The effects of high magnitude cyclic tensile load on cartilage matrix metabolism in cultured chondrocytes

Superficial zone protein affects boundary lubrication on the surface of mandibular condylar cartilage

We examined the localization and boundary lubricating function of superficial zone protein (SZP) on the surface of mandibular condylar cartilage. Chondrocytes were separated from the surface layer of mandibular condylar cartilage of 6- to 9-month-old female pigs. A cyclic tensile strain of 7% or 21% cell elongation was applied to the cultured chondrocytes. Gene expression levels of cartilage matrix proteins and secretory phospholipase A(2) (sPLA(2)) were quantified by real-time polymerase chain reaction analysis. The friction coefficient of the mandibular condylar surface was measured by a friction tester before and after treatment with 0.1 U/ml sPLA(2). Significantly higher mRNA levels of SZP and type I collagen were found in chondrocytes from the superficial layer than in those in the other layers. The SZP mRNA level was up-regulated by cyclic tensile strain of 7% and 21% cell elongation. Cyclic tensile strain of 21% cell elongation up-regulated the sPLA(2) mRNA level. The friction coefficient of the condylar surface was increased significantly by treatment with sPLA(2). The removal of SZP from the surface layer of mandibular condylar cartilage by sPLA(2) resulted in a significant increase in the friction coefficient on the surface of articular cartilage.

Modulation of hyaluronan catabolism in chondrocytes by mechanical stimuli

Hyaluronan (HA) is a component of the extracellular matrices of cartilage contributing to the structural and functional integrity. HA metabolism is regulated by both anabolic and catabolic processes; however, a great deal more of the detail has been unknown yet. The purpose of this study was to clarify the effect of excessive mechanical load on the expression and activity of hyaluronidase (HYAL) in chondrocytes with a special reference to the expressions of IL-1beta and tumor necrosis factor (TNF)-alpha. A cyclic tensile load of 22.8% cell elongation, regarded as an excessive mechanical stimulus, was applied to cultured rabbit knee articular chondrocytes. HYAL1, HYAL2, IL-1beta, and TNF-alpha mRNA levels were examined by quantitative real-time PCR analysis. The HYAL activity in culture medium was examined by HA zymography. Both HYAL1 and HYAL2 mRNA levels were upregulated significantly by the loading in cultured chondrocytes. HYAL activity was also enhanced as compared with unloaded controls. The IL-1beta mRNA level was upregulated significantly by the loading, and TNF-alpha mRNA level was slightly upregulated. HYAL1 and HYAL2 mRNA levels were upregulated significantly by IL-1beta treatment, resulting in a slight increase in HYAL activity. These results show that the expression of HYAL1 and HYAL2 in articular chondrocytes is enhanced by excessive mechanical stimuli and affected in part by induction of IL-1beta, leading to HA catabolism in articular cartilage.

Effects of mechanical stimuli on the synthesis of superficial zone protein in chondrocytes

Superficial zone protein (SZP) has been demonstrated to contribute to the boundary lubrication in synovial joints. This study was designed to clarify the modulation of SZP expression by mechanical stress in articular chondrocytes. Cyclic tensile strains of 7 and 21% cell elongation were applied to cultured chondrocytes obtained from porcine mandibular condylar cartilage. The mRNA levels of SZP, IL-1 beta, and TGF-beta1 were examined by a quantitative real-time PCR analysis. Protein level of SZP was examined by Western blotting. The SZP mRNA level was significantly upregulated after 12, 24, and 48 h by 7% elongation. Although SZP mRNA level was upregulated by 21% elongation after 12 h, it decreased to a lower level than the control after 48 h. The TGF-beta1 mRNA level exhibited an almost similar change to SZP. The IL-1 beta mRNA level was not changed markedly by 7% elongation. However, the IL-1 beta mRNA level was significantly increased by a 12-h application of 21% elongation. Western blot analysis revealed that the SZP expression was increased by 7% elongation, but decreased remarkably by 21% elongation. It is suggested from these findings that the SZP expression level in the chondrocytes is enhanced by optimal mechanical stimuli, but inhibited by excessive loading partly affected by TGF-beta1 and IL-1 beta, leading to the deterioration of joint lubrication.

Mechanical load inhibits IL-1 induced matrix degradation in articular cartilage

OBJECTIVE

Osteoarthritis is a disease process of cellular degradation of articular cartilage caused by mechanical loads and inflammatory cytokines. We studied the cellular response in native cartilage subjected to a mechanical load administered simultaneously with an inflammatory cytokine interleukin-1 (IL-1), hypothesizing that the combination of load and cytokine would result in accelerated extracellular matrix (ECM) degradation.

METHODS

Mature bovine articular cartilage was loaded for 3 days (stimulation) with 0.2 and 0.5 MPa stresses, with and without IL-1 (IL-1alpha, 10 ng/ml), followed by 3 days of no stimulation (recovery). Aggrecan and collagen loss were measured as well as aggrecan cleavage using monoclonal antibodies AF-28 and BC-3 for cleavage by aggrecanases (ADAMTS) and matrix metalloproteinases (MMPs), respectively.

RESULTS

Incubation with IL-1 caused aggrecan cleavage by aggrecanases and MMPs during the 3 days of stimulation. A load of 0.5 MPa inhibited the IL-1-induced aggrecan loss while no inhibition was found for the 0.2 MPa stress. There was no collagen loss during the treatments but upon load and IL-1 removal proteoglycan and collagen loss increased. Load itself under these conditions was found to have no effect when compared to the unloaded controls.

CONCLUSIONS

A mechanical load of sufficient magnitude can inhibit ECM degradation by chondrocytes when stimulated by IL-1. The molecular mechanisms involved in this process are not clear but probably involve altered mechanochemical signal transduction between the ECM and chondrocyte.

Biomechanical and biochemical characteristics of the mandibular condylar cartilage

The human masticatory system consists of a mandible which is able to move with respect to the skull at its bilateral temporomandibular joint (TMJ) through contractions of the masticatory muscles. Like other synovial joints, the TMJ is loaded mechanically during function. The articular surface of the mandibular condyle is covered with cartilage that is composed mainly of collagen fibers and proteoglycans. This construction results in a viscoelastic response to loading and enables the cartilage to play an important role as a stress absorber during function. To understand its mechanical functions properly, and to assess its limitations, detailed information about the viscoelastic behavior of the mandibular condylar cartilage is required. The purpose of this paper is to review the fundamental concepts of the biomechanical behavior of the mandibular condylar cartilage. This review consists of four parts. Part 1 is a brief introduction of the structure and function of the mandibular condylar cartilage. In Part 2, the biochemical composition of the mandibular condylar cartilage is summarized. Part 3 explores the biomechanical properties of the mandibular condylar cartilage. Finally, Part 4 relates this behavior to the breakdown mechanism of the mandibular condylar cartilage which is associated with the progression of osteoarthritis in the TMJ.

