Distinct expression and shared activities of members of the hedgehog gene family of Xenopus laevis

The Xenopus laevis teratogenesis assay for developmental toxicity of phthalate plasticizers and alternatives

Contamination of phthalate ester plasticizers threatens the wildlife as well as human health. To evaluate the developmental toxicity of commonly used phthalate esters and emerging alternatives, the frog embryo teratogenesis assay-Xenopus (FETAX) was conducted for dibutyl-phthalate (DBP), benzyl-butyl-phthalate (BBP), dioctyl-terephthalate (DOTP), di(2-propylheptyl)-phthalate (DPHP), diisononyl-phthalate (DINP), diisodecyl-phthalate (DIDP), diethyl hexyl cyclohexane (DEHCH), and diisononyl-cyclohexane-1,2-dicarboxylate (DINCH). The 96-hrs LC₅₀ for DBP, BBP, DOTP, DIDP, DINCH, DINP, DPHP, and DEHCH were 18.3, 20.1, 588.7, 718.0, 837.5, 859.3, 899.0, and 899.0 mg/L, respectively. The 96-hrs EC₅₀ of developmental abnormality of DBP, BBP, DPHP, DOTP, DINP, DEHCH, DINCH, and DIDP were 7.5, 18.2, 645.1, 653.6, 664.4, 745.6, 813.7, and 944.5 mg/L, respectively. The lowest observed effective concentration for embryonic survival, malformation, and growth was DINP, DBP, BBP, DIDP, DPHP, DINCH, DEHCH, and DOTP in increasing order. In tadpoles, DBP, BBP, DEHCH, DINP, and DIDP caused inositol-requiring enzyme 1 or protein kinase R-like endoplasmic reticulum kinase pathway endoplasmic reticulum stress (ERS) in order, and BBP, DBP, DOTP, DPHP, DINP, and DIDP caused long term ERS-related apoptosis or mitochondrial apoptosis in order. Together, in Xenopus embryos, the developmental toxicity and the cellular stress-inducing potential of tested plasticizers were DEHCH, DINCH, DPHP, DIDP, DINP, DOTP, BBP, and DBP in increasing order. In consideration of public as well as environmental health this information would be helpful for industrial choice of phthalate ester plasticizers and their alternatives.

Non-canonical Hedgehog signaling regulates spinal cord and muscle regeneration in Xenopus laevis larvae

Inducing regeneration in injured spinal cord represents one of modern medicine’s greatest challenges. Research from a variety of model organisms indicates that Hedgehog (Hh) signaling may be a useful target to drive regeneration. However, the mechanisms of Hh signaling-mediated tissue regeneration remain unclear. Here, we examined Hh signaling during post-amputation tail regeneration in Xenopus laevis larvae. We found that while Smoothened (Smo) activity is essential for proper spinal cord and skeletal muscle regeneration, transcriptional activity of the canonical Hh effector Gli is repressed immediately following amputation, and inhibition of Gli1/2 expression or transcriptional activity has minimal effects on regeneration. In contrast, we demonstrate that protein kinase A is necessary for regeneration of both muscle and spinal cord, in concert with and independent of Smo, respectively, and that its downstream effector CREB is activated in spinal cord following amputation in a Smo-dependent manner. Our findings indicate that non-canonical mechanisms of Hh signaling are necessary for spinal cord and muscle regeneration.

The hedgehog pathway and ocular developmental anomalies

Mutations in effectors of the hedgehog signaling pathway are responsible for a wide variety of ocular developmental anomalies. These range from massive malformations of the brain and ocular primordia, not always compatible with postnatal life, to subtle but damaging functional effects on specific eye components. This review will concentrate on the effects and effectors of the major vertebrate hedgehog ligand for eye and brain formation, Sonic hedgehog (SHH), in tissues that constitute the eye directly and also in those tissues that exert indirect influence on eye formation. After a brief overview of human eye development, the many roles of the SHH signaling pathway during both early and later morphogenetic processes in the brain and then eye and periocular primordia will be evoked. Some of the unique molecular biology of this pathway in vertebrates, particularly ciliary signal transduction, will also be broached within this developmental cellular context.

Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate

Background

The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken).

Results

Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation.

Conclusions

While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of "ventral to dorsal induction" of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction.

Identification of new regulators of embryonic patterning and morphogenesis in Xenopus gastrulae by RNA sequencing

During early vertebrate embryogenesis, cell fate specification is often coupled with cell acquisition of specific adhesive, polar and/or motile behaviors. In Xenopus gastrulae, tissues fated to form different axial structures display distinct motility. The cells in the early organizer move collectively and directionally toward the animal pole and contribute to anterior mesendoderm, whereas the dorsal and the ventral-posterior trunk tissues surrounding the blastopore of mid-gastrula embryos undergo convergent extension and convergent thickening movements, respectively. While factors regulating cell lineage specification have been described in some detail, the molecular machinery that controls cell motility is not understood in depth. To gain insight into the gene battery that regulates both cell fates and motility in particular embryonic tissues, we performed RNA sequencing (RNA-seq) to investigate differentially expressed genes in the early organizer, the dorsal and the ventral marginal zone of Xenopus gastrulae. We uncovered many known signaling and transcription factors that have been reported to play roles in embryonic patterning during gastrulation. We also identified many uncharacterized genes as well as genes that encoded extracellular matrix (ECM) proteins or potential regulators of actin cytoskeleton. Co-expression of a selected subset of the differentially expressed genes with activin in animal caps revealed that they had distinct ability to block activin-induced animal cap elongation. Most of these factors did not interfere with mesodermal induction by activin, but an ECM protein, EFEMP2, inhibited activin signaling and acted downstream of the activated type I receptor. By focusing on a secreted protein kinase PKDCC1, we showed with overexpression and knockdown experiments that PKDCC1 regulated gastrulation movements as well as anterior neural patterning during early Xenopus development. Overall, our studies identify many differentially expressed signaling and cytoskeleton regulators in different embryonic regions of Xenopus gastrulae and imply their functions in regulating cell fates and/or behaviors during gastrulation.

G protein-coupled receptors Flop1 and Flop2 inhibit Wnt/β-catenin signaling and are essential for head formation in Xenopus

Patterning of the vertebrate anterior-posterior axis is regulated by the coordinated action of growth factors whose effects can be further modulated by upstream and downstream mediators and the cross-talk of different intracellular pathways. In particular, the inhibition of the Wnt/β-catenin signaling pathway by various factors is critically required for anterior specification. Here, we report that Flop1 and Flop2 (Flop1/2), G protein-coupled receptors related to Gpr4, contribute to the regulation of head formation by inhibiting Wnt/β-catenin signaling in Xenopus embryos. Using whole-mount in situ hybridization, we showed that flop1 and flop2 mRNAs were expressed in the neural ectoderm during early gastrulation. Both the overexpression and knockdown of Flop1/2 resulted in altered embryonic head phenotypes, while the overexpression of either Flop1/2 or the small GTPase RhoA in the absence of bone morphogenetic protein (BMP) signaling resulted in ectopic head induction. Examination of the Flops' function in Xenopus embryo animal cap cells showed that they inhibited Wnt/β-catenin signaling by promoting β-catenin degradation through both RhoA-dependent and -independent pathways in a cell-autonomous manner. These results suggest that Flop1 and Flop2 are essential regulators of Xenopus head formation that act as novel inhibitory components of the Wnt/β-catenin signaling pathway.

