Gating kinetics of pH-activated membrane fusion of vesicular stomatitis virus with cells: stopped-flow measurements by dequenching of octadecylrhodamine fluorescence

To identify the initial stages of membrane fusion induced by vesicular stomatitis virus, we performed stopped-flow kinetic measurement with fluorescently labeled virus attached to human erythrocyte ghosts that contained symmetric bilayer distributions of phospholipids. Fusion was monitored spectrofluorometrically using an assay based on mixing of the lipid fluorophore octadecylrhodamine. At 37 degrees C and pH values near the threshold for fusion, a lag phase of 2 s was observed. The lag time decreased steeply as the pH decreased, while the initial rate of fusion showed the reverse functional dependence on pH. The observed rapid fluorescence changes resulted from fusion of virus bound to the target, and the time lags were not due to association-dissociation reactions between virus and target. For a given pH value, the temperature dependence of the lag time was similar to that of the initial rate of fusion. The results were fitted to a multistate model similar to that resulting from ion channel gating kinetics. The model allows testing of hypotheses concerning the role of cooperativity and conformational changes in viral spike glycoprotein-mediated membrane fusion.

Apical membrane vesicles of ADH-stimulated toad bladder are highly water permeable

Antidiuretic hormone (ADH) stimulation of the toad urinary bladder causes intracellular vesicles called aggrephores to fuse with the apical plasma membrane of granular cells. Aggrephore membranes contain particle aggregates. Particle aggregates are believed to be water channels that cause large increases in the water permeability (PF) of the granular cell apical membrane. Removal of ADH causes the retrieval of particle aggregate-containing apical membrane via endocytosis and a decline in PF. We have previously shown that fluid phase markers are sequestered in these particle aggregate-containing vesicles during retrieval of the apical membrane and that these vesicles can be recovered in cell homogenates. We have now loaded these vesicles with the self-quenching fluorophore carboxyfluorescein (CF) to measure and compare their PF with that of CF-loaded resealed human erythrocyte ghosts. The membranes of these retrieved vesicles have a very high water permeability. The minimum PF of 99% of these vesicles is 4.5 X 10(-2) cm/s. This PF is comparable with that of erythrocyte ghosts (5.4 X 10(-2) cm/s) measured under identical conditions. We conclude that these vesicles are highly permeable to water, and this is consistent with their postulated function of retrieving water channels that have been inserted into the apical membrane in response to ADH.

On the use of self-quenching fluorophores in the study of membrane fusion kinetics. The effect of slow probe redistribution

In glycoprotein-mediated pH-induced fusion of virus to animal cells, the mixing of materials between membranes or between cytoplasmic spaces occurs after the virus-cell complex has gone through a number of activation reactions. The monitoring of the fluorescence changes measured in a fusing system using self-quenching probes could reflect not only the kinetics of activation, but also the redistribution reaction of probes. For instance, time delay seen in the onset of fluorescence changes after triggering the fusion reaction (S.J. Morris, D.P. Sarkar, J.M. White and R. Blumenthal, J. Biol. Chem. (1989) 3972), could be due to rate-limiting probe redistribution kinetics. In this paper we examined in detail the effect of probe redistribution rates on fusion kinetics. Simulations were performed using a very simple model with two fusion-activation steps and an exponential probe redistribution kinetics. We conclude that if the rates of probe redistribution are faster than or equal to those of viral glycoprotein activation, the kinetics of the fusion reaction are not significantly affected.

Diminished corticosterone levels in nude mice implanted with MCF-7 or ZR-75-1 human breast tumor cells

Corticosteroid levels were studied in the plasma of athymic mice implanted with human breast tumor cells, either from MCF-7 or ZR-75-1 cell lines. There was a highly significant decrease in plasma corticosterone levels in the mice implanted with these tumor cells. There was no significant effect on corticosterone of GW 39 colon cancer cells, LS 174T colon cancer cells, or Calu-3 lung cancer cells.

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.

Kinetics of pH-dependent fusion between 3T3 fibroblasts expressing influenza hemagglutinin and red blood cells. Measurement by dequenching of fluorescence

