FXYD-11 associates with Na+-K+-ATPase in the gill of Atlantic salmon: regulation and localization in relation to changed ion-regulatory status

Tipsmark CK, Mahmmoud YA, Borski RJ, Madsen SS. FXYD-11 associates with Na+-K+-ATPase in the gill of Atlantic salmon: regulation and localization in relation to changed ion-regulatory status. Am J Physiol Regul Integr Comp Physiol 299: R1212-R1223, 2010. First published August 11, 2010; doi:10.1152/ajpregu.00015.2010.-The Na+-K+-ATPase is the primary electrogenic component driving transepithelial ion transport in the teleost gill; thus regulation of its level of activity is of critical importance for osmotic homeostasis. In the present study, we examined the dynamics of the gill-specific FXYD-11 protein, a putative regulatory subunit of the pump, in Atlantic salmon during seawater (SW) acclimation, smoltification, and treatment with cortisol, growth hormone, and prolactin. Dual-labeling immunohistochemistry showed that branchial FXYD-11 is localized in Na+-K+-ATPase immunoreactive cells, and coimmuno-precipitation experiments confirmed a direct association between FXYD-11 and the Na+-K+-ATPase alpha-subunit. Transfer of freshwater (FW)-acclimated salmon to SW induced a parallel increase in total alpha-subunit and FXYD-11 protein expression. A similar concurrent increase was seen during smoltification in FW. In FW fish, cortisol induced an increase in both alpha-subunit and FXYD-11 abundance, and growth hormone further stimulated FXYD-11 levels. In SW fish, prolactin induced a decrease in FXYD-11 and alpha-subunit protein levels. In vitro cortisol (18 h, 10 mu g/ml) stimulated FXYD-11, but not FXYD-9, mRNA levels in gills from FW and SW salmon. The data show that Na+-K+-ATPase expressed in branchial mitochondrion-rich cells is accompanied by FXYD-11, and that regulation of the two proteins is highly coordinated. The demonstrated association of FXYD-11 and alpha-subunit strengthens our hypothesis that FXYD-11 has a role in modulating the pump's kinetic properties. The presence of putative phosphorylation sites on the intracellular domain of FXYD-11 suggests the possibility that this protein also may transmit external signals that regulate Na+-K+-ATPase activity.