Compression of cartilage results in differential effects on biosynthetic pathways for aggrecan, link protein, and hyaluronan

Aggrecan and Hyaluronan: The Infamous Cartilage Polyelectrolytes - Then and Now

Cartilages are unique in the family of connective tissues in that they contain a high concentration of the glycosaminoglycans, chondroitin sulfate and keratan sulfate attached to the core protein of the proteoglycan, aggrecan. Multiple aggrecan molecules are organized in the extracellular matrix via a domain-specific molecular interaction with hyaluronan and a link protein, and these high molecular weight aggregates are immobilized within the collagen and glycoprotein network. The high negative charge density of glycosaminoglycans provides hydrophilicity, high osmotic swelling pressure and conformational flexibility, which together function to absorb fluctuations in biomechanical stresses on cartilage during movement of an articular joint. We have summarized information on the history and current knowledge obtained by biochemical and genetic approaches, on cell-mediated regulation of aggrecan metabolism and its role in skeletal development, growth as well as during the development of joint disease. In addition, we describe the pathways for hyaluronan metabolism, with particular focus on the role as a "metabolic rheostat" during chondrocyte responses in cartilage remodeling in growth and disease.Future advances in effective therapeutic targeting of cartilage loss during osteoarthritic diseases of the joint as an organ as well as in cartilage tissue engineering would benefit from 'big data' approaches and bioinformatics, to uncover novel feed-forward and feed-back mechanisms for regulating transcription and translation of genes and their integration into cell-specific pathways.

Mechanical Cues: Bidirectional Reciprocity in the Extracellular Matrix Drives Mechano-Signalling in Articular Cartilage

The composition and organisation of the extracellular matrix (ECM), particularly the pericellular matrix (PCM), in articular cartilage is critical to its biomechanical functionality; the presence of proteoglycans such as aggrecan, entrapped within a type II collagen fibrillar network, confers mechanical resilience underweight-bearing. Furthermore, components of the PCM including type VI collagen, perlecan, small leucine-rich proteoglycans—decorin and biglycan—and fibronectin facilitate the transduction of both biomechanical and biochemical signals to the residing chondrocytes, thereby regulating the process of mechanotransduction in cartilage. In this review, we summarise the literature reporting on the bidirectional reciprocity of the ECM in chondrocyte mechano-signalling and articular cartilage homeostasis. Specifically, we discuss studies that have characterised the response of articular cartilage to mechanical perturbations in the local tissue environment and how the magnitude or type of loading applied elicits cellular behaviours to effect change. In vivo, including transgenic approaches, and in vitro studies have illustrated how physiological loading maintains a homeostatic balance of anabolic and catabolic activities, involving the direct engagement of many PCM molecules in orchestrating this slow but consistent turnover of the cartilage matrix. Furthermore, we document studies characterising how abnormal, non-physiological loading including excessive loading or joint trauma negatively impacts matrix molecule biosynthesis and/or organisation, affecting PCM mechanical properties and reducing the tissue’s ability to withstand load. We present compelling evidence showing that reciprocal engagement of the cells with this altered ECM environment can thus impact tissue homeostasis and, if sustained, can result in cartilage degradation and onset of osteoarthritis pathology. Enhanced dysregulation of PCM/ECM turnover is partially driven by mechanically mediated proteolytic degradation of cartilage ECM components. This generates bioactive breakdown fragments such as fibronectin, biglycan and lumican fragments, which can subsequently activate or inhibit additional signalling pathways including those involved in inflammation. Finally, we discuss how bidirectionality within the ECM is critically important in enabling the chondrocytes to synthesise and release PCM/ECM molecules, growth factors, pro-inflammatory cytokines and proteolytic enzymes, under a specified load, to influence PCM/ECM composition and mechanical properties in cartilage health and disease.

Vibrometry as a noncontact alternative to dynamic and viscoelastic mechanical testing in cartilage

Physiological loading of knee cartilage is highly dynamic and may contribute to the progression of osteoarthritis. Thus, an understanding of cartilage's dynamic mechanical properties is crucial in cartilage research. In this study, vibrometry was used as a fast (2 h), noncontact and novel alternative to the slower (30 h), traditional mechanical and biochemical assays for characterization of cartilage from the condyle, patella, trochlear groove and meniscus. Finite-element models predicted tissue resonant frequencies and bending modes, which strongly correlated with experiments (R2 = 0.93). Vibrometry-based viscoelastic properties significantly correlated with moduli from stress relaxation and creep tests, with correlation strengths reaching up to 0.78. Loss modulus also strongly correlated with glycosoaminoglycan (GAG) content. Dynamic properties measured by vibrometry significantly differed among various knee cartilages, ranging between 6.1 and 56.4 MPa. Interestingly, meniscus viscoelastic properties suggest that contrary to common belief, it may lack shock absorption abilities; instead, condylar hyaline cartilage may be a better shock absorber. These data demonstrate for the first time that vibrometry is a noncontact approach to dynamic mechanical characterization of hyaline and fibrocartilage cartilage with concrete relationships to standard quasi-static mechanical testing and biochemical composition. Thus, with a single tool, vibrometry greatly facilitates meeting multiple regulatory recommendations for mechanical characterization of cartilage replacements.

Detrimental effects of long sedentary bouts on the biomechanical response of cartilage to sliding

Purpose/Aim: Epidemiological evidence suggests, contrary to popular mythos, that increased exercise/joint activity does not place articular cartilage at increased risk of disease, but instead promotes joint health. One explanation for this might be activity-induced cartilage rehydration; where joint articulation drives restoration of tissue hydration, thickness, and dependent tribomechanical outcomes (e.g., load support, stiffness, and lubricity) lost to joint loading. However, there have been no studies investigating how patterning of intermittent articulation influences the hydration and biomechanical functions of cartilage.Materials and Methods: Here we leveraged the convergent stationary contact area (cSCA) testing configuration and its unique ability to drive tribological rehydration, to elucidate how intermittency of activity affects the biomechanical functions of bovine stifle cartilage under well-controlled sliding conditions that have been designed to model a typical "day" of human joint activity.Results: For a fixed volume of "daily" activity (30 min) and sedentary time (60 min), breaking up intermittent activity into longer and less-frequent bouts (corresponding to longer continuous sedentary periods) resulted in the exposure of articular cartilage to markedly greater strains, losses of interstitial pressure, and friction coefficients.Conclusions: These results demonstrated that the regularity of ex vivo activity regimens, specifically the duration of sedentary bouts, had a dominant effect on the biomechanical functions of articular cartilage. In more practical terms, the results suggest that brief but regular movement patterns (e.g., every hour) may be biomechanically preferred to long and infrequent movement patterns (e.g., a long walk after a sedentary day) when controlling for daily activity volume (e.g., 30 min).

