Runx2 is essential for larval hyobranchial cartilage formation inXenopus laevis

Directionality of developing skeletal muscles is set by mechanical forces

Formation of oriented myofibrils is a key event in musculoskeletal development. However, the mechanisms that drive myocyte orientation and fusion to control muscle directionality in adults remain enigmatic. Here, we demonstrate that the developing skeleton instructs the directional outgrowth of skeletal muscle and other soft tissues during limb and facial morphogenesis in zebrafish and mouse. Time-lapse live imaging reveals that during early craniofacial development, myoblasts condense into round clusters corresponding to future muscle groups. These clusters undergo oriented stretch and alignment during embryonic growth. Genetic perturbation of cartilage patterning or size disrupts the directionality and number of myofibrils in vivo. Laser ablation of musculoskeletal attachment points reveals tension imposed by cartilage expansion on the forming myofibers. Application of continuous tension using artificial attachment points, or stretchable membrane substrates, is sufficient to drive polarization of myocyte populations in vitro. Overall, this work outlines a biomechanical guidance mechanism that is potentially useful for engineering functional skeletal muscle.

Developmental disorders caused by cefixime in the otic vesicles of zebrafish embryos or larvae

To explore the developmental toxicity of cefixime (CE) in the developmental disorder and toxicity mechanism of CE on otic vesicles, zebrafish embryos were used as an animal model. The results showed that CE increased mortality in a dose-dependent manner and decreased the hatching rate of zebrafish larva at 96 hpf. Interestingly, CE significantly reduced the area of the saccule and utricle, as well as the area of otic vesicles in zebrafish larvae (p < 0.001). Fibroblast growth factor 8a (Fgf8a) inhibitors and bone morphogenetic protein (BMP) inhibitors caused similar morphological changes. CE decreased the lateral hair cells of zebrafish larvae in a dose-dependent manner. Furthermore, CE caused the downregulation of cartilage and bone-related genes and Na⁺/K⁺-ATPase-related genes of zebrafish larvae at 72 hpf and 120 hpf according to RT-qPCR. A comparison with the control group revealed that 100 μg/mL CE also caused a decrease in Na⁺/K⁺-ATPase activity (p < 0.01). In addition, antibody staining verified that CE inhibited the expression of Na⁺/K⁺-ATPase in the otic vesicles and the nephridium of zebrafish larvae. The data obtained in this study suggested that CE has significant ototoxicity during embryonic development of zebrafish, which is closely related to Na⁺/K⁺-ATPase and the regulation of the Fgf8a/BMP signaling pathways. The effects and toxicity of CE on ear development in other animal models need to be further explored.

Retinoid-X receptor agonists increase thyroid hormone competence in lower jaw remodeling of pre-metamorphic Xenopus laevis tadpoles

Thyroid hormone (TH) signaling plays critical roles during vertebrate development, including regulation of skeletal and cartilage growth. TH acts through its receptors (TRs), nuclear hormone receptors (NRs) that heterodimerize with Retinoid-X receptors (RXRs), to regulate gene expression. A defining difference between NR signaling during development compared to in adult tissues, is competence, the ability of the organism to respond to an endocrine signal. Amphibian metamorphosis, especially in Xenopus laevis, the African clawed frog, is a well-established in vivo model for studying the mechanisms of TH action during development. Previously, we’ve used one-week post-fertilization X. laevis tadpoles, which are only partially competent to TH, to show that in the tail, which is naturally refractive to exogenous T3 at this stage, RXR agonists increase TH competence, and that RXR antagonism inhibits the TH response. Here, we focused on the jaw that undergoes dramatic TH-mediated remodeling during metamorphosis in order to support new feeding and breathing styles. We used a battery of approaches in one-week-old tadpoles, including quantitative morphology, differential gene expression and whole mount cell proliferation assays, to show that both pharmacologic (bexarotene) and environmental (tributyltin) RXR agonists potentiated TH-induced responses but were inactive in the absence of TH; and the RXR antagonist UVI 3003 inhibited TH action. Bex and TBT significantly potentiated cellular proliferation and the TH induction of runx2, a transcription factor critical for developing cartilage and bone. Prominent targets of RXR-mediated TH potentiation were members of the matrix metalloprotease family, suggesting that RXR potentiation may emphasize pathways responsible for rapid changes during development.

