Signaling by the Wnt family of secreted glycolipoproteins via the transcription co-activator β-catenin controls embryonic development and adult homeostasis. Here we review recent progresses in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists and antagonists and their interactions with Wnt receptors. We also dissect critical events that regulate β-catenin stability from Wnt receptors to the cytoplasmic β-catenin destruction complex, and nuclear machinery that mediates β-catenin-dependent transcription. Finally we highlight some key aspects of Wnt/β-catenin signaling in human diseases including congenital malformations, cancer and osteoporosis and potential therapeutic implications.

run in TBE £0.5 at room temperature for 2 h at 150 V. The following two (Q/q) 27-bp unmethylated oligonucleotides were used: 5 0 -GATCCTTCGCCTAGGCTC(A/G)CAGCG CGGGAGCGA-3 0 . A methylated q probe (q*) was generated by incorporating a methylated cytosine at the mutated CpG site during oligonucleotide synthesis. Transient transfection assayThe constructs contained 578 bp from IGF2 intron 3 (nucleotides 2868-3446), followed by the IGF2 P3 promoter (nucleotides 2222 to þ45 relative to the start of transcription) 12 and a luciferase reporter. C2C12 myoblast cells were grown to approximately 80% confluence. Cells were transiently co-transfected with the firefly luciferase reporter construct (4 mg) and a Renilla luciferase control vector (phRG-TK, Promega; 80 ng) using 10 mg Lipofectamine 2000 (Invitrogen). Cells were incubated for 25 h before lysis in 100 ml Triton lysis solution. Luciferase activities were measured using the Dual-Luciferase Reporter Assay System (Promega). The results are based on four triplicate experiments using two independent plasmid preparations for each construct. Statistical analysis was done with an analysis of variance.

Wnt regulation of beta-catenin degradation is essential for development and carcinogenesis. beta-catenin degradation is initiated upon amino-terminal serine/threonine phosphorylation, which is believed to be performed by glycogen synthase kinase-3 (GSK-3) in complex with tumor suppressor proteins Axin and adnomatous polyposis coli (APC). Here we describe another Axin-associated kinase, whose phosphorylation of beta-catenin precedes and is required for subsequent GSK-3 phosphorylation of beta-catenin. This "priming" kinase is casein kinase Ialpha (CKIalpha). Depletion of CKIalpha inhibits beta-catenin phosphorylation and degradation and causes abnormal embryogenesis associated with excessive Wnt/beta-catenin signaling. Our study uncovers distinct roles and steps of beta-catenin phosphorylation, identifies CKIalpha as a component in Wnt/beta-catenin signaling, and has implications to pathogenesis/therapeutics of human cancers and diabetes.

Mutation in the TSC2 tumor suppressor causes tuberous sclerosis complex, a disease characterized by hamartoma formation in multiple tissues. TSC2 inhibits cell growth by acting as a GTPase-activating protein toward Rheb, thereby inhibiting mTOR, a central controller of cell growth. Here, we show that Wnt activates mTOR via inhibiting GSK3 without involving beta-catenin-dependent transcription. GSK3 inhibits the mTOR pathway by phosphorylating TSC2 in a manner dependent on AMPK-priming phosphorylation. Inhibition of mTOR by rapamycin blocks Wnt-induced cell growth and tumor development, suggesting a potential therapeutic value of rapamycin for cancers with activated Wnt signaling. Our results show that, in addition to transcriptional activation, Wnt stimulates translation and cell growth by activating the TSC-mTOR pathway. Furthermore, the sequential phosphorylation of TSC2 by AMPK and GSK3 reveals a molecular mechanism of signal integration in cell growth regulation.

The Wnt family of secreted signalling molecules are essential in embryo development and tumour formation. The Frizzled (Fz) family of serpentine receptors function as Wnt receptors, but how Fz proteins transduce signalling is not understood. In Drosophila, arrow phenocopies the wingless (DWnt-1) phenotype, and encodes a transmembrane protein that is homologous to two members of the mammalian low-density lipoprotein receptor (LDLR)-related protein (LRP) family, LRP5 and LRP6 (refs 12-15). Here we report that LRP6 functions as a co-receptor for Wnt signal transduction. In Xenopus embryos, LRP6 activated Wnt-Fz signalling, and induced Wnt responsive genes, dorsal axis duplication and neural crest formation. An LRP6 mutant lacking the carboxyl intracellular domain blocked signalling by Wnt or Wnt-Fz, but not by Dishevelled or beta-catenin, and inhibited neural crest development. The extracellular domain of LRP6 bound Wnt-1 and associated with Fz in a Wnt-dependent manner. Our results indicate that LRP6 may be a component of the Wnt receptor complex.

