Compression-induced changes in the shape and volume of the chondrocyte nucleus

Computational investigation of in situ chondrocyte deformation and actin cytoskeleton remodelling under physiological loading

Previous experimental studies have determined local strain fields for both healthy and degenerate cartilage tissue during mechanical loading. However, the biomechanical response of chondrocytes in situ, in particular the response of the actin cytoskeleton to physiological loading conditions, is poorly understood. In the current study a three-dimensional (3-D) representative volume element (RVE) for cartilage tissue is created, comprising a chondrocyte surrounded by a pericellular matrix and embedded in an extracellular matrix. A 3-D active modelling framework incorporating actin cytoskeleton remodelling and contractility is implemented to predict the biomechanical behaviour of chondrocytes. Physiological and abnormal strain fields, based on the experimental study of Wong and Sah (J. Orthop. Res. 2010; 28: 1554-1561), are applied to the RVE. Simulations demonstrate that the presence of a focal defect significantly affects cellular deformation, increases the stress experienced by the nucleus, and alters the distribution of the actin cytoskeleton. It is demonstrated that during dynamic loading cyclic tension reduction in the cytoplasm causes continuous dissociation of the actin cytoskeleton. In contrast, during static loading significant changes in cytoplasm tension are not predicted and hence the rate of dissociation of the actin cytoskeleton is reduced. It is demonstrated that chondrocyte behaviour is affected by the stiffness of the pericellular matrix, and also by the anisotropy of the extracellular matrix. The findings of the current study are of particular importance in understanding the biomechanics underlying experimental observations such as actin cytoskeleton dissociation during the dynamic loading of chondrocytes.

Cell deformation behavior in mechanically loaded rabbit articular cartilage 4 weeks after anterior cruciate ligament transection

OBJECTIVE

Chondrocyte stresses and strains in articular cartilage are known to modulate tissue mechanobiology. Cell deformation behavior in cartilage under mechanical loading is not known at the earliest stages of osteoarthritis. Thus, the aim of this study was to investigate the effect of mechanical loading on volume and morphology of chondrocytes in the superficial tissue of osteoarthritic cartilage obtained from anterior cruciate ligament transected (ACLT) rabbit knee joints, 4 weeks after intervention.

METHODS

A unique custom-made microscopy indentation system with dual-photon microscope was used to apply controlled 2 MPa force-relaxation loading on patellar cartilage surfaces. Volume and morphology of chondrocytes were analyzed before and after loading. Also global and local tissue strains were calculated. Collagen content, collagen orientation and proteoglycan content were quantified with Fourier transform infrared microspectroscopy, polarized light microscopy and digital densitometry, respectively.

RESULTS

Following the mechanical loading, the volume of chondrocytes in the superficial tissue increased significantly in ACLT cartilage by 24% (95% confidence interval (CI) 17.2-31.5, P < 0.001), while it reduced significantly in contralateral group tissue by -5.3% (95% CI -8.1 to -2.5, P = 0.003). Collagen content in ACLT and contralateral cartilage were similar. PG content was reduced and collagen orientation angle was increased in the superficial tissue of ACLT cartilage compared to the contralateral cartilage.

CONCLUSIONS

We found the novel result that chondrocyte deformation behavior in the superficial tissue of rabbit articular cartilage is altered already at 4 weeks after ACLT, likely because of changes in collagen fibril orientation and a reduction in PG content.

The effect of moving point of contact stimulation on chondrocyte gene expression and localization in tissue engineered constructs

Tissue engineering is a promising approach for articular cartilage repair. However, using current technologies, the developed engineered constructs generally do not possess an organized superficial layer, which contributes to the tissue's durability and unique mechanical properties. In this study, we investigated the efficacy of applying a moving point of contract-type stimulation (MPS) to stimulate the production of a superficial-like layer in the engineered constructs. MPS was applied to chondrocyte-agarose hydrogels at a frequency of 0.5, 1 or 2 Hz, under a constant compressive load of 10 mN for durations between 5 and 60 min over 3 consecutive days. Expression and localization of superficial zone constituents was conducted by qRT-PCR and in situ hybridization. Finite element modeling was also constructed to gain insight into the relationship between the applied stimulus and superficial zone constituent expression. Gene expression of superficial zone markers were affected in a frequency dependent manner with a physiologic frequency of 1 Hz producing maximal expression of PRG4, biglycan, decorin and collagen II. In situ hybridization revealed that localization of these markers predominantly occurred at 500-1000 μm below the construct surface which correlated to sub-surface strains between 10 and 25% as determined by finite element modeling. These results indicate that while mechanical stimuli can be used to enhance the expression of superficial zone constituents in engineered cartilage constructs, the resultant subsurface loading is a critical factor for localizing expression. Future studies will investigate altering the applied stimulus to further localize superficial zone constituent expression at the construct surface.

Computational aspects in mechanical modeling of the articular cartilage tissue

This review focuses on the modeling of articular cartilage (at the tissue level), chondrocyte mechanobiology (at the cell level) and a combination of both in a multiscale computation scheme. The primary objective is to evaluate the advantages and disadvantages of conventional models implemented to study the mechanics of the articular cartilage tissue and chondrocytes. From monophasic material models as the simplest form to more complicated multiscale theories, these approaches have been frequently used to model articular cartilage and have contributed significantly to modeling joint mechanics, addressing and resolving numerous issues regarding cartilage mechanics and function. It should be noted that attentiveness is important when using different modeling approaches, as the choice of the model limits the applications available. In this review, we discuss the conventional models applicable to some of the mechanical aspects of articular cartilage such as lubrication, swelling pressure and chondrocyte mechanics and address some of the issues associated with the current modeling approaches. We then suggest future pathways for a more realistic modeling strategy as applied for the simulation of the mechanics of the cartilage tissue using multiscale and parallelized finite element method.

Chondrocyte deformation under extreme tissue strain in two regions of the rabbit knee joint

Articular cartilage and its native cells-chondrocytes-are exposed to a wide range of mechanical loading. Chondrocytes are responsible for maintaining the cartilage matrix, yet relatively little is known regarding their behavior under a complete range of mechanical loads or how cell mechanics are affected by region within the joint. The purpose of this study was to investigate chondrocyte deformations in situ under tissue loads ranging from physiological to extreme (0-80% nominal strain) in two regions of the rabbit knee joint (femoral condyles and patellae). Local matrix strains and cell compressive strains increased with increasing loads. At low loads the extracellular matrix (ECM) strains in the superficial zone were greater than the applied tissue strains, while at extreme loads, the local ECM strains were smaller than the applied strains. Cell compressive strains were always smaller than the applied tissue strains and, in our intact, in situ preparation, were substantially smaller than those previously found in hemi-cylindrical explants. This resulted in markedly different steady-state cell volume changes in the current study compared to those working with cartilage explants. Additionally, cells from different regions in the knee exhibited striking differences in deformation behavior under load. The current results suggest: (i) that the local extracellular and pericellular matrix environment is intimately linked to chondrocyte mechanobiology, protecting chondrocytes from potentially damaging strains at high tissue loads; and (ii) that cell mechanics are a function of applied load and local cartilage tissue structure.

