocranial development requires early Fgf and later Hh signaling. Collectively, our results provide a mechanistic understanding of early postchordal neurocranial development and demonstrate a hierarchy of signaling between Fgf and Hh in the development of this structure.

Pdgfra and Pdgfrb genetically interact during craniofacial development

BACKGROUND

One of the most prevalent congenital birth defects is cleft palate. The palatal skeleton is derived from the cranial neural crest and platelet-derived growth factors (Pdgf) are critical in palatogenesis. Of the two Pdgf receptors, pdgfra is required for neural crest migration and palatogenesis. However, the role pdgfrb plays in the neural crest, or whether pdgfra and pdgfrb interact during palatogenesis is unclear.

RESULTS

We find that pdgfrb is dispensable for craniofacial development in zebrafish. However, the palatal defect in pdgfra;pdgfrb double mutants is significantly more severe than in pdgfra single mutants. Data in mouse suggest this interaction is conserved and that neural crest requires both genes. In zebrafish, pdgfra and pdgfrb are both expressed by neural crest within the pharyngeal arches, and pharmacological analyses demonstrate Pdgf signaling is required at these times. While neither proliferation nor cell death appears affected, time-lapsed confocal analysis of pdgfra;pdgfrb mutants shows a failure of proper neural crest condensation during palatogenesis.

CONCLUSIONS

We provide data showing that pdgfra and pdgfrb interact during palatogenesis in both zebrafish and mouse. In zebrafish, this interaction affects proper condensation of maxillary neural crest cells, revealing a previously unknown interaction between Pdgfra and Pdgfrb during palate formation. Developmental Dynamics 245:641-652, 2016. © 2016 Wiley Periodicals, Inc.

Investigation of the effects of estrogen on skeletal gene expression during zebrafish larval head development

The development of craniofacial skeletal structures requires well-orchestrated tissue interactions controlled by distinct molecular signals. Disruptions in normal function of these molecular signals have been associated with a wide range of craniofacial malformations. A pathway mediated by estrogens is one of those molecular signals that plays role in formation of bone and cartilage including craniofacial skeletogenesis. Studies in zebrafish have shown that while higher concentrations of 17-β estradiol (E 2) cause severe craniofacial defects, treatment with lower concentrations result in subtle changes in head morphology characterized with shorter snouts and flatter faces. The molecular basis for these morphological changes, particularly the subtle skeletal effects mediated by lower E 2 concentrations, remains unexplored. In the present study we address these effects at a molecular level by quantitative expression analysis of sets of candidate genes in developing heads of zebrafish larvae treated with two different E 2 concentrations. To this end, we first validated three suitable reference genes, ppia2, rpl8 and tbp, to permit sensitive quantitative real-time PCR analysis. Next, we profiled the expression of 28 skeletogenesis-associated genes that potentially respond to estrogen signals and play role in craniofacial development. We found E 2 mediated differential expression of genes involved in extracellular matrix (ECM) remodelling, mmp2/9/13, sparc and timp2a, as well as components of skeletogenic pathways, bmp2a, erf, ptch1/2, rankl, rarab and sfrp1a. Furthermore, we identified a co-expressed network of genes, including cpn1, dnajc3, esr1, lman1, rrbp1a, ssr1 and tram1 with a stronger inductive response to a lower dose of E 2 during larval head development.

Pathogenesis of POLR1C-dependent Type 3 Treacher Collins Syndrome revealed by a zebrafish model

Treacher Collins Syndrome (TCS) is a rare congenital birth disorder (1 in 50,000 live births) characterized by severe craniofacial defects, including the downward slanting palpebral fissures, hypoplasia of the facial bones, and cleft palate (CP). Over 90% of patients with TCS have a mutation in the TCOF1 gene. However, some patients exhibit mutations in two new causative genes, POLR1C and POLR1D, which encode subunits of RNA polymerases I and III, that affect ribosome biogenesis. In this study, we examine the role of POLR1C in TCS using zebrafish as a model system. Our data confirmed that polr1c is highly expressed in the facial region, and dysfunction of this gene by knockdown or knock-out resulted in mis-expression of neural crest cells during early development that leads to TCS phenotype. Next generation sequencing and bioinformatics analysis of the polr1c mutants further demonstrated the up-regulated p53 pathway and predicted skeletal disorders. Lastly, we partially rescued the TCS facial phenotype in the background of p53 mutants, which supported the hypothesis that POLR1C-dependent type 3 TCS is associated with the p53 pathway.

