Intestinal FXYD12 and sodium-potassium ATPase: A comparative study on two euryhaline medakas in response to salinity changes.

FXYD proteins are the regulators of sodium-potassium ATPase (Na+/K+-ATPase, NKA). In teleosts, NKA is a primary driving force for the operation of many ion transport systems in the osmoregulatory organs (e.g. intestines). Hence, the purpose of this study was to determine the expression of FXYD proteins and NKA α-subunit in the intestines of two closely related medakas (Oryzias dancena and O. latipes), which came from different salinity habitats and have diverse osmoregulatory capabilities, to illustrate the association between NKA and FXYD proteins of two medaka species in response to salinity changes. The results showed that the fxyd12 mRNA was the most predominant in the intestines of both medakas. The association of FXYD12 and NKA in the intestines of the two medaka species was demonstrated via double immunofluorescent staining and co-immunoprecipitation. Upon salinity challenge, the localization of FXYD12 and NKA was similar in the intestines of the two medaka species. However, the expression profiles of intestinal FXYD12 and NKA (mRNA and protein levels), as well as NKA activity differed between the medakas. These results showed that FXYD12 may play a role in modulating NKA activity in the intestines of the two medakas following salinity changes in the maintenance of internal homeostasis. These findings contributed to knowledge of the expression and potential role of vertebrate FXYD12, the regulators of NKA, upon salinity challenge.