Detection of protein-free lipoprotein analogues with an apolar lipid core by freeze-etch electron microscopy

Freeze-fracture electron microscopy of protein-free lipoprotein analogues consisting of polar phospholipids with neutral lipid cores shows that these systems can exhibit novel, smooth crossfracture faces. Resolution of these features can be enhanced by etching techniques. This novel particle morphology provides a new procedure to characterize microemulsions composed of polar and neutral lipids.

A non-covalent method of attaching antibodies to liposomes

A novel non-covalent method of attaching antibodies to liposomes which exploits the high affinity of streptavidin for biotin, is described. The two-step coupling protocol involves the initial attachment of streptavidin to liposomes containing biotin PE, followed by the coupling of biotinated antibodies to streptavidin-liposomes. The association of streptavidin with liposomes containing biotinated PE is rapid (less than 5 min), resulting in a maximum association of 40 molecules of streptavidin per 100 nm vesicle. In the presence of equimolar cholesterol, the amount of streptavidin bound is twice that observed when biotin PE/egg PC liposomes are used. Irrespective of the mole ratio of biotin to antibody (e.g. for 1-6 biotins per antibody), or the molar ratio of antibody to streptavidin in the second incubation step, equimolar amounts of antibody bind to streptavidin. It is shown that anti-rat-erythrocyte IgG or F(ab')2 complexed to liposomes via the streptavidin linker bind specifically to rat erythrocytes but not to human erythrocytes. This coupling protocol can be readily extended to other biotinated antibodies.

Polyfunctional properties of hybridoma lupus anticoagulant antibodies

Lupus anticoagulant antibodies are antiphospholipid antibodies which are characterized by their ability to prolong the clotting time in in vitro coagulation assays measuring the partial thromboplastin time (PTT). In our study, we have analyzed 11 hybridoma lupus anticoagulant antibodies and studied their reactivity with hexagonal and lamellar phospholipids, DNA, IgG, cytoskeletal components, and intact platelets. Our results demonstrate that hexagonal phase phospholipids but not lamellar phospholipids were able to neutralize the lupus anticoagulant activity of all 11 hybridoma antibodies. However, the lupus anticoagulant antibodies differed from one another in their reactivity with DNA, the Fc fragment of IgG, cytoskeletal proteins and platelets. Some of the lupus anticoagulant antibodies reacted with all of these structures, while others reacted with only some or none of the antigens tested. Isoelectric focusing gel analysis confirmed that this polyreactivity was due to a single antibody. Direct binding of lupus anticoagulant antibodies to intact platelets was shown to correlate with the DNA binding activity of the antibodies. Lupus anticoagulant antibodies were also capable of inducing a morphological shape change in platelets. Our data suggest that hybridoma lupus anticoagulant antibodies may react as lupus anticoagulant antibodies only, anti-DNA antibodies, rheumatoid factors, anticytoskeletal antibodies, and antiplatelet antibodies. The degree of overlap in these subgroups and the epitopes responsible for these multiple reactivities remain to be determined.

Lipid polymorphism

The phase behavior of phospholipids may be monitored using 31P or 2H NMR techniques, which provide information concerning the motional properties of the lipid ensemble, which may then be correlated with structure. The lamellar/nonlamellar phase preferences of many lipids, either synthetic or naturally derived, may be controlled by factors such as variation in temperature, hydration, or of greater physiological relevance, pH, ionic strength, the presence of divalent cations such as calcium, or the presence of lipid soluble agents as anesthetics and alcohols. The ability of short-chain alcohols to stabilize a bilayer structure for egg PE may be rationalized in terms of the packing of lipids whose dynamic shapes are complementary, as illustrated in Figure 11. On the basis, short-chain alcohols would partition preferentially at the membrane/water interface and would thereby stabilize a lamellar structure. Larger-chain alcohols may partition deeper into the hydrophobic acyl chain region in order to minimize hydrocarbon/water contact and so may perturb the acyl chain packing, increasing the effective swept volume of the chains and so promoting hexagonal HII phase formation.

