MgATP activates the beta cell KATP channel by interaction with its SUR1 subunit

Glucose-dependent insulinotropic polypeptide (GIP)

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) was the first incretin identified and plays an essential role in the maintenance of glucose tolerance in healthy humans. Until recently GIP had not been developed as a therapeutic and thus has been overshadowed by the other incretin, glucagon-like peptide 1 (GLP-1), which is the basis for several successful drugs to treat diabetes and obesity. However, there has been a rekindling of interest in GIP biology in recent years, in great part due to pharmacology demonstrating that both GIPR agonism and antagonism may be beneficial in treating obesity and diabetes. This apparent paradox has reinvigorated the field, led to new lines of investigation, and deeper understanding of GIP.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GIP biology and discuss the therapeutic implications of GIPR signal modification on various diseases.

MAJOR CONCLUSIONS

Following its classification as an incretin hormone, GIP has emerged as a pleiotropic hormone with a variety of metabolic effects outside the endocrine pancreas. The numerous beneficial effects of GIPR signal modification render the peptide an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, drug-induced nausea and both bone and neurodegenerative disorders.

Dynamic duo: Kir6 and SUR in KATP channel structure and function

KATP channels are ligand-gated potassium channels that couple cellular energetics with membrane potential to regulate cell activity. Each channel is an eight subunit complex comprising four central pore-forming Kir6 inward rectifier potassium channel subunits surrounded by four regulatory subunits known as the sulfonylurea receptor, SUR, which confer homeostatic metabolic control of KATP gating. SUR is an ATP binding cassette (ABC) protein family homolog that lacks membrane transport activity but is essential for KATP expression and function. For more than four decades, understanding the structure-function relationship of Kir6 and SUR has remained a central objective of clinical significance. Here, we review progress in correlating the wealth of functional data in the literature with recent KATP cryoEM structures.

A plasma membrane-associated glycolytic metabolon is functionally coupled to KATP channels in pancreatic α and β cells from humans and mice

The ATP-sensitive K+ (KATP) channel is a key regulator of hormone secretion from pancreatic islet endocrine cells. Using direct measurements of KATP channel activity in pancreatic β cells and the lesser-studied α cells, from both humans and mice, we provide evidence that a glycolytic metabolon locally controls KATP channels on the plasma membrane. The two ATP-consuming enzymes of upper glycolysis, glucokinase and phosphofructokinase, generate ADP that activates KATP. Substrate channeling of fructose 1,6-bisphosphate through the enzymes of lower glycolysis fuels pyruvate kinase, which directly consumes the ADP made by phosphofructokinase to raise ATP/ADP and close the channel. We further show the presence of a plasma membrane-associated NAD+/NADH cycle whereby lactate dehydrogenase is functionally coupled to glyceraldehyde-3-phosphate dehydrogenase. These studies provide direct electrophysiological evidence of a KATP-controlling glycolytic signaling complex and demonstrate its relevance to islet glucose sensing and excitability.

KATP channels in focus: Progress toward a structural understanding of ligand regulation

KATP channels are hetero-octameric complexes of four inward rectifying potassium channels, Kir6.1 or Kir6.2, and four sulfonylurea receptors, SUR1, SUR2A, or SUR2B from the ABC transporter family. This unique combination enables KATP channels to couple intracellular ATP/ADP ratios, through gating, with membrane excitability, thus regulating a broad range of cellular activities. The prominence of KATP channels in human physiology, disease, and pharmacology has long attracted research interest. Since 2017, a steady flow of high-resolution KATP cryoEM structures has revealed complex and dynamic interactions between channel subunits and their ligands. Here, we highlight insights from recent structures that begin to provide mechanistic explanations for decades of experimental data and discuss the remaining knowledge gaps in our understanding of KATP channel regulation.

Mechanistic insights on KATP channel regulation from cryo-EM structures

Gated by intracellular ATP and ADP, ATP-sensitive potassium (KATP) channels couple cell energetics with membrane excitability in many cell types, enabling them to control a wide range of physiological processes based on metabolic demands. The KATP channel is a complex of four potassium channel subunits from the Kir channel family, Kir6.1 or Kir6.2, and four sulfonylurea receptor subunits, SUR1, SUR2A, or SUR2B, from the ATP-binding cassette (ABC) transporter family. Dysfunction of KATP channels underlies several human diseases. The importance of these channels in human health and disease has made them attractive drug targets. How the channel subunits interact with one another and how the ligands interact with the channel to regulate channel activity have been long-standing questions in the field. In the past 5 yr, a steady stream of high-resolution KATP channel structures has been published using single-particle cryo-electron microscopy (cryo-EM). Here, we review the advances these structures bring to our understanding of channel regulation by physiological and pharmacological ligands.

Decreased KATP Channel Activity Contributes to the Low Glucose Threshold for Insulin Secretion of Rat Neonatal Islets

Transitional hypoglycemia in normal newborns occurs in the first 3 days of life and has clinical features consistent with hyperinsulinism. We found a lower threshold for glucose-stimulated insulin secretion from freshly isolated embryonic day (E) 22 rat islets, which persisted into the first postnatal days. The threshold reached the adult level by postnatal day (P) 14. Culturing P14 islets also decreased the glucose threshold. Freshly isolated P1 rat islets had a lower threshold for insulin secretion in response to 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid, a nonmetabolizable leucine analog, and diminished insulin release in response to tolbutamide, an inhibitor of β-cell KATP channels. These findings suggested that decreased KATP channel function could be responsible for the lower glucose threshold for insulin secretion. Single-cell transcriptomic analysis did not reveal a lower expression of KATP subunit genes in E22 compared with P14 β cells. The investigation of electrophysiological characteristics of dispersed β cells showed that early neonatal and cultured cells had fewer functional KATP channels per unit membrane area. Our findings suggest that decreased surface density of KATP channels may contribute to the observed differences in glucose threshold for insulin release.

High-Resolution Structures of K+ Channels

Potassium channels are present in every living cell and essential to setting up a stable, non-zero transmembrane electrostatic potential which manifests the off-equilibrium livelihood of the cell. They are involved in other cellular activities and regulation, such as the controlled release of hormones, the activation of T-cells for immune response, the firing of action potential in muscle cells and neurons, etc. Pharmacological reagents targeting potassium channels are important for treating various human diseases linked to dysfunction of the channels. High-resolution structures of these channels are very useful tools for delineating the detailed chemical basis underlying channel functions and for structure-based design and optimization of their pharmacological and pharmaceutical agents. Structural studies of potassium channels have revolutionized biophysical understandings of key concepts in the field - ion selectivity, conduction, channel gating, and modulation, making them multi-modality targets of pharmacological regulation. In this chapter, I will select a few high-resolution structures to illustrate key structural insights, proposed allostery behind channel functions, disagreements still open to debate, and channel-lipid interactions and co-evolution. The known structural consensus allows the inference of conserved molecular mechanisms shared among subfamilies of K⁺ channels and makes it possible to develop channel-specific pharmaceutical agents.

