Tensegrity I. Cell structure and hierarchical systems biology

Cell mechanics of alveolar epithelial cells (AECs) and macrophages (AMs)

Cell mechanics provides an integrated view of many biological phenomena which are intimately related to cell structure and function. Because breathing constitutes a sustained motion synonymous with life, pulmonary cells are normally designed to support permanent cyclic stretch without breaking, while receiving mechanical cues from their environment. The authors study the mechanical responses of alveolar cells, namely epithelial cells and macrophages, exposed to well-controlled mechanical stress in order to understand pulmonary cell response and function. They discuss the principle, advantages and limits of a cytoskeleton-specific micromanipulation technique, magnetic bead twisting cytometry, potentially applicable in vivo. They also compare the pertinence of various models (e.g., rheological; power law) used to extract cell mechanical properties and discuss cell stress/strain hardening properties and cell dynamic response in relation to the structural tensegrity model. Overall, alveolar cells provide a pertinent model to study the biological processes governing cellular response to controlled stress or strain.

Extracellular matrix dynamics in development and regenerative medicine

The extracellular matrix (ECM) regulates cell behavior by influencing cell proliferation, survival, shape, migration and differentiation. Far from being a static structure, the ECM is constantly undergoing remodeling--i.e. assembly and degradation--particularly during the normal processes of development, differentiation and wound repair. When misregulated, this can contribute to disease. ECM assembly is regulated by the 3D environment and the cellular tension that is transmitted through integrins. Degradation is controlled by complex proteolytic cascades, and misregulation of these results in ECM damage that is a common component of many diseases. Tissue engineering strives to replace damaged tissues with stem cells seeded on synthetic structures designed to mimic the ECM and thus restore the normal control of cell function. Stem cell self-renewal and differentiation is influenced by the 3D environment within the stem cell niche. For tissue-engineering strategies to be successful, the intimate dynamic relationship between cells and the ECM must be understood to ensure appropriate cell behavior.

Microsystems for biomechanical measurements

The use of microtechnology to make biomechanical measurements allows for the study of cellular and subcellular scale mechanical forces. Forces generated by cells are in the few nanoNewton to several microNewton range and can change spatially over subcellular size scales. Transducing forces at such small size and force scales is a challenging task. Methods of microfabrication developed in the integrated circuit industry have allowed researchers to build platforms with cellular and subcellular scale parts with which individual cells can interact. These parts act as transducers of stresses and forces generated by the cell during migration or in the maintenance of physical equilibrium. Due to the size and sensitivity of such devices, quantitative studies of single cell and even single molecule biomechanics have become possible. In this review we focus on two classes of cellular force transducers: silicon-based devices and soft-polymer platforms. We concentrate on the biomechanical discoveries made with these devices and less so on the engineering behind their development because this is covered in great detail elsewhere.

Modeling mechanosensing and its effect on the migration and proliferation of adherent cells

The behavior of normal adherent cells is influenced by the stiffness of the substrate they are anchored to. Cells are able to detect substrate mechanical properties by actively generating contractile forces and use this information to migrate and proliferate. In particular, the speed and direction of cell crawling, as well as the rate of cell proliferation, vary with the substrate compliance and prestrain. In this work, we present an active mechanosensing model based on an extension of the classical Hill's model for skeletal muscle behavior. We also propose a thermodynamical approach to model cell migration regulated by mechanical stimuli and a proliferation theory also depending on the mechanical environment. These contributions give rise to a conceptually simple mathematical formulation with a straightforward and inexpensive computational implementation, yielding results consistent with numerous experiments. The model can be a useful tool for practical applications in biology and medicine in situations where cell-substrate interaction as well as substrate mechanical behavior play an important role, such as the design of tissue engineering applications.

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

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

From single cells to tissue architecture-a bottom-up approach to modelling the spatio-temporal organisation of complex multi-cellular systems

Collective phenomena in multi-cellular assemblies can be approached on different levels of complexity. Here, we discuss a number of mathematical models which consider the dynamics of each individual cell, so-called agent-based or individual-based models (IBMs). As a special feature, these models allow to account for intracellular decision processes which are triggered by biomechanical cell-cell or cell-matrix interactions. We discuss their impact on the growth and homeostasis of multi-cellular systems as simulated by lattice-free models. Our results demonstrate that cell polarisation subsequent to cell-cell contact formation can be a source of stability in epithelial monolayers. Stroma contact-dependent regulation of tumour cell proliferation and migration is shown to result in invasion dynamics in accordance with the migrating cancer stem cell hypothesis. However, we demonstrate that different regulation mechanisms can equally well comply with present experimental results. Thus, we suggest a panel of experimental studies for the in-depth validation of the model assumptions.

Can cancer be reversed by engineering the tumor microenvironment?

