Structural heterogeneity and environmentally regulated remodeling of Francisella tularensis subspecies novicida lipid A characterized by tandem mass spectrometry

Lipid A Structural Determination from a Single Colony

We describe an innovative use for the recently reported fast lipid analysis technique (FLAT) that allows for the generation of MALDI tandem mass spectrometry data suitable for lipid A structure analysis directly from a single Gram-negative bacterial colony. We refer to this tandem MS version of FLAT as FLATn. Neither technique requires sophisticated sample preparation beyond the selection of a single bacterial colony, which significantly reduces overall analysis time (∼1 h), as compared to conventional methods. Moreover, the tandem mass spectra generated by FLATn provides comprehensive information on fragments of lipid A, for example, ester bonded acyl chain dissociations, cross-ring cleavages, and glycosidic bond dissociations, all of which allow the facile determination of novel lipid A structures or confirmation of expected structures. In addition to generating tandem mass spectra directly from single colonies, we also show that FLATn can be used to analyze lipid A structures taken directly from a complex biological clinical sample without the need for ex vivo growth. From a urine sample from a patient with an E. coli infection, FLATn identified the organism and demonstrated that this clinical isolate carried the mobile colistin resistance-1 gene (mcr-1) that results in the addition of a phosphoethanolamine moiety and subsequently resistance to the antimicrobial, colistin (polymyxin E). Moreover, FLATn allowed for the determination of the existence of a structural isomer in E. coli lipid A that had either a 1- or 4'-phosphate group modification by phosphoethanolamine generated by a change of bacterial culture conditions.

Remodelling of the Gram-negative bacterial Kdo2-lipid A and its functional implications

The lipopolysaccharide (LPS) is a characteristic molecule of the outer leaflet of the Gram-negative bacterial outer membrane, which consists of lipid A, core oligosaccharide, and O antigen. The lipid A is embedded in outer membrane and provides an efficient permeability barrier, which is particularly important to reduce the permeability of antibiotics, toxic cationic metals, and antimicrobial peptides. LPS, an important modulator of innate immune responses ranging from localized inflammation to disseminated sepsis, displays a high level of structural and functional heterogeneity, which arise due to regulated differences in the acylation of the lipid A and the incorporation of non-stoichiometric modifications in lipid A and the core oligosaccharide. This review focuses on the current mechanistic understanding of the synthesis and assembly of the lipid A molecule and its most salient non-stoichiometric modifications.

Toll-Like Receptors (TLRs), NOD-Like Receptors (NLRs), and RIG-I-Like Receptors (RLRs) in Innate Immunity. TLRs, NLRs, and RLRs Ligands as Immunotherapeutic Agents for Hematopoietic Diseases

The innate immune system plays a pivotal role in the first line of host defense against infections and is equipped with patterns recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Several classes of PRRS, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-I-like receptors (RLRs) recognize distinct microbial components and directly activate immune cells. TLRs are transmembrane receptors, while NLRs and RLRs are intracellular molecules. Exposure of immune cells to the ligands of these receptors activates intracellular signaling cascades that rapidly induce the expression of a variety of overlapping and unique genes involved in the inflammatory and immune responses. The innate immune system also influences pathways involved in cancer immunosurveillance. Natural and synthetic agonists of TLRs, NLRs, or RLRs can trigger cell death in malignant cells, recruit immune cells, such as DCs, CD8+ T cells, and NK cells, into the tumor microenvironment, and are being explored as promising adjuvants in cancer immunotherapies. In this review, we provide a concise overview of TLRs, NLRs, and RLRs: their structure, functions, signaling pathways, and regulation. We also describe various ligands for these receptors and their possible application in treatment of hematopoietic diseases.

Species-Specific Endotoxin Stimulus Determines Toll-Like Receptor 4- and Caspase 11-Mediated Pathway Activation Characteristics

The innate immune system is the body’s first line of defense against pathogens and its protection against infectious diseases. On the surface of host myeloid cells, Toll-like receptor 4 (TLR4) senses lipopolysaccharide (LPS), the major outer membrane component of Gram-negative bacteria. Intracellularly, LPS is recognized by caspase 11 through the noncanonical inflammasome to induce pyroptosis—an inflammatory form of lytic cell death. While TLR4-mediated signaling perturbations result in secretion of cytokines and chemokines that help clear infection and facilitate adaptive immunity, caspase 11-mediated pyroptosis leads to the release of damage-associated molecular patterns and inflammatory mediators. Although the core signaling events and many associated proteins in the TLR4 signaling pathway are known, the complex signaling events and protein networks within the noncanonical inflammasome pathway remain obscure. Moreover, there is mounting evidence for pathogen-specific innate immune tuning. We characterized the major LPS structures from two different pathogens, modeled their binding to the surface receptors, systematically examined macrophage inflammatory responses to these LPS molecules, and surveyed the temporal differences in global protein secretion resulting from TLR4 and caspase 11 activation in macrophages using mass spectrometry (MS)-based quantitative proteomics. This integrated strategy, spanning functional activity assays, top-down structural elucidation of endotoxins, and secretome analysis of stimulated macrophages, allowed us to identify crucial differences in TLR4- and caspase 11-mediated protein secretion in response to two Gram-negative bacterial endotoxins.

IMPORTANCE Macrophages and monocytes are innate immune cells playing an important role in orchestrating the initial innate immune response to bacterial infection and the tissue damage. This response is facilitated by specific receptors on the cell surface and intracellularly. One of the bacterial molecules recognized is a Gram-negative bacteria cell wall component, lipopolysaccharide (LPS). The structure of LPS differs between different species. We have characterized the innate immune responses to the LPS molecules from two bacteria, Escherichia coli and Bordetella pertussis, administered either extracellularly or intracellularly, whose structures we first determined. We observed marked differences in the temporal dynamics and amounts of proteins secreted by the innate immune cells stimulated by any of these molecules and routes. This suggests that there is specificity in the first line of response to different Gram-negative bacteria that can be explored to tailor specific therapeutic interventions.

Rapid microbial identification and colistin resistance detection via MALDI-TOF MS using a novel on-target extraction of membrane lipids

Rapid infection diagnosis is critical to improving patient treatment and outcome. Recent studies have shown microbial lipids to be sensitive and selective biomarkers for identifying bacterial and fungal species and antimicrobial resistance. Practical procedures for microbial lipid biomarker analysis will therefore improve patient outcomes and antimicrobial stewardship. However, current lipid extraction methods require significant hands-on time and are thus not suited for direct adoption as a clinical assay for microbial identification. Here, we have developed a method for lipid extraction directly on the surface of stainless-steel matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) plates, termed fast lipid analysis technique or FLAT, which facilitates the identification of bacterial and fungal species using a sub-60-minute workflow. Additionally, our method detects lipid A modifications in Gram-negative bacteria that are associated with antimicrobial resistance, including to colistin.

