MEASUREMENT OF THE LATERAL COMPRESSIBILITY OF SEVERAL PHOSPHOLIPID BILAYERS

Lipid bilayer properties potentially contributed to the evolutionary disappearance of betaine lipids in seed plants

BACKGROUND

Many organisms rely on mineral nutrients taken directly from the soil or aquatic environment, and therefore, developed mechanisms to cope with the limitation of a given essential nutrient. For example, photosynthetic cells have well-defined responses to phosphate limitation, including the replacement of cellular membrane phospholipids with non-phosphorous lipids. Under phosphate starvation, phospholipids in extraplastidial membranes are replaced by betaine lipids in microalgae. In higher plants, the synthesis of betaine lipid is lost, driving plants to other strategies to cope with phosphate starvation where they replace their phospholipids by glycolipids.

RESULTS

The aim of this work was to evaluate to what extent betaine lipids and PC lipids share physicochemical properties and could substitute for each other. By neutron diffraction experiments and dynamic molecular simulation of two synthetic lipids, the dipalmitoylphosphatidylcholine (DPPC) and the dipalmitoyl-diacylglyceryl-N,N,N-trimethylhomoserine (DP-DGTS), we found that DP-DGTS bilayers are thicker than DPPC bilayers and therefore are more rigid. Furthermore, DP-DGTS bilayers are more repulsive, especially at long range, maybe due to unexpected unscreened electrostatic contribution. Finally, DP-DGTS bilayers could coexist in the gel and fluid phases.

CONCLUSION

The different properties and hydration responses of PC and DGTS provide an explanation for the diversity of betaine lipids observed in marine organisms and for their disappearance in seed plants.

The dilatable membrane of oleosomes (lipid droplets) allows their in vitro resizing and triggered release of lipids

It has been reported that lipid droplets (LDs), called oleosomes, have an inherent ability to inflate or shrink when absorbing or fueling lipids in the cells, showing that their phospholipid/protein membrane is dilatable. This property is not that common for membranes stabilizing oil droplets and when well understood, it could be exploited for the design of responsive and metastable droplets. To investigate the nature of the dilatable properties of the oleosomes, we extracted them from rapeseeds to obtain an oil-in-water emulsion. Initially, we added an excess of rapeseed oil in the dispersion and applied high-pressure homogenization, resulting in a stable oil-in-water emulsion, showing the ability of the molecules on the oleosome membrane to rearrange and reach a new equilibrium when more surface was available. To confirm the rearrangement of the phospholipids on the droplet surface, we used molecular dynamics simulations and showed that the fatty acids of the phospholipids are solubilized in the oil core and are homogeneously spread on the liquid-like membrane, avoiding clustering with neighbouring phospholipids. The weak lateral interactions on the oleosome membrane were also confirmed experimentally, using interfacial rheology. Finally, to investigate whether the weak lateral interactions on the oleosome membrane can be used to have a triggered change of conformation by an external force, we placed the oleosomes on a solid hydrophobic surface and found that they destabilise, allowing the oil to leak out, probably due to a reorganisation of the membrane phospholipids after their interaction with the hydrophobic surface. The weak lateral interactions on the LD membrane and their triggered destabilisation present a unique property that can be used for a targeted release in foods, pharmaceuticals and cosmetics.

Vesicles Balance Osmotic Stress with Bending Energy That Can Be Released to Form Daughter Vesicles

The bending energy of the lipid membrane is central to biological processes involving vesicles, such as endocytosis and exocytosis. To illustrate the role of bending energy in these processes, we study the response of single-component giant unilamellar vesicles (GUVs) subjected to external osmotic stress by glucose addition. For osmotic pressures exceeding 0.15 atm, an abrupt shape change from spherical to prolate occurs, showing that the osmotic pressure is balanced by the free energy of membrane bending. After equilibration, the external glucose solution was exchanged for pure water, yielding rapid formation of monodisperse daughter vesicles inside the GUVs through an endocytosis-like process. Our theoretical analysis shows that this process requires significant free energies stored in the deformed membrane to be kinetically allowed. The results indicate that bending energies stored in GUVs are much higher than previously implicated, with potential consequences for vesicle fusion/fission and the osmotic regulation in living cells.

Cell cycle dependence on the mevalonate pathway: Role of cholesterol and non-sterol isoprenoids

The mevalonate pathway is responsible for the synthesis of isoprenoids, including sterols and other metabolites that are essential for diverse biological functions. Cholesterol, the main sterol in mammals, and non-sterol isoprenoids are in high demand by rapidly dividing cells. As evidence of its importance, many cell signaling pathways converge on the mevalonate pathway and these include those involved in proliferation, tumor-promotion, and tumor-suppression. As well as being a fundamental building block of cell membranes, cholesterol plays a key role in maintaining their lipid organization and biophysical properties, and it is crucial for the function of proteins located in the plasma membrane. Importantly, cholesterol and other mevalonate derivatives are essential for cell cycle progression, and their deficiency blocks different steps in the cycle. Furthermore, the accumulation of non-isoprenoid mevalonate derivatives can cause DNA replication stress. Identification of the mechanisms underlying the effects of cholesterol and other mevalonate derivatives on cell cycle progression may be useful in the search for new inhibitors, or the repurposing of preexisting cholesterol biosynthesis inhibitors to target cancer cell division. In this review, we discuss the dependence of cell division on an active mevalonate pathway and the role of different mevalonate derivatives in cell cycle progression.

