Truncation of Kir6.2 produces ATP-sensitive K+ channels in the absence of the sulphonylurea receptor

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.

Gain of function mutation in K(ATP) channels and resulting upregulation of coupling conductance are partners in crime in the impairment of Ca2+ oscillations in pancreatic ß-cells

Gain of function mutations in the pore forming Kir6 subunits of the ATP sensitive K+ channels (K(ATP) channels) of pancreatic β-cells are the major cause of neonatal diabetes in humans. In this study, we show that in insulin secreting mouse β-cell lines, gain of function mutations in Kir6.1 result in a significant connexin36 (Cx36) overexpression, which form gap junctional connections and mediate electrical coupling between β-cells within pancreatic islets. Using computational modeling, we show that upregulation in Cx36 might play a functional role in the impairment of glucose stimulated Ca2+ oscillations in a cluster of β-cells with Kir6.1 gain of function mutations in their K(ATP) channels (GoF-K(ATP) channels). Our results show that without an increase in Cx36 expression, a gain of function mutation in Kir6.1 might not be sufficient to diminish glucose stimulated Ca2+ oscillations in a β-cell cluster. We also show that a reduced Cx36 expression, which leads to loss of coordination in a wild-type β-cell cluster, restores coordinated Ca2+ oscillations in a β-cell cluster with GoF-K(ATP) channels. Our results indicate that in a heterogenous β-cell cluster with GoF-K(ATP) channels, there is an inverted u-shaped nonmonotonic relation between the cluster activity and Cx36 expression. These results show that in a neonatal diabetic β-cell model, gain of function mutations in the Kir6.1 cause Cx36 overexpression, which aggravates the impairment of glucose stimulated Ca2+ oscillations.

Novel loss-of-function variants expand ABCC9-related intellectual disability and myopathy syndrome

Loss-of-function mutation of ABCC9, the gene encoding the SUR2 subunit of ATP sensitive-potassium (KATP) channels, was recently associated with autosomal recessive ABCC9-related intellectual disability and myopathy syndrome (AIMS). Here we identify nine additional subjects, from seven unrelated families, harbouring different homozygous loss-of-function variants in ABCC9 and presenting with a conserved range of clinical features. All variants are predicted to result in severe truncations or in-frame deletions within SUR2, leading to the generation of non-functional SUR2-dependent KATP channels. Affected individuals show psychomotor delay and intellectual disability of variable severity, microcephaly, corpus callosum and white matter abnormalities, seizures, spasticity, short stature, muscle fatigability and weakness. Heterozygous parents do not show any conserved clinical pathology but report multiple incidences of intra-uterine fetal death, which were also observed in an eighth family included in this study. In vivo studies of abcc9 loss-of-function in zebrafish revealed an exacerbated motor response to pentylenetetrazole, a pro-convulsive drug, consistent with impaired neurodevelopment associated with an increased seizure susceptibility. Our findings define an ABCC9 loss-of-function-related phenotype, expanding the genotypic and phenotypic spectrum of AIMS and reveal novel human pathologies arising from KATP channel dysfunction.

Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance

Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.

Structure of an open KATP channel reveals tandem PIP2 binding sites mediating the Kir6.2 and SUR1 regulatory interface

ATP-sensitive potassium (KATP) channels, composed of four pore-lining Kir6.2 subunits and four regulatory sulfonylurea receptor 1 (SUR1) subunits, control insulin secretion in pancreatic β-cells. KATP channel opening is stimulated by PIP2 and inhibited by ATP. Mutations that increase channel opening by PIP2 reduce ATP inhibition and cause neonatal diabetes. Although considerable evidence has implicated a role for PIP2 in KATP channel function, previously solved open-channel structures have lacked bound PIP2, and mechanisms by which PIP2 regulates KATP channels remain unresolved. Here, we report the cryoEM structure of a KATP channel harboring the neonatal diabetes mutation Kir6.2-Q52R, in the open conformation, bound to amphipathic molecules consistent with natural C18:0/C20:4 long-chain PI(4,5)P2 at two adjacent binding sites between SUR1 and Kir6.2. The canonical PIP2 binding site is conserved among PIP2-gated Kir channels. The non-canonical PIP2 binding site forms at the interface of Kir6.2 and SUR1. Functional studies demonstrate both binding sites determine channel activity. Kir6.2 pore opening is associated with a twist of the Kir6.2 cytoplasmic domain and a rotation of the N-terminal transmembrane domain of SUR1, which widens the inhibitory ATP binding pocket to disfavor ATP binding. The open conformation is particularly stabilized by the Kir6.2-Q52R residue through cation-π bonding with SUR1-W51. Together, these results uncover the cooperation between SUR1 and Kir6.2 in PIP2 binding and gating, explain the antagonistic regulation of KATP channels by PIP2 and ATP, and provide a putative mechanism by which Kir6.2-Q52R stabilizes an open channel to cause neonatal diabetes.

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.

Functional dissection of KATP channel structures reveals the importance of a conserved interface

ATP-sensitive potassium channels (KATP) are inhibited by ATP but activated by Mg-ADP, coupling the intracellular ATP/ADP ratio to the potassium conductance of the plasma membrane. Although there has been progress in determining the structure of KATP, the functional significance of the domain-domain interface in the gating properties of KATP channels remains incompletely understood. In this study, we define the structure of KATP as two modules: KATPcore and SURABC. Based on this model, we identified two functionally important interfaces between these two modules, namely interface I and interface II. Further structure-guided mutagenesis experiments indicate that destabilizing interface II by deleting ECL3 on the SUR1 subunit impairs KNtp-independent Mg-ADP activation, demonstrating the essential role of intramolecular interactions between KATPcore and SURABC in Mg-ADP activation. Additionally, interface II is functionally conserved between SUR1 and SUR2, and the hydrophobic residue F351 on ECL3 of SUR1 is crucial for maintaining the stability of this interface.

