Membrane stability

Pressure-sensitive multivesicular liposomes as a smart drug-delivery system for high-altitude pulmonary edema

Changes in bodily fluid pressures, such as pulmonary artery pressure, play key roles in high-altitude pulmonary edema (HAPE) and other disorders. Smart delivery systems releasing a drug in response to these pressures might facilitate early medical interventions. However, pressure-responsive delivery systems are unavailable. We here constructed hydrostatic pressure-sensitive multivesicular liposomes (PSMVLs) based on the incomplete filling of the internal vesicle space with neutral lipids. These liposomes were loaded with amlodipine besylate (AB), a next-generation calcium channel inhibitor, to treat HAPE on time. AB-loaded PSMVLs (AB-PSMVLs) were destroyed, and AB was released through treatment under hydrostatic pressure of at least 25 mmHg. At 25 mmHg, which is the minimum pulmonary artery pressure value in HAPE, 38.8% of AB was released within 1 h. In a mouse HAPE model, AB-PSMVLs concentrated in the lung and released AB to diffuse into the vascular wall. Intravenously injected AB-PSMVLs before HAPE modeling resulted in a stronger protection of lung tissues and respiratory function and lower occurrence of pulmonary edema than treatment with free drug or non-pressure-sensitive AB-loaded liposomes. This study offers a new strategy for developing smart drug delivery systems that respond to changes in bodily fluid pressures.

A Ready-to-Use Metal-Supported Bilayer Lipid Membrane Biosensor for the Detection of Phenol in Water

This work presents a novel metal-supported bilayer lipid membrane (BLM) biosensor built on tyrosinase to quantitate phenol. The detection strategy is based on the enzyme-analyte initial association and not the commonly adopted monitoring of the redox cascade reactions; such an approach has not been proposed in the literature to date and offers many advantages for environmental monitoring with regard to sensitivity, selectivity, reliability and assay simplicity. The phenol sensor developed herein showed good analytical and operational characteristics: the detection limit (signal-to-noise ratio = 3) was 1.24 pg/mL and the sensitivity was 33.45 nA per pg/mL phenol concentration. The shelf life of the tyrosinase sensor was 12 h and the lifetime (in consecutive assays) was 8 h. The sensor was reversible with bathing at pH 8.5 and could be used for eight assay runs in consecutive assays. The validation in real water samples showed that the sensor could reliably detect 2.5 ppb phenol in tap and river water and 6.1 ppb phenol in lake water, without sample pretreatment. The prospects and applicability of the proposed biosensor and the underlying technology are also discussed.

Centrifugation-induced release of ATP from red blood cells

Centrifugation is the primary preparation step for isolating red blood cells (RBCs) from whole blood, including for use in studies focused on transduction of adenosine triphosphate (ATP), an important vasodilatory signaling molecule. Despite the wide use of centrifugation, little work has focused on how the centrifugation itself affects release of ATP from RBCs prior to subsequent experimentation. Here we report that both the centrifugation force and duration have a pronounced impact on the concentration of ATP present in the packed RBCs following centrifugation. Multiple subsequent centrifugations yield extracellular ATP concentrations comparable to the amount released during the initial centrifugation, suggesting this effect is cumulative. Pairwise measurements of hemoglobin and ATP suggest the presence of ATP is primarily due to an increase in centrifugation-induced hemolysis. These results indicate that common centrifugation parameters, within the ranges explored here, can release ATP in quantities comparable to the low end of the range of values measured in typical ATP transduction experiments, potentially complicating experimental interpretation of those results.

Pro-apoptotic cBid and Bax exhibit distinct membrane remodeling activities: An AFM study

Bcl-2 proteins are key regulators of the mitochondrial outer membrane (MOM) permeabilization that mediates apoptosis. During apoptosis, Bid is cleaved (cBid) and translocates to the MOM, where it activates Bax. Bax then oligomerizes and induces MOM permeabilization. However, little is known about how these proteins affect membrane organization aside from pore formation. In previous studies, we have shown that both cBid and Bax are able to remodel membranes and stabilize curvature. Here, we dissected the independent effects of Bax and cBid on supported lipid structures mimicking the mitochondrial composition by means of atomic force spectroscopy. We show that cBid did not permeabilize the membrane but lowered the membrane breakthrough force. On the other hand, Bax effects were dependent on its oligomeric state. Monomeric Bax did not affect the membrane properties. In contrast, oligomeric Bax lowered the breakthrough force of the membrane, which in the context of pore formation, implies a lowering of the line tension at the edge of the pore.

