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CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR CHAPTER 27 – THE SULFONYLUREA RECEPTOR AN ABCC ACTS AS AN ION CHANNEL REGULATOR
551 THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR MICHINORI MATSUO, KAZUMITSU UEDA, TIMOTHY RYDER AND FRANCES ASHCROFT INTRODUCTION ATP-binding cassette (ABC) proteins constitute a large and diverse family of integral membrane proteins, which are found in both prokaryotes and eukaryotes (Dean et al., 2001) Most members of this diverse family are involved in the ATP-dependent transport of solutes across surface or intracellular membranes (Higgins, 1992; Holland and Blight, 1999) Unique functions, however, have been identified for two members of the ABCC subfamily The cystic fibrosis transmembrane conductance regulator (CFTR) functions as a chloride ion channel, and harnesses the energy of ATP hydrolysis to open and close the channel pore The sulfonylurea receptor (SUR), the topic of this review, serves as the regulatory subunit of the ATP-sensitive potassium (KATP) channel, endowing it with the ability to respond to changes in cell metabolism It is an open question whether or not SUR has an additional classical transport function, and thus in this review we confine ourselves to the role of SUR as a channel regulator ATP-sensitive potassium (KATP) channels are inwardly rectifying potassium channels, which are inhibited by ATP and activated by MgADP They link the metabolism of the cell to the membrane potential by sensing changes in intracellular adenine nucleotide concentrations KATP channels play important functional roles in ABC Proteins: From Bacteria to Man ISBN 0-12-352551-9 27 CHAPTER numerous tissues, including pancreatic -cells, neurons, cardiac muscle, skeletal muscle and smooth muscle For example, their activation leads to shortening of the cardiac action potential, relaxation of vascular smooth muscle, and inhibition of insulin secretion and neurotransmitter release The KATP channel is a hetero-octameric complex of four Kir6.x and four SUR subunits Kir6.x belongs to the family of inwardly rectifying Kϩ channels and assembles into tetramers to form the channel (Figure 27.1) Binding of ATP to the intracellular domains of Kir6.x produces channel inhibition Associated with each Kir6.x subunit is a regulatory subunit, the sulfonylurea receptor (SUR) Like other members of the ABC transporter family, SUR has two large intracellular domains, containing consensus sequences for nucleotide binding and hydrolysis, which are known as the nucleotidebinding domains (NBDs) Interaction of Mgnucleotides with these NBDs mediates activation of the KATP channel The SUR subunit also binds therapeutic drugs, such as the sulfonylureas, which inhibit KATP channel activity, and the KATP channel openers, which stimulate channel activity More than one isoform exists for both Kir6.x (Kir6.1, Kir6.2) and SUR (SUR1, SUR2A, SUR2B) and variation in the subunit composition of the KATP channel accounts for the different metabolic and drug sensitivities of KATP channels in different cells In most tissues, Kir6.2 serves as Copyright 2003 Elsevier Science Ltd All rights of reproduction in any form reserved ABC PROTEINS: FROM BACTERIA TO MAN NH2 Kir6.x SUR SU Ki r K SU ir R K SU ir R Outside SU Ki R r 552 R Inside NH2 WA WB WA WB SS SS NBD1 NBD2 COOH COOH Figure 27.1 KATP channels are formed from two different types of subunits The KATP channel is an octameric complex of four pore-forming Kir6.x subunits and four regulatory sulfonylurea receptor (SUR) subunits Like other inwardly rectifying Kϩ channels, Kir6.2 has only two transmembrane segments, and cytosolic N- and C-termini The cytosolic domains are involved in channel inhibition by ATP SUR has 17 transmembrane segments, arranged in groups of 5, and 6, and two cytosolic nucleotide-binding domains (NBDs), each of which contains a Walker A (WA) motif, a Walker B (WB) motif and an ABC signature sequence (SS) These motifs are involved in the activation of the KATP channel by Mg-nucleotides the pore-forming subunit, but it associates with different SUR subunits: for example, SUR1 in pancreas and brain, SUR2A in heart and skeletal muscle and SUR2B in a variety of tissues including brain and smooth muscle In some smooth muscles, the KATP channel comprises Kir6.1 in association with SUR2B In this review, we first focus on the physiological role of the KATP channel and how this is impaired in disease We then discuss the molecular composition of the KATP channel, and detail its regulation by nucleotides, metabolism and other signaling molecules Finally, we briefly summarize what is known of the pharmacological properties of the channel PHYSIOLOGICAL ROLE KATP CHANNELS IN THE PANCREAS The physiological role of the KATP channel is best understood in the pancreatic -cell, where metabolically induced changes in KATP channel activity play a key role in glucose-stimulated insulin secretion This is illustrated in the cartoon in Figure 27.2 At substimulatory glucose concentrations, KATP channels are open, and their activity serves to maintain the resting membrane potential at a hyperpolarized level Elevation of the blood glucose concentration increases glucose uptake and metabolism by the -cell, producing changes in cytosolic nucleotide concentrations that result in closure of the KATP channels This leads to depolarization of the -cell membrane potential, and thus to activation of voltage-gated calcium channels and Ca2ϩ influx The resulting rise in the intracellular Ca2ϩ concentration triggers insulin release The physiological importance of the KATP channel (Kir6.2/SUR1) in regulating insulin secretion is demonstrated by the fact that mutations in the SUR1 subunit (discussed below) have been found in patients with persistent hyperinsulinemic hypoglycemia of infancy (PHHI), a serious disorder characterized by excessive and unregulated insulin secretion Furthermore, defective metabolic regulation of the KATP channel results in diabetes mellitus (see Glossary, page 566) in both humans and transgenic animals KATP channels have also been described in the glucagon-secreting ␣-cells (Gopel et al., 2000b) and somatostatin-secreting ␦-cells (Gopel et al., 2000a) of the pancreas In both cell types, glucose metabolism results in KATP channel closure In ␦-cells, this causes hormonal secretion (as it does in -cells) In ␣-cells, however, KATP channel closure and the resulting membrane depolarization causes inactivation of the voltage-gated channels that participate in action THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR Figure 27.2 Model of a pancreatic -cell summarizing the roles of major players, including SUR1/Kir6.2, involved in regulating insulin secretion (Modified and reprinted by permission from Nature (Bell and Polonsky, 2001) copyright 2001 Macmillan Publishers Ltd.) ATP generated by glycolysis and the Krebs cycle can result in inhibition and closure of the SUR1/Kir6.2, ATP-sensitive channel The consequent reduction in ؉ ؉ potassium efflux results in membrane depolarization and Ca2؉ influx High intracellular Ca2؉ levels lead to the fusion of secretory granules containing insulin with the membrane and secretion of insulin into the circulation potential generation Consequently, glucagon secretion is inhibited Insights from genetically engineered mice Transgenic mice have proved to be valuable tools for analyzing the physiological role of the KATP channel (Seino et al., 2000) Miki and colleagues (1997) showed that mutation of a key glycine in the Kϩ channel selectivity sequence to serine (Kir6.2-G132S) has a dominant-negative effect on the KATP channel By expressing this transgene under the control of the insulin promoter, they were able to selectively delete KATP channel function in the pancreatic -cell Neonatal transgenic mice developed hypoglycemia with hyperinsulinemia, which progressed to hyperglycemia with hypoinsulinemia in later life A high frequency of apoptotic -cells was found prior to the appearance of hyperglycemia, suggesting that KATP channels may play a role in pancreatic -cell survival (Miki et al., 2001a) A Kir6.2 knockout mouse, in which Kir6.2 was disrupted genetically in all tissues, had a different phenotype (Miki et al., 1998) Despite showing defective glucose-induced insulin secretion, these mice had only a mild impairment of glucose tolerance This appeared to be due to an enhanced insulin sensitivity, which compensated for the reduced insulin secretion The -cell number decreased with age, but not as markedly as in the transgenic mice (Miki et al., 2001a) Kir6.2 knockout mice can develop fasting hyperglycemia and glucose intolerance with age, but only if they become obese This suggests that both the genetic defect and environmental factors are required for the mice to develop diabetes Like the Kir6.2 knockout mice, SUR1 knockout mice are normoglycemic, unless stressed (Seghers et al., 2000) However, they are not insulin hypersensitive They become more hyperglycemic when glucose-loaded, and more hypoglycemic when fasted, than wildtype mice Isolated islets from SUR1 knockout mice lack first phase insulin secretion and have an attenuated second phase secretion in response to glucose stimulation As expected, KATP currents are not found in -cells of Kir6.2 knockout or SUR1 knockout mice (Miki et al., 553 554 ABC PROTEINS: FROM BACTERIA TO MAN 2001a; Seghers et al., 2000) Because patients carrying loss-of-function mutations in the SUR1 gene can be severely hypoglycemic, unlike the SUR1 knockout mice, it is hypothesized that KATP-independent pathways controlling insulin secretion may be regulated differently in man and mouse (Seghers et al., 2000) The effect of reducing the KATP channel ATP sensitivity has also been explored An N-terminal truncated Kir6.2, which forms KATP channels with about 10-fold lower ATP sensitivity, was expressed under the control of the insulin promoter, so that overexpression was confined to the -cell (Koster et al., 2000) The mutation is expected to partially prevent the decrease in -cell KATP channel activity that occurs when plasma glucose levels rise, and thereby to decrease insulin resistance As predicted the mice showed a severe diabetic phenotype: they were hyperglycemic, hypoinsulinemic, and most of them died within days of birth Because histological analysis revealed that islet morphology, insulin localization, and ␣- and -cell distribution were normal, the diabetic phenotype may be caused by reduced insulin secretion An interesting and important finding was that a comparatively small reduction in ATP sensitivity (Ͻ10-fold) was sufficient to cause diabetes In conclusion, studies of genetically engineered mice demonstrate that KATP channels play a key role in both glucose-induced and sulfonylurea-induced insulin secretion They further show that defects in KATP channel activity can predispose the animal to diabetes, which is exacerbated by obesity PHHI Persistent hyperinsulinemic hypoglycemia of infancy (PHHI), also known as familial hyperinsulinism or nesidioblastosis, is an inherited disorder characterized by abnormally high levels of insulin secretion despite severe hypoglycemia (Glaser, 2000; Sharma et al., 2000) It presents at birth or in early childhood, and in the absence of clinical treatment may be lethal or result in irreversible neurological damage Mild cases of the disease can be treated with the KATP channel opener diazoxide (or even by supplementing the diet with glucose), but the more severe forms require subtotal pancreatectomy The frequency of PHHI in the general population is low but in inbred populations it may be as high as in 2500 live births In most families, PHHI is inherited in an autosomal recessive manner, but dominant forms of the disease have also been described (Glaser et al., 1998; Huopio et al., 2000) Sporadic cases have also been identified Histopathologically, PHHI can also be divided into two types (Glaser, 2000) One is a diffuse form of the disease, in which the morphological defect is found within all the islets throughout the pancreas and the size of islets is irregular The other is a focal type, in which the morphological defect is localized to a particular area within the pancreas The disease results from mutations in at least four different genes: the KATP channel genes Kir6.2 and SUR1, and the enzymes glucokinase (Glaser et al., 1998) and glutamate dehydrogenase (Stanley et al., 1998) Mutations in SUR1 are the most common cause of the disorder and account for about 50% of cases All mutations are thought to lead to loss of KATP channel function in the pancreatic -cell, as a consequence of defects either in the channel itself or in its metabolic regulation Because the KATP channel sets the -cell membrane potential, loss of channel activity produces a persistent membrane depolarization that leads to activation of voltage-gated Ca2ϩ influx and continuous insulin secretion, irrespective of the blood glucose level Mutations in Kir6.2 and SUR1 causing PHH1 In this review, we focus on the mutations in KATP channel subunits responsible for PHHI To date, three mutations have been identified in the Kir6.2 gene (Aguilar-Bryan and Bryan, 1999; Nestorowicz et al., 1997; Thomas et al., 1996b) and numerous mutations in the SUR1 gene (Dunne et al., 1997; Kane et al., 1996; Nestorowicz et al., 1996; Nichols et al., 1996; Otonkoski et al., 1999; Sharma et al., 2000; Shyng et al., 1998; Tanizawa et al., 2000; Thomas et al., 1995, 1996a; Verkarre et al., 1998) The Kir6.2 mutations comprise two missense mutations, L147P and W91R, and a nonsense mutation that truncates Kir6.2 after 12 amino acids Kir6.2 carrying these mutations does not reconstitute functional KATP channels when coexpressed with SUR1 (Aguilar-Bryan and Bryan, 1999; Nestorowicz et al., 1997; Thomas et al., 1996a) The SUR1 mutations are very heterogeneous They are found throughout the gene and they include nonsense, missense, frameshift and THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR splice site mutations Functionally, these mutations can be grouped into two broad classes: those in which the channel is not present in the surface membrane, and those in which the channel is present in the plasma membrane but is always closed, independent of the metabolic state of the cell The molecular mechanism of action of several SUR1 mutations has been analyzed in detail, both by examination of -cells obtained from therapeutic pancreatectomy (Kane et al., 1996) and by heterologous expression of the mutant channels (Cartier et al., 2001; Matsuo et al., 2000b; Nichols et al., 1996; Otonkoski et al., 1999; Shyng et al., 1997) Such studies have provided important insights into the structure–function relationships of the KATP channel, as well as its physiological role A number of mutations are associated with the absence of KATP channel activity even when exposed to nucleotide-free intracellular solutions, suggesting the channel may not be present in the surface membrane (e.g Cartier et al., 2001; Otonkoski et al., 1999; Shyng et al., 1997) Direct evidence for defective trafficking to the plasma membrane has been obtained for one of these mutations (deletion of F1388 in SUR1) (Cartier et al., 2001) Interestingly, the ⌬F1388 mutant channels have reduced ATP sensitivity and not respond to stimulation by MgADP or diazoxide, even when expressed at the surface membrane by mutations that correct the trafficking defect Other mutations influence nucleotide interactions with the SUR1 subunit Nichols and colleagues (1996) first reported that the G1479R mutation, which lies within NBD2, just upstream of the signature sequence, strongly reduced channel activation by MgADP Subsequently, many other mutations have also been found to reduce MgADP activation (e.g Huopio et al., 2000; Shyng et al., 1997) In these mutant channels, it appears that loss of MgADP activation underlies the inability of the channel to respond to metabolic inhibition In some cases, the mutant channel retains sensitivity to diazoxide, which accounts for the fact that some PHHI patients respond to this drug A missense mutation (R1420C) within NBD2 of SUR1 is associated with mild disease (Matsuo et al., 2000b) This mutation slightly reduced the affinity of nucleotide binding to NBD2, and shifted the concentration–response curve for channel activation by MgADP to higher concentrations It also reduced the surface expression of the channel These two effects may explain why the mutation causes PHHI Interestingly, the R1420C mutation impairs the ability of MgADP, acting at NBD2, to stabilize the binding of 8-azido-ATP at NBD1 Because MgADP activation was not abolished, this result suggests that nucleotide stabilization at NBD1 is not required for channel activation by MgADP All mutations in which SUR1 does not reach the surface membrane produce a severe form of PHHI Mutations that are associated with loss or reduction of MgADP activation appear to be somewhat less severe However, some mutations altered channel function only minimally in vitro but were associated with severe clinical disease Thus, the precise relationship between the individual