Preparation of unilamellar liposomes of intermediate size (0.1-0.2 mumol) by a combination of reverse phase evaporation and extrusion through polycarbonate membranes

Development of anti-p185HER2 immunoliposomes for cancer therapy

The product of the HER2 protooncogene, p185HER2, represents an attractive target for cancer immunotherapies. We have prepared anti-p185HER2 immunoliposomes in which Fab' fragments of a humanized anti-p185HER2 monoclonal antibody with antiproliferative properties (rhuMAb-HER2) were conjugated to either conventional or sterically stabilized liposomes. These immunoliposomes bind specifically to p185HER2-overexpressing breast cancer cells (SK-BR-3 and BT-474). High-affinity binding of anti-p185HER2 immunoliposomes is comparable to that of free rhuMAbHER2-Fab' or the intact antibody. Empty immunoliposomes inhibit the culture growth of p185HER2-overexpressing breast cancer cells, and this antiproliferative effect is superior to that of free rhuMAbHER2-Fab', indicating that liposomal anchoring of these anti-p185HER2 Fab' fragments enhances their biological activity. Efficient internalization of anti-p185HER2 immunoliposomes, demonstrated by light and electron microscopy, occurs by receptor-mediated endocytosis via the coated pit pathway and also possibly by membrane fusion. Doxorubicin-loaded anti-p185HER2 immunoliposomes are markedly and specifically cytotoxic against p185HER2-overexpressing tumor cells in vitro. Anti-p185HER2 immunoliposomes administered in vivo in Scid mice bearing human breast tumor (BT-474) xenografts can deliver doxorubicin to tumors. These results indicate that anti-p185HER2 immunoliposomes are a promising therapeutic vehicle for the treatment of p185HER2-overexpressing human cancers.

Self-association processes involving anthracene labeled phosphatidylcholines in model membrane

When studying lipid-lipid or lipid-protein interaction in membranes, the correct interpretation of data obtained when using fluorescent phospholipid probes requires the best possible knowledge of probe behaviour in phospholipid membranes. Analysis of the translational dynamics and photochemical properties of the anthracene-labeled phosphatidylcholine (EAPC) shows that a self-association process occurs with this probe in the membrane at the ground state. This anthracene self-association is characterized and leads to a hypochromic effect which has been studied by means of ultraviolet absorption spectroscopy in unilamellar egg-yolk phosphatidylcholine (EggPC) vesicles. A model with indefinite linear self-association, in which each step has the same equilibrium constant, best describes the data. The equilibrium constant was found to be in the 300-500 M(-1) range and the complex lateral distribution pattern of EAPC in model membranes, which results from this self-association process, is characterized and seems to be mainly controlled by the amount of EAPC incorporated into the lipid bilayer.

Liposome targeting to human immunodeficiency virus type 1-infected cells via recombinant soluble CD4 and CD4 immunoadhesin (CD4-IgG)

HIV-infected cells producing virions express the viral envelope glycoprotein gp120/gp41 on their surface. We examined whether liposomes coupled to recombinant soluble CD4 (sCD4, the ectodomain of CD4 which binds gp120 with high affinity) could specifically bind to HIV-infected cells. sCD4 was chemically coupled by 2 different methods to liposomes containing rhodamine-phosphatidylethanolamine in their membrane as a fluorescent marker. In one method, sCD4 was thiolated with N-succinimidyl acetylthioacetate (SATA) and coupled to liposomes via a maleimide-derivatised phospholipid. In the other method, the oligosaccharides on sCD4 were coupled to a sulfhydryl-derivatised phospholipid, utilizing the bifunctional reagent, 4-(4-N-maleimidophenyl)butyric acid hydrazide (MPBH). The association of the liposomes with HIV-1-infected or uninfected cells was examined by flow cytometry. CD4-coupled liposomes associated specifically to chronically infected H9/HTLV-IIIB cells, but not to uninfected H9 cells. CD4-coupled liposomes also associated specifically with monocytic THP-1 cells chronically infected with HIV-1 (THP-1/HIV-1IIIB). Control liposomes without coupled CD4 did not associate significantly with any of the cells, while free sCD4 could competitively inhibit the association of the CD4-coupled liposomes with the infected cells. The chimeric molecule CD4-immunoadhesin (CD4-IgG) could also be used as a ligand to target liposomes with covalently coupled Protein A (which binds the Fc region of the CD4-IgG) to H9/HTLV-IIIB cells. The CD4-liposomes inhibited the infectivity of HIV-1 in A3.01 cells, and also bound rgp120. Our results suggest that liposomes containing antiviral or cytotoxic agents may be targeted specifically to HIV-infected cells.

Trapping of dextran-coated colloids in liposomes by transient binding to aminophospholipid: preparation of ferrosomes

A procedure is described that allows to increase the efficiency of the loading of liposomes with dextran-stabilized iron oxides (MION). The method produces a preparation of liposomes (REVs) with high iron oxide content as a result of transient binding of oxidized dextran with amino groups of aminophospholipids. Phosphatidylethanolamine (PE)-containing lipid mixtures (PC/DOPE/CH or SM/DOPE/CH, 9:2:9 molar ratio) in organic phase were combined with oxidized MION at pH 8. Liposomes then were obtained by reversed-phase evaporation. Liposomes, 263 +/- 89 nm in diameter, contained up to 11.8 mol Fe/mol phospholipid (encapsulation yield 49%). 10.2% of liposome-associated iron was dissociated from liposomes upon changing the pH to 4.5. When lipid compositions of extracts prepared from liposomes incubated at pH 4.5 and pH 8.0 were compared, an increase of relative PE-content in extracts of liposomes incubated at lowered pH was detected. This indicates a dissociation of imine bonds between aldehydes on the MION surface and PE. The accessibility of liposomal PE for acylation was demonstrated by modification with an activated ester of methoxy poly(ethylene glycol) succinate. Control liposomes, containing no aminophospholipid, or PE-containing liposomes obtained in the presence of non-oxidized MION, were 3.5-5-fold less effective for MION encapsulation and showed extensive aggregation.

