Fusogenic capacities of divalent cations and effect of liposome size

In Vitro Synthesis and Reconstitution Using Mammalian Cell-Free Lysates Enables the Systematic Study of the Regulation of LINC Complex Assembly

Understanding the structure and structure-function relationships of membrane proteins is a fundamental problem in biomedical research. Given the difficulties inherent to performing mechanistic biochemical and biophysical studies of membrane proteins in vitro, we previously developed a facile HeLa cell-based cell-free expression (CFE) system that enables the efficient reconstitution of full-length (FL) functional inner nuclear membrane Sad1/UNC-84 (SUN) proteins (i.e., SUN1 and SUN2) in supported lipid bilayers. Here, we provide evidence that suggests that the reconstitution of CFE-synthesized FL membrane proteins in supported lipid bilayers occurs primarily through the fusion of endoplasmic reticulum-derived microsomes present within our CFE reactions with our supported lipid bilayers. In addition, we demonstrate the ease with which our synthetic biology platform can be used to investigate the impact of the chemical environment on the ability of CFE-synthesized FL SUN proteins reconstituted in supported lipid bilayers to interact with the luminal domain of the KASH protein nesprin-2. Moreover, we use our platform to study the molecular requirements for the homo- and heterotypic interactions between SUN1 and SUN2. Finally, we show that our platform can be used to simultaneously reconstitute three different CFE-synthesized FL membrane proteins in a single supported lipid bilayer. Overall, these results establish our HeLa cell-based CFE and supported lipid bilayer reconstitution platform as a powerful tool for performing mechanistic dissections of the oligomerization and function of FL membrane proteins in vitro. While our platform is not a substitute for cell-based studies, it does provide important mechanistic insights into the biology of difficult-to-study membrane proteins.

Membrane fusion of phospholipid bilayers under high pressure: Spherical and irreversible growth of giant vesicles

Membrane fusion of giant vesicles (GVs) for binary bilayers of unsaturated phospholipids, dioleoylphosphatidyl-ethanolamine (DOPE) having an ability to promote membrane fusion, and its homolog dioleoylphosphatidylcholine (DOPC) having an ability to form GV, was investigated under atmospheric and high pressure. While DOPC formed GVs in the presence of inorganic salts with a multivalent metal ion under atmospheric pressure, an equimolar mixture of DOPE and DOPC formed GVs both in the absence and the presence of LaCl3. We examined the change in size and shape of the GVs of this binary mixture in the absence and presence of LaCl3 as a function of time under atmospheric and high pressure. The size and shape of the GVs in the absence of LaCl3 under atmospheric and high pressure and those in the presence of LaCl3 under atmospheric pressure hardly changed with time. By contrast, the GV in the presence of LaCl3 under high pressure gradually changed in the size and shape with time on a time scale of several hours. Namely, the GV became larger than the original GV due to accelerated membrane fusion and its shape became more spherical. This pressure-induced membrane fusion was completely irreversible, and the growth rate was correlated with the applied pressure. The reason for the GV growth by applying pressure was considered on the basis of thermodynamic phase diagrams. We concluded that the growth is attributable to a closer packing of lipid molecules in the bilayer resulting from their preference of smaller volumes under high pressure. Furthermore, the molecular mechanism of the pressure-induced membrane fusion was explored by observing the fusion of two GVs with almost the same size. From their morphological changes, we revealed that the fusion is caused by the actions of Laplace and osmotic pressure.

Division and Regrowth of Phase-Separated Giant Unilamellar Vesicles*

Success in the bottom‐up assembly of synthetic cells will depend on strategies for the division of protocellular compartments. Here, we describe the controlled division of phase‐separated giant unilamellar lipid vesicles (GUVs). We derive an analytical model based on the vesicle geometry, which makes four quantitative predictions that we verify experimentally. We find that the osmolarity ratio required for division is 2 , independent of the GUV size, while asymmetric division happens at lower osmolarity ratios. Remarkably, we show that a suitable osmolarity change can be triggered by water evaporation, enzymatic decomposition of sucrose or light‐triggered uncaging of CMNB‐fluorescein. The latter provides full spatiotemporal control, such that a target GUV undergoes division whereas the surrounding GUVs remain unaffected. Finally, we grow phase‐separated vesicles from single‐phased vesicles by targeted fusion of the opposite lipid type with programmable DNA tags to enable subsequent division cycles.

