MATERNAL BETA-CATENIN ESTABLISHES A DORSAL SIGNAL IN EARLY XENOPUS EMBRYOS

In previous work, we demonstrated that maternally encoded beta-catenin , the vertebrate homolog of armadillo, is required for formation of do rsal axial structures in early Xenopus embryos (Heasman, J., Crawford, A., Goldstone, K., Garner-Hamrick, P., Gumbiner, B., Kintner, C., Yos hida-Noro, C. and Wylie, C. (1994). Cell 79, 791-803). Here we investi gated, firstly, the role(s) of beta-catenin in spatial terms, in diffe rent regions of the embryo, by injecting beta-catenin mRNA into indivi dual blastomeres of beta-catenin-depleted embryos at the 32 cell stage . The results indicate that beta-catenin can rescue the dorsal axial s tructures in a non-cell-autonomous way and without changing the fates of the injected cells. This suggests that cells overexpressing beta-ca tenin send a 'dorsal signal' to other cells. This was confirmed by sho wing that beta-catenin overexpressing animal caps did not cause wild-t ype caps to form mesoderm, but did cause isolated beta-catenin-deficie nt marginal zones to form dorsal mesoderm. Furthermore beta-catenin-de ficient vegetal masses treated with overexpressing caps regained their ability to act as Nieuwkoop Centers. Secondly, we studied the tempora l activity of beta-catenin. We showed that zygotic transcription of be ta-catenin starts after the midblastula transition (MBT), but does not rescue dorsal axial structures. We further demonstrated that the vege tal mass does not release a dorsal signal until after the onset of tra nscription, at the midblastula stage, suggesting that maternal beta-ca tenin protein is required at or before this time. Thirdly we investiga ted where, in relationship to other gene products known to be active i n axis formation, beta-catenin is placed. We find that BVg1, bFGF, tBR (the truncated form of BMP2/4R), siamois and noggin activities are al l downstream of beta-catenin, as shown by the fact that injection of t heir mRNAs rescues the effect of depleting maternally encoded beta-cat enin. Interference with the action of glycogen synthase kinase (GSK), a vertebrate homolog of the Drosophila gene product, zeste white 3 kin ase, does not rescue the effect, suggesting that it is upstream.