Haploinsufficiency of SF3B2 causes craniofacial microsomia

Craniofacial microsomia (CFM) is the second most common congenital facial anomaly, yet its genetic etiology remains unknown. We perform whole-exome or genome sequencing of 146 kindreds with sporadic (n = 138) or familial (n = 8) CFM, identifying a highly significant burden of loss of function variants in SF3B2 (P = 3.8 × 10^-10), a component of the U2 small nuclear ribonucleoprotein complex, in probands. We describe twenty individuals from seven kindreds harboring de novo or transmitted haploinsufficient variants in SF3B2. Probands display mandibular hypoplasia, microtia, facial and preauricular tags, epibulbar dermoids, lateral oral clefts in addition to skeletal and cardiac abnormalities. Targeted morpholino knockdown of SF3B2 in Xenopus results in disruption of cranial neural crest precursor formation and subsequent craniofacial cartilage defects, supporting a link between spliceosome mutations and impaired neural crest development in congenital craniofacial disease. The results establish haploinsufficient variants in SF3B2 as the most prevalent genetic cause of CFM, explaining ~3% of sporadic and ~25% of familial cases.

Metformin-Incorporated Gelatin/Hydroxyapatite Nanofiber Scaffold for Bone Regeneration

Tissue engineering and regenerative medicine has gradually evolved as a promising therapeutic strategy to the modern health care of aging and diseased population. In this study, we developed a novel nanofibrous scaffold and verified its application in the critical bone defect regeneration. The metformin-incorporated nano-gelatin/hydroxyapatite fibers (NGF) was produced by electrospinning, cross-linked, and then characterized by X-ray powder diffractometer and Fourier-transform infrared spectroscopy. Cytotoxicity, cell adhesion, cell differentiation, and quantitative osteogenic gene and protein expression were analyzed by bone marrow stem cells (BMSCs) from rat. Rat forearm critical bone defect model was performed for the in vivo study. The NGF were characterized by their porous structures with proper interconnectivity without significant cytotoxic effects; the adhesion of BMSCs on the NGF could be enhanced. The osteogenic gene and protein expression were upregulated. Postimplantation, the new regenerated bone in bone defect was well demonstrated in the NGF samples. We demonstrated that the metformin-incorporated NGF greatly improved healing potential on the critical-size bone defect. Although metformin-incorporated NGF had advantageous effectiveness during bone regeneration, further validation is required before it can be applied to clinical applications. Impact statement Bone is the structure that supports the rest of the human body. Critical-size bone defect hinders the regeneration of damaged bone tissues and compromises the mechanical strength of the skeletal system. Characterized by their porous structures with proper interconnectivity, the electrospinning nano-gelatin/hydroxyapatite fibrous scaffold developed in this study can greatly improve the healing potential on the critical-size bone defect. Further validation can validate its potential clinical applications.

Evolutionary repression of chondrogenic genes in the vertebrate osteoblast

Gene expression in extant animals might reveal how skeletal cells have evolved over the past 500 million years. The cells that make up cartilage (chondrocytes) and bone (osteoblasts) express many of the same genes, but they also have important molecular differences that allow us to distinguish them as separate cell types. For example, traditional studies of later-diverged vertebrates, such as mouse and chick, defined the genes Col2a1 and sex-determining region Y-box 9 as cartilage-specific. However, recent studies have shown that osteoblasts of earlier-diverged vertebrates, such as frog, gar, and zebrafish, express these 'chondrogenic' markers. In this review, we examine the resulting hypothesis that chondrogenic gene expression became repressed in osteoblasts over evolutionary time. The amphibian is an underexplored skeletal model that is uniquely positioned to address this hypothesis, especially given that it diverged when life transitioned from water to land. Given the relationship between phylogeny and ontogeny, a novel discovery for skeletal cell evolution might bolster our understanding of skeletal cell development.

Diminution of pharyngeal segmentation and the evolution of the amniotes

BACKGROUND

The pharyngeal arches are a series of bulges found on the lateral surface of the head of vertebrate embryos, and it is within these segments that components of the later anatomy are laid down. In most vertebrates, the post-otic pharyngeal arches will form the branchial apparatus, while in amniotes these segments are believed to generate the larynx. It has been unclear how the development of these segments has been altered with the emergence of the amniotes.

