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. The adult stem-cell marker Lgr5 and its family members encode orphan seven-transmembrane receptors that are similar to the receptors for the hormones FSH, LH and TSH, but their ligand and molecular function have been elusive. In experiments on intestinal epithelial crypt cells, binding of Lgr5 homologues to R-spondins — a family of secreted proteins linked to the Wnt signalling pathway — is shown to trigger downstream canonical Wnt signals through associated Frizzled–Lrp5/6 complexes. This work suggests that R-spondins have regenerative activity in tissues that express Lgr5 homologues.