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

Abstract

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.

Inhibition of Wnt activity induces heart formation from posterior mesoderm

“…Conversely, the presence of Wnt signals, in combination with BMP activity, is necessary for formation of primitive erythrocytes in posterior mesoderm. Head induction in Xenopus embryos requires the absence of both Wnt signals and BMP signals (Glinka et al 1997), and ectopic Wnt expression can block the formation of anterior neural tissue in Xenopus (Christian and Moon 1993). Together with our own findings, these observations suggest that induction of anterior cell fates in both ectoderm and mesoderm requires a blockade of Wnt signals.…”

“…Ectopic expression of FrzB, a Wnt-8 antagonist, expands cement gland and inhibits posterior development in Xenopus (Leyns et al 1997;Wang et al 1997). In contrast, zygotically transcribed XWnt-8 promotes convergent extension movements and the development of ventral and posterior structures, including blood and somites (Christian and Moon 1993;Hoppler et al 1996;Hoppler and Moon 1998). A second class of Wnt antagonists represented by Dkk-1 also inhibits Wnt-8 signaling at the extracellular level and has effects similar to those of FrzB on the Xenopus embryo (Glinka et al 1998).…”

IGFBP-4 is an inhibitor of canonical Wnt signalling required for cardiogenesis

“…Full-length LRP6, membrane-bound forms of LRP6 deletion mutants, and Dkk1 were from C. Niehrs 24 . pXwnt8 and pCSKA-Xwnt8 were from J. Christian 25 . pCS2-b-catenin was from D. Kimelman 26 .…”

Induction and modulation of smooth muscle differentiation in Xenopus embryonic cells

“…We tested whether this interaction also activate smooth muscle gene expression. Two-cell-stage embryos were injected with the plasmid CSKA-XWnt8 DNA where Wnt8 cDNA is placed under the control of the cytoskeletal actin promoter (CSKA) allowing the expression of Wnt8 protein after the midblastula transition (Christian and Moon, 1993). Animal caps derived from embryos injected with either 10 pg or 100 pg of CSKA-XWnt8 express SM-MHC and SM22␣ but not MLC1f (Fig.…”