Drosophila melanogaster NPC2 proteins bind bacterial cell wall components and may function in immune signal pathways

ML (MD-2 (myeloid differentiation factor-2)-related Lipid-recognition) is a conserved domain identified in MD-2, MD-1, NPC2 (Niemann-Pick disease type C2), and mite major allergen protein from animals, plants, and fungi. Vertebrate members of the ML family proteins, such as NPC2 and MD-2, play important roles in lipid metabolism and immune signaling pathway. MD-2 is an essential co-receptor in the lipopolysaccharide (LPS)/Toll-like receptor 4 (TLR4) signaling pathway. Insects contain multiple ML genes, arbitrarily named md-2- or npc2-like genes. However, whether insect ML genes have functions similar to vertebrate md-2 is unknown. In Drosophila melanogaster, there are eight npc2 genes (npc2a-h), and they can be further divided into three subgroups based on the numbers of cysteine residues (6, 7 and 8 Cys) in the mature proteins. The purpose of this study is to investigate whether any Drosophila npc2 genes may have functions in immune signaling pathways. We chose npc2a, npc2e and npc2h genes representing the three subgroups for this study. We showed that recombinant NPC2a, NPC2e and NPC2h not only bound to LPS and lipid A, but also bound to peptidoglycan (PG) and lipoteichoic acid (LTA), a property that has not been reported previously for vertebrate NPC2 or MD-2. More importantly, we showed that over-expression of NPC2a and NPC2e activated diptericin promoter reporter in S2 cells stimulated by PG, suggesting that NPC2e and NPC2a may play a role in the immune deficiency (Imd) pathway. This is the first in vitro study about NPC2 proteins in innate immunity of D. melanogaster.     

MD-2 binds to bacterial lipopolysaccharide

The exact roles and abilities of the individual components of the lipopolysaccharide (LPS) receptor complex of proteins remain unclear. MD-2 is a molecule found in association with toll-like receptor 4. We produced recombinant human MD-2 to explore its LPS binding ability and role in the LPS receptor complex. MD-2 binds to highly purified rough LPS derived from Salmonella minnesota and Escherichia coli in five different assays; one assay yielded an apparent KD of 65 nm. MD-2 binding to LPS did not require LPS-binding proteins LBP and CD14; in fact LBP competed with MD-2 for LPS. MD-2 enhanced the biological activity of LPS in toll-like receptor 4-transfected Chinese hamster ovary cells but inhibited LPS activation of U373 astrocytoma cells and of monocytes in human whole blood. These data indicate that MD-2 is a genuine LPS-binding protein and strongly suggest that MD-2 could play a role in regulation of cellular activation by LPS depending on its local availability.     

ML -- a conserved domain involved in innate immunity and lipid metabolism

ML (MD-2-related lipid-recognition) is a novel domain identified in MD-1, MD-2, GM2A, Npc2 and multiple proteins of unknown function in plants, animals and fungi. These single-domain proteins were predicted to form a beta-rich fold containing multiple strands, and to mediate diverse biological functions through interacting with specific lipids.     

The functional and structural properties of MD-2 required for lipopolysaccharide binding are absent in MD-1

MD-1 and MD-2 are secretory glycoproteins that exist on the cell surface in complexes with transmembrane proteins. MD-1 is anchored by radioprotective 105 (RP105), and MD-2 is associated with TLR4. In vivo studies revealed that MD-1 and MD-2 have roles in responses to LPS. Although the direct binding function of MD-2 to LPS has been observed, the physiological function of MD-1 remains unknown. In this study, we compared the LPS-binding functions of MD-1 and MD-2. LPS binding to cell surface complexes was detected for cells transfected with TLR4/MD-2. In contrast, binding was not observed for RP105/MD-1-transfected cells. When rMD-2 protein was expressed in Escherichia coli, it was purified in complexes containing LPS. In contrast, preparations of MD-1 did not contain LPS. When rMD-2 protein was prepared in a mutant strain lacking the lpxM gene, LPS binding disappeared. Therefore, the secondary myristoyl chain attached to the (R)-3-hydroxymyristoyl chain added by LpxM is required for LPS recognition by MD-2, under these conditions. An amphipathic cluster composed of basic and hydrophobic residues in MD-2 has been suggested to be the LPS-binding site. We specifically focused on two Phe residues (119 and 121), which can associate with fatty acids. A mutation at Phe(191) or Phe(121) strongly reduced binding activity, and a double mutation at these residues prevented any binding from occurring. The Phe residues are present in MD-2 and absent in MD-1. Therefore, the LPS recognition mechanism by RP105/MD-1 is distinct from that of TLR4/MD-2.     

