Target cell membrane sialic acid modulates both binding and fusion activity of influenza virus

The immunomodulatory effects of probiotics on respiratory viral infections: A hint for COVID-19 treatment?

Respiratory virus infections are among the most prevalent diseases in humans and contribute to morbidity and mortality in all age groups. Moreover, since they can evolve fast and cross the species barrier, some of these viruses, such as influenza A and coronaviruses, have sometimes caused epidemics or pandemics and were associated with more serious clinical diseases and even mortality. The recently identified Coronavirus Disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a Public Health Emergency of International concern and has been associated with rapidly progressive pneumonia. To ensure protection against emerging respiratory tract infections, the development of new strategies based on modulating the immune responses is essential. The use of probiotic components has substantially increased due to their effects on immune responses, in particular on those that occur in the upper/lower respiratory tract. Superinduction of inflammatory reaction, known as a cytokine storm, has been correlated directly with viral pneumonia and serious complications of respiratory infections. In this review, probiotics, as potential immunomodulatory agents, have been proposed to improve the host's response to respiratory viral infections. In addition, the effects of probiotics on different aspects of immune responses and their antiviral properties in both pre-clinical and clinical contexts have been described in detail.

Analysis of the Changes in Expression Levels of Sialic Acid on Influenza-Virus-Infected Cells Using Lectin-Tagged Polymeric Nanoparticles

Viral infections affect millions around the world, sometimes leading to severe consequences or even epidemics. Understanding the molecular dynamics during viral infections would provide crucial information for preventing or stopping the progress of infections. However, the current methods often involve the disruption of the infected cells or expensive and time-consuming procedures. In this study, fluorescent polymeric nanoparticles were fabricated and used as bioimaging nanoprobes that can monitor the progression of influenza viral infection through the changes in the expression levels of sialic acids expressed on the cell membrane. The nanoparticles were composed of a biocompatible monomer to prevent non-specific interactions, a hydrophobic monomer to form the core, a fluorescent monomer, and a protein-binding monomer to conjugate lectin, which binds sialic acids. It was shown that these lectin-tagged nanoparticles that specifically target sialic acids could track the changes in the expression levels of sialic acids caused by influenza viral infections in human lung epithelial cells. There was a sudden drop in the levels of sialic acid at the initial onset of virus infection (t = 0~1 h) and at approximately 4~5 h post-infection. The latter drop correlated with the production of viral proteins that was confirmed using traditional techniques. Thus, the accuracy, the rapidity and the efficacy of the nanoprobes were demonstrated. Such molecular bioimaging tools, which allow easy-handling and in situ monitoring, would be useful to directly observe and decipher the viral infection mechanisms.

Antigen-specific B-cell receptor sensitizes B cells to infection by influenza virus

Influenza A virus-specific B lymphocytes and the antibodies they produce protect against infection. However, the outcome of interactions between an influenza haemagglutinin-specific B cell via its receptor (BCR) and virus is unclear. Through somatic cell nuclear transfer we generated mice that harbour B cells with a BCR specific for the haemagglutinin of influenza A/WSN/33 virus (FluBI mice). Their B cells secrete an immunoglobulin gamma 2b that neutralizes infectious virus. Whereas B cells from FluBI and control mice bind equivalent amounts of virus through interaction of haemagglutinin with surface-disposed sialic acids, the A/WSN/33 virus infects only the haemagglutinin-specific B cells. Mere binding of virus is not sufficient for infection of B cells: this requires interactions of the BCR with haemagglutinin, causing both disruption of antibody secretion and FluBI B-cell death within 18 h. In mice infected with A/WSN/33, lung-resident FluBI B cells are infected by the virus, thus delaying the onset of protective antibody release into the lungs, whereas FluBI cells in the draining lymph node are not infected and proliferate. We propose that influenza targets and kills influenza-specific B cells in the lung, thus allowing the virus to gain purchase before the initiation of an effective adaptive response.

