Spontaneous Channel Activity of the Inositol 1,4,5-Trisphosphate (InsP3) Receptor (InsP3R). Application of Allosteric Modeling to Calcium and InsP3 Regulation of InsP3R Single-channel Gating

The InsP3R Ca2+ release channel has a biphasic dependence on cytoplasmic free Ca2+ concentration ([Ca2+]i). InsP3 activates gating primarily by reducing the sensitivity of the channel to inhibition by high [Ca2+]i. To determine if relieving Ca2+ inhibition is sufficient for channel activation, we examined single-channel activities in low [Ca2+]i in the absence of InsP3, by patch clamping isolated Xenopus oocyte nuclei. For both endogenous Xenopus type 1 and recombinant rat type 3 InsP3R channels, spontaneous InsP3-independent channel activities with low open probability P o (∼0.03) were observed in [Ca2+]i < 5 nM with the same frequency as in the presence of InsP3, whereas no activities were observed in 25 nM Ca2+. These results establish the half-maximal inhibitory [Ca2+]i of the channel to be 1.2–4.0 nM in the absence of InsP3, and demonstrate that the channel can be active when all of its ligand-binding sites (including InsP3) are unoccupied. In the simplest allosteric model that fits all observations in nuclear patch-clamp studies of [Ca2+]i and InsP3 regulation of steady-state channel gating behavior of types 1 and 3 InsP3R isoforms, including spontaneous InsP3-independent channel activities, the tetrameric channel can adopt six different conformations, the equilibria among which are controlled by two inhibitory and one activating Ca2+-binding and one InsP3-binding sites in a manner outlined in the Monod-Wyman-Changeux model. InsP3 binding activates gating by affecting the Ca2+ affinities of the high-affinity inhibitory sites in different conformations, transforming it into an activating site. Ca2+ inhibition of InsP3-liganded channels is mediated by an InsP3-independent low-affinity inhibitory site. The model also suggests that besides the ligand-regulated gating mechanism, the channel has a ligand-independent gating mechanism responsible for maximum channel P o being less than unity. The validity of this model was established by its successful quantitative prediction of channel behavior after it had been exposed to ultra-low bath [Ca2+].

[1]  M. Jackson Spontaneous openings of the acetylcholine receptor channel. , 1984, Proceedings of the National Academy of Sciences of the United States of America.

[2]  S. Siegelbaum,et al.  Constraining Ligand-Binding Site Stoichiometry Suggests that a Cyclic Nucleotide–Gated Channel Is Composed of Two Functional Dimers , 1998, Neuron.

[3]  A. M. Riley,et al.  Disaccharide polyphosphates based upon adenophostin A activate hepatic D-myo-inositol 1,4,5-trisphosphate receptors. , 1997, Biochemistry.

[4]  J. Watras,et al.  Inositol 1,4,5-Trisphosphate (InsP3) and Calcium Interact to Increase the Dynamic Range of InsP3 Receptor-dependent Calcium Signaling , 1997, The Journal of general physiology.

[5]  Colin W. Taylor,et al.  Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity , 1997, Current Biology.

[6]  D. Bers,et al.  How to make and use calcium-specific mini- and microelectrodes. , 1994, Methods in cell biology.

[7]  L. Missiaen,et al.  Molecular and Functional Evidence for Multiple Ca2+-binding Domains in the Type 1 Inositol 1,4,5-Trisphosphate Receptor* , 1997, The Journal of Biological Chemistry.

[8]  S Swillens,et al.  Transient inositol 1,4,5-trisphosphate-induced Ca2+ release: a model based on regulatory Ca(2+)-binding sites along the permeation pathway. , 1994, Proceedings of the National Academy of Sciences of the United States of America.

[9]  I. Moraru,et al.  Regulation of Type 1 Inositol 1,4,5-Trisphosphate–gated Calcium Channels by InsP3 and Calcium , 1999, The Journal of general physiology.

[10]  D. Mak,et al.  Single-channel recordings of recombinant inositol trisphosphate receptors in mammalian nuclear envelope. , 2001, Biophysical journal.

[11]  R. Nuccitelli A practical guide to the study of calcium in living cells , 1994 .

[12]  S. W. Jones,et al.  Commentary: a plausible model. , 1999, The Journal of general physiology.

[13]  J. Keizer,et al.  A single-pool inositol 1,4,5-trisphosphate-receptor-based model for agonist-stimulated oscillations in Ca2+ concentration. , 1992, Proceedings of the National Academy of Sciences of the United States of America.

[14]  James Watras,et al.  Bell-shaped calcium-response curves of lns(l,4,5)P3- and calcium-gated channels from endoplasmic reticulum of cerebellum , 1991, Nature.

[15]  L. Stryer,et al.  Calcium spiking. , 1991, Annual review of biophysics and biophysical chemistry.

[16]  D. Mak,et al.  Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor. , 1998, The American journal of physiology.

[17]  M. Iino,et al.  Functional and biochemical analysis of the type 1 inositol (1,4,5)-trisphosphate receptor calcium sensor. , 2003, Biophysical journal.

[18]  L. Stryer,et al.  Highly cooperative opening of calcium channels by inositol 1,4,5-trisphosphate. , 1988, Science.

[19]  J. Changeux,et al.  ON THE NATURE OF ALLOSTERIC TRANSITIONS: A PLAUSIBLE MODEL. , 1965, Journal of molecular biology.

