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Conformational Mechanics, Adsorption, and Normal Force Interactions of Lubricin and Hyaluronic Acid on Model Surfaces†
journal contribution
posted on 2008-02-19, 00:00 authored by Debby P. Chang, Nehal I. Abu-Lail, Farshid Guilak, Gregory D. Jay, Stefan ZauscherGlycoproteins, such as lubricin, and hyaluronic acid (HA) play a prominent role in the boundary lubrication mechanism
in diarthrodial joints. Although many studies have tried to elucidate the lubrication mechanisms of articular cartilage,
the molecular details of how lubricin and HA interact with cartilage surfaces and mediate their interaction still remain
poorly understood. Here we used model substrates, functionalized with self-assembled monolayers terminating in
hydroxyl or methyl groups, (1) to determine the effect of surface chemistry on lubricin and HA adsorption using surface
plasmon resonance (SPR) and (2) to study normal force interactions between these surfaces as a function of lubricin
and HA concentration using colloidal probe microscopy. We found that lubricin is amphiphilic and adsorbed strongly
onto both methyl- and hydroxyl-terminated surfaces. On hydrophobic surfaces, lubricin likely adopts a compact,
looplike conformation in which its hydrophobic domains at the N and C termini serve as surface anchors. On hydrophilic
surfaces, lubricin likely adsorbs anywhere along its hydrophilic central domain and adopts, with increasing solution
concentration, an extended tail-like conformation. Overall, lubricin develops strong repulsive interactions when
compressing two surfaces into contact. Furthermore, upon surface separation, adhesion occurs between the surfaces
as a result of molecular bridging and chain disentanglement. This behavior is in contrast to that of HA, which does
not adsorb appreciably on either of the model surfaces and does not develop significant repulsive interactions. Adhesive
forces, particularly between the hydrophobic surfaces, are large and not appreciably affected by HA. For a mixture
of lubricin and HA, we observed slightly larger adsorptions and repulsions than those found for lubricin alone. Our
experiments suggest that this interaction depends on unspecific physical rather than chemical interactions between
lubricin and HA. We speculate that in mediating interactions at the cartilage surface, an important role of lubricin,
possibly in conjunction with HA, is one of providing a protective coating on cartilage surfaces that maintains the
contacting surfaces in a sterically repulsive state.