Parallel Bilayers Research Articles

Computational characterization of claudin-15 strand flexibility

Claudins self-polymerize into tight junction strand networks to seal the paracellular pathway between adjacent cells and form selective barriers to control the passive transport of water and small molecules. These anastomosing networks of claudin strands are flexible and dynamic, capable of maintaining barrier function throughout the cellular movement and large-scale tissue rearrangements. To understand the flexibility of tight junction strands, we performed molecular dynamics (MD) simulations of claudin-15 strands ranging from 27nm to 225 nm in two parallel lipid bilayers and characterized their mechanical properties.

The Synaptotagmin-1 C2B Domain Is a Key Regulator in the Stabilization of the Fusion Pore.

Fusion pores serve as an effective mechanism to connect intracellular organelles and release vesicle contents during exocytosis. A complex lipid rearrangement takes place as membranes approximate, bend, fuse, and establish a traversing water channel to define the fusion pore, linking initially isolated chambers. Thermodynamically, the process is unfavorable and thought to be mediated by specialized proteins. In this work, we have developed a reaction coordinate to induce fusion pores from initially flat and parallel lipid bilayers and we have used it to describe the effects of the synaptotagmin-1 C2B domain during the process. We have obtained free-energy profiles of the whole lipid reorganization in biologically realistic membranes, going from planar and parallel bilayers through stalk hemifusion to water channel formation. Our results point to a lysine-rich polybasic region on synaptotagmin-1 C2B as the key to lipid reorganization control through the formation of phosphatidylinositol bisphosphate clusters that stabilize the fusion pore.

Microstructure of different chain length ionic liquids intercalated into montmorillonite: A molecular dynamics study

This study employs molecular dynamics modeling to examine the interlayer microstructures of montmorillonite intercalated with different chains of CnmimCl montmorillonites with varying chain lengths, intercalation amount, arrangement and energy. C4mim+ and C8mim+ arrange themselves in monolayers, while C12mim+ and C16mim+ in parallel bilayers or tilt aligning. The basal spacing of montmorillonite and the corresponding energy increased as the chain length of organic cations is increased. It is indicated that no bond has been formed for N+O due to the minimum distances of no less than 3.1Å and angles of 35°–45° between N+ in Cnmim+ and silicon–oxygen tetrahedron. These modeling results provide insights into the understanding of synthesized Cnmim+-montmorillonite microstructure and guidelines for relevant engineering applications.

Biaxial mechanics and inter‐lamellar shearing of stem‐cell seeded electrospun angle‐ply laminates for annulus fibrosus tissue engineering

The annulus fibrosus (AF) of the intervertebral disk plays a critical role in vertebral load transmission that is heavily dependent on the microscale structure and composition of the tissue. With degeneration, both structure and composition are compromised, resulting in a loss of AF mechanical function. Numerous tissue engineering strategies have addressed the issue of AF degeneration, but few have focused on recapitulation of AF microstructure and function. One approach that allows for generation of engineered AF with appropriate (+/-)30° lamellar microstructure is the use of aligned electrospun scaffolds seeded with mesenchymal stem cells (MSCs) and assembled into angle-ply laminates (APL). Previous work indicates that opposing lamellar orientation is necessary for development of near native uniaxial tensile properties. However, most native AF tensile loads are applied biaxially, as the disk is subjected to multi-axial loads and is constrained by its attachments to the vertebral bodies. Thus, the objective of this study was to evaluate the biaxial mechanical response of engineered AF bilayers, and to determine the importance of opposing lamellar structure under this loading regime. Opposing bilayers, which replicate native AF structure, showed a significantly higher modulus in both testing directions compared to parallel bilayers, and reached ∼60% of native AF biaxial properties. Associated with this increase in biaxial properties, significantly less shear, and significantly higher stretch in the fiber direction, was observed. These results provide additional insight into native tissue structure-function relationships, as well as new benchmarks for engineering functional AF tissue constructs.

