Ion ChannelsDiane Lipscombe, Brown University, Providence, Rhode Island, USA

Ion channels are membrane proteins that allow ions to cross cell membranes with high

speed. All cells of all organisms use ion channels to control many cellular functions.

What are Ion Channels and why do Cellsneed them?

Ion channels aremembrane proteins that support the rapidflow of ions across cell membranes (Figure 1). The nervoussystem offers the richest source of ion channels since theyalone are responsible for generating and propagating elec-trical signals. However, all cells of all organisms, multicel-lular to unicellular, and from plants to animals, use ionchannels to control a spectrum of essential cellular func-tions. See also: Membrane dynamics

Ions channels are found in every cell

. Ion channels receive information and communicate thisfrom one part of the cell to the other. Ion channels un-derlie the action potential that carries electrical signalswithin neurons over a distance of up to 1m or evengreater Ion channels also receive and relay informationbetween cells. They convert sensory stimuli such as heat,cold, light, sound, smell and touch into electrical signals.Ion channels form electrical junctions between mem-branes of neighbouring cells and regulate neurotrans-mitter and hormone release from neurons and secretorycells for cell–cell communication;

. Special ‘pace-making’ ion channels establish intrinsicrhythmic activity in electrically excitable cells. Pacemaking channels underlie spontaneous depolarizationsthat drive the heart beat, and rhythmic electrical activityin neuronal networks that control circadian rhythm(wake/sleep cycle) and cyclical hormone release.

. The activity of ion channels in T lymphocytes, mastcells and other cells of the immune system is essential formediating cellular responses against foreign pathogens.

. Ion channels are also present in membranes of intracel-lular organelles such as the endoplasmic reticulum, mi-tochondria and the nucleus. Ion channels coupleneuronal excitation to muscle contraction via the re-lease of calcium from the endoplasmic reticulum of skel-etal muscle (sacroplasmic reticulum). In mitochondria,adenosine triphosphate (ATP)-sensitive ion channels areimportant for volume regulation and, in the nuclearmembrane, ion channels regulate the local release ofcalcium that controls gene expression by modifying theactivity of transcription factors.

A large number of congenital disorders originate frommutations in genes that encode ion channels. Examples ofaffected ion channels and their associated disorders are inTable 1. See also: Action potential: generation andpropagation; Action potential: ionic mechanisms; Cellstructure; Circadian rhythms; Muscle contraction regula-tion; Muscle contraction mechanisms: use of synchrotronX-ray diffraction; Nervous control of movement; Neuro-transmitters; Sex hormones in vertebrates.Ion channels are also important targets of drugs and

general anaesthetics. Drugs used to treat high blood pres-sure, cardiac arrhythmias, epilepsy, pain, anxiety and typeII diabetes act on ion channels.

Article Contents

Introductory article

. What are Ion Channels and why do Cells need them?

. What Forces Act on an Ion as it Flows through an Ion

Channel?

. Visualizing the Current through a Single Ion Channel:

A Protein in Motion

. HowMany Different Kinds of Ion Channels are there?

. Ions Move through the Pores of Ion Channels at High

Rates

. The Molecular Basis of Selective Ion Permeation and

Rapid Ion Flow

. Many Stimuli Can Open Ion Channels

. The Molecular Basis of Gating

. Ion Channel Behaviour Can be Modulated

. Summary

doi: 10.1038/npg.els.0004070

Figure 1 View of an ion channel within the cell membrane. The lipidbilayer is shown surrounding the ion channel protein complex. The

location of the ion pore that spans the membrane is indicated.

1ENCYCLOPEDIA OF LIFE SCIENCES & 2005, John Wiley & Sons, Ltd. www.els.net

Which ions flow through ion channels?

Potassium (K+), sodium (Na+), chloride (Cl2) and cal-cium (Ca2+) are themajor inorganic ions that flow throughion channels. Hydrogen ions (‘protons’) (H+) also flowthrough some ion channels but to a more limited extent.Magnesium ions (Mg2+) are a special case. They can enterthe pores of several types of ion channels but in partbecause of relatively strong interactions with water, Mg2+

ions do not permeate well through most ion pores. Mg2+

ions play a critical role in modulating the flow of otherions through special ion channels including those involvedin memory formation. See also: Ion motive ATPases:V- and P-type ATPases; Sodium, calcium and potassiumchannels

What happens when ions flow across cellmembranes?