mutation and the severity of the clinical phenotype is not completely clear Diabetes Because KATP channels regulate insulin secretion (see Figure 27.2), it seems logical to postulate that gain-of-function mutations in the SUR1 or Kir6.2 genes might lead to diabetes mellitus Enhanced KATP channel activity in the presence of stimulatory glucose concentrations would impair -cell depolarization, resulting in reduced Ca2ϩ channel activation, less Ca2ϩ influx and less insulin release A larger KATP current could arise from a reduced ATP sensitivity of the channel (as in the case of the transgenic mice), enhanced sensitivity to the stimulatory effects of MgADP, a higher channel density in the plasma membrane, or altered concentrations of channel modulators (e.g ATP) Many linkage analyses and mutation screening studies of the Kir6.2 and SUR1 genes have been performed, but the results remain confusing Several groups have reported that genetic variation in SUR1 (Iwasaki et al., 1996; Lindner et al., 1997; Stirling et al., 1995) or Kir6.2 (Sakura et al., 1996) does not play a major role in susceptibility to type diabetes in diverse populations (Mexican-American, Japanese, Caucasian or non-Hispanic whites) On the other hand, some SUR1 (Hart et al., 2000; Inoue et al., 1996; Rissanen et al., 2000) or Kir6.2 (Hani et al., 1998) gene variants are reported to be associated with type diabetes in different populations, including Caucasians and Finns Meta-analysis of several studies showed that the polymorphism E23K, which lies in the N-terminus of Kir6.2, is associated with type diabetes in Caucasians (Hani et al., 1998) Our unpublished studies indicate that the E23K 555 556 ABC PROTEINS: FROM BACTERIA TO MAN mutation produces, if anything, only a small reduction in the ATP sensitivity of the KATP channel However, it is apparent from studies on genetically engineered mice (Koster et al., 2000) that small changes in KATP channel ATP sensitivity are sufficient to produce diabetes This is because the input resistance of the -cell is very high, so that small changes in KATP current exert large effects on the membrane potential In this regard, it is worth remembering that the ATP sensitivity of the KATP channel can also be reduced by lipid modulators, such as PIP2 (Baukrowitz et al., 1998; Fan and Makielski, 1997; Shyng and Nichols, 1998) and long-chain acyl-CoA (Bränström et al., 1997, 1998; Gribble et al., 1998a) Because plasma lipids, including long-chain acyl-CoA esters, are elevated in obesity (Prentki and Corkey, 1996; Prentki et al., 1997), this may be one mechanism by which obesity is linked to the development of impaired insulin secretion and diabetes Recent studies have suggested that PHHI may be self-limiting since affected -cells undergo apoptosis (Kassem et al., 2000), and there is accumulating evidence that some forms of PHHI lead to type diabetes in later life For example, the dominant E1506K mutation in the SUR1 gene (which lies immediately after the Walker B motif of NBD2), which leads to a reduction in the level, but not complete loss, of KATP channel function, is associated with the progressive development of a reduced insulin secretory capacity that predisposes to diabetes in adult life (Huopio et al., 2000) This may be because continuous -cell membrane depolarization, resulting from the loss of functional KATP channels, produces an increase in the intracellular Ca2ϩ concentration This, in turn, could activate apoptosis, so reducing -cell mass (Efanova et al., 1998) Studies of genetically engineered mice support the idea that loss of KATP channel function gives rise to -cell apoptosis (Miki et al., 2001a) Impaired metabolic regulation of KATP channels resulting from mutations in genes that influence -cell metabolism can also cause diabetes Maturity-onset diabetes of the young (MODY) is characterized by early onset and autosomal dominant inheritance Mutations in six different genes are known to cause MODY In French families (Froguel et al., 1993), around 60% of all MODY patients carry mutations in the gene encoding glucokinase, the glycolytic enzyme that catalyzes the conversion of glucose to glucose-6-phosphate in liver and -cells (Randle, 1993) Mutations in the transcription factor, hepatocyte nuclear factor-1␣, are the commonest cause of MODY in the UK population (Frayling et al., 1997) Although the molecular mechanism of this MODY variant is not known, there is clear evidence that impaired -cell metabolism is the basis of the disease, and that this causes reduced metabolic regulation of KATP channels (Dukes et al., 1998) Impaired mitochondrial metabolism can also give rise to diabetes, as in maternally inherited diabetes with deafness (MIDD), which results from a mutation at position 3243 of the mitochondrial DNA that encodes a leucine transfer RNA (Maassen and Kadowaki, 1996) All these mutations result in reduced KATP channel inhibition in response to glucose metabolism and consequently impaired insulin secretion Uncoupling proteins induce a proton leak in the mitochondria that leads to impaired ATP production Mice in which the uncoupling protein UCP2 was knocked out had higher islet ATP levels and increased insulin secretion, whereas rodents overexpressing UCP2 had impaired insulin secretion (Chan et al., 2001; Zhang et al., 2001) These results suggest that mutations in uncoupling proteins may also be involved in the development of diabetes and obesity in humans KATP CHANNELS IN THE BRAIN In situ hybridization and immunocytochemistry suggest that SUR1 and Kir6.2 are expressed in several regions of the brain (Karschin et al., 1997) Single-cell polymerase chain reaction (PCR) data are providing finer resolution of neuronal expression These studies have shown, for example, that gamma-aminobutyric acidergic (GABAergic) neurons in the substantia nigra (SN) selectively express SUR1 and Kir6.2, whereas dopaminergic SN neurons express Kir6.2 together with SUR1 or SUR2B, or both SURs (Liss et al., 1999) Genetically engineered mice have facilitated our understanding of the role of KATP channels in the brain In particular, they point to a role for KATP channels in seizure prevention Hypoxia inhibited the activity of substantia nigra reticulata (SNr) neurons in wild-type mice, but enhanced SNr neuronal activity in Kir6.2 knockout mice (Yamada et al., 2001) Furthermore, Kir6.2 knockout mice are susceptible to generalized seizures after brief hypoxia This suggests that the opening of KATP channels in SNr neurons protects against seizure propagation during THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR metabolic stress (Yamada et al., 2001) In support of this idea, transgenic mice that overexpress SUR1 in cortex, hippocampus, and striatum are more resistant to kainic acid-induced seizures than wild-type mice (Hernandez-Sanchez et al., 2001) The transgenic animals suffered no loss of hippocampal pyramidal neurons after kainic acid administration, whereas wild-type mice lost 70–80% of their pyramidal neurons These results indicate that the overexpression of SUR1 in forebrain protects mice from seizures and neuronal damage It is difficult to assess whether PHHI subjects carrying SUR1 mutations also suffer neurological problems as a result of decreased neuronal KATP channel function, because any brain damage that is present might be a consequence of low blood glucose levels experienced prior to diagnosis and treatment KATP channels also serve as glucose sensors in glucose-responsive neurons of the ventromedial hypothalamus, a role which is essential for the maintenance of glucose homeostasis (Miki et al., 2001b) Although pancreatic -cell function is intact in Kir6.2 knockout mice, the animals exhibit a severe defect in glucagon secretion in response to systemic hypoglycemia In addition, they show a complete loss of glucagon secretion, together with reduced food intake, in response to neuroglucopenia Thus hypothalamic and -cell KATP channels act in concert, as central and peripheral glucose sensors, to regulate glucose homeostasis KATP CHANNELS IN THE CARDIOVASCULAR SYSTEM In most tissues other than the pancreas, including cardiac and skeletal muscle, KATP channels are closed under normal conditions In the heart, KATP channels open when the intracellular concentration of ATP falls under ischemic stress (Nichols and Lederer, 1990) This serves to shorten the action potential duration and reduce Ca2ϩ influx, so decreasing contractile force and ATP consumption In this way, KATP channel activation helps protect the myocardium from ischemic injury Analysis of Kir6.2 knockout