Influence of cholesterol on the structural preferences of dioleoylphosphatidylethanolamine-dioleoylphosphatidylcholine systems: a phosphorus-31 and deuterium nuclear magnetic resonance study

Computational Insights into the Role of Cholesterol in Inverted Hexagonal Phase Stabilization and Endosomal Drug Release

Cholesterol is a major component of many lipid-based drug delivery systems, including cationic lipid nanoparticles. Despite its critical role in the drug release stage, the underlying molecular mechanism by which cholesterol assists in endosomal escape remains unclear. An efficient drug release from the endosome requires endosomal disruption. This disruption is believed to involve a lamellar-to-inverted hexagonal (L_α-H_II) phase transition upon fusion of the lipid nanoparticle with the endosomal membrane. We used molecular dynamics simulations to study the structural properties of H_II systems composed of an anionic lipid distearoyl phosphatidylserine (DSPS), an ionizable cationic lipid (KC2H), and cholesterol for several hydration levels and molar ratios. This system corresponds to the lipid mixtures in the hypothesized H_II structure formed upon fusion and is of interest for the rational design of ionizable cationic lipids, including KC2, for an optimal drug release. Simulations suggest a geometry- and symmetry-driven lipid sorting and cholesterol-DSPS co-location around the water cores. Cholesterol preferentially co-locates with negatively charged saturated DSPS lipids at interstitial angles. The observed cholesterol-DSPS co-location results in an overall increase in the DSPS acyl chains' order parameters, which we propose to assist in stabilizing the H_II phase by stretching the DSPS acyl chains for filling the voids formed by three adjacent lipid tubules. Furthermore, a systematic increase in the cholesterol concentration increased the lattice plane spacing and the water core radius but decreased the undulations along the lipid tubule axis. We propose that cholesterol and the degree of saturation/polyunsaturation of the lipid acyl chains, and not the lipid charge, are the main contributors in facilitating the L_α-H_II phase transition and stabilizing/destabilizing the formed H_II phase, whereas the positive charge of the ionizable cationic lipid promotes the LNP-endosomal membrane adhesion and assists in initiating the fusion process at the local contact area. We also propose that the effect of cholesterol on the H_II structure and curvature is the main underlying reason for the well-documented H_II stabilization and destabilization at low and high molar concentrations of cholesterol, respectively.

Raman Spectroscopic Study of Phase Coexistence in Binary Phospholipid Bilayers

Binary phospholipid bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) were studied by Raman spectroscopy and differential scanning calorimetry (DSC). We examined features in Raman scattering spectra that are sensitive to the lipid phase and, therefore, could indicate the phase coexistence. It was found that the low-frequency half-width of half-maximum (LHWHM) of the 2850 cm^-1 Raman line, corresponding to the symmetric CH₂ stretching vibrations, unequivocally reveals the coexisting phospholipids in ordered and disordered conformational states, which correspond to ordered and disordered phases coexistence, in the DPPC mole concentration range from 0.4 to 0.9. The phase coexistence in this concentration range was supported by the particular concentration behavior of the ratio between the intensities of the 2880 cm^-1 antisymmetric CH₂ vibration line and the 2850 cm^-1 symmetric one. It was also shown that the spectral shape of the 1300 cm^-1 Raman line, corresponding to the CH₂ twisting vibrations, is a good indicator for the phase state and phase coexistence in the phospholipid bilayers. Comparison with the DSC curves confirmed that in the DPPC mole concentration range from 0.4 to 0.9, the two phase transition peaks are observed in DSC curve, those positions are independent of the DPPC concentration. The outcome of the study is the robust label-free contactless approach for the detection of the lipid phase separation, which can be realized with the micrometer resolution.

How cholesterol could be drawn to the cytoplasmic leaf of the plasma membrane by phosphatidylethanolamine

In the mammalian plasma membrane, cholesterol can translocate rapidly between the exoplasmic and cytoplasmic leaves, so that its distribution between them should be given by the equality of its chemical potential in the leaves. Due to its favorable interaction with sphingomyelin, which is almost entirely in the outer leaf, one expects the great majority of cholesterol to be there also. Experimental results do not support this, implying that there is some mechanism attracting cholesterol to the inner leaf. We hypothesize that it is drawn there to reduce the bending free energy of the membrane caused by the presence of PE (phosphatidylethanolamine). It does this in two ways: first by simply diluting the amount of PE in the inner leaf, and second by ordering the tails of the PE to reduce its spontaneous curvature. Incorporating this mechanism into a model free energy for the bilayer, we find that between 50 and 60% of the total cholesterol should be in the inner leaf of human erythrocytes.

Structure and kinetics of lipid-nucleic acid complexes

The structure and function of lipid-based complexes (lipoplexes) have been widely investigated as cellular delivery vehicles for nucleic acids-DNA and siRNA. Transfection efficiency in applications such as gene therapy and gene silencing has been clearly linked to the local, nano-scale organization of the nucleic acid in the vehicle, as well as to the global properties (e.g. size) of the carriers. This review focuses on both the structure of DNA and siRNA complexes with cationic lipids, and the kinetics of structure evolution during complex formation. The local organization of the lipoplexes is largely set by thermodynamic, equilibrium forces, dominated by the lipid preferred phase. As a result, complexation of linear lambda-phage DNA, circular plasmid DNA, or siRNA with lamellae-favoring lipids (or lipid mixtures) forms multi-lamellar L(α)(C) liquid crystalline arrays. Complexes created with lipids that have bulky tail groups may form inverted hexagonal HII(C) phases, or bicontinuous cubic Q(II)(C) phases. The kinetics of complex formation dominates the large-scale, global structure and the properties of lipoplexes. Furthermore, the time-scales required for the evolution of the equilibrium structure may be much longer than expected. In general, the process may be divided into three distinct stages: An initial binding, or adsorption step, where the nucleic acid binds onto the surface of the cationic vesicles. This step is relatively rapid, occurring on time scales of order of milliseconds, and largely insensitive to system parameters. In the second step, vesicles carrying adsorbed nucleic acid aggregate to form larger complexes. This step is sensitive to the lipid characteristics, in particular the bilayer rigidity and propensity to rupture, and to the lipid to nucleic acid (L/D) charge ratio, and is characterized by time scales of order seconds. The last and final step is that of internal rearrangement, where the overall global structure remains constant while local adjustment of the nucleic acid/lipid organization takes place. This step may occur on unusually long time scales of order hours or longer. This rate, as well, is highly sensitive to lipid characteristics, including membrane fluidity and rigidity. While the three step process is consistent with many experimental observations to date, improving the performance of these non-viral vectors requires better understanding of the correlations between the parameters that influence lipoplexes' formation and stability and the specific rate constants i.e., the timescales required to obtain the equilibrium structures. Moreover, new types of cellular delivery agents are now emerging, such as antimicrobial peptide complexes with anionic lipids, and other proteins and small-molecule lipid carriers, suggesting that better understanding of lipoplex kinetics would apply to a variety of new systems in biotechnology and nanomedicine.

