The community effect is an interaction among a group of many nearby precursor cells, necessary for them to maintain tissue-specific gene expression and differentiate co-ordinately. During Xenopus myogenesis, the muscle precursor cells must be in group contact throughout gastrulation in order to develop into terminally differentiated muscle. The molecular basis of this community interaction has not to date been elucidated. We have developed an assay for testing potential community factors, in which isolated muscle precursor cells are treated with a candidate protein and cultured in dispersion. We have tested a number of candidate factors and we find that only eFGF protein is able to mediate a community effect, stimulating stable muscle-specific gene expression in demonstrably single muscle precursor cells. In contrast, Xwnt8, bFGF, BMP4 and TGF(β)2 do not show this capacity. We show that eFGF is expressed in the muscle precursor cells at the right time to mediate the community effect. Moreover, the time when the muscle precursor cells are sensitive to eFGF corresponds to the period of the endogenous community effect. Finally, we demonstrate that FGF signalling is essential for endogenous community interactions. We conclude that eFGF is likely to mediate the community effect in Xenopus myogenesis.
Wnts are secreted signaling molecules implicated in various developmental processes and frizzled proteins are the receptors for these Wnt ligands. To investigate the physiological roles of frizzled proteins, we isolated and characterized a novel mouse frizzled gene Fzd5. Fzd5 mRNA was expressed in the yolk sac, eye and lung bud at 9.5 days post coitum. Fzd5 specifically synergized with Wnt2, Wnt5a and Wnt10b in ectopic axis induction assays in Xenopus embryos. Using homologous recombination in embryonic stem cells, we have generated Fzd5 knockout mice. While the heterozygotes were viable, fertile and appeared normal, the homozygous embryos died in utero around 10.75 days post coitum, owing to defects in yolk sac angiogenesis. At 10.25 days post coitum, prior to any morphological changes, endothelial cell proliferation was markedly reduced in homozygous mutant yolk sacs, as measured by BrdU labeling. By 10.75 days post coitum, large vitelline vessels were poorly developed, and the capillary plexus was disorganized. At this stage, vasculogenesis in the placenta was also defective, although that in the embryo proper was normal. Because Wnt5a and Wnt10b co-localized with Fzd5 in the developing yolk sac, these two Wnts are likely physiological ligands for the Fzd5-dependent signaling for endothelial growth in the yolk sac.
We describe the identification of an enhancer sequence and a sequence-specific DNA-binding protein required for developmental expression of the Xenopus laevis GS17 gene. Using microinjection of recombinant plasmids into fertilized frog eggs, we have shown that a 14 base pair CT-rich sequence element, normally located about 700 bases upstream of the GS17 promoter, is sufficient to activate transcription of a heterologous reporter gene in gastrula stage embryos. This regulatory element has been called the OZ sequence. Sequences closely related to OZ are located in the promoter regions of several other genes expressed during Xenopus development. Extracts prepared from Xenopus embryos show the presence of a DNA-binding factor, OZ-1, that specifically recognizes the OZ sequence. Mutations within the OZ element that abolish OZ-1 binding also abolish enhancer activity. The OZ-1 factor contains at least two proteins of approximate M(r) 76 × 10(3) and 100 × 10(3). The sequence-specific binding activity accumulates during oogenesis and remains present at approximately constant levels throughout early development.