The Ion Channel and GPCR Toolkit of Brain Capillary Pericytes

Brain pericytes reside on the abluminal surface of capillaries, and their processes cover ~90% of the length of the capillary bed. These cells were first described almost 150 years ago (Eberth, 1871; Rouget, 1873) and have been the subject of intense experimental scrutiny in recent years, but their physiological roles remain uncertain and little is known of the complement of signaling elements that they employ to carry out their functions. In this review, we synthesize functional data with single-cell RNAseq screens to explore the ion channel and G protein-coupled receptor (GPCR) toolkit of mesh and thin-strand pericytes of the brain, with the aim of providing a framework for deeper explorations of the molecular mechanisms that govern pericyte physiology. We argue that their complement of channels and receptors ideally positions capillary pericytes to play a central role in adapting blood flow to meet the challenge of satisfying neuronal energy requirements from deep within the capillary bed, by enabling dynamic regulation of their membrane potential to influence the electrical output of the cell. In particular, we outline how genetic and functional evidence suggest an important role for G_s-coupled GPCRs and ATP-sensitive potassium (K_ATP) channels in this context. We put forth a predictive model for long-range hyperpolarizing electrical signaling from pericytes to upstream arterioles, and detail the TRP and Ca²⁺ channels and G_q, G_i/o, and G_12/13 signaling processes that counterbalance this. We underscore critical questions that need to be addressed to further advance our understanding of the signaling topology of capillary pericytes, and how this contributes to their physiological roles and their dysfunction in disease.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Role of Magnesium in Type 2 Diabetes Mellitus

Magnesium (in its ionized and biologically active form, Mg²⁺) is an essential trace element that participates in numerous physiologic processes. Abnormal Mg²⁺ homeostasis can lead to many metabolic disorders, including diabetes mellitus (DM) and its complications. Mg²⁺ participates in energy generation and is required for DNA and RNA synthesis, reproduction, and protein synthesis. Additionally, Mg²⁺ acts as a calcium antagonist and protects vascular endothelial cells from oxidative stress. Imbalances in Mg²⁺ status, more frequently hypomagnesemia, inhibit glucose transporter type 4 translocation, increase insulin resistance, affect lipid metabolism, induce oxidative stress, and impair the antioxidant system of endothelial cells, In these ways, hypomagnesemia contributes to the initiation and progression of DM and its macrovascular and microvascular complications. In this review, we summarize recent advances in knowledge of the mechanisms whereby Mg²⁺ regulates insulin secretion and sensitivity. In addition, we discuss the future prospects for research regarding the mechanisms whereby Mg²⁺ status impacts DM and its complications.

Nucleotide inhibition of the pancreatic ATP-sensitive K+ channel explored with patch-clamp fluorometry

Pancreatic ATP-sensitive K⁺ channels (K_ATP) comprise four inward rectifier subunits (Kir6.2), each associated with a sulphonylurea receptor (SUR1). ATP/ADP binding to Kir6.2 shuts K_ATP. Mg-nucleotide binding to SUR1 stimulates K_ATP. In the absence of Mg²⁺, SUR1 increases the apparent affinity for nucleotide inhibition at Kir6.2 by an unknown mechanism. We simultaneously measured channel currents and nucleotide binding to Kir6.2. Fits to combined data sets suggest that K_ATP closes with only one nucleotide molecule bound. A Kir6.2 mutation (C166S) that increases channel activity did not affect nucleotide binding, but greatly perturbed the ability of bound nucleotide to inhibit K_ATP. Mutations at position K205 in SUR1 affected both nucleotide affinity and the ability of bound nucleotide to inhibit K_ATP. This suggests a dual role for SUR1 in K_ATP inhibition, both in directly contributing to nucleotide binding and in stabilising the nucleotide-bound closed state.

Stimulus-Secretion Coupling in Beta-Cells: From Basic to Bedside

Insulin secretion in humans is usually induced by mixed meals, which upon ingestion, increase the plasma concentration of glucose, fatty acids, amino acids, and incretins like glucagon-like peptide 1. Beta-cells can stay in the off-mode, ready-mode or on-mode; the mode-switching being determined by the open state probability of the ATP-sensitive potassium channels, and the activity of enzymes like glucokinase, and glutamate dehydrogenase. Mitochondrial metabolism is critical for insulin secretion. A sound understanding of the intermediary metabolism, electrophysiology, and cell signaling is essential for comprehension of the entire spectrum of the stimulus-secretion coupling. Depolarization brought about by inhibition of the ATP sensitive potassium channel, together with the inward depolarizing currents through the transient receptor potential (TRP) channels, leads to electrical activities, opening of the voltage-gated calcium channels, and exocytosis of insulin. Calcium- and cAMP-signaling elicited by depolarization, and activation of G-protein-coupled receptors, including the free fatty acid receptors, are intricately connected in the form of networks at different levels. Activation of the glucagon-like peptide 1 receptor augments insulin secretion by amplifying calcium signals by calcium induced calcium release (CICR). In the treatment of type 2 diabetes, use of the sulfonylureas that act on the ATP sensitive potassium channel, damages the beta cells, which eventually fail; these drugs do not improve the cardiovascular outcomes. In contrast, drugs acting through the glucagon-like peptide-1 receptor protect the beta-cells, and improve cardiovascular outcomes. The use of the glucagon-like peptide 1 receptor agonists is increasing and that of sulfonylurea is decreasing. A better understanding of the stimulus-secretion coupling may lead to the discovery of other molecular targets for development of drugs for the prevention and treatment of type 2 diabetes.

Ion Channels of the Islets in Type 2 Diabetes

Ca²⁺ is an essential signal for pancreatic β-cell function. Ca²⁺ plays critical roles in numerous β-cell pathways such as insulin secretion, transcription, metabolism, endoplasmic reticulum function, and the stress response. Therefore, β-cell Ca²⁺ handling is tightly controlled. At the plasma membrane, Ca²⁺ entry primarily occurs through voltage-dependent Ca²⁺ channels. Voltage-dependent Ca²⁺ channel activity is dependent on orchestrated fluctuations in the plasma membrane potential or voltage, which are mediated via the activity of many ion channels. During the pathogenesis of type 2 diabetes the β-cell is exposed to stressful conditions, which result in alterations of Ca²⁺ handling. Some of the changes in β-cell Ca²⁺ handling that occur under stress result from perturbations in ion channel activity, expression or localization. Defective Ca²⁺ signaling in the diabetic β-cell alters function, limits insulin secretion and exacerbates hyperglycemia. In this review, we focus on the β-cell ion channels that control Ca²⁺ handling and how they impact β-cell dysfunction in type 2 diabetes.

ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate KATP channels

Neuroendocrine-type ATP-sensitive K⁺ (K_ATP) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2)₄, comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K⁺-selective inward rectifier (Kir6.2 or KCNJ11). Interactions between nucleotide-, agonist-, and antagonist-binding sites affect channel activity allosterically. Although it is hypothesized that opening these channels requires SUR1-mediated MgATP hydrolysis, we show here that ATP binding to SUR1, without hydrolysis, opens channels when nucleotide antagonism on Kir6.2 is minimized and SUR1 mutants with increased ATP affinities are used. We found that ATP binding is sufficient to switch SUR1 alone between inward- or outward-facing conformations with low or high dissociation constant, K_D , values for the conformation-sensitive channel antagonist [³H]glibenclamide ([³H]GBM), indicating that ATP can act as a pure agonist. Assembly with Kir6.2 reduced SUR1's K_D for [³H]GBM. This reduction required the Kir N terminus (KNtp), consistent with KNtp occupying a "transport cavity," thus positioning it to link ATP-induced SUR1 conformational changes to channel gating. Moreover, ATP/GBM site coupling was constrained in WT SUR1/WT Kir6.2 channels; ATP-bound channels had a lower K_D for [³H]GBM than ATP-bound SUR1. This constraint was largely eliminated by the Q1179R neonatal diabetes-associated mutation in helix 15, suggesting that a "swapped" helix pair, 15 and 16, is part of a structural pathway connecting the ATP/GBM sites. Our results suggest that ATP binding to SUR1 biases K_ATP channels toward open states, consistent with SUR1 variants with lower K_D values causing neonatal diabetes, whereas increased K_D values cause congenital hyperinsulinism.

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.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Cardiovascular pleiotropic effects of statins and new onset diabetes: is there a common link: do we need to evaluate the role of KATP channels?

INTRODUCTION

Statins are considered the main stay of treatment in the prevention of cardio-vascular morbidity and mortality. They have multiple pleiotropic effects, like stabilization of atherosclerotic plaques, inhibition of platelet aggregation, and vascular smooth muscle proliferation; in addition to their lipid lowering action. Statins manifest these pleiotropic effects because they activate KATP channels in the cardiac and vascular tissue. Simultaneous activation of the KATP channels by statins in β cells of pancreas may inhibit insulin release which may lead to diabetes. Areas covered: Literature published between 1980 and 2016 on cholesterol biosynthesis, new onset diabetes and on the pleiotropic effects of statins, was reviewed. A comprehensive search on PubMed, Embase and Cochrane databases was carried out. Expert opinion: Statins exert their beneficial pleiotropic effects on the cardiovascular system by activating KATP channels in the cardiac and vascular tissue. However, simultaneous activation of KATP channels in the beta cells of pancreas leads to inhibition of insulin release. This disturbs the carbohydrate metabolism and probably leads to diabetes. In our opinion, use of stains should be more judicious and restricted to secondary prevention only.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Cryo-EM structure of the ATP-sensitive potassium channel illuminates mechanisms of assembly and gating

K_ATP channels are metabolic sensors that couple cell energetics to membrane excitability. In pancreatic β-cells, channels formed by SUR1 and Kir6.2 regulate insulin secretion and are the targets of antidiabetic sulfonylureas. Here, we used cryo-EM to elucidate structural basis of channel assembly and gating. The structure, determined in the presence of ATP and the sulfonylurea glibenclamide, at ~6 Å resolution reveals a closed Kir6.2 tetrameric core with four peripheral SUR1s each anchored to a Kir6.2 by its N-terminal transmembrane domain (TMD0). Intricate interactions between TMD0, the loop following TMD0, and Kir6.2 near the proposed PIP₂ binding site, and where ATP density is observed, suggest SUR1 may contribute to ATP and PIP₂ binding to enhance Kir6.2 sensitivity to both. The SUR1-ABC core is found in an unusual inward-facing conformation whereby the two nucleotide binding domains are misaligned along a two-fold symmetry axis, revealing a possible mechanism by which glibenclamide inhibits channel activity.

DOI:http://dx.doi.org/10.7554/eLife.24149.001

The hormone insulin reduces blood sugar levels by encouraging fat, muscle and other body cells to take up sugar. When blood sugar levels rise following a meal, cells within the pancreas known as beta cells should release insulin. In people with diabetes, the beta cells fail to release insulin, meaning that the high blood sugar levels are not corrected.

When blood sugar levels are high, beta cells generate more energy in the form of ATP molecules. The increased level of ATP causes channels called ATP-sensitive potassium (K_ATP) channels in the membrane of the cell to close. This triggers a cascade of events that leads to the release of insulin.

Some treatments for diabetes alter how the K_ATP channels work. For example, a widely prescribed medication called glibenclamide (also known as glyburide in the United States) stimulates the release of insulin by preventing the flow of potassium through K_ATP channels. It remains unknown exactly how ATP and glibenclamide interact with the channel’s molecular structure to stop the flow of potassium ions.

K_ATP channels are made up of two proteins called SUR1 and Kir6.2. To investigate the structure of the K_ATP channel, Martin et al. purified channels made of the hamster form of the SUR1 protein and the mouse form of Kir6.2, which each closely resemble their human counterparts. The channels were purified in the presence of ATP and glibenclamide and were then rapidly frozen to preserve their structure, which allowed them to be visualized individually using electron microscopy. By analyzing the images taken from many channels, Martin et al. constructed a highly detailed, three-dimensional map of the K_ATP channel. The structure revealed by this map shows how SUR1 and Kir6.2 work together and provides insight into how ATP and glibenclamide interact with the channel to block the flow of potassium, and hence stimulate the release of insulin.

An important next step will be to improve the structure to more clearly identify where ATP and glibenclamide bind to the K_ATP channel. It will also be important to study the structures of channels that are bound to other regulatory molecules. This will help researchers to fully understand how K_ATP channels located throughout the body operate under healthy and diseased conditions. This knowledge will aid in the design of more effective drugs to treat several devastating diseases caused by defective K_ATP channels.

DOI:http://dx.doi.org/10.7554/eLife.24149.002

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.

Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells

Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly regulated. In this review, we focus on the principal ionic channels (KATP, Nav, and Cav channels) involved in GSIS and how classic and new proteins, hormones, and drugs regulate it. Moreover, we also discuss advances on how metabolic disorders such as metabolic syndrome and diabetes mellitus change channel activity leading to changes in insulin secretion.