Lipid Coated Microbubbles and Low Intensity Pulsed Ultrasound Enhance Chondrogenesis of Human Mesenchymal Stem Cells in 3D Printed Scaffolds

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters-excitation intensity, frequency and duty cycle-to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Cartilaginous constructs using primary chondrocytes from continuous expansion culture seeded in dense collagen gels

Cell-based therapies such as autologous chondrocyte implantation require in vitro cell expansion. However, standard culture techniques require cell passaging, leading to dedifferentiation into a fibroblast-like cell type. Primary chondrocytes grown on continuously expanding culture dishes (CE culture) limits passaging and protects against dedifferentiation. The authors tested whether CE culture chondrocytes were advantageous for producing mechanically competent cartilage matrix when three-dimensionally seeded in dense collagen gels. Primary chondrocytes, grown either in CE culture or passaged twice on static silicone dishes (SS culture; comparable to standard methods), were seeded in dense collagen gels and cultured for 3 weeks in the absence of exogenous chondrogenic growth factors. Compared with gels seeded with SS culture chondrocytes, CE chondrocyte-seeded gels had significantly higher chondrogenic gene expression after 2 and 3 weeks in culture, correlating with significantly higher aggrecan and type II collagen protein accumulation. There was no obvious difference in glycosaminoglycan content from either culture condition, yet CE chondrocyte-seeded gels were significantly thicker and had a significantly higher dynamic compressive modulus than SS chondrocyte-seeded gels after 3 weeks. Chondrocytes grown in CE culture and seeded in dense collagen gels produce more cartilaginous matrix with superior mechanical properties, making them more suitable than SS cultured cells for tissue engineering applications.

Effects of Dexamethasone on Mesenchymal Stromal Cell Chondrogenesis and Aggrecanase Activity: Comparison of Agarose and Self-Assembling Peptide Scaffolds

OBJECTIVE

Dexamethasone (Dex) is a synthetic glucocorticoid that has pro-anabolic and anti-catabolic effects in cartilage tissue engineering systems, though the mechanisms by which these effects are mediated are not well understood. We tested the hypothesis that the addition of Dex to chondrogenic medium would affect matrix production and aggrecanase activity of human and bovine bone marrow stromal cells (BMSCs) cultured in self-assembling peptide and agarose hydrogels.

DESIGN

We cultured young bovine and adult human BMSCs in (RADA)₄ self-assembling peptide and agarose hydrogels in medium containing TGF-β1±Dex and analyzed extracellular matrix composition, aggrecan cleavage products, and the effects of the glucocorticoid receptor antagonist RU-486 on proteoglycan content, synthesis, and catabolic processing.

RESULTS

Dex improved proteoglycan synthesis and retention in agarose hydrogels seeded with young bovine cells, but decreased proteoglycan accumulation in peptide scaffolds. These effects were mediated by the glucocorticoid receptor. Adult human BMSCs showed minimal matrix accumulation in agarose, but accumulated ~50% as much proteoglycan and collagen as young bovine BMSCs in peptide hydrogels. Dex reduced aggrecanase activity in (RADA)₄ and agarose hydrogels, as measured by anti-NITEGE Western blotting, for both bovine and human BMSC-seeded gels.

CONCLUSIONS

The effects of Dex on matrix production are dependent on cell source and hydrogel identity. This is the first report of Dex reducing aggrecanase activity in a tissue engineering culture system.

Biomechanical influence of cartilage homeostasis in health and disease

There is an urgent demand for long term solutions to improve osteoarthritis treatments in the ageing population. There are drugs that control the pain but none that stop the progression of the disease in a safe and efficient way. Increased intervention efforts, augmented by early diagnosis and integrated biophysical therapies are therefore needed. Unfortunately, progress has been hampered due to the wide variety of experimental models which examine the effect of mechanical stimuli and inflammatory mediators on signal transduction pathways. Our understanding of the early mechanopathophysiology is poor, particularly the way in which mechanical stimuli influences cell function and regulates matrix synthesis. This makes it difficult to identify reliable targets and design new therapies. In addition, the effect of mechanical loading on matrix turnover is dependent on the nature of the mechanical stimulus. Accumulating evidence suggests that moderate mechanical loading helps to maintain cartilage integrity with a low turnover of matrix constituents. In contrast, nonphysiological mechanical signals are associated with increased cartilage damage and degenerative changes. This review will discuss the pathways regulated by compressive loading regimes and inflammatory signals in animal and in vitro 3D models. Identification of the chondroprotective pathways will reveal novel targets for osteoarthritis treatments.

Periodic Nanomechanical Stimulation in a Biokinetics Model Identifying Anabolic and Catabolic Pathways Associated With Cartilage Matrix Homeostasis

Enhancing the available nanotechnology to describe physicochemical interactions during biokinetic regulation will strongly support cellular and molecular engineering efforts. In a recent mathematical model developed to extend the applicability of a statically loaded, single-cell biomechanical analysis, a biokinetic regulatory threshold was presented (Saha and Kohles, 2010, "A Distinct Catabolic to Anabolic Threshold Due to Single-Cell Static Nanomechanical Stimulation in a Cartilage Biokinetics Model," J. Nanotechnol. Eng. Med., 1(3), p. 031005). Results described multiscale mechanobiology in terms of catabolic to anabolic pathways. In the present study, we expand the mathematical model to continue exploring the nanoscale biomolecular response within a controlled microenvironment. Here, we introduce a dynamic mechanical stimulus for regulating cartilage molecule synthesis. Model iterations indicate the identification of a biomathematical mechanism balancing the harmony between catabolic and anabolic states. Relative load limits were defined to distinguish between "healthy" and "injurious" biomolecule accumulations. The presented mathematical framework provides a specific algorithm from which to explore biokinetic regulation.

A Distinct Catabolic to Anabolic Threshold Due to Single-Cell Static Nanomechanical Stimulation in a Cartilage Biokinetics Model

Understanding physicochemical interactions during biokinetic regulation will be critical for the creation of relevant nanotechnology supporting cellular and molecular engineering. The impact of nanoscale influences in medicine and biology can be explored in detail through mathematical models as an in silico testbed. In a recent single-cell biomechanical analysis, the cytoskeletal strain response due to fluid-induced stresses was characterized (Wilson, Z. D., and Kohles, S. S., 2010, "Two-Dimensional Modeling of Nanomechanical Strains in Healthy and Diseased Single-Cells During Microfluidic Stress Applications," J. Nanotech. Eng. Med., 1(2), p. 021005). Results described a microfluidic environment having controlled nanometer and piconewton resolution for explorations of multiscale mechanobiology. In the present study, we constructed a mathematical model exploring the nanoscale biomolecular response to that controlled microenvironment. We introduce mechanical stimuli and scaling factor terms as specific input values for regulating a cartilage molecule synthesis. Iterative model results for this initial multiscale static load application have identified a transition threshold load level from which the mechanical input causes a shift from a catabolic state to an anabolic state. Modeled molecule homeostatic levels appear to be dependent upon the mechanical stimulus as reflected experimentally. This work provides a specific mathematical framework from which to explore biokinetic regulation. Further incorporation of nanomechanical stresses and strains into biokinetic models will ultimately lead to refined mechanotransduction relationships at the cellular and molecular levels.