Functional activity of MD-2 polymorphic variant is significantly different in soluble and TLR4-bound forms: decreased endotoxin binding by G56R MD-2 and its rescue by TLR4 ectodomain

MD-2 is an essential component of endotoxin (LPS) sensing, binding LPS independently and when bound to the ectodomain of the membrane receptor TLR4. Natural variation of proteins involved in the LPS-recognition cascade such as the LPS-binding protein, CD14, and TLR4, as well as proteins involved in intracellular signaling downstream of LPS binding, affect the cellular response to endotoxin and host defense against bacterial infections. We now describe the functional properties of two nonsynonymous coding polymorphisms of MD-2, G56R and P157S, documented in HapMap. As predicted from the MD-2 structure, the P157S mutation had little or no effect on MD-2 function. In contrast, the G56R mutation, located close to the LPS-binding pocket, significantly decreased cellular responsiveness to LPS. Soluble G56R MD-2 showed markedly reduced LPS binding that was to a large degree rescued by TLR4 coexpression or presence of TLR4 ectodomain. Thus, cells that express TLR4 without MD-2 and whose response to LPS depends on ectopically produced MD-2 were most affected by expression of the G56R variant of MD-2. Coexpression of wild-type and G56R MD-2 yielded an intermediate phenotype with responses to LPS diminished to a greater extent than that resulting from expression of the D299G TLR4 polymorphic variant.     

髓样分化蛋白-2在天然免疫中的作用

Toll样受体 (Toll likereceptor ,TLR)家族作为模式识别受体 ,在天然免疫中具有重要作用。髓样分化蛋白 2 (myeloiddifferentialprotein 2 ,MD 2 )可能含有两个相对独立的功能结构域 ,既能与Toll样受体家族中的TLR4、TLR2结合 ,也能与多种配体结合 (包括lipopolysaccharide ,LPS)。这种特殊的结构可能与其三方面的主要功能有关 :(1)MD 2与TLR4结合 ,赋予TLR4对各种配体 (包括LPS)的反应性 ;(2 )MD 2与TLR2结合 ,赋予TLR2对LPS的反应性 ,并增强TLR2对细菌及其胞壁成分的反应性 ;(3)MD 2能促进TLR4和TLR2的表达 ,并且与TLR4在细胞内的分布密切相关。这表明MD 2可以通过两种方式直接或间接调控TLRs的功能 :与TLR2 /TLR4结合 ,或调控TLR2 /TLR4的表达与分布。因而MD 2不仅仅是TLR4的辅助分子 ,而且还是天然免疫中的调控分子 ,可能在感染、炎症、免疫等病理生理过程中具有更广泛的生物学功能     

利用荧光共振能量转移技术研究活细胞TLR4与MD-2作用结构域

脂多糖(LPS)的识别和信号转导是宿主发生防御反应的关键,Toll样受体4(TLR4)与髓样分化蛋白-2(MD-2)形成复合物在LPS的识别及其信号转导中发挥了重要作用．研究TLR4与MD-2结合的功能结构域,对于深入了解LPS信号转导机制及其内毒素休克的防治具有重要意义．运用基于强度的三通道荧光共振能量转移技术(FRET)及基因突变和转染技术,研究了活细胞TLR4与MD-2作用的结构域．结果表明：N端Glu²⁴～Met⁴¹缺失使TLR4与MD-2结合能力明显下降;LPS刺激后TLR4聚合迅速增加,而缺失Glu²⁴～Met⁴¹的TLR4不能聚合．上述结果提示,TLR4的Glu²⁴～Met⁴¹不仅是结合MD-2的区域,并且还参与了LPS刺激后TLR4的聚合作用．     