Design of hybrid lipid/retroviral-like particle gene delivery vectors

Recombinant retroviruses provide highly efficient gene delivery and the potential for stable gene expression. The retroviral envelope protein, however, is the source of significant disadvantages such as immunogenicity, poor stability (half-life of transduction activity of 5-7 h at 37 °C for amphotropic murine leukemia virus), and difficult production and purification. To address these problems, we report the construction of efficient hybrid vectors through the association of murine leukemia virus (MLV)-like particles (M-VLP) with synthetic liposomes comprising DOTAP, DOPE, and cholesterol (φ/M-VLP). We conclude that the lipid composition is a significant determinant of the transfection efficiency and uptake of φ/M-VLP in HEK293 cells with favorable compositions for transfections being those with low DOTAP, low DOPE, and high cholesterol content. Cellular uptake, however, was dependent on DOTAP content alone. By extrusion of liposomes prior to vector assembly, the size of these hybrid vectors could also be decreased to ≈300 nm, as confirmed via DLS and TEM. φ/M-VLP were also robust on storage in terms of vector size and transfection efficiency and provided stable transgene expression over a period of three weeks. We conclude that the noncovalent combination of biocompatible synthetic lipids with inactive retroviral particles to form a highly efficient hybrid vector is a significant extension to the development of novel gene delivery platforms.

Use of simple kinetic and reaction-order measurements for the evaluation of the mechanism of surfactant-liposome interactions

Surfactant-liposome interactions have been previously studied through different methods and techniques. We present here a classical physical chemistry study on liposome solutions added to destabilizing agents at concentrations well above the solubilization concentration, which enable us to draw useful and interesting conclusions about the mechanism of surfactant-induced liposomal breakdown by simply exploiting the kinetics and the reaction order of the liposomal content release. In such excess of surfactant, the mechanism of surfactant-induced rupture of the liposomes has been demonstrated to be different from that proposed for low surfactant concentrations. Thus, depending on the surfactant concentration, two prevailing processes have been evidenced: (i) a cooperative mechanism that implies the assembly of a critical number of surfactant molecules to trigger the formation of a channel and therefore the release of the liposomal content and (ii) a mechanism driven by direct interaction of the surfactant molecules with the lipids that causes the complete solubilization of the liposomes. The former mechanism occurs at low surfactant concentrations, whereas the latter occurs at higher concentrations and above the CMC of the surfactants. The effect of different guests embedded into the liposomal bilayer on the mechanism of surfactant-induced liposomal breakdown has been compared by using the second-order rate constants measured for the liposome breakdown process.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Quantitation of cholesterol incorporation into extruded lipid bilayers

Cholesterol incorporation into lipid bilayers, in the form of multilamellar vesicles or extruded large unilamellar vesicles, has been quantitated. To this aim, the cholesterol contents of bilayers prepared from phospholipid:cholesterol mixtures 33-75 mol% cholesterol have been measured and compared with the original mixture before lipid hydration. There is a great diversity of cases, but under most conditions the actual cholesterol proportion present in the extruded bilayers is much lower than predicted. A quantitative analysis of the vesicles is thus required before any experimental study is undertaken.

Liposome characterization by quartz crystal microbalance measurements and atomic force microscopy

This chapter reviews liposome characterization by quartz crystal microbalance (QCM) measurements and atomic force microscopy (AFM). In many studies, AFM imaging is simply used to image liposomes with resolution often that does not allow morphological analysis. Although liposome size can be obtained by processing AFM images, it is found that liposomes flatten upon surface adsorption or immobilization. Liposome stability and stiffness have been characterized by using AFM imaging or AFM force measurements, although the latter method, using a microsphere attached on the AFM cantilever, seems more appropriate to limit liposome damage and to obtain more quantitative analysis, such as the Young's modulus. Investigation of liposome layers by QCM revealed that liposomes can be detected from a combined analysis of frequency and bandwidth shifts. However, QCM by itself provides only limited information on liposomes. QCM can be used to assess the presence of a layer and also to discriminate between rigid and viscoelastic ones. Liposome properties have been derived from QCM curves, but often this requires making hypotheses that are difficult to assess. AFM and QCM analyses need to be combined with other techniques to provide complementary information.

Connexin channels and phospholipids: association and modulation

BACKGROUND

For membrane proteins, lipids provide a structural framework and means to modulate function. Paired connexin hemichannels form the intercellular channels that compose gap junction plaques while unpaired hemichannels have regulated functions in non-junctional plasma membrane. The importance of interactions between connexin channels and phospholipids is poorly understood.

RESULTS

Endogenous phospholipids most tightly associated with purified connexin26 or connexin32 hemichannels or with junctional plaques in cell membranes, those likely to have structural and/or modulatory effects, were identified by tandem electrospray ionization-mass spectrometry using class-specific interpretative methods. Phospholipids were characterized by headgroup class, charge, glycerol-alkyl chain linkage and by acyl chain length and saturation. The results indicate that specific endogenous phospholipids are uniquely associated with either connexin26 or connexin32 channels, and some phospholipids are associated with both. Functional effects of the major phospholipid classes on connexin channel activity were assessed by molecular permeability of hemichannels reconstituted into liposomes. Changes to phospholipid composition(s) of the liposome membrane altered the activity of connexin channels in a manner reflecting changes to the surface charge/potential of the membrane and, secondarily, to cholesterol content. Together, the data show that connexin26 and connexin32 channels have a preference for tight association with unique anionic phospholipids, and that these, independent of headgroup, have a positive effect on the activity of both connexin26 and connexin32 channels. Additionally, the data suggest that the likely in vivo phospholipid modulators of connexin channel structure-function that are connexin isoform-specific are found in the cytoplasmic leaflet. A modulatory role for phospholipids that promote negative curvature is also inferred.

CONCLUSION

This study is the first to identify (endogenous) phospholipids that tightly associate with connexin channels. The finding that specific phospholipids are associated with different connexin isoforms suggests connexin-specific regulatory and/or structural interactions with lipid membranes. The results are interpreted in light of connexin channel function and cell biology, as informed by current knowledge of lipid-protein interactions and membrane biophysics. The intimate involvement of distinct phospholipids with different connexins contributes to channel structure and/or function, as well as plaque integrity, and to modulation of connexin channels by lipophilic agents.

