Single cell mechanotransduction and its modulation analyzed by atomic force microscope indentation

Combining mechanical and optical approaches to dissect cellular mechanobiology

Mechanical force modulates a wide array of cell physiological processes. Cells sense and respond to mechanical stimuli using a hierarchy of structural complexes spanning multiple length scales, including force-sensitive molecules and cytoskeletal networks. Understanding mechanotransduction, i.e., the process by which cells convert mechanical inputs into biochemical signals, has required the development of novel biophysical tools that allow for probing of cellular and subcellular components at requisite time, length, and force scales and technologies that track the spatio-temporal dynamics of relevant biomolecules. In this review, we begin by discussing the underlying principles and recent applications of atomic force microscopy, magnetic twisting cytometry, and traction force microscopy, three tools that have been widely used for measuring the mechanical properties of cells and for probing the molecular basis of cellular mechanotransduction. We then discuss how such tools can be combined with advanced fluorescence methods for imaging biochemical processes in living cells in the context of three specific problem spaces. We first focus on fluorescence resonance energy transfer, which has enabled imaging of intra- and inter-molecular interactions and enzymatic activity in real time based on conformational changes in sensor molecules. Next, we examine the use of fluorescence methods to probe force-dependent dynamics of focal adhesion proteins. Finally, we discuss the use of calcium ratiometric signaling to track fast mechanotransductive signaling dynamics. Together, these studies demonstrate how single-cell biomechanical tools can be effectively combined with molecular imaging technologies for elucidating mechanotransduction processes and identifying mechanosensitive proteins.

Defining the role of syndecan-4 in mechanotransduction using surface-modification approaches

The ability of cells to respond to external mechanical stimulation is a complex and robust process involving a diversity of molecular interactions. Although mechanotransduction has been heavily studied, many questions remain regarding the link between physical stimulation and biochemical response. Of significant interest has been the contribution of the transmembrane proteins involved, and integrins in particular, because of their connectivity to both the extracellular matrix and the cytoskeleton. Here, we demonstrate the existence of a mechanically based initiation molecule, syndecan-4. We first demonstrate the ability of syndecan-4 molecules to support cell attachment and spreading without the direct extracellular binding of integrins. We also examine the distribution of focal adhesion-associated proteins through controlling surface interactions of beads with molecular specificity in binding to living cells. Furthermore, after adhering cells to elastomeric membranes via syndecan-4-specific attachments we mechanically strained the cells via our mechanical stimulation and polymer surface chemical modification approach. We found ERK phosphorylation similar to that shown for mechanotransductive response for integrin-based cell attachments through our elastomeric membrane-based approach and optical magnetic twisting cytometry for syndecan-4. Finally, through the use of cytoskeletal disruption agents, this mechanical signaling was shown to be actin cytoskeleton dependent. We believe that these results will be of interest to a wide range of fields, including mechanotransduction, syndecan biology, and cell-material interactions.

Tumor suppressor protein SMAR1 modulates the roughness of cell surface: combined AFM and SEM study

Background

Imaging tools such as scanning electron microscope (SEM) and atomic force microscope (AFM) can be used to produce high-resolution topographic images of biomedical specimens and hence are well suited for imaging alterations in cell morphology. We have studied the correlation of SMAR1 expression with cell surface smoothness in cell lines as well as in different grades of human breast cancer and mouse tumor sections.

Methods

We validated knockdown and overexpression of SMAR1 using RT-PCR as well as Western blotting in human embryonic kidney (HEK) 293, human breast cancer (MCF-7) and mouse melanoma (B16F1) cell lines. The samples were then processed for cell surface roughness studies using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The same samples were used for microarray analysis as well. Tumors sections from control and SMAR1 treated mice as well as tissues sections from different grades of human breast cancer on poly L-lysine coated slides were used for AFM and SEM studies.

Results

Tumor sections from mice injected with melanoma cells showed pronounced surface roughness. In contrast, tumor sections obtained from nude mice that were first injected with melanoma cells followed by repeated injections of SMAR1-P44 peptide, exhibited relatively smoother surface profile. Interestingly, human breast cancer tissue sections that showed reduced SMAR1 expression exhibited increased surface roughness compared to the adjacent normal breast tissue. Our AFM data establishes that treatment of cells with SMAR1-P44 results into increase in cytoskeletal volume that is supported by comparative gene expression data showing an increase in the expression of specific cytoskeletal proteins compared to the control cells. Altogether, these findings indicate that tumor suppressor function of SMAR1 might be exhibited through smoothening of cell surface by regulating expression of cell surface proteins.

Conclusion

Tumor suppressor protein SMAR1 might be used as a phenotypic differentiation marker between cancerous and non-cancerous cells.

Mechanical dynamics of single cells during early apoptosis

Dynamic mechanical properties of cells are becoming recognized as indicators and regulators of physiological processes such as differentiation, malignant phenotypes and mitosis. A key process in development and homeostasis is apoptosis and whilst the molecular control over this pathway is well studied, little is known about the mechanical consequences of cell death. Here, we study the caspase-dependent mechanical kinetics of single cells during early apoptosis initiated with the general protein-kinase inhibitor staurosporine. This results in internal remodelling of the cytoskeleton and nucleus which is reflected in dynamic changes in the mechanical properties of the cell. Utilizing simultaneous confocal and atomic force microscopy (AFM), we measured distinct mechanical dynamics in the instantaneous cellular Young's Modulus and longer timescale viscous deformation. This allowed us to visualize time-dependent nuclear and cytoskeletal control of force dissipation with fluorescent fusion proteins throughout the cell. This work reveals that the cell death program not only orchestrates biochemical dynamics but also controls the mechanical breakdown of the cell. Importantly, the consequences of mechanical disregulation during apoptosis may be a contributing factor to several human pathologies through the poorly timed release of dead cells and cell debris.

