Oxidized Phosphatidylcholines Promote Phase Separation of Cholesterol-sphingomyelin Domains

Lipid lateral segregation in the plasma membrane is believed to play an important role in cell physiology. Sphingomyelin (SM) and cholesterol (Chol)-enriched microdomains have been proposed as liquid-ordered phase platforms that serve to localize signaling complexes and modulate the intrinsic activities of the associated proteins. We modeled plasma membrane domain organization using Langmuir monolayers of ternary POPC/SM/Chol as well as DMPC/SM/Chol mixtures, which exhibit a surface-pressure-dependent miscibility transition of the coexisting liquid-ordered and -disordered phases. Using Brewster angle microscopy and Langmuir monolayer compression isotherms, we show that the presence of an oxidatively modified phosphatidylcholine, 1-palmitoyl-2-azelaoyl-sn-glydecero-3-phosphocholine, efficiently opposes the miscibility transition and stabilizes micron-sized domain separation at lipid lateral packing densities corresponding to the equilibrium lateral pressure of ∼32 mN/m that is suggested to prevail in bilayer membranes. This effect is ascribed to augmented hydrophobic mismatch induced by the oxidatively truncated phosphatidylcholine. To our knowledge, our results represent the first quantitative estimate of the relevant level of phospholipid oxidation that can potentially induce changes in cell membrane organization and its associated functions.