A commonly accepted model of Wnt/β-catenin signaling involves target gene activation by a complex of β-catenin with a TCF family member. TCF3 is a transcriptional repressor that has been implicated in Wnt signaling and plays key roles in embryonic axis specification and stem cell differentiation. Here we demonstrate that Wnt proteins stimulate TCF3 phosphorylation in gastrulating Xenopus embryos and mammalian cells. This phosphorylation event involves β-catenin-mediated recruitment of homeodomain-interacting protein kinase 2 (HIPK2) to TCF3 and culminates in the dissociation of TCF3 from a target gene promoter. Mutated TCF3 proteins resistant to Wnt-dependent phosphorylation function as constitutive inhibitors of Wnt-mediated activation of Vent2 and Cdx4 during anteroposterior axis specification. These findings reveal an alternative in vivo mechanism of Wnt signaling that involves TCF3 phosphorylation and subsequent derepression of target genes and link this molecular event to a specific developmental process.

The dorsoventral axis is established early in Xenopus development and may involve signaling by Wnts, a family of Wntl-protooncogene-related proteins. The protein kinase shaggy functions in the wingless/Wnt signaling pathway, which operates during Drosophila development. To assess the role of a closely related kinase, glycogen synthase kinase 3,B , in vertebrate embryogenesis, we cloned a cDNA encoding a Xenopus homolog of To assess a possible role for GSK-3,B in dorsoventral axis formation, we cloned a Xenopus homolog of GSK-3,B/shaggy (XGSK-313)t and examined its expression pattern in early embryos. We also constructed several mutated forms of rat GSK-3,3 and studied effects of their overexpression in Xenopus embryos. Since shaggy is a presumed negative regulator of the Wg pathway in Drosophila, we speculated that a dominant negative mutant of GSK-3P may mimic the effect of ectopic Wnt expression on embryonic axis formation. Consistent with these expectations, microinjection of an enzymatically inactive form of rat GSK-3f3 carrying a point mutation in the ATPbinding site triggered an ectopic body axis formation in early embryos. This effect could be suppressed by the wild-type XGSK-3,3. Our data strongly indicate that the endogenous

Signaling by the Wnt family of extracellular proteins is critical in a variety of developmental processes in which cell and tissue polarity are established [1-5]. Wnt signal transduction has been studied mostly by the genetic approach in Drosophila and Caenorhabditis elegans [1,2,5], but the biochemical mechanisms involved remain to be elucidated. The Wnt pathway also operates during axis determination in vertebrates [3,5]. Frizzled receptors transduce a signal to Dishevelled, leading to inactivation of glycogen synthase kinase 3 (GSK3) and regulation of gene expression by the complex of beta-catenin with LEF/TCF (lymphocyte enhancer factor/T-cell factor) transcription factors [3,5]. Axin is a negative regulator of Wnt signaling and dorsal axial development in vertebrates [6]. Here, we demonstrate that axin is associated with GSK3 in the Xenopus embryo and we localize the GSK3-binding domain to a short region of axin. Binding of GSK3 correlates with the ability of axin to inhibit axial development and with the axis-inducing activity of its dominant-negative form (delta RGS). We also find that wild-type axin, but not delta RGS, forms a complex with beta-catenin. Thus, axin may act as a docking station mediating negative regulation of beta-catenin by GSK3 during dorsoventral axis determination in vertebrate embryos.

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A series of synthetic heterosite-purpurite, (Mn y Fe 1-y )PO 4 (y < 0.8), with negligible disorder and impurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite-lithiophilite series, Li(Mn y Fe 1-y )PO 4 (ordered olivine structure, space group Pnma). Comparative magnetic and X-ray/ neutron powder diffraction investigations of the two solid-solution lines were performed as a function of Mn content to increase understanding of the electrochemical activity loss of Mn 3+ /Mn 2+ in the Li x (Mn y Fe 1-y )PO 4 electrode system. Introducing Mn ions into the 4c site did not cause significant change in the local geometry of M 2+ O 6 and PO 4 polyhedra, while the M 3+ O 6 octahedra became severely distorted with an increase in the number of Jahn-Teller active Mn 3+ ions. The edge-sharing geometry of M 3+ O 6 and PO 4 polyhedra fixed the shared O3′-O3′ interatomic distance, causing selective strong elongation of the M 3+ -O3′ distance with small shrinkage of other M 3+ -O1, M 3+ -O2, and M 3+ -O3 bond lengths. The overall distortion of the MO 6 octahedra with M ) Mn 3+ was much larger than the corresponding change in the unit-cell orthorombicity and significantly increased asymmetry in the M-O-M superexchange interaction. All samples exhibited antiferromagnetism; however, the trivalent series had more than a sevenfold larger decrease in Neel temperature T N (from ca. 130 K at y ) 0 to ca. 50 K at y ) 0.8) compared to the divalent series (from ca. 52 K at y ) 0 to ca. 35 K at y )1) as a function of the Mn content y.

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BackgroundDishevelled (Dsh) is a key component of multiple signaling pathways that are initiated by Wnt secreted ligands and Frizzled receptors during embryonic development. Although Dsh has been detected in a number of cellular compartments, the importance of its subcellular distribution for signaling remains to be determined.ResultsWe report that Dsh protein accumulates in cell nuclei when Xenopus embryonic explants or mammalian cells are incubated with inhibitors of nuclear export or when a specific nuclear-export signal (NES) in Dsh is disrupted by mutagenesis. Dsh protein with a mutated NES, while predominantly nuclear, remains fully active in its ability to stimulate canonical Wnt signaling. Conversely, point mutations in conserved amino-acid residues that are essential for the nuclear localization of Dsh impair the ability of Dsh to activate downstream targets of Wnt signaling. When these conserved residues of Dsh are replaced with an unrelated SV40 nuclear localization signal, full Dsh activity is restored. Consistent with a signaling function for Dsh in the nucleus, treatment of cultured mammalian cells with medium containing Wnt3a results in nuclear accumulation of endogenous Dsh protein.ConclusionsThese findings suggest that nuclear localization of Dsh is required for its function in the canonical Wnt/β-catenin signaling pathway. We discuss the relevance of these findings to existing models of Wnt signal transduction to the nucleus.

Signaling by the Wnt family of secreted proteins plays an important role in animal development and is often misregulated in carcinogenesis. Wnt signal transduction is controlled by the rate of degradation of ␤-catenin by a complex of proteins including glycogen synthase kinase 3 (GSK3), adenomatous polyposis coli, and Axin. Dishevelled is required for Wnt signal transduction, and its activation results in stabilization of ␤-catenin. However, the biochemical events underlying this process remain largely unclear. Here we show that Xenopus Dishevelled (Xdsh) interacts with a Xenopus Axin-related protein (XARP). This interaction depends on the presence of the Dishevelled-Axin (DIX) domains in both XARP and Xdsh. Moreover, the same domains are essential for signal transduction through Xdsh. Finally, our data point to a possible mechanism for signal transduction, in which Xdsh prevents ␤-catenin degradation by displacing GSK3 from its complex with XARP.