Frizzled and LRP5/6 receptors for Wnt/β-catenin signaling

Abstract

Frizzled and LRP5/6 are Wnt receptors that upon activation lead to stabilization of cytoplasmic β-catenin. In this study, we review the current knowledge of these two families of receptors, including their structures and interactions with Wnt proteins, and signaling mechanisms from receptor activation to the engagement of intracellular partners Dishevelled and Axin, and finally to the inhibition of β-catenin phosphorylation and ensuing β-catenin stabilization.

Figure 1.

Frizzled (FZD) and Dishevelled (DVL). (A) Phylogeny comparing human FZD proteins. (B) Topology of a generic FZD on the plasma membrane. The shape of CRD is shaded in green, with the 10 invariable cysteine (C) residues forming five disulfide bonds highlighted. (ψ) Potential N-glycosylation sites. Additional conserved cysteine and other residues in the linker and ECL1-3 in the extracellular space are indicated. Conserved residues in ICL1-3 and the carboxy-terminal domain in the intracellular space are also indicated, with invariable residues among all FZD proteins in bold. (* above the letter) Missense mutations found in Drosophila Dfz1 (Povelones et al. 2005) and human FZD4 (Robitaille et al. 2002); (underlined) residues tested via double alanine substitution scanning mutagenesis in FZD5 (Cong et al. 2004b). DVL protein is also shown schematically with DIX, PDZ, and DEP domains and their interaction partners highlighted. Two discontinuous regions of FZD ICL3 (motif I [pink]; motif II [green]) bind to the carboxy-terminal region of DVL. The FZD carboxyl tail containing the KTxxxW motif (blue) interacts strongly with the DVL PDZ domain and also the DEP domain, which also shows some interaction with motif II in ICL3. (C) Crystal structure of FZD8CRD. Residues in red (and to a lesser degree those in orange) are suggested to be a Wnt-binding interface from an alanine scanning mutagenesis (Hsieh et al. 1999; Dann et al. 2001), which partially overlaps with site 2 indentified in the Wnt8–FZD8CRD co-crystal structure. Residues in green when altered did not affect Wnt activity and areas in gray were not tested. Site 1 and site 2, which mediate contacts between Wnt8 and FZD8CRD in the crystal structure, are labeled. A shade of Wnt8 index finger contacting site 2 is sketched. (Panel C is derived from Fzd8 1IJY [PDB doi: 10.2210/pdb1ijy/pdb].) (D) Wnt8–FZD8CRD co-crystal structure, shown as a ribbon diagram superimposed on surface representation (Janda et al., 2012). Palmitoleic acid adduct from the Wnt8 thumb at site 1 (red); N-glycans of Wnt8 and FZD8CRD (yellow). Note that the FZD8CRD in C and D is viewed in different angles. N term, amino terminal; C term, carboxyl terminal.

Figure 2.

LRP5/6 and Axin. (A) Arrow, LRP5, and LRP6 are shown schematically together with LDLR. LRP6 binding to different Wnt proteins and antagonists SOST and DKK1 are shown. (B) Phylogeny of the four β-propellers (P1–4) in LRP5 and LRP6. (C,D) Side and top views of the atomic structure of LRP6 P3E3–P4E4 (Chen et al. 2011). (E) Wnt3a and DKK1 share an overlapping binding surface on the top of LRP6 P3. (F,G) Top and side views of a model of the entire LRP6 extracellular domain derived from electron microscopic staining (Chen et al. 2011). Orientation and relationship to the plasma membrane are speculative. Axin is shown schematically. (Dashed line) Indicates the unknown nature of the domain involved in LRP6-binding. Axin domains that interact with DVL, APC, β-catenin, GSK3, and CK1α are indicated. (DAX) The DIX domain of Axin. (C–G, From Chen et al. 2011; reprinted, with permission, from the author.)

Figure 3.

LRP6 phosphorylation and phosphorylation sites. (A) Wnt induces LRP6 phosphorylation in the FZD–LRP6 complex. Dominant-negative LRP6 mutants are generated by deleting the cytoplasmic domain or mutating all five PPPSPxS motifs (S to A). Constitutively activated LRP6ΔN and LDLRΔN-PPPSP are constitutively phosphorylated. (B) Alignment of the cytoplasmic domain of LRP6, LRP5, Arrow, and the Nematostella LRP5/6 homolog. Identified phosphorylation sites by various kinases in LRP6 are indicated.

Figure 4.

Models for LRP6 phosphorylation and signaling. (A) Initiation-amplification model. (B) Signalosome model. Only one Axin molecule is drawn for clarity. (C) Endosomal signaling model.

Figure 5.

Models for Wnt-induced inhibition of β-catenin phosphorylation. (A) Axin complex disassembly. (B) Axin protein degradation. (C) GSK3 inhibition by phosphorylated PPPSPxS motifs. (D) GSK3 inclusion by multivesicular bodies.