ATP-sensitive potassium channel
|potassium inwardly-rectifying channel, subfamily J, member 8|
|Locus||Chr. 12 p12.1|
|potassium inwardly-rectifying channel, subfamily J, member 11|
|Locus||Chr. 11 p15.1|
|ATP-binding cassette, sub-family C (CFTR/MRP), member 8|
|Locus||Chr. 11 p15.1|
|ATP-binding cassette, sub-family C (CFTR/MRP), member 9|
|Alt. symbols||SUR2A, SUR2B|
|Locus||Chr. 12 p12.1|
An ATP-sensitive potassium channel (or KATP channel) is a type of potassium channel that is gated by intracellular nucleotides, ATP and ADP. ATP-sensitive potassium channels are composed of Kir6.x-type subunits and sulfonylurea receptor (SUR) subunits, along with additional components. KATP channels are found in the plasma membrane; however some may also be found on subcellular membranes. These latter classes of KATP channels can be classified as being either sarcolemmal ("sarcKATP"), mitochondrial ("mitoKATP"), or nuclear ("nucKATP").
Discovery and structure
KATP channels were first identified in cardiac myocytes by the Akinori Noma group in Japan. They have also been found in pancreas where they control insulin secretion, but are in fact widely distributed in plasma membranes. SarcKATP are composed of eight protein subunits (octamer). Four of these are members of the inward-rectifier potassium ion channel family Kir6.x (either Kir6.1 or Kir6.2), while the other four are sulfonylurea receptors (SUR1, SUR2A, and SUR2B). The Kir subunits have two transmembrane spans and form the channel’s pore. The SUR subunits have three additional transmembrane domains, and contain two nucleotide-binding domains on the cytoplasmic side. These allow for nucleotide-mediated regulation of the potassium channel, and are critical in its roles as a sensor of metabolic status. These SUR subunits are also sensitive to sulfonylureas, MgATP, and some other pharmacological channel openers. While all sarcKATP are constructed of eight subunits in this 4:4 ratio, their precise composition varies with tissue type.
MitoKATP were first identified in 1991 by single-channel recordings of the inner mitochondrial membrane. The molecular structure of mitoKATP is less clearly understood than that of sarcKATP. Some reports indicate that cardiac mitoKATP consist of Kir6.1 and Kir6.2 subunits, but neither SUR1 nor SUR2. More recently, it was discovered that certain multiprotein complexes containing succinate dehydrogenase can provide activity similar to that of KATP channels.
Sensor of cell metabolism
Regulation of gene expression
Four genes have been identified as members of the KATP gene family. The sur1 and kir6.2 genes are located in chr11p15.1 while kir6.1 and sur2 genes reside in chr12p12.1. The kir6.1 and kir6.2 genes encode the pore-forming subunits of the KATP channel, with the SUR subunits being encoded by the sur1 (SUR1) gene or selective splicing of the sur2 gene (SUR2A and SUR2B).
Changes in the transcription of these genes, and thus the production of KATP channels, are directly linked to changes in the metabolic environment. High glucose levels, for example, induce a significant decrease in the kir6.2 mRNA level – an effect that can be reversed by lower glucose concentration. Similarly, 60 minutes of ischemia followed by 24 to 72 hours of reperfusion leads to an increase in kir6.2 transcription in left ventricle rat myocytes.
A mechanism has been proposed for the cell’s KATP reaction to hypoxia and ischemia. Low intracellular oxygen levels decrease the rate of metabolism by slowing the TCA cycle in the mitochondria. Unable to transfer electrons efficiently, the intracellular NAD+/NADH ratio decreases, activating phosphotidylinositol-3-kinase and extracellular signal-regulated kinases. This, in turn, upregulates c-jun transcription, creating a protein which binds to the sur2 promoter.
One significant implication of the link between cellular oxidative stress and increased KATP production is that overall potassium transport function is directly proportional to the membrane concentration of these channels. In cases of diabetes, KATP channels cannot function properly, and a marked sensitivity to mild cardiac ischemia and hypoxia results from the cells' inability to adapt to adverse oxidative conditions.
The degree to which particular compounds are able to regulate KATP channel opening varies with tissue type, and more specifically, with a tissue’s primary metabolic substrate.
