Proteoglycan 4 (PRG4) gene expression in synovial membrane (A) and cartilage (B) from normal equine carpal joints (n = 22) and equine carpal joints with naturally occurring OA (n = 36). Categories were numbered as normal (0), mild OA (1), moderate OA (2) and severe OA (3). Synovial membrane PRG4 expression was significantly increased (A) and cartilage PRG4 expression was significantly decreased (B) in the moderate OA group as compared to the control group. (C) Synovial fluid lubricin concentrations were significantly elevated in OA joints as compared to controls. Data are presented as mean ± S.E.M. Scatterplots showing the correlation between (D) synovial membrane PRG4 expression and synovial membrane ADAMTS5 (log transformed) (ρ = 0.488, P < 0.001) and (E) TNFα (log transformed) (ρ = 0.508, P < 0.001) expression. (F) Scatterplots showing the correlation between synovial fluid lubricin concentrations (log transformed) vs TNFα concentrations (log transformed) (ρ = 0.520, P < 0.001).