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

Cell attachment on poly(3-hydroxybutyrate)-poly(ethylene glycol) copolymer produced by Azotobacter chroococcum 7B

BACKGROUND

The improvement of biomedical properties, e.g. biocompatibility, of poly(3-hydroxyalkanoates) (PHAs) by copolymerization is a promising trend in bioengineering. We used strain Azotobacter chroococcum 7B, an effective producer of PHAs, for biosynthesis of not only poly(3-hydroxybutyrate) (PHB) and its main copolymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV), but also alternative copolymer, poly(3-hydroxybutyrate)-poly(ethylene glycol) (PHB-PEG).

RESULTS

In biosynthesis we used sucrose as the primary carbon source and valeric acid or poly(ethylene glycol) 300 (PEG 300) as additional carbon sources. The chemical structure of PHB-PEG and PHB-HV was confirmed by 1H nuclear-magnetic resonance (1H NMR) analysis. The physico-chemical properties (molecular weight, crystallinity, hydrophilicity, surface energy) and surface morphology of films from PHB copolymers were studied. To study copolymers biocompatibility in vitro the protein adsorption and COS-1 fibroblasts growth on biopolymer films by XTT assay were analyzed. Both copolymers had changed physico-chemical properties compared to PHB homopolymer: PHB-HV and PHB-PEG had less crystallinity than PHB; PHB-HV was more hydrophobic than PHB in contrast to PHB-PEG appeared to have greater hydrophilicity than PHB; whereas the morphology of polymer films did not differ significantly. The protein adsorption to PHB-PEG was greater and more uniform than to PHB and PHB-PEG copolymer promoted better growth of COS-1 fibroblasts compared with PHB homopolymer.

CONCLUSIONS

Thus, despite low EG-monomers content in bacterial origin PHB-PEG copolymer, this polymer demonstrated significant improvement in biocompatibility in contrast to PHB and PHB-HV copolymers, which may be coupled with increased protein adsorption and hydrophilicity of PEG-containing copolymer.

The terpolymer produced by Azotobacter chroococcum 7B: effect of surface properties on cell attachment

The copolymerization of poly(3-hydroxybutyrate) (PHB) is a promising trend in bioengineering to improve biomedical properties, e.g. biocompatibility, of this biodegradable polymer. We used strain Azotobacter chroococcum 7B, an effective producer of PHB, for biosynthesis of not only homopolymer and its main copolymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV), but also novel terpolymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-poly(ethylene glycol) (PHB-HV-PEG), using sucrose as the primary carbon source and valeric acid and poly(ethylene glycol) 300 (PEG 300) as additional carbon sources. The chemical structure of PHB-HV-PEG was confirmed by (1)H nuclear-magnetic resonance analysis. The physico-chemical properties (molecular weight, crystallinity, hydrophilicity, surface energy) of produced biopolymer, the protein adsorption to the terpolymer, and cell growth on biopolymer films were studied. Despite of low EG-monomers content in bacterial-origin PHB-HV-PEG polymer, the terpolymer demonstrated significant improvement in biocompatibility in vitro in contrast to PHB and PHB-HV polymers, which may be coupled with increased protein adsorption, hydrophilicity and surface roughness of PEG-containing copolymer.

Evolution of the hemifused intermediate on the pathway to membrane fusion

The pathway to membrane fusion in synthetic and biological systems is thought to pass through hemifusion, in which the outer leaflets are fused while the inner leaflets engage in a hemifusion diaphragm (HD). Fusion has been proposed to be completed by lysis of the expanded HD that matures from a localized stalklike initial connection. However, the process that establishes the expanded HD is poorly understood. Here we mathematically modeled hemifusion of synthetic vesicles, where hemifusion and fusion are most commonly driven by calcium and membrane tension. The model shows that evolution of the hemifused state is driven by these agents and resisted by interleaflet frictional and tensile stresses. Predicted HD growth rates depend on tension and salt concentration, and agree quantitatively with experimental measurements. For typical conditions, we predict that HDs expand at ~30 μm(2)/s, reaching a final equilibrium area ~7% of the vesicle area. Key model outputs are the evolving HD tension and area during the growth transient, properties that may determine whether HD lysis occurs. Applying the model to numerous published experimental studies that reported fusion, our results are consistent with a final fusion step in which the HD ruptures due to super-lysis HD membrane tensions.

Vesicles and vesicle fusion: coarse-grained simulations

Biological cells are highly dynamic, and continually move material around their own volume and between their interior and exterior. Much of this transport encapsulates the material inside phospholipid vesicles that shuttle to and from, fusing with, and budding from, other membranes. A feature of vesicles that is crucial for this transport is their ability to fuse to target membranes and release their contents to the distal side. In industry, some personal care products contain vesicles to help transport reagents across the skin, and research on drug formulation shows that packaging active compounds inside vesicles delays their clearance from the blood stream. In this chapter, we survey the biological role and physicochemical properties of phospholipids, and describe progress in coarse-grained simulations of vesicles and vesicle fusion. Because coarse-grained simulations retain only those molecular details that are thought to influence the large-scale processes of interest, they act as a model embodying our current understanding. Comparing the predictions of these models with experiments reveals the importance of the retained microscopic details and also the deficiencies that can suggest missing details, thereby furthering our understanding of the complex dynamic world of vesicles.

