The specification of anteroposterior ( AP ) axis is a fundamental and complex patterning process that sets up the embryonic polarity and shapes a multicellular organism. This process involves the integration of distinct signaling pathways to coordinate temporal–spatial gene expression and morphogenetic movements. In the frog Xenopus , extensive embryological and molecular studies have provided major advance in understanding the mechanism implicated in AP patterning. Following fertilization, cortical rotation leads to the transport of maternal determinants to the dorsal region and creates the primary dorsoventral ( DV ) asymmetry. The activation of maternal Wnt/ß‐catenin signaling and a high Nodal signal induces the formation of the Nieuwkoop center in the dorsal‐vegetal cells, which then triggers the formation of the Spemann organizer in the overlying dorsal marginal zone. It is now well established that the Spemann organizer plays a central role in building the vertebrate body axes because it provides patterning information for both DV and AP polarities. The antagonistic interactions between signals secreted in the Spemann organizer and the opposite ventral region pattern the mesoderm along the DV axis, and this DV information is translated into AP positional values during gastrulation. The formation of anterior neural tissue requires simultaneous inhibition of zygotic Wnt and bone morphogenetic protein ( BMP ) signals, while an endogenous gradient of Wnt, fibroblast growth factors ( FGFs ), retinoic acid ( RA ) signaling, and collinearly expressed Hox genes patterns the trunk and posterior regions. Collectively, DV asymmetry is mostly coupled to AP polarity, and cell–cell interactions mediated essentially by the same regulatory networks operate in DV and AP patterning. WIREs Dev Biol 2016, 5:150–168. doi: 10.1002/wdev.217 This article is categorized under: Early Embryonic Development > Development to the Basic Body Plan

Mesoderm and embryonic axis formation in vertebrates is mediated by maternal and zygotic factors that activate the expression of target genes. Transcriptional derepression plays an important role in the regulation of expression in different contexts; however, its involvement and possible mechanism in mesoderm and embryonic axis formation are largely unknown. Here we demonstrate that XDSCR6, a Xenopus homologue of human Down syndrome critical region protein 6 (DSCR6, or RIPPLY3), regulates mesoderm and embryonic axis formation through derepression of polycomb group (PcG) proteins. Xdscr6 maternal mRNA is enriched in the endoderm of the early gastrula and potently triggers the formation of dorsal mesoderm and neural tissues in ectoderm explants; it also dorsalises ventral mesoderm during gastrulation and induces a secondary embryonic axis. A WRPW motif, which is present in all DSCR6 homologues, is necessary and sufficient for the dorsal mesoderm- and axis-inducing activity. Knockdown of Xdscr6 inhibits dorsal mesoderm gene expression and results in head deficiency. We further show that XDSCR6 physically interacts with PcG proteins through the WRPW motif, preventing the formation of PcG bodies and antagonising their repressor activity in embryonic axis formation. By chromatin immunoprecipitation, we demonstrate that XDSCR6 releases PcG proteins from chromatin and allows dorsal mesoderm gene transcription. Our studies suggest that XDSCR6 might function to sequester PcG proteins and identify a novel derepression mechanism implicated in embryonic induction and axis formation.

RNA-binding proteins play an important role to post-transcriptionally regulate gene expression. During early development they exhibit temporally and spatially regulated expression pattern. The expression of Xenopus laevis Seb4 gene, also known as RBM24 in other vertebrates, is restricted to the lateral and ventral mesoderm during gastrulation and then localized to the somitic mesoderm, in a similar pattern as XMyoD gene. Using a hormone-inducible form of MyoD to identify potential direct MyoD target genes, we find that Seb4 expression is directly regulated by MyoD at the gastrula stage. We further show that a 0.65kb X. tropicalis RBM24 regulatory region contains multiple E boxes (CANNTG), which are potential binding sites for MyoD and other bHLH proteins. By injecting a RBM24 reporter construct into the animal pole of X. laevis embryos, we find that this reporter gene is indeed specifically activated by MyoD and repressed by a dominant negative MyoD mutant. Knockdown of Seb4 produces similar effects as those obtained by the dominant negative MyoD mutant, indicating that it is required for the expression of myogenic genes and myogenesis in the embryo. In cultured ectodermal explants, although overexpression of Seb4 has no obvious effect on myogenesis, knockdown of Seb4 inhibits the expression of myogenic genes and myogenesis induced by MyoD. These results reveal that Seb4 is a target of MyoD during myogenesis and is required for myogenic gene expression.

