Membrane fusion: Kinetics and mechanisms

Interaction between charged lipid vesicles and point- or rod-like trivalent ions

This work examines the influence of the charge distribution of trivalent cations on their interaction with soft anionic particles, using a combination of experimental measurements and theoretical modelling. In particular, we perform electrophoresis measurements to determine the zeta-potential of anionic liposomes in the presence of spermidine and lanthanum cations. We work in a range of electrolyte concentration where a reversal in the electrophoretic mobility of the liposomes is expected; however, unlike the case of lanthanum cations, spermidine does not induce mobility reversal of liposomes. As a result, the charge distribution within the counterion appears to be a key factor. This conclusion is supported by a theory that accounts for intra-ionic correlations, which has previously been successfully used to describe the colloidal electric double layer. It allows us to model spermidine as rod-like ions and lanthanum cations as point-like ions in order to test the importance of the ionic geometry in the interactions with soft particles such as lipid vesicles.

Biomimetic nano-surfactant stabilizes sub-50 nanometer phospholipid particles enabling high paclitaxel payload and deep tumor penetration

Sub-50 nm nanoparticles feature long circulation and deep tumor penetration. However, at high volume fractions needed for intravenous injection, safe, highly biocompatible phospholipids cannot form such nanoparticles due to the fluidity of phospholipid shells. Here we overcome this challenge using a nano-surfactant, a sterilized 18-amino-acid biomimetic of the amphipathic helical motif abundant in HDL-apolipoproteins. As it induces a nanoscale phase (glass) transition in the phospholipid monolayer, the peptide stabilizes 5-7 nm phospholipid micelles that do not fuse at high concentrations but aggregate into stable micellesomes exhibiting size-dependent penetration into tumors. In mice bearing human Her-2-positive breast cancer xenografts, high-payload paclitaxel encapsulated in 25 nm (diameter) micellesomes kills more cancer cells than paclitaxel in standard clinical formulation, as evidenced by the enhanced apparent diffusion coefficient of water determined by in vivo MR imaging. Importantly, the bio-inertness of this biomimetic nano-surfactant spares the nanoparticles from being absorbed by liver hepatocytes, making them more generally available for drug delivery.

Dynamics of Hydrogel-Assisted Giant Unilamellar Vesicle Formation from Unsaturated Lipid Systems

While current research is centered on observing biophysical properties and phenomena in giant unilamellar vesicles (GUVs), little is known about fabrication parameters that control GUV formation. Using different lipids and rehydration buffers, we directly observe varying dynamics of hydrogel-assisted GUV formation via fluorescence microscopy. We observe the effects of buffer ionic strength, osmolarity, agarose density, and pH on the formation of GUVs using neutral and charged lipids. We find that increasing rehydration buffer ionic strength correlates with increased vesicle size and rate of GUV formation. Increasing buffer acidity increased the rate of GUV formation, while more basic environments slowed the rate. For buffers containing 500 mM sucrose, GUV formation was overall inhibited and only tubules formed. Observations of GUV formation dynamics elucidate parametric effects of charge, ionic strength, pH, and osmolarity, demonstrating the versatility of this biomimetic platform.

Effect of calcium and magnesium on phosphatidylserine membranes: experiments and all-atomic simulations

It is known that phosphatidylserine (PS(-)) lipids have a very similar affinity for Ca(2+) and Mg(2+) cations, as revealed by electrokinetic and stability experiments. However, despite this similar affinity, experimental evidence shows that the presence of Ca(2+) or Mg(2+) induces very different aggregation behavior for PS(-) liposomes as characterized by their fractal dimensions. Also, turbidity measurements confirm substantial differences in aggregation behavior depending on the presence of Ca(2+) or Mg(2+) cations. These puzzling results suggest that although these two cations have a similar affinity for PS(-) lipids, they induce substantial structural differences in lipid bilayers containing each of these cations. In other words, these cations have strong ion-specific effects on the structure of PS(-) membranes. This interpretation is supported by all-atomic molecular-dynamics simulations showing that Ca(2+) and Mg(2+) cations have different binding sites and induce different membrane hydration. We show that although both ions are incorporated deep into the hydrophilic region of the membrane, they have different positions and configurations at the membrane. Absorbed Ca(2+) cations present a peak at a distance ~2 nm from the center of the lipid bilayer, and their most probable binding configuration involves two oxygen atoms from each of the charged moieties of the PS molecule (phosphate and carboxyl groups). In contrast, the distribution of absorbed Mg(2+) cations has two different peaks, located a few angstroms before and after the Ca(2+) peak. The most probable configurations (corresponding to these two peaks) involve binding to two oxygen atoms from carboxyl groups (the most superficial binding peak) or two oxygen atoms from phosphate groups (the most internal peak). Moreover, simulations also show differences in the hydration structure of the membrane: we obtained a hydration of 7.5 and 9 water molecules per lipid in simulations with Ca(2+) and Mg(2+), respectively.

Aggregation of liposomes induced by calcium: a structural and kinetic study

In this work, the calcium-induced aggregation of phosphatidylserine liposomes is probed by means of the analysis of the kinetics of such process as well as the aggregate morphology. This novel characterization of liposome aggregation involves the use of static and dynamic light-scattering techniques to obtain kinetic exponents and fractal dimensions. For salt concentrations larger than 5mM, a diffusion-limited aggregation regime is observed and the Brownian kernel properly describes the time evolution of the diffusion coefficient. For slow kinetics, a slightly modified multiple contact kernel is required. In any case, a time evolution model based on the numerical resolution of Smoluchowski's equation is proposed in order to establish a theoretical description for the aggregating system. Such a model provides an alternative procedure to determine the dimerization constant, which might supply valuable information about interaction mechanisms between phospholipid vesicles.

