Lubricin and hyaluronan are altered in multiple equine models of traumatic joint injury

Abstract

early after injury is mainly from dead or dying cells due to the trauma. HMGB1 detected in synovial fluid at later time points may be due to active release from immune cells and could be a potential contributor to disease progression. These results suggest that HMGB1 release into synovial fluid may be essential to the early inflammatory response to knee trauma and that HMGB1 is a potential target for future therapeutics.Purpose: Lubricin and hyaluronan (HA) are the primary lubricating molecules of articular cartilage. Lubricin is a boundary lubricant, while hyaluronic acid confers viscoelastic properties to synovial fluid. Profound loss of synovial fluid lubricin has been detected in human knee osteoarthritis (OA) and rodent destabilization models of knee OA; however, increased lubricin has recently been detected in equine synovial fluid following acute and chronic joint injury, including osteochondral fragmentation. Lubricin synovial fluid concentrations are also elevated following intra-articular fractures in humans. Therefore, it is unknown whether increased lubricin is specific to intra-articular fracture or other types of joint injury. Our objectives were to determine whether lubricin concentrations increased in non-fracture models of equine traumatic joint injury and to compare synovial fluid lubricin, HA, and HA molecular weight (MW) distribution over time post-injury. Methods: Synovial fluid samples were obtained from equine joints prior to and post-injury in three injury models: carpal osteochondral fragmentation (n¼8 horses), talar partial-thickness cartilage impact injury (n¼7 horses), and femoral lateral trochlear ridge full-thickness cartilage defects (n¼22 horses). Synovial fluid samples were obtained from the middle carpal (wrist) joint on days 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, and 70 post-arthroscopy for horses that underwent osteochondral fragmentation. Horses subjected to talar impact had synovial fluid sampled from the tarsocrural (ankle) joint on days 0, 4, 7, 14, 43, 71, and 168 after surgery. Femoropatellar (knee) joints subjected to full-thickness lateral trochlear ridge cartilage defects were sampled on days 0, 84, and 397 post-injury. Lubricin and HA were quantified using sandwich ELISAs. HA molecular weight (MW) distributions were determined using agarose gel electrophoresis and staining. Linear, mixed effect models were used to evaluate differences in synovial fluid biochemical parameters over time while controlling for the hierarchical nature of the data. Significance was set at p<0.05. Results: Lubricin concentrations increased post-injury in synovial fluid samples from all models (carpal fragmentation, p¼0.001; talar impact, p<0.0001; full thickness cartilage defect, p¼0.03) (Fig. 1, A-C). Conversely, HA concentrations decreased precipitously post-injury in both the carpal fragmentation (p<0.0001) and talar impact (P<0.0001) models and remained decreased for up to 168 days post-arthroscopy in the talar