Differential response of K(ATP) channels containing SUR2A or SUR2B subunits to nucleotides and pinacidil

Sulfonylurea receptor 2 (SUR2), intricate sensors for intracellular Mg-nucleotides

SUR2, similar to SUR1, is a regulatory subunit of the ATP-sensitive potassium channel (KATP), which plays a key role in numerous important physiological processes and is implicated in various diseases. Recent structural studies have revealed that, like SUR1, SUR2 can undergo ligand-dependent dynamic conformational changes, transitioning between an inhibitory inward-facing conformation and an activating occluded conformation. In addition, SUR2 possesses a unique inhibitory Regulatory helix (R helix) that is absent in SUR1. The binding of the activating Mg-ADP to NBD2 of SUR2 competes with the inhibitory Mg-ATP, thereby promoting the release of the R helix and initiating the activation process. Moreover, the signal generated by Mg-ADP binding to NBD2 might be directly transmitted to the TMD of SUR2, prior to NBD dimerization. Furthermore, the C-terminal 42 residues (C42) of SUR2 might allosterically regulate the kinetics of Mg-nucleotide binding on NBD2. These distinctive properties render SUR2 intricate sensors for intracellular Mg-nucleotides.

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Rapid Characterization of the Functional and Pharmacological Consequences of Cantú Syndrome KATP Channel Mutations in Intact Cells

Gain-of-function of KATP channels, resulting from mutations in either KCNJ8 (encoding inward rectifier sub-family 6 [Kir6.1]) or ABCC9 (encoding sulphonylurea receptor [SUR2]), cause Cantú syndrome (CS), a channelopathy characterized by excess hair growth, coarse facial appearance, cardiomegaly, and lymphedema. Here, we established a pipeline for rapid analysis of CS mutation consequences in Landing pad HEK 293 cell lines stably expressing wild type (WT) and mutant human Kir6.1 and SUR2B. Thallium-influx and cell membrane potential, reported by fluorescent Tl-sensitive Fluozin-2 and voltage-sensitive bis-(1,3-dibutylbarbituric acid)trimethine oxonol (DiBAC4(3)) dyes, respectively, were used to assess channel activity. In the Tl-influx assay, CS-associated Kir6.1 mutations increased sensitivity to the ATP-sensitive potassium (KATP) channel activator, pinacidil, but there was strikingly little effect of pinacidil for any SUR2B mutations, reflecting unexpected differences in the molecular mechanisms of Kir6.1 versus SUR2B mutations. Compared with the Tl-influx assay, the DiBAC4(3) assay presents more significant signal changes in response to subtle KATP channel activity changes, and all CS mutants (both Kir6.1 and SUR2B), but not WT channels, caused marked hyperpolarization, demonstrating that all mutants were activated under ambient conditions in intact cells. Most SUR2 CS mutations were markedly inhibited by <100 nM glibenclamide, but sensitivity to inhibition by glibenclamide, repaglinide, and PNU37883A was markedly reduced for Kir6.1 CS mutations. Understanding functional consequences of mutations can help with disease diagnosis and treatment. The analysis pipeline we have developed has the potential to rapidly identify mutational consequences, aiding future CS diagnosis, drug discovery, and individualization of treatment. SIGNIFICANCE STATEMENT: We have developed new fluorescence-based assays of channel activities and drug sensitivities of Cantú syndrome (CS) mutations in human Kir6.1/SUR2B-dependent KATP channels, showing that Kir6.1 mutations increase sensitivity to potassium channel openers, while SUR2B mutations markedly reduce K channel opener (KCO) sensitivity. However, both Kir6.1 and SUR2B CS mutations are both more hyperpolarized than WT cells under basal conditions, confirming pathophysiologically relevant gain-of-function, validating DiBAC4(3) fluorescence to characterize hyperpolarization induced by KATP channel activity under basal, non KCO-activated conditions.

The inhibition mechanism of the SUR2A-containing KATP channel by a regulatory helix

KATP channels are metabolic sensors for intracellular ATP/ADP ratios, play essential roles in many physiological processes, and are implicated in a spectrum of pathological conditions. SUR2A-containing KATP channels differ from other subtypes in their sensitivity to Mg-ADP activation. However, the underlying structural mechanism remains poorly understood. Here we present a series of cryo-EM structures of SUR2A in the presence of different combinations of Mg-nucleotides and the allosteric inhibitor repaglinide. These structures uncover regulatory helix (R helix) on the NBD1-TMD2 linker, which wedges between NBD1 and NBD2. R helix stabilizes SUR2A in the NBD-separated conformation to inhibit channel activation. The competitive binding of Mg-ADP with Mg-ATP to NBD2 mobilizes the R helix to relieve such inhibition, allowing channel activation. The structures of SUR2B in similar conditions suggest that the C-terminal 42 residues of SUR2B enhance the structural dynamics of NBD2 and facilitate the dissociation of the R helix and the binding of Mg-ADP to NBD2, promoting NBD dimerization and subsequent channel activation.

Personalized Therapeutics for KATP-Dependent Pathologies

Ubiquitously expressed throughout the body, ATP-sensitive potassium (K_ATP) channels couple cellular metabolism to electrical activity in multiple tissues; their unique assembly as four Kir6 pore-forming subunits and four sulfonylurea receptor (SUR) subunits has resulted in a large armory of selective channel opener and inhibitor drugs. The spectrum of monogenic pathologies that result from gain- or loss-of-function mutations in these channels, and the potential for therapeutic correction of these pathologies, is now clear. However, while available drugs can be effective treatments for specific pathologies, cross-reactivity with the other Kir6 or SUR subfamily members can result in drug-induced versions of each pathology and may limit therapeutic usefulness. This review discusses the background to K_ATP channel physiology, pathology, and pharmacology and considers the potential for more specific or effective therapeutic agents.

Structural identification of vasodilator binding sites on the SUR2 subunit

ATP-sensitive potassium channels (K_ATP), composed of Kir6 and SUR subunits, convert the metabolic status of the cell into electrical signals. Pharmacological activation of SUR2- containing K_ATP channels by class of small molecule drugs known as K_ATP openers leads to hyperpolarization of excitable cells and to vasodilation. Thus, K_ATP openers could be used to treat cardiovascular diseases. However, where these vasodilators bind to K_ATP and how they activate the channel remains elusive. Here, we present cryo-EM structures of SUR2A and SUR2B subunits in complex with Mg-nucleotides and P1075 or levcromakalim, two chemically distinct K_ATP openers that are specific to SUR2. Both P1075 and levcromakalim bind to a common site in the transmembrane domain (TMD) of the SUR2 subunit, which is between TMD1 and TMD2 and is embraced by TM10, TM11, TM12, TM14, and TM17. These K_ATP openers synergize with Mg-nucleotides to stabilize SUR2 in the NBD-dimerized occluded state to activate the channel.