Cell nanomechanics and focal adhesions are regulated by retinol and conjugated linoleic acid in a dose-dependent manner

Retinol and conjugated linoleic acid (CLA) have previously been shown to have an important role in gene expression and various cellular processes, including differentiation, proliferation and cell death. In this study we have investigated the effect of retinol and CLA, both individually and in combination, on the intracellular cytoskeleton, focal adhesions (FAs) and the nanomechanical properties of 3T3 fibroblasts. We observed a dose-dependent decrease in the formation of FAs following treatment with either compound, which was directly correlated to an increase in cell height (>30%) and a decrease in the measured Young's modulus (approximately 28%). Furthermore, treatments with both compounds demonstrated an increased effect and led to a reduction of >70% in the average number of FAs per cell and a decrease of >50% in average cell stiffness. These data reveal that retinol and CLA disrupt FA formation, leading to an increase in cell height and a significant decrease in stiffness. These results may broaden our understanding of the interplay between cell nanomechanics and cellular contact with the external microenvironment, and help to shed light on the important role of retinoids and CLA in health and disease.

Compaction of cell shape occurs before decrease of elasticity in CHO-K1 cells treated with actin cytoskeleton disrupting drug cytochalasin D

The actin filaments of the cytoskeleton form a highly dynamic polymer scaffold which is actively involved in many essential mechanisms such as cell migration, transport, mitosis, and mechanosensitivity. We treated CHO-K1 cells with different concentrations of the actin cytoskeleton disrupting drug cytochalasin D. Then investigating the cells' elastic behaviour by scanning force microscopy-based rheology we confirmed for high cytochalasin D concentrations (> or =1.5 microM) a significant decrease of mechanical stability. At lower concentrations we measured no significant softening, but flattening and a horizontal contraction was observable even at low concentrations (> or =0.3 microM) of cytochalasin D. The observed changes in cell shape resulted in a lower cell volume, showing that there is compensation by volume for small decreases in cytoskeletal strength resulting from reduced numbers or lengths of actin filaments. These results suggest that the characteristic functions defining a cell's mechanical stability such as mechanosensitivity can be maintained via small changes in cell volume in order to counter fluctuations in cytoskeletal composition.

Mechanics, malignancy, and metastasis: the force journey of a tumor cell

A cell undergoes many genetic and epigenetic changes as it transitions to malignancy. Malignant transformation is also accompanied by a progressive loss of tissue homeostasis and perturbations in tissue architecture that ultimately culminates in tumor cell invasion into the parenchyma and metastasis to distant organ sites. Increasingly, cancer biologists have begun to recognize that a critical component of this transformation journey involves marked alterations in the mechanical phenotype of the cell and its surrounding microenvironment. These mechanical differences include modifications in cell and tissue structure, adaptive force-induced changes in the environment, altered processing of micromechanical cues encoded in the extracellular matrix (ECM), and cell-directed remodeling of the extracellular stroma. Here, we review critical steps in this "force journey," including mechanical contributions to tissue dysplasia, invasion of the ECM, and metastasis. We discuss the biophysical basis of this force journey and present recent advances in the measurement of cellular mechanical properties in vitro and in vivo. We end by describing examples of molecular mechanisms through which tumor cells sense, process and respond to mechanical forces in their environment. While our understanding of the mechanical components of tumor growth, survival and motility remains in its infancy, considerable work has already yielded valuable insight into the molecular basis of force-dependent tumor pathophysiology, which offers new directions in cancer chemotherapeutics.

Nanostructure and nanomechanics analysis of lymphocyte using AFM: from resting, activated to apoptosis

The ultrastructural and mechanical properties of single resting, activated and apoptosis lymphocyte have been investigated by atomic force microscopy (AFM). Using topographic imaging, we showed that the surface of the resting lymphocyte is smooth, while lymphocyte activation and apoptosis are often accompanied by changes in cell morphology. The apoptosis lymphocyte is rougher than those of the two other morphotypes, and coated with many big particles. Using spatially resolved force-distance curves, we found that the valve of the activated lymphocyte is about two to three times stiffer (Young's modulus of approximately 20 kPa) than those of the two other morphotypes (5-11 kPa). These results can improve our understanding of the mechanical properties of cells during growth and differentiation.

A miniature probe for ultrasonic penetration of a single cell

Although ultrasound cavitation must be avoided for safe diagnostic applications, the ability of ultrasound to disrupt cell membranes has taken on increasing significance as a method to facilitate drug and gene delivery. A new ultrasonic resonance driving method is introduced to penetrate rigid wall plant cells or oocytes with springy cell membranes. When a reasonable design is created, ultrasound can gather energy and increase the amplitude factor. Ultrasonic penetration enables exogenous materials to enter cells without damaging them by utilizing instant acceleration. This paper seeks to develop a miniature ultrasonic probe experiment system for cell penetration. A miniature ultrasonic probe is designed and optimized using the Precise Four Terminal Network Method and Finite Element Method (FEM) and an ultrasonic generator to drive the probe is designed. The system was able to successfully puncture a single fish cell.

