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 varies with tissue type and environmental salinity; however, its localization in the gills has remained unresolved. Here, we determined the cellular localization of Kcnj15 in the gills and examined branchial gene expression in response to changes in environmental salinity. In fish acclimated to seawater (SW), Kcnj15 was localized to the basolateral membrane of Nka-positive and anti-Nkcc (T4)-positive ionocytes. Following acclimation to hypoosmotic brackish water (BW; 1 ppt), transcript levels of cftr and kcnj1a decreased, whereas kcnj15 transcript levels did not differ significantly among the experimental groups. In 1-ppt BW, Kcnj15 was also localized to the basolateral membrane of Nka-positive ionocytes. These results demonstrate the basolateral localization of Kcnj15 in branchial ionocytes under both SW and low-salinity conditions and suggest that Kcnj15 may contribute to a basolateral K+ leak pathway supporting ionocyte homeostasis.