Glycoprotein Methods and Protocols - P4

GI Adherent Mucous Gel Barrier 57

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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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The Gastrointestinal Adherent Mucous Gel Barrier

Adrian Allen and Jeffrey P. Pearson

1. Introduction

Three phases of production of mucin can be identified in the gastrointestinal (GI)

tract on the basis of their location in vivo: the stored, presecreted, intracellular mucin;

the gel phase adherent to the epithelial surfaces; and the viscous, mobile mucin, which

is largely in soluble form and mixes with the luminal contents. The layer of adherent

mucous gel that lines the epithelial surfaces throughout the gut from the stomach to

the colon marks the interface between the mucosal epithelium and the fluid luminal

environment, which is teeming with nutrients, bacteria, destructive hydrolases, for￾eign compounds and so on. The adherent mucous gel thus provides a protective barrier

and a stable unstirred layer with its own microenvironment, between the mucosal sur￾face and the lumen (1,2). In the mouth, where salivary mucins are secreted, and the

esophagus, there is no discernible adherent mucous gel layer.

A practical definition of the adherent mucous gel layer is that secretion of mucus

which remains attached to the mucosal surface after washing away the luminal con￾tents. This mucous gel layer is readily visible on unfixed, transverse sections of mu￾cosa as a translucent layer of variable thickness (5–200 µm), between the dense

mucosal surface and the bathing solution (3). Mucous gel scraped from the mucosal

surface has the rheological characteristics of a true viscoelastic gel, although it has the

ability to slowly flow (30–120 min) and reform when sectioned (4,5). The concentra￾tion of mucin in such gels scraped from the mucosal surface is high, e.g., ranging from

50 mg/mL in gastric mucus to 20 mg/mL in colonic mucus (6). The extent of the mu￾cus adherent to the mucosal surface in vivo is determined by a balance between the

rate of secretion by the underlying epithelium and the rate of erosion by mechanical

shear associated with the digestive processes and digestion by hydrolases, particularly

proteases (1,2). For full studies of mucous secretion in vivo, it is essential to quantitate

both the adherent mucous gel and mucin in the luminal solution since the latter can

arise by degradation of the former as well as by secretion, and changes in these two

phases do not necessarily parallel each other. Despite being the primary mucous bar-