Upregulation of basolateral small conductance potassium channels (KCNQ1/KCNE3) in ulcerative colitis

Adel Al-Hazza,John Linley,Qadeer Aziz,Malcolm Hunter,Geoffrey Sandle

doi:10.1016/j.bbrc.2015.12.086

Abstract

BackgroundBasolateral K+ channels hyperpolarize colonocytes to ensure Na+ (and thus water) absorption. Small conductance basolateral (KCNQ1/KCNE3) K+ channels have never been evaluated in human colon. We therefore evaluated KCNQ1/KCNE3 channels in distal colonic crypts obtained from normal and active ulcerative colitis (UC) patients. MethodsKCNQ1 and KCNE3 mRNA levels were determined by qPCR, and KCNQ1/KCNE3 channel activity in normal and UC crypts, and the effects of forskolin (activator of adenylate cyclase) and UC-related proinflammatory cytokines on normal crypts, studied by patch clamp recording. ResultsWhereas KCNQ1 and KCNE3 mRNA expression was similar in normal and UC crypts, single 6.8 pS channels were seen in 36% of basolateral patches in normal crypts, and to an even greater extent (74% of patches, P < 0.001) in UC crypts, with two or more channels per patch. Channel activity was 10-fold higher (P < 0.001) in UC crypts, with a greater contribution to basolateral conductance (5.85 ± 0.62 mS cm−2) than in controls (0.28 ± 0.04 mS cm−2, P < 0.001). In control crypts, forskolin and thromboxane A2 stimulated channel activity 30-fold and 10-fold respectively, while PGE2, IL-1β, and LTD4 had no effect. ConclusionsKCNQ1/KCNE3 channels make only a small contribution to basolateral conductance in normal colonic crypts, with increased channel activity in UC appearing insufficient to prevent colonic cell depolarization in this disease. This supports the proposal that defective Na+ absorption rather than enhanced Cl− secretion, is the dominant pathophysiological mechanism of diarrhea in UC.

Highlights

Basolateral potassium (Kþ) channels play a central role in maintaining colonic epithelial cells in a hyperpolarized state, which is a critical requirement for ionic movements across the apical and basolateral membranes
The present study shows for the first time that (i) both KCNQ1 mRNA and KCNE3 mRNA are present in control human colonic epithelial cells, (ii) low basal levels of KCNQ1/KCNE3 channel activity are present in the basolateral membranes of these cells, and (iii) these low basal levels of KCNQ1/KCNE3 channel activity are markedly stimulated by forskolin-induced cAMP-dependent phosphorylation
Based on the findings that basolateral KCNQ1/KCNE3 channels (i) co-exist with apical ClÀ (CFTR) channels in intestinal crypt cells [21], (ii) are stimulated by cAMP [8], and (iii) inhibited by chromanol 293B [8], it has become clear that these Kþ channels play a critical role during cAMP-activated ClÀ secretion, recycling Kþ ions across the basolateral membrane to maintain a favourable electrical gradient for apical ClÀ exit, making them a prime target for novel antidiarrheal drugs

Summary

Introduction

Basolateral potassium (Kþ) channels play a central role in maintaining colonic epithelial cells in a hyperpolarized state, which is a critical requirement for ionic movements across the apical and basolateral membranes. The possible existence of KCNQ1/KCNE3 channels and their contribution to the basolateral Kþ conductance of human colonic crypt cells, has never been explored. Small conductance basolateral (KCNQ1/KCNE3) Kþ channels have never been evaluated in human colon. Conclusions: KCNQ1/KCNE3 channels make only a small contribution to basolateral conductance in normal colonic crypts, with increased channel activity in UC appearing insufficient to prevent colonic cell depolarization in this disease. This supports the proposal that defective Naþ absorption rather than enhanced ClÀ secretion, is the dominant pathophysiological mechanism of diarrhea in UC

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