PEG as a tool to gain insight into membrane fusion

Thirty years ago, Klaus Arnold and others showed that the action of PEG in promoting cell-cell fusion was not due to such effects as surface absorption, cross-linking, solubilization, etc. Instead PEG acted simply by volume exclusion, resulting in an osmotic force driving membranes into close contact in a dehydrated region. This simple observation, based on a number of physical measurements and the use of PEG-based detergents that insert into membranes, spawned several important areas of research. One such area is the use of PEG to bring membranes into contact so that the role of different lipids and fusion proteins in membrane fusion can be examined in detail. We have summarized here insights into the fusion mechanism that have been obtained by this approach. This evidence indicates that fusion of model membranes (and probably cell membranes) occurs via severely bent lipidic structures formed at the point of sufficiently close contact between membranes of appropriate lipid composition. This line of research has also suggested that fusion proteins seem to catalyze fusion in part by reducing the free energy of hydrophobic interstices inherent to the lipidic fusion intermediate structures.

A soluble peripherin/Rds C-terminal polypeptide promotes membrane fusion and changes conformation upon membrane association

Photoreceptor rod cells contain a unique tetraspanin fusion protein known as peripherin/rds. This protein is important in membrane fusion events hypothesized to be essential to disk membrane morphogenesis and disk shedding. In vivo and in vitro fusogenic activity has been mapped to the C-terminal domain of peripherin/rds. Moreover, a fusion peptide domain localized to a 15 amino acid long region (residues 311-325) is essential for mediating lipid bilayer fusion of model membranes. To address the functional and structural properties required for peripherin/rds dependent membrane fusion, constructs of the entire C-terminal domain (residues 284-346) were generated and polypeptides expressed. A wild type-peripherin/rds C-terminal GST fusion construct that included the entire C-terminus (PERCTER) or a C-terminal truncation mutant (PERCTN) were engineered with a thrombin cleavage site. Protein expression was induced in E. coli with IPTG, expressed proteins cleaved from the GST with thrombin and purified to homogeneity on a Superdex 75 column. Purity was confirmed by SDS-PAGE and Western blot analysis. The purified wt C-terminal protein resolved as a monomer under reducing conditions on SDS-PAGE (15%) and was immunoreactive with anti peripherin/rds antibody 2B6 (gift from Dr R. Molday). The purified polypeptide promoted the requisite steps of fusion, membrane destabilization, lipid mixing and aqueous contents mixing. Conversely, the truncation mutant lacking a portion of the fusion domain was unable to promote these steps. A common feature of most membrane fusion proteins is a change in conformation upon membrane association. Structural changes in the C-terminal polypeptide were investigated using far UV CD. The far UV CD spectra of the purified C-terminal polypeptide indicated substantial alpha-helical content in the wt peptide in isotonic aqueous buffer. An increase in intensity of 208 and 222 nm CD bands upon addition of DPC vesicles indicated an increase in alpha-helical content of the polypeptide. These results demonstrate that a purified soluble form of the C-terminus of peripherin/rds can interact with biological phospholipids; moreover, this interaction promotes a conformational change that is most consistent with an increase in alpha-helical content.

Conformation change of horseradish peroxidase in lipid membrane

The electrochemical behavior of horseradish peroxidase (HRP) in the dimyristoyl phosphatidylcholine (DMPC) bilayer on the glassy carbon (GC) electrode was studied by cyclic voltammetry. The direct electron transfer of HRP was observed in the DMPC bilayer. Only a small cathodic peak was observed for HRP on the bare GC electrode. The electron transfer of HRP in the DMPC membrane is facilitated by DMPC membrane. UV-Vis and circular dichroism (CD) spectroscopy were used to study the interaction between HRP and DMPC membrane. On binding to the DMPC membrane the secondary structure of HRP remains unchanged while there is a substantial change in the conformation of the heme active site. Tapping mode atomic force microscopy (AFM) was first applied for the investigation on the structure of HRP adsorbed on supported phospholipid bilayer on the mica and on the bare mica. HRP molecules adsorb and aggregate on the mica without DMPC bilayer. The aggregation indicates an attractive interaction among the adsorbed molecules. The molecules are randomly distributed in the DMPC bilayer. The adsorption of HRP in the DMPC bilayer changes drastically the domains and defects in the DMPC bilayer due to a strong interaction between HRP and DMPC films.

