Reorganisation of the salivary mucin network by dietary components: insights from green tea polyphenols.

Heather S Davies,Pantelis Georgiades,David J Thornton,Ewan W Blanch,Nigel W Hodson,Paul D A Pudney,Thomas A Waigh,Caroline E Ridley,Vladimir N Uversky

doi:10.1371/journal.pone.0108372

Abstract

The salivary mucins that include MUC5B (gel-forming) and MUC7 (non-gel-forming) are major contributors to the protective mucus barrier in the oral cavity, and it is possible that dietary components may influence barrier properties. We show how one dietary compound, the green tea polyphenol epigallocatechin gallate (EGCG), can substantially alter the properties of both the polymeric MUC5B network and monomeric MUC7. Using rate-zonal centrifugation, MUC5B in human whole saliva and MUC5B purified from saliva sedimented faster in the presence of EGCG. The faster sedimentation by EGCG was shown to be greater with increasing MUC5B concentration. Particle tracking microrheology was employed to determine the viscosity of purified MUC5B solutions and showed that for MUC5B solutions of 200–1600 µg/mL, EGCG caused a significant increase in mucin viscosity, which was greater at higher MUC5B concentrations. Visualisation of the changes to the MUC5B network by EGCG was performed using atomic force microscopy, which demonstrated increased aggregation of MUC5B in a heterogeneous manner by EGCG. Using trypsin-resistant, high-molecular weight oligosaccharide-rich regions of MUC5B and recombinant N-terminal and C-terminal MUC5B proteins, we showed that EGCG causes aggregation at the protein domains of MUC5B, but not at the oligosaccharide-rich regions of the mucin. We also demonstrated that EGCG caused the majority of MUC7 in human whole saliva to aggregate. Furthermore, purified MUC7 also underwent a large increase in sedimentation rate in the presence of EGCG. In contrast, the green tea polyphenol epicatechin caused no change in the sedimentation rate of either MUC5B or MUC7 in human whole saliva. These findings have demonstrated how the properties of the mucin barrier can be influenced by dietary components. In the case of EGCG, these interactions may alter the function of MUC5B as a lubricant, contributing to the astringency (dry puckering sensation) of green tea.

Highlights

Saliva is the body’s first line of defence to ingested insults, such as pathogens and chemicals, and is paramount to the protection of hard and soft tissues in the oral cavity and alimentary canal [1,2,3,4]
We examined the interaction of epigallocatechin gallate (EGCG) with MUC5B and MUC7 in human whole saliva, and MUC5B and MUC7 purified from human whole saliva by isopycnic density gradient centrifugation
The majority of non-gel-forming mucin MUC7 in human whole saliva was aggregated by EGCG, and purified MUC7 showed a substantial increase in sedimentation rate in the presence of EGCG

Summary

Introduction

Saliva is the body’s first line of defence to ingested insults, such as pathogens and chemicals, and is paramount to the protection of hard and soft tissues in the oral cavity and alimentary canal [1,2,3,4] This complex barrier is composed of many components, including the high-molecular weight, heavily O-glycosylated gel-forming mucin MUC5B, which forms the viscoelastic network that is important for hydration, lubrication, pathogen exclusion and resistance to proteolytic digestion. In addition to disrupted saliva integrity during disease, the composition and properties of saliva varies in healthy individuals since the secretion and protein content of saliva have been shown to vary with time of the day and can be influenced by lifestyle [16,17] It is possible, that diet may alter the structure and function of salivary components. We explore this possibility with one type of dietary compound, green tea polyphenols

Methods

Results

Conclusion