Distinct structural mechanisms of LGR4 modulation by Norrin and RSPOs in Wnt/β-catenin signaling

Area of Science:

• Molecular biology
• Structural biology
• Cell signaling

Background:

• The Wnt/β-catenin pathway is crucial for development and homeostasis.
• Precise regulation of this pathway is essential, but structural mechanisms are not fully understood.
• Leucine-rich repeat-containing G-protein coupled receptor 4 (LGR4) plays a key role in modulating Wnt signaling.

Purpose of the Study:

• To elucidate the structural basis of LGR4's differential recognition of Norrin and R-spondins (RSPOs).
• To understand how LGR4's distinct binding modes lead to varied Wnt signaling outcomes.
• To identify potential therapeutic targets for Wnt-related diseases.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of LGR4 bound to Norrin.
• Structural analysis of the LGR4-Norrin complex and comparison with the LGR4-RSPO2-ZNRF3 complex.
• Mapping of disease-linked mutations to functional regions of LGR4.

Main Results:

• A distinct 2:2 LGR4-Norrin complex structure was revealed, different from the LGR4-RSPO complex.
• Norrin binding to LGR4 sterically hinders Frizzled4 receptor interaction, creating a signaling checkpoint.
• Mutations affecting LGR4 binding are linked to familial exudative vitreoretinopathy (FEVR), while Frizzled4 interaction mutations are linked to Norrie disease.
• A nanobody blocking both Norrin and RSPO binding to LGR4 was developed.

Conclusions:

• LGR4 achieves differential signaling through distinct recognition of Norrin and RSPOs.
• Mutually exclusive interactions mediated by LGR4's binding sites drive specific signaling pathways.
• Structural insights into LGR4 function provide a foundation for therapeutic strategies in Wnt-related disorders.