Wnt signals across the plasma membrane to activate the β-catenin pathway by forming oligomers containing its receptors, Frizzled and LRP

Abstract

Wnt-induced signaling via beta-catenin plays crucial roles in animal development and tumorigenesis. Both a seven-transmembrane protein in the Frizzled family and a single transmembrane protein in the LRP family (LDL-receptor-related protein 5/6 or Arrow) are essential for efficiently transducing a signal from Wnt, an extracellular ligand, to an intracellular pathway that stabilizes beta-catenin by interfering with its rate of destruction. However, the molecular mechanism by which these two types of membrane receptors synergize to transmit the Wnt signal is not known. We have used mutant and chimeric forms of Frizzled, LRP and Wnt proteins, small inhibitory RNAs, and assays for beta-catenin-mediated signaling and protein localization in Drosophila S2 cells and mammalian 293 cells to study transmission of a Wnt signal across the plasma membrane. Our findings are consistent with a mechanism by which Wnt protein binds to the extracellular domains of both LRP and Frizzled receptors, forming membrane-associated hetero-oligomers that interact with both Disheveled (via the intracellular portions of Frizzled) and Axin (via the intracellular domain of LRP). This model takes into account several observations reported here: the identification of intracellular residues of Frizzled required for beta-catenin signaling and for recruitment of Dvl to the plasma membrane; evidence that Wnt3A binds to the ectodomains of LRP and Frizzled; and demonstrations that a requirement for Wnt ligand can be abrogated by chimeric receptors that allow formation of Frizzled-LRP hetero-oligomers. In addition, the beta-catenin signaling mediated by ectopic expression of LRP is not dependent on Disheveled or Wnt, but can also be augmented by oligomerization of LRP receptors.

β-Catenin destruction complex: insights and questions from a structural perspective

“…Moreover, at least in certain overexpression assays, LRP5/6 can activate the canonical Wnt pathway independently of Frizzled and Dishevelled (Mao et al, 2001;Cong et al, 2004a, b). While normal Wnt signaling requires Frizzled, LRP6 and Dishevelled (Mao et al, 2001;Schweizer and Varmus, 2003;Cong et al, 2004b), the overexpression assays show that LRP5/6 can function without Frizzled or Dishevelled. So what is phosphorylated LRP5/6 doing?…”

“…One suggestion is that the binding of Dishevelled to Frizzled recruits Dishevelled to the membrane where it activates the Wnt co-receptor Lipoprotein receptor-related protein 5/6 (LRP5/6) using the Dishevelled-binding protein CK1e (Dale, 2006). Alternatively, or in addition, it has been suggested that the Wnt-dependent translocation of Dishevelled to the membrane is involved in recruiting Axin to the membrane since Dishevelled binds Axin through its DIX domain (Kishida et al, 1999;Li et al, 1999) and thus Dishevelled could tether Axin to Frizzled, which together with LRP5/6, inactivate the destruction b-Catenin destruction complex D Kimelman and W Xu complex (Cliffe et al, 2003;Schweizer and Varmus, 2003;Cong et al, 2004b). However, Dishevelled lacking its PDZ domain can still activate the Wnt pathway in at least some overexpression experiments (note that not all experiments produce this conclusion, but this may be due to the fact that some authors delete only a minimal PDZ domain whereas other authors delete adjacent sequences that might be essential Axelrod et al, 1998;Rothbacher et al, 2000;Cong et al, 2004a).…”

“…One attractive membrane localized component is Dishevelled bound to Frizzled. While this may well be an important part of the normal Wnt signaling pathway, the fact that LRP5/6 can function in a Frizzled and Dishevelled independent manner shows that this is not the only mechanism by which LRP5/6 functions to inhibit the complex Schweizer and Varmus, 2003;Cong et al, 2004b). Understanding how Wnt signaling affects the destruction complex will require understanding how Dishevelled and LRP5/6 are able to individually affect the function of the complex, and then how these molecules synergize to achieve maximal inhibition.…”

“…Much of the complication in understanding the canonical Wnt pathway may be because overexpression of any one component can obviate the need for the other components to be active. For example, as discussed above, under certain conditions LRP5/6 can function in a Frizzled and Dishevelled-independent manner whereas normally all of these components function synergistically Schweizer and Varmus, 2003;Cong et al, 2004b). Thus, while the overexpression studies are useful, understanding how the different components interact when expressed at lower levels is likely to be critical for unraveling the Wnt pathway.…”

LRP8 mediates Wnt/β-catenin signaling and controls osteoblast differentiation

“…(2) Low-density lipoprotein (LDL) receptor-related proteins 5 and 6 (LRP5/6), members of the low-density lipoprotein receptor (LDLR) family, are single-pass co-receptors that interact directly, albeit poorly, with Wnts, and form a Wnt-induced ternary complex with Fz that mediates b-catenin signaling. (2,(5)(6)(7)(8) LRP5/6 have an overall homology of 71%. (9) They are essential for Wnt/bcatenin signaling and are part of the LDL receptor family with a number of distinct conserved motifs, including complement-like cysteine-rich repeat motif (the LDL repeat), epidermal growth factor (EGF) receptor-like repeats, tyrosine-tryptophan-threonine-aspartate (YWTD) repeats that form a b-propeller structure critical for proper ligand binding, and proline-proline-prolineserine-proline (PPPSP) motifs that are phosphorylated and important for downstream pathway activation.…”