Probing lipid–cholesterol interactions in DOPC/eSM/Chol and DOPC/DPPC/Chol model lipid rafts with DSC and 13C solid-state NMR

Cholesterol Changes Interfacial Water Alignment in Model Cell Membranes

The nanoscopic layer of water that directly hydrates biological membranes plays a critical role in maintaining the cell structure, regulating biochemical processes, and managing intermolecular interactions at the membrane interface. Therefore, comprehending the membrane structure, including its hydration, is essential for understanding the chemistry of life. While cholesterol is a fundamental lipid molecule in mammalian cells, influencing both the structure and dynamics of cell membranes, its impact on the structure of interfacial water has remained unknown. We used surface-specific vibrational sum-frequency generation spectroscopy to study the effect of cholesterol on the structure and hydration of monolayers of the lipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and egg sphingomyelin (SM). We found that for the unsaturated lipid DOPC, cholesterol intercalates in the membrane without significantly changing the orientation of the lipid tails and the orientation of the water molecules hydrating the headgroups of DOPC. In contrast, for the saturated lipids DPPC and SM, the addition of cholesterol leads to clearly enhanced packing and ordering of the hydrophobic tails. It is also observed that the orientation of the water hydrating the lipid headgroups is enhanced upon the addition of cholesterol. These results are important because the orientation of interfacial water molecules influences the cell membranes' dipole potential and the strength and specificity of interactions between cell membranes and peripheral proteins and other biomolecules. The lipid nature-dependent role of cholesterol in altering the arrangement of interfacial water molecules offers a fresh perspective on domain-selective cellular processes, such as protein binding.

Assessing the negative impact of chlorantraniliprole, isoxaflutole, and simazine pesticides on phospholipid membrane models and tilapia gill tissues

The indiscriminate and, very often, incorrect use of pesticides in Brazil, as well as in other countries, results in severe levels of environmental pollution and intoxication of human life. Herein, we studied plasma membrane models (monolayer and bilayer) of the phospholipid Dioleoyl-sn-glycerol-3-phosphocholine (DOPC) using Langmuir films, and large (LUVs) and giant (GUVs) unilamellar vesicles, to determine the effect of the pesticides chlorantraniliprole (CLTP), isoxaflutole (ISF), and simazine (SMZ), used in sugarcane. CLTP affects the lipid organization of the bioinspired models of DOPC π-A isotherms, while ISF and SMZ pesticides significantly affect the LUVs and GUVs. Furthermore, the in vivo study of the gill tissue in fish in the presence of pesticides (2.0 × 10-10 mol/L for CLTP, 8.3 × 10-9 mol/L for ISF, and SMZ at 9.9 × 10-9 mol/L) was performed using optical and fluorescence images. This investigation was motivated by the gill lipid membranes, which are vital for regulating transporter activity through transmembrane proteins, crucial for maintaining ionic balance in fish gills. In this way, the presence of phospholipids in gills offers a model for understanding their effects on fish health. Histological results show that exposure to CLTP, ISF, and SMZ may interfere with vital gill functions, leading to respiratory disorders and osmoregulation dysfunction. The results indicate that exposure to pesticides caused severe morphological alterations in fish, which could be correlated with their impact on the bioinspired membrane models. Moreover, the effect does not depend on the exposure period (24h and 96h), showing that animals exposed to pesticides for a short period suffer irreparable damage to gill tissue. In summary, we can conclude that the harm caused by pesticides, both in membrane models and in fish gills, occurs due to contamination of the aquatic system with pesticides. Therefore, water quality is vital for the preservation of ecosystems.

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.

Thermodynamic and Structural Study of Budesonide-Exogenous Lung Surfactant System

The clinical benefits of using exogenous pulmonary surfactant (EPS) as a carrier of budesonide (BUD), a non-halogenated corticosteroid with a broad anti-inflammatory effect, have been established. Using various experimental techniques (differential scanning calorimetry DSC, small- and wide- angle X-ray scattering SAXS/WAXS, small- angle neutron scattering SANS, fluorescence spectroscopy, dynamic light scattering DLS, and zeta potential), we investigated the effect of BUD on the thermodynamics and structure of the clinically used EPS, Curosurf®. We show that BUD facilitates the Curosurf® phase transition from the gel to the fluid state, resulting in a decrease in the temperature of the main phase transition (Tm) and enthalpy (ΔH). The morphology of the Curosurf® dispersion is maintained for BUD < 10 wt% of the Curosurf® mass; BUD slightly increases the repeat distance d of the fluid lamellar phase in multilamellar vesicles (MLVs) resulting from the thickening of the lipid bilayer. The bilayer thickening (~0.23 nm) was derived from SANS data. The presence of ~2 mmol/L of Ca2+ maintains the effect and structure of the MLVs. The changes in the lateral pressure of the Curosurf® bilayer revealed that the intercalated BUD between the acyl chains of the surfactant's lipid molecules resides deeper in the hydrophobic region when its content exceeds ~6 wt%. Our studies support the concept of a combined therapy utilising budesonide-enriched Curosurf®.

Exosome-like genistein-loaded nanoparticles developed by thin-film hydration and 3D-printed Tesla microfluidic chip: A comparative study

Exosomes are naturally derived information carriers that present interest as drug delivery systems. However, their vague cargo and isolation difficulties hinder their use in clinical practice. To overcome these limitations, we developed exosome-like nanoparticles, consisted of the main lipids of exosomes, using two distinct methods: thin-film hydration and 3D-printed microfluidics. Our novel microfluidic device, fabricated through digital light processing printing, demonstrated a favorable architecture to produce exosome-like nanoparticles. We compared these two techniques by analyzing the physicochemical characteristics (size, size distribution, and ζ-potential) of both unloaded and genistein-loaded exosome-like nanoparticles, using dynamic and electrophoretic light scattering. Our findings revealed that the presence of small lipophilic molecules, cholesterol and/or genistein, influenced the characteristics of the final formulations differently based on the development approach. Regardless of the initial differences of the formulations, all exosome-like nanoparticles, whether loaded with genistein or not, exhibited remarkable colloidal stability over time. Furthermore, an encapsulation efficiency of over 87% for genistein was achieved in all cases. Additionally, thermal analysis uncovered the presence of metastable phases within the membranes, which could impact the drug delivery efficiency. In summary, this study provides a comprehensive comparison between conventional and innovative methods for producing complex liposomal nanosystems, exemplified by exosome-like nanoparticles.

