R-spondins and LGRs

Also see the page on Multiple Wnt receptors

Reviews by Jiang and Cong 2016, and Lehoczky and Tabin 2018

See figure

R-spondin (Rspo) proteins are  secreted proteins defined by two N-terminal furin domains and a thrombospondin domain (Kazanskaya et al, 2004; Kim et al, 2005; reviewed in  MacDonald and He (2012),  Niehrs (2012)). In various contexts, Rspo proteins work synergistically with Wnts to enhance signaling levels but do not activate in the absence of Wnts themselves. Unlike Wnts, which work locally, Rspo proteins have systemic effects. Rspo mutations have been found in several hereditary syndromes in humans (Table below). RSPO1 is disrupted in a recessive syndrome characterized by XX sex reversal, a skin abnormality called palmoplantar hyperkeratosis, and predisposition to squamous cell carcinomas. Mutations in the RSPO4 gene are linked to congenital anonychia, severe hypo- plasia of finger- and toenails. 

Rspo proteins use members of  the Lgr family as receptors  (Glinka et al, 2011, Carmon et al, 2011, De Lau et al, 2011). (see Figure), in particular in stem cells and thereby have input into the canonical Wnt pathway. The receptors are related to G-protein-coupled receptors for thyroid-stimulating hormone,  follicle-stimulating hormone and luteinizing hormone. These receptors contain a large N-terminal extracellular leucine-rich repeat domain that binds the glycoprotein hormones. Similarly, the Lgr proteins bind R-spondins through their N-terminal ectodomain, but current evidence indicates that they do not utilize G proteins (Carmon et al, 2011, De Lau et al, 2011). 

LGR receptors are thought to inhibit the ZNRF3 and RNF3 transmembrane molecules that by themselves down-regulate Wnt signaling. ZNRF3 and RNF3 have E3 ubiquitin ligase activity acting on the Frizzled molecules, leading to turn-over of these receptors (Hao et al, 2012, Koo et al, 2012). Binding of R-Spondin to ZNRF3 has been postulated to down-regulate the activity of the ZNRF3 activity, thereby enhancing Wnt signaling as the Frizzled receptors now become available (Hao et al, 2012). ZNRF3 and RNF3 are mutated in some tumors (Koo et al, 2012).

It should be noted though that Szenker-Ravi et al, (2018) have obtained results indicating  that RSPO2, without the LGR4/5/6 receptors, serves as a direct antagonistic ligand to RNF43 and ZNRF3. They established that the triple and ubiquitous knockout of Lgr4, Lgr5 and Lgr6 in mice did not recapitulate the known Rspo2 or Rspo3 loss-of-function phenotypes. Moreover, based on cell culture and biochemical experiments, Lebensohn and Rohatgi (2018)  have shown that R-spondins can potentiate WNT signaling without LGRs, possibly using HSPGs instead. (Reviewed by Lehoczky and Tabin 2018)

Lgr4 as well as Lgr5 mutant mice are neonatal lethal.  Lgr5 is a Wnt target gene in colon cancer and that it marks adult stem cells in a number of actively self-renewing organs, including the intestinal tract and the hair follicle (reviewed in Clevers and Nusse, 2012) The finding that the Lgr proteins act as receptors for Rspo molecules reinforces the connections between Wnt signaling and activation of adult stem cells. 

R-spondin mimetics have been developed to target specific cell types (Zhang et al, 2020).