Phosphotyrosine-dependent in vitro reconstitution of recombinant LAT-nucleated multiprotein signalling complexes on liposomes

Numerous cell surface receptors propagate activation signals to the interior of the cell via tyrosine phosphorylation of transmembrane proteins. This leads to the phosphotyrosine (PiY)-mediated recruitment of cytoplasmic signalling protein complexes which catalyze crucial biochemical signalling reactions. Here we describe the first in vitro reconstitution of such PiY-nucleated protein complexes on an artificial lipid membrane. A tyrosine phosphorylated recombinant variant of the transmembrane adaptor protein Linker for Activation of T cells (PiYLAT) was anchored in liposomes. These PiYLAT proteoliposomes specifically recruited cooperative high avidity signalling protein complexes from Jurkat cytosol. Nucleation of signalling protein assemblies readily occurred on PiYLAT liposomes composed of phosphatidylserine, but not on PiYLAT liposomes composed of phosphatidylcholine. Purified recombinant grb2 alone did not stably associate with tyrosine phosphorylated LAT proteoliposomes. However, when grb2 was presented to the PiYLAT proteoliposomes in the context of Jurkat cytosol it was incorporated into multiprotein signalling complexes. Together the data suggest that these reconstituted high-avidity signalling protein complexes represent a cooperative protein network. This novel in vitro approach offers a novel technology permitting biochemical, structural, and pharmacological analyses of plasma membrane receptor signalling complexes.

The Gaussian curvature elastic energy of intermediates in membrane fusion

The Gaussian curvature elastic energy contribution to the energy of membrane fusion intermediates has usually been neglected because the Gaussian curvature elastic modulus, kappa, was unknown. It is now possible to measure kappa for phospholipids that form bicontinuous inverted cubic (Q(II)) phases. Here, it is shown that one can estimate kappa for lipids that do not form Q(II) phases by studying the phase behavior of lipid mixtures. The method is used to estimate kappa for several lipid compositions in excess water. The values of kappa are used to compute the curvature elastic energies of stalks and catenoidal fusion pores according to recent models. The Gaussian curvature elastic contribution is positive and similar in magnitude to the bending energy contribution: it increases the total curvature energy of all the fusion intermediates by 100 units of k(B)T or more. It is important to note that this contribution makes the predicted intermediate energies compatible with observed lipid phase behavior in excess water. An order-of-magnitude fusion rate equation is used to estimate whether the predicted stalk energies are consistent with the observed rates of stalk-mediated processes in pure lipid systems. The current theory predicts a stalk energy that is slightly too large, by approximately 30 k(B)T, to rationalize the observed rates of stalk-mediated processes in phosphatidylethanolamine or N-monomethylated dioleoylphosphatidylethanolamine systems. Despite this discrepancy, the results show that models of fusion intermediate energy are accurate enough to make semiquantitative predictions about how proteins mediate biomembrane fusion. The same rate model shows that for proteins to drive biomembrane fusion at observed rates, they have to perform mediating functions corresponding to a reduction in the energy of a purely lipidic stalk by several tens of k(B)T. By binding particular peptide sequences to the monolayer surface, proteins could lower fusion intermediate energies by altering the elastic constants of the patches of lipid monolayer that form the stalk. Here, it is shown that if peptide binding changes kappa or some other combinations of local elastic constants by only tens of percents, the stalk energy and the energy of catenoidal fusion pores would decrease by tens of k(B)T relative to the pure lipid value. This is comparable to the required mediating effect. The curvature energies of stalks and catenoidal fusion pores have almost the same dependence on monolayer elastic constants as the curvature energies of the rhombohedral and Q(II) phases; respectively. The effects of isolated fusion-relevant peptides on the energies of these intermediates can be determined by studying the effects of the peptides on the stability of rhombohedral and Q(II) phases.

Influence of the lamellar phase unbinding energy on the relative stability of lamellar and inverted cubic phases

Based on curvature energy considerations, nonbilayer phase-forming phospholipids in excess water should form stable bicontinuous inverted cubic (Q(II)) phases at temperatures between the lamellar (L(alpha)) and inverted hexagonal (H(II)) phase regions. However, the phosphatidylethanolamines (PEs), which are a common class of biomembrane phospholipids, typically display direct L(alpha)/H(II) phase transitions and may form intermediate Q(II) phases only after the temperature is cycled repeatedly across the L(alpha)/H(II) phase transition temperature, T(H), or when the H(II) phases are cooled from T > T(H). This raises the question of whether models of inverted phase stability, which are based on curvature energy alone, accurately predict the relative free energy of these phases. Here we demonstrate the important role of a noncurvature energy contribution, the unbinding energy of the L(alpha) phase bilayers, g(u), that serves to stabilize the L(alpha) phase relative to the nonlamellar phases. The planar L(alpha) phase bilayers must separate for a Q(II) phase to form and it turns out that the work of their unbinding can be larger than the curvature energy reduction on formation of Q(II) phase from L(alpha) at temperatures near the L(alpha)/Q(II) transition temperature (T(Q)). Using g(u) and elastic constant values typical of unsaturated PEs, we show that g(u) is sufficient to make T(Q) > T(H) for the latter lipids. Such systems would display direct L(alpha) --> H(II) transitions, and a Q(II) phase might only form as a metastable phase upon cooling of the H(II) phase. The g(u) values for methylated PEs and PE/phosphatidylcholine mixtures are significantly smaller than those for PEs and increase T(Q) by only a few degrees, consistent with observations of these systems. This influence of g(u) also rationalizes the effect of some aqueous solutes to increase the rate of Q(II) formation during temperature cycling of lipid dispersions. Finally, the results are relevant to protocols for determining the Gaussian curvature modulus, which substantially affects the energy of intermediates in membrane fusion and fission. Recently, two such methods were proposed based on measuring T(Q) and on measuring Q(II) phase unit cell dimensions, respectively. In view of the effect of g(u) on T(Q) that we describe here, the latter method, which does not depend on the value of g(u), is preferable.

Determining the ratio of the Gaussian curvature and bending elastic moduli of phospholipids from Q(II) phase unit cell dimensions

A method is presented for measuring M, the ratio of the Gaussian (saddle splay) elastic modulus to the bending elastic modulus of a lipid monolayer. The ratio M is determined from measurements of the equilibrium bicontinuous inverted cubic (Q(II)) phase unit cell size in excess water as a function of temperature. The analysis includes the effect of a curvature elastic term that is second-order in the Gaussian curvature, K. Preliminary results using data on DOPE-Me validate the method. The fitted value of M is within 8% of the value estimated in an earlier treatment. The method can be used to measure changes in M due to addition of exogenous lipids and peptides to a host lipid system. The Gaussian elastic modulus has a substantial effect on the stability of fusion intermediates (stalks, hemifusion diaphragms, and fusion pores). Studying the effects of peptides and different lipids on M via this method may yield insights into how fusion protein moieties stabilize intermediates in membrane fusion in vivo. The contribution of the K2 curvature elastic term to the free energy of Q(II) phase and fusion pores explains some features of fusion pore stability and dynamics, and some peculiar observations concerning the mechanism of L(alpha)/Q(II) phase transitions.

