Olga Kazanskaya

LGR4 and LGR5 are R-spondin receptors mediating Wnt/β-catenin and Wnt/PCP signalling

R‐spondins are secreted Wnt signalling agonists, which regulate embryonic patterning and stem cell proliferation, but whose mechanism of action is poorly understood. Here we show that R‐spondins bind to the orphan G‐protein‐coupled receptors LGR4 and LGR5 by their Furin domains. Gain‐ and loss‐of‐function experiments in mammalian cells and Xenopus embryos indicate that LGR4 and LGR5 promote R‐spondin‐mediated Wnt/β‐catenin and Wnt/PCP signalling. R‐spondin‐triggered β‐catenin signalling requires Clathrin, while Wnt3a‐mediated β‐catenin signalling requires Caveolin‐mediated endocytosis, suggesting that internalization has a mechanistic role in R‐spondin signalling.

The Wnt signaling regulator R-spondin 3 promotes angioblast and vascular development

The vertebrate embryonic vasculature develops from angioblasts, which are specified from mesodermal precursors and develop in close association with blood cells. The signals that regulate embryonic vasculogenesis and angiogenesis are incompletely understood. Here, we show that R-spondin 3 (Rspo3), a member of a novel family of secreted proteins in vertebrates that activate Wnt/β-catenin signaling, plays a key role in these processes. In Xenopus embryos, morpholino antisense knockdown of Rspo3 induces vascular defects because Rspo3 is essential for regulating the balance between angioblast and blood cell specification. In mice, targeted disruption of Rspo3 leads to embryonic lethality caused by vascular defects. Specifically in the placenta, remodeling of the vascular plexus is impaired. In human endothelial cells, R-spondin signaling promotes proliferation and sprouting angiogenesis in vitro, indicating that Rspo3 can regulate endothelial cells directly. We show that vascular endothelial growth factor is an immediate early response gene and a mediator of R-spondin signaling. The results identify Rspo3 as a novel, evolutionarily conserved angiogenic factor in embryogenesis.