Lipid Cylinders Research Articles

Structural Properties of Inverted Hexagonal Phase: A Hybrid Computational and Experimental Approach.

Inverted/reverse hexagonal (HII) phases are of special interest in several fields of research, including nanomedicine. We used molecular dynamics (MD) simulation to study HII systems composed of dioleoylphosphatidylethanolamine (DOPE) and palmitoyloleoylphosphatidylethanolamine (POPE) at several hydration levels and temperatures. The effect of the hydration level on several HII structural parameters, including deuterium order parameters, was investigated. We further used MD simulations to estimate the maximum hydrations of DOPE and POPE HII lattices at several given temperatures. Finally, the effect of acyl chain unsaturation degree on the HII structure was studied via comparing the DOPE with POPE HII systems. In addition to MD simulations, we used deuterium nuclear magnetic resonance (2H NMR) and small-angle X-ray scattering (SAXS) experiments to measure the DOPE acyl chain order parameters, lattice plane distances, and the water core radius in HII phase DOPE samples at several temperatures in the presence of excess water. Structural parameters calculated from MD simulations are in excellent agreement with the experimental data. Dehydration decreases the radius of the water core. An increase in hydration level slightly increased the deuterium order parameter of lipids acyl chains, whereas an increase in temperature decreased it. Lipid cylinders undulated along the cylinder axis as a function of hydration level. The maximum hydration levels of PE HII phases at different temperatures were successfully predicted by MD simulations based on a single experimental measurement for the lattice plane distance in the presence of excess water. An increase in temperature decreases the maximum hydration and consequently the radius of the water core and lattice plane distances. Finally, DOPE formed HII structures with a higher curvature compared to POPE, as expected. We propose a general protocol for constructing computational HII systems that correspond to the experimental systems. This protocol could be used to study HII systems composed of molecules other than the PE systems used here and to improve and validate force field parameters by using the target data in the HII phase.

Inverse hexagonal phase of poly-unsaturated monogalactolipid: A computer model and analysis

Monogalactosylodiacylglycerol (MGDG) is the main lipid component of thylakoid membranes. In water, MGDG molecules pack preferentially in the hexagonal HII phase, which consists of water-filled lipid cylinders arranged in a hexagonal lattice. The HII phase can also form in the thylakoid membrane where it plays a key role in the photoprotection of the photosynthetic apparatus. In this study an all-atom computer model of the 16-cylinder MGDG HII phase was generated in a 3-μs molecular dynamics simulation and mathematical tools were developed to analyse its structural properties. The model-predicted values of the basic structural parameters, namely hexagonal spacing, circular and effective radii of the water channel, surface area/MGDG, and order parameters for the MGDG acyl chains, were compared with the experimental data available in the literature to validate the model. A stable computer model of the MGDG HII phase and the tools developed may serve further numerical experiments and investigations into the role of this phase in biological processes.

Fully deuterated magnetically oriented system based on fatty acid direct hexagonal phases.

There is strong demand in the field of NMR for simple oriented lipid supramolecular assemblies, the constituents of which can be fully deuterated, for specifically studying the structure of host protonated molecules (e.g., peptides, proteins...) in a lipid environment. Also, small-angle neutron scattering (SANS) in fully deuterated oriented systems is powerful for gaining information on protonated host molecules in a lipid environment by using the contrast proton/deuterium method. Here we report on a very simple system made of fatty acids (dodecanoic and tetradecanoic) and ethanolamine in water. All components of this system can be obtained commercially as perdeuterated. Depending on the molar ratio and the concentration, the system self-assembles at room temperature into a direct hexagonal phase that is oriented by moderate magnetic fields of a few tesla. The orientation occurs within the magnetic field upon cooling the system from its higher-temperature isotropic phase: the lipid cylinders of the hexagonal phase become oriented parallel to the field. This is shown by solid-state NMR using either perdeuterated fatty acids or ethanolamine. This system bears strong interest for studying host protonated molecules but also in materials chemistry for building oriented solid materials.

Synthesis of Stable Hybrid Silica−Lipid Cylinders with Nanoscale Helical Ripples

Self-assembled cylindrical lipid tubules have attracted attention as templates for synthesizing hybrid organic−inorganic materials. Here we report the synthesis of hybrid silica−lipid cylinders by sol−gel condensation of tetraethoxysilane (TEOS) on helical rippled tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC8,9PC). Transmission electron microscopy reveals that the nanoscale helical ripples of the lipid tubules are transcripted to the templated silica films. On the other hand, the deposition of the silica films substantially improves the thermal stability of lipid tubules. We find that the crystalline ordering of lipid bilayer walls confined in the hybrid silica−lipid cylinders is stable up to 110 °C.

