Characterization of the inhibitory effect of PEG-lipid conjugates on the intracellular delivery of plasmid and antisense DNA mediated by cationic lipid liposomes

Poly(ethylene glycol)-lipid (PEG-lipid) conjugates are widely used in the field of liposomal drug delivery to provide a polymer coat that can confer favorable pharmacokinetic characteristics on particles in the circulation. More recently these lipids have been employed as an essential component in the self-assembly of cationic and neutral lipids with polynucleic acids to form small, stable lipid/DNA complexes that exhibit long circulation times in vivo and accumulate at sites of disease. However, the presence of a steric barrier lipid might be expected to inhibit the transfection activity of lipid/DNA complexes by reducing particle-membrane contact. In this study we examine what effect varying the size of the hydrophobic anchor and hydrophilic head group of PEG-lipids has on both gene and antisense delivery into cells in culture. Lipid/DNA complexes were made using unilamellar vesicles composed of 5 mole% PEG-lipids in combination with equimolar dioleoylphosphatidylethanolamine and the cationic lipid dioleyldimethylammonium chloride. Using HeLa and HepG2 cells we show that under the conditions employed PEG-lipids had a minimal effect on the binding and subsequent endocytosis of lipid/DNA complexes but they severely inhibited active gene transfer and the endosomal release of antisense oligodeoxynucleotides into the cytoplasm. Decreasing the size of the hydrophobic anchor or the size of the grafted hydrophilic PEG moiety enhanced DNA transfer by the complexes.

Membrane perturbation and the mechanism of lipid-mediated transfer of DNA into cells

Mixtures of cationic lipids and unsaturated phosphatidylethanolamine are used extensively for the intracellular delivery of plasmids and antisense oligodeoxynucleotides (ODN) in vitro. However, the mechanism by which cytoplasmic delivery of these large molecules is achieved remains unclear. The common hypothesis is that phosphatidylethanolamine promotes fusion of lipid/DNA particles with endosomal membranes, but this is inconsistent with several reports that have failed to correlate the fusogenic activity of a wide variety of lipid/DNA particles, measured by lipid mixing techniques, with their transfection activity. To address this issue further we have conducted a detailed analysis of the lipid mixing and DNA transfer activity of two, physically similar but functionally different, lipid/DNA particles composed of equimolar dioleyldimethylammonium chloride (DODAC) and dioleoylphosphatidylethanolamine (DOPE) or dioleoylphosphatidylcholine (DOPC). In combination with DODAC both phospholipids form almost identical lipid/DNA particles, they are endocytosed by cells to the same extent and each undergoes equivalent lipid mixing with cell membranes after uptake. Despite this, DNA transfer is 10- to 100-fold more extensive for lipid/DNA particles containing DOPE. We conclude that lipid mixing between lipid-based delivery systems and endosomal membranes must occur for DNA transfer to occur. However, the potency of different lipid/DNA particles correlates better with the ability of the exogenous lipid to disrupt membrane integrity.

Cationic lipids, phosphatidylethanolamine and the intracellular delivery of polymeric, nucleic acid-based drugs (review)

Polymeric, nucleic acid drugs must be protected from endogenous nucleases and delivered to target cell nuclei in order to maximize their activity. Constructs expressing therapeutic genes, antisense oligonucleotides and ribozymes can be delivered into cells by viral vectors, but concerns over safety and clinical utility have led to research into the development of alternative, non-viral delivery systems. Antisense and ribozyme drug development has focused upon modifications to the natural oligonucleotide chemistry which make the molecules resistant to nuclease degradation. These novel oligonucleotides cannot be generated by transgenes and must be administered in similar fashion to conventional drugs. However, oligonucleotides cannot cross membranes by passive diffusion and intracellular delivery for these drugs is very inefficient. Here we review the recent advances in forming lipid-DNA particles designed to mimic viral delivery of DNA. Most evidence now supports the hypothesis that lipid-DNA drugs enter target cells by endocytosis and disrupt the endosomal membrane, releasing nucleic acid into the cytoplasm. The mechanisms of particle formation and endosome disruption are not well understood. Cationic lipids are employed to provide an electrostatic interaction between the lipid carrier and polyanionic nucleic acids, and they are critical for efficient packaging of the drugs into a form suitable for systemic administration. However, their role in endosome disruption and other aspects of successful delivery leading to gene expression or inhibition of mRNA translation are less clear. We discuss the propensity of lipid-nucleic acid particles to undergo lipid mixing and fusion with adjacent membranes, and how phosphatidylethanolamine and other lipids may act as factors capable of disrupting bilayer structure and the endosomal pathway. Finally, we consider the challenges that remain in bringing nucleic acid based drugs into the realm of clinical reality.

