Ion channels

Figure 1: Two ion channels in the lipid bilayer of the cell membrane. These channels are made from four subunits. The one on the right is cut away to allow a view of the pore inside.

Ion channels are ion-permeable pores in the lipid membranes of all cells. They open and close in response to stimuli, gating the flow of specific small ions. The ions flow downhill thermodynamically.

[edit] Overview of function and structure

[edit] Function

Ion channels serve three principal physiological roles (Hille, 2001; Levitan and Kaczmarek, 2002).

Ion channels set up the resting membrane potentials of all cells. Since the flow of ions moves charge and constitutes an electric current, channel opening and closing underlie all electrical signaling of electrically excitable cells such as nerve and muscle. Thus, when open, potassium ion-selective channels and anion channels hyperpolarize cells (cause the membrane potential to become more negative), whereas sodium- or calcium-selective channels and non-selective cation channels depolarize cells (cause the membrane potential to become more positive).
Flux of ions through ion channels contributes to the electrolyte movements required for volume regulation of single cells and for the net polarized transport of salt across epithelia like gut, kidney, or the choroid plexus.
A few ions, notably Ca²⁺, make regulatory signals inside cells. Cytoplasmic calcium signals are generated by the opening of Ca²⁺-permeable ion channels that let Ca²⁺ ions flow into the cytoplasm. The Ca²⁺ may come from the extracellular medium or from intracellular organelles. Entry from the outside is the primary mechanism for translation of electrical signals into chemical signals. It is how electrical signals in electrically excitable cells couple to hormone secretion, neurotransmitter release, muscle contraction, and changes in gene expression.

The ability of ion channels to accomplish these three physiological functions also requires the housekeeping operation of another class of membrane proteins, the transporters and pumps, to set up standing ion concentration gradients across cell membranes. Ion concentration gradients and electrical forces drive the flow of ions through channel pores.

Conceptually three significant functional domains of all ion channels are:

Ion conducting pore: An aqueous pathway for ions with a narrow selectivity filter that distinguishes among the ions that do go through and the ions that do not
Gates: a part of the channel that can open and close the conducting pore
Sensors: detectors of stimuli that respond to electrical potential changes or chemical signals. The sensors couple to the channel gates to control the probability that they open or close.

[edit] Structure

Ion channels are membrane proteins. Typically they are oligomeric complexes of several subunits. The majority of channels have three, four, or five homologous or identical subunits, arranged in circular symmetry, forming a single aqueous pore at the axial intersection ( Figure 2). However, one set of channels (ClC chloride channels) has two homologous subunits forming one pore in each of the subunits. There is much variety. Perhaps there are 500 genes for pore-forming and accessory subunits of channels. They fall into a few large families of closely related proteins and many small outlying families that lack any known evolutionary relationship to the others. Several of these structural families are ubiquitous and ancient, being found in bacteria, archaea, and eukaryotes alike--in all cellular life.

Figure 2: Pseudosymmetric architecture of ion channels formed from 2, 3, 4, or 5 protein subunits or multiple repeated domains in a single subunit. In addition many of these channels have smaller accessory subunits that typically do not contribute to the actual pore. The view is from outside and shows the pore as a hole.

[edit] Ion channels as proteins

Ion channels have many features of typical membrane proteins. They are synthesized and inserted into the membrane of the endoplasmic reticulum, glycosylated in the Golgi, and transported and inserted into target membranes by membrane fusion. They are regulated by trafficking, phosphorylation, ubiquitination, reversible interactions with other signaling proteins and second messengers, proteolytic cleavage, and other modifications. Like other signaling proteins, ion channels are flexible molecules that undergo conformational changes between open (active) and closed (inactive) states. They evolve and increase in number through phylogeny and can be placed in gene families and superfamilies according to their sequence similarities.

[edit] The voltage-gated channel super family

The largest superfamily of ion channels consists of tetrameric voltage-gated cation channels and their relatives (Hille, 2001). They are called voltage-gated because many of them are opened by changes in membrane potential. For most of them a membrane potential depolarization from rest favors opening.

Voltage-gated channels are built from four homologous modules ( Figure 1, Figure 2), each comprising a voltage-sensor domain and a pore-forming domain. The four pore-forming domains converge to line the single resulting central pore, whereas the four voltage sensors splay out laterally within the membrane lipid bilayer. Each voltage sensor has a polybasic region whose positive charges are pulled back and forth across the membrane in response to changes of the electric field in the membrane. Their movements, the elementary basis of voltage sensing, can be detected as tiny “gating currents” flowing transiently after a change of membrane potential (Armstrong, 1992).