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.

In-plane and out-of-plane gigahertz sound velocities of saturated and unsaturated phospholipid bilayers from cryogenic to room temperatures

Here, we examined the gigahertz sound velocities of hydrated multibilayers of saturated (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) and unsaturated (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) phospholipids by Brillouin spectroscopy. Out-of-plane and in-plane (lateral) phonons were studied independently of each other. Similar strong temperature dependences of the sound velocities were found for phonons of both types. The sound velocities in the low-temperature limit were two-fold higher than that at physiological temperatures; a significant part of the changes in sound velocity occurs in the solid-like gel phase. The factors that may be involved in the peculiar behavior of sound velocity include changes in the chain conformational state, relaxation susceptibility, changes in the elastic modulus at infinite frequencies, and lateral packing of molecules.

Phase-Separated Giant Liposomes for Stable Elevation of α-Hemolysin Concentration in Lipid Membranes

Staphylococcus aureus α-hemolysin (αHL) is one of the most popular proteins in nanopore experiments within lipid membranes. Higher concentrations of αHL within the lipid membrane are desirable to enhance the mass transport capacity through nanopores. However, the reconstitution of αHL at high concentrations is associated with the problem of membrane lytic disruption. In this study, we present a method that effectively increases αHL concentration while maintaining membrane stability. This method is achieved by using phase-separated giant liposomes, where coexisting liquid-disordered (Ld) and liquid-ordered phases (Lo) are enriched in unsaturated lipids and saturated lipids with cholesterol (Chol), respectively. Fluorescence observation of αHL in liposomes revealed that the presence of Chol facilitates αHL insertion into the membrane. Despite the preferential localization of αHL in the Ld phase rather than the Lo phase, the coexistence of both Lo and Ld phases prevents membrane disruption in the presence of concentrated αHL. We have explained this stabilization mechanism considering the lower membrane tension exhibited by phase-separated liposomes compared to homogeneous liposomes. Under hypertonic conditions, we have successfully increased the local concentration of αHL by invagination of the lipid-only region in the Ld phase, leaving αHL behind. This method exhibits potential for the reconstitution of various nanochannels and membrane proteins that prefer the Ld phase over the Lo phase, thus enabling the production of giant liposomes at high concentrations and the replication of the membrane-crowding condition observed in cells.

The crossroad of nanovesicles and oral delivery of insulin

INTRODUCTION

Diabetes mellitus is one of the challenging health problems worldwide. Multiple daily subcutaneous injection of insulin causes poor compliance in patients. Development of efficient oral formulations to improve the quality of life of such patients has been an important goal in pharmaceutical industry. However, due to serious issues such as low bioavailability and instability, it has not been achieved yet.

AREAS COVERED

Due to functional properties of the vesicles and the fact that hepatic-directed vesicles of insulin could reach the clinical phases, we focused on three main vesicular delivery systems for oral delivery of insulin: liposomes, niosomes, and polymersomes. Recent papers were thoroughly discussed to provide a broad overview of such oral delivery systems.

EXPERT OPINION

Although conventional liposomes are unstable in the presence of bile salts, their further modifications such as surface coating could increase their stability in the GI tract. Bilosomes showed good flexibility and stability in GI fluids. Also, niosomes were stable, but they could not induce significant hypoglycemia in animal studies. Although polymersomes were effective, they are expensive and there are some issues about their safety and industrial scale-up. Also, we believe that other modifications such as addition of a targeting agent or surface coating of the vesicles could significantly increase the bioavailability of insulin-loaded vesicles.

Biophysical studies of lipid nanodomains using different physical characterization techniques

For the past 50 years, evidence for the existence of functional lipid domains has been steadily accumulating. Although the notion of functional lipid domains, also known as "lipid rafts," is now widely accepted, this was not always the case. This ambiguity surrounding lipid domains could be partly attributed to the fact that they are highly dynamic, nanoscopic structures. Since most commonly used techniques are sensitive to microscale structural features, it is therefore, not surprising that it took some time to reach a consensus regarding their existence. In this review article, we will discuss studies that have used techniques that are inherently sensitive to nanoscopic structural features (i.e., neutron scatting, nuclear magnetic resonance, and Förster resonance energy transfer). We will also mention techniques that may be of use in the future (i.e., cryoelectron microscopy, droplet interface bilayers, inelastic x-ray scattering, and neutron reflectometry), which can further our understanding of the different and unique physicochemical properties of nanoscopic lipid domains.

Phase Heterogeneity in Cholesterol-Containing Ternary Phospholipid Lamellar Phases

Pseudo-ternary mixtures of lamellar phase phospholipids (DPPC and brain sphingomyelin with cholesterol) were studied below T _m while comparing the influence of cholesterol content, temperature, and the presence of small quantities of vitamin D binding protein (DBP) or vitamin D receptor (VDR). The measurements, conducted by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), cover a range of cholesterol concentrations (20% mol. wt to 40% mol. wt.) and physiologically relevant temperature range (294-314 K). In addition to rich intraphase behavior, data and modeling are used to approximate the lipids' headgroup location variations under the abovementioned experimental conditions.

Brillouin Spectroscopy of Binary Phospholipid-Cholesterol Bilayers

Multicomponent lipid bilayers are used as models for searching the origin of spatial heterogeneities in biomembranes called lipid rafts, implying the coexistence of domains of different phases and compositions within the lipid bilayer. The spatial organization of multicomponent lipid bilayers on a scale of a hundred nanometers remains unknown. Brillouin spectroscopy providing information about the acoustic phonons with the wavelength of several hundred nanometers has an unexplored potential for this problem. Here, we applied Brillouin spectroscopy for three binary bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. The Brillouin experiment for the oriented planar multibilayers was realized for two scattering geometries involving phonons for the lateral and normal directions of the propagation. The DPPC-DOPC mixtures known for the coexistence of the solid-ordered and liquid-disordered phases had bimodal Brillouin peaks, revealing the phase domains with sizes more than a hundred nanometers. Analysis of the Brillouin data for the binary mixtures concluded that the lateral phonons are preferable for testing the lateral homogeneity of the bilayers, while the phonons spreading across the bilayers are sensitive to the layered packing at the mesoscopic scale.

