Hexagonal phase forming propensity detected in phospholipid bilayers with fluorescent probes

Multidimensional Spectral Phasors of LAURDAN's Excitation-Emission Matrices: The Ultimate Sensor for Lipid Phases?

The impact of lipid diversity on the lateral organization of biological membranes remains a topic of debate. While the existence of domains in lamellar membranes is well-established, the nonlamellar phases occurring in biological systems are less explored due to technical constraints. Here, we present the measurement of the excitation-emission matrices (EEM) of LAURDAN in several lipid structures. LAURDAN is a fluorescence probe widely used for characterizing lipid assemblies. The EEMs were analyzed by multidimensional spectral phasors (MdSP), an approach that seizes information from both the excitation and emission spectra. We developed a computer algorithm to construct EEM data based on a model for LAURDAN's photophysics. The MdSP calculated from the simulated EEMs reveals that all feasible possibilities lie inside a universal triangle in the phasor's plot. We use this triangle to propose a ternary representation for the phasors, allowing a better assessment of LAURDAN's surroundings in terms of hydration, water mobility, and local electronic environment. Building upon this foundation, we constructed a theoretical "phase map" that can assess both lamellar and nonlamellar membranes. We thoroughly validated this theory using well-known lipid mixtures under different phase-state conditions and enzymatically generated systems. Our results confirm that the use of MdSP is a powerful tool for obtaining quantitative information on both lamellar and nonlamellar structures. This study not only advances our understanding of the impact of lipid diversity on membrane organization but also provides a robust and general framework for the assessment of fluorescence properties that can be further extended to other probes.

Water as a Link between Membrane and Colloidal Theories for Cells

This review is an attempt to incorporate water as a structural and thermodynamic component of biomembranes. With this purpose, the consideration of the membrane interphase as a bidimensional hydrated polar head group solution, coupled to the hydrocarbon region allows for the reconciliation of two theories on cells in dispute today: one considering the membrane as an essential part in terms of compartmentalization, and another in which lipid membranes are not necessary and cells can be treated as a colloidal system. The criterium followed is to describe the membrane state as an open, non-autonomous and responsive system using the approach of Thermodynamic of Irreversible Processes. The concept of an open/non-autonomous membrane system allows for the visualization of the interrelationship between metabolic events and membrane polymorphic changes. Therefore, the Association Induction Hypothesis (AIH) and lipid properties interplay should consider hydration in terms of free energy modulated by water activity and surface (lateral) pressure. Water in restricted regions at the lipid interphase has thermodynamic properties that explain the role of H-bonding networks in the propagation of events between membrane and cytoplasm that appears to be relevant in the context of crowded systems.

Sugar-based bactericides targeting phosphatidylethanolamine-enriched membranes

Anthrax is an infectious disease caused by Bacillus anthracis, a bioterrorism agent that develops resistance to clinically used antibiotics. Therefore, alternative mechanisms of action remain a challenge. Herein, we disclose deoxy glycosides responsible for specific carbohydrate-phospholipid interactions, causing phosphatidylethanolamine lamellar-to-inverted hexagonal phase transition and acting over B. anthracis and Bacillus cereus as potent and selective bactericides. Biological studies of the synthesized compound series differing in the anomeric atom, glycone configuration and deoxygenation pattern show that the latter is indeed a key modulator of efficacy and selectivity. Biomolecular simulations show no tendency to pore formation, whereas differential metabolomics and genomics rule out proteins as targets. Complete bacteria cell death in 10 min and cellular envelope disruption corroborate an effect over lipid polymorphism. Biophysical approaches show monolayer and bilayer reorganization with fast and high permeabilizing activity toward phosphatidylethanolamine membranes. Absence of bacterial resistance further supports this mechanism, triggering innovation on membrane-targeting antimicrobials.

Bacillus anthracis causes the infectious disease anthrax. Here, the authors synthesized deoxy glycosides that are effective against B. anthracis and related bacteria and found that these amphiphilic compounds kill bacteria via an unusual mechanism of action.

Triacontanol and jasmonic acid differentially modulate the lipid organization as evidenced by the fluorescent probe behavior and 31P nuclear magnetic resonance shifts in model membranes

Fluorescence resonance energy transfer (FRET), time-resolved fluorescence and anisotropy decays were determined in large unilamellar vesicles (LUVs) of egg phosphatidylcholine with the FRET pair N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dipalmitoyl-sn-glycero-3-phospho-ethanolamine as donor and lissamine rhodamine B 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine as acceptor, using 2-ps pulses from a Ti:sapphire laser on LUVs with incorporated plant growth regulators: triacontanol (TRIA) and jasmonic acid (JA). FRET efficiency, energy transfer rate, rotation correlation time, microviscosity, and diffusion coefficient of lateral diffusion of lipids were calculated from these results. It was observed that TRIA and JA differentially modulated all parameters studied. The effect of JA in such modulations was always partially reversed by TRIA. Also, the generalized polarization of laurdan fluorescence indicated that JA enhances the degree of hydration in lipid bilayers to a larger extent than does TRIA. Solid-state (31)P magic-angle spinning nuclear magnetic resonance spectra of LUVs showed two chemical shifts, at 0.009 and -11.988 ppm, at low temperatures (20 degrees C), while at increasing temperatures (20-60 degrees C) only one (at -11.988 ppm) was prominent and the other (0.009 ppm) gradually became obscure. However, LUVs with TRIA exhibited only one of the shifts at 0.353 ppm even at lower temperatures and JA did not affect the chemical shifts.

