The interaction of Sendai virus with negatively charged liposomes: virus-induced lysis of carboxyfluorescein-loaded small unilamellar vesicles

Enhanced immune responses induced by vaccine using Sendai virosomes as carrier

Sendai virosomes can deliver encapsulated contents into the cytoplasm directly in a virus fusion-dependent manner. In this paper, Sendai virosomes-formulated melanoma vaccine was constructed and its anti-tumor effects were investigated. The melanoma vaccine was prepared by encapsulating mixture antigen into the Sendai virosomes. The antigen, mixture proteins were extracted from B(16) melanoma cells. The cytotoxic T lymphocyte (CTL) response level was evaluated by (51)Cr release method, and the change of CD4(+) and CD8(+) expression as well as the concentration of IgG in serum of immunized mice was measured. The results showed that Sendai virosomes-formulated melanoma vaccine can effectively elicit not only systemic immune response but also strong CTL response. Sendai virosomes can be used as an effective vector for use in anti-tumor vaccine therapy.

Determination of the entrapped volume of liposomes: dilution method

A novel method was developed for the determination of the entrapped volume of liposomes. The obtained values of the entrapped volume by our "dilution method" agreed very well with those of the conventional "quenching method." The dilution method also offered the great advantages of simple procedure and high reproducibility. The principle and validity of our method are discussed.

Fusion of sendai virus with liposome depends on only F protein, but not HN protein

Sendai virus is able to fuse with liposomes even without virus receptors. To determine the roles of envelope protein, hemagglutinin-neuraminidase (HN) and fusion (F) protein, in Sendai virus-liposome fusion, we treated the virus with proteases and examined its fusion with liposomes and the conditions of HN and F protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blotting analysis showed that the virus treated with 150 units/ml of trypsin, which inactivated selectively hemolysis activity, maintained intact HN, F and partially digested F (32 kDa) protein, while virus treated with 15,000 units/ml of trypsin, which inactivated both hemolysis and neuraminidase activity, had only a 15-kDa digested HN protein and completely digested F protein. The former fused with liposomes, but the latter did not. In the virus treated with chymotrypsin, which lost both hemolysis and neuraminidase activity, F protein was intact, while HN protein was degraded to 15 kDa; in this case the virus fused with liposomes. As the virus with 15-kDa HN protein fused with liposomes and that with 20-kDa protein did not, HN protein does not appear to play any role in virus-liposome fusion. The virus that fused with liposomes had intact F protein. We conclude that Sendai virus-liposome fusion is strongly dependent on the presence of intact F protein, but not HN protein.

Effect of glycosaminoglycans and PEG on fusion of Sendai virus with phosphatidylserine vesicles

The fusion of Sendai virus with phosphatidylserine vesicles was monitored by a pyrene-phosphatidylcholine fluorescence assay. A strong influence of pH and ionic strength on the extent of fusion was observed. The negatively-charged polymers (dextran sulfate, heparin and chondroitin sulfate) inhibited the ability of the viruses to fuse with the liposomes. The extent of inhibition, for a given amount (w/v) of the polymers, was the greatest for dextran sulfate followed by heparin and chondroitin sulfate. The extent of inhibition depended on the pH and ionic strength of the solution; the lower the pH of the solution, the more effective the fusion inhibition by the polymers. The molecular weight of dextran sulfate (DS) influenced the inhibition effect, i.e., DS with higher molecular weight exhibited a stronger inhibition effect. The presence of sodium sulfate, even in excess concentration, had no inhibitory effect on fusion. On the other hand, PEG had an opposite effect on fusion compared to the negatively-charged polymers, and it decreased their inhibition effect when both were present in the same media. It is concluded that the inhibition of the fusion activity of Sendai virus results from the adherence of negatively-charged polymers to the virus surface preventing close contacts between the virus and liposome surface.

Liposomes as a model for the study of the mechanism of fish toxicity of sodium dodecyl sulfate in sea water

The mechanism underlying the shark repellency of SDS was studied by comparing it with the shark nonrepelling detergent, Triton X-100. The findings can be summarized as follows: (1) The effective concentration of SDS for termination of shark tonic immobility (an immediate and fast response) was close to its critical micellar concentration in sea water (70 microM). The fish lethal concentrations (LD50) were far below the CMC value for SDS, and at CMC level for Triton X-100. (2) In sea water SDS possesses a strong affinity for lipid membranes, expressed in a lipid sea water partition coefficient (Kp) of about 3000. (3) In liposomal systems examined by assays of turbidity, fluorescence resonance energy transfer and kinetics of carboxyfluorescein (CF) release, the pattern of SDS induced changes in the phospholipid bilayer suggests: (a) absence of vesicle-vesicle fusion; (b) occurrence of vesicle size increase, and (c) nonlytic gradual release of CF above and below its CMC values. In contrast, Triton X-100 above its CMC induces membrane solubilization. (4) Assays coupling CF release from liposomes to potassium diffusion potential induced by valinomycin indicate that SDS related CF release can also be attributed to a specific mechanism such as cation pore formation and not only to membrane solubilization. The hypothesis of pore formation by SDS is discussed.