Hedgehog receptor function during craniofacial development

The Hedgehog signalling pathway plays a fundamental role in orchestrating normal craniofacial development in vertebrates. In particular, Sonic hedgehog (Shh) is produced in three key domains during the early formation of the head; neuroectoderm of the ventral forebrain, facial ectoderm and the pharyngeal endoderm; with signal transduction evident in both ectodermal and mesenchymal tissue compartments. Shh signalling from the prechordal plate and ventral midline of the diencephalon is required for appropriate division of the eyefield and forebrain, with mutation in a number of pathway components associated with Holoprosencephaly, a clinically heterogeneous developmental defect characterized by a failure of the early forebrain vesicle to divide into distinct halves. In addition, signalling from the pharyngeal endoderm and facial ectoderm plays an essential role during development of the face, influencing cranial neural crest cells that migrate into the early facial processes. In recent years, the complexity of Shh signalling has been highlighted by the identification of multiple novel proteins that are involved in regulating both the release and reception of this protein. Here, we review the contributions of Shh signalling during early craniofacial development, focusing on Hedgehog receptor function and describing the consequences of disruption for inherited anomalies of this region in both mouse models and human populations.

Developmental basis of phenotypic integration in two Lake Malawi cichlids

Background

Cichlid fishes from the Rift Lakes of East Africa have undergone the most spectacular adaptive radiations in vertebrate history. Eco-morphological adaptations in lakes Victoria, Malawi and Tanganyika have resulted in a vast array of skull shapes and sizes, yet primary axes of morphological variation are conserved in all three radiations, prominently including the size of the preorbital region of the skull. This conserved pattern suggests that development may constrain the trajectories of cichlid head morphological evolution.

Results

Here, we (1) present a comparative analysis of adult head morphology in two sand-dweller cichlids from Lake Malawi with preorbital size differences representative of the main axis of variation among the three lakes and (2) analyze the ontogeny of shape and size differences by focusing on known developmental modules throughout the head. We find that (1) developmental differences between the two species correlate with known developmental modules; (2) differences in embryonic cartilage development result in phenotypically integrated changes among all bones derived from a single cartilage, while differences in dermal bone development tend to influence isolated regions within a bone; and lastly (3) species-specific morphologies appear in the embryo as subtle differences, which become progressively amplified throughout ontogeny. We propose that this amplification takes place at skeletal growth zones, the locations and shapes of which are patterned during embryogenesis.

Conclusions

This study is the most anatomically comprehensive analysis of the developmental differences underlying cichlid skull evolution in the Rift Lakes of East Africa. The scale of our analysis reveals previously unnoticed correlations between developmental modules and patterns of phenotypic integration. We propose that the primary axes of morphological variation among East African cichlid adaptive radiations are constrained by the hierarchical modularity of the teleost head skeleton.

The chick embryo as a model for the effects of prenatal exposure to alcohol on craniofacial development

Prenatal exposure to ethanol results in fetal alcohol spectrum disorder (FASD), a syndrome characterised by a broad range of clinical manifestations including craniofacial dysmorphologies and neurological defects. The characterisation of the mechanisms by which ethanol exerts its teratogenic effects is difficult due to the pleiotropic nature of its actions. Different experimental model systems have been employed to investigate the aetiology of FASD. Here, I will review studies using these different model organisms that have helped to elucidate how ethanol causes the craniofacial abnormalities characteristic of FASD. In these studies, ethanol was found to impair the prechordal plate-an important embryonic signalling centre-during gastrulation and to negatively affect the induction, migration and survival of the neural crest, a cell population that generates the cartilage and most of the bones of the skull. At the cellular level, ethanol appears to inhibit Sonic hedgehog signalling, alter levels of retionoic acid activity, trigger a Ca(2+)-CamKII-dependent pathway that antagonises WNT signalling, affect cytoskeletal dynamics and increase oxidative stress. Embryos of the domestic chick Gallus gallus domesticus have played a central role in developing a working model for the effects of ethanol on craniofacial development because they are easily accessible and because key steps in craniofacial development are particularly well established in the avian embryo. I will finish this review by highlighting some potential future avenues of fetal alcohol research.

Developmental mechanisms underlying variation in craniofacial disease and evolution

Craniofacial disease phenotypes exhibit significant variation in penetrance and severity. Although many genetic contributions to phenotypic variation have been identified, genotype-phenotype correlations remain imprecise. Recent work in evolutionary developmental biology has exposed intriguing developmental mechanisms that potentially explain incongruities in genotype-phenotype relationships. This review focuses on two observations from work in comparative and experimental animal model systems that highlight how development structures variation. First, multiple genetic inputs converge on relatively few developmental processes. Investigation of when and how variation in developmental processes occurs may therefore help predict potential genetic interactions and phenotypic outcomes. Second, genetic mutation is typically associated with an increase in phenotypic variance. Several models outlining developmental mechanisms underlying mutational increases in phenotypic variance are discussed using Satb2-mediated variation in jaw size as an example. These data highlight development as a critical mediator of genotype-phenotype correlations. Future research in evolutionary developmental biology focusing on tissue-level processes may help elucidate the "black box" between genotype and phenotype, potentially leading to novel treatment, earlier diagnoses, and better clinical consultations for individuals affected by craniofacial anomalies.

