Surface-immobilized fibronectin conformation influences synovial fluid adsorption and film formation

The articular cartilage extracellular matrix (ECM) is a complex network of biomolecules that includes fibronectin (FN). FN acts as an extracellular glue, controlling the assembly of other macromolecular constituents to the ECM. However, how FN participates in the binding and retention of synovial fluid components, the natural lubricant of articulated joints, to form a wear-protecting and lubricating film has not been established. This study reports on the role of FN and its molecular conformation in mediating macromolecular assembly of synovial fluid ad-layers. FN immobilized on functionalized gold surfaces, a model of FN's articular cartilage surface, formed films that retained different amounts of synovial fluid (SF) depending on FN conformation. FN conformational changes were induced by depositing FN from bulk solution at pH 7 (extended state) or at pH 4 (unfolded state) on self-assembled monolayers on gold-coated quartz crystals, followed by adsorption of diluted SF (25 %) onto FN precursor films. Mass density, thin film compliance, surface morphologies, and the secondary and tertiary structures of FN films reveal pH-induced differences. FN films deposited at pH 4 were thicker, more rigid, showed a more homogeneous morphology, and had altered α-helix and β-sheet content, compared to FN films deposited at pH 7. FN precursor films deposited at pH 7 adsorbed and retained more synovial fluid than those at pH 4, revealing the importance of FN conformation at the articular cartilage surface to bind and maintain a thin layer of synovial fluid constituents. This knowledge will enable a better understanding of the molecular interactions and synergies between the articular cartilage ECM components and SF.