Structure Of Biological Membranes Research Articles

Effect of Pulsed Electric Fields on Interface Properties of Phospholipids Dispersion

Effects of pulsed electric fields (PEF) treatment on interface properties of phospholipids dispersion were studied. Results showed that the conductivity of solutions with different concentration after PEF treatment increased while surface tension value decreased with increasing electric field strength. The absolute value of zeta potential firstly increased when the field intensity was within 0~20 kV/cm, and then decreased if the field intensity increased further. It can be inferred that the torque of phospholipids molecule may have changed and phospholipids molecule have rearranged after PEF treatment. Moreover, as a resemblance to biological membrane structure, the phospholipid molecules of cell membrane may generate micropores after rearrangement, which result in the cell electroporation.

Membrane Domains and the “Lipid Raft” Concept

The bulk structure of biological membranes consists of a bilayer of amphipathic lipids. According to the fluid mosaic model proposed by Singer and Nicholson, the glycerophospholipid bilayer is a two-dimensional fluid construct that allows the lateral movement of membrane components. Different types of lateral interactions among membrane components can take place, giving rise to multiple levels of lateral order that lead to highly organized structures. Early observations suggested that some of the lipid components of biological membranes may play active roles in the creation of these levels of order. In the late 1980s, a diverse series of experimental findings collectively gave rise to the lipid raft hypothesis. Lipid rafts were originally defined as membrane domains, i.e., ordered structures created as a consequence of the lateral segregation of sphingolipids and differing from the surrounding membrane in their molecular composition and properties. This definition was subsequently modified to introduce the notion that lipid rafts correspond to membrane areas stabilized by the presence of cholesterol within a liquid-ordered phase. During the past two decades, the concept of lipid rafts has become extremely popular among cell biologists, and these structures have been suggested to be involved in a great variety of cellular functions and biological events. During the same period, however, some groups presented experimental evidence that appeared to contradict the basic tenets that underlie the lipid raft concept. The concept is currently being re-defined, with greater consistency regarding the true nature and role of lipid rafts. In this article we will review the concepts, criticisms, and the novel confirmatory findings relating to the lipid raft hypothesis.

Membrane Domains and the Lipid Raft Concept

The bulk structure of biological membranes consists of a bilayer of amphipathic lipids. According to the fluid mosaic model proposed by Singer and Nicholson, the glycerophospholipid bilayer is a two-dimensional fluid construct that allows the lateral movement of membrane components. Different types of lateral interactions among membrane components can take place, giving rise to multiple levels of lateral order that lead to highly organized structures. Early observations suggested that some of the lipid components of biological membranes may play active roles in the creation of these levels of order. In the late 1980s, a diverse series of experimental findings collectively gave rise to the lipid raft hypothesis. Lipid rafts were originally defined as membrane domains, i.e., ordered structures created as a consequence of the lateral segregation of sphingolipids and differing from the surrounding membrane in their molecular composition and properties. This definition was subsequently modified to introduce the notion that lipid rafts correspond to membrane areas stabilized by the presence of cholesterol within a liquid-ordered phase. During the past two decades, the concept of lipid rafts has become extremely popular among cell biologists, and these structures have been suggested to be involved in a great variety of cellular functions and biological events. During the same period, however, some groups presented experimental evidence that appeared to contradict the basic tenets that underlie the lipid raft concept. The concept is currently being re-defined, with greater consistency regarding the true nature and role of lipid rafts. In this article we will review the concepts, criticisms, and the novel confirmatory findings relating to the lipid raft hypothesis.

Characterization of the motion of membrane proteins using high-speed atomic force microscopy

For cells to function properly, membrane proteins must be able to diffuse within biological membranes. The functions of these membrane proteins depend on their position and also on protein-protein and protein-lipid interactions. However, so far, it has not been possible to study simultaneously the structure and dynamics of biological membranes. Here, we show that the motion of unlabelled membrane proteins can be characterized using high-speed atomic force microscopy. We find that the molecules of outer membrane protein F (OmpF) are widely distributed in the membrane as a result of diffusion-limited aggregation, and while the overall protein motion scales roughly with the local density of proteins in the membrane, individual protein molecules can also diffuse freely or become trapped by protein-protein interactions. Using these measurements, and the results of molecular dynamics simulations, we determine an interaction potential map and an interaction pathway for a membrane protein, which should provide new insights into the connection between the structures of individual proteins and the structures and dynamics of supramolecular membranes.

