The effect of molecular weight on hyaluronan's cartilage boundary lubricating ability - alone and in combination with proteoglycan 4

Objectives: (1) assess the molecular weight dependence of hyaluronan's (HA) cartilage boundary lubricating ability, alone and in combination with proteoglycan 4 (PRG4), at physiological concentrations; (2) determine if HA and PRG4 interact in solution via electrophoretic mobility shift assay (EMSA). Methods: The cartilage boundary lubricating ability of a broad range of MW HA (20kDa, 132kDa, 780kDa, 1.5MDa, and 5MDa) at 3.33mg/ml, both alone and in combination with PRG4 at 450μg/ml, was assessed using a previously described cartilage-on-cartilage friction test. Static, μ _{static, Neq}, and kinetic, <μ _{kinetic, Neq}>, were calculated. An EMSA was conducted with PRG4 and monodisperse 150kDa and 1,000kDa HA. Results: Friction coefficients were reduced by HA, in a MW-dependent manner. Values of <μ _{kinetic, Neq}> in 20kDa HA, 0.098 (0.089, 0.108), were significantly greater compared to both 780kDa, 0.080 (0.072, 0.088), and 5MDa, 0.079 (0.070, 0.089). Linear regression showed a significant correlation between both μ _{static, Neq} and <μ _{kinetic, Neq}>, and log HA MW. Friction coefficients were also reduced by PRG4, and with subsequent addition of HA; however the synergistic effect was not dependent on HA MW. Values of <μ _{kinetic, Neq}> in PRG4, 0.080 (0.047, 0.113), were significantly greater than values of PRG4+various MW HA (similar in value, averaging 0.040 (0.033, 0.047)). EMSA indicated that migration of 150kDa and 1,000kDa HA was retarded when combined with PRG4 at high PRG4:HA ratios. Conclusions: These results suggest alterations in HA MW could significantly affect synovial fluid's cartilage boundary lubricating ability, yet this diminishment in function could be circumvented by physiological levels of PRG4 forming a complex, potentially in solution, with HA.