Intestinal Expression of Aquaporins in the European Eel (Anguilla anguilla): Effects of SW Acclimation on Isoform Expression

Five aquaporin (AQP) isoforms, comprising two homologues of mammalian aquaporin 1 (called AQP1 and AQP1 duplicate), a novel aqua-glyceroporin called AQPe and two homologues of the “unorthodox” or “atypical” mammalian aquaporin 8 (called AQP8a and AQP8b) and are expressed within the intestine of the European eel ( Anguilla anguilla ). All isoforms are expressed within anterior, mid and posterior sections of the intestine, although AQP1 expression predominates within the posterior/rectal segment. Expression of AQP1, AQP8a and AQP8b isoforms increased in all intestinal segments two days after transfer of fish from freshwater (FW) to sea water (SW). No consistent changes in intestinal expression were found with AQPe or AQP1 duplicate with the latter exhibiting highly variable levels of expression between animals within the same experimental groups. Although single mRNA species were identified for most isoforms, Northern blot analysis revealed three and possibly four different mRNA transcripts for AQP8b, ranging from 1.6 to 2.6 kb in length. The different transcripts appeared to be expressed in a random manner between fish in each experimental group although all were observed to up-regulate when fish were acclimated to SW. Immunohistochemistry identified AQP1 to be present within the endothelial cells of blood vessels within the intestine and also to the apical brush-border membrane of epithelial cells within the posterior and rectal segments. Antisera for AQP8b identified specific staining of intracellular vesicles/mitochondria in the perinuclear regions of epithelial cells along the whole length of the intestine. The possible physiological roles of these AQP isoforms within the GI tract will be discussed.