Topological properties of two cubic phases of a phospholipid:cholesterol:diacylglycerol aqueous system and their possible implications in the phospholipase C-induced liposome fusion

Phospholipase C hydrolysis of phospholipids in bilayers of mixed lipid compositions

Phosphatidylcholine phospholipase C (EC 3.1.4.3) from Bacillus cereus has been assayed with substrates in the form of large unilamellar vesicles. Phosphatidylcholine, phosphatidylethanolamine (also a substrate for the enzyme), sphingomyelin, and cholesterol have been mixed in various proportions, in binary, ternary, and quaternary mixtures. A lag period, followed by a burst of enzyme activity, has been found in all cases. The activity burst was always accompanied by an increase in turbidity of the vesicle suspension. Varying lipid compositions while keeping constant all the other parameters leads to a range of lag times extending over 2 orders of magnitude (from 0.13 to 38.0 min), and a similar variability is found in maximal enzyme rates (from 0.40 to 55.9 min-1). Meanwhile, the proportion of substrate that is hydrolyzed during the lag period remains relatively constant at 0.10% moles of total lipid, in agreement with the idea that enzyme activation is linked to vesicle aggregation through diacylglycerol-rich patches. Phosphatidylethanolamine and cholesterol enhance the enzyme activity in a dose-dependent way: they reduce the lag times and increase the maximal rates. The opposite is true of sphingomyelin. These lipids exert each its own peculiar effect, positive or negative, either alone or in combination, so that the susceptibility of a given mixture to the enzyme activity can be to some extent predicted from its composition. Phospholipase C activity is not directly influenced by the formation of nonlamellar structures. However, the presence of lipids with a tendency to form nonlamellar phases, such as phosphatidylethanolamine or cholesterol, stimulates the enzyme even under conditions at which purely lamellar phases exist. Conversely sphingomyelin, a well-known stabilizer of the lamellar phase, inhibits the enzyme. Thus phospholipase C appears to be regulated by the overall geometry and composition of the bilayer.

Self-evolving microstructured systems upon enzymatic catalysis

The consequences of cell microstructuration on enzyme functions is discussed in the framework of self-evolving microstructured systems. Molecular assemblies of amphiphiles or lipids are spontaneously formed by self-organisation. Among these different structures, reversed micelles, liquid crystalline mesophases and vesicles are hosts for enzymatic reaction studies. Inside a living cell, phospholipid metabolism is responsible for membrane structural modifications; the catalytic behaviour of lipolytic enzymes, mainly phospholipase (PL) A2, is described in relation with structural aspects of biological membranes. The implication in cellular regulation events of PLC and PLD is discussed in relation with the role of their reaction products as second messengers in membrane fusion processes. The in vitro synthesis of dialkyl phosphatidylcholines, via the enzymatic 'salvage pathway' which leads to the formation of vesicles upon phospholipid formation, is considered in relation with autopoiesis. More recent studies on self-evolving systems based on enzyme-surfactants reactions are detailed. The interactions between amphiphilic aggregates and enzymes allow to explore the OG/octanol/water phase diagram. Enzymatic formation of dipalmitoylphosphatidylcholine (DPPC) liposomes and non-ionic surfactant vesicles (NSV), starting from mixed micelles or open structures, finally sets an example of a biomimetic self-evolving system.

Synergistic effects of diacylglycerols and fatty acids on membrane structure and protein kinase C activity