A Novel Phase of Compressed Bilayers That Models the Prestalk Transition State of Membrane Fusion

The force model of protein-mediated membrane fusion hypothesizes that fusion is driven by mechanical forces exerted on the membranes, but many details are unknown. Here, we investigated by x-ray diffraction the consequence of applying compressive force on a stack of membranes against the hydration barrier. We found that as the osmotic pressure increased, the lamellar phase transformed first to a new phase of tetragonal lattice (T-phase) over a narrow range of relative humidity, and then to a phase of rhombohedral lattice. The unit cell structure changed from parallel bilayers to a bent configuration with a point contact between adjacent bilayers and then to the stalk hemifusion configuration. The T-phase is discussed as a possible transition state in the membrane merging pathway of fusion. We estimate the work required to form the T-phase and the subsequent hemifusion-stalk-resembling R-phase. The work for the formation of a stalk is compatible with the energy estimated to be released by several SNARE complexes.

Statistical mechanics of bilayer membranes in troubled aqueous media

We consider a bilayer membrane surrounded by small impurities, assumed to be attractive or repulsive. The purpose is a quantitative study of the effects of these impurities on the statistical properties of the supported membrane. Using the replica trick combined with a variational method, we compute the membrane mean-roughness and the height correlation function for almost-flat membranes, as functions of the primitive elastic constants of the membrane and some parameter that is proportional to the volume fraction of impurities and their interaction strength. As results, the attractive impurities increase the shape fluctuations due to the membrane undulations, while repulsive ones suppress these fluctuations. Second, we compute the equilibrium diameter of (spherical) vesicles surrounded by small random particles starting from the curvature equation. Third, the study is extended to a lamellar phase composed of two parallel fluid membranes, which are separated by a finite distance. This lamellar phase undergoes an unbinding transition. We demonstrate that the attractive impurities increase the unbinding critical temperature, while repulsive ones decrease this temperature. Finally, we say that the presence of small impurities in an aqueous medium may be a mechanism to suppress or to produce an unbinding transition, even the temperature and polarizability of the aqueous medium are fixed, in lamellar phases formed by parallel lipid bilayers.

Nanostructured Ultrathin Films Obtained by the Spreading of Diblock Copolymers on a Surface

We propose a theory of nanopattern formation in ultrathin films obtained by the spreading of A-B diblock copolymers on a surface. A blocks are strongly adsorbed (spread) on the substrate, and the strength of the adsorption can be varied. B blocks are incompatible with the substrate, but they can spread atop the layer of the A blocks. We predict disk-shaped micelles ordered with hexagonal symmetry and parallel stripe-shaped micelles and bilayers to be stable. The type of resulting structure and its geometrical parameters depend on the composition of the copolymer and interaction parameters. We interrelate these results with those obtained for the case of the spreading of both blocks on the substrate, and we construct a unified phase diagram.

A Defective Lamellar Phase in a Nonionic Surfactant Water System Studied by NMR Methods

Lamellar phases of some surfactant water system are not infinite, continuous lamellar planes but several experimental findings showed that they contain water-filled defects with highly curved edges. Here, the polydomain lamellar phase of the pentaethylene glycol dodecyl ether(C 12 E 5 )/decane/water system has been studied by 1 H-NMR pulsed field gradient experiments (PGSE) and 2 H-NMR line shapes of D 2 O, at a constant surfactant (S) to oil (O) weight ratio of 51.8/48.2. Within the lamellar phase of this system, which was observed at S+O weight fraction from 5 to 82 wt%, three types of bilayer organisation have been found. At low surfactant+oil concentrations, <45wt% S+O, the lamellar phase consists of a stack of flat parallel bilayers while within a intermediate-concentration regime water-filled bilayers exist, as demonstrated by the presence of three-dimensional water diffusion. At concentration higher than 68 wt% S+O this defected lamellar phase oriented in the magnetic field, giving a mono-domain structure.