Contribution of cryoelectron microscopy of vitreous sections to the understanding of biological membrane structure

Using cryoelectron microscopy of vitreous sections, we investigated in situ the ultrastructure of biological membranes, selected from several cell types for their diverse biological functions. Here we describe how to visualize the two membrane leaflets and tightly apposed membranes, lying as close as 1.1 nm apart, by tuning the imaging conditions. We show how defects in membrane stacks may be clues to resolving their structure. Details of membrane proteins are also resolved, as well as protein lattices with correlations between stacked membranes. Imaging the cell in its native hydrated state can now be done in the nanometer resolution range, which should open unique routes for investigating structure-function relationships.

Dose dependences of lipid microviscosity of biological membranes induced by synthetic antioxidant potassium phenosan salt

Earlier, we demonstrated for the first time that the natural antioxidant α�tocopherol in a wide concentra� tion range can modify the structure of different lipid regions in the endoplasmic reticulum and plasma membranes of hepatocytes in experiments in vitro (1, 2). Despite the biochemical and functional differences between endoplasmic reticulum and plasma mem� branes, they are characterized by similar polymodal nonlinear dose dependences of the effects of α�toco� pherol on the rigidity of superficial (~8 A) and micro� viscosity of buried (~20 A) lipid bilayer regions. Such patterns are typical for biologically active substances, showing an effect over a wide concentration range, including ultralow doses (1, 2). The polymodality of dose dependences is associated with the statistically significant effects of α�tocopherol in three concentra� tion ranges. It is assumed that three possible mecha� nisms of action may underlie the effect of this com� pound depending on its concentration: insertion into the membrane, interaction with specific binding sites, and the information transfer through the layers of water with special physical and chemical properties (1-3). It is known that the main physicochemical characteristic of antioxidants is their ability to inhibit lipid peroxidation (LPO) both in vitro and in vivo, the regulatory system of which includes the microviscosity of lipids (4). In view of above, a question arises as to what are the common and distinctive features of the effects of nat� ural and synthetic agents at ultralow doses and whether the relationship between the ability of antiox� idants to inhibit LPO and modify the structure of bio� logical membranes is retained when they are used at ultralow doses. To answer this question, we assessed the effect of potassium salt of β�(4 �hydroxy�3,5�diter� tbutylphenyl)�propionic acid (potassium phenosan), a synthetic antioxidant, in a wide concentration range on the lipid microviscosity of EPR and PM of hepato� cytes. The study was performed with 100

Lipid–lipid interactions in bilayer membranes: Married couples and casual liaisons

There is considerable interest in lateral domain structure in biological membranes not least because a variety of physiological processes are believed to require assembly and mutual organization of particular membrane components for their execution. Domain structure is known to be created by differences in physicochemical properties between membrane lipids such as phase transition temperature, intermolecular hydrogen bonding and ionic functional groups. Domains are also created by specific interactions between different membrane lipids to form stoichiometric complexes. Such complexes often form ordered structures referred to as membrane rafts. The present challenge is to define the balance of line tension between lateral membrane domains in individual leaflets of the bilayer and coupling forces operating at the midplane of the bilayer responsible for maintaining lipids in opposing domains on either side of the structure in register. A review of the current evidence relating to these questions is presented.

The Kupershtokh-Medvedev electrostrictive instability as possible mechanism of initiation of phase transitions, domains and pores in lipid membranes and influence of microwave irradiation on cell

One of the possible mechanisms of initiation of local phase transitions and formation of nonuniform structure of biological and model lipid membranes is suggested. It is based on anisotropic electrohydrodynamic instability of Kupershtokh and Medvedev in strong electric field relative to density perturbations. This mechanism may clarify initial stages of formation of membrane domains and pores, some aspects of cell signalization and influence of microwave irradiation of nonthermal intensity on living organisms.

