Endodermal Nodal-related signals and mesoderm induction in Xenopus

In Xenopus, mesoderm induction by endoderm at the blastula stage is well do cumented, but the molecular nature of the endogenous inductive signals rema ins unknown. The carboxy-terminal fragment of Cerberus, designated Cer-S, p rovides a specific secreted antagonist of mesoderm-inducing Xenopus Nodal-R elated (Xnr) factors. Cer-S does not inhibit signalling by other mesoderm i nducers such as Activin, Derriere, Vg1 and BMP4, nor by the neural inducer; Xnr3, In the present study we show that Cer-S blocks the induction of both dorsal and ventral mesoderm in animal-vegetal Nieuwkoop-type recombinants. During blastula stages Xnr1, Xnr2 and Xnr4 are expressed in a dorsal to ven tral gradient in endodermal cells. Dose-response experiments using cer-S mR NA injections support the existence of an endogenous activity gradient of X nrs, Xnr expression at blastula can be activated by the vegetal determinant s VegT and Vg1 acting in synergy with dorsal beta-catenin, The data support a modified model for mesoderm induction in Xenopus, in which mesoderm indu ction is mediated by a gradient of multiple Nodal-related signals released by endoderm at the blastula stage.