Phosphatidylcholine structure determines cholesterol solubility and lipid polymorphism

In the present work, we demonstrate that phosphatidylcholine with (16:1)9 acyl chains undergoes polymorphic rearrangements in mixtures with 0.6-0.8 mol fraction cholesterol. Studies were performed using differential scanning calorimetry, X-ray diffraction, cryo-electron microscopy, 31P NMR static powder patterns and 13C MAS/NMR. Mixtures of phosphatidylcholine with (16:1)9 acyl chains and 0.6 mol fraction cholesterol, after being heated to 100 degrees C, can form an ordered array with periodicity 14 nm which may be indicative of a cubic phase. Our results indicate that the formation of highly curved bilayer structures, such as those required for membrane fusion, can occur in mixtures of cholesterol with certain phosphatidylcholines that do not form non-lamellar structures in the absence of cholesterol. We also determine the polymorphic behavior of mixtures of symmetric phosphatidylcholines with cholesterol. Species of phosphatidylcholine with (20:1)11, (22:1)13 or (24:1)15 acyl chains in mixtures with 0.6-0.8 mol fraction cholesterol undergo a transition to the hexagonal phase at temperatures 70-80 degrees C. This is not the case for phosphatidylcholine with (18:1)6 acyl chains which remains in the lamellar phase up to 100 degrees C when mixed with as much as 0.8 mol fraction cholesterol. Thus, the polymorphic behavior of mixtures of phosphatidylcholine and cholesterol is not uncommon and is dependent on the intrinsic curvature of the phospholipid. Crystals of cholesterol can be detected in mixtures of all of these phosphatidylcholines at sufficiently high cholesterol mole fraction. What is unusual about the formation of these crystals in several cases is that cholesterol crystals are present in the monohydrate form in preference to the anhydrous form. Furthermore, after heating to 100 degrees C and recooling, the cholesterol crystals are again observed to be in the monohydrate form, although pure cholesterol crystals require many hours to rehydrate after being heated to 100 degrees C. Both the nature of the acyl chain as well as the mole fraction cholesterol determine whether cholesterol crystals in mixtures with the phospholipids will be in the monohydrate or in the anhydrous form.

Cardanol as a replacement for cholesterol into the lipid bilayer of POPC liposomes

Large unilamellar liposomes were prepared by hydration of 1-palmitoyl-2-oleylphosphatydilcholine (POPC) films and subsequent extrusion of the obtained liposomal suspension. Inclusion of cholesterol and cardanol brings about a stabilization of the membranes of the liposomes, as determined by their rates of release of entrapped 5(6)-carboxyfluorescein. The liposome breakdown was promoted by a non-ionic surfactant (Triton X-100) and the kinetic measurements were carried out by fluorimetry in water at 25 degrees C. Morphological analyses of giant POPC liposomes in the presence and in the absence of both guests were also performed. The results obtained suggest the use of cardanol (an easy available natural product) as a replacement for cholesterol as a new possibility for stabilizing liposomes in drug targetting.

Orientation and pore-forming mechanism of a scorpion pore-forming peptide bound to magnetically oriented lipid bilayers

The orientation and pore-forming mechanisms of pandinin 2 (pin2), an antimicrobial peptide isolated from venom of the African scorpion Pandinus imperator, bound to magnetically oriented lipid bilayers were examined by 31P and 13C solid-state, and 15N liquid-state NMR spectroscopy. 31P NMR measurements at various temperatures, under neutral and acidic conditions, showed that membrane lysis occurred only under acidic conditions, and at temperatures below the liquid crystal-gel phase transition of the lipid bilayers, after incubation for two days in the magnet. Differential scanning calorimetry measurements showed that pin2 induced negative curvature strain in lipid bilayers. The 13C chemical shift values of synthetic pin2 labeled at Gly3, Gly8, Leu12, Phe17, or Ser18 under static or slow magic-angle spinning conditions, indicate that pin2 penetrates the membrane with its average helical axis perpendicular to the membrane surface. Furthermore, amide H-D exchange experiments of 15N-Ala4, Gly8, and Ala9 triply-labeled pin2 suggest that this peptide forms oligomers and confirms that the N-terminal region creates membrane pores.

The gaussian curvature elastic modulus of N-monomethylated dioleoylphosphatidylethanolamine: relevance to membrane fusion and lipid phase behavior

The energy of intermediates in fusion of phospholipid bilayers is sensitive to kappa(m), the saddle splay (Gaussian curvature) elastic modulus of the lipid monolayers. The value kappa(m) is also important in understanding the stability of inverted cubic (Q(II)) and rhombohedral (R) phases relative to the lamellar (L(alpha)) and inverted hexagonal (H(II)) phases in phospholipids. However, kappa(m) cannot be measured directly. It was previously measured by observing changes in Q(II) phase lattice dimensions as a function of water content. Here we use observations of the phase behavior of N-mono-methylated dioleoylphosphatidylethanolamine (DOPE-Me) to determine kappa(m). At the temperature of the L(alpha)/Q(II) phase transition, T(Q), the partial energies of the two phases are equal, and we can express kappa(m) in terms of known lipid monolayer parameters: the spontaneous curvature of DOPE-Me, the monolayer bending modulus kappa(m), and the distance of the monolayer neutral surface from the bilayer midplane, delta. The calculated ratio kappa(m)/kappa(m) is -0.83 +/- 0.08 at T(Q) approximately 55 degrees C. The uncertainty is due primarily to uncertainty in the value of delta for the L(alpha) phase. This value of kappa(m)/kappa(m) is in accord with theoretical expectations, including recent estimates of the value required to rationalize observations of rhombohedral (R) phase stability in phospholipids. The value kappa(m) substantially affects the free energy of formation of fusion intermediates: more energy (tens of k(B)T) is required to form stalks and fusion pores (ILAs) than estimated solely on the basis of the bending elastic energy. In particular, ILAs are much higher in energy than previously estimated. This rationalizes the action of fusion-catalyzing proteins in stabilizing nascent fusion pores in biomembranes; a function inferred from recent experiments in viral systems. These results change predictions of earlier work on ILA and Q(II) phase stability and L(alpha)/Q(II) phase transition mechanisms. To our knowledge, this is the first determination of the saddle splay (Gaussian) modulus in a lipid system consisting only of phospholipids.