Phosphatidylserine-containing membranes alter the thermal stability of prothrombin's catalytic domain: a differential scanning calorimetric study

Denaturation profiles of bovine prothrombin and its isolated fragments were examined in the presence of Na2EDTA, 5 mM CaCl2, and CaCl2 plus membranes containing 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC) in combination with bovine brain phosphatidylserine (PS). We have shown previously [Lentz, B. R., Wu, J. R., Sorrentino, A. M., & Carleton, J. A. (1991) Biophys. J. 60, 70] that binding to PS/POPC (25/75) large unilamellar vesicles resulted in an enthalpy loss in the main endotherm of prothrombin denaturation (Tm approximately 57-58 degrees C) and a comparable enthalpy gain in a minor endotherm (Tm approximately 59 degrees C) accompanying an upward shift in peak temperature (Tm approximately 73 degrees C). This minor endotherm was also responsive to Ca2+ binding and, in the absence of PS/POPC membranes, corresponded to melting of the N-terminal, Ca2+ and membrane binding domain (fragment 1). Peak deconvolution analysis of the prothrombin denaturation profile and extensive studies of the denaturation of isolated prothrombin domains in the presence and absence of PS/POPC vesicles suggested that membrane binding induced changes in the C-terminal catalytic domain of prothrombin (prethrombin 2) and in a domain that links fragment 1 with the catalytic domain (fragment 2). Specifically, the results have confirmed that the fragment 2 domain interacts with the stabilizes the prethrombin 2 domain and also have shown that fragment 2 interacts directly with the membrane. In addition, the results have demonstrated a heretofore unrecognized interaction between the catalytic and membrane binding domains. This interaction can account for another portion of the denaturation enthalpy that appears at high temperatures in the presence of membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of N-(7-nitrobenz-2-oxa-1,3-diazole-4-yl)-labeled lipids in determining transmembrane lipid distribution

Transbilayer lipid distribution of small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs) was measured using 31P-nuclear magnetic resonance (NMR) spectroscopy, chemical modification with 2,4,6-trinitrobenzene sulfonic acid (TNBS) and dithionite reduction of N-(7-nitrobenz-2-oxa-1,3-diazole-4-yl)-labeled lipid (NBD-lipid). The dithionite assay was the most reproducible of the three assays, with 1.2% error for SUVs and 3.9% error for LUVs. The dithionite assay also agreed best with theoretical inner:outer leaflet ratios, based on vesicle diameters determined by electron microscopy (Thomas et al. (1989) Biochem. Biophys. Acta 978, 85-90). Dithionite assay measurements were within 2.7% of theoretical ratios for SUVs and 2.3% for LUVs, while the NMR assay for SUVs was 14% lower than theoretical ratios and 23% lower for LUVs. The accuracy of NBD-lipids as markers for total transbilayer lipid was investigated. NBD-labeled phosphatidylserine, phosphatidylcholine and phosphatidylglycerol were accurate markers for total transbilayer lipid distribution, as their distributions were in close agreement with theoretical ratios. However, NBD-labeled phosphatidylethanolamine displayed a slight preference for the inner leaflet at low mole fractions of phosphatidylethanolamine, while native phosphatidylethanolamine showed a preference for the outer leaflet at the same concentration. NBD-labeled phosphatidic acid also showed a slight preference for the inner leaflet. We conclude that although dithionite-based assessment of NBD-labeled lipids across membrane bilayers can be a powerful analytical tool, caution must be used in the interpretation of results.

Reconstitution of the myometrial oxytocin receptor into proteoliposomes. Dependence of oxytocin binding on cholesterol

The requirements for regaining high-affinity binding of the myometrial oxytocin receptor after detergent solubilization were investigated by reconstitution experiments. Large unilamellar liposomes were prepared by reverse-phase evaporation from different mixtures of phospholipids, cholesterol and cholesteryl hemisuccinate. In the presence of the oxytocin receptor solubilized from myometrial membranes from pregnant guinea pig uterus, liposomes were treated with 3-[(3-cholamidopropyl)-dimethylammonio]-2-hydroxy-1-propanesulfonate (Chapso) throughout the range of detergent concentrations that cause the transformation of lamellar structures to mixed micelles. Detergent removal was achieved using bio-beads SM-2 as adsorbent. The presence of cholesterol was a prerequisite for regaining high-affinity binding of [3H]oxytocin and 125I-oxytocin antagonist to reconstituted proteoliposomes. Binding of [3H]oxytocin but not of the antagonist was dependent on the presence of Mn2+ ions. Reconstitution after lectin chromatography and photoaffinity labeling of reconstituted vesicles resulted in the exclusive labeling of the oxytocin receptor with a molecular mass of 68-80 kDa.

Inhibition of membrane fusion by lysophosphatidylcholine

The ability of lysophosphatidylcholine to inhibit membrane fusion at subsolubilizing concentrations (between 1 and 9 mol % with respect to the membrane lipids) was examined. Fusion between N-methyldioleoylphosphatidylethanolamine (DOPE) large unilamellar vesicles (LUV) and fusion between Sendai virus and N-methyl-DOPE LUV were measured. A contents mixing fusion assay was used for LUV fusion (ANTS/DPX), and a lipid mixing assay (octadecylrhodamine B) was used for the virus fusion experiments. Lysophosphatidylcholine was effective at inhibiting both LUV fusion and Sendai virus/LUV fusion. Lysophosphatidylcholine also inhibited leakage from N-methyl-DOPE LUV, 31P nuclear magnetic resonance data were obtained of N-methyl-DOPE in the presence of lysophosphatidylcholine. Lysophosphatidylcholine stabilized the lamellar phase and reduced the incidence of nonlamellar structures at all temperatures. The destabilization of nonlamellar structures with a negative radius of curvature may be a mechanism for inhibition of fusion by lysophosphatidylcholine in these systems.

