Wikipedia

Ion channel

(Redirected fromIon channels)
Not to be confused withIon TelevisionorIon implantation.

Ion channelsare pore-formingmembrane proteinsthat allowionsto pass through the channel pore. Their functions include establishing aresting membrane potential,[1]shapingaction potentialsand other electrical signals bygatingthe flow ofionsacross thecell membrane,controlling the flow of ions acrosssecretoryandepithelial cells,and regulatingcellvolume. Ion channels are present in the membranes of all cells.[2][3]Ion channels are one of the two classes ofionophoricproteins, the other beingion transporters.[4]

Schematic diagram of an ion channel.1- channeldomains(typically four per channel),2- outer vestibule,3-selectivity filter,4- diameter of selectivity filter,5-phosphorylationsite,6-cell membrane.

The study of ion channels often involvesbiophysics,electrophysiology,andpharmacology,while using techniques includingvoltage clamp,patch clamp,immunohistochemistry,X-ray crystallography,fluoroscopy,andRT-PCR.Their classification as molecules is referred to aschannelomics.

Basic features

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Structure of the KcsApotassium channel(PDB: 1K4C). The two gray planes indicate thehydrocarbonboundaries of thelipid bilayerand were calculated with the ANVIL algorithm.[5]

There are two distinctive features of ion channels that differentiate them from other types of ion transporter proteins:[4]

  1. The rate of ion transport through the channel is very high (often 106ions per second or greater).
  2. Ions pass through channels down theirelectrochemical gradient,which is a function of ion concentration and membrane potential, "downhill", without the input (or help) of metabolic energy (e.g.ATP,co-transportmechanisms, oractive transportmechanisms).

Ion channels are located within themembraneof all excitable cells,[3]and of many intracellularorganelles.They are often described as narrow, water-filled tunnels that allow only ions of a certain size and/or charge to pass through. This characteristic is calledselective permeability.The archetypal channel pore is just one or two atoms wide at its narrowest point and is selective for specific species of ion, such assodiumorpotassium.However, some channels may be permeable to the passage of more than one type of ion, typically sharing a common charge: positive (cations) or negative (anions). Ions often move through the segments of the channel pore in a single file nearly as quickly as the ions move through the free solution. In many ion channels, passage through the pore is governed by a "gate", which may be opened or closed in response to chemical or electrical signals, temperature, or mechanical force.[citation needed]

Ion channels areintegral membrane proteins,typically formed as assemblies of several individual proteins. Such "multi-subunit"assemblies usually involve a circular arrangement of identical orhomologousproteins closely packed around a water-filled pore through the plane of the membrane orlipid bilayer.[6][7]For mostvoltage-gated ion channels,the pore-forming subunit(s) are called the α subunit, while the auxiliary subunits are denoted β, γ, and so on.

Biological role

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Because channels underlie thenerve impulseand because "transmitter-activated" channels mediate conduction across thesynapses,channels are especially prominent components of thenervous system.Indeed,numerous toxinsthat organisms have evolved for shutting down the nervous systems of predators and prey (e.g., the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails, and others) work by modulating ion channel conductance and/or kinetics. In addition, ion channels are key components in a wide variety of biological processes that involve rapid changes in cells, such ascardiac,skeletal,andsmooth musclecontraction,epithelialtransport of nutrients and ions,T-cellactivation, andpancreaticbeta-cellinsulinrelease. In the search for new drugs, ion channels are a frequent target.[8][9][10]

Diversity

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There are over 300 types of ion channels just in the cells of the inner ear.[11]Ion channels may be classified by the nature of theirgating,the species of ions passing through those gates, the number of gates (pores), and localization of proteins.[12]

Further heterogeneity of ion channels arises when channels with different constitutivesubunitsgive rise to a specific kind of current.[13]Absence or mutation of one or more of the contributing types of channel subunits can result in loss of function and, potentially, underlie neurologic diseases.[citation needed]

Classification by gating

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Ion channels may be classified by gating, i.e. what opens and closes the channels. For example, voltage-gated ion channels open or close depending on the voltage gradient across the plasma membrane, while ligand-gated ion channels open or close depending on binding ofligandsto the channel.[14]

Voltage-gated

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Main article:Voltage-gated ion channel

Voltage-gated ion channels open and close in response tomembrane potential.

