Membrane fusion activity of reconstituted vesicles of influenza virus hemagglutinin glycoproteins

Two DNA aptamers against avian influenza H9N2 virus prevent viral infection in cells

New antiviral therapy for pandemic influenza mediated by the H9N2 avian influenza virus (AIV) is increasingly in demand not only for the poultry industry but also for public health. Aptamers are confirmed to be promising candidates for treatment and prevention of influenza viral infections. Thus, we studied two DNA aptamers, A9 and B4, selected by capillary electrophoresis-based systemic evolution of ligands by exponential enrichment (CE-SELEX) procedure using H9N2 AIV purified haemagglutinin (HA) as target. Both aptamers had whole-virus binding affinity. Also, an enzyme-linked aptamer assay (ELAA) confirmed binding affinity and specificity against other AIV subtypes. Finally, we studied aptamer-inhibitory effects on H9N2 AIV infection in Madin-Darby canine kidney (MDCK) cells and quantified viral load in supernatant and in cell with quantitative PCR (qPCR). Our data provide a foundation for future development of innovative anti-influenza drugs.

Chapter 9 Fusion of Viral Envelopes with Cellular Membranes

This chapter reviews some characteristic features of membrane fusion activity for each virus and discusses the mechanisms of membrane fusion, especially low pH-induced membrane fusion. It concentrates on the interaction of the hydrophobic segment with the target cell membrane lipid bilayer and suggests the entrance of the segment into the lipid bilayer hydrophobic core as a key step in fusion. The envelope is a lipid bilayer membrane with the virus specific glycoproteins spanning it. The bilayer originates from the host cell membrane and has a lipid composition and transbilayer distribution quite similar to the host's. The viral glycoproteins have the functions of binding to the target cell surface and fusion with the cell membranes. The two functions are carried by a single glycoprotein in influenza virus (HA), vesicular stomatitis virus (VSV) G glycoprotein, and Semliki Forest virus SFV E glycoprotein. In Sendai virus (HVJ), the functions are carried by separate glycoproteins, hemagglutinin-neuraminidase (HN) for binding and fusion glycoprotein (F) for fusion. When viruses encounter target cells, they first bind to the cell surface through an interaction of the viral glycoprotein with receptors.

Membrane fusion by single influenza hemagglutinin trimers. Kinetic evidence from image analysis of hemagglutinin-reconstituted vesicles

Influenza hemagglutinin, the receptor-binding and membrane fusion protein of the virus, is a prototypic model for studies of biological membrane fusion in general. To elucidate the minimum number of hemagglutinin trimers needed for fusion, the kinetics of fusion induced by reconstituted vesicles of hemagglutinin was studied by using single-vesicle image analysis. The surface density of hemagglutinin fusion-activity sites on the vesicles was varied, while keeping the surface density of receptor-binding activity sites constant, by co-reconstitution of the fusogenic form of hemagglutinin, HA(1,2), and the non-fusogenic form, HA(0), at various HA(1,2):(HA(1,2) + HA(0)) ratios. The rate of fusion between the hemagglutinin vesicles containing a fluorescent lipid probe, octadecylrhodamine B, and red blood cell ghost membranes was estimated from the time distribution of fusion events of single vesicles observed by fluorescence microscopy. The best fit of a log-log plot of fusion rate versus the surface density of HA(1,2) exhibited a slope of 0.85, strongly supporting the hypothesis that single hemagglutinin trimers are sufficient for fusion. When only HA(1,2) (without HA(0)) was reconstituted on vesicles, the dependence of fusion rate on the surface density of HA(1,2) was distinct from that for the HA(1,2)-HA(0) co-reconstitution. The latter result suggested interference with fusion activity by hemagglutinin-receptor binding, without having to assume a fusion mechanism involving multiple hemagglutinin trimers.

