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

Abstract

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 FWacclimated 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.

Differential expression and novel permeability properties of three aquaporin 8 paralogs from seawater-challenged Atlantic salmon smolts

“…There are as yet no other reports of Aqp8 expression in the teleost spleen. The intestinal levels of aqp8ab mRNA tended to be elevated in long-term SW-acclimated animals, which is consistent with earlier findings (Cutler et al, 2009;Kim et al, 2010;Tipsmark et al, 2010). In contrast to the tandemly duplicated aqp8aa1 and -8ab transcripts, expression of aqp8b is more ubiquitous, with the highest expression levels occurring in the brain.…”

“…The increasing intestinal expression of the Aqp8ab protein upon SW challenge supports previous reports of elevated aqp8 mRNA levels in the intestine of various fish species exposed to SW (Cutler et al, 2009;Tipsmark et al, 2010;Kim et al, 2010). These findings underline the important role of this paralog in transforming the intestine into a water-absorptive organ when fish are exposed to hyperosmotic conditions.…”

“…Similar results were found for European and Japanese eels, where SWacclimated animals showed higher levels of aqp8 mRNA in the intestine (Cutler et al, 2009;Kim et al, 2010). The involvement of Aqp8 in the SW acclimation of Atlantic salmon was further supported by recent work in our laboratory, where a homologous antibody detected Aqp8ab in the brush borders and lateral membranes of enterocytes .…”

“…The presence of aqp8 mRNAs in intestinal segments has been observed earlier in different teleosts, although the particular paralog expressed varies between species. For example, eels may express the aqp8aa paralog (Cutler et al, 2009;Kim et al, 2010), zebrafish express both the aqp8aa and -8ab paralogs (TingaudSequeira et al, 2010), while Atlantic salmon expresses the aqp8ab and -8b paralogs (Tipsmark et al, 2010;present study). In Atlantic salmon, aqp8ab mRNA was also present in the spleen, where its apparent upregulation in SW-acclimated animals has not been reported previously.…”

“…In mammals, such as rats, only one AQP8 is expressed, and it is found in the proximal kidney tubules, hepatocytes, testes, salivary gland and intestine (Elkjaer et al, 2001). In teleosts such as the European eel (Anguilla anguilla), Japanese eel (Anguilla japonica), zebrafish and Atlantic salmon, several aqp8 transcripts have been located in some of the same tissues, although each paralog showed a distinct tissue mRNA expression pattern (Cutler et al, 2009;Kim et al, 2010;Tingaud-Sequeira et al, 2010;Tipsmark et al, 2010). Because of the extra round of genome duplication in salmonids, more aqp8s may exist in this species than in zebrafish and the present study probably does not reveal the full complement of aqp8s in Atlantic salmon.…”