Lipopolysaccharide Recognition in the Crossroads of TLR4 and Caspase-4/11 Mediated Inflammatory Pathways

The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors-TLR4/MD-2 complex and caspase-4/11, respectively-are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a "basic" hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very "narrow" specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

Identification and structural characterization of lipid A from Escherichia coli, Pseudomonas putida and Pseudomonas taiwanensis using liquid chromatography coupled to high-resolution tandem mass spectrometry

RATIONALE

Lipid A is a part of the lipopolysaccharide layer, which is a main component of the outer membrane from Gram-negative bacteria. It can be sensed by mammalians to identify the presence of Gram-negative bacteria in their tissues and plays a key role in the pathogenesis of bacterial infections. Lipid A is also used as an adjuvant in human vaccines, emphasizing the importance of its structural analysis.

METHODS

In order to distinguish and characterize various lipid A species, a liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) method was developed. Isolation of lipid A from different bacteria was carried out using a modified Bligh and Dyer extraction following a mild acid hydrolysis. Chromatography was performed using a bifunctional reversed-phase-based stationary phase. High-resolution MS using negative electrospray ionization was applied and MS/MS experiments utilizing high-energy collisional dissociation generated diagnostic product ions, which allowed the assignment of the side chains to distinct positions of the lipid A backbone.

RESULTS

The method was applied to lipid A isolations of Escherichia coli (E. coli), Pseudomonas putida (P. putida) and Pseudomonas taiwanensis (P. taiwanensis). Various lipid A species were identified by their accurate masses and their structures were characterized using MS/MS experiments. Previously described lipid A structures from E. coli were identified and their structures confirmed by MS/MS. For the biotechnologically relevant strains P. putida and P. taiwanensis, we confirmed species by MS/MS, which have previously only been analyzed using MS. In addition, several lipid A species were discovered that have not been previously described in the literature.

CONCLUSIONS

The combination of LC and MS/MS enabled the selective and sensitive identification and structural characterization of various lipid A species from Gram-negative bacteria. These species varied in their substituted side chains, speaking of fatty acids and phosphate groups. Characteristic product ions facilitated the assignment of side chains to distinct positions of the lipid A backbone.

Droplet delivery and nebulization system using surface acoustic wave for mass spectrometry

We present a piezoelectric transducer for standing wave surface acoustic wave nebulization (SW-SAWN). The transducer nebulizes nonvolatile analytes present in bulk fluid into ambient air after which the aerosolized drops are sampled by mass spectrometry (MS) for detection. Furthermore, we report for the first time integration of anisotropic ratchet conveyors (ARCs) on the SAWN transducer surfaces to automate the sample preparation and droplet delivery process. The ARCs employ micro-sized hydrophilic patterns on hydrophobic Cytop coatings. Moving, positioning, merging, and mixing of droplets at a designated nebulization location are demonstrated. To create the ARCs, we adopt parylene C as a stencil mask so that the hydrophobicity of the Cytop does not degrade during the microfabrication process. MS measurements with the SAWN chip are performed under different input frequencies. The SAWN transducer can provide a controllable nebulization rate by varying the input nebulization frequency while maintaining a reasonable signal to noise ratio for MS detection.

The UDP-GalNAcA biosynthesis genes gna-gne2 are required to maintain cell envelope integrity and in vivo fitness in multi-drug resistant Acinetobacter baumannii

Acinetobacter baumannii infects a wide range of anatomic sites including the respiratory tract and bloodstream. Despite its clinical importance, little is known about the molecular basis of A. baumannii pathogenesis. We previously identified the UDP-N-acetyl-d-galactosaminuronic acid (UDP-GalNAcA) biosynthesis genes, gna-gne2, as being critical for survival in vivo. Herein, we demonstrate that Gna-Gne2 are part of a complex network connecting in vivo fitness, cell envelope homeostasis and resistance to antibiotics. The ∆gna-gne2 mutant exhibits a severe fitness defect during bloodstream infection. Capsule production is abolished in the mutant strain, which is concomitant with its inability to survive in human serum. In addition, the ∆gna-gne2 mutant was more susceptible to vancomycin and unable to grow on MacConkey plates, indicating an alteration in cell envelope integrity. Analysis of lipid A by mass spectrometry showed that the hexa- and hepta-acylated species were affected in the gna-gne2 mutant. Finally, the ∆gna-gne2 mutant was more susceptible to several classes of antibiotics. Together, this study demonstrates the importance of UDP-GalNAcA in the pathobiology of A. baumannii. By interrupting its biosynthesis, we showed that this molecule plays a critical role in capsule biosynthesis and maintaining the cell envelope homeostasis.

Repurposing Eukaryotic Kinase Inhibitors as Colistin Adjuvants in Gram-Negative Bacteria

Kinase inhibitors comprise a diverse cohort of chemical scaffolds that are active in multiple biological systems. Currently, thousands of eukaryotic kinase inhibitors are commercially available, have well-characterized targets, and often carry pharmaceutically favorable toxicity profiles. Recently, our group disclosed that derivatives of the natural product meridianin D, a known inhibitor of eukaryotic kinases, modulated behaviors of both Gram-positive and Gram-negative bacteria. Herein, we expand our exploration of kinase inhibitors in Gram-negative bacilli utilizing three commercially available kinase inhibitor libraries and, ultimately, identify two chemical structures that potentiate colistin (polymyxin E) in multiple strains. We report IMD-0354, an inhibitor of IKK-β, as a markedly effective adjuvant in colistin-resistant bacteria and also describe AR-12 (OSU-03012), an inhibitor of pyruvate dehydrogenase kinase-1 (PDK-1), as a potentiator in colistin-sensitive strains. This report comprises the first description of the novel cross-reactivity of these molecules.

Francisella tularensis subsp. holarctica Releases Differentially Loaded Outer Membrane Vesicles Under Various Stress Conditions

Francisella tularensis is a Gram-negative, facultative intracellular bacterium, causing a severe disease called tularemia. It secretes unusually shaped nanotubular outer membrane vesicles (OMV) loaded with a number of virulence factors and immunoreactive proteins. In the present study, the vesicles were purified from a clinical isolate of subsp. holarctica strain FSC200. We here provide a comprehensive proteomic characterization of OMV using a novel approach in which a comparison of OMV and membrane fraction is performed in order to find proteins selectively enriched in OMV vs. membrane. Only these proteins were further considered to be really involved in the OMV function and/or their exceptional structure. OMV were also isolated from bacteria cultured under various cultivation conditions simulating the diverse environments of F. tularensis life cycle. These included conditions mimicking the milieu inside the mammalian host during inflammation: oxidative stress, low pH, and high temperature (42°C); and in contrast, low temperature (25°C). We observed several-fold increase in vesiculation rate and significant protein cargo changes for high temperature and low pH. Further proteomic characterization of stress-derived OMV gave us an insight how the bacterium responds to the hostile environment of a mammalian host through the release of differentially loaded OMV. Among the proteins preferentially and selectively packed into OMV during stressful cultivations, the previously described virulence factors connected to the unique intracellular trafficking of Francisella were detected. Considerable changes were also observed in a number of proteins involved in the biosynthesis and metabolism of the bacterial envelope components like O-antigen, lipid A, phospholipids, and fatty acids. Data are available via ProteomeXchange with identifier PXD013074.

