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

Hyper-osmotic stress induces volume change and calcium transients in chondrocytes by transmembrane, phospholipid, and G-protein pathways

Mechanical compression of cartilage is associated with a rise in the interstitial osmotic pressure, which can alter cell volume and activate volume recovery pathways. One of the early events implicated in regulatory volume changes and mechanotransduction is an increase of intracellular calcium ion ([Ca(2+)](i)). In this study, we tested the hypothesis that osmotic stress initiates intracellular Ca(2+) signaling in chondrocytes. Using laser scanning microscopy and digital image processing, [Ca(2+)](i) and cell volume were monitored in chondrocytes exposed to hyper-osmotic solutions. Control experiments showed that exposure to hyper-osmotic solution caused significant decreases in cell volume as well as transient increases in [Ca(2+)](i). The initial peak in [Ca(2+)](i) was generally followed by decaying oscillations. Pretreatment with gadolinium, a non-specific blocker of mechanosensitive ion channels, inhibited this [Ca(2+)](i) increase. Calcium-free media eliminated [Ca(2+)](i) increases in all cases. Pretreatment with U73122, thapsigargin, or heparin (blockers of the inositol phosphate pathway), or pertussis toxin (a blocker of G-proteins) significantly decreased the percentage of cells responding to osmotic stress and nearly abolished all oscillations. Cell volume decreased with hyper-osmotic stress and recovered towards baseline levels throughout the duration of the control experiments. The peak volume change with 550 mOsm osmotic stress, as well as the percent recovery of cell volume, was dependent on [Ca(2+)](i.) These findings indicate that osmotic stress causes significant volume change in chondrocytes and may activate an intracellular second messenger signal by inducing transient increases in [Ca(2+)](i).

An analysis of the effects of depth-dependent aggregate modulus on articular cartilage stress-relaxation behavior in compression

An accurate description of the mechanical environment around chondrocytes embedded within their dense extracellular matrix (ECM) is essential for the study of mechano-signal transduction mechanism(s) in explant experiments. New methods have been developed to determine the inhomogeneous strain distribution throughout the depth of the ECM during compression (Schinagl et al., 1996, Annals of Biomedical Engineering 24, 500-512; Schinagl et al 1997. Journal of Orthopaedics Research 15, 499-506) and the corresponding depth-dependent aggregate modulus distribution (Wang and Mow, 1998. Transactions of the Orthopaedics Research Society 23, 484; Chen and Sah, 1999. Transactions of the Orthopaedics Research Society 24, 635). These results provide the motivation for the current investigation to assess the influence of tissue inhomogeneity on the chondrocyte milieu in situ, e.g. stress, strain, fluid velocity and pressure fields within articular cartilage. To describe this inhomogeneity, we adopted the finite deformation biphasic constitutive law developed by Holmes and Mow (1990 Journal of Biomechanics 23, 1145-1156). Our calculations show that the mechanical environment inside an inhomogeneous tissue differs significantly from that inside a homogeneous tissue. Furthermore, our results indicate that the need to incorporate an inhomogeneous aggregate modulus. or an anisotropy, into the biphasic theory to describe articular cartilage depends largely on the motivation for the study.

Atomic force microscopy can be used to mechanically stimulate osteoblasts and evaluate cellular strain distributions

In this study, atomic force microscopy (AFM) was used to mechanically stimulate primary osteoblasts. In response to mechanical force applied by the AFM, the indented cell increased its intracellular calcium concentration. The material properties of the cell could be estimated and the membrane strains calculated. We proceeded to validate this technique experimentally and a 20% error was found between the predicted and the measured diameter of indentation. We also determined the strain distributions within the cell that result from AFM indentation using a simple finite element model. This enabled us to formulate hypotheses as to the mechanism through which cells may sense the applied mechanical strains. Finally, we report the effect of the Poisson ratio and the cell thickness on the strain distributions. Varying the Poisson ratio did not change the order of magnitude of the strains; whereas the cellular thickness dramatically changed the order of magnitude of the cellular strains. We conclude that AFM can be used for controlled mechanical stimulation of osteoblasts and that cellular strain distributions can be computed with a good accuracy when the cell is indented in its highest part.

