Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

Mechano-metabolism of adherent cells in 2D and 3D microenvironments

Cells regulate their shape and metabolic activity in response to the mechano-chemical properties of their microenvironment. To elucidate the impact of matrix stiffness and ligand density on a cell's bioenergetics, we developed a non-equilibrium, active chemo-mechanical model that accounts for mechanical energy of the cell and matrix, chemical energy from ATP hydrolysis, interfacial energy, and mechano-sensitive regulation of stress fiber assembly through signaling. By integrating the kinetics and energetics of these processes we introduce the concept of the metabolic potential of the cell that, when minimized, gives experimentally testable predictions of the cell contractility, shape, and the ATP consumption. Specifically, we show that MDA-MB-231 breast cancer cells in 3D collagen gels follow a spherical to spindle to spherical change in morphology with increasing matrix stiffness consistent with experimental observations. This biphasic transition in cell shape emerges from a competition between increased contractility accompanied by ATP hydrolysis enabled by mechano-sensitive signaling, which lowers the volumetric contribution to the metabolic potential of elongated cells and the interfacial energy which is lower for spherical shapes. On 2D hydrogels, our model predicts a hemispherical to spindle to disc shape transition with increasing gel stiffness. In both cases, we show that increasing matrix stiffness monotonically increases the cell's contractility as well as ATP consumption. Our model also predicts how the increased energy demand in stiffer microenvironments is met by AMPK activation, which is confirmed through experimental measurement of activated AMPK levels as a function of matrix stiffness carried out here in both 2D and 3D micro-environments. Further, model predictions of increased AMPK activation on stiffer micro-environments are found to correlate strongly with experimentally measured upregulation of mitochondrial potential, glucose uptake and ATP levels. The insights from our model can be used to understand mechanosensitive regulation of metabolism in physiological events such as metastasis and tumor progression during which cells experience dynamic changes in their microenvironment and metabolic state.

Strain-dependent elastography of cancer cells reveals heterogeneity and stiffening due to attachment

Because cells vary in thickness and in biomechanical properties, the use of a constant force trigger during atomic force microscopy (AFM) stiffness mapping produces a varied nominal strain that can obfuscate the comparison of local material properties. In this study, we measured the biomechanical spatial heterogeneity of ovarian and breast cancer cells by using an indentation-dependent pointwise Hertzian method. Force curves and surface topography were used together to determine cell stiffness as a function of nominal strain. By recording stiffness at a particular strain, it may be possible to improve comparison of the material properties of cells and produce higher contrast representations of cell mechanical properties. Defining a linear region of elasticity that corresponds to a modest nominal strain, we were able to clearly distinguish the mechanics of the perinuclear region of cells. We observed that, relative to the lamelopodial stiffness, the perinuclear region was softer for metastatic cancer cells than their nonmetastatic counterparts. Moreover, contrast in the strain-dependent elastography in comparison to conventional force mapping with Hertzian model analysis revealed a significant stiffening phenomenon in the thin lamellipodial region in which the modulus scales inversely and exponentially with cell thickness. The observed exponential stiffening is not affected by relaxation of cytoskeletal tension, but finite element modeling indicates it is affected by substrate adhesion. The novel cell mapping technique explores cancer cell mechanical nonlinearity that results from regional heterogeneity, which could help explain how metastatic cancer cells can show soft phenotypes while simultaneously increasing force generation and invasiveness.

How do cells stiffen?

Cell stiffness is an important characteristic of cells and their response to external stimuli. In this review, we survey methods used to measure cell stiffness, summarize stimuli that alter cell stiffness, and discuss signaling pathways and mechanisms that control cell stiffness. Several pathological states are characterized by changes in cell stiffness, suggesting this property can serve as a potential diagnostic marker or therapeutic target. Therefore, we consider the effect of cell stiffness on signaling and growth processes required for homeostasis and dysfunction in healthy and pathological states. Specifically, the composition and structure of the cell membrane and cytoskeleton are major determinants of cell stiffness, and studies have identified signaling pathways that affect cytoskeletal dynamics both directly and by altered gene expression. We present the results of studies interrogating the effects of biophysical and biochemical stimuli on the cytoskeleton and other cellular components and how these factors determine the stiffness of both individual cells and multicellular structures. Overall, these studies represent an intersection of the fields of polymer physics, protein biochemistry, and mechanics, and identify specific mechanisms involved in mediating cell stiffness that can serve as therapeutic targets.

