Colonic Anion Secretory Defects and Metabolic Acidosis in Mice Lacking the NBC1 Na+/HCO3- Cotransporter

Abstract

The NBC1 Na ؉ /HCO 3 ؊ cotransporter is expressed in many tissues, including kidney and intestinal epithelia. NBC1 mutations cause proximal renal tubular acidosis in humans, consistent with its role in HCO 3 ؊ absorption in the kidney. In intestinal and colonic epithelia, NBC1 localizes to basolateral membranes and is thought to function in anion secretion. To test the hypothesis that NBC1 plays a role in transepithelial HCO 3 ؊ secretion in the intestinal tract, null mutant (NBC1 /HCO 3 Ϫ cotransporters (1-3). NBC1 has two protein variants, which localize to basolateral membranes (4) and mediate electrogenic Na ϩ /HCO 3 Ϫ cotransport (2, 3). The kNBC1 variant is expressed in kidney epithelia and eye (4, 5), and the pNBC1 variant is expressed in pancreas, duodenum, colon, and several other tissues (4 -8). The stoichiometry of the transporter can be altered from 1NaϪ by phosphorylation of a residue near the carboxyl terminus (9). In the kidney, the ion stoichiometry and electrochemical driving forces for NBC1 result in Na ϩ and HCO 3 Ϫ extrusion across the basolateral membrane (2, 3, 9, 10); thus, in the kidney, NBC1 functions in HCO 3 Ϫ reabsorption in the proximal tubule (2, 3). In pancreas and the intestinal tract, the ion stoichiometry and driving forces for NBC1 appear to result in Na ϩ and HCO 3 Ϫ entry into the cell (2, 8); thus, in intestine and colon, NBC1 has been proposed to mediate HCO 3 Ϫ uptake across the basolateral membrane to support transepithelial anion secretion (8, 11).Human patients with proximal renal tubular acidosis resulting from mutations in NBC1 have been reported (12-16), thereby confirming a bicarbonate-absorptive role for NBC1 in kidney. The primary mutations were single amino acid substitutions (R298S, T485S, R510H, A799V, R881C, and S427L), which appeared to cause decreased function of the cotransporter rather than loss of function (12)(13)(14). One patient had an inactivating mutation in the unique N terminus of the kidney NBC1 variant (Q29X), but the pancreatic variant, which is expressed in many other tissues and at low levels in kidney (4), was intact (15). Only a single patient has been identified with a complete inactivating mutation, a nucleotide deletion that causes a frameshift at codon 721 (16). The pRTA resulting from NBC1 mutations clearly shows that this transporter is essential for renal HCO 3 Ϫ absorption; however, clinically significant intestinal disease has not been reported.NBC1 has been localized to the basolateral membrane of epithelial cells lining both the small and large intestine (8,17,18). In the colon, its expression was greatest in crypt cells, consist-* This work was supported by National Institutes of Health (NIH) Grants DK50594 and HL61974 (to G. E. S.), DK67749 (to L. R. G.), DK57552 (to J. N. L.), DK48816 (to L. L. C.), and T32-RR-07004 (to J. E. S.) and NIEHS, NIH, Grant ES06096 (to the Center for Environmental Genetics, Alvaro Puga PI). The costs of publication of this article were defrayed in part by the payment of page charges. This article mus...

TGF‐β signaling directly regulates transcription and functional expression of the electrogenic sodium bicarbonate cotransporter 1, NBCe1 (SLC4A4), via Smad4 in mouse astrocytes

“…Mice were sacrificed by cervical dislocation, and all efforts were made to minimize suffering. Slc4a4 and Tgf‐β2 deficient mice have been described earlier (Gawenis et al, 2007; Sanford et al, 1997). …”

Pathophysiological Roles of Mutations in the Electrogenic Na+-HCO - Cotransporter NBCe1

“…Consistent with this view, the homozygous inactivating mutations in NBCe1A cause severe pRTA with the blood bicarbonate concentration often less than 10 mM [3][4][5][6][7][8][9][10][11]. Functional deletion of NBCe1 in mice produces even more severe acidemia with the blood bicarbonate concentration around 5 mM [11,14]. By contrast, functional deletion of Cl -/HCO3 -exchanger AE1, which is responsible for a majority of basolateral bicarbonate exit from α-intercalated duct cells, produces only moderate acidemia in mice with the blood bicarbonate concentration around 17 mM [37].…”

“…Some pRTA patients also have migraine, suggesting that NBCe1 may also contribute to the pH regulation in the brain [10]. In addition, mice models for NBCe1 deficiency have been developed [11,14].…”

MicroRNA 224 Regulates Ion Transporter Expression in Ameloblasts To Coordinate Enamel Mineralization

“…These transporters control the uptake and efflux of many critical ions or substrates during amelogenesis and have been determined to result in enamel abnormalities if mutated. Mutations in SLC4A4 result in pigmentation differences and a propensity to fracture (21,(42)(43)(44)(45)(46). Patients with proximal renal tubular acidosis (pRTA) caused by mutations in SLC4A4 also exhibit dental abnormalities (5,42).…”

“…The enamel of the incisors of NBCe1 Ϫ/Ϫ mice appeared to be hypoplastic, with altered enamel microstructure (20,21). CFTR knockout mice also demonstrated enamel abnormalities, with a soft and chalky white appearance.…”