Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine

Stability of Erythrocyte-Derived Nanovesicles Assessed by Light Scattering and Electron Microscopy

Extracellular vesicles (EVs) are gaining increasing amounts of attention due to their potential use in diagnostics and therapy, but the poor reproducibility of the studies that have been conducted on these structures hinders their breakthrough into routine practice. We believe that a better understanding of EVs stability and methods to control their integrity are the key to resolving this issue. In this work, erythrocyte EVs (hbEVs) were isolated by centrifugation from suspensions of human erythrocytes that had been aged in vitro. The isolate was characterised by scanning (SEM) and cryo-transmission electron microscopy (cryo-TEM), flow cytometry (FCM), dynamic/static light scattering (LS), protein electrophoresis, and UV-V spectrometry. The hbEVs were exposed to various conditions (pH (4–10), osmolarity (50–1000 mOsm/L), temperature (15–60 °C), and surfactant Triton X-100 (10–500 μM)). Their stability was evaluated by LS by considering the hydrodynamic radius (R_h), intensity of scattered light (I), and the shape parameter (ρ). The morphology of the hbEVs that had been stored in phosphate-buffered saline with citrate (PBS–citrate) at 4 °C remained consistent for more than 6 months. A change in the media properties (50–1000 mOsm/L, pH 4–10) had no significant effect on the R_h (=100–130 nm). At pH values below 6 and above 8, at temperatures above 45 °C, and in the presence of Triton X-100, hbEVs degradation was indicated by a decrease in I of more than 20%. Due to the simple preparation, homogeneous morphology, and stability of hbEVs under a wide range of conditions, they are considered to be a suitable option for EV reference material.

Small-Angle X-ray Scattering Studies on the Structure of Disc-Shaped Bicelles Incorporated with Neutral PEGylated Lipids

In this study, small-angle X-ray scattering (SAXS) is successfully employed to investigate the structure of the DPPC/diC7PC disc-shaped bicelles incorporated with different amounts of C16-PEG2000-Ceramide lipids. The incorporation of the C16-PEG2000-Ceramide lipids could provide an antifouling capability to the bicelle for biomedical applications. However, traditionally it is believed that most of the incorporated PEGlylated lipids should lie in the rim of the disc-shaped bicelle. In this study, high sensitivity SAXS reveals the distribution of the added C16-PEG2000-Ceramide lipids in both the planar region and in the rim of the bicelle. The PEG brushes of C16-PEG2000-Ceramide lipids form a second shell outside the lipid headgroup shell of the bicelle. A double shell disc bicelle model is used in analyzing the SAXS data. The lipid density of C16-PEG2000-Ceramide in the rim is found to be about 1.7 times the C16-PEG2000-Ceramide lipid density in the planar region for all three C16-PEG2000-Ceramide concentrations, 1, 2, and 3 mM. Moreover, the bicelle core radius can be predicted well using the actual molecular ratio of lipids in the planar region to the lipids in the rim of the bicelles in the model calculation.

Multivariate Analysis of Mixed Lipid Aggregate Phase Transitions Monitored Using Raman Spectroscopy

The phase behavior of aqueous 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) mixtures between 8.0 ℃ and 41.0 ℃ were monitored using Raman spectroscopy. Temperature-dependent Raman matrices were assembled from series of spectra and subjected to multivariate analysis. The consensus of pseudo-rank estimation results is that seven to eight components account for the temperature-dependent changes observed in the spectra. The spectra and temperature response profiles of the mixture components were resolved by applying a variant of the non-negative matrix factorization (NMF) algorithm described by Lee and Seung (1999). The rotational ambiguity of the data matrix was reduced by augmenting the original temperature-dependent spectral matrix with its cumulative counterpart, i.e., the matrix formed by successive integration of the spectra across the temperature index (columns). Successive rounds of constrained NMF were used to isolate component spectra from a significant fluorescence background. Five major components exhibiting varying degrees of gel and liquid crystalline lipid character were resolved. Hydrogen-bonded water networks exhibiting varying degrees of organization are associated with the lipid components. Spectral parameters were computed to compare the chain conformation, packing, and hydration indicated by the resolved spectra. Based on spectral features and relative amounts of the components observed, four components reflect long chain lipid response. The fifth component could reflect the response of the short chain lipid, DHPC, but there were no definitive spectral features confirming this assignment. A minor component of uncertain assignment that exhibits a striking response to the DMPC pre-transition and chain melting transition also was recovered. While none of the spectra resolved exhibit features unequivocally attributable to a specific aggregate morphology or step in the gelation process, the results are consistent with the evolution of mixed phase bicelles (nanodisks) and small amounts of worm-like DMPC/DHPC aggregates, and perhaps DHPC micelles, at low temperature to suspensions of branched and entangled worm-like aggregates above the DMPC gel phase transition and perforated multi-lamellar aggregates at high temperature.

