Characterization of the nonlamellar cubic and HII structures of lipid A from Salmonella enterica serovar Minnesota by X-ray diffraction and freeze-fracture electron microscopy

Exploring the Molecular Dynamics of a Lipid-A Vesicle at the Atom Level: Morphology and Permeation Mechanism

Lipid-A was previously shown to spontaneously aggregate into a vesicle via the hybrid particle field approach. We assess the validity of the proposed vesiculation mechanism by simulating the resulting lipid-A vesicle at the atom level. The spatial confinement imposed by the vesicle geometry on the conformation and packing of lipid-A induces significant heterogeneity of physical properties in the inner and outer leaflets. It also induces tighter molecular packing and lower acyl chain order compared to the lamellar arrangement. Around 5% of water molecules passively permeates the vesicle membrane inward and outward. The permeation is facilitated by interactions with water molecules that are transported across the membrane by a network of electrostatic interactions with the hydrogen bond donors/acceptors in the N-acetylglucosamine ring and upper region of the acyl chains of lipid-A. The permeation process takes place at low rates but still at higher frequencies than observed for the lamellar arrangement of lipid-A. These findings not only substantiate the proposed lipid-A vesiculation mechanism but also reveal the complex structural dynamics of an important nonlamellar arrangement of lipid-A.

Bacterial lipids drive compartmentalization on the nanoscale

The design of cellular functions in synthetic systems, inspired by the internal partitioning of living cells, is a constantly growing research field that is paving the way to a large number of new remarkable applications. Several hierarchies of internal compartments like polymersomes, liposomes, and membranes are used to control the transport, release, and chemistry of encapsulated species. However, the experimental characterization and the comprehension of glycolipid mesostructures are far from being fully addressed. Lipid A is indeed a glycolipid and the endotoxic part of Gram-negative bacterial lipopolysaccharide; it is the moiety that is recognized by the eukaryotic receptors giving rise to the modulation of innate immunity. Herein we propose, for the first time, a combined approach based on hybrid Particle-Field (hPF) Molecular Dynamics (MD) simulations and Small Angle X-Ray Scattering (SAXS) experiments to gain a molecular picture of the complex supramolecular structures of lipopolysaccharide (LPS) and lipid A at low hydration levels. The mutual support of data from simulations and experiments allowed the unprecedented discovery of the presence of a nano-compartmentalized phase composed of liposomes of variable size and shape which can be used in synthetic biological applications.

Interest of Homodialkyl Neamine Derivatives against Resistant P. aeruginosa, E. coli, and β-Lactamases-Producing Bacteria-Effect of Alkyl Chain Length on the Interaction with LPS

Development of novel therapeutics to treat antibiotic-resistant infections, especially those caused by ESKAPE pathogens, is urgent. One of the most critical pathogens is P. aeruginosa, which is able to develop a large number of factors associated with antibiotic resistance, including high level of impermeability. Gram-negative bacteria are protected from the environment by an asymmetric Outer Membrane primarily composed of lipopolysaccharides (LPS) at the outer leaflet and phospholipids in the inner leaflet. Based on a large hemi-synthesis program focusing on amphiphilic aminoglycoside derivatives, we extend the antimicrobial activity of 3′,6-dinonyl neamine and its branched isomer, 3′,6-di(dimethyloctyl) neamine on clinical P. aeruginosa, ESBL, and carbapenemase strains. We also investigated the capacity of 3′,6-homodialkyl neamine derivatives carrying different alkyl chains (C7–C11) to interact with LPS and alter membrane permeability. 3′,6-Dinonyl neamine and its branched isomer, 3′,6-di(dimethyloctyl) neamine showed low MICs on clinical P. aeruginosa, ESBL, and carbapenemase strains with no MIC increase for long-duration incubation. In contrast from what was observed for membrane permeability, length of alkyl chains was critical for the capacity of 3′,6-homodialkyl neamine derivatives to bind to LPS. We demonstrated the high antibacterial potential of the amphiphilic neamine derivatives in the fight against ESKAPE pathogens and pointed out some particular characteristics making the 3′,6-dinonyl- and 3′,6-di(dimethyloctyl)-neamine derivatives the best candidates for further development.

Influence of disulfide bonds in human beta defensin-3 on its strain specific activity against Gram-negative bacteria

Antimicrobial peptides (AMPs) play an important role in the host defense against various microbes. One of the most efficient human AMPs is the human beta defensin-3 (hBD-3) which is produced by, e.g. keratinocytes and lung epithelial cells. However, the structure-function relationship for AMPs and in particular for defensins with their typical three disulfide bonds is still poorly understood. In this study the importance of the three disulfide bonds for the activity of the AMPs is investigated with biological assays and with biophysical experiments utilizing different membrane reconstitution systems. The activities of natural hBD-3, hBD-3-c (cyclic variant with one disulfide bond), and hBD-3-l (linear variant without disulfide bonds) and fragments thereof were tested against specific Gram-negative bacteria. Furthermore, hemolytic and cytotoxic activities were analyzed as well as the potency to neutralize immune cell stimulation of lipopolysaccharide (LPS). Experiments using reconstituted lipid matrices composed of phospholipids or LPS purified from the respective Gram-negative bacteria, showed that the membrane activity of all three hBD-3 peptides is decisive for their capability to kill bacteria and to neutralize LPS. In most of the test systems the linear hBD-3-l showed the highest activity. It was also the only peptide significantly active against polymyxin B-resistant Proteus mirabilis R45. However, the stability of hBD-3 against protease activity decreases with decreasing number of disulfide bonds. This study demonstrates that the refining of AMP structures can generate more active compounds against certain strains.

Aggregation of Lipid A Variants: A Hybrid Particle-Field Model

Lipid A is one of the three components of bacterial lipopolysaccharides constituting the outer membrane of Gram-negative bacteria, and is recognized to have an important biological role in the inflammatory response of mammalians. Its biological activity is modulated by the number of acyl-chains that are present in the lipid and by the dielectric medium, i.e., the type of counter-ions, through electrostatic interactions. In this paper, we report on a coarse-grained model of chemical variants of Lipid A based on the hybrid particle-field/molecular dynamics approach (hPF-MD). In particular, we investigate the stability of Lipid A bilayers for two different hexa- and tetra-acylated structures. Comparing particle density profiles along bilayer cross-sections, we find good agreement between the hPF-MD model and reference all-atom simulation for both chemical variants of Lipid A. hPF-MD models of constituted bilayers composed by hexa-acylated Lipid A in water are stable within the simulation time. We further validate our model by verifying that the phase behavior of Lipid A/counterion/water mixtures is correctly reproduced. In particular, hPF-MD simulations predict the correct self-assembly of different lamellar and micellar phases from an initially random distribution of Lipid A molecules with counterions in water. Finally, it is possible to observe the spontaneous formation and stability of Lipid A vesicles by fusion of micellar aggregates.

pH Responsiveness of Hexosomes and Cubosomes for Combined Delivery of Brucea javanica Oil and Doxorubicin

