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This Article Abstract Full Text (PDF) All Versions of this Article: 554/2/245    most recent jphysiol.2003.057703v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kurachi, Y. Articles by North, A. Search for Related Content PubMed PubMed Citation Articles by Kurachi, Y. Articles by North, A. Related Collections Review articles

¹ Department of Pharmacology, Graduate School of Medicine, Osaka University, Osaka 565-0871, Japan² Institute of Molecular Physiology, University of Sheffield, Sheffield S10 2TN, UK

	Abstract

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In the past two decades, investigations on ion channels have made remarkable progress. Since the development of patch clamp recording techniques in 1980s, investigators have been able to detect the real-time behaviour of single channels on many kinds of cell membrane. Rapid advances in molecular biology techniques led to the cloning and identification of a variety of ion channel genes, many of which have been the targets of knock-outs or disruptions. Recent significant progress in structural biology has enabled us to grasp the molecular architecture of some ion channels, and their structure–function relationship. In this special issue, knowledge from these various approaches are summarized and presented by seven specialists in the respective fields. This discussion took place on 24 March, 2003, during the joint annual scientific sessions of the 80th Japanese Physiological and the 76th Pharmacological Society meetings in Fukuoka, Japan. The symposium was held just after the outbreak of hostilities in Iraq, and some of the speakers originally invited could not participate. Fortunately, two excellent substitute speakers took part (Drs Zamponi and Findlay on Ca²⁺ channels).

Structure–function relationship of ion channels

In this year 2003, the Nobel Prize in Chemistry is dedicated to the discoveries concerning the structure–function relationship of ion and water channels in cell membranes. One laureate, Roderick MacKinnon, has been vigorously unravelling the molecular structures of several ion channels and providing structural insights into their biophysical properties. In this symposium, two experts in the field of structural biology discuss the structure–function relationship of ion and water channels. Eric Gouaux described the structure of the extracellular ligand-binding domain of ionotropic glutamate receptors (iGluR), and gave an account of the regulatory mechanisms of iGluRs, including desensitization, on a structural basis (Gouaux, 2003). Yoshinori Fujiyoshi has developed a powerful method for investigating 2D-crystal structure. In collaboration with Peter Agre, the second Nobel laureate, he has for the first time revealed the crystal structure of a water channel, aquaporin-1, and elegantly showed the permeation mechanism of water molecules. Although he provided an excellent lecture in the symposium, he was unable to contribute a review article to this series but aspects of this work are described in Agre et al. (2003). Some ion channels can be activated by intracellular ligands such as Ca²⁺. Although John Adelman could not participate in the symposium, he has contributed an excellent review on his work on small conductance Ca²⁺-activated K⁺ (SK) channels to this series (Maylie et al. 2003). Unlike the classical Ca²⁺-activated big conductance (BK) channel, SK channels are activated by binding of Ca²⁺ to calmodulin (CaM) covalently coupled with the channels. Using biochemical, molecular, electrophysiological and structural biological techniques, Adelman and colleagues have revealed the activation mechanism of SK channels by Ca²⁺–CaM.

Controversies of Ca²⁺ channel inactivation

Voltage-dependent Ca²⁺ channels (VDCCs) are the major pathway of Ca²⁺ entry via excitable membranes. Upon depolarization, VDCCs open transiently, and this is followed by inactivation which shuts off voltage-dependent Ca²⁺ influx. Thus the inactivation properties of VDCCs are quite important for avoiding excess Ca²⁺ entry. In this issue, Gerald Zamponi and colleagues present a novel model for the molecular mechanism of voltage-dependent inactivation of VDCCs (Stotz et al. 2003). They suggest that the intracellular I–II linker region acts as a ‘hinged-lid’ and directly blocks the pore, in the same way as the ‘ball-and-chain’ model proposed earlier for voltage-dependent potassium channels. They also highlighted a possible involvement of the auxiliary ß-subunit in voltage-dependent inactivation gating.

VDCCs undergo Ca²⁺-dependent inactivation (CDI), as well as voltage-dependent inactivation (VDI). Which kind of inactivation mechanism is mainly used to shut off the channel under physiological conditions? Recently, Ian Findlay has answered this question (Findlay, 2003). In native cardiomyocytes, it had been thought that VDI was relatively slow and CDI played a major role. Findlay and colleagues found that depolarization increased the probability that an ion channel shows rapid VDI before CDI. He also shows that VDI is modulated by protein kinase A-mediated phosphorylation. The degree of VDI is attenuated and thus CDI plays a dominant role in the control of Ca²⁺ channel inactivation during ß-adrenergic stimulation. The contribution of VDI and CDI to the decay of VDCCs is thus determined by turning on (by depolarization) or turning off (by adrenergic stimuli) of rapid VDI. These findings are intriguing for understanding physiological functions of VDCCs in action potential formation.

Diversity of inwardly rectifying potassium channels

Kir channels do not exhibit voltage-dependent gating mechanisms, but outward K⁺ flow is blocked by intracellular non-organic or organic cations, such as Mg²⁺ or polyamines; this results in inward rectification. Kir channels are subject to various modulators, such as G-proteins or intracellular ATP.

