The rate of lipid transfer during fusion depends on the structure of fluorescent lipid probes: a new chain-labeled lipid transfer probe pair

Sequential administration of PEG-Span 80 niosome enhances anti-tumor effect of doxorubicin-containing PEG liposome

To improve the anti-tumor effect of polyethylene glycol-modified liposome containing doxorubicin (DOX-PEG liposome), the effect of sequential administration of PEG-Span 80 niosome was investigated for Colon-26 cancer cells (C26)-bearing mice. The concept of the current study is as follows: Since both particulates would be accumulated in the tumor tissue due to the enhanced permeability and retention (EPR) effect, PEG-Span 80 niosome, mainly composed of synthetic surfactant (Span 80), would interact with DOX-PEG liposome and be a trigger to induce the release of DOX from the liposome within the tumor tissue, leading to the improvement of anti-tumor effect of DOX-PEG liposome. To find out an adequate liposome for this strategy, several PEG liposomes with different compositions were examined in terms of drug release enhancement and it was found that PEG-Span80 niosome could significantly enhance the release of calcein and DOX from a PEG liposome composed of 90% hydrogenated soybean phosphatidylcholine (HSPC) and 10% cholesterol. The sequential administration of PEG-Span 80 niosome at 24 or 48 h after dosing of DOX-PEG liposome provided a higher anti-tumor effect than the single dose of DOX-PEG liposome in the C26-bearing mice. Particularly, the 24 h-later dosing of PEG-Span 80 niosome has been found to be more effective than the 48 h-later dosing. It was also confirmed that the coexistence of PEG-Span 80 niosome with DOX-PEG liposome in 50% serum or in 50% supernatant of tumor tissue homogenate significantly increased DOX release from PEG liposome, suggesting that DOX release from DOX-PEG liposome within tumor tissue would be enhanced via the interaction with PEG-Span 80 niosome. This strategy would lead to the safer and more inexpensive chemotherapy, since it could make it possible to provide the better anti-tumor effect by utilizing the lower dose of DOX.

Study of membrane deformations induced by Hepatitis C protein NS4B and its terminal amphipathic peptides

Many viruses destabilize cellular membranous compartments to form their replication complexes, but the mechanism(s) underlying membrane perturbation remains unknown. Expression in eukaryotic cells of NS4B, a protein of the hepatitis C virus (HCV), alters membranous complexes and induces structures similar to the so-called membranous web that appears crucial to the formation of the HCV replication complex. As over-expression of the protein is lethal to both prokaryotic and eukaryotic cells, NS4B was produced in large quantities in a "cell-free" system in the presence of detergent, after which it was inserted into lipid membranes. X-ray diffraction revealed that NS4B modifies the phase diagram of synthetic lipid aqueous phases considerably, perturbing the transition temperature and cooperativity. Cryo-electron microscopy demonstrated that NS4B introduces significant disorder in the synthetic membrane as well as discontinuities that could be interpreted as due to the formation of pores and membrane merging events. C- and N-terminal fragments of NS4B are both able to destabilize liposomes. While most NS4B amphipathic peptides perforate membranes, one NS4B peptide induces membrane fusion. Cryo-electron microscopy reveals a particular structure that can be interpreted as arising from hemi-fusion-like events. Amphipathic domains are present in many proteins, and if exposed to the aqueous cytoplasmic medium are sufficient to destabilize membranes in order to form viral replication complexes. These domains have important functions in the viral replication cycle, and thus represent potential targets for the development of anti-viral molecules.

Phosphatidylserine-Dependent Catalysis of Stalk and Pore Formation by Synaptobrevin JMR-TMD Peptide

Although the importance of a SNARE complex in neurotransmitter release is widely accepted, there exist different views on how the complex promotes fusion. One hypothesis is that the SNARE complex's ability to bring membranes into contact is sufficient for fusion, another points to possible roles of juxtamembrane regions (JMRs) and transmembrane domains (TMDs) in catalyzing lipid rearrangement, and another notes the complex's presumed ability to bend membranes near the point of contact. Here, we performed experiments with highly curved vesicles brought into contact using low concentrations of polyethylene glycol (PEG) to investigate the influence of the synaptobrevin (SB) TMD with an attached JMR (SB-JMR-TMD) on the rates of stalk and pore formation during vesicle fusion. SB-JMR-TMD enhanced the rates of stalk and fusion pore (FP) formation in a sharply sigmoidal fashion. We observed an optimal influence at an average of three peptides per vesicle, but only with phosphatidylserine (PS)-containing vesicles. Approximately three SB-JMR-TMDs per vesicle optimally ordered the bilayer interior and excluded water in a similar sigmoidal fashion. The catalytic influences of hexadecane and SB-JMR-TMD on fusion kinetics showed little in common, suggesting different mechanisms. Both kinetic and membrane structure measurements support the hypotheses that SB-JMR-TMD 1) catalyzes initial intermediate formation as a result of its basic JMR disrupting ordered interbilayer water and permitting closer interbilayer approach, and 2) catalyzes pore formation by forming a membrane-spanning complex that increases curvature stress at the circumference of the hemifused diaphragm of the prepore intermediate state.

