Metabolic profiling of inflammatory synovial fluid

Abstract

Synovial fluid (SF) has unique properties regarding the maintenance of joint homeostasis. It consists mainly of proteins, but also small molecules, which are metabolic derivatives of these proteins. These molecules can be assessed by metabolic profiling using techniques such as nuclear magnetic resonance (NMR) spectroscopy and Mass Spectrometry (MS). A systematic review identified over 200 metabolites, from which 26 were putative biomarkers in osteoarthritis, inflammatory arthropathies and trauma. The molecular composition of osteoarthritic and infected SF was examined using NMR spectroscopy. Due to ethical constraints, it was not possible to collect samples from normal joints. It was hypothesised that SF of hip and knee joints have different metabolic characteristics. Osteoarthritic hip and knee samples were taken and assessed for differences in their metabolic composition. Four metabolites presented in significantly greater quantities in the knee compared to the hip group (N-acetylated molecules, glycosaminoglycans, citrate, glutamine). These are involved in collagen degradation, the tricarboxylic acid cycle and oxidative metabolism in diseased joints. These findings may represent a combination of intra and extra-articular factors. Another promising application was to examine infected versus non-infected SF. Three classes of metabolites presented in higher relative concentrations (lipids, cholesterol and N-acetylated molecules) and 13 in lower relative concentrations in the infected group. Metabolites in significantly greater concentrations in the infected cohort represented those having a defensive role against pathogenic microorganisms, a role in lipid metabolism and the inflammatory response. Those in significantly reduced concentrations were involved in carbohydrate (citrate, glucose and mannose) and nucleoside metabolism (glycine, glutamine and valine), the glutamate metabolic pathway (glutamine, proline and lysine), increased oxidative stress in the diseased state (dimethylsulfone) and reduced articular cartilage breakdown (glycosaminoglycans). Future studies should include larger cohorts to confirm these findings and validation of the identified metabolites using other metabolic profiling techniques with more targeted analysis, such as MS. This may ultimately lead to identifying putative biomarkers that form the bases of new diagnostic tests.