Editors' ChoiceDevelopment

Orphan GPCRs, R-Spondins, and Wnt Signaling

Science Signaling  19 Jul 2011:
Vol. 4, Issue 182, pp. ec198
DOI: 10.1126/scisignal.4182ec198

The G protein–coupled receptor (GPCR) family represents the largest receptor family in the human genome, but endogenous ligands and downstream signaling mechanisms are not known for all its members. Carmon et al. explored one subfamily of orphan GPCRs, the LGR (leucine-rich repeat containing GPCR) family composed of LGR4, LGR5, and LGR6, and identified R-spondins (RSPOs) as ligands for LGR4 and LGR5. Noting that R-spondin1 (RSPO1) has a strong proliferative effect on LGR5-positive stem cells of the intestinal crypt, the authors found that HEK293 transfected with Myc-tagged LGR4 or LGR5 bound Fc-tagged RSPO1 (by immunostaining, fluorescence-activated cell sorting, and competition assay) and that RSPO1 colocalized with internalized receptors (LGR4 and LGR5 exhibited constitutive internalization that did not depend on the presence of RSPO1). RSPOs had been previously reported to enhance signaling through the Wnt-β-catenin pathway, and RSPO1 treatment of cells transfected to coexpress LGR4 or LGR5 with a β-catenin reporter gene showed enhanced responsiveness to Wnt3a compared with cells only expressing the reporter. RSPO1 enhanced Wnt signaling even in HEK293 cells not overexpressing LGR4 or LGR5; mRNA analysis indicated that both of these receptors were endogenously expressed in these cells, and knockdown of LGR4 or LGR5 reduced the response to RSPO. RSPOs had been previously reported to be ligands of the Wnt co-receptor LRP6. However, Carmon et al. provide evidence that the potency of RSPO in potentiating Wnt signaling depended on the abundance of the LGR and was not affected by overexpression of LRP6, whereas the maximal efficiency of RSPO in enhancing Wnt signaling depended on the amount of Wnt3a and the abundance of LRP6. RSPO did not appear to stimulate LGR4 or LGR5 activation of G proteins or recruitment of β-arrestin; thus, the mechanism by which the RSPO-LGR pathway enhanced Wnt signaling remains unknown.

K. S. Carmon, X. Gong, Q. Lin, A. Thomas, Q. Liu, R-spondins function as ligands of the orphan receptors LGR4 and LGR5 to regulate Wnt/β-catenin signaling. Proc. Natl. Acad. Sci. U.S.A. 108, 11452–11457 (2011). [Abstract] [Full Text]