Prominent among these are the voltage-gated Na⁺, K⁺, and Ca²⁺ channels that underlie the action potentials and the rapid calcium signaling of electrically excitable cells. Fast voltage-dependent opening of Na⁺ channels accounts for all-or-nothing excitation initiated with a sharp threshold by depolarizing stimuli and for the regenerative spread of excitation as an action potential propagates along an axon or muscle fiber.

Several other ion channel families are clearly related to the well-known voltage-gated Na⁺, K⁺, and Ca²⁺ channels. Some are at best weakly voltage dependent, although they retain voltage-sensor domains with a few positive charges. Included among these are the cyclic nucleotide gated (CNG) and the TRP families of non-selective cation channels. The CNG channels serve vertebrate phototransduction and olfaction. The TRP family includes diverse forms serving in phototransduction of invertebrates and in sensory receptors detecting hot, cold, chili peppers, mustard, ginger, and possibly touch, pressure, and motion.

Another family of ion channels, the inwardly rectifying K⁺ channels, has no voltage-sensor domain but has a pore-forming domain with much sequence identity and ion selectivity identical to that of voltage-gated K⁺ channels. Such evolutionary diversification suggests an early origin of modular ion-selective pore domains for tetrameric channels and of voltage-sensor domains that could be appended to them. Inwardly rectifying channels take their name because they pass inward K⁺ current much better than outward current. They acquire this apparent voltage dependence by being blocked (plugged) by several cytoplasmic polyvalent cations that move into the inner pore whenever outward current would flow.

Also distantly related in part to other tetrameric channels are synaptic glutamate receptor channels (see next section). They have a pore-domain fold related to that of the other tetrameric channels.

[edit] Ligand-gated ion channel families

Several families of ion channels are gated by extracellular ligands (Hille, 2001). Prominent among these are the cysteine-loop channels of fast chemical synapses specialized as receptors (R) for the chemical neurotransmitter acetylcholine (ACh), glycine (gly), gamma-aminobutyric acid (GABA), or serotonin (5-HT). They serve both inhibitory and excitatory synaptic transmission. The first to be described was the nicotinic acetylcholine receptor. The single central pore is formed from five homologous subunits ( Figure 2). The pore opens within milliseconds after several of the subunits have bound extracellular ligand, and then small cations (for nAChR, 5HT₃R) or small anions (for glyR, GABA_AR) pass into the cell with little selectivity among ions of similar size and charge ( Figure 3).

Figure 3: Patch-clamp recording of unitary current steps from a single nicotinic acetylcholine receptor channel (nAChR). Openings are induced by a low concentration of ACh in the recording pipette. Dashed line is zero current (channel closed), and downward deflections signify inward cation current flowing when the channel is open. Recorded from an embryonic muscle fiber. (Hille, 2001)

Another major family of fast synaptic receptors is the ionotropic glutamate receptors (gluR). Although very similar in function, their architecture is quite distinct from that of the cysteine-loop synaptic receptors. They are tetramers of homologous subunits, forming a central pore strikingly resembling that of the voltage-gated superfamily, but with inverse topological orientation in the membrane. In the gluRs, the tetrameric pore module is appended to large extracellular glutamate-binding modules of separate origin. Again the pore opens within a millisecond of the binding of several glutamates and, in most cases, small cations pass into the cell with little selectivity among cations, generating a depolarization and excitation.

Additional families of ligand-gated channels include the purinergic receptor channels (P2XR), opened by extracellular nucleotides such as ATP, and the amiloride-blocked epithelial Na⁺ channel (ENaC) degenerin family. They have one central cation-preferring pore formed from three homologous subunits.

Ion channels were first recognized in the plasma membranes of cells, but they are present in all intracellular organelle membranes as well. For example, some members of the ClC family are prominent in endosomes and in plant vacuoles. The ClC gene family is unusual in that some members form anion channels and other members form proton-coupled Cl^- transporters. Such diversity reinforces the concept that ion channels and ion transporters (carriers) are formally and, at least sometimes, structurally related membrane proteins.

[edit] Bioelectricity results from currents in ion channels

Salts dissolved in water dissociate into negative anions and positive cations. In an applied electric field, the anions move toward the positive pole and the cations towards the negative pole. Both streams of ions contribute to electric current flow. By convention, current is said to flow in the direction that positive charges would move. In simple conductors the current (I, units amperes) is proportional to the electrical driving force (E, units volts), \(I = gE \ ,\) (called Ohm's Law). The proportionality constant g is called the conductance (units Siemens). Pure lipid bilayers have a conductance near zero. On the other hand real biological membranes have some conductance, all of which is contributed by the ion channels. The membrane conductance is the sum of the individual conductances of each of the channels (see Electrical properties of cell membranes). Hence the number of open channels is readily determined by an electrical measurement of the total conductance of the membrane. The single-channel conductance of typical ion channels ranges from 0.1 to 100 pS (picosiemens).