The effect of detergents on trimeric G-protein activity in isolated plasma membranes from rat brain cortex: correlation with studies of DPH and Laurdan fluorescence

The effect of non-ionic detergents on baclofen (GABAB-R agonist)-stimulated G-protein activity was measured as a [(35)S]GTPgammaS binding assay in the plasma membranes (PM) isolated from the brain tissue. The effect was clearly biphasic--a decrease in the activity was followed by an activation maximum and finally, at high concentrations, drastic inhibition of the G-protein activity was noticed. Contrarily, specific radioligand binding to GABAB-receptor was inhibited in the whole range of detergent concentrations step by step, i.e. it was strictly monophasic. The magnitude of both detergent effects was decreased in the same order of potency: Brij58>Triton X-100>Digitonin. The identical order was found when comparing detergents ability to alter fluorescence anisotropy of the membrane probe 1,6-diphenyl-1,3,5-hexatriene (rDPH) incorporated into the hydrophobic PM interior. Decrease of rDPH, in the order of Brij58>Triton X-100>Digitonin, was reflected as decrease of the S-order parameter and rotation correlation time phi paralleled by an increase of diffusion wobbling constant Dw (analysis by time-resolved fluorescence according to "wobble-in-cone" model). The influence of the detergents on the membrane organization at the polar headgroup region was characterized by Laurdan generalized polarization (GP). As before, the effect of detergents on GP parameters proceeded in the order: Brij58>Triton X-100>Digitonin.

Barotropic phase transition between the lamellar liquid crystal phase and the inverted hexagonal phase of dioleoylphosphatidylethanolamine

The phase transition between the lamellar liquid crystal (Lalpha) phase and the inverted hexagonal (H(II)) phase of dioleoylphosphatidylethanolamine (DOPE) in aqueous NaCl solutions was observed by means of differential scanning calorimetry (DSC) under ambient pressure and light-transmittance technique under high pressure. The pressure dependence of the transition temperature (dT/dp) and the thermodynamic quantities for the Lalpha/H(II) transition were compared with those of another phase transition found in the DOPE bilayer membrane, which is the transition from the lamellar crystal (Lc) phase to the Lalpha phase. The dT/dp value of the Lalpha/H(II) transition was about 3.5 times as large as that of the Lc/Lalpha transition while the thermodynamic quantities were significantly smaller than those of the latter to the contrary. Comparing the enthalpy and volume behavior of the Lalpha/H(II) transition with that of the Lc/Lalpha transition, we concluded that the Lalpha/H(II) transition can be regarded as the volume-controlled transition for the reconstruction of molecular packing.

Effects of cholesterol in chylomicron remnant models of lipid emulsions on apoE-mediated uptake and cytotoxicity of macrophages

Chylomicron remnants have been suggested to be involved in the development of atherosclerosis. To investigate the mechanisms of chylomicron remnant-induced atherosclerosis, we prepared cholesterol (Chol)-containing emulsion particles as models for chylomicron remnants. Chol markedly increased the apolipoprotein E (apoE) binding maximum of emulsions without changing the binding affinity and thereby promoted emulsion uptake by J774 macrophages. Fluorescence measurements showed that Chol increased acyl chain order and head group hydration of the surface phospholipid (PL) layer of emulsions. The binding maximum of apoE was closely correlated with the hydration and the increase in the PL head group separation at the emulsion surface. From experiments using inhibitors for lipoprotein receptors, heparan sulfate proteoglycans and low density lipoprotein receptor-related protein were found to be the major contributors to the uptake of Chol-containing emulsions. Trypan blue dye exclusion revealed that the uptake of Chol-containing emulsions induced cytotoxicity to J774 macrophages. This study proposes a mechanism of atherosclerosis induced by chylomicron remnants.

On the importance of the phosphocholine methyl groups for sphingomyelin/cholesterol interactions in membranes: a study with ceramide phosphoethanolamine