Directed Fusion of Mesenchymal Stem Cells with Cardiomyocytes via VSV-G Facilitates Stem Cell Programming

Mesenchymal stem cells (MSCs) spontaneously fuse with somatic cells in vivo, albeit rarely, and the fusion products are capable of tissue-specific function (mature trait) or proliferation (immature trait), depending on the microenvironment. That stem cells can be programmed, or somatic cells reprogrammed, in this fashion suggests that stem cell fusion holds promise as a therapeutic approach for the repair of damaged tissues, especially tissues not readily capable of functional regeneration, such as the myocardium. In an attempt to increase the frequency of stem cell fusion and, in so doing, increase the potential for cardiac tissue repair, we expressed the fusogen of the vesicular stomatitis virus (VSV-G) in human MSCs. We found VSV-G expressing MSCs (vMSCs) fused with cardiomyocytes (CMs) and these fusion products adopted a CM-like phenotype and morphology in vitro. In vivo, vMSCs delivered to damaged mouse myocardium via a collagen patch were able to home to the myocardium and fuse to cells within the infarct and peri-infarct region of the myocardium. This study provides a basis for the investigation of the biological impact of fusion of stem cells with CMs in vivo and illustrates how viral fusion proteins might better enable such studies.

Cellular mechanisms of plasmalemmal sealing and axonal repair by polyethylene glycol and methylene blue

Mammalian neurons and all other eukaryotic cells endogenously repair traumatic injury within minutes by a Ca²⁺-induced accumulation of vesicles that interact and fuse with each other and the plasmalemma to seal any openings. We have used uptake or exclusion of extracellular fluorescent dye to measure the ability of rat hippocampal B104 cells or rat sciatic nerves to repair (seal) transected neurites in vitro or transected axons ex vivo. We report that endogenous sealing in both preparations is enhanced by Ca²⁺-containing solutions and is decreased by Ca²⁺-free solutions containing antioxidants such as dithiothreitol (DTT), melatonin (MEL), methylene blue (MB), and various toxins that decrease vesicular interactions. In contrast, the fusogen polyethylene glycol (PEG) at 10-50 mM artificially seals the cut ends of B104 cells and rat sciatic axons within seconds and is not affected by Ca²⁺ or any of the substances that affect endogenous sealing. At higher concentrations, PEG decreases sealing of transected axons and disrupts the plasmalemma of intact cells. These PEG-sealing data are consistent with the hypothesis that lower concentrations of PEG directly seal a damaged plasmalemma. We have considered these and other data to devise a protocol using a well-specified series of solutions that vary in tonicity, Ca²⁺, MB, and PEG content. These protocols rapidly and consistently repair (PEG-fuse) rat sciatic axons in completely cut sciatic nerves in vivo rapidly and dramatically to restore long-lasting morphological continuity, action potential conduction, and behavioral functions.

Mouse embryonic stem cells inhibit murine cytomegalovirus infection through a multi-step process

In humans, cytomegalovirus (CMV) is the most significant infectious cause of intrauterine infections that cause congenital anomalies of the central nervous system. Currently, it is not known how this process is affected by the timing of infection and the susceptibility of early-gestational-period cells. Embryonic stem (ES) cells are more resistant to CMV than most other cell types, although the mechanism responsible for this resistance is not well understood. Using a plaque assay and evaluation of immediate-early 1 mRNA and protein expression, we found that mouse ES cells were resistant to murine CMV (MCMV) at the point of transcription. In ES cells infected with MCMV, treatment with forskolin and trichostatin A did not confer full permissiveness to MCMV. In ES cultures infected with elongation factor-1α (EF-1α) promoter-green fluorescent protein (GFP) recombinant MCMV at a multiplicity of infection of 10, less than 5% of cells were GFP-positive, despite the fact that ES cells have relatively high EF-1α promoter activity. Quantitative PCR analysis of the MCMV genome showed that ES cells allow approximately 20-fold less MCMV DNA to enter the nucleus than mouse embryonic fibroblasts (MEFs) do, and that this inhibition occurs in a multi-step manner. In situ hybridization revealed that ES cell nuclei have significantly less MCMV DNA than MEF nuclei. This appears to be facilitated by the fact that ES cells express less heparan sulfate, β1 integrin, and vimentin, and have fewer nuclear pores, than MEF. This may reduce the ability of MCMV to attach to and enter through the cellular membrane, translocate to the nucleus, and cross the nuclear membrane in pluripotent stem cells (ES/induced pluripotent stem cells). The results presented here provide perspective on the relationship between CMV susceptibility and cell differentiation.

Biological therapies for cardiac arrhythmias: can genes and cells replace drugs and devices?

Cardiac rhythm disorders reflect failures of impulse generation and/or conduction. With the exception of ablation methods that yield selective endocardial destruction, present therapies are nonspecific and/or palliative. Progress in understanding the underlying biology opens up prospects for new alternatives. This article reviews the present state of the art in gene- and cell-based therapies to correct cardiac rhythm disturbances. We begin with the rationale for such approaches, briefly discuss efforts to address aspects of tachyarrhythmia, and review advances in creating a biological pacemaker to cure bradyarrhythmia. Insights gained bring the field closer to a paradigm shift away from devices and drugs, and toward biologics, in the treatment of rhythm disorders.

