Step conductance increases in bilayer membranes induced by antibody-antigen-complement action.

A sharp rise in the electrical conductance of lipid bilayer membranes was observed following the addition of antigen (bovine serum), antibody (rabbit anti-bovine serum), and complement to the neighboring aqueous phases. At low concentrations, step increases in the conductivity occurred which are consistent with the appearance of about 2.2 nm holes in the membrane. Probably attack or lysis of the lipid bilayer by complement is responsible.     

Structural and dynamical aspects of membrane immunochemistry using model membranes.

Three different phospholipid haptens have been synthesized, in which the haptenic group is the paramagnetic nitroxide (spin-label) group. These lipid haptens differ from one another in the length and composition of the molecular chain linking the 2,2,6,6-tetramethylpiperidinyl-N-oxy moiety to the phosphodiester group of the lipid. These lipid haptens have been incorporated at low molar concentrations (0.01 to 0.5 mol %) in liposomes containing various proportions of cholesterol and dipalmitoylphosphatidylcholine (DPPC). A study has been made of specific antinitroxide IgG (and Fab) binding to these liposomes, and the fixation of complement. From these studies we conclude: (a) For lipid haptens whose possible extension above the bilayer plane is limited (e.g., approximately 10-20 A), antibody binding and complement fixation depend strongly on the hapten structure and host lipid composition, because of steric limitations on the accessibility of lipid haptens to the binding sites in the protein. (b) Complement fixation by specific IgG antibodies directed against the nitroxide group as part of a lipid hapten depends strongly on the lateral mobility of the lipid hapten when its molar concentration in the plane of the membrane is of the order of 0.1 mol % or less. It is likely that this conclusion applies to many lipid haptens, and possibly other membrane components. (c) The inclusion of cholesterol in lipid membranes has at least two distinct effects on complement fixation involving lipid haptens. Through a steric effect on bilayer structure (probably involving lateral molecular ordering) cholesterol in phosphatidylcholine bilayers can enhance hapten exposure to antibody binding sites, enhance antibody binding, and thereby enhance complement fixation. It is likely that cholesterol also affects complement fixation at low hapten concentrations through a modification of membrane fluidity.     

Interactions of liposomes with serum proteins

The interactions of serum proteins are diverse, complex and can lead to dramatic effects on liposome stability and in vivo behavior; conversely lipids can modify the biological activities of serum proteins. Serum lipoproteins can potentially destabilize bilayer membranes leading to vesicle disruption and loss of contents; irregularities in the lipid bilayer, such as those which exist at phase boundaries, promote the destabilizing effects of lipoproteins. Other serum components such as fibronectin, immunoglobulins and C reactive protein can modify the biological properties of liposomes by promoting interactions with reticuloendothelial cells and/or activation of the complement system. Liposomes can avidly bind certain serum clotting factors, a process which can lead to dramatic effects on the clotting cascade. Thus the interactions of liposomes with serum proteins can reciprocally effect both components involved.     

Pore formation by complement in the outer membrane of Gram-negative bacteria studied with asymmetric planar lipopolysaccharide/phospholipid bilayers

Summary The interaction of complement with an asymmetric planar lipopolysaccharide/phospholipid bilayer system as a model for the lipid matrix of the outer membrane of Gram-negative bacteria has been studied. The addition of whole human serum to the aqueous solution at the lipopolysaccharide side of the asymmetric membrane resulted in a rapid increase of the bilayer conductance in discrete steps, indicating the formation of transmembrane pores, which were not observed in the case of pure phospholipid membranes. The amplitudes of the discrete conductance steps varied over a range of more than one order of magnitude. The mean single step conductance was (0.39±0.24) nS for a subphase containing (inmm): 100 KCl, 5 MgCl₂ and 5 HEPES buffer. The steps were grouped into bursts of typically 9±3 events per burst and the conductance change within one burst was (8.25±4.00) nS.The pore-forming activity of serum at the asymmetric membrane system was independent of the presence of specific antibodies against the lipopolysaccharide but was dependent on calcium ions. Furthermore, the pore-forming activity required complement component C9.A model for the mode of pore formation by complement is proposed: The complement pore is generated in discrete steps by insertion of C9 monomers into the membrane and their irreversible aggregation to water-filled channels with a diameter of approximately 7 nm assuming a circular geometry.     

