Wnt Signaling: Multiple Pathways, Multiple Receptors, and Multiple Transcription Factors (2006)

Signaling pathways are an ever present force in every animal's life. During development, these pathways provide critical cell-cell communication required to coordinate the activities of vast numbers of cells. In adulthood, similar communication mechanisms are utilized to achieve tissue homeostasis and regeneration. Regulation of signaling is crucial; too much or too little activity from a given signal transduction pathway can cause devastating results such as developmental defects or, later in life, disease. The Wnts comprise a large family of highly conserved growth factors that are responsible for important developmental and homeostatic processes throughout the animal kingdom (for a more comprehensive review see Ref. 1Logan C.Y. Nusse R. Annu. Rev. Cell Dev. Biol. 2004; 20: 781-810Crossref PubMed Scopus (4196) Google Scholar). Their implication in a wide array of developmental events and human diseases has made Wnts and their signaling pathways the subject of intense investigation over the last two decades. This has never been truer than in the past few years, when the association of Wnt signaling with stem cell fate has added fuel to an already active field. Membership in the Wnt family is defined by amino acid sequence rather than functional properties. It is therefore not too surprising that Wnts have been associated with a number of different activities and downstream signaling pathways. Although the majority of work in the field to date has focused on β-catenin-dependent, or canonical, Wnt signaling, examples continue to accumulate in which Wnts and/or other key components of the canonical signaling cascade participate in β-catenin-independent processes (reviewed in Ref. 2Veeman M.T. Axelrod J.D. Moon R.T. Dev. Cell. 2003; 5: 367-377Abstract Full Text Full Text PDF PubMed Scopus (1150) Google Scholar). In this review, we will focus largely on the canonical pathway, paying particular attention to recent insights. We will then touch on some developments in β-catenin-independent signaling and discuss some issues that may be important to all Wnts, regardless of the signal they generate. The defining event in canonical Wnt signaling is the cytoplasmic accumulation of β-catenin and its subsequent nuclear translocation and activity (Fig. 1). Under unstimulated conditions, a β-catenin destruction complex formed by proteins that include Axin, adenomatous polyposis coli (APC), 2The abbreviations used are: APC, adenomatous polyposis coli; GSK, glycogen synthase kinase; LRP, low density lipoprotein receptor-related protein; TCF, T cell-specific transcription factor; LEF, lymphoid enhancer-binding factor 1; PCP, planar cell polarity; CE, convergence extension; RTK, receptor tyrosine kinase; CBP, CREB-binding protein; CREB, cAMP-response element-binding protein. and glycogen synthase kinase 3β (GSK-3) keeps cytoplasmic levels of β-catenin low through phosphorylation by GSK-3. Phosphorylated β-catenin becomes ubiquitylated and is targeted for degradation by the proteasome (3Zeng L. Fagotto F. Zhang T. Hsu W. Vasicek T.J. Perry 3rd, W.L. Lee J.J. Tilghman S.M. Gumbiner B.M. Costantini F. Cell. 1997; 90: 181-192Abstract Full Text Full Text PDF PubMed Scopus (785) Google Scholar). Following Wnt binding to a receptor complex composed of members of the Frizzled (Fz) family of seven transmembrane, serpentine receptors and low density lipoprotein receptor-related protein (LRP), the Axin·APC·GSK-3 complex is inhibited, leading to a block in β-catenin phosphorylation by GSK-3 (Fig. 1B). Hypophosphorylated β-catenin accumulates in the cytoplasm and is translocated to the nucleus where it regulates target gene expression through partnerships with the T cell-specific transcription factor/lymphoid enhancer-binding factor 1 (TCF/LEF) family of transcription factors (4Molenaar M. van de Wetering M. Oosterwegel M. Peterson-Maduro J. Godsave S. Korinek V. Roose J. Destree O. Clevers H. Cell. 1996; 86: 391-399Abstract Full Text Full Text PDF PubMed Scopus (1608) Google Scholar). The role of Fz in acting as a receptor for Wnts is long established; Fz carries an extracellular cysteine-rich domain that is sufficient to bind Wnt proteins, and addition of Fz to Wnt nonresponsive cells can render signaling competence (5Bhanot P. Brink M. Samos C.H. Hsieh J.C. Wang Y. Macke J.P. Andrew D. Nathans J. Nusse R. Nature. 