Role of PIKfyve in macrophage and neutrophil immune response

Solid particles such as pathogens, dying cells, and debris are engulfed by macrophages and neutrophils and sequestered into a phagosome. Phagosomes fuse with early and late endosomes and ultimately with lysosomes to mature into phagolysosomes, a process known as phagosome maturation. The formation of highly acidic and degradative phagolysosomes plays an important role in degradation of the internalized particle. We employed siRNA and pharmacological tools to demonstrate that phosphatidylinositol-3,5-bisphosphate [PI(3,5)P2], synthesized by the PIKfyve lipid kinase, is required for phagosome maturation. However, the mechanism by which PI(3,5)P2 controls phagosome maturation remained uncharacterized. We hypothesized that PI(3,5)P2 may control phagosome-lysosome fusion partly by stimulating TRPML1, a lysosomal Ca2+ channel gated by PI(3,5)P2. Upon opening of the channel, lysosomal Ca2+ would diffuse and trigger phagosome-lysosome fusion since Ca2+ is known to induce membrane fusion post-docking of SNARE proteins. In addition, we also demonstrated that the lipid kinase PIKfyve coordinates the neutrophils immune response by controlling phagosome maturation and regulating Rac GTPase activity. PIKfyve produces both PI(3,5)P2 and phosphatidylinositol-5-phosphate (PI5P); therefore, it might control phagosome maturation through production of PI(3,5)P2 and activation of TRPML1 as well as regulates ROS production and chemotaxis through synthesis of PI5P, which leads to the activation of Tiam1, and Rac GTPase.