Effects of mechanical loading on collagen propeptides processing in cartilage repair

Injured articular cartilage has poor reparative capabilities and if left untreated may develop into osteoarthritis. Unsatisfactory results with conventional treatment methods have brought as an alternative treatment the development of matrix autologous chondrocyte transplants (MACTs). Recent evidence proposes that the maintenance of the original phenotype by isolated chondrocytes grown in a scaffold transplant is linked to mechanical compression, because macromolecules, particularly collagen, of the extracellular matrix have the ability to 'self-assemble'. In load-bearing tissues, collagen is abundantly present and mechanical properties depend on the collagen fibre architecture. Study of the active changes in collagen architecture is the focus of diverse fields of research, including developmental biology, biomechanics and tissue engineering. In this review, the structural model of collagen assembly is presented in order to understand how scaffold geometry plays a critical role in collagen propeptide processing and chondrocyte development. When physical forces are applied to different cell-based scaffolds, the resulting specific twist of the scaffolds might be accompanied by changes in the fibril pattern synthesis of the new collagen. The alteration in the scaffolds due to mechanical stress is associated with cellular signalling communication and the preservation of N-terminus procollagen moieties, which would regulate both the collagen synthesis and the diameter of the fibre. The structural difference would also affect actin stabilization, cytoskeleton remodelling and proteoglycan assembly. These effects seemed to be dependent on the magnitude and duration of the physical stress. This review will contribute to the understanding of mechanisms for collagen assembly in both a natural and an artificial environment.

IGF-1 does not moderate the time-dependent transcriptional patterns of key homeostatic genes induced by sustained compression of bovine cartilage

OBJECTIVE

To determine changes in chondrocyte transcription of a range of anabolic, catabolic and signaling genes following simultaneous treatment of cartilage with Insulin-like growth factor-1 (IGF-1) and ramp-and-hold mechanical compression, and compare with effects on biosynthesis.

METHODS

Explant disks of bovine calf cartilage were slowly compressed (unconfined) over 3-min to their 1mm cut-thickness (0%-compression) or to 50%-compression with or without 300 ng/ml IGF-1. Expression of 24 genes involved in cartilage homeostasis was measured using qPCR at 2, 8, 24, 32, 48 h after compression +/-IGF-1. Clustering analysis was used to identify groups of co-expressed genes to further elucidate mechanistic pathways.

RESULTS

IGF-1 alone stimulated gene expression of aggrecan and collagen II, but simultaneous 24h compression suppressed this effect. Compression alone up-regulated expression of matrix metalloproteinase (MMP)-3, MMP-13, a disintegrin and metalloproteinase with thrombospondin motif (ADAMTS)-5 and transforming growth factor (TGF)-beta, an effect not reversed by simultaneous IGF-1 treatment. Temporal changes in expression following IGF-1 treatment were generally slower than that following compression. Clustering analysis revealed five distinct groups within which the pairings, tissue inhibitor of metalloproteinase (TIMP)-3 and ADAMTS-5, MMP-1 and IGF-2, and IGF-1 and Collagen II, were all robustly co-expressed, suggesting inherent regulation and feedback in chondrocyte gene expression. While aggrecan synthesis was transcriptionally regulated by IGF-1, inhibition of aggrecan synthesis by sustained compression appeared post-transcriptionally regulated.

CONCLUSION

Sustained compression markedly altered the effects of IGF-1 on expression of genes involved in cartilage homeostasis, while IGF-1 was largely unable to moderate the transcriptional effects of compression alone. The demonstrated co-expressed gene pairings suggest a balance of anabolic and catabolic activity following simultaneous mechanical and growth factor stimuli.

Cartilage stress-relaxation proceeds slower at higher compressive strains

Articular cartilage is the connective tissue which covers bone surfaces and deforms during in vivo activity. Previous research has investigated flow-dependent cartilage viscoelasticity, but relatively few studies have investigated flow-independent mechanisms. This study investigated polymer dynamics as an explanation for the molecular basis of flow-independent cartilage viscoelasticity. Polymer dynamics predicts that stress-relaxation will proceed more slowly at higher volumetric concentrations of polymer. Stress-relaxation tests were performed on cartilage samples after precompression to different strain levels. Precompression increases the volumetric concentration of cartilage biopolymers, and polymer dynamics predicts an increase in relaxation time constant. Stress-relaxation was slower for greater precompression. There was a significant correlation between the stress-relaxation time constant and cartilage volumetric concentration. Estimates of the flow-dependent timescale suggest that flow-dependent relaxation occurs on a longer timescale than presently observed. These results are consistent with polymer dynamics as a mechanism of cartilage viscoelasticity.

The effect of endogenous overexpression of hyaluronan synthases on material, morphological, and biochemical properties of uncrosslinked collagen biomaterials

Hyaluronan is an essential component of the native extracellular matrix that has often been added exogenously to biomaterials. The role of endogenously produced hyaluronan on soft tensile tissue mechanics, however, has been largely overlooked. To investigate this aspect of hyaluronan using a cell-mediated approach, cells overexpressing the hyaluronan synthases (has), namely has-1, has-2, has-3 or the empty vector control LXSN, were seeded within collagen gel scaffolds. The resulting engineered tissues were grown under static tension for 6 weeks. Following 6 weeks of culture, the samples were characterized to assess collagen gel contraction, matrix organization, production of hyaluronan, and tissue material properties. The engineered tissues containing cells transfected to overexpress one of the has isozymes had significantly increased retention of hyaluronan within the scaffold; elevated hyaluronan secretion into the culture medium (all but has-2); reduced contraction; reduced collagen density; and significantly altered material properties compared to the LXSN controls. These results indicate that the cell-mediated endogenous overproduction of hyaluronan within biomaterials alters their material, morphological and biochemical characteristics. This investigation, the first to examine the role of endogenously produced hyaluronan in engineered tissue mechanics, suggests that overproduction of hyaluronan in soft connective tissues can transform their biological and biomechanical functionality.

Enhanced matrix synthesis in de novo, scaffold free cartilage-like tissue subjected to compression and shear

Production of a de novo cartilage-like tissue construct is a goal for the repair of traumatic chondral defects. We aimed to enhance the matrix synthesis within a scaffold free, de novo cartilage-like tissue construct by way of mechanical load. A novel loading machine that enables the application of shear, as well as compression, was used to subject tissue engineered cartilage-like tissue to mechanical stress. The machine, which applies the load through a roller mechanism, can load up to 20 constructs with four different loading patterns simultaneously. The expression of mRNA encoding matrix products, and subsequent changes in matrix protein content, were analyzed after various loading regimes. The force applied to the immature tissue had a direct bearing on the short-term (first 4 h) response. A load of 0.5 N caused an increase in collagen II and aggrecan mRNA within an hour, with a peak at 2 h. This increased mRNA expression was translated into an increase of up to 60% in the glycosaminoglycan content of the optimally loaded constructs after 4 days of intermittent cyclical loading. Introducing pauses between load cycles reproducibly lead to an increase in GAG/DNA. In contrast, constant cyclical load, with no pause, lead to a decrease in the final glycosaminoglycan content compared with unloaded controls. Our data suggest that a protocol of mechanical stimulation, simulating in vivo conditions and involving shear and compression, may be a useful mechanism to enhance the properties of tissue engineered tissue prior to implantation.

Cell instructive polymers

Polymeric materials used in tissue engineering were initially used solely as delivery vehicles for transplanting cells. However, these materials are currently designed to actively regulate the resultant tissue structure and function. This control is achieved through spatial and temporal regulation of various cues (e.g., adhesion ligands, growth factors) provided to interacting cells from the material. These polymeric materials that control cell function and tissue formation are termed cell instructive polymers, and recent trends in their design are outlined in this chapter.

