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

Lipid-dependence of target membrane stability during influenza viral fusion

Although influenza kills about a half million people each year, even after excluding pandemics, there is only one set of antiviral drugs: neuraminidase inhibitors. By using a new approach utilizing giant unilamellar vesicles and infectious X-31 influenza virus, and testing for the newly identified pore intermediate of membrane fusion, we observed ∼30-87% poration, depending upon lipid composition. Testing the hypothesis that spontaneous curvature (SC) of the lipid monolayer controls membrane poration, our Poisson model and Boltzmann energetic considerations suggest a transition from a leaky to a non-leaky fusion pathway depending on the SC of the target membrane. When the target membrane SC is below approximately -0.20 nm^-1 fusion between influenza virus and target membrane is predominantly non-leaky while above that fusion is predominantly leaky, suggesting that influenza hemagglutinin (HA)-catalyzed topological conversion of target membranes during fusion is associated with a loss of membrane integrity.

Interaction of fluorescent phospholipids with cyclodextrins

Fluorescent analogs of phospholipids are often employed to investigate the structure and dynamics of lipids in membranes. Some of those studies have used cyclodextrins e.g., to modulate the lipid phase. However, the role of the fluorescence moiety of analogs for the interaction between cyclodextrins and fluorescent lipids has not been investigated so far in detail. Therefore, in the present study the interaction of various fluorescent phospholipid analogs with methylated α-, β- and γ- cyclodextrins was investigated. The analogs differed in their structure, in the length of the fatty acyl chain, in the position of the fluorescence group, and in the attached fluorescence moiety (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or dipyrrometheneboron difluoride (BODIPY)). In aqueous buffer, cyclodextrins bind fluorescent lipids disturbing the organization of the analogs. When incorporated into lipid vesicles, analogs are selectively extracted from the membrane upon addition of cyclodextrins. The results show that the interaction of cyclodextrins with fluorescent phospholipids depends on the cyclodextrin species, the fluorescence moiety and the phospholipid structure. The presented data should be of interest for studies using fluorescent phospholipids and cyclodextrins, since the interaction between the fluorescence group and the cyclodextrin may interfere with the process(es) under study.

pH-Controlled two-step uncoating of influenza virus

Upon endocytosis in its cellular host, influenza A virus transits via early to late endosomes. To efficiently release its genome, the composite viral shell must undergo significant structural rearrangement, but the exact sequence of events leading to viral uncoating remains largely speculative. In addition, no change in viral structure has ever been identified at the level of early endosomes, raising a question about their role. We performed AFM indentation on single viruses in conjunction with cellular assays under conditions that mimicked gradual acidification from early to late endosomes. We found that the release of the influenza genome requires sequential exposure to the pH of both early and late endosomes, with each step corresponding to changes in the virus mechanical response. Step 1 (pH 7.5-6) involves a modification of both hemagglutinin and the viral lumen and is reversible, whereas Step 2 (pH <6.0) involves M1 dissociation and major hemagglutinin conformational changes and is irreversible. Bypassing the early-endosomal pH step or blocking the envelope proton channel M2 precludes proper genome release and efficient infection, illustrating the importance of viral lumen acidification during the early endosomal residence for influenza virus infection.

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.

Novel lipid transfer property of two mitochondrial proteins that bridge the inner and outer membranes

This study provides evidence of a novel function for mitochondrial creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D). Both are basic peripheral membrane proteins with symmetrical homo-oligomeric structure, which in the case of MtCK was already shown to allow crossbridging of lipid bilayers. Here, different lipid dilution assays clearly demonstrate that both kinases also facilitate lipid transfer from one bilayer to another. Lipid transfer occurs between liposomes mimicking the lipid composition of mitochondrial contact sites, containing 30 mol % cardiolipin, but transfer does not occur when cardiolipin is replaced by phosphatidylglycerol. Ubiquitous MtCK, but not NDPK-D, shows some specificity in the nature of the lipids transferred and it is not active with phosphatidylcholine alone. MtCK can undergo reversible oligomerization between dimeric and octameric forms, but only the octamer can bridge membranes and promote lipid transfer. Cytochrome c, another basic mitochondrial protein known to bind to anionic membranes but not crosslinking them, is also incapable of promoting lipid transfer. The lipid transfer process does not involve vesicle fusion or loss of the internal contents of the liposomes.

