Physicochemical characterization of large unilamellar phospholipid vesicles prepared by reverse-phase evaporation

Morphology and phase behavior of two types of unilamellar vesicles prepared from synthetic phosphatidylcholines studied by freeze-fracture electron microscopy and calorimetry

Differential scanning calorimetry and freeze-fracture electron microscopy have been used to characterize the phase behavior and morphology of two types of unilamellar vesicles composed of synthetic phosphatidylcholines. The first type displayed an average diameter of roughly 100 nm and was formed by slow dilution and dialysis of octylglucoside-solubilized lipid. These large, unilamellar vesicles were termed dialyzed, octylglucoside vesicles and could be obtained as a fairly well defined and uniform population of vesicles. The second vesicle type was prepared by a unique procedure involving dialysis of deoxycholate-solubilized lipid at its pre-transition temperature. This procedure produced a much more heterogeneous distribution of vesicle sizes (500 to 4000 nm in diameter) and left some dilamellar and oligolamellar species which could not be conveniently separated from the giant, unilamellar vesicles constituting the major portion of the sample. Both populations of vesicles displayed phase behavior similar, but not identical to that of large, multilamellar vesicles (LMV). Fracture-face morphology of the gel phase was also observed to differ between the two unilamellar and the multilamellar species. LMV have previously been shown to have clear undulated or banded fracture-faces in the P beta phase, while octylglucoside vesicles are shown here to have facetted fracture-faces. Giant, unilamellar vesicles displayed a faint banded morphology similar to but less distinct than that of the LMV P beta phase. These results have demonstrated that bilayer apposition is not required to support the banded fracture-face morphology characteristic of the P beta phase but that a limiting curvature is necessary.

Production of large unilamellar vesicles by a rapid extrusion procedure: characterization of size distribution, trapped volume and ability to maintain a membrane potential

A technique for the rapid production of large unilamellar vesicles by repeated extrusion under moderate pressures (≤ 500 lb/in²) of multilamellar vesicles through polycarbonate filters (100 nm pore size) is demonstrated. In combination with freeze-thaw protocols where required, this procedure results in unilamellar vesicles with diameters in the range 60-100 nm and with trapped volumes in the region of 1-3 μl/μmol phospholipid. Advantages of this technique include the absence of organic solvents or detergents, the high lipid concentrations (up to 300 μmol/ml) that can be employed and the high trapping efficiencies (up to 30%) that can be achieved. Further, the procedure for generating these 'LUVET's' (large unilamellar vesicles by extrusion techniques) is rapid (≤ min preparation time) and can be employed to generate large unilamellar vesicles from a wide variety of lipid species and mixtures. As a particular illustration of the utility of this vesicle preparation, LUVET systems exhibiting a membrane potential (ΔΨ) in response to a transmembrane Na⁺/K⁺ gradient (K⁺ inside) have been characterized. By employing the lipophilic cation methyltriphenylphosphonium (MTPP⁺) it is shown that a K⁺ of diffusion potential (ΔΨ < -100 mV) forms rapidly in the presence of the K⁺ ionophore valinomycin for soya phosphatidylcholine (soya PC) LUVET's. The values of Δψ obtained correlate well with the K⁺ concentration gradient across the membrane, and it is demonstrated that the decay of Δψ with time depends on the flux of Na+ into the vesicles.

Modulation of membrane fusion by membrane fluidity: temperature dependence of divalent cation induced fusion of phosphatidylserine vesicles

We have investigated the temperature dependence of the fusion of phospholipid vesicles composed of pure bovine brain phosphatidylserine (PS) induced by Ca2+ or Mg2+. Aggregation of the vesicles was monitored by 90 degrees light-scattering measurements, fusion by the terbium/dipicolinic acid assay for mixing of internal aqueous volumes, and release of vesicle contents by carboxyfluorescein fluorescence. Membrane fluidity was determined by diphenylhexatriene fluorescence polarization measurements. Small unilamellar vesicles (SUV, diameter 250 A) or large unilamellar vesicles (LUV, diameter 1000 A) were used, and the measurements were done in 0.1 M NaCl at pH 7.4. The following results were obtained: (1) At temperatures (0-5 degrees C) below the phase transition temperature (Tc) of the lipid, LUV (PS) show very little fusion in the presence of Ca2+, although vesicle aggregation is rapid and extensive. With increasing temperature, the initial rate of fusion increases dramatically. Leakage of contents at the higher temperatures remains limited initially, but subsequently complete release occurs as a result of collapse of the internal aqueous space of the fusion products. (2) SUV (PS) are still in the fluid state down to 0 degree C, due to the effect of bilayer curvature, and fuse rapidly in the entire temperature range from 0 to 35 degrees C in the presence of Ca2+. The initial rate of leakage is low relative to the rate of fusion. At higher temperatures (15 degrees C and above), subsequent collapse of the vesicles' internal space causes complete release.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-sensitive liposomes mediate cytoplasmic delivery of encapsulated macromolecules

