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

Objective To describe proteoglycan 4 ( PRG4 ) expression, lubricin tissue distribution, and lubricin synovial fluid concentrations in healthy and injured equine flexor tendons and tenosynovial structures. Methods Synovial fluid and tissues were collected over the span of 1 year. Proteoglycan 4 expression was quantified using quantitative real-time PCR, lubricin synovial fluid concentrations were measured using ELISAs, and lubricin tissue distribution was evaluated with immunohistochemistry. Histology and quantitative real-time PCR data for healthy samples were analyzed using paired t tests, repeated-measures ANOVA, or nonparametric equivalents, and ELISA data for healthy and injured horses were evaluated using mixed linear models. Results Samples were obtained from 16 horses without tendon injury and 11 horses with tendon injury (n = 27 horses total). Proteoglycan 4 was expressed in equine superficial digital flexor tendon, deep digital flexor tendon, manica flexoria, and tendon sheath synovial membrane tissues, with the greatest expression in the synovial membrane and manica flexoria. Conclusions Lubricin immunohistochemical staining was similar between the healthy superficial digital flexor tendon and deep digital flexor tendon, with more intense staining in epitenon regions and intrasynovial locations. Synovial fluid lubricin concentrations were greater in sheaths with tendon injury (839.4 vs 184.7 µg/mL), and lubricin staining was greater at sites of tendon injury. Clinical Relevance Lubricin/ PRG4 is present in equine flexor tendons and tendon sheaths, with more intense immunostaining in intrasynovial locations and at sites of tendon injury as a possible reparative response to maintain tendon elasticity and gliding function.