Patterning of the neural ectoderm of Xenopus laevis by the amino-terminal product of hedgehog autoproteolytic cleavage

Membrane-bound full-length Sonic Hedgehog identifies cancer stem cells in human non-small cell lung cancer

The mechanism of Sonic Hedgehog (Shh) pathway activation in non-small cell lung cancer (NSCLC) is poorly described. Using an antibody against the Shh C-terminal domain, we found a small population of Shh-positive (Shh+) cells in NSCLC cells. The objective of this study was to characterize these Shh+ cells. Shh+ and Shh- cells were sorted by using Fluorescence Activated Cell Sorting (FACS) on 12 commercial NSCLC cell lines. Functional analyses on sorted cells were performed with gene expression assays (qRT-PCR and microarray) and cells were treated with cytotoxic chemotherapy and a targeted inhibitor of Shh signaling (GDC0449). We used in vivo models of nude mice inoculated with Shh+ and Shh- sorted cells and drug-treated cells. Finally, we confirmed our results in fresh human NSCLC samples (n=48) paired with normal lung tissue. We found that Shh+ cells produced an uncleaved, full-length Shh protein detected on the membranes of these cells. Shh+ cells exerted a paracrine effect on Shh- cells, inducing their proliferation and migration. Shh+ cells were chemo-resistant and showed features of cancer stem cells (CSCs) in vitro and in vivo. Pharmacological inhibition of the Shh pathway suppressed their CSC features. A high percentage of Shh+ cells was associated with poor prognosis in early-stage NSCLC patients. In conclusion, we describe for the first time the presence of an abnormal membrane-bound full-length Shh protein in human cancer cells that allows the identification of CSCs in vitro and in vivo.

The serpin PN1 is a feedback regulator of FGF signaling in germ layer and primary axis formation

Germ layer formation and primary axis development rely on Fibroblast growth factors (FGFs). In Xenopus, the secreted serine protease HtrA1 induces mesoderm and posterior trunk/tail structures by facilitating the spread of FGF signals. Here, we show that the serpin Protease nexin-1 (PN1) is transcriptionally activated by FGF signals, suppresses mesoderm and promotes head development in mRNA-injected embryos. An antisense morpholino oligonucleotide against PN1 has the opposite effect and inhibits ectodermal fate. However, ectoderm and anterior head structures can be restored in PN1-depleted embryos when HtrA1 and FGF receptor activities are diminished, indicating that FGF signals negatively regulate their formation. We show that PN1 binds to and inhibits HtrA1, prevents degradation of the proteoglycan Syndecan 4 and restricts paracrine FGF/Erk signaling. Our data suggest that PN1 is a negative-feedback regulator of FGF signaling and has important roles in ectoderm and head development.

Active signals, gradient formation and regional specificity in neural induction

The question of how the vertebrate embryo gives rise to a nervous system is of paramount interest in developmental biology. Neural induction constitutes the earliest step in this process and is tightly connected with development of the embryonic body axes. In the Xenopus embryo, perpendicular gradients of BMP and Wnt signals pattern the dorsoventral and anteroposterior body axes. Both pathways need to be inhibited to allow anterior neural induction to occur. FGF8 and IGF are active neural inducers that together with BMP and Wnt signals are integrated at the level of Smad 1/5/8 phosphorylation. Hedgehog (Hh) also contributes to anterior neural induction. Suppressor-of-fused plays an important role in intertwining the Hh and Wnt pathways. Distinct mechanisms are discussed that establish morphogen gradients and integrate retinoic acid and FGF signals during posterior development. These findings not only improve our understanding of regional specification in neural induction, but have profound implications for mammalian stem cell research and regenerative medicine.

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.

Processing and turnover of the Hedgehog protein in the endoplasmic reticulum

Autocatalytic processing of the Hedgehog ligand from its precursor protein relies on protein disulfide isomerase and ER-associated degradation.

The Hedgehog (Hh) signaling pathway has important functions during metazoan development. The Hh ligand is generated from a precursor by self-cleavage, which requires a free cysteine in the C-terminal part of the protein and results in the production of the cholesterol-modified ligand and a C-terminal fragment. In this paper, we demonstrate that these reactions occur in the endoplasmic reticulum (ER). The catalytic cysteine needs to form a disulfide bridge with a conserved cysteine, which is subsequently reduced by protein disulfide isomerase. Generation of the C-terminal fragment is followed by its ER-associated degradation (ERAD), providing the first example of an endogenous luminal ERAD substrate that is constitutively degraded. This process requires the ubiquitin ligase Hrd1, its partner Sel1, the cytosolic adenosine triphosphatase p97, and degradation by the proteasome. Processing-defective mutants of Hh are degraded by the same ERAD components. Thus, processing of the Hh precursor competes with its rapid degradation, explaining the impaired Hh signaling of processing-defective mutants, such as those causing human holoprosencephaly.

