Mechanical properties of L929 cells measured by atomic force microscopy: effects of anticytoskeletal drugs and membrane crosslinking

Nanomechanical analysis of cells from cancer patients

Change in cell stiffness is a new characteristic of cancer cells that affects the way they spread. Despite several studies on architectural changes in cultured cell lines, no ex vivo mechanical analyses of cancer cells obtained from patients have been reported. Using atomic force microscopy, we report the stiffness of live metastatic cancer cells taken from the body (pleural) fluids of patients with suspected lung, breast and pancreas cancer. Within the same sample, we find that the cell stiffness of metastatic cancer cells is more than 70% softer, with a standard deviation over five times narrower, than the benign cells that line the body cavity. Different cancer types were found to display a common stiffness. Our work shows that mechanical analysis can distinguish cancerous cells from normal ones even when they show similar shapes. These results show that nanomechanical analysis correlates well with immunohistochemical testing currently used for detecting cancer.

Atomic force microscopy probing of cell elasticity

Atomic force microscopy (AFM) has recently provided the great progress in the study of micro- and nanostructures including living cells and cell organelles. Modern AFM techniques allow solving a number of problems of cell biomechanics due to simultaneous evaluation of the local mechanical properties and the topography of the living cells at a high spatial resolution and force sensitivity. Particularly, force spectroscopy is used for mapping mechanical properties of a single cell that provides information on cellular structures including cytoskeleton structure. This entry is aimed to review the recent AFM applications for the study of dynamics and mechanical properties of intact cells associated with different cell events such as locomotion, differentiation and aging, physiological activation and electromotility, as well as cell pathology. Local mechanical characteristics of different cell types including muscle cells, endothelial and epithelial cells, neurons and glial cells, fibroblasts and osteoblasts, blood cells and sensory cells are analyzed in this paper.

Vascular adaptation and mechanical homeostasis at tissue, cellular, and sub-cellular levels

Blood vessels exhibit a remarkable ability to adapt throughout life that depends upon genetic programming and well-orchestrated biochemical processes. Findings over the past four decades demonstrate, however, that the mechanical environment experienced by these vessels similarly plays a critical role in governing their adaptive responses. This article briefly reviews, as illustrative examples, six cases of tissue level growth and remodeling, and then reviews general observations at cell-matrix, cellular, and sub-cellular levels, which collectively point to the existence of a "mechanical homeostasis" across multiple length and time scales that is mediated primarily by endothelial cells, vascular smooth muscle cells, and fibroblasts. In particular, responses to altered blood flow, blood pressure, and axial extension, disease processes such as cerebral aneurysms and vasospasm, and diverse experimental manipulations and clinical treatments suggest that arteries seek to maintain constant a preferred (homeostatic) mechanical state. Experiments on isolated microvessels, cell-seeded collagen gels, and adherent cells isolated in culture suggest that vascular cells and sub-cellular structures such as stress fibers and focal adhesions likewise seek to maintain constant a preferred mechanical state. Although much is known about mechanical homeostasis in the vasculature, there remains a pressing need for more quantitative data that will enable the formulation of an integrative mathematical theory that describes and eventually predicts vascular adaptations in response to diverse stimuli. Such a theory promises to deepen our understanding of vascular biology as well as to enable the design of improved clinical interventions and implantable medical devices.

Viscoelastic properties of human mesenchymally-derived stem cells and primary osteoblasts, chondrocytes, and adipocytes

The mechanical properties of single cells play important roles in regulating cell-matrix interactions, potentially influencing the process of mechanotransduction. Recent studies also suggest that cellular mechanical properties may provide novel biological markers, or "biomarkers," of cell phenotype, reflecting specific changes that occur with disease, differentiation, or cellular transformation. Of particular interest in recent years has been the identification of such biomarkers that can be used to determine specific phenotypic characteristics of stem cells that separate them from primary, differentiated cells. The goal of this study was to determine the elastic and viscoelastic properties of three primary cell types of mesenchymal lineage (chondrocytes, osteoblasts, and adipocytes) and to test the hypothesis that primary differentiated cells exhibit distinct mechanical properties compared to adult stem cells (adipose-derived or bone marrow-derived mesenchymal stem cells). In an adherent, spread configuration, chondrocytes, osteoblasts, and adipocytes all exhibited significantly different mechanical properties, with osteoblasts being stiffer than chondrocytes and both being stiffer than adipocytes. Adipose-derived and mesenchymal stem cells exhibited similar properties to each other, but were mechanically distinct from primary cells, particularly when comparing a ratio of elastic to relaxed moduli. These findings will help more accurately model the cellular mechanical environment in mesenchymal tissues, which could assist in describing injury thresholds and disease progression or even determining the influence of mechanical loading for tissue engineering efforts. Furthermore, the identification of mechanical properties distinct to stem cells could result in more successful sorting procedures to enrich multipotent progenitor cell populations.

Lipid rafts in membrane-cytoskeleton interactions and control of cellular biomechanics: actions of oxLDL

Membrane-cytoskeleton coupling is known to play major roles in a plethora of cellular responses, such as cell growth, differentiation, polarization, motility, and others. In this review, the authors discuss the growing amount of evidence indicating that membrane-cytoskeleton interactions are regulated by the lipid composition of the plasma membrane, suggesting that cholesterol-rich membrane domains (lipid rafts), including caveolae, are essential for membrane-cytoskeleton coupling. Several models for raft-cytoskeleton interactions are discussed. Also described is the evidence suggesting that raft-cytoskeleton interactions play key roles in several cytoskeleton-dependent processes, particularly in the regulation of cellular biomechanical properties. To address further the physiological significance of raft-cytoskeleton coupling, the authors focus on the impact of oxidized low density lipoproteins, one of the major cholesterol carriers and proatherogenic factors, on the integrity of lipid rafts/caveolae, and on the organization of the cytoskeleton. Finally, the authors review the recent studies showing that oxLDL and cholesterol depletion have similar impacts on the biomechanical properties of vascular endothelial cells, which in turn affect endothelial angiogenic potential.

Constitutive material modeling of cell: a micromechanics approach

The variations in mechanical properties of cells obtained from experimental and theoretical studies can be overcome only through the development of a sound mathematical framework correlating the derived mechanical property with the cellular structure. Such a formulation accounting for the inhomogeneity of the cytoplasm due to stress fibers and actin cortex is developed in this work. The proposed model is developed using the Mori-Tanaka method of homogenization by treating the cell as a fiber-reinforced composite medium satisfying the continuum hypothesis. The validation of the constitutive model using finite element analysis on atomic force microscopy (AFM) and magnetic twisting cytometry (MTC) has been carried out and is found to yield good correlation with reported experimental results. It is observed from the study that as the volume fraction of the stress fiber increases, the stiffness of the cell increases and it alters the force displacement behavior for the AFM and MTC experiments. Through this model, we have also been able to find the stress fiber as a likely cause of the differences in the derived mechanical property from the AFM and MTC experiments. The correlation of the mechanical behavior of the cell with the cell composition, as obtained through this study, is an important observation in cell mechanics.