mice confirms that sarcolemmal KATP channels containing Kir6.2 subunits mediate the depression of cardiac excitability and contractility produced by KATP channel openers and metabolic inhibition (Suzuki et al., 2001) Vascular smooth muscle KATP channels are thought to play a role in regulation of vessel tone (Quayle et al., 1997; Yokoshiki et al., 1998) They are regulated by a variety of neurotransmitters, some of which mediate their effects via protein kinase A (which enhances KATP channel activity) and/ or protein kinase C (which decreases channel activity) (Hayabuchi et al., 2001) Analysis of Kir6.2 knockout mice suggests that Kir6.2 does not contribute to the arterial KATP channel (Suzuki et al., 2001), and it seems likely that Kir6.1 serves this role (Inagaki et al., 1995b) In addition to KATP channels in the plasma membrane, it has been suggested that the mitochondrial membrane contains another type of KATP channel that may be involved in ischemic preconditioning in the heart (Grover and Garlid, 2000) As the molecular identity of this channel remains unknown, and its relationship to ABC transporters is unclear, it will not be considered further here MOLECULAR PROPERTIES MOLECULAR COMPOSITION As shown in Figure 27.1, the KATP channel is a hetero-octameric complex of Kir6.x and SUR subunits (Clement et al., 1997; Inagaki et al., 1997; Shyng and Nichols, 1997) Kir6.x belongs to the family of inwardly rectifying Kϩ channels and there are two isoforms: Kir6.1 and Kir6.2 (Inagaki et al., 1995a, 1995b; Sakura et al., 1995) Both isoforms have cytosolic N- and C-termini, and two transmembrane segments, linked by a region that is predicted to re-enter the membrane and serve as the selectivity filter for potassium ions In most tissues, Kir6.2 acts as the pore-forming subunit of the KATP channel, but in vascular smooth muscle Kir6.1 serves this role The site at which ATP binds to mediate channel inhibition lies within the cytosolic domains of Kir6.2 (Tanabe et al., 1999; Tucker et al., 1998) SUR1 is a member of the ABCC subfamily of the ABC proteins (Aguilar-Bryan et al., 1995), which also includes the multidrug resistance-associated protein (MRP), and the cystic fibrosis transmembrane conductance regulator (CFTR) (Dean et al., 2001) Although SUR1 was originally thought to have 13 transmembrane segments (Aguilar-Bryan et al., 1995), sequence alignment of SURs and MRPs suggested that there are 17 transmembrane segments (TMS), arranged in three groups of 5, and TMSs 557 558 ABC PROTEINS: FROM BACTERIA TO MAN (Tusnády et al., 1997) (see Figure 27.1) Evidence for the 17 TMS model was obtained subsequently by epitope mapping (RaabGraham et al., 1999) There are two large intracellular NBDs, one between TMS 11 and 12 and the other at the C-terminus Like other ABC transporters (Higgins, 1992), each NBD contains several highly conserved motifs These include a Walker A motif and a Walker B motif, a linker sequence (the ABC signature sequence, LSGGQ), and an invariant glutamine and histidine residue (sometimes equated with the Q-loop and H-loop, respectively) There are two subtypes of SUR: SUR1 (ABCC8) and SUR2 (ABCC9) (Inagaki et al., 1996) SUR2A shares 68% amino acid identity with SUR1, and Northern blotting has revealed that it is expressed at high levels in heart, skeletal muscle, and ovary Two splicing variants of SUR2A (SUR2B and SUR2C) have been identified (Chutkow et al., 1999; Yamada et al., 1997) SUR2B is identical to SUR2A except for its C-terminal 42 amino acids, which more closely resemble those of SUR1 SUR2C has a deletion of 35 amino acids near NBD1 of SUR2A (Chutkow et al., 1999) The Kir6.2 and SUR1 genes map to the same region of human chromosome 11p15.1, and both reading frames are aligned in the same direction (Inagaki et al., 1995a) The Kir6.1 and SUR2 genes are similarly clustered on human chromosome 12p (11.23-12.12) (Chutcow et al., 1996) STOICHIOMETRY There is good evidence that Kir6.2 co-assembles as a tetramer, as is the case for other Kir channels (Clement et al., 1997; Shyng and Nichols, 1997) (see Figure 27.1) The stoichiometry of the KATP channel has been determined using tandem constructs of SUR1 and Kir6.2 subunits (Clement et al., 1997; Inagaki et al., 1997) The SUR1–Kir6.2 construct showed similar properties to native channels, indicating that a 1:1 stoichiometry is sufficient for functional KATP channels In contrast, the SUR1–Kir6.2–Kir6.2 construct did not produce functional channels, although it could be rescued by coexpression with SUR1 This suggests that each Kir6.2 subunit requires an SUR partner The octameric (4 ϩ 4) nature of the KATP channel complex is supported by biochemical studies showing that the molecular mass of the purified Kir6.2/ SUR1 complex is approximately 950 kDa, which corresponds to four SUR1 and four Kir6.2 subunits (Clement et al., 1997) ASSEMBLY AND TRAFFICKING OF KATP CHANNELS Both Kir6.2 and SUR possess an endoplasmic reticulum retention motif (RKR) that prevents their surface expression in the absence of the other type of subunit (Zerangue et al., 1999) This ensures that only fully functional octameric complexes reach the plasma membrane The RKR motif in Kir6.2 lies within the C-terminal region of the protein and is masked in the presence of SUR1 Truncation of the last 26–36 amino acids of Kir6.2 deletes this retention signal and thus allows plasma membrane expression in the absence of SUR (Tucker et al., 1997) SUR1 also contains an RKR motif in a cytoplasmic loop between TMS11 and NBD1 (Zerangue et al., 1999) In addition to this ER retention signal, the C-terminus of SUR1 has been proposed to contain an anterograde signal, consisting of a dileucine motif and a downstream phenylalanine, which is required for KATP channels to exit the ER of mammalian cells (Sharma et al., 1999) In contrast, truncation of NBD2 of SUR1 does not prevent functional KATP channels from reaching the plasma membrane when expressed in Xenopus oocytes (Sakura et al., 1999) Whether this reflects a difference in KATP channel trafficking in oocytes and mammalian cells, or some other process, has not been ascertained SUR1 and Kir6.2 must be closely associated, since Kir6.2 can be photoaffinity labeled with a ligand that binds to SUR (Clement et al., 1997), and the subunits can be co-immunoprecipitated (Lorenz et al., 1998) The first transmembrane segment and the N-terminus of Kir6.2 are involved in KATP assembly (Giblin et al., 1999; Schwappach et al., 2000) A requirement for the C-terminus of Kir6.2 has also been reported (Giblin et al., 1999; Lorenz and Terzic, 1999) The regions of SUR that are involved in assembly and interaction with Kir6.2 have not been fully defined Using a trafficking-based assay for detection of such interactions, Schwappach and colleagues (2000) concluded that TMS 6–17 of SUR1 are required for interaction with Kir6.2 and that the NBDs are not needed for correct assembly In a different approach, TMS 12 and 13 were shown not to be required for SUR1 assembly, but TMS12 was found to be involved in interaction with Kir6.2 (Mikhailov et al., 2000) THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR REGULATION BY NUCLEOTIDES The nucleotide regulation of the KATP channel is complex, as channel activity is inhibited by nucleotide binding to Kir6.2 and activated by Mg-nucleotides binding to the two NBDs of SUR In addition, MgATP may activate lipid and protein kinases, thereby increasing the membrane concentration of phospholipids such as PIP2 (which modulates channel ATP sensitivity), or altering the phosphorylation state of the channel itself This makes it more difficult to elucidate the molecular mechanisms underlying nucleotide regulation of the KATP channel Two types of experiment have helped to determine the properties of the different binding sites: first, radiolabeled ATP binding to one type of subunit, expressed in the absence of the other; and secondly, electrophysiological studies of KATP channels containing mutant subunits THE KIR6.2 NUCLEOTIDE-BINDING SITE Truncation of the cytoplasmically located Cterminal 26–36 amino acids of Kir6.2 (Kir6.2⌬C), allows this subunit to reach the surface membrane in the absence of SUR This construct permitted the first demonstration that ATP binding to Kir6.2 causes channel closure (Tucker et al., 1997) Confirmation that Kir6.2 indeed binds ATP is provided by the fact that this subunit can be photoaffinity labeled with the ATP analogues 8-azido-ATP and ATP-[␥]4-azidoanilido (Tanabe et al., 1999, 2000) Kir6.2⌬C, expressed in the absence of SUR, has proved a useful tool for analyzing