The effects of osmotic stress on the viscoelastic and physical properties of articular chondrocytes

Using changes in hydrostatic and osmotic pressure to manipulate metabolic function in chondrocytes

Articular cartilage has distinct histological depth zones. In each zone, chondrocytes are subject to different hydrostatic (HP) and osmotic pressure (OP) due to weight-bearing and joint-loading. Previous in vitro studies of regeneration and pathophysiology in cartilage have failed to consider the characteristics of histological heterogeneity and the effects of combinations of changes in HP and OP. Thus, we have constructed molecular, biochemical, and histological profiles of anabolic and catabolic molecules produced by chondrocytes from each depth zone isolated from bovine articular cartilage in response to changes in HP and OP. We cultured the chondrocytes with combinations of loading or off-loading of HP at 0-0.5 MPa, 0.5 Hz, and changes in OP of 300-450 mosM over 1 wk, and evaluated mRNA expression and immunohistology of both anabolic and catabolic molecules and amounts of accumulated sulfated glycosaminoglycan. Any changes in HP and OP upregulated mRNA of anabolic and catabolic molecules in surface-, middle-, and deep-zone cells, in descending order of magnitude. Off-loading HP maintained the anabolic and reduced the catabolic mRNA; high OP retained upregulation of catabolic mRNA. These molecular profiles were consistent with immunohistological and biochemical findings. Changes in HP and OP are essential for simulating chondrocyte physiology and useful for manipulating phenotypes.

Nature of sterols affects plasma membrane behavior and yeast survival during dehydration

The plasma membrane (PM) is a main site of injury during osmotic perturbation. Sterols, major lipids of the PM structure in eukaryotes, are thought to play a role in ensuring the stability of the lipid bilayer during physicochemical perturbations. Here, we investigated the relationship between the nature of PM sterols and resistance of the yeast Saccharomyces cerevisiae to hyperosmotic treatment. We compared the responses to osmotic dehydration (viability, sterol quantification, ultrastructure, cell volume, and membrane permeability) in the wild-type (WT) strain and the ergosterol mutant erg6Δ strain. Our main results suggest that the nature of membrane sterols governs the mechanical behavior of the PM during hyperosmotic perturbation. The mutant strain, which accumulates ergosterol precursors, was more sensitive to osmotic fluctuations than the WT, which accumulates ergosterol. The hypersensitivity of erg6Δ was linked to modifications of the membrane properties, such as stretching resistance and deformation, which led to PM permeabilization during the volume variation during the dehydration-rehydration cycles. Anaerobic growth of erg6Δ strain with ergosterol supplementation restored resistance to osmotic treatment. These results suggest a relationship between hydric stress resistance and the nature of PM sterols. We discuss this relationship in the context of the evolution of the ergosterol biosynthetic pathway.

Response of the cell membrane-cytoskeleton complex to osmotic and freeze/thaw stresses

In order to develop successful cryopreservation protocols a better understanding of the freeze- and dehydration-induced changes occurring in the cell membrane and its underlying support, the actin cytoskeleton, is required. In this study, we compared the biophysical response of model mammalian cells (human foreskin fibroblasts) to hyperosmotic stress and freeze/thaw. Transmitted light, infrared spectroscopy, fluorescence- and cryo-microscopy were used to investigate the changes in the cell membrane and the actin cytoskeleton. We observed that a purely hyperosmotic challenge at room temperature resulted in bleb formation. A decrease in temperature abrogated the blebbing behavior, but was accompanied by a decrease in viability. These results suggested that cell survival depended on the availability of the membrane material to accommodate the volumetric expansion back to the original cell volume at isotonic conditions. Our data also showed that freeze/thaw stresses altered the cell membrane morphology resulting in a loss of membrane material. There was also a significantly lower incidence of blebbing after freeze/thaw as compared to isothermal osmotic stress experiments at room temperature. Significant depolymerization of the actin cytoskeleton was seen in cells whose membranes had been compromised by freeze/thaw stresses. Actin depolymerization using cytochalasin D affected the stability of the membrane against mechanical stress at isothermal conditions. This study shows that both the membrane and cytoskeleton, as a system, are involved in the osmotic and freeze/thaw-induced responses of the mammalian 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.

Cellular deformation and intracellular stress propagation during optical stretching

Experiments have shown that mechanical stress can regulate many cellular processes. However, in most cases, the exact regulatory mechanisms are still not well understood. One approach in improving our understanding of such mechanically induced regulation is the quantitative study of cell deformation under an externally applied stress. In this paper, an axisymmetric finite-element model is developed and used to study the deformation of single, suspended fibroblasts in an optical stretcher in which a stretching force is applied onto the surface of the cell. A feature of our physical model is a viscoelastic material equation whose parameters vary spatially to mimic the experimentally observed spatial heterogeneity of cellular material properties. Our model suggests that cell size is a more important factor in determining the maximal strain of the optically stretched fibroblasts compared to the thickness of the actin cortical region. This result could explain the higher deformability observed experimentally for malignant fibroblasts in the optical stretcher. Our model also shows that maximal stress propagates into the nuclear region for malignant fibroblasts whereas for normal fibroblasts, it does not. We discuss how this may impact the transduction of cancer signaling pathways.

Power-law rheology analysis of cells undergoing micropipette aspiration

Accurate quantification of the mechanical properties of living cells requires the combined use of experimental techniques and theoretical models. In this paper, we investigate the viscoelastic response of suspended NIH 3T3 fibroblasts undergoing micropipette aspiration using power-law rheology model. As an important first step, we examine the pipette size effect on cell deformation and find that pipettes larger than ~7 μm are more suitable for bulk rheological measurements than smaller ones and the cell can be treated as effectively continuum. When the large pipettes are used to apply a constant pressure to a cell, the creep deformation is better fitted with the power-law rheology model than with the liquid drop or spring-dashpot models; magnetic twisting cytometry measurement on the rounded cell confirms the power-law behavior. This finding is further extended to suspended cells treated with drugs targeting their cytoskeleton. As such, our results suggest that the application of relatively large pipettes can provide more effective assessment of the bulk material properties as well as support application of power-law rheology to cells in suspension.

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.