Mechanical compressive loading stimulates the activity of proximal region of human COL2A1 gene promoter in transfected chondrocytes

Previous studies have demonstrated that the mechanical compressive loading affects the biosynthesis of chondrocytes seeded in three dimensional scaffolds. In this study, the level of type II collagen mRNA expression was increased by a continuous dynamic compression at 10% compressive strain and 0.1 Hz in chondrocytes seeded in a biodegradable, elastomeric scaffold, poly(L-lactide-co-epsilon-caprolactone) (PLCL). To further examine this molecular mechanism, the promoter region of COL2A1 gene, which is encoding type II collagen, was analyzed using rabbit chondrocytes transfected with luciferase reporter vectors containing the 5'-flanking regions of human COL2A1 gene. A deletion mutant analysis revealed that the most active short promoter in response to continuous dynamic compression is in the region between -509 and -109 base pairs, where the transcription factor Sp1 is located. Additionally, an mRNA decay experiment using transcription inhibitor actinomycin D demonstrated that dynamic compression do not stabilize type II collagen mRNA. Our results indicate that mechanical compression increases the level of type II mRNA expression by transcriptional activation possibly through the Sp1 binding sites residing in the proximal region of the COL2A1 gene promoter.

Collagen biosynthesis of mechanically loaded articular cartilage explants

OBJECTIVE

The purpose of this study was to investigate systematically the effect of load amplitudes, frequencies and load durations of intermittently applied mechanical pressure on the biosynthesis of collagen and non-collagenous proteins (NCP) as well as on the water content of cultured bovine articular cartilage explants.

METHODS

Cyclic compressive pressure was applied using a sinusoidal waveform of 0.5 Hz frequency with a peak stress of 0.1, 0.5 or 1.0 MPa for a period of 10s followed by a load-free period of 10, 100 or 1000s. These intermittent loading protocols were repeated for a total duration of 1, 3 or 6 days. During the final 18 h of experiments, the incorporation of [(3)H]-proline into collagen and NCP, the content of water as well as the deformation of loaded explants were determined.

RESULTS

Intermittently applied, cyclic mechanical loading of articular cartilage explants consistently reduced the relative rate of collagen synthesis compared to load-free conditions. This reduced proportion of newly synthesized collagen among newly made proteins was independent of the mechanical stimuli applied. The release of newly synthesized collagen and NCP from loaded explants into the nutrient media was unaffected by any of the loading protocols applied. In addition, quantitative data are provided showing that only high amplitudes of loads and frequencies enhanced the water content of the explants.

CONCLUSIONS

Previous studies reporting that osteoarthritic cartilage in vivo can synthesize elevated amounts of collagen imply that the loading protocols chosen were inadequate for simulating in vitro osteoarthritic-like alterations of collagen synthesis. In our experiments the collagen biosynthesis of chondrocytes was only minor responsive to alterations in mechanical stimuli, applied over a wide range. Thus, our results imply that the synthesis of these structural macromolecules is under the strict control of normal chondrocytes enabling them to maintain the shape of this physical demanded tissue.

Mechanical injury of cartilage explants causes specific time-dependent changes in chondrocyte gene expression

OBJECTIVE

Joint injury in young adults leads to an increased risk of developing osteoarthritis (OA) later in life. This study was undertaken to determine if injurious mechanical compression of cartilage explants results in changes at the level of gene transcription that may lead to subsequent degradation of the cartilage.

METHODS

Cartilage was explanted from the femoropatellar groove of newborn calves. Levels of messenger RNA encoding matrix molecules, proteases, their natural inhibitors, transcription factors, and cytokines were assessed in free swelling control cultures as compared with cartilage cultures at 1, 2, 4, 6, 12, and 24 hours after application of a single injurious compression.

RESULTS

Gene-expression levels measured in noninjured, free swelling cartilage varied over 5 orders of magnitude. Matrix molecules were the most highly expressed of the genes tested, while cytokines, matrix metalloproteinases (MMPs), aggrecanases (ADAMTS-5), and transcription factors showed lower expression levels. Matrix molecules showed little change in expression after injurious compression, whereas MMP-3 increased approximately 250-fold, ADAMTS-5 increased approximately 40-fold, and tissue inhibitor of metalloproteinases 1 increased approximately 12-fold above the levels in free swelling cultures. Genes typically used as internal controls, GAPDH and beta-actin, increased expression levels approximately 4-fold after injury, making them unsuitable for use as normalization genes in this study. The expression levels of tumor necrosis factor alpha and interleukin-1beta, cytokines known to be involved in the progression of OA, did not change in the chondrocytes after injury.

CONCLUSION

Changes in the level of gene expression after mechanical injury are gene specific and time dependent. The quantity of specific proteins may be altered as a result of these changes in gene expression, which may eventually lead to degradation at the tissue level and cause a compromise in cartilage structure and function.

Chondrocyte mechanotransduction: effects of compression on deformation of intracellular organelles and relevance to cellular biosynthesis

OBJECTIVE

The effects of mechanical deformation of intact cartilage tissue on chondrocyte biosynthesis in situ have been well documented, but the mechanotransduction pathways that regulate such phenomena have not been elucidated completely. The goal of this study was to examine the effects of tissue deformation on the morphology of a range of intracellular organelles which play a major role in cell biosynthesis and metabolism.

DESIGN

Using chemical fixation, high pressure freezing, and electron microscopy, we imaged chondrocytes within mechanically compressed cartilage explants at high magnification and quantitatively and qualitatively assessed changes in organelle volume and shape caused by graded levels of loading.

RESULTS

Compression of the tissue caused a concomitant reduction in the volume of the extracellular matrix (ECM), chondrocyte, nucleus, rough endoplasmic reticulum, and mitochondria. Interestingly, however, the Golgi apparatus was able to resist loss of intraorganelle water and retain a portion of its volume relative to the remainder of the cell. These combined results suggest that a balance between intracellular mechanical and osmotic gradients govern the changes in shape and volume of the organelles as the tissue is compressed.

CONCLUSIONS

Our results lead to the interpretive hypothesis that organelle volume changes appear to be driven mainly by osmotic interactions while shape changes are mediated by structural factors, such as cytoskeletal interactions that may be linked to extracellular matrix deformations. The observed volume and shape changes of the chondrocyte organelles and the differential behavior between organelles during tissue compression provide evidence for an important mechanotransduction pathway linking translational and post-translational events (e.g., elongation and sulfation of glycosaminoglycans (GAGs) in the Golgi) to cell deformation.

Mechanical Compression of Cartilage Explants Induces Multiple Time-dependent Gene Expression Patterns and Involves Intracellular Calcium and Cyclic AMP

Chondrocytes are influenced by mechanical forces to remodel cartilage extracellular matrix. Previous studies have demonstrated the effects of mechanical forces on changes in biosynthesis and mRNA levels of particular extracellular matrix molecules, and have identified certain signaling pathways that may be involved. However, the broad extent and kinetics of mechano-regulation of gene transcription has not been studied in depth. We applied static compressive strains to bovine cartilage explants for periods between 1 and 24 h and measured the response of 28 genes using real time PCR. Compression time courses were also performed in the presence of an intracellular calcium chelator or an inhibitor of cyclic AMP-activated protein kinase A. Cluster analysis of the data revealed four main expression patterns: two groups containing either transiently up-regulated or duration-enhanced expression profiles could each be subdivided into genes that did or did not require intracellular calcium release and cyclic AMP-activated protein kinase A for their mechano-regulation. Transcription levels for aggrecan, type II collagen, and link protein were up-regulated approximately 2-3-fold during the first 8 h of 50% compression and subsequently down-regulated to levels below that of free-swelling controls by 24 h. Transcription levels of matrix metalloproteinases-3, -9, and -13, aggrecanase-1, and the matrix protease regulator cyclooxygenase-2 increased with the duration of 50% compression 2-16-fold by 24 h. Thus, transcription of proteins involved in matrix remodeling and catabolism dominated over anabolic matrix proteins as the duration of static compression increased. Immediate early genes c-fos and c-jun were dramatically up-regulated 6-30-fold, respectively, during the first 8 h of 50% compression and remained up-regulated after 24 h.