When an ion crosses a cell membrane unaccompanied byits counterion (e.g. Na+ without Cl2), a current is gener-ated. This current or charge movement in turn creates avoltage difference across the cell membrane of amagnitudethat is directly proportional to the rate of ionflow.Theflowof positive ions into (or negative ions out of) a cell depo-larizes the cell membrane. The flow of positive ions out of(or negative ions into) a cell hyperpolarizes the cell mem-brane. It is depolarization and hyperpolarization of the cellmembrane, triggered by the opening and closing of selec-tively permeable ion channels, that forms the basis of elec-trical signalling. See also: Membrane potential

Table 1 A selected list of congenital diseases and disorders linked to mutations in genes that encode ion channels that afflicthumans

Disease/symptoms Ion channel Location

Congenital myasthenic syndrome Nicotinic acetylcholine receptor Skeletal muscleStartle disease (hyperexplexia) Glycine receptor Central nervous systemTotal colour blindness Cyclic GMP-gated cation channel

(CNGA3)Retinal cone cells of the eye

Malignant hypothermia and central coredisease

Ryanodine calcium release channel(RYR1)

Sarcoplasmic reticulum ofskeletal muscle

Congenital kidney stones Cl2 channel (CLCN5) KidneyCystic fibrosis Cl2 channel (CFTR) Exocrine glands, including

lungs and pancreasThomsen disease (myotonia congenita) Cl2 channel (CIC-1) Skeletal muscleTimthly syndrome / voltage-gated ca2+

channel (Cav1.2/ Multiple organsHyperkalaemic periodic paralysis(episodic muscle paralysis)

Na+ channel Skeletal muscle

Congenital bilateral deafness (Jervell andLarge-Nielsen syndrome)

Voltage-gated K+ channel (KVLQT1) Inner ear

Congenital arrythmias or long QTsyndromes including Brugada(prolonged cardiac action potentials)

Voltage-gated K+ channel (KVLQT1,minK, HERG), Voltage-gated Na+

channel (SCN5A)

Heart

Stationary night blindness Voltage-gated Ca2+ channel (CaV1.4) RetinaIdiopathic epilepsies Voltgate-gated Na+ channel

(SCN1A, 2A, 1B)Central nervous system

K channels (KCNA1, Q2, Q3)Cl channel (CLCN2)GABA receptor (A1, G2)Neuronal nicotinic receptor

Timothy syndrome Voltage-gated Ca2+

channel (Cav1.2) Multiple organsHypokalaemic periodic paralysis

Voltage-gated Ca2+ channel (CaV1.1) Skeletal muscle

Familial hemiplegic migraine Voltage-gated Ca2+ channel (CaV2.1) Central nervous systemEpisodic ataxia type-2, spinocerebellarataxia 6

Voltage-gated Ca2+ channel (CaV2.1) Central nervous system

Lambert–Eaton myasthenic syndrome Voltage-gated Ca2+ channel (CaV2.1) Central nervous system

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Why are ion channels needed?

Cell membranes create a very hydrophobic (‘water-hat-ing’) impermeable barrier to all chargedmolecules nomat-ter how small. There is little incentive for inorganic ionslike Na+ to leave the aqueous environment inside (intra-cellular) or outside (extracellular) the cell, where they hap-pily exist surrounded by water molecules (hydrated), toenter the energetically unfavourable ‘oily’ environment ofthe cell membrane. Ion channels ‘lure’ ions across cellmembranes and pose as aqueous holes. Two importantproperties of ion channels, however, set them apart fromsimple water-filled holes in cell membranes: (1) the poresof most ion channels discriminate remarkably well amongdifferent ions (selectivity); and (2) pore opening and closing(gating) is a regulated process in most ion channels.See also: Membrane dynamics; Membrane proteins;Plasma membranes: methods for preparation

What Forces Act on an Ion as it Flowsthrough an Ion Channel?

Ion channels provide aqueous pores through which ionspassively diffuse across cell membranes. The intrinsic ge-ometry and chemical environment of the channel pore setlimits on the rate at which ions can pass. The amount ofintrinsic resistance that a pore imposes on ion flow varieswith the ion that passes and the direction inwhich itmoves.Two extrinsic forces act on the ion independent of thechannel through which it passes. One force depends on thetransmembrane concentration gradient of the permeantion and the other on the transmembrane electrical gradi-ent. The balance between these extrinsic forces (electro-chemical gradient) determines the direction and influencesthe rate of ion movement through a given ion channel.See also: Cell membranes: intracellular pH and electro-chemical potential; Ion transport across nonexcitablemembranes

