Alzheimer's disease (AD) is a neurodegenerative disease that results in memory loss and the impairment of cognitive skills. Several mechanisms of AD's pathogenesis were proposed, such as the ...progressive accumulation of amyloid-β (Aβ) and τ pathology. Nevertheless, the exact neurodegenerative mechanism of the Aβ remains complex and not fully understood. This paper proposes an alternative hypothesis of the mechanism based on maintaining the neuron membrane's mechanical balance. The incorporation of Aβ decreases the lipid membrane's elastic properties, which eventually leads to the impairment of membrane clustering, disruption of mechanical wave propagation, and change in gamma oscillations. The first two disrupt the neuron's ability to function correctly while the last one decreases sensory encoding and perception enabling. To begin discussing this mechanical-balance hypothesis, we measured the effect of two selected peptides, Aβ-40 and Aβ-42, as well as their fluorescently labeled modification, on membrane mechanical properties. The decrease of bending rigidity, consistent for all investigated peptides, was observed using molecular dynamic studies and experimental flicker-noise techniques. Additionally, wave propagation was investigated with molecular dynamic studies in membranes with and without incorporated neurodegenerative peptides. A change in membrane behavior was observed in the membrane system with incorporated Aβ.
Here, we describe how to extract tethers or lipid membrane nanotubes from the plasma membrane of cells using optical tweezers. This technique allows measuring the force required to hold the membrane ...tether at a constant length, which is related to the cell membrane tension. Following the evolution of this force during mechanical or chemical perturbations of the cell gives insight about the regulation of cell membrane tension. By pulling very long membrane tethers, one can also probe the membrane reservoir of a cell and a sudden rise in the tether force is usually due to the depletion of excess membranes stored in membrane folds or invaginations.
Recent experimental evidence points to the possibility that cell surface-associated caveolae may participate in mechanotransduction. The particular shape of caveolae suggests that these structures ...serve to prevent exposure of putative mechanosensors residing within these membrane invaginations to shear stresses at magnitudes associated with initiation of cell signaling. Accordingly, we numerically analyzed the fluid flow in and around caveolae using the equation of motion for flow of plasma at low Reynolds numbers and assuming no slip-condition on the membrane. The plasma velocity inside a typical caveola and the shear stress acting on its membrane are markedly reduced compared to the outside membrane. Computation of the diffusion field in the vicinity of a caveola under flow, however, revealed a rapid equilibration of agonist concentration in the fluid inside a caveola with the outside plasma. Western blots and immunocytochemistry support the role of caveolae as shear stress shelters for putative membrane-bound mechanoreceptors such as flk-1. Our results, therefore, suggest that caveolae serve to reduce the fluid shear stress acting on receptors in their interior, while allowing rapid diffusion of ligands into the interior. This mechanism may permit differential control of flow and ligand activation of flk-1 receptor in the presence of ligands.
•Fucosterol and desmosterol support liquid-ordered membrane phases.•Fucosterol and desmosterol induce liquid-ordered and liquid-disordered coexistence.•Acyl-chain order in fluid membranes: ...cholesterol > ergosterol > desmosterol > fucosterol.•Ordering capacity reflect different molecular structure of the sterols at the hydrocarbon tail.
Higher sterols are universally present in large amounts (20–30%) in the plasma membranes of all eukaryotes whereas they are universally absent in prokaryotes. It is remarkable that each kingdom of the eukaryotes has chosen, during the course of evolution, its preferred sterol: cholesterol in animals, ergosterol in fungi and yeast, phytosterols in higher plants, and e.g., fucosterol and desmosterol in algae. The question arises as to which specific properties do sterols impart to membranes and to which extent do these properties differ among the different sterols. Using a range of biophysical techniques, including calorimetry, fluorescence microscopy, vesicle-fluctuation analysis, and atomic force microscopy, we have found that fucosterol and desmosterol, found in red and brown macroalgae (seaweeds), similar to cholesterol support liquid-ordered membrane phases and induce coexistence between liquid-ordered and liquid-disordered domains in lipid bilayers. Fucosterol and desmosterol induce acyl-chain order in liquid membranes, but less effectively than cholesterol and ergosterol in the order: cholesterol>ergosterol>desmosterol>fucosterol, possibly reflecting the different molecular structure of the sterols at the hydrocarbon tail.
