Lipopolysaccharide lipid A: A promising molecule for new immunity-based therapies and antibiotics

Lipopolysaccharides (LPS) are the main components of the external leaflet of the Gram-negative outer membrane and consist of three different moieties: lipid A, core oligosaccharide, and O-polysaccharide. The lipid A is a glucosamine disaccharide with different levels of acylation and phosphorylation, beside carrying, in certain cases, additional substituents on the sugar backbone. It is also the main immunostimulatory part of the LPS, as its recognition by the host immune system represents a fundamental event for detection of perilous microorganisms. Moreover, an uncontrolled immune response caused by a large amount of circulating LPS can lead to dramatic outcomes for human health, such as septic shock. The immunostimulant properties of an LPS incredibly vary depending on lipid A chemical structure, and for this reason, natural and synthetic variants of the lipid A are under study to develop new drugs that mimic or antagonise its natural effects. Here, we review past and recent findings on the lipid A as an antibiotic target and immune-therapeutic molecule, with a special attention on the crucial role of the chemical structure and its exploitation for conceiving novel strategies for treatment of several immune-related pathologies.

Presence of 2-hydroxymyristate on endotoxins is associated with death in neonates with Enterobacter cloacae complex septic shock

Enterobacter cloacae complex species are involved in infections among critically ill patients. After a recent E.cloacae outbreak of fulminant neonatal septic shock, we conducted a study to determine whether septic shock severity and its lethal consequence are related to structural features of the endotoxin (lipopolysaccharide [LPS]) of the strains isolated from hospitalized infants and more specifically its lipid A region. It appeared that the LPSs are very heterogeneous, carrying fifteen different molecular species of lipid A. The virulence was correlated with a structural feature identified by matrix-assisted laser desorption ionization–time of flight mass spectrometry and gas chromatography coupled with mass spectrometry: the presence of 2-hydroxymyristic acid as a secondary substituent in lipid A. This is the first published evidence linking LPS structural moiety to neonatal sepsis outcome and opens the possibility of using this fatty acid marker as a detection tool for high-risk patients, which could help reduce their mortality.

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Fifteen different molecular species of lipid A is found in E. cloacae complex

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2-Hydroxymyristate moiety on Lipid A is a virulence marker of the E. cloacae complex

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Presence of 2-hydroxymyristate is associated with mortality in neonatal sepsis

Fnr and ArcA Regulate Lipid A Hydroxylation in Salmonella Enteritidis by Controlling lpxO Expression in Response to Oxygen Availability

Lipid A is the bioactive component of lipopolysaccharide, and presents a dynamic structure that undergoes modifications in response to environmental signals. Many of these structural modifications influence Salmonella virulence. This is the case of lipid A hydroxylation, a modification catalyzed by the dioxygenase LpxO. Although it has been established that oxygen is required for lipid A hydroxylation acting as substrate of LpxO in Salmonella, an additional regulatory role for oxygen in lpxO expression has not been described. The existence of this regulation could be relevant considering that Salmonella faces low oxygen tension during infection. This condition leads to an adaptive response by changing the expression of numerous genes, and transcription factors Fnr and ArcA are major regulators of this process. In this work, we describe for the first time that lipid A hydroxylation and lpxO expression are modulated by oxygen availability in Salmonella enterica serovar Enteritidis (S. Enteritidis). Biochemical and genetic analyses indicate that this process is regulated by Fnr and ArcA controlling the expression of lpxO. In addition, according to our results, this regulation occurs by direct binding of both transcription factors to specific elements present in the lpxO promoter region. Altogether, our observations revealed a novel role for oxygen acting as an environment signal controlling lipid A hydroxylation in S. Enteritidis.

Review: Lipopolysaccharide inner core oligosaccharide structure and outer membrane stability in human pathogens belonging to the Enterobacteriaceae

In the Enterobacteriaceae, the outer membrane is primarily comprised of lipopolysaccharides. The lipopolysaccharide molecule is important in mediating interactions between the bacterium and its environment and those regions of the molecule extending further away from the cell surface show a higher amount of structural diversity. The hydrophobic lipid A is highly conserved, due to its important role in the structural integrity of the outer membrane. Attached to the lipid A region is the core oligosaccharide. The inner core oligosaccharide (lipid A proximal) backbone is also well conserved. However, non-stoichiometric substitutions of the basic inner core structure lead to structural variation and microheterogeneity. These include the addition of negatively charged groups (phosphate or galacturonic acid), ethanolamine derivatives, and glycose residues (Kdo, rhamnose, galactose, glucosamine, N-acetylglucosamine, heptose, Ko). The genetics and biosynthesis of these substitutions is beginning to be elucidated. Modification of heptose residues with negatively charged molecules (such as phosphate in Escherichia coli and Salmonella and galacturonic acid in Klebsiella pneumoniae ) has been shown to be involved in maintaining membrane stability. However, the biological role(s) of the remaining substitutions is unknown.