Concentration gradients exist across the plasma mem-branes of all cells for each of the major inorganic ions(Table 2). In the absence of an electrical gradient, ions dif-fuse down their concentration gradient. When K+-perme-able ion channels open in a cell membrane, K+ ions, whichare 20-fold more concentrated inside the cell, immediatelyleave and in doing so make the inside of the cell morenegative with respect to the outside. The resulting excessnegative charge inside the cell slows the exit of K+ ions dueto electrostatic attraction (opposite charges attract). If thecell is only permeable toK+ ions, the finalmagnitude of theelectrical gradient or voltage difference that developsacross the membrane depends on the size of the K+ ionconcentration gradient. This voltage difference across thecell membrane is stable as long as the K+ channels remainopen and no other ion moves across the membrane.See also: Voltage-gated potassium channelsIn the general case, the final membrane potential that

develops across a membrane that is permeable to a singleion species is the equilibrium (Eion) or Nernst potential forthe ion and is dependent on the concentration gradient forthat ion. Equilibrium potentials for the major ions are inTable 2. For example, a cell that is only permeable to K+

ions will have a resting membrane potential equal to EK,about 280mV, whereas the membrane potential of a cellthat is only permeable to Na+ ions will be about+60mV .See also: Sodium channels

K+ selective ion channels underlie the restingmembrane potential of the cell

Themembranes ofmost cells, particularly neurons, containa variety of different ion channels that open in response tounique stimuli. Some ion channels are selectively permeableto one particular ion species (e.g. most voltage-gated ionchannels), while others allow the passage of only cations oronly anions (e.g. most ligand-gated ion channels). The rest-ingmembrane potential of most cells is close toEK, usuallybetween 240mV and 270 mV. This is because most cellmembranes containK+ selective ion channels that are open

Table 2 Approximate concentration gradients for ions in amammalian cell together with the calculated equilibrium potentials

Ion Inside (mmol L21

) Outside (mmol L21) Equilibrium potential

a(mV)

K+ 100 5 280Cl2 13b 150 265Na+ 15 150 +62Ca2+ 0.0002c 2 +123

aEion=(RT/zF) ln ([ion]out/([ion]in), where R, T, z and F are the gas constant (8.315 J mol21K21), temperature (K), ion valency and theFaraday constant (9.648� 1048Cmol21), respectively. The equilibrium potentials given are those predicted at body temperature of 378Cor310K.

bThebulk of intracellular and extracellular solutions are electrically neutral.Membrane-impermeable proteinsmake up the bulk of negativelycharged molecules inside cells.

cThe value given is the approximate concentration of free intracellular calcium. The total concentration of intracellular calcium is in the1–2mmolL21 range but it is bound to Ca2+ buffers and contained in intracellular calcium storage organelles.

Ion Channels

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in the absence of any stimulus making the membranerelatively permeable to K+ ions. Specific classes of K+

channels (one and two pore-domain structures see Figure 6)are the main contributors to the resting membrane poten-tial. Their activities are under the control of many differentagents including oxygen, pH, neurotransmitters, secondmessengers, temperature and stretch. See also: Ion chan-nels: ligand gated

Ion channels underlie the action potential

In addition to setting the resting membrane potential, ionchannels underlie rapid changes in themembrane potentialin response to various stimuli. The best-understood exam-ple of ion channel-mediated electrical signalling is the gen-eration of the action potential. In 1952, SirAndrewHuxleyand Sir Alan Hodgkin determined the ionic basis of theaction potential in their classic studies of the squid giantaxon. This work earned them the Nobel Prize in Physiol-ogy or Medicine in 1963, which they shared with Sir JohnEccles. Hodgkin and Huxley used voltage-clamp tech-niques to directly measure changes in the conductance orrelative permeability of the membrane of the squid axon toNa+ and potassium K+ ions (Figure 2). They deduced thatthe action potential, which is used by all nervous systemsfrom molluscs to man for rapid information transfer, wasgenerated by the precise, regulated opening and closing ofNa+ andK+ ion selective pores in the cell membrane of theaxon. During the rising phase of an action potential (rapidmembrane depolarization), the permeability of the axonalmembrane greatly increases forNa+ as a result of the rapid

opening of a large number of Na+ selective voltage-gatedion channels. At the peak of the action potential, theaxonal membrane potential approaches ENa (Table 2). Thecombination of Na+ channel inactivation and opening ofK+ selective voltage-gated channels triggers the fallingphase of the action potential (membrane hyperpolariza-tion). See also: Action potential: ionic mechanisms;Sodium, calcium and potassium channels