The shifting landscape of KATP channelopathies and the need for 'sharper' therapeutics

ATP-sensitive potassium (KATP) channels play fundamental roles in the regulation of endocrine, neural and cardiovascular function. Small-molecule inhibitors (e.g., sulfonylurea drugs) or activators (e.g., diazoxide) acting on SUR1 or SUR2 have been used clinically for decades to manage the inappropriate secretion of insulin in patients with Type 2 diabetes, hyperinsulinism and intractable hypertension. More recently, the discovery of rare disease-causing mutations in KATP channel-encoding genes has highlighted the need for new therapeutics for the treatment of certain forms of neonatal diabetes mellitus, congenital hyperinsulinism and Cantu syndrome. Here, we provide a high-level overview of the pathophysiology of these diseases and discuss the development of a flexible high-throughput screening platform to enable the development of new classes of KATP channel modulators.

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.

Sulfonylureas suppress the stimulatory action of Mg-nucleotides on Kir6.2/SUR1 but not Kir6.2/SUR2A KATP channels: a mechanistic study

Sulfonylureas suppress the stimulatory effect of Mg-nucleotides on recombinant β-cell (Kir6.2/SUR1) but not cardiac (Kir6.2/SUR2A) K_ATP channels.

Sulfonylureas, which stimulate insulin secretion from pancreatic β-cells, are widely used to treat both type 2 diabetes and neonatal diabetes. These drugs mediate their effects by binding to the sulfonylurea receptor subunit (SUR) of the ATP-sensitive K⁺ (K_ATP) channel and inducing channel closure. The mechanism of channel inhibition is unusually complex. First, sulfonylureas act as partial antagonists of channel activity, and second, their effect is modulated by MgADP. We analyzed the molecular basis of the interactions between the sulfonylurea gliclazide and Mg-nucleotides on β-cell and cardiac types of K_ATP channel (Kir6.2/SUR1 and Kir6.2/SUR2A, respectively) heterologously expressed in Xenopus laevis oocytes. The SUR2A-Y1206S mutation was used to confer gliclazide sensitivity on SUR2A. We found that both MgATP and MgADP increased gliclazide inhibition of Kir6.2/SUR1 channels and reduced inhibition of Kir6.2/SUR2A-Y1206S. The latter effect can be attributed to stabilization of the cardiac channel open state by Mg-nucleotides. Using a Kir6.2 mutation that renders the K_ATP channel insensitive to nucleotide inhibition (Kir6.2-G334D), we showed that gliclazide abolishes the stimulatory effects of MgADP and MgATP on β-cell K_ATP channels. Detailed analysis suggests that the drug both reduces nucleotide binding to SUR1 and impairs the efficacy with which nucleotide binding is translated into pore opening. Mutation of one (or both) of the Walker A lysines in the catalytic site of the nucleotide-binding domains of SUR1 may have a similar effect to gliclazide on MgADP binding and transduction, but it does not appear to impair MgATP binding. Our results have implications for the therapeutic use of sulfonylureas.

Genetic variations in magnesium-related ion channels may affect diabetes risk among African American and Hispanic American women

BACKGROUND

Prospective studies consistently link low magnesium intake to higher type 2 diabetes (T2D) risk.

OBJECTIVE

We examined the association of common genetic variants [single nucleotide polymorphisms (SNPs)] in genes related to magnesium homeostasis with T2D risk and potential interactions with magnesium intake.

METHODS

Using the Women's Health Initiative-SNP Health Association Resource (WHI-SHARe) study, we identified 17 magnesium-related ion channel genes (583 SNPs) and examined their associations with T2D risk in 7287 African-American (AA; n = 1949 T2D cases) and 3285 Hispanic-American (HA; n = 611 T2D cases) postmenopausal women. We performed both single- and multiple-locus haplotype analyses.

RESULTS

Among AA women, carriers of each additional copy of SNP rs6584273 in cyclin mediator 1 (CNNM1) had 16% lower T2D risk [OR: 0.84; false discovery rate (FDR)-adjusted P = 0.02]. Among HA women, several variants were significantly associated with T2D risk, including rs10861279 in solute carrier family 41 (anion exchanger), member 2 (SLC41A2) (OR: 0.54; FDR-adjusted P = 0.04), rs7174119 in nonimprinted in Prader-Willi/Angelman syndrome 1 (NIPA1) (OR: 1.27; FDR-adjusted P = 0.04), and 2 SNPs in mitochondrial RNA splicing 2 (MRS2) (rs7738943: OR = 1.55, FDR-adjusted P = 0.01; rs1056285: OR = 1.48, FDR-adjusted P = 0.02). Even with the most conservative Bonferroni adjustment, two 2-SNP-haplotypes in SLC41A2 and MRS2 region were significantly associated with T2D risk (rs12582312-rs10861279: P = 0.0006; rs1056285-rs7738943: P = 0.002). Among women with magnesium intake in the lowest 30% (AA: ≤0.164 g/d; HA: ≤0.185 g/d), 4 SNP signals were strengthened [rs11590362 in claudin 19 (CLDN19), rs823154 in SLC41A1, rs5929706 and rs5930817 in membra; HA: ≥0.313 g/d), rs6584273 in CNNM1 (OR: 0.71; FDR-adjusted P = 0.04) and rs1800467 in potassium inwardly rectifying channel, subfamily J, member 11 (KCNJ11) (OR: 2.50; FDR-adjusted P = 0.01) were significantly associated with T2D risk.

CONCLUSIONS

Our findings suggest important associations between genetic variations in magnesium-related ion channel genes and T2D risk in AA and HA women that vary by amount of magnesium intake.

Sulfonylurea receptors regulate the channel pore in ATP-sensitive potassium channels via an intersubunit salt bridge

ATP-sensitive potassium channels play key roles in many tissues by coupling metabolic status to membrane potential. In contrast with other potassium channels, the pore-forming Kir6 subunits must co-assemble in hetero-octameric complexes with ATP-binding cassette (ABC) family sulfonylurea receptor (SUR) subunits to facilitate cell surface expression. Binding of nucleotides and drugs to SUR regulates channel gating but how these responses are communicated within the complex has remained elusive to date. We have now identified an electrostatic interaction, forming part of a functional interface between the cytoplasmic nucleotide-binding domain-2 of SUR2 subunits and the distal C-terminus of Kir6 polypeptides that determines channel response to nucleotide, potassium channel opener and antagonist. Mutation of participating residues disrupted physical interaction and regulation of expressed channels, properties that were restored in paired charge-swap mutants. Equivalent interactions were identified in Kir6.1- and Kir6.2-containing channels suggesting a conserved mechanism of allosteric regulation.