RESULTS

In this study, we examined the development of pharyngeal arches in amniotes and show that the post-otic pharyngeal arches in this clade are greatly diminished. We find that the post-otic segments do not undergo myogenesis or skeletogenesis, but are remodelled before these processes occur. We also find that nested DLX expression, which is a feature of all the pharyngeal arches in anamniotes, is associated with the anterior segments but less so with the posterior arches in amniotes. We further show that the posterior arches of the mouse embryo fail to properly delineate, which demonstrates the lack of function of these posterior segments in later development.

CONCLUSION

In amniotes, there has been a loss of the ancestral "branchial" developmental programme that is a general feature of gnathostomes; myogenesis and skeletogenesis This is likely to have facilitated the emergence of the larynx as a new structure not constrained by the segmental organisation of the posterior pharyngeal region.

Mustn1: A Developmentally Regulated Pan-Musculoskeletal Cell Marker and Regulatory Gene

The Mustn1 gene encodes a small nuclear protein (~9.6 kDa) that does not belong to any known family. Its genomic organization consists of three exons interspersed by two introns and it is highly homologous across vertebrate species. Promoter analyses revealed that its expression is regulated by the AP family of transcription factors, especially c-Fos, Fra-2 and JunD. Mustn1 is predominantly expressed in the major tissues of the musculoskeletal system: bone, cartilage, skeletal muscle and tendon. Its expression has been associated with normal embryonic development, postnatal growth, exercise, and regeneration of bone and skeletal muscle. Moreover, its expression has also been detected in various musculoskeletal pathologies, including arthritis, Duchenne muscular dystrophy, other skeletal muscle myopathies, clubfoot and diabetes associated muscle pathology. In vitro and in vivo functional perturbation revealed that Mustn1 is a key regulatory molecule in myogenic and chondrogenic lineages. This comprehensive review summarizes our current knowledge of Mustn1 and proposes that it is a new developmentally regulated pan-musculoskeletal marker as well as a key regulatory protein for cell differentiation and tissue growth.

Bioelectric signalling via potassium channels: a mechanism for craniofacial dysmorphogenesis in KCNJ2-associated Andersen-Tawil Syndrome

KEY POINTS

Xenopus laevis craniofacial development is a good system for the study of Andersen-Tawil Syndrome (ATS)-associated craniofacial anomalies (CFAs) because (1) Kcnj2 is expressed in the nascent face; (2) molecular-genetic and biophysical techniques are available for the study of ion-dependent signalling during craniofacial morphogenesis; (3) as in humans, expression of variant Kcnj2 forms in embryos causes a muscle phenotype; and (4) variant forms of Kcnj2 found in human patients, when injected into frog embryos, cause CFAs in the same cell lineages. Forced expression of WT or variant Kcnj2 changes the normal pattern of Vmem (resting potential) regionalization found in the ectoderm of neurulating embryos, and changes the normal pattern of expression of ten different genetic regulators of craniofacial development, including markers of cranial neural crest and of placodes. Expression of other potassium channels and two different light-activated channels, all of which have an effect on Vmem , causes CFAs like those induced by injection of Kcnj2 variants. In contrast, expression of Slc9A (NHE3), an electroneutral ion channel, and of GlyR, an inactive Cl(-) channel, do not cause CFAs, demonstrating that correct craniofacial development depends on a pattern of bioelectric states, not on ion- or channel-specific signalling. Using optogenetics to control both the location and the timing of ion flux in developing embryos, we show that affecting Vmem of the ectoderm and no other cell layers is sufficient to cause CFAs, but only during early neurula stages. Changes in Vmem induced late in neurulation do not affect craniofacial development. We interpret these data as strong evidence, consistent with our hypothesis, that ATS-associated CFAs are caused by the effect of variant Kcnj2 on the Vmem of ectodermal cells of the developing face. We predict that the critical time is early during neurulation, and the critical cells are the ectodermal cranial neural crest and placode lineages. This points to the potential utility of extant, ion flux-modifying drugs as treatments to prevent CFAs associated with channelopathies such as ATS.