Regulation of transient receptor potential canonical channel 1 (TRPC1) by sphingosine 1-phosphate in C2C12 myoblasts and its relevance for a role of mechanotransduction in skeletal muscle differentiation

Transient receptor potential canonical (TRPC) channels provide cation and Ca2+ entry pathways, which have important regulatory roles in many physio-pathological processes, including muscle dystrophy. However, the mechanisms of activation of these channels remain poorly understood. Using siRNA, we provide the first experimental evidence that TRPC channel 1 (TRPC1), besides acting as a store-operated channel, represents an essential component of stretch-activated channels in C2C12 skeletal myoblasts, as assayed by whole-cell patch-clamp and atomic force microscopic pulling. The channel's activity and stretch-induced Ca2+ influx were modulated by sphingosine 1-phosphate (S1P), a bioactive lipid involved in satellite cell biology and tissue regeneration. We also found that TRPC1 was functionally assembled in lipid rafts, as shown by the fact that cholesterol depletion resulted in the reduction of transmembrane ion current and conductance. Association between TRPC1 and lipid rafts was increased by formation of stress fibres, which was elicited by S1P and abolished by treatment with the actin-disrupting dihydrocytochalasin B, suggesting a role for cytoskeleton in TRPC1 membrane recruitment. Moreover, TRPC1 expression was significantly upregulated during myogenesis, especially in the presence of S1P, implicating a crucial role for TRPC1 in myoblast differentiation. Collectively, these findings may offer new tools for understanding the role of TRPC1 and sphingolipid signalling in skeletal muscle regeneration and provide new therapeutic approaches for skeletal muscle disorders.

Cell fate regulation by coupling mechanical cycles to biochemical signaling pathways

Many aspects of cellular motility and mechanics are cyclic in nature such as the extension and retraction of lamellipodia or filopodia. Inherent to the cycles of extension and retraction that test the environment is the production of mechano-chemical signals that can alter long-term cell behavior, transcription patterns, and cell fate. We are just starting to define such cycles in several aspects of cell motility, including periodic contractions, integrin cycles of binding and release as well as the normal oscillations in motile activity. Cycles of local cell contraction and release are directly coupled to cycles of stressing and releasing extracellular contacts (matrix or cells) as well as cytoplasmic mechanotransducers. Stretching can alter external physical properties or sites exposed by matrix molecules as well as internal networks; thus, cell contractions can cause a secondary wave of mechano-regulated outside-in and internal cell signal changes. In some cases, the integration of both external and internal signals in space and time can stimulate a change in cell state from quiescence to growth or differentiation. In this review we will develop the basic concept of the mechano-chemical cycles and the ways in which they can be described and understood.

NUMERICAL SIMULATION OF NANOINDENTATION AND PATCH CLAMP EXPERIMENTS ON MECHANOSENSITIVE CHANNELS OF LARGE CONDUCTANCE IN ESCHERICHIA COLI

A hierarchical simulation framework that integrates information from all-atom simulations into a finite element model at the continuum level is established to study the mechanical response of a mechanosensitive channel of large conductance (MscL) in bacteria Escherichia Coli (E.coli) embedded in a vesicle formed by the dipalmitoylphosphatidycholine (DPPC) lipid bilayer. Sufficient structural details of the protein are built into the continuum model, with key parameters and material properties derived from molecular mechanics simulations. The multi-scale framework is used to analyze the gating of MscL when the lipid vesicle is subjective to nanoindentation and patch clamp experiments, and the detailed structural transitions of the protein are obtained explicitly as a function of external load; it is currently impossible to derive such information based solely on all-atom simulations. The gating pathways of E.coli-MscL qualitatively agree with results from previous patch clamp experiments. The gating mechanisms under complex indentation-induced deformation are also predicted. This versatile hierarchical multi-scale framework may be further extended to study the mechanical behaviors of cells and biomolecules, as well as to guide and stimulate biomechanics experiments.

Cyclic Hydraulic Pressure and Fluid Flow Differentially Modulate Cytoskeleton Re-Organization in MC3T3 Osteoblasts

Mechanical loads are essential towards maintaining bone mass and skeletal integrity. Such loads generate various stimuli at the cellular level, including cyclic hydraulic pressure (CHP) and fluid shear stress (FSS). To gain insight into the anabolic responses of osteoblasts to CHP and FSS, we subjected MC3T3-E1 preosteoblasts to either FSS (12 dynes/cm(2)) or CHP varying from 0 to 68 kPa at 0.5 Hz. As with FSS, CHP produced a significant increase in ATP release over static controls within 5 min of onset. Cell stiffness examined by atomic force microscopy increased after 15 min of either CHP or FSS stimulation, which was attenuated when extracellular ATP was hydrolyzed with apyrase. As previously shown FSS induced polymerization of actins into stress fibers. However, the microtubule network was completely disrupted under FSS. In contrast, CHP appeared to maintain strong microtubule and f-actin networks. The purinergic signaling was found to be involved in the remodeling of f-actin, but not microtubule. Both CHP and FSS applied for 1 hour increased expression of COX-2. These data indicate that, while CHP and FSS produce similar anabolic responses, these stimuli have very different effects on the cytoskeleton remodeling and could contribute to loss of mechanosensitivity with extended loading.

