Differential Activities and Mechanisms of the Four R-spondins in Potentiating Wnt/β-catenin Signaling

J Biol Chem. 2018 Jun 22;293(25):9759-9769. doi: 10.1074/jbc.RA118.002743. Epub 2018 May 11.

Abstract

The four R-spondins (RSPO1-4) strongly potentiate Wnt signaling and play critical roles in normal development, adult stem cell survival, and cancer development and aggressiveness. All four RSPOs have been suggested to potentiate Wnt signaling by binding to three related receptors, i.e. leucine-rich repeat-containing, G protein-coupled receptors 4, 5, and 6 (LGR4/5/6), and then inducing the clearance of two E3 ubiquitin ligases (RNF43 and ZNRF3) that otherwise would ubiquitinate Wnt receptors for degradation. Here, we show that RSPO1-4 have differential dependence on LGRs in potentiating Wnt/β-catenin signaling and that RSPO2 can enhance this pathway without any LGR. LGR4 knockout (LGR4KO) in HEK293 cells completely abrogated the Wnt/β-catenin signaling response to RSPO1 and RSPO4 and strongly impaired the response to RSPO3. RSPO2, however, retained robust activity albeit with decreased potency. Complete rescue of RSPO1-4 activity in LGR4KO cells required the seven-transmembrane domain of LGR4. Furthermore, an RSPO2 mutant with normal binding affinity to ZNRF3 but no or little binding to LGR4 or LGR5 still potentiated Wnt/β-catenin signaling in vitro, supported the growth of intestinal organoids ex vivo, and stimulated intestinal crypt growth in vivo Mechanistically, RSPO2 could increase Wnt receptor levels in the absence of any LGR without affecting ZNRF3 endocytosis and stability. These findings suggest that RSPO1-4 use distinct mechanisms in regulating Wnt and other signaling pathways, which have important implications for understanding the pleiotropic functions of RSPOs and LGRs in both normal and cancer development.

Keywords: 7-helix receptor; LGR4; LGR5; R-spondin; Wnt signaling; beta-catenin (β-catenin); cell signaling; organoids; stem cells.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • HEK293 Cells
  • Humans
  • Intercellular Signaling Peptides and Proteins / genetics
  • Intercellular Signaling Peptides and Proteins / metabolism
  • Intestinal Mucosa / cytology
  • Intestinal Mucosa / metabolism
  • Mice
  • Mice, Inbred C57BL
  • Organoids / cytology
  • Organoids / metabolism
  • Receptors, G-Protein-Coupled / antagonists & inhibitors
  • Receptors, G-Protein-Coupled / genetics
  • Receptors, G-Protein-Coupled / metabolism
  • Signal Transduction*
  • Thrombospondins / genetics
  • Thrombospondins / metabolism*
  • Wnt Proteins / genetics
  • Wnt Proteins / metabolism*
  • beta Catenin / genetics
  • beta Catenin / metabolism*

Substances

  • CTNNB1 protein, human
  • Intercellular Signaling Peptides and Proteins
  • LGR4 protein, human
  • R-spondin2 protein, human
  • RSPO1 protein, human
  • RSPO3 protein, human
  • RSPO4 protein, human
  • Receptors, G-Protein-Coupled
  • Thrombospondins
  • Wnt Proteins
  • beta Catenin