We report pH-responsive liquid crystalline lipid nanoparticles, which are dual-loaded by Brucea javanica oil (BJO) and doxorubicin hydrochloride (DOX) and display a pH-induced inverted hexagonal (pH = 7.4) to cubic (pH = 6.8) to emulsified microemulsion (pH = 5.3) phase transition with a therapeutic application in cancer inhibition. BJO is a traditional herbal medicine that strongly inhibits the proliferation and metastasis of various cancers. Doxorubicin is an antitumor drug, which prevents DNA replication and hampers protein synthesis through intercalation between the base pairs of the DNA helices. Its dose-dependent cardiotoxicity imposes the need for safe delivery carriers. Here, pH-induced changes in the structural and interfacial properties of designed multicomponent drug delivery (monoolein-oleic acid-BJO-DOX) systems are determined by synchrotron small-angle X-ray scattering and the Langmuir film balance technique. The nanocarrier assemblies display good physical stability in the studied pH range and adequate particle sizes and ζ-potentials. Their interaction with model lipid membrane interfaces is enhanced under acidic pH conditions, which mimic the microenvironment around tumor cells. In vitro cytotoxicity and apoptosis studies with BJO-DOX dual-loaded pH-switchable liquid crystalline nanoparticles are performed on the human breast cancer Michigan Cancer Foundation-7 (MCF-7) cell line and MCF-7 cells with doxorubicin resistance (MCF-7/DOX), respectively. The obtained pH-sensitive nanomedicines exhibit enhanced antitumor efficacy. The performed preliminary studies suggest a potential reversal of the resistance of the MCF-7/DOX cells to DOX. These results highlight the necessity for further understanding the link between the established pH-dependent drug release profiles of the nanocarriers and the role of their pH-switchable inverted hexagonal, bicontinuous cubic, and emulsified microemulsion inner organizations for therapeutic outcomes.

In situ phase transition of microemulsions for parenteral injection yielding lyotropic liquid crystalline carriers of the antitumor drug bufalin

In this work, we used the small angle X-ray scattering (SAXS) method for controlled preparation of in situ forming sustained-release carriers for the antitumor drug bufalin (BUF), which has very poor solubility and a considerable cardiotoxicity in a non-encapsulated state. To that aim, we exploited the pseudo-ternary phase diagram of an oil(O)/surfactant(S)/water(W) system containing medium chain capric/caprylic triglycerides (MCT) and a co-surfactant blend of Macrogol (15)-hydroxystearate (Solutol HS 15) and sorbitan monooleate (Span 80). Two compositions with different oil contents (sample B and C) were selected from the microemulsion region of the phase diagram in order to study the effect of the aqueous environment on their structural behavior. A phase transition from a microemulsion (ME) to a liquid crystalline phase (LC) was established by SAXS upon progressive dilution. The drug bufalin (BUF) was encapsulated in the microemulsions with low viscosity, whereas the release of the drug occurred from the in situ generated lamellar liquid crystalline structures. The formulations were characterized by SAXS, dynamic light scattering (DLS), cryogenic transmission electron microscopy (Cryo-TEM), rheology, drug loading and encapsulation efficiency, and in vitro release profiles. A correlation was suggested between the structures of the in situ phase-transition formed LCME formulations, the differences in their viscosities and drug release profiles. The performed cytotoxicity, cell apoptosis and pharmacokinetic experiments showed an enhanced bioavailability of BUF after encapsulation. These results suggest potential clinical applications for the obtained safe in situ phase-transition sustained-release formulations of BUF.

Construction and Characterization of a Novel Sustained-Release Delivery System for Hydrophobic Pesticides Using Biodegradable Polydopamine-Based Microcapsules

Microcapsule formulations have been highly desirable and widely developed for effective utilization of pesticides and environmental pollution reduction. However, commercial and traditional microcapsule formulations of λ-cyhalothrin (LC) were prepared by complicated synthesis procedures and thereby specific organic solvents were needed. In this work, LC was encapsulated into versatile, robust, and biodegradable polydopamine (PDA) microcapsules by self-polymerization of dopamine. LC-loaded PDA microcapsules were characterized by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and thermogravimetric analysis measurements (TGA). LC-loaded PDA microcapsules have uniform morphology with nanoscale, decent LC loading content (>50.0% w/w), and good physicochemical stability and sustained release properties. The bioassay against housefly ( Musca domestica) showed that the bioactivity and long-term efficiency of LC-loaded PDA microcapsules was superior to that of the commercial formulation. All of these results demonstrated that LC-loaded PDA microcapsules could be applied as a commercial LC microcapsule formulation with fewer environmental side effects and higher effective delivery.

Supramolecular structure of enterobacterial wild-type lipopolysaccharides (LPS), fractions thereof, and their neutralization by Pep19-2.5

Lipopolysaccharides (LPS) belong to the strongest immune-modulating compounds known in nature, and are often described as pathogen-associated molecular patterns (PAMPs). In particular, at higher concentrations they are responsible for sepsis and the septic shock syndrome associated with high lethality. Since most data are indicative that LPS aggregates are the bioactive units, their supramolecular structures are considered to be of outmost relevance for deciphering the molecular mechanisms of its bioactivity. So far, however, most of the data available addressing this issue, were published only for the lipid part (lipid A) and the core-oligosaccharide containing rough LPS, representing the bioactive unit. By contrast, it is well known that most of the LPS specimen identified in natural habitats contain the smooth-form (S-form) LPS, which carry additionally a high-molecular polysaccharide (O-chain). To fill this lacuna and going into a more natural system, here various wild-type (smooth form) LPS including also some LPS fractions were investigated by small-angle X-ray scattering with synchrotron radiation to analyze their aggregate structure. Furthermore, the influence of a recently designed synthetic anti-LPS peptide (SALP) Pep19-2.5 on the aggregate structure, on the binding thermodynamics, and on the cytokine-inducing activity of LPS were characterized, showing defined aggregate changes, high affinity binding and inhibition of cytokine secretion. The data obtained are suitable to refine our view on the preferences of LPS for non-lamellar structures, representing the highest bioactive forms which can be significantly influenced by the binding with neutralizing peptides such as Pep19-2.5.

Differential immune responses to HIV-1 envelope protein induced by liposomal adjuvant formulations containing monophosphoryl lipid A with or without QS21

Liposomes have shown promise as constituents of adjuvant formulations in vaccines to parasitic and viral diseases. A particular type of liposomal construct, referred to as Army Liposome Formulation (ALF), containing neutral and anionic saturated phospholipids, cholesterol, and monophosphoryl lipid A (MPLA), has been used as an adjuvant for many years. Here we investigated the effects of physical and chemical changes of ALF liposomes on adjuvanted immune responses to CN54 gp140, a recombinant HIV-1 envelope protein. While holding the total amounts of liposomal MPLA and the gp140 antigen constant, different liposome sizes and liposomal MPLA:phospholipid molar ratios, and the effect of adding QS21 to the liposomes were compared for inducing immune responses to the gp140. For liposomes lacking QS21, higher titers of IgG binding antibodies to gp140 were induced by small unilamellar vesicle (SUV) rather than by large multilamellar vesicle (MLV) liposomes, and the highest titers were obtained with SUV having the MPLA:phospholipid ratio of 1:5.6. ALF plus QS21 (ALFQ) liposomes induced the same maximal binding antibody titers regardless of the MPLA:phospholipid ratio. ALF MLV liposomes induced mainly IgG1 and very low IgG2a antibodies, while ALF SUV liposomes induced IgG1≥IgG2a>IgG2b antibodies. Liposomes containing QS21 induced IgG1>IgG2a>IgG2b>IgG3 antibodies. ELISPOT analysis of splenocytes from immunized mice revealed that ALF liposomes induced low levels of IFN-γ, but ALFQ induced high levels. ALF and ALFQ liposomes each induced approximately equivalent high levels of IL-4. Based on antibody subtypes and cytokine secretion, we conclude that ALF liposomes predominantly stimulate Th2, while ALFQ strongly induces both Th1 and Th2 immunity. When CN54 gp140 was adjuvanted with either ALF or ALFQ liposomes, antibodies were induced that neutralized two HIV-1 tier 1 clade C strain pseudoviruses.