Review: Conserved and variable structural features in the lipopolysaccharide of Pseudomonas aeruginosa

The review is devoted to recent progress in the structural elucidation of the lipopolysaccharide of the bacterium Pseudomonas aeruginosa, including O-antigen biological repeats, core oligosaccharide, and lipid A. Data on biosynthesis, genetics and serology of the lipopolysaccharide isolated from various P. aeruginosa O-serogroups are discussed in relation to the chemical structures.

Role of pagL and lpxO in Bordetella bronchiseptica lipid A biosynthesis

PagL and LpxO are enzymes that modify lipid A. PagL is a 3-O deacylase that removes the primary acyl chain from the 3 position, and LpxO is an oxygenase that 2-hydroxylates specific acyl chains in the lipid A. pagL and lpxO homologues have been identified in the genome of Bordetella bronchiseptica, but in the current structure for B. bronchiseptica lipid A the 3 position is acylated and 2-OH acylation is not reported. We have investigated the role of B. bronchiseptica pagL and lpxO in lipid A biosynthesis. We report a different structure for wild-type (WT) B. bronchiseptica lipid A, including the presence of 2-OH-myristate, the presence of which is dependent on lpxO. We also demonstrate that the 3 position is not acylated in the major WT lipid A structures but that mutation of pagL results in the presence of 3-OH-decanoic acid at this position, suggesting that lipid A containing this acylation is synthesized but that PagL removes most of it from the mature lipid A. These data refine the structure of B. bronchiseptica lipid A and demonstrate that pagL and lpxO are involved in its biosynthesis.

Salmonella-regulated lipopolysaccharide modifications

Salmonella enterica are Gram-negative enteric pathogens that cause typhoid fever and gastroenteritis in humans. Many bacteria, including Salmonella, use signal transduction cascades such as two-component regulatory systems to detect and respond to stimuli in the local microenvironment. During infection, environmental sensing allows bacteria to regulate gene expression to evade host immune defenses and thrive in vivo. Activation of the Salmonella two-component regulatory systems PhoP-PhoQ and PmrA-PmrB and the RcsC-RcsD-RcsB phosphorylay by specific environmental signals in the intestine and within host cells leads to several lipopolysaccharide modifications that promote bacterial survival, cationic antimicrobial peptide resistance and virulence. Many pathogens encode orthologs to Salmonella two-component regulatory systems and also modify the lipopolysaccharide to escape killing by the host immune response. However, these organisms often regulate their virulence genes, including those responsible for lipopolysaccharide modification, in ways that differ from Salmonella. Further examination of bacterial virulence gene regulation and lipopolysaccharide modifications may lead to improved antimicrobial therapies and vaccines.

beta-Hydroxylation of the aspartyl residue in the phytotoxin syringomycin E: characterization of two candidate hydroxylases AspH and SyrP in Pseudomonas syringae

The pseudomonal phytotoxin syringomycin E and related nonribosomal peptides contain an L- threo-beta-hydroxyaspartyl residue at the eighth position of the lipodepsipeptide backbone as part of a conserved nonproteinogenic tripeptide motif. Informatic analysis of the P. syringae genome suggests only one putative non-heme iron hydroxylase, AspH. On heterologous expression in Escherichia coli AspH shows robust catalytic activity with free L-Asp and L-Asp thioesters to make beta-OH-Asp but yields the erythro diastereomer rather than the threo configuration that is found in syringomycin. Further analysis of the Syr gene cluster indicated that SyrP, previously annotated as the gene regulatory protein for the five-gene Syr cluster, is actually homologous to the known non-heme mononuclear iron hydroxylase TauD. Indeed, purified SyrP acts on Asp tethered as the protein-bound S-pantetheinyl thioester on the eighth module of the SyrE megasynthetase. The hydroxylation gives the anticipated L- threo-3-OH-Asp diastereomer found in syringomycin. The knockout of syrP abolishes the production of the mature syringomycin E, while knockout of aspH has no effect on syringomycin production.

