Research Article

A molecular mechanism for Wnt ligand-specific signaling

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Science  17 Aug 2018:
Vol. 361, Issue 6403, eaat1178
DOI: 10.1126/science.aat1178

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How Wnt ligands achieve specificity

Wnt signaling is essential for development, tissue homeostasis, and disease. The 19 members of the Wnt family interact promiscuously with the 10 Frizzled receptors, raising the question of how ligand-specific discrimination is achieved in a biological context. Eubelen et al. used experiments in zebrafish to show that cells are equipped with decoding modules that bind Wnt with high specificity and trigger signal amplification via their recruitment into higher-order Frizzled signalosomes (see the Perspective by Kim and Goentoro). Thus, distinct Wnt ligand-receptor pairs can be targeted specifically for therapeutic purposes.

Science, this issue p. eaat1178; see also p. 643

Structured Abstract

INTRODUCTION

Wnt signaling is an ancient signaling pathway that has accompanied the emergence of metazoans and is key to many developmental, physiological, and disease processes. Similar to other signaling pathways, gene families for both Wnt ligand and its corresponding Frizzled receptor have undergone extensive expansion during metazoan evolution. Vertebrate genomes harbor 19 closely related Wnt genes as well as 10 Frizzled genes. Gene duplication is typically considered a major driving force in the evolution of new biological functions through neo- or subfunctionalization of emerging paralogs. How this functional diversification of Wnt ligands is structurally and molecularly organized, however, remains poorly understood. The Wnt/Frizzled molecular interaction is mediated by residues conserved across both families. This promiscuous interaction is incompatible with monospecific recognition and, accordingly, when tested in pair-wise combinations, multiple Wnt ligands compete for binding to various Frizzled receptors.

RATIONALE

These observations raise the questions of how Wnt ligands achieve functional diversification and how cells interpret the intermingled expression patterns of simultaneous and sometimes conflicting Wnt signals. In some biological settings, cells may integrate all signaling inputs nondiscriminately and trigger appropriate responses by considering their total net balance. However, other biological processes exhibit strict Wnt ligand selectivity, despite complex Wnt/Frizzled expression landscapes. A prototypical example is provided by the exclusive control of mammalian forebrain and ventral spinal cord angiogenesis by Wnt7a and Wnt7b.

Within this neurovascular unit, in order to respond to neural progenitor–derived Wnt7 by activating Wnt/β-catenin signaling, cerebral endothelial cells must express a membrane protein complex consisting of the adhesion G protein–coupled receptor (GPCR) Gpr124 and the glycosylphosphatidylinositol-anchored glycoprotein Reck. This Gpr124/Reck complex was recently reported to promote Wnt7-specific responses.

RESULTS

Using a combination of biophysical approaches and ligand-binding assays in genetically engineered cells, we demonstrate that ligand selectivity is conferred by Reck, which mediates Wnt7-specific binding in a Frizzled-independent manner. Reck orchestrates Wnt ligand discrimination by engaging the structurally disordered and highly divergent linker domain of Wnt7. The presence of Gpr124 is required to deliver Reck-bound Wnt7 to Frizzled by assembling higher-order Reck/Gpr124/Frizzled/Lrp5/6 complexes. This Gpr124 tethering function does not rely on its GPCR structure but instead on its combined capacity to interact with Reck extracellularly and recruit the Dishevelled scaffolding protein intracellularly. By bridging Gpr124 and Frizzled, Dishevelled recruits Wnt7, via its association with Reck, into dynamic Wnt/Frizzled signalosomes, resulting in increased local concentrations of ligand available for Frizzled signaling.

CONCLUSION

Our data reveal that cells are equipped with “Wnt-decoding modules” that distinguish between Wnt ligands that are otherwise very similar. They also reveal a critical role for the linker domain in Wnt ligand evolution and functional diversification. These mechanistic insights into the Wnt decoding capacities of vertebrate cells predict that additional Wnt decoding modules exist, enabling fine-tuning of cellular behaviors in response to other Wnt or Frizzled family members. These modules expand the diversity of proximal events in Wnt signaling, opening new therapeutic opportunities for conditions in which Wnt stimulation or inhibition are desirable at the membrane level. In particular, the mechanisms uncovered here provide an opportunity for the targeted treatment of human central nervous system neurovascular disorders.

Task sharing for orchestrated Wnt7-specific cellular responses.

(Top) Gpr124 and Reck cooperatively alter the cell’s perception of its Wnt microenvironment by selectively potentiating Wnt7 signals (cyan-tinted dots). (Bottom) Reck decodes Wnt ligands by establishing monospecific contacts with the highly divergent Wnt7 linker domain. Gpr124 links Reck-bound Wnt7 to Dishevelled. Dishevelled polymers by interacting simultaneously with Gpr124 and Fz assemble Wnt7-enriched signalosomes that trigger signaling through Fz receptors and Lrp5/6 co-receptors.

Abstract

Wnt signaling is key to many developmental, physiological, and disease processes in which cells seem able to discriminate between multiple Wnt ligands. This selective Wnt recognition or “decoding” capacity has remained enigmatic because Wnt/Frizzled interactions are largely incompatible with monospecific recognition. Gpr124 and Reck enable brain endothelial cells to selectively respond to Wnt7. We show that Reck binds with low micromolar affinity to the intrinsically disordered linker region of Wnt7. Availability of Reck-bound Wnt7 for Frizzled signaling relies on the interaction between Gpr124 and Dishevelled. Through polymerization, Dishevelled recruits Gpr124 and the associated Reck-bound Wnt7 into dynamic Wnt/Frizzled/Lrp5/6 signalosomes, resulting in increased local concentrations of Wnt7 available for Frizzled signaling. This work provides mechanistic insights into the Wnt decoding capacities of vertebrate cells and unravels structural determinants of the functional diversification of Wnt family members.

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