title: Hypertonic activation of the renal betaine/GABA transporter is microtubule dependent.
abstract: BACKGROUND
  Epithelial cells in the renal inner medulla accumulate osmolytes such as betaine to maintain normal cell volume during prolonged extracellular hypertonic stress. Betaine accumulation is the result of activation of transcription of the BGT1 transporter gene followed by increased betaine transport.
  
  
  METHODS
  We studied the possible role of microtubules in this adaptive mechanism using renal cells in culture. RESULTS.: In cultured renal cell lines [Madin-Darby canine kidney (MDCK) and mouse inner medullary collecting duct (mIMCD-3)], up-regulation of BGT1 activity was maximal after 24 to 30 hours in growth medium made hypertonic (510 mOsm/kg) by the addition of sucrose or NaCl. Up-regulation was reversed within 24 to 36 hours after returning cells to isotonic medium. Both cycloheximide (20 micromol/L) and nocodazole (20 micromol/L) blocked the hypertonic up-regulation of BGT1. Nocodazole was partially effective even when added 16 to 20 hours after the switch to hypertonic medium. Recovery from nocodazole action was rapid, and there was full activation of BGT1 transport within three to six hours after nocodazole removal, suggesting rapid trafficking to the cell surface once microtubules repolymerized. Hypertonic activation of BGT1 transport was detected in an isolated membrane fraction and was blocked by cycloheximide but not by nocodazole. Confocal microscopy confirmed the increased abundance of BGT1 proteins in the plasma membrane of hypertonic cells and showed that BGT1 remained intracellular during nocodazole treatment.
  
  
  CONCLUSIONS
  Hypertonic activation of BGT1 in renal cells requires de novo protein synthesis and microtubule-dependent trafficking of additional transporters to the cell surface. The apparent resistance of membrane BGT1 to nocodazole blockade is likely due to the presence in the membrane fraction of an increased intracellular pool of active BGT1 transporters.
authors.affiliation: Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana 46202-5120, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Basham
authors.forename: A
authors.initials: A
authors.lastname: Chabrerie
authors.forename: S A
authors.initials: SA
authors.lastname: Kempson
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D003078:Colchicine / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism*; D050485:GABA Plasma Membrane Transport Proteins; D006982:Hypertonic Solutions / Q000494:pharmacology; D007552:Isotonic Solutions / Q000494:pharmacology; D007685:Kidney Tubules, Collecting / Q000166:cytology*; D007686:Kidney Tubules, Distal / Q000166:cytology; D008565:Membrane Proteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D051379:Mice; D018613:Microscopy, Confocal; D008870:Microtubules / Q000187:drug effects / Q000378:metabolism*; D015739:Nocodazole / Q000494:pharmacology; D027361:Organic Anion Transporters; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D015854:Up-Regulation / Q000502:physiology; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D000970:Antineoplastic Agents; D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D006982:Hypertonic Solutions; D007552:Isotonic Solutions; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; D011500:Protein Synthesis Inhibitors; C072207:betaine plasma membrane transport proteins; D003513:Cycloheximide; D015739:Nocodazole; D003078:Colchicine