Wild-type and mutant hemagglutinin fusion peptides alter bilayer structure as well as kinetics and activation thermodynamics of stalk and pore formation differently: mechanistic implications

Viral fusion peptides are short N-terminal regions of type-1 viral fusion proteins that are critical for virus entry. Although the importance of viral fusion peptides in virus-cell membrane fusion is established, little is known about how they function. We report the effects of wild-type (WT) hemagglutinin (HA) fusion peptide and its G1S, G1V, and W14A mutants on the kinetics of poly(ethylene glycol)(PEG)-mediated fusion of small unilamellar vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine, sphingomyelin, and cholesterol (molar ratio of 35:30:15:20). Time courses of lipid mixing, content mixing, and content leakage were obtained using fluorescence assays at multiple temperatures and analyzed globally using either a two-step or three-step sequential ensemble model of the fusion process to obtain the rate constant and activation thermodynamics of each step. We also monitored the influence of peptides on bilayer interfacial order, acyl chain order, bilayer free volume, and water penetration. All these data were considered in terms of a recently published mechanistic model for the thermodynamic transition states for each step of the fusion process. We propose that WT peptide catalyzes Step 1 by occupying bilayer regions vacated by acyl chains that protrude into interbilayer space to form the Step 1 transition state. It also uniquely contributes a positive intrinsic curvature to hemi-fused leaflets to eliminate Step 2 and catalyzes Step 3 by destabilizing the highly stressed edges of the hemi-fused microstructures that dominate the ensemble of the intermediate state directly preceding fusion pore formation. Similar arguments explain the catalytic and inhibitory properties of the mutant peptides and support the hypothesis that the membrane-contacting fusion peptide of HA fusion protein is key to its catalytic activity.

Individual vesicle fusion events mediated by lipid-anchored DNA

Membrane fusion consists of a complex rearrangement of lipids and proteins that results in the merger of two lipid bilayers. We have developed a model system that employs synthetic DNA-lipid conjugates as a surrogate for the membrane proteins involved in the biological fusion reaction. We previously showed that complementary DNA-lipids, inserted into small unilamellar vesicles, can mediate membrane fusion in bulk. Here, we use a model membrane architecture developed in our lab to directly observe single-vesicle fusion events using fluorescence microscopy. In this system, a planar tethered membrane patch serves as the target membrane for incoming vesicles. This allows us to quantify the kinetics and characteristics of individual fusion events from the perspective of the lipids or the DNA-lipids involved in the process. We find that the fusion pathways are heterogeneous, with an arrested hemi-fusion state predominating, and we quantitate the outcome and rate of fusion events to construct a mechanistic model of DNA-mediated vesicle fusion. The waiting times between docking and fusion are distributed exponentially, suggesting that fusion occurs in a single step. Our analysis indicates that when two lipid bilayers are brought into close proximity, fusion occurs spontaneously, with little or no dependence on the number of DNA hybrids formed.

The effect of hydrophilic ionic liquids 1-ethyl-3-methylimidazolium lactate and choline lactate on lipid vesicle fusion

Ionic liquids (ILs) are room-temperature molten salts that have applications in both physical sciences and more recently in the purification of proteins and lipids, gene transfection and sample preparation for electron microscopy (EM) studies. Transfection of genes into cells requires membrane fusion between the cell membrane and the transfection reagent, thus, ILs may be induce a membrane fusion event. To clarify the behavior of ILs with cell membranes the effect of ILs on model membranes, i.e., liposomes, were investigated. We used two standard ILs, 1-ethyl-3-methylimidazolium lactate ([EMI][Lac]) and choline lactate ([Ch][Lac]), and focused on whether these ILs can induce lipid vesicle fusion. Fluorescence resonance energy transfer and dynamic light scattering were employed to determine whether the ILs induced vesicle fusion. Vesicle solutions at low IL concentrations showed negligible fusion when compared with the controls in the absence of ILs. At concentrations of 30% (v/v), both types of ILs induced vesicle fusion up to 1.3 and 1.6 times the fluorescence intensity of the control in the presence of [Ch][Lac] and [EMI][Lac], respectively. This is the first demonstration that [EMI][Lac] and [Ch][Lac] induce vesicle fusion at high IL concentrations and this observation should have a significant influence on basic biophysical studies. Conversely, the ability to avoid vesicle fusion at low IL concentrations is clearly advantageous for EM studies of lipid samples and cells. This new information describing IL-lipid membrane interactions should impact EM observations examining cell morphology.