Prevention of nonspecific lysis in liposomal and erythrocyte immunoassay systems by small lipid vesicles and erythrocyte ghosts

Large unilamellar liposomes prepared by the reverse-phase evaporation method (REVs) were made immunoreactive by incorporating dinitrophenylaminocaproyl-phosphatidylethanolamine (DNP-Cap-PE) or 8-(3-carboxypropyl)-theophylline-dipalmitoylphosphatidylethanolamine (Th-DPPE) into the phospholipid bilayer. Specific lysis in the presence of anti-DNP-BSA and goat anti-theophylline serum respectively, was induced by adding guinea pig serum as source for complement to these liposomes. However, specific lysis was found to be compromised by high levels of nonspecific lysis as monitored by the release of the fluorescent aqueous-space marker 6-carboxyfluorescein. Nonspecific lysis could be prevented without affecting specific lysis by pretreatment of complement or incubation of the reaction mixture with small unilamellar liposomes (SUVs). SUVs of various lipid compositions produced the desired effect; however, when the fraction of negative charge in the SUVs was increased to 30 mol%, specific lysis was inhibited as well. In a similar assay system consisting of hemolysin-sensitized sheep red blood cells it was also found that nonspecific lysis could be inhibited by addition of erythrocyte ghosts to the incubation medium, although specific lysis was somewhat depressed. However, SUVs or REVs of a composition similar to sheep erythrocytes were ineffective indicating a more selective nature of complement-mediated immunoreaction with erythrocyte membranes than with synthetic bilayer membranes.     

Human serum albumin supported lipid patterns for the targeted recognition of microspheres coated by membrane based on ss-DNA hybridization

Human serum albumin (HSA) patterns have been successfully fabricated for the deposition of lipid bilayer, 1,2-dimyristoyl-sglycerophosphate (DMPA), by making use of the micro-contact printing (microCP) technique and liposome fusion. Confocal laser scanning microscopy (CLSM) results indicate that lipid bilayer has been assembled in HSA patterns with a good stability. Such well-defined lipid patterns formed on HSA surface create possibility to incorporate specific components like channels or receptors for specific recognition. In view of this, microspheres coated with lipid membranes were immobilized in HSA-supported lipid patterns via the hybridization of complementary ss-DNAs. This procedure enables to transfer solid materials to a soft surface through a specific recognition.     

Single-channel analysis of the conductance fluctuations induced in lipid bilayer membranes by complement proteins C5b-9

Summary Single-channel analysis of electrical fluctuations induced in planar bilayer membranes by the purified human complement proteins C5b6, C7, C8, and C9 have been analyzed. Reconstitution experiments with lipid bilayer membranes showed that the C5b-9 proteins formed pores only if all proteins were present at one side of the membrane. The complement pores had an average single-channel conductance of 3.1 nS at 0.15m KCl. The histogram of the complement pores suggested a substantial variation of the size of the single channel. The linear relationship between single-channel conductance at fixed ionic strength and the aqueous mobility of the ions in the bulk aqueous phase indicated that the ions move inside the complement pore in a manner similar to the way they move in the aqueous phase. The minimum diameter of the pores as judged from the conductance data is approximately 3 nm. The complement channels showed no apparent voltage control or regulation up to transmembrane potentials of 100 mV. At neutral pH the pore is three to four times more permeable for alkali ions than for chloride, which may be explained by the existence of fixed negatively charged groups in or near the pore. The significance of these observations to current molecular models of the membrane lesion formed by these cytolytic serum proteins is considered.     

