Fusion of influenza virus membranes with liposomes at pH 7.5

Measuring single-virus fusion kinetics using an assay for nucleic acid exposure

The kinetics by which individual enveloped viruses fuse with membranes provide an important window into viral-entry mechanisms. We have developed a real-time assay using fluorescent probes for single-virus genome exposure than can report on stages of viral entry including or subsequent to fusion pore formation and prior to viral genome trafficking. We accomplish this using oxazole yellow nucleic-acid-binding dyes, which can be encapsulated in the lumen of target membranes to permit specific detection of fusion events. Since increased fluorescence of the dye occurs only when it encounters viral genome via a fusion pore and binds, this assay excludes content leakage without fusion. Using this assay, we show that influenza virus fuses with liposomes of different sizes with indistinguishable kinetics by both testing liposomes extruded through pores of different radii and showing that the fusion kinetics of individual liposomes are uncorrelated with the size of the liposome. These results suggest that the starting curvature of such liposomes does not control the rate-limiting steps in influenza entry.

High-Speed AFM Reveals Molecular Dynamics of Human Influenza A Hemagglutinin and Its Interaction with Exosomes

Influenza A hemagglutinin (HA) is one of the crucial virulence factors that mediate host tropism and viral infectivity. Presently, the mechanism of the fusogenic transition of HA remains elusive. Here, we used high-speed atomic force microscopy (HS-AFM) to decipher the molecular dynamics of HA and its interaction with exosomes. Our data reveal that the native conformation of HA in the neutral buffer is ellipsoidal, and HA undergoes a conformational change in an acidic buffer. Real-time visualization of the fusogenic transition by HS-AFM suggests that the mechanism is possibly fit to the "uncaging" model, and HA intermediate appears as Y-shaped. A firm interaction between the HA and exosome in an acidic buffer indicates the insertion of a fusion peptide into the exosomal layer and subsequently destabilizes the layer, resulting in the deformation or rupture of exosomes, releasing exosomal contents. In contrast, the HA-exosome interaction is weak in a neutral buffer because the interaction is mediated by weak bonds between the HA receptor-binding site and receptors on the exosome.

Cholesterol enhances influenza binding avidity by controlling nanoscale receptor clustering

Influenza virus infects cells by binding to sialylated glycans on the cell surface. While the chemical structure of these glycans determines hemagglutinin-glycan binding affinity, bimolecular affinities are weak, so binding is avidity-dominated and driven by multivalent interactions. Here, we show that membrane spatial organization can control viral binding. Using single-virus fluorescence microscopy, we demonstrate that the sterol composition of the target membrane enhances viral binding avidity in a dose-dependent manner. Binding shows a cooperative dependence on concentration of receptors for influenza virus, as would be expected for a multivalent interaction. Surprisingly, the ability of sterols to promote viral binding is independent of their ability to support liquid-liquid phase separation in model systems. We develop a molecular explanation for this observation via molecular dynamics simulations, where we find that cholesterol promotes small-scale clusters of glycosphingolipid receptors. We propose a model whereby cholesterol orders the monomeric state of glycosphingolipid receptors, reducing the entropic penalty of receptor association and thus favoring multimeric complexes without phase separation. This model explains how cholesterol and other sterols control the spatial organization of membrane receptors for influenza and increase viral binding avidity. A natural consequence of this finding is that local cholesterol concentration in the plasma membrane of cells may alter the binding avidity of influenza virions. Furthermore, our results demonstrate a form of cholesterol-dependent membrane organization that does not involve lipid rafts, suggesting that cholesterol's effect on cell membrane heterogeneity is likely the interplay of several different factors.

Small-angle neutron scattering studies of phospholipid-NSAID adducts

Nonsteroidal anti-inflammatory drugs (NSAIDs) are known to have strong interactions with lipid membranes. Using small-angle neutron scattering, the effect of ibuprofen, a prominent NSAID, on the radius of small unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and their bilayer structure was studied systematically as a function of pH (ranging from 2 to 8) and drug-to-lipid mole ratio (from 0/1 to 0.62/1 mol/mol). Ibuprofen with a pK(a) of approximately 4.6 was found to significantly affect the bilayer structure at all pH values, irrespective of the charge state of the drug. At low pH values, the drug reduces the bilayer thickness, induces fluid-like behavior, and changes headgroup hydration. The incorporation of the drug in the lipid bilayer while affecting the local bilayer structure and hydration of the lipid does not affect the overall stability of the vesicle dispersions over the pH range studied.

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.