Whole-mount and sectioned in situ hybridization was used to identify genes with restricted expression pattern in the presomitic and somitic mesoderm during Xenopus early development. Here we report the dynamic expression pattern of six distinct genes differentially expressed in these regions. These include Xenopus homologues of purine nucleoside phosphorylase, acetylcholine receptor, aspartate aminotransferase, glycine amidinotransferase and brain and muscle isoform creatine kinases. Purine nucleoside phosphorylase was initially expressed in the marginal zone at gastrula stage and then localized to the tail bud. Although the other genes showed no significantly localized expression at gastrula or neurula stages, they are progressively restricted in the somitic mesoderm as development proceeds. A common feature for these genes is that their deficiency in humans leads to an impairment of several tissue or cell functions. An analysis of their expression pattern could provide information regarding their implication in early development.

The Glypican family of heparan sulfate proteoglycans regulates Wnt signaling and convergent extension (CE) in vertebrate embryos. They are predicted to be glycosylphosphatidylinositol (GPI)-tethered membrane-bound proteins, but there is no functional evidence of their regulation by the GPI synthesis complex. Down syndrome critical region protein 5 (Dscr5, also known as Pigp) is a component of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex, and is associated with specific features of Down syndrome. Here we report that Dscr5 regulates CE movements through the non-canonical Wnt pathway. Both dscr5 overexpression and knockdown impaired convergence and extension movements. Dscr5 functionally interacted with Knypek/Glypican 4 and was required for its localization at the cell surface. Knockdown of dscr5 disrupted Knypek membrane localization and caused an enhanced Frizzled 7 receptor endocytosis in a Caveolin-dependent manner. Furthermore, dscr5 knockdown promoted specific Dishevelled degradation by the ubiquitin-proteosome pathway. These results reveal a functional link between Knypek/Glypican 4 and the GPI synthesis complex in the non-canonical Wnt pathway, and provide the new mechanistic insight that Dscr5 regulates CE in vertebrate embryos by anchoring different Wnt receptors at the cell surface and maintaining Dishevelled stability.

Stem cell biology is one of the most exciting subjects in life science nowadays. The major point in stem cell biology is the extraordinary capacity of these cells to self-renew and to give rise to different cell types. Nevertheless, major issues remain to be cleared and very few diseases can actually be cured based on stem cell therapy. Adult stem cells remain difficult to locate, isolate and amplify in a homogeneous fashion and, thus, limit their therapeutic application in clinical trial. Embryonic stem cells could represent a new hope in stem cell therapy but in addition to the scientific difficulties, over ethical and judiciary issues should be addressed. In order to cure routinely patients, controlled conditions for stem cell isolation, amplification, differentiation, and administration must be defined and effective tissue integration have to be established. In this review we will discuss these different aspects of stem cell biology.

The T-box transcription factor Tbx6 is required for somite formation and loss-of-function or reduced activity of Tbx6 result in absence of posterior paraxial mesoderm or disorganized somites, but how it is involved in a regulatory hierarchy during Xenopus early development is not clear. We show here that Tbx6 is expressed in the lateral and ventral mesoderm of early gastrula, and it is necessary and sufficient to directly and indirectly regulate the expression of a subset of early mesodermal and endodermal genes. Ectopic expression of Tbx6 inhibits early neuroectodermal gene expression by strongly inducing the expression of posterior mesodermal genes, and expands the mesoderm territory at the expense of neuroectoderm. Conversely, overexpression of a dominant negative Tbx6 mutant in the ventral mesoderm inhibits the expression of several mesodermal genes and results in neural induction in a dose-dependent manner. Using a hormone-inducible form of Tbx6, we have identified FGF8, Xwnt8 and XMyf5 as immediate early responsive genes of Tbx6, and the induction of these genes by Tbx6 is independent of Xbra and VegT. These target genes act downstream and mediate the function of Tbx6 in anteroposterior specification. Our results therefore identify a regulatory cascade governed by Tbx6 in the specification of posterior mesoderm during Xenopus early development.