Effect of the β-propiolactone treatment on the adsorption and fusion of influenza A/Brisbane/59/2007 and A/New Caledonia/20/1999 virus H1N1 on a dimyristoylphosphatidylcholine/ganglioside GM3 mixed phospholipids monolayer at the air-water interface

The production protocol of many whole cell/virion vaccines involves an inactivation step with β-propiolactone (BPL). Despite the widespread use of BPL, its mechanism of action is poorly understood. Earlier work demonstrated that BPL alkylates nucleotide bases, but its interaction with proteins has not been studied in depth. In the present study we use ellipsometry to analyze the influence of BPL treatment of two H1N1 influenza strains, A/Brisbane/59/2007 and A/New Caledonia/20/1999, which are used for vaccine production on an industrial scale. Analyses were conducted using a mixed lipid monolayer containing ganglioside GM3, which functions as the viral receptor. Our results show that BPL treatment of both strains reduces viral affinity for the mixed monolayer and also diminishes the capacity of viral domains to self-assemble. In another series of experiments, the pH of the subphase was reduced from 7.4 to 5 to provoke the pH-induced conformational change of hemagglutinin, which occurs following endocytosis into the endosome. In the presence of the native virus the pH decrease caused a reduction in domain size, whereas lipid layer thickness and surface pressure were increased. These observations are consistent with a fusion of the viral membrane with the lipid monolayer. Importantly, this fusion was not observed with adsorbed inactivated virus, which indicates that BPL treatment inhibits the first step of virus-membrane fusion. Our data also indicate that BPL chemically modifies hemagglutinin, which mediates the interaction with GM3.

N-linked glycosylation facilitates sialic acid-independent attachment and entry of influenza A viruses into cells expressing DC-SIGN or L-SIGN

It is widely recognized that sialic acid (SA) can mediate attachment of influenza virus to the cell surface, and yet the specific receptors that mediate virus entry are not known. For many viruses, a definitive demonstration of receptor function has been achieved when nonpermissive cells are rendered susceptible to infection following transfection of the gene encoding a putative receptor. For influenza virus, such approaches have been confounded by the abundance of SA on mammalian cells so that it has been difficult to identify cell lines that are not susceptible to infection. We examined influenza virus infection of Lec2 Chinese hamster ovary (CHO) cells, a mutant cell line deficient in SA. Lec2 CHO cells were resistant to influenza virus infection, and stable cell lines expressing either DC-SIGN or L-SIGN were generated to assess the potential of each molecule to function as SA-independent receptors for influenza A viruses. Virus strain BJx109 (H3N2) bound to Lec2 CHO cells expressing DC-SIGN or L-SIGN in a Ca(2+)-dependent manner, and transfected cells were susceptible to virus infection. Treatment of Lec2-DC-SIGN and Lec2-L-SIGN cells with mannan, but not bacterial neuraminidase, blocked infection, a finding consistent with SA-independent virus attachment and entry. Moreover, virus strain PR8 (H1N1) bears low levels of mannose-rich glycans and was inefficient at infecting Lec2 CHO cells expressing either DC-SIGN or L-SIGN, whereas other glycosylated H1N1 subtype viruses could infect cells efficiently. Together, these data indicate that human C-type lectins (DC-SIGN and L-SIGN) can mediate attachment and entry of influenza viruses independently of cell surface SA.

Construction and characterization of insect cell-derived influenza VLP: cell binding, fusion, and EGFP incorporation

We have constructed virus-like particles (VLPs) harboring hemagglutinin (HA), neuraminidase (NA), matrix protein 1 (M1) ,and proton channel protein (M2) using baculovirus as a vector in the SF9 insect cell. The size of the expressed VLP was estimated to be ~100 nm by light scattering experiment and transmission electron microscopy. Recognition of HA on the VLP surface by the HA2-specific monoclonal antibody IIF4 at acidic pH, as probed by surface plasmon resonance, indicated the pH-induced structural rearrangement of HA. Uptake of the particle by A549 mediated by HA-sialylose receptor interaction was visualized by the fluorescent-labeled VLP. The HA-promoted cell-virus fusion activity was illustrated by fluorescence imaging on the Jurkat cells incubated with rhodamine-loaded VLP performed at fusogenic pH. Furthermore, the green fluorescence protein (GFP) was fused to NA to produce VLP with a pH-sensitive probe, expanding the use of VLP as an antigen carrier and a tool for viral tracking.