1996; 382: 225-230Crossref PubMed Scopus (1219) Google Scholar). However, the discovery that members of the LRP family of single pass transmembrane proteins are also required for signal transduction immediately raised the possibility that Fz and LRP function as co-receptors for Wnt proteins (6Tamai K. Semenov M. Kato Y. Spokony R. Liu C. Katsuyama Y. Hess F. Saint-Jeannet J.P. He X. Nature. 2000; 407: 530-535Crossref PubMed Scopus (1081) Google Scholar, 7Wehrli M. Dougan S.T. Caldwell K. O'Keefe L. Schwartz S. Vaizel-Ohayon D. Schejter E. Tomlinson A. DiNardo S. Nature. 2000; 407: 527-530Crossref PubMed Scopus (717) Google Scholar). Indeed, binding has been seen between Wnt and LRP, and a ternary complex between a mouse Wnt and soluble forms of Fz and LRP extracellular domains has been reported (6Tamai K. Semenov M. Kato Y. Spokony R. Liu C. Katsuyama Y. Hess F. Saint-Jeannet J.P. He X. Nature. 2000; 407: 530-535Crossref PubMed Scopus (1081) Google Scholar). A model in which Wnt physically mediates an interaction between Fz and LRP is therefore appealing. Is such an interaction sufficient to induce signaling in the absence of Wnt binding? Early reports suggest that it is. Fusion of a heterologous ligand/receptor pair (NT3 and TrkN) to Fz and LRP, respectively, is sufficient to induce Wnt-independent signaling when the two molecules are co-overexpressed, as is a fusion of a natural LRP-binding protein, Dickkopf (DKK), to Fz (8Cong F. Schweizer L. Varmus H. Development. 2004; 131: 5103-5115Crossref PubMed Scopus (272) Google Scholar, 9Holmen S.L. Robertson S.A. Zylstra C.R. Williams B.O. Biochem. Biophys. Res. Commun. 2005; 328: 533-539Crossref PubMed Scopus (40) Google Scholar). Lastly, addition of the intracellular portion of LRP to the C terminus of Fz creates a constitutively active receptor (10Tolwinski N.S. Wehrli M. Rives A. Erdeniz N. DiNardo S. Wieschaus E. Dev. Cell. 2003; 4: 407-418Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar). These data argue that inducing the proximity of the Fz and LRP cytoplasmic domains is the key event in Wnt signal initiation, although direct evidence of its occurrence upon Wnt ligand reception has not yet been reported. There has been much interest in elucidating the events that bridge the activation of the Fz/LRP receptors and inhibition of the β-catenin destruction complex. A key intermediate in this process is Dishevelled (Dsh), a cytosolic phosphoprotein required upstream of Axin·APC·GSK-3 inhibition in both fly and mammalian cells. Although Dishevelled's requirement in Wnt signaling has been known for over a decade, the molecular events leading to Dsh activation by Frizzled and the manner in which Dsh transduces the Wnt signal to the inhibitory complex have remained murky. Several papers report on a physical association between Fz and Dsh or Wnt-dependent phosphorylation of Dsh (11Chen W. ten Berge D. Brown J. Ahn S. Hu L.A. Miller W.E. Caron M.G. Barak L.S. Nusse R. Lefkowitz R.J. Science. 