Signalling cascades in mechanotransduction: cell-matrix interactions and mechanical loading

Mechanical loading of articular cartilage stimulates the metabolism of resident chondrocytes and induces the synthesis of molecules to maintain the integrity of the cartilage. Mechanical signals modulate biochemical activity and changes in cell behavior through mechanotransduction. Compression of cartilage results in complex changes within the tissue including matrix and cell deformation, hydrostatic and osmotic pressure, fluid flow, altered matrix water content, ion concentration and fixed charge density. These changes are detected by mechanoreceptors on the cell surface, which include mechanosensitive ion channels and integrins that on activation initiate intracellular signalling cascades leading to tissue remodelling. Excessive mechanical loading also influences chondrocyte metabolism but unlike physiological stimulation leads to a quantitative imbalance between anabolic and catabolic activity resulting in depletion of matrix components. In this article we focus on the role of mechanical signalling in the maintenance of articular cartilage, and discuss how alterations in normal signalling can lead to pathology.

Modeling linear vibration of cell nucleus in low intensity ultrasound field

Therapeutic ultrasound of low to medium intensity is known to induce alterations in structure and functioning of cells and tissues, both in vivo and in vitro. Such effects, including excitation or inhibition of action potentials, enhanced angiogenesis rate, increased membrane permeability and changes in molecular expression, cannot be attributed in many cases to rising temperatures or the presence of gas bubbles. This study attempts to find a possible alternative explanation for the cases where neither thermal effects nor cavitation mechanisms count. We focus our attention on the complex and dense structure of cell cytoplasm, looking for periodic separating forces and relative motion between intracellular elements, such as the nucleus, and the structure in which they are embedded. It is hypothesized that relative oscillatory displacements between intracellular elements of different densities might appear in cells in response to low intensity therapeutic ultrasound (LITUS). Those displacements might induce alterations in cell structure and functioning. A linear model is constructed and solved for a spherical object, representing a typical organelle such as the nucleus, within a homogenous viscoelastic medium that vibrates uniformly. The structure in which the object is embedded is described by four different rheologic models, including viscous fluid, elastic solid, and Voigt and Maxwell viscoelastic constructs. It is found that cyclic intracellular displacements comparable with and even larger than the mean thermal fluctuations may be obtained due to LITUS irradiation in conditions where the relative motion of organelles is dominated by elastic response, or where the effective viscosity of the cytoplasm is low. Resonance frequency at which intracellular vibration of maximal amplitude is obtained is found to lie within the low LITUS frequency range, i.e., tens to hundreds of kHz. Local intracellular strain on the order of 0.5% is found for 1 microm organelle in 10 microm cell under typical LITUS settings. It is suggested that fatigue-like, cumulative effect underlie the transfer of the intracellular strain into biologic alterations.

Bioreactor for Biaxial Mechanical Stimulation to Tissue Engineered Constructs

The complex structure and properties of biological tissues as well as their in situ environment often make it difficult to self-heal. A suitable replacement tissue may be created in vitro through tissue engineering approaches and mechanical stimulation of tissue constructs. A new biaxial bioreactor was designed, constructed, and evaluated for the purposes of developing constructs with specific functional characteristics. Once constructed and assembled, the bioreactor was tested for position accuracy and application of strain. Additionally, a tissue construct was tested in the chamber and compared with a nonstimulated construct. Results showed high position accuracy, but some loss between applied strain via grip movement and strain experienced by the scaffold. The tested construct exhibited an increase in cells and matrix deposition in comparison to the nonstimulated construct. This biaxial bioreactor will be useful for mechanically stimulating tissue constructs in two perpendicular directions to create implants for tissues requiring preferred compressive and tensile resistances.

Synergistic effect of chondroitin sulfate and cyclic pressure on biochemical and morphological properties of chondrocytes from articular cartilage

OBJECTIVE

To investigate the synergistic effect of chondroitin sulfate (CS) and cyclic pressure on the biochemical and morphological properties of chondrocytes isolated from articular cartilage and cultured in alginate matrix.

METHODS

The chondrocytes obtained from articular cartilage of goat femoropatellar joint were isolated and cultured in alginate matrix. The cells were exposed to CS (100 microg/ml) along with cyclic pressure of 1.2 MPa and 2.4 MPa and biochemical analysis of DNA, proteoglycan, collagen and protease activity was carried out in different matrix fractions, i.e., cellular matrix (CM) and further removed matrix (FRM) and in culture medium. The morphological studies of chondrocytes were carried out using transmission electron microscopy (TEM).

RESULTS

The treatment of chondrocytes with CS along with cyclic pressure increased the rate of cell proliferation relative to control (without load and in the absence of CS) and CS alone (P<0.001). The proteoglycan content in CM increased in the presence of CS alone (P<0.05) as well as CS with cyclic pressure (P<0.001). The specific activity of protease in CM and FRM decreased in the presence of CS with cyclic pressure relative to control (P<0.001). The TEM images showed abundant CM, improved cell morphology and matrix organization in the presence of CS and cyclic load treatment.

CONCLUSIONS

The results of this study suggested that in the presence of CS along with cyclic loading, the cellular ability to utilize and incorporate exogenous CS as extracellular matrix improved, as compared to CS alone.

The effect of bioreactor induced vibrational stimulation on extracellular matrix production from human derived fibroblasts

To study the affect of mechanical stimuli on human laryngeal fibroblasts, we developed bioreactors capable of vibrating cell seeded substrates at frequencies and displacements comparable to measured phonation values in human subjects. In addition, we developed a means of harvesting the secreted matrix as a bulk biomaterial by removing the polymer foam using an organic solvent. Using the system human derived laryngeal fibroblasts were subjected to vibrational stimuli (100 Hz) for 1-21 days. Following mechanical conditioning, extracellular matrix and matrix related gene expression, cytokine production, matrix protein accumulation, and construct material properties were assessed with DNA microarray, enzyme linked immunosorbent, indirect immunofluorescent, and uni-axial tensile assays respectively. The results show that vocal fold-like vibrational stimuli is sufficient to influence the expression of several key matrix and matrix related genes, enhance the secretion of the profibrotic cytokine TGFbeta1, increase the accumulation of the extracellular matrix proteins, fibronectin and collagen type 1, as well as enhance construct stiffness compared to non-stimulated controls. Our results demonstrate that high frequency substrate vibration, like cyclic strain, can accelerate matrix deposition from human derived laryngeal fibroblasts. The study supports the notion that preconditioning regimens using human cells may be useful for producing cell derived biomaterials for therapeutic application.

Chondrogenesis, bone morphogenetic protein-4 and mesenchymal stem cells

OBJECTIVE

As adult cartilage has very limited potential to regenerate, cartilage repair is challenging. Available treatments have several disadvantages, including formation of fibrocartilage instead of hyaline-like cartilage, as well as eventual ossification of the newly formed tissue. The focus of this review is the application of bone morphogenetic protein-4 (BMP-4) and mesenchymal stem cells (MSCs) in cartilage repair, a combination that could potentially lead to the formation of permanent hyaline-like cartilage in the defect.

METHODS

This review is based on recent literature in the orthopaedic and tissue engineering fields, and is focused on MCSs and bone morphogenetic proteins (BMPs).