Visualizing the Current through a SingleIon Channel: A Protein in Motion

The studies ofHodgkin andHuxley on squid axonwere thefirst of a series of remarkable biophysical experiments thatformed the cornerstone of our understanding of ion chan-nel function. However, it was not until 1978, when ErwinNeher and Bert Sakmann developed a new voltage-clamprecording method called the patch clamp technique thatinvestigators had their first view of currents through singleion channels in real time (Figure 3). All previous recordingmethods measured the average current of ion channels in alimited number of cells. The patch clamp recording tech-nique relies upon electrically isolating a small patch of cellmembrane with a glass pipette and uses this to monitor thecurrent that flows through ion channels present in thepatch of membrane. In 1981, Neher, Sakmann and theircolleagues showed that the suction could improve the elec-trical seal between the glass patch pipette and the mem-brane surface, allowing the resolution of yet smallercurrents (� 0.5 pA). Single ion channels were seen to openand close on themicrosecond time scale and,when open, topass currents in the range of 0.5–10 pA (10212A), equiv-

alent to flow rates of 0.3� 107– 6� 107 monova-lent ions s21 (Figure 3). Ion channels opened to discreteconducting states and single channel current amplitudes, atfixed voltages, were relatively constant. The patch clamprecording method is also a powerful method for monitor-ing the overall activity of ion channels in very small cells,including neurons from the mammalian and human brainthat are otherwise inaccessible to electrophysiological re-cording methods. In 1991, Neher and Sakmann wereawarded the Nobel Prize in Physiology or Medicine fortheir discoveries concerning The Function of Single IonChannels in Cells.

How Many Different Kinds of IonChannels are there?

The human genome contains several hundred genes thatencode ion channels but each genemay generate thousandsof different protein products and each ion channel maycontain a combination of different proteins. There are too

Figure 2 The approximate time course and magnitude of the change in

membrane potential in an excitable cell during an action potential. Thevalue of themembrane potential is shownon the y-axis and the time on the

x-axis. During the actionpotential the value of the cellmembrane potentialrapidly sweeps from270 to +40mV and then returns to270mV in about

1ms. The relative permeabilities of the cell membrane to potassium (PK)and sodium (PNa) are indicated at the start, peak and end of the action

potential.

Ion Channels

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many different ion channels in humans to count. However,most ion channels fall into three basic structural motifsdefined by the number of like domains that associate toform the central ion pore (Figure 4).

Each domain is either a separate, independently synthe-sized protein (or subunit) or, in the case of voltage-gatedNa+ and Ca2+ channels, repeats of structurally similardomainswithin a protein. These domains or subunits cometogether in different multiples to create the complete ionchannel complex that has the ion pore at its centre. Forexample, four (tetramer), five (pentamer) and six (hex-amer) domain ion channels have been described (Figure 4).Within each domain, the protein snakes back and forthacross the membrane between 2 and 8 times. There are

regions in each domain that define the gate of the ionchannel and other regions that contribute to the ion pore.Each domainmay contain one- or two-pore forming struc-tures (Figure 5).Ion channels that form a tetrameric arrangement include

voltage-gated Na+, K+ and Ca2+ channels and inwardlyrectifying K+ channels. These channels have highly dis-criminating ion pores. High-resolution X-ray crystallo-graphy has confirmed the tetrameric structure of some K+

ion channels. Ionotropic glutamate receptors and severalsecond messenger-operated ion channels, including the cy-clic nucleotide-gated channels, are tetramers. These cation-permeable channels are, in general, less discriminating thanvoltage-gated ion channels. See also: AMPA receptors;Calcium channel diversity; Cyclic nucleotide-gated ionchannels; Olfactory receptor neurons; Sodium channelsPentameric ion channels are composed of a rosette of

five subunits and include some neurotransmitter receptor-operated channels. The nicotinic acetylcholine and 5-hy-droxytryptamine receptor type-3 channels contain cation-permeable pores, whereas glycine and g-aminobutyric acid(GABA) receptor ion channels are anion permeable. Crys-tallization of amollusc-derived acetylcholine-binding pep-tide with homology to the ligand-binding site of thenicotinic receptor supports pentameric assembly. See also:GABAA receptors; Glycine receptors; Neurotransmitters;Nicotinic acetylcholine receptors; Nicotinic acetylcholinereceptors in muscle; Nicotinic acetylcholine receptors inneurons

Tetrameric Pentameric Hexameric

Figure 4 Views of the three major classes of ion channels; tetrameric,pentameric and hexameric. In each case, the pore of the channel is located

through the centre of theprotein complex.Within each subunit theproteincrosses the membrane 2–6 times forming transmembrane spanning

regions.

