Crosstalk between cartilage and bone: when bone cytokines matter

The cartilage damage which characterizes osteoarthritis is often accompanied by bone lesions. Joint integrity results from the balance in the physiological interactions between bone and cartilage. Several local factors regulate the physiological remodeling of cartilage, the disequilibrium of these leading to a higher cartilage catabolism. Several cytokines secreted by bone cells can induce chondrocyte differentiation, which suggests their role in the dialogue between both cells. Accumulative in vivo evidence shows that increased bone resorption occurs at an early stage in the development of osteoarthritis and that blocking bone-resorbing cytokines prevents cartilage damage, confirming the role of bone factors in the crosstalk of both tissues. Recently, molecules of the Wnt pathway have emerged as key regulators of bone and cartilage. Activation of Wnt/βcatenin induces an imbalance in cartilage homeostasis, and agonists/antagonists of Wnt are potential candidates for this interaction. This review will summarize what is known about the contribution of bone cytokines to the physiological remodeling of cartilage and in the pathophysiology of osteoarthritis.     

Invited review: the mitochondrion in osteoarthritis

In a variety of tissues, cumulative oxidative stress, disrupted mitochondrial respiration, and mitochondrial damage promote aging, cell death, and ultimately, functional failure and degeneration. Because articular cartilage chondroyctes are highly glycolytic, mitochondrially mediated pathogenesis has not been previously applied in models for pathogenesis of osteoarthritis (OA), a cartilage degenerative disease that increases markedly in aging. However, chondrocyte mitochondria respire in vitro and they demonstrate swelling and changes in number in situ in the course of OA. Normal chondrocyte mitochondrial function is hypothesized to critically support adenosine triphosphate (ATP) reserves in functional stressed chondrocytes during OA evolution. In this model, disruption of chondrocyte respiration by nitric oxide, a mediator markedly up-regulated in OA cartilage, is centrally involved in chondrocyte functional compromise. Furthermore, mitochondrial dysfunction can mediate several specific pathogenic pathways implicated in OA. These include oxidative stress, inadequacy of chondrocyte biosynthetic and growth responses, up-regulated chondrocyte cytokine-induced inflammation and matrix catabolism, increased chondrocyte apoptosis, and pathologic cartilage matrix calcification. In addition, the direct, sublethal impairment of chondrocyte mitochondrial ATP synthesis in vitro decreases matrix synthesis and increases matrix calcification ('disease in a dish'). The weight of evidence reviewed herein strongly supports chondrocyte mitochondrial impairment as a mediator of the establishment and progression of OA.     

In situ chondrocyte deformation with physiological compression of the feline patellofemoral joint

The mechanical environment is an important factor affecting the maintenance and adaptation of articular cartilage, and thus the function of the joint and the progression of joint degeneration. Recent evidence suggests that cartilage deformation caused by mechanical loading is directly associated with deformation and volume changes of chondrocytes. Furthermore, in vitro experiments have shown that these changes in the mechanical states of chondrocytes correlate with a change in the biosynthetic activity of cartilage cells. The purpose of this study was to apply our knowledge of contact forces within the feline patellofemoral joint to quantify chondrocyte deformation in situ under loads of physiological magnitude. A uniform, static load of physiological magnitude was applied to healthy articular cartilage still fully intact and attached to its native bone. The compressed cartilage was then chemically fixed to enable the evaluation of cartilage strain, chondrocyte deformation and chondrocyte volumetric fraction. Patella and femoral groove articular cartilages differ in thickness, chondrocyte aspect ratio, and chondrocyte volumetric fraction in both magnitude and depth distribution. Furthermore, when subjected to the same compressive loads, changes to all of these parameters differ in magnitude and depth distribution between patellar and femoral groove articular cartilage. This evidence suggests that significant chondrocyte deformation likely occurs during in vivo joint loading, and may influence chondrocyte biosynthetic activity. Furthermore, we hypothesise that the contrasts between patella and femoral groove cartilages may explain, in part, the site-specific progression of osteoarthritis in the patellofemoral joint of the feline anterior cruciate ligament transected knee.     

