AE2 Cl⁻/HCO₃⁻exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Abstract

Gawenis LR, Bradford EM, Alper SL, Prasad V, Shull GE. AE2 Cl Ϫ /HCO 3 Ϫ exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon. Am J Physiol Gastrointest Liver Physiol 298: G493-G503, 2010. First published January 28, 2010 doi:10.1152/ajpgi.00178.2009.-Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 NaϪ cotransporter mediates basolateral Cl Ϫ uptake. However, additional evidence suggests that the AE2 Cl Ϫ /HCO 3 Ϫ exchanger, when coupled with the NHE1 Na ϩ /H ϩ exchanger or a Na ϩ -HCO 3 Ϫ cotransporter (NBC), contributes to HCO 3 Ϫ and/or Cl Ϫ uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (Isc) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2 Ϫ/Ϫ ) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated Isc was the same in WT and AE2 Ϫ/Ϫ colon. However, the Isc response in AE2 Ϫ/Ϫ colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO 3 Ϫ resulted in robust cAMP-stimulated Isc in both AE2Ϫ/Ϫ and WT colon that was largely mediated by NKCC1, whereas removal of Cl Ϫ resulted in sharply decreased cAMP-stimulated Isc in AE2 Ϫ/Ϫ colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated Isc in AE2 Ϫ/Ϫ colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2 Ϫ/Ϫ colon, Cl Ϫ secretion supported by basolateral NKCC1 is enhanced, whereas HCO 3 Ϫ secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.

Physiology of Epithelial Chloride and Fluid Secretion

“…AE2 is expressed at high levels in parietal cells of the gastric epithelium, and null mice display a dramatic gastric epithelial phenotype with abnormal canaliculae and reduced HCl secretion (Gawenis et al 2004). Other Cl-secreting epithelia are also affected, notably the proximal colon (Gawenis et al 2010); the role of AE2 has been difficult to establish in most tissues because it is electrically silent, there are no specific inhibitors available, and AE2-null mice die soon after birth. AE2 is expressed in the basolateral membrane of Calu-3 cells (Loffing et al 2000), and recent studies of a stable, AE2-deficient Calu-3 cell line under open-circuit conditions revealed that AE2 mediates most basolateral anion exchange ) and accounts for up to 70% of the net Cl 2 flux during cAMPstimulated fluid secretion .…”

Physiological relevance of cell-specific distribution patterns of CFTR, NKCC1, NBCe1, and NHE3 along the crypt-villus axis in the intestine

“…Functional studies in genetically engineered animal models that examined the Cl Ϫ /HCO 3 Ϫ anion exchanger 2 (AE2), the SLC26A6 and SLC26A3 Cl Ϫ /HCO 3 Ϫ exchangers, and the sodium/bicarbonate (Na ϩ /HCO 3 Ϫ ) cotransporter (NBC) family members revealed critical roles for bicarbonate secretion in specific intestinal segments along the proximal-distal axis of the intestine (3,10,11,36,76). In addition to its established central role in chloride secretion, the cystic fibrosis transmembrane conductance regulator (CFTR) is now recognized as a critical regulator of intestinal bicarbonate secretion (25,44,67,77).…”

CFTR is restricted to a small population of high expresser cells that provide a forskolin-sensitive transepithelial Cl– conductance in the proximal colon of the possum,Trichosurus vulpecula

“…This had no effect on the forskolin-stimulated ⌬I sc (⌬I sc forskolin control29±13Acm -2 and following incubation with acetazolamide ⌬I sc forskolin23±7Acm -2 , N4). Similarly, pre-treatment with acetazolamide and serosal DIDS (1mmoll -1 ), to prevent compensatory up regulation of alternative transporters (Gawenis et al, 2010), had no effect on the forskolin-stimulated I sc and nor did the NHE inhibitor EIPA (100moll -1 serosal, data not shown), demonstrating that the secretion of HCO 3 -produced by hydration of CO 2 does not contribute to the forskolin-stimulated ⌬I sc .…”