In this study, we have examined how the headgroup size and properties affect the membrane properties of sphingomyelin and interactions with cholesterol. We prepared N-palmitoyl ceramide phosphoethanolamine (PCPE) and compared its membrane behavior with D-erythro-N-palmitoyl-sphingomyelin (PSM), both in monolayers and bilayers. The pure PCPE monolayer did not show a phase transition at 22 degrees C (in contrast to PSM), but displayed a much higher inverse isothermal compressibility as compared to the PSM monolayer, indicating stronger intermolecular interactions between PCPEs than between PSMs. At 37 degrees C the PCPE monolayer was more expanded (than at 22 degrees C) and displayed a rather poorly defined phase transition. When cholesterol was comixed into the monolayer, a condensing effect of cholesterol on the lateral packing of the lipids in the monolayer could be observed. The phase transition from an ordered to a disordered state in bilayer membranes was determined by diphenylhexatriene steady-state anisotropy. Whereas the PSM bilayer became disordered at 41 degrees C, the PCPE bilayer main transition occurred around 64 degrees C. The diphenylhexatriene steady-state anisotropy values were similar in both PCPE and PSM bilayers before and after the phase transition, suggesting that the order in the hydrophobic core in both bilayer types was rather similar. The emission from Laurdan was blue shifted in PCPE bilayers in the gel phase when compared to the emission spectra from PSM bilayers, and the blue-shifted component in PCPE bilayers was retained also after the phase transition, suggesting that Laurdan molecules sensed a more hydrophobic environment at the PCPE interface compared to the PSM interface both below and above the bilayer melting temperature. Whereas PSM was able to form sterol-enriched domains in dominantly fluid bilayers (as determined from cholestatrienol dequenching experiments), PCPE failed to form such domains, suggesting that the size and/or properties of the headgroup was important for stabilizing sphingolipid/sterol interaction. In conclusion, our study has highlighted how the headgroup in sphingomyelin affect its membrane properties and interactions with cholesterol.

Fluorescence properties of Laurdan in cochleate phases

Cochleates are lipid-based delivery system that have found application in drug and gene delivery. They are precipitates, formed as a result of interaction between cations (e.g. Ca2+) and negatively charged phospholipids such as phosphatidylserine (PS). In the present study, we investigated the utility of fluorescent probe Laurdan (6-dodecanoyl-2-dimethylamino naphthalene) to monitor cochleate phase formation. Following addition of Ca2+ to Laurdan labeled lipid vesicles comprised of brain phosphatidylserine (BPS), a significant blue shift in the emission peak maximum of Laurdan was observed and the spectral features were distinct from those observed for the gel and liquid-crystalline (LC) phases. This is consistent with the formation of anhydrous cochleate cylinders that was further confirmed by electron microscopy studies. Due to dipolar relaxation, excitation and emission generalized polarization (GPEx and GPEm) indicate transition from a LC to a rigid and dehydrated (RD) cochleate phase. These spectral changes were utilized to monitor the influence of lipid composition, ionic strength and lamellarity on the formation of cochleate phase. The results indicated that the presence of phosphatidylcholine (PC) and bulk Na+ concentration influenced the formation of cochleate structures from small unilamellar vesicles (SUV) and multilamellar vesicles (MLV) composed of PS. The presence of PC and higher bulk Na+ concentration stabilized the PS vesicles against collapse and total loss of contents, intermediate molecular events in the formation of cochleate structures. From these studies, we conclude that Laurdan fluorescence is a sensitive and a rapid method to detect cochleate phase formation.

A new ratiometric fluorescence probe as strong sensor of surface charge of lipid vesicles and micelles

We report on the ability of a new fluorescent probe, 4-(2-pyren-1-yl-vinyl) pyridine, 1, to respond to micelles and phospholipid vesicles of different surface charge. The probe gets incorporated into micellar and membranous assemblies and shows a large red-shift in the fluorescence emission maxima especially when the surface charge of the organized media is anionic. The effect on the photo-excitation of the probe is very clear and pronounced as it can be easily visualized. The sample color upon photo-excitation changes from blue to orange/red once the probe experiences negatively charged vesicular or micellar surfaces. These results make the probe molecule useful as a reporter for sensing electrostatic environment in biological membranes and related organized assemblies.

Properties of palmitoyl phosphatidylcholine, sphingomyelin, and dihydrosphingomyelin bilayer membranes as reported by different fluorescent reporter molecules

The properties of vesicle membranes prepared from 16:0-SM, 16:0-DHSM, or DPPC were characterized using steady-state and time-resolved fluorescence spectroscopy and different fluorescent reporter molecules. The acyl-chain region was probed using free and phospholipid-bound 1,6-diphenyl-1,3,5-hexatriene. 16:0-DHSM was found to be the more ordered than both DPPC and 16:0-SM 5 degrees C below and above melting temperature. Interfacial properties of the phospholipid bilayers were examined using 6-dodecanoyl-2-dimethyl-aminonaphthalene (Laurdan), 6-propionyl-2-dimethyl-amino-naphthalene (Prodan), and dansyl-PE. Laurdan and Prodan reported that the two sphingomyelin (SM) membrane interfaces were clearly different from the DPPC membrane interface, whereas the two SM membrane interfaces had more similar properties (both in gel and liquid-crystalline phase). Prodan partition studies showed that membrane resistance to Prodan partitioning increased in the order: 16:0-SM < DPPC < 16:0-DHSM. The degree to which dansyl-PE is exposed to water reflects the structural properties of the membrane-water interface. By comparing the lifetime of dansyl-PE in water and deuterium oxide solution, we could show that the degree to which the dansyl moiety was exposed to water in the membranes increased in the order: 16:0-SM < DPPC < 16:0-DHSM. In conclusion, this study has shown that DHSM forms more ordered bilayers than acyl-chain matched SM or phosphatidylcholine, even in the liquid-crystalline state.

