Role of the chondrocyte cytoskeleton in health and disease

Abstract

Introduction: Articular cartilage comprises a dense extracellular matrix (ECM) of primarily collagen, proteoglycans and water interspersed with the cartilage cell- the chondrocyte. Osteoarthritis (OA) is a disease characterised by articular cartilage degradation and a change in chondrocyte phenotype. Increased or abnormal joint loading is a risk factor for OA and can regulate chondrocyte phenotype. The chondrocyte cytoskeleton comprises actin microfilaments, tubulin microtubules and vimentin intermediate filaments and has been implicated in the propagation of physical signals to the chondrocyte nucleus, termed 'mechanotransduction'. In addition, the organisation of chondrocyte cytoskeletal networks has been observed to differ in both human OA and in a rat model of OA when compared with normal chondrocytes. We hypothesise that dysregulation of cytoskeletal networks prevents normal ECM-chondrocyte signalling and promotes a catabolic phenotype as in OA. Results: When compared with normal human chondrocytes, OA chondrocytes exhibited differences in the gene expression of components of the cytoskeleton and in the spatial organisation and architecture of the cytoskeleton, both in situ and in vitro. In normal and OA human chondrocytes cultured in agarose hydrogels, disruption of each of the three main cytoskeletal elements resulted in gene expression changes in both normal and OA cells. A number of gene responses to cytoskeletal disruption were similar in normal and OA cells, such as SOX9, MMP14, TGFB1, CASP3 and PTGS2 (COX-2). Other genes responded differently to the same treatment in normal versus OA cells, including ADAMTS5, COMP, FGFR3 and NOS2A (iNOS). Cyclic compression (15% strain, 0.5 Hz) for up to 40 minutes induced cytoskeletal reorganisation in normal and OA human chondrocytes and up-regulation of p-tubulin and destrin mRNA expression. Recovery in free swelling conditions for five hours post-load showed the chondrocyte phenotype was enhanced in OA chondrocytes. Cyclic compression in the presence of cytoskeletal disruption altered the transcriptional response of the actin depolymerising proteins cofilin and destrin in normal and OA chondrocytes, and the transcriptional response of SOX9 and the actin sequestering protein thymosin p4 in OA chondrocytes. Conclusions: Changes in the cytoskeleton of OA chondrocytes are not simply a result of the altered mechanical environment in OA articular cartilage. Changes in the cytoskeleton can affect chondrocyte phenotype and the response of chondrocytes to cyclic compression, therefore the observed differences in organisation and expression could result in the altered phenotype of OA chondrocytes. Differences between the effect of cyclic compression on normal and OA human chondrocytes support the existence of different or divergent mechanotransduction pathways that are mediated in part by elements of the chondrocyte cytoskeleton