Mouse cristin/R-spondin family proteins are novel ligands for the Frizzled 8 and LRP6 receptors and activate beta-catenin-dependent gene expression.
Wnt signaling plays critical biological roles during normal embryonic development and homeostasis in adults. In the canonical pathway, binding of Wnt ligands to the Frizzled (Fzd) receptor and the low density lipoprotein-related receptor (LRP) 5 or LRP6 coreceptor initiates downstream signaling events leading to gene activation by beta-catenin and the T-cell factor (TCF)-lymphoid enhancer factor (LEF) family transcription factor complex. In this study, we provide several lines of evidence that the mouse Cristin/R-spondin family proteins function as Fzd8 and LRP6 receptor ligands and induce the canonical Wnt/beta-catenin signaling pathway, leading to TCF-dependent gene activation. First, conditioned medium containing Cristin/R-spondin proteins effectively induced reporter activity in a TCF-binding site-dependent manner. Second, stimulation of cells with Cristin/R-spondin was accompanied by stabilization of endogenous beta-catenin proteins and induction of canonical Wnt target genes. Third, Cristin/R-spondin proteins physically interacted with the extracellular domains of the LRP6 and Fzd8 receptors in vivo and in vitro. Interestingly, unlike canonical Wnt ligands, Cristin/R-spondin failed to form a ternary complex with both LRP6 and Fzd8 receptors, suggesting that R-spondin may activate the canonical Wnt signaling pathway by different mechanisms. Furthermore, Cristin/R-spondin proteins possess an intriguing positive modulatory activity on Wnt ligands, possibly through a direct interaction. Our findings expand the repertoire of ligands that induce beta-catenin/TCF-dependent gene activation and implicate the presence of active beta-catenin-dependent gene activation in a Wnt-free biological context.
Nam
Ju-Suk
JS
Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074.
Turcotte
Taryn J
TJ
Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074.
Smith
Peter F
PF
Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074.
Choi
Sangdun
S
Division of Biology, California Institute of Technology, Pasadena, California 91125.
Yoon
Jeong Kyo
JK
Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074. Electronic address: yoonje@mmc.org.
D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D018507:Gene Expression Regulation, Developmental; D006493:Heparin / Q000378:metabolism; D006801:Humans; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D051379:Mice; D008969:Molecular Sequence Data; D005810:Multigene Family; D010802:Phylogeny; D011485:Protein Binding; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D011973:Receptors, LDL / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D019699:Thrombospondins / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D051153:Wnt Proteins / Q000378:metabolism; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism*
D006493:Heparin; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D043562:Receptors, G-Protein-Coupled; D011973:Receptors, LDL; D019699:Thrombospondins; D051153:Wnt Proteins; D051176:beta Catenin; C495267:CTNNB1 protein, mouse; C555475:LRP6 protein, human; C555476:Lrp6 protein, mouse; C511610:Fzd8 protein, mouse