Spectral domain phase microscopy for local measurements of cytoskeletal rheology in single cells

We present spectral domain phase microscopy (SDPM) as a new tool for measurements at the cellular scale. SDPM is a functional extension of spectral domain optical coherence tomography that allows for the detection of cellular motions and dynamics with nanometer-scale sensitivity in real time. Our goal was to use SDPM to investigate the mechanical properties of the cytoskeleton of MCF-7 cells. Magnetic tweezers were designed to apply a vertical force to ligand-coated magnetic beads attached to integrin receptors on the cell surfaces. SDPM was used to resolve cell surface motions induced by the applied stresses. The cytoskeletal response to an applied force is shown for both normal cells and those with compromised actin networks due to treatment with Cytochalasin D. The cell response data were fit to several models for cytoskeletal rheology, including one- and two-exponential mechanical models, as well as a power law. Finally, we correlated displacement measurements to physical characteristics of individual cells to better compare properties across many cells, reducing the coefficient of variation of extracted model parameters by up to 50%.

Elastic properties of the cell surface and trafficking of single AMPA receptors in living hippocampal neurons

Although various approaches are routinely used to study receptor trafficking, a technology that allows for visualizing trafficking of single receptors at the surface of living cells remains lacking. Here we used atomic force microscope to simultaneously probe the topography of living cells, record the elastic properties of their surface, and examine the distribution of transfected alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA)-type glutamate receptors (AMPAR). On nonstimulated neurons, AMPARs were located in stiff nanodomains with high elasticity modulus relative to the remaining cell surface. Receptor stimulation with N-methyl-D-aspartate (NMDA) provoked a permanent disappearance of these stiff nanodomains followed by a decrease (53%) of the number of surface AMPARs. Blocking electrical activity before NMDA stimulation recruited the same number of AMPARs for internalization, preceded by the loss of the stiff nanodomains. However, in that case, the stiff nanodomains were recovered and AMPARs were reinserted into the membrane shortly after. Our results show that modulation of receptor distribution is accompanied by changes in the local elastic properties of cell membrane. We postulate, therefore, that the mechanical environment of a receptor might be critical to determine its specific distribution behavior in response to different stimuli.

Monitoring cell adhesion by using thickness shear mode acoustic wave sensors

The thickness shear mode (TSM) acoustic wave sensor attached with living cells has been shown to be an effective functional biosensing device to monitor the process of cell adhesion to a surface in real time. In this study, a multilayer sensor model that includes a quartz substrate, a cell-substrate interfacial layer and a cell layer was constructed based on the state of cell adhesion to the substrate. The dynamic process of cell adhesions as a function of cell seeding densities was monitored using the acoustic wave sensor. The mechanisms that are responsible for the frequency and resistance change are discussed according to the predictions of the acoustic wave sensor model. In addition, knowing that the actin cytoskeleton is important for cell adhesion, we investigated the motional resistance change caused by the disruption of actin cytoskeleton induced by fungal toxin cytochalasin D in the human skin fibroblasts. The results indicate that resistance changes are related to the disruption of actin cytoskeleton and dosage-dependent.

Distinct contributions of microtubule subtypes to cell membrane shape and stability

Microtubules (MTs) are linked to cell mechanobiology. "Stable" and "dynamically unstable" microtubule (MT) subtypes are differentially sensitive to growth and distribution in serum starved (SS) versus full serum (FS) conditions. Atomic Force and Immunofluorescence microscopies were used to study the nanomechanical properties of the cell membrane in response to serum conditions and nocodazole. Nanomechanical properties of the cell membrane remain unchanged under SS/FS conditions even though there are drastic MT changes. The cell membrane is shown to depend on unstable MTs and the intermediate filament (IF) networks to maintain local stiffness. Measurements of local membrane nanomechanics in response to nocodazole display characteristic serum dependent decays. The responses suggest that the cell exists in a mechanical transition state. Stiffness is shown to depend on the interplay between dynamically unstable MTs, stable MTs and IFs which all act to impart a distinct cellular type of transient "metastability".

Cytoskeletal changes of mesenchymal stem cells during differentiation

Mesenchymal stem cells (MSCs) are progenitors for tissues such as bone and cartilage. In this report, the actin cytoskeleton and nanomechanobiology of human mesenchymal stem cells (hMSCs) were studied using fluorescence microscopy and atomic force microscopy (AFM). Human MSCs were differentiated into chondrocytes and osteoblasts as per previous approaches. Cytochalasin D (CytD) was used to temporarily disrupt cytoskeleton in hMSCs, hMSC-chondrocytes (hMSC-Cys) and hMSC-osteoblasts (hMSC-Obs). Fluorescence microscopy revealed a dose-dependent response to CytD. Removal of CytD from the media of cytoskeleton-disrupted cells led to the recovery of the cytoskeletal structures, as confirmed by both fluorescence microscopy and AFM. Force-volume imaging by AFM evaluated the nanomechanics of all three cell types before, during, and after CytD treatment. Cytochalasin D disruption of cytoskeleton had marked effects on hMSCs and hMSC-Cys, in comparison with limited cytoskeleton disruption in hMSC-Obs, as confirmed qualitatively by fluorescence microscopy and quantitatively by AFM. Treatment with CytD resulted in morphology changes of all cell types, with significant decreases in the observed Young's Moduli of hMSCs and hMSC-Cys. These data suggest human mesenchymal stem cells alter their cytoskeletal components during differentiation. Additional studies will address the mechanisms of cytoskeletal changes using biochemical and biophysical methods.

Cellular chemomechanics at interfaces: sensing, integration and response

Living cells are complex entities whose remarkable, emergent capacity to sense, integrate, and respond to environmental cues relies on an intricate series of interactions among the cell's macromolecular components. Defects in mechanosensing, transduction,or responses underlie many diseases such as cancers, immune disorders, cardiac hypertrophy, genetic malformations, and neuropathies. Here, we highlight micro- and nanotechnology-based tools that have been used to study how chemical and mechanical cues modulate the responses of single cells in contact with the extracellular environment. Understanding the physical aspects of these complex processes at the micro- and nanometer scale could produce profound and fundamental new insights into how the processes of cell migration, metastasis, immune function and other areas which are regulated by mechanical forces.

Atomic force microscopy of mechanically trapped bacterial cells

This article presents a study on the influence of the protocol used for immobilization of bacterial cells onto surfaces by mechanically trapping them into a filter. In this sense, the surface and structure of trapped cells are analyzed. Bacteria can be present solely or with extracellular polymeric substances (EPS). To test the behavior of the EPS layer duing the filtering process, different strains of a well-known EPS-producer bacteria (Staphylococcus epidermidis), which produce an extracellular matrix clearly visible in AFM images, have been used. Results show that this immobilization method can cause severe structural and mechanical deformation to the cell membrane. This altered mechanical state may possibly influence the parameters derived from AFM force curves (which are micro/nano-mechanical tests). Also, our results suggest that the EPS layer might move during the filtering process and could accumulate at the upper part of the cell, thus favoring distorted data of adhesion/pull-off forces as measured by an AFM tip, especially in the case of submicron-sized microbial cells such as bacteria.

