Molecular organization in mixed SOPC and SDPC model membranes: Water permeability studies of polyunsaturated lipid bilayers

Concentration-Dependent Effects of Curcumin on Membrane Permeability and Structure

Growing evidence suggests that many bioactive molecules can nonspecifically modulate the physicochemical properties of membranes and influence the action of embedded membrane proteins. This study investigates the interactions of curcumin with protein-free model membranes consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DOPC with cholesterol (4/1 mol ratio). The focus is on the capability of curcumin to modify membrane barrier properties such as water permeability assayed through the droplet interface bilayer (DIB) model membrane. For pure DOPC, our findings show a concentration-dependent biphasic effect: a reduction in water permeability is observed at low concentrations (up to 2 mol %), whereas at high concentrations of curcumin, water permeability increases. In the presence of cholesterol, we observed an overall reduction in water permeability. A combination of complementary experimental methods, including phase transition parameters studied by differential scanning calorimetry (DSC) and structural properties measured by attenuated total reflectance (ATR)-FTIR, provides a deeper understanding of concentration-dependent interactions of curcumin with DOPC bilayers in the absence and presence of cholesterol. Our experimental findings align with a molecular mechanism of curcumin's interaction with model membranes, wherein its effect is contingent on its concentration. At low concentrations, curcumin binds to the lipid-water interface through hydrogen bonding with the phosphate headgroup, thereby obstructing the transport of water molecules. Conversely, at high concentrations, curcumin permeates the acyl chain region, inducing packing disorders and demonstrating evidence of phase separation. Enhanced knowledge of the impact of curcumin on membranes, which, in turn, can affect protein function, is likely to be beneficial for the successful translation of curcumin into effective medicine.

Differential Effects of Soy Isoflavones on the Biophysical Properties of Model Membranes

The effects that the main soy isoflavones, genistein and daidzein, have upon the biophysical properties of a model lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or DOPC with cholesterol (4 to 1 mol ratio) have been investigated by transbilayer water permeability, differential scanning calorimetry, and confocal Raman microspectroscopy. Genistein is found to increase water permeability, decrease phase transition temperature, reduce enthalpy of transition, and induce packing disorder in the DOPC membrane with an increasing concentration. On the contrary, daidzein decreases water permeability and shows negligible impact on thermodynamic parameters and packing disorder at comparable concentrations. For a cholesterol-containing DOPC bilayer, both genistein and daidzein exhibit an overall less pronounced effect on transbilayer water permeability. Their respective differential abilities to modify the physical and structural properties of biomembranes with varying lipid compositions signify a complex and sensitive nature to isoflavone interactions, which depends on the initial state of bilayer packing and the differences in the molecular structures of these soy isoflavones, and provide insights in understanding the interactions of these molecules with cellular membranes.

Aspirin Interacts with Cholesterol-Containing Membranes in a pH-Dependent Manner

Aspirin has been used for broad therapeutic treatment, including secondary prevention of cardiovascular disease associated with increased cholesterol levels. Aspirin and other nonsteroidal anti-inflammatory drugs have been shown to interact with lipid membranes and change their biophysical properties. In this study, mixed lipid model bilayers made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) or 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) comprising varying concentrations of cholesterol (10:1, 4:1, and 1:1 mole ratio of lipid:chol), prepared by the droplet interface bilayer method, were used to examine the effects of aspirin at various pH on transbilayer water permeability. The presence of aspirin increases the water permeability of POPC bilayers in a concentration-dependent manner, with a greater magnitude of increase at pH 3 compared to pH 7. In the presence of cholesterol, aspirin is similarly shown to increase water permeability; however, the extent of the increase depends on both the concentration of cholesterol and the pH, with the least pronounced enhancement in water permeability at high cholesterol levels at pH 7. A fusion of data from differential scanning calorimetry, confocal Raman microspectrophotometry, and interfacial tensiometric measurements demonstrates that aspirin can promote significant thermal, structural, and interfacial property perturbations in the mixed-lipid POPC or DOPC membranes containing cholesterol, indicating a disordering effect on the lipid membranes. Our findings suggest that aspirin fluidizes phosphocholine membranes in both cholesterol-free and cholesterol-enriched states and that the overall effect is greater when aspirin is in a neutral state. These results confer a deeper comprehension of the divergent effects of aspirin on biological membranes having heterogeneous compositions, under varying physiological pH and different cholesterol compositions, with implications for a better understanding of the gastrointestinal toxicity induced by the long term use of this important nonsteroidal anti-inflammatory molecule.