Physical confinement alters cytoskeletal contributions towards human mesenchymal stem cell migration

The in vivo microenvironment is critical for providing physico-chemical signaling cues which ultimately regulate human mesenchymal stem cell (hMSC) behavior in clinically-relevant applications. hMSCs experience mechanical confinement of the cell body and nucleus in three dimensional (3D) tissues during homing and in porous tissue engineered scaffolds, yet the effects of this mechanical cue on hMSC migration are not known. Here, we use a microchannel device to systematically examine the effect of confinement on hMSC migration and cytoskeletal organization. Notably, we show that hMSC actin and microtubules change from filamentous in unconfined spaces to a more diffuse network in confinement, and that confinement abrogates the presence of paxillin-rich focal adhesions seen in 2D. Furthermore, several morphological parameters of the hMSC body are altered in confinement. Interestingly, hMSC speed displays a biphasic trend as a function of confinement, and increasing hMSC passage number decreases speed in all but the narrowest microchannels. Confinement also alters the relative contributions of cytoskeletal (i.e., actin and microtubule) and contractile (i.e., myosin II and Rho kinase) machinery in hMSC migration in unconfined and confined spaces. These results provide an improved understanding of how hMSCs navigate mechanical confinement, which is a central component of complicated 3D microenvironments. Hence, this work may provide insight towards more effective control of hMSC localization in porous tissue engineered scaffolds and mobilization to distinct tissue sites during homing after clinical therapy.

Heterogeneous filament network formation by myosin light chain isoforms effects on contractile energy output of single cardiomyocytes derived from human induced pluripotent stem cells

Cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs) are expected to play an important role in heart therapies, in which hiPSC-CMs should generate sufficient contractile force to pump blood. However, recent studies have shown that the contractility of myocardial mimics composed of hiPSC-CMs is lower than that of adult human myocardium. To examine the mechanism by which contractile force output of hiPSC-CMs is weakened, we measured the contractile force of single hiPSC-CMs and observed the fibrous distribution of myosin II regulatory light chain (MRLC) of cardiac (contributes to beating) and non-cardiac (does not contribute to beating) isoforms. Single hiPSC-CMs were cultured on an extracellular matrix gel, and the contractile force and strain energy exerted on the gel were measured. Strain energy was not uniform between cells and ranged from 0.2 to 5.8 pJ. The combination of contractile force measurement and immunofluorescent microscopy for MRLC isoforms showed that cells with higher strain energy expressed the weakened non-cardiac myosin II fibers compared to those of cells with lower strain energy. Observation of cardiac and non-cardiac MRLC showed that the MRLC isoforms formed heterogeneous filament networks. These results suggest that strain energy output from single hiPSC-CMs depends both cardiac and non-cardiac myosin fibers, which prevent deformation of the cell body.

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The contractile force output of single hiPSC-CMs is uniform between cells.

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Cells that output high strain energy generally form weak non-cardiac myosin II fibers.

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Cardiac myosin II and non-cardiac myosin II construct heterogeneous fiber networks.

The Outflow Pathway: A Tissue With Morphological and Functional Unity

The trabecular meshwork (TM) plays an important role in high-tension glaucomas. Indeed, the TM is a true organ, through which the aqueous humor flows from the anterior chamber to Schlemm's canal (SC). Until recently, the TM, which is constituted by endothelial-like cells, was described as a kind of passive filter. In reality, it is much more. The cells delineating the structures of the collagen framework of the TM are endowed with a cytoskeleton, and are thus able to change their shape. These cells also have the ability to secrete the extracellular matrix, which expresses proteins and cytokines, and are capable of phagocytosis and autophagy. The cytoskeleton is attached to the nuclear membrane and can, in millionths of a second, send signals to the nucleus in order to alter the expression of genes in an attempt to adapt to biomechanical insult. Oxidative stress, as happens in aging, has a deleterious effect on the TM, leading eventually to cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility, and (ultimately) increased IOP. TM failure is the most relevant factor in the cascade of events triggering apoptosis in the inner retinal layers, including ganglion cells. J. Cell. Physiol. 231: 1876-1893, 2016. © 2016 Wiley Periodicals, Inc.

