ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies

Sulfonylurea receptor coupled conductances alter the performace of two central pattern generating circuits in Cancer borealis

Neuronal activity and energy supply must maintain a fine balance for neuronal fitness. Various channels of communication between the two could impact network output in different ways. Sulfonylurea receptors (SURs) are a modification of ATP-binding cassette proteins (ABCs) that confer ATP-dependent gating on their associated ion channels. They are widely expressed and link metabolic states directly to neuronal activity. The role they play varies in different circuits, both enabling bursting and inhibiting activity in pathological conditions. The crab, Cancer borealis, has central patterns generators (CPGs) that fire in rhythmic bursts nearly constantly and it is unknown how energy availability influences these networks. The pyloric network of the stomatogastric ganglion (STG) and cardiac ganglion (GC) control rhythmic contractions of the foregut and heart respectively. Pharmacological manipulation of SURs results in opposite effects in the two CPGs. Neuronal firing completely stops in the STG when SUR-associated channels are open, and firing increases when the channels are closed. This results from a decrease in the excitability of pyloric dilator (PD) neurons, which are a part of the pacemaker kernel. The neurons of the CG, paradoxically, increase firing within bursts when SUR-associated channels are opened, and bursting slows when SUR-associated channels are closed. The channel permeability and sensitivities analyses present novel SUR-conductance biophysics, which nevertheless change activity in ways reminiscent of the predominantly studied mammalian receptor/channels. We suggest that SUR-associated conductances allow different neurons to respond to energy states in different ways through a common mechanism.

DNA methylation regulates pancreatic gene expression and links maternal high-fat diet to the offspring glucose metabolism

Maternal high-fat diet (HFD) is related to an increased risk of glucose metabolism disorders throughout the whole life of offspring. The pancreas is a glucose homeostasis regulator. Accumulating evidence has revealed that maternal HFD affects offspring pancreas structure and function. However, the potential mechanism remains unclear. In this study, the mouse dam was fed with HFD or control diet (CD) during prepregnancy, pregnancy and lactation. The pancreatic insulin secretion function and islet genome methylome of offspring were analyzed. Pancreatic islet specific gene methylation was detected by using MeDIP qPCR. The results showed that body weight, blood glucose after oral glucose loads, fasting serum insulin, and HOMA-IR index values were significantly higher in male 12-week-old offspring from HFD dams than in the offspring from CD dams. Maternal HFD induced insulin secretion defects in male offspring. Compared with that in maternal CD group, methylation of the Abcc8 and Kcnj11 genes was increased in maternal HFD group in male offspring pancreatic islets. Furthermore, the expression levels of Abcc8 and Kcnj11 were downregulated by intrauterine exposure to a maternal HFD. In summary, maternal HFD results in a long-term functional disorder of the pancreas that is involved in insulin secretion-related gene DNA hypermethylation.

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

Regulation of insulin secretion by the post-translational modifications

Post-translational modification (PTM) has a significant impact on cellular signaling and function regulation. In pancreatic β cells, PTMs are involved in insulin secretion, cell development, and viability. The dysregulation of PTM in β cells is clinically associated with the development of diabetes mellitus. Here, we summarized current findings on major PTMs occurring in β cells and their roles in insulin secretion. Our work provides comprehensive insight into understanding the mechanisms of insulin secretion and potential therapeutic targets for diabetes from the perspective of protein PTMs.

Cyb5r3-based mechanism and reversal of secondary failure to sulfonylurea in diabetes

Sulfonylureas (SUs) are effective and affordable antidiabetic drugs. However, chronic use leads to secondary failure, limiting their utilization. Here, we identify cytochrome b5 reductase 3 (Cyb5r3) down-regulation as a mechanism of secondary SU failure and successfully reverse it. Chronic exposure to SU lowered Cyb5r3 abundance and reduced islet glucose utilization in mice in vivo and in ex vivo murine islets. Cyb5r3 β cell-specific knockout mice phenocopied SU failure. Cyb5r3 engaged in a glucose-dependent interaction that stabilizes glucokinase (Gck) to maintain glucose utilization. Hence, Gck activators can circumvent Cyb5r3-dependent SU failure. A Cyb5r3 activator rescued secondary SU failure in mice in vivo and restored insulin secretion in ex vivo human islets. We conclude that Cyb5r3 is a key factor in the secondary failure to SU and a potential target for its prevention, which might rehabilitate SU use in diabetes.

