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

Exploring Interfacial Hydrolysis of Artificial Neutral Lipid Monolayer and Bilayer Catalyzed by Phospholipase C

Phospholipase C (PLC) represents an important type of enzymes with the feature of hydrolyzing phospholipids at the position of the glycerophosphate bond, among which PLC extracted from Bacillus cereus (BC-PLC) has been extensively studied owing to its similarity to hitherto poorly characterized mammalian analogues. This study focuses on investigating the interfacial hydrolysis mechanism of phosphatidylcholine (PC) monolayer and bilayer membranes catalyzed by BC-PLC using sum frequency generation vibrational spectroscopy (SFG-VS) and laser scanning confocal microscopy (LSCM). We found that, upon interfacial hydrolysis, BC-PLC was adsorbed onto the lipid interface and catalyzed the lipolysis with no net orientation, as evidenced by the silent amide I band, indicating that ordered PLC alignment was not a prerequisite for the enzyme activity, which is very different from what we have reported for phospholipase A₁ (PLA₁) and phospholipase A₂ (PLA₂) [Kai, S. Phys. Chem. Chem. Phys. 2018, 20(1), 63-67; Wang, F. Langmuir 2019, 35(39), 12831-12838; Zhang, F. Langmuir 2020, 36(11), 2946-2953]. For the PC monolayer, one of the two hydrolysates, phosphocholine, desorbed from the interface into the aqueous phase, while the other one, diacylglycerol (DG), stayed well packed with high order at the interface. For the PC bilayer, phosphocholine dispersed into the aqueous phase too, similar to the monolayer case; however, DG, presumably formed clusters with the unreacted lipid substrates and desorbed from the interface. With respect to both the monolayer and bilayer cases, mechanistic schematics were presented to illustrate the different interfacial hydrolysis processes. Therefore, this model experimental study in vitro provides significant molecular-level insights and contributes necessary knowledge to reveal the lipolysis kinetics with respect to PLC and lipid membranes with monolayer and bilayer structures.

Interplay of Fusion, Leakage, and Electrostatic Lipid Clustering: Membrane Perturbations by a Hydrophobic Antimicrobial Polycation

Membrane active compounds are able to induce various types of membrane perturbations. Natural or biomimetic candidates for antimicrobial treatment or drug delivery scenarios are mostly designed and tested for their ability to induce membrane permeabilization, also termed leakage. Furthermore, the interaction of these usually cationic amphiphiles with negatively charged vesicles often causes colloidal instability leading to vesicle aggregation or/and vesicle fusion. We show the interplay of these modes of membrane perturbation in mixed phosphatidyl glycerol (PG)/phosphatidyl ethanolamine (PE) by the statistical copolymer MM:CO comprising, both, charged and hydrophobic subunits. MM:CO is a representative of partially hydrophobic, highly active, but less selective antimicrobial polycations. Cryo-electron microscopy indicates vesicle fusion rather than vesicle aggregation upon the addition of MM:CO to negatively charged PG/PE (1:1) vesicles. In a combination of fluorescence-based leakage and fusion assays, there is support for membrane permeabilization and pronounced vesicle fusion activity as distinct effects. To this end, membrane fusion and aggregation were prevented by including lipids with polyethylene glycol attached to their head groups (PEG-lipids). The leakage activity of MM:CO is very similar in the absence and presence of PEG-lipids. Vesicle aggregation and fusion however are largely suppressed. This strongly suggests that MM:CO induces leakage by asymmetric packing stress because of hydrophobically driven interactions which could lead to leakage. As a further membrane perturbation effect, MM:CO causes lipid clustering in model vesicles. We address potential artifacts and misinterpretations of experiments characterizing leakage and fusion. Additional to the leakage activity, the pronounced fusogenic activity of the polymer and potentially of many other similar compounds likely has implications for antimicrobial activity and beyond.

Molecular and mesoscopic geometries in autophagosome generation. A review

Autophagy is an essential process in cell self-repair and survival. The centre of the autophagic event is the generation of the so-called autophagosome (AP), a vesicle surrounded by a double membrane (two bilayers). The AP delivers its cargo to a lysosome, for degradation and re-use of the hydrolysis products as new building blocks. AP formation is a very complex event, requiring dozens of specific proteins, and involving numerous instances of membrane biogenesis and architecture, including membrane fusion and fission. Many stages of AP generation can be rationalised in terms of curvature, both the molecular geometry of lipids interpreted in terms of 'intrinsic curvature', and the overall mesoscopic curvature of the whole membrane, as observed with microscopy techniques. The present contribution intends to bring together the worlds of biophysics and cell biology of autophagy, in the hope that the resulting cross-pollination will generate abundant fruit.