Molecular mechanisms underlying physiological and receptor pleiotropic effects mediated by GLP-1R activation

The incidence of type 2 diabetes in developed countries is increasing yearly with a significant negative impact on patient quality of life and an enormous burden on the healthcare system. Current biguanide and thiazolidinedione treatments for type 2 diabetes have a number of clinical limitations, the most serious long-term limitation being the eventual need for insulin replacement therapy (Table 1). Since 2007, drugs targeting the glucagon-like peptide-1 (GLP-1) receptor have been marketed for the treatment of type 2 diabetes. These drugs have enjoyed a great deal of success even though our underlying understanding of the mechanisms for their pleiotropic effects remain poorly characterized even while major pharmaceutical companies actively pursue small molecule alternatives. Coupling of the GLP-1 receptor to more than one signalling pathway (pleiotropic signalling) can result in ligand-dependent signalling bias and for a peptide receptor such as the GLP-1 receptor this can be exaggerated with the use of small molecule agonists. Better consideration of receptor signalling pleiotropy will be necessary for future drug development. This is particularly important given the recent failure of taspoglutide, the report of increased risk of pancreatitis associated with GLP-1 mimetics and the observed clinical differences between liraglutide, exenatide and the newly developed long-acting exenatide long acting release, albiglutide and dulaglutide.

Glucose-stimulated single pancreatic islets sustain increased cytosolic ATP levels during initial Ca2+ influx and subsequent Ca2+ oscillations

In pancreatic islets, insulin secretion occurs via synchronous elevation of Ca(2+) levels throughout the islets during high glucose conditions. This Ca(2+) elevation has two phases: a quick increase, observed after the glucose stimulus, followed by prolonged oscillations. In these processes, the elevation of intracellular ATP levels generated from glucose is assumed to inhibit ATP-sensitive K(+) channels, leading to the depolarization of membranes, which in turn induces Ca(2+) elevation in the islets. However, little is known about the dynamics of intracellular ATP levels and their correlation with Ca(2+) levels in the islets in response to changing glucose levels. In this study, a genetically encoded fluorescent biosensor for ATP and a fluorescent Ca(2+) dye were employed to simultaneously monitor the dynamics of intracellular ATP and Ca(2+) levels, respectively, inside single isolated islets. We observed rapid increases in cytosolic and mitochondrial ATP levels after stimulation with glucose, as well as with methyl pyruvate or leucine/glutamine. High ATP levels were sustained as long as high glucose levels persisted. Inhibition of ATP production suppressed the initial Ca(2+) increase, suggesting that enhanced energy metabolism triggers the initial phase of Ca(2+) influx. On the other hand, cytosolic ATP levels did not fluctuate significantly with the Ca(2+) level in the subsequent oscillation phases. Importantly, Ca(2+) oscillations stopped immediately before ATP levels decreased significantly. These results might explain how food or glucose intake evokes insulin secretion and how the resulting decrease in plasma glucose levels leads to cessation of secretion.

A mouse model of human hyperinsulinism produced by the E1506K mutation in the sulphonylurea receptor SUR1

Loss-of-function mutations in the KATP channel genes KCNJ11 and ABCC8 cause neonatal hyperinsulinism in humans. Dominantly inherited mutations cause less severe disease, which may progress to glucose intolerance and diabetes in later life (e.g., SUR1-E1506K). We generated a mouse expressing SUR1-E1506K in place of SUR1. KATP channel inhibition by MgATP was enhanced in both homozygous (homE1506K) and heterozygous (hetE1506K) mutant mice, due to impaired channel activation by MgADP. As a consequence, mutant β-cells showed less on-cell KATP channel activity and fired action potentials in glucose-free solution. HomE1506K mice exhibited enhanced insulin secretion and lower fasting blood glucose within 8 weeks of birth, but reduced insulin secretion and impaired glucose tolerance at 6 months of age. These changes correlated with a lower insulin content; unlike wild-type or hetE1506K mice, insulin content did not increase with age in homE1506K mice. There was no difference in the number and size of islets or β-cells in the three types of mice, or evidence of β-cell proliferation. We conclude that the gradual development of glucose intolerance in patients with the SUR1-E1506K mutation might, as in the mouse model, result from impaired insulin secretion due a failure of insulin content to increase with age.

Molecular mechanism of sulphonylurea block of K(ATP) channels carrying mutations that impair ATP inhibition and cause neonatal diabetes

Sulphonylurea drugs are the therapy of choice for treating neonatal diabetes (ND) caused by mutations in the ATP-sensitive K(+) channel (KATP channel). We investigated the interactions between MgATP, MgADP, and the sulphonylurea gliclazide with KATP channels expressed in Xenopus oocytes. In the absence of MgATP, gliclazide block was similar for wild-type channels and those carrying the Kir6.2 ND mutations R210C, G334D, I296L, and V59M. Gliclazide abolished the stimulatory effect of MgATP on all channels. Conversely, high MgATP concentrations reduced the gliclazide concentration, producing a half-maximal block of G334D and R201C channels and suggesting a mutual antagonism between nucleotide and gliclazide binding. The maximal extent of high-affinity gliclazide block of wild-type channels was increased by MgATP, but this effect was smaller for ND channels; channels that were least sensitive to ATP inhibition showed the smallest increase in sulphonylurea block. Consequently, G334D and I296L channels were not fully blocked, even at physiological MgATP concentrations (1 mmol/L). Glibenclamide block was also reduced in β-cells expressing Kir6.2-V59M channels. These data help to explain why patients with some mutations (e.g., G334D, I296L) are insensitive to sulphonylurea therapy, why higher drug concentrations are needed to treat ND than type 2 diabetes, and why patients with severe ND mutations are less prone to drug-induced hypoglycemia.

Engineered Kir6.2 mutations that correct the trafficking defect of K(ATP) channels caused by specific SUR1 mutations

KATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. The molecular interactions between SUR1 and Kir6.2 that govern channel gating and biogenesis are incompletely understood. In a recent study, we showed that a SUR1 and Kir6.2 mutation pair, E203K-SUR1 and Q52E-Kir6.2, at the SUR1/Kir6.2 interface near the plasma membrane increases the ATP-sensitivity of the channel by nearly 100-fold. Here, we report the finding that the same mutation pair also suppresses channel folding/trafficking defects caused by select SUR1 mutations in the first transmembrane domain of SUR1. Analysis of the contributions from individual mutations, however, revealed that the correction effect is attributed largely to Q52E-Kir6.2 alone. Moreover, the correction is dependent on the negative charge of the substituting amino acid at the Q52 position in Kir6.2. Our study demonstrates for the first time that engineered mutations in Kir6.2 can correct the biogenesis defect caused by specific mutations in the SUR1 subunit.