Ultrastructure of the mineralizing metacarpophalangeal joint of progressive ankylosis (ank/ank) mice

The metacarpophalangeal joint of the progressive ankylosis mouse was examined at 4, 6 and 8 weeks of age by using electron microscopy and electron probe microanalysis. These studies describe the progression of the disorder, and they reveal three overlapping phases: pannus proliferation, chondrophyte formation and mineralization, and articular cartilage mineralization. The chondrophyte mineralized in a manner fairly similar to endochondrial bone, whereas articular cartilage mineralized by advancement of a calcification front across the cartilage. Synovial crystal phagocytosis was also observed. The progressive ankylosis mouse should be an important model for studying both crystal deposition and abnormal articular cartilage calcification.     

Inhibitory function of parathyroid hormone-related protein on chondrocyte hypertrophy: the implication for articular cartilage repair

Cartilage repair tissue is usually accompanied by chondrocyte hypertrophy and osseous overgrowths, and a role for parathyroid hormone-related protein (PTHrP) in inhibiting chondrocytes from hypertrophic differentiation during the process of endochondral ossification has been demonstrated. However, application of PTHrP in cartilage repair has not been extensively considered. This review systemically summarizes for the first time the inhibitory function of PTHrP on chondrocyte hypertrophy in articular cartilage and during the process of endochondral ossification, as well as the process of mesenchymal stem cell chondrogenic differentiation. Based on the literature review, the strategy of using PTHrP for articular cartilage repair is suggested, which is instructive for clinical treatment of cartilage injuries as well as osteoarthritis.     

Can exercise modulate the maturation of functionally different immature tendons in the horse?

Tendons can be considered in two functional groups, those contributing to energetics of locomotion and those acting solely to position the limb. The energy-storing tendons in both human and equine athletes have a high frequency of injury with similar pathophysiology. In previous studies, high-intensity exercise appears to induce a disruption of the matrix rather than functional adaptation in adults. Here we explore the hypothesis that the introduction of controlled exercise during growth would result in an adaptive response without deleterious effects. Young horses were given a controlled exercise program similar to that previously shown to induce matrix changes in energy-storing tendons of skeletally mature animals. The tendons were assessed in relation to mechanical properties, molecular composition, and morphology. Results showed a significant increase in cartilage oligomeric matrix protein (COMP) in the positional tendon but not in the energy-storing tendon. Other matrix properties and mechanical properties were not significantly changed. While the imposition of high-strain-rate exercise in immature horses failed to augment the development of the energy-storing tendon over and above that induced by normal pasture exercise, it did not induce deleterious changes, supporting an earlier introduction of athletic training in horses.     

Inflammation-induced chondrocyte hypertrophy is driven by receptor for advanced glycation end products

The multiligand receptor for advanced glycation end products (RAGE) mediates certain chronic vascular and neurologic degenerative diseases accompanied by low-grade inflammation. RAGE ligands include S100/calgranulins, a class of low-molecular-mass, calcium-binding polypeptides, several of which are chondrocyte expressed. Here, we tested the hypothesis that S100A11 and RAGE signaling modulate osteoarthritis (OA) pathogenesis by regulating a shift in chondrocyte differentiation to hypertrophy. We analyzed human cartilages and cultured human articular chondrocytes, and used recombinant human S100A11, soluble RAGE, and previously characterized RAGE-specific blocking Abs. Normal human knee cartilages demonstrated constitutive RAGE and S100A11 expression, and RAGE and S100A11 expression were up-regulated in OA cartilages studied by immunohistochemistry. CXCL8 and TNF-alpha induced S100A11 expression and release in cultured chondrocytes. Moreover, S100A11 induced cell size increase and expression of type X collagen consistent with chondrocyte hypertrophy in vitro. CXCL8-induced, IL-8-induced, and TNF-alpha-induced but not retinoic acid-induced chondrocyte hypertrophy were suppressed by treatment with soluble RAGE or RAGE-specific blocking Abs. Last, via transfection of dominant-negative RAGE and dominant-negative MAPK kinase 3, we demonstrated that S100A11-induced chondrocyte type X collagen expression was dependent on RAGE-mediated p38 MAPK pathway activation. We conclude that up-regulated chondrocyte expression of the RAGE ligand S100A11 in OA cartilage, and RAGE signaling through the p38 MAPK pathway, promote inflammation-associated chondrocyte hypertrophy. RAGE signaling thereby has the potential to contribute to the progression of OA.     