Pathophysiological Consequences of KATP Channel Overactivity and Pharmacological Response to Glibenclamide in Skeletal Muscle of a Murine Model of Cantù Syndrome

Cantù syndrome (CS) arises from mutations in ABCC9 and KCNJ8 genes that lead to gain of function (GOF) of ATP-sensitive potassium (KATP) channels containing SUR2A and Kir6.1 subunits, respectively, of KATP channels. Pathological consequences of CS have been reported for cardiac and smooth muscle cells but consequences in skeletal muscle are unknown. Children with CS show muscle hypotonia and adult manifest fatigability. We analyzed muscle properties of Kir6.1[V65M] CS mice, by measurements of forelimb strength and ultrasonography of hind-limb muscles, as well as assessing KATP channel properties in native Flexor digitorum brevis (FDB) and Soleus (SOL) fibers by the patch-clamp technique in parallel with histopathological, immunohistochemical and Polymerase Chain Reaction (PCR) analysis. Forelimb strength was lower in Kir6.1^wt/VM mice than in WT mice. Also, a significant enhancement of echodensity was observed in hind-limb muscles of Kir6.1^wt/VM mice relative to WT, suggesting the presence of fibrous tissue. There was a higher KATP channel current amplitude in Kir6.1^wt/VM FDB fibers relative to WT and a reduced response to glibenclamide. The IC₅₀ of glibenclamide to block KATP channels in FDB fibers was 1.3 ± 0.2 × 10⁻⁷ M in WT and 1.2 ± 0.1 × 10⁻⁶ M in Kir6.1^wt/VM mice, respectively; and it was 1.2 ± 0.4 × 10⁻⁷ M in SOL WT fibers but not measurable in Kir6.1^wt/VM fibers. The sensitivity of the KATP channel to MgATP was not modified in Kir6.1^wt/VM fibers. Histopathological/immunohistochemical analysis of SOL revealed degeneration plus regressive-necrotic lesions with regeneration, and up-regulation of Atrogin-1, MuRF1, and BNIP3 mRNA/proteins in Kir6.1^wt/VM mice. Kir6.1^wt/VM mutation in skeletal muscle leads to changes of the KATP channel response to glibenclamide in FDB and SOL fibers, and it is associated with histopathological and gene expression changes in slow-twitch muscle, suggesting marked atrophy and autophagy.

Central Role of Subthreshold Currents in Myotonia

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Role of the C-terminus of SUR in the differential regulation of β-cell and cardiac KATP channels by MgADP and metabolism

Key points

β‐Cell K_ATP channels are partially open in the absence of metabolic substrates, whereas cardiac K_ATP channels are closed.

Using cloned channels heterologously expressed in Xenopus oocytes we measured the effect of MgADP on the MgATP concentration–inhibition curve immediately after patch excision.

MgADP caused a far more striking reduction in ATP inhibition of Kir6.2/SUR1 channels than Kir6.2/SUR2A channels; this effect declined rapidly after patch excision.

Exchanging the final 42 amino acids of SUR was sufficient to switch the Mg‐nucleotide regulation of Kir6.2/SUR1 and Kir6.2/SUR2A channels, and partially switch their sensitivity to metabolic inhibition.

Deletion of the C‐terminal 42 residues of SUR abolished MgADP activation of both Kir6.2/SUR1 and Kir6.2/SUR2A channels.

We conclude that the different metabolic sensitivity of Kir6.2/SUR1 and Kir6.2/SUR2A channels is at least partially due to their different regulation by Mg‐nucleotides, which is determined by the final 42 amino acids.

Abstract

ATP‐sensitive potassium (K_ATP) channels couple the metabolic state of a cell to its electrical activity and play important physiological roles in many tissues. In contrast to β‐cell (Kir6.2/SUR1) channels, which open when extracellular glucose levels fall, cardiac (Kir6.2/SUR2A) channels remain closed. This is due to differences in the SUR subunit rather than cell metabolism. As ATP inhibition and MgADP activation are similar for both types of channels, we investigated channel inhibition by MgATP in the presence of 100 μm MgADP immediately after patch excision [when the channel open probability (P_O) is near maximal]. The results were strikingly different: 100 μm MgADP substantially reduced MgATP inhibition of Kir6.2/SUR1, but had no effect on MgATP inhibition of Kir6.2/SUR2A. Exchanging the final 42 residues of SUR2A with that of SUR1 switched the channel phenotype (and vice versa), and deleting this region abolished Mg‐nucleotide activation. This suggests the C‐terminal 42 residues are important for the ability of MgADP to influence ATP inhibition at Kir6.2. This region was also necessary, but not sufficient, for activation of the K_ATP channel in intact cells by metabolic inhibition (azide). We conclude that the ability of MgADP to impair ATP inhibition at Kir6.2 accounts, in part, for the differential metabolic sensitivities of β‐cell and cardiac K_ATP channels.

β‐Cell K_ATP channels are partially open in the absence of metabolic substrates, whereas cardiac K_ATP channels are closed.

Using cloned channels heterologously expressed in Xenopus oocytes we measured the effect of MgADP on the MgATP concentration–inhibition curve immediately after patch excision.

MgADP caused a far more striking reduction in ATP inhibition of Kir6.2/SUR1 channels than Kir6.2/SUR2A channels; this effect declined rapidly after patch excision.

Exchanging the final 42 amino acids of SUR was sufficient to switch the Mg‐nucleotide regulation of Kir6.2/SUR1 and Kir6.2/SUR2A channels, and partially switch their sensitivity to metabolic inhibition.

Deletion of the C‐terminal 42 residues of SUR abolished MgADP activation of both Kir6.2/SUR1 and Kir6.2/SUR2A channels.

We conclude that the different metabolic sensitivity of Kir6.2/SUR1 and Kir6.2/SUR2A channels is at least partially due to their different regulation by Mg‐nucleotides, which is determined by the final 42 amino acids.

Cryo-electron microscopy structures and progress toward a dynamic understanding of KATP channels

Puljung reviews recent cryo-EM K_ATP channel structures and proposes a mechanism by which ligand binding results in channel opening.

Adenosine triphosphate (ATP)–sensitive K⁺ (K_ATP) channels are molecular sensors of cell metabolism. These hetero-octameric channels, comprising four inward rectifier K⁺ channel subunits (Kir6.1 or Kir6.2) and four sulfonylurea receptor (SUR1 or SUR2A/B) subunits, detect metabolic changes via three classes of intracellular adenine nucleotide (ATP/ADP) binding site. One site, located on the Kir subunit, causes inhibition of the channel when ATP or ADP is bound. The other two sites, located on the SUR subunit, excite the channel when bound to Mg nucleotides. In pancreatic β cells, an increase in extracellular glucose causes a change in oxidative metabolism and thus turnover of adenine nucleotides in the cytoplasm. This leads to the closure of K_ATP channels, which depolarizes the plasma membrane and permits Ca²⁺ influx and insulin secretion. Many of the molecular details regarding the assembly of the K_ATP complex, and how changes in nucleotide concentrations affect gating, have recently been uncovered by several single-particle cryo-electron microscopy structures of the pancreatic K_ATP channel (Kir6.2/SUR1) at near-atomic resolution. Here, the author discusses the detailed picture of excitatory and inhibitory ligand binding to K_ATP that these structures present and suggests a possible mechanism by which channel activation may proceed from the ligand-binding domains of SUR to the channel pore.