A Kir6.2 pore mutation causes inactivation of ATP-sensitive potassium channels by disrupting PIP2-dependent gating

In the absence of intracellular nucleotides, ATP-sensitive potassium (KATP) channels exhibit spontaneous activity via a phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent gating process. Previous studies show that stability of this activity requires subunit-subunit interactions in the cytoplasmic domain of Kir6.2; selective mutagenesis and disease mutations at the subunit interface result in time-dependent channel inactivation. Here, we report that mutation of the central glycine in the pore-lining second transmembrane segment (TM2) to proline in Kir6.2 causes KATP channel inactivation. Unlike C-type inactivation, a consequence of selectivity filter closure, in many K(+) channels, the rate of inactivation in G156P channels was insensitive to changes in extracellular ion concentrations or ion species fluxing through the pore. Instead, the rate of G156P inactivation decreased with exogenous application of PIP2 and increased when PIP2-channel interaction was inhibited with neomycin or poly-L-lysine. These findings indicate the G156P mutation reduces the ability of PIP2 to stabilize the open state of KATP channels, similar to mutations in the cytoplasmic domain that produce inactivation. Consistent with this notion, when PIP2-dependent open state stability was substantially increased by addition of a second gain-of-function mutation, G156P inactivation was abolished. Importantly, bath application and removal of Mg(2+)-free ATP or a nonhydrolyzable analog of ATP, which binds to the cytoplasmic domain of Kir6.2 and causes channel closure, recover G156P channel from inactivation, indicating crosstalk between cytoplasmic and transmembrane domains. The G156P mutation provides mechanistic insight into the structural and functional interactions between the pore and cytoplasmic domains of Kir6.2 during gating.

A universally conserved residue in the SUR1 subunit of the KATP channel is essential for translating nucleotide binding at SUR1 into channel opening

The sulphonylurea receptor (SUR1) subunit of the ATP-sensitive potassium (KATP) channel is a member of the ATP-binding cassette (ABC) protein family. Binding of MgADP to nucleotide-binding domain 2 (NBD2) is critical for channel activation.We identified a residue in NBD2 (G1401) that is fully conserved among ABC proteins and whose functional importance is unknown. Homology modelling places G1401 on the outer surface of the protein, distant from the nucleotide-binding site. The ATPase activity of purified SUR1-NBD2-G1410R (bound to maltose-binding protein) was slightly inhibited when compared to the wild-type protein, but its inhibition by MgADP was unchanged, indicating that MgADP binding is not altered. However, MgADP activation of channel activity was abolished. This implies that the G1401R mutation impairs the mechanism by which MgADP binding to NBD2 is translated into opening of the KATP channel pore. The location of G1401 would be consistent with interaction of this residue with the pore-forming Kir6.2 subunit. Channel activity in the presence of MgATP reflects the balance between the stimulatory (at SUR1) and inhibitory (at Kir6.2) effects of nucleotides. Mutant channels were 2.5-fold less sensitive to MgATP inhibition and not activated by MgATP. This suggests that ATP block of the channel is reduced by the SUR1 mutation. Interestingly, this effect was dependent on the functional integrity of the NBDs. These results therefore suggest that SUR1 modulates both nucleotide inhibition and activation of the KATP channel.

ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 and Kir6.2 with diminished PIP2 sensitivity

ATP-sensitive potassium (K(ATP)) channels are inhibited by ATP and activated by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Both channel subunits Kir6.2 and sulfonylurea receptor 1 (SUR1) contribute to gating: while Kir6.2 interacts with ATP and PIP(2), SUR1 enhances sensitivity to both ligands. Recently, we showed that a mutation, E128K, in the N-terminal transmembrane domain of SUR1 disrupts functional coupling between SUR1 and Kir6.2, leading to reduced ATP and PIP(2) sensitivities resembling channels formed by Kir6.2 alone. We show here that when E128K SUR1 was co-expressed with Kir6.2 mutants known to disrupt PIP(2) gating, the resulting channels were surprisingly stimulated rather than inhibited by ATP. To explain this paradoxical gating behavior, we propose a model in which the open state of doubly mutant channels is highly unstable; ATP binding induces a conformational change in ATP-unbound closed channels that is conducive to brief opening when ATP unbinds, giving rise to the appearance of ATP-induced stimulation.

A conserved tryptophan at the membrane-water interface acts as a gatekeeper for Kir6.2/SUR1 channels and causes neonatal diabetes when mutated

We identified a novel heterozygous mutation, W68R, in the Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel, in a patient with transient neonatal diabetes. This tryptophan is absolutely conserved in mammalian Kir channels. The functional effects of mutations at residue 68 of Kir6.2 were studied by heterologous expression in Xenopus oocytes, and by homology modelling. We found the Kir6.2-W68R mutation causes a small reduction in ATP inhibition in the heterozygous state and an increase in the whole-cell KATP current. This can explain the clinical phenotype of the patient. The effect of the mutation was not charge or size dependent, the order of potency for ATP inhibition being W Collapse

Key Words Collapse

MESH Headings

• ATP-Binding Cassette Transporters/genetics
• ATP-Binding Cassette Transporters/metabolism
• Amino Acid Sequence
• Animals
• Cell Membrane/genetics
• Cell Membrane/metabolism
• Child
• Conserved Sequence/genetics
• Diabetes Mellitus/genetics
• Diabetes Mellitus/metabolism
• Female
• Genetic Carrier Screening
• Humans
• Infant, Newborn
• Infant, Newborn, Diseases/genetics
• Infant, Newborn, Diseases/metabolism
• Molecular Sequence Data
• Mutation/genetics
• Oocytes/metabolism
• Phenotype
• Potassium Channels, Inwardly Rectifying/genetics
• Potassium Channels, Inwardly Rectifying/metabolism
• Rats
• Receptors, Drug/genetics
• Receptors, Drug/metabolism
• Sulfonylurea Receptors
• Tryptophan/genetics
• Xenopus laevis

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Grants

• Wellcome Trust
• 084655 Wellcome Trust
• 089795 Wellcome Trust
• 076436 Wellcome Trust

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Affiliation(s)

Roope Männikkö Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
Phillip J Stansfeld
Alexandra S Ashcroft
Andrew T Hattersley
Mark S P Sansom
Sian Ellard
Frances M Ashcroft

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Mutations of the same conserved glutamate residue in NBD2 of the sulfonylurea receptor 1 subunit of the KATP channel can result in either hyperinsulinism or neonatal diabetes

OBJECTIVE

Two novel mutations (E1506D, E1506G) in the nucleotide-binding domain 2 (NBD2) of the ATP-sensitive K(+) channel (K(ATP) channel) sulfonylurea receptor 1 (SUR1) subunit were detected heterozygously in patients with neonatal diabetes. A mutation at the same residue (E1506K) was previously shown to cause congenital hyperinsulinemia. We sought to understand why mutations at the same residue can cause either neonatal diabetes or hyperinsulinemia.