Tensional homeostasis in single fibroblasts

Adherent cells generate forces through acto-myosin contraction to move, change shape, and sense the mechanical properties of their environment. They are thought to maintain defined levels of tension with their surroundings despite mechanical perturbations that could change tension, a concept known as tensional homeostasis. Misregulation of tensional homeostasis has been proposed to drive disorganization of tissues and promote progression of diseases such as cancer. However, whether tensional homeostasis operates at the single cell level is unclear. Here, we directly test the ability of single fibroblast cells to regulate tension when subjected to mechanical displacements in the absence of changes to spread area or substrate elasticity. We use a feedback-controlled atomic force microscope to measure and modulate forces and displacements of individual contracting cells as they spread on a fibronectin-patterned atomic-force microscope cantilever and coverslip. We find that the cells reach a steady-state contraction force and height that is insensitive to stiffness changes as they fill the micropatterned areas. Rather than maintaining a constant tension, the fibroblasts altered their contraction force in response to mechanical displacement in a strain-rate-dependent manner, leading to a new and stable steady-state force and height. This response is influenced by overexpression of the actin crosslinker α-actinin, and rheology measurements reveal that changes in cell elasticity are also strain- rate-dependent. Our finding of tensional buffering, rather than homeostasis, allows cells to transition between different tensional states depending on how they are displaced, permitting distinct responses to slow deformations during tissue growth and rapid deformations associated with injury.

Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

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.

PTEN inhibition improves wound healing in lung epithelia through changes in cellular mechanics that enhance migration

The phosphoinositide-3 kinase/Akt pathway is a vital survival axis in lung epithelia. We previously reported that inhibition of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a major suppressor of this pathway, results in enhanced wound repair following injury. However, the precise cellular and biomechanical mechanisms responsible for increased wound repair during PTEN inhibition are not yet well established. Using primary human lung epithelia and a related lung epithelial cell line, we first determined whether changes in migration or proliferation account for wound closure. Strikingly, we observed that cell migration accounts for the majority of wound recovery following PTEN inhibition in conjunction with activation of the Akt and ERK signaling pathways. We then used fluorescence and atomic force microscopy to investigate how PTEN inhibition alters the cytoskeletal and mechanical properties of the epithelial cell. PTEN inhibition did not significantly alter cytoskeletal structure but did result in large spatial variations in cell stiffness and in particular a decrease in cell stiffness near the wound edge. Biomechanical changes, as well as migration rates, were mediated by both the Akt and ERK pathways. Our results indicate that PTEN inhibition rapidly alters biochemical signaling events that in turn provoke alterations in biomechanical properties that enhance cell migration. Specifically, the reduced stiffness of PTEN-inhibited cells promotes larger deformations, resulting in a more migratory phenotype. We therefore conclude that increased wound closure consequent to PTEN inhibition occurs through enhancement of cell migration that is due to specific changes in the biomechanical properties of the cell.

Nano-mechanical properties of living cells expressing constitutively active RhoA effectors

Filamentous actin and myosin-II are major determinants of cell mechanics and are tightly regulated by a small guanosine triphosphatase, RhoA, and its downstream effectors. We examined the effects of constitutively active mutants of RhoA effectors, which have not been reported before, on cortical stiffness of living cells by using scanning probe microscopy, fluorescence microscopy, and truncated mutants of RhoA effectors labeled with a fluorescent protein. Our data indicated that expression of a constitutively active mutant of Dia1, a formin-family actin polymerizer, enhanced cortical stiffness and increased actin filament quantity in cells. Furthermore, expression of a constitutively active mutant of Rho-associated coiled-coil kinase, a myosin-II activator, softened the cell cortex but increased myosin-II activity. Our findings provide new insights into anomalous mechanics of cells, which is a topic of current interest in a variety of biological research fields.

On the modelling of biological patterns with mechanochemical models: Insights from analysis and computation

The diversity of biological form is generated by a relatively small number of underlying mechanisms. Consequently, mathematical and computational modelling can, and does, provide insight into how cellular level interactions ultimately give rise to higher level structure. Given cells respond to mechanical stimuli, it is therefore important to consider the effects of these responses within biological self-organisation models. Here, we consider the self-organisation properties of a mechanochemical model previously developed by three of the authors in Acta Biomater. 4, 613-621 (2008), which is capable of reproducing the behaviour of a population of cells cultured on an elastic substrate in response to a variety of stimuli. In particular, we examine the conditions under which stable spatial patterns can emerge with this model, focusing on the influence of mechanical stimuli and the interplay of non-local phenomena. To this end, we have performed a linear stability analysis and numerical simulations based on a mixed finite element formulation, which have allowed us to study the dynamical behaviour of the system in terms of the qualitative shape of the dispersion relation. We show that the consideration of mechanotaxis, namely changes in migration speeds and directions in response to mechanical stimuli alters the conditions for pattern formation in a singular manner. Furthermore without non-local effects, responses to mechanical stimuli are observed to result in dispersion relations with positive growth rates at arbitrarily large wavenumbers, in turn yielding heterogeneity at the cellular level in model predictions. This highlights the sensitivity and necessity of non-local effects in mechanically influenced biological pattern formation models and the ultimate failure of the continuum approximation in their absence.

