Membrane Elastic Properties Research Articles

Atomistic–continuum model for probing the biomechanical properties of human erythrocyte membrane under extreme conditions

A precise first attempt is performed to quantify the biomechanical properties of human erythrocyte membrane subjects to extreme temperature and loading conditions. An improved three-dimensional (3D) atomistic–continuum model based on the Cauchy–Born rule is proposed to investigate the elastic properties and biomechanical responses of the erythrocyte membrane. A membrane rigidity model is developed to estimate the membrane elastic properties over an extreme temperature range. Our computational results reveal that the membrane is able to sustain large strains up to a certain limit; beyond which, mechanically induced hemolysis may occur as exponential stress increment, fluctuations and multiple peaks were observed in the stress–strain curves. Additionally, we found that the overall deformability of the erythrocyte membrane significantly decreases as temperature increases. It is concluded that the observed increase in membrane rigidity may be attributed to the denaturation, structural remodeling and cross-linking of membrane cytoskeletal proteins.

Highly curved image sensors: a practical approach for improved optical performance.

The significant optical and size benefits of using a curved focal surface for imaging systems have been well studied yet never brought to market for lack of a high-quality, mass-producible, curved image sensor. In this work we demonstrate that commercial silicon CMOS image sensors can be thinned and formed into accurate, highly curved optical surfaces with undiminished functionality. Our key development is a pneumatic forming process that avoids rigid mechanical constraints and suppresses wrinkling instabilities. A combination of forming-mold design, pressure membrane elastic properties, and controlled friction forces enables us to gradually contact the die at the corners and smoothly press the sensor into a spherical shape. Allowing the die to slide into the concave target shape enables a threefold increase in the spherical curvature over prior approaches having mechanical constraints that resist deformation, and create a high-stress, stretch-dominated state. Our process creates a bridge between the high precision and low-cost but planar CMOS process, and ideal non-planar component shapes such as spherical imagers for improved optical systems. We demonstrate these curved sensors in prototype cameras with custom lenses, measuring exceptional resolution of 3220 line-widths per picture height at an aperture of f/1.2 and nearly 100% relative illumination across the field. Though we use a 1/2.3" format image sensor in this report, we also show this process is generally compatible with many state of the art imaging sensor formats. By example, we report photogrammetry test data for an APS-C sized silicon die formed to a 30° subtended spherical angle. These gains in sharpness and relative illumination enable a new generation of ultra-high performance, manufacturable, digital imaging systems for scientific, industrial, and artistic use.

A three-dimensional quasicontinuum approach for predicting biomechanical properties of malaria-infected red blood cell membrane

• A multiscale model for malaria-infected RBC biomechanical properties is proposed. • Vital elastic properties and mechanical responses are predicted for the first time. • Elastic properties except Poisson's ratio rise significantly as parasite matures. • Mechanical responses rise exponentially resulting in semi-log stress–strain curves. • Elastic properties increase as temperature rises in trophozoite infection stage. This paper presents the first attempt to comprehensively estimate the elastic properties and mechanical responses of malaria-infected red blood cell (iRBC) membrane when subjected to uniaxial, shear and isotropic area-dilation loading conditions. With the three-dimensional (3D) quasicontinuum approach, we predicted the biomechanical properties of the iRBC membrane for all infection stages. Effect of temperature on the membrane elastic properties during the trophozoite stage was also examined. It is found that a multifold increase in the elastic properties of the iRBC membrane occurs as infection progresses. The axial, shear and area stiffnesses of the iRBC membrane increase exponentially, resulting in semi-logarithmic stress–strain relationship curves. In addition, the rigidity of the iRBC membrane in the trophozoite stage increases as temperature rise. It is concluded that Plasmodium falciparum parasites significantly affect the biomechanical properties of the RBC membrane due to the structural remodeling of the iRBC membrane microstructure.