Mapping phosphate modifications of substituted lipid A via a targeted MS3 CID/UVPD strategy

Structural characterization of lipid A from Gram-negative bacteria remains a significant challenge, especially with respect to localizing modifications of the phosphate groups typically found on the reducing and non-reducing ends of the β-1',6-linked glucosamine disaccharide backbone of lipid A. As reported here, combining traditional collisional activated dissociation (CAD) and ultraviolet photodissociation (UVPD) in a hybrid MS3 approach facilitates identification and localization of substituents of the phosphate groups. The focus is on rapid identification and characterization of substituted lipid A species with specific emphasis on the modifications on the 1 and 4' phosphate moieties. Mapping these modifications, typically ones that modify the surface charges of lipopolysaccharides, is particularly important owing to the impact of these types of modifications on antibiotic resistance. The presence of phosphoethanolamine, aminoarabinose, and galactosamine moieties in hexaacylated and heptaacylated lipid A species, including ones from Enterobacter cloacae and Acinetobacter baumannii, are characterized using a targeted MS3 strategy to identify glycosidic product ions (1,5X1 and 0,4A2, typically) which allow localization of the substituents.

Mass Spectrometry-based Structural Analysis and Systems Immunoproteomics Strategies for Deciphering the Host Response to Endotoxin

One cause of sepsis is systemic maladaptive immune response of the host to bacteria and specifically, to Gram-negative bacterial outer-membrane glycolipid lipopolysaccharide (LPS). On the host myeloid cell surface, proinflammatory LPS activates the innate immune system via Toll-like receptor-4/myeloid differentiation factor-2 complex. Intracellularly, LPS is also sensed by the noncanonical inflammasome through caspase-11 in mice and 4/5 in humans. The minimal functional determinant for innate immune activation is the membrane anchor of LPS called lipid A. Even subtle modifications to the lipid A scaffold can enable, diminish, or abolish immune activation. Bacteria are known to modify their LPS structure during environmental stress and infection of hosts to alter cellular immune phenotypes. In this review, we describe how mass spectrometry-based structural analysis of endotoxin helped uncover major determinations of molecular pathogenesis. Through characterization of LPS modifications, we now better understand resistance to antibiotics and cationic antimicrobial peptides, as well as how the environment impacts overall endotoxin structure. In addition, mass spectrometry-based systems immunoproteomics approaches can assist in elucidating the immune response against LPS. Many regulatory proteins have been characterized through proteomics and global/targeted analysis of protein modifications, enabling the discovery and characterization of novel endotoxin-mediated protein translational modifications.

Top Down Tandem Mass Spectrometric Analysis of a Chemically Modified Rough-Type Lipopolysaccharide Vaccine Candidate

Recent advances in lipopolysaccharide (LPS) biology have led to its use in drug discovery pipelines, including vaccine and vaccine adjuvant discovery. Desirable characteristics for LPS vaccine candidates include both the ability to produce a specific antibody titer in patients and a minimal host inflammatory response directed by the innate immune system. However, in-depth chemical characterization of most LPS extracts has not been performed; hence, biological activities of these extracts are unpredictable. Additionally, the most widely adopted workflow for LPS structure elucidation includes nonspecific chemical decomposition steps before analyses, making structures inferred and not necessarily biologically relevant. In this work, several different mass spectrometry workflows that have not been previously explored were employed to show proof-of-principle for top down LPS primary structure elucidation, specifically for a rough-type mutant (J5) E. coli-derived LPS component of a vaccine candidate. First, ion mobility filtered precursor ions were subjected to collision induced dissociation (CID) to define differences in native J5 LPS v. chemically detoxified J5 LPS (dLPS). Next, ultra-high mass resolving power, accurate mass spectrometry was employed for unequivocal precursor and product ion empirical formulae generation. Finally, MS³ analyses in an ion trap instrument showed that previous knowledge about dissociation of LPS components can be used to reconstruct and sequence LPS in a top down fashion. A structural rationale is also explained for differential inflammatory dose-response curves, in vitro, when HEK-Blue hTLR4 cells were administered increasing concentrations of native J5 LPS v. dLPS, which will be useful in future drug discovery efforts. Graphical Abstract ᅟ.

Host-based lipid inflammation drives pathogenesis in Francisella infection

Mass spectrometry imaging (MSI) was used to elucidate host lipids involved in the inflammatory signaling pathway generated at the host-pathogen interface during a septic bacterial infection. Using Francisella novicida as a model organism, a bacterial lipid virulence factor (endotoxin) was imaged and identified along with host phospholipids involved in the splenic response in murine tissues. Here, we demonstrate detection and distribution of endotoxin in a lethal murine F. novicida infection model, in addition to determining the temporally and spatially resolved innate lipid inflammatory response in both 2D and 3D renderings using MSI. Further, we show that the cyclooxygenase-2-dependent lipid inflammatory pathway is responsible for lethality in F. novicida infection due to overproduction of proinflammatory effectors including prostaglandin E2. The results of this study emphasize that spatial determination of the host lipid components of the immune response is crucial to identifying novel strategies to effectively address highly pathogenic and lethal infections stemming from bacterial, fungal, and viral origins.

Small molecule adjuvants that suppress both chromosomal and mcr-1 encoded colistin-resistance and amplify colistin efficacy in polymyxin-susceptible bacteria

Bacterial resistance to polymyxin antibiotics has taken on a new and more menacing form. Common are genomically-encoded resistance mechanisms to polymyxins, specifically colistin (polymyxin E), however, the plasmid-borne mobile colistin resistance-1 (mcr-1) gene has recently been identified and poses a new threat to global public health. Within six months of initial identification in Chinese swine in November 2015, the first human clinical isolation in the US was reported (Apr. 2016). Herein we report successful reversion of mcr-1-driven colistin resistance in Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli with adjuvants we previously reported as modulators of chromosomally-encoded colistin resistance. Further screening of our in-house library of nitrogen-dense heterocycles has identified additional chemical scaffolds that actively attenuate colistin resistance. Ultimately, we present a diverse cohort of adjuvants that both sensitize colistin-resistant and colistin-susceptible bacteria to this antibiotic, thus providing a potential avenue to both reduce colistin dosage and toxicity, and overcome colistin resistance.

