Roles of Na-K ATPase α1-subunit isoforms in zebrafish ion regulation mechanism

Gill and skin ionocytes are the major cell types which play an essential role in the transepithelial transport of ions and maintenance of the acid-base balance in fish. Previous studies found three subtypes of ionocytes in zebrafish (Danio rerio), Na+-K+ ATPase-Rich Cells (NaR cells), H+-ATPase-Rich Cells (HR cells) and Na+-Cl- Co-transporter Cells (NCC cells). These subtypes are responsible for Ca2+ uptake, Na+ uptake/acid secretion and Cl- uptake, respectively. A recent study on zebrafish has reported three distinct Na+-K+ ATPase (NKA) α1-subunit genes, atp1a1a.1, atp1a1a.2, and atp1a1a.5, which are expressed in NaR cells, NCC cells and HR cells, respectively.
The present study aims to test the hypothesis of whether the NKA α1 isoforms play distinct roles in the ion regulation pathways in the three types of ionocytes. Knockdown experiments with specific morpholinos against the three NKA α1 genes were conducted.
The results showed that knockdown of atp1a1a.1 decreased the Ca2+ influx in the morphants. This indicates the role of atp1a1a.1 in Ca2+ uptake mechanism in NaR cells is important. Knockdown of atp1a1a.2 decreased the Cl- content and the mRNA expression of NCC in the morphants. These results suggest that loss-of-function of ATP1a1a.2 has may impact the intracellular electrochemistry and thus affect the transcription/translation of other transporters, consequently resulting in impairment to Cl- uptake function in NCC cells. Knockdown of atp1a1a.5 in zebrafish embryos showed a significant decrease of whole body Na+ content, implying an importance of ATP1a1a.5 in Na+ uptake mechanisms in HR cells. These results for the first time provide in vivo molecular physiological evidence for the different roles of NKA α1-subunit isoforms in zebrafish gill/skin ion regulation mechanisms.