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)

99mTc-labeling of lipid vesicles containing the lipophilic chelator PE-DTTA: effect of tin-to-chelate ratio, chelate content and surface polymer on labeling efficiency and biodistribution behavior

When injected intravenously, lipid vesicles labeled with 99mTc by means of a lipophilic chelator dipalmitoylphosphatidylethanolamine-diethylenetriaminetetraacetic acid (PE-DTTA) are rapidly accumulated by the mononuclear phagocytic system (MPS). By derivatizing the membrane surface with the lipid-polymer complex dipalmitoylphosphatidylethanolamine-monomethoxy polyethylene glycol 5000 (PE-MPEG), MPS uptake can be suppressed and loss of 99mTc label from the lipid surface reduced depending upon both PE-DTTA and PE-MPEG content. For vesicles containing 20% PE-DTTA, addition of PE-MPEG makes no difference to their rate of clearance from the circulation. However for vesicles containing 2% PE-DTTA, addition of more than 0.8% PE-MPEG increases circulation half-life, suppresses liver uptake and reduces renal clearance of the 99mTc label. The molar ratio of reducing agent (Sn) to chelator (PE-DTTA) is critical to efficient and reproducible labeling. For vesicles containing 2% PE-DTTA at a lipid concentration of 100 mM, a Sn/DTTA ratio of 0.35 gives close to optimal labeling. Variation in the Sn/DTTA ratio by a factor of two negatively impacts upon both labeling efficiency in vitro and circulation half-life in vivo. Potential uses for technetium-labeled lipid vesicles with extended circulation half-life are discussed.

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.

Polymer-derivatized technetium 99mTc-labeled liposomal blood pool agents for nuclear medicine applications

By using the lipophilic chelator, dipalmitoylphosphatidylethanolamine-diethylenetriaminetetraacetic acid (DPPE-DTTA), lipid vesicles may be prepared labeled on their surface with technetium 99m. When technetium-labeled vesicles were injected intravenously into rabbits, the half-life for clearance of the label from the circulation was less than 30 min. By further incorporating a synthetic phosphatidylethanolamine-monomethoxypoly(ethylene glycol) 5000 conjugate (PE-MPEG) the circulation half-life of the radiolabel was increased, liver uptake decreased and exchange of technetium from the vesicle surface suppressed, depending upon both the DPPE-DTTA and PE-MPEG content. For vesicles containing 20 mol% DPPE-DTTA, incorporation of PE-MPEG had no effect upon the circulation half-life of the radiolabel, however, for vesicles containing 2 mol% DPPE-DTTA, incorporation of more than 4 mol% PE-MPEG increased the circulation half-life of the label to more than 12 h. Less than 2 mol% PE-MPEG or 8 mol% ganglioside GM1 were, however, ineffective at increasing the circulation half-life of surface-bound technetium. It was shown that unilamellar lipid vesicles with DPPE-DTTA can be lyophilized in the presence of external sucrose, subsequently rehydrated with no change in vesicle size and labeled with technetium. It is suggested that polymer-derivatized, technetium-labeled vesicles may prove a useful substitute for technetium-labeled red blood cells as a vascular marker in various nuclear medicine procedures and that lyophilization/rehydration provides a possible route to realization of such vesicles in a pharmaceutically useful form.

Attachment of 99mTc to lipid vesicles containing the lipophilic chelate dipalmitoylphosphatidylethanolamine-DTTA

The binding of 99mTc to negatively-charged and neutral unilamellar lipid vesicles was investigated in the absence and presence of the ligand diethylenetriaminepentaacetic acid (DTPA) covalently attached to the headgroup of phosphatidylethanolamine at the surface of the membrane. Even in the absence of DTPA on the membrane surface, 99mTc reduced by Sn bound to the membrane surface but rapidly dissociated from the vesicles in the presence of plasma in vitro. When DTPA was present on the membrane surface, dissociation of 99mTc from the vesicle surface in plasma was much reduced. The dissociation of 99mTc from the surface of negatively-charged vesicles was less than for neutral vesicles in the absence of ligand but was markedly greater than for vesicles containing the ligand DTPA, suggesting that the binding of 99mTc to vesicles with surface-attached DTPA could not be explained solely on the basis of the negative charge provided by the DTPA. In vitro experiments using 14C-labeled lipids as well as in vivo imaging studies indicated that dissociation of 99mTc from the surface of the vesicle did not arise predominantly because of lipid exchange with plasma components or due to cleavage of Tc-DTPA from the vesicle surface. For vesicles with surface-attached DTPA, 99mTc dissociation from the vesicle surface in plasma was further reduced by addition of the antioxidant ascorbate.