RESEARCH DESIGN AND METHODS

Neonatal diabetic patients were sequenced for mutations in ABCC8 (SUR1) and KCNJ11 (Kir6.2). Wild-type and mutant K(ATP) channels were expressed in Xenopus laevis oocytes and studied with electrophysiological methods.

RESULTS

Oocytes expressing neonatal diabetes mutant channels had larger resting whole-cell K(ATP) currents than wild-type, consistent with the patients' diabetes. Conversely, no E1506K currents were recorded at rest or after metabolic inhibition, as expected for a mutation causing hyperinsulinemia. K(ATP) channels are activated by Mg-nucleotides (via SUR1) and blocked by ATP (via Kir6.2). All mutations decreased channel activation by MgADP but had little effect on MgATP activation, as assessed using an ATP-insensitive Kir6.2 subunit. Importantly, using wild-type Kir6.2, a 30-s preconditioning exposure to physiological MgATP concentrations (>300 µmol/L) caused a marked reduction in the ATP sensitivity of neonatal diabetic channels, a small decrease in that of wild-type channels, and no change for E1506K channels. This difference in MgATP inhibition may explain the difference in resting whole-cell currents found for the neonatal diabetes and hyperinsulinemia mutations.

CONCLUSIONS

Mutations in the same residue can cause either hyperinsulinemia or neonatal diabetes. Differentially altered nucleotide regulation by NBD2 of SUR1 can explain the respective clinical phenotypes.

Syntaxin-1A interacts with distinct domains within nucleotide-binding folds of sulfonylurea receptor 1 to inhibit beta-cell ATP-sensitive potassium channels

The ATP-sensitive potassium (K(ATP)) channel regulates pancreatic β-cell function by linking metabolic status to electrical activity. Syntaxin-1A (Syn-1A), a SNARE protein mediating exocytotic fusion, binds and inhibits the K(ATP) channel via the nucleotide-binding folds (NBFs) of its sulfonylurea receptor-1 (SUR1) regulatory subunit. In this study, we elucidated the precise regions within the NBFs required for Syn-1A-mediated K(ATP) inhibition, using in vitro binding assays, whole cell patch clamp and FRET assay. Specifically, NBF1 and NBF2 were each divided into three subregions, Walker A (W(A)), signature sequence linker, and Walker B (W(B)), to make GST fusion proteins. In vitro binding assays revealed that Syn-1A associates with W(A) and W(B) regions of both NBFs. Patch clamp recordings on INS-1 and primary rat β-cells showed that Syn-1A-mediated channel inhibition was reversed by co-addition of NBF1-W(B) (not NBF1-W(A)), NBF2-W(A), and NBF2-W(B). The findings were corroborated by FRET studies showing that these truncates disrupted Syn-1A interactions with full-length SUR1. To further identify the binding sites, series single-site mutations were made in the Walker motifs of the NBFs. Only NBF1-W(A) (K719M) or NBF2-W(A) (K1385M) mutant no longer bound to Syn-1A; K1385M failed to disrupt Syn-1A-mediated inhibition of K(ATP) channels. These data suggest that NBF1-W(A) (Lys-719) and NBF2-W(A) (Lys-1385) are critical for Syn-1A-K(ATP) channel interaction. Taken together, Syn-1A intimately and functionally associates with the SUR1-NBF1/2 dimer via direct interactions with W(A) motifs and sites adjacent to W(B) motifs of NBF1 and NBF2 but transduces its inhibitory actions on K(ATP) channel activity via some but not all of these NBF domains.

KATP channel mutations in congenital hyperinsulinism

Adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP) channels) have a central role in the regulation of insulin secretion in pancreatic β cells. They are octameric complexes organized around the central core constituted by the Kir6.2 subunits. The regulation of the channel itself takes place on the sulfonylurea receptor-1 subunit. The channel opens and closes according to the balance between adenine nucleotide ATP and adenosine diphosphate. Hyperinsulinemic hypoglycemia (also named congenital hyperinsulinism, or CHI) is associated with loss-of-function K(ATP) channel mutations. Their frequency depends on the histopathological form and the responsiveness of CHI patients to diazoxide. ABCC8/KCNJ11 defects are identified in approximately 80% of patients with CHI refractory to diazoxide. Within this group, focal forms are related to a paternally inherited KCNJ11 or ABCC8 mutation and the loss of the corresponding maternal allele in some pancreatic β cells leading to a focal lesion. Diffuse forms are mostly associated with recessively inherited mutations. Some patients with diffuse forms also carried a single K(ATP) channel mutation. In contrast, K(ATP) mutations are involved in 15% of diazoxide-responsive CHI cases that are either sporadic or dominantly inherited.

Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders K_ATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. K_ATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC₅₀ of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC₅₀ for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (P_O > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type K_ATP channels, we conclude that the MgADP sensitivity of the wild-type K_ATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg²⁺) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.

ATP modulates interaction of syntaxin-1A with sulfonylurea receptor 1 to regulate pancreatic beta-cell KATP channels

ATP-sensitive potassium (K(ATP)) channels are regulated by a variety of cytosolic factors (adenine nucleotides, Mg(2+), phospholipids, and pH). We previously reported that K(ATP) channels are also regulated by endogenous membrane-bound SNARE protein syntaxin-1A (Syn-1A), which binds both nucleotide-binding folds of sulfonylurea receptor (SUR)1 and 2A, causing inhibition of K(ATP) channel activity in pancreatic islet β-cells and cardiac myocytes, respectively. In this study, we show that ATP dose-dependently inhibits Syn-1A binding to SUR1 at physiological concentrations, with the addition of Mg(2+) causing a decrease in the ATP-induced inhibitory effect. This ATP disruption of Syn-1A binding to SUR1 was confirmed by FRET analysis in living HEK293 cells. Electrophysiological studies in pancreatic β-cells demonstrated that reduced ATP concentrations increased K(ATP) channel sensitivity to Syn-1A inhibition. Depletion of endogenous Syn-1A in insulinoma cells by botulinum neurotoxin C1 proteolysis followed by rescue with exogenous Syn-1A showed that Syn-1A modulates K(ATP) channel sensitivity to ATP. Thus, our data indicate that although both ATP and Syn-1A independently inhibit β-cell K(ATP) channel gating, they could also influence the sensitivity of K(ATP) channels to each other. These findings provide new insight into an alternate mechanism by which ATP regulates pancreatic β-cell K(ATP) channel activity, not only by its direct actions on Kir6.2 pore subunit, but also via ATP modulation of Syn-1A binding to SUR1.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

ATP-binding cassette proteins involved in glucose and lipid homeostasis

Glucose and lipids are essential to the body, but excess glucose or lipids lead to metabolic syndrome. ATP-binding cassette (ABC) proteins are involved in the homeostasis of glucose and lipid in that they regulate insulin secretion and remove excess cholesterol from the body. Sulfonylurea receptor (SUR) is a subunit of the ATP-sensitive potassium channels, which regulate insulin secretion from pancreatic beta-cells by sensing cellular metabolic levels. ABCG1 removes excess cholesterol from peripheral tissues and functions in reverse cholesterol transport to the liver. ABCG5 and ABCG8 suppress the absorption of cholesterol in the intestine and exclude cholesterol from the liver to the bile duct. ABCG1 and ABCG4, expressed in the central nervous system, play roles in lipid metabolism in the brain. These ABC proteins are targets of drugs and functional foods to cure and prevent diabetes, hyperlipidemia, and neurodegenerative diseases. In this review, recent knowledge of the physiological function and regulation of ABC proteins in the homeostasis of glucose and lipids is discussed.