Structural Impact of Selected Retinoids on Model Photoreceptor Membranes

Photoreceptor membranes have a unique lipid composition. They contain a high level of polyunsaturated fatty acids including the most unsaturated fatty acid in nature, docosahexaenoic acid (22:6), and are enriched in phosphatidylethanolamines. The phospholipid composition and cholesterol content of the subcellular components of photoreceptor outer segments enables to divide photoreceptor membranes into three types: plasma membranes, young disc membranes, and old disc membranes. A high degree of lipid unsaturation, extended exposure to intensive irradiation, and high respiratory demands make these membranes sensitive to oxidative stress and lipid peroxidation. Moreover, all-trans retinal (AtRAL), which is a photoreactive product of visual pigment bleaching, accumulates transiently inside these membranes, where its concentration may reach a phototoxic level. An elevated concentration of AtRAL leads to accelerated formation and accumulation of bisretinoid condensation products such as A2E or AtRAL dimers. However, a possible structural impact of these retinoids on the photoreceptor-membrane properties has not yet been studied. In this work we focused just on this aspect. The changes induced by retinoids, although noticeable, seem not to be significant enough to be physiologically relevant. This is, however, an positive conclusion because it can be assumed that accumulation of AtRAL in photoreceptor membranes will not affect the transduction of visual signals and will not disturb the interaction of proteins engaged in this process.

Sucrose Osmotic Self-Oscillation Drives Membrane Permeability

Molecular permeation through phospholipid membranes is a fundamental biological process for small molecules. Sucrose is one of the most widely used sweeteners and a key factor in the pathogenesis of obesity and diabetes, yet a detailed understanding of its mechanism involved in permeability into phospholipid membranes is still lacking. Here, using giant unimolecular vesicles (GUVs) reconstituting membrane properties, we compared the osmotic behavior of sucrose in GUVs and HepG2 cells to explore the effect of sucrose on membrane stability in the absence of protein enhancers. The results suggested that the particle size and potential of GUVs and the cellular membrane potential changed significantly with increasing the sucrose concentration (p < 0.05). In microscopic images of cells containing GUVs and sucrose, the fluorescence intensity of vesicles was 537 ± 17.69 after 15 min, and the value was significantly higher than that of microscopic images of cells without sucrose addition (p < 0.05). These changes suggested that the permeability of the phospholipid membrane became larger under a sucrose environment. This study provides a theoretical basis for better insight on the role of sucrose in the physiological environment.

Trans-Resveratrol Decreases Membrane Water Permeability: A Study of Cholesterol-Dependent Interactions

