title: Expression and subcellular localization of the AQP8 and AQP1 water channels in the mouse gall-bladder epithelium.
abstract: BACKGROUND
  Transepithelial transport of water is one of the most distinctive functions by which the gall-bladder rearranges its bile content. Water is reabsorbed from the gall-bladder lumen during fasting, whereas it is secreted into the lumen following meal ingestion. Nevertheless, the molecular mechanism by which water is transported across the gall-bladder epithelium remains mostly unclear.
  
  
  RESULTS
  In the present study, we investigate the presence and subcellular localization of AQP (aquaporin) water channels in the mouse gall-bladder epithelium. Considerable AQP8 mRNA was detected in the gall-bladder epithelium of mouse, calf, rabbit, guinea pig and man. Studies of subcellular localization were then addressed to the mouse gall-bladder where the transcript of a second AQP, AQP1, was also detected. Immunoblotting experiments confirmed the presence of AQP8 and AQP1 at a protein level. Immunohistochemistry showed intense expression of AQP8 and AQP1 in the gall-bladder epithelial cells where AQP8 was localized in the apical membrane, whereas AQP1 was seen both in the apical and basolateral membranes, and in vesicles located in the subapical cytoplasm.
  
  
  CONCLUSIONS
  The pattern of subcellular distribution of AQP8 and AQP1 strongly corroborates the hypothesis of a transcellular route for the movement of water across the gall-bladder epithelium. Osmotic water would cross the apical membrane through AQP8 and AQP1, although AQP1 would be the facilitated pathway for the movement of water across the basolateral membrane. The presence of two distinct AQPs in the apical membrane is an unusual finding and may relate to the membrane's ability both to absorb and secrete fluid. It is tempting to hypothesize that AQP1 is hormonally translocated to the gall-bladder apical membrane to secrete water as in the bile duct epithelium, a functional homologue of the gall-bladder epithelium, whereas apical AQP8 may account for the absorption of water from gall-bladder bile.
authors.affiliation: Department of General and Environmental Physiology, University of Bari, via Amendola, 165/A-I-70126 Bari, Italy. calamita@biologia.uniba.it
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Calamita
authors.forename: Domenico
authors.initials: D
authors.lastname: Ferri
authors.forename: Claudia
authors.initials: C
authors.lastname: Bazzini
authors.forename: Amelia
authors.initials: A
authors.lastname: Mazzone
authors.forename: Guido
authors.initials: G
authors.lastname: Bottà
authors.forename: Giuseppa E
authors.initials: GE
authors.lastname: Liquori
authors.forename: Markus
authors.initials: M
authors.lastname: Paulmichl
authors.forename: Piero
authors.initials: P
authors.lastname: Portincasa
authors.forename: Giuliano
authors.initials: G
authors.lastname: Meyer
authors.forename: M
authors.initials: M
authors.lastname: Svelto
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D051398:Aquaporin 1; D020346:Aquaporins / Q000096:biosynthesis* / Q000737:chemistry; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism*; D005704:Gallbladder / Q000378:metabolism*; D005786:Gene Expression Regulation; D006168:Guinea Pigs; D006728:Hormones / Q000378:metabolism; D015151:Immunoblotting; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007473:Ion Channels / Q000096:biosynthesis* / Q000737:chemistry; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution; D014867:Water / Q000378:metabolism
chemical_list: C496021:Aqp1 protein, mouse; D020346:Aquaporins; D006728:Hormones; D007473:Ion Channels; D012333:RNA, Messenger; C108330:aquaporin 8; D014867:Water; D051398:Aquaporin 1