Mechanisms of Signaling on Membrane Surfaces

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SUMMARY At the leading edge of a polarized macrophage, a membrane-based chemotaxis pathway directs cell mi- gration up attractant gradients to sites of infection, inflammation, or tissue damage. Upon arrival, a phagocyto- sis pathway controls the formation and internalization of a phagosome in which the pathogens or damaged tis- sue are engulfed and destroyed. Both the chemotaxis and phagocytosis pathways are regulated by PI3K lipid kinases that serve as regulatory hubs by integrating Ca2+, receptor, G protein, and other input signals while phosphorylating substrate lipids to produce output lipid signals. The potent lipid signals, in turn, activate multi- ple downstream protein kinases. In chemotaxis, the lipid signal controls actin and membrane remodeling to drive the leading edge up the attractant gradient. In phagocytosis, the lipid signal controls processing of the phagosome including the production of reactive oxygen species (ROS) to inactivate pathogens. Closely related PI3K pathways regulate other cell processes, notably including cell growth. When dysregulated, PI3K path- ways trigger or exacerbate a wide array of human diseases ranging from cancer and developmental disorders to defects in innate immunity, inflammation or autoimmunity. The two classes of PI3K lipid kinases targeted by this research program are Class 1 PI3-Kinases (PI3K1) that generate the signaling lipid PIP3 at the leading edge membrane of polarized macrophages, and Class 3 PI3-Kinases (PI3K3, specifically PI3K3 Complex II) that produce PI3P on the surface of the phagosome. The proposed research seeks to understand the regulation of both pathways by addressing fundamental, broad questions including: (i) How do PI3K1 and PI3K3 regulatory hubs integrate multiple inputs from Ca2+ channels, receptors, G proteins and other effectors, and do these inputs combine in additive, synergistic, or opposing fashions? (ii) How do the resulting PIP3 and PI3P output lipids activate downstream protein kinases, including some of the most important master kinases in the cell? (iii) How do drugs, potential therapeutics, and disease- linked mutations inhibit or superactivate key components and reaction steps to generate pathway perturbation or dysregulation? To answer these and other questions, the PI's laboratory has developed a unique, two-pronged approach combining innovative, in vitro single molecule methods with live cell imaging studies. The in vitro studies utilize single molecule TIRF to elucidate signaling mechanisms in a subsection of the pathway, or signaling module, that is reconstituted on a supported lipid bilayer under near physiological conditions. The live cell studies em- ploy fluorescent sensors and cell imaging to test key predictions of the in vitro mechanistic model for relevance in the cellular context. The PI has a strong track record and continues to play leadership roles in his research field, as well as the university and scientific communities. Overall, this research program is well positioned to continue generating fundamental advances with significant impacts on signaling biology and medicine.