Cantu syndrome-associated SUR2 (ABCC9) mutations in distinct structural domains result in KATP channel gain-of-function by differential mechanisms

The complex disorder Cantu syndrome (CS) arises from gain-of-function mutations in either KCNJ8 or ABCC9, the genes encoding the Kir6.1 and SUR2 subunits of ATP-sensitive potassium (K_ATP) channels, respectively. Recent reports indicate that such mutations can increase channel activity by multiple molecular mechanisms. In this study, we determined the mechanism by which K_ATP function is altered by several substitutions in distinct structural domains of SUR2: D207E in the intracellular L0-linker and Y985S, G989E, M1060I, and R1154Q/R1154W in TMD2. We engineered substitutions at their equivalent positions in rat SUR2A (D207E, Y981S, G985E, M1056I, and R1150Q/R1150W) and investigated functional consequences using macroscopic rubidium (⁸⁶Rb⁺) efflux assays and patch-clamp electrophysiology. Our results indicate that D207E increases K_ATP channel activity by increasing intrinsic stability of the open state, whereas the cluster of Y981S/G985E/M1056I substitutions, as well as R1150Q/R1150W, augmented Mg-nucleotide activation. We also tested the responses of these channel variants to inhibition by the sulfonylurea drug glibenclamide, a potential pharmacotherapy for CS. None of the D207E, Y981S, G985E, or M1056I substitutions had a significant effect on glibenclamide sensitivity. However, Gln and Trp substitution at Arg-1150 significantly decreased glibenclamide potency. In summary, these results provide additional confirmation that mutations in CS-associated SUR2 mutations result in K_ATP gain-of-function. They help link CS genotypes to phenotypes and shed light on the underlying molecular mechanisms, including consequences for inhibitory drug sensitivity, insights that may inform the development of therapeutic approaches to manage CS.

The Nucleotide-Binding Sites of SUR1: A Mechanistic Model

ATP-sensitive potassium (K_ATP) channels comprise four pore-forming Kir6.2 subunits and four modulatory sulfonylurea receptor (SUR) subunits. The latter belong to the ATP-binding cassette family of transporters. K_ATP channels are inhibited by ATP (or ADP) binding to Kir6.2 and activated by Mg-nucleotide interactions with SUR. This dual regulation enables the K_ATP channel to couple the metabolic state of a cell to its electrical excitability and is crucial for the K_ATP channel’s role in regulating insulin secretion, cardiac and neuronal excitability, and vascular tone. Here, we review the regulation of the K_ATP channel by adenine nucleotides and present an equilibrium allosteric model for nucleotide activation and inhibition. The model can account for many experimental observations in the literature and provides testable predictions for future experiments.

Myorelaxant action of fluorine-containing pinacidil analog, flocalin, in bladder smooth muscle is mediated by inhibition of L-type calcium channels rather than activation of KATP channels

Flocalin (FLO) is a new ATP-sensitive K(+) (KATP) channel opener (KCO) derived from pinacidil (PIN) by adding fluorine group to the drug's structure. FLO acts as a potent cardioprotector against ischemia-reperfusion damage in isolated heart and whole animal models primarily via activating cardiac-specific Kir6.2/SUR2A KATP channels. Given that FLO also confers relaxation on several types of smooth muscles and can partially inhibit L-type Ca(2+) channels, in this study, we asked what is the mechanism of FLO action in bladder detrusor smooth muscle (DSM). The actions of FLO and PIN on contractility of rat and guinea pig DSM strips and membrane currents of isolated DSM cells were compared by tensiometry and patch clamp. Kir6 and SUR subunit expression in rat DSM was assayed by reverse transcription PCR (RT-PCR). In contrast to PIN (10 μM), FLO (10 μM) did not produce glibenclamide-sensitive DSM strips' relaxation and inhibition of spontaneous and electrically evoked contractions. However, FLO, but not PIN, inhibited contractions evoked by high K(+) depolarization. FLO (40 μM) did not change the level of isolated DSM cell's background K(+) current, but suppressed by 20 % L-type Ca(2+) current. Determining various Kir6 and SUR messenger RNA (mRNA) expressions in rat DSM by RT-PCR indicated that dominant KATP channel in rat DSM is of vascular type involving association of Kir6.1 and SUR2B subunits. Myorelaxant effects of FLO in bladder DSM are explained by partial blockade of L-type Ca(2+) channel-mediated Ca(2+) influx rather than by hyperpolarization associated with increased K(+) permeability. Thus, insertion of fluorine group in PIN's structure made the drug more discriminative between Kir6.2/SUR2A cardiac- and Kir6.1/SUR2B vascular-type KATP channels and rendered it partial L-type Ca(2+) channel-blocking potency.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

ATP-dependent potassium channels and type 2 diabetes mellitus

Diabetes mellitus is a public health problem, which affects a millions worldwide. Most diabetes cases are classified as type 2 diabetes mellitus, which is highly associated with obesity. Type 2 diabetes is considered a multifactorial disorder, with both environmental and genetic factors contributing to its development. An important issue linked with diabetes development is the failure of the insulin releasing mechanism involving abnormal activity of the ATP-dependent potassium channel, KATP. This channel is a transmembrane protein encoded by the KCNJ11 and ABCC8 genes. Furthermore, polymorphisms in these genes have been linked to type 2 diabetes because of the role of KATP in insulin release. While several genetic variations have been reported to be associated with this disease, the E23K polymorphism is most commonly associated with this pathology, as well as to obesity. Here, we review the molecular genetics of the potassium channel and discusses its most described polymorphisms and their associations with type 2 diabetes mellitus.

Successful development and use of a thermodynamic stability screen for optimizing the yield of nucleotide binding domains

ATP sensitive potassium (KATP) channels consist of four copies of a pore-forming inward rectifying potassium channel (Kir6.1 or Kir6.2) and four copies of a sulfonylurea receptor (SUR1, SUR2A, or SUR2B). SUR proteins are members of the ATP-binding cassette superfamily of proteins. Binding of ATP to the Kir6.x subunit mediates channel inhibition, whereas MgATP binding and hydrolysis at the SUR NBDs results in channel opening. Mutations in SUR1 and SUR2A NBDs cause diseases of insulin secretion and cardiac disorders, respectively, underlying the importance of studying the NBDs. Although purification of SUR2A NBD1 in a soluble form is possible, the lack of long-term sample stability of the protein in a concentrated form has precluded detailed studies of the protein aimed at gaining a molecular-level understanding of how SUR mutations cause disease. Here we use a convenient and cost-effective thermodynamic screening method to probe stabilizing conditions for SUR2A NBD1. Results from the screen are used to alter the purification protocol to allow for significantly increased yields of the purified protein. In addition, the screen provides strategies for long-term storage of NBD1 and generating NBD1 samples at high concentrations suitable for NMR studies. NMR spectra of NBD1 with MgAMP-PNP are of higher quality compared to using MgATP, indicating that MgAMP-PNP be used as the ligand in future NMR studies. The screen presented here can be expanded to using different additives and can be employed to enhance purification yields, sample life times, and storage of other low stability nucleotide binding domains, such as GTPases.

The ATPase activities of sulfonylurea receptor 2A and sulfonylurea receptor 2B are influenced by the C-terminal 42 amino acids

Unusually among ATP-binding cassette proteins, the sulfonylurea receptor (SUR) acts as a channel regulator. ATP-sensitive potassium channels are octameric complexes composed of four pore-forming Kir6.2 subunits and four regulatory SUR subunits. Two different genes encode SUR1 (ABCC8) and SUR2 (ABCC9), with the latter being differentially spliced to give SUR2A and SUR2B, which differ only in their C-terminal 42 amino acids. ATP-sensitive potassium channels containing these different SUR2 isoforms are differentially modulated by MgATP, with Kir6.2/SUR2B being activated more than Kir6.2/SUR2A. We show here that purified SUR2B has a lower ATPase activity and a 10-fold lower K(m) for MgATP than SUR2A. Similarly, the isolated nucleotide-binding domain (NBD) 2 of SUR2B was less active than that of SUR2A. We further found that the NBDs of SUR2B interact, and that the activity of full-length SUR cannot be predicted from that of either the isolated NBDs or NBD mixtures. Notably, deletion of the last 42 amino acids from NBD2 of SUR2 resulted in ATPase activity resembling that of NBD2 of SUR2A rather than that of NBD2 of SUR2B: this might indicate that these amino acids are responsible for the lower ATPase activity of SUR2B and the isolated NBD2 of SUR2B. We suggest that the lower ATPase activity of SUR2B may result in enhanced duration of the MgADP-bound state, leading to channel activation.