Noble gas xenon is a novel adenosine triphosphate-sensitive potassium channel opener

BACKGROUND

Adenosine triphosphate-sensitive potassium (KATP) channels in brain are involved in neuroprotective mechanisms. Pharmacologic activation of these channels is seen as beneficial, but clinical exploitation by using classic K channel openers is hampered by their inability to cross the blood-brain barrier. This is different with the inhalational anesthetic xenon, which recently has been suggested to activate KATP channels; it partitions freely into the brain.

METHODS

To evaluate the type and mechanism of interaction of xenon with neuronal-type KATP channels, these channels, consisting of Kir6.2 pore-forming subunits and sulfonylurea receptor-1 regulatory subunits, were expressed in HEK293 cells and whole cell, and excised patch-clamp recordings were performed.

RESULTS

Xenon, in contrast to classic KATP channel openers, acted directly on the Kir6.2 subunit of the channel. It had no effect on the closely related, adenosine triphosphate (ATP)-regulated Kir1.1 channel and failed to activate an ATP-insensitive mutant version of Kir6.2. Furthermore, concentration-inhibition curves for ATP obtained from inside-out patches in the absence or presence of 80% xenon revealed that xenon reduced the sensitivity of the KATP channel to ATP. This was reflected in an approximately fourfold shift of the concentration causing half-maximal inhibition (IC50) from 26 +/- 4 to 96 +/- 6 microm.

CONCLUSIONS

Xenon represents a novel KATP channel opener that increases KATP currents independently of the sulfonylurea receptor-1 subunit by reducing ATP inhibition of the channel. Through this action and by its ability to readily partition across the blood-brain barrier, xenon has considerable potential in clinical settings of neuronal injury, including stroke.

Interaction between mutations in the slide helix of Kir6.2 associated with neonatal diabetes and neurological symptoms

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion from pancreatic beta-cells. Gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause neonatal diabetes. We report two novel mutations on the same haplotype (cis), F60Y and V64L, in the slide helix of Kir6.2 in a patient with neonatal diabetes, developmental delay and epilepsy. Functional analysis revealed the F60Y mutation increases the intrinsic channel open probability (Po(0)), thereby indirectly producing a marked decrease in channel inhibition by ATP and an increase in whole-cell K(ATP) currents. When expressed alone, the V64L mutation caused a small reduction in apparent ATP inhibition, by enhancing the ability of MgATP to stimulate channel activity. The V64L mutation also ameliorated the deleterious effects on the F60Y mutation when it was expressed on the same (but not a different) subunit. These data indicate that F60Y is the pathogenic mutation and reveal that interactions between slide helix residues can influence K(ATP) channel gating.

K(ATP) channelopathies in the pancreas

Adenosine-triphosphate-sensitive potassium channels (KATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic beta-cell. The critical involvement of KATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in KATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in KATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which KATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.

Adjacent mutations in the gating loop of Kir6.2 produce neonatal diabetes and hyperinsulinism

K(ATP) channels regulate insulin secretion from pancreatic beta-cells. Loss- and gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause hyperinsulinism of infancy and neonatal diabetes, respectively. We report two novel mutations in the gating loop of Kir6.2 which cause neonatal diabetes with developmental delay (T293N) and hyperinsulinism (T294M). These mutations increase (T293N) or decrease (T294M) whole-cell K(ATP) currents, accounting for the different clinical phenotypes. The T293N mutation increases the intrinsic channel open probability (Po((0))), thereby indirectly decreasing channel inhibition by ATP and increasing whole-cell currents. T294M channels exhibit a dramatically reduced Po((0)) in the homozygous but not in the pseudo-heterozygous state. Unlike wild-type channels, hetT294M channels were activated by MgADP in the absence but not in the presence of MgATP; however, they are activated by MgGDP in both the absence and presence of MgGTP. These mutations demonstrate the importance of the gating loop of Kir channels in regulating Po((0)) and further suggest that Mg-nucleotide interaction with SUR1 may reduce ATP inhibition at Kir6.2.

A cytosolic factor that inhibits KATP channels expressed in Xenopus oocytes by impairing Mg-nucleotide activation by SUR1

ATP-sensitive K(+) (K(ATP)) channels couple cell metabolism to cell electrical activity. Wild-type (Kir6.2/SUR1) K(ATP) channels heterologously expressed in Xenopus oocytes give rise to very small inward currents in cell-attached patches. A large increase in the current is observed on patch excision into zero ATP solution. This is presumably due to loss of intracellular ATP leading to unblock of K(ATP) channels. In contrast, channels containing Kir6.2 mutations associated with reduced ATP-sensitivity display non-zero cell-attached currents. Unexpectedly, these cell-attached currents are significantly smaller (by approximately 40%) than those observed when excised patches are exposed to physiological ATP concentrations (1-10 mm). Cramming the patch back into the oocyte cytoplasm restores mutant K(ATP) current amplitude to that measured in the cell-attached mode. This implies that the magnitude of the cell-attached current is regulated not only by intracellular ATP but also by another cytoplasmic factor/s. This factor seems to require the nucleotide-binding domains of SUR1 to be effective. Thus a mutant Kir6.2 (Kir6.2DeltaC-I296L) expressed in the absence of SUR1 exhibited currents of similar magnitude in cell-attached patches as in inside-out patches exposed to 10 mm MgATP. Similar results were found when Kir6.2-I296L was coexpressed with an SUR1 mutant that is insensitive to MgADP or MgATP activation. This suggests the oocyte contains a cytoplasmic factor that reduces nucleotide binding/hydrolysis at the NBDs of SUR1. In conclusion, our results reveal a novel regulatory mechanism for the K(ATP) channel. This was not evident for wild-type channels because of their high sensitivity to block by ATP.