Vesicle formation during electro-fusion of mesophyll protoplasts of Kalanchoë daigremontiana

Biophysical characterisation of electrofused giant HEK293-cells as a novel electrophysiological expression system

Giant HEK293 cells of 30-65 microm in diameter were produced by three-dimensional multi-cell electrofusion in 75 mOsm sorbitol media. These strong hypotonic conditions facilitated fusion because of the spherical shape and smooth membrane surface of the swollen cells. A regulatory volume decrease (RVD), as observed at higher osmolalities, did not occur at 75 mOsm. In contrast to field-treated, but unfused cells, the increase in volume induced by hypotonic shock was only partly reversible in the case of fused giant cells after their transfer into isotonic medium. The large size of the electrofused cells allowed the study of their electrophysiological properties by application of both whole-cell and giant excised patch-clamp techniques. Recordings on giant cells yielded a value of 1.1+/-0.1 microF/cm2 for the area-specific membrane capacitance. This value was consistent with that of the parental cells. The area-specific conductivity of giant cells (diameter > 50 microm) was found to be between 12.8 and 16.1 microS/cm2, which is in the range of that of the parental cells. Measurements with patch-pipettes containing fluorescein showed uniform dye uptake in the whole-cell configuration, but not in the cell-attached configuration. The diffusion-controlled uniform uptake of the dye into the cell interior excludes internal compartmentalisation. The finding of a homogeneous fusion was also supported by expression of the yellow fluorescent protein YFP (as part of the fusion-protein ChR2-YFP) in giant cells since no plasma-membrane bound YFP-mediated fluorescence was detected in the interior of the electrofused cells. Functional expression and the electrophysiological characterisation of the light-activated cation channel Channelrhodopsin 2 (ChR2) yielded similar results as for parental cells. Most importantly, the giant cells exhibited a comparable expression density of the channel protein in the plasma membrane as observed in parental cells. This demonstrates that electrofused cells can be used as a heterologous expression system.

Membrane microextension: a possible mechanism for establishing molecular contact in electrofusion

True cell membrane contact is an essential condition for electro-pulsed cell fusion, but initial morphological perturbation leading to true contact is still not clear. Dielectrophoresis mediated compression and fusogenic pulse induced compaction of cells led to rapid merger of tight membranes, and deprived direct microscopic view of surface membrane perturbation. Freely suspending cells with large and different cell-cell gaps may proceed to electrofusion with perturbed membrane and initiates fusion events at different time. These pulsed exposed cells can be used for capturing changes in the membrane surface and early electrofusion events. Early stage of fusion of freely suspended intact human erythrocytes exposed to single exponential decay pulse was studied by scanning electron microscopy (SEM). Field pulse induces small membrane bumps. Interaction of bumps on adjacent membranes lead to true membrane contact and form bridges between the membranes as microextension, combining both membranes into a topologically single structure. Some fusion products showed expanded fusion zones, which suggest indication of open lumen at contact area.

Membrane-membrane contact: involvement of interfacial instability in the generation of discrete contacts

The classical approach to understanding the closeness of approach of two membranes has developed from consideration of the net effect of an attractive van der Waals force and a repulsive electrostatic force. The repulsive role of hydration forces and stereorepulsion glycocalyx forces have been recently recognized and an analysis of the effect of crosslinking molecules has been developed. Implicit in these approaches is the idea of an intercellular water layer of uniform thickness which narrows but retains a uniform thickness as the cells move towards an equilibrium separation distance. Most recently an attempt has been made to develop a physical chemical approach to contact which accommodates the widespread occurrence of localized spatially separated point contacts between interacting cells and membranes. It is based on ideas drawn from analysis of the conditions required to destabilize thin liquid films so that thickness fluctuations develop spontaneously and grow as interfacial instabilities to give spatially periodic contact. Examples of plasma membrane behaviour which are consistent with the interfacial instability approach are discussed and experiments involving polycation, polyethylene glycol, dextran and lectin adhesion and agglutination of erythrocytes are reviewed.

Isolation of functional extensor and flexor protoplasts fromPhaseolus coccineus L. pulvini: potassium induced swelling

Methods are described for the isolation of functional protoplasts from secondary pulvinus tissue (flexor and extensor) and from leaf mesophyll tissue of primary leaves ofPhaseolus coccineus L. Integrity of the protoplasts was shown by vital staining and their ability to evolve oxygen in the light. Extensor-cell protoplasts increased their volume for up to 60% upon addition of 10 mM KCl. This K(+)-induced swelling was accompanied by increased rates of proton extrusion.

