MACF1

Abstract: The Wnts comprise a large class of secreted proteins that control essential developmental processes such as embryonic patterning, cell growth, migration, and differentiation. In the most well-understood “canonical” Wnt signaling pathway, Wnt binding to Frizzled receptors induces β-catenin protein stabilization and entry into the nucleus, where it complexes with T-cell factor/lymphoid enhancer factor transcription factors to affect the transcription of target genes. In addition to the canonical pathway, evidence for several other Wnt signaling pathways has accumulated, in particular for Wnt5a, which has therefore been classified as a noncanonical Wnt family member. To study the alternative mechanisms by which Wnt proteins signal, we purified the Wnt5a protein to homogeneity. We find that purified Wnt5a inhibits Wnt3a protein–induced canonical Wnt signaling in a dose-dependent manner, not by influencing β-catenin levels but by downregulating β-catenin–induced reporter gene expression. The Wnt5a signal is mediated by the orphan tyrosine kinase Ror2, is pertussis toxin insensitive, and does not influence cellular calcium levels. We show that in addition to its inhibitory function, Wnt5a can also activate β-catenin signaling in the presence of the appropriate Frizzled receptor, Frizzled 4. Thus, this study shows for the first time that a single Wnt ligand can initiate discrete signaling pathways through the activation of two distinct receptors. Based on these and additional observations, we propose a model wherein receptor context dictates Wnt signaling output. In this model, signaling by different Wnt family members is not intrinsically regulated by the Wnt proteins themselves but by receptor availability.

Abstract: Control of stability of β-catenin is central in the wnt signaling pathway. Here, the protein conductin was found to form a complex with both β-catenin and the tumor suppressor gene product adenomatous polyposis coli (APC). Conductin induced β-catenin degradation, whereas mutants of conductin that were deficient in complex formation stabilized β-catenin. Fragments of APC that contained a conductin-binding domain also blocked β-catenin degradation. Thus, conductin is a component of the multiprotein complex that directs β-catenin to degradation and is located downstream of APC. In Xenopus embryos, conductin interfered with wnt-induced axis formation.

Abstract: Signalling by the Wnt family of secreted lipoproteins has essential functions in development and disease1. The canonical Wnt/β-catenin pathway requires a single-span transmembrane receptor, low-density lipoprotein (LDL)-receptor-related protein 6 (LRP6)2,3,4, whose phosphorylation at multiple PPPSP motifs is induced upon stimulation by Wnt and is critical for signal transduction5. 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. Glycogen synthase kinase 3 (GSK3), which is known for its inhibitory role in Wnt signalling through the promotion of β-catenin phosphorylation and degradation, mediates the phosphorylation and activation of LRP6. We show that Wnt induces sequential phosphorylation of LRP6 by GSK3 and casein kinase 1, and this dual phosphorylation promotes the engagement of LRP6 with the scaffolding protein Axin. We show further that a membrane-associated form of GSK3, in contrast with cytosolic GSK3, stimulates Wnt signalling and Xenopus axis duplication. Our results identify two key kinases mediating Wnt co-receptor activation, reveal an unexpected and intricate logic of Wnt/β-catenin signalling, and illustrate GSK3 as a genuine switch that dictates both on and off states of a pivotal regulatory pathway.

Abstract: The Wnt signaling pathway controls cell proliferation and body patterning throughout development. A surprising number of cytoplasmic Wnt regulators (e.g., beta-catenin, Bcl-9/Lgs, APC, Axin) also appear, often transiently, in the nucleus. beta-Catenin is an integral component of E-cadherin complexes at intercellular adherens junctions, but also recruits chromatin remodeling complexes to activate transcription in the nucleus. The APC tumor suppressor is a part of the cytoplasmic beta-catenin destruction complex, yet also counteracts beta-catenin transactivation and histone H3K4 methylation at Wnt target genes. Furthermore, APC coordinates the cyclic exchange of Wnt coregulator complexes at the DNA. These opposing roles of APC and beta-catenin enable a rapid coordination of gene expression and cytoskeletal organization throughout the cell in response to signaling.