To advance cancer research in a transformative way, we must redefine the problem. Although epithelial cancers, such as breast cancer, may be caused by random somatic gene mutations, the reality is that this is only one of many ways to induce tumor formation. Cancers also can be produced in experimental systems in vitro and in vivo, for example, by inducing sustained alterations of extracellular matrix (ECM) structure. Moreover, certain epithelial cancers can be induced to 'reboot' and regenerate normal tissue morphology when combined with embryonic mesenchyme or exogenous ECM scaffolds that are produced through epithelial-stromal interactions. At the same time, work in the field of Mechanical Biology has revealed that many cell behaviors critical for cancer formation (e.g., growth, differentiation, motility, apoptosis) can be controlled by physical interactions between cells and their ECM adhesions that alter the mechanical force balance in the ECM, cell and cytoskeleton. Epithelial tumor progression also can be induced in vitro by changing ECM mechanics or altering cytoskeletal tension generation through manipulation of the Rho GTPase signaling pathway. Mechanical interactions between capillary cells and ECM that are mediated by Rho signaling similarly mediate control of capillary cell growth and angiogenesis, which are equally critical for cancer progression and metastasis. These findings question basic assumptions in the cancer field, and raise the intriguing possibility that cancer may be a reversible disease that results from progressive deregulation of tissue architecture, which leads to physical changes in cells and altered mechanical signaling. This perspective raises the possibility of developing a tissue engineering approach to cancer therapy in which biologically inspired materials that mimic the embryonic microenvironment are used to induce cancers to revert into normal tissues.

Mapping cell-matrix stresses during stretch reveals inelastic reorganization of the cytoskeleton

The mechanical properties of the living cell are intimately related to cell signaling biology through cytoskeletal tension. The tension borne by the cytoskeleton (CSK) is in part generated internally by the actomyosin machinery and externally by stretch. Here we studied how cytoskeletal tension is modified during stretch and the tensional changes undergone by the sites of cell-matrix interaction. To this end we developed a novel technique to map cell-matrix stresses during application of stretch. We found that cell-matrix stresses increased with imposition of stretch but dropped below baseline levels on stretch release. Inhibition of the actomyosin machinery resulted in a larger relative increase in CSK tension with stretch and in a smaller drop in tension after stretch release. Cell-matrix stress maps showed that the loci of cell adhesion initially bearing greater stress also exhibited larger drops in traction forces after stretch removal. Our results suggest that stretch partially disrupts the actin-myosin apparatus and the cytoskeletal structures that support the largest CSK tension. These findings indicate that cells use the mechanical energy injected by stretch to rapidly reorganize their structure and redistribute tension.

Cellular mechanobiology and cancer metastasis

The primary cause of cancer treatment failure is invasion and metastasis, and invading tumor cells utilize many of the motility patterns that have been documented for normal morphogenesis. Recently, the role of mechanical forces in guiding various tissue and cell movements in embryonic development has been systematically analyzed with new experimental and computational methods. The tissue and cellular mechanobiology approach also holds promise for increasing the understanding of tumor invasion. In fact, the mechanical stiffness of tumors has correlated with invasiveness, and manipulation of extracellular matrix (ECM) stiffness in vitro has suppressed the cancer phenotype. Several important signaling molecules reside on the cytoskeleton, which is affected by external stress imparted by the ECM, and deformation of the nucleus can trigger the activation of certain genes. All these observations suggest that a synthesis of the biology of cancer cell invasion and cellular mechanobiology may offer new targets for the treatment of malignant disease. Accordingly, sensitive and relevant in vivo models and methods to study cancer mechanobiology are needed.

Cytoskeletal mechanics in airway smooth muscle cells

Mechanical properties and contractility of airway smooth muscle tissue are largely responsible for airway narrowing and airway hyperresponsiveness in asthma. To explain these pathological phenomena, investigators have studied the mechanical behaviour of airway smooth muscle cells and its relationship to the underlying cellular biophysical and biochemical mechanisms. During the past decade, a growing body of evidence has indicated that a deformable intracellular polymer network, known as the cytoskeleton, plays a major role in transmitting and distributing mechanical forces within the cell and in their conversion into biochemical responses. We review here evidence suggesting that the tensed and crosslinked cytoskeletal lattice, the contractile apparatus, and the cytoskeleton-extracellular matrix interactions are key determinants of mechanical properties and mechanosensing of airway smooth muscle cells, with the mechanical distending stress of the cytoskeleton playing the central role.

Tensegrity-based mechanosensing from macro to micro

This article is a summary of a lecture on cellular mechanotransduction that was presented at a symposium on "Cardiac Mechano-Electric Feedback and Arrhythmias" that convened at Oxford, England in April 2007. Although critical mechanosensitive molecules and cellular components, such as integrins, stretch-activated ion channels, and cytoskeletal filaments, have been shown to contribute to the response by which cells convert mechanical signals into a biochemical response, little is known about how they function in the structural context of living cells, tissues and organs to produce orchestrated changes in cell behavior in response to stress. Here, studies are reviewed that suggest our bodies use structural hierarchies (systems within systems) composed of interconnected extracellular matrix and cytoskeletal networks that span from the macroscale to the nanoscale to focus stresses on specific mechanotransducer molecules. A key feature of these networks is that they are in a state of isometric tension (i.e., experience a tensile prestress), which ensures that various molecular-scale mechanochemical transduction mechanisms proceed simultaneously and produce a concerted response. These features of living architecture are the same principles that govern tensegrity (tensional integrity) architecture, and mathematical models based on tensegrity are beginning to provide new and useful descriptions of living materials, including mammalian cells. This article reviews how the use of tensegrity at multiple size scales in our bodies guides mechanical force transfer from the macro to the micro, as well as how it facilitates conversion of mechanical signals into changes in ion flux, molecular binding kinetics, signal transduction, gene transcription, cell fate switching and developmental patterning.

