Inward-rectifier potassium ion channel
|Inward rectifier potassium channel|
crystal structure of an inward rectifier potassium channel
|Inward rectifier potassium channel N-terminal|
Inwardly rectifying potassium channels (Kir, IRK) are a specific subset of potassium selective ion channels. To date, seven subfamilies have been identified in various mammalian cell types and they are also found in plants. They are the targets of multiple toxins, and malfunction of the channels has been implicated in several diseases.
Overview of inward rectification
A channel that is "inwardly-rectifying" is one that passes current (positive charge) more easily in the inward direction (into the cell) than in the outward direction (out of the cell). It is thought that this current may play an important role in regulating neuronal activity, by helping to establish the resting membrane potential of the cell.
By convention, inward current is displayed in voltage clamp as a downward deflection, while an outward current (positive charge moving out of the cell) is shown as an upward deflection. At membrane potentials negative to potassium's reversal potential, inwardly rectifying K+ channels support the flow of positively charged K+ ions into the cell, pushing the membrane potential back to the resting potential. This can be seen in figure 1: when the membrane potential is clamped negative to the channel's resting potential (e.g. -60 mV), inward current flows (i.e. positive charge flows into the cell). However, when the membrane potential is set positive to the channel's resting potential (e.g. +60 mV), these channels pass very little charge out of the cell. Simply put, this channel passes much more current in the inward direction than the outward one. Note that these channels are not perfect rectifiers, as they can pass some outward current in the voltage range up to about 30 mV above resting potential.
These channels differ from the potassium channels that are typically responsible for repolarizing a cell following an action potential, such as the delayed rectifier and A-type potassium channels. Those more "typical" potassium channels preferentially carry outward (rather than inward) potassium currents at depolarized membrane potentials, and may be thought of as "outwardly rectifying." When first discovered, inward rectification was named "anomalous rectification" to distinguish it from outward potassium currents.
Inward rectifiers also differ from tandem pore domain potassium channels, which are largely responsible for "leak" K+ currents. Some inward rectifiers, termed "weak inward rectifiers," carry measurable outward K+ currents at voltages positive to the K+ reversal potential (corresponding to, but larger than, the small currents above the 0 nA line in figure 1). They, along with the "leak" channels, establish the resting membrane potential of the cell. Other inwardly rectifying channels, termed "strong inward rectifiers," carry very little outward current at all, and are mainly active at voltages negative to the reversal potential, where they carry inward current (the much larger currents below the 0 nA line in figure 1).
Mechanism of inward rectification
The phenomenon of inward rectification of Kir channels is the result of high-affinity block by endogenous polyamines, namely spermine, as well as magnesium ions, that plug the channel pore at positive potentials, resulting in a decrease in outward currents. This voltage-dependent block by polyamines causes currents to be conducted well only in the inward direction. While the principal idea of polyamine block is understood, the specific mechanisms are still controversial.
Activation by PIP2
Kir channels require phosphatidylinositol 4,5-bisphosphate (PIP2) for activation. PIP2 binds to and directly activates Kir 2.2 with agonist-like properties. In this regard Kir channels are PIP2 ligand-gated ion channels.
Role of Kir channels
Kir channels are found in multiple cell types, including macrophages, cardiac and kidney cells, leukocytes, neurons, and endothelial cells. By mediating a small depolarizing K+ current at negative membrane potentials, they help establish resting membrane potential, and in the case of the Kir3 group, they help mediate inhibitory neurotransmitter responses, but their roles in cellular physiology vary across cell types:
|cardiac myocytes||Kir channels close upon depolarization, slowing membrane repolarization and helping maintain a more prolonged cardiac action potential. This type of inward-rectifier channel is distinct from delayed rectifier K+ channels, which help repolarize nerve and muscle cells after action potentials; and potassium leak channels, which provide much of the basis for the resting membrane potential.|
|endothelial cells||Kir channels are involved in regulation of nitric oxide synthase.|
|kidneys||Kir export surplus potassium into collecting tubules for removal in the urine, or alternatively may be involved in the reuptake of potassium back into the body.|
|neurons and in heart cells||G-protein activated IRKs (Kir3) are important regulators, modulated by neurotransmitters. A mutation in the GIRK2 channel leads to the weaver mouse mutation. "Weaver" mutant mice are ataxic and display a neuroinflammation-mediated degeneration of their dopaminergic neurons. Relative to non-ataxic controls, Weaver mutants have deficits in motor coordination and changes in regional brain metabolism. Weaver mice have been examined in labs interested in neural development and disease for over 30 years.|
|pancreatic beta cells||KATP channels (composed of Kir6.2 and SUR1 subunits) control insulin release.|
Classification of Kir channels
There are seven subfamilies of Kir channels, denoted as Kir1 - Kir7. Each subfamily has multiple members (i.e. Kir2.1, Kir2.2, Kir2.3, etc.) that have nearly identical amino acid sequences across known mammalian species.
