The influence of ions on the lubricative abilities of mucin and the role of sialic acids

Mucus reduces friction between epithelial surfaces by providing lubrication in the boundary and mixed regime. Mucins, the main macromolecule, are heavily glycosylated proteins that polymerise and retain water molecules, resulting in a hydrated biogel. It is assumed that positively charged ions can influence mucin film structure by screening the electrostatic repulsions between the negatively charged glycans on mucin moieties and draw in water molecules via hydration shells. The ionic concentration can vary significantly in different mucus systems and here we show that increasing the ionic concentration in mucin films leads to an increase in lubrication between two polydimethylsiloxane surfaces at sliding contact in a compliant oral mimic. Mucins were found to bind sodium ions in a concentration-dependent manner and increased ionic concentration appears to cause mucin films to swell when assessed by Quartz Crystal hiMicrobalance with Dissipation (QCM-D) analysis. Furthermore, we determined that the removal of negatively charged sialic acid moieties by sialidase digestion resulted in reduced adsorption to hydrophilic surfaces but did not affect the swelling of mucin films with increasing ionic concentrations. Moreover, the coefficient of friction was increased with sialic acid removal, but lubrication was still increased with increasing ionic concentrations. Taken together this suggests that sialic acids are important for lubrication and may exert this through the sacrificial layer mechanism. Ionic concentration appears to influence mucin films and their lubrication, and sialic acids, at least partly, may be important for ion binding.