Summary The liver, pancreas and lungs are induced from endoderm progenitors by a series of dynamic growth factor signals from the mesoderm, but how the temporal-spatial activity of these signals is controlled is poorly understood. We have identified an extracellular regulatory loop required for robust BMP signaling in the Xenopus foregut. We show that BMP signaling is required to maintain foregut progenitors and induce expression of the secreted frizzled related protein Sizzled (Szl) and the extracellular metalloprotease Tolloid-like 1 (Tll1). Szl negatively regulates Tll activity to control deposition of a Fibronectin (FN) matrix between the mesoderm and endoderm, which is required to maintain BMP signaling. Foregut-specific Szl depletion results in a loss of the FN matrix and failure to maintain robust pSmad1 levels causing a loss of foregut gene expression and organ agenesis. These results have implications for BMP signaling in diverse contexts and the differentiation of foregut tissue from stem cells.

Of the three Dishevelled (Dvl) genes, only Dvl2 and Dvl3 are maternally encoded in the frog, Xenopus laevis. We show here by loss of function analysis that single depletion of either Dvl2 or Dvl3 from the oocyte causes the same embryonic phenotype. We find that the effects of loss of function of Dvl2 and 3 together are additive, and that the proteins physically interact, suggesting that both are required in the same complex. We show that maternal Dvl2 and 3 are required for convergence extension movements downstream of the dorsally localized signaling pathway activated by Xnr3, but not downstream of the pathway activated by activin. Also, depletion of maternal Dvl2 and 3 mRNAs causes the up-regulation of a subset of zygotic ectodermal genes, including Foxi1e, with surprisingly no significant effect on the canonical Wnt direct target genes Siamois and Xnr3. We suggest that the likely reason for continued expression of the Wnt target genes in Dvl2/3-depleted embryos is that maternal Dvl mRNA depletion is insufficient to deplete stored punctae of Dvl protein in the oocyte cortex, which may transduce dorsal signaling after fertilization.

Cholesterol-rich membrane microdomains (CRMMs) are specialized structures that have recently gained much attention in cell biology because of their involvement in cell signaling and trafficking. However, few investigations, particularly those addressing embryonic development, have succeeded in manipulating and observing CRMMs in living cells. In this study, we performed a detailed characterization of the CRMMs lipid composition during early frog development. Our data showed that disruption of CRMMs through methyl-β-cyclodextrin (MβCD) cholesterol depletion at the blastula stage did not affect Spemann's organizer gene expression and inductive properties, but impaired correct head development in frog and chick embryos by affecting the prechordal plate gene expression and cellular morphology. The MβCD anterior defect phenotype was recapitulated in head anlagen (HA) explant cultures. Culture of animal cap expressing Dkk1 combined with MβCD-HA generated a head containing eyes and cement gland. Together, these data show that during Xenopus blastula and gastrula stages, CRMMs have a very dynamic lipid composition and provide evidence that the secreted Wnt antagonist Dkk1 can partially rescue anterior structures in cholesterol-depleted head anlagen.

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