  • Voltage-gated sodium channels:This family contains at least 9 members and is largely responsible foraction potentialcreation and propagation. The pore-forming α subunits are very large (up to 4,000amino acids) and consist of four homologous repeat domains (I-IV) each comprising six transmembrane segments (S1-S6) for a total of 24 transmembrane segments. The members of this family also coassemble with auxiliary β subunits, each spanning the membrane once. Both α and β subunits are extensivelyglycosylated.
  • Voltage-gated calcium channels:This family contains 10 members, though these are known to coassemble with α2δ, β, and γ subunits. These channels play an important role in both linking muscle excitation with contraction as well as neuronal excitation with transmitter release. The α subunits have an overall structural resemblance to those of the sodium channels and are equally large.
  • Voltage-gated potassium channels(KV): This family contains almost 40 members, which are further divided into 12 subfamilies. These channels are known mainly for their role in repolarizing the cell membrane followingaction potentials.The α subunits have six transmembrane segments, homologous to a single domain of the sodium channels. Correspondingly, they assemble astetramersto produce a functioning channel.
  • Sometransient receptor potential channels:This group of channels, normally referred to simply as TRP channels, is named after their role inDrosophilaphototransduction. This family, containing at least 28 members, is incredibly diverse in its method of activation. Some TRP channels seem to be constitutively open, while others are gated byvoltage,intracellularCa2+,pH, redox state, osmolarity, andmechanical stretch.These channels also vary according to the ion(s) they pass, some being selective for Ca2+while others are less selective, acting as cation channels. This family is subdivided into 6 subfamilies based on homology: classical (TRPC), vanilloid receptors (TRPV), melastatin (TRPM), polycystins (TRPP), mucolipins (TRPML), and ankyrin transmembrane protein 1 (TRPA).
  • Hyperpolarization-activatedcyclic nucleotide-gated channels:The opening of these channels is due tohyperpolarizationrather than the depolarization required for other cyclic nucleotide-gated channels. These channels are also sensitive to the cyclic nucleotidescAMPandcGMP,which alter the voltage sensitivity of the channel's opening. These channels are permeable to the monovalent cations K+and Na+.There are 4 members of this family, all of which form tetramers of six-transmembrane α subunits. As these channels open under hyperpolarizing conditions, they function aspacemakingchannels in the heart, particularly theSA node.
  • Voltage-gated proton channels:Voltage-gated proton channels open with depolarization, but in a strongly pH-sensitive manner. The result is that these channels open only when the electrochemical gradient is outward, such that their opening will only allow protons to leave cells. Their function thus appears to be acid extrusion from cells. Another important function occurs in phagocytes (e.g.eosinophils,neutrophils,macrophages) during the "respiratory burst." When bacteria or other microbes are engulfed by phagocytes, the enzymeNADPH oxidaseassembles in the membrane and begins to producereactive oxygen species(ROS) that help kill bacteria. NADPH oxidase is electrogenic, moving electrons across the membrane, and proton channels open to allow proton flux to balance the electron movement electrically.

Ligand-gated (neurotransmitter)

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Main article:Ligand-gated ion channel

Also known as ionotropicreceptors,this group of channels open in response to specific ligand molecules binding to the extracellular domain of the receptor protein.[15]Ligand binding causes a conformational change in the structure of the channel protein that ultimately leads to the opening of the channel gate and subsequent ion flux across the plasma membrane. Examples of such channels include the cation-permeablenicotinic acetylcholine receptors,ionotropic glutamate-gated receptors,acid-sensing ion channels(ASICs),[16]ATP-gated P2X receptors,and the anion-permeable γ-aminobutyric acid-gatedGABAAreceptor.

Ion channels activated by second messengers may also be categorized in this group, althoughligandsand second messengers are otherwise distinguished from each other.[citation needed]

Lipid-gated

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Main article:Lipid-gated ion channels

This group of channels opens in response to specificlipidmolecules binding to the channel's transmembrane domain typically near the inner leaflet of the plasma membrane.[17]Phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) are the best-characterized lipids to gate these channels.[18][19][20]Many of the leak potassium channels are gated by lipids including theinward-rectifier potassium channelsand two pore domain potassium channels TREK-1 and TRAAK.KCNQ potassium channel familyare gated by PIP2.[21]The voltage activated potassium channel (Kv) is regulated by PA. Its midpoint of activation shifts +50 mV upon PA hydrolysis, near resting membrane potentials.[22]This suggests Kv could be opened by lipid hydrolysis independent of voltage and may qualify this channel as dual lipid and voltage gated channel.

Other gating

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Gating also includes activation and inactivation bysecond messengersfrom the inside of thecell membrane– rather than from outside the cell, as in the case for ligands.

