Reductions of erythrocyte membrane viscoelastic coefficients reflect spectrin deficiencies in hereditary spherocytosis

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Constitutive Model of Erythrocyte Membranes with Distributions of Spectrin Orientations and Lengths

We present an analytical hyperelastic constitutive model of the red blood cell (erythrocyte) membrane based on recently improved characterizations of density and microscopic structure of its spectrin network from proteomics and cryo-electron tomography. The model includes distributions of both orientations and natural lengths of spectrin and updated copy numbers of proteins. By applying finite deformation to the spectrin network, we obtain the total free energy and stresses in terms of invariants of shear and area deformation. We generalize an expression of the initial shear modulus, which is independent of the number of molecular orientations within the network and also derive a simplified version of the model. We apply the model and its simplified version to analyze micropipette aspiration computationally and analytically and explore the effect of local cytoskeletal density change. We also explore the discrepancies among shear modulus values measured using different experimental techniques reported in the literature. We find that the model exhibits hardening behavior and can explain many of these discrepancies. Moreover, we find that the distribution of natural lengths plays a crucial role in the hardening behavior when the correct copy numbers of proteins are used. The initial shear modulus values we obtain using our current model (5.9-15.6 pN/μm) are close to the early estimates (6-9 pN/μm). This new, to our knowledge, constitutive model establishes a direct connection between the molecular structure of spectrin networks and constitutive laws and also defines a new picture of a much denser spectrin network than assumed in prior studies.

Mechanical Signature of Red Blood Cells Flowing Out of a Microfluidic Constriction Is Impacted by Membrane Elasticity, Cell Surface-to-Volume Ratio and Diseases

Despite the fact that Red Blood Cells (RBCs) have been intensively studied in the past 50 years to characterize mechanical phenotypes associated with both healthy and pathological states, only ektacytometry (i.e., laser diffractometry) is currently used by hematologists to screen for RBC membrane disorders. Therefore, the development of new diagnostic tools able to perform analysis at the scale of a single cell, over a statistically relevant population, would provide important complementary information. But these new diagnostic tools would have to be able to discriminate between different disorders causing a change in RBCs mechanical properties. We evaluated the mechanical response of artificially rigidified RBCs flowing through a microfluidic constriction. The geometry consists in a 50 μm wide channel with a succession of 14 tooth-like patterns, each composed of a 5 μm wide and 10 μm long constriction, associated with a 25 μm wide and 10 μm long enlargement. RBCs deformability was altered using two chemical treatments, known to affect RBCs membrane surface area and membrane deformability, lysolecithine (LPC) and diamide, respectively. Differences between samples were highlighted by the representation of the inverse of the shape recovery time (1/τ r ), versus the extension at the exit of the constriction, D out . The results demonstrate that our approach is able to provide a direct signature of RBCs membrane composition and architecture, as it allows discriminating the effect of changes in RBCs membrane surface area from changes in RBCs membrane deformability. Finally, in order to evaluate the potential of our microsystem to detect pathological cells, we have performed preliminary experiments on patients with Hereditary Spherocytosis (HS) or Sickle Cell Anemia (SCA).

Nanoscale membrane architecture of healthy and pathological red blood cells

Red blood cells feature remarkable mechanical properties while navigating through microcirculation vessels and during spleen filtration. An unusual combination of plasma membrane and cytoskeleton physical properties allows red blood cells to undergo extensive deformation. Here we used atomic force microscopy multiparametric imaging to probe how cellular organization influences nanoscale and global mechanical properties of cells in both physiological and pathological conditions. Our data obtained in native conditions confirmed that, compared to healthy cells, cells from patients with hereditary spherocytosis are stiffer. Through vertical segmentation of the cell elasticity, we found that healthy and pathological cells display nanoscale architecture with an increasing stiffness along the direction of the applied force. By decoupling the mechanical response of the plasma membrane from its underlying cytoskeleton, we find that both components show altered properties in pathological conditions. Nanoscale multiparametric imaging also revealed lipid domains that exhibit differential mechanical properties than the bulk membrane in both healthy and pathological conditions. Thanks to correlated AFM-fluorescence imaging, we identified submicrometric sphingomyelin-enriched lipid domains of variable stiffness at the red blood cell surface. Our experiments provide novel insights into the interplay between nanoscale organization of red blood cell plasma membrane and their nanomechanical properties. Overall, this work contributes to a better understanding of the complex relationship between cellular nanoscale organization, cellular nanomechanics and how this 3D organization is altered in pathological conditions.

Dynamic fatigue measurement of human erythrocytes using dielectrophoresis

Erythrocytes must undergo severe deformation to pass through narrow capillaries and submicronic splenic slits for several hundred thousand times in their normal lifespan. Studies of erythrocyte biomechanics have been mainly focused on cell deformability and rheology measured from a single application of stress and mostly under a static or quasi-static state using classical biomechanical techniques, such as optical tweezers and micropipette aspiration. Dynamic behavior of erythrocytes in response to cyclic stresses that contributes to the membrane failure in blood circulation is not fully understood. This paper presents a new experimental method for dynamic fatigue analysis of erythrocytes, using amplitude modulated electrokinetic force field in a microfluidic platform. We demonstrate the capability of this new technique using a low cycle fatigue analysis of normal human erythrocytes and ATP-depleted erythrocytes. Cyclic tensile stresses are generated to induce repeated uniaxial stretching and extensional recovery of single erythrocytes. Results of morphological and biomechanical parameters of individually tracked erythrocytes show strong correlations with the number of the loading cycles. Under a same strength of electric field, after 180 stress cycles, for normal erythrocytes, maximum stretch ratio decreases from 3.80 to 2.86, characteristic time of cellular extensional recovery increases from 0.16s to 0.37s, membrane shear viscosity increases from 1.0(µN/m)s to 1.6(µN/m)s. Membrane deformation in a small number of erythrocytes becomes irreversible after large deformation for about 200 cyclic loads. ATP-depleted cells show similar trends in decreased deformation and increased characteristic time with the loading cycles. These results show proof of concept of the new microfluidics technique for dynamic fatigue analysis of human erythrocytes.