Figure 3 Single-channel recording using the patch clamp technique. Examples of currents recorded from single voltage-gated calcium channels in a

neuronal cell membrane are shown. The positions of closed [C] and open [O] levels are shown. The probability that a channel will be open increases as themembrane potential is depolarized because the channel is voltage-gated. The amount of current that flows through the channel is the same at a given

membranepotential but decreases as themembranepotential is depolarizedbecause the driving force on the iondecreases as it approaches its equilibriumpotential.

Ion Channels

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Gap junction or connexin ion channels have a do-decameric structure formed by the end-to-end associationof two hexamers (six domains) arranged around a centralpore. These channels mediate cell–cell interactions and thepores of some of these channels allow the passage of notonly small inorganic ions but also relatively large mole-cules including certain second messengers. See also:GABAA receptors;Glycine receptors;Nicotinic acetylcho-line receptors; Nicotinic acetylcholine receptors in muscle;Nicotinic acetylcholine receptors in neurons

Adiverse numberof naming schemes have appearedwiththe discovery of new ion channels.Names have reflected theion that passes through the pore (e.g. Na+, Cl2, and K+);the stimulus that triggers pore opening (e.g. voltage, ligand

and acid); the function that the channel performs (e.g.gap-junction), and the pharmacological characteristics ofthe ion channel (e.g. N-methyl-D-aspartate (NMDA),ryanodine and nicotine). Nomenclatures based on genesequence are more informative and logical. Within a genefamily, numerical labels reflect sequence similarities. Forexample, CaV1.1, CaV1.2 and CaV3.2 are different genesthat encode four-domain subunits of threedifferent typesofvoltage-gated Ca2+ channels. The numbering system indi-cates that CaV1.1 and CaV1.2 genes are more homologousas compared to CaV3.2.

Ions Move through the Pores of IonChannels at High Rates

The rate of flow of ions through most ion channels exceeds106 ions s21, and is so fast through some ion channels (>107

ions s21) that it approaches diffusion-limited rates of ionmovement. It is as though the permeant ion is barely awareof the pore through which it moves. Ion channels achievethis high throughput by creating a chemical and physicalenvironment thatmost closely parallels aqueous solution tothe ion. Just as important as speed, however, is the ability ofion channels to discriminate among different ions. For ex-ample, a class of large conductance K+ channels supportsion flow rates in excess of 3� 107 ions s21, while excludingNa+ ions from their channel pores. The channel does thiswith extraordinary efficiency, allowing the passage of onlyone Na+ ion for approximately 1000K+ ions.

The Molecular Basis of Selective IonPermeation and Rapid Ion Flow

In 1998,RoderickMacKinnonand colleagues reported thefirst high-resolution crystal structure of a K+-selective ionchannel, offering unparalleled insights into the molecularstructures that underlie both ion selectivity and rapid ionflow. MacKinnon used bacterial-derived potassium chan-nels to produce sufficient quantities of protein to makewell-ordered crystals forX-ray analysis. In recognition ‘forstructural and mechanistic studies of ion channels’, Rode-rick MacKinnon received the 2003 Nobel Prize in Chem-istry. The presence of ion-binding sites within the channelpore was the basis of ion selectively, supporting theoriesdeveloped by several biophysicists who studied voltage-gated Ca2+ and K+ ion channels. The important featuresof the ion channel pore derived from the crystal structureinclude the following features (Figure 5)

. The presence of water-filled vestibules at the outer andinnermouths of the ionpore that are linedprimarilywithhydrophobic amino acids that limit ion contact with thepore and promote rapid ion flow. Rings of negative

Figure 5 Inside the pore of a K+-selective ion channel. This diagram is

based on the reported crystal structure of a K+-selective ion channel(MacKinnon et al.). The two balls locatedwithin the narrow selectivity filter

region of the pore represent two dehydrated K+ions. The inner and outervestibules of the pore are filled with water (1 A50.1nm).

Ion Channels

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countercharges are also present in the vestibules thatserve to ‘lure’ positively charged K+ ions into the pore.

. Aconstriction in the channel to only 3 A in diameter and12 A in length that connects the inner and outer vesti-bules (1 A5 0.1 nm). This narrowest part of the pore islong enough to accommodate two dehydrated K+ ionsin single file. Carbonyl oxygen atoms, from the peptidebackbone, substitute for water and perfectly coordinatethe dehydrated K+ ions. The geometric constraints onthe carbonyl oxygen atoms are perfect for coordinatingK+ ions (2.66 A diameter) but not other ions, includingNa+ ions that are smaller (1.90 A). This is the molecularbasis of ion selectivity.