Comparative genomic and expression analysis of the conserved NTPDase gene family in Xenopus

The purines, ATP and adenosine, are important signaling molecules in the nervous system. ATP is sequentially degraded to adenosine by the ectonucleotidase proteins. The NTPDase (or CD39) family is a subfamily of these enzymes, which consists of nine members in mammals. In Xenopus embryos, we have shown that ATP, and its antagonist adenosine, regulate the rundown of swimming and we therefore proposed that ectonucleotidase proteins are key regulators of locomotor activity. Here, we report the cloning of all nine members of the NTPDase family in Xenopus laevis and Xenopus tropicalis. Our phylogenetic analysis shows that this family is highly conserved between the frog species and also during vertebrate evolution. In the adult frog, NTPDase genes are broadly expressed. During development, all NTPDase genes, except for NTPDase8, are expressed and display a distinct specific expression pattern, suggesting potentially different functions of these proteins during embryogenesis of X. laevis.

A highly conserved amino-terminal region of sonic hedgehog is required for the formation of its freely diffusible multimeric form

Although members of the Hedgehog (Hh) family were initially described as morphogens, many of these early conclusions were based on experiments that used non-physiologically relevant forms of Hh. Native Hh is modified by cholesterol (HhNp) and palmitate. These hydrophobic modifications are responsible for the ability of Hh to associate with cellular membranes, a property that initially appeared inconsistent with its ability to act far from its site of synthesis. Although it is now clear that Hh family members are capable of acting directly in long-range signaling, the form of Hh capable of this activity remains controversial. We have previously provided evidence for a freely diffusible multimeric form of Sonic Hedgehog (Shh) termed s-ShhNp, which is capable of accumulating in a gradient fashion through a morphogenic field. Here, we provide further evidence that s-ShhNp is the physiologically relevant form of Shh. We show that the biological activity of freely diffusible ShhNp resides in its multimeric form and that this multimeric form is exceedingly stable, even to high concentrations of salt and detergent. Furthermore, we now validate the Shh-Shh interactions previously observed in the crystal structure of human Shh, showing that a highly conserved amino-terminal domain of Shh is important for the formation of s-ShhNp. We also conclusively show that palmitoylation is required for s-ShhNp formation. Thus, our results identify both protein-protein and protein-lipid interactions that are required for s-ShhNp formation, and provide the first structural analyses supporting the existence of Shh multimers.

Novel lipid modifications of secreted protein signals

Secreted signaling proteins function in a diverse array of essential patterning events during metazoan development, ranging from embryonic segmentation in insects to neural tube differentiation in vertebrates. These proteins generally are expressed in a localized manner, and they may elicit distinct concentration-dependent responses in the cells of surrounding tissues and structures, thus functioning as morphogens that specify the pattern of cellular responses by their tissue distribution. Given the importance of signal distribution, it is notable that the Hedgehog (Hh) and Wnt proteins, two of the most important families of such signals, are known to be covalently modified by lipid moieties, the membrane-anchoring properties of which are not consistent with passive models of protein mobilization within tissues. This review focuses on the mechanisms underlying biogenesis of the mature Hh proteins, which are dually modified by cholesteryl and palmitoyl adducts, as well as on the relationship between Hh proteins and the self-splicing proteins (i.e., proteins containing inteins) and the Hh-like proteins of nematodes. We further discuss the cellular mechanisms that have evolved to handle lipidated Hh proteins in the spatial deployment of the signal in developing tissues and the more recent findings that implicate palmitate modification as an important feature of Wnt signaling proteins.

What's your position? the Xenopus cement gland as a paradigm of regional specification

The correct positioning of organs during embryonic development requires multiple cues. The Xenopus cement gland is a mucus-secreting epithelium that is a simple model for organogenesis, allowing detailed analysis of this complex process. The cement gland forms at a conserved anterior position, where embryonic ectoderm and endoderm touch. In all deuterostomes, this region will form the stomodeum (primitive mouth) and, in some aquatic larva, will also form a cement gland. In recent years, a model has been put forward suggesting that an intermediate level of BMP signaling in the ectoderm leads to cement gland formation. We propose an alternative model whereby, during gastrulation, the cement gland (CG) is positioned by the overlap of three domains, corresponding to anterodorsal identity (AD), ventrolateral identity (VL), and ectodermal outer layer identity (EO), defining the equation (AD + VL + EO = CG). Anterodorsal identity requires a contribution by the transcription factor Otx2 while ventrolateral identity requires the BMP4 signaling pathway. These postional cues are integrated to activate cement gland differentiation. This integration appears to require intermediate steps, including expression of pitx genes, and members of the ATF/CREB and Ets transcription factor families.

Xbra3 elicits the production of neural tissue by a non-BMP-dependent mechanism in Xenopus sp

Ectoderm cells in animal caps from Xenopus embryos develop to form either epidermis or neural tissue depending upon their receipt of intercellular signals. To date, several secreted neural inducers have been identified which act through the local inhibition of bone morphogenetic protein (BMP) signaling, preventing differentiation to epidermis and resulting in adoption of neural fate. In this work, we have exploited an interspecies animal cap assay, which enables detection of the effects of signaling molecules produced by cells of one animal cap and influencing development in a second cap cultured in close apposition in a Holtfreter combination. We show that expression of the T-box protein, Xbra3, in one cap causes the production of a factor, which causes adoption of neural fate by cells of the other animal cap. The action of this factor is not inhibited by the over-expression of BMP in cells of the responding animal cap, or by the inhibition of Wnt signaling. These findings suggest the existence of a secreted signaling molecule that is able to induce ectodermal cells to adopt neural fate by a mechanism independent of the inhibition of the BMP or Wnt signaling pathways.