Structure of an open K ATP channel reveals tandem PIP 2 binding sites mediating the Kir6.2 and SUR1 regulatory interface

ATP-sensitive potassium (K ATP ) channels, composed of four pore-lining Kir6.2 subunits and four regulatory sulfonylurea receptor 1 (SUR1) subunits, control insulin secretion in pancreatic β-cells. K ATP channel opening is stimulated by PIP 2 and inhibited by ATP. Mutations that increase channel opening by PIP 2 reduce ATP inhibition and cause neonatal diabetes. Although considerable evidence has indicated PIP 2 in K ATP channel function, previously solved open-channel structures have lacked bound PIP 2 , and mechanisms by which PIP 2 regulates K ATP channels remain unresolved. Here, we report the cryoEM structure of a K ATP channel harboring the neonatal diabetes mutation Kir6.2-Q52R, bound to natural C18:0/C20:4 long-chain PIP 2 in an open conformation. The structure reveals two adjacent PIP 2 molecules between SUR1 and Kir6.2. The first PIP 2 binding site is conserved among PIP 2 -gated Kir channels. The second site forms uniquely in K ATP at the interface of Kir6.2 and SUR1. Functional studies demonstrate both binding sites determine channel activity. Kir6.2 pore opening is associated with a twist of the Kir6.2 cytoplasmic domain and a rotation of the N-terminal transmembrane domain of SUR1, which widens the inhibitory ATP binding pocket to disfavor ATP binding. The open conformation is particularly stabilized by the Kir6.2-Q52R residue through cation-π bonding with SUR1-W51. Together, these results uncover the cooperation between SUR1 and Kir6.2 in PIP 2 binding and gating, explain the antagonistic regulation of K ATP channels by PIP 2 and ATP, and provide the mechanism by which Kir6.2-Q52R stabilizes an open channel to cause neonatal diabetes.

Ion channels regulate energy homeostasis and the progression of metabolic disorders: Novel mechanisms and pharmacology of their modulators

The progression of metabolic diseases, featured by dysregulated metabolic signaling pathways, is orchestrated by numerous signaling networks. Among the regulators, ion channels transport ions across the membranes and trigger downstream signaling transduction. They critically regulate energy homeostasis and pathogenesis of metabolic diseases and are potential therapeutic targets for treating metabolic disorders. Ion channel blockers have been used to treat diabetes for decades by stimulating insulin secretion, yet with hypoglycemia and other adverse effects. It calls for deeper understanding of the largely elusive regulatory mechanisms, which facilitates the identification of new therapeutic targets and safe drugs against ion channels. In the article, we critically assess the two principal regulatory mechanisms, protein-channel interaction and post-translational modification on the activities of ion channels to modulate energy homeostasis and metabolic disorders through multiple novel mechanisms. Moreover, we discuss the multidisciplinary methods that provide the tools for elucidation of the regulatory mechanisms mediating metabolic disorders by ion channels. In terms of translational perspective, the mechanistic analysis of recently validated ion channels that regulate insulin resistance, body weight control, and adverse effects of current ion channel antagonists are discussed in details. Their small molecule modulators serve as promising new drug candidates to combat metabolic disorders.

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.

KATP Channels and the Metabolic Regulation of Insulin Secretion in Health and Disease: The 2022 Banting Medal for Scientific Achievement Award Lecture

Diabetes is characterized by elevation of plasma glucose due to an insufficiency of the hormone insulin and is associated with both inadequate insulin secretion and impaired insulin action. The Banting Medal for Scientific Achievement Commemorates the work of Sir Frederick Banting, a member of the team that first used insulin to treat a patient with diabetes almost exactly one hundred years ago on 11 January 1922. This article is based on my Banting lecture of 2022 and concerns the mechanism of glucose-stimulated insulin secretion from pancreatic β-cells, with an emphasis on the metabolic regulation of the KATP channel. This channel plays a central role in insulin release. Its closure in response to metabolically generated changes in the intracellular concentrations of ATP and MgADP stimulates β-cell electrical activity and insulin granule exocytosis. Activating mutations in KATP channel genes that impair the ability of the channel to respond to ATP give rise to neonatal diabetes. Impaired KATP channel regulation may also play a role in type 2 diabetes. I conjecture that KATP channel closure in response to glucose is reduced because of impaired glucose metabolism, which fails to generate a sufficient increase in ATP. Consequently, glucose-stimulated β-cell electrical activity is less. As ATP is also required for insulin granule exocytosis, both reduced exocytosis and less β-cell electrical activity may contribute to the reduction in insulin secretion. I emphasize that what follows is not a definitive review of the topic but a personal account of the contribution of my team to the field that is based on my Banting lecture.

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.

Venom Peptides, Polyphenols and Alkaloids: Are They the Next Antidiabetics That Will Preserve β-Cell Mass and Function in Type 2 Diabetes?

Improvement of insulin secretion by pancreatic β-cells and preservation of their mass are the current challenges that future antidiabetic drugs should meet for achieving efficient and long-term glycemic control in patients with type 2 diabetes (T2D). The successful development of glucagon-like peptide 1 (GLP-1) analogues, derived from the saliva of a lizard from the Helodermatidae family, has provided the proof of concept that antidiabetic drugs directly targeting pancreatic β-cells can emerge from venomous animals. The literature reporting on the antidiabetic effects of medicinal plants suggests that they contain some promising active substances such as polyphenols and alkaloids, which could be active as insulin secretagogues and β-cell protectors. In this review, we discuss the potential of several polyphenols, alkaloids and venom peptides from snake, frogs, scorpions and cone snails. These molecules could contribute to the development of new efficient antidiabetic medicines targeting β-cells, which would tackle the progression of the disease.

Structural Insights into ATP-Sensitive Potassium Channel Mechanics: A Role of Intrinsically Disordered Regions

Commonly used techniques, such as CryoEM or X-ray, are not able to capture the structural reorganizations of disordered regions of proteins (IDR); therefore, it is difficult to assess their functions in proteins based exclusively on experiments. To fill this gap, we used computational molecular dynamics (MD) simulation methods to capture IDR dynamics and trace biological function-related interactions in the Kir6.2/SUR1 potassium channel. This ATP-sensitive octameric complex, one of the critical elements in the insulin secretion process in human pancreatic β-cells, has four to five large, disordered fragments. Using unique MD simulations of the full Kir6.2/SUR1 channel complex, we present an in-depth analysis of the dynamics of the disordered regions and discuss the possible functions they could have in this system. Our MD results confirmed the crucial role of the N-terminus of the Kir6.2 fragment and the L0-loop of the SUR1 protein in the transfer of mechanical signals between domains that trigger insulin release. Moreover, we show that the presence of IDRs affects natural ligand binding. Our research takes us one step further toward understanding the action of this vital complex.