The compound DATEM inhibits respiratory syncytial virus fusion activity with epithelial cells

The effect of diacetyltartaric acid esters of mono and diglycerides (DATEM) on fusion of respiratory syncytial virus (RSV) with HEp-2 cells was studied using the R18 fluorescence dequenching fusion assay. At DATEM concentrations less than 2.0 microg/ml, the inhibition of fusion increased with the concentration of DATEM. At 2 microg/ml of DATEM, the fusion was suppressed by 80-90%. Studies examining possible mechanism of fusion-inhibition indicated that DATEM was likely adsorbed onto lipid membranes of both viral envelope and target cell membranes. Quantitative measurements of DATEM adsorption onto membranes were also performed using lipid monolayers and vesicles. The surface pressure of lipid monolayer formed at the air/aqueous interface increased as the concentration of DATEM in the monolayer subphase increased, suggesting that DATEM was inserted into the monolayer. As the concentration of DATEM in vesicle suspensions increased, electrophoretic mobility of initially uncharged lipid vesicles also increased, reflective of increased negative charge at vesicle surfaces. These results strongly suggest that the insertion of DATEM onto membranes inhibited viral fusion. DATEM may prove to be effective in limiting the infectivity of RSV by interference with the fusion of the viral envelope with target cell membranes.

Kinetics of influenza hemagglutinin-mediated membrane fusion as a function of technique

Reliable techniques are required to evaluate the plausibility of proposed membrane fusion mechanisms. Here we have studied the kinetics of establishing the lipidic connection between hemagglutinin-expressing cells (HA-cells) and red blood cells (RBC) labeled with octadecylrhodamine, R18, using three different experimental approaches: (1) the most common approach of monitoring the rate of the R18 dequenching in a cuvette with a suspension of RBC/HA-cell complexes; (2) video fluorescence microscopy (VFM) to detect the waiting times before the onset of R18 redistribution, not dequenching, for each RBC attached to an adherent HA-cell; and (3) a new approach based on blockage of RBC fusion to an adherent HA-cell at different time points by lysophosphatidylcholine (LPC), so that only the cell pairs which, at the time of LPC application, had fused or were irreversibly committed to fusion contributed to the final extent of lipid mixing. The LPC blockage and VFM gave very similar estimates for the fusion kinetics, with LPC monitoring also those sites committed to the lipid mixing process. In contrast, R18 dequenching in the cuvette was much slower, i.e., it monitors a much later stage of dye redistribution.

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.

Ca(2+) and Mg(2+)/ATP independently trigger homotypic membrane fusion in gastric secretory membranes

Exocytic activation of gastric parietal cells represents a massive transformation. We studied a step in this process, homotypic fusion of H,K-ATPase-containing tubulovesicles, using R18 dequenching. Ca(2+) and Mg(2+)/ATP each caused dramatic dequenching, reflecting a change in R18 distribution from 5% to 65-90% of the assay's membranes in 2.5 min. These stimuli also triggered fusion between tubulovesicles and liposomes. Independent confirmation that dequenching represented membrane fusion was established by separating tubulovesicle-liposome fusion products on density gradients. Only agents that trigger fusion allowed the transmembrane H,K-ATPase to move to low-density fractions along with R18. EC(50) for Ca(2+)-triggered fusion was 150 nm and for Mg(2+)/ATP-triggered fusion 1 mm, the latter having a Hill coefficient of 2.5. ATP-triggered fusion was specific for Mg(2+)/ATP, required ATP hydrolysis, and was insensitive to inhibition of NSF and/or H,K-ATPase. Fusion initiated by either trigger caused tubulovesicles to become resistant to subsequent challenge by either trigger. Ca(2+) and Mg(2+)/ATP-triggered fusion required protein component(s) in tubulovesicles, though this was required in only one of the fusing membranes since tubulovesicles fused well with liposomes containing no proteins. Our data suggest that exocytosis in parietal cells is triggered by separate but interacting pathways and is regulated by self-inhibition.

Infection of human airway epithelia with H1N1, H2N2, and H3N2 influenza A virus strains

Three subtypes of influenza A virus cause human disease: H1N1, H2N2, and H3N2. Although all result in respiratory illness, little is known about how these subtypes infect differentiated airway epithelia. Therefore, we assayed A/PR/8/34 (H1N1), A/Japan/305/57 (H2N2), and X31 (H3N2) influenza virus strains for binding and infection on fully differentiated primary cultures of airway epithelia isolated from human bronchus, grown on semiporous filters at an air-liquid interface. In this model system, viral infectivity was highest when virus was applied to the apical versus the basolateral surface; Japan was most infectious, followed by PR8. The X31 strain showed very low levels of infectivity. Confocal microscopy and fluorescence-resonance energy transfer studies indicated that Japan virus could enter and fuse with cellular membranes, while infection with X31 virions was greatly inhibited. Japan virus could also productively infect human trachea explant tissues. These data show that influenza viruses with SAalpha2,3Gal binding specificity, like Japan, productively infect differentiated human airway epithelia from the apical surface. These data are important to consider in the development of pseudotyped recombinant viral vectors for gene transfer to human airway epithelia for gene therapy.