Mechanical loading by fluid shear is sufficient to alter the cytoskeletal composition of osteoblastic cells

Many structural modifications have been observed as a part of the cellular response to mechanical loading in a variety of cell types. Although changes in morphology and cytoskeletal rearrangement have been widely reported, few studies have investigated the change in cytoskeletal composition. Measuring how the amounts of specific structural proteins in the cytoskeleton change in response to mechanical loading will help to elucidate cellular mechanisms of functional adaptation to the applied forces. Therefore, the overall hypothesis of this study was that osteoblasts would respond to fluid shear stress by altering the amount of specific cross-linking proteins in the composition of the cytoskeleton. Mouse osteoblats cell line MC3T3-E1 and human fetal osteoblasts (hFOB) were exposed to 2 Pa of steady fluid shear for 2 h in a parallel plate flow chamber, and then the amount of actin, vimentin, α-actinin, filamin, and talin in the cytoskeleton was measured using Western blot analyses. After mechanical loading, there was no change in the amount of actin monomers in the cytoskeleton, but the cross-linking proteins α-actinin and filamin that cofractionated with the cytoskeleton increased by 29% ( P < 0.01) and 18% ( P < 0.02), respectively. Localization of the cross-linking proteins by fluorescent microscopy revealed that they were more widely distributed throughout the cell after exposure to fluid shear. The amount of vimentin in the cytoskeleton also increased by 15% ( P < 0.01). These results indicate that osteoblasts responded to mechanical loading by altering the cytoskeletal composition, which included an increase in specific proteins that would likely enhance the mechanical resistance of the cytoskeleton.

Mitochondrial displacements in response to nanomechanical forces

Mechanical stress affects and regulates many aspects of the cell, including morphology, growth, differentiation, gene expression and apoptosis. In this study we show how mechanical stress perturbs the intracellular structures of the cell and induces mechanical responses. In order to correlate mechanical perturbations to cellular responses, we used a combined fluorescence-atomic force microscope (AFM) to produce well defined nanomechanical perturbations of 10 nN while simultaneously tracking the real-time motion of fluorescently labelled mitochondria in live cells. The spatial displacement of the organelles in response to applied loads demonstrates the highly dynamic mechanical response of mitochondria in fibroblast cells. The average displacement of all mitochondrial structures analysed showed an increase of approximately 40%, post-perturbation ( approximately 160 nm in comparison to basal displacements of approximately 110 nm). These results show that local forces can produce organelle displacements at locations far from the initial point of contact (up to approximately 40 microm). In order to examine the role of the cytoskeleton in force transmission and its effect on mitochondrial displacements, both the actin and microtubule cytoskeleton were disrupted using Cytochalasin D and Nocodazole, respectively. Our results show that there is no significant change in mitochondrial displacement following indentation after such treatments. These results demonstrate the role of the cytoskeleton in force transmission through the cell and on mitochondrial displacements. In addition, it is suggested that care must be taken when performing mechanical experiments on living cells with the AFM, as these local mechanical perturbations may have significant structural and even biochemical effects on the cell.

Over-expression of alpha-actinin with a GFP fusion protein is sufficient to increase whole-cell stiffness in human osteoblasts

Osteoblasts respond to shear stress by simultaneously increasing their whole-cell stiffness and up-regulating the cytoskeletal crosslinking protein alpha-actinin. The stiffness of reconstituted cytoskeletal networks increases following the addition of alpha-actinin, but the effect of alpha-actinin on whole-cell mechanical behavior has not been investigated. The hypothesis of this study was that increasing alpha-actinin in the cytoskeleton would be sufficient to increase whole-cell stiffness. hFOB osteoblasts were transfected with a plasmid for GFP-tagged alpha-actinin, resulting in a 150% increase in the amount of alpha-actinin. The GFP-alpha-actinin fusion protein co-fractionated with the cytoskeleton and co-localized to the same regions of the cytoskeleton as endogenous alpha-actinin. Whole-cell mechanical behavior was measured by atomic force microscopy using a 25 mum diameter microsphere as an indenter. The whole-cell stiffness of cells over-expressing GFP-alpha-actinin was 60% higher than cells expressing only endogenous alpha-actinin (p < 0.002), which was within the range of mechanical behavior observed in osteoblastic cells exposed to 1 and 2 Pa of fluid shear. These results indicate that the up-regulation of alpha-actinin synthesis in osteoblasts is sufficient to alter the whole-cell mechanical behavior and highlights the potential role of alpha-actinin to reinforce cells against mechanical loads.

Combined scanning probe and total internal reflection fluorescence microscopy

Combining scanning probe and optical microscopy represents a powerful approach for investigating structure-function relationships and dynamics of biomolecules and biomolecular assemblies, often in situ and in real-time. This platform technology allows us to obtain three-dimensional images of individual molecules with nanometer resolution, while simultaneously characterizing their structure and interactions though complementary techniques such as optical microscopy and spectroscopy. We describe herein the practical strategies for the coupling of scanning probe and total internal reflection fluorescence microscopy along with challenges and the potential applications of such platforms, with a particular focus on their application to the study of biomolecular interactions at membrane surfaces.