2003; 301: 1391-1394Crossref PubMed Scopus (291) Google Scholar, 12Wong H.C. Bourdelas A. Krauss A. Lee H.J. Shao Y. Wu D. Mlodzik M. Shi D.L. Zheng J. Mol. Cell. 2003; 12: 1251-1260Abstract Full Text Full Text PDF PubMed Scopus (391) Google Scholar). The kinases PAR-1 and CKII appear to directly phosphorylate Dsh, although the precise role of each in transducing the Wnt signal is not entirely clear nor is the specific activity bestowed on Dsh by phosphorylation (13Sun T.Q. Lu B. Feng J.J. Reinhard C. Jan Y.N. Fantl W.J. Williams L.T. Nat. Cell Biol. 2001; 3: 628-636Crossref PubMed Scopus (208) Google Scholar). A second component that appears to play a key role in directly linking receptor activation to inhibition of the cytoplasmic β-catenin destruction complex is a member of the complex itself, Axin. Once thought of as simply a scaffold protein linking together other members of the complex, Axin now appears to play a more dynamic role in signal activation by binding directly to the cytoplasmic tail of LRP in response to Wnt reception (14Mao J. Wang J. Liu B. Pan W. Farr 3rd, G.H. Flynn C. Yuan H. Takada S. Kimelman D. Li L. Wu D. Mol. Cell. 2001; 7: 801-809Abstract Full Text Full Text PDF PubMed Scopus (686) Google Scholar). The recruitment of Axin to LRP is mediated by phosphorylation of LRP on key residues by the kinases CK1γ and GSK-3 (15Zeng X. Tamai K. Doble B. Li S. Huang H. Habas R. Okamura H. Woodgett J. He X. Nature. 2005; 438: 873-877Crossref PubMed Scopus (649) Google Scholar, 16Davidson G. Wu W. Shen J. Bilic J. Fenger U. Stannek P. Glinka A. Niehrs C. Nature. 2005; 438: 867-872Crossref PubMed Scopus (462) Google Scholar). Surprisingly, overexpression of a membrane-tethered form of the LRP intracellular domain can induce β-catenin accumulation, even in the absence of Fz or Dsh, suggesting that membrane recruitment of Axin is sufficient to activate signaling (17Brennan K. Gonzalez-Sancho J.M. Castelo-Soccio L.A. Howe L.R. Brown A.M. Oncogene. 2004; 23: 4873-4884Crossref PubMed Scopus (90) Google Scholar). Presumably this occurs through titration of Axin away from the Axin·APC·GSK-3 complex, thereby compromising the ability of the complex to phosphorylate β-catenin. Once in the nucleus, β-catenin partners with members of the LEF/TCF family of transcription factors to activate the transcription of Wnt target genes (18Brunner E. Peter O. Schweizer L. Basler K. Nature. 1997; 385: 829-833Crossref PubMed Scopus (444) Google Scholar, 19van de Wetering M. Cavallo R. Dooijes D. van Beest M. van Es J. Loureiro J. Ypma A. Hursh D. Jones T. Bejsovec A. Peifer M. Mortin M. Clevers H. Cell. 1997; 88: 789-799Abstract Full Text Full Text PDF PubMed Scopus (1058) Google Scholar). TCF provides sequence-specific binding activity and, in the absence of nuclear β-catenin, partners with the transcriptional repressor Groucho and histone deacetylases to form a repressive complex and block transcription of Wnt target genes (20Cavallo R.A. Cox R.T. Moline M.M. Roose J. Polevoy G.A. Clevers H. Peifer M. Bejsovec A. Nature. 1998; 395: 604-608Crossref PubMed Scopus (594) Google Scholar, 21Chen G. Fernandez J. Mische S. Courey A.J. Genes Dev. 1999; 13: 2218-2230Crossref PubMed Scopus (353) Google Scholar). When β-catenin enters the nucleus, it directly replaces Groucho from its binding of TCF and converts the complex to a transcriptional activator, thereby effecting the transcription of Wnt target genes (22Daniels D.L. Weis W.I. Nat. Struct. Mol. Biol. 2005; 12: 364-371Crossref PubMed Scopus (424) Google Scholar). Other members of this activating complex are the histone acetylase CBP/p300 and the SWI/SNF complex member Brg-1, both of which may act through the remodeling of chromatin surrounding TCF binding sites (23Hecht A. Vleminckx K. Stemmler M.P. van Roy F. Kemler R. EMBO J. 2000; 19: 1839-1850Crossref PubMed Google Scholar, 24Takemaru K.I. Moon R.T. J. Cell Biol. 2000; 149: 249-254Crossref PubMed Scopus (402) Google Scholar). There are a number of other factors that bind to the TCF·β-catenin complex and are necessary for transcriptional activation, including Legless (Lgs), Pygopus (Pygo), and most recently discovered hyrax/parafibromin, a member of the polymerase-associated factor 1 (PAF1) complex (25Thompson B. Townsley F. Rosin-Arbesfeld R. Musisi H. Bienz M. Nat. Cell Biol. 2002; 4: 367-373Crossref PubMed Scopus (244) Google Scholar, 26Kramps T. Peter O. Brunner E. Nellen D. Froesch B. Chatterjee S. Murone M. Zullig S. Basler K. Cell. 2002; 109: 47-60Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar, 27Parker D.S. Jemison J. Cadigan K.M. Development. 