Plastid movement in statocytes of the arg1 (altered response to gravity) mutant

The ability of a plant to respond to gravity is crucial for growth and development throughout the life cycle. A key player in the cellular mechanisms of gravitropism is ARG1 (altered response to gravity), a DnaJ-like protein that associates with components of the vesicular trafficking pathway and carries a C-terminal domain with similarities to cytoskeleton-associated proteins. The arg1-2 mutant of Arabidopsis thaliana has reduced and delayed gravitropism in roots, shoots, and inflorescence stems when grown in the light or dark. We performed light microscopic studies of plastid movement in the gravity-perceiving statocytes (endodermal cells) of hypocotyls of arg1-2 and WT light-grown seedlings following reorientation to better characterize the role of ARG1 in gravitropism. Cryofixation/freeze substitution procedures were used because they provide a reliable indication of rapid cellular events within the statocytes. Our results suggest that ARG1 affects gravitropism by reducing plastid movement/sedimentation, a process known to be essential for early phases of signaling cascades in the statocytes.

An integrated systems biology approach to understanding the rules of keratinocyte colony formation

Closely coupled in vitro and in virtuo models have been used to explore the self-organization of normal human keratinocytes (NHK). Although it can be observed experimentally, we lack the tools to explore many biological rules that govern NHK self-organization. An agent-based computational model was developed, based on rules derived from literature, which predicts the dynamic multicellular morphogenesis of NHK and of a keratinocyte cell line (HaCat cells) under varying extracellular Ca++ concentrations. The model enables in virtuo exploration of the relative importance of biological rules and was used to test hypotheses in virtuo which were subsequently examined in vitro. Results indicated that cell-cell and cell-substrate adhesions were critically important to NHK self-organization. In contrast, cell cycle length and the number of divisions that transit-amplifying cells could undergo proved non-critical to the final organization. Two further hypotheses, to explain the growth behaviour of HaCat cells, were explored in virtuo-an inability to differentiate and a differing sensitivity to extracellular calcium. In vitro experimentation provided some support for both hypotheses. For NHKs, the prediction was made that the position of stem cells would influence the pattern of cell migration post-wounding. This was then confirmed experimentally using a scratch wound model.

Chapter 19: Mechanical response of cytoskeletal networks

The cellular cytoskeleton is a dynamic network of filamentous proteins, consisting of filamentous actin (F-actin), microtubules, and intermediate filaments. However, these networks are not simple linear, elastic solids; they can exhibit highly nonlinear elasticity and a thermal dynamics driven by ATP-dependent processes. To build quantitative mechanical models describing complex cellular behaviors, it is necessary to understand the underlying physical principles that regulate force transmission and dynamics within these networks. In this chapter, we review our current understanding of the physics of networks of cytoskeletal proteins formed in vitro. We introduce rheology, the technique used to measure mechanical response. We discuss our current understanding of the mechanical response of F-actin networks, and how the biophysical properties of F-actin and actin cross-linking proteins can dramatically impact the network mechanical response. We discuss how incorporating dynamic and rigid microtubules into F-actin networks can affect the contours of growing microtubules and composite network rigidity. Finally, we discuss the mechanical behaviors of intermediate filaments.

Cell culture models in microfluidic systems

Microfluidic technology holds great promise for the creation of advanced cell culture models. In this review, we discuss the characterization of cell culture in microfluidic systems, describe important biochemical and physical features of the cell microenvironment, and review studies of microfluidic cell manipulation in the context of these features. Finally, we consider the integration of analytical elements, ways to achieve high throughput, and the design constraints imposed by cell biology applications.

Comparison of quantitative methods for cell-shape analysis

Morphology is an important large-scale manifestation of the global organizational and physiological state of cells, and is commonly used as a qualitative or quantitative measure of the outcome of various assays. Here we evaluate several different basic representations of cell shape - binary masks, distance maps and polygonal outlines - and different subsequent encodings of those representations - Fourier and Zernike decompositions, and the principal and independent components analyses - to determine which are best at capturing biologically important shape variation. We find that principal components analysis of two-dimensional shapes represented as outlines provide measures of morphology which are quantitative, biologically meaningful, human interpretable and work well across a range of cell types and parameter settings.

