Expression of aquaporins 3, 8 and 10 in the intestines of freshwater- and seawater-acclimated Japanese eels Anguilla japonica

Euryhaline teleosts possess excellent adaptability to a wide range of environmental salinities. Although intestinal water absorption is important for seawater (SW) adaptation, little is known about the molecular mechanisms of the water-transporting pathway in the intestine. We have cloned three homologs of the mammalian aquaporins (AQPs) 3, 8 and 10 from the intestine of the SW-acclimated Japanese eel. The deduced amino-acid sequences shared 47–98% homology with other known respective AQP isoforms. Topology prediction of the identified sequences showed six membrane-spanning domains, intracellular N- and C-terminal tails, and two NPA (asparagine–proline–alanine) motifs in the second and fifth connecting loops, all of which are highly conserved among known AQPs. Reverse transcription PCR analysis revealed that AQP3 was expressed in various tissues, whereas the expression of AQP 8 and 10 mRNAs was detected predominantly in the intestine. The expression levels of AQPs 1, 3, 8 and 10 in the anterior and posterior intestines and the rectum were determined by real-time quantitative PCR and compared for FW (freshwater)- and SW-acclimated eels. AQP1 expression levels in the posterior intestine and rectum were significantly higher in the SW-acclimated eel than in the FW-acclimated fish. AQP3 expression in the SW-acclimated eel was only higher in the rectum than that in the FW-acclimated eel. Expression levels of AQPs 8 and 10 in the intestinal segments tended to be higher in the SW-acclimated eel than in the FW-acclimated eel. These results showed that intestinal AQP expression is closely related to SW adaptation, suggesting the presence of a water-absorbing mechanism associated with multiple AQP isoforms in the intestinal tract.