Study of the cholesterol–GM3 ganglioside interaction by surface pressure measurements and fluorescence microscopy

The nature of the cholesterol/glycolipid interaction in rafts being poorly understood, the interaction of cholesterol with the GM3 ganglioside has been studied by surface pressure measurements and fluorescence microscopy. Results have been compared to those obtained with sphingomyelin (SM)-cholesterol and palmitoyl-oleoyl-phosphatidylcholine (POPC)–cholesterol monolayers. The analysis of (π–A) isotherms of mixed monolayers show a condensing effect of cholesterol on GM3 molecules, in the same range than the effect observed with POPC and higher than the effect on SM. This is likely due to the similar state of GM3 and POPC, since both molecules are in liquid expanded phases in our experimental conditions. The study of the cholesterol desorption induced by β-cyclodextrin suggests also that the GM3–cholesterol interaction is rather weak as in the case of POPC–cholesterol interaction, and clearly lower than SM–cholesterol one. This lack of interaction is discussed in terms of nature of lipid chains and molecular shape, and suggests that no hydrogen bond is formed between GM3 and cholesterol polar heads. Fluorescence microscopy performed on mixed GM3–cholesterol monolayers shows the presence, at surface pressure higher than 10 mN/m, of particular blurring patterns without defined boundary, which could be due to a partial solubilization in one phase of different phases observed at lower surface pressure, whereas SM–cholesterol and POPC–cholesterol monolayers are homogeneous at the lateral resolution of our microscopy set-up.