Structural Transformation in Vesicles upon Hydrolysis of Phosphatidylethanolamine and Phosphatidylcholine with Phospholipase C

This study provides insights into dynamic nanostructural changes in phospholipid systems during hydrolysis with phospholipase C, the fate of the hydrolysis products, and the kinetics of lipolysis. The effect of lipid restructuring of the vesicle was investigated using small-angle X-ray scattering and cryogenic scanning electron microscopy. The rate and extent of phospholipid hydrolysis were quantified using nuclear magnetic resonance. Hydrolysis of two phospholipids, phosphatidylethanolamine (PE) and phosphatidylcholine (PC), results in the cleavage of the molecular headgroup, causing two strikingly different changes in lipid self-assembly. The diacylglycerol product of PC escapes the lipid bilayer, whereas the diacylglycerol product adopts a different configuration within the lipid bilayer of the PE vesicles. These results are then discussed concerning the change of the lipid configuration upon the lipid membrane and its potential implications in vivo, which is of significant importance for the detailed understanding of the fate of lipidic particles and the rational design of enzyme-responsive lipid-based drug delivery systems.

Membrane curvature allosterically regulates the phosphatidylinositol cycle, controlling its rate and acyl-chain composition of its lipid intermediates

Signaling events at membranes are often mediated by membrane lipid composition or membrane physical properties. These membrane properties could act either by favoring the membrane binding of downstream effectors or by modulating their activity. Several proteins can sense/generate membrane physical curvature (i.e. shape). However, the modulation of the activity of enzymes by a membrane's shape has not yet been reported. Here, using a cell-free assay with purified diacylglycerol kinase ϵ (DGKϵ) and liposomes, we studied the activity and acyl-chain specificity of an enzyme of the phosphatidylinositol (PI) cycle, DGKϵ. By systematically varying the model membrane lipid composition and physical properties, we found that DGKϵ has low activity and lacks acyl-chain specificity in locally flat membranes, regardless of the lipid composition. On the other hand, these enzyme properties were greatly enhanced in membrane structures with a negative Gaussian curvature. We also found that this is not a consequence of preferential binding of the enzyme to those structures, but rather is due to a curvature-mediated allosteric regulation of DGKϵ activity and acyl-chain specificity. Moreover, in a fine-tuned interplay between the enzyme and the membrane, DGKϵ favored the formation of structures with greater Gaussian curvature. DGKϵ does not bear a regulatory domain, and these findings reveal the importance of membrane curvature in regulating DGKϵ activity and acyl-chain specificity. Hence, this study highlights that a hierarchic coupling of membrane physical property and lipid composition synergistically regulates membrane signaling events. We propose that this regulatory mechanism of membrane-associated enzyme activity is likely more common than is currently appreciated.

Human ATG3 binding to lipid bilayers: role of lipid geometry, and electric charge

Specific protein-lipid interactions lead to a gradual recruitment of AuTophaGy-related (ATG) proteins to the nascent membrane during autophagosome (AP) formation. ATG3, a key protein in the movement of LC3 towards the isolation membrane, has been proposed to facilitate LC3/GABARAP lipidation in highly curved membranes. In this work we have performed a biophysical study of human ATG3 interaction with membranes containing phosphatidylethanolamine, phosphatidylcholine and anionic phospholipids. We have found that ATG3 interacts more strongly with negatively-charged phospholipid vesicles or nanotubes than with electrically neutral model membranes, cone-shaped anionic phospholipids (cardiolipin and phosphatidic acid) being particularly active in promoting binding. Moreover, an increase in membrane curvature facilitates ATG3 recruitment to membranes although addition of anionic lipid molecules makes the curvature factor relatively less important. The predicted N-terminus amphipathic α-helix of ATG3 would be responsible for membrane curvature detection, the positive residues Lys 9 and 11 being essential in the recognition of phospholipid negative moieties. We have also observed membrane aggregation induced by ATG3 in vitro, which could point to a more complex function of this protein in AP biogenesis. Moreover, in vitro GABARAP lipidation assays suggest that ATG3-membrane interaction could facilitate the lipidation of ATG8 homologues.

Interaction of phospholipase C with liposome: A conformation transition of the enzyme is critical and specific to liposome composition for burst hydrolysis and fusion in concert

Phospholipase C (PLC)¹ is known to help the pathogen B. cereus entry to the host cell and human PLC is over expressed in multiple cancers. Knowledge of dynamic activity of the enzyme PLC while in action on membrane lipids is essential and helpful to drug design and delivery. In view of this, interactions of PLC with liposome of various lipid compositions have been visualized by testing enzyme activity and microenvironments around the intrinsic fluorophores of the enzyme. Overall change of the protein's conformation has been monitored by fluorescence spectroscopy and circular dichroism (CD). Liposome aggregation and fusion were predicted by increase in turbidity and vesicle size. PLC in solution has high fluorescence and exhibit appreciable shift in its emission maxima, upon gradual change in excitation wavelength towards the red edge of the absorption band. REES fluorescence studies indicated that certain Trp fluorophores of inactive PLC are in motionally restricted compact/rigid environments in solution conformation. PLC fluorescence decreased in association with liposome and Trps loosed rigidity where liposome aggregation and fusion occurred. We argue that the structural flexibility is the cause of decrease of fluorescence, mostly to gain optimum conformation for maximum activity of the enzyme PLC. Further studies deciphered that the enzyme PLC undergoes change of conformation when mixed to LUVs prepared with specific lipids. CD data at the far-UV and near-UV regions of PLC in solution are in excellent agreement with the previous reports. CD analyses of PLC with LUVs, showed significant reduction of α-helices, increase of β-sheets; and confirmed dramatic change of orientations of Trps. In case of liposome composed of lipid raft like composition, the enzyme binds very fast, hydrolyze PC with higher rate, exhibit highest structural flexibility and promote vesicle fusion. These data strongly suggest marked differences in conformation transition induced PLC activation and liposome fusion on the lipid composition.