The G-protein-gated inwardly rectifying K⁺ (K_G, GIRK, Kir 3.1/3.4)) channel, which is directly activated by G-protein ß subunits released from pertussis toxin-sensitive G-proteins, is responsible for acetylcholine (ACh)-induced slowing of the heart, and for neurotransmitter-evoked slow inhibitory postsynaptic potentials in many different neurones. The native cardiac K_G channel current has been known to exhibit a characteristic voltage-dependent behaviour, called ‘relaxation.’ Yoshihisa Kurachi reported a series of studies showing that this behaviour reflects the apparent voltage-dependent control of the trimeric G-protein cycle by a family of regulators of G-protein signalling (RGS) proteins (Kurachi & Ishii, 2003). Their findings indicate that the control may not be restricted to K_G channels, but may also affect other G-protein effects such as adenylyl cyclase and phospholipase C. Thus it may be of general importance in the control of G-protein-mediated signalling systems.

The ATP-sensitive inwardly rectifying K⁺ (K_ATP) channel, which is characterized by its inhibition by intracellular ATP, is distributed in various tissues, including pancreatic ß-cells, heart, skeletal muscle, vascular smooth muscle and brain. This channel couples the cell metabolic state to membrane excitability. The K_ATP channel is a hetero-octamer composed of four pore-forming Kir6.x subunits (Kir6.1 or Kir6.2) and four regulatory sulphonylurea receptor subunits (SUR1, 2A and 2B). Susumu Seino has shown, by generating mice with one or other gene ablated, the physiological relevance of K_ATP channels containing Kir6.1 or Kir6.2 in various tissues (Seino & Miki, 2003). He reported that Kir6.2-containing K_ATP channels play a crucial role in the control of glucose metabolism, not only in pancreatic insulin secretion but also in glucose uptake by skeletal muscle, and in the control of glucagon secretion by the central nervous system. He and colleagues also showed that Kir6.2-containing K_ATP channels play crucial roles in protecting various organs including heart and brain again ischaemia. It is also noteworthy that their Kir6.1-knock-out mouse shows cardiac sudden death due to vasospasm, which confirms and establishes the importance of Kir6.1-containing K_ATP channels in the regulation of vascular tone, as proposed previously.

P2X receptor ion channels – their roles in nociception

ATP released from injured tissues is known to induce pain. The target of this ATP action is the P2X receptor, which constitutes a cation channel; activation by ATP causes a depolarization of peripheral afferent neurones. Alan North and colleagues have investigated the regulatory mechanisms of P2X receptor ion channels, since isolating the seven subunit cDNA clones (North, 2003). They are especially interested in channels consisting of heteromultimers of P2X₂ and P2X₃ subunits, for which studies in P2X₃-selective antagonists, antisense oligonucleotides and gene knock-outs support a role in neuropathic pain. By extensive use of the substituted cysteine accessibility method, they have deduced regions of the molecule that contribute to ion permeation and ligand binding. Experiments with intersubunit engineered disulphides indicate the arrangement of subunits in the heteromultimer channel.

	Conclusions

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This series of reviews on ion and water channels has concisely described current knowledge of their properties from atomic structure to function. Ion and water channels are mandatory for both homeostasis and the dynamic activity of essentially all types of cells. Progress in basic research at various levels on these membrane proteins will facilitate our understanding of the beauty of life and will enable us to develop new ways to control cell functions to treat various diseases.

	References

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Agre P, King LS, Yasui M, Guggino WB, Ottersen OP, Fujiyoshi Y, Engel A & Nielsen S (2003). Aquaporin water channels – from atomic structure to clinical medicine. J Physiol 542, 3–16.[CrossRef]

Findlay I (2003). Physiological modulation of inactivation in L-type Ca²⁺ channels: one switch.J Physiol 554, 275–283.[CrossRef][Medline]

Gouaux E (2003). Structure and function of AMPA receptors. J Physiol 554, 249–243.[CrossRef][Medline]

Kurachi Y & Ishii M (2003). Cell signal control of the G protein-gated potassium channel and its subcellular localization. J Physiol 554, 245–247[CrossRef]

Maylie J, Bond CT, Herson PS, Lee W-S & Adelman JP (2003). Small conductance Ca²⁺-activated K⁺ channels and calmodulin. J Physiol 554, 255–261.[CrossRef][Medline]

North RA (2003). P2X3 receptors and peripheral pain mechanisms. J Physiol 554, 301–308.[CrossRef][Medline]

Seino S & Miki T (2003). Gene targeting approach to clarification of ion channel function: studies of Kir6.x null mice. J Physiol 554, 295–300.[CrossRef][Medline]

Stotz SC, Jarvis SE & Zamponi GW (2003). Functional roles of cytoplasmic loops and pore lining transmembrane helices in the voltage-dependent inactivation of HVA calcium channels. J Physiol 554, 263–273.[CrossRef][Medline]

This Article Abstract Full Text (PDF) All Versions of this Article: 554/2/245    most recent jphysiol.2003.057703v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kurachi, Y. Articles by North, A. Search for Related Content PubMed PubMed Citation Articles by Kurachi, Y. Articles by North, A. Related Collections Review articles