A modified micropipette aspiration technique and its application to tether formation from human neutrophils

Cell Membrane Tension Gradients, Membrane Flows, and Cellular Processes

Cell membrane tension affects and is affected by many fundamental cellular processes, yet it is poorly understood. Recent experiments show that membrane tension can propagate at vastly different speeds in different cell types, reflecting physiological adaptations. Here we briefly review the current knowledge about membrane tension gradients, membrane flows, and their physiological context.

Changes in the mechanical properties of human mesenchymal stem cells during differentiation

A thorough understanding of the changes in mechanical property behind intracellular biophysical and biochemical processes during differentiation of human mesenchymal stem cells (hMSCs) is helpful to direct and enhance the commitment of cells to a particular lineage. In this study, displacement creep of the mesenchymal cell lineages (osteogenic, chondrogenic and adipogenic hMSCs) were determined by using atomic force microscopy, which was then used to determine their mechanical properties. We found that at any stages of differentiation, the mesenchymal cell lineages are linear viscoelastic materials and well matched with a simple power-law creep compliance. In addition, the viscoelasticity of mesenchymal cell lineages showed different trends during differentiation. The adipogenic hMSCs showed continuous softening at all stages. The osteogenic and chondrogenic hMSCs only continuously soften and become more fluid-like in the early stage of differentiation, and get stiffened and less fluid-like in the later stage. These findings will help more accurately imitate cellular biomechanics in the microenvironment, and provided an important reference in the biophysics biomimetic design of stem cell differentiation.

Biomechanics of Neutrophil Tethers

Leukocytes, including neutrophils, propelled by blood flow, can roll on inflamed endothelium using transient bonds between selectins and their ligands, and integrins and their ligands. When such receptor–ligand bonds last long enough, the leukocyte microvilli become extended and eventually form thin, 20 µm long tethers. Tether formation can be observed in blood vessels in vivo and in microfluidic flow chambers. Tethers can also be extracted using micropipette aspiration, biomembrane force probe, optical trap, or atomic force microscopy approaches. Here, we review the biomechanical properties of leukocyte tethers as gleaned from such measurements and discuss the advantages and disadvantages of each approach. We also review and discuss viscoelastic models that describe the dependence of tether formation on time, force, rate of loading, and cell activation. We close by emphasizing the need to combine experimental observations with quantitative models and computer simulations to understand how tether formation is affected by membrane tension, membrane reservoir, and interactions of the membrane with the cytoskeleton.

Recent Advances on the Model, Measurement Technique, and Application of Single Cell Mechanics

Since the cell was discovered by humans, it has been an important research subject for researchers. The mechanical response of cells to external stimuli and the biomechanical response inside cells are of great significance for maintaining the life activities of cells. These biomechanical behaviors have wide applications in the fields of disease research and micromanipulation. In order to study the mechanical behavior of single cells, various cell mechanics models have been proposed. In addition, the measurement technologies of single cells have been greatly developed. These models, combined with experimental techniques, can effectively explain the biomechanical behavior and reaction mechanism of cells. In this review, we first introduce the basic concept and biomechanical background of cells, then summarize the research progress of internal force models and experimental techniques in the field of cell mechanics and discuss the latest mechanical models and experimental methods. We summarize the application directions of cell mechanics and put forward the future perspectives of a cell mechanics model.

A practical review on the measurement tools for cellular adhesion force

Cell-cell and cell-matrix adhesions are fundamental in all multicellular organisms. They play a key role in cellular growth, differentiation, pattern formation and migration. Cell-cell adhesion is substantial in the immune response, pathogen-host interactions, and tumor development. The success of tissue engineering and stem cell implantations strongly depends on the fine control of live cell adhesion on the surface of natural or biomimetic scaffolds. Therefore, the quantitative and precise measurement of the adhesion strength of living cells is critical, not only in basic research but in modern technologies, too. Several techniques have been developed or are under development to quantify cell adhesion. All of them have their pros and cons, which has to be carefully considered before the experiments and interpretation of the recorded data. Current review provides a guide to choose the appropriate technique to answer a specific biological question or to complete a biomedical test by measuring cell adhesion.

Hypotonic Challenge of Endothelial Cells Increases Membrane Stiffness with No Effect on Tether Force

Regulation of cell volume is a fundamental property of all mammalian cells. Multiple signaling pathways are known to be activated by cell swelling and to contribute to cell volume homeostasis. Although cell mechanics and membrane tension have been proposed to couple cell swelling to signaling pathways, the impact of swelling on cellular biomechanics and membrane tension have yet to be fully elucidated. In this study, we use atomic force microscopy under isotonic and hypotonic conditions to measure mechanical properties of endothelial membranes including membrane stiffness, which reflects the stiffness of the submembrane cytoskeleton complex, and the force required for membrane tether formation, reflecting membrane tension and membrane-cytoskeleton attachment. We find that hypotonic swelling results in significant stiffening of the endothelial membrane without a change in membrane tension/membrane-cytoskeleton attachment. Furthermore, depolymerization of F-actin, which, as expected, results in a dramatic decrease in the cellular elastic modulus of both the membrane and the deeper cytoskeleton, indicating a collapse of the cytoskeleton scaffold, does not abrogate swelling-induced stiffening of the membrane. Instead, this swelling-induced stiffening of the membrane is enhanced. We propose that the membrane stiffening should be attributed to an increase in hydrostatic pressure that results from an influx of solutes and water into the cells. Most importantly, our results suggest that increased hydrostatic pressure, rather than changes in membrane tension, could be responsible for activating volume-sensitive mechanisms in hypotonically swollen cells.