Inhibition of protein kinase C by resveratrol

Evidence is emerging that resveratrol (RV), a polyphenolic phytoaxelin present in dietary sources including red wine, may protect against atherosclerosis and cardiovascular disease by enhancing the integrity of the endothelium. In this study, the possibility that such beneficial effects of RV may arise from a modulation of protein kinase C (PKC)-mediated signaling was investigated by determining the effects of RV on the in vitro activities of PKC isozymes. It was found that the Ca(2+)-dependent activities of membrane-associated PKCalpha induced by either phorbol ester or diacylglycerol were potently inhibited by RV, each with an IC(50) of approximately 2 microM. The inhibitory effect of RV was also observed for conventional PKCbetaI, whereas the activities of novel PKC epsilon and atypical PKCzeta were each unaffected. The inhibition of PKCalpha activity was found to be competitive with respect to phorbol ester concentration but noncompetitive with respect to Ca(2+) and phosphatidylserine concentrations, suggesting that the RV may compete for phorbol ester-binding to the C1 domains. Supporting this, it was found that RV bound to a fusion peptide containing the C1A and C1B domains of PKCalpha. Similar to the effects of diacylglycerol and phorbol ester, the interaction of RV with the C1 domains induced the association of PKCalpha with membrane lipid vesicles, although this did not result in activation. Overall, the results suggest that the inhibitory effect of RV on PKC activity, and therefore on the associated signaling networks, may, in part, underlie the mechanism(s) by which this agent exerts its beneficial effects on endothelial and cardiovascular function. Furthermore, the effects of RV on these signaling networks are predicted to differ according to the cellular localization and the regulating PKC isozyme.

Interaction of quinine with model lipid membranes of different compositions

The binding of a drug such as Quinine with neutral and negatively charged small unilamellar lipid vesicles at pH 7 and 37 degrees C was investigated. Changes in the fluorescence properties of the drug after association with the liposomes were used to obtain binding isotherms over a range of phospholipid compositions at different ionic strengths. Under such conditions, the association was strongly enhanced by the negative net charge of the bilayer but diminished by the increasing presence of electrolytes in the aqueous media. Binding data were analyzed in terms of a surface partition equilibrium without and after correcting for electrostatic effects by means of the Gouy-Chapman theory. The intrinsic (hydrophobic) constant, obtained after charge correction, was smaller than the apparent binding constant determined without taking into account such an effect. A new analysis considering the two components not fully dissociated and affected by an identical screening factor has been introduced. It yielded rather similar hydrophobic partition coefficients for all conditions, independent of both the surface charge density of the lipid vesicles and the ionic strength, with an average value estimated to be (3 +/- 1) x 10(3) M(-1). All the findings suggested that the association of Quinine to liposomes is controlled primarily through electrostatic attractions, and, in a lesser extent, by hydrophobic forces. Because electrostatic and hydrophobic interactions play a crucial role in both the drug-membrane affinity and the location of the drug, their quantitative evaluation can shed light on the mechanism for a next therapeutic "action."

The contrasting kinetics of peroxidation of vitamin E-containing phospholipid unilamellar vesicles and human low-density lipoprotein

It is well established that alpha-tocopherol, TocH, is an outstanding lipid-soluble, peroxyl radical trapping antioxidant in homogeneous systems. It is also well established that TocH functions as a prooxidant in human low-density lipoprotein, LDL, subjected to attack by peroxyl radicals generated in the aqueous phase by, for example, thermal decomposition of the azo compound, ABAP. This tocopherol-mediated peroxidation, TMP, of LDL involves a three-step chain reaction, one step being hydrogen atom abstraction from the LDL lipids by the tocopheroxyl radical, Toc*. The occurrence of TMP has been attributed to three factors, (i) translocation by TocH of radical character from the aqueous phase into LDL lipid, (ii) isolation of the water-insoluble Toc* in the LDL particle in which it is formed for times sufficient to permit it to react with the lipid, and (iii) the small lipid volume of LDL which ensures that no particle can contain more than a single radical for a significant length of time. This consensus view of TMP implies that it should occur in any TocH-containing dispersion of small lipid particles. However, the present examination of the kinetics of the ABAP-initiated peroxidation of small unilamellar vesicles, SUVs, made from palmitoyllinoleoylphosphatidylcholine and cholesterol with a composition designed to mimic the surface coat of LDL, has shown that TocH functions as an antioxidant in such systems and that TMP does not occur under conditions where it would have occurred if the particles had been LDL. Several possible reasons for the kinetic differences between SUVs and LDL have been considered and ruled out by experiment. It is concluded that TMP can occur in LDL because these particles contain a lipid core in which the Toc* radical "hides" for much of its lifetime well away from the peroxyl radicals in the aqueous phase. In contrast, because SUVs have no lipid core, the Toc(*) radical is always "exposed" and available to aqueous peroxyl radicals with which it reacts rapidly and is destroyed before it can abstract a hydrogen atom from the lipid.