Fibroblast mechanics in 3D collagen matrices

Connective tissues provide mechanical support and frameworks for the other tissues of the body. Type 1 collagen is the major protein component of ordinary connective tissue, and fibroblasts are the cell type primarily responsible for its biosynthesis and remodeling. Research on fibroblasts interacting with collagen matrices explores all four quadrants of cell mechanics: pro-migratory vs. pro-contractile growth factor environments on one axis; high tension vs. low tension cell-matrix interactions on the other. The dendritic fibroblast - probably equivalent to the resting tissue fibroblast - can be observed only in the low tension quadrant and generally has not been appreciated from research on cells incubated with planar culture surfaces. Fibroblasts in the low tension quadrant require microtubules for formation of dendritic extensions, whereas fibroblasts in the high tension quadrant require microtubules for polarization but not for spreading. Ruffling of dendritic extensions rather than their overall protrusion or retraction provides the mechanism for remodeling of floating collagen matrices, and floating matrix remodeling likely reflects a model of tissue mechanical homeostasis.

Microfabrication procedure of PDMS microbeam array using photolithography for laminin printing and piconewton force transduction on axons

The purpose of this paper is to introduce our design for transducing forces on the order of tens of piconewtons by optically measuring deflection of a microfabricated beam tip as it pulls on an array of flexible structures such as axons in an array of laminin-printed neurons. To achieve this we have designed polymeric beams with spring constants on the order of 10 pN/microm. We have fabricated circular microbeams with Sylgard polydimethylsiloxane (PDMS). The elastic modulus of PDMS was determined experimentally using a microscale and a micrometer at different concentrations of curing agent and base agent and found to be on the order of 100 kPa. The designed geometry is a 100x100 tapered microcone array with each beam having a length of 100 microm, and a base diameter of 10 microm. A SU-8 negative photoresist is etched using photolithography and used as a mold for PDMS soft lithography. PDMS was injected into the mold and the array peeled from the mold.

The emergence of ECM mechanics and cytoskeletal tension as important regulators of cell function

The ability to harvest and maintain viable cells from mammalian tissues represented a critical advance in biomedical research, enabling individual cells to be cultured and studied in molecular detail. However, in these traditional cultures, cells are grown on rigid glass or polystyrene substrates, the mechanical properties of which often do not match those of the in vivo tissue from which the cells were originally derived. This mechanical mismatch likely contributes to abrupt changes in cellular phenotype. In fact, it has been proposed that mechanical changes in the cellular microenvironment may alone be responsible for driving specific cellular behaviors. Recent multidisciplinary efforts from basic scientists and engineers have begun to address this hypothesis more explicitly by probing the effects of ECM mechanics on cell and tissue function. Understanding the consequences of such mechanical changes is physiologically relevant in the context of a number of tissues in which altered mechanics may either correlate with or play an important role in the onset of pathology. Examples include changes in the compliance of blood vessels associated with atherosclerosis and intimal hyperplasia, as well as changes in the mechanical properties of developing tumors. Compelling evidence from 2-D in vitro model systems has shown that substrate mechanical properties induce changes in cell shape, migration, proliferation, and differentiation, but it remains to be seen whether or not these same effects translate to 3-D systems or in vivo. Furthermore, the molecular "mechanotransduction" mechanisms by which cells respond to changes in ECM mechanics remain unclear. Here, we provide some historical context for this emerging area of research, and discuss recent evidence that regulation of cytoskeletal tension by changes in ECM mechanics (either directly or indirectly) may provide a critical switch that controls cell function.

Capillary-induced contact guidance

Topographical features are known to impose capillary forces on liquid droplets, and this phenomenon is exploited in applications such as printing, coatings, textiles and microfluidics. Surface topographies also influence the behavior of biological cells (i.e., contact guidance), with implications ranging from medicine to agriculture. An accurate physical description of how cells detect and respond to surface topographies is necessary in order to move beyond a purely heuristic approach to optimizing the topographies of biomaterial interfaces. Here, we have used a combination of Langmuir-Blodgett lithography and nanoimprinting to generate a range of synthetic microstructured surfaces with grooves of subcellular dimensions in order to investigate the influence of capillary forces on the biological process of contact guidance. The physical-chemical properties of these surfaces were assessed by measuring the anisotropic spreading of sessile water droplets. Having established the physical properties of each surface, we then investigated the influence of capillary forces on the processes of cellular contact guidance in biological organisms, using mammalian osteoblasts and germinating fungal spores as tester organisms. Our results demonstrate that capillary effects are present in topographical contact guidance and should therefore be considered in any physical model that seeks to predict how cells will respond to a particular surface topography.

Coupling of transverse and longitudinal response in stiff polymers

The time-dependent transverse response of stiff inextensible polymers is well understood on the linear level, where transverse and longitudinal displacements evolve independently. We show that for times beyond a characteristic time tf, longitudinal friction considerably weakens the response compared to the widely used linear response predictions. The corresponding feedback mechanism is explained by scaling arguments and quantified by a systematic theory. Our scaling laws and exact solutions for the transverse response apply to cytoskeletal filaments as well as DNA under tension.

