Membrane lipids are asymmetrically distributed between the inner and the outer leaflet of the plasma membrane, and this asymmetric distribution of lipids is an important factor for many signaling events. The lipids investigated in this study, phosphoinositides, are found in the inner leaflet of the plasma membrane and have been shown to mediate a wide variety of important physiological processes by affecting the activity and/or localization of membrane associated proteins. Phosphoinositide properties are largely determined by the characteristics of their headgroup, which at physiological pH is highly charged but is also capable of hydrogen bond formation. For phosphoinositide mediated signaling events to occur, it requires the local enrichment of phosphoinositides, which depend on the interchange between attractive and repulsive forces. Factors expected to affect mutual phosphoinositide interaction are pH as well as the presence of cations or positively charged proteins.
The second aim of this study was to investigate the interactions between phosphoinositides and cholesterol. Our goal was to gain further insight into this interaction and to investigate the possibility that cholesterol may enhance phosphoinositide co-localization and subsequent domain formation. We studied lipid monolayer systems in the presence of cholesterol as well as in the presence of cholesterol derivatives to observe changes in phosphoinositide interaction and domain formation and we found that: 1) cholesterol had a condensing effect on the phosphoinositide monolayer films and that an increase in the molar concentration of cholesterol in the lipid system further condensed these monolayers. 2) Each of the investigated phosphoinositide derivatives was not able to form domains without the presence of cholesterol. 3) Modifications of the hydroxyl group position of cholesterol lead to an altered interaction with phosphoinositides. Our results underscored that cholesterol induces phosphoinsitide domain formation and the results highlighted the importance of the hydroxyl group of cholesterol for the putative phosphoinositide/cholesterol interaction.
Kent State University