Surface behavior of myelin monolayers

Reduced cholesterol alters the biophysical properties of repaired myelin

The myelin sheath is a lipid-rich membrane that ensheathes axons and is required for healthy and efficient signal transduction. Myelin is damaged in neurological diseases like multiple sclerosis, but remyelination can occur through the action of oligodendrocyte precursor cells (OPCs), which differentiate into mature oligodendrocytes that wrap axons to form repaired myelin. In this study, a genetic-based mouse model of demyelination was used, which features near-complete demyelination followed by robust remyelination in the brain. Lipid mass spectrometry on isolated myelin from the remyelinated brain revealed a decrease in the percent mole fraction of cholesterol when compared to healthy myelin. Biophysical studies on monomolecular lipid films formed using repaired myelin lipid extracts showed changes in the surface behavior of the lipid films, compared to the healthy myelin lipids. Films formed using the remyelinated lipid extracts resulted in lower surface pressures and lower compressional moduli when compared to healthy controls, suggesting that repaired myelin membranes have lower lateral molecular packing within the lipid film. Synthetically prepared model membranes, based on the major lipid compositions of the healthy and diseased extracts, revealed that changes in cholesterol levels were the primary contributor to the changes in biophysical properties. Supplementation of the diseased lipid extracts with cholesterol led to a robust improvement in membrane surface pressures and compressibility. Together, these results suggest that high cholesterol levels are required for myelin membrane stability and that reduced cholesterol in repaired myelin may have a profound impact on the biophysical properties of the myelin membrane.

Nanoarchitectonic approaches for measuring the catalytic behavior of a membrane anchored enzyme. From Langmuir-Blodgett to a novel Langmuir-Schaefer based nanofilm building device

Self-organized lipid monolayers at the air-water interface (Langmuir films, LF) are commonly used for measuring the catalytic properties of membrane-bound enzymes. This methodology allows to provide a consistent flat topography molecular density, packing defects and thickness. The aim of the present work was to show the methodological advantages of using the horizontal transfer method (Langmuir-Schaefer) with respect to the vertical transfer method (Langmuir-Blodgett) when mounting a device to measure catalytic activity of membrane enzymes. Based on the results obtained we can conclude that it is possible to prepare stable Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films from Bovine Erythrocyte Membranes (BEM) preserving the catalytic activity of its native Acetylcholinesterase (BEA). In comparison, the LS films showed Vmax values more similar to the enzyme present in the vesicles of natural membranes. In addition, it was much easier to produce large amounts of transferred areas with the horizontal transfer methodology. It was possible to decrease the time required to mount an assay with numerous activity points, such as building activity curves as a function of substrate concentration. The present results show that LSBEM provides a proof of concept for the development of biosensors based on transferred purified membrane for the screening of new products acting on an enzyme embedded on its natural milieu. In the case of BEA, the application of these enzymatic sensors could have medical interest, providing drug screening tools for the treatment of Alzheimer's disease.

Nanoscale Structure of Langmuir–Blodgett Film of Bent-Core Molecules

Bent-core mesogens (BCMs) are a class of thermotropic liquid crystals featuring several unconventional properties. However, the interpretation and technological exploitation of their unique behavior have been hampered by the difficulty of controlling their anchoring at surfaces. To tackle this issue, we report the nanoscale structural characterization of BCM films prepared using the Langmuir–Blodgett technique. Even though BCMs are quite different from typical amphiphilic molecules, we demonstrate that stable molecular films form over water, which can then be transferred onto silicon substrates. The combination of Brewster angle microscopy, atomic force microscopy, and X-ray reflectivity measurements shows that the molecules, once transferred onto a solid substrate, form a bilayer structure with a bottom layer of flat molecules and an upper layer of upright molecules. These results suggest that Langmuir–Blodgett films of BCMs can provide a useful means to control the alignment of this class of liquid crystals.

Purified myelin lipids display a critical mixing point at low surface pressure

Lipids extracted from Purified Myelin Membranes (LPMM) were spread as monomolecular films at the air/aqueous interface. The films were visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (π) and ionic environments. Coexistence of Liquid-Expanded (LE) and cholesterol-enriched (CE) rounded domains persisted up to π ≈ 5 mN/m but the monolayers became homogeneous at higher surface pressures. Before mixing, the domains distorted to non-rounded domains. We experimentally measured the line tension (λ) for the lipid monolayers at the domain borders by a shape relaxation technique using non-homogeneous electric fields. Regardless of the subphase conditions, the obtained line tensions are of the order of pN and tended to decrease as lateral pressure increased toward the mixing point. From the mean square displacement of nested trapped domains, we also calculated the dipole density difference between phases (μ). A non-linear drop was detected in this parameter as the mixing point is approached. Here we quantitively evaluated the π-dependance of both parameters with proper power laws in the vicinity of the critical mixing surface pressure, and the exponents showed to be consistent with a critical phenomenon in the two-dimensional Ising universality class. This idea of bidimensionality was found to be compatible only for simplified lipidic systems, while for whole myelin monolayers, that means including proteins, no critical mixing point was detected. Finally, the line tension values were related with the thickness differences between phases (Δt) near the critical point.

Lipopolysaccharides at Solid and Liquid Interfaces: Models for Biophysical Studies of the Gram-negative Bacterial Outer Membrane

Lipopolysaccharides (LPSs) are a constitutive element of the cell envelope of Gram-negative bacteria, representing the main lipid in the external leaflet of their outer membrane (OM) lipid bilayer. These unique surface-exposed glycolipids play a central role in the interactions of Gram-negative organisms with their surrounding environment and represent a key element for protection against antimicrobials and the development of antibiotic resistance. The biophysical investigation of a wide range of different types of in vitro model membranes containing reconstituted LPS has revealed functional and structural properties of these peculiar membrane lipids, providing molecular-level details of their interaction with antimicrobial compounds. LPS assemblies reconstituted at interfaces represent a versatile tool to study the properties of the Gram-negative OM by exploiting several surface-sensitive techniques, in particular X-ray and neutron scattering, which can probe the structure of thin films with sub-nanometer resolution. This review provides an overview of different approaches employed to investigate structural and biophysical properties of LPS, focusing on studies on Langmuir monolayers of LPS at the air/liquid interface and a range of supported LPS-containing model membranes reconstituted at solid/liquid interfaces.

