Cb. Chang et A. Hemmati-brivanlou, A post-mid-blastula transition requirement for TGF beta signaling in earlyendodermal specification, MECH DEVEL, 90(2), 2000, pp. 227-235
In Xenopus, endodermal cell fate is determined gradually from late blastula
to early gastrula stages, cell-cell interaction plays an important role in
this process. Here we use a cell dissociation assay to show that extracell
ular signaling is required continuously before endoderm determination. Acti
vin and Vg1, but not BMP2 or basic FGF, rescue the expression of endodermal
markers in dissociated cells when provided at the mid-blastula transition
(MBT, the time in which zygotic transcription begins). Removal of exogenous
ly added activin or Vg1 before MBT results in reduction of endodermal gene
expression in dissociated vegetal cells. In vivo, endogenous endodermal mar
kers are reduced in vegetal explants when activin-like signaling is blocked
with dominant negative receptors. VegT, a maternal transcription factor sh
own to be critical for endoderm specification, relies on an active TGF beta
pathway to induce endoderm in animal caps. These results indicate that TGF
beta signaling may be activated by the maternally expressed VegT to partic
ipate in endoderm determination. In addition, VegT function seems to be req
uired in parallel with the TGF beta pathway, as overexpression of activin d
oes not relieve endoderm repression by a dominant negative VegT mutant in v
egetal cells. Our data suggest that maternal VegT first activates a zygotic
TGF beta signal, then cooperates with this signal to determine the endoder
mal cell fate. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.