Chapter 10 Sendai Virus-Mediated Cell Fusion

Optimization study of plasmonic cell fusion

Artificial cell fusion often serves as a valuable tool for studying different applications in biology and medicine, including natural development, immune response, cancer metastasis and production of therapeutic molecules. Plasmonic cell fusion, a technique that uses specific cell labeling by gold nanoparticles and resonant femtosecond pulse irradiation for fusing neighboring cells, has been demonstrated useful for such applications, allowing high cell specificity and an overall low toxicity. Despite these advantages, the numerous experimental factors contributing to plasmonic fusion have often led to subpar fusion efficiencies, requiring repeated experiments and extensive calibration protocols for achieving optimal results. In this work we present a study that aims to improve the overall performance of plasmonic cell fusion in terms of fusion efficiency and cell viability. By varying the pulse fluence, nanoparticle concentration, incubation times, and culture handling protocols, we demonstrate up to 100% fusion of malignant epithelial cells across the entire irradiated area of the culture. We also show that some of the smaller cells may stay viable for up to several days. The results would allow plasmonic fusion to play a key role in numerous studies and applications that require specific, high-efficiency cell-cell fusion.

Microfluidic Surgery in Single Cells and Multicellular Systems

Microscale surgery on single cells and small organisms has enabled major advances in fundamental biology and in engineering biological systems. Examples of applications range from wound healing and regeneration studies to the generation of hybridoma to produce monoclonal antibodies. Even today, these surgical operations are often performed manually, but they are labor intensive and lack reproducibility. Microfluidics has emerged as a powerful technology to control and manipulate cells and multicellular systems at the micro- and nanoscale with high precision. Here, we review the physical and chemical mechanisms of microscale surgery and the corresponding design principles, applications, and implementations in microfluidic systems. We consider four types of surgical operations: (1) sectioning, which splits a biological entity into multiple parts, (2) ablation, which destroys part of an entity, (3) biopsy, which extracts materials from within a living cell, and (4) fusion, which joins multiple entities into one. For each type of surgery, we summarize the motivating applications and the microfluidic devices developed. Throughout this review, we highlight existing challenges and opportunities. We hope that this review will inspire scientists and engineers to continue to explore and improve microfluidic surgical methods.

Reciprocal regulation of AKT and MAP kinase dictates virus-host cell fusion

Viruses of the Paramyxoviridae family bind to their host cells by using hemagglutinin-neuraminidase (HN), which enhances fusion protein (F)-mediated membrane fusion. Although respiratory syncytial virus and parainfluenza virus 5 of this family are suggested to trigger host cell signaling during infection, the virus-induced intracellular signals dictating virus-cell fusion await elucidation. Using an F- or HN-F-containing reconstituted envelope of Sendai virus, another paramyxovirus, we revealed the role and regulation of AKT1 and Raf/MEK/ERK cascades during viral fusion with liver cells. Our observation that extracellular signal-regulated kinase (ERK) activation promotes viral fusion via ezrin-mediated cytoskeletal rearrangements, whereas AKT1 attenuates fusion by promoting phosphorylation of F protein, indicates a counteractive regulation of viral fusion by reciprocal activation of AKT1 and mitogen-activated protein kinase (MAPK) cascades, establishing a novel conceptual framework for a therapeutic strategy.

Novel Mucosal Insulin Delivery Systems Based on Fusogenic Liposomes

PURPOSE

Fusogenic liposomes (FLs) are unique delivery vehicles capable of introducing their contents directly into the cytoplasm with the aid of envelope glycoproteins of Sendai virus (SeV). The objective of this study was to evaluate the potential of FL to improve the mucosal absorption of insulin from rat intestinal membranes.

METHOD

The FLs containing insulin were prepared by fusing insulin-loaded liposomes with inactivated SeV particles and were administered directly into the ileal, the colonic, and the rectal loops (10 IU/kg).