Membrane heterogeneities and fusogenicity in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

The role of pH-dependent lipid heterogeneities on the fusogenicity of membranes was evaluated on model lipid bilayers in the form of unilamellar vesicles composed of lipid pairs at a fixed equimolar ratio of phosphatidylcholine (PC) and phosphatidic acid (PA) headgroups. The pH and the hydrophobic composition (lipid acyl tails) of membranes were systematically altered, and their effect on vesicle aggregation, membrane fusogenicity, content release, and content mixing was evaluated. Lowering pH increases the extent of protonated PA headgroups forming phase-separated PA-rich heterogeneities and giving rise to molecular packing defects originating at the phase boundaries. Phase boundaries within the membrane's hydrophobic portion are affected by the lipid acyl-tail dynamics (fluidity) and the matching or nonmatching lengths of the acyl tails of lipid pairs comprising the membrane. The aggregates' size increases with lowering pH and is independent of the membrane's hydrophobic composition. Contrary to aggregation, the initial rates of lipid mixing are proportional to pH and also depend on membrane's hydrophobic composition. The apparent lipid-mixing rate constants are greater for membranes containing lipid pairs with mismatched acyl-tail lengths, followed by pairs with matching acyl tails in the gel state and by pairs with matching tails where one lipid is close to its transition temperature. Addition of cholesterol conserves the independence of vesicle aggregation from the membrane's hydrophobic composition. However, it decreases the aggregation rates and inverts the tendency for fusion, among the three types of hydrophobic compositions, suggesting a role of cholesterol's preferential partition in different regions of membrane's heterogeneities. We propose a phenomenological model where the membrane phase boundaries containing molecular packing defects act as "sticking points" on the vesicle exterior via which vesicles aggregate upon contact followed by defect merging via intervesicle lipid exchange and mixing. Such heterogeneous bilayers in the form of drug encapsulating liposomes may potentially improve the therapeutic efficacy by fusing with endosomal membranes, thus increasing drug bioavailability.

Liposomes tethered to omega-functional PEG brushes and induced formation of PEG brush supported planar lipid bilayers

Self-assembly of planar supported lipid bilayers on top of hydrophilic polymer brushes is a desirable alternative to solid supported lipid bilayers and covalently tethered lipid bilayers for applications like sensing on transmembrane proteins which require a large aqueous volume between membrane and substrate. We present a simple dip-and-rinse method to produce poly(ethylene glycol) (PEG) brushes with sparse positively charged hydrophobic tethers, using poly(l-lysine)-graft-poly(ethylene glycol)-quaternary ammonium compound copolymers. The interaction of such polymer coatings with liposomes of different compositions and the conditions for formation of planar lipid bilayers of extraordinarily high fluidity on top of the >10 nm thick reservoir by liposome self-assembly and sequentially triggered rupture are investigated.

Liposome-hydrogel bead complexes prepared via biotin-avidin conjugation

Liposomes immobilized onto polymeric hydrogel microbeads have potential advantages both in tissue engineering applications and as drug delivery vehicles. Here we demonstrate, quantify, and optimize lipid vesicle binding to polymeric hydrogel microbeads via the avidin-biotin conjugation system and characterize the stability of the resulting microgel-bound liposomes. Microgels consisting of a copolymer of N-isopropylacrylamide (NIPAM) and acrylic acid (AA), cross-linked with bis-acrylamide, that is, p(NIPAM-co-AA), were biotinylated using aqueous carbodiimide chemistry. Extruded liposomes consisting of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) plus a small fraction of a biotin-derivatized phosphatidylethanolamine (B-PE) were saturated with avidin and allowed to bind to biotinylated hydrogel beads. Using a combination of fluorescence spectroscopy, quenching, and microscopy and 31P NMR static and magic angle spinning (MAS) spectroscopies, we demonstrate conditions for near-quantitative liposome binding to p(NIPAM-co-AA) microbeads and show that liposome fusion does not occur under such conditions, that the liposomes remain intact and impermeable when so bound, and that they can function as slow release vehicles for entrapped aqueous species.

High-purity discrete PEG-oligomer crystals allow structural insight

To great (monodisperse) lengths: An improved synthesis of purer ethylene glycol (EG) oligomers allows access to 16- and 32-mers pure enough for multiple incorporation, and also to the longest (48-mer) discrete EG oligomer yet reported. The high purity enables the first crystallizations and hence the first glimpses of secondary 3(10)-helical PEG structures.

Experimental evidence to support a theory of lipid membrane fusion

Membrane fusion between two lipid membranes with different curvatures was measured by using a fluorescence fusion assay for lipid vesicle systems and was also obtained by measuring lipid monolayer surface tension upon the fusion of vesicles to monolayer membranes. For such membrane systems, it was found that when lysolipid was incorporated only in the membrane with a greater curvature, membrane fusion was more suppressed than those for the case where the same amount (molar ratio of lysolipid to non-lysolipids) of lysolipid was incorporated only in the membrane with a lower curvature. When lysolipid was incorporated only in a flat membrane (e.g., monolayer) and the fusion of small vesicles (SUV) to the monolayer was measured, suppression of membrane fusion by lysolipid was minimal. It is known that lysolipid lowers the surface energy of curved membranes, which stabilizes energetically such membrane surfaces, and thus suppresses membrane fusion. Our results support our theory of lipid membrane fusion where the membrane fusion occurs through the most curved membrane region at the contact area of two interacting membranes.

Biological pacemakers as a therapy for cardiac arrhythmias

PURPOSE OF REVIEW

Cardiac rhythm disorders are caused by malfunctions of impulse generation or conduction. Malfunctions of impulse generation, that is, defects in pacemaking, are often life-threatening. Present therapies span a wide array of approaches, but remain largely palliative. Recent progress in understanding of the underlying biology of pacemaking opens up new prospects for better alternatives to the present routine. Specifically, development and use of biological pacemakers could prove to be advantageous to the conventional approaches.