ABSTRACT

Variants in potassium channel KCNJ2 cause Andersen-Tawil Syndrome (ATS); the induced craniofacial anomalies (CFAs) are entirely unexplained. We show that KCNJ2 is expressed in Xenopus and mouse during the earliest stages of craniofacial development. Misexpression in Xenopus of KCNJ2 carrying ATS-associated mutations causes CFAs in the same structures affected in humans, changes the normal pattern of membrane voltage potential regionalization in the developing face and disrupts expression of important craniofacial patterning genes, revealing the endogenous control of craniofacial patterning by bioelectric cell states. By altering cells' resting potentials using other ion translocators, we show that a change in ectodermal voltage, not tied to a specific protein or ion, is sufficient to cause CFAs. By adapting optogenetics for use in non-neural cells in embryos, we show that developmentally patterned K(+) flux is required for correct regionalization of the resting potentials and for establishment of endogenous early gene expression domains in the anterior ectoderm, and that variants in KCNJ2 disrupt this regionalization, leading to the CFAs seen in ATS patients.

Dissection of Xenopus laevis neural crest for in vitro explant culture or in vivo transplantation

The neural crest (NC) is a transient dorsal neural tube cell population that undergoes an epithelium-to-mesenchyme transition (EMT) at the end of neurulation, migrates extensively towards various organs, and differentiates into many types of derivatives (neurons, glia, cartilage and bone, pigmented and endocrine cells). In this protocol, we describe how to dissect the premigratory cranial NC from Xenopus laevis embryos, in order to study NC development in vivo and in vitro. The frog model offers many advantages to study early development; abundant batches are available, embryos develop rapidly, in vivo gain and loss of function strategies allow manipulation of gene expression prior to NC dissection in donor and/or host embryos. The NC explants can be plated on fibronectin and used for in vitro studies. They can be cultured for several days in a serum-free defined medium. We also describe how to graft NC explants back into host embryos for studying NC migration and differentiation in vivo.

Summarizing craniofacial genetics and developmental biology (SCGDB)

This overview article highlights active areas of research in craniofacial genetics and developmental biology as reflected in presentations given at the 34th annual meeting of the Society of Craniofacial Genetics and Developmental Biology (SCGDB) in Montreal, Quebec on October 11, 2011. This 1-day meeting provided a stimulating occasion that demonstrated the present status of research in craniofacial genetics and developmental biology and where the field is heading. To accompany the abstracts published in this issue I have selected several themes that emerged from the meeting. After discussing the basis on which craniofacial defects/syndromes are classified and investigated, I address the multi-gene basis of craniofacial syndromes with an examination of the roles of Sox9 and FGF receptors in normal and abnormal craniofacial development. I then turn to the knowledge being gained from population-wide and longitudinal cohort studies and from the discovery of new signaling centers that regulate craniofacial development.

Quantitative analysis of orofacial development and median clefts in Xenopus laevis

Xenopus has become a useful tool to study the molecular mechanisms underlying orofacial development. However, few quantitative analyses exist to describe the anatomy of this region. In this study we combine traditional facial measurements with geometric morphometrics to describe anatomical changes in the orofacial region during normal and abnormal development. Facial measurements and principal component (PC) analysis indicate that during early tadpole development the face expands primarily in the midface region accounting for the development of the upper jaw and primary palate. The mouth opening correspondingly becomes flatter and wider as it incorporates the jaw elements. A canonical variate analysis of orofacial and mouth opening shape emphasized that changes in the orofacial shape occur gradually. Orofacial anatomy was quantified after altered levels of retinoic acid using all-trans retinoic acid or an inhibitor of retinoic acid receptors or by injecting antisense oligos targeting RALDH2. Such perturbations resulted in major decreases in the width of the midface and the mouth opening illustrated in facial measurements and a PC analysis. The mouth opening shape also had a gap in the primary palate resulting in a median cleft in the mouth opening that was only illustrated quantitatively in the morphometric analysis. Finally, canonical and discriminant function analysis statistically distinguished the orofacial and mouth opening shape changes among the different modes used to alter retinoic acid signaling levels. By combining quantitative analyses with molecular studies of orofacial development we will be better equipped to understand the complex morphogenetic processes involved in palate development and clefting.