. The twoK+ ions in the selectivity filter region of the poreare close enough that there is substantial electrostaticrepulsion between the ions sufficient to overwhelm theirdesire to remain in the pore. Electrostatic repulsion pro-motes rapid ion flow.

In most ion channels, such as voltage-gated cation chan-nels, the ion conducting pathways and gating machineryare structurally distant. The CLC chloride ion channel isdifferent. There is a functional link between the gate andthe ion pore of this anion channel. The chloride ion thatpasses through the pore influences the opening and closingof the channel. See also: Chloride channels; Macromole-cular structure determined by X-ray crystallography;Voltage-gated potassium channels

Many Stimuli Can Open Ion Channels

Ion channel opening and closing is under tight cellularcontrol. One or more number of stimuli can trigger thegating of specific classes of ion channels and include thefollowing:

. Achange in themembrane voltage. For example, voltage-gated Na+, Ca2+ and K+ channels that open when themembrane depolarizes as well as certain cation channelsthat open when the membrane hyperpolarizes. Hyper-polarization-activated cation channels are present inrhythmically active cells in brain and heart, where theysupport repetitive firing.

. The binding of a neurotransmitter. For example, nicotinicacetylcholine receptor cation channels, ionotropic gluta-mate receptor cation channels and glycine receptor an-ion channels. Ligand-gated ion channels onpostsynaptic membranes respond to transmitter re-leased from presynaptic nerve terminals.

. A change in pH levels. For example, acid-sensitive ionchannels (ASIC) in neurons of the heart open when pHlevels drop during ischemia and trigger the transmissionof pain.

. The binding of a second messenger. For example, cyclicadenosine monophosphate (AMP) and cyclic guanosine

monophosphate (GMP)nucleotide-gated cation channelsand inositol triphosphate (IP3)-gated cation channels. Inthe retina, the gating of cyclic GMP-activated cationchannels are critical for visual transduction.

. Mechanical stretching. For example, stretch-activatedion channels in the heart transduce information aboutmechanical stress. Mechano-transducing channels con-verted movement in hair cells of the inner ear into elec-trical signals. In addition, volume-regulated anionchannels help restore cell volume after swelling.

. Achange in temperature. For example, transient receptorpotential ion channels (Trp) in sensory neurons activatewith noxious cold temperatures, whereas othermembersof the same gene family are heat-activated.

In the absence of the appropriate trigger, the probabilitythat a channel will be open is low (e.g.>0.1%of the time).However, the binding of a ligand (for a neurotransmitter orsecondmessenger-gated channel) or a change inmembranepotential (for a voltage-gated channel; Figure 3) induces arapid global conformational change in the ion channelprotein that increases the likelihood that it will be open(Figure 7). The total amount of time that a channel is opendepends on the strength of the stimulus, although even inthe presence of a large excess of stimulus an ion channelmay be open most of the time but never 100% of the time.Since themoment-to-moment gating of ion channels is a

random or stochastic process governed by the laws ofthermodynamics it is not possible to predict if a channelwill open at a fixed point in time. The probability that achannel is open can be calculated. See also: Thermody-namics in biochemistry

The Molecular Basis of Gating

The sequences of molecular rearrangements that link thesensing part of an ion channel to its ion pore are underintense investigation. MacKinnon’s group has presented astructure of a potassium channel locked in the open state.The structure shows the inner helices that obstruct the porein the closed state,move apart in the opened state (Figure7).These changes require large conformational changes in theprotein.Structural elements that sense particular stimuli are

identified in some ion channels:

. The voltage-sensor region of voltage-gated ion channelsis thought to reside in the fourth (S4) of six transmem-brane spanning segments that make up one of the fourstructurally homologous domains of the channel protein(see six transmembrane one pore-domain structure inFigure 6). Each transmembrane-spanning segment iscomposed of a string of about 20 lipophilic (‘lipid-lov-ing’) amino acids with the exception of S4, which con-tains positively charged hydrophilic (‘water-loving’)

Ion Channels

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amino acids at every third or fourth position. There aredifferent models to explain how the S4 transmembranesegments, of which there are four in voltage-gated ionchannels, trigger the global conformational changes thatpromote channel opening.

. The sensor for neurotransmitter receptor-gated ionchannels is located in a large extracellular domain ofthe protein dedicated to creating a high-affinity bindingsite for neurotransmitter. Typically, at least two of the

subunits in the complex contain unique binding pocketsthat select for the appropriate neurotransmitter, e.g.GABA, glycine, 5-hydroxytryptamine, acetylcholine orglutamate.