the properties of the Kir6.2 ATPbinding site Unlike many classical ATP-binding sites, Mg2ϩ is not required for the action of the nucleotide, suggesting that channel inhibition does not require nucleotide hydrolysis Inhibition is extremely sensitive to the structure of the adenine base moiety Even ITP, which differs from ATP at only two positions, blocks the channel approximately 50-fold less effectively than ATP (Tucker et al., 1998) In contrast, the terminal (␥)-phosphate group of ATP appears not to be required since ADP blocks almost as effectively as ATP (Tucker et al., 1998) Furthermore, diadenosine polyphosphates, in which extra phosphate groups and an additional adenosine moiety are attached to the ␥-phosphate, inhibit channel activity as effectively as ATP AMP, however, in which the -phosphate group is removed, is a poor inhibitor of channel activity (Tucker et al., 1998) Hence, it appears that both the adenine base and the -phosphate group are necessary for high-affinity channel inhibition Although the site to which ATP binds lies on Kir6.2, coexpression with SUR enhances the potency of ATP inhibition by about 10-fold (Tucker et al., 1997), by a mechanism that is not understood There is evidence that each Kir6.2 monomer in the KATP channel complex has its own ATPbinding site, so that there are four per channel (Markworth et al., 2000) However, binding of ATP to just one subunit is sufficient to induce channel closure The precise location of the ATP-binding site on Kir6.2 remains unclear There are no obvious ATP-binding consensus sequences and mutagenesis studies require careful interpretation because a mutation may affect the channel ATP sensitivity in several ways For example, it may influence ATP binding, impair the ability of the channel to close, or interfere with the transduction mechanism by which ATP binding induces channel closure If ATP binds to a particular state of the channel (e.g the closed state), even binding studies may not be able to distinguish between these possibilities However, several mutations have been found that reduce the channel ATP sensitivity without altering the single-channel kinetics in the absence of ATP These lie within both the N- and C-terminus and include residues R50, K185, I182, R201A and G334 (Drain et al., 1998; Shyng et al., 2000; Tucker et al., 1997, 1998) Both R50G and K185Q mutations have been directly shown to decrease ATP binding (Tanabe et al., 2000) THE SUR NUCLEOTIDE-BINDING SITE It is well established that MgADP stimulates KATP channel activity Because MgADP blocks Kir6.2⌬C but stimulates Kir6.2/SUR1 (Tucker et al., 1997), it appears that MgADP stimulation is mediated by the SUR subunit Mutations within the NBDs of SUR have confirmed that this is the case and demonstrated that MgADP mediates its stimulatory effects by interaction with the NBDs (Gribble et al., 1997a; Nichols et al., 1996; Shyng et al., 1997 ) Thus, mutation of the conserved Walker A lysine or the Walker B aspartate abolished MgADP activation and unmasked an inhibitory action of the nucleotide on Kir6.2 (Gribble et al., 1997a) 559 560 ABC PROTEINS: FROM BACTERIA TO MAN In most ABC transporters, MgATP is the major ligand, and its hydrolysis to MgADP provides the energy required for substrate transport A difficulty with trying to investigate the effects of MgATP at the NBDs of SUR is that the nucleotide also produces a potent block of the channel via Kir6.2 To circumvent this problem, Gribble et al (1998b) coexpressed SUR1 with an ATP-insensitive pore mutant, Kir6.2-R50G Addition of MgATP stimulated the activity of these channels (Gribble et al., 1998b), an effect that was abolished by simultaneous mutation of the Walker A lysine in both NBDs of SUR1 Thus, like MgADP, MgATP stimulates KATP channel activity via interaction with the NBDs of SUR1 There is growing evidence, however, that MgADP is the effective ligand, and that MgATP must be hydrolyzed to MgADP at the NBDs of SUR1 before it is able to enhance KATP channel activity (Zingman et al., 2001) Nucleotide binding by the NBDs The nucleotide-binding properties of the two NBDs of SURs have been examined directly, using the photoaffinity analogue 8-azido-ATP (Ueda et al., 1997) 8-Azido-ATP binding to SUR1 is both high affinity and very stable This made it possible to investigate the cooperative interaction between the two NBDs (Ueda et al., 1999) Prebound 8-azido-ATP did not dissociate at 0°C and dissociated only slowly at 37°C, provided that Mg2ϩ was present Addition of MgADP or MgATP markedly stabilized prebound 8-azido-ATP, whereas the slowly hydrolyzable ATP analogue ATP-␥S did not Mutations in the Walker A and B motifs of NBD2 had almost no effect on 8-azido-ATP binding itself, but abolished the ability of MgADP to stabilize prebound 8-azido-ATP These results suggest that MgADP, either by directly binding to NBD2 or by hydrolysis of bound MgATP, induces a conformational change at NBD2 that results in stabilization of ATP binding at NBD1 A similar cooperativity was found for nucleotide binding to the two NBDs of SUR2A and SUR2B (Matsuo et al., 2000a) The functional significance of this effect is unclear, however, because a mutation in SUR1 (R1420C, located between the Walker A and the LSGG motifs in NBD2) that abolished cooperativity did not impair the ability of either MgATP or MgADP to stimulate channel activity (Matsuo et al., 2000b) To examine the nucleotide binding of two NBDs in more detail, SURs photoaffinity labeled with 8-azido-[32P]ATP were digested mildly with trypsin and tryptic fragments immunoprecipitated with antibodies against NBD1 or NBD2 (Matsuo et al., 1999, 2000a) The ATP-binding properties of the tryptic fragments indicated that NBD1 of SUR binds 8-azido-ATP in a Mg2ϩ-independent manner, and that NBD2 binds 8-azido-ATP in a Mg2ϩdependent manner Because KATP channels are activated by ADP only in the presence of Mg2ϩ (Gribble et al., 1997a), NBD2 seems to be primarily responsible for the channel activation by MgADP Mutations in the signature sequence of NBD2 of SUR1 (e.g G1479D, G1479R, G1485D, G1485R, Q1486H and D1506A) abolish KATP channel activation by MgADP, whereas similar mutations in NBD1 (e.g G827D, G827R and Q834H) not (Gribble et al., 1997a; Shyng et al., 1997) Furthermore, mutation of the Walker A lysine in NBD2 of SUR2A (K1348A) abolishes channel activation by MgADP, but the corresponding mutation in NBD1 (K707A) does not (Reimann et al., 2000) These results support the idea that NBD2 of SUR is essential for MgADP activation and that NBD1 may play a secondary role Although mutation of the Walker A lysine in NBD1 of SUR1 (K719A) abolishes the ability of MgADP to stimulate KATP channel activity (Gribble et al., 1997a), we have found that the mutation of this residue (K719M) abolishes 8-azido-ATP binding at both NBDs of SUR1 (Matsuo et al., unpublished data) Thus, it is not clear yet if ATP binding to NBD1 is required for MgADP activation of the KATP channel It remains possible, however, that nucleotide binding to NBD1 facilitates the action of MgATP (or indeed MgADP) at NBD2: for example by enhancing nucleotide hydrolysis or transduction of nucleotide binding/hydrolysis into channel gating The nucleotide-binding affinities of the NBD1 of SUR1 are significantly higher than those of SUR2A and SUR2B, particularly in the case of ATP (Matsuo et al., 2000a) Interestingly, the affinity of NBD1 of SUR2B for ATP is higher than that of SUR2A, and the affinities of NBD2 of SUR2B for both ATP and ADP are greater than those of SUR2A Because SUR2A and SUR2B share the same amino acid sequence except for their C-terminal 42 amino acids, the C-terminal ‘tail’ may affect the nucleotidebinding properties of both NBDs The different nucleotide-binding affinities of the various NBDs THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR may explain, in part, the differential nucleotide regulation of KATP channel subtypes For example, the fact that higher concentrations of MgADP are needed to activate Kir6.2/SUR2A channels than Kir6.2/SUR1 or Kir6.2/SUR2B channels, when tested in the presence of MgATP (Matsuoka et al., 2000) ATP hydrolysis by the NBDs Unlike many other ABC transporters, where both NBDs are believed to hydrolyze ATP, NBD2 of SUR appears to be more efficient at hydrolyzing ATP than NBD1 (but see Chapter 29) Thus, under conditions permitting ATP hydrolysis, NBD2 of all three SUR subtypes was photoaffinity labeled with 8-azido-[␣-32P]ATP but not with 8-azido-[␥-32P]ATP, whereas NBD1 was photoaffinity labeled with both ligands (Matsuo et al., 2000a) This suggests