Ionic environment, thickness and line tension as determinants of phase separation in whole Purified Myelin Membranes monolayers

Purified myelin membranes (PMM) were spread as monomolecular films at the air/aqueous solution interface, and visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (π) on three specific aqueous solutions: absence of salts, physiological conditions and presence of calcium. Coexistence of Liquid-Expanded (LE) and Liquid Ordered (LO) phases persisted up to collapse in the presence of salts, whereas monolayers became homogeneous at π ≥ 35-40 mN/m when salts are absent. This PMM phase-mixing behavior in monolayers is similar to the previously reported behavior of PMM multilamellar vesicles. Reflectivities (R_p) of p-polarized light from both phases were assessed throughout the whole π -range, and film thicknesses (t) were calculated from the R_p values and measured film refractive indices (n). The LO phase was found to be more reflective and thicker than the LE phase at π ≤ 15 mN/m, but less reflective and thinner at higher π. We also determined the line tension (λ) of PMM monolayers at the domain boundaries from the rate of domain shape relaxation, which turned out to be of the order of picoNewtons (pN) and decreased as π increased. A correlation between λ and thickness differences (Δt) was found, suggesting that Δt is a molecular determinant for λ in PMM monolayers. Both λ and Δt were found to increase markedly when calcium was present in the subphase. This result corroborates the concept of divalent cations as a stabilizing factor for phase separation, in line with earlier studies on this mixture forming multilamellar membrane arrangements.

Interaction of Two Amphipathic α-Helix Bundle Proteins, ApoLp-III and ApoE 3, with the Oil-Aqueous Interface

Protein-lipid interactions govern the structure and function of lipoprotein particles, which transport neutral lipids and other hydrophobic cargo through the blood stream. Apolipoproteins cover the surface of lipoprotein particles, including low-density (LDL) and high-density (HDL) lipoproteins, and determine their function. Previous work has focused on small peptides derived from these apolipoproteins or used such artificial lipid systems as Langmuir monolayers or the lipid disc assay to determine how apolipoproteins interact with the neutral lipid interface. Here, we focus on a recurring protein domain found in many neutral lipid-binding proteins, the amphipathic α-helix bundle. We use liquid droplet tensiometry to investigate protein-lipid interactions on an oil droplet, which mimics the real lipoprotein interface. The N-terminus of apoE 3 and full-length apoLp-III serve as model proteins. We find that each protein interacts with lipid monolayers at the oil-aqueous interface in unique ways. For the first time, we show that helix bundle unfolding is critical for proper protein insertion into the lipid monolayer at the oil-aqueous interface and that specific membrane lipids promote the rebinding of protein upon fluctuation in droplet size. These results shed new light on how amphipathic apolipoprotein α-helix bundles interact with neutral lipid particles.

How the replacement of cholesterol by 25-hydroxycholesterol affects the interactions with sphingolipids: The Langmuir Monolayer Study complemented with theoretical calculations

In this paper, a representative of chain-oxidized sterols, 25-hydroxycholesterol (25-OH), has been studied in Langmuir monolayers mixed with the sphingolipids sphingomyelin (SM) and ganglioside (GM₁) to build lipid rafts. A classical Langmuir monolayer approach based on thermodynamic analysis of interactions was complemented with microscopic visualization of films (Brewster angle microscopy), surface-sensitive spectroscopy (polarization modulation-infrared reflection-absorption spectroscopy) and theoretical calculations (density functional theory modelling and molecular dynamics simulations). Strong interactions between 25-OH and both investigated sphingolipids enabled the formation of surface complexes. As known from previous studies, 25-OH in pure monolayers can be anchored to the water surface with a hydroxyl group at either C(3) or C(25). In this study, we investigated how the presence of additional strong interactions with sphingolipids modifies the surface arrangement of 25-OH. Results have shown that, in the 25-OH/GM₁ system, there are no preferences regarding the orientation of the 25-OH molecule in surface complexes and two types of complexes are formed. On the other hand, SM enforces one specific orientation of 25-OH: being anchored with the C(3)-OH group to the water. The strength of interactions between the studied sphingolipids and 25-OH versus cholesterol is similar, which indicates that cholesterol may well be replaced by oxysterol in the lipid raft system. In this way, the composition of lipid rafts can be modified, changing their rheological properties and, as a consequence, influencing their proper functioning.

Molecular insight into neurodegeneration - Langmuir monolayer study on the influence of oxysterols on model myelin sheath

Systematic studies on the influence of selected ring-oxidized (7α-hydroxycholesterol, 7α-OH; 7β-hydroxycholesterol, 7β-OH; 7-ketocholesterol, 7-K) and chain-oxidized (25-OH) sterols on lipid layer of myelin were performed. Myelin sheath was modeled as five-component Langmuir monolayer (Chol:PE:SM:PS:PC 50:20:12:9:9). Particular oxysterols have been incorporated into the model myelin sheath by replacing cholesterol totally or partially (1:1). The effect of oxysterol incorporation was characterized with surface pressure and electric surface potential - area isotherms and visualized with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). It has been noticed that model myelin loses its homogeneous structure (due to the appearance of domains) at physiological bilayer conditions (30-35 mN/m). In the presence of oxysterols, the fluidity of myelin model increases and the organization of lipids is altered, which is reflected in the decrease of electric surface potential changes (ΔV). The strongest myelin/oxysterol interactions have been observed for 7-K and 25-OH, being the most cytotoxic oxysterols found in biological tests.

Sensing molecular organizational changes through the catalytic activity of acetylcholinesterase from erythrocyte membranes in Langmuir-Blodgett films

Langmuir films prepared from bovine erythrocyte membranes (LF_BEM) were studied and transferred to alkylated glasses (Langmuir-Blodgett films, LB_BEM) in order to assess the effects of membrane molecular packing on Bovine Erythrocyte Acetylcholinesterase (BEA) catalytic activity. Surface pressure (π) vs Area isotherms showed three 2D-transitions at ~7, ~18 and ~44 mN/m and a collapse pressure at π_c = 49 mN/m. The 0-12-0 mN/m compression-decompression cycles resulted reversible while those 0-40-0 mN/m exhibited a significant hysteresis. Taken together, EFM, BAM and AFM images and the stability of the film after 3C-D cycles, we can suggest that over the air-water interface as well as over the silanized glass substrate the surface is mostly covered by a monolayer with a few particles dispersed. Acetylthiocholine hydrolysis was assayed with BEA in bovine erythrocyte membrane suspensions (S_BEM) and in LB_BEM packed at 10 (LB_BEM,10) and 35 mN/m (LB_BEM,35), which gave the following kinetic parameters: V_max = 3.41 ± 0.15, 0.021 ± 0.002 and 0.030 ± 0.003 nmol.min^-1·μg prot^-1 and K_M = 0.11 ± 0.02, 0.047 ± 0.017 and 0.026 ± 0.017 mM, respectively. Although from S_BEM to LB_BEM we lost active enzyme, the catalytic efficiency (V_max/K_M) increased ~750 times. Eugenol and 1,8-cineol inhibited BEA catalytic activity in LB_BEM,35. Our results demonstrate the transmission of information between the membrane and the environment within the subphase immediately below the membrane, where anchored proteins are hosted. This was reflected by the membrane packing-induced modulation of BEA catalytic activity. Furthermore, LB_BEM provides a proof of concept for the development of biosensors to screen new green pesticides acting through BEA interaction.