The synergistic effects of diacylglycerol (DAG) and fatty acid (FA) in activating protein kinase C have been investigated by correlating their individual and combined effects on enzymatic activity and on membrane structure in phosphatidylcholine/phosphatidylserine (4:1) lipid mixtures using a combination of specific enzymatic assays and 31P and 2H NMR. Addition of DAGs and unsaturated FAs to the bilayers synergistically increased the tendency of the lipids to form nonbilayer phases with a concomitant increase in PKC activity until a maximum was achieved. Further increases in the DAG/FA concentration led to the formation of the nonbilayer lipid phases under the conditions of the PKC activity assays and correlated with decreased activity. The nonbilayer lipid phases still supported PKC activity, although with less than 50% efficiency as compared with the bilayer lipids. Long-chain saturated FA increased DAG-induced PKC activity by causing a lateral phase separation of gel (Lbeta) and liquid-crystalline (Lalpha) domains. Due to the preferential partitioning of DAGs into liquid-crystalline domains, the local DAG concentration increased in these domains, leading to an increase in PKC activity. Because a wide range of lipophilic compounds is capable of altering curvature stress, and therefore the tendency for nonbilayer phase formation in cellular membranes, these compounds would be expected to modulate PKC activity and the activities of a number of other membrane-associated enzymes that are sensitive to biophysical properties of lipid membranes.

Modulation of lipid polymorphism by the feline leukemia virus fusion peptide: implications for the fusion mechanism

The structural effects of the fusion peptide of feline leukemia virus (FeLV) on lipid polymorphism were studied, using differential scanning calorimetry (DSC), 31P nuclear magnetic resonance (NMR), and time-resolved X-ray diffraction. This peptide lowers the bilayer to inverted hexagonal phase transition temperature, TH, of dipalmitoleoylphosphatidylethanolamine (DiPoPE) at peptide mole fractions of up to 1.5 x 10(-3) at pH 5.0 and at pH 7.4. The temperature at which isotropic 31P NMR signals for monomethyldioleoylphosphatidylethanolamine (MeDOPE) first occurred is lowered by the FeLV peptide. The amount of isotropic signal seen at 40 degrees C is directly correlated to the peptide:lipid molar ratio. In the peptide-containing samples, more lipid remains in the isotropic state over the whole recorded temperature range. Isotropic 31P NMR signals were observed for DiPoPE in the presence of the FeLV peptide for the entire recorded temperature range of 35-50 degrees C, while pure DiPoPE showed no significant amount of isotropic signal. X-ray studies of DiPoPE show the formation of a new lipid phase with peptide, which is not seen in the pure lipid samples. Disordering of the Lalpha phase is evidenced by broadening of the diffraction peaks, and the hexagonal cell parameter is decreased with peptide present. Our results suggest that the FeLV peptide is increasing the negative curvature of the lipid system, which is thought to be crucial to the formation of highly bent, high-energy structural fusion intermediates, such as the "stalk" model. Fusion activity for this putative fusogenic peptide was also demonstrated, using a resonance energy transfer (RET) lipid mixing assay. To our knowledge, this work provides the first published experimental evidence of both fusogenic activity and effects on lipid polymorphism for the FeLV fusion peptide.

Effect of single chain lipids on phospholipase C-promoted vesicle fusion. A test for the stalk hypothesis of membrane fusion

The effect of low proportions (up to 5 mol %) of single-chain lipids on phospholipase C-promoted fusion of large unilamellar vesicles has been investigated with the aim of testing the so-called stalk model of membrane fusion. This model is known in two main versions, the one originally published by Kozlov and Markin [Kozlov, M. M. and Markin, V. S. (1983) Biofizika 28, 255-261] and what is known as the "modified stalk model" [Siegel, D. P. (1993) Biophys. J. 65, 2124-2140], that differ in a number of predictions. In the view of the latter author, hydrocarbons or other nonpolar lipids should help fusion by decreasing the interstitial energy of the stalk connecting the two apposed bilayers. We show that small amounts of hexadecane or squalene increase significantly the fusion rates in our system. Changes in monolayer curvature are the object of different predictions by the original and modified stalk theories. According to the original form, fusion would be promoted by lipids inducing a negative curvature in the closest (cis) monolayers of the fusing membranes and inhibited by the same lipids in the trans monolayers; the opposite would happen with lipids inducing a positive curvature. The modified stalk model predicts that fusion is helped by increasing the negative curvature of both monolayers. In our system, symmetrically distributed arachidonic acid, which increases the negative curvature, enhances lipid and content mixing, and the opposite is found with symmetrically distributed lysophosphatidylcholine or palmitoylcarnitine, which facilitate a positive monolayer curvature. In addition, fluorescence polarization and 31P NMR studies of the lamellar-to-isotropic (Q224 cubic) thermotropic transition of a lipid mixture corresponding to our liposomal composition reveal that all lipids that facilitate fusion decrease the transition temperature, while fusion inhibitors increase the transition temperature. Moreover, fusion (content mixing) rates show a maximum at the lamellar-to-isotropic transition temperature. These observations support the involvement of inverted lipid structures, as occurring in the inverted cubic phases, in membrane fusion. All these data are in full agreement with the stalk model of membrane fusion, particularly in its modified version.