Potassium channels in articular chondrocytes

Chondrocytes are the resident cells of cartilage, which synthesize and maintain the extracellular matrix. The range of known potassium channels expressed by these unique cells is continually increasing. Since chondrocytes are non-excitable, and do not need to be repolarized following action potentials, the function of potassium channels in these cells has, until recently, remained completely unknown. However, recent advances in both traditional physiology and “omic” technologies have enhanced our knowledge and understanding of the chondrocyte channelome. A large number of potassium channels have been identified and a number of putative, but credible, functions have been proposed. Members of each of the potassium channel sub-families (calcium activated, inward rectifier, voltage-gated and tandem pore) have all been identified. Mechanotransduction, cell volume regulation, apoptosis and chondrogenesis all appear to involve potassium channels. Since evidence suggests that potassium channel gene transcription is altered in osteoarthritis, future studies are needed that investigate potassium channels as potential cellular biomarkers and therapeutic targets for treatment of degenerative joint conditions.

Continuum modeling of a neuronal cell under blast loading

Traumatic brain injuries have recently been put under the spotlight as one of the most important causes of accidental brain dysfunctions. Significant experimental and modeling efforts are thus underway to study the associated biological, mechanical and physical mechanisms. In the field of cell mechanics, progress is also being made at the experimental and modeling levels to better characterize many of the cell functions, including differentiation, growth, migration and death. The work presented here aims to bridge both efforts by proposing a continuum model of a neuronal cell submitted to blast loading. In this approach, the cytoplasm, nucleus and membrane (plus cortex) are differentiated in a representative cell geometry, and different suitable material constitutive models are chosen for each one. The material parameters are calibrated against published experimental work on cell nanoindentation at multiple rates. The final cell model is ultimately subjected to blast loading within a complete computational framework of fluid-structure interaction. The results are compared to the nanoindentation simulation, and the specific effects of the blast wave on the pressure and shear levels at the interfaces are identified. As a conclusion, the presented model successfully captures some of the intrinsic intracellular phenomena occurring during the cellular deformation under blast loading that potentially lead to cell damage. It suggests, more particularly, that the localization of damage at the nucleus membrane is similar to what has already been observed at the overall cell membrane. This degree of damage is additionally predicted to be worsened by a longer blast positive phase duration. In conclusion, the proposed model ultimately provides a new three-dimensional computational tool to evaluate intracellular damage during blast loading.

The effect of remodelling and contractility of the actin cytoskeleton on the shear resistance of single cells: a computational and experimental investigation

The biomechanisms that govern the response of chondrocytes to mechanical stimuli are poorly understood. In this study, a series of in vitro tests are performed, in which single chondrocytes are subjected to shear deformation by a horizontally moving probe. Dramatically different probe force-indentation curves are obtained for untreated cells and for cells in which the actin cytoskeleton has been disrupted. Untreated cells exhibit a rapid increase in force upon probe contact followed by yielding behaviour. Cells in which the contractile actin cytoskeleton was removed exhibit a linear force-indentation response. In order to investigate the mechanisms underlying this behaviour, a three-dimensional active modelling framework incorporating stress fibre (SF) remodelling and contractility is used to simulate the in vitro tests. Simulations reveal that the characteristic force-indentation curve observed for untreated chondrocytes occurs as a result of two factors: (i) yielding of SFs due to stretching of the cytoplasm near the probe and (ii) dissociation of SFs due to reduced cytoplasm tension at the front of the cell. In contrast, a passive hyperelastic model predicts a linear force-indentation curve similar to that observed for cells in which the actin cytoskeleton has been disrupted. This combined modelling-experimental study offers a novel insight into the role of the active contractility and remodelling of the actin cytoskeleton in the response of chondrocytes to mechanical loading.

Multiscale mechanics of articular cartilage: potentials and challenges of coupling musculoskeletal, joint, and microscale computational models

Articular cartilage experiences significant mechanical loads during daily activities. Healthy cartilage provides the capacity for load bearing and regulates the mechanobiological processes for tissue development, maintenance, and repair. Experimental studies at multiple scales have provided a fundamental understanding of macroscopic mechanical function, evaluation of the micromechanical environment of chondrocytes, and the foundations for mechanobiological response. In addition, computational models of cartilage have offered a concise description of experimental data at many spatial levels under healthy and diseased conditions, and have served to generate hypotheses for the mechanical and biological function. Further, modeling and simulation provides a platform for predictive risk assessment, management of dysfunction, as well as a means to relate multiple spatial scales. Simulation-based investigation of cartilage comes with many challenges including both the computational burden and often insufficient availability of data for model development and validation. This review outlines recent modeling and simulation approaches to understand cartilage function from a mechanical systems perspective, and illustrates pathways to associate mechanics with biological function. Computational representations at single scales are provided from the body down to the microstructure, along with attempts to explore multiscale mechanisms of load sharing that dictate the mechanical environment of the cartilage and chondrocytes.

Mechanical regulation of nuclear structure and function

Mechanical loading induces both nuclear distortion and alterations in gene expression in a variety of cell types. Mechanotransduction is the process by which extracellular mechanical forces can activate a number of well-studied cytoplasmic signaling cascades. Inevitably, such signals are transduced to the nucleus and induce transcription factor-mediated changes in gene expression. However, gene expression also can be regulated through alterations in nuclear architecture, providing direct control of genome function. One putative transduction mechanism for this phenomenon involves alterations in nuclear architecture that result from the mechanical perturbation of the cell. This perturbation is associated with direct mechanical strain or osmotic stress, which is transferred to the nucleus. This review describes the current state of knowledge relating the nuclear architecture and the transfer of mechanical forces to the nucleus mediated by the cytoskeleton, the nucleoskeleton, and the LINC (linker of the nucleoskeleton and cytoskeleton) complex. Moreover, remodeling of the nucleus induces alterations in nuclear stiffness, which may be associated with cell differentiation. These phenomena are discussed in relation to the potential influence of nuclear architecture-mediated mechanoregulation of transcription and cell fate.