Solid-state NMR analysis of the orientation and dynamics of epigallocatechin gallate, a green tea polyphenol, incorporated into lipid bilayers

Catechins are the principle polyphenolic compounds in green tea; the four major compounds identified are epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECg) and epigallocatechin gallate (EGCg). Tea catechins tend to attach externally to their targets, such as viral envelopes, cell membranes, or the surface of low-density lipoproteins. In order to further our understanding of the molecular mobility of these compounds in cells, we examined the interaction of tea catechins with lipid membranes using solid-state NMR techniques. Our previous work indicated that the EGCg molecule is incorporated into lipid bilayers in a unique orientation. However, the detailed configuration, orientation, and dynamics of EGCg in lipid bilayers have not been well-characterized. Here, we investigated the orientation and dynamics of EGCg incorporated into multi-lamellar vesicles (MLVs) and bicelles using solid-state NMR spectroscopy.

NMR of molecules interacting with lipids in small unilamellar vesicles

Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.

Effects of lipid chain length and unsaturation on bicelles stability. A phosphorus NMR study

Most studies reported until now on the magnetically alignable system formed by the binary mixtures of long- and short-chain lipids were based on the mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (D14PC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (D6PC) lipids. We have recently shown that a large part of the phase diagrams of this lipid mixture could be understood by taking into account the partial miscibility between the long-chain lipids and the short-chain lipids when the sample was heated above the melting transition temperature (Tm) of the long-chain lipids. In this work, we show by modifying the chain length of either one of the two lipids that it is possible to control their miscibility and thus the intervals of temperature and composition where spontaneous alignment is observed in a magnetic field. By using 31P NMR, we demonstrate that the very special properties of such binary lipid mixtures are correlated with the propensity for short-chain lipids to diffuse into the bilayer regions. We also show that lipid mixtures with comparable properties can be formed with unsaturated lipids such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC).

Reinvestigation by phosphorus NMR of lipid distribution in bicelles

Mixtures of dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl-phosphatidylcholine (DHPC) in water form disks also called bicelles and different bilayer organizations when the mol ratio of the two lipids and the temperature are varied. The spontaneous alignment in a magnetic field of these bilayers above the transition temperature T(m) of DMPC is an attractive property that was successfully used to investigate protein structure by NMR. In this article, we have attempted to give an overview of all structural transformations of DMPC/DHPC mixtures that can be inferred from broad band (31)P-NMR spectroscopy between 5 and 60 degrees C. We show that above a critical temperature, T(v), perforated vesicles progressively replace alignable structures. The holes in these vesicles disappear above a new temperature threshold, T(h). The driving force for these temperature-dependent transformations that has been overlooked in previous studies is the increase of DHPC miscibility in the bilayer domain above T(m). Accordingly, we propose a new model (the "mixed bicelle" model) that emphasizes the consequence of the mixing. This investigation shows that the various structures of DMPC in the presence of increasing mol ratios of the short-chain DHPC is reminiscent of the observation put forward by several laboratories investigating solubilization and reconstitution of biological membranes.

Rapid proton fat-water spectroscopy for the characterization of non-CNS lesions in vivo

The presence or absence of fat in lesions can have important diagnostic implications. Current MR techniques for the evaluation of fat within lesions in the body rely on indirect imaging methods. The goal of this study was to develop a rapid clinically practical proton spectroscopy procedure for the direct observation of a localized fat-water signal within the body. The technique developed reliably determined fat-water ratios in phantoms and from lesions in vivo in 6 s with single voxel sizes as small as 0.125 cc.

Structural evaluation of phospholipid bicelles for solution-state studies of membrane-associated biomolecules

Several complementary physical techniques have been used to characterize the aggregate structures formed in solutions containing dimyristoylphosphatidylcholine (DMPC)/dihexanoylphosphatidylcholine (DHPC) at ratios of < or =0.5 and to establish their morphology and lipid organization as that of bicelles. (31)P NMR studies showed that the DMPC and DHPC components were highly segregated over a wide range of DMPC/DHPC ratios (q = 0.05-0.5) and temperatures (15 degrees C and 37 degrees C). Only at phospholipid concentrations below 130 mM did the bicelles appear to undergo a change in morphology. These results were corroborated by fluorescence data, which demonstrated the inverse dependence of bicelle size on phospholipid concentration as well as a distinctive change in phospholipid arrangement at low concentrations. In addition, dynamic light scattering and electron microscopy studies supported the hypothesis that the bicellar phospholipid aggregates are disk-shaped. The radius of the planar domain of the disk was found to be directly proportional to the ratio of DMPC/DHPC and inversely proportional to the total phospholipid concentration when the DMPC/DHPC ratio was held constant at 0.5. Taken together, these results suggest that bicelles with low q retain the morphology and bilayer organization typical of their liquid-crystalline counterparts, making them useful membrane mimetics.

