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Imaging and Shape Analysis of GUVs as Model Plasma Membranes: Effect of Trans DOPC on Membrane Properties

Department of Physics and Astronomy, University of Maine, Orono, Maine

Correspondence: Address reprint requests to Prof. Samuel T. Hess, Dept. of Physics and Astronomy, University of Maine, Orono, ME 04469. Tel.: 207-581-1036; Fax: 207-581-3410; E-mail: sam.hess{at}umit.maine.edu.

Unsaturated trans fatty acids have been linked to a higher incidence of coronary artery disease, but not enough is known about the effect of trans lipids on membrane properties. Liquid-ordered (l_o) and liquid-disordered (l_d) membrane domains are implicated in various biological processes, such as endocytosis, adhesion, signaling, protein transport, apoptosis, and disease pathogenesis. The physical forces that induce domain formation and thus orchestrate cell function need to be further addressed and quantified. Here, we test the effect of trans DOPC (dielaidoyl phosphatidylcholine or DEPC) on the morphology of giant unilamellar vesicles (GUVs, used as a biomembrane model) made by electroformation with varying compositions of egg sphingomyelin, trans DOPC, cis DOPC, and cholesterol. GUVs were imaged by confocal fluorescence microscopy and then analyzed for changes in membrane morphology and properties such as l_o/l_d phase coexistence and area fractions, distribution of meridional curvature, and fluorescent-probe intensity distribution. BODIPY-FL-C₁₂-sphingomyelin, Lissamine rhodamine B dioleoylphosphatidylethanolamine and BODIPY-TR-C₁₂-sphingomyelin were used as fluorescent probes to differentially label the l_o and l_d phases. Trans DOPC induces some vesicles to form multidomain, invaginated morphologies that differ from the typical two-domain circular and truncated spherical shapes observed in its absence. Trans DOPC also alters the membrane curvature distribution; this is more pronounced in the l_o phase near the phase boundary, where significantly negative curvatures (<–0.5 µm^–1) are observed. A narrower distribution of meridional curvatures in GUVs with trans DOPC is suggestive of higher membrane bending rigidity. The ratio of average fluorescent intensities in the l_d/l_o phases indicates a greater concentration or brightness of the probes BODIPY-FL-C₁₂-sphingomyelin and BODIPY-TR-C₁₂-sphingomyelin in the l_o phase in the presence of trans DOPC. Addition of trans DOPC does not alter the l_o/l_d area fractions, indicating that it does not act like egg sphingomyelin, a saturated lipid. These changes in membrane properties seen in the presence of trans lipids could significantly impact cell function.