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

Local membrane deformation and micro-injury lead to qualitatively different responses in osteoblasts

Micro-damage of bone tissue is known to regulate bone turnover. However, it is unknown if individual bone cells can differentiate between membrane deformation and micro-injury. We generated osteoblasts from mouse bone marrow or bone morphogenetic protein 2-transfected C2C12 cells. Single cells were mechanically stimulated by indentation with the atomic force microscopy probe with variable force load either resulting in membrane deformation only, or leading to membrane penetration and micro-injury. Changes in the cytosolic free calcium concentration ([Ca (2+)] i) in fluo4-AM loaded cells were analyzed. When deformation only was induced, it resulted in an immediate elevation of [Ca (2+)] i which was localized to the probe periphery. Multiple consecutive local Ca (2+) responses were induced by sequential application of low level forces, with characteristic recovery time of ~2 s. The duration of [Ca (2+)] i elevations was directly proportional to the tip-cell contact time. In contrast, cell micro-injury resulted in transient global elevations of [Ca (2+)] i, the magnitude of which was independent of the tip-cell contact time. Sequential micro-injury of the same cell did not induce Ca (2+) response within 30 s of the first stimulation. Both local and global Ca (2+)elevations were blocked in Ca (2+)-free media or in the presence of stretch-activated channel blocker Gd (3+). In addition, amount of Ca (2+) released during global responses was significantly reduced in the presence of PLC inhibitor Et-18-OCH 3. Thus, we found qualitative differences in calcium responses to mechanical forces inducing only membrane deformation or deformation leading to micro-injury.

Materials as stem cell regulators

The stem cell/material interface is a complex, dynamic microenvironment in which the cell and the material cooperatively dictate one another's fate: the cell by remodelling its surroundings, and the material through its inherent properties (such as adhesivity, stiffness, nanostructure or degradability). Stem cells in contact with materials are able to sense their properties, integrate cues via signal propagation and ultimately translate parallel signalling information into cell fate decisions. However, discovering the mechanisms by which stem cells respond to inherent material characteristics is challenging because of the highly complex, multicomponent signalling milieu present in the stem cell environment. In this Review, we discuss recent evidence that shows that inherent material properties may be engineered to dictate stem cell fate decisions, and overview a subset of the operative signal transduction mechanisms that have begun to emerge. Further developments in stem cell engineering and mechanotransduction are poised to have substantial implications for stem cell biology and regenerative medicine.

Insights into the alteration of osteoblast mechanical properties upon adhesion on chitosan

Cell adhesion on substrates is accompanied by significant changes in shape and cytoskeleton organization, which affect subsequent cellular and tissue responses, determining the long-term success of an implant. Alterations in osteoblast stiffness upon adhesion on orthopaedic implants with different surface chemical composition and topography are, thus, of central interest in the field of bone implant research. This work aimed to study the mechanical response of osteoblasts upon adhesion on chitosan-coated glass surfaces and to investigate possible correlations with the level of adhesion, spreading, and cytoskeleton reorganization. Using the micropipette aspiration technique, the osteoblast elastic modulus was found higher on chitosan-coated than on uncoated control substrates, and it was found to increase in the course of spreading for both substrates. The cell-surface contact area was measured throughout several time points of adhesion to quantify cell spreading kinetics. Significant differences were found between chitosan and control surfaces regarding the response of cell spreading, while both groups displayed a sigmoidal kinetical behavior with an initially elevated spreading rate which stabilizes in the second hour of attachment. Actin filament structural changes were confirmed after observation with confocal microscope. Biomaterial surface modification can enhance osteoblast mechanical response and induce favorable structural organization for the implant integration.

Hydraulic Pressure during Fluid Flow Regulates Purinergic Signaling and Cytoskeleton Organization of Osteoblasts

During physiological activities, osteoblasts experience a variety of mechanical forces that stimulate anabolic responses at the cellular level necessary for the formation of new bone. Previous studies have primarily investigated the osteoblastic response to individual forms of mechanical stimuli. However in this study, we evaluated the response of osteoblasts to two simultaneous, but independently controlled stimuli; fluid flow-induced shear stress (FSS) and static or cyclic hydrostatic pressure (SHP or CHP, respectively). MC3T3-E1 osteoblasts-like cells were subjected to 12dyn/cm² FSS along with SHP or CHP of varying magnitudes to determine if pressure enhances the anabolic response of osteoblasts during FSS. For both SHP and CHP, the magnitude of hydraulic pressure that induced the greatest release of ATP during FSS was 15 mmHg. Increasing the hydraulic pressure to 50 mmHg or 100 mmHg during FSS attenuated the ATP release compared to 15 mmHg during FSS. Decreasing the magnitude of pressure during FSS to atmospheric pressure reduced ATP release to that of basal ATP release from static cells and inhibited actin reorganization into stress fibers that normally occurred during FSS with 15 mmHg of pressure. In contrast, translocation of nuclear factor kappa B (NFκB) to the nucleus was independent of the magnitude of hydraulic pressure and was found to be mediated through the activation of phospholipase-C (PLC), but not src kinase. In conclusion, hydraulic pressure during FSS was found to regulate purinergic signaling and actin cytoskeleton reorganization in the osteoblasts in a biphasic manner, while FSS alone appeared to stimulate NFκB translocation. Understanding the effects of hydraulic pressure on the anabolic responses of osteoblasts during FSS may provide much needed insights into the physiologic effects of coupled mechanical stimuli on osteogenesis.

Nanobiomechanics of living cells: a review

Nanobiomechanics of living cells is very important to understand cell-materials interactions. This would potentially help to optimize the surface design of the implanted materials and scaffold materials for tissue engineering. The nanoindentation techniques enable quantifying nanobiomechanics of living cells, with flexibility of using indenters of different geometries. However, the data interpretation for nanoindentation of living cells is often difficult. Despite abundant experimental data reported on nanobiomechanics of living cells, there is a lack of comprehensive discussion on testing with different tip geometries, and the associated mechanical models that enable extracting the mechanical properties of living cells. Therefore, this paper discusses the strategy of selecting the right type of indenter tips and the corresponding mechanical models at given test conditions.