Lipids regulate the hydrolysis of membrane bound glucosylceramide by lysosomal β-glucocerebrosidase

Glucosylceramide (GlcCer) is the primary storage lipid in the lysosomes of Gaucher patients and a secondary one in Niemann-Pick disease types A, B, and C. The regulatory roles of lipids on the hydrolysis of membrane bound GlcCer by lysosomal β-glucocerebrosidase (GBA1) was probed using a detergent-free liposomal assay. The degradation rarely occurs at uncharged liposomal surfaces in the absence of saposin (Sap) C. However, anionic lipids stimulate GlcCer hydrolysis at low pH by up to 1,000-fold depending on the nature and position of the negative charges in their head groups while cationic lipids inhibit the degradation, thus showing the importance of electrostatic interactions between the polycationic GBA1 and the negatively charged vesicle surfaces at low pH. Ceramide, fatty acids, monoacylglycerol, and diacylglycerol also stimulate GlcCer hydrolysis while SM, sphingosine, and sphinganine play strong inhibitory roles, thereby explaining the secondary storage of GlcCer in Niemann-Pick diseases. Surprisingly, cholesterol stimulates GlcCer degradation in the presence of bis(monoacylglycero)phosphate (BMP). Sap C strongly stimulates GlcCer hydrolysis even in the absence of BMP and the regulatory roles of the intraendolysosomal lipids on its activity is discussed. Our data suggest that these strong modifiers of GlcCer hydrolysis affect the genotype-phenotype correlation in several cases of Gaucher patients independent of the types.

Decoding P4-ATPase substrate interactions

Cellular membranes display a diversity of functions that are conferred by the unique composition and organization of their proteins and lipids. One important aspect of lipid organization is the asymmetric distribution of phospholipids (PLs) across the plasma membrane. The unequal distribution of key PLs between the cytofacial and exofacial leaflets of the bilayer creates physical surface tension that can be used to bend the membrane; and like Ca2+, a chemical gradient that can be used to transduce biochemical signals. PL flippases in the type IV P-type ATPase (P4-ATPase) family are the principle transporters used to set and repair this PL gradient and the asymmetric organization of these membranes are encoded by the substrate specificity of these enzymes. Thus, understanding the mechanisms of P4-ATPase substrate specificity will help reveal their role in membrane organization and cell biology. Further, decoding the structural determinants of substrate specificity provides investigators the opportunity to mutationally tune this specificity to explore the role of particular PL substrates in P4-ATPase cellular functions. This work reviews the role of P4-ATPases in membrane biology, presents our current understanding of P4-ATPase substrate specificity, and discusses how these fundamental aspects of P4-ATPase enzymology may be used to enhance our knowledge of cellular membrane biology.

Comparative study of the inverse versus normal bicontinuous cubic phases of the β-d-glucopyranoside water-driven self-assemblies using fluorescent probes

This work investigates the head group region of the inverse and normal bicontinuous cubic phases (Ia3d space group) of the glucopyranoside/water system using 2-(2′-hydroxyphenyl)benzoxazole and its derivatives as fluorescent probes.

Thermally-induced aggregation and fusion of protein-free lipid vesicles

Membrane fusion is an important phenomenon in cell biology and pathology. This phenomenon can be modeled using vesicles of defined size and lipid composition. Up to now fusion models typically required the use of chemical (polyethyleneglycol, cations) or enzymatic catalysts (phospholipases). We present here a model of lipid vesicle fusion induced by heat. Large unilamellar vesicles consisting of a phospholipid (dioleoylphosphatidylcholine), cholesterol and diacylglycerol in a 43:57:3 mol ratio were employed. In this simple system, fusion was the result of thermal fluctuations, above 60 °C. A similar system containing phospholipid and cholesterol but no diacylglycerol was observed to aggregate at and above 60 °C, in the absence of fusion. Vesicle fusion occurred under our experimental conditions only when (31)P NMR and cryo-transmission electron microscopy of the lipid mixtures used in vesicle preparation showed non-lamellar lipid phase formation (hexagonal and cubic). Non-lamellar structures are probably the result of lipid reassembly of the products of individual fusion events, or of fusion intermediates. A temperature-triggered mechanism of lipid reassembly might have occurred at various stages of protocellular evolution.