Bioengineered chondrocyte sheets may be potentially useful for the treatment of partial thickness defects of articular cartilage

Some treatments for full thickness defects of articular cartilage, such as cultured chondrocyte transplantation, have already been done. However, to overcome osteoarthritis, we must further study the partial thickness defect of articular cartilage. It is much more difficult to repair a partial thickness defect because few repairing cells can address such injured sites. We herein show that bioengineered layered chondrocyte sheets using temperature-responsive culture dishes may be a potentially useful treatment for partial thickness defects. We evaluated the property of these sheets using real-time PCR and histological findings, and allografted these sheets to evaluate the effect of treatment using a rabbit partial model. In conclusion, layered chondrocyte sheets were able to maintain the cartilageous phenotype, and could be attached to the sites of cartilage damage which acted as a barrier to prevent a loss of proteoglycan from these sites and to protect them from catabolic factors in the joint.     

Hypoxia-inducible factor-2α regulates Fas-mediated chondrocyte apoptosis during osteoarthritic cartilage destruction

Apoptosis of articular chondrocytes is associated with the pathogenesis of osteoarthritis (OA). Recently, we demonstrated that hypoxia-inducible factor (HIF)-2α, encoded by Epas1, causes OA cartilage destruction by regulating the expression of various matrix-degrading enzymes. Here, we investigated the involvement of HIF-2α in chondrocyte apoptosis and OA cartilage destruction. HIF-2α levels in human and mouse OA chondrocytes were markedly elevated in association with increased apoptosis of articular chondrocytes. Overexpression or knockdown of HIF-2α alone did not cause chondrocyte apoptosis. However, HIF-2α expression markedly increased chondrocyte apoptosis in the presence of an agonistic anti-Fas (CD95) antibody. HIF-2α enhanced Fas expression and potentiated downstream signaling pathways, increasing the activity of initiator and executioner caspases. Overexpression of HIF-2α in mouse cartilage tissue, either by intra-articular injection of Epas1 adenovirus (Ad-Epas1) or in the context of chondrocyte-specific Epas1 transgenic mice, increased chondrocyte apoptosis and cartilage destruction. In contrast, chondrocyte-specific knockout of Epas1 in mice suppressed DMM (destabilization of the medial meniscus)-induced chondrocyte apoptosis and inhibited OA cartilage destruction. Moreover, Fas-deficient mice exhibited diminished chondrocyte apoptosis and OA cartilage destruction in response to Ad-Epas1 injection or DMM surgery. Taken together, our results demonstrate that HIF-2α potentiates Fas-mediated chondrocyte apoptosis, which is associated with OA cartilage destruction.     

Transamidation by transglutaminase 2 transforms S100A11 calgranulin into a procatabolic cytokine for chondrocytes

In osteoarthritis (OA), low-grade joint inflammation promotes altered chondrocyte differentiation and cartilage catabolism. S100/calgranulins share conserved calcium-binding EF-hand domains, associate noncovalently as homodimers and heterodimers, and are secreted and bind receptor for advanced glycation end products (RAGE). Chondrocyte RAGE expression and S100A11 release are stimulated by IL-1beta in vitro and increase in OA cartilage in situ. Exogenous S100A11 stimulates chondrocyte hypertrophic differentiation. Moreover, S100A11 is covalently cross-linked by transamidation catalyzed by transglutaminase 2 (TG2), itself an inflammation-regulated and redox stress-inducible mediator of chondrocyte hypertrophic differentiation. In this study, we researched mouse femoral head articular cartilage explants and knee chondrocytes, and a soluble recombinant double point mutant (K3R/Q102N) of S100A11 TG2 transamidation substrate sites. Both TG2 and RAGE knockout cartilage explants retained IL-1beta responsiveness. The K3R/Q102N mutant of S100A11 retained the capacity to bind to RAGE and chondrocytes but lost the capacity to signal via the p38 MAPK pathway or induce chondrocyte hypertrophy and glycosaminoglycans release. S100A11 failed to induce hypertrophy, glycosaminoglycan release, and appearance of the aggrecanase neoepitope NITEGE in both RAGE and TG2 knockout cartilages. We conclude that transamidation by TG2 transforms S100A11 into a covalently bonded homodimer that acquires the capacity to signal through the p38 MAPK pathway, accelerate chondrocyte hypertrophy and matrix catabolism, and thereby couple inflammation with chondrocyte activation to potentially promote OA progression.     