P2Y2 receptors regulate osteoblast mechanosensitivity during fluid flow

Mechanical stimulation of osteoblasts activates many cellular mechanisms including the release of ATP. Binding of ATP to purinergic receptors is key to load-induced osteogenesis. Osteoblasts also respond to fluid shear stress (FSS) with increased actin stress fiber formation (ASFF) that we postulate is in response to activation of the P2Y2 receptor (P2Y2R). Furthermore, we predict that ASFF increases cell stiffness and reduces the sensitivity to further mechanical stimulation. We found that small interfering RNA (siRNA) suppression of P2Y2R attenuated ASFF in response to FSS and ATP treatment. In addition, RhoA GTPase was activated within 15 min after the onset of FSS or ATP treatment and mediated ASFF following P2Y2R activation via the Rho kinase (ROCK)1/LIM kinase 2/cofilin pathway. We also observed that ASFF in response to FSS or ATP treatment increased the cell stiffness and was prevented by knocking down P2Y2R. Finally, we confirmed that the enhanced cell stiffness and ASFF in response to RhoA GTPase activation during FSS drastically reduced the mechanosensitivity of the osteoblasts based on the intracellular Ca(2+) concentration ([Ca(2+)]i) response to consecutive bouts of FSS. These data suggest that osteoblasts can regulate their mechanosensitivity to continued load through P2Y2R activation of the RhoA GTPase signaling cascade, leading to ASFF and increased cell stiffness.

Integrated micro/nanoengineered functional biomaterials for cell mechanics and mechanobiology: a materials perspective

The rapid development of micro/nanoengineered functional biomaterials in the last two decades has empowered materials scientists and bioengineers to precisely control different aspects of the in vitro cell microenvironment. Following a philosophy of reductionism, many studies using synthetic functional biomaterials have revealed instructive roles of individual extracellular biophysical and biochemical cues in regulating cellular behaviors. Development of integrated micro/nanoengineered functional biomaterials to study complex and emergent biological phenomena has also thrived rapidly in recent years, revealing adaptive and integrated cellular behaviors closely relevant to human physiological and pathological conditions. Working at the interface between materials science and engineering, biology, and medicine, we are now at the beginning of a great exploration using micro/nanoengineered functional biomaterials for both fundamental biology study and clinical and biomedical applications such as regenerative medicine and drug screening. In this review, an overview of state of the art micro/nanoengineered functional biomaterials that can control precisely individual aspects of cell-microenvironment interactions is presented and they are highlighted them as well-controlled platforms for mechanistic studies of mechano-sensitive and -responsive cellular behaviors and integrative biology research. The recent exciting trend where micro/nanoengineered biomaterials are integrated into miniaturized biological and biomimetic systems for dynamic multiparametric microenvironmental control of emergent and integrated cellular behaviors is also discussed. The impact of integrated micro/nanoengineered functional biomaterials for future in vitro studies of regenerative medicine, cell biology, as well as human development and disease models are discussed.

Mechanosensing through focal adhesion-anchored intermediate filaments

Integrin-based mechanotransduction involves a complex focal adhesion (FA)-associated machinery that is able to detect and respond to forces exerted either through components of the extracellular matrix or the intracellular contractile actomyosin network. Here, we show a hitherto unrecognized regulatory role of vimentin intermediate filaments (IFs) in this process. By studying fibroblasts in which vimentin IFs were decoupled from FAs, either because of vimentin deficiency (V0) or loss of vimentin network anchorage due to deficiency in the cytolinker protein plectin (P0), we demonstrate attenuated activation of the major mechanosensor molecule FAK and its downstream targets Src, ERK1/2, and p38, as well as an up-regulation of the compensatory feedback loop acting on RhoA and myosin light chain. In line with these findings, we show strongly reduced FA turnover rates in P0 fibroblasts combined with impaired directional migration, formation of protrusions, and up-regulation of "stretched" high-affinity integrin complexes. By exploiting tension-independent conditions, we were able to mechanistically link these defects to diminished cytoskeletal tension in both P0 and V0 cells. Our data provide important new insights into molecular mechanisms underlying cytoskeleton-regulated mechanosensing, a feature that is fundamental for controlled cell movement and tumor progression.

Resiliency of the plasma membrane and actin cortex to large-scale deformation

The tight coupling between the plasma membrane and actin cortex allows cells to rapidly change shape in response to mechanical cues and during physiological processes. Mechanical properties of the membrane are critical for organizing the actin cortex, which ultimately governs the conversion of mechanical information into signaling. The cortex has been shown to rapidly remodel on timescales of seconds to minutes, facilitating localized deformations and bundling dynamics that arise during the exertion of mechanical forces and cellular deformations. Here, we directly visualized and quantified the time-dependent deformation and recovery of the membrane and actin cortex of HeLa cells in response to externally applied loads both on- and off-nucleus using simultaneous confocal and atomic force microscopy. The local creep-like deformation of the membrane and actin cortex depends on both load magnitude and duration and does not appear to depend on cell confluency. The membrane and actin cortex rapidly recover their initial shape after prolonged loading (up to 10 min) with large forces (up to 20 nN) and high aspect ratio deformations. Cytoplasmic regions surrounding the nucleus are shown to be more resistant to long-term creep than nuclear regions. These dynamics are highly regulated by actomyosin contractility and an intact actin cytoskeleton. Results suggest that in response to local deformations, the nucleus does not appear to provide significant resistance or play a major role in cell shape recovery. The membrane and actin cortex clearly possess remarkable mechanical stability, critical for the transduction of mechanical deformation into long term biochemical signals and cellular remodeling.

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

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

Atomic force microscopy application in biological research: a review study

Atomic force microscopy (AFM) is a three-dimensional topographic technique with a high atomic resolution to measure surface roughness. AFM is a kind of scanning probe microscope, and its near-field technique is based on the interaction between a sharp tip and the atoms of the sample surface. There are several methods and many ways to modify the tip of the AFM to investigate surface properties, including measuring friction, adhesion forces and viscoelastic properties as well as determining the Young modulus and imaging magnetic or electrostatic properties. The AFM technique can analyze any kind of samples such as polymers, adsorbed molecules, films or fibers, and powders in the air whether in a controlled atmosphere or in a liquid medium. In the past decade, the AFM has emerged as a powerful tool to obtain the nanostructural details and biomechanical properties of biological samples, including biomolecules and cells. The AFM applications, techniques, and -in particular- its ability to measure forces, are not still familiar to most clinicians. This paper reviews the literature on the main principles of the AFM modality and highlights the advantages of this technique in biology, medicine, and- especially- dentistry. This literature review was performed through E-resources, including Science Direct, PubMed, Blackwell Synergy, Embase, Elsevier, and Scholar Google for the references published between 1985 and 2010.