Self-Organisation, Thermotropic and Lyotropic Properties of Glycolipids Related to their Biological Implications

Glycolipids are amphiphilic molecules which bear an oligo- or polysaccharide as hydrophilic head group and hydrocarbon chains in varying numbers and lengths as hydrophobic part. They play an important role in life science as well as in material science. Their biological and physiological functions are quite diverse, ranging from mediators of cell-cell recognition processes, constituents of membrane domains or as membrane-forming units. Glycolipids form an exceptional class of liquid-crystal mesophases due to the fact that their self-organisation obeys more complex rules as compared to classical monophilic liquid-crystals. Like other amphiphiles, the supra-molecular structures formed by glycolipids are driven by their chemical structure; however, the details of this process are still hardly understood. Based on the synthesis of specific glycolipids with a clearly defined chemical structure, e.g., type and length of the sugar head group, acyl chain linkage, substitution pattern, hydrocarbon chain lengths and saturation, combined with a profound physico-chemical characterisation of the formed mesophases, the principles of the organisation in different aggregate structures of the glycolipids can be obtained. The importance of the observed and formed phases and their properties are discussed with respect to their biological and physiological relevance. The presented data describe briefly the strategies used for the synthesis of the used glycolipids. The main focus, however, lies on the thermotropic as well as lyotropic characterisation of the self-organised structures and formed phases based on physico-chemical and biophysical methods linked to their potential biological implications and relevance.

Fusion of Legionella pneumophila outer membrane vesicles with eukaryotic membrane systems is a mechanism to deliver pathogen factors to host cell membranes

The formation and release of outer membrane vesicles (OMVs) is a phenomenon observed in many bacteria, including Legionella pneumophila. During infection, this human pathogen primarily invades alveolar macrophages and replicates within a unique membrane-bound compartment termed Legionella-containing vacuole. In the current study, we analysed the membrane architecture of L. pneumophila OMVs by small-angle X-ray scattering and biophysically characterized OMV membranes. We investigated the interaction of L. pneumophila OMVs with model membranes by Förster resonance energy transfer and Fourier transform infrared spectroscopy. These experiments demonstrated the incorporation of OMV membrane material into liposomes composed of different eukaryotic phospholipids, revealing an endogenous property of OMVs to fuse with eukaryotic membranes. Cellular co-incubation experiments showed a dose- and time-dependent binding of fluorophore-labelled OMVs to macrophages. Trypan blue quenching experiments disclosed a rapid internalization of OMVs into macrophages at 37 and 4 °C. Purified OMVs induced tumour necrosis factor-α production in human macrophages at concentrations starting at 300 ng ml(-1). Experiments on HEK293-TLR2 and TLR4/MD-2 cell lines demonstrated a dominance of TLR2-dependent signalling pathways. In summary, we demonstrate binding, internalization and biological activity of L. pneumophila OMVs on human macrophages. Our data support OMV membrane fusion as a mechanism for the remote delivery of virulence factors to host cells.

Mixture of nonionic/ionic surfactants for the formulation of nanostructured lipid carriers: effects on physical properties

The objective of the present work was to investigate the effects of the mixture of nonionic/ionic surfactants on nanostructured lipid carriers (NLCs). Nonionic surfactant (polyethylene-poly(propylene glycol), Pluronic F68) and ionic surfactant (octenylsuccinic acid modified gum arabic, GA-OSA) were chosen as emulsifier for NLCs. The NLCs systems, which were composed of lipid matrix, modified 4-dedimethylaminosancycline (CMT-8), and various emulsifier agents, were characterized with dynamic light scattering (DLS), high performance liquid chromatography (HPLC), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), in vitro release, and phagocytosis assay. This mixture of nonionic/ionic surfactants showed significant effects on physical properties including particle size, polydispersity index (PDI), entrapment efficiency, and particle morphology. Compared with single stabilizer, this mixed nonionic/ionic surfactant system provided NLCs with better drug carrier properties including prolonged release profile and low phagocytosis by phagocyte. We expect that these explorations can provide a new strategy for the development of lipid nanoparticles as drug delivery.

Pulmonary surfactant protein A-induced changes in the molecular conformation of bacterial deep-rough LPS lead to reduced activity on human macrophages

The lung is constantly exposed to immune stimulation by LPS from inhaled microorganisms. A primary mechanism to maintain immune homeostasis is based on anti-inflammatory regulation by surfactant protein A (SP-A), a secreted component of lung innate immunity. The architecture of LPS aggregates is strongly associated with biological activity. We therefore investigated whether SP-A affects the physico-chemical properties of LPS. Determination of the three-dimensional aggregate structure of LPS by small-angle X-ray scattering demonstrated that SP-A induced the formation of multi-lamellar aggregate structures. Determination of the acyl-chain-fluidity of LPS aggregates by Fourier transform infrared (FTIR) spectroscopy showed that the phase transition temperature of LPS was reduced in the presence of SP-A. The phosphate groups at the diglucosamine backbone of LPS represent important functional groups for the bioactivity of LPS. FTIR analysis revealed changes in the vibrational bands νas PO-(2), indicating an interaction of SP-A with the 1-phosphate, but not with the 4'-phosphate. The physico-chemical changes induced by SP-A were associated with up to 90% reduction in LPS-induced TNF-α-production by human macrophages. In conclusion, our data demonstrate that the SP-A/LPS interaction induces conformational changes in LPS aggregates leading to biologically less active structures, thereby providing a new molecular mechanism of immune modulation by SP-A.

Preparation and characterization of 4-dedimethylamino sancycline (CMT-3) loaded nanostructured lipid carrier (CMT-3/NLC) formulations

Chemically modified tetracyclines (CMTs) have been reported to strongly inhibit proliferation and metastasis of various cancers, but their efficacy is restricted by poor water solubility. In the present study, a hydrophilic 4-dedimethylamino sancycline (CMT-3) loaded nanostructured lipid carrier (CMT-3/NLC) was produced by high pressure homogenization (HPH). The physical properties of CMT-3/NLC formulations were characterized by dynamic light scattering (DLS), high efficiency liquid chromatography (HPLC), atomic force microscopy (AFM), scanning electron microscopy (SEM), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS) and wide-angle X-ray powder diffraction (XRD). The lipid and surfactant ingredients, as well as drug/lipid concentrations (m/m) were optimized to produce stable and sustained NLC formulations. In vitro cytotoxicity of CMT-3/NLC against HeLa cells was evaluated by MTT assay. The diameter of CMT-3/NLC was found to increase from 153.1±3.0 nm to a maximum of 168.5±2.0 nm after 30 days of storage, while the entrapment efficiency remained constant at >90%. CMT-3/NLC demonstrated a burst-sustained release profile in release media with different pH, a property attributed to the 3-dimensional structure of CMT-3/NLC. Cell uptake and localization studies indicated that NLC reached the cytoplasm and could thereby facilitate CMT-3 entry into HeLa cells.