The homeobox gene Otx2 is expressed during gastrulation in the anterior domain of the vertebrate embryo and is involved in neural and head induction during Xenopus early development. It also prevents convergent extension movements in trunk and posterior mesoderm. Insulin-like growth factors (IGFs) were shown to have similar function. However, whether they interact and the mechanism by which they affect convergent extension remain unclear. We show that IGF pathway specifically induces the expression of Otx2 in the early gastrula and blocks convergent extension of neuroectoderm and mesoderm through the transcriptional activation of Otx2 gene. Otx2 represses the expression of Xbra and Xwnt-11, and the effects of IGF on gastrulation movements can be partially rescued by antisense Otx2 morpholino oligonucleotide. These indicate that IGF pathway interacts with Otx2 to restrict Xbra and Xwnt-11 expression in the trunk and posterior regions. Consistent with this, we show that inhibition of IGF signaling or Otx2 function induces Xbra and Xwnt11 expression and convergent extension in ectodermal cells. Furthermore, the blockade of convergent extension by IGF-I and Otx2 can be rescued by coexpression of Xwnt-11 or a constitutively active Jun N-terminal kinase (JNK). Because Xbra and Xwnt-11 are required for convergent extension movements and Xwnt-11 activates the non-canonical Wnt-11/JNK pathway, our results reveal a mutually exclusive function between IGF and Wnt-11/JNK pathways in regulating cell behaviours during vertebrate head and trunk development.

Wnt signaling has an important role in cell-fate determination, tissue patterning and tumorigenesis. Wnt proteins signal through seven-pass transmembrane receptors of the frizzled family to activateβ-catenin-dependent transcription of target genes. Using early Xenopus embryos, we show that frizzled receptors can dimerize and that dimerization is correlated with activation of the Wnt/β-catenin pathway. Co-immunoprecipitation studies revealed that the receptor Xfz3 exists as a dimer when expressed in Xenopus embryos, and it has been shown to activate the Wnt/β-catenin pathway as revealed by expression of the target gene siamois. Xfz3 dimerization requires intramolecular and/or intermolecular disulfide linkages, and the N-terminal extracellular region of the receptor, including the cysteine-rich domain (CRD), is sufficient for dimerization. The receptor Xfz7 behaves differently from Xfz3 when overexpressed in the embryo as Xfz7 is monomeric and is unable to directly activate the Wnt/β-catenin pathway. However, activation of this pathway can be achieved by artificially forcing Xfz7 dimerization. These results provide the first direct evidence for the dimerization of frizzled receptors and suggest that dimerization contributes to transducing the Wnt/β-catenin signal.

TEL is a frequent target of chromosomal translocations in human cancer and an alleged tumor suppressor gene. TEL encodes two isoforms: a major TEL-M1 isoform as well as TEL-M43, which lacks the first 42 amino acid residues of TEL-M1. Both isoforms are potent transcriptional repressors that can inhibit RAS-induced transformation. Here we show that the v-SRC proteintyrosine kinase relieves the repressive activity of TEL-M1, an activity that is associated with the v-SRC-induced delocalization of TEL-M1 from the nucleus to the cytoplasm. TEL-M1 delocalization requires the kinase activity of v-SRC and is not induced by oncogenic RAS or AKT. Cytoplasmic delocalization of TEL-M1 in response to v-SRC critically depends upon its unique amino-terminal domain (SRCD domain) because (i) v-SRC did not inhibit the repressive properties of TEL-M43, nor affected TEL-M43 nuclear localization; (ii) fusion of the first 52 amino acid residues of TEL-M1 to FLI-1, an ETS protein insensitive to v-SRC-induced delocalization, is sufficient to confer v-SRC-induced delocalization to this TEL/FLI-1 chimeric protein. The v-SRC-induced nucleo-cytoplasmic delocalization of TEL-M1 does not involve phosphorylation of the SRCD and does not require TEL self-association and repressive domains. Finally, enforced expression of the v-SRC-insensitive TEL-M43, but not of TEL-M1, inhibits v-SRC-induced transformation of NIH3T3 fibroblasts. These results identify a regulatory domain in TEL that specifically impinges on the subcellular localization of its major TEL-M1 isoform. They, furthermore, indicate that inhibition of TEL-M1 nuclear function is required for v-SRC to induce cellular transformation.

linkages, and the N-terminal extracellular region of the receptor, including the cysteine-rich domain (CRD), is sufficient for dimerization. The receptor Xfz7 behaves differently from Xfz3 when overexpressed in the embryo as Xfz7 is monomeric and is unable to directly activate the Wnt/β-catenin pathway. However, activation of this pathway can be achieved by artificially forcing Xfz7 dimerization. These results provide the first direct evidence for the dimerization of frizzled receptors and suggest that dimerization contributes to transducing the Wnt/β-catenin signal. Summary