Mapping correlated membrane pulsations and fluctuations in human cells

The cell membrane and cytoskeleton are dynamic structures that are strongly influenced by the thermo-mechanical background in addition to biologically driven mechanical processes. We used atomic force microscopy (AFM) to measure the local membrane motion of human foreskin fibroblasts (HFFs) which were found to be governed by random and non-random correlated mechanical processes. Interphase cells displayed distinct membrane pulsations in which the membrane was observed to slowly contract upwards followed by a recovery to its initial position. These pulsations occurred one to three times per minute with variable amplitudes (20-100 pN) separated by periods of random baseline fluctuations with amplitudes of <20 pN. Cells were exposed to actin and microtubule (MT) destabilizing drugs and induced into early apoptosis. Mechanical pulsations (20-80 pN) were not prevented by actin or MT depolymerization but were prevented in early apoptotic cells which only displayed small amplitude baseline fluctuations (<20 pN). Correlation analysis revealed that the cell membrane motion is largely random; however several non-random processes, with time constants varying between approximately 2 and 35 s are present. Results were compared to measured cardiomyocyte motion which was well defined and highly correlated. Employing automated positioning of the AFM tip, interphase HFF correlation time constants were also mapped over a 10 microm2 area above the nucleus providing some insights into the spatial variability of membrane correlations. Here, we are able to show that membrane pulsations and fluctuations can be linked to physiological state and cytoskeletal dynamics through distinct sets of correlation time constants in human cells.

A theoretical model for F-actin remodeling in vascular smooth muscle cells subjected to cyclic stretch

A constrained mixture theory model was developed and used to estimate remodeling of F-actin in vascular smooth muscle cells that were subjected to 10% equibiaxial stretching for up to 30min. The model was based on a synthesis of data on time-dependent changes in atomic force microscopy measured cell stiffness and immunofluorescence measured focal adhesion associated vinculin as well as data on stress fiber stiffness and pre-stretch. Results suggest that an observed acute (after 2min of stretching) increase in cell stiffness is consistent with an increased stretch of the originally present F-actin plus an assembly of new F-actin having nearly homeostatic values of stretch. Moreover, the subsequent (after 30min of stretching) decrease in cell stiffness back towards the baseline value is consistent with a replacement of the overstretched original filaments with the new (reassembled), less stretched filaments. That is, overall cell response is consistent with a recently proposed concept of "tensional homeostasis" whereby cells seek to maintain constant certain mechanical factors via a remodeling of intracellular and transmembrane proteins. Although there is a need to refine the model based on more comprehensive data sets, using multiple experimental approaches, the present results suggest that a constrained mixture theory can capture salient features of the dynamics of F-actin remodeling and that it offers some advantages over many past methods of modeling, particularly those based on classical linearized viscoelasticity.

Frequency response of a viscoelastic tensegrity model: Structural rearrangement contribution to cell dynamics

In an attempt to understand the role of structural rearrangement onto the cell response during imposed cyclic stresses, we simulated numerically the frequency-dependent behavior of a viscoelastic tensegrity structure (VTS model) made of 24 elastic cables and 6 rigid bars. The VTS computational model was based on the nonsmooth contact dynamics (NSCD) method in which the constitutive elements of the tensegrity structure are considered as a set of material points that mutually interact. Low amplitude oscillatory loading conditions were applied and the frequency response of the overall structure was studied in terms of frequency dependence of mechanical properties. The latter were normalized by the homogeneous properties of constitutive elements in order to capture the essential feature of spatial rearrangement. The results reveal a specific frequency-dependent contribution of elastic and viscous effects which is responsible for significant changes in the VTS model dynamical properties. The mechanism behind is related to the variable contribution of spatial rearrangement of VTS elements which is decreased from low to high frequency as dominant effects are transferred from mainly elastic to mainly viscous. More precisely, the elasticity modulus increases with frequency while the viscosity modulus decreases, each evolution corresponding to a specific power-law dependency. The satisfactorily agreement found between present numerical results and the literature data issued from in vitro cell experiments suggests that the frequency-dependent mechanism of spatial rearrangement presently described could play a significant and predictable role during oscillatory cell dynamics.

Linearity and time-scale invariance of the creep function in living cells

We report here the creep function measured in three cell types, after a variety of interventions, and over three time decades (from 3 ms to 3.2 s). In each case the response conformed to a power law, implying that no distinct molecular relaxation times or time constants could characterize the response. These results add to a growing body of evidence that stands in contrast to widely used viscoelastic models featuring at most a few time constants. We show instead that the ability of the matrix to deform is time-scale invariant and characterized by only one parameter: the power law exponent that controls the transition between solid-like and liquid-like behaviour. Moreover, we validate linearity by comparison of measurements in the time and frequency domains.

Effects of heat stress on Young's modulus of outer hair cells in mice

Intense sound exposure causes permanent hearing loss due to hair cell and cochlear damage. Prior conditioning with sublethal stressors, such as nontraumatic sound, heat stress and restraint protects the ear from acoustic injury. However, the mechanisms underlying conditioning-related cochlear protection remain unknown. In this paper, Young's modulus and the amount of filamentous actin (F-actin) of outer hair cells (OHCs) with/without heat stress were investigated by atomic force microscopy and confocal laser scanning microscopy, respectively. Conditioning with heat stress resulted in a statistically significant increase in Young's modulus of OHCs at 3-6 h after application, and such modulus then began to decrease by 12 h and returned to pre-conditioning level at 48 h after heat stress. The amount of F-actin began to increase by 3 h after heat stress and peaked at 12 h. It then began to decrease by 24 h and returned to the pre-conditioning level by 48-96 h after heat stress. These time courses are consistent with a previous report in which heat stress was shown to suppress permanent threshold shift (PTS). In addition, distortion product otoacoustic emissions (DPOAEs) were confirmed to be enhanced by heat stress. These results suggest that conditioning with heat stress structurally modifies OHCs so that they become stiffer due to an increase in the amount of F-actin. As a consequence, OHCs possibly experience less strain when they are exposed to loud noise, resulting in protection of mammalian hearing from traumatic noise exposure.

Assessing micromechanical properties of cells with atomic force microscopy: importance of the contact point

Mechanical properties are obtainable from atomic force microscopy (AFM) indentation force-depth curves, which are calculated from relationships between tip deflection and cantilever position, i.e. deflection curves. Indentation depth is the difference between tip deflections on a rigid and a soft material for the same amount of cantilever advancement, after contact is made. Since the contact point cannot be unequivocally identified from experimental data, there is some uncertainty in estimating material properties. Using simulations, this study examines some important issues related to the influence of contact point identification on estimated material properties. Simulations for linear materials using a typical stiffness for an AFM cantilever demonstrate that certain portions of the post-contact region of deflection curves for soft and very stiff materials can be approximated by quadratic and linear functions, respectively. Based on these findings, we first develop and verify an objective, automatic method to identify the contact point for materials with linear properties. We then assess the effect of misidentifying the contact point, with and without noise. If the contact point is missed by <50 nm, material properties for small indentations are erroneous but the error decreases asymptotically beyond 200 nm of indentation and the correct estimate of material stiffness is obtained. If the contact point is missed by >100 nm, however, the true material properties cannot be estimated accurately. Noise adds to uncertainty in material properties at small indentations but the combined effect of missing the contact point and noise is dominated by the former. Even though the algorithm was developed for linear materials, it is also suitable for certain nonlinear materials making it more generally applicable.