Dynamic compression of chondrocyte-seeded fibrin gels: effects on matrix accumulation and mechanical stiffness

OBJECTIVE

Various strategies have been tested to direct and control matrix synthesis in tissue engineered cartilage, including mechanical stimulation of the construct both before and after implantation. This study examined the effects of oscillatory compression on chondrocytes in a fibrin-based tissue engineered cartilage.

DESIGN

Chondrocyte-seeded fibrin gels were cultured under unconfined mechanical compression for 10 or 20 days (free-swelling, 10% static, or 10+/-4% at 0.1 or 1Hz). During the culture period, accumulation of nitrite, sGAG, and proteolytic enzymes in the culture media were monitored. Following culture, the mechanical stiffness and biochemical content of the gels (DNA, sGAG, and hydroxyproline content and GAG Delta-disaccharide composition) were assessed.

RESULTS

Compared to free-swelling conditions, static compression had little effect on the mechanical stiffness or biochemical content of the gels. Compared to static compression, oscillatory compression produced softer gels, inhibited sGAG and hydroxyproline accumulation in the gels, and stimulated accumulation of nitrite and sGAG in the culture media. Minimal differences were observed in DNA content and Delta-disaccharide composition across treatment conditions.

CONCLUSIONS

In this study, oscillatory compression inhibited formation of cartilage-like tissues by chondrocytes in fibrin gels. These results suggest that the effects of mechanical stimuli on tissue engineered cartilage may vary substantially between different scaffold systems.

Mechanisms and kinetics of glycosaminoglycan release following in vitro cartilage injury

OBJECTIVE

Acute joint injury leads to increased risk for osteoarthritis (OA). Although the mechanisms underlying this progression are unclear, early structural, metabolic, and compositional indicators of OA have been reproduced using in vitro models of cartilage injury. This study was undertaken to determine whether glycosaminoglycan (GAG) loss following in vitro cartilage injury is mediated by cellular biosynthesis, activation of enzymatic activity, or mechanical disruption of the cartilage extracellular matrix.

METHODS

Immature bovine cartilage was cultured for up to 10 days. After 3 days, groups of samples were subjected to injurious mechanical compression (single uniaxial unconfined compression to 50% thickness, strain rate 100% per second). GAG release to the medium was measured, and levels were compared with those in location-matched, uninjured controls. The effects of medium supplementation with inhibitors of biosynthesis (cycloheximide), of matrix metalloproteinase (MMP) activity (CGS 27023A or GM 6001), and of aggrecanase activity (SB 703704) on GAG release after injury were assessed.

RESULTS

GAG release from injured cartilage was highest during the first 4 hours after injury, but remained higher than that in controls during the first 24 hours postinjury, and was not affected by inhibitors of biosynthesis or degradative enzymes. GAG release during the period 24-72 hours postinjury was similar to that in uninjured controls, but the MMP inhibitor CGS 27023A reduced cumulative GAG loss from injured samples between 1 day and 7 days postinjury. Other inhibitors of enzymatic degradation or biosynthesis had no significant effect on GAG release.

CONCLUSION

Injurious compression of articular cartilage induces an initially high rate of GAG release from the tissue, which could not be inhibited, consistent with mechanical damage. However, the finding that MMP inhibition reduced GAG loss in the days following injury suggests a potential therapeutic intervention.

Mechanical regulation of mitogen-activated protein kinase signaling in articular cartilage

Articular chondrocytes respond to mechanical forces by alterations in gene expression, proliferative status, and metabolic functions. Little is known concerning the cell signaling systems that receive, transduce, and convey mechanical information to the chondrocyte interior. Here, we show that ex vivo cartilage compression stimulates the phosphorylation of ERK1/2, p38 MAPK, and SAPK/ERK kinase-1 (SEK1) of the JNK pathway. Mechanical compression induced a phased phosphorylation of ERK consisting of a rapid induction of ERK1/2 phosphorylation at 10 min, a rapid decay, and a sustained level of ERK2 phosphorylation that persisted for at least 24 h. Mechanical compression also induced the phosphorylation of p38 MAPK in strictly a transient fashion, with maximal phosphorylation occurring at 10 min. Mechanical compression stimulated SEK1 phosphorylation, with a maximum at the relatively delayed time point of 1 h and with a higher amplitude than ERK1/2 and p38 MAPK phosphorylation. These data demonstrate that mechanical compression alone activates MAPK signaling in intact cartilage. In addition, these data demonstrate distinct temporal patterns of MAPK signaling in response to mechanical loading and to the anabolic insulin-like growth factor-I. Finally, the data indicate that compression coactivates distinct signaling pathways that may help define the nature of mechanotransduction in cartilage.

The role of cell seeding density and nutrient supply for articular cartilage tissue engineering with deformational loading

OBJECTIVE

Functional tissue engineering (FTE) of articular cartilage involves the use of physiologically relevant mechanical signals to encourage the growth of engineered constructs. The goal of this study was to determine the utility of deformational loading in enhancing the mechanical properties of chondrocyte-seeded agarose hydrogels, and to investigate the role of initial cell seeding density and nutrient supply in this process.

DESIGN

Chondrocyte-seeded agarose hydrogels were cultured in free-swelling conditions or with intermittent deformational loading (10% deformation, 1 Hz, 1 h on/ 1 h off, 3 h per day, five days per week) over a two-month culture period. Disks were seeded at lower (10 million cells/ml) and higher (60 million cells/ml) seeding densities in the context of a greater medium supply than previous studies (decreasing the number of cells/ml feed medium/day) and with an increasing concentration of fetal bovine serum (10 or 20% FBS).

RESULTS

Under these more optimal nutrient conditions, at higher seeding densities and high serum concentration (20% FBS), dynamically loaded constructs show >2-fold increases in material properties relative to free-swelling controls. After two months of culture, dynamically loaded constructs achieved a Young's modulus of approximately 185 kPa and a dynamic modulus (at 1 Hz) of approximately 1.6 MPa, with a frequency dependent response similar to that of the native tissue. These values represent approximately 3/4 and approximately 1/4 the values measured for the native tissue, respectively. While significant differences were found in mechanical properties, staining and bulk measurements of both proteoglycan and collagen content of higher seeding density constructs revealed no significant differences between free-swelling and loading groups. This finding indicates that deformational loading may act to increase material properties via differences in the structural organization, the production of small linker ECM molecules, or by modulating the size of macromolecular proteoglycan aggregates.

CONCLUSIONS

Taken together, these results point to the utility of dynamic deformational loading in the mechanical preconditioning of engineered articular cartilage constructs and the necessity for increasing feed media volume and serum supplementation with increasing cell seeding densities.

Hydrostatic pressure modulates proteoglycan metabolism in chondrocytes seeded in agarose

OBJECTIVE

To investigate the effect of isolated hydrostatic pressure on proteoglycan metabolism in chondrocytes.