  • Some potassium channels:
    • Inward-rectifier potassium channels:These channels allow potassium ions to flow into the cell in an "inwardly rectifying" manner: potassium flows more efficiently into than out of the cell. This family is composed of 15 official and 1 unofficial member and is further subdivided into 7 subfamilies based on homology. These channels are affected by intracellularATP,PIP2,andG-proteinβγ subunits. They are involved in important physiological processes such as pacemaker activity in the heart, insulin release, and potassium uptake inglial cells.They contain only two transmembrane segments, corresponding to the core pore-forming segments of the KVand KCachannels. Their α subunits form tetramers.
    • Calcium-activated potassium channels:This family of channels is activated by intracellular Ca2+and contains 8 members.
    • Tandem pore domain potassium channel:This family of 15 members form what are known asleak channels,and they displayGoldman-Hodgkin-Katz(open)rectification.Contrary to their common name of 'Two-pore-domain potassium channels', these channels have only one pore but two pore domains per subunit.[23][24]
  • Two-pore channelsinclude ligand-gated and voltage-gated cation channels, so-named because they contain two pore-forming subunits. As their name suggests, they have two pores.[25][26][27][28][29]
  • Light-gated channelslikechannelrhodopsinare directly opened byphotons.
  • Mechanosensitive ion channelsopen under the influence of stretch, pressure, shear, and displacement.
  • Cyclic nucleotide-gated channels:This superfamily of channels contains two families: the cyclic nucleotide-gated (CNG) channels and the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. This grouping is functional rather than evolutionary.
    • Cyclic nucleotide-gated channels: This family of channels is characterized by activation by either intracellularcAMPorcGMP.These channels are primarily permeable to monovalent cations such as K+and Na+.They are also permeable to Ca2+,though it acts to close them. There are 6 members of this family, which is divided into 2 subfamilies.
    • Hyperpolarization-activatedcyclic nucleotide-gated channels
  • Temperature-gated channels: Members of thetransient receptor potential ion channelsuperfamily, such asTRPV1orTRPM8,are opened either by hot or cold temperatures.

Classification by type of ions

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Classification by cellular localization

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Ion channels are also classified according to their subcellular localization. The plasma membrane accounts for around 2% of the total membrane in the cell, whereas intracellular organelles contain 98% of the cell's membrane. The major intracellular compartments areendoplasmic reticulum,Golgi apparatus,andmitochondria.On the basis of localization, ion channels are classified as:

  • Plasma membrane channels
    • Examples: Voltage-gated potassium channels (Kv), Sodium channels (Nav), Calcium channels (Cav) and Chloride channels (ClC)
  • Intracellular channels, which are further classified into different organelles
    • Endoplasmic reticulumchannels: RyR, SERCA, ORAi
    • Mitochondrial channels: mPTP, KATP, BK, IK, CLIC5, Kv7.4 at the inner membrane and VDAC and CLIC4 as outer membrane channels.

Other classifications

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Some ion channels are classified by the duration of their response to stimuli:

  • Transient receptor potential channels:This group of channels, normally referred to simply as TRP channels, is named after their role inDrosophilavisual phototransduction. This family, containing at least 28 members, is diverse in its mechanisms of activation. Some TRP channels remain constitutively open, while others are gated byvoltage,intracellular Ca2+,pH,redoxstate,osmolarity,andmechanical stretch.These channels also vary according to the ion(s) they pass, some being selective for Ca2+while others are less selective cation channels. This family is subdivided into 6 subfamilies based on homology: canonical TRP (TRPC), vanilloid receptors (TRPV), melastatin (TRPM), polycystins (TRPP), mucolipins (TRPML), and ankyrin transmembrane protein 1 (TRPA).

Detailed structure

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Channels differ with respect to the ion they let pass (for example,Na+,K+,Cl−), the ways in which they may be regulated, the number of subunits of which they are composed and other aspects of structure.[32]Channels belonging to the largest class, which includes the voltage-gated channels that underlie the nerve impulse, consist of four or sometimes five[33]subunits with sixtransmembrane heliceseach. On activation, these helices move about and open the pore. Two of these six helices are separated by a loop that lines the pore and is the primary determinant of ion selectivity and conductance in this channel class and some others.[citation needed]

