Figure 3. Opposite effects of Nodal and Oep on rasl11b expression.

(A) Scheme of the Nodal cascade. The Oep coreceptor is necessary for the binding of the Nodal ligand Cyc and Sqt to the TGFβ receptors (containing the type I receptor Taram-A, the zebrafish Alk4 orthologue) that in turn phosphorylate Smad2. Smad2-P is translocated to the nucleus and triggers the transcription of a first set of genes encoding the transcription factors, Mixer, Gata5 and Mezzo. This set is required for the expression of the Sox factor Casanova that in turn initiates the transcription of the endodermal markers sox17 and axial/foxa2. (B–M, Dorsal view of gastrulae save C, I, sagittal sections and G, lateral view) (B, C) In wild-type (WT) embryos rasl11b is expressed at the animal pole (arrows) and at the dorsal margin (black arrowheads). (D) This marginal expression is lost in MZoep mutants (star), devoid of maternal and zygotic oep, consequently devoid of Nodal signaling, and so, unable to form most of the mesendoderm. (E, F) rasl11b dorso-marginal expression is normal in the cyclops Nodal mutant and largely reduced in the MZsquint Nodal mutant. (G) Activation of the Nodal signal by injection of a constitutively activated form of the Nodal type I receptor Taram-A (tar*) leads to a duplication of the rasl11b marginal expression domain (likely by inducing a second organizer, white arrowheads) (H, I) In Zoep mutants, devoid of zygotic oep, this mesendodermal expression domain is extended (white arrowheads). (J–M) A large series of embryos expressing different levels of Nodal signal was generated by injecting between blastula cells increasing doses of the recombinant Lefty protein, a Nodal pathway extracellular inhibitor. Here, only four representative doses are displayed. A progressive decrease of rasl11b marginal expression but no expansion was observed, even with concentrations of Lefty able to mimic a Zoep-like phenotype.