The mechanism of lamellar-to-inverted hexagonal phase transitions in phosphatidylethanolamine: implications for membrane fusion mechanisms

We studied the mechanism of the lamellar-to-inverted hexagonal (L alpha/H[II]) phase transition, using time-resolved cryotransmission electron microscopy (TRC-TEM), 31P-NMR, and differential scanning calorimetry. The transition was initiated in dispersions of large unilamellar vesicles of dipalmitoleoyl phosphatidylethanolamine (DiPoPE). We present evidence that the transition proceeds in three steps. First, many small connections form between apposed membranes. Second, the connections aggregate within the planes of the bilayers, forming arrays with hexagonal order in some projections. Third, these quasihexagonal structures elongate into small domains of H(II) phase, acquiring lipid molecules by diffusion from contiguous bilayers. A previously proposed membrane fusion mechanism rationalizes these results. The modified stalk theory predicts that the L alpha/H(II) phase transition involves some of the same intermediate structures as membrane fusion. The small interbilayer connections observed via TRC-TEM are compatible with the structure of a critical intermediate in the modified stalk mechanism: the trans monolayer contact (TMC). The theory predicts that 1) TMCs should form starting at tens of degrees below TH; 2) when TMCs become sufficiently numerous, they should aggregate into transient arrays like the quasihexagonal arrays observed here by TRC-TEM; and 3) these quasihexagonal arrays can then elongate directly into H(II) phase domains. These predictions rationalize the principal features of our data, which are incompatible with the other transition mechanisms proposed to date. Thus these results support the modified stalk mechanism for both membrane fusion and the L alpha/H(II) phase transition. We also discuss some implications of the modified stalk theory for fusion in protein-containing systems. Specifically, we point out that recent data on the effects of hydrophobic peptides and viral fusion peptides on lipid phase behavior are consistent with an effect of the peptides on TMC stability.

Poly(ethylene glycol)-lipid conjugates inhibit phospholipase C-induced lipid hydrolysis, liposome aggregation and fusion through independent mechanisms

Poly(ethylene glycol)-phosphatidylethanolamine (PEG-PE) conjugates have been introduced in liposomal compositions. The resulting large unilamellar vesicles were subjected to the action of phospholipase C. Enzyme-promoted vesicle aggregation and fusion were assayed in liposomes containing various proportions of PEG-PE. At PEG-PE concentrations above 1 mol% the rate of phospholipid hydrolysis decreases, perhaps because the PEG moiety hinders the enzyme from reaching the membrane surface. At concentrations above 0.1 mol% vesicle aggregation occurs at a slower rate, presumably because of the repulsive barrier properties or surface-grafted PEG. Lipid mixing decreases in parallel with vesicle aggregation. Finally, liposomal fusion rates measured as mixing of vesicle aqueous contents are decreased at or even below 0.1 mol%. The latter inhibition is due, apart from the reduced rates of lipid hydrolysis, vesicle aggregation and lipid mixing, to a PEG-PE-based stabilization of the lipid bilayer structure. Thus the observed low rates of contents mixing arise from three combined and independent inhibitory effects of PEG-PE.