Evolutionary genomics and adaptive evolution of the Hedgehog gene family (Shh, Ihh and Dhh) in vertebrates

The Hedgehog (Hh) gene family codes for a class of secreted proteins composed of two active domains that act as signalling molecules during embryo development, namely for the development of the nervous and skeletal systems and the formation of the testis cord. While only one Hh gene is found typically in invertebrate genomes, most vertebrates species have three (Sonic hedgehog – Shh; Indian hedgehog – Ihh; and Desert hedgehog – Dhh), each with different expression patterns and functions, which likely helped promote the increasing complexity of vertebrates and their successful diversification. In this study, we used comparative genomic and adaptive evolutionary analyses to characterize the evolution of the Hh genes in vertebrates following the two major whole genome duplication (WGD) events. To overcome the lack of Hh-coding sequences on avian publicly available databases, we used an extensive dataset of 45 avian and three non-avian reptilian genomes to show that birds have all three Hh paralogs. We find suggestions that following the WGD events, vertebrate Hh paralogous genes evolved independently within similar linkage groups and under different evolutionary rates, especially within the catalytic domain. The structural regions around the ion-binding site were identified to be under positive selection in the signaling domain. These findings contrast with those observed in invertebrates, where different lineages that experienced gene duplication retained similar selective constraints in the Hh orthologs. Our results provide new insights on the evolutionary history of the Hh gene family, the functional roles of these paralogs in vertebrate species, and on the location of mutational hotspots.

FoxA4 favours notochord formation by inhibiting contiguous mesodermal fates and restricts anterior neural development in Xenopus embryos

In vertebrates, the embryonic dorsal midline is a crucial signalling centre that patterns the surrounding tissues during development. Members of the FoxA subfamily of transcription factors are expressed in the structures that compose this centre. Foxa2 is essential for dorsal midline development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate. The related gene foxA4 is only present in amphibians. Expression begins in the blastula -chordin and -noggin expressing centre (BCNE) and is later restricted to the dorsal midline derivatives of the Spemann's organiser. It was suggested that the early functions of mammalian foxa2 are carried out by foxA4 in frogs, but functional experiments were needed to test this hypothesis. Here, we show that some important dorsal midline functions of mammalian foxa2 are exerted by foxA4 in Xenopus. We provide new evidence that the latter prevents the respecification of dorsal midline precursors towards contiguous fates, inhibiting prechordal and paraxial mesoderm development in favour of the notochord. In addition, we show that foxA4 is required for the correct regionalisation and maintenance of the central nervous system. FoxA4 participates in constraining the prospective rostral forebrain territory during neural specification and is necessary for the correct segregation of the most anterior ectodermal derivatives, such as the cement gland and the pituitary anlagen. Moreover, the early expression of foxA4 in the BCNE (which contains precursors of the whole forebrain and most of the midbrain and hindbrain) is directly required to restrict anterior neural development.

Hedgehog activity controls opening of the primary mouth

To feed or breathe, the oral opening must connect with the gut. The foregut and oral tissues converge at the primary mouth, forming the buccopharyngeal membrane (BPM), a bilayer epithelium. Failure to form the opening between gut and mouth has significant ramifications, and many craniofacial disorders have been associated with defects in this process. Oral perforation is characterized by dissolution of the BPM, but little is known about this process. In humans, failure to form a continuous mouth opening is associated with mutations in Hedgehog (Hh) pathway members; however, the role of Hh in primary mouth development is untested. Here, we show, using Xenopus, that Hh signaling is necessary and sufficient to initiate mouth formation, and that Hh activation is required in a dose-dependent fashion to determine the size of the mouth. This activity lies upstream of the previously demonstrated role for Wnt signal inhibition in oral perforation. We then turn to mouse mutants to establish that SHH and Gli3 are indeed necessary for mammalian mouth development. Our data suggest that Hh-mediated BPM persistence may underlie oral defects in human craniofacial syndromes.

Development of the annelid axochord: insights into notochord evolution

The origin of chordates has been debated for more than a century, with one key issue being the emergence of the notochord. In vertebrates, the notochord develops by convergence and extension of the chordamesoderm, a population of midline cells of unique molecular identity. We identify a population of mesodermal cells in a developing invertebrate, the marine annelid Platynereis dumerilii, that converges and extends toward the midline and expresses a notochord-specific combination of genes. These cells differentiate into a longitudinal muscle, the axochord, that is positioned between central nervous system and axial blood vessel and secretes a strong collagenous extracellular matrix. Ancestral state reconstruction suggests that contractile mesodermal midline cells existed in bilaterian ancestors. We propose that these cells, via vacuolization and stiffening, gave rise to the chordate notochord.

Stabilization of speckle-type POZ protein (Spop) by Daz interacting protein 1 (Dzip1) is essential for Gli turnover and the proper output of Hedgehog signaling

The Hedgehog (Hh) pathway is essential for embryonic development and adult tissue homeostasis. The Gli/Cubitus interruptus (Ci) family of transcription factors acts at the downstream end of the pathway to mediate Hh signaling. Both Hh-dependent and -independent Gli regulatory mechanisms are important for the output of Hh signaling. Daz interacting protein 1 (Dzip1) has bipartite positive and negative functions in the Hh pathway. The positive Hh regulatory function appears to be attributed to a requirement for Dzip1 during ciliogenesis. The mechanism by which Dzip1 inhibits Hh signaling, however, remains largely unclear. We recently found that Dzip1 is required for Gli turnover, which may account for its inhibitory function in Hh signaling. Here, we report that Dzip1 regulates Gli/Ci turnover by preventing degradation of speckle-type POZ protein (Spop), a protein that promotes proteasome-dependent turnover of Gli proteins. We provide evidence that Dzip1 regulates the stability of Spop independent of its function in ciliogenesis. Partial knockdown of Dzip1 to levels insufficient for perturbing ciliogenesis, sensitized Xenopus embryos to Hh signaling, leading to phenotypes that resemble activation of Hh signaling. Importantly, overexpression of Spop was able to restore proper Gli protein turnover and rescue phenotypes in Dzip1-depleted embryos. Consistently, depletion of Dzip1 in Drosophila S2 cells destabilized Hh-induced BTB protein (HIB), the Drosophila homolog of Spop, and increased the level of Ci. Thus, Dzip1-dependent stabilization of Spop/HIB is evolutionarily conserved and essential for proper regulation of Gli/Ci proteins in the Hh pathway.

An intact brachyury function is necessary to prevent spurious axial development in Xenopus laevis

We have previously shown that the member of the HES family hairy2 induces the ectopic expression of dorsal markers when it is overexpressed in the ventral side of Xenopus embryos. Intriguingly, hairy2 represses the mesoderm transcription factor brachyury (bra) throughout its domain in the marginal zone. Here we show that in early gastrula, bra and hairy2 are expressed in complementary domains. Overexpression of bra repressed hairy2. Interference of bra function with a dominant-negative construct expanded the hairy2 domain and, like hairy2 overexpression, promoted ectopic expression of dorsal axial markers in the ventral side and induced secondary axes without head and notochord. Hairy2 depletion rescued the ectopic dorsal development induced by interference of bra function. We concluded that an intact bra function is necessary to exclude hairy2 expression from the non-organiser field, to impede the ectopic specification of dorsal axial fates and the appearance of incomplete secondary axes. This evidence supports a previously unrecognised role for bra in maintaining the dorsal fates inhibited in the ventral marginal zone, preventing the appearance of trunk duplications.

