Evidence supporting the postulated role of a Wnt ligand in the establishment of planar cell polarity has been elusive, but recent studies show that the movement of epithelial cells during vertebrate gastrulation or Drosophila dorsal closure depends on both a Wnt ligand and the planar cell polarity pathway

McEwen, Donald

Peifer, Mark

English

Carolina Digital Repository

R562 DispatchWnt signaling: Moving in a new directionDonald G. McEwen* and Mark Peifer*†‡Evidence supporting the postulated role of a Wnt ligandin the establishment of planar cell polarity has beenelusive, but recent studies show that the movement ofepithelial cells during vertebrate gastrulation orDrosophila dorsal closure depends on both a Wntligand and the planar cell polarity pathway.Addresses: *Lineberger Comprehensive Cancer Center, †Department ofBiology, ‡Curriculum in Genetics and Molecular Biology, The University ofNorth Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280, USA.E-mail: mcewen@email.unc.eduCurrent Biology 2000, 10:R562–R5640960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.The differentiation of multicellular animals from single-cell zygotes depends upon both a series of morphogeneticmovements that generate the body plan and the elaborationof cell-fate decisions. Both processes are regulated byintercellular signals, many of which have been conservedacross phyla. Classic studies have demonstrated thatWingless signaling functions in Drosophila to establishvarious cell fates throughout development; however, morerecent studies in both invertebrates and vertebrates havedemonstrated that known components of the canonicalWingless/Wnt signaling pathway can function indepen-dently of Armadillo/β-catenin to establish planar cellpolarity, and thus influence morphogenesis.Planar cell polarity is the process whereby cells within anepithelium orient themselves relative to the body axes(reviewed in [1]). In Drosophila, the classic example ofplanar cell polarity involves the proximal–distal orientationof the actin-based wing hairs, which project in acoordinated fashion from the distal-most vertex of each cell(Figure 1a). To date, about a dozen proteins thatparticipate in the establishment of planar cell polarity inDrosophila have been identified through genetic screens.Of these, Frizzled and Dishevelled are known to functionas part of the Wingless/Wnt signaling pathway (Figure 1b).More downstream components of the Wingless/Wntsignaling pathway, such as Zeste-white 3 (the Drosophilahomolog of the vertebrate enzyme glycogen synthasekinase 3β), Armadillo, and dTCF, do not appear to beinvolved in the establishment of planar cell polarity [2,3].Instead, the Jun N-terminal kinase (JNK) signaling cascadeseems to function downstream of Frizzled and Dishevelledto specify planar cell polarity (reviewed in [4]).The role of Wingless/Wnt signaling in regulating cellasymmetries is not limited to Drosophila. Many of theupstream components of the Wnt signaling pathway in thenematode Caenorhabditis elegans, such as the frizzledhomolog mom-5, are required to orient the mitotic spindlesof early embryonic cells, whereas wrm-1 and pop-1, whichencode homologs of the downstream componentsArmadillo and TCF, respectively, are not [5]. Presumably,such regulation occurs at the level of the cytoskeleton, asgene transcription is not required to orient the spindle [5].The requirement for Frizzled in the generation of planarcell polarity hints at a role for a Wingless/Wnt ligand, butdirect evidence for this has been either elusive or ambigu-ous in both Drosophila and C. elegans.Dishevelled proteins from various phyla have threeconserved domains which play distinct roles in canonicalWingless/Wnt signaling and planar cell polarity(Figure 1b). Two of the domains, the DIX and PDZdomains, are required for Wingless/Wnt signaling,whereas the DEP domain is dispensable (reviewed in[1]). In contrast, characterization of the dsh1 mutation,which causes a planar-cell-polarity-specific phenotype,identified a single amino-acid substitution in the DEPdomain [2,3]. This alteration compromises Dishevelled’sability to activate the JNK signaling cascade [2] as well ascoordinate planar cell polarity [2,3]. Therefore, the DEPdomain is not required for Wingless/Wnt signaling, yet itis absolutely required to establish planar cell polarity.Together, these data suggest that Dishevelled functionsas a binary switch, deciding whether to activate thecanonical Wingless/Wnt signaling pathway or the JNKsignaling pathway.Given the roles of Frizzled and Dishevelled in establish-ing cell polarity in flies and worms, as well as the highlyconserved nature of this signal transduction pathwaybetween phyla, many of us have been eagerly awaiting ademonstration that the Wingless/Wnt signaling pathwayregulates planar cell polarity in vertebrates. Using twomodel systems and a variety of experimental approaches,three recent papers have provided just such evidence[6–8]. Together, these papers demonstrate that Wntsignaling, acting through Dishevelled, affects planar cellpolarity during gastrulation.In a genetic screen for mutations that influence forebrainpatterning in zebrafish, Heisenberg et al. [9] identified thesilberblick mutation. During normal gastrulation, lateralcells migrate towards the midline, intercalating