Macrophage receptors for influenza A virus: role of the macrophage galactose-type lectin and mannose receptor in viral entry

Although sialic acid has long been recognized as the primary receptor determinant for attachment of influenza virus to host cells, the specific receptor molecules that mediate viral entry are not known for any cell type. For the infection of murine macrophages by influenza virus, our earlier study indicated involvement of a C-type lectin, the macrophage mannose receptor (MMR), in this process. Here, we have used direct binding techniques to confirm and characterize the interaction of influenza virus with the MMR and to seek additional macrophage surface molecules that may have potential as receptors for viral entry. We identified the macrophage galactose-type lectin (MGL) as a second macrophage membrane C-type lectin that binds influenza virus and is known to be endocytic. Binding of influenza virus to MMR and MGL occurred independently of sialic acid through Ca(2+)-dependent recognition of viral glycans by the carbohydrate recognition domains of the two lectins; influenza virus also bound to the sialic acid on the MMR. Multivalent ligands of the MMR and MGL inhibited influenza virus infection of macrophages in a manner that correlated with expression of these receptors on different macrophage populations. Influenza virus strain A/PR/8/34, which is poorly glycosylated and infects macrophages poorly, was not recognized by the C-type lectin activity of either the MMR or the MGL. We conclude that lectin-mediated interactions of influenza virus with the MMR or the MGL are required for the endocytic uptake of the virus into macrophages, and these lectins can thus be considered secondary or coreceptors with sialic acid for infection of this cell type.

Immunological study of HA1 domain of hemagglutinin of influenza H5N1 virus

The neutralization titer of a hemagglutinin (HA)-specific neutralizing antibody against new isolates reflect both the antigenic drift and the conformation status of HA protein in these new influenza viruses. Since most antigenic sites are in the HA1 domain of HA, using HA1 domain of influenza virus as antigen is of great importance in vaccine development. In this study, we investigate different purification processes for optimizing the immunological properties of an Escherichia coli-expressed HA1 domain (rH5HA1) of influenza H5N1 virus. rH5HA1 was expressed as inclusion bodies and extracted with 6M guanidine hydrochloride (GnHCl)/PBS buffer. The best condition for generating HA1-specific neutralization determinants is on-column oxidative refolding procedures with GSH/GSSG and l-arginine buffer. Others refolding procedures such as using high-pH buffer and/or different detergent solubilizations were found to be ineffective producing neutralization epitope recognized by a HA1-specific neutralizing monoclonal antibody that was raised against H5N1 virus.

[Identification of a new carbohydrate-binding site of influenza virus]

It has recently been shown that the influenza virus can specifically bind the residue of a nonsialylated sulfated oligosaccharide Gal(6SO(3)H)beta1-4GlcNAcbeta (6'SLacNAc). To identify by photoaffinity labeling the virion component that binds 6'SLacNAc, we synthesized a carbohydrate probe containing a (125)I labeled diazocyclopentadien-2-yl carbonyl group as an aglycone. According to the electrophoretic data, the labeled areas corresponded to a large hemagglutinin subunit, a nucleocapsid protein, and neuraminidase (NA). Probing in the presence of an excess of 6'SLacNAcbeta-OCH(2)CH(2)NHAc glycoside resulted in redistribution of the labeling intensity, with the maximum inhibition being observed for NA. The data obtained indicate that NA is a viral 6'SLacNAc-binding protein.