Tet3 CXXC domain and dioxygenase activity cooperatively regulate key genes for Xenopus eye and neural development

Ten-Eleven Translocation (Tet) family of dioxygenases dynamically regulates DNA methylation and has been implicated in cell lineage differentiation and oncogenesis. Yet their functions and mechanisms of action in gene regulation and embryonic development are largely unknown. Here, we report that Xenopus Tet3 plays an essential role in early eye and neural development by directly regulating a set of key developmental genes. Tet3 is an active 5mC hydroxylase regulating the 5mC/5hmC status at target gene promoters. Biochemical and structural studies further demonstrate that the Tet3 CXXC domain is critical for specific Tet3 targeting. Finally, we show that the enzymatic activity and CXXC domain are both crucial for Tet3's biological function. Together, these findings define Tet3 as a transcription regulator and reveal a molecular mechanism by which the 5mC hydroxylase and DNA binding activities of Tet3 cooperate to control target gene expression and embryonic development.

Antagonistic cross-regulation between Wnt and Hedgehog signalling pathways controls post-embryonic retinal proliferation

Continuous neurogenesis in the adult nervous system requires a delicate balance between proliferation and differentiation. Although Wnt/β-catenin and Hedgehog signalling pathways are thought to share a mitogenic function in adult neural stem/progenitor cells, it remains unclear how they interact in this process. Adult amphibians produce retinal neurons from a pool of neural stem cells localised in the ciliary marginal zone (CMZ). Surprisingly, we found that perturbations of the Wnt and Hedgehog pathways result in opposite proliferative outcomes of neural stem/progenitor cells in the CMZ. Additionally, our study revealed that Wnt and Hedgehog morphogens are produced in mutually exclusive territories of the post-embryonic retina. Using genetic and pharmacological tools, we found that the Wnt and Hedgehog pathways exhibit reciprocal inhibition. Our data suggest that Sfrp-1 and Gli3 contribute to this negative cross-regulation. Altogether, our results reveal an unexpected antagonistic interplay of Wnt and Hedgehog signals that may tightly regulate the extent of neural stem/progenitor cell proliferation in the Xenopus retina.

Palmitoylation of Hedgehog proteins

Hedgehog (Hh) proteins are secreted signaling proteins that contain amide-linked palmitate at the N-terminus and cholesterol at the C-terminus. Palmitoylation of Hh proteins is critical for effective long- and short-range signaling. The palmitoylation reaction occurs during transit of Hh through the secretory pathway, most likely in the lumen of the ER. Attachment of palmitate to Hh proteins is independent of cholesterol modification and autoprocessing and is catalyzed by Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane bound O-acyltransferase (MBOAT) family, a subgroup of multipass membrane proteins that catalyze transfer of fatty acyl groups to lipids and proteins. Several classes of secreted proteins have recently been shown to be substrates for MBOAT acyltransferases, including Hh proteins and Spitz (palmitoylated by Hhat), Wg/Wnt proteins (modified with palmitate and/or palmitoleate by Porcupine) and ghrelin (octanoylated by ghrelin O-acyltransferase). These findings highlight protein fatty acylation as a mechanism that not only influences membrane binding of intracellular proteins but also regulates the signaling range and efficacy of secreted proteins.

The dual regulator Sufu integrates Hedgehog and Wnt signals in the early Xenopus embryo

Hedgehog (Hh) and Wnt proteins are important signals implicated in several aspects of embryonic development, including the early development of the central nervous system. We found that Xenopus Suppressor-of-fused (XSufu) affects neural induction and patterning by regulating the Hh/Gli and Wnt/β-catenin pathways. Microinjection of XSufu mRNA induced expansion of the epidermis at the expense of neural plate tissue and caused enlargement of the eyes. An antisense morpholino oligonucleotide against XSufu had the opposite effect. Interestingly, both gain- and loss-of-function experiments resulted in a posterior shift of brain markers, suggesting a biphasic effect of XSufu on anteroposterior patterning. XSufu blocked early Wnt/β-catenin signaling, as indicated by the suppression of XWnt8-induced secondary axis formation in mRNA-injected embryos, and activation of Wnt target genes in XSufu-MO-injected ectodermal explants. We show that XSufu binds to XGli1 and Xβ-catenin. In Xenopus embryos and mouse embryonic fibroblasts, Gli1 inhibits Wnt signaling under overexpression of β-catenin, whereas β-catenin stimulates Hh signaling under overexpression of Gli1. Notably, endogenous Sufu is critically involved in this crosstalk. The results suggest that XSufu may act as a common regulator of Hh and Wnt signaling and contribute to intertwining the two pathways.

Adult-type myogenesis of the frog Xenopus laevis specifically suppressed by notochord cells but promoted by spinal cord cells in vitro

Larval-to-adult myogenic conversion occurs in the dorsal muscle but not in the tail muscle during Xenopus laevis metamorphosis. To know the mechanism for tail-specific suppression of adult myogenesis, response character was compared between adult myogenic cells (Ad-cells) and larval tail myogenic cells (La-cells) to a Sonic hedgehog (Shh) inhibitor, notochord (Nc) cells, and spinal cord (SC) cells in vitro. Cyclopamine, an Shh inhibitor, suppressed the differentiation of cultured Ad (but not La) cells, suggesting the significance of Shh signaling in promoting adult myogenesis. To test the possibility that Shh-producing axial elements (notochord and spinal cord) regulate adult myogenesis, Ad-cells or La-cells were co-cultured with Nc or SC cells. The results showed that differentiation of Ad-cells were strongly inhibited by Nc cells but promoted by SC cells. If Ad-cells were "separately" co-cultured with Nc cells without direct cell-cell interactions, adult differentiation was not inhibited but rather promoted, suggesting that Nc cells have two roles, one is a short-range suppression and another is a long-range promotion for adult myogenesis. Immunohistochemical analysis showed both notochord and spinal cord express the N-terminal Shh fragment throughout metamorphosis. The "spinal cord-promotion" and long-range effect by Nc cells on adult myogenesis is thus involved in Shh signaling, while the signaling concerning the short-range "Nc suppression" will be determined by future studies. Interestingly, these effects, "Nc suppression" and "SC promotion" were not observed for La-cells. Situation where the spinal cord/notochord cross-sectional ratio is quite larger in tadpole trunk than in the tail seems to contribute to trunk-specific promotion and tail-specific suppression of adult myogenesis during Xenopus metamorphosis.

Xenopus sonic hedgehog guides retinal axons along the optic tract

The role of classic morphogens such as Sonic hedgehog (Shh) as axon guidance cues has been reported in a variety of vertebrate organisms (Charron and Tessier-Lavigne [2005] Development 132:2251-2262). In this work, we provide the first evidence that Xenopus sonic hedgehog (Xshh) signaling is involved in guiding retinal ganglion cell (RGC) axons along the optic tract. Xshh is expressed in the brain during retinal axon extension, adjacent to these axons in the ventral diencephalon. Retinal axons themselves express Patched 1 and Smoothened co-receptors during RGC axon growth. Blocking Shh signaling causes abnormal ventral pathfinding, and targeting errors at the optic tectum. Misexpression of exogenous N-Shh peptide in vivo also causes pathfinding errors. Retinal axons grown in culture respond to N-Shh in a dose-dependent manner, either by decreasing extension at lower concentrations, or retracting axons in the presence of higher doses. These data suggest that Shh signaling is required for normal RGC axon pathfinding and tectal targeting in the developing visual system of Xenopus. We propose that Shh serves as a ventral optic tract repellent that helps to define the caudal boundary for retinal axons in the diencephalon, and that this signaling is also required for initial target recognition at the optic tectum.

