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

1：真核细胞在广泛的趋化剂浓度梯度中产生化学税，因此需要抑制过程来终止细胞反应以达到适应，同时保持对较高浓度刺激的敏感性。然而，抑制过程背后的分子机制仍然知之甚少。在这里，我们揭示了一个局部控制的抑制过程中的信号网络的gpr介导的趋化性盘基网柄菌。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适应激活并对其起着至关重要的作用。我们发现C2和GAP结构域都是C2GAP1膜靶向所必需的，GPCR触发的RAS激活从胞浆中招募C2GAP1并将其保留在膜上，以局部抑制RAS信号转导。Ras激活的改变导致c2gap1基因敲除(c2gap1-)细胞的梯度感觉受损和F-肌动蛋白过度聚合，导致趋化缺陷。值得注意的是，c2gap1-细胞以一种浓度依赖的方式表现出细胞反应改变、定向感觉受损和趋化缺陷。因此，我们发现了一种新的适应和远程化学税所需的抑制机制(Xu等人，在修订中)。 2：人的吞噬细胞，包括中性粒细胞和巨噬细胞，是天然免疫的重要组成部分。在细菌感染后，中性粒细胞离开循环并迁移到感染部位以对抗病原体。他们通过检测细菌感染部位产生的趋化性物质来寻找细菌，并通过趋化性来追逐它们。一旦到达这个部位，它们就会通过吞噬作用识别细菌表面的信号，从而结合并摄取细菌。盘基鞭毛虫是一种专业的吞噬细胞，它通过趋化作用追踪细菌，并通过吞噬作用捕获和摄取细菌作为食物。对盘形藻这一简单模式生物的研究对我们目前理解吞噬细胞趋化和吞噬的分子机制做出了巨大的贡献。最近，我们发现Discoideum阿米巴使用一种趋化剂GPCRfAR1来检测细菌释放的叶酸，这既是为了趋化捕获细菌，也是为了吞噬细菌(潘等人，2016)。这一发现向我们表明，趋化因子GPCR介导的信号网络控制着肌动蛋白细胞骨架的趋化和吞噬重组，这代表了免疫学中一个改变范式的新概念。因此，对盘状芽孢杆菌趋化和(或)吞噬作用的分子成分的研究将继续揭示控制免疫细胞迁移和免疫细胞吞噬作用以消除人体内细菌病原体的分子机制。最近，我们发现甲酰肽受体(FPR)与异三聚体GI蛋白偶联，介导了fMLP包被颗粒和大肠杆菌的趋化和吞噬作用。我们的研究揭示了一种进化上保守的机制，即引导专业吞噬细胞通过趋化作用向细菌迁移，并促进它们通过表面吞噬作用吞噬细菌，这是天然免疫的重要组成部分。

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.