Molecular Physiological Evidence for the Role of Na + -Cl - Co-Transporter in Branchial Na + Uptake in Freshwater Teleosts

The gills are the major organ for Na uptake in teleosts. It was proposed that freshwater (FW) teleosts adopt Na /H exchanger 3 (Nhe3) as the primary transporter for Na uptake and Na -Cl co-transporter (Ncc) as the backup transporter. However, convincing molecular physiological evidence to support the role of Ncc in branchial Na uptake is still lacking due to the limitations of functional assays in the gills. Thus, this study aimed to reveal the role of branchial Ncc in Na uptake with an in vivo detection platform (scanning ion-selective electrode technique, SIET) that has been recently established in fish gills. First, we identified that Ncc2-expressing cells in zebrafish gills are a specific subtype of ionocyte (NCC ionocytes) by using single-cell transcriptome analysis and immunofluorescence. After a long-term low-Na FW exposure, zebrafish increased branchial Ncc2 expression and the number of NCC ionocytes and enhanced gill Na uptake capacity. Pharmacological treatments further suggested that Na is indeed taken up by Ncc, in addition to Nhe, in the gills. These findings reveal the uptake roles of both branchial Ncc and Nhe under FW and shed light on osmoregulatory physiology in adult fish.