Sending and transducing Hedgehog and Wnt signals - Project summary The main focus of our lab is on deciphering key mechanisms involved in cell-cell signaling through the vertebrate Hedgehog (Hh) and Wnt signaling pathways, which are essential for embryogenesis, adult stem cell maintenance, and are deeply involved in cancer and birth defects. A secondary focus is the development of chemical probes for various biological molecules (DNA, RNA, proteins, lipids) in cells and tissues; these probes are critical in our work on cell signaling, but they are useful beyond. In Hh and Wnt signaling, we primarily study the events at or near cellular membranes, because of the larger questions still unanswered in this area. We are interested in understanding how cells send Hh and Wnt signals (proteins referred to as Hh and Wnt ligands), how the signals travel to faraway cells, and how the target cells receive and interpret these signals. Both Hh and Wnt ligands are attached to lipids, a feature that makes them stick to cellular membranes, raising the question of how they are released from producing cells. The same lipids are also essential for the signaling activity of the ligands, by mediating binding to receptors on target cells; however, it has been unknown how the lipid-modified ligands are delivered to their receptors and how they activate signaling. In recent work, we discovered that both Hh and Wnt ligands have dedicated release pathways, resulting in the formation of complexes between ligands and specific carrier proteins, that shield the water-repelling lipids from the extracellular environment. We found dedicated pathways that receive these carrier-ligand complexes on the surface of target cells, to allow ligand-receptor binding. We also discovered a unique mechanism by which cholesterol plays a critical role in Hh signaling, and built novel chemical tools to study how cholesterol functions in this context. Our findings open several fundamental questions, which we propose to answer in the next 5 years, using a combination of biochemistry, structural, chemical and cell biology. We propose to elucidate how the lipid modifications of Hh and Wnt ligands are handled during the packaging of carrier-ligand complexes, which is currently unknown. We will also determine how the target cells unpack the carrier-ligand complexes, and how they promote ligand-receptor binding, to trigger signaling. Finally, we will dissect the role of cholesterol in Hh signaling, which has remained mysterious. These studies are important for the following reasons: A) They will advance understanding of the Hh and Wnt pathways, by elucidating critical molecular mechanisms involved in signaling; B) They will identify novel targets for blocking oncogenic Hh and Wnt signaling; and C) Our novel chemical probes will be broadly applicable to study cholesterol in health and in disease.
