title: Na+, Cl-, and pH dependence of the human choline transporter (hCHT) in Xenopus oocytes: the proton inactivation hypothesis of hCHT in synaptic vesicles.
abstract: The recent cloning of the human choline transporter (hCHT) has allowed its expression in Xenopus laevis oocytes and the simultaneous measurement of choline transport and choline-induced current under voltage clamp. hCHT currents and choline transport are evident in cRNA-injected oocytes and significantly enhanced by the hCHT trafficking mutant L530A/V531A. The charge/choline ratio of hCHT varies from 10e/choline at -80 mV to 3e/choline at -20 mV, in contrast with the reported fixed stoichiometry of the Na+-coupled glucose transporter in the same gene family. Ion substitution shows that the choline uptake and choline-induced current are Na+ and Cl- dependent; however, the reversal potential of the induced current suggests a Na+-selective mechanism, consigning Cl- to a regulatory role rather than a coupled, cotransported-ion role. The hCHT-specific inhibitor hemicholinium-3 (HC-3) blocks choline uptake and choline-induced current; in addition, HC-3 alone reveals a constitutive, depolarizing leak current through hCHT. We show that external protons reduce hCHT current, transport, and binding with a similar pKa of 7.4, suggesting proton titration of residue(s) that support choline binding and transport. Given the localization of the choline transporter to synaptic vesicles, we propose that proton inactivation of hCHT prevents acetylcholine and proton leakage from the acidic interior of cholinergic synaptic vesicles. This mechanism would allow cholinergic, activity-triggered delivery of silent choline transporters to the plasma membrane, in which normal pH would reactivate the transporters for choline uptake and subsequent acetylcholine synthesis.
authors.affiliation: Department of Pharmacology and Center for Molecular Neuroscience, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548, USA.
authors.forename: Hideki
authors.initials: H
authors.lastname: Iwamoto
authors.forename: Randy D
authors.initials: RD
authors.lastname: Blakely
authors.forename: Louis J
authors.initials: LJ
authors.lastname: De Felice
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002712:Chlorides / Q000494:pharmacology*; D002794:Choline / Q000378:metabolism*; D005260:Female; D006426:Hemicholinium 3 / Q000494:pharmacology; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008959:Models, Neurological; D009865:Oocytes; D018408:Patch-Clamp Techniques; D011522:Protons; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*; D027981:Symporters; D013572:Synaptic Vesicles / Q000378:metabolism / Q000502:physiology*; D014982:Xenopus laevis
chemical_list: D002712:Chlorides; D026901:Membrane Transport Proteins; D011522:Protons; D011993:Recombinant Fusion Proteins; C510731:SLC5A7 protein, human; D027981:Symporters; D006426:Hemicholinium 3; D012964:Sodium; D002794:Choline; D000109:Acetylcholine