The Mutation Spectrum of Rare Variants in the Gene of Adenosine Triphosphate (ATP)-Binding Cassette Subfamily C Member 8 in Patients with a MODY Phenotype in Western Siberia

During differential diagnosis of diabetes mellitus, the greatest difficulties are encountered with young patients because various types of diabetes can manifest themselves in this age group (type 1, type 2, and monogenic types of diabetes mellitus, including maturity-onset diabetes of the young (MODY)). The MODY phenotype is associated with gene mutations leading to pancreatic-β-cell dysfunction. Using next-generation sequencing technology, targeted sequencing of coding regions and adjacent splicing sites of MODY-associated genes (HNF4A, GCK, HNF1A, PDX1, HNF1B, NEUROD1, KLF11, CEL, PAX4, INS, BLK, KCNJ11, ABCC8, and APPL1) was carried out in 285 probands. Previously reported missense variants c.970G>A (p.Val324Met) and c.1562G>A (p.Arg521Gln) in the ABCC8 gene were found once each in different probands. Variant c.1562G>A (p.Arg521Gln) in ABCC8 was detected in a compound heterozygous state with a pathogenic variant of the HNF1A gene in a diabetes patient and his mother. Novel frameshift mutation c.4609_4610insC (p.His1537ProfsTer22) in this gene was found in one patient. All these variants were detected in available family members of the patients and cosegregated with diabetes mellitus. Thus, next-generation sequencing of MODY-associated genes is an important step in the diagnosis of rare MODY subtypes.

Methylene Blue Inhibits Cromakalim-Activated K+ Currents in Follicle-Enclosed Oocytes

The effects of methylene blue (MB) on cromakalim-induced K⁺ currents were investigated in follicle-enclosed Xenopus oocytes. In concentrations ranging from 3-300 μM, MB inhibited K⁺ currents (IC₅₀: 22.4 μM) activated by cromakalim, which activates K_ATP channels. MB inhibited cromakalim-activated K⁺ currents in a noncompetitive and voltage-independent manner. The respective EC₅₀ and slope values for cromakalim-activation of K⁺ currents were 194 ± 21 µM and 0.91 for controls, and 206 ± 24 µM and 0.87 in the presence of 30 μM MB. The inhibition of cromakalim-induced K⁺ currents by MB was not altered by pretreatment with the Ca²⁺ chelator BAPTA, which suggests that MB does not influence Ca²⁺-activated second messenger pathways. K⁺ currents mediated through a C-terminally deleted form of Kir6.2 (KirΔC26), which does not contain the sulfonylurea receptor, were still inhibited by MB, indicating direct interaction of MB with the channel-forming Kir6.2 subunit. The binding characteristics of the K_ATP ligand [³H]glibenclamide are not altered by MB in a concentration range between 1 μM-1 mM, as suggested by radioligand binding assay. The presence of a membrane permeable cGMP analogue (8-Br-cGMP, 100 µM) and a guanylate cyclase activator (BAY 58-2667, 3 µM) did not affect the inhibitory effects of MB, suggesting that MB does not inhibit cromakalim-activated K⁺ currents through guanylate cyclase. Collectively, these results suggest that MB directly inhibits cromakalim-activated K⁺ currents in follicular cells of Xenopus oocytes.

From Crosstalk to Synergism: The Combined Effect of Cholesterol and PI(4,5)P2 on Inwardly Rectifying Potassium Channels

Inwardly rectifying potassium (Kir) channels are integral membrane proteins that control the flux of potassium ions across cell membranes and regulate membrane permeability. All eukaryotic Kir channels require the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for activation. In recent years, it has become evident that the function of many members of this family of channels is also mediated by another essential lipid-cholesterol. Here, we focus on members of the Kir2 and Kir3 subfamilies and their modulation by these two key lipids. We discuss how PI(4,5)P₂ and cholesterol bind to Kir2 and Kir3 channels and how they affect channel activity. We also discuss the accumulating evidence indicating that there is interplay between PI(4,5)P₂ and cholesterol in the modulation of Kir2 and Kir3 channels. In particular, we review the crosstalk between PI(4,5)P₂ and cholesterol in the modulation of the ubiquitously expressed Kir2.1 channel and the synergy between these two lipids in the modulation of the Kir3.4 channel, which is primarily expressed in the heart. Additionally, we demonstrate that there is also synergy in the modulation of Kir3.2 channels, which are expressed in the brain. These observations suggest that alterations in the relative levels PI(4,5)P₂ and cholesterol may fine-tune Kir channel activity.

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.

Ligand-mediated Structural Dynamics of a Mammalian Pancreatic KATP Channel

Regulation of pancreatic KATP channels involves orchestrated interactions of their subunits, Kir6.2 and SUR1, and ligands. Previously we reported KATP channel cryo-EM structures in the presence and absence of pharmacological inhibitors and ATP, focusing on the mechanisms by which inhibitors act as pharmacological chaperones of KATP channels (Martin et al., 2019). Here we analyzed the same cryo-EM datasets with a focus on channel conformational dynamics to elucidate structural correlates pertinent to ligand interactions and channel gating. We found pharmacological inhibitors and ATP enrich a channel conformation in which the Kir6.2 cytoplasmic domain is closely associated with the transmembrane domain, while depleting one where the Kir6.2 cytoplasmic domain is extended away into the cytoplasm. This conformational change remodels a network of intra- and inter-subunit interactions as well as the ATP and PIP2 binding pockets. The structures resolved key contacts between the distal N-terminus of Kir6.2 and SUR1's ABC module involving residues implicated in channel function and showed a SUR1 residue, K134, participates in PIP2 binding. Molecular dynamics simulations revealed two Kir6.2 residues, K39 and R54, that mediate both ATP and PIP2 binding, suggesting a mechanism for competitive gating by ATP and PIP2.