RESULTS

The FL successfully enhanced the insulin absorption and induced a significant hypoglycemic effect following the colonic and the rectal administration without detectable mucosal damage. This enhancing effect of insulin absorption was further improved by increasing the amount of insulin loaded in the FL and by coencapsulating insulin-degrading enzyme inhibitor. In contrast, the insulin absorption was not increased by the ileal administration of FL because the mucous/glycocalyx layers overlaid on the ileal epithelium impede the fusion of FL to the intestinal mucosa.

CONCLUSION

Our results indicated that FL is a useful carrier for improving the absorption of poorly absorbable drugs, such as insulin, via the intestinal tract.

Histidylated lipid-modified Sendai viral envelopes mediate enhanced membrane fusion and potentiate targeted gene delivery

Recent studies have demonstrated that covalent grafting of a single histidine residue into a twin-chain aliphatic hydrocarbon compound enhances its endosome-disrupting properties and thereby generates an excellent DNA transfection system. Significant increase in gene delivery efficiencies has thus been obtained by using endosome-disrupting multiple histidine functionalities in the molecular architecture of various cationic polymers. To take advantage of this unique feature, we have incorporated L-histidine (N,N-di-n-hexadecylamine) ethylamide (L(H)) in the membrane of hepatocyte-specific Sendai virosomes containing only the fusion protein (F-virosomes (Process for Producing a Targeted Gene (Sarkar, D. P., Ramani, K., Bora, R. S., Kumar, M., and Tyagi, S. K. (November 4, 1997) U. S. Patent 5,683,866))). Such L(H)-modified virosomal envelopes were four times more (p < 0.001) active in terms of fusion with its target cell membrane. On the other hand, the presence of L(H) in reconstituted influenza and vesicular stomatitis virus envelopes failed to enhance spike glycoprotein-induced membrane fusion with host cell membrane. Circular dichroism and limited proteolysis experiments with F-virosomes indicated that the presence of L(H) leads to conformational changes in the F protein. The molecular mechanism associated with the increased membrane fusion induced by L(H) has been addressed in the light of fusion-competent conformational change in F protein. Such enhancement of fusion resulted in a highly efficient gene delivery system specific for liver cells in culture and in whole animals.

Enhanced cytotoxicity of doxorubicin encapsulated in liposomes with reconstituted Sendai F-proteins

Sendai F-virosomes, a novel type of liposome with reconstituted Sendai F-proteins, have been tested as a delivery system for various bioactive materials. However, encapsulation limitations and difficulties in controlling their constituents were drawbacks for further application to therapeutic purposes. We have tried to control virosomal constituents and have enhanced drug encapsulation efficiency into the virosomes. In vitro cytotoxicity of doxorubicin encapsulated in the F-virosomes were compared with free doxorubicin and doxorubicin in conventional liposomes. The F-virosomes were spontaneously prepared by detergent dialysis, a reconstitution process of Sendai F-proteins into liposomes. The reconstitution density of F-proteins affected the vesicle size of virosomes prepared by detergent dialysis; the larger amount of F-proteins made a smaller size of virosomes. There was little variation of size with time at physiological conditions, whilst the vesicle size of virosomes increased at acidic storage conditions (pH 5.5). Doxorubicin encapsulated in the F-virosomes exhibited a lower IC50 against B16BL6 mouse melanoma cells and Chang human hepatocarcinoma cells than that in conventional liposomes. The F-virosomes also exhibited higher cellular uptake than conventional liposomes. Addition of dioleoylphophatidylethanolamine, a fusogenic phospholipid, into the F-virosome further increased the cellular uptake as well as in vitro cytotoxicity. These types of virosome formulations can be clinically applicable as versatile vesicles for the efficient delivery of various therapeutic drugs, including genetic materials.

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.