STATEMENT OF SIGNIFICANCE

Red blood cells (RBCs) experience a tremendous number of deformation in blood circulation before losing their mechanical deformability and eventually being degraded in the reticuloendothelial system. Prior efforts in RBC biomechanics have mostly focused on a single-application of stress, or quasi-static loading through physical contact to deform cell membranes, thus with limited capabilities in probing cellular dynamic responses to cyclic stresses. We present a unique electrokinetic microfluidic system for the study of dynamic fatigue behavior of RBCs subjected to cyclic loads. Our work shows quantitatively how the cyclic stretching loads cause membrane mechanical degradation and irreversibly deformed cells. This new technique can be useful to identify biomechanical markers for prediction of the mechanical stability and residual lifespan of circulating RBCs.

Fast Stiffness Mapping of Cells Using High-Bandwidth Atomic Force Microscopy

The cytoskeleton controls cellular morphology and mediates the mechanical interactions between a cell and its environment. Atomic force microscopy (AFM) has the unique capability to map cytoskeletal mechanics and structures with nanometer resolution. However, whole-cell cytomechanical imaging with conventional AFM techniques is limited by low imaging speed. Here, we present fast nanomechanical mapping of cells using high-bandwidth AFM (HB-AFM), where >10(6) nanoindentation measurements were acquired in ∼10 min-a task that would take weeks to finish using conventional AFM. High-bandwidth measurements enabled capture of the entire tip-sample interaction for each tap on cells, engendering a new measurement ("force phase") that exceeds the contrast of conventional tapping mode and enabling spectral visualization of >10 harmonics. The abundance of measurements allowed discovery of subtle cytomechanical features, including the stiffness of fibers of the cellular spectrin network in situ. This approach bridges HB-AFM and high-harmonic imaging and opens future opportunities for measuring the dynamic mechanical properties of living cells.

Profiling individual human red blood cells using common-path diffraction optical tomography

Due to its strong correlation with the pathophysiology of many diseases, information about human red blood cells (RBCs) has a crucial function in hematology. Therefore, measuring and understanding the morphological, chemical, and mechanical properties of individual RBCs is a key to understanding the pathophysiology of a number of diseases in hematology, as well as to opening up new possibilities for diagnosing diseases in their early stages. In this study, we present the simultaneous and quantitative measurement of the morphological, chemical, and mechanical parameters of individual RBCs employing optical holographic microtomography. In addition, it is demonstrated that the correlation analyses of these RBC parameters provide unique information for distinguishing and understanding diseases.

Biomechanical properties of red blood cells in health and disease towards microfluidics

Red blood cells (RBCs) possess a unique capacity for undergoing cellular deformation to navigate across various human microcirculation vessels, enabling them to pass through capillaries that are smaller than their diameter and to carry out their role as gas carriers between blood and tissues. Since there is growing evidence that red blood cell deformability is impaired in some pathological conditions, measurement of RBC deformability has been the focus of numerous studies over the past decades. Nevertheless, reports on healthy and pathological RBCs are currently limited and, in many cases, are not expressed in terms of well-defined cell membrane parameters such as elasticity and viscosity. Hence, it is often difficult to integrate these results into the basic understanding of RBC behaviour, as well as into clinical applications. The aim of this review is to summarize currently available reports on RBC deformability and to highlight its association with various human diseases such as hereditary disorders (e.g., spherocytosis, elliptocytosis, ovalocytosis, and stomatocytosis), metabolic disorders (e.g., diabetes, hypercholesterolemia, obesity), adenosine triphosphate-induced membrane changes, oxidative stress, and paroxysmal nocturnal hemoglobinuria. Microfluidic techniques have been identified as the key to develop state-of-the-art dynamic experimental models for elucidating the significance of RBC membrane alterations in pathological conditions and the role that such alterations play in the microvasculature flow dynamics.

Mechanical control of the sense of touch by β-spectrin

The ability to sense and respond to mechanical stimuli emanates from sensory neurons and is shared by most, if not all, animals. Exactly how such neurons receive and distribute mechanical signals during touch sensation remains mysterious. Here, we show that sensation of mechanical forces depends on a continuous, pre-stressed spectrin cytoskeleton inside neurons. Mutations in the tetramerization domain of Caenorhabditis elegans β-spectrin (UNC-70), an actin-membrane crosslinker, cause defects in sensory neuron morphology under compressive stress in moving animals. Through atomic force spectroscopy experiments on isolated neurons, in vivo laser axotomy and fluorescence resonance energy transfer imaging to measure force across single cells and molecules, we show that spectrin is held under constitutive tension in living animals, which contributes to elevated pre-stress in touch receptor neurons. Genetic manipulations that decrease such spectrin-dependent tension also selectively impair touch sensation, suggesting that such pre-tension is essential for efficient responses to external mechanical stimuli.

