An NMR spectroscopy and molecular mechanics study of the molecular basis for the supramolecular structure of lipopolysaccharides

A model of Neisseria meningitidis vaccine based on LPS micelles detoxified by synthetic antiendotoxin peptides

We describe a model of vaccine based on detoxified endotoxin (LPS) conserving the supramolecular structure of micelles. Detoxification of LPS from Neisseria meningitidis group A, strain A1 (LPS A1), has been achieved by complex formation with a synthetic anti-endotoxin peptide (SAEP 2) binding to the lipid A moiety of LPS A1 with high affinity. Following subcutaneous injection in SW mice, LPS A1/SAEP 2 complex induced high titers of boostable IgG antibodies against the immunotype determinants of LPS A1, cross-reactive with group B LPS in either purified or cell-associated form. These antibodies were able to functionally fix and activate homologous and heterologous species of complement after binding to LPS A1-coated sheep erythrocytes. None of the IgG antibodies induced were specific for lipid A or SAEP 2 and none of the IgG antibodies cross-reacted with heterologous LPS. The purified IgG polyclonal antibodies significantly inhibited serum TNF production in CD1 mice intravenously challenged by homologous but not heterologous LPS. The immunogenic properties of LPS A1/SAEP 2 complex, investigated by the kinetic, magnitude and sub-isotype composition of the polyclonal antibodies induced, were comparable to those of a glycoconjugate obtained by covalent binding of LPS A1, detoxified by SAEP 2, to BSA working as a T-cell dependent carrier protein. The results obtained suggest that LPS behaves in vivo as a T-cell dependent antigen. The strategy of properly delivering to the immune system of mammalians, non-toxic LPS fully expressing its supramolecular antigenic structure, represents a novel approach for development of a new generation of R- and S-LPS/SAEP complex-based vaccines for prophylaxis of specific Gram-negative infections leading to sepsis and endotoxemia.

New synthesis and conformational analysis of lipid A: biological activity and supramolecular assembly

In order to investigate precise structural requirements for expression of the biological activity of lipid A, new structural analogs were synthesized via improved, highly efficient preparative routes. The number and chain length of the fatty acyl groups proved to have crucial influence on the biological activity. For the study of conformation relevant to activity of lipid A by means of NMR spectrometry, lipid A analogs [13C]-labeled at the 6-position were then synthesized starting from 6-[13C]-glucose. Precise analysis of J CH and NOE data disclosed a particular molecular conformation and characteristic supramolecular assembly in an aqueous SDS micelle solution of the tetraacyl biosynthetic precursor molecule. The observed mode of assembly may reflect the actual behavior of lipid A molecules in the cell surface outer membrane of living bacteria. A biologically inactive, artificial, tetra-acyl analog having shorter acyl chains exhibits, by contrast, distinct conformations with no sign of supramolecule formation. The acyl moieties in lipid A are thus found to play an important role in regulating the overall conformation of the hydrophilic backbone.

Chemistry of lipid A: at the heart of innate immunity

In many Gram-negative bacteria, lipopolysaccharide (LPS) and its lipid A moiety are pivotal for bacterial survival. Depending on its structure, lipid A carries the toxic properties of the LPS and acts as a potent elicitor of the host innate immune system via the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex. It often causes a wide variety of biological effects ranging from a remarkable enhancement of the resistance to the infection to an uncontrolled and massive immune response resulting in sepsis and septic shock. Since the bioactivity of lipid A is strongly influenced by its primary structure, a broad range of chemical syntheses of lipid A derivatives have made an enormous contribution to the characterization of lipid A bioactivity, providing novel pharmacological targets for the development of new biomedical therapies. Here, we describe and discuss the chemical aspects regarding lipid A and its role in innate immunity, from the (bio)synthesis, isolation and characterization to the molecular recognition at the atomic level.

Comparison of lipopolysaccharide structures of Bordetella pertussis clinical isolates from pre- and post-vaccine era

Endotoxins are lipopolysaccharides (LPS), and major constituents of the outer membrane of Gram-negative bacteria. Bordetella pertussis LPS were the only major antigens, of this agent of whooping-cough, that were not yet analyzed on isolates from the pre- and post-vaccination era. We compared here the LPS structures of four clinical isolates with that of the vaccine strain BP 1414. All physico-chemical analyses, including SDS-PAGE, TLC, and different MALDI mass spectrometry approaches were convergent. They helped demonstrating that, on the contrary to some other B. pertussis major antigens, no modification occurred in the dodecasaccharide core structure, as well as in the whole LPS molecules. These results are rendering these major antigens good potential vaccine components. Molecular modeling of this conserved LPS structure also confirmed the conclusions of previous experiments leading to the production of anti-LPS monoclonal antibodies and defining the main epitopes of these major antigens.