The effect of fructan on membrane lipid organization and dynamics in the dry state

Fructans are a group of fructose-based oligo- and polysaccharides, which appear to be involved in membrane preservation during dehydration by interacting with the membrane lipids. To get further understanding of the protective mechanism, the consequences of the fructan-membrane lipid interaction for the molecular organization and dynamics in the dry state were studied. POPC and DMPC were investigated in the dry state by (2)H, (31)P NMR, and Fourier transform infrared spectroscopy using two types of fructan and dextran. The order-disorder transition temperature of dry POPC was reduced by 70 degrees C in the presence of fructan. Fructan increased the mobility of the acyl chains, but immobilized the lipid headgroup region. Most likely, fructans insert between the headgroups of lipids, thereby spacing the acyl chains. This results in a much lower phase transition temperature. The headgroup is immobilized by the interaction with fructan. The location of the interaction with the lipid headgroup is different for the inulin-type fructan compared to the levan-type fructan, since inulin shows interaction with the lipid phosphate group, whereas levan does not. Dextran did not influence the phase transition temperature of dry POPC showing that reduction of this temperature is not a general property of polysaccharides.

Triggered release of doxorubicin following mixing of cationic and anionic liposomes

In many applications, an ability of liposomes to retain drug and then rapidly release it at some later time would be of benefit. In this work, we investigate the ability of cationic large unilamellar vesicles (LUV) to promote rapid release of doxorubicin from anionic LUV. It is shown that the addition of cationic liposomes containing cholesterol, dioleoylphosphatidylethanolamine (DOPE), distearoylphosphatidylcholine (DSPC) and the cationic lipid N,N-dioleyl-N,N-dimethylammonium chloride (DODAC) to doxorubicin-containing LUV composed of cholesterol, DOPE, DSPC and the anionic lipid dioleoyphosphatidylglycerol (DOPG) can result in release of more than 90% of the drug in times of 30 s or less. Further, it is shown that these release characteristics are exquisitely dependent on the presence of DOPE and cholesterol. In the absence of DOPE, much slower release rates are observed, with maximum release levels of 50% after a 2-h incubation at 20 degrees C. Remarkably, threshold levels of more than 10 mol% cholesterol are required before any appreciable release is observed. [31P]NMR spectroscopy and freeze-fracture electron microscopy studies reveal that systems giving rise to rapid release of doxorubicin exhibit limited formation of inverted hexagonal (H(II)) phase, suggesting that these lipids facilitate drug release by formation of local regions of non-bilayer structure. It is concluded that drug release triggered by mixing anionic and cationic liposomes could be of utility in drug delivery applications.

Sphingomyelin and cholesterol promote HIV-1 gp41 pretransmembrane sequence surface aggregation and membrane restructuring

The interfacial sequence DKWASLWNWFNITNWLWYIK, preceding the transmembrane anchor of gp41 glycoprotein subunit, has been shown to be essential for fusion activity and incorporation into virions. HIV(c), a peptide representing this region, formed lytic pores in liposomes composed of the main lipids occurring in the human immunodeficiency virus, type 1 (HIV-1), envelope, i.e. 1-palmitoyl-2-oleoylphosphatidylcholine (POPC):sphingomyelin (SPM):cholesterol (Chol) (1:1:1 mole ratio), at low (>1:10,000) peptide-to-lipid mole ratio, and promoted the mixing of vesicular lipids at >1:1000 peptide-to-lipid mole ratios. Inclusion of SPM or Chol in POPC membranes had different effects. Whereas SPM sustained pore formation, Chol promoted fusion activity. Even if partitioning into membranes was not affected in the absence of both SPM and Chol, HIV(c) had virtually no effect on POPC vesicles. Conditions described to disturb occurrence of lateral separation of phases in these systems reproduced the high peptide-dose requirements for leakage as found in pure POPC vesicles and inhibited fusion. Surface aggregation assays using rhodamine-labeled peptides demonstrated that SPM and Chol promoted HIV(c) self-aggregation in membranes. Employing head-group fluorescent phospholipid analogs in planar supported lipid layers, we were able to discern HIV(c) clusters associated to ordered domains. Our results support the notion that the pretransmembrane sequence may participate in the clustering of gp41 monomers within the HIV-1 envelope, and in bilayer architecture destabilization at the loci of fusion.

Phase behavior, DNA ordering, and size instability of cationic lipoplexes. Relevance to optimal transfection activity

Mechanisms of cationic lipid-based nucleic acid delivery are receiving increasing attention, but despite this the factors that determine high or low activity of lipoplexes are poorly understood. This study is focused on the fine structure of cationic lipid-DNA complexes (lipoplexes) and its relevance to transfection efficiency. Monocationic (N-(1-(2,3-dioleoyloxy)propyl),N,N,N-trimethylammonium chloride, N-(1-(2,3-dimyristyloxypropyl)-N,N-dimethyl-(2-hydroxyethyl)ammonium bromide) and polycationic (2,3-dioleyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propanammonium trifluoroacetate) lipid-based assemblies, with or without neutral lipid (1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine, 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine, cholesterol) were used to prepare lipoplexes of different L(+)/DNA(-) charge ratios. Circular dichroism, cryogenic-transmission electron microscopy, and static light scattering were used for lipoplex characterization, whereas expression of human growth hormone or green fluorescent protein was used to quantify transfection efficiency. All monocationic lipids in the presence of inverted hexagonal phase-promoting helper lipids (1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine, cholesterol) induced appearance of Psi(-) DNA, a chiral tertiary DNA structure. The formation of Psi(-) DNA was also dependent on cationic lipid-DNA charge ratio. On the other hand, monocationic lipids either alone or with 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine as helper lipid, or polycationic 2,3-dioleyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propanammonium trifluoroacetate-based assemblies, neither of which promotes a lipid-DNA hexagonal phase, did not induce the formation of Psi(-) DNA. Parallel transfection studies reveal that the size and phase instability of the lipoplexes, and not the formation of Psi(-) DNA structure, correlate with optimal transfection.

On the mechanism whereby cationic lipids promote intracellular delivery of polynucleic acids

The mechanism whereby cationic lipids destabilize cell membranes to facilitate the intracellular delivery of macromolecules such as plasmid DNA or antisense oligonucleotides is not well understood. Here, we show that cationic lipids can destabilize lipid bilayers by promoting the formation of nonbilayer lipid structures. In particular, we show that mixtures of cationic lipids and anionic phospholipids preferentially adopt the inverted hexagonal (H(II)) phase. Further, the presence of 'helper' lipids such as dioleoylphosphatidylethanolamine or cholesterol, lipids that enhance cationic lipid-mediated transfection of cells also facilitate the formation of the H(II)phase. It is suggested that the ability of cationic lipids to promote nonbilayer structures in combination with anionic phospholipids leads to disruption of the endosomal membrane following uptake of nucleic acid-cationic lipid complexes into cells, thus facilitating cytoplasmic release of the plasmid or oligonucleotide.