Isolation of microtubules and microtubule proteins

This unit describes various protocols for the isolation and purification of the main constituents of microtubules, chiefly alpha- and beta-tubulin, and the most significant microtubule associated proteins (MAPs), specifically MAP1A, MAP1B, MAP2, and tau. We include a classical isolation method for soluble tubulin heterodimer as the first basic purification protocol. In addition, we show how to analyze the tubulin and MAPs obtained after a phosphocellulose chromatography purification procedure. This unit also details a powerful and simple method to determine the native state of the purified tubulin based on one-dimensional electrophoresis under nondenaturing conditions (UNIT 6.5). The last protocol describes the application of a new technique that allows visualizing the quality of polymerized microtubules based on atomic force microscopy (AFM).

Gating mechanisms of mechanosensitive channels of large conductance, II: systematic study of conformational transitions

Part II of this study is based on the continuum mechanics-based molecular dynamics-decorated finite element method (MDeFEM) framework established in Part I. In Part II, the gating pathways of Escherichia coli-MscL channels under various basic deformation modes are simulated. Upon equibiaxial tension (which is verified to be the most effective mode for gating), the MDeFEM results agree well with both experiments and all-atom simulations in literature, as well as the analytical continuum models and elastic network models developed in Part I. Different levels of model sophistication and effects of structural motifs are explored in detail, where the importance of mechanical roles of transmembrane helices, cytoplasmic helices, and loops are discussed. The conformation transitions under complex membrane deformations are predicted, including bending, torsion, cooperativity, patch clamp, and indentation. Compared to atom-based molecular dynamics simulations and elastic network models, the MDeFEM framework is unusually well-suited for simulating complex deformations at large length scales. The versatile hierarchical framework can be further applied to simulate the gating transition of other mechanosensitive channels and other biological processes where mechanical perturbation is important.

Noncontact measurement of the local mechanical properties of living cells using pressure applied via a pipette

Mechanosensitivity in living biological tissue is a study area of increasing importance, but investigative tools are often inadequate. We have developed a noncontact nanoscale method to apply quantified positive and negative force at defined positions to the soft responsive surface of living cells. The method uses applied hydrostatic pressure (0.1-150 kPa) through a pipette, while the pipette-sample separation is kept constant above the cell surface using ion conductance based distance feedback. This prevents any surface contact, or contamination of the pipette, allowing repeated measurements. We show that we can probe the local mechanical properties of living cells using increasing pressure, and hence measure the nanomechanical properties of the cell membrane and the underlying cytoskeleton in a variety of cells (erythrocytes, epithelium, cardiomyocytes and neurons). Because the cell surface can first be imaged without pressure, it is possible to relate the mechanical properties to the local cell topography. This method is well suited to probe the nanomechanical properties and mechanosensitivity of living cells.

Mechanotransduction in human bone: in vitro cellular physiology that underpins bone changes with exercise

Bone has a remarkable ability to adjust its mass and architecture in response to a wide range of loads, from low-level gravitational forces to high-level impacts. A variety of types and magnitudes of mechanical stimuli have been shown to influence human bone cell metabolism in vitro, including fluid shear, tensile and compressive strain, altered gravity and vibration. Therefore, the current article aims to synthesize in vitro data regarding the cellular mechanisms underlying the response of human bone cells to mechanical loading. Current data demonstrate commonalities in response to different types of mechanical stimuli on the one hand, along with differential activation of intracellular signalling on the other. A major unanswered question is, how do bone cells sense and distinguish between different types of load? The studies included in the present article suggest that the type and magnitude of loading may be discriminated by overlapping mechanosensory mechanisms including (i) ion channels; (ii) integrins; (iii) G-proteins; and (iv) the cytoskeleton. The downstream signalling pathways identified to date appear to overlap with known growth factor and hormone signals, providing a mechanism of interaction between systemic influences and the local mechanical environment. Finally, the data suggest that exercise should emphasize the amount of load rather than the number of repetitions.

Stimulation of nitric oxide mechanotransduction in single osteoblasts using atomic force microscopy

Nitric oxide (NO) released from mechanosensitive bone cells plays a key role in the adaptation of bone structure to its mechanical usage. Despite its importance in bone, the mechanisms involved in NO mechanotransduction at the cellular level remain unknown. Using combined atomic force microscopy and fluorescence microscopy, we report both stimulation and real-time monitoring of NO responses in single osteoblasts induced by application of quantified periodic indenting forces to the osteoblast membrane. Peak forces ranging from 17 to 50 nN stimulated three distinct NO responses in the indented osteoblasts: (1) a rapid and sustained diffusion of NO from the perinuclear region, (2) diffusion of NO from localized pools throughout the osteoblast, and (3) an initial increase and subsequent drop in intracellular NO. Force-indentation characteristics showed considerable interosteoblast variation in elasticity. NO responses were associated with application of force to more rigid membrane sites, suggesting cytoskeletal involvement in mechanotransduction.