Lipid membranes in external electric fields: kinetics of large pore formation causing rupture

About 40 years ago, Helfrich introduced an elastic model to explain shapes and shape transitions of cells (Z Naturforsch C, 1973; 28:693). This seminal article stimulated numerous theoretical as well as experimental investigations and created new research fields. In particular, the predictive power of his approach was demonstrated in a large variety of lipid model system. Here in this review, we focus on the development with respect to planar lipid membranes in external electric fields. Stimulated by the early work of Helfrich on electric field forces acting on liposomes, we extended his early approach to understand the kinetics of lipid membrane rupture. First, we revisit the main forces determining the kinetics of membrane rupture followed by an overview on various experiments. Knowledge on the kinetics of defect formation may help to design stable membranes or serve for novel mechanism for controlled release.

Electropermeabilization of the cell membrane

Membrane electropermeabilization is the observation that the permeability of a cell membrane can be transiently increased when a micro-millisecond external electric field pulse is applied on a cell suspension or on a tissue. Applicative aspects for the transfer of foreign molecules (macromolecules) into the cytoplasm are routinely used. But only a limited knowledge about what is really occurring in the cell and its membranes at the molecular levels is available. This chapter is a critical attempt to report the present state of the art and to point out some of the still open problems. The experimental facts associated to membrane electropermeabilization are firstly reported. They are valid on biological and model systems. Secondly, soft matter approaches give access to the bioelectrochemical description of the thermodynamical constraints supporting the destabilization of simplified models of the biological membrane. It is indeed described as a thin dielectric leaflet, where a molecular transport takes place by electrophoresis and then diffusion. This naïve approach is due to the lack of details on the structural aspects affecting the living systems as shown in a third part. Membranes are part of the cell machinery. The critical property of cells as being an open system from the thermodynamical point of view is almost never present. Computer simulations are now contributing to our knowledge on electropermeabilization. The last part of this chapter is a (very) critical report of all the efforts that have been performed. The final conclusion remains that we still do not know all the details on the reversible structural and dynamical alterations of the cell membrane (and cytoplasm) supporting its electropermeabilization. We have a long way in basic and translational researches to reach a pertinent description.

A methodological combined framework for roadmapping biosensor research: a fault tree analysis approach within a strategic technology evaluation frame

Biosensor technology began in the 1960s to revolutionize instrumentation and measurement. Despite the glucose sensor market success that revolutionized medical diagnostics, and artificial pancreas promise currently the approval stage, the industry is reluctant to capitalize on other relevant university-produced knowledge and innovation. On the other hand, the scientific literature is extensive and persisting, while the number of university-hosted biosensor groups is growing. Considering the limited marketability of biosensors compared to the available research output, the biosensor field has been used by the present authors as a suitable paradigm for developing a methodological combined framework for "roadmapping" university research output in this discipline. This framework adopts the basic principles of the Analytic Hierarchy Process (AHP), replacing the lower level of technology alternatives with internal barriers (drawbacks, limitations, disadvantages), modeled through fault tree analysis (FTA) relying on fuzzy reasoning to count for uncertainty. The proposed methodology is validated retrospectively using ion selective field effect transistor (ISFET) - based biosensors as a case example, and then implemented prospectively membrane biosensors, putting an emphasis on the manufacturability issues. The analysis performed the trajectory of membrane platforms differently than the available market roadmaps that, considering the vast industrial experience in tailoring and handling crystallic forms, suggest the technology path of biomimetic and synthetic materials. The results presented herein indicate that future trajectories lie along with nanotechnology, and especially nanofabrication and nano-bioinformatics, and focused, more on the science-path, that is, on controlling the natural process of self-assembly and the thermodynamics of bioelement-lipid interaction. This retained the nature-derived sensitivity of the biosensor platform, pointing out the differences between the scope of academic research and the market viewpoint.