Molecular dynamics simulation of the evolution of hydrophobic defects in one monolayer of a phosphatidylcholine bilayer: relevance for membrane fusion mechanisms

The spontaneous formation of the phospholipid bilayer underlies the permeability barrier function of the biological membrane. Tears or defects that expose water to the acyl chains are spontaneously healed by lipid lateral diffusion. However, mechanical barriers, e.g., protein aggregates held in place, could sustain hydrophobic defects. Such defects have been postulated to occur in processes such as membrane fusion. This gives rise to a new question in bilayer structure: What do the lipids do in the absence of lipid lateral diffusion to minimize the free energy of a hydrophobic defect? As a first step to understand this rather fundamental question about bilayer structure, we performed molecular dynamic simulations of up to 10 ns of a planar bilayer from which lipids have been deleted randomly from one monolayer. In one set of simulations, approximately one-half of the lipids in the defect monolayer were restrained to form a mechanical barrier. In the second set, lipids were free to diffuse around. The question was simply whether the defects caused by removing a lipid would aggregate together, forming a large hydrophobic cavity, or whether the membrane would adjust in another way. When there are no mechanical barriers, the lipids in the defect monolayer simply spread out and thin with little effect on the other intact monolayer. In the presence of a mechanical barrier, the behavior of the lipids depends on the size of the defect. When 3 of 64 lipids are removed, the remaining lipids adjust the lower one-half of their chains, but the headgroup structure changes little and the intact monolayer is unaffected. When 6 to 12 lipids are removed, the defect monolayer thins, lipid disorder increases, and lipids from the intact monolayer move toward the defect monolayer. Whereas this is a highly simplified model of a fusion site, this engagement of the intact monolayer into the fusion defect is strikingly consistent with recent results for influenza hemagglutinin mediated fusion.

Cationic triple-chain amphiphiles facilitate vesicle fusion compared to double-chain or single-chain analogues

Cationic, triple-chain amphiphiles promote vesicle fusion more than structurally related double-chain or single-chain analogues. Two types of vesicle fusion experiments were conducted, mixing of oppositely charged vesicles and acid-triggered self-fusion of vesicles composed of cationic amphiphile and anionic cholesteryl hemisuccinate (CHEMS). Vesicle fusion was monitored by standard fluorescence assays for intermembrane lipid mixing, aqueous contents mixing and leakage. Differential scanning calorimetry was used to show that triple-chain amphiphiles lower the lamellar-inverse hexagonal (L(alpha)-H(II)) phase transition temperature for dipalmitoleoylphosphatidylethanolamine. The triple-chain amphiphiles may enhance vesicle fusion because they can stabilize the inversely curved membrane surfaces of the fusion intermediates, however, other factors such as extended conformation, packing defects, chain motion, or surface dehydration may also contribute. From the perspective of drug delivery, the results suggest that vesicles containing cationic, triple-chain amphiphiles (and cationic, cone-shaped amphiphiles in general) may be effective as fusogenic delivery capsules.

Electron microscopic investigation of the morphology and calcium-induced fusion of lipid vesicles with an oligomerised inner leaflet

The lipid head groups in the inner leaflet of unilamellar bilayer vesicles of the synthetic lipids DHPBNS and DDPBNS can be selectively oligomerised. Earlier studies have established that these vesicles fuse much slower and less extensively upon oligomerisation of the lipid head groups. The morphology and calcium-induced fusion of vesicles of DHPBNS and DDPBNS were investigated using cryo-electron microscopy. DHPBNS vesicles are not spherical but flattened, ellipsoidal structures. Upon addition of CaCl(2), DHPBNS vesicles with an oligomerised inner leaflet were occasionally observed in an arrested hemifused state. However, the evidence for hemifusion is not equivocal due to potential artefacts of sample preparation. DDPBNS vesicles show the expected spherical morphology. Upon addition of excess CaCl(2), DDPBNS vesicles fuse into dense aggregates that show a regular spacing corresponding to the bilayer width. Upon addition of EDTA, the aggregates readily disperse into large unilamellar vesicles. At low concentration of calcium ion, DDPBNS vesicles with an oligomerised inner leaflet form small multilamellar aggregates, in which a spacing corresponding to the bilayer width appears. Addition of excess EDTA results in slow dispersal of the Ca2+-lipid aggregates into a heterogeneous mixture of bilamellar, spherical vesicles and networks of thread-like vesicles. These lipid bilayer rearrangements are discussed within the context of shape transformations and fusion of lipid membranes.

Greasing membrane fusion and fission machineries

Biological membrane fusion is a local-point event, extremely fast, and under strict control. Proteins are responsible for the mutual recognition of the fusion partners and for the initiation of biomembrane fusion, and thus determine where and when fusion occurs. However, the central event during membrane fusion is the merger of two membranes, which requires a transient reorganization of membrane lipids into highly curved fusion intermediates. This review focuses on the potential role of lipids in the generation of membrane curvature, and thus in the regulation of membrane fusion and fission.

Membrane fusion by proline-rich Rz1 lipoprotein, the bacteriophage lambda Rz1 gene product

The fusogenic properties of Rz1, the proline-rich lipoprotein that is the bacteriophage lambda Rz1 gene product, were studied. Light scattering was used to monitor Rz1-induced aggregation of artificial neutral (dipalmitoylphosphatidylcholine/cholesterol) and negatively charged (dipalmitoylphosphatidylcholine/cholesterol/dioleoylphosphatidylserin e) liposomes. Fluorescence assays [the resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine and N-(lissamine rhodamine B sulfonyl)dihexadecanol-sn-glycero-3-phosphoethanolamine lipid fluorescent probes, as well as fluorescent complex formation between terbium ions and dipicolinic acid encapsulated in two liposome populations and calcein fluorescence] were used to monitor Rz1-induced lipid mixing, contents mixing and leakage of neutral and negatively charged liposomes. The results demonstrated that Rz1 caused adhesion of neutral and negatively charged liposomes with concomitant lipid mixing; membrane distortion, leading to the fusion of liposomes and hence their internal content mixing; and local destruction of the membrane accompanied by leakage of the liposome contents. The use of artificial membranes showed that Rz1 induced the fusion of membranes devoid of any proteins. This might mean that the proline stretch of Rz1 allowed interaction with membrane lipids. It is suggested that Rz1-induced liposome fusion was mediated primarily by the generation of local perturbation in the bilayer lipid membrane and to a lesser extent by electrostatic forces.

