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Cholesterol stabilizes fluid phosphoinositide domains.

ABSTRACT: Local accumulation of phosphoinositides (PIPs) is an important factor for a broad range of cellular events including membrane trafficking and cell signaling. The negatively charged phosphoinositide headgroups can interact with cations or cationic proteins and this electrostatic interaction has been identified as the main phosphoinositide clustering mechanism. However, an increasing number of reports show that phosphoinositide-mediated signaling events are at least in some cases cholesterol dependent, suggesting other possible contributors to the segregation of phosphoinositides. Using fluorescence microscopy on giant unilamellar vesicles and monolayers at the air/water interface, we present data showing that cholesterol stabilizes fluid phosphoinositide-enriched phases. The interaction with cholesterol is observed for all investigated phosphoinositides (PI(4)P, PI(3,4)P2, PI(3,5)P2, PI(4,5)P2 and PI(3,4,5)P3) as well as phosphatidylinositol. We find that cholesterol is present in the phosphoinositide-enriched phase and that the resulting phase is fluid. Cholesterol derivatives modified at the hydroxyl group (cholestenone, cholesteryl ethyl ether) do not promote formation of phosphoinositide domains, suggesting an instrumental role of the cholesterol hydroxyl group in the observed cholesterol/phosphoinositide interaction. This leads to the hypothesis that cholesterol participates in an intermolecular hydrogen bond network formed among the phosphoinositide lipids. We had previously reported that the intra- and intermolecular hydrogen bond network between the phosphoinositide lipids leads to a reduction of the charge density at the phosphoinositide phosphomonoester groups (Kooijman et al., 2009). We believe that cholesterol acts as a spacer between the phosphoinositide lipids, thereby reducing the electrostatic repulsion, while participating in the hydrogen bond network, leading to its further stabilization. To illustrate the effect of phosphoinositide segregation on protein binding, we show that binding of the tumor suppressor protein PTEN to PI(5)P and PI(4,5)P2 is enhanced in the presence of cholesterol. These results provide new insights into how phosphoinositides mediate important cellular events.

SUBMITTER: Jiang Z

PROVIDER: S-EPMC4117703 | biostudies-literature | 2014 Sep

REPOSITORIES: biostudies-literature

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Cholesterol stabilizes fluid phosphoinositide domains.

Jiang Zhiping Z Redfern Roberta E RE Isler Yasmin Y Ross Alonzo H AH Gericke Arne A

Chemistry and physics of lipids 20140217

Local accumulation of phosphoinositides (PIPs) is an important factor for a broad range of cellular events including membrane trafficking and cell signaling. The negatively charged phosphoinositide headgroups can interact with cations or cationic proteins and this electrostatic interaction has been identified as the main phosphoinositide clustering mechanism. However, an increasing number of reports show that phosphoinositide-mediated signaling events are at least in some cases cholesterol depen ...[more]

PMID: 24556334

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Project description:A uniquely sensitive method for ceramide domain detection allowed us to study in detail cholesterol-ceramide interactions in lipid bilayers with low (physiological) ceramide concentrations, ranging from low or no cholesterol (a situation similar to intracellular membranes, such as endoplasmic reticulum) to high cholesterol (similar to mammalian plasma membrane). Diverse fluorescence spectroscopy and microscopy experiments were conducted showing that for low cholesterol amounts ceramide segregates into gel domains that disappear upon increasing cholesterol levels. This was observed in different raft (sphingomyelin/cholesterol-containing) and non-raft (sphingomyelin-absent) membranes, i.e. mimicking different types of cell membranes. Cholesterol-ceramide interactions have been described mainly as raft sphingomyelin-dependent. Here sphingomyelin independence is demonstrated. In addition, ceramide-rich domains re-appear when either cholesterol is converted by cholesterol oxidase to cholestenone or the temperature is decreased. Ceramide is more soluble in cholesterol-rich fluid membranes than in cholesterol-poor ones, thereby increasing the chemical potential of cholesterol. Ceramide solubility depends on the average gel-fluid transition temperature of the remaining membrane lipids. The inability of cholestenone-rich membranes to dissolve ceramide gel domains shows that the cholesterol ordering and packing properties are fundamental to the mixing process. We also show that the solubility of cholesterol in ceramide domains is low. The results are rationalized by a ternary phospholipid/ceramide/cholesterol phase diagram, providing the framework for the better understanding of biochemical phenomena modulated by cholesterol-ceramide interactions such as cholesterol oxidase activity, lipoprotein metabolism, and lipid targeting in cancer therapy. It also suggests that the lipid compositions of different organelles are such that ceramide gel domains are not formed unless a stress or pathological situation occurs.

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Cholesterol stabilizes fluid phosphoinositide domains.

Publications

Cholesterol stabilizes fluid phosphoinositide domains.

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