Conformational changes in Sindbis virions resulting from exposure to low pH and interactions with cells suggest that cell penetration may occur at the cell surface in the absence of membrane fusion

Alphaviruses have the ability to induce cell-cell fusion after exposure to acid pH. This observation has served as an article of proof that these membrane-containing viruses infect cells by fusion of the virus membrane with a host cell membrane upon exposure to acid pH after incorporation into a cell endosome. We have investigated the requirements for the induction of virus-mediated, low pH-induced cell-cell fusion and cell-virus fusion. We have correlated the pH requirements for this process to structural changes they produce in the virus by electron cryo-microscopy. We found that exposure to acid pH was required to establish conditions for membrane fusion but that membrane fusion did not occur until return to neutral pH. Electron cryo-microscopy revealed dramatic changes in the structure of the virion as it was moved to acid pH and then returned to neutral pH. None of these treatments resulted in the disassembly of the virus protein icosahedral shell that is a requisite for the process of virus membrane-cell membrane fusion. The appearance of a prominent protruding structure upon exposure to acid pH and its disappearance upon return to neutral pH suggested that the production of a "pore"-like structure at the fivefold axis may facilitate cell penetration as has been proposed for polio (J. Virol. 74 (2000) 1342) and human rhino virus (Mol. Cell 10 (2002) 317). This transient structural change also provided an explanation for how membrane fusion occurs after return to neutral pH. Examination of virus-cell complexes at neutral pH supported the contention that infection occurs at the cell surface at neutral pH by the production of a virus structure that breaches the plasma membrane bilayer. These data suggest an alternative route of infection for Sindbis virus that occurs by a process that does not involve membrane fusion and does not require disassembly of the virus protein shell.

Structural characterizations of fusion peptide analogs of influenza virus hemagglutinin. Implication of the necessity of a helix-hinge-helix motif in fusion activity

Infection by enveloped viruses initially involves membrane fusion between viral and host cell membranes. The fusion peptide plays a crucial role in triggering this reaction. To clarify how the fusion peptide exerts this specific function, we carried out biophysical studies of three fusion peptide analogs of influenza virus hemagglutinin HA2, namely E5, G13L, and L17A. E5 exhibits an activity similar to the native fusion peptide, whereas G13L and L17A, which are two point mutants of the E5 analog, possess much less fusion activity. Our CD data showed that the conformations of these three analogs in SDS micelles are pH-dependent, with higher alpha-helical contents at acidic pH. Tryptophan fluorescence emission experiments indicated that these three analogs insert deeper into lipid bilayers at acidic pH. The three-dimensional structure of the E5 analog in SDS micelles at pH 4.0 revealed that two segments, Leu(2)-Glu(11) and Trp(14)-Ile(18), form amphipathic helical conformations, with Gly(12)-Gly(13) forming a hinge. The hydrophobic residues in the N- and C-terminal helices form a hydrophobic cluster. At neutral pH, however, the C-terminal helix of Trp(14)-Ile(18) reduces dramatically, and the hydrophobic core observed at acidic pH is severely disrupted. We suggest that the disruption of the C-terminal helix renders the E5 analog fusion-inactive at neutral pH. Furthermore, the decrease of the hinge and the reduction of fusion activity in G13L reveal the importance of the hinge in fusion activity. Also, the decrease in the C-terminal helix and the reduction of fusion activity in L17A demonstrates the importance of the C-terminal helix in fusion activity. Based on these biophysical studies, we propose a model that illustrates the structural change of the HA2 fusion peptide analog and explains how the analog interacts with the lipid bilayer at different pH values.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

Entry of animal viruses and macromolecules into cells

The entry of animal viruses into cells is mediated by conformational changes in certain virion-particle components. These changes are triggered by the binding of virions to receptors and are influenced by low pH during receptor-mediated endocytosis. These conformational alterations promote the interaction of some viral proteins with cellular membranes thereby leading to transient pore formation and the disruption of ionic and pH gradients. The entry of toxins that do not possess receptors on the cell surface is promoted during the translocation of the virus genome or the nucleocapsid to the cytoplasm. A model is now presented which indicates that efficient virus translocation through cellular membranes requires energy, that may be generated by a protonmotive force. The entry of some animal viruses, as promoted by low pH, should thus only take place when a pH gradient and/or a membrane potential exist, but will not take place if these are dissipated, even if virion particles are present in an acidic environment.

Concanamycin A blocks influenza virus entry into cells under acidic conditions

The selective inhibitor of the vacuolar proton-ATPase, concanamycin A, powerfully blocks influenza virus entry into cells, if present during the initial times of virus infection. Attachment of virus particles to cells is not prevented by concanamycin A, rather the exit of influenza virus from endosomes is the step blocked by this macrolide antibiotic. Inhibition of influenza virus entry into cells by concanamycin A or by nigericin takes place under acidic conditions. Moreover, if the pH gradient is abolished by pre-incubation of cells in acidic pH, influenza virus entry does not occur even in the absence of any inhibitors. These results indicate that acidic conditions per se are not sufficient to promote virus entry into cells; rather this step of virus infection requires a pH gradient.

