Soft picture of lateral heterogeneity in biomembranes

Diffusion coefficients and MSD measurements on curved membranes and porous media

We study some geometric aspects that influence the transport properties of particles that diffuse on curved surfaces. We compare different approaches to surface diffusion based on the Laplace-Beltrami operator adapted to predict concentration along entire membranes, confined subdomains along surfaces, or within porous media. Our goal is to summarize, firstly, how diffusion in these systems results in different types of diffusion coefficients and mean square displacement measurements, and secondly, how these two factors are affected by the concavity of the surface, the shape of the possible barriers or obstacles that form the available domains, the sinuosity, tortuosity, and constrictions of the trajectories and even how the observation plane affects the measurements of the diffusion. In addition to presenting a critical and organized comparison between different notions of MSD, in this review, we test the correspondence between theoretical predictions and numerical simulations by performing finite element simulations and illustrate some situations where diffusion theory can be applied. We briefly reviewed computational schemes for understanding surface diffusion and finally, discussed how this work contributes to understanding the role of surface diffusion transport properties in porous media and their relationship to other transport processes.

Electron Paramagnetic Resonance Gives Evidence for the Presence of Type 1 Gonadotropin-Releasing Hormone Receptor (GnRH-R) in Subdomains of Lipid Rafts

This study investigated the effect of type 1 gonadotropin releasing hormone receptor (GnRH-R) localization within lipid rafts on the properties of plasma membrane (PM) nanodomain structure. Confocal microscopy revealed colocalization of PM-localized GnRH-R with GM₁-enriched raft-like PM subdomains. Electron paramagnetic resonance spectroscopy (EPR) of a membrane-partitioned spin probe was then used to study PM fluidity of immortalized pituitary gonadotrope cell line αT3-1 and HEK-293 cells stably expressing GnRH-R and compared it with their corresponding controls (αT4 and HEK-293 cells). Computer-assisted interpretation of EPR spectra revealed three modes of spin probe movement reflecting the properties of three types of PM nanodomains. Domains with an intermediate order parameter (domain 2) were the most affected by the presence of the GnRH-Rs, which increased PM ordering (order parameter (S)) and rotational mobility of PM lipids (decreased rotational correlation time (τc)). Depletion of cholesterol by methyl-β-cyclodextrin (methyl-β-CD) inhibited agonist-induced GnRH-R internalization and intracellular Ca²⁺ activity and resulted in an overall reduction in PM order; an observation further supported by molecular dynamics (MD) simulations of model membrane systems. This study provides evidence that GnRH-R PM localization may be related to a subdomain of lipid rafts that has lower PM ordering, suggesting lateral heterogeneity within lipid raft domains.

Effect of flavonoid structure on the fluidity of model lipid membranes

We investigated how the structural properties of (+)-catechin, (-)-epicatechin, (-)-epigallocatechin (EGC) and (-)-epigallocatechin-3-gallate (EGCG) and butylated hydroxytoluene (BHT) correlate with structural changes of phosphatidylcholine plus sphingomyelin (2.4:1) model lipid membranes. Changes were measured by fluorescence anisotropy, electron paramagnetic resonance, and differential scanning calorimetry. Two fluorophores and two spin probes were used to monitor membrane characteristics close to water-lipid interface and in the middle of the bilayer. The data obtained were correlated to the amount of bounded compounds, the number of H-bonds, and the topological polar surface area (TPSA) of the compounds. These correlations reflect the behaviours of (+)-catechin, (-)-epicatechin, EGC, EGCG and BHT. Our results confirm that phenolics studied here are bounded to a membrane surface predominantly via hydrogen bonds, while BTH is inserted into the lipid bilayer.

Influence of cancerostatic perifosine on membrane fluidity of liposomes and different cell lines as measured by electron paramagnetic resonance

Aim

To test whether membrane fluidity and its changes are important for the sensitivity of cells to the action of perifosine (OPP), a new anticancer drug targeting cell membrane and not DNA.

Method

Influence of OPP on the membrane structure of OPP-resistant MCF7, and OPP-sensitive MT3 breast cancer cell lines, as well as of mouse fibroblasts (L929) cell lines, and model cells (liposomes) was investigated by electron paramagnetic resonance, using spin labeled derivative of OPP (P5) and 5-doxylpalmitoyl methylester (MeFASL(10,3)) as spin probes.