RESULTS

BMP-4, a stimulator of chondrogenesis, both in vitro and in vivo, is a potential therapeutic agent for cartilage regeneration. BMP-4 delivery can improve the healing process of an articular cartilage defect by stimulating the synthesis of the cartilage matrix constituents: type II collagen and aggrecan. BMP-4 has also been shown to suppress chondrogenic hypertrophy and maintain regenerated cartilage. Use of an appropriate carrier for BMP-4 is crucial for successful reconstruction of cartilage defects. Due to the relatively short half-life in vivo of BMP-4, there is a need to localize and maintain the delivery of BMP-4 to the injury site. Additionally, the delivery of MSCs to the wound site could improve cartilage regeneration; therefore, the carrier should function both as a cell and a protein delivery vehicle.

CONCLUSION

The role of BMP-4 in chondrogenesis is significant, and successful methods to deliver BMP-4, with or without MSCs, to the cartilage defect site are a promising therapy to treat cartilage defects.

Physiologic deformational loading does not counteract the catabolic effects of interleukin-1 in long-term culture of chondrocyte-seeded agarose constructs

An interplay of mechanical and chemical factors is integral to cartilage maintenance and/or degeneration. Interleukin-1 (IL-1) is a pro-inflammatory cytokine that is present at elevated concentrations in osteoarthritic joints and initiates the rapid degradation of cartilage when cultured in vitro. Several short-term studies have suggested that applied dynamic deformational loading may have a protective effect against the catabolic actions of IL-1. In the current study, we examine whether the long-term (42 days) application of dynamic deformational loading on chondrocyte-seeded agarose constructs can mitigate these catabolic effects. Three studies were carried out using two IL-1 isoforms (IL-1alpha and IL-1beta) in chemically defined medium supplemented with a broad range of cytokine concentrations and durations. Physiologic loading was unable to counteract the long-term catabolic effects of IL-1 under any of the conditions tested, and in some cases led to further degeneration over unloaded controls.

Intra-articular injection of interleukin-4 decreases nitric oxide production by chondrocytes and ameliorates subsequent destruction of cartilage in instability-induced osteoarthritis in rat knee joints

OBJECTIVE

To investigate the in vitro and in vivo effects of interleukin (IL)-4 on mechanical stress-induced nitric oxide (NO) expression by chondrocytes, and destruction of cartilage and NO production in an instability-induced osteoarthritis (OA) model in rat knee joints, respectively.

MATERIALS AND METHODS

Cyclic tensile stress (CTS; 0.5Hz and 7% elongation) was applied to cultured normal rat chondrocytes with or without pre-incubation with recombinant rat IL-4 (rrIL-4). Inducible NO synthase (iNOS) mRNA expression and NO production were examined with real-time polymerase chain reaction and the Griess reaction, respectively. OA was induced in rat knee joints by transection of the anterior cruciate and medial collateral ligaments and resection of the medial meniscus. rrIL-4 (10, 50, and 100 ng/joint/day) was injected intra-articularly, and knee joint samples were collected 2, 4, and 6 weeks after surgery. Cartilage destruction was evaluated by the modified Mankin score and Osteoarthritis Research Society International scoring system on paraffin-embedded sections stained with safranin O. Cleavage of aggrecan and NO production were examined by immunohistochemistry for aggrecan neoepitope (NITEGE) and of nitrotyrosine (NT), respectively.

RESULTS

rrIL-4 down-regulated CTS-induced iNOS mRNA expression and NO production by chondrocytes. The intra-articular injection of rrIL-4 gave rise to a limited, but significant amelioration of cartilage destruction, prevention of loss of aggrecan, and decrease in the number of NT-positive chondrocytes, an effect that was not dose-dependent.

CONCLUSION

The present study suggests that IL-4 may exert chondroprotective properties against mechanical stress-induced cartilage destruction, at least in part, by inhibiting NO production by chondrocytes.

Development of a cell culture system loading cyclic mechanical strain to chondrogenic cells

Mechanical stimulation is considered to be one of the major epigenetic factors regulating the metabolism, proliferation, survival and differentiation of cells in the skeletal tissues. It is generally accepted that the cytoskeleton can undergo remodeling in response to mechanical stimuli such as tensile strain or fluid flow. Mechanically induced cell deformation is one of the possible mechanotransduction pathways by which chondrocytes sense and respond to changes in their mechanical environment. Mechanical strain has a variety of effects on the structure and function of their cells in the skeletal tissues, such as chondrocytes, osteoblasts and fibroblasts. However, little is known about the effect of the quality and quantity of mechanical strain and the timing of mechanical loading on the differentiation of these cells. The present study was designed to investigate the effect of the deformation of chondrogenic cells, and cyclic compression using a newly developed culture device, by analyzing mechanobiological response to the differentiating chondrocytes. Cyclic compression between 0 and 22% strains, at 23 microHz was loaded on chondrogenic cell line ATDC5 by seeding in a mass mode on PDMS membrane, assuming direct transfer of cyclic deformation from the membrane to the cells at the same frequency. The compressive strain, induced within the membrane, was characterized based on the analysis of the finite element modeling (FEM). The results showed that the tensile strain inhibits the chondrogenic differentiation of ATDC5 cells, whereas the compressive strain enhances the chondrogenic differentiation, suggesting that the differentiation of the chondrogenic cells could be controlled by the amount and the mode of strain. In conclusion, we have developed a unique strain loading culture system to analyze the effect of various types of mechanical stimulation on various cellular activities.

Cyclic loading inhibits expression of MMP-3 but not MMP-1 in an in vitro rabbit flexor tendon model

BACKGROUND

Gene expression analysis is useful for assessing cellular behavior and may improve our understanding of the initial cellular response to mechanical load leading to tendon degeneration. This study assessed gene expression of MMP-1 and MMP-3, genes associated with matrix degradation, in tendons exposed to cyclic loads within physiologic range.

METHODS

Six flexor tendons from each of ten New Zealand White rabbits were harvested and randomly assigned to one of the following six groups: load deprived for 18h; cyclically loaded for 18h to a peak stress of 2MPa; 3MPa; 4MPa; 5MPa; or snap frozen in liquid nitrogen. MMP-1, MMP-3 and 18s mRNA expression was measured by qRT-PCR.

FINDINGS

No significant differences in MMP-1 mRNA expression levels were found between loading groups. MMP-3 expression was significantly inhibited (57%) in tendons cyclically loaded to a peak stress of 4MPa in comparison to load deprived tendons, however, when peak stress was increased to 5MPa, expression was no longer significantly lower compared to stress shielded tendons.

INTERPRETATION

The results suggest a 'U' shape relationship between load and MMP-3 expression. The lack of change in MMP-1 expression with loading was unexpected as inhibition of MMP-1 in response to mechanical load has been demonstrated in previous studies. In conclusion, we demonstrate that MMP-3 expression is modulated by cyclic load and is sensitive to load magnitude. MMP-1 mRNA expression is not significantly modulated by cyclic load in this model.