External surface and lamellarity of lipid vesicles: a practice-oriented set of assay methods

Three methods are presented for the determination of external surface of large lipid vesicles of different lamellarity with 2% absolute accuracy. These methods (referred to as EPR, NBD and TNBS assays) use different marker lipids which provide signals (electron paramagnetic resonance, fluorescence of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) residues, and UV absorption increase of 2,4,6-trinitrobenzenesulfonic acid after reaction with aminolipids, respectively). The signals change upon addition of different membrane-impermeant reagents due to reaction with marker lipids at the external vesicle surface. They were applied to the same vesicle samples, including unilamellar and multilamellar vesicles, both at two different lipid compositions. External surface data matched for the three assays within 2%, but only after appropriate redesign or adaptation of so far published procedures. Main improvements related to slow influx of reagents (TNBS and NBD assays) or to redistribution of marker lipids (EPR assay), obscuring determination of outer vesicle surface from fast reaction between reagent and readily accessible marker lipids. Furthermore, suitable strategies were found to obtain accurate 100% values (reaction of all marker lipids present), required to relate external vesicle surface to total surface. This included corrections for light scattering (NBD assay) and for turbidity (TNBS assay). These three methods appear to close a gap in the methodology to determine external surface of vesicles for typical practical needs. In particular, the reliability range of the NBD assay could be extended to marker lipid densities as low as 1 marker lipid per 3000 lipids.

Analysis of the particle size distribution and internal volume of liposomal preparations

In this work we studied the particle size distribution of three liposomal preparations by quasi-elastic light scattering spectroscopy. Sized unilamellar vesicles of small diameter (s-SUV) were prepared by ultrasonication and subsequent centrifugation followed by extrusion through polycarbonate membranes of 0.2-micron pore size. Large unilamellar vesicles were obtained by reversed-phase evaporation (REV) and extrusion through polycarbonate filters with or without preliminary freezing-thawing cycles (VETI and VETII, respectively). After preparation, REV were sized to small diameter REV (s-REV) by extrusion through 0.4- and 0.2-micron polycarbonate membranes. According to the results, the s-SUV preparations were made up of two subpopulations, the major of which consisted of vesicles that were 26 nm in mean diameter and accounted for 95% of the overall s-SUV population. The s-REV dispersions always resolved into two populations centered at 120 and 380 nm, the relative proportions of which depended on the pore size of the filters used. VET structures were composed of a single population of vesicles that were approximately 100 nm in mean diameter. Cholesterol inclusion into the bilayer composition extended the distribution without altering its mean value. On the other hand, the internal volumes calculated from mean diameters or assuming a Gaussian distribution were inconsistent with experimental data obtained by usual techniques.

Size distribution of liposomes by flow field-flow fractionation

The applicability of field-low fractionation (FFF) to the characterization of liposomes is discussed and theoretically described. Because of fundamental differences in their driving forces, sedimentation FFF and flow FFF measure different vesicle properties. Sedimentation FFF, although used previously to measure vesicle sizes and size distributions, is fundamentally a technique that measures the effective mass and mass distribution of particles. It is sensitive to small changes in the effective mass of either the biomembrane or its encapsulated load and thus is likely to be useful in characterizing such properties as drug loading, biomembrane volumes and areas, and distributions of these properties. Size characterization by sedimentation FFF can only be done by deducing size from effective mass. Flow FFF, by contrast, provides a direct measurement of vesicle size and size distribution. After demonstrating the high resolution and relative accuracy of size measurement of flow FFF by the separation of polystyrene latex standards, flow FFF was applied to two preparations of DSPC-DSPA liposomes that were sonicated under different temperature conditions. Fractograms and size distributions are reported as a function of sonication time. The rapid elimination of a large diameter tail to the distribution is shown to constitute a major mechanism for distribution narrowing. Finally, results are provided bearing on the reproducibility of size distribution measurements by flow FFF.

Annexin I interactions with human neutrophil specific granules: fusogenicity and coaggregation with plasma membrane vesicles

The interactions of annexin I with specific granules isolated from human neutrophils were investigated. Unfractionated cytosol induced Ca(2+)-dependent granule self-aggregation and fusion of granules with model phospholipid vesicles. High Ca2+ concentrations were required for these processes (500-600 microM for the half-maximal rate of granule self-aggregation; 100-200 microM for the half-maximal rate of fusion with phospholipid vesicles). These activities were inhibited by a monoclonal antibody specific for annexin I and immunodepletion of cytosol by this antibody greatly reduced activity, implicating annexin I as the major mediator of these processes in neutrophil cytosol. The fact that the Ca2+ concentration dependences differed for different membranes suggests that specificity may be controlled by the type of intracellular membrane involved and the local Ca2+ concentration. Trypsin treatment of granules enhanced the rate of fusion of phospholipid vesicles with granules, suggesting that access to phospholipids in the granule membrane may be modulated by granule proteins or that a fusogenic protein factor in the granule membrane is activated by trypsin treatment. Coaggregation of specific granules with plasma membrane vesicles mediated by Ca2+ and annexin I was suggested by the fact that granules preincubated with Ca2+, cytosol and plasma membrane vesicles blocked the fusion of subsequently added phospholipid vesicles with the plasma membrane vesicles. These data suggest a role for annexin I as part of a multiprotein system involved in membrane-membrane contact necessary for exocytosis of specific granules in human neutrophils.