RECENT FINDINGS

We review the current state of the art in gene and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, embryonic as well as adult stem cell therapies, novel synthetic pacemaker channels, and adult somatic cell-fusion approach.

SUMMARY

Biological pacemaking can be achieved by modulating ionic currents by gene transfer or by delivering engineered pacemaker cells into normally quiescent myocardium. The present state of development is proof-of-concept; we are now working on reducing to practice a stable, reliable biological product as an alternative to electronic pacemakers.

Calculation of free energy barriers to the fusion of small vesicles

The fusion of small vesicles, either with a planar bilayer or with one another, is studied using a microscopic model in which the bilayers are composed of hexagonal- and lamellar-forming amphiphiles. The free energy of the system is obtained within the self-consistent field approximation. We find that the free energy barrier to form the initial stalk is hardly affected by the radius of the vesicle, but that the barrier to expand the hemifusion diaphragm and form a fusion pore decreases rapidly as the radius decreases. As a consequence, once the initial barrier to stalk formation is overcome, one which we estimate at 13 k(B)T for biological membranes, fusion involving small vesicles should proceed with little or no further input of energy.

Productive hemifusion intermediates in fast vesicle fusion driven by neuronal SNAREs

An in vitro fusion assay uses fluorescence microscopy of labeled lipids to monitor single v-SNARE vesicle docking and fusion events on a planar lipid bilayer containing t-SNAREs. For vesicles and bilayer comprising phosphatidylcholine (POPC, 84-85% by mol) and phosphatidylserine (DOPS, 15% by mol), previous work demonstrated prompt, full fusion (tau(fus) = 25 ms). Substitution of 20-60% phosphatidylethanolamine (DOPE) for phosphatidylcholine in the v-SNARE vesicle with either 0 or 20% DOPE included in the t-SNARE bilayer gives rise to hemifusion events. Labeled lipids diffuse into the planar bilayer as two temporally distinct waves, presumably hemifusion of the outer leaflet followed by inner leaflet (core) fusion. The fusion kinetics with DOPE is markedly heterogeneous. Some vesicle/docking site pairs exhibit prompt, full fusion while others exhibit hemifusion. Hemifusion events are roughly half productive (leading to subsequent core fusion within 20 s) and half dead-end. In qualitative accord with expectations from studies of protein-free vesicle-vesicle fusion, the hemifusion rate k(hemi) is 15-20 times faster than the core fusion rate k(core), and the fraction of hemifusion events increases with increasing percentage of DOPE. This suggests similar underlying molecular pathways for protein-free and neuronal SNARE-driven fusion. Removal of phosphatidylserine from the v-SNARE vesicle has no effect on docking or fusion.

Glycolipid Membranes through Atomistic Simulations: Effect of Glucose and Galactose Head Groups on Lipid Bilayer Properties

Though glycolipids are involved in a multitude of cellular functions, the understanding of their atom-scale properties in lipid membranes has remained very limited due to the lack of atomistic simulations. In this work, we employ extensive simulations to characterize one-component membranes comprised of glycoglycerolipids, focusing on two common glyco head groups, namely glucose and galactose. The properties of these two glycoglycerolipid bilayers are compared in a systematic manner with membranes consisting of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) lipids, whose structures aside from the head group are identical with those of the two glycolipids. We find that the glycolipid systems are characterized by a substantial number of hydrogen bonds in the head group region, leading to membrane packing that is stronger than in a PC but less significant than that in a PE bilayer. The role played by the glyco head group is especially evident in the electrostatic membrane potential, which is particularly large in the glycolipid membranes. For the same reason, the interfacial forces near glycolipid bilayers are significantly different from those found in PC and PE bilayers, affecting, e.g., the ordering of water close to the membrane. These effects are particularly important for the case of galactose, an important component in thylacoids.

Creation of a Biological Pacemaker by Cell Fusion

As an alternative to electronic pacemakers, we explored the feasibility of converting ventricular myocytes into pacemakers by somatic cell fusion. The idea is to create chemically induced fusion between myocytes and syngeneic fibroblasts engineered to express HCN1 pacemaker channels (HCN1-fibroblasts). HCN1-fibroblasts were fused with freshly isolated guinea pig ventricular myocytes using polyethylene-glycol 1500. In vivo fused myocyte-HCN1-fibroblast cells exhibited spontaneously oscillating action potentials; the firing frequency increased with beta-adrenergic stimulation. The heterokaryons created ectopic ventricular pacemaker activity in vivo at the site of cell injection. Coculture of nonfused HCN1-fibroblasts and myocytes without polyethylene-glycol 1500 revealed no evidence of dye transfer, demonstrating that the I(f)-mediated pacemaker activity arises from heterokaryons rather than electrotonic coupling. This nonviral, non-stem cell approach enables autologous, adult somatic cell therapy to create biopacemakers.

Rapid membrane fusion of individual virus particles with supported lipid bilayers

Many enveloped viruses employ low-pH-triggered membrane fusion during cell penetration. Solution-based in vitro assays in which viruses fuse with liposomes have provided much of our current biochemical understanding of low-pH-triggered viral membrane fusion. Here, we extend this in vitro approach by introducing a fluorescence assay using single particle tracking to observe lipid mixing between individual virus particles (influenza or Sindbis) and supported lipid bilayers. Our single-particle experiments reproduce many of the observations of the solution assays. The single-particle approach naturally separates the processes of membrane binding and membrane fusion and therefore allows measurement of details that are not available in the bulk assays. We find that the dynamics of lipid mixing during individual Sindbis fusion events is faster than 30 ms. Although neither virus binds membranes at neutral pH, under acidic conditions, the delay between membrane binding and lipid mixing is less than half a second for nearly all virus-membrane combinations. The delay between binding and lipid mixing lengthened only for Sindbis virus at the lowest pH in a cholesterol-dependent manner, highlighting the complex interaction between lipids, virus proteins, and buffer conditions in membrane fusion.

