A role for the vegetally expressed Xenopus gene Mix. 1 in endoderm formation and in the restriction of mesoderm to the marginal zone (original) (raw)

Endoderm Specification and Differentiation in Xenopus Embryos

Developmental Biology, 2001

It is known from work with amniote embryos that regional specification of the gut requires cell-cell signalling between the mesoderm and the endoderm. In recent years, much of the interest in Xenopus endoderm development has focused on events that occur before gastrulation and this work has led to a different model whereby regional specification of the endoderm is autonomous. In this paper, we examine the specification and differentiation of the endoderm in Xenopus using neurula and tail-bud-stage embryos and we show that the current hypothesis of stable autonomous regional specification is not correct. When the endoderm is isolated alone from neurula and tail bud stages, it remains fully viable but will not express markers of regional specification or differentiation. If mesoderm is present, regional markers are expressed. If recombinations are made between mesoderm and endoderm, then the endodermal markers expressed have the regional character of the mesoderm. Previous results with vegetal explants had shown that endodermal differentiation occurs cell-autonomously, in the absence of mesoderm. We have repeated these experiments and have found that the explants do in fact show some expression of mesoderm markers associated with lateral plate derivatives. We believe that the formation of mesoderm cells by the vegetal explants accounts for the apparent autonomous development of the endoderm. Since the fate map of the Xenopus gut shows that the mesoderm and endoderm of each level do not come together until tail bud stages, we conclude that stable regional specification of the endoderm must occur quite late, and as a result of inductive signals from the mesoderm.

Novel gene expression domains reveal early patterning of the Xenopus endoderm

Gene Expression Patterns, 2003

The endoderm gives rise the respiratory and digestive tract epithelia as well as associated organs such as the liver, lungs and pancreas. Investigations examining the molecular basis of embryonic endodermal patterning and organogenesis have been hampered by the lack of regionally expressed molecular markers in the early endoderm. By differentially screening an arrayed cDNA library, combined with an in situ hybridization screen we identified 13 new genes regionally expressed in the early tailbud endoderm of the Xenopus embryo. The putative proteins encoded by these cDNAs include a cell surface transporter, secreted proteins, a protease, a protease inhibitor, an RNA-binding protein, a phosphatase inhibitor and several enzymes. We find that the expression of these genes falls into one of three re-occurring domains in the tailbud embryo; (1) a ventral midgut, (2) posterior to the midgut and (3) in the dorsal endoderm beneath the notochord. Several of these genes are also regionally expressed at gastrula and neurula stages and appear to mark territories that were previously only predicted by the endoderm fate map. This indicates that there is significant positional identity in the early endoderm long before stages 28 -32 when regional specification of the endoderm is thought to occur. These new genes provide valuable tools for studying endodermal patterning and organogenesis in Xenopus. q

Refinement of gene expression patterns in the early Xenopus embryo

Development, 2004

During blastula and gastrula stages of Xenopus development, cells become progressively and asynchronously committed to a particular germ layer. We have analysed the expression of genes normally expressed in ectoderm,mesoderm or endoderm in individual cells from early and late gastrula embryos,by both in situ hybridization and single-cell RT-PCR. We show that at early gastrula stages, individual cells in the same region may express markers of two or more germ layers, and `rogue' cells that express a marker outside its canonical domain are also observed at these stages. However, by the late gastrula stage, individual cells express markers that are more characteristic of their position in the embryo, and `rogue' cells are seen less frequently. These observations exemplify at the gene expression level the observation that cells of the early gastrula are less committed to one germ layer than are cells of the late gastrula embryo. Ectodermal cells induced to form mesendoderm by th...

Inhibition of FGF signaling converts dorsal mesoderm to ventral mesoderm in early Xenopus embryos

Differentiation, 2011

In early vertebrate development, mesoderm induction is a crucial event regulated by several factors including the activin, BMP and FGF signaling pathways. While the requirement of FGF in Nodal/activininduced mesoderm formation has been reported, the fate of the tissue modulated by these signals is not fully understood. Here, we examined the fate of tissues when exogenous activin was added and FGF signaling was inhibited in animal cap explants of Xenopus embryos. Activin-induced dorsal mesoderm was converted to ventral mesoderm by inhibition of FGF signaling. We also found that inhibiting FGF signaling in the dorsal marginal zone, in vegetal-animal cap conjugates or in the presence of the activin signaling component Smad2, converted dorsal mesoderm to ventral mesoderm. The expression and promoter activities of a BMP responsive molecule, PV.1 and a Spemann organizer, noggin, were investigated while FGF signaling was inhibited. PV.1 expression increased, while noggin decreased. In addition, inhibiting BMP-4 signaling abolished ventral mesoderm formation induced by exogenous activin and FGF inhibition. Taken together, these results suggest that the formation of dorso-ventral mesoderm in early Xenopus embryos is regulated by a combination of FGF, activin and BMP signaling.