Lipid vesicles are valuable mesoscale molecular confinement vessels for studying membrane mechanics and lipid–protein interactions, and they have found utility among bio-inspired technologies, ...including drug delivery vehicles. While vesicle morphology can be modified by changing the lipid composition and introducing fusion or pore-forming proteins and detergents, the influence of extramembrane crowding on vesicle morphology has remained under-explored owing to a lack of experimental tools capable of capturing morphological changes on the nanoscale. Here, we use biocompatible polymers to simulate molecular crowding
in vitro
, and through combinations of FRET spectroscopy, lifetime analysis, dynamic light scattering, and single-vesicle imaging, we characterize how crowding regulates vesicle morphology. We show that both freely diffusing and surface-tethered vesicles fluorescently tagged with the DiI and DiD FRET pair undergo compaction in response to modest concentrations of sorbitol, polyethylene glycol, and Ficoll. A striking observation is that sorbitol results in irreversible compaction, whereas the influence of high molecular weight PEG-based crowders was found to be reversible. Regulation of molecular crowding allows for precise control of the vesicle architecture
in vitro
, with vast implications for drug delivery and vesicle trafficking systems. Furthermore, our observations of vesicle compaction may also serve to act as a mechanosensitive readout of extramembrane crowding.
•Provides a historical perspective for micropipet aspiration/fluorescence microscopy techniques applied to giant vesicles.•Presents and discusses some relevant contributions to our understanding of ...lipid bilayer membranes.•Outlines studies that would utilize both techniques simultaneously on the same vesicle to the level of a single membrane.
This manuscript discusses basic methodological aspects of optical microscopy and micromanipulation methods to study membranes and reviews methods to generate giant unilamellar vesicles (GUVs). In particular, we focus on the use of fluorescence microscopy and micropipet manipulation techniques to study composition–structure–property materials relationships of free-standing lipid bilayer membranes. Because their size (∼5–100μm diameter) that is well above the resolution limit of regular light microscopes, GUVs are suitable membrane models for optical microscopy and micromanipulation experimentation. For instance, using different fluorescent reporters, fluorescence microscopy allows strategies to study membrane lateral structure/dynamics at the level of single vesicles of diverse compositions. The micropipet manipulation technique on the other hand, uses Hoffman modulation contrast microscopy and allows studies on the mechanical, thermal, molecular exchange and adhesive-interactive properties of compositionally different membranes under controlled environmental conditions. The goal of this review is to (i) provide a historical perspective for both techniques; (ii) present and discuss some of their most important contributions to our understanding of lipid bilayer membranes; and (iii) outline studies that would utilize both techniques simultaneously on the same vesicle thus bringing the ability to characterize structure and strain responses together with the direct application of well-defined stresses to a single membrane or observe the effects of adhesive spreading. Knowledge gained by these studies has informed several applications of lipid membranes including their use as lung surfactants and drug delivery systems for cancer.
Correctly formulated continuum models for lipid-bilayer membranes present a significant challenge to computational mechanics. In particular, the mid-surface behavior is that of a 2-dimensional fluid, ...while the membrane resists bending much like an elastic shell. Here we consider a well-known “Helfrich–Cahn–Hilliard” model for two-phase lipid-bilayer vesicles, incorporating mid-surface fluidity, curvature elasticity and a phase field. We present a systematic approach to the direct computation of vesicle configurations possessing icosahedral symmetry, which have been observed in experiment and whose mathematical existence has recently been established. We first introduce a radial-graph formulation to overcome the difficulties associated with fluidity within a conventional Lagrangian description. We use the so-called subdivision surface finite element method combined with an icosahedral-symmetric mesh. The resulting discrete equations are well-conditioned and inherit equivariance properties under a representation of the icosahedral group. We use group-theoretic methods to obtain a reduced problem that captures all icosahedral-symmetric solutions of the full problem. Finally we explore the behavior of our reduced model, varying numerous physical parameters present in the mathematical model.