The latent-TGFbeta-binding-protein-1 (LTBP-1) is expressed in the organizer and regulates nodal and activin signaling

The latent TGF-beta binding proteins (LTBP) are believed to control the availability of TGF-beta in the extracellular milieu. To gain insight into the potential roles of LTBP in early development, we isolated the Xenopus LTBP-1 (xLTBP-1) cDNA. The cDNA encodes a protein similar to the mammalian LTBP-1 in both size and domain structure. In addition, we found a novel longer splice isoform of xLTBP. The RNAs for both forms of xLTBP displayed temporal regulation and the shorter transcript is expressed maternally. Both transcripts also display spatial regulation and are found in the dorsal mesoderm of the organizer. In animal cap experiments, LTBP-1 potentiates the activity of activin and nodal. The activity of LTBP-1 did not appear to require covalent association with activin as the addition of medium containing activin and LTBP-1 to animal caps enhanced the activin effect. These results indicate that LTBP-1 may be part of the regulatory system that establishes the threshold of morphogen activity for activins and nodals in the dorsal side of the embryo during gastrulation.

Conserved expression control and shared activity between cognate T-box genes Tbx2 and Tbx3 in connection with Sonic hedgehog signaling during Xenopus eye development

Tbx2 and Tbx3 are considered to be cognate genes within a Tbx2/3/4/5 subfamily of T-box genes and are expressed in closely overlapping areas in a variety of tissues, including the eye. Herein, we show that misexpression of Tbx2 and Tbx3 in Xenopus embryos gave rise to defective eye morphogenesis, which was reminiscent of the defect caused by attenuated Sonic hedgehog (Shh) signaling. Indeed, Tbx2/3 misexpression suppressed Gli1, Gli2, Ptc2 and Pax2, mediators or targets of Hedgehog (Hh) signals. From these data, Tbx2/3 may have a shared function in inhibiting Gli-dependent Shh signaling during eye development. Conversely, the expression of Tbx2/3 was severely affected by both Shh and a putative dominant negative form of Hh, as well as by both transactivator and transrepressor forms of Gli-fusion proteins, suggesting that the expression of Tbx2/3 may be regulated by a Gli-dependent Hh signal transduction pathway. Because the Shh signal has been considered to play crucial roles in the formation of the proximal-distal and dorsal-ventral axes in the eyes, these findings about the mutual regulatory mechanism between Tbx2/3 and Gli-dependent Hh signaling provide valuable insight into the cause of the localized expression of Tbx2/3 and their role during the formation of these axes. In addition, our findings also imply the conserved regulation and shared activity between the cognate genes of Tbx2 and Tbx3.

Xpbx1b and Xmeis1b play a collaborative role in hindbrain and neural crest gene expression in Xenopus embryos

Pbx1 is a homeodomain protein that functions in complexes with other homeodomain-containing proteins to regulate gene expression during embryogenesis and oncogenesis. Pbx proteins bind DNA cooperatively as heterodimers or higher order complexes with Meis family members and Hox proteins and are believed to specify cell identity during development. Here, we present evidence that Pbx1, in partnership with Meis1b, can regulate posterior neural markers and neural crest marker genes during Xenopus development. A Xenopus homolog of the Pbx1b homeodomain protein was isolated and shown to be expressed throughout embryogenesis. Xpbx1b expression overlaps with Xmeis1 in several areas, including the lateral neural folds, caudal branchial arch, hindbrain, and optic cup. When ectopically expressed, Xpbx1b can synergize with Xmeis1b to promote posterior neural and neural crest gene expression in ectodermal explants. Further, a physical interaction between these two homeodomain proteins is necessary for induction of these genes in embryonic tissue. In addition, coexpression of Xmeis1b and Xpbx1b leads to a prominent shift in the localization of Xmeis1b from the cytoplasm to the nucleus, suggesting that nuclear transport or retention of Xmeis1b may depend upon Xpbx1b. Finally, expression of a mutant construct in which Xpbx1b protein is fused to the repressor domain from Drosophila Engrailed inhibits posterior neural and neural crest gene expression. These data indicate that Xpbx1b and its partner, Xmeis1b, function in a transcriptional activation complex during hindbrain and neural crest development.

Sonic Hedgehog as a mediator of long-range signaling

The ability of Hedgehog (Hh) proteins to exert their biological effects is regulated by a series of post-translational processes. These processes include an intramolecular cleavage, covalent addition of cholesterol and/or palmitate, and conversion into a multimeric freely diffusible form. The processing of Hh proteins affects their trafficking, potency, and ability to signal over many cell diameters. Accordingly, the loss of gene products required for these processes abrogates the Hh proteins' abilities to exert their effects, which can be long range, short range, or both. We review here recent evidence demonstrating that Hh proteins are directly responsible for their long-range biological effects. Additionally, we integrate both genetic and biochemical data to delineate a model illustrating how the unusual biochemistry of Hh family members may allow them to act as morphogens, signaling over both short and long distances.