The dynamic interplay of PIP2 and ATP in the regulation of the KATP channel

ATP-sensitive potassium (KATP ) channels couple the intracellular ATP concentration to insulin secretion. KATP channel activity is inhibited by ATP binding to the Kir6.2 tetramer and activated by phosphatidylinositol 4,5-bisphosphate (PIP2 ). Here, we use molecular dynamics simulation, electrophysiology and fluorescence spectroscopy to show that ATP and PIP2 occupy different binding pockets that share a single amino acid residue, K39. When both ligands are present, simulations suggest that K39 shows a greater preference to co-ordinate with PIP2 than with ATP. They also predict that a neonatal diabetes mutation at K39 (K39R) increases the number of hydrogen bonds formed between K39 and PIP2 , potentially accounting for the reduced ATP inhibition observed in electrophysiological experiments. Our work suggests that PIP2 and ATP interact allosterically to regulate KATP channel activity. KEY POINTS: The KATP channel is activated by the binding of phosphatidylinositol 4,5-bisphosphate (PIP2 ) lipids and inactivated by the binding of ATP. K39 has the potential to bind to both PIP2 and ATP. A mutation to this residue (K39R) results in neonatal diabetes. This study uses patch-clamp fluorometry, electrophysiology and molecular dynamics simulation. We show that PIP2 competes with ATP for K39, and this reduces channel inhibition by ATP. We show that K39R increases channel affinity to PIP2 by increasing the number of hydrogen bonds with PIP2 , when compared with the wild-type K39. This therefore decreases KATP channel inhibition by ATP.

Kir6.1 and SUR2B in Cantú syndrome

Kir6.1 and SUR2 are subunits of ATP-sensitive potassium (KATP) channels expressed in a wide range of tissues. Extensive study has implicated roles of these channel subunits in diverse physiological functions. Together they generate the predominant KATP conductance in vascular smooth muscle and are the target of vasodilatory drugs. Roles for Kir6.1/SUR2 dysfunction in disease have been suggested based on studies of animal models and human genetic discoveries. In recent years, it has become clear that gain-of-function (GoF) mutations in both genes result in Cantú syndrome (CS)-a complex, multisystem disorder. There is currently no targeted therapy for CS, but studies of mouse models of the disease reveal that pharmacological reversibility of cardiovascular and gastrointestinal pathologies can be achieved by administration of the KATP channel inhibitor, glibenclamide. Here we review the function, structure, and physiological and pathological roles of Kir6.1/SUR2B channels, with a focus on CS. Recent studies have led to much improved understanding of the underlying pathologies and the potential for treatment, but important questions remain: Can the study of genetically defined CS reveal new insights into Kir6.1/SUR2 function? Do these reveal new pathophysiological mechanisms that may be important in more common diseases? And is our pharmacological armory adequately stocked?

Inward and outward currents of native and cloned K(ATP) channels (Kir6.2/SUR1) share single-channel kinetic properties

Background

Methods

Results

Conclusions

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Outward K(ATP) channel currents are not grouped in bursts regardless of membrane potential.

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No differences in the number of kinetic states could be seen for single channels between native K(ATP) channels, Kir6.2ΔC26 alone and co-expressed with SUR1 for outward currents.

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K(ATP) channel open time for outward currents corresponds to burst duration for inward currents.

Structural insights into the mechanism of pancreatic KATP channel regulation by nucleotides

ATP-sensitive potassium channels (K_ATP) are metabolic sensors that convert the intracellular ATP/ADP ratio to the excitability of cells. They are involved in many physiological processes and implicated in several human diseases. Here we present the cryo-EM structures of the pancreatic K_ATP channel in both the closed state and the pre-open state, resolved in the same sample. We observe the binding of nucleotides at the inhibitory sites of the Kir6.2 channel in the closed but not in the pre-open state. Structural comparisons reveal the mechanism for ATP inhibition and Mg-ADP activation, two fundamental properties of K_ATP channels. Moreover, the structures also uncover the activation mechanism of diazoxide-type K_ATP openers.

K_ATP channels are energy sensors. Here, authors report the Cryo-EM structures of pancreatic K_ATP in both the closed state and the pre-open state. These structures illuminate the mechanism of K_ATP channel regulation by the intracellular nucleotides.

Early Effects of Metabolic Syndrome on ATP-Sensitive Potassium Channels from Rat Pancreatic Beta Cells

Metabolic syndrome (MS) is a cluster of metabolic signs that increases the risk of developing type 2 two diabetes mellitus and cardiovascular diseases. MS leads to pancreatic beta cell exhaustion and decreased insulin secretion through unknown mechanisms in a time-dependent manner. ATP-sensitive potassium channels (K_ATP channels), common targets of anti-diabetic drugs, participate in the glucose-stimulated insulin secretion, coupling the metabolic status and electrical activity of pancreatic beta cells. We investigated the early effects of MS on the conductance, ATP and glybenclamide sensitivity of the K_ATP channels. We used Wistar rats fed with a high-sucrose diet (HSD) for 8 weeks as a MS model. In excised membrane patches, control and HSD channels showed similar unitary conductance and ATP sensitivity pancreatic beta cells in their K_ATP channels_. In contrast, MS produced variability in the sensitivity to glybenclamide of K_ATP channels. We observed two subpopulations of pancreatic beta cells, one with similar (Gly1) and one with increased (Gly2) glybenclamide sensitivity compared to the control group. This study shows that the early effects of MS produced by consuming high-sugar beverages can affect the pharmacological properties of K_ATP channels to one of the drugs used for diabetes treatment.

Blocking Kir6.2 channels with SpTx1 potentiates glucose-stimulated insulin secretion from murine pancreatic β cells and lowers blood glucose in diabetic mice

ATP-sensitive K⁺ (K_ATP) channels in pancreatic β cells are comprised of pore-forming subunits (Kir6.2) and modulatory sulfonylurea receptor subunits (SUR1). The ATP sensitivity of these channels enables them to couple metabolic state to insulin secretion in β cells. Antidiabetic sulfonylureas such as glibenclamide target SUR1 and indirectly suppress Kir6.2 activity. Glibenclamide acts as both a primary and a secondary secretagogue to trigger insulin secretion and potentiate glucose-stimulated insulin secretion, respectively. We tested whether blocking Kir6.2 itself causes the same effects as glibenclamide, and found that the Kir6.2 pore-blocking venom toxin SpTx1 acts as a strong secondary, but not a strong primary, secretagogue. SpTx1 triggered a transient rise of plasma insulin and lowered the elevated blood glucose of diabetic mice overexpressing Kir6.2 but did not affect those of nondiabetic mice. This proof-of-concept study suggests that blocking Kir6.2 may serve as an effective treatment for diabetes and other diseases stemming from K_ATP hyperactivity that cannot be adequately suppressed with sulfonylureas.

