Proteoglycan 4 is expressed in equine flexor tendons and tendon sheaths, with greater synovial fluid lubricin in tendon sheaths with tendon injury.
Authors: Matthew J Woodman, M. Thomas, Marshall J Colville, S. Durgam, S. McDonough, Heidi L. Reesink
Journal: American journal of veterinary research
Summary
# Editorial Summary: Lubricin Expression in Equine Flexor Tendons and Injury Response Lubricin, the protein product of the PRG4 gene, is well recognised for its role in joint lubrication, yet its function within the specialised environment of flexor tendon sheaths remains poorly characterised in horses. Woodman and colleagues investigated PRG4 expression patterns across healthy and injured digital flexor tendons and their surrounding synovial structures in 27 horses, employing quantitative PCR to measure gene expression, immunohistochemistry to map protein distribution, and ELISA to quantify synovial fluid concentrations. The researchers found that PRG4 is actively expressed throughout flexor tendon tissues, with particularly robust expression in the synovial membrane and manica flexoria, whilst lubricin staining concentrated in epitenon regions and intrasynovial locations—areas critical for tendon gliding mechanics. Most striking was the marked elevation in synovial fluid lubricin concentrations in injured sheaths (839.4 µg/mL versus 184.7 µg/mL in healthy sheaths), alongside increased immunostaining at sites of tendon damage, suggesting an upregulated protective response. These findings indicate that lubricin serves as both a baseline lubricant maintaining tendon function and an adaptively-expressed molecule responding to injury, which has implications for understanding tendon healing biology and potentially for developing targeted therapeutics to support the reparative process and restore gliding capacity in horses with flexor tendon injuries.
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Practical Takeaways
- •Elevated lubricin in synovial fluid may serve as a biomarker for flexor tendon sheath involvement; synovial fluid analysis could help assess tendon injury severity
- •The body's natural upregulation of lubricin at injury sites supports conservative management approaches that preserve tendon sheath function and allow healing
- •Understanding that lubricin is a protective response reinforces the importance of controlled exercise and early mobilisation in flexor tendon rehabilitation to maintain tendon gliding
Key Findings
- •PRG4 (lubricin) is expressed in equine flexor tendons, manica flexoria, and tendon sheath synovial membranes, with greatest expression in synovial membrane and manica flexoria
- •Synovial fluid lubricin concentrations were 4.5-fold higher in injured tendon sheaths (839.4 µg/mL) compared to healthy sheaths (184.7 µg/mL)
- •Lubricin immunostaining was more intense in epitenon regions, intrasynovial locations, and at sites of active tendon injury
- •Increased lubricin at injury sites suggests a protective reparative response to maintain tendon gliding function and elasticity