Target-cell specificity of fusogenic liposomes: membrane fusion-mediated macromolecule delivery into human blood mononuclear cells

Fusogenic liposome, a unique vector prepared by fusing ultraviolet-inactivated Sendai virus and liposome, is known to efficiently deliver content into various animal cells through membrane fusion. In this study, we examined the target-cell specificity of fusogenic liposome (FL)-mediated macromolecule delivery into human blood cells using diphtheria toxin fragment A (DTA) as a probe. Among the peripheral blood mononuclear cells (PBMC), FL was able to deliver its encapsulates into CD14+ monocytes and CD4-/CD8- T-cells, but not into CD19+ B-lymphocytes, CD4+ T-cells or CD8+ T-cells. The susceptibility of human leukemia cell lines to FL was similar to that of PBMC; the order of the reactivity was U937 (monoblastic leukemia)>MOLT4, Jurkat (T-lymphoma)>Daudi, BALL1 (B-lymphoma)>K562 (erythroblastic leukemia). Interestingly, FL showed similar binding activity to all of these leukemia cell lines. These findings indicate that, among blood cells, monocytes, monoblastic leukemia cells, CD4-/CD8- T-cells and T-lymphoma cells are preferable targets for FL-mediated macromolecule delivery. This is the first demonstration of the existence of non-permissive cells against FL. Our results also suggest that some molecules on target-cells other than the binding targets of SV-derived protein may participate in fusion between FL and cells.

Effect of substitution of hemagglutinin-neuraminidase with influenza hemagglutinin on Sendai virus F protein mediated membrane fusion

Recombinant virosomes containing fusion protein (F) of Sendai virus and the envelope glycoproteins hemagglutinin (HA) and neuraminidase (NA) of influenza virus within the same membrane were prepared. Such hybrid vesicles were found to hemolyse red blood cells both at pH 7.4 and pH 5.0. Hemolysis induced by hybrid vesicles was much higher than seen with F-virosomes in the presence of WGA, but was about two-fold less than the hemolysis caused by F,HN-virosomes. Reconstituted influenza virus envelopes and F-virosomes failed to induce hemolysis at pH 7.4. Using a fluorescence probe-based lipid mixing fusion assay, hybrid virosomes were found to fuse with cultured HeLa cells both at pH 7.4 as well as pH 5.0. The data indicate that the presence of Sendai virus HN protein in the virosomal membrane is not absolutely essential for the virosome cell fusion process.

Reconstituted Sendai virus envelopes as biological carriers: dual role of F protein in binding and fusion with liver cells

We have assessed the potential of reconstituted Sendai viral envelopes containing only the fusion protein (F-virosomes) as biological carriers for the delivery of drugs and macromolecules. [125I]lysozyme entrapped in F-virosome is used to study its distribution in various organs of Balb/c mouse in vivo as a function of dose and time. F-virosomes injected intravenously are rapidly cleared from circulation. A major percentage (55-60%) of vesicle contents is delivered to liver at 15 min after injection, showing thereby the liver to be the major site for the accumulation of vesicles. Uptake of virosomes by liver is found to reach a near saturation level at a dose of 0.5 mg F-protein associated with virosomes. In competition studies, the inhibitory effect of asialofetuin on the uptake of F-virosomes suggests the involvement of asialoglycoprotein receptor in its recognition by hepatic parenchymal cells. Incorporation of asialoganglioside-GM1 in the F-virosomes enhanced the uptake by about 1.6-fold. The observed specific interaction of hepatic receptor with F-protein containing a terminal galactose moiety is further supported by degalactosylation of F-virosomes with hard-shelled clam exoglycosidase. The uptake of degalactosylated F-virosomes by liver is found to be significantly reduced. The subcellular radioactivity profile in liver cells exhibits a considerable decrease in cytosolic localisation of the degalactosylated F-virosomal contents with a concomitant increase in their accumulation in lysosomal/mitochondrial fraction as compared to the untreated virosomes. Trypsinized and heat-treated F-virosomes also reflect similar subcellular distribution profile as that of degalactosylated virosomes. Moreover, F-virosomes are able to interact and deliver [125I]lysozyme to the HepG2 cells in culture in the presence of a potent inhibitor of endocytotic process. These results indicate the involvement of specific binding of F-proteins with hepatic receptors followed by their fusion with the membrane of liver cells in the delivery of [125I]lysozyme. The findings reported here open up the possibility of using F-virosomes with defined specificity as fusogenic vehicles for efficient delivery of drugs and biologically active macromolecules both in vivo and in vitro.