Expression of truncated Kir6.2 promotes insertion of functionally inverted ATP-sensitive K+ channels

ATP-sensitive K⁺ (K_ATP) channels couple cellular metabolism to electrical activity in many cell types. Wild-type K_ATP channels are comprised of four pore forming (Kir6.x) and four regulatory (sulfonylurea receptor, SURx) subunits that each contain RKR endoplasmic reticulum retention sequences that serve to properly translocate the channel to the plasma membrane. Truncated Kir6.x variants lacking RKR sequences facilitate plasma membrane expression of functional Kir6.x in the absence of SURx; however, the effects of channel truncation on plasma membrane orientation have not been explored. To investigate the role of truncation on plasma membrane orientation of ATP sensitive K⁺ channels, three truncated variants of Kir6.2 were used (Kir6.2ΔC26, 6xHis-Kir6.2ΔC26, and 6xHis-EGFP-Kir6.2ΔC26). Oocyte expression of Kir6.2ΔC26 shows the presence of a population of inverted inserted channels in the plasma membrane, which is not present when co-expressed with SUR1. Immunocytochemical staining of intact and permeabilized HEK293 cells revealed that the N-terminus of 6xHis-Kir6.2ΔC26 was accessible on both sides of the plasma membrane at roughly equivalent ratios, whereas the N-terminus of 6xHis-EGFP-Kir6.2Δ26 was only accessible on the intracellular face. In HEK293 cells, whole-cell electrophysiological recordings showed a ca. 50% reduction in K⁺ current upon addition of ATP to the extracellular solution for 6xHis-Kir6.2ΔC26, though sensitivity to extracellular ATP was not observed in 6xHis-EGFP-Kir6.2ΔC26. Importantly, the population of channels that is inverted exhibited similar function to properly inserted channels within the plasma membrane. Taken together, these data suggest that in the absence of SURx, inverted channels can be formed from truncated Kir6.x subunits that are functionally active which may provide a new model for testing pharmacological modulators of Kir6.x, but also indicates the need for added caution when using truncated Kir6.2 mutants.

From Bench to Biomolecular Simulation: Phospholipid Modulation of Potassium Channels

Potassium (K+) ion channels are crucial in numerous cellular processes as they hyperpolarise a cell through K+ conductance, returning a cell to its resting potential. K+ channel mutations result in multiple clinical complications such as arrhythmia, neonatal diabetes and migraines. Since 1995, the regulation of K+ channels by phospholipids has been heavily studied using a range of interdisciplinary methods such as cellular electrophysiology, structural biology and computational modelling. As a result, K+ channels are model proteins for the analysis of protein-lipid interactions. In this review, we will focus on the roles of lipids in the regulation of K+ channels, and how atomic-level structures, along with experimental techniques and molecular simulations, have helped guide our understanding of the importance of phospholipid interactions.

Molecular mechanisms of centipede toxin SsTx-4 inhibition of inwardly rectifying potassium channels

Inwardly rectifying potassium channels (Kirs) are important drug targets, with antagonists for the Kir1.1, Kir4.1, and pancreatic Kir6.2/SUR1 channels being potential drug candidates for treating hypertension, depression, and diabetes, respectively. However, few peptide toxins acting on Kirs are identified and their interacting mechanisms remain largely elusive yet. Herein, we showed that the centipede toxin SsTx-4 potently inhibited the Kir1.1, Kir4.1, and Kir6.2/SUR1 channels with nanomolar to submicromolar affinities and intensively studied the molecular bases for toxin–channel interactions using patch-clamp analysis and site-directed mutations. Other Kirs including Kir2.1 to 2.4, Kir4.2, and Kir7.1 were resistant to SsTx-4 treatment. Moreover, SsTx-4 inhibited the inward and outward currents of Kirs with different potencies, possibly caused by a K⁺ “knock-off” effect, suggesting the toxin functions as an out pore blocker physically occluding the K⁺-conducting pathway. This conclusion was further supported by a mutation analysis showing that M137 located in the outer vestibule of the Kir6.2/ΔC26 channel was the key residue mediating interaction with SsTx-4. On the other hand, the molecular determinants within SsTx-4 for binding these Kir channels only partially overlapped, with K13 and F44 being the common key residues. Most importantly, K11A, P15A, and Y16A mutant toxins showed improved affinity and/or selectivity toward Kir6.2, while R12A mutant toxin had increased affinity for Kir4.1. To our knowledge, SsTx-4 is the first characterized peptide toxin with Kir4.1 inhibitory activity. This study provides useful insights for engineering a Kir6.2/SUR1 channel–specific antagonist based on the SsTx-4 template molecule and may be useful in developing new antidiabetic drugs.

Control of Biophysical and Pharmacological Properties of Potassium Channels by Ancillary Subunits

Potassium channels facilitate and regulate physiological processes as diverse as electrical signaling, ion, solute and hormone secretion, fluid homeostasis, hearing, pain sensation, muscular contraction, and the heartbeat. Potassium channels are each formed by either a tetramer or dimer of pore-forming α subunits that co-assemble to create a multimer with a K⁺-selective pore that in most cases is capable of functioning as a discrete unit to pass K⁺ ions across the cell membrane. The reality in vivo, however, is that the potassium channel α subunit multimers co-assemble with ancillary subunits to serve specific physiological functions. The ancillary subunits impart specific physiological properties that are often required for a particular activity in vivo; in addition, ancillary subunit interaction often alters the pharmacology of the resultant complex. In this chapter the modes of action of ancillary subunits on K⁺ channel physiology and pharmacology are described and categorized into various mechanistic classes.

