The purposes of this study were 1) to quantify the proteoglycan 4 (PRG4) and hyaluronan (HA) content in synovial fluid (SF) from normal donors and from patients with chronic osteoarthritis (OA) and 2) to assess the cartilage boundary-lubricating ability of PRG4-deficient OA SF as compared to that of normal SF, with and without supplementation with PRG4 and/or HA.
OA SF was aspirated from the knee joints of patients with symptomatic chronic knee OA prior to therapeutic injection. PRG4 concentrations were measured using a custom sandwich enzyme-linked immunosorbent assay (ELISA), and HA concentrations were measured using a commercially available ELISA. The molecular weight distribution of HA was measured by agarose gel electrophoresis. The cartilage boundary-lubricating ability of PRG4-deficient OA SF, PRG4-deficient OA SF supplemented with PRG4 and/or HA, and normal SF was assessed using a cartilage-on-cartilage friction test. Two friction coefficients (μ) were calculated: static (μ(static, Neq) ) and kinetic (<μ(kinetic, Neq) >) (where N(eq)  represents equilibrium axial load and angle brackets indicate that the value is an average).
The mean ± SEM PRG4 concentration in normal SF was 287.1 ± 31.8 μg/ml. OA SF samples deficient in PRG4 (146.5 ± 28.2 μg/ml) as compared to normal were identified and selected for lubrication testing. The HA concentration in PRG4-deficient OA SF (mean ± SEM 0.73 ± 0.08 mg/ml) was not significantly different from that in normal SF (0.54 ± 0.09 mg/ml). In PRG4-deficient OA SF, the molecular weight distribution of HA was shifted toward the lower range. The cartilage boundary-lubricating ability of PRG4-deficient OA SF was significantly diminished as compared to normal (mean ± SEM <μ(kinetic, Neq) > = 0.043 ± 0.008 versus 0.025 ± 0.002; P < 0.05) and was restored when supplemented with PRG4 (<μ(kinetic, Neq) > = 0.023 ± 0.003; P < 0.05).
These results indicate that some OA SF may have decreased PRG4 levels and diminished cartilage boundary-lubricating ability as compared to normal SF and that PRG4 supplementation can restore normal cartilage boundary lubrication function to these OA SF.