Membrane Fusion Biophysical Analysis of Fusogenic Liposomes

Liposomes represent important drug carrier vehicles in biological systems. A fusogenic liposomal system composed of equimolar mixtures of the cationic lipid DOTAP and the phospholipid DOPE showed high fusion and delivery efficiencies with cells and lipid vesicles. However, aspects of the thermodynamics involving the interaction of these fusogenic liposomes and biomimetic systems remain unclear. Here, we investigate the fusion of this system with large unilamellar vesicles (LUVs) composed of the zwitterionic lipid POPC and increasing fractions of the anionic lipid POPG and up to 30 mol % cholesterol. The focus here is to concomitantly follow changes in size, zeta-potential, and enthalpy binding upon membrane interaction and fusion. Isothermal titration calorimetry (ITC) data showed that membrane fusion in our system is an exothermic process in the absence of cholesterol, suggesting that electrostatic attraction is the driving force for fusion. An endothermic component appeared and eventually dominated the titration at 30 mol % cholesterol, which we propose is caused by membrane fluidification when cholesterol is diluted upon fusion. The inflection points of the ITC data occurred around 0.5-0.7 POPG/DOTAP for all systems, the same stoichiometry for which zeta-potential and dynamic light scattering measurements showed an increase in size coupled with charge neutralization of the system, which is consistent with the fact that fusion in our system is charge-mediated. Microscopy observations of the final mixtures revealed the presence of giant vesicles, which is a clear indication of fusion, coexisting with intermediate-sized objects that could be the result of both fusion and/or aggregation. The results show that the fusion efficiency of the DOTAP:DOPE fusogenic system is modulated by the charge and membrane packing of the acceptor membrane and explain why the system fuses very efficiently with cells.

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.

Development of Icephilic ACTIVE Glycopeptides for Cryopreservation of Human Erythrocytes

Ice formation and recrystallization exert severe impairments to cellular cryopreservation. In light of cell-damaging washing procedures in the current glycerol approach, many researches have been devoted to the development of biocompatible cryoprotectants for optimal bioprotection of human erythrocytes. Herein, we develop a novel ACTIVE glycopeptide of saccharide-grafted ε-poly(L-lysine), that can be credited with adsorption on membrane surfaces, cryopreservation with trehalose, and icephilicity for validity of human erythrocytes. Then, by Borch reductive amination or amidation, glucose, lactose, maltose, maltotriose, or trehalose was tethered to ε-polylysine. The synthesized ACTIVE glycopeptides with intrinsic icephilicity could localize on the membrane surface of human erythrocytes and improve cryopreservation with trehalose, so that remarkable post-thaw cryosurvival of human erythrocytes was achieved with a slight variation in cell morphology and functions. Human erythrocytes (∼50% hematocrit) in cryostores could maintain high cryosurvival above 74%, even after plunged in liquid nitrogen for 6 months. Analyses of differential scanning calorimetry, Raman spectroscopy, and dynamic ice shaping suggested that this cryopreservation protocol combined with the ACTIVE glycopeptide and trehalose could enhance the hydrogen bond network in nonfrozen solutions, resulting in inhibition of recrystallization and growth of ice. Therefore, the ACTIVE glycopeptide can be applied as a trehalose-associated "chaperone", providing a new way to serve as a candidate in glycerol-free human erythrocyte cryopreservation.

Co-Encapsulation of Fisetin and Cisplatin into Liposomes for Glioma Therapy: From Formulation to Cell Evaluation

(1) Background: Glioblastoma (GBM) is the most frequent cerebral tumor. It almost always relapses and there is no validated treatment for second-line GBM. We proposed the coencapsulation of fisetin and cisplatin into liposomes, aiming to (i) obtain a synergistic effect by combining the anti-angiogenic effect of fisetin with the cytotoxic effect of cisplatin, and (ii) administrate fisetin, highly insoluble in water. The design of a liposomal formulation able to encapsulate, retain and deliver both drugs appeared a challenge. (2) Methods: Liposomes with increasing ratios of cholesterol/DOPC were prepared and characterized in term of size, PDI and stability. The incorporation of fisetin was explored using DSC. The antiangiogneic and cytotoxic activities of the selected formulation were assayed in vitro. (3) Results: We successfully developed an optimized liposomal formulation incorporating both drugs, composed by DOPC/cholesterol/DODA-GLY-PEG2000 at a molar ratio of 75.3/20.8/3.9, with a diameter of 173 ± 8 nm (PDI = 0.12 ± 0.01) and a fisetin and cisplatin drug loading of 1.7 ± 0.3% and 0.8 ± 0.1%, respectively, with a relative stability over time. The maximum incorporation of fisetin into the bilayer was determined at 3.2% w/w. Then, the antiangiogenic activity of fisetin was maintained after encapsulation. The formulation showed an additive effect of cisplatin and fisetin on GBM cells; (4) Conclusions: The developed co-loaded formulation was able to retain the activity of fisetin, was effective against GBM cells and is promising for further in vivo experimentations.

Toxicity and membrane perturbation properties of the ribotoxin-like protein Ageritin

Ageritin is the prototype of a new ribotoxin-like protein family, which has been recently identified also in basidiomycetes. The protein exhibits specific RNase activity through the cleavage of a single phosphodiester bond located at sarcin/ricin loop of the large rRNA, thus inhibiting protein biosynthesis at early stages. Conversely to other ribotoxins, its activity requires the presence of divalent cations. In the present study, we report the activity of Ageritin on both prokaryotic and eukaryotic cells showing that the protein has a prominent effect on cancer cells viability and no effects on eukaryotic and bacterial cells. In order to rationalize these findings, the ability of the protein to interact with various liposomes mimicking normal, cancer and bacterial cell membranes was explored. The collected results indicate that Ageritin can interact with DPPC/DPPS/Chol vesicles, used as a model of cancer cell membranes, and with DPPC/DPPG vesicles, used as a model of bacterial cell membranes, suggesting a selective interaction with anionic lipids. However, a different perturbation of the two model membranes, mediated by cholesterol redistribution, was observed and this might be at the basis of Ageritin selective toxicity towards cancer cells.

