Cloning and expression of three aquaporin homologues from the European eel (Anguilla anguilla): effects of seawater acclimation and cortisol treatment on renal expression

Abstract

The European eel (Anguilla anguilla) is able to osmoregulate over a wide range of environmental salinities from FW (freshwater) to hyperconcentrated SW (seawater). Successful acclimation is associated with strict regulation of ion and water transport pathways within key osmoregulatory epithelia to enable animals to survive the dehydrating or oedematous conditions. These observations suggested that homologues of the AQP (aquaporin) water channel family were expressed in the eel and that these proteins may contribute to the water transport and osmoregulation in all euryhaline teleosts. Complementary DNAs encoding a homologue of the mammalian aquaglyceroporins (termed AQPe) and two homologues of mammalian aquaporin-1 [termed AQP1 and AQP1dup (aquaporin-1 duplicate)] were isolated from the European eel. Northern-blot analysis revealed (i) two AQP1 transcripts exhibiting a wide tissue distribution, (ii) a single AQP1dup mRNA transcript found in the kidney and the oesophagus, and (iii) a single AQPe mRNA detectable mainly in the kidney and the intestine. The relative expression of isoforms within the kidney was AQP1dup>AQPe>AQP1. SW acclimation significantly reduced the abundance of AQP1, AQP1dup and AQPe transcripts in the kidney of yellow eels by approx. 72, 66 and 34% respectively, whereas the expression levels in silver eels were independent of salinity and equivalent to those observed in yellow SW-acclimated fish. AQP1 protein expression was primarily located within the vascular endothelium in yellow eels and the epithelial apical brush border in some renal tubules in silver eels. Infusion of cortisol into FW eels had no effect on AQPe mRNA expression, but induced significant decreases in AQP1 and AQP1dup mRNA levels in the kidney of yellow eels. Cortisol infusion had no effect on the expression of any isoform in the silver eels. These results suggest that SW-acclimation or cortisol infusion induces a down-regulation of renal AQP expression in yellow eels. However, the lower levels of aquaporin expression found within the silver eel kidney were not further reduced by salinity transfer or steroid infusion. These differences in mRNA expression were accompanied by changes in the cellular distribution of the AQP1 protein between vascular endothelial and tubular epithelial cells.