Force meets chemistry: Analysis of mechanochemical conversion in focal adhesions using fluorescence recovery after photobleaching

Effects of Titanium Implant Surface Topology on Bone Cell Attachment and Proliferation in vitro

Purpose

Titanium is commonly used for implants because of its corrosion resistance and osseointegration capability. It is well known that surface topology affects the response of bone tissue towards implants. In vivo studies have shown that in weeks or months, bone tissue bonds more efficiently to titanium implants with rough surfaces compared to smooth surfaces. In addition, stimulating early endosseous integration increases the long-term stability of bone-implants and hence their clinical outcome. Here, we evaluated the response of human MG-63 osteoblast-like cells to flat and solid, compared to rough and porous surface topologies in vitro 1–6 days post seeding. We compared the morphology, proliferation, and attachment of cells onto three smooth surfaces: tissue culture (TC) plastic or microscope cover glasses, machined polyether-ether-ketone (PEEK), and machined solid titanium, to cells on a highly porous (average R_a 22.94 μm) plasma-sprayed titanium surface (composite Ti-PEEK spine implants).

Methods

We used immuno-fluorescence (IF) and scanning electron microscopy (SEM), as well as Live/Dead and WST-1 cell proliferation assays.

Results

SEM analyses confirmed the rough topology of the titanium implant surface, compared to the smooth surface of PEEK, solid titanium, TC plastic and cover glasses. In addition, SEM analyses revealed that MG-63 cells seeded onto smooth surfaces (solid titanium, PEEK) adopted a flat, planar morphology, while cells on the rough titanium surface adopted an elongated morphology with numerous filopodial and lamellipodial extensions interacting with the substrate. Finally, IF analyses of focal adhesions (vinculin, focal adhesion kinase), as well as proliferation assays indicate that MG-63 cells adhere less and proliferate at a slower rate on the rough than on a smooth titanium surface.

Conclusion

These observations suggest that bone-forming osteoblasts adhere less strongly and proliferate slower on rough compared to smooth titanium surfaces, likely promoting cell differentiation, which is in agreement with other porous implant materials.

Correlation of cellular traction forces and dissociation kinetics of adhesive protein zyxin revealed by multi-parametric live cell microscopy

Cells exert traction forces on the extracellular matrix to which they are adhered through the formation of focal adhesions. Spatial-temporal regulation of traction forces is crucial in cell adhesion, migration, cellular division, and remodeling of the extracellular matrix. By cultivating cells on polyacrylamide hydrogels of different stiffness we were able to investigate the effects of substrate stiffness on the generation of cellular traction forces by Traction Force Microscopy (TFM), and characterize the molecular dynamics of the focal adhesion protein zyxin by Fluorescence Correlation Spectroscopy (FCS) and Fluorescence Recovery After Photobleaching (FRAP). As the rigidity of the substrate increases, we observed an increment of both, cellular traction generation and zyxin residence time at the focal adhesions, while its diffusion would not be altered. Moreover, we found a positive correlation between the traction forces exerted by cells and the residence time of zyxin at the substrate elasticities studied. We found that this correlation persists at the subcellular level, even if there is no variation in substrate stiffness, revealing that focal adhesions that exert greater traction present longer residence time for zyxin, i.e., zyxin protein has less probability to dissociate from the focal adhesion.

Live cell imaging reveals focal adhesions mechanoresponses in mammary epithelial cells under sustained equibiaxial stress

Mechanical stimuli play a key role in many cell functions such as proliferation, differentiation and migration. In the mammary gland, mechanical signals such as the distension of mammary epithelial cells due to udder filling are proposed to be directly involved during lactation and involution. However, the evolution of focal adhesions -specialized multiprotein complexes that mechanically connect cells with the extracellular matrix- during the mammary gland development, as well as the influence of the mechanical stimuli involved, remains unclear. Here we present the use of an equibiaxial stretching device for exerting a sustained normal strain to mammary epithelial cells while quantitatively assessing cell responses by fluorescence imaging techniques. Using this approach, we explored changes in focal adhesion dynamics in HC11 mammary cells in response to a mechanical sustained stress, which resembles the physiological stimuli. We studied the relationship between a global stress and focal adhesion assembly/disassembly, observing an enhanced persistency of focal adhesions under strain as well as an increase in their size. At a molecular level, we evaluated the mechanoresponses of vinculin and zyxin, two focal adhesion proteins postulated as mechanosensors, observing an increment in vinculin molecular tension and a slower zyxin dynamics while increasing the applied normal strain.