Mechanical properties of the high cholesterol-containing membrane: An AFM study

Cholesterol (Chol) content in most cellular membranes does not exceed 50 mol%, only in the eye lens's fiber cell plasma membrane, its content surpasses 50 mol%. At this high concentration, Chol induces the formation of pure cholesterol bilayer domains (CBDs), which coexist with the surrounding phospholipid-cholesterol domain (PCD). Here, we applied atomic force microscopy to study the mechanical properties of Chol/phosphatidylcholine membranes where the Chol content was increased from 0 to 75 mol%, relevant to eye lens membranes. The surface roughness of the membrane decreases with an increase of Chol content until it reaches 60 mol%, and roughness increases with a further increment in Chol content. We propose that the increased roughness at higher Chol content results from the formation of CBDs. Force spectroscopy on the membrane with Chol content of 50 mol% or lesser exhibited single breakthrough events, whereas two distinct puncture events were observed for membranes with the Chol content greater than 50 mol%. We propose that the first puncture force corresponds to the membranes containing coexisting PCD and CBDs. In contrast, the second puncture force corresponds to the "CBD water pocket" formed due to coexisting CBDs and PCD. Membrane area compressibility modulus (K_A) increases with an increase in Chol content until it reaches 60 mol%, and with further increment in Chol content, CBDs are formed, and K_A starts to decrease. Our results report the increase in membrane roughness and decrease K_A at very high Chol content (>60 mol%) relevant to the eye lens membrane.

Anomalous Salt Dependence Reveals an Interplay of Attractive and Repulsive Electrostatic Interactions in α-synuclein Fibril Formation

α-Synuclein (α-syn) is an intrinsically disordered protein with a highly asymmetric charge distribution, whose aggregation is linked to Parkinson’s disease. The effect of ionic strength was investigated at mildly acidic pH (5.5) in the presence of catalytic surfaces in the form of α-syn seeds or anionic lipid vesicles using thioflavin T fluorescence measurements. Similar trends were observed with both surfaces: increasing ionic strength reduced the rate of α-syn aggregation although the surfaces as well as α-syn have a net negative charge at pH 5.5. This anomalous salt dependence implies that short-range attractive electrostatic interactions are critical for secondary nucleation as well as heterogeneous primary nucleation. Such interactions were confirmed in Monte Carlo simulations of α-syn monomers interacting with surface-grafted C-terminal tails, and found to be weakened in the presence of salt. Thus, nucleation of α-syn aggregation depends critically on an attractive electrostatic component that is screened by salt to the extent that it outweighs the screening of the long-range repulsion between negatively charged monomers and negative surfaces. Interactions between the positively charged N-termini of α-syn monomers on the one hand, and the negatively C-termini of α-syn on fibrils or vesicles surfaces on the other hand, are thus critical for nucleation.

Correlation between the hydration of acyl chains and phosphate groups in lipid bilayers: Effect of phase state, head group, chain length, double bonds and carbonyl groups

This paper demonstrates by means of FTIR/ATR analysis that water molecules intercalate at different extents in the acyl chain region of lipid membranes in correlation with the hydration of the phosphate groups. This correlation is sensible to the chain length, the presence of double bonds and the phase state of the lipid membrane. The presence of carbonyl groups CO modifies the profile of hydration of the two regions as observed from the comparison of DMPC and 14:0 Diether PC. The different water populations in lipid interphases would give arrangements with different free energy states that could drive the interaction of biological effectors with membranes.

Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Dynamic Morphologies and Stability of Droplet Interface Bilayers

We develop a theoretical framework for understanding dynamic morphologies and stability of droplet interface bilayers (DIBs), accounting for lipid kinetics in the monolayers and bilayer, and droplet evaporation due to imbalance between osmotic and Laplace pressures. Our theory quantitatively describes distinct pathways observed in experiments when DIBs become unstable. We find that when the timescale for lipid desorption is slow compared to droplet evaporation, the lipid bilayer will grow and the droplets approach a hemispherical shape. In contrast, when lipid desorption is fast, the bilayer area will shrink and the droplets eventually detach. Our model also suggests there is a critical size below which DIBs can become unstable, which may explain experimental difficulties in miniaturizing the DIB platform.