A multi-structural single cell model of force-induced interactions of cytoskeletal components

Several computational models based on experimental techniques and theories have been proposed to describe cytoskeleton (CSK) mechanics. Tensegrity is a prominent model for force generation, but it cannot predict mechanics of individual CSK components, nor explain the discrepancies from the different single cell stimulating techniques studies combined with cytoskeleton-disruptors. A new numerical concept that defines a multi-structural 3D finite element (FE) model of a single-adherent cell is proposed to investigate the biophysical and biochemical differences of the mechanical role of each cytoskeleton component under loading. The model includes prestressed actin bundles and microtubule within cytoplasm and nucleus surrounded by the actin cortex. We performed numerical simulations of atomic force microscopy (AFM) experiments by subjecting the cell model to compressive loads. The numerical role of the CSK components was corroborated with AFM force measurements on U2OS-osteosarcoma cells and NIH-3T3 fibroblasts exposed to different cytoskeleton-disrupting drugs. Computational simulation showed that actin cortex and microtubules are the major components targeted in resisting compression. This is a new numerical tool that explains the specific role of the cortex and overcomes the difficulty of isolating this component from other networks in vitro. This illustrates that a combination of cytoskeletal structures with their own properties is necessary for a complete description of cellular mechanics.

Force nanoscopy of cell mechanics and cell adhesion

Cells are constantly exposed to mechanical stimuli in their environment and have several evolved mechanisms to sense and respond to these cues. It is becoming increasingly recognized that many cell types, from bacteria to mammalian cells, possess a diverse set of proteins to translate mechanical cues into biochemical signalling and to mediate cell surface interactions such as cell adhesion. Moreover, the mechanical properties of cells are involved in regulating cell function as well as serving as indicators of disease states. Importantly, the recent development of biophysical tools and nanoscale methods has facilitated a deeper understanding of the role that physical forces play in modulating cell mechanics and cell adhesion. Here, we discuss how atomic force microscopy (AFM) has recently been used to investigate cell mechanics and cell adhesion at the single-cell and single-molecule levels. This knowledge is critical to our understanding of the molecular mechanisms that govern mechanosensing, mechanotransduction, and mechanoresponse in living cells. While pushing living cells with the AFM tip provides a means to quantify their mechanical properties and examine their response to nanoscale forces, pulling single surface proteins with a functionalized tip allows one to understand their role in sensing and adhesion. The combination of these nanoscale techniques with modern molecular biology approaches, genetic engineering and optical microscopies provides a powerful platform for understanding the sophisticated functions of the cell surface machinery, and its role in the onset and progression of complex diseases.

Segmentation and morphometric analysis of cells from fluorescence microscopy images of cytoskeletons

We developed a method to reconstruct cell geometry from confocal fluorescence microscopy images of the cytoskeleton. In the method, region growing was implemented twice. First, it was applied to the extracellular regions to differentiate them from intracellular noncytoskeletal regions, which both appear black in fluorescence microscopy imagery, and then to cell regions for cell identification. Analysis of morphological parameters revealed significant changes in cell shape associated with cytoskeleton disruption, which offered insight into the mechanical role of the cytoskeleton in maintaining cell shape. The proposed segmentation method is promising for investigations on cell morphological changes with respect to internal cytoskeletal structures.

Atomic force microscopy-coupled microcoils for cellular-scale nuclear magnetic resonance spectroscopy

We present the coupling of atomic force microscopy (AFM) and nuclear magnetic resonance (NMR) technologies to enable topographical, mechanical, and chemical profiling of biological samples. Here, we fabricate and perform proof-of-concept testing of radiofrequency planar microcoils on commercial AFM cantilevers. The sensitive region of the coil was estimated to cover an approximate volume of 19.4 × 10³ μm³ (19.4 pl). Functionality of the spectroscopic module of the prototype device is illustrated through the detection of ¹Η resonance in deionized water. The acquired spectra depict combined NMR capability with AFM that may ultimately enable biophysical and biochemical studies at the single cell level.

Spatiotemporal properties of intracellular calcium signaling in osteocytic and osteoblastic cell networks under fluid flow

Mechanical stimuli can trigger intracellular calcium (Ca(2+)) responses in osteocytes and osteoblasts. Successful construction of bone cell networks necessitates more elaborate and systematic analysis for the spatiotemporal properties of Ca(2+) signaling in the networks. In the present study, an unsupervised algorithm based on independent component analysis (ICA) was employed to extract the Ca(2+) signals of bone cells in the network. We demonstrated that the ICA-based technology could yield higher signal fidelity than the manual region of interest (ROI) method. Second, the spatiotemporal properties of Ca(2+) signaling in osteocyte-like MLO-Y4 and osteoblast-like MC3T3-E1 cell networks under laminar and steady fluid flow stimulation were systematically analyzed and compared. MLO-Y4 cells exhibited much more active Ca(2+) transients than MC3T3-E1 cells, evidenced by more Ca(2+) peaks, less time to the 1st peak and less time between the 1st and 2nd peaks. With respect to temporal properties, MLO-Y4 cells demonstrated higher spike rate and Ca(2+) oscillating frequency. The spatial intercellular synchronous activities of Ca(2+) signaling in MLO-Y4 cell networks were higher than those in MC3T3-E1 cell networks and also negatively correlated with the intercellular distance, revealing faster Ca(2+) wave propagation in MLO-Y4 cell networks. Our findings show that the unsupervised ICA-based technique results in more sensitive and quantitative signal extraction than traditional ROI analysis, with the potential to be widely employed in Ca(2+) signaling extraction in the cell networks. The present study also revealed a dramatic spatiotemporal difference in Ca(2+) signaling for osteocytic and osteoblastic cell networks in processing the mechanical stimulus. The higher intracellular Ca(2+) oscillatory behaviors and intercellular coordination of MLO-Y4 cells provided further evidences that osteocytes may behave as the major mechanical sensor in bone modeling and remodeling processes.