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Signalling by the Wnt family of secreted lipoproteins plays essential roles in development and disease 1 . The canonical Wnt/β-catenin pathway requires a single-span transmembrane receptor, LDL receptor related protein 6 (LRP6) 2-4 , whose phosphorylation at multiple PPPSP motifs is induced upon Wnt stimulation and critical for signal transduction 5 . The kinase responsible for LRP6 phosphorylation has not been identified. Here we provide biochemical and genetic evidence for a 'dual-kinase' mechanism for LRP6 phosphorylation and activation. Surprisingly, glycogen synthase kinase 3 (GSK3), which is known for its inhibitory role in Wnt signalling via promoting β-catenin phosphorylation and degradation, mediates LRP6 phosphorylation and activation. We demonstrate that Wnt induces sequential phosphorylation of LRP6 by GSK3 and casein kinase 1 (CK1), and this dual-phosphorylation promotes the engagement of LRP6 with the scaffolding protein Axin. We further show that a membrane-associated form of GSK3, contrary to cytosolic GSK3, stimulates Wnt signalling and Xenopus axis duplication. Our results identify two key kinases mediating Wnt coreceptor activation, reveal an unexpected and intricate logic of Wnt/β-catenin signalling, and illustrate GSK3 as a bona fide switch dictating both on and off states of a pivotal regulatory pathway.Canonical Wnt signalling operates through regulating phosphorylation and degradation of the transcription co-activator β-catenin 1,6 . Without Wnt stimulation, β-catenin is assembled into the Axin complex, in which β-catenin is sequentially phosphorylated by CK1 and GSK3 and earmarked for degradation 7-9 . Wnt stimulation leads to inhibition of β-catenin phosphorylation/degradation. This signal transduction is initiated at the plasma membrane by two distinct receptors, a Frizzled serpentine receptor and LRP6 or LRP5, which together may form a Wnt-induced Frizzled-LRP6 (or LRP5) complex 3,6,10-12 . While the mechanism by which this receptor pair initiates signalling remains to be understood, Wnt-induced LRP6 phosphorylation at a PPPSP motif, which is reiterated five times in LRP5/6 cytoplasmic domain (Fig. 1a), plays a critical role 5 . (For simplicity we use PPPSP to represent PPPSP or PPPTP). Indeed, LRP6 mutants lacking these motifs or harbouring substitutions at the S/T residues are inactive in signalling 5,13 . Conversely, a single PPPSP motif upon transfer to a heterologous receptor is sufficient to initiate β-catenin signalling 5 . As the phosphorylated PPPSP motif mediates LRP5/6-Axin interaction 5,14 , we proposed a model 5 in which Wnt-induced LRP6 Figs. 1 and 2). In vivo GSK3 overexpression promoted LRP6 PPPSP phosphorylation ( Fig. 1b) whereas LiCl and SB216763 prevented Wnt-induced LRP6 PPPSP phosphorylation (Fig. 1c). Importantly, Wnt-induced LRP6 phosphorylation was abolished in mouse embryo fibroblasts (MEFs) that harbour genetic deletions of Gsk3α and Gsk3β genes ( Fig. 1d). These data show that GSK3 is involved in LRP6 PPPSP phosphorylation.GSK3 phosphorylation of m...

Wnt signaling via the Frizzled (Fz) receptor controls cell polarity and movement during development, but the molecular nature of Wnt/Fz polarity signal transduction remains poorly defined. Here we report that in human cells and during Xenopus embryogenesis, Wnt/Fz signaling activates the small GTPase Rho, a key regulator of cytoskeleton architecture. Wnt/Fz activation of Rho requires the cytoplasmic protein Dishevelled (Dvl) and a novel Formin homology protein Daam1. Daam1 binds to both Dvl and Rho, and mediates Wnt-induced Dvl-Rho complex formation. Inhibition or depletion of Daam1 prevents Wnt/Fz activation of Rho and of Xenopus gastrulation, but not of beta-catenin signaling. Our study illustrates a molecular pathway from Wnt/Fz signaling to Rho activation in cell polarity signal transduction.