Insulin granule biogenesis, trafficking and exocytosis

It is becoming increasingly apparent that beta cell dysfunction resulting in abnormal insulin secretion is the essential element in the progression of patients from a state of impaired glucose tolerance to frank type 2 diabetes (Del Prato, 2003; Del Prato and Tiengo, 2001). Although extensive studies have examined the molecular, cellular and physiologic mechanisms of insulin granule biogenesis, sorting, and exocytosis the precise mechanisms controlling these processes and their dysregulation in the developed of diabetes remains an area of important investigation. We now know that insulin biogenesis initiates with the synthesis of preproinsulin in rough endoplastic reticulum and conversion of preproinsulin to proinsulin. Proinsulin begins to be packaged in the Trans-Golgi Network and is sorting into immature secretory granules. These immature granules become acidic via ATP-dependent proton pump and proinsulin undergoes proteolytic cleavage resulting the formation of insulin and C-peptide. During the granule maturation process, insulin is crystallized with zinc and calcium in the form of dense-core granules and unwanted cargo and membrane proteins undergo selective retrograde trafficking to either the constitutive trafficking pathway for secretion or to degradative pathways. The newly formed mature dense-core insulin granules populate two different intracellular pools, the readily releasable pools (RRP) and the reserved pool. These two distinct populations are thought to be responsible for the biphasic nature of insulin release in which the RRP granules are associated with the plasma membrane and undergo an acute calcium-dependent release accounting for first phase insulin secretion. In contrast, second phase insulin secretion requires the trafficking of the reserved granule pool to the plasma membrane. The initial trigger for insulin granule fusion with the plasma membrane is a rise in intracellular calcium and in the case of glucose stimulation results from increased production of ATP, closure of the ATP-sensitive potassium channel and cellular depolarization. In turn, this opens voltage-dependent calcium channels allowing increased influx of extracellular calcium. Calcium is thought to bind to members of the fusion regulatory proteins synaptogamin that functionally repressors the fusion inhibitory protein complexin. Both complexin and synaptogamin interact as well as several other regulatory proteins interact with the core fusion machinery composed of the Q- or t-SNARE proteins syntaxin 1 and SNAP25 in the plasma membrane that assembles with the R- or v-SNARE protein VAMP2 in insulin granules. In this chapter we will review the current progress of insulin granule biogenesis, sorting, trafficking, exocytosis and signaling pathways that comprise the molecular basis of glucose-dependent insulin secretion.

Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator

SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

The cardiovascular system operates under a wide scale of demands, ranging from conditions of rest to extreme stress. How the heart muscle matches rates of ATP production with utilization is an area of active investigation. ATP-sensitive potassium (K(ATP)) channels serve a critical role in the orchestration of myocardial energetic well-being. K(ATP) channel heteromultimers consist of inwardly-rectifying K(+) channel 6.2 and ATP-binding cassette sulfonylurea receptor 2A that translates local ATP/ADP levels, set by ATPases and phosphotransfer reactions, to the channel pore function. In cells in which the mobility of metabolites between intracellular microdomains is limited, coupling of phosphotransfer pathways with K(ATP) channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This K(ATP) channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varying workloads. Mutations of K(ATP) channels result in disruption of the nucleotide signaling network and generate a stress-vulnerable phenotype with excessive susceptibility to injury, development of cardiomyopathy, and arrhythmia. Solving the mechanisms underlying the integration of K(ATP) channels into the cellular energy network will advance the understanding of endogenous cardioprotection and the development of strategies for the management of cardiovascular injury and disease progression.

Virtual Screening for Novel Openers of Pancreatic KATPChannels

Ligand-based virtual screening approaches were applied to search for new chemotype KCOs activating Kir6.2/SUR1 KATP channels. A total of 65 208 commercially available compounds, extracted from the ZINC archive, served as database for screening. In a first step, pharmacokinetic filtering via VolSurf reduced the initial database to 1913 compounds. Afterward, six molecules were selected as templates for similarity searches: similarity scores, obtained toward these templates, were calculated with the GRIND, FLAP, and TOPP approaches, which differently encode structural information into potential pharmacophores. In this way, we obtained 32 hit candidates, 16 via GRIND and eight each via FLAP and TOPP. For biological testing of the hit candidates, their effects on membrane potentials in HEK 293 cells expressing Kir6.2/SUR1 were studied. GRIND, FLAP, and TOPP all yielded hits, but no method top-ranked all the actives. Thus, parallel application of different approaches probably improves hit detection.

A key role for the subunit SUR2B in the preferential activation of vascular KATP channels by isoflurane

BACKGROUND AND PURPOSE

It has been postulated that isoflurane, a volatile anaesthetic, produces vasodilatation through activation of ATP-sensitive K+ (KATP) channels. However, there is no direct evidence for the activation of vascular KATP channels by isoflurane. This study was conducted to examine the effect of isoflurane on vascular KATP channels and compare it with that on cardiac KATP channels.

EXPERIMENTAL APPROACH

Effects of isoflurane on KATP channels were examined in aortic smooth muscle cells and cardiomyocytes of the mouse using patch clamp techniques. Effects of the anaesthetic on the KATP channels with different combinations of the inward rectifier pore subunits (Kir6.1 and Kir6.2) and sulphonylurea receptor subunits (SUR2A and SUR2B) reconstituted in a heterologous expression system were also examined.

KEY RESULTS

Isoflurane increased the coronary flow in Langendorff-perfused mouse hearts in a concentration-dependent manner, which was abolished by 10 microM glibenclamide. In enzymically-dissociated aortic smooth muscle cells, isoflurane evoked a glibenclamide-sensitive current (i.e. KATP current). In isolated mouse ventricular cells, however, isoflurane failed to evoke the KATP current unless the KATP current was preactivated by the K+ channel opener pinacidil. Although isoflurane readily activated the Kir6.1/SUR2B channels (vascular type), the volatile anesthetic could not activate the Kir6.2/SUR2A channels (cardiac type) expressed in HEK293 cells. Isoflurane activated a glibenclamide-sensitive current in HEK293 cells expressing Kir6.2/SUR2B channels.

CONCLUSION AND IMPLICATIONS

Isoflurane activates KATP channels in vascular smooth muscle cells and produces coronary vasodilation in mouse hearts. SUR2B may be important for the activation of vascular-type KATP channels by isoflurane.