METHODS

Bovine articular chondrocytes cultured in agarose gels were subjected to 5 MPa hydrostatic pressure for 4 hours in either a static or a pulsatile (1 Hz) mode, and changes in glycosaminoglycan (GAG) synthesis, hydrodynamic size, and aggregation properties of proteoglycans and aggrecan messenger RNA (mRNA) levels were determined.

RESULTS

The application of 5 MPa static pressure caused a significant increase in GAG synthesis of 11% (P < 0.05). Column chromatography showed that this increase in GAG synthesis was associated with large proteoglycans. In addition, semiquantitative reverse transcriptase-polymerase chain reaction showed a 4-fold increase in levels of aggrecan mRNA (P < 0.01).

CONCLUSION

Hydrostatic pressure in isolation, which does not cause cell deformation, can affect proteoglycan metabolism in chondrocytes cultured in agarose gels, indicating an important role of hydrostatic pressure in the regulation of extracellular matrix turnover in articular cartilage.

Fibronectin metabolism of cartilage explants in response to the frequency of intermittent loading

Chondrocytes within articular cartilage experience complete unloading between loading cycles and in so doing utilize mechanical signals to regulate their own metabolic activities. A strongly elevated fibronectin content is an early feature in osteoarthritis and appears to be related to increases in both the synthesis and retention of this glycoprotein. The objectives of this study were to investigate systematically whether the frequency of intermittently applied cyclic mechanical loading of cartilage explants alters the biosynthesis and retention of fibronectin, and to assess whether it is possible to induce in vitro osteoarthritic-like changes of this metabolic parameter by mechanical means over a period of 6 days. Cartilage plugs consisting of viability-checked chondrocytes were exposed to sinusoidal cyclic compressive pressure alterations of 0.1, 0.5 or 1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5, 10 or 20 s, followed by an unloading period of 10, 100 or 1000 s, and compared to unloaded reference plugs from the same joint and topographic origin. The incorporation of radioactive precursor into fibronectin during the last 18 h, the content of fibronectin, and the viability of chondrocytes were determined. Our data revealed that (a) the fibronectin synthesis was selectively, but non-linearly affected by the frequency of intermittent loads applied (as defined by the frequency of the applied force, the duration of the loading cycle and the duration of the force-free period between each loading cycle), and that (b) the retention of endogenous fibronectin and proteins within loaded cartilage explants is strongly elevated. These data support our hypothesis that the mechanical factor "frequency of intermittent loading" seems to be the crucial mechanical parameter controlling the metabolism of chondrocytes. The effect of the frequency of intermittent loading cannot be described by a simple statistical correlation, so that no specific predictions are possible. However, our results imply that distinct loading protocols have been established that can induce alterations of the fibronectin metabolism similar to those observed in human and animal osteoarthritis.

The sulfation pattern of chondroitin sulfate from articular cartilage explants in response to mechanical loading

Chondrocytes within articular cartilage experience complete unloading between loading cycles thereby utilizing mechanical signals to regulate their own anabolic and catabolic activities. Structural alterations of proteoglycans (PGs) during aging and the development of osteoarthritis (OA) have been reported; whether these can be attributed to altered load or compression is largely unknown. We report here on experiments in which the effect of intermittent loading on the fine structure of newly synthesized chondroitin sulfate (CS) in bovine articular cartilage explants was examined. Tissues were subjected for 6 days to cyclic compressive pressure using a sinusoidal waveform of 0.1, 0.5 or 1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5, 10 or 20 s, followed by an unloading period lasting 10, 100 or 1000 s. During the final 18 h of the culture, cartilage explants were radiolabeled with 50 microCi/ml D-6-[3H]glucosamine, and newly synthesized as well as endogenous CS chains were isolated after proteinase solubilization of the tissue. CS chains were depolymerized with chondroitinase ABC and ACII, and the 3H-digestion products were quantified after fractionation by high-performance anion-exchange chromatography using a CarboPac PA1 column. Intermittently applied cyclic mechanical loading did not affect the proportion of 4- and 6-sulfated disaccharide repeats, but caused a significant decrease in the abundance of the 4,6-disulfated nonreducing terminal galNAc residues. In addition, loading induced elongation of CS chains. Taken together, these data provide evidence for the first time that long-term in vitro loading results in marked and reproducible changes in the fine structure of newly synthesized CS, and that accumulation of such chains may in turn modify the physicochemical and biological response of articular cartilage. Moreover, data presented here suggest that in vitro dynamic compression of cartilage tissue can induce some of the same alterations in CS sulfation that have previously been shown to occur during the development of degenerative joint diseases such as OA.

Cyclic hydrostatic pressure enhances the chondrogenic phenotype of human mesenchymal progenitor cells differentiated in vitro

Much attention has been given to the influences of bioactive factors on mesenchymal progenitor cell differentiation and proliferation, but few studies have examined the effect of mechanical factors on these cells. This study examined the effects of cyclic hydrostatic pressure on human bone marrow-derived mesenchymal progenitor cells undergoing chondrogenic differentiation. Aggregates of bone marrow-derived mesenchymal progenitor cells were cultured in a defined chondrogenic medium and were subjected to cyclic hydrostatic pressure. Aggregates were loaded at various time points: single (day 1 or 3) or multiple (days 1-7). At 14 and 28 days, aggregates were harvested for histology, immunohistochemistry, and quantitative DNA and matrix macromolecule analysis. The aggregates loaded for a single day did not demonstrate significant changes in proteoglycan and collagen contents compared with the non-loaded controls. In contrast, for the multi-day loaded aggregates, statistically significant increases in proteoglycan and collagen contents were found on both day 14 and day 28. Aggregates loaded for seven days were larger and histological staining indicated a greater matrix/cell ratio. This study indicates that hydrostatic pressure enhances the cartilaginous matrix formation of mesenchymal progenitor cells differentiated in vitro, and suggests that mechanical forces may play an important role in cartilage repair and regeneration in vivo.

Rapid regulation of collagen but not metalloproteinase 1, 3, 13, 14 and tissue inhibitor of metalloproteinase 1, 2, 3 expression in response to mechanical loading of cartilage explants in vitro

This study analyzes the molecular response of articular chondrocytes to short-term mechanical loading with a special focus on gene expression of molecules relevant for matrix turnover. Porcine cartilage explants were exposed to static and dynamic unconfined compression and viability of chondrocytes was assessed to define physiologic loading conditions. Cell death in the superficial layer correlated with mechanical loading and occurred at peak stresses >or=6 MPa and a cartilage compression above 45%. Chondrocytes in native cartilage matrix responded to dynamic loading by rapid and highly specific suppression of collagen expression. mRNA levels dropped 11-fold (collagen 2; 6 MPa, P=0.009) or 14-fold (collagen 1; 3 and 6 MPa, P=0.009) while levels of aggrecan, tenascin-c, matrix metalloproteinases (MMP1, 3, 13, 14), and their inhibitors (TIMP1-3) did not change significantly. Thus, dynamic mechanical loading rapidly shifted the balance between collagen and aggrecan/tenascin/MMP/TIMP expression. A better knowledge of the chondrocyte response to mechanical stress may improve our understanding of mechanically induced osteoarthrits.

