Osmotic correction to elastic area compressibility measurements on red cell membrane

Deformability of mouse erythrocytes in different diluents measured using optical tweezers

Optical tweezers are widely used to measure the mechanical properties of erythrocytes, which is crucial to the study of pathology and clinical diagnosis of disease. During the measurement, the blood sample is diluted and suspended in an exogenous physiological fluid, which may affect the elastic properties of the cells in vitro. Here, we investigate the effect of different diluents on the elastic properties of mouse erythrocytes by quantitatively evaluating their elastic constants using optical tweezers. The diluents are plasma extracted from mouse blood, veterinary blood diluent (V-52D), Dulbecco's modified Eagle's medium (DMEM), phosphate-buffered saline (PBS), and normal saline (NS). To create an environment that closely resembles in vivo conditions, the experiment is performed at 36.5 °C. The results show that the spring constant of mouse erythrocytes in plasma is 6.23 ± 0.41 μN m-1. The elasticity of mouse erythrocytes in V-52D and DMEM is 8.21 ± 0.91 and 6.95 ± 0.85 μN m-1, which are higher than that in plasma extracted from blood, whereas, the elasticity in PBS and NS is 4.23 ± 0.85 and 4.68 ± 0.79 μN m-1, which are less than that in plasma extracted from blood. At last, we observe the size and circularity of erythrocytes in different diluents, and consider that the erythrocyte diameter and circularity may affect cell deformability. Our results provide a reference of the diluent choice for measuring the mechanical properties of erythrocytes in vitro.

Activation effects on the physical characteristics of T lymphocytes

The deformability of leukocytes is relevant to a wide array of physiological and pathophysiological behaviors. The goal of this study is to provide a detailed, quantitative characterization of the mechanical properties of T cells and how those properties change with activation. We tested T cells and CD8⁺ cells isolated from peripheral blood samples of healthy donors either immediately (naïve population) or after 7 days of activation in vitro. Single-cell micropipette aspiration was used to test the mechanical properties. T cells exhibit the general characteristics of a highly viscous liquid drop with a cortical "surface" tension, T _cort. The time course of each cell entry into the micropipette was measured at two different aspiration pressures to test for shear thinning behavior. The data were analyzed in the framework of an approximate mechanical model of the cell deformation to determine the cortical tension, the cell volume, the magnitude of the initial cell entry, the characteristic viscosity μ _o, and the shear thinning coefficient, b. Activation generally caused increases in cellular resistance to deformation and a broadening of the distribution of cell properties. The cell volume increased substantially upon cell activation from ∼200 μm³ to ∼650 μm³. Naive and activated T cells had similar mean cortical tension (∼150 pN/μm). However, compared to naïve CD8⁺ cells, the cortical tension of activated CD8⁺ cells increased significantly to ∼250 pN/μm. Dynamic resistance of naive CD8⁺ T cells, as reflected in their characteristic viscosity, was ∼870 Pa and significantly increased to 1,180 Pa after in vitro activation. The magnitude of the instantaneous projection length as the cell enters the pipette (L _init) was more than doubled for activated vs. naive cells. All cell types exhibited shear thinning behavior with coefficients b in the range 0.5-0.65. Increased cell size, cortical tension, and characteristic viscosity all point to increased resistance of activated T cells to passage through the microvasculature, likely contributing to cell trapping. The increased initial elastic response of cells after activation was unexpected and could point to instability in the cell that might contribute to spontaneous cell motility.

Structural and mechanical properties of the red blood cell’s cytoplasmic membrane seen through the lens of biophysics

Red blood cells (RBCs) are the most abundant cell type in the human body and critical suppliers of oxygen. The cells are characterized by a simple structure with no internal organelles. Their two-layered outer shell is composed of a cytoplasmic membrane (RBC_cm) tethered to a spectrin cytoskeleton allowing the cell to be both flexible yet resistant against shear stress. These mechanical properties are intrinsically linked to the molecular composition and organization of their shell. The cytoplasmic membrane is expected to dominate the elastic behavior on small, nanometer length scales, which are most relevant for cellular processes that take place between the fibrils of the cytoskeleton. Several pathologies have been linked to structural and compositional changes within the RBC_cm and the cell’s mechanical properties. We review current findings in terms of RBC lipidomics, lipid organization and elastic properties with a focus on biophysical techniques, such as X-ray and neutron scattering, and Molecular Dynamics simulations, and their biological relevance. In our current understanding, the RBC_cm’s structure is patchy, with nanometer sized liquid ordered and disordered lipid, and peptide domains. At the same time, it is surprisingly soft, with bending rigidities κ of 2–4 k_BT. This is in strong contrast to the current belief that a high concentration of cholesterol results in stiff membranes. This extreme softness is likely the result of an interaction between polyunsaturated lipids and cholesterol, which may also occur in other biological membranes. There is strong evidence in the literature that there is no length scale dependence of κ of whole RBCs.

Membrane tension controls the phase equilibrium in fusogenic liposomes

Fusogenic liposomes have been widely used for molecule delivery to cell membranes and cell interior. However, their physicochemical state is still little understood. We tested mechanical material behavior by micropipette aspiration of giant vesicles from fusogenic lipid mixtures and found that the membranes of these vesicles are fluid and under high mechanical tension even before aspiration. Based on this result, we developed a theoretical framework to determine the area expansion modulus and membrane tension of such pre-tensed vesicles from aspiration experiments. Surprisingly high membrane tension of 2.1 mN m^-1 and very low area expansion modulus of 63 mN m^-1 were found. We interpret these peculiar material properties as the result of a mechanically driven phase transition between the usual lamellar phase and an, as of now, not finally determined three dimensional phase of the lipid mixture. The free enthalpy of transition between these phases is very low, i.e. on the order of the thermal energy.

Amphiphilic Aminoglycosides as Medicinal Agents

The conjugation of hydrophobic group(s) to the polycationic hydrophilic core of the antibiotic drugs aminoglycosides (AGs), targeting ribosomal RNA, has led to the development of amphiphilic aminoglycosides (AAGs). These drugs exhibit numerous biological effects, including good antibacterial effects against susceptible and multidrug-resistant bacteria due to the targeting of bacterial membranes. In the first part of this review, we summarize our work in identifying and developing broad-spectrum antibacterial AAGs that constitute a new class of antibiotic agents acting on bacterial membranes. The target-shift strongly improves antibiotic activity against bacterial strains that are resistant to the parent AG drugs and to antibiotic drugs of other classes, and renders the emergence of resistant Pseudomonas aeruginosa strains highly difficult. Structure–activity and structure–eukaryotic cytotoxicity relationships, specificity and barriers that need to be crossed in their development as antibacterial agents are delineated, with a focus on their targets in membranes, lipopolysaccharides (LPS) and cardiolipin (CL), and the corresponding mode of action against Gram-negative bacteria. At the end of the first part, we summarize the other recent advances in the field of antibacterial AAGs, mainly published since 2016, with an emphasis on the emerging AAGs which are made of an AG core conjugated to an adjuvant or an antibiotic drug of another class (antibiotic hybrids). In the second part, we briefly illustrate other biological and biochemical effects of AAGs, i.e., their antifungal activity, their use as delivery vehicles of nucleic acids, of short peptide (polyamide) nucleic acids (PNAs) and of drugs, as well as their ability to cleave DNA at abasic sites and to inhibit the functioning of connexin hemichannels. Finally, we discuss some aspects of structure–activity relationships in order to explain and improve the target selectivity of AAGs.

