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 negativefeedback 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.

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.

Single-cell migration is a key process in development, homeostasis, and disease. Nevertheless, the control over basic cellular mechanisms directing cells into motile behavior in vivo is largely unknown. Here, we report on the identification of a minimal set of parameters the regulation of which confers proper morphology and cell motility. Zebrafish primordial germ cells rendered immotile by knockdown of Dead end, a negative regulator of miRNA function, were used as a platform for identifying processes restoring motility. We have defined myosin contractility, cell adhesion, and cortex properties as factors whose proper regulation is sufficient for restoring cell migration of this cell type. Tight control over the level of these cellular features, achieved through a balance between miRNA-430 function and the action of the RNA-binding protein Dead end, effectively transforms immotile primordial germ cells into polarized cells that actively migrate relative to cells in their environment.