Membrane permeabilization induced by sphingosine: effect of negatively charged lipids

Sphingosine [(2S, 3R, 4E)-2-amino-4-octadecen-1, 3-diol] is the most common sphingoid long chain base in sphingolipids. It is the precursor of important cell signaling molecules, such as ceramides. In the last decade it has been shown to act itself as a potent metabolic signaling molecule, by activating a number of protein kinases. Moreover, sphingosine has been found to permeabilize phospholipid bilayers, giving rise to vesicle leakage. The present contribution intends to analyze the mechanism by which this bioactive lipid induces vesicle contents release, and the effect of negatively charged bilayers in the release process. Fluorescence lifetime measurements and confocal fluorescence microscopy have been applied to observe the mechanism of sphingosine efflux from large and giant unilamellar vesicles; a graded-release efflux has been detected. Additionally, stopped-flow measurements have shown that the rate of vesicle permeabilization increases with sphingosine concentration. Because at the physiological pH sphingosine has a net positive charge, its interaction with negatively charged phospholipids (e.g., bilayers containing phosphatidic acid together with sphingomyelins, phosphatidylethanolamine, and cholesterol) gives rise to a release of vesicular contents, faster than with electrically neutral bilayers. Furthermore, phosphorous 31-NMR and x-ray data show the capacity of sphingosine to facilitate the formation of nonbilayer (cubic phase) intermediates in negatively charged membranes. The data might explain the pathogenesis of Niemann-Pick type C1 disease.

The basic structure and dynamics of cell membranes: an update of the Singer-Nicolson model

The fluid mosaic model of Singer and Nicolson (1972) is a commonly used representation of the cell membrane structure and dynamics. However a number of features, the result of four decades of research, must be incorporated to obtain a valid, contemporary version of the model. Among the novel aspects to be considered are: (i) the high density of proteins in the bilayer, that makes the bilayer a molecularly "crowded" space, with important physiological consequences; (ii) the proteins that bind the membranes on a temporary basis, thus establishing a continuum between the purely soluble proteins, never in contact with membranes, and those who cannot exist unless bilayer-bound; (iii) the progress in our knowledge of lipid phases, the putative presence of non-lamellar intermediates in membranes, and the role of membrane curvature and its relation to lipid geometry, (iv) the existence of lateral heterogeneity (domain formation) in cell membranes, including the transient microdomains known as rafts, and (v) the possibility of transient and localized transbilayer (flip-flop) lipid motion. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Imaging the early stages of phospholipase C/sphingomyelinase activity on vesicles containing coexisting ordered-disordered and gel-fluid domains

The binding and early stages of activity of a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa on giant unilamellar vesicles (GUV) have been monitored using fluorescence confocal microscopy. Both the lipids and the enzyme were labeled with specific fluorescent markers. GUV consisted of a mixture of phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, and cholesterol in equimolar ratios, to which 5-10 mol% of the enzyme end-product ceramide and/or diacylglycerol were occasionally added. Morphological examination of the GUV in the presence of enzyme reveals that, although the enzyme diffuses rapidly throughout the observation chamber, detectable enzyme binding appears to be a slow, random process, with new bound-enzyme-containing vesicles appearing for several minutes. Enzyme binding to the vesicles appears to be a cooperative process. After the initial cluster of bound enzyme is detected, further binding and catalytic activity follow rapidly. After the activity has started, the enzyme is not released by repeated washing, suggesting a "scooting" mechanism for the hydrolytic activity. The enzyme preferentially binds the more disordered domains, and, in most cases, the catalytic activity causes the disordering of the other domains. Simultaneously, peanut- or figure-eight-shaped vesicles containing two separate lipid domains become spherical. At a further stage of lipid hydrolysis, lipid aggregates are formed and vesicles disintegrate.

End-products diacylglycerol and ceramide modulate membrane fusion induced by a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa

A phospholipase C/sphingomyelinase from Pseudomonas aeruginosa has been assayed on vesicles containing phosphatidylcholine, sphingomyelin, phosphatidylethanolamine and cholesterol at equimolar ratios. The enzyme activity modifies the bilayer chemical composition giving rise to diacylglycerol (DAG) and ceramide (Cer). Assays of enzyme activity, enzyme-induced aggregation and fusion have been performed. Ultrastructural evidence of vesicle fusion at various stages of the process is presented, based on cryo-EM observations. The two enzyme lipidic end-products, DAG and Cer, have opposite effects on the bilayer physical properties; the former abolishes lateral phase separation, while the latter generates a new gel phase [Sot et al., FEBS Lett. 582, 3230-3236 (2008)]. Addition of either DAG, or Cer, or both to the liposome mixture causes an increase in enzyme binding to the bilayers and a decrease in lag time of hydrolysis. These two lipids also have different effects on the enzyme activity, DAG enhancing enzyme-induced vesicle aggregation and fusion, Cer inhibiting the hydrolytic activity. These effects are explained in terms of the different physical properties of the two lipids. DAG increases bilayers fluidity and decreases lateral separation of lipids, thus increasing enzyme activity and substrate accessibility to the enzyme. Cer has the opposite effect mainly because of its tendency to sequester sphingomyelin, an enzyme substrate, into rigid domains, presumably less accessible to the enzyme.