High-Throughput Monitoring of Single Vesicle Fusion Using Freestanding Membranes and Automated Analysis

In vivo membrane fusion primarily occurs between highly curved vesicles and planar membranes. A better understanding of fusion entails an accurate in vitro reproduction of the process. To date, supported bilayers have been commonly used to mimic the planar membranes. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that induce membrane fusion usually have limited fluidity when embedded in supported bilayers. This alters the kinetics and prevents correct reconstitution of the overall fusion process. Also, observing content release across the membrane is hindered by the lack of a second aqueous compartment. Recently, a step toward resolving these issues was achieved by using membranes spread on holey substrates. The mobility of proteins was preserved but vesicles were prone to bind to the substrate when reaching the edge of the hole, preventing the observation of many fusion events over the suspended membrane. Building on this recent advance, we designed a method for the formation of pore-spanning lipid bilayers containing t-SNARE proteins on Si/SiO₂ holey chips, allowing the observation of many individual vesicle fusion events by both lipid mixing and content release. With this setup, proteins embedded in the suspended membrane bounced back when they reached the edge of the hole which ensured vesicles did not bind to the substrate. We observed SNARE-dependent membrane fusion with the freestanding bilayer of about 500 vesicles. The time between vesicle docking and fusion is ∼1 s. We also present a new multimodal open-source software, Fusion Analyzer Software, which is required for fast data analysis.

Direct detection of SERCA calcium transport and small-molecule inhibition in giant unilamellar vesicles

We have developed a charge-mediated fusion method to reconstitute the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) in giant unilamellar vesicles (GUV). Intracellular Ca²⁺ transport by SERCA controls key processes in human cells such as proliferation, signaling, and contraction. Small-molecule effectors of SERCA are urgently needed as therapeutics for Ca²⁺ dysregulation in human diseases including cancer, diabetes, and heart failure. Here we report the development of a method for efficiently reconstituting SERCA in GUV, and we describe a streamlined protocol based on optimized parameters (e.g., lipid components, SERCA preparation, and activity assay requirements). ATP-dependent Ca²⁺ transport by SERCA in single GUV was detected directly using confocal fluorescence microscopy with the Ca²⁺ indicator Fluo-5F. The GUV reconstitution system was validated for functional screening of Ca²⁺ transport using thapsigargin (TG), a small-molecule inhibitor of SERCA currently in clinical trials as a prostate cancer prodrug. The GUV system overcomes the problem of inhibitory Ca²⁺ accumulation for SERCA in native and reconstituted small unilamellar vesicles (SUV). We propose that charge-mediated fusion provides a widely-applicable method for GUV reconstitution of clinically-important membrane transport proteins. We conclude that GUV reconstitution is a technological advancement for evaluating small-molecule effectors of SERCA.

Dynamic microPET imaging of ultrasound contrast agents and lipid delivery

Interest in ultrasound contrast agents (lipid-shelled microbubbles) as delivery vehicles is increasing; however, the biodistribution of these agents remains uncharacterized, both with and without ultrasound. In this study, an (18)F-labeled lipid ([(18)F]fluorodipalmitin), incorporated in microbubble shells, was used as a dynamic microPET probe for quantitative 90-minute biodistribution measurements in male Fischer 344 rats (n=2). The spleen retained the highest concentration of radioactive lipid at approximately 2.6%-injected dose per cubic centimeter (% ID/cc) and the liver demonstrated the largest total accumulation (approximately 17% ID). The microbubble pharmacokinetic profile differed from free lipid, which is rapidly cleared from blood, and liposomes, which remain in circulation. Additionally, region of interest (ROI) analysis over 60 minutes (post-ultrasound treatment) quantified the delivery of lipid by therapeutic ultrasound from microbubbles to kidney tissue (n=8). The ultrasound sequence consisted of a 200 kPa, 5.3 MHz radiation force pulse followed by a 1.6 MPa, 1.4 MHz fragmentation pulse and was applied to one kidney, while the contralateral kidney served as a control. ROI-estimated activity in treated kidneys was slightly but significantly greater at 0 and 60 min than in untreated kidneys (p=0.0012 and 0.0035, respectively). This effect increased with the number of microbubbles injected (p=0.006). In summary, [(18)F]fluorodipalmitin was used to characterize the biodistribution of contrast microbubble shells and the deposition of lipid was shown to be locally increased after insonation.

Acoustically-active microbubbles conjugated to liposomes: characterization of a proposed drug delivery vehicle

A new acoustically-active delivery vehicle was developed by conjugating liposomes and microbubbles, using the high affinity interaction between avidin and biotin. Binding between microbubbles and liposomes, each containing 5% DSPE-PEG2kBiotin, was highly dependent on avidin concentration and observed above an avidin concentration of 10 nM. With an optimized avidin and liposome concentration, we measured and calculated as high as 1000 to 10,000 liposomes with average diameters of 200 and 100 nm, respectively, attached to each microbubble. Replacing avidin with neutravidin resulted in 3-fold higher binding, approaching the calculated saturation level. High-speed photography of this new drug delivery vehicle demonstrated that the liposome-bearing microbubbles oscillate in response to an acoustic pulse in a manner similar to microbubble contrast agents. Additionally, microbubbles carrying liposomes could be spatially concentrated on a monolayer of PC-3 cells at the focal point of ultrasound beam. As a result of cell-vehicle contact, the liposomes fused with the cells and internalization of NBD-cholesterol occurred shortly after incubation at 37 degrees C, with internalization of NBD-cholesterol substantially enhanced in the acoustic focus.