Stabilization of large multilamellar liposomes by human serum in vitro

The leakage of 5,6-carboxyfluorescein from large multilamellar liposomes prepared from dipalmitoylphosphatidylcholine (without or with cholesterol) was investigated in vitro in the presence of human serum. Below the phospholipid phase transition temperature, the rate of dye release is retarted 3–8-fold in the presence of up to 25% human serum in the incubation medium, as compared to the release in isotonic phosphate-buffered saline. This effect is significantly augmented by incorporation of 50 mol% cholesterol into the lipid bilayer. At and above the phase transition temperature, the initial rapid dye leakage in the presence of serum is followed by a slow long-term release. Incubation of the liposomes with serum is assumed to result in the association of serum proteins with the outermost lipid bilayer which in turn will lead to their stabilization, while the inner lamellae are not immediately accessible to the serum proteins. The permeability of the outer protein-rich lipid bilayer appears to be restricted, as concluded from the decreased dye release in the presence of serum. Massive leakage from multilamellar liposomes appears to be primarily due to bilayer defects occurring in the lipid transition region rather than being caused by protein-lipid interactions. The results of our in vitro experiments are discussed in terms of the potential usefulness of multilamellar liposomes as drug carriers in vivo for local and topical applications.     

Binding and reorientation of melittin in a POPC bilayer: Computer simulations

We performed, using an all-atom force field, molecular dynamics computer simulations to study the binding of melittin to the POPC bilayer and its subsequent reorientation in this bilayer. The binding process involves a simultaneous folding and adsorption of the peptide to the bilayer, followed by the creation of a "U shaped" conformation. The reorientation of melittin from the parallel to the perpendicular conformation requires charged residues to cross the hydrophobic core of the bilayer. This is accomplished by a creation of defects in the bilayer that are filled out with water. The defects are caused by peptide charged residues dragging the lipid headgroup atoms along with them, as they reorient. With increased concentration of melittin water defects form stable pores; this makes it easier for the peptide N-terminus to reorient. Our results complement experimental and computational observations of the melittin/lipid bilayer interaction.     

A thermodynamic study of the effects of cholesterol on the interaction between liposomes and ethanol

The association of ethanol with unilamellar dimyristoyl phosphatidylcholine (DMPC) liposomes of varying cholesterol content has been investigated by isothermal titration calorimetry over a wide temperature range (8-45 degrees C). The calorimetric data show that the interaction of ethanol with the lipid membranes is endothermic and strongly dependent on the phase behavior of the mixed lipid bilayer, specifically whether the lipid bilayer is in the solid ordered (so), liquid disordered (ld), or liquid ordered (lo) phase. In the low concentration regime (<10 mol%), cholesterol enhances the affinity of ethanol for the lipid bilayer compared to pure DMPC bilayers, whereas higher levels of cholesterol (>10 mol%) reduce affinity of ethanol for the lipid bilayer. Moreover, the experimental data reveal that the affinity of ethanol for the DMPC bilayers containing small amounts of cholesterol is enhanced in the region around the main phase transition. The results suggest the existence of a close relationship between the physical structure of the lipid bilayer and the association of ethanol with the bilayer. In particular, the existence of dynamically coexisting domains of gel and fluid lipids in the transition temperature region may play an important role for association of ethanol with the lipid bilayers. Finally, the relation between cholesterol content and the affinity of ethanol for the lipid bilayer provides some support for the in vivo observation that cholesterol acts as a natural antagonist against alcohol intoxication.     

Determining Peptide Partitioning Properties via Computer Simulation

The transfer of polypeptide segments into lipid bilayers to form transmembrane helices represents the crucial first step in cellular membrane protein folding and assembly. This process is driven by complex and poorly understood atomic interactions of peptides with the lipid bilayer environment. The lack of suitable experimental techniques that can resolve these processes both at atomic resolution and nanosecond timescales has spurred the development of computational techniques. In this review, we summarize the significant progress achieved in the last few years in elucidating the partitioning of peptides into lipid bilayer membranes using atomic detail molecular dynamics simulations. Indeed, partitioning simulations can now provide a wealth of structural and dynamic information. Furthermore, we show that peptide-induced bilayer distortions, insertion pathways, transfer free energies, and kinetic insertion barriers are now accurate enough to complement experiments. Further advances in simulation methods and force field parameter accuracy promise to turn molecular dynamics simulations into a powerful tool for investigating a wide range of membrane active peptide phenomena.     

