Xenopus Embryo: β‐catenin and Dorsal–Ventral Axis Formation

Randall T Moon, University of Washington and Howard Hughes Medical Institute, Seattle, Washington, USA

Published online: September 2005

DOI: 10.1038/npg.els.0004185

Abstract

During embryonic development of the amphibian, Xenopus laevis, the dorsal side of the embryo forms approximately opposite the sperm entry point, and the anterior and posterior axes are linked to this through the process of gastrulation. The mechanisms responsible for specification of the dorsal axis include a post‐fertilization cortical rotation that promotes a dorsal accumulation of β‐catenin, and the activation of a cascade of regulatory genes by the complexes of β‐catenin and HMG box transcription factors that start the process of gastrulation.

Keywords: Xenopus; embryo; gene expression; axis; β‐catenin

Figure 1.

Overview of how fertilization leads to establishment of the dorsal axis in Xenopus embryos. (a) In the unfertilized egg there is no dorsal–ventral polarity, (b) Fertilization anywhere within the animal (upper) hemisphere stimulates a rotation of approximately 30° of the cortex relative to the cytoplasm. This is dependent upon microtubule tracks in the vegetal pole. Vesicles, likely with associated Dsh and GBP, move from the vegetal pole 60–90° to the future dorsal side, opposite the sperm entry point, (c) GSK‐3 activity is suppressed on the dorsal side. While it is unknown whether the regulation of GSK‐3 activity upon fertilization involves any Wnt and Fz signalling from the membrane, it is thought to be dependent upon the dorsal accumulation of Dsh and GBP, (d) β‐catenin is synthesized throughout the egg, but is degraded on the ventral side in response to phosphorylation of β‐catenin by CK1 and GSK‐3, which targets it to ubiquitination and degradation by the proteosome, (e) Since β‐catenin is more stable on the prospective dorsal side of the embryo, it accumulates in the cytoplasm, (f) As β‐catenin accumulates, it enters nuclei, where it forms complexes with the HMG box transcription factor Tcf, (g) Complexes of β‐catenin and Tcf bind to the promoter of the homeobox gene siamois, directly inducing its expression on the dorsal side, leading to induction of additional genes required to form the Spemann organizer. (Circles denote cells expressing siamosis.)

Figure 2.

Wnt/β‐catenin signalling pathway in vertebrates. In the absence of Wnt signals (left), newly synthesized β‐catenin either binds to cadherins at the plasma membrane, or is phosphorylated by CK1 then GSK‐3, probably in a degradation complex containing Axin and APC, leading to its being ubiquitinated and degraded by the proteosome. In the presence of Wnt signals, the Wnt receptor Fz and LRP‐5/6 co‐receptor signalling leads to activation of Dishevelled (Dsh), which then acts at least in part via GBP to suppress the activity of GSK‐3. With GSK‐3 suppressed, β‐catenin accumulates, enters nuclei and forms complexes with Tcf and LEF family members (X) which then regulate gene expression (arrows in nucleus denotes gene activation). α, α‐catenin; β, β‐catenin; P, phosphate. (Adapted with permission from Miller and Moon ; more complete model at http://www.stanford.edu/%7Ernusse/wntwindow.html. Copyright © 1996 Cold Spring Harbor Laboratory Press.)

Figure 3.

Dorsal accumulation of β‐catenin in Xenopus embryos. Confocal microscopic image of β‐catenin in a four‐cell Xenopus embryo. β‐catenin (orange) accumulates in the cytoplasm on the prospective dorsal side of the embryo (right) opposite the sperm entry point (left). Green is autofluorescence of yolk. (Reproduced from Larabell et al.. Copyright, The Rockerfeller University Press.)

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References

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Miller JR, Rowning BA, Larabell CA, et al. (1999) Establishment of the dorsal–ventral axis in Xenopus embryos coincides with the dorsal enrichment of Dishevelled that is dependent on cortical rotation. Journal of Cell Biology 146: 427–437.

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Further Reading

Cadigan KM and Nusse R (1997) Wnt signaling: a common theme in animal development. Genes and Development 11: 3286–3305.

Heasman J (1997) Patterning the Xenopusblastula. Development 12: 2155–2164.

Moon RT and Kimelman D (1998) From cortical rotation to organizer gene expression: toward a molecular explanation of axis specification in Xenopus. BioEssays 20: 536–545.

Weaver C, Farr GH, Pan W, et al. (2003) GBP binds kinesin light chain and translocates during cortical rotation in Xenopus eggs. Development 130: 5425–5436.

Wnt homepage http://www.stanford.edu/%7Ernusse/wntwindow.html

Related Sub-Topics:

Cleavage and Gastrulation | Determination of Cell Fate | Genes, Molecules and Cellular Processes
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Article Title: Xenopus Embryo: β‐catenin and Dorsal–Ventral Axis Formation
 
How to Cite close
Moon, Randall T(Sep 2005) Xenopus Embryo: β‐catenin and Dorsal–Ventral Axis Formation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0004185]