| Summary: | Cell-cell communication involving the Wnt family of secreted signalling molecules is fundamental to animal development and homeostasis, whilst dysregulation of Wnt signalling causes many human diseases including cancer and osteoporosis. Low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) co-operates with members of the Frizzled family of seven- pass transmembrane proteins to transduce Wnt signalling across the cell membrane. This thesis first reviews the Wnt signalling pathway from a molecular perspective, and then describes the methods employed in this doctoral work. Next, structural and functional studies of the LRP6 extracellular domain that reveal a cell surface platform for Wnt signalling are reported. Finally, this thesis presents results preliminary to the structural characterisation of the Dickkopf (Dkk) family of secreted Wnt modulators and their complexes with LRP receptors. The LRP6 ectodomain comprises four tandem six- bladed Tyr- Trp- Thr-Asp B- propeller-epidermal growth factor-like domain (PE) pairs which harbour binding sites for Wnt morphogens and their antagonists including Dkkl. To understand how these multiple interactions are integrated, crystallographic analysis of the third and fourth PE pairs was combined with electron microscopy to determine the complete ectodomain structure. An extensive inter-pair interface, conserved for the first-to-second and third-to-fourth PE interactions, contributes to a compact platform-like architecture, which is disrupted by mutations implicated in developmental diseases. Electron microscopy reconstruction of the LRP6 platform bound to chaperone Mesd (mesoderm development) exemplifies a binding mode spanning PE pairs. Cellular ar d binding assays identify overlapping Wnt3a- and Dkkl- binding surfaces on the third PE pair, consistent with steric competition, but also suggest a model in which the platform structure supports interplay of ligands through multiple interaction sites. The major discoveries of this work have been published as a research article in the journal Developmental Cell.
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