Multi-scale analysis of the toraymyxin adsorption cartridge. Part I: molecular interaction of polymyxin B with endotoxins

Endotoxins or lipopolysaccharides are the main constituents of the outer leaflet of Gram-negative bacteria membrane and play a central role in the pathogenesis of the septic shock. Polymyxin B has both antibacterial and antiendotoxin capability; indeed it is able to destroy the bacterial outer membrane and bind endotoxin neutralizing its toxic effects. Cartridges containing polymyxin B-immobilized fibers (Toraymyxin PMX-F, Toray Industries, Japan) are used in extracorporeal hemoperfusion to remove circulating endotoxin. The aim of this study is the characterization of the polymyxin B-endotoxin system at the molecular level, thus providing quantitative evaluation of the binding forces exerted in the molecular complex. Polymyxin B was interfaced with five molecular models of lipopolysaccharides differing in their structure and molecular mechanics simulations were performed at different intermolecular distances aimed at calculating the interaction energies of the complex. Binding forces were calculated by fitting interaction energies data. Results show that in the short range the polymyxin B-endotoxin complex is mediated by hydrophobic forces and in the long range the complex is driven by ionic forces only. From a mechanical standpoint, polymyxin B-endotoxin complex is characterized by maximum binding forces ranging between 1.39 nN to 3.79 nN. The knowledge of the binding force behavior at different intermolecular distances allows further investigations at higher scale level (Part II).

Mitogenicity of a spread film of monophosphoryl lipid A

When spread at the air-water interface, monophosphoryl lipid A (MPLA) forms stable insoluble monolayers that collapse at approximately 55 dyn/cm. At collapse, the exclusion area of each molecule is approximately 119 Angstrom(2), consistent with the cross-sectional area of the lipid's 6 acyl chains. The nominal thickness of such films is approximately 22 Angstrom, determined, presumably, by the length of the acyl chains. For biological modeling of MPLA films, a system was developed in which monolayers of the lipid are supported by monodisperse hydrophobic beads of microscopic dimensions. Beads coated with MPLA monolayers within which the nominal area of each molecule is approximately equivalent to the "take-off" area of the lipid at the air-water interface, 280 Angstrom(2), are mitogenic for spleen cells. Given the natural occurrence of lipid A in the bacterial cell wall as well as the inherent stability of lipid A films, it seems reasonable to assume that at least some of the biological activities attributed to the lipid derive from its presentation/operation at an interface, i.e., on a surface. We propose beads coated with adsorbed films of lipid A will prove useful tools for modeling the activities of the lipid both in vitro and in vivo, and for elucidating the surface dependency and structural requirements of those activities.

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.

Helical disposition of proteins and lipopolysaccharide in the outer membrane of Escherichia coli

In bacteria, several physiological processes once thought to be the products of uniformly dispersed reactions are now known to be highly asymmetric, with some exhibiting interesting geometric localizations. In particular, the cell envelope of Escherichia coli displays a form of subcellular differentiation in which peptidoglycan and outer membrane proteins at the cell poles remain stable for generations while material in the lateral walls is diluted by growth and turnover. To determine if material in the side walls was organized in any way, we labeled outer membrane proteins with succinimidyl ester-linked fluorescent dyes and then grew the stained cells in the absence of dye. Labeled proteins were not evenly dispersed in the envelope but instead appeared as helical ribbons that wrapped around the outside of the cell. By staining the O8 surface antigen of E. coli 2443 with a fluorescent derivative of concanavalin A, we observed a similar helical organization for the lipopolysaccharide (LPS) component of the outer membrane. Fluorescence recovery after photobleaching indicated that some of the outer membrane proteins remained freely diffusible in the side walls and could also diffuse into polar domains. On the other hand, the LPS O antigen was virtually immobile. Thus, the outer membrane of E. coli has a defined in vivo organization in which a subfraction of proteins and LPS are embedded in stable domains at the poles and along one or more helical ribbons that span the length of this gram-negative rod.