Cholesterol stabilizes hemifused phospholipid bilayer vesicles

Cholesterol was found to inhibit full fusion of oppositely charged phospholipid bilayer vesicles by stabilizing the contacting membranes at the stage of the hemifused intermediate. Vesicles of opposite charge containing different amounts of cholesterol were prepared using cationic (1,2-dioleoyl-sn-glycero-3-ethylphosphocholine) and anionic (dioleoylphosphatidylglycerol) phospholipids. Pairwise interactions between such vesicles were observed by fluorescence video microscopy in real time after electrophoretically maneuvering the vesicles into contact. Hemifusion accounted for more than 80% of the observed events when the vesicles contained 33-50 mole% cholesterol. In contrast, vesicles containing only a small proportion of cholesterol (</=10 mole%), underwent full fusion in approx. 70% of the interactions monitored. The role of cholesterol is explained both as favoring the formation of the hemifused intermediate according to the adhesion-condensation mechanism of bilayer fusion and as disfavoring the transition from hemifusion to full fusion on the basis of reduced tension in the vesicle bilayers.

Lipid phase separation correlates with activation in platelets during chilling

When human platelets are chilled below 22 degrees C, they spontaneously activate, a phenomenon that severely limits their storage life. It has previously been proposed that there is a correlation between cold-induced platelet activation and passage of the membranes through a liquid-crystalline to gel phase transition. Because animal models are essential for developing methods for cold storage of platelets, it is necessary to investigate such a correlation in animal platelets. In this work, horse platelets were used as a model, and it was found that cold-induced morphological activation is related to the lipid phase transition. Using fluorescence microscopy with the lipophilic fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (Dil-C18), and Fourier transform infrared spectroscopy (FTIR), it was found that lipid phase separation occurs during cooling and low temperature storage. Furthermore, removal of cholesterol from the plasma membrane also induced a phase separation, possibly between specific phospholipid classes. Steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and trimethylammonium-DPH (TMA-DPH) were compared in cells and multilamellar vesicles (MLV) composed of platelet lipids. Cholesterol depletion led to a decrease in the fluorescence anisotropy of the two probes, which can be explained by changes in the order of the phospholipid molecules. In addition, the lipid composition and fatty acid profile of the cellular phospholipids were determined. Based of the similarities between horse and human platelets, it is suggested that horse platelets may be used as a model for studying cold-stored platelets. The results are discussed in relation to the possible role of phase separation during cell signalling.

Interactions of peptides with liposomes: pore formation and fusion

Leakage from liposomes induced by several peptides is reviewed and a pore model is described. According to this model peptide molecules become incorporated into the vesicle bilayer and aggregate reversibly or irreversibly within the surface. When a peptide aggregate reaches a critical size, peptide translocation can occur and a pore is formed. With the peptide GALA the pores are stable and persist for at least 10 minutes. The model predicts that for a given lipid/peptide ratio, the extent of leakage should decrease as the vesicle diameter decreases, and for a given amount of peptide bound per vesicle less leakage would be observed at higher temperatures due to the increase in reversibility of surface aggregates of the peptide. Effect of membrane composition on pore formation is reviewed. When cholesterol was included in the liposomes the efficiency of inducation of leakage by the peptide GALA was reduced due to reduced binding and increased reversibility of surface aggregation of the peptide. Phospholipids which contain less ordered acyl-chains and have a slightly wedge-like shape, can better accommodate peptide surface aggregates, and consequently insertion and translocation of the peptide may be less favored. Demonstrations of antagonism between pore formation and fusion are presented. The choice of factors which promote vesicle aggregation, e.g., larger peptides, increased vesicle and peptide concentration results in enhanced vesicle fusion at the expense of formation of intravesicular pores. FTIR studies with HIV-1 fusion peptides indicate that in systems where extensive vesicle fusion occurred the beta conformation of the peptides was predominant, whereas the alpha conformation was exhibited in cases where leakage was the main outcome. Antagonism between leakage and fusion was exhibited by 1-palmitoyl-2-oleoylphosphatidylglycerol vesicles, where the order of addition of peptide (HIV(arg)) or Ca(2+)dictated whether pore formation or vesicle fusion would occur. The current study emphasizes that the addition of Ca(2+), which promotes vesicle aggregation can also reduce peptide translocation in isolated vesicles.

The events relating to lanthanide ions enhanced permeability of human erythrocyte membrane: binding, conformational change, phase transition, perforation and ion transport

The binding and uptake of Gd3+ ions by human erythrocytes in vitro were studied by determining the Gd contents in membrane and in cytosol by means of particle-induced X-ray emission (PIXE) spectrometry. Results obtained from varied incubation time revealed that the Gd3+ ions bind to the membrane proteins and lipids at first. Gd3+ binding to the membrane lipids and proteins lasts 0 approximately 20 and 20 approximately 100 ms respectively, as shown by the stopped-flow studies. Then a fraction of Gd3+ ions diffuses through the membrane. The kinetics of Gd3+ binding indicates that the binding to phospholipids is prior to that to the membrane proteins, but a portion of the lipid-bound Gd3+ redistributed later to the proteins. PIXE studies showed that the entry of Gd3+ increased the influx of Ca2+ and Cl-. By monitoring the changes in fluorescence of proteins and that of the Ln3+, the uptake of La3+, Eu3+, Gd3+ and Tb3+ was shown to be a process comprising a series of events. Binding to the membrane molecules induces the phase transition of lipid bilayer and conformational changes and aggregation of membrane proteins. Conformational changes of the proteins were characterized by Fourier transform IR spectroscopy (FT-IR) deconvolved spectra, i.e. alpha-helix content decreases while beta-sheet increases. ESR spectra of MSL-labeled proteins reflect the aggregation state related with the conformational change. [31P]NMR spectra of membrane lipid bilayer revealed the Ln3+ ions induced hexagonal (H(II)) phase formation. Phase transition and aggregation of membrane proteins cause the formation of domain structure and perforation in the membrane. These alterations in membrane structure are responsible for the Ln3+ enhanced membrane permeability. Thus the previous Ln3+ binding will facilitate the across-membrane transport of other Ln3+ ions through the membrane.