Novel method for preparing spheroplasts from cells with an internal cellulosic cell wall

Protoplast and spheroplast preparations allow the transfer of macromolecules into cells and provide the basis for the generation of engineered organisms. Crypthecodinium cohnii cells harvested from polyethylene glycol-containing agar plates possessed significantly lower levels of cellulose in their cortical layers, which facilitated the delivery of fluorescence-labeled oligonucleotides into these cells.

Creation of a biological pacemaker by gene- or cell-based approaches

Cardiac rhythm-associated disorders are caused by mal-functions of impulse generation and conduction. Present therapies for the impulse generation span a wide array of approaches but remain largely palliative. The progress in the understanding of the biology of the diseases with related biological tools beckons for new approaches to provide better alternatives to the present routine. Here, we review the current state of the art in gene- and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, stem cell therapies, adult somatic cell-fusion approach, novel synthetic pacemaker channel, and creating a self-contained pacemaker activity in non-excitable cells. We then conclude by discussing advantages and disadvantages of the new possibilities.

Evidence of cholesterol accumulated in high curvature regions: implication to the curvature elastic energy for lipid mixtures

Recent experiments suggested that cholesterol and other lipid components of high negative spontaneous curvature facilitate membrane fusion. This is taken as evidence supporting the stalk-pore model of membrane fusion in which the lipid bilayers go through intermediate structures of high curvature. How do the high-curvature lipid components lower the free energy of the curved structure? Do the high-curvature lipid components modify the average spontaneous curvature of the relevant monolayer, thereby facilitate its bending, or do the lipid components redistribute in the curved structure so as to lower the free energy? This question is fundamental to the curvature elastic energy for lipid mixtures. Here we investigate the lipid distribution in a monolayer of a binary lipid mixture before and after bending, or more precisely in the lamellar, hexagonal, and distorted hexagonal phases. The lipid mixture is composed of 2:1 ratio of brominated di18:0PC and cholesterol. Using a newly developed procedure for the multiwavelength anomalous diffraction method, we are able to isolate the bromine distribution and reconstruct the electron density distribution of the lipid mixture in the three phases. We found that the lipid distribution is homogenous and uniform in the lamellar and hexagonal phases. But in the distorted hexagonal phase, the lipid monolayer has nonuniform curvature, and cholesterol almost entirely concentrates in the high curvature region. This finding demonstrates that the association energies between lipid molecules vary with the curvature of membrane. Thus, lipid components in a mixture may redistribute under conditions of nonuniform curvature, such as in the stalk structure. In such cases, the spontaneous curvature depends on the local lipid composition and the free energy minimum is determined by lipid distribution as well as curvature.

Bacterial osmosensing transporters

Cells faced with dehydration because of increasing extracellular osmotic pressure accumulate solutes through synthesis or transport. Water follows, restoring cellular hydration and volume. Prokaryotes and eukaryotes possess arrays of osmoregulatory genes and enzymes that are responsible for solute accumulation under osmotic stress. In bacteria, osmosensing transporters can detect increasing extracellular osmotic pressure and respond by mediating the uptake of organic osmolytes compatible with cellular functions ("compatible solutes"). This chapter reviews concepts and methods critical to the identification and study of osmosensing transporters. Like some experimental media, cytoplasm is a "nonideal" solution so the estimation of key solution properties (osmotic pressure, osmolality, water activity, osmolarity, and macromolecular crowding) is essential for studies of osmosensing and osmoregulation. Because bacteria vary widely in osmotolerance, techniques for its characterization provide an essential context for the elucidation of osmosensory and osmoregulatory mechanisms. Powerful genetic, molecular biological, and biochemical tools are now available to aid in the identification and characterization of osmosensory transporters, the genes that encode them, and the osmoprotectants that are their substrates. Our current understanding of osmosensory mechanisms is based on measurements of osmosensory transporter activity performed with intact cells, bacterial membrane vesicles, and proteoliposomes reconstituted with purified transporters. In the quest to elucidate the structural mechanisms of osmosensing and osmoregulation, researchers are now applying the full range of available biophysical, biochemical, and molecular biological tools to osmosensory transporter prototypes.

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.

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.

Chain packing in the inverted hexagonal phase of phospholipids: a study by X-ray anomalous diffraction on bromine-labeled chains

Although lipid phases are routinely studied by X-ray diffraction, construction of their unit cell structures from the diffraction data is difficult except for the lamellar phases. This is due to the well-known phase problem of X-ray diffraction. Here we successfully applied the multiwavelength anomalous dispersion (MAD) method to solve the phase problem for an inverted hexagonal phase of a phospholipid with brominated chains. Although the principle of the MAD method for all systems is the same, we found that for lipid structures it is necessary to use a procedure of analysis significantly different from that used for protein crystals. The inverted hexagonal phase has been used to study the chain packing in a hydrophobic interstice where three monolayers meet. Hydrophobic interstices are of great interest, because they occur in the intermediate states of membrane fusion. It is generally believed that chain packing in such a region is energy costly. Consequently, it has been speculated that the inverted lipid tube is likely to deviate from a circular shape, and the chain density distribution might be nonuniform. The bromine distribution obtained from the MAD analysis provides the information for the chain packing in the hexagonal unit cell. The intensity of the bromine distribution is undulated around the unit cell. The analysis shows that the lipid chains pack the hexagonal unit cell at constant volume per chain, with no detectable effect from a high-energy interstitial region.