Facile L-Glutamine delivery to erythrocytes via DOPC-DPPG mixed liposomes

Sickle cell disease (SCD) is a mortal erythrocyte-based disease which is hard to treat effectively. Development of a treatment method that can prevent deoxygenation of erythrocytes or reduce the oxidative stress of sickle erythrocytes is one of the important issues towards SCD. Among a wide variety of potential drug carriers, liposomes are advantageous and preferable with their easy preparation and biocompatibility. In this study, L-Glutamine (Gln) loaded liposomes were prepared with 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-Dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG). Liposomes were characterized via zeta potential, size measurements, differential scanning calorimetry, Fourier Transform Infra-red Spectroscopy and they were visualized via transmission electron microscopy and scanning electron microscopy. Effect of the encapsulated amount of Gln was investigated by encapsulating Gln at three different concentrations (i.e0.20 mM, 40 mM and 60 mM). Drug encapsulation and release studies were implemented with high pressure liquid chromatography (HPLC). The encapsulation efficiency of Gln was determined to be the higher than the ones reported in the literature: 83.6%, 87.1% and 84.9% for 20 mM, 40 mM and 60 mM Gln, respectively. It was found that after 6 hours, liposomes loaded with 60 mM of Gln had released 45.7% of Gln. Optical microscopy images of the erythrocytes after 3 hours of incubation and haemolysis measurements proved that presence of liposomes did not cause any structural changes on the erythrocyte shape. Overall, it was concluded that L-Gln loaded PC/PG liposomes provide promising results in terms of developing a new drug delivery platform for SCD.

Molecular Study of Ultrasound-Triggered Release of Fluorescein from Liposomes

Several investigations have suggested that ultrasound triggers the release of drugs encapsulated into liposomes at acoustic pressures low enough to avoid cavitation or high hyperthermia. However, the mechanism leading to this triggered release as well as the adequate composition of the liposome membrane remains unknown. Here, we investigate the ultrasound-triggered release of fluorescein disodium salt encapsulated into liposomes made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or 1,2-distearoylphosphatidyl-ethanolamine (DSPC) lipids with various concentrations of cholesterol (from 0 to 44 mol %). The passive release of encapsulated fluorescein was first characterized. It was observed to be higher when the membrane is in a fluid phase and increased with temperature but decreased upon addition of cholesterol. Next, the release of fluorescein was measured at different acoustic frequencies (0.8, 1.1, and 3.3 MHz) and peak-to-peak pressures (0, 2, 2.5, 5, and 8 MPa). Measurements were performed at temperatures where DOPC and DSPC liposomes were, respectively, in the fluid or gel phase. We found that the release rate did not depend on the ultrasound frequency. For DOPC liposomes, the ultrasound-triggered release of fluorescein decreased with increasing concentration of cholesterol in liposomes, while the behavior was more complex for DSPC liposomes. Overall, the triggered release from DSPC liposomes was up to ten times less than DOPC liposomes. Molecular dynamics simulations performed on a pure DOPC membrane showed that a membrane experiences, under a directional pressure of ±2.4 MPa, various changes in properties such as the area per lipid (APL). An increase in the APL was notably observed when the simulation box was laterally stretched or perpendicularly compressed, which was accompanied by an increase in the number of water molecules crossing the membrane. This suggests that ultrasound most probably enhances the diffusion of encapsulated molecules at small acoustic pressures by increasing the distance between lipids.

Characterization of a New Biofunctional, Exolytic Alginate Lyase from Tamlana sp. s12 with High Catalytic Activity and Cold-Adapted Features

Alginate, a major acidic polysaccharide in brown algae, has attracted great attention as a promising carbon source for biorefinery systems. Alginate lyases, especially exo-type alginate lyase, play a critical role in the biorefinery process. Although a large number of alginate lyases have been characterized, few can efficiently degrade alginate comprised of mannuronate (M) and guluronate (G) at low temperatures by means of an exolytic mode. In this study, the gene of a new exo-alginate lyase-Alys1-with high activity (1350 U/mg) was cloned from a marine strain, Tamlana sp. s12. When sodium alginate was used as a substrate, the recombinant enzyme showed optimal activity at 35 °C and pH 7.0-8.0. Noticeably, recombinant Alys1 was unstable at temperatures above 30 °C and had a low melting temperature of 56.0 °C. SDS and EDTA significantly inhibit its activity. These data indicate that Alys1 is a cold-adapted enzyme. Moreover, the enzyme can depolymerize alginates polyM and polyG, and produce a monosaccharide as the minimal alginate oligosaccharide. Primary substrate preference tests and identification of the final oligosaccharide products demonstrated that Alys1 is a bifunctional alginate lyase and prefers M to G. These properties make Alys1 a valuable candidate in both basic research and industrial applications.

Membrane active Janus-oligomers of β3-peptides

Self-assembly of an acyclic β³-hexapeptide with alternating side chain chirality, into nanometer size oligomeric bundles showing membrane activity and hosting capacity for hydrophobic small molecules.

Self-assembling peptides offer a versatile set of tools for bottom-up construction of supramolecular biomaterials. Among these compounds, non-natural peptidic foldamers experience increased focus due to their structural variability and lower sensitivity to enzymatic degradation. However, very little is known about their membrane properties and complex oligomeric assemblies – key areas for biomedical and technological applications. Here we designed short, acyclic β³-peptide sequences with alternating amino acid stereoisomers to obtain non-helical molecules having hydrophilic charged residues on one side, and hydrophobic residues on the other side, with the N-terminus preventing formation of infinite fibrils. Our results indicate that these β-peptides form small oligomers both in water and in lipid bilayers and are stabilized by intermolecular hydrogen bonds. In the presence of model membranes, they either prefer the headgroup regions or they insert between the lipid chains. Molecular dynamics (MD) simulations suggest the formation of two-layered bundles with their side chains facing opposite directions when compared in water and in model membranes. Analysis of the MD calculations showed hydrogen bonds inside each layer, however, not between the layers, indicating a dynamic assembly. Moreover, the aqueous form of these oligomers can host fluorescent probes as well as a hydrophobic molecule similarly to e.g. lipid transfer proteins. For the tested, peptides the mixed chirality pattern resulted in similar assemblies despite sequential differences. Based on this, it is hoped that the presented molecular framework will inspire similar oligomers with diverse functionality.