Influence of the partitioning of osmolytes by the cytoplasm on the passive response of cells to osmotic loading

Due to the dense organization of organelles, cytoskeletal elements, and protein complexes that make up the intracellular environment, it is likely that membrane-permeant solutes may be excluded from a fraction of the interstitial space of the cytoplasm via steric restrictions, electrostatic interactions, and other long-range intermolecular forces. This study investigates the hypothesis that the intracellular partitioning of membrane-permeant solutes manifests itself as a partial volume recovery in response to hyperosmotic loading, based on prior theoretical and biomimetic experimental studies. Osmotic loading experiments are performed on immature bovine chondrocytes using culture conditions where regulatory volume responses are shown to be insignificant. Osmotic loading with membrane-permeant glycerol (92 Da) and urea (60 Da) are observed to produce partial volume recoveries consistent with the proposed hypothesis, whereas loading with 1,2-propanediol (76 Da) produces complete volume recovery. Combining these experimental results with the previous theoretical framework produces a measure for the intracellular partition coefficient of each of these solutes. At 1000 mOsm, 1,2-propanediol is the only osmolyte to yield a partition coefficient not statistically different from unity, kappa(p)(i) = 1.00 +/- 0.02. For glycerol, the partition coefficient increases with osmolarity from kappa(p)(i) = 0.48 +/- 0.19 at 200 mOsm to kappa(p)(i) = 0.80 +/- 0.07 at 1000 mOsm; urea exhibits no such dependence, with an average value of kappa(p)(i) = 0.87 +/- 0.07 for all osmolarities from 200 to 1000 mOsm. The finding that intracellular partitioning of membrane-permeant solutes manifests itself as a partial volume recovery under osmotic loading offers a simple method for characterizing the partition coefficient. These measurements suggest that significant partitioning may occur even for small membrane-permeant osmolytes. Furthermore, a positive correlation is observed, suggesting that a solute's cytoplasmic partition coefficient increases with increasing hydrophobicity.

Lateral reorganization of plasma membrane is involved in the yeast resistance to severe dehydration

In this study, we investigated the kinetic and the magnitude of dehydrations on yeast plasma membrane (PM) modifications because this parameter is crucial to cell survival. Functional (permeability) and structural (morphology, ultrastructure, and distribution of the protein Sur7-GFP contained in sterol-rich membrane microdomains) PM modifications were investigated by confocal and electron microscopy after progressive (non-lethal) and rapid (lethal) hyperosmotic perturbations. Rapid cell dehydration induced the formation of many PM invaginations followed by membrane internalization of low sterol content PM regions with time. Permeabilization of the plasma membrane occurred during the rehydration stage because of inadequacies in the membrane surface and led to cell death. Progressive dehydration conducted to the formation of some big PM pleats without membrane internalization. It also led to the modification of the distribution of the Sur7-GFP microdomains, suggesting that a lateral rearrangement of membrane components occurred. This event is a function of time and is involved in the particular deformations of the PM during a progressive perturbation. The maintenance of the repartition of the microdomains during rapid perturbations consolidates this assumption. These findings highlight that the perturbation kinetic influences the evolution of the PM organization and indicate the crucial role of PM lateral reorganization in cell survival to hydric perturbations.

Quantitative phase microscopy of articular chondrocyte dynamics by wide-field digital interferometry

We experimentally implement label-free phase microscopy using wide-field digital interferometry (WFDI) techniques to retrieve quantitative volumetric data of articular chondrocyte dynamics. Using the scanless interferometric system, we visualize chondrocyte swelling and bursting induced by hypo-osmotic pressure. Reconstructed images are obtained by an efficient digital process. We use the resulting images to calculate quantitative temporal-spatial morphological parameters of the cell, with the observed dynamics limited only by the true frame rate of the camera. To show the utility of WFDI in recording articular chondrocyte dynamics, we also provide an experimental comparison of WFDI and differential interference contrast microscopy.

Functional characterization of TRPV4 as an osmotically sensitive ion channel in porcine articular chondrocytes

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable channel that can be gated by tonicity (osmolarity) and mechanical stimuli. Chondrocytes, the cells in cartilage, respond to their osmotic and mechanical environments; however, the molecular basis of this signal transduction is not fully understood. This study was undertaken to demonstrate the presence and functionality of TRPV4 in chondrocytes.

METHODS

TRPV4 protein expression was measured by immunolabeling and Western blotting. In response to TRPV4 agonist/antagonists, osmotic stress, and interleukin-1 (IL-1), changes in Ca(2+) signaling, cell volume, and prostaglandin E(2) (PGE(2)) production were measured in porcine chondrocytes using fluorescence microscopy, light microscopy, or immunoassay, respectively.

RESULTS

TRPV4 was expressed abundantly at the RNA and protein levels. Exposure to 4alpha-phorbol 12,13-didecanoate (4alphaPDD), a TRPV4 activator, caused Ca(2+) signaling in chondrocytes, which was blocked by the selective TRPV4 antagonist, GSK205. Blocking TRPV4 diminished the chondrocytes' response to hypo-osmotic stress, reducing the fraction of Ca(2+) responsive cells, the regulatory volume decrease, and PGE(2) production. Ca(2+) signaling was inhibited by removal of extracellular Ca(2+) or depletion of intracellular stores. Specific activation of TRPV4 restored the defective regulatory volume decrease caused by IL-1. Chemical disruption of the primary cilium eliminated Ca(2+) signaling in response to either 4alphaPDD or hypo-osmotic stress.

CONCLUSION

Our findings indicate that TRPV4 is present in articular chondrocytes, and chondrocyte response to hypo-osmotic stress is mediated by this channel, which involves both an extracellular Ca(2+) and intracellular Ca(2+) release. TRPV4 may also be involved in modulating the production or influence of proinflammatory molecules in response to osmotic stress.

Composition of the pericellular matrix modulates the deformation behaviour of chondrocytes in articular cartilage under static loading

The aim was to assess the role of the composition changes in the pericellular matrix (PCM) for the chondrocyte deformation. For that, a three-dimensional finite element model with depth-dependent collagen density, fluid fraction, fixed charge density and collagen architecture, including parallel planes representing the split-lines, was created to model the extracellular matrix (ECM). The PCM was constructed similarly as the ECM, but the collagen fibrils were oriented parallel to the chondrocyte surfaces. The chondrocytes were modelled as poroelastic with swelling properties. Deformation behaviour of the cells was studied under 15% static compression. Due to the depth-dependent structure and composition of cartilage, axial cell strains were highly depth-dependent. An increase in the collagen content and fluid fraction in the PCMs increased the lateral cell strains, while an increase in the fixed charge density induced an inverse behaviour. Axial cell strains were only slightly affected by the changes in PCM composition. We conclude that the PCM composition plays a significant role in the deformation behaviour of chondrocytes, possibly modulating cartilage development, adaptation and degeneration. The development of cartilage repair materials could benefit from this information.

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.