Fourier transform infrared and dielectric study of water–C12E4 liquid crystals

Fourier transform infrared (FT-IR) spectroscopy and time domain reflectometry (TDR) have been employed to study nano-confinement effects in water–tetraethylene glycol monododecyl ether (C12E4) system as a function of the water to surfactant molar ratio (R) at 25 °C. The study emphasises that, at low R values, the vibrational dynamics of water entrapped between equidistant parallel bilayers of oriented surfactant molecules is significantly different from that of pure water whereas at R&amp;gt;7 it becomes practically indistinguishable. On the other hand, even at the higher R values investigated, the dielectric properties of the aqueous domain confined in water–C12E4 liquid crystals are found to be strongly affected by confinement effects. The observed single relaxation processes with a symmetrical distribution of relaxation time are discussed in terms of effects due to interfacial interactions and topological restrictions on water-surfactant head group dynamics in liquid crystals. Correlation between the structural and dynamical properties of water–C12E4 liquid crystals and thermal effects due to microwave irradiation on these highly viscous microheterogeneous systems are also discussed.

Inorganic Polymerization in a Non-Ionic Lyotropic Lamellar Phase

Silicon tetramethoxide was polymerized within a nonionic lyotropic lamellar phase. After gelation, the birefringence of the sample is preserved and polarized light optical microscope observations reveal no segregation. High Resolution Small Angle X-ray Scattering patterns of the gel shows Bragg peaks characteristic of a layered structure. Freeze fracture experiments on the lamellar gel show stacks of parallel bilayers. The experimental observations indicate that the silica is located in the lamellar structure.

Evidence for Multilamellar Vesicles in the Lamellar Phase of an Electrostatic Lyotropic Ternary System. A Solid State 2H-NMR and Freeze Fracture Electron Microscopy Study

Deuterium solid state NMR and freeze fracture electron microscopy experiments have been carried out in the lamellar Lα phase of the water−sodium dodecyl sulfate−octanol system. Within the lamellar phase two types of bilayer organizations have been found. At high surfactant and alcohol concentrations, the lamellar phase is made of a stack of flat parallel bilayers while in the dilute part, it consists of multilayered vesicles of large radius (ca. 10 000 Å). These latter structures, commonly called spherulites, which appear spontaneously at low octanol contents and/or for high water dilution, can be described as textural defects of the lamellar phase. The location of this onion-like structure region is discussed within the framework of the membrane elasticity theory. As a side result, the presence of 33% glycerol in some of the freezefracture experiments is shown to barely affect the bending modulus of the film but rather disorder the molecular packing of the bilayers.

Interlayer Josephson coupling of thermally excited vortices in Bi2Sr2CaCu2O8-y.

Measurements of the zero-field resistive transition of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8] single crystals suggest that a bulk zero-resistance state develops through a sequential process characterized by two transition temperatures. The upper transition corresponds to Josephson coupling of parallel CuO bilayers, and the lower transition to the conventional Kosterlitz-Thouless dissociation of bound vortex excitations. Measurements using a flux transformer'' electrode geometry indicate a pronounced peak in the secondary voltage, which is suppressed by an external magnetic field.

Morphological characteristics of tumours formed by Lewis lung carcinoma-derived cloned cell lines with different metastatic potentials: structural differences in their basement membranes formed in vivo