Expression of Transient Receptor Potential Vanilloid (TRPV) channels in different passages of articular chondrocytes

Ion channels play important roles in chondrocyte mechanotransduction. The transient receptor potential vanilloid (TRPV) subfamily of ion channels consists of six members. TRPV1-4 are temperature sensitive calcium-permeable, relatively non-selective cation channels whereas TRPV5 and TRPV6 show high selectivity for calcium over other cations. In this study we investigated the effect of time in culture and passage number on the expression of TRPV4, TRPV5 and TRPV6 in articular chondrocytes isolated from equine metacarpophalangeal joints. Polyclonal antibodies raised against TRPV4, TRPV5 and TRPV6 were used to compare the expression of these channels in lysates from first expansion chondrocytes (P0) and cells from passages 1–3 (P1, P2 and P3) by western blotting. TRPV4, TRPV5 and TRPV6 were expressed in all passages examined. Immunohistochemistry and immunofluorescence confirmed the presence of these channels in sections of formalin fixed articular cartilage and monolayer cultures of methanol fixed P2 chondrocytes. TRPV5 and TRPV6 were upregulated with time and passage in culture suggesting that a shift in the phenotype of the cells in monolayer culture alters the expression of these channels. In conclusion, several TRPV channels are likely to be involved in calcium signaling and homeostasis in chondrocytes.

Cytoskeletal dissolution blocks oxidant release and cell death in injured cartilage

The mechanisms by which articular surface impact causes post-traumatic osteoarthritis are not well understood, but studies of cartilage explants implicate the mitochondrial electron transport chain as a source of oxidants that cause chondrocyte death from mechanical injury. The linkage of mitochondria to the cytoskeleton suggests that they might release oxidants in response to mechanical strain, an effect that disrupting the cytoskeleton would prevent. To test this we investigated the effects of agents that promote the dissolution of microfilaments (cytochalasin B) or microtubules (nocodazole) on oxidant production and chondrocyte death following impact injury. Osteochondral explants treated with cytochalasin B or nocodazole for 4 h were impacted (7 J/cm(2)) and stained for oxidant production directly after impact and for cell viability 24 h after impact. Surfaces within and outside impact sites were then imaged by confocal microscopy. Both agents significantly reduced impact-induced oxidant release (p < 0.05); however, cytochalasin B was more effective than nocodazole (>60% reduction vs. 40% reduction, respectively). Both agents also prevented impact induced cell death. Dissolution of the cytoskeleton by both drugs was confirmed by phalloidin staining and confocal microscopy. These findings show that chondrocyte mortality from impact injury depends substantially on mitochondrial-cytoskeletal linkage, suggesting new approaches to stem mechanically induced cartilage degeneration.

Multiscale strain analysis of tissue equivalents using a custom-designed biaxial testing device

Mechanical signals transferred between a cell and its extracellular matrix play an important role in regulating fundamental cell behavior. To further define the complex mechanical interactions between cells and matrix from a multiscale perspective, a biaxial testing device was designed and built. Finite element analysis was used to optimize the cruciform specimen geometry so that stresses within the central region were concentrated and homogenous while minimizing shear and grip effects. This system was used to apply an equibiaxial loading and unloading regimen to fibroblast-seeded tissue equivalents. Digital image correlation and spot tracking were used to calculate three-dimensional strains and associated strain transfer ratios at macro (construct), meso, matrix (collagen fibril), cell (mitochondria), and nuclear levels. At meso and matrix levels, strains in the 1- and 2-direction were statistically similar throughout the loading-unloading cycle. Interestingly, a significant amplification of cellular and nuclear strains was observed in the direction perpendicular to the cell axis. Findings indicate that strain transfer is dependent upon local anisotropies generated by the cell-matrix force balance. Such multiscale approaches to tissue mechanics will assist in advancement of modern biomechanical theories as well as development and optimization of preconditioning regimens for functional engineered tissue constructs.

Transport phenomena in articular cartilage cryopreservation as predicted by the modified triphasic model and the effect of natural inhomogeneities

Knowledge of the spatial and temporal distribution of cryoprotective agent (CPA) is necessary for the cryopreservation of articular cartilage. Cartilage dehydration and shrinkage, as well as the change in extracellular osmolality, may have a significant impact on chondrocyte survival during and after CPA loading, freezing, and thawing, and during CPA unloading. In the literature, Fick's law of diffusion is commonly used to predict the spatial distribution and overall concentration of the CPA in the cartilage matrix, and the shrinkage and stress-strain in the cartilage matrix during CPA loading are neglected. In this study, we used a previously described biomechanical model to predict the spatial and temporal distributions of CPA during loading. We measured the intrinsic inhomogeneities in initial water and fixed charge densities in the cartilage using magnetic resonance imaging and introduced them into the model as initial conditions. We then compared the prediction results with the results obtained using uniform initial conditions. The simulation results in this study demonstrate the presence of a significant mechanical strain in the matrix of the cartilage, within all layers, during CPA loading. The osmotic response of the chondrocytes to the cartilage dehydration during CPA loading was also simulated. The results reveal that a transient shrinking occurs to different levels, and the chondrocytes experience a significant decrease in volume, particularly in the middle and deep zones of articular cartilage, during CPA loading.

Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.

QUANTIFICATION OF CHONDROCYTE MORPHOLOGY BY CONFOCAL ARTHROSCOPY

The purpose of this study was to determine the effectiveness of a novel Laser Scanning Confocal Arthroscope (LSCA) for the morphological quantification of articular cartilage chondrocytes.

Healthy and debrided regions of the knee articular cartilage of six (6) New Zealand White rabbits were imaged during open follow-up surgery. Quantitative morphological analysis of chondrocyte cell populations was performed and compared to known parameters. Optical histology images were compared to conventional histology of similar sites.

Optical histology revealed viable cells in normal hyaline cartilage tissue and enabled the visualization of fibro-cartilage in defect tissue. Morphological analysis was able to characterize the in vivo two-dimensional equivalent-area-diameter of chondrocytes. Significant differences (P<0.05) were seen between the morphology of chondrocytes observed in optical and conventional histology.

This study concludes that the LSCA is capable of illustrating the surface and sub-surface appearance of healthy and defect articular cartilage, thereby providing a non-destructive method for assessing cartilage condition in vivo. In this role the LSCA may find application in the investigation of cartilage pathologies or repair techniques.