Visualization of Chondrocyte Intercalation and Directional Proliferation via Zebrabow Clonal Cell Analysis in the Embryonic Meckel's Cartilage

Development of the vertebrate craniofacial structures requires precise coordination of cell migration, proliferation, adhesion and differentiation. Patterning of the Meckel's cartilage, a first pharyngeal arch derivative, involves the migration of cranial neural crest (CNC) cells and the progressive partitioning, proliferation and organization of differentiated chondrocytes. Several studies have described CNC migration during lower jaw morphogenesis, but the details of how the chondrocytes achieve organization in the growth and extension of Meckel's cartilage remains unclear. The sox10 restricted and chemically induced Cre recombinase-mediated recombination generates permutations of distinct fluorescent proteins (RFP, YFP and CFP), thereby creating a multi-spectral labeling of progenitor cells and their progeny, reflecting distinct clonal populations. Using confocal time-lapse photography, it is possible to observe the chondrocytes behavior during the development of the zebrafish Meckel's cartilage. Multispectral cell labeling enables scientists to demonstrate extension of the Meckel's chondrocytes. During extension phase of the Meckel's cartilage, which prefigures the mandible, chondrocytes intercalate to effect extension as they stack in an organized single-cell layered row. Failure of this organized intercalating process to mediate cell extension provides the cellular mechanistic explanation for hypoplastic mandible that we observe in mandibular malformations.

Zebrafish Craniofacial Development: A Window into Early Patterning

The formation of the face and skull involves a complex series of developmental events mediated by cells derived from the neural crest, endoderm, mesoderm, and ectoderm. Although vertebrates boast an enormous diversity of adult facial morphologies, the fundamental signaling pathways and cellular events that sculpt the nascent craniofacial skeleton in the embryo have proven to be highly conserved from fish to man. The zebrafish Danio rerio, a small freshwater cyprinid fish from eastern India, has served as a popular model of craniofacial development since the 1990s. Unique strengths of the zebrafish model include a simplified skeleton during larval stages, access to rapidly developing embryos for live imaging, and amenability to transgenesis and complex genetics. In this chapter, we describe the anatomy of the zebrafish craniofacial skeleton; its applications as models for the mammalian jaw, middle ear, palate, and cranial sutures; the superior imaging technology available in fish that has provided unprecedented insights into the dynamics of facial morphogenesis; the use of the zebrafish to decipher the genetic underpinnings of craniofacial biology; and finally a glimpse into the most promising future applications of zebrafish craniofacial research.

Cdon promotes neural crest migration by regulating N-cadherin localization

Neural crest cells (NCCs) are essential embryonic progenitor cells that are unique to vertebrates and form a remarkably complex and coordinated system of highly motile cells. Migration of NCCs occurs along specific pathways within the embryo in response to both environmental cues and cell-cell interactions within the neural crest population. Here, we demonstrate a novel role for the putative Sonic hedgehog (Shh) receptor and cell adhesion regulator, cdon, in zebrafish neural crest migration. cdon is expressed in developing premigratory NCCs but is downregulated once the cells become migratory. Knockdown of cdon results in aberrant migration of trunk NCCs: crestin positive cells can emigrate out of the neural tube but stall shortly after the initiation of migration. Live cell imaging analysis demonstrates reduced directedness of migration, increased velocity and mispositioned cell protrusions. In addition, transplantation analysis suggests that cdon is required cell-autonomously for directed NCC migration in the trunk. Interestingly, N-cadherin is mislocalized following cdon knockdown suggesting that the role of cdon in NCCs is to regulate N-cadherin localization. Our results reveal a novel role for cdon in zebrafish neural crest migration, and suggest a mechanism by which Cdon is required to localize N-cadherin to the cell membrane in migratory NCCs for directed migration.

prdm1a functions upstream of itga5 in zebrafish craniofacial development

Cranial neural crest cells are specified and migrate into the pharyngeal arches where they subsequently interact with the surrounding environment. Signaling and transcription factors, such as prdm1a regulate this interaction, but it remains unclear which specific factors are required for posterior pharyngeal arch development. Previous analysis suggests that prdm1a is required for posterior ceratobranchial cartilages in zebrafish and microarray analysis between wildtype and prdm1a mutants at 25 h post fertilization demonstrated that integrin α5 (itga5) is differentially expressed in prdm1a mutants. Here, we further investigate the interaction between prdm1a and itga5 in zebrafish craniofacial development. In situ hybridization for itga5 demonstrates that expression of itga5 is decreased in prdm1a mutants between 18 and 31 h post fertilization and itga5 expression overlaps with prdm1a in the posterior arches, suggesting a temporal window for interaction. Double mutants for prdm1a;itga5 have an additive viscerocranium phenotype more similar to prdm1a mutants, suggesting that prdm1a acts upstream of itga5. Consistent with this, loss of posterior pharyngeal arch expression of dlx2a, ceratobranchial cartilages 2-5, and cell proliferation in prdm1a mutants can be rescued with itga5 mRNA injection. Taken together, these data suggest that prdm1a acts upstream of itga5 and are both necessary for posterior pharyngeal arch development in zebrafish.