Cholesterol-dependent nanomechanical stability of phase-segregated multicomponent lipid bilayers

Cholesterol is involved in endocytosis, exocytosis, and the assembly of sphingolipid/cholesterol-enriched domains, as has been demonstrated in both model membranes and living cells. In this work, we explored the influence of different cholesterol levels (5-40 mol%) on the morphology and nanomechanical stability of phase-segregated lipid bilayers consisting of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/Chol) by means of atomic force microscopy (AFM) imaging and force mapping. Breakthrough forces were consistently higher in the SM/Chol-enriched liquid-ordered domains (Lo) than in the DOPC-enriched fluid-disordered phase (Ld) at a series of loading rates. We also report the activation energies (DeltaEa) for the formation of an AFM-tip-induced fracture, calculated by a model for the rupture of molecular thin films. The obtained DeltaEa values agree remarkably well with reported values for fusion-related processes using other techniques. Furthermore, we observed that within the Chol range studied, the lateral organization of bilayers can be categorized into three distinct groups. The results are rationalized by fracture nanomechanics of a ternary phospholipid/sphingolipid/cholesterol mixture using correlated AFM-based imaging and force mapping, which demonstrates the influence of a wide range of cholesterol content on the morphology and nanomechanical stability of model bilayers. This provides fundamental insights into the role of cholesterol in the formation and stability of sphingolipid/cholesterol-enriched domains, as well as in membrane fusion.

Electroporation: an arsenal of application

Electroporation is a way to induce nanometersized membrane pore for exogenous substances delivery into cytoplasm using an artificial electric field. Now it was widely used for molecules transfer especially in molecular experiments and genetic aspects. In recent years, modern electroporation on the embryo was developed, whose most important point is that it adopts low energy and rectangular pulse that could obtain high transfection efficiency and low damage to the embryo. This paper reviewed on the pool of application: from lab works to human clinical treatments.

Pore formation in a lipid bilayer under a tension ramp: modeling the distribution of rupture tensions

The rupture of fluid membrane vesicles with a steady ramp of micropipette suction has been shown to produce a distribution of breakage tensions, with a mean that rises rapidly with tension rate. Starting from a lattice model that incorporates the essential features of the lipid bilayers held together with hydrophobic forces, and developing it to handle varying tension rates, we reproduce the main features of the experimental results. In essence, we show that the rupture kinetics are driven by the nucleation and growth of pores, with two limiting kinetics-growth-limited and nucleation-limited. The model has been extended to address the role of peptides in solution that can adsorb and insert themselves into the bilayer. At concentrations below those required to spontaneously rupture the membrane, the effect of the peptides is to lower the rupture tensions systematically for all tension rates.

Simulation of electroporated cell by chronopotentiometry

Chronopotentiometry on planar lipid bilayer (BLM) is proposed as a method for modeling the electrical phenomena in electroporated cell. Two techniques are discussed: constant-current and linear-current chronopotentiometry. It is proposed that the constant-current chronopotentiometry may provide basis for modeling the electroporated cell shortly after the removal of the electric field, when activity of cellular pumps counteracts ionic fluxes through the electropore and ionic channels. The linear-current method can be considered for modeling the cell in the later stage after electroporation, when energetical resources of the cell are gradually getting exhausted and the activity of pumps decreases. Based on this idea, it may be postulated that the electropore in the cell has fluctuating dynamics whose stochastic characteristics, similarly as biological channels, shows 1/f noise. The model implies that the fluctuations would disappear leaving the electropore with a constant resistance when efficiency of the pumps becomes very small. The results of chronopotentiometry also may suggest that opening time, conductivity and selectivity of the electropore can be controlled by the cell environment or membrane composition.