The mutation Y1206S increases the affinity of the sulphonylurea receptor SUR2A for glibenclamide and enhances the effects of coexpression with Kir6.2

1. ATP-sensitive K(+) channels (K(ATP) channels) are tetradimeric complexes of inwardly rectifying K(+) channels (Kir6.x) and sulphonylurea receptors (SURs). The SURs SUR2A (cardiac) and SUR2B (smooth muscle) differ only in the last 42 amino acids. In SUR2B, the mutation Y1206S, located at intracellular loop 8, increases the affinity for glibenclamide (GBC) about 10-fold. Here, we examined whether the mutation Y1206S in SUR2A had effects similar to those in SUR2B.2. GBC bound to SUR2A with K(D)=20 nM; the mutation increased affinity approximately 5 x. 3. In cells, coexpression of SUR2A with Kir6.2 increased the affinity for GBC approximately 3 x; with the mutant, the increase was 9 x. 4. The mutation did not affect the affinity of SUR2A for openers; coexpression with Kir6.2 reduced opener affinity of wild-type and mutant SUR2A by about 2 x. 5. The negative allosteric interaction between the opener, P1075, and GBC at wild-type and mutant SUR2A was markedly affected by the presence of MgATP and by coexpression with Kir6.2. 6. In inside-out patches, GBC inhibited the wild-type Kir6.2/SUR2A and 2B channels with IC(50) values of 27 nM; the mutation shifted the IC(50) values to approximately 1 nM. 7. The data show that the mutation Y1206S increased the affinity of SUR2A for GBC and modulated the effects of coexpression. Overall, the changes were similar to those observed with SUR2B(Y1206S), suggesting that the differences in the last 42 carboxy-terminal amino acids of SUR2A and 2B are of limited influence on the binding of GBC and P1075 to the SUR2 isoforms.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

Inhibition of ATP-sensitive potassium channels by haloperidol

Chronic haloperidol treatment has been associated with an increased incidence of glucose intolerance and type-II diabetes mellitus. We studied the effects of haloperidol on native ATP-sensitive potassium (K(ATP)) channels in mouse pancreatic beta cells and on cloned Kir6.2/SUR1 channels expressed in HEK293 cells. The inhibitory effect of haloperidol on the K(ATP) channel was not mediated via the D2 receptor signaling pathway, as both D2 agonists and antagonists blocked the channel. K(ATP) currents were studied using the patch-clamp technique in whole-cell and outside-out patch configurations. Addition of haloperidol to the extracellular solution inhibited the K(ATP) conductance immediately, in a reversible and voltage-independent manner. Haloperidol did not block the channel when applied intracellularly in whole-cell recordings. Haloperidol blocked cloned Kir6.2/SUR1 and Kir6.2DeltaC36 K(ATP) channels expressed in HEK cells. This suggests that the drug interacts with the Kir6.2 subunit of the channel. The IC(50) for inhibition of the K(ATP) current by haloperidol was 1.6 microM in 2 mM extracellular K(+) concentration ([K(+)](o)) and increased to 23.9 microM in 150 mM [K(+)](o). The Hill coefficient was close to unity, suggesting that the binding of a single molecule of haloperidol is sufficient to close the channel. Haloperidol block of K(ATP) channels may contribute to the side effects of this drug when used therapeutically.

Taurine block of cloned ATP-sensitive K+ channels with different sulfonylurea receptor subunits expressed in Xenopus laevis oocytes

Taurine has been found to inhibit ATP-sensitive K+ (KATP) channels in rat pancreatic beta-cells [Park et al., Biochem Pharmacol 2004;67:1089-1096] which could be due to its interaction with a benzamido-binding site on SUR1. In present study, we further evaluated the mechanism of taurine action on the KATP-channel inhibition, using cloned KATP-channels with different types of SUR subunit expressed in Xenopus laevis oocytes. The oocytes were coinjected with Kir6.2 mRNA, and mRNA encoding SUR1, SUR2A or SUR2B. Macroscopic currents were recorded from giant excised inside-out patches. The binding of glibenclamide to SUR1 was assessed by using a glibenclamide-fluorescent probe. Intracellular taurine inhibited all three types of KATP-channels to a similar extent. They were fit to the Hill equation, showing IC50 of 11.0 mM for Kir6.2/SUR1, 10.9 mM for Kir6.2/SUR2A, and 9.0 mM for Kir6.2/SUR2B currents. Taurine at the concentration of 10 mM enhanced the high-affinity bindings of glibenclamide and repaglinide on all types of SUR, whereas the low-affinity binding on Kir6.2 was not affected. The intensity of glibenclamide fluorescence was higher in the plasma membrane of taurine-pretreated oocytes. The high-affinity binding of tolbutamide or gliclazide on SUR was not modified by taurine. These results suggest that the taurine inhibition of KATP-channels is mediated by an interaction with the site on SUR where the benzamido group is bound. Therefore, intracellular concentrations of taurine in different tissues may be more important in determining taurine modulation of the KATP-channel rather than distinct types of SUR subunit.

The nucleotide-binding domains of sulfonylurea receptor 2A and 2B play different functional roles in nicorandil-induced activation of ATP-sensitive K+ channels

Nicorandil activates ATP-sensitive K(+) channels composed of Kir6.2 and either sulfonylurea receptor (SUR) 2A or 2B. Although SUR2A and SUR2B differ only in their C-terminal 42 amino acids (C42) and possess identical drug receptors and nucleotide-binding domains (NBDs), nicorandil more potently activates SUR2B/Kir6.2 than SUR2A/Kir6.2 channels. Here, we analyzed the roles of NBDs in these channels' response to nicorandil with the inside-out configuration of the patch-clamp method. Binding and hydrolysis of nucleotides by NBDs were impaired by mutations in the Walker A motif of NBD1 (K708A) and NBD2 (K1349A) and in the Walker B motif of NBD2 (D1470N). Experiments were done with internal ATP (1 mM). In SUR2A/Kir6.2 channels, the K708A mutation abolished, and the K1349A but not D1470N mutation reduced the sensitivity to nicorandil. ADP (100 microM) significantly increased the wild-type channels' sensitivity to nicorandil, which was abolished by the K1349A or D1470N mutation. Thus, the SUR2A/Kir6.2 channels' response to nicorandil critically depends on ATP-NBD1 interaction and is facilitated by interactions of ATP or ADP with NBD2. In SUR2B/Kir6.2 channels, either the K708A or K1349A mutation partially suppressed the response to nicorandil, and double mutations abolished it. The D1470N mutation also significantly impaired the response. ADP did not sensitize the channels. Thus, NBD2 hydrolyzes ATP, and NBD1 and NBD2 equally contribute to the response by interacting with ATP and ADP, accounting for the higher nicorandil sensitivity of SUR2B/Kir6.2 than SUR2A/Kir6.2 channels in the presence of ATP alone. Thus, C42 modulates the interaction of both NBDs with intracellular nucleotides.