Mechano-electrochemical properties of articular cartilage: their inhomogeneities and anisotropies

In this chapter, the recent advances in cartilage biomechanics and electromechanics are reviewed and summarized. Our emphasis is on the new experimental techniques in cartilage mechanical testing, new experimental and theoretical findings in cartilage biomechanics and electromechanics, and emerging theories and computational modeling of articular cartilage. The charged nature and depth-dependent inhomogeneity in mechano-electrochemical properties of articular cartilage are examined, and their importance in the normal and/or pathological structure-function relationships with cartilage is discussed, along with their pathophysiological implications. Developments in theoretical and computational models of articular cartilage are summarized, and their application in cartilage biomechanics and biology is reviewed. Future directions in cartilage biomechanics and mechano-biology research are proposed.

Perfusion increases cell content and matrix synthesis in chondrocyte three-dimensional cultures

This work examines the effect of perfusion on the cell content and sulfated glycosaminoglycan synthesis of ovine articular chondrocytes cultured on polyglycolic acid (PGA) scaffolds. Ovine chondrocytes were seeded onto the scaffolds and cultured for up to 9 days. During this time the cells were subjected to perfusion at velocities of up to 170 microm/s. The samples were radiolabeled with (35)SO(4) to quantify the overall synthesis of sulfated glycosaminoglycans (S-GAGs) and the retention of S-GAGs in the construct. The constructs were also analyzed for DNA as a measure of cellular content. Constructs subjected to perfusion during culture had significantly higher DNA contents than those cultured statically. Matrix metabolism was also modulated by perfusion, with this modulation depending on culture duration. Nine days of continuous perfusion increased S-GAG synthesis and deposition by approximately 40% when compared with static controls. However, perfusion at early time points (during the initial 3-day culture period) suppressed the synthesis and retention of S-GAGs when compared with controls. This work demonstrates the effects of perfusion on cartilage growth in vitro, illustrating the use of perfusion to modulate the growth of tissue-engineered cartilage constructs, and potentially enhance tissue growth in vitro.

Static and dynamic compression modulate matrix metabolism in tissue engineered cartilage

Static and dynamic compression are known to modulate the metabolism of articular cartilage. The present study focused on determining the effects of compressive loading on the metabolism of sulfated glycosaminoglycans (S-GAG) and protein in tissue engineered cartilage constructs. Cartilage constructs were subjected to static or dynamic compression for 24 h and radiolabeled with 35SO4 and 3H-proline to assess the total synthesis and percentage retention of S-GAG and total protein, respectively. Static compression at an amplitude of 50% suppressed the synthesis of both S-GAG and protein by 35% and 57%, respectively. Dynamic compression at an amplitude of 5% had stimulatory effects on synthesis that were dependent on the static offset compression amplitude (10% or 50%) and dynamic compression frequency (0.001 or 0.1 Hz). Thus, tissue engineered cartilage demonstrated the ability to respond to mechanical loading in a manner similar to that observed with articular cartilage. Mechanical loading may therefore potentially be used to modulate the growth of cartilaginous tissues in vitrd, potentially facilitating the culture of functional cartilage tissues suitable for implantation.

From lab bench to market: critical issues in tissue engineering

Revolutionary advances in tissue engineering are redefining approaches to tissue repair and transplantation through the creation of replacement tissues that remain biointeractive after implantation, imparting physiologic functions as well as structure to the tissue or organ damaged by disease or trauma.(1,2) Over the last decade this field has moved from "science fiction" to "science fact" with the research-oriented acceptance of its potential to regulatory approvals allowing commercial products to be available for use in many countries. The maintenance of tissue integrity, functionality, and viability from cell seeding through product manufacture, shipping, and end-use has been accomplished through innovations in design and scale-up of both tissue growth and preservation processes. These unique systems have enabled the delivery of tissue-engineered products that are uniform inter- and intra-lot, readily available as off-the-shelf products, easy to use, and efficacious. Skin replacement products are the most advanced, with several tissue-engineered wound care materials on the market in the U.S. and in several international communities.(3-5) The potential impact of this field is far broader, offering novel solutions to the medical field for drug screening and development, genetic engineering, and total tissue and organ replacement.

Solid stress facilitates spheroid formation: potential involvement of hyaluronan

When neoplastic cells grow in confined spaces in vivo, they exert a finite force on the surrounding tissue resulting in the generation of solid stress. By growing multicellular spheroids in agarose gels of defined mechanical properties, we have recently shown that solid stress inhibits the growth of spheroids and that this growth-inhibiting stress ranges from 45 to 120 mmHg. Here we show that solid stress facilitates the formation of spheroids in the highly metastatic Dunning R3327 rat prostate carcinoma AT3.1 cells, which predominantly do not grow as spheroids in free suspension. The maximum size and the growth rate of the resulting spheroids decreased with increasing stress. Relieving solid stress by enzymatic digestion of gels resulted in gradual loss of spheroidal morphology in 8 days. In contrast, the low metastatic variant AT2.1 cells, which grow as spheroids in free suspension as well as in the gels, maintained their spheroidal morphology even after stress removal. Histological examination revealed that most cells in AT2.1 spheroids are in close apposition whereas a regular matrix separates the cells in the AT3.1 gel spheroids. Staining with the hyaluronan binding protein revealed that the matrix between AT3.1 cells in agarose contained hyaluronan, while AT3.1 cells had negligible or no hyaluronan when grown in free suspension. Hyaluronan was found to be present in both free suspensions and agarose gel spheroids of AT2.1. We suggest that cell-cell adhesion may be adequate for spheroid formation, whereas solid stress may be required to form spheroids when cell-matrix adhesion is predominant. These findings have significant implications for tumour growth, invasion and metastasis.

Mechanical compression alters gene expression and extracellular matrix synthesis by chondrocytes cultured in collagen I gels

Articular cartilage responds to its mechanical environment through altered cell metabolism and matrix synthesis. In this study, isolated articular chondrocytes were cultured in collagen type I gels and exposed to uniaxial static compression of 0%, 25%, or 50% of original thickness for 0.5, 4, and 24 h, and to oscillatory (25 +/- 4%, 1 Hz) compression for 24 h. The cellular response was assessed through competitive and real-time RT-PCR to quantify expression of genes for collagen type I, collagen type II, and aggrecan core protein, and through radiolabelled proline and sulfate incorporation to quantify protein and proteoglycan synthesis rates. Static compression for 24 h inhibited expression of collagen I and II mRNAs and inhibited 3H-proline and 35S-sulfate incorporation. The mRNA expression exhibited transient fluctuations at intermediate time points. Oscillatory compression had no effect upon mRNA expression, and 24 h after release from static compression, there was no difference in collagen II or aggrecan mRNA, while there was an inhibition of collagen I. We conclude that the chondrocytes maintained some aspects of their ability to sense and respond to static compression, despite a biochemical and mechanical environment which is different from that in tissue. This suggests that mechanical stimuli may be useful in modulating chondrocyte metabolism in tissue engineering systems using fibrillar protein scaffolds.

Cartilage tissue remodeling in response to mechanical forces

Recent studies suggest that there are multiple regulatory pathways by which chondrocytes in articular cartilage sense and respond to mechanical stimuli, including upstream signaling pathways and mechanisms that may lead to direct changes at the level of transcription, translation, post-translational modifications, and cell-mediated extracellular assembly and degradation of the tissue matrix. This review focuses on the effects of mechanical loading on cartilage and the resulting chondrocyte-mediated biosynthesis, remodeling, degradation, and repair of this tissue. The effects of compression and tissue shear deformation are compared, and approaches to the study of mechanical regulation of gene expression are described. Of particular interest regarding dense connective tissues, recent experiments have shown that mechanotransduction is critically important in vivo in the cell-mediated feedback between physical stimuli, the molecular structure of newly synthesized matrix molecules, and the resulting macroscopic biomechanical properties of the tissue.