Magnetic microposts for mechanical stimulation of biological cells: fabrication, characterization, and analysis

Cells use force as a mechanical signal to sense and respond to their microenvironment. Understanding how mechanical forces affect living cells requires the development of tool sets that can apply nanoscale forces and also measure cellular traction forces. However, there has been a lack of techniques that integrate actuation and sensing components to study force as a mechanical signal. Here, we describe a system that uses an array of elastomeric microposts to apply external forces to cells through cobalt nanowires embedded inside the microposts. We first biochemically treat the posts' surfaces to restrict cell adhesion to the posts' tips. Then by applying a uniform magnetic field (B<0.3 T), we induce magnetic torque on the nanowires that is transmitted to a cell's adhesion site as an external force. We have achieved external forces of up to 45 nN, which is in the upper range of current nanoscale force-probing techniques. Nonmagnetic microposts, similarly prepared but without nanowires, surround the magnetic microposts and are used to measure the traction forces and changes in cell mechanics. We record the magnitude and direction of the external force and the traction forces by optically measuring the deflection of the microposts, which linearly deflect as cantilever springs. With this approach, we can measure traction forces before and after force stimulation in order to monitor cellular response to forces. We present the fabrication methods, magnetic force characterization, and image analysis techniques used to achieve the measurements.

Hydrostatic pressure sensation in cells: integration into the tensegrity model

Hydrostatic pressure (HP) is a mechanical stimulus that has received relatively little attention in the field of the cell biology of mechanotransduction. Generalized models, such as the tensegrity model, do not provide a detailed explanation of how HP might be detected. This is significant, because HP is an important mechanical stimulus, directing cell behaviour in a variety of tissues, including cartilage, bone, airways, and the vasculature. HP sensitivity may also be an important factor in certain clinical situations, as well as under unique environmental conditions such as microgravity. While downstream cellular effects have been well characterized, the initial HP sensation mechanism remains unclear. In vitro evidence shows that HP affects cytoskeletal polymerization, an effect that may be crucial in triggering the cellular response. The balance between free monomers and cytoskeletal polymers is shifted by alterations in HP, which could initiate a cellular response by releasing and (or) activating cytoskeleton-associated proteins. This new model fits well with the basic tenets of the existing tensegrity model, including mechanisms in which cellular HP sensitivity could be tuned to accommodate variable levels of stress.

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.

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.

New technologies for dissecting the arteriolar myogenic response

The arteriolar myogenic response is crucial for the setting of vascular resistance and for providing a level of tone upon which vasodilators can act. Despite its physiological importance, questions remain regarding the underlying signaling mechanisms of the arteriolar myogenic response. Does an increase in pressure within an arteriole exert its effects via the extracellular matrix, an action on cell membranes and/or deformation of cytoskeletal structures? Recent advances in methodology, particularly involving sophisticated imaging approaches, are enabling the study of forces at single-cell and even subcellular levels. Atomic force microscopy (AFM) not only enables detection of cell morphology and stiffness but also allows discrete forces to be applied to single smooth muscle cells and subsequent responses to be observed. Importantly, the repertoire of approaches involving AFM can be expanded by using it in combination with other imaging approaches - including fluorescence imaging for cellular signals such as Ca(2+), and total internal reflectance fluorescence, fluorescence resonance energy transfer and confocal microscopy for probing cellular contact function. Combinations of these advanced imaging and nanomechanical approaches will be instructive to studies of intact vessels and the circulatory system in general.

Chemomechanical mapping of ligand-receptor binding kinetics on cells

The binding kinetics between cell surface receptors and extracellular biomolecules is critical to all intracellular and intercellular activity. Modeling and prediction of receptor-mediated cell functions are facilitated by measurement of the binding properties on whole cells, ideally indicating the subcellular locations or cytoskeletal associations that may affect the function of bound receptors. This dual need is particularly acute vis à vis ligand engineering and clinical applications of antibodies to neutralize pathological processes. Here, we map individual receptors and determine whole-cell binding kinetics by means of functionalized force imaging, enabled by scanning probe microscopy and molecular force spectroscopy of intact cells with biomolecule-conjugated mechanical probes. We quantify the number, distribution, and association/dissociation rate constants of vascular endothelial growth factor receptor-2 with respect to a monoclonal antibody on both living and fixed human microvascular endothelial cells. This general approach to direct receptor imaging simultaneously quantifies both the binding kinetics and the nonuniform distribution of these receptors with respect to the underlying cytoskeleton, providing spatiotemporal visualization of cell surface dynamics that regulate receptor-mediated behavior.

Osteoblast cytoskeletal modulation in response to compressive stress at physiological levels

Biomechanical force is one of the major epigenetic factors that determine the form and differentiation of skeletal tissues. In this study, osteoblastic cells UMR-106 were exposed to compressive forces at 1000 mustrain and 4000 mustrain via a four-point bending system, and analyzed by MTT and LSCM techniques. Cell proliferation activity decreased shortly after loading but recovered to normal levels within 24 h. And the cytoskeleton depolymerized at first, but then gradually repolymerized. To find out the role of cytoskeleton in mechanotransduction, we examined the relationship between cytoskeleton construction and c-fos expression. A transient stress-induced upregulation in c-fos mRNA and c-Fos protein was discovered when cells were exposed to physiological forces. And the upregulation in c-fos expression was blocked by cytochalasin D (Depolymerizing agent of microfilament). It gave clues that the organization of cytoskeleton was an important link in transcriptional control in response to low-mechanical stimulation.