Static compression of single chondrocytes catabolically modifies single-cell gene expression

Previous work has established that mechanical forces can lead to quantifiable alterations in cell function. However, how forces change gene expression in a single cell and the mechanisms of force transmission to the nucleus are poorly understood. Here we demonstrate that the gene expression of proteins related to the extracellular matrix in single articular chondrocytes is modified by compressive forces in a dosage-dependent manner. Increasing force exposure catabolically shifts single-cell mRNA levels of aggrecan, collagen IIa, and tissue inhibitor of metalloproteinase-1. Cytohistochemistry reveals that the majority of strain experienced by the cell is also experienced by the nucleus, resulting in considerable changes in nuclear volume and structure. Transforming growth factor-beta1 and insulin-like growth factor-I offer mechanoprotection and recovery of gene expression of aggrecan and metalloproteinase-1. These results suggest that forces directly influence gene transcription and may do so by changing chromatin conformation.

Cyclic equibiaxial tensile strain induces both anabolic and catabolic responses in articular chondrocytes

Mechanical disturbance is directly implicated in the development of osteoarthritis (OA) but the precise mode for degenerative changes is still largely unknown because of the complexity of the biomechanical and biochemical milieu in the articular joint. To investigate the effects of tensile strain on articular cartilage, cyclic equibiaxial tensile strain (CTS, 0.5 Hz, 10% strain) was applied to monolayer cultures of porcine articular chondrocytes by using a Flexercell strain unit. Overproduction of proinflammatory mediators and imbalanced expression of anabolic and catabolic genes were induced. The cellular secretion of nitric oxide (NO) and prostaglandin E(2) (PGE(2)), as well as the mRNA level of cyclooxygenase-2 (COX-2) were up-regulated in response to mechanical stimuli. Additionally, CTS resulted in an initial peak of anabolic response at 3 h of stretch with respect to the expression of type II collagen and aggrecan. After 12 h of CTS, the expression for these two cartilage-specific matrix proteins fell to control levels. A distinct catabolic response developed after 24 h of stretch with an increase in matrix metalloproteinase-1 (MMP-1). Interestingly, a parallel increase in transforming growth factor (TGF) beta3 was associated with the anabolic changes while an increase in expression of TGF beta1, the predominant isoform of the TGF family, appeared at 24 h. The expression at 24 h of MMP-1, an enzyme that degrades interstitial collagens as well as other cartilage matrix proteins and TGF beta1, may signify a shift towards matrix remodeling and potentially a change in matrix composition as a consequence of continuous CTS.

Pushing the limit: masticatory stress and adaptive plasticity in mammalian craniomandibular joints

Excessive, repetitive and altered loading have been implicated in the initiation of a series of soft- and hard-tissue responses or ;functional adaptations' of masticatory and locomotor elements. Such adaptive plasticity in tissue types appears designed to maintain a sufficient safety factor, and thus the integrity of given element or system, for a predominant loading environment(s). Employing a mammalian species for which considerable in vivo data on masticatory behaviors are available, genetically similar domestic white rabbits were raised on diets of different mechanical properties so as to develop an experimental model of joint function in a normal range of physiological loads. These integrative experiments are used to unravel the dynamic inter-relationships among mechanical loading, tissue adaptive plasticity, norms of reaction and performance in two cranial joint systems: the mandibular symphysis and temporomandibular joint (TMJ). Here, we argue that a critical component of current and future research on adaptive plasticity in the skull, and especially cranial joints, should employ a multifaceted characterization of a functional system, one that incorporates data on myriad tissues so as to evaluate the role of altered load versus differential tissue response on the anatomical, cellular and molecular processes that contribute to the strength of such composite structures. Our study also suggests that the short-term duration of earlier analyses of cranial joint tissues may offer a limited notion of the complex process of developmental plasticity, especially as it relates to the effects of long-term variation in mechanical loads, when a joint is increasingly characterized by adaptive and degradative changes in tissue structure and composition. Indeed, it is likely that a component of the adaptive increases in rabbit TMJ and symphyseal proportions and biomineralization represent a compensatory mechanism to cartilage degradation that serves to maintain the overall functional integrity of each joint system. Therefore, while variation in cranial joint anatomy and performance among sister taxa is, in part, an epiphenomenon of interspecific differences in diet-induced masticatory stresses characterizing the individual ontogenies of the members of a species, this behavioral signal may be increasingly mitigated in over-loaded and perhaps older organisms by the interplay between adaptive and degradative tissue responses.

Matrix metalloprotease activity is an essential link between mechanical stimulus and mesenchymal stem cell behavior

Progenitor cells are involved in the regeneration of the musculoskeletal system, which is known to be influenced by mechanical boundary conditions. Furthermore, matrix metalloproteases (MMPs) and tissue-specific inhibitors of metalloproteases (TIMPs) are crucial for matrix remodelling processes that occur during regeneration of bone and other tissues. This study has therefore investigated whether MMP activity affects mesenchymal stem cell (MSC) behavior and how MMP activity is influenced by the mechanical stimulation of these cells. Broad spectrum inhibition of MMPs altered the migration, proliferation, and osteogenic differentiation of MSCs. Expression analysis detected MMP-2, -3, -10, -11, -13, and -14, as well as TIMP-2, in MSCs at the mRNA and protein levels. Mechanical stimulation of MSCs led to an upregulation of their extracellular gelatinolytic activity, which was consistent with the increased protein levels seen for MMP-2, -3, -13, and TIMP-2. However, mRNA expression levels of MMPs/TIMPs showed no changes in response to mechanical stimulation, indicating an involvement of post-transcriptional regulatory processes such as alterations in MMP secretion or activation. One potential regulatory molecule might be the furin protease. Specific inhibition of MMP-2, -3, and -13 showed MMP-13 to be involved in osteogenic differentiation. The results of this study suggest that MSC function is controlled by MMP activity, which in turn is regulated by mechanical stimulation of cells. Thus, MMP/TIMP balance seems to play an essential role in transferring mechanical signals into MSC function. Disclosure of potential conflicts of interest is found at the end of this article.

The effects of radial shock waves on the metabolism of equine cartilage explantsin vitro

AIM

To investigate, in vitro, the effects of radial shock waves on the release of nitric oxide (NO) and synthesis of prostaglandin E2 (PGE2) and glycosaminoglycan (GAG), and liberation of GAG, from equine articular cartilage explants.

METHODS

Equine cartilage from normal metacarpophalangeal and metatarsophalangeal joints was exposed to radial shock waves at various impulse doses and then maintained as explants in culture for 48 h. Shock waves were delivered at 1,876 Torr pressure and a frequency of 10 Hz. Treatment groups consisted of a negative control group, or application of 500, 2,000, or 4,000 impulses by use of either a convex handpiece (Group A) or concave handpiece (Group B). Synthesis of GAG was measured using incorporation of 35S-labelled sodium sulphate. Additionally, the synthesis of NO and PGE2, and content of GAG of the explants and media were determined.

RESULTS

No significant effects (p>0.05) of radial shock-wave treatment were evident on the synthesis of NO or PGE2, or release of GAG by cartilage explants. However, radial shock waves decreased synthesis of GAG measured 48 h after exposure for all treatment groups other than the 500-impulse Group-A explants (p<0.05).

