Properties Of Biological Membranes Research Articles

AFM Determination of the Elasticity of Model Biomembranes

The utility of AFM (Atomic Force Microscopy) to study surface morphologies and molecular level forces is well known. It is also possible to apply well-defined small forces on soft surfaces using the AFM. The latter is being used here to determine the elasticity (Young's modulus) of model membrane lipid bilayers. The mechanical properties of biological membranes are of interest in understanding various biologically related processes like cell hemolysis, viral fusion and pore formation, etc. and are closely related to the cell stability. The method involves determination of the force curves by compressing the lipid bilayer by indenting the AFM tip on the bilayer surface, and the resulting elastic response of the bilayer to the applied force (load) is analysed using theoretical models for the tip geometry. We will discuss the sample preparation technique and the results of our experiments on coexisting fluid-phase domains in typical ‘lipid raft’ mixtures, as well as the effects of varying the level of unsaturation of the lipid carbon chains.

Water Adsorption Isotherms of Lipids

Hydration of bilayer lipids is a fundamental property of biological membranes. The available database of lipid hydration isotherms is fitted over the entire range of water activities by using a statistical mechanical approach that is an extension of the common Brunauer-Emmett-Teller model, to include differential energies of association for water molecules beyond the first strongly bound layer. Three-parameter fits are obtained that can be used to represent the experimental isotherms to a good degree of accuracy over the complete range of water-binding activities. Fits are also made in terms of the hydration pressure and correlation length of water ordering, by using the polarization theory of lipid hydration. The relationship of the latter approach to measurements of hydration forces between lipid bilayers is discussed.

Interfacial tension of bilayer lipid membranes

Abstract Interfacial tension is an important characteristic of a biological membrane because it determines its rigidity, thus affecting its stability. It is affected by factors such as medium pH and by the presence of certain substances, for example cholesterol, other lipids, fatty acids, amines, amino acids, or proteins, incorporated in the lipid bilayer. Here, the effects of various parameters to on interfacial tension values of bilayer lipid membranes are discussed. The mathematically derived and experimentally confirmed results presented in this paper are of importance to the interpretation of phenomena occurring in lipid bilayers. These results can lead to a better understanding of the physical properties of biological membranes. The simple interfacial tension method proposed herein may be successfully used to determine the interfacial tension values of 1:1 lipid-lipid, lipid-cholesterol, lipid-fatty acid, lipid-amine, and lipid-amino acid systems.

Analysis of lipid surface area in protein–membrane systems combining voronoi tessellation and monte carlo integration methods

All-atom molecular dynamics (MD) simulation has become a powerful research tool to investigate structural and dynamical properties of biological membranes and membrane proteins. The lipid structures of simple membrane systems in recent MD simulations are in good agreement with those obtained by experiments. However, for protein-membrane systems, the complexity of protein-lipid interactions makes investigation of lipid structure difficult. Although the area per lipid is one of the essential structural properties in membrane systems, the area in protein-membrane systems cannot be computed easily by conventional approaches like the Voronoi tessellation method. To overcome this limitation, we propose a new method combining the two-dimensional Voronoi tessellation and Monte Carlo integration methods. This approach computes individual surface areas of lipid molecules not only in bulk lipids but also in proximity to membrane proteins. We apply the method to all-atom MD trajectories of the sarcoplasmic reticulum Ca(2+)-pump and the SecY protein-conducting channel. The calculated lipid surface area is in agreement with experimental values and consistent with other structural parameters of lipid bilayers. We also observe changes in the average area per lipid induced by the conformational transition of the SecY channel. Our method is particularly useful for examining equilibration of lipids around membrane proteins and for analyzing the time course of protein-lipid interactions.