Isolation of intact bacteria from blood by selective cell lysis in a microfluidic porous silica monolith

Rapid and efficient isolation of bacteria from complex biological matrices is necessary for effective pathogen identification in emerging single-cell diagnostics. Here, we demonstrate the isolation of intact and viable bacteria from whole blood through the selective lysis of blood cells during flow through a porous silica monolith. Efficient mechanical hemolysis is achieved while providing passage of intact and viable bacteria through the monoliths, allowing size-based isolation of bacteria to be performed following selective lysis. A process for synthesizing large quantities of discrete capillary-bound monolith elements and millimeter-scale monolith bricks is described, together with the seamless integration of individual monoliths into microfluidic chips. The impact of monolith morphology, geometry, and flow conditions on cell lysis is explored, and flow regimes are identified wherein robust selective blood cell lysis and intact bacteria passage are achieved for multiple gram-negative and gram-positive bacteria. The technique is shown to enable rapid sample preparation and bacteria analysis by single-cell Raman spectrometry. The selective lysis technique presents a unique sample preparation step supporting rapid and culture-free analysis of bacteria for the point of care.

Mechanics and Dynamics of Bacterial Cell Lysis

Membrane lysis, or rupture, is a cell death pathway in bacteria frequently caused by cell wall-targeting antibiotics. Although previous studies have clarified the biochemical mechanisms of antibiotic action, a physical understanding of the processes leading to lysis remains lacking. Here, we analyze the dynamics of membrane bulging and lysis in Escherichia coli, in which the formation of an initial, partially subtended spherical bulge ("bulging") after cell wall digestion occurs on a characteristic timescale of 1 s and the growth of the bulge ("swelling") occurs on a slower characteristic timescale of 100 s. We show that bulging can be energetically favorable due to the relaxation of the entropic and stretching energies of the inner membrane, cell wall, and outer membrane and that the experimentally observed timescales are consistent with model predictions. We then show that swelling is mediated by the enlargement of wall defects, after which cell lysis is consistent with both the inner and outer membranes exceeding characteristic estimates of the yield areal strains of biological membranes. These results contrast biological membrane physics and the physics of thin, rigid shells. They also have implications for cellular morphogenesis and antibiotic discovery across different species of bacteria.

Synergistic Integration of Laboratory and Numerical Approaches in Studies of the Biomechanics of Diseased Red Blood Cells

In red blood cell (RBC) disorders, such as sickle cell disease, hereditary spherocytosis, and diabetes, alterations to the size and shape of RBCs due to either mutations of RBC proteins or changes to the extracellular environment, lead to compromised cell deformability, impaired cell stability, and increased propensity to aggregate. Numerous laboratory approaches have been implemented to elucidate the pathogenesis of RBC disorders. Concurrently, computational RBC models have been developed to simulate the dynamics of RBCs under physiological and pathological conditions. In this work, we review recent laboratory and computational studies of disordered RBCs. Distinguished from previous reviews, we emphasize how experimental techniques and computational modeling can be synergically integrated to improve the understanding of the pathophysiology of hematological disorders.

Negatively Charged Lipids as a Potential Target for New Amphiphilic Aminoglycoside Antibiotics: A BIOPHYSICAL STUDY

Bacterial membranes are highly organized, containing specific microdomains that facilitate distinct protein and lipid assemblies. Evidence suggests that cardiolipin molecules segregate into such microdomains, probably conferring a negative curvature to the inner plasma membrane during membrane fission upon cell division. 3',6-Dinonyl neamine is an amphiphilic aminoglycoside derivative active against Pseudomonas aeruginosa, including strains resistant to colistin. The mechanisms involved at the molecular level were identified using lipid models (large unilamellar vesicles, giant unilamelllar vesicles, and lipid monolayers) that mimic the inner membrane of P. aeruginosa The study demonstrated the interaction of 3',6-dinonyl neamine with cardiolipin and phosphatidylglycerol, two negatively charged lipids from inner bacterial membranes. This interaction induced membrane permeabilization and depolarization. Lateral segregation of cardiolipin and membrane hemifusion would be critical for explaining the effects induced on lipid membranes by amphiphilic aminoglycoside antibiotics. The findings contribute to an improved understanding of how amphiphilic aminoglycoside antibiotics that bind to negatively charged lipids like cardiolipin could be promising antibacterial compounds.

Pressure-driven occlusive flow of a confined red blood cell

When red blood cells (RBCs) move through narrow capillaries in the microcirculation, they deform as they flow. In pathophysiological processes such as sickle cell disease and malaria, RBC motion and flow are severely restricted. To understand this threshold of occlusion, we use a combination of experiment and theory to study the motion of a single swollen RBC through a narrow glass capillary of varying inner diameter. By tracking the movement of the squeezed cell as it is driven by a controlled pressure drop, we measure the RBC velocity as a function of the pressure gradient as well as the local capillary diameter, and find that the effective blood viscosity in this regime increases with both decreasing RBC velocity and tube radius by following a power-law that depends upon the length of the confined cell. Our observations are consistent with a simple elasto-hydrodynamic model and highlight the role of lateral confinement in the occluded pressure-driven slow flow of soft confined objects.

Continuum modeling of deformation and aggregation of red blood cells

In order to gain better understanding for rheology of an isolated red blood cell (RBC) and a group of multiple RBCs, new continuum models for describing mechanical properties of cellular structures of an RBC and inter-cellular interactions among multiple RBCs are developed. The viscous property of an RBC membrane, which characterizes dynamic behaviors of an RBC under stress loading and unloading processes, is determined using a generalized Maxwell model. The present model is capable of predicting stress relaxation and stress-strain hysteresis, of which prediction is not possible using the commonly used Kelvin-Voigt model. Nonlinear elasticity of an RBC is determined using the Yeoh hyperelastic material model in a framework of continuum mechanics using finite-element approximation. A novel method to model inter-cellular interactions among multiple adjacent RBCs is also developed. Unlike the previous modeling approaches for aggregation of RBCs, where interaction energy for aggregation is curve-fitted using a Morse-type potential function, the interaction energy is analytically determined. The present aggregation model, therefore, allows us to predict various effects of physical parameters such as the osmotic pressure, the thickness of a glycocalyx layer, the penetration depth, and the permittivity, on the depletion and electrostatic energy among RBCs. Simulations for elongation and recovery deformation of an RBC and for aggregation of multiple RBCs are conducted to evaluate the efficacy of the present continuum modeling methods.