Deformation and nano-rheology of red blood cells: An AFM investigation

Interaction forces, deformation and nano-rheology of individual red blood cells in physiologically relevant solution conditions have been determined by colloid probe atomic force microscopy (AFM). On approach of the physically immobilised cell and silica glass spherical probe surfaces, deformation of the red blood cell was observed in the force curves. At low levels of deformation, spring constants were determined in the range 3-6 m Nm(-1), whereas for higher levels of deformation, the forces increase non-linearly and on retraction, significant force curve hysteresis is observed (i.e. lower forces upon retraction). The extent of force curve hysteresis was dependent on both the drive velocity and loading force, typical of a viscoelastic system. The response of the red blood cell has been described by viscoelastic theory, where the short and long time scale elastic moduli and relaxation times are determined, i.e. the cell's nano-rheological properties elucidated. In addition to a time independent elastic modulus of 4.0 x 10(3)Nm(-2) at low levels of deformation, time-dependent elastic moduli ranges are observed (3.5 x 10(4) to 5.5 x 10(4)Nm(-2) at intermediate levels of deformation and 1.5 x 10(5) to 3.0 x 10(5)Nm(-2) at higher levels of deformation). That is, one elastic and more than one viscoelastic response to the red blood cell deformation is evident, which is considered to reflect the cellular structure.

Measurement and characterization of whole-cell mechanical behavior

An understanding of whole-cell mechanical behavior can provide insight into cellular responses to mechanical loading and diseases in which such responses are altered. However, this aspect of cellular mechanical behavior has received limited attention. In this study, we used the atomic force microscope (AFM) in conjunction with several mechanical characterization methods (Hertz contact theory, an exponential equation, and a parallel-spring recruitment model) to establish a mechanically rigorous method for measuring and characterizing whole-cell mechanical behavior in the deformation range 0-500 nm. Using MC3T3-E1 osteoblasts, measurement repeatability was assessed by performing multiple loading cycles on individual cells. Despite variability in measurements, repeatability of the measurement technique was statistically confirmed. The measurement technique also proved acceptable since only 5% of the total variance across all measurements was due to variations within measurements for a single cell. The parallel-spring recruitment model, a single-parameter model, accurately described the measured nonlinear force-deformation response (R2>0.99) while providing a mechanistic explanation of whole-cell mechanical behavior. Taken together, the results should improve the capabilities of the AFM to probe whole-cell mechanical behavior. In addition, the success of the parallel-spring recruitment model provides insight into the micromechanical basis of whole-cell behavior.

The effects of morphology, confluency, and phenotype on whole-cell mechanical behavior

Emerging evidence indicates that cellular mechanical behavior can be altered by disease, drug treatment, and mechanical loading. To effectively investigate how disease and mechanical or biochemical treatments influence cellular mechanical behavior, it is imperative to determine the source of large inter-cell differences in whole-cell mechanical behavior within a single cell line. In this study, we used the atomic force microscope to investigate the effects of cell morphological parameters and confluency on whole-cell mechanical behavior for osteoblastic and fibroblastic cells. For nonconfluent cells, projected nucleus area, cell area, and cell aspect ratio were not correlated with mechanical behavior (p>or=0.46), as characterized by a parallel-spring recruitment model. However, measured force-deformation responses were statistically different between osteoblastic and fibroblastic cells (p<0.001) and between confluent and nonconfluent cells (p<0.001). Osteoblastic cells were 2.3-2.8 times stiffer than fibroblastic cells, and confluent cells were 1.5-1.8 times stiffer than nonconfluent cells. The results indicate that structural differences related to phenotype and confluency affect whole-cell mechanical behavior, while structural differences related to global morphology do not. This suggests that cytoskeleton structural parameters, such as filament density, filament crosslinking, and cell-cell and cell-matrix attachments, dominate inter-cell variability in whole-cell mechanical behavior.

Adhesion force between calcium oxalate monohydrate crystal and kidney epithelial cells and possible relevance for kidney stone formation

AFM interaction force measurements have been performed between calcium oxalate monohydrate crystal (COM) colloidal probes and monolayers of renal epithelial cells (on a polymer substrate) in artificial urine (AU) solutions. The adhesion force was measured for the COM/MDCK cell interaction, while no adhesion force was found for the COM/LLC-PK(1) cell interaction. Long-range repulsive forces for both lines of cells were measured in the range of 2-3 mum. After removal of the cell from the substrate by the AU flow, the basal membrane (BM), with a thickness of 100-200 nm, remained on the substrate. In this case, the shorter-range repulsive forces were found on the extending (approaching) portion of force/indentation curves. Similar to the COM/MDCK cell interaction, the retracting portions of curves for COM/basal membranes have shown the existence of the attractive force of adhesion for the interaction of COM with a BM of MDCK cells, while no adhesion was found for COM/BM LLC-PK(1) cells interaction. No adhesion force was found for the interaction of a BM (of any cells) with the silicon nitride tip. Besides the hydrodynamic reasons, the adhesion difference between LLC-PK(1) and MDCK cells possibly explains the preferential deposition of crystals only in collecting ducts (lined with MDCK-type cells) and the lack of the crystal deposition in the proximal tubules (lined with LLC-PK(1)-type cells). Previous treatments of cells with oxalate alone increased the adhesion force COM/BM MDCK; however, even after oxalate treatment there was small or no adhesion between COM and BM LLC-PK(1) cells. Note that the adhesion force for COM/BM MDCK is practically independent of the probe velocity, i.e., does not have the viscous origin. Evaluation of the adhesion energy shows that this force should be related to the ionic or hydrogen bonds of samples.

Stiffness of normal and pathological erythrocytes studied by means of atomic force microscopy

During recent years, atomic force microscopy has become a powerful technique for studying the mechanical properties (such as stiffness, viscoelasticity, hardness and adhesion) of various biological materials. The unique combination of high-resolution imaging and operation in physiological environment made it useful in investigations of cell properties. In this work, the microscope was applied to measure the stiffness of human red blood cells (erythrocytes). Erythrocytes were attached to the poly-L-lysine-coated glass surface by fixation using 0.5% glutaraldehyde for 1 min. Different erythrocyte samples were studied: erythrocytes from patients with hemolytic anemias such as hereditary spherocytosis and glucose-6-phosphate-dehydrogenase deficiency patients with thalassemia, and patients with anisocytosis of various causes. The determined Young's modulus was compared with that obtained from measurements of erythrocytes from healthy subjects. The results showed that the Young's modulus of pathological erythrocytes was higher than in normal cells. Observed differences indicate possible changes in the organization of cell cytoskeleton associated with various diseases.