Cholesterol affects the interaction between an ionic liquid and phospholipid vesicles. A study by differential scanning calorimetry and nanoplasmonic sensing

The present work aims at studying the interactions between cholesterol-rich phosphatidylcholine-based lipid vesicles and trioctylmethylphosphonium acetate ([P₈₈₈₁][OAc]), a biomass dissolving ionic liquid (IL). The effect of cholesterol was assayed by using differential scanning calorimetry (DSC) and nanoplasmonic sensing (NPS) measurement techniques. Cholesterol-enriched dipalmitoyl-phosphatidylcholine vesicles were exposed to different concentrations of the IL, and the derived membrane perturbation was monitored by DSC. The calorimetric data could suggest that the binding and infiltration of the IL are delayed in the vesicles containing cholesterol. To clarify our findings, NPS was applied to quantitatively follow the resistance of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine incorporating 0, 10, and 50mol% of cholesterol toward the IL exposure over time. The membrane perturbation induced by different concentrations of IL was found to be a concentration dependent process on cholesterol-free lipid vesicles. Moreover, our results showed that lipid depletion in cholesterol-enriched lipid vesicles is inversely proportional to the increasing amount of cholesterol in the vesicles. These findings support that cholesterol-rich lipid bilayers are less susceptible toward membrane disrupting agents as compared to membranes that do not incorporate any sterols. This probably occurs because cholesterol tightens the phospholipid acyl chain packing of the plasma membranes, increasing their resistance and reducing their permeability.

Surfactant films in lyotropic lamellar (and related) phases: Fluctuations and interactions

The analogy between soap films thinning under border capillary suction and lamellar stacks of surfactant bilayers dehydrated by osmotic stress is explored, in particular in the highly dehydrated limit where the soap film becomes a Newton black film. The nature of short-range repulsive interactions between surfactant-covered interfaces and acting across water channels in both cases will be discussed.

Wavelike charge density fluctuations and van der Waals interactions at the nanoscale

Recent experiments on noncovalent interactions at the nanoscale have challenged the basic assumptions of commonly used particle- or fragment-based models for describing van der Waals (vdW) or dispersion forces. We demonstrate that a qualitatively correct description of the vdW interactions between polarizable nanostructures over a wide range of finite distances can only be attained by accounting for the wavelike nature of charge density fluctuations. By considering a diverse set of materials and biological systems with markedly different dimensionalities, topologies, and polarizabilities, we find a visible enhancement in the nonlocality of the charge density response in the range of 10 to 20 nanometers. These collective wavelike fluctuations are responsible for the emergence of nontrivial modifications of the power laws that govern noncovalent interactions at the nanoscale.

Supported lipid bilayer membrane arrays on micro-patterned ITO electrodes

Lipid bilayer arrays were formed on micropatterned ITO electrodes. With this bilayer array platform both the fluorescence microscopy and electrochemical detection can be realized to explore the biophysical properties of cell membrane.

Dynamic morphologies of microscale droplet interface bilayers

Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers (μDIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the μDIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and fission: when forming μDIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform shrinking: when using the lipid-in method (lipids in water phase) to form μDIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and unzipping: finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces.

Phospholipid membrane protection by sugar molecules during dehydration-insights into molecular mechanisms using scattering techniques

Scattering techniques have played a key role in our understanding of the structure and function of phospholipid membranes. These techniques have been applied widely to study how different molecules (e.g., cholesterol) can affect phospholipid membrane structure. However, there has been much less attention paid to the effects of molecules that remain in the aqueous phase. One important example is the role played by small solutes, particularly sugars, in protecting phospholipid membranes during drying or slow freezing. In this paper, we present new results and a general methodology, which illustrate how contrast variation small angle neutron scattering (SANS) and synchrotron-based X-ray scattering (small angle (SAXS) and wide angle (WAXS)) can be used to quantitatively understand the interactions between solutes and phospholipids. Specifically, we show the assignment of lipid phases with synchrotron SAXS and explain how SANS reveals the exclusion of sugars from the aqueous region in the particular example of hexagonal II phases formed by phospholipids.

Time-resolved surface-enhanced ellipsometric contrast imaging for label-free analysis of biomolecular recognition reactions on glycolipid domains

We have applied surface-enhanced ellipsometry contrast (SEEC) imaging for time-resolved label-free visualization of biomolecular recognition events on spatially heterogeneous supported lipid bilayers (SLB). Using a conventional inverted microscope equipped with total internal reflection (TIR) illumination, biomolecular binding events were monitored with a lateral resolution near the optical diffraction limit at an acquisition rate of ~1 Hz with a sensitivity in terms of surface coverage of ~1 ng/cm(2). Despite the significant improvement in spatial resolution compared to alternative label-free surface-based imaging technologies, the sensitivity remains competitive with surface plasmon resonance (SPR) imaging and imaging ellipsometry. The potential of the technique to discriminate local differences in protein binding kinetics was demonstrated by time-resolved imaging of anti-GalCer antibodies binding to phase-separated lipid bilayers consisting of phosphatidylcholine (POPC) and galactosylceramide (GalCer). A higher antibody binding capacity was observed on the GalCer-diluted fluid region in comparison to the GalCer-rich gel phase domains. This observation is tentatively attributed to differences in the presentation of the GalCer epitope in the two phases, resulting in differences in availability of the ligand for antibody binding. The complementary information obtained by swiftly switching between SEEC and fluorescence (including TIR fluorescence) imaging modes was used to support the data interpretation. The simplicity and generic applicability of the concept is discussed in terms of microfluidic applications.