Computation of the effective mechanical response of biological networks accounting for large configuration changes

The asymptotic homogenization technique is involved to derive the effective elastic response of biological membranes viewed as repetitive beam networks. Thereby, a systematic methodology is established, allowing the prediction of the overall mechanical properties of biological membranes in the nonlinear regime, reflecting the influence of the geometrical and mechanical micro-parameters of the network structure on the overall response of the equivalent continuum. Biomembranes networks are classified based on nodal connectivity, so that we analyze in this work 3, 4 and 6-connectivity networks, which are representative of most biological networks. The individual filaments of the network are described as undulated beams prone to entropic elasticity, with tensile moduli determined from their persistence length. The effective micropolar continuum evaluated as a continuum substitute of the biological network has a kinematics reflecting the discrete network deformation modes, involving a nodal displacement and a microrotation. The statics involves the classical Cauchy stress and internal moments encapsulated into couple stresses, which develop internal work in duality to microcurvatures reflecting local network undulations. The relative ratio of the characteristic bending length of the effective micropolar continuum to the unit cell size determines the relevant choice of the equivalent medium. In most cases, the Cauchy continuum is sufficient to model biomembranes. The peptidoglycan network may exhibit a re-entrant hexagonal configuration due to thermal or pressure fluctuations, for which micropolar effects become important. The homogenized responses are in good agreement with FE simulations performed over the whole network. The predictive nature of the employed homogenization technique allows the identification of a strain energy density of a hyperelastic model, for the purpose of performing structural calculations of the shape evolutions of biomembranes.

Mechanosensitive channels are activated by stress in the actin stress fibres, and could be involved in gravity sensing in plants

Mechanosensitive (MS) channels are expressed in a variety of cells. The molecular and biophysical mechanism involved in the regulation of MS channel activities is a central interest in basic biology. MS channels are thought to play crucial roles in gravity sensing in plant cells. To date, two mechanisms have been proposed for MS channel activation. One is that tension development in the lipid bilayer directly activates MS channels. The second mechanism proposes that the cytoskeleton is involved in the channel activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. We tested whether tension in the cytoskeleton activates MS channels. Mammalian endothelial cells were microinjected with phalloidin-conjugated beads, which bound to stress fibres, and a traction force to the actin cytoskeleton was applied by dragging the beads with optical tweezers. MS channels were activated when the force was applied, demonstrating that a sub-pN force to the actin filaments activates a single MS channel. Plants may use a similar molecular mechanism in gravity sensing, since the cytoplasmic Ca(2+) concentration increase induced by changes in the gravity vector was attenuated by potential MS channel inhibitors, and by actin-disrupting drugs. These results support the idea that the tension increase in actin filaments by gravity-dependent sedimentation of amyloplasts activates MS Ca(2+) -permeable channels, which can be the molecular mechanism of a Ca(2+) concentration increase through gravistimulation. We review recent progress in the study of tension sensing by actin filaments and MS channels using advanced biophysical methods, and discuss their possible roles in gravisensing.

Cellular pressure and volume regulation and implications for cell mechanics

In eukaryotic cells, small changes in cell volume can serve as important signals for cell proliferation, death, and migration. Volume and shape regulation also directly impacts the mechanics of cells and tissues. Here, we develop a mathematical model of cellular volume and pressure regulation, incorporating essential elements such as water permeation, mechanosensitive channels, active ion pumps, and active stresses in the cortex. The model can fully explain recent experimental data, and it predicts cellular volume and pressure for several models of cell cortical mechanics. Moreover, we show that when cells are subjected to an externally applied load, such as in an atomic force microscopy indentation experiment, active regulation of volume and pressure leads to a complex cellular response. Instead of the passive mechanics of the cortex, the observed cell stiffness depends on several factors working together. This provides a mathematical explanation of rate-dependent response of cells under force.

Optical measurement of biomechanical properties of individual erythrocytes from a sickle cell patient

Sickle cell disease (SCD) is characterized by the abnormal deformation of red blood cells (RBCs) in the deoxygenated condition, as their elongated shape leads to compromised circulation. The pathophysiology of SCD is influenced by both the biomechanical properties of RBCs and their hemodynamic properties in the microvasculature. A major challenge in the study of SCD involves accurate characterization of the biomechanical properties of individual RBCs with minimum sample perturbation. Here we report the biomechanical properties of individual RBCs from a SCD patient using a non-invasive laser interferometric technique. We optically measure the dynamic membrane fluctuations of RBCs. The measurements are analyzed with a previously validated membrane model to retrieve key mechanical properties of the cells: bending modulus; shear modulus; area expansion modulus; and cytoplasmic viscosity. We find that high cytoplasmic viscosity at ambient oxygen concentration is principally responsible for the significantly decreased dynamic membrane fluctuations in RBCs with SCD, and that the mechanical properties of the membrane cortex of irreversibly sickled cells (ISCs) are different from those of the other types of RBCs in SCD.

Quantification of embryonic atrioventricular valve biomechanics during morphogenesis

Tissue assembly in the developing embryo is a rapid and complex process. While much research has focused on genetic regulatory machinery, understanding tissue level changes such as biomechanical remodeling remains a challenging experimental enigma. In the particular case of embryonic atrioventricular valves, micro-scale, amorphous cushions rapidly remodel into fibrous leaflets while simultaneously interacting with a demanding mechanical environment. In this study we employ two microscale mechanical measurement systems in conjunction with finite element analysis to quantify valve stiffening during valvulogenesis. The pipette aspiration technique is compared to a uniaxial load deformation, and the analytic expression for a uniaxially loaded bar is used to estimate the nonlinear material parameters of the experimental data. Effective modulus and strain energy density are analyzed as potential metrics for comparing mechanical stiffness. Avian atrioventricular valves from globular Hamburger-Hamilton stages HH25-HH34 were tested via the pipette method, while the planar HH36 leaflets were tested using the deformable post technique. Strain energy density between HH25 and HH34 septal leaflets increased 4.6±1.8 fold (±SD). The strain energy density of the HH36 septal leaflet was four orders of magnitude greater than the HH34 pipette result. Our results establish morphological thresholds for employing the micropipette aspiration and deformable post techniques for measuring uniaxial mechanical properties of embryonic tissues. Quantitative biomechanical analysis is an important and underserved complement to molecular and genetic experimentation of embryonic morphogenesis.

