title: Osmoregulatory inositol transporter SMIT1 modulates electrical activity by adjusting PI(4,5)P2 levels.
abstract: Myo-inositol is an important cellular osmolyte in autoregulation of cell volume and fluid balance, particularly for mammalian brain and kidney cells. We find it also regulates excitability. Myo-inositol is the precursor of phosphoinositides, key signaling lipids including phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. However, whether myo-inositol accumulation during osmoregulation affects signaling and excitability has not been fully explored. We found that overexpression of the Na(+)/myo-inositol cotransporter (SMIT1) and myo-inositol supplementation enlarged intracellular PI(4,5)P2 pools, modulated several PI(4,5)P2-dependent ion channels including KCNQ2/3 channels, and attenuated the action potential firing of superior cervical ganglion neurons. Further experiments using the rapamycin-recruitable phosphatase Sac1 to hydrolyze PI(4)P and the P4M probe to visualize PI(4)P suggested that PI(4)P levels increased after myo-inositol supplementation with SMIT1 expression. Elevated relative levels of PIP and PIP2 were directly confirmed using mass spectrometry. Inositol trisphosphate production and release of calcium from intracellular stores also were augmented after myo-inositol supplementation. Finally, we found that treatment with a hypertonic solution mimicked the effect we observed with SMIT1 overexpression, whereas silencing tonicity-responsive enhancer binding protein prevented these effects. These results show that ion channel function and cellular excitability are under regulation by several "physiological" manipulations that alter the PI(4,5)P2 setpoint. We demonstrate a previously unrecognized linkage between extracellular osmotic changes and the electrical properties of excitable cells.
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, WA 98195;
authors.forename: Gucan
authors.initials: G
authors.lastname: Dai
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, WA 98195;
authors.forename: Haijie
authors.initials: H
authors.lastname: Yu
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, WA 98195;
authors.forename: Martin
authors.identifier: 0000-0003-4301-8385
authors.initials: M
authors.lastname: Kruse
authors.affiliation: ATK Innovation Analytics & Discovery, North Bend, WA 98045; Division of Gastroenterology, Department of Medicine, University of Washington, Seattle, WA 98195.
authors.forename: Alexis
authors.initials: A
authors.lastname: Traynor-Kaplan
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, WA 98195; hille@uw.edu.
authors.forename: Bertil
authors.identifier: 0000-0002-7266-1671
authors.initials: B
authors.lastname: Hille
mesh_terms: D000200:Action Potentials; D020013:Calcium Signaling; D002460:Cell Line; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels / Q000378:metabolism; D057809:HEK293 Cells; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007294:Inositol / Q000378:metabolism; D051658:KCNQ2 Potassium Channel / Q000235:genetics / Q000378:metabolism; D051659:KCNQ3 Potassium Channel / Q000235:genetics / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D064587:Osmoregulation; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D017783:Superior Cervical Ganglion / Q000166:cytology / Q000378:metabolism; D027981:Symporters / Q000235:genetics / Q000378:metabolism*; D050053:TRPM Cation Channels / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D006360:Heat-Shock Proteins; C094401:KCNJ6 protein, human; D051658:KCNQ2 Potassium Channel; C496914:KCNQ2 protein, human; D051659:KCNQ3 Potassium Channel; C496916:KCNQ3 protein, human; C118125:NFAT5 protein, human; D019269:Phosphatidylinositol 4,5-Diphosphate; D011994:Recombinant Proteins; D027981:Symporters; D050053:TRPM Cation Channels; D014157:Transcription Factors; C073884:SLC5A3 protein, human; D007294:Inositol; D017346:Protein Serine-Threonine Kinases; C486583:TRPM7 protein, human
keywords: PIP2; SMIT1; ion channels; myo-inositol; phosphoinositide