Laser tweezers in the study of mechanobiology in live cells

The study of how cells respond to mechanical stimuli has recently leaped into the forefront of cell science with recent advances in molecular probes facilitating real-time measurements of cell signaling. In this chapter, we will detail the development of a "real-time" molecular probe designed to report the current fractional activated state of Src kinase by changing its spectral output in accordance to local Src states. Src kinase is widely understood to be a key player in the transduction of mechanical stimuli transduced through cell adhesions and focal complexes. To study the local and long-range Src response to localized stresses, an experimental protocol was developed whereby ligand-coated microspheres were adhered to the cell surface and pulled laterally by laser tweezers. This chapter contains a practical discussion of system design considerations and force calibration. Image processing, background subtraction, and the construction of an unbiased ratio image are discussed. Methods of analyzing the distribution of activated Src molecules are detailed with examples of cells with varying degrees of mechanostimulation.

Probing intracellular force distributions by high-resolution live cell imaging and inverse dynamics

Highly coordinated molecular regulation of mechanical processes is central to numerous cell processes. A key challenge in cell biophysics is, therefore, to probe intracellular force distributions and mechanical properties of live cells with high spatial and temporal resolution. This chapter describes a passive (i.e. nonperturbing) approach to map intracellular force distributions with submicron spatial resolution, and on a timescale of seconds. On the basis of a continuum mechanical interpretation of the cell cytoskeleton, this approach performs an inverse reconstruction of intracellular forces from cytoskeletal flows measured in high-resolution live cell images acquired by quantitative fluorescent speckle microscopy (qFSM). Our inverse algorithm can robustly reconstruct the relative force distribution even in the absence of a quantitative profile of network elasticity. In addition, we also propose an emerging technique for probing the in vivo actin network compliance based on correlation analysis of the same data set. We demonstrate the force reconstruction on migrating epithelial cells, where the reconstructed intracellular force field indicates spatial and temporal coordination of force generation by cytoskeleton assembly, contraction and focal adhesion resistance, and its functional output in the form of cell edge movements. This technique will potentially allow the analysis of intracellular force regulation in numerous other cell functions.

Modulation of cellular mechanics during osteogenic differentiation of human mesenchymal stem cells

Recognition of the growing role of human mesenchymal stem cells (hMSC) in tissue engineering and regenerative medicine requires a thorough understanding of intracellular biochemical and biophysical processes that may direct the cell's commitment to a particular lineage. In this study, we characterized the distinct biomechanical properties of hMSCs, including the average Young's modulus determined by atomic force microscopy (3.2 +/- 1.4 kPa for hMSC vs. 1.7 +/- 1.0 kPa for fully differentiated osteoblasts), and the average membrane tether length measured with laser optical tweezers (10.6 +/- 1.1 microm for stem cells, and 4.0 +/- 1.1 microm for osteoblasts). These differences in cell elasticity and membrane mechanics result primarily from differential actin cytoskeleton organization in these two cell types, whereas microtubules did not appear to affect the cellular mechanics. The membrane-cytoskeleton linker proteins may contribute to a stronger interaction of the plasma membrane with F-actins and shorter membrane tether length in osteoblasts than in stem cells. Actin depolymerization or ATP depletion caused a two- to threefold increase in the membrane tether length in osteoblasts, but had essentially no effect on the stem-cell membrane tethers. Actin remodeling in the course of a 10-day osteogenic differentiation of hMSC mediates the temporally correlated dynamical changes in cell elasticity and membrane mechanics. For example, after a 10-day culture in osteogenic medium, hMSC mechanical characteristics were comparable to those of mature bone cells. Based on quantitative characterization of the actin cytoskeleton remodeling during osteodifferentiation, we postulate that the actin cytoskeleton plays a pivotal role in determining the hMSC mechanical properties and modulation of cellular mechanics at the early stage of stem-cell osteodifferentiation.

Action and perception at the level of synergies

Meeting the challenge of assembling coherent organizations of very many muscles characterizes a functional level of biological movement systems referred to as the level of muscular-articular links or synergies. The present article examines the issues confronting the forming, regulating, and ordering of synergies and the hypothesized principles, both classical and contemporary, which resolve them. A primary goal of the article is to highlight the abstractness of the concepts and tools required to understand the level's action-perception competence. Coverage is given to symmetry groups, task space, order parameters, metastability, biotensegrity, allometric scaling, and impredicative definitions.

Mechanotransduction of keratinocytes in culture and in the epidermis

The epidermis, like many other tissues, reacts to mechanical stress by increasing cell proliferation. Mechanically stressed skin regions often develop thicker skin and hyperkeratosis. Interestingly, a large number of skin diseases are accompanied by epidermal proliferation and hyperkeratosis even under normal mechanical stress conditions. Although, some of the molecular pathways of mechanical signaling involving integrins, the epidermal growth factor receptor and mitogen-activated protein kinases are known it is still unclear, how mechanical force is sensed and transformed into the molecular signals that induce cell proliferation. This review focuses on the molecules and pathways known to play a role in mechanotransduction in epidermal keratinocytes and discusses the pathways identified in other well-studied cell types.

