The status of Wnt signalling regulates neural and epidermal fates in the chick embryo

The acquisition of neural fate by embryonic ectodermal cells is a fundamental step in the formation of the vertebrate nervous system. Neural induction seems to involve signalling by fibroblast growth factors (FGFs) and attenuation of the activity of bone morphogenetic protein (BMP). But FGFs, either alone or in combination with BMP antagonists, are not sufficient to induce neural fate in prospective epidermal ectoderm of amniote embryos. These findings suggest that additional signals are involved in the specification of neural fate. Here we show that the state of Wnt signalling is a critical determinant of neural and epidermal fates in the chick embryo. Continual Wnt signalling blocks the response of epiblast cells to FGF signals, permitting the expression and signalling of BMP to direct an epidermal fate. Conversely, a lack of exposure of epiblast cells to Wnt signals permits FGFs to induce a neural fate.

Wnt signaling regulates B lymphocyte proliferation through a LEF-1 dependent mechanism

Lymphocyte enhancer factor-1 (LEF-1) is a member of the LEF-1/TCF family of transcription factors, which have been implicated in Wnt signaling and tumorigenesis. LEF-1 was originally identified in pre-B and T cells, but its function in B lymphocyte development remains unknown. Here we report that LEF-1-deficient mice exhibit defects in pro-B cell proliferation and survival in vitro and in vivo. We further show that Lef1-/- pro-B cells display elevated levels of fas and c-myc transcription, providing a potential mechanism for their increased sensitivity to apoptosis. Finally, we establish a link between Wnt signaling and normal B cell development by demonstrating that Wnt proteins are mitogenic for pro-B cells and that this effect is mediated by LEF-1.

A Drosophila Axin homolog, Daxin, inhibits Wnt signaling

The vertebrate Axin protein, the product of the mouse fused gene, binds to beta-catenin to inhibit Wnt signaling. We have identified a homolog of Axin in Drosophila, Daxin. Using double-stranded RNA interference, we generated loss-of-function phenotypes that are similar to overexpression of the Drosophila Wnt gene wingless (wg). Overexpression of Daxin produces phenotypes similar to loss of wg. In addition, we show that Daxin overexpression can modify phenotypes elicited by wg and another Drosophila Wnt gene, DWnt-2. Using immunoprecipitation of endogenous Daxin protein from embryos we show that Daxin interacts with Armadillo and Zeste-white 3. The loss-of-function and overexpression phenotypes show that Daxin, like its mammalian counterpart, acts as a negative regulator of wg/Wnt signaling.

Wnt-induced dephosphorylation of axin releases beta-catenin from the axin complex

The stabilization of beta-catenin is a key regulatory step during cell fate changes and transformations to tumor cells. Several interacting proteins, including Axin, APC, and the protein kinase GSK-3beta are implicated in regulating beta-catenin phosphorylation and its subsequent degradation. Wnt signaling stabilizes beta-catenin, but it was not clear whether and how Wnt signaling regulates the beta-catenin complex. Here we show that Axin is dephosphorylated in response to Wnt signaling. The dephosphorylated Axin binds beta-catenin less efficiently than the phosphorylated form. Thus, Wnt signaling lowers Axin's affinity for beta-catenin, thereby disengaging beta-catenin from the degradation machinery.

WNT signaling in the control of hair growth and structure

Characterization of the molecular pathways controlling differentiation and proliferation in mammalian hair follicles is central to our understanding of the regulation of normal hair growth, the basis of hereditary hair loss diseases, and the origin of follicle-based tumors. We demonstrate that the proto-oncogene Wnt3, which encodes a secreted paracrine signaling molecule, is expressed in developing and mature hair follicles and that its overexpression in transgenic mouse skin causes a short-hair phenotype due to altered differentiation of hair shaft precursor cells, and cyclical balding resulting from hair shaft structural defects and associated with an abnormal profile of protein expression in the hair shaft. A putative effector molecule for WNT3 signaling, the cytoplasmic protein Dishevelled 2 (DVL2), is normally present at high levels in a subset of cells in the outer root sheath and in precursor cells of the hair shaft cortex and cuticle which lie immediately adjacent to Wnt3-expressing cells. Overexpression of Dvl2 in the outer root sheath mimics the short-hair phenotype produced by overexpression of Wnt3, supporting the hypothesis that Wnt3 and Dvl2 have the potential to act in the same pathway in the regulation of hair growth. These experiments demonstrate a previously unrecognized role for WNT signaling in the control of hair growth and structure, as well as presenting the first example of a mammalian phenotype resulting from overexpression of a Dvl gene and providing an accessible in vivo system for analysis of mammalian WNT signaling pathways.