Hypertrophic differentiation of chondrocytes in osteoarthritis: the developmental aspect of degenerative joint disorders

Osteoarthritis is characterized by a progressive degradation of articular cartilage leading to loss of joint function. The molecular mechanisms regulating pathogenesis and progression of osteoarthritis are poorly understood. Remarkably, some characteristics of this joint disease resemble chondrocyte differentiation processes during skeletal development by endochondral ossification. In healthy articular cartilage, chondrocytes resist proliferation and terminal differentiation. By contrast, chondrocytes in diseased cartilage progressively proliferate and develop hypertrophy. Moreover, vascularization and focal calcification of joint cartilage are initiated. Signaling molecules that regulate chondrocyte activities in both growth cartilage and permanent articular cartilage during osteoarthritis are thus interesting targets for disease-modifying osteoarthritis therapies.     

Post-traumatic osteoarthritis: the role of accelerated chondrocyte senescence

Joint injuries frequently lead to progressive joint degeneration that causes the clinical syndrome of post-traumatic osteoarthritis. The pathogenesis of osteoarthritis remains poorly understood, but patient age is a significant risk factor for progressive joint degeneration. We have found that articular cartilage chondrocytes show strong evidence of senescence with increasing age, including synthesis of smaller more irregular aggrecans; increased expression of lysosomal beta-galactosidase and telomere erosion; and decreased proteoglycan synthesis, response to the anabolic cytokine IGF-I, proliferative capacity, and mitochondrial function. These observations help explain the strong association between age and joint degeneration, but they do not explain how joint injury increases the risk of joint degeneration in younger individuals. We hypothesized that excessive loading of articular surfaces due to acute joint trauma or post-traumatic joint instability, incongruity or mal-alignment increases release of reactive oxygen species, and that the increased oxidative stress on chondrocytes accelerates chondrocyte senescence thereby decreasing the ability of the cells to maintain or restore the tissue. To test this hypothesis, we exposed human articular cartilage chondrocytes from young adults to mechanical and oxidative stress. We found that shear stress applied to cartilage explants in a triaxial pressure vessel increased release of reactive oxygen species and oxidative stress induced chondrocyte senescence (as measured by expression of lysosomal beta-galactosidase, nuclear and mitochondrial DNA damage and decreased mitochondrial function). These observations support the hypothesis that joint injury accelerates chondrocyte senescence and that this acceleration plays a role in the joint degeneration responsible for post-traumatic osteoarthritis.     

The influence of ageing and exercise on tendon growth and degeneration--hypotheses for the initiation and prevention of strain-induced tendinopathies

Strain-induced tendinopathy is a common injury in both human and equine athletes, with increasing incidence associated with greater involvement in sport and an increasingly aged population. This paper reviews our studies on the abundant non-collagenous protein, cartilage oligomeric matrix protein (COMP), in equine tendons. Its variation between tendon type and site, age and exercise has provided an insight into how age and exercise influence tendon growth and maturation. Tendons can be broadly divided into two types, reflecting their different matrix composition and function: the energy-storing tendons used for weight-bearing and locomotion, which suffer a high incidence of strain-induced tendinopathy, and positional tendons involved in limb placement or manipulative skills. It would appear that while energy-storing tendon can respond to the mechanical forces applied to it during growth, there is no evidence that it can do so after skeletal maturity. Instead, cumulative fatigue damage causes degeneration at the molecular level, potentially weakening it and increasing the risk of clinical injury. Appropriate exercise regimes early in life may help to improve the quality of growing tendon, thereby reducing the incidence of injury during ageing or subsequent athletic career.     