A dual reporter gene based system to quantitate the cell fusion of avian influenza virus H5N1

Membrane fusion is central to the entry of influenza virus into host cells. To quantitatively determine the fusion activity of hemagglutinin (HA) of avian influenza virus H5N1, we established a cell fusion assay based on a dual luciferase reporter gene. The HA fusion activity was assayed by measuring luciferase expression in fused cells, allowing a rapid, sensitive, and quantitative comparison of HA fusion activities at various pHs and in different cells types. The simplicity and the quantitative nature of this novel assay are ideally suited for identifying viral receptors or screening for inhibitors of viral entry in the future.

Search for additional influenza virus to cell interactions

Sialyl oligosaccharides have long been considered to be the sole receptors for influenza virus. However, according to [1] some viruses are able to grow in sialic-free MDCK cells. Here we attempted to reveal a possible second, non-sialic receptor, hypothesizing the involvement of additional carbohydrate lectin recognition in influenza virus reception process, first of all in situations when a lectin of the host cell could recognize the viral carbohydrate ligand. We tested the presence of galactose- and sialic acid-binding lectins, as well as mannoside- and sulfo-N-acetyllactosamine-recognizing properties of MDCK and Vero cells using polyacrylamide neoglycoconjugates and antibodies. MDCK cells bind galactoside probes stronger than Vero cells, whereas Vero cells bind preferentially sialoside, mannoside and various sulfo-oligosaccharide probes. The probing of viruses with the neoglycoconjugates revealed specific 6'-HSO (3) LacNAc (but not other sulfated oligosaccharides) binding property of A and B human strains. Affinity of 6'-HSO (3) LacNAc probe was comparable with affinity of 6'-SiaLac probe but the binding was not inhibited by the sialooligosaccharide.

Liposome composition effects on lipid mixing between cells expressing influenza virus hemagglutinin and bound liposomes

The involvement of contacting and distal lipid monolayers in different stages of protein-mediated fusion was studied for fusion mediated by influenza virus hemagglutinin. Inclusion of non-bilayer lipids in the composition of the liposomes bound to hemagglutinin-expressing cells affects fusion triggered by low pH. Lysophosphatidylcholine added to the outer membrane monolayers inhibits fusion. The same lipid added to the inner monolayer of the liposomes promotes both lipid and content mixing. In contrast to the inverted cone-shaped lysophosphatidylcholine, lipids of the opposite effective shape, oleic acid or cardiolipin with calcium, present in the inner monolayers inhibit fusion. These results along with fusion inhibition by a bipolar lipid that does not support peeling of one monolayer of the liposomal membrane from the other substantiate the hypothesis that fusion proceeds through a local hemifusion intermediate. The transition from hemifusion to the opening of an expanding fusion pore allows content mixing and greatly facilitates lipid mixing between liposomes and cells.

Influenza virus entry and infection require host cell N-linked glycoprotein

A widely held view of influenza virus infection is that the viral receptor consists of cell surface carbohydrate sialic acid, which can be present as glycoprotein or glycolipid. Here, we examined influenza virus entry and infection in Lec1 cells, a mutant CHO cell line deficient in terminal N-linked glycosylation caused by a mutation in the N-acetylglucosaminyltransferase I (GnT1) gene. We show that influenza virus cannot infect Lec1 cells, despite having full capacity to undergo virus binding and fusion. Lec1 cells also show no virus replication defect, and infection was restored in Lec1 cells expressing wild-type GnT1. Viruses were apparently arrested at the level of internalization from the plasma membrane and were not endocytosed. Lec1 cells were refractory to infection by several strains of influenza virus, including H1 and H3 strains of influenza A, as well as influenza B virus. Finally, cleavage of N-glycans from wild-type CHO cells markedly reduced infection by influenza virus. We suggest that influenza virus specifically requires N-linked glycoprotein for entry into cells, and that sialic acid, although acting as an efficient attachment factor, is not sufficient as an influenza virus receptor in vivo.

