Synovial fluid (SF) is the viscous fluid present within articular joints that contributes to load bearing and lubrication functions. Proteoglycan 4 (PRG4) and hyaluronan (HA) in SF contribute synergistically to cartilage boundary lubrication. However, changes in SF PRG4 and HA content with osteoarthritis (OA) and associated effects on cartilage boundary lubricating function are not fully understood. Furthermore, the effects of PRG4+HA interaction on solution viscosity have not been thoroughly characterized.

The objectives of this thesis were to 1) investigate the relationship between PRG4 and HA composition and boundary lubricating function of normal and OA SF, and 2) to investigate how the concentration and structure of PRG4 contributes to interactions with itself and HA, and subsequently the boundary lubricating and rheological properties of SF.

Novel and previously characterized biochemical and biomechanical methods were used to evaluate boundary lubricant composition and lubricating ability of SF. While not all OA SF samples had low PRG4, samples that had low PRG4 concentration and decreased HA molecular weight (MW) demonstrated decreased cartilage boundary lubricating ability in vitro, which could be restored by addition of PRG4. SF aspirated after a flare reaction to intra-articular injection that had low PRG4 and an approximately normal HA MW distribution demonstrated normal cartilage boundary lubricating ability. In purified solutions of PRG4 and HA, decreased PRG4 or decreased high MW HA limited cartilage boundary lubricating ability. PRG4 and recombinant human PRG4 increased the viscosity of HA solutions at low concentrations, but decreased the viscosity of high concentration HA solutions. The intra- and inter-molecular disulfide bonded structure of PRG4 was observed to be important for its contributions to both PRG4+HA cartilage boundary lubricating ability and PRG4+HA solution viscosity.

These results demonstrate that alterations in both PRG4 and HA content in SF may have negative effects on SF cartilage boundary lubricating and rheological function, and are consistent with a non-covalent, crowding mechanism of interaction. They suggest that maintaining PRG4 and HA content in SF during injury and disease, through the development of new PRG4±HA biotherapeutic treatments, may be able to both protect cartilage from degeneration and restore SF viscosity in vivo.