MD-2

Lysophosphatidylcholine Acetyltransferase 2 (LPCAT2) Influences the Gene Expression of the Lipopolysaccharide Receptor Complex in Infected RAW264.7 Macrophages, Depending on the E. coli Lipopolysaccharide Serotype

Escherichia coli (E. coli) is a frequent gram-negative bacterium that causes nosocomial infections, affecting more than 100 million patients annually worldwide. Bacterial lipopolysaccharide (LPS) from E. coli binds to toll-like receptor 4 (TLR4) and its co-receptor's cluster of differentiation protein 14 (CD14) and myeloid differentiation factor 2 (MD2), collectively known as the LPS receptor complex. LPCAT2 participates in lipid-raft assembly by phospholipid remodelling. Previous research has proven that LPCAT2 co-localises in lipid rafts with TLR4 and regulates macrophage inflammatory response. However, no published evidence exists of the influence of LPCAT2 on the gene expression of the LPS receptor complex induced by smooth or rough bacterial serotypes. We used RAW264.7-a commonly used experimental murine macrophage model-to study the effects of LPCAT2 on the LPS receptor complex by transiently silencing the LPCAT2 gene, infecting the macrophages with either smooth or rough LPS, and quantifying gene expression. LPCAT2 only significantly affected the gene expression of the LPS receptor complex in macrophages infected with smooth LPS. This study provides novel evidence that the influence of LPCAT2 on macrophage inflammatory response to bacterial infection depends on the LPS serotype, and it supports previous evidence that LPCAT2 regulates inflammatory response by modulating protein translocation to lipid rafts.

Integrated analysis of RNA-seq datasets reveals novel targets and regulators of COVID-19 severity

During the COVID-19 pandemic, RNA-seq datasets were produced to investigate the virus-host relationship. However, much of these data remains underexplored. To improve the search for molecular targets and biomarkers, we performed an integrated analysis of multiple RNA-seq datasets, expanding the cohort and including patients from different countries, encompassing severe and mild COVID-19 patients. Our analysis revealed that severe COVID-19 patients exhibit overexpression of genes coding for proteins of extracellular exosomes, endomembrane system, and neutrophil granules (e.g., S100A9, LY96, and RAB1B), which may play an essential role in the cellular response to infection. Concurrently, these patients exhibit down-regulation of genes encoding components of the T cell receptor complex and nucleolus, including TP53, IL2RB, and NCL Finally, SPI1 may emerge as a central transcriptional factor associated with the up-regulated genes, whereas TP53, MYC, and MAX were associated with the down-regulated genes during COVID-19. This study identified targets and transcriptional factors, lighting on the molecular pathophysiology of syndrome coronavirus 2 infection.

Cross talk between bacterial and human gene networks enriched using ncRNAs in IBD disease

Inflammatory bowel disease (IBD) is a long-term inflammatory immune-mediated gut illness with several extra-intestinal complications. The aims of this study were to identify a novel network-based meta-analysis approach on the basis of the combinations of the differentially expressed genes (DEGs) from microarray data, to enrich the functional modules from human protein-protein interaction (PPI) and gene ontology (GO) data, and to profile the ncRNAs on the genes involved in IBD. The gene expression profiles of GSE126124, GSE87473, GSE75214, and GSE95095 are obtained from the Gene Expression Omnibus (GEO) database based on the study criteria between 2017 and 2022. The DEGs were screened by the R software. DEGs were then used to examine gene ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. The ncRNAs including the miRNAs and ceRNAs were predicted on the PPIs visualized using Cytoscape. Enrichment analysis of genes with differential expression (n = 342) using KEGG and GO showed that the signaling pathways related with staphylococcus aureus and pertussis bacterial infections may stimulate the immune system and exacerbate IBD via the interaction with human proteins including Fibrinogen gamma chain (FGG), Keratin 10 (KRT10), and Toll like receptor 4 (TLR4). By building a ceRNA network, lncRNA XIST and NEAT1 were determined by affecting common miRNAs, hsa-miR-6875-5p, hsa-miR-1908-5p, hsa-miR-186-5p, hsa-miR-6763-5p, hsa-miR-4436a, and hsa-miR-520a-5p. Additionally, the chromosome regions including NM_001039703 and NM_006267, which produce the most potent circRNAs play a significant role in the ceRNA network of IBD. Also, we predicted the siRNAs that would be most effective against the bacterial genes in staphylococcus aureus and pertussis infections. These findings suggested that three genes (FGG, KRT10, and TLR4), six miRNAs (hsa-miR-6875-5p, hsa-miR-1908-5p, hsa-miR-186-5p, hsa-miR-4436a, hsa-miR-520a-5p, and hsa-miR-6763-5p), two lncRNAs (XIST and NEAT1), and chromosomal regions including NM_001039703 and NM_006267 with the production of the most effective circRNAs are involved in the ncRNA-associated ceRNA network of IBD. These ncRNA profiles are related to the described gene functions and may play therapeutic targets in controlling inflammatory bowel disease.

Transcriptome Analysis Reveals Early Hemocyte Responses upon In Vivo Stimulation with LPS in the Stick Insect Bacillus rossius (Rossi, 1788)

Simple Summary

Non-model insect species such as B. rossius suffer from a profound gap of knowledge regarding the temporal progression of physiological responses following the challenge with bacterial pathogens or cell wall components thereof. The reason for this mostly lies in the lack of genomic/transcriptomic resources, which would provide an unparalleled in-depth capacity in the analysis of molecular, biochemical, and metabolic mechanisms. We present a high-quality transcriptome obtained from high-coverage sequencing of hemocytes harvested from adult stick insect specimens both pre- and post-LPS stimulation. Such a resource served as the basis for a stringent differential gene expression and functional enrichment analyses, the results of which were characterized and discussed in depth. Selected transcripts encoding for C-type lectins and ML-domain containing proteins were further investigated from a phylogenetic perspective. Overall, these findings shed light on the physiological responses driven by a short-term LPS stimulation in the European stick insect.

Abstract

Despite a growing number of non-model insect species is being investigated in recent years, a greater understanding of their physiology is prevented by the lack of genomic resources. This is the case of the common European stick insect Bacillus rossius (Rossi, 1788): in this species, some knowledge is available on hemocyte-related defenses, but little is known about the physiological changes occurring in response to natural or experimental challenges. Here, the transcriptional signatures of adult B. rossius hemocytes were investigated after a short-term (2 h) LPS stimulation in vivo: a total of 2191 differentially expressed genes, mostly involved in proteolysis and carbohydrate and lipid metabolic processes, were identified in the de novo assembled transcriptome and in-depth discussed. Overall, the significant modulation of immune signals—such as C-type lectins, ML domain-containing proteins, serpins, as well as Toll signaling-related molecules—provide novel information on the early progression of LPS-induced responses in B. rossius.

Potential Anti-Inflammatory Effects of a New Lyophilized Formulation of the Conditioned Medium Derived from Periodontal Ligament Stem Cells

The mesenchymal stem cells’ (MSCs) secretome includes the bioactive molecules released in the conditioned medium (CM), such as soluble proteins, free nucleic acids, lipids and extracellular vesicles. The secretome is known to mediate some of the beneficial properties related to MSCs, such as anti-inflammatory, anti-apoptotic and regenerative capacities. In this work, we aim to evaluate the anti-inflammatory potential of a new lyophilized formulation of CM derived from human periodontal ligament stem cells (hPDLSCs). With this aim, we treat hPDLSCs with lipopolysaccharide (LPS) and test the anti-inflammatory potential of lyophilized CM (LYO) through the evaluation of wound closure, transcriptomic and immunofluorescence analysis. LPS treatment increased the expression of TLR4 and of genes involved in its signaling and in p38 and NF-κB activation, also increasing the expression of cytokines and chemokines. Interestingly, LYO downregulated the expression of genes involved in Toll-like receptor 4 (TLR-4), nuclear factor kappa light chain enhancer of activated B cells (NF-κB) and p38 signaling. As a consequence, the genes encoding for cytokines and chemokines were also downregulated. Immunofluorescence acquisitions confirmed the downregulation of TLR-4 and NF-κB with the LYO treatment. Moreover, the LYO treatment also increased hPDLSCs’ migration. LYO was demonstrated to contain transforming growth factor (TGF)-β3 and vascular endothelial growth factor (VEGF). These results suggest that LYO represents an efficacious formulation with anti-inflammatory potential and highlights lyophilization as a valid method to produce stable formulations of MSCs’ secretome.