A novel assay for detecting fusion pore formation: implications for the fusion mechanism

Membrane fusion is broadly envisioned as a two- or three-step process proceeding from contacting bilayers through one or two semistable, nonlamellar lipidic intermediate structures to a fusion pore. A true fusion event requires mixing of contents between compartments and is monitored by the movement of soluble molecules between trapped compartments. We have used poly(ethylene glycol) (PEG) to rapidly generate an ensemble aggregated state A that proceeds sequentially through intermediates (I₁ and/or I₂) to a final fusion pore state (FP) with rate constants k₁, k₂, and k₃. Movement of moderately sized solutes (e.g., Tb³⁺/dipicolinic acid) has been used to detect pores assigned to intermediate states as well as to the final state (FP). Analysis of ensemble kinetic data has required that mixing of contents occurs with defined probabilities (αi) in each ensemble state, although it is unclear whether pores that form in different states are different. We introduce here a simple new assay that employs fluorescence resonance energy transfer (FRET) between a 6-carboxyfluorescein (donor) and tetramethylrhodamine (acceptor), which are covalently attached to complementary sequences of 10 bp oligonucleotides. Complementary sequences of fluorophore-labeled oligonucleotides were incorporated in vesicles separately, and the level of FRET increased in a simple exponential fashion during PEG-mediated fusion. The resulting rate constant corresponded closely to the slow rate constant of FP formation (k₃) derived from small molecule assays. Additionally, the total extent of oligonucleotide mixing corresponded to the fraction of content mixing that occurred in state FP in the small molecule assay. The results show that both large "final pores" and small (presumably transient) pores can form between vesicles throughout the fusion process. The implications of this result for the mechanism of membrane fusion are discussed.

Activation thermodynamics of poly(ethylene glycol)-mediated model membrane fusion support mechanistic models of stalk and pore formation

Membrane fusion, essential to eukaryotic life, is broadly envisioned as a three-step process proceeding from contacting bilayers through two semistable, nonlamellar lipidic intermediate states to a fusion pore. Here, we introduced a new, to our knowledge, experimental approach to gain insight into the nature of the transition states between initial, intermediate, and final states. Recorded time courses of lipid-mixing, content-mixing, and content-leakage associated with fusion of 23 nm vesicles in the presence of poly(ethylene glycol) at multiple temperatures were fitted globally to a three-step sequential model to yield rate constants and thereby activation thermodynamics for each step of the process, as well as probabilities of occurrence of lipid-mixing, content-mixing, or content-leakage in each state. Experiments with membranes containing hexadecane, known to reduce interstice energy in nonlamellar structures, provided additional insight into the nature of fusion intermediates and transition states. The results support a hypothesis for the mechanism of stalk formation (step-1) that involves acyl chain protrusions into the interbilayer contact region, a hypothesis for a step-2 mechanism involving continuous interconversion of semistable nonlamellar intermediates, and a hypothesis for step-3 (pore formation) mechanism involving correlated movement of whole lipid molecules into hydrophobic spaces created by geometry mismatch between intermediate structures.

Phase separation and fluctuations in mixtures of a saturated and an unsaturated phospholipid

We describe quantitatively the interactions in a mixture of a saturated and an unsaturated phospholipid, and their consequences to the phase behavior at macroscopic and microscopic levels. This type of lipid-lipid interaction is fundamental in determining the organization and physical behavior of biological membranes. Mixtures of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) are examined in detail by multiple experimental approaches (differential scanning calorimetry (DSC), fluorescence resonance energy transfer, and confocal fluorescence microscopy) in combination with Monte Carlo simulations in a lattice. The interactions between all possible pairs of lipid species and states are determined by matching the heat capacity calculated through Monte Carlo simulations to that measured experimentally by DSC. Only for one other lipid system, a mixture between two saturated phosphatidylcholines, is a similar quantitative description available. The interactions in the two systems and different representations used to model them are compared. Phase separation occurs in DPPC/POPC at about the center of the phase diagram mapped by DSC, but not at all compositions and temperatures in the coexistence region. Close to the extremes of composition, the phase behavior is best described by large fluctuations. At the heat capacity maxima in the mixtures, the domain size distributions change remarkably; large domains disappear and cooperative fluctuations increase.

Distinct structural rearrangements of the VSV glycoprotein drive membrane fusion

Electron microscopy reveals that the flat base of the vesicular stomatitis virus is a privileged site for membrane fusion and that the glycoproteins located outside form regular arrays required at late stages of the fusion process.

