title: Differential expression and novel permeability properties of three aquaporin 8 paralogs from seawater-challenged Atlantic salmon smolts.
abstract: Aquaporins may facilitate transepithelial water absorption in the intestine of seawater (SW)-acclimated fish. Here we have characterized three full-length aqp8 paralogs from Atlantic salmon (Salmo salar). Bayesian inference revealed that each paralog is a representative of the three major classes of aqp8aa, aqp8ab and aqp8b genes found in other teleosts. The permeability properties were studied by heterologous expression in Xenopus laevis oocytes, and the expression levels examined by qPCR, immunofluorescence and immunoelectron microscopy, and immunoblotting of membrane fractions from intestines of SW-challenged smolts. All three Aqp8 paralogs were permeable to water and urea, whereas Aqp8ab and -8b were, surprisingly, also permeable to glycerol. The mRNA tissue distribution of each paralog was distinct, although some tissues such as the intestine showed redundant expression of more than one paralog. Immunofluorescence microscopy localized Aqp8aa(1+2) to intracellular compartments of the liver and intestine, and Aqp8ab and Aqp8b to apical plasma membrane domains of the intestinal epithelium, with Aqp8b also in goblet cells. In a control experiment with rainbow trout, immunoelectron microscopy confirmed abundant labeling of Aqp8ab and -8b at apical plasma membranes of enterocytes in the middle intestine and also in subapical vesicular structures. During SW challenge, Aqp8ab showed significantly increased levels of protein expression in plasma-membrane-enriched fractions of the intestine. These data indicate that the Atlantic salmon Aqp8 paralogs have neofunctionalized on a transcriptional as well as a functional level, and that Aqp8ab may play a central role in the intestinal transcellular uptake of water during SW acclimation.
authors.affiliation: Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark.
authors.forename: Morten B
authors.initials: MB
authors.lastname: Engelund
authors.forename: François
authors.initials: F
authors.lastname: Chauvigné
authors.forename: Birgitte Mønster
authors.initials: BM
authors.lastname: Christensen
authors.forename: Roderick Nigel
authors.initials: RN
authors.lastname: Finn
authors.forename: Joan
authors.initials: J
authors.lastname: Cerdà
authors.forename: Steffen S
authors.initials: SS
authors.lastname: Madsen
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D020346:Aquaporins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002463:Cell Membrane Permeability; D003001:Cloning, Molecular; D005455:Fluorescent Antibody Technique; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000166:cytology / Q000648:ultrastructure; D008969:Molecular Sequence Data; D009928:Organ Specificity / Q000235:genetics; D010802:Phylogeny; D021381:Protein Transport; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020317:Salmo salar / Q000378:metabolism*; D012623:Seawater; D017386:Sequence Homology, Amino Acid; D013347:Subcellular Fractions / Q000378:metabolism; D013997:Time Factors; D014982:Xenopus laevis
chemical_list: D020346:Aquaporins; D012333:RNA, Messenger; C108330:aquaporin 8
keywords: aquaporin; evolution; intestine; neofunctionalization; osmoregulation; salmon