Bio-inspired supramolecular self-assembly towards soft nanomaterials

Supramolecular self-assembly has proven to be a reliable approach towards versatile nanomaterials based on multiple weak intermolecular forces. In this review, the development of bio-inspired supramolecular self-assembly into soft materials and their applications are summarized. Molecular systems used in bio-inspired "bottom-up self-assembly" involve small organic molecules, peptides or proteins, nucleic acids, and viruses. Self-assembled soft nanomaterials have been exploited in various applications such as inorganic nanomaterial synthesis, drug or gene delivery, tissue engineering, and so on.

Morphology, size distribution, and aggregate structure of lipopolysaccharide and lipid A dispersions from enterobacterial origin

Lipopolysaccharides (LPSs) from Gram-negative bacteria are strong elicitors of the human immune systems. There is strong evidence that aggregates and not monomers of LPS play a decisive role at least in the initial stages of cell activation of immune cells such as mononuclear cells. In previous reports, it was shown that the biologically most active part of enterobacterial LPS, hexa-acyl bisphosphorylated lipid A, adopts a particular supramolecular conformation, a cubic aggregate structure. However, little is known about the size and morphology of these aggregates, regarding the fact that LPS may have strong variations in the length of the saccharide chains (various rough mutant and smooth-form LPS). Thus, in the present paper, several techniques for the determination of details of the aggregate morphology such as freeze-fracture and cryo-electron microscopy, analytical ultracentrifugation, laser backscattering analysis, and small-angle X-ray scattering were applied for various endotoxin (lipid A and different LPS) preparations. The data show a variety of different morphologies not only for different endotoxins but also when comparing different applied techniques. The data are interpreted with respect to the suitability of the single techniques, in particular on the basis of available literature data.

Influence of serum on the immune recognition of a synthetic lipopeptide mimetic of the 19-kDa lipoprotein from Mycobacterium tuberculosis

The innate immune response provides a critical first-line defense against Mycobacterium tuberculosis, an intracellular pathogen that represents a major health threat world-wide. A synthetic lipopeptide (LP) mimicking the lipid moiety of the cell-wall associated 19-kDa lipoprotein from M. tuberculosis has recently been assigned an important role in the induction of an antibacterial immune response in host macrophages. Here, we present experimental data on the biological activities and the biophysical mechanisms underlying cell activation by synthetic 19-kDa M. tuberculosis-derived lipopeptide (Mtb-LP). Investigation of the geometry of the LP (i.e. the molecular conformation and supramolecular aggregate structure) and the preference for membrane intercalation provide an explanation for the biological activities of the mycobacterial LP. Cell activation by low concentrations of Mtb-LP was enhanced by the lipopolysaccharide—binding protein and CD14. However, surprisingly, we found that activation of human macrophages to induce pro- as well as antiinflammatory mediators (tumor necrosis factor(TNF)-α, Interleukin(IL)-6, IL-8, and IL-10) in response to the Mtb-LP is strongly reduced in the presence of serum. This observation could be confirmed for the immune response of murine macrophages which showed a strongly enhanced TNF-α release in the absence of serum, suggesting that the molecular mechanisms of immune recognition of the Mtb-LP are tailored to the ambient conditions of the lung.

Physicochemical characterization and biological activity of lipooligosaccharides and lipid A from Neisseria meningitidis

Meningococcal endotoxin is the major contributor to the pathogenesis of fulminant sepsis and meningitis of meningococcal disease and is a potent activator of the MyD88-dependent and MyD88-independent pathways via the MD-2/TLR4 receptor. To understand better the biological properties of meningococcal endotoxin that initiates these events, the physicochemical structure of Neisseria meningitidis lipopoly(oligo)saccharide (LOS) of the serogroup B wild-type strain NMB (NeuNAc-Gal beta-GlcNAc-Gal beta-Glc beta-Hep2(GlcNAc,Glc alpha)PEA-Kdo2-lipid A, 1,4'-bisphosphorylated +/- PEA, PEtN) and the genetically-defined mutants (gmhB, Kdo2 -lipid A; kdtA, meningococcal lipid A; gmhB-lpxL1, Kdo2penta-acylated lipid A and NMB-lpx1, penta-acylated meningococcal LOS) were assessed in relation to bioactivity. Confirming previous work, Kdo2lipid A was the minimal structure required for optimal activation of the MD-2/TLR4 pathway of human macrophages. Meningococcal lipid A alone was a very weak agonist in stimulating human macrophages, even at high doses. Penta-acylated LOS structures demonstrated a moderate reduction in TLR4/MyD88-dependent signaling and a dramatic decrease in TLR4-TRIF-dependent signaling. For a better understanding of these results, we have performed an analysis of physicochemical parameters of the LOS structures such as the gel-to-liquid crystalline phase transition of the acyl chains, the inclination angle of the diglucosamine backbone with respect to the membrane surface, and the aggregate structure, and have found a very significant correlation of these parameters with biological activities extending our concept of endotoxicity.

Determination of pyrophosphorylated forms of lipid A in Gram-negative bacteria using a multivaried mass spectrometric approach

Lipid A isolated from several bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, and various strains of Yersinia) showed abundant formation of pyrophosphate anions upon ion dissociation. Pyrophosphate [H(3)P(2)O(7)](-) and/or [HP(2)O(6)](-) anions were observed as dominant fragments from diphosphorylated lipid A anions regardless of the ionization mode (matrix-assisted laser desorption ionization or electrospray ionization), excitation mode (collisional activation or infrared photoexcitation), or mass analyzer (time-of-flight/time-of-flight, tandem quadrupole, Fourier transform-ion cyclotron resonance mass spectrometry). Dissociations of anions from model lipid phosphate, pyrophosphate, and hexose diphosphates confirmed that pyrophosphate fragments were formed abundantly only in the presence of an intact pyrophosphate group in the analyte molecule and were not due to intramolecular rearrangement upon ionization, ion-molecule reactions, or rearrangement following activation. This indicated that pyrophosphate groups are present in diphosphorylated lipid A from a variety of Gram-negative bacteria.

Structural polymorphism of hydrated monoacylated maltose glycolipids

The physico-chemical properties of three fully hydrated monoacyl maltoside glycolipids were investigated with Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). The different synthesized maltoside glycoconjugates vary in the length and saturation of the fatty acid moiety, whereas the constant head group region contains a beta-linked maltose with a OC(2)-NH spacer. The compounds with saturated acyl chains showed a complex pattern of temperature-dependent behaviour, regarding the adopted three-dimensional aggregate structure of the molecules and the main phase transition from the gel to liquid crystalline phase of the acyl chains. A substitution of the saturated acyl chain with an unsaturated acyl chain led to a complete change of the structural preferences, from a high ordered stacking of the bilayers to an unilamellar arrangement of completely disordered and fluid membranes. The presence of the NH group in the spacer, compared to the compounds lacking the NH group allows the formation of a hydrogen bonding network, which influences the observed phase properties. The results of these studies of the hydrated monoacylated maltose glycolipids are discussed in relation to the thermotropic phase properties of the pure compounds in the absence of water.

