Investigations on Membrane Asymmetry and the Eect of Antidepressants on Lipid Microdomains

For researchers in biological membranes, compositional and structural information contributes to the understanding of their fundamental cellular functions. This thesis is focused on two important structures of the cell′s plasma membrane: compositional asymmetry across the lipid bilayer and heterogeneous lateral organization in the membrane. Building asymmetric model membranes and studying the passive movement of lipids will provide insights into the energy toll to maintain the asymmetry. In this work, a non-perturbative approach to fabricate asymmetric model membranes is provided. The approach is using the simple diffusion of lipids through the aqueous medium, by exchanging lipids between silica nanoparticles coated with a lipid bilayer and lipid vesicles having different lipid compositions, to build asymmetry in lipid vesicles. The vesicles can be isolated after they have acquired an asymmetric distribution of lipids by removing the silica nanoparticles using centrifugation. Two asymmetric systems are prepared and investigated. The results with phospholipids agree with recent work on the same system, but where vesicle asymmetry is achieved using cyclodextrin. The results with cholesterol suggest that cholesterol can be energetically placed asymmetrically across the cell plasma membrane. In addition to membrane asymmetry, I also investigate how the physical properties of lipid microdomains in model membranes may be affected by external molecules like antidepressants. The results from microscopy and steady-state laurdan fluorescence emission measurements show that antidepressants (active and inactive) promote raft formation and increase the membrane packing in synthetic model membranes.