Scaffolding role of TcpB in disrupting TLR4-Mal interactions: Three to tango

Toll/interleukin-1 like receptors (TLRs) are membrane-spanning proteins crucially involved in innate immunity. On activation, the cytoplasmic toll/interleukin-1 receptor (TIR) domains of these receptors undergo homo- or heterodimerization. Brucella sp. are bacterial pathogens that affect the immune system by suppressing the TLR signaling pathway. They enact this by encoding a TIR domain–containing protein, TcpB, which suppresses NF-κB activation and proinflammatory cytokine secretion mediated by TLR4 receptors. TcpB has been shown to target the Mal-mediated pathway to suppress TLR signaling. The recent identification of its mechanism of interference with TLR4 signaling involving Mal prompted us to further study the structural aspects of TcpB binding with TLR4 and Mal. Our triprotein model displays the overall scaffolding role of TcpB in anchoring TLR4 and Mal thereby inhibiting their interaction leading to the attenuation of the TLR4 pathway.     

An in silico approach towards the identification of novel inhibitors of the TLR-4 signaling pathway

Precise functioning and fine-tuning of Toll-like receptor 4 (TLR4) signaling is a critical requirement for the smooth functioning of the innate immune system, since aberrant TLR4 activation causes excessive production of pro-inflammatory cytokines and interferons. This can result in life threatening conditions such as septic shock and other inflammatory disorders. The TRIF-related adaptor molecule (TRAM) adaptor protein is unique to the TLR4 signaling pathway and abrogation of TRAM-mediated TLR4 signaling is a promising strategy for developing therapeutics aimed at disrupting TRAM interactions with other components of the TLR4 signaling complex. The VIPER motif from the vaccinia virus-producing protein, A46 has been reported to disrupt TRAM-TLR4 interactions. We have exploited this information, in combination with homology modeling and docking approaches, to identify a potential binding site on TRAM lined by the BB loop and αC helix. Virtual screening of commercially available small molecules targeting the binding site enabled to short-list 12 small molecules to abrogate TRAM-mediated TLR4 signaling. Molecular dynamics and molecular mechanics calculations have been performed for the analysis of these receptor-ligand interactions.     

Association of interacting genes in the toll-like receptor signaling pathway and the antibody response to pertussis vaccination

Background

Activation of the Toll-like receptor (TLR) signaling pathway through TLR4 may be important in the induction of protective immunity against Bordetella pertussis with TLR4-mediated activation of dendritic and B cells, induction of cytokine expression, and reversal of tolerance as crucial steps. We examined whether single nucleotide polymorphisms (SNPs) in genes of the TLR4 pathway and their interaction are associated with the response to whole-cell vaccine (WCV) pertussis vaccination in 490 one-year-old children.

Methodology/Principal Findings

We analyzed associations of 75 haplotype-tagging SNPs in genes in the TLR4 signaling pathway with pertussis toxin (PT)-IgG titers. We found significant associations between the PT-IgG titer and SNPs in CD14, TLR4, TOLLIP, TIRAP, IRAK3, IRAK4, TICAM1, and TNFRSF4 in one or more of the analyses. The strongest evidence for association was found for two SNPs (rs5744034 and rs5743894) in TOLLIP that were almost completely in linkage disequilibrium, provided statistically significant associations in all tests with the lowest p-values, and displayed a dominant mode of inheritance. However, none of these single gene associations would withstand correction for multiple testing. In addition, Multifactor Dimensionality Reduction Analysis, an approach that does not need correction for multiple testing, showed significant and strong two and three locus interactions between SNPs in TOLLIP (rs4963060), TLR4 (rs6478317) and IRAK1 (rs1059703).

Conclusions/Significance

We have identified significant interactions between genes in the TLR pathway in the induction of vaccine-induced immunity. These interactions underline that these genes are functionally related and together form a true biological relationship in a protein-protein interaction network. Practically all our findings may be explained by genetic variation in directly or indirectly interacting proteins at the extra- and intracytoplasmic sites of the cell membrane of antigen-presenting cells, B cells, or both. Fine tuning of interacting proteins in the TLR pathway appears important for the induction of an optimal vaccine response.     