PS exposure increases the susceptibility of cells to fusion with DOTAP liposomes

Cationic liposomes are used as efficient carriers for gene delivery into mammalian cells due to their ability to bind nucleic acids, adsorb onto the cell surface and fuse with negatively charged membranes. This last property enables the release and escape of their cargo from endosomal compartments. The efficiency of this fusion mainly depends on the surface charge of the target membranes. Here, we report that cells of two different lines, epithelial adenocarcinoma HeLa and lymphocytic leukemia Jurkat T, which externalize PS, are more susceptible to fusion with DOTAP liposomes than control cells. We compared the ability to undergo fusion of untreated and apoptotic cells. Apoptosis was induced by various pro-apoptotic agents and treatments, namely: incubation in the presence of MnCl(2), cytostatic drugs fludarabine and mitoxantrone, staurosporine and serum depletion in the case of HeLa cells. Jurkat T cells were treated similarly except apoptosis was additionally induced by incubation in the presence of 4% EtOH. Epithelial cells fused with the highest efficiencies of lipid mixing, when pretreated with staurosporine. Jurkat T cells were less susceptible to fusion, but they also displayed an increase in fusion efficiency after the induction of apoptosis. Alternatively, we treated the cells with metabolic inhibitors causing ATP-depletion in order to inactivate aminophospholipid translocase. After ATP-depletion, HeLa and Jurkat T cells fused with DOTAP liposomes with higher efficiencies than control cells. Our conclusion is that the lipid asymmetry of natural membranes may limit fusion with cationic liposomes.

Pore formation in fluctuating membranes

We study the nucleation of a single pore in a fluctuating lipid membrane, specifically taking into account the membrane fluctuations, as well as the shape fluctuations of the pore. For large enough pores, the nucleation free energy is well-described by shifts in the effective membrane surface tension and the pore line tension. Using our framework, we derive the stability criteria for the various pore formation regimes. In addition to the well-known large-tension regime from the classical nucleation theory of pores, we also find a low-tension regime in which the effective line and surface tensions can change sign from their bare values. The latter scenario takes place at sufficiently high temperatures, where the opening of a stable pore of finite size is entropically favorable.

Influence of cholesterol on electroporation of bilayer lipid membranes: chronopotentiometric studies

This paper presents the results of constant-current (chronopotentiometric) measurements of the egg yolk phosphatidylcholine (PC) bilayer membrane without and with cholesterol. The experiments were performed on planar bilayer lipid membrane (BLM) formed by the Mueller-Rudin method. It is demonstrated that the constant-intensity current flow through bilayer membranes generated fluctuating pores in their structure. The presence of cholesterol in the membrane caused an increase in the value of the breakdown potential. It is postulated that greater stability of the bilayer with cholesterol can result from an increased critical pore radius (at which the bilayer would undergo irreversible rupture). This confirms that cholesterol has a stabilizing effect on BLM. Besides, our results suggest that addition of cholesterol causes shift in the distribution of pore conductance towards a smaller value. It is suggested that this can be connected with the phenomenon of domain formation in the membranes containing high concentration of cholesterol. Moreover, it is shown that chronopotentiometry with programmable current intensity is a promising method for observation of the membrane recovery process.

Lattice model for the kinetics of rupture of fluid bilayer membranes

We have constructed a model for the kinetics of rupture of membranes under tension, applying physical principles relevant to lipid bilayers held together by hydrophobic interactions. The membrane is characterized by the bulk compressibility (for expansion) K, the thickness 2h(t) of the hydrophobic part of the bilayer, the hydrophobicity sigma, and a parameter gamma characterizing the tail rigidity of the lipids. The model is a lattice model which incorporates strain relaxation, and considers the nucleation of pores at constant area, constant temperature, and constant particle number. The particle number is conserved by allowing multiple occupancy of the sites. An equilibrium "phase diagram" is constructed as a function of temperature and strain with the total pore surface and distribution as the order parameters. A first-order rupture line is found with increasing tension, and a continuous increase in protopore concentration with rising temperature till instability. The model explains current results on saturated and unsaturated phosphatidylcholine lipid bilayers and thicker artificial bilayers made of diblock copolymers. Pore size distributions are presented for various values of area expansion and temperature, and the fractal dimension of the pore edge is evaluated.

Inactivation of bacteria in seawater by low-amperage electric current

Seawater used in mariculture has been suspected of being a potential source of infection. In this study, the lethal effects of low-amperage electric treatment on microorganisms were examined in natural seawater and in seawater inoculated with Vibrio parahaemolyticus. In both cases, bacteria including V. parahaemolyticus in seawater were completely eliminated in 100 ms by a 0.5-A, 12-V direct current. Electron microscopic investigation of the electrically treated bacteria revealed substantial structural damage at the cellular level. In conclusion, our results indicate that low-amperage electric treatment is effective for rapid inactivation of microorganisms in seawater.