Structural study of the relationship between the rate of membrane fusion and the ability of the fusion peptide of influenza virus to perturb bilayers

The amino-terminal segment of the HA2 protein of influenza virus (fusion peptide) has been identified as an important region for membrane fusion. The wild type virus can fuse to membranes more rapidly at pH 5 than at pH 7.4. It has been demonstrated that there is a relationship between the ability of the peptide to promote the formation of inverted phases and the fusogenicity of the intact virus. In this work, we use small-angle X-ray diffraction to study the mechanism of the structural effect of the peptide, at different pHs, on lipid systems characterized by each having a different spontaneous radius of curvature. The overall results show that the action of the peptide on the polymorphism of the lipid systems investigated is strongly pH-dependent. In particular, a rapid formation of cubic phases at pH 5.0 is observed in the presence of this fusion peptide. The ability of the fusion peptide to promote cubic phases exhibits the same dependence on the pH as does the fusogenicity of the intact virus. It is proposed that the peptide promotes cubic phases at pH 5.0 by changing the kinetics of the lamellar to inverted phase transitions.

Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles: a cryo-transmission electron microscopy study of liposome fusion

Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the "honeycomb structure") that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure ("stalk") would act as a structural intermediate in membrane fusion. Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.

Origin of the lag period in the phospholipase C cleavage of phospholipids in membranes. Concomitant vesicle aggregation and enzyme activation

When phospholipase C is added to a suspension of large unilamellar vesicles of egg phosphatidylcholine, maximal rates of hydrolysis occur only after a latency period. No lag period is seen when the substrate is in the form of small (sonicated) vesicles, or of short-chain phosphatidylcholine monomers. For a given vesicle concentration, the lag time may vary as a function of Ca2+, enzyme concentration, or temperature, but activation occurs at a fixed molar fraction of diacylglycerol produced. Lag times decrease gradually with vesicle size, and also with the amount of diacylglycerol present in the bilayers when it is mixed with phospholipid prior to enzyme addition. Parallel recordings of enzyme activity and suspension turbidity reveal that in all cases the latency period ends concomitantly with the start of a process of vesicle aggregation. Both the lag time and the amount of diacylglycerol formed before activation decrease with vesicle concentration, suggesting that enzyme activation is somehow related to vesicle aggregation. The latency period of phospholipase C may be explained in terms of a hypothesis according to which (a) full enzyme activity requires the presence of membrane surface irregularities or defects, (b) the diacylglycerol generated in the lag phase produces some kind of phase separation, with the formation of diacylglycerol-rich "patches" or domains, (c) vesicles aggregate through contacts between those patches, and (d) aggregation causes (and/or increases, and/or stabilizes) the surface inhomogeneities that allow fast enzyme activity. These data and suggestions may be relevant to the process of model membrane fusion promoted by phospholipase C.

Measured effects of diacylglycerol on structural and elastic properties of phospholipid membranes

Diacylglycerol, a biological membrane second messenger, is a strong perturber of phospholipid planar bilayers. It converts multibilayers to the reverse hexagonal phase (HII), composed of highly curved monolayers. We have used x-ray diffraction and osmotic stress of the HII phase to measure structural dimensions, spontaneous curvature, and bending moduli of dioleoylphosphatidylethanolamine (DOPE) monolayers doped with increasing amounts of dioleoylglycerol (DOG). The diameter of the HII phase cylinders equilibrated in excess water decreases significantly with increasing DOG content. Remarkably, however, all structural dimensions at any specific water/lipid ratio that is less than full hydration are insensitive to DOG. By plotting structural parameters of the HII phase with changing water content in a newly defined coordinate system, we show that the elastic deformation of the lipid monolayers can be described as bending around a pivotal plane of constant area. This dividing surface includes 30% of the lipid volume independent of the DOG content (polar heads and a small fraction of hydrocarbon chains). As the mole fraction of DOG increases to 0.3, the radius of spontaneous curvature defined for the pivotal surface decreases from 29 A to 19 A, and the bending modulus increases from approximately 11 to 14 (+/-0.5) kT. We derive the conversion factors and estimate the spontaneous curvatures and bending moduli for the neutral surface which, unlike the pivotal plane parameters, are intrinsic properties that apply to other deformations and geometries. The spontaneous curvature of the neutral surface differs from that of the pivotal plane by less than 10%, but the difference in the bending moduli is up to 40%. Our estimate shows that the neutral surface bending modulus is approximately 9kT and practically does not depend on the DOG content.