Сучасні уявлення про структуру і динаміку біологічних мембран

Сучасні уявлення про структуру і динаміку біологічних мембран

Probing Dielectric and Hydrogen Bonding Gradients in Biological Membranes

Nitroxide spin-labeling in combination with EPR spectroscopy has found many applications in studying structure and dynamics of proteins and biological membranes. Recently, there has been a substantial interest in utilizing EPR to characterize local effects of polarity and hydrogen bonding in proteins and biological membrane systems. Here we report on employing an arsenal of advanced spin-labeling EPR methods to profile heterogeneous dielectric and hydrogen bonding environment along the α-helical chain of an alanine-rich WALP peptide that is anchored in a lipid bilayer in a transmembrane orientation. A series of WALP cysteine mutants was labeled with a pH-sensitive nitroxide IMSTL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester) that is similar in molecular volume to phenylalanine. The protonation state of this nitroxide could be directly observed by EPR allowing us to follow proton gradient across the membrane in the vicinity of the WALP α-helix, and, thus, to reconstruct the gradient in the effective dielectric constant. These experiments were complemented by assessing local polarity from characteristic changes in EPR spectra that were enhanced by the use of perdeuterated and 15N-substituted nitroxides and high field EPR at 130 GHz (D-band). Formation of hydrogen bonds between the nitroxides and membrane-penetrating water molecules was observed directly in HYSCORE X-band experiments. Such measurements allowed us to derive experimental profiles of heterogeneous dielectric and hydrogen bonding environment along a typical transmembrane α-helix. Supported by: NSF-0843632 to TIS and NIH 1R01GM072897 to AIS.

Simulating Pores in Saturated Phosphatidylcholine Lipid Bilayers

Lipid bilayers form the basic structure of cellular membranes. There is a large degree of diversity in the structure and composition of biological membranes. While one of the most important functions of membranes is to prohibit polar molecules from crossing the membrane, pore formation is crucial in a number of biological processes. We have used atomistic simulations to investigate the thermodynamics and kinetics of pore formation and dissipation in three saturated phosphatidylcholine bilayers, DLPC, DMPC, and DPPC. Pore formation has a large free energy cost, which increases as the tails length increases: 16 kJ/mol (DLPC), 40 kJ/mol (DMPC), and 80 kJ/mol (DPPC). We find that pore formation has a large unfavorable entropic contribution, possibly due to the constriction of water within the pore. The large unfavorable entropic contribution is compensated by a favorable enthalpic contribution to pore formation. Once formed, pores in the shorter lipid bilayers are larger and more stable than pores in bilayers with longer lipids. These results have broad implications on biological processes involving pore formation, such as lipid flip-flop, antimicrobial peptides, and cell penetrating peptides.

Structure and Function of Biological Membranes: The Impact of Nanoparticles

Structure and Function of Biological Membranes: The Impact of Nanoparticles

Effect of α-tocopherol concentrations on the self-organization, physicochemical properties of solutions, and the structure of biological membranes

Effect of α-tocopherol concentrations on the self-organization, physicochemical properties of solutions, and the structure of biological membranes

Ca2+induces clustering of membrane proteins in the plasma membrane via electrostatic interactions

Membrane proteins and membrane lipids are frequently organized in submicron-sized domains within cellular membranes. Factors thought to be responsible for domain formation include lipid-lipid interactions, lipid-protein interactions and protein-protein interactions. However, it is unclear whether the domain structure is regulated by other factors such as divalent cations. Here, we have examined in native plasma membranes and intact cells the role of the second messenger Ca(2+) in membrane protein organization. We find that Ca(2+) at low micromolar concentrations directly redistributes a structurally diverse array of membrane proteins via electrostatic effects. Redistribution results in a more clustered pattern, can be rapid and triggered by Ca(2+) influx through voltage-gated calcium channels and is reversible. In summary, the data demonstrate that the second messenger Ca(2+) strongly influences the organization of membrane proteins, thus adding a novel and unexpected factor that may control the domain structure of biological membranes.

Imaging Fluorescence Cross-Correlation Spectroscopy as a Tool to Study Cell-Membrane Organization

The structure of biological membranes has been investigated for many years. However, progress is hindered by the fact that putative domains are highly dynamic and their size is smaller than the optical diffraction limit and thus direct observations are difficult. Therefore, there is a need to develop new biophysical tools which can infer the existence of domains within membranes and can follow their development over time. We have introduced in the past a method called Imaging Total Internal Reflection-Fluorescence Correlation Spectroscopy (ITIR-FCS) using EMCCD or sCMOS cameras. ITIR-FCS allows the measurement of a large number (up to ∼0.5 million) correlation curves at contiguous locations on cell membranes of live cells with millisecond time resolution. The spatial information within the data can be used to obtain information on the structure and organization of the membranes. This is achieved by calculating differences between the forward and backward cross-correlations between adjacent pixels A and B (CCFAB - CCFBA) or A, B, and C (CCFAB - CCFCB). The results can be depicted as histograms referred to as ΔCCF distributions. In this work we conduct measurements on supported lipid bilayers and cell membranes and perform simulations to demonstrate how ΔCCF distributions change characteristically with membrane complexity and structure. In particular, we demonstrate that domains with sizes below the diffraction limit have a characteristic broadening effect on the ΔCCF distributions. As an example we show that changes in membrane structure and organization of live neuroblastoma cells can be followed over the time course of an hour or more by way of ΔCCF distributions. To deal with large amount of data collected we developed an open source software, ImFCS, to calculate and fit the auto- and cross-correlation functions and depict the results in an imaging format.