Interaction of peptidomimetics with bilayer membranes: biophysical characterization and cellular uptake

Enzymatically stable cell-penetrating α-peptide/β-peptoid peptidomimetics constitute promising drug delivery vehicles for the transport of therapeutic biomacromolecules across membrane barriers. The aim of the present study was to elucidate the mechanism of peptidomimetic-lipid bilayer interactions. A series of peptidomimetics consisting of alternating cationic and hydrophobic residues displaying variation in length and N-terminal end group were applied to fluid-phase, anionic lipid bilayers, and their interaction was investigated using isothermal titration calorimetry (ITC) and ellipsometry. Titration of lipid vesicles into solutions of peptidomimetics resulted in exothermic adsorption processes, and the interaction of all studied peptidomimetics with anionic lipid membranes was found to be enthalpy-driven. The enthalpy and Gibbs free energy (ΔG) proved more favorable with increasing chain length. However, not all charges contribute equally to the interaction, as evidenced by the charge-normalized ΔG being inversely correlated to the sequence length. Ellipsometry data suggested that the hydrophobic residues also played an important role in the interaction process. Furthermore, ΔG extracted from ellipsometry data showed good agreement with that obtained with ITC. To further elucidate their interaction with biological membranes, quantitative uptake and cellular distribution were studied in proliferating HeLa cells by flow cytometry and confocal microscopy. The cellular uptake of carboxyfluorescein-labeled peptidomimetics showed a similar ranking as that obtained from the adsorbed amount, and binding energy to model membranes demonstrated that the initial interaction with the membrane is of key importance for the cellular uptake.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: a statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Fluctuations and the rate-limiting step of peptide-induced membrane leakage

Peptide-induced vesicle leakage is a common experimental test for the membrane-perturbing activity of antimicrobial peptides. The leakage kinetics is usually very slow, requiring minutes to hours for complete release of vesicle contents, and exhibits a biphasic behavior. We report here that, in the case of the peptaibol trichogin GA IV, all processes involved in peptide-membrane interaction, such as peptide-membrane association, peptide aggregation, and peptide translocation, take place on a timescale much shorter than the leakage kinetics. On the basis of these findings, we propose a stochastic model in which the leakage kinetics is determined by the discrete nature of a vesicle suspension: peptides are continuously exchanging among vesicles, producing significant fluctuations over time in the number of peptide molecules bound to each vesicle, and in the formation of pores. According to this model, the fast initial leakage is caused by vesicles that contain at least one pore after the peptides are randomly distributed among the liposomes, whereas the slower release is associated with the time needed to occasionally reach in an intact vesicle the critical number of bound peptides necessary for pore formation. Fluctuations due to peptide exchange among vesicles therefore represent the rate-limiting step of such a slow mechanism.

DNA binding to zwitterionic model membranes

This study shows that DNA (linearized plasmid, 4331 base pairs and salmon sperm, 2000 base pairs, respectively) adsorbs to model membranes of zwitterionic liquid crystalline phospholipid bilayers in solutions containing divalent Ca(2+) cations, and also in solutions containing monovalent Na(+). The interaction between DNA and surface-supported model membranes was followed in situ using null ellipsometry, quartz crystal microbalance with dissipation, as well as neutron reflectometry. In the presence of Na(+) (in the absence of multivalent ions), DNA adopts an extended coil conformation upon adsorption. The solvent content in the adsorbed layer is high, and DNA is positioned on top of the membrane. In the presence of divalent Ca(2+), the driving force for the adsorption of DNA is electrostatic, and the adsorbed DNA film is not as dilute as in a solution containing Na(+). Cryo-TEM and SANS were further used to investigate the interaction in bulk solution using vesicles as model membrane systems. DNA adsorption could not be identified in the presence of Na(+) using SANS, but cryo-TEM indicates the presence of DNA between neighboring unilamellar vesicles. In the presence of Ca(2+), DNA induces the formation of multilamellar vesicles in which DNA intercalates the lamellae. Possible electrostatic and hydrophobic mechanisms for the adsorption of DNA in solutions containing monovalent salt are discussed and compared to the observations in divalent salt.

