The dual regulator Sufu integrates Hedgehog and Wnt signals in the early Xenopus embryo.

Hedgehog (Hh) and Wnt proteins are important signals implicated in several aspects of embryonic development, including the early development of the central nervous system. We found that Xenopus Suppressor-of-fused (XSufu) affects neural induction and patterning by regulating the Hh/Gli and Wnt/ß-catenin pathways. Microinjection of XSufu mRNA induced expansion of the epidermis at the expense of neural plate tissue and caused enlargement of the eyes. An antisense morpholino oligonucleotide against XSufu had the opposite effect. Interestingly, both gain- and loss-of-function experiments resulted in a posterior shift of brain markers, suggesting a biphasic effect of XSufu on anteroposterior patterning. XSufu blocked early Wnt/ß-catenin signaling, as indicated by the suppression of XWnt8-induced secondary axis formation in mRNA-injected embryos, and activation of Wnt target genes in XSufu-MO-injected ectodermal explants. We show that XSufu binds to XGli1 and Xß-catenin. In Xenopus embryos and mouse embryonic fibroblasts, Gli1 inhibits Wnt signaling under overexpression of ß-catenin, whereas ß-catenin stimulates Hh signaling under overexpression of Gli1. Notably, endogenous Sufu is critically involved in this crosstalk. The results suggest that XSufu may act as a common regulator of Hh and Wnt signaling and contribute to intertwining the two pathways.

Retinol dehydrogenase 10 is a feedback regulator of retinoic acid signalling during axis formation and patterning of the central nervous system.

Retinoic acid (RA) is an important morphogen that regulates many biological processes, including the development of the central nervous system (CNS). Its synthesis from vitamin A (retinol) occurs in two steps, with the second reaction--catalyzed by retinal dehydrogenases (RALDHs)--long considered to be crucial for tissue-specific RA production in the embryo. We have recently identified the Xenopus homologue of retinol dehydrogenase 10 (XRDH10) that mediates the first step in RA synthesis from retinol to retinal. XRDH10 is specifically expressed in the dorsal blastopore lip and in other domains of the early embryo that partially overlap with XRALDH2 expression. We show that endogenous RA suppresses XRDH10 gene expression, suggesting negative-feedback regulation. In mRNA-injected Xenopus embryos, XRDH10 mimicked RA responses, influenced the gene expression of organizer markers, and synergized with XRALDH2 in posteriorizing the developing brain. Knockdown of XRDH10 and XRALDH2 by specific antisense morpholino oligonucleotides had the opposite effects on organizer gene expression, and caused a ventralized phenotype and anteriorization of the brain. These data indicate that the conversion of retinol into retinal is a developmentally controlled step involved in specification of the dorsoventral and anteroposterior body axes, as well as in pattern formation of the CNS. We suggest that the combinatorial gene expression and concerted action of XRDH10 and XRALDH2 constitute a ;biosynthetic enzyme code' for the establishment of a morphogen gradient in the embryo.

The secreted serine protease xHtrA1 stimulates long-range FGF signaling in the early Xenopus embryo.

We found that the secreted serine protease xHtrA1, expressed in the early embryo and transcriptionally activated by FGF signals, promotes posterior development in mRNA-injected Xenopus embryos. xHtrA1 mRNA led to the induction of secondary tail-like structures, expansion of mesoderm, and formation of ectopic neurons in an FGF-dependent manner. An antisense morpholino oligonucleotide or a neutralizing antibody against xHtrA1 had the opposite effects. xHtrA1 activates FGF/ERK signaling and the transcription of FGF genes. We show that Xenopus Biglycan, Syndecan-4, and Glypican-4 are proteolytic targets of xHtrA1 and that heparan sulfate and dermatan sulfate trigger posteriorization, mesoderm induction, and neuronal differentiation via the FGF signaling pathway. The results are consistent with a mechanism by which xHtrA1, through cleaving proteoglycans, releases cell-surface-bound FGF ligands and stimulates long-range FGF signaling.