Control of embryonic Xenopus morphogenesis by a Ral-GDS/Xral branch of the Ras signalling pathway.
Ras proteins mediate biological responses through various effectors and play a key role in relaying the Fibroblast Growth Factor (FGF) mesoderm induction signal during embryogenesis of the frog, Xenopus laevis. One Ras effector pathway involves the activation of the small G protein Ral. In the present study, we have investigated the role of key components in the Ral branch of FGF and Ras signalling during early Xenopus development. Treatment of animal caps with bFGF, which converts prospective ectoderm to mesoderm, activates Xral. The Ras mutant 12V37G, which can bind to Ral-GDS but not Raf, also activates Xral as well as causing developmental defects and cortical F-actin disassembly. A similar phenotype is induced by Ral-GDS itself. FGF-induced expression of several signature mesodermal genes, by contrast, is independent of Xral signalling. This and other data suggest that the RalB branch of Ras and FGF signalling regulates the actin cytoskeleton and morphogenesis in a transcriptionally independent manner. We also find Xral to be specifically activated in the marginal zone of Xenopus embryos, and find that disruption of the Ral pathway in this region prevents closure of the blastopore during gastrulation. We conclude that Ral signalling is autonomously required by mesodermal cells to effect essential morphogenetic changes during Xenopus gastrulation.
Institut Jacques Monod, CNRS, Universités Paris VI et Paris VII, Mécanismes Moléculaires du Développement, 2 Place Jussieu, 75251 Paris cedex 05, France.
Stéphanie
S
Lebreton
Laurent
L
Boissel
Jacques
J
Moreau
D000818:Animals; D003001:Cloning, Molecular; D004625:Embryo, Nonmammalian / Q000378:metabolism; D004789:Enzyme Activation; D005346:Fibroblast Growth Factors / Q000378:metabolism*; D005775:Gastrula / Q000166:cytology / Q000378:metabolism; D008648:Mesoderm / Q000166:cytology / Q000378:metabolism; D018613:Microscopy, Confocal; D009024:Morphogenesis / Q000502:physiology*; D009154:Mutation; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D014981:Xenopus / Q000196:embryology / Q000378:metabolism*; D020725:ral GTP-Binding Proteins / Q000378:metabolism; D020726:ral Guanine Nucleotide Exchange Factor / Q000378:metabolism*; D018631:ras Proteins / Q000378:metabolism*
D020726:ral Guanine Nucleotide Exchange Factor; D005346:Fibroblast Growth Factors; D020725:ral GTP-Binding Proteins; D018631:ras Proteins