Universal behavior of the osmotically compressed cell and its analogy to the colloidal glass transition

Mechanical robustness of the cell under different modes of stress and deformation is essential to its survival and function. Under tension, mechanical rigidity is provided by the cytoskeletal network; with increasing stress, this network stiffens, providing increased resistance to deformation. However, a cell must also resist compression, which will inevitably occur whenever cell volume is decreased during such biologically important processes as anhydrobiosis and apoptosis. Under compression, individual filaments can buckle, thereby reducing the stiffness and weakening the cytoskeletal network. However, the intracellular space is crowded with macromolecules and organelles that can resist compression. A simple picture describing their behavior is that of colloidal particles; colloids exhibit a sharp increase in viscosity with increasing volume fraction, ultimately undergoing a glass transition and becoming a solid. We investigate the consequences of these 2 competing effects and show that as a cell is compressed by hyperosmotic stress it becomes progressively more rigid. Although this stiffening behavior depends somewhat on cell type, starting conditions, molecular motors, and cytoskeletal contributions, its dependence on solid volume fraction is exponential in every instance. This universal behavior suggests that compression-induced weakening of the network is overwhelmed by crowding-induced stiffening of the cytoplasm. We also show that compression dramatically slows intracellular relaxation processes. The increase in stiffness, combined with the slowing of relaxation processes, is reminiscent of a glass transition of colloidal suspensions, but only when comprised of deformable particles. Our work provides a means to probe the physical nature of the cytoplasm under compression, and leads to results that are universal across cell type.

Developmental and osteoarthritic changes in Col6a1-knockout mice: biomechanics of type VI collagen in the cartilage pericellular matrix

OBJECTIVE

Chondrocytes, the sole cell type in articular cartilage, maintain the extracellular matrix (ECM) through a homeostatic balance of anabolic and catabolic activities that are influenced by genetic factors, soluble mediators, and biophysical factors such as mechanical stress. Chondrocytes are encapsulated by a narrow tissue region termed the "pericellular matrix" (PCM), which in normal cartilage is defined by the exclusive presence of type VI collagen. Because the PCM completely surrounds each cell, it has been hypothesized that it serves as a filter or transducer for biochemical and/or biomechanical signals from the cartilage ECM. The present study was undertaken to investigate whether lack of type VI collagen may affect the development and biomechanical function of the PCM and alter the mechanical environment of chondrocytes during joint loading.

METHODS

Col6a1(-/-) mice, which lack type VI collagen in their organs, were generated for use in these studies. At ages 1, 3, 6, and 11 months, bone mineral density (BMD) was measured, and osteoarthritic (OA) and developmental changes in the femoral head were evaluated histomorphometrically. Mechanical properties of articular cartilage from the hip joints of 1-month-old Col6a1(-/-), Col6a1(+/-), and Col6a1(+/+) mice were assessed using an electromechanical test system, and mechanical properties of the PCM were measured using the micropipette aspiration technique.

RESULTS

In Col6a1(-/-) and Col6a1(+/-) mice the PCM was structurally intact, but exhibited significantly reduced mechanical properties as compared with wild-type controls. With age, Col6a1(-/-) mice showed accelerated development of OA joint degeneration, as well as other musculoskeletal abnormalities such as delayed secondary ossification and reduced BMD.

CONCLUSION

These findings suggest that type VI collagen has an important role in regulating the physiology of the synovial joint and provide indirect evidence that alterations in the mechanical environment of chondrocytes, due to either loss of PCM properties or Col6a1(-/-)-derived joint laxity, can lead to progression of OA.

An atomic force microscope operating at hypergravity for in situ measurement of cellular mechano-response

We present a novel atomic force microscope (AFM) system, operational in liquid at variable gravity, dedicated to image cell shape changes of cells in vitro under hypergravity conditions. The hypergravity AFM is realized by mounting a stand-alone AFM into a large-diameter centrifuge. The balance between mechanical forces, both intra- and extracellular, determines both cell shape and integrity. Gravity seems to be an insignificant force at the level of a single cell, in contrast to the effect of gravity on a complete (multicellular) organism, where for instance bones and muscles are highly unloaded under near weightless (microgravity) conditions. However, past space flights and ground based cell biological studies, under both hypogravity and hypergravity conditions have shown changes in cell behaviour (signal transduction), cell architecture (cytoskeleton) and proliferation. Thus the role of direct or indirect gravity effects at the level of cells has remained unclear. Here we aim to address the role of gravity on cell shape. We concentrate on the validation of the novel AFM for use under hypergravity conditions. We find indications that a single cell exposed to 2 to 3 x g reduces some 30-50% in average height, as monitored with AFM. Indeed, in situ measurements of the effects of changing gravitational load on cell shape are well feasible by means of AFM in liquid. The combination provides a promising technique to measure, online, the temporal characteristics of the cellular mechano-response during exposure to inertial forces.

Matrix strains induced by cells: Computing how far cells can feel

Many tissue cells exert contractile forces that mechanically couples them to elastic matrices and that influence cell adhesion, cytoskeletal organization, and even cell differentiation. However, strains within the depths of matrices are often unclear and are likely relevant not only to the fact that some matrices such as so-called basement membranes are thin relative to cell dimensions but also to defining how far cells can 'feel'. Here we briefly present experimental results for cell spreading on thin, ligand-coated gels and for prestress in stem cells in relation to gel stiffness. We then introduce a finite element computation in which a cell is placed on an elastic matrix, while matrix elasticity and thickness are varied in order to compute and compare elastostatic deformations within the matrix. Average interfacial strains between cell and matrix show large deviations only when soft matrices are a fraction of the height and width of a cell, proving consistent with experiments. Three-dimensional (3D) cell morphologies that model stem cell-derived neurons, myoblasts, and osteoblasts show that a cylinder-shaped myoblast induces the highest strains, consistent with the prominent contractility of muscle. Groups of such cells show a weak crosstalk in matrix strains, but the cells must be much closer than a cell-width. Cells thus feel on length scales closer to that of adhesions than on cellular scales or higher.

A neural network model for cell classification based on single-cell biomechanical properties

The potential success of tissue engineering or other cell-based therapies is dependent on factors such as the purity and homogeneity of the source cell populations. The ability to enrich cell harvests for specific phenotypes can have significant effects on the overall success of such therapies. While most techniques for cell sorting or enrichment have relied on cell surface markers, recent studies have shown that single-cell mechanical properties can serve as identifying markers of phenotype. In this study, a neural network modeling approach was developed to classify mesenchymal-derived primary and stem cells based on their biomechanical properties. Cell sorting was simulated using previously published data characterizing the mechanical properties of several different cell types as measured by atomic force microscopy. Neural networks were trained using combined data sets, with the resultant groupings analyzed for their purity, efficiency, and enrichment. Heterogeneous populations of zonal chondrocytes, chondrosarcoma cells, and mesenchymal-lineage cells, respectively, could all be classified into enriched subpopulations. Additionally, adult stem cells (adipose-derived or bone marrow-derived) separated disproportionately into nodes associated with the three primary mesenchymal lineages examined. These findings suggest that mathematical approaches such as neural network modeling, in combination with novel measures of cell properties, may provide a means of classifying and eventually sorting mixed populations of cells that are otherwise difficult to identify using more established techniques. In this respect, the identification of biomechanically based cell properties that increase the percentage of stem cells capable of differentiating into predictable lineages may improve the overall success of cell-based therapies.