Dynamic response of micropipettes during piezo-assisted intracytoplasmic sperm injection

In the intracytoplasmic sperm injection (ICSI) process, a piezoelectric actuator is commonly used to assist the piercing of cell membrane. The longitudinal pulses that are performed by the piezo actuator, however, cause undesired lateral vibrations at the drawn tip of the injection micropipette. This mechanism is not well understood, despite its critical role in piezo-assisted cellular microinjection. We provide an analytical model to characterize the micropipette tip vibrations under assumed base excitation arising from the piezoelectric pulses. The resulting dynamic response is determined by using the Duhamel integral method. This study quantifies the effect of fluid damping, embedded mercury, and the apparent cell membrane elasticity. We found that, in practice, a small mercury droplet filled in pipette essentially creates higher shear forces at the membrane-pipette interface. The increased shear due to underdamped eigenmodes is conceived to assist the piercing of the cell membrane.

A nonlinear characteristic regime of biomembrane force probe

A linear relation between stiffness and aspiration pressure is the basis for biomembrane force probe (BFP), a widely used technique to measure minuscule forces. Here we perform finite element simulations and semi-analytical modeling of the BFP operation to show that, at low aspiration pressures, there exists a characteristic regime in which the relation between stiffness and aspiration pressure is actually nonlinear. We find that this nonlinear characteristic regime arises from a transition in configuration of a partially aspirated biomembrane force probe under increasing aspiration pressure. We discuss both the conditions for the transition and the characteristics of the nonlinear characteristic regime, as well as its potential applications.

Influence of power-law rheology on cell injury during microbubble flows

The reopening of fluid-occluded pulmonary airways generates microbubble flows which impart complex hydrodynamic stresses to the epithelial cells lining airway walls. In this study we used boundary element solutions and finite element techniques to investigate how cell rheology influences the deformation and injury of cells during microbubble flows. An optimized Prony-Dirichlet series was used to model the cells' power-law rheology (PLR) and results were compared with a Maxwell fluid model. Results indicate that membrane strain and the risk for cell injury decreases with increasing channel height and bubble speed. In addition, the Maxwell and PLR models both indicate that increased viscous damping results in less cellular deformation/injury. However, only the PLR model was consistent with the experimental observation that cell injury is not a function of stress exposure duration. Correlation of our models with experimental observations therefore highlights the importance of using PLR in computational models of cell mechanics/deformation. These computational models also indicate that altering the cell's viscoelastic properties may be a clinically relevant way to mitigate microbubble-induced cell injury.

Image-based finite element modeling of alveolar epithelial cell injury during airway reopening

The acute respiratory distress syndrome (ARDS) is characterized by fluid accumulation in small pulmonary airways. The reopening of these fluid-filled airways involves the propagation of an air-liquid interface that exerts injurious hydrodynamic stresses on the epithelial cells (EpC) lining the airway walls. Previous experimental studies have demonstrated that these hydrodynamic stresses may cause rupture of the plasma membrane (i.e., cell necrosis) and have postulated that cell morphology plays a role in cell death. However, direct experimental measurement of stress and strain within the cell is intractable, and limited data are available on the mechanical response (i.e., deformation) of the epithelium during airway reopening. The goal of this study is to use image-based finite element models of cell deformation during airway reopening to investigate how cell morphology and mechanics influence the risk of cell injury/necrosis. Confocal microscopy images of EpC in subconfluent and confluent monolayers were used to generate morphologically accurate three-dimensional finite element models. Hydrodynamic stresses on the cells were calculated from boundary element solutions of bubble propagation in a fluid-filled parallel-plate flow channel. Results indicate that for equivalent cell mechanical properties and hydrodynamic load conditions, subconfluent cells develop higher membrane strains than confluent cells. Strain magnitudes were also found to decrease with increasing stiffness of the cell and membrane/cortex region but were most sensitive to changes in the cell's interior stiffness. These models may be useful in identifying pharmacological treatments that mitigate cell injury during airway reopening by altering specific biomechanical properties of the EpC.

High fluidity and soft elasticity of the inner membrane of Escherichia coli revealed by the surface rheology of model Langmuir monolayers

We have studied the equilibrium and linear mechanical properties of model membranes of Escherichia coli built up as Langmuir monolayers of a native lipid extract using surface thermodynamics, fluorescence microscopy, and surface rheology measurements. The experimental study has been carried out at different temperatures across the physiological operative range 15-37 degrees C. Lipid phase coexistence has been revealed over a broad pressure range by fluorescence microscopy. The presence of ordered domains has been invoked to explain the emergence of shear elasticity accompanying the hydrostatic compression elasticity typical of fluid monolayers. The surface rheology measurements point out the soft character of E. coli membranes; i.e., upon deformation they react as a near-ideal compliant body with minimal energy dissipation, thus optimizing the effectiveness of external stresses in producing membrane deformations. These mechanical features appear to be independent of temperature, suggesting the existence of a passive thermoregulation mechanism.

Actin stress fibers transmit and focus force to activate mechanosensitive channels

Mechanosensitive (MS) channels are expressed in various cells in a wide range of phylogenetic lineages from bacteria to humans. Understanding the molecular and biophysical mechanisms of their activation is an important research pursuit. It is controversial whether eukaryotic MS channels need accessory proteins – typically cytoskeletal structures – for activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. Here we demonstrate that direct mechanical stimulation (stretching) of an actin stress fiber using optical tweezers can activate MS channels in cultured human umbilical vein endothelial cells (HUVECs). Furthermore, by using high-speed total internal reflection microscopy, we visualized spots of Ca2+ influx across individual MS channels distributed near focal adhesions in the basal surface of HUVECs. This study provides the first direct evidence that the cytoskeleton works as a force-transmitting and force-focusing molecular device to activate MS channels in eukaryotic cells.

Theoretical analysis of an iron mineral-based magnetoreceptor model in birds

Sensing the magnetic field has been established as an essential part of navigation and orientation of various animals for many years. Only recently has the first detailed receptor concept for magnetoreception been published based on histological and physical results. The considered mechanism involves two types of iron minerals (magnetite and maghemite) that were found in subcellular compartments within sensory dendrites of the upper beak of several bird species. But so far a quantitative evaluation of the proposed receptor is missing. In this article, we develop a theoretical model to quantitatively and qualitatively describe the magnetic field effects among particles containing iron minerals. The analysis of forces acting between these subcellular compartments shows a particular dependence on the orientation of the external magnetic field. The iron minerals in the beak are found in the form of crystalline maghemite platelets and assemblies of magnetite nanoparticles. We demonstrate that the pull or push to the magnetite assemblies, which are connected to the cell membrane, may reach a value of 0.2 pN -- sufficient to excite specific mechanoreceptive membrane channels in the nerve cell. The theoretical analysis of the assumed magnetoreceptor system in the avian beak skin clearly shows that it might indeed be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.