Correlative optical and scanning probe microscopies for mapping interactions at membranes

Innovative approaches for real-time imaging on molecular-length scales are providing researchers with powerful strategies for characterizing molecular and cellular structures and dynamics. Combinatorial techniques that integrate two or more distinct imaging modalities are particularly compelling as they provide a means for overcoming the limitations of the individual modalities and, when applied simultaneously, enable the collection of rich multi-modal datasets. Almost since its inception, scanning probe microscopy has closely associated with optical microscopy. This is particularly evident in the fields of cellular and molecular biophysics where researchers are taking full advantage of these real-time, in situ, tools to acquire three-dimensional molecular-scale topographical images with nanometer resolution, while simultaneously characterizing their structure and interactions though conventional optical microscopy. The ability to apply mechanical or optical stimuli provides an additional experimental dimension that has shown tremendous promise for examining dynamic events on sub-cellular length scales. In this chapter, we describe recent efforts in developing these integrated platforms, the methodology for, and inherent challenges in, performing coupled imaging experiments, and the potential and future opportunities of these research tools for the fields of molecular and cellular biophysics with a specific emphasis on the application of these coupled approaches for the characterization of interactions occurring at membrane interfaces.

Mechanical cues in cellular signalling and communication

Multicellular organisms comprise an organized array of individual cells surrounded by a meshwork of biomolecules and fluids. Cells have evolved various ways to communicate with each other, so that they can exchange information and thus fulfil their specified and unique functions. At the same time, cells are also physical entities that are subjected to a variety of local and global mechanical cues arising in the microenvironment. Cells are equipped with several different mechanisms to sense the physical properties of the microenvironment and the mechanical forces arising within it. These mechanical cues can elicit a variety of responses that have been shown to play a crucial role in vivo. In this review, we discuss the current views and understanding of cell mechanics and demonstrate the emerging evidence of the interplay between physiological mechanical cues and cell-cell communication pathways.

Magnetic nanoparticle-mediated massively parallel mechanical modulation of single-cell behavior

We report a technique for generating controllable, time-varying and localizable forces on arrays of cells in a massively parallel fashion. To achieve this, we grow magnetic nanoparticle-dosed cells in defined patterns on micromagnetic substrates. By manipulating and coalescing nanoparticles within cells, we apply localized nanoparticle-mediated forces approaching cellular yield tensions on the cortex of HeLa cells. We observed highly coordinated responses in cellular behavior, including the p21-activated kinase-dependent generation of active, leading edge-type filopodia and biasing of the metaphase plate during mitosis. The large sample size and rapid sample generation inherent to this approach allow the analysis of cells at an unprecedented rate: in a single experiment, potentially tens of thousands of cells can be stimulated for high statistical accuracy in measurements. This technique shows promise as a tool for both cell analysis and control.

The effect of osteocyte apoptosis on signalling in the osteocyte and bone lining cell network: a computer simulation

Osteocytes play a critical role in the regulation of bone remodelling by translating strain due to mechanical loading into biochemical signals transmitted through the interconnecting lacuno-canalicular network to bone lining cells (BLCs) on the bone surface. This work aims to examine the effects of disruption of that intercellular communication by simulation of osteocyte apoptosis in the bone matrix. A model of a uniformly distributed osteocyte network has been developed that simulates the signalling through the network to the BLCs based on strain level. Bi-directional and asymmetric communication between neighbouring osteocytes and BLCs is included. The effect of osteocyte apoptosis is examined by preventing signalling at and through the affected cells. The simulation shows that apoptosis of only 3% of the osteocyte cells leads to a significant reduction in the peak signal at the BLCs. Furthermore, experiments with the model confirm how important the location and density of the apoptotic osteocytes are to the signalling received at the bone surface. With 5% and 9% osteocyte apoptosis, the mean peak BLC levels were reduced by 25% and 37% respectively. Such a significant reduction in the signal at the BLCs may explain a possible mechanism that leads to the increased remodelling and eventual bone loss observed with osteoporosis. More generally, it provides a unique framework for a broader exploration of the role of osteocyte and bi-directional and asymmetric cell-cell communication in mechanotransduction, and the effects of disruption to that communication.

Evidence of interlinks between bioelectromagnetics and biomechanics: from biophysics to medical physics

A vast literature on electromagnetic and mechanical bioeffects at the bone and soft tissue level, as well as at the cellular level (osteoblasts, osteoclasts, keratinocytes, fibroblasts, chondrocytes, nerve cells, endothelial and muscle cells) has been reviewed and analysed in order to show the evident connections between both types of physical energies. Moreover, an intimate link between the two is suggested by transduction phenomena (electromagnetic-acoustic transduction and its reverse) occurring in living matter, as a sound biophysical literature has demonstrated. However, electromagnetic and mechanical signals are not always interchangeable, depending on their respective intensity. Calculations are reported in order to show in which cases (read: for which values of electric field in V/m and of mechanical pressure in Pa) a given electromagnetic or mechanical bioeffect is only due to the directly impinging energy or even to the indirect transductional energy. The relevance of the treated item for the applications of medical physics to regenerative medicine is stressed.

An integrated fiber-optic microfluidic device for detection of muscular force generation of microscopic nematodes

This paper reports development of an integrated fiber-optic microfluidic device for measuring muscular force of small nematode worms with high sensitivity, high data reliability, and simple device structure. A moving nematode worm squeezed through multiple detection points (DPs) created between a thinned single mode fiber (SMF) cantilever and a sine-wave channel with open troughs. The SMF cantilever was deflected by the normal force imposed by the worm, reducing optical coupling from the SMF to a receiving multimode fiber (MMF). Thus, multiple force data could be obtained for the worm-SMF contacts to verify with each other, improving data reliability. A noise equivalent displacement of the SMF cantilever was 0.28 μm and a noise equivalent force of the device was 143 nN. We demonstrated the workability of the device to detect muscular normal forces of the parasitic nematodes Oesophagotomum dentatum L3 larvae on the SMF cantilever. Also, we used this technique to measure force responses of levamisole-sensitive (SENS) and resistant (LERV) O. dentatum isolates in response to different doses of the anthelmintic drug, levamisole. The results showed that both of the isolates generated a larger muscular normal force when exposed to a higher concentration of levamisole. We also noticed muscular force phenotype differences between the SENS and LERV worms: the SENS muscles were more sensitive to levamisole than the LERV muscles. The ability to quantify the muscular forces of small nematode worms will provide a new approach for screening mutants at single animal resolution. Also, the ability to resolve small differences in muscular forces in different environmental conditions will facilitate phenotyping different isolates of nematodes. Thus, the present technology can potentially benefit and advance the current whole animal assays.