An inner membrane dioxygenase that generates the 2-hydroxymyristate moiety of Salmonella lipid A

The lipid A residues of certain Gram-negative bacteria, including most strains of Salmonella and Pseudomonas, are esterified with one or two secondary S-2-hydroxyacyl chains. The S-2 hydroxylation process is O 2-dependent in vivo, but the relevant enzymatic pathways have not been fully characterized because in vitro assays have not been developed. We previously reported that expression of the Salmonella lpxO gene confers upon Escherichia coli K-12 the ability to synthesize 2-hydroxymyristate modified lipid A ( J. Biol. Chem. (2000) 275, 32940-32949). We now demonstrate that inactivation of lpxO, which encodes a putative Fe (2+)/O 2/alpha-ketoglutarate-dependent dioxygenase, abolishes S-2-hydroxymyristate formation in S. typhimurium. Membranes of E. coli strains expressing lpxO are able to hydroxylate Kdo 2-[4'- (32)P]-lipid A in vitro in the presence of Fe (2+), O 2, alpha-ketoglutarate, ascorbate, and Triton X-100. The Fe (2+) chelator 2,2'-bipyridyl inhibits the reaction. The product generated in vitro is a monohydroxylated Kdo 2-lipid A derivative. The [4'- (32)P]-lipid A released by mild acid hydrolysis from the in vitro product migrates with authentic S-2-hydroxlyated lipid A isolated from (32)P-labeled S. typhimurium cells. Electrospray ionization mass spectrometry and gas chromatography/mass spectrometry of the in vitro product are consistent with the 2-hydroxylation of the 3'-secondary myristoyl chain of Kdo 2-lipid A. LpxO contains two predicted trans-membrane helices (one at each end of the protein), and its active site likely faces the cytoplasm. LpxO is an unusual example of an integral membrane protein that is a member of the Fe (2+)/O 2/alpha-ketoglutarate-dependent dioxygenase family.

Species-specific functioning of the Pseudomonas XcpQ secretin: role for the C-terminal homology domain and lipopolysaccharide

Secretins are oligomeric proteins that mediate the export of macromolecules across the bacterial outer membrane. The members of the secretin superfamily possess a C-terminal homology domain that is important for oligomerization and channel formation, while their N-terminal halves are thought to be involved in system-specific interactions. The XcpQ secretin of Pseudomonas spp. is a component of the type II secretion pathway. XcpQ from Pseudomonas alcaligenes is not able to functionally replace the secretin of the closely related species Pseudomonas aeruginosa. By analysis of chimeric XcpQ proteins, a region important for species-specific functioning was mapped between amino acid residues 344 and 478 in the C-terminal homology domain. Two chromosomal suppressor mutations were obtained that resulted in the proper functioning in P. aeruginosa of P. alcaligenes XcpQ and inactive hybrids. These mutations caused a defect in the synthesis of the lipopolysaccharide (LPS) outer core region. Subsequent analysis of different LPS mutants showed that changes in the outer core and not the loss of O antigen caused the suppressor phenotype. High concentrations of divalent cations in the growth medium also allowed P. alcaligenes XcpQ and inactive hybrids to function properly in P. aeruginosa. Since divalent cations are known to affect the structure of LPS, this observation supports the hypothesis that LPS has a role in the functioning of secretins.

Phosphoethanolamine substitution in the lipid A of Escherichia coli O157 : H7 and its association with PmrC