Human immunodeficiency virus type 1 (HIV-1) fusion with model membranes: kinetic analysis and the role of lipid composition, pH and divalent cations

The kinetics and extent of HIV-1 fusion with model membranes was studied. HIV-1 was labeled with octadecyl rhodamine B chloride, and fusion was monitored continuously as the dilution of the probe into target membranes. The results were analyzed by a mass action model which yielded good simulations and predictions for the kinetics and final extents of fluorescence increase. The model determined the percent of virions capable of fusing and rate constants of fusion, aggregation and dissociation. Ultrastructural analysis of the virus and reaction products by electron microscopy also provided evidence of HIV-1 fusion with membranes lacking CD4. HIV-1 fusion activity depends on the target membrane lipid composition according to the sequence: cardiolipin (CL) > > phosphatidylinositol > CL/dioleoylphosphatidylcholine (DOPC) (3:7), phosphatidic acid > phosphatidylserine (PS), PS/cholesterol (2:1) > PS/PC (1:1), PS/phosphatidylethanolamine (1:1) > DOPC, erythrocyte ghosts. Reduction of pH from 7.5 generally enhances the rate and extent of HIV-1 fusion. Physiologically relevant concentrations of calcium stimulate HIV-1 fusion with several liposome compositions and with erythrocyte ghost membranes. The fusion products of HIV-1 with liposomes consist of a single virus and several liposomes. The mass action analysis revealed that, compared to intact virions, the fusion products show a striking reduction in the fusion rate constant. Like influenza and Sendai viruses, HIV-1 fusion with membranes containing its own envelope glycoprotein(s) is strongly inhibited. Unlike these viruses, HIV-1 fusion is promoted by physiological levels of calcium. HIV-1 fusion with liposomes is qualitatively similar to simian immunodeficiency virus fusion.

23Na-NMR study of ion transport across vesicle membranes facilitated by phenylalanine analogs of gramicidin

The transport of Na+ ions across phosphatidylcholine/phosphatidylglycerol large unilamellar vesicle membranes facilitated by phenylalanine analogs of gramicidin A has been studied using 23Na-NMR spectroscopy. The four analogs studied were Phe9-, Phe11-, Phe13-and Phe15-gramicidin A. These analogs were found to transport Na+ ions in the following order Phe15 > Phe13 > Phe11 > Phe9. The entropy and enthalpy of activation for the transport of Na+ ions were determined for each analog. A correlation is made between the activation enthalpies and the single channel conductance values of the analogs.

Aggregation of hapten-bearing liposomes mediated by specific antibodies

We studied specific membrane-membrane interactions mediated by ligand-receptor binding in a model system, which consisted of (a) FG3P, the fluorescein hapten attached to a phospholipid by a peptidyl spacer as described previously (Petrossian, A., A.B. Kantor, and J.C. Owicki. 1985. J. Lipid Res. 26:767-773), (b) antifluorescein monoclonal antibodies (MAbs), and (c) phospholipid vesicles (liposomes) into which the FG3P was incorporated. The aggregation of the hapten-bearing liposomes by four MAbs was studied by differential centrifugation. The ability of the MAbs to induce vesicle aggregation varied considerably and correlated inversely with affinity. Aggregation by one of the MAbs was studied in more detail by turbidimetry and freeze-fracture electron microscopy of samples frozen throughout the course of the aggregation. Rapid freezing was achieved with a double propane-jet apparatus. The aggregate morphologies and the time evolution of the aggregate size distribution were obtained from the two-dimensional fracture views with a stereological correction. The aggregation kinetics were simulated by considering dynamical aggregation according to a mass-action model with two parameters, the rate constants for antibody-mediated vesicle aggregation and disaggregation. Both rate constants were orders of magnitude lower than the rate constants for the corresponding interactions of antibodies with haptens either in solution or on vesicles under nonaggregating conditions.

Osmotic properties of large unilamellar vesicles prepared by extrusion

We have examined the morphology and osmotic properties of large unilamellar vesicles (LUVs) prepared by extrusion. Contrary to expectations, we observe by cryo-electron microscopy that such vesicles, under isoosmotic conditions, are non-spherical. This morphology appears to be a consequence of vesicle passage through the filter pores during preparation. As a result when such LUVs are placed in a hypoosmotic medium they are able to compensate, at least partially, for the resulting influx of water by "rounding up" and thereby increasing their volume with no change in surface area. The increase in vesicle trapped volume associated with these morphological changes was determined using the slowly membrane-permeable solute [3H]-glucose. This allowed calculation of the actual osmotic gradient experienced by the vesicle membrane for a given applied differential. When LUVs were exposed to osmotic differentials of sufficient magnitude lysis occurred with the extent of solute release being dependent on the size of the osmotic gradient. Surprisingly, lysis was not an all-or-nothing event, but instead a residual osmotic differential remained after lysis. This differential value was comparable in magnitude to the minimum osmotic differential required to trigger lysis. Further, by comparing the release of solutes of differing molecular weights (glucose and dextran) a lower limit of about 12 nm diameter can be set for the bilayer defect created during lysis. Finally, the maximum residual osmotic differentials were compared for LUVs varying in mean diameter from 90 to 340 nm. This comparison confirmed that these systems obey Laplace's Law relating vesicle diameter and lysis pressure. This analysis also yielded a value for the membrane tension at lysis of 40 dyn cm-1 at 23 degrees C, which is in reasonable agreement with previously published values for giant unilamellar vesicles.

