Identification of Synovium Secretome Factor(s) for the Prevention of Osteoarthritis. Mejia S, Begum L, Keller LE, Zhang S, Fu Q, Fortier LA. Cornell University, Ithaca, NY.
Previous studies suggested that unknown bioactive factors secreted from synoviocytes protect cartilage from interleukin-1 beta (IL-1β)-induced catabolism. We hypothesized that using unbiased bottom-up proteomics would identify chondroprotective factors in synoviocyte secretome. Matched cartilage explants and synovium were collected from horses (n = 4). Six groups were established: cartilage, synoviocytes, and cartilage + synoviocytes (co-culture), all ± IL-1β. The catabolic effect of IL-1β was verified by glycosaminoglycan (GAG) released from cartilage into media by 1,9-dimethyl-methylene blue assay and cartilage toluidine blue histochemistry. Conditioned media from co-cultures ± IL-1β were submitted for bottom-up proteomic analysis. Synoviocyte gene expression was evaluated using RT-qPCR for proteins of interest. Retention of matrix metachromasia of cartilage in co-cultures treated with IL-1β confirmed the chondroprotective effects of synoviocytes. Proteomics identified 34 proteins that were upregulated in at least 50% of the IL-1β treated co-cultures, 14 of which were nonstructural proteins of interest. Metalloproteinase inhibitor 3 precursor (TIMP3), insulin-like growth factor-binding protein 2 (IGFBP2), and tumor necrosis factor receptor superfamily member 11B (OPG) were selected for synoviocyte gene expression analysis because of their known chondroprotective functions. Expression of TIMP3 and OPG were increased four- and 11-fold, respectively, indicating that these proteins were contributing component of synoviocyte secretome. Expression of IGFBP2 did not change, indicating it did not originate from synoviocytes. TIMP3 inhibits MMPs and aggrecanases providing broad chondroprotective functions, and OPG binds the anti-inflammatory/anti-apoptotic protein TRAIL; however, its role in OA is unknown. The proteins identified require further evaluation to explore them as potential disease modifying therapeutics for OA.