Phase Diagram of Purified CNS Myelin Reveals Continuous Transformation between Expanded and Compacted Lamellar States

Purified myelin membranes (PMMs) are the starting material for biochemical studies, from individual components up to the isolation of detergent-resistant membrane (DRM) fractions or detergent-insoluble glycosphingolipid (DIG) fractions, which are commonly believed to resemble physiological lipid rafts. The normal DIG isolation protocol involves the extraction of lipids under moderate cooling. The isolation of PMMs also involves the cooling of myelin as well as exposure to low ionic strength (IS). Here, we addressed the combined influence of cooling and IS on the structure of PMMs. The phase behaviour was investigated by small angle X-ray diffraction. Analysis of the diffraction peaks revealed the lamellar periodicity (d), the number of periodically correlated bilayers (N), and the relatives fractions of each phase. Departure from physiological conditions induced a phase separation in myelin. The effect of monovalent and divalent ions was also compared at equivalent IS, showing a differential effect, and phase diagrams for both ion types were established—Ca²⁺ induced the well-known over-compacted phase, but additionally we also found an expanded phase at low IS. Na⁺ promoted phase separation, and also induced over-compaction at sufficiently high IS. Finally, exploring the whole phase diagram, we found evidence for the direct isothermal transformation from the expanded to the compacted phase, suggesting that both phases could in fact originate from the identical primary lateral phase separation, whereas the apparent difference lies in the inter-bilayer interaction that is modulated by the ionic milieu.

Effect of Cholesterol and Myelin Basic Protein (MBP) Content on Lipid Monolayers Mimicking the Cytoplasmic Membrane of Myelin

Myelin basic protein (MBP) is located in the insulating covers of nerve cells in the brain and spinal cord. By interacting with lipid membranes, it is responsible for compaction of the myelin sheath in the central nervous system, which is weakened in demyelinating diseases. The lipid composition of the myelin leaflet has a high impact on the interaction between the membrane and MBP. Cholesterol is present in the cytoplasmic leaflet with a rather high amount of 44% (mol%). In this study, the focus is on the effect of cholesterol, mainly by varying its content, on the interaction of MBP with a lipid monolayer. Therefore, Langmuir lipid monolayers mimicking the cytoplasmic membrane of myelin and monolayers with variations of cholesterol content between 0% and 100% were measured at the air/water interface with additional imaging by fluorescence microscopy. All experiments were performed with and without bovine MBP to study the dependence of the interaction of the protein with the monolayers on the cholesterol content. The native amount of 44% cholesterol in the monolayer combines optima in the order of the monolayer (presumably correlating to compaction and thermodynamic stability) and protein interaction and shows unique features in comparison to lower or higher cholesterol contents.

Psychosine remodels model lipid membranes at neutral pH

β-Galactosylsphingosine or psychosine (PSY) is a single chain sphingolipid with a cationic group, which is degraded in the lysosome lumen by β-galactosylceramidase during sphingolipid biosynthesis. A deficiency of this enzyme activity results in Krabbe's disease and PSY accumulation. This favors its escape to extralysosomal spaces, with its pH changing from acidic to neutral. We studied the interaction of PSY with model lipid membranes in neutral conditions, using phospholipid vesicles and monolayers as classical model systems, as well as a complex lipid mixture that mimics the lipid composition of myelin. At pH 7.4, when PSY is mainly neutral, it showed high surface activity, self-organizing into large structures, probably lamellar in nature, with a CMC of 38 ± 3 μM. When integrated into phospholipid membranes, PSY showed preferential partition into disordered phases, shifting phase equilibrium. The presence of PSY reduces the compactness of the membrane, making it more easily compressible. It also induces lipid domain disruption in vesicles composed of the main myelin lipids. The surface electrostatics of lipid membranes was altered by PSY in a complex manner. A shift to positive zeta potential values evidenced its presence in the vesicles. Furthermore, the increase of surface potential and surface water structuring observed may be a consequence of its location at the interface of the positively charged layer. As Krabbe's disease is a demyelinating process, PSY alteration of the membrane phase state, lateral lipid distribution and surface electrostatics appears important to the understanding of myelin destabilization at the supramolecular level.

Cooling induces phase separation in membranes derived from isolated CNS myelin

Purified myelin membranes (PMMs) are the starting material for biochemical analyses such as the isolation of detergent-insoluble glycosphingolipid-rich domains (DIGs), which are believed to be representatives of functional lipid rafts. The normal DIGs isolation protocol involves the extraction of lipids under moderate cooling. Here, we thus address the influence of cooling on the structure of PMMs and its sub-fractions. Thermodynamic and structural aspects of periodic, multilamellar PMMs are examined between 4°C and 45°C and in various biologically relevant aqueous solutions. The phase behavior is investigated by small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). Complementary neutron diffraction (ND) experiments with solid-supported myelin multilayers confirm that the phase behavior is unaffected by planar confinement. SAXS and ND consistently show that multilamellar PMMs in pure water become heterogeneous when cooled by more than 10–15°C below physiological temperature, as during the DIGs isolation procedure. The heterogeneous state of PMMs is stabilized in physiological solution, where phase coexistence persists up to near the physiological temperature. This result supports the general view that membranes under physiological conditions are close to critical points for phase separation. In presence of elevated Ca²⁺ concentrations (> 10 mM), phase coexistence is found even far above physiological temperatures. The relative fractions of the two phases, and thus presumably also their compositions, are found to vary with temperature. Depending on the conditions, an “expanded” phase with larger lamellar period or a “compacted” phase with smaller lamellar period coexists with the native phase. Both expanded and compacted periods are also observed in DIGs under the respective conditions. The observed subtle temperature-dependence of the phase behavior of PMMs suggests that the composition of DIGs is sensitive to the details of the isolation protocol.