Interaction of a novel dihydropyridine K+ channel opener, A-312110, with recombinant sulphonylurea receptors and KATP channels: comparison with the cyanoguanidine P1075

1. ATP-sensitive K(+) channels (K(ATP) channels) are composed of pore-forming subunits (Kir6.x) and of regulatory subunits, the sulphonylurea receptors (SURx). Synthetic openers of K(ATP) channels form a chemically heterogeneous class of compounds that are of interest in several therapeutic areas. We have investigated the interaction of a novel dihydropyridine opener, A-312110 ((9R)-9-(4-fluoro-3-iodophenyl)-2,3,5,9-tetrahydro-4H-pyrano[3,4-b]thieno [2,3-e]pyridin-8(7H)-one-1,1-dioxide), with SURs and Kir6/SUR channels in comparison to the cyanoguanidine opener P1075. 2. In the presence of 1 mM MgATP, A-312110 bound to SUR2A (the SUR in cardiac and skeletal muscle) and to SUR2B (smooth muscle) with K(i) values of 14 and 18 nM; the corresponding values for P1075 were 16 and 9 nM, respectively. Decreasing the MgATP concentration reduced the affinity of A312110 binding to SUR2A significantly more than that to SUR2B; for P1075, the converse was true. At SUR1 (pancreatic beta-cell), both openers showed little binding up to 100 microM. 3. In the presence of MgATP, both openers inhibited [(3)H]glibenclamide binding to the SUR2 subtypes in a biphasic manner. In the absence of MgATP, the high-affinity component of the inhibition curves was absent. 4. In inside-out patches, the two openers activated the Kir6.2/SUR2A and Kir6.2/SUR2B channels with similar potency (approximately 50 nm). Both were almost 2 x more efficacious in opening the Kir6.2/SUR2B than the Kir6.2/SUR2A channel. 5. The results show that the novel dihydropyridine A-312110 is a potent K(ATP) channel opener with binding and channel-opening properties similar to those of P1075.

The distinct nucleotide binding states of the transporter associated with antigen processing (TAP) are regulated by the nonhomologous C-terminal tails of TAP1 and TAP2

The transporter associated with antigen processing (TAP) delivers peptides into the lumen of the endoplasmic reticulum for binding onto major histocompatibility complex class I molecules. TAP comprises two polypeptides, TAP1 and TAP2, each with an N-terminal transmembrane domain and a C-terminal cytosolic nucleotide binding domain (NBD). The two NBDs have distinct intrinsic nucleotide binding properties. In the resting state of TAP, the NBD1 has a much higher binding activity for ATP than the NBD2, while the binding of ADP to the two NBDs is equivalent. To attribute the different nucleotide binding behaviour of NBD1 and NBD2 to specific sequences, we generated chimeric TAP1 and TAP2 polypeptides in which either the nonhomologous C-terminal tails downstream of the Walker B motif, or the core NBDs which are enclosed by the conserved Walker A and B motifs, were reciprocally exchanged. Our biochemical and functional studies on the different TAP chimeras show that the distinct nucleotide binding behaviour of TAP1 and TAP2 is controlled by the nonhomologous C-terminal tails of the two TAP chains. In addition, our data suggest that the C-terminal tail of TAP2 is required for a functional transporter by regulating ATP binding. Further experiments indicate that ATP binding to NBD2 is important because it prevents simultaneous uptake of ATP by TAP1. We propose that the C-terminal tails of TAP1 and TAP2 play a crucial regulatory role in the coordination of nucleotide binding and ATP hydrolysis by TAP.

Sulphonylurea action revisited: the post-cloning era

Hypoglycaemic agents such as sulphonylureas and the newer group of "glinides" stimulate insulin secretion by closing ATP-sensitive potassium (K(ATP)) channels in pancreatic beta cells, but have varying cross-reactivity with related channels in extrapancreatic tissues such as heart, vascular smooth and skeletal muscle. Experiments on the structure-function relationships of recombinant K(ATP) channels and the phenotypes of mice deficient in different K(ATP) channel subunits have provided important insights into the mechanisms underlying sulphonylurea selectivity, and the potential consequences of K(ATP) channel blockade outside the pancreatic beta cell. The different pharmacological properties of K(ATP) channels from beta cells compared with those from cardiac, smooth and skeletal muscle, are accounted for by the expression of alternative types of sulphonylurea receptor, with non-identical drug binding sites. The sulphonylureas and glinides are found to fall into two groups: one exhibiting selectivity for beta cell sulphonylurea receptors (SUR1), and the other blocking cardiovascular and skeletal muscle sulphonylurea receptors (SUR2) with potencies similar to their action on SUR1. In seeking potential side effects of K(ATP) channel inhibitors in humans, it is essential to take these drug differences into account, along with the probability (suggested by the studies on K(ATP) channel knockout mice) that the effects of extrapancreatic K(ATP) channel inhibition might be either subtle or rare. Further studies are still required before a final decision can be made on whether non-selective agents are appropriate for the therapy of Type 2 diabetes.

Binding and effect of K ATP channel openers in the absence of Mg2+

1 Openers of ATP-sensitive K(+) channels (K(ATP) channels) are thought to act by enhancing the ATPase activity of sulphonylurea receptors (SURs), the regulatory channel subunits. At higher concentrations, some openers activate K(ATP) channels also in the absence of MgATP. Here, we describe binding and effect of structurally diverse openers in the absence of Mg(2+) and presence of EDTA. 2 Binding of openers to SUR2B was measured using a mutant with high affinity for [(3)H]glibenclamide ([(3)H]GBC). In the absence of Mg(2+), 'typical' openers (benzopyrans, cyanoguanidines and aprikalim) inhibited [(3)H]GBC binding with K(i) values approximately 200 x higher than in the presence of MgATP. Minoxidil sulphate and nicorandil were inactive, whereas binding of diazoxide was unaffected by MgATP. 3 In the absence/presence of MgATP, N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) activated the Kir6.2/SUR2B channel in inside-out patches with EC(50)=2000/67nM and E(max)=32/134%. In the absence of Mg(2+), responses were variable with only a small part of the variability being explained by a decrease in channel responsiveness with time after patch excision and to differences in the ATP sensitivity between patches. 4 The rank order of efficacy of the openers was P1075>rilmakalim approximately nicorandil>diazoxide>minoxidil sulphate. 5 The data show that structurally diverse openers are able to bind to, and to activate the Kir6.2/SUR2B channel by a pathway independent of ATP hydrolysis. These effects are observed at concentrations used to define the biochemical mechanism of the openers in the presence of MgATP and allow the openers to be classified into 'typical' and 'atypical' KCOs with diazoxide standing apart.

Modulation of the trafficking efficiency and functional properties of ATP-sensitive potassium channels through a single amino acid in the sulfonylurea receptor

Mutations in the sulfonylurea receptor 1 (SUR1), a subunit of ATP-sensitive potassium (K(ATP)) channels, cause familial hyperinsulinism. One such mutation, deletion of phenylalanine 1388 (DeltaPhe-1388), leads to defects in both trafficking and MgADP response of K(ATP) channels. Here we investigated the biochemical features of Phe-1388 that control the proper trafficking and function of K(ATP) channels by substituting the residue with all other 19 amino acids. Whereas surface expression is largely dependent on hydrophobicity, channel response to MgADP is governed by multiple factors and involves the detailed architecture of the amino acid side chain. Thus, structural features in SUR1 required for proper channel function are distinct from those required for correct protein trafficking. Remarkably, replacing Phe-1388 by leucine profoundly alters the physiological and pharmacological properties of the channel. The F1388L-SUR1 channel has increased sensitivity to MgADP and metabolic inhibition, decreased sensitivity to glibenclamide, and responds to both diazoxide and pinacidil. Because this conservative amino acid substitution occurs in the SUR2A and SUR2B isoforms, the mutation provides a mechanism by which functional diversities in K(ATP) channels are generated.