An MD-2-related lipid-recognition protein is required for insect reproduction and integument development

The myeloid differentiation factor 2 (MD-2)-related lipid-recognition protein is involved in immune responses through recognizing bacteria lipopolysaccharide in mammals, arthropods and plants. However, the physiological roles of MD-2 in other biological processes are largely unknown. Here, we identified three homologue MD-2 genes (NlML1, NlML2 and NlML3) by searching the genome and transcriptome databases of the brown planthopper Nilaparvata lugens, a hemipteran insect species. Temporospatial analysis showed that the NlML1 gene was highly expressed in the fat body but much less so in the other tissues, while the NlML2 and NlML3 genes were highly expressed in the testis or digestive tract. RNA interference-mediated depletion of the NlML1 gene significantly downregulated the transcription of numerous integument protein genes. The NlML1 knockdown led to moulting failure and mortality at the nymph-adult transition phase, impaired egg laying and hatching, and reduced 20-hydroxyecdysone (20E) production in the nymphs. 20E could rescue the deficient moulting phenotypes derived from dsNlML1 RNAi. These novel findings indicate that NlML1 is required for nymphal moulting and female reproductive success as it plays an important role in regulating 20E synthesis, lipid and chitin metabolisms in N. lugens, thus contributing to our understanding of developmental and reproductive mechanisms in insects.

MD2 contributes to the pathogenesis of perioperative neurocognitive disorder via the regulation of α5GABAA receptors in aged mice

BACKGROUND

Perioperative neurocognitive disorder (PND) is a long-term postoperative complication in elderly surgical patients. The underlying mechanism of PND is unclear, and no effective therapies are currently available. It is believed that neuroinflammation plays an important role in triggering PND. The secreted glycoprotein myeloid differentiation factor 2 (MD2) functions as an activator of the Toll-like receptor 4 (TLR4) inflammatory pathway, and α5GABA_A receptors (α5GABA_ARs) are known to play a key role in regulating inflammation-induced cognitive deficits. Thus, in this study, we aimed to investigate the role of MD2 in PND and determine whether α5GABA_ARs are involved in the function of MD2.

METHODS

Eighteen-month-old C57BL/6J mice were subjected to laparotomy under isoflurane anesthesia to induce PND. The Barnes maze was used to assess spatial reference learning and memory, and the expression of hippocampal MD2 was assayed by western blotting. MD2 expression was downregulated by bilateral injection of AAV-shMD2 into the hippocampus or tail vein injection of the synthetic MD2 degrading peptide Tat-CIRP-CMA (TCM) to evaluate the effect of MD2. Primary cultured neurons from brain tissue block containing cortices and hippocampus were treated with Tat-CIRP-CMA to investigate whether downregulating MD2 expression affected the expression of α5GABA_ARs. Electrophysiology was employed to measure tonic currents. For α5GABA_ARs intervention experiments, L-655,708 and L-838,417 were used to inhibit or activate α5GABA_ARs, respectively.

RESULTS

Surgery under inhaled isoflurane anesthesia induced cognitive impairments and elevated the expression of MD2 in the hippocampus. Downregulation of MD2 expression by AAV-shMD2 or Tat-CIRP-CMA improved the spatial reference learning and memory in animals subjected to anesthesia and surgery. Furthermore, Tat-CIRP-CMA treatment decreased the expression of membrane α5GABA_ARs and tonic currents in CA1 pyramidal neurons in the hippocampus. Inhibition of α5GABA_ARs by L-655,708 alleviated cognitive impairments after anesthesia and surgery. More importantly, activation of α5GABA_ARs by L-838,417 abrogated the protective effects of Tat-CIRP-CMA against anesthesia and surgery-induced spatial reference learning and memory deficits.

CONCLUSIONS

MD2 contributes to the occurrence of PND by regulating α5GABA_ARs in aged mice, and Tat-CIRP-CMA is a promising neuroprotectant against PND.

Myeloid differentiation protein-2 has a protective role in house dust mite-mediated asthmatic characteristics with the proinflammatory regulation of airway epithelial cells and dendritic cells

BACKGROUND

Myeloid differentiation protein-2 (MD-2) is a lipopolysaccharide-binding protein involved in lipopolysaccharide signalling via Toll-like receptor 4 (TLR4). TLR4 plays an essential role in HDM-mediated allergic airway inflammation. Moreover, MD-2 is structurally similar to Der f 2, a major allergen from house dust mite (HDM).

OBJECTIVES

We aimed to clarify the role of MD-2 in the pathogenesis of HDM-mediated allergic airway inflammation.

METHODS

Wild-type (WT), TLR4 knockout and MD-2 knockout mice were subjected to intranasal instillation of HDM extract, and asthmatic features were evaluated. We also evaluated gene sets regulated by MD-2 in HDM-treated airway epithelial cells and examined the function of dendritic cells from lymph nodes and from lungs.

RESULTS

Aggravated allergic airway inflammation with increased airway hyperresponsiveness was observed in MD-2 knockout mice compared with WT and TLR4 knockout mice. Global gene expression analysis revealed an MD-2 regulated proinflammatory response and reconstituted TLR4 signalling in airway epithelial cells. The ability of dendritic cells to evoke an allergic immune response was enhanced in MD-2 knockout mice.

CONCLUSIONS & CLINICAL RELEVANCE

MD-2 plays a protective role in HDM-induced airway allergy with the proinflammatory regulation of airway epithelial cells and dendritic cells. MD-2 may serve as a therapeutic target in the treatment of asthma.

An MD2-perturbing peptide has therapeutic effects in rodent and rhesus monkey models of stroke

Studies have failed to translate more than 1000 experimental treatments from bench to bedside, leaving stroke as the second leading cause of death in the world. Thrombolysis within 4.5 hours is the recommended therapy for stroke and cannot be performed until neuroimaging is used to distinguish ischemic stroke from hemorrhagic stroke. Therefore, finding a common and critical therapeutic target for both ischemic and hemorrhagic stroke is appealing. Here, we report that the expression of myeloid differentiation protein 2 (MD2), which is traditionally regarded to be expressed only in microglia in the normal brain, was markedly increased in cortical neurons after stroke. We synthesized a small peptide, Trans-trans-activating (Tat)-cold-inducible RNA binding protein (Tat-CIRP), which perturbed the function of MD2 and strongly protected neurons against excitotoxic injury in vitro. In addition, systemic administration of Tat-CIRP or genetic deletion of MD2 induced robust neuroprotection against ischemic and hemorrhagic stroke in mice. Tat-CIRP reduced the brain infarct volume and preserved neurological function in rhesus monkeys 30 days after ischemic stroke. Tat-CIRP efficiently crossed the blood-brain barrier and showed a wide therapeutic index for stroke because no toxicity was detected when high doses were administered to the mice. Furthermore, we demonstrated that MD2 elicited neuronal apoptosis and necroptosis via a TLR4-independent, Sam68-related cascade. In summary, Tat-CIRP provides robust neuroprotection against stroke in rodents and gyrencephalic nonhuman primates. Further efforts should be made to translate these findings to treat both ischemic and hemorrhagic stroke in patients.

Identification of a Heme Activation Site on the MD-2/TLR4 Complex

Myeloid differentiation factor-2 (MD-2) binds lipopolysaccharide (LPS) and initiates toll-like receptor-4 (TLR4) pro-inflammatory signaling. Heme also activates TLR4 signaling, but it is unknown if heme interacts with MD-2. Therefore, we examined MD-2 for a potential heme activation site. Heme-agarose and biotin-heme/streptavidin-agarose pulled down recombinant MD-2, which was inhibited by excess free heme. UV/visible spectroscopy confirmed MD-2-heme binding. To determine whether MD-2 was required for heme-mediated TLR4 signaling, HEK293 cells were transfected with MD-2, TLR4, CD14, and an NF-κB luciferase reporter, and then stimulated with heme or LPS. Heme or LPS treatment elicited robust reporter activity. Absence of MD-2, TLR4 or CD14 plasmid abolished NF-κB reporter responses to heme or LPS. In silico analysis identified two potential heme docking sites on MD-2 near conserved amino acids W23/S33/Y34 and Y36/C37/I44. Heme-induced NF-κB activity was reduced by 39 and 78% in HEK293 cells transfected with MD-2 mutants W23A and Y34A, respectively, compared to WT-MD-2. NF-κB activation by LPS was not affected by the same mutants. Biotinyl-heme/streptavidin-agarose pulled down 68% less W23A and 80% less W23A/S33A/Y34A mutant MD-2 than WT-MD-2. In contrast, at the Y36/C37/I44 MD-2 site, heme-induced NF-κB activity was significantly increased by mutants Y36A (191% of WT-MD-2) and unchanged by mutants C37A and I44A (95 and 92%, respectively, of WT-MD-2). In conclusion, these data suggest that heme binds and activates TLR4 signaling at amino acids W23 and Y34 on MD-2.