Membrane reserves and hypotonic cell swelling

To accommodate expanding volume (V) during hyposmotic swelling, animal cells change their shape and increase surface area (SA) by drawing extra membrane from surface and intracellular reserves. The relative contributions of these processes, sources and extent of membrane reserves are not well defined. In this study, the SA and V of single substrate-attached A549, 16HBE14o(-), CHO and NIH 3T3 cells were evaluated by reconstructing cell three-dimensional topology based on conventional light microscopic images acquired simultaneously from two perpendicular directions. The size of SA reserves was determined by swelling cells in extreme 98% hypotonic (approximately 6 mOsm) solution until membrane rupture; all cell types examined demonstrated surprisingly large membrane reserves and could increase their SA 3.6 +/- 0.2-fold and V 10.7 +/- 1.5-fold. Blocking exocytosis (by N-ethylmaleimide or 10 degrees C) reduced SA and V increases of A549 cells to 1.7 +/- 0.3-fold and 4.4 +/- 0.9-fold, respectively. Interestingly, blocking exocytosis did not affect SA and V changes during moderate swelling in 50% hypotonicity. Thus, mammalian cells accommodate moderate (<2-fold) V increases mainly by shape changes and by drawing membrane from preexisting surface reserves, while significant endomembrane insertion is observed only during extreme swelling. Large membrane reserves may provide a simple mechanism to maintain membrane tension below the lytic level during various cellular processes or acute mechanical perturbations and may explain the difficulty in activating mechanogated channels in mammalian cells.

Epigenetic reversion of breast carcinoma phenotype is accompanied by changes in DNA sequestration as measured by AluI restriction enzyme

The importance of microenvironment and context in regulation of tissue-specific genes is well established. DNA exposure to or the sequestration from nucleases detects differences in higher order chromatin structure in intact cells without disturbing cellular or tissue architecture. To investigate the relationship between chromatin organization and tumor phenotype, we used an established three-dimensional assay in which normal and malignant human breast cells can be easily distinguished by the morphology of the structures they make (acinus-like versus tumor-like, respectively). We show that these phenotypes can be distinguished also by sensitivity to AluI digestion in which the malignant cells resist digestion relative to nonmalignant cells. Treatment of T4-2 breast cancer cells in three-dimensional culture with cAMP analogs or a phosphatidylinositol 3-kinase inhibitor not only reverted their phenotype from nonpolar to polar acinar-like structures but also enhanced chromatin sensitivity to AluI. By using different cAMP analogs, we show that cAMP-induced phenotypic reversion, polarization, and shift in DNA organization act through a cAMP-dependent protein-kinase A-coupled signaling pathway. Importantly, inhibitory antibody to fibronectin produced the same effect. These experiments underscore the concept that modifying the tumor microenvironment can alter the organization of tumor cells and demonstrate that architecture and global chromatin organization are coupled and highly plastic.

Functional taxonomy of bacterial hyperstructures

The levels of organization that exist in bacteria extend from macromolecules to populations. Evidence that there is also a level of organization intermediate between the macromolecule and the bacterial cell is accumulating. This is the level of hyperstructures. Here, we review a variety of spatially extended structures, complexes, and assemblies that might be termed hyperstructures. These include ribosomal or "nucleolar" hyperstructures; transertion hyperstructures; putative phosphotransferase system and glycolytic hyperstructures; chemosignaling and flagellar hyperstructures; DNA repair hyperstructures; cytoskeletal hyperstructures based on EF-Tu, FtsZ, and MreB; and cell cycle hyperstructures responsible for DNA replication, sequestration of newly replicated origins, segregation, compaction, and division. We propose principles for classifying these hyperstructures and finally illustrate how thinking in terms of hyperstructures may lead to a different vision of the bacterial cell.

Mechanotransduction from the ECM to the genome: Are the pieces now in place?

A multitude of biochemical signaling processes have been characterized that affect gene expression and cellular activity. However, living cells often need to integrate biochemical signals with mechanical information from their microenvironment as they respond. In fact, the signals received by shape alone can dictate cell fate. This mechanotrasduction of information is powerful, eliciting proliferation, differentiation, or apoptosis in a manner dependent upon the extent of physical deformation. The cells internal "prestressed" structure and its "hardwired" interaction with the extra-cellular matrix (ECM) appear to confer this ability to filter biochemical signals and decide between divergent cell functions influenced by the nature of signals from the mechanical environment. In some instances mechanical signaling through the tissue microenvironment has been shown to be dominant over genomic defects, imparting a normal phenotype on cells that otherwise have transforming genetic lesions. This mechanical control of phenotype is postulated to have a central role in embryogenesis, tissue physiology as well as the pathology of a wide variety of diseases, including cancer. We will briefly review studies showing physical continuity between the external cellular microenvironment and the interior of the cell nucleus. Newly characterized structures, termed nuclear envelope lamina spanning complexes (NELSC), and their interactions will be described as part of a model for mechanical transduction of extracellular cues from the ECM to the genome.

