Inward-rectifier potassium ion channel: Wikis

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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.[1] They are the targets of multiple toxins, and malfunction of the channels has been implicated in several diseases.[2]

Contents

Overview of inward rectification

Figure 1. Whole-cell current recordings of Kir2 inwardly-rectifying potassium channels expressed in an HEK293 cell. (This is a strongly inwardly rectifying current. Downward deflections are inward currents, upward deflections outward currents, and the x-axis is time in seconds.) There are 13 responses superimposed in this image. The bottom-most trace is current elicited by a voltage step to negative 60mV, and the top-most to positive 60mV, relative to the resting potential, which is close to the K+ reversal potential in this experimental system. Other traces are in 10mV increments between the two.

A channel that is "inwardly-rectifying" is one that passes current (positive charge) more easily in the inward direction (into 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 the 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.[3]

Inward rectifiers also differ from tandem pore domain potassium channels, which are largely responsible for "leak" K+ currents.[4] 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).[5]

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.

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 hyperpolarizing K+ current at negative membrane potentials, they help establish resting membrane potential, and in the case of the K ir3 group, they help mediate inhibitory neurotransmitter responses, but their roles in cellular physiology vary across cell types:

Location Function
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.[6] 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.

Biochemistry of Kir channels

There are seven subfamilies of Kir channels, denoted as Kir1 - Kir7.[1] 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 (ie Kir2.1 and Kir2.3) when the channels are overexpressed.

Diversity

Gene Protein Aliases Associated subunits
KCNJ1 Kir1.1 ROMK1 NHERF2
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
KCNJ15 Kir4.2 Kir1.3
KCNJ16 Kir5.1 BIR 9
KCNJ8 Kir6.1 KATP SUR2B
KCNJ11 Kir6.2 KATP SUR1, SUR2A, and SUR2B
KCNJ13 Kir7.1 Kir1.4

Diseases related to 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).

See also

References

  1. ^ a b Kubo Y, Adelman JP, Clapham DE, Jan LY, Karschin A, Kurachi Y, Lazdunski M, Nichols CG, Seino S, Vandenberg CA (2005). "International Union of Pharmacology. LIV. Nomenclature and molecular relationships of inwardly rectifying potassium channels". Pharmacol Rev 57 (4): 509–26. doi:10.1124/pr.57.4.11. PMID 16382105.  
  2. ^ Abraham MR, Jahangir A, Alekseev AE, Terzic A (November 1, 1999). "Channelopathies of inwardly rectifying potassium channels". Faseb J 13 (14): 1901–10. PMID 10544173. http://www.fasebj.org/cgi/content/full/13/14/1901.  
  3. ^ Bertil Hille (2001). Ion Channels of Excitable Membranes 3rd ed. (Sinauer: Sunderland, MA), p. 151. ISBN 0878933212.
  4. ^ Hille, p. 155.
  5. ^ Hille, p. 153.
  6. ^ 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.  

Further reading

Bertil Hille (2001). Ion Channels of Excitable Membranes 3rd ed. (Sinauer: Sunderland, MA), pp. 149–154. ISBN 0878933212.

External links

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