Modeling Membrane Tubules with Lipid Droplets and Migrasomes

Membrane tubules of few tens of nanometer cross-sectional diameters and micron-scale lengths represent a basic structural component of intra-cellular organelles, such as endoplasmic reticulum and Golgi Complex, and emerge from plasma membranes in the course of cell crawling on extra-cellular matrices and substrates. Besides barrier functions, the tubular membranes serve as platforms for formation of peculiar cell organelles, Lipid Droplets and Migrasomes, whose properties are to be understood in terms of simple physics. Lipid Droplet can be regarded as lenses of hydrophobic substance (triacylglycerol, sterol esters) growing up between the two membrane leaflets into micron-large buds, which, possibly, detach from the membrane to form emulsion-like droplets. Migrasomes are micron-size spherical bodies discovered, recently, to form on membrane tethers pulled out of the cell body in the course of cell migration. We address the micromechanics of the two organelles to gain understanding of physics behind their formation and evolution. We analyze the shape and energy of a membrane tubule containing a lipid droplet in dependence of the droplet size and the elastic properties of the tubular membrane and the lipid monolayers covering the droplet surface. We determine the conditions of the droplet detachment from the tubule. We suggest a physical model of Migrasome as a rigid membrane domain, which swells within a membrane tubule into a sphere-like body as a result of interplay between the membrane bending rigidity and lateral tension. By computing the domain shapes for varying membrane elastic properties and the domain sizes and comparing the results with the available images, we suggest the criteria of Migrasome formation.

The Water to Solute Permeability Ratio Governs the Osmotic Volume Dynamics in Beetroot Vacuoles.

Plant cell vacuoles occupy up to 90% of the cell volume and, beyond their physiological function, are constantly subjected to water and solute exchange. The osmotic flow and vacuole volume dynamics relies on the vacuole membrane -the tonoplast- and its capacity to regulate its permeability to both water and solutes. The osmotic permeability coefficient (Pf) is the parameter that better characterizes the water transport when submitted to an osmotic gradient. Usually, Pf determinations are made in vitro from the initial rate of volume change, when a fast (almost instantaneous) osmolality change occurs. When aquaporins are present, it is accepted that initial volume changes are only due to water movements. However, in living cells osmotic changes are not necessarily abrupt but gradually imposed. Under these conditions, water flux might not be the only relevant driving force shaping the vacuole volume response. In this study, we quantitatively investigated volume dynamics of isolated Beta vulgaris root vacuoles under progressively applied osmotic gradients at different pH, a condition that modifies the tonoplast Pf. We followed the vacuole volume changes while simultaneously determining the external osmolality time-courses and analyzing these data with mathematical modeling. Our findings indicate that vacuole volume changes, under progressively applied osmotic gradients, would not depend on the membrane elastic properties, nor on the non-osmotic volume of the vacuole, but on water and solute fluxes across the tonoplast. We found that the volume of the vacuole at the steady state is determined by the ratio of water to solute permeabilites (Pf/Ps), which in turn is ruled by pH. The dependence of the permeability ratio on pH can be interpreted in terms of the degree of aquaporin inhibition and the consequently solute transport modulation. This is relevant in many plant organs such as root, leaves, cotyledons, or stems that perform extensive rhythmic growth movements, which very likely involve considerable cell volume changes within seconds to hours.

The Effect of Proteins and Lipids on Membrane Stiffness

The dynamical and elastic properties of membranes are important for a wide range of biological functions, such as endo- and exocytosis, autophagy and tabulation. The key elastic parameter of a membrane, as defined by Helfrich-Canham theory, is the bending modulus [1]. Proteins that can either sense regions with a low bending modulus or can locally reduce the bending modulus, leading to increased membrane curvature, such as BAR domains, have been extensively studied. Little is known, however, about what effect homeostatic integral membrane proteins, such as ion channels and pores, have on the elastic properties of membranes. For example the bending modulus is reduced when SERCA1a, a calcium ATPase, is inserted into GUVs [2]. Measuring the bending modulus experimentally is difficult. We therefore have simulated the dynamics of a range of large (50,000+ lipids) unary, binary and ternary membranes using a coarse-grained description, MARTINI. Our computer simulations suggest that adding integral membrane proteins (such as Kir2.2, Aqp0 or BtuB) at physiological concentrations reduces the bending rigidity of the membrane. Furthermore, increasing the proportion of cholesterol in the ternary lipid mixture increases the bending rigidity. Finally, we demonstrate that MARTINI describes the dynamics of the membranes by a careful comparison to a long atomistic simulation of 1,500 lipids.1. Brown FLH (2008) Elastic modeling of biomembranes and lipid bilayers. Ann Rev Phys Chem 59:6852. Girard P, Prost J, Bassereau P (2005) Passive or active fluctuations in membranes containing proteins Phys Rev Lett 94:2