Rupture of molecular thin films observed in atomic force microscopy. I. Theory

In atomic force microscope studies of molecular thin films, a defined jump of the tip through the film is often observed once a certain threshold force has been exceeded. Here, we present a theory to describe this film rupture and to relate microscopic parameters to measurable quantities. We assume that the tip has to overcome an activation energy before the film ruptures. A universal relation between the force dependence of the activation energy and the approaching velocity of the tip is derived. Two complementary models for calculating the activation energy are presented: a continuum nucleation model and a discrete molecular model. Both models predict a narrow distribution of yield forces in agreement with experimental results.

Phagocytosis by Acanthamoeba castellanii: ionic strength dependence of the probability of cell attachment; ingestion and contact seam morphology

The phagocytosis of glutaraldehyde-fixed horse erythrocytes by Acanthamoeba castellanii has been examined in iso-osmolal phosphate buffered saline/sucrose suspending phases of ionic strength, I, ranging from 0.17 to 0.0017. The erythrocytes were exposed, at a ratio of 15:1, to 5x10(6) amoeba in 0.2 ml volumes. The average number of erythrocytes forming a contact with an amoeba over 30 min (T(30)) was well described by T(30)=5.2 exp(-0.112xI(-0.5)). The index of the exponential 'probability of attachment' term may also be expressed in terms of either surface potential (psi(0)) or the Debye length (kappa(-1)). The probability term is formally similar to a Bolzmann factor. Electron microscopy showed that contact spreading of the amoeba over the erythrocyte took place by formation of discrete contacts and that the lateral separation distance between contacts was 0.66, 1.36 and 1.59 &mgr;m for ionic strengths 0.17, 0.052 and 0.0017, respectively. The direction of change in lateral contact separation distance was consistent with published changes in focal contact separation when amoeba move over glass or when human erythrocyte-erythrocyte adhesion occurs in different ionic strength media. The direction was also consistent with interfacial instability theory predictions for the dependence of localised membrane contact formation on interaction potential. The proportion of attached cells that were subsequently ingested correlated more strongly with the number of contacts formed along the cell-cell contact region (seam) than with the seam length at different ionic strengths.

Electrofusion: a biophysical modification of cell membrane and a mechanism in exocytosis

The molecular bases of the exocytosis process remain poorly known. Many proteins have been recognized to play key roles in the machinery. Their functions are well characterized in the specificity of the docking processes. Forces involved in the merging of the two partners must take into account the physics of membrane interfaces. The target membrane and the vesicle are both electrically charged interfaces. Strong electrostatic fields are triggered when they are brought in close neighborhood. These fields are high enough to induce an electropermeabilisation process. It is now well known that when applied on a cell, an external field induces a modulation of the transmembrane potential difference. When high enough the transmembrane potential may induce a membrane destabilisation. This results in a free exchange of polar molecules across well defined parts of the cell surface. Furthermore, when permeabilization is present on two cells, if those parts of the cell surfaces are brought in close contact, membrane merging occurs spontaneously. Cell fusion results from this membrane coalescence. The similarity with what is taking place in exocytosis is striking. The present review describes the state-of-the-art in the knowledge on electrofusion. It is emphasized that it results from electropermeabilisation and not from a direct effect of the external field. A local destabilisation of the vesicle membrane results from electrostatic interactions while keeping unaffected its viability. Such processes appear relevant for what takes place during exocytosis.

Ionic strength dependence of localized contact formation between membranes: nonlinear theory and experiment

Erythrocyte membrane surface or suspending phase properties can be experimentally modified to give either spatially periodic local contacts or continuous contact along the seams of interacting membranes. Here, for cells suspended in a solution of the uncharged polysaccharide dextran, the average lateral separation between localized contacts in spatially periodic seams at eight ionic strengths, decreasing from 0.15 to 0.065, increased from 0.65 to 3.4 micrometers. The interacting membranes and intermembrane aqueous layer were modeled as a fluid film, submitted to a disjoining pressure, responding to a displacement perturbation either through wave growth resulting in spatially periodic contacts or in perturbation decay, to give a plane continuous film. Measured changes of lateral contact separations with ionic strength change were quantitatively consistent with analytical predictions of linear theory for an instability mechanism dependent on the membrane bending modulus. Introduction of a nonlinear approach established the consequences of the changing interaction potential experienced by different parts of the membrane as the disturbance grew. Numerical solutions of the full nonlinear governing equations correctly identified the ionic strength at which the bifurcation from continuous seam to a stationary periodic contact pattern occurred and showed a decrease in lateral contact and wave crest separation with increasing ionic strength. The nonlinear approach has the potential to recognize the role of nonspecific interactions in initiating the localized approach of membranes, and then incorporate the contribution of specific molecular interactions, of too short a range to influence the beginning of perturbation growth. This new approach can be applied to other biological processes such as neural cell adhesion, phagocytosis, and the acrosome reaction.