Lipid asymmetry induced by transmembrane pH gradients in large unilamellar vesicles

We have investigated the influence of transmembrane pH gradients across large unilamellar vesicle membranes on the transbilayer distributions of simple lipids with weak base and weak acid characteristics. Trinitrobenzenesulfonic acid labeling results consistent with a rapid and complete migration of stearylamine and sphingosine to the inner monolayer of the large unilamellar vesicles are observed when the large unilamellar vesicles' interior is acidic. Alternatively, when the vesicle interior is basic, oleic and stearic acid cannot be removed by external bovine serum albumin, indicating a localization in the inner monolayer. Moreover, effects corresponding to the decrease in external surface charge predicted upon the migration of stearylamine or stearic acid to the inner monolayer are readily detected employing ion exchange chromatography. These results are consistent with transbilayer distributions of these agents dictated by a Henderson-Hasselbach equilibrium. The possible implications for metabolic regulation by pH gradients, as well as factors giving rise to phospholipid transbilayer asymmetry, are discussed.

Human hybridoma lupus anticoagulants distinguish between lamellar and hexagonal phase lipid systems

Antibodies to phospholipids may have important physiological and biological functions. Lupus anticoagulants represent a subclass of anti-phospholipid antibodies which are characterized by their ability to prolong the clotting time in in vitro coagulation assays measuring partial thromboplastin time (PTT) (Thiagarajan, P., Shapiro, S. S., and DeMarco, L. (1980) J. Clin. Invest. 66, 397-405). In the present study, we produced hybridomas by fusing lymphocytes from 13 systemic lupus erythematosus patients with the GM 4672 lymphoblastoid line. Of the resulting 67 hybridoma autoantibodies, 14 (21%) were found to prolong a modified PTT assay, and 11 of these antibodies were analyzed further. Competition experiments, using a modified PTT assay, demonstrated that hexagonal phase phospholipids, including natural and synthetic forms of phosphatidylethanolamine, were able to neutralize the lupus anticoagulant activity of all 11 hybridoma antibodies. In contrast, lamellar phospholipids, such as phosphatidylcholine and synthetic lamellar forms of phosphatidylethanolamine, had no effect on the anticoagulant activity. Thus, these antibodies are capable of recognizing phospholipids on purely structural criteria. The demonstration that anti-phospholipid antibodies are able to distinguish between different structural arrangements of phospholipid may have important implications regarding the immunoregulation of autoimmunity.

Vesicles of variable sizes produced by a rapid extrusion procedure

Previous studies from this laboratory have shown that large unilamellar vesicles can be efficiently produced by extrusion of multilamellar vesicles through polycarbonate filters with a pore size of 100 nm (Hope, M.J., Bally, M.B., Webb, G. and Cullis, P.R. (1985) Biochim. Biophys. Acta 812, 55-65). In this work it is shown that similar procedures can be employed for the production of homogeneously sized unilamellar or plurilamellar vesicles by utilizing filters with pore sizes ranging from 30 to 400 nm. The unilamellarity and trapping efficiencies of these vesicles can be significantly enhanced by freezing and thawing the multilamellar vesicles prior to extrusion. This procedure is particularly applicable when very high lipid concentrations (400 mg/ml) are used, where extrusion of the frozen and thawed multilamellar vesicles through 100 and 400 nm filters results in trapping efficiencies of 56 and 80%, respectively. Freeze-fracture electron microscopy revealed that vesicles produced at these lipid concentrations exhibit size distributions and extent of multilamellar character comparable to systems produced at lower lipid levels. These results indicate that the freeze-thaw and extrusion process is the technique of choice for the production of vesicles of variable sizes and high trapping efficiency.