Advances in Micropipette Aspiration: Applications in Cell Biomechanics, Models, and Extended Studies

With five decades of sustained application, micropipette aspiration has enabled a wide range of biomechanical studies in the field of cell mechanics. Here, we provide an update on the use of the technique, with a focus on recent developments in the analysis of the experiments, innovative microaspiration-based approaches, and applications in a broad variety of cell types. We first recapitulate experimental variations of the technique. We then discuss analysis models focusing on important limitations of widely used biomechanical models, which underpin the urge to adopt the appropriate ones to avoid misleading conclusions. The possibilities of performing different studies on the same cell are also considered.

Membrane Tension Acts Through PLD2 and mTORC2 to Limit Actin Network Assembly During Neutrophil Migration

For efficient polarity and migration, cells need to regulate the magnitude and spatial distribution of actin assembly. This process is coordinated by reciprocal interactions between the actin cytoskeleton and mechanical forces. Actin polymerization-based protrusion increases tension in the plasma membrane, which in turn acts as a long-range inhibitor of actin assembly. These interactions form a negative feedback circuit that limits the magnitude of membrane tension in neutrophils and prevents expansion of the existing front and the formation of secondary fronts. It has been suggested that the plasma membrane directly inhibits actin assembly by serving as a physical barrier that opposes protrusion. Here we show that efficient control of actin polymerization-based protrusion requires an additional mechanosensory feedback cascade that indirectly links membrane tension with actin assembly. Specifically, elevated membrane tension acts through phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin nucleation. In the absence of this pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling suggests roles for both the direct (mechanical) and indirect (biochemical via PLD2 and mTORC2) feedback loops in organizing cell polarity and motility—the indirect loop is better suited to enable competition between fronts, whereas the direct loop helps spatially organize actin nucleation for efficient leading edge formation and cell movement. This circuit is essential for polarity, motility, and the control of membrane tension.

A mechanosensory biochemical cascade involving phospholipase D2 and mTORC2 coordinates physical forces and cytoskeletal rearrangements to allow efficient polarization and migration of neutrophils.

How cells regulate the size and number of their protrusions for efficient polarity and motility is a fundamental question in cell biology. We recently found that immune cells known as neutrophils use physical forces to regulate this process. Actin polymerization-based protrusion stretches the plasma membrane, and this increased membrane tension acts as a long-range inhibitor of actin-based protrusions elsewhere in the cell. Here we investigate how membrane tension limits protrusion. We demonstrate that the magnitude of actin network assembly in neutrophils is determined by a mechanosensory biochemical cascade that converts increases in membrane tension into decreases in protrusion. Specifically, we show that increasing plasma membrane tension acts through a pathway containing the phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin network assembly. Without this negative feedback pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling indicates that this feedback circuit is a favorable topology to enable competition between protrusions during neutrophil polarization. Our work shows how biochemical signals, physical forces, and the cytoskeleton can collaborate to generate large-scale cellular organization.

Endothelial Surface Protrusion by a Point Force

During leukocyte rolling on the endothelium, surface protrusion and membrane tether extraction occur consecutively on leukocytes. Both surface protrusion and tether extraction of leukocytes stabilize leukocyte rolling. Tethers can also be extracted from endothelial cells (ECs), but surface protrusion of ECs has never been confirmed to exist. In this study, we examined EC surface protrusion with the micropipette aspiration technique. We found that, like leukocytes, surface protrusion on an EC did exist when a point force was imposed. Both the protrusional stiffness and the crossover force of EC surface protrusion were dependent on the force loading rate and the cytoskeletal integrity, but neither of them was dependent on tumor necrosis factor α stimulation. Temperature (37°C) affected the protrusional stiffness only at small force loading rates. When a neutrophil was employed to directly impose the pulling force on the EC, simultaneous surface protrusion from both cells occurred, and it can be modeled as two springs connected in series, although the spring constants should be adjusted according to the force loading rate. Therefore, EC surface protrusion is an important aspect of leukocyte rolling, and it should not be ignored when leukocyte rolling stability is studied systematically.

Analyses of movement and contact of two nucleated cells using a gas-driven micropipette aspiration technique

Adhesion between two nucleated cells undergoes specific significances in immune responses and tumor metastasis since cellular adhesive molecules usually express on two apposed cell membranes. However, quantification of the interactions between two nucleated cells is still challenging in microvasculature. Here distinct cell systems were used, including three types of human cells (Jurkat cell or PMN vs. MDA-MB-231 cell) and two kinds of murine native cells (PMN vs. liver sinusoidal endothelial cell). Cell movement, compression to, and relaxation from the counterpart cell were quantified using an in-house developed gas-driven micropipette aspiration technique (GDMAT). This assay is robust to quantify this process since cell movement and contact inside a pipette are independent of the repeated test cycles. Measured approaching or retraction velocity follows well a normal distribution, which is independent on the cycle period. Contact area or duration also fits a Gaussian distribution and moreover contact duration is linearly correlated with the cycle period. Cell movement is positively related to gas flux but negatively associated to medium viscosity. Cell adhesion tends to reach an equilibrium state with increase of cycle period or contact duration. These results further the understanding in the dynamics of cell movement and contact in microvasculature.