Functional analysis of SPECC1L in craniofacial development and oblique facial cleft pathogenesis

BACKGROUND

Oblique facial clefts, also known as Tessier clefts, are severe orofacial clefts, the genetic basis of which is poorly understood. Human genetics studies revealed that disruption in SPECC1L resulted in oblique facial clefts, demonstrating that oblique facial cleft malformation has a genetic basis. An important step toward innovation in treatment of oblique facial clefts would be improved understanding of its genetic pathogenesis. The authors exploit the zebrafish model to elucidate the function of SPECC1L by studying its homolog, specc1lb.

METHODS

Gene and protein expression analysis was carried out by reverse-transcriptase polymerase chain reaction and immunohistochemistry staining. Morpholino knockdown, mRNA rescue, lineage tracing and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assays were performed for functional analysis.

RESULTS

Expression of specc1lb was detected in epithelia juxtaposed to chondrocytes. Knockdown of specc1lb resulted in bilateral clefts between median and lateral elements of the ethmoid plate, structures analogous to the frontonasal process and the paired maxillary processes. Lineage tracing analysis revealed that cranial neural crest cells contributing to the frontonasal prominence failed to integrate with the maxillary prominence populations. Cells contributing to lower jaw structures were able to migrate to their destined pharyngeal segment but failed to converge to form mandibular elements.

CONCLUSIONS

These results demonstrate that specc1lb is required for integration of frontonasal and maxillary elements and convergence of mandibular prominences. The authors confirm the role of SPECC1L in orofacial cleft pathogenesis in the first animal model of Tessier cleft, providing morphogenetic insight into the mechanisms of normal craniofacial development and oblique facial cleft pathogenesis.

Holmgren's principle of delamination during fin skeletogenesis

During fin morphogenesis, several mesenchyme condensations occur to give rise to the dermal skeleton. Although each of them seems to create distinctive and unique structures, they all follow the premises of the same morphogenetic principle. Holmgren's principle of delamination was first proposed to describe the morphogenesis of skeletal elements of the cranium, but Jarvik extended it to the development of the fin exoskeleton. Since then, some cellular or molecular explanations, such as the "flypaper" model (Thorogood et al.), or the evolutionary description by Moss, have tried to clarify this topic. In this article, we review new data from zebrafish studies to meet these criteria described by Holmgren and other authors. The variety of cell lineages involved in these skeletogenic condensations sheds light on an open discussion of the contributions of mesoderm- versus neural crest-derived cell lineages to the development of the head and trunk skeleton. Moreover, we discuss emerging molecular studies that are disclosing conserved regulatory mechanisms for dermal skeletogenesis and similarities during fin development and regeneration, which may have important implications in the potential use of the zebrafish fin as a model for regenerative medicine.

Wnt signaling interacts with bmp and edn1 to regulate dorsal-ventral patterning and growth of the craniofacial skeleton

Craniofacial development requires signals from epithelia to pattern skeletogenic neural crest (NC) cells, such as the subdivision of each pharyngeal arch into distinct dorsal (D) and ventral (V) elements. Wnt signaling has been implicated in many aspects of NC and craniofacial development, but its roles in D-V arch patterning remain unclear. To address this we blocked Wnt signaling in zebrafish embryos in a temporally-controlled manner, using transgenics to overexpress a dominant negative Tcf3, (dntcf3), (Tg(hsp70I:tcf3-GFP), or the canonical Wnt inhibitor dickkopf1 (dkk1), (Tg(hsp70i:dkk1-GFP) after NC migration. In dntcf3 transgenics, NC cells in the ventral arches of heat-shocked embryos show reduced proliferation, expression of ventral patterning genes (hand2, dlx3b, dlx5a, msxe), and ventral cartilage differentiation (e.g. lower jaws). These D-V patterning defects resemble the phenotypes of zebrafish embryos lacking Bmp or Edn1 signaling, and overexpression of dntcf3 dramatically reduces expression of a subset of Bmp receptors in the arches. Addition of ectopic BMP (or EDN1) protein partially rescues ventral development and expression of dlx3b, dlx5a, and msxe in Wnt signaling-deficient embryos, but surprisingly does not rescue hand2 expression. Thus Wnt signaling provides ventralizing patterning cues to arch NC cells, in part through regulation of Bmp and Edn1 signaling, but independently regulates hand2. Similarly, heat-shocked dkk1+ embryos exhibit ventral arch reductions, but also have mandibular clefts at the ventral midline not seen in dntcf3+ embryos. Dkk1 is expressed in pharyngeal endoderm, and cell transplantation experiments reveal that dntcf3 must be overexpressed in pharyngeal endoderm to disrupt D-V arch patterning, suggesting that distinct endodermal roles for Wnts and Wnt antagonists pattern the developing skeleton.

