Osmotic Pressure-Induced Lipid Membrane Phase Separation within Macromolecular Environments

Lipid membrane phase separation, which is a thermodynamic physical process, has attracted attention as a dynamic function of cellular interfaces. As membrane tension influences the phase separation of cellular membranes under isothermal conditions, it is essential to clarify the physicochemical mechanism involved. Living cells contain numerous macromolecules that can generate osmotic pressure across the membrane due to the semipermeable nature of the lipid bilayer. In this research, we examined how macromolecular surroundings and the transmembrane osmotic pressure influence membrane phase separation, utilizing model membranes like giant lipid vesicles. We generated osmotic pressure across the membrane using dextran (molecular weights 40,000 and 200,000) and polyethylene glycol (molecular weight 6,000) as model macromolecules. Microscopic observations represented the changes in the percentage of phase-separated vesicles and miscibility temperature caused by osmotic tension, indicating that macromolecular surroundings tend to suppress membrane phase separation, while macromolecular osmotic pressure across the membrane markedly induces phase separation. Lipid membrane phase separation can be regulated by the macromolecular concentration and osmotic pressure across the membrane. This finding offers new insights into the formation and regulation of membrane domains within the macromolecularly crowded environments of living cells.