Effects of cellular viscoelasticity in lifetime extraction of single receptor-ligand bonds

Single-molecule force spectroscopy is widely used to determine kinetic parameters of dissociation by analyzing bond rupture data obtained via applying mechanical force to cells, capsules, and beads that are attached to an intermolecular bond. The bond rupture data are obtained in experiments either at a constant force or at a constant loading rate. We explore the effect of cellular viscoelasticity in constant-force experiments. Specifically, we perform Monte Carlo simulations of bond rupture at a given constant force to obtain the bond lifetime as a function of force in the absence and in the presence of bond force modulation due to cellular viscoelasticity, to explore its effect on the bond lifetime.

Nuclear deformability constitutes a rate-limiting step during cell migration in 3-D environments

Cell motility plays a critical role in many physiological and pathological settings, ranging from wound healing to cancer metastasis. While cell migration on 2-dimensional (2-D) substrates has been studied for decades, the physical challenges cells face when moving in 3-D environments are only now emerging. In particular, the cell nucleus, which occupies a large fraction of the cell volume and is normally substantially stiffer than the surrounding cytoplasm, may impose a major obstacle when cells encounter narrow constrictions in the interstitial space, the extracellular matrix, or small capillaries. Using novel microfluidic devices that allow observation of cells moving through precisely defined geometries at high spatial and temporal resolution, we determined nuclear deformability as a critical factor in the cells' ability to pass through constrictions smaller than the size of the nucleus. Furthermore, we found that cells with reduced levels of the nuclear envelope proteins lamins A/C, which are the main determinants of nuclear stiffness, passed significantly faster through narrow constrictions during active migration and passive perfusion. Given recent reports that many human cancers have altered lamin expression, our findings suggest a novel biophysical mechanism by which changes in nuclear structure and composition may promote cancer cell invasion and metastasis.

A novel pneumatic micropipette aspiration method using a balance pressure model

This paper presents a novel micropipette aspiration (MA) method based on a common pneumatic micro-injection system. This method is the first to quantify the influence of capillary effect on aspiration pressure using a balance pressure model, and in return, uses the capillary effect to quantify the aspiration pressure. Subsequently, the seal between the cell and the micropipette is detected to judge and exclude the ineffective MA attempts. The rationality of the balance pressure model is validated by the designed micropipette-filling experiments. Through applied to elasticity-determination of the cells with different sizes, the feasibility and versatility of this MA method are proved. With abilities to quantify aspiration pressures and detect the seam between the cell and the micropipette, our method is expected to advance the application of the commercial pneumatic injector in the MA of cells. Moreover, with the quantified volume of the liquid entering into the micropipette during MA process, our method also has a potential applicability to the study of the permeability of the cell membrane in the future.

Tangential tether extraction and spontaneous tether retraction of human neutrophils

Membrane tethers are extracted when neutrophils roll on the endothelium to initiate their transendothelial migration. Tether extraction from both neutrophils and endothelial cells stabilizes neutrophil rolling, so it has been studied extensively and the force-velocity relationship for tether extraction is of great interest. Due to limitations of the techniques used in previous studies, this relationship has been obtained only from tethers perpendicular to the cell surface. Here, with the microcantilever technique, where latex beads affixed on silicon cantilevers were used as the force transducer, we extracted tethers either perpendicular or tangential to the neutrophil surface. We found that the force-velocity relationship was not sensitive to tether pulling direction. Little movement of the tether-cell junction was observed during tangential tether extraction, and no coalescence was observed during multiple tether extraction. Following adhesion rupture, spontaneous tether retraction was visualized by membrane staining, which revealed two phases: one was fast and exponential, whereas the other was slow and linear. Both phases can be reproduced with a mechanical model. These results show for the first time, to our knowledge, how neutrophil tethers shorten upon instantaneous force removal, and furthermore, they illustrate how membrane tethers contribute to neutrophil rolling stability during the inflammatory response.

Rupture of single receptor-ligand bonds: a new insight into probability distribution function

Single molecule force spectroscopy is widely used to determine kinetic parameters of dissociation by analyzing bond rupture data obtained via applying mechanical force to cells, capsules, and beads that are attached to an intermolecular bond. The current analysis assumes that the intermolecular bond force is equal to the externally applied mechanical force. We confirm that viscous drag alone or in combination with cellular deformation resulting in viscoelasticity modulates bond force so that the instantaneous intermolecular bond force is not equivalent to the applied force. The bond force modulation leads to bond rupture time and force histograms that differ from those predicted by probability distribution function (PDF) using the current approach. A new methodology that accounts for bond force modulation in obtaining PDF is presented. The predicted histograms from the new methodology are in excellent agreement with the respective histograms obtained from Monte Carlo simulation.

Use the force: membrane tension as an organizer of cell shape and motility

Many cell phenomena that involve shape changes are affected by the intrinsic deformability of the plasma membrane (PM). Far from being a passive participant, the PM is now known to physically, as well as biochemically, influence cell processes ranging from vesicle trafficking to actin assembly. Here we review current understanding of how changes in PM tension regulate cell shape and movement, as well as how cells sense PM tension.

Membrane tension maintains cell polarity by confining signals to the leading edge during neutrophil migration

Little is known about how neutrophils and other cells establish a single zone of actin assembly during migration. A widespread assumption is that the leading edge prevents formation of additional fronts by generating long-range diffusible inhibitors or by sequestering essential polarity components. We use morphological perturbations, cell-severing experiments, and computational simulations to show that diffusion-based mechanisms are not sufficient for long-range inhibition by the pseudopod. Instead, plasma membrane tension could serve as a long-range inhibitor in neutrophils. We find that membrane tension doubles during leading-edge protrusion, and increasing tension is sufficient for long-range inhibition of actin assembly and Rac activation. Furthermore, reducing membrane tension causes uniform actin assembly. We suggest that tension, rather than diffusible molecules generated or sequestered at the leading edge, is the dominant source of long-range inhibition that constrains the spread of the existing front and prevents the formation of secondary fronts.