Cellular membrane composition defines A beta-lipid interactions

Alzheimer's disease pathology has demonstrated amyloid plaque formation associated with plasma membranes and the presence of intracellular amyloid-beta (A beta) accumulation in specific vesicular compartments. This suggests that lipid composition in different compartments may play a role in A beta aggregation. To test this hypothesis, we have isolated cellular membranes from human brain to evaluate A beta 40/42-lipid interactions. Plasma, endosomal, lysosomal, and Golgi membranes were isolated using sucrose gradients. Electron microscopy demonstrated that A beta fibrillogenesis is accelerated in the presence of plasma and endosomal and lysosomal membranes with plasma membranes inducing an enhanced surface organization. Alternatively, interaction of A beta with Golgi membranes fails to progress to fibril formation, suggesting that A beta-Golgi head group interaction stabilizes A beta. Fluorescence spectroscopy using the environment-sensitive probes 1,6-diphenyl-1,3,5-hexatriene, laurdan, N-epsilon-dansyl-L-lysine, and merocyanine 540 demonstrated variations in the inherent lipid properties at the level of the fatty acyl chains, glycerol backbone, and head groups, respectively. Addition of A beta 40/42 to the plasma and endosomal and lysosomal membranes decreases the fluidity not only of the fatty acyl chains but also the head group space, consistent with A beta insertion into the bilayer. In contrast, the Golgi bilayer fluidity is increased by A beta 40/42 binding which appears to result from lipid head group interactions and the production of interfacial packing defects.

Inhibition of lipoprotein lipase activity by sphingomyelin: role of membrane surface structure

We have recently shown that sphingomyelin (SM) strongly inhibits lipoprotein lipase (LPL)-mediated lipolysis in monolayers and emulsion particles. To further evaluate how SM modulates LPL activity on the emulsion surface, the relationship between membrane surface structure and LPL activity was investigated. We measured fluorescence anisotropy of 1-palmitoyl-2-[3-(diphenylhexatrienyl)propionyl]-sn-3-phosphati dylcho line, probing surface acyl chain fluidity, and fluorescence lifetime of N-(5-dimethylaminonaphthalene-1-sulfonyl)dipalmitoylphosphatidylethan olamine in H(2)O and D(2)O buffer, assessing the degree of hydration in the head group region. The results revealed that incorporation of egg SM into triolein-egg phosphatidylcholine emulsions markedly increased acyl chain order and decreased head group hydration of the surface monolayers. In contrast, cholesterol was shown to increase head group hydration despite a strong increase in acyl chain order. The close correlation between the apparent K(m) values of LPL and the degree of head group hydration indicated that LPL interacts with the head group region rather than with the hydrophobic interior of the surface monolayers. However, apparent V(max) did not show a simple correlation with any surface structure, and the finding in which SM had no effect on apparent V(max) of medium-chain triglyceride emulsions suggested that the hydrophobic interaction between acyl chains of SM and triglyceride at the emulsion surface is important for determining the apparent V(max). These results showed conclusively that SM inhibits LPL activity mainly by changing the emulsion surface structure and not by a specific interaction between SM and LPL.

Fluorescent probes used to monitor membrane interfacial polarity

The polarity of the interface between a lipid bilayer membrane and bulk water is an important physical parameter of the membrane. It is likely that several membrane-dependent biological functions are modulated by this property. However, interfacial polarity can be difficult to define because of an imprecise knowledge of the molecular nature of the interface. Nevertheless, attempts have been made to measure this quantity with the use of fluorescent probes which are sensitive to the solvent polarity. Often, however, other factors, such as the rate of solvent relaxation must be known in order to interpret the fluorescent properties in terms of the dielectric constant. In addition, the spatial orientation and location of the fluorophore are often not known precisely. Nevertheless, there have been successful efforts to gain a more accurate knowledge of this aspect of membrane physical properties and its relationship to biological phenomena is discussed.

Merocyanine 540 as a fluorescence indicator for molecular packing stress at the onset of lamellar-hexagonal transition of phosphatidylethanolamine bilayers

The fluorescence of Merocyanine 540 (MC 540) is sensitive to the molecular packing of membrane lipids. Therefore, the fluorescence of MC 540 is expected to be sensitive to the curvature-related packing stress at the onset of the lamellar-hexagonal phase transition. We measured the fluorescence intensity of MC 540 when the temperatures of lipid bilayers approached their lamellar-hexagonal phase transitions. The fluorescence of MC 540 in the presence of egg and dioleoylphosphatidylethanolamine bilayers increased at the respective lamellar-hexagonal phase transitions of these lipids. Furthermore, increases in fluorescence intensity were also observed at temperatures just below their phase transitions. The enhanced fluorescence was not due to the specific interaction of the dye with the ethanolamine headgroup, because no such increase was observed when the probe was exposed to phosphatidylethanolamines which do not form hexagonal phase within the range of applied temperature. In addition, when the temperature of the lamellar-hexagonal phase transition was shifted, by the addition of a small amount of phosphatidylcholine, the dependence of the fluorescence intensity on temperature was modified accordingly. We postulate that the change of MC 540 fluorescence intensity at temperatures approaching the lamellar-hexagonal phase transition reflects changes in the partition of MC 540 into the fluid lipid phase. The change in partition is influenced by the curvature stress in bilayers at temperatures just below the lamellar-hexagonal phase transition.