HISTOLOGICAL ASSESSMENT OF THE CHONDRAL AND CONNECTIVE TISSUES OF THE KNEE BY CONFOCAL ARTHROSCOPE

Conventional histological assessment of tissue requires the removal of tissue from its physiological environment for pathological processing. In this study we report on the development and application of a laser scanning confocal arthroscope (LSCA) capable of in situ histological assessment by confocal microscopy.

The knee joints of three euthanasied adult Merino sheep were imaged using the LSCA and a combination of fluorescent contrasting agents. LSCA images of various tissues were examined for common histological characteristics.

The confocal histology images from the ovine knee acquired during the course of our study illustrated some of the major histological features of chondral and connective tissues.

Our study demonstrates the efficacy of the LSCA for the histological assessment of the chondral and connective tissues of the ovine knee. The LSCA provides an easy and rapid method for assessing the in situ structure of cartilage, muscle, tendon, ligament, meniscus and synovium in their native and unaltered physiological environment. By obviating the need for mechanical biopsy, the LSCA provides a method for the non-destructive assessment of the chondral and connective tissues of the knee, and may be of great benefit in the investigation of orthopedic diseases or in the assessment of newly emerging cartilage repair techniques.

Topographic modulation of the orientation and shape of cell nuclei and their influence on the measured elastic modulus of epithelial cells

The influence of nucleus shape and orientation on the elastic modulus of epithelial cells was investigated with atomic force microscopy. The shape and orientation were controlled by presenting the epithelial cells with anisotropic parallel ridges and grooves of varying pitch at the cell substratum. As the cells oriented to the underlying topography, the volume of the nucleus increased as the pitch of the topography increased from 400 nm to 2000 nm. The increase in nucleus volume was reflected by an increase in the measured elastic modulus of the topographically aligned cells. Significant alterations in the shape of the nucleus, by intimate contact with the topographic ridge and grooves of the underlying cell, were also observed via confocal microscopy, indicating that the nucleus may also act as a direct mechanosensor of substratum topography.

Cyclic stretch increases splicing noise rate in cultured human fibroblasts

Background

Mechanical forces are known to alter the expression of genes, but it has so far not been reported whether they may influence the fidelity of nucleus-based processes. One experimental approach permitting to address this question is the application of cyclic stretch to cultured human fibroblasts. As a marker for the precision of nucleus-based processes, the number of errors that occur during co-transcriptional splicing can then be measured. This so-called splicing noise is found at low frequency in pre-mRNA splicing.

Findings

The amount of splicing noise was measured by RT-qPCR of seven exon skips from the test genes AATF, MAP3K11, NF1, PCGF2, POLR2A and RABAC1. In cells treated by altered uniaxial cyclic stretching for 18 h, a uniform and significant increase of splicing noise was found for all detectable exon skips.

Conclusion

Our data demonstrate that application of cyclic stretch to cultured fibroblasts correlates with a reduced transcriptional fidelity caused by increasing splicing noise.

Remodeling of actin cytoskeleton in mouse periosteal cells under mechanical loading induces periosteal cell proliferation during bone formation

Background

The adaptive nature of bone formation under mechanical loading is well known; however, the molecular and cellular mechanisms in vivo of mechanical loading in bone formation are not fully understood. To investigate both mechanisms at the early response against mechanotransduction in vivo, we employed a noninvasive 3-point bone bending method for mouse tibiae. It is important to investigate periosteal woven bone formation to elucidate the adaptive nature against mechanical stress. We hypothesize that cell morphological alteration at the early stage of mechanical loading is essential for bone formation in vivo.

Principal Findings

We found the significant bone formation on the bone surface subjected to change of the stress toward compression by this method. The histological analysis revealed the proliferation of periosteal cells, and we successively observed the appearance of ALP-positive osteoblasts and increase of mature BMP-2, resulting in woven bone formation in the hypertrophic area. To investigate the mechanism underlying the response to mechanical loading at the molecular level, we established an in-situ immunofluorescence imaging method to visualize molecules in these periosteal cells, and with it examined their cytoskeletal actin and nuclei and the extracellular matrix proteins produced by them. The results demonstrated that the actin cytoskeleton of the periosteal cells was disorganized, and the shapes of their nuclei were drastically changed, under the mechanical loading. Moreover, the disorganized actin cytoskeleton was reorganized after release from the load. Further, inhibition of onset of the actin remodeling blocked the proliferation of the periosteal cells.

Conclusions

These results suggest that the structural change in cell shape via disorganization and remodeling of the actin cytoskeleton played an important role in the mechanical loading-dependent proliferation of cells in the periosteum during bone formation.

Mechanical behavior of human embryonic stem cell pellet under unconfined compression

As a prelude to the understanding of mechanotransduction in human embryonic stem cell (hESC) differentiation, the mechanical behavior of hESCs in the form of cell pellet is studied. The pellets were tested after 3 or 5 weeks of cell culture in order to demonstrate the effect of the duration of cell culture on the mechanical properties of the pellets. A micromechanical tester was used to conduct unconfined compression on hESC pellet, and experimental, numerical, and analytical methods were combined to determine the mechanical properties of hESC pellet. It is assumed that the mechanical behavior of hESC pellets can be described by an isotropic, linear viscoelastic model consisting of a spring and two Maxwell units in parallel, and the Poisson's ratio of the hESC pellet is constant based on pellet deformation in the direction perpendicular to the compression direction. Finite element method (FEM) simulation was adopted to determine the values of Poisson's ratio and the five parameters contained in the viscoelastic model. The variations of Poisson's ratio and the initial elastic modulus are found to be larger compared with those of the four other parameters. Results show that longer duration of cell culture leads to higher modulus of hESC pellet. The effect of pellet size error on the values of mechanical parameters determined is studied using FEM simulation, and it is found that the effect of size error on Poisson's ratio and initial elastic modulus is much larger than that on the other parameters.

Osmotic stress alters chromatin condensation and nucleocytoplasmic transport

Osmotic stress is a potent regulator of biological function in many cell types, but its mechanism of action is only partially understood. In this study, we examined whether changes in extracellular osmolality can alter chromatin condensation and the rate of nucleocytoplasmic transport, as potential mechanisms by which osmotic stress can act. Transport of 10 kDa dextran was measured both within and between the nucleus and the cytoplasm using two different photobleaching methods. A mathematical model was developed to describe fluorescence recovery via nucleocytoplasmic transport. As osmolality increased, the diffusion coefficient of dextran decreased in the cytoplasm, but not the nucleus. Hyper-osmotic stress decreased nuclear size and increased nuclear lacunarity, indicating that while the nucleus was getting smaller, the pores and channels interdigitating the chromatin had expanded. The rate of nucleocytoplasmic transport was increased under hyper-osmotic stress but was insensitive to hypo-osmotic stress, consistent with the nonlinear osmotic properties of the nucleus. The mechanism of this osmotic sensitivity appears to be a change in the size and geometry of the nucleus, resulting in a shorter effective diffusion distance for the nucleus. These results may explain physical mechanisms by which osmotic stress can influence intracellular signaling pathways that rely on nucleocytoplasmic transport.