Craniofacial abnormalities are among the most common birth defects. Understanding the molecular mechanisms underlying craniofacial disorders is crucial for developing treatment strategies. Much of the craniofacial skeleton arises from specialized embryonic structures known as pharyngeal arches. Patterning of these arches requires precise spatial and temporal expression of multiple genes, which is coordinated between tissues by secreted signals. Wnts are secreted ligands expressed throughout the pharyngeal arches yet their role in craniofacial patterning remains unclear. In this study we examine the role of Wnts in craniofacial patterning using transgenic zebrafish to inhibit downstream Wnt signaling. We show that Wnt signaling deficient embryos have lower jaw specific defects, which strongly resembles loss-of-function phenotypes in both the Bmp and Edn1 signaling pathways. Through rescue experiments we find that Wnts are upstream regulators of both Bmp and Edn1 signaling. We thus have uncovered a crucial requirement for Wnt signaling in craniofacial patterning.

Gene-ethanol interactions underlying fetal alcohol spectrum disorders

Fetal alcohol spectrum disorders (FASD) is an umbrella term that describes a diverse set of ethanol-induced defects. The phenotypic variation is generated by numerous factors, including timing and dosage of ethanol exposure as well as genetic background. We are beginning to learn about how the concentration, duration, and timing of ethanol exposure mediate variability within ethanol teratogenesis. However, little is known about the genetic susceptibilities in FASD. Studies of FASD animal models are beginning to implicate a number of susceptibility genes that are involved in various pathways. Here we review the current literature that focuses on the genetic predispositions in FASD.

Analyzing craniofacial morphogenesis in zebrafish using 4D confocal microscopy

Time-lapse imaging is a technique that allows for the direct observation of the process of morphogenesis, or the generation of shape. Due to their optical clarity and amenability to genetic manipulation, the zebrafish embryo has become a popular model organism with which to perform time-lapse analysis of morphogenesis in living embryos. Confocal imaging of a live zebrafish embryo requires that a tissue of interest is persistently labeled with a fluorescent marker, such as a transgene or injected dye. The process demands that the embryo is anesthetized and held in place in such a way that healthy development proceeds normally. Parameters for imaging must be set to account for three-dimensional growth and to balance the demands of resolving individual cells while getting quick snapshots of development. Our results demonstrate the ability to perform long-term in vivo imaging of fluorescence-labeled zebrafish embryos and to detect varied tissue behaviors in the cranial neural crest that cause craniofacial abnormalities. Developmental delays caused by anesthesia and mounting are minimal, and embryos are unharmed by the process. Time-lapse imaged embryos can be returned to liquid medium and subsequently imaged or fixed at later points in development. With an increasing abundance of transgenic zebrafish lines and well-characterized fate mapping and transplantation techniques, imaging any desired tissue is possible. As such, time-lapse in vivo imaging combines powerfully with zebrafish genetic methods, including analyses of mutant and microinjected embryos.

Mutations in CSPP1 cause primary cilia abnormalities and Joubert syndrome with or without Jeune asphyxiating thoracic dystrophy

Joubert syndrome (JBTS) is a recessive ciliopathy in which a subset of affected individuals also have the skeletal dysplasia Jeune asphyxiating thoracic dystrophy (JATD). Here, we have identified biallelic truncating CSPP1 (centrosome and spindle pole associated protein 1) mutations in 19 JBTS-affected individuals, four of whom also have features of JATD. CSPP1 mutations explain ∼5% of JBTS in our cohort, and despite truncating mutations in all affected individuals, the range of phenotypic severity is broad. Morpholino knockdown of cspp1 in zebrafish caused phenotypes reported in other zebrafish models of JBTS (curved body shape, pronephric cysts, and cerebellar abnormalities) and reduced ciliary localization of Arl13b, further supporting loss of CSPP1 function as a cause of JBTS. Fibroblasts from affected individuals with CSPP1 mutations showed reduced numbers of primary cilia and/or short primary cilia, as well as reduced axonemal localization of ciliary proteins ARL13B and adenylyl cyclase III. In summary, CSPP1 mutations are a major cause of the Joubert-Jeune phenotype in humans; however, the mechanism by which these mutations lead to both JBTS and JATD remains unknown.