This study shows that lipid A ofEscherichia coliO157 : H7 differs from that ofE. coliK-12 in that it has a phosphoform at the C-1 position, which is distinctively modified by a phosphoethanolamine (PEtN) moiety, in addition to the diphosphoryl form. ThepmrCgene responsible for the addition of PEtN to the lipid A ofE. coliO157 : H7 was inactivated and the changes in lipid A profiles were assessed. ThepmrCnull mutant still produced PEtN-modified lipid A species, albeit in a reduced amount, indicating that PmrC was not the only enzyme that could be used to add PEtN to lipid A. Natural PEtN substitution was shown to be present in the lipid A of other serotypes of enterohaemorrhagicE. coliand absent from the lipid A ofE. coliK-12. However, the clonedpmrCO157gene in a high-copy-number plasmid generated a large amount of PEtN-substituted lipid A species inE. coliK-12. The occurrence of PEtN-substituted lipid A species was associated with a slight increase in the MICs of cationic peptide antibiotics, suggesting that the lipid A modification with PEtN would be beneficial for survival ofE. coliO157 : H7 in certain environmental niches. However, PEtN substitution in the lipid A profiles was not detected when putative inner-membrane proteins (YhbX/YbiP/YijP/Ecf3) that show significant similarity with PmrC in amino acid sequence were expressed from high-copy-number plasmids inE. coliK-12. This suggests that these potential homologues are not responsible for the addition of PEtN to lipid A in thepmrCmutant ofE. coliO157 : H7. When cells were treated with EDTA, the amount of palmitoylated lipid A from the cells carrying a high-copy-number plasmid clone ofpmrCO157that resulted in significant increase of PEtN substitution was unchanged compared with cells without PEtN substitution, suggesting that the PEtN moiety substituted in lipid A does not compensate for the loss of divalent cations required for bridging neighbouring lipid A molecules.

Kdo2-Lipid A of Escherichia coli, a defined endotoxin that activates macrophages via TLR-4

The LIPID MAPS Consortium (www.lipidmaps.org) is developing comprehensive procedures for identifying all lipids of the macrophage, following activation by endotoxin. The goal is to quantify temporal and spatial changes in lipids that occur with cellular metabolism and to develop bioinformatic approaches that establish dynamic lipid networks. To achieve these aims, an endotoxin of the highest possible analytical specification is crucial. We now report a large-scale preparation of 3-deoxy-D-manno-octulosonic acid (Kdo)(2)-Lipid A, a nearly homogeneous Re lipopolysaccharide (LPS) sub-structure with endotoxin activity equal to LPS. Kdo(2)-Lipid A was extracted from 2 kg cell paste of a heptose-deficient Escherichia coli mutant. It was purified by chromatography on silica, DEAE-cellulose, and C18 reverse-phase resin. Structure and purity were evaluated by electrospray ionization/mass spectrometry, liquid chromatography/mass spectrometry and (1)H-NMR. Its bioactivity was compared with LPS in RAW 264.7 cells and bone marrow macrophages from wild-type and toll-like receptor 4 (TLR-4)-deficient mice. Cytokine and eicosanoid production, in conjunction with gene expression profiling, were employed as readouts. Kdo(2)-Lipid A is comparable to LPS by these criteria. Its activity is reduced by >10(3) in cells from TLR-4-deficient mice. The purity of Kdo(2)-Lipid A should facilitate structural analysis of complexes with receptors like TLR-4/MD2.

The Pseudomonas aeruginosa proteome during anaerobic growth

Isotope-coded affinity tag analysis and two-dimensional gel electrophoresis followed by tandem mass spectrometry were used to identify Pseudomonas aeruginosa proteins expressed during anaerobic growth. Out of the 617 proteins identified, 158 were changed in abundance during anaerobic growth compared to during aerobic growth, including proteins whose increased expression was expected based on their role in anaerobic metabolism. These results form the basis for future analyses of alterations in bacterial protein content during growth in various environments, including the cystic fibrosis airway.

Phenotypic and transcriptional characterization of the meningococcal PhoPQ system, a magnesium-sensing two-component regulatory system that controls genes involved in remodeling the meningococcal cell surface

We previously identified and characterized a two-component regulatory system in the meningococcus with homology to the phoP-phoQ system in salmonella and showed that allele replacement of the NMB0595 regulator gene led to loss of virulence, sensitivity to antimicrobial peptides, perturbed protein expression, and magnesium-sensitive growth. On the basis of these findings we proposed that the system should be designated the meningococcal PhoPQ system. Here we further characterized the NMB0595 mutant and demonstrated that it had increased membrane permeability and was unable to form colonies on solid media with low magnesium concentrations, features that are consistent with disruption of PhoPQ-mediated modifications to the lipooligosaccharide structure. We examined the transcriptional profiles of wild-type and NMB0595 mutant strains and found that magnesium-regulated changes in gene expression are completely abrogated in the mutant, indicating that, similar to the salmonella PhoPQ system, the meningococcal PhoPQ system is regulated by magnesium. Transcriptional profiling of the mutant indicated that, also similar to the salmonella PhoPQ system, the meningococcal system is involved in control of virulence and remodeling of the bacterial cell surface in response to the host environment. The results are consistent with the hypothesis that the PhoP homologue plays a role in the meningococcus similar to the role played by PhoP in salmonella. Elucidating the role that the PhoPQ system and PhoPQ-regulated genes play in the response of the meningococcus to the host environment may provide new insights into the pathogenic process.