Selective targeting of the TLR4 co-receptor, MD2, prevents colon cancer growth and lung metastasis

Toll-like receptor (TLR) signaling is an emerging pathway in tumor cell invasion and metastasis. Myeloid differentiation protein-2 (MD2) contributes to ligand recognition and activation of TLRs in response to exogenous microbial insults or endogenous agents. We hypothesized that blocking MD2 using a specific inhibitor would prevent TLR4-mediated inflammatory responses and metastatic cancer growth. Here, we report that a MD2 inhibitor, L6H21, inhibited migration and invasion of LPS-activated colon cancer CT26.WT cells. These activities were accompanied by inhibition of nuclear factor-κB (NF-κB) activation, and thereby inhibition of the production of pro-inflammatory cytokines and adhesive molecules in colon cancer cells. Furthermore, L6H21 inhibited CT26.WT metastasis to the lung in BALB/c mice as well as suppressed colitis-induced colon cancer induced by azoxymethane/dextran sulfate sodium (AOM/DSS). Taken together, our results demonstrated that L6H21 suppressed tumor invasion and metastasis through blocking TLR4-MD2/NF-κB signaling axis. These findings reveal that inhibition of MD2 may be an important target for the development of colon cancer therapies.

Exploring the biological functional mechanism of the HMGB1/TLR4/MD-2 complex by surface plasmon resonance

BACKGROUND

High Mobility Group Box 1 (HMGB1) was first identified as a nonhistone chromatin-binding protein that functions as a pro-inflammatory cytokine and a Damage-Associated Molecular Pattern molecule when released from necrotic cells or activated leukocytes. HMGB1 consists of two structurally similar HMG boxes that comprise the pro-inflammatory (B-box) and the anti-inflammatory (A-box) domains. Paradoxically, the A-box also contains the epitope for the well-characterized anti-HMGB1 monoclonal antibody "2G7", which also potently inhibits HMGB1-mediated inflammation in a wide variety of in vivo models. The molecular mechanisms through which the A-box domain inhibits the inflammatory activity of HMGB1 and 2G7 exerts anti-inflammatory activity after binding the A-box domain have been a mystery. Recently, we demonstrated that: 1) the TLR4/MD-2 receptor is required for HMGB1-mediated cytokine production and 2) the HMGB1-TLR4/MD-2 interaction is controlled by the redox state of HMGB1 isoforms.

METHODS

We investigated the interactions of HMGB1 isoforms (redox state) or HMGB1 fragments (A- and B-box) with TLR4/MD-2 complex using Surface Plasmon Resonance (SPR) studies.

RESULTS

Our results demonstrate that: 1) intact HMGB1 binds to TLR4 via the A-box domain with high affinity but an appreciable dissociation rate; 2) intact HMGB1 binds to MD-2 via the B-box domain with low affinity but a very slow dissociation rate; and 3) HMGB1 A-box domain alone binds to TLR4 more stably than the intact protein and thereby antagonizes HMGB1 by blocking HMGB1 from interacting with the TLR4/MD-2 complex.

CONCLUSIONS

These findings not only suggest a model whereby HMGB1 interacts with TLR4/MD-2 in a two-stage process but also explain how the A-box domain and 2G7 inhibit HMGB1.

Recent progress in the discovery of myeloid differentiation 2 (MD2) modulators for inflammatory diseases

Myeloid differentiation protein 2 (MD2), together with Toll-like receptor 4 (TLR4), binds lipopolysaccharide (LPS) with high affinity, inducing the formation of the activated homodimer LPS-MD2-TLR4. MD2 directly recognizes the Lipid A domain of LPS, leading to the activation of downstream signaling of cytokine and chemokine production, and initiation of inflammatory and immune responses. However, excessive activation and potent host responses generate severe inflammatory syndromes such as acute sepsis and septic shock. MD2 is increasingly being considered as an attractive pharmacological target for the development of potent anti-inflammatory agents. In this Keynote review, we provide a comprehensive overview of the recent advances in the structure and biology of MD2, and present MD2 modulators as promising agents for anti-inflammatory intervention.

Cellular uptake of exogenous calcineurin B is dependent on TLR4/MD2/CD14 complexes, and CnB is an endogenous ligand of TLR4

Our previous research showed that recombinant calcineurin B (rhCnB) stimulates cytokine secretion by immune cells, probably through TLR4. Exogenous CnB can be incorporated into many different tumour cells in vitro, but the mode of uptake and receptors required remain unknown. Here, we report that exogenous CnB is taken up by cells in a time- and concentration-dependent manner via clathrin-dependent receptor-mediated internalization. Our findings further confirm that uptake is mediated by the TLR4/MD2 complex together with the co-receptor CD14. The MST results revealed a high affinity between CnB and the TLR4 receptor complex. No binding was detected between CnB and LPS. CnB inhibited the uptake of LPS, and LPS also inhibited the uptake of CnB. These results indicate that the uptake of exogenous CnB did not occur through LPS and that CnB was not a chaperone of LPS. Thus, we conclude that TLR4 receptor complexes were required for the recognition and internalization of exogenous CnB. CnB could be a potential endogenous ligand of TLR4 and function as an agonist of TLR4. These properties of CnB support its potential for development as an anti-cancer drug.

Lactoferrin: A Roadmap to the Borderland between Caries and Periodontal Disease

Lactoferrin is one of a number of multifunctional proteins that are present in or on all mucosal surfaces throughout the body. Levels of lactoferrin are consistently elevated in inflammatory diseases such as arthritis, inflammatory bowel diseases, corneal disease, and periodontitis. Single-nucleotide polymorphisms (SNPs) in lactoferrin have been shown to be present in individuals susceptible to Escherichia coli-induced travelers' diarrhea and in tear fluid derived from virally associated corneal disease. Here, we review data showing a lactoferrin SNP in amino acid position 29 in the antimicrobial region of lactoferrin that acts against caries associated bacteria. This SNP was initially discovered in African American subjects with localized aggressive periodontitis (LAP) who had proximal bone loss but minimal proximal caries. Results were confirmed in a genetic association study of children from Brazil with this same SNP who showed a reduced level of caries. In vitro data indicate that lactoferrin from whole saliva derived from subjects with this SNP, recombinant human lactoferrin containing this SNP, or an 11-mer peptide designed for this SNP kills mutans streptococci associated with caries by >1 log. In contrast, the SNP has minimal effect on Gram-negative species associated with periodontitis. Moreover, periodontally healthy subjects homozygous for this lysine (K) SNP have lactoferrin in their saliva that kills mutans streptococci and have reduced proximal decay. The review summarizes data supporting the ecologic plaque hypothesis and suggests that a genetic variant in lactoferrin with K in position 29 when found in saliva and crevice fluid can influence community biofilm composition. We propose that, for caries, this SNP is ethnicity independent and protective by directly killing caries-provoking bacteria (reducing proximal decay). However, the clinical effect of this SNP in LAP is ethnicity dependent, destructive (increases LAP incidence), and complex with mechanisms still to be determined.

Association of neonatal necrotizing enterocolitis with myeloid differentiation-2 and GM2 activator protein genetic polymorphisms

The aim of the present study was to investigate the association of neonatal necrotizing enterocolitis (NEC) with myeloid differentiation-(MD-2) and GM2 activator protein (GM2A) genetic polymorphisms. Gene resequencing of the MD-2 and GM2A gene exons was performed on 42 neonates, diagnosed with NEC (NEC group), as well as in the rs11465996 locus, located in the MD-2 gene promoter region. The aim was to detect the genetic polymorphisms present in the neonates with NEC and compare the functional polymorphic loci with 83 neonates without NEC (control group), who had been born during the same period. A polymorphic locus with abnormal frequency was detected in the exon region of the MD-2 gene. In the NEC group, the frequency of genotypes carrying the low frequency allele (G) in the rs11465996 locus (MD-2 promoter region) was significantly higher compared with the control group (χ(2)=4.388, P=0.036). Furthermore, the frequencies of genotypes carrying the low frequency A and C alleles in the rs1048719 (GM2A gene exon 1) and rs2075783 loci (GM2A intron), respectively, were significantly higher in the NEC group compared with the control group (χ(2)=4.316, P=0.038; and χ(2)=13.717, P=0.000, respectively). In addition, the rs11465996 polymorphism in the MD-2 gene promoter region was found to be associated with the severity of NEC. Furthermore, the rs2075783 polymorphism in the GM2A gene exon 1 and the rs1048719 polymorphism in the intron region of this gene, were associated with the occurrence of NEC. The present study demonstrated that gene polymorphisms of MD-2 and GM2A were associated with the occurrence or severity of NEC; however, further in-depth exploration is required to clarify the associations between genetic predispositions to polymorphisms, and NEC.

