Tresset G (2009), The multiple faces of self-assembled lipidic sy...

ECB-ART-41108

PMC Biophys 2009 Apr 17;21:3. doi: 10.1186/1757-5036-2-3.

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The multiple faces of self-assembled lipidic systems.

Tresset G .

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Lipids, the building blocks of cells, common to every living organisms, have the propensity to self-assemble into well-defined structures over short and long-range spatial scales. The driving forces have their roots mainly in the hydrophobic effect and electrostatic interactions. Membranes in lamellar phase are ubiquitous in cellular compartments and can phase-separate upon mixing lipids in different liquid-crystalline states. Hexagonal phases and especially cubic phases can be synthesized and observed in vivo as well. Membrane often closes up into a vesicle whose shape is determined by the interplay of curvature, area difference elasticity and line tension energies, and can adopt the form of a sphere, a tube, a prolate, a starfish and many more. Complexes made of lipids and polyelectrolytes or inorganic materials exhibit a rich diversity of structural morphologies due to additional interactions which become increasingly hard to track without the aid of suitable computer models. From the plasma membrane of archaebacteria to gene delivery, self-assembled lipidic systems have left their mark in cell biology and nanobiotechnology; however, the underlying physics is yet to be fully unraveled.PACS Codes: 87.14.Cc, 82.70.Uv.

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Genes referenced: LOC115919910 LOC115925415 tefl tubgcp2