Sphingomyelins and ent-Sphingomyelins Form Homophilic Nano-Subdomains within Liquid Ordered Domains

Sphingomyelin (SM), a major component of small domains (or lipid rafts) in mammalian cell membranes, forms a liquid-ordered phase in the presence of cholesterol (Cho). However, the nature of molecular interactions within the ordered SM/Cho phase remains elusive. We previously revealed that stearoyl-SM (SSM) and its enantiomer (ent-SSM) separately form nano-subdomains within the liquid-ordered phase involving homophilic SSM-SSM and ent-SSM-ent-SSM interactions. In this study, the details of the subdomain formation by SSMs at the nanometer range were examined using Förster resonance energy transfer (FRET) measurements in lipid bilayers containing SSM and ent-SSM, dioleoyl-phosphatidylcholine and Cho. Although microscopy detected a stereochemical effect on partition coefficient favoring stereochemically homophilic interactions in the liquid-ordered state, it showed no significant difference in large-scale liquid-ordered domain formation by the two stereoisomers. In contrast to the uniform domains seen microscopy, FRET analysis using fluorescent donor- and acceptor-labeled SSM showed distinct differences in SM and ent-SM colocalization within nanoscale distances. Donor- and acceptor-labeled SSM showed significantly higher FRET efficiency than did donor-labeled SSM and acceptor-labeled ent-SSM in lipid vesicles composed of “racemic” (1:1) mixtures of SSM/ent-SSM with dioleoylphosphatidylcholine and Cho. The difference in FRET efficiency indicated that SSM and ent-SSM assemble to form separate nano-subdomains. The average size of the subdomains decreased as temperature increased, and at physiological temperatures, the subdomains were found to have a single-digit nanometer radius. These results suggest that (even in the absence of ent-SM) SM-SM interactions play a crucial role in forming nano-subdomains within liquid-ordered domains and may be a key feature of lipid microdomains (or rafts) in biological membranes.

Principles and Applications of Biological Membrane Organization

Many critical biological events, including biochemical signaling, membrane traffic, and cell motility, originate at membrane surfaces. Each such event requires that members of a specific group of proteins and lipids rapidly assemble together at a specific site on the membrane surface. Understanding the biophysical mechanisms that stabilize these assemblies is critical to decoding and controlling cellular functions. In this article, we review progress toward a quantitative biophysical understanding of the mechanisms that drive membrane heterogeneity and organization. We begin from a physical perspective, reviewing the fundamental principles and key experimental evidence behind each proposed mechanism. We then shift to a biological perspective, presenting key examples of the role of heterogeneity in biology and asking which physical mechanisms may be responsible. We close with an applied perspective, noting that membrane heterogeneity provides a novel therapeutic target that is being exploited by a growing number of studies at the interface of biology, physics, and engineering.

Lipid chain mobility and packing in DOPC bilayers at cryogenic temperatures

Low-temperature molecular mobility and packing in biological tissues are important for their survival upon cryopreservation. Electron paramagnetic resonance (EPR) in its pulsed version of electron spin echo (ESE) allows studying stochastic librations of spin-labeled molecules, the type of motion which dominates at low temperatures. These librations are characterized by the parameter <α²>τ_c where <α²> is the mean squared angular amplitude and τ_c is the correlation time for the motion. This parameter is known to be larger for higher temperature and for looser intermolecular structure. In this work, ESE data for the bilayers comprised of doubly-unsaturated DOPC (dioleoyl-glycero-phosphocholine) lipids and mono-unsaturated POPC (palmitoyl-oleoyl-glycero-phosphocholine) lipids with spin-labeled stearic acids added were obtained in the temperature range between 80 and 210 K; the results were compared also with the previously obtained data for fully-saturated DPPC (dipalmitoyl-glycero-phosphocholine) lipid bilayers [J. Phys. Chem. B2014, 118, 12,478-12,485; Appl. Magn. Reson. 2018, 49, 1369-1383]. It turned out that for DOPC bilayers the <α²>τ_c values are of intermediate magnitude between those for POPC and DPPC bilayers, which implies an intermediate density of lipid packing. A possible explanation of this result could be rearrangement at cryogenic temperatures of the DOPC lipid tails, with their terminal segments folding cooperatively. This interpretation is also in agreement with the known thermodynamic properties of gel-fluid transition for DOPC bilayer.

Bioinspired Liposomes for Oral Delivery of Colistin to Combat Intracellular Infections by Salmonella enterica

Bacterial invasion into eukaryotic cells and the establishment of intracellular infection has proven to be an effective means of resisting antibiotic action, as anti-infective agents commonly exhibit a poor permeability across the host cell membrane. Encapsulation of anti-infectives into nanoscaled delivery systems, such as liposomes, is shown to result in an enhancement of intracellular delivery. The aim of the current work is, therefore, to formulate colistin, a poorly permeable anti-infective, into liposomes suitable for oral delivery, and to functionalize these carriers with a bacteria-derived invasive moiety to enhance their intracellular delivery. Different combinations of phospholipids and cholesterol are explored to optimize liposomal drug encapsulation and stability in biorelevant media. These liposomes are then surface-functionalized with extracellular adherence protein (Eap), derived from Staphylococcus aureus. Treatment of HEp-2 and Caco-2 cells infected with Salmonella enterica using colistin-containing, Eap-functionalized liposomes resulted in a significant reduction of intracellular bacteria, in comparison to treatment with nonfunctionalized liposomes as well as colistin alone. This indicates that such bio-invasive carriers are able to facilitate intracellular delivery of colistin, as necessary for intracellular anti-infective activity. The developed Eap-functionalized liposomes, therefore, present a promising strategy for improving the therapy of intracellular infections.