Alternatively spliced myeloid differentiation protein-2 inhibits TLR4-mediated lung inflammation

We previously identified a novel alternatively spliced isoform of human myeloid differentiation protein-2 (MD-2s) that competitively inhibits binding of MD-2 to TLR4 in vitro. In this study, we investigated the protective role of MD-2s in LPS-induced acute lung injury by delivering intratracheally an adenovirus construct that expressed MD-2s (Ad-MD-2s). After adenovirus-mediated gene transfer, MD-2s was strongly expressed in lung epithelial cells and readily detected in bronchoalveolar lavage fluid. Compared to adenovirus serotype 5 containing an empty vector lacking a transgene control mice, Ad-MD-2s delivery resulted in significantly less LPS-induced inflammation in the lungs, including less protein leakage, cell recruitment, and expression of proinflammatory cytokines and chemokines, such as IL-6, keratinocyte chemoattractant, and MIP-2. Bronchoalveolar lavage fluid from Ad-MD-2s mice transferred into lungs of naive mice before intratracheal LPS challenge diminished proinflammatory cytokine levels. As house dust mite (HDM) sensitization is dependent on TLR4 and HDM Der p 2, a structural homolog of MD-2, we also investigated the effect of MD-2s on HDM-induced allergic airway inflammation. Ad-MD-2s given before HDM sensitization significantly inhibited subsequent allergic airway inflammation after HDM challenge, including reductions in eosinophils, goblet cell hyperplasia, and IL-5 levels. Our study indicates that the alternatively spliced short isoform of human MD-2 could be a potential therapeutic candidate to treat human diseases induced or exacerbated by TLR4 signaling, such as Gram-negative bacterial endotoxin-induced lung injury and HDM-triggered allergic lung inflammation.

MD-2 is required for disulfide HMGB1-dependent TLR4 signaling

Yang et al. show that a disulfide isoform of HMGB1, with a role in TLR4 signaling, physically interacts with and binds MD-2. MD-2 deficiency in macrophage cell lines or in primary mouse macrophages stimulated with HMGB1 implicates MD-2 in TLR4 signaling. They also identify an HGMB1 peptide inhibitor, P5779, which when administered in vivo can protect mice from acetaminophen-induced hepatoxicity, ischemia/reperfusion injury, and sepsis.

Innate immune receptors for pathogen- and damage-associated molecular patterns (PAMPs and DAMPs) orchestrate inflammatory responses to infection and injury. Secreted by activated immune cells or passively released by damaged cells, HMGB1 is subjected to redox modification that distinctly influences its extracellular functions. Previously, it was unknown how the TLR4 signalosome distinguished between HMGB1 isoforms. Here we demonstrate that the extracellular TLR4 adaptor, myeloid differentiation factor 2 (MD-2), binds specifically to the cytokine-inducing disulfide isoform of HMGB1, to the exclusion of other isoforms. Using MD-2–deficient mice, as well as MD-2 silencing in macrophages, we show a requirement for HMGB1-dependent TLR4 signaling. By screening HMGB1 peptide libraries, we identified a tetramer (FSSE, designated P5779) as a specific MD-2 antagonist preventing MD-2–HMGB1 interaction and TLR4 signaling. P5779 does not interfere with lipopolysaccharide-induced cytokine/chemokine production, thus preserving PAMP-mediated TLR4–MD-2 responses. Furthermore, P5779 can protect mice against hepatic ischemia/reperfusion injury, chemical toxicity, and sepsis. These findings reveal a novel mechanism by which innate systems selectively recognize specific HMGB1 isoforms. The results may direct toward strategies aimed at attenuating DAMP-mediated inflammation while preserving antimicrobial immune responsiveness.

TLR9 agonist regulates angiogenesis and inhibits corneal neovascularization

Myeloid cells are highly adaptable and may positively or negatively regulate angiogenesis dependent on the cognate and soluble signals they receive. Toll-like receptors (TLRs) initiate immune responses, orchestrate adaptive immune responses, and regulate vascular endothelial growth factor (VEGF)-mediated angiogenesis during wound healing. We investigated the possible role of TLR ligands in attenuation of new vessel growth via regulation of expression of VEGF or soluble fms-like tyrosine kinase-1 (sFlt-1) in both an aortic ring assay and a model of suture-induced corneal angiogenesis. The TLR3 ligand [poly(I:C)] markedly suppressed VEGF secretion and stimulated sFlt-1 release from macrophages. The aortic ring assay demonstrated that new vessels were promoted by the TLR2 ligand (heat killed Listeria monocytogenes) and the TLR4 ligand (lipopolysaccharide), concomitant with increased VEGF and matrix metalloproteinase 9 secretion. In contrast, the TLR9 ligand [oligodeoxynucleotide (ODN)1826] stimulated sFlt-1 secretion from macrophages and reduced the number of aortic ring vessel sprouts. ODN1826 also significantly reduced the length and volume of both hemangiogenesis and lymphangiogenesis in the suture-induced corneal angiogenesis model. Furthermore, 53 angiogenic factors were examined via protein array and compared between ODN1826- and water-treated corneas to interrogate the pathway of ODN1826 inhibition, demonstrating an up-regulation of Serpin E1 signal. Further clinical and IHC analyses of the aortic ring assay indicated that TLR9 suppressed tip cell migration and recruitment of mural cells and adventitial macrophages.

Atractylenolide-I sensitizes human ovarian cancer cells to paclitaxel by blocking activation of TLR4/MyD88-dependent pathway

Paclitaxel, a known TLR4 ligand, leads to activation of TLR4/MyD88-dependent pathway that mediates chemoresistance and tumor progression in epithelial ovarian carcinoma (EOC). Atractylenolide-I (AO-I), a novel TLR4-antagonizing agent, inhibits TLR4 signaling by interfering with the binding of LPS or paclitaxel to membrane TLR4 of human leukocytes. In this study, AO-I was found to attenuate paclitaxel-induced protein expression of IL-6, VEGF and survivin, and to enhance early apoptosis and growth inhibition in MyD88⁺ EOC cells; AO-I was shown to fit into the hydrophobic pocket of human MD-2 and to partially overlap with the binding site of paclitaxel by docking simulations, suggesting that AO-I may block the MD-2-mediated TLR4/MyD88-dependent paclitaxel signaling in MyD88⁺ EOC cells. Therefore, AO-I could significantly sensitize the response of MyD88⁺ EOC cells to paclitaxel by blocking MD-2-mediated TLR4/MyD88 signaling, and that AO-I-paclitaxel combination could be a promising strategy for the treatment of EOC with a functional TLR4/MyD88/NF-κB pathway.

Nitrogen catabolite repressible GAP1 promoter, a new tool for efficient recombinant protein production in S. cerevisiae

BACKGROUND

Decades of work requiring heterologous expression of eukaryotic proteins have shown that no expression system can be considered as the panacea and the appropriate expression strategy is often protein-dependent. In a large number of cases, yeasts have proven to be reliable organisms for heterologous protein expression by combining eukaryotic cellular organization with the ease of use of simpler microorganisms.

RESULTS

During this work, a novel promoter system based on the nitrogen catabolite regulation has been developed to produce the general amino acid permease (Gap1) in its natural host, the yeast Saccharomyces cerevisiae. A simple purification protocol was also established that allows to purify milligrams of Gap1 from cells cultivated in a five liters bio-reactor. In order to test the ability of the system to be used for expression of other proteins, the yeast specific transporter of γ-aminobutyric acid (Uga4), a human vesicular transporter of glutamate (Vglut1) and a small secreted glycoprotein (MD-2) were also expressed using the nitrogen catabolite regulation. All proteins were fused to GFP and their presence and localization were confirmed by western blot analysis and fluorescence microscopy.

CONCLUSIONS

Our work shows that the nitrogen catabolite repressible GAP1 promoter can be used to obtain high levels of recombinant protein while allowing for large biomass production in S. cerevisiae. This approach can be used to express membrane and soluble proteins from higher eukaryotes (from yeast to human). Therefore, this system stands as a promising alternative to commonly used expression procedure in yeasts.

