Surface exposure of phosphatidylserine is associated with the swelling and osmotically-induced fusion of human erythrocytes in the presence of Ca2+

Efficient encapsulation of functional proteins into erythrocytes by controlled shear-mediated membrane deformation

Red blood cells (RBCs) are attractive carriers of biomolecular payloads due to their biocompatibility and the ability to shelter their encapsulated cargo. Commonly employed strategies to encapsulate payloads into RBCs, such as hypotonic shock, membrane fusion or electroporation, often suffer from low throughput and unrecoverable membrane impairment. This work describes an investigation of a method to encapsulate protein payloads into RBCs by controlling membrane deformation either transiently or extendedly in a microfluidic channel. Under the optimized conditions, the loading efficiency of enhanced green fluorescent protein into mouse RBCs increased was about 2.5- and 4-fold compared to that with osmotic entrapment using transient and extended deformation, respectively. Significantly, mouse RBCs loaded with human arginase exhibit higher enzymatic activity and membrane integrity compared to their counterparts loaded by osmotic entrapment. These features together with the fact that this shear-mediated encapsulation strategy allows loading with physiological buffers highlight the key advantages of this approach compared to traditional osmotic entrapment.

The effect of PS content on the ability of natural membranes to fuse with positively charged liposomes and lipoplexes

Supramolecular aggregates containing cationic lipids have been widely used as transfection mediators due to their ability to interact with negatively charged DNA molecules and biological membranes. First steps of the process leading to transfection are partly electrostatic, partly hydrophobic interactions of liposomes/lipoplexes with cell and/or endosomal membrane. Negatively charged compounds of biological membranes, namely glycolipids, glycoproteins and phosphatidylserine (PS), are responsible for such events as adsorption, hemifusion, fusion, poration and destabilization of natural membranes upon contact with cationic liposomes/lipoplexes. The present communication describes the dependence of interaction of cationic liposomes with natural and artificial membranes on the negative charge of the target membrane, charges which in most cases were generated by charging the PS content or its exposure. The model for the target membranes were liposomes of variable content of PS or PG (phosphatidylglycerol) and erythrocyte membranes in which the PS and other anionic compound content/exposure was modified in several ways. Membranes of increased anionic phospholipid content displayed increased fusion with DOTAP (1,2-dioleoyl-3-trimethylammoniumpropane) liposomes, while erythrocyte membranes partly depleted of glycocalix, its sialic acid, in particular, showed a decreased fusion ability. The role of the anionic component is also supported by the fact that erythrocyte membrane inside-out vesicles fused easily with cationic liposomes. The data obtained on erythrocyte ghosts of normal and disrupted asymmetry, in particular, those obtained in the presence of Ca(2+), indicate the role of lipid flip-flop movement catalyzed by scramblase. The ATP-depletion of erythrocytes also induced an increased sensitivity to hemoglobin leakage upon interactions with DOTAP liposomes. Calcein leakage from anionic liposomes incubated with DOTAP liposomes was also dependent on surface charge of the target membranes. In all experiments with the asymmetric membranes the fusion level markedly increased with an increase of temperature, which supports the role of membrane lipid mobility. The decrease in positive charge by binding of plasmid DNA and the increase in ionic strength decreased the ability of DOTAP liposomes/lipoplexes to fuse with erythrocyte ghosts. Lower pH promotes fusion between erythrocyte ghosts and DOTAP liposomes and lipoplexes. The obtained results indicate that electrostatic interactions together with increased mobility of membrane lipids and susceptibility to form structures of negative curvature play a major role in the fusion of DOTAP liposomes with natural and artificial membranes.

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.

