Proteoglycan 4 is expressed in equine flexor tendons and tendon sheaths, with greater synovial fluid lubricin in tendon sheaths with tendon injury.

Overview

• This study investigated the presence and role of proteoglycan 4 (PRG4), also known as lubricin, in the tendons and tendon sheaths of horses, focusing on differences between healthy and injured tissues.
• Researchers measured how much PRG4 was expressed in the tissues, how lubricin was distributed, and the concentrations of lubricin in synovial fluid, particularly examining changes associated with tendon injuries.

Background and Purpose

• Proteoglycan 4 (PRG4), or lubricin, is a glycoprotein known for its lubricating properties in joints, helping reduce friction during movement.
• The study aimed to characterize the expression of PRG4 and lubricin distribution in equine flexor tendons and surrounding structures, and to compare levels between healthy and injured states.
• The focus was on various tendon components including the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), manica flexoria (a synovial fold within the tendon sheath), and tendon sheath synovial membrane.

Materials and Methods

• Samples were collected over one year from 27 horses: 16 healthy and 11 with tendon injuries.
• Proteoglycan 4 expression in tissues was quantified using quantitative real-time PCR (qRT-PCR), a precise method to measure RNA levels indicative of gene expression.
• Lubricin concentrations in synovial fluid were measured using enzyme-linked immunosorbent assays (ELISAs), allowing quantification of protein levels.
• Immunohistochemistry was used to visualize lubricin distribution directly in tissue samples, highlighting locations within tendons and sheaths.
• Statistical analyses included paired t tests, repeated-measures ANOVA, and mixed linear models to compare data between healthy and injured groups and across tissue types.

Key Findings: PRG4 Expression and Lubricin Distribution

• PRG4 gene expression was detected in all tendon-related tissues examined: SDFT, DDFT, manica flexoria, and tendon sheath synovial membrane.
• The highest PRG4 expression was found in the synovial membrane of the tendon sheath and the manica flexoria, suggesting these structures have an important role in producing lubricin.
• Immunohistochemistry showed lubricin staining was present in both the SDFT and DDFT tendons.
• The staining was more intense in the epitenon (outer layer of the tendon) regions and in the intrasynovial locations inside tendon sheaths, indicating higher lubricin localization in areas involved in tendon gliding and lubrication.

Changes in Lubricin Levels With Tendon Injury

• Synovial fluid lubricin concentrations were significantly higher in tendon sheaths that had tendon injuries compared to healthy tendons (average 839.4 µg/mL vs. 184.7 µg/mL).
• At sites of tendon injury, lubricin immunostaining was more intense, suggesting an upregulation or accumulation of lubricin in response to damage.
• This increase may represent a reparative mechanism aimed at restoring or maintaining tendon elasticity and smooth gliding function in the presence of injury.

Implications and Conclusions

• The presence of PRG4/lubricin in tendons and tendon sheaths confirms that these proteins likely play a key role in tendon health by providing lubrication and protecting tissues from friction-related damage.
• The elevated lubricin levels in injured tendons and synovial fluid suggest that lubricin might be part of the tendon’s natural response to injury, potentially helping in repair and functional maintenance.
• These findings underline the importance of lubricin as a potential therapeutic target or biomarker for tendon injury diagnosis and treatment strategies in horses and potentially other species.