Determining beta2-integrin and intercellular adhesion molecule 1 binding kinetics in tumor cell adhesion to leukocytes and endothelial cells by a gas-driven micropipette assay

Interactions between polymorphonuclear neutrophils (PMNs) and tumor cells have been reported to facilitate the adhesion and subsequent extravasation of tumor cells through the endothelium under blood flow, both of which are mediated by binding β(2)-integrin to intercellular adhesion molecule 1 (ICAM-1). Here the adhesions between human WM9 metastatic melanoma cells, PMNs, and human pulmonary microvascular endothelial cells (HPMECs) were quantified by a gas-driven micropipette aspiration technique (GDMAT). Our data indicated that the cellular binding affinity of PMN-WM9 pair was 3.9-fold higher than that of the PMN-HPMEC pair. However, the effective binding affinities per molecular pair were comparable between the two cell pairs no matter whether WM9 cells or HPMECs were quiescent or cytokine-activated, indicating that the stronger adhesion between PMN-WM9 pair is mainly attributed to the high expression of ICAM-1 on WM9 cells. These results proposed an alternative mechanism, where WM9 melanoma cells adhere first with PMNs near vessel-wall regions and then bind to endothelial cells via PMNs under blood flow. In contrast, the adhesions between human MDA-MB-231 metastatic breast carcinoma cells and PMNs showed a comparable cellular binding affinity to PMN-HPMEC pair because the ICAM-1 expressions on MDA-MB-231 cells and HPMECs are similar. Furthermore, differences were observed in the intrinsic forward and reverse rates of the β(2)-integrin-ICAM-1 bond between PMN-TC and PMN-EC pairs. This GDMAT assay enables us to quantify the binding kinetics of cell adhesion molecules physiologically expressed on nucleated cells. The findings also further the understanding of leukocyte-facilitated tumor cell adhesion from the viewpoint of molecular binding kinetics.

Effect of cell and microvillus mechanics on the transmission of applied loads to single bonds in dynamic force spectroscopy

Receptor-ligand interactions that mediate cellular adhesion are often subjected to forces that regulate their detachment via modulating off-rates. Although the dynamics of detachment is primarily controlled by the physical chemistry of adhesion molecules, cellular features such as cell deformability and microvillus viscoelasticity have been shown to affect the rolling velocity of leukocytes in vitro through experiments and simulation. In this work, we demonstrate via various micromechanical models of two cells adhered by a single (intramolecular) bond that cell deformability and microvillus viscoelasticity modulate transmission of an applied external load to an intramolecular bond, and thus the dynamics of detachment. Specifically, it is demonstrated that the intermolecular bond force is not equivalent to the instantaneous applied force and that the instantaneous bond force decreases with cellular and microvillus compliance. As cellular compliance increases, not only does the time lag between the applied load and the bond force increase, an initial response time is observed during which cell deformation is observed without transfer of force to the bond. It is further demonstrated that following tether formation the instantaneous intramoleular bond force increases linearly at a rate dependent on microvillus viscosity. Monte Carlo simulations with fixed kinetic parameters predict that both cell and microvillus compliance increase the average rupture time, although the average rupture force based on bond length remains nearly unchanged.

The constitutive equation for membrane tether extraction

Membrane tethers or nanotubes play a critical role in a variety of cellular and subcellular processes such as leukocyte rolling and intercellular mass transport. The current constitutive equations that describe the relationship between the pulling force and the tether velocity during tether extraction have serious limitations. In this article, we propose a new phenomenological constitutive equation that captures all known characteristics of nanotube formation, including nonlinearity, nonzero threshold force, and possible negative tether velocity. We used tether extraction from endothelial cells as a prototype to illustrate how to obtain the material constants in the constitutive equation. With the micropipette aspiration technique, we measured tether pulling forces at both positive and negative tether velocities. We also determined the threshold force of 55 pN experimentally for the first time. This new constitutive equation unites two established ones and provides us a unified platform to better understand not only the physiological role of tether extraction during leukocyte rolling and intercellular or intracellular transport, but also the physics of membrane tether growth or retraction.

Multi-scale simulation of L-selectin-PSGL-1-dependent homotypic leukocyte binding and rupture

L-selectin-PSGL-1-mediated polymorphonuclear (PMN) leukocyte homotypic interactions potentiate the extent of PMN recruitment to endothelial sites of inflammation. Cell-cell adhesion is a complex phenomenon involving the interplay of bond kinetics and hydrodynamics. As a first step, a 3-D computational model based on the Immersed Boundary Method is developed to simulate adhesion-detachment of two PMN cells in quiescent conditions. Our simulations predict that the total number of bonds formed is dictated by the number of available receptors (PSGL-1) when ligands (L-selectin) are in excess, while the excess amount of ligands influences the rate of bond formation. Increasing equilibrium bond length results in a higher number of receptor-ligand bonds due to an increased intercellular contact area. On-rate constants determine the rate of bond formation, while off-rates control the average number of bonds by modulating bond lifetimes. Application of an external pulling force leads to time-dependent on- and off-rates and causes bond rupture. Moreover, the time required for bond rupture in response to an external force is inversely proportional to the applied load and decreases with increasing off-rate.