Influence of Charge on the Elastic Properties of Lipid Membranes

Lipid membrane elastic properties play an important role in the membrane deformations and dynamic morphological transitions necessary for cell function. A key elastic property underlying these dynamics is the bending rigidity, motivating significant research efforts to quantify the effects of lipid structure and additives on the membrane bending modulus. To date, the majority of experimental research has focused on the dynamics of model membrane systems composed of zwitterionic lipids; however, most biomembranes are negatively charged at physiological conditions due to the presence of charged lipid headgroups. Here we study the bending dynamics of negatively-charged phosphatidylglycerol (PG) bilayers using neutron spin echo spectroscopy. Our results show that the charged lipid bilayers are softer than analogous zwitterionic phosphatidylcholine (PC) bilayers in both the gel and fluid phases at low ionic strength conditions. Interestingly, theoretical predictions indicate that the opposite should be true and that the bending rigidity should increase with increasing surface charge. We propose that this discrepancy is due to charged-related differences in the area per headgroup and lipid hydration between PG and PC bilayers that are not considered by existing theoretical models. Our results provide new insights into the influence of charge on the membrane elastic properties and demonstrate how these dynamics are coupled to charge effects on the membrane structure.

Influence of a Microwave Irradiation on the Swelling and Permeation Properties of a Nafion Membrane

The effect of a microwave irradiation at 2450 MHz on the swelling and permeation properties of a Nafion membrane in water and methanol media has been studied. The influence of the irradiation power and the exposure time has been analyzed. The results found show that the irradiation hardly affects the membrane liquid uptake, but it affects the expansion properties of the membrane. The hydraulic permeability coefficient of the unmodified and the irradiated membranes has been experimentally determined. Higher hydraulic permeability has been obtained for the irradiated membranes in both water and methanol, but the degree of increment in permeability coefficient with microwave depends on kinds of permeation liquid. The results have been discussed considering the degradation effect occurring on the membrane hydrophobic matrix, which affects to the membrane elastic properties.

Raft Formation in Lipid Bilayers Coupled to Curvature

We present computer simulations of a membrane in which the local composition is coupled to the local membrane curvature. At high temperatures (i.e., above the temperature of macroscopic phase separation), finite-sized transient domains are observed, reminiscent of lipid rafts. The domain size is in the range of hundred nanometers, and set by the membrane elastic properties. These findings are in line with the notion of the membrane as a curvature-induced microemulsion. At low temperature, the membrane phase separates. The transition to the phase-separated regime is continuous and belongs to the two-dimensional Ising universality class when the coupling to curvature is weak, but becomes first-order for strong curvature-composition coupling.

Buffers Affect the Bending Rigidity of Model Lipid Membranes

In biophysical and biochemical studies of lipid bilayers the influence of the used buffer is often ignored or assumed to be negligible on membrane structure, elasticity, or physical properties. However, we here present experimental evidence, through bending rigidity measurements performed on giant vesicles, of a more complex behavior, where the buffering molecules may considerably affect the bending rigidity of phosphatidylcholine bilayers. Furthermore, a synergistic effect on the bending modulus is observed in the presence of both salt and buffer molecules, which serves as a warning to experimentalists in the data interpretation of their studies, since typical lipid bilayer studies contain buffer and ion molecules.

