Bloom AK, et al. - In this study, researchers used confocal fluorescence recovery after photobleaching (FRAP) to determine the specific and dose-dependent effect of proteoglycan 4 (PRG4) on hyaluronan (HA) solutions of different molecular weight. In addition, they investigated how reduction and alkylation (R/A) of PRG4 influence its effects on HA solutions. The diffusion coefficient of fluorescein isothiocyanate (FITC)-dextran tracer (Dt) through 1500 kDa and 500 kDa HA solutions (0–3.3 mg/ml) ± PRG4 or a control protein, bovine serum albumin (BSA), at physiological (450 μg/ml) or pathophysiological (45 μg/ml) concentrations were determined using confocal FRAP. At both 45 and 450 μg/ml, PRG4 led to the slowing of diffusion of the FITC-dextran tracer for all concentrations of HA and a decrease in the apparent mesh size within the HA solution. Findings thereby suggest that the solution properties of HA can be significantly influenced by PRG4; the permeability of the HA network was essentially reduced by PRG4. PRG4 entangling HA molecules through binding and/or HA crowding PRG4 molecules into a self-assembled network may explain this effect.