Effect of membrane stiffness and cytoskeletal element density on mechanical stimuli within cells: an analysis of the consequences of ageing in cells

A finite element model of a single cell was created and used to compute the biophysical stimuli generated within a cell under mechanical loading. Major cellular components were incorporated in the model: the membrane, cytoplasm, nucleus, microtubules, actin filaments, intermediate filaments, nuclear lamina and chromatin. The model used multiple sets of tensegrity structures. Viscoelastic properties were assigned to the continuum components. To corroborate the model, a simulation of atomic force microscopy indentation was performed and results showed a force/indentation simulation with the range of experimental results. A parametric analysis of both increasing membrane stiffness (thereby modelling membrane peroxidation with age) and decreasing density of cytoskeletal elements (thereby modelling reduced actin density with age) was performed. Comparing normal and aged cells under indentation predicts that aged cells have a lower membrane area subjected to high strain as compared with young cells, but the difference, surprisingly, is very small and may not be measurable experimentally. Ageing is predicted to have a more significant effect on strain deep in the nucleus. These results show that computation of biophysical stimuli within cells are achievable with single-cell computational models; correspondence between computed and measured force/displacement behaviours provides a high-level validation of the model. Regarding the effect of ageing, the models suggest only small, although possibly physiologically significant, differences in internal biophysical stimuli between normal and aged cells.

Life cycle-dependent cytoskeletal modifications in Plasmodium falciparum infected erythrocytes

Plasmodium falciparum infection of human erythrocytes is known to result in the modification of the host cell cytoskeleton by parasite-coded proteins. However, such modifications and corresponding implications in malaria pathogenesis have not been fully explored. Here, we probed the gradual modification of infected erythrocyte cytoskeleton with advancing stages of infection using atomic force microscopy (AFM). We reported a novel strategy to derive accurate and quantitative information on the knob structures and their connections with the spectrin network by performing AFM-based imaging analysis of the cytoplasmic surface of infected erythrocytes. Significant changes on the red cell cytoskeleton were observed from the expansion of spectrin network mesh size, extension of spectrin tetramers and the decrease of spectrin abundance with advancing stages of infection. The spectrin network appeared to aggregate around knobs but also appeared sparser at non-knob areas as the parasite matured. This dramatic modification of the erythrocyte skeleton during the advancing stage of malaria infection could contribute to the loss of deformability of the infected erythrocyte.

Stretch-induced membrane damage in muscle: comparison of wild-type and mdx mice

One component of stretch-induced muscle damage is an increase in the permeability of the cell membrane. As a result soluble myoplasmic proteins leak out of the muscle into the plasma, extracellular proteins can enter the muscle, and extracellular ions, including calcium, are driven down their electrochemical gradient into the myoplasm. In Duchenne muscular dystrophy, caused by the absence of the cytoskeletal protein dystrophin, stretch-induced membrane damage is much more severe. The most popular theory to explain the occurrence of stretch-induced membrane damage is that stretched-contractions cause transient mechanically-induced defects in the membrane (tears or rips). Dystrophin, which is part of a mechanical link between the contractile machinery and the extracellular matrix, is thought to contribute to membrane strength so that in its absence mechanically-induced defects are worse. In our view the evidence that stretch-induced muscle damage causes increased membrane permeability is overwhelming but the evidence that the increased permeability is caused by mechanically-induced defects is weak. Instead we review the substantial evidence that the membrane permeability is a secondary consequence of the mechanical events in which elevated intracellular calcium and reactive oxygen species are important intermediaries.

Integral protein linkage and the bilayer-skeletal separation energy in red blood cells

Stabilization of the lipid bilayer membrane in red blood cells by its association with an underlying membrane-associated cytoskeleton has long been recognized as critical for proper red blood cell function. One of the principal connections between skeleton and bilayer is via linkages between band 3, the integral membrane protein that transports anions across the cell surface, and membrane skeletal elements including ankyrin, adducin, spectrin, and the junctional complex of the skeleton. Here, we use membrane tether formation coupled with fluorescent labeling of membrane components to examine the importance of band 3 in stabilizing the bilayer-skeletal association. In membranes from a patient deficient in band 3, the energy associated with the bilayer skeleton is approximately zero, whereas when band 3 is immobilized by ligation with the monoclonal antibody R10, the energy of association approximately doubles. Fluorescence images of tethers reveal that approximately 40% of the band 3 on the normal cell surface can be pulled into the tether, confirming a lateral segregation of membrane components during tether formation. These results validate a critical role for band 3 in stabilizing the bilayer-skeletal association in red cells.

The plasma membrane of erythrocytes plays a fundamental role in the transport of oxygen, carbon dioxide and nitric oxide and in the maintenance of the reduced state of the heme iron

Here we review new insights into the role of the erythrocyte membrane and the implications of its architecture on the several functions accomplished by the red blood cells. The picture which emerges highlights the capability of Hb and band 3 to modulate erythrocyte metabolism and to meet the needs of the cell.

A hybrid model for erythrocyte membrane: a single unit of protein network coupled with lipid bilayer

To investigate the nanomechanics of the erythrocyte membrane we developed a hybrid model that couples the actin-spectrin network to the lipid bilayer. This model features a Fourier space Brownian dynamics model of the bilayer, a Brownian dynamics model of the actin protofilament, and a modified wormlike-chain model of the spectrin (including a cable-dynamics model to predict the oscillation in tension). This model enables us to predict the nanomechanics of single or multiple units of the protein network, the lipid bilayer, and the effect of their interactions. The present work is focused on the attitude of the actin protofilament at the equilibrium states coupled with the elevations of the lipid bilayer through their primary linkage at the suspension complex in deformations. Two different actin-spectrin junctions are considered at the junctional complex. With a point-attachment junction, large pitch angles and bifurcation of yaw angles are predicted. Thermal fluctuations at bifurcation may lead to mode-switching, which may affect the network and the physiological performance of the membrane. In contrast, with a wrap-around junction, pitch angles remain small, and the occurrence of bifurcation is greatly reduced. These simulations suggest the importance of three-dimensional molecular junctions and the lipid bilayer/protein network coupling on cell membrane mechanics.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

Shape Recovery of an Optically Trapped Vesicle: Effect of Flow Velocity and Temperature

A new biophysical approach based on optical tweezers is developed to measure the time-dependent shape transformation and recovery of a single liposome, which is induced by the sudden stop of a moving liposome from various flow velocities at constant temperature. A simple viscoelastic model has been applied to correlate the temporal geometric parameter of the deformed liposome with a characteristic time constant, i.e., the ratio of membrane viscosity to elasticity. Our results show that membrane viscosity becomes dominant in governing the shape recovery rate when sample temperature goes beyond the main phase transition temperature of the phospholipid bilayer. More importantly, flow speed and vesicle size are demonstrated as key physical determinants for the shape recovery of liposome.