Natural variation in embryo mechanics: gastrulation in Xenopus laevis is highly robust to variation in tissue stiffness

How sensitive is morphogenesis to the mechanical properties of embryos? To estimate an upper bound on the sensitivity of early morphogenetic movements to tissue mechanical properties, we assessed natural variability in the apparent stiffness among gastrula-stage Xenopus laevis embryos. We adapted micro-aspiration methods to make repeated, nondestructive measurements of apparent tissue stiffness in whole embryos. Stiffness varied by close to a factor of 2 among embryos within a single clutch. Variation between clutches was of similar magnitude. On the other hand, the direction of change in stiffness over the course of gastrulation was the same in all embryos and in all clutches. Neither pH nor salinity--two environmental factors we predicted could affect variability in nature--affected tissue stiffness. Our results indicate that gastrulation in X. laevis is robust to at least twofold variation in tissue stiffness.

Gene expression profiles of dynamically compressed single chondrocytes and chondrons

A chondrocyte produces a hydrated pericellular matrix (PCM); together they form a chondron. Previous work has shown that the presence of the PCM influences the biological response of chondrocytes to loading. The objective of this study was to determine the gene expression profiles of enzymatically isolated single chondrocytes and chondrons in response to dynamic compression. Cartilage specific extracellular matrix components and transcription factors were examined. Following dynamic compression, chondrocytes and chondrons showed variations in gene expression profiles. Aggrecan, Type II collagen and osteopontin gene expression were significantly increased in chondrons. Lubricin gene expression decreased in both chondrons and chondrocytes. Dynamic compression had no effect on SOX9 gene expression. Our results demonstrate a clear role for the PCM in interfacing the mechanical signalling in chondrocytes in response to dynamic compression. Further investigation of single chondrocytes and chondrons from different zones within articular cartilage may further our understanding of cartilage mechanobiology.

Nonlinear osmotic properties of the cell nucleus

In the absence of active volume regulation processes, cell volume is inversely proportional to osmolarity, as predicted by the Boyle Van't Hoff relation. In this study, we tested the hypothesis that nuclear volume has a similar relationship with extracellular osmolarity in articular chondrocytes, cells that are exposed to changes in the osmotic environment in vivo. Furthermore, we explored the mechanism of the relationships between osmolarity and nuclear size and shape. Nuclear size was quantified using two independent techniques, confocal laser scanning microscopy and angle-resolved low coherence interferometry. Nuclear volume was osmotically sensitive but this relationship was not linear, showing a decline in the osmotic sensitivity in the hypo-osmotic range. Nuclear shape was also influenced by extracellular osmolarity, becoming smoother as the osmolarity decreased. The osmotically induced changes in nuclear size paralleled the changes in nuclear shape, suggesting that shape and volume are interdependent. The osmotic sensitivity of shape and volume persisted after disruption of the actin cytoskeleton. Isolated nuclei contracted in response to physiologic changes in macromolecule concentration but not in response to physiologic changes in ion concentration, suggesting solute size has an important influence on the osmotic pressurization of the nucleus. This finding in turn implies that the diffusion barrier that causes osmotic effects is not a semi-permeable membrane, but rather due to size constraints that prevent large solute molecules from entering small spaces in the nucleus. As nuclear morphology has been associated previously with cell phenotype, these findings may provide new insight into the role of mechanical and osmotic signals in regulating cell physiology.

Interleukin-1 inhibits osmotically induced calcium signaling and volume regulation in articular chondrocytes

OBJECTIVE

Articular chondrocytes respond to osmotic stress with transient changes in cell volume and the intracellular concentration of calcium ion ([Ca(2+)](i)). The goal of this study was to examine the hypothesis that interleukin-1 (IL-1), a pro-inflammatory cytokine associated with osteoarthritis, influences osmotically induced Ca(2+) signaling.

METHODS

Fluorescence ratio imaging was used to measure [Ca(2+)](i) and cell volume in response to hypo- or hyper-osmotic stress in isolated porcine chondrocytes, with or without pre-exposure to 10-ng/ml IL-1alpha. Inhibitors of IL-1 (IL-1 receptor antagonist, IL-1Ra), Ca(2+) mobilization (thapsigargin, an inhibitor of Ca-ATPases), and cytoskeletal remodeling (toxin B, an inhibitor of the Rho family of small GTPases) were used to determine the mechanisms involved in increased [Ca(2+)](i), F-actin remodeling, volume adaptation and active volume recovery.

RESULTS

In response to osmotic stress, chondrocytes exhibited transient increases in [Ca(2+)](i), generally followed by decaying oscillations. Pre-exposure to IL-1 significantly inhibited regulatory volume decrease (RVD) following hypo-osmotic swelling and reduced the change in cell volume and the time to peak [Ca(2+)](i) in response to hyper-osmotic stress, but did not affect the peak magnitudes of [Ca(2+)](i) in those cells that did respond. Co-treatment with IL-1Ra, thapsigargin, or toxin B restored these responses to control levels. The effects were associated with alterations in F-actin organization.

CONCLUSIONS

IL-1 alters the normal volumetric and Ca(2+) signaling response of chondrocytes to osmotic stress through mechanisms involving F-actin remodeling via small Rho GTPases. These findings provide further insights into the mechanisms by which IL-1 may interfere with normal physiologic processes in the chondrocyte, such as the adaptation or regulatory responses to mechanical or osmotic loading.

Dependence of zonal chondrocyte water transport properties on osmotic environment

OBJECTIVE: The increasing concentration of proteoglycans from the surface to the deep zone of articular cartilage produces a depth-dependent gradient in fixed charge density, and therefore extracellular osmolarity, which may vary with loading conditions, growth and development, or disease. In this study we examine the relationship between in situ variations in osmolarity on chondrocyte water transport properties. Chondrocytes from the depth-dependent zones of cartilage, effectively preconditioned in varying osmolarities, were used to probe this relationship. DESIGN: First, depth variation in osmolarity of juvenile bovine cartilage under resting and loaded conditions was characterized using a combined experimental/theoretical approach. Zonal chondrocytes were isolated into two representative "baseline" osmolarities chosen from this analysis to reflect in situ conditions. Osmotic challenge was then used as a tool for determination of water transport properties at each of these baselines. Cell calcium signaling was monitored simultaneously as a preliminary examination of osmotic baseline effects on cell signaling pathways. RESULTS: Osmotic baseline exhibits a significant effect on the cell membrane hydraulic permeability of certain zonal subpopulations but not on cell water content or incidence of calcium signaling. CONCLUSIONS: Chondrocyte properties can be sensitive to changes in baseline osmolarity, such as those occurring during OA progression (decrease) and de novo tissue synthesis (increase). Care should be taken in comparing chondrocyte properties across zones when cells are tested in vitro in non-physiologic culture media.