Force transduction and strain dynamics in actin stress fibres in response to nanonewton forces

It is becoming clear that mechanical stimuli are crucial factors in regulating the biology of the cell, but the short-term structural response of a cell to mechanical forces remains relatively poorly understood. We mechanically stimulated cells transiently expressing actin-EGFP with controlled forces (0-20 nN) in order to investigate the structural response of the cell. Two clear force-dependent responses were observed: a short-term (seconds) local deformation of actin stress fibres and a long-term (minutes) force-induced remodelling of stress fibres at cell edges, far from the point of contact. By photobleaching markers along stress fibres we were also able to quantify strain dynamics occurring along the fibres throughout the cell. The results reveal that the cell exhibits complex heterogeneous negative and positive strain fluctuations along stress fibres in resting cells that indicate localized contraction and stretch dynamics. The application of mechanical force results in the activation of myosin contractile activity reflected in an ~50% increase in strain fluctuations. This approach has allowed us to directly observe the activation of myosin in response to mechanical force and the effects of cytoskeletal crosslinking on local deformation and strain dynamics. The results demonstrate that force application does not result in simplistic isotropic deformation of the cytoarchitecture, but rather a complex and localized response that is highly dependent on an intact microtubule network. Direct visualization of force-propagation and stress fibre strain dynamics have revealed several crucial phenomena that take place and ultimately govern the downstream response of a cell to a mechanical stimulus.

Stability enhancement of an atomic force microscope for long-term force measurement including cantilever modification for whole cell deformation

Atomic force microscopy (AFM) is widely used in the study of both morphology and mechanical properties of living cells under physiologically relevant conditions. However, quantitative experiments on timescales of minutes to hours are generally limited by thermal drift in the instrument, particularly in the vertical (z) direction. In addition, we demonstrate the necessity to remove all air-liquid interfaces within the system for measurements in liquid environments, which may otherwise result in perturbations in the measured deflection. These effects severely limit the use of AFM as a practical tool for the study of long-term cell behavior, where precise knowledge of the tip-sample distance is a crucial requirement. Here we present a readily implementable, cost effective method of minimizing z-drift and liquid instabilities by utilizing active temperature control combined with a customized fluid cell system. Long-term whole cell mechanical measurements were performed using this stabilized AFM by attaching a large sphere to a cantilever in order to approximate a parallel plate system. An extensive examination of the effects of sphere attachment on AFM data is presented. Profiling of cantilever bending during substrate indentation revealed that the optical lever assumption of free ended cantilevering is inappropriate when sphere constraining occurs, which applies an additional torque to the cantilevers "free" end. Here we present the steps required to accurately determine force-indentation measurements for such a scenario. Combining these readily implementable modifications, we demonstrate the ability to investigate long-term whole cell mechanics by performing strain controlled cyclic deformation of single osteoblasts.

Effect of matrix on cardiomyocyte viscoelastic properties in 2D culture

Cardiomyocyte phenotype changes significantly in 2D culture systems depending on the substrate composition and organization. Given the variety of substrates that are used both for basic cardiac cell culture studies and for regenerative medicine applications, there is a critical need to understand how the different matrices influence cardiac cell mechanics. In the current study, the mechanical properties of neonatal rat cardiomyocytes cultured in a subconfluent layer upon aligned and unaligned collagen and fibronectin matrices were assessed over a two week period using atomic force microscopy. The elastic modulus was estimated by fitting the Hertz model to force curve data and the percent relaxation was determined from stress relaxation curves. The Quasilinear Viscoelastic (QLV) and Standard Linear Solid (SLS) models were fit to the stress relaxation data. Cardiomyocyte cellular mechanical properties were found to be highly dependent on matrix composition and organization as well as time in culture. It was observed that the cells stiffened and relaxed less over the first 3 to 5 days in culture before reaching a plateau in their mechanical properties. After day 5, cells on aligned matrices were stiffer than cells on unaligned matrices and cells on fibronectin matrices were stiffer than cells on collagen matrices. No such significant trends in percent relaxation measurements were observed but the QLV model fit the data very well. These results were correlated with observed changes in cellular structure associated with culture on the different substrates and analyzed for cell-to-cell variability.

Deconvolution of calcium fluorescent indicator signal from AFM cantilever reflection

Atomic force microscopy (AFM) can be combined with fluorescence microscopy to measure the changes in intracellular calcium levels (indicated by fluorescence of Ca²⁺ sensitive dye fluo-4) in response to mechanical stimulation performed by AFM. Mechanical stimulation using AFM is associated with cantilever movement, which may interfere with the fluorescence signal. The motion of the AFM cantilever with respect to the sample resulted in changes of the reflection of light back to the sample and a subsequent variation in the fluorescence intensity, which was not related to changes in intracellular Ca²⁺ levels. When global Ca²⁺ responses to a single stimulation were assessed, the interference of reflected light with the fluorescent signal was minimal. However, in experiments where local repetitive stimulations were performed, reflection artifacts, correlated with cantilever motion, represented a significant component of the fluorescent signal. We developed a protocol to correct the fluorescence traces for reflection artifacts, as well as photobleaching. An added benefit of our method is that the cantilever reflection in the fluorescence recordings can be used for precise temporal correlation of the AFM and fluorescence measurements.

In situ mechanical analysis of myofibrillar perturbation and aging on soft, bilayered Drosophila myocardium

Drosophila melanogaster is a genetically malleable organism with a short life span, making it a tractable system in which to study mechanical effects of genetic perturbation and aging on tissues, e.g., impaired heart function. However, Drosophila heart-tube studies can be hampered by its bilayered structure: a ventral muscle layer covers the contractile cardiomyocytes. Here we propose an atomic force microscopy-based analysis that uses a linearized-Hertz method to measure individual mechanical components of soft composite materials. The technique was verified using bilayered polydimethylsiloxane. We further demonstrated its biological utility via its ability to resolve stiffness changes due to RNA interference to reduce myofibrillar content or due to aging in Drosophila myocardial layers. This protocol provides a platform to assess the mechanics of soft biological composite systems and, to our knowledge, for the first time, permits direct measurement of how genetic perturbations, aging, and disease can impact cardiac function in situ.