Towards controlled release of BDNF — Manufacturing strategies for protein-loaded lipid implants and biocompatibility evaluation in the brain

It was the aim of this study to establish triglyceride matrices as potential carriers for long-term release of brain-derived neurotrophic factor (BDNF), a potential therapeutic for Huntington's disease. First, four different manufacturing strategies were investigated with lysozyme as a model substance: either lyophilized protein was mixed with lipid powder, or suspended in organic solution thereof (s/o). Or else, an aqueous protein solution was dispersed by w/o emulsion in organic lipid solution. Alternatively, a PEG co-lyophilization was performed prior to dispersing solid protein microparticles in organic lipid solution. After removal of the solvent(s), the resulting powder formulations were compressed at 250 N to form mini-cylinders of 2 mm diameter, 2.2 mm height and 7 mg weight. Protein integrity after formulation and release was evaluated from an enzyme activity assay and SDS-PAGE. Confocal microscopy revealed that the resulting distribution of FITC-lysozyme within the matrices depended strongly on the manufacturing method, which had an important impact on matrix performance: matrices with a very fine and homogeneous protein distribution (PEG co-lyophilization) continually released protein for 2 months. The other methods did not guarantee a homogeneous distribution and either failed in sustaining release for more than 1 week (powder mixture), completely liberating the loading (s/o dispersion) or preserving protein activity during manufacturing (w/o emulsion, formation of aggregates and 25% activity loss). Based on these results, miniature-sized implants of 1 mm diameter, 0.8 mm height and 1 mg weight were successfully loaded by the PEG co-lyophilization method with 2% BDNF and 2% PEG. Release studies in phosphate buffer pH 7.4 at 4 and 37 °C revealed a controlled release of either 20 or 60% intact protein over one month as determined by ELISA. SDS-PAGE detected only minor aggregates in the matrix during release at higher temperature. In vivo evaluation of lipid cylinders in the striatum of rat brains revealed a biocompatibility comparable to silicone reference cylinders.

Chiral Self-Assembly of Nanotubules and Ribbons from Phospholipid Mixtures

Nanoscale structures have been found to self-assemble in a binary mixture of a long-chain, diacetylenic phospholipid and a short chain, saturated lipid. We observe unusual chiral-optical signatures of nanotubule, twisted ribbon, and microtubule morphologies that can be used to monitor their temporal and thermal stability. Circular dichroism results suggest that chiral packing of the lipids drives formation of these aggregates. The ease of formation, stability, and robust behavior of these lipid cylinders suggest that they have potential use in applications requiring high aspect ratio nanomaterials.

Structural organisation and lipid composition of the epicuticular accessory layer of infective larvae of Trichinella spiralis

The epicuticle of infective larvae of Trichinella spiralis represents the interface between this intracellular nematode parasite and the cytosol of mammalian skeletal muscle. The macromolecular structures that make up the epicuticle were studied by freeze-fracture electron microscopy and compositional analysis. Three fracture planes were observed: one with a typical plasma membrane-type bilayer organisation which was overlaid by two extended layers of lipid in an inverted cylindrical configuration. This overall structure remained unchanged in response to variations in temperature between 20°C and 45°C. The lipid cylinders were on average 6.8 nm in diameter, with randomly-associated particles that were not dissociated by high-salt treatment, indicative of hydrophobically associated proteins. The majority of the lipids were non-polar, consisting of cholesterol, cholesterol esters, mono- and tri-glycerides, and free fatty acids. Three major classes of phospholipids were identified: phosphatidylethanolamine, phosphatidylglycerol and phosphatidylcholine. Total lipid extracts did not adopt an inverted cylindrical or micellar configuration on isolation, but formed flat sheets of lamellae as did the purified polar and non-polar fractions of the lipids. Isolated lipids did not undergo thermally-induced polymorphism between 20°C and 60°C and there was no pH dependency of the structures adopted. The fatty acid saturation levels of the phospholipids were compatible with the observation that they did not form polymorphic structures on isolation. We suggest that this unusual configuration is probably stabilised by the associated (glyco)proteins and may be required for selective permeation of nutrients from the host cell cytosol and/or for maintaining the high curvature of the parasite within the cell.

Phase transition from a gel to a fluid phase of cubic symmetry in dimyristoylphosphatidylcholine/myristic acid (1:2, mol/mol) bilayers

Aqueous dispersions (pH 4.0) of a 2:1 (mol/mol) mixture of myristic acid with dimyristoylphosphatidylcholine undergo a sharp transition at 45–47°C from a lamellar gel phase to a fluid phase which is optically isotropic. This fluid phase gives rise to 31P-NMR spectra, and 2H-NMR spectra of the chain-deuterated components, which are also isotropic. X-ray diffraction studies of the fluid phase at 49°C, reveal reflections with spacings in the ratio √2: (√3): √4: √6: √8, accompanied by a strong diffuse scatter. These reflections index on a cubic lattice of primitive space group Pn3 or Pn3m, or possibly the body-centered group Im3m, with a lattice constant of 21.2 nm. The dimensions of the phase are consistent with a structure composed of two systems of tetrahedrally (octahedrally) oriented inverted lipid cylinders, found for other cubic lipid phases with Pn3m (Im3m) symmetry. At higher temperatures the cubic phase gradually converts, with increasing temperature, to a coexisting inverted hexagonal phase.

