Requirement of energy for the cell fusion reaction of Ehrlich ascites tumor cells by HVJ

Mouse C2 myoblast cells resist HVJ (Sendai virus)-mediated cell fusion in the proliferating stage but become capable of fusion after differentiation

To investigate the mechanism of myoblast fusion, we attempted to prepare artificial myotubes of mouse C2 myoblast cells using the hemagglutinating virus of Japan (HVJ, Sendai virus). Proliferating C2 cells showed strong resistance to HVJ-mediated cell fusion and remained morphologically unchanged even though massive numbers of virions adsorbed onto their surface. They showed no membrane disruption, which occurs in the early stage of cell fusion induced by HVJ. These observations suggest that proliferating C2 cells are resistant to HVJ-mediated cell fusion. However, upon induction of differentiation, C2 cells gradually became capable of fusion induced by HVJ and then even generated heterokaryons with Ehrlich ascites tumor cells. When differentiated C2 cells that had become fusion-sensitive were treated with HVJ in the presence of EDTA, they did not fuse but degenerated, suggesting that their cell membranes were transiently disrupted by interaction with HVJ. These results suggest that the cell membranes of myoblasts change to a fusion-capable state during the process of differentiation.

Sendai virus-mediated lysis of liposomes requires cholesterol

Vesicles were constituted with glycophorin, the Sendai virus receptor of human erythrocytes, and loaded with calcein, a polar derivative of fluorescein, at self-quenching concentrations. On exposure to Sendai virus and mild hypo-osmotic stress, vesicles of the appropriate composition released a significant portion of their internal contents, as indicated by an increase in calcein fluorescence. Susceptible liposomes were not induced to leak by heat-inactivated virus or by trypsin-treated virus. The response of the vesicles to virus attachment is thus analogous to virus-induced hemolysis and presumably involves fusion of the vesicle and virus membranes. In addition to glycophorin and phosphatidylcholine, cholesterol was absolutely required for the lytic response to the virus. The need for cholesterol was not attributable to inactivation of the virus by liposomes without cholesterol. The presence of gangliosides increased the encapsulated volume of the liposomes, but gangliosides did not effectively substitute for glycophorin. Thin-layer chromatography of lipid extracted from incubated virus and liposomes containing a small amount of a fluorescent phosphatidylcholine indicated that phosphatidylcholine in the vesicle is not chemically altered by functional interaction with the virus.

Ageing and the fusion sensitivity potential of human cells in culture: relation to tissue origin, donor age, and in vitro culture level and condition

Cell fusion was induced by inactivated Sendai virus in different human diploid cell lines. These were derived from the kidney, lung or skin originating from embryos or adult donors and representing predominantly epithelioid cells (kidney) and fibroblastic cells (lung and skin). The fusion sensitivity (FS) potentials of these cell lines were determined and related to various aspects of cell ageing. In case of the fibroblastic lines, an inverse relation was demonstrated both to the culture age in vitro and to the donor age. The FS potential of embryonal fibroblasts decreased some 40-50% during the in vitro cultivation. In comparison to exponentially growing fibroblasts, the FS potentials were higher in cells in the stationary phase of growth. This was shown to correlate well with the fact that the life-span in calendar time also increased in cultures predominantly grown in the stationary phase. In the case of the kidney cells, the FS potentials were some 50% higher than those of fibroblasts. Since the cellular manifestation of the FS potential most likely primarily resides in the cell membrane-cytoskeleton structure, the results emphasize the importance of this system in relation to ageing.

Requirement of carbohydrates for cell fusion induced by vesicular stomatitis virus

The fusion of BHK-21-KB cells by vesicular stomatitis virus was not induced in Eagle's minimal essential medium without glucose. In medium containing glucose, the rate of polykaryocyte formation decreased as the concentration of glucose was reduced below 5 mM. However, no reduction in virus production 24 hr after infection was seen under this condition. Addition of pyruvate or mannose to the culture medium caused a reversal of cell fusing activity. Cell fusion and virus growth were significantly suppressed by sodium azide and 2,4-dinitrophenol.