Physico-chemical and biophysical study of the interaction of hexa- and heptaacyl lipid A from Erwinia carotovora with magainin 2-derived antimicrobial peptides

The neutralization of endotoxin structures such as the active 'endotoxic principle' lipid A by suitable compounds has been shown to be a key step in the treatment of infectious diseases, in particular in the case of Gram-negative bacteria which frequently may lead to the septic shock syndrome. An effective antimicrobial peptide, originally found in the skin of an African frog, is magainin 2. Here, the interaction of magainin 2-amide and a peptide derived thereof, M2V, with chemically defined and homogeneous hexaacyl and heptaacyl lipids A isolated from LPS of Erwinia carotovora, was investigated. By using Fourier-transform infrared spectroscopy, the gel to liquid crystalline phase transition of the acyl chains of lipid A and the conformation of their phosphate groups due to peptide binding was investigated. The former parameter was also determined by using differential scanning calorimetry. The electrophoretic mobility of lipid A aggregates under the influence of the peptides was studied to determine the Zeta potential, and small-angle X-ray scattering was applied for the elucidation of the types of aggregate structures in the absence and presence of the peptides. The lipid A-induced cytokine production in human mononuclear cells shows that the ability of the two peptides to inhibit a tumor necrosis factor-alpha production correlates with characteristic changes of the biophysical parameters. These are much stronger expressed for the peptide M2V than for magainin 2-amide, which apparently is connected with the higher number of positive as well as more hydrophobic amino acids, leading to a stronger amphiphilicity necessary to neutralize the amphiphilic lipid A aggregates.

Self-assembly properties of alkyloxyethyl beta-glycosides with different types of carbohydrate headgroups

The effect of alkyl chain length on micelle formation in aqueous solutions of synthetic alkyloxyethyl glycosides containing an ethyl spacer with different conformations of the disaccharide headgroups was investigated. The molecular shape was systematically changed from a wedge-shaped to a rodlike geometry by changing the type of carbohydrate headgroup. The lipophilic part consists of dodecyl or tetradecyl chains. The adsorption at the liquid-air interface was investigated by surface tension measurements. The micellar phase region (L1) was studied using small-angle neutron scattering. We have observed a strong influence of the linkage between the sugar moieties in the disaccharide headgroup and the ethyl spacer on the micellar structure: the transformation from spherical to disklike aggregates was observed for compounds with a rodlike shape, but only spherical aggregates were formed by the wedge-shaped molecules.

Mechanism of interaction of optimized Limulus-derived cyclic peptides with endotoxins: thermodynamic, biophysical and microbiological analysis

On the basis of formerly investigated peptides corresponding to the endotoxin-binding domain from LALF [Limulus anti-LPS (lipopolysaccharide) factor], a protein from Limulus polyphemus, we have designed and synthesized peptides of different lengths with the aim of obtaining potential therapeutic agents against septic shock syndrome. For an understanding of the mechanisms of action, we performed a detailed physicochemical and biophysical analysis of the interaction of rough mutant LPS with these peptides by applying FTIR (Fourier-transform infrared) spectroscopy, SAXS (small-angle X-ray scattering), calorimetric techniques [DSC (differential scanning calorimetry) and ITC (isothermal titration calorimetry)] and FFTEM (freeze-fracture transmission electron microscopy). Also, the action of the peptides on bacteria of different origin in microbial assays was investigated. Using FTIR and DSC, our results indicated a strong fluidization of the lipid A acyl chains due to peptide binding, with a decrease in the endothermic melting enthalpy change of the acyl chains down to a complete disappearance in the 1:0.5 to 1:2 [LPS]:[peptide] molar ratio range. Via ITC, it was deduced that the binding is a clearly exothermic process which becomes saturated at a 1:0.5 to 1:2 [LPS]:[peptide] molar ratio range. The results obtained with SAXS indicated a drastic change of the aggregate structures of LPS into a multilamellar stack, which was visualized in electron micrographs as hundreds of lamellar layers. This can be directly correlated with the inhibition of the LPS-induced production of tumour necrosis factor alpha in human mononuclear cells, but not with the action of the peptides on bacteria.

Physicochemical and biological analysis of synthetic bacterial lipopeptides: validity of the concept of endotoxic conformation

The importance of the biological function and activity of lipoproteins from the outer or cytoplasmic membranes of Gram-positive and Gram-negative bacteria is being increasingly recognized. It is well established that they are like the endotoxins (lipopolysaccharide (LPS)), which are the main amphiphilic components of the outer membrane of Gram-negative bacteria, potent stimulants of the human innate immune system, and elicit a variety of proinflammatory immune responses. Investigations of synthetic lipopeptides corresponding to N-terminal partial structures of bacterial lipoproteins defined the chemical prerequisites for their biological activity and in particular the number and length of acyl chains and sequence of the peptide part. Here we present experimental data on the biophysical mechanisms underlying lipopeptide bioactivity. Investigation of selected synthetic diacylated and triacylated lipopeptides revealed that the geometry of these molecules (i.e. the molecular conformations and supramolecular aggregate structures) and the preference for membrane intercalation provide an explanation for the biological activities of the different lipopeptides. This refers in particular to the agonistic or antagonistic activity (i.e. their ability to induce cytokines in mononuclear cells or to block this activity, respectively). Biological activity of lipopeptides was hardly affected by the LPS-neutralizing antibiotic polymyxin B, and the biophysical interaction characteristics were found to be in sharp contrast to that of LPS with polymyxin B. The analytical data show that our concept of "endotoxic conformation," originally developed for LPS, can be applied also to the investigated lipopeptide and suggest that the molecular mechanisms of cell activation by amphiphilic molecules are governed by a general principle.

Thermodynamic analysis of the lipopolysaccharide-dependent resistance of gram-negative bacteria against polymyxin B

Cationic antimicrobial cationic peptides (CAMP) have been found in recent years to play a decisive role in hosts' defense against microbial infection. They have also been investigated as a new therapeutic tool, necessary in particular due to the increasing resistance of microbiological populations to antibiotics. The structural basis of the activity of CAMPs has only partly been elucidated and may comprise quite different mechanism at the site of the bacterial cell membranes or in their cytoplasm. Polymyxin B (PMB) is a CAMP which is effective in particular against Gram-negative bacteria and has been well studied with the aim to understand its interaction with the outer membrane or isolated membrane components such as lipopolysaccharide (LPS) and to define the mechanism by which the peptides kill bacteria or neutralize LPS. Since PMB resistance of bacteria is a long-known phenomenon and is attributed to structural changes in the LPS moiety of the respective bacteria, we have performed a thermodynamic and biophysical analysis to get insights into the mechanisms of various LPS/PMB interactions in comparison to LPS from sensitive strains. In isothermal titration calorimetric (ITC) experiments considerable differences of PMB binding to sensitive and resistant LPS were found. For sensitive LPS the endothermic enthalpy change in the gel phase of the hydrocarbon chains converts into an exothermic reaction in the liquid crystalline phase. In contrast, for resistant LPS the binding enthalpy change remains endothermic in both phases. As infrared data show, these differences can be explained by steric changes in the headgroup region of the respective LPS.

