Context-Dependent Regulation of Wnt Signaling Through the Primary Cilium
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BRIEF REVIEW www.jasn.org Context-Dependent Regulation of Wnt Signaling through the Primary Cilium † †‡ Edwin C. Oh* and Nicholas Katsanis* *Center for Human Disease Modeling, Duke University, Durham, North Carolina; and Departments of †Cell Biology and ‡Pediatrics, Duke University, Durham, North Carolina ABSTRACT The primary cilium is a highly conserved environmental sensor and modulator of fluid canonical (b-catenin dependent) or movement in tubular structures. The growing recognition of mutations among its noncanonical (b-catenin independent). many components has led to the discovery of new disorders collectively called cil- This classification is, however, an over- iopathies. Ciliary dysfunction disturbs a variety of signaling pathways along its basal simplification given the overlap of re- body and axoneme that are critical for embryonic development and cell and organ ceptorsandeffectorsinbotharmsof homeostasis. Among the many pathways, here we discuss the emerging role of Wnt Wnt signaling. proteins in morphogenic signaling and ciliary biology during health and disease. In mammals there are at least 19 se- creted Wnt glycoproteins and 10 frizzled J Am Soc Nephrol 24: 10–18, 2013. doi: 10.1681/ASN.2012050526 receptors.22 In the absence of canonical Wnt stimulation, b-catenin is targeted for degradation by a destruction complex composed of GSK3b/Axin2/adenomato- The primary cilium is an evolutionarily signaling; we evaluate empirical, some- sis polyposis coli (Figure 1). Upon binding conservedstructureresponsibleforthegen- times contradictory, evidence for the of a Wnt ligand to a complex of an Fz re- fl fl eration of uid ow and the perception links between Wnt signaling and ciliary ceptor and a single-pass transmembrane 1–3 of sensory information. Although once function and dysfunction, and discuss coreceptor, LDL receptor–related protein, considered a vestigial structure, the recent context-dependent regulation of mor- Dvl is recruited to Fz through its PDZ do- association of ciliary dysfunction with a phogenetic signaling. main and promotes the polymerization of host of genetic disorders known collec- Our current understanding of the the receptor complex. The resultant scaf- 2 tively as ciliopathies has highlighted an Wnt signal transduction pathway is the fold provides a platform for Axin and . array for roles for this organelle critical result of work performed 25 years ago GSK3b binding, dissolving the destruc- for both development and homeostasis. demonstrating that Drosophila wingless tion complex. Free cytoplasmic b-catenin In the kidney, dysfunction of primary cil- (wg) and mouse Int1 are gene products then translocates to the nucleus, where it 4 ium results in a variety of ciliopathies and of a conserved family of signaling pro- displaces Groucho, a family of Wnt re- 5–8 21 modulates cystogenesis and planar cell teins. Early studies revealed that muta- pressor proteins and dimerizes with the 6,9,10 b polarity (PCP). tions in a key effector protein, -catenin, T-cell factor family of transcription fac- 21 An emerging class of functions involves were enriched in some cancers, sug- tors to transactivate target genes. b signaling receptors and/or effectors de- gesting that Wnt/ -catenin signaling re- The noncanonical branch also utilizes a 21,22 livered to the ciliary axoneme by special- gulates cell proliferation and survival. Fz/LDL receptor–related protein receptor ized transport machinery and sequestered An additional layer of complexity was complex and Dvl; however, a major to regulate morphogenetic signaling path- added when it was recognized that the ways. There are currently at least seven establishment of polarity during hair signaling cascades linked to a functional orientation or gastrulation requires a Published online ahead of print. Publication date cilium, defects in which can be associated paracrine signal through a Frizzled available at www.jasn.org. causally with some phenotypic aspects of (Fz) transmembrane family of Wnt re- Correspondence: Dr. Nicholas Katsanis, Center for ciliopathies. These pathways include sonic ceptors and the downstream effector, Human Disease Modeling, Department of Cell Bi- ology, 466 Nanaline Building, Duke University, 11 12 13 b hedgehog (Shh), PDGF, Wnt, Dishevelled (Dvl), in a -catenin inde- Durham, NC 27710. Email: nicholas.katsanis@duke. G-protein signaling,14–16 fibroblast growth pendent manner. Taken together, these edu 17,18 19 b 20 fi factor, Notch, and TGF- . Here, ndings led to a broad categorization Copyright © 2013 by the American Society of we focus on one of these pathways, Wnt of the Wnt signaling pathway as either Nephrology 10 ISSN : 1046-6673/2401-10 J Am Soc Nephrol 24: 10–18, 2013 