Membrane fusion and the lamellar-to-inverted-hexagonal phase transition in cardiolipin vesicle systems induced by divalent cations

The polymorphic phase behavior of bovine heart cardiolipin (CL) in the presence of different divalent cations and the kinetics of CL vesicle fusion induced by these cations have been investigated. (31)P-NMR measurements of equilibrium cation-CL complexes showed the lamellar-to-hexagonal (L(alpha)-H(II)) transition temperature (T(H)) to be 20-25 degrees C for the Sr(2+) and Ba(2+) complexes, whereas in the presence of Ca(2+) or Mg(2+) the T(H) was below 0 degrees C. In the presence of Sr(2+) or Ba(2+), CL large unilamellar vesicles (LUVs) (0.1 microm diameter) showed kinetics of destabilization, as assessed by determination of the release of an aqueous fluorescent dye, which strongly correlated with the L(alpha)-H(II) transition of the final complex: at temperatures above the T(H), fast and extensive leakage, mediated by vesicle-vesicle contact, was observed. On the other hand, mixing of vesicle contents was limited and of a highly transient nature. A different behavior was observed with Ca(2+) or Mg(2+): in the temperature range of 0-50 degrees C, where the H(II) configuration is the thermodynamically favored phase, relatively nonleaky fusion of the vesicles occurred. Furthermore, with increasing temperature the rate and extent of leakage decreased, with a concomitant increase in fusion. Fluorescence measurements, involving incorporation of N-NBD-phosphatidylethanolamine in the vesicle bilayer, demonstrated a relative delay in the L(alpha)-H(II) phase transition of the CL vesicle system in the presence of Ca(2+). Freeze-fracture electron microscopy of CL LUV interaction products revealed the exclusive formation of H(II) tubes in the case of Sr(2+), whereas with Ca(2+) large fused vesicles next to H(II) tubes were seen. The extent of binding of Ca(2+) to CL in the lamellar phase, saturating at a binding ratio of 0.35 Ca(2+) per CL, was close to that observed for Sr(2+) and Ba(2+). It is concluded that CL LUVs in the presence of Ca(2+) undergo a transition that favors nonleaky fusion of the vesicles over rapid collapse into H(II) structures, despite the fact that the equilibrium Ca(2+)-CL complex is in the H(II) phase. On the other hand, in the presence of Sr(2+) or Ba(2+) at temperatures above the T(H) of the respective cation-CL complexes, CL LUVs rapidly convert to H(II) structures with a concomitant loss of vesicular integrity. This suggests that the nature of the final cation-lipid complex does not primarily determine whether CL vesicles exposed to the cation will initially undergo a nonleaky fusion event or collapse into nonvesicular structures.

Poly(ethylene glycol) (PEG)-mediated fusion between pure lipid bilayers: a mechanism in common with viral fusion and secretory vesicle release?

Membrane fusion is fundamental to the life of eukaryotic cells. Cellular trafficking and compartmentalization, import of food stuffs and export of waste, inter-cellular communication, sexual reproduction, and cell division are all dependent on this basic process. Yet, little is known about the molecular mechanism(s) by which fusion occurs. It is known that fusing membranes must somehow be docked and brought into close contact. Specific proteins, many of which have been identified within the past decade, accomplish this. An electrical connection or 'fusion pore' is established between compartments surrounded by the fusing membranes. Three primary views of the mechanism of pore formation during secretory and viral fusion have been proposed within the past decade. In one view, a protein ring forms an initial transient connection that expands slowly by recruiting lipid so as to form a lipidic junction. In another view, the initial fusion pore consists of a protein-lipid complex that transforms slowly until the fusion proteins dissociate from the complex to form an irreversible lipidic pore. In a third view, the initial pore is a transient lipid pore that fluctuates between open and closed states before either expanding irreversibly or closing. Recent work has helped define the mechanism by which poly(ethylene glycol) (PEG) mediates fusion of highly curved model membranes composed only of synthetic phospholipids. PEG is a highly hydrated polymer that can bring vesicle membranes to near molecular contact by making water between them thermodynamically unfavourable. Disrupted packing in the contacting monolayers of these vesicle membranes is necessary to induce fusion. The time course and sequence of molecular events of the ensuing fusion process have also been defined. This sequence of events involves the formation of an initial, transient intermediate in which outer leaflet lipids have mixed and small transient pores join fusing compartments ('stalk'). The transient intermediate transforms in 1-3 min to a fusion-committed, second intermediate ('septum') that then 'pops' to form the fusion pore. Inner leaflet mixing, which is shown to be distinct from outer leaflet mixing, accompanies contents mixing that marks formation of the fusion pore. Both the sequence of events and the activation energies of these events correspond well to those observed in viral membrane fusion and secretory granule fusion. These results strongly support the contention that both viral and secretory fusion events occur by lipid molecule rearrangements that can be studied and defined through the use of PEG-mediated vesicle fusion as a model system. A possible mechanism by which fusion proteins might mediate this lipidic process is described.

