Canonical and noncanonical Wnt signaling: Multilayered mediators, signaling mechanisms and major signaling crosstalk

Abstract

Wnt signaling plays a major role in regulating cell proliferation and differentiation. The Wnt ligands are a family of 19 secreted glycoproteins that mediate their signaling effects via binding to Frizzled receptors and LRP5/6 coreceptors and transducing the signal either through β-catenin in the canonical pathway or through a series of other proteins in the noncanonical pathway. Many of the individual components of both canonical and noncanonical Wnt signaling have additional functions throughout the body, establishing the complex interplay between Wnt signaling and other signaling pathways. This crosstalk between Wnt signaling and other pathways gives Wnt signaling a vital role in many cellular and organ processes. Dysregulation of this system has been implicated in many diseases affecting a wide array of organ systems, including cancer and embryological defects, and can even cause embryonic lethality. The complexity of this system and its interacting proteins have made Wnt signaling a target for many therapeutic treatments. However, both stimulatory and inhibitory treatments come with potential risks that need to be addressed. This review synthesized much of the current knowledge on the Wnt signaling pathway, beginning with the history of Wnt signaling. It thoroughly described the different variants of Wnt signaling, including canonical, noncanonical Wnt/PCP, and the noncanonical Wnt/Ca²⁺ pathway. Further description involved each of its components and their involvement in other cellular processes. Finally, this review explained the various other pathways and processes that crosstalk with Wnt signaling.

Keywords: Canonical Wnt; Noncanonical Wnt; Signal transduction; Signaling crosstalk; β-catenin.

Figure 1

The canonical Wnt signaling pathway. The left panel (A) demonstrates the activated Wnt signaling cascade, while the right side portrays the inhibited Wnt signaling cascade. Wnt binds to the Fz receptor and LRP5/6 co-receptor. This activates Dvl to cause the dissociation of Axin from the destruction complex, causing β-catenin to be stabilized and enter the nucleus. β-Catenin can then displace the inhibitory TLE/Groucho complexes, enabling TCF/LEF to transcribe the target genes. PP2A can also enhance Wnt signaling by dephosphorylating β-catenin, APC, and Axin. The result is the preservation of β-catenin by preventing ubiquitination and proteasomal breakdown. In the absence of Wnt signaling (B), the destruction complex breaks down β-catenin and inhibits gene transcription. Several other proteins also contribute to the inhibition of Wnt signaling. Dkk1 associates with Krm1 or Krm2 and LRP5/6, causing endocytosis of the LRP5/6 co-receptor. Wise/sclerostin binds to LRP5/6 to inhibit proper Wnt association with the coreceptor. xCer-L and WIF-1 both bind to Wnt ligands to inhibit signaling. IGFBP-4 functions as a competitive inhibitor of Wnt signaling by associating with LRP6 and Fz8, while sFRPs complex with Fz receptors to prevent Wnt ligand binding. The illustration was inspired by and created in BioRender.

Figure 2

The noncanonical Wnt/PCP pathway. The binding of Wnt ligands leads to the phosphorylation of Dvl, which recruits Invs, Par6, and Smurf. Smurf ubiquitinates the inhibitory protein Prickle, targeting it for destruction. Dvl can then associate with DAAM, activating Rac1, profilin, and RhoA. Rac1 activates JNK, which phosphorylates c-Jun and CapZIP. c-Jun then goes to the nucleus to stimulate gene transcription. RhoA activates ROCK and DIA1, with the latter activating MRLC. CapZIP, MRLC, DIA1, and profilin all stimulate actin polymerization. Celsr1 stimulates Dvl due to Wnt binding like the Fz receptor. Wnt binding to the Vangl2 receptor causes dissociation of Prickle and Intu from Dvl, which can then bind to Invs. The illustration was inspired by and created in BioRender.

