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BBA - Biomembranes (v.1798, #4)

Editorial Board (pp. i).

A surface view on membrane structure, dynamics and applications by Jesús Pérez-Gil (pp. 701-702).

Surface topography of membrane domains by Marie-Cécile Giocondi; Daisuke Yamamoto; Eric Lesniewska; Pierre-Emmanuel Milhiet; Toshio Ando; Christian Le Grimellec (pp. 703-718).

Elucidating origin, composition, size, and lifetime of microdomains in biological membranes remains a major issue for the understanding of cell biology. For lipid domains, the lack of a direct access to the behaviour of samples at the mesoscopic scale has constituted for long a major obstacle to their characterization, even in simple model systems made of immiscible binary mixtures. By its capacity to image soft surfaces with a resolution that extends from the molecular to the microscopic level, in air as well as under liquid, atomic force microscopy (AFM) has filled this gap and has become an inescapable tool in the study of the surface topography of model membrane domains, the first essential step for the understanding of biomembranes organization. In this review we mainly focus on the type of information on lipid microdomains in model systems that only AFM can provide. We will also examine how AFM can contribute to understand data acquired by a variety of other techniques and present recent developments which might open new avenues in model and biomembrane AFM applications.

Keywords: Membrane domain; Langmuir Blodgett film; Supported lipid bilayer; Atomic force microscopy

Silica xerogel/aerogel-supported lipid bilayers: Consequences of surface corrugation by Emel I. Goksu; Matthew I. Hoopes; Barbara A. Nellis; Chenyue Xing; Roland Faller; Curtis W. Frank; Subhash H. Risbud; Joe H. Satcher Jr.; Marjorie L. Longo (pp. 719-729).

The objective of this paper was to review our recent investigations of silica xerogel and aerogel-supported lipid bilayers. These systems provide a format to observe relationships between substrate curvature and supported lipid bilayer formation, lipid dynamics, and lipid mixtures phase behavior and partitioning. Sensitive surface techniques such as quartz crystal microbalance and atomic force microscopy are readily applied to these systems. To inform current and future investigations, we review the experimental literature involving the impact of curvature on lipid dynamics, lipid and phase-separated lipid domain localization, and membrane–substrate conformations and we review our molecular dynamics simulations of supported lipid bilayers with the atomistic and molecular information they provide.

Keywords: Atomic force microscopy; Quartz crystal microbalance; Membrane raft; Coarse-graining; Porous material; Membrane bending

Surface view of the lateral organization of lipids and proteins in lung surfactant model systems—A ToF-SIMS approach by Mohammed Saleem; Hans-Joachim Galla (pp. 730-740).

The lateral organization of domain structures is an extremely significant aspect of biomembrane research. Chemical imaging by mass spectrometry with its recent advancement in sensitivity and lateral resolution has become a highly promising tool in biological research. In this review, we focus briefly on the instrumentation, working principle and important concepts related to time-of-flight secondary ion mass spectrometry followed by an overview of lipid/protein fragmentation patterns and chemical mapping. The key issues addressed are the applications of time-of-flight secondary ion mass spectrometry in biological membrane research. Additionally, we briefly review our recent investigations based on time-of-flight secondary ion mass spectrometry to unravel the lateral distribution of lipids and surfactant proteins in lung surfactant model systems as an example that highlights the importance of fluidity and ionic conditions on lipid phase behavior and lipid–protein interactions.

Keywords: Time-of-flight secondary ion mass spectrometry; Chemical imaging; Surfactant protein B; Lung surfactant; Lipid–protein interaction; Lateral organization of membrane

Nanomechanics of lipid bilayers by force spectroscopy with AFM: A perspective by Sergi Garcia-Manyes; Fausto Sanz (pp. 741-749).