In pancreatic beta cells, ATP is the primary metabolic source, and the ATP/ADP ratio determines KATP channel activity. Under resting conditions, the weakly inwardly rectifying KATP channels in pancreatic beta cells are spontaneously active, allowing potassium ions to flow out of the cell and maintaining a negative resting membrane potential (slightly more positive than the K+ reversal potential). In the presence of higher glucose metabolism, and consequently increased relative levels of ATP, the KATP channels close, causing the membrane potential of the cell to depolarize, activating voltage-gated calcium channels, and thus promoting the calcium-dependent release of insulin. The change from one state to the other happens quickly and synchronously, due to C-terminus multimerization among proximate KATP channel molecules.
Cardiomyocytes, on the other hand, derive the majority of their energy from long-chain fatty acids and their acyl-CoA equivalents. Cardiac ischemia, as it slows the oxidation of fatty acids, causes an accumulation of acyl-CoA and induces KATP channel opening while free fatty acids stabilize its closed conformation. This variation was demonstrated by examining transgenic mice, bred to have ATP-insensitive potassium channels. In the pancreas, these channels were always open, but remained closed in the cardiac cells.
Mitochondrial KATP and the regulation of aerobic metabolism
Upon the onset of a cellular energy crisis, mitochondrial function tends to decline. This is due to alternating inner membrane potential, imbalanced trans-membrane ion transport, and an overproduction of free radicals, among other factors. In such a situation, mitoKATP channels open and close to regulate both internal Ca2+ concentration and the degree of membrane swelling. This helps restore proper membrane potential, allowing further H+ outflow, which continues to provide the proton gradient necessary for mitochondrial ATP synthesis. Without aid from the potassium channels, the depletion of high energy phosphate would outpace the rate at which ATP could be created against an unfavorable electrochemical gradient.
Nuclear and sarcolemmal KATP channels also contribute to the endurance of and recovery from metabolic stress. In order to conserve energy, sarcKATP open, reducing the duration of the action potential while nucKATP-mediated Ca2+ concentration changes within the nucleus favor the expression of protective protein genes.
Cardiovascular KATP channels and protection from ischemic injury
Cardiac ischemia, while not always immediately lethal, often leads to delayed cardiomyocyte death by necrosis, causing permanent injury to the heart muscle. One method, first described by Keith Reimer in 1986, involves subjecting the affected tissue to brief, non-lethal periods of ischemia (3–5 minutes) before the major ischemic insult. This procedure is known as ischemic preconditioning ("IPC"), and derives its effectiveness, at least in part, from KATP channel stimulation.
Both sarcKATP and mitoKATP are required for IPC to have its maximal effects. Selective mitoKATP blockade with 5-hydroxydecanoic acid (“5-HD”) or MCC-134 completely inhibits the cardioprotection afforded by IPC, and genetic knockout of sarcKATP genes. in mice has been shown to increase the basal level of injury compared to wild type mice. This baseline protection is believed to be a result of sarcKATP’s ability to prevent cellular Ca2+ overloading and depression of force development during muscle contraction, thereby conserving scarce energy resources.
Absence of sarcKATP, in addition to attenuating the benefits of IPC, significantly impairs the myocyte’s ability to properly distribute Ca2+, decreasing sensitivity to sympathetic nerve signals, and predisposing the subject to arrhythmia and sudden death. Similarly, sarcKATP regulates vascular smooth muscle tone, and deletion of the kir6.2 or sur2 genes leads to coronary artery vasospasm and death.
Upon further exploration of sarcKATP’s role in cardiac rhythm regulation, it was discovered that mutant forms of the channel, particularly mutations in the SUR2 subunit, were responsible for dilated cardiomyopathy, especially after ischemia/reperfusion. It is still unclear as to whether opening of KATP channels has completely pro- or antiarrhythmic effects. Increased potassium conductance should stabilize membrane potential during ischemic insults, reducing the extent infarct and ectopic pacemaker activity. On the other hand, potassium channel opening accelerates repolarization of the action potential, possibly inducing arrhythmic reentry.
- Stephan D, Winkler M, Kühner P, Russ U, Quast U (September 2006). "Selectivity of repaglinide and glibenclamide for the pancreatic over the cardiovascular K(ATP) channels". Diabetologia. 49 (9): 2039–48. doi:10.1007/s00125-006-0307-3. PMID 16865362.
