LGR6 Is a High Affinity Receptor of R-Spondins and Potentially Functions as a Tumor Suppressor

BACKGROUND: LGR6 (leucine-rich repeat containing, G protein-coupled receptor 6) is a member of the rhodopsin-like seven transmembrane domain receptor superfamily with the highest homology to LGR4 and LGR5. LGR6 was found as one of the novel genes mutated in colon cancer through total exon sequencing and its promoter region is hypermethylated in 20-50% of colon cancer cases. In the skin, LGR6 marks a population of stem cells that can give rise to all cell lineages. Recently, we and others demonstrated that LGR4 and LGR5 function as receptors of R-spondins to potentiate Wnt/β-catenin signaling. However, the binding affinity and functional response of LGR6 to R-spondins, and the activity of colon cancer mutants of LGR6 have not been determined. PRINCIPAL FINDINGS: We found that LGR6 also binds and responds to R-spondins 1-3 with high affinity to enhance Wnt/β-catenin signaling through increased LRP6 phosphorylation. Similar to LGR4 and LGR5, LGR6 is not coupled to heterotrimeric G proteins or to β-arrestin following R-spondin stimulation. Functional and expression analysis of three somatic mutations identified in colon cancer samples indicates that one mutant fails to bind and respond to R-spondin (loss-of-function), but the other two have no significant effect on receptor function. Overexpression of wild-type LGR6 in HeLa cells leads to increased cell migration following co-treatment with R-spondin1 and Wnt3a when compared to vector control cells or cells overexpressing the loss-of-function mutant. CONCLUSIONS: LGR6 is a high affinity receptor for R-spondins 1-3 and potentially functions as a tumor suppressor despite its positive effect on Wnt/β-catenin signaling

Expression pattern of the orphan receptor LGR4/GPR48 gene in the mouse.

Leucine-rich G-protein-coupled Receptors (LGR) constitute a subfamily of receptors related to glycoprotein hormone receptors. Amongst them, LGR4, LGR5 and LGR6 form a cluster for which natural agonists are still unknown. By an extensive gene trapping approach, Leighton et al. (2001) obtained a mouse line in which the LGR4 gene is disrupted by a trap vector carrying two biological markers, beta-geo (a fusion between bacterial beta-galactosidase and neomycin phosphotransferase) and a placental alkaline phosphatase (PLAP). Due to perinatal lethality, characterization of adult mice homozygous for this insertion has been impaired. In the present study we have investigated LacZ and PLAP activity patterns in heterozygous mice as a marker for LGR4 natural expression at both macroscopic and histological levels. We present a detailed atlas of LGR4 expression, which displays very wide expression with particularly strong activity in cartilages, kidneys, reproductive tracts and nervous system cells.Journal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe

Lgr5 homologues associate with Wnt receptors and mediate R-spondin signalling.

The adult stem cell marker Lgr5 and its relative Lgr4 are often co-expressed in Wnt-driven proliferative compartments. We find that conditional deletion of both genes in the mouse gut impairs Wnt target gene expression and results in the rapid demise of intestinal crypts, thus phenocopying Wnt pathway inhibition. Mass spectrometry demonstrates that Lgr4 and Lgr5 associate with the Frizzled/Lrp Wnt receptor complex. Each of the four R-spondins, secreted Wnt pathway agonists, can bind to Lgr4, -5 and -6. In HEK293 cells, RSPO1 enhances canonical WNT signals initiated by WNT3A. Removal of LGR4 does not affect WNT3A signalling, but abrogates the RSPO1-mediated signal enhancement, a phenomenon rescued by re-expression of LGR4, -5 or -6. Genetic deletion of Lgr4/5 in mouse intestinal crypt cultures phenocopies withdrawal of Rspo1 and can be rescued by Wnt pathway activation. Lgr5 homologues are facultative Wnt receptor components that mediate Wnt signal enhancement by soluble R-spondin proteins. These results will guide future studies towards the application of R-spondins for regenerative purposes of tissues expressing Lgr5 homologues