Ras-mediated FGF signaling is required for the formation of posterior but not anterior neural tissue in Xenopus laevis

Fibroblast growth factor (FGF) has been proposed to be involved in the spec ification and patterning of the developing vertebrate nervous system. There is conflicting evidence, however, concerning the requirement for FGF signa ling in these processes. To provide insight into the signaling mechanisms t hat are important for neural induction and anterior-posterior neural patter ning, we have employed the dominant negative Ras mutant, N17Ras, in additio n to a truncated FGF receptor (XFD). Both N17Ras and XFD, when expressed in Xenopus laevis animal cap ectoderm, inhibit the ability of FGF to generate neural pattern. They also block induction of posterior neural tissue by XB F2 and XMeis3. However, neither XFD nor N17Ras inhibits noggin, neurogenin, or XBF2 induction of anterior neural markers. MAP kinase activation has be en proposed to be necessary for neural induction, yet N17Ras inhibits the p hosphorylation of MAP kinase that usually follows explantation of explants. In whole embryos, Pas-mediated FGF signaling is critical for the formation of posterior neural tissues but is dispensable for neural induction. (C) 2 000 Academic Press.