Isorhamnetin attenuates osteoarthritis by inhibiting osteoclastogenesis and protecting chondrocytes through modulating reactive oxygen species homeostasis

Increasing evidence indicates that osteoarthritis (OA) is a musculoskeletal disease affecting the whole joint, including both cartilage and subchondral bone. Reactive oxygen species (ROS) have been demonstrated to be one of the important destructive factors during early‐stage OA development. The objective of this study was to investigate isorhamnetin (Iso) treatment on osteoclast formation and chondrocyte protection to attenuate OA by modulating ROS. Receptor activator of nuclear factor‐kappa B ligand (RANKL) was used to establish the osteoclast differentiation model in bone marrow macrophages (BMMs) in vivo. H₂O₂ was used to induce ROS, which could further cause chondrocyte apoptosis. We demonstrated that Iso suppressed RANKL‐induced ROS generation, which could mediate osteoclastogenesis. Moreover, we found that Iso inhibited osteoclast formation and function by suppressing the expression of osteoclastogenesis‐related genes and proteins. We proved that Iso inhibited RANKL‐induced activation of mitogen‐activated protein kinase activation of mitogen‐activated protein kinase (MAPK), nuclear factor‐kappa B (NF‐κB) and AKT signalling pathways in BMMs. In addition, Iso inhibited ROS‐induced chondrocyte apoptosis by regulating apoptosis‐related proteins. Moreover, Iso was administered to an anterior cruciate ligament transection (ACLT)‐induced OA mouse model. The results indicated that Iso exerted beneficial effects on inhibiting excessive osteoclast activity and chondrocyte apoptosis, which further remedied cartilage damage. Overall, our data showed that Iso is an effective candidate for treating OA.     

Potential involvement of oxidative stress in cartilage senescence and development of osteoarthritis: oxidative stress induces chondrocyte telomere instability and downregulation of chondrocyte function

Oxidative stress leads to increased risk for osteoarthritis (OA) but the precise mechanism remains unclear. We undertook this study to clarify the impact of oxidative stress on the progression of OA from the viewpoint of oxygen free radical induced genomic instability, including telomere instability and resulting replicative senescence and dysfunction in human chondrocytes. Human chondrocytes and articular cartilage explants were isolated from knee joints of patients undergoing arthroplastic knee surgery for OA. Oxidative damage and antioxidative capacity in OA cartilage were investigated in donor-matched pairs of intact and degenerated regions of tissue isolated from the same cartilage explants. The results were histologically confirmed by immunohistochemistry for nitrotyrosine, which is considered to be a maker of oxidative damage. Under treatment with reactive oxygen species (ROS; 0.1 μmol/l H₂O₂) or an antioxidative agent (ascorbic acid: 100.0 μmol/l), cellular replicative potential, telomere instability and production of glycosaminoglycan (GAG) were assessed in cultured chondrocytes. In tissue cultures of articular cartilage explants, the presence of oxidative damage, chondrocyte telomere length and loss of GAG to the medium were analyzed in the presence or absence of ROS or ascorbic acid. Lower antioxidative capacity and stronger staining of nitrotyrosine were observed in the degenerating regions of OA cartilages as compared with the intact regions from same explants. Immunostaining for nitrotyrosine correlated with the severity of histological changes to OA cartilage, suggesting a correlation between oxidative damage and articular cartilage degeneration. During continuous culture of chondrocytes, telomere length, replicative capacity and GAG production were decreased by treatment with ROS. In contrast, treatment with an antioxidative agent resulted in a tendency to elongate telomere length and replicative lifespan in cultured chondrocytes. In tissue cultures of cartilage explants, nitrotyrosine staining, chondrocyte telomere length and GAG remaining in the cartilage tissue were lower in ROS-treated cartilages than in control groups, whereas the antioxidative agent treated group exhibited a tendency to maintain the chondrocyte telomere length and proteoglycan remaining in the cartilage explants, suggesting that oxidative stress induces chondrocyte telomere instability and catabolic changes in cartilage matrix structure and composition. Our findings clearly show that the presence of oxidative stress induces telomere genomic instability, replicative senescence and dysfunction of chondrocytes in OA cartilage, suggesting that oxidative stress, leading to chondrocyte senescence and cartilage ageing, might be responsible for the development of OA. New efforts to prevent the development and progression of OA may include strategies and interventions aimed at reducing oxidative damage in articular cartilage.     