Dynamics of raft molecules in the cell and artificial membranes: approaches by pulse EPR spin labeling and single molecule optical microscopy

Lipid rafts in the plasma membrane, domains rich in cholesterol and sphingolipids, have been implicated in a number of important membrane functions. Detergent insolubility has been used to define membrane "rafts" biochemically. However, such an approach does not directly contribute to the understanding of the size and the lifetime of rafts, dynamics of the raft-constituent molecules, and the function of rafts in the membrane in situ. To address these issues, we have developed pulse EPR spin labeling and single molecule tracking optical techniques for studies of rafts in both artificial and cell membranes. In this review, we summarize our results and perspectives obtained by using these methods. We emphasize the importance of clearly distinguishing small/unstable rafts (lifetime shorter than a millisecond) in unstimulated cells and stabilized rafts induced by liganded and oligomerized (GPI-anchored) receptor molecules (core receptor rafts, lifetime over a few minutes). We propose that these stabilized rafts further induce temporal, greater rafts (signaling rafts, lifetime on the order of a second) for signaling by coalescing other small/unstable rafts, including those in the inner leaflet of the membrane, each containing perhaps one molecule of the downstream effector molecules. At variance with the general view, we emphasize the importance of cholesterol segregation from the liquid-crystalline unsaturated bulk-phase membrane for formation of the rafts, rather than the affinity of cholesterol and saturated alkyl chains. In the binary mixture of cholesterol and an unsaturated phospholipid, cholesterol is segregated out from the bulk unsaturated liquid-crystalline phase, forming cholesterol-enriched domains or clustered cholesterol domains, probably due to the lateral nonconformability between the rigid planar transfused ring structure of cholesterol and the rigid bend of the unsaturated alkyl chain at C9-C10. However, such cholesterol-rich domains are small, perhaps consisting of only several cholesterol molecules, and are short-lived, on the order of 1-100 ns. We speculate that these cholesterol-enriched domains may be stabilized by the presence of saturated alkyl chains of sphingomyelin or glycosphingolipids, and also by clustered raft proteins. In the influenza viral membrane, one of the simplest forms of a biological membrane, the lifetime of a protein and cholesterol-rich domain was evaluated to be on the order of 100 micro, again showing the short lifetime of rafts in an unstimulated state. Finally, we propose a thermal Lego model for rafts as the basic building blocks for signaling pathways in the plasma membrane.

Fusogenic activity of reconstituted newcastle disease virus envelopes: a role for the hemagglutinin-neuraminidase protein in the fusion process

Enveloped viruses, such as newcastle disease virus (NDV), make their entry into the host cell by membrane fusion. In the case of NDV, the fusion step requires both transmembrane hemagglutinin-neuraminidase (HN) and fusion (F) viral envelope glycoproteins. The HN protein should show fusion promotion activity. To date, the nature of HN-F interactions is a controversial issue. In this work, we aim to clarify the role of the HN glycoprotein in the membrane fusion step. Four types of reconstituted detergent-free NDV envelopes were used, on differing in their envelope protein contents. Fusion of the different virosomes and erythrocyte ghosts was monitored using the octadecyl rhodamine B chloride assay. Only the reconstituted envelopes having the F protein, even in the absence of HN protein, displayed residual fusion activity. Treatment of such virosomes with denaturing agents affecting the F protein abolished fusion, indicating that the fusion detected was viral protein-dependent. Interestingly, the rate of fusion in the reconstituted systems was similar to that of intact viruses in the presence of the inhibitor of HN sialidase activity 2,3-dehydro-2-deoxy-N-acetylneuraminic acid. The results show that the residual fusion activity detected in the reconstituted systems was exclusively due to F protein activity, with no contribution from the fusion promotion activity of HN protein.

Structure of influenza haemagglutinin at neutral and at fusogenic pH by electron cryo-microscopy

The three-dimensional structures of the complete haemagglutinin (HA) of influenza virus A/Japan/305/57 (H2N2) in its native (neutral pH) and membrane fusion-competent (low pH) form by electron cryo-microscopy at a resolution of 10 A and 14 A, respectively, have been determined. In the fusion-competent form the subunits remain closely associated preserving typical overall features of the trimeric ectodomain at neutral pH. Rearrangements of the tertiary structure in the distal and the stem parts are associated with the formation of a central cavity through the entire ectodomain. We suggest that the cavity is essential for relocation of the so-called fusion sequence of HA towards the target membrane.