Cartilage viability and catabolism in the intact porcine knee following transarticular impact loading with and without articular fracture

Posttraumatic arthritis commonly develops following articular fracture. The objective of this study was to develop a closed joint model of transarticular impact with and without creation of an articular fracture that maintains the physiologic environment during loading. Fresh intact porcine knees were preloaded and impacted at 294 J via a drop track. Osteochondral cores were obtained from the medial and lateral aspects of the femoral condyles and tibial plateau. Chondrocyte viability was assessed at days 0, 3, and 5 postimpact in sham, impacted nonfractured, and impacted fractured joints. Total matrix metalloproteinase (MMP) activity, aggrecanase (ADAMTS-4) activity, and sulfated glycosaminoglycan (S-GAG) release were measured in culture media from days 3 and 5 posttrauma. No differences were observed in chondrocyte viability of impacted nonfractured joints (95.9 ± 6.9%) when compared to sham joints (93.8 ± 7.7%). In impacted fractured joints, viability of the fractured edge was 40.5 ± 27.6% and significantly lower than all other sites, including cartilage adjacent to the fractured edge (p < 0.001). MMP and aggrecanase activity and S-GAG release were significantly increased in specimens from the fractured edge. This study showed that joint impact resulting in articular fracture significantly decreased chondrocyte viability, increased production of MMPs and aggrecanases, and enhanced S-GAG release, whereas the same level of impact without fracture did not cause such changes.

A systems biology approach to cancer: fractals, attractors, and nonlinear dynamics

Cancer begins to be recognized as a highly complex disease, and advanced knowledge of the carcinogenic process claims to be acquired by means of supragenomic strategies. Experimental data evidence that tumor emerges from disruption of tissue architecture, and it is therefore consequential that the tissue level should be considered the proper level of observation for carcinogenic studies. This paradigm shift imposes to move from a reductionistic to a systems biology approach. Indeed, cell phenotypes are emergent modes arising through collective nonlinear interactions among different cellular and microenvironmental components, generally described by a phase space diagram, where stable states (attractors) are embedded into a landscape model. Within this framework cell states and cell transitions are generally conceived as mainly specified by the gene-regulatory network. However, the system's dynamics cannot be reduced to only the integrated functioning of the genome-proteome network, and the cell-stroma interacting system must be taken into consideration in order to give a more reliable picture. As cell form represents the spatial geometric configuration shaped by an integrated set of cellular and environmental cues participating in biological functions control, it is conceivable that fractal-shape parameters could be considered as "omics" descriptors of the cell-stroma system. Within this framework it seems that function follows form, and not the other way around.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Nuclear mechanics during cell migration

During cell migration, the movement of the nucleus must be coordinated with the cytoskeletal dynamics at the leading edge and trailing end, and, as a result, undergoes complex changes in position and shape, which in turn affects cell polarity, shape, and migration efficiency. We here describe the steps of nuclear positioning and deformation during cell polarization and migration, focusing on migration through three-dimensional matrices. We discuss molecular components that govern nuclear shape and stiffness, and review how nuclear dynamics are connected to and controlled by the actin, tubulin and intermediate cytoskeleton-based migration machinery and how this regulation is altered in pathological conditions. Understanding the regulation of nuclear biomechanics has important implications for cell migration during tissue regeneration, immune defence and cancer.

Tissue and cellular morphological changes in growth plate explants under compression

The mechanisms by which mechanical loading may alter bone development within growth plates are still poorly understood. However, several growth plate cell or tissue morphological parameters are associated with both normal and mechanically modulated bone growth rates. The aim of this study was to quantify in situ the three-dimensional morphology of growth plate explants under compression at both cell and tissue levels. Growth plates were dissected from ulnae of immature swine and tested under 15% compressive strain. Confocal microscopy was used to image fluorescently labeled chondrocytes in the three growth plate zones before and after compression. Quantitative morphological analyses at both cell (volume, surface area, sphericity, minor/major radii) and tissue (cell/matrix volume ratio) levels were performed. Greater chondrocyte bulk strains (volume decrease normalized to the initial cell volume) were found in the proliferative (35.4%) and hypertrophic (41.7%) zones, with lower chondrocyte bulk strains (24.7%) in the reserve zone. Following compression, the cell/matrix volume ratio decreased in the reserve and hypertrophic zones by 24.3% and 22.6%, respectively, whereas it increased by 35.9% in the proliferative zone. The 15% strain applied on growth plate explants revealed zone-dependent deformational states at both tissue and cell levels. Variations in the mechanical response of the chondrocytes from different zones could be related to significant inhomogeneities in growth plate zonal mechanical properties. The ability to obtain in situ cell morphometry and monitor the changes under compression will contribute to a better understanding of mechanisms through which abnormal growth can be triggered.

Mechano-topographic modulation of stem cell nuclear shape on nanofibrous scaffolds

Stem cells transit along a variety of lineage-specific routes towards differentiated phenotypes. These fate decisions are dependent not just on the soluble chemical cues that are encountered or enforced in vivo and in vitro, but also on physical cues from the cellular microenvironment. These physical cues can consist of both nano- and micro-scale topographical features, as well as mechanical inputs provided passively (from the base properties of the materials to which they adhere) or actively (from extrinsic applied mechanical deformations). A suitable tool to investigate the coordination of these cues lies in nanofibrous scaffolds, which can both dictate cellular and cytoskeletal orientation and facilitate mechanical perturbation of seeded cells. Here, we demonstrate a coordinated influence of scaffold architecture (aligned vs. randomly organized fibers) and tensile deformation on nuclear shape and orientation. Sensitivity of nuclear morphology to scaffold architecture was more pronounced in stem cell populations than in terminally differentiated fibrochondrocytes. Tension applied to the scaffold elicited further alterations in nuclear morphology, greatest in stem cells, that were mediated by the filamentous actin cytoskeleton, but not the microtubule or intermediate filament network. Nuclear perturbations were time and direction dependent, suggesting that the modality and direction of loading influenced nuclear architecture. The present work may provide additional insight into the mechanisms by which the physical microenvironment influences cell fate decisions, and has specific application to the design of new materials for regenerative medicine applications with adult stem cells.

