Xenopus Vg1, a transforming growth factor β (Tgfβ)family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg1 transcript does not rescue Vg1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg1 function,and shows it to be an essential maternal regulator of embryonic patterning.

The Gli proteins are critical components of multiple processes in development, homeostasis and disease, including neurogenesis and tumorigenesis. However, it is unclear how the Gli code, the sum of their combinatorial positive and negative functions, dictates cell fate and behavior. Using an antisense approach to knockdown gene function in vivo, we find that each of the three Gli proteins is required for the induction of all primary neurons in the amphibian neural plate and regulates the bHLH/Notch neurogenic cascade. Analyses of endogenous Gli function in Gli-mediated neurogenesis and tumorigenesis, and in animal cap assays, reveal specific requirements that are context specific. Nuclear colocalization and binding studies suggest the formation of complexes, with the first two zinc fingers of the Gli five zinc-finger domain acting as a protein-protein interaction site. The Gli proteins therefore appear to form a dynamic physical network that underlies cooperative function, greatly extending the combinatorial possibilities of the Gli code, which may be further fine-tuned in cell fate specification by co-factor function.

Epiboly, the spreading of the blastoderm over the large yolk cell, is the first morphogenetic movement of the teleost embryo. Examining this movement as a paradigm of vertebrate morphogenesis, we have focused on the epiboly arrest mutant half baked ( hab ), which segregates as a recessive lethal, including alleles expressing zygotic-maternal dominant (ZMD) effects. Here we show that hab is a mutation in the zebrafish homolog of the adhesion protein E-cadherin. Whereas exclusively recessive alleles of hab produce truncated proteins, dominant alleles all contain transversions in highly conserved amino acids of the extracellular domains,suggesting these alleles produce dominant-negative effects. Antisense oligonucleotides that create specific splicing defects in the hab mRNA phenocopy the recessive phenotypes and, surprisingly, some of the ZMD phenotypes as well. In situ analyses show that during late epiboly hab is expressed in a radial gradient in the non axial epiblast, from high concentrations in the exterior layer of the epiblast to low concentrations in the interior layer of the epiblast . During epiboly,using an asymmetric variant of radial intercalation, epiblast cells from the interior layer sequentially move into the exterior layer and become restricted to that layer; there they participate in subtle cell shape changes that further expand the blastoderm. In hab mutants, when cells intercalate into the exterior layer, they tend to neither change cell shape nor become restricted, and many of these cells `de-intercalate' and move back into the interior layer. Cell transplantation showed all these defects to be cell-autonomous. Hence, as for the expansion of the mammalian trophoblast at a similar developmental stage, hab /E-cadherin is necessary for the cell rearrangements that spread the teleost blastoderm over the yolk.

FoxH1 (Fast1) was first characterized as the transcriptional partner for Smad proteins. Together with Smad2/4, it forms the activin response factor(ARF) that binds to the Mix.2 promoter in Xenopus embryos. Foxh1 is expressed maternally in Xenopus . Depletion of maternal Foxh1 mRNA results in abnormalities of head and dorsal axis formation. We show that FoxH1 is required, together with XTcf3/β catenin,to activate the zygotic expression of the nodal gene, Xnr3 in a Smad2-independent manner. In contrast, maternal FoxH1 acts as an inhibitor of Xnr5 and 6 transcription, preventing their upregulation on the ventral side of the embryo, by the maternal T-box transcription factor VegT. We conclude that maternal FoxH1 has essential, context-dependent roles in regulating the pattern of zygotic gene expression in the early embryo.