betweenone another and thus lengthening the embryo along theanterior–posterior axis (Figure 1c). In the silberblickmutant, such convergent extension fails to occur [6].When genetic mapping revealed that silberblick mappednear wnt11, it became the obvious candidate locus of themutation as earlier work from the Moon lab had demon-strated that ectopic expression of Wnts could affect con-vergent extension movements during Xenopus gastrulation(for example, see [10,11]). Characterization of two differ-ent silberblick alleles identified two independent nonsensemutations in wnt11, while determination of silberblickexpression in paraxial head mesoderm and neurectoderm,two tissues that participate in the gastrulation movementsin question, substantiated the connection [6].Heisenberg et al. [6] next probed the signal transductionpathway that coordinates convergent extension. Theycould not test the canonical Wnt signaling pathway, asperturbations of β-catenin or TCF profoundly affect thedorsal–ventral axis prior to gastrulation. Previous worksuggested that Dishevelled regulates gastrulation [12], soHeisenberg et al. [6] took advantage of work on DrosophilaDishevelled [3] to create a mutant form of the Dishev-elled protein predicted to block canonical Wnt signalingbut not planar cell polarity. When overexpressed, thisconstruct rescued silberblick-associated convergent exten-sion defects. In contrast, misexpression in wild-typeembryos of a dominant-negative form of Dishevelledthought to selectively block its planar-cell-polarityfunction mimicked the silberblick phenotype, consistentwith a requirement for planar cell polarity signaling inconvergent extension. In a complementary study, Wallingford et al. [7] examinedthe mechanism whereby Dishevelled influences gastrula-tion in Xenopus. Cells that are undergoing convergentextension polarize their actin cytoskeletons, such thatlamellipodia predominantly extend along the medial–lateralaxis and then become stabilized [13]. Wallingford et al. [7]found that Xenopus embryos expressing a dominant-nega-tive form of Dishevelled still extend lamellipodia, yet thesestructures fail to be stabilized and their medial–lateral biasis lost. In contrast, over-expression of wild-type Dishev-elled had no effect on lamellipodial stabilization but didabolish the medial-lateral bias. By using a series of Dishevelled mutants, as well asblocking the canonical Wnt pathway with a dominant-negative form of glycogen synthase kinase 3β, Wallingfordet al. [7] demonstrated a specific requirement for theplanar-cell-polarity pathway in convergent extension.Finally, these authors overexpressed Dishevelled linkedto the green fluorescent protein (GFP) to examine thelocalization of Dishevelled in cells undergoing convergentextension. Building on previous work [3], they demon-strate that Dishevelled is diffusely cytoplasmic in cellsthat are not undergoing convergent extension, but as con-vergent extension proceeds, Dishevelled is recruited tothe plasma membrane.Tada and Smith [8] also examined convergent extension inXenopus, but they approached the subject by a differentDispatch R563Figure 1Proximal DistalDIX PDZ DEP(a) (b) (c)Medial LateralWg/Wnt AnteriorPosterior  Current BiologyFz/dFz2DshRho, RacZw3 APCAxinArmGenetranscriptionPlanarcell polarityCytoskeleton JNK cascadeTCF(a) The establishment of planar polarity throughout the wing ofDrosophila relies upon the upstream components of the Winglesssignaling pathway. Both Frizzled and Dishevelled are required to orientthe actin-based hairs protruding from the distal-most vertex of eachwing cell. In their absence, the proximal–distal orientation of the winghairs is disrupted. (b) Two distinct pathways function downstream ofFrizzled and Dishevelled. Stabilization of Armadillo in the cytoplasmdepends on the canonical Wingless signaling pathway, which includesZeste white-3, dAPC/2 and dAXIN, and results in transcriptionalactivation by dTCF and Armadillo. The regulation of planar cell polaritydepends on a distinct set of intracellular effectors, including theJNK signaling cascade. Such effectors are thought to affect planarcell polarity by regulating the cytoskeleton directly as well as genetranscription independent of Armadillo/β-catenin. (c) Convergentextension is the process whereby mesodermal cells intercalate andthereby lengthen the embryo along the anterior–posterior axis.Recent work by Heisenberg et al. [6], Wallingford et al. [7] and Tadaand Smith [8] has shown that the regulation of convergent extensiondepends on the planar-cell polarity pathway downstream of theDEP domain of Dishevelled (see text for details).R564 Current Biology Vol 10 No 15route. They identified wnt11 as a target gene of themesodermal transcription factor Brachyury, a knownregulator of convergent extension that functions down-stream of Activin, a cell–cell signalling protein of the trans-forming growth factor β (TGF-β) family. They found that adominant-negative form of XWnt11 blocks convergentextension both in embryos and in Activin-treated explants.Using a series of dominant-negative and partially func-tional Dishevelled constructs, they also demonstrated thatDishevelled functions to promote convergent extension. These new papers represent a major advance in our under-standing of the Wingless/Wnt signaling