Validation, In-Depth Analysis, and Modification of the Micropipette Aspiration Technique

The micropipette aspiration technique (MAT) has been successfully applied to many studies in cell adhesion such as leukocyte-endothelium interactions. However, this technique has never been validated experimentally and it has been only employed to impose constant forces. In this study, we validated the force measurement of the MAT with the optical trap and analyzed two technical issues of the MAT, force-transducer offset and cell-micropipette gap, with finite element simulation. We also modified the MAT so that increasing or decreasing forces can be applied. With the modified MAT, we studied tether extraction from endothelial cells by pulling single tethers at increasing velocities and constant force loading rates. Before the onset of tether extraction, an apparently-linear surface protrusion of a few hundred nanometers was observed, which is likely related to membrane receptors pulling on the underlying cytoskeleton. The strength of the modified MAT lies in its capability and consistency to apply a wide range of force loading rates from several piconewtons per second up to thousands of piconewtons per second. With this modification, the MAT becomes more versatile in the study of single molecule and single cell biophysics.

A direct micropipette-based calibration method for atomic force microscope cantilevers

In this report, we describe a direct method for calibrating atomic force microscope (AFM) cantilevers with the micropipette aspiration technique (MAT). A closely fitting polystyrene bead inside a micropipette is driven by precisely controlled hydrostatic pressures to apply known loads on the sharp tip of AFM cantilevers, thus providing a calibration at the most functionally relevant position. The new method is capable of calibrating cantilevers with spring constants ranging from 0.01 to hundreds of newtons per meter. Under appropriate loading conditions, this new method yields measurement accuracy and precision both within 10%, with higher performance for softer cantilevers. Furthermore, this method may greatly enhance the accuracy and precision of calibration for colloidal probes.

Surface-bound selectin-ligand binding is regulated by carrier diffusion

Two-dimensional (2D) kinetics of receptor-ligand interactions governs cell adhesion in many biological processes. While the dissociation kinetics of receptor-ligand bond is extensively investigated, the association kinetics has much less been quantified. Recently receptor-ligand interactions between two surfaces were investigated using a thermal fluctuation assay upon biomembrane force probe technique (Chen et al. in Biophys J 94:694-701, 2008). The regulating factors on association kinetics, however, are not well characterized. Here we developed an alternative thermal fluctuation assay using optical trap technique, which enables to visualize consecutive binding-unbinding transition and to quantify the impact of microbead diffusion on receptor-ligand binding. Three selectin constructs (sLs, sPs, and PLE) and their ligand P-selectin glycoprotein ligand 1 were used to conduct the measurements. It was indicated that bond formation was reduced by enhancing the diffusivity of selectin-coupled carrier, suggesting that carrier diffusion is crucial to determine receptor-ligand binding. It was also found that 2D forward rate predicted upon first-order kinetics was in the order of sPs > sLs > PLE and bond formation was history-dependent. These results further the understandings in regulating association kinetics of surface-bound receptor-ligand interactions.

Human neutrophil surface protrusion under a point load: location independence and viscoelasticity

Mechanical properties of neutrophils have been recognized as key contributors to stabilizing neutrophil rolling on the endothelium during the inflammatory response. In particular, accumulating evidence suggests that surface protrusion and tether extraction from neutrophils facilitate stable rolling by relieving the disruptive forces on adhesive bonds. Using a customized optical trap setup, we applied piconewton-level pulling forces on targeted receptors that were located either on the microvillus tip (CD162) or intermicrovillus surface of neutrophils (CD18 and CD44). Under a constant force-loading rate, there always occurred an initial tent-like surface protrusion that was terminated either by rupture of the adhesion or by a "yield" or "crossover" to tether extraction. The corresponding protrusional stiffness of neutrophils was found to be between 0.06 and 0.11 pN/nm, depending on the force-loading rate and the cytoskeletal integrity, but not on the force location, the medium osmolality, nor the temperature increase from 22 degrees C to 37 degrees C. More importantly, we found that neutrophil surface protrusion was accompanied by force relaxation and hysteresis. In addition, the crossover force did not change much in the range of force-loading rates studied, and the protrusional stiffness of lymphocytes was similar to that of neutrophils. These results show that neutrophil surface protrusion is essentially viscoelastic, with a protrusional stiffness that stems primarily from the actin cortex, and the crossover force is independent of the receptor-cytoskeleton interaction.

Enrichment of distinct microfilament-associated and GTP-binding-proteins in membrane/microvilli fractions from lymphoid cells

Lymphocyte microvilli mediate initial adhesion to endothelium during lymphocyte transition from blood into tissue but their molecular organization is incompletely understood. We modified a shear-based procedure to prepare biochemical fractions enriched for membrane/microvilli (MMV) from both human peripheral blood T-lymphocytes (PBT) and a mouse pre-B lymphocyte line (300.19). Enrichment of proteins in MMV relative to post nuclear lysate was determined by LC/MS/MS analysis and label-free quantitation. Subsequent analysis emphasized the 291 proteins shared by PBT and 300.19 and estimated by MS peak area to be highest abundance. Validity of the label-free quantitation was confirmed by many internal consistencies and by comparison with Western blot analyses. The MMV fraction was enriched primarily for subsets of cytoskeletal proteins, transmembrane proteins and G-proteins, with similar patterns in both lymphoid cell types. The most enriched cytoskeletal proteins were microfilament-related proteins NHERF1, Ezrin/Radixin/Moesin (ERMs), ADF/cofilin and Myosin1G. Other microfilament proteins such as talin, gelsolin, myosin II and profilin were markedly reduced in MMV, as were intermediate filament- and microtubule-related proteins. Heterotrimeric G-proteins and some small G-proteins (especially Ras and Rap1) were enriched in the MMV preparation. Two notable general observations also emerged. There was less overlap between the two cells in their transmembrane proteins than in other classes of proteins, consistent with a special role of plasma membrane proteins in differentiation. Second, unstimulated primary T-lymphocytes have an unusually high concentration of actin and other microfilament related proteins, consistent with the singular role of actin-mediated motility in the immunological surveillance performed by these primary cells.