The role of chromatin structure in cell migration

Chromatin dynamics play a major role in regulating genetic processes. Now, accumulating data suggest that chromatin structure may also affect the mechanical properties of the nucleus and cell migration. Global chromatin organization appears to modulate the shape, the size and the stiffness of the nucleus. Directed-cell migration, which often requires nuclear reshaping to allow passage of cells through narrow openings, is dependent not only on changes in cytoskeletal elements but also on global chromatin condensation. Conceivably, during cell migration a physical link between the chromatin and the cytoskeleton facilitates coordinated structural changes in these two components. Thus, in addition to regulating genetic processes, we suggest that alterations in chromatin structure could facilitate cellular reorganizations necessary for efficient migration.

Mechanical stress as a regulator of cytoskeletal contractility and nuclear shape in embryonic epithelia

The mechano-sensitive responses of the heart and brain were examined in the chick embryo during Hamburger and Hamilton stages 10-12. During these early stages of development, cells in these structures are organized into epithelia. Isolated hearts and brains were compressed by controlled amounts of surface tension (ST) at the surface of the sample, and microindentation was used to measure tissue stiffness following several hours of culture. The response of both organs was qualitatively similar, as they stiffened under reduced loading. With increased loading, however, the brain softened while heart stiffness was similar to controls. In the brain, changes in nuclear shape and morphology correlated with these responses, as nuclei became more elliptical with decreased loading and rounder with increased loading. Exposure to the myosin inhibitor blebbistatin indicated that these changes in stiffness and nuclear shape are likely caused by altered cytoskeletal contraction. Computational modeling suggests that this behavior tends to return peak tissue stress back toward the levels it has in the intact heart and brain. These results suggest that developing cardiac and neural epithelia respond similarly to changes in applied loads by altering contractility in ways that tend to restore the original mechanical stress state. Hence, this study supports the view that stress-based mechanical feedback plays a role in regulating epithelial development.

Calcium regulates cyclic compression-induced early changes in chondrocytes during in vitro cartilage tissue formation

A single application of cyclic compression (1kPa, 1Hz, 30min) to bioengineered cartilage results in improved tissue formation through sequential catabolic and anabolic changes mediated via cell shape changes that are regulated by α5β1 integrin and membrane-type metalloprotease (MT1-MMP). To determine if calcium was involved in this process, the role of calcium in regulating cell shape changes, MT1-MMP expression and integrin activity in response to mechanical stimulation was examined. Stimulation-induced changes in cell shape and MT1-MMP expression were abolished by chelation of extracellular calcium, and this effect was reversed by re-introduction of calcium. Spreading was inhibited by blocking stretch-activated channels (with gadolinium), while retraction was prevented by blocking the L-Type voltage-gated channel (with nifedipine); both compounds inhibited MT1-MMP upregulation. Calcium A23187 ionophore restored cellular response further supporting a role for these channels. Calcium regulated the integrin-mediated signalling pathway, which was facilitated through Src kinase. Both calcium- and integrin-mediated pathways converged on ERK-MAPK in response to stimulation. While both integrins and calcium signalling mediate chondrocyte mechanotransduction, calcium appears to play the major regulatory role. Understanding the underlying molecular mechanisms involved in chondrocyte mechanotransduction may lead to the development of improved bioengineered cartilage.

Matrix accumulation by articular chondrocytes during mechanical stimulation is influenced by integrin-mediated cell spreading

We have shown previously that cyclic compression of newly forming bioengineered cartilage in vitro results in improved tissue formation via changes in expression of matrix metalloproteases, such as, MT1-MMP (membrane type metalloprotease), and increased synthesis of matrix molecules. Several studies have suggested an association between MT1-MMP and integrins, which are known to influence cell shape. Thus, the objectives of this study were to determine the effect of compressive mechanical stimulation on cell shape and the role of integrins and MT1-MMP in mediating these changes and influencing matrix accumulation. Bovine articular chondrocytes were grown on the surface of a porous ceramic substrate for 72 h and then cyclically compressed for 30 min. Scanning electron microscopy and morphometric analysis demonstrated that compression induced a rapid, transient increase in chondrocyte spreading by 10 min, followed by a retraction to prestimulated size within 6 h. This was associated with increased accumulation of newly synthesized proteoglycans, as determined by quantification of radioisotope incorporation. Blocking the alpha5beta1 integrin, or its beta1 subunit, inhibited cell spreading and resulted in a partial inhibition of compression-induced increase in matrix accumulation. Knockdown of MT1-MMP expression partially inhibited cell retraction and resulted in a reduced matrix accumulation as well. These results suggest that chondrocyte spreading and retraction following cyclic compression in vitro regulates matrix accumulation. Understanding the mechanisms that regulate chondrocyte mechanotransduction may ultimately lead to the design of improved repair tissue for cartilage damage. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Nuclear morphology and deformation in engineered cardiac myocytes and tissues

Cardiac tissue engineering requires finely-tuned manipulation of the extracellular matrix (ECM) microenvironment to optimize internal myocardial organization. The myocyte nucleus is mechanically connected to the cell membrane via cytoskeletal elements, making it a target for the cellular response to perturbation of the ECM. However, the role of ECM spatial configuration and myocyte shape on nuclear location and morphology is unknown. In this study, printed ECM proteins were used to configure the geometry of cultured neonatal rat ventricular myocytes. Engineered one- and two-dimensional tissue constructs and single myocyte islands were assayed using live fluorescence imaging to examine nuclear position, morphology and motion as a function of the imposed ECM geometry during diastolic relaxation and systolic contraction. Image analysis showed that anisotropic tissue constructs cultured on microfabricated ECM lines possessed a high degree of nuclear alignment similar to that found in vivo; nuclei in isotropic tissues were polymorphic in shape with an apparently random orientation. Nuclear eccentricity was also increased for the anisotropic tissues, suggesting that intracellular forces deform the nucleus as the cell is spatially confined. During systole, nuclei experienced increasing spatial confinement in magnitude and direction of displacement as tissue anisotropy increased, yielding anisotropic deformation. Thus, the nature of nuclear displacement and deformation during systole appears to rely on a combination of the passive myofibril spatial organization and the active stress fields induced by contraction. Such findings have implications in understanding the genomic consequences and functional response of cardiac myocytes to their ECM surroundings under conditions of disease.