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.

Reconstitution of membrane fusion between pancreatic islet secretory granules and plasma membranes: catalysis by a protein constituent recognized by monoclonal antibodies directed against glyceraldehyde-3-phosphate dehydrogenase

An isoform of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) isolated and purified from rabbit brain cytosol has previously been demonstrated to catalyze membrane fusion (Glaser and Gross, Biochemistry 33 (1994) 5805-5812; Glaser and Gross, Biochemistry 34 (1995) 12193-12203). Herein, we provide evidence suggesting that this GAPDH isoform can reconstitute in vitro protein-catalyzed fusion between naturally occurring subcellular membrane fractions involved in insulin exocytosis. Utilizing purified rat pancreatic beta-cell plasma membranes and secretory granules, we show that a brain cytosolic factor catalyzed the rapid and efficient fusion of these two purified membrane fractions which could be inhibited by a monoclonal antibody directed against the brain isoform of GAPDH. Moreover, the brain cytosolic factor also catalyzed the fusion of reconstituted vesicles prepared from lipid extracts of islet plasma membranes and secretory granules. Although the brain cytosolic factor rapidly catalyzed membrane fusion between islet plasma membranes and secretory granules, it did not catalyze fusion between one secretory granule population with another. To identify the potential importance of brain cytosolic factor catalyzed membrane fusion in islet cells, we examined extracts of hamster insulinoma tumor cells (HIT cells) for fusion-catalyzing activity. A protein constituent was present in HIT cell cytosol which was immunologically similar to the rabbit brain GAPDH isoform. Although native HIT cell cytosol did not catalyze membrane fusion, removal of an endogenous protein inhibitor unmasked the presence of the protein which catalyzed membrane fusion activity and such fusion was ablated by a monoclonal antibody directed against the brain isoform of GAPDH. Collectively, these results suggest the possibility that an isoform of brain GAPDH, also evident in HIT cells, can catalyze fusion between the two naturally occurring subcellular membrane compartments involved in insulin secretion and suggest a novel paradigm potentially coupling glycolytic flux with insulin release.

Physicochemical characterisation of human stratum corneum lipid liposomes

Liposomes were prepared from an extract of all human stratum corneum lipids (hSCL) and characterised in terms of temperature and the presence of Ca2+ by different physicochemical methods. Vesicle aggregation and lateral phase separation were induced by divalent cations with Ca2+ being more efficient than Mg2+. At 24.1 degrees C, i.e. well below physiological temperatures the suspensions consisted of a lamellar phase and crystalline cholesterol. At and above 37 degrees C, this cholesterol surplus was dissolved in the hSCL membranes. However, melting of the hSCL was not completed up to 60 degrees C. The presence of Ca2+ (> or = 9 mM) induced lateral phase separation and fusion of vesicles into extended multilamellar lipid sheets (MLLS) at and above 32.5 degrees C. Upon a subsequent cooling cycle recrystallisation of cholesterol occurred within the MLLS. Finally, membrane mixing of hSCL liposomes with vesicles made of synthetic lipids was investigated. No mixing was observed between either of DPPE/oleic acid, DPPC/DPPE, DPPC/lyso-PC and hSCL liposomes. Mixtures of DPPC/cholesterol hemisuccinate showed a temperature-dependent membrane mixing behaviour, whilst hSCL liposomes and phosphatidylserine liposomes fused temperature-independently with hSCL liposomes.

Protein lipid interaction in bile: effects of biliary proteins on the stability of cholesterol-lecithin vesicles

The nucleation of cholesterol crystals is an obligatory precursor to cholesterol gallstone formation. Nucleation, in turn, is believed to be preceded by aggregation and fusion of cholesterol-rich vesicles. We have investigated the effects of two putative pro-nucleating proteins, a concanavalin A-binding protein fraction and a calcium-binding protein, on the stability of sonicated small unilamellar cholesterol-lecithin vesicles. Vesicle aggregation is followed by monitoring absorbance, and upon addition of the concanavalin A-binding protein fraction the absorbance of a vesicle dispersion increases continuously with time. Vesicle fusion is probed by a fluorescence contents-mixing assay. Vesicles apparently fuse slowly after the addition of the concanavalin A-binding protein, although inner filter effects confound the quantitative measurement of fusion rates. The rates of change of absorbance and fluorescence increase with the concentration of the protein, and the second-order dimerization rate constant increases with both the protein concentration and the cholesterol content of the vesicles. On the other hand, the calcium-binding protein has no effect on the stability of the vesicle dispersion. This protein may therefore affect cholesterol crystal formation not by promoting the nucleation process, but by enhancing crystal growth and packaging. Our results demonstrate that biliary proteins can destabilize lipid vesicles and that different proteins play different roles in the mechanism of cholesterol gallstone formation.