Lipid bilayers determine the architecture of cell membranes and regulate a myriad of distinct processes that are highly dependent on the lateral organization of the phospholipid molecules that compose the membrane. Indeed, the mechanochemical properties of the membrane are strongly correlated with the function of several membrane proteins, which demand a very specific, highly localized physicochemical environment to perform their function. Several mesoscopic techniques have been used in the past to investigate the mechanical properties of lipid membranes. However, they were restricted to the study of the ensemble properties of giant bilayers. Force spectroscopy with AFM has emerged as a powerful technique able to provide valuable insights into the nanomechanical properties of supported lipid membranes at the nanometer/nanonewton scale in a wide variety of systems. In particular, these measurements have allowed direct measurement of the molecular interactions arising between neighboring phospholipid molecules and between the lipid molecules and the surrounding solvent environment. The goal of this review is to illustrate how these novel experiments have provided a new vista on membrane mechanics in a confined area within the nanometer realm, where most of the specific molecular interactions take place. Here we report in detail the main discoveries achieved by force spectroscopy with AFM on supported lipid bilayers, and we also discuss on the exciting future perspectives offered by this growing research field.

Keywords: Force spectroscopy; Atomic force microscopy; Lipid bilayer; Nanomechanics

Nanoscale analysis of supported lipid bilayers using atomic force microscopy by Karim El Kirat; Sandrine Morandat; Dufrene Yves F. Dufrêne (pp. 750-765).

During the past 15years, atomic force microscopy (AFM) has opened new opportunities for imaging supported lipid bilayers (SLBs) on the nanoscale. AFM offers a means to visualize the nanoscale structure of SLBs in physiological conditions. A unique feature of AFM is its ability to monitor dynamic events, like bilayer alteration, remodelling or digestion, upon incubation with various external agents such as drugs, detergents, proteins, peptides, nanoparticles, and solvents. Here, we survey recent progress made in the area.

Keywords: Atomic force microscopy; Biomembranes; Nanoscale organization; Real-time imaging; Supported lipid bilayers

Surface analysis of membrane dynamics by Ana J. García-Sáez; Petra Schwille (pp. 766-776).

Cellular membranes change shape and composition in a controlled way in order for life to prosper. They are set apart from other cellular structures by their two-dimensional nature, which governs their properties and the processes happening in or on them. During the last decade, a boom of new techniques has allowed the study of the surface of biological membranes in unprecedented ways and has greatly advanced our understanding of its molecular organization. In this review, we first describe the principles of state-of-the-art microscopy methods and the suitable model membranes for the study of dynamic processes. Then, we explain how they can be used to investigate lipid dynamics, the lateral organization of membrane domains and the dynamic structure of cellular membranes.

Keywords: Membrane dynamic; Giant unilamellar vesicle; Supported lipid bilayer; FCS; AFM; Single particle tracking; FRAP; TIRF microscopy; Image correlation spectroscopy; Membrane organization; Membrane domain

A nanometer scale optical view on the compartmentalization of cell membranes by Thomas S. van Zanten; Alessandra Cambi; Maria F. Garcia-Parajo (pp. 777-787).

For many years, it was believed that the laws of diffraction set a fundamental limit to the spatial resolution of conventional light microscopy. Major developments, especially in the past few years, have demonstrated that the diffraction barrier can be overcome both in the near- and far-field regime. Together with dynamic measurements, a wealth of new information is now emerging regarding the compartmentalization of cell membranes. In this review we focus on optical methods designed to explore the nanoscale architecture of the cell membrane, with a focal point on near-field optical microscopy (NSOM) as the first developed technique to provide truly optical super-resolution beyond the diffraction limit of light. Several examples illustrate the unique capabilities offered by NSOM and highlight its usefulness on cell membrane studies, complementing the palette of biophysical techniques available nowadays.

Keywords: Membrane nanodomain; Lipid raft; Single molecule detection; Near-field scanning optical microscopy; Super-resolution optical microscopy

Infrared reflection–absorption spectroscopy: Principles and applications to lipid–protein interaction in Langmuir films by Richard Mendelsohn; Guangru Mao; Carol R. Flach (pp. 788-800).

