Genome-wide identification and quantification of salinity-responsive Na+/K+-ATPase α-subunits in three salmonids

Na+/K+-ATPase (NKA) is a ubiquitous ion-transporting pump that functions to maintain intracellular homeostasis. We performed a comprehensive assessment of salinity response in terms of genome-wide identification, evolutionary footprint, structural characterization, expression patterns of NKA α-subunit genes and NKA activity in three salmonids, including rainbow trout (Oncorhynchus mykiss), steelhead trout (O. mykiss irideus), and Atlantic salmon (Salmo salar). From the O. mykiss and S. salar genomes, a total of 48 NKAα genes were identified and classified into nine isoforms: NKAα1 (a/b/b2/c), NKAα2 (a/b), and NKAα3 (a/b/b2). The intergenomic homologous genes across O. mykiss and S. salar exhibited multicollinearity relationships, with two tandem duplicate events occurring mainly in NKAα1b/b2 and α1c. The predicted motifs and domains of NKAα genes were highly conserved, accompanied by unequal transmembrane domains (TMDs) that contained specific functional sites (Na+-K+/ATP/phosphorylation). The expression profiles of the nine NKAα isoforms in three salmonids were tissue-specific, and the expression levels of NKAα1 except α1c were the highest in the osmoregulatory organs (e.g. gill and mesonephros), while those of NKAα2/α3 were mainly in the excitable tissues (e.g. muscle and brain). Exploration of potential indicators for three salmonids as follows: In the rainbow trout, NKAα2a and NKAα1b/b2 expression were extremely sensitive to salinity change in the gill following transfer to seawater. In the steelhead trout, NKAα3 (a/b/b2) responded more significantly in the heart. In the Atlantic salmon, NKAα1b/b2 isoforms were more prominent in the muscle exposed to seawater. These changes in expression of NKAα were associated with increased NKA activity in the gill and muscle and exhibited salinity-dependent variation and differences between the osmoregulatory capacity of parr and smolt. These results provide insights into the molecular mechanisms of NKAα-mediated salinity sensitivity as well as deliver effective indicators for elucidating the osmoregulation strategy of salmonids.