Triggering and visualizing the aggregation and fusion of lipid membranes in microfluidic chambers

We present a method that makes it possible to trigger, observe, and quantify membrane aggregation and fusion of giant liposomes in microfluidic chambers. Using electroformation from spin-coated films of lipids on transparent indium tin oxide electrodes, we formed two-dimensional networks of closely packed, surface-attached giant liposomes. We investigated the effects of fusogenic agents by simply flowing these molecules into the chambers and analyzing the resulting shape changes of more than 100 liposomes in parallel. We used this setup to quantify membrane fusion by several well-studied mechanisms, including fusion triggered by Ca2+, polyethylene glycol, and biospecific tethering. Directly observing many liposomes simultaneously proved particularly useful for studying fusion events in the presence of low concentrations of fusogenic agents, when fusion was rare and probabilistic. We applied this microfluidic fusion assay to investigate a novel 30-mer peptide derived from a recently identified human receptor protein, B5, that is important for membrane fusion during the entry of herpes simplex virus into host cells. This peptide triggered fusion of liposomes at an approximately 6 times higher probability than control peptides and caused irreversible interactions between adjacent membranes; it was, however, less fusogenic than Ca2+ at comparable concentrations. Closely packed, surface-attached giant liposomes in microfluidic chambers offer a method to observe membrane aggregation and fusion in parallel without requiring the use of micromanipulators. This technique makes it possible to characterize rapidly novel fusogenic agents under well-defined conditions.

SNARE-mediated lipid mixing depends on the physical state of the vesicles

Reconstitution experiments have suggested that N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins constitute a minimal membrane fusion machinery but have yielded contradictory results, and it is unclear whether the mechanism of membrane merger is related to the stalk mechanism that underlies physiological membrane fusion. Here we show that reconstitution of solubilized neuronal SNAREs into preformed 100 nm liposomes (direct method) yields proteoliposomes with more homogeneous sizes and protein densities than the standard reconstitution method involving detergent cosolubilization of proteins and lipids. Standard reconstitutions yield slow but efficient lipid mixing at high protein densities and variable amounts of lipid mixing at moderate protein densities. However, the larger, more homogenous proteoliposomes prepared by the direct method yield almost no lipid mixing at moderate protein densities. These results suggest that the lipid mixing observed for standard reconstitutions is dominated by the physical state of the membrane, perhaps due to populations of small vesicles (or micelles) with high protein densities and curvature stress created upon reconstitution. Accordingly, changing membrane spontaneous curvature by adding lysophospholipids inhibits the lipid mixing observed for standard reconstitutions. Our data indicate that the lipid mixing caused by high SNARE densities and/or curvature stress occurs by a stalk mechanism resembling the mechanism of fusion between biological membranes, but the neuronal SNAREs are largely unable to induce lipid mixing at physiological protein densities and limited curvature stress.

Ion regulation of homotypic vacuole fusion in Saccharomyces cerevisiae

Biological membrane fusion employs divalent cations as protein cofactors or as signaling ligands. For example, Mg2+ is a cofactor for the N-ethylmaleimide-sensitive factor (NSF) ATPase, and the Ca2+ signal from neuronal membrane depolarization is required for synaptotagmin activation. Divalent cations also regulate liposome fusion, but the role of such ion interactions with lipid bilayers in Rab- and soluble NSF attachment protein receptor (SNARE)-dependent biological membrane fusion is less clear. Yeast vacuole fusion requires Mg2+ for Sec18p ATPase activity, and vacuole docking triggers an efflux of luminal Ca2+. We now report distinct reaction conditions where divalent or monovalent ions interchangeably regulate Rab- and SNARE-dependent vacuole fusion. In reactions with 5 mm Mg2+, other free divalent ions are not needed. Reactions containing low Mg2+ concentrations are strongly inhibited by the rapid Ca2+ chelator BAPTA. However, addition of the soluble SNARE Vam7p relieves BAPTA inhibition as effectively as Ca2+ or Mg2+, suggesting that Ca2+ does not perform a unique signaling function. When the need for Mg2+, ATP, and Sec18p for fusion is bypassed through the addition of Vam7p, vacuole fusion does not require any appreciable free divalent cations and can even be stimulated by their chelators. The similarity of these findings to those with liposomes, and the higher ion specificity of the regulation of proteins, suggests a working model in which ion interactions with bilayer lipids permit Rab- and SNARE-dependent membrane fusion.

Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

Previously, we showed that the proton permeability of small unilamellar vesicles (SUVs) composed of polar lipid fraction E (PLFE) from the thermoacidophilic archaeon Sulfolobus acidocaldarius was remarkably low and insensitive to temperature (Komatsu and Chong 1998). In this study, we used photon correlation spectroscopy to investigate the time dependence of PLFE SUV size as a function of Ca2+ concentration. In the absence of Ca2+, vesicle diameter changed little over 6 months. Addition of Ca2+, however, immediately induced formation of vesicle aggregates with an irregular shape, as revealed by confocal fluorescence microscopy. Aggregation was reversible upon addition of EDTA; however, the reversibility varied with temperature as well as incubation time with Ca2+. Freeze-fracture electron microscopy showed that, after a long period of incubation (2 weeks) with Ca2+, the PLFE vesicles had not just aggregated, but had fused or coalesced. The initial rate of vesicle aggregation varied sigmoidally with Ca2+ concentration. At pH 6.6, the threshold calcium concentration (Cr) for vesicle aggregation at 25 and 40 degrees C was 11 and 17 mM, respectively. At pH 3.0, the Cr at 25 degrees C increased to 25 mM. The temperature dependence of Cr may be attributable to changes in membrane surface potential, which was -22.0 and -13.2 mV at 25 and 40 degrees C, respectively, at pH 6.6, as determined by 2-(p-toluidinyl)naphthalene-6-sulfonic acid fluorescence. The variation in surface potential with temperature is discussed in terms of changes in lipid conformation and membrane organization.