MD-2 is involved in the stimulation of matrix metalloproteinase-1 expression by interferon-γ and high glucose in mononuclear cells - a potential role of MD-2 in Toll-like receptor 4-independent signalling

We reported recently that treatment of diabetic apolipoprotein E-deficient mice with the Toll-like receptor 4 (TLR4) antagonist Rs-LPS, a lipopolysaccharide isolated from Rhodobacter sphaeroides, inhibited atherosclerosis. Since it is known that Rs-LPS antagonizes TLR4 by targeting TLR4 co-receptor MD-2, this finding indicates that MD-2 is a potential target for the treatment of atherosclerosis. In this study, we determined if MD-2 is involved in the gene expression regulated by signalling pathways independent of TLR4. Given that interferon-γ (IFNγ) and hyperglycaemia play key roles in atherosclerosis, we determined if MD-2 is involved in IFN-γ and high-glucose-regulated gene expression in mononuclear cells. Results showed that IFN-γ and high glucose synergistically stimulated matrix metalloproteinase 1 (MMP-1), a proteinase essential for vascular tissue remodelling and atherosclerosis, in U937 mononuclear cells, but Rs-LPS inhibited the MMP-1 stimulation. To provide more evidence for a role of MD-2 in IFN-γ-stimulated MMP-1, studies using antibodies and small interfering RNA demonstrated that MD-2 blockade or knockdown attenuated the effect of IFN-γ on MMP-1. Furthermore, studies using PCR arrays showed that MD-2 blockade had a similar effect as IFN-γ receptor blockade on the inhibition of IFN-γ-stimulated pro-inflammatory molecules. Although these findings indicate the involvement of MD-2 in IFN-γ signalling, we also observed that MD-2 was up-regulated by IFN-γ and high glucose. We found that MD-2 up-regulation by IFN-γ played an essential role in the synergistic effect of IFN-γ and LPS on MMP-1 expression. Taken together, these findings indicate that MD-2 is involved in IFN-γ signalling and IFN-γ-augmented MMP-1 up-regulation by LPS.

The many faces of HMGB1: molecular structure-functional activity in inflammation, apoptosis, and chemotaxis

HMGB1 is a ubiquitous nuclear protein present in almost all cell types. In addition to its intracellular functions, HMGB1 can be extracellularly released, where it mediates activation of innate immune responses, including chemotaxis and cytokine release. HMGB1 contains three conserved redox-sensitive cysteines (C23, C45, and C106); modification of these cysteines determines the bioactivity of extracellular HMGB1. Firstly, the cytokine-stimulating activity of HMGB1 requires C23 and C45 to be in a disulfide linkage, at the same time that C106 must remain in its reduced form as a thiol. This distinctive molecular conformation enables HMGB1 to bind and signal via the TLR4/MD-2 complex to induce cytokine release in macrophages. Secondly, for HMGB1 to act as a chemotactic mediator, all three cysteines must be in the reduced form. This all-thiol HMGB1 exerts its chemotactic activity to initiate inflammation by forming a heterocomplex with CXCL12; that complex binds exclusively to CXCR4 to initiate chemotaxis. Thirdly, binding of the HMGB1 to CXCR4 or to TLR4 is completely prevented by all-cysteine oxidation. Also, the initial post-translational redox modifications of HMGB1 are reversible processes, enabling HMGB1 to shift from acting as a chemotactic factor to acting as a cytokine and vice versa. Lastly, post-translational acetylation of key lysine residues within NLSs of HMGB1 affects HMGB1 to promote inflammation; hyperacetylation of HMGB1 shifts its equilibrium from a predominant nuclear location toward a cytosolic and subsequent extracellular presence. Hence, post-translational modifications of HMGB1 determine its role in inflammation and immunity.

Reviewing and identifying amino acids of human, murine, canine and equine TLR4 / MD-2 receptor complexes conferring endotoxic innate immunity activation by LPS/lipid A, or antagonistic effects by Eritoran, in contrast to species-dependent modulation by lipid IVa

There is literature evidence gathered throughout the last two decades reflecting unexpected species differences concerning the immune response to lipid IVa which provides the opportunity to gain more detailed insight by the molecular modeling approach described in this study. Lipid IVa is a tetra-acylated precursor of lipid A in the biosynthesis of lipopolysaccharide (LPS) in Gram-negative bacteria. Lipid A of the prototypic E. coli-type is a hexa-acylated structure that acts as an agonist in all tested mammalian species by innate immunorecognition via the Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) receptor complex. In contrast, lipid IVa is proinflammatory in mouse cells (agonism) but it remains inactive to human macrophages and even antagonizes the action of potent agonists like E. coli-type lipid A. This particular ambivalent activity profile of lipid IVa has been confirmed in other mammalian species: in equine cells Lipid IVa also acts in a weak agonistic manner, whereas being inactive and antagonizing the lipid A-induced activation of canine TLR4/MD-2. Intriguingly, the respective TLR4 amino acid sequences of the latter species are more identical to the human (67%, 68%) than to the murine (62%, 58%) ortholog. In order to address the unpaired activity-sequence dualism for human, murine, canine and equine species regarding the activity of lipid IVa as compared to LPS and lipid A and, we review the literature and computationally pinpoint the differential biological effects of lipid IVa versus LPS and lipid A to specific amino acid residues. In contrast to lipid IVa the structurally related synthetic compound Eritoran (E5564) acts consistently in an antagonistic manner in these mammalian species and serves as a reference ligand for molecular modeling in this study. The combined evaluation of data sets provided by prior studies and in silico homology mapping of differential residues of TLR4/MD-2 complexes lends detailed insight into the driving forces of the characteristic binding modes of the lipid A domain in LPS and the precursor structure lipid IVa to the receptor complex in individual mammalian species.

Circulating levels of HMGB1 are correlated strongly with MD2 in HIV-infection: possible implication for TLR4-signalling and chronic immune activation

Progressive HIV infection is characterized by profound enterocyte damage, microbial translocation and chronic immune activation. We aimed to test whether High Mobility Group Box protein 1(HMGB1), a marker of cell death, alone, or in combination with LPS, might contribute to HIV-associated immune activation and progression. Altogether, 29 untreated HIV-infected individuals, 25 inflammatory bowel disease (IBD) patients and 30 controls were included. HIV-infected patients had lower plasma LPS levels than IBD patients, but higher levels of soluble CD14 and Myeloid Differentiation (MD) 2, which interacts with TLR4 to initiate LPS-signalling. Furthermore, plasma levels of HMGB1 and MD2 were correlated directly within the HIV-infected cohort (r = 0.89, P < 0.001) and the IBD-cohort (r = 0.85, P < 0.001), implying HMGB1 signalling through the MD2/TLR4-pathway. HMGB1 and LPS, although not inter-correlated, were both moderately (r = 0.4) correlated with CD38 density on CD8+ T cells in HIV progressors. The highest levels of CD38 density and MD2 were found in progressors with plasma levels of both LPS and HMGB1 above the fiftieth percentile. Our results could imply that, in some patients, immune activation is triggered by microbial translocation, in some by cell death and in some by HMGB1 in complex with bacterial products through activation of the MD2/TLR4-pathway.

Respiratory syncytial virus fusion protein-induced toll-like receptor 4 (TLR4) signaling is inhibited by the TLR4 antagonists Rhodobacter sphaeroides lipopolysaccharide and eritoran (E5564) and requires direct interaction with MD-2

Respiratory syncytial virus (RSV) is a leading cause of infant mortality worldwide. Toll-like receptor 4 (TLR4), a signaling receptor for structurally diverse microbe-associated molecular patterns, is activated by the RSV fusion (F) protein and by bacterial lipopolysaccharide (LPS) in a CD14-dependent manner. TLR4 signaling by LPS also requires the presence of an additional protein, MD-2. Thus, it is possible that F protein-mediated TLR4 activation relies on MD-2 as well, although this hypothesis has not been formally tested. LPS-free RSV F protein was found to activate NF-κB in HEK293T transfectants that express wild-type (WT) TLR4 and CD14, but only when MD-2 was coexpressed. These findings were confirmed by measuring F-protein-induced interleukin 1β (IL-1β) mRNA in WT versus MD-2(-/-) macrophages, where MD-2(-/-) macrophages failed to show IL-1β expression upon F-protein treatment, in contrast to the WT. Both Rhodobacter sphaeroides LPS and synthetic E5564 (eritoran), LPS antagonists that inhibit TLR4 signaling by binding a hydrophobic pocket in MD-2, significantly reduced RSV F-protein-mediated TLR4 activity in HEK293T-TLR4-CD14-MD-2 transfectants in a dose-dependent manner, while TLR4-independent NF-κB activation by tumor necrosis factor alpha (TNF-α) was unaffected. In vitro coimmunoprecipitation studies confirmed a physical interaction between native RSV F protein and MD-2. Further, we demonstrated that the N-terminal domain of the F1 segment of RSV F protein interacts with MD-2. These data provide new insights into the importance of MD-2 in RSV F-protein-mediated TLR4 activation. Thus, targeting the interaction between MD-2 and RSV F protein may potentially lead to novel therapeutic approaches to help control RSV-induced inflammation and pathology.

