Expression of activated MAP kinase in Xenopus laevis embryos: Evaluating the roles of FGF and other signaling pathways in early induction and patterning

FGF signaling has been implicated in germ layer formation and axial determi nation. An antibody specific for the activated form of mitogen-activated pr otein kinase ((MAPK) was used to monitor FGF signaling in vivo during early Xenopus development. Activation of MAPK in young embryos is abolished by i njection of a dominant negative FGF receptor (XFD) RNA, suggesting that MAP K is activated primarily by EGF in this context. A transition from cytoplas mic to nuclear localization of activated MAPK occurs in morula/blastula sta ge embryo animal and marginal zones coinciding with the proposed onset of m esodermal competence. Activated MAPK delineates the region of the dorsal ma rginal zone before blastopore formation and persists in this region during gastrulation, indicating an early role for FGF signaling in dorsal mesoderm . Activated MAPK was also found in posterior neural tissue from late gastru lation onward. Inhibition of PGE signaling does not block posterior neural gene expression (HoxB9) or activation of MAPK; however, inhibition of FGF s ignaling does cause a statistically significant decrease in the level of ac tivated MAPK. These results point toward the involvement of other receptor tyrosine kinase signaling pathways in posterior neural patterning. (C) 2000 Academic Press.