Membrane effects of trifluoperazine, dibucaine and praziquantel on human erythrocytes

Trifluoperazine (TFP) is a potent antipsychotic agent, dibucaine (DBC) is a local anaesthetic and praziquantel (PZQ) is a highly effective agent against schistosomiasis. The present work was conducted to (i) investigate the cytotoxic effects of TFP, DBC and PZQ on human erythrocyte membranes; and (ii) compare the alterations induced by the cationic drugs (TFP and DBC) with those induced by the uncharged compound (PZQ), in an attempt to have a better insight on the pathways of each drug-membrane interaction. The erythrocyte morphological alterations induced by sublytic concentrations of TFP, DBC and PZQ were evaluated by scanning electron microscopy and expressed quantitatively by the morphological index. Haemolysis and release of membrane lipids (phospholipids and cholesterol) produced by selected concentrations of TFP, DBC and PZQ, were compared with those resulting from the corresponding triple concentrations of each drug. Our results showed that the uncharged molecule of PZQ induces the same morphological alterations (stomatocytosis) as the cationic drugs TFP and DBC. Haemolysis was shown to vary with the drug used and to be concentration-dependent, with values approximately 10-fold more elevated for TFP and DBC than for PZQ, which revealed a maximum of 6% haemolysis for the highest concentration tested. Different concentration-response curves were obtained for lipid elution, although the profiles of cholesterol and phospholipids released were similar for all drugs. Nevertheless, at a fixed rate of 50% haemolysis, TFP induced a approximately 2-fold increment in the elution of cholesterol when compared with that produced by DBC (P<0. 05). The different effects induced by TFP, DBC and PZQ on erythrocyte morphology, haemolysis and lipid exfoliation are related to the physical and chemical characteristics of each compound. These results suggest that distinct cell membrane interaction pathways lead to drug-specific mechanisms of cytotoxicity.

Phosphatidylserine-mediated adhesion of T-cells to endothelial cells

Phosphatidylserine was exposed on the surface of human umbilical endothelial cells (ECV304) a few minutes after adding thrombin in vitro, as monitored by prothrombinase assays with and without annexin V. Jurkat T cells adhered to the thrombin-treated cells. The adhesion was inhibited by annexin V, indicating that it was mediated by exposed phosphatidylserine on the endothelial cells.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.

Interactions between metal ions and poly(ethylene glycol) in the fusion of human erythrocytes

Diffusion of the fluorescent membrane probe, Dil-C16 (3), from labelled to unlabelled human erythrocytes has been employed to monitor hemi-fusion (membrane fusion) in monolayers of cells exposed to poly(ethylene glycol) (PEG). Diffusion of the cytoplasmic probe, 6-carboxyfluorescein, was used similarly to monitor cell fusion (cytoplasmic mixing). Hemi-fusion, which is normally seen when erythrocytes are exposed to dehydrating concentrations of commercial PEG 6000, did not occur when the PEG was pretreated with Chelex 100 resin to remove metal ions. Cytoplasmic mixing, which is normally observed when the dehydrated erythrocytes are substantially rehydrated, also failed to occur when both PEG 6000 and the rehydrating buffer had been treated with Chelex 100. The re-addition to Chelex-treated PEG of components removed by the resin, and the addition of 10 mu mM concentrations of La3+ or Al3+, restored its ability to induce hemi-fusion and cell fusion. Higher concentrations of several other metals, including Ca2+, were also effective. These observations show that metal ions are required for hemi-fusion with erythrocytes in the presence of PEG, and that dehydration alone is insufficient to induce hemi-fusion. Phosphatidylserine was apparently not accessible in erythrocytes treated with PEG 6000 until the cells were rehydrated. This indicates that metal ions do not assist the hemi-fusion of erythrocytes by forming trans complexes with surface phosphatidylserine when the cells are dehydrated by PEG.(ABSTRACT TRUNCATED AT 250 WORDS)

Human neonatal red blood cells. An experimental model for the study of calcium-activated potassium channel refractoriness

To study some characteristics of calcium-activated potassium channels free from plasma membrane lipid perturbations, neonatal red blood cells were selected. To this end, cells not previously treated (ATP depletion and/or reversible lysis) were exposed to the ionophore A23187 in a NO3 medium with calcium. The net efflux of potassium from the cells was studied. Preincubation in a medium devoid of potassium induced a significant decrease only in the rate constant of potassium flux. The data suggest that, in the absence of plasma membrane lipid impairment, changes in internal sites reactivity with calcium ions of channel proteins would be involved in the refractoriness. This would be at variance with channel density modifications.