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	Figure 1. Lipid bilayer. (A) A flat membrane of lipids assembled into bilayer. (B) Chemical structure of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, a common phospholipid with p ~1 and which thereby forms flat bilayers. A schematic lipid molecule is depicted in (C), with the hydrophilic headgroup represented in red and the double hydrophobic alkyl chain in grey. The molecule fits to a cylinder making its packing parameter p close to 1. (D) Lamellar phase Lα of fluid lipid bilayers.
	Figure 2. Three-dimensional atomic force microscopy image of raft microdomains. A binary mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (black) and sphingomyelin (orange) forming a bilayer is immobilized on a mica substrate and exhibits a lipid phase separation. The height of the raft microdomains is ~7 Å. The yellow peaks correspond to a glycosylphosphatidylinositol-anchored protein which is located preferentially in the raft microdomains. The width of the scan is ~2 μm. Adapted from reference [52]. Used with permission.
	Figure 3. Illustration of hexagonal lipid phases. Inverted hexagonal (HII) and micellar hexagonal (HI) phases. The lipids are represented with the same conventions as on Figure 1, the hydrophilic headgroup in red and the hydrocarbon chains in grey.
	Figure 4. Schematic structures of lipid cubic phases. Ia3d, Pn3m and Im3m are the inverse bicontinuous cubic phases reported so far experimentally. Fd3m is an inverse micellar cubic phase found with mixtures of DOPC and glycerol. The two types of inverse micelle (open and grey spheres) with their polyhedral shapes are indicated on each site of the cubic lattice. Adapted from reference [73]. Reproduced by permission of the PCCP Owner Societies.
	Figure 5. Cryo-Transmission Electron Microscopy micrographs of non-lamellar lipid nanoparticles. Inverse bicontinuous cubic phase nanoparticles viewed along the [001] (A) and [111] (B) directions. The Fourier transforms of magnified areas shown in the right-lower inserts of each micrographs are consistent with a body centered cubic phase Im3m. The nanoparticles are made up of a dispersion of glycerol monooleate (GMO), surfactants and polymeric stabilizers. (C) Sponge phase L3 nanoparticles containing diglycerol monooleate (DGMO) and glycerol dioleate (GDO). (D) Inverted hexagonal HII nanoparticles also based on DGMO and GDO but with a different fraction of stabilizer. Adapted from reference [75] with permission. Copyright 2005 by the American Chemical Society.
	Figure 6. Liposome morphologies. Liposomes are presented with respect to the shape, size and number of bilayers. SUV = small unilamellar vesicle, LUV = large unilamellar vesicle, MLV = multilamellar vesicle, GUV = giant unilamellar vesicle, OVV = oligovesicular vesicle. The short-range structure of the lipid bilayers is shown in the magnified view. The GUV on the fluorescence picture is made of a mixture of phospholipids and fluorescent dye, and contains three red-fluorescent 200-nm polystyrene spheres which can move freely within the vesicle. The scale bar is 5 μm. Adapted from reference [99] with permission. Copyright 2005 by the American Chemical Society.
	Figure 7. Exotic vesicles. (A) Prolate, (B) stomatocyte, and (C) starfish vesicles made of a ternary mixture of saturated and unsaturated phospholipids and cholesterol, in presence of ~1 mM of sorbitol at 60°C. Reprinted figure with permission from reference [114]. Copyright 2008 by the American Physical Society. DOI: 10.1103/PhysRevLett.100.148102 (D) Three-dimensional confocal microscopy image of a giant vesicle labeled with two distinct fluorescent dyes staining the liquid-ordered and liquid-disordered phases of a ternary mixture of lipids. Reproduced from reference [123] by permission of the PCCP Owner Societies.
	Figure 8. Transmission electron micrograph of cochleates. Transmission electron micrograph after freeze-fracture of cochleate cylinders prepared from anionic phosphatidylserine (PS) and calcium ions. The layered structure is clearly visible. The scale bar is 200 nm. Reproduced from reference [138] with permission. Copyright 2002 by Elsevier Science.
	Figure 9. Morphologies of glycolipid nanotubes. Transmission electron microscope images of (A) twisted, (B) coiled, and (C) tubular one-dimensional assemblies of glycolipids. Reproduced from reference [128] with permission. Copyright 2005 by the American Chemical Society. (D) Schematic cross-section of the interdigitated lamellar layers of single hydrocarbon chain glycolipids in the nanotube membrane. The glycolipid headgroup is represented in red and the hydrocarbon chain in grey.
	Figure 10. Lipoplex phases. Schematic representations of the phases of lipid-DNA complexes: complexed lamellar , complexed inverted hexagonal , and complexed micellar hexagonal . The lipids are depicted in red (headgroup) and grey (hydrocarbon chain), while the DNA rods are in blue.
	Figure 11. Lipid-microtubule nanotube. Complexed nanotube consisting of a microtubule (~26 nm in diameter) made of tubulin protein subunits (red-blue-yellow-green spheres) coated by a cationic lipid bilayer (headgroups in green and white, hydrocarbon tails in yellow). A third layer of tubulin oligomers actually wraps up the bilayer in the plane perpendicular to the nanotube axis. The arrangement is deduced from x-ray data and transmission electron microscopy images. Reproduced from reference [175] with permission. Copyright 2007 by the Biophysical Society.
	Figure 12. Lipid bilayers templated on carbon nanotubes. Transmission electron microscopy image of self-assembled single-chained lysophospholipids on single-walled carbon nanotubes. The assemblies display a striation periodicity of ~4.5 nm. The scale bar is 15 nm. Reproduced from reference [189] with permission. Copyright 2006 by the American Chemical Society.
	Figure 13. Computer simulation of a self-assembled lipidic complex. This water-free Monte Carlo simulation represents a hexagonal complex of zwitterionic lipids (headgroups in red and hydrocarbon chains in grey), divalent cations (yellow spheres) and DNA rods (blue). The DNA rods are maintained fixed on a hexagonal lattice over the simulation. Lipids and cations are randomly distributed at the initial stage (inset). Reproduced from reference [165] with permission. Copyright 2007 by the American Chemical Society.
	Figure 14. Overview of the structures formed by self-assembled lipidic systems. The structures described in this mini-review are summarized along with the physical factors enabling to get from one structural category to another. p denotes the lipid packing parameter, c0 and κ are the spontaneous curvature and the mean curvature modulus of membranes, Tc is the critical temperature of lipid phase transition, ΔΠ is the difference of osmotic pressure between the interior of vesicles and the bulk solution, and CM stands for the local curvature of a material to associate. Each structural category is illustrated by a schematic of a typical lipidic structure: inverted hexagonal phase HII, fluid and gel lamellar phases Lα and Lβ, raft microdomains, liposomes and starfish vesicles, complexed inverted hexagonal phase , lipid vesicles on a surface, and lipid-coated carbon nanotubes.

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