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A 47-year-old man presented to an urgent care ambulatory clinic with a 3-day history of swelling around his left eye and a sensation of tightness in his throat. It had become difficult for him to swallow solids, and he felt as though food was sticking in his throat.

Objective The pathophysiology of the most common joint disease, osteoarthritis (OA), remains poorly understood. Since synovial fluid (SF) bathes joint cartilage and synovium, we reasoned that a comparative analysis of its protein constituents in health and OA could identify pathways involved in joint damage. We undertook this study to perform a proteomic analysis of knee SF from OA patients and control subjects and to compare the results to microarray expression data from cartilage and synovium. Methods Age-matched knee SF samples from 10 control subjects, 10 patients with early-stage OA, and 10 patients with late-stage OA were compared using 2-dimensional difference-in-gel electrophoresis and mass spectrometry (MS). MS with a multiplexed peptide selected reaction monitoring assay was used to confirm differential expression of a subset of proteins in an independent OA patient cohort. Proteomic results were analyzed by Ingenuity Pathways Analysis and compared to published synovial tissue and cartilage messenger RNA profiles. Results Sixty-six proteins were differentially present in healthy and OA SF. Three major pathways were identified among these proteins: the acute-phase response signaling pathway, the complement pathway, and the coagulation pathway. Differential expression of 5 proteins was confirmed by selected reaction monitoring assay. A focused analysis of transcripts corresponding to the differentially present proteins indicated that both synovial and cartilage tissues may contribute to the OA SF proteome. Conclusion Proteins involved in the acute-phase response signaling pathway, the complement pathway, and the coagulation pathway are differentially regulated in SF from OA patients, suggesting that they contribute to joint damage. Validation of these pathways and their utility as biomarkers or therapeutic targets in OA is warranted.

Osteoarthritis, characterized by the breakdown of articular cartilage in synovial joints, has long been viewed as the result of 'wear and tear'. Although low-grade inflammation is detected in osteoarthritis, its role is unclear. Here we identify a central role for the inflammatory complement system in the pathogenesis of osteoarthritis. Through proteomic and transcriptomic analyses of synovial fluids and membranes from individuals with osteoarthritis, we find that expression and activation of complement is abnormally high in human osteoarthritic joints. Using mice genetically deficient in complement component 5 (C5), C6 or the complement regulatory protein CD59a, we show that complement, specifically, the membrane attack complex (MAC)-mediated arm of complement, is crucial to the development of arthritis in three different mouse models of osteoarthritis. Pharmacological modulation of complement in wild-type mice confirmed the results obtained with genetically deficient mice. Expression of inflammatory and degradative molecules was lower in chondrocytes from destabilized joints from C5-deficient mice than C5-sufficient mice, and MAC induced production of these molecules in cultured chondrocytes. Further, MAC colocalized with matrix metalloprotease 13 (MMP13) and with activated extracellular signal-regulated kinase (ERK) around chondrocytes in human osteoarthritic cartilage. Our findings indicate that dysregulation of complement in synovial joints has a key role in the pathogenesis of osteoarthritis.

Osteoarthritis (OA) was once viewed originally as a mechanical disease of "wear and tear," but advances made during the past two decades suggest that abnormal biomechanics contribute to active dysregulation of chondrocyte biology, leading to catabolism of the cartilage matrix. A number of signaling and transcriptional mechanisms have been studied in relation to the regulation of this catabolic program, but how they specifically regulate the initiation or progression of the disease is poorly understood. Here, we demonstrate that cartilage-specific ablation of Nuclear factor of activated T cells c1 (Nfatc1) in Nfatc2(-/-) mice leads to early onset, aggressive OA affecting multiple joints. This model recapitulates features of human OA, including loss of proteoglycans, collagen and aggrecan degradation, osteophyte formation, changes to subchondral bone architecture, and eventual progression to cartilage effacement and joint instability. Consistent with the notion that NFATC1 is an OA-suppressor gene, NFATC1 expression was significantly down-regulated in paired lesional vs. macroscopically normal cartilage samples from OA patients. The highly penetrant, early onset, and severe nature of this model make it an attractive platform for the preclinical development of treatments to alter the course of OA. Furthermore, these findings indicate that NFATs are key suppressors of OA, and regulating NFATs or their transcriptional targets in chondrocytes may lead to novel disease-modifying OA therapies.