Structural features of membrane fusion between influenza virus and liposome as revealed by quick-freezing electron microscopy

The structure of membrane fusion intermediates between the A/PR/8(H1N1) strain of influenza virus and a liposome composed of egg phosphatidylcholine, cholesterol, and glycophorin was studied using quick-freezing electron microscopy. Fusion by viral hemagglutinin protein was induced at pH 5.0 and 23 degrees C. After a 19-s incubation under these conditions, small protrusions with a diameter of 10-20 nm were found on the fractured convex faces of the liposomal membranes, and small pits complementary to the protrusions were found on the concave faces. The protrusions and pits corresponded to fractured parts of outward bendings of the lipid bilayer or "microprotrusions of the lipid bilayer." At the loci of the protrusions and pits, liposomal membranes had local contacts with viral membranes. In many cases both the protrusions and the pits were aligned in regular polygonal arrangements, which were thought to reflect the array of hemagglutinin spikes on the viral surface. These structures were induced only when the medium was acidic with the virus present. Based on these observations, it was concluded that the microprotrusions of the lipid bilayer are induced by hemagglutinin protein. Furthermore, morphological evidence for the formation of the "initial fusion pore" at the microprotrusion was obtained. The protrusion on the convex face sometimes had a tiny hole with a diameter of <4 nm in the center. The pits transformed into narrow membrane connections <10 nm in width, bridging viruses and liposomes. The structures of the fusion pore and fusion neck with larger sizes were also observed, indicating growth of the protrusions and pits to distinct fusion sites. We propose that the microprotrusion of the lipid bilayer is a fusion intermediate induced by hemagglutinin protein, and suggest that the extraordinarily high curvature of this membrane structure is a clue to the onset of fusion. The possible architecture of the fusion intermediate is discussed with regard to the localization of intramembrane particles at the microprotrusion.

Transient changes of the conformation of hemagglutinin of influenza virus at low pH detected by time-resolved circular dichroism spectroscopy

Membrane fusion of influenza virus is mediated by a conformational change of the viral membrane protein hemagglutinin (HA) triggered by low pH. By near UV CD spectroscopy, which is sensitive to the arrangement and mobility of aromatic amino acids in proteins, we have monitored continuously with a time resolution of 5 s the kinetics of structural alterations of the ectodomain of HA isolated from different influenza virus strains (H1 (A/PR 8/34), H2 (A/Japan), and H3 (X31)). To establish a functional correlation to structural alterations of the HA ectodomain reflected by the CD, we have measured the kinetics of the virus-erythrocyte fusion and of the inactivation of fusion by low pH preincubation of viruses. At acidic pH we found a multiphasic behavior of the CD signal recorded at 283 nm. Upon lowering the pH we detected first an increase of the CD amplitude, which is associated with the formation of a fusion-competent state of HA. The initial increase was followed by a continuous decline of CD amplitude, which can be ascribed to a transformation into a fusion-inactivated conformation that is in its early phase reversible as found for A/Japan. The half-time of the different phases of the CD signal depended on the virus strain, the temperature, and the acidic pH. The results support recent hypotheses that the fusion-competent conformation is an intermediate of the fusion-inactivated structure of HA.

Membrane fusion of influenza virus with phosphatidylcholine liposomes containing viral receptors

pH-dependent membrane fusion of influenza virus with liposomes made of phosphatidylcholine was studied by the spin-labelling method. Efficiency of viral fusion with liposomes composed of dimyristoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine was considerably lower compared to dioleoyl phosphatidyl choline or egg yolk phosphatidylcholine, suggesting importance of unsaturation of acyl chains of lipid bilayers. Reconstitution of specific viral receptors such as Glycophorin or sialylparagloboside strongly enhanced fusion with liposomes composed of dimyristoyl phosphatidylcholine. A direct comparison between the activities of the receptors showed that Glycophorin was about 50 times more effective than sialyparagloboside at the same receptor/phosphatidylcholine molar ratio.