Visualization of craniofacial development in the sox10: kaede transgenic zebrafish line using time-lapse confocal microscopy

Vertebrate palatogenesis is a highly choreographed and complex developmental process, which involves migration of cranial neural crest (CNC) cells, convergence and extension of facial prominences, and maturation of the craniofacial skeleton. To study the contribution of the cranial neural crest to specific regions of the zebrafish palate a sox10: kaede transgenic zebrafish line was generated. Sox10 provides lineage restriction of the kaede reporter protein to the neural crest, thereby making the cell labeling a more precise process than traditional dye or reporter mRNA injection. Kaede is a photo-convertible protein that turns from green to red after photo activation and makes it possible to follow cells precisely. The sox10: kaede transgenic line was used to perform lineage analysis to delineate CNC cell populations that give rise to maxillary versus mandibular elements and illustrate homology of facial prominences to amniotes. This protocol describes the steps to generate a live time-lapse video of a sox10: kaede zebrafish embryo. Development of the ethmoid plate will serve as a practical example. This protocol can be applied to making a time-lapse confocal recording of any kaede or similar photoconvertible reporter protein in transgenic zebrafish. Furthermore, it can be used to capture not only normal, but also abnormal development of craniofacial structures in the zebrafish mutants.

Pdgfra protects against ethanol-induced craniofacial defects in a zebrafish model of FASD

Human birth defects are highly variable and this phenotypic variability can be influenced by both the environment and genetics. However, the synergistic interactions between these two variables are not well understood. Fetal alcohol spectrum disorders (FASD) is the umbrella term used to describe the wide range of deleterious outcomes following prenatal alcohol exposure. Although FASD are caused by prenatal ethanol exposure, FASD are thought to be genetically modulated, although the genes regulating sensitivity to ethanol teratogenesis are largely unknown. To identify potential ethanol-sensitive genes, we tested five known craniofacial mutants for ethanol sensitivity: cyp26b1, gata3, pdgfra, smad5 and smoothened. We found that only platelet-derived growth factor receptor alpha (pdgfra) interacted with ethanol during zebrafish craniofacial development. Analysis of the PDGF family in a human FASD genome-wide dataset links PDGFRA to craniofacial phenotypes in FASD, prompting a mechanistic understanding of this interaction. In zebrafish, untreated pdgfra mutants have cleft palate due to defective neural crest cell migration, whereas pdgfra heterozygotes develop normally. Ethanol-exposed pdgfra mutants have profound craniofacial defects that include the loss of the palatal skeleton and hypoplasia of the pharyngeal skeleton. Furthermore, ethanol treatment revealed latent haploinsufficiency, causing palatal defects in ∼62% of pdgfra heterozygotes. Neural crest apoptosis partially underlies these ethanol-induced defects in pdgfra mutants, demonstrating a protective role for Pdgfra. This protective role is mediated by the PI3K/mTOR pathway. Collectively, our results suggest a model where combined genetic and environmental inhibition of PI3K/mTOR signaling leads to variability within FASD.

Full transcriptome analysis of early dorsoventral patterning in zebrafish

Understanding the molecular interactions that lead to the establishment of the major body axes during embryogenesis is one of the main goals of developmental biology. Although the past two decades have revolutionized our knowledge about the genetic basis of these patterning processes, the list of genes involved in axis formation is unlikely to be complete. In order to identify new genes involved in the establishment of the dorsoventral (DV) axis during early stages of zebrafish embryonic development, we employed next generation sequencing for full transcriptome analysis of normal embryos and embryos lacking overt DV pattern. A combination of different statistical approaches yielded 41 differentially expressed candidate genes and we confirmed by in situ hybridization the early dorsal expression of 32 genes that are transcribed shortly after the onset of zygotic transcription. Although promoter analysis of the validated genes suggests no general enrichment for the binding sites of early acting transcription factors, most of these genes carry “bivalent” epigenetic histone modifications at the time when zygotic transcription is initiated, suggesting a “poised” transcriptional status. Our results reveal some new candidates of the dorsal gene regulatory network and suggest that a plurality of the earliest upregulated genes on the dorsal side have a role in the modulation of the canonical Wnt pathway.

Requirement for frzb and fzd7a in cranial neural crest convergence and extension mechanisms during zebrafish palate and jaw morphogenesis

Regulation of convergence and extension by wnt-frizzled signaling is a common theme in embryogenesis. This study examines the functional requirements of frzb and fzd7a in convergence and extension mechanisms during craniofacial development. Using a morpholino knockdown approach, we found that frzb and fzd7a are dispensable for directed migration of the bilateral trabeculae, but necessary for the convergence and extension of the palatal elements, where the extension process is mediated by chondrocyte proliferation, morphologic change and intercalation. In contrast, frzb and fzd7a are required for convergence of the mandibular prominences, where knockdown of either frzb or fzd7a resulted in complete loss of lower jaw structures. Further, we found that bapx1 was specifically downregulated in the wnt9a/frzb/fzd7a morphants, while general neural crest markers were unaffected. In addition, expression of wnt9a and frzb was also absent in the edn-/- mutant. Notably, over-expression of bapx1 was sufficient to partially rescue mandibular elements in the wnt9a/frzb/fzd7a morphants, demonstrating genetic epistasis of bapx1 acting downstream of edn1 and wnt9a/frzb/fzd7a in lower jaw development. This study underscores the important role of wnt-frizzled signaling in convergence and extension in palate and craniofacial morphogenesis, distinct regulation of upper vs. lower jaw structures, and integration of wnt-frizzled with endothelin signaling to coordinate shaping of the facial form.