nexus. Theyexpand the scope of events in which the planar-cell-polaritypathway participates by demonstrating that the convergentextension movements that accompany gastrulation dependon the DEP domain of Dishevelled. Furthermore, theyshow that the establishment of planar cell polarity in verte-brates depends on Wnt signaling. These data, in conjunc-tion with previous data in Drosophila and culturedmammalian cells, are consistent with a model wherebyDishevelled acts as a ‘decision maker’ mediating the activa-tion of either the canonical Armadillo/TCF pathway or theplanar-cell-polarity pathway, which in Drosophila has beenshown to involve the JNK signaling cascade.The JNK and Wingless signaling pathways were previouslythought to act in distinct processes, but the reality may bemore complex. McEwen et al. [14] recently identified anunexpected connection between JNK and Wingless signal-ing in Drosophila. JNK signaling is well known for its role indorsal closure in Drosophila (reviewed in [4]), a dramaticmorphogenetic event in which epithelial sheets undergocoordinated migration toward the dorsal midline. In doingso, the JNK pathway is thought to influence the cytoskele-ton, perhaps via Rac and Rho, as well as modulating geneexpression. One target gene of the JNK pathway is decapen-taplegic, which encodes a TGF-β family member thought toact as a secondary signal to more lateral cells. McEwen et al.[14] demonstrated that Wingless also plays a role in dorsalclosure. They found that wingless mutants fail to expressdecapentaplegic and lack the coordinated cell shape changesthat normally accompany dorsal closure. Surprisingly, muta-tions that affect dishevelled, armadillo or dTCF also blockdecapentaplegic expression. At the very least, these data suggest that the canonicalWingless and JNK signaling pathways operate in parallelto influence the morphogenetic movements that directdorsal closure. One speculative possibility is that Winglesssignaling, via Dishevelled, can trigger both pathwayssimultaneously. Together with the observations outlinedabove, these data raise the possibility that Wingless/Wntsignaling influences a broader set of cellular events bymodulating cell shape changes and cell migration in avariety of contexts. Consistent with this, Wnt pathwaymutants also affect cell migration in C. elegans [15]. Thesenew results move an already complex field in a new direc-tion, and promise further revelations to come.References1. Mlodzik M: Planar polarity in the Drosophila eye: a multifacetedview of signaling specificity and cross-talk. EMBO J 1999,18:6873-6879.2. Boutros M, Paricio N, Strutt DI, Mlodzik M: Dishevelled activatesJNK and discriminates between JNK pathways in planar polarityand Wingless signaling. Cell 1998, 94:109-118.3. Axelrod JD, Miller JR, Shulman JM, Moon RT, Perrimon N: Differentialrecruitment of Dishevelled provides signaling specificity in theplanar cell polarity and Wingless signaling pathways. Genes Dev1998, 12:2610-2622.4. Noselli S, Agnes F: Roles of the JNK signaling pathway inDrosophila morphogenesis. Curr Opin Genet Dev 1999,9:466-472.5. Schlesinger A, Shelton CA, Maloof JN, Meneghini M,  Bowerman B:Wnt pathway components orient a mitotic spindle in the earlyCaenorhabditis elegans embryo without requiring genetranscription in the responding cell. Genes Dev 1999,13:2028-2038.6. Heisenberg CP, Tada M, Rauch GJ, Saude L, Concha ML, Geisler R,Stemple DL, Smith JC, Wilson SW: Silberblick/Wnt11 mediatesconvergent extension movements during zebrafish gastrulation.Nature 2000, 405:76-81.7. Wallingford JB, Rowning BA, Vogeli KM, Rothbacher U, Fraser SE,Harland RM: Dishevelled controls cell polarity during Xenopusgastrulation. Nature 2000, 405:81-85.8. Tada M, Smith JC: Xwnt11 is a target of Xenopus Brachyury:regulation of gastrulation movements via Dishevelled, but notthrough the canonical Wnt pathway. Development 2000,127:2227-2238.9. Heisenberg CP, Brand M, Jiang YJ, Warga RM, Beuchle D, van EedenFJ, Furutani-Seiki M, Granato M, Haffter P, Hammerschmidt M, et al.:Genes involved in forebrain development in the zebrafish, Daniorerio. Development 1996, 123: 191-203.10. Moon RT, Campbell RM, Christian JL, McGrew LL, Shih J, Fraser S:Xwnt-5A: a maternal Wnt that affects morphogenetic movementsafter overexpression in embryos of Xenopus laevis. Development1993, 119:97-111.11. Du SJ, Purcell SM, Christian JL, McGrew LL, Moon RT: Identificationof distinct classes and functional domains of Wnts throughexpression of wild-type and chimeric proteins in Xenopusembryos. Mol Cell Biol 1995, 15:2625-2634.12. Sokol SY: Analysis of Dishevelled signaling pathways duringXenopus development. Curr Biol 1996, 6:1456-1467.13. Shih J, Keller R: Cell motility driving mediolateral intercalation inexplants of Xenopus laevis. Development 1992, 116:901-914.14. McEwen DG, Cox RT, Peifer MA: The canonical Wg and JNKsignaling cascades collaborate to promote both dorsal closureand ventral patterning. Development 2000, in press.15. Whangbo J,  Kenyon C: A Wnt signaling system that specifies twopatterns of cell migration in C. elegans. Mol Cell 1999, 4:851-858.

Wnt signaling: Moving in a new direction

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Wnt signaling: Moving in a new direction

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