Journal of Cell Science 113, 911-919 (2000) 911 Printed in Great Britain © The Company of Biologists Limited 2000 JCS0743 COMMENTARY Cross-regulation of the Wnt signalling pathway: a role of MAP kinases Jürgen Behrens Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Str. 10, 13122 Berlin, Germany *Author for correspondence (e-mail: [email protected]) Published on WWW 21 February 2000 SUMMARY The Wnt signal transduction pathway regulates various Wnt-controlled cell fate decision in the early aspects of embryonal development and is involved in cancer Caenorhabditis elegans embryo. Moreover, TAK1 activates formation. Wnts induce the stabilisation of cytosolic β- NLK, which phosphorylates TCFs bound to β-catenin. This catenin, which then associates with TCF transcription blocks nuclear localization and DNA binding of TCFs. factors to regulate expression of Wnt-target genes. At Since TAK1 is activated by TGF-β and various cytokines, various levels the Wnt pathway is subject to cross- it might provide an entry point for regulation of the Wnt regulation by other components. Recent evidence suggests system by other pathways. In addition, alterations in that a specific MAP kinase pathway involving the MAP TAK1-NLK might play a role in cancer. kinase kinase kinase TAK1 and the MAP kinase NLK counteract Wnt signalling. In particular, homologues of TAK1 and NLK, MOM-4 and LIT-1, negatively regulate Key words: Wnt pathway, MAP kinase, TAK1, NLK INTRODUCTION THE Wnt SIGNAL TRANSDUCTION PATHWAY A variety of biochemical pathways that relay information Wnts are a family of secreted glycoproteins that act on from the extracellular milieu to the cytosol and cause neighbouring cells in a paracrine fashion and thereby mediate alterations in gene expression have been identified in recent cellular interactions during development (Cadigan and Nusse, years. In most cases, secreted factors bind to and activate 1997). The Wnt pathway is strikingly conserved in different specific receptors on the cell surface, which couple to adaptor species and has been studied in C. elegans, Drosophila proteins in the cytoplasm; these interact with additional melanogaster, Zebrafish, Xenopus laevis, chicken, mouse and downstream components in a sequential fashion. Most of the human cancer cells (Bienz, 1998; Cox and Peifer, 1998; Han, pathways end up in the cell nucleus and activate or repress 1997; Miller and Moon, 1996; Moon and Miller, 1997; the function of specific transcription factors that regulate Perrimon, 1994). Wnts are expressed in a tissue-specific target genes responsible for the ultimate biological response. manner, and mice that have mutations in genes whose products Although the individual pathways can be studied are involved in Wnt signalling show defects in the development independently of each other in separate experimental of a variety of organs (for a review see Cadigan and Nusse, systems, it is becoming increasingly clear that they are 1997). In Xenopus embryos, ectopic expression of activating frequently interconnected in the cell. This creates a highly components of the pathway induces dorsalisation of the complex network of signalling pathways. Although an embryo, which results in the formation of a secondary body increasing number of points at which cross-talk occurs are axis. Conversely, inhibitory components of the pathway being identified, their biological significance is only promote ventralisation and prevent axis formation. In beginning to be understood. Drosophila, the homologous Wingless pathway is involved in Here I concentrate on the Wnt signalling pathway and the establishment of segment polarity, wing formation and discuss cross-regulatory effectors that have been identified differentiation of the endoderm (Cadigan and Nusse, 1997). recently. A major focus is on the connection between Wnt The hierarchy of the Wnt pathway has been mainly established signalling and a MAP kinase pathway that involves the TAK1 through genetic epistasis experiments in Drosophila as well as and NLK serine/threonine kinases. I first introduce the two through overexpression studies in Xenopus. The Wnt system in pathways separately and then go on to describe the recent Caenorhabditis elegans is discussed in more detail below. evidence that indicates a connection between the two systems. In the overview given here, I use the mammalian names of Finally, I discuss the implications of the results as well as the Wnt pathway components (Fig. 1). In brief, Wnts act in a future research directions. paracrine fashion by binding to frizzled receptors, which are a 912 J. Behrens ubiquitination by phosphorylation of specific serine and AB threonine residues in its N-terminal domain. Phosphorylated β- Porcupine MOM-1 catenin is recognized by the F-box protein slimb/βTrCP, which attracts ubiquitinating enzymes that transfer polyubiquitin Wnt MOM-2 chains to β-catenin (Hart et al., 1999; Jiang and Struhl, 1998; Frizzled MOM-5 Kitagawa et al., 1999). Mutations of the phosphorylation sites result in