Investigation of blood neutrophil structural and functional characteristics in patients with chronic obstructive pulmonary disease and pulmonary hypertension using atomic force microscopy

Aim. The aim of this study was to investigate structural and functional characteristics (morphology, membrane stiffness, adhesion) of blood neu trophils in patients with COPD and pulmonary hypertension (PH) using atomic force microscopy. Methods. This was a local open comparative study involving 30 in patients with severe COPD. Of them, 15 patients had PH and 15 patients had normal pulmonary artery pressure. Complete blood cell count was performed in the 1st day of admission. Pulse oxymetry, spirometry, echocardiographic examination were used in all patients. Neutrophils were separated from venous blood. Surfaces of 10–12 cells were scanned and lateral section curves were constructed to meas ure the cell height and maximal height of cytoplasmic granules. Nuclear and cell areas were also calculated using Pico Image Basic 5.1 software. The membrane elastic properties and adhesion strength of native neutrophils were evaluated using force spectrometry. Results. Neutrophil adhesion strength was significantly higher in patients with COPD + PH compared to this parameter in patients with COPD and normal pulmonary artery pressure. In patients with COPD + PH neutrophils had less nuclear segments and significantly higher cell phasic height, increased cytoplasm granularity adhesion strength and membrane stiffness. Conclusion. The results demonstrated that structural and functional changes in blood neutrophils were more significant in patients with COPD and PH.

Membrane elastic properties and cell function.

Recent studies indicate that the cell membrane, interacting with its attached cytoskeleton, is an important regulator of cell function, exerting and responding to forces. We investigate this relationship by looking for connections between cell membrane elastic properties, especially surface tension and bending modulus, and cell function. Those properties are measured by pulling tethers from the cell membrane with optical tweezers. Their values are determined for all major cell types of the central nervous system, as well as for macrophage. Astrocytes and glioblastoma cells, which are considerably more dynamic than neurons, have substantially larger surface tensions. Resting microglia, which continually scan their environment through motility and protrusions, have the highest elastic constants, with values similar to those for resting macrophage. For both microglia and macrophage, we find a sharp softening of bending modulus between their resting and activated forms, which is very advantageous for their acquisition of phagocytic functions upon activation. We also determine the elastic constants of pure cell membrane, with no attached cytoskeleton. For all cell types, the presence of F-actin within tethers, contrary to conventional wisdom, is confirmed. Our findings suggest the existence of a close connection between membrane elastic constants and cell function.

Cholesterol-Induced Buckling in Physisorbed Polymer-Tethered Lipid Monolayers

The influence of cholesterol concentration on the formation of buckling structures is studied in a physisorbed polymer-tethered lipid monolayer system using epifluorescence microscopy (EPI) and atomic force microscopy (AFM). The monolayer system, built using the Langmuir-Blodgett (LB) technique, consists of 3 mol % poly(ethylene glycol) (PEG) lipopolymers and various concentrations of the phospholipid, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and cholesterol (CHOL). In the absence of CHOL, AFM micrographs show only occasional buckling structures, which is caused by the presence of the lipopolymers in the monolayer. In contrast, a gradual increase of CHOL concentration in the range of 0–40 mol % leads to fascinating film stress relaxation phenomena in the form of enhanced membrane buckling. Buckling structures are moderately deficient in CHOL, but do not cause any notable phospholipid-lipopolymer phase separation. Our experiments demonstrate that membrane buckling in physisorbed polymer-tethered membranes can be controlled through CHOL-mediated adjustment of membrane elastic properties. They further show that CHOL may have a notable impact on molecular confinement in the presence of crowding agents, such as lipopolymers. Our results are significant, because they offer an intriguing prospective on the role of CHOL on the material properties in complex membrane architecture.