Recovery of elasticity of aged human epithelial cells in vitro

We recently found a considerable increase in rigidity of human epithelial cells during aging in vitro. This is important because the loss in elasticity of epithelial tissues with aging contributes to many human diseases. We also found that cultured cells had three distinct regions of rigidity and that the increase in rigidity correlated with an increase in density of cytoskeletal fibers. However, it was not clear which type of fiber was important. Atomic force microscopy and immunofluorescence microscopy were used in this study to characterize aging human epithelial cells in vitro, both before and after treatment with cytochalasin B. We found that the fibers associated with increased rigidity were mostly F-actin microfilaments. Furthermore, using cytochalasin B, a chemical that inhibits polymerization of F-actin, we restored the rigidity of old cells to the young level in all three areas of rigidity simultaneously. These results clarify how the cell mechanics changes during aging in vitro, and they may be relevant for treatment of age-related loss of elasticity in epithelial tissues.

Prestressed F-actin networks cross-linked by hinged filamins replicate mechanical properties of cells

We show that actin filaments, shortened to physiological lengths by gelsolin and cross-linked with recombinant human filamins (FLNs), exhibit dynamic elastic properties similar to those reported for live cells. To achieve elasticity values of comparable magnitude to those of cells, the in vitro network must be subjected to external prestress, which directly controls network elasticity. A molecular requirement for the strain-related behavior at physiological conditions is a flexible hinge found in FLNa and some FLNb molecules. Basic physical properties of the in vitro filamin-F-actin network replicate the essential mechanical properties of living cells. This physical behavior could accommodate passive deformation and internal organelle trafficking at low strains yet resist externally or internally generated high shear forces.

Force microscopy of nonadherent cells: a comparison of leukemia cell deformability

Atomic force microscopy (AFM) has become an important tool for quantifying mechanical properties of biological materials ranging from single molecules to cells and tissues. Current AFM techniques for measuring elastic and viscoelastic properties of whole cells are based on indentation of cells firmly adhered to a substrate, but these techniques are not appropriate for probing nonadherent cells, such as passive human leukocytes, due to a lateral instability of the cells under load. Here we present a method for characterizing nonadherent cells with AFM by mechanically immobilizing them in microfabricated wells. We apply this technique to compare the deformability of human myeloid and lymphoid leukemia cells and neutrophils at low deformation rates, and we find that the cells are well described by an elastic model based on Hertzian mechanics. Myeloid (HL60) cells were measured to be a factor of 18 times stiffer than lymphoid (Jurkat) cells and six times stiffer than human neutrophils on average (E(infinity) = 855 +/- 670 Pa for HL60 cells, E(infinity) = 48 +/- 35 Pa for Jurkat cells, E(infinity) = 156 +/- 87 for neutrophils, mean +/- SD). This work demonstrates a simple method for extending AFM mechanical property measurements to nonadherent cells and characterizes properties of human leukemia cells that may contribute to leukostasis, a complication associated with acute leukemia.

Local mechanical properties of mouse outer hair cells: atomic force microscopic study

OBJECTIVES

Outer hair cells (OHCs) are capable of altering their cell length in response to changes in membrane potential. Due to this electromotility, OHCs probably subject the basilar membrane to force, resulting in cochlear amplification. To understand the mechanism of such amplification, knowledge of the mechanical properties of OHCs is required since the force produced by OHC electromotility is thought to depend on such properties. Various studies have been conducted to investigate the mechanical properties of guinea pig OHCs. With regard to mice, however, although various kinds of transgenic and knockout mice possess great potential as research models, the mechanical properties of mouse OHCs have not as yet been reported since the cells and/or tissues in the mouse hearing organ are relatively small and vulnerable to external stimuli, rendering sample preparation difficult. In this study, therefore, to establish indicators of the mechanical properties of OHCs in mice, such properties were measured by atomic force microscopy (AFM).

METHODS

CBA/JNCrj strain male mice aged 10-12 weeks (25-30 g) were used. Cochleae were dissected out from the animal and both the basilar membrane and the organ of Corti were simultaneously unwrapped from the modiolus with forceps. Dissected coiled tissue was then incubated with an enzymatic digestion medium for 15 min. After digestion, OHCs were isolated by gently triturating the coiled tissue. Local mechanical properties of the OHCs were then measured by an indentation test using an AFM.

RESULTS

Young's modulus and stiffness of the OHC in the apical turn of the mouse cochlea were 2.1+/-0.5 kPa and 4.4+/-1.2 mN/m, respectively.

CONCLUSIONS

Young's modulus of the OHC in the apical turn of the cochlea in mice was roughly the same as that in the apical turn of the cochlea in guinea pigs; however, the stiffness of the former was about two times greater than that of the latter because the cell length of the former was shorter than that of the latter.

Colloid probe AFM investigation of the influence of cross-linking on the interaction behavior and nano-rheology of colloidal droplets

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.

Non-Hertzian Approach to Analyzing Mechanical Properties of Endothelial Cells Probed by Atomic Force Microscopy

Detailed measurements of cell material properties are required for understanding how cells respond to their mechanical environment. Atomic force microscopy (AFM) is an increasingly popular measurement technique that uniquely combines subcellular mechanical testing with high-resolution imaging. However, the standard method of analyzing AFM indentation data is based on a simplified “Hertz” theory that requires unrealistic assumptions about cell indentation experiments. The objective of this study was to utilize an alternative “pointwise modulus” approach, that relaxes several of these assumptions, to examine subcellular mechanics of cultured human aortic endothelial cells (HAECs). Data from indentations in 2‐to5‐μm square regions of cytoplasm reveal at least two mechanically distinct populations of cellular material. Indentations colocalized with prominent linear structures in AFM images exhibited depth-dependent variation of the apparent pointwise elastic modulus that was not observed at adjacent locations devoid of such structures. The average pointwise modulus at an arbitrary indentation depth of 200nm was 5.6±3.5kPa and 1.5±0.76kPa (mean±SD, n=7) for these two material populations, respectively. The linear structures in AFM images were identified by fluorescence microscopy as bundles of f-actin, or stress fibers. After treatment with 4μM cytochalasin B, HAECs behaved like a homogeneous linear elastic material with an apparent modulus of 0.89±0.46kPa. These findings reveal complex mechanical behavior specifically associated with actin stress fibers that is not accurately described using the standard Hertz analysis, and may impact how HAECs interact with their mechanical environment.