Phosphatidylinositol metabolism and membrane fusion

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

Self-assembly properties of alkyloxyethyl beta-glycosides with different types of carbohydrate headgroups

The effect of alkyl chain length on micelle formation in aqueous solutions of synthetic alkyloxyethyl glycosides containing an ethyl spacer with different conformations of the disaccharide headgroups was investigated. The molecular shape was systematically changed from a wedge-shaped to a rodlike geometry by changing the type of carbohydrate headgroup. The lipophilic part consists of dodecyl or tetradecyl chains. The adsorption at the liquid-air interface was investigated by surface tension measurements. The micellar phase region (L1) was studied using small-angle neutron scattering. We have observed a strong influence of the linkage between the sugar moieties in the disaccharide headgroup and the ethyl spacer on the micellar structure: the transformation from spherical to disklike aggregates was observed for compounds with a rodlike shape, but only spherical aggregates were formed by the wedge-shaped molecules.

Structural and rheological investigation of Fd3m inverse micellar cubic phases

In the present study we demonstrate that a bulk inverse micellar cubic phase of Fd3m structure can be obtained by adding a hydrophobic component, such as the food-grade limonene, to the binary system monolinolein/water in a well-defined composition. The Fd3m structure studied in this work had a very slow kinetics of formation, as a consequence of partitioning of water into two types of micelle populations with different sizes. The Fd3m structure formed at a ratio of limonene oil to total lipids of alpha = 0.4 is stable in the bulk up to a maximum hydration of 12.68 wt % water, beyond which it starts to coexist with dispersed water. At full hydration, by combining small-angle X-ray scattering and available topological models, the inverse micellar cubic phase of Fd3m structure was shown to be formed by 16 small micelles and 8 larger micelles per cubic lattice cell (Q227 group), with radii of the micellar polar cores ranging between 1 and 3 nm and 149-168 monolinolein molecules per micelle depending on the water content. The temperature dependence of the structural and rheological properties of the Fd3m mesophase was investigated using SAXS, rheology, and turbidimetry. It appeared that the Fd3m phase underwent crystallization below 18 degrees C and began melting in an inverse microemulsion (L2 phase) coexisting with water above 28.5 degrees C with complete melting obtained at 40-45 degrees C, as evidenced by SAXS and rheology. Macroscopic phase separation between the L2 phase and excess water was observed with time at higher temperatures. The investigation of the viscoelastic properties of the Fd3m inverse discrete micellar cubic phase revealed a rheological signature similar to that of the bicontinuous cubic phases Pn3m and Ia3d observed in homologous binary systems. However, the Fd3m phase presented a complex set of slower relaxation mechanisms leading to a shift by 1 order of magnitude of the dominant relaxation times and whole relaxation spectrum, as compared to those of inverse bicontinuous cubic phases. These findings have been tentatively explained by (i) the multiple relaxation of micelles upon deformation, (ii) the small hydration level of the Fd3m phase, and (iii) the low temperature at which this phase can be observed.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Leakage-free membrane fusion induced by the hydrolytic activity of PlcHR(2), a novel phospholipase C/sphingomyelinase from Pseudomonas aeruginosa

PlcHR(2) is the paradigm member of a novel phospholipase C/phosphatase superfamily, with members in a variety of bacterial species. This paper describes the phospholipase C and sphingomyelinase activities of PlcHR(2) when the substrate is in the form of large unilamellar vesicles, and the subsequent effects of lipid hydrolysis on vesicle and bilayer stability, including vesicle fusion. PlcHR(2) cleaves phosphatidylcholine and sphingomyelin at equal rates, but is inactive on phospholipids that lack choline head groups. Calcium in the millimolar range does not modify in any significant way the hydrolytic activity of PlcHR(2) on choline-containing phospholipids. The catalytic activity of the enzyme induces vesicle fusion, as demonstrated by the concomitant observation of intervesicular total lipid mixing, inner monolayer-lipid mixing, and aqueous contents mixing. No release of vesicular contents is detected under these conditions. The presence of phosphatidylserine in the vesicle composition does not modify significantly PlcHR(2)-induced liposome aggregation, as long as Ca(2+) is present, but completely abolishes fusion, even in the presence of the cation. Each of the various enzyme-induced phenomena have their characteristic latency periods, that increase in the order lipid hydrolysis<vesicle aggregation<total lipid mixing<inner lipid mixing<contents mixing. Concomitant measurements of the threshold diacylglyceride+ceramide concentrations in the bilayer show that late events, e.g. lipid mixing, require a higher concentration of PlcHR(2) products than early ones, e.g. aggregation. When the above results are examined in the context of the membrane effects of other phospholipid phosphocholine hydrolases it can be concluded that aggregation is necessary, but not sufficient for membrane fusion to occur, that diacylglycerol is far more fusogenic than ceramide, and that vesicle membrane permeabilization occurs independently from vesicle fusion.