Chemistry and biology of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-labeled lipids: fluorescent probes of biological and model membranes

Lipids that are covalently labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group are widely used as fluorescent analogues of native lipids in model and biological membranes to study a variety of processes. The fluorescent NBD group may be attached either to the polar or the apolar regions of a wide variety of lipid molecules. Synthetic routes for preparing the lipids, and spectroscopic and ionization properties of these probes are reviewed in this report. The orientation of various NBD-labeled lipids in membranes, as indicated by the location of the NBD group, is also discussed. The NBD group is uncharged at neutral pH in membranes, but loops up to the surface if attached to acyl chains of phospholipids. These lipids find applications in a variety of membrane-related studies which include membrane fusion, lipid motion and dynamics, organization of lipids and proteins in membranes, intracellular lipid transfer, and bilayer to hexagonal phase transition in liposomes. Use of NBD-labeled lipids as analogues of natural lipids is critically evaluated.

Phosphatidylserine vesicle lysis by Sendai virus at low pH is not due to virus-vesicle fusion

As a model of the fusion of Sendai virus with red cells, the interaction of the virus with phosphatidylserine (PS) vesicles at pH 5 was quantitated by the release of a trapped marker from target vesicles and by mixing of lipids of the virus and the vesicles. Release of the marker was measured as dequenching of calcein trapped at a self-quenched concentration and lipid mixing was measured as a decrease in energy transfer between fluorescent phospholipid analogs in the target membrane. At comparable virus:vesicle ratios both calcein release and lipid mixing were maximal at pH 5 and significantly reduced after trypsin, but not chymotrypsin, treatment. In contrast, these two effects differed in their PS dependence, time course, and temperature dependence, indicating that calcein release is not a consequence of the fusion of a permeable virus membrane with an impermeable target membrane. Vesicles composed of 25 to 100% PS released similar amounts of calcein, whereas fusion increased linearly as a function of PS content of the target vesicles. The half-time was 15 s for calcein release but 1.5 min for fusion. The temperature coefficient of fusion was at least three times greater than that of calcein release. These results indicate that calcein release at pH 5 may signify an interaction of the virus with PS target membranes which precedes but does not necessarily culminate in fusion, given too low a temperature or an inappropriate target membrane composition.

Interaction of the fluorescent probe N-(lissamine Rhodamine B sulfonyl)dipalmitoylphosphatidylethanolamine with phosphatidylcholine bilayers

The surface density of the fluorescent probe N-(lissamine Rhodamine B sulfonyl)dipalmitoylphosphatidylcholine is the same in the two lipid leaflets of phosphatidylcholine bilayers containing the probe. In the liquid-crystalline state, the probe molecules aggregate above a threshold amount, approximately 0.2 mol/mol phospholipids. Above this threshold value, the surface density of the free probe molecules is constant, and all probe molecules added are incorporated in the aggregated form. The aggregation of the probe increases by approximately 20% when the medium pH is lowered to 4. In the gel state, the probe aggregation is higher than that in the liquid-crystalline state, and the free probe molecules distribute unevenly in the bilayer surface. Even though the results obtained in our model system cannot be directly extrapolated to all model systems, we point out that care is to be taken in the use of the probe. In fact, only in membranes in the liquid-crystalline state in which the amount of probe molecules to phospholipid molecules is lower than 1:7 the fluorescence response of the probe is independent of the pH changes and of the molecular aggregation.

Active function of membrane receptors for enveloped viruses. I. Specific requirement for liposome-associated sialoglycolipids, but not sialoglycoproteins, to allow lysis of phospholipid vesicles by reconstituted Sendai viral envelopes

