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 alpha- and beta-subunits. To gain insight into the potential function of nka genes in salinity adaptation, 5 nkaa genes (nka alpha 1a, nka alpha 1b, nka alpha 2, nka alpha 3a, nka alpha 3b) and 7 nka beta genes (nka beta 1a, nka beta 1b, nka beta 2a, nka beta 2b, nka beta 3a, nka beta 3b and nka beta 4) 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 (nka alpha 1a, nka alpha 1b, nka alpha 3a, nka alpha 3b, nka beta 1b and nka beta 2a) 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 nka alpha/beta-isoforms showed their diverse responses to salinity changes and the expression of nka genes including nka alpha 1a, nka alpha 3b, nka beta 1b in gill, nka alpha 3a in kidney and nka beta 2a in intestine were transcriptionally regulated by altered salinity. Notably, the expression patterns of nka alpha 1a and nka beta 1b in gill showed similar variation trend with NKA activity, suggesting that nka alpha 1a/beta 1b 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.