A homeostatic function of CXCR2 signalling in articular cartilage

Abstract

ObjectiveELR+ CXC chemokines are heparin-binding cytokines signalling through the CXCR1 and CXCR2 receptors. ELR+ CXC chemokines have been associated with inflammatory arthritis due to their capacity to attract inflammatory cells. Here, we describe an unsuspected physiological function of these molecules in articular cartilage homeostasis.MethodsChemokine receptors and ligands were detected by immunohistochemistry, western blotting and RT-PCR. Osteoarthritis was induced in wild-type and CXCR2−/− mice by destabilisation of the medial meniscus (DMM). CXCR1/2 signalling was inhibited in vitro using blocking antibodies or siRNA. Chondrocyte phenotype was analysed using Alcian blue staining, RT-PCR and western blotting. AKT phosphorylation and SOX9 expression were upregulated using constitutively active AKT or SOX9 plasmids. Apoptosis was detected by terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay.ResultsCXCL6 was expressed in healthy cartilage and was retained through binding to heparan sulfate proteoglycans. CXCR2−/− mice developed more severe osteoarthritis than wild types following DMM, with increased chondrocyte apoptosis. Disruption of CXCR1/2 in human and CXCR2 signalling in mouse chondrocytes led to a decrease in extracellular matrix production, reduced expression of chondrocyte differentiation markers and increased chondrocyte apoptosis. CXCR2-dependent chondrocyte homeostasis was mediated by AKT signalling since forced expression of constitutively active AKT rescued the expression of phenotypic markers and the apoptosis induced by CXCR2 blockade.ConclusionsOur study demonstrates an important physiological role for CXCR1/2 signalling in maintaining cartilage homeostasis and suggests that the loss of ELR+ CXC chemokines during cartilage breakdown in osteoarthritis contributes to the characteristic loss of chondrocyte phenotypic stability.

Cartilage damage in osteoarthritis and rheumatoid arthritis—two unequal siblings

“…Conversely, transfection of CXCR2deficient chondrocytes with a SOX9expressing plasmid results in the rescue of COL2A1 mRNA expression to levels com parable with those of wildtype chondrocytes. 4 Notably, the CXCR2deficient mice develop OA only after injury, not spontaneously, suggesting that the protective func tion of SOX9 is only required in differentiated chondro cytes in response to challenge, and not for homeostasis in unchallenged cartilage. 4 Further evidence of the protection of cartilage integ rity by the binding of soluble factors comes from the demonstration that fibroblast growth factor 2 (FGF2), which binds to cartilage matrix in the growth plate 51 and in adult articular cartilage, 52 acts as an endogenous chondroprotective molecule that suppresses ADAMTS5 expression.…”

“…50 In addition to directly regulating SOX9 expression, the ECM might also act as a signalling scaffold, harbour ing SOX9regulating chemokines that are needed in adult cartilage to keep chondrocytes in their stable, resting state. 4 Degradation of heparan sulfate proteo glycans within the cartilage matrix in response to osteo arthritic injury leads to the release of chemokines such as CXCL6, which interacts with the chemokine receptor CXCR2. The injuryinduced OA is more severe in geneti cally modified mice lacking CXCR2 than in mice that express this receptor.…”

“…4 Among the factors that regulate chondrocyte differentiation, transcription factor SOX9 has been studied most intensively, and is not only essential for early chondrogenesis but is also closely linked to the later fate of chondrocytes in the growth plate and in adult articular cartilage. SOX9 is needed for functional growthplate formation, and inhibits the progression of chondrocytes from prolif eration to prehypertrophy, as well as the acquisition of an osteoblastic phenotype.…”

“…1 An important point to note is that the ECM not only provides the structural basis for the gliding and shockabsorbing properties of articular cartilage, but also constitutes a 'signalling scaf fold' . Through direct interactions with the embedded chondrocytes, as well as by retaining soluble mediators, this scaffold has a critical contribution to the develop ment of cartilage and bone during embryogenesis, 2,3 to the maintenance of the phenotypic stability of chondro cytes 4 and to their sitespecific metabolic activities. Any damage to cartilage integrity not only affects its function and the ability to move a damaged joint, but can also result in profound effects on the state-and metabolic activities-of the chondrocytes themselves, as well as on neighbouring cells and tissues, such as the synovium.…”

Chemokine receptors and their therapeutic opportunities in diseased lung: Far beyond leukocyte trafficking

“…The interaction between chemokines and their specific receptors leads to activation of intracellular signaling and subsequent chemotactic migration of the targeted cells, which then contribute to the restoration of tissue structure, function, and homeostasis (80,143). Chemokine receptors and their ligands are involved in the protection of the lung against external damaging agents (31,145,162,163).…”