A Novel Mechanism for Wnt Activation of Canonical Signaling through the LRP6 Receptor

Abstract

LDL receptor-related protein 6 (LRP6) is a Wnt coreceptor in the canonical signaling pathway, which plays essential roles in embryonic development. We demonstrate here that wild-type LRP6 forms an inactive dimer through interactions mediated by epidermal growth factor repeat regions within the extracellular domain. A truncated LRP6 comprising its transmembrane and cytoplasmic domains is expressed as a constitutively active monomer whose signaling ability is inhibited by forced dimerization. Conversely, Wnts are shown to activate canonical signaling through LRP6 by inducing an intracellular conformational switch which relieves allosteric inhibition imposed on the intracellular domains. Thus, Wnt canonical signaling through LRP6 establishes a novel mechanism for receptor activation which is opposite to the general paradigm of ligand-induced receptor oligomerization.The Wnt family of secreted signaling molecules is essential in embryonic induction, cell polarity generation, and cell fate specification (46). Deregulation of Wnt signaling results in defects in development and growth control. The canonical Wnt pathway involves activation of ␤-catenin-dependent transcription and is evolutionarily conserved from Caenorhabditis elegans to humans. Mutations in components, which constitutively activate canonical signaling, have been identified in several tumor types, including colorectal cancer (34). Wnts bind to two coreceptors, the Frizzled-type seven-transmembrane-domain receptor (5,17,19,49) and the low-density receptor-related protein (LRP) 5/6 (35, 41) or the Drosophila melanogaster ortholog Arrow (44). These interactions cause ␤-catenin stabilization through inhibition of its phosphorylation by glycogen synthase kinase 3␤ (GSK3␤), which is assembled in a large cytoplasmic complex that includes Dishevelled, casein kinase I, Axin, APC, and Frat (36). As a consequence, stabilized cytoplasmic ␤-catenin is translocated to the nucleus and forms a complex with a family of high-mobility group-like transcription factors, including leukocyte enhancer factor-1 (LEF-1) and T-cell factors (TCF), activating transcription of target genes (4).Frizzled family members have been shown to possess various affinities for different Wnt ligands. For example, Drosophila frizzled, Dfz1 and Dfz2, bind to Wingless (Wg), the Drosophila orthologue of Wnt, to activate the canonical pathway. However, Dfz2 has a 10-fold higher affinity for Wg than Dfz1 and plays a predominant role in transducing the Wg canonical signal in vivo (37). Frizzled family members also signal through the planar polarity pathway, which similarly involves Dsh (25) but is mediated through JNK and RhoA rather than ␤-catenin stabilization (7). In Drosophila, Dfz1 but not Dfz2 appears to regulate planar polarity signaling (8, 37). LRP5, LRP6, and arrow receptors specifically function in the canonical pathway. Inactivation of arrow in Drosophila results in a phenotype similar to that of the wingless mutant (41), and mice deficient in LRP6 exhibit developmental defects resembling ...

LRP6 expression promotes cancer cell proliferation and tumorigenesis by altering β-catenin subcellular distribution

“…Thus, the mechanisms by which LRP5/6 modulates the Wnt signal are complex. It was proposed that, under normal conditions when Wnt ligand and receptors are at the physiological levels, both frizzled and LRP5/6 receptors are required to achieve efficient intracellular coupling to Wnt/b-catenin signaling (Liu et al, 2003). However, under certain pathological conditions such as in malignancy, high levels of LRP5/6 alone might be able to induce Wnt/b-catenin signaling by modulating subcellular b-catenin distribution.…”

“…Recent studies also showed that LRP5 or LRP6 truncated from the amino-terminal ends (i.e. lacking the extracellular domain) are capable of mediating Wnt/b-catenin signaling independent of either a Wnt ligand or frizzled receptor (Mao et al, 2001;Liu et al, 2003;Brennan et al, 2004). Furthermore, it was demonstrated that a PPPSP motif, which is reiterated five times in the LRP6 intracellular domain, is necessary and sufficient to trigger Wnt/b-catenin signaling (Brennan et al, 2004;Tamai et al, 2004).…”

Crystal structure of 12-oxophytodienoate reductase 3 from tomato: Self-inhibition by dimerization

“…Many cases of intramolecular self-inhibition (autoinhibition) are known (29,30), but examples of intermolecular inhibition by homodimerization have been reported as well (31)(32)(33)(34)(35)(36). Proteolytic cleavage, phosphorylation, ligand binding, and even lightinduced conformational changes represent different possibilities to initiate or abrogate self-inhibition.…”

Wnts as ligands: processing, secretion and reception

“…LRP5 and -6 proteins possess a relatively small intracellular domain and a large extracellular domain containing several potential protein interaction domains (He et al, 2004). Truncated proteins that lack the extracellular portion of the protein, but still contain the transmembrane and intracellular domains, produce a constitutively active canonical Wnt signal (Liu et al, 2003). By contrast, LRP proteins lacking only the intracellular domain serve as dominant negative proteins (Tamai et al, 2000).…”