Different effects of enzyme-generated ceramides and diacylglycerols in phospholipid membrane fusion and leakage

When large unilamellar vesicles consisting of sphingomyelin:phosphatidylethanolamine:cholesterol (2:1:1 molar ratio) are treated with sphingomyelinase, production of ceramides in the bilayer is accompanied by leakage of vesicle aqueous contents and by vesicle aggregation in the absence of lipid mixing or vesicle fusion. This is in contrast to the situation of phosphatidylcholine:phosphatidylethanolamine:cholesterol (2:1:1 molar ratio) liposomes when treated with phospholipase C. In that case, in situ generation of diacylglycerol leads to vesicle aggregation followed by vesicle fusion in the absence of leakage (Nieva, J. L., Goñi, F. M., and Alonso, A. (1989) Biochemistry 28, 7364-7367). Moreover, when ceramides (5-10 mol %) are included in the formulation of the phosphatidylcholine-containing vesicles, they reduce the lag time of phospholipase C-induced fusion, although they are less active than diacylglycerols in this respect. 31P NMR studies of aqueous lipid dispersions show that diacylglycerols as well as ceramides induce a thermotropic lamellar to non-lamellar phase transition in both phospholipid:cholesterol mixtures under study although sphingomyelin-containing bilayers are more stable than those containing phosphatidylcholine, and ceramide is less active than diacylglycerol in promoting non-lamellar phase formation. These observations are relevant to both the physiological role of ceramides and the current views on the mechanism of membrane fusion.

Correlation between bilayer lipid dynamics and activity of the diglucosyldiacylglycerol synthase from Acholeplasma laidlawii membranes

In the single membrane of Acholeplasma laidlawii a specific glucosyltransferase synthesize the major, lamellar-forming lipid diglucosyldiacylglycerol (DGlcDAG) from the major, nonlamellar-prone monoglucosyldiacylglycerol (MGlcDAG). This is crucial for the maintenance of phase equilibria close to a bilayer-nonbilayer transition and a nearly constant spontaneous curvature in the membrane lipid bilayer. Acyl chain order is also affected, but not kept constant. Phosphatidylglycerol (PG) is an essential activator, needed in substantial amounts by the DGlcDAG synthase, and likely to affect bilayer properties. A potential connection was investigated between the (i) lateral diffusion, (ii) domain formation of the PG activator and (iii) bilayer chain ordering (i.e., the hydrocarbon free volume), revealed in unilamellar liposomes by lipid probes containing one or two (fluorescent) pyrene acyl chains, and (iv) activity of the DGlcDAG synthase. Different activator, nonbilayer perturbant, and bilayer matrix conditions were employed. Diffusion of PG was substantially slower in a DGlcDAG compared to a phosphatidylcholine (PC) matrix with 18:1c chains but increased with the PG content in both. No obvious correlation between diffusion and enzyme activity, and no local concentration of PG as a function of chain ordering or curvature, was detected. However, an enrichment of PG activator into domains could be induced by a chain length mismatch between 18:1c-PG and 14:1c-PC (but not 22:1c-PC), even at small PG fractions. Patching was sufficient to stimulate enzyme activity 4-fold in relation to the activities normally valid at low PG concentrations. Chain order was substantially lower (i.e., free volumes larger) in bilayers of DGlcDAG than in bilayers of PC and increased in an additive fashion in both by the content of especially the nonbilayer-prone 1,3-18:1c-DAG but also by PG. At physiological concentrations of PG in DGlcDAG bilayers (approximately 20%) a good correlation was evident between increased DAG content and chain ordering and strongly enhanced enzyme activities, with maxima close to a bilayer-nonbilayer transition. It is concluded that regulation of packing conditions in A. laidlawii membranes by the DGlcDAG synthase seems to be governed not by the absolute extent of chain order but more by the spontaneous curvature within a certain range of conditions. Domain formation of the essential PG activator due to bilayer conditions is a second mechanism, potentially overriding the curvature effects.