The role of N-acetylneuraminic (sialic) acid in the pH dependence of influenza virion fusion with planar phospholipid membranes

It is known that fusion of influenza virus to host cell membranes is strongly promoted by acidic pH. We have determined conditions required to obtain pH-dependent fusion of influenza virus to planar bilayer membranes. The rate of viral fusion was determined from the flash rate of R18-labeled virions delivered to the surface of the planar membrane by pressure-ejection from a pipette. For a bilayer formed only of phospholipids and cholesterol, the fusion rate was independent of pH and unaffected by the phospholipid composition. When the gangliosides GD1a + GT1b were included in the planar membrane, however, the fusion rate varied steeply with pH. The rate at pH 7.4 in the presence of the gangliosides was about an order of magnitude less than in their absence. At pH less than approximately 5.5, the rate was about an order of magnitude greater in the presence of gangliosides than in their absence. The fusion rate with planar membranes containing globoside, a ceramide-backboned glycolipid, was also independent of pH, indicating that the pH dependence required sialic acid on the carbohydrate moiety of the glycolipid. The gangliosides GM1a and GM3, both of which possess sialic acid, produced the same pH-dependent fusion rate as seen with GD1a + GT1b, indicating that the presence, but not the location, of terminal sialic acids is critical. Incubating virus with soluble sialyllactose blocked fusion to both ganglioside-free and ganglioside-containing planar membranes. These results show that the pH dependence of influenza virion fusion arises from the interaction of the sialic acid receptor with the influenza hemagglutinin. A model for sialic acid-hemagglutinin interactions accounting for pH-dependent fusion is presented.

Kinetics of fusion and lipid transfer between virus receptor containing liposomes and influenza viruses as measured with the octadecylrhodamine B chloride assay

Octadecylrhodamine B chloride (R18) and ganglioside GD1a (virus receptor) were incorporated into small unilamellar liposomes [Hoekstra et al. (1984) Biochemistry 23, 5675-5681]. Upon interaction of these liposomes with PR8 influenza viruses without prebinding, two types of dequenching were observed at 37 degrees C, both second-order processes: a fast reaction at pH 5.3, 2k = 17.53 x 10(-3) (Q.s)-1, and a slow reaction at pH 7.4, 2k = 0.335 x 10(-3) (Q.s)-1. The maximal level of dequenching was the same for both. Upon prebinding of liposomes to PR8 viruses (30 min, 0 degrees C, pH 7.4) at high concentrations, a very fast dequenching occurred when the prebinding mixture was diluted into prewarmed (37 degrees C) 10 mM PBS, pH 5.3. For the initial phase, a first-order rate constant of 0.5 s-1 could be extrapolated. After a quick drop in velocity during the first 30 s, the reaction was kinetically indistinguishable from the one found without prebinding. A second-order process with 2k = 16.52 x 10(-3) (Q.s)-1 became rate-limiting. The fast reactions at pH 5.3 can be abolished by inactivation or removal of the virus hemagglutinin. We conclude that the reaction at pH 5.3 reflects the hemagglutinin-dependent fusion process known to occur between influenza viruses and partner membranes at low pH; however, second-order kinetics indicate that specific binding rather than fusion is the rate-limiting step. For the slow dequenching, which is not affected by prebinding, the rate constant is 20 times lower than for the fast reaction, and the process is independent of viral hemagglutinin.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane fusion activity of influenza virus. Effects of gangliosides and negatively charged phospholipids in target liposomes

Fusion of influenza virus with liposomes composed of negatively charged phospholipids differs from fusion with biological membranes or zwitterionic liposomes with ganglioside receptors [Stegmann, T., Hoekstra, D., Scherphof, G., & Wilschut, J. (1986) J. Biol. Chem. 261, 10966-10969]. In this study, we investigated how the kinetics and extent of fusion of influenza virus, monitored with a fluorescence resonance energy-transfer assay, are influenced by the surface charge and the presence of receptors on liposomal membranes. The results were analyzed in terms of mass action kinetic model, providing separate rate constants for the initial virus-liposome adhesion, or aggregation, and for the actual fusion reaction. Incorporation of increasing amounts of cardiolipin (CL) or phosphatidylserine (PS) into otherwise zwitterionic phosphatidylcholine (PC)/phosphatidylethanolamine (PE) vesicles results in a gradual shift of the pH threshold of fusion to neutral, relative to the pH threshold obtained with PC/PE vesicles containing the ganglioside GD1a, while also the rate of fusion increases. This indicates the emergence of a fusion mechanism not involving the well-documented conformational change in the viral hemagglutinin (HA). However, only with pure CL liposomes this nonphysiological fusion reaction dominates the overall fusion process; with pure PS or with zwitterionic vesicles containing CL or PS, the contribution of the nonphysiological fusion reaction is small. Accordingly, preincubation of the virus alone at low pH results in a rapid inactivation of the viral fusion capacity toward all liposome compositions studied, except pure CL liposomes. The results of the kinetic analyses show that with pure CL liposomes the rates of both virus-liposome adhesion and fusion are considerably higher than with all other liposome compositions studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Virus entry into animal cells

