Abstract. Sequence analysis of chromosome IX of Saccharomyces cerevisiae revealed an open reading frame of 166 residues, designated TPM2, having 64.5 % sequence identity to

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Fibronectin (FN) is a glycoprotein found in the superficial zone of cartilage; however, its role in the lubrication and the wear protection of articular joints is unknown. In this work, we have investigated the molecular interactions between FN and various components of the synovial fluid such as lubricin (LUB), hyaluronan (HA), and serum albumin (SA), which are all believed to contribute to joint lubrication. Using a Surface Forces Apparatus, we have measured the normal (adhesion/repulsion) and lateral (friction) forces across layers of individual synovial fluid components physisorbed onto FN-coated mica substrates. Our chief findings are (i) FN strongly tethers LUB and HA to mica, as indicated by high and reversible long-range repulsive normal interactions between surfaces, and (ii) FN and LUB synergistically enhance wear protection of surfaces during shear, as suggested by the structural robustness of FN+LUB layers under pressures up to about 4 MPa. These findings provide new insights into the role of FN in the lubricating properties of synovial fluid components sheared between ideal substrates and represent a significant step forward in our understanding of cartilage damage involved in diseases such as osteoarthritis.

The synovial fluid (SF) that lubricates articular joints exhibits complex rheological and tribological properties due to the interactions and behaviors of its various molecular components. Under shear, SF films abruptly thicken by more than 300% and large, dense aggregates form within the fluid. In this study, we used the Surface Force Apparatus to elucidate which SF components are involved in this shear-induced transformation by (i) determining which (if any) of all major SF components replicate the behavior of SF under shear and (ii) observing the effect of removing implicated components from SF by enzymatic digestion. While most previous studies of SF have focused on the tribological roles of lubricin or hyaluronic acid, our results indicate that albumin is a key contributor to the formation of aggregates in SF under shear. Our results also suggest that SF aggregation is associated with efficient surface protection against wear. As our findings are based on experiments involving rigid, nonporous surfaces, they may be used to investigate shear-mediated aggregation mechanisms occurring during the lubrication of artificial joints, ultimately advancing our current vision of implant design.

Lubricin (LUB), a major mucinous glycoprotein of mammalian synovial fluids, is believed to provide excellent lubrication to cartilage surfaces. Consequently, when joint disease or replacement leads to increased friction and surface damage in the joint, robust synthetic LUB alternatives that could be used therapeutically to improve lubrication and surface protection are needed. Here, we report the characterization of a lubricating multiblock bottlebrush polymer whose architecture was inspired by LUB, and we investigate the role of fibronectin (FN), a glycoprotein found in the superficial zone of cartilage, in mediating the tribological properties of the polymer upon shear between mica surfaces. Our surface forces apparatus (SFA) normal force measurements indicate that the lubricin-mimetic (mimLUB) could be kept anchored between mica surfaces, even under high contact pressures, when an intermediate layer of FN was present. Additional SFA friction measurements show that FN would also extend the wearless friction regime of the polymer up to pressures of 3.4 MPa while ensuring stable friction coefficients (μ ≈ 0.28). These results demonstrate synergistic interactions between mimLUB and FN in assisting the lubrication and wear protection of ideal (mica) substrates upon shear. Collectively, these findings suggest that our proposed mimLUB might be a promising alternative to LUB, as similar mechanisms could potentially facilitate the interaction between the polymer and cartilage surfaces in articular joints and prosthetic implants in vivo.

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