Membrane perturbation by the antimicrobial peptide PMAP-23: a fluorescence and molecular dynamics study

Several bioactive peptides exert their biological function by interacting with cellular membranes. Structural data on their location inside lipid bilayers are thus essential for a detailed understanding of their mechanism of action. We propose here a combined approach in which fluorescence spectroscopy and molecular dynamics (MD) simulations were applied to investigate the mechanism of membrane perturbation by the antimicrobial peptide PMAP-23. Fluorescence spectra, depth-dependent quenching experiments, and peptide-translocation assays were employed to determine the location of the peptide inside the membrane. MD simulations were performed starting from a random mixture of water, lipids and peptide, and following the spontaneous self-assembly of the bilayer. Both experimental and theoretical data indicated a peptide location just below the polar headgroups of the membrane, with an orientation essentially parallel to the bilayer plane. These findings, together with experimental results on peptide-induced leakage from large and giant vesicles, lipid flip-flop and peptide exchange between vesicles, support a mechanism of action consistent with the "carpet" model. Furthermore, the atomic detail provided by the simulations suggested the occurrence of an additional, more specific and novel mechanism of bilayer destabilization by PMAP-23, involving the unusual insertion of charged side chains into the hydrophobic core of the membrane.

Mechanism and regulation of the violaxanthin cycle: the role of antenna proteins and membrane lipids

The violaxanthin cycle describes the reversible conversion of violaxanthin to zeaxanthin via the intermediate antheraxanthin. This light-dependent xanthophyll conversion is essential for the adaptation of plants and algae to different light conditions and allows a reversible switch of photosynthetic light-harvesting complexes between a light-harvesting state under low light and a dissipative state under high light. The photoprotective functions of zeaxanthin have been intensively studied during the last decade, but much less attention has been directed to the mechanism and regulation of xanthophyll conversion. In this review, an overview is given on recent progress in the understanding of the role of (i) xanthophyll binding by antenna proteins and of (ii) the lipid properties of the thylakoid membrane in the regulation of xanthophyll conversion. The consequences of these findings for the mechanism and regulation of xanthophyll conversion in the thylakoid membrane will be discussed.

Alamethicin interaction with lipid membranes: a spectroscopic study on synthetic analogues

Alamethicin (Alm) is one of the most extensively studied membrane-active antibiotic peptides, but several aspects of its mechanism of action are still under debate. In this study, synthetic analogues of natural Alm F50/5 (Alm-N), labeled with a 9H-fluoren-9-yl group at the N- (F-Alm) or C-terminus (Alm-F), were employed to investigate the position and orientation of this peptide in the membrane environment. Depth-dependent fluorescence quenching and polarized ATR-FT-IR experiments demonstrated that, in the absence of a transmembrane potential, Alm inserts its N-terminus into the membrane, while the C-terminus is exposed to the outer aqueous phase. We also found that the peptaibol populates different orientations with respect to the membrane normal. Furthermore, fluorescence resonance-energy transfer (FRET) indicated that no peptide translocation to the inner leaflet of lipid bilayers occurs. The mechanism of action of Alm is discussed on the basis of these findings. Two other Alm analogues, Alm-P and Alm-S, were exploited to investigate the role of specific Alm residues in terms of membrane-perturbing activity. Substitution of two or three Gln (E) residues (the only polar amino acids in the alamethicin sequence) by gamma-methyl glutamate (Glu(OMe)) residues induced marked variations in the aggregation and partition behaviors of the peptaibols, which, in turn, modulate their membrane activity. In particular, substitution of Gln(18) and Gln(19) caused a six-fold increase in membrane-perturbing activity, thus demonstrating that these residues are not essential for the stabilization of Alm pores.

Effects of monovalent anions of the hofmeister series on DPPC lipid bilayers Part II: modeling the perpendicular and lateral equation-of-state

The effects of Hofmeister anions on the perpendicular and lateral equation-of-state (EOS) of the dipalmitoylphosphatidylcholine lamellar phase discussed in the companion article are here examined using appropriate free energy models for the intra- and interbilayer interactions. Minimizing the free energy with respect to the two basic geometrical parameters of the lamellar phase, which are the interbilayer water thickness, d(w), and the lipid headgroup area, a(L), provides the perpendicular (osmotic pressure balance) and lateral EOS. Standard models were used for the hydration, undulation, and Van der Waals attractive force between the bilayers in the presence of electrolytes whereas two alternative treatments of electrostatic interactions were used to obtain "binding" or "partitioning" constants of anions to the lipid bilayers both in the absence and in the presence of sodium binding. The computed binding constants depend on anion type and follow the Hofmeister series, but were found to increase with electrolyte concentration, implying that the local binding approximation cannot fit bilayer repulsion data. The partitioning model was also found inadequate at high electrolyte concentrations. The fitting attempts revealed two additional features worthy of future investigation. First, at maximum swelling in the presence of electrolytes the osmotic pressure of the bilayer system cannot be set equal to zero. Second, at high salt concentrations an additional repulsion appears to come into effect in the presence of strongly adsorbing anions such as I(-) or SCN(-). Both these phenomena may reflect an inconsistent treatment of the ion-surface interactions, which have an impact on the osmotic pressure. Alternatively, they may arise from bulk solution nonidealities that cannot be handled by the classical Poisson-Boltzmann formalism. The inability of current models to explain the "lateral" EOS by fitting the area per lipid headgroup as a function of salt type and concentration shows that current understanding of phospholipid-ion interactions is still very incomplete.