Hereditary spherocytosis—defects in proteins that connect the membrane skeleton to the lipid bilayer

The molecular causes of hereditary spherocytosis (HS) have been unraveled in the past decade. No frequent defect is found, and nearly every family has a unique mutation. In dominant HS, nonsense and frameshift mutations of ankyrin, band 3, and beta-spectrin predominate. Recessive HS is most often due to compound heterozygosity of defects in ankyrin, alpha-spectrin, or protein 4.2. Common combinations include a defect in the promoter or 5'-untranslated region of ankyrin paired with a missense mutation, a low expression allele of alpha-spectrin plus a missense mutation, and various mutations in the gene for protein 4.2. In most patients' red cells, no abnormal protein is present. Only rare missense mutations, like ankyrin Walsrode (V463I) or beta-spectrin Kissimmee (W202R), have given any insight into the functional domains of the respective proteins. Although the eminent role of the spleen in the premature hemolysis of red cells in HS is unquestioned, the molecular events that cause splenic conditioning of spherocytes are unclear. Electron micrographs show that small membrane vesicles are shed during the formation of spherocytes. Animal models give further insight into the pathogenetic consequences of membrane protein defects as well as the causes of the variability of disease severity.

Ultrastructural alterations in red blood cell membranes exposed to shear stress

In the mechanism of damage to red blood cells (RBCs) caused by a centrifugal pump, the prolonged effects to the RBC membrane caused by exposure to shear stress remain unclear. We focused on the band 3 protein (B3), one of the major proteins in the membrane skeleton, and investigated the ultrastructural alterations of the RBC membrane with loaded shear stress. Using flow cytometry, the relative amount of B3 was examined in relation to RBC deformability. The results, with continuous exposure to low shear stress, showed cell downsizing, an increase in B3 density, and a decrease in the deformability of the RBC membrane. Exposure to high shear stress does not appear to exert any influence on the membrane skeleton of the RBC. Therefore, in addition to conventional processes including the instantaneous destruction of a cell due to intense shear stresses, the results of the present study indicate the presence of another process based on changes in membrane proteins leading to cell fragmentation. Under low shear stress, the RBC membrane skeleton shows delayed destruction, which is exhibited as a disorder of B3 distribution, and the related membrane dysfunction includes decreases in RBC deformability and stability.

Molecular pathophysiology of myofiber injury in deficiencies of the dystrophin-glycoprotein complex

Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin, a 427 kd protein normally found at the cytoplasmic face of the sarcolemma. In normal muscle, dystrophin is associated with a multimolecular glycoprotein complex. Primary mutations in the genes encoding members of this glycoprotein complex are also associated with muscular dystrophy. The dystrophin-glycoprotein complex provides a physical linkage between the internal cytoskeleton of myofibers and the extracellular matrix, but the precise functions of the dystrophin-glycoprotein complex remain uncertain. In this review, five potential pathogenetic mechanisms implicated in the initiation of myofiber injury in dystrophin-glycoprotein complex deficiencies are discussed: (1) mechanical weakening of the sarcolemma, (2) inappropriate calcium influx, (3) aberrant cell signaling, (4) increased oxidative stress, and (5) recurrent muscle ischemia. Particular emphasis is placed on the multifunctional nature of the dystrophin-glycoprotein complex and the fact that the above mechanisms are in no way mutually exclusive and may interact with one another to a significant degree.

Deformation-enhanced fluctuations in the red cell skeleton with theoretical relations to elasticity, connectivity, and spectrin unfolding

To assess local elasticity in the red cell's spectrin-actin network, nano-particles were tethered to actin nodes and their constrained thermal motions were tracked. Cells were both immobilized and controllably deformed by aspiration into a micropipette. Since the network is well-appreciated as soft, thermal fluctuations even in an unstressed portion of network were expected to be many tens of nanometers based on simple equipartition ideas. Real-time particle tracking indeed reveals such root-mean-squared motions for 40-nm fluorescent beads either tethered to actin directly within a cell ghost or connected to actin from outside a cell via glycophorin. Moreover, the elastically constrained displacements are significant on the scale of the network's internodal distance of approximately 60-80 nm. Surprisingly, along the aspirated projection-where the network is axially extended by as much as twofold or more-fluctuations in the axial direction are increased by almost twofold relative to motions in the unstressed network. The molecular basis for such strain softening is discussed broadly in terms of force-driven transitions. Specific considerations are given to 1) protein dissociations that reduce network connectivity, and 2) unfolding kinetics of a localized few of the red cell's approximately 10(7) spectrin repeats.

