Direct and indirect regulation of derrière,a Xenopus mesoderm-inducing factor,by VegT

One candidate for an endogenous mesoderm-inducing factor in Xenopus is derrière, a member of the TGFbeta family closely related to Vg1. In this paper we first show that derrière is able to exert long-range effects in the early Xenopus embryo, reinforcing the view that it functions as a secreted factor required for proper formation of posterior structures. Analysis of the derrière promoter shows that expression of the gene is controlled through a complex inductive network involving VegT and TGFbeta-related molecules and also, perhaps, FGF family members. The work confirms that derrière plays an important role in mesoderm formation and it illustrates the complex regulation to which inducing factors are subject.     

Neural Induction in Xenopus: Requirement for Ectodermal and Endomesodermal Signals via Chordin, Noggin, β-Catenin, and Cerberus

The origin of the signals that induce the differentiation of the central nervous system (CNS) is a long-standing question in vertebrate embryology. Here we show that Xenopus neural induction starts earlier than previously thought, at the blastula stage, and requires the combined activity of two distinct signaling centers. One is the well-known Nieuwkoop center, located in dorsal-vegetal cells, which expresses Nodal-related endomesodermal inducers. The other is a blastula Chordin- and Noggin-expressing (BCNE) center located in dorsal animal cells that contains both prospective neuroectoderm and Spemann organizer precursor cells. Both centers are downstream of the early β-Catenin signal. Molecular analyses demonstrated that the BCNE center was distinct from the Nieuwkoop center, and that the Nieuwkoop center expressed the secreted protein Cerberus (Cer). We found that explanted blastula dorsal animal cap cells that have not yet contacted a mesodermal substratum can, when cultured in saline solution, express definitive neural markers and differentiate histologically into CNS tissue. Transplantation experiments showed that the BCNE region was required for brain formation, even though it lacked CNS-inducing activity when transplanted ventrally. Cell-lineage studies demonstrated that BCNE cells give rise to a large part of the brain and retina and, in more posterior regions of the embryo, to floor plate and notochord. Loss-of-function experiments with antisense morpholino oligos (MO) showed that the CNS that forms in mesoderm-less Xenopus embryos (generated by injection with Cerberus-Short [CerS] mRNA) required Chordin (Chd), Noggin (Nog), and their upstream regulator β-Catenin. When mesoderm involution was prevented in dorsal marginal-zone explants, the anterior neural tissue formed in ectoderm was derived from BCNE cells and had a complete requirement for Chd. By injecting Chd morpholino oligos (Chd-MO) into prospective neuroectoderm and Cerberus morpholino oligos (Cer-MO) into prospective endomesoderm at the 8-cell stage, we showed that both layers cooperate in CNS formation. The results suggest a model for neural induction in Xenopus in which an early blastula β-Catenin signal predisposes the prospective neuroectoderm to neural induction by endomesodermal signals emanating from Spemann's organizer.     

Maternal xNorrin, a canonical Wnt signaling agonist and TGF-β antagonist, controls early neuroectoderm specification in Xenopus

Dorsal-ventral specification in the amphibian embryo is controlled by β-catenin, whose activation in all dorsal cells is dependent on maternal Wnt11. However, it remains unknown whether other maternally secreted factors contribute to β-catenin activation in the dorsal ectoderm. Here, we show that maternal Xenopus Norrin (xNorrin) promotes anterior neural tissue formation in ventralized embryos. Conversely, when xNorrin function is inhibited, early canonical Wnt signaling in the dorsal ectoderm and the early expression of the zygotic neural inducers Chordin, Noggin, and Xnr3 are severely suppressed, causing the loss of anterior structures. In addition, xNorrin potently inhibits BMP- and Nodal/Activin-related functions through direct binding to the ligands. Moreover, a subset of Norrin mutants identified in humans with Norrie disease retain Wnt activation but show defective inhibition of Nodal/Activin-related signaling in mesoderm induction, suggesting that this disinhibition causes Norrie disease. Thus, xNorrin is an unusual molecule that acts on two major signaling pathways, Wnt and TGF-β, in opposite ways and is essential for early neuroectoderm specification.     

Reconstitution Of β-catenin Degradation In Xenopus Egg Extract

Xenopus laevis egg extract is a well-characterized, robust system for studying the biochemistry of diverse cellular processes. Xenopus egg extract has been used to study protein turnover in many cellular contexts, including the cell cycle and signal transduction pathways^1-3. Herein, a method is described for isolating Xenopus egg extract that has been optimized to promote the degradation of the critical Wnt pathway component, β-catenin. Two different methods are described to assess β-catenin protein degradation in Xenopus egg extract. One method is visually informative ([³⁵S]-radiolabeled proteins), while the other is more readily scaled for high-throughput assays (firefly luciferase-tagged fusion proteins). The techniques described can be used to, but are not limited to, assess β-catenin protein turnover and identify molecular components contributing to its turnover. Additionally, the ability to purify large volumes of homogenous Xenopus egg extract combined with the quantitative and facile readout of luciferase-tagged proteins allows this system to be easily adapted for high-throughput screening for modulators of β-catenin degradation.     