The entry of enveloped viruses into cells requires the fusion of viral and cellular membranes, driven by conformational changes in viral glycoproteins. Many studies have shown that fusion involves the cooperative action of a large number of these glycoproteins, but the underlying mechanisms are unknown. We used electron microscopy and tomography to study the low pH–induced fusion reaction catalyzed by vesicular stomatitis virus glycoprotein (G). Pre- and post-fusion crystal structures were observed on virions at high and low pH, respectively. Individual fusion events with liposomes were also visualized. Fusion appears to be driven by two successive structural rearrangements of G at different sites on the virion. Fusion is initiated at the flat base of the particle. Glycoproteins located outside the contact zone between virions and liposomes then reorganize into regular arrays. We suggest that the formation of these arrays, which have been shown to be an intrinsic property of the G ectodomain, induces membrane constraints, achieving the fusion reaction.

Lipid membrane adhesion and fusion driven by designed, minimally multivalent hydrogen-bonding lipids

Cyanuric acid (CA) and melamine (M) functionalized lipids can form membranes that exhibit robust hydrogen-bond driven surface recognition in water, facilitated by multivalent surface clustering of recognition groups and variable hydration at the lipid-water interface. Here we describe a minimal lipid recognition cluster: three CA or M recognition groups are forced into proximity by covalent attachment to a single lipid headgroup. This trivalent lipid system guides recognition at the lipid-water interface using cyanurate-melamine hydrogen bonding when incorporated at 0.1-5 mol percent in fluid phospholipid membranes, inducing both vesicle-vesicle binding and membrane fusion. Fusion was accelerated when the antimicrobial peptide magainin was used to anchor trivalent recognition, or when added exogenously to a preassembled lipid vesicle complex, underscoring the importance of coupling recognition with membrane disruption in membrane fusion. Membrane apposition and fusion were studied in vesicle suspensions using light scattering, FRET assays for lipid mixing, surface plasmon resonance, and cryo-electron microscopy. Recognition was found to be highly spatially selective as judged by vesicular adhesion to surface patterned supported lipid bilayers (SLBs). Fusion to SLBs was also readily observed by fluorescence microscopy. Together, these studies indicate effective and functional recognition of trivalent phospholipids, despite low mole percentage concentration, solvent competition for hydrogen bond donor/acceptor sites, and simplicity of structure. This novel designed molecular recognition motif may be useful for directing aqueous-phase assembly and biomolecular interactions.

Internal lipid synthesis and vesicle growth as a step toward self-reproduction of the minimal cell

One of the major properties of the semi-synthetic minimal cell, as a model for early living cells, is the ability to self-reproduce itself, and the reproduction of the boundary layer or vesicle compartment is part of this process. A minimal bio-molecular mechanism based on the activity of one single enzyme, the FAS-B (Fatty Acid Synthase) Type I enzyme from Brevibacterium ammoniagenes, is encapsulated in 1-palmitoyl-2oleoyl-sn-glycero-3-phosphatidylcholine (POPC) liposomes to control lipid synthesis. Consequently molecules of palmitic acid released from the FAS catalysis, within the internal lumen, move toward the membrane compartment and become incorporated into the phospholipid bilayer. As a result the vesicle membranes change in lipid composition and liposome growth can be monitored. Here we report the first experiments showing vesicles growth by catalysis of one enzyme only that produces cell boundary from within. This is the prototype of the simplest autopoietic minimal cell.

Toxicological investigation and antinociceptive property of potassium thiophene-3-trifluoroborate

The aim of the present study was to evaluate pharmacological and toxicological properties of potassium thiophene-3-trifluoroborate (RBF(3)K). The acute effect of RBF(3)K was evaluated on mice. To this end, mice received a single dose of RBF(3)K (25, 50, and 100 mg/kg, by oral route, p.o.) and after 72 hrs, blood, liver, and kidney samples were collected. delta-Aminolevulinate dehydratase, catalase and glutathione-S-transferase activities, thiobarbituric acid-reactive substances and vitamin C levels, as well as plasma aspartate and alanine aminotransferase activities and creatinine levels were determined. Hepatic and renal lipid peroxidation levels in treated mice did not differ from those in control mice. No significant differences between treated and control mice were detected in hepatic and renal delta-aminolevulinate dehydratase activity. Aspartate and alanine aminotransferase activities as well as urea and creatinine levels were similar among the groups. In contrast, results obtained from in vivo experiments revealed that RBF(3)K, orally administered, reduced peritoneovisceral pain induced by acetic acid administered i.p. Doses of 1, 5, 10, 25, 50, and 100 mg/kg of RBF(3)K were assessed in the antinociceptive investigation and the effect was significantly different than control groups from 5 mg/kg. It was observed that alpha(2-)adrenergic and serotonergic, but not opioidergic, receptors appear to be involved in orally administered RBF(3)K. Mice treated with RBF(3)K did not reveal any motor impairment in the open field. This is a promising compound for more detailed pharmacological studies involving organotrifluoroborate compounds.