Model of cell electrofusion. Membrane electroporation, pore coalescence and percolation

High electric field impulses (1-20 kV/cm, 1-20 microseconds) may trigger fusion between adhering cells or lipid vesicles (electrofusion). In this paper a qualitative model of electrofusion is proposed consistent with both electron and light microscopic data. Electrofusion is considered as a multistep process comprising tight membrane-contact formation, membrane electroporation as well as an alternating series of subsequent fast collective and slow diffusive fusion stages. The following sequence of steps is suggested: The electric field pulse enforces (via polarization) a tight contact between the membranes of the cells or vesicles to be fused. During tight-contact formation between the opposing membrane surfaces the membrane-adherent water layers are partially squeezed out from the intermembraneous space. Pores are formed in the double membrane contact area (electroporation) involving lateral diffusion and rotation of the lipid molecules in both adhering membrane parts. With increasing pore density, pore-pore interactions lead to short-range coalescence of double membrane pores resulting in ramified cracks; especially small tongues and loops are formed. At supercritical pore density long-range coalescence of the pores occurs (percolation) producing one large double membrane loop (or tongue) and subsequently one large hole in the contact area. After switching off the electric field, the smaller pores, tongues and loops reseal and water flows back into the intermembraneous space of the double membrane in the contact area. As a consequence of the increasing membrane-membrane separation due to water backflow, cooperative rounding of the edges of remaining larger tongues and holes occurs. This results in the formation of an intercellular cytoplasm bridge (channel) concomitant with the disappearance of the contact line between the fusing cells. The membrane parts surrounded by continuous loop-like cracks may separate from the system and may finally form vesicles. Our electrofusion model comprises a strong linkage between the membrane pore formation by high electric fields (electroporation) and the process of electrofusion. Additionally, both pore-pore interactions as well as protein-protein interactions in the contact area of the fusing cells are explicitly introduced. The model provides a qualitative molecular description of basic experimental observations such as the production of membrane fragments, of smaller inside-out vesicles and the formation of larger intercellular cytoplasm bridges.

Kinetics of ultrastructural changes during electrically induced fusion of human erythrocytes

The sequence of events during the electrically induced fusion of human erythrocytes was studied by rapid quench freeze-fracture electron microscopy. A single electric field pulse was used to induce fusion of human erythrocytes treated with pronase and closely positioned by dielectrophoresis. The electronic circuit was coupled to a rapid freezing mechanism so that ultrastructural changes of the membrane could be preserved at given time points. Pronase treatment enabled adjacent cells to approach each other within 15 nm during dielectrophoresis. The pulse caused a brief disruption of the aqueous boundaries which separated the cells. Within 100 msec following pulse application, the fracture faces exhibited discontinuous areas which were predominantly free of intramembranous particles. At 2 sec after the pulse, transient point defects attributed to intercellular contact appeared in the same membrane areas and replaced the discontinuous areas as the predominant membrane perturbation. At 10 sec after the pulse, the majority of the discontinuous areas and point defects disappeared as the intercellular distance returned to approximately 15 to 25 nm, except at sites of cytoplasmic bridge formation. Intramembranous particle clearing was observed at 60 sec following pulse application in discrete zones of membrane fusion.

Membrane stability

Rupture and buckling of artificial and biological membranes is an important part of many biological processes. In this review, we present some of the main experimental facts and their analysis. Recent theoretical work, in particular thin film models and nucleation mechanisms of membrane instability, are discussed in detail. Possible applications to membrane adhesion and fusion are pointed out. Attempts are made to explain biological phenomena and experimental results for biological membranes based on a rigorous physicochemical approach developed previously for thin films in colloid systems.

Electro-fusion of cells: principles and potential for the future

Exposure of cells or liposomes to a brief pulse of a strong electrical field can result in a reversible breakdown of the outer membrane. Such breakdown results in an increase in permeability of the plasmalemma, which however re-seals after a short incubation (i.e. the original impermeability is restored). Two or more cells in contact can be made to fuse by this process, provided that the contact is close enough and that the pulse of the electrical field is short enough not to damage the cells. Methods of achieving this contact by electrical and magnetic fields are described. The magnetic method does not demand the use of the low conductivity media used earlier. Other possible modifications of this flexible technique are also described, and used to show how the technique can be modified in future, and how it may be applied to the fields of membrane research, medicine and plant breeding.

Electro-fusion of mesophyll protoplasts ofAvena sativa : Determination of the cellular adenylate-level of hybrids and its influence on the fusion process

In order to assay the viability of electrically fused mesophyll protoplasts ofAvena sativa a technique was developed to determine adenylate levels in single protoplasts and fusion products. The results demonstrate that the intracellular ATP/ADP ratios are identical before and after fusion (values between 1.4 and 1.8) and that the time of the rounding up process is directly related to the ATP level of the hybrid. This was shown by the manipulation of the intracellular ATP/ADP ratio in the light using different effectors. Hybrids with an ATP/ADP ratio of 2.3 needed 54 s to round up completely; in the presence of antimycin (inhibition of both oxidative and light-dependent cyclic electron flow: ATP/ADP=1.1) or dibromothymoquinone (plastoquinone antagonist: ATP/ADP=1.0) the time for rounding up was slightly increased (64 s and 76 s respectively), whereas after preincubation with antimycin, dichlorophenyldimethylurea (inhibition of oxidative and light-dependent electron flow) or uncouplers (ATP/ADP=0.19-0.32) this process needed 128-153s for completion. These results are discussed in relation to the viability of electrically induced fusion products and to energy-dependent events involved in the process of fusion.