Nonequilibrium Patterns of Cholesterol-Rich Chemical Heterogenieties within Single Fluid Supported Phospholipid Bilayer Membranes

Abstract

We have developed a simple method to introduce cholesterol- and sphingomyelin-rich chemical heterogeneities into controlled densities and concentrations within predetermined regions of another distinct fluid phospholipid bilayer supported on a solid substrate. A contiguous primary phase--a fluid POPC bilayer displaying a well-defined array of lipid-free voids (e.g., 20-100 microm squares)--was first prepared on a clean glass surface by microcontact printing under water using a poly(dimethylsiloxane) stamp. The aqueous-phase primary bilayer pattern was subsequently incubated with secondary-phase small unilamellar vesicles composed of independent chemical compositions. Backfilling by comparable vesicles resulted in gradual mixing between the primary- and secondary-phase lipids, effacing the pattern. When the secondary vesicles consisted of phase-separating mixtures of cholesterol, sphingomyelin, and a phospholipid (2:1:1 POPC/sphingomyelin/cholesterol or 1:1:1 DOPC/sphingomyelin/cholesterol), well-defined spatial patterns of fluorescence, chemical compositions, and fluidities emerged. We conjecture that these patterns form because of the differences in the equilibration rates of the secondary liquid-ordered and liquid-disordered phases with the primary fluid POPC phase. The pattern stability depended strongly on the ambient-phase temperature, cholesterol concentration, and miscibility contrast between the two phases. When cholesterol concentration in the secondary vesicles was below 20 mol %, secondary intercalants gradually diffused within the primary POPC bilayer phase, ultimately dissolving the pattern in several minutes and presumably forming a new quasi-equilibrated lipid mixture. These phase domain micropatterns retain some properties of biological rafts including detergent resistance and phase mixing induced by selective cholesterol extraction. These patterns enable direct comparisons of cholesterol- and sphingomyelin-rich phase domains and fluid phospholipid phases for their functional preferences and may be useful for developing simple, parallelized assays for phase and chemical composition-dependent membrane functionalities.