Kinetics of influenza virus fusion with the endosomal and plasma membranes of cultured cells. Effect of temperature

We performed a detailed kinetic analysis of influenza virus fusion with the endosomal and plasma membranes of Madin Darby canine kidney (MDCK) cells and provided a comparison of the kinetic parameters obtained for both cases at 20 degrees C and 37 degrees C. Using our mass action kinetic model, we determined that the fusion rate constant, f, for influenza virus with the endosomal membrane was 0.02 s(-1) at 37 degrees C and 0.0035 s(-1) at 20 degrees C. The analysis of the fusion kinetics of influenza virus with the plasma membrane yielded that the fusion rate constants were close to those deduced with the endosomal membrane. The systematic kinetic analysis performed in this study provides for the first time a biophysical support for studies on influenza virus-cell fusion where the acidic endosomal internal environment is simulated artificially by lowering the pH of the medium.

Tight binding of influenza virus hemagglutinin to its receptor interferes with fusion pore dilation

Deletion of oligosaccharide side chains near the receptor binding site of influenza virus A/USSR/90/77 (H1N1) hemagglutinin (HA) enhanced the binding of HA to erythrocyte receptors, as was also observed with A/FPV/Rostock/34 (H7N1). Correlated with the enhancement of binding activity, the cell fusion activity of HA was reduced. A mutant HA in which three oligosaccharide side chains were deleted showed the highest level of binding and the lowest level of fusion among the HAs tested. The cell fusion activity of the oligosaccharide deletion mutant of HA, however, was drastically elevated when the binding activity was reduced by deletion of four amino acids adjacent to the receptor binding site. Thus, a reciprocal relationship was observed between the receptor binding and the cell fusion activities of H1/USSR HA. No difference was observed, however, in lipid mixing activity, so-called hemifusion, between wild-type (WT) and oligosaccharide deletion mutant HAs. Soluble dye transfer testing showed that even the HA with the lowest cell fusion activity was able to form fusion pores through which a small molecule such as calcein could pass. However, electron microscopic studies revealed that a large molecule such as hemoglobin hardly passed through the fusion pores formed by the mutant HA, whereas hemoglobin did efficiently pass through those formed by the WT HA. These results suggested that interference in the process of dilation of fusion pores occurs when the binding of HA to the receptor is too tight. Since the viral nucleocapsid is far larger than hemoglobin, appropriate receptor binding affinity is important for virus entry.

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

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

Stochastic simulation of hemagglutinin-mediated fusion pore formation

Studies on fusion between cell pairs have provided evidence that opening and subsequent dilation of a fusion pore are stochastic events. Therefore, adequate modeling of fusion pore formation requires a stochastic approach. Here we present stochastic simulations of hemagglutinin (HA)-mediated fusion pore formation between HA-expressing cells and erythrocytes based on numerical solutions of a master equation. The following elementary processes are taken into account: 1) lateral diffusion of HA-trimers and receptors, 2) aggregation of HA-trimers to immobilized clusters, 3) reversible formation of HA-receptor contacts, and 4) irreversible conversion of HA-receptor contacts into stable links between HA and the target membrane. The contact sites between fusing cells are modeled as superimposed square lattices. The model simulates well the statistical distribution of time delays measured for the various intermediates of fusion pore formation between cell-cell fusion complexes. In particular, these are the formation of small ion-permissive and subsequent lipid-permissive fusion pores detected experimentally (R. Blumenthal, D. P. Sarkar, S. Durell, D. E. Howard, and S. J., J. Cell Biol. 135:63-71). Moreover, by averaging the simulated individual stochastic time courses across a larger population of cell-cell-complexes the model also provides a reasonable description of kinetic measurements on lipid mixing in cell suspensions (T. Danieli, S. L. Pelletier, Y.I. Henis, and J. M. White, 1996, J. Cell Biol. 133:559-569).

