WNT7A and WNT7B, secreted by neural cells, are essential regulators of developmental brain angiogenesis and blood-brain barrier integrity. In brain endothelial cells, WNT7 proteins activate β-catenin signaling through the ligand-specific receptor complex GPR124-RECK and classical WNT receptors of the FZD and LRP families. Previous studies suggested that WNT7 isoforms assemble a GPR124-RECK-FZD-LRP5/6 multi-receptor complex for signaling. However, direct biochemical evidence for this complex and its signaling mechanisms remains elusive. Here, we investigated the formation and signaling mechanisms of WNT7 co-receptor complexes in brain endothelial cells using CRISPR/Cas9, biochemical analyses, and cell-based assays. Unexpectedly, cells with knockout of all FZD isoforms retained ∼25% of WNT7 responsiveness, whereas classical WNT3A signaling was completely abolished. Similarly, knockout of all Dvl paralogs, key mediators of FZD signaling, preserved ∼50% of WNT7 signaling activity but fully blocked WNT3A responses. In contrast, knockout of Gpr124, Reck, or Lrp5/6 completely abrogated WNT7 signaling. While both WNT7A and WNT3A triggered LRP6 phosphorylation, only WNT3A induced DVL2 phosphorylation. Biochemical analyses revealed WNT7-dependent recruitment of LRP5/6, but not FZD, to the GPR124-RECK heterodimer, forming a GPR124-RECK-WNT7-LRP5/6 complex. In GPR124-deficient cells, WNT7 proteins still assembled a RECK-WNT7-LRP5/6 core complex, yet this complex lacked signaling activity and LRP6 phosphorylation. Clustering of RECK-WNT7-LRP5/6 complexes with recombinant dimeric GPR124 ectodomain or a RECK antibody partially restored signaling, suggesting that GPR124 mediates formation of higher-order complexes. Our findings indicate that WNT7 signaling in brain endothelial cells is driven by distinct co-receptor complexes: a FZD-independent GPR124-RECK-LRP5/6 complex and FZD-dependent complexes that likely act in synergy.
Heiden et al. (Mon,) studied this question.