CONCLUSIONS

Radial shock waves impact the metabolism of GAG in chondrocytes in equine articular cartilage. Further studies will be required to fully investigate the impact of this effect on the health of joints, and to elucidate the clinical impact.

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.

Differential expression of type X collagen in a mechanically active 3-D chondrocyte culture system: a quantitative study

Objective

Mechanical loading of cartilage influences chondrocyte metabolism and gene expression. The gene encoding type X collagen is expressed specifically by hypertrophic chondrocytes and up regulated during osteoarthritis. In this study we tested the hypothesis that the mechanical microenvironment resulting from higher levels of local strain in a three dimensional cell culture construct would lead to an increase in the expression of type X collagen mRNA by chondrocytes in those areas.

Methods

Hypertrophic chondrocytes were isolated from embryonic chick sterna and seeded onto rectangular Gelfoam sponges. Seeded sponges were subjected to various levels of cyclic uniaxial tensile strains at 1 Hz with the computer-controlled Bio-Stretch system. Strain distribution across the sponge was quantified by digital image analysis. After mechanical loading, sponges were cut and the end and center regions were separated according to construct strain distribution. Total RNA was extracted from the cells harvested from these regions, and real-time quantitative RT-PCR was performed to quantify mRNA levels for type X collagen and a housing-keeping gene 18S RNA.

Results

Chondrocytes distributed in high (9%) local strain areas produced more than two times type X collagen mRNA compared to the those under no load conditions, while chondrocytes located in low (2.5%) local strain areas had no appreciable difference in type X collagen mRNA production in comparison to non-loaded samples. Increasing local strains above 2.5%, either in the center or end regions of the sponge, resulted in increased expression of Col X mRNA by chondrocytes in that region.

Conclusion

These findings suggest that the threshold of chondrocyte sensitivity to inducing type X collagen mRNA production is more than 2.5% local strain, and that increased local strains above the threshold results in an increase of Col X mRNA expression. Such quantitative analysis has important implications for our understanding of mechanosensitivity of cartilage and mechanical regulation of chondrocyte gene expression.

NAD(P)H oxidase activity of Nox4 in chondrocytes is both inducible and involved in collagenase expression

Reactive oxygen species (ROS) are regulators of redox-sensitive cell signaling pathways. In osteoarthritis, human interleukin-1beta is implicated in cartilage destruction through an ROS-dependent matrix metalloproteinase production. To determine the molecular source of ROS production in the human IL-1beta (hIL-1beta)-sensitive chondrocyte immortalized cell line C-20/A4, transfected cells were constructed that overexpress NAD(P)H oxidases. First, RT-PCR analysis showed that the C-20/A4 cell line expressed Nox2, Nox4, p22( phox ), and p67( phox ), but not p47( phox ). It was found that ROS production by C-20/A4 chondrocytes does not depend on PMA and ionomycin activation. This indicates that Nox2 was not involved in the production of ROS. In C- 20/A4 cells that overexpress Nox4, hIL-1beta stimulated ROS production three times more than the normal production of C-20/A4 cells. Moreover, there was a fourfold increase in the production of collagenase (MMP-1) by chondrocytes that overexpress Nox4. Interestingly, MMP-1 production in cells that overexpress Nox2 was not sensitive to hIL-1beta. These data suggest that under hIL-1beta stimulation, C-20/A4 chondrocytes produce MMP-1 through a Nox4-mediated, ROS-dependent pathway.

Cyclic tensile stretch load and oxidized low density lipoprotein synergistically induce lectin-like oxidized ldl receptor-1 in cultured bovine chondrocytes, resulting in decreased cell viability and proteoglycan synthesis

Mechanical stimulation is known to be an essential factor in the regulation of cartilage metabolism. We tested the hypothesis that expression of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) can be modulated by cyclic tensile stretch load in chondrocytes. Cyclic loading of repeated stretch stress at 10 cycles per minute with 10 kPa of stress for 6 h induced expression of LOX-1 to 2.6 times control in cultured bovine articular chondrocytes, equivalent to the addition of 10 microg/mL oxidized low density lipoprotein (ox-LDL) (2.4 times control). Application of the cyclic load to the chondrocytes along with 10 microg/mL ox-LDL resulted in synergistically increased LOX-1 expression to 6.3 times control. Individual application of cyclic loading and 10 microg/mL ox-LDL significantly suppressed chondrocytes viability (84.6% +/- 3.4% and 80.9% +/- 3.2% of control at 24 h, respectively; n = 3; p < 0.05) and proteoglycan synthesis [81.0% +/- 7.1% and 85.7% +/- 5.2% of control at 24 h, respectively; p < 0.05 when compared with 94.6% +/- 4.6% for native-LDL (n = 3)]. Cyclic loading and 10 microg/mL ox-LDL synergistically affected cell viability and proteoglycan synthesis, which were significantly suppressed to 45.6% +/- 4.9% and 48.7% +/- 6.7% of control at 24 h, respectively (n = 3; p < 0.01 when compared with individual application of cyclic loading or 10 microg/mL ox-LDL). In this study, we demonstrated synergistic effects of cyclic tensile stretch load and ox-LDL on cell viability and proteoglycan synthesis in chondrocytes, which may be mediated through enhanced expression of LOX-1 and which has important implications in the progression of cartilage degeneration in osteoarthritis.

Osteoblast responses one hour after load-induced fluid flow in a three-dimensional porous matrix

When bone is loaded, substrate strain is generated by the external force and this strain induces fluid flow that creates fluid shear stress on bone cells. Our current understanding of load-driven gene regulation of osteoblasts is based primarily on in vitro studies on planer two-dimensional tissue culture substrates. However, differences between a flat layer of cells and cells in 3-dimensional (3D) ECM are being recognized for signal transduction. Proliferation and differentiation of osteoblasts are affected by substrate geometry. Here we developed a novel 3D culture system that would mimic physiologically relevant substrate strain as well as strain-induced fluid flow in a 3D porous collagen matrix. The system allowed us to evaluate the responses of osteoblasts in a 3D stress-strain environment similar to a mechanical field to which bone is exposed. Using MC3T3-E1 osteoblasts grown in the 3D collagen matrix with and without hydroxyapatite deposition, we tested the role of strain and the strain-induced fluid flow in the expression of the load-responsive genes such as c-fos, egr1, cox2, osteopontin, and mmp1B involved in transcriptional regulation, osteogenesis, and rearrangement of ECM. Strain-induced fluid flow was visualized with a microspheres approximately 3 microm in diameter in real time, and three viscoelastic parameters were determined. The results obtained by semi-quantitative PCR, immunoblot assay, enzymatic activity assays for collagenase and gelatinase, and mechanical characterization of collagen matrices supported the dominant role of strain-induced fluid flow in expression of the selected genes one hour after the mechanical treatment.

