Molecular cloning, genomic organization, and functional expression of Na+/H+ exchanger isoform 5 (NHE5) from human brain

To isolate a cDNA encoding Na+/H+ exchanger isoform 5 (NHE5), we screened a human spleen library using exon sequences of the NHE5 gene. Clones spannin g 2.9 kilobase pairs were isolated; however, they contained several introns and were missing coding sequences at both the 5' and 3' ends. The missing 5' sequences were obtained by 5'-rapid amplification of cDNA ends and by an alysis of an NHE5 genomic clone, and the missing 3' sequences were obtained by 3'-rapid amplification of cDNA ends. Polymerase chain reaction amplific ation of brain cDNA yielded products in which each of the introns had been correctly excised, whereas the introns were retained in products from splee n and testis, suggesting that the NHE5 transcripts expressed in these organ s do not encode a functional transporter. The intron/exon organization of t he NHE5 gene was analyzed and found to be very similar to that of the NHE3 gene. The NHE5 cDNA, which encodes an 896-amino acid protein that is most c losely related to NHE3, was expressed in Na+/H+ exchanger-deficient fibrobl asts and shown to mediate Na+/H+ exchange activity. Northern blot analysis demonstrated that the mRNA encoding NHE5 is expressed in multiple regions o f the brain, including hippocampus, consistent with the possibility that it regulates intracellular pH in hippocampal and other neurons.