Influence of Cholesterol on Lateral Segregation in Bilayers Containing Different Sphingomyelins and Unsaturated Phospholipids

Unsaturated and saturated phospholipids (PLs) may laterally segregate into ordered and disordered domains. Cholesterol is known to influence this lateral domain formation in model membranes, which likely resembles the formation of cholesterol rich nanodomains in biological membranes. Cholesterol prefers interacting with saturated PLs over monounsaturated and especially polyunsaturated PLs. Cholesterol also favors interacting with sphingomyelin (SM) over saturated phosphatidylcholines. We have earlier showed that the relative cholesterol affinity for unsaturated and saturated PL can determine to what degree cholesterol promotes lateral domain formation. The fluorescence decays of trans-parinaric acid (tPA) was analyzed to detect how much SM was required to form ordered domains in a fluid bilayer. We determined lateral segregation in bilayers containing 0 or 20 mol% cholesterol to compare to what degree cholesterol promoted lateral segregation. Monounsaturated and polyunsaturated PLs were chosen as fluid lipids. As ordered lipids SMs with different N-acyl chain lengths and saturation were chosen. These SMs were compared to SM mixtures, both specifically chosen mixtures and biological mixtures. Sterol partitioning experiments were performed to determine the relative cholesterol affinity for the saturated and unsaturated PLs. Differential scanning calorimetry was used to determine if cholesterol could stabilize domain thermostability in complex bilayers. The preliminary results showed that cholesterol promoted lateral segregation for all SMs studied, including the mixtures. General observations about how cholesterol influence domain formation will be presented. These, can be of importance for understanding dynamics of nanodomains in biological membranes.