Human AQP1 Is a Constitutively Open Channel that Closes by a Membrane-Tension-Mediated Mechanism

This work presents experimental results combined with model-dependent predictions regarding the osmotic-permeability regulation of human aquaporin 1 (hAQP1) expressed in Xenopus oocyte membranes. Membrane elastic properties were studied under fully controlled conditions to obtain a function that relates internal volume and pressure. This function was used to design a model in which osmotic permeability could be studied as a pressure-dependent variable. The model states that hAQP1 closes with membrane-tension increments. It is important to emphasize that the only parameter of the model is the initial osmotic permeability coefficient, which was obtained by model-dependent fitting. The model was contrasted with experimental records from emptied-out Xenopus laevis oocytes expressing hAQP1. Simulated results reproduce and predict volume changes in high-water-permeability membranes under hypoosmotic gradients of different magnitude, as well as under consecutive hypo- and hyperosmotic conditions. In all cases, the simulated permeability coefficients are similar to experimental values. Predicted pressure, volume, and permeability changes indicate that hAQP1 water channels can transit from a high-water-permeability state to a closed state. This behavior is reversible and occurs in a cooperative manner among monomers. We conclude that hAQP1 is a constitutively open channel that closes mediated by membrane-tension increments.

Modulation of Elastic Properties through Unsaturated Lipid Content as a Mechanism for Inducing Resistance to Amphiphilic Antimicrobial Peptides

We explore whether changes in fatty acid composition in bacterial membrane models lead to inhibition of amphiphilic antimicrobial peptide-(AMPs) through the modulation of the membrane elastic properties. Specifically, we examined whether changes in the unsaturated/saturated fatty acid ratio, which is controlled by bacteria, inhibit AMP function. In phosphatidylglycerol (PG) lipid bilayers we observe a 10-fold increase in resistance to Magainin-2 (MAG) in POPG (16:0/18:1) vs. DPPG (16:0/16:0) bilayers and a 20-fold increase in resistance in DOPG (18:1/18:1) vs. DPPG bilayers, as measured by calcein release at 45 C (where lipids are in the liquid-crystalline phase). By analyzing the leakage kinetics, we find that the activation energy for pore formation in DPPG/POPG mixtures increases linearly with POPG content, confirming that unsaturation is energetically unfavorable for pore formation. Laurdan polarization (GP) measurements correlate well with MAG resistance, suggesting that head group spacing is an indicator of resistance (increased head-group spacing could allow for more peptides to partition into the head group region, instead of becoming inserted into the bilayer as bilayer-spanning monomers). The changes in potency cannot be explained solely by the increased head group spacing, however. We therefore performed leakage experiments using known modifiers of membrane elastic properties (Triton-X, capsaicin) that both reduce bilayer stiffness but cause opposite changes in curvature. Bilayer softening plays a role in decreasing MAG's potency, whereas the curvature alters only the leakage kinetics, not potency. Based on previous studies, we propose that membrane softening somehow makes it more difficult for MAG to reach a critical concentration necessary to produce pore formation⎯maybe because the energetics of lateral association among bilayer-spanning MAG monomers favors the monomers in softer bilayers. Unsaturated lipids may therefore influence MAG potency by modulating membrane stiffness.

Physisorbed Polymer-Tethered Lipid Bilayer with Lipopolymer Gradient

Physisorbed polymer-tethered lipid bilayers consisting of phospholipids and lipopolymers represent an attractive planar model membrane platform, in which bilayer fluidity and membrane elastic properties can be regulated through lipopolymer molar concentration. Herein we report a method for the fabrication of such a planar model membrane system with a lateral gradient of lipopolymer density. In addition, a procedure is described, which leads to a sharp boundary between regions of low and high lipopolymer molar concentrations. Resulting gradients and sharp boundaries are visualized on the basis of membrane buckling structures at elevated lipopolymer concentrations using epifluorescence microscopy and atomic force microscopy. Furthermore, results from spot photobleaching experiments are presented, which provide insight into the lipid lateral fluidity in these model membrane architectures. The presented experimental data highlight a planar, solid-supported membrane characterized by fascinating length scale-dependent dynamics and elastic properties with remarkable parallels to those observed in cellular membranes.