Xenopus Enhancer of Zeste (XEZ); an anteriorly restricted polycomb gene with a role in neural patterning

We have identified the Xenopus homologue of Drosophila Enhancer of Zeste using a differential display strategy designed to identify genes involved in early anterior neural differentiation. XEZ codes for a protein of 748 amino acids that is very highly conserved in evolution and is 96% identical to both human and mouse EZ(H)2. In common with most other Xenopus Pc-G genes and unlike mammalian Pc-G genes, XEZ is anteriorly restricted. Zygotic expression of XEZ commences during gastrulation, much earlier than other anteriorly localized Pc-G genes; expression is restricted to the anterior neural plate and is confined later to the forebrain, eyes and branchial arches. XEZ is induced in animal caps overexpressing noggin; up-regulation of XEZ therefore represents a response to inhibition of BMP signalling in ectodermal cells. We show that the midbrain/hindbrain junction marker En-2,and hindbrain marker Krox-20, are target genes of XEZ and that XEZ functions to repress these anteroposterior marker genes. Conversely, XEZ does not repress the forebrain marker Otx-2. XEZ overexpression results in a greatly thickened floor of the forebrain. These results implicate an important role for XEZ in the patterning of the nervous system.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

The role of prechordal mesendoderm in neural patterning

Prechordal mesendoderm is formed in response to Nodal and maternal beta-Catenin signaling and is regulated by signals from anterior endoderm and chordamesoderm. Prechordal mesendodermal cells are involved in neural induction and in anteroposterior and dorsoventral neural patterning. Inhibitors of Wnt and BMP growth factors secreted by prechordal mesendoderm mediate neural induction and anteroposterior and dorsoventral patterning, whereas SHH and TGF betas mediate dorsoventral patterning.

Cholesterol modification of proteins

The demonstration over 30 years ago that inhibitors of cholesterol biosynthesis disrupt animal development suggested an intriguing connection between fundamental cellular metabolic processes and the more global processes of embryonic tissue patterning. Adding a new dimension to this relationship is the more recent finding that the Hedgehog family of tissue patterning factors are covalently modified by cholesterol. Here we review the mechanism of the Hedgehog autoprocessing reaction that results in this modification, and compare this reaction to that undergone by other autoprocessing proteins. We also discuss the biological consequences of cholesterol modification, in particular the use of cholesterol as a molecular handle in the spatial deployment of the protein signal in developing tissues. Finally, the developmental consequences of chemical and genetic disruption of cholesterol homeostasis are summarized, along with the potential importance of cholesterol-rich lipid rafts in production of and response to the Hh signal.

Pattern formation in the cerebellar cortex

The cerebellar cortex is subdivided rostrocaudally and mediolaterally into a reproducible array of zones and stripes. This makes the cerebellum a valuable model for studying pattern formation in the vertebrate central nervous system. The structure of the adult mouse cerebellar cortex and the series of embryological events that generate the topography are reviewed.Key words: zebrin, Hsp25, Purkinje cells.

A role for xGCNF in midbrain-hindbrain patterning in Xenopus laevis

Cells in the presumptive neural ectoderm of Xenopus are committed to neural fate through a process called neural induction, which may involve proteins that antagonize BMP signaling pathways. To identify genes that are induced by the BMP antagonists and that may be involved in subsequent neural patterning, we used a suppression PCR-based subtraction screen. Here we investigate the prospective activities and functions of one of the genes, a nuclear orphan receptor previously described as xGCNF. In animal cap assays, xGCNF synergizes with ectopic chordin to induce the midbrain-hindbrain marker engrailed-2 (En-2). In Keller explants, which rely on endogenous factors for neural induction, similar increases in En-2 are observed. Expression in embryos of a dominant interfering form of xGCNF reduces the expression of endogenous En-2 and Krox-20. These gain-of-function and prospective loss-of-function experiments, taken with the observation that xGCNF is expressed in the early neural plate and is elevated in the prospective midbrain-hindbrain region, which subsequently expresses En-2, suggest that xGCNF may play a role in regulating En-2 and thus midbrain-hindbrain identity.

Cytochalasin B inhibits morphogenetic movement and muscle differentiation of activin-treated ectoderm in Xenopus

Xenopus ectodermal explants (animal caps) begin to elongate after treatment with the mesoderm inducing factor activin A. This phenomenon mimics the convergent extension of dorsal mesoderm during gastrulation. To analyze the relationship between elongation movement and muscle differentiation, animal caps were treated with colchicine, taxol, cytochalasin B and hydroxyurea (HUA)/aphidicolin following activin treatment. Cytochalasin B disrupted the organization of actin filaments and inhibited the elongation of the activin-treated explants. Muscle differentiation was also inhibited in these explants at the histologic and molecular levels. Colchicine and taxol, which are known to affect microtubule organization, had little effect on elongation of the activin-treated exp ants. Co-treatment with HUA and aphidicolin caused serious damage on the explants and they did not undergo elongation. These results suggest that actin filaments play an important role in the elongation movement that leads to muscle differentiation of activin-treated explants.