Quantitative analysis of liposome-cell interactions in vitro: rate constants of binding and endocytosis with suspension and adherent J774 cells and human monocytes

We have characterized the parameters describing the total association (uptake) of liposomes with murine macrophage-like cell line J774 cells and human peripheral blood monocytes at 4 degrees C and at 37 degrees C with or without inhibitors of endocytosis. The uptake of neutral liposomes composed of phosphatidylcholine (PC)/cholesterol (Chol) (2:1 mole ratio) is about 10-fold lower than that of negatively charged liposomes composed of phosphatidylserine (PS)/PC/Chol (1:1:1). However, the rate of uptake of PC/Chol liposomes at 37 degrees C is still 10-fold higher than that by fluid-phase pinocytosis. The uptake of liposomes, which is mediated by high-affinity binding to the cell surface binding sites and subsequent endocytosis, could be simulated and predicted by model calculations employing mass action kinetics. The number of binding sites, affinity constants of binding at 37 degrees C and 4 degrees C, on- and off-rate constants of binding, and endocytic rate constants for both types of liposomes were determined. The number of binding sites and the binding constants for PS/PC/Chol liposomes binding to J774 cells is severalfold to an order of magnitude higher than that for PC/Chol liposomes, but the rate constants at which they are endocytosed following binding to the cells are similar for both liposome types. The binding of liposomes, especially PS/PC/Chol, to J774 cells and monocytes is greatly enhanced by adherence of cells to plastic substratum and is also increased by maturation/differentiation in the case of monocytes. Our quantitative analysis indicates that the binding and endocytosis of liposomes, especially PS-containing liposomes, is mediated by binding sites that have strong affinity, comparable to or about an order of magnitude smaller than other known particle-cell interactions with specific receptors such as virus and lipoproteins binding to cells.

Efficacies of liposome-encapsulated streptomycin and ciprofloxacin against Mycobacterium avium-M. intracellulare complex infections in human peripheral blood monocyte/macrophages

Current treatments of disseminated infection caused by the Mycobacterium avium-M. intracellulare complex (MAC) are generally ineffective. Liposome-mediated delivery of antibiotics to MAC-infected tissues in vivo can enhance the efficacy of the drugs (N. Düzgüneş, V. K. Perumal, L. Kesavalu, J. A. Goldstein, R. J. Debs, and P. R. J. Gangadharam, Antimicrob. Agents Chemother. 32:1404-1411, 1988; N. Düzgüneş, D. A. Ashtekar, D. L. Flasher, N. Ghori, R. J. Debs, D. S. Friend, and P. R. J. Gangadharam, J. Infect. Dis. 164:143-151, 1991). We investigated the therapeutic efficacies of liposome-encapsulated streptomycin and ciprofloxacin against growth of the MAC inside human peripheral blood monocyte/macrophages. Treatment was initiated 24 h after infection of macrophages with the MAC and stopped after 20 h, and the cells were incubated for another 7 days. The antimycobacterial activity of streptomycin was enhanced when the drug was delivered to macrophages in liposome-encapsulated form, reducing the CFU about threefold more than the free drug did throughout the concentration range studied (10 to 50 micrograms/ml). With 50 micrograms of encapsulated streptomycin per ml, the CFU were reduced to 11% of the initial level of infection. Liposome-encapsulated ciprofloxacin was at least 50 times more effective against the intracellular bacteria than was the free drug: at a concentration of 0.1 microgram/ml, liposome-encapsulated ciprofloxacin had greater antimycobacterial activity than the free drug at 5 microgram/ml. With liposome-encapsulated ciprofloxacin at 5 micrograms/ml, the CFU were reduced by more than 1,000-fold at the end of the 7-day incubation period, compared with untreated controls. These results suggest that liposome-encapsulated ciprofloxacin or other fluoroquinolones may be effective against MAC infections in vivo.

Retinal and retinol promote membrane fusion

Disk membranes from the bovine retinal rod outer segments (ROS) were found to fuse with vesicles made of lipids extracted from unbleached ROS disk membranes, using a lipid mixing assay for membrane fusion (relief of self-quenching of R18, octadecylrhodamine B chloride). If the retinal chromophore of rhodopsin was reductively linked to opsin before lipid extraction, the vesicles made of the extracted lipids were not suitable targets for fusion of the disk membranes. The addition of retinal and retinol to these vesicles restored their ability to fuse. Therefore, the presence of all-trans retinal was implicated in promoting membrane fusion in this system. To test this possibility, the ability of retinal and retinol to influence the phase behavior and the fusion capability of large unilamellar vesicles (LUV) of N-methyl dioleoylphosphatidylethanolamine (N-methyl-DOPE) was examined. Both retinal and retinol stimulated the fusion of vesicles of N-methyl-DOPE (contents mixing with ANTS, 1-aminonaphthalene-3,6,8-trisulfonic acid; DPX, p-xylylene bis(pyridinium bromide)). Both compounds reduced the onset temperature for isotropic resonances in the 31P-NMR spectra of N-methyl-DOPE dispersions and the onset temperature, TH, for formation of hexagonal II phase. These results were consistent with previous studies in which the onset temperature for the 31P-NMR isotropic resonances were correlated with stimulation of membrane fusion. These data suggested that both retinal and retinol may stimulate membrane fusion by destabilizing the bilayers of membranes.

Evidence that the scavenger receptor is not involved in the uptake of negatively charged liposomes by cells

Scavenger receptors have a broad ligand specificity, ranging from modified low-density lipoproteins to a variety of high-molecular-weight poly-anions. A recent report by Nishikawa et al. (J. Biol. Chem. (1990) 265, 5226-5231) suggested that this receptor is also involved in the binding and endocytosis of liposomes containing negatively charged phospholipids. The mechanism by which liposomes are taken up by cells is of interest because liposomes are promising versatile carriers for macromolecules and drugs both in vitro and in vivo. In this report, we re-examine the role of the scavenger receptor in the uptake of liposomes using both Chinese hamster ovary cells transfected with the type I or type II bovine scavenger receptor, and smooth muscle cells induced to increase scavenger receptor expression by phorbol ester treatment. Expression of both types of scavenger receptors by Chinese hamster ovary cells induced an increase in the uptake of chemically modified low-density lipoproteins, but not the uptake of negatively charged liposomes. In smooth muscle cells treated with phorbol ester, scavenger receptor expression was upregulated and the uptake of chemically modified low-density lipoproteins was enhanced dramatically, but there was no effect on the uptake of negatively charged liposomes. We conclude that the existing evidence does not support the suggestion that the scavenger receptor is involved in the uptake of anionic liposomes by cells.