Nisin promotes the formation of non-lamellar inverted phases in unsaturated phosphatidylethanolamines

Nisin, a peptide used as a food preservative, is shown, by 31P-nuclear magnetic resonance and infrared spectroscopy, to perturb the structure of membranes formed of unsaturated phosphatidylethanolamine (PE) and to induce the formation of inverted non-lamellar phases. In the case of dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), nisin promotes the formation of inverted hexagonal phase. Similarly, the peptide induces the formation of an isotropic phase, most likely a cubic phase, with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). It is proposed that the insertion of the peptide in the bilayer shifts the amphiphilic balance by increasing the hydrophobic contribution and is at the origin of the changes in the polymorphic propensities of PE. This is supported by the fact that the presence of cholesterol in the PE bilayer inhibits the power of nisin to perturb the membrane structure, most likely because the peptide insertion is difficult in the fluid ordered phase. This finding provides insight into possible antibacterial mechanisms of nisin.

Transacylase formation of bis(monoacylglycerol)phosphate

Recent work within our laboratory has focused on the enzymes we hypothesize are involved in the biosynthesis of bis(monoacylglycerol)phosphate from phosphatidylglycerol. Here we describe a transacylase, active at acidic pH values, isolated from a macrophage-like cell line, RAW 264.7. This enzyme acylates the head group glycerol of sn-3:sn-1' lysophosphatidylglycerol to form sn-3:sn-1' bis(monoacylglycerol)phosphate. Here we demonstrate that this enzyme uses two lysophosphatidylglycerol molecules, one as an acyl donor and another as an acyl acceptor, and that the acyl contributions from all other lipids tested are comparatively minor. This enzyme prefers saturated acyl chains to monounsaturates, 16 and 18 carbon fatty acids over 14 carbon fatty acids, and saturated acyl chains at the sn-1 position to monounsaturated acyl chains on the sn-2 carbon of lysophosphatidylglycerol. We present data which show the transacylase activity depends on the presence of a lipid-water interface and the lipid polymorphic state.

Calorimetric and spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylethanolamine bilayers

Aqueous dispersions of cholesterol-containing phosphatidylethanolamine (PE) bilayers were examined by a combination of high-sensitivity differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and 31P-nuclear magnetic resonance spectroscopy. Regardless of hydrocarbon chain length, the incorporation of low levels of cholesterol into these bilayers causes progressive reductions in the temperature, enthalpy and overall cooperativity of the lipid hydrocarbon chain-melting phase transition. Moreover, at low cholesterol levels, the heating and cooling thermograms observed for the cholesterol/PE binary mixtures are similar, indicating comparable levels of lateral miscibility of cholesterol with PE bilayers in the gel and liquid-crystalline states. However, at higher levels of cholesterol incorporation, marked differences between the heating and cooling thermograms are noted. Upon heating, complex multicomponent thermograms are observed in PE bilayers containing large amounts of cholesterol, and the temperature and overall enthalpy values increase discontinuously from the pattern of monotonic decrease observed at lower cholesterol levels. Moreover, these discontinuities begin to emerge at progressively lower cholesterol concentrations as PE hydrocarbon chain length increases. Upon cooling, a simpler pattern of thermotropic behavior is observed, and the measured temperature and enthalpy values continue to decrease monotonically with increases in cholesterol content. These results suggest that at higher concentrations cholesterol exhibits a decreased degree of lateral miscibility in the gel or crystalline as compared to the liquid-crystalline states of PE bilayers, particularly in the case of the longer-chain PEs. Our FTIR and 31P-nuclear magnetic resonance spectroscopic studies also show that the thermotropic events observed with mixtures of low cholesterol content are analogous to the gel/liquid-crystalline phase transitions exhibited by the pure PEs. However, lamellar crystalline phases readily form when mixtures of high cholesterol content are cooled to low temperatures. Moreover, these crystalline phases are spectroscopically indistinguishable from those formed by the pure PEs, indicating that cholesterol is excluded from such phases. Upon subsequent heating, the melting of these crystalline phases gives rise to the complex thermograms detected by DSC and to the discontinuities in the phase transition temperature and enthalpy noted above. This pattern of behavior differs markedly from that observed with the corresponding phosphatidylcholines (PCs), where comparable degrees of cholesterol miscibility are observed in the gel and liquid-crystalline states even at high cholesterol concentrations, and where cholesterol inhibits rather than facilitates the formation of lamellar crystalline phases. We also find that the presence of cholesterol does not result in the hydrophobic mismatch-dependent shifts in the phase transition temperature in PE bilayers previously observed in PC bilayers of varying thickness. We attribute these differences in the effects of cholesterol on phospholipid thermotropic phase behavior to stronger electrostatic and hydrogen bonding interactions at the surfaces of PE and compared to PC bilayers.

Resonance energy transfer study of hemoglobin and cytochrome c complexes with lipids

The complexes of hemoglobin and cytochrome c with liposomes composed of phosphatidylcholine and its mixtures with cardiolipin and cholesterol have been studied by monitoring resonance energy transfer between fluorescent probe 3-methoxybenzanthrone as donor and heme groups of the proteins as acceptors. By analyzing experimental data within the framework of the model of energy transfer in two-dimensional systems, the limits of the range of possible heme positions with respect to lipid bilayer have been assessed. The distance of heme group of hemoglobin from the membrane center was found to increase in the presence of cardiolipin or cholesterol. The results obtained for cytochrome c complexes with cardiolipin-containing model membranes suggest the existence of preferential protein orientation relative to the lipid bilayer, and provide evidence for the protein penetration in the membrane interior.

Tracking phospholipid populations in polymorphism by sideband analyses of 31P magic angle spinning NMR

A method was developed to track the distributional preferences of phospholipids in polymorphism based on sideband analyses of the 31P magic angle spinning nuclear magnetic resonance spectra. The method was applied to lipid mixtures containing phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradecane, as well as mixtures containing the anionic phosphatidylmethanol, phosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (Lalpha) and inverted hexagonal (HII) phases remained constant throughout the Lalpha --> HII transition in all mixtures, except those that contained saturated PtdCho and unsaturated PtdEtn in the presence of cholesterol-mixtures that are known to be microimmiscible because of favored associations between Chol and saturated acyl chains. In the latter mixture, saturated PtdCho was enriched in the planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved HII structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity among coexisting phases was observed, it appeared that preexisting lateral microheterogeneities led to compositionally distinct transitional clusters, such that the distributional preferences that resulted were not those of the individual phospholipids.