Cholesterol stabilizes hemifused phospholipid bilayer vesicles

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

A novel N-acyl phosphatidylethanolamine-containing delivery vehicle for spermine-condensed plasmid DNA

A unique method for formulation of plasmid DNA with phospholipids has been devised for the purpose of producing vehicles that can mediate gene delivery and transfection of living cells. The polycation, spermine, was used to condense plasmid DNA within a water-in-chloroform emulsion stabilized by phospholipids. After organic solvent removal, the particles formed could be extruded to a number average size of about 200 nm and retained DNA that was protected from nuclease digestion. This resulted in a relatively high protected DNA-to-lipid ratio of approximately 1 microg DNA/micromol lipid. The size distribution of the preparation was relatively homogeneous as judged by light microscopy and quasi-elastic light scattering. Electron microscopic studies showed structural heterogeneity, but suggested that at least some of the plasmid DNA in this preparation was in the form of the previously observed spermine-condensed bent rods and toroids and was encapsulated within liposomal membranes. Preparations with the fusogenic phospholipid composition, 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-dodecanoyl/ 1, 2-dioleoyl-sn-glycero-3-phosphocholine, showed transfection activity for several cells lines, particularly OVCAR-3 cells. The transfection activity sedimented with the lipid during centrifugation, confirming the association of active plasmid DNA with phospholipids. Transfection efficiency in culture was found to be of the same order of magnitude as cationic lipoplexes but much less toxic to the cells. Significant transfection of OVCAR-3 cells in tissue culture could also be observed, even in the presence of the intraperitoneal fluid from a mouse with an OVCAR-3 ascites tumor. These data indicate a new type of liposomal gene delivery system devoid of cationic lipids, phosphatidylethanolamine, cationic polymers and viral components.

Phosphate-enhanced transfection of cationic lipid-complexed mRNA and plasmid DNA

Cationic lipid-mediated gene transfer has been shown to be a competent albeit inefficient mechanism of promoting cellular gene transfer. One way to improve the efficacy of cationic lipid-mediated transgene expression is to optimize conditions for complex formation between the lipids and nucleic acids. In this report we describe the beneficial effects of using phosphate buffer to precondition lipofectin (a 1:1 (w/w) mixture of N-[1-(2,3-dioleyloxy)propyl]-n,n, n-trimethylammonium chloride (DOTMA), and dioleoyl phosphatidylethanolamine (DOPE)) prior to complexing with plasmid DNA or mRNA. Under such optimized conditions we studied the kinetics of DNA- and RNA-mediated transgene expression in a human osteosarcoma cell line (HOS). Preincubation of lipofectin in phosphate buffer resulted in up to 26- and 56-fold increases in luciferase expression from plasmid DNA and mRNA, respectively. Addition of chloroquine (50 microM), which enhanced plasmid-mediated gene delivery 3-fold, was synergistic with phosphate resulting in an additional 46-fold increase in luciferase expression. The preincubation with phosphate shortened both the time required for cellular uptake and the time to achieve maximal transgene expression. Optimal transfection was achieved in the presence of 30-80 mM phosphate, at pH 5.6-6.8 under which the phosphate anion is divalent. The effect of phosphate anion was specific in that monovalent Cl- and acetate anions were not stimulatory. These results demonstrate that divalent phosphate anion plays a stimulatory role during complex formation and transfection when cationic lipids come in contact with negatively charged nucleic acids and cell membranes. These findings delineate specific conditions which dramatically enhance transfection efficiency for both DNA and mRNA, and provide an effective procedure for gene transfection studies.

Polyphosphoinositide inclusion in artificial lipid bilayer vesicles promotes divalent cation-dependent membrane fusion

Recent studies suggest that phosphoinositide kinases may participate in intracellular trafficking or exocytotic events. Because both of these events ultimately require fusion of biological membranes, the susceptibility of membranes containing polyphosphoinositides (PPIs) to divalent cation-induced fusion was investigated. Results of these investigations indicated that artificial liposomes containing PPI or phosphatidic acid required lower Ca2+ concentrations for induction of membrane fusion than similar vesicles containing phosphatidylserine, phosphatidylinositol, or phosphatidylcholine. This trend was first observed in liposomes composed solely of one type of phospholipid. In addition, however, liposomes designed to mimic the phospholipid composition of the endofacial leaflet of plasma membranes (i.e., liposomes composed of combinations of PPI, phosphatidylethanolamine, and phosphatidylcholine) also required lower Ca2+ concentrations for induction of aggregation and fusion. Liposomes containing PPI and phosphatidic acid also had increased sensitivity to Mg(2+)-induced fusion, an observation that is particularly intriguing given the intracellular concentration of Mg2+ ions. Moreover, the fusogenic effects of Ca2+ and Mg2+ were additive in vesicles containing phosphatidylinositol bisphosphate. These data suggest that enzymatic modification of the PPI content of intracellular membranes could be an important mechanism of fusion regulation.