Infrared reflection–absorption spectroscopy (IRRAS) of lipid/protein monolayer films in situ at the air/water interface provides unique molecular structure and orientation information from the film constituents. The technique is thus well suited for studies of lipid/protein interaction in a physiologically relevant environment. Initially, the nature of the IRRAS experiment is described and the molecular structure information that may be obtained is recapitulated. Subsequently, several types of applications, including the determination of lipid chain conformation and tilt as well as elucidation of protein secondary structure are reviewed. The current article attempts to provide the reader with an understanding of the current capabilities of IRRAS instrumentation and the type of results that have been achieved to date from IRRAS studies of lipids, proteins, and lipid/protein films of progressively increasing complexity. Finally, possible extensions of the technology are briefly considered.

Keywords: Infrared spectroscopy; Lipid conformation; Aqueous monolayers; Langmuir films; Protein secondary structure; Pulmonary surfactant

Overcoming rapid inactivation of lung surfactant: Analogies between competitive adsorption and colloid stability by Joseph A. Zasadzinski; Patrick C. Stenger; Ian Shieh; Prajna Dhar (pp. 801-828).

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air–liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air–water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.

Keywords: Charge reversal; Chitosan; Inhibition; Depletion attraction; Albumin; Polyethylene glycol; Debye length

RNA and DNA interactions with zwitterionic and charged lipid membranes — A DSC and QCM-D study by Agnes Michanek; Nora Kristen; Hook Fredrik Höök; Tommy Nylander; Emma Sparr (pp. 829-838).

The aim of the present study is to establish under which conditions tRNA associates with phospholipid bilayers, and to explore how this interaction influences the lipid bilayer. For this purpose we have studied the association of tRNA or DNA of different sizes and degrees of base pairing with a set of model membrane systems with varying charge densities, composed of zwitterionic phosphatidylcholines (PC) in mixtures with anionic phosphatidylserine (PS) or cationic dioctadecyl-dimethyl-ammoniumbromide (DODAB), and with fluid or solid acyl-chains (oleoyl, myristoyl and palmitoyl). To prove and quantify the attractive interaction between tRNA and model-lipid membrane we used quartz crystal microbalance with dissipation (QCM-D) monitoring to study the tRNA adsorption to deposit phospholipid bilayers from solutions containing monovalent (Na⁺) or divalent (Ca²⁺) cations. The influence of the adsorbed polynucleic acids on the lipid phase transitions and lipid segregation was studied by means of differential scanning calorimetry (DSC). The basic findings are: i) tRNA adsorbs to zwitterionic liquid-crystalline and gel-phase phospholipid bilayers. The interaction is weak and reversible, and cannot be explained only on the basis of electrostatic attraction. ii) The adsorbed amount of tRNA is higher for liquid-crystalline bilayers compared to gel-phase bilayers, while the presence of divalent cations show no significant effect on the tRNA adsorption. iii) The adsorption of tRNA can lead to segregation in the mixed 1,2-dimyristoyl-sn-glycerol-3-phosphatidylcholine (DMPC)-1,2-dimyristoyl-sn-glycero-3-phosphatidylserine (DMPS) and DMPC–DODAB bilayers, where tRNA is likely excluded from the anionic DMPS-rich domains in the first system, and associated with the cationic DODAB-rich domains in the second system. iv) The addition of shorter polynucleic acids influence the chain melting transition and induce segregation in a mixed DMPC–DMPS system, while larger polynucleic acids do not influence the melting transition in these system. The results in this study on tRNA–phospholipid interactions can have implications for understanding its biological function in, e.g., the cell nuclei, as well as in applications in biotechnology and medicine.

Keywords: DSC; QCM-D; RNA; DNA; Phospholipids; Phospholipid phase behavior; Polynucleic acid adsorption

Templating membrane assembly, structure, and dynamics using engineered interfaces by Ann E. Oliver; Atul N. Parikh (pp. 839-850).

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.

Keywords: Supported lipid bilayer; Lipid monolayer; Membrane interface; Vesicle fusion; Membrane curvature; Carbohydrate supported bilayer; Cell patterning; Trehalose

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BBA - Biomembranes (v.1798, #4)

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BBA - Biomembranes (v.1798, #4)