The plasma membrane lipid composition affects fusion between cells and model membranes.

Investigations were carried out on the effect of plasma membrane lipid modifications on the fusogenic capacity of control and ras-transformed fibroblasts. The plasma membrane lipid composition was modified by treatment of cells with exogenous phospholipases C and D, sphingomyelinase and cyclodextrin. The used enzymes hydrolyzed definite membrane lipids thus inducing specific modifications of the lipid composition while cyclodextrin treatment reduced significantly the level of cholesterol. The cells with modified membranes were used for assessment of their fusogenic capacity with model membranes with a constant lipid composition. Treatment with phospholipases C and D stimulated the fusogenic potential of both cell lines whereas the specific reduction of either sphingomyelin or cholesterol induced the opposite effect. The results showed that all modifications of the plasma membrane lipid composition affected the fusogenic capacity irrespective of the initial differences in the membrane lipid composition of the two cell lines. These results support the notion that the lipid composition plays a significant role in the processes of membrane-membrane fusion. This role could be either direct or through modulation of the activity of specific proteins which regulate membrane fusion.

Cholesterol distribution in plasma membranes of beta1 integrin-expressing and beta1 integrin-deficient fibroblasts.

The effect of integrin receptors on the level and transmembrane localization of cholesterol molecules was investigated in beta1 integrin-expressing (beta1) and beta1 integrin-deficient (beta1 null) cells. We found that the content of specific raft components-cholesterol, sphingomyelin, and caveolin-was increased in integrin-expressing cells. Integrin presence affected as well the transmembrane distribution of cholesterol-a higher percent was found in the plasma membrane outer monolayer of beta1 compared to beta1 null cells. Sphingomyelin depletion reduced the presence of cholesterol in the outer membrane monolayer of both cell lines, but the differences in cholesterol asymmetry, observed between beta1 and beta1 null cells before sphingomyelinase treatment were preserved. These findings implied that integrin receptors affected the non-random transmembrane distribution of cholesterol. Finally, a higher percent of detergent-resistant membranes was obtained from beta1 integrin-expressing cells, suggesting that the presence of these receptors in the membranes influenced the formation and/or stabilization of lipid raft domains.