NHE3: a Na+/H+ exchanger isoform of renal brush border

Abstract

Na+/H+ exchangers in the brush-border (luminal, apical) membrane of renal proximal tubules are responsible for active, transcellular reabsorption of NaHCO3 and NaCl. Although well characterized kinetically, the protein that mediates Na+/H+ exchange in the renal brush border has not been identified. Several Na+/H+ exchanger genes, including NHE1, NHE2, NHE3, and NHE4, are expressed in the kidney. To identify the NHE3 gene product and to determine its cellular and subcellular localization in the rabbit kidney, an NHE3-isoform-specific antibody was prepared. Guinea pigs were immunized with purified fusion protein containing the carboxy-terminal 40 amino acids of NHE3 (fpNHE3-C40). After affinity purification, immune sera demonstrated specific reactivity to the NHE3 sequence within the fusion protein as well as to an 80-kDa polypeptide expressed in NHE3-transfected LAP1 cells. Western blot analysis showed that anti-fpNHE3-C40 specifically reacted with an 80-kDa protein that is relatively enriched in renal brush-border membrane compared with basolateral membrane. Immunocytochemical studies confirmed that the Na+/H+ exchanger isoform NHE3 is expressed along the microvillar membrane of the brush border of proximal tubule cells in the rabbit kidney.

Na+/H+ Exchangers of Mammalian Cells

“…NHE1 contains both N-and O-glycosylated residues, and mutation of Asn-75 abolishes the N-linked glycosylation (13). In contrast, rabbit NHE2 exhibits only O-linked glycosylation (14) and NHE3 appears not to be glycosylated (13,15) (Fig. 1).…”

“…Its precise physiological roles are unclear, but when transfected into mutagenized cells devoid of endogenous NHE activity, NHE2 is capable of regulating pH i , cellular volume, and proliferation in a manner resembling NHE1 (21). Immunological studies have localized NHE3 to the apical membranes of renal proximal tubule (15) and intestinal (20,22) epithelia, implicating this isoform in Na ϩ absorption. The accompanying luminal secretion of H ϩ is essential for HCO 3 Ϫ reabsorption in renal tubules.…”

Regulation of Na+/H+ Exchanger Isoform 3 by Protein Kinase A in the Renal Proximal Tubule

“…Several lines of evidence strongly support the conclusion that NHE3 is the principal NHE isoform responsible for apical membrane Na + /H + exchange in the proximal tubule. First, studies using isoform-specific antibodies have demonstrated that NHE3 is expressed on the brush border membrane of proximal tubule cells (Biemesderfer et al, 1993b;Biemesderfer et al, 1997). Second, the pattern of sensitivity to different inhibitors of Na + /H + exchange in renal brush border vesicles and of bicarbonate absorption in microperfused proximal tubules is most consistent with the properties of NHE3 among the known NHE isoforms (Vallon et al, 2000).…”

“…Thus far, five sodium proton exchangers (NHE1, NHE2, NHE3, NHE4 and NHE8) have been identified in plasma membrane of renal tubular cells (Biemesderfer et al, 1992;Biemesderfer et al, 1993a;Amemiya et al, 1995;Chambrey et al, 1997;Chambrey et al, 1998). Of these, NHE3, the most abundant NHE isoform in renal tissue, is confined to the apical membrane of proximal tubule and thin and thick ascending limb.…”

RhoA and Rho Kinase Regulate the Epithelial Na+/H+ Exchanger NHE3

“…Other isoforms, in contrast, have a more restricted tissue distribution and are believed to mediate specialized functions. These include NHE3, which is the predominant isoform found on the apical membranes of epithelial cells in the kidney and gastrointestinal tract (4,5). In these tissues, NHE3 catalyzes the entry of luminal Na ϩ into the cells, which in turn drives the transport of salt and osmotically obliged water across the epithelium.…”