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.

Low pH fusion of mouse liver nuclei with liposomes bearing covalently bound lysozyme

Lysozyme covalently bound to liposomes induces the fusion of liposomes with isolated mouse liver nuclei. The fusion behavior is very similar to the case of erythrocyte ghosts (Arvinte, T., Hildenbrand, K., Wahl, P. and Nicolau, C. (1986) Proc. Natl. Acad. Sci. USA 83, 962-966). Kinetic studies showed that membrane lipid mixing was completed within 15 min, as indicated from the resonance energy transfer (RET) measurements. For the resonance energy transfer kinetic measurements the liposomes contained L-alpha-dipalmitoylphosphatidylethanolamine (DPPE), labeled at the free amino group with the energy donor 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or with the energy acceptor tetramethylrhodamine. The lipid mixing at equilibrium was studied by the fluorescence recovery after photobleaching technique (FRAP). Liposomes (with/without lysozyme) containing Rh-labeled DPPE in their membranes were incubated with nuclei at 37 degrees C, pH 5.2, for 30 min. After washing of nuclei by three centrifugations, 60-70% of the initial amount of labeled DPPE was associated with the nuclei in the case of liposomes bearing lysozyme and only 7-10% in the case of liposomes without lysozyme. For the nuclei incubated with liposomes having lysozyme, about 70% of the total Rh-labeled lipids present in the nuclei diffused in the nuclear membrane(s) (lateral diffusion constant of D = (1.4 +/- 0.5) X 10(-9) cm2/s). By encapsulating fluorescein isothiocyanate-labeled dextran of 150 kDa molecular mass into the liposomes and using a microfluorimetric method, it was shown that after the fusion a part of the liposome contents is found in the nuclei interior. In this lysozyme-induced fusion process between liposomes and nuclei or erythrocyte ghosts, the binding of lysozyme to the glycoconjugates contained in the biomembranes at acidic pH seems to be the determining step which explains the high fusogenic property of the liposomes bearing lysozyme.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.

Lysozyme-induced fusion of liposomes with erythrocyte ghosts at acidic pH

Lysozyme that was covalently bound to the outer surface of sonicated vesicles induced fusion of the vesicles with human white erythrocyte ghosts. The kinetics of membrane mixing were evaluated by the resonance-energy-transfer method using L-alpha-dipalmitoyl phosphatidylethanolamine labeled at the free amino group with the energy donor 7-nitro-2,1,3-benzoxadiazol-4-yl or with the energy acceptor tetramethylrhodamine. The equilibrium state after fusion was characterized by using fluorescence photobleaching and recovery techniques. Rates and equilibrium percentages of fusion were maximal at the pH optimum of the enzyme, and rates were strongly reduced by the addition of N,N',N''-triacetylchitotriose, a competitive inhibitor of lysozyme. An apparent activation energy of 28 +/- kcal/mol was obtained for the lipid-mixing process. At 37 degrees C, the fusion half-time was 0.5 min. After 30 min at 37 degrees C, 40% of the labeled lipids initially present in the fusion mixture had a lateral diffusion constant, D, of 1.1 +/- 0.5 X 10(-9) cm2 X sec-1 in the ghost membrane. The strong induction of fusion at the lysozyme pH optimum was not observed in the absence of lysozyme or when free lysozyme was added to the solution. Bound lysozyme did not induce fusion of electrically neutral liposomes with each other. These observations indicate that it is the liposome-bound lysozyme that induces fusion between liposomes and erythrocyte ghosts.