Lipid peroxidation modifies the picture of membranes from the “Fluid Mosaic Model” to the “Lipid Whisker Model”

The “Fluid Mosaic Model”, described by Singer and Nicolson, explain both how a cell membrane preserves a critical barrier function while it concomitantly facilitates rapid lateral diffusion of proteins and lipids within the planar membrane surface. However, the lipid components of biological plasma membranes are not regularly distributed. They are thought to contain “rafts” – nano-domains enriched in sphingolipids and cholesterol that are distinct from surrounding membranes of unsaturated phospholipids. Cholesterol and fatty acids adjust the transport and diffusion of molecular oxygen in membranes. The presence of cholesterol and saturated phospholipids decreases oxygen permeability across the membrane. Alpha-tocopherol, the main antioxidant in biological membranes, partition into domains that are enriched in polyunsaturated phospholipids increasing the concentration of the vitamin in the place where it is most required. On the basis of these observations, it is possible to assume that non-raft domains enriched in phospholipids containing PUFAs and vitamin E will be more accessible by molecular oxygen than lipid raft domains enriched in sphingolipids and cholesterol. This situation will render some nano-domains more sensitive to lipid peroxidation than others. Phospholipid oxidation products are very likely to alter the properties of biological membranes, because their polarity and shape may differ considerably from the structures of their parent molecules. Addition of a polar oxygen atom to several peroxidized fatty acids reorients the acyl chain whereby it no longer remains buried within the membrane interior, but rather projects into the aqueous environment “Lipid Whisker Model”. This exceptional conformational change facilitates direct physical access of the oxidized fatty acid moiety to cell surface scavenger receptors.

Fatty Acid and Peptide Profiles in Plasma Membrane and Membrane Rafts of PUFA Supplemented RAW264.7 Macrophages

The eukaryotic cell membrane possesses numerous complex functions, which are essential for life. At this, the composition and the structure of the lipid bilayer are of particular importance. Polyunsaturated fatty acids may modulate the physical properties of biological membranes via alteration of membrane lipid composition affecting numerous physiological processes, e.g. in the immune system. In this systematic study we present fatty acid and peptide profiles of cell membrane and membrane rafts of murine macrophages that have been supplemented with saturated fatty acids as well as PUFAs from the n-3, the n-6 and the n-9 family. Using fatty acid composition analysis and mass spectrometry-based peptidome profiling we found that PUFAs from both the n-3 and the n-6 family have an impact on lipid and protein composition of plasma membrane and membrane rafts in a similar manner. In addition, we found a relation between the number of bis-allyl-methylene positions of the PUFA added and the unsaturation index of plasma membrane as well as membrane rafts of supplemented cells. With regard to the proposed significance of lipid microdomains for disease development and treatment our study will help to achieve a targeted dietary modulation of immune cell lipid bilayers.

Aqueous Solutions at the Interface with Phospholipid Bilayers

In a sense, life is defined by membranes, because they delineate the barrier between the living cell and its surroundings. Membranes are also essential for regulating the machinery of life throughout many interfaces within the cell's interior. A large number of experimental, computational, and theoretical studies have demonstrated how the properties of water and ionic aqueous solutions change due to the vicinity of membranes and, in turn, how the properties of membranes depend on the presence of aqueous solutions. Consequently, understanding the character of aqueous solutions at their interface with biological membranes is critical to research progress on many fronts. The importance of incorporating a molecular-level description of water into the study of biomembrane surfaces was demonstrated by an examination of the interaction between phospholipid bilayers that can serve as model biological membranes. The results showed that, in addition to well-known forces, such as van der Waals and screened Coulomb, one has to consider a repulsion force due to the removal of water between surfaces. It was also known that physicochemical properties of biological membranes are strongly influenced by the specific character of the ions in the surrounding aqueous solutions because of the observation that different anions produce different effects on muscle twitch tension. In this Account, we describe the interaction of pure water, and also of aqueous ionic solutions, with model membranes. We show that a symbiosis of experimental and computational work over the past few years has resulted in substantial progress in the field. We now better understand the origin of the hydration force, the structural properties of water at the interface with phospholipid bilayers, and the influence of phospholipid headgroups on the dynamics of water. We also improved our knowledge of the ion-specific effect, which is observed at the interface of the phospholipid bilayer and aqueous solution, and its connection with the Hofmeister series. Nevertheless, despite substantial progress, many issues remain unresolved. Thus, for example, we still cannot satisfactorily explain the force of interaction between phospholipid bilayers immersed in aqueous solutions of NaI. Although we try to address many issues here, the scope of the discussion is limited and does not cover such important topics as the influence of ionic solutions on phases of bilayers, the influence of salts on the properties of Langmuir monolayers containing lipid molecules, or the influence of aqueous solutions on bilayers containing mixtures of lipids. We anticipate that the future application of more powerful experimental techniques, in combination with more advanced computational hardware, software, and theory, will produce molecular-level information about these important topics and, more broadly, will further illuminate our understanding of interfaces between aqueous solutions and biological membranes.

