Lipopolysaccharide of the Phototrophic BacteriumRhodospirillum fulvum

Characterization of lipid A profiles from Shigella flexneri variant X lipopolysaccharide

RATIONALE

In developing countries, Shigella flexneri (Sf) is the major causative agent of the endemic shigellosis (bacillary dysentery) responsible annually for one million fatalities mostly among infants. Lipopolysaccharides (LPSs) are characteristic components of the outer membrane of the overwhelming majority of Gram-negative bacteria. Since lipid A is essential for the viability of the Gram-negative bacteria, it is subject to extensive chemical studies with new analytical techniques.

METHODS

Lipid A was released by mild acid hydrolysis from the lipopolysaccharide which was obtained via the phenol/water extraction, purified and analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and matrix-assisted laser desorption/ionization laser-induced dissociation tandem mass spectrometry (MALDI-LID-MS/MS).

RESULTS

A detailed structural study of the whole lipid A obtained from S. flexneri variant X was carried out for the first time. Thus, we have shown that lipid A is a heterogeneous mixture having different numbers of acylated and phosphoethanolamine groups attached to the diglucosamine backbone. Furthermore, we found in the phenol phase an unusual hepta-acylated lipid A species, although the abundance was very low.

CONCLUSIONS

MALDI-TOF-MS allowed us to unravel the lipid A heterogeneity, which was not previously reported in Sf LPS. It is well known that slight variations of the chemical structure of lipid A may change its biological activity. Thus, the knowledge of the detailed chemical structure represents an essential step for further development of new preventive or therapeutically active compounds.

Structure of a novel lipid A obtained from the lipopolysaccharide of Caulobacter crescentus

Caulobacter crescentus CB15 is a dimorphic bacterium that is best known as a prokaryotic model for cell development. However, it is also being exploited in biotechnology, where the crystalline surface (S-layer) protein secretion system has been adapted for heterologous protein display or secretion. Because the S-layer attaches to the cell surface via lipopolysaccharide (LPS) and since the LPS represents a potential endotoxin contaminant of recombinant proteins, the lipid A component was examined in detail. LPS was acid hydrolyzed to obtain crude lipid A, which was methylated and purified by HPLC. HPLC peak fractions were analyzed by mass spectrometry and nuclear magnetic resonance spectroscopy. The structure of the major lipid A of C. crescentus comprised the tetrasaccharide backbone α-D-GalpA-(1→4)-β-D-DAG-(1→6)-α-D-DAG-(1→1)-α-D-GalpA substituted with six fatty acids, and a molecular mass of 1875 (GalpA, galactopyranuronic acid; DAG, 2,3-diamino-2,3-dideoxyglucopyranose). No phosphate residues were detected. The major lipid A component had 12:0[3-O[Δ5-12:1(3-OH)]] and 12:0[3-O(Δ5-12:1)] fatty acyl chains at either the 3′- or the 2′ positions of the distal subunit DAG B, and 12:0(3OH) and 12:0[3,6-(OH) 2] fatty acyl chains at 3- and 2- positions of the reducing end subunit DAG A, respectively. In addition, several other variations in the structure were observed. The LPS was evaluated for TNF-α inducing activity and consistent with its unusual lipid A structure (relative to that of enteric bacteria), the activity was reduced by greater than 100-fold as compared to Escherichia coli ReLPS. This and other evidence suggests the potential application of this lipid A as a vaccine adjuvant or the suitability of Caulobacter displaying antigens for formulation of whole cell vaccines.

Structure of bacterial lipopolysaccharides

Bacterial lipopolysaccharides are the major components of the outer surface of Gram-negative bacteria They are often of interest in medicine for their immunomodulatory properties. In small amounts they can be beneficial, but in larger amounts they may cause endotoxic shock. Although they share a common architecture, their structural details exert a strong influence on their activity. These molecules comprise: a lipid moiety, called lipid A, which is considered to be the endotoxic component, a glycosidic part consisting of a core of approximately 10 monosaccharides and, in "smooth-type" lipopolysaccharides, a third region, named O-chain, consisting of repetitive subunits of one to eight monosaccharides responsible for much of the immunospecificity of the bacterial cell.

Biological activities of lipopolysaccharides are determined by the shape of their lipid A portion

Lipopolysaccharide (LPS) represents a major virulence factor of Gram-negative bacteria ('endotoxin') that can cause septic shock in mammals including man. The lipid anchor of LPS to the outer membrane, lipid A, has a peculiar chemical structure, harbours the 'endotoxic principle' of LPS and is responsible for the expression of pathophysiological effects. Chemically modified lipid A can be endotoxically inactive, but may express strong antagonistic activity against LPS, a property that can be utilized in antisepsis treatment. We show here that these different biological activities are directly correlated with the molecular shape of lipid A. Only (hexaacyl) lipid A with a conical/concave shape, the cross-section of the hydrophobic region being larger than that of the hydrophilic region, exhibited strong interleukin-6 (IL-6)-inducing capacity. Most strikingly, a correlation between a cylindrical molecular shape of lipid A and antagonistic activity was established: IL-6 induction by enterobacterial LPS was inhibited by cylindrically shaped lipid A except for compounds with reduced headgroup charge. The antagonistic action is interpreted by assuming that lipid A molecules intercalate into the cytoplasmic membrane of mononuclear cells, and subsequently blocking of the putative signaling protein by the lipid A with cylindrical shape.

The Charge of Endotoxin Molecules Influences Their Conformation and IL-6-Inducing Capacity

The activation of cells by endotoxin (LPS) is one of the early host responses to infections with Gram-negative bacteria. The lipid A part of LPS molecules is known to represent the endotoxic principle; however, the specific requirements for the expression of biologic activity are still not fully understood. We previously found that a specific molecular conformation (endotoxic conformation) is a prerequisite for lipid A to be biologically active. In this study, we have investigated the interdependence of molecular charge and conformation of natural and chemically modified LPS and lipid A and its transport and intercalation into phospholipid membranes mediated by human LPS-binding protein, as well as IL-6 production after stimulation of whole blood or PBMCs. We found that the number, nature, and location of negative charges strongly modulate the molecular conformation of endotoxin. In addition, the LPS-binding protein-mediated transport of LPS into phospholipid membranes depends on the presence of net negative charge, yet charge is only a necessary, but not a sufficient, prerequisite for transport and intercalation. The biologic activity is determined mainly by the molecular conformation: only conical molecules are highly biologically active, whereas cylindrical ones are largely inactive. We could demonstrate that the net negative charge of the lipid A component and its distribution within the hydrophilic headgroup strongly influence the molecular conformation and, therefore, also the biologic activity.