Inhibition by gangliosides of Bacillus cereus phospholipase C activity against monolayers, micelles and bilayer vesicles

The effect of complex glycosphingolipids (gangliosides) on the activity of phospholipase C from Bacillus cereus was studied using lipid monolayers, mixed micelles and small unilamellar vesicles containing phosphatidylcholine as substrate. In all artificial membrane systems assayed, gangliosides exhibit qualitatively similar inhibitory properties. Gangliosides decrease the enzyme activity irrespective of the aggregation structure in which the substrate is offered to B. cereus phospholipase C, and they do not affect the adsorption process of the enzyme. The modulatory effect of gangliosides occurs at the level of the interface, affecting both the maximum rate of catalysis of the enzyme already adsorbed and the availability of the substrate in a suitable organization for enzyme catalysis to take place.

Dual inhibitory effect of gangliosides on phospholipase C-promoted fusion of lipidic vesicles

The effect of a variety of gangliosides has been tested on the phospholipase C-induced fusion of large unilamellar vesicles. Bilayer composition was phosphatidylcholine:phosphatidylethanolamine: cholesterol (2:1:1 mole ratio) plus the appropriate amounts of glycosphingolipids. Enzyme phosphohydrolase activity, vesicle aggregation, mixing of bilayer lipids and mixing of liposomal aqueous contents were separately assayed. Small amounts ( < 1 mol %) of gangliosides in the lipid bilayer produce a significant inhibition of the above processes. The inhibitory effect of gangliosides increases with the size of the oligosaccharide chain in the polar head group. Inhibition depends in a nonlinear manner on the ganglioside proportion, and is complete at approximately 5 mol %. Inhibition is not due to ganglioside-dependent changes in vesicle curvature or size. Ganglioside inhibition of vesicle fusion is due to two different effects: inhibition of phospholipase C activity and stabilization of the lipid lamellar phase. Enzyme inhibition leads to a parallel decrease of vesicle aggregation and lipid mixing rates. Mixing of aqueous contents, though, is depressed beyond the enzyme inhibition levels. This is explained in terms of the fusion pore requiring a local destabilization of the lipid bilayer, the lamellar structure being stabilized by gangliosides. 31P-NMR and DSC experiments confirm the inhibitory effect of gangliosides in various lamellar-to-nonlamellar transitions.

Diacylglycerol and the promotion of lamellar-hexagonal and lamellar-isotropic phase transitions in lipids: implications for membrane fusion

Changes in steady-state fluorescence anisotropy of 1 -(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene TMA-DPH) are applied to the detection of lamellar-hexagonal transitions in egg phosphatidylethanolamine. Even low (2 mole%) proportions of diacylglycerol decrease the hexagonal transition temperature considerably, as confirmed by differential scanning calorimetry. Diacylglycerol is also found to promote a lamellar to "isotropic" (Q(224) cubic) transition in mixtures of phosphatidylcholine: phosphatidylethanolamine:cholesterol. This nonreversible transition is also observed by (31)P nuclear magnetic resonance and detected as a large increase in TMA-DPH steady-state anisotropy. The same technique reveals as well that lysophosphatidylcholine counteracts the effect of diacylglycerol and stabilizes the lamellar phase in both transitions. Diacylglycerol and lysophosphatidylcholine are known to respectively promote and inhibit membrane fusion in a variety of systems. These data are interpreted in support of the hypothesis of a highly bent structural fusion intermediate ("stalk"). They also show the interest of lipid-phase studies in predicting and rationalizing membrane fusion mechanisms.