Tunable cell-surface mimetics as engineered cell substrates

Most recent breakthroughs in understanding cell adhesion, cell migration, and cellular mechanosensitivity have been made possible by the development of engineered cell substrates of well-defined surface properties. Traditionally, these substrates mimic the extracellular matrix (ECM) environment by the use of ligand-functionalized polymeric gels of adjustable stiffness. However, such ECM mimetics are limited in their ability to replicate the rich dynamics found at cell-cell contacts. This review focuses on the application of cell surface mimetics, which are better suited for the analysis of cell adhesion, cell migration, and cellular mechanosensitivity across cell-cell interfaces. Functionalized supported lipid bilayer systems were first introduced as biomembrane-mimicking substrates to study processes of adhesion maturation during adhesion of functionalized vesicles (cell-free assay) and plated cells. However, while able to capture adhesion processes, the fluid lipid bilayer of such a relatively simple planar model membrane prevents adhering cells from transducing contractile forces to the underlying solid, making studies of cell migration and cellular mechanosensitivity largely impractical. Therefore, the main focus of this review is on polymer-tethered lipid bilayer architectures as biomembrane-mimicking cell substrate. Unlike supported lipid bilayers, these polymer-lipid composite materials enable the free assembly of linkers into linker clusters at cellular contacts without hindering cell spreading and migration and allow the controlled regulation of mechanical properties, enabling studies of cellular mechanosensitivity. The various polymer-tethered lipid bilayer architectures and their complementary properties as cell substrates are discussed.

Probing Activated and Non-Activated Single Calmodulin Molecules under a Piconewton Compressive Force

Interrogating the protein structure-function inter-relationship under a piconewton force manipulation has been highly promising and informative. Although protein conformational changes under pulling force manipulations have been extensively studied, protein conformational changes under a compressive force have not been explored in detail. Using our home-modified sensitive and high signal-to-noise atomic force microscopy (AFM) approach, we have applied a piconewton compressive force, manipulating a Calmodulin (CaM) molecule to characterize two different forms of CaM, the Ca²⁺-ligated activated form and the Ca²⁺ free non-activated form (apo-CaM). We observed sudden and spontaneous structural rupture of apo-CaM under compressive force applied by an AFM tip, though no such events were recorded in the case of Ca²⁺-ligated activated CaM form. The sudden spontaneous structural rupture under a piconewton force compression has never been reported before, which presents an unexplored function that is likely important for protein-protein interactions and cell signaling functions.

Revealing Abrupt and Spontaneous Ruptures of Protein Native Structure under picoNewton Compressive Force Manipulation

Manipulating protein conformations for exploring protein structure-function relationship has shown great promise. Although protein conformational changes under pulling force manipulation have been extensively studied, protein conformation changes under a compressive force have not been explored quantitatively. The latter is even more biologically significant and relevant in revealing protein functions in living cells associated with protein crowdedness, distribution fluctuations, and cell osmotic stress. Here we report our experimental observations on abrupt ruptures of protein native structures under compressive force, demonstrated and studied by single-molecule AFM-FRET spectroscopic nanoscopy. Our results show that the protein ruptures are abrupt and spontaneous events occurred when the compressive force reaches a threshold of 12-75 pN, a force amplitude accessible from thermal fluctuations in a living cell. The abrupt ruptures are sensitive to local environment, likely a general and important pathway of protein unfolding in living cells.