Lactoferrin activates macrophages via TLR4-dependent and -independent signaling pathways

Lactoferrin (LF) is a component of innate immunity and is known to interact with accessory molecules involved in the TLR4 pathway, including CD14 and LPS binding protein, suggesting that LF may activate components of the TLR4 pathway. In the present study, we have asked whether bovine LF (bLF)-induced macrophage activation is TLR4-dependent. Both bLF and LPS stimulated IL-6 production and CD40 expression in RAW 264.7 macrophages and in BALB/cJ peritoneal exudate macrophages. However, in macrophages from congenic TLR4(-/-) C.C3-Tlr4(lps-d) mice, CD40 was not expressed while IL-6 secretion was increased relative to wild-type cells. The signaling components NF-kappaB, p38, ERK and JNK were activated in RAW 264.7 cells and BALB/cJ macrophages after bLF or LPS stimulation, demonstrating that the TLR4-dependent bLF activation pathway utilizes signaling components common to LPS activation. In TLR4 deficient macrophages, bLF-induced activation of NF-kappaB, p38, ERK and JNK whereas LPS-induced cell signaling was absent. We conclude from these studies that bLF induces limited and defined macrophage activation and cell signaling events via TLR4-dependent and -independent mechanisms. bLF-induced CD40 expression was TLR4-dependent whereas bLF-induced IL-6 secretion was TLR4-independent, indicating potentially separate pathways for bLF mediated macrophage activation events in innate immunity.     

Hippo信号通路结构生物学研究进展

生物体内存在多种信号转导通路参与发育调控和组织稳态维持等重要过程,其信号异常与多种疾病特别是癌症的发生和发展密切相关。进化上高度保守的Hippo信号通路在个体发育和稳态平衡中发挥极为关键的作用。Hippo信号通路主要通过一系列相关激酶的相互作用和级联磷酸化来传递信号,能抑制细胞增殖并促进凋亡,在很多组织器官中控制细胞数量和器官大小。Hippo信号通路在一系列恶性肿瘤中出现显著异常,被认为是癌症治疗和再生医学的重要靶标。目前,Hippo信号通路中大部分关键组分已经确定,而其具体信号调控机制及功能正在完善之中。本文总结了目前已知的Hippo信号通路各蛋白成员的结构信息,重点从结构生物学角度对其信号的转导与调控机制进行分析,并对已有的Hippo信号通路靶向小分子及多肽抑制剂进行梳理,以期深化人们对该通路关键蛋白质机器的理解,并进一步促进相关的功能研究和潜在的治疗干预研发。     

Role of MyD88 in route-dependent susceptibility to vesicular stomatitis virus infection

TLRs are important components of the innate immune response. The role of the TLR signaling pathway in host defense against a natural viral infection has been largely unexplored. We found that mice lacking MyD88, an essential adaptor protein in TLR signaling pathway, were extremely sensitive to intranasal infection with vesicular stomatitis virus, and this susceptibility was dose dependent. We demonstrated that this increased susceptibility correlates with the impaired production of IFN-alpha and defective induction and maintenance of neutralizing Ab. These studies outline the important role of the TLR signaling pathway in nasal mucosae-respiratory tracts-neuroepithelium environment in the protection against microbial pathogen infections. We believe that these results explain how the route of infection, probably by virtue of activating different cell populations, can lead to entirely different outcomes of infection based on the underlying genetics of the host.     

