Renal expression of novel Na⁺/H⁺exchanger isoform NHE8

Abstract

Although Na(+)/H(+) exchanger isoform 3 (NHE3) mediates most Na(+)/H(+) exchange in the proximal tubule, studies of NHE3/NHE2 null mice suggest residual Na(+)-dependent proton secretion (Choi JY, Shah M, Lee MG, Schultheis PJ, Shull GE, Muallem S, and Baum M. J Clin Invest 105: 1141-1146, 2000). To characterize additional NHE isoforms that might be expressed in the kidney, we identified the partial sequence of a novel NHE. PCR was used to define the 5'- and 3'-ends, and a cDNA encoding the complete open reading frame was amplified from mouse kidney. The predicted protein of 576 amino acids, which we have named NHE8, has 30-35% amino acid identity to known mammalian isoforms (NHE1-7) but has >50% identity to Drosophila melanogaster "NHE1," suggesting it is the mammalian ortholog of this ancient invertebrate isoform. Northern blot of mouse tissues revealed ubiquitous expression. Western blot using anti-NHE8 antibodies demonstrated protein expression in apical membranes purified from rat renal cortex by divalent cation precipitation. In situ hybridization revealed that NHE8 message was present in both cortex and medulla. In the cortex, NHE8 was present in the majority of cortical tubules, consistent with proximal tubule (S1 and S2) localization. In the medulla, NHE8 message was most highly expressed in the proximal tubules (S3) of the outer stripe of the outer medulla. Thus NHE8 is expressed in the proximal tubule, where it may contribute to apical membrane ion transport.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

“…In search of an additional NHE in the kidney, Aronson et al cloned NHE8 by searching EST databases [54]. NHE8 is expressed ubiquitously in mouse tissues at the RNA level with a higher level of expression in the kidney, liver, skeletal muscle and testis.…”

The Basolateral NHE1 Na+/H+ Exchanger Regulates Transepithelial HCO3− Absorption through Actin Cytoskeleton Remodeling in Renal Thick Ascending Limb

“…At least eight NHE isoforms (NHE1 to -8) have been identified in mammalian cells (1)(2)(3)(4)(5). These differ in their tissue distribution, membrane localization, inhibitor sensitivity, and physiological functions.…”

Reactive oxygen species-mediated regulation of the Na+–H+ exchanger 1 gene expression connects intracellular redox status with cells' sensitivity to death triggers

“…NHE-1, the first isoform cloned, is ubiquitous in its distribution, while other isoforms have more restricted or specialized functions. [13][14][15]16 Interestingly, in addition to its role as a principal regulator of pHi and cell volume, NHE-1 has been implicated in cell proliferation, survival and transformation. Growth factors such as serum, epidermal growth factor (EGF) and phorbol ester myristyl acetate (PMA), have all been shown to activate NHE-1 promoter activity.…”