Lipopolysaccharide recognition protein, MD-2, facilitates cellular uptake of E. coli-derived plasmid DNA in synovium

BACKGROUND: Several cell types are susceptible to transfection in vivo using naked plasmid DNA. The mechanisms involved in mediating in vivo transfection are incompletely known, but evidence suggests that receptor-mediated endocytosis is important for specific types of cells. In this study we tested the hypothesis that residual Escherichia coli lipopolysaccharide (LPS) forms a non-covalent complex with expression plasmid DNA, and host-cell-derived soluble LPS-binding proteins bind to the DNA-LPS complexes in order to facilitate receptor-mediated endocytosis. METHODS: Cells from the murine synovial lining were used as an in vivo model system and in vivo luciferase imaging was used to quantify levels of transgene expression. Using a series of gene-deleted mice, the roles of LPS recognition complex proteins, lipopolysaccharide-binding protein (LBP), CD14 and MD-2, in the process of in vivo transfection were determined. RESULTS: Luciferase expression assays revealed that mice lacking LBP or CD14 had increased luciferase expression (p < 0.023 and < 0.165, respectively), while mice deleted of MD-2 had significant reductions in luciferase expression (p < 0.001). Gene deletion of hyaluronic acid binding protein CD44 was used as a control and had no statistically significant effect on transgene expression in vivo. In muscle tissue, where neither cell surface nor soluble MD-2 is expressed, no MD-2 dependence of plasmid transfection was identified, suggesting the role of MD-2 is tissue or cell type specific. Additionally, depleting mice of macrophages showed that luciferase expression is occurring within fibroblast-like synoviocytes. CONCLUSIONS: Our data support a physical association between LPS and E. coli-derived plasmid DNA, and that in vivo transfection of fibroblast-like synoviocytes is dependent on the soluble form of the LPS-binding protein MD-2.     

NMR study on the major mite allergen Der f 2: its refined tertiary structure, epitopes for monoclonal antibodies and characteristics shared by ML protein group members

Group 2 major mite allergens Der f 2 and Der p 2 are classified into the recently identified group of MD-2-related lipid-recognition (ML) proteins, but the ligands and biological functions of these allergens are unknown. We have obtained a high-quality NMR structure for Der f 2, and found that it is more similar to the crystal structure of NPC2, a distant homologue, than to that of Der p 2, in terms of the separation and angle between the two major beta-sheets. This made us propose that ML proteins undergo clamshell-like motions that change the sizes of ligand-binding spaces inside their immunoglobulin-fold beta-sandwich to accommodate lipid molecules. This type of motion in lipopolysaccaride recognition of MD-2 is suggested to be likely as well by structural models. We also report the applicability of NMR differential exchange broadening experiments for complexes of intact monoclonal antibodies and antigens; using this technique, we have detected the conformational epitopes for monoclonal antibodies 15E11 and 13A4 as two separate surface patches.     

Interaction of soluble form of recombinant extracellular TLR4 domain with MD-2 enables lipopolysaccharide binding and attenuates TLR4-mediated signaling

TLRs have been implicated in recognition of pathogen-associated molecular patterns. TLR4 is a signaling receptor for LPS, but requires MD-2 to respond efficiently to LPS. The purposes of this study were to examine the interactions of the extracellular TLR4 domain with MD-2 and LPS. We generated soluble forms of rTLR4 (sTLR4) and TLR2 (sTLR2) lacking the putative intracellular and transmembrane domains. sTLR4 consisted of Glu(24)-Lys(631). MD-2 bound to sTLR4, but not to sTLR2 or soluble CD14. BIAcore analysis demonstrated the direct binding of sTLR4 to MD-2 with a dissociation constant of K(D) = 6.29 x 10(-8) M. LPS-conjugated beads precipitated MD-2, but not sTLR4. However, LPS beads coprecipitated sTLR4 and MD-2 when both proteins were coincubated. The addition of sTLR4 to the medium containing the MD-2 protein significantly attenuated LPS-induced NF-kappaB activation and IL-8 secretion in wild-type TLR4-expressing cells. These results indicate that the extracellular TLR4 domain-MD-2 complex is capable of binding LPS, and that the extracellular TLR4 domain consisting of Glu(24)-Lys(631) enables MD-2 binding and LPS recognition to TLR4. In addition, the use of sTLR4 may lead to a new therapeutic strategy for dampening endotoxin-induced inflammation.     