Equivalent mechanical properties of biological membranes from lattice homogenization

The goal of this manuscript is to set up a novel methodology for the calculation of the effective mechanical properties of biological membranes viewed as repetitive networks of elastic filaments, based on the discrete asymptotic homogenization method. We will show that for some lattice configurations, flexional effects due to internal structure mechanisms at the unit cell scale lead to additional flexional effects at the continuum scale, accounted for by an internal length associated to a micropolar behavior. Thereby, a systematic methodology is established, allowing the prediction of the overall mechanical properties of biological membranes for a given network topology, as closed form expressions of the geometrical and mechanical micro-parameters. The peptidoglycan and the erythrocyte have been analyzed using this methodology, and their effective moduli are calculated and recorded versus the geometrical and mechanical lattice parameters. A classification of lattices with respect to the choice of the equivalent continuum model is proposed: The Cauchy continuum and a micropolar continuum are adopted as two possible effective medium, for a given beam model. The relative ratio of the characteristic length of the micropolar continuum to the unit cell size determines the relevant choice of the equivalent medium. In most cases, the Cauchy continuum is sufficient to model membranes in most of their configurations. The peptidoglycan network may exhibit a re-entrant hexagonal lattice, for which micropolar effects become important. This is attested by the characteristic length becoming larger than the beam length for such configurations. The homogenized moduli give accurate results for both membranes, as revealed by comparison with experimental measurements or simulation results from the literature at the network scale. A first insight into the nonlinear mechanical behavior of the hexagonal and triangular networks is lastly investigated using a perturbative method.

Model answers to lipid membrane questions.

Ever since it was discovered that biological membranes have a core of a bimolecular sheet of lipid molecules, lipid bilayers have been a model laboratory for investigating physicochemical and functional properties of biological membranes. Experimental and theoretical models help the experimental scientist to plan experiments and interpret data. Theoretical models are the theoretical scientist's preferred toys to make contact between membrane theory and experiments. Most importantly, models serve to shape our intuition about which membrane questions are the more fundamental and relevant ones to pursue. Here we review some membrane models for lipid self-assembly, monolayers, bilayers, liposomes, and lipid-protein interactions and illustrate how such models can help answering questions in modern lipid cell biology.

A thermodynamic analysis of the Collander equation and establishment of a reference solvent for use in drug partitioning studies.

A thermodynamic analysis of the Collander equation, ln PI = a + b ln PII (I and II refer to two different partitioning systems with partition coefficients PI and PII, respectively), is given and applied to three forms of correlation. The intercept, a, is shown to have no general fundamental significance whereas the slope, b is shown to reflect differences in non-aqueous solvent properties; b is also shown to be of use in scaling solvent behaviour to select solvents which closely represent biological membrane properties for use in partitioning studies. Laboratory and literature data are subjected to the analysis.

Effect of Amide-Linked Acyl Chain Length on the Sphingomyelin-Cholesterol Interactions

Sphingolipids are important component of eukaryotic membrane and are crucial for several physical as well as biological processes in cells. Sphingolipids play key role in the lateral domain formations which further assists in cell signalling and various significant cellular phenomena. Even though sphingolipids display an extensive diversity in their structures both in the head group and hydrophobic region, sphingosine (D-erythro-2-amino-trans-4-octadecene-1, 3-diol) is the prevalent backbone in most mammalian sphingolipids, including sphingomyelin (SM). The aim of this study was to examine how variation in the chain length in SM affect their bilayer properties as well as their interactions with cholesterol. We have synthesized SM analogs with different amide- linked acyl chains (14:0, 15:0, 16:0, 17:0, 18:0, 19:0, 20:0, 22:0 and 24:0). The main transition temperature and the degree of acyl chain order in pure SM bilayers was determined from the steady-state anisotropy of diphenylhexatriene. Ordered domain formation and sterol interaction with SM analogs was determined from the quenching susceptibility of dihenylhexatriene and cholestatrienol. Sterol affinity to fluid bilayer membranes containing saturated SM analogs was determined using a cholestatrienol equilibrium partitioning assay. We found that all SM analogs formed ordered domains ina fluid bilayer. Sterol was always included in these ordered domains, albeit to a varying degree. The thermostability of the ordered domains was remarkably similar, even though the SM acyl chaing length varied (16-24 carbons). Only 14:0 and 15:0 SM formed less thermostable ordered domains. We hope our results will enable us to better understand the behavior of asymmetric SM molecules in complex membranes, and to predict how they influence properties of biological membranes.