A revised model of Xenopus dorsal midline development: differential and separable requirements for Notch and Shh signaling

The development of the vertebrate dorsal midline (floor plate, notochord, and hypochord) has been an area of classical research and debate. Previous studies in vertebrates have led to contrasting models for the roles of Shh and Notch signaling in specification of the floor plate, by late inductive or early allocation mechanisms, respectively. Here, we show that Notch signaling plays an integral role in cell fate decisions in the dorsal midline of Xenopus laevis, similar to that observed in zebrafish and chick. Notch signaling promotes floor plate and hypochord fates over notochord, but has variable effects on Shh expression in the midline. In contrast to previous reports in frog, we find that Shh signaling is not required for floor plate vs. notochord decisions and plays a minor role in floor plate specification, where it acts in parallel to Notch signaling. As in zebrafish, Shh signaling is required for specification of the lateral floor plate in the frog. We also find that the medial floor plate in Xenopus comprises two distinct populations of cells, each dependent upon different signals for its specification. Using expression analysis of several midline markers, and dissection of functional relationships, we propose a revised allocation mechanism of dorsal midline specification in Xenopus. Our model is distinct from those proposed to date, and may serve as a guide for future studies in frog and other vertebrate organisms.

Visualization of Gli activity in craniofacial tissues of hedgehog-pathway reporter transgenic zebrafish

Background

The Hedgehog (Hh)-signaling pathway plays a crucial role in the development and maintenance of multiple vertebrate and invertebrate organ systems. Gli transcription factors are regulated by Hh-signaling and act as downstream effectors of the pathway to activate Hh-target genes. Understanding the requirements for Hh-signaling in organisms can be gained by assessing Gli activity in a spatial and temporal fashion.

Methodology/Principal Findings

We have generated a Gli-dependent (Gli-d) transgenic line, Tg(Gli-d:mCherry), that allows for rapid and simple detection of Hh-responding cell populations in both live and fixed zebrafish. This transgenic line expresses a mCherry reporter under the control of a Gli responsive promoter, which can be followed by using fluorescent microscopy and in situ hybridization. Expression of the mCherry transgene reporter during embryogenesis and early larval development faithfully replicated known expression domains of Hh-signaling in zebrafish, and abrogating Hh-signaling in transgenic fish resulted in the suppression of reporter expression. Moreover, ectopic shh expression in Tg(Glid:mCherry) fish led to increased transgene production. Using this transgenic line we investigated the nature of Hh-pathway response during early craniofacial development and determined that the neural crest skeletal precursors do not directly respond to Hh-signaling prior to 48 hours post fertilization, suggesting that earlier requirements for pathway activation in this population of facial skeleton precursors are indirect.

Conclusion/Significance

We have determined that early Hh-signaling requirements in craniofacial development are indirect. We further demonstrate the Tg(Gli-d:mCherry) fish are a highly useful tool for studying Hh-signaling dependent processes during embryogenesis and larval stages.

Expression of components of Wnt and Hedgehog pathways in different tissue layers during lung development in Xenopus laevis

Although Wnt and Hedgehog (Hh) signaling pathways play important roles in mouse lung development, these have not been explored in the development of Xenopus lung. This may be due to the lack of specific molecular markers for different layers of tissue in Xenopus lung and/or insufficient knowledge on expression patterns of Wnt and Hh signaling components in Xenopus lung. In this study, we first described the early morphogenesis of Xenopus laevis lung by using surfactant protein C (sftpc) as a marker of lung epithelium and compared it with the expression patterns of several genes of Wnt and Hh pathways in Xenopus lungs. Our data showed that wnt7b was expressed in the entire lung epithelium from stage 37 to stage 45, while two other Wnt signaling components, wnt5a and wif1 (wnt inhibitory factor 1), were expressed in the mesenchyme layer of the entire lungs through stages 39-41. We also found that sonic hedgehog (shh) was expressed at stage 41 only in the anterior, but not in the posterior part of the lungs. These results show the expression of wnt5a, wnt7b, wif1 and shh in different layers of tissue of Xenopus lungs at early developmental stages, which implies different roles of these genes in the early development of Xenopus lungs. Our study for the first time defined specific molecular markers for description of early lung development in Xenopus, as well as provided information about expression of components of Wnt and Hh pathways in early Xenopus lungs, which should be useful for future functional studies.

Sonic hedgehog expression during Xenopus laevis forebrain development

We have analyzed the developing expression pattern of x-Shh in the Xenopus forebrain, interpreting the results within the framework of the neuromeric model to assess evolutionary trends and clues. To achieve this goal, we have characterized phenotypically the developing x-Shh expressing forebrain subdivisions and neurons by means of the combination of in situ hybridization for x-Shh and immunohistochemistry for the detection of forebrain essential regulators and markers, such as the homeodomain transcription factors Islet 1, Orthopedia, NKX2.1 and NKX2.2 and tyrosine hydroxylase. Substantial evidence was found for x-Shh expression in the telencephalic commissural preoptic area and this is strongly correlated with the presence of a pallidum and/or a basal telencephalic cholinergic system. In the diencephalon, x-Shh was demonstrated in the zona limitans intrathalamica and the x-Shh expressing cells were extended into the prethalamus. Throughout development and in the adult hypothalamic x-Shh expression was strong in basal regions but, in addition, in the alar suprachiasmatic region. The findings obtained in the forebrain of Xenopus revealed a largely conserved pattern of Shh expression among tetrapods. However, interesting differences were also noted that could be related to evolutive changes in forebrain organization.

Paracrine Hedgehog signaling in stomach and intestine: new roles for hedgehog in gastrointestinal patterning

BACKGROUND & AIMS

Hedgehog signaling is critical in gastrointestinal patterning. Mice deficient in Hedgehog signaling exhibit abnormalities that mirror deformities seen in the human VACTERL (vertebral, anal, cardiac, tracheal, esophageal, renal, limb) association. However, the direction of Hedgehog signal flow is controversial and the cellular targets of Hedgehog signaling change with time during development. We profiled cellular Hedgehog response patterns from embryonic day 10.5 (E10.5) to adult in murine antrum, pyloric region, small intestine, and colon.

METHODS

Hedgehog signaling was profiled using Hedgehog pathway reporter mice and in situ hybridization. Cellular targets were identified by immunostaining. Ihh-overexpressing transgenic animals were generated and analyzed.

RESULTS

Hedgehog signaling is strictly paracrine from antrum to colon throughout embryonic and adult life. Novel findings include the following: mesothelial cells of the serosa transduce Hedgehog signals in fetal life; the hindgut epithelium expresses Ptch but not Gli1 at E10.5; the 2 layers of the muscularis externa respond differently to Hedgehog signals; organogenesis of the pyloric sphincter is associated with robust Hedgehog signaling; dramatically different Hedgehog responses characterize stomach and intestine at E16; and after birth, the muscularis mucosa and villus smooth muscle consist primarily of Hedgehog-responsive cells and Hh levels actively modulate villus core smooth muscle.

CONCLUSIONS

These studies reveal a previously unrecognized association of paracrine Hedgehog signaling with several gastrointestinal patterning events involving the serosa, pylorus, and villus smooth muscle. The results may have implications for several human anomalies and could potentially expand the spectrum of the human VACTERL association.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

Cell-cell interactions during remodeling of the intestine at metamorphosis in Xenopus laevis

Amphibian metamorphosis is accompanied by extensive intestinal remodeling. This process, mediated by thyroid hormone (TH) and its nuclear receptors, affects every cell type. Gut remodeling in Xenopus laevis involves epithelial and mesenchymal proliferation, smooth muscle thickening, neuronal aggregation, formation of intestinal folds, and shortening of its length by 75%. Transgenic tadpoles expressing a dominant negative TH receptor (TRDN) controlled by epithelial-, fibroblast-, and muscle-specific gene promoters were studied. TRDN expression in the epithelium caused abnormal development of virtually all cell types, with froglet guts displaying reduced intestinal folds, thin muscle and mesenchyme, absence of neurons, and reduced cell proliferation. TRDN expression in fibroblasts caused abnormal epithelia and mesenchyme development, and expression in muscle produced fewer enteric neurons and a reduced inter-muscular space. Gut shortening was inhibited only when TRDN was expressed in fibroblasts. Gut remodeling results from both cell-autonomous and cell-cell interactions.