- Noma, A. (1983). "ATP-regulated K+ channels in cardiac muscle". Nature. 305 (5930): 147–148. doi:10.1038/305147a0. PMID 6310409.
- Babenko, A. P.; Aguilar-Bryan, L.; Bryan, J. (1998). "A View of Sur/kir6.x, Katpchannels". Annual Review of Physiology. 60: 667–687. doi:10.1146/annurev.physiol.60.1.667. PMID 9558481.
- Inagaki N, Gonoi T, Clement JP, Namba N, Inazawa J, Gonzalez G, Aguilar-Bryan L, Seino S, Bryan J (November 1995). "Reconstitution of IKATP: an inward rectifier subunit plus the sulfonylurea receptor". Science. 270 (5239): 1166–70. doi:10.1126/science.270.5239.1166. PMID 7502040.
- Seino S, Miki T (February 2003). "Physiological and pathophysiological roles of ATP-sensitive K+ channels". Prog. Biophys. Mol. Biol. 81 (2): 133–76. doi:10.1016/S0079-6107(02)00053-6. PMID 12565699.
- Zhuo ML, Huang Y, Liu DP, Liang CC (April 2005). "KATP channel: relation with cell metabolism and role in the cardiovascular system". Int. J. Biochem. Cell Biol. 37 (4): 751–64. doi:10.1016/j.biocel.2004.10.008. PMID 15694835.
- Inoue I, Nagase H, Kishi K, Higuti T (July 1991). "ATP-sensitive K+ channel in the mitochondrial inner membrane". Nature. 352 (6332): 244–7. doi:10.1038/352244a0. PMID 1857420.
- Lacza Z, Snipes JA, Miller AW, Szabó C, Grover G, Busija DW (November 2003). "Heart mitochondria contain functional ATP-dependent K+ channels". J. Mol. Cell. Cardiol. 35 (11): 1339–47. doi:10.1016/S0022-2828(03)00249-9. PMID 14596790.
- Mironova GD; Grigoriev SM; Skarga YuYu; Negoda AE; Kolomytkin OV (1997). "ATP-dependent potassium channel from rat liver mitochondria: inhibitory analysis, channel clusterization". Membr Cell Biol. 10 (5): 583–91. PMID 9225262.
- Ardehali H, Chen Z, Ko Y, Mejía-Alvarez R, Marbán E (August 2004). "Multiprotein complex containing succinate dehydrogenase confers mitochondrial ATP-sensitive K+ channel activity". Proc. Natl. Acad. Sci. U.S.A. 101 (32): 11880–5. doi:10.1073/pnas.0401703101. PMC 511068. PMID 15284438.
- Quesada I, Rovira JM, Martin F, Roche E, Nadal A, Soria B (July 2002). "Nuclear KATP channels trigger nuclear Ca(2+) transients that modulate nuclear function". Proc. Natl. Acad. Sci. U.S.A. 99 (14): 9544–9. doi:10.1073/pnas.142039299. PMC 123177. PMID 12089327.
- Aguilar-Bryan L, Clement JP, Gonzalez G, Kunjilwar K, Babenko A, Bryan J (January 1998). "Toward understanding the assembly and structure of KATP channels". Physiol. Rev. 78 (1): 227–45. PMID 9457174.
- Moritz W, Leech CA, Ferrer J, Habener JF (January 2001). "Regulated expression of adenosine triphosphate-sensitive potassium channel subunits in pancreatic beta-cells". Endocrinology. 142 (1): 129–38. doi:10.1210/en.142.1.129. PMID 11145575.
- Akao M, Ohler A, O'Rourke B, Marbán E (June 2001). "Mitochondrial ATP-sensitive potassium channels inhibit apoptosis induced by oxidative stress in cardiac cells". Circ. Res. 88 (12): 1267–75. doi:10.1161/hh1201.092094. PMID 11420303.
- Crawford RM, Jovanović S, Budas GR, Davies AM, Lad H, Wenger RH, Robertson KA, Roy DJ, Ranki HJ, Jovanović A (August 2003). "Chronic mild hypoxia protects heart-derived H9c2 cells against acute hypoxia/reoxygenation by regulating expression of the SUR2A subunit of the ATP-sensitive K+ channel". J. Biol. Chem. 278 (33): 31444–55. doi:10.1074/jbc.M303051200. PMC 2134977. PMID 12791696.