The existence and mechanism for ion selectivity was first postulated in the late 1960s byBertil HilleandClay Armstrong.[34][35][36][37][38]The idea of the ionic selectivity for potassium channels was that the carbonyl oxygens of the protein backbones of the "selectivity filter" (named byBertil Hille) could efficiently replace the water molecules that normally shield potassium ions, but that sodium ions were smaller and cannot be completely dehydrated to allow such shielding, and therefore could not pass through. This mechanism was finally confirmed when the first structure of an ion channel was elucidated. A bacterial potassium channel KcsA, consisting of just the selectivity filter, "P" loop, and two transmembrane helices was used as a model to study the permeability and the selectivity of ion channels in the Mackinnon lab. The determination of the molecular structure of KcsA byRoderick MacKinnonusingX-ray crystallographywon a share of the 2003Nobel Prize in Chemistry.[39]

Because of their small size and the difficulty of crystallizing integral membrane proteins for X-ray analysis, it is only very recently that scientists have been able to directly examine what channels "look like." Particularly in cases where the crystallography required removing channels from their membranes with detergent, many researchers regard images that have been obtained as tentative. An example is the long-awaited crystal structure of a voltage-gated potassium channel, which was reported in May 2003.[40][41]One inevitable ambiguity about these structures relates to the strong evidence that channels change conformation as they operate (they open and close, for example), such that the structure in the crystal could represent any one of these operational states. Most of what researchers have deduced about channel operation so far they have established throughelectrophysiology,biochemistry,genesequence comparison andmutagenesis.

Channels can have single (CLICs) to multiple transmembrane (K channels, P2X receptors, Na channels) domains which span plasma membrane to form pores. Pore can determine the selectivity of the channel. Gate can be formed either inside or outside the pore region.

Pharmacology

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Main article:Channel modulator

Chemical substances can modulate the activity of ion channels, for example by blocking or activating them.

Ion channel blockers

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Main article:Ion channel blocker

A variety ofion channel blockers(inorganic and organic molecules) can modulate ion channel activity and conductance. Some commonly used blockers include:

Ion channel activators

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Main article:Ion channel opener

Several compounds are known to promote the opening or activation of specific ion channels. These are classified by the channel on which they act:

Diseases

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There are a number of disorders which disrupt normal functioning of ion channels and have disastrous consequences for the organism. Genetic and autoimmune disorders of ion channels and their modifiers are known aschannelopathies.SeeCategory:Channelopathiesfor a full list.

History

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The fundamental properties of currents mediated by ion channels were analyzed by the BritishbiophysicistsAlan HodgkinandAndrew Huxleyas part of theirNobel Prize-winning research on theaction potential,published in 1952. They built on the work of other physiologists, such as Cole and Baker's research into voltage-gated membrane pores from 1941.[44][45]The existence of ion channels was confirmed in the 1970s byBernard KatzandRicardo Milediusing noise analysis[citation needed].It was then shown more directly with anelectrical recording techniqueknown as the "patch clamp",which led to a Nobel Prize toErwin NeherandBert Sakmann,the technique's inventors. Hundreds if not thousands of researchers continue to pursue a more detailed understanding of how these proteins work. In recent years the development ofautomated patch clamp deviceshelped to increase significantly the throughput in ion channel screening.

The Nobel Prize in Chemistry for 2003 was awarded toRoderick MacKinnonfor his studies on the physico-chemical properties of ion channel structure and function, includingx-ray crystallographicstructurestudies.

Culture

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Birth of an Idea(2007) byJulian Voss-Andreae.The sculpture was commissioned byRoderick MacKinnonbased on the molecule's atomic coordinates that were determined by MacKinnon's group in 2001.

Roderick MacKinnoncommissionedBirth of an Idea,a 5-foot (1.5 m) tall sculpture based on theKcsA potassium channel.[46]The artwork contains a wire object representing the channel's interior with a blown glass object representing the main cavity of the channel structure.

See also

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References

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  45. ^ Cole KS, Baker RF (July 1941)."Longitudinal Impedance of the Squid Giant Axon".The Journal of General Physiology.24(6). The Rockefeller University Press: 771–88.doi:10.1085/jgp.24.6.771.PMC2238007.PMID19873252.Describes what happens when you stick agiant squid axonwith electrodes and pass through an alternating current, and then notice that sometimes the voltage rises with time, and sometimes it decreases. The inductive reactance is a property of the axon and requires that it contain an inductive structure. The variation of the impedance with interpolar distance indicates that the inductance is in the membrane
  46. ^Ball P (March 2008)."The crucible: Art inspired by science should be more than just a pretty picture".Chemistry World.5(3): 42–43.Retrieved2009-01-12.
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