Cephalic hedgehog expression is regulated directly by Sox17 in endoderm development of Xenopus laevis

In early development of animals, hedgehog (Hh) genes function as morphogen in the axis determination and the organ formation. In Xenopus, three hedgehog genes, sonic (shh), banded (bhh), and cephalic (chh), were identified and might organize various tissues and organs in embryogenesis. Here, we report the spatial and temporal regulation of Xchh which is expressed in endoderm cells differentiating to digestive organs. Xchh expression in endoderm was inhibited by ectopic expression of the dominant-negative activin receptor, tAR. Moreover, a maternally inherited transcription factor VegT and its downstream regulators activated Xchh expression. These indicates that Xchh is regulated by the factor involved in the cascade originated from VegT via activin/nodal signals. Using the Sox17alpha-VP16-GR construct, we showed that Xchh expression might be induced directly by transcription factor Sox17.

Retinoid signaling can repress blastula Wnt signaling and impair dorsal development in Xenopus embryo

Vitamin A derivatives (retinoids) are actively involved during vertebrate embryogenesis. However, exogenous retinoids have also long been known as potent teratogens. The defects caused by retinoid treatment are complex. Here, we provided evidence that RAR-mediated retinoid signaling can repress Xenopus blastula Wnt signaling and impair dorsal development. Exogenous retinoic acid (RA) could antagonize the dorsalizing effects of lithium chloride-mediated Wnt activation in blastula embryos. The Wnt-responsive reporter gene transgenesis and luciferase assay showed that excess RA can repress the Wnt signaling in blastula embryos. In addition, the downstream target genes of the Wnt signaling that direct embryonic dorsal development, were also down-regulated in the RA-treated embryos. Mechanically, RA did not interfere with the stability of beta-catenin, but promoted its nuclear accumulation. The inverse agonist of retinoic acid receptors (RAR) rescued the Wnt signaling repression by RA and relieved the RA-induced nuclear accumulation of beta-catenin. Our results explain one of the reasons for the complicated teratogenic effects of retinoids and shed light on the endogenous way of interactions between two developmentally important signaling pathways.

Xenopus Tbx6 mediates posterior patterning via activation of Wnt and FGF signalling

In vertebrates, the patterning of anterior-posterior (AP) axis is a fundamental process during embryogenesis. Wnt and FGF signalling pathways play important roles in regulating the patterning of embryo AP axis. Mouse Tbx6 encodes a transcription factor that has been demonstrated to be involved in the specification of the posterior tissue in mouse embryonic body. Here, we prove that morpholino-induced knockdown of XTbx6 impairs posterior development, indicating the requirement of XTbx6 in this process. Meanwhile, gain of XTbx6 function is sufficient to induce ectopic posterior structures in Xenopus embryos. Furthermore, XTbx6 activates the expression of Xwnt8 and FGF8, which are two mediators of posterior development, suggesting a mechanism by which XTbx6 modulates posterior patterning via Wnt and FGF signalling pathway activation.

The Hedgehog gene family of the cnidarian, Nematostella vectensis, and implications for understanding metazoan Hedgehog pathway evolution

Hedgehog signaling is an important component of cell-cell communication during bilaterian development, and abnormal Hedgehog signaling contributes to disease and birth defects. Hedgehog genes are composed of a ligand ("hedge") domain and an autocatalytic intein ("hog") domain. Hedgehog (hh) ligands bind to a conserved set of receptors and activate downstream signal transduction pathways terminating with Gli/Ci transcription factors. We have identified five intein-containing genes in the anthozoan cnidarian Nematostella vectensis, two of which (NvHh1 and NvHh2) contain definitive hedgehog ligand domains, suggesting that to date, cnidarians are the earliest branching metazoan phylum to possess definitive Hh orthologs. Expression analysis of NvHh1 and NvHh2, the receptor NvPatched, and a downstream transcription factor NvGli (a Gli3/Ci ortholog) indicate that these genes may have conserved roles in planar and trans-epithelial signaling during gut and germline development, while the three remaining intein-containing genes (NvHint1,2,3) are expressed in a cell-type-specific manner in putative neural precursors. Metazoan intein-containing genes that lack a hh ligand domain have previously only been identified within nematodes. However, we have identified intein-containing genes from both Nematostella and in two newly annotated lophotrochozoan genomes. Phylogenetic analyses suggest that while nematode inteins may be derived from an ancestral true hedgehog gene, the newly identified cnidarian and lophotrochozoan inteins may be orthologous, suggesting that both true hedgehog and hint genes may have been present in the cnidarian-bilaterian ancestor. Genomic surveys of N. vectensis suggest that most of the components of both protostome and deuterostome Hh signaling pathways are present in anthozoans and that some appear to have been lost in ecdysozoan lineages. Cnidarians possess many bilaterian cell-cell signaling pathways (Wnt, TGFbeta, FGF, and Hh) that appear to act in concert to pattern tissues along the oral-aboral axis of the polyp. Cnidarians represent a diverse group of animals with a predominantly epithelial body plan, and perhaps selective pressures to pattern epithelia resulted in the ontogeny of the hedgehog pathway in the common ancestor of the Cnidaria and Bilateria.

Two alloalleles of Xenopus laevis hairy2 gene--evolution of duplicated gene function from a developmental perspective

Gene duplication is a fundamental source of a new gene in the process of evolution. A duplicated gene is able to accept many kinds of mutations that could lead to loss of function or novel phenotypic diversity. Alternatively, the duplicated genes complementarily lose part of their functions to play original roles as a set of genes, a process called subfunctionalization. Pseudotetraploid frog Xenopus laevis has four sets of genes, and it is generally thought that the alloalleles in X. laevis have mutually indistinguishable functions. In this paper, we report differences and similarities between Xhairy2a and Xhairy2b in the neural crest, floor plate, and prechordal plate. Knockdown studies showed that Xhairy2a seems not to function in the neural crest, although both of them are required in the floor plate and the prechordal plate. Temporal expression pattern analysis revealed that Xhairy2a is a maternal factor having lower zygotic expression than Xhairy2b, while Xhairy2b is not loaded in the egg but has high zygotic expression. Spatial expression pattern analysis demonstrated that future floor plate expression is shared by both alloalleles, but Xhairy2b expression in the neural crest is much higher than Xhairy2a expression, consistent with the results of individual knockdown experiments. Therefore, our data suggest that subfunctionalization occurs in Xhairy2.

