Lactosylceramide (LacCer) is a key intermediate in glycosphingolipid metabolism and is highly enriched in detergent-resistant biomembrane fractions associated with microdomains, i.e., rafts and caveolae. Here, the lateral interactions of cholesterol with LacCers containing various homogeneous saturated (8:0, 16:0, 18:0, 24:0) or monounsaturated acyl chains (18:1, 24:1) have been characterized using a Langmuir-type film balance. Cholesterol-induced changes in lateral packing were assessed by measuring changes in average molecular area, i.e., area condensations, and in lateral elasticity, i.e., surface compressional moduli (CS-S) with emphasis on high surface pressures (≥30 mN/m) that mimic biomembrane conditions. Cholesterol most dramatically affected the lateral packing elasticity of LacCers with long saturated acyl chains at sterol mole fractions ≥0.3, consistent with liquid-ordered (LO) phase formation. The lateral elasticity within the LacCer-cholesterol LO-phase was much lower than that observed within pure LacCer condensed, i.e., gel, phase. The magnitude of the cholesterol-induced reduction in lateral elasticity was strongly mitigated by cis monounsaturation in the LacCer acyl chain. At identical high sterol mole fractions, higher lateral elasticity was observed within LacCer-cholesterol mixtures compared with galactosylceramide-cholesterol and sphingomyelin-cholesterol mixtures. The results show how changes to sphingolipid headgroup and acyl chain structure contribute to the modulation of lateral packing elasticity in sphingolipid-cholesterol LO-phases.
Bibliographical noteFunding Information:
We gratefully acknowledge the support of United States Public Health Service grants HL49180 (H.L.B.) and GM45928 (R.E.B.), and the Hormel Foundation. Portions of this investigation were presented in preliminary form at the Biophysical Society Discussion: Probing Membrane Microdomains held in Asilomar, CA (Oct. 28–31, 2004) and at the 48th Annual Meeting of the Biophysical Society held in Baltimore, MD (Feb 14–18, 2004) and have been published in abstract form.