The interferon regulatory factor, IRF5, is a central mediator of toll-like receptor 7 signaling

Interferon regulatory factors (IRFs) are critical components of virus-induced immune activation and type I interferon regulation. IRF3 and IRF7 are activated in response to a variety of viruses or after engagement of Toll-like receptor (TLR) 3 and TLR4 by double-stranded RNA and lipopolysaccharide, respectively. The activation of IRF5, is much more restricted. Here we show that in contrast to IRF3 and IRF7, IRF5 is not a target of the TLR3 signaling pathway but is activated by TLR7 or TLR8 signaling. We also demonstrate that MyD88, interleukin 1 receptor-associated kinase 1, and tumor necrosis factor receptor-associated factor 6 are required for the activation of IRF5 and IRF7 in the TLR7 signaling pathway. Moreover, ectopic expression of IRF5 enabled type I interferon production in response to TLR7 signaling, whereas knockdown of IRF5 by small interfering RNA reduced type I interferon induction in response to the TLR7 ligand, R-848. IRF5 and IRF7, therefore, emerge from these studies as critical mediators of TLR7 signaling.     

Dioscorin isolated from Dioscorea alata activates TLR4-signaling pathways and induces cytokine expression in macrophages

The Toll-like receptor 4 (TLR4)-signaling pathway is crucial for activating both innate and adaptive immunity. TLR4 is a promising molecular target for immune-modulating drugs, and TLR4 agonists are of therapeutic potential for treating immune diseases and cancers. Several medicinal herb-derived components have recently been reported to act via TLR4-dependent pathways, suggesting that medicinal plants are potential resources for identifying TLR4 activators. We have applied a screening procedure to systematically identify herbal constituents that activate TLR4. To exclude possible LPS contamination in these plant-derived components, a LPS inhibitor, polymyxin B, was added during screening. One of the plant components we identified from the screening was dioscorin, the glycoprotein isolated from Dioscorea alata. It induced TLR4-downstream cytokine expression in bone marrow cells isolated from TLR4-functional C3H/HeN mice but not from TLR4-defective C3H/HeJ mice. Dioscorin also stimulated multiple signaling molecules (NF-kappaB, ERK, JNK, and p38) and induced the expression of cytokines (TNF-alpha, IL-1beta, and IL-6) in murine RAW 264.7 macrophages. Furthermore, the ERK, p38, JNK, and NF-kappaB-mediated pathways are all involved in dioscorin-mediated TNF-alpha production. In summary, our results demonstrate that dioscorin is a novel TLR4 activator and induces macrophage activation via typical TLR4-signaling pathways.     

Structural insights of resveratrol with its binding partners in the toll-like receptor 4 pathway

The benefits associated with resveratrol (Resv; 3,4′,5-trihydroxy-trans-stilbene) are known for a long time. The therapeutic properties of Resv are observed in diseases like cancer, neurological disorders, atherosclerosis, aging, inflammation, etc. Multiple studies suggest that the beneficial properties of Resv are due to its binding to targets in multiple pathways. The same has been reflected in inflammation, where Resv has been shown to inhibit nuclear factor κ light-chain enhancer of activated B cells in the toll-like receptor 4 (TLR4) pathway. There are multiple cellular targets which bind to Resv, however the mode and the key interactions involved remain elusive for many of them. In the current work, we have investigated the structural insights of Resv with three of its binding partners involved in the inflammatory TLR4 signaling pathway. Through a structure-based modelling and molecular dynamics study, we have unraveled the molecular and atomic interactions involved in the Resv-binary complexes of inhibitor of κB kinase, cyclooxygeanse-2, and tank-binding kinase I, all three of which are key players in TLR4 inflammatory signaling. This study is the latest addition to the investigations of the structural partners of Resv and its molecular interactions.     

IKK-i signals through IRF3 and NFkappaB to mediate the production of inflammatory cytokines

IKK-i and TBK1 were recently identified as IKK-related kinases that are activated by toll-like receptors TLR3 and TLR4. These kinases were identified as essential components of the virus-activated as well as LPS-MyD88 independent kinase complex that phosphorylates IRF3 and results in the production of cytokines involved in innate immunity. Both IKK-i and TBK1 have also been implicated in the activation of the NFkappaB pathway but the precise mechanism is not clear. Although the literature to date suggests that IKK-i and TBK1 play redundant roles in TLR3 and TLR4 signaling, recent data suggest that there may be subtle differences in the signaling pathways affected by these kinases. We have generated tetracycline-inducible stable cell lines that express a wild type or kinase-inactive mutant form of IKK-i. Our data suggest that expression of IKK-i can activate both NFkappaB and IRF3, leading to the production of several cytokines including interferon beta. IKK-i most likely acts upstream of IKK2 to activate NFkappaB in these cells since expression of the kinase-inactive version of IKK-i did not inhibit TNFalpha mediated production of inflammatory cytokines. The data suggest that IKK-i is not involved in TNF-alpha mediated signaling but instead could likely play a role in activating IKK2 downstream of Toll-like receptor signaling. We also identified STAT1, Tyk2, and JAK1 as secondary mediators of IKK-i signaling as a result of interferon beta production in these cells.     