Cartilage on the move: cartilage lineage tracing during tadpole metamorphosis

The reorganization of cranial cartilages during tadpole metamorphosis is a set of complex processes. The fates of larval cartilage-forming cells (chondrocytes) and sources of adult chondrocytes are largely unknown. Individual larval cranial cartilages may either degenerate or remodel, while many adult cartilages appear to form de novo during metamorphosis. Determining the extent to which adult chondrocytes/cartilages are derived from larval chondrocytes during metamorphosis requires new techniques in chondrocyte lineage tracing. We have developed two transgenic systems to label cartilage cells throughout the body with fluorescent proteins. One system strongly labels early tadpole cartilages only. The other system inducibly labels forming cartilages at any developmental stage. We examined cartilages of the skull (viscero- and neurocranium), and identified larval cartilages that either resorb or remodel into adult cartilages. Our data show that the adult otic capsules, tecti anterius and posterius, hyale, and portions of Meckel's cartilage are derived from larval chondrocytes. Our data also suggest that most adult cartilages form de novo, though we cannot rule out the potential for extreme larval chondrocyte proliferation or de- and re-differentiation, which could dilute our fluorescent protein signal. The transgenic lineage tracing strategies developed here are the first examples of inducible, skeleton-specific, lineage tracing in Xenopus.

The protein kinase MLTK regulates chondrogenesis by inducing the transcription factor Sox6

Sox9 acts together with Sox5 or Sox6 as a master regulator for chondrogenesis; however, the inter-relationship among these transcription factors remains unclear. Here, we show that the protein kinase MLTK plays an essential role in the onset of chondrogenesis through triggering the induction of Sox6 expression by Sox9. We find that knockdown of MLTK in Xenopus embryos results in drastic loss of craniofacial cartilages without defects in neural crest development. We also find that Sox6 is specifically induced during the onset of chondrogenesis, and that the Sox6 induction is inhibited by MLTK knockdown. Remarkably, Sox6 knockdown phenocopies MLTK knockdown. Moreover, we find that ectopic expression of MLTK induces Sox6 expression in a Sox9-dependent manner. Our data suggest that p38 and JNK pathways function downstream of MLTK during chondrogenesis. These results identify MLTK as a novel key regulator of chondrogenesis, and reveal its action mechanism in chondrocyte differentiation during embryonic development.

Evolution of the bone gene regulatory network

Current fossil, embryological and genetic data shed light on the evolution of the gene regulatory network (GRN) governing bone formation. The key proteins and genes involved in skeletogenesis are well accepted. We discuss when these essential components of the GRN evolved and propose that the Runx genes, master regulators of skeletogenesis, functioned in early cartilages well before they were co-opted to function in the making of bone. Two rounds of whole genome duplication, together with additional tandem gene duplications, created a genetic substrate for segregation of one GRN into several networks regulating the related tissues of cartilage, bone, enamel, and dentin. During this segregation, Runx2 assumed its position at the top of the bone GRN, and Sox9 was excluded from bone, retaining its ancient role in cartilage.

Mustn1 is essential for craniofacial chondrogenesis during Xenopus development

Mustn1 is a vertebrate-specific protein that, in vitro, was showed to be essential for prechondrocyte function and thus it has the potential to regulate chondrogenesis during embryonic development. We use Xenopus laevis as a model to examine Mustn1 involvement in chondrogenesis. Previous work suggests that Mustn1 is necessary but not sufficient for chondrogenic proliferation and differentiation, as well as myogenic differentiation in vitro. Mustn1 was quantified and localized in developing Xenopus embryos using RT-PCR and whole mount in situ hybridization. Xenopus embryos were injected with either control morpholinos (Co-MO) or one designed against Mustn1 (Mustn1-MO) at the four cell stage. Embryos were scored for morphological defects and Sox9 was visualized via in situ hybridization. Finally, Mustn1-MO-injected embryos were co-injected with Mustn1-MO resistant mRNA to confirm the specificity of the observed phenotype. Mustn1 is expressed from the mid-neurula stage to the swimming tadpole stages, predominantly in anterior structures including the pharyngeal arches and associated craniofacial tissues, and the developing somites. Targeted knockdown of Mustn1 in craniofacial and dorsal axial tissues resulted in phenotypes characterized by small or absent eye(s), a shortened body axis, and tail kinks. Further, Mustn1 knockdown reduced cranial Sox9 mRNA expression and resulted in the loss of differentiated cartilaginous head structures (e.g. ceratohyal and pharyngeal arches). Reintroduction of MO-resistant Mustn1 mRNA rescued these effects. We conclude that Mustn1 is necessary for normal craniofacial cartilage development in vivo, although the exact molecular mechanism remains unknown.