CNS myelin structural modification induced in vitro by phospholipases A2

Myelin is the self-stacked membrane surrounding axons; it is also the target of several pathological and/or neurodegenerative processes like multiple sclerosis. These processes involve, among others, the hydrolytic attack by phospholipases. In this work we describe the changes in isolated myelin structure after treatment with several secreted PLA2 (sPLA2), by using small angle x-ray scattering (SAXS) measurements. It was observed that myelin treated with all the tested sPLA2s (from cobra and bee venoms and from pig pancreas) preserved the lamellar structure but displayed an enlarged separation between membranes in certain zones. Additionally, the peak due to membrane asymmetry was clearly enhanced. The coherence length was also lower than the non-treated myelin, indicating increased disorder. These SAXS results were complemented by Langmuir film experiments to follow myelin monolayer hydrolysis at the air/water interface by a decrease in electric surface potential at different surface pressures. All enzymes produced hydrolysis with no major qualitative difference between the isoforms tested.

Synaptosomal membrane-based Langmuir-Blodgett films: a platform for studies on γ-aminobutyric acid type A receptor binding properties

In this work we used Langmuir-Blodgett films (LB) as model membranes to study the effect of molecular packing on the flunitrazepam (FNZ) accessibility to the binding sites at the GABAA receptor (GABAA-R). Ligand binding data were correlated with film topography analysis by atomic force microscopy images (AFM) and SDS-PAGE. Langmuir films (LF) were prepared by the spreading of synaptosomal membranes (SM) from bovine brain cortex at the air-water interface. LBs were obtained by the transference, at 15 or 35 mN/m constant surface pressure (π), of one (LB15/1c and LB35/1c) or two (LB35/2c) LFs to a film-free hydrophobic alkylated substrate (CONglass). Transference was performed in a serial manner, which allowed the accumulation of a great number of samples. SDS-PAGE clearly showed a 55 kDa band characteristic of GABAA-R subunits. Detrended fluctuation analysis of topographic data from AFM images exhibited a single slope value (self-similarity parameter α) in CONglass and a discontinuous slope change in the α value at an autocorrelation length of ∼100 nm in all LB samples, supporting the LF transference to the substrate. AFM images of CONglass and LB15/1c exhibited roughness and average heights that were similar between measurements and significantly lower than those of LB35/1c and LB35/2c, suggesting that the substrate coverage in the latter was more stable than in LB15/1c. While [(3)H]FNZ binding in LB15/1c did not reach saturation, in LB35/1c the binding kinetics became sigmoid with a binding affinity lower than in the SM suspension. Our results highlight the π dependence of both binding and topological data and call to mind the receptor mechanosensitivity. Thus, LB films provide a tool for bionanosensing GABAA-R ligand binding as well as GABAA-R activity modulation induced by the environmental supramolecular organization.

Self-segregation of myelin membrane lipids in model membranes

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are multilamellar, lipid-rich membranes produced by oligodendrocytes in the central nervous system. To act as an insulator, myelin has to form a stable and firm membrane structure. In this study, we have analyzed the biophysical properties of myelin membranes prepared from wild-type mice and from mouse mutants that are unable to form stable myelin. Using C-Laurdan and fluorescence correlation spectroscopy, we find that lipids are tightly organized and highly ordered in myelin isolated from wild-type mice, but not from shiverer and ceramide synthase 2 null mice. Furthermore, only myelin lipids from wild-type mice laterally segregate into physically distinct lipid phases in giant unilamellar vesicles in a process that requires very long chain glycosphingolipids. Taken together, our findings suggest that oligodendrocytes exploit the potential of lipids to self-segregate to generate a highly ordered membrane for electrical insulation of axons.

Coupling between GABA(A)-R ligand-binding activity and membrane organization in β-cyclodextrin-treated synaptosomal membranes from bovine brain cortex: new insights from EPR experiments

Correlations between GABA(A) receptor (GABA(A)-R) activity and molecular organization of synaptosomal membranes (SM) were studied along the protocol for cholesterol (Cho) extraction with β-cyclodextrin (β-CD). The mere pre-incubation (PI) at 37°C accompanying the β-CD treatment was an underlying source of perturbations increasing [(3)H]-FNZ maximal binding (70%) and K (d) (38%), plus a stiffening of SMs' hydrocarbon core region. The latter was inferred from an increased compressibility modulus (K) of SM-derived Langmuir films, a blue-shifted DPH fluorescence emission spectrum and the hysteresis in DPH fluorescence anisotropy (A (DPH)) in SMs submitted to a heating-cooling cycle (4-37-4°C) with A (DPH,heating) < A (DPH,cooling). Compared with PI samples, the β-CD treatment reduced B (max) by 5% which correlated with a 45%-decrement in the relative Cho content of SM, a decrease in K and in the order parameter in the EPR spectrum of a lipid spin probe labeled at C5 (5-SASL), and significantly increased A (TMA-DPH). PI, but not β-CD treatment, could affect the binding affinity. EPR spectra of 5-SASL complexes with β-CD-, SM-partitioned, and free in solution showed that, contrary to what is usually assumed, β-CD is not completely eliminated from the system through centrifugation washings. It was concluded that β-CD treatment involves effects of at least three different types of events affecting membrane organization: (a) effect of PI on membrane annealing, (b) effect of residual β-CD on SM organization, and (c) Cho depletion. Consequently, molecular stiffness increases within the membrane core and decreases near the polar head groups, leading to a net increase in GABA(A)-R density, relative to untreated samples.

Equivalent aqueous phase modulation of domain segregation in myelin monolayers and bilayer vesicles

Purified myelin can be spread as monomolecular films at the air/aqueous interface. These films were visualized by fluorescence and Brewster angle microscopy, showing phase coexistence at low and medium surface pressures (<20-30 mN/m). Beyond this threshold, the film becomes homogeneous or not, depending on the aqueous subphase composition. Pure water as well as sucrose, glycerol, dimethylsulfoxide, and dimethylformamide solutions (20% in water) produced monolayers that become homogeneous at high surface pressures; on the other hand, the presence of salts (NaCl, CaCl(2)) in Ringer's and physiological solution leads to phase domain microheterogeneity over the whole compression isotherm. These results show that surface heterogeneity is favored by the ionic milieu. The modulation of the phase-mixing behavior in monolayers is paralleled by the behavior of multilamellar vesicles as determined by small-angle and wide-angle x-ray scattering. The correspondence of the behavior of monolayers and multilayers is achieved only at high surface pressures near the equilibrium adsorption surface pressure; at lower surface pressures, the correspondence breaks down. The equilibrium surface tension on all subphases corresponds to that of the air/alkane interface (27 mN/m), independently on the surface tension of the clean subphase.