Membrane instability in late-stage erythropoiesis

During maturation of the red blood cell (RBC) from the nucleated normoblast stage to the mature biconcave discocyte, both the structure and mechanical properties of the cell undergo radical changes. The development of the mechanical stability of the membrane reflects underlying changes in the organization of membrane-associated cytoskeletal proteins, and so provides an assessment of the time course of the development of membrane structural organization. Membrane stability in maturing erythrocytes was assessed by measuring forces required to form thin, tubular, lipid strands (tethers) from the surfaces of mononuclear cells obtained from fresh human marrow samples, marrow reticulocytes, circulating reticulocytes, and mature erythrocytes. Cells were biotinylated and manipulated with a micropipette to form an adhesive contact with a glass microcantilever, which gave a measure of the tethering force. The cell was withdrawn at controlled velocity and aspiration pressure to form a tether from the cell surface. The mean force required to form tethers from marrow reticulocytes and normoblasts was 27 +/- 9 pN, compared to 54 +/- 14 pN for mature cells. The energy of dissociation of the bilayer from the underlying skeleton increases 4-fold between the marrow reticulocyte stage and the mature cell, demonstrating that the mechanical stability of the membrane is not completely established until the very last stages of RBC maturation.

Actin protofilament orientation in deformation of the erythrocyte membrane skeleton

The red cell's spectrin-actin network is known to sustain local states of shear, dilation, and condensation, and yet the short actin filaments are found to maintain membrane-tangent and near-random azimuthal orientations. When calibrated with polarization results for single actin filaments, imaging of micropipette-deformed red cell ghosts has allowed an assessment of actin orientations and possible reorientations in the network. At the hemispherical cap of the aspirated projection, where the network can be dilated severalfold, filaments have the same membrane-tangent orientation as on a relatively unstrained portion of membrane. Likewise, over the length of the network projection pulled into the micropipette, where the network is strongly sheared in axial extension and circumferential contraction, actin maintains its tangent orientation and is only very weakly aligned with network extension. Similar results are found for the integral membrane protein Band 3. Allowing for thermal fluctuations, we deduce a bound for the effective coupling constant, alpha, between network shear and azimuthal orientation of the protofilament. The finding that alpha must be about an order of magnitude or more below its tight-coupling value illustrates how nanostructural kinematics can decouple from more macroscopic responses. Monte Carlo simulations of spectrin-actin networks at approximately 10-nm resolution further support this conclusion and substantiate an image of protofilaments as elements of a high-temperature spin glass.

Adaptation and survival of surface-deprived red blood cells in mice

The consequences of lost membrane area for long-term erythrocyte survival in the circulation were investigated. Mouse red blood cells were treated with lysophosphatidylcholine to reduce membrane area, labeled fluorescently, reinfused into recipient mice, and then sampled periodically for 35 days. The circulating fraction of the modified cells decreased on an approximately exponential time course, with time constants ranging from 2 to 14 days. The ratio of volume to surface area of the surviving cells, measured using micropipettes, decreased rapidly over the first 5 days after infusion to within 5% of normal. This occurred by both preferential removal of the most spherical cells and modification of others, possibly due to membrane stress developed during transient trapping of cells in the microvasculature. After 5 days, the cell area decreased with time in the circulation, but the ratio of volume to surface area remained essentially constant. These results demonstrate that the ratio of cell volume to surface area is a major determinant of the ability of erythrocytes to circulate properly.

A putative spectrin-containing membrane skeleton in hyphal tips of Neurospora crassa

The apical plasma membrane (PM) is important in hyphal tip growth, where it may regulate tip extensibility via its association with an appropriate membrane skeleton (MS). By cell fractionation and immunocytochemistry we show that proteins with characteristics of actin, spectrin, and integrin are associated in a MS-like manner with the PM of Neurospora crassa hyphae. The spectrin-like protein in particular is highly concentrated at the PM in the region of maximum apical expansion. This protein shares with other spectrins immunoreactivity, molecular weight, PM association, and actin binding capacity. Its distribution in hyphae suggests that it is a dominant component of the MS in true fungi and is critical to hyphal tip growth.

Elasticity of the red cell membrane and its relation to hemolytic disorders: an optical tweezers study

We have used optical tweezers to study the elasticity of red cell membranes; force was applied to a bead attached to a permeabilized spherical ghost and the force-extension relation was obtained from the response of a second bead bound at a diametrically opposite position. Interruption of the skeletal network by dissociation of spectrin tetramers or extraction of the actin junctions engendered a fourfold reduction in stiffness at low applied force, but only a twofold change at larger extensions. Proteolytic scission of the ankyrin, which links the membrane skeleton to the integral membrane protein, band 3, induced a similar effect. The modified, unlike the native membranes, showed plastic relaxation under a prolonged stretch. Flaccid giant liposomes showed no measurable elasticity. Our observations indicate that the elastic character is at least as much a consequence of the attachment of spectrin as of a continuous membrane-bound network, and they offer a rationale for formation of elliptocytes in genetic conditions associated with membrane-skeletal perturbations. The theory of Parker and Winlove for elastic deformation of axisymmetric shells (accompanying paper) allows us to determine the function BH(2) for the spherical saponin-permeabilized ghost membranes (where B is the bending modulus and H the shear modulus); taking the literature value of 2 x 10(-19) Nm for B, H then emerges as 2 x 10(-6) Nm(-1). This is an order of magnitude higher than the value reported for intact cells from micropipette aspiration. Reasons for the difference are discussed.