Characterization of Lipid A Variants by Energy-Resolved Mass Spectrometry: Impact of Acyl Chains

Lipid A molecules consist of a diglucosamine sugar core with a number of appended acyl chains that vary in their length and connectivity. Because of the challenging nature of characterizing these molecules and differentiating between isomeric species, an energy-resolved MS/MS strategy was undertaken to track the fragmentation trends and map genealogies of product ions originating from consecutive cleavages of acyl chains. Generalizations were developed based on the number and locations of the primary and secondary acyl chains as well as variations in preferential cleavages arising from the location of the phosphate groups. Secondary acyl chain cleavage occurs most readily for lipid A species at the 3' position, followed by primary acyl chain fragmentation at both the 3' and 3 positions. In the instances of bisphosphorylated lipid A variants, phosphate loss occurs readily in conjunction with the most favorable primary and secondary acyl chain cleavages. Graphical Abstract ᅟ.

Structural Modification of Lipopolysaccharide Conferred by mcr-1 in Gram-Negative ESKAPE Pathogens

mcr-1 was initially reported as the first plasmid-mediated colistin resistance gene in clinical isolates of Escherichia coli and Klebsiella pneumoniae in China and has subsequently been identified worldwide in various species of the family Enterobacteriaceaemcr-1 encodes a phosphoethanolamine transferase, and its expression has been shown to generate phosphoethanolamine-modified bis-phosphorylated hexa-acylated lipid A in E. coli Here, we investigated the effects of mcr-1 on colistin susceptibility and on lipopolysaccharide structures in laboratory and clinical strains of the Gram-negative ESKAPE (Enterococcus faecium, Staphylococcus aureus, K. pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens, which are often treated clinically by colistin. The effects of mcr-1 on colistin resistance were determined using MIC assays of laboratory and clinical strains of E. coli, K. pneumoniae, A. baumannii, and P. aeruginosa Lipid A structural changes resulting from MCR-1 were analyzed by mass spectrometry. The introduction of mcr-1 led to colistin resistance in E. coli, K. pneumoniae, and A. baumannii but only moderately reduced susceptibility in P. aeruginosa Phosphoethanolamine modification of lipid A was observed consistently for all four species. These findings highlight the risk of colistin resistance as a consequence of mcr-1 expression among ESKAPE pathogens, especially in K. pneumoniae and A. baumannii Furthermore, the observation that lipid A structures were modified despite only modest increases in colistin MICs in some instances suggests more sophisticated surveillance methods may need to be developed to track the dissemination of mcr-1 or plasmid-mediated phosphoethanolamine transferases in general.

Lipid A structural modifications in extreme conditions and identification of unique modifying enzymes to define the Toll-like receptor 4 structure-activity relationship

Strategies utilizing Toll-like receptor 4 (TLR4) agonists for treatment of cancer, infectious diseases, and other targets report promising results. Potent TLR4 antagonists are also gaining attention as therapeutic leads. Though some principles for TLR4 modulation by lipid A have been described, a thorough understanding of the structure-activity relationship (SAR) is lacking. Only through a complete definition of lipid A-TLR4 SAR is it possible to predict TLR4 signaling effects of discrete lipid A structures, rendering them more pharmacologically relevant. A limited 'toolbox' of lipid A-modifying enzymes has been defined and is largely composed of enzymes from mesophile human and zoonotic pathogens. Expansion of this 'toolbox' will result from extending the search into lipid A biosynthesis and modification by bacteria living at the extremes. Here, we review the fundamentals of lipid A structure, advances in lipid A uses in TLR4 modulation, and the search for novel lipid A-modifying systems in extremophile bacteria. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Rapid lipid a structure determination via surface acoustic wave nebulization and hierarchical tandem mass spectrometry algorithm

RATIONALE

Surface acoustic wave nebulization (SAWN) is an easy to use sample transfer method for rapid mass spectrometric analysis. A new standing wave (SW) SAWN chip, with higher ionization efficiency than our previously reported design, is used for rapid analysis of lipids.

METHODS

The crude, yet fast, Caroff protocol was used for lipid A extraction from Francisella novicida. SW-SAWN with a Waters Synapt G2S quadrupole time-of-flight (QTOF) mass spectrometer was used to generate lipid A ions. Quadrupole collision-induced dissociation (Q-CID) of lipid A at varying CID energies was used to approximate the ion trap MSⁿ data required for our hierarchical tandem mass spectrometry (HiTMS) algorithm. Structural hypotheses can be obtained directly from the HiTMS algorithm to identify species-specific lipid A molecules.

RESULTS

SW-SAWN successfully generated ions from lipid A extracted from Francisella novicida using the faster Caroff method. In addition, varying collision energies were used to generate tandem mass spectra similar to MS³ and MS⁴ spectra from an ion trap. The Q-CID spectra are compatible with our HiTMS algorithm and offer an improvement over lipid A tandem mass spectra acquired in an ion trap.

CONCLUSIONS

Combining SW-SAWN and Q-CID enabled more structural assignments than previously reported in half the time. The ease of generating spectra by SAWN tandem MS in combination with HiTMS interpretation offers high-throughput lipid A structural analysis and thereby rapid detection of pathogens based on lipid fingerprinting. Copyright © 2016 John Wiley & Sons, Ltd.

Characterization of complex, heterogeneous lipid A samples using HPLC-MS/MS technique I. Overall analysis with respect to acylation, phosphorylation and isobaric distribution

We established a new reversed phase-high performance liquid chromatography method combined with electrospray ionization quadrupole time-of-flight tandem mass spectrometry for the simultaneous determination and structural characterization of different lipid A types in bacteria (Escherichia coli O111, Salmonella adelaide O35 and Proteus morganii O34) showing serological cross-reactivity. The complex lipid A mixtures (obtained by simple extraction and acid hydrolysis of the outer membrane lipopolysaccharides) were separated and detected without phosphate derivatization. Several previously unidentified ions were detected, which differed in the number and type of acyl chains and number of phosphate groups. In several cases, we observed the different retention of isobaric lipid A species, which had different secondary fatty acyl distribution at the C2' or the C3' sites. The fragmentation of the various, C4' monophosphorylated lipid A species in deprotonated forms provided structural assignment for each component. Fragmentation pathways of the tri-acylated, tetra-acylated, penta-acylated, hexa-acylated and hepta-acylated lipid A components and of the lipid A partial structures are suggested. As standards, the hexa-acylated ion at m/z 1716 with the E. coli-type acyl distribution and the hepta-acylated ion at m/z 1954 with the Salmonella-type acyl distribution were used. The results confirmed the presence of multiple forms of lipid A in all strains analyzed. In addition, the negative-ion mode MS permitted efficient detection for non-phosphorylated lipid A components, too. Copyright © 2016 John Wiley & Sons, Ltd.