Probing mechanical properties of living cells by atomic force microscopy with blunted pyramidal cantilever tips

Atomic force microscopy (AFM) allows the acquisition of high-resolution images and the measurement of mechanical properties of living cells under physiological conditions. AFM cantilevers with blunted pyramidal tips are commonly used to obtain images of living cells. Measurement of mechanical properties with these tips requires a contact model that takes into account their blunted geometry. The aim of this work was to develop a contact model of a blunted pyramidal tip and to assess the suitability of pyramidal tips for probing mechanical properties of soft gels and living cells. We developed a contact model of a blunted pyramidal tip indenting an elastic half-space. We measured Young's modulus (E) and the complex shear modulus (G*= G' +i G" ) of agarose gels and A549 alveolar epithelial cells with pyramidal tips and compared them with those obtained with spherical tips. The gels exhibited an elastic behavior with almost coincident loading and unloading force curves and negligible values of G". E fell sharply with indentation up to approximately 300 nm , showing a linear regime for deeper indentations. A similar indentation dependence of E with twofold lower values at the linear regime was obtained with the spherical tip fitted with Hertz's model. The dependence of E on indentation in cells paralleled that found in gels. Cells exhibited viscoelastic behavior with G"/G' approximately 1/4 . Pyramidal tips commonly used for AFM imaging are suitable for probing mechanical properties of soft gels and living cells.

Osteoblast Elastic Modulus Measured by Atomic Force Microscopy Is Substrate Dependent

The actin and microtubule cytoskeleton have been found to contribute to the elastic modulus of cells, which may be modulated by adhesion to extracellular matrix (ECM) proteins and subsequent alterations in the cytoskeleton. In this study, the apparent elastic modulus (Eapp) of osteoblast-like MC3T3-E1 cells adhered to fibronectin (FN), vitronectin (VN), type I collagen (COLI), fetal bovine serum (FBS), or poly-l-lysine (PLL), and bare glass were determined using an atomic force microscope (AFM). The E(app) of osteoblasts adhered to ECM proteins (FN, VN, COLI, and FBS) that bind cells via integrins were higher compared to cells on glass and PLL, which adhere cells through nonspecific binding. Also, osteoblasts adhered to FN, VN, COLI, and FBS had F-actin stress fiber formation, while osteoblasts on glass and PLL showed few F-actin fibers. Disruption of the actin cytoskeleton decreased E(app) of osteoblasts plated on FN to the level of osteoblasts plated on glass, while microtubule disruption had no significant effect. This suggests that the elevated modulus of osteoblasts adhered to FN was due to remodeling of the actin cytoskeleton upon adhesion to ECM proteins. Modulation of cell stiffness upon adhesion to various substrates may influence mechanosignal transduction in osteoblasts.

Human epithelial cells increase their rigidity with ageing in vitro: direct measurements

The decrease in elasticity of epithelial tissues with ageing contributes to many human diseases. This change was previously attributed to increased crosslinking of extracellular matrix proteins. Here we show that individual human epithelial cells also become significantly more rigid during ageing in vitro. Using atomic force microscopy (AFM), we found that the Young's modulus of viable cells was consistently increased two- to four-fold in older versus younger cells. Direct visualization of the cytoskeleton using a novel method involving the AFM suggested that increased rigidity of ageing cells was due to a higher density of cytoskeletal fibres. Our results identify a unique mechanism that might contribute to the age-related loss of elasticity in epithelial tissues.

Estimating the sensitivity of mechanosensitive ion channels to membrane strain and tension

Bone adapts to its environment by a process in which osteoblasts and osteocytes sense applied mechanical strain. One possible pathway for the detection of strain involves mechanosensitive channels and we sought to determine their sensitivity to membrane strain and tension. We used a combination of experimental and computational modeling techniques to gain new insights into cell mechanics and the regulation of mechanosensitive channels. Using patch-clamp electrophysiology combined with video microscopy, we recorded simultaneously the evolution of membrane extensions into the micropipette, applied pressure, and membrane currents. Nonselective mechanosensitive cation channels with a conductance of 15 pS were observed. Bleb aspiration into the micropipette was simulated using finite element models incorporating the cytoplasm, the actin cortex, the plasma membrane, cellular stiffening in response to strain, and adhesion between the membrane and the micropipette. Using this model, we examine the relative importance of the different cellular components in resisting suction into the pipette and estimate the membrane strains and tensions needed to open mechanosensitive channels. Radial membrane strains of 800% and tensions of 5 10(-4) N.m(-1) were needed to open 50% of mechanosensitive channels. We discuss the relevance of these results in the understanding of cellular reactions to mechanical strain and bone physiology.

Expression of prostate specific membrane antigen in androgen-independent prostate cancer cell line PC-3

During the progression of prostate cancer from androgen-dependence or sensitivity to androgen-independence, the overall expression of prostate specific membrane antigen (PSMA) increases with its appearance in plasma membrane. However, surprisingly some androgen-independent metastatic prostate cancer cell lines do not express this protein. Estradiol (E2) and basic fibroblast growth factor (bFGF) due to their recognized and strong involvement in prostate growth, development, and pathology were selected with the aim of restoring the expression of PSMA in markedly dedifferentiated prostate cancer PC-3 cells and in Du 145. E2 (10(-7)-10(-11)M) and bFGF (10ng/ml) stimulated the expression of mRNAs for PSMA (2- to 4-fold increase) that apparently were further translated and processed to its membrane form in LNCaP, PC-3, and Du 145 cells. The values of interaction force between the same anti-PSMA antibodies and all studied cells were almost identical (45-64pN), indicating antigenic similarity of the membrane form of PSMA expressed in LNCaP, PC-3, and Du 145 cells.

Unconfined creep compression of chondrocytes

The study of single cell mechanics offers a valuable tool for understanding cellular milieus. Specific knowledge of chondrocyte biomechanics could lead to elucidation of disease etiologies and the biomechanical factors most critical to stimulating regenerative processes in articular cartilage. Recent studies in our laboratory have suggested that it may be acceptable to approximate the shape of a single chondrocyte as a disc. This geometry is easily utilized for generating models of unconfined compression. In this study, three continuum mechanics models of increasing complexity were formulated and used to fit unconfined compression creep data. Creep curves were obtained from middle/deep zone chondrocytes (n = 15) and separately fit using the three continuum models. The linear elastic solid model yielded a Young's modulus of 2.55+/-0.85 kPa. The viscoelastic model (adapted from the Kelvin model) generated an instantaneous modulus of 2.47+/-0.85 kPa, a relaxed modulus of 1.48+/-0.35 kPa, and an apparent viscosity of 1.92+/-1.80 kPa-s. Finally, a linear biphasic model produced an aggregate modulus of 2.58+/-0.87 kPa, a permeability of 2.57 x 10(-12)+/-3.09 m(4)/N-s, and a Poisson's ratio of 0.069+/-0.021. The results of this study demonstrate that similar values for the cell modulus can be obtained from three models of increasing complexity. The elastic model provides an easy method for determining the cell modulus, however, the viscoelastic and biphasic models generate additional material properties that are important for characterizing the transient response of compressed chondrocytes.

