Assays for gene function in developing Xenopus embryos

Interactions between Xwnt-8 and Spemann organizer signaling pathways generate dorsoventral pattern in the embryonic mesoderm of Xenopus

This study analyzes the hierarchy of signals that spatially restrict expression of Xenopus Xwnt-8 to mesodermal cells outside of the Spemann organizer field and examines the potential role that endogenous Xwnt-8 may play in dorsoventral patterning of the embryonic mesoderm. The effects of ectopic introduction of a Nieuwkoop center-like activity or of ectopic expression of goosecoid, on the distribution of endogenous Xwnt-8 transcripts were analyzed. The results of these studies are consistent with the hypothesis that maternally derived signals from the Nieuwkoop center function to positively regulate expression of the homeo box gene goosecoid in Spemann organizer cells, leading to a subsequent repression of Xwnt-8 expression in these cells. This exclusion of Xwnt-8 from cells of the organizer field may be important for normal dorsal development, in that ectopic expression of Xwnt-8 in organizer cells after the midblastula stage, by injection of plasmid DNA, ventralizes the fate of these cells. This is distinct from the previously observed dorsalizing effect of Xwnt-8 when expressed prior to the midblastula stage by injection of RNA. The effects of plasmid-derived Xwnt-8 on isolated blastula animal cap ectoderm were also analyzed. Expression of Xwnt-8 in animal pole ectoderm after the midblastula stage ventralizes the response of dorsal animal pole cells to activin and allows naive ectodermal cells to differentiate as ventral mesoderm in the absence of added growth factors. Collectively, these data are consistent with the hypothesis that Xwnt-8 plays a role in the mesodermal differentiation of ventral marginal zone cells during normal development. Furthermore, endogenous Xwnt-8 may ventralize the response of lateral mesodermal cells to dorsalizing signals from the organizer, thus contributing to the graded nature of the final body pattern.

Responses of embryonic Xenopus cells to activin and FGF are separated by multiple dose thresholds and correspond to distinct axes of the mesoderm

The potent mesoderm-inducing factors activin and FGF are present as maternally synthesized proteins in embryos of X. laevis. We show that activin can act on explanted blastomeres to induce at least five different cell states ranging from posterolateral mesoderm to dorsoanterior organizer mesoderm. Each state is induced in a narrow dose range bounded by sharp thresholds. By contrast, FGF induces only posterolateral markers and does so over relatively broad dose ranges. FGF can modulate the actions of activin, potentiating them and broadening the threshold-bounded dose windows. Our results indicate that orthogonal gradients of activin and FGF would be sufficient to specify the main elements of the body plan.

Nuclear transport and phosphorylation of the RNA binding Xenopus zinc finger protein XFG 5-1

XFG 5-1 is a Krüppel-type Xenopus zinc finger protein with specific RNA homopolymer binding activity in vitro. In the oocyte, the protein is distributed between nucleus and cytoplasm; the nuclear fraction, not the cytoplasm, contains phosphorylated isoform(s) of XFG 5-1. In vitro phosphorylation by use of oocyte/egg extracts or purified casein kinase II is specific to the amino-terminal portion of the protein. The carboxy-terminal zinc finger domain contains a signal sufficient for nuclear transport. Overexpression of either full length XFG 5-1 or of the carboxy-terminal portion alone, which maintains RNA binding and nuclear import activities, was achieved in Xenopus embryos by mRNA injection. This treatment did not result in impaired regulation of development, suggesting that XFG 5-1 functions in a way distinct from the mode of action exemplified in the Drosophila zinc finger protein Krüppel.