Membrane changes during viral infection

Changes in membrane permeability during Semliki Forest virus induced cell fusion

The infection of Aedes albopictus cells by Semliki Forest virus (SFV) is a non lytic event. Exposure of infected cells to mildly acidic pH (less than 6.2) leads to syncytium formation. This polykaryon formation is accompanied by an influx of protons into the cells (Kempf et al. Biosci. Rep. 7, 761-769, 1987). We have further investigated this permeability change using various fluorescent or radiolabeled compounds. A significant, pH dependent increase of the membrane permeability to low molecular weight compounds (M(r) less than 1000) was observed when infected cells were exposed to a pH less than 6.2. The pH dependence of the permeability change was very similar to the pH dependence of cell-cell fusion. The permeability change was sensitive to divalent cations, protons and anionic antiviral drugs such as trypan blue. The nature of this virus induced, pH dependent permeability change is discussed.

Infection of cultured human airway epithelial cells by influenza A virus

The lack of an adequate in vitro model has hampered study of the cellular basis by which influenza A virus causes disease in the human airway. We report in vitro infection of human airway epithelial cells by influenza A virus. Fetal and adult human tracheal and bronchial epithelial cells cultured from explants and SV40 transformed adult human tracheal epithelial cells were exposed to a recently isolated strain of influenza A virus (H1N1) and a laboratory passaged strain (WSN) of influenza A virus at similar multiplicity of infection. All cultures derived from explants showed hemadsorption (approximately 30% of the cells) with the H1N1 virus. No hemadsorption was detected with the WSN virus. One of two transformed cell lines showed a 5-10% hemadsorption to cells after H1N1 exposure and none following exposure to WSN. Immunofluorescent staining for influenza A-specific antigens in virus-exposed, explant-derived cells indicated viral infection and replication in these cells. Hemagglutinating material in the growth medium of infected, explant-derived cell lines, increased as a function of time, indicating the production of virus proteins. Exposure of rhesus monkey kidney cells and new human tracheal epithelial cultures to supernatant from these cells resulted in hemadsorption, indicating the presence of infectious virus in the supernatant. Light microscopic examination of virally infected bronchial epithelial cells demonstrated that the common types of cytopathic changes were rarely seen while cell proliferation continued over time. The data indicate that influenza A virus can infect, replicate, and produce infectious virus in cultured human tracheal and bronchial epithelial cells.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Parainfluenza virus infection of cultured airway epithelial cells

Studies of the cellular effects of respiratory viruses have generally used cultures of non-airway (particularly renal) epithelial cells. This requires the assumption that, despite the marked differences between renal epithelium and airway epithelium, the virus-host cell interactions in cultures of renal epithelium will be relevant to those in airway epithelium. To study viral infection of airway epithelial cells, we removed the epithelial cells from ferret tracheas using 0.1% pronase solution, and plated them at a density of 5 X 10(5) cells/cm2 in collagen-coated plastic tissue culture wells. Cultures grew to confluence after 5-7 days. Viral inocula, consisting of supernatants from parainfluenza type 1-infected rhesus monkey kidney cell monolayers, were added to the culture medium in a concentration 10(3) times that sufficient to produce infection in 50% of rhesus monkey kidney monolayers (TCID50). Cytopathic changes, consisting of cellular elongation and detachment, became apparent after 3-6 days, at which time the medium contained 5 X 10(8) TCID50/ml. The monolayer appeared to be uniformly infected as revealed by adsorption of guinea pig erythrocytes. Specific immunofluorescence revealed uniformly positive staining for parainfluenza type 1 antigens. The ability to infect pure cultures of airway epithelial cells with viruses will allow us to examine the effects of these viruses on epithelial cell function, and to study virus-host cell interactions in cell cultures derived from the natural host cell.

Initial stages of influenza hemagglutinin-induced cell fusion monitored simultaneously by two fluorescent events: cytoplasmic continuity and lipid mixing

We have monitored the mixing of both aqueous intracellular and membrane-bound fluorescent dyes during the fusion of human red blood cells to influenza hemagglutinin-expressing fibroblasts using fluorescence spectroscopy and low light, image-enhanced video microscopy. The water-soluble fluorescent dye, N-(7-nitrobenzofurazan-4-yl)taurine, was incorporated into intact human red blood cells. The fluorescence of the dye in the intact red blood cell was partially quenched by hemoglobin. The lipid fluorophore, octadecylrhodamine, was incorporated into the membrane of the same red blood cell at self-quenching concentrations (Morris, S. J., D. P. Sarkar, J. M. White, and R. Blumenthal. 1989. J. Biol. Chem. 264: 3972-3978). Fusion, which allowed movement of the water-soluble dye from the cytoplasm of the red blood cell into the hemagglutinin-expressing fibroblasts, and movement of octadecylrhodamine from membranes of red blood cell to the plasma membrane of the fibroblasts, was observed by fluorescence microscopy as a spatial relocation of dyes, and monitored by spectrofluorometry as an increase in fluorescence. Upon lowering the pH below 5.4, fluorescence increased after a delay of about 30 s at 37 degrees C, reaching a maximum within 3 min. The kinetics, pH profile, and temperature dependence were similar for both fluorescent events measured simultaneously, indicating that influenza hemagglutinin-induced fusion rapidly establishes bilayer continuity and exchange of cytoplasmic contents.