Role of nuclear lamina-cytoskeleton interactions in the maintenance of cellular strength

The response of individual cells to cellular stress is vital for cellular functioning. A large network of physically interconnected cellular components, starting from the structural components of the cells' nucleus, via cytoskeleton filaments to adhesion molecules and the extracellular matrix, constitutes an integrated matrix that functions as a scaffold allowing the cell to cope with mechanical stress. Next to a role in mechanical properties, this network also has a mechanotransductional function in the response to mechanical stress. This signaling route does not only regulate a rapid reorganization of structural components such as actin filaments, but also stimulates for example gene activation via NFkappaB and other transcription factors. The importance of an intact mechano-signaling network is illustrated by the physiological consequences of several genetic defects of cellular network components e.g. actin, dystrophin, desmin and lamins. These give rise to an impaired response of the affected cells to mechanical stress and often result in dystrophy of the affected tissue. Recently, the importance of the cell nucleus in cellular strength has been established. Several new interconnecting proteins, such as the nesprins that link the nuclear lamina to the cytoskeleton, have been identified. Furthermore, the function of nuclear lamins in determining cellular strength and nuclear stability was illustrated in lamin-knock-out cells. Absence of the A-type lamins or mutations in these structural components of the nuclear lamina lead to an impaired cellular response to mechanical stress and disturbances in cytoskeletal organization. In addition, laminopathies show clinical phenotypes comparable to those seen for diseases resulting from genetic defects in cytoskeletal components, further indicating that lamins play a central role in maintaining the mechanical properties of the cell.

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

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

The effect of combined simulated microgravity and microgrooved surface topography on fibroblasts

This study evaluated in vitro the differences in morphological behaviour between fibroblast cultured on smooth and microgrooved substrata (groove depth: 0.5 microm, width: 1, 2, 5, and 10 microm), which were subjected to simulated microgravity. The aim of the study was to clarify which of these parameters was more dominant to determine cell behaviour. Morphological characteristics were investigated using scanning electron microscopy and fluorescence microscopy in order to obtain qualitative information on cell alignment and area. Confocal laser scanning microscopy visualised distribution of actin filaments and focal adhesion points. Finally, expression of collagen type I, fibronectin, and alpha1- and beta1-integrin were investigated by PCR. Microscopy and image analysis showed that the fibroblasts aligned along the groove direction on all textured surfaces. On the smooth substrata, cells had spread out in a random fashion. The alignment of cells cultured on grooved surfaces decreased under simulated microgravity, especially after 24 h of culturing. Cell surface area on grooved substrata were significantly smaller than on smooth substrata, but simulated microgravity on the grooved groups resulted in an enlargement of cell area. ANOVA was performed on all main parameters: topography, gravity force, and time. In this analysis, all parameters proved significant. In addition, gene levels were reduced by microgravity particularly those of beta1-integrin and fibronectin. From our data it is concluded that the fibroblasts primarily adjust their shape according to morphological environmental cues like substratum surface whilst a secondary, but significant, role is played by microgravity conditions.

Endothelial cell migration during angiogenesis

Endothelial cell migration is essential to angiogenesis. This motile process is directionally regulated by chemotactic, haptotactic, and mechanotactic stimuli and further involves degradation of the extracellular matrix to enable progression of the migrating cells. It requires the activation of several signaling pathways that converge on cytoskeletal remodeling. Then, it follows a series of events in which the endothelial cells extend, contract, and throw their rear toward the front and progress forward. The aim of this review is to give an integrative view of the signaling mechanisms that govern endothelial cell migration in the context of angiogenesis.

Microtubule function in fibroblast spreading is modulated according to the tension state of cell-matrix interactions

Mechanical and physical features of the extracellular environment dramatically impact cell shape. Fibroblasts interacting with 3D relaxed collagen matrices appear much different from cells on 2D collagen-coated surfaces and form dendritic cell extensions that contain microtubule cores and actin-rich tips. We found that interfering with cellular microtubules caused cells in relaxed matrices to remain round and unable to form dendritic extensions, whereas fibroblasts on coverslips formed lamellipodial extensions and were spread completely without microtubules but were unable to become polarized. Fibroblasts in relaxed collagen matrices lack stress fibers, focal adhesions, and focal adhesion signaling. Fibroblasts on collagen-coated coverslips that were unable to develop stress fibers and focal adhesions, because of either adding blebbistatin to the cells or use of soft coverslips, also formed microtubule-dependent dendritic extensions. Conversely, fibroblasts interacting with precontracted collagen matrices developed stress fibers and lamellipodial extensions and required microtubules for polarization but not spreading. Our findings demonstrate an unexpected relationship between the role of microtubules in cell spreading and the tension state of cell-matrix interactions. At a low tension state (absence of stress fibers and focal adhesions) typical of fibroblasts in relaxed collagen matrices, cells spread with dendritic extensions whose formation requires microtubules; at a high tension state (stress fibers and focal adhesions) typical of cells on coverslips, cells spread with lamellipodial extensions and microtubules are required for cell polarization but not for spreading.