Effects of fluoride on expression of bone-specific genes in developing Xenopus laevis larvae

The effect of fluoride treatment on the expression of a panel of osteogenic and stress markers in Stage 55 premetamorphic Xenopus larvae was examined at the precise onset of replacement of the larval cartilaginous skeleton with bone. A dosing regimen of 10 mmol/L sodium fluoride over 8 days was followed, during which time larvae developed to Stage 58, when the process of progressive ossification takes place in the vertebral column and membranous bones of the skull, pelvic, and pectoral girdles and portions of the appendicular skeleton. Markers of bone formation, including COL1A1, the transcription factors Osterix, RUNX2-II, and matrix metalloproteinases MMP1 and MMP13, decreased relative to age-matched controls, though the osteoblast marker BGLAP was not significantly altered. Expression of the pro-osteoclastogenic factor RANKL decreased, whereas expression of the anti-osteoclastogenic factor osteoprotegerin increased. Altered expression of oxidative stress markers, with the exception of superoxide dismutase, was generally not observed. These data demonstrate the potent effects of fluoride on the expression of factors required for osteoblast and osteoclast differentiation, as well as on the expression of osteoblast products, including MMP1 and collagen. Importantly, these effects were observed in the absence of significant changes in the expression of oxidative stress markers. The results provide the first molecular insights into the mechanisms underlying skeletal fluorosis in a whole organism developmental model.

A new mechanistic scenario for the origin and evolution of vertebrate cartilage

The appearance of cellular cartilage was a defining event in vertebrate evolution because it made possible the physical expansion of the vertebrate "new head". Despite its central role in vertebrate evolution, the origin of cellular cartilage has been difficult to understand. This is largely due to a lack of informative evolutionary intermediates linking vertebrate cellular cartilage to the acellular cartilage of invertebrate chordates. The basal jawless vertebrate, lamprey, has long been considered key to understanding the evolution of vertebrate cartilage. However, histological analyses of the lamprey head skeleton suggest it is composed of modern cellular cartilage and a putatively unrelated connective tissue called mucocartilage, with no obvious transitional tissue. Here we take a molecular approach to better understand the evolutionary relationships between lamprey cellular cartilage, gnathostome cellular cartilage, and lamprey mucocartilage. We find that despite overt histological similarity, lamprey and gnathostome cellular cartilage utilize divergent gene regulatory networks (GRNs). While the gnathostome cellular cartilage GRN broadly incorporates Runx, Barx, and Alx transcription factors, lamprey cellular cartilage does not express Runx or Barx, and only deploys Alx genes in certain regions. Furthermore, we find that lamprey mucocartilage, despite its distinctive mesenchymal morphology, deploys every component of the gnathostome cartilage GRN, albeit in different domains. Based on these findings, and previous work, we propose a stepwise model for the evolution of vertebrate cellular cartilage in which the appearance of a generic neural crest-derived skeletal tissue was followed by a phase of skeletal tissue diversification in early agnathans. In the gnathostome lineage, a single type of rigid cellular cartilage became dominant, replacing other skeletal tissues and evolving via gene cooption to become the definitive cellular cartilage of modern jawed vertebrates.

Sox9 function in craniofacial development and disease

The Sox family of transcriptional regulators has been implicated in the control of a broad array of developmental processes. One member of this family SOX9 was first identified as a candidate gene for campomelic dysplasia (CD), a human syndrome affecting skeletal, and testis development. In these patients most endochondral bones of the face fail to develop resulting in multiple defects such as micrognathia, cleft palate, and facial dysmorphia. In this review we describe Sox9 expression during embryonic development and summarize loss of function experiments in frog, fish, and mouse embryos highlighting the role of Sox9 in regulating morphogenesis of the face. We also discuss the mutations in and around SOX9 responsible for craniofacial defects in CD patients.