www.jasn.org BRIEF REVIEW Figure 1. Wnt signaling is regulated through the primary cilium. Under normal resting conditions, Bardet-biedl syndrome proteins participate in the degradation of b-catenin through the proteasome. Jouberin can also limit b-catenin nuclear entry by forming a complex in the cytoplasm. In the presence of the Wnt ligand, b-catenin enters the nucleus to activate T-cell factor/lymphoid enhancer factor signal transduction. Upon ablation of Dnchc2, short cilia are present in the limb bud, metanephros, and developing lung and dampened Wnt activity is observed. When the cilium is ablated or when basal body proteins are defective, the Wnt response is augmented in part due to an accumulation of nuclear b-catenin. distinction from canonical Wnt signaling resolve the question of whether observed knock down of Inversin in Xenopus em- is the activation of small Rho GTPases, changes in levels of b-catenin were a fun- bryos gave rise to defects in CE move- JNK, calcium signaling, and actin remod- damental result of ciliary dysfunction or ments, revealing a role for Inversin in eling. Over the past 10 years, noncanoni- reflected nonciliary functions of IFT88 Wnt/PCP. Similar to these studies, modu- cal Wnt signaling has been implicated in a and KIF3a, the authors hypothesized lating the expression of another ciliary process by which cells orient with respect that cilia might regulate canonical Wnt protein, NPHP3 blocked Dvl1-induced to an axis along a plane of an organ, signaling. Subsequent work on, inversin, activation of canonical Wnt signaling termed PCP.Dysregulation in this process mutations in which cause one of the cili- and resulted in defective cell movements has wide-ranging effects on tissue homeo- opathy phenotypes, nephronophthisis during gastrulation in zebrafish.28 To- stasis and disease because PCP mouse (NPH),4 showed that Inversin/NPHP2, gether, the studies highlighted a role for mutants show defective convergent exten- localized primarily to the base of the cil- the cilium in constraining canonical Wnt sion (CE) movements during gastrula- ium and could negatively regulate canon- signaling through Dvl. tion,23 misoriented stereociliary bundles ical Wnt signaling while potentiating non- Wnt/PCP mutant phenotypes were in the organ of Corti, neural tube defects, canonical Wnt targets.27 Using in vitro soon discovered in other genes associated open eyelids, and randomization of fur cotransfection assays in a HEK293 cell with the primary cilium and basal body. stroke direction24 (Figure 2). line, the authors observed that Inversin Knockout mice lacking Bbs1, Bbs4, or blocks Dvl1-mediated activation of a ca- Bbs6 recapitulated a constellation of nonical Wnt-responsive reporter con- phenotypes observed in patients with WNT SIGNALING THROUGH struct by targeting cytoplasmic Dvl for Bardet-Biedl syndrome, including reti- THE PRIMARY CILIUM degradation. Moreover, whereas Dvl- nal degeneration, anosmia, and kidney injected Xenopus embryos displayed ca- defects.29,30 Upon further examination Initial evidence suggesting a link between nonical Wnt defects such as a secondary of the mutant mouse models, PCP mutant the primary cilium and Wnt signaling body axis, coinjection of Inversin RNA phenotypes including neural tube closure was described in two separate reports, inhibited this process, suggesting that In- defects and misoriented stereociliary hair showing increased cytoplasmic b-catenin versin inhibits canonical Wnt signaling in bundle phenotypes in the inner ear were expression in epithelial kidney cells from vitro and in vivo. Regulation of Dvl is a identified in several Bbs mutant lines (Fig- kidney-specific Kif3a knockout mice25 shared process between the canonical ure 2). To test whether Bbs genes function and in pancreatic ducts derived from IF- and noncanonical branches of Wnt signal- in the same signaling pathway as PCP T88orpk mice,26 respectively (Figure 2). ing; Simons et al. next demonstrated that genes, genetic interaction studies were Although these studies could not fully both overexpression and morpholino- performed and Bbs1 or Bbs6 was removed J Am Soc Nephrol 24: 10–18, 2013 Wnt Signaling through the Primary Cilium 11 BRIEF REVIEW www.jasn.org of the liver, pancreas, and gut. The au- thors found that dub mayinpartbe regulated by its phosphorylation status and they demonstrated that serine to alanine mutants recapitulated dub loss of function phenotypes. In addition, dub interacted genetically with Fz2, suggesting that Wnt/PCP mutant phe- notypes were not limited to the pertur- bation in basal body function, but could also be driven by defective primary cilia formation. The connection between the primary cilium and canonical Wnt was bolstered further with two additional studies. Gerdes et al. utilized a similar HEK293 cell line system as Simons