Human immunodeficiency virus (HIV-1) cytotoxicity: perturbation of the cell membrane and depression of phospholipid synthesis

The molecular mechanism(s) by which human immunodeficiency virus (HIV-1) injures a T-cell line was studied. A pathological role for viral env proteins, which are inserted into the plasma membrane, has been previously demonstrated for HIV as well as other retroviruses which are cytopathic. We therefore initiated studies examining whether perturbations of the cell membrane or membrane-associated biochemical events may be occurring in cells acutely infected with HIV and whether such perturbations, if present, may be responsible for cytopathology. A human T-cell line (ERIC), which is sensitive to the cytopathic effects of HIVs, was infected with HTLV-IIIB and its membrane permeability to cations and its lipid metabolism were studied coincident with the peak expression of viral p24 and with the first sign of cytopathology (slowing of cell division) 72 to 96 hr after infection. It was found that the rate of influx of Ca2+ into the cell increased over that of uninfected cells and that phospholipid synthesis, primarily phosphatidylcholine, became depressed. Diacylglycerol, which serves both as an intermediate for synthesis of phospholipids and as a second-messenger for lymphocyte activation, was also greatly reduced. However, triglyceride synthesis was enhanced, indicating that not all lipid metabolic pathways were being shut down. This decreased membrane-synthetic ability and reduced second-messenger for cell division are likely to be important causes of HIV-1 cytopathology in ERIC cells. This hypothesis was supported by our finding that HIV cytopathology of ERIC cells could be partially prevented by treatment with compounds (diacylglyceride or PMA and transiently by oleic acid) which either replenish diacylglycerol in the infected cell and/or activate protein kinase C or phosphocholine cytidyltransferase, the latter being the rate-limiting step in synthesis of the major structural phospholipid in most cells.

Increased sensitivity of virus-infected cells to inhibitors of protein synthesis does not correlate with changes in plasma membrane permeability

Semliki Forest virus-infected BHK cells or herpes simplex virus-infected Vero cells were incubated with the protein synthesis inhibitors hygromycin B and gougerotin. Infected cells take up no more [3H]hygromycin or [3H]gougerotin than do mock-infected cells, at a time p.i. at which either compound is more inhibitory to protein synthesis in infected, than in mock-infected cells. The concentrations of hygromycin and gougerotin required to inhibit protein synthesis in intact cells (irrespective of whether they are infected or not) are several orders of magnitude higher than those required in either permeabilized cells or in cell-free systems. Infected cells take up 86Rb+ at the same rate as mock-infected cells, their intracellular content of K+ is the same, and the activity of the Na+ pump is the same. It is concluded that the increased efficacy of hygromycin and gougerotin in virus-infected cells is a consequence of altered intracellular compartmentation and that increases in permeability of the plasma membrane, if any, are so small as to be undetectable by direct methods.

Oxonol dyes as monitors of membrane potential: the effect of viruses and toxins on the plasma membrane potential of animal cells in monolayer culture and in suspension

Optical indicators of the cationic, cyanine and anionic oxonol classes were used to evaluate the plasma membrane potential of animal cells in suspension and in monolayer culture. The optical signals were calibrated by using diffusion potentials either of K+ (in the presence of valinomycin) or of H+ (in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone; FCCP); both classes of dye gave similar values of plasma membrane potential, in the range -40 to -90 mV for different cell types. Addition of haemolytic Sendai virus or Staphylococcus aureus alpha-toxin depolarizes cells and causes them to leak monovalent cations; these effects are antagonized by extracellular Ca2+. Cells infected with vesicular stomatitis or Semliki Forest virus become depolarized during an infectious cycle; infection with other viruses was without affect on plasma membrane potential.

Protein synthesis in cells infected with Semliki Forest virus is not controlled by intracellular cation changes

Treatment of BHK cells with 1 microM nigericin results in a 55% decrease in K+ and a 3.3-fold increase in intracellular Na+; protein synthesis under these conditions is depressed by 35%. In BHK cells infected with Semliki Forest virus (SFV), protein synthesis is depressed by 76% 6.5 h after infection; intracellular K+ is unchanged, and intracellular Na+ is increased 1.8-fold at this time. These results suggest that the increase in intracellular Na+ in SFV-infected BHK cells does not adequately account for the decrease in protein synthesis, and makes it likely that an increased Na+ concentration is a consequence, not a cause, of alterations in protein synthesis in virally-infected cells. No evidence was obtained for the purported [Alonso, M. A. and Carrasco, L. (1980) Eur. J. Biochem. 109, 535-540; (1981) Eur. J. Biochem. 118, 289-294; (1981) FEBS Lett. 127, 112-114] ability of 1 microM nigericin to permeabilize' cells.

Myxoviruses do not induce non-specific alterations in membrane permeability early on in infection

The permeability characteristics of cells infected with myxoviruses have been studied by measuring the concentrative uptake of nutrients, the concentration of intracellular K+, and the maintenance of the Na+ gradient across the plasma membrane. Cells either show no change at all (Sendai virus-infected BHK cells and measles virus-infected Vero cells) or they show a decreased ability to concentrate nutrients, while intracellular K+ and the Na+ gradient remain unchanged (Sendai and influenza virus-infected L-1210 cells, measles virus-infected lymphocytes and mumps virus-infected L-41 cells). In no case, therefore, was a change observed that resembles the non-specific increase in membrane permeability induced by haemolytic paramyxoviruses (35, 42) or the non-specific membrane leakiness postulated to take place in infected cells (8, 9). A preliminary account of some of these findings has been presented (39).

Membrane-mediated alterations of intracellular Na+ and K+ in lytic-virus-infected and retrovirus-transformed cells

Infection of chick-embryo fibroblasts and other cells by certain animal viruses results in alterations in the intracellular concentrations of Na+ and K+. Dramatic alterations in monovalent-cation concentrations of lytic-virus-infected cells may favor the synthesis of viral proteins over cellular proteins. More subtle alterations in retrovirus-transformed cells may result in the expression of many morphological and biochemical changes associated with the transformed phenotype.