Evaluation of fusion between liposomes and erythrocytes for intracellular derivatization of amino acids in cells

Erythrocytes were fused with liposome for intracellular derivatization of amino acids in cells. The fusion efficiency was evaluated with capillary electrophoresis (CE) and laser-induced fluorescence (LIF) detection. Reagent fluorescein isothiocyanate (FITC) was enveloped in liposomes and introduced into erythrocytes by fusion between liposomes and erythrocytes. The amino acids in the fused cells were derivated by the introduced FITC and the derivated amino acids were extracted for detection by capillary electrophoresis equipped with laser-induced fluorescence detector. The fusion conditions were investigated. It was found that incubation of liposome and erythrocytes in the presence of 13% polyethylene glycol 6000 (PEG 6000) for 15min produced the highest fusion efficiency and kept the erythrocytes stability.

Human cytomegalovirus entry into epithelial and endothelial cells depends on genes UL128 to UL150 and occurs by endocytosis and low-pH fusion

Human cytomegalovirus (HCMV) replication in epithelial and endothelial cells appears to be important in virus spread, disease, and persistence. It has been difficult to study infection of these cell types because HCMV laboratory strains (e.g., AD169 and Towne) have lost their ability to infect cultured epithelial and endothelial cells during extensive propagation in fibroblasts. Clinical strains of HCMV (e.g., TR and FIX) possess a cluster of genes (UL128 to UL150) that are largely mutated in laboratory strains, and recent studies have indicated that these genes facilitate replication in epithelial and endothelial cells. The mechanisms by which these genes promote infection of these two cell types are unclear. We derived an HCMV UL128-to-UL150 deletion mutant from strain TR, TRdelta4, and studied early events in HCMV infection of epithelial and endothelial cells, and the role of genes UL128 to UL150. Analysis of wild-type TR indicated that HCMV enters epithelial and endothelial cells by endocytosis followed by low-pH-dependent fusion, which is different from the pH-independent fusion with the plasma membrane observed with human fibroblasts. TRdelta4 displayed a number of defects in early infection processes. Adsorption and entry of TRdelta4 on epithelial cells were poor compared with those of TR, but these defects could be overcome with higher doses of virus and the use of polyethylene glycol (PEG) to promote fusion between virion and cellular membranes. High multiplicity and PEG treatment did not promote infection of endothelial cells by TRdelta4, yet virus particles were internalized. Together, these data indicate that genes UL128 to UL150 are required for HCMV adsorption and penetration of epithelial cells and to promote some early stage of virus replication, subsequent to virus entry, in endothelial cells.

MyD88 expression is required for efficient cross-presentation of viral antigens from infected cells

While virus-infected dendritic cells (DCs) in certain instances have the capacity to activate naive T cells by direct priming, cross-priming by DCs via the uptake of antigens from infected cells has lately been recognized as another important pathway for the induction of antiviral immunity. During cross-priming, danger and stranger signals play important roles in modulating immune responses. Analogous to what has been shown for other microbial infections, virally infected cells may contain several pathogen-associated molecular patterns that are recognized by Toll-like receptors (TLRs). We analyzed whether the efficient presentation of antigens derived from infected cells requires the usage of MyD88, which is a common adaptor molecule used by all TLRs. For this study, we used murine DCs that were wild type or deficient in MyD88 expression and fibroblasts that were infected with an alphavirus replicon to answer this question. Our results show that when DCs are directly infected, they are able to activate antigen-specific CD8(+) T cells in a MyD88-independent manner. In contrast, a strict requirement of MyD88 for cross-priming was observed when virally infected cells were used as a source of antigen in vitro and in vivo. This indicates that the effects of innate immunity stimulation via the MyD88 pathway control the efficiency of cross-presentation, but not direct presentation or DC maturation, and have important implications in the development of cytotoxic T lymphocyte responses against alphaviral replicon infections.

Energetics of vesicle fusion intermediates: comparison of calculations with observed effects of osmotic and curvature stresses

We reported previously the effects of both osmotic and curvature stress on fusion between poly(ethylene glycol)-aggregated vesicles. In this article, we analyze the energetics of fusion of vesicles of different curvature, paying particular attention to the effects of osmotic stress on small, highly curved vesicles of 26 nm diameter, composed of lipids with negative intrinsic curvature. Our calculations show that high positive curvature of the outer monolayer "charges" these vesicles with excess bending energy, which then releases during stalk expansion (increase of the stalk radius, r(s)) and thus "drives" fusion. Calculations based on the known mechanical properties of lipid assemblies suggest that the free energy of "void" formation as well as membrane-bending free energy dominate the evolution of a stalk to an extended transmembrane contact. The free-energy profile of stalk expansion (free energy versus r(s)) clearly shows the presence of two metastable intermediates (intermediate 1 at r(s) approximately 0 - 1.0 nm and intermediate 2 at r(s) approximately 2.5 - 3.0 nm). Applying osmotic gradients of +/-5 atm, when assuming a fixed trans-bilayer lipid mass distribution, did not significantly change the free-energy profile. However, inclusion in the model of an additional degree of freedom, the ability of lipids to move into and out of the "void", made the free-energy profile strongly dependent on the osmotic gradient. Vesicle expansion increased the energy barrier between intermediates by approximately 4 kT and the absolute value of the barrier by approximately 7 kT, whereas compression decreased it by nearly the same extent. Since these calculations, which are based on the stalk hypothesis, correctly predict the effects of both membrane curvature and osmotic stress, they support the stalk hypothesis for the mechanism of membrane fusion and suggest that both forms of stress alter the final stages, rather than the initial step, of the fusion process, as previously suggested.