Effect of physical constraints on the mechanisms of membrane fusion: bolaform lipid vesicles as model systems

Bolaform lipid vesicles were used to study the effect of physical constraints on membrane fusion. In these vesicles the membrane is organized in a single monolayer, because of the presence of covalent bonds in its middle plane. Therefore, the formation of fusion intermediates is subject to higher energy barriers and greater geometrical constraints than is usual in bilayer membranes. Bolaform lipids were extracted from the thermophilic archaeon Sulfolobus solfataricus. These lipids can be divided into two classes, the monosubstituted molecules, in which one of the polar heads is glycerol, and the bisubstituted molecules, endowed with two complex polar heads. The fusion process in vesicles composed of different mixtures of monosubstituted/bisubstituted molecules was studied by means of fluorescence techniques. Ca2+ or poly(ethylene glycol) was employed as a fusogenic agent. We found that fusion of such constrained membranes is still possible, provided that molecules able to mediate a structural rearrangement of the membrane are present. This condition is fulfilled by monosubstituted molecules, which are able to partition the glycerol headgroup in the apolar moiety. In addition, the presence of traces (approximately 5%) of the monopolar compound diphytanylglycerol is an important factor for fusion to occur. On the contrary, vesicles formed by bisubstituted molecules are unable to fuse, irrespective of the fusogen employed.

Development of an aqueous-space mixing assay for fusion of granules and plasma membranes from human neutrophils

Several models have been developed to study neutrophil degranulation. At the most basic level, phospholipid vesicles have been used to investigate the lipid interactions occurring during membrane fusion. The two major forms of assays used to measure phospholipid vesicle fusion are based either on the dilution of tagged phospholipids within the membrane of the two fusing partners or the mixing of the aqueous contents of the vesicles. Although problems exist with both methods, the latter is considered to be more accurate and representative of true fusion. Using 8-aminonaphthalene-1,3,6-trisulphonic acid (ANTS) as a fluorescent marker, we have taken advantage of the quenching properties of p-xylenebispyridinium bromide ('DPX') to develop a simple aqueous-space mixing assay that can be used with any sealed vesicle. We compared our new assay with more conventional assays using liposomes composed of phosphatidic acid (PA) and phosphatidylethanolamine (PE), obtaining comparable results with respect to Ca2+-dependent fusion. We extended our studies to measure the fusion of neutrophil plasma-membrane vesicles as well as azurophil and specific granules with PA/PE (1:3) liposomes. Both specific granules and plasma-membrane vesicles fused with PA/PE liposomes at [Ca2+] as low as 500 microM, while azurophil granules showed no fusion at [Ca2+] as high as 12 mM. These differences in the ability of Ca2+ to induce fusion may be related to differences observed in whole cells with respect to secretion.

Quenching and dequenching of octadecyl Rhodamine B chloride fluorescence in Ca(2+)-induced fusion of phosphatidylserine vesicles: effects of poly(ethylene glycol)

Ca(2+)-induced fusion of SUV and LUV composed of ox brain phosphatidylserine (PS) was studied as a function of temperature and concentration of Ca2+ using octadecyl Rhodamine B chloride (R-18). Ca2+ was added to a 1:1 mixture of labelled (8 mol%) and unlabelled vesicles (assay conditions) or to samples containing only labelled liposomes (control conditions). Both, in SUV and LUV the dependence of differences in fluorescence between assay and control samples on temperature can be divided into three regions. At temperatures lower than 20 degrees C the differences in fluorescence increase only slightly in SUV or remain unchanged in LUV after the addition of Ca2+. At 28 degrees C and higher temperatures the differences of fluorescence intensities increase much more drastically, whereby SUV exhibit higher fusion rates than LUV. Between 20 degrees C and 28 degrees C exists an intermediate region for both SUV and LUV. Here the fluorescence changes continuously from one behaviour to the other independent of the concentration of Ca2+. A drastic quenching of R-18 fluorescence occurs in LUV composed of PS below 10 degrees C, where the lipids are in the gel state. In SUV the fluorescence is only weakly changed in this temperature region. It is assumed that a demixing between dye and phospholipid molecules occurs below phase transition. During fusion the phase transition of PS is shifted from 8-10 degrees C to about 24-28 degrees C as revealed by polarization measurements using diphenylhexatriene. Because the differences in R-18 fluorescence between assay and control samples depend strongly on temperature we assume that the shift in phase transition temperature of PS occurs immediately after the addition of Ca2+ to SUV or LUV. Poly(ethylene glycol) 6000 accelerates fusion in both SUV and LUV under all conditions where a fusion takes place. Further, the threshold concentration of Ca2+ to induce fusion is diminished from about 1 mmol/l without polymers to about 0.5 mmol/l in the presence of 10% (w/v) PEG 6000. The intermediate region of changes in fluorescence properties of R-18 in the Ca(2+)-induced fusion of PS is not changed by PEG.