Dynamics of neutrophil membrane compliance and microstructure probed with a micropipet-based piconewton force transducer

A novel biointerface probe was implemented to study the deformability of the neutrophil membrane and cortical cytoskeleton. Piconewton scale forces are applied to the cell using an ultrasensitive and tunable force transducer comprised of an avidin-coated microsphere attached to a biotinylated and swollen red blood cell. Deformations of freshly isolated human neutrophils were observed on the stage of an inverted phase contrast microscope. Force versus probe indentation curves over a cycle of contact, indentation, and retraction revealed three distinct material responses. Small probe deformations (approximately 500 nm) tested over a range of rates (e.g. 100-500 nm/s) revealed predominantly an elastic response. An initial low-slope region in the force-indentation curves (approximately 0.005 pN/nm), typically extending 0.5-1.0 microm from the cell surface was interpreted as probe contact with microvilli extensions. Further deformation yielded a slope of 0.054+/-0.006 pN/nm, indicative of a stiffer cortical membrane. Disrupting cytoskeletal actin organization by pretreatment with cytochalasin D, reduced the slope by 40% to 0.033+/-0.007 pN/nm and introduced hysteresis in the recovery phase. Modeling the neutrophil as a liquid drop with constant surface tension yielded values of cortical tension of 0.035 pN/nm for resting and 0.02 pN/nm for cytochalasin-treated neutrophils. These data demonstrate the utility of the biointerface probe for measuring local surface compliance and microstructure of living cells.

Mechanical properties of the cell nucleus and the effect of emerin deficiency

Nuclear structure and mechanics are gaining recognition as important factors that affect gene expression, development, and differentiation in normal function and disease, yet the physical mechanisms that govern nuclear mechanical stability remain unclear. Here we examined the physical properties of the cell nucleus by imaging fluorescently labeled components of the inner nucleus (chromatin and nucleoli) and the nuclear envelope (lamins and membranes) in nuclei deformed by micropipette aspiration (confocal imaged microdeformation). We investigated nuclei, both isolated and in intact, living cells, and found that nuclear volume significantly decreased by 60-70% during aspiration. While nuclear membranes exhibited blebbing and fluid characteristics during aspiration, the nuclear lamina exhibited behavior of a solid-elastic shell. Under large deformations of GFP-lamin A-labeled nuclei, we observed a decay of fluorescence intensity into the tip of the deformed tongue that we interpreted in terms of nonlinear, two-dimensional elasticity theory. Here we applied this method to study nuclear envelope stability in disease and found that mouse embryo fibroblasts lacking the inner nuclear membrane protein, emerin, had a significantly decreased ratio of the area expansion to shear moduli (K/mu) compared to wild-type cells (2.1 +/- 0.2 versus 5.1 +/- 1.3). These data suggest that altered nuclear envelope elasticity caused by loss of emerin could contribute to increased nuclear fragility in Emery-Dreifuss muscular dystrophy patients with mutations in the emerin gene. Based on our experimental results and theoretical considerations, we present a model describing how the nucleus is stabilized in the pipette. Such a model is essential for interpreting the results of any micropipette study of the nucleus and porous materials in general.

A mixture theory analysis for passive transport in osmotic loading of cells

The theory of mixtures is applied to the analysis of the passive response of cells to osmotic loading with neutrally charged solutes. The formulation, which is derived for multiple solute species, incorporates partition coefficients for the solutes in the cytoplasm relative to the external solution, and accounts for cell membrane tension. The mixture formulation provides an explicit dependence of the hydraulic conductivity of the cell membrane on the concentration of permeating solutes. The resulting equations are shown to reduce to the classical equations of Kedem and Katchalsky in the limit when the membrane tension is equal to zero and the solute partition coefficient in the cytoplasm is equal to unity. Numerical simulations demonstrate that the concentration-dependence of the hydraulic conductivity is not negligible; the volume response to osmotic loading is very sensitive to the partition coefficient of the solute in the cytoplasm, which controls the magnitude of cell volume recovery; and the volume response is sensitive to the magnitude of cell membrane tension. Deviations of the Boyle-van't Hoff response from a straight line under hypo-osmotic loading may be indicative of cell membrane tension.

Theoretical and experimental investigation of the behaviour of ultrasound contrast agent particles in whole blood

The majority of the existing models for the behaviour of ultrasound (US) contrast agents consider a single contrast agent particle (CAP) surrounded by an infinite, homogeneous and Newtonian fluid. In vivo, however, CAPs are suspended within the confines of blood vessels, in fluid containing both other CAPs and a high volume fraction of cells of comparable size. The aim of this work was to investigate the influence of blood cells upon CAP acoustic response to determine how existing models should be modified for the purposes of improving CAP design. A new model for a CAP surrounded by a cluster of cells was derived and solved numerically. Broadband US attenuation measurements were then made in suspensions of Optison (Amersham PLC, Bucks, UK) in plasma and in whole blood. Both the theoretical and experimental results indicate that the presence of blood cells has a relatively small effect upon CAP dynamics and hence acoustic response. This implies that it is justifiable to model blood as homogeneous and Newtonian for the purposes of CAP design.

X-ray scattering study of pike olfactory nerve: elastic, thermodynamic and physiological properties of the axonal membrane

The effects of several agents, sugars, isotonic KCl, and a variety of drugs, on the structure of the axonal membranes of unmyelinated pike olfactory nerve have been studied by synchrotron radiation X-ray scattering experiments. The main effects of the sugars are: (i) to increase the electron density of the extra-axonal space and thereby yield the absolute scale of the electron density profile; (ii) to osmotically stress the membrane and thus yield its elastic modulus of area compressibility, since the related strain, thickness dilation, is directly determined by the X-ray scattering experiments. Exposure to isotonic KCl, a depolarizing agent, induces membrane thickness to increase. The energy liberated in this process is a function of the amplitude of the dilation and of the elastic modulus of the membrane. This energy turns out to be close to the thermal energy liberated by the pike olfactory nerve during the initial phase of action potential that has previously been measured by others. Electrical depolarization thus seems to be accompanied by a thickness dilation of the axonal membrane. Another effect of isotonic KCl is to induce a large fraction of the membranes to pair by tight apposition of their extra-axonal faces. Local anaesthetics and some drugs have the effect of altering membrane thickness. All these observations are interpreted in terms of a modulation of the conformational disorder of the hydrocarbon chains of the lipid molecules.

Contraction transitions of F1-F0 ATPase during catalytic turnover

Strong acoustic pressure was applied to submitochondrial particles (SMP) from bovine heart in order to drive ATP synthesis by F1-F0 complex for the account of sound waves. We observed a net ATP production at two narrow frequency ranges, about 170 Hz and about 340 Hz, that corresponds to the resonance oscillations of experimental cuvette when the acoustic pressure had a magnitude of 100 kPa. The results can be explained quantitatively by contractive conformational changes of F1-F0 complex during catalytic turnover. Negative staining electron microscopy of SMP preparations was used to visualize the ADP(Mg2+)-induced conformational changes of F1-F0 complex. In the particles with high ATPase activity in the presence of phosphate the factors F1 and F0 formed a congregated domain plunged into the membrane without any observable stalk in between. The presence of ADP(Mg2+) caused a structural rearrangement of F1-F0 to the essentially different conformation: the domains F1 and F0 were dislodged distinctly from each other and connected by a long thin stalk. The latter conformation resembled well the usual bipartite profile of ATPase. The data indicate that besides rotation, the catalytic turnover of ATP synthase is also accompanied by stretch transitions of F1-F0 complex.