SNAREs in mammalian sperm: possible implications for fertilization

Soluble N-ethylmalameide-sensitive factor attachment protein receptor (SNARE) proteins are present in mammalian sperm and could be involved in critical membrane fusion events during fertilization, namely the acrosome reaction. Vesicle-associated membrane protein/synaptobrevin, a SNARE on the membrane of a vesicular carrier, and syntaxin 1, a SNARE on the target membrane, as well as the calcium sensor synaptotagmin I, are present in the acrosome of mammalian sperm (human, rhesus monkey, bull, hamster, mouse). Sperm SNAREs are sloughed off during the acrosome reaction, paralleling the release of sperm membrane vesicles and acrosomal contents, and SNARE antibodies inhibit both the acrosome reaction and fertilization, without inhibiting sperm-egg binding. In addition, sperm SNAREs may be responsible, together with other sperm components, for the asynchronous male DNA decondensation that occurs following intracytoplasmic sperm injection, an assisted reproduction technique that bypasses normal sperm-egg surface interactions. The results suggest the participation of sperm SNAREs during membrane fusion events at fertilization in mammals.

Conformational intermediates and fusion activity of influenza virus hemagglutinin

Three strains of influenza virus (H1, H2, and H3) exhibited similar characteristics in the ability of their hemagglutinin (HA) to induce membrane fusion, but the HAs differed in their susceptibility to inactivation. The extent of inactivation depended on the pH of preincubation and was lowest for A/Japan (H2 subtype), in agreement with previous studies (A. Puri, F. Booy, R. W. Doms, J. M. White, and R. Blumenthal, J. Virol. 64:3824-3832, 1990). While significant inactivation of X31 (H3 subtype) was observed at 37 degrees C at pH values corresponding to the maximum of fusion (about pH 5.0), no inactivation was seen at preincubation pH values 0.2 to 0.4 pH units higher. Surprisingly, low-pH preincubation under those conditions enhanced the fusion rates and extents of A/Japan as well as those of X31. For A/PR 8/34 (H1 subtype), neither a shift of the pH (to >5.0) nor a decrease of the temperature to 20 degrees C was sufficient to prevent inactivation. We provide evidence that the activated HA is a conformational intermediate distinct from the native structure and from the final structure associated with the conformational change of HA, which is implicated by the high-resolution structure of the soluble trimeric fragment TBHA2 (P. A. Bullough, F. M. Hughson, J. J. Skehel, and D. C. Wiley, Nature 371:37-43, 1994).

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.

pH-induced destabilization of lipid bilayers by a lipopeptide derived from influenza hemagglutinin

A synthetic twenty-one amino acid peptide (AcE4K) based on the amino acid sequence of the influenza HA2 fusion peptide was coupled to a distearoylglycerol lipid anchor by amidation of an N-terminal lysine side chain. The secondary structure of Lipo-AcE4K incorporated into POPC (1-palmitoyl-2-oleoyl-sn-phosphatidylcholine) liposomes was not measurably affected by pH, but increased membrane penetration was indicated by tryptophan fluorescence. At outer monolayer concentrations up to 10 mol%, Lipo-AcE4K formed stable liposomes with POPC and EPC/Chol (egg phosphatidylcholine/cholesterol) (55:45) at pH 7.5. Acid-induced destabilization and fusion of these vesicles were demonstrated by fluorescent lipid mixing and contents leakage assays, and by freeze-fracture electron microscopy. Membrane destabilization increased with increasing lipopeptide concentrations, decreasing pH, inclusion of cholesterol, and incorporation of lipopeptide into the inner monolayer as well as the outer monolayer of the liposomes. Fusion of liposomes bearing Lipo-AcE4K with erythrocyte ghosts was demonstrated by lipid mixing and fluorescence microscopy.

Voltage-dependent translocation of R18 and DiI across lipid bilayers leads to fluorescence changes

We show that the lipophilic, cationic fluorescent dyes R18 and Dil translocate from one monolayer of a phospholipid bilayer membrane to the other in a concentration and voltage-dependent manner. When the probes were incorporated into voltage-clamped planar membranes and potentials were applied, displacement currents resulted. The charged probes sensed a large fraction of the applied field. When these probes were added to only one monolayer, displacement currents were symmetrical around 0 mV, indicating that the probes distributed equally between the two monolayers. Charge translocation required that the bilayer be fluid. When membranes were in a condensed gel phase, displacement currents were not observed; raising the temperature to above the gel-liquid crystalline transition restored the currents. Translocation of R18 was also shown by fluorescence measurements. When R18 was in the bilayer at high, self-quenching concentrations, voltage pulses led to voltage-dependent fluorescence changes. The kinetics of the fluorescence changes and charge translocations correlated. Adding the quencher I- to one aqueous phase caused fluorescence to decrease or increase when voltage moved R18 toward or away from the quencher at low, nonquenching concentrations of R18. In contrast to R18, Dil incorporated into bilayers was a carrier fo I-, and hence I- altered Dil currents. Voltage-driven translocations allow R18 and Dil to be used to probe membrane potential changes.

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.