A mechanism of pressure-induced interdigitation of lipid bilayers

Cholesterol-induced microdomain formation in lipid bilayer membranes consisting of completely miscible lipids

Recently, we reported that a ternary lipid bilayer comprising phosphatidylethanolamine (PE), phosphatidylcholine (PC), which were both derived from chicken egg, and cholesterol (Chol) generates microdomains that function as specific fusion sites for proteoliposomes. Chol-induced microdomain formation in a completely miscible lipid bilayer is an exceptional phenomenon. Numerous studies have elucidated the formation of domains in liquid ordered (L_o) and liquid disordered (L_d) phases of ternary bilayers, which comprise two partially miscible lipids and Chol. Herein, we investigated the composition and mechanism of formation of these unique microdomains in supported lipid bilayers (SLBs) using a fluorescence microscope and an atomic force microscope (AFM). We prepared ternary SLBs using egg-derived PC (eggPC), Chol and three different types of PE: egg-derived PE, 1-palmitoyl-2-oleoyl-PE, and 1,2-didocosahexaenoyl-PE (diDHPE). Fluorescence microscopy observations revealed that fluid and continuous SLBs were formed at PE concentrations (C_PE) of ≥6 mol%. Fluorescence recovery after photobleaching measurement revealed that the microdomain was more fluid than the surrounding region that showed typical diffusion coefficient of the L_o phase. The microdomains were observed as depressions in the AFM topographies. Their area fraction (θ) increased with C_PE, and diDHPE produced a significantly large θ among the three PEs. The microdomains in the PE+eggPC+Chol-SLBs were rich in polyunsaturated PE and were in the L_d-like phase. Associating eggPC and Chol caused polyunsaturated PE to segregate, resulting in a microdomain formation by conferring the umbrella effect on Chol, entropic effect of disordered acyl chains, and π-π interactions in the hydrophobic core.

Effect of an Anionic Lipid on the Barotropic Behavior of a Ternary Bicellar Mixture

Dispersions of lipid mixtures comprising long- and short-chain phospholipids (bicellar mixtures) can form small isotropically reorienting particles (bilayered micelles), magnetically orientable stuctures, or unorientable lamellar structures. Application of hydrostatic pressure can also induce interdigitation of the long-chain lipid components. In this work, variable-pressure ²H NMR was used to study the effect of head group charge on the barotropic behavior of bicellar mixtures. Observations at pressures up to 152 MPa and temperatures up to 64 °C were combined with earlier observations at lower pressure and lower temperature to obtain a pressure-temperature phase diagram for DMPC-d₅₄/DMPG/DHPC (3:1:1). In this phase diagram, a region corresponding to small, isotropically reorienting particles at lower temperature and higher pressure is separated from a region corresponding to unorientable lamellar organization, at higher temperature and lower pressure, by a band in which the magnetically orientable phase is stable below ∼100 MPa and in which an interdigitated gel phase is stable above ∼120 MPa. From ∼46 to ∼52 °C, the dispersion transforms directly from the unorientable lamellar to isotropically reorienting particle phases upon isothermal pressurization. The extent to which this behavior reflects the presence of anionic lipid in the long-chain fraction of this mixture is illustrated by comparison with spectral series obtained during isothermal pressurization of DMPC-d₅₄/DHPC (4:1) and DMPC-d₅₄/DMPG/DHPC (2.7:1.3:1) at selected temperatures. These observations show how electrostatic interactions at a bilayer surface can affect the balance between hydrophobic and hydrophilic interactions that is reflected by a dispersion's barotropic phase behavior.

Location and Effects of an Antitumoral Catechin on the Structural Properties of Phosphatidylethanolamine Membranes

Green tea catechins exhibit high diversity of biological effects including antioncogenic properties, and there is enormous interest in their potential use in the treatment of a number of pathologies. It is recognized that the mechanism underlying the activity of catechins relay in part in processes related to the membrane, and many studies revealed that the ability of catechins to interact with lipids plays a probably necessary role in their mechanism of action. We present in this work the characterization of the interaction between an antitumoral synthetically modified catechin (3-O-(3,4,5-trimethoxybenzoyl)-(-)-catechin, TMCG) and dimiristoylphosphatidyl-ethanolamine (DMPE) membranes using an array of biophysical techniques which include differential scanning calorimetry, X-ray diffraction, infrared spectroscopy, atomic force microscopy, and molecular dynamics simulations. We found that TMCG incorporate into DMPE bilayers perturbing the thermotropic transition from the gel to the fluid state forming enriched domains which separated into different gel phases. TMCG does not influence the overall bilayer assembly of phosphatidylethanolamine systems but it manages to influence the interfacial region of the membrane and slightly decrease the interlamellar repeat distance of the bilayer. TMCG seems to be located in the interior of the phosphatidylethanolamine bilayer with the methoxy groups being in the deepest position and some portion of the molecule interacting with the water interface. We believe that the reported interactions are significant not only from the point of view of the known antitumoral effect of TMCG, but also might contribute to understanding the basic molecular mechanism of the biological effects of the catechins found at the membrane level.

Response to hydrostatic pressure of bicellar dispersions containing an anionic lipid: pressure-induced interdigitation

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54), 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG), and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), with molar ratios of 3:1:1, were studied using variable-pressure (2)H NMR spectroscopy at hydrostatic pressures up to 125 MPa. Upon warming of the dispersions, spectra at ambient pressure indicated a progressive coalescence from small bilayered disks undergoing isotropic reorientation to more extended micellar structures in which spectra indicated anisotropic reorientation and, under some conditions, magnetic orientation and finally to randomly oriented lamellae or multilamellar vesicles. Temperatures for the onsets of anisotropic reorientation and random lamellar orientation increased with pressure at rates of 0.22 and 0.15 °C/MPa, respectively. In the 3.5-T magnetic field used for this work, magnetic orientation within the intermediate phase was not observed at 83 MPa or higher pressures. Comparison of spectra obtained at fixed pressure showed significant asymmetry between behaviors upon warming and cooling. For samples of DMPC-d54/DMPG/DHPC (3:1:1), but not DMPC-d54/DHPC (4:1), a persistent interdigitated phase was formed after repeated cooling from high temperature at 83 MPa. This is likely a metastable phase and might reflect kinetic trapping of the short-chain lipid component, DHPC, in a nonequilibrium spatial distribution as temperature is lowered at high pressure. Bicellar dispersions typically behave differently upon warming and cooling, and these observations could provide some insight into the observed behaviors in such systems. This work also suggests the possibility of trapping bicellar dispersions in persistent nonequilibrium morphologies.