title: Head inducer Dickkopf-1 is a ligand for Wnt coreceptor LRP6.
abstract: BACKGROUND
  Dickkopf-1 (Dkk-1) is a head inducer secreted from the vertebrate head organizer and induces anterior development by antagonizing Wnt signaling. Although several families of secreted antagonists have been shown to inhibit Wnt signal transduction by binding to Wnt, the molecular mechanism of Dkk-1 action is unknown. The Wnt family of secreted growth factors initiates signaling via the Frizzled (Fz) receptor and its candidate coreceptor, LDL receptor-related protein 6 (LRP6), presumably through Fz-LRP6 complex formation induced by Wnt. The significance of the Fz-LRP6 complex in signal transduction remains to be established.
  
  
  RESULTS
  We report that Dkk-1 is a high-affinity ligand for LRP6 and inhibits Wnt signaling by preventing Fz-LRP6 complex formation induced by Wnt. Dkk-1 binds neither Wnt nor Fz, nor does it affect Wnt-Fz interaction. Dkk-1 function in head induction and Wnt signaling inhibition strictly correlates with its ability to bind LRP6 and to disrupt the Fz-LRP6 association. LRP6 function and Dkk-1 inhibition appear to be specific for the Wnt/Fz beta-catenin pathway.
  
  
  CONCLUSIONS
  Our results demonstrate that Dkk-1 is an LRP6 ligand and inhibits Wnt signaling by blocking Wnt-induced Fz-LRP6 complex formation. Our findings thus reveal a novel mechanism for Wnt signal modulation. LRP6 is a Wnt coreceptor that appears to specify Wnt/Fz signaling to the beta-catenin pathway, and Dkk-1, distinct from Wnt binding antagonists, may be a specific inhibitor for Wnt/beta-catenin signaling. Our findings suggest that Wnt-Fz-LRP6 complex formation, but not Wnt-Fz interaction, triggers Wnt/beta-catenin signaling.
authors.affiliation: Division of Neuroscience, Children's Hospital, Department of Neurology, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA.
authors.forename: M V
authors.initials: MV
authors.lastname: Semënov
authors.forename: K
authors.initials: K
authors.lastname: Tamai
authors.forename: B K
authors.initials: BK
authors.lastname: Brott
authors.forename: M
authors.initials: M
authors.lastname: Kühl
authors.forename: S
authors.initials: S
authors.lastname: Sokol
authors.forename: X
authors.initials: X
authors.lastname: He
mesh_terms: D000818:Animals; D002460:Cell Line; D003598:Cytoskeletal Proteins / Q000378:metabolism; D051157:Frizzled Receptors; D006801:Humans; D015151:Immunoblotting; D036341:Intercellular Signaling Peptides and Proteins; D026502:LDL-Receptor Related Proteins; D008024:Ligands; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D008845:Microinjections; D008954:Models, Biological; D009865:Oocytes / Q000502:physiology; D011233:Precipitin Tests; D011485:Protein Binding; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D012313:RNA / Q000378:metabolism; D011973:Receptors, LDL / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics* / Q000502:physiology; D015534:Trans-Activators; D051153:Wnt Proteins; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology / Q000502:physiology; D029961:Zebrafish Proteins; D051176:beta Catenin
chemical_list: C495266:CTNNB1 protein, Xenopus; C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; C501126:DKK1 protein, human; D051157:Frizzled Receptors; D036341:Intercellular Signaling Peptides and Proteins; D026502:LDL-Receptor Related Proteins; C555482:LRP5 protein, human; C555475:LRP6 protein, human; D008024:Ligands; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D011506:Proteins; D011518:Proto-Oncogene Proteins; D011973:Receptors, LDL; D011994:Recombinant Proteins; D015534:Trans-Activators; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; C110182:dkk1 protein, Xenopus; C501128:dkk1b protein, zebrafish; D012313:RNA