Integrin and cadherin clusters: A robust way to organize adhesions for cell mechanics

Recent studies at the nanometer scale have revealed that relatively uniform clusters of adhesion proteins (50-100 nm) constitute the modular units of cell adhesion sites in both cell-matrix and cell-cell adhesions. Super resolution microscopy and membrane protein diffusion studies both suggest that even large focal adhesions are formed of 100 nm clusters that are loosely aggregated. Clusters of 20-50 adhesion molecules (integrins or cadherins) can support large forces through avidity binding interactions but can also be disassembled or endocytosed rapidly. Assembly of the clusters of integrins is force-independent and involves gathering integrins at ligand binding sites where they are stabilized by cytoplasmic adhesion proteins that crosslink the integrin cytoplasmic tails plus connect the clusters to the cell cytoskeleton. Cooperative-signaling events can occur in a single cluster without cascading to other clusters. Thus, the clusters appear to be very important elements in many cellular processes and can be considered as a critical functional module.

Local 3D matrix microenvironment regulates cell migration through spatiotemporal dynamics of contractility-dependent adhesions

The physical properties of two-dimensional (2D) extracellular matrices (ECMs) modulate cell adhesion dynamics and motility, but little is known about the roles of local microenvironmental differences in three-dimensional (3D) ECMs. Here we generate 3D collagen gels of varying matrix microarchitectures to characterize their regulation of 3D adhesion dynamics and cell migration. ECMs containing bundled fibrils demonstrate enhanced local adhesion-scale stiffness and increased adhesion stability through balanced ECM/adhesion coupling, whereas highly pliable reticular matrices promote adhesion retraction. 3D adhesion dynamics are locally regulated by ECM rigidity together with integrin/ECM association and myosin II contractility. Unlike 2D migration, abrogating contractility stalls 3D migration regardless of ECM pore size. We find force is not required for clustering of activated integrins on 3D native collagen fibrils. We propose that efficient 3D migration requires local balancing of contractility with ECM stiffness to stabilize adhesions, which facilitates the detachment of activated integrins from ECM fibrils.

Little is known about how the physical properties of three dimensional (3D) extracellular matrices modulate cell adhesion dynamics. Here Doyle et al. generate 3D collagen gels of varying microarchitecture and quantify the effect on adhesion dynamics and cell motility.

Gravitational Effects on Human Physiology

Physical working capacity decreases with age and also in microgravity. Regardless of age, increased physical activity can always improve the physical adaptability of the body, although the mechanisms of this adaptability are unknown. Physical exercise produces various mechanical stimuli in the body, and these stimuli may be essential for cell survival in organisms. The cytoskeleton plays an important role in maintaining cell shape and tension development, and in various molecular and/or cellular organelles involved in cellular trafficking. Both intra and extracellular stimuli send signals through the cytoskeleton to the nucleus and modulate gene expression via an intrinsic property, namely the "dynamic instability" of cytoskeletal proteins. αB-crystallin is an important chaperone for cytoskeletal proteins in muscle cells. Decreases in the levels of αB-crystallin are specifically associated with a marked decrease in muscle mass (atrophy) in a rat hindlimb suspension model that mimics muscle and bone atrophy that occurs in space and increases with passive stretch. Moreover, immunofluorescence data show complete co-localization of αB-crystallin and the tubulin/microtubule system in myoblast cells. This association was further confirmed in biochemical experiments carried out in vitro showing that αB-crystallin acts as a chaperone for heat-denatured tubulin and prevents microtubule disassembly induced by calcium. Physical activity induces the constitutive expression of αB-crystallin, which helps to maintain the homeostasis of cytoskeleton dynamics in response to gravitational forces. This relationship between chaperone expression levels and regulation of cytoskeletal dynamics observed in slow anti-gravitational muscles as well as in mammalian striated muscles, such as those in the heart, diaphragm and tongue, may have been especially essential for human evolution in particular. Elucidation of the intrinsic properties of the tubulin/microtubule and chaperone αB-crystallin protein complex systems is expected to provide valuable information for high-pressure bioscience and gravity health science.