Sonic hedgehog is not required for the induction of medial floor plate cells in the zebrafish

Sonic hedgehog (Shh) is a secreted protein that is involved in the organization and patterning of several tissues in vertebrates. We show that the zebrafish sonic-you (syu) gene, a member of a group of five genes required for somite patterning, is encoding Shh. Embryos mutant for a deletion of syu display defects in patterning of the somites, the lateral floor plate cells, the pectoral fins, the axons of motorneurons and the retinal ganglion cells. In contrast to mouse embryos lacking Shh activity, syu mutant embryos do form medial floor plate cells and motorneurons. Since ectopic overexpression of shh in zebrafish embryos does not induce ectopic medial floor plate cells, we conclude that shh is neither required nor sufficient to induce this cell type in the zebrafish.

The Xenopus Emx genes identify presumptive dorsal telencephalon and are induced by head organizer signals

We have isolated and studied the expression pattern of Xemx1 and Xemx2 genes in Xenopus laevis. Xemx genes are the homologues of mouse Emx genes, related to Drosophila empty spiracles. They are expressed in selected regions of the developing brain, particularly in the telencephalon, and, outside the brain, in the otic vesicles, olfactory placodes, visceral arches and the developing excretory system. We also report on experiments concerning the tissue and molecular signals responsible for their activation in competent ectoderm. Xemx genes are activated in ectoderm conjugated with head organizer tissue, but not with tail organizer tissue. Furthermore, they are not activated in animal cap either by noggin or by Xnr3, thus suggesting that a different inducer or the integration of several signals may be responsible for their activation.

Floor plate and the subcommissural organ are the source of secretory compounds of related nature: comparative immunocytochemical study

The subcommissural organ of vertebrates secretes glycoproteins into the third ventricle that condense to form Reissner's fiber (RF). Antibodies raised against the bovine RF-glycoproteins reacted with the floor plate (FP) cells of two teleost (Oncorhynchus kisutch, Sparus aurata) and two amphibian (Xenopus laevis, Batrachyla taeniata) species. At the ultrastructural level, the immunoreactivity was confined to secretory granules, mainly concentrated at the apical cell pole. In the rostro-caudal axis, a clear zonation of the FP was distinguished, with the hindbrain FP being the most, or the only (Batrachyla taeniata), immunoreactive region of the FP. In all the species studied, the caudal FP lacked immunoreactivity. Both the chemical nature of the immunoreactive material and the rostro-caudal zonation of the FP appear to be conservative features. Evidence was obtained that the FP secretes into the cerebrospinal fluid a material chemically related to the RF-glycoproteins secreted by the subcommissural organ. Thus, in addition to being the source of contact-mediated and diffusible signals, the FP might also secrete compounds into the cerebrospinal fluid that may act on distant targets.

Anterior specification of embryonic ectoderm: the role of the Xenopus cement gland-specific gene XAG-2

In a search for novel developmental genes expressed in a spatially restricted pattern in dorsal ectoderm of Xenopus we have identified XAG-2, a cement gland-specific gene with a putative role in ectodermal patterning. XAG-2 encodes a secreted protein, which is expressed in the anterior region of dorsal ectoderm from late gastrula stages onwards. Activation of XAG-2 transcription is observed in response to organizer-secreted molecules including the noggin, chordin, follistatin and cerberus gene products. Overexpression of XAG-2 but not of the related cement gland marker XAG-1 induces both cement gland differentiation and expression of anterior neural marker genes in the absence of mesoderm formation. Further, we show that XAG-2 signaling depends on an intact fibroblast growth factor (FGF) signal transduction pathway and that XAG-2-induced anterior neural fate of ectodermal cells can be transformed to a more posterior character by retinoic acid. Based on these findings we propose a role for XAG-2 in the specification of dorsoanterior ectodermal fate, i.e. in the formation of cement gland and induction of forebrain fate of Xenopus.

Patterning of the embryonic forebrain

The vertebrate forebrain is derived from the anterior neural plate, where anteroposterior, dorsoventral and local patterning mechanisms specify regional identify. The recent identification of genetic regulators of these processes has opened the way to elucidating how the major forebrain regions (i.e. cerebral cortex, basal ganglia, thalamus, and hypothalamus) are formed, and how molecular lesions in these processes cause human birth defects.

Neural induction and patterning by fibroblast growth factor, notochord and somite tissue in Xenopus

Two natural neural inducing sources have been used, the notochord and the somites together with the growth factor bFGF, to investigate the anterior/posterior patterning of neural tissue in an animal cap explant model in Xenopus laevis. Notochord and somite tissue from stages 12.5/13 and 16, respectively, were manually isolated, and combined heterochronically with responding animal cap ectoderm aged to gastrula stages. Somite recombinants were also constructed with animal caps injected with noggin mRNA. The responses of the ectoderm were analyzed by reverse transcription polymerase chain reaction (RT-PCR) detection of marker gene expression, and in some cases by in situ hybridization. The requirement for FGF receptor function was analyzed using the dominant negative FGF receptor (XFD). The experiments showed that bFGF is capable of direct neural induction in caps aged to stage 10.5. It was also shown that notochords are capable of inducing anterior neural tissue in gastrula stage animal cap ectoderm, and this induction is sensitive to XFD in the responding tissue. Injection of noggin mRNA results in the induction of anterior neural differentiation, and it was demonstrated that this induction was insensitive to the expression of XFD in the responding tissue. It was also shown that somite tissue recombined with gastrula stage animal cap ectoderm, can induce both anterior and posterior nervous tissue and can also posteriorize noggin-induced anterior neural tissue when combined with noggin-injected animal cap ectoderm. This response is partially sensitive to XFD expression. The results shed light on the role of competence of animal cap ectoderm and the signals from postgastrulation axial and paraxial mesoderm in the patterning of the amphibian nervous system.