Effects of linker sequences on vesicle fusion mediated by lipid-anchored DNA oligonucleotides

Synthetic lipid-oligonucleotide conjugates inserted into lipid vesicles mediate fusion when one population of vesicles displays the 5'-coupled conjugate and the other the 3'-coupled conjugate, so that anti-parallel hybridization allows the membrane surfaces to come into close proximity. Improved assays show that lipid mixing proceeds more quickly and to a much greater extent than content mixing, suggesting the latter is rate limiting. To test the effect of membrane-membrane spacing on fusion, a series of conjugates was constructed by adding 2-24 noncomplementary bases at the membrane-proximal ends of two complementary sequences. Increasing linker lengths generally resulted in progressively reduced rates and extents of lipid and content mixing, in contrast to higher vesicle docking rates. The relatively flexible, single-stranded DNA linker facilitates docking but allows greater spacing between the vesicles after docking, thus making the transition into fusion less probable, but not preventing it altogether. These experiments demonstrate the utility of DNA as a model system for fusion proteins, where sequence can easily be modified to systematically probe the effect of distance between bilayers in the fusion reaction.

Synthesis and characterization of stable fluorocarbon nanostructures as drug delivery vehicles for cytolytic peptides

The therapeutic potential of cytolytic peptides is plagued by nonspecificity and enzymatic degradation. We report the first stable incorporation of melittin (a 26 amino acid amphipathic peptide) into an outer lipid monolayer of perfluorocarbon nanoparticles. Melittin binds avidly to the nanoparticles (dissociation constant approximately 3.27 nM) and retains its pore-forming activity after contact-mediated delivery to model bilayer membrane (liposomes) thereby demonstrating the effectiveness of perfluorocarbon nanoparticles as unique nanocarriers for cytolytic peptides.

A Mycobacterium tuberculosis-derived lipid inhibits membrane fusion by modulating lipid membrane domains

Tuberculosis is an infectious and potentially fatal disease caused by the acid-fast bacillus Mycobacterium tuberculosis (MTB). One hallmark of a tuberculosis infection is the ability of the bacterium to subvert the normal macrophage defense mechanism of the host immune response. Lipoarabinomannan (LAM), an integral component of the MTB cell wall, is released when MTBs are taken into phagosomes and has been reported to be involved in the inhibition of phago-lysosomal (P-L) fusion. However, the physical chemistry of the effects of LAM on lipid membrane structure relative to P-L fusion has not been studied. We produced membranes in vitro composed of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol to simulate phagosomal lipid membranes and quantified the effects of the addition of LAM to these membranes, using fluorescence resonance energy transfer assays and atomic force microscopy. We found that LAM inhibits vesicle fusion and markedly alters lipid membrane domain morphology and sphingomyelin-chollesterol/dioleoylphosphatidylcholine ratios. These data demonstrate that LAM induces a dramatic reorganization of lipid membranes in vitro and clarifies the role of LAM in the inhibition of P-L fusion and the survival of the MTB within the macrophage.

Analysis of membrane fusion as a two-state sequential process: evaluation of the stalk model

We propose a model that accounts for the time courses of PEG-induced fusion of membrane vesicles of varying lipid compositions and sizes. The model assumes that fusion proceeds from an initial, aggregated vesicle state ((A) membrane contact) through two sequential intermediate states (I(1) and I(2)) and then on to a fusion pore state (FP). Using this model, we interpreted data on the fusion of seven different vesicle systems. We found that the initial aggregated state involved no lipid or content mixing but did produce leakage. The final state (FP) was not leaky. Lipid mixing normally dominated the first intermediate state (I(1)), but content mixing signal was also observed in this state for most systems. The second intermediate state (I(2)) exhibited both lipid and content mixing signals and leakage, and was sometimes the only leaky state. In some systems, the first and second intermediates were indistinguishable and converted directly to the FP state. Having also tested a parallel, two-intermediate model subject to different assumptions about the nature of the intermediates, we conclude that a sequential, two-intermediate model is the simplest model sufficient to describe PEG-mediated fusion in all vesicle systems studied. We conclude as well that a fusion intermediate "state" should not be thought of as a fixed structure (e.g., "stalk" or "transmembrane contact") of uniform properties. Rather, a fusion "state" describes an ensemble of similar structures that can have different mechanical properties. Thus, a "state" can have varying probabilities of having a given functional property such as content mixing, lipid mixing, or leakage. Our data show that the content mixing signal may occur through two processes, one correlated and one not correlated with leakage. Finally, we consider the implications of our results in terms of the "modified stalk" hypothesis for the mechanism of lipid pore formation. We conclude that our results not only support this hypothesis but also provide a means of analyzing fusion time courses so as to test it and gauge the mechanism of action of fusion proteins in the context of the lipidic hypothesis of fusion.