Mapping the cytoskeletal prestress

Cell mechanical properties on a whole cell basis have been widely studied, whereas local intracellular variations have been less well characterized and are poorly understood. To fill this gap, here we provide detailed intracellular maps of regional cytoskeleton (CSK) stiffness, loss tangent, and rate of structural rearrangements, as well as their relationships to the underlying regional F-actin density and the local cytoskeletal prestress. In the human airway smooth muscle cell, we used micropatterning to minimize geometric variation. We measured the local cell stiffness and loss tangent with optical magnetic twisting cytometry and the local rate of CSK remodeling with spontaneous displacements of a CSK-bound bead. We also measured traction distributions with traction microscopy and cell geometry with atomic force microscopy. On the basis of these experimental observations, we used finite element methods to map for the first time the regional distribution of intracellular prestress. Compared with the cell center or edges, cell corners were systematically stiffer and more fluidlike and supported higher traction forces, and at the same time had slower remodeling dynamics. Local remodeling dynamics had a close inverse relationship with local cell stiffness. The principal finding, however, is that systematic regional variations of CSK stiffness correlated only poorly with regional F-actin density but strongly and linearly with the regional prestress. Taken together, these findings in the intact cell comprise the most comprehensive characterization to date of regional variations of cytoskeletal mechanical properties and their determinants.

Probing the mechanical architecture of the vertebrate meiotic spindle

Accurate chromosome segregation during meiosis depends on the assembly of a microtubule-based spindle of proper shape and size. Current models for spindle-size control focus on reaction diffusion-based chemical regulation and balance in activities of motor proteins. Although several molecular perturbations have been used to test these models, controlled mechanical perturbations have not been possible. Here we report a piezoresistive dual cantilever-based system to test models for spindle-size control and examine the mechanical features, such as deformability and stiffness, of the vertebrate meiotic spindle. We found that meiotic spindles prepared in Xenopus laevis egg extracts were viscoelastic and recovered their original shape in response to small compression. Larger compression resulted in plastic deformation, but the spindle adapted to this change, establishing a stable mechanical architecture at different sizes. The technique we describe here may also be useful for examining the micromechanics of other cellular organelles.

Wide range scanning probe microscopy for probing mechanical effects on cellular function

Scanning probe microscopy (SPM) provides a range of strategies for studying biological phenomena due to its ability to image surfaces under liquids. However, some cellular events, such as cell migration, exceed the maximum measurable range of SPM. Recently, we have developed a wide range scanning probe microscope (WR-SPM) to investigate cellular events which exceed the range of the conventional SPM. In this review, we introduce the instrumentation of the WR-SPM, which can measure a sample for 400 µm in the xy directions and 23 µm in the z direction. We then show the application of the WR-SPM to studies of the stiffness response of epithelial cells to an external loading force and demonstrat that the stiffness of the epithelial cells increases under stretched conditions. We also showed the results on the mesh structure on the surface of a melanoma cell as well as the regulatory mechanism of the cellular contractile force by the combined use of topographical and mechanical modes of the WR-SPM. These findings indicate that the WR-SPM is very useful for studying the functions of a cell in relation to the surface structure and mechanical properties of that cell.

Mechanical properties of plasma membrane and nuclear envelope measured by scanning probe microscope

Atomic force microscopy has been used to visualize nano-scale structures of various cellular components and to characterize mechanical properties of biomolecules. In spite of its ability to measure non-fixed samples in liquid, the application of AFM for living cell manipulation has been hampered by the lack of knowledge of the mechanical properties of living cells. In this study, we successfully combine AFM imaging and force measurement to characterize the mechanical properties of the plasma membrane and the nuclear envelope of living HeLa cells in a culture medium. We examine cantilevers with different physical properties (spring constant, tip angle and length) to find out the one suitable for living cell imaging and manipulation. Our results of elasticity measurement revealed that both the plasma membrane and the nuclear envelope are soft enough to absorb a large deformation by the AFM probe. The penetrations of the plasma membrane and the nuclear envelope were possible when the probe indents the cell membranes far down close to a hard glass surface. These results provide useful information to the development of single-cell manipulation techniques.