Isoprenoid metabolism in the vertebrate retina

Herein, studies concerning the biosynthesis, intracellular transport and utilization of isoprenoid lipids in vertebrate retinas are reviewed, with particular regard to rod photoreceptor cells and the assembly of rod outer segment (ROS) disk membranes. Initial in vitro studies with bovine retinas showed that [3H]mevalonate is metabolized primarily to squalene and 'methylated' sterols, rather than to cholesterol. Subsequently, similar results were obtained with frog retinas using [3H]acetate as a precursor, and the absolute rate of the sterol pathway was determined in vitro with 3H2O. With the aid of vesicular transport inhibitors, energy poisons, and reduced temperature, it was demonstrated that lipid and protein trafficking mechanisms in the rod cell are separate and independent from one another. In vivo, the majority of newly synthesized squalene in the frog retina is not metabolized to sterols; rather, it is transported to the ROS, where it turns over in parallel with the disk membranes. The remaining squalene is converted slowly to cholesterol, much of which becomes incorporated into the ROS. In contrast, the in vivo metabolism of [3H]acetate to cholesterol in the rat retina is relatively efficient and rapid. However, in both frog and rat, retinal cholesterol turnover is slow (> 60 days), suggesting the existence of a retention mechanism that minimizes the need for de novo biosynthesis. The use of pharmacological approaches to assess the biological roles of isoprenoid lipids and protein prenylation in the retina and the mechanism of retinal cholesterol homeostasis are discussed.

Influence of membrane components on the stability and drug release properties of reverse phase evaporation vesicles (REVs): light sensitive all-trans retinal, negatively charged phospholipid dicetylphosphate and cholesterol

Incorporation of a negatively charged phospholipid, dicetylphosphate, initially increased encapsulation efficiency (from 12 to 24%) but beyond 5% (molar) a detrimental effect was observed. Rate of drug release from REVs was, for most cases, found to be bi-phasic implying partitioning between the lipid bilayer and the aqueous compartment. It was not possible to prepare liposomes with more than 1% (molar) all-trans retinal (ATR) as a membrane component. When ATR was reduced to 0.5% (molar), encapsulation efficiency increased to 7.76%. Upon exposure to long wave UV (365 nm), release from ATR containing REVs was increased and this was attributed to the formation of 13-cis isomer as indicated by HPLC and UV spectroscopy data.

Temperature- and pH-controlled fusion between complex lipid membranes. Examples with the diacylphosphatidylcholine/fatty acid mixed liposomes

The fusion capability of complex lipid bilayers and its pH as well as temperature sensitivity have been studied by optical and spectroscopic means. The aggregation and fusion efficiency of such lipid membranes can be optimized by controlling the phase characteristics of the individual membrane components. For a practically relevant illustration, the stoichiometric 1:2 (mol/mol) mixtures of phosphatidylcholines and fatty acids are used. Perhaps the most interesting liposomes of this kind, which are made of dipalmitoylphosphatidylcholine/elaidic acid (DPPC/ELA-COOH (1:2)), undergo a chain-melting phase transition between 42 degrees C and 48 degrees C, depending on the bulk pH value. The highest chain-melting phase transition temperatures are measured with the fully protonated fatty acids at pH < or = 5.5 and involve a change into the non-bilayer high-temperature state. Upon increasing pH, this transition reverts into an ordinary gel-to-fluid lamellar phase change and occurs at 42 degrees C, by and large. Simultaneously, the rate and the efficacy of fusion between the PC/FA and PC/FA- mixed vesicles decreases. The fusion efficacy of the PC/FA(-) mixed liposomes at pH > or = pK(FA) approximately 7.5 is practically negligible. This is largely due to the increased interbilayer repulsion and to the relatively high water-solubility of the deprotonated fatty acid molecules at high pH. While the pH-variability chiefly affects the efficacy of the intermembrane aggregation, the vesicle fusion itself is more sensitive to temperature variations. It is more likely that the temperature dependence of the intramembrane defect density is chiefly responsible for this. Optimal conditions for the fusion between DPPC/ELA-COOH (1:2) mixed vesicles are thus 3.5 < or = pH < or = 5.5 (6.3) (aggregation maximum) and T > or = 41.5 degrees C = Tm(DPPC) (defect density and fusion maximum). Under such conditions the average size of PC/FA (1:2) mixed vesicles in a 1 mM suspension increases by a factor of 10 over a period of 10 min. Interbilayer fusion can also be catalyzed by the mechanically induced local membrane defects. Freshly made liposomes thus always fuse more avidly than aged vesicles. This permits estimates of the kinetics of membrane defects annihilation based on the measured temporal dependence of the maximum fusion-rate. From such studies, a quasi-exponential decay on the time scale of 1.2 h is found for the thermolabile fusogenic DPPC/ELA-COOH liposomes.

Polymer-induced membrane fusion: potential mechanism and relation to cell fusion events

Poly(ethylene glycol) (PEG) is used widely to mediate cell-cell fusion in the production of somatic cell hybrids and in the fusion injection of macromolecules into cultured cells from erythrocytes or liposomes. However, little is known about the mechanisms by which PEG induces fusion of cell membranes, making its use much more an art than a science. This article considers possible molecular events involved in biomembrane fusion and summarizes what we have learned about these in recent years from studies of fusion of well-defined model membranes. In addition, it recounts observations made over the past several years about the process of PEG-mediated fusion of model membranes. These observations have defined the process to an extent sufficient to allow us to propose a model for the molecular events involved in the process. It is suggested that dehydration leads to asymmetry in the lipid packing pressure in the two leaflets of the membrane bilayer leading to formation of a single bilayer septum at a point of close apposition of two membranes. The single bilayer septum then decays during formation of the initial fusion pore. Agents that enhance or alleviate the dehydration-induced asymmetric packing stress will favor or inhibit fusion. Although the proposed picture is consistent with much accumulated data, it is not yet proven; experiments must now be devised to test its details. Finally, the proposed model is discussed in terms of potential implications for the mechanisms available to a cell in controlling more complex in vivo cell fusion processes such as endocytosis, exocytosis, protein sorting/transport, and viral budding/infection.

