foxD5a, a Xenopus winged helix gene, maintains an immature neural ectodermvia transcriptional repression that is dependent on the C-terminal domain

Xenopus foxD5a, the full-length fork head gene previously described as a PC R fragment (XFLIP), is first detectable at stage II of oogenesis. Low-abund ance maternal transcripts are localized to the animal hemisphere of the cle avage embryo, and protein can be translocated to the nucleus prior to the o nset of zygotic transcription. Zygotic expression is strongest in the presu mptive neural ectoderm at gastrula and neural plate stages, but there is mi nor paraxial mesodermal expression during primary gastrulation that becomes significant in the tail bud during secondary gastrulation. Expression of f oxD5a in animal cap explants induces elongation and expression of mesoderma l, neural-inducing, and early neural-specifying genes, indicating a role in dorsal axis formation. Zygotic foxD5a expression is induced strongly by si amois, moderately by cerberus, weakly by Wnt8 and noggin, and not by chordi n in animal cap explants. Expression of foxD5a in whole embryos has differe ntial dorsal and ventral effects. Ventral mRNA injection induces partial se condary axes composed of expanded mesodermal and epidermal tissues, but doe s not induce ectopic neural tissues. Dorsal mRNA injection causes hypertrop hy of the neural plate and expansion of early neural genes (sox3 and otx2), but this is not the result of increased proliferation or expanded neural-i nducing mesoderm. The neural plate appears to be maintained in an immature state because otx2 expression is expanded and expression of en2, Krox20, pr oneural genes (Xnrgn1, neuroD) and a neural differentiation gene (In-tubuli n) is repressed in foxD5a-expressing cells. These results indicate that fox D5a maintains an undifferentiated neural ectoderm after neural induction. E xpression of foxD5a constructs fused with the engrailed repressor domain or with the VP16 activation domain demonstrates that FoxD5a acts as a transcr iptional repressor in axis formation and neural plate expansion. Deletion c onstructs indicate that this activity requires the C-terminal domain of the protein. (C) 2001 Academic Press.