Nuclear magnetic relaxation dispersion and 31P-NMR studies of the effect of covalent modification of membrane surfaces with poly(ethylene glycol)

Covalent attachment of methoxypoly(ethylene glycol) (MPEG) 5000 to the surface of unilamellar liposomes composed of egg phosphatidylcholine and dioleoylphosphatidylethanolamine (DOPE) (8:2) containing paramagnetic chelates, either entrapped within the interior volume of the liposomes, or associated with the membrane surface, had no effect upon the measured spin-lattice relaxation rates (1/T1) for water in these systems. 31P-NMR studies indicate no destabilization of dioleoylphosphatidylcholine (DOPC)/(DOPE) (1:1) vesicles following attachment of MPEG. However, in DOPC/DOPE (1:3) mixtures, covalent modification with MPEG results in a destabilization of multilamellar vesicles into smaller vesicular structures. These results indicate that covalent attachment of poly(ethylene glycol) to liposomal magnetic resonance agents may prove a useful method for increasing their utility as vascular MR agents by extending their lifetime in the circulation, without decreasing the relaxivity of paramagnetic species associated with the liposome, but that the presence of PEG covalently attached to the membrane surface may modify the polymorphic phase behavior of the lipid system to which it is covalently linked.

The design of liposomal paramagnetic MR agents: effect of vesicle size upon the relaxivity of surface-incorporated lipophilic chelates

The 1/T1 NMRD profiles of lipid vesicles with the paramagnetic ion Gd attached via a chelate covalently linked to the membrane surface show a peak at approximately 20 MHz indicating that fluctuations of approximately 10(-8) s contribute to the form of the dispersion profile. If the correlation time for fluctuations of the paramagnetic chelate on the membrane surface is much less than the correlation time for rotation of the lipid vesicle, it would be expected that the measured 1/T1 relaxation rate for solvent protons should be invariant with vesicle size above a certain minimum vesicle diameter. We show that this is indeed the case for vesicles in the size range 50 to 400 nm average diameter and discuss general design considerations for the preparation of vesicle-associated MR contrast agents based upon paramagnetic chelates either trapped within the vesicle interior or attached to the membrane surface.

Permeability of liposomal membranes to water: results from the magnetic field dependence of T1 of solvent protons in suspensions of vesicles with entrapped paramagnetic ions

The diffusive permeability to water molecules, Pd, of lipid vesicles with entrapped paramagnetic solute ions can be determined rapidly from analysis of the magnetic field dependence (nuclear magnetic relaxation dispersion, or NMRD profile) of T1 of exterior solvent water protons. Such data yield tau, the mean lifetime of solvent molecules inside the vesicles, from tau = (fT1Para) - T1Ves, where f is the volume fraction of entrapped water, T1Para is the observed T1 corrected for buffer background, and T1Ves is the relaxation time of water protons in the entrapped solution. For small spherical unilamellar vesicles of inner radius R, Pd = R/3 tau, f can be obtained accurately from knowledge of both the concentration of Gd(DTPA)2- in the solution in which the vesicles were formed and the average concentration of ions in the final sample. At low temperatures, in the limit of slow exchange, T1Para becomes independent of field and tau = fT1Para; the observation of a field-independent profile is a control that confirms that no paramagnetic material is external to the vesicles. We have measured T1Para, using a field-cycling relaxometer, for suspensions of POPC (1-palmitoyl-2-oleoyl-lecithin) vesicles with 100-500 mM entrapped Gd(DTPA)2- and membrane concentrations of cholesterol ranging from 0 to 40 mol %. These profiles, which span the field range 0.01-50 MHz proton Larmor frequency, were taken at 5, 15, 25, and 35 degrees C. Concentrations of Gd(DTPA)2- were determined independently by both ICP analyses and NMRD methods. Values for Pd for vesicles with 100 mM Gd(DTPA)2- and outer diameters 100 nm +/- 20%, as determined by quasielastic light scattering, are 63, 47, 24, 16, and 8.7 x 10(-4) cm s-1, at 25 degrees C, for cholesterol concentrations of 0, 10, 20, 30, and 40%, respectively. The corresponding activation enthalpies are 14, 14, 14, 17, and 17 kcal/M. Comparison with 2H NMR studies of deuterated POPC vesicles with no cholesterol at 20 degrees C, and with 10% at 40 degrees C, which yielded the same order parameter for the palmitoyl acyl chains, gives no indication of a correlation between order parameter and permeability.

