Surrogate Wnt agonists that phenocopy canonical Wnt and β-catenin signalling

The authors describe water-soluble surrogate Wnt agonists, with specificity towards some frizzled (FZD) receptors, which can maintain human intestinal organoid cultures and have effects on the mouse liver in vivo. Wnt ligands interact with FZDand Lrp5/6-type receptors to influence diverse developmental, homeostatic and pathologic processes through β-catenin-dependent signalling. The promiscuity of Wnt ligands towards several receptors and the fact that Wnts can be hydrophobic make it difficult to produce therapeutic recombinant Wnts. Chris Garcia and colleagues have developed surrogate water-soluble Wnt agonists that have specificity towards certain FZDs.The new agonists act similarly to Wnt3 in differentiation assays towards the osteogenic lineage in vitro, can maintain intestinal organoid cultures, and have in vivo effects on the mouse liver. Elsewhere in this issue, Calvin Kuo and colleagues use these novel water-soluble Wnt agonists in the mouse intestinal stem-cell niche to dissect the respective roles of R-spondin and Wnt ligands, both of which activate similar signalling receptors and pathways. They find that Lgr5+ intestinal stem cells normally differentiate unless both R-spondin and Wnt ligands are present. However, on their own, each ligand acts non-redundantly and in cooperation with Wnt agonists, activating R-spondin receptors to maintain stem-cell competency and these receptors are in turn activated in the presence of R-spondin to drive stem-cell expansion. These water-soluble Wnt agonists could be used in a range of assays to understand this signalling pathway and modulate it in therapeutical applications. Wnt proteins modulate cell proliferation and differentiation and the self-renewal of stem cells by inducing β-catenin-dependent signalling through the Wnt receptor frizzled (FZD) and the co-receptors LRP5 and LRP6 to regulate cell fate decisions and the growth and repair of several tissues1. The 19 mammalian Wnt proteins are cross-reactive with the 10 FZD receptors, and this has complicated the attribution of distinct biological functions to specific FZD and Wnt subtype interactions. Furthermore, Wnt proteins are modified post-translationally by palmitoylation, which is essential for their secretion, function and interaction with FZD receptors2,3,4. As a result of their acylation, Wnt proteins are very hydrophobic and require detergents for purification, which presents major obstacles to the preparation and application of recombinant Wnt proteins. This hydrophobicity has hindered the determination of the molecular mechanisms of Wnt signalling activation and the functional importance of FZD subtypes, and the use of Wnt proteins as therapeutic agents. Here we develop surrogate Wnt agonists, water-soluble FZD–LRP5/LRP6 heterodimerizers, with FZD5/FZD8-specific and broadly FZD-reactive binding domains. Similar to WNT3A, these Wnt agonists elicit a characteristic β-catenin signalling response in a FZD-selective fashion, enhance the osteogenic lineage commitment of primary mouse and human mesenchymal stem cells, and support the growth of a broad range of primary human organoid cultures. In addition, the surrogates can be systemically expressed and exhibit Wnt activity in vivo in the mouse liver, regulating metabolic liver zonation and promoting hepatocyte proliferation, resulting in hepatomegaly. These surrogates demonstrate that canonical Wnt signalling can be activated by bi-specific ligands that induce receptor heterodimerization. Furthermore, these easily produced, non-lipidated Wnt surrogate agonists facilitate functional studies of Wnt signalling and the exploration of Wnt agonists for translational applications in regenerative medicine.