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.

Annexin-mediated membrane fusion of human neutrophil plasma membranes and phospholipid vesicles

Membrane fusion was studied using human neutrophil plasma membrane preparations and phospholipid vesicles approximately 0.15 microns in diameter and composed of phosphatidylserine and phosphatidylethanolamine in a ratio of 1 to 3. Liposomes were labeled with N-(7-nitrobenzo-2-oxa-1,3-diazol-4-yl (NBD) and lissamine rhodamine B derivatives of phospholipids. Apparent fusion was detected as an increase in fluorescence of the resonance energy transfer donor, NBD, after dilution of the probes into unlabeled membranes. 0.5 mM Ca2+ alone was sufficient to cause substantial fusion of liposomes with a plasma membrane preparation but not with other liposomes. Both annexin I and des(1-9)annexin I caused a substantial increase in the rate of fusion under these conditions while annexin V inhibited fusion. Fusion mediated by des(1-9)annexin I was observed at Ca2+ concentrations as low as approximately 5 microM, suggesting that the truncated form of this protein may be active at physiologically low Ca2+ concentrations. Trypsin treated plasma membranes were incapable of fusion with liposomes, suggesting that plasma membrane proteins may mediate fusion. Liposomes did not fuse with whole cells at any Ca2+ concentration, indicating that the cytoplasmic side of the membrane is involved. These results suggest that annexin I and unidentified plasma membrane proteins may play a role in Ca(2+)-dependent degranulation of human neutrophils.

A dynamical model for phase transitions of lipids induced by calcium ions

Lipid membranes exist essentially in two different phases. A phase transition can be triggered off either by changing the temperature or, isothermally, by varying an external factor such as ionic concentration, pH, organic solvents, etc. Since the isothermal transition may be induced at physiological temperature, it may play an important regulatory role in diverse cellular functions. Based on the Landau-Ginsburg theory, the thermotropic transitions of lipids has been described by a number of models. In the present work a dynamical model for an isothermal phase transition of phospholipids induced by ionic binding is proposed. The properties of the model show that by ionic binding, phospholipids may form spatial heterogeneous distributions of lipids in fluid and crystalline phases. This heterogeneity possibly being the cause of membrane instabilities, which favour enhanced vesicle fusions observed in the presence of Ca2+.

Molecular mechanisms of calcium-induced membrane fusion

We have reviewed studies on calcium-induced fusion of lipid bilayer membranes and the role of synexin and other calcium-binding proteins (annexins) in membrane fusion. We have also discussed the roles of other cations, lipid phase transitions, long chain fatty acids and other fusogenic molecules. Finally, we have presented a simple molecular model for the mechanism of lipid membrane fusion, consistent with the experimental evidence and incorporating various elements proposed previously.

Monovalent cation-induced fusion of acidic phospholipid vesicles

Fusion of small unilamellar vesicles (SUV) consisting of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and phosphatidylglycerol (PG) from egg yolk, dipalmitoylphosphatidylserine (DPPS) and phosphatidylserine (PS) from bovine brain was studied as a function of monovalent cation concentration. Fusion was detected by measuring the changes in the excimer to monomer fluorescence intensity ratio (IE/M) of pyrene-labeled phospholipid analogues upon fusion of the pyrene-labeled and unlabeled vesicles. No fusion was observed from vesicles consisting of DMPC, PS from bovine brain or PG from egg yolk upon addition of NaCl (up to 1 M). However, considerable fusion was evident for vesicles consisting of DMPG or DPPS upon addition of monovalent cations (300 mM to 1 M). Fusion kinetics were fast reaching a plateau after 5 min of addition of cations. The order of efficiency of different monovalent cations to induce the fusion of DMPG vesicles as judged by the changes of the IE/M ratio was Li+ greater than Na+ greater than K+ greater than Cs+. DSC-scan of sonicated DMPG vesicles showed, in the absence of salt, a phase transition at 19.2 degrees C with enthalpy of 1.1 kcal.mol-1. After incubation in the presence of 600 mM NaCl the DSC scan showed a narrow phase transition at 24.1 degrees C with enthalpy of 6.9 kcal.mol-1 and a pronounced pretransition, both supporting that the fusion of the vesicles had occurred in the presence of NaCl. The results indicate that sonicated vesicles consisting of acidic phospholipids with fully saturated fatty acids fuse in the presence of monovalent cations, whereas those containing unsaturated fatty acids do not.