Mechano-electrical vibrations of microtubules--link to subcellular morphology

Spontaneous mechanical oscillations were predicted and experimentally proven on almost every level of cellular structure. Besides morphogenetic potential of oscillatory mechanical force, oscillations may drive vibrations of electrically polar structures or these structures themselves may oscillate on their own natural frequencies. Vibrations of electric charge will generate oscillating electric field, role of which in morphogenesis is discussed in this paper. This idea is demonstrated in silico on the conformation of two growing microtubules.

Multi-layer phase analysis: quantifying the elastic properties of soft tissues and live cells with ultra-high-frequency scanning acoustic microscopy

Scanning acoustic microscopy is potentially a powerful tool for characterizing the elastic properties of soft biological tissues and cells. In this paper, we present a method, multi-layer phase analysis (MLPA), which can be used to extract local speed of sound values, for both thin tissue sections mounted on glass slides and cultured cells grown on cell culture plastic, with a resolution close to 1 μm. The method exploits the phase information that is preserved in the interference between the acoustic wave reflected from the substrate surface and internal reflections from the acoustic lens. In practice, a stack of acoustic images are captured beginning with the acoustic focal point 4 μm above the substrate surface and moving down in 0.1-μm increments. Scanning parameters, such as acoustic wave frequency and gate position, were adjusted to obtain optimal phase and lateral resolution. The data were processed offline to extract the phase information with the contribution of any inclination in the substrate removed before the calculation of sound speed. Here, we apply this approach to both skin sections and fibroblast cells, and compare our data with the V(f) (voltage versus frequency) method that has previously been used for characterization of soft tissues and cells. Compared with the V(f) method, the MPLA method not only reduces signal noise but can be implemented without making a priori assumptions with regards to tissue or cell parameters.

How do control-based approaches enter into biology?

Control is intrinsic to biological organisms, whose cells are in a constant state of sensing and response to numerous external and self-generated stimuli. Diverse means are used to study the complexity through control-based approaches in these cellular systems, including through chemical and genetic manipulations, input-output methodologies, feedback approaches, and feed-forward approaches. We first discuss what happens in control-based approaches when we are not actively examining or manipulating cells. We then present potential methods to determine what the cell is doing during these times and to reverse-engineer the cellular system. Finally, we discuss how we can control the cell's extracellular and intracellular environments, both to probe the response of the cells using defined experimental engineering-based technologies and to anticipate what might be achieved by applying control-based approaches to affect cellular processes. Much work remains to apply simplified control models and develop new technologies to aid researchers in studying and utilizing cellular and molecular processes.

A power-law rheology-based finite element model for single cell deformation

Physical forces can elicit complex time- and space-dependent deformations in living cells. These deformations at the subcellular level are difficult to measure but can be estimated using computational approaches such as finite element (FE) simulation. Existing FE models predominantly treat cells as spring-dashpot viscoelastic materials, while broad experimental data are now lending support to the power-law rheology (PLR) model. Here, we developed a large deformation FE model that incorporated PLR and experimentally verified this model by performing micropipette aspiration on fibroblasts under various mechanical loadings. With a single set of rheological properties, this model recapitulated the diverse micropipette aspiration data obtained using three protocols and with a range of micropipette sizes. More intriguingly, our analysis revealed that decreased pipette size leads to increased pressure gradient, potentially explaining our previous counterintuitive finding that decreased pipette size leads to increased incidence of cell blebbing and injury. Taken together, our work leads to more accurate rheological interpretation of micropipette aspiration experiments than previous models and suggests pressure gradient as a potential determinant of cell injury.

Effects of mechanical forces and stretch on intercellular gap junction coupling

Mechanical forces provide fundamental physiological stimulus in living organisms. Recent investigations demonstrated how various types of mechanical load, like strain, pressure, shear stress, or cyclic stretch can affect cell biology and gap junction intercellular communication (GJIC). Depending on the cell type, the type of mechanical load and on strength and duration of application, these forces can induce hypertrophic processes and modulate the expression and function of certain connexins such as Cx43, while others such as Cx37 or Cx40 are reported to be less mechanosensitive. In particular, not only expression but also subcellular localization of Cx43 is altered in cardiomyocytes submitted to cyclic mechanical stretch resulting in the typical elongated cell shape with an accentuation of Cx43 at the cell poles. In the heart both cardiomyocytes and fibroblasts can alter their GJIC in response to mechanical load. In the vasculature both endothelial cells and smooth muscle cells are subject to strain and cyclic stretch resulting from the pulsatile flow. In addition, vascular endothelial cells are mainly affected by shear stress resulting from the blood flow parallel to their surface. These mechanical forces lead to a regulation of GJIC in vascular tissue. In bones, osteocytes and osteoblasts are coupled via gap junctions, which also react to mechanical forces. Since gap junctions are involved in regulation of cell growth and differentiation, the mechanosensitivity of the regulation of these channels might open new perspectives to explain how cells can respond to mechanical load, and how stretch induces self-organization of a cell layer which might have implications for embryology and the development of organs. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Application of atomic force microscopy measurements on cardiovascular cells

The atomic force microscope (AFM) is a state-of-the-art tool that can analyze and characterize samples on a scale from angstroms to 100 μm by physical interaction between AFM cantilever tip and sample surface. AFM imaging has been used incrementally over last decade in living cells in cardiovascular research. Beyond its high resolution 3D imaging, AFM allows the quantitative assessments on the structure and function of the underlying cytoskeleton and cell organelles, binding probability, adhesion forces, and micromechanical properties of the cell by "sensing" the cell surface with mechanical sharp cantilever tip. AFM measurements have enhanced our understanding of cell mechanics in normal physiological and pathological states.