Toward a comprehensive model for induced endoreduplication

Both the biological significance and the molecular mechanism of endoreduplication (END) have been debated for a long time by cytogeneticists and researchers into cell cycle enzymology and dynamics alike. Mainly due to the fact that a wide variety of agents have been reported as able to induce endoreduplication and the diversity of cell types where it has been described, until now no clear or unique mechanism of induction of this phenomenon, rare in animals but otherwise quite common in plants, has been proposed. DNA topoisomerase II (topo II), plays a major role in mitotic chromosome segregation after DNA replication. The classical topo II poisons act by stabilizing the enzyme in the so-called cleavable complex and result in DNA damage as well as END, while the true catalytic inhibitors, which are not cleavable-complex-stabilizers, do induce END without concomitant DNA and chromosome damage. Taking into account these observations on the induction of END by drugs that interfere with topo II, together with our recently obtained evidence that the nature of DNA plays an important role for chromosome segregation [Cortes, F., Pastor, N., Mateos, S., Dominguez, I., 2003. The nature of DNA plays a role in chromosome segregation: endoreduplication in halogen-substituted chromosomes. DNA Repair 2, 719-726.], a straightforward model is proposed in which the different mechanisms leading to induced END are considered.

On atomic force microscopy and the constitutive behavior of living cells

Atomic force microscopy (AFM) is one of many new technologies available to study the mechanical properties and mechanobiological responses of living cells. Despite the widespread usage of this technology, there has been little attempt to develop new theoretical frameworks to interpret the associated data. Rather, most analyses rely on the classical Hertz solution for the indentation of an elastic half-space within the context of linearized elasticity. In contrast, we propose a fully nonlinear, constrained mixture model for adherent cells that allows one to account separately for the contributions of the three primary structural constituents of the cytoskeleton. Moreover, we extend a prior solution for a small indentation superimposed on a finite equibiaxial extension by incorporating in this mixture model for the special case of an initially random distribution of constituents (actin, intermediate filaments, and microtubules). We submit that this theoretical framework will allow an improved interpretation of indentation force-depth data from a sub-class of atomic force microscopy tests and will serve as an important analytical check for future finite element models. The latter will be necessary to exploit further the capabilities of both atomic force microscopy and nonlinear mixture theories for cell behavior.

Cholesterol depletion increases membrane stiffness of aortic endothelial cells

This study has investigated the effect of cellular cholesterol on membrane deformability of bovine aortic endothelial cells. Cellular cholesterol content was depleted by exposing the cells to methyl-beta-cyclodextrin or enriched by exposing the cells to methyl-beta-cyclodextrin saturated with cholesterol. Control cells were treated with methyl-beta-cyclodextrin-cholesterol at a molar ratio that had no effect on the level of cellular cholesterol. Mechanical properties of the cells with different cholesterol contents were compared by measuring the degree of membrane deformation in response to a step in negative pressure applied to the membrane by a micropipette. The experiments were performed on substrate-attached cells that maintained normal morphology. The data were analyzed using a standard linear elastic half-space model to calculate Young elastic modulus. Our observations show that, in contrast to the known effect of cholesterol on membrane stiffness of lipid bilayers, cholesterol depletion of bovine aortic endothelial cells resulted in a significant decrease in membrane deformability and a corresponding increase in the value of the elastic coefficient of the membrane, indicating that cholesterol-depleted cells are stiffer than control cells. Repleting the cells with cholesterol reversed the effect. An increase in cellular cholesterol to a level higher than that of normal cells, however, had no effect on the elastic properties of bovine aortic endothelial cells. We also show that although cholesterol depletion had no apparent effect on the intensity of F-actin-specific fluorescence, disrupting F-actin with latrunculin A abrogated the stiffening effect. We suggest that cholesterol depletion increases the stiffness of the membrane by altering the properties of the submembrane F-actin and/or its attachment to the membrane.

Myotubes differentiate optimally on substrates with tissue-like stiffness: pathological implications for soft or stiff microenvironments

Contractile myocytes provide a test of the hypothesis that cells sense their mechanical as well as molecular microenvironment, altering expression, organization, and/or morphology accordingly. Here, myoblasts were cultured on collagen strips attached to glass or polymer gels of varied elasticity. Subsequent fusion into myotubes occurs independent of substrate flexibility. However, myosin/actin striations emerge later only on gels with stiffness typical of normal muscle (passive Young's modulus, E approximately 12 kPa). On glass and much softer or stiffer gels, including gels emulating stiff dystrophic muscle, cells do not striate. In addition, myotubes grown on top of a compliant bottom layer of glass-attached myotubes (but not softer fibroblasts) will striate, whereas the bottom cells will only assemble stress fibers and vinculin-rich adhesions. Unlike sarcomere formation, adhesion strength increases monotonically versus substrate stiffness with strongest adhesion on glass. These findings have major implications for in vivo introduction of stem cells into diseased or damaged striated muscle of altered mechanical composition.

Mechanical properties of sensory and supporting cells in the organ of Corti of the guinea pig cochlea--study by atomic force microscopy

Mammalian hearing is refined by amplification of the motion of the cochlear partition. To understand the cochlear amplification, mechanical models of the cochlea have been used. When the dynamic behavior of the cochlea is analyzed by a model, elastic properties of the cells in the organ of Corti must be determined in advance. Recently, elastic properties of outer hair cells (OHCs) and pillar cells have been elucidated. However, those of other cells have not yet been clarified. Therefore, in this study, using an atomic force microscope (AFM), elastic properties of Hensen's cells, Deiters' cells and inner hair cells (IHCs) in the apical turn and those in the basal and second turns were estimated. As a result, slopes indicative of cell elastic properties were (8.9 +/- 5.8) x 10(3) m(-1) for Hensen's cells (n = 30), (5.5 +/- 5.3) x 10(3) m(-1) for Deiters' cells (n = 20) and (3.8 +/- 2.6) x 10(3) m(-1) for IHCs (n = 20), and Young's modulus were 0.69 +/- 0.45 kPa for Hensen's cells and 0.29 +/- 0.20 kPa for IHCs. There was no significant difference between elastic properties of each type of cell in the apical turn and those in the basal and second turns. However, it was found that there is a significant difference between Young's moduli of cells estimated in this study and those of the OHCs and pillar cells reported previously.

Listeria monocytogenes actin-based motility varies depending on subcellular location: a kinematic probe for cytoarchitecture

Intracellular Listeria monocytogenes actin-based motility is characterized by significant individual variability, which can be influenced by cytoarchitecture. L. monocytogenes was used as a probe to transmit information about structural variation among subcellular domains defined by mitochondrial density. By analyzing the movement of a large population of L. monocytogenes in PtK2 cells, we found that mean speed and trajectory curvature were significantly larger for bacteria moving in mitochondria-containing domains (generally perinuclear) than for bacteria moving in mitochondria-free domains (generally peripheral). Analysis of bacteria that traversed both mitochondria-containing and mitochondria-free domains revealed that these motile differences were not intrinsic to bacteria themselves. Disruption of mitochondrial respiration did not affect bacterial mean speed, speed persistence, or trajectory curvature. In contrast, microtubule depolymerization lead to decreased mean speed per bacterium and increased mean speed persistence of L. monocytogenes moving in mitochondria-free domains compared with untreated cells. L. monocytogenes were also observed to physically collide with mitochondria and push them away from the bacterial path of motion, causing bacteria to slow down before rapidly resuming their speed. Our results show that subcellular domains along with microtubule depolymerization may influence the actin cytoskeleton to affect L. monocytogenes speed, speed persistence, and trajectory curvature.

