A novel role for the TLR signaling adaptor TRAM

描述(申请人提供)：Toll样受体(TLRs)是一种膜受体，其胞外结构域可以识别入侵微生物(如内毒素、CpG等)释放的保守分子。TLR结合提醒宿主注意潜在病原体的存在，并激活先天免疫。TLR信号通路的遗传突变可导致免疫缺陷和免疫病理。TLRs通过与细胞质适配器相互作用来向细胞发出信号，后者招募下游信号分子，并导致促炎细胞因子和I型干扰素的诱导。目前的TLR范式认为，除TLR3外，所有13种哺乳动物的TLR都通过接头MyD88向宿主细胞发出信号，而TLR3通过另一种接头TRIF通过“MyD88不依赖”的途径发出信号。TLR4也可以通过MyD88不依赖的途径发出信号，因此通过交替使用两个不同的桥接接头之一：TIRAP/MAL来招募MyD88或TRAM来招募TRIF，从而通过MyD88和TRIF来唯一地发出信号。在最近对革兰氏阴性杆菌的免疫反应的研究中，我们在体内发现了一条新的信号通路，它需要TRAM，但似乎不涉及“MyD88非依赖的”TRIF信号通路。我们在单核细胞增多性李斯特菌感染的小鼠模型中观察到了类似的途径。这些结果令人惊讶，因为TRAM唯一已知的功能是在经典的MyD88非依赖途径中连接TLR4和TRIF。重要的是，TLR2，而不是TLR4，是识别和控制图拉氏丝虫病和单核细胞增多症小鼠感染所必需的。因此，根据我们的初步研究，我们认为TRAM可以将TLR4以外的TLR(如TLR2)与免疫激活联系起来，从而在TLR信号转导中发挥比已知更广泛的作用。为了验证这一假设，我们将1)阐明TRAM依赖的、TLR4不依赖的信号转导调节肺图拉热症小鼠宿主保护反应的机制(S)，2)确定TRAM依赖的新的信号通路的分子组成、细胞内定位和下游靶点。总之，这些研究将为一个重要的TLR信号分子定义一个新的角色，揭示TLR特异性是如何控制的，并扩展目前的TLR范式。拟议的研究具有创新性，因为它们将为关键的TLR信号分子定义新的角色，并修改当前的TLR范式。它们具有重要意义，因为该途径是体内关键的免疫激活途径，其阐明可能为TLR信号转导和促炎作用的治疗操作提供新的靶点。

DESCRIPTION (provided by applicant): The Toll-like receptors (TLRs) are membrane receptors whose extracellular domains can recognize conserved molecules released from invading microbes (e.g. LPS, CpG, etc). TLR binding alerts the host to the presence of potential pathogens and activates innate immunity. Inherited mutations in TLR signaling pathways can lead to immunodeficiency and immunopathology. TLRs signal cells by interacting with cytoplasmic adaptors that recruit downstream signaling molecules and lead to induction of proinflammatory cytokines and type I interferons. The current TLR paradigm asserts that all 13 mammalian TLRs, except TLR3, signal host cells via the adaptor MyD88, whereas TLR3 signals through a "MyD88-independent" pathway via the alternate adaptor TRIF. TLR4 can also signal via the MyD88-independent pathway and thus uniquely signals through both MyD88 and TRIF by alternate use of one of two different bridging adaptors: TIRAP/Mal to recruit MyD88 or TRAM to recruit TRIF. In recent studies of the immune response to the Gram-negative bacterium Francisella tularensis, we have uncovered a novel signaling pathway in vivo that requires TRAM but does not appear to involve the "MyD88-independent" TRIF signaling pathway. We have observed a similar pathway in a mouse model of Listeria monocytogenes infection. These results were surprising since TRAM's only known function has been to bridge TLR4 to TRIF in the classic MyD88-independent pathway. Importantly, TLR2, and not TLR4, is required for recognition and control of F. tularensis and L. monocytogenes infections in mice. Based on our preliminary studies, therefore, we propose that TRAM can link TLRs other than TLR4 (e.g. TLR2) to immune activation, and thus plays a broader role in TLR signaling than previously known. To test this hypothesis, we will 1) elucidate the mechanism(s) by which TRAM-dependent, TLR4- independent signaling regulates the host protective response in a mouse model of pulmonary tularemia, and 2) define the molecular components, intracellular location and downstream targets of the novel TRAM-dependent signaling pathway. Together, these studies will define a new role for an important TLR signaling molecule, shed new light on how TLR specificity is controlled, and expand the current TLR paradigm. The proposed studies are innovative because they will define a new role for a key TLR signaling molecule and modify the current TLR paradigm. They are significant because this pathway is a critical immune activation pathway in vivo, and its elucidation may provide new targets for therapeutic manipulation of TLR signaling and the pro-inflammatory.