Phosphatidylethanolamine-induced cholesterol domains chemically identified with mass spectrometric imaging

Coexisting liquid phases of model membrane systems are chemically identified using imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS). The systems studied were Langmuir-Blodgett (LB) model membranes of cholesterol (CH) with two different phospholipids, one a major component in the outer plasma membrane bilayer leaflet (dipalmitoylphosphatidylcholine (PC)) and the other a major component in the inner leaflet (dipalmitoylphosphatidylethanolamine (PE)). Binary mixtures of CH with each of the phospholipids were investigated, as well as a ternary system. A single homogeneous phase is evident for PC/CH, whereas both systems containing PE show lateral heterogeneity with phospholipid-rich and CH-rich regions. The interaction between CH and the two phospholipids differs due to the disparity between the phospholipid headgroups. Imaging TOF-SIMS offers a novel opportunity to chemically identify and differentiate the specific membrane locations of CH and phospholipid in membrane regions without the use of fluorescent dyes. This unique imaging method has been used to demonstrate the formation of micrometer-size CH domains in phosphatidylethanolamine-rich systems and is further evidence suggesting that CH may facilitate transport and signaling across the two leaflets of the plasma membrane.