Results

OPP increased membrane fluidity of all cell lines at concentrations higher than 50 µM (on the level of P ≤ 0.05, t test). In cells, the differences were observed only by P5 and not by MeFASL(10,3). Average order parameter S_eff decreased for about 12% in MCF7 and L929 and only for 8% in OPP-sensitive MT3 cells, showing that there was no correlation between membrane fluidity changes and sensitivity of cells to OPP. The only correlation we found was between OPP sensitivity and the cell growth rate. In liposomes, both spin probes were sensitive to the action of OPP. S_eff decreased with increasing concentration of OPP. For MeFASL(10,3) a significant decrease was observed at 4 mol% OPP, while for P5 it was observed at 8 mol%.

Conclusion

Influence of OPP on plasma membrane fluidity of breast cancer cells is not the determining factor in the sensitivity of cells to OPP.

Effect of growth medium pH of Aeropyrum pernix on structural properties and fluidity of archaeosomes

The influence of pH (6.0; 7.0; 8.0) of the growth medium of Aeropyrum pernix K1 on the structural organization and fluidity of archaeosomes prepared from a polar-lipid methanol fraction (PLMF) was investigated using fluorescence anisotropy and electron paramagnetic resonance (EPR) spectroscopy. Fluorescence anisotropy of the lipophilic fluorofore 1,6-diphenyl-1,3,5-hexatriene and empirical correlation time of the spin probe methylester of 5-doxylpalmitate revealed gradual changes with increasing temperature for the pH. A similar effect has been observed by using the trimethylammonium-6-diphenyl-1,3,5-hexatriene, although the temperature changes were much smaller. As the fluorescence steady-state anisotropy and the empirical correlation time obtained directly from the EPR spectra alone did not provide detailed structural information, the EPR spectra were analysed by computer simulation. This analysis showed that the archaeosome membranes are heterogeneous and composed of several regions with different modes of spin-probe motion at temperatures below 70°C. At higher temperatures, these membranes become more homogeneous and can be described by only one spectral component. Both methods indicate that the pH of the growth medium of A. pernix does not significantly influence its average membrane fluidity. These results are in accordance with TLC analysis of isolated lipids, which show no significant differences between PLMF isolated from A. pernix grown in medium with different pH.

GLOBAL PROPERTIES OF BIOMIMETIC MEMBRANES: PERSPECTIVES ON MOLECULAR FEATURES

Global properties of biological model membranes such as, e.g., structure or elasticity, are known to be closely related to their local features. If a membrane active compound interacts with the membrane assembly, the membrane will primarily be affected on the local, molecular level. The local perturbation may than, through some coupling, translate into a global adjustment of the membrane. In order to address this coupling x-ray and neutron diffraction data analysis techniques have been developed that allow accurate monitoring of changes in global properties. This offers new perspectives on molecular membrane features that in combination with complementary techniques, such as differential scanning calorimetry, spectroscopy or dynamic scattering lead to a better understanding of biomimetic membranes. The present article reviews these aspects giving application examples for single- and multicomponent membranes, respectively.

Extremophilic yeasts: plasma-membrane fluidity as determinant of stress tolerance

Our aim was to investigate the response of selected yeasts and yeast-like fungi from extreme environments to various temperatures at the level of their plasma membranes, in order to elucidate the connections between their plasma-membrane fluidity (measured by electron paramagnetic resonance spectroscopy - EPR), growth temperature range, stress tolerance, and ecological distribution. Although all studied fungi can be considered mesophilic according to their growth temperature profiles, their plasma-membrane fluidity indicated otherwise. Arctic yeast Rhodosporidium diobovatum could be classified as psychrotolerant due to its higher average membrane fluidity. Extremely halotolerant black yeast-like fungus Hortaea werneckii isolated from solar salterns, on the other hand, is not adapted to low temperature, which is reflected in the higher average rigidity of its plasma membrane and as a consequence its inability to grow at temperatures lower than 10°C. The plasma membrane of Aureobasidium sp. isolated so far exclusively from an Arctic glacier with its intermediate fluidity and high fluidity variation at different temperatures may indicate the specialization of this yeast-like fungus to the specific glacial environment. Similar behaviour of plasma membrane was detected in the reference yeast, non-extremophilic Saccharomyces cerevisiae. Its membranes of intermediate fluidity and with high fluidity fluctuation at different temperatures may reflect the specialization of this yeast to mesophilic environments and prevent its colonization of extreme environments. Halotolerant Aureobasidium pullulans from salterns, and Arctic Cryptococcus liquefaciens and Rhodotorula mucilaginosa with moderately fluctuating plasma membranes of intermediate fluidity are representatives of globally distributed generalistic and stress-tolerant species that can thrive in a variety of environments. Keeping the membranes stable and flexible is one of the necessities for the microorganisms to survive changes in extreme habitats. Our data suggest that plasma-membrane fluidity can be used as an indicator of fitness for survival in the extreme environments. In addition to the average fluidity of plasma membrane, the fluctuation of fluidity is an important determinant of stress tolerance: high absolute fluidity fluctuation is tied to decreased survival. The fluidity and its variation therefore reflect survival strategy and fitness in extreme environments and are good indicators of the adaptability of microorganisms.

