Structure-function studies of mechanisms underlying the modulation of WNT signalling by R-spondins

The WNT signalling pathway regulates patterning and morphogenesis during embryonic development and promotes the renewal of stem cells to maintain tissue homeostasis in adults. Aberrant WNT signalling also drives many types of cancer. Some WNT responses in vertebrates depend on a second signal provided by the R-spondin family of four secreted proteins, RSPO1-4. RSPOs markedly amplify target cell sensitivity to WNT ligands by neutralizing two transmembrane ubiquitin ligases, ZNRF3 and RNF43, which reduce the cell surface levels of WNT receptors. RSPOs can simultaneously engage ZNRF3/RNF43 and the 7-pass transmembrane receptors LGR4, 5 or 6 to trigger the clearance of ZNRF3/RNF43 from the cell surface, followed by lysosomal degradation. RSPO2 and RSPO3 can also engage heparan sulfate proteoglycans (HSPGs) such as glypicans or syndecans to promote ZNRF3/RNF43 clearance in the presence or absence of LGRs. In both cases, ZNRF3/RNF43 clearance results in increased WNT receptor levels at the cell surface and higher sensitivity to WNT ligands.

The molecular mechanism whereby binding of RSPOs to their LGR and/or HSPG receptors and to their ZNRF3/RNF43 effectors promotes the clearance of the trimeric or tetrameric complexes from the cell surface has remained elusive. In this project, students will combine genetic, cell biological, biochemical, biophysical and structural approaches in the laboratories of Dr. Jones at Oxford University and Dr. Lebensohn at the National Cancer Institute to elucidate the underlying mechanisms.