Can you hear me now? Understanding vertebrate middle ear development

The middle ear is a composite organ formed from all three germ layers and the neural crest. It provides the link between the outside world and the inner ear, where sound is transduced and routed to the brain for processing. Extensive classical and modern studies have described the complex morphology and origin of the middle ear. Non-mammalian vertebrates have a single ossicle, the columella. Mammals have three functionally equivalent ossicles, designated the malleus, incus and stapes. In this review, I focus on the role of genes known to function in the middle ear. Genetic studies are beginning to unravel the induction and patterning of the multiple middle ear elements including the tympanum, skeletal elements, the air-filled cavity, and the insertion point into the inner ear oval window. Future studies that elucidate the integrated spatio-temporal signaling mechanisms required to pattern the middle ear organ system are needed. The longer-term translational benefits of understanding normal and abnormal ear development will have a direct impact on human health outcomes.

Early cranial patterning in the direct-developing frog Eleutherodactylus coqui revealed through gene expression

Genetic and developmental alterations associated with the evolution of amphibian direct development remain largely unexplored. Specifically, little is known of the underlying expression of skeletal regulatory genes, which may reveal early modifications to cranial ontogeny in direct-developing species. We describe expression patterns of three key skeletal regulators (runx2, sox9, and bmp4) along with the cartilage-dominant collagen 2alpha1 gene (col2a1) during cranial development in the direct-developing anuran, Eleutherodactylus coqui. Expression patterns of these regulators reveal transient skeletogenic anlagen that correspond to larval cartilages, but which never fully form in E. coqui. Suprarostral anlagen in the frontonasal processes are detected through runx2, sox9, and bmp4 expression. Previous studies have described these cartilages as missing from Eleutherodactylus cranial ontogeny. These transcriptionally active suprarostral anlagen fuse to the more posterior cranial trabeculae before they are detectable with col2a1 staining or with the staining techniques used in earlier studies. Additionally, expression of sox9 fails to reveal an early anterior connection between the palatoquadrate and the neurocranium, which is detectable through sox9 staining in Xenopus laevis embryos (a metamorphosing species). Absence of this connection validates an instance of developmental repatterning, where the larval quadratocranial commissure cartilage is lost in E. coqui. Expression of runx2 reveals dermal-bone precursors several developmental stages before their detection with alizarin red. This early expression of runx2 correlates with the accelerated embryonic onset of bone formation characteristic of E. coqui and other direct-developing anurans, but which differs from the postembryonic bone formation of most metamorphosing species. Together these results provide an earlier depiction of cranial patterning in E. coqui by using earlier markers of skeletogenic cell differentiation. These data both validate and modify previously reported instances of larval recapitulation and developmental repatterning associated with the evolution of anuran direct development.

Development of the ethmoidal structures of the endocranium in Discoglossus pictus (Anura: Discoglossidae)

We use histological techniques and computer-aided three-dimensional reconstructions made from serial histological sections to describe the ontogeny of the ethmoidal endocranium of discoglossid frog Discoglossus pictus. We identify a pattern of development for the suprarostral cartilage that differs from previous findings and probably represents the ancestral anuran pattern. The nasal cartilages, including the inferior prenasal cartilage, are de novo adult structures. The only larva-derived structures of the adult nasal capsules are the posterior aspects of the solum nasi and septum nasi. We also identify patterns of development for the ethmoid plate and postnasal wall that occur during early in ontogenesis. These patterns are associated with development events during metamorphic climax. The pattern of timing of chondrification of the anterior nasal cartilages more closely coincides with that of the neobatrachian species than that recorded for the pelobatid frog Spea. In addition, this study supports a sister taxon relationship between Discoglossus and Alytes.

Expression analysis of Runx3 and other Runx family members during Xenopus development

Runx genes encode a family of proteins defined by the highly conserved runt DNA-binding domain. Studies in several organisms have shown that these transcription factors regulate multiple aspects of embryonic development and are responsible for the pathogenesis of several human diseases. Here we report the cloning and expression of Runx3 during Xenopus development and compare its expression pattern to other Runx family members, Runx1 and Runx2, and to Cbfbeta, the obligatory binding partner of Runx proteins. Using in situ hybridization in the whole embryo and on sections we show that Runx3 is co-expressed with Runx1 in the hematopoietic lineage and in Rohon-Beard sensory neurons. In contrast Runx3 and Runx2 are co-expressed in craniofacial cartilage elements. Runx3 shows also unique expression domains in a number of derivatives of the neurogenic placodes, including the ganglia of the anteroposterior and middle lateral line nerves, and ganglia of the trigeminal, glossopharyngeal, facial and vagal nerves. These observations suggest a critical role for Runx3 in the development of cranial sensory neurons, while in other tissues its co-expression with Runx1 or Runx2 may signify functional redundancy between these family members.