A mechanism underlying stimulation and inhibition of protein kinase C by lyso-PC: A role of membrane physical state

Lysophosphatidylcholine (lyso-PC) biphasically regulates the diacylglycerol-induced activation of protein kinase C (PKC). In common parlance, lyso-PC stimulates PKC at low concentrations, but, conversely, inhibits it at high concentrations. The activity of purified PKC from rat brains was measured in the vesicles made up of dipalmitoylphosphatidylserine (DPPS), 1, 2-sn-diolein (DOG) and different molar ratios of 1-palmitoyl-sn-glycerol-3-phosphoryl-choline (C16:0 lyso-PC). The effect, i. e. stimulation or inhibition on PKC by C16:0 lyso-PC, depends on DPPS and DOG concentrations as well as its own concentration. When the concentration of DOG is stable, this C16:0 lyso-PC action depends on C16:0 lyso-PCIDPPS molar ratio. Differential scanning calorimetry (DSC), two fluorescence probes and light scattering were used to analyze the physical characteristics of membrane, including thermotropic phase behavior, the turbidity, the lipid molecular acyl chains packing and the head group spacing. The more adulteration of C16:0 lyso-PC in liposome bilayer membrane, the looser acyl chains pack, and the broader head group spacing. DSC results show that there are two immiscible lipid areas in the membrane: C16:0 lyso-PC-rich area and C16: 0 lyso-PC-poor area. When C16:0 lyso-PC/DPPS molar ratio was 0.234, the two areas had the broadest boundary and the activation of PKC was the highest. When the ratio was over 0.434, the phase transition of DPPS disappeared; micelle tended to substitute the structure of bilayer; the activity of PKC was inhibited completely. DOG can stabilize the bilayer structure of membrane, so the C16:0 lyso-PC/DPPS molar ratios to inhibit PKC in lipid mixture with DOG are higher than that without DOG. The ability of C16:0 lyso-PC to change the physical properties and the structure of membrane plays an important role in its effect on PKC activation.

Structural characterisation of the natural membrane-bound state of melittin: a fluorescence study of a dansylated analogue

The binding of a dansylated analogue of melittin (DNC-melittin) to natural membranes is described. The cytolytic peptide from honey bee venom melittin was enzymatically labelled in its glutamine-25 with the fluorescent probe monodansylcadaverine using guinea pig liver transglutaminase. The labelled peptide was characterised functionally in cytolytic assays, and spectroscopically by circular dichroism and fluorescence. The behaviour of DNC-melittin was, in all respects, indistinguishable from that of the naturally occurring peptide. We used resonance energy transfer to measure the state of aggregation of melittin on the membrane plane in synthetic and natural lipid bilayers. When bound to erythrocyte ghost membranes, the extent of energy transfer was found to be equivalent to when bound to small unilamellar vesicles of phosphatidylcholine. Our results correlate best with a proposed model in which the initial interaction between melittin and the red blood cells could be merely electrostatic and the peptide remains in a low alpha-helical conformation. The next step would be a peptide stabilisation in the membrane in a monomeric alpha-helical conformation that would imply the collapse of the membrane structure and liberation of the cell contents.

Binding of basic amphipathic peptides to neutral phospholipid membranes: a thermodynamic study applied to dansyl-labeled melittin and substance P analogues

A thermodynamic approach is proposed to quantitatively analyze the binding isotherms of peptides to model membranes as a function of one adjustable parameter, the actual peptide charge in solution z(p)+. The main features of this approach are a theoretical expression for the partition coefficient calculated from the molar free energies of the peptide in the aqueous and lipid phases, an equation proposed by S. Stankowski [(1991) Biophysical Journal, Vol. 60, p. 341] to evaluate the activity coefficient of the peptide in the lipid phase, and the Debye-Hückel equation that quantifies the activity coefficient of the peptide in the aqueous phase. To assess the validity of this approach we have studied, by means of steady-state fluorescence spectroscopy, the interaction of basic amphipathic peptides such as melittin and its dansylcadaverine analogue (DNC-melittin), as well as a new fluorescent analogue of substance P, SP (DNC-SP) with neutral phospholipid membranes. A consistent quantitative analysis of each binding curve was achieved. The z(p)+ values obtained were always found to be lower than the physical charge of the peptide. These z(p)+ values can be rationalized by considering that the peptide charged groups are strongly associated with counterions in buffer solution at a given ionic strength. The partition coefficients theoretically derived using the z(p)+ values were in agreement with those deduced from the Gouy-Chapman formalism. Ultimately, from the z(p)+ values the molar free energies for the free and lipid-bound states of the peptides have been calculated.