Structural derivation of lipid A from Cronobacter sakazakii using tandem mass spectrometry

RATIONALE

Cronobacter sakazakii is a Gram-negative opportunistic pathogen that can cause necrotizing enterocolitis, bacteremia, and meningitis. Lipid A, the glycolipid membrane anchor of lipopolysaccharide (LPS), is a potential virulence factor for C. sakazakii. Given the potential importance of this molecule in infection and virulence, structural characterization of lipid A was carried out.

METHODS

The structural characterization of lipid A extracted from C. sakazakii was performed using electrospray ionization and collision-induced dissociation in a linear ion trap mass spectrometer. Specifically, for detailed structural characterization, hierarchical tandem mass spectrometry was performed on the dominant ions present in the precursor ion mass spectra. By comparing the C. sakazakii fragmentation pathways to those of the known structure of E. coli lipid A, a structure of C. sakazakii lipid A was derived.

RESULTS

The precursor ion at m/z 1796 from C. sakazakii is produced from a lipid A molecule where the acyl chains between the 2'b (C14) and 3'b (C12) positions are reversed as compared to E. coli lipid A. Additionally, the precursor ion at m/z 1824 from C. sakazakii corresponds to an E. coli structure with the same acyl chain at the 2'b position (C14), but a longer acyl chain (C14) at the 3'b position versus m/z 1796.

CONCLUSIONS

Two lipid A structures were derived for the C. sakazakii ions at m/z 1796 and 1824. They differed in composition at the 2'b and 3'b acyl chain substituents, which may be a result of differences in substrate specificity of the two lipid A acyl chain transferases: LpxL and LpxM. Copyright © 2016 John Wiley & Sons, Ltd.

Norharmane Matrix Enhances Detection of Endotoxin by MALDI-MS for Simultaneous Profiling of Pathogen, Host, and Vector Systems

The discovery of novel pathogenic mechanisms engaged during bacterial infections requires the evolution of advanced techniques. Here, we evaluate the dual polarity matrix norharmane (NRM) to improve detection of bacterial lipid A (endotoxin), from host and vector tissues infected withFrancisella novicida (Fn). We evaluated NRM for improved detection and characterization of a wide range of lipids in both positive and negative polarities, including lipid A and phospholipids across a range of matrix-assisted laser desorption-ionization-coupled applications. NRM matrix improved the limit of detection (LOD) for monophosphoryl lipid A (MPLA) down to picogram level representing a 10-fold improvement of LOD versus 2,5-dihydroxybenzoic acid and 100-fold improvement of LOD versus 9-aminoacridine (9-AA). Improved LOD for lipid A subsequently facilitated detection of theFn lipid A major ion (m/z 1665) from extracts of infected mouse spleen and the temperature-modifiedFn lipid A atm/z 1637 from infectedDermacentor variabilis ticks. Finally, we simultaneously mapped bacterial phospholipid signatures within anFn-infected spleen along with an exclusively host-derived inositol-based phospholipid (m/z 933) demonstrating coprofiling of the host-pathogen interaction. Expanded use of NRM matrix in other infection models and endotoxin-targeting imaging experiments will improve our understanding of the lipid interactions at the host-pathogen interface.

Characterization of a Novel d-Glycero-d-talo-oct-2-ulosonic acid-substituted Lipid A Moiety in the Lipopolysaccharide Produced by the Acetic Acid Bacterium Acetobacter pasteurianus NBRC 3283

Acetobacter pasteurianus is an aerobic Gram-negative rod that is used in the fermentation process used to produce the traditional Japanese black rice vinegar kurozu. Previously, we found that a hydrophobic fraction derived from kurozu stimulates Toll-like receptors to produce cytokines. LPSs, particularly LPS from A. pasteurianus, are strong candidates for the immunostimulatory component of kurozu. The LPS of A. pasteurianus remains stable in acidic conditions during the 2 years of the abovementioned fermentation process. Thus, we hypothesized that its stability results from its structure. In this study, we isolated the LPS produced by A. pasteurianus NBRC 3283 bacterial cells and characterized the structure of its lipid A component. The lipid A moiety was obtained by standard weak acid hydrolysis of the LPS. However, the hydrolysis was incomplete because a certain proportion of the LPS contained acid-stable d-glycero-d-talo-oct-2-ulosonic acid (Ko) residues instead of the acid-labile 3-deoxy-d-manno-oct-2-ulosonic acid residues that are normally found in typical LPS. Even so, we obtained a Ko-substituted lipid A with a novel sugar backbone, α-Man(1-4)[α-Ko(2-6)]β-GlcN3N(1-6)α-GlcN(1-1)α-GlcA. Its reducing end GlcN(1-1)GlcA bond was also found to be quite acid-stable. Six fatty acids were attached to the backbone. Both the whole LPS and the lipid A moiety induced TNF-α production in murine cells via Toll-like receptor 4, although their activity was weaker than those of Escherichia coli LPS and lipid A. These results suggest that the structurally atypical A. pasteurianus lipid A found in this study remains stable and, hence, retains its immunostimulatory activity during acetic acid fermentation.

Evidence Suggesting That Francisella tularensis O-Antigen Capsule Contains a Lipid A-Like Molecule That Is Structurally Distinct from the More Abundant Free Lipid A

Francisella tularensis, the Gram-negative bacterium that causes tularemia, produces a high molecular weight capsule that is immunologically distinct from Francisella lipopolysaccharide but contains the same O-antigen tetrasaccharide. To pursue the possibility that the capsule of Francisella live vaccine strain (LVS) has a structurally unique lipid anchor, we have metabolically labeled Francisella with [¹⁴C]acetate to facilitate highly sensitive compositional analysis of capsule-associated lipids. Capsule was purified by two independent methods and yielded similar results. Autoradiographic and immunologic analysis confirmed that this purified material was largely devoid of low molecular weight LPS and of the copious amounts of free lipid A that the Francisellae accumulate. Chemical hydrolysis yielded [¹⁴C]-labeled free fatty acids characteristic of Francisella lipid A but with a different molar ratio of 3-OH C18:0 to 3-OH C16:0 and different composition of non-hydroxylated fatty acids (mainly C14:0 rather than C16:0) than that of free Francisella lipid A. Mild acid hydrolysis to induce selective cleavage of KDO-lipid A linkage yielded a [¹⁴C]-labeled product that partitioned during Bligh/Dyer extraction and migrated during thin-layer chromatography like lipid A. These findings suggest that the O-antigen capsule of Francisella contains a covalently linked and structurally distinct lipid A species. The presence of a discrete lipid A-like molecule associated with capsule raises the possibility that Francisella selectively exploits lipid A structural heterogeneity to regulate synthesis, transport, and stable bacterial surface association of the O-antigen capsular layer.