Measuring the elastic properties of living cells with atomic force microscopy indentation

Atomic force microscopy (AFM) is a powerful and versatile tool for probing the mechanical properties of biological samples. This chapter describes the procedures for using AFM indentation to measure the elastic moduli of living cells. We include step-by-step instructions for cantilever calibration and data acquisition using a combined AFM/optical microscope system, as well as a detailed protocol for data analysis. Our protocol is written specifically for the BioScope™ Catalyst™ AFM system (Bruker AXS Inc.); however, most of the general concepts can be readily translated to other commercial systems.

Atomic force microscopy as nanorobot

Atomic force microscopy (AFM) is a powerful and widely used imaging technique that can visualize single molecules under physiological condition at the nanometer scale. In this chapter, an AFM-based nanorobot for biological studies is introduced. Using the AFM tip as an end effector, the AFM can be modified into a nanorobot that can manipulate biological objects at the single-molecule level. By functionalizing the AFM tip with specific antibodies, the nanorobot is able to identify specific types of receptors on the cell membrane. It is similar to the fluorescent optical microscopy but with higher resolution. By locally updating the AFM image based on interaction force information and objects' model during nanomanipulation, real-time visual feedback is obtained through the augmented reality interface. The development of the AFM-based nanorobotic system enables us to conduct in situ imaging, sensing, and manipulation simultaneously at the nanometer scale (e.g., protein and DNA levels). The AFM-based nanorobotic system offers several advantages and capabilities for studying structure-function relationships of biological specimens. As a result, many biomedical applications can be achieved by the AFM-based nanorobotic system.

Cell types can be distinguished by measuring their viscoelastic recovery times using a micro-fluidic device

We introduce a simple micro-fluidic device containing an actuated flexible membrane, which allows the viscoelastic characterization of cells in small volumes of suspension by loading them in compression and observing the cell deformation in time. From this experiment, we can determine the characteristic time constant of recovery of the cell. To validate the device, two cell types known to have different cytoskeletal structures, 3T3 fibroblasts and HL60 cells, are tested. They show a substantially different response in the device and can be clearly distinguished on the basis of the measured characteristic recovery time constant. Also, the effect of breaking down the actin network, a main mechanical component of the cytoskeleton, by a treatment with Cytochalasin D, results in a substantial increase of the measured characteristic recovery time constant. Experimental variations in loading force, loading time, and surface treatment of the device also influence the measured characteristic recovery time constant significantly. The device can therefore be used to distinguish between cells with different mechanical structure in a quantitative way, and makes it possible to study changes in the mechanical response due to cell treatments, changes in the cell’s micro-environment, and mechanical loading conditions.

Bone cell elasticity and morphology changes during the cell cycle

The mechanical properties of cells are reported to be regulated by a range of factors including interactions with the extracellular environment and other cells, differentiation status, the onset of pathological states, as well as the intracellular factors, for example, the cytoskeleton. The cell cycle is considered to be a well-ordered sequence of biochemical events. A number of processes reported to occur during its progression are inherently mechanical and, as such, require mechanical regulation. In spite of this, few attempts have been made to investigate the putative regulatory role of the cell cycle in mechanobiology. In the present study, Atomic Force Microscopy (AFM) was employed to investigate the elastic modulus of synchronised osteoblasts. The data obtained confirm that osteoblast elasticity is regulated by cell cycle phase; specifically, cells in S phase were found to have a modulus approximately 1.7 times that of G1 phase cells. Confocal microscopy studies revealed that aspects of osteoblast morphology, namely F-actin expression, were also modulated by the cell cycle, and tended to increase with phase progression from G0 onwards. The data obtained in this study are likely to have implications for the fields of tissue- and bio-engineering, where prior knowledge of cell mechanobiology is essential for the effective replacement and repair of tissue. Furthermore, studies focused on biomechanics and the biophysical properties of cells are important in the understanding of the onset and progression of disease states, for example cancer at the cellular level. Our study demonstrates the importance of the combined use of traditional and relatively novel microscopy techniques in understanding mechanical regulation by crucial cellular processes, such as the cell cycle.

Rapid cytoskeletal response of epithelial cells to force generation by type IV pili

Many bacterial pathogens interfere with cellular functions including phagocytosis and barrier integrity. The human pathogen Neissieria gonorrhoeae generates grappling hooks for adhesion, spreading, and induction of signal cascades that lead to formation cortical plaques containing f-actin and ezrin. It is unclear whether high mechanical forces generated by type IV pili (T4P) are a direct signal that leads to cytoskeletal rearrangements and at which time scale the cytoskeletal response occurs. Here we used laser tweezers to mimic type IV pilus mediated force generation by T4P-coated beads on the order of 100 pN. We found that actin-EGFP and ezrin-EGFP accumulated below pilus-coated beads when force was applied. Within 2 min, accumulation significantly exceeded controls without force or without pili, demonstrating that T4P-generated force rapidly induces accumulation of plaque proteins. This finding adds mechanical force to the many strategies by which bacteria modulate the host cell cytoskeleton.

Finite element method (FEM), mechanobiology and biomimetic scaffolds in bone tissue engineering

Techniques of bone reconstructive surgery are largely based on conventional, non-cell-based therapies that rely on the use of durable materials from outside the patient's body. In contrast to conventional materials, bone tissue engineering is an interdisciplinary field that applies the principles of engineering and life sciences towards the development of biological substitutes that restore, maintain, or improve bone tissue function. Bone tissue engineering has led to great expectations for clinical surgery or various diseases that cannot be solved with traditional devices. For example, critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of bone tissue engineering is to apply engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. The total market for bone tissue regeneration and repair was valued at $1.1 billion in 2007 and is projected to increase to nearly $1.6 billion by 2014.Usually, temporary biomimetic scaffolds are utilized for accommodating cell growth and bone tissue genesis. The scaffold has to promote biological processes such as the production of extra-cellular matrix and vascularisation, furthermore the scaffold has to withstand the mechanical loads acting on it and to transfer them to the natural tissues located in the vicinity. The design of a scaffold for the guided regeneration of a bony tissue requires a multidisciplinary approach. Finite element method and mechanobiology can be used in an integrated approach to find the optimal parameters governing bone scaffold performance.In this paper, a review of the studies that through a combined use of finite element method and mechano-regulation algorithms described the possible patterns of tissue differentiation in biomimetic scaffolds for bone tissue engineering is given. Firstly, the generalities of the finite element method of structural analysis are outlined; second, the issues related to the generation of a finite element model of a given anatomical site or of a bone scaffold are discussed; thirdly, the principles on which mechanobiology is based, the principal theories as well as the main applications of mechano-regulation models in bone tissue engineering are described; finally, the limitations of the mechanobiological models and the future perspectives are indicated.

