Behavior of sulfatide/cholesterol mixed monolayers at the air/water interface
Introduction
Recent studies suggest that lipid domains, the so-called “lipid rafts”, have been postulated to perform critical roles in a number of normal cellular processes, such as signal transduction [1], [2], [3], [4], membrane fusion [5], [6], organization of the cytoskeleton [7], [8], lipid sorting [9], [10], and protein trafficking/recycling [11], [12], [13], as well as pathological events such as the cellular invasion of influenza, Ebola, and human immunodeficiency-1 viruses [14], [15], [16], [17], and formation of the plaques associated with Alzheimer's disease [18], [19]. However, the raft field is still quite controversial. Currently, there are fundamental unsolved questions concerning microdomains in membranes, including their size and stability [20], [21], [22], their physical properties [23], [24], mechanisms of their formation [21], [22], [25], [26], [27], and even their very existence [23], [28], [29], [30]. These questions are currently being addressed by a number of sophisticated biophysical, biochemical, and computational methodologies. Detergent-resistant membranes (DRMs) extracted from cells, thought to be related to rafts, are enriched in cholesterol and sphingomyelin (SM) [31], [32]. Detergent extraction procedures were instrumental in the discovery and analysis of rafts, and still are an important tool in determining the composition of membrane microdomains in cells [32], [33], [34], [35], [36]. However, this method involves breaking up the membrane and has limitations in terms of defining the size, properties, and dynamics of intact microdomains [24], [28], [37], [38], [39].
In the past several years, many studies have shown that rafts do indeed spontaneously form in lipid bilayers with lipid compositions approximating those found in trans-Golgi or plasma membranes. Thus, lipid–lipid interactions are thought to play key roles in the formation of membrane rafts. Need less to say, the lipid rafts are the so-called mixed systems and the structure constructed is not monolayer but bilayer. In order to examine the properties of bilayers comprising mixtures of different types of ammphiphilic substances of lipid rafts, monolayer studies can give us fundamental but indispensable information about such mixed systems. As a matter of fact, the monolayer behavior of many such mixtures has been investigated as functions of chemical species structure and their mixing ratios [40], [41], [42], [43], [44], [45]. Depending upon temperature and total composition, the domains have liquid-like or solid-like properties and can vary in size over orders of magnitude (50 nm to 50 mm). Importantly, the size, shape and composition of these domains appear to be functionally relevant [46]. Langmuir monolayers at the air/liquid interface are ideal model system to study the thermodynamic behavior of binary or ternary lipid systems [47], [48]. According to π–A isotherm, we can calculate many parameters (mean molecular area, excess Gibbs energy, compressibility coefficient, and so on) to evaluate the miscibility and stability of the monolayers. It has been well established that atomic force microscopy (AFM) is an effective technique for investigating the topography and assembly of lateral domains at molecular level [49], [50], [51].
It has reported that sulfatides, together with other sphingolipids, participate in lateral domain formation in the detergent insoluble fraction of the myelin membranes [52], [53]. Sulfatide is a major constituent of brain lipids, constituting 4–6 mole% of the total lipids in adult brain myelin [42], [43]. The content of sulfatide has important effect on diverse biological processes including the regulation of cell growth, protein trafficking, signal transduction, cell adhesion, neuronal plasticity and morphogenesis [41]. Therefore, the understanding of the thermodynamic and assembly behavior of sulfatide is hypothesized to play an important role for formation of lipid rafts in biomembrane.
In this study, the miscibility and thermodynamic behavior of mixed sulfatide/cholesterol monolayers were investigated by means of calculating mean molecular area, excess Gibbs energy and compressibility coefficient over the whole composition range. Our results give rise to a better understanding of the interaction between sulfatide and cholesterol at a molecular level. To this end, AFM was employed to characterize the LB monolayers, based on the thermodynamic study. We aim to provide valuable information on the mechanical properties of lateral domain formation, in particular in studying the nanomechanical stability of the monolayers.
Section snippets
Preparation of mixed monolayers
Sulfatides from bovine brain and cholesterol were purchased from Sigma Chemical Co. (St. Louis, MO, USA). The preparation of the monolayers was performed in a computer-controlled device (Minitrough; KSV, Helsinki, Finland). For all the experiments, the subphase was water (18.2 MΩ), obtained from a Millipore purification system.
The lipids were dissolved in chloroform–methanol (3:1, v/v) to a final concentration of 0.5 mg mL−1. Each 50 μL of sample solution was spread on the subphase with a Hamilton
Thermodynamic analysis of mixed sulfatide/cholesterol monolayers
The surface pressure (π) vs. mean molecular area (A) isotherms at various mole fractions of sulfatide/cholesterol monolayers at the air–water interface are shown in Fig. 1. In the case of cholesterol alone, no phase transition is observed. The monolayer of cholesterol gives a typical condensed monolayer with a limiting area of about 38 Å2/molecule (estimated by extrapolation of the isotherm to zero surface pressure) and the lift-off area of surface pressure is at the mean molecular area of 40.6 Å2
Conclusion
In this work the property of sulfatide/cholesterol monolayers are studied at the air–water interface. The thermodynamic analysis indicates miscibility for the two systems with negative deviation from the ideal behavior, as evidenced from the results of excess area and excess Gibbs energy analysis. The elasticity of cholesterol alone system is much larger than the other systems due to the rigid molecular structure of cholesterol. AFM images show the cholesterol monolayer tends to form the net
Acknowledgments
The work was supported by Grant No. 20772077 from the National Natural Science Foundation of China and Grant No. 104167 from the National Basic Research Priorities Program of China.
References (65)
- et al.
J. Biol. Chem.
(1997) - et al.
J. Biol. Chem.
(2005) Curr. Opin. Cell Biol.
(2001)- et al.
J. Biol. Chem.
(2003) - et al.
J. Clin. Virol.
(2001) - et al.
J. Biol. Chem.
(2004) J. Lipid Res.
(2003)- et al.
Cell
(2005) Cell
(2003)- et al.
Curr. Opin. Colloid Interf. Sci.
(2004)