IMPORTANCE

This study shows for the first time that the fusion (F) protein of respiratory syncytial virus (RSV), a major cause of bronchiolitis and death, particularly in infants and young children, physically interacts with the Toll-like receptor 4 (TLR4) coreceptor, MD-2, through its N-terminal domain. We show that F protein-induced TLR4 activation can be blocked by lipid A analog antagonists. This observation provides a strong experimental rationale for testing such antagonists in animal models of RSV infection for potential use in people.

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.

Anopheles gambiae antiviral immune response to systemic O'nyong-nyong infection

Background

Mosquito-borne viral diseases cause significant burden in much of the developing world. Although host-virus interactions have been studied extensively in the vertebrate host, little is known about mosquito responses to viral infection. In contrast to mosquitoes of the Aedes and Culex genera, Anopheles gambiae, the principal vector of human malaria, naturally transmits very few arboviruses, the most important of which is O'nyong-nyong virus (ONNV). Here we have investigated the A. gambiae immune response to systemic ONNV infection using forward and reverse genetic approaches.

Methodology/Principal Findings

We have used DNA microarrays to profile the transcriptional response of A. gambiae inoculated with ONNV and investigate the antiviral function of candidate genes through RNAi gene silencing assays. Our results demonstrate that A. gambiae responses to systemic viral infection involve genes covering all aspects of innate immunity including pathogen recognition, modulation of immune signalling, complement-mediated lysis/opsonisation and other immune effector mechanisms. Patterns of transcriptional regulation and co-infections of A. gambiae with ONNV and the rodent malaria parasite Plasmodium berghei suggest that hemolymph immune responses to viral infection are diverted away from melanisation. We show that four viral responsive genes encoding two putative recognition receptors, a galectin and an MD2-like receptor, and two effector lysozymes, function in limiting viral load.

Conclusions/Significance

This study is the first step in elucidating the antiviral mechanisms of A. gambiae mosquitoes, and has revealed interesting differences between A. gambiae and other invertebrates. Our data suggest that mechanisms employed by A. gambiae are distinct from described invertebrate antiviral immunity to date, and involve the complement-like branch of the humoral immune response, supressing the melanisation response that is prominent in anti-parasitic immunity. The antiviral immune response in A. gambiae is thus composed of some key conserved mechanisms to target viral infection such as RNAi but includes other diverse and possibly species-specific mechanisms.

Mosquito-borne viral diseases are found across the globe and are responsible for numerous severe human infections. In order to develop novel methods for prevention and treatment of these diseases, detailed understanding of the biology of viral infection and transmission is required. Little is known about invertebrate responses to infection in mosquito hosts. In this study we used a model system of Anopheles gambiae mosquitoes and O'nyong-nyong virus to study mosquito immune responses to infection. We examined the global transcriptional responses of A. gambiae to viral infection of the mosquito blood equivalent (the hemolymph) identifying a number of genes with immune functions that are switched on or off in response to infection, including complement-like proteins that circulate in the mosquito hemolymph. The switching on of these genes combined with co-infection experiments with malaria parasites suggests that viral infection inhibits the melanisation pathway. Through silencing the function of a selection of viral responsive genes, we identified four genes that have roles in A. gambiae anti-viral immunity; two putative recognition receptors (a galectin and an MD2-like receptor); two effector lysozymes. These molecules have previously non-described roles in antiviral immunity, and suggest uncharacterised mechanisms for targeting viral infection in A. gambiae mosquitoes.

Host transcription factors in the immediate pro-inflammatory response to the parasitic mite Psoroptes ovis

Background

Sheep scab, caused by infestation with the ectoparasitic mite Psoroptes ovis, results in the rapid development of cutaneous inflammation and leads to the crusted skin lesions characteristic of the disease. We described previously the global host transcriptional response to infestation with P. ovis, elucidating elements of the inflammatory processes which lead to the development of a rapid and profound immune response. However, the mechanisms by which this response is instigated remain unclear. To identify novel methods of intervention a better understanding of the early events involved in triggering the immune response is essential. The objective of this study was to gain a clearer understanding of the mechanisms and signaling pathways involved in the instigation of the immediate pro-inflammatory response.

Results

Through a combination of transcription factor binding site enrichment and pathway analysis we identified key roles for a number of transcription factors in the instigation of cutaneous inflammation. In particular, defined roles were elucidated for the transcription factors NF-kB and AP-1 in the orchestration of the early pro-inflammatory response, with these factors being implicated in the activation of a suite of inflammatory mediators.

Conclusions

Interrogation of the host temporal response to P. ovis infestation has enabled the further identification of the mechanisms underlying the development of the immediate host pro-inflammatory response. This response involves key regulatory roles for the transcription factors NF-kB and AP-1. Pathway analysis demonstrated that the activation of these transcription factors may be triggered following a host LPS-type response, potentially involving TLR4-signalling and also lead to the intriguing possibility that this could be triggered by a P. ovis allergen.

The structural biology of Toll-like receptors

The membrane-bound Toll-like receptors (TLRs) trigger innate immune responses after recognition of a wide variety of pathogen-derived compounds. Despite the wide range of ligands recognized by TLRs, the receptors share a common structural framework in their extracellular, ligand-binding domains. These domains all adopt horseshoe-shaped structures built from leucine-rich repeat motifs. Typically, on ligand binding, two extracellular domains form an "m"-shaped dimer sandwiching the ligand molecule bringing the transmembrane and cytoplasmic domains in close proximity and triggering a downstream signaling cascade. Although the ligand-induced dimerization of these receptors has many common features, the nature of the interactions of the TLR extracellular domains with their ligands varies markedly between TLR paralogs.

Partially glycosylated dendrimers block MD-2 and prevent TLR4-MD-2-LPS complex mediated cytokine responses

The crystal structure of the TLR4-MD-2-LPS complex responsible for triggering powerful pro-inflammatory cytokine responses has recently become available. Central to cell surface complex formation is binding of lipopolysaccharide (LPS) to soluble MD-2. We have previously shown, in biologically based experiments, that a generation 3.5 PAMAM dendrimer with 64 peripheral carboxylic acid groups acts as an antagonist of pro-inflammatory cytokine production after surface modification with 8 glucosamine molecules. We have also shown using molecular modelling approaches that this partially glycosylated dendrimer has the flexibility, cluster density, surface electrostatic charge, and hydrophilicity to make it a therapeutically useful antagonist of complex formation. These studies enabled the computational study of the interactions of the unmodified dendrimer, glucosamine, and of the partially glycosylated dendrimer with TLR4 and MD-2 using molecular docking and molecular dynamics techniques. They demonstrate that dendrimer glucosamine forms co-operative electrostatic interactions with residues lining the entrance to MD-2's hydrophobic pocket. Crucially, dendrimer glucosamine interferes with the electrostatic binding of: (i) the 4'phosphate on the di-glucosamine of LPS to Ser118 on MD-2; (ii) LPS to Lys91 on MD-2; (iii) the subsequent binding of TLR4 to Tyr102 on MD-2. This is followed by additional co-operative interactions between several of the dendrimer glucosamine's carboxylic acid branches and MD-2. Collectively, these interactions block the entry of the lipid chains of LPS into MD-2's hydrophobic pocket, and also prevent TLR4-MD-2-LPS complex formation. Our studies have therefore defined the first nonlipid-based synthetic MD-2 antagonist using both animal model-based studies of pro-inflammatory cytokine responses and molecular modelling studies of a whole dendrimer with its target protein. Using this approach, it should now be possible to computationally design additional macromolecular dendrimer based antagonists for other Toll Like Receptors. They could be useful for treating a spectrum of infectious, inflammatory and malignant diseases.

Expression of Toll-like receptors 2, 3, 4, 6, 9, and MD-2 in the normal equine cornea, limbus, and conjunctiva

OBJECTIVE

Human corneal cells have detectable levels of TLRs 1-10. TLRs 2 and 4 are the major corneal receptors, recognizing the PAMPs associated with fungal invasion in humans. The conjunctiva and limbus contain TLRs 2, 4, and 9. Our purpose was to determine the expression of TLRs 2, 3, 4, 6, 9, and MD-2 in the normal equine cornea, conjunctiva, and limbus.