p38 MAPK and COX2 inhibition modulate human chondrocyte response to TGF-beta

These studies compare actions of p38 MAPK inhibition and COX2 inhibition to modulate human arthritic chondrocyte responses to TGF-beta and FCS under basal and IL-1 activated conditions. Chondrocytes isolated from arthritic human femoral condyle cartilage obtained at total knee replacement were grown to 80% confluence. Proteoglycan synthesis and proliferation were measured with and without IL-1 activation in the presence and absence of growth factors and with and without inhibition of p38 MAPK or COX2 activity. Experiments to evaluate TIMP-1 production under these conditions were done using cartilage organ cultures. Neither p38 MAPK inhibitors nor COX2 inhibition affected basal proliferation. However both inhibitors enhanced the proliferative response to TGF-beta and FCS in IL-1 activated chondrocytes. TGF-beta stimulated proteoglycan synthesis was decreased by p38 MAPK inhibition, however COX2 inhibition restored the response to TGF-beta in IL-1 activated cells. In contrast, COX2 inhibition did not modulate TIMP-1 production while p38 MAPK inhibitors potentiated TGF-beta stimulated production of TIMP-1 in IL-1 activated cartilage. p38 MAPK inhibition and COX2 inhibition have unique and similar abilities to counteract some of the effects of IL-1 on human chondrocyte/cartilage metabolism. Both will partially restore the proliferative response to growth factors. p38 MAPK inhibition blunts TGF-beta stimulation of proteoglycan synthesis, but increases TIMP-1 synthesis. COX2 inhibition can restore the proteoglycan synthetic response to TGF-beta, but has no effect on cartilage TIMP-1 production. Use of these inhibitors to minimize cartilage damage in arthritic and mechanically stressed joints should reflect these characteristics.

Bovine primary chondrocyte culture in synthetic matrix metalloproteinase-sensitive poly(ethylene glycol)-based hydrogels as a scaffold for cartilage repair

A poly(ethylene glycol) (PEG)-based hydrogel was used as a scaffold for chondrocyte culture. Branched PEG-vinylsulfone macromers were end-linked with thiol-bearing matrix metalloproteinase (MMP)-sensitive peptides (GCRDGPQGIWGQDRCG) to form a three-dimensional network in situ under physiologic conditions. Both four- and eight-armed PEG macromer building blocks were examined. Increasing the number of PEG arms increased the elastic modulus of the hydrogels from 4.5 to 13.5 kPa. PEG-dithiol was used to prepare hydrogels that were not sensitive to degradation by cell-derived MMPs. Primary bovine calf chondrocytes were cultured in both MMP-sensitive and MMP-insensitive hydrogels, formed from either four- or eight-armed PEG. Most (>90%) of the cells inside the gels were viable after 1 month of culture and formed cell clusters. Gel matrices with lower elastic modulus and sensitivity to MMP-based matrix remodeling demonstrated larger clusters and more diffuse, less cell surface-constrained cell-derived matrix in the chondron, as determined by light and electron microscopy. Gene expression experiments by real-time RT-PCR showed that the expression of type II collagen and aggrecan was increased in the MMP-sensitive hydrogels, whereas the expression level of MMP-13 was increased in the MMP-insensitive hydrogels. These results indicate that cellular activity can be modulated by the composition of the hydrogel. This study represents one of the first examples of chondrocyte culture in a bioactive synthetic material that can be remodeled by cellular protease activity.

Mechanical regulation of terminal chondrocyte differentiation via RGD-CAP/beta ig-h3 induced by TGF-beta

RGD-CAP (beta ig-h3), initially cloned as a transforming growth factor (TGF)-beta inducible gene in human lung adenocarcinoma cells, was demonstrated to have a negative regulatory function in mineralization in hypertrophic chondrocytes, and the expression was shown to be associated with mechanical stimulation. We hypothesized that mechanical stimulation may regulate the terminal chondrocyte differentiation through the TGF-beta pathway by enhancing the RGD-CAP expression. To test this hypothesis, we investigated the effects of mechanical strain on the terminal differentiation and mineralization of growth-plate chondrocytes and assessed the mechanical regulation of TGF-ss and RGD-CAP expression. A cyclic mechanical strain of 12% elongation was applied to the cultured pre-hypertrophic chondrocytes isolated from the rib cartilage of 4-week-old male rats at 30 cycles/min (loading and relaxation on every alternate second). The terminal differentiation and mineralization of chondrocytes were assessed by alkaline phosphatase (ALP) activity assay and alizarin red staining. The gene expressions of TGF-ss and RGD-CAP, as well as chondrocytic terminal differentiation markers such as type X collagen and ALP, were examined with real-time RT-PCR. Cyclic mechanical strain decreased the ALP activity and intensity of alizarin red staining in pre-hypertrophic chondrocytes, as well as the gene expressions of type X collagen and ALP. TGF-ss and RGD-CAP were upregulated in the pre-hypertrophic chondrocytes subjected to mechanical strain, whereas the level of PTHrP receptor mRNA was not affected by the mechanical strain. The neutralizing antibody for TGF-ss suppressed the reduction of the mineralization of chondrocyte cultures with the downregulation of RGD-CAP. These results suggest that mechanical strain negatively regulates the terminal differentiation of chondrocytes through the signal pathway of TGF-ss with the induction of RGD-CAP.

Mechanical stretch induces matrix metalloproteinase 1 production in human hepatic stellate cells

In increasing portal blood flow, hepatic stellate cells (HSC) may be lengthened in response to mechanical stretch stimulation and their function may be changed. However, little is known about the influence of mechanical stretch on hepatic stellate cells. We examined production of matrix metalloproteinases (MMP), tissue inhibitors of metalloproteinases (TIMP), and extracellular matrix by hepatic stellate cells to investigate the relationship between mechanical stretch and hepatic fibrosis. LI90 cells, human hepatic stellate cells, were stretched cyclically using the Flexer cell strain unit. Concentrations of MMP1, MMP2, TIMP1, TIMP2, type I collagen C-telopeptide (1CTP), procollagen III propeptide (PIIIP), and hyaluronic acid in culture supernatants were determined. MMP1, MMP2, and TIMP1 mRNA expression was measured by reverse transcription-polymerase chain reaction (RT-PCR). In stretched LI90 cells, concentration of MMP1 showed an increase relative to unstretched cells, but concentrations of MMP2, TIMPl, and TIMP2 showed a decrease. MMP1/TIMP1 ratio and MMP1 mRNA expression showed an increase in stretched cells. Our finding suggested that in the early phase of portal hypertension, hepatic stellate cells increase production of MMPl and decrease production of TIMP1 and TIMP2, activated by mechanical stretch.

Matrix metalloproteinases and their clinical applications in orthopaedics

Imbalance in the expression of matrix metalloproteinases and their inhibitors contribute considerably to abnormal connective tissue degradation prevalent in various orthopaedic joint diseases such as rheumatoid arthritis and osteoarthritis. Matrix metalloproteinase expression has been detected in ligament, tendon, and cartilage tissues in the joint. They are known to contribute to the development, remodeling, and maintenance of healthy tissue through their ability to cleave a wide range of extracellular matrix substrates. Their role has been extended to cell growth, migration, differentiation, and apoptosis. In orthopaedics, their clinical applications constantly are being explored. The multiple steps in matrix metalloproteinase regulation offer potential targets for inhibition, useful in drug therapy. The correlation between matrix metalloproteinases and progression in joint erosion presents potential prognostic and diagnostic tools in rheumatoid arthritis. Matrix metalloproteinases also can be incorporated into scaffold design to control the degradation rate of engineered tissue constructs. This current review aims to summarize and emphasize the importance of matrix metalloproteinases and their natural inhibitors in the maturation of musculoskeletal tissue through matrix remodeling and, therefore, in the generation of a new clinical potential in orthopaedics.