An Inward-Rectifier Potassium Channel Coordinates the Properties of Biologically Derived Membranes

KirBac1.1 is a prokaryotic inward-rectifier K⁺ channel from Burkholderia pseudomallei. It shares the common inward-rectifier K⁺ channel fold with eukaryotic channels, including conserved lipid-binding pockets. Here, we show that KirBac1.1 changes the phase properties and dynamics of the surrounding bilayer. KirBac1.1 was reconstituted into vesicles composed of ¹³C-enriched biological lipids. Two-dimensional liquid-state and solid-state NMR experiments were used to assign lipid ¹H and ¹³C chemical shifts as a function of lipid identity and conformational degrees of freedom. A solid-state NMR temperature series reveals that KirBac1.1 lowers the primary thermotropic phase transition of Escherichia coli lipid membranes while introducing both fluidity and internal lipid order into the fluid phases. In B. thailandensis liposomes, the bacteriohopanetetrol hopanoid, and potentially ornithine lipids, introduce a similar primary lipid-phase transition and liquid-ordered properties. Adding KirBac1.1 to B. thailandensis lipids increases B. thailandensis lipid fluidity while preserving internal lipid order. This synergistic effect of KirBac1.1 in bacteriohopanetetrol-rich membranes has implications for bilayer dynamic structure. If membrane proteins can anneal lipid translational degrees of freedom while preserving internal order, it could offer an explanation to the nature of liquid-ordered protein-lipid organization in vivo.

Effects of artepillin C on model membranes displaying liquid immiscibility

It has been hypothesized that the therapeutic effects of artepillin C, a natural compound derived from Brazilian green propolis, are likely related to its partition in the lipid bilayer component of biological membranes. To test this hypothesis, we investigated the effects of the major compound of green propolis, artepillin C, on model membranes (small and giant unilamelar vesicles) composed of ternary lipid mixtures containing cholesterol, which display liquid-ordered (l_o) and liquid-disordered (l_d) phase coexistence. Specifically, we explored potential changes in relevant membrane parameters upon addition of artepillin C presenting both neutral and deprotonated states by means of small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and confocal and multiphoton excitation fluorescence microscopy. Thermotropic analysis obtained from DSC experiments indicated a loss in the lipid cooperativity of l_o phase at equilibrium conditions, while at similar conditions spontaneous formation of unilamellar vesicles from SAXS experiments showed that deprotonated artepillin C preferentially located at the surface of the membrane. Time-resolved experiments using fluorescence microscopy showed that at doses above 100 µM, artepillin C in its neutral state interacted with both liquid-ordered and liquid-disordered phases, inducing curvature stress and promoting dehydration at the membrane interface.

The on-fibrillation-pathway membrane content leakage and off-fibrillation-pathway lipid mixing induced by 40-residue β-amyloid peptides in biologically relevant model liposomes

Disruption of the synaptic plasma membrane (SPM) induced by the aggregation of β-amyloid (Aβ) peptides has been considered as a potential mechanism for the neurotoxicity of Aβ in Alzheimer's disease (AD). However, the molecular basis of such membrane disruption process remains unclear, mainly because of the severe systematic heterogeneity problem that prevents the high-resolution studies. Our previous studies using a two-component phosphatidylcholine (PC)/phosphatidylglycerol (PG) model liposome showed the presence of Aβ-induced membrane disruptions that were either on the pathway or off the pathway of fibril formation. The present study focuses on a more biologically relevant model membrane with compositions that mimic the outer leaflet of SPMs. The main findings are: (1) the two competing membrane disruption effects discovered in PC/PG liposomes and their general peptide-to-lipid-molar-ratio dependence persist in the more complicated membrane models; (2) the SPM-mimic membrane promotes the formation of certain "on-fibrillation-pathway" intermediates with higher α-helical structural population, which lead to more rapid and significant of membrane content leakage; (3) although the "on-fibrillation-pathway" intermediate structures show dependence on membrane compositions, there seems to be a common final fibril structure grown from different liposomes, suggesting that there may be a predominant fibril structure for 40-residue Aβ (i.e. Aβ40) peptides in biologically-relevant membranes. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Molecular AFM imaging of Hsp70-1A association with dipalmitoyl phosphatidylserine reveals membrane blebbing in the presence of cholesterol

Hsp70-1A-the major stress-inducible member of the HSP70 chaperone family-is being implicated in cancer diseases with the development of resistances to standard therapies. In normal cells, the protein is purely cytosolic, but in a growing number of tumor cells, a significant fraction can be identified on to the cell surface. The anchoring mechanism is still under debate, as Hsp70-1A lacks conventional signaling sequences for translocation from the cytosol to exoplasmic leaflet of the plasma membrane and common membrane binding domains. Recent reports propose a lipid-mediated anchoring mechanism based on a specific interaction with charged, saturated lipids such as dipalmitoyl phosphatidylserine (DPPS). Here, we prepared planar supported lipid bilayers (SLBs) to visualize the association of Hsp70-1A directly and on the single molecule level by atomic force microscopy (AFM). The single molecule sensitivity of our approach allowed us to explore the low concentration range of 0.05 to 1.0 μg/ml of Hsp70-1A which was not studied before. We compared the binding of the protein to bilayers with 20% DPPS lipid content both in the absence and presence of cholesterol. Hsp70-1A inserted exclusively into DPPS domains and assembled in clusters with increasing protein density. A critical density was reached for incubation with 0.5 μg/ml (7 nM); at higher concentrations, membrane defects were observed that originated from cluster centers. In the presence of cholesterol, this critical concentration leads to the formation of membrane blebs, which burst at higher concentrations supporting a previously proposed non-classical pathway for the export of Hsp70-1A by tumor cells. In the discussion of our data, we attempt to link the lipid-mediated plasma membrane localization of Hsp70-1A to its potential involvement in the development of resistances to radiation and chemotherapy based on our own findings and the current literature.