Convenient Method To Access New 4,4-Dialkoxy- and 4,4-Diaryloxy-diaza-s-indacene Dyes: Synthesis and Spectroscopic Evaluation

A straightforward method for the synthesis of original 4,4-dialkoxy- or 4,4-diaryloxy-diaza-s-indacenes (BODIPY) derivatives obtained by treatment of BODIPY 1 with various alcohols in the presence of AlCl3 is described. The novel compounds are characterized by spectroscopic properties similar to those of the parent BODIPY 1, absorption and emission spectra with similar band shapes, high molar absorption coefficients (epsilon lambda max approximately 80,000 M(-1) cm(-1)), and for most of them high fluorescence quantum yields (Phi exp from 0.52 to 0.71). Among all of the new compounds synthesized, the dye 2 h exhibits higher fluorescence quantum yield (0.71) and lifetime (4.09 ns) than compound 1 and a good chemical stability toward conditions compatible with biological cell-based assays.

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.

Effect of electrostatic interactions on binding and retention of DNA oligomers to PNA liposomes assessed by FRET measurements

A FRET-based method is used to observe the desorption of di-alkyl peptide nucleic acid amphiphiles (PNAA) from liposomes occurring on binding of complementary DNA oligomers. PNA liposomes were prepared containing fluorescein-labeled PNAA and rhodamine-labeled dipalmitoylphosphoethanolamine (DPPE). These liposomes showed efficient energy transfer from the fluorescein to rhodamine, with an average donor-to-acceptor distance of 5.91nm. In low-ionic-strength buffer (50mM Tris-HCl, pH 8.0), the FRET signal was maintained in the presence of a stoichiometric amount of 10- and 20-mers DNA complements, but the signal attenuated for 40-mer complements, indicating that DNA first binds the PNAA before the PNAA/DNA duplex desorbs from the lipid bilayer. The FRET signal was maintained in the presence of 10-, 20-, 40-, and 60-mer DNA in high ionic-strength buffer, showing that the driving force for the desorption is electrostatic repulsion between the bound DNA oligomer and the liposome surface. This conclusion is corroborated by comparison of the PNA/DNA binding energy, the energy of adsorption of the di-alkyl PNAA to the lipid bilayer, and a calculation of the DNA/lipid bilayer electrostatic repulsion using the linearized Poisson-Boltzmann equation.

Neuronal SNAREs do not trigger fusion between synthetic membranes but do promote PEG-mediated membrane fusion

At low surface concentrations that permit formation of impermeable membranes, neuronal soluble N-ethyl maleimide sensitive factor attachment protein receptor (SNARE) proteins form a stable, parallel, trans complex when vesicles are brought into contact by a low concentration of poly(ethylene glycol) (PEG). Surprisingly, formation of a stable SNARE complex does not trigger fusion under these conditions. However, neuronal SNAREs do promote fusion at low protein/lipid ratios when triggered by higher concentrations of PEG. Promotion of PEG-triggered fusion required phosphatidylserine and depended only on the surface concentration of SNAREs and not on the formation of a trans SNARE complex. These results were obtained at protein surface concentrations reported for synaptobrevin in synaptic vesicles and with an optimally fusogenic lipid composition. At a much higher protein/lipid ratio, vesicles joined by SNARE complex slowly mixed lipids at 37 degrees C in the absence of PEG, in agreement with earlier reports. However, vesicles containing syntaxin at a high protein/lipid ratio (>or=1:250) lost membrane integrity. We conclude that the neuronal SNARE complex promotes fusion by joining membranes and that the individual proteins syntaxin and synaptobrevin disrupt membranes so as to favor formation of a stalk complex and to promote conversion of the stalk to a fusion pore. These effects are similar to the effects of viral fusion peptides and transmembrane domains, but they are not sufficient by themselves to produce fusion in our in vitro system at surface concentrations documented to occur in synaptic vesicles. Thus, it is likely that proteins or factors other than the SNARE complex must trigger fusion in vivo.

Membrane destabilizing properties of cell-penetrating peptides

Although cell-penetrating peptides (CPPs), also denoted protein transduction domains (PTDs), have been widely used for intracellular delivery of large and hydrophilic molecules, the mechanism of uptake is still poorly understood. In a recent live cell study of the uptake of penetratin and tryptophan-containing analogues of Tat(48-60) and oligoarginine, denoted TatP59W, TatLysP59W and R(7)W, respectively, it was found that both endocytotic and non-endocytotic uptake pathways are involved [Thoren et al., Biochem. Biophys. Res. Commun. 307 (2003) 100-107]. Non-endocytotic uptake was only observed for the arginine-rich peptides TatP59W and R(7)W. In this paper, the interactions of penetratin, R(7)W, TatP59W and TatLysP59W with phospholipid vesicles are compared in the search for an understanding of the mechanisms for cellular uptake. While R(7)W, TatP59W and TatLysP59W are found to promote vesicle fusion, indicated by mixing of membrane components, penetratin merely induces vesicle aggregation. Studies of the leakage from dye-loaded vesicles indicate that none of the peptides forms membrane pores and that vesicle fusion is not accompanied by leakage of the aqueous contents of the vesicles. These observations are important for a proper interpretation of future experiments on the interactions of these peptides with model membranes. We suggest that the discovered variations in propensity to destabilize phospholipid bilayers between the peptides investigated, in some cases sufficient to induce fusion, may be related to their different cellular uptake properties.