EPR and FTIR studies reveal the importance of highly ordered sterol-enriched membrane domains for ostreolysin activity

Ostreolysin is a cytolytic protein from the edible oyster mushroom (Pleurotus ostreatus), which recognizes specifically and binds to raft-like sterol-enriched membrane domains that exist in the liquid-ordered phase. Its binding can be abolished by micromolar concentrations of lysophospholipids and fatty acids. The membrane activity of ostreolysin, however, does not completely correlate with the ability of a certain sterol to induce the formation of a liquid-ordered phase, suggesting that the protein requires an additional structural organization of the membrane to exert its activity. The aim of this study was to further characterize the lipid membranes that facilitate ostreolysin binding by analyzing their lipid phase domain structure. Fourier-transformed infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR) were used to analyze the ordering and dynamics of membrane lipids and the membrane domain structure of a series of unilamellar liposomes prepared by systematically changing the lipid components and their ratios. Our results corroborate the earlier conclusion that the average membrane fluidity of ostreolysin-susceptible liposomes alone cannot account for the membrane activity of the protein. Combined with previous data computer-aided interpretation of EPR spectra strongly suggests that chemical properties of membrane constituents, their specific distribution, and physical characteristics of membrane nanodomains, resulting from the presence of sterol and sphingomyelin (or a highly ordered phospholipid, dipalmitoylphosphatidylcholine), are essential prerequisites for ostreolysin membrane binding and pore-formation.

Site-directed spin-labeling study of the light-harvesting complex CP29

The topology of the long N-terminal domain (approximately 100 amino-acid residues) of the photosynthetic Lhc CP29 was studied using electron spin resonance. Wild-type protein containing a single cysteine at position 108 and nine single-cysteine mutants were produced, allowing to label different parts of the domain with a nitroxide spin label. In all cases, the apoproteins were either solubilized in detergent or they were reconstituted with their native pigments (holoproteins) in vitro. The spin-label electron spin resonance spectra were analyzed in terms of a multicomponent spectral simulation approach, based on hybrid evolutionary optimization and solution condensation. These results permit to trace the structural organization of the long N-terminal domain of CP29. Amino-acid residues 97 and 108 are located in the transmembrane pigment-containing protein body of the protein. Positions 65, 81, and 90 are located in a flexible loop that is proposed to extend out of the protein from the stromal surface. This loop also contains a phosphorylation site at Thr81, suggesting that the flexibility of this loop might play a role in the regulatory mechanisms of the light-harvesting process. Positions 4, 33, 40, and 56 are found to be located in a relatively rigid environment, close to the transmembrane protein body. On the other hand, position 15 is located in a flexible region, relatively far away from the transmembrane domain.

An ESR characterization of micelles and vesicles formed in aqueous decanoic acid/sodium decanoate systems using different spin labels

Aqueous decanoic acid/sodium decanaote systems were studied as a function of pH and concentration, up to 0.3 M decanoic acid/sodium decanoate, by electron spin resonance (ESR) spectroscopy using three different amphiphilic spin labels. The distribution of the spin labels between vesicles and micelles as well as their dynamic properties were determined by quantitative analysis of the ESR spectra using two novel simulation software packages. Rotational correlation time of the labels in micelles was found to increase with decreasing pH, with substantial increase in the region where vesicles were formed (7.8<pH<8). In the interval 6.5<pH<7.8, the coexistence of vesicles and micelles was observed. Presence of vesicles was confirmed by the captured aqueous volume, determined independently with a hydrophilic spin label. The ESR measurements indicate that decanoic acid vesicle formation observed as the concentration is increased between 0.01 M and 0.03 M at pH 7.0 most likely occurs via the formation of micelles which remain in coexistence with the vesicles, even if the concentration is well above these values.