that NBD2 has ATPase activity and that NBD1 has little or none It has also been shown that a construct consisting of NBD2 of SUR2 (lacking the C-terminal tail) fused to the maltose-binding protein has twice the ATPase activity of a similar fusion protein containing NBD1 However, it should be noted that these ATPase activities are very low: Vmax ϭ 0.018 and 0.009 mol minϪ1 mgϪ1, for NBD2 and NBD1, respectively (Bienengraeber et al., 2000; Zingman et al., 2001) For comparison, the NBD1 of CFTR fused to the maltosebinding protein and the purified recombinant CFTR from Sf9 cells showed ATPase activity of about 0.03 and 0.05 mol minϪ1 mgϪ1 of purified protein, respectively (Bear et al., 1997; Ko and Pederson, 1995; Li et al., 1996) The lower ATPase activity of SUR is consistent with its role as a channel regulator rather than a transporter Interestingly, the ATPase activity of NBD2 is stimulated by KATP channel openers, such as rilmakalim, pinacidil, cromakalim, diazoxide and nicorandil (Bienengraeber et al., 2000) METABOLIC REGULATION OF THE KATP CHANNEL KATP channels couple cellular metabolism to electrical excitability, by sensing changes in intracellular ATP and ADP concentrations The intracellular ATP concentration ([ATP]i) is estimated to lie between and mM, even during metabolic inhibition (Gribble et al., 1997b, 2000; Himmelreich and Dobson, 2000), whereas free MgADP concentrations are thought to be less than 100 M (Askenasy and Navon, 1997; Ghosh et al., 1991; Himmelreich and Dobson, 2000) Even under metabolic stress, changes in cytosolic ADP concentration only occur within the 100 M range (Weiss and Venkatesh, 1993) This suggests that metabolically induced changes in MgADP not modulate KATP channel activity by directly competing with MgATP for binding at NBD2 Rather, an ATP hydrolysis cycle at NBD2 generates bound MgADP and changes in cell metabolism influence KATP channel activity by modulating the length of time that NBD2 remains in the MgADP-bound (active) state (Zingman et al., 2001) When metabolic activity is high, cytosolic ADP levels will be low so that MgADP dissociates more rapidly from NBD2, causing channel activity to decrease In contrast, when metabolic activity declines, the rise in MgADP will slow the off-rate of MgADP, and promote channel opening In this way, SUR monitors changes in intracellular MgADP concentration In addition to their stimulatory effects, mediated via NBD2 of SUR, nucleotides (ATP, ADP) also inhibit the KATP channel via Kir6.2 The metabolic response of the KATP channel will reflect all of these processes It will also be affected by the metabolic rate of the cell, and the extent to which changes in submembrane nucleotide concentrations are buffered, e.g by creatine kinase and adenylate kinase (Carrasco et al., 2001) Native KATP channels exhibit different sensitivities to metabolic inhibition KATP channels in pancreatic -cells and microvascular coronary endothelial cells open when extracellular glucose levels fall (Ashcroft et al., 1984; Langheinrich and Daut, 1997), whereas cardiac channels remain closed in zero glucose solutions and only open in response to ischaemia or metabolic inhibition (Nichols and Lederer, 1991) There is evidence that KATP channels in vascular smooth muscle contribute to the regulation of coronary flow under both normoxic and hypoxic conditions (Daut et al., 1994) These differences in metabolic sensitivity reflect both differences in cell metabolism and differences in the metabolic sensitivities of the channels themselves Thus, Kir6.2/SUR2A channels are less activated by MgATP than either Kir6.2/SUR2B or Kir6.2/SUR1 channels 561 562 ABC PROTEINS: FROM BACTERIA TO MAN (Reimann et al., 2000) There is evidence that this may relate to differences in nucleotide handling at NBD2 (Song and Ashcroft, unpublished) COMPARISON WITH OTHER ABC TRANSPORTERS Most ABC transporters are active transporters that expend the energy of ATP hydrolysis to transport compounds against a chemical gradient (Dean et al., 2001; Higgins, 1992) In these proteins, a catalytic ATP hydrolysis cycle is coupled to substrate transport (Senior and Gadsby, 1997) In contrast, SUR serves as the regulatory subunit of the KATP channel and it is therefore not perhaps surprising that the nucleotide-binding properties of the NBDs of SUR differ from those of other ABC proteins The archetypal ABC transporter MDR1 can be photoaffinity labeled by 8-azido-ATP in the presence of orthovanadate (Senior et al., 1995; Ueda et al., 1997; Urbatsch et al., 1995) This vanadate-induced nucleotide trapping occurs at both NBDs, although not simultaneously, and is abolished by mutations in the Walker A or Walker B motif of either NBD (Azzaria et al., 1989; Loo and Clarke, 1995a; Muller et al., 1996) or by modification of either NBD with N-ethylmaleimide (NEM) (Al-Shawi et al., 1994; Liu and Sharom, 1996; Loo and Clarke, 1995b; Takada et al., 1998) This indicates that the ATPase activity of the two NBDs of MDR1 is highly cooperative Some interaction also occurs between the NBDs of SUR, because MgADP binding to NBD2 stabilizes ATP binding at NBD1 However, the very low ATP hydrolysis rate at NBD1 of SUR in the intact protein (Matsuo et al., 1999, 2000a) suggests that the ATP hydrolysis cycle is not coupled in the same way as found for MDR Mg2ϩ is required for ATP binding and hydrolysis at both NBDs of MDR1 (Ueda et al., 1997) By analogy, we may assume that although ATP binding to NBD1 of SUR does not require Mg2ϩ, the cation will be necessary if there are conditions under which NBD1 is able to hydrolyze MgATP Photoaffinity labeling of MRP1, a xenobiotic exporter that also belongs to the ABCC subfamily, demonstrated that NBD1 binds 8-azidoATP strongly in the presence of Mg2ϩ and that NBD2 shows vanadate-induced nucleotide trapping (Gao et al., 2000; Hou et al., 2000; Nagata et al., 2000) In contrast, NBD1 of SUR binds 8-azido-ATP strongly even in the absence of Mg2ϩ (Matsuo et al., 1999, 2000a; Ueda et al., 1997) Although NBD2 did not show vanadate-induced nucleotide trapping under the conditions examined in these experiments, other studies are consistent with the presence of nucleotide trapping at NBD2 of SUR2A (Zingman et al., 2001) Beryllium fluoride, which mimics a prehydrolytic ADPbound state, caused KATP channel closure, whereas orthovanadate, which mimics a posthydrolytic ADP-bound state, induced channel opening ATP hydrolysis has been implicated in the regulation of chloride flux through the CFTR Cl channel (Baukrowitz et al., 1994), although its precise role is somewhat controversial It was originally proposed that channel gating was entirely dependent on ATP hydrolysis (Hwang et al., 1994; Weinreich et al., 1999) However, the finding that ATP can support channel activity in the absence of Mg suggests this not the case NBD1 of CFTR is photoaffinity labeled by 8-azido-ATP with high affinity Binding is Mg2ϩ and temperature dependent, and is very stable, with nucleotide occlusion predicted to take place during the ATP hydrolysis cycle of CFTR (Szabo et al., 1999) ATP binding to NBD1 of SUR1 is also temperature dependent (Matsuo et al., 1999), but does not require Mg2ϩ (Ueda et al., 1997) Thus, although nucleotide occlusion produced by a conformational change at NBD1 of SUR1 might take place, it does not so as part of an ATP hydrolysis cycle Szabo et al (1999) predicted that NBD1 of CFTR rapidly enters an occluded nucleotide state, whereas the complete ATP hydrolytic cycle takes much longer In contrast, both ATP hydrolysis and ADP release are much faster at NBD2, which explains why vanadate is required for stabilizing the transient state of NBD2 Mg-independent nucleotide binding at NBD1 and slow ATP hydrolysis at NBD2 seem to be unique features of SUR compared with other ABC proteins However, it is not clear what causes these differences Hrycyna et al (1999) reported that the context of the NBD, rather than its exact sequence, is an important determinant of ATP binding This suggests that the three-dimensional structure of the NBDs, which will be determined not only by their amino acid sequence but also by their interaction with membrane domains, serves as a major determinant of the nucleotide-binding properties of SUR THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR EFFECTS OF OTHER KATP CHANNEL REGULATORS REGULATION BY PHOSPHOLIPIDS Membrane phospholipids such as PIP2 and PIP3 interact with KATP channels to increase their probability of being open and to reduce their ATP sensitivity (Baukrowitz and Fakler, 2000; Baukrowitz et al., 1998; Fan and Makielski, 