Examination of a palatogenic gene program in zebrafish

Developmental and organ-specific toxicity of cucurbit[7]uril: in vivo study on zebrafish models

The macrocyclic Cucurbit[7]uril was evaluated for its in vivo toxicity profile, including developmental toxicity and organ-specific toxicities using zebrafish models.

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.

A gene expression map of the larval Xenopus laevis head reveals developmental changes underlying the evolution of new skeletal elements

The morphology of the vertebrate head skeleton is highly plastic, with the number, size, shape, and position of its components varying dramatically between groups. While this evolutionary flexibility has been key to vertebrate success, its developmental and genetic bases are poorly understood. The larval head skeleton of the frog Xenopus laevis possesses a unique combination of ancestral tetrapod features and anuran-specific novelties. We built a detailed gene expression map of the head mesenchyme in X. laevis during early larval development, focusing on transcription factor families with known functions in vertebrate head skeleton development. This map was then compared to homologous gene expression in zebrafish, mouse, and shark embryos to identify conserved and evolutionarily flexible aspects of vertebrate head skeleton development. While we observed broad conservation of gene expression between X. laevis and other gnathostomes, we also identified several divergent features that correlate to lineage-specific novelties. We noted a conspicuous change in dlx1/2 and emx2 expression in the second pharyngeal arch, presaging the differentiation of the reduced dorsal hyoid arch skeletal element typical of modern anamniote tetrapods. In the first pharyngeal arch we observed a shift in the expression of the joint inhibitor barx1, and new expression of the joint marker gdf5, shortly before skeletal differentiation. This suggests that the anuran-specific infrarostral cartilage evolved by partitioning of Meckel's cartilage with a new paired joint. Taken together, these comparisons support a model in which early patterning mechanisms divide the vertebrate head mesenchyme into a highly conserved set of skeletal precursor populations. While subtle changes in this early patterning system can affect skeletal element size, they do not appear to underlie the evolution of new joints or cartilages. In contrast, later expression of the genes that regulate skeletal element differentiation can be clearly linked to the evolution of novel skeletal elements. We posit that changes in the expression of downstream regulators of skeletal differentiation, like barx1 and gdf5, is one mechanism by which head skeletal element number and articulation are altered during evolution.

Developmental age strengthens barriers to ethanol accumulation in zebrafish

Fetal Alcohol Spectrum Disorders (FASD) describes a wide range of phenotypic defects affecting facial and neurological development associated with ethanol teratogenicity. It affects approximately 1 in 100 children born in the United States each year. Genetic predisposition along with timing and dosage of ethanol exposure are critical in understanding the prevalence and variability of FASD. The zebrafish attributes of external fertilization, genetic tractability, and high fecundity make it a powerful tool for FASD studies. However, a lack of consensus of ethanol treatment paradigms has limited the interpretation of these various studies. Here we address this concern by examining ethanol tissue concentrations across timing and genetic background. We utilize headspace gas chromatography to determine ethanol concentration in the AB, fli1:EGFP, and Tu backgrounds. In addition, we treated these embryos with ethanol over two different developmental time windows, 6-24 h post fertilization (hpf) and 24-48 hpf. Our analysis demonstrates that embryos rapidly equilibrate to a sub-media level of ethanol. Embryos then maintain this level of ethanol for the duration of exposure. The ethanol tissue concentration level is independent of genetic background, but is timing-dependent. Embryos exposed from 6 to 24 hpf were 2.7-4.2-fold lower than media levels, while embryos were 5.7-6.2-fold lower at 48 hpf. This suggests that embryos strengthen one or more barriers to ethanol as they develop. In addition, both the embryo and, to a lesser extent, the chorion, surrounding the embryo are barriers to ethanol. Overall, this work will help tighten ethanol treatment regimens and strengthen zebrafish as a model of FASD.

Fish is Fish: the use of experimental model species to reveal causes of skeletal diversity in evolution and disease

Fishes are wonderfully diverse. This variety is a result of the ability of ray-finned fishes to adapt to a wide range of environments, and has made them more specious than the rest of vertebrates combined. With such diversity it is easy to dismiss comparisons between distantly related fishes in efforts to understand the biology of a particular fish species. However, shared ancestry and the conservation of developmental mechanisms, morphological features and physiology provide the ability to use comparative analyses between different organisms to understand mechanisms of development and physiology. The use of species that are amenable to experimental investigation provides tools to approach questions that would not be feasible in other 'non-model' organisms. For example, the use of small teleost fishes such as zebrafish and medaka has been powerful for analysis of gene function and mechanisms of disease in humans, including skeletal diseases. However, use of these fish to aid in understanding variation and disease in other fishes has been largely unexplored. This is especially evident in aquaculture research. Here we highlight the utility of these small laboratory fishes to study genetic and developmental factors that underlie skeletal malformations that occur under farming conditions. We highlight several areas in which model species can serve as a resource for identifying the causes of variation in economically important fish species as well as to assess strategies to alleviate the expression of the variant phenotypes in farmed fish. We focus on genetic causes of skeletal deformities in the zebrafish and medaka that closely resemble phenotypes observed both in farmed as well as natural populations of fishes.

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.

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.

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.

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.