Ablation studies on the developing inner ear reveal a propensity for mirror duplications

The inner ear develops from a simple ectodermal thickening known as the otic placode. Classic embryological manipulations rotating the prospective placode tissue found that the anteroposterior axis was determined before the dorsoventral axis. A small percentage of such rotations also resulted in the formation of mirror duplicated ears, or enantiomorphs. We demonstrate a different embryological manipulation in the frog Xenopus: the physical removal or ablation of either the anterior or posterior half of the placode, which results in an even higher percentage of mirror image ears. Removal of the posterior half results in mirror anterior duplications, whereas removal of the anterior half results in mirror posterior duplications. In contrast, complete extirpation results in more variable phenotypes but never mirror duplications. By the time the otocyst separates from the surface ectoderm, complete extirpation results in no regeneration. To test for a dosage response, differing amounts of the placode or otocyst were ablated. Removal of one third of the placode resulted in normal ears, whereas two-thirds ablations resulted in abnormal ears, including mirror duplications. Recent studies in zebrafish have demonstrated a role for the hedgehog (Hh) signaling pathway in anteroposterior patterning of the developing ear. We have used overexpression of Hedgehog interacting protein (Hip) to block Hh signaling and find that this strategy resulted in mirror duplications of anterior structures, consistent with the results in zebrafish.

Hedgehog signaling regulates the amount of hypaxial muscle development during Xenopus myogenesis

Hedgehog (Hh) signaling is proposed to have different roles on differentiation of hypaxial myoblasts of amniotes. Within the somitic environment, Hh signals restrict hypaxial development and promote epaxial muscle formation. On the other hand, in the limb bud, Hh signaling represses hypaxial myoblast differentiation. This poses the question of whether differences in response to Hh signaling are due to variations in local environment or are intrinsic differences between pre- and post-migratory hypaxial myoblasts. We have approached this question by examining the role of Hh signaling on myoblast development in Xenopus laevis, which, due to its unique mode of hypaxial muscle development, allows us to examine myoblast development in vivo in the absence of the limb environment. Cyclopamine and sonic hedgehog (shh) mRNA overexpression were used to inhibit or activate the Hh pathway, respectively. We find that hypaxial myoblasts respond similarly to Hh manipulations regardless of their location, and that this response is the same for epaxial myoblasts. Overexpression of shh mRNA causes a premature differentiation of the dermomyotome, subsequently inhibiting all further growth of the epaxial and hypaxial myotome. Cyclopamine treatment has the opposite effect, causing an increase in dermomyotome and a shift in myoblast fate from epaxial to hypaxial, eventually leading to an excess of hypaxial body wall muscle. Cyclopamine treatment before stage 20 can rescue the effects of shh overexpression, indicating that early Hh signaling plays an essential role in maintaining the balance between epaxial and hypaxial muscle mass. After stage 20, the premature differentiation of the dermomyotome caused by shh overexpression cannot be rescued by cyclopamine, and no further embryonic muscle growth occurs.

Early molecular effects of ethanol during vertebrate embryogenesis

Fetal alcohol spectrum disorder (FASD) is the combination of developmental, morphological, and neurological defects that result from exposing human embryos to ethanol (EtOH). Numerous embryonic structures are affected, leading to a complex viable phenotype affecting among others, the anterior/posterior axis, head, and eye formation. Recent studies have provided evidence suggesting that EtOH teratogenesis is mediated in part through a reduction in retinoic acid (RA) levels, targeting mainly the embryonic organizer (Spemann's organizer) and its subsequent functions. EtOH-treated Xenopus embryos were subjected to an analysis of gene expression patterns. Analysis of organizer-specific genes revealed a transient delay in the invagination of gsc- and chordin-positive cells that eventually reach their normal rostro-caudal position. Dorsal midline genes show defects along the rostro-caudal axis, lacking either their rostral (Xbra and Xnot2) or caudal (FoxA4b and Shh) expression domains. Head-specific markers like Otx2, en2, and Shh show abnormal expression patterns. Otx2 exhibits a reduction in expression levels, while en2 becomes restricted along the dorsal/ventral axis. During neurula stages, Shh becomes up-regulated in the rostral region and it is expressed in an abnormal pattern. These results and histological analysis suggest the existence of malformations in the brain region including a lack of the normal fore brain ventricle. An increase in the size of both the prechordal plate and the notochord was observed, while the spinal cord is narrower. The reduction in head and eye size was accompanied by changes in the eye markers, Pax6 and Tbx3. Our results provide evidence for the early molecular changes induced by EtOH exposure during embryogenesis, and may explain some of the structural changes that are part of the EtOH teratogenic phenotype also in FASD individuals.

PP2A:B56ε is required for eye induction and eye field separation

Eye induction and eye field separation are the earliest events during vertebrate eye development. Both of these processes occur much earlier than the formation of optic vesicles. The insulin-like growth factor (IGF) pathway appears to be essential for eye induction, yet it remains unclear how IGF downstream pathways are involved in eye induction. As a consequence of eye induction, a single eye anlage is specified in the anterior neural plate. Subsequently, this single eye anlage is divided into two symmetric eye fields in response to Sonic Hedgehog (Shh) secreted from the prechordal mesoderm. Here, we report that B56epsilon regulatory subunit of protein phosphatase 2A (PP2A) is involved in Xenopus eye induction and subsequent eye field separation. We provide evidence that B56epsilon is required for the IGF/PI3K/Akt pathway and that interfering with the PI3K/Akt pathway inhibits eye induction. In addition, we show that B56epsilon regulates the Hedgehog (Hh) pathway during eye field separation. Thus, B56epsilon is involved in multiple signaling pathways and plays critical roles during early development.

The anuran Bauplan: a review of the adaptive, developmental, and genetic underpinnings of frog and tadpole morphology

Anurans (frogs, toads, and their larvae) are among the most morphologically derived of vertebrates. While tightly conserved across the order, the anuran Bauplan (body plan) diverges widely from that of other vertebrates, particularly with respect to the skeleton. Here we address the adaptive, ontogenetic, and genetic bases of three such hallmark anuran features: (1) the absence of discrete caudal vertebrae, (2) a truncated axial skeleton, and (3) elongate hind limbs. We review the functional significance of each as it relates to the anuran lifestyle, which includes locomotor adaptations to both aquatic and terrestrial environments. We then shift our focus to the proximal origins of each feature, namely, ontogeny and its molecular regulation. Drawing on relatively limited data, we detail the development of each character and then, by extrapolating from comparative vertebrate data, propose molecular bases for these processes. Cast in this light, the divergent morphology of anurans emerges as a product of evolutionary modulation of the generalised vertebrate developmental machinery. Specifically, we hypothesise that: (1) the formation of caudal vertebrae is precluded due to a failure of sclerotomes to form cartilaginous condensations, perhaps resulting from altered expression of a suite of genes, including Pax1, Pax9, Msx1, Uncx-4.1, Sonic hedgehog, and noggin; (2) anteriorised Hox gene expression in the paraxial mesoderm has led to a rostral shift of morphological boundaries of the vertebral column; and, (3) spatial and temporal shifts in Hox expression may underlie the expanded tarsal elements of the anuran hind limb. Technology is currently in place to investigate each of these scenarios in the African clawed frog Xenopus. Experimental corroboration will further our understanding of the molecular regulation of the anuran Bauplan and provide insight into the origin of vertebrate morphological diversity as well as the role of development in evolution.