Ohm's law gives an approximate description of ion currents in an open ion channel except that for real channels the electrical driving force is usually not zero when the membrane potential E is zero. Because of the operation of ion pumps and coupled transporters there are standing concentration gradients of Na⁺, K⁺, Ca²⁺, and Cl^- ions across biological membranes. Therefore ions will flow down their concentration gradients through open channels even in the absence of an overt electrical potential difference E across the membrane. In this sense ion channels act as tiny batteries that can generate electrical currents and potentials across the cell membrane.

One needs to know what the zero-current potential for an ion-selective channel is in order to write Ohm's law correctly for that channel. Consider two cases, either only one ion is permeant in the channel (the simplest case) or several ions are permeant. Walther Nernst derived the formula from equilibrium thermodynamics for the zero-current potential or equilibrium potential when only one ion is permeant and it is driven by a concentration gradient. In that case the voltage of the ion channel "battery" is given by the Nernst Equation. Goldman, Hodgkin, and Katz derived an equation for the zero-current potential when several ions are permeant, a non-equilibrium empirical formula. In that case the ion channel "battery" is given by the GHK Equation. In either case, if the potential of the channel "battery" obtained from such equations is called E_ions, the revised Ohm's law may be written:

\[ I_{channel} = g_{channel} (E - E_{ions}) \]

This equation says there is no net ionic current in the channel when the membrane potential is E_ions. Thus, if only this channel type is open, the cell membrane potential will be brought quickly to the value of E_ions for that channel.

Even this revised Ohm's law does not describe current in open ion channels perfectly. Ohm's law is by definition a linear relationship. Because the structure of the pore of ion channels often has intrinsic asymmetries and because the concentrations of the permeant ions (and any blocking ion) on either side differ, the current-voltage relation of real channels may be curved or rectifying depending on the direction of ion flow.

[edit] Early origin of the ion channel concept

By the mid 1800s biophysical studies of osmosis and urine filtration led to the hypothesis that there are pores of molecular diameter in biological membranes. This concept was presented in textbooks of physiology from then on as one of several unproven possibilities. At the time the word membrane was applied without distinction to sheets of tissue such as epithelia and to the then hypothetical envelope of cells that is known today as the plasma membrane.

Functional studies during the period 1945 to 1970 revealed voltage-gated and ligand-gated channels in nerve and muscle plasma membranes and finally proved that they are aqueous pores. Their use of voltage clamp allowed Alan L. Hodgkin and Andrew F. Huxley to determine kinetic rules for the voltage-dependent opening of Na⁺ and K⁺ channels in the squid giant axon (Hodgkin and Huxley, 1952). Bernard Katz and colleagues (Katz, 1966) determined kinetic rules for ACh-gated opening of nAChRs at the frog neuromuscular junction. Subsequently patch clamp showed that the opening and closing transitions of individual channel molecules are sudden, all-or-nothing events as the flexible channel protein snaps from one conformation to another. Modern arguments for an aqueous pore include: the very high throughput rate for ions (~10⁷ ions per channel per second, Figure 2), the permeability to only small ions, block by slightly larger ions, and now definitively the crystal structures that show the water-filled transmembrane pathway with several ions in it.

[edit] References

Armstrong CM (1992) Voltage-dependent ion channels and their gating. Physiol Rev 72(4 Suppl): S5-S13. Review.

Gouaux E, Mackinnon R (2005) Principles of selective ion transport in channels and pumps. Science 310: 1461-1465.

Hille B (2001) Ion Channels of Excitable Membranes, 3rd Ed. Sinauer Associates, Sunderland, Mass.

Hodgkin AL, Huxley AF (1952) A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol 117: 500-544.

Katz B (1966) Nerve, Muscle, and Synapse. McGraw Hill, New York.

Levitan IB, Kaczmarek LK (2002) The Neuron: Cell and Molecular Biology. Oxford University Press, Oxford.

Internal references

Ernst Niebur (2008) Electrical properties of cell membranes. Scholarpedia, 3(6):7166.

Eugene M. Izhikevich (2007) Equilibrium. Scholarpedia, 2(10):2014.

Eugene Roberts (2007) Gamma-aminobutyric acid. Scholarpedia, 2(10):3356.

Rodolfo Llinas (2008) Neuron. Scholarpedia, 3(8):1490.

Pierre-Jean Corringer and Jean-Pierre Changeux (2008) Nicotinic acetylcholine receptors. Scholarpedia, 3(1):3468.

John W. Moore (2007) Voltage clamp. Scholarpedia, 2(9):3060.