Detailed toxicity studies of liposomal gadolinium-DTPA

Acute, subacute, and delayed toxicity testing was assessed in mice for liposomal gadolinium-DTPA (Gd-DTPA), blank liposomes, and nonliposomal Gd-DTPA. In the subacute experiments mice were injected intravenously (IV) with 0.3 mmol/kg Gd-DTPA per day for 30 days in the form of either free Gd-DTPA, liposomal Gd-DTPA, or an equivalent amount of lipid in blank liposomes without Gd-DTPA. The interpolated acute LD50 of liposomal and nonliposomal Gd-DTPA, estimated as a means of identifying the approximate level, was similar (LD50 = 5.7 mmol/kg Gd-DTPA). In subacute toxicity testing, prolonged high doses of liposomal Gd-DTPA caused splenomegaly, cardiomegaly, lymphocytopenia and hypergammaglobulinemia (P less than .05). Nonliposomal Gd-DTPA caused mild cardiomegaly and altered liver enzymes (P less than .05). Blank liposomes caused relatively mild splenomegaly (P less than .05) but few other changes. Delayed testing three months after the subacute testing showed that most of the changes caused by the liposomal Gd-DTPA were reversible.

Polymeric gastrointestinal MR contrast agents

Combining either paramagnetic (gadolinium chelates) or superparamagnetic (ferrite) contrast agents with polymers such as polyethylene glycol or cellulose, or with simple sugars such as dextrose, results in mixtures that exhibit improved T1 and/or T2 relaxivity compared with that of the contrast agent alone. It is suggested that the addition of such inexpensive and nontoxic polymers or saccharides may improve the effectiveness and decrease the cost of enteric contrast agents.

Membrane fusion without cytoplasmic fusion (hemi-fusion) in erythrocytes that are subjected to electrical breakdown

There are many reports of hemi-fusion in phospholipid vesicles but few published studies on hemi-fusion in cells. We report evidence from both fluorescence microscopy and freeze-fracture electron microscopy for hemi-fusion in the electrofusion of human erythrocytes. We have also characterised the conditions that favour hemi-fusion as opposed to complete fusion, and discuss the possibility that hemi-fusion might precede complete electrically-induced cell fusion. A membrane probe (DiIC16) and a cytoplasmic probe (6-carboxyfluorescein) were used to investigate the behaviour of doubly-labelled human erythrocytes which were aligned in chains by dielectrophoresis and then exposed to high voltage breakdown pulses. Some of the cells were fused by the pulses, as shown by diffusion of both membrane and cytoplasmic probes from labelled to unlabelled cells. With other cells, the membrane probe diffused into unlabelled cells after the breakdown pulses, without the cytoplasmic probe diffusing into unlabelled cells or leaking into the medium. Membrane fusion (hemi-fusion) thus occurred without cytoplasmic fusion in these erythrocytes. Such cells were irreversibly, but fragilely, attached to their neighbours by the breakdown pulses. There was an inverse relationship between conditions that permit complete fusion and those that favour hemi-fusion, with respect to breakdown pulse length, breakdown voltage and, in particular, osmolarity and temperature. The incidence of hemi-fusion in 250 mM erythritol was twice that in 150 mM erythritol, and hemi-fusion was 5-fold greater at 25 degrees C than at 20 degrees C. Hemi-fused erythrocytes occasionally fused completely on heating to 50 degrees C, demonstrating that hemi-fusion can proceed to complete cell fusion. Freeze fracture electron micrographs of preparations of hemi-fused cells revealed long-lived, complementary depressions and protrusions on the E- and P-fracture faces, respectively, of tightly apposed cells that may mediate hemi-fusion. The possibility that the fusion of closely adjacent human erythrocytes by electrical breakdown pulses may involve an intermediate, shared bilayer structure, which is stable in certain conditions but which can be ruptured by osmotic swelling of the permeabilised cells, is discussed.