Polarity of lipid bilayers. A fluorescence investigation

Through steady-state and time-resolved fluorescence experiments, the polarity of the bilayers of egg phosphatidylcholine vesicles was studied by means of the solvatochromic 2-anthroyl fluorophore which we have recently introduced for investigating the environmental micropolarity of membranes and which was incorporated synthetically in phosphatidylcholine molecules (anthroyl-PC) in the form of 8-(2-anthroyl)octanoic acid. Fluorescence quenching experiments carried out with N,N-dimethylaniline and 12-doxylstearic acid as quenchers showed that the 2-anthroyl chromophore was located in depth in the hydrophobic region of the lipid bilayer corresponding to the C9-C16 segment of the acyl chains. Steady-state fluorescence spectroscopy revealed a nonstructured and red-shifted (lambda em(max) = 464 nm) spectrum for the probe in egg-PC bilayers, which greatly differed from the structured and blue (lambda em(max) = 404 nm) spectrum the fluorophore was shown to display in n-hexane. While the fluorescence decays of the fluorophore in organic solvents were monoexponential, three exponentials were required to account for the fluorescence decays of anthroyl-PC in egg-PC vesicles, with average characteristic times of 1.5 ns, 5.5 ns, and 20 ns. These lifetime values were independent of the emission wavelength used. Addition of cholesterol to the lipid did not alter these tau values. One just observed an increase in the fractional population of the 1.5-ns short-living species detrimental to the population of the 20-ns long-living ones. These observations enabled time-resolved fluorescence spectroscopy measurements to be achieved in the case of the 1/1 (mol/mol) egg-PC/cholesterol mixture. Three distinct decay associated spectra (DAS) were recorded, with maximum emission wavelengths, respectively, of 410 nm, 440 nm, and 477 nm for the 1.5-ns, 6-ns, and 20-ns lifetimes found in this system. On account of the properties and the polarity scale previously established for the 2-anthroyl chromophore in organic solvents, these data strongly suggest the occurrence of three distinct excited states for anthroyl-PC in egg-PC bilayers, corresponding to three environments for the 2-anthroyl chromophore, differing in polarity. The lifetime of 1.5 ns and the corresponding structured and blue (lambda em(max) = 410 nm) DAS account for a hydrophobic environment, with an apparent dielectric constant of 2, which is that expected for the hydrophobic core of the lipid bilayer.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane destabilization by N-terminal peptides of viral envelope proteins

The fusion of lipid enveloped viruses with cellular membranes is thought to be mediated by the insertion into the target membrane of the N-terminal polypeptides of viral spike glycoproteins. Since membrane destabilization is a necessary step in membrane fusion, we investigated whether synthetic peptides with amino acid sequences corresponding to the N-termini of influenza virus hemagglutinin (HA2), vesicular stomatitis virus G-protein and Sendai virus F-protein, induce the destabilization and fusion of phospholipid vesicles. Membrane destabilization by the peptides was monitored by the release of aqueous contents of large unilamellar phospholipid vesicles. Aggregation was detected by a resonance energy transfer assay. Membrane fusion was followed by means of assays for the intermixing of phospholipids and of aqueous contents. The 17-amino acid HA2 peptide (HA2.17) destabilized phosphatidylcholine (PC) vesicles even at neutral pH, but the rate and extent of destabilization increased at lower pH. This peptide did not mediate appreciable release of contents from phosphatidylserine (PS) vesicles. HA2.17 induced neither aggregation nor fusion of PC or PS vesicles. In contrast, the 7-amino acid N-terminal peptide of G-protein (G.7) destabilized PS-containing membranes and not pure PC vesicles. Although G.7 caused aggregation of and lipid mixing between PS vesicles, it did not mediate any detectable intermixing of aqueous contents. The presence of cholesterol in PC membranes did not affect the destabilization caused by the N-terminal peptide of Sendai virus F-protein (F1.7), suggesting that cholesterol is not necessary for the effective interaction of this peptide with membranes, contrary to earlier proposals. Our results support the hypothesis that the hydrophobic N-terminal region of certain viral envelope proteins insert into and destabilize target membranes.

The local anesthetic tetracaine destabilizes membrane structure by interaction with polar headgroups of phospholipids

The effect of the local anesthetic tetracaine at less than 10 mM on the water permeability of the phospholipid membrane was examined using liposomes composed of various molar ratios of negatively charged cardiolipin to electrically neutral phosphatidylcholine by monitoring their osmotic shrinkage in hypertonic glucose solution at 30 degrees C. The concentration of tetracaine causing the maximum velocity of shrinkage of liposomes increased with increase in the molar ratio of cardiolipin. Tetracaine increased the zeta-potential of the negatively charged liposomal membrane toward the positive side due to the binding of its cationic form to the negatively charged polar headgroups in the membrane. The maximum velocity of water permeation induced by osmotic shock was observed at essentially the same tetracaine concentration giving a zeta-potential of the liposomal membrane of 0 mV. These concentrations were not affected by change in the sort of acyl-chain of phospholipids in the liposomes when their negative charges were the same. These results suggests that the membrane integrity is governed mainly by the electrical charge of phospholipid polar headgroups when phospholipid bilayers are in the highly fluid state, and that positively charged tetracaine molecules neutralize the negative surface charge, lowering the barrier for water permeation through phospholipid bilayers.

Excretion of glutamate from Corynebacterium glutamicum triggered by amine surfactants

Corynebacterium glutamicum is used for the industrial production of glutamate. Excretion of the amino acid may be induced by various means. We have analyzed the characteristics of glutamate excretion induced by two amine surfactants, dodecylammonium acetate (DA) and dodecyltrimethylammonium bromide (DTA). Addition of these surfactants induced an immediate efflux of internal glutamate. It also induced a perturbation of the energetic parameters of the cell (decrease of delta mu H, decrease of the internal ATP concentration). The efflux was not the result of these perturbations: glutamate is taken up by the cells via an ATP-dependent unidirectional active transport system and no efflux took place as a consequence of an artificial decrease of the energetic parameters. In addition, amine surfactants also induced an excretion of other species, in particular potassium. We have tested the possibility that the effluxes result from a permeabilization of the lipid bilayer by analyzing the interactions between the surfactants and liposomes.