Plasmenylethanolamine facilitates rapid membrane fusion: a stopped-flow kinetic investigation correlating the propensity of a major plasma membrane constituent to adopt an HII phase with its ability to promote membrane fusion

A critical step in membrane fusion involves the formation of a lipid intermediate which shares a conformational similarity with an inverted hexagonal phase (HII). Since plasmenylethanolamines possess a marked propensity for hexagonal phase formation and represent a major lipid constituent of several membrane systems which undergo rapid membrane fusion (e.g., plasma membranes and synaptic vesicle membranes), we compared the relative fusogenicity of lipid vesicles containing plasmenylethanolamine to that of vesicles containing their diacyl phospholipid counterpart (i.e., phosphatidylethanolamine). Vesicles comprised of equimolar mixtures of phosphatidylcholine and phosphatidylethanolamine fused slowly with phosphatidylserine vesicles in the presence of 10 mM CaCl2, as assessed either by lipid mixing (dequenching of octadecyl rhodamine fluorescence, 7.4 Fmax% s-1) or internal contents mixing (fluorescence enhancement from the resultant Tb/dipicolinic acid charge transfer complex, 8.7Fmax% s-1). In stark contrast, vesicles comprised of equimolar mixtures of phosphatidylcholine and plasmenylethanolamine fused three times more rapidly, as assessed by both lipid mixing (22.1 Fmax% s-1) and internal contents mixing (21.4Fmax% s-1) assays. The importance of an HII-like intermediate in membrane fusion was further substantiated by demonstration that plasmenylethanolamines containing arachidonic acid at the sn-2 position (which demonstrate a greater propensity for HII phase formation) exhibited the most rapid rate of membrane fusion (five times greater than phosphatidylethanolamine containing oleic acid at the sn-2 position). Furthermore, vesicles containing plasmenylethanolamines in physiologic ratios with other phospholipids (i.e., PC/PE/PS, 45:45:10, mol/mol) underwent fusion six times more rapidly (4.4Fmax% min-1) than corresponding vesicles in which plasmenylethanolamine was replaced with phosphatidylethanolamine (0.7Fmax% min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Divalent cation-induced lipid mixing between phosphatidylserine liposomes studied by stopped-flow fluorescence measurements: effects of temperature, comparison of barium and calcium, and perturbation by DPX

To understand the mechanism of membrane fusion, it is important to study the processes that mix the lipids of two apposed membranes. We measured the rates of divalent cation-induced aggregation and lipid mixing of bovine brain phosphatidylserine (BBPS) LUV, using light scattering and a resonance energy transfer assay. The lipid and divalent cation solutions were combined by stopped-flow mixing, which permitted measuring the half-times of aggregation and lipid mixing between pairs of liposomes. The collisional quencher DPX [p-xylene-bis(pyridinium bromide)], used in a liposome contents-mixing assay, lowered the main transition temperature (Tm) of BBPS by about 10 degrees C and decreased the temperature threshold for lipid mixing. Since DPX was inside the liposomes for the latter measurements, this implies that perturbations to the inner monolayer affect the reactivity of the liposome. When palmitoyl-oleoyl-PS (POPS) was substituted for BBPS, little or no lipid mixing occurred. Ca(2+)- and Ba(2+)-induced BBPS aggregation and lipid mixing were compared as a function of temperature and divalent cation concentration. Aggregation rates were nearly insensitive to temperature and correlated with the percent of PS bound to either Ba2+ or Ca2+. Above Tm, lipid-mixing rates increased with the Ba2+ and Ca2+ concentrations and temperature, even above the Tm of the Ba2+/PS complex. Arrhenius plots were linear for both ions. The temperature dependence was greater for Ca(2+)- than Ba(2+)-induced reactions, and the slopes were independent of divalent cation concentration. When equivalent fractions of PS were bound with divalent cation at, and above, 20 degrees C, the lipid-mixing rate was greater with Ca2+ than with Ba2+. The faster rate may reflect greater activation entropies and/or greater attempt frequencies at one or more steps in the Ca(2+)-induced process. We conclude that stopped-flow mixing permits better characterization of initial interaction between liposomes, that small changes in the acyl chain region of the PS bilayer or the inner monolayer can have large effects on lipid-mixing rates, and that the differences between Ba(2+)- and Ca(2+)-induced interactions may be related to qualitative differences in the destabilization step.

Effect of diacylglycerols on the activity of cobra venom, bee venom, and pig pancreatic phospholipases A2