Kir channels are formed from as homotetrameric membrane proteins. Each of the four identical protein subunits is composed of two membrane-spanning alpha helices (M1 and M2). Heterotetramers can form between members of the same subfamily (i.e. Kir2.1 and Kir2.3) when the channels are overexpressed.
|KCNJ2||Kir2.1||IRK1||Kir2.2, Kir4.1, PSD-95, SAP97, AKAP79|
|KCNJ12||Kir2.2||IRK2||Kir2.1 and Kir2.3 to form heteromeric channel, auxiliary subunit: SAP97, Veli-1, Veli-3, PSD-95|
|KCNJ4||Kir2.3||IRK3||Kir2.1 and Kir2.3 to form heteromeric channel, PSD-95, Chapsyn-110/PSD-93|
|KCNJ14||Kir2.4||IRK4||Kir2.1 to form heteromeric channel|
|KCNJ3||Kir3.1||GIRK1, KGA||Kir3.2, Kir3.4, Kir3.5, Kir3.1 is not functional by itself|
|KCNJ6||Kir3.2||GIRK2||Kir3.1, Kir3.3, Kir3.4 to form heteromeric channel|
|KCNJ9||Kir3.3||GIRK3||Kir3.1, Kir3.2 to form heteromeric channel|
|KCNJ5||Kir3.4||GIRK4||Kir3.1, Kir3.2, Kir3.3|
|KCNJ10||Kir4.1||Kir1.2||Kir4.2, Kir5.1, and Kir2.1 to form heteromeric channels|
|KCNJ11||Kir6.2||KATP||SUR1, SUR2A, and SUR2B|
- Persistent hyperinsulinemic hypoglycemia of infancy is related to autosomal recessive mutations in Kir6.2. Certain mutations of this gene diminish the channel's ability to regulate insulin secretion, leading to hypoglycemia.
- Bartter's syndrome can be caused by mutations in Kir channels. This condition is characterized by the inability of kidneys to recycle potassium, causing low levels of potassium in the body.
- Andersen's syndrome is a rare condition caused by multiple mutations of Kir2.1. Depending on the mutation, it can be dominant or recessive. It is characterized by periodic paralysis, cardiac arrhythmias and dysmorphic features. (See also KCNJ2)
- Barium poisoning is likely due to its ability to block Kir channels.
- Atherosclerosis (heart disease) may be related to Kir channels. The loss of Kir currents in endothelial cells is one of the first known indicators of atherogenesis (the beginning of heart disease).
- Thyrotoxic hypokalaemic periodic paralysis has been linked to altered Kir2.6 function.
- EAST/SeSAME syndrome may be caused by mutations of KCNJ10.
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- Bertil Hille (2001). Ion Channels of Excitable Membranes 3rd ed. (Sinauer: Sunderland, MA), p. 151. ISBN 0-87893-321-2.
- Hille, p. 155.
- Hille, p. 153.
- Hansen, S. (2011). "Structural basis of PIP2 activation of the classical inward rectifier K+ channel Kir2.2.". Nature 477 (7365): 495–498. PMID 21874019.
- Peng J, Xie L, Stevenson FF et al. (2006). "Nigrostriatal dopaminergic neurodegeneration in the weaver mouse is mediated via neuroinflammation and alleviated by minocycline administration". J. Neurosci. 26 (45): 11644–51. doi:10.1523/JNEUROSCI.3447-06.2006. PMID 17093086.
- Strazielle C, Deiss V, Naudon L, Raisman-Vozari R, Lalonde R. (2006). "Regional brain variations of cytochrome oxidase activity and motor coordination in Girk2(Wv) (Weaver) mutant mice.". Neuroscience 142 (2): 437–49. doi:10.1016/j.neuroscience.2006.06.011. PMID 16844307.
- Inward Rectifier Potassium Channels at the US National Library of Medicine Medical Subject Headings (MeSH).
- "Inwardly Recifying Potassium Channels". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.
- UMich Orientation of Proteins in Membranes families/family-85 - Spatial positions of inward rectifier potassium channels in membranes.