Recognition of liposomes by cells: in vitro binding and endocytosis mediated by specific lipid headgroups and surface charge density

We investigated the interaction of liposomes of different surface properties with two mammalian cell lines, CV1, an African green monkey kidney cell line, and J774, a murine macrophage-like cell line. Cell surface binding and endocytosis of liposomes were quantified by fluorometry, using the liposome-encapsulated pH-sensitive fluorescent dye, pyranine, and the lipid marker rhodamine-PE. The liposome uptake was dependent both on the surface properties of the liposomes and on the cell line. Negatively charged phospholipids incorporated into egg phosphatidylcholine (PC)/cholesterol (C) (2:1) liposomes were recognized by the two cell lines to different extents depending on the lipid headgroup and its charge density in the liposome bilayer. Inclusion of 9% phosphatidylserine (PS), phosphatidylglycerol (PG), or phosphatidic acid (PA) promoted the uptake by CV1 cells more than 20-fold. Increasing the content of these negatively charged lipids beyond 9% did not further enhance the uptake. In contrast, 9% monosialoganglioside GM1, phosphatidylinositol (PI), or phosphatidylethanolamine conjugated to poly(ethylene glycol) (PEG-PE) did not promote the uptake. Inclusion of 9% PS, PG, PA or PI in PC/C liposomes did not enhance the uptake by J774 cells, but a drastic enhancement was observed when increasing concentrations of these anionic lipids were incorporated in the liposome bilayer. At least 50% PS, PG, or PI was needed to reach the level of uptake seen with CV1 cells. The uptake of liposomes containing 50% PS by J774 cells was inhibited by poly-anions which are the competing ligands for scavenger receptors, but the uptake by CV1 was not inhibited. Different mechanisms of liposome uptake by these two cell lines are suggested from the different patterns of uptake and the competition with various poly-anions. The differences observed in the uptake rate of liposomes with different lipid compositions seemed to be primarily due to the differences in the binding between liposomes and cell membrane components. The in vitro interaction of various liposomes with these cell lines, especially CV1 cells, shows significant similarities to the in vivo clearance rates of the liposomes.

Sequence of critical events involved in fusion of phospholipid vesicles induced by clathrin

Membrane fusion induced by clathrin is accompanied by several events such as conformational change, membrane binding and association of clathrin, and membrane aggregation (Maezawa et al. (1989) Biochemistry 28, 1422-1428; Maezawa and Yoshimura (1990) Biochem. Biophys. Res. Commun. 173, 134-140). To clarify the sequence of these events, we examined their time-courses by reducing the pH of the medium from 7.4 to a given pH in the range of 3.5-5.0 at 25 degrees C or 10 degrees C. Large unilamellar vesicles composed of phosphatidylserine and phosphatidylcholine were used in most experiments. The half-time for conformational change of clathrin was less than those for membrane binding and association of clathrin. The half-times and the initial rates of membrane binding and association of clathrin were similar order of magnitude, although the pH-profiles of the initial rates of the two events were somewhat different. Membrane aggregation started after membrane binding of clathrin. A lag phase was observed in the time-course of membrane fusion, whereas there was no lag phase in membrane binding and association of clathrin and membrane aggregation. Moreover, the lag time before fusion was independent of the clathrin concentration, although the initial rates of these three events were dependent on it, suggesting that the three reactions are not responsible for the lag phase before fusion, and that there is some other event(s) in the lag time. On the other hand, there was a threshould-pH in the pH profile of the lag-time and the threshold-pH coincided with the critical pH at which the final associated state of clathrin was apparently reversed in the presence and absence of liposomes, suggesting that the event(s) in the lag phase may be related to this final associated state of clathrin molecules on the liposome membranes. These results indicate that clathrin-induced fusion of liposomes is initiated through the following sequential events: conformational change of clathrin, membrane binding and association of clathrin, which occur simultaneously but independently, membrane aggregation, an event(s) in the lag phase, and actual fusion.

Light microscopic localization of silver-enhanced liposome-entrapped colloidal gold in mouse tissues

Silver-enhanced liposome-entrapped colloidal gold was developed for light microscopic localization of liposomes. Preparation of colloidal gold entrapped in liposomes was achieved by a modified method of Hong, et al. (1983) Biochim. Biophys. Acta 732, 320-323). In this report, a gold chloride/citrate solution of low pH (3.4) was used to inhibit the formation of gold granules during the liposome preparation. The diameter of most liposomes ranged from 80 to 100 nm. Following liposome preparation, the pH was adjusted to 6, and the temperature increased to 55 degrees C. The majority of the liposomes contained one to three gold particles. Liposomes were injected into mice via tail vein; 24 h later, tissues were collected. Sections were processed for silver enhancement of the gold particles and examined by light microscopy. Silver-enhanced gold particles were clearly observed in both liver and implanted tumor. Localization was confirmed by electron and fluorescence microscopy. Thus, we have shown that silver enhancement of colloidal gold liposomes is a direct and sensitive method for tracing the fate of liposomes in vivo, providing minimal background interference and a good definition of various cell types.