Measurement of lipid forces by X-ray diffraction and osmotic stress

Lipid suspensions in aqueous solutions most often form multilamellar vesicles of uniformly spaced bilayers. Interlamellar spacing is determined by the balance of attractive van der Waals (charge fluctuation) and repulsive forces. This balance of forces, as well as membrane elasticity, can be probed by applied osmotic stress. We describe how osmotic stress can be imposed on multilamellar lipid samples to study lipid interactions.

Rhodamine 123 permeability through the catfish intestinal wall: Relationship to thermal acclimation and acute temperature change

Temperature is known to influence xenobiotic retention in fish. The effect of acute and acclimatory temperature change upon Rhodamine 123 (Rho123) permeability through an in vitro catfish multi-segment (3) everted sac intestinal wall model was examined in a 9 cell matrix of acclimation and assay temperatures (10, 20 and 30 degrees C). Changes in Rho123 permeability were examined in context with membrane fluidity, xenobiotic solubility and intestinal morphology. When assayed at the acclimation temperature greater Rho123 permeability was noted at warmer acclimation temperatures for the proximal and middle intestinal segments, while the distal segment exhibited little change and apparent compensation across temperatures. Rho123 permeability was increased as assay temperatures were elevated above the acclimation temperature for most comparisons. Cold acclimation significantly increased total intestinal length (43.2%) and proximal intestine weights while total body weights did not differ. Brush border membranes (BBM) increased fluidity with increased assay temperatures, however, composite anisotropy lines were not significantly different between acclimation treatments. In an additive manner, the membrane probe DPH exhibited increased solubility in BBM with increases in acclimation and assay temperatures. Compositely, these results suggest that acclimation and acute temperature change may differentially influence xenobiotic permeability among intestinal segments with interacting mechanisms.

Controlling the Passage of Light through Metal Microchannels by Nanocoatings of Phospholipids

The flow of polarized light through a metal film with an array of microchannels is controlled by the phase of an optically active, phospholipid nanocoating, even though the coating does not cover the open area of the microchannels. The molecular details of the assembly (DPPC phospholipid monolayer/bilayer on a hexadecanethiol monolayer on a copper- or nickel-coated microarray) were determined using the infrared, surface-plasmon-mediated, extraordinary transmission of the metal microarrays. Infrared absorption spectra with greatly enhanced absorptions by comparison to literature were recorded and used as a diagnostic for the phase, composition, and molecular geometry of these nanocoatings. This approach presents new tools for nanoscale construction in constricted microspaces, which may ultimately be useful with individual microchannels.

Folding kinetics of the outer membrane proteins OmpA and FomA into phospholipid bilayers

The folding mechanism of outer membrane proteins (OMPs) of Gram-negative bacteria into lipid bilayers has been studied using OmpA of E. coli and FomA of F. nucleatum as examples. Both, OmpA and FomA are soluble in unfolded form in urea and insert and fold into phospholipid bilayers upon strong dilution of the denaturant urea. OmpA is a structural protein and forms a small ion channel, composed of an 8-stranded transmembrane beta-barrel domain. FomA is a voltage-dependent porin, predicted to form a 14 stranded beta-barrel. Both OMPs fold into a range of model membranes of very different phospholipid compositions. Three membrane-bound folding intermediates of OmpA were discovered in folding studies with dioleoylphosphatidylcholine bilayers that demonstrated a highly synchronized mechanism of secondary and tertiary structure formation of beta-barrel membrane proteins. A study on FomA folding into lipid bilayers indicated the presence of parallel folding pathways for OMPs with larger transmembrane beta-barrels.

The major outer membrane protein of Fusobacterium nucleatum (FomA) folds and inserts into lipid bilayers via parallel folding pathways

Membrane protein insertion and folding was studied for the major outer membrane protein of Fusobacterium nucleatum (FomA), which is a voltage-dependent general diffusion porin. The transmembrane domain of FomA forms a beta-barrel that is predicted to consist of 14 beta-strands. Here, unfolded FomA is shown to insert and fold spontaneously and quantitatively into phospholipid bilayers upon dilution of the denaturant urea, which was shown previously only for outer membrane protein A (OmpA) of Escherichia coli. Folding of FomA is demonstrated by circular dichroism and fluorescence spectroscopy, by SDS-polyacrylamide gel electrophoresis, and by single-channel recordings. Refolded FomA had a single-channel conductance of 1.1 nS at 1 M KCl, in agreement with the conductance of FomA isolated from membranes in native form. In contrast to OmpA, which forms a smaller eight-stranded beta-barrel domain, folding kinetics of the larger FomA were slower and provided evidence for parallel folding pathways of FomA into lipid bilayers. Two pathways were observed independent of membrane thickness with two different lipid bilayers, which were either composed of dicapryl phosphatidylcholine or dioleoyl phosphatidylcholine. This is the first observation of parallel membrane insertion and folding pathways of a beta-barrel membrane protein from an unfolded state in urea into lipid bilayers. The kinetics of both folding pathways depended on the chain length of the lipid and on temperature with estimated activation energies of 19 kJ/mol (dicapryl phosphatidylcholine) and 70 kJ/mol (dioleoyl phosphatidylcholine) for the faster pathways.