Low-temperature-induced changes in composition and fluidity of lipopolysaccharides in the antarctic psychrotrophic bacterium Pseudomonas syringae

The Antarctic psychrotrophic bacterium Pseudomonas syringae was more sensitive to polymyxin B at a lower (4 degrees C) temperature of growth than at a higher (22 degrees C) temperature. The amount of hydroxy fatty acids in the lipopolysaccharides (LPS) also increased at the lower temperature. These changes correlated with the increase in fluidity of the hydrophobic phase of lipopolysaccharide aggregates in vitro.

The core oligosaccharide component from Mannheimia (Pasteurella) haemolytica serotype Al lipopolysaccharide contains L-glycero-D-manno- and D-glycero-D-manno-heptoses: Analysis of the structure and conformation by high-resolution NMR spectroscopy

Previous studies from our laboratory have indicated that the lipopolysaccharide (LPS) from Mannheimia haemolytica serotype A1 contains both L-glycero-D-manno-heptose and D-glycero-D-manno-heptose residues. NMR methods making use of 1D 1H selective excitation and 2D (1H, 13C) and (1H, 31P) heteronuclear experiments were used for the structural determination of the major core oligosaccharide components of the deacylated low-molecular-mass LPS obtained following sequential treatment with anhydrous hydrazine and aq KOH. The core oligosaccharide region was found to be composed of a branched octasaccharide linked to the deacylated lipid A moiety via a 3-deoxy-4-phospho-D-manno-oct-2-ulosonate residue having the structure,[Formula: see text]Heterogeneity was found to be present at several linkages. NMR methods were devised to distinguish between the diastereomeric forms of the heptose residues. Synthesized monosaccharides of L-D- and D-D-heptose were used as model compounds for analysis of the 1H and 13C NMR chemical shifts and proton coupling constants. Molecular modeling using a Monte Carlo method for conformational analysis of saccharides was used to determine the conformation of the inner core of the oligosaccharide and to establish the stereochemical relationships between the heptoses.Key words: LPS, NMR, conformation, oligosaccharide, heptose.

Structural determination of lipid A of the lipopolysaccharide from Pseudomonas reactans. A pathogen of cultivated mushrooms

The chemical structure of lipid A from the lipopolysaccharide of the mushroom-associated bacterium Pseudomonas reactans, a pathogen of cultivated mushroom, was elucidated by compositional analysis and spectroscopic methods (MALDI-TOF and two-dimensional NMR). The sugar backbone was composed of the beta-(1'-->6)-linked d-glucosamine disaccharide 1-phosphate. The lipid A fraction showed remarkable heterogeneity with respect to the fatty acid and phosphate composition. The major species are hexacylated and pentacylated lipid A, bearing the (R)-3-hydroxydodecanoic acid [C12:0 (3OH)] in amide linkage and a (R)-3-hydroxydecanoic [C10:0 (3OH)] in ester linkage while the secondary fatty acids are present as C12:0 and/or C12:0 (2-OH). A nonstoichiometric phosphate substitution at position C-4' of the distal 2-deoxy-2-amino-glucose was detected. Interestingly, the pentacyl lipid A is lacking a primary fatty acid, namely the C10:0 (3-OH) at position C-3'. The potential biological meaning of this peculiar lipid A is also discussed.

Computer simulation of the rough lipopolysaccharide membrane of Pseudomonas aeruginosa

Lipopolysaccharides (LPSs) form the major constituent of the outer membrane of Gram-negative bacteria, and are believed to play a key role in processes that govern microbial metal binding, microbial adsorption to mineral surfaces, and microbe-mediated oxidation/reduction reactions at the bacterial exterior surface. A computational modeling capability is being developed for the study of geochemical reactions at the outer bacterial envelope of Gram-negative bacteria. A molecular model for the rough LPS of Pseudomonas aeruginosa has been designed based on experimentally determined structural information. An electrostatic model was developed based on Hartree-Fock SCF calculations of the complete LPS molecule to obtain partial atomic charges. The exterior of the bacterial membrane was assembled by replication of a single LPS molecule and a single phospholipid molecule. Molecular dynamics simulations of the rough LPS membrane of P. aeruginosa were carried out and trajectories were analyzed for the energetic and structural factors that determine the role of LPS in processes at the cell surface.