The chondrocyte cytoskeleton in mature articular cartilage: structure and distribution of actin, tubulin, and vimentin filaments

We investigated the structure of the chondrocyte cytoskeleton in intact tissue sections of mature bovine articular cartilage using confocal fluorescence microscopy complemented by protein extraction and immunoblotting analysis. Actin microfilaments were present inside the cell membrane as a predominantly cortical structure. Vimentin and tubulin spanned the cytoplasm from cell to nuclear membrane, the vimentin network appearing finer compared to tubulin. These cytoskeletal structures were present in chondrocytes from all depth zones of the articular cartilage. However, staining intensity varied from zone to zone, usually showing more intense staining for the filament systems at the articular surface compared to the deeper zones. These results obtained on fluorescently labeled sections were also corroborated by protein contents extracted and observed by immunoblotting. The observed cytoskeletal structures are compatible with some of the proposed cellular functions of these systems and support possible microenvironmental regulation of the cytoskeleton, including that due to physical forces from load-bearing, which are known to vary through the depth layers of articular cartilage.

Patellar cartilage deformation in vivo after static versus dynamic loading

The objective of this study was to test the hypothesis that static loading (squatting at a 90 degrees angle) and dynamic loading (30 deep knee bends) cause different extents and patterns of patellar cartilage deformation in vivo. The two activities were selected because they imply different types of joint loading and reflect a realistic and appropriate range of strenuous activity. Twelve healthy volunteers were examined and the volume and thickness of the patellar cartilage determined before and from 90 to 320s after loading, using a water excitation gradient echo MR sequence and a three-dimensional (3D) distance transformation algorithm. Following knee bends, we observed a residual reduction of the patellar cartilage volume (-5.9+/-2.1%; p<0.01) and of the maximal cartilage thickness (-2.8+/-2.6%), the maximal deformation occurring in the superior lateral and the medial patellar facet. Following squatting, the change of patellar cartilage volume was -4.7+/-1.6% (p<0.01) and that of the maximal cartilage thickness -4.9+/-1.4% (p<0.01), the maximal deformation being recorded in the central aspect of the lateral patellar facet. The volume changes were significantly lower after squatting than after knee bends (p<0.05), but the maximal thickness changes higher (p<0.05). The results obtained in this study can serve to validate computer models of joint load transfer, to guide experiments on the mechanical regulation of chondrocyte biosynthesis, and to estimate the magnitude of deformation to be encountered by tissue-engineered cartilage within its target environment.

Comparison of knee joint cartilage thickness in triathletes and physically inactive volunteers based on magnetic resonance imaging and three-dimensional analysis

The objective of this study was to employ quantitative magnetic resonance imaging for the analysis of knee joint cartilage thickness in triathletes and physically inactive volunteers. The right knee joints of nine male triathletes (10 hours training per week for at least 3 years) and nine inactive male volunteers (<1 hour of physical activity per week throughout life) were imaged with a previously validated fat-suppressed gradient echo sequence. The cartilage plates were reconstructed three-dimensionally, and the cartilage thickness was computed independently of the original section orientation with a three-dimensional Euclidian distance transformation. There was a high interindividual variability of the mean and the maximal cartilage thickness values in all surfaces, both in the triathletes and in the inactive volunteers. In the patella, the femoral trochlea, and the lateral femoral condyle, the mean and maximal cartilage thickness values were slightly higher in the triathletes, but they were somewhat lower in the medial femoral condyle, and in the medial and lateral tibial plateau. However, the differences did not attain statistical significance. These results are unexpected in view of the functional adaptation observed in other musculoskeletal tissues, such as muscle and bone, in which a more obvious relationship with the magnitude of the applied mechanical stress has been observed.

Viscoelastic properties of the cell nucleus

Mechanical factors play an important role in the regulation of cell physiology. One pathway by which mechanical stress may influence gene expression is through a direct physical connection from the extracellular matrix across the plasma membrane and to the nucleus. However, little is known of the mechanical properties or deformation behavior of the nucleus. The goal of this study was to quantify the viscoelastic properties of mechanically and chemically isolated nuclei of articular chondrocytes using micropipet aspiration in conjunction theoretical viscoelastic model. Isolated nuclei behaved as viscoelastic solid materials similar to the cytoplasm, but were 3-4 times stiffer and nearly twice as viscous as the cytoplasm. Quantitative information of the biophysical properties and deformation behavior of the nucleus may provide further insight on the relationships between the stress-strain state of the nucleus and that of the extracellular matrix, as well as potential mechanisms of mechanical signal transduction.