Simultaneous tether extraction from endothelial cells and leukocytes: observation, mechanics, and significance

It has been hypothesized, from earlier studies on single-tether extraction from individual leukocytes and human umbilical vein endothelial cells, that during rolling of leukocytes on the endothelium, simultaneous extraction of membrane nanotubes (tethers) occurs, resulting in enhancement of the force decrease on the adhesive bond. In this study, using the micropipette aspiration technique and fluorescence microscopy, we show that tethers are indeed extracted simultaneously when an endothelial cell and a leukocyte are separated after brief contact and adhesion, and the endothelial cell contributes much more to the composite tether length. In addition, the constitutive relationship for simultaneous tether extraction is determined with neutrophils and T-lymphocytes as force transducers, and cytokine-stimulated human umbilical vein and dermal microvascular endothelial cells as substrates, respectively. This relationship is consistent with that derived theoretically from the constitutive equations for single-tether extraction from either cell alone. Moreover, we show that simultaneous tether extraction was likely terminated by receptor-ligand bond dissociation. With a biomechanical model of leukocyte rolling, we predict the force history of the adhesive receptor-ligand bond and show that it is remarkably similar for different leukocyte-endothelial cell pairs. Simultaneous tether extraction therefore represents a generic mechanism for stabilizing leukocyte rolling on the endothelium.

Effect of temperature on tether extraction, surface protrusion, and cortical tension of human neutrophils

Neutrophil rolling on endothelial cells, the initial stage of its migrational journey to a site of inflammation, is facilitated by tether extraction and surface protrusion. Both phenomena have been studied extensively at room temperature, which is considerably lower than human body temperature. It is known that temperature greatly affects cellular mechanical properties such as viscosity. Therefore, we carried out tether extraction, surface protrusion, and cortical tension experiments at 37 degrees C with the micropipette aspiration technique. The experimental temperature was elevated using a custom-designed microscope chamber for the micropipette aspiration technique. To evaluate the constant temperature assumption in our experiments, the temperature distribution in the whole chamber was computed with finite element simulation. Our simulation results showed that temperature variation around the location where our experiments were performed was less than 0.2 degrees C. For tether extraction at 37 degrees C, the threshold force required to pull a tether (40 pN) was not statistically different from the value at room temperature (51 pN), whereas the effective viscosity (0.75 pN.s/microm) decreased significantly from the value at room temperature (1.5 pN.s/microm). Surface protrusion, which was modeled as a linear deformation, had a slightly smaller spring constant at 37 degrees C (40 pN/microm) than it did at room temperature (56 pN/microm). However, the cortical tension at 37 degrees C (5.7+/-2.2 pN/microm) was substantially smaller than that at room temperature (23+/-8 pN/microm). These data clearly suggest that neutrophils roll differently at body temperature than they do at room temperature by having distinct mechanical responses to shear stress of blood flow.

The effect of cellular cholesterol on membrane-cytoskeleton adhesion

Whereas recent studies suggest that cholesterol plays important role in the regulation of membrane proteins, its effect on the interaction of the cell membrane with the underlying cytoskeleton is not well understood. Here, we investigated this by measuring the forces needed to extract nanotubes (tethers) from the plasma membrane, using atomic force microscopy. The magnitude of these forces provided a direct measure of cell stiffness, cell membrane effective surface viscosity and association with the underlying cytoskeleton. Furthermore, we measured the lateral diffusion constant of a lipid analog DiIC12, using fluorescence recovery after photobleaching, which offers additional information on the organization of the membrane. We found that cholesterol depletion significantly increased the adhesion energy between the membrane and the cytoskeleton and decreased the membrane diffusion constant. An increase in cellular cholesterol to a level higher than that in control cells led to a decrease in the adhesion energy and the membrane surface viscosity. Disassembly of the actin network abrogated all the observed effects, suggesting that cholesterol affects the mechanical properties of a cell through the underlying cytoskeleton. The results of these quantitative studies may help to better understand the biomechanical processes accompanying the development of atherosclerosis.

Tether extrusion from red blood cells: integral proteins unbinding from cytoskeleton

We investigate the mechanical strength of adhesion and the dynamics of detachment of the membrane from the cytoskeleton of red blood cells (RBCs). Using hydrodynamical flows, we extract membrane tethers from RBCs locally attached to the tip of a microneedle. We monitor their extrusion and retraction dynamics versus flow velocity (i.e., extrusion force) over successive extrusion-retraction cycles. Membrane tether extrusion is carried out on healthy RBCs and ATP-depleted or -inhibited RBCs. For healthy RBCs, extrusion is slow, constant in velocity, and reproducible through several extrusion-retraction cycles. For ATP-depleted or -inhibited cells, extrusion dynamics exhibit an aging phenomenon through extrusion-retraction cycles: because the extruded membrane is not able to retract properly onto the cell body, each subsequent extrusion exhibits a loss of resistance to tether growth over the tether length extruded at the previous cycle. In contrast, the additionally extruded tether length follows healthy dynamics. The extrusion velocity L depends on the extrusion force f according to a nonlinear fashion. We interpret this result with a model that includes the dynamical feature of membrane-cytoskeleton association. Tether extrusion leads to a radial membrane flow from the cell body toward the tether. In a distal permeation regime, the flow passes through the integral proteins bound to the cytoskeleton without affecting their binding dynamics. In a proximal sliding regime, where membrane radial velocity is higher, integral proteins can be torn out, leading to the sliding of the membrane over the cytoskeleton. Extrusion dynamics are governed by the more dissipative permeation regime: this leads to an increase of the membrane tension and a narrowing of the tether, which explains the power law behavior of L(f). Our main result is that ATP is necessary for the extruded membrane to retract onto the cell body. Under ATP depletion or inhibition conditions, the aging of the RBC after extrusion is interpreted as a perturbation of membrane-cytoskeleton linkage dynamics.