Temperature- and pH-Induced Structural Changes in the Membrane of the Hyperthermophilic Archaeon Aeropyrum pernix K1

The influence of pH and temperature on the structural organization, fluidity and permeability of the hyperthermophilic archaeon membrane was investigated in situ by a combination of electron paramagnetic resonance (EPR) and fluorescence emission spectroscopy. For EPR measurements, Aeropyrum pernix cells, after growing at different pHs, were spin-labeled with the doxyl derivative of palmitic acid methylester (MeFASL(10,3)). From the EPR spectra maximal hyperfine splitting (2A (max)) and empirical correlation time (tau (emp)), which are related to mean membrane fluidity, were determined. The mean membrane fluidity increases with temperature and depends on the pH of the growth medium. Computer simulation of the EPR spectra shows that membrane of A. pernix is heterogeneous and consists of the regions characterized with three different types of motional characteristics, which define three types of membrane domains. Order parameter and proportion of the spin probes in the three types of domains define mean membrane fluidity. The fluidity changes of the membrane with pH and temperature correlate well with the ratio between the fluorescence emission intensity of the first and third bands in the vibronic spectra of pyrene, I(1)/I(3). At pH 7.0 a decrease of I(1)/I(3) from 2.0 to 1.2, due to the penetration of pyrene into the nonpolar membrane region, is achieved at temperatures above 65 degrees C, the lower temperature limit of A. pernix growth.

Evidence for a role of caveolin-1 in neurokinin-1 receptor plasma-membrane localization, efficient signaling, and interaction with beta-arrestin 2

This study was focused on the relationship between the plasma-membrane localization of neurokinin-1 receptor (NK1-R) and its endocytic and signaling properties. First, we employed electron paramagnetic resonance (EPR) to study the domain structure of HEK-293 cells and NK1-R microlocalization. EPR spectra and the GHOST condensation routine demonstrated that NK1-R was distributed in a well-ordered domain of HEK-293 cells possibly representing lipid raft/caveolae microdomains, whereas the impairment of caveolae changed the NK1-R plasma-membrane distribution. Internalization and second messenger assays combined with bioluminescence resonance energy transfer were employed subsequently to evaluate the functional importance of the NK1-R microlocalization in lipid raft/caveolae microdomains. The internalization pattern was delineated through the use of dominant-negative mutants (DNM) of caveolin-1 S80E (Cav1 S80E), dynamin-1 K44A (Dyn K44A), and beta-arrestin (beta-arr 319-418) and by means of cell lines that expressed various endogenous levels of beta-arrestins. NK1-R displayed rapid internalization that was substantially reduced by DNMs of dynamin-1 and beta-arrestin and even more profoundly in cells lacking both beta-arrestin1 and beta-arrestin2. These internalization data were highly suggestive of the predominant use of the clathrin-mediated pathway by NK1-R, even though NK1-R tended to reside constitutively in lipid raft/caveolae microdomains. Evidence was also obtained that the proper clustering of the receptor in these microdomains was important for effective agonist-induced NK1-R signaling and for its interaction with beta-arrestin2.

Rigidification of neutral lipid bilayers in the presence of salts

We studied the influence of sodium and calcium chloride on the global and local membrane properties of fluid palmitoyl-oleoyl phosphatidylcholine bilayers, applying synchrotron small-angle x-ray diffraction, spin-labeling electron paramagnetic resonance spectroscopy, and differential scanning calorimetry, as well as simultaneous density and acoustic measurements. The salt concentration was varied over a wide range from 0 to 5 M. We found that NaCl leads to a continuous swelling of the bilayers, whereas the behavior of the bilayer separation dW in the presence of CaCl2 is more complex, showing an initial large dW value, which decreased upon further addition of salt and finally increased again in the high concentration regime. This can be understood by a change of balance between electrostatic and van der Waals interactions. We were further able to show that both salts lead to a significant increase of order within the lipid bilayer, leading to a decrease of bilayer elasticity and shift of main phase transition temperature. This effect is more pronounced for Ca2+, and occurs mainly in the high salt-concentration regime. Thus, we were able to reconcile previous controversies between molecular dynamics simulations and x-ray diffraction experiments regarding the effect of salts on neutral lipid bilayers.