Atomic force microscopy analysis of cell volume regulation

Cells swell in response a hypoosmotic challenge. By converting osmotic pressure to hydrostatic pressure at the cell membrane via van't Hoff's law, and converting that to tension via Laplace's law one predicts that the cell membrane should stretch and become stiff. We tested this prediction using the atomic force microscopy. During osmotic swelling cells did not become stiff and generally became softer. This result contradicts the assumption of the cell membrane as the constraining element in osmotic stress but is consistent with the cytoskeleton acting as a cross-linked gel. Models of the cells' response to osmotic stress must include energy terms for three-dimensional stresses.

Importance of collagen orientation and depth-dependent fixed charge densities of cartilage on mechanical behavior of chondrocytes

The collagen network and proteoglycan matrix of articular cartilage are thought to play an important role in controlling the stresses and strains in and around chondrocytes, in regulating the biosynthesis of the solid matrix, and consequently in maintaining the health of diarthrodial joints. Understanding the detailed effects of the mechanical environment of chondrocytes on cell behavior is therefore essential for the study of the development, adaptation, and degeneration of articular cartilage. Recent progress in macroscopic models has improved our understanding of depth-dependent properties of cartilage. However, none of the previous works considered the effect of realistic collagen orientation or depth-dependent negative charges in microscopic models of chondrocyte mechanics. The aim of this study was to investigate the effects of the collagen network and fixed charge densities of cartilage on the mechanical environment of the chondrocytes in a depth-dependent manner. We developed an anisotropic, inhomogeneous, microstructural fibril-reinforced finite element model of articular cartilage for application in unconfined compression. The model consisted of the extracellular matrix and chondrocytes located in the superficial, middle, and deep zones. Chondrocytes were surrounded by a pericellular matrix and were assumed spherical prior to tissue swelling and load application. Material properties of the chondrocytes, pericellular matrix, and extracellular matrix were obtained from the literature. The loading protocol included a free swelling step followed by a stress-relaxation step. Results from traditional isotropic and transversely isotropic biphasic models were used for comparison with predictions from the current model. In the superficial zone, cell shapes changed from rounded to elliptic after free swelling. The stresses and strains as well as fluid flow in cells were greatly affected by the modulus of the collagen network. The fixed charge density of the chondrocytes, pericellular matrix, and extracellular matrix primarily affected the aspect ratios (height/width) and the solid matrix stresses of cells. The mechanical responses of the cells were strongly location and time dependent. The current model highlights that the collagen orientation and the depth-dependent negative fixed charge densities of articular cartilage have a great effect in modulating the mechanical environment in the vicinity of chondrocytes, and it provides an important improvement over earlier models in describing the possible pathways from loading of articular cartilage to the mechanical and biological responses of chondrocytes.

Reversible disassembly of the actin cytoskeleton improves the survival rate and developmental competence of cryopreserved mouse oocytes

Effective cryopreservation of oocytes is critically needed in many areas of human reproductive medicine and basic science, such as stem cell research. Currently, oocyte cryopreservation has a low success rate. The goal of this study was to understand the mechanisms associated with oocyte cryopreservation through biophysical means using a mouse model. Specifically, we experimentally investigated the biomechanical properties of the ooplasm prior and after cryopreservation as well as the consequences of reversible dismantling of the F-actin network in mouse oocytes prior to freezing. The study was complemented with the evaluation of post-thaw developmental competence of oocytes after in vitro fertilization. Our results show that the freezing-thawing process markedly alters the physiological viscoelastic properties of the actin cytoskeleton. The reversible depolymerization of the F-actin network prior to freezing preserves normal ooplasm viscoelastic properties, results in high post-thaw survival and significantly improves developmental competence. These findings provide new information on the biophysical characteristics of mammalian oocytes, identify a pathophysiological mechanism underlying cryodamage and suggest a novel cryopreservation method.

Dynamic effects of Hg2+-induced changes in cell volume

Using a microfluidic volume sensor, we studied the dynamic effects of Hg2+ on hypotonic stress-induced volume changes in CHO cells. A hypotonic challenge to control cells caused them to swell but did not evoke a significant regulatory volume decrease (RVD). Treatment with 100 muM HgCl2 caused a substantial increase in the steady-state volume following osmotic stress. Continuous hypotonic challenge following a single 10-min exposure to HgCl2 produced a biphasic volume increase with a steady-state volume 100% larger than control cells. Repeated hypotonic challenges to cells exposed once to Hg2+ resulted in a sequential approach to the same steady-state volume. Stimulation after reaching steady state caused a reduction in peak cell volume. Repeated stimulation was different than continuous stimulation resulting in a more rapid approach to steady state. Substituting extracellular Na+ with impermeant NMDG+ in the hypotonic solution produced a rapid RVD-like volume decrease and eliminated the Hg2+-induced excess swelling. The volume decrease in the presence of Hg2+ was inhibited by tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium, blockers of K+ and Cl(-) channels, respectively, suggesting that part of the Hg2+ effect was increasing NaCl influx over KCl efflux. The presence of multiple phases of steady-state volume and their sensitivity to the stimulation history suggests that factors beyond solute fluxes, such as modification of mechanical stress within the cytoskeleton also plays a role in the response to hypotonic stress.

Label-free, high-throughput measurements of dynamic changes in cell nuclei using angle-resolved low coherence interferometry

Accurate measurements of nuclear deformation, i.e., structural changes of the nucleus in response to environmental stimuli, are important for signal transduction studies. Traditionally, these measurements require labeling and imaging, and then nuclear measurement using image analysis. This approach is time-consuming, invasive, and unavoidably perturbs cellular systems. Light scattering, an emerging biophotonics technique for probing physical characteristics of living systems, offers a promising alternative. Angle-resolved low-coherence interferometry (a/LCI), a novel light scattering technique, was developed to quantify nuclear morphology for early cancer detection. In this study, a/LCI is used for the first time to noninvasively measure small changes in nuclear morphology in response to environmental stimuli. With this new application, we broaden the potential uses of a/LCI by demonstrating high-throughput measurements and by probing aspherical nuclei. To demonstrate the versatility of this approach, two distinct models relevant to current investigations in cell and tissue engineering research are used. Structural changes in cell nuclei due to subtle environmental stimuli, including substrate topography and osmotic pressure, are profiled rapidly without disrupting the cells or introducing artifacts associated with traditional measurements. Accuracy > or = 3% is obtained for the range of nuclear geometries examined here, with the greatest deviations occurring for the more complex geometries. Given the high-throughput nature of the measurements, this deviation may be acceptable for many biological applications that seek to establish connections between morphology and function.