[20]  K. Mikoshiba,et al.  Inositol trisphosphate receptor and Ca2+ signalling. , 1993, Philosophical transactions of the Royal Society of London. Series B, Biological sciences.

[21]  J. Foskett,et al.  Novel Regulation of Calcium Inhibition of the Inositol 1,4,5-trisphosphate Receptor Calcium-release Channel , 2003, The Journal of general physiology.

[22]  Sreenivas Devidas,et al.  Cystic Fibrosis Transmembrane Conductance Regulator–associated ATP Release Is Controlled by a Chloride Sensor , 1998, The Journal of cell biology.

[23]  N. Vardi,et al.  Identification of a family of calcium sensors as protein ligands of inositol trisphosphate receptor Ca2+ release channels , 2002, Proceedings of the National Academy of Sciences of the United States of America.

[24]  J. Foskett,et al.  Atp-Dependent Adenophostin Activation of Inositol 1,4,5-Trisphosphate Receptor Channel Gating , 2001, The Journal of general physiology.

[25]  S. Swillens,et al.  Stochastic simulation of a single inositol 1,4,5-trisphosphate-sensitive Ca2+ channel reveals repetitive openings during 'blip-like' Ca2+ transients. , 1998, Cell calcium.

[26]  K. Mikoshiba,et al.  Mutational Analysis of the Ligand Binding Site of the Inositol 1,4,5-Trisphosphate Receptor* , 1996, The Journal of Biological Chemistry.

[27]  S. Snyder,et al.  Characterization of inositol trisphosphate receptor binding in brain. Regulation by pH and calcium. , 1987, The Journal of biological chemistry.

[28]  M. Iino,et al.  Biphasic Ca2+ dependence of inositol 1,4,5-trisphosphate-induced Ca release in smooth muscle cells of the guinea pig taenia caeci , 1990, The Journal of general physiology.

[29]  P. Cullen,et al.  Heparin inhibits the inositol 1,4,5‐trisphosphate‐induced Ca2+ release from rat liver microsomes , 1988, FEBS letters.

[30]  K. Magleby Kinetic Gating Mechanisms for Bk Channels , 2001, The Journal of general physiology.

[31]  C. Lin,et al.  Heteroligomers of Type-I and Type-III Inositol Trisphosphate Receptors in WB Rat Liver Epithelial Cells (*) , 1995, The Journal of Biological Chemistry.

[32]  L. Missiaen,et al.  Characterization of a Cytosolic and a Luminal Ca2+ Binding Site in the Type I Inositol 1,4,5-Trisphosphate Receptor* , 1996, The Journal of Biological Chemistry.

[33]  E. Toescu Temporal and spatial heterogeneities of Ca2+ signaling: mechanisms and physiological roles. , 1995, The American journal of physiology.

[34]  Don-On Daniel Mak,et al.  ATP Regulation of Type 1 Inositol 1,4,5-Trisphosphate Receptor Channel Gating by Allosteric Tuning of Ca2+ Activation* , 1999, The Journal of Biological Chemistry.

[35]  Don-On Daniel Mak,et al.  Inositol 1,4,5-tris-phosphate activation of inositol tris-phosphate receptor Ca2+ channel by ligand tuning of Ca2+ inhibition , 1998 .

[36]  Don-On Daniel Mak,et al.  Single-Channel Kinetics, Inactivation, and Spatial Distribution of Inositol Trisphosphate (IP3) Receptors in Xenopus Oocyte Nucleus , 1997, The Journal of general physiology.

[37]  J. Foskett,et al.  Regulation by Ca2+ and Inositol 1,4,5-Trisphosphate (Insp3) of Single Recombinant Type 3 Insp3 Receptor Channels , 2001, The Journal of general physiology.

[38]  M. Berridge Inositol trisphosphate and calcium signalling , 1993, Nature.

[39]  S. Takahashi,et al.  Adenophostins, newly discovered metabolites of Penicillium brevicompactum, act as potent agonists of the inositol 1,4,5-trisphosphate receptor. , 1994, The Journal of biological chemistry.

[40]  S. Caenepeel,et al.  Single-channel function of recombinant type 2 inositol 1,4, 5-trisphosphate receptor. , 2000, Biophysical journal.

[41]  J. I. Korenbrot,et al.  Spontaneous, ligand‐independent activity of the cGMP‐gated ion channels in cone photoreceptors of fish. , 1995, The Journal of physiology.

[42]  J. Foskett,et al.  Atp Regulation of Recombinant Type 3 Inositol 1,4,5-Trisphosphate Receptor Gating , 2001, The Journal of general physiology.

[43]  S. Gordon,et al.  Cooperativity and cooperation in cyclic nucleotide-gated ion channels. , 2000, Biochemistry.

[44]  S. Patel,et al.  Molecular properties of inositol 1,4,5-trisphosphate receptors. , 1999, Cell calcium.

[45]  Don-On Daniel Mak,et al.  Single-Channel Properties in Endoplasmic Reticulum Membrane of Recombinant Type 3 Inositol Trisphosphate Receptor , 2000, The Journal of general physiology.

[46]  J. Changeux,et al.  Allosteric receptors after 30 years , 1998, Neuron.

[47]  D. Mak,et al.  Single-channel inositol 1,4,5-trisphosphate receptor currents revealed by patch clamp of isolated Xenopus oocyte nuclei. , 1994, The Journal of biological chemistry.

[48]  C. Adkins,et al.  Lateral inhibition of inositol 1,4,5-trisphosphate receptors by cytosolic Ca2+ , 1999, Current Biology.