Regulation of α-cell glucagon secretion: The role of second messengers

Glucagon is a potent glucose‐elevating hormone that is secreted by pancreatic α‐cells. While well‐controlled glucagon secretion plays an important role in maintaining systemic glucose homeostasis and preventing hypoglycaemia, it is increasingly apparent that defects in the regulation of glucagon secretion contribute to impaired counter‐regulation and hyperglycaemia in diabetes. It has therefore been proposed that pharmacological interventions targeting glucagon secretion/signalling can have great potential in improving glycaemic control of patients with diabetes. However, despite decades of research, a consensus on the precise mechanisms of glucose regulation of glucagon secretion is yet to be reached. Second messengers are a group of small intracellular molecules that relay extracellular signals to the intracellular signalling cascade, modulating cellular functions. There is a growing body of evidence that second messengers, such as cAMP and Ca²⁺, play critical roles in α‐cell glucose‐sensing and glucagon secretion. In this review, we discuss the impact of second messengers on α‐cell electrical activity, intracellular Ca²⁺ dynamics and cell exocytosis. We highlight the possibility that the interaction between different second messengers may play a key role in the glucose‐regulation of glucagon secretion.

Structure based analysis of KATP channel with a DEND syndrome mutation in murine skeletal muscle

Developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome, the most severe end of neonatal diabetes mellitus, is caused by mutation in the ATP-sensitive potassium (K_ATP) channel. In addition to diabetes, DEND patients present muscle weakness as one of the symptoms, and although the muscle weakness is considered to originate in the brain, the pathological effects of mutated K_ATP channels in skeletal muscle remain elusive. Here, we describe the local effects of the K_ATP channel on muscle by expressing the mutation present in the K_ATP channels of the DEND syndrome in the murine skeletal muscle cell line C2C12 in combination with computer simulation. The present study revealed that the DEND mutation can lead to a hyperpolarized state of the muscle cell membrane, and molecular dynamics simulations based on a recently reported high-resolution structure provide an explanation as to why the mutation reduces ATP sensitivity and reveal the changes in the local interactions between ATP molecules and the channel.

Production and purification of ATP-sensitive potassium channel particles for cryo-electron microscopy

ATP-sensitive potassium (K_ATP) channels are multimeric protein complexes made of four inward rectifying potassium channel (Kir6.x) subunits and four ABC protein sulfonylurea receptor (SURx) subunits. Kir6.x subunits form the potassium ion conducting pore of the channel, and SURx functions to regulate Kir6.x. Kir6.x and SURx are uniquely dependent on each other for expression and function. In pancreatic β-cells, channels comprising SUR1 and Kir6.2 mediate glucose-stimulated insulin secretion and are the targets of antidiabetic sulfonylureas. Mutations in genes encoding SUR1 or Kir6.2 are linked to insulin secretion disorders, with loss- or gain-of-function mutations causing congenital hyperinsulinism or neonatal diabetes mellitus, respectively. Defects in the K_ATP channel in other tissues underlie human diseases of the cardiovascular and nervous systems. Key to understanding how channels are regulated by physiological and pharmacological ligands and how mutations disrupt channel assembly or gating to cause disease is the ability to observe structural changes associated with subunit interactions and ligand binding. While recent advances in the structural method of single-particle cryo-electron microscopy (cryoEM) offers direct visualization of channel structures, success of obtaining high-resolution structures is dependent on highly concentrated, homogeneous K_ATP channel particles. In this chapter, we describe a method for expressing K_ATP channels in mammalian cell culture, solubilizing the channel in detergent micelles and purifying K_ATP channels using an affinity tag to the SURx subunit for cryoEM structural studies.

The ligand-bound state of a G protein-coupled receptor stabilizes the interaction of functional cholesterol molecules

Cholesterol is a major component of mammalian plasma membranes that not only affects the physical properties of the lipid bilayer but also is the function of many membrane proteins including G protein-coupled receptors. The oxytocin receptor (OXTR) is involved in parturition and lactation of mammals and in their emotional and social behaviors. Cholesterol acts on OXTR as an allosteric modulator inducing a high-affinity state for orthosteric ligands through a molecular mechanism that has yet to be determined. Using the ion channel-coupled receptor technology, we developed a functional assay of cholesterol modulation of G protein-coupled receptors that is independent of intracellular signaling pathways and operational in living cells. Using this assay, we discovered a stable binding of cholesterol molecules to the receptor when it adopts an orthosteric ligand-bound state. This stable interaction preserves the cholesterol-dependent activity of the receptor in cholesterol-depleted membranes. This mechanism was confirmed using time-resolved FRET experiments on WT OXTR expressed in CHO cells. Consequently, a positive cross-regulation sequentially occurs in OXTR between cholesterol and orthosteric ligands.

Association between KCNJ11 rs5219 variant and alcohol consumption on the effect of insulin secretion in a community-based Korean cohort: a 12-year follow-up study

Chronic alcohol consumption is known to be associated with type 2 diabetes (T2D), which is developed by two underlying mechanisms, β-cell dysfunction and insulin resistance. Identification of genetic variants in association with the development of T2D may help explain the genetic risk factors of T2D. In this study, we tried to find out some genetic variations, which interact with alcohol consumption and also are associated with β-cell function through 12 year’s follow-up study in Korean population. We performed a genotype association study using the community-based Ansung-Ansan Cohort data (baseline n = 3120; follow-up n = 433). Genotype association analyses of the baseline data showed that alcohol consumption is associated with the decreases of blood insulin levels and insulin secretion in participants with the KCNJ11 rs5219 risk allele. Moreover, multivariate logistic regression analyses revealed that the risk allele group is vulnerable to impairment of β-cell function in response to alcohol consumption (OR 1.450; 95% CI 1.061–1.982). Furthermore, 12-year’ follow-up results showed that alcohol consumption synergistically decreases insulin secretion in participants with KCNJ11 rs5219 risk alleles. Our findings demonstrate that the KCNJ11 rs5219 risk allele in combination with alcohol consumption could be a potential risk factor of β-cell dysfunction. We hope that this new findings could be helpful to further understand the development of T2D depending on individual genetic background in association with alcohol consumption.

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.

Evaluating inositol phospholipid interactions with inward rectifier potassium channels and characterising their role in disease

Membrane proteins are frequently modulated by specific protein-lipid interactions. The activation of human inward rectifying potassium (hKir) channels by phosphoinositides (PI) has been well characterised. Here, we apply a coarse-grained molecular dynamics free-energy perturbation (CG-FEP) protocol to capture the energetics of binding of PI lipids to hKir channels. By using either a single- or multi-step approach, we establish a consistent value for the binding of PIP2 to hKir channels, relative to the binding of the bulk phosphatidylcholine phospholipid. Furthermore, by perturbing amino acid side chains on hKir6.2, we show that the neonatal diabetes mutation E179K increases PIP2 affinity, while the congenital hyperinsulinism mutation K67N results in a reduced affinity. We show good agreement with electrophysiological data where E179K exhibits a reduction in neomycin sensitivity, implying that PIP2 binds more tightly E179K channels. This illustrates the application of CG-FEP to compare affinities between lipid species, and for annotating amino acid residues.