Expression pattern of Irx1 and Irx2 during mouse digit development 159 C. Barrett, S. Guthrie (UK) Expression patterns of the netrin receptor UNC5H1 among developing motor neurons in the embryonic rat hindbrain 163 Embryonic expression of Kru Èppel-like factor 6 in neural and non-neural tissues 167 Dynamic expression patterns of the new protocadherin families CNRs and Pcdh-g during mouse odontogenesis: comparison with reelin expression 181 Abdominal B-type Hox gene expression in Xenopus laevis 191 Nato3 is an evolutionarily conserved bHLH transcription factor expressed in the CNS of Drosophila and mouse 197

Transcription factors of the signal transducer and activator of transcription (STAT) family are required for cellular responses to multiple signalling molecules. After ligand binding-induced activation of cognate receptors, STAT proteins are phosphorylated, hetero- or homodimerize, and enter the nucleus. STAT dimers bind to specific DNA elements and alter the transcriptional activity of the signal-responsive genes. We report the cloning and developmental pattern of expression of XSTAT5, a Xenopus laevis member of the STAT family, closely related to the mammalian STAT5A and STAT5B. XSTAT5 is expressed maternally and zygotically. With the onset of neurulation, XSTAT5 RNA are clearly localized in the anterior neural plate and subsequently in the neural structures of the developing eye, the pineal gland and the cement gland anlage. At late tailbud stages, a faint expression is detected in a ventral location that might correspond to the ventral blood islands.

We previously reported a fusion between TEL and JAK2 in a t(9;12)(p24;p13) chromosomal translocation in childhood acute T-cell leukemia. This fusion gene encodes a TEL-JAK2 chimeric protein in which the 336 amino-terminal residues of TEL, including its specific self-association domain, are fused to the kinase domain of JAK2. TEL-JAK2 exhibits constitutive activation of its tyrosine kinase activity which, in turn, confers growth factor-independent proliferation to the interleukin-3-dependent Ba/F3 hematopoietic cell line. To elucidate the properties of TEL-JAK2 in primary cells and to create an animal model for TEL-JAK2-induced leukemia, we generated transgenic mice in which the TEL-JAK2 complementary DNA was placed under the transcriptional control of the EmuSRalpha enhancer/promoter. TEL-JAK2 founder mice and their transgenic progeny developed fatal leukemia at 4 to 22 weeks of age. Selective amplification of CD8-positive T cells was observed in blood, lymph nodes, thymus, spleen, and bone marrow. Expression of a tyrosine-phosphorylated TEL-JAK2 protein and activation of STAT1 and STAT5 (signal transducer and activator of transcription) were detected in leukemic tissues. TEL-JAK2 diseased mice also displayed invasion of nonhematopoietic organs, including liver, brain, lung, and kidney, by leukemic T cells. Leukemic organs of founder and transgenic progeny contained a monoclonal/oligoclonal T-cell population as analyzed by the rearrangement of the TCRbeta locus. Transplantation of TEL-JAK2 leukemic cells in nude mice confirmed their invasive nature. We conclude that the TEL-JAK2 fusion is an oncogene in vivo and that its expression in lymphoid cells results in the preferential expansion of CD8-positive T cells. (Blood. 2000;95:3891-3899)

TEL is a gene frequently involved in specific chromosomal translocations in human leukemia and sarcoma that encodes a member of the ETS family of transcriptional regulators. TEL is unusual among other ETS proteins by its ability to self-associatein vivo, a property that is essential to the oncogenic activation of TEL-derived fusion proteins. We show here that TEL is a sequence-specific transcriptional repressor of ETS-binding site-driven transcription of model and natural promoters. Deletion of the oligomerization domain of TEL or its substitution by the homologous region of monomeric ETS1 impaired the ability of TEL to repress. In contrast, substitution of the oligomerization domain of TEL by unrelated oligomerization domains resulted in an active repressor, showing that the ability of TEL to repress depends on its ability to self-associate. The study of the properties of TEL fusions to the heterologous DNA binding domain of Gal4 identified two autonomous repression domains in TEL, distinct from its oligomerization domain, that are essential to the ability of TEL to repress ETS-binding site-containing promoters. These results have implications for the normal function of TEL, its relation to other ETS proteins, and its role in leukemogenesis.