Eukaryotic membrane tethers revisited using magnetic tweezers

Membrane nanotubes, under physiological conditions, typically form en masse. We employed magnetic tweezers (MTW) to extract tethers from human brain tumor cells and compared their biophysical properties with tethers extracted after disruption of the cytoskeleton and from a strongly differing cell type, Chinese hamster ovary cells. In this method, the constant force produced with the MTW is transduced to cells through super-paramagnetic beads attached to the cell membrane. Multiple sudden jumps in bead velocity were manifest in the recorded bead displacement-time profiles. These discrete events were interpreted as successive ruptures of individual tethers. Observation with scanning electron microscopy supported the simultaneous existence of multiple tethers. The physical characteristics, in particular, the number and viscoelastic properties of the extracted tethers were determined from the analytic fit to bead trajectories, provided by a standard model of viscoelasticity. Comparison of tethers formed with MTW and atomic force microscopy (AFM), a technique where the cantilever-force transducer is moved at constant velocity, revealed significant differences in the two methods of tether formation. Our findings imply that extreme care must be used to interpret the outcome of tether pulling experiments performed with single molecular techniques (MTW, AFM, optical tweezers, etc). First, the different methods may be testing distinct membrane structures with distinct properties. Second, as soon as a true cell membrane (as opposed to that of a vesicle) can attach to a substrate, upon pulling on it, multiple nonspecific membrane tethers may be generated. Therefore, under physiological conditions, distinguishing between tethers formed through specific and nonspecific interactions is highly nontrivial if at all possible.

Double-tether extraction from human umbilical vein and dermal microvascular endothelial cells

Multiple tethers are very likely extracted when leukocytes roll on the endothelium under high shear stress. Endothelial cells have been predicted to contribute more significantly to simultaneous tethers and thus to the overall rolling stabilization. We therefore extracted and quantified double tethers from endothelial cells with the micropipette aspiration technique. We show that the constitutive parameters (threshold force (F0) and effective viscosity (etaeff)) for double-tether extraction are twice those for single-tether extraction and are remarkably similar for human neonatal (F0=105+/-5 pN; etaeff=1.0+/-0.1 pN.s/microm) and adult (F0=118+/-13 pN; etaeff=1.3+/-0.2 pN.s/microm) dermal microvascular, and human umbilical vein (F0=99+/-3 pN; etaeff=1.0+/-0.1 pN.s/microm) endothelial cells. Additionally, these parameters are also independent of surface receptor type, cytokine stimulation, and attachment state of the endothelial cell. We also introduce a novel correlation between the cell-substrate contact stress and gap width, with which we can predict the apparent cell-substrate separation range to be 0.01-0.1 microm during leukocyte rolling. With a biomechanical model of leukocyte rolling, we calculate the force history on the receptor-ligand bond during tether extraction and predict maximum stabilization for the double simultaneous tether extraction case.

Single membrane tether extraction from adult and neonatal dermal microvascular endothelial cells

Membrane tethers were found to be extracted from leukocytes and macrovascular endothelial cells (e.g., human umbilical vein endothelial cells or HUVECs) when a point pulling force was exerted. These tethers stabilize leukocyte rolling on the endothelium during the inflammatory response. However, little is known about tether extraction from other vascular cells like microvascular endothelial cells (MECs). In this study, we extracted tethers from both adult and neonatal dermal MECs with the micropipette aspiration technique. We found a linear relationship between the pulling force and tether growth velocity for both cell lines. This constitutive relationship is mainly determined by the membrane mechanical property and the underlying actin-based cytoskeleton for both attached and suspended endothelial cells. It is independent of cell surface receptor type, attachment state, cytokine stimulation, or cell lineage. For both types of MECs, the threshold forces are approximately 50 pN and the effective viscosities are around 0.5 pN x s/microm. These results, which are close to what was obtained from HUVECs, indicate that homogeneity is preserved in terms of tether extraction among different types of endothelial cells, and simultaneous tethers are likely extracted when leukocytes roll on either microvascular or macrovascular surfaces.

