Effects of bilayer phases on phospholipid-poloxamer interactions

Poloxamers are amphiphilic copolymers capable of interacting with biological membranes, while the fundamental mechanism of the interactions is not yet fully understood. Using liposomes as model membranes, we have investigated the interactions of the poloxamer with phospholipids bilayers by isothermal titration calorimetry (ITC) and electron cryomicroscopy (cryo-EM). The results suggest that the phase structure of the lipid bilayer plays a critical role in regulating poloxamer insertion into lipid bilayers. ITC shows that the poloxamer is incorporated into the liposome at temperatures (T) above the bilayer main phase transition temperature (T-m) but the incorporation is completely inhibited otherwise. This distinct effect from the phase structure of the lipid bilayer determines the concentration of poloxamer incorporated into the membrane and affects the morphology of the self-assembled structure of lipid-poloxamer mixtures. When poloxamers are introduced at concentrations above the critical micelle concentration to pre-formed liposomes, liposomes are disrupted into flat discs at temperatures above T-m but remain as spherical shells at temperatures below T-m, as evidenced by cryo-EM. With relatively low concentrations of poloxamers introduced during liposome formation, spherical poloxamer-lipid vesicles are formed at T > T-m; The spherical-shell structure of binary poloxamer-lipid liposomes changes to flat discs over a short time scale when the temperature is dropped below T-m. These flat discs are capable of reverting to the spherical vesicular structure when the temperature is raised above T-m, though a much longer time is needed. Understanding the effects of the bilayer phase in lipid-poloxamer interactions can help improve the design of poloxamers for pharmaceutical use.