Fluorescent probes of membrane surface properties

We have studied the properties of two new fluorescent probes, 7-dimethylaminocoumarin derivatives, 4-[N, N-dimethyl-N-(n-tetradecyl)ammoniummethyl]-7-(N,N-dimethylamino)co umarin chloride (TAMAC) and 4-(n-dodecylthiomethyl)-7-(N,N-dimethylamino)coumarin (DTMAC) in model membrane systems. Both probes are sensitive to solvent polarity. The TAMAC probe has a quaternary ammonium function to position it at a fixed location with respect to the membrane interface. In membranes of dipalmitoleoylphosphatidylethanolamine (DiPoPE), both probes detect marked increases in surface hydrophobicity as the bilayer to hexagonal phase transition temperature is approached. This does not occur when the probes are embedded in dipalmitoleoylphosphatidylcholine (DiPoPC) in which case the fluorescence emission is found to be largely independent of temperature. A nitroxide quencher covalently linked to the 5 position of the sn-2 acyl chain of phosphatidylcholine quenches the fluorescence of DTMAC in DiPoPC more than in DiPoPE, indicating the deeper insertion of this probe in DiPoPC. As the temperature is increased the DTMAC fluorophore moves even further out of the membrane. These findings indicate that DTMAC, which does not contain a group to fix its location along the bilayer normal, adjusts its position to small changes in environment polarity, so as to maintain an environment of a fixed dielectric constant. However, with greater changes in membrane interfacial polarity the environment of the probe will be altered. Thus, in addition to the sensitivity of these probes to solvent polarity, the ability of a fixed nitroxide to quench DTMAC becomes another parameter with which to characterize membrane properties with these probes.

Stability and pH sensitivity of sulfatide-containing phosphatidylethanolamine small unilamellar vesicles

The bilayer stabilization effect of sulfatide and the pH sensitivity of sulfatide-containing 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) small unilamellar vesicles were examined by light scattering and the release of entrapped calcein. At 30 mol% sulfatide, stable DOPE/sulfatide vesicles were formed at the physiological pH and their stability was preserved in the presence of human plasma. These vesicles were found to be pH-sensitive and became leaky at pH 6.0 or when there was a pH-gradient across the membrane bilayer. Under such conditions, the amount of calcein released after 24 h incubation at 37 degrees C was increased by one-fold compared to that found at pH 7.4. Our results suggest that the hydration and partial dehydration of the headgroup of sulfatide upon changing pH play an essential role in determining the pH sensitivity of DOPE/sulfatide vesicles, while the importance of the condensing effect of the glycolipid on membrane bilayer is less significant.

Monovalent cations differentially affect membrane surface properties and membrane curvature, as revealed by fluorescent probes and dynamic light scattering

The effects of monovalent cations on the interfacial electrostatic potential (psi d), hydrodynamic shear boundary distance (ds), and membrane curvature were studied in large unilamellar phospholipid and galacto/sulfolipid liposomes containing different fractions of negatively charged lipids. The differential effects of alkali metal ions on psi d could be accurately determined at physiological surface charge densities with a surface-anchored fluorescent probe. Li+ and Na+ more effectively decrease psi d and exhibit higher association constants (Kas) than K+ and Cs+. These two groups of cations display qualitatively different perturbations of the interfacial structure. Combining Kas values with the electrokinetic (zeta) potentials yielded the respective ds values. At low ionic strength ds more substantially increases with Li+ or Na+ than with K+ or Cs+. Increasing surface charge density causes increased membrane curvature in the presence of K+ or Cs+, but this is largely prevented by Li+ or Na+. Membrane binding of the amphiphilic cation acridine orange decreases surface charge and membrane curvature more extensively than H3O+, Li+, and Na+. The differential interface-perturbing behavior of monovalent cations is discussed with regard to their different hydration tendencies that will modulate the extent and stability of the hydrogen-bond network along the charged membrane surface.

The effects of non-lamellar forming lipids on membrane protein-lipid interactions

The role of lipid polymorphism in the regulation of membrane-associated protein function is examined, based on recent studies which showed that changes in the levels of phosphatidylethanolamine (PE), cholesterol and phospholipid unsaturation, modulate the activity of the key signal transduction enzyme, protein kinase C (PKC). It is shown that effects of membrane compositional changes on PKC activity involve a perturbation of protein-lipid interactions with the head group region rather than with the hydrophobic interior of the bilayer. A key determinant in the perturbation of these interactions is suggested to be an elastic curvature energy, termed curvature stress, which results from the unfavorable packing of non-lamellar forming lipids in a planar bilayer. PKC activity is shown to be a biphasic function of curvature stress, with an optimum value of this parameter corresponding to an optimally active PKC conformation. Thus, it is shown that the maximal activity of conformationally distinct PKC isoforms may require a different optimum value of curvature stress. Furthermore, it is hypothesized that curvature stress may have differing effects on the conformation of membrane-associated PKC activity induced by diacylglycerols, phorbol esters or other activators, based on recent studies showing that these agents induce the formation of disparate active conformers of the enzyme.

