Arthritis affects millions of people and costs billions to the global economy. Although joint pain is the leading patient-reported symptom of those with arthritis, current analgesics often fail to produce consistent pain relief and, with chronic use, are associated with detrimental side effects. Osteoarthritis (OA) is the most common form of arthritis, with the knee being among the joints most frequently affected. Understanding the molecular basis for OA joint pain is important in seeking to develop targeted, efficacious analgesic therapies, which produce fewer side effects. There is also a pressing clinical need for biomarkers of OA that detect the disease in its early stages, where the progression of the disease may still be responsive to pharmacotherapy. One route to gaining insight into future therapeutic targets is to use human samples, such as synovial fluid (SF), in pre-clinical research. SF lubricates healthy synovial joints, like the knee, and has been shown to have elevated levels of various inflammatory mediators, such as peptide nerve growth factor (NGF), which correlates with pain during OA. A further important class of inflammatory mediators are lipids, which can be pro- or anti- inflammatory. Previous lipidomic studies have also identified that various sphingolipid (SL) species are present at elevated levels in OA-SF, but their contribution to pain is often unknown. My research goal was to investigate the possible involvement of certain mediators, within human OA-SF samples, in pain mechanisms by using various novel in vitro and in vivo murine OA models.

Experimental animal models of arthritis do not fully recapitulate the human disease and hence there is a need to develop models that utilise human clinical samples to bridge the translational gap between bench and bedside. I developed a novel, in vivo translational model to study OA pain by injecting mouse knees with human SF obtained from OA patients or post-mortem donors with no known joint disease. Although joint inflammation was consistently induced, no consistent pain behaviour phenotype was observed. I also used an in vitro model to investigate if OA-SF samples and fibroblast condition media from patients at different stages and severity of knee OA differentially regulated sensory neuron excitability, but no significant differences were observed. Finally, based on human and rodent studies identifying the specific SL, sphingomyelin 34:1 (SM34:1), as being present at elevated levels in OA-SF and positively correlating with pain and joint degeneration, I used a novel in vivo joint inflammation model to determine if SM 34:1 induces pain behaviours. We found that SM 34:1 does seem to have a role in arthritic pain, such that intraarticular knee injection caused joint inflammation, which correlated with a decreased pain threshold in spontaneous and evoked pain measurements. Further functional studies need to be carried out to understand the exact role of SM 34:1 in pain and its potential as an early biomarker and/or therapeutic target in OA.

Findings from this Thesis highlight multiple ways to identify drivers of inflammatory knee pain and open the door for further screening of other possible biomarkers and therapeutic targets, in particular SLs, for OA pain.