Nuclear architecture and chromatin dynamics revealed by atomic force microscopy in combination with biochemistry and cell biology

The recent technical development of atomic force microscopy (AFM) has made nano-biology of the nucleus an attractive and promising field. In this paper, we will review our current understanding of nuclear architecture and dynamics from the structural point of view. Especially, special emphases will be given to: (1) How to approach the nuclear architectures by means of new techniques using AFM, (2) the importance of the physical property of DNA in the construction of the higher-order structures, (3) the significance and implication of the linker and core histones and the nuclear matrix/scaffold proteins for the chromatin dynamics, (4) the nuclear proteins that contribute to the formation of the inner nuclear architecture. Spatio-temporal analyses using AFM, in combination with biochemical and cell biological approaches, will play important roles in the nano-biology of the nucleus, as most of nuclear structures and events occur in nanometer, piconewton and millisecond order. The new applications of AFM, such as recognition imaging, fast-scanning imaging, and a variety of modified cantilevers, are expected to be powerful techniques to reveal the nanostructure of the nucleus.

Development of a device to stretch tissue-like materials and to measure their mechanical properties by scanning probe microscopy

We have developed a new stretch device to investigate the biomechanical responses to an external loading force on a tissue-like material consisting of cells and a collagen gel. Collagen gel, a typical matrix found abundantly in the connective tissue, was attached to an elastic chamber that was precoated with a thin layer of collagen. Madin-Darby canine kidney cells that were cultured on the collagen gel were stretched in a uniaxial direction via deformation of the elastic chamber. Changes in the morphology and stiffness of the tissue-like structure were measured before and after the stretch using wide-range scanning probe microscopy (WR-SPM). The change in cellular morphology was heterogeneous, and there was a twofold increase in the intercellular junction due to the stretch. In addition to the WR-SPM measurements, this device enables observation of the spatial distribution of cytoskeletal proteins such as vimentin and alpha-catenin using immunofluorescent microscopy. We concluded that the stretch device we have reported in this paper is useful for measuring the mechanical response of a tissue-like material over a range of cell sizes when exposed to an external loading force.

Diphosphorylation of the myosin regulatory light chain enhances the tension acting on stress fibers in fibroblasts

Regulation of the contractile force is crucial for cell migration, cell proliferation, and maintenance of cell morphology. Phosphorylation of the myosin II regulatory light chain (MRLC) is involved in these processes. To show whether the diphosphorylation of MRLC increases the tension acting on stress fibers, changes in the stiffness of fibroblasts expressing wild-type MRLC and a mutant type, which cannot be diphosphorylated, on treatment with lysophosphatidic acid (LPA) were examined by a mechanical-scanning probe microscope (M-SPM). The LPA treatment increased cellular stiffness in the wild-type MRLC expressing cells, while it had no effect on the mutated cells. Immunostaining showed that LPA stimulation induced the diphosphorylation of MRLC. These results suggest that the diphosphorylation of MRLC enhances the tension acting on stress fibers.

A formylated hexapeptide ligand mimics the ability of Wnt-5a to impair migration of human breast epithelial cells

Loss of Wnt-5a protein expression is associated with shorter recurrence-free survival in breast carcinoma patients and increased motility in mammary cell lines. Based on sequence analysis of Wnt-5a, we identified 14 peptide fragments and investigated their ability to mimic the effects of Wnt-5a on mammary cell adhesion and migration. Two of these peptides significantly increased adhesion and impaired migration in the non-tumorigenic HB2 breast epithelial cell line and in the MDA-MB-468 breast cancer cell line, both of which show little endogenous expression of the Wnt-5a protein. We removed two amino acids at a time from the N terminus of the shorter of these two peptides to identify the shortest peptide that still inhibited migration. The influence on tumor cell adhesion was gradually lost and was no longer detectable when only six amino acids remained. However, formylation of the N-terminal methionine of this hexapeptide restored its effect on adhesion and reduced tumor cell motility via a Frizzled-5 receptor-dependent mechanism, even at a low pH such as encountered in breast tumor tissue. This formylated hexapeptide ligand induced a rapid cytosolic calcium signal, whereas it did not affect the cellular levels of unphosphorylated beta-catenin or active JNK. The novel formyl-Met-Asp-Gly-Cys-Glu-Leu peptide ligand is not only a valuable experimental tool but has also a potential role in antimetastatic treatment of the 50% of human breast cancer patients that have reduced endogenous Wnt-5a protein expression.