. Second-messenger-gated ion channels are overall struc-turally similar to the voltage-gated ion channel family(see six transmembrane one pore-domain structure inFigure6). They are tetramers, but each subunit has a largeintracellular domain devoted to forming an appropriatehigh-affinity, second-messenger binding site. One classof calcium-activated potassium channel, maxiK, is ac-tivated both by voltage and by intracellular calcium.

The stimuli that gate different ion channels (e.g. voltage,calcium, stretch, and neurotransmitter) have little in com-mon. Similarly, the domains on different ion channel fam-ilies responsible for detecting these different stimuli arestructurally distinct. Remarkably, however, each set ofstimuli can trigger the gating of an ion pore with effectivelythe same breathtaking speed.

Ion Channel Behaviour Can beModulated

The activity of almost every ion channel studied to date issubject to modulation. In the nervous system, modulation

Six transmembraneOne pore-domain

P

+++

Four transmembraneTwo pore-domain

PP

Four transmembraneOne pore-domain

P

Three transmembraneOne pore-domain

P

Two transmembraneOne pore-domain

P

S4

Figure 6 Architecture of five different transmembrane spanning patterns in individual ion channel subunits and domains. The vertical cylindricalstructures represent the transmembrane alpha helices (TM) that span the lipid bilayer and shorter helices that contribute to the ion pore region (P). Six-TM

one-pore domain channels include voltage-gatedNa+, Ca2+ and K+ channels, IP3 receptors, cyclic nucleotide-gated and TRP channels. The fourth TMalphahelix (S4) of the domain is the voltage sensor, it contains a series of basic amino acids shownby the positive symbols. These domains associate as tetramers

(Figure 4); inwardly rectifying K+ channels comprised two-TM one-pore domains. These domains associate as tetramers: KCNK channels contain four-TMtwo-pore domains; ionotropic glutamate receptors contain three-TM one-pore domains that associate as tetramers; and nicotinic receptors, 5-HT3 and

glycine channels four-TM one-pore domains that associate as pentamers.

Figure 7 One way to look at ion channel gating. In this model, thechannel twists betweena closed andopen conformation. The physical gate

regulating ion flux in K+ selective ion channels lies deep in the pore beyondthe narrow selectivity filter at the cytoplasmic face.

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of ion channels underlies the ability of our brains toaccommodate to a constantly changing external environ-ment. Modulation of ion channel activity is also at theheart of the ‘adrenaline rush’ that we feel when anxious,excited or scared. A modulator may be a neurotransmit-ter, lipid, hormone, drug or second messenger that altersthe activity of an ion channel in a way that is not part ofthe normal mechanism used to gate the channel. The ef-fects of a modulator of ion channel activity can last for afew milliseconds to hours, days or even perhaps a life-time. There are in general two ways to modulate channelactivity:

. A modulator can influence the gating of a channel, forexample by altering the voltage range or neurotrans-mitter concentration over which an ion channel acti-vates.

. Amodulator can influence the permeation pathway, forexample by altering the rate at which an ion flowsthrough the channel pore.

. A modulator can influence the efficiency of protein as-sembly, trafficking and stability.

A common mechanism for modulating ion channel func-tion involves activation of a G protein-coupled neuro-transmitter receptor. Receptors couple either directly, viaactivated G protein binding to the target ion channel, orindirectly, via activationof secondmessenger cascades thatusually culminates in protein kinase-mediated phosphor-ylation of the channel. See also: G protein-coupledreceptors; G proteins

Examples of neurotransmitter receptor-mediated mod-ulation of ion channel function include the following.See also: Acetylcholine; Adrenergic receptors

. The slowing of the heart beat following vagal nervestimulation. The neurotransmitter acetylcholine that isreleased from the vagal nerve acts on muscarinic recep-tors that couple to the activation of a K+ channel via Gprotein binding that renders the muscle cell less excit-able.

. Augmentation of cardiac muscle contraction followingexcessive sympathetic nerve stimulation. The neuro-transmitter noradrenaline (norepinephrine) that is re-leased from sympathetic nerves acts on b-adrenergicreceptors in the heart. The receptor then couples to ac-tivation of voltage-gated Ca2+ channels via activationof adenylyl cyclase, increased cAMP, activation of acAMP-dependent protein kinase and finally phosphor-ylation and increased activity of the Ca2+ channel.cAMP also acts directly on hyperpolarization-activatedcation channels in pacemaker cells of the heart, facili-tating their activity at negativemembrane potentials andpromoting membrane depolarization.

