Ion uptake pathways in European sea bass Dicentrarchus labrax

Ion uptake mechanisms are diverse in fish species, certainly linked to duplication events that have led to the presence of a multitude of paralogous genes. In fish, Na⁺ uptake involves several ion transporters expressed in different ionocyte subtypes. In the European sea bass Dicentrarchus labrax, several key transporters potentially involved in Na⁺ uptake have been investigated in seawater (SW) and following a 2 weeks freshwater (FW) acclimation. Using gel electrophoresis, we have shown that the Na⁺/H⁺-exchanger 3 (nhe3, slc9a3) is expressed in gills and kidney at both salinities. Quantitative realtime PCR analysis showed a significantly higher nhe3 expression in fresh water (FW) compared to SW. Its apical localization in a subset of gill ionocytes in freshwater-acclimated fish supports the role of NHE3 in Na⁺ uptake. Interestingly, NHE3-immunopositive cells also express basolateral Na⁺/K⁺/2Cl^- cotransporter 1 (NKCC1) and are mainly localized in gill lamella. Among the three nhe2 (slc9a2) paralogs, only nhe2c shows differential branchial expression levels with higher mRNA levels in SW than in FW. The increased branchial expression of the ammonia transporter rhcg1 (Rhesus protein), nhe3 and cytoplasmic carbonic anhydrase (cac) in FW could indicate the presence of a functional coupling between ion transporters to form a Na⁺/NH4⁺ exchange complex. Acid-sensing ion channel 4 (asic4) seems not to be expressed in sea bass gills. Na⁺/Cl^- cotransporter (ncc2a or ncc-like) is about three times more expressed in FW compared to SW suggesting coupled Na⁺ and Cl^- uptake in a subset of gill ionocytes. Besides the main pump Na⁺/K⁺-ATPase, branchial NCC2a and NHE3 may be key players in ion uptake in sea bass following a long-term freshwater challenge.