The mechanisms underlying the GPCR-mediated chemotaxis in D. discoideum

1：真核细胞在广泛的化学引诱剂浓度梯度下趋化，因此需要抑制过程来终止细胞反应以达到适应，同时保持对高浓度刺激的敏感性。然而，抑制过程的分子机制仍然知之甚少。在这里，我们揭示了gpcr介导的Dictyostelium disideum趋化性信号网络中的局部控制抑制过程。我们发现了一种新的Ras信号负调控因子C2GAP1，它位于趋化细胞的前沿，被gpcr介导的Ras适应激活并对其至关重要。我们发现C2GAP1的膜靶向需要C2和GAP结构域，并且gpcr触发的Ras激活从细胞质中招募C2GAP1并将其保留在膜上以局部抑制Ras信号传导。Ras激活的改变导致c2gap1敲除（c2gap1-）细胞中梯度感知受损和F-actin过度聚合，导致趋化性缺陷。值得注意的是，c2gap1-细胞表现出细胞反应改变、定向传感受损和趋化性缺陷，这些缺陷以趋化剂浓度依赖的方式存在。因此，我们发现了一种新的抑制机制，需要适应和长期的化学税。（Xu et al.，在修订中）。

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.