Confocal analysis of cytoskeletal organisation within isolated chondrocyte sub-populations cultured in agarose

This study reports the cytoskeletal organisation within chondrocytes, isolated from the superficial and deep zones of articular cartilage and seeded into agarose constructs. At day 0, marked organisation of actin microfilaments was not observed in cells from both zones. Partial or clearly organised microtubules and vimentin intermediate filaments cytoskeletal components were present, however, in a proportion of cells. Staining for microtubules and vimentin intermediate filaments was less marked after 1 day in culture however than on initial seeding. For all three cytoskeletal components there was a dramatic increase in organisation between days 3 and 14 and, in general, organisation was greater within deep zone cells. Clear organisation for actin microfilaments was characterised by a cortical network and punctate staining around the periphery of the cell, while microtubules and vimentin intermediate filaments formed an extensive fibrous network. Cytoskeletal organisation within chondrocytes in agarose appears, therefore, to be broadly similar to that described in situ. Variations in the organisation of actin microfilaments between chondrocytes cultured in agarose and in monolayer are consistent with a role in phenotypic modulation. Vimentin intermediate filaments and microtubules form a link between the plasma membrane and the nucleus and may play a role in the mechanotransduction process.

Chondrocyte deformation within compressed agarose constructs at the cellular and sub-cellular levels

Mechanotransduction events in articular cartilage may be resolved into extracellular components followed by intracellular signalling events, which finally lead to altered cell response. Cell deformation is one of the former components, which has been examined using a model involving bovine chondrocytes seeded in agarose constructs. Viable fluorescent labels and confocal laser scanning microscopy were used to examine cellular and sub-cellular morphology. It was observed that cell size increased up to day 6 in culture, associated with an increase in the contents of proteoglycan and collagen. In addition, the organisation of the cytoskeleton components, described using a simple scoring scale, revealed temporal changes for actin fibres, microtubules and vimentin intermediate filaments. The constructs on day 1 were also subjected to unconfined compressive strains. A series of confocal scans through the centre of individual cells revealed a change from a spherical to an elliptical morphology. This was demonstrated by a change in diameter ratio, from a mean value of 1.00 at 0% strain to 0.60 at 25% strain. Using simple equations, the volume and surface areas were also estimated from the scans. Although the former revealed little change with increasing construct strain, surface area appeared to increase significantly. However further examination, using transmission electron microscopy to reveal fine ultrastructural detail at the cell periphery, suggest that this increase may be due to an unravelling of folds at the cell membrane. Cell deformation was associated with a decrease in the nuclear diameter, in the direction of the applied strain. The resulting nuclear strain in one direction increased in constructs compressed at later time points, although its values at all three assessment times were less than the corresponding values for cell strain. It is suggested that the nuclear behaviour may be a direct result of temporal changes observed in the organisation of the cytoskeleton. The study demonstrated that the chondrocyte-agarose model provides a useful system for the examination of compression events at both cellular and sub-cellular levels.

Some bioengineering considerations for tissue engineering of articular cartilage

The mechanism(s) by which chondrocytes convert physical stimuli to intracellular signals, which in turn direct cell activities, represents an area of intense current orthopaedic tissue engineering research. This report is aimed at providing an overview of some biomechanical engineering factors that are required for pursuing this type of research. Two specific aspects of cartilage are addressed: (1) how does the tissue function biomechanically; and (2) what is the nature of physical stimuli inside articular cartilage. By focusing on the effects of inhomogeneities of material properties, a description of some of the mechanical and electrochemical events (the physical stimuli) that would occur in cartilage during loading is presented. Two simple and common tests are considered: permeation and confined compression. Theoretical analyses using appropriate constitutive laws (the biphasic and triphasic theories) reveal the details of how surface loadings are converted to mechanical and electrochemical signals by the extracellular matrix to hydraulic and osmotic pressures, fluid, solute and ion flows, matrix deformations, and electrical fields. The material inhomogeneities are shown to be able to significantly change the mechanical and electrochemical events within the extracellular matrix, and thus the environments around chondrocytes. Material inhomogeneities arising from the flow of interstitial fluid through the porous and permeable extracellular matrix also are discussed. In the authors' view, the charged extracellular matrix, together with the associated interstitial fluid and ions, collectively can be thought of as a signal transducer. Knowledge of the nature of the mechanical and electrochemical events in the extracellular matrix, and their variations with time and location during and after loading, is essential in the understanding of the mechanical signal transduction mechanism(s) in chondrocytes and articular cartilage.

