Effect of hydroperoxides on red blood cell membrane mechanical properties

Copyright © 2011 Biophysical SocietyWe 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

Ferromagnetic microswimmers

Copyright © 2008 The American Physical SocietyWe propose a model for a novel artificial low Reynolds number swimmer, based on the magnetic interactions of a pair of ferromagnetic particles: one with hard and the other with soft magnetic properties, connected by a linear spring. Using a computational model, we analyze the behavior of the system and demonstrate that for realistic values of the parameters involved, the swimmer is capable of self-propelling with average speeds of the order of hundreds of micrometers per second

The micromechanics of the superficial zone of articular cartilage.

Journal ArticleOBJECTIVE: To investigate the relationships between the unique mechanical and structural properties of the superficial zone of articular cartilage on the microscopic scale. DESIGN: Fresh unstained equine metacarpophalangeal cartilage samples were mounted on tensile and compressive loading rigs on the stage of a multiphoton microscope. Sequential image stacks were acquired under incremental loads together with simultaneous measurements of the applied stress and strain. Second harmonic generation was used to visualise the collagen fibre network, while two photon fluorescence was used to visualise elastin fibres and cells. The changes visualised by each modality were tracked between successive loads. RESULTS: The deformation of the cartilage matrix was heterogeneous on the microscopic length scale. This was evident from local strain maps, which showed shearing between different regions of collagen under tensile strain, corrugations in the articular surface at higher tensile strains and a non-uniform distribution of compressive strain in the axial direction. Chondrocytes elongated and rotated under tensile strain and were compressed in the axial direction under compressive load. The magnitude of deformation varied between cells, indicating differences in either load transmission through the matrix or the mechanical properties of individual cells. Under tensile loading the reorganisation of the elastin network differed from a homogeneous elastic response, indicating that it forms a functional structure. CONCLUSIONS: This study highlights the complexity of superficial zone mechanics and demonstrates that the response of the collagen matrix, elastin fibres and chondrocytes are all heterogeneous on the microscopic scale.Arthritis Research U

Theory of ferromagnetic microswimmers

Copyright © 2011 Oxford University Press. This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The Quarterly Journal of Mechanics and Applied Mathematics following peer review. The definitive publisher-authenticated version [Volume 64, Issue 3, pp. 239-263] is available online at: http://qjmam.oxfordjournals.org/content/64/3/239This paper considers the dynamics of a microscale swimmer based on two magnetic beads that are elastically coupled together. A time-varying external magnetic field is imposed that has two principal effects: one is to exert a torque on the magnetic beads. The second is to change the orientation of the magnetic field dipoles in one or both beads, depending on their ferromagnetic properties. This then creates an attraction or repulsion between the two dipoles. The combination of dipole attraction/repulsion, moderated by the elastic coupling, and torque gives motions that are not generally time reversible and can lead to unidirectional swimming, that is persistent motion in one direction, in a Stokes flow regime. The equations of motion for the swimmer are set up using a Lagrangian formulation and supplemented by equations giving the dipole orientation of the magnetic fields of the beads in the external field. The equations are non-dimensionalized and key parameters determined. Numerical simulations reveal a number of regimes that are studied using simplified models and multiple scale analysis. Approximate thresholds are obtained above which the swimmer moves in a closed path and below which the orientation is `trapped' giving unidirectional motion. Three mechanisms for such trapping are isolated and discussed

The structure and micromechanics of elastic tissue.

Journal ArticleReviewElastin is a major component of tissues such as lung and blood vessels, and endows them with the long-range elasticity necessary for their physiological functions. Recent research has revealed the complexity of these elastin structures and drawn attention to the existence of extensive networks of fine elastin fibres in tissues such as articular cartilage and the intervertebral disc. Nonlinear microscopy, allowing the visualization of these structures in living tissues, is informing analysis of their mechanical properties. Elastic fibres are complex in composition and structure containing, in addition to elastin, an array of microfibrillar proteins, principally fibrillin. Raman microspectrometry and X-ray scattering have provided new insights into the mechanisms of elasticity of the individual component proteins at the molecular and fibrillar levels, but more remains to be done in understanding their mechanical interactions in composite matrices. Elastic tissue is one of the most stable components of the extracellular matrix, but impaired mechanical function is associated with ageing and diseases such as atherosclerosis and diabetes. Efforts to understand these associations through studying the effects of processes such as calcium and lipid binding and glycation on the mechanical properties of elastin preparations in vitro have produced a confusing picture, and further efforts are required to determine the molecular basis of such effects.British Heart FoundationArthritis U

Chemically specific imaging and in-situ chemical analysis of articular cartilage with stimulated Raman scattering