An improved method for the preparation of liposomal gadolinium-DTPA. Ionophore-mediated active entrapment of gadolinium

We have developed an improved method for the production of liposomal gadolinium-DTPA (Gd-DTPA). The ionophore A23187 facilitates the uptake of externally added Gd into the interior aqueous space of a unilamellar lipid vesicle, where it is chelated by passively entrapped DTPA to form the Gd-DTPA chelate in situ. The presence of a pH gradient across the vesicle membrane is not essential for Gd uptake, the extent of which apparently is limited only by the interior concentration of the chelator. Once formed internally, the Gd-DTPA complex is retained within the vesicles for at least several days at room temperature. Biodistribution studies in mice indicate that liposomal Gd-DTPA prepared by this ionophoretic loading procedure exhibits biodistribution and clearance characteristics similar to 153Gd-DTPA-labeled liposomes prepared by means of passive entrapment of the preformed chelate.

Localized osmotic swelling and cell fusion in erythrocytes: possible implications for exocytosis

Factors that govern the experimentally induced fusion of erythrocytes with one another may generally be relevant to whether or not osmotic forces drive membrane fusion in exocytosis because, under appropriate conditions, osmotic swelling can drive the fusion of erythrocytes. It is now reported that these cells fuse when they are subjected to osmotic swelling caused by exposure to small permeant molecules. The behaviour of erythrocytes in fusion induced by treatment with a concentrated solution of high molecular weight poly(ethylene glycol) (PEG) is also of specific interest in relation to exocytosis because the haemoglobin of erythrocytes that are dehydrated by concentrated solutions of the polymer may be regarded as a model for the tightly packed, dehydrated contents of the granules in secretory cells. We have observed that, under certain conditions of rehydration, the swelling of aqueous microdroplets between the dehydrated haemoglobin and the plasma membrane is closely associated with the fusion of partially rehydrated but still shrunken, PEG-treated erythrocytes. It is therefore apparent that osmotic forces, acting locally at the sites of aqueous microdroplets, can drive the fusion of membranes that encapsulate a dehydrated, concentrated protein, even though gross osmotic swelling at the level of the light microscope is absent. This finding is consistent with the possibility that osmotic swelling may play a role in exocytotic membrane fusion if it is restricted to a small zone immediately under the granule membrane.

Movements of fluorescent probes in the mechanism of cell fusion induced by poly(ethylene glycol)

It has been claimed that purified poly(ethylene glycol) (PEG) is able only to aggregate cells and not to fuse them. In our hands, purified PEG 6000 (recrystallized/dialysed) induces both aggregation and fusion of human erythrocytes, and the mechanism of fusion by the purified polymer has been investigated with fluorescent probes. No movement of a carbocyanine probe or of octadecyl rhodamine B chloride from labelled to unlabelled cells occurred in the absence of PEG or with cells treated with concanavalin A, protamine or spermine. With 40% PEG, however, both probes immediately started to diffuse into the membranes of unlabelled cells. This indicates that continuity between the phospholipid bilayer membranes of adjacent erythrocytes (i.e. membrane fusion) is established within seconds in concentrated solutions of the polymer, and precedes the cell fusion event that is induced by purified PEG. These observations are consistent with the idea that micro-regions of shared phospholipid bilayer may be formed in the membranes of cells when they are forced together as a consequence of the dehydrating action of PEG. Intact erythrocytes were cytoplasmically labelled with 6-carboxyfluorescein to avoid the possibility that loading the cells with a cytoplasmic marker by hypotonic haemolysis might modify their response to PEG. Unlike the lipid probes, carboxyfluorescein did not diffuse from labelled to unlabelled cells in the presence of 40% PEG, and there was little diffusion on subsequent dilution of the polymer solution to 13%. However, after the PEG solution had been replaced by an isotonic buffer, a rapid transfer of the cytoplasmic fluorophore to unlabelled cells often occurred. This is considered to be more consistent with the osmotic rupture of a membranous barrier, such as a shared bilayer, between the labelled and unlabelled cells than with the return of cytoplasmic viscosity to normal when the PEG is removed. Possible reasons are discussed for the reported inability of purified PEG to fuse fibroblasts with hypotonically loaded human erythrocytes.

