Role of target membrane structure in fusion with influenza virus: effect of modulating erythrocyte transbilayer phospholipid distribution

Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling

Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.

Conformational change of influenza virus hemagglutinin is sensitive to ionic concentration

The homotrimeric spike glycoprotein hemagglutinin (HA) of influenza virus undergoes a low pH-mediated conformational change which mediates the fusion of the viral envelope with the target membrane. Previous approaches predict that the interplay of electrostatic interactions between and within HA subunits, HA 1 and HA2, are essential for the metastability of the HA ectodomain. Here, we show that suspension media of low ionic concentration promote fusion of fluorescent labelled influenza virus X31 with erythrocyte ghosts and with ganglioside containing liposomes. By measuring the low pH mediated inactivation of the fusion competence of HA and the Proteinase K sensitivity of low pH incubated HA we show that the conformational change is promoted by low ionic concentration. We surmise that electrostatic attraction within the HA ectodomain is weakened by lowering the ionic concentration facilitating the conformational change at low pH.

Modulation of entry of enveloped viruses by cholesterol and sphingolipids (Review)

Enveloped animal viruses infect host cells by fusion of viral and target membranes. This crucial fusion event occurs either with the plasma membrane of the host cells at the physiological pH or with the endosomal membranes at low pH and is triggered by specific glycoproteins in the virus envelope. Both lipids and proteins play critical and co-operative roles in the fusion process. Interactions of viral proteins with their receptors direct which membranes fuse and viral fusion proteins then drive the process. These fusion proteins operate on lipid assemblies, whose physical and mechanical properties are equally important to the proper functioning of the process. Lipids contribute to the viral fusion process by virtue of their distinct chemical structure, composition and/or their preferred partitioning into specific microdomains in the plasma membrane called 'rafts'. An involvement of lipid rafts in viral entry and membrane fusion has been examined recently. However, the mechanism(s) by which lipids as dynamic raft components control viral envelope-glycoprotein-triggered fusion is not clear. This paper will review literature findings on the contribution of the two raft-associated lipids, cholesterol and sphingolipids in viral entry.

Dilation of the influenza hemagglutinin fusion pore revealed by the kinetics of individual cell-cell fusion events

We have monitored kinetics of fusion between cell pairs consisting of a single influenza hemaglutinin (HA)-expressing cell and a single erythrocyte (RBC) that had been labeled with both a fluorescent lipid (Dil) in the membrane and a fluorescent solute (calcein) in the aqueous space. Initial fusion pore opening between the RBC and HA-expressing cell produced a change in RBC membrane potential (delta psi) that was monitored by a decrease in Dil fluorescence. This event was followed by two distinct stages of fusion pore dilation: the flux of fluorescent lipid (phi L) and the flux of a large aqueous fluorescent dye (phi s). We have analyzed the kinetics of events that occur as a result of transitions between a fusion pore (FP) and a solute permissive fusion pore (FPs). Our data are consistent with a fusion pore comprising six HA trimers.

Non-bilayer lipids and biological fusion intermediates

Disparate biological fusion reactions and fusion of purely lipid bilayers are similarly influenced by 'non-bilayer' lipids (lipids which do not form lipid bilayers in water by themselves). Lipid composition of membranes affects biological fusion at a stage downstream of activation of fusion proteins and prior to fusion pore formation. These data suggest that actual merger of membrane lipid bilayers in different fusion reactions proceeds via the same pathway. The effects of non-bilayer lipids specifically correlate with their ability to bend lipid monolayers in different directions, and appear to be consistent with the specific hypothesis of membrane fusion suggesting that fusion proceeds through highly bent intermediates--stalks, local connections between contacting monolayers of fusing membranes.

Analysis of delay times of hemagglutinin-mediated fusion between influenza virus and cell membranes

We have studied the kinetics of low pH-induced fusion between influenza virus A/PR 8/34 and human erythrocyte membranes in suspension by using an assay based on fluorescence dequenching (FDQ) of the lipophilic dye octadecylrhodamine B chloride (R 18). As shown previously (Clague et al. 1991) the onset of FDQ is preceded by a characteristic lag time (tlag) following pH reduction. Whereas tlag represents only a subpopulation of fusing viruses with the shortest delay time we suggest here that a representative mean lag time mu lag of virus-cell fusion can be deduced from the R 18-assay. Kinetics of FDQ reflects the cumulative distribution function of lag times tau lag of single fusion events with the mean value mu lag. We show that tau lag obtained from the onset of FDQ does not always reflect the fusion behaviour of the whole population of fusing viruses. While both lag times, taulag and mu lag, exhibit a similar temperature dependence we found a significantly different dependence of both delay times on virus inactivation by low pH-pretreatment. We conclude that the mean lag time mu lag appears to be a more appropriate parameter describing the kinetics of virus-cell fusion. The analysis of delay times offers a new approach to test the validity of different kinetic models of HA-mediated fusion and to gain valuable information about HA-mediated fusion. The analysis confirms that the inactivation process proceeds via steps of the formation of the fusion pore. Although the increase of lag times can be explained by a depletion of fusion competent HA's, our data suggest that intermediate structures of HA along the inactivation pathway can still transform into a fusion site.