Direct measures of large, anisotropic strains in deformation of the erythrocyte cytoskeleton

The erythrocyte's spectrin-actin membrane skeleton is directly shown to be capable of sustaining large, anisotropic strains. Photobleaching of an approximately 1-micrometer stripe in rhodamine phalloidin-labeled actin appears stable up to at least 37 degrees C, and is used to demonstrate large in-surface stretching during elastic deformation of the skeleton. Principal extension or stretch ratios of at least approximately 200% and contractions down to approximately 40%, both referenced to an essentially undistorted cell, are visually demonstrated in micropipette-imposed deformation. Such anisotropic straining is seen to be consistent at a qualitative level with now classic analyses (Evans. 1973. Biophys. J. 13:941-954) and is generally nonhomogeneous though axisymmetric down to the submicron scale. Local, direct measurements of stretching prove quantitatively consistent (within approximately 10%) with integrated estimates that are based simply on a measured relative density distribution of actin. The measurements are also in close agreement with direct computation of mean spectrin chain extension in full statistical mechanical simulations of a coarse-grained network held in a micropipette. Finally, as a cell thermally fragments near approximately 48 degrees C, the patterned photobleaching demonstrates a destructuring of the surface network in a process that is more readily attributable to transitions in spectrin than in F-actin.

Actin protofilament orientation at the erythrocyte membrane

The short actin filaments in the erythrocyte's membrane skeleton are shown to be largely oriented tangent to the lipid bilayer. Actin "proto"-filaments have previously been described as junctional centers intertriangulated by spectrin; however, the protofilaments may simultaneously serve as pinning centers between the network and the overlying bilayer. The latter function now seems of particular importance because near-normal network assembly has been reported with transgenic mouse sphero-erythrocytes that lack the primary linkage protein Band 3. To assess possible physical constraints on actin protofilaments in intact membranes, fluorescence polarization microscopy (FPM) has been used to study rhodamine phalloidin-labeled red cell ghosts. A basis for interpreting FPM images of cells is provided by FPM applied to isolated actin filaments. These are labeled with the same rhodamine probes and imaged at various orientations with respect to the polarizers, including filament orientations perpendicular to the image plane. High aperture and fluorophore conjugation effects are found to be minimal, enabling development of a simple, semi-empirical model which indicates that protofilaments are generally within approximately 20 degrees of the membrane tangent plane.

Deformability and geometry of neonatal erythrocytes with irregular shapes

More stomatocytes, spherocytes, and erythrocytes with spicules and protrusions are found in blood from newborn infants than in blood from adults. The objective of this study was to measure volume, surface area, membrane rigidity, and viscosity of red blood cells (RBC) with irregular shapes in preterm and full-term neonates. RBC from 13 full-term and 12 preterm (25-36 wk of gestation) infants were studied by means of a micropipette system. In full-term neonates and in preterm infants, 19 and 26% of RBC, respectively, showed shapes different from those of discocytes. Keratocytes, spherocytes, akanthocytes (in full-term infants), and elliptocytes (in preterm infants) had smaller volumes than did discocytes (p < 0.05). Spherocytes showed decreased and elliptocytes had increased excess surface area and swelling capacity compared with discocytes, indicating both volume and membrane loss for spherocytes and only volume loss for elliptocytes. Immature RBC and knizocytes had similar large volumes and surface areas, suggesting that knizocytes in neonates are relatively young RBC. Increased membrane rigidity was found only in stomatocytes. Membrane viscosity was decreased in immature RBC and knizocytes and increased in stomatocytes, keratocytes, akanthocytes, and elliptocytes when compared with discocytes. There were inverse relationships between RBC volume and membrane rigidity (r=-0.42) and between RBC volume and membrane viscosity (r=-0.72). This suggests that volume loss of neonatal RBC is associated with impaired membrane deformability. Moreover, we conclude that the geometry and membrane deformability of some of the irregularly shaped neonatal RBC resemble properties of RBC in hemolytic anemias (e.g. spherocytosis).

The molecular basis of activity-induced muscle injury in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common of the human muscular dystrophies, affecting approximately 1 in 3500 boys. Most DMD patients die in their late teens or early twenties due to involvement of the diaphragm and other respiratory muscles by the disease. The primary abnormality in DMD is an absence of dystrophin, a 427 kd protein normally found at the cytoplasmic face of the muscle cell surface membrane. Based upon the predicted structure and location of the protein, it has been proposed that dystrophin plays an important role in providing mechanical reinforcement to the sarcolemmal membrane of muscle fibers. Therefore, dystrophin could help to protect muscle fibers from potentially damaging tissue stresses developed during muscle contraction. In the present paper, the nature of mechanical stresses placed upon myofibers during various forms of muscle contraction are reviewed, along with current lines of evidence supporting a critical role for dystrophin as a subsarcolemmal membrane-stabilizing protein in this setting. In addition, the implications of these findings for exercise programs and other potential forms of therapy in DMD are discussed.

Biophysical properties of human erythrocyte spectrin at alkaline pH: implications for spectrin structure, function, and association

The effects of pH 6-13 on the conformation and assembly of spectrin were studied by means of analytical ultracentrifugation, circular dichroism (CD), 1H NMR, and UV spectrophotometry. Sedimentation velocity analysis showed that spectrin oligomers dissociate cooperatively into component alpha- and beta-subunits above pH 9.5, and that spectrin tetramers, heterodimers, and monomers adopt more extended and/or expanded shapes above this pH. The dissociation to monomers is mostly completed by pH 10.5 and is used as the basis for purifying the subunits [see Fujita et al. (1998) Biochemistry 37, 272-280]. Along with the dissociation, biphasic unfolding of spectrin was observed above pH 9.5 as detected by CD. The first phase of the transition occurred between pH 9.5 and 11, and the second phase between pH 11 and 13. A similar biphasic dependence was observed for the upfield shift of lysine epsilon-CH2 resonances detected by spin-echo 1H NMR and the spectrophotometric titration of the absorbance at 294 nm. These data indicate that deprotonation of tyrosine and lysine residues is closely correlated with (i) the dissociation of spectrin oligomers into heterodimers, (ii) the dissociation of heterodimers into monomers, and (iii) the unfolding of spectrin. Taken together, our data suggest that hydrophobic and electrostatic interactions involving tyrosine and lysine residues play a critical role in the formation of the alpha-helix of spectrin and assembly of physiologically relevant spectrin oligomers from the two component subunits.