Wnt and FGF pathways cooperatively pattern anteroposterior neural ectoderm in Xenopus

Previous gain-of-function assays in Xenopus have demonstrated that Xwnt-3a can pattern neural tissue by reducing the expression of anterior neural genes, and elevating the expression of posterior neural genes. To date, no loss-of-function studies have been conducted in Xenopus to show a requirement of endogenous Wnt signaling for patterning of the neural ectoderm along the anteroposterior axis. We report that expression of a dominant negative Wnt in Xenopus embryos causes a reduction in the expression of posterior neural genes, and an elevation in the expression of anterior neural genes, thereby confirming the involvement of endogenous Wnt signaling in patterning the neural axis. We further demonstrate that the ability of Xwnt-3a to decrease expression of anterior neural genes in noggin-treated explants is dependent upon a functional FGF signaling pathway, while the elevation of expression of posterior neural genes does not require FGF signaling. The previously reported ability of FGF to elevate the expression of posterior neural genes in noggin-treated explants was found to be dependent on endogenous Wnt signaling. We conclude that neural induction occurs initially in a Wnt-independent manner, but that generation of complete anteroposterior neural pattern requires the cooperative actions of Wnt and FGF pathways.

Daughters against dpp modulates dpp organizing activity in Drosophila wing development

The family of TGF-beta signalling molecules play inductive roles in various developmental contexts. One member of this family, Drosophila Decapentaplegic (Dpp) serves as a morphogen that patterns both the embryo and adult. We have now isolated a gene, Daughters against dpp (Dad), whose transcription is induced by Dpp. Dad shares weak homology with Drosophila Mad (Mothers against dpp), a protein required for transduction of Dpp signals. In contrast to Mad or the activated Dpp receptor, whose overexpression hyperactivates the Dpp signalling pathway, overexpression of Dad blocks Dpp activity. Expression of Dad together with either Mad or the activated receptor rescues phenotypic defects induced by each protein alone. Dad can also antagonize the activity of a vertebrate homologue of Dpp, bone morphogenetic protein, as evidenced by induction of dorsal or neural fate following overexpression in Xenopus embryos. We conclude that the pattern-organizing mechanism governed by Dpp involves a negative-feedback circuit in which Dpp induces expression of its own antagonist, Dad. This feedback loop appears to be conserved in vertebrate development.

Crystal structure of a Hedgehog autoprocessing domain: homology between Hedgehog and self-splicing proteins

The approximately 25 kDa carboxy-terminal domain of Drosophila Hedgehog protein (Hh-C) possesses an autoprocessing activity that results in an intramolecular cleavage of full-length Hedgehog protein and covalent attachment of a cholesterol moiety to the newly generated amino-terminal fragment. We have identified a 17 kDa fragment of Hh-C (Hh-C17) active in the initiation of autoprocessing and report here its crystal structure. The Hh-C17 structure comprises two homologous subdomains that appear to have arisen from tandem duplication of a primordial gene. Residues in the Hh-C17 active site have been identified, and their role in Hedgehog autoprocessing probed by site-directed mutagenesis. Aspects of sequence, structure, and reaction mechanism are conserved between Hh-C17 and the self-splicing regions of inteins, permitting reconstruction of a plausible evolutionary history of Hh-C and the inteins.

Identification of a novel sonic hedgehog response element in the chicken ovalbumin upstream promoter-transcription factor II promoter

Sonic hedgehog (Shh) is a secreted morphogen that regulates dorso-ventral patterning within the neural tube during embryonic development. It is well established that Shh can induce motor-neuron differentiation that coincides with the appearance of specific motor-neuron markers including chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) and Isl1. However, the mechanism of Shh-induced signaling pathway in vertebrates is not clearly defined. In this report we have identified COUP-TFII as a target gene for Shh. In addition we have used a 1.6-kb region of the COUP-TFII promoter to identify a target element that mediates the Shh-induced activity. Extensive deletions introduced within this region have further enabled us to identify a novel sonic hedgehog response element (ShhRE) in the COUP-TFII promoter. Point mutations introduced within the ShhRE reveal some key nucleotides that are essential for protein(s)-binding activity. Finally, the ShhRE is capable of functioning as a true enhancer element and can mediate Shh-induced transactivation of reporter gene via a heterologous promoter.