Micropipet-based pico force transducer: in depth analysis and experimental verification

Measurements of forces in the piconewton range are very important for the study of molecular adhesion and mechanics. Recently, a micropipet-based force transducer for this type of experiment was presented (E. Evans, K. Ritchie, and R. Merkel, 1995, Biophys. J., 68:2580-2587). In the present article we give a detailed mechanical analysis of this transducer, including nonlinear effects. An analytical expression for the transducer stiffness at small elongations is given. Using magnetic tweezers (F. Ziemann, J. Rädler, and E. Sackmann, 1994, Biophys. J., 66:2210-2216), we were able to determine the force displacement relation of this transducer experimentally. Forces from approximately 10 pN to 500 pN were applied. Theoretical predictions and experimental results coincide remarkably well.

Surface area and volume changes during maturation of reticulocytes in the circulation of the baboon

Changes in the surface area and volume of reticulocytes were measured in vivo during late stage maturation. Baboons were treated with erythropoietin to produce mild reticulocytosis. Reticulocyte-rich cohorts of cells were obtained from whole blood by density gradient centrifugation. The cohorts were labeled with biotin, reinfused into the animal, and recovered from whole blood samples by panning on avidin supports. Changes in the surface area, volume, and membrane deformability were measured using micropipettes during the 2 to 6 weeks subsequent to reinfusion. For the entire cohort, the membrane area decreased by 10% to 15% and the cell volume decreased by approximately 8.5%, mostly within 24 hours after reinfusion. Estimates of the cellular dimensions of the reticulocyte subpopulation within this cohort indicated larger reductions in the mean cell area (12% to 30%) and mean cell volume (approximately 15%) of the reticulocytes themselves. Two weeks after reinfusion, the distribution of cell size for the cohort was indistinguishable from that of whole blood. There was evidence of slightly elevated membrane shear rigidity in some reticulocytes before reinfusion, but this slight increase disappeared within 24 hours after reinfusion. These are the first direct measurements of changes in the membrane physical properties of an identifiable cohort of reticulocytes as they mature in vivo.

Effects of lost surface area on red blood cells and red blood cell survival in mice

The effects of removing area from mouse red blood cells on the fate of the cells after reinfusion were investigated. When cells were made nearly spherical (by reducing cell area by approximately 35%) and then reinfused into the animal, most were cleared from the circulation within 1-2 h, although approximately 20% of the cells survived for 4 h or longer. When only 20% of the area was removed (leaving a 15% excess), more than 90% of the cells continued to circulate for 4 h. After reinfusion, the mean surface area of the surviving cells remained constant (73-75 microns2), but the mean volume decreased, from 56.6 +/- 2.1 to 19.1 +/- 1.5 microns3 (+/- SD of 5 replicates) over 4 h. These changes did not occur in cells suspended in plasma but not reinfused into the animal. Thus a loss of surface area results in a decrease in cell volume, as if to maintain a requisite degree of deformability. The results support the hypothesis that the increase in cell density associated with increasing cell age may be a consequence of surface area loss.

Tensile strength and dilatational elasticity of giant sarcolemmal vesicles shed from rabbit muscle

1. Mechanical properties of the surface membrane of skeletal muscle were determined on sarcolemmal vesicles (mean diameter, 71 microns) shed by rabbit psoas muscle swelling in 140 mM KC1 containing collagenase. 2. Vesicles were stressed by partial aspiration into parallel bore pipettes. The isotropic membrane tension so created caused an increase in membrane area which expresses itself in an elongation of the vesicle projection into the pipette. 3. For individual vesicles, a linear relationship between membrane tension and membrane area increase was found up to the point when the vesicle burst, i.e. sarcolemmal vesicles behaved as perfectly elastic structures. 4. The maximum tension sarcolemmal vesicles could sustain before bursting was 12.4 +/- 0.2 mN m-1 (median +/- 95% confidence interval), and the corresponding fractional increase in membrane area was 0.026 +/- 0.005 (median +/- 95% confidence interval). The elastic modulus of area expansion was 490 +/- 88 mN m-1 (mean +/- S.D.). 5. In conformity with cited comparable work on red blood cells and artificial lipid vesicles, the strength and area elasticity of the skeletal muscle membrane are considered properties of the fluid lipid matrix of the membrane and of the degree to which the bilayer is perturbed by lipid-protein interaction.

Measurement of erythrocyte membrane elasticity by flicker eigenmode decomposition

We have studied the flickering of erythrocytes at wavelengths comparable to the cell dimension. To do this we have analyzed the edge fluctuations of the cell to a resolution of 5 nm by combining phase contrast microscopy with fast image processing. By measuring the edge excitations simultaneously at four orthogonal positions around the cell, the eigenmodes of equal azimuthal mode numbers m = 0,1,2 could be separated. From a continuous time sequence of 100 s of video frames taken at 40 ms time intervals, we determined the time-auto correlation function for the modes m = 0,1,2 and calculated their mean square amplitudes <delta n2m> as well as their decay times tau m. To explain the results we also present the theoretically calculated energy eigenmodes of an erythrocyte, accounting for the constraint that the cell is in contact with the substrate along an annular ring, which agreed well with the experimental findings. We found that the softest mode is a "hindered translational" mode with m = 1 of the adhered cell, which is almost insensitive to the shear elastic modulus. Comparison of the calculated and measured amplitudes yielded an average value for the bending stiffness of kc = 4 x 10(-19) J, which is much larger than the value obtained by flicker analysis at short wavelengths (kc = 2.3 x 10(-20) J). It would, however, agree well with the value expected from the red cell membrane area compressibility modulus of K = 4.5 x 10(-1)N/m, which corresponds to a lipid bilayer containing approximately 50 mol % of cholesterol. In contradiction to our theoretical expectations we found that the flicker eigenmodes seemed not to be influenced by the membrane shear elasticity, which will be discussed in terms of an unusual coupling between the lipid bilayer and the cytoskeleton.