The effect of cyclic mechanical strain on activation of dendritic cells cultured on adhesive substrates

Dendritic cells (DCs), key regulators of tolerance and immunity, have been found to reside in mechanically active tissues such as the interior layers of the arterial wall, which experience cyclic radial wall strain due to pulsatile blood flow. Although experimentally difficult to determine in vivo, it is reasonable to postulate DCs experience the mechanical forces in such mechanically active tissues. However, it is currently unknown how DCs respond to cyclic mechanical strain. In order to explore the hypothesis that DCs are responsive to mechanical strain, DCs were cultured in vitro on pre-adsorbed adhesive proteins (e.g., laminin, collagen, fibrinogen) and 1 Hz cyclic strain was applied for various durations and strain magnitudes. It was determined that a strain magnitude of 10% and 24 h duration adversely affected DC viability compared to no-strain controls, but culture on certain adhesive substrates provided modest protection of viability under this harsh strain regime. In contrast, application of 1 h of 1 Hz cyclic 3% strain did not affect DC viability and this strain regime was used for the remaining experiments for quantifying DC activation and T-cell priming capability. Application of 3% strain increased expression of stimulatory (MHC-II) and costimulatory molecules (CD86, CD40), and this effect was generally increased by culture on pre-coated adhesive substrates. Interestingly, the cytokine secretion profile of DCs was not significantly affected by strain. Lastly, strained DCs demonstrated increased stimulation of allogeneic T-cell proliferation, in a manner that was independent of the adhesive substrate. These observations indicate generation of a DC consistent with what has been described as a semi-mature phenotype. This work begins elucidating a potential role for DCs in tissue environments exposed to cyclic mechanical forces.

Force-driven aggregation of specific bonds on compliant substrates

A thermodynamic model was employed to understand the underlying physics of the force-dependent size of cell's focal adhesions. The model describes the specific adhesion of an elastic membrane to a compliant substrate such that the adhesion site is subjected to the force of a constant pulling traction. The membrane contains mobile adhesion receptors with specific energetic affinities for complimentary ligands on the substrate. The adhesion is resisted by a disjoining pressure induced by a squeezed layer of glycocalyx. The model demonstrates that the enlargement of adhesion area with increasing pulling traction is possible due to a spontaneous response of the adhesion site to attain the state of minimum free energy. The correlation between the force and the adhesion area strongly depends on the rigidity of the substrate.

Hybrid mechanoresponsive polymer wires under force activation

Force activation is triggered in a stretched polymer wire with color changes produced as a consequence of the molecules undergoing structural and conformational changes. A markedly increased efficiency of force activation is achieved by decreasing the diameter of the wires. The hybrid mechanosensitive polymer wire can function as micro- and nanoscale force sensor.

Key molecular mechanisms in lung cancer invasion and metastasis: a comprehensive review

Lung cancer remains one of the most common and malignant cancers worldwide. It is most often diagnosed at late stages, when it has already presented local invasion and distal metastases. The basic stages of invasion and metastasis involve the detachment of tumor cells from the extracellular matrix, invasion of surrounding tissues and basal lamina, intravasation into the blood stream, survival and transport through the blood stream, migration, arrest and extravasation at a distal site and formation of a metastatic lesion. These steps require fundamental mechanisms such as angiogenesis, degradation of matrix barriers, disruption of cell-cell and cell-matrix adhesion and inducement of cellular motility. Genes that regulate functions like unlimited growth potential, survival, genomic instability, angiogenesis, epithelial to mesenchymal transition and apoptosis evasion, are involved in giving lung cancer tumors invasive and metastatic competence. Improving of understanding of the underlying molecular and cellular mechanisms remains an urgent and essential issue, in order to develop new more effective strategies in preventing and treating lung cancer.

Emergent complexity of the cytoskeleton: from single filaments to tissue

Despite their overwhelming complexity, living cells display a high degree of internal mechanical and functional organization which can largely be attributed to the intracellular biopolymer scaffold, the cytoskeleton. Being a very complex system far from thermodynamic equilibrium, the cytoskeleton's ability to organize is at the same time challenging and fascinating. The extensive amounts of frequently interacting cellular building blocks and their inherent multifunctionality permits highly adaptive behavior and obstructs a purely reductionist approach. Nevertheless (and despite the field's relative novelty), the physics approach has already proved to be extremely successful in revealing very fundamental concepts of cytoskeleton organization and behavior. This review aims at introducing the physics of the cytoskeleton ranging from single biopolymer filaments to multicellular organisms. Throughout this wide range of phenomena, the focus is set on the intertwined nature of the different physical scales (levels of complexity) that give rise to numerous emergent properties by means of self-organization or self-assembly.