Neonatal red blood cell lysis induced by hypertonic low ionic strength media

Human neonatal red cells (placental blood) incubated in hypertonic sucrose media showed a significative lytic process in a relatively short time interval. The addition of sodium chloride into the sucrose media reduced the extent of hemolysis. In contrast, the addition of calcium chloride enhanced the hemolysis in these red cells. Calcium-membrane components complex formation that destabilize membrane's bilayer structure would explain the calcium effect above mentioned (on account of the low ionic strength media used and exposed fixed negative charges) This study intends to clarify, in neonatal red cells, the relation between surface charges and cellular stability.

Divalent cations, phospholipid asymmetry and osmotic swelling in electrically-induced lysis, cell fusion and giant cell formation with human erythrocytes

We have previously reported that acidic phospholipids are exposed at the surface of human erythrocytes when the cells are subjected to electrical breakdown. It has now been shown that the prothrombinase assay, which was used previously for the determination of acidic phospholipids, is specific for phosphatidylserine under the conditions of our experiments. In the light of this finding, we have investigated and characterised factors that govern cell lysis, cell fusion, and the formation of giant cells induced by electrical breakdown with human erythrocytes in media of low ionic strength. Divalent cations (1.1 mM) protected the cells against haemolysis, in the order Mn2+ > Ca2+ > Ba2+ > Mg2+ >> Zn2+, whereas about 99% of the cells lysed immediately on breakdown in the presence of Na+ or K+ (2.1 mM), or Al3+ (0.95 mM). The lengths of pearl chains of fused erythrocytes formed was similarly greatest with Mn2+ and decreased progressively with Ba2+, Zn2+, Ca2+ and Mg2+. No cell fusion occurred with Na+, K+, or Al3+. It is suggested that interactions with phosphatidylserine, which is exposed at the cell surface by electrical breakdown, may enable Mn2+, Ba2+ and Ca2+ ions to inhibit cell lysis (via membrane resealing) and facilitate cell fusion. Following electrically-induced cell fusion, erythrocytes round-up into giant cells. It has previously been proposed that Ca2+ ions accelerate the rounding-up process. However, data are presented which show that, as with erythrocytes treated with Sendai virus, the formation of rounded, giant cells following cell fusion depends on the osmotic swelling properties of permeabilised erythrocytes. Osmotic swelling may also have induced any hemi-fused cells present to fuse completely. Zn2+ ions anomalously enabled erythrocytes to round-up very rapidly into giant cells following electrical breakdown. This phenomenon may result from an interaction of Zn2+ ions with cysteine groups in membrane proteins, which decreases the immediate loss of ions that occurs when erythrocytes are subjected to electrical breakdown in low-ionic-strength media.

Swelling-induced O2- generation in guinea-pig neutrophils

Without the addition of any exogenous stimuli, neutrophils generated O2- and then ceased in a reversible manner that correlated with cellular swelling and contraction. The nature of the possible mechanism responsible for this O2- generation was studied and compared with that observed in the triggering of stimulant-dependent O2- generation (respiratory burst). The swelling-induced O2- generation was inhibited by diphenyliodonium, and was independent of the functional distortion of mitochondrial and/or microsomal electron transport and xanthine oxidase. This suggested that such generation was involved in respiratory-burst oxidase activation; however, this generation was not accompanied by any new phosphorylation of the 47-kDa protein or of tyrosine proteins. Dihydrocytochalasin B potentiated the O2- generation. The cellular swelling produced a priming effect on the triggering of respiratory burst with different stimuli. Cellular contraction, conversely, suppressed the respiratory burst. The structural specificity of the swelling-induced plasma membrane modulation for the O2- generation was suggested by the finding that modulation of plasma membrane structures by various non-ionic detergents per se inhibited O2- generation. Lipophilic and positively-charged agents inhibited the generation and this inhibition was abrogated by negatively-charged, but not by non-ionic agents. Negatively-charged agents potentiated the O2- generation. These results suggest that both the interaction of the plasma membrane with the cytoskeleton and an increase in net negative charges at the plasma membrane play important role in evoking O2- generation; this is discussed and compared with the signal transduction reported previously for respiratory burst.