Hemifusion and fusion of giant vesicles induced by reduction of inter-membrane distance

Proteins involved in membrane fusion, such as SNARE or influenza virus hemagglutinin, share the common function of pulling together opposing membranes in closer contact. The reduction of inter-membrane distance can be sufficient to induce a lipid transition phase and thus fusion. We have used functionalized lipids bearing DNA bases as head groups incorporated into giant unilamellar vesicles in order to reproduce the reduction of distance between membranes and to trigger fusion in a model system. In our experiments, two vesicles were isolated and brought into adhesion by the mean of micromanipulation; their evolution was monitored by fluorescence microscopy. Actual fusion only occurred in about 5% of the experiments. In most cases, a state of "hemifusion" is observed and quantified. In this state, the outer leaflets of both vesicles' bilayers merged whereas the inner leaflets and the aqueous inner contents remained independent. The kinetics of the lipid probes redistribution is in good agreement with a diffusion model in which lipids freely diffuse at the circumference of the contact zone between the two vesicles. The minimal density of bridging structures, such as stalks, necessary to explain this redistribution kinetics can be estimated.

Analysis of host range phenotypes of primate hepadnaviruses by in vitro infections of hepatitis D virus pseudotypes

Hepatitis B virus (HBV) and woolly monkey hepatitis B virus (WMHBV) have natural host ranges that are limited to closely related species. The barrier for infection of primates seems to be at the adsorption and/or entry steps of the viral replication cycle, since a human hepatoma cell line is permissive for HBV and WMHBV replication following transfection of cloned DNA. We hypothesized that the HBV and WMHBV envelope proteins contain the principal viral determinants of host range. As previously shown by using the hepatitis D virus (HDV) system, recombinant HBV-HDV particles were infectious in chimpanzee as well as human hepatocytes. We extended the HDV system to include HDV particles pseudotyped with the WMHBV envelope. In agreement with the natural host ranges of HBV and WMHBV, in vitro infections demonstrated that HBV-HDV and WM-HDV particles preferentially infected human and spider monkey cells, respectively. Previous studies have implicated the pre-S1 region of the large (L) envelope protein in receptor binding and host range; therefore, recombinant HDV particles were pseudotyped with the hepadnaviral envelopes containing chimeric L proteins with the first 40 amino acids from the pre-S1 domain exchanged between HBV and WMHBV. Surprisingly, addition of the human amino terminus to the WMHBV L protein increased infectivity on spider monkey hepatocytes but did not increase infectivity for human hepatocytes. Based upon these data, we discuss the possibility that the L protein may be comprised of two domains that affect infectivity and that sequences downstream of residue 40 may influence host range and receptor binding or entry.

Architecture of the influenza hemagglutinin membrane fusion site

The mechanism of influenza hemagglutinin (HA) mediated membrane fusion has been intensively studied for over 20 years after the bromelain-released ectodomain of HA at neutral pH was first crystallized. Nearly 10 years ago, the low-pH-induced "spring coiled" conformational change of HA was predicted from peptide chemistry and confirmed by crystallography. Other work has yielded a wealth of knowledge on the observed changes in HA fusion/hemifusion phenotypes as a function of site-specific mutations of HA, or added amphipathic molecules or particular IgGs. It is becoming clear that the conformational changes predicted by the crystallography are necessary to cause fusion and that interfering with these changes can block fusion or reduce it to hemifusion. What is not known is how the conformational changes cause fusion. In particular, while it is generally agreed that fusion requires an aggregate of HAs, how the aggregate may act to transduce the energy of the HA conformational changes to creating the initial fusion defect is not known. We have used a comprehensive mass action kinetic model of HA-mediated fusion to carry out a "meta-analysis" of several key data sets, using HA-expressing cells and using virions. The consensus result of these detailed kinetic studies was that the fusion site of influenza hemagglutinin (HA) is an aggregate with at least eight HAs. The high-energy conformational change of only two of these HAs within the aggregate permits the formation of the first fusion pore. This "8 and 2" result was required to best fit all the data. We review these studies and how this kinetic result can guide and constrain HA fusion models. The kinetic analysis suggests that the sequence of fusion intermediates starts with protein control and ends with lipid control, which makes sense. While curvature intermediates, e.g. the lipid stalk, are almost certainly within the fusion sequence, the "8 and 2" result does not suggest that they are the first step after HA aggregation. The stabilized hydrophobic defect model we have proposed as a precursor to the lipid stalk can form and is consistent with the "8 and 2" result.

Protein-Lipid Interplay in Fusion and Fission of Biological Membranes

Disparate biological processes involve fusion of two membranes into one and fission of one membrane into two. To formulate the possible job description for the proteins that mediate remodeling of biological membranes, we analyze the energy price of disruption and bending of membrane lipid bilayers at the different stages of bilayer fusion. The phenomenology and the pathways of the well-characterized reactions of biological remodeling, such as fusion mediated by influenza hemagglutinin, are compared with those studied for protein-free bilayers. We briefly consider some proteins involved in fusion and fission, and the dependence of remodeling on the lipid composition of the membranes. The specific hypothetical mechanisms by which the proteins can lower the energy price of the bilayer rearrangement are discussed in light of the experimental data and the requirements imposed by the elastic properties of the bilayer.