Effect of surface nanoscale topography on elastic modulus of individual osteoblastic cells as determined by atomic force microscopy

Mechanical stimulation of osteoblasts by fluid flow promotes a variety of pro-differentiation effects and improving the efficiency of these mechanical signals could encourage specific differentiation pathways. One way this could be accomplished is by altering mechanical properties of osteoblasts. In this study, murine osteoblastic MC3T3-E1 cells were cultured on surfaces covered with nanometer-sized islands to examine the hypothesis that the elastic modulus of osteoblastic cells is affected by nanoscale topography. Nanoislands were produced by polymer demixing of polystyrene and poly(bromostyrene), which leads to a segregated polymer system and formation of nanometer-sized topographical features. The elastic modulus of MC3T3-E1 cells was determined using atomic force microscopy in conjunction with the Hertz mathematical model. Osteoblastic cells cultured on nanotopographic surfaces (11-38 nm high islands) had a different distribution of cellular modulus values, e.g., the distribution shifted toward higher modulus values, relative to cells on flat control surfaces. There were also differences in cell modulus distribution between two flat controls as surface chemistry was changed between polystyrene and glass. Taken together, our results demonstrate that both surface nanotopography and chemistry affect the mechanical properties of cells and may provide new methods for altering the response of cells to external mechanical signals.

Nanotopographical stimulation of mechanotransduction and changes in interphase centromere positioning

We apply a recently developed method for controlling the spreading of cultured cells using electron beam lithography (EBL) to create polymethylmethacrylate (PMMA) substrata with repeating nanostructures. There are indications that the reduced cell spreading on these substrata, compared with planar PMMA, results from a reduced adhesivity since there are fewer adhesive structures and fewer of their associated stress fibres. The reduced cell spreading also results in a reduced nuclear area and a closer spacing of centrosomes within the nucleus, suggesting that the tension applied to the nucleus is reduced as would be expected from the reduction in stress fibres. In order to obtain further evidence for this, we have used specific inhibitors of components of the cytoskeleton and have found effects comparable with those induced by the new substrata. We have also obtained evidence that these subtrata result in downregulation of gene expression which suggests that this may be due to the changed tension on the nucleus: an intriguing possibility that merits further investigation.

Nanomechanotransduction and Interphase Nuclear Organization influence on genomic control

The ability of cells to alter their genomic regulation in response to mechanical conditioning or through changes in morphology and the organization of the interphase nuclei are key questions in cell biology. Here, two nanotopographies have been used as a model surfaces to change cell morphology in order to investigate spatial genomic changes within the nuclei of fibroblasts. Initially, centromeres for chromosome pairs were labeled and the average distance on different substrates calculated. Further to this, Affymetrix whole genome GeneChips were used to rank genomic changes in response to topography and plot the whereabouts on the chromosomes these changes were occurring. It was seen that as cell spreading was changed, so were the positions along the chromosomes that gene regulations were being observed. We hypothesize that as changes in cell and thus nuclear morphology occur, that this may alter the probability of transcription through opening or closing areas of the chromosomes to transcription factors.

Cellular response to low adhesion nanotopographies

This review focuses on how cells respond to low-adhesion nanotopographies. In order to do this, fabrication techniques, how cells may locate nanofeatures through the use of filopodia and possible mechanotransductive mechanisms are discussed. From this, examples of low-adhesion topographies and sizes and arrangements that may lead to low-adhesion are discussed. Finally, it is hypothesized as to how specifically low-adhesion materials may fit into the outlined mechanotransductive mechanisms.

Adaptation of cellular mechanical behavior to mechanical loading for osteoblastic cells

Numerous cellular biochemical responses to mechanical loading are transient, indicating a cell's ability to adapt its behavior to a new mechanical environment. Since load-induced cellular deformation can initiate these biochemical responses, the overall goal of this study was to investigate the adaptation of global, or whole-cell, mechanical behavior, i.e., cellular deformability, in response to mechanical loading for osteoblastic cells. Confluent cell cultures were subjected to 1 or 2 Pa flow-induced shear stress for 2 h. Whole-cell mechanical behavior was then measured for individual cells using an atomic force microscope. Compared to cells maintained under static conditions, whole-cell stiffness was 1.36-fold (p=0.006) and 1.70-fold (p<0.001) greater for cells exposed to 1 and 2 Pa shear loading, respectively. The increase in shear stress magnitude from 1 to 2 Pa also caused a statistically significant, 1.25-fold increase in cell stiffness (p=0.02). Increases in cell stiffness were not altered in either flow group for 70 min after flow was terminated (p=0.15). Flow-induced rearrangement of the actin cytoskeleton was also maintained for at least 90 min after flow was terminated. Taken together, these findings support the hypothesis that cells become mechanically adapted to their mechanical environment via cytoskeletal modifications. Accordingly, cellular mechanical adaptation may play a key role in regulation of cellular mechanosensitivity and the related effects on tissue structure and function.