Identification of LPS-binding peptide fragment of MD-2, a toll-receptor accessory protein

Members of the toll-like receptor family are crucial in recognition of microbial pathogens as part of innate immune response. MD-2, an accessory protein to TLR4, present on the extracellular side of the membrane is needed to initiate the signal transduction. We have identified a 15 amino acid region of human MD-2 that contains several features of other lipopolysaccharide (LPS) binding proteins and peptides. In vitro LPS neutralization by this peptide was observed and confirmed by 2D transferred NOESY NMR experiments. NMR experiments have also shown binding of the MD-2 peptide to lipoteichoic acid (LTA) but not to peptidoglycan. Furthermore this peptide inhibited growth of gram-negative and to a lower extent of some gram-positive bacteria. Our results indicate that this region of MD-2 might be responsible for binding of LPS and confirms the role of MD-2 as an accessory protein in LPS signaling bestowing the Toll receptors their specificity.     

Preparation and characterization of truncated human lipopolysaccharide-binding protein in Escherichia coli

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria, and is the causative agent of endotoxin shock. LPS induces signal transduction in immune cells when it is recognized by the cell surface complex of toll-like receptor 4 (TLR4) and MD-2. The complex recognizes the lipid A structure in LPS, which is buried in the membrane of the outer envelope. To present the Lipid A structure to the TLR4/MD-2, processing of LPS by LPS-binding protein (LBP) and CD14 is required. In previous studies, we expressed recombinant proteins of human MD-2 and CD14 as fusion proteins with thioredoxin in Escherichia coli, and demonstrated their specific binding abilities to LPS. In this study, we prepared a recombinant fusion protein containing 212 amino terminal residues of human LBP (HLB212) by using the same expression system. The recombinant protein expressed in E. coli was purified as a complex form with host LPS. The binding was not affected by high concentrations of salt, but was prevented by low concentrations of various detergents. Both rough-type LPS lacking the O antigen and smooth-type LPS with the antigen bound to HLBP212. Therefore, oligosaccharide repeats appeared to be unnecessary for the binding. A nonpathogenic penta-acylated LPS also bound to HLBP212, but the binding was weaker than that of the wild type. The hydrophobic interaction between the LBP and acyl chains of lipid A appears to be important for the binding. The recombinant proteins of LPS-binding molecules would be useful for analyzing the defense mechanism against infections.     

Mutational analysis of membrane and soluble forms of human MD-2

Toll-like receptor 4 and MD-2 form a receptor for lipopolysaccharide (LPS), a major constituent of Gram-negative bacteria. MD-2 is a 20-25-kDa extracellular glycoprotein that binds to Tolllike receptor 4 (TLR4) and LPS and is a critical part of the LPS receptor. Here we have shown that the level of MD-2 expression regulates TLR4 activation by LPS. Using site-directed mutagenesis, we have found that glycosylation has no effect on MD-2 function as a membrane receptor for LPS. We used alanine-scanning mutagenesis to identify regions of human MD-2 that are important for TLR4 and LPS binding. We found that mutation in the N-terminal 46 amino acids of MD-2 did not substantially diminish LPS activation of Chinese hamster ovary (CHO) cells co-transfected with TLR4 and mutant MD-2. The residues 46-50 were important for LPS activation but not LPS binding. The residues 79-83, 121-124, and 125-129 are identified as important in LPS activation but not surface expression of membrane MD-2. The function of soluble MD-2 is somewhat more sensitive to mutation than membrane MD-2. Our results suggest that the 46-50 and 127-131 regions of soluble MD-2 bind to TLR4. The region 79-120 is not involved in LPS binding but affects monomerization of soluble MD-2 as well as TLR4 binding. We define the LPS binding region of monomeric soluble MD-2 as a cluster of basic residues 125-131. Studies on both membrane and soluble MD-2 suggest that domains of MD-2 for TLR4 and LPS binding are separate as well as overlapping. By mapping these regions on a three-dimensional model, we show the likely binding regions of MD-2 to TLR4 and LPS.     