Molecular dynamics study on lipid A from Escherichia coli: insights into its mechanism of biological action

Structural properties of the Escherichia coli lipid A moiety were analysed by means of molecular mechanics and molecular dynamics simulations and compared to synthetic monophospho and dephospho analogues with different biological activities in the Limulus assay. The conformation of glucosamine disaccharide headgroup, order and packing of fatty acid chains, solvation of phosphate groups, coordination by water molecules, sodium counterions and models of cationic amino acid side chains were described in terms of mean values, mean residence times, radial distribution functions, coordination numbers, solvation and interaction energies. Solvation and polar interactions of the phosphate groups were correlated to known biological activities the lipid A variants. The observed relationship between the biological effect and the number and position of the phosphate groups were explained with the help of simple mechanistic models of lipid A action. The possible mechanism of action involving specific binding of lipid A disaccharide headgroup to cationic residues of a receptor model was compared with an alternative mechanism, which assumes a relationship between the ability to adopt non-lamellar supramolecular structures and the biological activity. Conclusions are drawn about the probable mode of lipid A action. Implications for rational drug design of endotoxin-neutralising agents are discussed.

Interpretation of biological activity data of bacterial endotoxins by simple molecular models of mechanism of action

Lipid A moiety has been identified as the bioactive component of bacterial endotoxins (lipopolysaccharides). However, the molecular mechanism of biological activity of lipid A is still not fully understood. This paper contributes to understanding of the molecular mechanism of action of bacterial endotoxins by comparing molecular modelling results for two possible mechanisms with the underlying experimental data. Mechanisms of action involving specific binding of lipid A to a protein receptor as well as nonspecific intercalation into phospholipid membrane of a host cell were modelled and analysed. As the cellular receptor for endotoxin has not been identified, a model of a peptidic pseudoreceptor was proposed, based on molecular structure, symmetry of the lipid A moiety and the observed character of endotoxin-binding sites in proteins. We have studied the monomeric form of lipid A from Escherichia coli and its seven synthetic analogues with varying numbers of phosphate groups and correlated them with known biological activities determined by the Limulus assay. Gibbs free energies associated with the interaction of lipid A with the pseudoreceptor model and intercalation into phospholipid membrane calculated by molecular mechanics and molecular dynamics methods were used to compare the two possible mechanisms of action. The results suggest that specific binding of lipid A analogues to the peptidic pseudoreceptor carrying an amphipathic cationic binding pattern BHPHB (B, basic; H, hydrophobic; P, polar residue, respectively) is energetically more favourable than intercalation into the phospholipid membrane. In addition, binding affinities of lipid A analogues to the best minimum binding sequence KFSFK of the pseudoreceptor correlated with the experimental Limulus activity parameter. This correlation enabled us to rationalize the observed relationship between the number and position of the phosphate groups in the lipid A moiety and its biological activity in terms of specific ligand-receptor interactions. If lipid A-receptor interaction involves formation of phosphate-ammonium ion-pair(s) with cationic amino-acid residues, the specific mechanism of action was fully consistent with the underlying experimental data. As a consequence, recognition of lipid A variants by an amphipathic binding sequence BHPHB of a host-cell protein receptor might represent the initial and/or rate-determining molecular event of the mechanism of action of lipid A (or endotoxin). The insight into the molecular mechanism of action and the structure of the lipid A-binding pattern have potential implications for rational drug design strategies of endotoxin-neutralizing agents or binding factors.

Pathobiology of lipopolysaccharide

Lipopolysaccharide is a component of the gram-negative bacterial cell wall that is responsible for 25,000-50,000 deaths in the United States each year. The sequelae of gram-negative infection and septicemia leading to death include fever, hypotension with inadequate tissue perfusion, and disseminated intravascular coagulation. It is clear that different cell types respond differently to lipopolysaccharide. Furthermore, various autacoids and cytokines are released that can affect cellular function even in cell types that do not recognize lipopolysaccharide. Despite advances made in the etiology of septic shock and organ failure, therapy is still for the most part supportive and largely ineffective. The aim of this review is to summarize the current understanding of the role of lipopolysaccharide in the development of septicemia by examining signal transduction and therapeutic approaches.