Temperature-Dependent Gentamicin Resistance of Francisella tularensis is Mediated by Uptake Modulation

Gentamicin (Gm) is an aminoglycoside commonly used to treat bacterial infections such as tularemia – the disease caused by Francisella tularensis. In addition to being pathogenic, F. tularensis is found in environmental niches such as soil where this bacterium likely encounters Gm producers (Micromonospora sp.). Here we show that F. tularensis exhibits increased resistance to Gm at ambient temperature (26°C) compared to mammalian body temperature (37°C). To evaluate whether F. tularensis was less permeable to Gm at 26°C, a fluorescent marker [Texas Red (Tr)] was conjugated with Gm, yielding Tr-Gm. Bacteria incubated at 26°C showed reduced fluorescence compared to those at 37°C when exposed to Tr-Gm suggesting that uptake of Gm was reduced at 26°C. Unconjugated Gm competitively inhibited uptake of Tr-Gm, demonstrating that this fluorescent compound was taken up similarly to unconjugated Gm. Lysates of F. tularensis bacteria incubated with Gm at 37°C inhibited the growth of Escherichia coli significantly more than lysates from bacteria incubated at 26°C, further indicating reduced uptake at this lower temperature. Other facultative pathogens (Listeria monocytogenes and Klebsiella pneumoniae) exhibited increased resistance to Gm at 26°C suggesting that the results generated using F. tularensis may be generalizable to diverse bacteria. Regulation of the uptake of antibiotics provides a mechanism by which facultative pathogens survive alongside antibiotic-producing microbes in nature.

Top-Down Strategies for the Structural Elucidation of Intact Gram-negative Bacterial Endotoxins

Re-modelling of lipopolysaccharides, which are the primary constituent of the outer cell membrane of Gram-negative bacteria, modulates pathogenesis and resistance to microbials. Reported herein is the characterization of intact Gram-negative bacterial lipooligosaccharides (LOS) via a new strategy utilizing online liquid chromatography (LC) coupled with ultraviolet photodissociation (UVPD) mass spectrometry. Compared to collision-based MS/MS methods, UVPD and UVPD/HCD promoted a greater array of cleavages within both the glycan and lipid moieties, including C-C, C-N, C-O cleavages in the acyl chains as well as glycosidic and cross-ring cleavages, thus providing the most far-reaching structural characterization of LOS. This LC-MS/MS strategy affords a robust analytical method to structurally characterize complex mixtures of bacterial endotoxins that maintains the integrity of the core oligosaccharide and lipid A domains of LOS, providing direct feedback about the cell envelope architectures and LOS modification strategies involved in resistance host innate immune defense.

Endotoxin structures in the psychrophiles Psychromonas marina and Psychrobacter cryohalolentis contain distinctive acyl features

Lipid A is the essential component of endotoxin (Gram-negative lipopolysaccharide), a potent immunostimulatory compound. As the outer surface of the outer membrane, the details of lipid A structure are crucial not only to bacterial pathogenesis but also to membrane integrity. This work characterizes the structure of lipid A in two psychrophiles, Psychromonas marina and Psychrobacter cryohalolentis, and also two mesophiles to which they are related using MALDI-TOF MS and fatty acid methyl ester (FAME) GC-MS. P. marina lipid A is strikingly similar to that of Escherichia coli in organization and total acyl size, but incorporates an unusual doubly unsaturated tetradecadienoyl acyl residue. P. cryohalolentis also shows structural organization similar to a closely related mesophile, Acinetobacter baumannii, however it has generally shorter acyl constituents and shows many acyl variants differing by single methylene (-CH2-) units, a characteristic it shares with the one previously reported psychrotolerant lipid A structure. This work is the first detailed structural characterization of lipid A from an obligate psychrophile and the second from a psychrotolerant species. It reveals distinctive structural features of psychrophilic lipid A in comparison to that of related mesophiles which suggest constitutive adaptations to maintain outer membrane fluidity in cold environments.

The Influence of Ziegler-Natta and Metallocene Catalysts on Polyolefin Structure, Properties, and Processing Ability

50 years ago, Karl Ziegler and Giulio Natta were awarded the Nobel Prize for their discovery of the catalytic polymerization of ethylene and propylene using titanium compounds and aluminum-alkyls as co-catalysts. Polyolefins have grown to become one of the biggest of all produced polymers. New metallocene/methylaluminoxane (MAO) catalysts open the possibility to synthesize polymers with highly defined microstructure, tacticity, and steroregularity, as well as long-chain branched, or blocky copolymers with excellent properties. This improvement in polymerization is possible due to the single active sites available on the metallocene catalysts in contrast to their traditional counterparts. Moreover, these catalysts, half titanocenes/MAO, zirconocenes, and other single site catalysts can control various important parameters, such as co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, and branching of the polymer. However, in most cases research in this area has reduced academia as olefin polymerization has seen significant advancements in the industries. Therefore, this paper aims to further motivate interest in polyolefin research in academia by highlighting promising and open areas for the future.

Kdo2 -lipid A: structural diversity and impact on immunopharmacology

3-deoxy-d-manno-octulosonic acid-lipid A (Kdo₂-lipid A) is the essential component of lipopolysaccharide in most Gram-negative bacteria and the minimal structural component to sustain bacterial viability. It serves as the active component of lipopolysaccharide to stimulate potent host immune responses through the complex of Toll-like-receptor 4 (TLR4) and myeloid differentiation protein 2. The entire biosynthetic pathway of Escherichia coli Kdo₂-lipid A has been elucidated and the nine enzymes of the pathway are shared by most Gram-negative bacteria, indicating conserved Kdo₂-lipid A structure across different species. Yet many bacteria can modify the structure of their Kdo₂-lipid A which serves as a strategy to modulate bacterial virulence and adapt to different growth environments as well as to avoid recognition by the mammalian innate immune systems. Key enzymes and receptors involved in Kdo₂-lipid A biosynthesis, structural modification and its interaction with the TLR4 pathway represent a clear opportunity for immunopharmacological exploitation. These include the development of novel antibiotics targeting key biosynthetic enzymes and utilization of structurally modified Kdo₂-lipid A or correspondingly engineered live bacteria as vaccines and adjuvants. Kdo₂-lipid A/TLR4 antagonists can also be applied in anti-inflammatory interventions. This review summarizes recent knowledge on both the fundamental processes of Kdo₂-lipid A biosynthesis, structural modification and immune stimulation, and applied research on pharmacological exploitations of these processes for therapeutic development.

193 nm ultraviolet photodissociation mass spectrometry for the structural elucidation of lipid A compounds in complex mixtures

Here we implement ultraviolet photodissociation (UVPD) in an online liquid chromatographic tandem mass spectrometry (MS/MS) strategy to support analysis of complex mixtures of lipid A combinatorially modified during development of vaccine adjuvants. UVPD mass spectrometry at 193 nm was utilized to characterize the structures and fragment ion types of lipid A from Escherichia coli, Vibrio cholerae, and Pseudomonas aeruginosa using an Orbitrap mass spectrometer. The fragment ions generated by UVPD were compared to those from collision induced dissociation (CID) and higher energy collision dissociation (HCD) with respect to the precursor charge state. UVPD afforded the widest array of fragment ion types including acyl chain C–O, C–N, and C–C bond cleavages and glycosidic C–O and cross ring cleavages, thus providing the most comprehensive structural analysis of the lipid A. UVPD exhibited virtually no dependence on precursor ion charge state and was best at determining lipid A structure including acyl chain length and composition, giving it an advantage over collision based methods. UVPD was incorporated into an LC–MS/MS methodology for the analysis of a number of structural variants in a complex mixture of combinatorially engineered Escherichia coli lipid A.