Biodegradable polymeric scaffolds for musculoskeletal tissue engineering

Biodegradable scaffolds have played an important role in a number of tissue engineering attempts over the past decade. The goal of this review article is to provide a brief overview of some of the important issues related to scaffolds fabricated from synthetic biodegradable polymers. Various types of such materials are available; some are commercialized and others are still in the laboratories. The properties of the most common of these polymers are discussed here. A variety of fabrication techniques were developed to fashion polymeric materials into porous scaffolds, and a selection of these is presented. The very important issue of scaffold architecture, including the topic of porosity and permeability, is discussed. Other areas such as cell growth on scaffolds, surface modification, scaffold mechanics, and the release of growths factors are also reviewed. A summary outlining the common themes in scaffold-related science that are found in the literature is presented.

Beta-1 integrin expression by human nasal chondrocytes in microcarrier spinner culture

Beta-1 integrin plays a major role in cell attachment and is believed to be involved in mediating the interactions of chondrocytes with their environment. We previously reported that articular chondrocytes propagated in microcarrier spinner culture proliferated and reexpressed their chondrocytic protein. The goal of the present study was to investigate the expression of beta-1 integrin by chondrocytes growing on the surface of microcarriers. Nasal chondrocytes (4 x 10(3)/cm(2)) were seeded on microcarriers and incubated at 37 degrees C, 5% CO(2), 60 rpm. Expression of chondrocyte markers and beta-1 integrin was determined using reverse transcriptase-polymerase chain reaction and immunocytochemical analyses. De novo synthesis of sulfate-containing proteoglycans was studied using 35SO(4) incorporation techniques. Like articular chondrocytes propagated in microcarrier spinner culture, nasal chondrocytes expressed high levels of collagen type II mRNA, whereas collagen type I mRNA levels were low. Aggrecan mRNA was detectable and levels of de novo 35SO(4) incorporation were high. Chondrocytes immunostained intensely for collagen type II and keratan sulfate but did not stain for collagen type I. beta-1 integrin mRNA levels were high, and the protein was immunolocalized to regions of cell-to-cell or cell-to-microcarrier contact. The fact the chondrocytes expressed high levels of beta-1 integrin raises the possibility that this integrin molecule has a role in the maintenance of the chondrocytic phenotype.

Mature full-thickness articular cartilage explants attached to bone are physiologically stable over long-term culture in serum-free media

Mature tissue explants containing the entire depth of articular cartilage, calcified and uncalcified, attached to a thin layer of subchondral bone were isolated from bovine humeral heads of 1-2-year-old steers. These explants were placed in defined serum-free culture medium for a period of 3 weeks to investigate their biological and mechanical stability and thus to determine their potential utility in studies of cartilage physiology. Tissue mass remained constant over the culture period and no evident tissue swelling or distortion was observed. Chondrocytes were viable in all zones at the time of tissue isolation and throughout the culture period, with the exception of a thin layer of cells at the articular surface and the cut radial edge of the disks. Proteoglycan metabolism attained a steady state after 5 days of culture when the rate of loss of proteoglycan to culture media was compensated by new synthesis to maintain a stable proteoglycan content. Collagen metabolism was also stable with a constant content of type II collagen and a constant content of denatured collagen II throughout culture; the content of the C-propeptide of type II procollagen as a measure of procollagen synthesis, dropped slightly during the first week to attain a steady state after that time. Dynamic and equilibrium mechanical properties of these explant disks were also stable confirming maintenance of these tissue properties during long-term culture. In addition, the disk geometry of the system, with the cut surface in the bone parallel to the intact articular surface, is well-suited to study tissue regulation by mechanical load. Taken together, the stability of these indicators of tissue physiology indicates the maintenance in serum-free conditions of normal metabolism for organ cultures containing full-depth mature articular cartilage attached to bone.

The role of microtubules in the regulation of proteoglycan synthesis in chondrocytes under hydrostatic pressure

Chondrocytes of the articular cartilage sense mechanical factors associated with joint loading, such as hydrostatic pressure, and maintain the homeostasis of the extracellular matrix by regulating the metabolism of proteoglycans (PGs) and collagens. Intermittent hydrostatic pressure stimulates, while continuous high hydrostatic pressure inhibits, the biosynthesis of PGs. High continuous hydrostatic pressure also changes the structure of cytoskeleton and Golgi complex in cultured chondrocytes. Using microtubule (MT)-affecting drugs nocodazole and taxol as tools we examined whether MTs are involved in the regulation of PG synthesis in pressurized primary chondrocyte monolayer cultures. Disruption of the microtubular array by nocodazole inhibited [(35)S]sulfate incorporation by 39-48%, while MT stabilization by taxol caused maximally a 17% inhibition. Continuous hydrostatic pressure further decreased the synthesis by 34-42% in nocodazole-treated cultures. This suggests that high pressure exerts its inhibitory effect through mechanisms independent of MTs. On the other hand, nocodazole and taxol both prevented the stimulation of PG synthesis by cyclic 0. 5 Hz, 5 MPa hydrostatic pressure. The drugs did not affect the structural and functional properties of the PGs, and none of the treatments significantly affected cell viability, as indicated by the high level of PG synthesis 24-48 h after the release of drugs and/or high hydrostatic pressure. Our data on two-dimensional chondrocyte cultures indicate that inhibition of PG synthesis by continuous high hydrostatic pressure does not interfere with the MT-dependent vesicle traffic, while the stimulation of synthesis by cyclic pressure does not occur if the dynamic nature of MTs is disturbed by nocodazole. Similar phenomena may operate in cartilage matrix embedded chondrocytes.

Ruthenium hexaammine trichloride chemography for aggrecan mapping in cartilage is a sensitive indicator of matrix degradation

We developed a new quantitative histochemical method for mapping aggrecan content in articular cartilage and applied it to models of cartilage degradation. Ruthenium hexaammine trichloride (RHT) forms co-precipitates with aggrecan, the main proteoglycan component of cartilage, and was previously found to be a good fixative in aiding the maintenance of chondrocyte morphology. We show that these RHT-aggrecan precipitates generate a positive chemographic signal on autoradiographic emulsions, in the absence of any radioactivity in the tissue section, via a process similar to the autometallographic process used previously for localization of trace metals ions in tissues. By exploiting the inherent depth-dependence of aggrecan concentration in adult articular cartilage, we demonstrated that the density of silver grains produced by RHT-derived chemography on autoradiographic emulsions correlated with locally measured aggrecan concentration as determined by the dimethylmethylene blue assay of microdissected tissue from these different depths of cartilage. To explore the benefits of this new method in monitoring tissue degradation, cartilage explants were degraded during culture using interleukin-1 (IL-1) or digested after culture using chondroitinase and keratinase. The RHT chemographic signal derived from these samples, compared to controls, showed sensitivity to loss of aggrecan and distinguished cell-mediated loss (IL-1) from degradation due to addition of exogenous enzymes. The RHT-derived chemographic grain density represents an interesting new quantitative tool for histological analysis of cartilage in physiology and in arthritis.