Cholesterol affects divalent cation-induced fusion and isothermal phase transitions of phospholipid membranes

The influence of cholesterol on divalent cation-induced fusion and isothermal phase transitions of large unilamellar vesicles composed of phosphatidylserine (PS) was investigated. Vesicle fusion was monitored by the terbium/dipicolinic acid assay for the intermixing of internal aqueous contents, in the temperature range 10-40 degrees C. The fusogenic activity of the cations decreases in the sequence Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mg2+ for cholesterol concentrations in the range 20-40 mol%, and at all temperatures. Increasing the cholesterol concentration decreases the initial rate of fusion in the presence of Ca2+ and Ba2+ at 25 degrees C, reaching about 50% of the rate for pure PS at a mole fraction of 0.4. From 10 to 25 degrees C, Mg2+ is ineffective in causing fusion at all cholesterol concentrations. However, at 30 degrees C, Mg2+-induced fusion is observed with vesicles containing cholesterol. At 40 degrees C, Mg2+ induces slow fusion of pure PS vesicles, which is enhanced by the presence of cholesterol. Increasing the temperature also causes a monotonic increase in the rate of fusion induced by Ca2+, Ba2+ and Sr2+. The enhancement of the effect of cholesterol at high temperatures suggests that changes in hydrogen bonding and interbilayer hydration forces may be involved in the modulation of fusion by cholesterol. The phase behavior of PS/cholesterol membranes in the presence of Na+ and divalent cations was studied by differential scanning calorimetry. The temperature of the gel-liquid crystalline transition (Tm) in Na+ is lowered as the cholesterol content is increased, and the endotherm is broadened. Addition of divalent cations shifts the Tm upward, with a sequence of effectiveness Ba2+ greater than Sr2+ greater than Mg2+. The Tm of these complexes decreases as the cholesterol content is increased. Although the transition is not detectable for cholesterol concentrations of 40 and 50 mol% in the presence of Na+, Sr2+ or Mg2+, the addition of Ba2+ reveals endotherms with Tm progressively lower than that observed at 30 mol%. Although the presence of cholesterol appears to induce an isothermal gel-liquid crystalline transition by decreasing the Tm, this change in membrane fluidity does not enhance the rate of fusion, but rather decreases it. The effect of cholesterol on the fusion of PS/phosphatidylethanolamine (PE) vesicles was investigated by utilizing a resonance energy transfer assay for lipid mixing. The initial rate of fusion of PS/PE and PS/PE/cholesterol vesicles is saturated at high Mg2+ concentrations. With Ca2+, saturation is not observed for cholesterol-containing vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)

Monolayer coupling in phosphatidylserine bilayers: distinct phase transitions induced by magnesium interacting with one or both monolayers

We have investigated the thermotropic behavior of phospatidylserine bilayers interacting with Mg2+ either on one side or both sides, using differential scanning calorimetry. Large unilamellar vesicles (LUV) of phosphatidylserine exposed to Mg2+ on the external side only displayed an upward shift of the gel-liquid transition temperature (Tm) of about 6-8 degrees C relative to the Tm of LUV in Na+. Mg2+ was shown not to enter the vesicle interior, by means of fluorescence measurements on encapsulated 8-hydroxyquinoline-5-sulfonate. Multilamellar vesicles prepared in the presence of Mg2+, or vesicles prepared by Mg2+-induced fusion of small unilamellar vesicles, had Tm values that were shifted upward by about 16-17 C degrees. When the latter preparation was treated with EDTA to produce vesicles with Mg2+ inside and Na+ outside, the Tm was found to be shifted again by only 6-8 degrees C. These observations indicate that the monolayer interacting with Na+ fluidizes the monolayer interacting with Mg2+, and that the latter tends to solidify the former. The two monolayers thus appear to be coupled, possibly by hydrocarbon chain interdigitation.

La3+-induced fusion of phosphatidylserine liposomes. Close approach, intermembrane intermediates, and the electrostatic surface potential

The fusion of large unilamellar phosphatidylserine liposomes (PS LUV) induced by La3+ has been monitored using the 1-aminoapthalene-3,6,8-trisulfonic acid/p-xylenebis(pyridinium bromide) (ANTS/DPX) fluorescence assay for the mixing of aqueous contents. The fusion event is extensive and nonleaky, with up to 95% mixing of contents in the fused liposomes. However, addition of excess EDTA leads to disruption of the fusion products in a way that implies the existence of metastable intermembrane contact sites. The maximal fusion activity occurs between 10 and 100 microM La3+ and fusion can be terminated rapidly, without loss of contents, by the addition of excess La3+, e.g., 1 mM La3+ at pH 7.4. This observation is explained by the very large intrinsic binding constant (approximately 10(5) M-1) of La3+ to the PS headgroup, as measured by microelectrophoresis. Addition of 1 mM La3+ causes charge reversal of the membrane and a large positive surface potential. La3+ binding to PS causes the release of a proton. These data can be explained if La3+ can chelate to PS at two sites, with one of the sites being the primary amino group. This binding model successfully predicts that at pH 4.5 fusion occurs up to 2 mM La3+, due to reduced La3+ binding at low pH. We conclude that the general mechanism of membrane fusion includes three kinetic steps. In addition to (a) aggregation, there is (b) the close approach of the surfaces, or thinning of the hydration layer, and (c) the formation of intermembrane intermediates which determine the extent to which membrane destabilization leads to fusion (mixing of aqueous contents), as opposed to lysis. The lifetime of these intermembrane intermediates appears to depend upon La3+ binding to both PS sites.