Lysines 128 and 132 enable lipopolysaccharide binding to MD-2, leading to Toll-like receptor-4 aggregation and signal transduction

Three cell-surface proteins have been recognized as components of the mammalian signaling receptor for bacterial lipopolysaccharide (LPS): CD14, Toll-like receptor-4 (TLR4), and MD-2. Biochemical and visual studies shown here demonstrate that the role of CD14 in signal transduction is to enhance LPS binding to MD-2, although its expression is not essential for cellular activation. These studies clarify how MD-2 functions: we found that MD-2 enables TLR4 binding to LPS and allows the formation of stable receptor complexes. MD-2 must be bound to TLR4 on the cell surface before binding can occur. Consequently, TLR4 clusters into receptosomes (many of which are massive) that recruit intracellular toll/IL-1/resistance domain-containing adapter proteins within minutes, thus initiating signal transduction. TLR4 activation correlates with the ability of MD-2 to bind LPS, as MD-2 mutants that still bind TLR4, but are impaired in the ability to bind LPS, conferred a greatly blunted LPS response. These findings help clarify the earliest events of TLR4 triggering by LPS and identify MD-2 as an attractive target for pharmacological intervention in endotoxin-mediated diseases.     

Lipopolysaccharide is in close proximity to each of the proteins in its membrane receptor complex. transfer from CD14 to TLR4 and MD-2

The structural features of some proteins of the innate immune system involved in mediating responses to microbial pathogens are highly conserved throughout evolution. Examples include members of the Drosophila Toll (dToll) and the mammalian Toll-like receptor (TLR) protein families. Activation of Drosophila Toll is believed to occur via an endogenous peptide rather than through direct binding of microbial products to the Toll protein. In mammals there is a growing consensus that lipopolysaccharide (LPS) initiates its biological activities through a heteromeric receptor complex containing CD14, TLR4, and at least one other protein, MD-2. LPS binds directly to CD14 but whether LPS then binds to TLR4 and/or MD-2 is not known. We have used transient transfection to express human TLRs, MD-2, or CD14 alone or in different combinations in HEK 293 cells. Interactions between LPS and these proteins were studied using a chemically modified, radioiodinated LPS containing a covalently linked, UV light-activated cross-linking group ((125)I-ASD-Re595 LPS). Here we show that LPS is cross-linked specifically to TLR4 and MD-2 only when co-expressed with CD14. These data support the contention that LPS is in close proximity to the three known proteins of its membrane receptor complex. Thus, LPS binds directly to each of the members of the tripartite LPS receptor complex.     

Pulmonary surfactant protein D inhibits lipopolysaccharide (LPS)-induced inflammatory cell responses by altering LPS binding to its receptors

Pulmonary surfactant protein D (SP-D) is a member of the collectin family that plays an important role in regulating innate immunity of the lung. We examined the mechanisms by which SP-D modulates lipopolysaccharide (LPS)-elicited inflammatory cell responses. SP-D bound to a complex of recombinant soluble forms of Toll-like receptor 4 (TLR4) and MD-2 with high affinity and down-regulated tumor necrosis factor-alpha secretion and NF-kappaB activation elicited by rough and smooth LPS, in alveolar macrophages and TLR4/MD-2-transfected HEK293 cells. Cell surface binding of both serotypes of LPS to TLR4/MD-2-expressing cells was attenuated by SP-D. In addition, SP-D significantly reduced MD-2 binding to both serotypes of LPS. A chimera containing the N-terminal region and the collagenous domain of surfactant protein A, and the coiled-coil neck and lectin domains of SP-D, was a weak inhibitor of LPS-induced cell responses and MD-2 binding to LPS, compared with native SP-D. The collagenase-resistant fragment consisting of the neck plus the carbohydrate recognition domain of SP-D also was a very weak inhibitor of LPS activation. This study demonstrates that SP-D down-regulates LPS-elicited inflammatory responses by altering LPS binding to its receptors and reveals the importance of the correct oligomeric structure of the protein in this process.     