Increase in fluidity in the membrane of MT3 breast cancer cells correlates with enhanced cell adhesion in vitro and increased lung metastasis in NOD/SCID mice

To study whether membrane fluidity of tumor cells have an influence on metastasis, MT3 breast cancer cells harvested during exponential growth and under confluent conditions were compared. Electron paramagnetic resonance (EPR) data revealed that, in comparison to growing cells, confluent cells have a significant higher fluidity in their membrane related to a higher relative portion of disordered domains and a reduced portion of the most ordered domains. Further, sialyl Lewis X and/or A ligand-mediated adhesion of these cells was 2-fold enhanced. Confocal laser scanning microscopy further demonstrated a higher motility of ligands in the membrane of confluent cells, together with an accumulation of these ligands in distinct areas. Both facts are suggested to be responsible for an enhanced cell adhesion observed. Finally, an increased number of large distinct metastatic foci was registered in lungs of mice after i.v. inoculation of confluent cells. The results indicate that domain organization and fluidity of the cell membrane affect tumor cell adhesion and can have in this way also an impact on the malignancy of breast cancer cells.

Motional restrictions of membrane proteins: a site-directed spin labeling study

Site-directed mutagenesis was used to produce 27 single cysteine mutants of bacteriophage M13 major coat protein spanning the whole primary sequence of the protein. Single-cysteine mutants were labeled with nitroxide spin labels and incorporated into phospholipid bilayers with increasing acyl chain length. The SDSL is combined with ESR and CD spectroscopy. CD spectroscopy provided information about the overall protein conformation in different mismatching lipids. The spin label ESR spectra were analyzed in terms of a new spectral simulation approach based on hybrid evolutionary optimization and solution condensation. This method gives the residue-level free rotational space (i.e., the effective space within which the spin label can wobble) and the diffusion constant of the spin label attached to the protein. The results suggest that the coat protein has a large structural flexibility, which facilitates a stable protein-to-membrane association in lipid bilayers with various degrees of hydrophobic mismatch.

Speeding up a genetic algorithm for EPR-based spin label characterization of biosystem complexity

Complexity of biological systems is one of the toughest problems for any experimental technique. Complex biochemical composition and a variety of biophysical interactions governing the evolution of a state of a biological system imply that the experimental response of the system would be superimposed of many different responses. To obtain a reliable characterization of such a system based on spin-label Electron Paramagnetic Resonance (EPR) spectroscopy, multiple Hybrid Evolutionary Optimization (HEO) combined with spectral simulation can be applied. Implemented as the GHOST algorithm this approach is capable of handling the huge solution space and provides an insight into the "quasicontinuous" distribution of parameters that describe the biophysical properties of an experimental system. However, the analysis procedure requires several hundreds of runs of the evolutionary optimization routine making this algorithm extremely computationally demanding. As only the best parameter sets from each run are assumed to contribute into the final solution, this algorithm appears far from being optimized. The goal of this study is to modify the optimization routine in a way that 20-40 runs would be enough to obtain qualitatively the same characterization. However, to keep the solution diversity throughout the HEO run, fitness sharing and newly developed shaking mechanisms are applied and tested on various test EPR spectra. In addition, other evolutionary optimization parameters such as population size and probability of genetic operators were also varied to tune the algorithm. According to the testing examples a speed-up factor of 5-7 was achieved.

Membrane switch hypothesis. 1. Cell density influences lateral domain structure of tumor cell membranes

The domain structure of human cancer cells membranes was investigated by electron paramagnetic resonance (EPR) in different phases of cell growth, and the results were compared to those obtained for nonmalignant cells. On the basis of computer simulation of the EPR spectra using a newly developed GHOST condensation routine it was suggested that plasma membranes of cancer cells have less lateral lipid domain types at confluent conditions than in the exponential growing phase, while in nonmalignant cells the domain structure does not change significantly during cell growth. In accordance to our experimental data we propose a membrane switch hypothesis: disappearance of certain membrane domain types might act as a switch promoting the clustering of membrane constituents into the active units in a common lipid membrane domain and thus influencing the physiology of cells.