Improved Characterization of EV Preparations Based on Protein to Lipid Ratio and Lipid Properties

In recent years the study of extracellular vesicles has gathered much scientific and clinical interest. As the field is expanding, it is becoming clear that better methods for characterization and quantification of extracellular vesicles as well as better standards to compare studies are warranted. The goal of the present work was to find improved parameters to characterize extracellular vesicle preparations. Here we introduce a simple 96 well plate-based total lipid assay for determination of lipid content and protein to lipid ratios of extracellular vesicle preparations from various myeloid and lymphoid cell lines as well as blood plasma. These preparations included apoptotic bodies, microvesicles/microparticles, and exosomes isolated by size-based fractionation. We also investigated lipid bilayer order of extracellular vesicle subpopulations using Di-4-ANEPPDHQ lipid probe, and lipid composition using affinity reagents to clustered cholesterol (monoclonal anti-cholesterol antibody) and ganglioside GM1 (cholera toxin subunit B). We have consistently found different protein to lipid ratios characteristic for the investigated extracellular vesicle subpopulations which were substantially altered in the case of vesicular damage or protein contamination. Spectral ratiometric imaging and flow cytometric analysis also revealed marked differences between the various vesicle populations in their lipid order and their clustered membrane cholesterol and GM1 content. Our study introduces for the first time a simple and readily available lipid assay to complement the widely used protein assays in order to better characterize extracellular vesicle preparations. Besides differentiating extracellular vesicle subpopulations, the novel parameters introduced in this work (protein to lipid ratio, lipid bilayer order, and lipid composition), may prove useful for quality control of extracellular vesicle related basic and clinical studies.     

Bilayer lipid membranes (BLM): Study of antigen-antibody interactions

Previous work of del Castillo and co-workers has shown that bilayer lipid membranes (BLM) can be used as transducers for detection of antigen-antibody reactions. The present experiments extend the previous work by incorporating complement into the BLM system. The results indicate that the antigen-antibody complex or the complement has no ability to affect the BLM system separately, but when carefully combined they will destabilize the BLM even at a much reduced concentration. Further development using the BLM as a tool for investigating immunological reactions is suggested.     

Vitamin A in liposomes. Inhibition of complement binding and alteration of membrane structure.

Incorporation of vitamin A aldehyde (retinal) into liposomes had an inhibitory effect on the amount of human complement protein bound in the presence of specific antiserum. The total membrane-bound protein was directly measured on liposomes which were washed after incubation in antiserum and fresh human serum (complement). At every concentration of complement, decreased protein binding was found with liposomes which contained retinal. Binding of the third component of complement (C3) was also measured directly on washed liposomes and was found to be decreased in the presence of retinal. The diminution in protein binding due to retinal was not caused by differences in the amount of antibody bound and this was shown by two experiments. First, specific antibody protein binding to liposomes was directly measured and was essentially unaffected by retinal. Second, liposomes were prepared from lipid extracts of sheep erythrocytes. These liposomes were used as as immunoadsorbants to remove antisheep erythrocyte antibodies. The immunoadsorbant capacity was the same in both the presence and the absence of retinal. A further conclusion from these experiments was that retinal did not change the number of liposomal glycolipid antigen molecules available for antibody binding and thus presumably did not change the total number of lipid molecules present on the outer surface of the liposomes. Retinal did have an effect on the geometric structure of the liposomes. Size distribution measurements were performed in the diameter range of 1-6.35 mum by using an electronic particle size analyzer (Coulter Counter). Liposomes containing retinal were shifted toward smaller sizes and had less total surface area and volume. It was suggested that retinal-containing liposomes may have had a tighter packing of the molecules in the phospholipid bilayer. This effect of retinal on liposomal structure may have been responsible for the observed decreased binding of C3 and total complement protein.     

