Activator of the canonical Wnt signaling pathway by acting as a ligand for LGR4-6 receptors, which acts as a key regulator of angiogenesis (PubMed:16543246, PubMed:21693646, PubMed:26766444). Upon binding to LGR4-6 (LGR4, LGR5 or LGR6), LGR4-6 associate with phosphorylated LRP6 and frizzled receptors that are activated by extracellular Wnt receptors, triggering the canonical Wnt signaling pathway to increase expression of target genes. Also regulates the canonical Wnt/beta-catenin-dependent pathway and non-canonical Wnt signaling by acting as an inhibitor of ZNRF3, an important regulator of the Wnt signaling pathway. Acts as a ligand for frizzled FZD8 and LRP6. May negatively regulate the TGF-beta pathway (PubMed:16543246, PubMed:21693646). Acts as a key regulator of angiogenesis by controlling vascular stability and pruning: acts by activating the non-canonical Wnt signaling pathway in endothelial cells (PubMed:26766444, PubMed:16543246, PubMed:21693646). Can also amplify Wnt signaling pathway independently of LGR4-6 receptors, possibly by acting as a direct antagonistic ligand to RNF43 and ZNRF3 (By similarity). Interacts with the extracellular domain of FZD8 and LRP6 (PubMed:16543246). It however does not form a ternary complex with FZD8 and LRP6 (PubMed:16543246). Interacts with WNT1 (PubMed:16543246). Binds heparin. Interacts with LGR4, LGR5 and LGR6 (PubMed:21693646). Highly expressed in endothelial cells (PubMed:26766444). Expressed from day 7.5 in the primitive streak. At day 9.5, it is expressed in various neural and mesodermal derivatives, mainly along dorsal neural tube, diencephalon, somites and tailbud mesoderm. Strongly expressed in limb buds, particularly in the morphogenetically active region such as the apical ectodermal ridge (AER). The FU repeats are required for activation and stabilization of beta-catenin. Embryonic lethality due to vascular remodeling defects (PubMed:26766444). Conditional knockout in endothelial cells results in impaired developmental and tumor vascular remodeling. Mice show endothelial cell apoptosis and vascular pruning, leading to reduced microvessel density (PubMed:26766444). Belongs to the R-spondin family.