Na+/H+ exchanger isoform 3 expressed in apical membrane of gill mitochondrion-rich cells in Mozambique tilapia Oreochromis mossambicus

A cDNA encoding a homolog of mammalian Na(+)/H(+) exchanger isoform 3 (NHE3) has been cloned from gills of Mozambique tilapia Oreochromis mossambicus using reverse transcription-polymerase chain reaction (RT-PCR). The deduced amino acid sequence shared 48-56% homology with other vertebrate NHE3 homologs. RT-PCR revealed that tilapia NHE3 was expressed in the gill, kidney and intestine in tilapia adapted to fresh water (FW) and sea water (SW), suggesting its involvement in osmoregulation. We also examined NHE3 mRNA expression levels in the gills of FW and SW fish, using real-time quantitative PCR. NHE3 mRNA was highly expressed in the gills of FW fish, compared with those of SW fish. Whole-mount immunohistochemistry of the gill filaments revealed that tilapia NHE3 was localized in the apical region of mitochondrion-rich (MR) cells in FW- and SW-adapted tilapia. Abundance of NHE3 mRNA in FW gills and apical localization of NHE3 in gill MR cells suggest that NHE3 is involved in FW adaptation and acid-base regulation, in which Na(+) is absorbed from external environments in exchange for H(+) secretion.