Fusion between membranes of 3T3 fibroblasts expressing hemagglutinin (HA) from the Japan strain of influenza virus and human red blood cells (RBC) was measured using an assay for lipid mixing based on the relief of self-quenching (dequenching) of fluorescence of the lipid probe octadecylrhodamine (R18). The probe was incorporated into the membrane of intact RBC at self-quenching concentrations, and the RBCs were bound to the 3T3 cells. Fusion, which allowed movement of R18 into 3T3 cell membranes, was monitored by spectrofluorometry as an increase in fluorescence. Upon lowering the pH below 5.4, the fluorescence increased after a delay of about 30 s at 37 degrees C, and leveled off within 2 min. In control experiments where R18 RBCs bound to 3T3 cells expressing the uncleaved precursor hemagglutinin (HA0) were incubated at 37 degrees C and low pH, no fluorescence increase was observed. This indicated that the R18 dequenching occurred as a result of HA-induced fusion of plasma membranes. Fusion showed a very steep pH dependence with a threshold at pH 5.4 and a maximum at pH 5.0, similar to HA-induced fusion seen previously using cell biological techniques. The fusion rate increased and the delay for the onset of fusion decreased as the temperature was raised above 20 degrees C. Low pH activation of the fusion process at 37 degrees C could be partially arrested by raising the pH after 2-10 s, but not after 15 s, indicating that the irreversible pH-activated conformational change of HA necessary for fusion was complete within about 15 s. Analysis of the data indicates that the pH-induced membrane fusion activity of HA is a highly cooperative event.

pH-dependent fusion of reconstituted vesicular stomatitis virus envelopes with Vero cells. Measurement by dequenching of fluorescence

Reconstituted vesicular stomatitis virus (VSV) envelopes were formed by solubilization of the viral envelope with Triton X-100 followed by removal of detergent by direct addition of SM2 biobeads. We provide direct demonstration of fusion of reconstituted VSV with cells using fluorescent lipid and aqueous probes incorporated into the VSV virosomes during reconstitution. We show a direct comparison of the kinetics and pH profile of fusion with cells between reconstituted VSV and fluorescently labeled intact virus. With this preparation it is now possible to gain additional information about the role of cooperativity in viral protein-mediated fusion, and to permit construction of efficient vehicles for delivery of drugs and other materials into cells.

Composition of octyl glucoside-phosphatidylcholine mixed micelles

The composition of mixed micelles of egg phosphatidylcholine (PC) and octyl glucoside was studied by a novel technique based on measuring resonance energy-transfer efficiency between two fluorescent lipid probes present in trace amounts. Equations were derived for calculating the stoichiometry of the composition of mixed micelles from the energy-transfer measurements. These were applied to determining the average number of lipid molecules in the octyl glucoside-egg PC mixed micelle as a function of detergent concentration. The average number of detergent molecules in these mixed micelles was independent of lipid concentration in the range studied (0-500 microM). The dependence of mixed micelle stoichiometry on the concentration of aqueous (monomeric) octyl glucoside is consistent with the assumptions of ideal mixing of the two amphiphiles in the mixed micelles and that mixed micelles can be treated as a distinct phase.

Activation of vesicular stomatitis virus fusion with cells by pretreatment at low pH

Fusion of vesicular stomatitis virus (VSV) with Vero cells was measured after exposure of the virus to low pH under a variety of experimental conditions. The method of relief of fluorescence self-quenching of the probe octadecylrhodamine was used to monitor fusion. Incubation of the virus at pH 5.5 prior to binding to cells led to significant enhancement of fusion at the plasma membrane, whereas fusion via the endocytic pathway was inhibited. Fusion of pH 5.5-pretreated VSV showed a similar pH threshold for fusion as nontreated virus, and it was blocked by antibody to VSV G protein. Activation of VSV by pretreatment at low pH was only slightly dependent on temperature. In contrast, when VSV was first bound to target cells and subsequently exposed at 4 degrees C to the low pH, activation of the fusion process did not occur. The pH 5.5-mediated activation of VSV could be reversed by returning the pH to neutral in the absence of target membranes. The low pH pretreatment also led to aggregation of virus; large aggregates could be pelleted by low speed centrifugation and only the effects of the supernatant, which consist of single virions and/or microaggregates, were considered. The data were analyzed in the framework of an allosteric model according to which viral spike glycoproteins undergo a pH-dependent conformational transition to an active (fusion-competent) state. Based on that analysis we conclude that the conformational transition to the active state is rate-limiting for fusion and that the viral spike glycoproteins are fusion-competent only in their protonated form.

