Low concentrations of macromolecular solutes significantly affect electrofusion yield in erythrocyte ghosts

Agents facilitating the electric field-induced fusion of intact rabbit erythrocytes

In the recent years, the role of specific membrane active agents in the electrofusion process has started to draw attention, and it has been found that the presence of various substances in the cell medium can affect the fusion process either in a positive or negative way. In this work, the effect of several proteins, bivalent cations and antibiotics was tested with respect to their ability to protect intact erythrocytes from hemolysis and facilitate the fusion process. The effect of different sugars was also studied. Among the different proteins, pronase and proteinase were found to be the most effective. With respect to bivalent cations, Ca2+ and Mn2+ were more effective while Mg2+ was less important. From the antibiotics, penicillin caused a negative effect while streptomycin acted positively. Finally, glucose medium was found to be the most effective compared to all sugars tested. The results indicated that there are strong differentiations of the induced effects caused by each substance, and some possible mechanisms of action of these agents on the erythrocyte membrane were discussed.

Dynamics of oscillating erythrocyte doublets after electrofusion

Erythrocytes were electrofused with multiple rectangular voltage pulses to show an oscillatory movement, divided into swell phases and pump events. During each swell phase, which lasted from 0.5 s to more than 180 s, the fused cells' (doublets') volume increased by colloid osmotic swelling, and the membrane area was expanded until rupture. Membrane rupture initiated the pump event, where the doublets' volume and membrane area decreased with an almost exponential time course and time constants between 2 ms and 8 ms. Simultaneously, a portion of cytosolic hemoglobin solution was ejected into extracellular space ("jet"). Pump event time constants and swell phase durations decreased with rising chamber temperature, indicating that both parts of the oscillatory movements were determined by physical properties of membrane and liquids. Relative volume change developments express a gradual loss of membrane elasticity during the oscillation, decreasing the elastic forces stored in the membrane. Evidence is given that the first rupture causes a weakening of the membrane at the rupture site. Heat treatment up to 45 degrees C had a negligible effect on swell times, pump time constants, and relative volume changes. A heat treatment of 50 degrees C prevented oscillatory movements. The rupture location accorded with theories of potential induced membrane electropermeabilization.

Membrane skeleton restraint of surface shape change during fusion of erythrocyte membranes: evidence from use of osmotic and dielectrophoretic microforces as probes

The role of the spectrin-based membrane skeleton in cell fusion was studied by following the condition-dependent diameter versus time expansion signature of the fusion zone in electrofused pairs of erythrocyte ghost membranes. Previous work showed that the presence of the dielectrophoresis-inducing alternating electric field, which is used to bring membranes into contact through pearl chain formation, had a detectable promoting effect on fusion zone expansion. Two new dielectrophoresis protocols were used in the present work to utilize this externally generated and controllable microforce field to probe the forces intrinsic to the system that drives the expansion of the fusion zone. First, fusion zones expanded to a greater diameter in a strong AC field compared to a weak AC field, and they expanded to a greater diameter if erythrocyte ghosts received a prior heat treatment (42 degrees C, 20 min). Furthermore, flat diaphragm fusion zones broke down into open lumen fusion zones sooner (i.e., had shorter lifetimes) when they were expanding more quickly. Second, changing the AC field strength at specific times during the fusion zone expansion led to an immediate visco-elastic response. However, shifting the AC field strength to zero after 5 s of fusion zone expansion resulted in a subsequent decrease in the average fusion zone diameter. This suggests not only that the spectrin-based membrane skeleton actually tends to prevent the rounding up process but that it may be capable of generating an antirounding force, which has broad implications for the role of the membrane skeleton in cell fusion. These results are consistent with the hypothesis that flat diaphragm fusion zones induced in heat-treated membranes were very easily stretched and that membrane-based forces that control or drive the expansion process must originate from membrane area that is outside rather than inside the fusion zone. Lastly, when an outward-directed osmotic pressure-based microforce was present at the time that erythrocyte ghosts were fused, the fusion zone diameter underwent a greater expansion in the 0-1 s interval after fusion. This suggests that an osmotic pressure-based microforce can be used to experimentally calibrate the dielectrophoretic force.

Surface shape change during fusion of erythrocyte membranes is sensitive to membrane skeleton agents

We previously reported that the induction of membrane fusion between pairs of erythrocyte ghosts is accompanied by the formation of a multipore fusion zone that undergoes an area expansion with condition-dependent characteristics. These characteristics allowed us to hypothesize substantial, if not major, involvement of the spectrin-based membrane skeleton in controlling this expansion. It was also found that the fusion zone, which first appears in phase optics as a flat diaphragm, has a lifetime that is also highly condition-dependent. We report here that 2,3-diphosphoglycerate, wheat germ agglutinin, diamide, and N-ethylmaleimide, all known to have binding sites primarily on skeleton components (including spectrin), have condition-dependent effects on specific components of the fusion zone diameter versus time expansion curve and the flat diaphragm lifetime. We also report a pH/ionic strength condition that causes a dramatic stabilization of flat diaphragms in a manner consistent with the known pH/ionic strength dependence of the spectrin calorimetric transition, thus further supporting the hypothesis of spectrin involvement. Our data suggest that the influence of the membrane skeleton on cell fusion is to restrain the rounding up that takes place after membrane fusion and that it may have variable, rather than fixed, mechanical properties. Data show that WGA, a known ligand for sialic acid, and DPG, a known metabolite, influences the flat diaphragm stability and late period expansion rates, raising the possibility that some of these mechanical properties are biologically regulated.