- Ren Y, Xu X, Wang X (December 2003). "Altered mRNA expression of ATP-sensitive and inward rectifier potassium channel subunits in streptozotocin-induced diabetic rat heart and aorta". J. Pharmacol. Sci. 93 (4): 478–83. doi:10.1254/jphs.93.478. PMID 14737020.
- Craig TJ, Ashcroft FM, Prokes P (July 2008). "How ATP Inhibits the open KATP Channel". J. Gen. Physiol. 132 (1): 131–144. doi:10.1085/jgp.200709874. PMC 2442177. PMID 18591420.
- Markworth E, Schwanstecher C, Schwanstecher M (September 2000). "ATP4- mediates closure of pancreatic beta-cell ATP-sensitive potassium channels by interaction with 1 of 4 identical sites". Diabetes. 49 (9): 1413–8. doi:10.2337/diabetes.49.9.1413. PMID 10969823.
- Koster, J.; Marshall, B.A.; Ensor, N.; Corbett, J.A.; Nichols, C.G. (2000). "Targeted Overactivity of β Cell KATP Channels Induces Profound Neonatal Diabetes". Cell. 100 (6): 645–654. doi:10.1016/S0092-8674(00)80701-1.
- Koster, J. C.; Knopp, A.; Flagg, T. P.; Markova, K. P.; Sha, Q.; Enkvetchakul, D.; Betsuyaku, T.; Yamada, K. A.; Nichols, C. G. (2001). "Tolerance for ATP-Insensitive KATP Channels in Transgenic Mice". Circulation Research. 89 (11): 1022–9. doi:10.1161/hh2301.100342. PMID 11717159.
- Xu M, Wang Y, Ayub A, Ashraf M (September 2001). "Mitochondrial K(ATP) channel activation reduces anoxic injury by restoring mitochondrial membrane potential". Am. J. Physiol. Heart Circ. Physiol. 281 (3): H1295–303. PMID 11514300.
- Mubagwa K, Flameng W (October 2001). "Adenosine, adenosine receptors and myocardial protection: an updated overview". Cardiovasc. Res. 52 (1): 25–39. doi:10.1016/S0008-6363(01)00358-3. PMID 11557231.
- Suzuki M, Saito T, Sato T, Tamagawa M, Miki T, Seino S, Nakaya H (February 2003). "Cardioprotective effect of diazoxide is mediated by activation of sarcolemmal but not mitochondrial ATP-sensitive potassium channels in mice". Circulation. 107 (5): 682–5. doi:10.1161/01.CIR.0000055187.67365.81. PMID 12578868.
- Gong B, Miki T, Seino S, Renaud JM (November 2000). "A K(ATP) channel deficiency affects resting tension, not contractile force, during fatigue in skeletal muscle". Am. J. Physiol., Cell Physiol. 279 (5): C1351–8. PMID 11029282.
- Zingman LV, Hodgson DM, Bast PH, Kane GC, Perez-Terzic C, Gumina RJ, Pucar D, Bienengraeber M, Dzeja PP, Miki T, Seino S, Alekseev AE, Terzic A (October 2002). "Kir6.2 is required for adaptation to stress". Proc. Natl. Acad. Sci. U.S.A. 99 (20): 13278–83. doi:10.1073/pnas.212315199. PMC 130624. PMID 12271142.
- Chutkow WA, Pu J, Wheeler MT, Wada T, Makielski JC, Burant CF, McNally EM (July 2002). "Episodic coronary artery vasospasm and hypertension develop in the absence of Sur2 K(ATP) channels". J. Clin. Invest. 110 (2): 203–8. doi:10.1172/JCI15672. PMC 151064. PMID 12122112.
- Bienengraeber M, Olson TM, Selivanov VA, Kathmann EC, O'Cochlain F, Gao F, Karger AB, Ballew JD, Hodgson DM, Zingman LV, Pang YP, Alekseev AE, Terzic A (April 2004). "ABCC9 mutations identified in human dilated cardiomyopathy disrupt catalytic KATP channel gating". Nat. Genet. 36 (4): 382–7. doi:10.1038/ng1329. PMC 1995438. PMID 15034580.