Microextraction of bacterial lipid A: easy and rapid method for mass spectrometric characterization

Endotoxins (lipopolysaccharides) are the main components of Gram-negative bacterial outer membranes. A quick and simple way to isolate their lipid region (lipid A) directly from whole bacterial cells was devised. This method using hot ammonium-isobutyrate solvent was applied to small quantities of cells and proved to be indispensable when a rapid characterization of lipid A structure by mass spectrometry was required. Biological activities of endotoxins are directly related to the lipid A structures, which vary greatly with cell growth conditions. This method is suitable for rough- and smooth-type bacteria and very efficient for screening variations in lipid A structures. Data are acquired in a few hours and avoid the use of phenol in extraction.

MsbA-dependent Translocation of Lipids across the Inner Membrane of Escherichia coli

MsbA is an essential ABC transporter in Escherichia coli required for exporting newly synthesized lipids from the inner to the outer membrane. It remains uncertain whether or not MsbA catalyzes trans-bilayer lipid movement (i.e. flip-flop) within the inner membrane. We now show that newly synthesized lipid A accumulates on the cytoplasmic side of the inner membrane after shifting an E. coli msbA missense mutant to the non-permissive temperature. This conclusion is based on the selective inhibition of periplasmic, but not cytoplasmic, covalent modifications of lipid A that occur in polymyxin-resistant strains of E. coli. The accessibility of newly synthesized phosphatidylethanolamine to membrane impermeable reagents, like 2,4,6-trinitrobenzene sulfonic acid, is also reduced severalfold. Our data showed that MsbA facilitates the rapid translocation of some lipids from the cytoplasmic to the periplasmic side of the inner membrane in living cells.

Identification of an outer membrane protein required for the transport of lipopolysaccharide to the bacterial cell surface

Lipopolysaccharide (LPS), also known as endotoxin due to its severe pathophysiological effects in infected subjects, is an essential component of the outer membrane (OM) of most Gram-negative bacteria. LPS is synthesized in the bacterial inner membrane, a process that is now well understood. In contrast, the mechanism of its transport to the outer leaflet of the OM has remained enigmatic. We demonstrate here that the OM protein, known as increased membrane permeability (Imp) or organic solvent tolerance protein, is involved in this process. An Imp-deficient mutant of Neisseria meningitidis was viable and produced severely reduced amounts of LPS. The limited amount of LPS that was still produced was not accessible to LPS-modifying enzymes expressed in the OM or added to the extracellular medium. We conclude therefore that Imp mediates the transport of LPS to the cell surface. The role of Imp in LPS biogenesis and its high conservation among Gram-negative bacteria make it an excellent target for the development of novel antibacterial compounds.

The PhoP-PhoQ two-component regulatory system of Photorhabdus luminescens is essential for virulence in insects

Photorhabdus luminescens is a symbiont of entomopathogenic nematodes. Analysis of the genome sequence of this organism revealed a homologue of PhoP-PhoQ, a two-component system associated with virulence in intracellular bacterial pathogens. This organism was shown to respond to the availability of environmental magnesium. A mutant with a knockout mutation in the regulatory component of this system (phoP) had no obvious growth defect. It was, however, more motile and more sensitive to antimicrobial peptides than its wild-type parent. Remarkably, the mutation eliminated virulence in an insect model. No insect mortality was observed after injection of a large number of the phoP bacteria, while very small amounts of parental cells killed insect larvae in less than 48 h. At the molecular level, the PhoPQ system mediated Mg(2+)-dependent modifications in lipopolysaccharides and controlled a locus (pbgPE) required for incorporation of 4-aminoarabinose into lipid A. Mg(2+)-regulated gene expression of pbgP1 was absent in the mutant and was restored when phoPQ was complemented in trans. This finding highlights the essential role played by PhoPQ in the virulence of an entomopathogen.