R-Spondin2 Is a Secreted Activator of Wnt/β-Catenin Signaling and Is Required for Xenopus Myogenesis

Abstract

We have carried out a small pool expression screen for modulators of the Wnt/beta-catenin pathway and identified Xenopus R-spondin2 (Rspo2) as a secreted activator of this cascade. Rspo2 is coexpressed with and positively regulated by Wnt signals and synergizes with Wnts to activate beta-catenin. Analyses of functional interaction with components of the Wnt/beta-catenin pathway suggest that Rspo2 functions extracellularly at the level of receptor ligand interaction. In addition to activating the Wnt/beta-catenin pathway, Rspo2 overexpression blocks Activin, Nodal, and BMP4 signaling in Xenopus, raising the possibility that it may negatively regulate the TGF-beta pathway. Antisense Morpholino experiments in Xenopus embryos and RNAi experiments in HeLa cells reveal that Rspo2 is required for Wnt/beta-catenin signaling. In Xenopus embryos depleted of Rspo2, the muscle markers myoD and myf5 fail to be activated and later muscle development is impaired. Thus, Rspo2 functions in a positive feedback loop to stimulate the Wnt/beta-catenin cascade.

Molecular regulation of kidney development: is the answer blowing in the Wnt?

“…A third possibility is that reporter activation is Wnt independent. Recent data suggest that a number of non-Wnt factors are able to activate the β-catenin pathway, including the secreted molecules norrin, dickopf 2, Sfrp1 and the R-spondins [72][73][74][75][76]. Although several of these molecules are expressed in the embryonic kidney, their roles in the development of this organ are unclear, although sFrp-1 has been suggested to have an inhibitory role in tubule formation [77].…”

Human SRY inhibits β-catenin-mediated transcription

“…RSPO1 is a small secreted factor that is able to stimulate the canonical Wnt/β-catenin signaling pathway by increasing the levels of a Wnt co-receptor, LRP6 (low density lipoprotein receptorrelated protein 6) at the membrane of the cell (Binnerts et al, 2007, Kazanskaya et al, 2004, Kim et al, 2005, Kim et al, 2006a, Nam et al, 2006. Importantly, the Wnt/β-catenin pathway is active in the early developing ovary and inactive in the early developing testis (Chassot et al, 2008).…”

R-Spondins in Cutaneous Biology: Nails and Cancer

“…26 In addition, the expression of R-spondins1-3 are reported to be deregulated in a number of tumors analyzed. 1 Therefore, as seen with misregulation of the Wnt signaling pathway, R-spondins also appear to play a role in tumorigenesis though their altered expression compared to normal tissue and this may reflect a change in the transcriptome rather than a causative role in the etiology of the cancer. However, the link with germline RSPO1 mutation and cutaneous SCC is more compelling.…”

“…2 Xenopus Rspo2 and human Rspo1, 2 and 3 have all been shown to promote stabilization of b-catenin in HEK 293T cells. 1,3,4 Furthermore, both human Rspo1 and Rspo3 can physically interact with the extracellular domains of the Wnt signaling pathway receptors LRP6 and Fzd8. 4 Wnts also appear to be involved in regulating R-spondin expression, as seen with the induction of R-spondin expression in Xenopus injected with Wnts 1 and the significant down-regulation of R-spondins in Wnt1/3a double knockout mice.…”

“…However, unlike Wnts, the cysteine residues found in R-spondins are organised into two adjacent furin-like domains, which have been suggested to be sufficient for inducing b-catenin stabilization. 1 All four members of the R-spondin family are expressed in mouse during development. 2 Xenopus Rspo2 and human Rspo1, 2 and 3 have all been shown to promote stabilization of b-catenin in HEK 293T cells.…”