Changes in surface topologies of chondrocytes subjected to mechanical forces: an AFM analysis

The cartilage is composed of chondrocytes embedded in a matrix of collagen fibrils interspersed within a network of proteoglycans and is constantly exposed to biomechanical forces during normal joint movement. Characterization of the surface morphology, cytoskeletal structure, adherance and elastic properties of these mechanosensitive cells are crucial in understanding the effects of mechanical forces around a cell and how a cell responds to changes in its physical environment. In this work, we employed the atomic force microscope (AFM) to image cultured chondrocytes before and after subjecting them to mechanical forces in the presence or absence of interleukin-1beta to mimic inflammatory conditions. Nanoscale imaging and quantitative measurements from AFM data revealed that there are distinct changes in cell-surface topology and cytoskeleton arrangement in the cells following treatment with mechanical forces, IL-1beta or both. Our findings for the first time demonstrate that cultured chondrocytes are amenable to high-resolution AFM imaging and dynamic tensile forces may help overcome the effect of inflammatory factors on chondrocyte response.

Control of chondrocyte regulatory volume decrease (RVD) by [Ca2+]i and cell shape

OBJECTIVES

Optimal matrix metabolism by articular chondrocytes is controlled by the 'set-point' volume which is determined mainly by membrane transporters. The signal transduction pathway(s) for the key membrane transporter which responds to cell swelling ('osmolyte channel') and mediates regulatory volume decrease (RVD) is poorly understood, so here the role of Ca2+ and the effects of 2D culture have been clarified.

METHODS

Changes to the volume and intracellular calcium levels ([Ca2+]i) of freshly isolated and 2D cultured bovine articular chondrocytes subjected to hypotonic challenge using a 43% reduction in medium osmolarity were studied by single-cell fluorescence microscopy. The effects of ethylene glycol tetraacetic acid (EGTA), REV5901 and Gd(3+) were studied and the role of Ca2+ influx determined by Mn2+ quench.

RESULTS

In freshly isolated cells, approximately 50% of chondrocytes exhibited 'robust RVD' (6[120]). RVD was inhibited by REV 5901 (4+/-2% responding) (3[23]) and 2 mM EGTA (18+/-5% responding) (4[166]) whereas Gd3+ had no effect (3[89]). The hypotonic challenge resulted in a Gd3+-insensitive rise in [Ca2+]i that did not correlate with RVD in all cells. Following 2D culture, chondrocytes also demonstrated Gd3+-insensitive RVD, but in contrast, the [Ca2+]i rise was blocked by this agent.

CONCLUSIONS

The data suggested that in freshly isolated and 2D cultured chondrocytes, the rise in [Ca2+]i occurring during hypotonic challenge could be related to RVD, but only in some cells. However, with 2D culture, the Ca2+ response switched to being Gd3+-sensitive, suggesting that as a result of changes to chondrocyte shape, stretch-activated cation channels although present, do not appear to play a role in volume regulation.

Effects of TGF-beta1 and IGF-I on the compressibility, biomechanics, and strain-dependent recovery behavior of single chondrocytes

The responses of articular chondrocytes to physicochemical stimuli are intimately linked to processes that can lead to both degenerative and regenerative processes. Toward understanding this link, we examined the biomechanical behavior of single chondrocytes in response to growth factors (IGF-I and TGF-beta1) and a range of compressive strains. The results indicate that the growth factors alter the biomechanics of the cells in terms of their stiffness coefficient ( approximately two-fold increase over control) and compressibility, as measured by an apparent Poisson's ratio ( approximately two-fold increase over control also). Interestingly, the compressibility decreased significantly with respect to the applied strain. Moreover, we have again detected a critical strain threshold in chondrocytes at approximately 30% strain in all treatments. Overall, these findings demonstrate that cellular biomechanics change in response to both biochemical and biomechanical perturbations. Understanding the underlying biomechanics of chondrocytes in response to such stimuli may be useful in understanding various aspects of cartilage, including the study of osteoarthritis and the development of tissue-engineering strategies.

A new technique for calculating individual dermal fibroblast contractile forces generated within collagen-GAG scaffolds

Cell-mediated contraction plays a critical role in many physiological and pathological processes, notably organized contraction during wound healing. Implantation of an appropriately formulated (i.e., mean pore size, chemical composition, degradation rate) three-dimensional scaffold into an in vivo wound site effectively blocks the majority of organized wound contraction and results in induced regeneration rather than scar formation. Improved understanding of cell contraction within three-dimensional constructs therefore represents an important area of study in tissue engineering. Studies of cell contraction within three-dimensional constructs typically calculate an average contractile force from the gross deformation of a macroscopic substrate by a large cell population. In this study, cellular solids theory has been applied to conventional column buckling relationships to quantify the magnitude of individual cell contraction events within a three-dimensional, collagen-glycosaminoglycan scaffold. This new technique can be used for studying cell mechanics with a wide variety of porous scaffolds that resemble low-density, open-cell foams. It extends previous methods for analyzing cell buckling of two-dimensional substrates to three-dimensional constructs. From data available in the literature, the mean contractile force (Fc) generated by individual dermal fibroblasts within the collagen-glycosaminoglycan scaffold was calculated to range between 11 and 41 nN (Fc=26+/-13 nN, mean+/-SD), with an upper bound of cell contractility estimated at 450 nN.

Spectral domain phase microscopy for local measurements of cytoskeletal rheology in single cells

We present spectral domain phase microscopy (SDPM) as a new tool for measurements at the cellular scale. SDPM is a functional extension of spectral domain optical coherence tomography that allows for the detection of cellular motions and dynamics with nanometer-scale sensitivity in real time. Our goal was to use SDPM to investigate the mechanical properties of the cytoskeleton of MCF-7 cells. Magnetic tweezers were designed to apply a vertical force to ligand-coated magnetic beads attached to integrin receptors on the cell surfaces. SDPM was used to resolve cell surface motions induced by the applied stresses. The cytoskeletal response to an applied force is shown for both normal cells and those with compromised actin networks due to treatment with Cytochalasin D. The cell response data were fit to several models for cytoskeletal rheology, including one- and two-exponential mechanical models, as well as a power law. Finally, we correlated displacement measurements to physical characteristics of individual cells to better compare properties across many cells, reducing the coefficient of variation of extracted model parameters by up to 50%.