Micelle-vesicle transition of egg phosphatidylcholine and octyl glucoside

The dissolution and formation of egg phosphatidylcholine (PC) vesicles by the detergent octyl glucoside were examined systematically by using resonance energy transfer between fluorescent lipid probes, turbidity, and gel filtration chromatography. Resonance energy transfer was exquisitely sensitive to the intermolecular distance when the lipids were in the lamellar phase and to the transitions leading to mixed micelles. Turbidity measurements provided information about the aggregation of lipid and detergent. Several reversible discrete transitions between states of the PC-octyl glucoside system were observed by both methods during dissolution and vesicle formation. These states could be described as a series of equilibrium structures that took the forms of vesicles, open lamellar sheets, and mixed micelles. As detergent was added to an aqueous suspension of vesicles, the octyl glucoside partitioned into the vesicles with a partition coefficient of 63. This was accompanied by leakage of small molecules and vesicle swelling until the mole fraction of detergent in the vesicles was just under 50% (detergent:lipid ratio of 1:1). Near this point, a transition was observed by an increase in turbidity and release of large molecules like inulin, consistent with the opening of vesicles. Both a turbidity maximum and a sharp increase in fluorescence were observed at a detergent to lipid mole ratio of 2.1:1. This was interpreted as the lower boundary of a region where both lamellar sheets and micelles are at equilibrium. At a detergent:lipid ratio of 3.0:1, another sharp change in resonance energy transfer and clarification of the suspension were observed, demarcating the upper boundary of this two-phase region. This latter transition is commonly referred to as solubilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Family attitudes and patient social adjustment in a longitudinal study of outpatient schizophrenics receiving low-dose neuroleptics: the family's view

Adverse effects of neuroleptic medication have led to the attempt to develop alternative strategies for the treatment of schizophrenia, but it is generally conceded that these strategies may have their own negative outcomes in the form of symptom exacerbation, reduced social performance and worsened family interactions. This paper examines the effect of one such strategy, low doses of medication, on the social adjustment of and family response to chronic schizophrenic outpatients. Patients who were randomly assigned to either a low-dose or standard-dose condition were rated by their families on various aspects of social adjustment. Despite a considerably higher relapse rate in the low-dose condition, families reported patients in the low-dose condition to be no poorer in their social adjustment than standard-dose patients. In addition, families of low-dose patients were more satisfied with their patients' overall level of adjustment and were no more rejecting at endpoint than families of standard-dose patients. Low-dose patients were viewed even more favorably when patients who relapsed were excluded from the analysis. Negative family attitudes, particularly rejection, measured at study entry, were found to predict time to relapse in the low-dose group. Implications for treatment and family intervention are discussed.

Cooperativity in viral fusion

A simple model for membrane fusion mediated by vial spike glycoproteins is presented. The viral proteins are considered to be allosteric proteins that undergo concerted conformational transitions when they bind the ligand. The ligand in this case is H+. The effect of the conformational transition is to bring membranes together and induce their fusion. An equation is derived for the dependence of fusion rates on ligand concentration, for a given dissociation constant (Kd), equilibrium constant for the conformational change (L), and number of cooperating subunits (n). Curves generated by this equation provide a reasonable fit to data on the rates of fusion of Vesicular Stomatitis virus with cells for a pKd of 6.3, L = 1000 and n = 6.

Secretion of a growth inhibitory factor by ZR-75-1 human breast cancer cells

Cultures of the human mammary carcinoma line ZR-75-1 secrete a growth inhibitory factor (GIF) that, when diluted, slows the growth of MDA-MB-231 and MCF-7 cells. Undiluted "conditioned" media prevents cell division from occurring in both human breast cancer lines. ZR-75-1 cells are unaffected by this factor. The amount of GIF in the culture media is related to the confluency of the ZR-75-1 cells. The activity of this GIF is not altered by DNAse or RNAse but is destroyed by heating or trypsin. Growth inhibition is 85-90% reversible if conditioned media is replaced with fresh media.

pH-dependent fusion of vesicular stomatitis virus with Vero cells. Measurement by dequenching of octadecyl rhodamine fluorescence

We have studied fusion between membranes of vesicular stomatitis virus (VSV) and Vero cells using an assay for lipid mixing based on the relief of self-quenching of octadecylrhodamine (R18) fluorescence. We could identify the two pathways of fusion by the kinetics of R18 dequenching, effects of inhibitors, temperature dependence, and dependence on osmotic pressure. Fusion at the plasma membrane began immediately after lowering the pH below 6 and showed an approximately exponential time course, whereas fusion via the endocytic pathway (pH 7.4) became apparent after a time delay of about 2 min. Fusion via the endocytic pathway was attenuated by treating cells with metabolic inhibitors and agents that raise the pH of the endocytic vesicle. A 10-fold excess of unlabeled virus arrested R18VSV entry via the endocytic pathway, whereas R18 dequenching below pH 6 (fusion at the plasma membrane) was not affected by the presence of unlabeled virus. The temperature dependence for fusion at pH 7.4 (in the endosome) was much steeper than that for fusion at pH 5.9 (with the plasma membrane). Fusion via the endocytic pathway was attenuated at hypo-osmotic pressures, whereas fusion at the plasma membrane was not affected by this treatment. The pH profile of Vero-VSV fusion at the plasma membrane, as measured by the dequenching method, paralleled that observed for VSV-induced cell-cell fusion. Fusion was blocked by adding neutralizing antibody to the Vero-VSV complexes. Activation of the fusion process by lowering the pH was reversible, in that the rate of fusion was arrested by raising the pH back to 7.4. The observation that pH-dependent fusion occurred at similar rates with fragments and with intact cells indicates that pH, voltage, or osmotic gradients are not required for viral fusion.