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.

Electropermeabilization of cell membranes

A position dependent modulation of the membrane potential difference is induced when an electric field is applied to a cell. When cells are submitted to short lived electric field pulses with an overcritical intensity, a local membrane alteration is induced, which may reseal. Its molecular definition remains unknown. A free exchange of hydrophilic molecules takes place across the membrane. A leakage of cytosolic metabolites is present. However, a loading of polar drugs into the cytoplasm is obtained. A short description of the processes affecting the cell membrane organization is given. Lipids appear as the primary target of the field effect as in the case of liposomes. Nevertheless membrane proteins appear to be affected by a direct or by a back effect. The permeabilized state is long lived. The cell metabolism plays indeed a critical role in the recovery. The cell viability can be nevertheless preserved.

Alteration of cell membrane by stress waves in vitro

Experiments on the biological effects of laser-induced stress waves indicate that there is a transient increase in the permeability of the cell membrane. A cell viability assay (propidium iodide exclusion) shows that mouse breast sarcoma cells are viable after a stress wave. The kinetics of this transient membrane permeability are measured using time-resolved fluorescence imaging. The efflux of a membrane-impermeable fluorescent probe (calcein) following the application of a 300-bar stress wave implies that there is an increase in the membrane permeability. This efflux ceases within 80 s after a stress wave, suggesting that the membrane is no longer permeable to the fluorescent probe. Fitting the observed kinetics to a simple diffusion model yields an average initial diffusion constant of 2.2 +/- 1.3 x 10(-7) cm2/s for mouse breast sarcoma cells following the application of a laser-induced stress wave.

Intermembrane lipid transfer during Trypanosoma cruzi-induced erythrocyte membrane destabilization

The ability of Trypanosoma cruzi to induce erythrocyte membrane destabilization in vitro was studied. Epimastigote forms adhered to human erythrocytes and caused fusion or lysis of the red cells, depending on the conditions of the interaction. Red cells were fused in the presence of calcium, while haemolysis was induced in the absence of the cation. Dextran 60 C facilitated fusion but delayed lysis. Optimum pH and temperature for fusion were 7.4 and 37 degrees C, respectively. Lipid alterations were produced in the plasma membrane of the red cell during the interaction with the parasite. A Ca(2+)-independent increase of lysophospholipids and free fatty acids was common to both the lysis and fusion processes. A relative increase of 1,2-diacylglycerides was unique to the fusion process and these changes were dependent on Ca2+. The transfer of free fatty acids and lysophospholipids from T. cruzi to erythrocyte membranes was demonstrated using parasites pre-labelled with radioactive phospholipids. Pre-treatment of parasites with exogenous phospholipase A2 abolished the fusogenicity, while lysis was increased. Neither fusion nor haemolysis occurred when the parasites were pre-treated with fatty acid free albumin, phospholipase A2 inhibitors or when these compounds were present in the medium during the parasite-erythrocyte interaction. Our results suggest that T. cruzi induces erythrocyte membrane destabilization in vitro by transfer of lipid material in a calcium independent manner and that this ion is necessary for other membrane alterations that lead to erythrocyte fusion.