METHODS

Corneal, limbal, and conjunctival tissues were collected from seven euthanized horses having no evidence of ocular disease. RNA extraction with DNase-1 digestion was performed followed by RT-PCR to determine expression of TLRs 2, 3, 4, 6, 9, and MD-2. Products were resolved by electrophoresis on 1.5% agarose gels and visualized using ethidium bromide staining.

RESULTS

Expression of TLRs 2, 3, 4, 6, 9, and MD-2 was present in the cornea, limbus, and conjunctiva of each horse, except one horse, where TLR3 expression was unable to be demonstrated in the dorsal and ventral conjunctiva.

CONCLUSIONS

Confirming the expression of TLRs in equine ocular tissues is an initial step in identifying how they play a role in infectious keratitis, particularly fungal. The results further support the use of equine ocular tissues as a model for human fungal keratitis. Studies of the TLR expression together with their cytokine profile induced during equine fungal keratitis may help further clarify the pathogenesis of the disease and possibly lead to the development of new treatment protocols for both equines and humans.

Deficiency in myeloid differentiation factor-2 and toll-like receptor 4 expression attenuates nonalcoholic steatohepatitis and fibrosis in mice

Toll-like receptor 4 (TLR4) and its coreceptor, myeloid differentiation factor-2 (MD-2), are key in recognition of lipopolysaccharide (LPS) and activation of proinflammatory pathways. Here we tested the hypothesis that TLR4 and its coreceptor MD-2 play a central role in nonalcoholic steatohepatitis (NASH) and liver fibrosis in nonalcoholic fatty liver disease. Mice of control genotypes and those deficient in MD-2 or TLR4 [knockout (KO)] received methionine choline-deficient (MCD) or methionine choline-supplemented (MCS) diet. In mice of control genotypes, MCD diet resulted in NASH, liver triglycerides accumulation, and increased thiobarbituric acid reactive substances, a marker of lipid peroxidation, compared with MCS diet. These features of NASH were significantly attenuated in MD-2 KO and TLR4 KO mice. Serum alanine aminotransferase, an indicator of liver injury, was increased in MCD diet-fed genotype controls but was attenuated in MD-2 KO and TLR4 KO mice. Inflammatory activation, indicated by serum TNF-α and nictoinamide adenine dinucleotide phosphate oxidase complex mRNA expression and activation, was significantly lower in MCD diet-fed MD-2 KO and TLR4 KO compared with corresponding genotype control mice. Markers of liver fibrosis [collagen by Sirius red and α-smooth muscle actin (SMA) staining, procollagen-I, transforming growth factor-β1, α-SMA, matrix metalloproteinase-2, and tissue inhibitor of matrix metalloproteinase-1 mRNA] were attenuated in MD-2 and TLR4 KO compared with their control genotype counterparts. In conclusion, our results demonstrate a novel, critical role for LPS recognition complex, including MD-2 and TLR4, through NADPH activation in liver steatosis, and fibrosis in a NASH model in mice.

Transcriptomic analysis of the temporal host response to skin infestation with the ectoparasitic mite Psoroptes ovis

Background

Infestation of ovine skin with the ectoparasitic mite Psoroptes ovis results in a rapid cutaneous immune response, leading to the crusted skin lesions characteristic of sheep scab. Little is known regarding the mechanisms by which such a profound inflammatory response is instigated and to identify novel vaccine and drug targets a better understanding of the host-parasite relationship is essential. The main objective of this study was to perform a combined network and pathway analysis of the in vivo skin response to infestation with P. ovis to gain a clearer understanding of the mechanisms and signalling pathways involved.

Results

Infestation with P. ovis resulted in differential expression of 1,552 genes over a 24 hour time course. Clustering by peak gene expression enabled classification of genes into temporally related groupings. Network and pathway analysis of clusters identified key signalling pathways involved in the host response to infestation. The analysis implicated a number of genes with roles in allergy and inflammation, including pro-inflammatory cytokines (IL1A, IL1B, IL6, IL8 and TNF) and factors involved in immune cell activation and recruitment (SELE, SELL, SELP, ICAM1, CSF2, CSF3, CCL2 and CXCL2). The analysis also highlighted the influence of the transcription factors NF-kB and AP-1 in the early pro-inflammatory response, and demonstrated a bias towards a Th2 type immune response.

Conclusions

This study has provided novel insights into the signalling mechanisms leading to the development of a pro-inflammatory response in sheep scab, whilst providing crucial information regarding the nature of mite factors that may trigger this response. It has enabled the elucidation of the temporal patterns by which the immune system is regulated following exposure to P. ovis, providing novel insights into the mechanisms underlying lesion development. This study has improved our existing knowledge of the host response to P. ovis, including the identification of key parallels between sheep scab and other inflammatory skin disorders and the identification of potential targets for disease control.

Distinct roles for Dectin-1 and TLR4 in the pathogenesis of Aspergillus fumigatus keratitis

Aspergillus species are a major worldwide cause of corneal ulcers, resulting in visual impairment and blindness in immunocompetent individuals. To enhance our understanding of the pathogenesis of Aspergillus keratitis, we developed a murine model in which red fluorescent protein (RFP)-expressing A. fumigatus (Af293.1RFP) conidia are injected into the corneal stroma, and disease progression and fungal survival are tracked over time. Using Mafia mice in which c-fms expressing macrophages and dendritic cells can be induced to undergo apoptosis, we demonstrated that the presence of resident corneal macrophages is essential for production of IL-1beta and CXCL1/KC, and for recruitment of neutrophils and mononuclear cells into the corneal stroma. We found that beta-glucan was highly expressed on germinating conidia and hyphae in the cornea stroma, and that both Dectin-1 and phospho-Syk were up-regulated in infected corneas. Additionally, we show that infected Dectin-1(-/-) corneas have impaired IL-1beta and CXCL1/KC production, resulting in diminished cellular infiltration and fungal clearance compared with control mice, especially during infection with clinical isolates expressing high beta-glucan. In contrast to Dectin 1(-/-) mice, cellular infiltration into infected TLR2(-/-), TLR4(-/-), and MD-2(-/-) mice corneas was unimpaired, indicating no role for these receptors in cell recruitment; however, fungal killing was significantly reduced in TLR4(-/-) mice, but not TLR2(-/-) or MD-2(-/-) mice. We also found that TRIF(-/-) and TIRAP(-/-) mice exhibited no fungal-killing defects, but that MyD88(-/-) and IL-1R1(-/-) mice were unable to regulate fungal growth. In conclusion, these data are consistent with a model in which beta-glucan on A.fumigatus germinating conidia activates Dectin-1 on corneal macrophages to produce IL-1beta, and CXCL1, which together with IL-1R1/MyD88-dependent activation, results in recruitment of neutrophils to the corneal stroma and TLR4-dependent fungal killing.

Alcoholic liver disease and the gut-liver axis

Alcoholic liver disease (ALD) is one of the leading causes of liver diseases and liver-related death worldwide. Of the many factors that contribute to the pathogenesis of ALD, gut-derived lipopolysaccharide (LPS) plays a central role in induction of steatosis, inflammation, and fibrosis in the liver. In this review, we discuss the mechanisms by which alcohol contributes to increased gut permeability, the activation of Kupffer cells, and the inflammatory cascade by LPS. The role of the Toll-like receptor 4 (TLR4) complex in LPS recognition and the importance of the TLR4-induced signaling pathways are evaluated in ALD.

Toll-like receptor 4 decoy, TOY, attenuates gram-negative bacterial sepsis

Lipopolysaccharide (LPS), the Gram-negative bacterial outer membrane glycolipid, induces sepsis through its interaction with myeloid differentiation protein-2 (MD-2) and Toll-like receptor 4 (TLR4). To block interaction between LPS/MD-2 complex and TLR4, we designed and generated soluble fusion proteins capable of binding MD-2, dubbed TLR4 decoy receptor (TOY) using ‘the Hybrid leucine-rich repeats (LRR) technique’. TOY contains the MD-2 binding ectodomain of TLR4, the LRR motif of hagfish variable lymphocyte receptor (VLR), and the Fc domain of IgG1 to make it soluble, productive, and functional. TOY exhibited strong binding to MD-2, but not to the extracellular matrix (ECM), resulting in a favorable pharmacokinetic profile in vivo. TOY significantly extended the lifespan, when administered in either preventive or therapeutic manners, in both the LPS- and cecal ligation/puncture-induced sepsis models in mice. TOY markedly attenuated LPS-triggered NF-κB activation, secretion of proinflammatory cytokines, and thrombus formation in multiple organs. Taken together, the targeting strategy for sequestration of LPS/MD-2 complex using the decoy receptor TOY is effective in treating LPS- and bacteria-induced sepsis; furthermore, the strategy used in TOY development can be applied to the generation of other novel decoy receptor proteins.