A role for Indian hedgehog in extraembryonic endoderm differentiation in F9 cells and the early mouse embryo

Hedgehog genes in Drosophila and vertebrates control patterning of a number of different structures during embryogenesis. They code for secreted signaling proteins that are cleaved into an active aminopeptide and a carboxypeptide. The aminopeptide can mediate local and long range events and can act as a morphogen, inducing differentiation of distinct cell types in a concentration-dependent manner. We demonstrate here that the expression of Indian hedgehog mRNA and protein is upregulated dramatically as F9 cells differentiate in response to retinoic acid, into either parietal endoderm or embryoid bodies, containing an outer visceral endoderm layer. The ES cell line D3 forms embryoid bodies in suspension culture without addition of retinoic acid and also upregulates Indian hedgehog expression. RT-PCR analysis of blastocyst outgrowth cultures demonstrates that whereas little or no Indian hedgehog message is present in blastocysts, significant levels appear upon subsequent days of culture, coincident with the emergence of parietal endoderm cells. In situ hybridization analysis for Indian hedgehog mRNA expression demonstrates the presence of elevated levels of message in the outer visceral endoderm cells relative to the core cells in mature embryoid bodies and in the visceral endoderm of Day 6.5 embryos. Whole-mount in situ hybridization analysis of Day 7.5 and 8.5 embryos indicates that Indian hedgehog expression is highest in the visceral yolk sac at this stage. F9 cell lines expressing a full length Indian hedgehog cDNA express a number of characteristics of differentiated cells, in the absence of retinoic acid. Taken together, these data suggest that Indian hedgehog is involved in mediating differentiation of extraembryonic endoderm during early mouse embryogenesis.

Xwnt-2b is a novel axis-inducing Xenopus Wnt, which is expressed in embryonic brain

Xwnt-2b is a novel member of the Wnt gene family and is 73-74% similar to human and mouse Wnt-2 proteins. Starting from stage 15, Xwnt-2b transcripts are localized to a non-contiguous stripe in the anterior neural plate of the Xenopus embryo. In the tailbud, Xwnt-2b is expressed along the dorsoanterior side of the prosencephalon-mesencephalon boundary. At the tadpole stages, the brain-specific expression fades, but the total amount of Xwnt-2b mRNA does not decline due to activation of its expression in non-brain areas. Microinjection of Xwnt-2b mRNA into a ventral blastomere of 4-8-cell embryos results in the formation of complete secondary body axes. These results suggest that Xwnt-2b is a member of the axis-inducing Wnts and that it is involved in brain development and in later organogenesis.

The world according to hedgehog

Members of the Hedgehog family of signaling molecules mediate many important short- and long-range patterning processes during invertebrate and vertebrate development. In the fly, a single hedgehog gene regulates segmental and imaginal disc patterning. In contrast, in vertebrates a hedgehog gene family is involved in the control of left-right asymmetry, polarity in the central nervous system (CNS), somites and limb, organogenesis, chondrogenesis and spermatogenesis. Here, we review recent experiments addressing the function of the various Hedgehog members during invertebrate and vertebrate development.

The hypothalamic-pituitary axis: co-development of two organs

Development of the anterior pituitary gland ultimately leads to the appearance of five distinct cell types that are defined by the trophic hormones which they produce, providing an instructive model system for elucidating the molecular mechanisms that underlie the determination of distinct cell phenotypes within an organ from a common precursor lineage. The recent identification of several homeodomain transcription factors expressed specifically in the anterior pituitary gland has revealed a transcriptional cascade orchestrating a developmental program that leads to the determination of the five mature cell types. Recent data from gene-targeting experiments in mice further imply that the execution of this program is dependent on inductive signals originating in the floor of the diencephalon.

Cholesterol modification of hedgehog signaling proteins in animal development

Hedgehog (Hh) proteins comprise a family of secreted signaling molecules essential for patterning a variety of structures in animal embryogenesis. During biosynthesis, Hh undergoes an autocleavage reaction, mediated by its carboxyl-terminal domain, that produces a lipid-modified amino-terminal fragment responsible for all known Hh signaling activity. Here it is reported that cholesterol is the lipophilic moiety covalently attached to the amino-terminal signaling domain during autoprocessing and that the carboxyl-terminal domain acts as an intramolecular cholesterol transferase. This use of cholesterol to modify embryonic signaling proteins may account for some of the effects of perturbed cholesterol biosynthesis on animal development.

Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function

Targeted gene disruption in the mouse shows that the Sonic hedgehog (Shh) gene plays a critical role in patterning of vertebrate embryonic tissues, including the brain and spinal cord, the axial skeleton and the limbs. Early defects are observed in the establishment or maintenance of midline structures, such as the notochord and the floorplate, and later defects include absence of distal limb structures, cyclopia, absence of ventral cell types within the neural tube, and absence of the spinal column and most of the ribs. Defects in all tissues extend beyond the normal sites of Shh transcription, confirming the proposed role of Shh proteins as an extracellular signal required for the tissue-organizing properties of several vertebrate patterning centres.