Polyunsaturated phosphatidylinositol and diacylglycerol substantially modify the fluidity and polymorphism of biomembranes: a solid-state deuterium NMR study

One of the main biological systems that can be used as a model for studying the molecular mechanisms involved in membrane fusion is the formation of the nuclear envelope (NE). NE assembly to form the male pronucleus at fertilization occurs by binding of NE membrane precursor vesicles to chromatin and their fusion. MV1 is an NE precursor vesicle population of low density, highly enriched in [18:0/20:4]PI. The modification of [18:0/20:4]PI to [18:0/20:4]DAG leads to NE formation, and the depletion of MV1 from the total membrane precursors results in the inhibition of NE assembly. Here we show by 2H NMR studies of various physiologically relevant model membranes made of [18:0/20:4]PI, [18:0/20:4]DAG, and saturated and unsaturated PC that membranes of composition similar to MV1 exhibit dramatically enhanced fluidity and non-lamellar structures, thus providing a possible explanation for the essential role of MV1 and the modification of PI to DAG in membrane precursor vesicles during NE assembly.

Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide, some closely related molecules (eicosasphingosine, phytosphingosine), and their phosphorylated compounds (sphingosine-1-phosphate, ceramide-1-phosphate), are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. This paper reviews the mechanisms by which these sphingolipid signals, sphingosine and ceramide in particular, are able to modify the properties of cell membranes.

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.

Evidence for uncorrelated tilted layer structure and electrically polarized bilayers in amphiphilic glycolipids

A strong low-frequency dielectric relaxation mode spanning from below 100 Hz at low temperatures to 100 kHz at high temperature, x-ray diffraction studies, and optical microscopic observations show, contrary to the currently accepted models, that the glycolipid molecules are tilted with respect to the layer normal in the smectic phase but the tilt direction is not correlated between the bilayers. The tilted glycolipid bilayers are electrically polarized. The tilted structure and the electric polarization of amphiphilic glycolipids may play an important role in biological cell membrane.

Transmembrane peptides stabilize inverted cubic phases in a biphasic length-dependent manner: implications for protein-induced membrane fusion

WALP peptides consist of repeating alanine-leucine sequences of different lengths, flanked with tryptophan "anchors" at each end. They form membrane-spanning alpha-helices in lipid membranes, and mimic protein transmembrane domains. WALP peptides of increasing length, from 19 to 31 amino acids, were incorporated into N-monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) at concentrations up to 0.5 mol % peptide. When pure DOPE-Me is heated slowly, the lamellar liquid crystalline (L(alpha)) phase first forms an inverted cubic (Q(II)) phase, and the inverted hexagonal (H(II)) phase at higher temperatures. Using time-resolved x-ray diffraction and slow temperature scans (1.5 degrees C/h), WALP peptides were shown to decrease the temperatures of Q(II) and H(II) phase formation (T(Q) and T(H), respectively) as a function of peptide concentration. The shortest and longest peptides reduced T(Q) the most, whereas intermediate lengths had weaker effects. These findings are relevant to membrane fusion because the first step in the L(alpha)/Q(II) phase transition is believed to be the formation of fusion pores between pure lipid membranes. These results imply that physiologically relevant concentrations of these peptides could increase the susceptibility of biomembrane lipids to fusion through an effect on lipid phase behavior, and may explain one role of the membrane-spanning domains in the proteins that mediate membrane fusion.

Asymmetric addition of ceramides but not dihydroceramides promotes transbilayer (flip-flop) lipid motion in membranes

Transbilayer lipid motion in membranes may be important in certain physiological events, such as ceramide signaling. In this study, the transbilayer redistribution of lipids induced either by ceramide addition or by enzymatic ceramide generation at one side of the membrane has been monitored using pyrene-labeled phospholipid analogs. When added in organic solution to preformed liposomes, egg ceramide induced transbilayer lipid motion in a dose-dependent way. Short-chain (C6 and C2) ceramides were less active than egg ceramide, whereas dihydroceramides or dioleoylglycerol were virtually inactive in promoting flip-flop. The same results (either positive or negative) were obtained when ceramides, dihydroceramides, or diacylglycerols were generated in situ through the action of a sphingomyelinase or of a phospholipase C. The phenomenon was dependent on the bilayer lipid composition, being faster in the presence of lipids that promote inverted phase formation, e.g., phosphatidylethanolamine and cholesterol; and, conversely, slower in the presence of lysophosphatidylcholine, which inhibits inverted phase formation. Transbilayer motion was almost undetectable in bilayers composed of pure phosphatidylcholine or pure sphingomyelin. The use of pyrene-phosphatidylserine allowed detection of flip-flop movement induced by egg ceramide in human red blood cell membranes at a rate comparable to that observed in model membranes. The data suggest that when one membrane leaflet becomes enriched in ceramides, they diffuse toward the other leaflet. This is counterbalanced by lipid movement in the opposite direction, so that net mass transfer between monolayers is avoided. These observations may be relevant to the physiological mechanism of transmembrane signaling via ceramides.