Redundant roles of PRDM family members in zebrafish craniofacial development

BACKGROUND

PRDM proteins are evolutionary conserved Zn-Finger transcription factors that share a characteristic protein domain organization. Previous studies have shown that prdm1a is required for the specification and differentiation of neural crest cells in the zebrafish.

RESULTS

Here we examine other members of this family, specifically prdm3, 5, and 16, in the differentiation of the zebrafish craniofacial skeleton. prdm3 and prdm16 are strongly expressed in the pharyngeal arches, while prdm5 is expressed specifically in the area of the forming neurocranium. Knockdown of prdm3 and prdm16 results in a reduction in the neural crest markers dlx2a and barx1 and defects in both the viscerocranium and the neurocranium. The knockdown of prdm3 and prdm16 in combination is additive in the neurocranium, but not in the viscerocranium. Injection of sub-optimal doses of prdm1a with prdm3 or prdm16 Morpholinos together leads to more severe phenotypes in the viscerocranium and neurocranium. prdm5 mutants have defects in the neurocranium and prdm1a and prdm5 double mutants also show more severe phenotypes.

CONCLUSIONS

Overall, our data reveal that prdm3, 5, and 16 are involved in the zebrafish craniofacial development and that prdm1a may interact with prdm3, 5, and 16 in the formation of the craniofacial skeleton in zebrafish.

Defective neural crest migration revealed by a Zebrafish model of Alx1-related frontonasal dysplasia

Frontonasal dysplasia (FND) refers to a class of midline facial malformations caused by abnormal development of the facial primordia. The term encompasses a spectrum of severities but characteristic features include combinations of ocular hypertelorism, malformations of the nose and forehead and clefting of the facial midline. Several recent studies have drawn attention to the importance of Alx homeobox transcription factors during craniofacial development. Most notably, loss of Alx1 has devastating consequences resulting in severe orofacial clefting and extreme microphthalmia. In contrast, mutations of Alx3 or Alx4 cause milder forms of FND. Whilst Alx1, Alx3 and Alx4 are all known to be expressed in the facial mesenchyme of vertebrate embryos, little is known about the function of these proteins during development. Here, we report the establishment of a zebrafish model of Alx-related FND. Morpholino knock-down of zebrafish alx1 expression causes a profound craniofacial phenotype including loss of the facial cartilages and defective ocular development. We demonstrate for the first time that Alx1 plays a crucial role in regulating the migration of cranial neural crest (CNC) cells into the frontonasal primordia. Abnormal neural crest migration is coincident with aberrant expression of foxd3 and sox10, two genes previously suggested to play key roles during neural crest development, including migration, differentiation and the maintenance of progenitor cells. This novel function is specific to Alx1, and likely explains the marked clinical severity of Alx1 mutation within the spectrum of Alx-related FND.

New perspectives on pharyngeal dorsoventral patterning in development and evolution of the vertebrate jaw

Patterning of the vertebrate facial skeleton involves the progressive partitioning of neural-crest-derived skeletal precursors into distinct subpopulations along the anteroposterior (AP) and dorsoventral (DV) axes. Recent evidence suggests that complex interactions between multiple signaling pathways, in particular Endothelin-1 (Edn1), Bone Morphogenetic Protein (BMP), and Jagged-Notch, are needed to pattern skeletal precursors along the DV axis. Rather than directly determining the morphology of individual skeletal elements, these signals appear to act through several families of transcription factors, including Dlx, Msx, and Hand, to establish dynamic zones of skeletal differentiation. Provocatively, this patterning mechanism is largely conserved from mouse and zebrafish to the jawless vertebrate, lamprey. This implies that the diversification of the vertebrate facial skeleton, including the evolution of the jaw, was driven largely by modifications downstream of a conversed pharyngeal DV patterning program.

Hh signaling regulates patterning and morphogenesis of the pharyngeal arch-derived skeleton

The proper function of the craniofacial skeleton requires the proper shaping of many individual skeletal elements. Neural crest cells generate much of the craniofacial skeleton and morphogenesis of skeletal elements occurs in transient, reiterated structures termed pharyngeal arches. The shape of individual elements depends upon intrinsic patterning within the neural crest as well as extrinsic signals to the neural crest from adjacent tissues within the arches. Hedgehog (Hh) signaling is known to play roles in craniofacial development, yet its involvement in intrinsic and extrinsic patterning of the craniofacial skeleton is still not well understood. Here, we show that morphogenetic movements of the pharyngeal arches and patterning of the neural crest require Hh signaling. Loss of Hh signaling, in smoothened (smo) mutants, disrupts the expression of some Dlx genes as well as other markers of dorsal/ventral patterning of the neural crest. Transplantation of wild-type neural crest cells into smo mutants rescues this defect, demonstrating that the neural crest requires reception of Hh signals for proper patterning. Despite the rescue, morphogenesis of the facial skeleton is not fully recovered. Through transplant analyses, we find two additional requirements for Hh signaling. The endoderm requires the reception of Hh signals for proper morphogenetic movements of the pharyngeal arches and the neural crest require the reception of Hh signaling for the activity of a reverse signal that maintains sonic hedgehog expression in the endoderm. Collectively, these results demonstrate that Hh signaling is essential to establish intrinsic and extrinsic patterning information for the craniofacial skeleton.

Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.