Negatively charged liposomes are endocytosed by the coated vesicle system and accumulate in acidic intracellular vesicles. Liposomes that become unstable at acidic pH improve cytoplasmic delivery of membrane-impermeant macromolecules such as calcein (CAL) and FITC dextran (18 or 40 kDa). Oleic acid (OA): phosphatidylethanolamine (PE) (3:7 mole ratio) liposomes become permeable to CAL at pH less than 7.0. Control liposomes of phosphatidylserine:PE or OA:phosphatidylcholine are stable at pH 4-8. OA:PE liposomes promote cytoplasmic delivery of encapsulated CAL to CV-1 cells, as evidenced by the emergence of diffuse, cytoplasmic CAL fluorescence. Delivery requires metabolic energy and is partially inhibited by chloroquine or monensin, which raise the pH of intracellular vesicles.

Fluorescence method for measuring the kinetics of fusion between biological membranes

An assay is presented that allows continuous and sensitive monitoring of membrane fusion in both artificial and biological membrane systems. The method relies upon the relief of fluorescence self-quenching of octadecyl Rhodamine B chloride. When the probe is incorporated into a lipid bilayer at concentrations up to 9 mol% with respect to total lipid, the efficiency of self-quenching is proportional to its surface density. Upon fusion between membranes labeled with the probe and nonlabeled membranes, the decrease in surface density of the fluorophore results in a concomitant, proportional increase in fluorescence intensity, allowing kinetic and quantitative measurements of the fusion process. The kinetics of fusion between phospholipid vesicles monitored with this assay were found to be the same as those determined with a fusion assay based on resonance energy transfer [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099]. Octadecyl Rhodamine B chloride can be readily inserted into native biological membranes by addition of an ethanolic solution of the probe. Evidence is presented showing that the dilution of the fluorophore, occurring when octadecyl Rhodamine containing influenza virus is mixed with phospholipid vesicles at pH 5.0, but not pH 7.4, resulted from virus-vesicle fusion and was not related to processes other than fusion. Furthermore, by use of this method, the kinetics of fusion between Sendai virus and erythrocyte ghosts and virus-induced fusion of ghosts were readily revealed. Dilution of the probe was not observed upon prior treatment of fluorescently labeled Sendai virus with trypsin.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton-induced membrane fusion. Role of phospholipid composition and protein-mediated intermembrane contact

Glycolipid-phospholipid vesicles containing phosphatidate and phosphatidylethanolamine were found to undergo proton-induced fusion upon acidification of the suspending medium from pH 7.4 to pH 6.5 or lower, as determined by an assay for lipid intermixing based on fluorescence resonance energy transfer. Lectin-mediated contact between the vesicles was required for fusion. Incorporation of phosphatidylcholine in the vesicles inhibited proton-induced fusion. Vesicles in which phosphatidate was replaced by phosphatidylserine underwent fusion only when pH was reduced below 4.5, while no significant fusion occurred (pH greater than or equal to 3.5) when the anionic phospholipid was phosphatidylinositol. It is suggested that partial protonation of the polar headgroup of phosphatidate and phosphatidylserine, respectively, causes a sufficient reduction in the polarity and hydration of the vesicle surface to trigger fusion at sites of intermembrane contact.

Purification of lamellar bodies from human amniotic fluid

Lamellar bodies were purified from human amniotic fluid obtained from pregnancies at 17, 31, and 40 weeks' gestation by means of Sephacryl S-300 column chromatography. Fractions were labeled with diphenylhexatriene and two peaks of phospholipid were identified at each gestational age. The size of peak I increased relative to that of peak II with advancing gestational age. Further study of the two peaks from 40-week pregnancies showed that peak I contained lamellar bodies that could be identified by electron microscopy. The ratio of the concentration of protein to phospholipid for peak I varied from 0.2 to 1.1. There were four proteins which could be identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (130K, 80K, 50K, and 20K), and there was 90-degree scattering of 400 nm light. Peak II also contained phospholipid. However, bilayer structures could not be visualized by electron microscopy, and there was no 90-degree scattering of 400 nm light. Peak II had a much higher ratio of protein to phospholipid (approximately 100) and two broad bands of protein on sodium dodecyl sulfate electrophoresis (80K, 50K). The anisotropy of diphenylhexatriene in both peaks decreased with advancing gestational age. The anisotropy in peak I was always lower than that in peak II, indicating that the microenvironment in peak I was more fluid. Peak I may represent mature surfactant and peak II, precursors of surfactant.