Do changes in the mechanical properties of articular cartilage promote catabolic destruction of cartilage and osteoarthritis?

Osteoarthritis (OA) is a joint disease characterized by cartilage degeneration, a thickening of subchondral bone, and formation of marginal osteophytes. Previous mechanical characterization of cartilage in our laboratory suggests that energy storage and dissipation is reduced in osteoarthritis as the extent of fibrillation and fissure formation increases. It is not clear whether the loss of energy storage and dissipation characteristics is a result of biochemical and/or biophysical changes that occur to hyaline cartilage in joints. The purpose of this study is to present data, on the strain rate dependence of the elastic and viscous behaviors of cartilage, in order to further characterize changes that occur in the mechanical properties that are associated with OA. We have previously hypothesized that the changes seen in the mechanical properties of cartilage may be due to altered mechanochemical transduction by chondrocytes. Results of incremental tensile stress-strain tests at strain rates between 100%/min and 10,000%/min conducted on OA cartilage indicate that the slope of the elastic stress-strain curve increases with increasing strain rate, unlike the reported behavior of skin and self-assembled collagen fibers. It is suggested that the strain-rate dependence of the elastic stress-strain curve is due to the presence of large quantities of proteoglycans (PGs), which protect articular cartilage by increasing the apparent stiffness. The increased apparent stiffness of articular cartilage at high strain rates may limit the stresses borne and prolong the onset of OA. It is further hypothesized that increased compressive loading of chondrocytes in the intermediate zone of articular cartilage occurs as a result of normal wear to the superficial zone or from excessive impact loading. Once the superficial zone of articular cartilage is worn away, the tension is decreased throughout all cartilage zones leading to increased chondrocyte compressive loading and up-regulation of mechanochemical transduction processes that elaborate catabolic enzymes.

Effect of pulse rate on collagen deposition in the tissue-engineered blood vessel

The effect of mechanical stimulation on the development of tissue-engineered blood vessels was examined. In particular, three different rates of radial distension were chosen to produce a nonpulsed environment (0 beats per minute [bpm]), an adult heart rate (90 bpm), and a fetal heart rate (165 bpm). Engineered vessels were cultured for an average of 7 weeks. Vessel walls were then analyzed for collagen content and distribution. In addition, extracellular matrix remodeling was assessed through measurement of active matrix metalloproteinase type 1 (MMP-1) and tissue inhibitor of metalloproteinases type 1 (TIMP-1) levels. Vessels grown at a distension rate of 165 bpm had significantly higher collagen levels than those grown under static conditions. MMP-1 and TIMP-1 levels were also higher under pulsed conditions as compared with nonpulsed conditions. For the 90- and 165-bpm conditions, collagen and MMP-1 levels were not significantly different. TIMP-1 levels were significantly elevated at 165 bpm, indicating an increased cellular response to mechanical stimulation. Mechanical forces and their transduction represent a means to enhance the physical properties of artificial blood vessels, possibly by affecting the rate of extracellular matrix deposition and remodeling.

CITED2-mediated regulation of MMP-1 and MMP-13 in human chondrocytes under flow shear

CITED2 (CBP/p300-interacting transactivator with ED-rich tail 2) is a member of the Cited family of nuclear regulators, previously known as mrg1 (melanocyte-specific gene-related gene 1). CITED2 is inducible by varying stimuli including lipopolysaccharide, hypoxia, and cytokines such as interleukin 9 and interferon gamma. Using the immortalized human chondrocyte cell line, C-28/I2, we investigated whether CITED2 could be responsive to mechanical stimuli, and if so, whether CITED2 could mediate shear-driven regulation of matrix metalloproteinase (MMP) genes. The C-28/I2 cells were cultured under flow shear at 1-20 dyn/cm2, and the role of CIT-ED2 in regulation of MMPs was examined using the plasmids encoding sense and antisense CITED2 DNA sequences. The results showed that flow shear at 5 dyn/cm2 increased CITED2 mRNA and protein levels and down-regulated MMP-1 and MMP-13 mRNA and protein levels as well as enzyme activities. Consistent with the coordinated expression patterns of CITED2 and MMPs, overexpression of CITED2 repressed MMP-1 and MMP-13 mRNA levels and activities, whereas antisense CITED2 plasmids prevented the shear-induced down-regulation of MMP expression. Interleukin-1beta induced the formation of p300-Ets-1 complexes without affecting expression of CITED2. Transforming growth factor-beta as well as flow shear at 5 dyn/cm2 stimulated not only the expression of CITED2 but also the association of CIT-ED2 with p300 by dissociating Ets-1 from p300. These results indicate that CITED2 plays a major role in shear-induced down-regulation of MMP-1 and MMP-13 via a transforming growth factor-beta-dependent pathway.

Chondrocyte response to growth factors is modulated by p38 mitogen-activated protein kinase inhibition

Inhibitors of p38 mitogen-activated protein kinase (MAPK) diminish inflammatory arthritis in experimental animals. This may be effected by diminishing the production of inflammatory mediators, but this kinase is also part of the IL-1 signal pathway in articular chondrocytes. We determined the effect of p38 MAPK inhibition on proliferative and synthetic responses of lapine chondrocytes, cartilage, and synovial fibroblasts under basal and IL-1-activated conditions.

Basal and growth factor-stimulated proliferation and proteoglycan synthesis were determined in primary cultures of rabbit articular chondrocytes, first-passage synovial fibroblasts, and cartilage organ cultures. Studies were performed with or without p38 MAPK inhibitors, in IL-1-activated and control cultures. Media nitric oxide and prostaglandin E₂ were assayed.

p38 MAPK inhibitors blunt chondrocyte and cartilage proteoglycan synthesis in response to transforming growth factor beta; responses to insulin-like growth factor 1 (IGF-1) and fetal calf serum (FCS) are unaffected. p38 MAPK inhibitors significantly reverse inhibition of cartilage organ culture proteoglycan synthesis by IL-1. p38 MAPK inhibition potentiated basal, IGF-1-stimulated and FCS-stimulated chondrocyte proliferation, and reversed IL-1 inhibition of IGF-1-stimulated and FCS-stimulated DNA synthesis. Decreases in nitric oxide but not prostaglandin E₂ synthesis in IL-1-activated chondrocytes treated with p38 MAPK inhibitors are partly responsible for this restoration of response. Synovial fibroblast proliferation is minimally affected by p38 MAPK inhibition.

p38 MAPK activity modulates chondrocyte proliferation under basal and IL-1-activated conditions. Inhibition of p38 MAPK enhances the ability of growth factors to overcome the inhibitory actions of IL-1 on proliferation, and thus could facilitate restoration and repair of diseased and damaged cartilage.