2002; 129: 2565-2576Crossref PubMed Google Scholar, 28Mosimann C. Hausmann G. Basler K. Cell. 2006; 125: 327-341Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar). Lgs and Pygo are an intriguing pair of proteins that associate with β-catenin in the nucleus. Lgs binds directly to the N terminus of β-catenin and serves as an adaptor to attach Pygo to the complex (25Thompson B. Townsley F. Rosin-Arbesfeld R. Musisi H. Bienz M. Nat. Cell Biol. 2002; 4: 367-373Crossref PubMed Scopus (244) Google Scholar). However, the essential function of Pygo is controversial; initially thought of as a nuclear cofactor that enhances transcriptional activation by the TCF·β-catenin complex (25Thompson B. Townsley F. Rosin-Arbesfeld R. Musisi H. Bienz M. Nat. Cell Biol. 2002; 4: 367-373Crossref PubMed Scopus (244) Google Scholar, 26Kramps T. Peter O. Brunner E. Nellen D. Froesch B. Chatterjee S. Murone M. Zullig S. Basler K. Cell. 2002; 109: 47-60Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar, 27Parker D.S. Jemison J. Cadigan K.M. Development. 2002; 129: 2565-2576Crossref PubMed Google Scholar), a subsequent report suggested that Pygo affects transcription by recruiting otherwise cytoplasmic β-catenin to the nucleus (29Townsley F.M. Cliffe A. Bienz M. Nat. Cell Biol. 2004; 6: 626-633Crossref PubMed Scopus (180) Google Scholar), a claim that has since been refuted in favor of the original model (30Hoffmans R. Stadeli R. Basler K. Curr. Biol. 2005; 15: 1207-1211Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). Additionally, the activity of TCF itself may be modulated by signaling from the mitogen-activated protein kinase cascade composed of TAK1 and NLK/Nemo (31Ishitani T. Ninomiya-Tsuji J. Matsumoto K. Mol. Cell. Biol. 2003; 23: 1379-1389Crossref PubMed Scopus (183) Google Scholar), and TCF/LEF-independent signaling by β-catenin can occur through interactions with other DNA-binding proteins, including Pitx2 and Prop1 (32Kioussi C. Briata P. Baek S.H. Rose D.W. Hamblet N.S. Herman T. Ohgi K.A. Lin C. Gleiberman A. Wang J. Brault V. Ruiz-Lozano P. Nguyen H.D. Kemler R. Glass C.K. Wynshaw-Boris A. Rosenfeld M.G. Cell. 2002; 111: 673-685Abstract Full Text Full Text PDF PubMed Scopus (473) Google Scholar, 33Olson L.E. Tollkuhn J. Scafoglio C. Krones A. Zhang J. Ohgi K.A. Wu W. Taketo M.M. Kemler R. Grosschedl R. Rose D. Li X. Rosenfeld M.G. Cell. 2006; 125: 593-605Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar). The latter case is further complicated by the observation that the Prop1·β-catenin complex can act both as an activator of transcription and also as a repressor, a function mediated by binding to the co-repressor Groucho (33Olson L.E. Tollkuhn J. Scafoglio C. Krones A. Zhang J. Ohgi K.A. Wu W. Taketo M.M. Kemler R. Grosschedl R. Rose D. Li X. Rosenfeld M.G. Cell. 2006; 125: 593-605Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar). In all, the emerging complexity of β-catenin nuclear activity may speak to one of the fundamental questions in the field, which is how different cell types respond to Wnt signaling by transcribing different (although often overlapping) sets of target genes. As its name denotes, the first fz gene described was not discovered for its role in Wnt signaling, but rather its function in organizing the bristles and hairs of the adult Drosophila cuticle (34Vinson C.R. Conover S. Adler P.N. Nature. 1989; 338: 263-264Crossref PubMed Scopus (302) Google Scholar). In flies mutant for fz, the normally uniform direction of hairs and bristles on the wings and thorax are perturbed, a result of disrupting what is now known as the planar cell polarity (PCP) pathway (reviewed in Ref. 35Strutt D. Development. 