The role of extracellular calcium ions in HVJ (Sendai virus)-induced cell fusion

The biochemical and biophysical roles of extracellular calcium ions in HVJ (Sendai virus)-induced cell fusion were studied. (1) Various kinds of cell, such as Ehrlich ascites tumor cells, mouse melanoma cells (B16-CW1 cells) and human epidermoid carcinoma cells (KB cells), could fuse in Ca2+-free medium containing a cheletor, glycoletherdiaminetetraacetic acid, in the same way as in Ca2+-containing medium. (2) The ATP content in Ehrlich ascites tumor cells decreased rapidly when the cells were treated with the virus in Ca2+-free medium but not in Ca2+-containing medium. (3) Intracellular adenine nucleotides leaked out into the reaction medium when the cells were treated with the virus in Ca2+-free medium but not in Ca2+-containing medium. (4) On addition of the virus, O2 consumption of Ehrlich ascites tumor cells decreased in Ca2+-free medium, but not in Ca2+-containing medium. (5) HVJ (Sendai virus) did not affect production of lactate by Ehrlich ascites tumor cells in both Ca2+-free medium and Ca2+-containing medium. These observations suggest that the role of extracellular Ca2+ in virus-induced cell fusion is to maintain the ATP and other intracellular metabolite contents at normal levels instead of triggering the fusion reaction itself.

Lipid vesicle-cell interactions. II. Induction of cell fusion

The ability of lipid vesicles of simple composition (lecithin, lysolecithin, and stearylamine) to induce cells of various types to fuse has been investigated. One in every three or four cells in monolayer cultures can be induced to fuse with a vesicle dose of about 100 per cell. At such dosages and for exposures of 15 min to 1 h, vesicles have essentially no effect on cell viability. Under anaerobic conditions, these cells lyse rather than fuse. Avian erythrocytes are readily fused with lipid vesicles in the presence of dextran. Fusion indices increase linearly with the zeta potential of the vesicles (increasing stearylamine content), indicating that contact between vesicle and cell membrane is required. Fusion indices increase sublinearly with increasing lysolecithin content. Divalent cations increase fusion indices at high vesicle doses. The data presented are consistent with the hypothesis that cell fusion occurs via simultaneous fusion of a vesicle with two adhering cell membranes.

Early events in cell-animal virus interactions

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Immunological methods for the agglutination of protoplasts from cell suspension cultures of different genera

Protoplasts from cell suspension cultures of Vicia hajastana Grossh., soybean (Glycine max L.) and brome grass (Bromus inermis Leyss.) were tightly agglutinated by immune sera prepared against them in rabbits. After incubation, the aggregated protoplasts became adpressed over a considerable area of their surface. Antibody prepared against Vicia protoplasts agglutinated both Vicia and soybean protoplasts alone, as well as a mixture of the two. Soybean and bromegrass antibody likewise cross-reacted with and agglutinated Vicia protoplasts. The heterologous reactions were nearly as strong as, and in some cases stronger than, the homologous. When sheep anti-rabbit globulin was reacted with a mixture of the protoplasts previously coated with homologous antibody, agglutination occurred much more quickly and the aggregates could not be dispersed without physical damage. Carbol-fuchsin staining of nuclei showed that Vicia and soybean protoplasts were randomly mixed in the aggregate. The protoplasts were viable and underwent division after the antibody treatment. The immune serum, which presumably contained complement, lysed the protoplasts unless it was heat-treated prior to use.