Phospholipid liposomes composed of phosphatidylcholine (PC) and cholesterol (chol), bearing the sialoglycoprotein glycophorin (GP), are able to effectively bind Sendai virus particles, but not to be lysed by them. Incorporation of gangliosides (gangl) into the above phospholipid vesicles (yielding liposomes composed of PC/chol/gangl/GP), although not increasing their ability to interact with Sendai virions, rendered them susceptible to the viral lytic activity. This was inferred from the ability of the virus to induce release of carboxyfluorescein (CF) upon interaction at 37 degrees C with liposomes composed of PC/chol/gangl/GP. Lysis of liposomes required the presence of the two viral envelope glycoproteins, namely the hemagglutinin/neuraminidase (HN) and the fusion (F) polypeptides, and was inhibited by phenylmethyl sulfonylfluoride (PMSF), dithiothreitol (DTT) and trypsin, showing that virus-induced lysis of PC/chol/gangl/GP liposomes reflects the fusogenic activity of the virus. Incubation of Sendai virus particles with liposomes containing the acidic phospholipid dicetylphosphate (DCP) but lacking sialic acid containing receptors, also resulted in release of the liposome content. Lysis of these liposomes was due to the activity of the viral HN glycoprotein, therefore not reflecting the natural viral fusogenic activity. Fluorescence dequenching studies, using fluorescently labeled reconstituted Sendai virus envelopes (RSVE), have shown that the viral envelopes are able to fuse with neutral, almost to the same extent, as with negatively charged liposomes. However, fusion with negatively charged liposomes, as opposed to fusion with neutral liposomes, was mediated by the viral HN glycoprotein and not by the viral fusion polypeptide.

Membrane vesicles containing the Sendai virus binding glycoprotein, but not the viral fusion protein, fuse with phosphatidylserine liposomes at low pH

Membrane vesicles containing the Sendai virus hemagglutinin/neuraminidase (HN) glycoprotein were able to induce carboxyfluorescein (CF) release from loaded phosphatidylserine (PS) but not loaded phosphatidylcholine (PC) liposomes. Similarly, fluorescence dequenching was observed only when HN vesicles, bearing self-quenched N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (N-NBD-PE), were incubated with PS but not PC liposomes. Thus, fusion between Sendai virus HN glycoprotein vesicles and the negatively charged PS liposomes is suggested. Induction of CF release and fluorescence dequenching were not observed when Pronase-treated HN vesicles were incubated with the PS liposomes. On the other hand, the fusogenic activity of the HN vesicles was not inhibited by treatment with dithiothreitol (DTT) or phenylmethanesulfonyl fluoride (PMSF), both of which are known to inhibit the Sendai virus fusogenic activity. Fusion was highly dependent on the pH of the medium, being maximal after an incubation of 60-90 s at pH 4.0. Electron microscopy studies showed that incubation at pH 4.0 of the HN vesicles with PS liposomes, both of which are of an average diameter of 150 nm, resulted in the formation of large unilamellar vesicles, the average diameter of which reached 450 nm. The relevance of these observations to the mechanism of liposome-membrane and virus-membrane fusion is discussed.

Fusion of Sendai virus with negatively charged liposomes as studied by pyrene-labelled phospholipid liposomes

Sendai virus particles fuse with negatively charged liposomes but not with vesicles made of zwitterionic phospholipids. The liposome-virus fusion process was studied by dilution of the concentration-dependent excimer-forming fluorophore 2-pyrenyldodecanoylphosphatidylcholine contained in the liposomes by the viral lipids. The data were analyzed in the framework of a mass action kinetic model. This provided analytical solutions for the final levels of probe dilution and numerical solutions for the kinetics of the overall fusion process, in terms of rate constants for the liposome-virus adhesion, deadhesion and fusion. This analysis led to the following conclusions: At neutral pH and 37 degrees C, only 15% of the virus particles can fuse with the phospholipid vesicles, although all the virions may aggregate with the liposomes. The rate constants for aggregation, fusion and deadhesion are of the orders of magnitude of 10(7) M-1 X s-1, 10(-3) s-1 and 10(-2), s-1, respectively. The fraction of active virus increases with temperature. At acidic pH, both the fraction of 'fusable' virus and the rate of fusion increase markedly. The optimal pH for fusion is 3-4, where most of the virus particles are active. At higher pH values, an increasing fraction of the virus particles become inactive, probably due to ionization of viral glycoproteins, whereas at pH values below 3.0 the fusion is markedly reduced, most likely due to protonation of the negatively charged vesicles. While only 15% of the virions fuse with the liposomes at pH 7.4 and 37 degrees C, all the liposomes lose their content (Amselem, S., Loyter, A. Lichtenberg, D. and Barenholz, Y. (1985) Biochim. Biophys. Acta 820, 1-10). We therefore propose that release of entrapped solutes is due to liposome-virus aggregation, and not to fusion. Both trypsinization and heat inactivation of the virus particles inhibit not only the fusion process but also the release of carboxyfluorescein. This demonstrates the obligatory role of viral membrane proteins in liposome-virus aggregation. Reconstituted vesicles made of the viral lipid and the hemagglutinin/neuraminidase (HN) glycoprotein fuse with negatively charged liposomes similar to the intact virions. This suggests that the fusion of virions with negatively charged vesicles, unlike the fusion of the virus with biological membranes, requires only the HN and not the fusion glycoprotein.