Simvastatin Inhibits Toll-like Receptor 8 (TLR8) Signaling in Primary Human Monocytes and Spontaneous Tumor Necrosis Factor Production from Rheumatoid Synovial Membrane Cultures

Simvastatin has been shown to have antiinflammatory effects that are independent of its serum cholesterol lowering action, but the mechanisms by which these antiinflammatory effects are mediated have not been elucidated. To explore the mechanism involved, the effect of simvastatin on toll-like receptor (TLR) signaling in primary human monocytes was investigated. A short pretreatment with simvastatin dose-dependently inhibited the production of tumor necrosis factor (TNF)-α in response to TLR8 activation (but not TLR2, -4 or -5). Statins are known inhibitors of the cholesterol biosynthetic pathway, but, intriguingly, TLR8 inhibition could not be reversed by addition of mevalonate or geranylgeranyl pyrophosphate, downstream products of cholesterol biosynthesis. TLR8 signaling was examined in HEK 293 cells stably expressing TLR8, where simvastatin inhibited I kappa B kinase (IKK)α/β phosphorylation and subsequent nuclear factor (NF)-κB activation without affecting the pathway to activating protein-1 (AP-1). Because simvastatin has been reported to have antiinflammatory effects in RA patients and TLR8 signaling contributes to TNF production in human RA synovial tissue in culture, simvastatin was tested in these cultures. Simvastatin significantly inhibited the spontaneous release of TNF in this model, which was not reversed by mevalonate. Together, these results demonstrate a hitherto unrecognized mechanism of simvastatin inhibition of TLR8 signaling that may in part explain its beneficial antiinflammatory effects.     

Toll-like signaling and the cytokine IL-6 regulate histone deacetylase dependent neuronal survival

Histone deacetylase (HDAC) proteins have a role in promoting neuronal survival in vitro, but the mechanism underlying this function has not been identified. Here we provide evidence that components of the neuronal microenvironment, including non-neuronal cells and defined culture media, can mitigate midbrain neuronal cell death induced by HDAC inhibitor treatment. Using microarrays we further identified gene expression changes taking place in non-neuronal cells as a result of HDAC inhibition. This analysis demonstrated that HDAC inhibitor treatment results in the down-regulation of immunity related signaling factors, in particular the Toll-like receptors (TLR). TLR signaling is active in cultured midbrain cells, yet blocking TLR receptors is not sufficient to cause neuronal cell death. In contrast, selective activation of this pathway using TLR ligands can modestly block the effects of HDAC inhibition. Furthermore, we observed that the negative effects of HDAC inhibitor treatment on neuronal survival could be more substantially blocked by the cytokine Interleukin-6 (IL-6), which is a major downstream target of TLR signaling. These data suggest that HDACs function to promote neuronal survival by activating a TLR and IL-6 dependent pathway.     

Structure and function of Toll-like receptor proteins

Beginning in 1997 with the identification of the first human homologue of the Drosophila protein Toll, a family of related molecules have been identified in both humans and other mammals. These Toll-like receptor (TLR) proteins appear to represent a conserved family of innate immune recognition receptors. TLR proteins share extended homology with receptors for the cytokines interleukin 1 (IL-1) and interleukin 18 (IL-18). These receptors are coupled to a signaling pathway that is conserved in mammals, insects, and plants, resulting in cellular activation, thereby stimulating innate immune defenses. A variety of bacterial and fungal products have been identified that serve as TLR ligands, and more recent studies have identified the first endogenous protein ligands for TLR proteins. While TLR signaling is likely to be a key feature of innate immune responses, these proteins may also regulate homeostasis via interaction with endogenous protein ligands.     