Lipid mixing during membrane aggregation and fusion: why fusion assays disagree

The kinetics of lipid mixing during membrane aggregation and fusion was monitored by two assays employing resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) and N-(lissamine Rhodamine B sulfonyl)phosphatidylethanolamine (Rh-PE). For the "probe mixing" assay, NBD-PE and Rh-PE were incorporated into separate populations of phospholipid vesicles. For the "probe dilution" assay, both probes were incorporated into one population of vesicles, and the assay monitored the dilution of the molecules into the membrane of unlabeled vesicles. The former assay was found to be very sensitive to aggregation, even when the internal aqueous contents of the vesicles did not intermix. Examples of this case were large unilamellar vesicles (LUV) composed of phosphatidylserine (PS) in the presence of Mg2+ and small unilamellar vesicles (SUV) composed of phosphatidylserine in the presence of high concentrations of Na+. No lipid mixing was detected in these cases by the probe dilution assay. Under conditions where membrane fusion (defined as the intermixing of aqueous contents with concomitant membrane mixing) was observed, such as LUV (PS) in the presence of Ca2+, the rate of probe mixing was faster than that of probe dilution, which in turn was faster than the rate of contents mixing. Two assays monitoring the intermixing of aqueous contents were also compared. The Tb/dipicolinic acid assay reported slower fusion rates than the 1-aminonaphthalene-3,6,8-trisulfonic acid/N,N'-p-xylylene-bis(pyridinium bromide) assay for PS LUV undergoing fusion in the presence of Ca2+. These observations point to the importance of utilizing contents mixing assays in conjunction with lipid mixing assays to obtain the rates of membrane destabilization and fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Fusion of phosphatidylethanolamine-containing liposomes and mechanism of the L alpha-HII phase transition

The initial kinetics of fusion and leakage of liposomes composed of N-methylated dioleoylphosphatidylethanolamine (DOPE-Me) have been correlated with the phase behavior of this lipid. Gagné et al. [Gagné, J., Stamatatos, L., Diacovo, T., Hui, S. W., Yeagle, P., & Silvius, J. (1985) Biochemistry 24, 4400-4408] have shown that this lipid is lamellar (L alpha) below 20 degrees C, is hexagonal (HII) above 70 degrees C, and shows isotropic 31P NMR resonances at intermediate temperatures. This isotropic state is also characterized by complex morphological structures. We have prepared DOPE-Me liposomes at pH 9.5 and monitored the temperature dependence of the mixing of aqueous contents, leakage, and changes in light scattering upon reduction of the pH to 4.5. At and below 20 degrees C, where the lipid is in the L alpha phase, there is very little aggregation or destabilization of the liposomes. Between 30 and 60 degrees C, i.e., where the lipid is in the isotropic state, the initial rates of liposome fusion (mixing of aqueous contents) and leakage increase. At temperatures approaching that where the hexagonal HII phase transition occurs, the initial rates and extents of fusion decrease, whereas leakage is enhanced. Similar results were found for dioleoylphosphatidylethanolamine/dioleoylphosphatidylcholine (2:1) liposomes. These results clearly establish a common mechanism between the appearance of the isotropic state (between the L alpha and HII phases) and the promotion of liposome fusion. We propose a simple model to explain both the observed behavior of phosphatidylethanolamine-containing membranes with respect to liposome fusion and/or lysis and the beginning of the L alpha-HII phase transition.

Membrane fusion: lipid vesicles as a model system

In many cellular functions the process of membrane fusion is of vital importance. It occurs in a highly specific and strictly controlled fashion. Proteins are likely to play a key role in the induction and modulation of membrane fusion reactions. Aimed at providing insight into the molecular mechanisms of membrane fusion, numerous studies have been carried out on model membrane systems. For example, the divalent-cation induced aggregation and fusion of vesicles consisting of negatively charged phospholipids, such as phosphatidylserine (PS) or cardiolipin (CL), have been characterized in detail. It is important to note that these systems largely lack specificity and control. Therefore conclusions derived from their investigation can not be extrapolated directly to a seemingly comparable counterpart in biology. Yet, the study of model membrane systems does reveal the general requirements of lipid bilayer fusion. The most prominent barrier to molecular contact between two apposing bilayers appears to be due to the hydration of the polar groups of the lipid molecules. Thus, dehydration of the bilayer surface and fluctuations in lipid packing, allowing direct hydrophobic interactions, are critical to the induction of membrane fusion. These membrane alterations are likely to occur only locally, at the site of intermembrane contact. Current views on the way membrane proteins may induce fusion under physiological conditions also emphasize the notion of local surface dehydration and perturbation of lipid packing, possibly through penetration of apolar amino acid segments into the hydrophobic membrane interior.