Analysis of the differential modulation of sulphonylurea block of beta-cell and cardiac ATP-sensitive K+ (K(ATP)) channels by Mg-nucleotides

Sulphonylureas stimulate insulin secretion by binding with high-affinity to the sulphonylurea receptor (SUR) subunit of the ATP-sensitive potassium (K(ATP)) channel and thereby closing the channel pore (formed by four Kir6.2 subunits). In the absence of added nucleotides, the maximal block is around 60-80 %, indicating that sulphonylureas act as partial antagonists. Intracellular MgADP modulated sulphonylurea block, enhancing inhibition of Kir6.2/SUR1 (beta-cell type) and decreasing that of Kir6.2/SUR2A (cardiac-type) channels. We examined the molecular basis of the different response of channels containing SUR1 and SUR2A, by recording currents from inside-out patches excised from Xenopus oocytes heterologously expressing wild-type or chimeric channels. We used the benzamido derivative meglitinide as this drug blocks Kir6.2/SUR1 and Kir6.2/SUR2A currents, reversibly and with similar potency. Our results indicate that transfer of the region containing transmembrane helices (TMs) 8-11 and the following 65 residues of SUR1 into SUR2A largely confers a SUR1-like response to MgADP and meglitinide, whereas the reverse chimera (SUR128) largely endows SUR1 with a SUR2A-type response. This effect was not specific for meglitinide, as tolbutamide was also unable to prevent MgADP activation of Kir6.2/SUR128 currents. The data favour the idea that meglitinide binding to SUR1 impairs either MgADP binding or the transduction pathway between the NBDs and Kir6.2, and that TMs 8-11 are involved in this modulatory response. The results provide a basis for understanding how beta-cell K(ATP) channels show enhanced sulphonylurea inhibition under physiological conditions, whereas cardiac K(ATP) channels exhibit reduced block in intact cells, especially during metabolic inhibition.

Effect of glimepiride on insulin-stimulated glycogen synthesis in cultured human skeletal muscle cells: a comparison to glibenclamide

OBJECTIVE

To examine the effect of glimepiride on insulin-stimulated glycogen synthesis in cultured human skeletal muscle cells in comparison with glibenclamide.

RESEARCH DESIGN AND METHODS

Myotubes derived from glucose-tolerant subjects were incubated with glimepiride or glibenclamide (0-100 micro mol/l) for 4 h and with or without insulin (100 nmol/l) for 2 h, and subsequently glycogen synthesis was determined.

RESULTS

Glimepiride had no significant effect on basal glycogen synthesis; in contrast, glimepiride caused a dose-dependent increase of insulin-stimulated glycogen synthesis, with a maximal effect of 39.97 +/- 8.4% (mean +/- SEM, n = 4, P < 0,02). The time course of this glimepiride effect on insulin-stimulated glycogen synthesis showed a peak after 12 h incubation with a half maximal effect after 4 h. Preincubation of the myotubes with wortmannin (100 nmol/l), an inhibitor of phosphatidylinositol (PI)- 3 kinase, caused an inhibition of this glimepiride effect on insulin-stimulated glycogen synthesis. In contrast to glimepiride, incubation of myotubes with glibenclamide (0-100nmol/l), a second generation sulfonylurea, had no significant effect on basal or insulin-stimulated glycogen synthesis.

CONCLUSIONS

Incubation of cultured human skeletal muscle cells derived from glucose-tolerant subjects with glimepiride caused a dose-dependent increase of insulin-stimulated glycogen synthesis using therapeutic glimepiride concentrations. This glimepiride effect seems to be mediated via the PI3 kinase pathway. In contrast to glimepiride, glibenclamide had no significant effect on basal or insulin-stimulated glycogen synthesis. These results suggest that glimepiride, beside its well-known effect to stimulate insulin secretion, possess an insulin-sensitizing action in cultured human skeletal muscle cells in support of the concept of an extrapancreatic action of glimepiride.

6-Chloro-3-alkylamino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide derivatives potently and selectively activate ATP sensitive potassium channels of pancreatic beta-cells

6-Chloro-3-alkylamino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide derivatives were synthesized and characterized as activators of adenosine 5'-triphosphate (ATP) sensitive potassium (K(ATP)) channels in the beta-cells by measuring effects on membrane potential and insulin release in vitro. The effects on vascular tissue in vitro were measured on rat aorta and small mesenteric vessels. Selected compounds were characterized as competitive inhibitors of [(3)H]glibenclamide binding to membranes of HEK293 cells expressing human SUR1/Kir6.2 and as potent inhibitors of insulin release in isolated rat islets. 6-Chloro-3-(1-methylcyclobutyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (54) was found to bind and activate the SUR1/Kir6.2 K(ATP) channels in the low nanomolar range and to be at least 1000 times more potent than the reference compound diazoxide with respect to inhibition of insulin release from rat islets. Several compounds, e.g., 3-propylamino- (30), 3-isopropylamino- (34), 3-(S)-sec-butylamino- (37), and 3-(1-methylcyclopropyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (53), which were found to be potent and beta-cell selective activators of K(ATP) channels in vitro, were found to inhibit insulin secretion in rats with minimal effects on blood pressure and to exhibit good oral pharmacokinetic properties.

Mutations in the linker domain of NBD2 of SUR inhibit transduction but not nucleotide binding

ATP-sensitive potassium (K(ATP)) channels are composed of an ATP-binding cassette (ABC) protein (SUR1, SUR2A or SUR2B) and an inwardly rectifying K(+) channel (Kir6.1 or Kir6.2). Like other ABC proteins, the nucleotide binding domains (NBDs) of SUR contain a highly conserved "signature sequence" (the linker, LSGGQ) whose function is unclear. Mutation of the conserved serine to arginine in the linker of NBD1 (S1R) or NBD2 (S2R) did not alter the ability of ATP or ADP (100 microM) to displace 8-azido-[(32)P]ATP binding to SUR1, or abolish ATP hydrolysis at NBD2. We co-expressed Kir6.2 with wild-type or mutant SUR in Xenopus oocytes and recorded the resulting currents in inside-out macropatches. The S1R mutation in SUR1, SUR2A or SUR2B reduced K(ATP) current activation by 100 microM MgADP, whereas the S2R mutation in SUR1 or SUR2B (but not SUR2A) abolished MgADP activation completely. The linker mutations also reduced (S1R) or abolished (S2R) MgATP-dependent activation of Kir6.2-R50G co-expressed with SUR1 or SUR2B. These results suggest that the linker serines are not required for nucleotide binding but may be involved in transducing nucleotide binding into channel activation.

Glibenclamide binding to sulphonylurea receptor subtypes: dependence on adenine nucleotides

1: ATP-sensitive K(+) channels are composed of pore-forming subunits Kir6.2 and of sulphonylurea receptors (SURs); the latter are the target of the hypoglycaemic sulphonylureas like glibenclamide. Here, we report on the negative allosteric modulation by MgATP and MgADP of glibenclamide binding to SUR1 and to SUR2 mutants with high glibenclamide affinity, SUR2A(Y1206S) and SUR2B(Y1206S). 2: ATP, in the presence of an ATP-regenerating system to oppose hydrolysis during incubation, inhibited glibenclamide binding to SUR1 and SUR2B(Y1206S) by approximately 60%, to SUR2A(Y1206S) by 21%). Inhibition curves for the SUR2(Y1206S) isoforms were monophasic with IC(50) values of 5-10 microM; the curve for SUR1 was biphasic (IC(50) values 4.7 and 1300 microM). 3: Glibenclamide inhibition curves for ADP, performed in the presence of an ATP-consuming system to oppose ATP formation from ADP, were generally shifted rightwards and showed positive cooperativity, in particular with the SUR2(Y1206S) isoforms. 4: In the absence of the coupled enzyme systems, inhibition curves of MgATP or MgADP were generally shifted leftwards. This indicated synergy of MgATP and MgATP in acting together. 5: Coexpression of SUR1 and SUR2B(Y1206S) with Kir6.2 reduced both potency and efficacy of ATP in inhibiting glibenclamide binding; this was particularly marked for Kir6.2/SUR1. 6: The data show (a) that the inhibitory effects of ATP and ADP on glibenclamide binding differ from one another, (b) that they depend on the SUR subtype, and (c) that they are weakened by coexpression with Kir6.2.