Lipid polymorphism and the roles of lipids in membranes

The reasons for lipid diversity in membranes are not understood. Here we review evidence supporting the proposal that factors related to the polymorphic capabilities of lipids provide a rationale for lipid diversity. In particular, the ability of lipids to adopt different polymorphic phases appears to be related to a generalized shape property, where lipids with a cylindrical geometry preferentially adopt the bilayer phase whereas 'cone' shaped lipids adopt the hexagonal HII phase. Lipid diversity may then be considered to satisfy three demands. The first is obviously a need for bilayer forming lipids to provide the basic permeability barrier, whereas the second concerns a need for non-bilayer lipids and associated structures for fusion and related membrane contact phenomena to proceed. A third, and less obvious demand satisfied by nonbilayer lipids concerns the ability of lipids of different shapes to modulate the order in the hydrocarbon region when constrained to a bilayer organization. These possibilities are summarized in a metamorphic mosaic model of membranes.

Techniques for encapsulating bioactive agents into liposomes

As a prerequisite for the use of liposomes for delivery of biologically active agents, techniques are required for the efficient and rapid entrapment of such agents in liposomes. Here we review the variety of procedures available for trapping hydrophilic and hydrophobic compounds. Considerations which are addressed include factors influencing the choice of a particular liposomal system and techniques for the passive entrapment of drugs in multilamellar vesicles and unilamellar vesicles. Attention is also paid to active trapping procedures relying on the presence of (negatively) charged lipid or transmembrane ion gradients. Such gradients are particularly useful for concentrating lipophilic cationic drugs inside liposomes, allowing trapping efficiencies approaching 100%.

Uptake of adriamycin into large unilamellar vesicles in response to a pH gradient

Previous work has shown that adriamycin can be accumulated into large unilamellar vesicle (LUV) systems in response to K+ diffusion potential established by valinomycin. It is demonstrated here that adriamycin can also be rapidly and efficiently accumulated into egg phosphatidylcholine (egg PC) and egg PC-cholesterol (1:1) LUVs in response to a transmembrane pH gradient (interior acidic) in the absence of ionophores. This 'active' loading gives rise to trapping efficiencies as high as 98%, interior drug concentrations as high as 100 mM and significantly enhances drug retention within the vesicles. This procedure may be of general utility for loading liposomal systems for in vivo drug delivery.

Influence of vesicle size on complement-dependent immune damage to liposomes

Complement-dependent antibody-mediated damage to multilamellar lipid vesicles (MLVs) normally results in a maximum release of 50-60% of trapped aqueous marker. The most widely accepted explanation for this is that only the outermost lamellae of MLVs are attacked by complement. To test this hypothesis, complement damage to two different types of large unilamellar vesicles (LUVs), large unilamellar vesicles prepared by the reverse-phase evaporation procedure (REVs) and large unilamellar vesicles prepared by extrusion techniques (LUVETs), were determined. In the presence of excess antibody and complement the LUVs released a maximum of only approx. 25 to 40% of trapped aqueous marker, instead of close to 100% that would be expected. Since small unilamellar vesicles apparently differ from LUVs in that they can release 100% of trapped aqueous marker it appeared that the size of the vesicles was an important factor. Because of these observations the influence of MLV size on marker release was examined. Three populations of MLVs of different sizes were separated by a fluorescence activated cell sorter. Assays of the separated MLV populations showed that the degree of complement-dependent marker release was inversely related to MLV size. No detectable glucose was taken up by MLVs when glucose was present only outside the liposomes during complement lysis. Our results can all be explained by the closing, or loss, of complement channels. We conclude that complement channels are only transiently open in liposomes, and that loss of channel patency may be due to either channel closing or to loss of channels.