Magnesium and polyamines are special kindsof modulators

Some ion channels depend on the action of amodulator fortheir normal function. Two examples are the family of po-tassium selective ion channels known as inward rectifiers,which help to set the resting membrane potential of manyneurons, and a subtype of glutamate receptor, the NMDAreceptor ion channel that underlies certain forms of synap-tic plasticity in the brain. Ion flow through both of thesechannels is modulated by the action of Mg2+ ions or po-lyamines that are highly positive charged molecules andnormal constituents of the cell.See also:NMDAreceptors;Synaptic plasticity: short termIn the case of the inwardly rectifying K+ channel, Mg2+

and polyamines preferentially block the outward, but notinward, flow of K+ ions by binding to and occluding theinner entrance to the pore of the channel. Inhibition ofoutward flow of K+ ions through the pore of the inwardrectifier is crucial for limiting the range overwhich they caninfluence the membrane potential of the cell.The NMDA receptor ion channel allows the flow of

cations to flow including calcium. It is activated by gluta-mate and expressed in a number of neurons that modulatebrain function during development, learning andmemory.Normally, the inward flow of Ca2+ through the NMDAreceptor channel is blocked by Mg2+, which binds to andoccludes the outer vestibule of the channel pore. However,membrane depolarization that accompanies periods of in-tense activity, destabilizesMg2+ block of the channel poreand, under these conditions, when glutamate activates theNMDA receptor, Ca2+ enters the cell. Ca2+ is an impor-tant second messenger that triggers long-term changes inthe responsiveness of the cell to subsequent stimulation.Scientists believe that activity-dependent changes such asthese underlie some forms ofmemory. See also: Glutamateas a neurotransmitter; Glutamatergic synapses: molecularorganization; Learning and memory; Neuronal firingpattern modulation; NMDA receptors; Receptor adapta-tion mechanisms

Summary

Ion channels are critical for many cellular functions but,most importantly, they underlie all forms of electrical sig-nalling. Mutations in ion channel genes link to numeroushuman disorders and ion channels are important drug tar-gets in the treatment of epilepsy, hypertension anddiabetes.Ion channels provide a high-speed thoroughfare for ions totraverse the otherwise unfavourable environment of the cellmembrane. Different classes of ion channels pass differentions and open and close with different stimuli. Hundreds ofgenes encode ion channels, as more are discovered so new

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roles for ion channels emerge. Studies of ion channels havereaped immeasurable benefits from the widespread use ofthe patch clamp recording technique. Investigators can‘visualize’ the moment-to-moment gating of a single ionchannel by measuring the current that flows through thechannel when it is open. Crystallographic studies of ionchannels have provided unparalleled views of the ion poreand the molecular mechanisms responsible for ion selec-tion. Future structures will reveal the steps that couplestimulus sensing to the gating of the ion channel pore.

Further Reading

Blaustein RO and Miller C (2004) Ion channels: shake, rattle or roll?

Nature 427: 499–500.

ClaphamDE (2003) TRP channels as cellular sensors.Nature 426: 517–

524.

Doyle DA, Cabral JM, Pfuetzner RA et al. (1998) The structure of the

potassium channel: molecular basis of K conduction and selectivity.

Science 280: 69–77.

Hamill OP, Marty A, Neher E, Sakmann B and Sigworth FJ (1981)

Improved patch-clamp techniques for high-resolution current record-

ing from cells and cell-free membrane patches. Pflugers Archives 391:

85–100.

Hille B (2001) Ionic Channels of Excitable Membranes, 3rd edn. Berlin:

Springer-Verlag.

Hodgkin AL and Huxley AF (1952) A quantitative description of mem-

brane currents and its application to conductance and excitation in

nerve. Journal of Physiology 117: 500–544.

Jiang Y, Lee A, Chen J et al. (2003) X-ray structure of a voltage-de-

pendent K+ channel. Nature 423: 33–41.

NeherE (1992)Nobel Lecture. Ion channels for communication between

and within cells. Neuron 8: 605–612.

Noda M, Furutani Y, Takahashi H et al. (1983) Cloning and seq-

uence analysis of calf cDNA and human genomic DNA encoding

a-subunit precursor of muscle acetylcholine receptor. Nature 305:

818–823.

Sakmann B (1992) Nobel Lecture. Elementary steps in synaptic trans-

mission revealed by currents through single ion channels. Neuron 8:

613 –629.

Sakmann B andNeher E (1997) Single-channel Recording, 2nd edn. New

York: Plenum Press.

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