The effects of a series of diacylglycerols (DAGs) with varying acyl chain lengths and degree of unsaturation on the activity of cobra venom, bee venom, and pig pancreatic phospholipases A2 (PL-A2S) were studied using two lipid substrates: dipalmitoylphosphatidylcholine (DPPC) or bovine liver phosphatidylcholine (BL-PC). The activities of the phospholipases critically depended on the chain length and degree of unsaturation of the added DAGs and on the chemical composition of the substrate. The effects of DAGs on cobra or bee venom PL-A2S were similar, but significantly different from the pig pancreatic PL-A2. The data, taken together with our previous NMR studies on physicochemical effects of these DAGs on lipid bilayer structure [De Boeck, H., & Zidovetzki, R. (1989) Biochemistry 28, 7439; (1992) Biochemistry 31, 623], allowed detailed correlation of the type of a bilayer perturbation induced by DAG with the activation or inhibition of the phospholipase on the same system. In general, the activation of the phospholipases correlated with the DAG-induced defects of the lipid bilayer structure. The results, however, argue against general designation of DAGs as "activators" or "inhibitors" of PL-A2S. Thus, for example, diolein activated phospholipases with the BL-PC lipid substrate, but inhibited them with the DPPC substrate. Dihexanoylglycerol and dioctanoylglycerol inhibited pig pancreatic PL-A2 with both lipid substrates and inhibited cobra or been venom PL-A2 with the DPPC substrate, but activated the latter two enzymes with the BL-PC substrate. Longer-chain DAGs (C greater than 12), which induce lateral phase separation of the bilayers into the regions of different fluidities, activated all PL-A2S with both lipid substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-induced fusion of phospholipid vesicles containing free fatty acids: modulation by transmembrane pH gradients

The influence of a transmembrane pH gradient on the Ca(2+)-induced fusion of phospholipid vesicles, containing free fatty acids, has been investigated. Large unilamellar vesicles composed of an equimolar mixture of cardiolipin, dioleoylphosphatidylcholine, and cholesterol, containing 20 mol % oleic acid, were employed. Fusion was measured using a kinetic assay for lipid mixing, based on fluorescence resonance energy transfer. At pH 7.5, but not at pH 6.0, in the absence of a pH gradient, oleic acid stimulates the fusion of the vesicles by shifting the Ca2+ threshold concentration required for aggregation and fusion of the vesicles from about 13 mM to 10 mM. In the presence of a pH gradient (at an external pH of 7.5 and a vesicle interior pH of 10.5), the vesicles exhibit fusion characteristics similar to vesicles that do not contain oleic acid at all, consistent with an effective sequestration of the fatty acid to the inner monolayer of the vesicle bilayer induced by the imposed pH gradient. The kinetics of the fusion process upon simultaneous generation of the pH gradient across the vesicle bilayer and initiation of the fusion reaction show that the inward movement of oleic acid in response to the pH gradient is extremely fast, occurring well within 1 s. Conversely, dissipation of an imposed pH gradient, by addition of a proton ionophore during the course of the fusion process, results in a rapid enhancement of the rate of fusion due to reequilibration of the oleic acid between the two bilayers leaflets.

Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N-terminal peptides with phospholipid vesicles

We have investigated the interaction of a number of synthetic 20-residue peptides, corresponding to the HA2 N-terminus of the influenza virus hemagglutinin (X31 strain), with phospholipid vesicles and monolayers. Besides the wild-type sequence, two peptides were studied with mutations corresponding to those previously studied in entire HA's expressed in transfected cells [Gething et al., (1986) J. Cell. Biol. 102, 11-23]. These mutations comprised a single Glu replacement for Gly at the N-terminus ("El" mutant) or at position 4 ("E4") of the HA2 subunit and were shown to produce striking alterations in virus-induced hemolysis and syncytia formation, especially for E1. The X31 "wild-type" (wt) peptide and its E4 variant are shown here to have the capacity to insert into phosphatidylcholine (POPC) large unilamellar vesicle (LUV) membranes in a strictly pH-dependent manner, penetration being marginal at pH 7.4 and significant at pH 5.0. Bilayer insertion was evident from a shift in the intrinsic Trp fluorescence of the wt and E4 peptides and from the induction of calcein leakage from POPC LUV and correlated well with the peptides' ability at pH 5.0 to penetrate into POPC monolayers at initial surface pressures higher than 30 mN/m. By contrast, the E1 peptide was found, at pH 5.0, to bind less tightly to vesicles (assessed by a physical separation method) and to cause much less leakage of POPC LUV than the wt, even under conditions where the peptides were bound to approximately the same extent. Consistent with the correlation between leakage and penetration observed for the wt peptide at pH 5 versus 7, the E1 peptide, even at low pH, showed much less lipid-vesicle-induced shift of its Trp fluorescence than wt, caused a much slower rate of leakage of vesicle contents, and did not insert into POPC monolayers at surface pressures beyond 28.5 mN/m. Circular dichroism spectroscopy measurements of peptides in POPC SUV showed that the conformations of all three peptides are sensitive to pH, but only the wt and E4 peptides became predominantly alpha-helical at acid pH.

On the reversibility of the phase transitions in lipid-water systems

Heating and cooling phase sequences observed in phospholipid and glycolipid dispersions in excess water have been summarized. Differences between heating and cooling sequences and also with respect to a reference phase sequence "subgel-gel-lamellar liquid crystalline-cubic-inverted hexagonal" have been pointed out. Together with kinetic data obtained by alternating current (AC) calorimetry, these data have been used for a discussion on the reversibility of the lipid phase transitions. Several typical symptoms of irreversible behavior are (i) undercooling of stable phases; (ii) formation of phases which are metastable over the whole range of their existence; (iii) slow formation of the nascent phase requiring isothermal annealing out of the transition region; (iv) different nonconvergent transition pathways in heating and cooling. These phenomena are related to the appearance of slow rearrangement modes during the phase transitions with characteristic times longer than experimental time scales. Similarly slow relaxations supporting the existence of long-lived non-equilibrium lipid states in the biomembranes may have also certain physiological significance.