title: Alternative splicing of the rat sodium/bile acid transporter changes its cellular localization and transport properties.
abstract: Bile secretion involves the structural and functional interplay of hepatocytes and cholangiocytes, the cells lining the intrahepatic bile ducts. Hepatocytes actively secrete bile acids into the canalicular space and cholangiocytes then transport bile acids in a vectorial manner across their apical and basolateral plasma membranes. The initial step in the transepithelial transport of bile acids across rat cholangiocytes is apical uptake by a Na(+)-dependent bile acid transporter (ASBT). To date, the molecular basis of the obligate efflux mechanism for extrusion of bile acids across the cholangiocyte basolateral membrane remains unknown. We have identified an exon-2 skipped, alternatively spliced form of ASBT, designated t-ASBT, expressed in rat cholangiocytes, ileum, and kidney. Alternative splicing causes a frameshift that produces a 154-aa protein. Antipeptide antibodies detected the approximately 19 kDa t-ASBT polypeptide in rat cholangiocytes, ileum, and kidney. The t-ASBT was specifically localized to the basolateral domain of cholangiocytes. Transport studies in Xenopus oocytes revealed that t-ASBT can function as a bile acid efflux protein. Thus, alternative splicing changes the cellular targeting of ASBT, alters its functional properties, and provides a mechanism for rat cholangiocytes and other bile acid-transporting epithelia to extrude bile acids. Our work represents an example in which a single gene appears to encode via alternative splicing both uptake and obligate efflux carriers in a bile acid-transporting epithelial cell.
authors.affiliation: Center for Basic Research in Digestive Diseases, Division of Gastroenterology and Hepatology, Department of Internal Medicine, Mayo Medical School, Clinic, and Foundation, Rochester, MN 55905, USA.
authors.forename: K N
authors.initials: KN
authors.lastname: Lazaridis
authors.forename: P
authors.initials: P
authors.lastname: Tietz
authors.forename: T
authors.initials: T
authors.lastname: Wu
authors.forename: S
authors.initials: S
authors.lastname: Kip
authors.forename: P A
authors.initials: PA
authors.lastname: Dawson
authors.forename: N F
authors.initials: NF
authors.lastname: LaRusso
mesh_terms: D017398:Alternative Splicing / Q000502:physiology*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001692:Biological Transport / Q000502:physiology; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D002451:Cell Compartmentation; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D029363:Organic Anion Transporters, Sodium-Dependent; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D012964:Sodium / Q000378:metabolism; D027981:Symporters
chemical_list: D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D029363:Organic Anion Transporters, Sodium-Dependent; D020033:Protein Isoforms; D027981:Symporters; C071908:sodium-bile acid cotransporter; D012964:Sodium