Macrophage-melanocyte heterokaryons. II. The activation of macrophage DNA synthesis. Studies with inhibitors of RNA synthesis

Mouse peritoneal macrophages, which do not synthesize DNA in vitro, were fused with melanocytes, a mouse cell strain which proliferates rapidly in vitro. DNA synthesis was induced in macrophage nuclei 2-3 hr after fusion and occurred irrespective of the number of macrophage nuclei present per melanocyte nucleus in each heterokaryon. 50-80% of macrophage nuclei initiated DNA synthesis in the 3-7 hr period after fusion. The activation of most 11-12-day chick red cell nuclei in melanocyte cytoplasm took longer than 10 hr. The lag before DNA synthesis may reflect the heterochromatin content of each nucleus. Studies with actinomycin showed that heterokaryon RNA synthesis was essential for subsequent macrophage DNA synthesis. This RNA was synthesized 1-4 hr before the DNA and was unlikely to be ribosomal RNA, since it was insensitive to <0.1 microg/ml actinomycin. Melanocytes and macrophages were treated before fusion with actinomycin and bromotubercidin to bring about a more selective inhibition of RNA synthesis. Macrophages pretreated for 1 hr with 5 microg/ml of actinomycin showed less than 20% of control RNA synthesis in the first 4 hr after fusion, but a normal activation of macrophage DNA synthesis. Pretreatment of melanocytes for 3-7 hr with 5 microg/ml bromotubercidin, a reversible inhibitor of RNA synthesis, prevented macrophage DNA synthesis without affecting macrophage RNA synthesis in the heterokaryons (81% of control). These studies showed that only melanocyte RNA synthesis was essential for the production of macrophage DNA. The exposure of one cell partner to actinomycin before fusion caused cross-toxicity of the untreated nucleus after fusion. Bromotubercidin, an adenosine analogue which is incorporated into RNA, did not give rise to such cross-toxicity after fusion. Once the macrophage nucleus becomes activated in the heterokaryon it becomes less sensitive to the action of actinomycin.

Controlled production of proliferating somatic cell hybrids

The techniques described permit the controlled production of large numbers of proliferating somatic cell hybrids in a relatively short period of time. Sendai virus is used to promote cell hybridization. beta-propriolactone is employed as the inactivating agent of Sendai virus since it produces complete loss of viral infectivity while preserving viral fusion capacity. Cells are fused in monolayer, instead of in suspension, since fixing cells in two dimensions permits one to control cell contacts during the fusion event through the expedient of varying multiplicities of the parental cells and the total cell density. Under the conditions described, a several hundred fold increase in the number of hybrid clones obtained is seen as compared to the controls.

A quantitative comparison of formation of spontaneous and virus-produced viable hybrids

Ultraviolet-inactivated Sendai virus used in conjunction with a selective system reproducibly gives high yields of viable hybrid cell lines. With two different crosses, the frequency of hybrid colonies obtained after virus treatment was found to be 100 times greater than the frequency of spontaneous hybrids, and this increase was found to show little variation between 33 and 1000 hemagglutinating units of virus. No differences have been found between the properties of hybrids obtained after Sendai treatment and those obtained from untreated mixed cultures of parental cells.

Interactions between Sendai virus and human erythrocytes

Concentrated Sendai virus, when adsorbed to erythrocytes at 4 C, caused invaginations in the plasma membrane. Following elevation of the temperature to 37 C, the plasma membrane became fused with the viral envelope before dissolution of the virions and rupture of the cells. Cell lysis was accompanied by rapid and total loss of hemoglobin to the extracellular space. Following aqueous pyridine extraction, the hemoglobin-free ghosts remaining were found to be devoid of N-acetylneuraminic acid and to have solubility properties different from those of normal erythrocyte ghosts. By the action of viral neuraminidase, bound N-acetylneuraminic acid was also liberated from purified virus receptor substance whose electrophoretic mobility was thereby substantially reduced. Cu(++) selectively inhibited hemolysis and neuraminidase without interfering with hemagglutination and attachment. Neuraminidase appeared to be essential for Sendai virus hemolysis; viral particle size may also be a critical factor in this process.

Structure and development of viruses as observed in the electron microscope. IX. Entry of parainfluenza I (Sendai) virus

After attachment, the uncoating of Sendai virus, which was accompanied by dissolution of the plasma membrane and fusion of virus to cell, proceeded quickly. Nucleoprotein filaments were found at stages of transit from virus to cytoplasm.