Purification of erythrocyte spectrin alpha- and beta-subunits at alkaline pH and structural and hydrodynamic properties of the isolated subunits

A new method for the isolation of the alpha- and beta-subunits of human erythrocyte spectrin was developed, and structural properties and association behavior of the isolated subunits were studied by means of CD, nondenaturing gel electrophoresis, and analytical ultracentrifugation. The alpha- and beta-subunits were isolated using ion-exchange FPLC (pH 11) followed by size-exclusion FPLC (pH 7.5), having shown that alkaline pH dissociates spectrin polymers to their monomers [see Fujita et al. (1998) Biochemistry 37, 264-271]. The isolated subunits had alpha-helical content and thermal stability almost equivalent to those of native spectrin and reassembled to form heterodimers and tetramers which were indistinguishable from native spectrin with respect to secondary structure content, thermal stability, migration pattern on nondenaturing gels, and sedimentation coefficients. Thus, our data show that the increase in the structural stability of a heterodimer by association of the two monomers is very small. Sedimentation coefficients for the isolated alpha- and beta-subunits were 6.3 and 5.7 S, respectively. The similar frictional ratios (f/f0) of the isolated alpha-subunit (2.42) and the beta-subunit (2.45) indicate that the flexibility of both these wormlike chains and the range of shapes they can adopt in solution are very similar. The f/f0 value for spectrin dimer (2.41) indicates that its flexibility is somewhat, but not grossly, reduced compared to that of the individual subunits. Consequently, the folded repeat units of the subunits and the flexible connections between them are probably "in register" along the length of the dimer.

Excessive arterial thrombus in spherocytosis. A case report

Spherocytosis is the most common of the hereditary hemolytic anemias, with characteristically shaped erythrocytes. An unusually large amount of arterial thrombus was documented in a dissected artery after angioplasty in a patient with spherocytosis. It is hypothesized that the excessive arterial thrombus may have been linked to the spherocytosis.

Energy of dissociation of lipid bilayer from the membrane skeleton of red blood cells

The association between the lipid bilayer and the membrane skeleton is important to cell function. In red blood cells, defects in this association can lead to various forms of hemolytic anemia. Although proteins involved in this association have been well characterized biochemically, the physical strength of this association is only beginning to be studied. Formation of a small cylindrical strand of membrane material (tether) from the membrane involves separation of the lipid bilayer from the membrane skeleton. By measuring the force required to form a tether, and knowing the contribution to the force due to the deformation of a lipid bilayer, it is possible to calculate the additional contribution to the work of tether formation due to the separation of membrane skeleton from the lipid bilayer. In the present study, we measured the tethering force during tether formation using a microcantilever (a thin, flexible glass fiber) as a force transducer. Numerical calculations of the red cell contour were performed to examine how the shape of the contour affects the calculation of tether radius, and subsequently separation work per unit area W(sk) and bending stiffness k(c). At high aspiration pressure and small external force, the red cell contour can be accurately modeled as a sphere, but at low aspiration pressure and large external force, the contour deviates from a sphere and may affect the calculation. Based on an energy balance and numerical calculations of the cell contour, values of the membrane bending stiffness k(c) = 2.0 x 10(-19) Nm and the separation work per unit area W(sk) = 0.06 mJ/m2 were obtained.

Influence of network topology on the elasticity of the red blood cell membrane skeleton

A finite-element network model is used to investigate the influence of the topology of the red blood cell membrane skeleton on its macroscopic mechanical properties. Network topology is characterized by the number of spectrin oligomers per actin junction (phi a) and the number of spectrin dimers per self-association junction (phi s). If it is assumed that all associated spectrin is in tetrameric form, with six tetramers per actin junction (i.e., phi a = 6.0 and phi s = 2.0), then the topology of the skeleton may be modeled by a random Delaunay triangular network. Recent images of the RBC membrane skeleton suggest that the values for these topological parameters are in the range of 4.2 < phi a < 5.5 and 2.1 < phi s < 2.3. Model networks that simulate these realistic topologies exhibit values of the shear modulus that vary by more than an order of magnitude relative to triangular networks. This indicates that networks with relatively sparse nontriangular topologies may be needed to model the RBC membrane skeleton accurately. The model is also used to simulate skeletal alterations associated with hereditary spherocytosis and Southeast Asian ovalocytosis.

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.

Anion exchanger 1 (band 3) is required to prevent erythrocyte membrane surface loss but not to form the membrane skeleton

The red blood cell (RBC) membrane protein AE1 provides high affinity binding sites for the membrane skeleton, a structure critical to RBC integrity. AE1 biosynthesis is postulated to be required for terminal erythropoiesis and membrane skeleton assembly. We used targeted mutagenesis to assess AE1 function in vivo. RBCs lacking AE1 spontaneously shed membrane vesicles and tubules, leading to severe spherocytosis and hemolysis, but the levels of the major skeleton components, the synthesis of spectrin in mutant erythroblasts, and skeletal architecture are normal or nearly normal. The results indicate that AE1 does not regulate RBC membrane skeleton assembly in vivo but is essential for membrane stability. We postulate that stabilization is achieved through AE1-lipid interactions and that loss of these interactions is a key pathogenic event in hereditary spherocytosis.