Langmuir films from human placental membranes: preparation, rheology, transfer to alkylated glasses, and sigmoidal kinetics of alkaline phosphatase in the resultant Langmuir-Blodgett film

In the present study, we studied the activity of human placental alkaline phosphatase (PLAP) constraint in a planar surface in controlled molecular packing conditions. For the first time, Langmuir films (LFs) were prepared by the spreading of purified placental membranes (PPM) on the air-water interface and their stability and rheological properties were studied. LFs exhibited a collapse pressure pi(C) = 48 mN/m, hysteresis during the compression-decompression cycle (C-D), indicating a plastic deformation, and a compressibility modulus (K) compatible with liquid-expanded phases. A phase transition point appeared at pi(T) = 28 mN/m and, following successive C-D, it moved toward lower surface areas and higher K, suggesting the lost of some non-PLAP proteins as components of vesicles that might protrude from the monolayer (confirmed by combining lipid/protein molar ratio analysis, PAGE-SDS and V(max)). LFs were transferred at 35 mN/m to alkylated glasses to obtain Langmuir-Blodgett films (LB(35)) the stability of which was confirmed by AFM. The kinetics of p-nitrophenyl phosphate (pNPP) hydrolysis at 37 degrees C catalyzed by PPM was Michaelian and exhibited the thermostability at 60 degrees C typical of PLAP. In LB(35), PLAP exhibited a sigmoidal kinetics which resembled the behavior of the partially metalated enzyme but might become from a cross-talk between protein and membrane structures.

Molecular packing tunes the activity of Kluyveromyces lactis beta-galactosidase incorporated in Langmuir-Blodgett films

Functional consequences of constraining beta-Gal in bidimensional space were studied at defined molecular packing densities and constant topology. Langmuir-Blodgett films, LB15 and LB35 composed of dipalmitoyl phosphatidylcholine and K. lactis beta-Gal, were obtained by transferring Langmuir films (L) initially packed at 15 and 35 mN/m, respectively, to alkylated glasses. The beta-Gal-monolayer binding equilibrium, mainly the adsorption rate and affinity, depended on the initial monolayer's surface pressure (lower for higher pi i). At pi i = 15 and 35 mN/m, the surface excess (Gamma) followed downward parabolic and power-law tendencies, respectively, as a function of subphase protein concentration. Gamma values in L roughly reflected the protein surface density chemically determined in LBs (0-7.5 ng/mm2 at pi i = 0-35 mN/m and [beta-Gal] subphase = 0-100 microg/mL). The beta-Gal-catalyzed hydrolysis of o-nitrophenyl-galactopyranoside showed a Michaelian kinetics in solution as well as in LB15. KM, KM,LB15, Vmax, and Vmax,LB15 were 5.15 +/- 2.2 and 9.25 +/- 6 mM and 39.63 and 0.0096 +/- 0.0027 micromol/min/mg protein, respectively. The sigmoidal kinetics observed with LB35 was evaluated by Hill's model (K0.5 = 9.55 +/- 0.4 mM, Vmax,35 = 0.0021 micromol/min/mg protein, Hill coefficient n = 9) and Savageau's fractal model (fractal constant K f = 9.84 mM; reaction order for the substrate gs = 9.06 and for the enzyme ge = 0.62). Fractal reaction orders would reflect the fractal organization of the environment, demonstrated by AFM images, more than the molecularity of the reaction. Particular dynamics of the protein-lipid structural coupling in each molecular packing condition would have led to the different kinetic responses.

Paleontological evidence for membrane fusion between a unit membrane and a half-unit membrane

Membrane fusion is of fundamental importance for many biological processes and has been a topic of intensive research in past decades with several models being proposed for it. Fossils had previously not been considered relevant to studies on membrane fusion. But here two different membrane fusion patterns are reported in the same well-preserved fossil plant from the Miocene (15-20 million years old) at Clarkia, Idaho, US. Scanning electron microscope, transmission electron microscope, and traditional studies reveal the vesicles in various states (even transient semi-fusion) of membrane fusion, and thus shed new light on their membrane structure and fusion during exocytoses. The new evidence suggests that vesicles in plant cells may have not only a unit membrane but also a half-unit membrane, and that a previously overlooked membrane fusion pattern exists in plant cells. This unexpected result from an unexpected material not only marks the first evidence of on-going physiological activities in fossil plants, but also raises questions on membrane fusion in recent plants.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Lateral reorganization of myelin lipid domains by myelin basic protein studied at the air-water interface

It has been speculated that adsorption of myelin basic protein (MBP) to the myelin lipid membrane leads to lateral reorganization of the lipid molecules within the myelin membrane. This hypothesis was tested in this study by surface pressure measurement and fluorescent imaging of a monolayer composed of a myelin lipid mixture. The properties of the lipid monolayer before and after addition of MBP into the subphase were monitored. Upon addition of MBP to the monolayer subphase, the surface pressure rose and significant rearrangement of the lipid domains was observed. These results suggest that binding and partial insertion of MBP into the lipid monolayer led to dramatic rearrangement and morphological changes of the lipid domains. A model of adsorption of MBP to the lipid domains and subsequent domain fusion promoted by minimization of electrostatic repulsion between the domains was proposed to account for the experimental observations. The significance of these results in light of the role of MBP in maintaining the myelin structural integrity is discussed.

Protein-induced surface structuring in myelin membrane monolayers

Monolayers prepared from myelin conserve all the compositional complexity of the natural membrane when spread at the air-water interface. They show a complex pressure-dependent surface pattern that, on compression, changes from the coexistence of two liquid phases to a viscous fractal phase embedded in a liquid phase. We dissected the role of major myelin protein components, myelin basic protein (MBP), and Folch-Lees proteolipid protein (PLP) as crucial factors determining the structural dynamics of the interface. By analyzing mixtures of a single protein with the myelin lipids we found that MBP and PLP have different surface pressure-dependent behaviors. MBP stabilizes the segregation of two liquid phases at low pressures and becomes excluded from the film under compression, remaining adjacent to the interface. PLP, on the contrary, organizes a fractal-like pattern at all surface pressures when included in a monolayer of the protein-free myelin lipids but it remains mixed in the MBP-induced liquid phase. The resultant surface topography and dynamics is regulated by combined near to equilibrium and out-of-equilibrium effects. PLP appears to act as a surface skeleton for the whole components whereas MBP couples the structuring to surface pressure-dependent extrusion and adsorption processes.