•A systematic computational framework for capturing icosahedron symmetric equilibria of lipid bilayer vesicles is proposed.•Subdivision surface finite element method and symmetry reduction method are at the core of our framework.•Parameter spaces are explored using numerical bifurcation/continuation methods.
Composite membranes composed of highly conductive and selective layer-by-layer (LbL) films and electrospun fiber mats were fabricated and characterized for mechanical strength and electrochemical ...selectivity. The LbL component consists of a proton-conducting, methanol-blocking poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (PDAC/sPPO) thin film. The electrospun fiber component consists of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) fibers in a nonwoven mat of 60–90% porosity. The bare mats were annealed to improve their mechanical properties, which improvements are shown to be retained in the composite membranes. Spray LbL assembly was used as a means for the rapid formation of proton-conducting films that fill the void space throughout the porous electrospun matrix and create a fuel-blocking layer. Coated mats as thin as 15 μm were fabricated, and viable composite membranes with methanol permeabilities 20 times lower than Nafion and through-plane proton selectivity five and a half times greater than Nafion are demonstrated. The mechanical properties of the spray coated electrospun mats are shown to be superior to the LbL-only system and possess intrinsically greater dimensional stability and lower mechanical hysteresis than Nafion under hydrated conditions. The composite proton exchange membranes fabricated here were tested in an operational direct methanol fuel cell. The results show the potential for higher open circuit voltages (OCV) and comparable cell resistances when compared to fuel cells based on Nafion.
Membrane proteins are essential for the diverse biological functions of the cells and intercellular communication in living organisms. With the recent developments in the methodologies, the research ...on membrane proteins has been undergoing a major transformation. In this informative book, the biological and dynamic behaviour of membrane proteins are introduced, discussed, and reviewed by some of the leading researchers in the field. The main objective of this compendium is to present the recent research in the fundamental and advanced concepts and methodologies used for studying membrane proteins. Membrane protein purification and reconstitution, protein–lipid interaction, ion/substrate transport, conformational and functional dynamics, the interaction of infectious agents, cell death, and organelle morphology are among the topics that are covered. This reprint is intended for a broad range of novice and experienced scientists with different levels of experience, from biophysicists and biochemists to microbiologists, cell biologists, and physiologists.
Sterilisation and preservation of vesicle formulations are important considerations for their viable manufacture for industry applications, particular those intended for medicinal use. Here, we ...undertake an initial investigation of the stability of hybrid lipid-block copolymer vesicles to common sterilisation and preservation processes, with particular interest in how the block copolymer component might tune vesicle stability. We investigate two sizes of polybutadiene-
-poly(ethylene oxide) polymers (PBd
-PEO
and PBd
-PEO
) mixed with the phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) considering the encapsulation stability of a fluorescent cargo and the colloidal stability of vesicle size distributions. We find that autoclaving and lyophilisation cause complete loss of encapsulation stability under the conditions studied here. Filtering through 200 nm pores appears to be viable for sterilisation for all vesicle compositions with comparatively low release of encapsulated cargo, even for vesicle size distributions which extend beyond the 200 nm filter pore size. Freeze-thaw of vesicles also shows promise for the preservation of hybrid vesicles with high block copolymer content. We discuss the process stability of hybrid vesicles in terms of the complex mechanical interplay between bending resistance, stretching elasticity and lysis strain of these membranes and propose strategies for future work to further enhance the process stability of these vesicle formulations.
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