Wnt6 activates endoderm in the sea urchin gene regulatory network

Author:

Croce Jenifer1,Range Ryan2,Wu Shu-Yu3,Miranda Esther3,Lhomond Guy1,Peng Jeff Chieh-fu4,Lepage Thierry1,McClay David R.3

Affiliation:

1. Biologie du Développement, Observatoire Océanologique de Villefranche-sur-Mer, UPMC, UMR7009 CNRS, 06230 Villefranche-sur-Mer, France.

2. National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892, USA.

3. Department of Biology, French Family Science Center, Duke University, Durham, NC 27708, USA.

4. Department of Biology, Cox Science Center, University of Miami, Coral Gables, FL 33124, USA.

Abstract

In the sea urchin, entry of β-catenin into the nuclei of the vegetal cells at 4th and 5th cleavages is necessary for activation of the endomesoderm gene regulatory network. Beyond that, little is known about how the embryo uses maternal information to initiate specification. Here, experiments establish that of the three maternal Wnts in the egg, Wnt6 is necessary for activation of endodermal genes in the endomesoderm GRN. A small region of the vegetal cortex is shown to be necessary for activation of the endomesoderm GRN. If that cortical region of the egg is removed, addition of Wnt6 rescues endoderm. At a molecular level, the vegetal cortex region contains a localized concentration of Dishevelled (Dsh) protein, a transducer of the canonical Wnt pathway; however, Wnt6 mRNA is not similarly localized. Ectopic activation of the Wnt pathway, through the expression of an activated form of β-catenin, of a dominant-negative variant of GSK-3β or of Dsh itself, rescues endomesoderm specification in eggs depleted of the vegetal cortex. Knockdown experiments in whole embryos show that absence of Wnt6 produces embryos that lack endoderm, but those embryos continue to express a number of mesoderm markers. Thus, maternal Wnt6 plus a localized vegetal cortical molecule, possibly Dsh, is necessary for endoderm specification; this has been verified in two species of sea urchin. The data also show that Wnt6 is only one of what are likely to be multiple components that are necessary for activation of the entire endomesoderm gene regulatory network.

Publisher

The Company of Biologists

Subject

Developmental Biology,Molecular Biology

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