Regulation of Toll-like receptor 4-associated MD-2 in intestinal epithelial cells: a comprehensive analysis

The intestinal epithelium maintains a state of controlled inflammation despite continuous contact with Gram-negative commensal bacteria and lipopolysaccharide (LPS) on its luminal surface. Recognition of LPS by the Toll-like receptor (TLR) 4/MD-2 complex results in pro-inflammatory gene expression and cytokine secretion in intestinal epithelial cells (IECs). We have shown that IECs express low levels of MD-2 and TLR4 and are poorly responsive to LPS. In this study, we did a comprehensive analysis to understand the immune-mediated and epigenetic mechanisms by which IECs down-regulate MD-2 expression. Expression of MD-2 and TLR4 mRNA was examined in human inflammatory bowel disease and intestinal epithelial cell lines (T84, HT-29, Caco-2). Nuclear factor-kappaB transcriptional activation was used as a measure of LPS responsiveness. Intestinal epithelial cells in patients with inflammatory bowel disease exhibited increased expression of MD-2 and TLR4 mRNA. Lipopolysaccharide responsiveness in IECs was polarized to the basolateral membrane. Bisulfite sequencing of the MD-2 promoter demonstrated methylation of CpG dinucleotides. Inhibition of methylation by 5-azacytidine and histone de-actylation by trichostatin A, two forms of epigenetic silencing, resulted in increased mRNA expression of MD-2 in IECs. These results demonstrate various molecular mechanisms by which IECs down-regulate MD-2 and, thereby, protect against dysregulated inflammation to commensal bacteria in the intestinal lumen.

Innate microbial sensors and their relevance to allergy

The innate immune system oversees the gateway to immunity with its microbial sensors. Innate microbial sensors are germ line-encoded receptors with genetically predetermined specificities for microbes. The readiness and effectiveness of the innate immune system to provide immediate and appropriate responses at the host-environment interface is dependent on its sensitive and comprehensive microbial detection systems. The purpose of this review is to provide an overview of innate microbial sensors, our growing understanding of their diverse repertoire, and their elegant structural and functional approaches to microbial recognition. Their relevance to allergic disease is also discussed: the potential recognition and uptake of allergens by some of these receptors, inhibited expression of other microbial sensors by allergic immune responses and inflammation, and their upregulation by microbial exposures in early life that may help to protect against the development of allergic immune responses and disease.

A Drosophila protein family implicated in pheromone perception is related to Tay-Sachs GM2-activator protein

Low volatility, lipid-like cuticular hydrocarbon pheromones produced by Drosophila melanogaster females play an essential role in triggering and modulating mating behavior, but the chemosensory mechanisms involved remain poorly understood. Recently, we showed that the CheB42a protein, which is expressed in only 10 pheromone-sensing taste hairs on the front legs of males, modulates progression to late stages of male courtship behavior in response to female-specific cuticular hydrocarbons. Here we report that expression of all 12 genes in the CheB gene family is predominantly or exclusively gustatory-specific, and occurs in many different, often non-overlapping patterns. Only the Gr family of gustatory receptor genes displays a comparable variety of gustatory-specific expression patterns. Unlike Grs, however, expression of all but one CheB gene is sexually dimorphic. Like CheB42a, other CheBs may therefore function specifically in gustatory perception of pheromones. We also show that CheBs belong to the ML superfamily of lipid-binding proteins, and are most similar to human GM2-activator protein (GM2-AP). In particular, GM2-AP residues involved in ligand binding are conserved in CheBs but not in other ML proteins. Finally, CheB42a is specifically secreted into the inner lumen of pheromone-sensing taste hairs, where pheromones interact with membrane-bound receptors. We propose that CheB proteins interact directly with lipid-like Drosophila pheromones and modulate their detection by the gustatory signal transduction machinery. Furthermore, as loss of GM2-AP in Tay-Sachs disease prevents degradation of GM2 gangliosides and results in neurodegeneration, the function of CheBs in pheromone response may involve biochemical mechanisms critical for lipid metabolism in human neurons.

Toll-like receptors at the ocular surface

The Toll-like receptor (TLR) family of pathogen recognition molecules has an important role in recognizing microbial pathogens and microbial breakdown products. Activation of TLRs in the corneal epithelium induces CXC chemokine production and recruitment of neutrophils to the corneal stroma. Although essential for pathogen killing, neutrophils can cause extensive tissue damage, leading to visual impairment and blindness. In this review, we examine the role of TLRs in microbial keratitis and in noninfectious corneal inflammation, most commonly associated with contact lens wear. we present recent findings on TLR signaling pathways in the cornea, including MyD88- and TRIF-dependent responses and discuss the role of resident macrophages and dendritic cells. Finally, we examine the potential for targeting the TLR pathway as a potential therapeutic intervention for microbial keratitis and contact lens-associated corneal inflammation.

Analysis of proteinase-activated receptor 2 and TLR4 signal transduction: a novel paradigm for receptor cooperativity

Proteinase-activated receptor 2 (PAR2), a seven-transmembrane G protein-coupled receptor, is activated at inflammatory sites by proteolytic cleavage of its extracellular N terminus by trypsin-like enzymes, exposing a tethered, receptor-activating ligand. Synthetic agonist peptides (AP) that share the tethered ligand sequence also activate PAR2, often measured by Ca2+ release. PAR2 contributes to inflammation through activation of NF-kappaB-regulated genes; however, the mechanism by which this occurs is unknown. Overexpression of human PAR2 in HEK293T cells resulted in concentration-dependent, PAR2 AP-inducible NF-kappaB reporter activation that was protein synthesis-independent, yet blocked by inhibitors that uncouple Gi proteins or sequester intracellular Ca2+. Because previous studies described synergistic PAR2- and TLR4-mediated cytokine production, we hypothesized that PAR2 and TLR4 might interact at the level of signaling. In the absence of TLR4, PAR2-induced NF-kappaB activity was inhibited by dominant negative (DN)-TRIF or DN-TRAM constructs, but not by DN-MyD88, findings confirmed using cell-permeable, adapter-specific BB loop blocking peptides. Co-expression of TLR4/MD-2/CD14 with PAR2 in HEK293T cells led to a synergistic increase in AP-induced NF-kappaB signaling that was MyD88-dependent and required a functional TLR4, despite the fact that AP exhibited no TLR4 agonist activity. Co-immunoprecipitation of PAR2 and TLR4 revealed a physical association that was AP-dependent. The response to AP or lipopolysaccharide was significantly diminished in TLR4(-/-) and PAR2(-/-) macrophages, respectively, and SW620 colonic epithelial cells exhibited synergistic responses to co-stimulation with AP and lipopolysaccharide. Our data suggest a unique interaction between two distinct innate immune response receptors and support a novel paradigm of receptor cooperativity in inflammatory responses.

Increased expression and internalization of the endotoxin coreceptor CD14 in enterocytes occur as an early event in the development of experimental necrotizing enterocolitis

BACKGROUND

The early signaling events in the development of necrotizing enterocolitis (NEC) remain undefined. We have recently shown that the endotoxin (lipopolysaccharide [LPS]) receptor toll-like receptor 4 (TLR4) on enterocytes is critical in the pathogenesis of experimental NEC. Given that the membrane receptor CD14 is known to facilitate the activation of TLR4, we now hypothesize that endotoxemia induces an early upregulation of CD14 in enterocytes and that this participates in the early intestinal inflammatory response in the development of NEC.

METHODS

IEC-6 enterocytes were treated with LPS (50 microg/mL), and the subcellular localization of CD14 and TLR4 was assessed by confocal microscopy. C57/Bl6 or CD14-/- mice were treated with LPS (5 mg/kg), whereas experimental NEC was induced using a combination of gavage formula feeding and intermittent hypoxia. CD14 expression was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and reverse transcriptase-polymerase chain reaction, and interleukin 6 was quantified by enzyme-linked immunosorbent assay and reverse transcriptase-polymerase chain reaction.

RESULTS

Exposure of IEC-6 enterocytes to LPS led to an initial, transient increase in CD14 expression. The early increase in CD14 expression was associated with internalization of CD14 to a perinuclear compartment where increased colocalization with TLR4 was noted. The in vivo significance of these findings is suggested as treatment of mice with LPS led to an early increase in CD14 expression in the intestinal mucosa, whereas the persistent endotoxemia of experimental NEC was associated with decreased CD14 expression within enterocytes.

CONCLUSIONS

LPS signaling in the enterocyte is marked by an early, transient increase in expression of CD14 and redistribution of the receptor. This process may contribute to the early activation of the intestinal inflammatory response that is observed in the development of NEC.