Identification and quantification of mucin expression

Extraction and Verification of Mouse and Human Mucins from Tissue and Fecal Material

Gel-forming mucins are highly O-glycosylated polymeric glycoproteins which have critical roles in tissue protection from environmental insult. To understand their biochemical properties, these samples must be extracted and enriched from biological samples. Here we describe how to extract and semi-purify human and murine mucins from intestinal scrapings or fecal material. As mucins have high molecular weights, traditional gel electrophoresis methods are unable to effectively separate these glycoproteins for analysis. We describe the procedure for making composite sodium dodecyl sulfate urea agarose-polyacrylamide (SDS-UAgPAGE) gels, which allows for accurate verification and band separation of extracted mucins.

The Reduction-insensitive Bonds of the MUC2 Mucin Are Isopeptide Bonds

The main structural component of the mucus in the gastrointestinal tract is the MUC2 mucin. It forms large networks that in colon build the loose outer mucous layer that provides the habitat for the commensal flora and the inner mucous layer that protects the epithelial cells by being impenetrable to bacteria. The epithelial cells in mice lacking MUC2 are not adequately protected from bacteria, resulting in inflammation and the development of colon cancer as found in human ulcerative colitis. Correct processing of the MUC2 mucin is the basis for the building of these protective networks. During the biosynthesis of the MUC2 mucin, post-translational modifications are formed resulting in reduction-insensitive bonds between MUC2 monomers. By the use of γ-glutamyltranspeptidase and isopeptidase activity in leech saliva, we could show that the molecular nature of these reduction-insensitive bonds is isopeptide bonds formed between side chains of lysine and glutamine. Transglutaminase 2 has an affinity to the MUC2 CysD2 domain in the nanomolar range and can catalyze its cross-linking. By using mass spectrometry, we identified MUC2 residues involved in this cross-linking. This shows for the first time that transamidation is not only stabilizing the skin and the fibrin clot, but is also important for the correct intracellular processing of MUC2 to generate protective mucus.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Large molecule bioanalysis using Q-TOF without predigestion and its data processing challenges

Background: Recent developments in LC–MS have turned it into a viable and valid alternative to ligand-binding assays. Large molecule bioanalysis by LC–MS is generally performed by tryptic digestion, purification and detection of one or more small signature peptides. High-resolution MS instruments offer quantification of intact small proteins or peptides and are able to increase the selectivity, while maintaining sensitivity. Results: Unlike multiple reaction monitoring assays, several factors affecting data processing were presented and the optimal parameters to consider during quantification method building were also demonstrated. MUC5AC-13 (MW 1709.8 Da), human hepcidin/LEAP-1 (MW 2797.4 Da), porcine calcitonin (MW 3604 Da) and chicken lysozyme (MW 14.3 kDa) were selected as model compounds and the possibility of intact peptide and small protein quantification, without tryptic digestion, was demonstrated. Conclusion: Selectivity and sensitivity were improved using different scan modes, such as TOF-MS and TOF-MS/MS.