This paper describes the earliest asymmetries in the stability of the β-Catenin protein. Establishment of the dorso-ventral axis in Xenopus embryos is presaged by early asymmetries in β-Catenin that are modulated by the Wnt signaling pathway. The dorsalizing signal affects a wide region of the embryo. β-Catenin translocation into nuclei demarcates the dorsalizing centers in frog and fish embryos. Schneider, S., Steinbeisser, H., Warga, R. Xenopus chordin: a novel dorsalizing factor activated by organizer-specific homeobox genes. Sasai, Y., Lu, B., Steinbeisser, H., Geissert, D., Gont, L. Expression cloning of noggin, a new dorsalizing factor localized to the Spemann organizer in Xenopus embryos. The original demonstration that nodal –related genes induce mesoderm and completely rescue Xenopus embryos that have been ventralized by ultraviolet radiation. Nodal-related signals induce axial mesoderm and dorsal mesoderm during gastrulation. Mechanism of Wnt signaling in development. The mouse Fused locus encodes Axin, an inhibitor of the Wnt signaling pathway that regulates embryonic axis formation. XTcf-3 transcription factor mediates β-catenin-induced axis formation in Xenopus embryos. From cortical rotation to organizer gene expression: toward a molecular explanation of axis specification in Xenopus. Formation and function of Spemann's Organizer. High frequency twinning of Xenopus laevis embryos from eggs centrifuged before first cleavage. in Gastrulation: Movements, Patterns, and Molecules (eds Keller, R. An old enigma: the gray crescent of amphibian eggs. A magnificent account of the experiments that have shaped embryological thinking in the twentieth century.īrachet, J. The Heritage of Experimental Embryology: Hans Spemann and the Organizer (Oxford Univ. Many extracellular proteins contain CR domains of the Chordin type, raising the possibility that the studies in embryos may provide a general model for the fine regulation of signalling by growth factors in the extracellular space. XTsg is a BMP-binding molecule that can form ternary complexes with full-length Chordin and BMP, thereby converting full-length Chordin into an even better BMP antagonist.Īfter Chordin is cleaved by Xolloid, xTsg displaces the binding of BMP to CRs, thereby promoting, rather than preventing, signalling through BMP receptors. The inhibition of BMP by Chordin can be reversed by the combined action of Xolloid, a zinc metalloprotease, and Twisted-gastrulation (xTsg). The organizer secretes a cocktail of growth factor antagonists such as Noggin, Chordin, Cerberus, Frzb-1 and Dkk-1, which further refine the pattern.Ĭhordin binds bone morphogenetic proteins (BMPs) through cysteine-rich modules, designated CRs. High Xnr levels lead to the induction of Spemann's organizer tissue in the overlying dorsal mesoderm. The inhibition of β-Catenin degradation in the dorsal side is the first step in a biochemical cascade that leads to dorsal development.Īt the blastula stage, a gradient of Nodal-related TGF-β factors (Xnrs) is formed in the endoderm, which induces both ventral and dorsal mesoderm. Transport of these cytoplasmic determinants is required for the differentiation of neural tissue, notochord, somite and kidney. Dorso-ventral asymmetry in the fertilized Xenopus egg starts when a set of parallel cortical microtubules transport small membrane vesicles, which bind Dishevelled protein on their cytoplasmic side, towards the dorsal aspect of the fertilized egg.
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