Membrane skeleton involvement in cell fusion kinetics: a parameter that correlates with erythrocyte osmotic fragility

Spectrin levels in erythrocytes have been related to several biomechanical and biophysical membrane properties essential to the survival and function of the cell. Populations of erythrocytes display a natural and finite range of sensitivities to osmotic shock that has been directly correlated, in studies from other laboratories, to the presence of spectrin. We used a procedure to isolate subpopulations of 1) the osmotically most sensitive and 2) the osmotically most resistant erythrocyte membranes in an attempt to select for membranes enriched and depleted in spectrin (and/or a related component). The mechanical function of the spectrin-based membrane skeleton was further explored in these two subpopulations by searching for any effect on the time-dependent increase in fusion zone diameter in pairs of electrofused erythrocyte ghosts as a model for cell fusion. The results clearly show that the diameter expansions in fusions of membranes from osmotically resistant erythrocytes are faster in the early stage (up to 9 to 10 s after fusion) but do not thereafter expand as far as in fusions of membranes from osmotically sensitive membranes.

Remodeling the shape of the skeleton in the intact red cell

The role of the membrane skeleton in determining the shape of the human red cell was probed by weakening it in situ with urea, a membrane-permeable perturbant of spectrin. Urea by itself did not alter the biconcave disk shape of the red cell; however, above threshold conditions (1.5 M, 37 degrees C, 10 min), it caused an 18% reduction in the membrane elastic shear modulus. It also potentiated the spiculation of cells by lysophosphatidylcholine. These findings suggest that the contour of the resting cell is not normally dependent on the elasticity of or tension in the membrane skeleton. Rather, the elasticity of the skeleton stabilizes membranes against deformation. Urea treatment also caused the projections induced both by micropipette aspiration and by lysophosphatidylcholine to become irreversible. Furthermore, urea converted the axisymmetric conical spicules induced by lysophosphatidylcholine into irregular, curved and knobby spicules; i.e., echinocytosis became acanthocytosis. Unlike controls, the ghosts and membrane skeletons obtained from urea-generated acanthocytes were imprinted with spicules. These data suggest that perturbing interprotein associations with urea in situ allowed the skeleton to evolve plastically to accommodate the contours imposed upon it by the overlying membrane.

An elastic network model based on the structure of the red blood cell membrane skeleton

A finite element network model has been developed to predict the macroscopic elastic shear modulus and the area expansion modulus of the red blood cell (RBC) membrane skeleton on the basis of its microstructure. The topological organization of connections between spectrin molecules is represented by the edges of a random Delaunay triangulation, and the elasticity of an individual spectrin molecule is represented by the spring constant, K, for a linear spring element. The model network is subjected to deformations by prescribing nodal displacements on the boundary. The positions of internal nodes are computed by the finite element program. The average response of the network is used to compute the shear modulus (mu) and area expansion modulus (kappa) for the corresponding effective continuum. For networks with a moderate degree of randomness, this model predicts mu/K = 0.45 and kappa/K = 0.90 in small deformations. These results are consistent with previous computational models and experimental estimates of the ratio mu/kappa. This model also predicts that the elastic moduli vary by 20% or more in networks with varying degrees of randomness. In large deformations, mu increases as a cubic function of the extension ratio lambda 1, with mu/K = 0.62 when lambda 1 = 1.5.

Integration and regulation of hyphal tip growth

Hyphal tip growth is an exquisitely controlled process that forms developmentally regulated, species-specific, even-diameter tubes at rates of up to about 50 μm/min. The traditional view is that this process results from the balance between the expansive force of turgor pressure and the controlled extensibility of the apical cell wall. While these elements are involved, the model places regulation into either the global domain (turgor pressure) or the extracellular environment (the cell wall), neither of which seem well suited to the level of control evinced. Recent evidence suggests that F-actin-rich elements of the cytoskeleton are important in tip morphogenesis. Our current models propose that tip expansion is regulated (restrained under normal turgor pressure and protruded under low turgor) by a peripheral network of F-actin that is attached to the plasmalemma and the cell wall by integrin-containing linkages, thus placing control in the cytoplasm where it is accessible to normal intracellular regulatory systems. The F-actin system also functions in cytoplasmic and organelle motility; control of plasmalemma-located, stretch-activated, Ca2+-transporting, ion channel distribution; vectoral vesicle transport; and exocytosis. Regulation of the system may involve Ca2+, the concentration of which is influenced by the tip-high gradient of the stretch-activated channels, thus suggesting a possible feedback regulation mechanism. Key words: tip growth, fungi, stretch-activated channels, F-actin, Ca2+, hyphae.

Physical measurements of bilayer-skeletal separation forces

Bilayer membranes are intrinsically fluid in character and require stabilization by association with an underlying cytoskeleton. Instability either in the membrane-associated cytoskeleton or in the association between the bilayer and the skeleton can lead to loss of membrane bilayer and premature cell death. In this report measurements of the physical strength of the association between membrane bilayer and the membrane-associated skeleton in red blood cells are reported. These measurements involve the mechanical formation of long, thin cylinders of membrane bilayer (tethers) from the red cell surface. Ultrastructural evidence is presented indicating that these tethers do not contain membrane skeleton and, furthermore, that they are deficient in at least some integral membrane proteins. By measuring the forces on the cell as the tether is formed and the dimensions of the tether, the energy associated with its formation can be calculated. The minimum force to form a tether was found to be approximately 50 pN corresponding to an energy of dissociation of 0.2-0.3 mJ/m2. Such measurements enable critical evaluation of potential physical mechanisms for the stabilization of the membrane bilayer by the underlying cytoskeleton. It is postulated that an important contribution to the energy of association between bilayer and skeleton comes from the increase in chemical potential due to the lateral segregation of lipids and integral proteins.