Delay of influenza hemagglutinin refolding into a fusion-competent conformation by receptor binding: a hypothesis

Two subunits of influenza hemagglutinin (HA), HA1 and HA2, represent one of the best-characterized membrane fusion machines. While a low pH conformation of HA2 mediates the actual fusion, HA1 establishes a specific connection between the viral and cell membranes via binding to the sialic acid-containing receptors. Here we propose that HA1 may also be involved in modulating the kinetics of HA refolding. We hypothesized that binding of the HA1 subunit to its receptor restricts the major refolding of the low pH-activated HA to a fusion-competent conformation and, in the absence of fusion, to an HA-inactivated state. Dissociation of the HA1-receptor connection was considered to be a slow kinetic step. To verify this hypothesis, we first analyzed a simple kinetic scheme accounting for the stages of dissociation of the HA1/receptor bonds, inactivation and fusion, and formulated experimentally testable predictions. Second, we verified these predictions by measuring the extent of fusion between HA-expressing cells and red blood cells. Three experimental approaches based on 1) the temporal inhibition of fusion by lysophosphatidylcholine, 2) rapid dissociation of the HA1-receptor connections by neuraminidase treatment, and 3) substitution of membrane-anchored receptors by a water-soluble sialyllactose all provided support for the proposed role of the release of HA1-receptor connections. Possible biological implications of this stage in HA refolding and membrane fusion are being discussed.

Influenza virus infection of desialylated cells

Sialic acid has long been considered to be the sole receptor for influenza virus. The viral hemagglutinin (HA) is known to bind cell surface sialic acid, and sialic acids on viral glyco-proteins are cleaved by the viral neuraminidase (NA) to promote efficient release of progeny virus particles. However, NWS-Mvi, a mutant virus completely lacking NA, grows well in MDCK cells continuously treated with exogenous neuraminidase (sialidase). Exogenous sialidase quantitatively releases all sialic acids from purified glycoproteins and glycolipids of MDCK cells and efficiently removes surface sialic acid from intact cells. Binding of NWS-Mvi and parent influenza viruses to MDCK cells is indistinguishable, and is only partially reduced by sialidase treatment of the cells. Both mutant and wild-type viruses enter enzymatically desialylated cells and initiate transcription. The ability of influenza A reassortant viruses to infect desialylated cells is shared by recent H3N2 clinical isolates, suggesting that this may be a general property of influenza A viruses. We propose that influenza virus infection can result from sialic acid-independent receptors, either directly or in a multistage process. When sialic acid is present, it may act to enhance virus binding to the cell surface to increase interaction with secondary receptors to mediate entry. Understanding virus entry will be critical to further efforts in infection control and prevention.

Involvement of the mannose receptor in infection of macrophages by influenza virus

Influenza viruses A/PR/8/34 (PR8; H1N1), A/Aichi/68 X-31 (HKx31; H3N2), and A/Beijing/89 X-109 (BJx109; H3N2) show marked differences in their ability to infect murine macrophages, including resident alveolar and peritoneal macrophages as well as the macrophage-derived cell line J774. The hierarchy in infectivity of the viruses (PR8 < HKx31 < BJx109) resembles that of their reactivity with mannose-binding lectins of the collectin family. Since the macrophage mannose receptor recognizes the same spectrum of monosaccharides as the collectins do, we investigated the possible involvement of this receptor in infection of macrophages by influenza virus. In competitive binding studies, the binding of (125)I-labeled mannosylated bovine serum albumin to macrophages was inhibited by the purified hemagglutinin and neuraminidase (HANA) glycoproteins of influenza virus but not by HANA that had been treated with periodate to oxidize its oligosaccharide side chains. The inhibitory activity of HANA from the three strains of virus differed markedly and correlated with the infectivity of each virus for macrophages. Infection of macrophages, but not MDCK cells, by influenza virus was inhibited by yeast mannan. A variant line of J774 cells, J774E, which expresses elevated levels of the mannose receptor, was more readily infected than J774, and the sensitivity of J774E cells to infection was greatly reduced by culture in the presence of D-mannose, which down-modulated mannose receptor expression. Together, the data implicate the mannose receptor as a major endocytic receptor in the infectious entry of influenza virus, and perhaps other enveloped viruses, into murine macrophages.

