Characterization of ion channel and transporter mRNA expressions in isolated gill chloride and pavement cells of seawater acclimating eels.

Ion channels and transporters (i.e. cystic fibrosis transmembrane regulator (CFTR), inward rectifier potassium channel (eKir), Na/K-ATPase, Na/K/Cl2 co-transporter (NKCC), aquaporin-3 (AQP-3), and Na/H exchanger-1 (NHE-1)) are known to be expressed in gill epithelia of teleost fish. Owing to the anatomical complexity of gill structures, their temporal expression profile in seawater acclimating gill pavement (PVCs) and chloride cells (CCs) are limited. In this study, we isolated the gill PVCs and CCs from seawater acclimating Japanese eels to address the issue. In the gill epithelia of freshwater adapted eels, CCs expressed the highest mRNA and/or protein levels of Na/K-ATPase, NKCC, and eKir as demonstrated by real-time PCR and/or immunohistochemical staining. AQP-3 mRNA was highly expressed in freshwater PVCs and its protein was in general expressed in all gill cells. The NHE-1 transcripts were expressed in similar levels in both PVCs and CCs. CFTR mRNA transcript was almost undetectable in all the freshwater gill cell samples. Seawater acclimation induced the transcript and/or protein levels of Na/K-ATPase, NKCC, CFTR, and eKir in CCs. The upregulation and the coexpression of these transporters in CCs suggested their cohort function in mediating Na+, K+, and Cl- transport. The expression of CFTR was found to be tightly regulated as its expression was restricted only in "seawater CCs". AQP-3 transcript and protein levels in PVCs reduced significantly during the acclimation. Interestingly immunocytochemical (ICC) staining of seawater gill epithelia revealed that AQP-3 immunoreactivities were mainly localized in seawater CCs. In the acclimation, there was no significant reduction of NHE-1 mRNA in both PVCs and CCs, however its protein level dropped significantly in the seawater condition. The present study is the first to demonstrate the activation of the mRNA transcripts for the ion channels and transporters in isolated gill CCs during seawater acclimation. The activating mechanism is found to be confined primarily in CCs. These results indicated that in addition to the increase in size and number of CCs, the molecular remodeling and the functional plasticity of CCs were essential in the ion transport process during seawater acclimation.