Proteoglycan 4 (PRG4) and hyaluronan (HA) are key synovial fluid constituents that contribute synergistically to cartilage boundary lubrication; however, the effects of their concentrations as well as their structure, both of which can be altered in osteoarthritis, on this functional synergism are unknown. The objectives of this study were to evaluate cartilage boundary lubricating ability of 1) PRG4 + HA in solution at constant HA concentration in a range of PRG4 concentrations, 2) constant PRG4 concentration in a range of HA concentrations, 3) HA + reduced/alkylated (R/A) PRG4, and 4) hylan G-F 20 + PRG4.
Static and kinetic friction coefficients (μstatic,Neq, <μkinetic,Neq>) were measured using a previously characterized cartilage-cartilage boundary mode friction test for the following concentrations of purified PRG4 and HA: Test 1: HA (1.5 MDa, 3.3 mg/mL) + PRG4 from 4.5 – 1500 μg/mL; Test 2: PRG4 (450, 150, 45 μg/mL) + HA (1.5 MDa) from 0.3 – 3.3 mg/mL. Test 3: hylan G-F 20 (3. 3 mg/mL) + PRG4 (450 μg/mL). Test 4: HA (3.3 mg/mL) + R/A PRG4 (450 μg/mL). ANOVA was used to compare lubricants within (comparing 6 lubricants of interest) and between (comparing 3 lubricants of interest) test sequences, with Tukey and Fishers post-hoc testing respectively.
This study demonstrates that both PRG4 and HA concentration, as well as PRG4 disulfide-bonded structure, can alter the cartilage boundary lubricating ability of PRG4 + HA solutions. The boundary lubricating ability of high MW HA + PRG4 solutions was limited by very low concentrations of PRG4. Decreased concentrations of high MW HA also limited the cartilage boundary lubricating ability of HA + PRG4 solutions, with the effect exacerbated by low PRG4 concentrations. The reduction of friction by addition of PRG4 to a cross-linked HA viscosupplement product, but not with addition of R/A PRG4 to HA, is consistent with a non-covalent mechanism of interaction where tertiary and quaternary PRG4 structure are important.
Collectively, these results demonstrate that deficiency of either or both PRG4 and HA, or alterations in PRG4 structure, may be detrimental to SF cartilage boundary lubricating function. This study provides further insight into the nature of cartilage boundary lubrication and advancement towards potential formulation of new intra-articular biotherapeutic treatments for osteoarthritis using PRG4 ± HA.