Effect of temperature on surface properties of cervical tissue homogenate and organic phase monolayers

The temperature dependence of Langmuir monolayers of normal and cancerous human cervical tissues and their organic phases between temperatures of 37 and 45 degrees C was evaluated. Analysis of the surface pressure-area isotherms revealed significantly different increase in fluidity of the cancerous cervical tissue monolayer at 42 degrees C as opposed to the normal cervical tissue monolayers (p<0.05). Similarly, in the case of cervical cancerous organic phase monolayers significant increase of fluidity was observed at 40 degrees C whereas no such change was observed in the normal cervical organic phase monolayers. The effect of temperature was found to be different in cancerous and normal cervical tissues and this may be due to the different lipid profiles in them. Cancerous cervical tissues had 1.8-fold higher total lipids as compared to the normals. Similarly, the PC, PE, PI, PG, SM and PS levels in cancerous cervical tissues were 3.6, 2.0, 2.3, 4.7, 1.7 and 2.2 times higher than those of normal cervical tissues, respectively. Significant cancer-normal difference in minimum surface tension and hysteresis area was found at all temperatures studied for both tissue homogenates and organic phases. For example, cancerous tissue homogenates showed minimum surface tensions of 51.9+/-4.6, 54.4+/-5.9, 57.6+/-6.0 and 51.9+/-5.6mN/m at temperatures 37, 40, 42 and 45 degrees C whereas the corresponding values for normal cervical tissue homogenates were 39.3+/-3.6, 39.2+/-3.7, 39.2+/-3.8 and 39.1+/-3.6, respectively. The fluidity change at hyperthermic range of temperature can be correlated to the increased efficiency of drug on combination therapy with hyperthermia. These results may have implications in manipulating the fluidity of cervical cancer tissue membranes for better permeability thereby leading to better therapeutic strategies for cervical cancer.

Molecular recognition on the supported and on the air/water interface-spread protein monolayers

Targeting of proteins at interfaces via affinity ligands or specific antibodies is important for the understanding of protein functioning in biological membranes. This review brings together a great number of research works accomplished in this field in the past decade by a variety of analytical methods. It highlights two simple in situ techniques of monitoring molecular recognition processes at interfaces recently developed in the author's laboratory. The first of these techniques is based on the measurements of surface pressure increments of a protein monolayer spread at the air/water interface at a constant area resulting from the interaction with its specific ligands injected into the aqueous subphase beneath the preformed protein monolayer. The second technique takes advantage of the feature of [(14)C]-labeled proteins that enable in situ measurements of surface density changes of adsorbed protein molecules on a solid support resulting from the interaction with its specific antibody.

Fengycin interaction with lipid monolayers at the air-aqueous interface-implications for the effect of fengycin on biological membranes

In this study, we investigated the interaction of fengycin, a lipopeptide produced by Bacillus subtilis, with lipid monolayers using the Langmuir trough technique in combination with Brewster angle microscopy. Thermodynamic analyses were performed to get further information about the mixing behavior and the molecular interactions between the two components. The effect of fengycin on the structural and morphological characteristics of DPPC monolayers, as a simple model of biological membranes, depends on the fengycin molar ratio. With a small proportion of fengycin (X(f)0.1), the compressibility of the monolayer is modified but the morphological characteristics of the DPPC are not significantly affected. At an intermediate molar ratio (0.1<X(f)0.5), fengycin has a fluidizing effect on the DPPC monolayer by interacting partially with DPPC molecules. At higher molar ratio (X(f)=0.66), fengycin totally dissolves the ordered phase of the lipid. These results highlight the capacity of fengycin to perturb the DPPC organization and are discussed in relation to fengycin capacity to affect biological membranes.

Protein-mediated surface structuring in biomembranes

The lipids and proteins of biomembranes exhibit highly dissimilar conformations, geometrical shapes, amphipathicity, and thermodynamic properties which constrain their two-dimensional molecular packing, electrostatics, and interaction preferences. This causes inevitable development of large local tensions that frequently relax into phase or compositional immiscibility along lateral and transverse planes of the membrane. On the other hand, these effects constitute the very codes that mediate molecular and structural changes determining and controlling the possibilities for enzymatic activity, apposition and recombination in biomembranes. The presence of proteins constitutes a major perturbing factor for the membrane sculpturing both in terms of its surface topography and dynamics. We will focus on some results from our group within this context and summarize some recent evidence for the active involvement of extrinsic (myelin basic protein), integral (Folch-Lees proteolipid protein) and amphitropic (c-Fos and c-Jun) proteins, as well as a membrane-active amphitropic phosphohydrolytic enzyme (neutral sphingomyelinase), in the process of lateral segregation and dynamics of phase domains, sculpturing of the surface topography, and the bi-directional modulation of the membrane biochemical reactivity.

Interfacial properties as biophysical markers of cervical cancer

Monolayers at air-liquid interfaces offer a convenient model for understanding the behavior of many natural systems like biological membranes. Langmuir monolayers were used to characterize the interfacial properties of tissue homogenates, organic phases and aqueous phases of tissue biopsy samples from 30 patients of cervical cancer and 15 normals. Our results reveal that the tensiometric parameters can differentiate between cancer and normal tissues obtained from human cervix and were statistically significant using t-test (P<0.05). The minimum surface tension of the cancer tissue monolayer was 52.9+/-4.4 mN/m, 1.4-folds greater than the normal cervical tissue homogenate value of 38.5+/-2.6 mN/m. The normal tissue homogenate isotherm had a hysteresis area of 90.3 microJ, which was approximately 6.2 times greater than that of the cervical cancer tissue monolayer. The total lipid and phospholipid contents of the cancerous cervical tissue were roughly double that of the normal cervical tissue and the surface activity was also in line with this observation. The difference in hysteresis of the cancerous and normal tissues indicates a decreased stability of the cancerous tissue film as compared to normal. The difference in surface activity denotes alterations in the molecular packing of the tissues in the cancerous state, which may have implications in terms of drug permeability and responsiveness. Further, differences in surface activity may play a role in altered cell adhesion and metastasis. This study is the first to evaluate surface properties of cancerous tissues and can lead to the development of a biophysical marker of cervical cancer based on interfacial properties.

c-Jun interacts with phospholipids and c-Fos at the interface

We describe c-Jun, a widely studied transcription factor that participates in cell proliferation, differentiation, and tumorigenesis, as amphitropic. We show that c-Jun forms stable monolayers and interacts favorably, although in a nonselective manner, with phospholipids at the interface. The surface activity of c-Jun, together with that of c-Fos, its common partner in AP-1 transcription heterodimers, drives interfacial complex formation. We show that AP-1 is very stable at the air-water interface and suggest that AP-1 may not be substantially formed in solution as a stable equimolar association of both proteins.

