Not just a fluidifying effect: omega-3 phospholipids induce formation of non-lamellar structures in biomembranes
The polyunsaturated omega-3 fatty acid docosaHexaenoic Acid (DHA) is found in very high concentration in a few peculiar tissues suggesting it must have a specialized role. DHA was proposed to affect the function of the cell membrane and related proteins through an indirect mechanism of action, based on the DHA-phospholipid effects on the lipid bilayer structure. In this respect, most studies have focused on its influence on lipid-rafts, somehow neglecting the analysis of effects on liquid disordered phases that constitute most of the cell membranes, by reporting in these cases only a general fluidifying effect. Here, by combining Neutron Reflectivity, Cryo-Transmission Electron Microscopy, Small Angle Neutron Scattering, Dynamic Light Scattering and Electron Paramagnetic Resonance spectroscopy, we characterize liquid disordered bilayers formed by the naturally abundant 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and different contents of a di-DHA glycero-phosphocholine, 22:6-22:6PC, from both a molecular/microscopic and supramolecular/mesoscopic viewpoint. We show that, below a threshold concentration of about 40% molar percent, incorporation of 22:6-22:6PC in the membrane increases lipid dynamics slightly but sufficiently to promote membrane deformation and increase of multilamellarity. Notably, beyond this threshold, 22:6-22:6PC disfavours formation of lamellar phases, leading to a phase separation consisting mostly of small spherical particles that coexist with a minority portion of a lipid blob with water-filled cavities. Concurrently, from a molecular viewpoint, the polyunsaturated acyl chains tend to fold and expose the termini to the aqueous medium. We propose that this peculiar tendency is a key feature of the DHA-phospholipids making them able to modulate the local morphology of biomembranes.