Raman spectra of planar supported lipid bilayers

Raman scattering has been used to obtain high quality vibrational spectra of planar supported lipid bilayers (pslb's) at the silica/water interface without the use of resonance or surface enhancement. A total internal reflection geometry was used both to increase the bilayer signal and to suppress the water background. Polarization control permits the determination of four components of the Raman tensor, of which three are independent for a uniaxial film. Spectra are reported of the phospholipids DMPC, DPPC, and POPC, in the C-H stretching region and the fingerprint region. The temperature-dependent polarized spectra of POPC show only small changes over the range 14-41 degrees C. The corresponding spectra of DMPC and DPPC bilayers show large thermal changes consistent with a decreasing tilt angle from the surface normal and increasing chain ordering at lower temperatures. The thermal behavior of DMPC pslb's is similar to that of vesicles of the same lipid in bulk suspension. In contrast to calorimetry, which shows a sharp phase transition (L alpha-L beta') with decreasing temperature, the changes in the Raman spectra occur over a temperature range of ca. 10 degrees C commencing at the calorimetric phase transition temperature.

Mechanism of membrane activity of the antibiotic trichogin GA IV: a two-state transition controlled by peptide concentration

Synthetic fluorescent analogs of the natural lipopeptide trichogin GA IV were used to investigate the peptide position and orientation in model membranes. A translocation assay based on Forster energy transfer indicates that trichogin is associated to both the outer and inner leaflet of the membrane, even at low concentration, when it is not active. Fluorescence quenching measurements, performed by using water soluble quenchers and quenchers positioned in the membrane at different depths, indicate that at low membrane-bound peptide/lipid ratios trichogin lies close to the region of polar headgroups. By increasing peptide concentration until membrane leakage takes place, a cooperative transition occurs and a significant fraction of the peptide becomes deeply buried into the bilayer. Remarkably, this change in peptide position is strictly coupled with peptide aggregation. Therefore, the mechanism of trichogin action can be envisaged as based on a two-state transition controlled by peptide concentration. One state is the monomeric, surface bound and inactive peptide, and the other state is a buried, aggregated form, which is responsible for membrane leakage and bioactivity.

The effect of pH on the electrophoretic behaviour of a new class of liposomes: arsonoliposomes

Herein we report the effect of pH on the surface charge of a new class of liposomes: arsonoliposomes. Plain or mixed arsonoliposomes with cholesterol (Chol) and distearoyl-phosphatidylcholine (DSPC) in 1:1 molar ratio were prepared with lauryl-(C12), myristoyl-(C14) and palmitoyl-(C16) acyl side chain arsonolipids. The one step hydration method was used for vesicle preparation and zeta potential measurements were performed in the pH range from 3 to 9. The results revealed that these lipids hold a negative surface charge at all pH values investigated. The presence of cholesterol in 1:1 molar ratio results in higher zeta potential compared with plain arsonoliposomes with the exception of palmitoyl-(C16) acyl chain arsonolipids in neutral and slightly basic pH values. Oppositely, the DSPC (1:1 molar ratio) containing arsonoliposomes had lower values of zeta potential compared with plain arsonoliposomes. Concluding, the experimental results reveal that zeta potential of arsonoliposomes is indeed modified when the vesicles are incubated in environments with different acidity. In most cases these changes are in accordance with the ionization pattern of the arsonolipid headgroup, while some peculiar deviations may be connected with the known difference in the structure between some of the vesicle types studied.

Hemifusion and fusion of giant vesicles induced by reduction of inter-membrane distance

Proteins involved in membrane fusion, such as SNARE or influenza virus hemagglutinin, share the common function of pulling together opposing membranes in closer contact. The reduction of inter-membrane distance can be sufficient to induce a lipid transition phase and thus fusion. We have used functionalized lipids bearing DNA bases as head groups incorporated into giant unilamellar vesicles in order to reproduce the reduction of distance between membranes and to trigger fusion in a model system. In our experiments, two vesicles were isolated and brought into adhesion by the mean of micromanipulation; their evolution was monitored by fluorescence microscopy. Actual fusion only occurred in about 5% of the experiments. In most cases, a state of "hemifusion" is observed and quantified. In this state, the outer leaflets of both vesicles' bilayers merged whereas the inner leaflets and the aqueous inner contents remained independent. The kinetics of the lipid probes redistribution is in good agreement with a diffusion model in which lipids freely diffuse at the circumference of the contact zone between the two vesicles. The minimal density of bridging structures, such as stalks, necessary to explain this redistribution kinetics can be estimated.