A morpholino-based screen to identify novel genes involved in craniofacial morphogenesis

BACKGROUND

The regulatory mechanisms underpinning facial development are conserved between diverse species. Therefore, results from model systems provide insight into the genetic causes of human craniofacial defects. Previously, we generated a comprehensive dataset examining gene expression during development and fusion of the mouse facial prominences. Here, we used this resource to identify genes that have dynamic expression patterns in the facial prominences, but for which only limited information exists concerning developmental function.

RESULTS

This set of ∼80 genes was used for a high-throughput functional analysis in the zebrafish system using Morpholino gene knockdown technology. This screen revealed three classes of cranial cartilage phenotypes depending upon whether knockdown of the gene affected the neurocranium, viscerocranium, or both. The targeted genes that produced consistent phenotypes encoded proteins linked to transcription (meis1, meis2a, tshz2, vgll4l), signaling (pkdcc, vlk, macc1, wu:fb16h09), and extracellular matrix function (smoc2). The majority of these phenotypes were not altered by reduction of p53 levels, demonstrating that both p53-dependent and -independent mechanisms were involved in the craniofacial abnormalities.

CONCLUSIONS

This Morpholino-based screen highlights new genes involved in development of the zebrafish craniofacial skeleton with wider relevance to formation of the face in other species, particularly mouse and human.

Ahsa1 and Hsp90 activity confers more severe craniofacial phenotypes in a zebrafish model of hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR)

The severity of most human birth defects is highly variable. Our ability to diagnose, treat and prevent defects relies on our understanding of this variability. Mutation of the transcription factor GATA3 in humans causes the highly variable hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR) syndrome. Although named for a triad of defects, individuals with HDR can also exhibit craniofacial defects. Through a forward genetic screen for craniofacial mutants, we isolated a zebrafish mutant in which the first cysteine of the second zinc finger of Gata3 is mutated. Because mutation of the homologous cysteine causes HDR in humans, these zebrafish mutants could be a quick and effective animal model for understanding the role of gata3 in the HDR disease spectrum. We demonstrate that, unexpectedly, the chaperone proteins Ahsa1 and Hsp90 promote severe craniofacial phenotypes in our zebrafish model of HDR syndrome. The strengths of the zebrafish system, including rapid development, genetic tractability and live imaging, make this an important model for variability.

Zebrafish Zic2a and Zic2b regulate neural crest and craniofacial development

Holoprosencephaly (HPE), the most common malformation of the human forebrain, is associated with defects of the craniofacial skeleton. ZIC2, a zinc-finger transcription factor, is strongly linked to HPE and to a characteristic set of dysmorphic facial features in humans. We have previously identified important functions for zebrafish Zic2 in the developing forebrain. Here, we demonstrate that ZIC2 orthologs zic2a and zic2b also regulate the forming zebrafish craniofacial skeleton, including the jaw and neurocranial cartilages, and use the zebrafish to study Zic2-regulated processes that may contribute to the complex etiology of HPE. Using temporally controlled Zic2a overexpression, we show that the developing craniofacial cartilages are sensitive to Zic2 elevation prior to 24hpf. This window of sensitivity overlaps the critical expansion and migration of the neural crest (NC) cells, which migrate from the developing neural tube to populate vertebrate craniofacial structures. We demonstrate that zic2b influences the induction of NC at the neural plate border, while both zic2a and zic2b regulate NC migratory onset and strongly contribute to chromatophore development. Both Zic2 depletion and early ectopic Zic2 expression cause moderate, incompletely penetrant mispatterning of the NC-derived jaw precursors at 24hpf, yet by 2dpf these changes in Zic2 expression result in profoundly mispatterned chondrogenic condensations. We attribute this discrepancy to an additional role for Zic2a and Zic2b in patterning the forebrain primordium, an important signaling source during craniofacial development. This hypothesis is supported by evidence that transplanted Zic2-deficient cells can contribute to craniofacial cartilages in a wild-type background. Collectively, these data suggest that zebrafish Zic2 plays a dual role during craniofacial development, contributing to two disparate aspects of craniofacial morphogenesis: (1) neural crest induction and migration, and (2) early patterning of tissues adjacent to craniofacial chondrogenic condensations.

Neural crest cells pattern the surface cephalic ectoderm during FEZ formation

BACKGROUND

Multiple fibroblast growth factor (Fgf) ligands are expressed in the forebrain and facial ectoderm, and vascular endothelial growth factor (VEGF) is expressed in the facial ectoderm. Both pathways activate the MAP kinase cascade and can be suppressed by SU5402. We placed a bead soaked in SU5402 into the brain after emigration of neural crest cells was complete.