Many length scales surface fractality in monomolecular films of whole myelin lipids and proteins

Monomolecular films prepared with all the lipid and protein components of myelin were spread at the air/aqueous buffer interface from isolated bovine spinal cord myelin fully dissolved in chloroform:methanol (2:1) or by surface free energy shock of myelin membrane microvesicles. These monolayers show indistinguishable surface behavior, with similar compositional phase coexistence through all the compression isotherm on several subphase conditions. The domains were observed through epifluorescence and Brewster angle microscopy on the air/water interface and on Langmuir-Blodgett films. Their thickness was measured ellipsometrically. Under molecular packing conditions resembling those found in the natural membrane, the morphology and size of the domains are highly self-similar, displaying no characteristic length scale. These properties are the hallmark of fractal objects. The fractality extends at least three orders of magnitudes, from the micrometer to the millimeter range, the fractal dimension being about 1.7. A possible implication of fractality in membrane structure and/or function is demonstrated through the high fluctuation of the propagation of signals through constrained diffusion in corrals formed by domains in the plane of the monolayer, which restricts the diffusion of a fluorescent probe over many length scale domains.

The Folch-Lees proteolipid induces phase coexistence and transverse reorganization of lateral domains in myelin monolayers

Solvent solubilized myelin membranes spread as monomolecular layers at the air-water interface show a heterogeneous pattern at all surface pressures. In order to asses the role of myelin protein and lipid components in the surface structuring we compared the topography, as seen by Brewster angle microscopy (BAM) and epifluorescence microscopy, of monolayers made from mixtures containing all myelin lipids (except gangliosides) and variable proportions of Folch-Lees proteolipid protein (PLP, the major protein component of myelin). The presence of the single PLP, in the absence of the other myelin proteins, can reproduce the surface pattern of the whole myelin extract films in a concentration-dependant manner. Moreover, a threshold mole fraction of PLP is necessary to induce the lipid-protein component reorganization leading to the appearance of a rigid (gray) phase, acting as a surface skeleton, at low surface pressures and of fractal clusters at high surface pressures. The average size of those clusters is also dependent on the PLP content in the monolayer and on the time elapsed from the moment of film spreading, as they apparently result from an irreversible lateral aggregation process. The transverse rearrangement of the monolayer occurring under compression was different in films with the highest and lowest PLP mole fractions tested.

Dynamic surface tensiometry of tissues using Langmuir films

Langmuir monolayers are useful models of biomembranes as they allow simulation of biological conditions and rigorous thermodynamic analysis. This technique was used to characterize tissues at body temperature for the first time in our study. The organs studied include liver, kidney, stomach, testis, heart and brain from goat and certain human cancerous as well as their corresponding normal biopsies to reveal the potential of the tissue monolayer technique. Monolayers were formed on the surface of deionized water by spreading monolayer amounts of the tissue homogenates. The parameters calculated were minimum surface tension, relative lift off area, relative limiting area, compressibility and hysteresis area. Our results reveal that the parameters can differentiate between tissues obtained from different organs and were statistically significant using one-way ANOVA and Newman Keul's test (P<0.05). For example goat's stomach tissue had the lowest hysteresis area (DeltaG) value (27.6 microJ) whereas brain DeltaG value was nine folds higher than stomach value. Brain had the lowest minimum surface tension of 30.3+/-1.0 mN/m whereas stomach had a value of 40.5+/-0. 2 mN/m. Interestingly, the DeltaG values of human normal neck and esophageal tissues were 3.4 and 3.2 folds greater than that of their respective cancer tissues whereas the DeltaG values of vulval and breast cancer tissues were 4.6 and 4 folds greater than that of their respective normal tissues. While the gammamin values of neck cancer tissue showed 95% increase from normal tissue values, those of vulval and breast cancer tissues were 46 and 50% less compared to their respective normal tissue values. Though all the surface tensiometric parameters showed significant changes, minimum surface tension and hysteresis area were the most sensitive indicators of tissue types and diseased states. Further, the effects of therapeutics could also be monitored by this technique. This is evidenced by the post-radiotherapy tissue isotherms of neck and vulval cancers, where clinical radio-sensitivity was associated with a shift in the tensiometry towards their respective normal isotherms. The small sample amounts required, precision of the technique, very low within group variability, organ specificity and sensitivity to detect changes in diseased states make it a promising tool for prognostic evaluation of diseased states and monitoring effects of therapeutics. Further research is warranted in this promising and hitherto unexplored field of tissue tensiometry.

Surface behavior, microheterogeneity and adsorption equilibrium of myelin at the air-water interface

Interfacial films of whole myelin membrane adsorb at the air-water interface from myelin vesicles. The films show a liquid state and their equilibrium spreading pressure is equal to the collapse pressure (about 47 mN/m). The films appear microheterogeneous as seen by epifluorescence microscopy, consisting in two liquid phases over all the adsorption isotherm, starting with rounded liquid expanded domains (low surface pressure) immersed in a cholesterol enriched phase and reaching a fractal pattern at high surface pressure similar to those previously observed by compressing the film. Vesicles adsorb to the interfacial film mainly at the lateral interfaces. The high surface pressure at equilibrium (almost equal to the collapse pressure) indicates the formation of surface multilayers, also shown by fluorescence microscopy.

Biochemical and structural information transduction at the mesoscopic level in biointerfaces containing sphingolipids

In this work we describe two aspects of molecular and supramolecular information transduction. The first is the biochemical and structural information content and transduction associated with sphingomyelinase activity. The results disclose a lipid-mediated cross-communication between the sphingomyelinase and phospholipase A2 pathways. In addition, the two-dimensional degradation of sphingomyelin by sphingomyelinase affects the surface topography and the latter modulates the enzyme activity. The second is the information contained in the compositionally driven lateral organization of whole glial and neuronal membrane interfaces. The myelin monolayer exhibits microheterogeneous topographical structuring and nonhomogeneous lateral thickness of phase separated regions, depending dynamically on the lateral surface pressure. On the other hand, the differential response of functional living cells depends on information contained in the molecular organization of the contacting membrane interface.

Compositional domain immiscibility in whole myelin monolayers at the air-water interface and Langmuir-Blodgett films

Monomolecular layers of whole myelin membrane can be formed at the air-water interface from vesicles or from solvent solution of myelin. The films appear microheterogeneous as seen by epifluorescence and Brewster angle microscopy. The pattern consists mainly of two coexisting liquid phases over the whole compression isotherm. The liquid nature of the phases is apparent from the fluorescent probe behavior, domain mobility, deformability and boundary relaxation due to the line tension of the surface domains. The monolayers were transferred to alkylated glass and fluorescently labeled against myelin components. The immunolabeling of two major proteins of myelin (myelin basic protein, proteolipid-DM20) and of 2',3'-cyclic nucleotide 3'-phosphodiesterase shows colocalization with probes partitioning preferentially in liquid-expanded lipid domains also containing ganglioside G(M1). A different phase showing an enrichment in cholesterol, galactocerebroside and phosphatidylserine markers is also found. The distribution of components is qualitatively independent of the lateral surface pressure and is generally constituted by one phase enriched in charged components in an expanded state coexisting with another phase enriched in non-charged constituents of lower compressibility. The domain immiscibility provides a physical basis for the microheterogeneity found in this membrane model system.