Force and Compliance Measurements on Living Cells Using Atomic Force Microscopy (AFM)

We describe the use of atomic force microscopy (AFM) in studies of cell adhesion and cell compliance. Our studies use the interaction between leukocyte function associated antigen-1 (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) as a model system. The forces required to unbind a single LFA-1/ICAM-1 bond were measured at different loading rates. This data was used to determine the dynamic strength of the LFA-1/ICAM-1 complex and characterize the activation potential that this complex overcomes during its breakage. Force measurements acquired at the multiple- bond level provided insight about the mechanism of cell adhesion. In addition, the AFM was used as a microindenter to determine the mechanical properties of cells. The applications of these methods are described using data from a previous study.

Mechanisms governing the visco-elastic responses of living cells assessed by foam and tensegrity models

The visco-elastic properties of living cells, measured to date by various authors, vary considerably, depending on the experimental methods and/or on the theoretical models used. In the present study, two mechanisms thought to be involved in cellular visco-elastic responses were analysed, based on the idea that the cytoskeleton plays a fundamental role in cellular mechanical responses. For this purpose, the predictions of an open unit-cell model and a 30-element visco-elastic tensegrity model were tested, taking into consideration similar properties of the constitutive F-actin. The quantitative predictions of the time constant and viscosity modulus obtained by both models were compared with previously published experimental data obtained from living cells. The small viscosity modulus values (10(0)-10(3) Pa x s) predicted by the tensegrity model may reflect the combined contributions of the spatially rearranged constitutive filaments and the internal tension to the overall cytoskeleton response to external loading. In contrast, the high viscosity modulus values (10(3)-10(5) Pa x s) predicted by the unit-cell model may rather reflect the mechanical response of the cytoskeleton to the bending of the constitutive filaments and/or to the deformation of internal components. The present results suggest the existence of a close link between the overall visco-elastic response of micromanipulated cells and the underlying architecture.

Contributions of molecular binding events and cellular compliance to the modulation of leukocyte adhesion

The interaction of leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) is central to the regulation of adhesion in leukocytes. In this report, we investigated the mechanisms by which phorbol myristate acetate (PMA) promotes LFA-1-dependent cell adhesion. The adhesion of PMA-stimulated cells to immobilized ICAM-1 was quantified in direct force measurements acquired by atomic force microscopy (AFM). Enhanced adhesion of PMA-stimulated cells to immobilized ICAM-1 stemmed from an increase in the number of LFA-1-ICAM-1 complexes formed between the two apposing surfaces on contact, rather than by affinity modulation of LFA-1. Single molecule force measurements revealed that the force spectrum of the LFA-1-ICAM-1 complex formed by PMA-stimulated cells is identical to the force spectrum of the complex formed by resting cells. Thus, PMA stimulation does not modify the mechanical strength of the individual LFA-1-ICAM-1 interaction. Instead, the enhanced cell adhesion of PMA-stimulated cells appears to be a complex process that correlates with changes in the mechanical properties of the cell. We estimate that changes in the elasticity of the cell gave rise to a more than 10-fold increase in cell adhesion.

Creep indentation of single cells

An apparatus for creep indentation of individual adherent cells was designed, developed, and experimentally validated. The creep cytoindentation apparatus (CCA) can perform stress-controlled experiments and measure the corresponding deformation of single anchorage-dependent cells. The apparatus can resolve forces on the order of 1 nN and cellular deformations on the order of 0.1 micron. Experiments were conducted on bovine articular chondrocytes using loads on the order of 10 nN. The experimentally observed viscoelastic behavior of these cells was modeled using the punch problem and standard linear solid. The punch problem yielded a Young's modulus of 1.11 +/- 0.48 kPa. The standard linear solid model yielded an instantaneous elastic modulus of 8.00 +/- 4.41 kPa, a relaxed modulus of 1.09 +/- 0.54 kPa, an apparent viscosity of 1.50 +/- 0.92 kPa-s, and a time constant of 1.32 +/- 0.65 s. To our knowledge, this is the first time that stress-controlled indentation testing has been applied at the single cell level. This methodology represents a new tool in understanding the mechanical nature of anchorage-dependent cells and mechanotransductional pathways.

Microrheology of human lung epithelial cells measured by atomic force microscopy

Lung epithelial cells are subjected to large cyclic forces from breathing. However, their response to dynamic stresses is poorly defined. We measured the complex shear modulus (G(*)(omega)) of human alveolar (A549) and bronchial (BEAS-2B) epithelial cells over three frequency decades (0.1-100 Hz) and at different loading forces (0.1-0.9 nN) with atomic force microscopy. G(*)(omega) was computed by correcting force-indentation oscillatory data for the tip-cell contact geometry and for the hydrodynamic viscous drag. Both cell types displayed similar viscoelastic properties. The storage modulus G'(omega) increased with frequency following a power law with exponent approximately 0.2. The loss modulus G"(omega) was approximately 2/3 lower and increased similarly to G'(omega) up to approximately 10 Hz, but exhibited a steeper rise at higher frequencies. The cells showed a weak force dependence of G'(omega) and G"(omega). G(*)(omega) conformed to the power-law model with a structural damping coefficient of approximately 0.3, indicating a coupling of elastic and dissipative processes within the cell. Power-law behavior implies a continuum distribution of stress relaxation time constants. This complex dynamics is consistent with the rheology of soft glassy materials close to a glass transition, thereby suggesting that structural disorder and metastability may be fundamental features of cell architecture.

Measuring the elastic properties of living cells by the atomic force microscope

In this chapter I discussed the possibility of measuring elastic properties of living cells by AFM. One reason for using the AFM for this purpose is its ability to both measure locally the mechanics of a cell and to distinguish different regions of the cell. Since the AFM can be operated under physiological conditions cellular processes can be followed, for example, cytokinesis and the investigation of the migration of cells.

Determination of cellular strains by combined atomic force microscopy and finite element modeling

Many organs adapt to their mechanical environment as a result of physiological change or disease. Cells are both the detectors and effectors of this process. Though many studies have been performed in vitro to investigate the mechanisms of detection and adaptation to mechanical strains, the cellular strains remain unknown and results from different stimulation techniques cannot be compared. By combining experimental determination of cell profiles and elasticities by atomic force microscopy with finite element modeling and computational fluid dynamics, we report the cellular strain distributions exerted by common whole-cell straining techniques and from micromanipulation techniques, hence enabling their comparison. Using data from our own analyses and experiments performed by others, we examine the threshold of activation for different signal transduction processes and the strain components that they may detect. We show that modulating cell elasticity, by increasing the F-actin content of the cytoskeleton, or cellular Poisson ratio are good strategies to resist fluid shear or hydrostatic pressure. We report that stray fluid flow in some substrate-stretch systems elicits significant cellular strains. In conclusion, this technique shows promise in furthering our understanding of the interplay among mechanical forces, strain detection, gene expression, and cellular adaptation in physiology and disease.