Hyperosmotic stress-induced apoptotic signaling pathways in chondrocytes

Articular chondrocytes have a well-developed osmoregulatory system that enables cells to survive in a constantly changing osmotic environment. However, osmotic loading exceeding that occurring under physiological conditions severely compromises chondrocyte function and leads to degenerative changes. The aim of the present study was to investigate the form of cell death and changes in apoptotic signaling pathways under hyperosmotic stress using a primary chondrocyte culture. Cell viability and apoptosis assays performed with annexin V and propidium iodide staining showed that a highly hyperosmotic medium (600 mOsm) severely reduced chondrocyte viability and led mainly to apoptotic cell death, while elevating osmotic pressure within the physiological range caused no changes compared to isosmotic conditions. Western blot analysis revealed that a 600 mOsm hyperosmotic environment induced the activation of proapoptotic members of the mitogen-activated protein kinase family such as c-Jun N-terminal kinase (JNK) and p38, and led to an increased level of extracellular signal regulated kinase (ERK1/2). Hyperosmotic stress also induced the activation of caspase-3. In summary, our results show that hyperosmotic stress leads to mainly apoptotic cell death via the involvement of proapoptotic signaling pathways in a primary chondrocyte culture.

New disposable tubes for rapid and precise biomass assessment for suspension cultures of mammalian cells

We present a new approach for biomass assessment in cell culture using a disposable microcentrifuge tube. The specially designed tube is fitted with an upper chamber for sample loading and a lower 5 microL capillary for cell collection during centrifugation. The resulting packed cell volume (PCV) can be quantitatively expressed as the percentage of the total volume of the sample. The present study focused on the validation of the method with mammalian cell lines that are widely used in bioprocessing. Using several examples, the PCV method was shown to be more precise, rapid, and reproducible than manual cell counting.

A thin-layer model for viscoelastic, stress-relaxation testing of cells using atomic force microscopy: do cell properties reflect metastatic potential?

Atomic force microscopy has rapidly become a valuable tool for quantifying the biophysical properties of single cells. The interpretation of atomic force microscopy-based indentation tests, however, is highly dependent on the use of an appropriate theoretical model of the testing configuration. In this study, a novel, thin-layer viscoelastic model for stress relaxation was developed to quantify the mechanical properties of chondrosarcoma cells in different configurations to examine the hypothesis that viscoelastic properties reflect the metastatic potential and invasiveness of the cell using three well-characterized human chondrosarcoma cell lines (JJ012, FS090, 105KC) that show increasing chondrocytic differentiation and decreasing malignancy, respectively. Single-cell stress relaxation tests were conducted at 2 h and 2 days after plating to determine cell mechanical properties in either spherical or spread morphologies and analyzed using the new theoretical model. At both time points, JJ012 cells had the lowest moduli of the cell lines examined, whereas FS090 typically had the highest. At 2 days, all cells showed an increase in stiffness and a decrease in apparent viscosity compared to the 2-h time point. Fluorescent labeling showed that the F-actin structure in spread cells was significantly different between FS090 cells and JJ012/105KC cells. Taken together with results of previous studies, these findings indicate that cell transformation and tumorigenicity are associated with a decrease in cell modulus and apparent viscosity, suggesting that cell mechanical properties may provide insight into the metastatic potential and invasiveness of a cell.

Chondrocyte intracellular calcium, cytoskeletal organization, and gene expression responses to dynamic osmotic loading

While chondrocytes in articular cartilage experience dynamic stimuli from joint loading activities, few studies have examined the effects of dynamic osmotic loading on their signaling and biosynthetic activities. We hypothesize that dynamic osmotic loading modulates chondrocyte signaling and gene expression differently than static osmotic loading. With the use of a novel microfluidic device developed in our laboratory, dynamic hypotonic loading (−200 mosM) was applied up to 0.1 Hz and chondrocyte calcium signaling, cytoskeleton organization, and gene expression responses were examined. Chondrocytes exhibited decreasing volume and calcium responses with increasing loading frequency. Phalloidin staining showed osmotic loading-induced changes to the actin cytoskeleton in chondrocytes. Real-time PCR analysis revealed a stimulatory effect of dynamic osmotic loading compared with static osmotic loading. These studies illustrate the utility of the microfluidic device in cell signaling investigations, and their potential role in helping to elucidate mechanisms that mediate chondrocyte mechanotransduction to dynamic stimuli.

Effects of interleukin-1 on calcium signaling and the increase of filamentous actin in isolated and in situ articular chondrocytes

OBJECTIVE

To determine whether interleukin-1 (IL-1) initiates transient changes in the intracellular concentration of [Ca2+]i and the organization of filamentous actin (F-actin) in articular chondrocytes.

METHODS

Articular chondrocytes within cartilage explants and enzymatically isolated chondrocytes were loaded with Ca(2+)-sensitive fluorescence indicators, and [Ca2+]i was measured using confocal fluorescence ratio imaging during exposure to 10 ng/ml IL-1alpha. Inhibitors of Ca2+ mobilization (Ca(2+)-free medium, thapsigargin [inhibitor of Ca-ATPases], U73122 [inhibitor of phospholipase C], and pertussis toxin [inhibitor of G proteins]) were used to determine the mechanisms of increased [Ca2+]i. Cellular F-actin was quantified using fluorescently labeled phalloidin. Toxin B was used to determine the role of the Rho family of small GTPases in F-actin reorganization.

RESULTS

In isolated cells on glass and in in situ chondrocytes within explants, exposure to IL-1 induced a transient peak in [Ca2+]i that was generally followed by a series of decaying oscillations. Thapsigargin, U73122, and pertussis toxin inhibited the percentage of cells responding to IL-1. IL-1 increased F-actin content in chondrocytes in a manner that was inhibited by toxin B.

CONCLUSION

Both isolated and in situ chondrocytes respond to IL-1 with transient increases in [Ca2+]i via intracellular Ca2+ release mediated by the phospholipase C and inositol trisphosphate pathways. The influx of Ca2+ from the extracellular space and the activation of G protein-coupled receptors also appear to contribute to these mechanisms. These findings suggest that Ca2+ mobilization may be one of the first signaling events in the response of chondrocytes to IL-1.

The pericellular matrix as a transducer of biomechanical and biochemical signals in articular cartilage

The pericellular matrix (PCM) is a narrow tissue region surrounding chondrocytes in articular cartilage, which together with the enclosed cell(s) has been termed the "chondron." While the function of this region is not fully understood, it is hypothesized to have important biological and biomechanical functions. In this article, we review a number of studies that have investigated the structure, composition, mechanical properties, and biomechanical role of the chondrocyte PCM. This region has been shown to be rich in proteoglycans (e.g., aggrecan, hyaluronan, and decorin), collagen (types II, VI, and IX), and fibronectin, but is defined primarily by the presence of type VI collagen as compared to the extracellular matrix (ECM). Direct measures of PCM properties via micropipette aspiration of isolated chondrons have shown that the PCM has distinct mechanical properties as compared to the cell or ECM. A number of theoretical and experimental studies suggest that the PCM plays an important role in regulating the microenvironment of the chondrocyte. Parametric studies of cell-matrix interactions suggest that the presence of the PCM significantly affects the micromechanical environment of the chondrocyte in a zone-dependent manner. These findings provide support for a potential biomechanical function of the chondrocyte PCM, and furthermore, suggest that changes in the PCM and ECM properties that occur with osteoarthritis may significantly alter the stress-strain and fluid environments of the chondrocytes. An improved understanding of the structure and function of the PCM may provide new insights into the mechanisms that regulate chondrocyte physiology in health and disease.

