Utilization of liposomes in vesicular transport studies

Use of liposomes to study vesicular transport

Liposomes have been utilized for variety of membrane transport studies including clathrin-mediated endocytosis. Here we introduce clathrin-coated structures that are generated on large unilamellar liposomes by incubating with clathrin coat proteins. Large unilamellar liposomes are also used to reconstitute vesicle formation in a cell-free system, and the vesicle formation can be quantified by using dynamic light scattering (DLS). Furthermore, phagocytosis assay using liposome-conjugated styrene beads is demonstrated.

A fluorescence-based technique to construct size distributions from single-object measurements: application to the extrusion of lipid vesicles

We report a novel approach to quantitatively determine complete size distributions of surface-bound objects using fluorescence microscopy. We measure the integrated intensity of single particles and relate it to their size by taking into account the object geometry and the illumination profile of the microscope, here a confocal laser scanning microscope. Polydisperse (as well as monodisperse) size distributions containing objects both below and above the optical resolution of the microscope are recorded and analyzed. The data is collected online within minutes, which allows the user to correlate the size of an object with the response from any given fluorescence-based biochemical assay. We measured the mean diameter of extruded fluorescently labeled lipid vesicles using the proposed method, dynamic light scattering, and cryogenic transmission electron microscopy. The three techniques were in excellent agreement, measuring the same values within 7-9%. Furthermore we demonstrated here, for the first time that we know of, the ability to determine the full size distribution of polydisperse samples of nonextruded lipid vesicles. Knowledge of the vesicle size distribution before and after extrusion allowed us to propose an empirical model to account for the effect of extrusion on the complete size distribution of vesicle samples.

A method to evaluate the effect of liposome lipid composition on its interaction with the erythrocyte plasma membrane

Lipid aggregates are considered promising carriers for macromolecules and toxic drugs. In order to fulfill this function, aggregates should have properties that ensure the efficient delivery of their cargo to the desired location. One of these properties is their stability in blood when accumulating in the targeted tissue. This stability may be affected by a number of factors, including enzymatic activity, protein adsorption, and non-specific lipid exchange between the aggregate and morphological blood components. Since blood cells in the majority consist of erythrocytes, their interaction with aggregates should be carefully analyzed. In this paper, we present a method that allows the exchange of lipid between liposomes and the erythrocyte plasma membrane to be evaluated. The extent of this exchange was measured in terms of the toxicity of a cationic lipid (DOTAP) incorporated into the liposome lipid bilayer, evaluated by plasma membrane mechanical properties. After liposomes were formed from DOTAP/PC or DOTAP/PE mixtures, erythrocyte plasma membranes were destabilized in a manner dependent on DOTAP concentration. A constant quantity of DOTAP mixed with various proportions of SM caused no such effect, indicating very limited lipid exchange with the cell membrane for such liposome formulations.

Ultrasound-mediated cavitation thresholds of liquid perfluorocarbon droplets in vitro

This study was undertaken to measure the ultrasound (US)-mediated cavitation threshold of microdroplets as a function of its content and US parameters (frequency, amplitude and burst length). Albumin-coated droplets were prepared with perfluoropropane, perfluorohexane or perfluoromethylcyclohexane contents. The filtered suspensions were diluted to 1:1000 (v) and compared with Optison. The formulations were injected into an acoustically transparent vessel and sonicated with a single focused transducer. The frequencies employed were 0.74, 1.1, 2.18 and 3.3 MHz and the burst length and acoustic pressure were varied. The inertial cavitation threshold for each experiment was monitored through passive acoustic detection. The formation of droplet emulsion of the perfluorocarbon increased the natural boiling point of the perfluorocarbon. However, perfluorocarbon droplets having contents with higher molecular weights and boiling points did not have detectably higher inertial cavitation thresholds and, thus, the droplets do not need to be in a superheated state to be cavitated by US bursts. Therefore, higher boiling point perfluorocarbons should be investigated for this purpose and may prove to be useful for both imaging and therapy. The inertial cavitation threshold of perfluorocarbon droplets increases with frequency, and was approximately 0.7 MPa at 0.74 MHz and 1.75 MPa at 3.3 MHz. Optison, already in a gaseous state, has the lowest cavitation threshold of all formulations studied. Results show that, for the frequencies tested, there is no dependence between inertial cavitation threshold and burst lengths between 20 and 100 ms. As a conclusion, the inertial cavitation threshold of albumin-coated microdroplets of several perfluorocarbons was determined in vitro. The results indicate that the physical properties of these droplets are such that they may be useful for localized US therapies.