Mobilization and aggregation of integral membrane proteins in erythrocytes induced by interaction with influenza virus at acidic pH

Effect of influenza virus on erythrocyte membranes was investigated by electron microscopy and fluorescence photobleaching recovery measurements. The virus induced mobilization of integral proteins in erythrocyte membrane at acidic pH, where it fused with the cell membrane to cause hemolysis and also cell fusions but not at neutral pH. At lower temperatures (e.g., 4 degrees C), the proteins aggregated in the membrane and, consequently, large protein-free lipid bilayer area was produced. At higher temperatures (e.g., 37 degrees C) the protein distribution became randomized. Spectrin meshwork underneath the erythrocyte membrane was also markedly modified by the virus at acidic pH. Diffuse fibril structure was converted into dense spots and the membrane area lacking the fibril structure was produced. Isolated hemagglutinin rosettes also caused mobilization and aggregation of the integral proteins at acidic pH but to smaller extent than that induced by virus. The membrane perturbation detected as the protein mobilization by the action of hemagglutinin was assigned to be the cause for envelope fusion.

Enhancement of influenza virus hemolysis by physical and serological treatments

The mode of hemolysis by influenza A virus was compared with that of Sendai virus. The WSN strain of influenza virus grown in either eggs or MDCK cells expressed hardly any hemolytic activity by itself. Treatment of the MDCK cell-grown WSN virus with sonication or freezing and thawing moderately enhanced the hemolytic activity, but the maximum level attainable was considerably lower than that of Sendai virus. A high level of hemolytic activity comparable to that of Sendai virus was obtained only after treatment of the virus with antibody and complement. An electron microscopic study revealed that non- or low-hemolytic WSN virions were not permeable to uranyl acetate stain in contrast with the hemolytic virions obtained after treatment with antibody and complement, indicating that the hemolytic virions had sustained some injury to their envelopes. These phenomena were comparable to those found with Sendai virus, showing that damage to the envelope is also responsible for the hemolysis of influenza virus. The influenza viruses, however, remained spherical after every treatment and the stain did not penetrate into the core of the virion. These observations suggest that the envelope of influenza virus is more rigid than that of Sendai virus but that the hemolytic process of influenza virus is nevertheless mediated through envelope-membrane fusion as in the case of Sendai virus.

Interaction of influenza virus haemagglutinin with cell membranes

Influenza virus causes susceptible cells to undergo haemagglutination, haemolysis and oxygen radical formation. Each activity is the result of an interaction between the haemagglutinin (HA) glycoprotein and the cell plasma membrane, and appears to involve three discrete functions of the HA glycoprotein.

The activity of membranes reconstituted from HVJ envelope proteins and lipids to induce hemolysis and fusion between liposomes and erythrocytes

A simple method for preparation of lipid-free envelope proteins (HN protein and F protein) of HVJ (Sendai virus) was developed. Reconstituted 'envelopes' were then prepared from envelope proteins and various lipids by the detergent dialysis method, and the activity to induce hemolysis and fusion between liposome and erythrocyte was studied. Lipid-free envelope protein aggregates could induce hemolysis and liposome-erythrocyte fusion. The activity was however greatly augmented by incorporation of envelope proteins into membrane of viral total lipids. Hemolytic and fusogenic activity was somewhat augmented by incorporation of envelope proteins into dipalmitoylphosphatidylcholine/cholesterol (1:1, molar ratio) and dimyristoylphosphatidylcholine/cholesterol (1:1), though the augmentation was lower than that observed with viral total lipids. When 'envelopes' were reconstituted with the proteins and viral total lipids supplemented with phosphatidylethanolamine, two kinds of 'envelopes' were prepared; one was permeable to Dextran (Mr 75000) and hemolytic, and the other was impermeable to Dextran and nonhemolytic. The latter acquired hemolytic activity after subjection to freezing and thawing, and its barrier function was lost concomitantly. The study suggests that envelope proteins (HN protein and F protein) could function without lipids but their activity was greatly influenced by not only the composition of additional lipids but also mode of arrangement of components on the reconstituted membranes.