title: Cholesterol and Sphingomyelin-Containing Model Condensed Lipid Monolayers: Heterogeneities Involving Ordered Microdomains Assessed by Two Cholesterol Derivatives.
abstract: Lipid monolayers are often considered as model membranes, but they are also the physiologic lipid part of the peripheral envelope of lipoproteins and cytosolic lipid bodies. However, their structural organization is still rather elusive, in particular when both cholesterol and sphingomyelin are present. To investigate such structural organization of hemimembranes, we measured, using alternative current voltammetry, the differential capacitance of condensed phosphatidylcholine-based monolayers as a function of applied potential, which is sensitive to their lipid composition and molecular arrangement. Especially, monolayers containing both sphingomyelin and cholesterol, at 15% w/w, presented specific characteristics of the differential capacitance versus potential curves recorded, which was indicative of specific interactions between these two lipid components. We then compared the behavior of two cholesterol derivatives (at 15% w/w), 21-methylpyrenyl-cholesterol (Pyr-met-Chol) and 22-nitrobenzoxadiazole-cholesterol (NBD-Chol), with that of cholesterol when present in model monolayers. Indeed, these two probes were chosen because of previous findings reporting opposite behaviors within bilayer membranes regarding their interaction with ordered lipids, with only Pyr-met-Chol mimicking cholesterol well. Remarkably, in monolayers containing sphingomyelin or not, Pyr-met-Chol and NBD-Chol presented contrasting behaviors, and Pyr-met-Chol mimicked cholesterol only in the presence of sphingomyelin. These two observations (i.e., optimal amounts of sphingomyelin and cholesterol, and the ability to discriminate between Pyr-met-Chol and NBD-Chol) can be interpreted by the existence of heterogeneities including ordered patches in sphingomyelin- and cholesterol-containing monolayers. Since such monolayer lipid arrangement shares some properties with the raft-type lipid microdomains well-described in sphingomyelin- and cholesterol-containing bilayer membranes, our data thus strongly suggest the existence of compact and ordered microdomains in model lipid monolayers.
authors.affiliation: INSERM U563, Faculté de Médecine de Rangueil, 31062 Toulouse, France.
authors.forename: Marie-France
authors.initials: MF
authors.lastname: Lecompte
authors.affiliation: INSERM U563, CHU Purpan, 31024 Toulouse cedex 3, France.|SB2SM and UMR8221/9198 CNRS, IBiTec-Saclay, CEA, 91191 Gif-sur-Yvette cedex, France.
authors.forename: Gérald
authors.initials: G
authors.lastname: Gaibelet
authors.affiliation: INRA UMR 1331, 31027 Toulouse cedex 3, France.
authors.forename: Chantal
authors.initials: C
authors.lastname: Lebrun
authors.affiliation: INSERM U1048, Université Toulouse III, UMR 1048, 31400 Toulouse, France.
authors.forename: François
authors.initials: F
authors.lastname: Tercé
authors.affiliation: INSERM U1048, Université Toulouse III, UMR 1048, 31400 Toulouse, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Collet
authors.affiliation: INSERM U563, CHU Purpan, 31024 Toulouse cedex 3, France.|SB2SM and UMR8221/9198 CNRS, IBiTec-Saclay, CEA, 91191 Gif-sur-Yvette cedex, France.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Orlowski
mesh_terms: D002784:Cholesterol / Q000737:chemistry*; D008055:Lipids / Q000737:chemistry*; D008956:Models, Chemical; D013109:Sphingomyelins / Q000737:chemistry*
chemical_list: D008055:Lipids; D013109:Sphingomyelins; D002784:Cholesterol