Positive and negative signals modulate formation of the Xenopus cement gland

The cement gland is a simple secretory organ that marks the anterior-most dorsal ectoderm in Xenopus embryos. In this study, we examine the timing of cement gland induction and the cell interactions that contribute to cement gland formation. Firstly, we show that the outer ectodermal layer, from which the cement gland arises, becomes specified as cement gland by mid-gastrula. Curiously, at early gastrula, the inner layer of the dorsal ectoderm, which does not contribute to the mature cement gland, is strongly and transiently specified as cement gland. Secondly, we show that the mid-gastrula dorsoanterior yolky endoderm, which comes to underlie the cement gland primordium, is a potent inducer of cement gland formation and patterning. The cement gland itself has an anteroposterior pattern, with the gene XA expressed only posteriorly. Dorsoanterior yolky endoderm greatly enhances formation of large, patterned cement glands in partially induced anterodorsal ectoderm, but is unable to induce cement gland in naive animal caps. Neural tissue is induced less frequently than cement gland by the dorsoanterior yolky endoderm, suggesting that the endoderm induces cement gland directly. Thirdly, we demonstrate that the ventral ectoderm adjacent to the cement gland attenuates cement gland differentiation late during gastrulation. The more distant ventral mesendoderm is also a potent inhibitor of cement gland formation. These are the first data showing that normal ventral tissues can inhibit cement gland differentiation and suggest that cement gland size and position may be partly regulated by negative signals. Previous work has shown that cement gland can be induced by neural plate and by dorsal mesoderm. Together, these data suggest that cement gland induction is a complex process regulated by multiple positive and negative cell interactions.

A sticky problem: the Xenopus cement gland as a paradigm for anteroposterior patterning

The cement gland is a mucus-secreting organ found at the extreme anterior of frog embryos. It attaches the embryo to a solid support before swimming and feeding begin, and also serves a related sensory function that stops the embryo from moving once it is attached. Cement gland is an extremely useful anterior marker, whose study continues to yield fundamental information concerning vertebrate axial patterning. Cement gland arises from the outer layer of the embryonic ectoderm and, in Xenopus, forms a cone of columnar epithelium. It is the first ectodermal organ to differentiate, beginning to do so by late gastrula. A battery of genes expressed in the developing and mature cement gland serve as useful markers. Cement gland development can be influenced by both stimulatory and inhibitory cell interactions. Stimulatory signals arise from the anterior neural plate, head endoderm, and the dorsal mesoderm. Inhibitory signals are present in the posterior dorsal mesoderm and in ventral ectoderm and mesoderm. Further, signalling between the ectodermal layers may restrict cement gland differentiation to the outer ectodermal cells. Several secreted molecules are able to induce or repress cement gland formation: these include noggin, follistatin, hedgehog, chordin, retinoic acid, embryonic fibroblast growth factor (eFGF), Bone Morphogenetic Protein-4 (BMP-4), and Xwnt-8. Several of these factors alter expression of the homeodomain gene Xotx2, which may be a transcriptional activator of cement gland differentiation genes. The significance of the cell interactions and factors described in positioning cement gland at the front of the embryo is explored.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

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

The hedgehog family of signaling proteins is associated with a variety of spatial patterning activities in insects and vertebrates. Here we show that new members of this family isolated from Xenopus laevis are expressed embryonically in patterns suggestive of roles in patterning in the ectoderm, nervous system and somites. Banded hedgehog is expressed throughout the neural plate and subsequently in both the nervous system and in the dermatome of somites. Cephalic hedgehog is expressed in anterior ectoderm and endodermal structures, and sonic hedgehog is expressed in patterns which parallel those in other species. Injection of RNAs encoding Xenopus hedgehogs induces ectopic cement gland formation in embryos. Similar to reported activities of noggin and follistatin, Xenopus hedgehogs share a common ability to induce cement glands in animal cap explants. However, hedgehog activities in naive ectoderm appear capable of acting independently of noggin and follistatin since, although all three are induced by activin in animal cap explants, X-hh expression does not induce noggin or follistatin.

Patterning activities of vertebrate hedgehog proteins in the developing eye and brain

BACKGROUND

The hedgehog (hh) family of secreted signaling proteins is responsible for developmental patterning in a variety of systems, including the neural tube, limbs and somites. Within the neural tube, at the level of the spinal cord, products of the vertebrate gene sonic hedgehog (shh) are proposed to function as a ventral patterning influence, with the capability of inducing floor plate and motor neurons.

RESULTS

We report the isolation of tiggy-winkle hedgehog (twhh), a novel member of the zebrafish hh gene family. Both twhh and shh are expressed in the ventral midline of the embryonic zebrafish neural tube and brain, but twhh expression becomes limited to the neural tube, whereas shh is also expressed in the notochord. Both genes are expressed in the developing brain, in domains that include a discrete region in the floor of the diencephalon, located between the sites of the future optic stalks. Using pax-2 and pax-6 as markers of proximo-distal fate within the developing eye, we found that ectopic expression of either hh gene promoted proximal fates and suppressed distal fates. In contrast, proximal fates were lost in cyclops mutant embryos, which lack twhh- and shh-expressing forebrain cells. Both twhh and shh proteins undergo autoproteolytic processing in vivo; a fragment corresponding to the amino-terminal cleavage product was sufficient to carry out all signaling activities associated with twhh in eye and brain development.

CONCLUSIONS

These findings suggest that secreted signals encoded by members of the hedgehog gene family, emanating from the ventral midline of the neural tube, not only play important roles in dorso-ventral patterning of the brain but also appear to constitute an early patterning activity along the proximo-distal axis of the developing eyes.