Tumour basement membrane (BM) is an extracellular matrix produced by tumour cells of epithelial origin. We examined the structure and function of the tumour BM of tumour tissues formed by Lewis lung carcinoma-derived cloned cell lines (P29, LM12-3 and LM60-D6 cells) with low, medium and high metastatic potentials, respectively. Immunohistochemical staining of major BM constituents laminin and type IV collagen demonstrated that all the cell lines produced and deposited these materials extracellularly in vivo. However, the continuity of the tumour BM composed of these materials was much greater in the higher metastatic LM12-3 and LM60-D6 tumours than in those with the low metastatic P29 tumour. Electron microscopic examination revealed that in the higher metastatic tumours, especially the LM60-D6 tumour, the tumour BM had a highly organized structure consisting of lamina densa and lamina rara. Parallel bilayers of BM and their fusion were often observed and tumour cells were in direct contact with the BM. In the vicinity of tumour blood vessels, similar interactions between the tumour BM and the vascular BM were observed, and the tumour cells rested on their own BM, the fused BM or the vascular BM. In contrast, in the low metastatic tumour in which the tumour BM was not clearly defined, this close contact between tumour cells and the vascular BM was not observed. In vitro studies showed that the higher metastatic cells adhered more firmly than the LMP cells to a subendothelial matrix. These results suggest that the adhesiveness of tumour cells to the vascular BM in vivo is correlated with their ability to form an integrated BM in vivo, and that this adhesiveness of the tumour cells may be mediated in part by the tumour BM via BM fusion.

Lamellar versus isotropic structures in dilute phases of fluid membranes

In recent years, considerable attention has been focused on dilute phases of fluid membranes in surfactant systems. At the present time, the structure of two different phases have been well characterized. The lamellar phase (L α) shows long range smectic order and consists of a regular stack of parallel bilayers. At high dilution, its long range order has been shown to be stabilized by the steric interaction between adjacent undulating membranes. The other phase (L 3) is isotropic and shows no long range positional order. Scattering patterns and transport properties strongly suggest that its structure consists of a randomly multiconnected bilayer separating two equivalent subvolumes of solvent. We here discuss the relative stability of these two structures in connection with the elastic properties of the amphiphilic membrane. A general scaling law for the free energy of the L 3 phase as function of the degree of dilution is proposed and is checked against experimental measurements of some of its static and dynamic physical properties.

Electron microscopy of cholesterol

Negative staining and platinum-carbon shadowing have been used to prepare electron microscope specimens from aqueous colloidal suspensions of cholesterol microscrystals and from crystalline suspensions in methanol and ethanol. Microcrystals prepared by injection of alcoholic solutions of cholesterol into water exhibit angular conformations of varying regularity which contain a number of parallel cholesterol bilayers. The electron optical images of the cholesterol microcrystals, oriented horizontally and ‘on-edge’, obtained by both negative staining and metal shadowing, are in good agreement. Metal shadowing does, however, reveal greater detail within microcrystal clusters than does negative staining, as well as of the bilayer steps at microcrystal edges. The needle-like crystals (from methanol) and plate-like crystals (from ethanol) present considerable difficulties for the negative staining technique, because of their thickness and the consequent depth of the surrounding negative stain. Small crystals are, nevertheless, shown to possess multiple cholesterol bilayers. Platinum-carbon shadowing of cholesterol crystals taken directly from methanol and ethanol provides more satisfactory images than negative staining. The large depth of focus of the transmission electron microscope enables the stacked cholesterol bilayers to be clearly defined at the edges of crystals. The results obtained are discussed in relation to the physicochemical and biological properties of cholesterol, which underlie the fundamental difficulty encountered when fixing and staining cholesterol for thin sectioning, and also the role of cholesterol insolubility in the formation of gallstones.

Divalent cation and hydrogen ion effects on the structure and surface activity of pulmonary surfactant