Lipid transfer between cationic vesicles and lipid-DNA lipoplexes: Effect of serum

Differential scanning calorimetry was used to examine the lipid exchange between model lipid systems, including vesicles of the cationic lipoids ethyldimyristoylphosphatidylcholine (EDMPC), ethyldipalmitoylphosphatidylcholine (EDPPC) or their complexes with DNA (lipoplexes), and the zwitterionic lipids (DMPC, DPPC). The changes of the lipid phase transition parameters (temperature, enthalpy, and cooperativity) upon consecutive temperature scans was used as an indication of lipid mixing between aggregates. A selective lipid transfer of the shorter-chain cationic lipoid EDMPC into the longer-chain aggregates was inferred. In contrast, transfer was hindered when EDMPC (but not EDPPC) was bound to DNA in the lipoplexes. These data support a simple molecular lipid exchange mechanism, but not lipid bilayer fusion. Exchange via lipid monomers is considerably more facile for the cationic ethylphosphatidylcholines than for zwitterionic phosphatidylcholines, presumably due to the higher monomer solubility of the charged lipids. With the cationic liposomes, lipid transfer was strongly promoted by the presence of serum in the dispersing medium. Serum proteins are presumed to be responsible for the accelerated transfer, since the effect was strongly reduced upon heating the serum to 80 °C. The effect of serum indicates that even though much lipoplex lipid is inaccessible due to the multilayered structure, the barrier due to buried lipid can be easily overcome. Serum did not noticeably promote the lipid exchange of zwitterionic liposomes. The phenomenon is of potential importance for the application of cationic liposomes to nonviral gene delivery, which often involves the presence of serum in vitro, and necessarily involves serum contact in vivo.     

Preparation and stability of lipid-coated nanocapsules of cisplatin: anionic phospholipid specificity

Cisplatin nanocapsules represent a novel lipid formulation of the anti-cancer drug cis-diamminedichloroplatinum(II) (cisplatin), in which nanoprecipitates of cisplatin are coated by a phospholipid bilayer consisting of a 1:1 mixture of zwitterionic phosphatidylcholine (PC) and negatively charged phosphatidylserine (PS). Cisplatin nanocapsules are characterized by an unprecedented cisplatin-to-lipid ratio and exhibit increased in vitro cytotoxicity compared to the free drug [Nat. Med. 8, (2002) 81]. In the present study, the stability of the cisplatin nanocapsules was optimized by varying the lipid composition of the bilayer coat and monitoring in vitro cytotoxicity and the release of contents during incubations in water and in mouse serum. The release of cisplatin from the PC/PS (1:1) nanocapsules in water increased with increasing temperature with a t(1/2) of 6.5 h at 37 degrees C. At 4 degrees C, cisplatin was retained in the nanocapsules for well over 8 days. Replacement of PS by either phosphatidylglycerol or phosphatidic acid revealed that nanocapsules prepared of PS were more stable, which was found to be due to the ability of PS to form a stable cisplatin-PS coordination complex. Mouse serum had a strong destabilizing effect on the cisplatin nanocapsules. The PC/PS formulation lost over 80% of cisplatin within minutes after resuspension in serum. Incorporation of poly(ethylene glycol 2000) (PEG)-derivatized phosphatidylethanolamine and cholesterol in the bilayer coat extended the lifetime of the cisplatin nanocapsules in mouse serum to almost an hour. The results demonstrate that specificity in the interaction of cisplatin with anionic phospholipids is an important criterium for the formation and stability of cisplatin nanocapsules.     