Electrical properties of cell pellets and cell electrofusion in a centrifuge

A new approach is proposed for studying cell deformability by centrifugal force, electrical properties of cell membranes in a high electric field, and for performing efficient cell electrofusion. Suspensions of cells (L929 and four other cell types examined) are centrifuged in special chambers, thus forming compact cell pellets in the gap between the electrodes. The setup allows measurement of the pellet resistance and also the high-voltage pulse application during centrifugation. The pellet resistance increases sharply with the centripetal acceleration, which correlates with reduction of the cell pellet porosity due to cell compression and deformation. Experiments with cells pretreated with cytochalasin B or colcemid showed that cell deformability depends significantly on the state of cytoskeleton. When the voltage applied to the cell pellet exceeds a 'critical' value, electrical breakdown (poration) of cell membranes occurs. This is seen as a deflection in the I(V) curve for the cell pellet. The electropores formed during the breakdown reseal in several stages: the fastest takes 0.5-1 ms while the whole process completes in minutes. A novel effect of colloid-osmotic compression of cell pellets after electric cell permeabilization is described. Supercritical pulse application to the cell pellet during intensive centrifugation leads to massive cell fusion. The fusion index grows with the increase of centripetal acceleration, and drops drastically when the pulse is applied after the centrifuge is stopped. The colloid-osmotic pellet compression enhances the fusion efficiency. No fusion occurs when cells are brought in contact after the pulse treatment. The data suggest that tight intermembrane contact formed prior to pulse application is a prerequisite condition for efficient cell electrofusion. The capacities of the technique proposed and the mechanism of membrane electrofusion are discussed.

Tension-stabilized pores in giant vesicles: determination of pore size and pore line tension

We present the first observations of giant, long-existing, stabilized pores in vesicle membranes. Using a new experimental technique for studying the electro-permeabilization of lipid membranes, giant liposomes (from 25 to 56 microns in diameter) were subjected to single, square, electric pulses (duration 150 microseconds and electric field strength from 63 to 126 kV/m). The liposomes were held by a micropipet and small membrane tensions were created by controlling the pipet suction pressure. The liposomes were loaded with media having different refractive index from the outside solution, and, under these conditions, the formation of pores in the pressurized liposome could be visualized by the jet of inside solution that flowed out from the membrane pore. By adjusting the membrane tension, pores were kept open, and pore lifetimes could be varied from tenths of a second to several seconds. The pore size was determined from the volumetric flow in the pore region and the measured pressure differences across the bilayer. It was clear from the experiments that only one pore remained opened after the pulse. The estimated pore radii were on the order of one micrometer. The pores were in a quasi-stationary state and when they closed they did so spontaneously in a quick process (in milliseconds). The isotropic membrane tension was determined for the same measurements and from determinations of both pore size and dynamic membrane tension the pore line tension was found. The line tension of the pore region was determined for two lipid compositions, stearoyl-oleoylphosphatidylcholine and stearoyl-oleoylphosphatidylcholine with 50 mol% cholesterol, and the obtained values for single bilayers were (0.92 +/- 0.07) x 10(-11) N and (3.05 +/- 0.12) x 10(-11) N, respectively.

Influence of polymer concentration and molecular weight and of enzymic glycocalyx modification on erythrocyte interaction in dextran solutions

Erythrocytes adhere to each other when suspended in supra-threshold concentrations of dextran of molecular mass of 40 kD or greater. The plasma membranes are parallel to each other over the entire length of the contact seam at the lower effective polymer concentrations. When cells are pretreated with the proteolytic enzyme pronase or the sialidase neuraminidase the membranes are not parallel but make contact at spatially periodic locations along the membrane surface. Pronase induced reduction of cell electrophoretic mobility rapidly reaches a limiting value. Nevertheless, prolonged pre-exposure to enzyme leads to a continuing reduction in contact separations. This result taken with the observation that, for equal loss of electrophoretic mobility, a shorter contact separation results from pronase rather than neuraminidase pre-treatment implies that a non-electrostatic consequence of pronase pre-treatment dominates membrane interaction in the experimental regimes examined here. The average lateral contact separation for different enzyme regimes lay in the range 3.3 microns to a limiting lower value of about 0.7 micron. There was a good correlation between the logarithm of a contact separation index (the approach of separation distance to its limiting value) against the logarithm of a derived index related to net attractive interaction for a wide range of experimental conditions. Treatments which increased attraction or decreased repulsion (e.g. increased dextrans concentration or enzyme pre-treatment) lead to shorter lateral contact separation. This result is qualitatively consistent with the predicted behaviour for the dominant wavelength arising from interfacial instability of a thin aqueous film between adjacent membranes.