Interaction of goosecoid and brachyury in Xenopus mesoderm patterning

Mechanisms of Development, 1997

Detailed in situ analyses reveal overlapping expression of gsc and Xbra in the early Spemann's organizer. Coexpression is lost during gastrulation suggesting an interaction between these genes. Ectopic expression of gsc ventrally suppresses endogenous Xbra expression and transcription from Xbra promoter reporter gene constructs. Suppression is mediated, at least partially, by a gsc-binding site within the first 349 bp of the promoter. Xbra reporter gene transcription is also suppressed in the region of endogenous gsc expression, whereas high-level ectopic Xbra expression has no effect on endogenous gsc expression. We suggest that early patterning of the vertebrate mesoderm, like early patterning of the Drosophila embryo, occurs by first establishing broad domains of gene expression which are subsequently refined by intergenic interactions to further delimit tissue boundaries.

Mesoderm induction in Xenopus laevis: a quantitative study using a cell lineage label and tissue-specific antibodies

Development, 1985

We have compared the development of the animal pole (AP) region of early Xenopus embryos in normal development, in isolation, and in combination with explants of tissue from the vegetal pole (VP) region. For the grafts and the combinations the animal pole tissue was lineage labelled with FLDx in order to ascertain the provenance of the structures formed. The normal fate of the AP region was determined by orthotopic grafts at stages (early blastula), 8 (mid blastula) and 10 (early gastrula). At later stages most of the labelled cells were found in ectodermal tissues such as epidermis, head mesenchyme and neural tube (the last from stages and 8 only). However, in stage- and stage-8 grafts some of the labelled cells were also found in the myotomes and lateral mesoderm. In isolated explants the AP region of all three stages differentiated only as epidermis assessed both histologically and by immunofluorescence using an antibody to epidermal keratin. The fate of labelled cells in AP-VP c...

The Xenopus Brachyury promoter is activated by FGF and low concentrations ofactivinandsuppressed by high concentrationsof activin and by paired-type homeodomain proteins

Genes & Development, 1997

The mesoderm of Xenopus laevis arises through an inductive interaction in which signals from the vegetal hemisphere of the embryo act on overlying equatorial cells. One candidate for an endogenous mesoderm-inducing factor is activin, a member of the TGF␤ superfamily. Activin is of particular interest because it induces different mesodermal cell types in a concentration-dependent manner, suggesting that it acts as a morphogen. These concentration-dependent effects are exemplified by the response of Xbra, expression of which is induced in ectodermal tissue by low concentrations of activin but not by high concentrations. Xbra therefore offers an excellent paradigm for studying the way in which a morphogen gradient is interpreted in vertebrate embryos. In this paper we examine the trancriptional regulation of Xbra2, a pseudoallele of Xbra that shows an identical response to activin. Our results indicate that 381 bp 5 of the Xbra2 transcription start site are sufficient to confer responsiveness both to FGF and, in a concentration-dependent manner, to activin. We present evidence that the suppression of Xbra expression at high concentrations of activin is mediated by paired-type homeobox genes such as goosecoid, Mix.1, and Xotx2.

Identification of novel genes affecting mesoderm formation and morphogenesis through an enhanced large scale functional screen in Xenopus

Mechanisms of Development, 2005

The formation of mesoderm is an important developmental process of vertebrate embryos, which can be broken down into several steps; mesoderm induction, patterning, morphogenesis and differentiation. Although mesoderm formation in Xenopus has been intensively studied, much remains to be learned about the molecular events responsible for each of these steps. Furthermore, the interplay between mesoderm induction, patterning and morphogenesis remains obscure. Here, we describe an enhanced functional screen in Xenopus designed for largescale identification of genes controlling mesoderm formation. In order to improve the efficiency of the screen, we used a Xenopus tropicalis unique set of cDNAs, highly enriched in full-length clones. The screening strategy incorporates two mesodermal markers, Xbra and Xmyf-5, to assay for cell fate specification and patterning, respectively. In addition we looked for phenotypes that would suggest effects in morphogenesis, such as gastrulation defects and shortened anterior-posterior axis. Out of 1728 full-length clones we isolated 82 for their ability to alter the phenotype of tadpoles and/or the expression of Xbra and Xmyf-5. Many of the clones gave rise to similar misexpression phenotypes (synphenotypes) and many of the genes within each synphenotype group appeared to be involved in similar pathways. We determined the expression pattern of the 82 genes and found that most of the genes were regionalized and expressed in mesoderm. We expect that many of the genes identified in this screen will be important in mesoderm formation. q

Region-specific activation of the Xenopus brachyury promoter involves active repression in ectoderm and endoderm: a study using transgenic frog embryos

Development, 2000

Tissue specification in the early embryo requires the integration of spatial information at the promoters of developmentally important genes. Although several response elements for signalling pathways have been identified in Xenopus promoters, it is not yet understood what defines the sharp borders that restrict expression to a specific tissue. Here we use transgenic frog embryos to study the spatial and temporal regulation of the Xbra promoter. Deletion analysis and point mutations in putative transcription factor-binding sites identified two repressor modules, which exert their main effects at different stages during gastrulation. One module is defined by a bipartite binding site for a Smad-interacting protein (SIP1) of the deltaEF1 repressor family and acts to confine expression to the marginal zone early in gastrulation. The other module is defined by two homeodomain-binding sites and is responsible for repression in dorsal mesoderm and ectoderm at mid-gastrula stages. In additi...