Volatile anesthetics inhibit sodium channels without altering bulk lipid bilayer properties

Although general anesthetics are clinically important and widely used, their molecular mechanisms of action remain poorly understood. Volatile anesthetics such as isoflurane (ISO) are thought to alter neuronal function by depressing excitatory and facilitating inhibitory neurotransmission through direct interactions with specific protein targets, including voltage-gated sodium channels (Na_v). Many anesthetics alter lipid bilayer properties, suggesting that ion channel function might also be altered indirectly through effects on the lipid bilayer. We compared the effects of ISO and of a series of fluorobenzene (FB) model volatile anesthetics on Na_v function and lipid bilayer properties. We examined the effects of these agents on Na_v in neuronal cells using whole-cell electrophysiology, and on lipid bilayer properties using a gramicidin-based fluorescence assay, which is a functional assay for detecting changes in lipid bilayer properties sensed by a bilayer-spanning ion channel. At clinically relevant concentrations (defined by the minimum alveolar concentration), both the FBs and ISO produced prepulse-dependent inhibition of Na_v and shifted the voltage dependence of inactivation toward more hyperpolarized potentials without affecting lipid bilayer properties, as sensed by gramicidin channels. Only at supra-anesthetic (toxic) concentrations did ISO alter lipid bilayer properties. These results suggest that clinically relevant concentrations of volatile anesthetics alter Na_v function through direct interactions with the channel protein with little, if any, contribution from changes in bulk lipid bilayer properties. Our findings further suggest that changes in lipid bilayer properties are not involved in clinical anesthesia.     

Molecular dynamics simulations of charged and neutral lipid bilayers: treatment of electrostatic interactions

Molecular dynamics (MD) simulations complement experimental methods in studies of the structure and dynamics of lipid bilayers. The choice of algorithms employed in this computational method represents a trade-off between the accuracy and real calculation time. The largest portion of the simulation time is devoted to calculation of long-range electrostatic interactions. To speed-up evaluation of these interactions, various approximations have been used. The most common ones are the truncation of long-range interactions with the use of cut-offs, and the particle-mesh Ewald (PME) method. In this study, several multi-nanosecond cut-off and PME simulations were performed to establish the influence of the simulation protocol on the bilayer properties. Two bilayers were used. One consisted of neutral phosphatidylcholine molecules. The other was a mixed lipid bilayer consisting of neutral phosphatidylethanolamine and negatively charged phosphatidylglycerol molecules. The study shows that the cut-off simulation of a bilayer containing charge molecules generates artefacts; in particular the mobility and order of the charged molecules are vastly different from those determined experimentally. In the PME simulation, the bilayer properties are in general agreement with experimental data. The cut-off simulation of bilayers containing only uncharged molecules does not generate artefacts, nevertheless, the PME simulation gives generally better agreement with experimental data.     

Lipid peroxidation-induced changes in physical properties of annular lipids in rat brain synaptosomal membranes

The effects of lipid peroxidation (LPO) on the physical state (fluidity) of the rat brain synaptosomal lipid bilayer matrix and the annular lipid domains were investigated using the fluorescent probe pyrene. The parameters of pyrene fluorescence intensity alpha = IE/IM were measured at excitation wavelengths 280 nm and 340 nm (alpha 280 and alpha 340), reflecting fluidity of lipid bilayer matrix and annular lipids, respectively. LPO induction was shown to result in changes of fluidity of both the bilayer and annular lipids. Upon reducing formation of LPO products by carnosine, fluidity changes of both the lipid bilayer matrix and annular lipids were diminished. Conformational changes of the annular lipid domain by LPO may therefore be considered as a possible cause of the functional changes in the receptor mediated responses and of the inactivation of membrane-bound enzymes by oxidative stress.     

Effects of bilayer surface charge density on molecular adsorption and transport across liposome bilayers

Second harmonic generation (SHG) was used to study both the adsorption of malachite green (MG), a positively charged organic dye, onto liposomes of different lipid compositions, and the transport kinetics of MG across the liposome bilayer in real time. We found that the dye adsorption increased linearly with the fraction of negatively charged lipids in the bilayer. Similarly, the transport rate constant for crossing the bilayer increased linearly with the fraction of charged lipid in the bilayer.