The atypical lipopolysaccharide of Francisella

Bacterial lipopolysaccharides (LPSs) are ubiquitous molecules that are prominent components of the outer membranes of most gram-negative bacteria. Genetic and structural characterizations of Francisella LPS have revealed substantial differences when compared to more commonly studied LPSs of the Enterobacteriaceae. This review discusses both the general characteristics and the unusual features of Francisella LPS.

Site-specific acylation changes in the lipid A of Escherichia coli lpxL mutants grown at high temperatures

LPS is a major component of the outer membrane of Gram-negative bacteria. The lipid A region of LPS mediates stimulation of the immune system. In E. coli, the gene (formerly htrB) codes for a late lauroyltransferase (LpxL) in lipid A biosynthesis. E. coli lpxL mutants have been described previously with impaired growth above 33°C in rich media. However, we were able to grow lpxL mutants at 30°C, 37°C and 42°C, and investigate their lipid A moieties to gain insight into changes and regulatory effects in lipid A biosynthesis. Multiple-stage mass spectrometry was used to decipher unusual lipid A structures produced by lpxL mutant bacteria at high temperatures that rescue the temperature-sensitive phenotype. At 37°C and 42°C, E. coli lpxL mutants appear to activate different acyltransferases or biosynthetic pathways that generate atypical penta- and hexaacyl lipid A structures by incorporating longer fatty acids, such as a secondary palmitoleic acid (2'-O-position, distal) and a secondary palmitic acid (2-O-position, proximal) respectively. However, we observed no changes in these structures through various growth curve stages. This study indicates that E. coli (lpxL) lipid A biosynthesis, and specifically the 'late' acylation of lipid A, is temperature dependent, thus suggesting a highly regulated process.

Metabolic labeling to characterize the overall composition of Francisella lipid A and LPS grown in broth and in human phagocytes

A hallmark of Francisella tularensis, a highly virulent Gram-negative bacterium, is an unusual LPS that possesses both structural heterogeneity and characteristics that may contribute to innate immune evasion. However, none of the methods yet employed has been sufficient to determine the overall LPS composition of Francisella. We now demonstrate that metabolic labeling of francisellae with [(14)C]acetate, combined with fractionation of [(14)C]acetate-labeled lipids by ethanol precipitation rather than hot phenol-water extraction, permits a more sensitive and quantitative appraisal of overall compositional heterogeneity in lipid A and LPS. The majority of lipid A of different francisellae strains grown in diverse bacteriologic media and within human phagocytes accumulated as very hydrophobic species, including free lipid A, with <10% of the lipid A molecules substituted with O-Ag polysaccharides. The spectrum of lipid A and LPS species varied in a medium- and strain-dependent fashion, and growth in THP-1 cells yielded lipid A species that were not present in the same bacteria grown in brain heart infusion broth. In summary, metabolic labeling with [(14)C]acetate greatly facilitates assessment of the effect of genotypic and/or environmental variables on the synthesis and accumulation of lipid A and LPS by Francisella, including during growth within the cytosol of infected host cells.

Structural characterization of bacterial lipopolysaccharides with mass spectrometry and on- and off-line separation techniques

The focus of this review is the application of mass spectrometry to the structural characterization of bacterial lipopolysaccharides (LPSs), also referred to as "endotoxins," because they elicit the strong immune response in infected organisms. Recently, a wide variety of MS-based applications have been implemented to the structure elucidation of LPS. Methodological improvements, as well as on- and off-line separation procedures, proved the versatility of mass spectrometry to study complex LPS mixtures. Special attention is given in the review to the tandem mass spectrometric methods and protocols for the analyses of lipid A, the endotoxic principle of LPS. We compare and evaluate the different ionization techniques (MALDI, ESI) in view of their use in intact R- and S-type LPS and lipid A studies. Methods for sample preparation of LPS prior to mass spectrometric analysis are also described. The direct identification of intrinsic heterogeneities of most intact LPS and lipid A preparations is a particular challenge, for which separation techniques (e.g., TLC, slab-PAGE, CE, GC, HPLC) combined with mass spectrometry are often necessary. A brief summary of these combined methodologies to profile LPS molecular species is provided.

NaxD is a deacetylase required for lipid A modification and Francisella pathogenesis

Modification of specific Gram-negative bacterial cell envelope components, such as capsule, O-antigen and lipid A, are often essential for the successful establishment of infection. Francisella species express lipid A molecules with unique characteristics involved in circumventing host defences, which significantly contribute to their virulence. In this study, we show that NaxD, a member of the highly conserved YdjC superfamily, is a deacetylase required for an important modification of the outer membrane component lipid A in Francisella. Mass spectrometry analysis revealed that NaxD is essential for the modification of a lipid A phosphate with galactosamine in Francisella novicida, a model organism for the study of highly virulent Francisella tularensis. Significantly, enzymatic assays confirmed that this protein is necessary for deacetylation of its substrate. In addition, NaxD was involved in resistance to the antimicrobial peptide polymyxin B and critical for replication in macrophages and in vivo virulence. Importantly, this protein is also required for lipid A modification in F. tularensis as well as Bordetella bronchiseptica. Since NaxD homologues are conserved among many Gram-negative pathogens, this work has broad implications for our understanding of host subversion mechanisms of other virulent bacteria.

Surface acoustic wave nebulization facilitating lipid mass spectrometric analysis

Surface acoustic wave nebulization (SAWN) is a novel method to transfer nonvolatile analytes directly from the aqueous phase to the gas phase for mass spectrometric analysis. The lower ion energetics of SAWN and its planar nature make it appealing for analytically challenging lipid samples. This challenge is a result of their amphipathic nature, labile nature, and tendency to form aggregates, which readily precipitate clogging capillaries used for electrospray ionization (ESI). Here, we report the use of SAWN to characterize the complex glycolipid, lipid A, which serves as the membrane anchor component of lipopolysaccharide (LPS) and has a pronounced tendency to clog nano-ESI capillaries. We also show that unlike ESI SAWN is capable of ionizing labile phospholipids without fragmentation. Lastly, we compare the ease of use of SAWN to the more conventional infusion-based ESI methods and demonstrate the ability to generate higher order tandem mass spectral data of lipid A for automated structure assignment using our previously reported hierarchical tandem mass spectrometry (HiTMS) algorithm. The ease of generating SAWN-MS(n) data combined with HiTMS interpretation offers the potential for high throughput lipid A structure analysis.