Characterisation of membrane elastic properties of capsules flowing in a square-section microfluidic channel

Characterisation of membrane elastic properties of capsules flowing in a square-section microfluidic channel

Nanoscale characterization of the biomechanical hardening of bovine zona pellucida

The zona pellucida (ZP) is an extracellular membrane surrounding mammalian oocytes. The so-called zona hardening plays a key role in fertilization process, as it blocks polyspermy, which may also be caused by an increase in the mechanical stiffness of the ZP membrane. However, structural reorganization mechanisms leading to ZP's biomechanical hardening are not fully understood yet. Furthermore, a correct estimate of the elastic properties of the ZP is still lacking. Therefore, the aim of the present study was to investigate the biomechanical behaviour of ZP membranes extracted from mature and fertilized bovine oocytes to better understand the mechanisms involved in the structural reorganization of the ZP that may lead to the biomechanical hardening of the ZP. For that purpose, a hybrid procedure is developed by combining atomic force microscopy nanoindentation measurements, nonlinear finite element analysis and nonlinear optimization. The proposed approach allows us to determine the biomechanical properties of the ZP more realistically than the classical analysis based on Hertz's contact theory, as it accounts for the nonlinearity of finite indentation process, hyperelastic behaviour and material heterogeneity. Experimental results show the presence of significant biomechanical hardening induced by the fertilization process. By comparing various hyperelastic constitutive models, it is found that the Arruda-Boyce eight-chain model best describes the biomechanical response of the ZP. Fertilization leads to an increase in the degree of heterogeneity of membrane elastic properties. The Young modulus changes sharply within a superficial layer whose thickness is related to the characteristic distance between cross-links in the ZP filamentous network. These findings support the hypothesis that biomechanical hardening of bovine ZP is caused by an increase in the number of inter-filaments cross-links whose density should be higher in the ZP inner side.

Influence of Particle Shape on Bending Rigidity of Colloidal Monolayer Membranes and Particle Deposition during Droplet Evaporation in Confined Geometries

We investigate the influence of particle shape on the bending rigidity of colloidal monolayer membranes (CMMs) and on evaporative processes associated with these membranes. Aqueous suspensions of colloidal particles are confined between glass plates and allowed to evaporate. Confinement creates ribbonlike air-water interfaces and facilitates measurement and characterization of CMM geometry during drying. Interestingly, interfacial buckling events occur during evaporation. Extension of the description of buckled elastic membranes to our quasi-2D geometry enables the determination of the ratio of CMM bending rigidity to its Young's modulus. Bending rigidity increases with increasing particle anisotropy, and particle deposition during evaporation is strongly affected by membrane elastic properties. During drying, spheres are deposited heterogeneously, but ellipsoids are not. Apparently, increased bending rigidity reduces contact line bending and pinning and induces uniform deposition of ellipsoids. Surprisingly, suspensions of spheres doped with a small number of ellipsoids are also deposited uniformly.

Effect of Hydroperoxides on Red Blood Cell Membrane Mechanical Properties

We investigate the effect of oxidative stress on red blood cell membrane mechanical properties in vitro using detailed analysis of the membrane thermal fluctuation spectrum. Two different oxidants, the cytosol-soluble hydrogen peroxide and the membrane-soluble cumene hydroperoxide, are used, and their effects on the membrane bending elastic modulus, surface tension, strength of confinement due to the membrane skeleton, and 2D shear elastic modulus are measured. We find that both oxidants alter significantly the membrane elastic properties, but their effects differ qualitatively and quantitatively. While hydrogen peroxide mainly affects the elasticity of the membrane protein skeleton (increasing the membrane shear modulus), cumene hydroperoxide has an impact on both membrane skeleton and lipid bilayer mechanical properties, as can be seen from the increased values of the shear and bending elastic moduli. The biologically important implication of these results is that the effects of oxidative stress on the biophysical properties, and hence the physiological functions, of the cell membrane depend on the nature of the oxidative agent. Thermal fluctuation spectroscopy provides a means of characterizing these different effects, potentially in a clinical milieu.