biostudies-literatureUnknownGranados AJNIGMS NIH HHSNIH HHSNorrin is an atypical ligand that regulates retinal angiogenesis through the Wnt/β-catenin pathway by triggering heterodimerization of Frizzled-4 (FZD4) with Low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6), leading to downstream β-catenin stabilization. Unlike Wnt ligands, Norrin requires the tetraspanin Tspan12 for signaling amplification, but it is not understood why. Here, we report a 3.4-angstrom structure of Tspan12 in complex with FZD4 determined by cryo-electron microscopy. The structure reveals that FZD4 and Tspan12 form a direct complex in the absence of Norrin, with a contact interface between the lower hinge of FZD4 and the extracellular loops of Tspan12 that enables FZD4 to respond with a more gradual response to a wider range of Norrin concentrations. The tranbioRxiv : the preprint server for biology2025.09.25.678640https://www.ebi.ac.uk/biostudies/studies/S-EPMC12485846biostudies-literatureStructural basis for regulation of Frizzled-4 signaling by the co-receptor Tetraspanin-12.PMC12485846DP5 OD036136R25 GM056847Alamo KAESusa KJGranados AJPratap PPfalseStructural basis for regulation of Frizzled-4 signaling by the co-receptor Tetraspanin-12.Norrin is an atypical ligand that regulates retinal angiogenesis through the Wnt/β-catenin pathway by triggering heterodimerization of Frizzled-4 (FZD4) with Low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6), leading to downstream β-catenin stabilization. Unlike Wnt ligands, Norrin requires the tetraspanin Tspan12 for signaling amplification, but it is not understood why. Here, we report a 3.4-angstrom structure of Tspan12 in complex with FZD4 determined by cryo-electron microscopy. The structure reveals that FZD4 and Tspan12 form a direct complex in the absence of Norrin, with a contact interface between the lower hinge of FZD4 and the extracellular loops of Tspan12 that enables FZD4 to respond with a more gradual response to a wider range of Norrin concentrations. The tran2025-01-01T00:00:00Z2025 Sep2026-05-08T03:16:16.218Z2026-05-08T03:10:31.191ZS-EPMC124858464104041110.1101/2025.09.25.678640