Resveratrol (RSV), a biologically active plant phenol, has been extensively investigated for cancer prevention and treatment due to its ability to regulate intracellular targets and signaling pathways which affect cell growth and metastasis. The non-specific interactions between RSV and cell membranes can modulate physical properties of membranes, which in turn can affect the conformation of proteins and perturb membrane-hosted biological functions. This study examines non-specific interactions of RSV with model membranes having varying concentrations of cholesterol (Chol), mimicking normal and cancerous cells. The perturbation of the model membrane by RSV is sensed by changes in water permeability parameters, using Droplet Interface Bilayer (DIB) models, thermotropic properties from Differential Scanning Calorimetry, and structural properties from confocal Raman spectroscopy, all of which are techniques not complicated by the use of probes which may themselves perturb the membrane. The nature and extent of interactions greatly depend on the presence and absence of Chol as well as the concentration of RSV. Our results indicate that the presence of RSV decreases water permeability of lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), indicating a capability for RSV in stiffening fluidic membranes. When Chol is present, however, (at 4:1 and 2:1 mol ratio DOPC to cholesterol), the addition of RSV has no significant effect upon the water permeability. DSC thermograms show that RSV interacts with DOPC and DOPC/Chol bilayers and influences their thermotropic phase behavior in a concentration-dependent manner, by decreasing the main phase transition temperature and enthalpy, with a phase separation shown at the higher concentrations of RSV. Raman spectroscopic studies indicate an ordering effect of RSV on DOPC supported bilayer, with a lesser extent of ordering in the presence of Chol. Combined results from these investigations highlight a differential effect of RSV on Chol-free and Chol-enriched membranes, respectively, which results constitute a bellwether for increased understanding and effective use of resveratrol in disease therapy including cancer.

Differential Interaction of Cannabidiol with Biomembranes Dependent on Cholesterol Concentration

Cannabidiol (CBD), the major nonpsychoactive component of plant-derived cannabinoids, has been reported to have a broad range of potential beneficial pharmacological effects on the central nervous system (CNS). In this study, the droplet interface bilayer, a model cell membrane, is used to examine the effects of CBD on passive water permeability, a fundamental membrane biophysical property. The presence of CBD decreases the water permeability of model lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and at low concentrations of cholesterol (Chol) (20 mol %) in DOPC, whereas when higher concentrations of Chol are present (33 mol %), CBD has an opposing effect, increasing water permeability. The diametric effect in water permeability change upon addition of CBD to Chol-low and Chol-high bilayers signifies a variant interaction of CBD, depending on the initial state of bilayer packing and fluidity. Additionally, differential scanning calorimetry studies provide evidence that there are selective changes in thermotropic behavior for CBD with DOPC and with DOPC/Chol membranes, respectively, supportive of these varying membrane interactions of CBD dependent upon cholesterol. The intriguing ability of CBD to sensitively respond to membrane Chol concentrations in modifying physical properties highlights the significant impact that CBD can have on heterogeneous biomembranes including those of the CNS, the neurons of which are enriched in Chol to a point where up to a quarter of the body's total Chol is in the brain, and defective brain Chol homeostasis is implicated in neurodegenerative diseases.

Ibuprofen and the Phosphatidylcholine Bilayer: Membrane Water Permeability in the Presence and Absence of Cholesterol

The interactions between drugs and cell membranes can modulate the structural and physical properties of membranes. The resultant perturbations of the membrane integrity may affect the conformation of the proteins inserted within the membrane, disturbing the membrane-hosted biological functions. In this study, the droplet interface bilayer (DIB), a model cell membrane, is used to examine the effects of ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), on transbilayer water permeability, which is a fundamental membrane biophysical property. Our results indicate that the presence of neutral ibuprofen (pH 3) increases the water permeability of the lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). When cholesterol is present with the DOPC, however, the water permeability is not influenced by addition of ibuprofen, regardless of the cholesterol content in DOPC. Given the fact that cholesterol is generally considered to impact packing in the hydrocarbon chain regions, our findings suggest that a potential competition between opposing effects of ibuprofen molecules and cholesterol on the hydrocarbon core environment of the phospholipid assembly may influence the overall water transport phenomena. Results from confocal Raman microspectroscopy and interfacial tensiometry show that ibuprofen molecules induce substantial structural and dynamic changes in the DOPC lipid bilayer. These results, demonstrating that the presence of ibuprofen increases the water permeability of pure DOPC but not that of DOPC-cholesterol mixtures, provide insight into the differential effect of a representative NSAID on heterogeneous biological membranes, depending upon the local composition and structure, results which will signal increased understanding of the gastrointestinal damage and toxicity induced by these molecules.