The Mechanism by Which Luteolin Disrupts the Cytoplasmic Membrane of Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most versatile human pathogens. Luteolin (LUT) has anti-MRSA activity by disrupting the MRSA cytoplasmic membrane. However, the mechanism by which luteolin disrupts the membrane remains unclear. Here, we performed differential scanning calorimetry (DSC) and all-atomic molecular dynamics (AA-MD) simulations to investigate the interactions and effects of LUT on model membranes composed of phosphatidylcholine (PC) and phosphatidylglycerol (PG). We detected the transition thermodynamic parameters of dipalmitoylphosphatidylcholine (DPPC) liposomes, dipalmitoylphosphatidylglycerol (DPPG) liposomes, and liposomes composed of both DPPC and DPPG at different LUT concentrations and showed that LUT molecules were located between polar heads and the hydrophobic region of DPPC/DPPG. In the MD trajectories, LUT molecules ranging from 5 to 50 had different effects on the membranes thickness, fluidity and ordered property of lipids, and lateral pressure of lipid bilayers composed of dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylglycerol (DOPG). Also, most LUT molecules were distributed in the region between the phosphorus atoms and C9 atoms of DOPC and DOPG. On the basis of the combination of these results, we conclude that the distinct effects of LUT on lipid bilayers composed of PCs and PGs may elucidate the mechanism by which LUT disrupts the cytoplasmic membrane of MRSA.

Pathways for creation and annihilation of nanoscale biomembrane domains reveal alpha and beta-toxin nanopore formation processes

Raft-like functional domains with putative sizes of 20–200 nm and which are evolving dynamically are believed to be the most crucial regions in cellular membranes which determine cell signaling and various functions of cells.

The potent effect of mycolactone on lipid membranes

Mycolactone is a lipid-like endotoxin synthesized by an environmental human pathogen, Mycobacterium ulcerans, the causal agent of Buruli ulcer disease. Mycolactone has pleiotropic effects on fundamental cellular processes (cell adhesion, cell death and inflammation). Various cellular targets of mycolactone have been identified and a literature survey revealed that most of these targets are membrane receptors residing in ordered plasma membrane nanodomains, within which their functionalities can be modulated. We investigated the capacity of mycolactone to interact with membranes, to evaluate its effects on membrane lipid organization following its diffusion across the cell membrane. We used Langmuir monolayers as a cell membrane model. Experiments were carried out with a lipid composition chosen to be as similar as possible to that of the plasma membrane. Mycolactone, which has surfactant properties, with an apparent saturation concentration of 1 μM, interacted with the membrane at very low concentrations (60 nM). The interaction of mycolactone with the membrane was mediated by the presence of cholesterol and, like detergents, mycolactone reshaped the membrane. In its monomeric form, this toxin modifies lipid segregation in the monolayer, strongly affecting the formation of ordered microdomains. These findings suggest that mycolactone disturbs lipid organization in the biological membranes it crosses, with potential effects on cell functions and signaling pathways. Microdomain remodeling may therefore underlie molecular events, accounting for the ability of mycolactone to attack multiple targets and providing new insight into a single unifying mechanism underlying the pleiotropic effects of this molecule. This membrane remodeling may act in synergy with the other known effects of mycolactone on its intracellular targets, potentiating these effects.

Lipid Pore-Filled Silica Thin-Film Membranes for Biomimetic Recovery of Dilute Carbohydrates

Selectively permeable biological membranes containing lipophilic barriers inspire the design of biomimetic carrier-mediated membranes for aqueous solute separation. The recovery of glucose, which can reversibly bind to boronic acid (BA) carriers, is examined in lipid pore-filled silica thin-film composite membranes with accessible mesopores. The successful incorporation of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and BA carriers (4-((N-Boc-amino)methyl)phenylboronic acid, BAMP-BA) in the pores of mesoporous silica (∼10 nm pore diameter) through evaporation deposition is verified by confocal microscopy and differential scanning calorimetry. In the absence of BA carriers, lipids confined inside the pores of silica thin films (∼200 nm thick) provide a factor of 14 increase in diffusive transport resistance to glucose, relative to traditional supported lipid bilayers formed by vesicle fusion on the porous surface. The addition of lipid-immobilized BAMP-BA (59 mol % in DPPC) facilitates the transport of glucose through the membrane; glucose flux increases from 45 × 10^-8 to 225 × 10^-8 mol/m²/s in the presence of BAMP-BA. Furthermore, the transport can be improved by environmental factors including pH gradient (to control the binding and release of glucose) and temperature (to adjust lipid bilayer fluidity). The successful development of biomimetic nanocomposite membranes demonstrated here is an important step toward the efficient dilute aqueous solute upgrading or separations, such as the processing of carbohydrates from lignocellulose hydrolysates, using engineered carrier/catalyst/support systems.

Mechanical Properties of Membranes Composed of Gel-Phase or Fluid-Phase Phospholipids Probed on Liposomes by Atomic Force Spectroscopy

In many liposome applications, the nanomechanical properties of the membrane envelope are essential to ensure, e.g., physical stability, protection, or penetration into tissues. Of all factors, the lipid composition and its phase behavior are susceptible to tune the mechanical properties of membranes. To investigate this, small unilamellar vesicles (SUV; diameter < 200 nm), referred to as liposomes, were produced using either unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in aqueous buffer at pH 6.7. The respective melting temperatures of these phospholipids were -20 and 41 °C. X-ray diffraction analysis confirmed that at 20 °C DOPC was in the fluid phase and DPPC was in the gel phase. After adsorption of the liposomes onto flat silicon substrates, atomic force microscopy (AFM) was used to image and probe the mechanical properties of the liposome membrane. The resulting force-distance curves were treated using an analytical model based on the shell theory to yield the Young's modulus (E) and the bending rigidity (k_C) of the curved membranes. The mechanical investigation showed that DPPC membranes were much stiffer (E = 116 ± 45 MPa) than those of DOPC (E = 13 ± 9 MPa) at 20 °C. The study demonstrates that the employed methodology allows discrimination of the respective properties of gel- or fluid-phase membranes when in the shape of liposomes. It opens perspectives to map the mechanical properties of liposomes containing both fluid and gel phases or of biological systems.