Paracrystalline inclusions associated with the disk membranes of frog retinal rod outer segments

Freeze-fracture replicas were prepared from freshly isolated, ultra-rapidly frozen frog retinal rod outer segments (ROSs), which were neither fixed nor exposed to antifreeze agents prior to freezing. Embedded within the lamellar array of ROS disk membranes, a series of rare paracrystalline inclusions were observed which displayed both irregular and cylindrical geometries. These inclusions appear to be formed from elongated lipid cylinders similar to those present in hexagonal (H II) lipid phases. In thin sections through the ROS, corresponding images have not been detected. However, lacunae with similar shapes and dimensions have been found. We believe these paracrystalline inclusions provide the first case in which contiguous bilayer and non-bilayer organizations of lipid have been visualized within a naturally occurring membrane system. The inclusions illustrate a unique and unpredicted mode of structural interaction between several adjacent ROS disks. The absence of freeze-fracture particles within such inclusions further suggests that only the lipid phases of these disks may be structurally continuous. Although the functional significance of these inclusions is unknown, they may have relevance to the enhanced rates of lipid redistribution and turnover in ROSs, compared to those of disk membrane proteins.

Cochleate lipid cylinders: formation by fusion of unilamellar lipid vesicles

Freeze-fracture electron microscopy was used to study the morphological changes occurring following the addition of Ca 2+ to sonicated preparations of phosphatidylserine in aqueous NaCl buffer. Before the addition of Ca 2+, preparations contained only small (200–500 Å diameter) spheroidal vesicles. After the addition of Ca 2+ (10 mM) and incubation for 1 h at 37°C preparations contained only large (2000–10 000 Å) apparently multilamellar structures many of which were cylindrical in shape. The lamellae in these cylinders appear to be folded in a spiral configuration. Addition of EDTA to these preparations produced large, closed, spherical, unilamellar vesicles. We suggest the name cochleate lipid cylinders for the spiral structures and propose that they are formed by fusion of unilamellar vesicles into large sheets which fold spirally to form cylinders.

Structure des phases liquide-cristallines de différents phospholipides, monoglycérides, sphingolipides, anhydres ou en présence d'eau

X-Ray diffraction studies have been carried out on some lipids of various chemical compositions: two monoglycerides (glyceryl-monodecanoate and glyceryl-monododecanoate), several phospholipids (synthetic di-palmitoyllecithin, egg lecithin, lysolecithin derived from the latter, egg phosphatidylethanolamine) and sphingolipids (sphingomyelin and cerebrosides isolated from beef brain) have been studied. The structure of these lipid-water systems depends on concentration and temperature: crystalline phases are formed at low temperatures, whereas higher temperatures and high lipid content promote the formation of liquid-crystalline phases. At low lipid content and high temperature, only lysolecithin forms a micellar solution in water; for the other lipids, a suspension of a liquid-crystalline phase in excess water is observed. Four liquid-crystalline phases, already described in other soap or lipid systems, have been observed here. 1. (a) A lamellar phase occurs in the following systems: water-monoglyceride (monodecanoate: 25 °C; monododecanoate: 45 °C); water-sphingomyelin (45 °C); water-cerebrosides (72 °C); water-phosphatidylethanolamine (20 °C); water-egg lecithin (5 to 35 °C) and anhydrous lecithin at high temperature. 2. (b) A hexagonal phase, made of a regular array of lipid cylinders, is observed in the water-lysolecithin system at 35 °C. 3. (c) An inverted hexagonal phase (cylinders made of water) occurs in the water-phosphatidylethanolamine system at 55 °C. 4. (d) A cubic phase is observed in anhydrous lecithin at high temperature. The structural parameters of these phases and the polymorphism of the various lipids are discussed.

Hexagonal Patterns in Cell Membranes

Dourmashkin, Dougherty and Harris1 demonstrated a hexagonal pattern on some viral and mammalian membranes treated with saponin. They took the view that the pattern was revealed by the removal of certain lipid components and suggested that their observation supported the mosaic structure proposed for cell membranes by Parpart and Ballentine2 in which lipid cylinders, 90 A in diameter and spaced at 150 A centre to centre, are enclosed in a protein mesh. Muir3 has produced the same effect in the plasma membrane of rat intestinal epithelium by in vivo treatment with saponin.