Overview: imaging in the study of integrins

Integrins play critical adhesion and signaling roles during development, wound healing, immunity, and cancer. Central to their function is a unique ability to dynamically modulate their adhesiveness and signaling properties through changes in conformation, both homo- and heterotypic protein-protein interactions and cellular distribution. Genetic, biochemical and structural studies have been instrumental in uncovering overall functions, describing ligand and regulatory protein interactions and elucidating the molecular architecture of integrins. However, such approaches alone are inadequate to describe how dynamic integrin behaviors are orchestrated in intact cells. To fill this void, a wide array of distinct light microscopy (largely fluorescence-based) imaging approaches have been developed and employed. Various microscopy technologies, including wide-field, optical sectioning (laser-scanning confocal, spinning-disk confocal, and multiphoton), TIRF and range of novel "Super-Resolution" techniques have been used in combination with diverse imaging modalities (such as IRM, FRET, FRAP, CALI, and fluorescence speckle imaging) to address distinct aspects of integrin function and regulation. This chapter provides an overview of these imaging approaches and how they have advanced our understanding of integrins.

Focal Adhesion Induction at the Tip of a Functionalized Nanoelectrode

Cells dynamically interact with their physical micro-environment through the assembly of nascent focal contacts and focal adhesions. The dynamics and mechanics of these contact points are controlled by transmembrane integrins and an array of intracellular adaptor proteins. In order to study the mechanics and dynamics of focal adhesion assembly, we have developed a technique for the timed induction of a nascent focal adhesion. Bovine aortic endothelial cells were approached at the apical surface by a nanoelectrode whose position was controlled with a resolution of 10s of nanometers using changes in electrode current to monitor distance from the cell surface. Since this probe was functionalized with fibronectin, a focal contact formed at the contact location. Nascent focal adhesion assembly was confirmed using time-lapse confocal fluorescent images of red fluorescent protein (RFP) - tagged talin, an adapter protein that binds to activated integrins. Binding to the cell was verified by noting a lack of change of electrode current upon retraction of the electrode. This study demonstrates that functionalized nanoelectrodes can enable precisely-timed induction and 3-D mechanical manipulation of focal adhesions and the assay of the detailed molecular kinetics of their assembly.

Mechanical failure, stress redistribution, elastase activity and binding site availability on elastin during the progression of emphysema

Emphysema is a disease of the lung parenchyma with progressive alveolar tissue destruction that leads to peripheral airspace enlargement. In this review, we discuss how mechanical forces can contribute to disease progression at various length scales. Airspace enlargement requires mechanical failure of alveolar walls. Because the lung tissue is under a pre-existing tensile stress, called prestress, the failure of a single wall results in a redistribution of the local prestress. During this process, the prestress increases on neighboring alveolar walls which in turn increases the probability that these walls also undergo mechanical failure. There are several mechanisms that can contribute to this increased probability: exceeding the failure threshold of the ECM, triggering local mechanotransduction to release enzymes, altering enzymatic reactions on ECM molecules. Next, we specifically discuss recent findings that stretching of elastin induces an increase in the binding off rate of elastase to elastin as well as unfolds hidden binding sites along the fiber. We argue that these events can initiate a positive feedback loop which generates slow avalanches of breakdown that eventually give rise to the relentless progression of emphysema. We propose that combining modeling at various length scales with corresponding biological assays, imaging and mechanics data will provide new insight into the progressive nature of emphysema. Such approaches will have the potential to contribute to resolving many of the outstanding issues which in turn may lead to the amelioration or perhaps the treatment of emphysema in the future.