Hyperosmotic Stimulus Down-regulates 1alpha, 25-dihydroxyvitamin D(3)-induced Osteoclastogenesis by Suppressing the RANKL Expression in a Co-culture System

The hyperosmotic stimulus is regarded as a mechanical factor for bone remodeling. However, whether the hyperosmotic stimulus affects 1alpha, 25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3))-induced osteoclastogenesis is not clear. In the present study, the effect of the hyperosmotic stimulus on 1alpha,25(OH)(2)D(3)-induced osteoclastogenesis was investigated in an osteoblast-preosteoclast co-culture system. Serial doses of sucrose were applied as a mechanical force. These hyperosmotic stimuli significantly evoked a reduced number of 1alpha,25(OH)(2)D(3)-induced tartrate-resistant acid phosphatase-positive multinucleated cells and 1alpha,25(OH)(2)D(3)-induced bone-resorbing pit area in a co-culture system. In osteoblastic cells, receptor activator of nuclear factor kappaB ligand (RANKL) and Runx2 expressions were down-regulated in response to 1alpha,25(OH)(2)D(3). Knockdown of Runx2 inhibited 1alpha,25(OH)(2)D(3)-induced RANKL expression in osteoblastic cells. Finally, the hyperosmotic stimulus induced the overexpression of TonEBP in osteoblastic cells. These results suggest that hyperosmolarity leads to the down-regulation of 1alpha,25(OH)(2)D(3)-induced osteoclastogenesis, suppressing Runx2 and RANKL expression due to the TonEBP overexpression in osteoblastic cells.

Biomechanical properties of single chondrocytes and chondrons determined by micromanipulation and finite-element modelling

A chondrocyte and its surrounding pericellular matrix (PCM) are defined as a chondron. Single chondrocytes and chondrons isolated from bovine articular cartilage were compressed by micromanipulation between two parallel surfaces in order to investigate their biomechanical properties and to discover the mechanical significance of the PCM. The force imposed on the cells was measured directly during compression to various deformations and then holding. When the nominal strain at the end of compression was 50 per cent, force relaxation showed that the cells were viscoelastic, but this viscoelasticity was generally insignificant when the nominal strain was 30 per cent or lower. The viscoelastic behaviour might be due to the mechanical response of the cell cytoskeleton and/or nucleus at higher deformations. A finite-element analysis was applied to simulate the experimental force-displacement/time data and to obtain mechanical property parameters of the chondrocytes and chondrons. Because of the large strains in the cells, a nonlinear elastic model was used for simulations of compression to 30 per cent nominal strain and a nonlinear viscoelastic model for 50 per cent. The elastic model yielded a Young's modulus of 14 ± 1 kPa (mean ± s.e.) for chondrocytes and 19 ± 2 kPa for chondrons, respectively. The viscoelastic model generated an instantaneous elastic modulus of 21 ± 3 and 27 ± 4 kPa, a long-term modulus of 9.3 ± 0.8 and 12 ± 1 kPa and an apparent viscosity of 2.8 ± 0.5 and 3.4 ± 0.6 kPa s for chondrocytes and chondrons, respectively. It was concluded that chondrons were generally stiffer and showed less viscoelastic behaviour than chondrocytes, and that the PCM significantly influenced the mechanical properties of the cells.

Metabolism and cell shape in cancer: a fractal analysis

Fractal analysis in cancer cell investigation provided meaningful insights into the relationship between morphology and phenotype. Some reports demonstrated that changes in cell shape precede and trigger dramatic modifications in both gene expression and enzymatic function. Nonetheless, metabolomic pattern in cells undergoing shape changes have been not still reported. Our study was aimed to investigate if modifications in cancer cell morphology are associated to relevant transition in tumour metabolome, analyzed by nuclear magnetic resonance spectroscopy and principal component analysis. MCF-7 and MDA-MB-231 breast cancer cells, exposed to an experimental morphogenetic field, undergo a dramatic change in their membrane profiles. Both cell lines recover a more rounded shape, loosing spindle and invasive protrusions, acquiring a quite "normal" morphology. This result, quantified by fractal analysis, shows that normalized bending energy (a global shape characterization expressing the amount of energy needed to transform a specific shape into its lowest energy state) decreases after 48 h. Later on, a significant shift from a high to a low glycolytic phenotype was observed on both cell lines: glucose flux begins to drop off at 48 h, leading to reduced lactate accumulation, and fatty acids and citrate synthesis slow-down after 72 h. Moreover, de novo lipidogenesis is inhibited and nucleotide synthesis is reduced, as indicated by the positive correlation between glucose and formate. In conclusion, these data indicate that the reorganization of cell membrane architecture, induced by environmental cues, is followed by a relevant transition of the tumour metabolome, suggesting cells undergo a dramatic phenotypic reversion.

Characterization of a stretch-activated potassium channel in chondrocytes

Chondrocytes possess the capacity to transduce load-induced mechanical stimuli into electrochemical signals. The aim of this study was to functionally characterize an ion channel activated in response to membrane stretch in isolated primary equine chondrocytes. We used patch-clamp electrophysiology to functionally characterize this channel and immunohistochemistry to examine its distribution in articular cartilage. In cell-attached patch experiments, the application of negative pressures to the patch pipette (in the range of 20-200 mmHg) activated ion channel currents in six of seven patches. The mean activated current was 45.9 +/- 1.1 pA (n = 4) at a membrane potential of 33 mV (cell surface area approximately 240 microm(2)). The mean slope conductance of the principal single channels resolved within the total stretch-activated current was 118 +/- 19 pS (n = 6), and reversed near the theoretical potassium equilibrium potential, E(K+), suggesting it was a high-conductance potassium channel. Activation of these high-conductance potassium channels was inhibited by extracellular TEA (K(d) approx. 900 microM) and iberiotoxin (K(d) approx. 40 nM). This suggests that the current was largely carried by BK-like potassium (MaxiK) channels. To further characterize these BK-like channels, we used inside-out patches of chondrocyte membrane: we found these channels to be activated by elevation in bath calcium concentration. Immunohistochemical staining of equine cartilage samples with polyclonal antibodies to the alpha1- and beta1-subunits of the BK channel revealed positive immunoreactivity for both subunits in superficial zone chondrocytes. These experiments support the hypothesis that functional BK channels are present in chondrocytes and may be involved in mechanotransduction and chemotransduction.

Deformation of stem cell nuclei by nanotopographical cues

Cells sense cues in their surrounding microenvironment. These cues are converted into intracellular signals and transduced to the nucleus in order for the cell to respond and adapt its function. Within the nucleus, structural changes occur that ultimately lead to changes in the gene expression. In this study, we explore the structural changes of the nucleus of human mesenchymal stem cells as an effect of topographical cues. We use a controlled nanotopography to drive shape changes to the cell nucleus, and measure the changes with both fluorescence microscopy and a novel light scattering technique. The nucleus changes shape dramatically in response to the nanotopography, and in a manner dependent on the mechanical properties of the substrate. The kinetics of the nuclear deformation follows an unexpected trajectory. As opposed to a gradual shape change in response to the topography, once the cytoskeleton attains an aligned and elongation morphology on the time scale of several hours, the nucleus changes shape rapidly and intensely.