The structure and surface activity of the extracellular fraction of pulmonary surfactant known as tubular myelin are Ca2+ dependent. Previous studies have demonstrated surfactant-specific proteins with monomeric molecular weights of 28,000-36,000 (SP28-36) are associated with this fraction. In reassembled lipoprotein mixtures, SP28-36 promotes the Ca2+-induced aggregation and surface activity of surfactant lipids, but the detailed interactions between Ca2+, SP28-36, and surfactant lipids have not been established. In this study, we investigated the effect of various cations on the aggregation of surfactant lipid liposomes in the presence of SP28-36. SP28-36 reduced the threshold ion concentration for liposome aggregation from greater than 10 to 0.5 mM for Ca2+, Ba2+, and Sr2+ but not Mg2+ or Mn2+. The liposome aggregation was reversed by ethylenediaminetetraacetic acid and not associated with leakage of carboxyfluorescein. SP28-36 promoted similar liposome aggregation at pH less than 5 in the absence of divalent cations. Surfactant lipids adsorbed slowly to an air-fluid interface in all ionic conditions unless SP28-36 was present. Both Ca2+ and H+ induced rapid lipid adsorption in the presence of SP28-36. The surface activity of native surfactant had a similar ion dependence. Electron micrographs of native surfactant showed typical tubular myelin structures at pH 7.4 only in the presence of Ca2+. At pH 4.4 in the absence of Ca2+, similar but not identical structures were seen. In the reconstituted system, SP28-36 in the presence of Ca2+ induced the formation of larger multilayered structures including parallel bilayers and small areas of squares and triangles with dimensions similar to structures found in the native material.(ABSTRACT TRUNCATED AT 250 WORDS)

Freeze fracture imaging and computer simulation of liposome structure: Membrane geometry and defects

At low weight fractions, many surfactant and biological amphiphiles form dispersions of lamellar liquid crystalline liposomes in water. Amphiphile molecules tend to align themselves in parallel bilayers which are free to bend. Bilayers must form closed surfaces to separate hydrophobic and hydrophilic domains completely. Continuum theory of liquid crystals requires that the constant spacing of bilayer surfaces be maintained except at singularities of no more than line extent. Maxwell demonstrated that only two types of closed surfaces can satisfy this constraint: concentric spheres and Dupin cyclides. Dupin cyclides (Figure 1) are parallel closed surfaces which have a conjugate ellipse (r1) and hyperbola (r2) as singularities in the bilayer spacing. Any straight line drawn from a point on the ellipse to a point on the hyperbola is normal to every surface it intersects (broken lines in Figure 1). A simple example, and limiting case, is a family of concentric tori (Figure 1b).To distinguish between the allowable arrangements, freeze fracture TEM micrographs of representative biological (L-α phosphotidylcholine: L-α PC) and surfactant (sodium heptylnonyl benzenesulfonate: SHBS)liposomes are compared to mathematically derived sections of Dupin cyclides and concentric spheres.

The three-dimensional aspect of the mammalian lung surfactant myelin figure

Rodent and primate lung surfactant was studied at the ultrastructural level utilizing procedures that retained most of the carbohydrates and lipids in thin section. The three-dimensional aspect of tubular myelin surfactant was observed to be four, lipid bilayer membranes oriented at right angles so that in cross-section it was square. In longitudinal section it appeared as two parallel lipid bilayers. Inside the tubular myelin was a homogeneous matrix material that completely filled the tubule except for a small, central area. A single multilamellar body, after it expanded and rearranged lamellae to form tubular myelin surfactant, still retained its basic morphology so that it was possible to determine the number and orientation of bodies that comprised a given surfactant area. This enabled quantification of surfactant by serial sectioning. Each transformed multilamellar body was observed to contain from 2 to 13 groups of tubular myelin, oriented at angles within the transformed body. With three-dimensional understanding, many of the areas previously reported to be homogeneous were determined to actually be oblique cross or longitudinal sections through tubular myelin surfactant. Five distinct layers characterized tubular myelin surfactant: (1) Unexpanded layer—up to 63 recently secreted multilamellar bodies. (2) Formation layerp̄aired lamellae expanding and rearranging to form tubules. (3) Mature layer—tubular myelin surfactant. (4) Air-surfactant interface layer—usually a single lipid bilayer which was the outermost layer of tubular myelin of from 1 to 12 transformed multilamellar bodies. (5) Degraded surfactant layer—lipid bilayer spheres were formed at the interface and degraded in the alveolar space.