1997, 1999; Hilgemann and Ball, 1996; Shyng and Nichols, 1998) This has been shown by direct application of PIPs to KATP channels in excised patches In addition, overexpression of PI5kinase, which enhances PIP2 levels, reduces the ATP sensitivity of the channel in membrane patches (Shyng et al., 2000), whereas breakdown of PIP2 by phospholipase C enhances the KATP channel ATP sensitivity (Xie et al., 1999) The effects of PIPs are mediated principally though the Kir6.2 subunit, because Kir6.2⌬C expressed in the absence of SUR also shows a reduced ATP sensitivity to applied PIP2 (Baukrowitz et al., 1998) and direct binding of PIP2 to Kir channels has been demonstrated (Huang et al., 1998) However, SUR may also modulate the PIP2 sensitivity, perhaps by changing the probability of channel opening (Song and Ashcroft, 2001) It appears that PIP2 may have both a direct effect on the channel ATP sensitivity and an indirect effect that is a consequence of altered channel gating (Fan and Makielski, 1999) The rundown of both native and cloned KATP channels that occurs in excised membrane patches is reversed by application of MgATP (Findlay and Dunne, 1986; Ohno-Shosaku et al., 1987; Xie et al., 1999) and PIP2 has also been implicated in such an effect (Xie et al., 1999) It is postulated that PIP2 is produced in the plasma membrane by serial phosphorylation of phosphatidylinositol (PI) and that this process is sequentially catalyzed by PI4-kinase and PIP-kinase MgATP generation of phospholipids therefore has multiple, but related, effects on KATP channel activity REGULATION BY PHOSPHORYLATION Phosphorylation of the R-domain of CFTR by protein kinase A (PKA) is required for channel activity (Gadsby and Nairn, 1999) Interestingly, SUR has a string of negatively charged amino acids in an equivalent position, raising the possibility that they may serve a similar role to the phosphorylated R-domain The activity of KATP channels is also regulated by protein kinase phosphorylation, although the effects are not so dramatic as for CFTR Protein kinase A (PKA) stimulates activity of both native and recombinant KATP channels (Light, 1996), while protein kinase C (PKC) reduces the activity of smooth muscle KATP channels Phosphorylation of Kir6.2 by PKA or PKC has been shown to influence the ATP sensitivity and/or open probability of the KATP channel (Béguin et al., 1999; Light et al., 2000; Lin et al., 2000) There is also evidence that phosphorylation of SUR may be involved in channel regulation Human SUR1 has four consensus sites for phosphorylation by PKA (Béguin et al., 1999) PKA phosphorylation of SUR1 (at S1571) decreases the burst duration, interburst interval and open probability of Kir6.2/SUR1 channels (Béguin et al., 1999) REGULATION BY G-PROTEINS There are several reports that KATP channel activity is upregulated by trimeric GTP-binding (G) proteins (Sánchez et al., 1998; Wada et al., 2000) Thus, G␣ was found to stimulate both Kir6.2/SUR1 and Kir6.2/SUR2A channels, whereas G␥ subunits had no effect (Sánchez et al., 1998) In contrast, another study reported that G␥ subunits cause a reduction of ATPinduced inhibition of Kir6.2/SUR2A channels that is mediated by G␥ interaction with SUR2A (Wada et al., 2000) ENDOGENOUS LIGANDS The high affinity with which sulfonylureas bind to SUR1 led to the search for endogenous ligands of the KATP channel Two peptides isolated from porcine brain, ␣- and -endosulfine, were found to inhibit sulfonylurea binding in vitro (Virsolvy-Vergine et al., 1988) Cloning of human ␣-endosulfine (Heron et al., 1998) and analysis of the recombinant protein revealed that it inhibits Kir6.2/SUR1 channel activity and stimulates insulin secretion in the absence of glucose Its functional significance has not yet been established 563 564 ABC PROTEINS: FROM BACTERIA TO MAN PHARMACOLOGY CHANNEL BLOCKERS Inhibitors of KATP channel activity fall into two groups: those that interact with Kir6.2 and those that interact with SUR1 All these drugs stimulate insulin secretion Imidazolines such as phentolamine and cibenzoline block KATP channels by binding to Kir6.2 (Mukai et al., 1998; Proks and Ashcroft, 1997) In contrast, sulfonylureas (tolbutamide, gliclazide, glimepiride) and benzamido derivatives (meglitinide) close KATP channels by binding with high affinity to SUR1 (for review, see Ashcroft and Gribble, 1999) Many of these drugs are used therapeutically to stimulate insulin secretion in type diabetes Sulfonylureas also interact with low affinity with Kir6.2 (Gribble et al., 1997b, 1998c), but this effect is of no clinical relevance, as the concentrations required to inhibit the Kir6.2 subunit are much higher than those found in the plasma of patients treated with the drugs Recent studies of cloned channels have revealed that low concentrations of tolbutamide block KATP channels containing SUR1, but not SUR2A or SUR2B, subunits (Gribble et al., 1998c) In contrast, meglitinide blocks all three types of cloned KATP channel This has been interpreted to indicate that sulfonylureas (such as tolbutamide) interact with a binding site that is specific to SUR1, whereas benzamido compounds (such as meglitinide) interact with a site that is common to all SUR subtypes (for review, see Ashcroft and Gribble, 1999) Glibenclamide, which contains both sulfonylurea and benzamido moieties, is postulated to interact with both sites on SUR1 but only a single (benzamido) on SUR2 This may account for the slow off-rate of the drug in electrophysiological experiments (it has to unbind at both sites) The different affinities of Kir6.2/SUR1 and Kir6.2/SUR2 channels for tolbutamide were exploited to determine the tolbutamide-binding site, using a chimeric approach (Ashfield et al., 1999; Babenko et al., 1999) These studies showed that TMS 14–16 of SUR1 are required for high-affinity tolbutamide inhibition and that serine-1237, which lies in the predicted cytoplasmic loop between TMS 15 and 16, is crucial for high-affinity block This is consistent with earlier studies suggesting an intracellular site of action for sulfonylureas (Lee et al., 1994; Schwanstecher et al., 1994) Recent studies, using a different approach, have shown that only the TMS 5–6 and TMS 15–16 cytosolic loops are needed for [3H]-glibenclamide binding (Mikhailov et al., 2001) This raises the possibility that the TMS 5–6 loop corresponds to the benzamido-binding site and the TMS 15–16 loop to the tolbutamide-binding site It also suggests that TMSs 14–16 may be involved in transducing binding at the ‘sulfonylurea’ site into channel inhibition MgADP modulates the inhibitory action of sulfonylureas by enhancing the apparent block of -cell KATP channels and reducing that of cardiac channels (Zunkler et al., 1988) Studies of recombinant Kir6.2/SUR1 channels have shown that sulfonylureas prevent the stimulatory effect of MgADP at SUR1, and unmask its inhibitory effect on Kir6.2 (Gribble et al., 1997b) This produces an apparent enhancement of the sulfonylurea block in the presence of MgADP, because the inhibitory effect of MgADP adds to that of the sulfonylurea It is not known whether the interaction between MgADP and sulfonylureas results because the sulfonylurea displaces MgADP binding to the NBDs, or because it blocks transduction of the stimulatory action of MgADP bound to the NBDs However, nanomolar concentrations of glibenclamide not change the affinity of the NBDs of SUR1 for either ATP or ADP (Matsuo et al., unpublished result) Although glibenclamide destabilizes ATP binding at NBD1 of SUR1 in the presence of MgATP or MgADP (Ueda et al., 1999), it is not clear if this is directly connected with high-affinity glibenclamide inhibition of KATP channels The mechanism of the reduced sulfonylurea block of Kir6.2/SUR2 channels in the presence of MgADP is also not certain However, it seems likely that this is a consequence of the enhanced channel activity produced by MgADP, which results in the channel spending less time in the interburst closed state Because sulfonylureas appear to act by stabilizing the interburst closed states (Gillis et al., 1989), the blocking potency of the drug will be reduced by MgADP (Alekseev et al., 1997) Thus, one can think of the intraburst state(s) as ligand-insensitive states and the interburst state(s) as ligand-sensitive states A similar effect would presumably operate in the case of Kir6.2/SUR1 channels, were it not for the ability of sulfonylureas to prevent the stimulatory effect of MgADP in this channel POTASSIUM CHANNEL OPENERS Potassium channel openers (KCOs) are a structurally diverse group of drugs