Abstract
BackgroundAirway surface liquid, often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs. Airway mucus contains various macromolecules, the most abundant being mucin glycoproteins, which contribute to its defensive function. Airway epithelial cells cultured in vitro secrete mucins and nonmucin proteins from their apical surface that mimics mucus production in vivo. The current study was undertaken to identify the polypeptide constituents of human airway epithelial cell secretions to gain a better understanding of the protein composition of respiratory mucus.ResultsFifty-five proteins were identified in the high molecular weight fraction of apical secretions collected from in vitro cultures of well-differentiated primary human airway epithelial cells and isolated under physiological conditions. Among these were MUC1, MUC4, MUC5B, and MUC16 mucins. By proteomic analysis, the nonmucin proteins could be classified as inflammatory, anti-inflammatory, anti-oxidative, and/or anti-microbial.ConclusionsBecause the majority of the nonmucin proteins possess molecular weights less than that selected for analysis, it is theoretically possible that they may associate with the high molecular weight and negatively charged mucins to form a highly ordered structural organization that is likely to be important for maintaining the proper defensive function of airway mucus.
Highlights
Airway surface liquid, often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs
Protein identification by nano-LC-IT mass spectrometry (MS) The human airway epithelial cell secretions (hAECS) sample resolved by gel filtration chromatography and SDS-PAGE was divided into ten 0.5 cm gel slices, each of which was processed as described in the Methods
While MUC5AC was not identified by proteome analysis, this mucin was readily detected by Western blotting of hAECS (Figure 3)
Summary
Often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs. Airway mucus contains various macromolecules, the most abundant being mucin glycoproteins, which contribute to its defensive function. Airway epithelial cells cultured in vitro secrete mucins and nonmucin proteins from their apical surface that mimics mucus production in vivo. The current study was undertaken to identify the polypeptide constituents of human airway epithelial cell secretions to gain a better understanding of the protein composition of respiratory mucus. Mucins are secreted or inserted to the apical surface membrane of epithelial cells via a membranespanning hydrophobic domain [2,3]. Secreted mucins constitute the most abundant glycoprotein component of mucus, a gel-like substance that covers all wet-surfaced epithelia, including those of the respiratory, gastrointestinal, and genitourinary tracts [4,5,6,7].
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