stabilisation of β-catenin (Morin et al., 1997; Yost et al., 1996). Dishevelled The phosphorylation of β-catenin appears to take place in a multi-protein complex assembled by the scaffolding CK I KIN-19 component axin or the related protein conductin (Behrens et ?? GSK3β SGG-1 al., 1998; Ikeda et al., 1998; Kishida et al., 1998; Zeng et al., APCAxin/Conductin APR-1 1997). These proteins contain separate binding sites for the TAB1 TAP-1 tumor-suppressor protein APC (adenomatous polyposis coli), β-Catenin TAK1 WRM-1 MOM-4 the serine/threonine kinase GSK3β (glycogen synthase kinase 3β) and β-catenin (Fig. 1). Several lines of evidence point to a TCF NLK POP-1 LIT-1 role for these factors in the degradation of β-catenin. Mutations of APC, which occur in about 80% of sporadic colon Vertebrates/Drosophila C. elegans carcinomas and in the inherited disease familial adenomatosis β Fig. 1. Overview of the Wnt pathway and cross-talk to the polyposis, are correlated with increased levels of -catenin TAK1/NLK pathway in vertebrates and Drosophila (A) and C. (Munemitsu et al., 1995; Polakis, 1997). The mutations lead to elegans (B). Functional relationships between the components, rather deletions of the axin/conductin-binding SAMP repeats and of than biochemical interactions, are indicated. Homologous proteins 20 amino acid repeats that bind to β-catenin. Introduction of are shown in the same colour. Note that GSK3β and APC are wild-type APC into colon carcinoma cells induces degradation negative regulatory components in vertebrates and Drosophila and of β-catenin (Munemitsu et al., 1995). positive regulators in C. elegans. TCFs are activated by β-catenin in GSK3β can phosphorylate β-catenin, and phosphorylation is vertebrates and Drosophila but inactivated in C. elegans. CKI (casein stimulated by axin. This indicates that axin/conductin act by kinase I)/KIN-19 has been positioned between dishevelled and β β β bringing GSK3 into close proximity to -catenin (Ikeda et al., GSK3 in both vertebrates and C. elegans. Axin and conductin are 1998). GSK3β can also phosphorylate APC as well as related proteins showing similar domain structure and biochemical interactions. Both proteins bind to β-catenin, APC and GSK3β by conductin/axin (Rubinfeld et al., 1996; Willert et al., 1999), which increases the affinity of these components for β-catenin. using separate domains and are probably functionally redundant. An β β axin/conductin homologue has not been found in the genome A dominant-negative mutant of GSK3 stabilises -catenin sequence of C. elegans (Ruvkun and Hobert, 1998). In the C. elegans and induces double-axis formation in Xenopus (Yost et al., pathway, LIT-1 is activated by WRM-1, which suggests that both 1996). Both axin and conductin can induce β-catenin proteins cooperate, possibly in a complex with POP-1, to degradation in colon cancer cell lines and block Wnt-induced downregulate repressive function of POP-1. NLK/LIT-1 is activated stabilisation of β-catenin and axis formation in Xenopus by TAK1/MOM-4 and might thus be regulated by upstream stimuli embryos (Behrens et al., 1998; Hart et al., 1998; Ikeda et al., that act on these components (indicated by ‘?’). In Drosophila, it is 1998; Zeng et al., 1997). Mutation of the axin gene in mice unknown whether the TAK1-NLK system plays a role in Wnt leads to duplication of the body axis (Zeng et al., 1997). signalling. Drosophila homologues of axin and APC that have roles in Wnt signalling have also been described (Hamada et al., 1999; family of seven-transmembrane-span proteins (Bhanot et al., Yu et al., 1999). In summary, conductin/axin, APC and GSK3β 1996). Activation of these receptors leads to the stabilisation are negative regulators of the Wnt pathway, which act by of the cytosolic component β-catenin, which then enters the inducing the degradation of β-catenin. nucleus and interacts with HMG-box transcription factors of Little is known about signal transmission from frizzled the LEF-1/TCF family (below collectively referred to as TCF). receptors to the axin/conductin complexes (Fig. 1). It is clear The TCF-β-catenin complexes transmit the Wnt signal into the that the cytoplasmic phosphoprotein dishevelled becomes nucleus and act as transcriptional regulators of Wnt-target activated by frizzled receptors; this might involve genes (Behrens et al., 1996; Huber et al., 1996; Molenaar et phosphorylation of dishevelled, but the mechanism of al., 1996). β-Catenin not only is involved in Wnt signalling but activation is unknown. Activated dishevelled inhibits GSK3β, was originally identified in mammalian cells as a component possibly by interacting with the axin/conductin complex (Li et associated with cadherin cell adhesion molecules. It serves as al., 1999a). β-Catenin becomes hypophosphorylated and is no a link between the cytoplasmic domain of cadherins and the longer recognized by slimb/βTrCP and the ubiquitination cytoskeleton-associated protein α-catenin (Behrens, 1999; machinery, and consequently accumulates in the cells.