Microbial Pattern Recognition and Signaling by the Adhesion GPCR BAI1

描述（由申请人提供）：先天免疫系统形成了针对入侵微生物病原体的特异性宿主防御的第一道防线。固有免疫应答的组成部分是模式识别受体（PRR），其识别由广泛的病原微生物表达的保守决定簇。其中包括Toll样受体，其响应于病原体识别而促进炎症信号传导和吞噬细胞募集，以及吞噬PRR的几个家族，包括清道夫受体和C型凝集素受体，其介导微生物从感染组织的吞噬清除。每个受体家族都包含多个成员，这些成员识别独特的微生物组分谱，并且它们共同提供针对广泛的感染性生物体的保护。我们已经确定了一种新的模式识别受体，BAI 1，这是在巨噬细胞上表达，并识别革兰氏阴性菌的表面脂多糖（LPS）使用的血小板反应蛋白重复序列（TSRs）在其细胞外结构域的阵列。在几种分泌蛋白质中的TSR已被证明结合微生物产物，但这是跨膜受体使用TSR介导特定病原体的吞噬清除的第一个证据。细菌与BAI 1的结合通过一种机制触发吞噬作用，该机制涉及通过与BAI 1直接相互作用的埃尔莫/Dock 180 GEF复合物激活小GTbac Rac。一个新兴的主题是，模式识别受体往往相互合作，以增强下游信号反应。我们的初步数据表明，BAI 1与Toll样受体TLR 4协同作用，促进炎症细胞因子/趋化因子的合成。BAI 1是一种孤儿GPCR样受体，除了它激活Rac的能力外，几乎没有人知道它如何将信号传递到细胞内部或哪些信号通路响应于BAI 1连接而被激活。这项研究的总体目标是确定BAI 1在宿主防御细菌感染中的作用。在目标1中，我们将使用生物化学、生物物理学和结构方法的组合来定义LPS与BAI 1 TSRs的相互作用模式。在目标2中，我们将确定与BAI 1相互作用的信号机制的组件，定义响应于LPS或细菌激活受体而激活的信号通路，并阐明BAI 1和TLR 4之间的串扰机制。在目标3中，我们将使用沙门氏菌感染的小鼠模型来表征BAI 1在体内宿主防御中的作用。这些研究将为一类新的模式识别受体提供独特的结构，生物化学和生理学见解，这些受体可能在对细菌病原体的先天免疫反应中具有重要功能。

DESCRIPTION (provided by applicant): The innate immune system forms the first line of specific host defense against invading microbial pathogens. Integral to the innate immune response are Pattern Recognition Receptors (PRRs) that recognize conserved determinants expressed by a broad range of pathogenic microorganisms. Among these are the Toll-like receptors, which promote inflammatory signaling and phagocyte recruitment in response to pathogen recognition, and several families of phagocytic PRRs, including scavenger receptors and C-type lectin receptors that mediate phagocytic clearance of microbes from infected tissues. Each receptor family contains multiple members that recognize a unique spectrum of microbial components, and collectively they provide protection against a wide range of infectious organisms. We have identified a novel pattern recognition receptor, BAI1, that is expressed on macrophages and recognizes the surface lipopolysaccharide (LPS) of Gram-negative bacteria using an array of thrombospondin repeats (TSRs) in its extracellular domain. TSRs in several secreted proteins have been shown to bind microbial products, but this is the first evidence of a transmembrane receptor that uses TSRs to mediate the phagocytic clearance of specific pathogens. Binding of bacteria to BAI1 triggers phagocytosis through a mechanism involving activation of the small GTPase Rac by the Elmo/Dock180 GEF complex, which interacts directly with BAI1. An emerging theme is that pattern recognition receptors often cooperate with each other to enhance downstream signaling responses. Our preliminary data suggest that BAI1 acts synergistically with the Toll-like receptor TLR4 to promote the synthesis of inflammatory cytokines/chemokines. BAI1 is an orphan GPCR-like receptor and apart from its ability to activate Rac, almost nothing is known about how it transmits signals to the cell interior or which signaling pathways are activated in response to BAI1 ligation. The overall goal of the proposed research is to define the role of BAI1 in host defense against bacterial infection. In Aim 1, we will define the mode of interaction of LPS with the BAI1 TSRs, using a combination of biochemical, biophysical and structural approaches. In Aim 2, we will identify components of the signaling machinery that interact with BAI1, define the signaling pathways activated in response to LPS or bacterial activation of the receptor, and elucidate the mechanisms of crosstalk between BAI1 and TLR4. In Aim 3, we will characterize the role of BAI1 in host defense in vivo, using a mouse model of Salmonella infection. Together these studies will provide unique structural, biochemical and physiological insights into a new class of pattern recognition receptor that may have important functions in the innate immune response to bacterial pathogens.