Structures and mechanisms in flavivirus fusion

This chapter focuses on the work carried out with tick-borne encephalitis (TBE) virus, the structurally best characterized of the flaviviruses. The data is related to those obtained with other flaviviruses, which are assumed to have a conserved structural organization, and compare the characteristics of flavivirus fusion to those of other enveloped viruses. Fusion proteins from several different virus families, including Orthomyxoviridae , Paramyxoviridae , Retroviridae , and Filoviridae have been shown to exhibit striking structural similarities; they all use a common mechanism for inducing membrane fusion, and the same general model applies to all of these cases. The flavivirus genome is a positive-stranded RNA molecule consisting of a single, long open reading frame of more than 10,000 nucleotides flanked by noncoding regions at the 5′ and 3′ ends. The fusion properties of flaviviruses have been investigated using several different assay systems, including virus-induced cell–cell fusion and virus–liposome fusion. All of these studies indicate that flaviviruses require an acidic pH for fusion, consistent with their proposed mode of entry.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

Membrane destabilizing activity of influenza virus hemagglutinin-based synthetic peptide: implications of critical glycine residue in fusion peptide

Peptide III is a 20-residue synthetic model peptide based on the fusion peptide of influenza virus A/PR/8/34 strain and takes a secondary structure similar to the original peptide. While conserving the amphiphilic helical nature, 20 peptides to modify the bulkiness of side chains of peptide III were synthesized, and acid-induced membrane destabilization was assessed by aqueous content leakage from large unilamellar vesicles. Substitutions on the hydrophobic side decreased activity but showed less effect on the hydrophilic side, which confirmed the importance of the hydrophobic side for interaction with the membrane. Interestingly, substitution at the 13th Gly residue enhanced the amphiphilic helical nature but severely reduced activity. Correlation between alpha-helical content at acidic pH and the activity was not recognized, suggesting rather that the importance of this site was due to helix termination by glycine which allows N-terminal and C-terminal halves to behave as different secondary structural units.

Role of a Transbilayer pH Gradient in the Membrane Fusion Activity of the Influenza Virus Hemagglutinin: Use of the R18 Assay to Monitor Membrane Merging

It had been suggested that influenza virus-mediated membrane fusion might be dependent on a pH gradient across a target membrane. We have designed experiments in which this issue could be addressed. Two populations of liposomes were prepared, both simulating the plasma membrane of target cells, but with the pH of the internal aqueous medium buffered either at pH 7.4 (physiological cytosol pH) or at pH 5.0 (endosomal pH at which influenza virus displays maximal fusion activity). By monitoring fusion using the R18 assay, we found that the internal pH of the target liposomes did not influence membrane merging as mediated by the influenza virus hemagglutinin, thus demonstrating that a transmembrane pH gradient is not required in this fusion process.

Costimulatory activity of inactivated influenza virus in response of human peripheral blood mononuclear cells to phytohemagglutinin A and anti-CD3 antibody

We tested the effect of inactivated influenza A virus on the response of human T lymphocytes to several stimuli. Our results demonstrate that the response to both phytohemagglutinin A (PHA) and anti-CD3 was substantially enhanced by pre-exposure of peripheral blood mononuclear cells (PBMC) to heat-inactivated viral particles, whereas no effect was noted on the proliferative response of PBMC to IL-2. In addition to cell proliferation, exposure of PBMC to viral particles increased the secretion of interferon (IFN)-gamma during stimulation with anti-CD3 antibody and PHA. The costimulatory effect of influenza A virus was reduced by 40% in the presence of 0.1 mM sialic acid. The possible mechanisms involved in the costimulatory effects of influenza A virus are discussed.