Influence of the spectrin network on fusion of influenza virus with red blood cells

We examined the influence of the physical state of the membrane skeleton on low pH fusion of influenza virus A/PR 8/34 with intact human red blood cells. Spectrin, the major component of the skeleton, is known to become denaturated at 50 degrees C. After heat treatment of erythrocytes at 50 degrees C we observed an enhanced kinetics of fusion monitored spectrofluorometrically by the octadecylrhodamine fluorescence dequenching assay, while the extent of fusion was not affected. The accelerated fusion of influenza virus after preincubation of red blood cells at 50 degrees C is not mediated by alterations of the lipid phase of the target. From ESR measurements using spin-labelled phospholipids we conclude that heat-induced alterations of the spectrin network did not affect either the phospholipid asymmetry or the fluidity of the exoplasmic and the cytoplasmic leaflets of the erythrocyte membrane. Moreover, as deduced from our previous investigations, the swelling behaviour of red blood cells could not be responsible for the observed effect. Possible mechanisms for the spectrin effect include a change in the ability of the target membrane to bend locally, and a change in the rate of formation and development of the fusion pore.

Protein-dependent translocation of aminophospholipids and asymmetric transbilayer distribution of phospholipids in the plasma membrane of ram sperm cells

We have investigated the transbilayer movement of phospholipids in the plasma membrane of ram sperm cells using spin- and fluorescence-labeled lipid analogues. After incorporation into the outer leaflet, phosphatidylcholine (PC) and sphingomyelin (SM) moved slowly to the inner cytoplasmic leaflet, whereas phosphatidylserine (PS) and phosphatidylethanolamine (PE) rapidly disappeared from the exoplasmic monolayer. Variation of the initial velocity of the relocation kinetics vs the amount of analogue incorporated into the membrane suggests a saturability of the transbilayer movement of aminophospholipids. ATP depletion or pretreatment with N-ethylmaleimide of ram sperm cells reduced the fast inward motion of PS and PE, indicating a protein-mediated aminophospholipid translocation. The results suggest for the plasma membrane of ram sperm cells the presence of an aminophospholipid translocase and an asymmetric transversal lipid distribution with aminophospholipids preferentially located in the inner leaflet and choline-containing phospholipids in the outer leaflet. The relevance of the transversal segregation of phospholipids for membrane fusion processes occurring during fertilization is discussed.

pH-dependent binding of the fluorophore bis-ANS to influenza virus reflects the conformational change of hemagglutinin

Binding of the fluorophore 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) to influenza virus A/PR 8/34 is strongly enhanced at low pH. Binding is accompanied by a significant increase in fluorescence intensity. The binding and the fluorescence increase are associated with the low-pH induced conformational change of the viral spike protein, hemagglutinin, exposing hydrophobic binding sites. The data indicate that in addition to the hydrophobic N-terminus of HA2 other hydrophobic sequences of the HA ectodomain become accessible to bis-ANS at low pH. It is shown that the time course of the fluorescence increase of bis-ANS at low pH is determined by the conformational change of HA. The application of this assay for continuously monitoring the kinetics of the structural alteration in HA is discussed and its relevance for elucidating the temporal relationship between the conformational change of HA and virus-membrane fusion is outlined.

On the validity of lipid dequenching assays for estimating virus fusion kinetics

Octadecylrhodamine (R18) has often been used to measure membrane fusion of enveloped viruses by fluorescence dequenching. In order to see whether non-specific R18 exchange between non-fused membranes occurs we have measured fusion of influenza virus with erythrocyte membranes by utilizing dequenching of the non-exchangeable lipid analogue N-(lissamine-rhodamine B-sulfonyl)diacylphosphatidylethanolamine (N-Rh-PE). Rather low concentration of N-Rh-PE (< 0.1 mol%) were required to assess fusion since self-quenching in the influenza virus membrane was more efficient in comparison to R18. For both markers we observed the same kinetics as well as the same extent of fluorescence dequenching upon triggering low pH-induced fusion. Non-specific marker transfer was not observed. Haemolysis was not affected by either type of fluorophore. Our results confirm that R18 is a valuable tool to investigate membrane fusion of enveloped viruses in a quantitative manner. Differences in the efficiency of self-quenching of both markers are discussed.

Enhancement of viral fusion by nonadsorbing polymers

Nonadsorbing polymers such as dextran and poly(ethylene glycol) enhance binding as well as extents of fusion of influenza virus with erythrocytes. Kinetics and extent of viral membrane fusion were measured using an assay based on lipid mixing of a fluorescent dye. The effects of nonadsorbing polymers were in the concentration range from 0 to 10 wt%, far below the concentration required to overcome hydration repulsion forces. The enhancing effects were dependent on the molecular weight of nonadsorbing polymer, and only occurred at molecular weight > 1500; this links the phenomena we observe to the so-called "excluded volume effect" of nonadsorbing polymers. The time delay between triggering and the onset of influenza virus fusion was significantly reduced in the presence of nonadsorbing polymers. High molecular weight poly(ethylene glycol) also induced fusion of vesicular stomatitis virus with intact erythrocytes, which do not serve as target of vesicular stomatitis virus fusion in the absence of the polymer. The forces between membranes which determine rate-limiting processes in viral fusion and how they are affected by nonadsorbing polymers are discussed.