Sodium Concentration Influences the Structure and Function of Salivary Mucins

Objectives: The objectives of this study are to determine the role that sodium concentration plays on the structure of MUC5B and the lubrication and film properties of MUC5B compared to saliva. Moreover, we aim to understand the role of sialic acids play in MUC5B structure and function.
Methods: Negative-stain electron microscopy was used to determine the effect of sodium on the structure of MUC5B. An oral tribology mimic was used to measure the lubrication of MUC5B and saliva with increasing sodium concentration and QCM-D was used to determine the film properties. Neuraminidase digestion was used to remove sialic acids. Lastly, a novel ion binding assay and a colorimetric assay was used to determine sodium binding.
Results: A high salt environment was found to condense the mucin structure. Moreover, removing the sialic acids at a low salt environment was found to straighten the mucin into a straight rod. Increasing sodium concentrations improves the lubrication of both saliva and MUC5B with saliva lubricating 10-fold higher than MUC5B alone. Moreover, increasing ionic concentrations was found to cause MUC5B films to swell, suggesting increased hydration. Salivary films were not found to swell and it was determined that the adsorbed salivary proteins were washed away over time. Finally, MUC5B and saliva was found to bind sodium in a concentration dependent manner, however MUC5B reaches binding capacity at approximately 3.1 mmol/mg protein.
Conclusions: This research demonstrates that sodium ions bind to MUC5B and salivary proteins, enhancing lubricating properties despite the compacting of the mucin structure. It also suggests that sialic acids and the presence of sodium ions are important for the structural integrity of MUC5B. It was also determined that MUC5B films swell with increasing ionic concentrations, suggesting the improved lubrication with increasing sodium concentrations is due to an increase in hydration due to hydration shells.