Glycoprotein Methods and Protocols - P5

Biosynthesis and Secretion of Mucin 65

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From: Methods in Molecular Biology, Vol. 125: Glycoprotein Methods and Protocols: The Mucins

Edited by: A. Corfield © Humana Press Inc., Totowa, NJ

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Quantitation of Biosynthesis

and Secretion of Mucin Using Metabolic Labeling

Jan Dekker, B. Jan-Willem Van Klinken,

Hans A. Büller, and Alexandra W. C. Einerhand

1. Introduction

Most epithelial mucins are secretory glycoproteins. The mucin-producing cells are

characterized by large intracellular stores of these very large and complex glycopro￾teins (1,2). These secretory mucins form mucous layers on the apical side of the cells,

protecting the vulnerable epithelium, while allowing selective interactions with the

apical environments, which is typically the lumen of an organ that is continuous with

the outer world. Secretion from mucin-producing cells is regulated. Normally, mucins

are constitutively secreted in relatively low amounts, which are sufficient under nor￾mal conditions to sustain the thickness of the mucous layer. On acute threats, the accu￾mulated mucins may be secreted in bulk amounts to provide mucus as an effective, yet

temporary, means of epithelial protection (1,2). Both types of secretion require syn￾thesis of mucin: constitutive secretion demands a continuous low level of biosynthesis,

whereas stimulated secretion requires massive synthesis to replenish the diminished

resources.

In particular pathological conditions, mucous production seems either to fall dra￾matically or to rise excessively, but systematic measurements of the actual changes in

mucin production at the various levels of regulation are most often not conducted. For

instance, in the chronic inflammatory bowel disease ulcerative colitis, it was debated

for many years whether mucous production actually dropped during the inflammation

(3). Only recently have researchers been able to show that MUC2 is the predominant

mucin in normal colon as well as in colon affected by ulcerative colitis, and that MUC2

production is actually decreased during active inflammation in ulcerative colitis (4–6).

Many researchers in the mucin field may therefore wish to quantify mucin synthe￾sis and secretion in health and disease, in order to determine the sequence and the

regulation of events. Only in this way will investigators be able fully to appreciate

mucin functions and hope to find ways to interfere in the production and secretion of