Loading of healing bone, fibrous tissue, and muscle: implications for orthopaedic practice

One of the most important concepts in orthopaedics in this century is the understanding that loading accelerates healing of bone, fibrous tissue, and skeletal muscle. Basic scientific and clinical investigations have shown that these tissues respond to certain patterns of loading by increasing matrix synthesis and in many instances by changing the composition, organization, and mechanical properties of their matrices. Although new approaches to facilitate bone and fibrous tissue healing have shown promise (e.g., the use of cytokines, cell transplants, and gene therapy), none has been proved to offer beneficial effects comparable to those produced by loading of healing tissues. For these reasons, patients with musculoskeletal injuries and those who have recently undergone surgery are now being treated with controlled physical activity that loads their healing tissues. Evaluation of new approaches to the promotion of healing of bone, fibrous tissue, and muscle should include consideration of the effects of loading on tissue repair and remodeling.

Modelling of location- and time-dependent deformation of chondrocytes during cartilage loading

Experimental evidence suggests that the biosynthetic activity of chondrocytes is regulated primarily by the mechanical environment. In order to study the mechanisms underlying remodeling, adaptation, and degeneration of articular cartilage in a joint subjected to changing loads, it is important to know the time-dependent fluid pressure and stress-strain state in chondrocytes. The purpose of the present study was to develop a theoretical model to simulate the mechanical behaviour of articular cartilage and to describe the time-dependent stress-strain state and fluid pressure distribution in chondrocytes during cartilage deformation. It was assumed that the volume occupied by the chondrocytes is small and that cartilage can be treated as a macroscopically homogenized material with effective material properties which depend on the material properties of the cells and matrix and the volumetric fraction of the cells. Model predictions on the time-dependent distribution of fluid pressure and stress and on the time-dependent cell deformation during confined and unconfined compression tests agree with previous theoretical predictions and experimental observations. The proposed model supplies the tools to study the mechanisms of degeneration, adaptation and remodelling of cartilage associated with cell loading and deformation.

Mechanically induced calcium waves in articular chondrocytes are inhibited by gadolinium and amiloride

Chondrocytes in articular cartilage utilize mechanical signals from their environment to regulate their metabolic activity. However, the sequence of events involved in the transduction of mechanical signals to a biochemical signal is not fully understood. It has been proposed that an increase in the concentration of intracellular calcium ion ([Ca2+]i) is one of the earliest events in the process of cellular mechanical signal transduction. With use of fluorescent confocal microscopy, [Ca2+]i was monitored in isolated articular chondrocytes subjected to controlled deformation with the edge of a glass micropipette. Mechanical stimulation resulted in an immediate and transient increase in [Ca2+]i. The initiation of Ca2+ waves was abolished by removing Ca2+ from the extracellular media and was significantly inhibited by the presence of gadolinium ion (10 microM) or amiloride (1 mM), which have previously been reported to block mechanosensitive ion channels. Inhibitors of intracellular Ca2+ release (dantrolene and 8-diethylaminooctyl 3,4,5-trimethoxybenzoate hydrochloride) or cytoskeletal disrupting agents (cytochalasin D and colchicine) had no significant effect on the characteristics of the Ca2+ waves. These findings suggest that a possible mechanism of Ca2+ mobilization in this case is a self-reinforcing influx of Ca2+ from the extracellular media, initiated by a Ca2+-permeable mechanosensitive ion channel. Our results indicate that a transient increase in intracellular Ca2+ concentration may be one of the earliest events involved in the response of chondrocytes to mechanical stress and support the hypothesis that deformation-induced Ca2+ waves are initiated through mechanosensitive ion channels.

Alterations in the Young's modulus and volumetric properties of chondrocytes isolated from normal and osteoarthritic human cartilage

The mechanical environment of the chondrocyte is an important factor that influences the maintenance of the articular cartilage extracellular matrix. Previous studies have utilized theoretical models of chondrocytes within articular cartilage to predict the stress-strain and fluid flow environments around the cell, but little is currently known regarding the cellular properties which are required for implementation of these models. The objectives of this study were to characterize the mechanical behavior of primary human chondrocytes and to determine the Young's modulus of chondrocytes from non-osteoarthritic ('normal') and osteoarthritic cartilage. A second goal was to quantify changes in the volume of isolated chondrocytes in response to mechanical deformation. The micropipette aspiration technique was used to measure the deformation of a single chondrocyte into a glass micropipette in response to a prescribed pressure. The results of this study indicate that the human chondrocyte behaves as a viscoelastic solid. No differences were found between the Young's moduli of normal (0.65+/-0.63 kPa, n = 44) and osteoarthritic chondrocytes (0.67+/-0.86 kPa, n = 69, p = 0.93). A significant difference in cell volume was observed immediately and 600 s after complete aspiration of the cell into the pipette (p < 0.001), and the magnitude of this volume change between normal (11+/-11%, n = 40) and osteoarthritic (20+/-11%, n = 41) chondroctyes was significantly different at both time points (p < 0.002). This finding suggests that chondrocytes from osteoarthritic cartilage may have altered volume regulation capabilities in response to mechanical deformation. The mechanical and volumetric properties determined in this study will be of use in analytical and finite element models of chondrocyte-matrix interactions in order to better predict the mechanical environment of the cell in vivo.