Osmotic forces in artificially induced cell fusion

The importance of cell swelling in the fusion of erythrocytes by three different chemical treatments has been investigated with cells that were cytoplasmically labelled with 6-carboxyfluorescein. Hen erythrocytes, which had been pre-incubated with ionophore A23187 and 5 mM Ca2+ to cause a proteolytic breakdown of the membrane skeleton, were induced to fuse by applying an osmotic shock. Human erythrocytes that had been incubated in an isotonic salt/buffer solution, which was progressively diluted and which contained 0.5 mM La3+ to minimise cell lysis, were also fused. In addition, the fusion of human erythrocytes by 40% poly(ethylene glycol) began only when the poly(ethylene glycol) was diluted, and it mostly occurred when the diluted polymer solution was subsequently replaced by isotonic buffer. In related experiments, the effect of an osmotic gradient on electrically induced cell fusion has been studied. Human erythrocytes in 150 mM erythritol fused more readily than less swollen cells in 200-400 mM erythritol when subjected to a 20 microseconds pulse of 3.5 kV X cm-1, indicating that the extent of cell fusion induced by the breakdown pulse is governed by the combined electrical-compressive and osmotic forces. Since osmotic phenomena are already known to be important in exocytosis, we suggest that these observations on cell fusion indicate that osmotic forces may provide the driving force for many membrane fusion reactions in biological systems.

An osmotic model for the fusion of biological membranes

A molecular model for fusion-fission reactions in membranes is proposed that is based on data from studies on artificially induced cell fusion and on the behaviour of phospholipid bilayers: it is put forward as a framework for further investigations into this fundamental property of biological systems.

Inhibition of the formation of myotubes in vitro by inhibitors of transglutaminase

The effects of competitive inhibitors of transglutaminase on the formation of myotubes by the fusion of myoblasts in vitro has been investigated. Myotube formation was inhibited when myoblasts from 11-day-old chick embryos were cultured in vitro in the presence of 10 mM histamine or 0.2 mM dansyl cadaverine. The inhibitions observed were reversed when the treated cells were subsequently cultured in normal medium. Glycine methyl ester also inhibited myotube formation but sarcosine methyl ester, which is not a competitive inhibitor of transglutaminase, had little if any inhibitory action. The formation of myotubes was not inhibited by cultivation in normal medium adjusted to pH 8.0-8.1, indicating that the observed effects of histamine and of dansyl cadaverine were not mediated by a lysosomotropic effect. Inhibition of myotube formation in the presence of histamine was accompanied by the production of abnormal multinucleated cells, indicating that myoblast fusion occurred in the treated cultures but that the fused cells failed to elongate into normal myotubes. Transglutaminase activity has been found in cell-free lysates of embryonic chick myoblasts and it is concluded that a transglutaminase enzyme, activated by an increase in the concentration of intracellular Ca2+, plays an important role in stabilising the cytoskeletal network of developing myotubes.

Is purified poly(ethylene glycol) able to induce cell fusion?

Preparations of poly(ethylene glycol) 6000 (PEG) from five different commercial sources have been purified, and their ability to fuse hen erythrocytes has been investigated. Quantitative assessments of cell fusion showed that before purification one of the preparations (PEG Wako), was able to induce limited fusion (5-6%) of erythrocytes with conditions (1 min incubation with 50% w/w PEG) under which the other four unpurified preparations of PEG were inactive. On purification, PEG (Wako) became inactive. By contrast, when erythrocytes were incubated with 45% w/w PEG for 15 min, extensive fusion (23-27%) occurred with all five unpurified preparations of PEG. Under these conditions, the fusogenic properties of four of the preparations of PEG were unaffected by purification: fusion induced by PEG (Wako) was, however, decreased on purification from 27% to 19%. It appears that polymeric poly(ethylene glycol) is itself able to fuse cells, but that some commercial preparations, e.g. PEG (Wako), have enhanced fusogenic properties resulting from the presence of contaminating substances. No relationship between the absorbance at 290 nm of PEG and its fusogenic properties was found in this study. The addition of small quantities of fusogenic lipid-soluble compounds to PEG was, however, observed to enhance cell fusion by up to 50%.