Über sezernierende Zellelemente im Nephron von Xenopus laevis

Ohne Zusammenfassung     

Is there a Root effect in Xenopus hemoglobin?

The reaction of Xenopus hemoglobin with oxygen and carbon monoxide has been reinvestigated over the pH range 8.5-6.0, in the absence and presence of organic phosphates (2,3-diphosphoglycerate or inositol hexakisphosphate), to establish if the tetramer can be stabilized in a T-quaternary state by protons and polyphosphate; the equilibrium and kinetic data indicate that Xenopus hemoglobin does exhibit a Root effect. These new results are discussed with reference to those reported by Bridges et al. [(1985) Resp. Physiol. 61, 125-136] on Xenopus blood and, more generally, to the molecular definition and the structural basis of the Root effect as an extreme form of the Bohr effect.     

Gastrulation in Xenopus laevis: involution—a current view

In this article, we describe some of the morphogenetic movements reshaping the Xenopus laevis embryo during gastrulation. We have learned a great deal about these movements in recent years through advances made in explant culture techniques. Here, we will focus on involution, the process by which mesoderm is internalized and placed in between ectoderm and endoderm. Our aim is to present our current view of how involution takes place in the dorsal involuting marginal zone of the Xenopus embryos.     

非洲爪蟾（Xenopus laevis）囊胚,神经胚两个cDNA文库的建立

随着分子发育生物学的发展,非洲爪蟾(Xenopus laevis)作为发育生物学,尤其是胚胎诱导、分化机制研究的重要实验材料,受到人们的重视。作者建立的非洲爪蟾囊胚(分期8)和神经胚(分期14)两个不同发育时期胚胎的cDNA文库,为进一步筛选有关的发育基因创造了条件。     

Assay, purification, properties and mechanism of action of γ-glutamylcysteine synthetase from the liver of the rat and Xenopus laevis

1. An improved radioassay for glutathione synthetase and gamma-glutamylcysteine synthetase was developed. 2. Xenopus laevis liver gamma-glutamylcysteine synthetase was purified 324-fold by saline-bicarbonate extraction, protamine sulphate precipitation, CM-cellulose and DEAE-cellulose column chromatography, and gel filtration. 3. Rat liver gamma-glutamylcysteine synthetase was purified 11400-fold by a procedure similar to that employed for the Xenopus laevis enzyme. 4. Rat liver gamma-glutamylcysteine synthetase activity was inhibited by GSH and activated by glycine. These effects, which were not found in the enzyme from Xenopus laevis, may have a regulatory significance. 5. Isotope-exchange experiments revealed fundamental differences in the partial reactions catalysed by the rat and Xenopus laevis synthetases. The enzyme from Xenopus laevis appears to follow a Bi Bi Uni Uni Ping Pong mechanism, with glutamyl-enzyme as intermediate before the addition of cysteine and the release of gamma-glutamylcysteine. The results for the rat liver enzyme are consistent with a Tri Tri sequential mechanism.     

ATP and the processivity of Xenopus laevis DNA polymerase α

We use specific restriction fragments as defined primers for DNA synthesis on single-stranded circular phage fd DNA. These structures are relatively poor templates for a highly purified DNA polymerase α from Xenopus laevis eggs. However, DNA synthesis is stimulated about 5-fold by addition of ATP to the reaction mixture. We show that the deoxynucleotide polymers, synthesized in the presence of ATP, are significantly longer than those produced in the absence of ATP. We also show that this effect is due to a more tenacious binding of DNA polymerase α to DNA and conclude that ATP increases the processivity of the enzyme.     

cDNA cloning and functional characterization of Xenopus laevis DNase γ

We report here the cDNA cloning and functional analysis of Xenopus DNase γ (xDNase γ). Two forms of cDNAs are isolated from adult spleen: one composing a 933 bp open reading frame for the enzymatically active xDNase γ protein, and the other encoding an inactive short alternative form. Northern blot analysis revealed that the xDNase γ mRNA is expressed in spleen, liver, testis, and ovary. xDNase γ expression is scarcely detected in the tail muscle of tadpoles; however, it increases during metamorphosis and reaches a maximum during the late metamorphic climax. The ectopic expression of xDNase γ results in the appearance of extensive DNA fragmentation in C2C12 myoblasts after the induction of apoptosis. In contrast, Xenopus DNase I fails to induce apoptotic DNA ladder formation under the same conditions. Our results suggest a possible involvement of xDNase γ in apoptosis during amphibian metamorphosis. The nucleotide sequence of Xenopus DNase γ has been submitted to DDBJ/EMBL/GenBank database under the accession number AF059612