Cyclic loading opens hemichannels to release ATP as part of a chondrocyte mechanotransduction pathway

The process of chondrocyte mechanotransduction is poorly understood. However, recent studies suggest the involvement of a purinergic calcium signaling pathway although the mechanism of ATP release has not been identified. The present study tests the hypothesis that cyclic compression opens hemichannels thereby triggering the release of ATP into the extracellular milieu activating P2 receptors. The well-established chondrocyte-agarose model was utilized enabling chondrocytes to be subjected to a 40-min period of cyclic compression at 0-15% strain and 1 Hz. The opening of hemichannels was determined using Lucifer yellow (LY) incorporation and fluorescence microscopy, whereas the release of ATP into the surrounding media was quantified using the luciferin-luciferase assay. Results indicated that cyclic compression activated hemichannels such that the percentage of cells showing LY incorporation increased from 50 to 70%. This was associated with a sevenfold increase in the release of ATP. Both LY incorporation and ATP release in response to mechanical loading were blocked by the hemichannel inhibitor, flufenamic acid. Treatment with apyrase or P2 receptor antagonists, suramin or oxidated-ATP, did not prevent the mechanically induced response. In conclusion, mechanical loading triggers release of ATP via hemichannels. Hence, this study provides the first evidence of hemichannel involvement in chondrocyte mechanobiology.

Tissue stretch induces nuclear remodeling in connective tissue fibroblasts

Studies in cultured cells have shown that nuclear shape is an important factor influencing nuclear function, and that mechanical forces applied to the cell can directly affect nuclear shape. In a previous study, we demonstrated that stretching of whole mouse subcutaneous tissue causes dynamic cytoskeletal remodeling with perinuclear redistribution of alpha-actin in fibroblasts within the tissue. We have further shown that the nuclei of these fibroblasts have deep invaginations containing alpha-actin. In the current study, we hypothesized that tissue stretch would cause nuclear remodeling with a reduced amount of nuclear invagination, measurable as a change in nuclear concavity. Subcutaneous areolar connective tissue samples were excised from 28 mice and randomized to either tissue stretch or no stretch for 30 min, then examined with histochemistry and confocal microscopy. In stretched tissue (vs. non-stretched), fibroblast nuclei had a larger cross-sectional area (P < 0.001), smaller thickness (P < 0.03) in the plane of the tissue, and smaller relative concavity (P < 0.005) indicating an increase in nuclear convexity. The stretch-induced loss of invaginations may have important influences on gene expression, RNA trafficking and/or cell differentiation.

The effects of osmotic stress on the structure and function of the cell nucleus

Osmotic stress is a potent regulator of the normal function of cells that are exposed to osmotically active environments under physiologic or pathologic conditions. The ability of cells to alter gene expression and metabolic activity in response to changes in the osmotic environment provides an additional regulatory mechanism for a diverse array of tissues and organs in the human body. In addition to the activation of various osmotically- or volume-activated ion channels, osmotic stress may also act on the genome via a direct biophysical pathway. Changes in extracellular osmolality alter cell volume, and therefore, the concentration of intracellular macromolecules. In turn, intracellular macromolecule concentration is a key physical parameter affecting the spatial organization and pressurization of the nucleus. Hyper-osmotic stress shrinks the nucleus and causes it to assume a convoluted shape, whereas hypo-osmotic stress swells the nucleus to a size that is limited by stretch of the nuclear lamina and induces a smooth, round shape of the nucleus. These behaviors are consistent with a model of the nucleus as a charged core/shell structure pressurized by uneven partition of macromolecules between the nucleoplasm and the cytoplasm. These osmotically-induced alterations in the internal structure and arrangement of chromatin, as well as potential changes in the nuclear membrane and pores are hypothesized to influence gene transcription and/or nucleocytoplasmic transport. A further understanding of the biophysical and biochemical mechanisms involved in these processes would have important ramifications for a range of fields including differentiation, migration, mechanotransduction, DNA repair, and tumorigenesis.

Contribution of the cytoskeleton to the compressive properties and recovery behavior of single cells

The cytoskeleton is known to play an important role in the biomechanical nature and structure of cells, but its particular function in compressive characteristics has not yet been fully examined. This study focused on the contribution of the main three cytoskeletal elements to the bulk compressive stiffness (as measured by the compressive modulus), volumetric or apparent compressibility changes (as further indicated by apparent Poisson's ratio), and recovery behavior of individual chondrocytes. Before mechanical testing, cytochalasin D, acrylamide, or colchicine was used to disrupt actin microfilaments, intermediate filaments, or microtubules, respectively. Cells were subjected to a range of compressive strains and allowed to recover to equilibrium. Analysis of the video recording for each mechanical event yielded relevant compressive properties and recovery characteristics related to the specific cytoskeletal disrupting agent and as a function of applied axial strain. Inhibition of actin microfilaments had the greatest effect on bulk compressive stiffness ( approximately 50% decrease compared to control). Meanwhile, intermediate filaments and microtubules were each found to play an integral role in either the diminution (compressibility) or retention (incompressibility) of original cell volume during compression. In addition, microtubule disruption had the largest effect on the "critical strain threshold" in cellular mechanical behavior (33% decrease compared to control), as well as the characteristic time for recovery ( approximately 100% increase compared to control). Elucidating the role of the cytoskeleton in the compressive biomechanical behavior of single cells is an important step toward understanding the basis of mechanotransduction and the etiology of cellular disease processes.

Characterization of cell mechanical properties by computational modeling of parallel plate compression

A substantial body of work has been reported in which the mechanical properties of adherent cells were characterized using compression testing in tandem with computational modeling. However, a number of important issues remain to be addressed. In the current study, using computational analyses, the effect of cell compressibility on the force required to deform spread cells is investigated and the possibility that stiffening of the cell cytoplasm occurs during spreading is examined based on published experimental compression test data. The effect of viscoelasticity on cell compression is considered and difficulties in performing a complete characterization of the viscoelastic properties of a cell nucleus and cytoplasm by this method are highlighted. Finally, a non-linear force-deformation response is simulated using differing linear viscoelastic properties for the cell nucleus and the cell cytoplasm.