Calcium ions are required for cell fusion mediated by the CD4-human immunodeficiency virus type 1 envelope glycoprotein interaction

Calcium ions are required for fusion of a wide variety of artificial and biological membranes. To examine the role of calcium ions for cell fusion mediated by interactions between CD4 and the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41), we used two experimental systems: (i) cells expressing gp120-gp41 and its receptor CD4, both encoded by recombinant vaccinia viruses, and (ii) chronically infected cells producing low levels of HIV-1. Fusion was measured by counting the number of syncytia and by monitoring the redistribution of fluorescence dyes by video microscopy. Syncytia did not form in solutions without calcium ions. Addition of calcium ions partially restored the formation of syncytia. EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] blocked syncytium formation in culture media containing calcium ions. Membrane fusion as monitored by fluorescence dye redistribution also required calcium ions. Cell fusion increased with an increase in calcium ion concentration from 100 microM to 10 mM but was not affected by magnesium ions in the concentration range from 0 to 30 mM. Fibrinogen and fibronectin did not promote fusion in the absence or presence of Ca2+. Binding of soluble CD4 to gp120-gp41-expressing cells was not affected by Ca2+ and Mg2+. We conclude that Ca2+ is involved in postbinding steps in cell fusion mediated by the CD4-HIV-1 envelope glycoprotein interaction.

Membrane destabilization by N-terminal peptides of viral envelope proteins

The fusion of lipid enveloped viruses with cellular membranes is thought to be mediated by the insertion into the target membrane of the N-terminal polypeptides of viral spike glycoproteins. Since membrane destabilization is a necessary step in membrane fusion, we investigated whether synthetic peptides with amino acid sequences corresponding to the N-termini of influenza virus hemagglutinin (HA2), vesicular stomatitis virus G-protein and Sendai virus F-protein, induce the destabilization and fusion of phospholipid vesicles. Membrane destabilization by the peptides was monitored by the release of aqueous contents of large unilamellar phospholipid vesicles. Aggregation was detected by a resonance energy transfer assay. Membrane fusion was followed by means of assays for the intermixing of phospholipids and of aqueous contents. The 17-amino acid HA2 peptide (HA2.17) destabilized phosphatidylcholine (PC) vesicles even at neutral pH, but the rate and extent of destabilization increased at lower pH. This peptide did not mediate appreciable release of contents from phosphatidylserine (PS) vesicles. HA2.17 induced neither aggregation nor fusion of PC or PS vesicles. In contrast, the 7-amino acid N-terminal peptide of G-protein (G.7) destabilized PS-containing membranes and not pure PC vesicles. Although G.7 caused aggregation of and lipid mixing between PS vesicles, it did not mediate any detectable intermixing of aqueous contents. The presence of cholesterol in PC membranes did not affect the destabilization caused by the N-terminal peptide of Sendai virus F-protein (F1.7), suggesting that cholesterol is not necessary for the effective interaction of this peptide with membranes, contrary to earlier proposals. Our results support the hypothesis that the hydrophobic N-terminal region of certain viral envelope proteins insert into and destabilize target membranes.

Significance of basolateral domain of polarized MDCK cells for Sendai virus-induced cell fusion

Fusion (fusion from within) of polarized MDCK monolayer cells grown on porous membranes was examined after infection with Sendai viruses. Wild-type virus, that buds at the apical membrane domain, did not induce cell fusion even when the F glycoprotein expressed at the apical domain was activated with trypsin. On the other hand, a protease activation mutant, F1-R, with F protein in the activated form and that buds bipolarly at the apical and basolateral domains, caused syncytia formation in the absence of exogenous protease. Anti-Sendai virus antibodies added to the basolateral side, but not at the apical side, inhibited cell fusion induced by F1-R. In addition, T-9, a mutant with bipolar budding phenotype of F1-R but with an uncleavable F protein phenotype like wild-type virus, induced cell fusion exclusively when trypsin was added to the basolateral medium. By electron microscopy, cell-to-cell fusion was shown to occur at the lateral domain of the plasma membrane. These results indicate that in addition to proteolytic activation of the F protein, basolateral expression of Sendai virus envelope glycoproteins is required to induce cell fusion.