LPS remodeling is an evolved survival strategy for bacteria

Maintenance of membrane function is essential and regulated at the genomic, transcriptional, and translational levels. Bacterial pathogens have a variety of mechanisms to adapt their membrane in response to transmission between environment, vector, and human host. Using a well-characterized model of lipid A diversification (Francisella), we demonstrate temperature-regulated membrane remodeling directed by multiple alleles of the lipid A-modifying N-acyltransferase enzyme, LpxD. Structural analysis of the lipid A at environmental and host temperatures revealed that the LpxD1 enzyme added a 3-OH C18 acyl group at 37 °C (host), whereas the LpxD2 enzyme added a 3-OH C16 acyl group at 18 °C (environment). Mutational analysis of either of the individual Francisella lpxD genes altered outer membrane (OM) permeability, antimicrobial peptide, and antibiotic susceptibility, whereas only the lpxD1-null mutant was attenuated in mice and subsequently exhibited protection against a lethal WT challenge. Additionally, growth-temperature analysis revealed transcriptional control of the lpxD genes and posttranslational control of the LpxD1 and LpxD2 enzymatic activities. These results suggest a direct mechanism for LPS/lipid A-level modifications resulting in alterations of membrane fluidity, as well as integrity and may represent a general paradigm for bacterial membrane adaptation and virulence-state adaptation.

A variety of novel lipid A structures obtained from Francisella tularensis live vaccine strain

F. tularensis is a Gram-negative coccobacillus that causes tularemia. Its LPS has nominal biological activity. Currently, there is controversy regarding the structure of the lipid A obtained from F. tularensis live vaccine strain (LVS). Therefore, to resolve this controversy, the purification and structural identification of this LPS was crucial. To achieve this, LPS from F. tularensis LVS was acid hydrolyzed to obtain crude lipid A that was methylated and purified by HPLC and the fractions were analyzed by MALDI-TOF MS. The structure of the major lipid A species was composed of a glucosamine disaccharide backbone substituted with four fatty acyl groups and a phosphate (1-position) with a molecular mass of 1505. The major lipid A component contained 18:0[3-O(16:0)] in the distal subunit and two 18:0(3-OH) fatty acyl chains at the 2- or 3-positions of the reducing subunit. Additional variations in the lipid A species include: heterogeneity in fatty acyl groups, a phosphate or a phosphoryl galactosamine at the 1-position, and a hexose at the 4' or 6' position, some of which have not been previously described for F. tularensis LVS. This analysis revealed that lipid A from F. tularensis LVS is far more complex than originally believed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Free lipid A isolated from Porphyromonas gingivalis lipopolysaccharide is contaminated with phosphorylated dihydroceramide lipids: recovery in diseased dental samples

Recent reports indicate that Porphyromonas gingivalis mediates alveolar bone loss or osteoclast modulation through engagement of Toll-like receptor 2 (TLR2), though the factors responsible for TLR2 engagement have yet to be determined. Lipopolysaccharide (LPS) and lipid A, lipoprotein, fimbriae, and phosphorylated dihydroceramides of P. gingivalis have been reported to activate host cell responses through engagement of TLR2. LPS and lipid A are the most controversial in this regard because conflicting evidence has been reported concerning the capacity of P. gingivalis LPS or lipid A to engage TLR2 versus TLR4. In the present study, we first prepared P. gingivalis LPS by the Tri-Reagent method and evaluated this isolate for contamination with phosphorylated dihydroceramide lipids. Next, the lipid A prepared from this LPS was evaluated for the presence of phosphorylated dihydroceramide lipids. Finally, we characterized the lipid A by the matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray-MS methods in order to quantify recovery of lipid A in lipid extracts from diseased teeth or subgingival plaque samples. Our results demonstrate that both the LPS and lipid A derived from P. gingivalis are contaminated with phosphorylated dihydroceramide lipids. Furthermore, the lipid extracts derived from diseased teeth or subgingival plaque do not contain free lipid A constituents of P. gingivalis but contain substantial amounts of phosphorylated dihydroceramide lipids. Therefore, the free lipid A of P. gingivalis is not present in measurable levels at periodontal disease sites. Our results also suggest that the TLR2 activation of host tissues attributed to LPS and lipid A of P. gingivalis could actually be mediated by phosphorylated dihydroceramides.

A rapid one-step method for the characterization of membrane lipid remodeling in Francisella using matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry

Lipids are essential components of all bacterial membranes. The most common membrane-associated lipids in Gram-negative bacteria are phospholipids and lipid A, the hydrophobic anchor of lipopolysaccharide. Diversity in these lipids arises through structural modifications that include changes in the length and location of fatty acids, and the addition of phosphate and carbohydrate moieties. Analysis of individual structural modifications normally requires large quantities of starting material and multiple methods for the isolation, hydrolysis, and analysis. In this study, we developed a novel one-step protocol for the combined isolation of phospholipids and lipid A from Francisella subspecies followed by analysis using matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry. The total time for lipid isolation and analysis was approximately 15 min and with a lower limit of detection of approximately 100 ng of purified lipid. This protocol identified the major lipid structures using both wild-type Ft subspecies strains and lipid A biosynthesis mutants. We also determined the relative levels of individual lipid A and phospholipids after growth under conditions that mimic the mammalian infection process. This analysis showed that the bacterial membranes remodeled rapidly to adapt to changes in environmental growth conditions and may be important for Francisella pathogenesis.

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.

IR and UV photodissociation as analytical tools for characterizing lipid A structures

The utility of 193-nm ultraviolet photodissociation (UVPD) and 10.6-μm infrared multiphoton dissociation (IRMPD) for the characterization of lipid A structures was assessed in an ion trap mass spectrometer. The fragmentation behavior of lipid A species was also evaluated by activated-electron photodetachment (a-EPD), which uses 193-nm photons to create charge reduced radicals that are subsequently dissociated by collisional activation. In contrast to collision-induced dissociation (CID), IRMPD offered the ability to selectively differentiate product ions with varying degrees of phosphorylation because of the increased photoabsorption cross sections and thus dissociation of phosphate-containing species. Both 193-nm UVPD and a-EPD yielded higher abundances and a larger array of product ions arising from C-C cleavages, as well as cross-ring and inter-ring glucosamine cleavages, compared to CID and IRMPD, because of high energy, single-photon absorption, and/or radical-directed dissociation. UVPD at 193 nm also exhibited enhanced cleavage between the amine and carbonyl groups on the 2- and 2'-linked primary acyl chains. Lastly, UVPD of phosphorylethanolamine-modified lipid A species resulted in preferential cleavage of the C-O bond between ethanolamine and phosphate, enabling the selective identification of this modification.