The kinetics of non-lamellar phase formation in DOPE-Me: relevance to biomembrane fusion

The mechanism of the lamellar/inverted cubic (QII) phase transition is related to that of membrane fusion in lipid systems. N-Monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) exhibits this transition and is commonly used to investigate the effects of exogenous substances, such as viral fusion peptides, on the mechanism of membrane fusion. We studied DOPE-Me phase behavior as a first step in evaluating the effects of membrane-spanning peptides on inverted phase formation and membrane fusion. These measurements show that: a) the onset temperatures for QII and inverted hexagonal (HII) phase formation both are temperature scan rate-dependent; b) longer pre-incubation times at low temperature and lower temperature scan rates favor formation of the QII phase; and c) in temperature-jump experiments between 61 and 65 degrees C, the meta-stable HII phase forms initially, and disappears slowly while the QII phase develops. These observations are rationalized in the context of a mechanism for both the lamellar/non-lamellar phase transition and the related process of membrane fusion.

Membrane Fusion Induced by the Catalytic Activity of a Phospholipase C/Sphingomyelinase fromListeria monocytogenes†

Listeria monocytogenes is a bacterium responsible for localized and generalized infections in humans and animals. It has the ability to spread from the cytoplasm of an infected cell to neighboring cells without becoming exposed to the extracellular space. The bacterium secretes a phospholipase C (PLC(LM)) that is active on glycerophospholipids, e.g., phosphatidylcholine, and on sphingomyelin; thus, PLC(LM) should be described more appropriately as a phospholipase C/sphingomyelinase. We have obtained PLC(LM) free from a frequent contaminant, listeriolysin O, using an improved purification procedure. PLC(LM) has been assayed on large unilamellar liposomes of defined lipid composition. The enzyme is activated by K(+) and Mg(2+), and readily degrades phospholipids in bilayer form, in the absence of detergents. Enzyme activity is accompanied by important changes in the structure of the phospholipid vesicles, namely, vesicle aggregation, intervesicular mixing of lipids, and mixing of aqueous contents, with very low leakage of vesicular contents. The data are interpreted as indicative of PLC(LM)-induced vesicle fusion. This is confirmed by the demonstration of intervesicular mixing of inner monolayer lipids, using a novel procedure. The observation of PLC(LM)-induced membrane fusion suggests a mechanism for the cell-to-cell propagation of the bacterium, which requires disruption of a double-membrane vacuole.

Crystallization of membrane proteins from media composed of connected-bilayer gels

The use of hydrated-lipid gels in which the bilayer is an infinitely periodic (or at least continuous), three-dimensional structure offers a relatively new approach for the crystallization of membrane proteins. While excellent crystals of the Halobacterial rhodopsins have been obtained with such media, success remains poor in extending their use to other membrane proteins. Experience with crystallization of bacteriorhodopsin has led us to recognize a number of improvements that can be made in the use of such hydrated-gel media, which may now prove to be of general value for the crystallization of other membrane proteins.

Diacylglycerol effects on phosphatidylinositol-specific phospholipase C activity and vesicle fusion

Diacylglycerol increased the hydrolytic activity of phosphatidylinositol-specific phospholipase C on large unilamellar vesicles containing 5-40% phosphatidylinositol. Moreover, diacylglycerol increased the rate and extent of vesicle fusion (contents mixing) induced by the enzyme. Kinetic studies of intervesicular lipid mixing revealed that fusion was limited by the frequency of contacts involving two diacylglycerol-rich domains.

Leaky vesicle fusion induced by phosphatidylinositol-specific phospholipase C: observation of mixing of vesicular inner monolayers

Large unilamellar vesicles containing phosphatidylinositol (PI), neutral phospholipids, and cholesterol are induced to fuse by the catalytic activity of phosphatidylinositol-specific phospholipase C (PI-PLC). PI cleavage by PI-PLC is followed by vesicle aggregation, intervesicular lipid mixing, and mixing of vesicular aqueous contents. An average of 2-3 vesicles merge into a large one in the fusion process. Vesicle fusion is accompanied by leakage of vesicular contents. A novel method has been developed to monitor mixing of lipids located in the inner monolayers of the vesicles involved in fusion. Using this method, the mixing of inner monolayer lipids and that of vesicular aqueous contents are seen to occur simultaneously, thus giving rise to the fusion pore. Kinetic studies show, for fusing vesicles, second-order dependence of lipid mixing on diacylglycerol concentration in the bilayer. Varying proportions of PI in the liposomal formulation lead to different physical effects of PI-PLC. Specifically, 30-40 mol % PI lead to vesicle fusion, while with 5-10 mol % PI only hemifusion is detected, i.e., mixing of outer monolayer lipids without mixing of aqueous contents. However, when diacylglycerol is included in the bilayers containing 5 mol % PI, PI-PLC activity leads to complete fusion.

