1: Eukaryotic cells chemotax in a wide range of chemoattractant concentration gradients, and thus need inhibitory processes that terminate cell responses to reach adaptation while maintaining sensitivity to higher-concentration stimuli. However, the molecular mechanisms underlying inhibitory processes are still poorly understood. Here, we reveal a locally controlled inhibitory process in a GPCR-mediated signaling network for chemotaxis in Dictyostelium discoideum. We discover a novel negative regulator of Ras signaling, C2GAP1, which localizes at the leading edge of chemotaxing cells, is activated by and is essential for GPCR-mediated Ras adaptation. We show that both C2 and GAP domains are required for the membrane targeting of C2GAP1, and that GPCR-triggered Ras activation recruits C2GAP1 from cytosol and retains it on the membrane to locally inhibit Ras signaling. The altered Ras activation results in impaired gradient sensing and excessive polymerization of F-actin in c2gap1 knockout (c2gap1-) cells, leading to chemotaxis defects. Remarkably, c2gap1- cells display altered cell response, impaired directional sensing, and chemotaxis defects in a chemoattractant concentration-dependent fashion. Thus, we have uncovered a novel inhibitory mechanism required for the adaptation and long-range chemotax.(Xu et al., in revision). 2: Human phagocytes, including neutrophils and macrophage, are an essential part of innate immunity. Upon bacterial infection, neutrophils leaves circulation and migration to infection sites to fight pathogens. They seek bacteria by detecting chemoattractants generated from the bacterial infection site and chase them via chemotaxis. Once reaching the site, they bind and ingest bacteria by recognizing signals form the bacterial surface via phagocytosis. Dictyostelium discoideum amoeba are professional phagocytes that track down bacteria by chemotaxis and capture and ingest them as food through phagocytosis. Studies in the simple model organism of D. discoideum have made tremendous contribution to our current understanding of molecular mechanisms underlying chemotaxis and phagocytosis of phagocytes. Recently, we discovered that D. discoideum amoeba use a chemoattractant GPCR fAR1 to detect folic acid released from bacteria for both chemotaxis to catch bacteria and phagocytosis to ingest them (Pan et al, 2016). This finding suggest to us that a chemoattractant GPCR-mediated signaling network controls reorganization of the actin cytoskeleton for both chemotaxis and phagocytosis, which represents a paradigm-shifting new concept in immunology. Thus, investigation of molecular components involved in chemotaxis and (or) phagocytosis in D. discoideum will continually shed light on the molecular mechanisms controlling migration of immune cells and as well as phagocytosis by immune cells to eliminate bacterial pathogens from human body. Recently, we discovered that formyl-peptide receptors (fPR GPCR) coupled with heterotrimeric Gi proteins mediate chemotaxis as well as phagocytosis of fMLP-coated particles and E. coli. Our studies revealed an evolutionarily conserved mechanism that directs professional phagocytes migrating toward bacteria via chemotaxis and promotes them to engulf bacterial via surface phagocytosis as an essential part of innate immunity.
Showing the most recent 10 out of 21 publications