Spin label EPR-based characterization of biosystem complexity

Following the widely spread EPR spin-label applications for biosystem characterization, a novel approach is proposed for EPR-based characterization of biosystem complexity. Hereto a computational method based on a hybrid evolutionary optimization (HEO) is introduced. The enormous volume of information obtained from multiple HEO runs is reduced with a novel so-called GHOST condensation method for automatic detection of the degree of system complexity through the construction of two-dimensional solution distributions. The GHOST method shows the ability of automatic quantitative characterization of groups of solutions, e.g. the determination of average spectral parameters and group contributions. The application of the GHOST condensation algorithm is demonstrated on four synthetic examples of different complexity and applied to two physiologically relevant examples--the determination of domains in biomembranes (lateral heterogeneity) and the study of the low-resolution structure of membrane proteins.

EPR study of lipid phase in renal cortical membrane organelles from intact and cadmium-intoxicated rats

Numerous studies have demonstrated various structure/function correlations at the level of transport proteins in the kidney cell membranes and various intracellular organelles. However, characterization of the lipid phase of these membranes is rare. Here, we report the differences in lipid organization and dynamics of the brush-border membranes (BBM), basolateral membranes (BLM) and endocytotic vesicles (EV), isolated from the kidney cortex of intact rats, studied with the EPR spectroscopy of the spin-labeled membrane lipids. The EPR spectra were analyzed by comparing experimentally observed line shapes with the line shapes calculated according to the theoretical model developed for liquid crystals. In the fitting procedure, three different lipid domains were assumed, which revealed clear differences in the lipid ordering and rotational correlation times, as well as in the lipid partition of these domains in each of the three types of membranes. A similar approach, used to compare the spectroscopic characteristics of BBM from control and cadmium-intoxicated rats, showed significantly changed ordering and increased molecular mobility in the lipid phase of BBM from Cd-treated animals. As tested by an established fluorescence assay, the Cd-induced changes in the lipid mobility co localized with approximately 5-fold higher conductance of BBM for potassium, with unchanged conductance for protons.

Membrane Switch Hypothesis. 2. Domain Structure of Phagocytes in Horses with Recurrent Airway Obstruction

The mechanism of recurrent airway obstruction (RAO) in horses was investigated by measuring the membrane domain structure and oxy-redoxy activity in phagocytes isolated from bronchoalveolar lavage fluid (BAL) and from the blood of healthy and RAO horses by electron paramagnetic resonance (EPR). Differences in the activity of intracellular antioxidant enzymes CAT, GPx, and SOD measured in phagocytes of RAO horses in comparison to healthy horses showed that the phagocytes were affected by oxidative stress. In comparison with polymorphonuclear leukocytes (phagocytes) from the blood of healthy horses the reduction mechanisms in BAL were faster and coincided with the merging of disordered membrane domains, while in horses with RAO the reduction and membrane domain structure remained unchanged. We assume that the merging of lipid domains observed in phagocytes from BAL of healthy horses could promote cluster formation of membrane proteins or ligands, which could trigger the activation process in phagocytes of healthy horses and consequently the physiological response that probably did not happen in phagocytes of RAO horses.

Synthesis and Biological Evaluation of Spin-Labeled Alkylphospholipid Analogs

Alkylphospholipid analogues of perifosine and miltefosine bearing a nitroxide moiety at different positions on an alkyl chain were synthesized as electron paramagnetic resonance (EPR) probes. Their amphiphilic properties were characterized by determining their critical micelle concentration (cmc) and hemolytic activity on erythrocytes both in free and liposomal form. Spin-labeled analogues as membrane components of large unilamellar liposomes containing cholesterol and dicetyl phosphate or in free solution were evaluated using the MTT assay to determine growth inhibition on MT1, MT3, and MCF7 breast cancer cell lines. 4a (IC50 = 56.4 microM) was found to be significantly more active than the perifosine against the MCF-7 cell line. Its high cmc (194.03 microM) and low hemolytic activity shows that its cytotoxic activity might be more specific; therefore, 4a can be an important molecular tool for further EPR investigations.