Na-K-ATPase and nka genes in spotted sea bass (Lateolabrax maculatus) and their involvement in salinity adaptation.

Euryhaline teleosts can survive in a wide salinity range via alteration of the molecular mechanisms to maintain internal ionic and osmotic balance in osmoregulatory organs such as gill，kidney and intestine. Na/K-ATPase (NKA), plays a crucial role in sustaining intracellular homeostasis and is characterized by association of multiple isoforms of α- and β-subunits. To gain insight into the potential function of nka genes in salinity adaptation, 5 nkaα genes (nkaα1a, nkaα1b, nkaα2, nkaα3a, nkaα3b) and 7 nkaβ genes (nkaβ1a, nkaβ1b, nkaβ2a, nkaβ2b, nkaβ3a, nkaβ3b and nkaβ4) were identified from transcriptomic and genomic databases of Lateolabrax maculatus. The annotation and evolutionary footprint of these nka genes was revealed via the analysis of phylogenetic tree, gene synteny, copy numbers, exon-intron structures and motif compositions. The expressions of 12 nka genes in spotted sea bass was tested in ten tissues (kidney, gonad, stomach, intestine, gill, muscle, heart, spleen, liver and brain) and 6 genes (nkaα1a, nkaα1b, nkaα3a, nkaα3b, nkaβ1b and nkaβ2a) showed high expression in osmoregulatory organs. Furthermore, the responses of NKA and potential salinity-sensitive nka genes were examined under different salinity treatment (0 ppt, 12 ppt, 30 ppt, 45 ppt). Results showed that the enzyme activity of NKA was highest in gill and exhibited salinity dependent variation, with the highest activity identified in 45 ppt. Different nkaα/β-isoforms showed their diverse responses to salinity changes and the expression of nka genes including nkaα1a, nkaα3b, nkaβ1b in gill, nkaα3a in kidney and nkaβ2a in intestine were transcriptionally regulated by altered salinity. Notably, the expression patterns of nkaα1a and nkaβ1b in gill showed similar variation trend with NKA activity, suggesting that nkaα1a/β1b could be the major function isoforms involved in primary ion transport during salinity adaptation. Our results provided insights into the roles of nkas in osmotic regulation and a theoretical basis for future studies that focus on detailed molecular mechanisms in salinity adaptation of euryhaline teleosts.