Nuclear magnetic resonance studies of lipid hydration in monomethyldioleoylphosphatidylethanolamine dispersions

Solid-state proton nuclear magnetic resonance has been used to examine surface hydration in suspensions of monomethyldioleoylphosphatidylethanolamine (MeDOPE). The magic-angle spinning (MAS) 1H spectra for aqueous suspensions of MeDOPE in the L alpha phase exhibited two resonances of roughly equal intensity that could be ascribed to water protons, but both their spin-lattice relaxation times and chemical shifts converged upon conversion to the hexagonal phase. Only a single water peak was observed for analogous samples of dioleoylphosphatidylcholine (DOPC). MAS-assisted two-dimensional nuclear Overhauser effect spectroscopy (NOESY) was conducted for multibilayers of both MeDOPE and DOPC. Through-space interactions were identified between pairs of lipid protons, as expected from their chemical structure. For lamellar suspensions of MeDOPE, positive NOESY cross-peaks were observed between the downfield-shifted water resonance (only) and both CH2N and NH2CH3+ protons of the lipid headgroup. These cross-peaks were not observed in the NOESY spectra of MeDOPE in its hexagonal or cubic phases or for lamellar DOPC reference samples. Taken together, the observation of two water peaks, spin-lattice relaxation behavior, and NOESY connectivities in MeDOPE suspensions support the interpretation that the low-field water peak corresponds to hydrogen-bonded interlamellar water interacting strongly with the lipid. Such a population of water molecules exists in association with MeDOPE in the lamellar phase but not for its inverted phases or for lamellar dispersions of DOPC.

Effect of propensity of hexagonal II phase formation on the activity of mitochondrial ubiquinol-cytochrome c reductase and H(+)-ATPase

The propensity of hexagonal II phase formation plays an important role in the activity of mitochondrial ubiquinol-cytochrome c reductase or H(+)-ATPase. The respiratory control ratio of reconstituted ubiquinol-cytochrome c reductase or the ATP-induced membrane potential of reconstituted H(+)-ATPase became higher as the non-bilayer phospholipid phosphatidylethanolamine content of proteoliposomes increased. The highest respiratory control ratio or ATP-induced membrane potential was obtained in the case of 60-80% phosphatidylethanolamine-containing proteoliposomes. Dioleoylphosphatidylethanolamine could significantly enhance the respiratory control ratio of ubiquinol-cytochrome c reductase and ATP-induced membrane potential of H(+)-ATPase, while no obvious change could be observed when dielaidoylphosphatidylethanolamine was used. The bilayer to hexagonal II phase transition temperature of ubiquinol-cytochrome c reductase-containing proteoliposomes reconstituted with phosphatidylcholine+phosphatidylethanolamine increases with decreasing content of phosphatidylethanolamine. Several additives such as the bilayer stabilizers, cholesterol 3-sulfate and carbobenzoxy-D-Phe-L-PheGly, or hexagonal II phase-forming promoters, such as diolein or eicosane, can decrease or increase the activity of these two enzyme complexes.

The effect of N-acyl ethanolamines on phosphatidylethanolamine phase transitions studied by laurdan generalised polarisation

The effect of N-lauroylethanolamine (N-LEA) and N-oleoylethanolamine (N-OEA) on the thermal behaviour of fully hydrated egg phosphatidylethanolamine (TPE) was investigated by the steady-state fluorescence of 2-dimethylamino-(6-lauroyl)-naphtalene (laurdan) and 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The parameter generalised polarisation (GP), calculated by exciting laurdan at 340 and 410 nm, revealed the gel to liquid crystalline lamellar (L alpha) as well as the L alpha to inverse hexagonal (HII) phase transitions of TPE. The L alpha to HII phase transition was not detected in TPE/N-OEA system, probably because of the formation of an intermediate Q224 cubic phase. The formation of Q224 phase in TPE/N-OEA and TPE/N-LEA systems was previously demonstrated by X-ray diffraction, but neither laurdan generalised polarisation nor TMA-DPH steady-state fluorescence anisotropy measurements revealed the presence of this phase. It is suggested that the lack of detection of the cubic phase is probably due to the similarity in dynamic characteristics and hydration levels of phospholipid headgroups in the bilayer and cubic phases.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

A comparison of membrane properties and composition between cell lines selected and transfected for multi-drug resistance

Cell lines selected (CHRC5) and transfected (LR-73-1A) for multi-drug resistance have total lipid compositions which are indistinguishable between resistant and parental cells. Lipid composition was evaluated by 1H NMR and the total fatty acid content by GLC. No change in surface hydrophobicity, as measured with the fluorescent probe dansyl-PE, was observed as a result of transfection of CHO cells with the mdr1 gene. However, the selected cell line, CHRC5, showed a decreased surface hydrophobicity. This decreased surface hydrophobicity was indicated by an 8 nm increase in the fluorescence emission of dansyl-PE in the CHRC5 cell line compared with the AB1. Both resistant cell lines showed an increase in the polarisation of the fluorescent probe, TMA-DPH in the plasma membranes corresponding to a 14% and a 24% change in fluorescence polarisation for the selected and transfected cell lines, respectively. This is indicative of reduced mobility of the acyl chains in the resistant cell lines. Both the CHRC5 and the transfected cell lines showed almost a 2-fold increase in the initial rate of membrane cycling. The membrane cycling could be inhibited by a known bilayer stabiliser, the N-carbobenzoxy-D-Phe-L-Phe-Gly. These results indicate that the properties of the plasma membrane from resistant cells are altered compared with their parental cell line.