A biophysical study of the interaction of the lipopeptide antibiotic iturin A with aqueous phospholipid bilayers

Iturin A is a lipopeptide extracted from the culture media of Bacillus subtilis which shows a strong antifungal action. The interaction of iturin A with multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) induced structures which did not sediment during centrifugation. Electron microscopy after negative staining showed that, at 30 mol%, iturin A/DMPC vesicles were visible but smaller than those formed by pure DMPC. Thermograms of DMPC/iturinA obtained after differential scanning calorimetry, at low concentrations of iturin A, were interpreted as indicating the presence of two laterally separated phases, one formed by pure phospholipid and the other by lipopeptide-phospholipid complexes, these two separated phases being already detected even at low concentrations such as 2 mol%. Fluorescence quenching experiments showed that the D-Tyr residue of the lipopeptide was fully accessible to the aqueous medium, indicating that the polar part of iturin A is located outside of the membrane hydrophobic palisade. It was concluded that the membrane barrier properties are likely to be damaged in the area where the lipid complexes are accumulated, due to structural fluctuations, and this may be one of the bases of its biological activity. Iturin-A was also able to greatly destabilize dielaidoylphosphatidylethanolamine (DEPE) membranes in the fluid form, producing a new structure which had a poor correlation in X-ray diffraction, and in 31P NMR spectroscopy gave rise to a spectrum containing a double isotropic signal. Iturin A was shown to induce DEPE to adopt phases other than H(II) inverted hexagonal, underlining that this lipopeptide is capable of modifying the curvature of the membrane, which may also be important in explaining the tendency of iturin A to create small vesicles and which may be another of the bases of its biological activity.

Lateral inhomogeneous lipid membranes: theoretical aspects

The physical concepts underlying the lateral distribution of the components forming a lamellar assembly of amphiphiles are discussed in this review. The role of amphiphiles' molecular structure and/or aqueous environment (ionic strength, water soluble substances) on formation and stability of lateral patterns is investigated. A considerable effort is devoted to the analysis of the properties of patterned structure which can be different from those of randomly mixed multi-component lamellae. Examples include adhesion and fusion among laterally inhomogeneous bilayers, enhanced interfacial adsorption of ions and polymers, enhanced transport across the bilayer, modified mechanical properties, local stabilization of non-planar geometries (pores, edges) and related phenomena (electroporation, budding transition and so on). Furthermore, an analysis of chemical reactivity within or at the water interface of a laterally inhomogeneous bilayer is briefly discussed. A link between these concepts and experimental findings taken from the biological literature is attempted throughout the review.

Polymerizable phosphatidylcholines: importance of phospholipid motions for optimum phospholipase A2 and C activity

Cross-linkable short-chain phosphatidylcholines with thiols at the chain terminus have been synthesized and characterized. These micelle-forming species were used to investigate two water-soluble phospholipases. When reduced, the thiol lipids were excellent substrates for phospholipase A2. Once cross-linked, they became extremely poor substrates. This is consistent with a mechanism in which a key step is the partial extraction of the substrate phosphatidylcholine from an aggregate. In contrast, phospholipase C activity was slightly enhanced if the product diglyceride was tethered to the aggregate through disulfide formation. For this enzyme such a kinetic effect is consistent with the hydrophobic diglyceride biasing the enzyme to the interface.

Substrate requirements of bacterial phosphatidylinositol-specific phospholipase C

A series of symmetric short-chain phosphatidylinositols (PI), including dihexanoyl-PI, diheptanoyl-PI (racemic as well as D and L forms), and 2-methoxy inositol-substituted diheptanoyl-PI, have been synthesized, characterized, and used to investigate key mechanistic questions about phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis. Key results include the following: (i) bacterial PI-PLC exhibits a 5-6-fold "interfacial activation" when its substrate is present in an interface as opposed to existing as a monomer in solution (in fact, the similarity to the activation observed with nonspecific PLC enzymes suggests a similarity in activation mechanisms); (ii) the 2-OH must be free since the enzyme cannot hydrolyze diheptanoyl-2-O-methyl-PI (this is most consistent with the formation of inositol cyclic 1,2-phosphate as a necessary step in catalysis); (iii) the inositol ring must have the D stereochemistry (the L-inositol attached to the lipid moiety is neither a substrate nor an inhibitor); and (iv) the presence of noninhibitory L-PI with the D-PI substrate relieves the diacylglycerol product inhibition detected at approximately 30% hydrolysis.

Assays of phospholipases on short-chain phospholipids

Short-chain phospholipids are extremely useful compounds for analysis of interfacial and substrate requirements of water-soluble phospholipases. A wide variety of PCs are easily prepared and can be characterized in detail to the point where at a given PC concentration, the micelle size distribution is known, and an area per head group can be estimated. Since they can be presented to enzymes as monomers, micelles, or in bilayer structures, short-chain phospholipids are ideal for examining kinetic preferences of lipolytic enzymes. In the future detailed studies of short-chain phospholipids with different head groups should facilitate kinetics with phospholipases which show a preference for anionic substrates.