Abstract
IntroductionArticular cartilage functions in withstanding mechanical loads and provides a lubricating surface for frictionless movement of joints. Osteoarthritis, characterised by cartilage degeneration, develops due to the progressive erosion of structural integrity and eventual loss of functional performance. Osteoarthritis is a multi-factorial disorder; two important risk factors are abnormal mechanical load and genetic predisposition. A single nucleotide polymorphism analysis demonstrated an association of hip osteoarthritis with an Arg324Gly substitution mutation in FrzB, a Wnt antagonist. The purpose of this study was two-fold: to assess whether mechanical stimulation modulates β-catenin signalling and catabolic gene expression in articular chondrocytes, and further to investigate whether there is an interplay of mechanical load and Wnt signalling in mediating a catabolic response.MethodsChondrocytes were pre-stimulated with recombinant Wnt3A for 24 hours prior to the application of tensile strain (7.5%, 1 Hz) for 30 minutes. Activation of Wnt signalling, via β-catenin nuclear translocation and downstream effects including the transcriptional activation of c-jun, c-fos and Lef1, markers of chondrocyte phenotype (type II collagen (col2a1), aggrecan (acan), SOX9) and catabolic genes (MMP3, MMP13, ADAMTS-4, ADAMTS-5) were assessed.ResultsPhysiological tensile strain induced col2a1, acan and SOX9 transcription. Load-induced acan and SOX9 expression were repressed in the presence of Wnt3A. Load induced partial β-catenin nuclear translocation; there was an additive effect of load and Wnt3A on β-catenin distribution, with both extensive localisation in the nucleus and cytoplasm. Immediate early response (c-jun) and catabolic genes (MMP3, ADAMTS-4) were up-regulated in Wnt3A stimulated chondrocytes. With load and Wnt3A there was an additive up-regulation of c-fos, MMP3 and ADAMTS-4 transcription, whereas there was a synergistic interplay on c-jun, Lef1 and ADAMTS-5 transcription.ConclusionOur data suggest that load and Wnt, in combination, can repress transcription of chondrocyte matrix genes, whilst enhancing expression of catabolic mediators. Future studies will investigate the respective roles of abnormal loading and genetic predisposition in mediating cartilage degeneration.
Highlights
Articular cartilage functions in withstanding mechanical loads and provides a lubricating surface for frictionless movement of joints
With load and Wnt3A there was an additive up-regulation of c-fos, MMP3 and ADAMTS-4 transcription, whereas there was a synergistic interplay on c-jun, Lef1 and ADAMTS-5 transcription
Abnormal loading induced by joint misalignment, trauma or zero gravity inhibits extracellular matrix (ECM) synthesis promoting catabolism via induction of proteolytic enzymes including collagen-degrading matrix metalloproteinases (MMPs) and the aggrecan-degrading aggrecanases (ADAMTSs) [1,3]
Summary
Articular cartilage functions in withstanding mechanical loads and provides a lubricating surface for frictionless movement of joints. Osteoarthritis, characterised by cartilage degeneration, develops due to the progressive erosion of structural integrity and eventual loss of functional performance. A single nucleotide polymorphism analysis demonstrated an association of hip osteoarthritis with an Arg324Gly substitution mutation in FrzB, a Wnt antagonist. The progressive erosion of structural integrity and eventual loss of functional performance is mediated by the MMPs, ADAMTSs and other matrix proteases. Single nucleotide polymorphism (SNP) analysis demonstrated an association of hip OA with a functional SNP resulting in an Arg324Gly substitution in the encoded secreted frizzled-related protein 3 (sFRP3), a Wnt antagonist [4]. Continuous activation of the canonical Wnt/b-catenin pathway can elicit matrix degradation [6]; in chondrocyte cultures, use of either Wnt3A conditioned media or forced expression of constitutive-active b-catenin stimulated transcription of MMP3, MMP13, ADAMTS-4 and ADAMTS-5 and increased proteoglycan loss [5,9]
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