The axolotl limb: a model for bone development, regeneration and fracture healing

Among vertebrates, urodele amphibians (e.g., axolotls) have the unique ability to perfectly regenerate complex body parts after amputation. The limb has been the most widely studied due to the presence of three defined axes and its ease of manipulation. Hence, the limb has been chosen as a model to study the process of skeletogenesis during axolotl development, regeneration and to analyze this animal's ability to heal bone fractures. Extensive studies have allowed researchers to gain some knowledge of the mechanisms controlling growth and pattern formation in regenerating and developing limbs, offering an insight into how vertebrates are able to regenerate tissues. In this study, we report the cloning and characterization of two axolotl genes; Cbfa-1, a transcription factor that controls the remodeling of cartilage into bone and PTHrP, known for its involvement in the differentiation and maturation of chondrocytes. Whole-mount in situ hybridization and immunohistochemistry results show that Cbfa-1, PTHrP and type II collagen are expressed during limb development and regeneration. These genes are expressed during specific stages of limb development and regeneration which are consistent with the appearance of skeletal elements. The expression pattern for Cbfa-1 in late limb development was similar to the expression pattern found in the late stages of limb regeneration (i.e. re-development phase) and it did not overlap with the expression of type II collagen. It has been reported that the molecular mechanisms involved in the re-development phase of limb regeneration are a recapitulation of those used in developing limbs; therefore the detection of Cbfa-1 expression during regeneration supports this assertion. Conversely, PTHrP expression pattern was different during limb development and regeneration, by its intensity and by the localization of the signal. Finally, despite its unsurpassed abilities to regenerate, we tested whether the axolotl was able to regenerate non-union bone fractures. We show that while the axolotl is able to heal a non-stabilized union fracture, like other vertebrates, it is incapable of healing a bone gap of critical dimension. These results suggest that the axolotl does not use the regeneration process to repair bone fractures.

Manipulation of hedgehog signaling in Xenopus by means of embryo microinjection and application of chemical inhibitors

Xenopus embryos provide a powerful model system to investigate the complex molecular mechanisms, which are controlled by or control the activity of the Hedgehog (Hh) signaling pathway. The use of synthetic mRNA or antisense oligonucleotide (morpholino) microinjection into blastomeres of early embryos or by simply treating the embryos with small organic inhibitors, has already led to an idea of the network in which the Hh pathway is embedded. More needs to be done in order to achieve a detailed understanding of how the different players of the Hh signaling pathway are integrated to control different genetic programs, such as axis formation in early embryos or cell differentiation during retinogenesis.

Hedgehog signaling and the retina: insights into the mechanisms controlling the proliferative properties of neural precursors

Hedgehog signaling has been linked to cell proliferation in a variety of systems; however, its effects on the cell cycle have not been closely studied. In the vertebrate retina, Hedgehog's effects are controversial, with some reports emphasizing increased proliferation and others pointing to a role in cell cycle exit. Here we demonstrate a novel role for Hedgehog signaling in speeding up the cell cycle in the developing retina by reducing the length of G1 and G2 phases. These fast cycling cells tend to exit the cell cycle early. Conversely, retinal progenitors with blocked Hedgehog signaling cycle more slowly, with longer G1 and G2 phases, and remain in the cell cycle longer. Hedgehog may modulate cell cycle kinetics through activation of the key cell cycle activators cyclin D1, cyclin A2, cyclin B1, and cdc25C. These findings support a role for Hedgehog in regulating the conversion from slow cycling stem cells to fast cycling transient amplifying progenitors that are closer to cell cycle exit.

Genomics and expression profiles of the Hedgehog and Notch signaling pathways in sea urchin development

The Hedgehog (Hh) and Notch signal transduction pathways control a variety of developmental processes including cell fate choice, differentiation, proliferation, patterning and boundary formation. Because many components of these pathways are conserved, it was predicted and confirmed that pathway components are largely intact in the sea urchin genome. Spatial and temporal location of these pathways in the embryo, and their function in development offer added insight into their mechanistic contributions. Accordingly, all major components of both pathways were identified and annotated in the sea urchin Strongylocentrotus purpuratus genome and the embryonic expression of key components was explored. Relationships of the pathway components, and modifiers predicted from the annotation of S. purpuratus, were compared against cnidarians, arthropods, urochordates, and vertebrates. These analyses support the prediction that the pathways are highly conserved through metazoan evolution. Further, the location of these two pathways appears to be conserved among deuterostomes, and in the case of Notch at least, display similar capacities in endomesoderm gene regulatory networks. RNA expression profiles by quantitative PCR and RNA in situ hybridization reveal that Hedgehog is produced by the endoderm beginning just prior to invagination, and signals to the secondary mesenchyme-derived tissues at least until the pluteus larva stage. RNA in situ hybridization of Notch pathway members confirms that Notch functions sequentially in the vegetal-most secondary mesenchyme cells and later in the endoderm. Functional analyses in future studies will embed these pathways into the growing knowledge of gene regulatory networks that govern early specification and morphogenesis.

The left-right axis is regulated by the interplay of Coco, Xnr1 and derrière in Xenopus embryos

Formation of the left-right axis involves a symmetry-breaking signal originating in the node or its equivalents, which increases TGF-beta signaling on the left side of the embryo and ultimately leads to asymmetric patterning of the viscera. DAN domain proteins are extracellular inhibitors of TGF-beta ligands, and are involved in regulating the left-right axis in chick, mouse and zebrafish. We find that Coco, a Xenopus DAN family member, and two TGF-beta ligands, Xnr1 and derrière, are coexpressed in the posterior paraxial mesoderm at neurula stage. Side-specific protein depletion demonstrated that left-right patterning requires Coco exclusively on the right side, and Xnr1 and derrière exclusively on the left, despite their bilateral expression pattern. In the absence of Coco, the TGF-beta signal is bilateral. Interactions among the three proteins show that derrière is required for normal levels of Xnr1 expression, while Coco directly inhibits both ligands. We conclude that derrière, Xnr1, and Coco define a posttranscriptionally regulated signaling center, which is a necessary link in the signaling chain leading to an increased TGF-beta signal on the left side of the embryo.

The presumptive floor plate (notoplate) induces behaviors associated with convergent extension in medial but not lateral neural plate cells of Xenopus

In previous work (Elul, T., Keller, R., 2000. Monopolar protrusive activity: a new morphogenic cell behavior in the neural plate dependent on vertical interactions with the mesoderm in Xenopus. Dev. Biol. 224, 3-19; Ezin, A.M., Skoglund, P. Keller, R. 2003. The midline (notochord and notoplate) patterns the cell motility underlying convergence and extension of the Xenopus neural plate. Dev. Biol. 256, 100-114), the midline tissues of notochord and overlying notoplate were found to induce the monopolar, medially directed protrusive activity of deep neural cells. This behavior is thought to drive the mediolateral intercalation and convergent extension of the neural plate in Xenopus. Here we address the issue of whether the notochord, the notoplate, or both is essential for this induction. Our strategy was to remove the notochord, leaving the overlying notoplate intact, and determine whether it alone can induce the monopolar, medially directed cell behavior. We first establish that the notoplate (presumptive floor plate), when separated from the underlying notochord in the early neurula (stages 13-14), will independently mature into a floor plate as assayed three criteria: (1) continued expression of an early marker, sonic hedgehog, and a later, marker, F-spondin; (2) the display of the notoplate/floor plate-specific randomly oriented protrusive activity; (3) the characteristic lack of mixing of cells between the notoplate and lateral neural plate. Under these conditions, in the presence of a mature notoplate/floor plate and in the absence of the notochord, the characteristic monopolar, medially directed behavior occurred, but only locally near the midline. These results show that the notoplate/floor plate capacity to induce the medially directed motility is limited in range, and they suggest that the notochord is necessary for the normally observed longer range induction in lateral neural plate cells. This work helps to further the understanding of molecular and tissue interactions required for convergent extension.

