FIGURE 10.2. Paraxial mesoderm caudalizes the neural plate via Wnt signals.


Paraxial mesoderm caudalizes the neural plate via Wnt signals. (A) The capability of paraxial mesoderm to induce neural caudalization was shown, when cells from the paraxial margins, 90o from the shield (brown staining), were transplanted into the animal region of zebrafish embryos. These cells induced ectopic anterior Krox20 expression and perturbed forebrain development in the transplanted embryos (right panel). In contrast, transplanted cells from the shield-level margins at 0o (brown staining), did not induce posterior marker expression or alter forebrain formation (bottom panel) (Woo and Fraser, 1997). (B) Quail paraxial mesoderm (green) transforms juxtaposed chick anterior neural plate explants (red) to more posterior fates, inducing Gbx2 and Krox20 while suppressing Otx2 expression (upper row). This transformation is Wnt-dependent as treatment of these recombinant explants with Frz8-CRD protein abolished its induction (bottom row) (Nordstrom et al., 2002). (C) Identification of the source of the Wnt ligand, as Wnt3a, in the paraxial mesoderm. Xenopus dorsolateral marginal mesoderm explants (red) transformed adjacent anterior neural ectoderm to more posterior fates, inducing hindbrain (Krox20), primary neurons (N-Tub), and neural crest (Slug) cell fate markers (blue-purple), and repressing anterior-neural Otx2 expression (middle column). Early expression of the hindbrain-promoting Wnt/β-catenin target mediator, Meis3, was also induced. However, dorsolateral mesoderm knocked down for Wnt3a could not activate Meis3 expression and subsequent posterior neural cell fate inductions,while still developing as a normal paraxial mesoderm, because normal levels of muscle actin were expressed (red staining) in Wnt3a morphant mesoderm explants. These results show that a neural caudalizing Wnt3a ligand emanates from the paraxial mesoderm to caudalize anterior neural ectoderm. Reconfirming the observations in zebrafish (A), dorsal marginal mesoderm did not alter the anterior nature of the neural ectoderm explants (left column) (Elkouby et al., 2010). (D) A model for posterior induction (Elkouby et al., 2010). Upon neural induction by axial-derived BMP antagonists (Da), paraxial mesoderm secretes the Wnt3a ligand to the overlying neural plate, where the activated Wnt/β-catenin pathway directly induces Meis3 gene expression (Db).

From: Chapter 10, The Role of Mesoderm and Specific Wnt Ligands in Neural Patterning

Cover of Wnt/β-Catenin Signaling in Vertebrate Posterior Neural Development
Wnt/β-Catenin Signaling in Vertebrate Posterior Neural Development.
Elkouby YM, Frank D.
San Rafael (CA): Morgan & Claypool Life Sciences; 2010.
Copyright © 2010 by Morgan & Claypool Life Sciences.

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