In addition to its many other functions, the plasma membrane of eukaryotic cells serves as a barrier against invading parasites and viruses. It is not permeable to ions and to low molecular weight solutes, let alone to proteins and polynucleotides. Yet it is clear that viruses are capable of transferring their genome and accessory proteins into the cytosol or into the nucleus, and thus infect the cell. While the detailed mechanisms remain unclear for most animal viruses, a general theme is apparent like other stages in the replication cycle; their entry depends on the activities of the host cell. In order to take up nutrients, to communicate with other cells, to control the intracellular ion balance, and to secrete substances, cells have a variety of mechanisms for bypassing and modifying the barrier properties imposed by their plasma membrane. It is these mechanisms, and the molecules involved in them, that viruses exploit.

Interaction of influenza virus with gangliosides and liposomes containing gangliosides

It has already been shown that influenza virus binds unspecifically to liposomes containing ganglioside GM1 wheras with gangliosides GD1b and GT1b binding occurs in a specific and saturable manner [Slepushkin et al. (1986) Biol. Membr. 3, 229-235]. In the present study the mode of interaction between influenza virus and various gangliosides or phospholipid liposomes containing cholesterol and gangliosides has been investigated. The influence of exogenous gangliosides on the structure of the viral envelope was studied using fluorescent and photoactivatable phospholipids incorporated into the viral membrane. With both types of probes maximal effects of gangliosides were caused by GT1b. Addition of that ganglioside resulted in a marked decrease in the fluorescence polarization (P) of fluorescent labeled virus as well as in substantial changes in the binding of photoactivatable analogues of sphingomyelin and phosphatidylcholine to virus proteins, mainly hemagglutinin. The effects of GT1b and GD1b on P value were comparable, whereas gangliosides with other oligosaccharide chains caused much smaller changes in P. Furthermore GT1b but not GM1 influenced phospholipid-hemagglutinin cross-linking. Interaction of the virus with large unilamellar liposomes was monitored by two fluorescence assays based on resonance-energy transfer from the tryptophans and tyrosines of viral proteins to vesicles labeled with a triacylglycerol (anthrylvinyldioleoylglycerol) or from these labeled vesicles to virions labeled with a perylenoyl derivative of galactosylcerebroside (PGalSph). A third fluorescence assay was based on relief of self-quenching in PGalSph-labeled virions, upon low-pH-induced virus-liposome fusion. With all three fusion assays the changes of fluorescence caused by GT1b were more pronounced than those induced by GM1. On the other hand, virus-induced release of [14C]glucose from multilamellar liposomes was enhanced by GM1 but not by GT1b or GD1b. It is concluded that the interaction of GT1b or GD1b with virus hemagglutinin induces a rearrangement of the viral lipids rendering lipid bilayer areas of the viral envelope significantly fluid, which in turn promotes fusion of the virus with target membranes. Probably virus-liposome fusion and virus-induced liposome leakage are brought about by different mechanisms depending on specific or unspecific binding of the virions to the target.

Ficoll and dextran enhance adhesion of Sendai virus to liposomes containing receptor (ganglioside GD1a)

Previous work has shown that high-speed centrifugation (300,000 g) of Sendai virus and liposomes in 40% (w/v) sucrose layered under a discontinuous sucrose gradient removes Sendai virus bound to liposomes containing the ganglioside GD1a, a Sendai virus receptor. Centrifugation also removes virus bound to liposomes containing other negatively charged lipids. This work shows that centrifugation of virus through a discontinuous ficoll gradient does not remove virus bound to liposomes containing GD1a but does remove virus from liposomes containing various other negatively charged lipids including the ganglioside GM1, which is not a Sendai virus receptor. The amount of virus that adheres to liposomes increases with increasing content of GD1a in the liposomes. The adhesion of virus to receptor-containing liposomes during centrifugation through a ficoll gradient results from the presence of ficoll and increases with increasing ficoll concentration. Virus also adheres to receptor-containing liposomes during centrifugation in the presence of dextran. These data indicate that caution should be used in interpreting associations demonstrated by centrifugation through dextran and ficoll gradients. They also indicate that binding of virus by ganglioside receptors can be modulated by carbohydrate polymers, which are thought not to have any specific interaction with either viruses or gangliosides.