The use of semifluorinated alkanes in blood-substitutes

Semifluorinated alkanes are useful for blood-substitutes in two different ways: as co-surfactant to stabilize emulsions with perfluorocarbons as oxygen-carrier and as oxygen-carrier instead of perfluorocarbons. Semifluorinated alkanes act as co-surfactants in low concentrations because they are enriched at the interface perfluorodecalin/water. Emulsions with semifluorinated alkanes dissolve about the same amount of oxygen as emulsions with perfluorocarbons. The stability of these emulsions depends on the nature of the alkyl and the perfluoroalkyl chain. Semifluorinated alkanes do not eliminate hydrogen fluoride under clean-up conditions of perfluorocarbons. According to toxicity tests against human carcinoma cells semifluorinated alkanes with alkyl chains are harmless in the examined range from C6 to C10.

Kinetics of pore size during irreversible electrical breakdown of lipid bilayer membranes

The kinetics of pore formation followed by mechanical rupture of lipid bilayer membranes were investigated in detail by using the charge-pulse method. Membranes of various compositions were charged to a sufficiently high voltage to induce mechanical breakdown. The subsequent decrease of membrane voltage was used to calculate the conductance. During mechanical breakdown, which was probably caused by the widening of one single pore, the membrane conductance was a linear and not exponential function of time after the initial starting process. In a large number of experiments using various lipids and electrolytes, the characteristic opening process of the pore turned out to be independent of the actual membrane potential and electrolyte concentration. Our theoretical analysis of the pore formation suggested that the voltage-induced irreversible breakdown is due to a decrease in edge energy when the pore had formed. After initiation of the pore, the electrical contribution to surface tension is negligible. The time course of the increase of pore size shows that our model of the irreversible breakdown is in good agreement with mechanical properties of membranes reported elsewhere.

Kinetics and mechanism of cell membrane electrofusion

A new quantitative approach to study cell membrane electrofusion has been developed. Erythrocyte ghosts were brought into close contact using dielectrophoresis and then treated with one square or even exponentially decaying fusogenic pulse. Individual fusion events were followed by lateral diffusion of the fluorescent lipid analogue 1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) from originally labeled to unlabeled adjacent ghosts. It was found that ghost fusion can be described as a first-order rate process with corresponding rate constants; a true fusion rate constant, k(f), for the square waveform pulse and an effective fusion rate constant, k(ef), for the exponential pulse. Compared with the fusion yield, the fusion rate constants are more fundamental characteristics of the fusion process and have implications for its mechanisms. Values of k(f) for rabbit and human erythrocyte ghosts were obtained at different electric field strength and temperatures. Arrhenius k(f) plots revealed that the activation energy of ghost electrofusion is in the range of 6-10 kT. Measurements were also made with the rabbit erythrocyte ghosts exposed to 42 degrees C for 10 min (to disrupt the spectrin network) or 0.1-1.0 mM uranyl acetate (to stabilize the bilayer lipid matrix of membranes). A correlation between the dependence of the fusion and previously published pore-formation rate constants for all experimental conditions suggests that the cell membrane electrofusion process involve pores formed during reversible electrical breakdown. A statistical analysis of fusion products (a) further supports the idea that electrofusion is a stochastic process and (b) shows that the probability of ghost electrofusion is independent of the presence of Dil as a label as well as the number of fused ghosts.

The lateral separation of contacts on erythrocytes agglutinated by polylysine

The form of contact seam (whether a continuous parallel seam or membranes in spatially periodic contact) has been characterized for normal and for neuraminidase pretreated human erythrocytes following adhesion in solutions of polylysine in the molecular mass range 10-225 kDa at concentrations from 0.5 to 1.0 mg/mL. The adhesion contact seam was spatially periodic for all normal control cells in polylysine. The lateral separation of contacts decreased from 1.6 to 0.8 microns as the concentration of 225 kDa polylysine was increased threefold from the adhesion threshold value. The separation distance did not change further even at high polymer concentrations that increased the electrophoretic velocity to positive values over twice the modulus of the velocity of control cells. The probability of cell adhesion decreased at these high polymer concentrations. The lateral contact separation increased and cell adhesion decreased for cells pretreated with neuraminidase. Cell adhesion did not occur when neuraminidase reduced the cell electrophoretic velocity modulus by 30%. Following neuraminidase pretreatments that allowed a small amount of adhesion, the cell contact seam was continuous rather than spatially periodic. The results show that a procedure that increases (e.g., polymer concentration increase) or decreases (e.g., enzyme removal of polycation crosslinking site) attraction leads to shorter (to a limiting value) or longer lateral contact separation, respectively.