Change of motion and localization of cholesterol molecule during L(alpha)-H(II) transition

Formation of the inverted hexagonal (H(II)) phase from the lamellar (L(alpha)) phase of bovine brain-extracted phosphatidylcholine (BBPC) and phosphatidylethanolamine (BBPE) was investigated using 31P-NMR with or without cholesterol. When the ratio of BBPC to BBPE was 1:1, the H(II) formation was observed in the presence of 33 mol% cholesterol (i.e., BBPC:BBPE:cholesterol = 1:1:1) at 47 degrees C. The fraction of the H(II) phase in the BBPC/BBPE/cholesterol system could be controlled by the addition of dioleoylglycerol. The change of molecular motion of cholesterol affected by the H(II) formation was measured at various ratios of the L(alpha) to H(II) phase with the time-resolved fluorescence depolarization method, using dehydroergosterol as a fluorescent probe. It is observed that the motion of cholesterol became vigorous in the mixture state of the L(alpha) and the H(II) phases compared to that in the L(alpha) or the H(II) phase only. These facts show that cholesterol has the strong ability to induce the H(II) phase, probably by special molecular motion, which includes change of its location from the headgroup area to the acyl-chain area.

Novel cationic liposomes for DNA-transfection with high efficiency and low toxicity

Liposomes containing the natural cationic amphiphile, sphingosine and some of its derivatives were used for transfection of DNA in vitro. Multilamellar liposomes comprised of dioleoylphosphatidylethanolamine (DOPE), different sphingosine derivatives, and diacylglycerols with varying fatty acid chains, preincubated with DNA, transfected efficiently the KK-1 murine granulosa cells. Most efficient transfection on this cell line was achieved with liposomes composed of phytosphingosine, DOPE, and dioctanoylglycerol (DC8G) (64:31:4.8, molar stoichiometry), which gave expression of the transfected gene 2-10-fold higher than the commercial reagent Lipofectin. At higher doses the new liposomes also caused markedly less cell death of KK-1 cells. On COS-7 cells these liposomes showed slightly, but significantly lower transfection, of approximately 70%, of that gained with Lipofectin. The murine Sertoli cells, MSC-1, selectively resisted transfection by the sphingosine derivative based liposomes tested, giving only 11-14% of the expression detected in Lipofectin transfected cells of the same line. In conclusion, the novel liposomes formulated offer an effective, technically easy and economical method of transfection for a variety of cultured cell lines.

The influence of cold acclimation on the lipid composition and cryobehaviour of the plasma membrane of isolated rye protoplasts

Destabilization of the plasma membrane, which is a primary cause of freezing injury, is a consequence of freeze-induced osmotic stresses and cell dehydration. However, the mechanism of injury depends on the magnitude of the osmotic stress and the extent of cell dehydration. Over the range of 0 to — 5 °C, destabilization of the plasma membrane in protoplasts isolated from non-acclimated rye leaves is a result of osmotic excursions, because freeze-induced osmotic contraction results in endocytotic vesiculation of the plasma membrane and sufficiently large area reductions are irreversible. At lower temperatures, the protoplasts are subjected to extremely large osmotic pressures ( — 12 MPa at —10 °C), and there are several changes in the ultrastructure of the plasma membrane, including the formation of aparticulate domains and lamellar- to hexagonaln-phase transitions. These changes, which are manifestations of demixing of the membrane components, are predicted by a theory of bilayer interactions at low levels of hydration. During cold acclimation, the cryobehaviour of the plasma membrane is altered; osmotic contraction results in the reversible formation of exocytotic extrusions and the propensity for dehydration-induced demixing and lamellar- to hexagonaln-phase transitions is decreased. In both cases, the differential behaviour is also observed in liposomes prepared from plasma membrane lipids isolated from non-acclimated and cold-acclimated leaves. However, as no lipid species are unique to the plasma membrane of either non-acclimated or cold-acclimated leaves, the differential behaviour is caused by altered lipid-lipid interactions because of different proportions of the lipid species. Hence the behaviour of the plasma membrane can be altered by using a protoplast-liposome fusion procedure to selectively modify the lipid composition of the plasma membrane. These studies provide direct evidence that the increased cryostability of the plasma membrane is a consequence of alterations in its lipid composition.

Effect of unsaturation on the chain order of phosphatidylcholines in a dioleoylphosphatidylethanolamine matrix

The properties of phosphatidylcholines (PCs) having a perdeuterated stearic acid, 18:0d35, in the sn-1 position and the fatty acid 18:0, 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 at the sn-2 position were investigated in a matrix of dioleoylphosphatidylethanolamine (DOPE) by 2H and 31P NMR spectroscopy. At a mole ratio of DOPE/PC = 5:1, the lipids form liquid crystalline lamellar phases below 40 degrees C and coexisting lamellar, inverse hexagonal (Hll), and cubic phases at higher temperatures. The sn-1 chain of the PCs in a DOPE matrix is appreciably more ordered than in pure PCs, corresponding to an increase in the hydrophobic bilayer thickness of approximately 1 A. Distearoylphosphatidylcholine in the DOPE matrix has a higher sn-1 chain order than the unsaturated PCs. We observed distinct differences in the lipid order of upper and lower sections of the hydrocarbon chains caused by changes of temperature, unsaturation, headgroups, and ethanol. Unsaturation lowers chain order, mostly in the lower third of the hydrocarbon chains. By contrast, the increase in chain order caused by the DOPE matrix and the decrease in order with increasing temperature have a constant magnitude for the upper two-thirds of the chain and are smaller for the lower third. Addition of 2 M ethanol reduced order parameters, in effect reversing the increase in chain order caused by the DOPE matrix.

Effects of cholesterol on the lamellar and the inverted hexagonal phases of dielaidoylphosphatidylethanolamine

Effects of cholesterol on the lamellar and the inverted hexagonal (HII ) phases of dielaidoylphosphatidylethanolamine (DEPE) were studied by means of not only differential scanning calorimetry (DSC) but also simultaneous X-ray diffraction and DSC (XDDSC). XDDSC shows that structural changes are related to thermotropic events of the mixtures. Addition of cholesterol to DEPE induces to broaden the transition from the lamellar gel (L beta) to lamellar liquid-crystalline (L alpha) phase. In fact, in the broad transition region, a coexistence of two lamellar X-ray diffraction peaks of the L beta and L alpha phases take place. In samples containing above 30 mol% cholesterol, no peak at the L beta-L alpha phase transition was observed in the DSC thermogram. On the other hand, cholesterol causes biphasic effects on the L alpha-HII phase transition: At low cholesterol concentrations below 20 mol%, the incorporation of cholesterol reduces the transition temperature and at high cholesterol concentrations about 30 mol%, the transition temperature increases by addition of cholesterol. Based upon the results of X-ray diffraction, the thermal expansion coefficients of lattice spacings, i.e., the temperature dependence of lattice spacings, were calculated in each phase. Addition of cholesterol reduces the thermal expansion coefficients of the lamellar phases and, in contrast, increases that of the HII phase. From the above results it is suggested that cholesterol in cell membranes works in keeping the bilayer membrane nature notwithstanding the change of external conditions.