Stressing Lipid Membranes: Effects of Polymers on Membrane Structural Integrity

Abstract

Cell membrane dysfunction due to loss of structure integrity is the pathology of tissue death in trauma, muscular dystrophies, reperfusion injuries and some common diseases. It is now established that certain poly(ethylene oxide) (PEO)-based biocompatible polymers, such as Poloxamer 188, Poloxamine 1107, and PEO homopolymers, are effective in sealing of injured cell membranes, and thus prevent acute necrosis. Despite the highly potential application of PEO-based polymers for these medical problems, the fundamental mechanism of how these polymers interact with cell membranes are still under debate. Here, the effects of these polymers on structural integrity of lipid vesicles were explored under osmotic and oxidative stress. Through fluorescence leakage assays, time-lapse fluorescence microscopy, dynamic light scattering and isothermal titration calorimetry, we identified that the surface-adsorbed hydrophilic polymers efficiently inhibits the loss of structural integrity of lipid vesicles under external stimuli, while the insertion of the hydrophobic polymers increases membrane permeability. To elucidate the mechanism by which hydrophilic polymers help restore membrane integrity while their hydrophobic counterparts disrupt it, 1H Overhauser Dynamic Nuclear Polarization (ODNP)-NMR spectroscopy, a newly developed NMR technique that is highly effective in differentiating weak surface adsorption versus translocation of polymers to membranes, was employed to detect polymer-lipid membrane interactions through the modulation of local hydration dynamics in lipid membranes. Our study shows that P188, the most hydrophilic poloxamer known as a membrane sealant, weakly adsorbs onto the membrane surface, yet effectively retards membrane hydration dynamics. Contrarily, P181, the most hydrophobic poloxamer known as a membrane permeabilizer, initially penetrates past lipid headgroups and enhances intrabilayer water diffusivity. Consequently, our results illustrate that the relative hydrophilic/hydrophobic ratio of the polymer dictates its functions.