A miniature Couette to generate shear for flow cytometry: studying real-time modulation of intracellular calcium in monocytic cells

Extracellular hydrodynamic forces may be transmitted to the interior of cells through the alteration of integrin conformation and affinity. Integrin activation regulates leukocyte recruitment, cell activation, and transmigration. The cellular and molecular mechanisms for integrin activation are not precisely known, although intracellular calcium signaling is involved. Flow cytometry offers a versatile way to study intracellular calcium signaling in real-time. We report a novel method to generate defined shear by using a miniature Couette. Testing involved measuring shear-induced intracellular calcium signals of human monoblastoid U937 cells in suspension. The Couette was connected externally to a flow cytometer and pressurized at 6 PSI (4.1 N/m(2) ). Cells were subjected to a well-defined shear between 0 and 1,000 s(-1) and delivered continuously within 10 s to a FACScan at 1 μl/s. Intracellular calcium levels and the percentage of cells activated increased as shear increased in duration and intensity.

Mechanical forces regulate elastase activity and binding site availability in lung elastin

Many fundamental cellular and extracellular processes in the body are mediated by enzymes. At the single molecule level, enzyme activity is influenced by mechanical forces. However, the effects of mechanical forces on the kinetics of enzymatic reactions in complex tissues with intact extracellular matrix (ECM) have not been identified. Here we report that physiologically relevant macroscopic mechanical forces modify enzyme activity at the molecular level in the ECM of the lung parenchyma. Porcine pancreatic elastase (PPE), which binds to and digests elastin, was fluorescently conjugated (f-PPE) and fluorescent recovery after photobleach was used to evaluate the binding kinetics of f-PPE in the alveolar walls of normal mouse lungs. Fluorescent recovery after photobleach indicated that the dissociation rate constant (k(off)) for f-PPE was significantly larger in stretched than in relaxed alveolar walls with a linear relation between k(off) and macroscopic strain. Using a network model of the parenchyma, a linear relation was also found between k(off) and microscopic strain on elastin fibers. Further, the binding pattern of f-PPE suggested that binding sites on elastin unfold with strain. The increased overall reaction rate also resulted in stronger structural breakdown at the level of alveolar walls, as well as accelerated decay of stiffness and decreased failure stress of the ECM at the macroscopic scale. These results suggest an important role for the coupling between mechanical forces and enzyme activity in ECM breakdown and remodeling in development, and during diseases such as pulmonary emphysema or vascular aneurysm. Our findings may also have broader implications because in vivo, enzyme activity in nearly all cellular and extracellular processes takes place in the presence of mechanical forces.

Mechanical control of tissue and organ development

Many genes and molecules that drive tissue patterning during organogenesis and tissue regeneration have been discovered. Yet, we still lack a full understanding of how these chemical cues induce the formation of living tissues with their unique shapes and material properties. Here, we review work based on the convergence of physics, engineering and biology that suggests that mechanical forces generated by living cells are as crucial as genes and chemical signals for the control of embryological development, morphogenesis and tissue patterning.

Combining mechanical and optical approaches to dissect cellular mechanobiology

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

Overview of confocal microscopy

Born out of the need to overcome an imaging problem in the 1950s, confocal microscopes today allow researchers to go beyond simple imaging and ask biochemical questions. This chapter provides background information on the development of modern confocal microscopes, their uses and applications. Sample preparation and observation are also discussed. Information is also provided about more advanced applications such as FRAP, FRET and 2-photon imaging. The requirements for setting up a confocal laboratory and the instrumentation needs are also discussed.

Multi-scale models of cell and tissue dynamics

Cell and tissue movement are essential processes at various stages in the life cycle of most organisms. The early development of multi-cellular organisms involves individual and collective cell movement; leukocytes must migrate towards sites of infection as part of the immune response; and in cancer, directed movement is involved in invasion and metastasis. The forces needed to drive movement arise from actin polymerization, molecular motors and other processes, but understanding the cell- or tissue-level organization of these processes that is needed to produce the forces necessary for directed movement at the appropriate point in the cell or tissue is a major challenge. In this paper, we present three models that deal with the mechanics of cells and tissues: a model of an arbitrarily deformable single cell, a discrete model of the onset of tumour growth in which each cell is treated individually, and a hybrid continuum-discrete model of the later stages of tumour growth. While the models are different in scope, their underlying mechanical and mathematical principles are similar and can be applied to a variety of biological systems.