Complement protein C9 labeled with fluorescein isothiocyanate can be used to monitor C9 polymerization and formation of the cytolytic membrane lesion

Human complement protein C9 was covalently labeled with the fluorescent chromophore fluorescein isothiocyanate (FITC) with only a small reduction in the cytolytic activity of the protein. Polymerization of the labeled protein--either by incubating with lipid vesicles treated with complement proteins C5b-8 (activating the C5b-9 membrane lesion) or by heating the protein [Tschopp, J., Muller-Eberhard, H.J., & Podack, E.R. (1982) Nature (London) 298, 534]--resulted in a 40-60% decrease in the fluorescence emission from FITC. The decrease in total fluorescence was accompanied by an increase in the steady-state anisotropy following activation and polymerization of FITC-C9 by C5b-8 membranes, while heat-induced aggregation of the protein resulted in a dramatic depolarization of fluorescence. Only small changes in either the absorbance spectrum or fluorescence lifetime of the chromophore were detected upon FITC-C9 polymerization. Evidence is presented that the measured changes in FITC fluorescence upon C9 activation are due to self energy transfer between closely apposed fluorescein chromophores which occur in the polymerized form of the protein. The significance of these observations to the molecular structure of the assembled C5b-9 complex is discussed, as are the potential applications of this fluorescent derivative of C9.

Thermodynamic characterization of the pretrasition of unilamellar dipalmitoyl-phosphatidylcholine vesicles

High-sensitivity differential scanning calorimetry of small unilamellar vesicles (SUV) made by sonication of dipalmitoylphosphatidylcholine (DPPC) reveals that the gel-liquid crystalline transition at 37 C is preceded by a pretransition at 28 C that is relatively slow (t1/2 approximately equal to 2-4 min) and has an enthalpy change of ca. 0.2 kcal/mol. On incubation at 4 C, these SUV fuse spontaneously into large unilamellar vesicles (LUV). LUV also exhibit both a pretransition and a main-phase transition. The temperature of the main transition (Tm) and the enthalpy change of both the pretransition and main transition of these fused vesicles are similar to those of large multilamellar vesicles (MLV). The enthalpy change associated with the transition at 28 C decreases in SUV in a manner directly correlated to the decrease in the apparent enthalpy change of the 37 C main transition, indicating that the smaller (low temperature) transition is indeed a pretransition that is an inherent property of SUV. Therefore, unilamellar vesicles of DPPC appear to exhibit a pretransition at a temperature that varies from 28 C for the small vesicles to 35 C for the much larger vesicles.

Phase behavior of large unilamellar vesicles composed of synthetic phospholipids

Large unilamellar vesicles (LUV) have been prepared by three procedures from several synthetic and natural phosphatidylcholines. Reverse-phase evaporation vesicles (REV) and fusion vesicles were prepared by established procedures. A published procedure for the preparation of dialyzed octyl glucoside vesicles (DOV) was modified to allow its use with synthetic phospholipids. Negative-staining and freeze--fracture electron microscopy was used to determine the vesicle size distribution (mean diameters 800-1000 A) and extent of oligolamellar contamination in DOV preparations. Trapping of 6-carboxyfluorescein yielded measurements of the internal volume (2.6 +/- 0.3 microL/mumol of Pi) consistent with the size distributions determined by electron microscopy. An upper limit of less than 3 mol % oligolamellar vesicle contamination was indicated by calorimetric heat capacity profiles. The phase behaviors of large multilamellar vesicles and all three types of LUV were compared by using high-sensitivity differential scanning calorimetry and fluorescence depolarization of the membrane probe diphenylhexatriene. The most remarkable feature was the increased breadth of the main transition of DOV and of REV relative to the multilamellar species and to fusion vesicles. Both the main transition and the pretransition occurred at nearly the same temperatures in unilamellar and multilamellar species, but the unilamellar pretransition involved less than half the enthalpy observed in the multilamellar transition. Additional experiments indicated that the broadened main phase transition associated with DOV and REV reflected bilayer impurities resulting from preparation. It is concluded that LUV prepared by procedures that avoid impurities undergo a highly cooperative phase transition, as demonstrated here for fusion vesicles.