This is the pre-peer reviewed version of the following article: Mansfield, J., Moger, J., Green, E., Moger, C. and Winlove, C. P. (2013), Chemically specific imaging and in-situ chemical analysis of articular cartilage with stimulated Raman scattering. J. Biophoton., 6: 803–814. doi: 10.1002/jbio.201200213, which has been published in final form at http://dx.doi.org/10.1002/jbio.201200213.© 2013 by WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimStimulated Raman scattering (SRS) has been applied to unstained samples of articular cartilage enabling the investigation of living cells within fresh tissue. Hyperspectral SRS measurements over the CH vibrational region showed variations in protein and lipid content within the cells, pericellular matrix and interterritorial matrix. Changes in the cells and pericellular matrix were investigated as a function of depth into the cartilage. Lipid was detected in the pericellular matrix of superficial zone chondrocytes. The spectral profile of lipid droplets within the chondrocytes indicated that they contained predominantly unsaturated lipids. The mineral content has been imaged by using the PO₄³⁻ vibration at 959 cm⁻¹ and the CO₃²⁻ vibration at 1070 cm⁻¹. Both changes in cells and mineralization are known to be important factors in the progression of osteoarthritis. SRS enables these to be visualized in fresh unstained tissue and consequently should benefit osteoarthiritis research

Red blood cell thermal fluctuations: comparison between experiment and molecular dynamics simulations

Copyright © 2013 Royal Society of ChemistryWe outline a new method of analysis of thermal shape fluctuations of red blood cells, based on comparison between experiments and coarse-grained molecular dynamics simulations. The fluctuations of 2D equatorial contours of red blood cells are recorded experimentally using fast phase-contrast video microscopy, from which the fluctuation spectrum is calculated. The spectrum is compared to the corresponding contour fluctuation spectrum obtained from a finite-temperature particle-dynamics simulation, modelling a cell with bending and shear elasticity and conserved volume and surface area. We demonstrate that the simulation correctly describes the mean cell shape as well as the membrane thermal fluctuations, returning physically sound values for the relevant membrane elastic moduli

α-Tocopherols modify the membrane dipole potential leading to modulation of ligand binding by P-glycoprotein

Journal ArticleThis is the author accepted manuscript. The final version is available from ASBMB via the DOI in this record.α-Tocopherol (vitamin E) has attracted considerable attention as a potential protective or palliative agent. In vitro, its free radical-scavenging antioxidant action has been widely demonstrated. In vivo, however, vitamin E treatment exhibits negligible benefits against oxidative stress. α-Tocopherol influences lipid ordering within biological membranes and its derivatives have been suggested to inhibit the multi-drug efflux pump, P-glycoprotein (P-gp). This study employs the fluorescent membrane probe, 1-(3-sulfonatopropyl)-4-[β[2-(di-n-octylamino)-6-naphthyl]vinyl] pyridinium betaine, to investigate whether these effects are connected via influences on the membrane dipole potential (MDP), an intrinsic property of biological membranes previously demonstrated to modulate P-gp activity. α-Tocopherol and its non-free radical-scavenging succinate analog induced similar decreases in the MDP of phosphatidylcholine vesicles. α-Tocopherol succinate also reduced the MDP of T-lymphocytes, subsequently decreasing the binding affinity of saquinavir for P-gp. Additionally, α-tocopherol succinate demonstrated a preference for cholesterol-treated (membrane microdomain enriched) cells over membrane cholesterol-depleted cells. Microdomain disruption via cholesterol depletion decreased saquinavir's affinity for P-gp, potentially implicating these structures in the influence of α-tocopherol succinate on P-gp. This study provides evidence of a microdomain dipole potential-dependent mechanism by which α-tocopherol analogs influence P-gp activity. These findings have implications for the use of α-tocopherol derivatives for drug delivery across biological barriers

Clostridium perfringens α-toxin interaction with red cells and model membranes.

The effects of Clostridium perfringens α-toxin on host cells have previously been studied extensively but the biophysical processes associated with toxicity are poorly understood. The work reported here shows that the initial interaction between the toxin and lipid membrane leads to measurable changes in the physical properties and morphology of the membrane. A Langmuir monolayer technique was used to assess the response of different lipid species to toxin. Sphingomyelin and unsaturated phosphatidylcholine showed the highest susceptibility to toxin lypolitic action, with a two stage response to the toxin (an initial, rapid hydrolysis stage followed by the insertion and/or reorganisation of material in the monolayer). Fluorescence confocal microscopy on unsaturated phosphatidylcholine vesicles shows that the toxin initially aggregates at discrete sites followed by the formation of localised "droplets" accumulating the hydrolysis products. This process is accompanied by local increases in the membrane dipole potential by about 50 (±42) mV. In contrast, red blood cells incubated with the toxin suffered a decrease of the membrane dipole potential by 50 (±40) mV in areas of high toxin activity (equivalent to a change in electric field strength of 10(7) V m(-1)) which is sufficient to affect the functioning of the cell membrane. Changes in erythrocyte morphology caused by the toxin are presented, and the early stages of interaction between toxin and membrane are characterised using thermal shape fluctuation analysis of red cells which revealed two distinct regimes of membrane-toxin interaction.Royal Society University Research Fellowshi