Secondary and tertiary structure formation of the beta-barrel membrane protein OmpA is synchronized and depends on membrane thickness

The mechanism of membrane insertion and folding of a beta-barrel membrane protein has been studied using the outer membrane protein A (OmpA) as an example. OmpA forms an eight-stranded beta-barrel that functions as a structural protein and perhaps as an ion channel in the outer membrane of Escherichia coli. OmpA folds spontaneously from a urea-denatured state into lipid bilayers of small unilamellar vesicles. We have used fluorescence spectroscopy, circular dichroism spectroscopy, and gel electrophoresis to investigate basic mechanistic principles of structure formation in OmpA. Folding kinetics followed a second-order rate law and is strongly depended on the hydrophobic thickness of the lipid bilayer. When OmpA was refolded into model membranes of dilaurylphosphatidylcholine, fluorescence kinetics were characterized by a rate constant that was about fivefold higher than the rate constants of formation of secondary and tertiary structure, which were determined by circular dichroism spectroscopy and gel electrophoresis, respectively. The formation of beta-sheet secondary structure and closure of the beta-barrel of OmpA were correlated with the same rate constant and coupled to the insertion of the protein into the lipid bilayer. OmpA, and presumably other beta-barrel membrane proteins therefore do not follow a mechanism according to the two-stage model that has been proposed for the folding of alpha-helical bundle membrane proteins. These different folding mechanisms are likely a consequence of the very different intramolecular hydrogen bonding and hydrophobicity patterns in these two classes of membrane proteins.

Normal and lateral forces between lipid covered solids in solution: correlation with layer packing and structure

We report on the normal and lateral forces between controlled-density mono- and bilayers of phospholipid co-adsorbed onto hydrophobic and hydrophilic solid supports, respectively. Interactions between 1,2-dioleoyl-sn-glycero-3-phosphocholine layers were measured using an atomic force microscope. Notable features of the normal force curves (barrier heights and widths) were found to correlate with the thickness and density of the supported lipid layers. The friction and normal force curves were also found interrelated. Thus, very low friction values were measured as long as the supported layer(s) resisted the normal pressure of the tip. However, as the applied load exceeded the critical value needed for puncturing the layers, the friction jumped to values close to those recorded between bare surfaces. The lipid layers were self-healing between measurements, but a significant hysteresis was observed in the force curves measured on approach and retraction, respectively. The study shows the potential of using atomic force microscopy for lipid layer characterization both with respect to structure and interactions. It further shows the strong lubricating effect of adsorbed lipid layers and how this varies with surface density of lipids. The findings may have important implications for the issue of joint lubrication.

Responding phospholipid membranes--interplay between hydration and permeability

Osmotic forces are important in regulating a number of physiological membrane processes. The effect of osmotic pressure on lipid phase behavior is of utmost importance for the extracellular lipids in stratum corneum (the outer part of human skin), due to the large gradient in water chemical potential between the water-rich tissue on the inside, and the relative dry environment on the outside of the body. We present a theoretical model for molecular diffusional transport over an oriented stack of two-component lipid bilayers in the presence of a gradient in osmotic pressure. This gradient serves as the driving force for diffusional motion of water. It also causes a gradient in swelling and phase transformations, which profoundly affect the molecular environment and thus the local diffusion properties. This feedback mechanism generates a nonlinear transport behavior, which we illustrate by calculations of the flux of water and solute (nicotine) through the bilayer stack. The calculated water flux shows qualitative agreement with experimental findings for water flux through stratum corneum. We also present a physical basis for the occlusion effect. Phase behavior of binary phospholipid mixtures at varying osmotic pressures is modeled from the known interlamellar forces and the regular solution theory. A first-order phase transformation from a gel to a liquid--crystalline phase can be induced by an increase in the osmotic pressure. In the bilayer stack, a transition can be induced along the gradient. The boundary conditions in water chemical potential can thus act as a switch for the membrane permeability.

Statistical analysis of predicted transmembrane alpha-helices

Statistical analyses were undertaken for putative transmembrane alpha-helices obtained from a database representing the subset of membrane proteins available in Swiss-Prot. The average length of a transmembrane alpha-helix was found to be 22-21 amino acids with a large variation around the mean. The transfer free energy from water to oil of a transmembrane alpha-helix in bitopic proteins, -48 kcal/mol, is higher than that in polytopic proteins, -39 kcal/mol, and is nearly identical to that obtained by assuming a random distribution of solely hydrophobic amino acids in the alpha-helix. The amino acid composition of hydrophobic residues is similar in bitopic and polytopic proteins. In contrast, the more polar the amino acids are, the less likely they are to be found in bitopic proteins compared to polytopic ones. This most likely reflects the ability of alpha-helical bundles to shield the polarity of residues from the hydrophobic bilayer. One half of all amino acids were distributed nonrandomly in both bitopic and polytopic proteins. A preference was found for tyrosine and tryptophan residues to be at the ends of transmembrane alpha-helices. Correlated distribution analysis of amino acid pairs indicated that most amino acids are independently distributed in each helix. Exceptions are cysteine, tyrosine, and tryptophan which appear to cluster closely to one another and glycines which are preferentially found on the same side of alpha-helices.