Regulatory roles for MD-2 and TLR4 in ligand-induced receptor clustering

LPS, a principal membrane component in Gram-negative bacteria, is recognized by a receptor complex consisting of TLR4 and MD-2. MD-2 is an extracellular molecule that is associated with the extracellular domain of TLR4 and has a critical role in LPS recognition. MD-2 directly interacts with LPS, and the region from Phe(119) to Lys(132) (Arg(132) in mice) has been shown to be important for interaction between LPS and TLR4/MD-2. With mouse MD-2 mutants, we show in this study that Gly(59) was found to be a novel critical amino acid for LPS binding outside the region 119-132. LPS signaling is thought to be triggered by ligand-induced TLR4 clustering, which is also regulated by MD-2. Little is known, however, about a region or an amino acid in the MD-2 molecule that regulates ligand-induced receptor clustering. MD-2 mutants substituting alanine for Phe(126) or Gly(129) impaired LPS-induced TLR4 clustering, but not LPS binding to TLR4/MD-2, demonstrating that ligand-induced receptor clustering is differentially regulated by MD-2 from ligand binding. We further show that dissociation of ligand-induced receptor clustering and of ligand-receptor interaction occurs in a manner dependent on TLR4 signaling and requires endosomal acidification. These results support a principal role for MD-2 in LPS recognition.     

Dysregulation of LPS-induced Toll-like receptor 4-MyD88 complex formation and IL-1 receptor-associated kinase 1 activation in endotoxin-tolerant cells

Prior exposure to LPS induces a transient state of cell refractoriness to subsequent LPS restimulation, known as endotoxin tolerance. Induction of LPS tolerance has been reported to correlate with decreased cell surface expression of the LPS receptor complex, Toll-like receptor 4 (TLR4)/MD-2. However, other results have underscored the existence of mechanisms of LPS tolerance that operate downstream of TLR4/MD-2. In the present study we sought to delineate further the molecular basis of LPS tolerance by examining the TLR4 signaling pathway in endotoxin-tolerant cells. Pretreatment of human monocytes with LPS decreased LPS-mediated NF-kappaB activation, p38 mitogen-activated protein kinase phosphorylation, and TNF-alpha gene expression, documenting the induction of endotoxin tolerance. FACS and Western blot analyses of LPS-tolerant monocytes showed increased TLR2 expression, whereas TLR4 expression levels were not affected. Comparable levels of mRNA and protein for myeloid differentiation factor 88 (MyD88), IL-1R-associated kinase 1 (IRAK-1), and TNFR-associated factor-6 were found in normal and LPS-tolerant monocytes, while MD-2 mRNA expression was slightly increased in LPS-tolerant cells. LPS induced the association of MyD88 with TLR4 and increased IRAK-1 activity in medium-pretreated cells. In LPS-tolerant monocytes, however, MyD88 failed to be recruited to TLR4, and IRAK-1 was not activated in response to LPS stimulation. Moreover, endotoxin-tolerant CHO cells that overexpress human TLR4 and MD-2 also showed decreased IRAK-1 kinase activity in response to LPS despite the failure of LPS to inhibit cell surface expression of transfected TLR4 and MD-2 proteins. Thus, decreased TLR4-MyD88 complex formation with subsequent impairment of IRAK-1 activity may underlie the LPS-tolerant phenotype.     

Multiple-mutation at a potential ligand-binding region decreased allergenicity of a mite allergen Der f 2 without disrupting global structure

We assessed the effect of multiple-mutations within one IgE-binding area on allergenicity of Der f 2. The triple-mutant of Der f 2, P34/95/99A, exhibited the most significant reduction of allergenicity and circular dichroism analysis showed that the global structure of Der f 2 was maintained in P34/95/99A. These results indicate that such a strategy is effective when designing allergen-vaccines, which achieve less allergenicity for a broad population of patients without disrupting the global structure. Structurally, Der f 2 is a member of the MD-2 related lipid-recognition proteins. The sites for the triple-mutation located on the characteristically charged entrance of a cavity and corresponded to the regions critical to ligand-binding in the Niemann-Pick type 2 disease protein and MD-2.