Degradation of MUC7 and MUC5B in human saliva

PLoS One. 2013 Jul 18;8(7):e69059. doi: 10.1371/journal.pone.0069059. Print 2013.

Abstract

Background: Two types of mucins, MUC7 and MUC5B constitute the major salivary glycoproteins, however their metabolic turnover has not been elucidated in detail to date. This study was conducted to examine turnover of MUC7 and MUC5B in saliva, by focusing on the relationship between their deglycosylation and proteolysis.

Methodology/principal findings: Whole saliva samples were collected from healthy individuals and incubated at 37°C in the presence of various protease inhibitors, sialidase, or a sialidase inhibitor. General degradation patterns of salivary proteins and glycoproteins were examined by SDS-polyacrylamide-gel-electrophoresis. Furthermore, changes of molecular sizes of MUC7 and MUC5B were examined by Western blot analysis. A protein band was identified as MUC7 by Western blot analysis using an antibody recognizing an N-terminal epitope. The MUC7 signal disappeared rapidly after 20-minutes of incubation. In contrast, the band of MUC7 stained for its carbohydrate components remained visible near its original position for a longer time indicating that the rapid loss of Western blot signal was due to the specific removal of the N-termimal epitope. Pretreatment of saliva with sialidase facilitated MUC7 protein degradation when compared with samples without treatment. Furthermore, addition of sialidase inhibitor to saliva prevented proteolysis of N-terminus of MUC7, suggesting that the desialylation is a prerequisite for the degradation of the N-terminal region of MUC7. The protein band corresponding to MUC5B detected in both Western blotting and glycoprotein staining showed little sign of significant degradation upon incubation in saliva up to 9 hours.

Conclusions/significance: MUC7 was highly susceptible to specific proteolysis in saliva, though major part of MUC5B was more resistant to degradation. The N-terminal region of MUC7, particularly sensitive to proteolytic degradation, has also been proposed to have distinct biological function such as antibacterial activities. Quick removal of this region may have biologically important implication.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Blotting, Western
  • Electrophoresis, Polyacrylamide Gel
  • Glycosylation
  • Humans
  • Mucin-5B / metabolism*
  • Mucins / metabolism*
  • Neuraminidase / antagonists & inhibitors
  • Neuraminidase / metabolism*
  • Proteolysis
  • Saliva / metabolism*
  • Salivary Proteins and Peptides / metabolism*

Substances

  • MUC5B protein, human
  • MUC7 protein, human
  • Mucin-5B
  • Mucins
  • Salivary Proteins and Peptides
  • Neuraminidase

Grant support

This work was supported by Japanese Government. This work was partly supported by JSPS Grant-in-Aid for Scientific Research (C) grant number 25463236. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.