Structural basis of Wnt signaling inhibition by Dickkopf binding to LRP5/6

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) are crucial coreceptors for Wnt growth factors.
• LRP5/6 also interact with Dickkopf (Dkk) proteins, which are secreted inhibitors of Wnt signaling.
• The LRP5/6 ectodomain comprises four repeats, with distinct regions binding different Wnt variants.

Purpose of the Study:

• To elucidate the structural basis of Dkk1 inhibition of Wnt signaling via LRP5/6.
• To determine how Dkk1 interacts with different domains of the LRP5/6 ectodomain.
• To understand the overall conformation of the LRP5/6 ectodomain when bound to inhibitors or other ligands.

Main Methods:

• X-ray crystallography was used to determine the structure of the Dkk1 C-terminal domain bound to LRP6(3-4).
• Small-angle X-ray scattering (SAXS) was employed to analyze the conformation of LRP6(1-4) in complex with Dkk1 or an antibody fragment.
• Biochemical assays were used to assess the binding interactions between Dkk1, LRP5/6 domains, and Wnts.

Main Results:

• The crystal structure revealed Dkk1's C-terminal domain binding to the LRP6(3-4) repeats.
• The N-terminal domain of Dkk1 was shown to bind the LRP6(1-2) repeats, indicating a single Dkk1 molecule bridges both regions.
• SAXS analysis showed that the LRP5/6 ectodomain adopts a curved conformation, positioning the repeats similarly relative to the cell membrane regardless of ligand binding.

Conclusions:

• A single Dkk1 molecule can simultaneously bind to distinct regions of the LRP5/6 ectodomain, providing a mechanism for inhibiting diverse Wnt ligands.
• The curved conformation of the LRP5/6 ectodomain influences the spatial presentation of bound Wnts to Frizzled coreceptors.
• These findings offer insights into the regulation of Wnt signaling pathways by Dkk proteins and LRP5/6 coreceptors.