Fusion activity of the influenza virus hemagglutinin does not require a transbilayer pH gradient

Following reports suggesting that membrane fusion mediated by the influenza virus hemagglutinin might be dependent on a pH gradient across a putative target membrane, we have designed experiments in which this issue could be addressed directly. Accordingly, we have prepared two populations of liposomes, both simulating the plasma membrane of target cells, but with the pH of the internal aqueous medium buffered either at pH 7.4 (physiological cytosol pH) or pH 5.0 (endosomal pH at which influenza virus displays maximal fusion activity). Monitoring fusion as the relief in self-quenching of the fluorescent probe octadecylrhodamine B chloride we have found that the internal pH of the target liposomes did not influence membrane merging as mediated by the influenza virus hemagglutinin, thus demonstrating that a transmembrane pH gradient is not required for the fusion process to take place.

MUC1 and other sialoglycoconjugates inhibit adenovirus-mediated gene transfer to epithelial cells

Recombinant adenoviruses are currently being evaluated as gene transfer vectors for the treatment of airway diseases. Recent evidence indicates that gene transfer to differentiated airway epithelial cells is inefficient. We hypothesized that apical membrane glycoconjugates, such as the transmembrane mucin MUC1, reduce the efficiency of adenovirus-mediated gene transfer. To address this, studies were performed in primary bronchial epithelial and Madin Darby canine kidney (MDCK) cells transduced to express human MUC1. Colocalization of MUC1 and an adenoviral lacZ transgene in the bronchial epithelial cells revealed that at several multiplicities of infection, the percentage of cells expressing lacZ was five-fold less in MUC1-expressing cells. Moreover, lacZ expression was three- to eight-fold lower in MUC1-expressing than in control MDCK cells, demonstrating that MUC1 interferes with gene transfer and is not merely a phenotypic marker of a cell that is refractory to adenovirus infection. Neuraminidase pretreatment of cells to remove sialic acid residues prior to viral adsorption increased the efficiency of gene transfer two- to five-fold in human airway and MDCK cells, and in a xenograft model of human airway. This effect was also observed in cultured cells that do not express MUC1, suggesting that other sialylated glycoconjugates impact on the efficiency of gene transfer. An inhibitory effect of negatively charged glycoconjugates on adenovirus binding was further supported by the finding that adsorption of adenovirus with a polycation significantly increased gene transfer efficiency. These data demonstrate for the first time that sialoglycoconjugates on epithelial cells reduce the efficiency of adenovirus-mediated gene transfer.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

Evidence that synaptobrevin is involved in fusion between synaptic vesicles and synaptic plasma membrane vesicles

We have developed a model system, consisting of rat brain synaptic vesicles and rat brain synaptic plasma membrane vesicles, to study the fusion process associated with the exocytotic release of neurotransmitters. Our results show a significant increase in the extent of fusion when the reaction takes place in cytosol compared to that obtained when fusion is carried out in buffer. This effect is mediated by cytosolic proteins, although N-ethylmaleimide-sensitive factor does not play a role in fusion. We also registered an almost complete inhibition of fusion when synaptic vesicles were pre-incubated with botulinum toxin B, indicating that synaptobrevin plays an important role in the coalescence of membrane lipids of the interacting membranes.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Fusion of reconstituted influenza virus envelopes with liposomes mediated by streptavidin/biotin interactions

Reconstituted influenza virus envelopes (virosomes) containing the viral hemagglutinin (HA) represent an efficient fusogenic cellular delivery system. By interaction of HA with its natural receptors, sialylated lipids (gangliosides) or proteins, virosomes bind to cells and, following endocytic uptake, deliver their contents to the cytosol through fusion from within acidic endosomes. Here, we show that binding to sialic acid is not necessary for fusion. In the presence of streptavidin, virosomes containing a biotinylated lipid fused with liposomes lacking sialic acid if these liposomes also had a biotinylated lipid in their membranes. Moreover, fusion characteristics corresponded well with fusion of virosomes with ganglioside-containing liposomes.