Polymorphic phase behavior of unsaturated lysophosphatidylethanolamines: a 31P NMR and X-ray diffraction study

The polymorphic phase behavior of aqueous dispersions of 1-oleoyl-, 1-linoleoyl-, and 1-linolenoyl-sn-3-glycerophosphoethanolamine (1-C18:1c-PE, 1-C18:2c-PE, and 1-C18:3c-PE, respectively) has been investigated by 31P NMR, small-angle and wide-angle X-ray diffraction, and freeze-fracture techniques in response to changes in temperature and pH. Between -20 and 0 degrees C at pH 7, NMR and X-ray data indicate that 1-C18:1c-PE adopts a lamellar phase. Above 20 degrees C, the X-ray diffraction from 1-C18:1c-PE reveals no long-range lattice order, whereas the NMR data indicate lamellar structure to 90 degrees C. Freeze-fracture electron microscopy shows that 1-C18:1c-PE at pH 8.2 forms closed multilamellar vesicles upon dispersion and also that large unilamellar vesicles produced by extrusion techniques (LUVETs) can be made from 1-C18:1c-PE at pH 7. Such LUVETs can trap [3H]inulin and support a K+ diffusion potential for up to 4 h. At pH 8.5 and above, 1-C18:1c-PE forms optically clear, fluid dispersions with NMR and X-ray characteristics consistent with a micellar (noninverted) phase structure. Attempts to prepare LUVETs from 1-C18:1c-PE at pH 9 result in structures that can neither trap [3H]inulin nor support a membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Preparation of reconstituted cytochrome oxidase vesicles with defined trans-membrane protein orientations employing a cytochrome c affinity column

Reconstituted cytochrome oxidase systems in which the majority of the vesicles contain a single oxidase dimer can be prepared. It is shown that, when these are passed through a cytochrome c affinity column, only those vesicles oriented outwards (such that the active site is available to external cytochrome c) are bound to the support matrix. Protein-free vesicles and vesicles containing an inwardly oriented enzyme are eluted in the void volume. Subsequently, vesicles containing an outwardly oriented enzyme can be eluted from the column at high salt concentrations. This protocol has been used successfully to resolve vesicles of either oxidase orientation when the enzyme is reconstituted with a variety of lipid mixtures. The recovery of oxidase activity from the column ranged between 75 and 94%.

Solute distributions and trapping efficiencies observed in freeze-thawed multilamellar vesicles

It has recently been observed (Gruner, Lenk, Janoff and Ostro (1985) Biochemistry, in the press) that mechanical dispersion of dry lipid in an aqueous buffer to form multilamellar vesicle (MLV) systems does not result in equilibrium trans-membrane distributions of solute. In particular, the entrapped buffer exhibits reduced solute concentrations. Here we demonstrate that egg phosphatidylcholine MLV systems dispersed in the presence of Mn2+ also exhibit non-equilibrium solute distributions, and that repetitive freeze-thawing cycles can remove such solute heterogeneity. Further, the resulting freeze-thawed MLVs exhibit dramatically enhanced trapped volumes and trapping efficiencies. At 400 mg phospholipid per ml, for example, the trapping efficiencies can be as high as 90%. This is associated with a remarkable change in MLV morphology where large inter-bilayer separations are commonly observed.

Protection of large unilamellar vesicles by trehalose during dehydration: retention of vesicle contents

The ability of trehalose and other sugars to maintain the integrity of large unilamellar vesicles subjected to dehydration and rehydration has been investigated. It is shown, employing freeze-fracture techniques, that large unilamellar vesicles prepared in the presence of trehalose at 125 mM or higher concentration do not exhibit significant structural changes during the dehydration-rehydration cycle. Further, up to 90% of entrapped 22Na or [3H]inulin is retained during this process. Other sugars also exhibited similar protective effects where trehalose was most effective, followed by sucrose, maltose, glucose and lactose. It is demonstrated that proton or Na+/K+ electrochemical gradients can be maintained during the dehydration-rehydration process, which can subsequently be used to drive the uptake of lipophilic cationic drugs such as adriamycin. The implications for long-term storage of liposomal systems for use in drug-delivery protocols are discussed.

