Mutations in the SLC26A3 (DRA (down-regulated in adenoma)) gene constitute the molecular etiology of congenital chloride-losing diarrhea in humans. To ascertain its role in intestinal physiology, gene targeting was used to prepare mice lacking slc26a3. slc26a3-deficient animals displayed postpartum lethality at low penetrance. Surviving dra-deficient mice exhibited high chloride content diarrhea, volume depletion, and growth retardation. In addition, the large intestinal loops were distended, with colonic mucosa exhibiting an aberrant growth pattern and the colonic crypt proliferative zone being greatly expanded in slc26a3-null mice. Apical membrane chloride/base exchange activity was sharply reduced, and luminal content was more acidic in slc26a3-null mouse colon. The epithelial cells in the colon displayed unique adaptive regulation of ion transporters; NHE3 expression was enhanced in the proximal and distal colon, whereas colonic H,K-ATPase and the epithelial sodium channel showed massive up-regulation in the distal colon. Plasma aldosterone was increased in slc26a3-null mice. We conclude that slc26a3 is the major apical chloride/base exchanger and is essential for the absorption of chloride in the colon. In addition, slc26a3 regulates colonic crypt proliferation. Deletion of slc26a3 results in chloride-rich diarrhea and is associated with compensatory adaptive up-regulation of ion-absorbing transporters.The SLC26A3 or DRA (down-regulated in adenoma) gene was originally identified in a subtractive hybridization screen comparing the mRNAs expressed in colon cancer and normal colon tissue (1). DRA is expressed in normal colonic epithelium, but is absent or reduced in adenomas and adenocarcinomas (1). Subsequent studies identified SLC26A3 (DRA) as a member of a large conserved family of anion exchangers (SLC26) that encompass at least 10 distinct genes (2-20). Except for SLC26A5 (prestin), all function as anion exchangers with versatility with respect to transported anions (2-20). Immunohistochemical studies localized SLC26A3 on the apical membrane of colonic mucosa, with lower levels in the small intestine (4, 25). In humans, SLC26A3 encodes a 764-amino acid protein and is located on chromosome 7 in a head-to-tail arrangement with SLC26A4 (pendrin), indicating ancient gene duplication.Genetic analysis studies linked mutations in DRA (SLC26A3) to congenital chloride-losing diarrhea (CLD 5 ; OMIM accession number 214700), a disease manifested by enhanced chloride loss in the stool and volume depletion (4). Functional studies in vitro have demonstrated that SLC26A3 can mediate multiple anion exchange modes, including Cl Ϫ /HCO 3 Ϫ , Cl Ϫ /oxalate, and Cl Ϫ /hydroxyl, and possibly sulfate/hydroxyl exchanges (6, 21-26). Similar anion exchange activities have been described previously in apical membranes of the colon (27, 28), the site of abundant DRA expression.To initiate an investigation into the role of DRA in an in vivo model, we created slc26a3 (dra) gene-targeted mice that are null for expression of the slc2...

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 identity of the transporter responsible for fructose absorption in the intestine in vivo and its potential role in fructose-induced hypertension remain speculative. Here we demonstrate that Glut5 (Slc2a5) deletion reduced fructose absorption by ϳ75% in the jejunum and decreased the concentration of serum fructose by ϳ90% relative to wild-type mice on increased dietary fructose. When fed a control (60% starch) diet, Glut5 ؊/؊ mice had normal blood pressure and displayed normal weight gain. However, whereas Glut5 ؉/؉ mice showed enhanced salt absorption in their jejuna in response to luminal fructose and developed systemic hypertension when fed a high fructose (60% fructose) diet for 14 weeks, Glut5 ؊/؊ mice did not display fructose-stimulated salt absorption in their jejuna, and they experienced a significant impairment of nutrient absorption in their intestine with accompanying hypotension as early as 3-5 days after the start of a high fructose diet. Examination of the intestinal tract of Glut5 ؊/؊ mice fed a high fructose diet revealed massive dilatation of the caecum and colon, consistent with severe malabsorption, along with a unique adaptive up-regulation of ion transporters. In contrast to the malabsorption of fructose, Glut5 ؊/؊ mice did not exhibit an absorption defect when fed a high glucose (60% glucose) diet. We conclude that Glut5 is essential for the absorption of fructose in the intestine and plays a fundamental role in the generation of fructose-induced hypertension. Deletion of Glut5 results in a serious nutrient-absorptive defect and volume depletion only when the animals are fed a high fructose diet and is associated with compensatory adaptive up-regulation of ion-absorbing transporters in the colon.

The Na-Cl cotransporter (NCC), which is the target of inhibition by thiazides, is located in close proximity to the chloride-absorbing transporter pendrin in the kidney distal nephron. Single deletion of pendrin or NCC does not cause salt wasting or excessive diuresis under basal conditions, raising the possibility that these transporters are predominantly active during salt depletion or in response to excess aldosterone. We hypothesized that pendrin and NCC compensate for loss of function of the other under basal conditions, thereby masking the role that each plays in salt absorption. To test our hypothesis, we generated pendrin/NCC double knockout (KO) mice by crossing pendrin KO mice with NCC KO mice. Pendrin/NCC double KO mice displayed severe salt wasting and sharp increase in urine output under basal conditions. As a result, animals developed profound volume depletion, renal failure, and metabolic alkalosis without hypokalemia, which were all corrected with salt replacement. We propose that the combined inhibition of pendrin and NCC can provide a strong diuretic regimen without causing hypokalemia for patients with fluid overload, including patients with congestive heart failure, nephrotic syndrome, diuretic resistance, or generalized edema.diuretics | kidney tubules | nephrogenic diabetes insipidus T he thiazide-sensitive Na-Cl cotransporter (NCC) (SLC12A3) and the Cl − /HCO 3 − exchanger pendrin (SLC26A4) are expressed on apical membranes of distal cortical nephron segments and mediate salt absorption, with pendrin working in tandem with the epithelial Na channel and NCC working by itself (1-6). Pendrin is expressed on the apical membrane of intercalated cells in late distal convoluted tubule (DCT), connecting tubule (CNT), and the cortical collecting duct (CCD) (7-9). The thiazide-sensitive NCC is primarily expressed on the apical membrane of DCT cells (10,11).Single deletion of pendrin or NCC does not cause salt wasting or excessive diuresis under basal conditions (5,(12)(13)(14)(15). Indeed, even a mild degree of salt wasting has not been demonstrated in these two genetically engineered mouse models at steady state. Kidney functions, including sodium and chloride excretion, urine output, and blood urea nitrogen (BUN) levels in mutant mice are comparable to wild-type (WT) animals (12-15). Both pendrin KO and NCC KO mice, however, show signs of volume depletion or develop hypotension during salt restriction (12,14). These findings have led investigators to conclude that pendrin and NCC are predominantly active during salt depletion (and or in response to increased aldosterone levels), and their contribution to salt reabsorption at baseline conditions is small.We hypothesized that NCC and pendrin may compensate for loss of the other under basal conditions, thereby masking the role that each plays in salt reabsorption. To test this hypothesis, we generated double knockout of pendrin and NCC mice by crossing animals with single deletion for NCC and pendrin. The double KO mice show significant salt and flui...