Atomic force microscopy of biological samples

The ability to evaluate structural-functional relationships in real time has allowed scanning probe microscopy (SPM) to assume a prominent role in post genomic biological research. In this mini-review, we highlight the development of imaging and ancillary techniques that have allowed SPM to permeate many key areas of contemporary research. We begin by examining the invention of the scanning tunneling microscope (STM) by Binnig and Rohrer in 1982 and discuss how it served to team biologists with physicists to integrate high-resolution microscopy into biological science. We point to the problems of imaging nonconductive biological samples with the STM and relate how this led to the evolution of the atomic force microscope (AFM) developed by Binnig, Quate, and Gerber, in 1986. Commercialization in the late 1980s established SPM as a powerful research tool in the biological research community. Contact mode AFM imaging was soon complemented by the development of non-contact imaging modes. These non-contact modes eventually became the primary focus for further new applications including the development of fast scanning methods. The extreme sensitivity of the AFM cantilever was recognized and has been developed into applications for measuring forces required for indenting biological surfaces and breaking bonds between biomolecules. Further functional augmentation to the cantilever tip allowed development of new and emerging techniques including scanning ion-conductance microscopy (SICM), scanning electrochemical microscope (SECM), Kelvin force microscopy (KFM) and scanning near field ultrasonic holography (SNFUH).

Rescuing loading induced bone formation at senescence

The increasing incidence of osteoporosis worldwide requires anabolic treatments that are safe, effective, and, critically, inexpensive given the prevailing overburdened health care systems. While vigorous skeletal loading is anabolic and holds promise, deficits in mechanotransduction accrued with age markedly diminish the efficacy of readily complied, exercise-based strategies to combat osteoporosis in the elderly. Our approach to explore and counteract these age-related deficits was guided by cellular signaling patterns across hierarchical scales and by the insight that cell responses initiated during transient, rare events hold potential to exert high-fidelity control over temporally and spatially distant tissue adaptation. Here, we present an agent-based model of real-time Ca(2+)/NFAT signaling amongst bone cells that fully described periosteal bone formation induced by a wide variety of loading stimuli in young and aged animals. The model predicted age-related pathway alterations underlying the diminished bone formation at senescence, and hence identified critical deficits that were promising targets for therapy. Based upon model predictions, we implemented an in vivo intervention and show for the first time that supplementing mechanical stimuli with low-dose Cyclosporin A can completely rescue loading induced bone formation in the senescent skeleton. These pre-clinical data provide the rationale to consider this approved pharmaceutical alongside mild physical exercise as an inexpensive, yet potent therapy to augment bone mass in the elderly. Our analyses suggested that real-time cellular signaling strongly influences downstream bone adaptation to mechanical stimuli, and quantification of these otherwise inaccessible, transient events in silico yielded a novel intervention with clinical potential.

Distinct mechanosensitive Ca2+ influx mechanisms in human primary synovial fibroblasts

Synovial cells are exposed to continually changing dynamic forces and are implicated in the maintenance of joint homeostasis. However, the mechanisms of synovial cell responses to mechanical stress are unclear. In this study, we investigated the difference between the mechanosensitive channels of human primary synovial fibroblasts (SFBs) and human primary dermal fibroblasts (DFBs) in response to mechanical stretch by uniaxial cyclic stretching and mechanical cell membrane deformation in vitro. Cyclic stretching induced orientation of SFBs and DFBs perpendicularly to the stretching direction. Furthermore, uniaxial stretching increased intracellular Ca(2+) levels in both cell types. The perpendicular orientation of DFBs was blocked by gadolinium (III) chloride (Gd(3+), a mechanosensitive Ca(2+) channel blocker) or ruthenium red (RR, a nonselective Ca(2+) channel blocker). However, Gd(3+) did not block the stretch-induced perpendicular orientation in SFBs, while RR inhibited this orientation. Similarly, Ca(2+) influx in DFBs induced by uniaxial stretching and membrane deformation was inhibited by Gd(3+), RR, and GsMTx-4 (another mechanosensitive Ca(2+) channel blocker), while only RR inhibited Ca(2+) influx in SFBs. Our results suggest that SFBs respond to mechanical stretch through mechanosensitive channels that are distinct from those of DFBs.

Nanoneedle: a multifunctional tool for biological studies in living cells

Studying biology in living cells is methodologically challenging but highly beneficial. Recent advances in nanobiotechnology offer exciting new opportunities to address this challenge. The nanoneedle technology, as an emerging technology that uses a cell membrane-penetrating nanoneedle to probe and manipulate biological processes in living cells, is expected to play an important role in this endeavor. Here we review the recent development and future direction of the nanoneedle technology for biological studies in living cells. The nanoneedle technology is shown to be powerful and versatile, and can offer numerous new ways to explore biological processes and biophysical properties of living cells with high spatial and temporal precision potentially reaching molecular resolution.

A biophysical view of the interplay between mechanical forces and signaling pathways during transendothelial cell migration

The vascular endothelium is exposed to an array of physical forces, including shear stress via blood flow, contact with other cells such as neighboring endothelial cells and leukocytes, and contact with the basement membrane. Endothelial cell morphology, protein expression, stiffness and cytoskeletal arrangement are all influenced by these mechanochemical forces. There are many biophysical tools that are useful in studying how forces are transmitted in endothelial cells, and these tools are also beginning to be used to investigate biophysical aspects of leukocyte transmigration, which is a ubiquitous mechanosensitive process. In particular, the stiffness of the substrate has been shown to have a significant impact on cellular behavior, and this is true for both endothelial cells and leukocytes. Thus, the stiffness of the basement membrane as an endothelial substrate, as well as the stiffness of the endothelium as a leukocyte substrate, is relevant to the process of transmigration. In this review, we discuss recent work that has related the biophysical aspects of endothelial cell interactions and leukocyte transmigration to the biochemical pathways and molecular interactions that take place during this process. Further use of biophysical tools to investigate the biological process of leukocyte transmigration will have implications for tissue engineering, as well as atherosclerosis, stroke and immune system disease research.