Pathological mechanism of chondrocytes and the surrounding environment during osteoarthritis of temporomandibular joint

Temporomandibular joint (TMJ) osteoarthritis is a common chronic degenerative disease of the TMJ. In order to explore its aetiology and pathological mechanism, many animal models and cell models have been constructed to simulate the pathological process of TMJ osteoarthritis. The main pathological features of TMJ osteoarthritis include chondrocyte death, extracellular matrix (ECM) degradation and subchondral bone remodelling. Chondrocyte apoptosis accelerates the destruction of cartilage. However, autophagy has a protective effect on condylar chondrocytes. Degradation of ECM not only changes the properties of cartilage but also affects the phenotype of chondrocytes. The loss of subchondral bone in the early stages of TMJ osteoarthritis plays an aetiological role in the onset of osteoarthritis. In recent years, increasing evidence has suggested that chondrocyte hypertrophy and endochondral angiogenesis promote TMJ osteoarthritis. Hypertrophic chondrocytes secrete many factors that promote cartilage degeneration. These chondrocytes can further differentiate into osteoblasts and osteocytes and accelerate cartilage ossification. Intrachondral angiogenesis and neoneurogenesis are considered to be important triggers of arthralgia in TMJ osteoarthritis. Many molecular signalling pathways in endochondral osteogenesis are responsible for TMJ osteoarthritis. These latest discoveries in TMJ osteoarthritis have further enhanced the understanding of this disease and contributed to the development of molecular therapies. This paper summarizes recent cognition on the pathogenesis of TMJ osteoarthritis, focusing on the role of chondrocyte hypertrophy degeneration and cartilage angiogenesis.     

Chondrocyte acting as phagocyte to internalize polyethylene wear particles and leads to the elevations of osteoarthritis associated NO and PGE2

It remains a mystery about the role of chondrocyte or cartilage on the co-existence of ultra-high molecular weight polyethylene (UHMWPE) wear particles from partial joint arthroplasty. An inverted co-culture system was performed to investigate the interactions between chondrocytes and UHMWPE wear particles. It was first time observed that chondrocytes can engulf UHMWPE particles and release osteoarthritis associated pro-inflammatory factors. TEM observation and flow cytometric analysis demonstrated the phagocytosis of particles by chondrocytes. It was found that polyethylene particles may reduce the viability of chondrocytes, and enhance the secretion of nitric oxide (NO) and PGE₂. In conclusion, all these phenomena may contribute to further cartilage degeneration after partial joint arthroplasty surgery. It is first identified in this study that the chondrocyte acts as phagocyte to internalize wear particles and leads to the elevations of precursor mediators of osteoarthritis.     

Mechanical stress protects against osteoarthritis via regulation of the AMPK/NF-κB signaling pathway

Mechanical stress plays a key role in regulating cartilage degradation in osteoarthritis (OA). The aim of this study was to evaluate the effects and mechanisms of mechanical stress on articular cartilage. A total of 80 male Sprague-Dawley rats were randomly divided into eight groups (n = 10 for each group): control group (CG), OA group (OAG), and CG or OAG subjected to low-, moderate-, or high-intensity treadmill exercise (CL, CM, CH, OAL, OAM, and OAH, respectively). Chondrocytes were obtained from the knee joints of rats; they were cultured on Bioflex 6-well culture plates and subjected to different durations of cyclic tensile strain (CTS) with or without exposure to interleukin-1β (IL-1β). The results of the histological score, immunohistochemistry, enzyme-linked immunosorbent assay, and western-blot analyses indicated that there were no differences between CM and CG, but OAM showed therapeutic effects compared with OAG. However, CH and OAH experienced more cartilage damage than CG and OAG, respectively. CTS had no therapeutic effects on collagen II of normal chondrocytes, which is consistent with findings after treadmill exercise. However, CTS for 4 hr could alleviate the chondrocyte damage induced by IL-1β by activating AMP-activated protein kinase (AMPK) phosphorylation and suppressing nuclear translocation of nuclear factor (NF)-κB p65. Our findings indicate that mechanical stress had no therapeutic effects on normal articular cartilage and chondrocytes; mechanical stress only caused damage with excessive stimulation. Still, moderate biomechanical stress could reduce sensitization to the inflammatory response of articular cartilage and chondrocytes through the AMPK/NF-κB signaling pathway.