Figure 3

The noncanonical Wnt/Ca²⁺ pathway. Wnt binding to the Fz receptor leads to G protein-mediated activation of PLC. PLC cleaves PIP2 into IP3 and DAG. IP3 binds to IP3 receptors (InsP3R) on the ER membrane to stimulate Ca²⁺ release. STIM1/2 detects this decrease in ER Ca²⁺ levels and activates Orai proteins in the plasma membrane to bring more Ca²⁺ into the cell, where SERCA can pump Ca²⁺ back into the ER. DAG can then activate PKC in the presence of Ca²⁺ and PKC can stimulate Cdc42 to enhance actin polymerization. The elevated intracellular Ca²⁺ level also stimulates calcineurin and CAMKII. Calcineurin activates NFAT, causing gene transcription. CAMKII activates TAK1, which activates NLK, when then phosphorylates TCF, preventing β-catenin-mediated gene transcription. The illustration was inspired by and created in BioRender.

Figure 4

β-Catenin protein interactions. Some of these proteins were not discussed in this paper due to space constraints. Among those that have been discussed in this paper, there are notable inhibitors and activators. GSK-3β, CK1, APC, Axin, and β-TrCP are inhibitors of β-catenin as part of the destruction complex. PP2A has dual effects; it can dephosphorylate β-catenin to prevent ubiquitination and stabilize β-catenin, while it can also dephosphorylate GSK-3β, which can then inhibit β-catenin. YAP/TAZ is another inhibitor, as it can either bind to and suppress β-catenin without affecting its levels or associate with the destruction complex. Smad7 and Smurf2 can complex with β-catenin to ubiquitinate and degrade the protein. SUFU can export β-catenin from the nucleus. On the other hand, there are several activators of β-catenin, proteins that are activated by β-catenin, and proteins that assist with the functions of β-catenin. Smad3 is a chaperone protein that transports β-catenin into the nucleus. TCF/LEF are transcription factors that are activated by β-catenin. α- and γ-catenin join with β-catenin to link CAMs like E-cadherin to cytoskeletal structures, strengthening cell adhesion. It is noteworthy that some of the protein interactions are species-, tissue-, and/or context-dependent. The illustration was inspired by the Wnt homepage created and maintained by the Nusse Lab at Stanford University (http://web.stanford.edu/group/nusselab/cgi-bin/wnt/protein_interactions) and reference .

Figure 5

Crosstalk between Wnt signaling and other major signaling pathways. The (A) left, (B) middle, and (C) right panels show the Wnt/BMP crosstalk, Wnt/Notch crosstalk, and the Wnt/Hippo crosstalk, respectively. Note that not all crosstalk interactions between each of these pathways are represented in the image due to space constraints. In Wnt/BMP crosstalk, the absence of Wnt signaling allows GSK-3β to phosphorylate Smad1, which is ubiquitinated and degraded. The activation of Wnt signaling inhibits GSK-3β, stabilizing Smad1 and enhancing BMP signaling. BMP2 can up-regulate Wnt3a and enhance β-catenin binding with Smad4. Wnt3a can also up-regulate BMP2 expression. Smad7 can either recruit Smurf2 to ubiquitinate β-catenin for degradation or cause Axin to dissociate from β-catenin, stabilizing it. For Wnt/Notch crosstalk, Notch can complex with β-catenin and promote its degradation. TCF/LEF can regulate the expression of the DLL1 Notch receptor ligand. Notch2 and Wnt4 are involved in a positive feedback loop. β-Catenin can enhance Hes1 transcription and inhibit the degradation of Notch1. β-Catenin can also up-regulate transcription of the JAG1 Notch ligand. Notch1 can complex with β-catenin and Lamp1 to promote lysosomal degradation of β-catenin. Notch can increase both Fz receptor expression and transcription of TCF1. Finally, in Wnt/Hippo crosstalk, there are many interactions as well. LATS1/2 can phosphorylate the HXRXXS motifs of YAP to signal to CK1 to phosphorylate the DSGXXS destruction motifs, leading to the recruitment of β-TrCP-mediated degradation of YAP. MST1/2 can sequester CK1, preventing phosphorylation of Dvl and inhibiting Wnt signaling. YAP/TAZ can bind to and suppress β-catenin while preserving its stability, although other reports indicate that YAP/TAZ is associated with Axin as part of the destruction complex. YAP/TAZ can also inhibit Dvl, reducing Wnt signaling. However, YAP can also interact with SHP2 to enhance β-catenin activity in the nucleus. Wnt signaling can cause YAP/TAZ to transcribe Hippo pathway target genes. The illustration was inspired by and created in BioRender.