No abstract

The SLC26 gene family encodes anion transporters with diverse functional attributes: (a) anion exchanger, (b) anion sensor and (c) anion conductance (likely channel). We have cloned and studied Slc26a9, a paralog expressed mostly in lung and stomach. Immunohistochemistry shows that Slc26a9 is present at apical and intracellular membranes of lung and stomach epithelia. Using expression in Xenopus laevis oocytes and ion-sensitive microelectrodes, we discovered that Slc26a9 has a novel function not found in any other Slc26 proteins -cation coupling. Intracellular pH and voltage measurements show that Slc26a9 is a nCl --HCO 3 -exchanger, suggesting roles in gastric HCl secretion or pulmonary HCO 3 -secretion; Na + electrodes and uptakes reveal that Slc26a9 has a cationdependence. Single channel measurements indicate that Slc26a9 displays discrete open and close states. These experiments show that Slc26a9 has three discrete physiological modes: nCl --HCO 3 -exchanger, Cl -channel, and Na + -anion cotransporter. Thus, the Slc26a9 transporter-channel is uniquely suited for dynamic and tissue-specific physiology or regulation in epithelial tissues.

SLC26 proteins function as anion exchangers, channels, and sensors. Previous cellular studies have shown that Slc26a3 and Slc26a6 interact with the R-region of the cystic fibrosis transmembrane conductance regulator (CFTR), (R)CFTR, via theSlc26 genes and proteins have attracted the attention of physiologists and geneticists. Why? Slc26a1 (Sat-1) was characterized as a Na ϩ -independent SO 4 2Ϫ transporter (1). Given the transport characteristics of the founding member of the gene family, Slc26 proteins were assumed to be sulfate transporters. Disease phenotypes, clone characterization, and family additions demonstrate that the Slc26 proteins are anion transporters or channels (2-4). These proteins have varied tissue expression patterns. At one extreme, Slc26a5 in mammals is found in the hair cells of the inner ear (5), whereas Slc26a2 (DTDST) is virtually ubiquitous in epithelial tissues (2).Several Slc26 proteins are found in the epithelia of the lung, intestine, stomach, pancreas, and kidney, usually in apical membranes. Interestingly these are also tissues and membranes in which the cystic fibrosis transmembrane conductance regulator (CFTR) 5 has been found functionally or by immunohistochemistry. Ko and co-workers (6 -8) examined the distribution of Slc26a3 and Slc26a6 in HCO 3 Ϫ secretory epithelia, and asked if an interaction might occur between these Slc26 proteins and CFTR. In particular, these studies indicate that in expression systems, there is a reciprocal-stimulatory interaction of the STAS (sulfate transporter anti-sigma) domains of Slc26a3 and Slc26a6 with the regulatory region (R-region) of CFTR. These investigators hypothesized that this stimulatory interaction could account for the differences in pancreatic insufficiency and sufficiency observed in cystic fibrosis patients. Nevertheless, knock-out Slc26a6 mouse studies reveal more complicated cell and tissue physiology (see "Discussion").Slc26a9 has been reported to be a Cl Ϫ -HCO 3 Ϫ exchanger (9, 10) or a large Cl Ϫ conductance (3,11,12). Loriol and co-workers (12) indicated that SLC26A9 has a Cl Ϫ conductance that may be stimulated by HCO 3 Ϫ . Two other groups have indicated that the Cl Ϫ conductance is not affected by the presence of HCO 3 Ϫ (10, 11). We have recently demonstrated that Slc26a9 functions as both an electrogenic nCl Ϫ -HCO 3 Ϫ exchanger and a Cl Ϫ channel (10). Dorwart and colleagues (11) found that WNK kinases inhibited the SLC26A9 Cl Ϫ conductance but that this effect was independent of kinase activity. One group has a preliminary report indicating that WNK3 decreased Cl Ϫ uptake,