No abstract

SLC26A6 (PAT1, CFEX) is an anion exchanger that is expressed on the apical membrane of the kidney proximal tubule and the small intestine. Modes of transport mediated by SLC26A6 include Cl−/formate exchange, Cl−/HCO3− exchange, and Cl−/oxalate exchange. To study its role in kidney and intestinal physiology, gene targeting was used to prepare mice lacking Slc26a6. Homozygous mutant Slc26a6−/− mice appeared healthy and exhibited a normal blood pressure, kidney function, and plasma electrolyte profile. In proximal tubules microperfused with a low-HCO3−/high-Cl− solution, the baseline rate of fluid absorption ( Jv), an index of NaCl transport under these conditions, was the same in wild-type and null mice. However, the stimulation of Jv by oxalate observed in wild-type mice was completely abolished in Slc26a6-null mice ( P < 0.05). Formate stimulation of Jv was partially reduced in null mice, but the difference from the response in wild-type mice did not reach statistical significance. Apical membrane Cl−/base exchange activity, assayed with the pH-sensitive dye BCPCF in microperfused proximal tubules, was decreased by 58% in Slc26a6−/− animals ( P < 0.001 vs. wild types). In the duodenum, the baseline rate of HCO3− secretion measured in mucosal tissue mounted in Ussing chambers was decreased by ∼30% ( P < 0.03), whereas the forskolin-stimulated component of HCO3− secretion was the same in wild-type and Slc26a6−/− mice. We conclude that Slc26a6 mediates oxalate-stimulated NaCl absorption, contributes to apical membrane Cl−/base exchange in the kidney proximal tubule, and also plays an important role in HCO3− secretion in the duodenum.

The Na+/H+ exchanger isoform NHE2 is highly expressed in the intestinal tract, but its physiological role has remained obscure. The aim of this study was to define its expression, location, and regulatory properties in murine colon and to look for the compensatory changes in NHE2 (−/−) colon that allow normal histology and absorptive function. To this end, we measured murine proximal colonic surface and crypt cell NHE1, NHE2, and NHE3 expression levels, transport rates in response to acid, hyperosmolarity and cAMP in murine proximal colonic crypts, as well as changes in transcript levels and acid-activated NHE activity in NHE2 (−/−) crypts. We found that NHE2 was expressed most abundantly in crypts, NHE1 equally in crypts and surface cells, and NHE3 much stronger in surface cells. NHE2, like NHE1, was activated by low intracellular pH (pHi), hyperosmolarity, and cAMP, whereas NHE3 was activated only by low pHi. Crypts isolated from NHE2 (−/−) mice displayed increased acid-activated NHE1- and NHE3-attributable Na+/H+ exchange activity, no change in NHE1 expression, and NHE3 expression levels twice as high as in normal littermates. No change in cellular ultrastructure was found in NHE2 (−/−) colon. Our results demonstrate high NHE2 expression in the crypts and suggest a role for NHE2 in cryptal pHi and volume homeostasis.

The epithelial anion channel CFTR interacts with multiple PDZ domain-containing proteins. Heterologous expression studies have demonstrated that the Na + /H + exchanger regulatory factors, NHERF1, NHERF2, and PDZK1 (NHERF3), modulate CFTR membrane retention, conductivity, and interactions with other transporters. To study their biological roles in vivo, we investigated CFTR-dependent duodenal HCO 3 -secretion in mouse models of Nherf1, Nherf2, and Pdzk1 loss of function. We found that Nherf1 ablation strongly reduced basal as well as forskolin-stimulated (FSK-stimulated) HCO 3 -secretory rates and blocked β 2 -adrenergic receptor (β 2 -AR) stimulation. Conversely, Nherf2 -/-mice displayed augmented FSK-stimulated HCO 3 -secretion. Furthermore, although lysophosphatidic acid (LPA) inhibited FSK-stimulated HCO 3 -secretion in WT mice, this effect was lost in Nherf2 -/-mice. Pdzk1 ablation reduced basal, but not FSK-stimulated, HCO 3 -secretion. In addition, laser microdissection and quantitative PCR revealed that the β 2 -AR and the type 2 LPA receptor were expressed together with CFTR in duodenal crypts and that colocalization of the β 2 -AR and CFTR was reduced in the Nherf1 -/-mice. These data suggest that the NHERF proteins differentially modulate duodenal HCO 3 -secretion: while NHERF1 is an obligatory linker for β 2 -AR stimulation of CFTR, NHERF2 confers inhibitory signals by coupling the LPA receptor to CFTR.

Slc26a9 is a recently identified anion transporter that is abundantly