Structural basis of the interaction between Norrin-Tspan12

As an atypical Wnt ligand, Norrin initiates β-catenin signaling through the receptor Frizzled 4 (FZD4), the co-receptors Low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6), and Tetraspanin 12 (Tspan12). Tspan12 interacts with Norrin via its large extracellular loop (LEL). However, the molecular mechanism by which Tspan12 enhances Norrin signaling has remained unclear. Here, we determined the cryo-EM structure of the Norrin-Tspan12 LEL complex at 3.78 Å resolution, which reveals that a Norrin dimer binds two Tspan12 molecules and defines the Norrin-Tspan12 interface. We show that Tspan12 binds directly to Norrin without enhancing the binding affinity between Norrin and FZD4. Our results support a model in which Norrin, FZD4, LRP5/6, and Tspan12 form a quaternary complex. As mutations in these proteins can lead to familial exudative vitreoretinopathy (FEVR), these findings are important for targeted therapy development.