Effects of poly(ethylene glycol) on liposomes and erythrocytes. Permeability changes and membrane fusion

Poly(ethylene glycol) 6000 induced a concentration-dependent, time-dependent decrease in the latency of the reaction between Arsenazo III sequestered in liposomes and extraliposomal Ca2+. This was mediated by a gross change in liposomal permeability, i.e. by a release of Arsenazo III from liposomes rather than simply by an entry of Ca2+. The loss of latency was strongly temperature-dependent, and it was markedly diminished on increasing the cholesterol content of the liposomes. It was apparently not due to an osmotic stress of the polymer. The high activation energy found (63 kJ . mol-1) is thought to indicate that the loss of latency resulted from local discontinuities in the lipid bilayers, caused by dehydration, rather than from partial or total lysis. Related microscopy experiments indicated that the polymer also caused the liposomes to fuse, and it is suggested that membrane fusion may have occurred at the sites of dehydration-induced discontinuities in adjacent bilayers. In addition, the polymer was found to enhance the permeability of hen erythrocytes of Ca2+ in a manner that was comparable to its effect on liposomal latency, and it is proposed that gel fusion induced by poly(ethylene glycol) may occur at the sites of similarly induced discontinuities in the phospholipid bilayers of two closely adjacent cells.

Calcium-activated thiol-proteinase activity in the fusion of rat erythrocytes induced by benzyl alcohol

1. Rat erythrocytes were fused by incubation with benzyl alcohol and Ca2+. 2. Cell fusion was inhibited by EGTA, N-ethylmaleimide, tetrathionate, iodoacetamide, cystamine, Tos-Lys-CH2Cl, and to a lesser extent by Tos-Phe-CH2Cl. Phenylmethanesulphonyl fluoride, Tos-Arg-OMe and histamine did not inhibit cell fusion. 3. Gel electrophoresis of membrane proteins from "ghosts" of the erythrocytes treated with benzyl alcohol showed that a high-molecular-weight polymer was present: this was consistent with the entry into the cells of Ca2+ and the activation of a transglutaminase enzyme. 4. In the treated cells the proteins corresponding to bands 2 and 3 in human erythrocytes were decreased, and a polypeptide with a slightly greater mobility than band 3 was produced. 5. These changes were inhibited by EGTA, N-ethylmaleimide, tetrathionate, iodoacetamide, cystamine, and Tos-Lys-CH2Cl, but not by phenylmethanesulphonyl fluoride, Tos-Arg-OMe, or histamine. 6. The intramembraneous particles of the P-fracture face of cells treated with benzyl alcohol to induce fusion were decreased in number and were susceptible to cold-induced aggregation; both of these phenomena were markedly inhibited to EGTA, and partially inhibited by Tos-Lys-CH2Cl and N-ethylmaleimide. 7. These several observations indicate that a Ca2+-activated thiol-proteinase, which acts to degrade membrane proteins and to give freedom of lateral movement to intramembranous particles, may be essential feature of membrane fusion in this system. 8. It is suggested that this proteinase may act to degrade spectrin-binding proteins that attach band-3 protein to the erythrocyte cytoskeleton.

Membrane proteins in human erythrocytes during cell fusion induced by oleoylglycerol

1. The fusion of human erythrocytes into multicellular bodies that is induced by microdroplets of oleoylglycerol was investigated by optical and electron microscopy, and by gel electrophoresis of membrane proteins. 2. At the highest concentrations of oleoylglycerol and Ca(2+) used, at least 80% of the cells fused after 30min at 37 degrees C and only about 5% of the cells had completely lysed; the shapes of fused multicellular bodies were usually retained in ;ghosts' prepared by hypo-osmotic lysis. 3. The rate of cell fusion was related to the concentration of Ca(2+), although some cells fused when no exogenous Ca(2+) was present. 4. Interactions of microdroplets of oleoylglycerol with the cells led to abnormalities in the structural appearance of the erythrocyte membrane; subsequent membrane fusion occurred, at least in some instances, at the sites of the microdroplets. 5. The intramembranous particles on the P-fracture face of the treated cells were more randomly distributed, but not significantly increased in number by comparison with the control cells. 6. Gel electrophoresis of the proteins of ;ghosts' prepared from fused human erythrocytes showed a production of material of very high molecular weight, the development of a new component in the band-3 region, an increased staining of bands 4.3 and 4.5, and a new component moving slightly faster than band 6. 7. Bands 2.1-2.3 were altered, band 3 was decreased and band 4.1 was lost. 8. Most, but not all, of the changes in the membrane proteins appeared to result from the entry of Ca(2+) into the cell. 9. 1-Chloro-4-phenyl-3-l-toluene-p-sulphonamidobutan-2-one partially inhibited both cell fusion and the associated decrease in band-3 protein. 10. The possibility that proteolytic degradation of membrane proteins may be involved in cell fusion induced by oleoylglycerol is considered, and some implications of this possibility are discussed.