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.

New insights into KATP channel gene mutations and neonatal diabetes mellitus

The ATP-sensitive potassium channel (K_ATP channel) couples blood levels of glucose to insulin secretion from pancreatic β-cells. K_ATP channel closure triggers a cascade of events that results in insulin release. Metabolically generated changes in the intracellular concentrations of adenosine nucleotides are integral to this regulation, with ATP and ADP closing the channel and MgATP and MgADP increasing channel activity. Activating mutations in the genes encoding either of the two types of K_ATP channel subunit (Kir6.2 and SUR1) result in neonatal diabetes mellitus, whereas loss-of-function mutations cause hyperinsulinaemic hypoglycaemia of infancy. Sulfonylurea and glinide drugs, which bind to SUR1, close the channel through a pathway independent of ATP and are now the primary therapy for neonatal diabetes mellitus caused by mutations in the genes encoding K_ATP channel subunits. Insight into the molecular details of drug and nucleotide regulation of channel activity has been illuminated by cryo-electron microscopy structures that reveal the atomic-level organization of the K_ATP channel complex. Here we review how these structures aid our understanding of how the various mutations in the genes encoding Kir6.2 (KCNJ11) and SUR1 (ABCC8) lead to a reduction in ATP inhibition and thereby neonatal diabetes mellitus. We also provide an update on known mutations and sulfonylurea therapy in neonatal diabetes mellitus.

The Structural Basis for the Binding of Repaglinide to the Pancreatic KATP Channel

Repaglinide (RPG) is a short-acting insulin secretagogue widely prescribed for the treatment of type 2 diabetes. It boosts insulin secretion by inhibiting the pancreatic ATP-sensitive potassium channel (K_ATP). However, the mechanisms by which RPG binds to the K_ATP channel are poorly understood. Here, we describe two cryo-EM structures: the pancreatic K_ATP channel in complex with inhibitory RPG and adenosine-5'-(γ-thio)-triphosphate (ATPγS) at 3.3 Å and a medium-resolution structure of a RPG-bound mini SUR1 protein in which the N terminus of the inward-rectifying potassium channel 6.1 (Kir6.1) is fused to the ABC transporter module of the sulfonylurea receptor 1 (SUR1). These structures reveal the binding site of RPG in the SUR1 subunit. Furthermore, the high-resolution structure reveals the complex architecture of the ATP binding site, which is formed by both Kir6.2 and SUR1 subunits, and the domain-domain interaction interfaces.

A computational structural biology study to understand the impact of mutation on structure-function relationship of inward-rectifier potassium ion channel Kir6.2 in human

Type 2 diabetes (T2D) is clinically characterized via hyperglycemia. Polymorphism rs5219 in the KCNJ11 gene is a risk factor for developing T2D in humans. KCNJ11 encodes the 'inward-rectifier potassium ion channel (Kir6.2)'. However, because of the absence of the complete crystal/NMR structures of Kir6.2 proteins, insight into its structure and function and its interaction with diverse ligands remain elusive to date. Therefore, a computational approach was employed for predicting the best plausible 'three-dimensional' structure of Kir6.2 as well as for studying the influence of mutation (p. GLU23LYS) on both architectures as well as the function of Kir6.2 employing simulation studies. Results obtained revealed that though, with increased time, 'Gibbs free energy' becomes positive, residues in wild type Kir6.2 experiences less random movement as compared to mutant Kir6.2. The less random movement of residues in wild type Kir6.2 represents the standard coupling between open and closing of 'K_ATP channel' and thus the normal secretion of insulin. The more dispersed motion of mutant Kir6.2 residues represents 'overactivity' of the 'K_ATP channel' and thus insulin 'under-secretion'. Further, molecular docking and simulation studies identified two phytochemicals/drugs, namely, A-348441 and chushizisin I, which retains the wild type property of Kir6.2 after binding with mutant protein. Unlike A-348441, this is for the first time, the present study is reporting about the plausible anti-diabetic property of chushizisin I. As these two phytochemicals/drugs, namely, A-348441 and chushizisin I, have passed ADMET test, in the near future, they may be utilized as anti-diabetic drugs after further investigation.Communicated by Ramaswamy H. Sarma.

Kir6.2-D323 and SUR2A-Q1336: an intersubunit interaction pairing for allosteric information transfer in the KATP channel complex

ATP-sensitive potassium (K_ATP) channels are widely expressed and play key roles in many tissues by coupling metabolic state to membrane excitability. The SUR subunits confer drug and enhanced nucleotide sensitivity to the pore-forming Kir6 subunit, and so information transfer between the subunits must occur. In our previous study, we identified an electrostatic interaction between Kir6 and SUR2 subunits that was key for allosteric information transfer between the regulatory and pore-forming subunit. In this study, we demonstrate a second putative interaction between Kir6.2-D323 and SUR2A-Q1336 using patch clamp electrophysiological recording, where charge swap mutation of the residues on either side of the potential interaction compromise normal channel function. The Kir6.2-D323K mutation gave rise to a constitutively active, glibenclamide and ATP-insensitive K_ATP complex, further confirming the importance of information transfer between the Kir6 and SUR2 subunits. Sensitivity to modulators was restored when Kir6.2-D323K was co-expressed with a reciprocal charge swap mutant, SUR-Q1336E. Importantly, equivalent interactions have been identified in both Kir6.1 and Kir6.2 suggesting this is a second important interaction between Kir6 and the proximal C terminus of SUR.

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.

