Effects of concentration and structure on proteoglycan 4 rheology and interaction with hyaluronan

BACKGROUND: The contribution of proteoglycan 4 (PRG4) to synovial fluid and hyaluronan (HA) solution rheology are poorly understood. The effects of PRG4 disulfide-bonded structure on viscosity and viscosity of newly available full-length recombinant human PRG4 (rhPRG4) have not previously been reported. OBJECTIVE: This study determined the viscosity of PRG4 and rhPRG4, R/A (reduced and alkylated) PRG4 and rhPRG4, and PRG4 and rhPRG4 + HA solutions. METHODS: Steady shear viscosities of 1.5 MDa HA, PRG4 from bovine cartilage explant culture, rhPRG4 and (rh) PRG4 + HA solutions were measured with 40 mm parallel plate fixtures. RESULTS: PRG4 demonstrated shear-dependent viscosity at high concentrations, but Newtonian behaviour at low concentrations and when disulfide-bonded/multimeric structure was disrupted by R/A. rhPRG4 demonstrated Newtonian behaviour over all concentrations tested and upon R/A. At high HA concentrations, rhPRG4 reduced solution viscosity, suggesting a binding interaction. At low HA concentrations, solution viscosity was increased relative to HA alone, possibly due to self-association of rhPRG4. Effects of PRG4 on HA solution viscosity were dependent on PRG4's disulfide-bonded structure. CONCLUSIONS: The finding that rhPRG4 can increase the viscosity of low concentration HA solutions suggests that supplementation with rhPRG4 may help mitigate the loss in synovial fluid viscosity experienced with decreased HA concentration in osteoarthritis.