Modelling cartilage mechanobiology

The growth, maintenance and ossification of cartilage are fundamental to skeletal development and are regulated throughout life by the mechanical cues that are imposed by physical activities. Finite element computer analyses have been used to study the role of local tissue mechanics on endochondral ossification patterns, skeletal morphology and articular cartilage thickness distributions. Using single-phase continuum material representations of cartilage, the results have indicated that local intermittent hydrostatic pressure promotes cartilage maintenance. Cyclic tensile strains (or shear), however, promote cartilage growth and ossification. Because single-phase material models cannot capture fluid exudation in articular cartilage, poroelastic (or biphasic) solid/fluid models are often implemented to study joint mechanics. In the middle and deep layers of articular cartilage where poroelastic analyses predict little fluid exudation, the cartilage phenotype is maintained by cyclic fluid pressure (consistent with the single-phase theory). In superficial articular layers the chondrocytes are exposed to tangential tensile strain in addition to the high fluid pressure. Furthermore, there is fluid exudation and matrix consolidation, leading to cell 'flattening'. As a result, the superficial layer assumes an altered, more fibrous phenotype. These computer model predictions of cartilage mechanobiology are consistent with results of in vitro cell and tissue and molecular biology experiments.

Effect of biomechanical conditioning on cartilaginous tissue formation in vitro

BACKGROUND

Although tissue engineering of articular cartilage is a promising approach for cartilage repair, it has been difficult to develop cartilaginous tissue in vitro that mimics the properties of native cartilage. Isolated chondrocytes grown in culture typically do not accumulate enough extracellular matrix, and the generated tissue possesses only a fraction of the mechanical properties of native cartilage. One potential explanation for this might be that the cells are grown in an environment that lacks the mechanical stimuli to which the chondrocytes are exposed in vivo. In this study, we compared the long-term effects of both dynamic compressive and shearing forces on cartilaginous tissue formation in vitro.

METHODS

Bovine articular chondrocytes were grown on the surface of porous ceramic substrates and were maintained under static, free-swelling conditions for a period of four weeks. Cultures were then subjected to six minutes of mechanical stimulation every other day, in either compression or shear, for an additional four-week period.

RESULTS

Cartilaginous tissues cultured in the presence of intermittent compression or shear were significantly thicker (p < 0.05) and had accumulated more extracellular matrix (p < 0.01) compared with the unstimulated controls. However, when normalized by the wet weight of the tissue, cultures stimulated in the presence of shearing forces contained more proteoglycans and collagen compared with compression-stimulated cultures. These cultures also displayed the largest increase in mechanical properties, with a threefold increase in equilibrium stress and a fivefold increase in equilibrium modulus.

CONCLUSIONS AND CLINICAL RELEVANCE

The results of this study demonstrate that a brief application of mechanical forces applied periodically over a long duration can improve the quality of cartilaginous tissue formed in vitro. However, the changes in tissue composition and mechanical properties were dependent on the specific mode of the applied mechanical forces, with shear stimulation eliciting the greater effect. This finding suggests that chondrocytes may respond differently to different modes of applied forces.

Physiologic weight-bearing increases new vessel formation during distraction osteogenesis: a micro-tomographic imaging study

During distraction osteogenesis, large volumes of new bone are formed through the slow distraction of fracture callus. The newly formed bone is closely linked to angiogenesis and positively influenced by physiologic loading. In this study, a rat model was used to explore the correlation between these two observations. Unilateral femoral lengthenings were performed in 18 male Sprague-Dawley rats (400-500 g, age<6 months). Half of the animals were allowed to bear weight freely (WB) while the remaining animals were made non-weight-bearing via a through-knee amputation (NWB). After a seven-day latency period, femurs were lengthened 7 mm over 21 days. Animals were sacrificed at 7, 21, 35, and 49 days (0, 4.7, 7, and 7 mm of distraction) at which time lower extremity vessels were perfused with a 60% (w/v) barium sulfate suspension. High-resolution three-dimensional images of the vascular architecture were generated using a fan-beam micro-tomography machine by digitally separating the contrast-filled vessels from surrounding tissue. For two subvolumes, VOI(1), which included vessels in the medullary canal, cortex, and periosteum, and VOI(2), which included only vessels in the canal and cortex, the total tissue volume (TV), the volume of vessels (VV), and vessel diameter (VD) were determined. For the larger subvolume (VOI(1)), VV and vessel density (VV/TV) increased as a function of time (p<0.001) in WB animals. In NWB animals, VV increased significantly with time (p=0.029), while VV/TV did not (p=0.36). Increases in VV and VV/TV were significantly greater in WB animals than in NWB animals (p<0.01 and 0.05, respectively). VD was similar in both groups and did not change with time. Our data suggest that weight bearing stimulates new vessel formation during distraction osteogenesis.

Insulin-like growth factor 1-induced interleukin-1 receptor II overrides the activity of interleukin-1 and controls the homeostasis of the extracellular matrix of cartilage

OBJECTIVE

We examined the effect of the insulin-like growth factor 1 (IGF-1)/IGF receptor I (IGFRI) autocrine/paracrine anabolic pathway on the extracellular matrix (ECM) of human chondrocytes and the mechanism by which IGF-1 reverses the catabolic effects of interleukin-1 (IL-1).

METHODS

Phenotypically stable human articular cartilage cells were obtained from normal cartilage and maintained in culture in alginate beads for 1 week to reach equilibrium of accumulated cell-associated matrix (CAM) compounds. Levels of CAM components aggrecan and type II collagen (CII) and levels of intracellular IGF-1, IL-1alpha, and IL-1beta and their respective plasma membrane-bound receptors IGFRI, IL-1 receptor I (IL-1RI), and the decoy receptor IL-1RII were assayed using flow cytometry to investigate the relationship between the autocrine/paracrine pathways and the homeostasis of ECM molecules in the CAM. The effects of IGF-1 on the expression of IGF-1, IL-1alpha, and IL-1beta and their respective receptor systems, the aggrecan core protein, and CII were determined by flow cytometry.

RESULTS

Cause-effect relationship experiments showed that IGF-1 up-regulates the levels of IGF-1, IGFRI, aggrecan, and CII in the CAM. No effects on the expression of IL-1alpha and IL-1beta and their signaling receptor IL-1RI were observed. However, IGF-1 was able to reverse IL-1beta-mediated degradation of aggrecan and the repression of the aggrecan synthesis rate. Interestingly, levels of aggrecan and CII in the CAM strongly correlated not only with IGF-1, but also with IL-1RII, which acts as a decoy receptor for IL-1alpha and IL-1beta. This suggests that IGF-1 and IL-1RII may cooperate in regulating ECM homeostasis. Additional experiments demonstrated that IGF-1 up-regulated IL-1RII, thereby overriding the catabolic effects of IL-1.

CONCLUSION

These findings reveal a new paradigm by which IGF-1 influences chondrocyte metabolism, by reversing the IL-1-mediated catabolic pathway through up-regulation of its decoy receptor.