Spectrofluorometric assay for the quantitation of cell-tissue electrofusion

Fluorometric assay for quantitating electrofusion, FAQE, was developed to measure the number of cells fused to intact tissue. The fluorescent vital dye hydroethidine is used in this method. The fluorescent intensity detected in the cell tissue hybrids is proportional to the number of individual cells fused. The number of cells fused was determined after fusion by lysing the epithelial layer with 0.2% sodium dodecyl sulfate and the supernatant fluids were measured in a spectrofluorometer and compared to established standard curves. The mean number of cells fused, in five separate experiments, was determined to be approximately 5000. All the experimental corneas had approximately the same number of fused cells with less than 10% variation. In addition, the technique was used to demonstrate an increase in the number of cells fused when multiple fusions were applied to the cell-tissue electrofusion system. These results demonstrate that FAQE can be utilized to quantitatively analyze the fusion yields.

Determination of electric field threshold for electrofusion of erythrocyte ghosts. Comparison of pulse-first and contact-first protocols

Rabbit erythrocyte ghosts were fused by means of electric pulses to determine the electrofusion thresholds for these membranes. Two protocols were used to investigate fusion events: contact-first, and pulse-first. Electrical capacitance discharge (CD) pulses were used to induce fusion. Plots of fusion yield vs peak field strength yielded curves that intersected the field strength axis at positive values (pseudothresholds) which depended on the protocol and decay half time of the pulses. It was found that plots of pseudothreshold vs reciprocal half time were linear for each protocol; when extrapolated to reciprocal half time = 0 (i.e., t----infinity), these lines intersected the ordinate at values of the field strength considered to be the true electrofusion thresholds. In this fashion, the contact-first protocol gave an electrofusion threshold of 46.5 +/- 11.5 V/mm for hemoglobin-free ghosts (white ghosts) and 40.9 +/- 8.8 V/mm for ghosts with fractional hemoglobin (pink ghosts), while the threshold for the pulse-first protocol applied to pink ghosts was determined to be 93.4 +/- 11.0 V/mm. Although the thresholds depended on the electrofusion protocol, plots of critical field strength vs reciprocal time had the same slopes, i.e., approximately 24 Vs/mm. The results suggest that the fusogenic state induced by an electric pulse in either the contact-first protocol or the pulse-first protocol (long-lived fusogenic state) may in fact share a common mechanism, if the two states are not actually identical.

Evidence that the spectrin network and a nonosmotic force control the fusion product morphology in electrofused erythrocyte ghosts

The conversion of the membrane area in the "contact zones" shared by erythrocyte ghosts held in contact by dielectrophoresis into a fusion product by electrofusion was studied by both light and electron microscopy. Fusion products fell into two categories: (a) those with a freely expanding open lumen which ended in the "giant cell morphology" and with considerable internal vesicle membrane fragments, and (b) linear chains of polyghosts with long term stability but having planar diaphragms at the ghost-ghost junctions. Thin section electron microscopy showed each of these planar diaphragms to be a double membrane septum multiply-perforated with fusion pores. Heat and low ionic strength treatments known to denature or detach spectrin caused the stable planar diaphragms to dissolve, thereby quickly converting the polyghost chains to the giant cell morphology, thereby suggesting that spectrin restricts fusion zone diameter expansion if it is intact. Other indications suggest that the expansion of the open lumens appears to take place as a result of one or more membrane-specific forces with a nonosmotic origin but this tendency to expansion can be overcome if the spectrin network on only one side of a contact zone is intact.

A delay in membrane fusion: lag times observed by fluorescence microscopy of individual fusion events induced by an electric field pulse

Low light level video microscopy of the fusion of DiI- (1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) labeled rabbit erythrocyte ghosts with unlabeled rabbit erythrocyte ghosts, held in stable apposition by dielectrophoresis in sodium phosphate buffers, showed reproducible time intervals (delays) between the application of a single fusogenic electric pulse and the earliest detection of fluorescence in the unlabeled adjacent membranes. The delay increased over the range 0.3-4 s with a decrease in (i) the electric field strength of the fusion-inducing pulse from 1000 to 250 V/mm, (ii) the decay half-time of the fusogenic pulse in the range 1.8-0.073 ms, and (iii) the dielectrophoretic force which brings the membranes into close apposition. A change in the buffer viscosity from 1.8 to 10 mP.s caused the delay to increase from 0.36 to 3.7 s (in glycerol solutions) or to 5.2 s (in sucrose solutions). The delay decreased 2-3 times with an increase in temperature from 21 to 37 degrees C. It did not differ significantly for "white" ghosts [0.013 mM hemoglobin (Hb)] or "red" ghosts (0.15 mM Hb) or buffer strength over the range 5-60 mM (sodium phosphate, pH 8.5). The calculated activation energy, 17 kcal/mol, does not depend on the field strength. The yield of fused cells was high when the delay was short. The delay in electrofusion resembles the delays in pH-dependent fusion of vesicular stomatitis viruses with erythrocyte ghosts [Clague, M. J., Schoch, C., Zech, L., & Blumenthal, R. (1990) Biochemistry 29, 1303-1308] and of fibroblasts expressing influenza hemagglutinin and red blood cells [Morris, S. J., Sarkar, D.P., White, J. M., & Blumenthal, R. (1989) J. Biol. Chem. 264, 3972-3978].(ABSTRACT TRUNCATED AT 250 WORDS)