IRAK—M在TLRs信号转导中负性调节作用的研究进展

白细胞介素1受体相关激酶（interleukin-1 receptor-associatd kinase,IRAK）家族被认为是TLR／IL-1R信号通路中重要的信号分子。迄今为止,已发现4个IRAK家族成员,其中IRAK-1和IRAK．4有激酶活性,IRAK-2和IRAK-M无激酶活性。最近的研究发现IRAK-M参与负性调控TLR信号通路和具有天然免疫的作用。本文就IRAK—M的结构特点及在TLR信号转导的分子机制和免疫耐受中的作用进行综述。     

鱼类Toll样受体及其信号传导的研究进展

鱼类是脊椎动物中的一个重要类群, 在其生存与进化的过程中, 免疫系统担负着保护鱼类免受病原感染的重任, 其中Toll样受体家族等介导的先天性免疫是鱼类抗病免疫的第一道防线, 并在连接先天性免疫与获得性免疫反应中起着桥梁作用. 虽然从无脊椎动物到高等脊椎动物, Toll样受体家族内多数成员在蛋白质结构与功能上都较为保守, 但是鱼类作为最低等的脊椎动物, 在其进化过程中又形成了一些特有Toll样受体分子, 其剪接类型也更丰富; 鱼类Toll样受体家族介导的免疫识别、免疫信号传导、激活和调控方式与高等脊椎动物也不尽相同. 文章主要综述了鱼类Toll样受体的结构、种类、功能、多样性、免疫信号传导及其调控特点, 为深入了解鱼类的免疫反应奠定基础.       

Contributions of Unique Intracellular Domains to Switchlike Biosensing by Toll-like Receptor 4

Toll-like receptors (TLRs) mediate immune recognition of both microbial infections and tissue damage. Aberrant TLR signaling promotes disease; thus, understanding the regulation of TLR signaling is of medical relevance. Although downstream mediators of TLR signaling have been identified, the detailed mechanism by which ligand binding-mediated dimerization induces downstream signaling remains poorly understood. Here, we investigate this question for TLR4, which mediates responsiveness to bacterial LPS and drives inflammatory disease. TLR4 exhibits structural and functional features that are unique among TLRs, including responsiveness to a wide variety of ligands. However, the connection between these structural features and the regulation of signaling is not clear. Here, we investigated how the unique intracellular structures of TLR4 contribute to receptor signaling. Key conclusions include the following. 1) The unique intracellular linker of TLR4 is important for achieving LPS-inducible signaling via Toll/IL-1 receptor (TIR) domain-containing adapter-inducing interferon-β (TRIF) but less so for signaling via myeloid differentiation primary response 88 (MyD88). 2) Membrane-bound TLR4 TIR domains were sufficient to induce signaling. However, introducing long, flexible intracellular linkers neither induced constitutive signaling nor ablated LPS-inducible signaling. Thus, the initiation of TLR4 signaling is regulated by a mechanism that does not require tight geometric constraints. Together, these observations necessitate refining the model of TLR4 signal initiation. We hypothesize that TLR4 may interact with an inhibitory partner in the absence of ligand, via both TIR and extracellular domains of TLR4. In this speculative model, ligand binding induces dissociation of the inhibitory partner, triggering spontaneous, switchlike TIR domain homodimerization to initiate downstream signaling.     