TEL est un gene de la famille ETS qui est frequemment altere dans les leucemies humaines sous forme de translocations chromosomiques specifiques. La proteine TEL se distingue de la plupart des proteines de cette famille par le fait qu’elle est un represseur transcriptionnel dont l’activite depend de sa liaison a des sequences d’ADN specifiques et qu’elle forme des oligomeres homotypiques, une propriete qui est importante pour ses proprietes transrepressives. Les translocations chromosomiques affectant TEL dans les leucemies generent le plus souvent des proteines de fusion. Celles-ci peuvent etre regroupees en deux grandes categories. La premiere comprend des proteines chimeriques dans lesquelles la region amino-terminale de TEL (incluant son domaine d’oligomerisation) est fusionnee au domaine catalytique de differentes proteines tyrosine kinases (PDGFRbeta; ABL ; JAK2 ; TRKC). La seconde inclut des proteines de fusion entre TEL et d’autres regulateurs transcriptionnels (AML1 ; MN1). Ces proteines de fusion peuvent, selon les cas, maintenir (MN1-TEL) ou ne pas maintenir (TEL-AML1) le domaine de liaison a l’ADN de TEL (domaine ETS). Bien qu’affectant des voies intracellulaires de transduction du signal differentes, ces proteines de fusion apparaissent toutes activer leurs proprietes oncogeniques en exploitant et detournant de leur fonction normale plusieurs domaines fonctionnels remarquables de TEL. Le nombre croissant de translocations chromosomiques affectant TEL, le plus souvent dans des leucemies, la diversite des partenaires impliques dans ces translocations et le role presume de la perte de l’allele non rearrange de TEL dans les LAL-B presentant une t(12;21) designent TEL comme une cible importante dans les processus oncogeniques.

TEL is a gene of the ETS family which is frequently altered in human leukemia as the result of specific chromosomal translocations. The TEL protein has features which distinguish it from other members of the ETS family. First, unlike most other ETS proteins, TEL is a sequence-specific transcriptional repressor. Second, TEL self-associates to form homotypic oligomers, a property which is important for its trans-repressing properties. The chromosomal translocations which affect TEL in leukemia generate fusion proteins. These can be grouped into two major categories. The first comprises chimeric proteins in which the amino-terminal region of TEL (including its self-association domain) is fused to the catalytic domain of protein tyrosine kinases (PDGFRbeta ; ABL ; JAK2 ; TRKC). The second includes fusion proteins between TEL and other transcriptional regulators (AML1 ; MN1). The latter can either maintain (TEL-MN1 fusions) or not maintain (TEL-AML1) the DNA binding domain (ETS domain) of TEL. Although deregulating different intracellular signaling pathways, these fusion proteins all appear to activate their oncogenic properties by using and subverting from their normal function several remarkable functional domains of TEL. The increasing number of chromosomal translocations affecting TEL, most often in leukemia, the diversity of the partners involved in these translocations and the presumed role of the loss of the non rearranged TEL allele in B-ALL with t(12 ; 21), clearly point to TEL as an important target in oncogenic processes.

TEL is a novel member of the ETS family of transcriptional regulators which is frequently involved in human leukemias as the result of specific chromosomal translocations. We show here by co-immunoprecipitation and GST chromatography analyses that TEL and TEL-derived fusion proteins form homotypic oligomers in vitro and in vivo. Deletion mutagenesis identifies the TEL oligomerization domain as a 65 amino acid region which is conserved in a subset of the ETS proteins including ETS-1, ETS-2, FLI-1, ERG-2 and GABP alpha in vertebrates and PNTP2, YAN and ELG in Drosophila. TEL-induced oligomerization is shown to be essential for the constitutive activation of the protein kinase activity and mitogenic properties of TEL-platelet derived growth factor receptor beta (PDGFR beta), a fusion oncoprotein characteristic of the leukemic cells of chronic myelomonocytic leukemia harboring a t(5;12) chromosomal translocation. Swapping experiments in which the TEL oligomerization domain was exchanged by the homologous domains of representative vertebrate ETS proteins including ETS-1, ERG-2 and GABP alpha show that oligomerization is a specific property of the TEL amino-terminal conserved domain. These results indicate that the amino-terminal domain conserved in a subset of the ETS proteins has evolved to generate a specialized protein-protein interaction interface which is likely to be an important determinant of their specificity as transcriptional regulators.