Negative regulation of Activin/Nodal signaling by SRF during Xenopus gastrulation.
2007
Activin/Nodal signaling is essential for germ-layer formation and axial
patterning during embryogenesis. Recent evidence has demonstrated that the
intra- or extracellular inhibition of this signaling is crucial for ectoderm
specification and correct positioning of mesoderm and endoderm. Here, we
analyzed the function of Xenopus serum response factor (XSRF) in
establishing germ layers during early development. XSRF transcripts are
restricted to the animal pole ectoderm in Xenopus early embryos.
Ectopic expression of XSRF RNA suppresses mesoderm induction, both in the
marginal zone in vivo and caused by Activin/Nodal signals in animal caps.
Dominant-negative mutant or antisense morpholino oligonucleotide-mediated
inhibition of XSRF function expands the expression of mesendodermal genes
toward the ectodermal territory and enhances the inducing activity of the
Activin signal. SRF interacts with Smad2 and FAST-1, and inhibits the
formation of the Smad2-FAST-1 complex induced by Activin. These results
suggest that XSRF might act to ensure proper mesoderm induction in the
appropriate region by inhibiting the mesoderm-inducing signals during early
embryogenesis.
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