Light-scattering technique for the study of orientation and deformation of red blood cells in a concentrated suspension

The backscattered and transmitted diagrams of He-Ne laser light illuminating a concentrated suspension of red blood cells (RBC's) are investigated. The shapes of these diagrams are closely related to the state of the suspension (at rest or submitted to a simple shear flow) and to the parameters that govern the non-Newtonian behavior of the blood suspension (such as the viscosity of the suspending medium and the volume concentration of the cells). An asymmetry in the backscattering diagram, which is absent on transmitted diagrams, is observed when the suspension is in a simple shear flow. This asymmetry is related to the deformation and orientation of the RBC's. The propagation of light through the suspension is modeled and a set of Monte Carlo simulations is performed to substantiate the inference that the relative variation of the backscattered flux is proportional to the gradients of deformation of the RBC's, and that such gradients must be known in order to apply a rheological model describing the non-Newtonian behavior of RBC membranes.

Osmotic properties of large unilamellar vesicles prepared by extrusion

We have examined the morphology and osmotic properties of large unilamellar vesicles (LUVs) prepared by extrusion. Contrary to expectations, we observe by cryo-electron microscopy that such vesicles, under isoosmotic conditions, are non-spherical. This morphology appears to be a consequence of vesicle passage through the filter pores during preparation. As a result when such LUVs are placed in a hypoosmotic medium they are able to compensate, at least partially, for the resulting influx of water by "rounding up" and thereby increasing their volume with no change in surface area. The increase in vesicle trapped volume associated with these morphological changes was determined using the slowly membrane-permeable solute [3H]-glucose. This allowed calculation of the actual osmotic gradient experienced by the vesicle membrane for a given applied differential. When LUVs were exposed to osmotic differentials of sufficient magnitude lysis occurred with the extent of solute release being dependent on the size of the osmotic gradient. Surprisingly, lysis was not an all-or-nothing event, but instead a residual osmotic differential remained after lysis. This differential value was comparable in magnitude to the minimum osmotic differential required to trigger lysis. Further, by comparing the release of solutes of differing molecular weights (glucose and dextran) a lower limit of about 12 nm diameter can be set for the bilayer defect created during lysis. Finally, the maximum residual osmotic differentials were compared for LUVs varying in mean diameter from 90 to 340 nm. This comparison confirmed that these systems obey Laplace's Law relating vesicle diameter and lysis pressure. This analysis also yielded a value for the membrane tension at lysis of 40 dyn cm-1 at 23 degrees C, which is in reasonable agreement with previously published values for giant unilamellar vesicles.

Is the surface area of the red cell membrane skeleton locally conserved?

The incompressibility of the lipid bilayer keeps the total surface area of the red cell membrane constant. Local conservation of membrane surface area requires that each surface element of the membrane skeleton keeps its area when its aspect ratio is changed. A change in area would require a flow of lipids past the intrinsic proteins to which the skeleton is anchored. in fast red cell deformations, there is no time for such a flow. Consequently, the bilayer provides for local area conservation. In quasistatic deformations, the extent of local change in surface area is the smaller the larger the isotropic modulus of the skeleton in relation to the shear modulus. Estimates indicate: (a) the velocity of relative flow between lipid and intrinsic proteins is proportional to the gradient in normal tension within the skeleton and inversely proportional to the viscosity of the bilayer; (b) lateral diffusion of lipids is much slower than this flow; (c) membrane tanktreading at frequencies prevailing in vivo as well as the release of a membrane tongue from a micropipette are fast deformations; and (d) the slow phase in micropipette aspiration may be dominated by a local change in skeleton surface.

Electric fields induce reversible changes in the surface to volume ratio of micropipette-aspirated erythrocytes

Micropipette-aspirated erythrocytes exhibit reversible changes in sphericity (surface-to-volume ratio) in response to applied electric fields. The potentials were applied between the shaft of the pipette and the bathing medium using Ag-AgCl electrodes and current clamping electronics. The change in surface-to-volume ratio is evidenced as a reversible change in the length of the cell projection in the pipette at constant aspiration pressure and changing voltage. The magnitude of the changes decreased in proportion to the inverse of the solute concentration indicating that the change in sphericity was due to a change in cell volume. Reversible changes in projection length equivalent to a 10% change in cell volume were observed to occur over times on the order of 10 s. The magnitude and time course of the effect were not affected by the removal of intracellular hemoglobin or inhibition of anion exchange. The effect was reduced by the presence of lanthanum and other multivalent cations in the suspending solution, suggesting that surface charge may play a role in mediating the effect.

Alterations of the apparent area expansivity modulus of red blood cell membrane by electric fields

Red blood cell membrane exhibits a large resistance to changes in surface area. This resistance is characterized by the area expansivity modulus K, which relates the isotropic membrane force resultant, T, to the fractional change in membrane surface area delta A/Ao. The experimental technique commonly used to determine K is micropipette aspiration. Using this method, E. A. Evans and R. Waugh (1977, Biophys. J. 20:307-313) obtained a value of 450 dyn/cm for the modulus. In the present report, it is shown that the value of K, as determined using this method, is affected by electric potential differences applied across the tip of the pipette. Using Ag-AgCl electrodes and current clamping electronics, we obtained values for K ranging from 150 dyn/cm with -1.0 V applied, to 1,500 dyn/cm with 1.0 V applied. At 0.0 V the modulus obtained was approximately 500 dyn/cm. A reversible, voltage- and pressure-dependent change in the cell volume probably accounts for the effect of the voltage on the calculated value of the modulus. The use of lanthanum chloride or increasing the extra- and intracellular solute concentrations reduced the voltage dependence of the measurements. It was also found that when dissimilar metals were used to "ground" the pipette to the chamber to prevent lysis of cells by static charge, values for K ranged from 121 to 608 dyn/cm. Based on measurements made at zero applied volts, in the presence of 0.4 mM lanthanum and at high solute concentration, we conclude that the true value of the modulus is approximately 500 dyn/cm.

Electro-mechanical permeabilization of lipid vesicles. Role of membrane tension and compressibility

A simple micropipet technique was used to determine the critical electric field strength for membrane breakdown as a function of the applied membrane tension for three different reconstituted membranes: stearoyloleoylphosphatidylcholine (SOPC), red blood cell (RBC) lipid extract, and SOPC cholesterol (CHOL), 1:1. For these membranes the elastic area expansivity modulus increases from approximately 200 to 600 dyn/cm, and the tension at lysis increases from 5.7 to 13.2 dyn/cm, i.e., the membranes become more cohesive with increasing cholesterol content. The critical membrane voltage, Vc, required for breakdown was also found to increase with increasing cholesterol from 1.1 to 1.8 V at zero membrane tension. We have modeled the behavior in terms of the bilayer expansivity. Membrane area can be increased by either tensile or electrocompressive stresses. Both can store elastic energy in the membrane and eventually cause breakdown at a critical area dilation or critical energy. The model predicts a relation between tension and voltage at breakdown and this relation is verified experimentally for the three reconstituted membrane systems studied here.