Moving forward moving backward: directional sorting of chemotactic cells due to size and adhesion differences

Differential movement of individual cells within tissues is an important yet poorly understood process in biological development. Here we present a computational study of cell sorting caused by a combination of cell adhesion and chemotaxis, where we assume that all cells respond equally to the chemotactic signal. To capture in our model mesoscopic properties of biological cells, such as their size and deformability, we use the Cellular Potts Model, a multiscale, cell-based Monte Carlo model. We demonstrate a rich array of cell-sorting phenomena, which depend on a combination of mescoscopic cell properties and tissue level constraints. Under the conditions studied, cell sorting is a fast process, which scales linearly with tissue size. We demonstrate the occurrence of "absolute negative mobility", which means that cells may move in the direction opposite to the applied force (here chemotaxis). Moreover, during the sorting, cells may even reverse the direction of motion. Another interesting phenomenon is "minority sorting", where the direction of movement does not depend on cell type, but on the frequency of the cell type in the tissue. A special case is the cAMP-wave-driven chemotaxis of Dictyostelium cells, which generates pressure waves that guide the sorting. The mechanisms we describe can easily be overlooked in studies of differential cell movement, hence certain experimental observations may be misinterpreted.

Viscoelastic properties of zonal articular chondrocytes measured by atomic force microscopy

OBJECTIVE

Articular chondrocytes respond to chemical and mechanical signals depending on their zone of origin with respect to distance from the tissue surface. However, little is known of the zonal variations in cellular mechanical properties in cartilage. The goal of this study was to determine the zonal variations in the elastic and viscoelastic properties of porcine chondrocytes using atomic force microscopy (AFM), and to validate this method against micropipette aspiration.

METHODS

A theoretical solution for stress relaxation of a viscoelastic, incompressible, isotropic surface indented with a hard, spherical indenter (5 microm diameter) was derived and fit to experimental stress-relaxation data for AFM indentation of chondrocytes isolated from the superficial or middle/deep zones of cartilage.

RESULTS

The instantaneous moduli of chondrocytes were 0.55+/-0.23 kPa for superficial cells (S) and 0.29+/-0.14 kPa for middle/deep cells (M/D) (P<0.0001), and the relaxed moduli were 0.31+/-0.15 kPa (S) and 0.17+/-0.09 kPa (M/D) (P<0.0001). The apparent viscosities were 1.15+/-0.66 kPas (S) and 0.61+/-0.69 kPa-s (M/D) (P<0.0001). Results from the micropipette aspiration test showed similar cell moduli but higher apparent viscosities, indicating that mechanical properties measured by these two techniques are similar.

CONCLUSION

Our findings suggest that chondrocyte biomechanical properties differ significantly with the zone of origin, consistent with previous studies showing zonal differences in chondrocyte biosynthetic activity and gene expression. Given the versatility and dynamic testing capabilities of AFM, the ability to conduct stress-relaxation measurements using this technique may provide further insight into the viscoelastic properties of isolated cells.

Finite element modeling predictions of region-specific cell-matrix mechanics in the meniscus

The knee meniscus exhibits significant spatial variations in biochemical composition and cell morphology that reflect distinct phenotypes of cells located in the radial inner and outer regions. Associated with these cell phenotypes is a spatially heterogeneous microstructure and mechanical environment with the innermost regions experiencing higher fluid pressures and lower tensile strains than the outer regions. It is presently unknown, however, how meniscus tissue mechanics correlate with the local micromechanical environment of cells. In this study, theoretical models were developed to study mechanics of inner and outer meniscus cells with varying geometries. The results for an applied biaxial strain predict significant regional differences in the cellular mechanical environment with evidence of tensile strains along the collagen fiber direction of approximately 0.07 for the rounded inner cells, as compared to levels of 0.02-0.04 for the elongated outer meniscus cells. The results demonstrate an important mechanical role of extracellular matrix anisotropy and cell morphology in regulating the region-specific micromechanics of meniscus cells, that may further play a role in modulating cellular responses to mechanical stimuli.

A mechano-chemical model for the passive swelling response of an isolated chondron under osmotic loading

The chondron is a distinct structure in articular cartilage that consists of the chondrocyte and its pericellular matrix (PCM), a narrow tissue region surrounding the cell that is distinguished by type VI collagen and a high glycosaminoglycan concentration relative to the extracellular matrix. We present a theoretical mechano-chemical model for the passive volumetric response of an isolated chondron under osmotic loading in a simple salt solution at equilibrium. The chondrocyte is modeled as an ideal osmometer and the PCM model is formulated using triphasic mixture theory. A mechano-chemical chondron model is obtained assuming that the chondron boundary is permeable to both water and ions, while the chondrocyte membrane is selectively permeable to only water. For the case of a neo-Hookean PCM constitutive law, the model is used to conduct a parametric analysis of cell and chondron deformation under hyper- and hypo-osmotic loading. In combination with osmotic loading experiments on isolated chondrons, model predictions will aid in determination of pericellular fixed charge density and its relative contribution to PCM mechanical properties.

Strain-dependent Recovery Behavior of Single Chondrocytes

One of the challenges facing researchers studying chondrocyte mechanobiology is determining the range of mechanical forces pertinent to the problems they study. One possible way to deal with this problem is to quantify how the biomechanical behavior of cells varies in response to changing mechanical forces. In this study, the compressibility and recovery behaviors of single chondrocytes were determined as a function of compressive strains from 6 to 63%. Bovine articular chondrocytes from the middle and deep zones were subjected to this range of strains, and digital videocapture was used to track changes in cell dimensions during and after compression. The normalized volume change, apparent Poisson's ratio, residual strain after recovery, cell volume fraction after recovery, and characteristic recovery time constant were analyzed with respect to axial strain. Normalized volume change varied as a function of strain, demonstrating that chondrocytes exhibited compressibility. The mean Poisson's ratio of chondrocytes was found to be 0.29 +/- 0.14, and did not vary with axial strain. In contrast, residual strain, recovered volume fraction, and recovery time constant all depended on axial strain. The dependence of residual strain and recovered volume fraction on axial strain showed a change in behavior around 25-30% strain, opening up the possibility that this range of strains represents a critical value for chondrocytes. Quantifying the mechanical behavior of cells as a function of stress and strain is a potentially useful approach for identifying levels of mechanical stimulation that may be germane to normal cartilage physiology, functional tissue engineering of cartilage, and the etiopathogenesis of osteoarthritis.