Cell-to-cell spread of HIV-1 occurs within minutes and may not involve the participation of virus particles

Although virus infections have been classically studied with "cell-free" virion preparations, many animal viruses are able to spread both in vitro and in vivo by inducing cell-cell fusion. An efficient system to monitor the cell-to-cell spread of HIV-1 has been developed employing chronically infected H9 donor cells. Under appropriate conditions of cocultivation with uninfected cells, the synthesis of unintegrated viral DNA, monitored by Southern blot hybridization, occurred between 2 and 4 hr following infection; viral proteins were detected 8 to 12 hr following cocultivation and progeny virions were released into the medium by 16 hr. The use of metabolic inhibitors or specific envelope/receptor antibodies revealed that the cell-to-cell spread of HIV required: (1) gp120-CD4 interaction and (2) reverse transcription. Light and electron microscopy, fluorescent dye redistribution, and soluble CD4 competition experiments all demonstrated that the HIV-induced cell-cell fusion began within 10 to 30 min of cocultivation. Surprisingly, the electron microscopic analyses also suggested that budding or mature virus particles did not participate in this process. Thus the virus-induced cell-cell fusion observed is very likely the result of gp120/gp41 proteins, on the surface of infected cells, interacting with CD4 molecules on uninfected cells. These findings are of immediate importance in understanding the mechanism(s) of HIV-1 transmission in vivo and for the design of effective vaccines and antiviral agents.

Single cell fusion events induced by influenza hemagglutinin: studies with rapid-flow, quantitative fluorescence microscopy

Fusion of individual human erythrocytes to fibroblasts expressing the influenza virus hemagglutinin Cells were attached to coverslips fitted in a specially designed flow chamber mounted on a microscope stage, and fusion was triggered by rapid acidification to pH less than 5.2. Fusion between single cell pairs was monitored by a fluorescence increase due to redistribution of fluorescent dyes between either membrane or cytoplasmic compartments of fusing cells. The single cell fusion events were broadly heterogenous in lag times, rise times, and overall shape of the curves. Lag times obtained with a water-soluble dye were within the range obtained with a water-soluble dye were within the range obtained with the membrane-bound fluorophores, (10-160 s). Fusion was both all-or-nothing and irreversible, in that once dye redistribution in any cell commenced, it completed, regardless of pH. Short pulses of pH 4.9 for 6-10 s led to about half of the cell pairs fusing, but pulses greater than 14 s were as effective as constant low pH. Pulses that were too short to trigger fusion did not partially activate nor deactivate the fusion process, as shown by the ability of a second acidification to cause fusion of the same cells, with similar lag times. These results indicate that the overall hemagglutinin-mediated fusion process is composed of at least two stages, one required for commitment of the hemagglutinin to a fusogenic state that is pH-dependent and a maturation stage that is pH-independent.

Monoclonal antibodies to the peplomer glycoprotein of coronavirus mouse hepatitis virus identify two subunits and detect a conformational change in the subunit released under mild alkaline conditions

Monoclonal antibodies (MAbs) directed against the E2 glycoprotein of mouse hepatitis virus (MHV) have been classified according to their ability to bind to either of the two purified 90,000-molecular-weight subunits (90K subunits) of the 180K peplomeric glycoprotein E2. Correlation with previously reported information about these MAbs suggest that both of the subunits of E2 are important for viral infectivity and cell fusion. Incubation of trypsin-treated virions at pH 8.0 and 37 degrees C released only the E2N subunit from virions. The pattern of MAb reactions suggested that a conformational change occurred in the E2N subunit in association with its release from virions under mildly alkaline conditions at 37 degrees C, the same conditions which are optimal for coronavirus-induced cell fusion.