Gene expression and protein localization of Cl− transporters, Slc26a6 and Cftr, in the gill ionocytes of rainbow trout

Euryhaline fishes acclimate to various osmotic environments by changing the direction of water and ion transport between body fluids and environmental waters. Ionocytes in the gills are one of the most important cells for the active ion transport. This study aimed to identify the molecules responsible for apical Cl⁻ transport in rainbow trout ionocytes. Tissue distribution and time-course changes after seawater transfer were analyzed for mRNA expression of slc26a6, cftr1, and cftr2. slc26a6 was specifically expressed in the freshwater gills and decreased after seawater transfer. Both cftr genes were expressed in the gills and higher in seawater; however, the magnitude of expression increase after seawater transfer was greater in cftr1 than in cftr2. These results suggest that Cftr1 is mainly functioned in hypo-osmoregulation and that Cftr2 may also be involved in ion transport under freshwater conditions, such as acid–base regulation. Slc26a6 was localized at the apical membrane of Nkcc1-negative ionocytes only in freshwater-acclimated trout. Apical Cftr1 localization was also identified in most of ionocytes in seawater-acclimated fish. These results indicate that Slc26a6 in freshwater and Cftr1 in seawater contribute to osmoregulatory Cl⁻ transport across the apical membrane of ionocytes in rainbow trout.