Evolution of the interaction between Runx2 and VDR, two transcription factors involved in osteoblastogenesis

Background

The mineralized skeleton is a major evolutionary novelty that has contributed to the impressive morphological diversifications of the vertebrates. Essential to bone biology is the solidified extracellular matrix secreted by highly specialized cells, the osteoblasts. We now have a rather complete view of the events underlying osteogenesis, from a cellular, molecular, genetic, and epigenetic perspective. Because this knowledge is still largely restricted to mammals, it is difficult, if not impossible, to deduce the evolutionary history of the regulatory network involved in osteoblasts specification and differentiation. In this study, we focused on the transcriptional regulators Runx2 and VDR (the Vitamin D Receptor) that, in mammals, directly interact together and stabilize complexes of co-activators and chromatin remodellers, thereby allowing the transcriptional activation of target genes involved in extracellular matrix mineralization. Using a combination of functional, biochemical, and histological approaches, we have asked if the interaction observed between Runx2 and VDR represents a recent mammalian innovation, or if it results from more ancient changes that have occurred deep in the vertebrate lineage.

Results

Using immunohistochemistry and in situ hybridization in developing embryos of chick, frog and teleost fishes, we have revealed that the co-expression of Runx2 and VDR in skeletal elements has been particularly strengthened in the lineage leading to amniotes. We show that the teleost Runx2 orthologue as well as the three mammalian Runx1, Runx2 and Runx3 paralogues are able to co-immunoprecipitate with the VDR protein present in nuclear extracts of rat osteoblasts stimulated with 1α,25-dihydroxyvitamin D₃. In addition, the teleost Runx2 can activate the transcription of the mammalian osteocalcin promoter in transfection experiments, and this response can be further enhanced by 1α,25-dihydroxyvitamin D₃. Finally, using pull-down experiments between recombinant proteins, we show that the VDR homologue from teleosts, but not from ascidians, is able to directly interact with the mammalian Runx2 homologue.

Conclusions

We propose an evolutionary scenario for the assembly of the molecular machinery involving Runx2 and VDR in vertebrates. In the last common ancestor of actinopterygians and sacropterygians, the three Runx paralogues possessed the potential to physically and functionally interact with the VDR protein. Therefore, 1α,25-dihydroxyvitamin D₃ might have been able to modulate the transcriptional activity of Runx1, Runx2 or Runx3 in the tissues expressing VDR. After the split from amphibians, in the lineage leading to amniotes, Runx2 and VDR became robustly co-expressed in developing skeletal elements, and their regulatory interaction was incorporated in the genetic program involved in the specification and differentiation of osteoblasts.

Dual transcriptional regulation by runx2 of matrix Gla protein in Xenopus laevis

Matrix Gla protein (MGP) is an extracellular mineral-binding protein expressed in several tissues but it only accumulates in bone and calcified cartilage under physiological conditions. Available evidence indicates that it acts as a physiological inhibitor of mineralization. Runx2 is a transcription factor essential for bone formation in mammals, affecting osteoblast and chondrocyte differentiation by regulating key genes crucial for bone and cartilage development. Being an important cartilage-associated gene, MGP is a potential target for Runx2, and thus we have investigated the possible functional interactions between them. In A6 cells, Runx2 was found to modulate MGP transcription and deletion analysis of MGP distal and proximal promoter-luciferase constructs identified cis-regulatory regions. Interestingly, we have also identified a runx2-binding site that mediates transcriptional repression of XlMGP. Mutation of this element, located between -54 and +33 bp, results in 18-fold up-regulation of transcription. Furthermore, and in addition to the previously reported Xlrunx2 types I and II, we have identified three transcripts encoding novel, truncated Xlrunx2 isoforms. Although only type I and type II could transactivate XlMGP, the truncated isoforms identified in this study, which result from alternative splicing, could be involved in negative regulation of MGP expression, as described for other RUNX2 truncated isoforms acting in other target genes. In vivo microinjection of XlMGP promoter constructs and runx2 mRNA confirmed that those promoters are targets for this transcription factor. These data also indicate that MGP is under dual regulation by runx2 through the use of various isoforms and context-dependent formation of transcriptional complexes.