Distinct contributions of microtubule subtypes to cell membrane shape and stability

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

Changes in cholesterol levels in the plasma membrane modulate cell signaling and regulate cell adhesion and migration on fibronectin

The number and distribution of lipid molecules, including cholesterol in particular, in the plasma membrane, may play a key role in regulating several physiological processes in cells. We investigated the role of membrane cholesterol in regulating cell shape, adhesion and motility. The acute depletion of cholesterol from the plasma membrane of cells that were well spread and motile on fibronectin caused the rounding of these cells and decreased their adhesion to and motility on fibronectin. These modifications were less pronounced in cells plated on laminin, vitronectin or plastic, indicating that cholesterol-mediated changes in adhesion and motility are more specific for adhesion mediated by fibronectin-specific integrins, such as alpha5beta1. These changes were accompanied by remodeling of the actin cytoskeleton, the spatial reorganization of paxillin in the membrane, and changes to the dynamics of alpha5 integrin and paxillin-rich focal adhesions. Levels of tyrosine phosphorylation at position 576/577 of FAK and Erk1/Erk2 MAP-kinase activity levels were both lower in cholesterol-depleted than in control cells. These levels normalized only on fibronectin when cholesterol was reincorporated into the cell membrane. Thus, membrane cholesterol content has a specific effect on certain signaling pathways specifically involved in regulating cell motility on fibronectin and organization of the actin cytoskeleton.

On the mechanism of wing size determination in fly development

A fundamental and unresolved problem in animal development is the question of how a growing tissue knows when it has achieved its correct final size. A widely held view suggests that this process is controlled by morphogen gradients, which adapt to tissue size and become flatter as tissue grows, leading eventually to growth arrest. Here, we present evidence that the decapentaplegic (Dpp) morphogen distribution in the developing Drosophila wing imaginal disk does not adapt to disk size. We measure the distribution of a functional Dpp-GFP transgene and the Dpp signal transduced by phospho-Mad and show that the characteristic length scale of the Dpp profile remains approximately constant during growth. This finding suggests an alternative scenario of size determination, where disk size is determined relative to the fixed morphogen distribution by a certain threshold level of morphogen required for growth. We propose that when disk boundary reaches the threshold the arrest of cell proliferation throughout the disk is induced by mechanical stress in the tissue. Mechanical stress is expected to arise from the nonuniformity of morphogen distribution that drives growth. This stress, through a negative feedback on growth, can compensate for the nonuniformity of morphogen, achieving uniform growth with the rate that vanishes when disk boundary reaches the threshold. The mechanism is demonstrated through computer simulations of a tissue growth model that identifies the key assumptions and testable predictions. This analysis provides an alternative hypothesis for the size determination process. Novel experimental approaches will be needed to test this model.

Cartilage tissue engineering and bioreactor systems for the cultivation and stimulation of chondrocytes

Damage to and degeneration of articular cartilage is a major health issue in industrialized nations. Articular cartilage has a particularly limited capacity for auto regeneration. At present, there is no established therapy for a sufficiently reliable and durable replacement of damaged articular cartilage. In this, as well as in other areas of regenerative medicine, tissue engineering methods are considered to be a promising therapeutic component. Nevertheless, there remain obstacles to the establishment of tissue-engineered cartilage as a part of the routine therapy for cartilage defects. One necessary aspect of potential tissue engineering-based therapies for cartilage damage that requires both elucidation and progress toward practical solutions is the reliable, cost effective cultivation of suitable tissue. Bioreactors and associated methods and equipment are the tools with which it is hoped that such a supply of tissue-engineered cartilage can be provided. The fact that in vivo adaptive physical stimulation influences chondrocyte function by affecting mechanotransduction leads to the development of specifically designed bioreactor devices that transmit forces like shear, hydrostatic pressure, compression, and combinations thereof to articular and artificial cartilage in vitro. This review summarizes the basic knowledge of chondrocyte biology and cartilage dynamics together with the exploration of the various biophysical principles of cause and effect that have been integrated into bioreactor systems for the cultivation and stimulation of chondrocytes.

Nanotopographical stimulation of mechanotransduction and changes in interphase centromere positioning

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

Cellular chemomechanics at interfaces: sensing, integration and response

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

Dimensions of systems biology

Systems biology, possibly the latest sub-discipline of biology, has arisen as a result of the shockwave of genomic and proteomic data that has appeared in the past few years. However, despite ubiquitous initiatives that carry this label, there is no precise definition of systems biology other than the implication of a new, all-encompassing, multidisciplinary endeavor. Here we propose that systems biology is more than the integration of biology with methods of the physical and computational sciences, and also more than the expansion of the single-pathway approach to embracing genome-scale networks. It is the discipline that specifically addresses the fundamental properties of the complexity that living systems represent. To facilitate the discussion, we dissect and project the multifaceted systems complexity of living organisms into five dimensions: (1) molecular complexity; (2) structural complexity; (3) temporal complexity; (4) abstraction and emergence; and (5) algorithmic complexity. This "five-dimensional space" may provide a framework for comparing, classifying, and complementing the vast diversity of existing systems biology programs and their goals, and will also give a glimpse of the magnitude of the scientific problems associated with unraveling the ultimate mysteries of life.

Nanomechanotransduction and Interphase Nuclear Organization influence on genomic control

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