Proteomic analysis of synovial fluid as an analytical tool to detect candidate biomarkers for knee osteoarthritis

We conducted research to detect the proteomic profiles in synovial fluid (SF) from knee osteoarthritis (OA) patients to better understand the pathogenesis and aetiology of OA. Our long-term goal is to identify reliable candidate biomarkers for OA in SF. The SF proteins obtained from 10 knee OA patients and 10 non-OA patients (9 of whom were patients with a meniscus injury in the knee; 1 had a discoid meniscus in the knee, and all exhibited intact articular cartilage) were separated by two-dimensional electrophoresis (2-DE). The repeatability of the obtained protein spots regarding their intensity was tested via triplicate 2-DE of selected samples. The observed protein expression patterns were subjected to statistical analysis, and differentially expressed protein spots were identified via matrix-assisted laser desorption/ionisation-time of flight/time of flight mass spectrometry (MALDI-TOF/TOF MS). Our analyses showed low intrasample variability and clear intersample variation. Among the protein spots observed on the gels, there were 29 significant differences, of which 22 corresponded to upregulation and 7 to downregulation in the OA group. One of the upregulated protein spots was confirmed to be haptoglobin by mass spectrometry, and the levels of haptoglobin in SF are positively correlated with the severity of OA (r = 0.89, P < 0.001). This study showed that 2-DE could be used under standard conditions to screen SF samples and identify a small subset of proteins in SF that are potential markers associated with OA. Spots of interest identified by mass spectrometry, such as haptoglobin, may be associated with OA severity.