Interbilayer lipid mixing induced by the human immunodeficiency virus type-1 fusion peptide on large unilamellar vesicles: the nature of the nonlamellar intermediates

A peptide corresponding to the 23 N-terminal amino acid residues of the human immunodeficiency virus type-1 (HIV-1) gp41 has the capacity to induce intervesicular lipid mixing in large unilamellar liposomes composed of dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE) and cholesterol (CHOL) (molar ratio, 1:1:1). Cryo-transmission electron microscopy (cryo-TEM) of diluted vesicles to which peptides has been externally added reveals a morphology that is compatible with the formation of nonlamellar lipidic aggregates during the time-course of lipid mixing. 31P-nuclear magnetic resonance and 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMADPH) steady-state anisotropy data at equilibrium indicate that the peptide is able to modulate the lipid polymorphism in pelletted membranes by: (i) promoting the thermotropic formation of inverted phases; and (ii) driving the lamellar-to-nonlamellar transition towards the formation of isotropic phases. Therefore, our combined morphological and spectroscopic data reveal the existence of a direct correlation between the ability of the externally added peptide to induce lipid-mixing in dilute liposome samples and its capacity to modulate lipid polymorphism in stacked bilayers.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

Simultaneous X-ray diffraction and calorimetric study of metastable- to-stable solid phase transformation of 1,2-dipalmitoyl-sn-glycerol

The thermal behaviour and structural changes associated with the phase transformation of 1,2-dipalmitoyl-sn-glycerol (DPG) were studied by means of simultaneous X-ray diffraction and differential scanning calorimetry. Metastable DPG solid phases are crystallized from the melted sample by thermal quenching. The metastable phase (alpha-phase) formed initially is converted into a stable phase (beta' phase) at approximately 50 degrees C on heating. It was found that the behaviour of the alpha- to beta'-phase transformation depends on the thermal history. DPG solid samples incubated at approximately 3 degrees C for more than 10 h after cooling transformed directly into the beta'-phase with heat release. On the other hand, in the solid samples without incubation, the alpha-phase once melted and then the crystallization of the beta'-phase occurred successively from the melted state.

Ceramides in phospholipid membranes: effects on bilayer stability and transition to nonlamellar phases

The effects of ceramides of natural origin on the gel-fluid and lamellar-inverted hexagonal phase transitions of phospholipids (mainly dielaidoylphosphatidylethanolamine) have been studied by differential scanning calorimetry, with additional support from infrared and 31P nuclear magnetic resonance (NMR) spectroscopy. In the lamellar phase, ceramides do not mix ideally with phospholipids, giving rise to the coexistence of domains that undergo the gel-fluid transition at different temperatures. The combination of differential scanning calorimetry and infrared spectroscopy, together with the use of deuterated lipids, allows the demonstration of independent melting temperatures for phospholipid and ceramide in the mixtures. In the lamellar-hexagonal phase transitions, ceramides (up to 15 mol %) decrease the transition temperature, without significantly modifying the transition enthalpy, thus facilitating the inverted hexagonal phase formation. 31P-NMR indicates the coexistence, within a certain range of temperatures, of lamellar and hexagonal phases, or hexagonal phase precursors. Ceramides from egg or from bovine brain are very similar in their effects on the lamellar-hexagonal transition. They are also comparable to diacylglycerides in this respect, although ceramides are less potent. These results are relevant in the interpretation of certain forms of interfacial enzyme activation and in the regulation and dynamics of the bilayer structure of cell membranes.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

Lipid polymorphism and protein-lipid interactions

Non-lamellar-forming lipids play an important role in determining the physical properties of membranes. They affect the activity of membrane proteins and peptides. In addition, peptides which lyse membranes as well as those which promote membrane fusion facilitate the formation of non-lamellar phases, either micelles, cubic or hexagonal phases. The relationship of these diverse effects on membrane curvature is discussed in relation to the function of certain peptides and proteins. Specific examples of ionophoric peptides, cytotoxic peptides and viral fusion peptides are given. In addition, we compare the modulation of the rate of photoisomerisation of an integral membrane protein, rhodopsin, by non-lamellar-forming lipids with the effects of these lipids on an amphitropic protein, protein kinase C. Among these diverse systems it is frequently observed that the modulation of biological activity can be described in terms of the effect of the peptide or protein on the relative stability of lamellar and non-lamellar structures.

Vesicle membrane fusion induced by the concerted activities of sphingomyelinase and phospholipase C

When vesicles composed of an equimolar mixture of sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, and cholesterol are treated with phospholipase C, phospholipid hydrolysis occurs without major changes in vesicle architecture. In the same way, addition of sphingomyelinase leads only to sphingomyelin cleavage. However, when both enzymes are added together, their joint hydrolytic activities give rise to leakage-free vesicle aggregation, lipid mixing, and aqueous contents mixing, i.e. vesicle fusion. The contribution of both enzymes is unequal, the main role of sphingomyelinase being the production of relatively large amounts of ceramide that will facilitate the lamellar-to-nonlamellar transition in the formation of the fusion pore, whereas phospholipase C provides mainly a localized, asymmetric, high concentration of diacylglycerol that constitutes the trigger for the fusion process. The lipidic end-products of both enzymes cooperate in destabilizing and fusing the membranes in a way that is never achieved through the action of any of the enzymes individually, nor by the products themselves when premixed with the other lipids during liposome preparation. Thus the enzymes appear to be coupled through their reaction products. This is the first observation of membrane fusion induced by the concerted activities of two enzymes. Besides, considering that both diacylglycerol and ceramide are important metabolites involved in cell signaling, it may also provide new ideas in the exploration of "cross-talk" phenomena between different signal transduction pathways.