Signaling pathway of autophagy associated with innate immunity

Autophagy has recently been shown to be an important component of the innate immune response. The signaling pathways leading to activation of autophagy in innate immunity are not well studied. Our recent study shows that Toll-like receptor 4 (TLR 4) serves as an environmental sensor for autophagy. We define a new molecular pathway in which lipopolysaccharide (LPS) induces autophagy in human and murine macrophages by a pathway regulated through Toll-interleukin 1 receptor domain-containing adaptor-inducing interferon-beta (TRIF)-dependent, myeloid differentiation factor 88 (MyD88)-independent TLR4 signaling. Receptor-interacting protein (RIP1) and p38 mitogen-activated protein-kinase (MAPK) are downstream components of this pathway. This signaling pathway does not affect cell viability, indicating that it is distinct from an autophagic death signaling pathway. We further show that LPS-induced autophagy can enhance mycobacterial co-localization with the autophagosomes. The above study raises important questions. (1) What is the complete signaling pathway for LPS-induced autophagy? (2) Does TLR3 mediate autophagy? (3) What are the mechanisms that determine whether autophagy acts as a pro-death or pro-survival pathway? (4) What are the physiological functions of LPS-induced autophagosomes? Future studies examining the above questions should provide us with important clues as to how autophagy is regulated in innate immunity, and how autophagy can be utilized in pathogen clearance.     

Poly(I-C)-induced Toll-like receptor 3 (TLR3)-mediated activation of NFkappa B and MAP kinase is through an interleukin-1 receptor-associated kinase (IRAK)-independent pathway employing the signaling components TLR3-TRAF6-TAK1-TAB2-PKR

Recent studies show that a member of the interleukin-1 (IL-1)/Toll receptor superfamily, Toll-like receptor 3 (TLR3), recognizes double-stranded RNA (dsRNA). Because of the similarity in their cytoplasmic domains, IL-1/Toll receptors share signaling components that associate with the IL-1 receptor, including IL-1 receptor-associated kinase (IRAK), MyD88, and TRAF6. However, we find that, in response to dsRNA, TLR3 can mediate the activation of both NFkappaB and mitogen-activated protein (MAP) kinases in IL-1-unresponsive mutant cell lines, including IRAK-deficient I1A and I3A cells, which are defective in a component that is downstream of IL-1R but upstream of IRAK. These results clearly indicate that TLR3 does not simply share the signaling components employed by the IL-1 receptor. Through biochemical analyses we have identified an IRAK-independent TLR3-mediated pathway. Upon binding of dsRNA to TLR3, TRAF6, TAK1, and TAB2 are recruited to the receptor to form a complex, which then translocates to the cytosol where TAK1 is phosphorylated and activated. The dsRNA-dependent protein kinase (PKR) is also detected in this signal-induced TAK1 complex. Kinase inactive mutants of TAK1 (TAK1DN) and PKR (PKRDN) inhibit poly(dI.dC)-induced TLR3-mediated NFkappaB activation, suggesting that both of these kinases play important roles in this pathway.     

TLR2 protects cisplatin-induced acute kidney injury associated with autophagy via PI3K/Akt signaling pathway

Toll-like receptors (TLRs), which are essential components of the innate immune response, play an important role in acute kidney injury (AKI). Toll-like receptor 2 (TLR2) is constitutively expressed in tubular epithelial cells of the kidney and participates in cisplatin-induced AKI. The autophagy is a dynamic catabolic process that maintains intracellular homeostasis, which is involved in the pathogenesis of AKI. Recent studies demonstrate that PI3K/Akt signaling pathway regulates autophagy in response to various stimuli. Therefore, we propose that cisplatin might activate TLR2, which subsequently phosphorylates PI3K/Akt, leading to enhanced autophagy of renal tubular epithelial cells and protecting cisplatin-induced AKI. We found that TLR2 expression was significantly increased in the kidney after the cisplatin treatment. TLR2-deficient mice exacerbated renal injury in cisplatin-induced AKI, with higher serum creatinine and blood urea nitrogen, more severe morphological injury compared with that of wild-type mice. In vitro, we found that inhibition of TLR2 reduced tubular epithelial cell autophagy after the cisplatin treatment. Mechanistically, TLR2 inhibited autophagy via activating PI3K/Akt signaling pathway in renal tubular epithelial cells after the cisplatin treatment. Take together, these results suggest that TLR2 may protect cisplatin-induced AKI by activating autophagy via PI3K/Akt signaling pathway.