Abstract
Cardiolipin (CL) has a critical role in maintaining mitochondrial inner membrane structure. In several conditions such as heart failure and aging, there is loss of CL content and remodeling of CL acyl chains, which are hypothesized to impair mitochondrial inner membrane biophysical organization. Therefore, this study discriminated how CL content and acyl chain composition influenced select properties of simple and complex mitochondrial mimicking model membranes. We focused on monolayer excess area/molecule (a measure of lipid miscibility), bilayer phase transitions, and microdomain organization. In monolayer compression studies, loss of tetralinoleoyl [(18:2)(4)] CL content decreased the excess area/molecule. Replacement of (18:2)(4)CL acyl chains with tetraoleoyl [(18:1)(4)] CL or tetradocosahexaenoyl [(22:6)(4)] CL generally had little influence on monolayer excess area/molecule; in contrast, replacement of (18:2)(4)CL acyl chains with tetramyristoyl [(14:0)(4)] CL increased monolayer excess area/molecule. In bilayers, calorimetric studies showed that substitution of (18:2)(4)CL with (18:1)(4)CL or (22:6)(4)CL lowered the phase transition temperature of phosphatidylcholine vesicles whereas (14:0)(4)CL had no effect. Finally, quantitative imaging of giant unilamellar vesicles revealed differential effects of CL content and acyl chain composition on microdomain organization, visualized with the fluorescent probe Texas Red DHPE. Notably, microdomain areas were decreased by differing magnitudes upon lowering of (18:2)(4)CL content and substitution of (18:2)(4)CL with (14:0)(4)CL or (22:6)(4)CL. Conversely, exchanging (18:2)(4)CL with (18:1)(4)CL increased microdomain area. Altogether, these data demonstrate that CL content and fatty acyl composition differentially target membrane physical properties, which has implications for understanding how CL regulates mitochondrial activity and the design of CL-specific therapeutics.