Synovial fluid alpha-2-macroglobulin, gelsolin and lubricin distinguish between osteoarthritic and healthy equine joints.

Abstract

Background: Synovial fluid (SF) is an ideal sentinel fluid for osteoarthritis (OA) diagnosis and prognostication due to its critical homeostatic role, proximity to articular tissues and immune cell composition. Untargeted proteomics enable identification of soluble markers for diagnostic and therapeutic applications while minimising bias.

Objectives: To use liquid chromatography-tandem mass spectrometry (LC-MS/MS) to define the SF proteome in horses with and without carpal OA. The goal was to identify differentially regulated proteins in mild-moderate carpal joint disease compared with healthy joints.

Study design: Cross-sectional study.

Methods: Synovial fluid was obtained from horses undergoing arthroscopic treatment for carpal fragmentation and from horses with healthy carpal joints. LC-MS/MS proteomics was performed on a subset of joints (n = 8 OA, n = 8 healthy). Total protein (TP), gelsolin, lubricin and prostaglandin E₂ (PGE₂) concentrations were quantified via biochemical or immunoassays (n = 58 OA, n = 25 healthy), and synovial membrane histology was graded (n = 16 OA, n = 6 healthy). Univariate and multivariate modelling were used to compare the predictive capacities of soluble factors between healthy and OA joints.

Results: Of 119 proteins identified, 14 were increased and 10 decreased in OA. Two of the most upregulated proteins included pregnancy zone protein and alpha-2-macroglobulin. In OA, gelsolin/TP was decreased (OA median: 0.48 μg/mL/mg [95% confidence interval 0.27-0.73]; healthy: 0.89 μg/mL/mg [0.63-1.17]) and lubricin/TP increased (OA: 27.98 μg/mL/mg, [21.71-45.79]; healthy: 6.77 μg/mL/mg [4.36-9.94]) when measured by immunoassay. Multivariate modelling including gelsolin/TP, lubricin/TP and PGE₂ was superior to univariate models for differentiating between OA and healthy joints.

Main limitations: Synovial membrane was available for a limited number of joints and most healthy samples were collected following euthanasia.

Conclusions: The upregulation of pregnancy zone protein and alpha-2-macroglobulin in OA joints motivates investigation into their function in OA and development of reagents for quantification. Several proteins with differential abundance in OA SF, including gelsolin and lubricin, were measured and may have diagnostic utility.