Cholesterol Rich Domains Are Essential for Interaction of Liposomes with Cells

Cholesterol is a molecule, which plays essential structural role in eucaryotic cells; which includes 30 – 50 mol % of cholesterol. It increases the orientational order of membrane lipid chains and decreases membrane permeability while maintaining membrane fluidity. When added to the phospholipids in model membranes at concentrations higher than 30 mol % it maintains the membrane in liquid ordered state through the large temperature range and in this way protects the cell membrane from drastic permeability changes, which could occur at phase transitions from solid to liquid phases [Bloom, M. Physics in Canada 48 (1992) 7]. As the biological membranes are heterogeneous with the regions with different modes of molecular motions (domains) cholesterol distributes within these domain according to their dynamic characteristics. It prefers to distribute in the regions with saturated hydrocarbon chains and forms separate liquid ordered phases, which coexist with liquid-disordered phases. Cholesterol is essential for the maintenance of these domain types, which determine the boundaries for lateral diffusion of proteins in each phase. By modulating lipid concentrations and external conditions the domains could merge together, what may play the role in regulating membrane properties. In combination with sphigolipids it forms special types of liquid ordered domains – rafts. The role of lipid rafts is to segregate and concentrate membrane proteins and are involved in sorting and distributing lipids and proteins to the cell surface [Simons, K. and Ikonen, E. Science 290 (2000) 1721]. With respect to the mentioned role of cholesterol in biological membranes it is important to study how cholesterol is distributed within the membrane and how this distribution between membrane domains influences the interaction of membranes with cells and proteins. In this work the changes in the domain structure of liposomes, models of cell membranes, with cholesterol concentration will be measured. The electron paramagnetic resonance (EPR) with the spin probe methylester of palmitic acid (MeFASL(10,3)) will be used. The information about the membrane domain structure will be obtained by computer simulation of the EPR spectra line-shape, taking into account that the membrane is heterogeneous, composed of several membrane domains with different motional characteristics. To obtain best fit of calculated to the experimental spectra the evolutionary optimization method (HEO) was used. It combines stochastic and population-based genetic algorithm, which is good at finding promising regions in complex search space, with Simplex Downhill for fine-tuning. In order to get a reasonable characterization