What do we really measure in AFM punch-through experiments on supported lipid bilayers?

Nowadays, there is much experimental evidence that the mechanical properties of biological membranes affect membrane protein functions. A very convenient technique to study these properties on a spatial scale relevant to that of single proteins is represented by Atomic Force Spectroscopy (AFS). In this study we measured the force the AFM tip has to apply on a supported lipid bilayer to punch-through it as a function of different environmental parameters. We observed that this force is reduced when the lipid bilayer is in its phase transition region. We interpreted our results on the basis of thermodynamical considerations and we stressed their biological relevance. In particular, the reduced punch-through force in the phase transition region could be relevant for the function of membrane proteins which operates by conformational changes at the protein/lipid interface. We also suggest that the presence of a transmembrane voltage drop can affect the measured punch-through force.

Lipophilicity and Its Relationship with Passive Drug Permeation

In this review, we first summarize the structure and properties of biological membranes and the routes of passive drug transfer through physiological barriers. Lipophilicity is then introduced in terms of the intermolecular interactions it encodes. Finally, lipophilicity indices from isotropic solvent systems and from anisotropic membrane-like systems are discussed for their capacity to predict passive drug permeation across biological membranes such as the intestinal epithelium, the blood-brain barrier (BBB) or the skin. The broad evidence presented here shows that beyond the predictive power of lipophilicity parameters, the various intermolecular forces they encode allow a mechanistic interpretation of passive drug permeation.

Biomimetic Membranes with Aqueous Nano Chanels but without Proteins: Impedance of Impregnated Cellulose Ester Filters

Earlier we have shown that many important properties of ionic aqueous channels in biological membranes can be imitated using simple biomimetic membranes. These membranes are composed of mixed cellulose ester-based filters, impregnated with isopropyl myristate or other esters of fatty acids, and can be used for high-throughput drug screening. If the membrane separates two aqueous solutions, combination of relatively hydrophilic polymer support with immobilized carboxylic groups results in the formation of thin aqueous layers covering inner surface of the pores, while the pore volume is filled by lipid-like substances. Because of these aqueous layers biomimetic membranes even without proteins have a cation/anion ion selectivity and specific (per unit of thickness) electrical properties, which are similar to typical properties of biological membranes. Here we describe frequency-dependent impedance of the isopropyl myristate-impregnated biomimetic membranes in the 4-electrode arrangement and present the results as Bode and Nyquist diagrams. When the membranes are placed in deionized water, it is possible to observe three different dispersion processes in the frequency range 0.1 Hz to 30 kHz. Only one dispersion is observed in 5 mM KH(2)PO(4) solution. It is suggested that these three dispersion features are determined by (a) conductivity in aqueous structures/channels, formed near the internal walls of the filter pores at high frequencies, (b) dielectric properties of the whole membrane at medium frequencies, determined by polymer support, aqueous layers and impregnating oil, and, finally, (c) by the processes in hydrated liquid crystal structures formed in pores by impregnating oil in contact with water at low frequencies.

Molecular dynamics investigation of myelin basic protein stability on lipid membranes

Simulation of proteins and membranes composed of synthetic lipids on computer clusters provides molecular information that complements experimental data. This paper describes molecular dynamics (MD) approaches to study the properties of biological membranes and proteins using the freely available GROMACS package on the C-terminal α-helical peptide of myelin basic protein (MBP). We simulated a mixed membrane – consisting of 2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPC/DMPE), and a pure DMPC membrane, composed of 188 and 248 lipids, respectively – for 200 ns at 309K. The DMPC membrane was approximately three times more fluid compared to the DMPC/DMPE system, with the diffusion coefficients (D) being 0.0207x10-5 cm2/s and 0.0068x10-5 cm2/s, respectively. In addition, we simulated the 14-residue peptide representing the C-terminal α-helical region of murine MBP, with sequence NH2-A141YDAQGTLSKIFKL154-COOH, in both membrane systems for 200 ns. The negatively-charged N-terminal end of the peptide penetrated further into the DMPC bilayer than into the mixed DMPC/DMPE bilayer. Reduced peptide accessibility to a formal positive charge of the DMPC amine ‘N’ atom surrounded by methyl and methylene groups may be the cause [1]. The peptide lost its α-helical structure in DMPC/DMPE but not in the DMPC bilayer. These findings show that membrane composition affects MBP’s interaction with it, a phenomenon that provides insights into myelin structure – and that may eventually be relevant to understanding the pathogenesis of multiple sclerosis (MS).