Reassembly of contractile actin cortex in cell blebs

Contractile actin cortex is involved in cell morphogenesis, movement, and cytokinesis, but its organization and assembly are poorly understood. During blebbing, the membrane detaches from the cortex and inflates. As expansion ceases, contractile cortex re-assembles under the membrane and drives bleb retraction. This cycle enabled us to measure the temporal sequence of protein recruitment to the membrane during cortex reassembly and to explore dependency relationships. Expanding blebs were devoid of actin, but proteins of the erythrocytic submembranous cytoskeleton were present. When expansion ceased, ezrin was recruited to the membrane first, followed by actin, actin-bundling proteins, and, finally, contractile proteins. Complete assembly of the contractile cortex, which was organized into a cagelike mesh of filaments, took approximately 30 s. Cytochalasin D blocked recruitment of actin and alpha-actinin, but had no effect on membrane association of ankyrin B and ezrin. Ezrin played no role in actin nucleation, but was essential for tethering the membrane to the cortex. The Rho pathway was important for cortex assembly in blebs.

Design of a novel MEMS platform for the biaxial stimulation of living cells

Micromechanical systems are increasingly being used as tools in biological applications, since their characteristic dimensions permit to operate at the same length scale of the structures under investigation. Here, we present a methodology for the design, fabrication and operation of a tool for the assessment of mechanical properties of single cells. In particular, we describe a microsystems platform to study bio-mechanical response of single living cells to in-plane biaxial stretching. The proposed device employs a new linkage design in order to obtain the displacement of the quadrants of a sliced circular plate in mutually-orthogonal directions using just one linear actuator. With this linkage geometry, the whole device has only one degree of freedom. This results in a very predictable and reliable mechanical behaviour, thereby allowing use a simple and easily available control electronics. Results of this study have relevance for the design of a powerful yet simple BioMEMS platform for the characterization of living cells as in-plane bi-axial loading simulated the conditions experienced by cells in vivo more realistically than a uniaxial stretching.

Atomic force microscopy-based cell nanostructure for ligand-conjugated quantum dot endocytosis

While it has been well demonstrated that quantum dots (QDs) play an important role in biological labeling both in vitro and in vivo, there is no report describing the cellular nanostructure basis of receptor-mediated endocytosis. Here, nanostructure evolution responses to the endocytosis of transferrin (Tf)-conjugated QDs were characterized by atomic force microscopy (AFM). AFM-based nanostructure analysis demonstrated that the Tf-conjugated QDs were specifically and tightly bound to the cell receptors and the nanostructure evolution is highly correlated with the cell membrane receptor-mediated transduction. Consistently, confocal microscopic and flow cytometry results have demonstrated the specificity and dynamic property of Tf-QD binding and internalization. We found that the internalization of Tf-QD is linearly related to time. Moreover, while the nanoparticles on the cell membrane increased, the endocytosis was still very active, suggesting that QD nanoparticles did not interfere sterically with the binding and function of receptors. Therefore, ligand-conjugated QDs are potentially useful in biological labeling of cells at a nanometer scale.

Mechanotransduction involving multimodular proteins: converting force into biochemical signals

Cells can sense and transduce a broad range of mechanical forces into distinct sets of biochemical signals that ultimately regulate cellular processes, including adhesion, proliferation, differentiation, and apoptosis. Deciphering at the nanoscale the design principles by which sensory elements are integrated into structural protein motifs whose conformations can be switched mechanically is crucial to understand the process of transduction of force into biochemical signals that are then integrated to regulate mechanoresponsive pathways. While the major focus in the search for mechanosensory units has been on membrane proteins such as ion channels, integrins, and associated cytoplasmic complexes, a multimodular design of tandem repeats of various structural motifs is ubiquitously found among extracellular matrix proteins, as well as cell adhesion molecules, and among many intracellular players that physically link transmembrane proteins to the contractile cytoskeleton. Single-molecule studies have revealed an unexpected richness of mechanosensory motifs, including force-regulated conformational changes of loop-exposed molecular recognition sites, intermediate states in the unraveling pathway that might either expose cryptic binding or phosphorylation sites, or regions that display enzymatic activity only when unmasked by force. Insights into mechanochemical signal conversion principles will also affect various technological fields, from biotechnology to tissue engineering and drug development.

Nanomechanical properties of individual chondrocytes and their developing growth factor-stimulated pericellular matrix

The nanomechanical properties of individual cartilage cells (chondrocytes) and their aggrecan and collagen-rich pericellular matrix (PCM) were measured via atomic force microscope nanoindentation using probe tips of two length scales (nanosized and micron-sized). The properties of cells freshly isolated from cartilage tissue (devoid of PCM) were compared to cells that were cultured for selected times (up to 28 days) in 3-D alginate gels which enabled PCM assembly and accumulation. Cells were immobilized and kept viable in pyramidal wells microfabricated into an array on silicon chips. Hertzian contact mechanics and finite element analyses were employed to estimate apparent moduli from the force versus depth curves. The effects of culture conditions on the resulting PCM properties were studied by comparing 10% fetal bovine serum to medium containing a combination of insulin growth factor-1 (IGF-1)+osteogenic protein-1 (OP-1). While both systems showed increases in stiffness with time in culture between days 7 and 28, the IGF-1+OP-1 combination resulted in a higher stiffness for the cell-PCM composite by day 28 and a higher apparent modulus of the PCM which is compared to the FBS cultured cells. These studies give insight into the temporal evolution of the nanomechanical properties of the pericellar matrix relevant to the biomechanics and mechanobiology of tissue-engineered constructs for cartilage repair.