Aqp1aa and Aqp4a mediate collecting duct water permeability in a marine teleost

Regulation of water permeability in the collecting duct is important for osmoregulatory acclimation in teleost fish. In hyperosmotic environments such as seawater (SW), the teleost kidney functions as a site of divalent ion excretion. The collecting ducts reabsorb Na⁺, Cl^-, and water, thereby reducing urine volume and producing small amounts of isotonic urine with high concentrations of divalent ions. In hypoosmotic environments such as freshwater (FW) or low-salinity water (LS), the kidney produces large volumes of hypotonic urine and serves as a site of water excretion; under these conditions, the collecting ducts reabsorb Na⁺ and Cl^- but not water. To identify aquaporins (Aqps) involved in regulating collecting duct water permeability, we analyzed renal Aqp expression in a euryhaline marine fish, the Japanese pufferfish (Takifugu rubripes), which possesses 16 Aqp genes in its genome, seven of which (Aqp1aa, 1ab, 3a, 4a, 7, 8bb, and 11a) are expressed in the kidney. Quantitative RT-PCR analysis showed that Aqp1aa and Aqp4a were highly expressed in collecting duct tissues, and that Aqp1aa expression was markedly reduced in fish acclimated to LS (1 ppt). Immunohistochemistry revealed apical localization of Aqp1aa and basolateral localization of Aqp4a in collecting duct cells, with apical Aqp1aa downregulated in LS. These results suggest that Aqp1aa and Aqp4a mediate water reabsorption in SW and that downregulation of Aqp1aa contributes to hypotonic urine production in under LS conditions.