Structural Insights into the Inhibitory Mechanism of Insulin Secretagogues on the Pancreatic ATP-Sensitive Potassium Channel

Sulfonylureas and glinides are commonly used oral insulin secretagogues (ISs) that act on the pancreatic ATP-sensitive potassium (K_ATP) channel to promote insulin secretion in order to lower the blood glucose level. Physiologically, K_ATP channels are inhibited by intracellular ATP and activated by Mg-ADP. Therefore, they sense the cellular energy status to regulate the permeability of potassium ions across the plasma membrane. The pancreatic K_ATP channel is composed of the pore-forming Kir6.2 subunits and the regulatory SUR1 subunits. Previous electrophysiological studies have established that ISs bind to the SUR1 subunit and inhibit the channel activity primarily by two mechanisms. First, ISs prevent Mg-ADP activation. Second, ISs inhibit the channel activity of Kir6.2 directly. Several cryo-EM structures of the pancreatic K_ATP channel determined recently have provided remarkable structural insights into these two mechanisms.

A family of orthologous proteins from centipede venoms inhibit the hKir6.2 channel

Inhibitors targeting ion channels are useful tools for studying their functions. Given the selectivity of any inhibitor for a channel is relative, more than one inhibitor of different affinities may be used to help identify the channel in a biological preparation. Here, we describe a family of small proteins in centipede venoms that inhibit the pore (hKir6.2) of a human ATP-sensitive K⁺ channel (hK_ATP). While the traditional peptide-sequencing service gradually vanishes from academic institutions, we tried to identify the sequences of inhibitory proteins purified from venoms by searching the sequences of the corresponding transcriptomes, a search guided by the key features of a known hKir6.2 inhibitor (SpTx1). The candidate sequences were cross-checked against the masses of purified proteins, and validated by testing the activity of recombinant proteins against hKir6.2. The four identified proteins (SsdTx1-3 and SsTx) inhibit hK_ATP channels with a K_d of <300 nM, compared to 15 nM for SpTx1. SsTx has previously been discovered to block human voltage-gated KCNQ K⁺ channels with a 2.5 μM K_d. Given that SsTx inhibits hKir6.2 with >10-fold lower K_d than it inhibits hKCNQ, SsTx may not be suitable for probing KCNQ channels in a biological preparation that also contains more-SsTx-sensitive K_ATP channels.

Endocytosis of KATP Channels Drives Glucose-Stimulated Excitation of Pancreatic β Cells

Insulin secretion from pancreatic β cells in response to high glucose (HG) critically depends on the inhibition of K_ATP channel activity in HG. It is generally believed that HG-induced effects are mediated by the increase in intracellular ATP, but here, we showed that, in INS-1 cells, endocytosis of K_ATP channel plays a major role. Upon HG stimulation, resting membrane potential depolarized by 30.6 mV (from -69.2 to -38.6 mV) and K_ATP conductance decreased by 91% (from 0.243 to 0.022 nS/pF), whereas intracellular ATP was increased by only 47%. HG stimulation induced internalization of K_ATP channels, causing a significant decrease in surface channel density, and this decrease was completely abolished by inhibiting endocytosis using dynasore, a dynamin inhibitor, or a PKC inhibitor. These drugs profoundly inhibited HG-induced depolarization. Our results suggest that the control of K_ATP channel surface density plays a greater role than ATP-dependent gating in regulating β cell excitability.

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

Venom from mammals, amphibians, snakes, arachnids, sea anemones and insects provides diverse sources of peptides with different potential medical applications. Several of these peptides have already been converted into drugs and some are still in the clinical phase. Diabetes type 2 is one of the diseases with the highest mortality rate worldwide, requiring specific attention. Diverse drugs are available (e.g., Sulfonylureas) for effective treatment, but with several adverse secondary effects, most of them related to the low specificity of these compounds to the target. In this context, the search for specific and high-affinity compounds for the management of this metabolic disease is growing. Toxins isolated from animal venom have high specificity and affinity for different molecular targets, of which the most important are ion channels. This review will present an overview about the electrical activity of the ion channels present in pancreatic β cells that are involved in the insulin secretion process, in addition to the diversity of peptides that can interact and modulate the electrical activity of pancreatic β cells. The importance of prospecting bioactive peptides for therapeutic use is also reinforced.

Activation mechanism of ATP-sensitive K+ channels explored with real-time nucleotide binding

The response of ATP-sensitive K⁺ channels (K_ATP) to cellular metabolism is coordinated by three classes of nucleotide binding site (NBS). We used a novel approach involving labeling of intact channels in a native, membrane environment with a non-canonical fluorescent amino acid and measurement (using FRET with fluorescent nucleotides) of steady-state and time-resolved nucleotide binding to dissect the role of NBS2 of the accessory SUR1 subunit of K_ATP in channel gating. Binding to NBS2 was Mg²⁺-independent, but Mg²⁺ was required to trigger a conformational change in SUR1. Mutation of a lysine (K1384A) in NBS2 that coordinates bound nucleotides increased the EC₅₀ for trinitrophenyl-ADP binding to NBS2, but only in the presence of Mg²⁺, indicating that this mutation disrupts the ligand-induced conformational change. Comparison of nucleotide-binding with ionic currents suggests a model in which each nucleotide binding event to NBS2 of SUR1 is independent and promotes K_ATP activation by the same amount.

Rab8a is involved in membrane trafficking of Kir6.2 in the MIN6 insulinoma cell line

Although ATP-sensitive K⁺ (K_ATP) channels play an important role in the secretion of insulin by pancreatic beta cells, the mechanisms that regulate the intracellular transport of K_ATP channel subunit proteins (i.e., Kir6.2 and sulfonylurea receptor 1 (SUR1)) to the plasma membrane remain uncharacterized. We investigated the possibility that an interaction between K_ATP channel subunit proteins and Rab8a protein, a member of the RAS superfamily, may be involved in the membrane trafficking of K_ATP channels. Co-immunoprecipitation and immunostaining experiments using co-expression systems with fluorescent protein-tagged Kir6.2 were carried out to identify the coupling of K_ATP channels and Rab8a proteins in the insulin-secreting cell line, MIN6. Rab8a protein co-localized with Kir6.2 protein, a channel pore subunit (in a granular pattern), and with insulin. Knockdown of the Rab8a gene with RNA interference using small interfering RNA systems caused reductions in the amount of total K_ATP and plasma membrane surface K_ATP channels without decreasing the messenger RNA transcription of the K_ATP channel subunits. Rab8a gene knockdown also enhanced glucose-induced insulin secretion. These results suggest that Rab8a may be involved in membrane trafficking of K_ATP channels and the maintenance of normal insulin secretion in the MIN6 pancreatic beta cell line.