Effects of snake venom phospholipase A2 toxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis) on aminophospholipid asymmetry in rat cerebrocortical synaptosomes

The effects of snake venom phospholipase A2 (PLA2) toxins (beta-bungarotoxin, notexin) and PLA2 enzymes (Naja nigricollis, Naja naja atra) on aminophospholipid asymmetry in rat cerebrocortical synaptic plasma membranes (SPM) were examined. Incubation of intact synaptosomes with 2 mM 2,4,6-trinitrobenzene sulfonic acid (TNBS) for 40 min, under non-penetrating conditions, followed by SPM isolation, allowed us to calculate the percentage of phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the outer leaflet of the SPM, while incubation with disrupted synaptosomes provided total labeling values with the difference representing labeling of the inner leaflet. We found that 30% of the PE and 2% of the PS were in the outer leaflet, with 54% of the PE and 80% of the PS in the inner leaflet; 16% of the PE and 18% of the PS was inaccessible to TNBS. PLA2 toxins and enzymes increased in a concentration-dependent manner the percentage of PS and, to a lesser extent, the percentage of PE in the outer leaflet of the SPM, due to a redistribution from the inner to the outer leaflet. There was no correlation between the PLA2 enzymatic activities and the increased percentage of PS in the outer leaflet of the SPM induced by the PLA2 toxins and enzymes. Alteration of aminophospholipid asymmetry does not explain the greater presynaptic specificity and potencies of the PLA2 toxins as compared to the PLA2 enzymes, but may be associated with the increased acetylcholine release from synaptosomes induced by both the toxins and enzymes.

Rapid loss and restoration of lipid asymmetry by different pathways in resealed erythrocyte ghosts

The normal asymmetric distribution of phospholipids across the plasma membrane of erythrocytes can be abolished by lysing and resealing cells in the presence of Ca2+. In the present study, using flow cytometric analysis of the binding of merocyanine 540 to monitor transbilayer phospholipid distribution, Ca(2+)-induced loss of asymmetry is shown to be independent from the aminophospholipid translocase which catalyzes movement of normally internal phospholipids from the outer to the inner leaflet of the membrane. Loss of asymmetry is rapid, temperature-sensitive, and occurs in an uninterrupted, intact bilayer, rather than by diffusion of lipids through the hemolytic pore. Addition of ATP during lysis reverses loss of asymmetry, and this restoration can be blocked by inhibitors of the aminophospholipid translocase. These results suggest that the ATP-dependent translocase is essential for recovery of asymmetry, in turn suggesting that separate mechanisms mediate the loss and the recovery of lipid asymmetry in erythrocytes.

Relationships between the surface exposure of acidic phospholipids and cell fusion in erythrocytes subjected to electrical breakdown

The procoagulant activity of human erythrocytes, which provides a measure of the translocation of acidic phospholipids from the inner to the outer monolayer of the plasma membrane, has been compared with the percentage cell fusion in experiments on the effects of electrical breakdown pulses under differing experimental conditions. After treatment with breakdown pulses of 20 microseconds or longer (5 kV cm-1), the plasma membranes of erythrocytes in 250 mM sucrose exhibited an almost complete loss of asymmetry with respect to acidic phospholipids. As the breakdown voltage was increased from 2 to 5 kV cm-1 (with breakdown pulses of 99 microseconds), the surface exposure of acidic phospholipids and cell fusion increased approximately in parallel. Furthermore, with 99 microseconds pulses and a voltage of 3 kV cm-1, a decrease in the osmolarity from 250 to 150 mM of the sucrose medium was accompanied by an increase in both the surface exposure of acidic phospholipids and the extent of cell fusion. Breakdown pulses of 2-5 microseconds were sufficient to cause a marked loss of asymmetry, but no cell fusion was observed unless the pulse length was at least 20 microseconds. Kinetic experiments indicated that exposure of the acidic phospholipids at the cell surface was more likely to be due to a direct effect of the electric field pulses on plasma membrane structure than to secondary effects, such as the action of endogenous proteinases on the membrane skeleton. It seems possible that a localised, surface exposure of acidic phospholipids may contribute to the 'long-lived fusogenic state' (Sowers, A.E. (1986) J. Cell Biol. 102, 1358-1362) and the 'transient permeant structures' (Teissié, J. and Rols, M.P. (1986) Biochem. Biophys. Res. Commun. 140, 258-266) that enable cell fusion to occur when contact between cells is established after they have been subjected to field pulses. Our observations also provide circumstantial support for the concept that changes in the phospholipid asymmetry of membranes may be important in physiologically-occurring instances of biomembrane fusion.