Correlated AFM and NanoSIMS imaging to probe cholesterol-induced changes in phase behavior and non-ideal mixing in ternary lipid membranes

Cholesterol is believed to be an important component in compositionally distinct lipid domains in the cellular plasma membrane, which are referred to as lipid rafts. Insight into how cholesterol influences the interactions that contribute to plasma membrane organization can be acquired from model lipid membranes. Here we characterize the lipid mixing and phase behavior exhibited by ¹⁵N-dilaurolyphosphatidycholine (¹⁵N-DLPC)/deuterated distearoylphosphatiylcholine (D₇₀-DSPC) membranes with various amounts of cholesterol (0, 3, 7, 15 or 19 mol%) at room temperature. The microstructures and compositions of individual membrane domains were determined by imaging the same membrane locations with both atomic force microscopy (AFM) and high-resolution secondary ion mass spectrometry (SIMS) performed with a Cameca NanoSIMS 50. As the cholesterol composition increased from 0 to 19 mol%, the circular ordered domains became more elongated, and the amount of ¹⁵N-DLPC in the gel-phase domains remained constant at 6-7 mol%. Individual and micron-sized clusters of nanoscopic domains enriched in D ₇₀-DSPC were abundant in the 19 mol% cholesterol membrane. AFM imaging showed that these lipid domains had irregular borders, indicating that they were gel-phase domains, and not non-ideally mixed lipid clusters or nanoscopic liquid-ordered domains.