2003; 130: 4501-4513Crossref PubMed Scopus (212) Google Scholar). In addition to Fz, another Wnt pathway component, Dsh, is also required for this process, and genetic analysis of the fly dsh gene has revealed mutations that separate its function in PCP and canonical Wnt signal transduction (36Penton A. Wodarz A. Nusse R. Genetics. 2002; 161: 747-762Crossref PubMed Google Scholar). Interestingly, mutations in the Dsh DEP (Dishevelled-Egl20-Pleckstrin) domain, which is absolutely required for PCP but dispensable for β-catenin signaling, have also been found to affect the process of convergence extension (CE) in vertebrates (37Heisenberg C.P. Tada M. Rauch G.J. Saude L. Concha M.L. Geisler R. Stemple D.L. Smith J.C. Wilson S.W. Nature. 2000; 405: 76-81Crossref PubMed Scopus (831) Google Scholar, 38Tada M. Smith J.C. Development. 2000; 127: 2227-2238Crossref PubMed Google Scholar). CE is also disrupted by loss of function in vertebrate homologs of the dedicated PCP genes strabismus, flamingo, and prickle, suggesting that CE and PCP are controlled by homologous pathways (39Wallingford J.B. Nat. Cell Biol. 2004; 6: 687-689Crossref PubMed Scopus (11) Google Scholar). All evidence in flies and vertebrates points toward a complete separation of β-catenin signaling and PCP signaling downstream of Dsh. However, there is a long outstanding question of what lies upstream of Fz in PCP, and more specifically, whether it is a Wnt. There are no known Wnt gene mutations in the fly that disrupt PCP but mutations in the zebrafish wnt5 and/or wnt11 genes cause CE phenotypes, suggesting that Wnts do act as ligands in this system (37Heisenberg C.P. Tada M. Rauch G.J. Saude L. Concha M.L. Geisler R. Stemple D.L. Smith J.C. Wilson S.W. Nature. 2000; 405: 76-81Crossref PubMed Scopus (831) Google Scholar, 40Kilian B. Mansukoski H. Barbosa F.C. Ulrich F. Tada M. Heisenberg C.P. Mech. Dev. 2003; 120: 467-476Crossref PubMed Scopus (260) Google Scholar). However, there is no direct evidence that Wnts act as directional cues in CE, rather than simply permissive signals. This is not to say that Wnts cannot act as directional cues; during the development of the vertebrate and invertebrate central nervous system, sources or gradients of Wnt protein can provide directional guidance to extending axons by transducing a signal through Ryk, an atypical member of the receptor tyrosine kinase (RTK) family (41Yoshikawa S. McKinnon R.D. Kokel M. Thomas J.B. Nature. 2003; 422: 583-588Crossref PubMed Scopus (341) Google Scholar, 42Liu Y. Shi J. Lu C.C. Wang Z.B. Lyuksyutova A.I. Song X.J. Zou Y. Nat. Neurosci. 2005; 8: 1151-1159Crossref PubMed Scopus (224) Google Scholar, 43Schmitt A.M. Shi J. Wolf A.M. Lu C.C. King L.A. Zou Y. Nature. 2006; 439: 31-37Crossref PubMed Scopus (233) Google Scholar). Just as PCP is most likely an example of Wnt-independent Fz function, the association of Wnt and RTKs may illustrate a Fz-independent function for Wnt (Fig. 2). Certain atypical members of the RTK family have an extracellular Wnt binding domain, cysteine-rich domains in the case of Ror1 and Ror2 and a WIF (Wnt inhibitory factor) domain in the case of Ryk (44Halford M.M. Stacker S.A. Bioessays. 2001; 23: 34-45Crossref PubMed Google Scholar). The Drosophila homolog of Ryk, Derailed, is an axon guidance receptor that mediates repulsion from Wnt5 during embryonic central nervous system patterning, a process that apparently does not involve Dsh or downstream canonical signaling components (41Yoshikawa S. McKinnon R.D. Kokel M. Thomas J.B. Nature. 2003; 422: 583-588Crossref PubMed Scopus (341) Google Scholar). Similarly, a number of Wnts, including Wnt5a (a mammalian ortholog of Drosophila Wnt5), have been shown to act through Ryk as axon repellents during mammalian central nervous system development (42Liu Y. Shi J. Lu C.C. Wang Z.B. Lyuksyutova A.I. Song X.J. Zou Y. Nat. Neurosci. 