The role of the target membrane structure in fusion with Sendai virus

Fusion between membranes of Sendai virus and liposomes or human erythrocytes ghosts was studied using an assay for lipid mixing based on the relief of self-quenching of octadecylrhodamine (R18) fluorescence. We considered only viral fusion that reflects the biological activity of the viral spike glycoproteins. The liposomes were made of phosphatidylcholine, and the effects of including cholesterol, the sialoglycolipid GD1a, and/or the sialoglycoprotein glycophorin as receptors were tested. Binding of Sendai virus to those liposomes at 37 degrees C was very weak. Fusion with the erythrocyte membranes occurred at a 30-fold faster rate than with the liposomes. Experiments with biological and liposomal targets of different size indicated that size did not account for differences in fusion efficiency.

Polylysine induces pH-dependent fusion of acidic phospholipid vesicles: a model for polycation-induced fusion

Polylysine induced aggregation and fusion of negatively charged small unilamellar phosphatidylcholine vesicles containing at least 10% anionic lipid. Aggregation was followed by absorbance changes and fusion was assayed both by electron microscopy and by fluorescence energy transfer between lipid probes. A method for preparing asymmetric vesicles, where the fluorescent probes were present only in the inner monolayer of the vesicle membrane, was developed. These vesicles were used to distinguish the inner and outer monolayer when measuring lipid mixing between vesicles. Since polylysine induced lipid mixing of both monolayers equally, fusion of these vesicles did occur. The extent of fusion was dependent on the charge ratio between bound polylysine and phosphatidylserine (PS) in the outer monolayer and was optimal at a ratio of about 1:1. Excess polylysine inhibited fusion. At a given concentration of polypeptide, fusion increased as the pH was lowered toward 3 with an apparent pKa near 4. Since this value is close to the pKa of the PS-carboxyl groups and far from the pKa of the lysine epsilon-amino groups, the pH dependence observed for fusion resides in the lipids rather than in the peptide. Fusion was dependent on the available lysine and not the size or molarity of the polypeptide. The data indicate that there must be sufficient sites on the vesicles and sufficient polypeptide to achieve effective aggregation. For fusion to occur after aggregation, charges on the vesicles must be neutralized either by polypeptide-PS interaction or by protonation of the PS carboxyl groups. Optimal conditions for fusion occur when charge neutralization is possible without completely covering the vesicles with polypeptide. The results are consistent with the notion that the polypeptide is necessary for fusion because of requirements for crosslinking, but limits fusion by steric inhibition.

pH-dependent lysis of liposomes by adenovirus

Purified adenovirus induced a dose-dependent release of the water-soluble markers calcein and carboxyfluorescein from liposomes. Marker release was strongly dependent on pH, and at temperatures below 5 degrees C, the rate of release showed an optimum at a pH of about 6. This pH dependence parallels disruption of endocytic vesicles by adenovirus and the permeabilization that adenovirus induces on the cell surface. There did not seem to be a striking dependence on the lipid composition of the liposomes. Electron microscopy using a negative stain shows liposomes bound to adenovirus. In some cases, the liposomes were still intact, but many liposomes, which were attached to the vertices of the virus, appeared lysed. These data support the notion that adenovirus, which enters the host cell by receptor-mediated endocytosis, gains access to the cytoplasm by a subsequent pH-dependent disruption of the membrane of the endocytic vesicle.

Sizing and separation of liposomes, biological vesicles, and viruses by high-performance liquid chromatography

The ability of an HPLC gel exclusion column (TSK G6000PW) to separate lipid vesicles, viruses, and biological vesicles according to size was tested and compared with separations on Sephacryl S1000. The columns were calibrated using vesicular Stokes radii determined by quasielastic light scattering. The vesicles separated according to size on both types of column and remained intact during elution. Viruses of known diameters and clathrin-coated vesicles were also eluted as a function of size. The TSK G6000PW column was able to separate larger particles (greater than 500 nm) than the Sephacryl S1000, and, when used in combination with the TSK G5000PW column, gave more discrete separations of smaller particles (10 to 30 nm diameter). Moreover, the HPLC columns can be run significantly faster (10-20 min vs several hours) and give more precise results than Sephacryl S1000. Therefore, HPLC using a G6000PW column alone, or in combination with a G5000PW column, provides a rapid and accurate means of sizing and selecting specifically sized biological and artificial vesicles.