Two members of the Tcf family implicated in Wnt/beta-catenin signaling during embryogenesis in the mouse

Tcf transcription factors interact with beta-catenin and Armadillo to mediate Wnt/Wingless signaling. We now report the characterization of genes encoding two murine members of the Tcf family, mTcf-3 and mTcf-4. mTcf-3 mRNA is ubiquitously present in embryonic day 6.5 (E6.5) mouse embryos but gradually disappears over the next 3 to 4 days. mTcf-4 expression occurs first at E10.5 and is restricted to di- and mesencephalon and the intestinal epithelium during embryogenesis. The mTcf-3 and mTcf-4 proteins bind a canonical Tcf DNA motif and can complex with the transcriptional coactivator beta-catenin. Overexpression of Wnt-1 in a mammary epithelial cell line leads to the formation of a nuclear complex between beta-catenin and Tcf proteins and to Tcf reporter gene transcription. These data demonstrate a direct link between Wnt stimulation and beta-catenin/Tcf transcriptional activation and imply a role for mTcf-3 and -4 in early Wnt-driven developmental decisions in the mouse embryo.

Beta-catenin: a key mediator of Wnt signaling

Beta-catenin is a pivotal player in the signaling pathway initiated by Wnt proteins, mediators of several developmental processes. beta-catenin activity is controlled by a large number of binding partners that affect the stability and the localization of beta-catenin and is thereby able to participate in such varying processes as gene expression and cell adhesion. Activating mutations in beta-catenin and in components regulating its stability can contribute to the formation of certain tumors.

Casein kinase 2 associates with and phosphorylates dishevelled

The dishevelled (dsh) gene of Drosophila melanogaster encodes a phosphoprotein whose phosphorylation state is elevated by Wingless stimulation, suggesting that the phosphorylation of Dsh and the kinase(s) responsible for this phosphorylation are integral parts of the Wg signaling pathway. We found that immunoprecipitated Dsh protein from embryos and from cells in tissue culture is associated with a kinase activity that phosphorylates Dsh in vitro. Purification and peptide sequencing of a 38 kDa protein co-purifying with this kinase activity showed it to be identical to Drosophila Casein Kinase 2 (CK2). Tryptic phosphopeptide mapping indicates that identical peptides are phosphorylated by CK2 in vitro and in vivo, suggesting that CK2 is at least one of the kinases that phosphorylates Dsh. Overexpression of Dfz2, a Wingless receptor, also stimulated phosphorylation of Dsh, Dsh-associated kinase activity, and association of CK2 with Dsh, thus suggesting a role for CK2 in the transduction of the Wg signal.

The Wnt-1 proto-oncogene induces changes in morphology, gene expression, and growth factor responsiveness in PC12 cells

The product of the Wnt-1 proto-oncogene is a secreted glycoprotein that is normally produced in regions of the embryonic neural tube. We show here that expression of mouse Wnt-1 cDNA in the rat PC12 pheochromocytoma cell line causes a dramatic conversion from a round to a flat cell morphology. In addition, PC12 cells expressing Wnt-1 (PC12/Wnt-1) fail to extend neurites after treatment with NGF, despite the presence and activation of high affinity NGF receptors encoded by the trk gene and the induction of early response genes. Furthermore, PC12/Wnt-1 cells fail to express several neuron- and chromaffin-specific genes, indicating that PC12/Wnt-1 cells have assumed a new phenotype. Although NGF and FGF utilize similar signal transduction pathways in PC12 cells, only FGF is capable of inducing a morphological response and synthesis of transin mRNA in PC12/Wnt-1 cells.

Efficient incorporation of human CD4 protein into avian leukosis virus particles

Virus envelope (Env) proteins are thought to contain specific signals for selective uptake by virus particles. In the course of attempting to define these signals by testing virus incorporation of CD4-Env chimeric proteins, normal human CD4 was found to be efficiently and selectively assembled into avian leukosis virus particles in quail cells. Viruses bearing CD4 at their surface may be useful reagents in the design of retrovirus-mediated gene therapy for the acquired immune deficiency syndrome.