2005; 8: 1151-1159Crossref PubMed Scopus (224) Google Scholar). Whether the fly and mouse processes are homologous is not known, and an answer will require more detailed knowledge of the other protein components mediating these signals. Moreover, Wnt5a can block canonical signaling in a process that depends on another RTK, Ror2 (45Mikels A.J. Nusse R. PLoS Biol. 2006; 4: e115Crossref PubMed Scopus (1002) Google Scholar). Clearly, there is much work still to be done in defining the signal transduction pathways downstream of Wnt-binding RTKs and sorting out the specificities of each ligand/receptor(s) interaction in modulating axon guidance or β-catenin stability. By reading even a cursory review such as this one, it is easy to appreciate the enormous complexity of Wnt signaling. The large number and diversity of components utilized in transduction of Wnt signals is and the seen in the a particular cell to a given Wnt. Wnt signaling such diversity and of the answer may from the recent discovery that each Wnt gene has a large of to of the have Wnt genes through can be B. N. G. A. M. Curr. Biol. 2002; 12: Full Text Full Text PDF PubMed Scopus Google Scholar, R. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). The surprising observation is that these are not the result of recent of these are present in the a that from the last of in A. K. C. M. J. U. A. B. Nature. 2005; PubMed Scopus Google Scholar). This that the of the Wnt was an development in the of and likely A. K. C. M. J. U. A. B. Nature. 2005; PubMed Scopus Google Scholar). was the for of the Wnt family and its over of The Wnt long another from amino acid sequence there are of The recent discovery that Wnt proteins can be to for some to this and has already raised one K. Brown J.D. E. T. 3rd, Nusse R. Nature. 2003; PubMed Scopus Google Scholar). known to be soluble in it was discovered that Wnts are by the of a more than analysis of the sequence K. Brown J.D. E. T. 3rd, Nusse R. Nature. 2003; PubMed Scopus Google Scholar). This observation a to the protein which was known to be required for of Wnt from cells T. E. J. K. N. Genes Dev. 1996; PubMed Scopus Google Scholar). It is thought that which an by may be the responsible for addition of the to Wnt and that this is required for Similarly, a second gene required for the of Wnts has recently been discovered C. D. C. P. Hausmann G. Basler K. Cell. 2006; 125: Full Text Full Text PDF PubMed Scopus Google Scholar, K. N. D. M. Cell. 2006; 125: Full Text Full Text PDF PubMed Scopus Google Scholar). The molecular of how Wnts are and then and/or targeted for by are but provide more that may be to in the Wnt signaling pathway to Wnts are in on over of which in the past such intense interest are a number of the of the Wnt family and the Wnt signaling pathways have for and evidence that of Wnt signaling is a factor in a number of human diseases to The most example is that of APC, of which adenomatous a that to Ref. J. 2006; PubMed Scopus Google Scholar). However, mutations in Wnts or Wnt signaling components have been associated with diseases that a wide of from to Ref. R.T. A. Nat. Rev. 2004; 5: PubMed Scopus Google Scholar). Lastly, Wnts appear to or stem cell K. Brown J.D. E. T. 3rd, Nusse R. Nature. 2003; PubMed Scopus Google Scholar). This observation has made Wnts more than for and Wnt proteins as to cells in and provide a key in stem tissue as of the Wnt pathway to there are a number of important questions that In of signal the of how signaling is upon Wnt binding to Fz and LRP to be as does the by which the β-catenin destruction complex is is the question of how is in the nuclear activity of β-catenin and its of target does cell and the of other signals the of genes upon Wnt signal this molecules that target the proteins in specific Additionally, as interest in Wnt ligands in more it will be important to more the of these proteins and the of each family it will be to see whether we can the large of we have on canonical Wnt signaling with the number of examples of β-catenin-independent signaling. there that are to the activities of all a more complete of this complex family of signaling proteins will to the