Membrane binding induces lipid-specific changes in the denaturation profile of bovine prothrombin. A scanning calorimetry study

Prothrombin denaturation was examined in the presence of Na2EDTA, 5mM CaCl2, and CaCl2 plus membranes containing 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC) in combination with either bovine brain phosphatidylserine (PS) or 1,2-dioleoyl-phosphatidylglycerol (DOPG). Heating denaturation of prothrombin produced thermograms showing two peaks, a minor one at approximately 59 degrees C previously reported to correspond to denaturation of the fragment 1 region (Ploplis, V. A., D. K. Strickland, and F. J. Castellino 1981. Biochemistry. 20:15-21), and a main one at approximately 57-58 degrees C, reportedly due to denaturation of the rest of the molecule (prethrombin 1). The main peak was insensitive to the presence of 5mM Ca2+ whereas the minor peak was shifted to higher temperature (Tm approximately 65 degrees C) by Ca2+. Sufficient concentrations of POPC/bovPS (75/25) large unilamellar vesicles to guarantee binding of 95% of prothrombin resulted in an enthalpy loss in the main endotherm and a comparable enthalpy gain in the minor endotherm accompanying an upward shift in peak temperature (Tm approximately 73 degrees C). Peak deconvolution analysis on the prothrombin denaturation profile and comparison with isolated prothrombin fragment 1 denaturation endotherms suggested that the change caused by POPC/PS vesicles reflected a shift of a portion of the enthalpy of the prethrombin 1 domain to higher temperature (Tm approximately 77 degrees C). The enthalpy associated with this high-temperature endotherm increased in proportion to the surface concentration of PS. By contrast, POPC/DOPG (50/50) membranes shifted the prethrombin 1 peak by 4 degrees C to a lower temperature and the fragment 1 peak by 5 degrees C to a higher temperature. The data lead to a hypothesis that the fragment 1 and prethrombin 1 domains of prothrombin do not denature quite independently and that binding of prothrombin to acidic-lipid membranes disrupts the interaction between these domains. It is further hypothesized that PS containing membranes exert the additional specific effect of decoupling the denaturation of two subdomains of the prethrombin 1 domain of prothrombin.

Annexin-mediated membrane fusion of human neutrophil plasma membranes and phospholipid vesicles

Membrane fusion was studied using human neutrophil plasma membrane preparations and phospholipid vesicles approximately 0.15 microns in diameter and composed of phosphatidylserine and phosphatidylethanolamine in a ratio of 1 to 3. Liposomes were labeled with N-(7-nitrobenzo-2-oxa-1,3-diazol-4-yl (NBD) and lissamine rhodamine B derivatives of phospholipids. Apparent fusion was detected as an increase in fluorescence of the resonance energy transfer donor, NBD, after dilution of the probes into unlabeled membranes. 0.5 mM Ca2+ alone was sufficient to cause substantial fusion of liposomes with a plasma membrane preparation but not with other liposomes. Both annexin I and des(1-9)annexin I caused a substantial increase in the rate of fusion under these conditions while annexin V inhibited fusion. Fusion mediated by des(1-9)annexin I was observed at Ca2+ concentrations as low as approximately 5 microM, suggesting that the truncated form of this protein may be active at physiologically low Ca2+ concentrations. Trypsin treated plasma membranes were incapable of fusion with liposomes, suggesting that plasma membrane proteins may mediate fusion. Liposomes did not fuse with whole cells at any Ca2+ concentration, indicating that the cytoplasmic side of the membrane is involved. These results suggest that annexin I and unidentified plasma membrane proteins may play a role in Ca(2+)-dependent degranulation of human neutrophils.

Inhibition of Sendai virus fusion with phospholipid vesicles and human erythrocyte membranes by hydrophobic peptides

Hydrophobic di- and tripeptides which are capable of inhibiting enveloped virus infection of cells are also capable of inhibiting at least three different types of membrane fusion events. Large unilamellar vesicles (LUV) of N-methyl dioleoylphosphatidylethanolamine (N-methyl DOPE), containing encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and/or p-xylene bis(pyridinium bromide) (DPX), were formed by extrusion. Vesicle fusion (contents mixing) and leakage were then monitored with the ANTS/DPX fluorescence assay. Sendai virus fusion with lipid vesicles and Sendai virus fusion with human erythrocyte membranes were measured by following the relief of fluorescence quenching of virus labeled with octadecylrhodamine B chloride (R18), a lipid mixing assay for fusion. This study found that the effectiveness of the peptides carbobenzoxy-L-Phe-L-Phe (Z-L-Phe-L-Phe), Z-L-Phe, Z-D-Phe, and Z-Gly-L-Phe-L-Phe in inhibiting N-methyl DOPE LUV fusion or fusion of virus with N-methyl DOPE LUV also paralleled their reported ability to block viral infectivity. Furthermore, Z-D-Phe-L-PheGly and Z-Gly-L-Phe inhibited Sendai virus fusion with human erythrocyte membranes with the same relative potency with which they inhibited vesicle-vesicle and virus-vesicle fusion. The evidence suggests a mechanism by which these peptides exert their inhibition of plaque formation by enveloped viruses. This class of inhibitors apparently acts by inhibiting fusion of the viral envelope with the target cell membrane, thereby preventing viral infection. The physical pathway by which these peptides inhibit membrane fusion was investigated. 31P nuclear magnetic resonance (NMR) of proposed intermediates in the pathway for membrane fusion in LUV revealed that the potent fusion inhibitor Z-D-Phe-L-PheGly selectively altered the structure (or dynamics) of the hypothesized fusion intermediates and that the poor inhibitor Z-Gly-L-Phe did not. One possible interpretation of these 31P NMR results was that the inhibitory peptide stabilized a membrane structure with a large radius of curvature, when the fusion pathway demanded a membrane defect with a small radius of curvature. This hypothesis was tested by determining the influence of an inhibitory and a noninhibitory peptide on the formation of membraneous structures with small radii of curvature, through ultrasonic irradiation of phospholipid dispersions. The inhibitory peptide prevented the formation of membrane structures with small radii of curvature, while the noninhibitory peptide did not prevent the formation of such structures.(ABSTRACT TRUNCATED AT 400 WORDS)