RESULTS

Within 24 hr we observed reduced pMEK and pERK staining that persisted for at least 48 hr. This was accompanied by significant apoptosis in the face. By day 15, the upper beaks were truncated. Molecular changes in the FNP were also apparent. Normally, Shh is expressed in the frontonasal ectodermal zone and controls patterned growth of the upper jaw. In treated embryos, Shh expression was reduced. Both the structural and molecular deficits were mitigated after transplantation of FNP-derived mesenchymal cells.

CONCLUSIONS

Thus, mesenchymal cells actively participate in signaling interactions of the face, and the absence of neural crest cells in neurocristopathies may not be merely structural.

Bmp and Shh signaling mediate the expression of satb2 in the pharyngeal arches

In human, mutation of the transcription factor SATB2 causes severe defects to the palate and jaw. The expression and sequence of SATB2 is highly conserved across vertebrate species, including zebrafish. We sought to understand the regulation of satb2 using the zebrafish model system. Due to the normal expression domains of satb2, we analyzed satb2 expression in mutants with disrupted Hh signaling or defective ventral patterning. While satb2 expression appears independent of Edn1 signaling, appropriate expression requires Shha, Smo, Smad5 and Hand2 function. Transplantation experiments show that neural crest cells receive both Bmp and Hh signaling to induce satb2 expression. Dorsomorphin- and cyclopamine-mediated inhibition of Bmp and Hh signaling, respectively, suggests that proper satb2 expression requires a relatively earlier Bmp signal and a later Hh signal. We propose that Bmp signaling establishes competence for the neural crest to respond to Hh signaling, thus inducing satb2 expression.

Development of head and trunk mesoderm in the dogfish, Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures

Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance for understanding the developmental body plan of vertebrates and its evolutionary origin. In particular, the segmentation of the mesoderm is central to the problem. The shark embryo has provided a canonical morphological scheme of the head, with its epithelialized coelomic cavities (head cavities), which have often been regarded as head somites. To understand the evolutionary significance of the head cavities, the embryonic development of the mesoderm was investigated at the morphological and histological levels in the shark, Scyliorhinus torazame. Unlike somites and some enterocoelic mesodermal components in other vertebrates, the head cavities in S. torazame appeared as irregular cyst(s) in the originally unsegmented mesenchymal head mesoderm, and not via segmentation of an undivided coelom. The mandibular cavity appeared first in the paraxial part of the mandibular mesoderm, followed by the hyoid cavity, and the premandibular cavity was the last to form. The prechordal plate was recognized as a rhomboid roof of the preoral gut, continuous with the rostral notochord, and was divided anteroposteriorly into two parts by the growth of the hypothalamic primordium. Of those, the posterior part was likely to differentiate into the premandibular cavity, and the anterior part disappeared later. The head cavities and somites in the trunk exhibited significant differences, in terms of histological appearance and timing of differentiation. The mandibular cavity developed a rostral process secondarily; its homology to the anterior cavity reported in some elasmobranch embryos is discussed.

Distinct requirements for wnt9a and irf6 in extension and integration mechanisms during zebrafish palate morphogenesis

Development of the palate in vertebrates involves cranial neural crest migration, convergence of facial prominences and extension of the cartilaginous framework. Dysregulation of palatogenesis results in orofacial clefts, which represent the most common structural birth defects. Detailed analysis of zebrafish palatogenesis revealed distinct mechanisms of palatal morphogenesis: extension, proliferation and integration. We show that wnt9a is required for palatal extension, wherein the chondrocytes form a proliferative front, undergo morphological change and intercalate to form the ethmoid plate. Meanwhile, irf6 is required specifically for integration of facial prominences along a V-shaped seam. This work presents a mechanistic analysis of palate morphogenesis in a clinically relevant context.

Rbms3 functions in craniofacial development by posttranscriptionally modulating TGF-β signaling

Rbms3 regulates TGF-βr signaling, a critical pathway for chondrogenesis, by binding and stabilizing Smad2 transcripts.

Cranial neural crest cells form much of the facial skeleton, and abnormalities in their development lead to severe birth defects. In a novel zebrafish protein trap screen, we identified an RNA-binding protein, Rbms3, that is transiently expressed in the cytoplasm of condensing neural crest cells within the pharyngeal arches. Morphants for rbms3 displayed reduced proliferation of prechondrogenic crest and significantly altered expression for chondrogenic/osteogenic lineage markers. This phenotype strongly resembles cartilage/crest defects observed in Tgf-βr2:Wnt1-Cre mutants, which suggests a possible link with TGF-β signaling. Consistent with this are the findings that: (a) Rbms3 stabilized a reporter transcript with smad2 3′ untranslated region, (b) RNA immunoprecipitation with full-length Rbms3 showed enrichment for smad2/3, and (c) pSmad2 levels were reduced in rbms3 morphants. Overall, these results suggest that Rbms3 posttranscriptionally regulates one of the major pathways that promotes chondrogenesis, the transforming growth factor β receptor (TGF-βr) pathway.