Early steps of supported bilayer formation probed by single vesicle fluorescence assays

We have developed a single vesicle assay to study the mechanisms of supported bilayer formation. Fluorescently labeled, unilamellar vesicles (30-100 nm diameter) were first adsorbed to a quartz surface at low enough surface concentrations to visualize single vesicles. Fusion and rupture events during the bilayer formation, induced by the subsequent addition of unlabeled vesicles, were detected by measuring two-color fluorescence signals simultaneously. Lipid-conjugated dyes monitored the membrane fusion while encapsulated dyes reported on the vesicle rupture. Four dominant pathways were observed, each exhibiting characteristic two-color fluorescence signatures: 1) primary fusion, in which an unlabeled vesicle fuses with a labeled vesicle on the surface, is signified by the dequenching of the lipid-conjugated dyes followed by rupture and final merging into the bilayer; 2) simultaneous fusion and rupture, in which a labeled vesicle on the surface ruptures simultaneously upon fusion with an unlabeled vesicle; 3) no dequenching, in which loss of fluorescence signal from both dyes occur simultaneously with the final merger into the bilayer; and 4) isolated rupture (pre-ruptured vesicles), in which a labeled vesicle on the surface spontaneously undergoes content loss, a process that occurs with high efficiency in the presence of a high concentration of Texas Red-labeled lipids. Vesicles that have undergone content loss appear to be more fusogenic than intact vesicles.

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

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

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

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

Probing the mechanism of fusion in a two-dimensional computer simulation

A two-dimensional (2D) model of lipid bilayers was developed and used to investigate a possible role of membrane lateral tension in membrane fusion. We found that an increase of lateral tension in contacting monolayers of 2D analogs of liposomes and planar membranes could cause not only hemifusion, but also complete fusion when internal pressure is introduced in the model. With a certain set of model parameters it was possible to induce hemifusion-like structural changes by a tension increase in only one of the two contacting bilayers. The effect of lysolipids was modeled as an insertion of a small number of extra molecules into the cis or trans side of the interacting bilayers at different stages of simulation. It was found that cis insertion arrests fusion and trans insertion has no inhibitory effect on fusion. The possibility of protein participation in tension-driven fusion was tested in simulation, with one of two model liposomes containing a number of structures capable of reducing the area occupied by them in the outer monolayer. It was found that condensation of these structures was sufficient to produce membrane reorganization similar to that observed in simulations with "protein-free" bilayers. These data support the hypothesis that changes in membrane lateral tension may be responsible for fusion in both model phospholipid membranes and in biological protein-mediated fusion.

Osmotic and curvature stress affect PEG-induced fusion of lipid vesicles but not mixing of their lipids

Poly (ethylene glycol) (PEG) in the external environment of membrane vesicles creates osmotic imbalance that leads to mechanical stress in membranes and may induce local membrane curvature. To determine the relative importance of membrane stress and curvature in promoting fusion, we monitored contents mixing (CM) and lipid mixing (LM) between different sized vesicles under a variety of osmotic conditions. CM between highly curved vesicles (SUV, 26 nm diameter) was up to 10 times greater than between less curved vesicles (LUV, 120 nm diameter) after 5 min incubation at a low PEG concentration (<10 wt%), whereas LM was only approximately 30% higher. Cryo-electron microscopy showed that PEG at 10 wt% did not create high curvature contacts between membranes in LUV aggregates. A negative osmotic gradient (-300 mOs/kg, hypotonic inside) increased CM two- to threefold for both types of vesicles, but did not affect LM. A positive gradient (+220 mOs/kg, hypertonic inside) nearly eliminated CM and had no effect on LM. Hexadecane added to vesicles had no effect on LM but enhanced CM and reduced the inhibitory effect on CM of a positive osmotic gradient, but had little influence on results obtained under a negative osmotic gradient. We conclude that the ability of closely juxtaposed bilayers to form an initial intermediate ("stalk") as soon as they come into close contact was not influenced by osmotic stress or membrane curvature, although pore formation was critically dependent on these stresses. The results also suggest that hexadecane affects the same part of the fusion process as osmotic stress. We interpret this result to suggest that both a negative osmotic gradient and hexadecane reduce the unfavorable free energy of hydrophobic interstices associated with the intermediates of the fusion process.

Fractional occurrence of defects in membranes and mechanically driven interleaflet phospholipid transport

The picture of biological membranes as uniform, homogeneous bileaflet structures has been revised in recent times due to the growing recognition that these structures can undergo significant fluctuations both in local curvature and in thickness. In particular, evidence has been obtained that a temporary, localized disordering of the lipid bilayer structure (defects) may serve as a principal pathway for movement of lipid molecules from one leaflet of the membrane to the other. How frequently these defects occur and how long they remain open are important unresolved questions. In this report, we calculate the rate of molecular transport through a transient defect in the membrane and compare this result to measurements of the net transbilayer flux of lipid molecules measured in an experiment in which the lipid flux is driven by differences between the mechanical stress in the two leaflets of the membrane bilayer. Based on this comparison, we estimate the frequency of defect occurrence in the membrane. The occurrence of defects is rare: the probability of finding a defect in 1.0 microm2 of a lecithin membrane is estimated to be approximately 6.0x10(-6). Based on this fractional occurrence of defects, the free energy of defect formation is estimated to be approximately 1.0x10(-19) J. The calculations provide support for a model in which interleaflet transport in membranes is accelerated by mechanically driven lipid flow.