Flexible selection of diversified Na(+)/K(+)-ATPase α-subunit isoforms for osmoregulation in teleosts

Multiple Na+/K+-ATPase (NKA) α-subunit isoforms express differentially in response to salinity transfer in teleosts but we observed that the isoform nomenclature is inconsistent with the phylogenetic relationship of NKA α-genes. We cloned the catalytic NKA α-subunit isoforms in eels and medaka, analyzed the time course of their expressions in osmoregulatory tissues after transfer from freshwater (FW) to seawater (SW), and performed phylogenetic analyses to deduce an evolutionary scenario that illustrates how various duplication events have led to the current genomic arrangement of NKA α-genes in teleosts.

Five and six α-subunits were cloned in eels and medaka respectively. In eels, the commonly-reported α1a and α1b isoforms were absent while the α1c isoform was diversified instead (α1c-1, α1c-2, α1c-3, α2, and α3 in eels). Phylogenetic estimation indicated that the α1a and α1b isoforms from salmon, tilapia, and medaka were generated by independent duplication events and thus they are paralogous isoforms. Re-examination of expression changes of known isoforms after salinity challenge revealed that the isoforms selected as predominant SW-types varied among teleost lineages. Diversification of α1 isoforms occurred by various types of gene duplication, or by alternative transcription among tandem genes to form chimeric transcripts, but there is no trend for more α1 copies in euryhaline species. Our data suggest that the isoform switching between FW (α1a predominates) and SW (α1b predominates) that occurs in salmonids is not universal in teleosts. Instead, in eels, α1c-1 was the major α-subunit upregulated gill, intestine, and kidney in SW. Localization of both NKA mRNA and protein showed consistent upregulation in gill and intestine in SW eels, but not in renal distal and collecting tubules, where low transcript expression levels were accompanied by high protein levels, suggesting a tissue-specific translational regulation that determines and fine-tunes the NKA expression. In medaka, α1b was upregulated in SW in anterior intestine while most other α-subunit isoforms were less responsive to salinity changes.

By integrating gene expression and phylogenetic results, we propose that the major NKA α-subunits for SW acclimation were not ancestrally selected, but rather were flexibly determined in lineage-specific fashion in teleosts.