Basolateral localization of Kcnj15 in branchial ionocytes of the seawater pufferfish Takifugu rubripes

In marine teleosts, gill ionocytes play central roles in the excretion of excess monovalent ions (Na+, Cl-, and K+). Basolateral K+ channels are considered essential for sustaining ion secretion by maintaining K+ homeostasis and membrane potential; however, the molecular identity of basolateral K+ channels in teleost gill ionocytes remains incompletely understood. Kcnj15 is a K+ channel expressed in multiple osmoregulatory tissues of the Japanese pufferfish (Takifugu rubripes), and its localization changes depending on tissue type and osmotic conditions; however, its localization in the gills has remained unresolved. Here, we determined the cellular localization of Kcnj15 in the gills and examined its expression in response to changes in environmental salinity. In seawater-acclimated fish, Kcnj15 was localized to the basolateral membrane of Nka- and Nkcc1-positive ionocytes. Following acclimation to hypoosmotic brackish water (1 ppt), transcript levels of cftr, nkcc1, and kcnj1a decreased, whereas kcnj15 expression remained unchanged. Under this condition, Kcnj15 was localized to the basolateral membrane of Nka-positive ionocytes. These results demonstrate that Kcnj15 is localized to the basolateral membrane of gill ionocytes and suggest that it may function as a basolateral K+ leak channel supporting ionocyte homeostasis under both seawater and low-salinity conditions.