Kcnj15, but not Kcnj1, localizes with Nkcc2 in the apical membranes of renal tubular and intestinal epithelial cells of marine pufferfish, Takifugu rubripes.

In mammals, Na+-K+-2Cl- cotransporter 2 (Nkcc2) is localized in the apical membrane of renal tubule and contributes to salt reabsorption. Nkcc2 localizes with K+ channels, such as the potassium inwardly rectifying channel subfamily J member 1 (Kcnj1), which provide K+ to the luminal fluid. In marine teleosts, Nkcc2 is expressed in the kidneys and intestine; however, the K+ channels localizing with it remains ambiguous. Here, tissue distribution analysis revealed that the paralogs of kcnj1 were expressed in the gills and other organs, but not in the kidneys and intestines of the marine pufferfish, Takifugu rubripes. In contrast, kcnj15 was highly expressed in the kidneys, intestine, gills, and urinary bladder of the pufferfish. Immunohistochemical analysis showed that Kcnj15 localizes to the apical region with Nkcc2 and to the basolateral region with Na+/K+-ATPase (Nka) in the renal tubules and intestinal epithelial cells of marine pufferfish. The expression of kcnj15, nkcc2, and Na+-Cl- cotransporter ncc in the kidney was significantly increased in brackish water (1 ppt salinity) compared to that in seawater (35 ppt). In brackish water, Kcnj15 was localized to the basolateral side with Nka in the renal tubules. In the urinary bladder, Kcnj15 was localized to the basolateral side along with Nka in Ncc-positive epithelial cells in both seawater and brackish water. These results suggest that Kcnj15 is involved in the salt (re)absorption systems of the urinary and digestive systems in pufferfish in a manner dependent on the expressing cells and habitat salinity.