Molecular determinants of the N-terminal acetyltransferase Naa60 anchoring to the Golgi membrane

Nα-Acetyltransferase 60 (Naa60 or NatF) was recently identified as an unconventional N-terminal acetyltransferase (NAT) because it localizes to organelles, in particular the Golgi apparatus, and has a preference for acetylating N termini of the transmembrane proteins. This knowledge challenged the prevailing view of N-terminal acetylation as a co-translational ribosome-associated process and suggested a new mechanistic functioning for the enzymes responsible for this increasingly recognized protein modification. Crystallography studies on Naa60 were unable to resolve the C-terminal tail of Naa60, which is responsible for the organellar localization. Here, we combined modeling, in vitro assays, and cellular localization studies to investigate the secondary structure and membrane interacting capacity of Naa60. The results show that Naa60 is a peripheral membrane protein. Two amphipathic helices within the Naa60 C terminus bind the membrane directly in a parallel position relative to the lipid bilayer via hydrophobic and electrostatic interactions. A peptide corresponding to the C terminus was unstructured in solution and only folded into an α-helical conformation in the presence of liposomes. Computational modeling and cellular mutational analysis revealed the hydrophobic face of two α-helices to be critical for membranous localization. Furthermore, we found a strong and specific binding preference of Naa60 toward membranes containing the phosphatidylinositol PI(4)P, thus possibly explaining the primary residency of Naa60 at the PI(4)P-rich Golgi. In conclusion, we have defined the mode of cytosolic Naa60 anchoring to the Golgi apparatus, most likely occurring post-translationally and specifically facilitating post-translational N-terminal acetylation of many transmembrane proteins.

Changes in physical and chemical properties of niosome membrane induced by cholesterol: a promising approach for niosome bilayer intervention

Recently, the self-assembly property of nonionic surfactants has been utilized to create vesicles as alternatives to liposomes.

Macroscopic and Nanoscopic Heterogeneous Structures in a Three-Component Lipid Bilayer Mixtures Determined by Atomic Force Microscopy

Much of lipid raft properties can be inferred from phase behavior of multicomponent lipid membranes. We use liquid compatible atomic force microscopy (AFM) to study a three-component system composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), egg sphingomyelin (eSM), and cholesterol. Specifically, we obtain macroscopic and nanoscopic heterogeneous structures in a broad compositional space of DOPC/eSM/cholesterol (23 °C). In the macroscopic liquid coexisting region, we quantify area fraction of the coexisting phases and determine a set of thermodynamic tie-lines. When lipid compositions are near the critical point, we obtain fluctuation-like nanoscopic structures. We also use AFM height images to explore the hypothetical three-phase coexisting region. Finally, we use fluorescence microscopy to compare the phase behavior from our AFM measurements to that in free-floating giant unilamellar vesicles (GUVs). Our results highlight the role of lipid composition in mediating lipid domain formation and stability.

Communication: Orientational self-ordering of spin-labeled cholesterol analogs in lipid bilayers in diluted conditions

Lipid-cholesterol interactions are responsible for different properties of biological membranes including those determining formation in the membrane of spatial inhomogeneities (lipid rafts). To get new information on these interactions, electron spin echo (ESE) spectroscopy, which is a pulsed version of electron paramagnetic resonance (EPR), was applied to study 3β-doxyl-5α-cholestane (DCh), a spin-labeled analog of cholesterol, in phospholipid bilayer consisted of equimolecular mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine. DCh concentration in the bilayer was between 0.1 mol.% and 4 mol.%. For comparison, a reference system containing a spin-labeled 5-doxyl-stearic acid (5-DSA) instead of DCh was studied as well. The effects of "instantaneous diffusion" in ESE decay and in echo-detected (ED) EPR spectra were explored for both systems. The reference system showed good agreement with the theoretical prediction for the model of spin labels of randomly distributed orientations, but the DCh system demonstrated remarkably smaller effects. The results were explained by assuming that neighboring DCh molecules are oriented in a correlative way. However, this correlation does not imply the formation of clusters of cholesterol molecules, because conventional continuous wave EPR spectra did not show the typical broadening due to aggregation of spin labels and the observed ESE decay was not faster than in the reference system. So the obtained data evidence that cholesterol molecules at low concentrations in biological membranes can interact via large distances of several nanometers which results in their orientational self-ordering.

Interaction of cholesterol in ternary lipid mixtures investigated using single-molecule fluorescence

Fluorescence measurements of the sterol analog 23-(dipyrrometheneboron difluoride)-24-norcholesterol (BODIPY-cholesterol) are used to compare the effects of cholesterol (Chol) in monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Chol and chicken egg sphingomyelin (SM)/DOPC/Chol. Monolayers are formed using the Langmuir-Blodgett technique and compared at surface pressures of 8 and 30 mN/m. In particular, these ternary lipid mixtures are compared using both ensemble and single-molecule fluorescence measurements of BODIPY-cholesterol. In mixed monolayers incorporating 0.10 mol % BODIPY-cholesterol, fluorescence microscopy measurements as a function of cholesterol added reveal similar trends in monolayer phase structure for both DPPC/DOPC/Chol and SM/DOPC/Chol films. With a probe concentration reduced to ∼10(-8) mol % BODIPY-cholesterol, single-molecule fluorescence measurements using defocused polarized total internal reflection microscopy are used to characterize the orientations of BODIPY-cholesterol in the monolayers. Population histograms of the BODIPY emission dipole tilt angle away from the membrane normal reveal distinct insertion geometries with a preferred angle observed near 78°. The measured angles and populations are relatively insensitive to added cholesterol and changes in surface pressure for monolayers of SM/DOPC/Chol. For monolayers of DPPC/DOPC/Chol, however, the single-molecule measurements reveal significant changes in the BODIPY-cholesterol insertion geometry when the surface pressure is increased to 30 mN/m. These changes are discussed in terms of a squeeze-out mechanism for BODIPY-cholesterol in these monolayers and provide insight into the partitioning and arrangement of BODIPY-cholesterol in ternary lipid mixtures.

Milk sphingomyelin domains in biomimetic membranes and the role of cholesterol: morphology and nanomechanical properties investigated using AFM and force spectroscopy

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.