Determination of bilayer membrane bending stiffness by tether formation from giant, thin-walled vesicles

The curvature elastic modulus (bending stiffness) of stearoyloleoyl phosphatidylcholine (SOPC) bilayer membrane is determined from membrane tether formation experiments. R. E. Waugh and R. M. Hochmuth 1987. Biophys. J. 52:391-400) have shown that the radius of a bilayer cylinder (tether) is inversely related to the force supported along its axis. The coefficient that relates the axial force on the tether to the tether radius is the membrane bending stiffness. Thus, the bending stiffness can be calculated directly from measurements of the tether radius as a function of force. Giant (10-50-microns diam) thin-walled vesicles were aspirated into a micropipette and a tether was pulled out of the surface by gravitational forces on small glass beads that had adhered to the vesicle surface. Because the vesicle keeps constant surface area and volume, formation of the tether requires displacement of material from the projection of the vesicle in the pipette. Tethers can be made to grow longer or shorter or to maintain equilibrium by adjusting the aspiration pressure in the micropipette at constant tether force. The ratio of the change in the length of the tether to the change in the projection length is proportional to the ratio of the pipette radius to the tether radius. Thus, knowing the density and diameter of the glass beads and measuring the displacement of the projection as a function of tether length, independent determinations of the force on the tether and the tether radius were obtained. The bending stiffness for an SOPC bilayer obtained from these data is approximately 2.0 x 10(-12) dyn cm, for tether radii in the range of 20-100 nm. An equilibrium relationship between pressure and tether force is derived which closely matches experimental observation.

Structure and deformation properties of red blood cells: concepts and quantitative methods

The lamellar configuration of the red cell membrane includes a (liquid) superficial bilayer of amphiphilic molecules supported by a (rigid) subsurface protein meshwork. Because of this composite structure, the red cell membrane exhibits very large resistance to changes in surface density or area with very low resistance to in-plane extension and bending deformations. The primary extrinsic factor in cell deformability is the surface area-to-volume ratio which establishes the minimum-caliber vessel into which a cell can deform (without rupture). Within the restriction provided by surface area and volume, the intrinsic properties of the membrane and cytoplasm determine the deformability characteristics of the red cell. Since the cytoplasm is liquid, the static rigidity of the cell is determined by membrane elastic constants. These include an elastic modulus for area compressibility in the range of 300-600 dyn/cm, an elastic modulus for in-plane extension or shear (at constant area) of 5-7 X 10(-3) dyn/cm, and a curvature or bending elastic modulus on the order of 10(-12) dyn.cm. Even though small, the surface rigidity of the cell membrane is sufficient to return the membrane capsule to a discoid shape after deformation by external forces. Viscous dissipation in the peripheral protein structure (cytoskeleton) dominates the dynamic response of the cell to extensional forces. Based on a time constant for recovery after extensional deformation on the order of 0.1 sec, the coefficient of surface viscosity is on the order of 10(-3) dyn.sec/cm. On the other hand, the dynamic resistance to folding of the cell appears to be limited by viscous dissipation in the cytoplasmic and external fluid phases. Dynamic rigidities for both extensional and folding deformations are important factors in the distribution of flow in the small microvessels. Although the red cell membrane normally behaves as a resilient viscoelastic shell, which recovers its conformation after deformation, structural relaxation and failure lead to break-up and fragmentation of the red cell. The levels of membrane extensional force which is two orders of magnitude less than the level of tension necessary to lyse vesicles by rapid area dilation. Each of the material properties ascribed to the red cell membrane plays an important role in the deformability and survivability of the red cell in the circulation over its several-month life span.

Changes in the rate of haemolysis during the early stage after burns in the rabbit

The kinetics of erythrocyte haemolysis in blood samples taken from normal and burned rabbits were investigated by nephelometry. The rate of haemolysis decreased significantly from 30 min to 2 h after severe burns, when many abnormal red cell forms were found in blood films; there was also an increase in osmotic fragility. The results confirm that the rate of haemolysis is strongly related to structural changes of the red cell membrane but do not parallel osmotic fragility. Differences in haemolysis rate and osmotic fragility between erythrocytes taken from burned animals and those heated in vitro suggest that erythrocyte changes following burning injury involve complex kinetic processes in which factors other than the direct effect of heat play important roles.

Mechanical equilibrium of thick, hollow, liquid membrane cylinders

The mechanical equilibrium of bilayer membrane cylinders is analyzed. The analysis is motivated by the observation that mechanically formed membrane strands (tethers) can support significant axial loads and that the tether radius varies inversely with the axial force. Previously, thin shell theory has been used to analyze the tether formation process, but this approach is inadequate for describing and predicting the equilibrium state of the tether itself. In the present work the membrane is modeled as two adjacent, thick, anisotropic liquid shells. The analysis predicts an inverse relationship between axial force and tether radius, which is consistent with experimental observation. The area expansivity modulus and bending stiffness of the tether membrane are calculated using previously measured values of tether radii. These calculated values are consistent with values of membrane properties measured previously. Application of the analysis to precise measurements of the relationship between tether radius and axial force will provide a novel method for determining the mechanical properties of biomembrane.

Alternative interpretation for the osmotic response of human erythrocytes

In a recent publication, Heubusch et al. (J Cell. Physiol, 122:266-272, 1985) reported changes of erythrocyte volume measured by the Coulter counter technique over a wide range of osmolalities (160 to 3000 m0sm). Their results showed a partially hindered, nonlinear response, in contrast to classical observations made over more restricted osmolality ranges, using other methods. The authors suggested the underlying cause of this behavior to be a mechanical resistance of the membrane cytoskeleton. In this paper, we wish to offer a different interpretation of their results on erythrocyte osmotic behavior, based on similar experiments carried out in our laboratory, and supported by previous analyses from the literature. In particular, it is shown that the shape-factor correction to the electronic sizing measurement can correctly account for the observed deviations from linearity in the hypotonic range. In contrast, increased chemical nonideality and eventual hemolysis are the likely factors responsible for the behavior in the hypertonic range.

Measurement of biophysical properties of red blood cells by resistive pulse spectroscopy: volume, shape, surface area, and deformability

This paper presents a simple, new approach to the determination of size, shape, surface area, and deformability information for cells, notably red blood cells. The results are obtained by combining experimental measurements from resistive pulse spectroscopy (an extension of electronic cell-sizing methodology) with theoretical calculations for model cell systems. Assuming constancy of surface area and approximating red cell shapes by both prolate and oblate ellipsoids of revolution, values are determined for cell shape factor and volume under a variety of conditions. For red blood cells under low-stress conditions, shape factor, volume, and surface area results are found to be consistent with those available from the literature, when the oblate model is used. The applicability of this approach for determination of red cell properties under altered conditions is demonstrated by results for cell volume, at varying osmotic pressure and mechanical shear (tensile) stress. By quantitating the change in cell shape with stress, a new numerical scale for measuring cell deformability is also obtained, and data are presented on its variation for red cells at different osmolalities, over the range of 140 to 500 mOsm.