The amphiphilic character of glycogenin

This study describes for the first time the amphiphilicity of the protein moiety of proteoglycogen. Glycogenin but not proteoglycogen associates to phospholipid vesicles and forms by itself stable Gibbs and Langmuir monolayers at the air-buffer interface. The adsorption free energy (-6.7 kcal/mol) and the glycogenin collapse pressure (47 mN/m) are indicative of its high surface activity which can thermodynamically drive and retain the protein at the membrane interface to a maximum equilibrium adsorption surface pressure of 21 mN/m. The marked surface activity of glycogenin is further enhanced by its thermodynamically favorable penetration into zwitterionic and anionic phospholipids with a high cut-off surface pressure point above 30 mN/m. The strong association to phospholipid vesicles and the marked surface activity of glycogenin correspond to a high amphiphilic character which supports its spontaneous association to membrane interfaces, in which the de novo biosynthesis of glycogen was proposed to initiate.

Molecular characteristics of astaxanthin and beta-carotene in the phospholipid monolayer and their distributions in the phospholipid bilayer

The molecular characteristics of the monolayers of astaxanthin with polar group on the beta-ionone ring in the molecule and beta-carotene without polar group and their interactions in mixed carotenoid-phospholipid monolayers and the effects of carotenoids on the phase behavior of the phospholipid bilayers were examined by the monolayer technique and differential scanning calorimetry (DSC). We found from the monolayer study that beta-carotene had an amphiphilic nature. The molecular assembly of astaxanthin in the monolayer at the hydrophobic/hydrophilic interface was more stable than that of beta-carotene. Dimyristoylphosphatidylcholine (DMPC) in the monolayer was miscible with astaxanthin in the range of 0-0.4 mol fractions of astaxanthin, but not fully miscible with beta-carotene even at low concentrations below 0.1 mol fraction of beta-carotene. Surface potential and compression/expansion cycles of beta-carotene monolayer indicated the formation of molecular aggregates by itself. DSC study showed that when small amount of astaxanthin was added, the transition temperature of dipalmitoylphosphatidylcholine (DPPC) was markedly shifted to lower temperatures and that the transition peak was asymmetrically broadened, indicative of a significant depression in cooperativity of the gel to liquid-crystalline transition. The asymmetric DSC endothermic bands of DPPC incorporating small amounts of astaxanthin were well fit by deconvolution into two to three domains containing different concentrations of astaxanthin. On the contrary, the incorporation of beta-carotene resulted in a small depression of the main transition temperature with a slight broadening of the transition peak, suggesting a small miscibility of beta-carotene with the phospholipid bilayer or a formation of aggregates of beta-carotene in the membranes. These results suggest that there would be a high localized concentration in the phase separated membrane for astaxanthin or beta-carotene to function effectively as scavenger.

Segregation of gangliosides GM1 and GD3 on cell membranes, isolated membrane rafts, and defined supported lipid monolayers

Lateral assemblies of sphingolipids, glycosphingolipids and cholesterol, termed rafts, are postulated to be present in biological membranes and to function in important cellular phenomena. We probed whether rafts are heterogeneous by determining the relative distribution of two gangliosides, GM1 and GD3, in artificial supported monolayers, in intact rat primary cerebellar granule neurones, and in membrane rafts isolated from rat cerebellum. Fluorescence resonance energy transfer (FRET) using fluorophore-labelled cholera toxin B subunit (which binds GM1) and mAb R24 (which binds GD3) revealed that GM1 spontaneously self-associates but does not co-cluster with GD3 in supported monolayers and on intact neurones. Cholera toxin and immunocytochemical labelling of isolated membrane rafts from rat cerebellum further demonstrated that GM1 does not co-localise with GD3. Furthermore, whereas the membrane raft resident proteins Lyn and caveolin both co-localise with GD3 in isolated membrane rafts, GM1 appears in separate and distinct aggregates. These data support prior reports that membrane rafts are heterogeneous, although the mechanisms for establishing and maintaining such heterogeneity remain to be determined.

c-Fos is surface active and interacts differentially with phospholipid monolayers

The transcription factor c-Fos forms stable Gibbs and Langmuir monolayers at the air-buffer interface. Its marked surface activity is enhanced by penetration into phospholipid films above the protein's own maximum adsorption surface pressure to a lipid-free interface. The protein-phospholipid stabilizing interactions at the interface depend on the lipid polar head group and the increases of lateral surface pressure generated are comparable to those of membrane-active proteins. The surface activity of c-Fos is strong enough to thermodynamically drive and retain c-Fos at the membrane interface where it may exert direct or indirect effects.

Epifluorescence microscopy of surface domain microheterogeneity in myelin monolayers at the air-water interface

Myelin lipids form liquid-expanded monolayers at the air-water interface, with no evidence of surface pressure-induced two-dimensional phase transition. However, the film doped with 2 mole % of the fluorescent probe N-(7-nitro-2-1,3-benzoxadiazol-4-yl) Diacyl Phosphatidyl-ethanolamine (NBD-PE) shows an irregular pattern of coexisting laterally segregated surface domains with diffuse boundaries that change from smooth patterns to fractal-like structures depending on surface pressure. Successive expansion-recompression cycles lead to more defined domains, with a general reorganization occurring at surface pressures of about 20 mN/m. At least two coexisting phases occur over almost all the compression isotherms. The presence of proteins in whole myelin monolayers induces defined domain textures with relatively sharp boundaries. The patterns during compression and expansion are quite similar and, after the first cycle, little changes occur under recompression. The patterns observed provide topographical evidence for the existence of dynamic domain microheterogeneity in the surface of myelin interfaces.

Lipid monolayers: why use half a membrane to characterize protein-membrane interactions?

Variants of membrane-active proteins and peptides are increasingly available through synthesis and molecular engineering. When determining the effects of structural changes upon the interaction of these proteins with lipid membranes, monomolecular films of lipids at the air-water interface have significant advantages over bilayers and other lipid dispersions. In the past year, a variety of protein-lipid interactions has been characterized successfully using relatively simple surface measurements.