Biophysical characterization of synthetic rhamnolipids

Synthetic rhamnolipids, derived from a natural diacylated glycolipid, RL-2,2(14), produced by Burkholderia (Pseudomonas) plantarii, were analyzed biophysically. Changes in the chemical structures comprised variations in the length, the stereochemistry and numbers of the lipid chains, numbers of rhamnoses, and the occurrence of charged or neutral groups. As relevant biophysical parameters, the gel (beta) to liquid crystalline (alpha) phase behavior of the acyl chains of the rhamnoses, their three-dimensional supramolecular aggregate structure, and the ability of the compounds to intercalate into phospholipid liposomes in the absence and presence of lipopolysaccharide-binding protein were monitored. Their biological activities were examined as the ability to induce cytokines in human mononuclear cells and to induce chemiluminescence in monocytes. Depending on the particular chemical structures, the physicochemical parameters as well as the biological test systems show large variations. This relates to the acyl chain fluidity, aggregate structure, and intercalation ability, as well as the bioactivity. Most importantly, the data extend our conformational concept of endotoxicity, based on the intercalation of naturally originating amphiphilic virulence factors into membranes from immune cells. This 'endotoxin conformation', produced by amphiphilic molecules with a hydrophilic charged backbone and apolar hydrophobic moiety, and adopting inverted cubic aggregate structures, causes high mechanical stress in target immune cells on integral proteins, eventually leading to cell activation. Furthermore, biologically inactive rhamnolipids with lamellar aggregate structures antagonize the endotoxin-induced activity in a way similar to lipid A-derived antagonists.

In situ characterization of lipid A interaction with antimicrobial peptides using surface X-ray scattering

Lipid A structure at the air-aqueous interface has been studied using pressure-area isotherm methods coupled with the surface X-ray scattering techniques of X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD). Lipid A monolayers were formed at the air-aqueous interface to represent the lipid moiety of the outer membrane of Gram-negative bacteria. Lipid A structure was characterized at surface pressures between 10 and 35 mN/m. Interactions of alpha-helical antimicrobial peptides LL-37, SMAP-29 and D2A22 with lipid A monolayers were subsequently studied. Although insertion into the lipid A monolayers was observed with the alpha-helical peptides, little change was seen from the X-ray data, suggesting that the lipid A hydrocarbon chains are involved in reorientation during insertion and that the hydrocarbon chains have a relatively rigid structure.

The lipopolysaccharide of Brucella abortus BvrS/BvrR mutants contains lipid A modifications and has higher affinity for bactericidal cationic peptides

The two-component BvrS/BvrR system is essential for Brucella abortus virulence. It was shown previously that its dysfunction abrogates expression of some major outer membrane proteins and increases bactericidal peptide sensitivity. Here, we report that BvrS/BvrR mutants have increased surface hydrophobicity and susceptibility to killing by nonimmune serum. The bvrS and bvrR mutant lipopolysaccharides (LPSs) bound more polymyxin B, chimeras constructed with bvrS mutant cells and parental LPS showed augmented polymyxin B resistance, and, conversely, parental cells and bvrS mutant LPS chimeras were more sensitive and displayed polymyxin B-characteristic outer membrane lesions, implicating LPS as being responsible for the phenotype of the BvrS/BvrR mutants. No qualitative or quantitative changes were detected in other envelope and outer membrane components examined: periplasmic beta(1-2) glucans, native hapten polysaccharide, and phospholipids. The LPS of the mutants was similar to parental LPS in O-polysaccharide polymerization and fine structure but showed both increased underacylated lipid A species and higher acyl-chain fluidity that correlated with polymyxin B binding. These lipid A changes did not alter LPS cytokine induction, showing that in contrast to other gram-negative pathogens, recognition by innate immune receptors is not decreased by these changes in LPS structure. Transcription of Brucella genes required for incorporating long acyl chains into lipid A (acpXL and lpxXL) or implicated in lipid A acylation control (bacA) was not affected. We propose that in Brucella the outer membrane homeostasis depends on the functioning of BvrS/BvrR. Accordingly, disruption of BvrS/BvrR damages the outer membrane, thus contributing to the severe attenuation manifested by bvrS and bvrR mutants.

Divalent cations affect chain mobility and aggregate structure of lipopolysaccharide from Salmonella minnesota reflected in a decrease of its biological activity

The physicochemical properties and biological activities of rough mutant lipopolysaccharides Re (LPS Re) as preformed divalent cation (Mg2+, Ca2+, Ba2+) salt form or as natural or triethylamine (Ten+)-salt form under the influence of externally added divalent cations were investigated using complementary methods: Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopic (FT-IR) measurements for the beta <--> alpha gel to liquid crystalline phase behaviour of the acyl chains of LPS, synchrotron radiation X-ray diffraction studies for their aggregate structures, electron density calculations of the LPS bilayer systems, and LPS-induced cytokine (interleukin-6) production in human mononuclear cells. The divalent cation salt forms of LPS exhibit considerable changes in physicochemical parameters such as acyl chain mobility and aggregate structures as compared to the natural or monovalent cation salt forms. Concomitantly, the biological activity was much lower in particular for the Ca2+- and Ba2+-salt forms. This decrease in activity results mainly from the conversion of the unilamellar/cubic aggregate structure of LPS into a multilamellar one. The reduced activity also clearly correlates with the higher order--lower mobility--of the lipid A acyl chains. Both effects can be understood by an impediment of the interactions of LPS with binding proteins such as lipopolysaccharide-binding protein (LBP) and CD14 due to the action of the divalent cations.

Enhancement of endotoxin neutralization by coupling of a C12-alkyl chain to a lactoferricin-derived peptide

Antibacterial peptide acylation, which mimics the structure of the natural lipopeptide polymyxin B, increases antimicrobial and endotoxin-neutralizing activities. The interaction of the lactoferricin-derived peptide LF11 and its N-terminally acylated analogue, lauryl-LF11, with different chemotypes of bacterial lipopolysaccharide (LPS Re, Ra and smooth S form) was investigated by biophysical means and was related to the peptides' biological activities. Both peptides exhibit high antibacterial activity against the three strains of Salmonella enterica differing in the LPS chemotype. Lauryl-LF11 has one order of magnitude higher activity against Re-type, but activity against Ra- and S-type bacteria is comparable with that of LF11. The alkyl derivative peptide lauryl-LF11 shows a much stronger inhibition of the LPS-induced cytokine induction in human mononuclear cells than LF11. Although peptide-LPS interaction is essentially of electrostatic nature, the lauryl-modified peptide displays a strong hydrophobic component. Such a feature might then explain the fact that saturation of the peptide binding takes place at a much lower peptide/LPS ratio for LF11 than for lauryl-LF11, and that an overcompensation of the negative LPS backbone charges is observed for lauryl-LF11. The influence of LF11 on the gel-to-liquid-crystalline phase-transition of LPS is negligible for LPS Re, but clearly fluidizing for LPS Ra. In contrast, lauryl-LF11 causes a cholesterol-like effect in the two chemotypes, fluidizing in the gel and rigidifying of the hydrocarbon chains in the liquid-crystalline phase. Both peptides convert the mixed unilamellar/non-lamellar aggregate structure of lipid A, the 'endotoxic principle' of LPS, into a multilamellar one. These data contribute to the understanding of the mechanisms of the peptide-mediated neutralization of endotoxin and effect of lipid modification of peptides.