Evidence that rabies virus forms different kinds of fusion machines with different pH thresholds for fusion

Fusion of rabies virus with membranes is triggered at a low pH and is mediated by a viral glycoprotein (G). Fusion of rabies virus with liposomes was monitored by using a lipid mixing assay based on fluorescence resonance energy transfer. Fusion was detected below pH 6.4, and its extent increased with H(+) concentrations to be maximal around pH 6.15. The origin of the partial fusion activity of rabies virus under suboptimal pH conditions (i.e., between pH 6.15 and 6.4) was investigated. We demonstrate unambiguously that fusion at a suboptimal pH is distinct from the phenomenon of low-pH-induced inactivation and that it is not due to heterogeneity of the virus population. We also show that viruses that do not fuse under suboptimal pH conditions are indeed bound to the target liposomes and that the fusion complexes they have formed are blocked at an early stage of the fusion pathway. Our conclusion is that along the fusion reaction, different kinds of fusion machines with different pH thresholds for fusion can be formed. Possible explanations of this difference of pH sensitivity are discussed.

Rpe65 Is a Retinyl Ester Binding Protein That Presents Insoluble Substrate to the Isomerase in Retinal Pigment Epithelial Cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

Experimental models of primitive cellular compartments: encapsulation, growth, and division

The clay montmorillonite is known to catalyze the polymerization of RNA from activated ribonucleotides. Here we report that montmorillonite accelerates the spontaneous conversion of fatty acid micelles into vesicles. Clay particles often become encapsulated in these vesicles, thus providing a pathway for the prebiotic encapsulation of catalytically active surfaces within membrane vesicles. In addition, RNA adsorbed to clay can be encapsulated within vesicles. Once formed, such vesicles can grow by incorporating fatty acid supplied as micelles and can divide without dilution of their contents by extrusion through small pores. These processes mediate vesicle replication through cycles of growth and division. The formation, growth, and division of the earliest cells may have occurred in response to similar interactions with mineral particles and inputs of material and energy.

Lipid bilayer vesicle fusion: intermediates captured by high-speed microfluorescence spectroscopy

The fusion of lipid bilayers can be visualized under the fluorescence microscope, but the process is very fast and requires special techniques for its study. It is reported here that vesicle fusion is susceptible to analysis by microspectrofluorometry and that for the first time, the entire fusion process has been captured. In the case of giant (>10- micro m diameter) bilayer vesicles having a high density of opposite charge, fusion proceeds through stages of adhesion, flattening, hemifusion, elimination of the intervening septum, and uptake of excess membrane to generate a spherical product very rapidly. These investigations became possible with a fluorescence microscope that was modified for recording of images simultaneously with the collection of fluorescence emission spectra from many (>100) positions along the fusion axis. Positively-charged vesicles, composed of O-ethylphosphatidylcholine and dioleoylphosphatidylcholine, were labeled with a carbocyanine fluorophore. Negatively-charged vesicles, composed of dioleoylphosphatidylglycerol and dioleoylphosphatidylcholine, were labeled with a rhodamine fluorophore that is a resonance energy transfer acceptor from the carbocyanine fluorophore. An electrophoretic chamber allowed selection of pairs of vesicles to be brought into contact and examined. Spectral changes along the axis of fusion were captured at high speed (a few ms/frame) by operating a sensitive digital camera in the virtual-chip mode, a software/hardware procedure that permits rapid readout of selected regions of interest and by pixel binning along the spectral direction. Simultaneously, color images were collected at video rates (30 frame/s). Comparison of the spectra and images revealed that vesicle fusion typically passes through a hemifusion stage and that the time from vesicle contact to fusion is <10 ms. Fluorescence spectra are well suited to rapid collection in the virtual-chip mode because spectra (in contrast to images) are accurately characterized with a relatively small number of points and interfering signals can be removed by judicious choice of barrier filters. The system should be especially well-suited to phenomena exhibiting rapid fluorescence change along an axis; under optimal conditions, it is possible to obtain sets of spectra (wavelength range of approximately 150 nm) at >100 positions along a line at rates >1000 frames/s with a spectral resolution of approximately 10 nm and spatial resolution at the limit of the light microscope ( approximately 0.2 micro m).

