Expression Profiles of Branchial FXYD Proteins in the Brackish Medaka <em>Oryzias dancena</em>: A Potential Saltwater Fish Model for Studies of Osmoregulation

<div><p>FXYD proteins are novel regulators of Na⁺-K⁺-ATPase (NKA). In fish subjected to salinity challenges, NKA activity in osmoregulatory organs (e.g., gills) is a primary driving force for the many ion transport systems that act in concert to maintain a stable internal environment. Although teleostean FXYD proteins have been identified and investigated, previous studies focused on only a limited group of species. The purposes of the present study were to establish the brackish medaka (<em>Oryzias dancena</em>) as a potential saltwater fish model for osmoregulatory studies and to investigate the diversity of teleostean FXYD expression profiles by comparing two closely related euryhaline model teleosts, brackish medaka and Japanese medaka (<em>O. latipes</em>), upon exposure to salinity changes. Seven members of the FXYD protein family were identified in each medaka species, and the expression of most branchial <em>fxyd</em> genes was salinity-dependent. Among the cloned genes, <em>fxyd11</em> was expressed specifically in the gills and at a significantly higher level than the other <em>fxyd</em> genes. In the brackish medaka, branchial <em>fxyd11</em> expression was localized to the NKA-immunoreactive cells in gill epithelia. Furthermore, the FXYD11 protein interacted with the NKA α-subunit and was expressed at a higher level in freshwater-acclimated individuals relative to fish in other salinity groups. The protein sequences and tissue distributions of the FXYD proteins were very similar between the two medaka species, but different expression profiles were observed upon salinity challenge for most branchial <em>fxyd</em> genes. Salinity changes produced different effects on the FXYD11 and NKA α-subunit expression patterns in the gills of the brackish medaka. To our knowledge, this report is the first to focus on FXYD expression in the gills of closely related euryhaline teleosts. Given the advantages conferred by the well-developed Japanese medaka system, we propose the brackish medaka as a saltwater fish model for osmoregulatory studies.</p>