The use of microwave resonance therapy in the combined treatment of chronic pyelonephritis

The inclusion of microwave resonance therapy with 'fluctuating' frequency and a varied wave range in the combined treatment of patients with chronic pyelonephritis is well-founded pathogenetically since this method allows for the correction of lipid peroxidation level and parameters of the prooxidative-antioxidative activity in the blood serum. Moreover, it has beneficial effect on regulation of physico-chemical properties of biological membranes.

Effects of natural flavones on membrane properties and citotoxicity of HeLa cells

The aim of this study was to determine whether eupafolin and hispidulin, flavones extracted from Eupatorium littorale Cabrera, Asteraceae, have the ability to change properties of biological membranes and promote cytotoxic effects. Eupafolin (50-200 µM) decreased approximately 30% the rate and total amplitude of valinomycin induced swelling and 60-100% the energy-dependent mitochondrial swelling. Moreover, eupafolin (200 µM) reduced 35% the mitochondrial permeability transition, and hispidulin did not change this parameter in any of the doses tested. The evaluation of phase transition of DMPC liposomes with the probe DPH demonstrated that hispidulin and eupafolin affect gel and fluid phase. With mitochondrial membrane as model, hispidulin increased the polarization of fluorescence when used DPH-PA probe. Eupafolin and hispidulin (100 µM) promoted a reduction of 40% in cellular viability of HeLa cells in 24 h. Our results suggest that eupafolin and hispidulin have cytotoxic effects that can be explained, in part, by alterations promoted on biological membranes properties and mitochondrial bioenergetics.

Templating membrane assembly, structure, and dynamics using engineered interfaces

The physical and chemical properties of biological membranes are intimately linked to their bounding aqueous interfaces. Supported phospholipid bilayers, obtained by surface-assisted rupture, fusion, and spreading of vesicular microphases, offer a unique opportunity, because engineering the substrate allows manipulation of one of the two bilayer interfaces as well. Here, we review a collection of recent efforts, which illustrates deliberate substrate-membrane coupling using structured surfaces exhibiting chemical and topographic patterns. Vesicle fusion on chemically patterned substrates results in co-existing lipid phases, which reflect the underlying pattern of surface energy and wettability. These co-existing bilayer/monolayer morphologies are useful both for fundamental biophysical studies (e.g., studies of membrane asymmetry) as well as for applied work, such as synthesizing large-scale arrays of bilayers or living cells. The use of patterned, static surfaces provides new models to design complex membrane topographies and curvatures. Dynamic switchable-topography surfaces and sacrificial trehalose based-substrates reveal abilities to dynamically introduce membrane curvature and change the nature of the membrane-substrate interface. Taken together, these studies illustrate the importance of controlling interfaces in devising model membrane platforms for fundamental biophysical studies and bioanalytical devices.

Self-Assembly Simulations of Membranes Containing Phospholipid Oxidation Products

Products of phospholipid oxidation (OXPLs) are involved with the genesis or pathology of several diseases. OXPLs can modify the physical properties of biological membranes, thereby possibly altering several biological processes near membranes including signaling pathways. We have used atomistic and course grained simulations to investigate the properties of OXPL-containing lipid bilayers. We ran self assembly simulations of mixtures of palmitoyl-oleoyl-phosphatidylcholine (POPC) with two different OXPLs: PazePC, which is anionic, and PoxnoPC, which is zwitterionic. The total sampling time exceeds 1 millisecond. Despite having shortened and polar acyl chains, the two OXPLs POPC self assemble into stable lipid bilayers with POPC. The bilayers can accommodate at least 25% OXPL, although such bilayers have a lower area compressibility modulus. As as example of the modification of a membrane-associated biological process, we show that KALP-23 peptides partition differently in POPC-OXPL and POPC bilayers. The peptides adopt a transmembrane orientation more easily when OXPLs are present in the bilayers.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.