which share the THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR common property that they activate KATP channels, thereby hyperpolarizing the plasma membrane and reducing electrical excitability (Ashcroft and Gribble, 2000) They include diazoxide, cromakalim, pinacidil and nicorandil None of these drugs is in widespread clinical use, although diazoxide is used to suppress excessive insulin secretion, minoxidil sulfate is used topically to stimulate hair growth, and nicorandil is currently in clinical trials for the treatment of angina Different SUR subtypes confer varying sensitivities to KATP channel openers Thus, in the presence of cytosolic MgATP, Kir6.2/SUR1 channels are stimulated by diazoxide but not by pinacidil or cromakalim, whereas Kir6.2/SUR2A channels are stimulated by pinacidil and cromakalim but not by diazoxide, and Kir6.2/SUR2B channels are stimulated by pinacidil, cromakalim and diazoxide (Babenko et al., 1998; D’hahan et al., 1999a, 1999b; Gribble et al., 1998c; Inagaki et al., 1995a, 1996; Isomoto et al., 1996) KATP channel openers interact with nucleotides in a complex fashion In particular, ATP is required for binding of the pinacidil analogue [3H]-P1075 (Hambrock et al., 1998, 1999), although channel activity can be stimulated by P1075 in the absence of added nucleotide (Gribble et al., 2000; Reimann et al., 2000; Terzic et al., 1995) The faster off-rate of the drug in the absence of MgATP (Gribble et al., 2000) may mean that it is more difficult to measure P1075 binding in nucleotide-free solutions Alternatively, ATP may remain bound at NBD1 for some time after patch excision, in electrophysiological experiments Recent evidence indicates that KATP channel openers, such as pinacidil, stimulate ATP hydrolysis at NBD2 (Bienengraeber et al., 2000) and that they promote channel opening by stabilizing the channel in the Mg-nucleotide bound state (Zingman et al., 2001) Conversely, agents that enhance the removal of MgADP, such as creatine kinase, promote channel closure (Zingman et al., 2001) The fact that KCOs stimulate the ATPase activity of SURs suggests the possibility that these ABC proteins may transport KCOs across the cell membrane, because MDR1 and MRP1 substrates also induce ATPase activity in those transporters However, no transport function has yet been demonstrated for SUR The binding site for most SUR2-specific KATP channel openers (e.g cromakalim, pinacidil, P1076) was first shown to lie within the third set of transmembrane segments (Babenko et al., 2000; D’hahan et al., 1999a; Uhde et al., 1999), and was subsequently narrowed down to TMS 17 Two residues in this TMS were found to be of key importance: L1249 and T1253 of SUR2A (Moreau et al., 2000) Interestingly, TMSs 12–17 are also involved in substrate recognition by MRPs and MDRs (Hafkemeyer et al., 1998; Ito et al., 2001a, 2001b; Loo and Clarke, 2001) The location of the binding site for diazoxide is still uncertain This drug binds to both SUR1 and SUR2 receptors, as it can activate Kir6.2/SUR1 and Kir6.2/SUR2B channels when MgATP is present (D’hanan et al., 1999b) Moreover, although it does not activate Kir6.2/SUR2A channels under these conditions, it is able to increase channel activity in the presence of MgADP, implying that the drug also binds to SUR2A This indicates that the diazoxide-binding site is common to all three SURs It further suggests that MgADP binding, probably at NBD2, is required for action of the drug The differential effects of MgATP on diazoxide activation are consistent with the idea that this drug stimulates ATPase activity at NBD2 of SUR1 and SUR2B but not SUR2A CONCLUSION AND PERSPECTIVES The sulfonylurea receptor differs from other ABC proteins in that its primary function is that of an ion channel regulator rather than an ATPdependent transporter Although SUR retains an ATPase cycle at NBD2, this is not apparently used to power substrate transport against a concentration gradient Instead, Kϩ ions move through the Kir6.2 pore along their electrochemical gradient, (as in other Kϩ channels), and the ATPase cycle at NBD2 has been adapted to serve as a sensor of cellular metabolism This may explain why the rate of ATP hydrolysis is relatively slow, compared to other ABC proteins The KATP channel is a valuable tool for studying the function of the NBDs of an ABC protein, because nucleotide interaction with the NBDs is coupled to gating of Kir6.2 Thus, potassium fluxes through the Kir6.2 pore can be used to monitor NBD function with high time resolution Furthermore, binding of MgADP to the NBDs of SUR is sufficient to stimulate channel activity, which enables the ATP hydrolysis step to be bypassed, and the 565 566 ABC PROTEINS: FROM BACTERIA TO MAN effects of ligand binding and transduction examined in isolation Many issues remain to be resolved As in the case of some other ABC proteins, the NBDs of SUR are not functionally equivalent However, the precise role of NBD1 is not yet clear Likewise, the role of the ABC signature sequence in SUR, as in other ABC proteins, remains controversial and it is not certain whether it is involved in ATP binding and/or hydrolysis or in signal transduction Nucleotidebinding studies, coupled with electrophysiological recordings, may help to resolve these questions The way in which conformational changes at the NBDs are transduced into changes in the transmembrane domains is not yet known Indeed, it is not clear whether regulation of Kir6.2 gating by SUR is mediated via the TMDs of SUR or through the cytosolic domains Increasingly, it is apparent that many ion channels possess regulatory (or beta) subunits and are modulated by a variety of cytosolic agents that interact with either the pore-forming or regulatory subunit The KATP channel appears to be subject to more extensive modulation than most channels, perhaps because it possesses two very different types of subunit Precisely how channel regulators such as nucleotides, sulfonylureas and KATP channel openers achieve their functional effects is likely to take some time to sort out Elucidation of the threedimensional structure of this large and complex hetero-octameric channel (either the whole complex or its constituent parts), however, would be a major step towards this goal GLOSSARY Diabetes mellitus A metabolic disorder characterized by elevation of the fasting blood glucose concentration There are two major forms of the disease: type and type Type diabetes results from the autoimmune destruction of the pancreatic -cells Type diabetes is characterized by the inability of the pancreatic -cells to secrete sufficient insulin and by a reduced efficacy of insulin action Type diabetes is far more common and can affect up to 40% of elderly adults in some populations It is treated by diet, drugs (such as sulfonylureas) and, if these fail, insulin Glucose intolerance A pre-diabetic condition in which the blood glucose concentration remains elevated for an abnormally long time after a meal (or glucose challenge) Hyperglycemia A higher than normal blood glucose concentration A fasting blood glucose concentration of above 5.5 mM is usually regarded as hyperglycemic Hyperinsulinemia A higher than normal blood insulin concentration Hypoxia Low oxygen level Ischemia Interruption of the blood supply First and second phase insulin secretion Insulin secretion shows a biphasic response to a glucose challenge: a rapid, large and transient response (first phase) that is followed by a smaller but sustained response (second phase) Seizure An abnormal discharge of electrical activity in the brain ACKNOWLEDGMENTS This work was supported by grants from the Royal Society, the Wellcome Trust, the Japan Ministry of Education, Science, Sports and Culture FMA is the Royal Society GlaxoSmithKline Research Professor REFERENCES Aguilar-Bryan, L and Bryan, J (1999) Molecular biology of adenosine triphosphate-sensitive potassium channels Endocr Rev 20, 101–135 Aguilar-Bryan, L., Nichols, C.G., Wechsler, S.W., Clement IV, J.P., Boyd III, A.E., González, G., Herrera-Sosa, H., Nguy, K., Bryan, J and Nelson, D.A (1995) Cloning of the -cell high-affinity sulfonylurea receptor: A regulator of insulin secretion Science 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THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR EFFECTS OF OTHER KATP CHANNEL REGULATORS REGULATION BY PHOSPHOLIPIDS Membrane phospholipids such as PIP2 and... membrane depolarization causes inactivation of the voltage-gated channels that participate in action THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR Figure 27. 2