Uptake of antineoplastic agents into large unilamellar vesicles in response to a membrane potential

Many drugs exhibit lipophilic and cationic (basic) characteristics. Previous studies have shown that lipophilic cations can be accumulated into model membrane 'liposomal' (vesicular) systems in response to establishing a membrane potential (inside negative) across the vesicle membrane. We demonstrate here that the anticancer drugs, adriamycin and vinblastine, can be rapidly accumulated into egg phosphatidylcholine large unilamellar vesicles in response to a valinomycin-dependent K+ diffusion potential (delta psi) to achieve high effective interior concentrations. Further, trapping efficiencies approaching 100% can be easily achieved. The influence of lipid composition and the requirement for valinomycin have been examined for adriamycin. Equimolar cholesterol levels inhibit the uptake process at 20 degrees C. However, incubation at higher temperature results in enhanced uptake. Similarly, the presence of egg phosphatidylserine or incubation at elevated temperatures results in significant adriamycin uptake in the absence of valinomycin. It is shown that the adriamycin retention time in the vesicles is enhanced by an order of magnitude or more when actively trapped by the presence of a membrane potential in comparison to passive trapping procedures. It is suggested that such active trapping procedures may be of use for loading liposomal systems for drug delivery applications, and may provide avenues for controlled release of encapsulated material.

Orientation of cytochrome c oxidase molecules in the two populations of reconstituted vesicles resolved by column chromatography on DEAE-Sephacryl

Vesicles reconstituted with bovine heart cytochrome c oxidase and dioleoylphosphatidylcholine can be resolved into two populations by column chromatography in DEAE-Sephacryl (Madden, T.D. and Cullis, P.R. (1984) J. Biol. Chem. 259, 7655-7658). These two fractions (I and II) were treated with two proteases. These are trypsin, which has been found to cleave subunit IV in the M domain of the cytochrome c oxidase molecule, and chymotrypsin, which has been found to cleave subunit III in the C domain. These studies show that fraction I vesicles contain cytochrome c oxidase orientation with the M domain outside, i.e., in the same topology as in submitochondrial particles, while fraction II vesicles contain enzyme molecules with their C domain outside, and thus in the same orientation as in mitochondria.

Uptake of dibucaine into large unilamellar vesicles in response to a membrane potential

Local amine anesthetics appear to exert their effects in the charged (protonated) form on the cytoplasmic side of excitable membranes. Two features of interest are the mechanism whereby these drugs move across the membrane to the inner monolayer and the actual membrane concentrations achieved. In this work, we have investigated the influence of a K+ diffusion potential, delta psi, on the transmembrane distribution and concentration of the local anesthetic dibucaine employing large unilamellar vesicle systems. It is demonstrated that egg phosphatidylcholine large unilamellar vesicles exhibiting a delta psi (interior negative) actively accumulate dibucaine to achieve high interior concentrations. 31P and 13C nuclear magnetic resonance studies show that the internalized drug is localized to the vesicle inner monolayer, and suggest that the protonated form of the anesthetic is the species that is actively transported. The inner monolayer anesthetic concentrations thus achieved can be an order of magnitude or more larger than predicted on the basis of anesthetic lipid-water partition coefficients. It is suggested that these effects may be related to the mechanisms whereby local anesthetics are localized and concentrated at their sites of action in nerve membranes.

Phosphatidic acid as a calcium ionophore in large unilamellar vesicle systems

The ionophoretic capabilities of dioleoylphosphatidic acid (DOPA) for transporting calcium across phospholipid bilayers have been investigated. Calcium uptake by large unilamellar vesicles is shown to depend on the presence of DOPA. This uptake is sensitive to the nature and concentration of calcium chelators in the vesicle interior, indicating that accumulation results from DOPA-mediated translocation of calcium across the membrane. Further, it is shown that characteristics of DOPA-mediated Ca2+ uptake are similar to those observed for the fungal calcium ionophore, A23187.