Neutrophil-bead collision assay: pharmacologically induced changes in membrane mechanics regulate the PSGL-1/P-selectin adhesion lifetime

Visualization of flowing neutrophils colliding with adherent 1-mum-diameter beads presenting P-selectin allowed the simultaneous measurement of collision efficiency (epsilon), membrane tethering fraction (f), membrane tether growth dynamics, and PSGL-1/P-selectin binding lifetime. For 1391 collisions analyzed over venous wall shear rates from 25 to 200 s(-1), epsilon decreased from 0.17 to 0.004, whereas f increased from 0.15 to 0.70, and the average projected membrane tether length, L(tether)(m), increased from 0.35 mum to approximately 2.0 mum over this shear range. At all shear rates tested, adhesive collisions lacking membrane tethers had average bond lifetimes less than those observed for collisions with tethers. For adhesive collisions that failed to form membrane tethers, the regressed Bell parameters (consistent with single bond Monte Carlo simulation) were zero-stress off-rate, k(off)(0) = 0.56 s(-1) and reactive compliance, r = 0.10 nm, similar to published atomic force microscopy (AFM) measurements. For all adhesion events (+/- tethers), the bond lifetime distributions were more similar to those obtained by rolling assay and best simulated by Monte Carlo with the above Bell parameters and an average of 1.48 bonds (n = 1 bond (67%), n = 2 (22%), and n = 3-5 (11%)). For collisions at 100 s(-1), pretreatment of neutrophils with actin depolymerizing agents, latrunculin or cytochalasin D, had no effect on epsilon, but increased L(tether)(m) by 1.74- or 2.65-fold and prolonged the average tether lifetime by 1.41- or 1.65-fold, respectively. Jasplakinolide, an actin polymerizing agent known to cause blebbing, yielded results similar to the depolymerizing agents. Conversely, cholesterol-depletion with methyl-beta-cyclodextrin or formaldehyde fixation had no effect on epsilon, but reduced L(tether)(m) by 66% or 97% and reduced the average tether lifetime by 30% or 42%, respectively. The neutrophil-bead collision assay combines advantages of atomic force microscopy (small contact zone), aggregometry (discrete interactions), micropipette manipulation (tether visualization), and rolling assays (physiologic flow loading). Membrane tether growth can be enhanced or reduced pharmacologically with consequent effects on PSGL-1/P-selectin lifetimes.

Effect of microvillus deformability on leukocyte adhesion explored using adhesive dynamics simulations

Leukocyte rolling on the endothelium via selectin molecules is an important step in the adhesion cascade, which allows leukocytes in the bloodstream to reach sites of infection. We improve upon Adhesive Dynamics simulations by incorporating deformable microvilli on which adhesion molecules are clustered. As determined in micropipette experiments, microvilli deform like an elastic spring at small forces and a combination of yield and viscous dissipation at high forces. First, we create a modified version of the state diagram for adhesion which includes microvillus deformation, and find four adhesion states-firmly bound; landing; rolling; and no-adhesion. Then, we simulate the effects of receptor clustering on the tips of microvilli, number of adhesion molecules on the cell, and the spring constant of the bonds, within the context of deformable microvilli. We also explore how the microvillus rheology itself controls the dynamics of adhesion. A minimum in rolling velocity occurs at an intermediate value of the microvillus membrane viscosity, remarkably close to the reported physiological value, suggesting that the mechanics of microvilli have evolved ideally for rolling and adhesion of leukocytes. We find that a larger degree of association between the membrane and cytoskeleton leads to slower rolling, and stiffer microvilli result in faster rolling. Decreasing the overall deformability of the microvilli greatly reduces a simulated cell's ability to roll. A comparison to experimental results of in vitro cell rolling agrees with the simulation at low shear rates. Furthermore, simulated rolling trajectories of cells with deformable microvilli display periods of rolling interdispersed with pauses, consistent with that seen in experiments where microvilli were observed to stretch.

Double tether extraction from human neutrophils and its comparison with CD4+ T-lymphocytes

The initial arrest and subsequent rolling of a leukocyte on the vascular endothelium is believed to be facilitated by the extraction of tethers, which are narrow membranous tubes drawn from the leukocyte. Although single tether extraction from neutrophils has been studied thoroughly, the relationship between the tether force (F) and tether-growth velocity (U(t)) is still unknown for double tethers drawn from neutrophils. In this study, we have determined this relationship with the micropipette-aspiration technique. As a comparison, tether extraction from CD4+ T-lymphocytes was also studied. The threshold force and effective viscosity for single tether extraction from passive CD4+ T-lymphocytes were found to be 46 pN and 1.55 pN x s/microm, respectively. These values were modulated by stimulation with phorbol myristate acetate (PMA), but not interleukin-8 (IL-8). More importantly, for both types of leukocyte, the threshold force and effective viscosity for double tether extraction are about twice as large as those corresponding to single tether extraction. Neither IL-8 nor PMA stimulation had any effect on this correlation. These results indicate that double tethers are highly localized on cellular surfaces and independent of each other during the rolling process.

Membrane tether extraction from human umbilical vein endothelial cells and its implication in leukocyte rolling

During the rolling of human neutrophils on the endothelium, tethers (cylindrical membrane tubes) are likely extracted from the neutrophil. Tether extraction reduces the force imposed on the adhesive bond between the neutrophil and endothelium, thereby facilitating the rolling. However, whether tethers can be extracted from the endothelium is still unknown. Here, with the micropipette-aspiration technique, we show that tethers can be extracted from either suspended or attached human umbilical vein endothelial cells. We also show that a linear relationship between the pulling force and tether growth velocity exists and this relationship does not depend on the receptor type (used to impose point forces), tumor necrosis factor-alpha stimulation, or cell attachment state. With linear regression, we determined that the threshold force was 50 pN and the effective viscosity was 0.50 pN.s/microm. Therefore, tethers might be simultaneously extracted from the neutrophil and endothelial cell during the rolling and, more importantly, the endothelial cell might contribute much more to the total composite tether length than the neutrophil. Compared with tether extraction from the neutrophil alone, simultaneous tether extraction results in a larger increase in the lifetime of the adhesive bond, and thus further stabilizes the rolling of neutrophils under high physiological shear stresses.