Synthesis and mesomorphic properties of glycosyl dialkyl- and diacyl-glycerols bearing saturated, unsaturated and methyl branched fatty acid and fatty alcohol chains

The biophysical properties of a series of glycosyl dialkyl- and diacyl-glycerols bearing unsaturated or chiral methyl branched chains in the tail, and di- and trisaccharide carbohydrate headgroups are described. Thermotropism was investigated by polarising microscopy, the lyotropism was investigated by small angle X-ray diffraction and by the contact preparation method, and the gel to liquid crystalline phase transition by FT-IR-spectroscopy. The compounds displayed thermotropic Smectic A (SmA), cubic and columnar phases, whereas in the lyotropic phase diagram lamellar, hexagonal and cubic phases are found. The introduction of unsaturated or methyl branched chains leads to liquid crystallinity at ambient temperature. The difference between the 1,3-oleyl-glycerol maltoside and the corresponding 1,2-oleoyl-glycerol maltoside is small.

Temperature dependence of the binding of endotoxins to the polycationic peptides polymyxin B and its nonapeptide

The interaction between endotoxins-free lipid A and various lipopolysaccharide (LPS) chemotypes with different sugar chain lengths-and the polycationic peptides polymyxin B and polymyxin nonapeptide has been investigated by isothermal titration calorimetry between 20 and 50 degrees C. The results show a strong dependence of the titration curves on the phase state of the endotoxins. In the gel phase (<30 degrees C for LPS and <45 degrees C for lipid A), an endothermic reaction is observed, for which the driving force is an entropically driven endotoxin-polymyxin interaction, due to disruption of the ordered water structure and cation assembly in the lipid A backbone and adjacent molecules. In the liquid crystalline phase (>35 degrees C for LPS and >47 degrees C for lipid A) an exothermic reaction takes place, which is mainly due to the strong electrostatic interaction of the polymyxins with the negative charges of the endotoxins, i.e., the entropic change DeltaS is much lower than in the gel phase. For endotoxins with short sugar chains (lipid A, LPS Re, LPS Rc) the stoichiometry of the polymyxin binding corresponds to pure charge neutralization; for the compounds with longer sugar chains (LPS Ra, LPS S-form) this is no longer valid. This can be related to the lower susceptibility of the corresponding bacterial strains to antibiotics.

Elucidation of the molecular structure of lipid A isolated from both a rough mutant and a wild strain of Aeromonas salmonicida lipopolysaccharides using electrospray ionization quadrupole time-of-flight tandem mass spectrometry

The chemical structure of lipid A, isolated by mild acid hydrolysis from a rough mutant and a wild strain of Aeromonas salmonicida lipopolysaccharide, was investigated using electrospray ionization quadrupole time-of-flight (QqToF) hybrid tandem mass spectrometry and showed a great degree of microheterogeneity. The chemical structure of the main constituent of this heterogeneous mixture was identified as a beta-D-(1 --> 6) linked D-glucosamine disaccharide substituted by two phosphate groups, one being bound to the non-reducing end at position O-4' and the other to the position O-1 of the reducing end of the D-glucosamine disaccharide. The location of the fatty acids linked to the disaccharide backbone was established by identifying diagnostic ions in the conventional QqToF-MS scan. Low-energy collision tandem mass spectrometry analysis of the selected precursor diagnostic ions confirmed, unambiguously, their proposed molecular structures. We have established that myristyloxylauric (C14:0(3-O(12:0))) acid residues were both N-2' and O-3' linked to the non-reducing end of the D-GlcN residue, and that two 3-hydroxymyristic (C14:0(3-OH)) acid chains acylated the remaining positions of the reducing end. The MS and MS/MS data obtained allowed us to determine the complex molecular structure of lipid A. The QqToF-MS/MS instrument has shown excellent superiority over a conventional quadrupole-hexapole-quadrupole tandem instrument which failed to fragment the selected precursor ion.

Physicochemical characterization of carboxymethyl lipid A derivatives in relation to biological activity

Lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria belongs to the most potent activators of the mammalian immune system. Its lipid moiety, lipid A, the 'endotoxic principle' of LPS, carries two negatively charged phosphate groups and six acyl chain residues in a defined asymmetric distribution (corresponding to synthetic compound 506). Tetraacyl lipid A (precursor IVa or synthetic 406), which lacks the two hydroxylated acyl chains, is agonistically completely inactive, but is a strong antagonist to bioactive LPS when administered to the cells before LPS addition. The two negative charges of lipid A, represented by the two phosphate groups, are essential for agonistic as well as for antagonistic activity and no highly active lipid A are known with negative charges other than phosphate groups. We hypothesized that the phosphate groups could be substituted by other negatively charged groups without changing the endotoxic properties of lipid A. To test this hypothesis, we synthesized carboxymethyl (CM) derivatives of hexaacyl lipid A (CM-506 and Bis-CM-506) and of tetraacyl lipid A (Bis-CM-406) and correlated their physicochemical with their endotoxic properties. We found that, similarly to compounds 506 and 406, also for their carboxymethyl derivatives a particular molecular ('endotoxic') conformation and with that, a particular aggregate structure is a prerequisite for high cytokine-inducing capacity and antagonistic activity, respectively. In other parameters such as acyl chain melting behaviour, antibody binding, activity in the Limulus lysate assay, and partially the binding of 3-deoxy-D-manno-oct-2-ulosonic acid transferase, strong deviations from the properties of the phosphorylated compounds were observed. These data allow a better understanding of endotoxic activity and its structural prerequisites.

Structural polymorphism and endotoxic activity of synthetic phospholipid-like amphiphiles

The physicochemical characteristics and in vitro biological activity of various synthetic hexaacyl phospholipid dimers were compared with the respective behavior of bacterial endotoxins (lipopolysaccharide, LPS). The structural variations of the synthetic amphiphiles include different stereochemical (R,S) configurations about their ester- and amide-linkages for the acyl chains and differences in the length of the serine backbone spacer. The temperature of the gel to liquid crystalline phase transition of the acyl chains (T(c)) lies between 10 and 15 degrees C for the compounds with the shortest backbone and decreases rapidly for the compounds with longer backbones. The phase transition enthalpies (8-16 kJ x mol(-1)) are considerably lower than those of lipid A from hexaacyl endotoxins (28-35 kJ x mol(-1)). In contrast, the dependence of T(c) on Mg(2+) and water content shows a behavior typical for endotoxins: a significant increase with increasing Mg(2+) and decreasing water concentrations. The aggregate structure is sensitively dependent not only on the length of the backbone spacer but also on the different stereochemical variations. It can be directly correlated with the biological activity of the compounds. Thus, as with natural lipid A, the capacity to induce cytokine production in mononuclear cells is directly related to the affinity to form nonlamellar cubic or inverted hexagonal H(II) aggregate structures. Together with the data on the transport and intercalation of the dimers into phospholipid liposomes mediated by the lipopolysaccharide-binding protein (LBP), our conformational concept of endotoxicity and cell activation can be applied to these non-LPS structures: endotoxically active compounds incorporate into membranes of immune cells and cause conformational changes at the site of signaling proteins such as Toll-like receptors or K(+)-channels due to their conical molecular shape.

