Microbial Pattern Recognition and Signaling by the Adhesion GPCR BAI1

描述(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定宿主防御的第一道防线。模式识别受体(PRRs)是先天性免疫反应的组成部分，它识别由广泛的病原微生物表达的保守决定因素。其中包括Toll样受体，它促进炎症信号和吞噬细胞募集，以响应病原体识别；以及几个吞噬PRR家族，包括清道夫受体和C型凝集素受体，它们介导对感染组织中微生物的吞噬清除。每个受体家族包含多个成员，它们识别独特的微生物成分，它们共同提供保护，以抵御广泛的感染性生物。我们已经鉴定出一种新的模式识别受体BAI1，它表达在巨噬细胞上，并利用其胞外区的一系列凝血酶敏感蛋白重复序列(TSRs)识别革兰氏阴性细菌的表面脂多糖(LPS)。几种分泌蛋白中的TSRs已被证明与微生物产物结合，但这是第一次有跨膜受体的证据，它使用TSRs来介导特定病原体的吞噬清除。细菌与BAI1的结合通过一种机制触发吞噬作用，该机制涉及Elmo/Dock180环境基金复合体激活小GTP酶Rac，该复合体直接与BAI1相互作用。一个新兴的主题是，模式识别受体经常相互合作，以增强下游信号反应。我们的初步数据表明，BAI1与Toll样受体TLR4协同作用，促进炎性细胞因子/趋化因子的合成。BAI1是一种孤立的GPCR样受体，除了激活RAC的能力外，几乎不知道它是如何向细胞内部传递信号的，也不知道BAI1连接后激活了哪些信号通路。这项拟议研究的总体目标是确定BAI1在宿主抵御细菌感染中的作用。在目标1中，我们将使用生化、生物物理和结构方法相结合的方法来定义内毒素与BAI1 TSR的相互作用模式。在目标2中，我们将确定与BAI1相互作用的信号机制的组件，定义响应内毒素或细菌激活受体的信号通路，并阐明BAI1和TLR4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BAI1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、生化和生理见解，这种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.