Poly(ethylene glycol), surface potential and cell fusion

Poly(ethylene glycol), glycerol and dimethyl sulphoxide markedly decrease the surface potentials of monolayers of phosphatidylcholine and phosphatidylethanolamine. This finding is discussed in relation to the properties of hen erythrocytes undergoing fusion induced by poly(ethylene glycol).

Changes in the distribution of intramembranous particles in hen erythrocytes during cell fusion induced by the bivalent-cation ionophore A23187

Incubation of hen erythrocytes with Ca2+ and the bivalent-cation ionophore A23187 induced slight cell fusion in 1 h at 37 degrees C, and extensive fusion during a subsequent 15 min at 47 degrees C. Redistributions of intramembranous particles were observed, possibly involving interactions between Ca2+ and phospholipids, which are discussed in relation to molecular mechanimss of cell fusion.

The fusion of erythrocytes by fatty acids, esters, retinol and alpha-tocopherol

1. The ability of a number of carboxylic acids, their esters, retinol and alpha-tocopherol to induce fusion of hen erythrocytes in vitro was investigated. 2. Some 30 different fat-soluble substances (100mug/ml) were found to cause the formation of multinucleated erythrocytes with a suspension of 3x10(8) erythrocytes/ml. The most effective agents induced fusion within 5-10min at 37 degrees C; some substances required about 1h. 3. Inclusion of Dextran 60C in the test medium minimized colloid osmotic lysis caused by exogenous lipids that induce cell fusion. 4. Cell swelling, followed by cell adhesion, was then seen to precede cell fusion. 5. Fusion occurred with C(10)-C(14) saturated carboxylic acids, with unsaturated, longer-chain carboxylic acids and their mono-esters; retinol, and to a lesser extent alpha-tocopherol, also caused cell fusion. 6. C(6)-C(9), C(15), C(16) and C(18) saturated carboxylic acids did not induce fusion within 4h; glyceryl dioleate was only weakly active, and glyceryl trioleate was inactive in the test system. 7. Fusion was facilitated by a high ratio of chemical agents to cell number and by incubation between pH5 and 6. It was inhibited by EDTA and by serum albumin. 8. Glyceryl mono-oleate caused both a similar fusion of several species of mammalian erythrocyte and the interspecific fusion of human and chicken erythrocytes. 9. The term ;fusogenic' is proposed to describe chemical, viral and physical agents that cause membranes to fuse. 10. The biochemical mechanisms involved and the possible biological significance of membrane fusion by fusogenic lipids are discussed.

Studies on chemically induced cell fusion

Hen erythrocytes that were fixed after treatment with lysolecithin in aqueous solution for 30 s at 37 °C showed evidence of bridge formation between adjacent lysed cells. Generally, the homokaryons that were produced using lysolecithin in this way contained large numbers of nuclei. These giant syncytia had damaged nuclear membranes and unstable plasma membranes; complete disintegration of the syncytia occurred within 1 min of adding lysolecithin to the erythrocytes.

In order to localize the action of lysolecithin, the fusing agent was incorporated into microdroplets of lipid. Cell fusion following the addition of lysolecithin in an aqueous glyceridelecithin emulsion was slower than with lysolecithin in aqueous solution, taking 10-30 min, and it was accompanied by considerably less damage to the plasma and nuclear membranes. The fused erythrocytes, which usually contained only two or three nuclei, lysed slowly during the 45 min following fusion, and lysis could be arrested by cooling the fused cells. The plasma membranes of lysed, multinucleated cells remained intact at 37°C for at least 90 h.

Mouse fibroblast-hen erythrocyte heterokaryons formed with the aid of the emulsion were more stable than those produced with lysolecithin in solution, but the hybrid cells nevertheless had damaged subcellular organelles. Viable clones of hybrid mouse-hamster fibroblast cells were obtained using the emulsion although, possibly owing to reduced viability of the lysolecithin-treated cells, only at twice the frequency of spontaneously produced hybrids.