Physicochemical characterization and biological activity of a glycoglycerolipid from Mycoplasma fermentans

We report a comprehensive physicochemical characterization of a glycoglycerolipid from Mycoplasma fermentans, MfGl-II, in relation to its bioactivity and compared this with the respective behaviors of phosphatidylcholine (PC) and a bacterial glycolipid, lipopolysaccharide (LPS) from deep rough mutant Salmonella minnesota strain R595. The beta left arrow over right arrow alpha gel-to-liquid crystalline phase transition behavior of the hydrocarbon chains with Tc = 30 degrees C for MfGl-II as well as for LPS exhibits high similarity between the two glycolipids. A lipopolysaccharide-binding protein (LBP)-mediated incorporation into negatively charged liposomes is observed for both glycolipids. The determination of the supramolecular aggregate structure confirms the existence of a mixed unilamellar/cubic structure for MfGl-II, similar to that observed for the lipid A moiety of LPS. The biological data clearly show that MfGl-II is able to induce cytokines such as tumor necrosis factor-alpha (TNF-alpha) in human mononuclear cells, although to a significantly lower degree than LPS. In contrast, in the Limulus amebocyte lysate test, MfGl-II is completely inactive, and in the CHO reporter cell line it does not indicate any reactivity with the Toll-like receptors TLR-2 and -4, in contrast to control lipopeptides and LPS. These data confirm the applicability of our conformational concept of endotoxicity to nonlipid A structures: an amphiphilic molecule with a nonlamellar cubic aggregate structure corresponding to a conical conformation of the single molecules and a sufficiently high negative charge density in the backbone.

Effects of carbohydrate headgroups on the stability of induced cubic phases in binary mixtures of glycolipids

This paper is part in a series of papers, investigating the influence of carbohydrate headgroups on the mesogenic properties of glycolipids. While previous papers focussed on the synthesis and mesogenic properties of the pure compounds, we will discuss here our results obtained with binary mixtures. Mixtures of compounds, one forming a lamellar phase and the other one a columnar phase in their pure state, displayed always an induced cubic phase. The stability of this induced cubic phase depends significantly on the structure of the carbohydrate headgroup of both components. Thus it was possible to derive structure-property relationships by comparison of the phase diagrams that have been obtained, if the carbohydrate headgroup of one component was changed systematically. We observed an interesting effect of galactose headgroups which might be of great biological importance. Furthermore, the observed kind of kinetic of the S(A)-->cub transition might also be of great biological relevance.

Pore formation and function of phosphoporin PhoE of Escherichia coli are determined by the core sugar moiety of lipopolysaccharide

The lipid matrix of the outer membrane of Gram-negative bacteria is an asymmetric bilayer composed of a phospholipid inner leaflet and a lipopolysaccharide outer leaflet. Incorporated into this lipid matrix are, among other macromolecules, the porins, which have a sieve-like function for the transport or exclusion of hydrophilic substances. It is known that a reduced amount of porins is found in the outer membrane of rough mutants as compared with wild-type bacteria. This observation was discussed to be caused by a reduced number of insertion sites in the former. We performed electrical measurements on reconstituted planar bilayers composed of lipopolysaccharide on one side and a phospholipid mixture on the other side using lipopolysaccharide from various rough mutant strains of Salmonella enterica serovar Minnesota. We found that pore formation by PhoE trimers that were added to the phospholipid side of the bilayers increased with the increasing length of the lipopolysaccharide core sugar moiety. These results allow us to conclude that the length of the sugar moiety of lipopolysaccharide is the parameter governing pore formation and that no particular insertion sites are required. Furthermore, we found that the voltage gating of the porin channels is strongly dependent on the composition of the lipid matrix.

Biophysical characterization of triacyl monosaccharide lipid a partial structures in relation to bioactivity

Synthetic triacyl glucosamine monosaccharide lipid A part structures corresponding to the non-reducing moiety of enterobacterial lipid A with an acyloxyacyl chain linked to position 3 of the glucosamine and an unbranched chain linked to position 2 (group 1) and vice versa (group 2) were analyzed biophysically: Fourier-transform infrared spectroscopy was performed to characterize the gel-to-liquid crystalline phase transition, the phosphate band contour, and the orientation of the glucosamine with respect to the membrane surface. Small-angle x-ray diffraction was applied for the elucidation of the supramolecular aggregate structure and, with that, of the molecular shape. With fluorescence resonance energy transfer the lipopolysaccharide-binding protein (LBP)-mediated intercalation of the lipid A partial structures into phospholipid liposomes was monitored. The physical data clearly exhibit a classification of the synthetic compounds into two groups: group 1 compounds have sharp phase transitions, indicating dense acyl chain packing and an inclination of the glucosamine backbone with respect to the membrane surface of 30 degrees with the phosphate buried in the membrane. Group 2 compounds have a very broad phase transition, indicating poorly packed acyl chains, and an inclination of -30 degrees with the phosphate group sticking outward. For the first group unilamellar phases are observed superimposed by a non-lamellar structure, and for the second one only multilamellar aggregate structures. The cytokine-inducing capacity in human mononuclear cells is relatively high for the first group and low or absent for the second group. Based on these data a model of the intra and intermolecular conformations is proposed which also extends the concept of "endotoxic conformation."

Biophysical characterization of lipopolysaccharide and lipid A inactivation by lactoferrin

The interaction of bacterial endotoxins (LPS Re and lipid A, the 'endotoxic principle' of LPS) with the endogenous antibiotic lactoferrin (LF) was investigated using various physical techniques and biological assays. By applying Fourier-transform infrared (FTIR) spectroscopy, we find that LF binds to the phosphate group within the lipid A part and induces a rigidification of the acyl chains of LPS. The secondary structure of the protein - as monitored by the amide I band - is, however, not changed. Concomitant with the IR data, scanning calorimetric data indicate a sharpening of the acyl chain phase transition. From titration calorimetric and zeta potential data, saturation of LF binding to LPS was found to lie at a [LF]:[LPS] ratio of 1:3 to 1:5 M from the former and 1:10 M from the latter technique. X-ray scattering data indicate a change of the lipid A aggregate structure from inverted cubic to multilamellar, and with fluorescence (FRET) spectroscopy, LF is shown to intercalate by itself into phospholipid liposomes and may also block the lipopolysaccharide-binding protein (LBP)-induced intercalation of LPS. The LPS-induced cytokine production of human mononuclear cells exhibits a decrease due to LF binding, whereas the coagulation of amebocyte lysate in the Limulus test exhibited concentration-dependent changes. Based on these results, a model for the mechanisms of endotoxin inactivation by LF is proposed.