Negative regulation of Hedgehog signaling by the cholesterogenic enzyme 7-dehydrocholesterol reductase

Cholesterol regulates Hedgehog (Hh) signaling during early vertebrate development. Smith-Lemli-Opitz syndrome (SLOS) is caused by defects in 7-dehydrocholesterol reductase (DHCR7), an enzyme catalyzing the final step of cholesterol biosynthesis. Many developmental malformations attributed to SLOS occur in tissues and organs where Hh signaling is required for development, but the precise role of DHCR7 deficiency in this disease remains murky. We report that DHCR7 and Sonic Hedgehog (Shh) are co-expressed during midline development in Xenopus embryos. DHCR7 has previously been implicated to function as a positive regulator of Hh signaling that acts to regulate the cholesterol adduction of Hh ligand or to affect Hh signaling in the responding cell. We present gain- and loss-of-function analyses suggesting that DHCR7 functions as a negative regulator of Hh signaling at the level or downstream of Smoothened (Smo) and affects intracellular Hh signaling. Our analysis also raises the possibility that the human condition SLOS is caused not only by disruption of the enzymatic role of DHCR7 as a reductase in cholesterol biosynthesis, but may also involve defects in DHCR7 resulting in derepression of Shh signaling.

FGF is essential for both condensation and mesenchymal-epithelial transition stages of pronephric kidney tubule development

The pronephros is a transient embryonic kidney that is essential for the survival of aquatic larvae. It is also absolutely critical for adult kidney development, as the pronephric derivative the wolffian duct forms the ductal system of the adult kidney and also triggers the condensation of metanephric mesenchyme into the adult nephrons. While exploring Xenopus pronephric patterning, we observed that epidermally delivered hedgehog completely suppresses pronephric kidney tubule development but does not effect development of the pronephric glomus, the equivalent of the mammalian glomerulus or corpuscle. This effect is not mediated by apoptosis. Microarray analysis of microdissected primordia identified FGF8 as one of the potential mediators of hedgehog action. Further investigation demonstrated that SU5402-sensitive FGF signaling plays a critical role in the very earliest stages of pronephric tubule development. Modulation of FGF8 activity using a morpholino has a later effect that blocks condensation of pronephric mesenchyme into the pronephric tubule. Together, these data show that FGF signaling plays a critical role at two stages of embryonic kidney development, one in the condensation of the pronephric primordium from the intermediate mesoderm and a second in the later epithelialization of this mesenchyme into the pronephric nephron. The data also show that in Xenopus, development of the glomus/glomerulus can be uncoupled from nephron formation via ectopic hedgehog expression and provides an experimental avenue for investigating glomerulogenesis in the complete absence of tubules.

Ciliogenesis defects in embryos lacking inturned or fuzzy function are associated with failure of planar cell polarity and Hedgehog signaling

The vertebrate planar cell polarity (PCP) pathway has previously been found to control polarized cell behaviors rather than cell fate. We report here that disruption of Xenopus laevis orthologs of the Drosophila melanogaster PCP effectors inturned (in) or fuzzy (fy) affected not only PCP-dependent convergent extension but also elicited embryonic phenotypes consistent with defective Hedgehog signaling. These defects in Hedgehog signaling resulted from a broad requirement for Inturned and Fuzzy in ciliogenesis. We show that these proteins govern apical actin assembly and thus control the orientation, but not assembly, of ciliary microtubules. Finally, accumulation of Dishevelled and Inturned near the basal apparatus of cilia suggests that these proteins function in a common pathway with core PCP components to regulate ciliogenesis. Together, these data highlight the interrelationships between cell polarity, cellular morphogenesis, signal transduction and cell fate specification.

Cooperative non-cell and cell autonomous regulation of Nodal gene expression and signaling by Lefty/Antivin and Brachyury in Xenopus

Dynamic spatiotemporal expression of the nodal gene and its orthologs is involved in the dose-dependent induction and patterning of mesendoderm during early vertebrate embryogenesis. We report loss-of-function studies that define a high degree of synergistic negative regulation on the Xenopus nodal-related genes (Xnrs) by extracellular Xenopus antivin/lefty (Xatv/Xlefty)-mediated functional antagonism and Brachyury-mediated transcriptional suppression. A strong knockdown of Xlefty/Xatv function was achieved by mixing translation- and splicing-blocking morpholino oligonucleotides that target both the A and B alloalleles of Xatv. Secreted and cell-autonomous inhibitors of Xnr signaling were used to provide evidence that Xnr-mediated induction was inherently long-range in this situation in the large amphibian embryo, essentially being capable of spreading over the entire animal hemisphere. There was a greater expansion of the Organizer and mesendoderm tissues associated with dorsal specification than noted in previous Xatv knockdown experiments in Xenopus, with consequent exogastrulation and long-term maintenance of expanded axial tissues. Xatv deficiency caused a modest animal-ward expansion of the marginal zone expression territory of the Xnr1 and Xnr2 genes. In contrast, introducing inhibitory Xbra-En(R) fusion constructs into Xatv-deficient embryos caused a much larger increase in the level and spatial extent of Xnr expression. However, in both cases (Xatv/Xlefty-deficiency alone, or combined with Xbra interference), Xnr2 expression was constrained to the superficial cell layer, suggesting a fundamental tissue-specific competence in the ability to express Xnrs, an observation with direct implications regarding the induction of endodermal vs. mesodermal fates. Our experiments reveal a two-level suppressive mechanism for restricting the level, range, and duration of Xnr signaling via extracellular inhibition by Xatv/Xlefty coupled with potent indirect transcriptional repression by Xbra.

Of chicken wings and frog legs: a smorgasbord of evolutionary variation in mechanisms of tetrapod limb development

The tetrapod limb, which has served as a paradigm for the study of development and morphological evolution, is becoming a paradigm for developmental evolution as well. In its origin and diversification, the tetrapod limb has undergone a great deal of remodeling. These morphological changes and other evolutionary phenomena have produced variation in mechanisms of tetrapod limb development. Here, we review that variation in the four major clades of limbed tetrapods. Comparisons in a phylogenetic context reveal details of development and evolution that otherwise may have been unclear. Such details include apparent differences in the mechanisms of dorsal-ventral patterning and limb identity specification between mouse and chick and mechanistic novelties in amniotes, anurans, and urodeles. As we gain a better understanding of the details of limb development, further differences among taxa will be revealed. The use of appropriate comparative techniques in a phylogenetic context thus sheds light on evolutionary transitions in limb morphology and the generality of developmental models across species and is therefore important to both evolutionary and developmental biologists.

Xenopus aristaless-related homeobox (xARX) gene product functions as both a transcriptional activator and repressor in forebrain development

Mutations in the aristaless-related homeobox (ARX) gene have been found in patients with a variety of X-linked mental retardation syndromes with forebrain abnormalities, including lissencephaly. Arx is expressed in the developing mouse, Xenopus, and zebrafish forebrain. We have used whole-mount in situ hybridization, overexpression, and loss-of-function studies to investigate the involvement of xArx in Xenopus brain development. We verified that xArx is expressed in the prospective diencephalon, as the forebrain is patterned and specified during neural plate stages. Expression spreads into the ventral and medial telencephalon as development proceeds through neural tube and tadpole stages. Overexpression of xArx resulted in morphological abnormalities in forebrain development, including loss of rostral midline structures, syn- or anophthalmia, dorsal displacement of the nasal organ, and ventral neural tube hyperplasia. Additionally, there is a delay in expression of many molecular markers of brain and retinal development. However, expression of some markers, dlx5 and wnt8b, was enhanced in xArx-injected embryos. Loss-of-function experiments indicated that xArx was necessary for normal forebrain development. Expansion of wnt8b expression depended on xArx function as a transcriptional repressor, whereas ectopic expression of dlx5, accompanied by development of ectopic otic structures, depended on function of Arx as a transcriptional activator. These results suggest that Arx acts as a bifunctional transcriptional regulator in brain development.