The novel diazoxide analog 3-isopropylamino-7-methoxy-4H-1,2,4-benzothiadiazine 1,1-dioxide is a selective Kir6.2/SUR1 channel opener

ATP-sensitive K(+) (K(ATP)) channels are activated by a diverse group of compounds known as potassium channel openers (PCOs). Here, we report functional studies of the Kir6.2/SUR1 Selective PCO 3-isopropylamino-7-methoxy-4H-1,2,4-benzothiadiazine 1,1-dioxide (NNC 55-9216). We recorded cloned K(ATP) channel currents from inside-out patches excised from Xenopus laevis oocytes heterologously expressing Kir6.2/SUR1, Kir6.2/SUR2A, or Kir6.2/SUR2B, corresponding to the beta-cell, cardiac, and smooth muscle types of the K(ATP) channel. NNC 55-9216 reversibly activated Kir6.2/SUR1 currents (EC(50) = 16 micromol/l). This activation was dependent on intracellular MgATP and was abolished by mutation of a single residue in the Walker A motifs of either nucleotide-binding domain of SUR1. The drug had no effect on Kir6.2/SUR2A or Kir6.2/SUR2B currents. We therefore used chimeras of SUR1 and SUR2A to identify regions of SUR1 involved in the response to NNC 55-9216. Activation was completely abolished and significantly reduced by swapping transmembrane domains 8-11. The reverse chimera consisting of SUR2A with transmembrane domains 8-11 and NBD2 consisting SUR1 was activated by NNC 55-9216, indicating that these SUR1 regions are important for drug activation. [(3)H]glibenclamide binding to membranes from HEK293 cells transfected with SUR1 was displaced by NNC 55-9216 (IC(50) = 105 micromol/l), and this effect was impaired when NBD2 of SUR1 was replaced by that of SUR2A. These results suggest NNC 55-9216 is a SUR1-selective PCO that requires structural determinants, which differ from those needed for activation of the K(ATP) channel by pinacidil and cromakalim. The high selectivity of NNC 55-9216 may prove to be useful for studies of the molecular mechanism of PCO action.

Intramolecular interaction of SUR2 subtypes for intracellular ADP-Induced differential control of K(ATP) channels

ATP-sensitive K+ (K(ATP)) channels are composed of sulfonylurea receptors (SURs) and inwardly rectifying Kir6.2-channels. The C-terminal 42 amino acid residues (C42) of SURs are responsible for ADP-induced differential activation of K(ATP) channels in SUR-subtypes. By examining ADP-effect on K(ATP) channels containing various chimeras of SUR2A and SUR2B, we identified a segment of 7 residues at central portion of C42 critical for this phenomenon. A 3-D structure model of the region containing the second nucleotide-binding domain (NBD2) of SUR and C42 was developed based on the structure of HisP, a nucleotide-binding protein forming the bacterial Histidine transporter complex. In the model, the polar and charged residues in the critical segment located within a distance that allows their electrostatic interaction with Arg1344 at the Walker-A loop of NBD2. Therefore, the interaction might be involved in the control of ADP-induced differential activation of SUR2-subtype K(ATP) channels.

Interaction of K(ATP) channel modulators with sulfonylurea receptor SUR2B: implication for tetramer formation and allosteric coupling of subunits

Sulfonylurea receptors (SURs) are subunits of ATP-sensitive K(+) channels (K(ATP) channels); they mediate the channel-closing effect of sulfonylureas such as glibenclamide and the channel-activating effect of K(ATP) channel openers such as the pinacidil analog P1075. We investigated the inhibition by MgATP and P1075 of glibenclamide binding to SUR2B, the SUR subtype in smooth muscle. To increase specific binding, experiments were also performed using SUR2B(Y1206S), a mutant with higher affinity for glibenclamide than for the wild-type (K(D )= 4 versus 22 nM, respectively) but otherwise exhibiting similar pharmacological properties. In the absence of MgATP, [(3)H]glibenclamide binding to both SURs was homogenous. MgATP inhibited [(3)H]glibenclamide binding to both SURs to 25% by reducing the apparent number of glibenclamide binding sites, leaving the affinity unchanged. In the absence of MgATP, P1075 inhibited [(3)H]glibenclamide binding in a monophasic manner with K(i) approximately 1 microM. In the presence of MgATP (1 mM), inhibition was biphasic with one K(i) value resembling the true affinity of P1075 for SUR2B (2-6 nM) and the other resembling K(i) in the absence of MgATP (approximately 1 microM). The data show that (1) MgATP induces heterogeneity in the glibenclamide sites; (2) the high-affinity glibenclamide sites remaining with MgATP are linked to two classes of P1075 sites; and (3) P1075 interacts specifically with SUR2B also in the absence of MgATP. The data are discussed with the assumption that SUR2B, expressed alone, forms tetramers; that MgATP induces allosteric interactions between the subunits; and that mixed SUR2B-glibenclamide-P1075 complexes can exist at equilibrium.

Structural basis for the interference between nicorandil and sulfonylurea action

Nicorandil is a new antianginal agent that potentially may be used to treat the cardiovascular side effects of diabetes. It is both a nitric oxide donor and an opener of ATP-sensitive K(+) (K(ATP)) channels in muscle and thereby causes vasodilation of the coronary vasculature. The aim of this study was to investigate the domains of the K(ATP) channel involved in nicorandil activity and to determine whether nicorandil interacts with hypoglycemic sulfonylureas that target K(ATP) channels in pancreatic beta-cells. K(ATP) channels in muscle and beta-cells share a common pore-forming subunit, Kir6.2, but possess alternative sulfonylurea receptors (SURs; SUR1 in beta-cells, SUR2A in cardiac muscle, and SUR2B in smooth muscle). We expressed recombinant K(ATP) channels in Xenopus oocytes and measured the effects of drugs and nucleotides by recording macroscopic currents in excised membrane patches. Nicorandil activated Kir6.2/SUR2A and Kir6.2/SUR2B but not Kir6.2/SUR1 currents, consistent with its specificity for cardiac and smooth muscle K(ATP) channels. Drug activity depended on the presence of intracellular nucleotides and was impaired when the Walker A lysine residues were mutated in either nucleotide-binding domain of SUR2. Chimeric studies showed that the COOH-terminal group of transmembrane helices (TMs), especially TM 17, is responsible for the specificity of nicorandil for channels containing SUR2. The splice variation between SUR2A and SUR2B altered the off-rate of the nicorandil response. Finally, we showed that nicorandil activity was unaffected by gliclazide, which specifically blocks SUR1-type K(ATP) channels, but was severely impaired by glibenclamide and glimepiride, which target both SUR1 and SUR2-type K(ATP) channels.