Abstract
Background
Osteoarthritis (OA), a common degenerative disorder of synovial joints, is characterized by disruption of the extracellular matrix (ECM) homeostasis with an overall misbalance towards cartilage catabolism. Integrins are alpha/beta heterodimeric transmembrane proteins transmitting chemical and biomechanical signals into the cells. There is a growing consensus that changes of ECM composition by proteolytic degradation of matrix constituents, or alteration of the biomechanical microenvironment of chondrocytes caused by chronic stress or injury significantly increase the risk of OA through the perturbation of integrin signaling. In order to further investigate the role of the b1 integrin subfamily in OA, we have challenged hip cartilage explants dissected for mice lacking beta1 integrins in chondrocytes by cytokines, ECM degradation products or mechanical stimulation.
Methods
Femoral articular cartilages were avulsed from hip joints of 6 weeks old wild type (WT) and b1fl/fl-PrxCre mutant (MT) mice. For the chemically-induced OA-like stimulation, femoral caps were cultured for 3 days in serum-free DMEM/F12 with or without the supplementation of interleukin-1a (IL1a), 120kDa cell-binding fibronectin fragments (120FNf), or tumor necrosis factor-alpha (TNFa) + oncostatin M (OM). Sulphated glycosaminoglycan (sGAG) release of the explants were measured in the supernatants by the 1,9-dimethylmethlene blue (DMMB) assay. Proteoglycan loss was monitored by Safranin-O (SO) staining on cryo-sections of the explants. For the cartilage injury model, avulsed femoral caps were either directly snap-frozen or kept in serum-free DMEM/F12 for 4 hours before snap-freezing. Gene expression changes were analyzed by quantitative RT-PCR using a pre-determined set of genes regulated by injury.
Results
Articular cartilages of MT mice were found to have consistently higher release of GAGs when exposed to cytokines or 120FNf. IL-1a exerted the highest catabolic stimulation. The ex vivo biochemical analysis was further verified by SO staining demonstrating more pronounced proteoglycan loss on MT sections compared to WT. Assessing the mRNA of articular cartilages subjected to the injury model, revealed expression changes in genes which have been previously implicated in OA: Il1a (interleukin 1, alpha) and Ptgs2 (prostaglandin-endoperoxide synthase 2) were upregulated in MT mice; whereas Il1rl1 (interleukin 1 receptor-like 1) and Nos2 (nitric oxide synthase 2) expression levels were significantly reduced in MT compared to WT.
Conclusion
The data imply that b1 integrins play a protective role against cytokine- and fibronectin fragment-induced cartilage degradation. Our findings also suggest that b1 integrins modulate the expression of catabolic factors upon mechanical insults.