Biomembrane models and drug-biomembrane interaction studies: Involvement in drug design and development

Contact with many different biological membranes goes along the destiny of a drug after its systemic administration. From the circulating macrophage cells to the vessel endothelium, to more complex absorption barriers, the interaction of a biomolecule with these membranes largely affects its rate and time of biodistribution in the body and at the target sites. Therefore, investigating the phenomena occurring on the cell membranes, as well as their different interaction with drugs in the physiological or pathological conditions, is important to exploit the molecular basis of many diseases and to identify new potential therapeutic strategies. Of course, the complexity of the structure and functions of biological and cell membranes, has pushed researchers toward the proposition and validation of simpler two- and three-dimensional membrane models, whose utility and drawbacks will be discussed. This review also describes the analytical methods used to look at the interactions among bioactive compounds with biological membrane models, with a particular accent on the calorimetric techniques. These studies can be considered as a powerful tool for medicinal chemistry and pharmaceutical technology, in the steps of designing new drugs and optimizing the activity and safety profile of compounds already used in the therapy.

Alec Douglas Bangham. 10 November 1921 — 9 March 2010

A first meeting with Alec Bangham would leave an impression of a larger-than-life scientist with a penetrating curiosity and formidable features characteristically portrayed in the dramatic portrait painted by his nephew, Humphrey Bangham, which currently hangs in one of the Royal Society’s rooms. Or, as Jeffrey Watkins (FRS) experienced on reaching the Institute of Animal Physiology at Babraham in September 1963, ‘at the moment of my arrival he was avidly peering down a microscope and excitedly proclaiming the wonders of myelin figures as viewed through crossed polaroids … [and that] they might prove useful as model systems for the study of structure and function in biological membranes. It was decided that I should work immediately along these lines.’ So what were the formative events that led to these first impressions? What had contributed to the formation of a scientist who became known worldwide as the father of liposomes, the inventor of Artificial Lung Expanding Compound (ALEC)—and someone who never stopped thinking about membranes, surfactant, anaesthetics, cricket, vegetables, family and friends?

Importance of the Sphingosine Base Double-Bond Geometry for the Structural and Thermodynamic Properties of Sphingomyelin Bilayers

The precise role of the sphingosine base trans double bond for the unique properties of sphingomyelins (SMs), one of the main lipid components in raftlike structures of biological membranes, has not been fully explored. Several reports comparing the hydration, lipid packing, and hydrogen-bonding behaviors of SM and glycerophospholipid bilayers found remarkable differences overall. However, the atomic interactions linking the double-bond geometry with these thermodynamic and structural changes remained elusive. A recent report on ceramides, which differ from SMs only by their hydroxyl headgroup, has shown that replacing the trans double bond of the sphingosine base by cis weakens the hydrogen-bonding potential of these lipids and thereby alters their biological activity. Based on data from extensive (a total 0.75 μs) atomistic molecular dynamics simulations of bilayers composed of all- trans, all- cis, and a trans/ cis (4:1 ratio) racemic mixture of sphingomyelin lipids, here we show that the trans configuration allows for the formation of significantly more hydrogen bonds than the cis. The extra hydrogen bonds enabled tighter packing of lipids in the all- trans and trans/ cis bilayers, thus reducing the average area per lipid while increasing the chain order and the bilayer thickness. Moreover, fewer water molecules access the lipid-water interface of the all- trans bilayer than of the all- cis bilayer. These results provide the atomic basis for the importance of the natural sphingomyelin trans double-bond conformation for the formation of ordered membrane domains.

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.

ChemInform Abstract: Polyelectrolyte-Sensitized Phospholipid Vesicles

Control of the structure and permeability of biological membranes is one of the most basic and least understood problems in contemporary biophysics. Diverse cellular processes require exquisite control of the membrane structure, including endocytosis and exocytosis, maintenance of the Golgi apparatus and the endoplasmic reticulum, and mitosis and meiosis, to mention just a few. Recently, significant progress has been made in controlling the permeability, and often the structures, of synthetic bilayer vesicles. These studies are especially important in that they help elucidate previously unimagined mechanisms for membrane control. Although no single model system is likely to utilize all of the rich capabilities exploited in nature, the synthetic systems can serve to identify concepts and mechanisms likely to be useful in a biological context. In addition, control of vesicular membrane systems may usher in novel chemistries for use in imaging, sensing, and therapeutic applications.