A method for quantifying time dependent changes in MR signal intensity of articular cartilage as a function of tissue deformation in intact joints

A method is proposed to determine accurately the signal intensity changes of the articular cartilage from sectional MR images and its related cartilage deformation under compression in an intact joint. Image processing methods are developed to delineate and register the cartilage boundaries in consecutive MR images in order to track corresponding tissue sectors during the loading experiment. Regions of interest can then be defined and traced during the compression, making a spatial and temporal analysis of signal intensity changes possible. In addition, the cartilage deformation is calculated in the respective tissue sectors and is related to the MR signal changes. Using a fat-suppressed FLASH 3D sequence, the preliminary results showed location-dependent slight changes of the signal intensity varying from individual to individual. The quantitative analysis of the signal intensity changes as a function of cartilage deformation with magnetic resonance imaging (MRI) aims to characterize microstructural properties of the articular cartilage that may lead to a better understanding of degenerative joint disease.

The influence of elaborated pericellular matrix on the deformation of isolated articular chondrocytes cultured in agarose

This study investigates the mechanical influence of pericellular matrix on the deformation of isolated articular chondrocytes compressed within 3% agarose specimens. After 1 day in culture, the cells were associated with minimal amounts of sulphated glycosaminoglycan (GAG) and hydroxyproline and exhibited substantial deformation from a spherical to an oblate ellipsoid morphology when subjected to 20% gross compressive strain. However, over the 6 day culture period, there was a reduction in cell deformation associated with an increase in matrix content. Treatment with testicular hyaluronidase at days 3 and 6 reduced sulphated GAG content to levels observed in untreated specimens at day 1. At day 3, the resulting cell deformation during 20% compression was equivalent to that in specimens compressed at day 1. However, at day 6 cell deformation was only partially restored, suggesting the presence of additional structural matrix components, other than sulphated GAG, which were not present at day 3. Dual scanning confocal microscopy indicated that the elaborated matrix formed a pericellular shell which did not deform during compression and was therefore stiffer than the 3% agarose substrate. Therefore, the elaboration of a mechanically functional pericellular matrix within 6 days, effectively limits the potential involvement of cell deformation in mechanotransduction within cell seeded systems such as those employed for cartilage repair.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Load-controlled compression of articular cartilage induces a transient stimulation of aggrecan gene expression

The effects of short- and long-term load-controlled compression on the levels of aggrecan mRNA have been determined. Results show that a compressive stress of 0.1 MPa on bovine articular cartilage explants for 1, 4, 12, and 24 h produces a transient up-regulation of aggrecan mRNA synthesis. At 1 h, aggrecan mRNA levels in loaded explants were increased 3.2-fold compared to control explants. At longer times (>/=4 h), the levels of aggrecan mRNA returned to baseline values or stayed slightly higher. There is a dose dependence in the response of the explant to increasing levels of compressive stress (0-0.5 MPa) for 1 h. Aggrecan mRNA levels increased 2- to 3-fold at 0-0.25 MPa. At 0.5 MPa, the level of aggrecan mRNA was lower than those at 0.1 and 0.25 MPa. This dose-dependent effect suggests a reversal of the stimulatory effects of compression on aggrecan gene expression at higher loads. After 24 h of compression, the levels of aggrecan mRNA in explants subjected to any of the stress levels were not significantly different from those in control explants. The stimulatory effect of 0.1 MPa compressive stress on aggrecan mRNA levels was blocked by Rp-cAMP and U-73122, indicating the involvement of the classical signal transduction pathways in the mechanical modulation of aggrecan gene expression. The responses of link protein mRNA to compression paralleled those of aggrecan, while there was no significant change in expression of the gene for the housekeeping protein elongation factor-1 alpha. The results indicate that articular cartilage chondrocytes can respond to short-term compressive loads by transiently up-regulating expression of the aggrecan gene. The fact that long-term compression did not significantly alter aggrecan mRNA levels suggests that previously observed inhibitory effects of prolonged static compression on proteoglycan synthesis in articular cartilage may be, for the most part, mediated through mechanisms other than suppression of aggrecan mRNA levels.