Synthesis of new, pyrene-containing, metal-chelating lipids and sensing of cupric ions

The syntheses of several saturated, pyrene-containing, metal-chelating lipids are described. These lipids are capable of strongly binding to transition metal ions employing the metal-chelating headgroup. The excimer-to-monomer ratio of the pyrene groups changes with addition of cupric ions to the liposomes. Three other transition metal ions (Zn(2+), Ni(2+), and Hg(2+)) did not cause any appreciable changes in the excimer-to-monomer ratio. [reaction--see text]

Optimizing and quantifying fusion of liposomes to mammalian sperm using resonance energy transfer and flow cytometric methods

BACKGROUND

Liposomes are used to carry pharmaceutical agents and to alter the lipid composition of cell membranes. This study compared resonance energy transfer (RET), fluorescence dequenching, and flow cytometry as monitors and quantifiers of fusion between liposomes and mammalian spermatozoa.

METHODS

Preliminary experiments used RET to determine the optimum sperm concentration for fusion of DL-alpha-phosphatidylcholine dipalmitoyl (PC)/DL-alpha-phosphatidylethanolamine dipalmitoyl (PE) liposomes at 35 degrees C +/- 5 mM Ca2+. Microscopy confirmed the fusion of liposomes, not just adhesion (n = 3). Dequenching tested the time-dependent fusion of liposomes of two different lipid compositions to sperm, both, (n = 3) +/- 1 mM Ca2+ and (n = 3) without Ca2+ at two sperm concentrations. Finally, flow cytometry absolutely quantified the percentage of sperm fusing to liposomes at different liposome-to-sperm ratios (n = 4) and with sperm from different donors (n = 3).

RESULTS

RET detected fusion of liposomes with sperm and microscopy confirmed the interaction to be true fusion. Dequenching detected more fusion of liposomes with sperm at 100 x 10(6) sperm per milliliter than at lower concentrations (P < 0.05). Fusion dynamics differed with lipid composition but Ca2+ had no effect. Flow cytometry reliably quantified the percentage of sperm fusing with liposomes, which varied from bull to bull (P < 0.05).

CONCLUSION

Liposome fusion with mammalian sperm membranes can be quantified cytometrically and varies with lipid composition, sperm-to-liposome ratio, and individual animals.

Kinetics of influenza hemagglutinin-mediated membrane fusion as a function of technique

Reliable techniques are required to evaluate the plausibility of proposed membrane fusion mechanisms. Here we have studied the kinetics of establishing the lipidic connection between hemagglutinin-expressing cells (HA-cells) and red blood cells (RBC) labeled with octadecylrhodamine, R18, using three different experimental approaches: (1) the most common approach of monitoring the rate of the R18 dequenching in a cuvette with a suspension of RBC/HA-cell complexes; (2) video fluorescence microscopy (VFM) to detect the waiting times before the onset of R18 redistribution, not dequenching, for each RBC attached to an adherent HA-cell; and (3) a new approach based on blockage of RBC fusion to an adherent HA-cell at different time points by lysophosphatidylcholine (LPC), so that only the cell pairs which, at the time of LPC application, had fused or were irreversibly committed to fusion contributed to the final extent of lipid mixing. The LPC blockage and VFM gave very similar estimates for the fusion kinetics, with LPC monitoring also those sites committed to the lipid mixing process. In contrast, R18 dequenching in the cuvette was much slower, i.e., it monitors a much later stage of dye redistribution.

Fluorescent probes for monitoring virus fusion kinetics: comparative evaluation of reliability

Fluorescence assays for viral membrane fusion employ lipidic probes whose kinetics of fluorescence dequenching should mimic the actual kinetics of membrane merging. We examined the fusion of influenza virus with CEM cells, erythrocyte ghosts or liposomes by monitoring the fluorescence dequenching of each one of the three probes, octadecylrhodamine B chloride (R18), N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (Rh-PE), or rac-2,3-dioleoylglycerol ester of rhodamine B (DORh-B), inserted into the virus membrane. Experimental conditions were designed to allow a clear distinction between membrane mixing and non-specific probe transfer. Fluorescence dequenching observed with Rh-PE was much slower than with R18, unless a particular experimental procedure was used. Using liposomes as a target membrane, the kinetics and extent of the decrease in resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) and Rh-PE, initially embedded in the liposome membrane, were matched by that of the dequenching of viral R18, but not of viral Rh-PE. DORh-B was found not to be appropriate to follow membrane merging. Our results indicate that on a time scale of several minutes R18 more accurately reflects the kinetics of membrane fusion. Nevertheless, control experiments should be performed to evaluate non-specific probe transfer of R18 molecules, whose contribution to fluorescence dequenching can become significant after long incubation times.