Integrated microscopy for real-time imaging of mechanotransduction studies in live cells

Mechanical force is an important stimulus and determinant of many vascular smooth muscle cell functions including contraction, proliferation, migration, and cell attachment. Transmission of force from outside the cell through focal adhesions controls the dynamics of these adhesion sites and initiates intracellular signaling cascades that alter cellular behavior. To understand the mechanism by which living cells sense mechanical forces, and how they respond and adapt to their environment, a critical first step is to develop a new technology to investigate cellular behavior at subcellular level that integrates an atomic force microscope (AFM) with total internal reflection fluorescence (TIRF) and fast-spinning disk (FSD) confocal microscopy, providing high spatial and temporal resolution. AFM uses a nanosensor to measure the cell surface topography and can apply and measure mechanical force with high precision. TIRF microscopy is an optical imaging technique that provides high-contrast images with high z-resolution of fluorescently labeled molecules in the immediate vicinity of the cell-coverslip interface. FSD confocal microscopy allows rapid 3-D imaging throughout the cell in real time. The integrated system is broadly applicable across a wide range of molecular dynamic studies in any adherent live cells, allowing direct optical imaging of cell responses to mechanical stimulation in real time.

Significant roles of microtubules in mature striated muscle deduced from the correlation between tubulin and its molecular chaperone alphaB-crystallin in rat muscles

To elucidate the significance of cytoskeletal microtubule networks in striated muscles, we analyzed correlation between the content of tubulin (building block of microtubules) and alphaB-crystallin (a molecular chaperone for tubulin) in a variety of striated muscles expressing different myosin heavy-chain (MHC) isoforms. The content of both tubulin and alphaB-crystallin was larger in MHC-I dominant soleus muscle and in MHC-alpha dominant cardiac (atrium and ventricle) muscles; intermediate in MHC-IId dominant masseter, tongue, and diaphragm muscles; and smaller in MHC-IIb dominant plantaris, gastrocnemius, psoas, extensor digitorum longus, and tibialis anterior muscles. Since the muscles of slow-type MHC (MHC-I/alpha) show the most economical features in their function and metabolism, which suit for continuous activity required to sustain posture and blood pumping, the present results afforded additional support to our hypothesis that microtubule networks transduce mechanical environmental demands to morphological and biochemical responses that eventually evolve adaptive transformation in the function and metabolism of the mature muscles. The comparison of tubulin/alphaB-crystalline ratios across the muscles of varied MHC isoforms further suggested that mechanical stress fluctuating at the rhythmic frequency of walking and breathing efficiently activates the hypothesized dynamic function of microtubules.

JNK-mediated phosphorylation of paxillin in adhesion assembly and tension-induced cell death by the adenovirus death factor E4orf4

The adenovirus type 2 Early Region 4 ORF4 (E4orf4) protein induces a caspase-independent death program in tumor cells involving changes in actin dynamics that are functionally linked to cell killing. Because an increase in myosin II-based contractility is needed for the death of E4orf4-expressing cells, we have proposed that alteration of cytoskeletal tension is part of the signals engaging the death pathway. Yet the mechanisms involved are poorly defined. Herein, we show that the Jun N-terminal kinase JNK is activated in part through a pathway involving Src, Rho, and ROCK (Rho kinase) and contributes to dysregulate adhesion dynamics and to kill cells in response to E4orf4. JNK supports the formation of atypically robust focal adhesions, which are bound to the assembly of the peculiar actomyosin network typifying E4orf4-induced cell death and which are required for driving nuclear condensation. Remarkably, the dramatic enlargement of focal adhesions, actin remodeling, and cell death all rely on paxillin phosphorylation at Ser-178, which is induced by E4orf4 in a JNK-dependent way. Furthermore, we found that Ser-178-paxillin phosphorylation is necessary to decrease adhesion turnover and to enhance the time residency of paxillin at focal adhesions, promoting its recruitment from an internal pool. Our results indicate that perturbation of tensional homeostasis by E4orf4 involves JNK-regulated changes in paxillin adhesion dynamics that are required to engage the death pathway. Moreover, our findings support a role for JNK-mediated paxillin phosphorylation in adhesion growth and stabilization during tension signaling.