title: Gating properties of the P2X2a and P2X2b receptor channels: experiments and mathematical modeling.
abstract: Adenosine triphosphate (ATP)-gated P2X2 receptors exhibit two opposite activation-dependent changes, pore dilation and pore closing (desensitization), through a process that is incompletely understood. To address this issue and to clarify the roles of calcium and the C-terminal domain in gating, we combined biophysical and mathematical approaches using two splice forms of receptors: the full-size form (P2X2aR) and the shorter form missing 69 residues in the C-terminal domain (P2X2bR). Both receptors developed conductivity for N-methyl-D-glucamine within 2-6 s of ATP application. However, pore dilation was accompanied with a decrease rather than an increase in the total conductance, which temporally coincided with rapid and partial desensitization. During sustained agonist application, receptors continued to desensitize in calcium-independent and calcium-dependent modes. Calcium-independent desensitization was more pronounced in P2X2bR, and calcium-dependent desensitization was more pronounced in P2X2aR. In whole cell recording, we also observed use-dependent facilitation of desensitization of both receptors. Such behavior was accounted for by a 16-state Markov kinetic model describing ATP binding/unbinding and activation/desensitization. The model assumes that naive receptors open when two to three ATP molecules bind and undergo calcium-independent desensitization, causing a decrease in the total conductance, or pore dilation, causing a shift in the reversal potential. In calcium-containing media, receptor desensitization is facilitated and the use-dependent desensitization can be modeled by a calcium-dependent toggle switch. The experiments and the model together provide a rationale for the lack of sustained current growth in dilating P2X2Rs and show that receptors in the dilated state can also desensitize in the presence of calcium.
authors.affiliation: Laboratory of Biological Modeling, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Anmar
authors.initials: A
authors.lastname: Khadra
authors.forename: Zonghe
authors.initials: Z
authors.lastname: Yan
authors.forename: Claudio
authors.initials: C
authors.lastname: Coddou
authors.forename: Melanija
authors.initials: M
authors.lastname: Tomić
authors.forename: Arthur
authors.initials: A
authors.lastname: Sherman
authors.forename: Stanko S
authors.initials: SS
authors.lastname: Stojilkovic
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D004594:Electrophysiology; D057809:HEK293 Cells; D006801:Humans; D015640:Ion Channel Gating / Q000502:physiology*; D008390:Markov Chains; D008954:Models, Biological; D008962:Models, Theoretical; D020033:Protein Isoforms / Q000502:physiology*; D058476:Receptors, Purinergic P2X2 / Q000502:physiology*; D014162:Transfection
chemical_list: D020033:Protein Isoforms; D058476:Receptors, Purinergic P2X2; D000255:Adenosine Triphosphate