Cell-Membrane-Anchored DNA Logic-Gated Nanoassemblies for In Situ Extracellular Bioimaging

Extracellular K⁺ and adenosine triphosphate (ATP) levels are significantly elevated in the tumor microenvironment (TME) and can be used as biomarkers for early cancer detection and tumor localization. Most reported TME sensors only respond to single abnormal factors, resulting in a lack of accuracy and specificity for the detection of complex environments. Thus, precisely locating the TME remains challenging. In this work, we aimed to develop an intelligent DNA nanoassembly controlled by a "YES-AND" logic circuit using a bimolecular G-quadruplex (G4) and ATP aptamer as logical control units. As a proof of concept, in the presence of K⁺ (input 1) and ATP (input 2), the YES-AND Boolean operator returned a true value and the output was the fluorescence resonance energy transfer (FRET) signal, indicating high sensitivity and selectivity. After being anchored to living cell surfaces, this logic nanosensor imaged extracellular K⁺ and ATP present at abnormal levels in situ. Owing to diverse disease markers in the TME, this novel logic sensor might hold great promise for the targeted delivery of intelligent anticancer drugs and Boolean logic-controlled treatment.

Nitrergic modulation of ion channel function in regulating neuronal excitability

Nitric oxide (NO) signaling in the brain provides a wide range of functional properties in response to neuronal activity. NO exerts its effects through different signaling pathways, namely, through the canonical soluble guanylyl cyclase-mediated cGMP production route and via post-translational protein modifications. The latter pathways comprise cysteine S-nitrosylation and 3-nitrotyrosination of distinct tyrosine residues. Many ion channels are targeted by one or more of these signaling routes, which leads to their functional regulation under physiological conditions or facilities their dysfunction leading to channelopathies in many pathologies. The resulting alterations in ion channel function changes neuronal excitability, synaptic transmission, and action potential propagation. Transient and activity-dependent NO production mediates reversible ion channel modifications via cGMP and S-nitrosylation signaling, whereas more pronounced and longer-term NO production during conditions of elevated oxidative stress leads to increasingly cumulative and irreversible protein 3-nitrotyrosination. The complexity of this regulation and vast variety of target ion channels and their associated functional alterations presents a challenging task in assessing and understanding the role of NO signaling in physiology and disease.

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.

Transcriptional profiles of genes related to electrophysiological function in Scn5a+/- murine hearts

The Scn5a gene encodes the major pore-forming Nav 1.5 (α) subunit, of the voltage-gated Na+ channel in cardiomyocytes. The key role of Nav 1.5 in action potential initiation and propagation in both atria and ventricles predisposes organisms lacking Scn5a or carrying Scn5a mutations to cardiac arrhythmogenesis. Loss-of-function Nav 1.5 genetic abnormalities account for many cases of the human arrhythmic disorder Brugada syndrome (BrS) and related conduction disorders. A murine model with a heterozygous Scn5a deletion recapitulates many electrophysiological phenotypes of BrS. This study examines the relationships between its Scn5a+/- genotype, resulting transcriptional changes, and the consequent phenotypic presentations of BrS. Of 62 selected protein-coding genes related to cardiomyocyte electrophysiological or homeostatic function, concentrations of mRNA transcribed from 15 differed significantly from wild type (WT). Despite halving apparent ventricular Scn5a transcription heterozygous deletion did not significantly downregulate its atrial expression, raising possibilities of atria-specific feedback mechanisms. Most of the remaining 14 genes whose expression differed significantly between WT and Scn5a+/- animals involved Ca2+ homeostasis specifically in atrial tissue, with no overlap with any ventricular changes. All statistically significant changes in expression were upregulations in the atria and downregulations in the ventricles. This investigation demonstrates the value of future experiments exploring for and clarifying links between transcriptional control of Scn5a and of genes whose protein products coordinate Ca2+ regulation and examining their possible roles in BrS.

Cyclic nucleotide-dependent ionic currents in olfactory receptor neurons of the hawkmoth Manduca sexta suggest pull-push sensitivity modulation

Olfactory receptor neurons (ORNs) of the hawkmoth Manduca sexta sensitize via cAMP- and adapt via cGMP-dependent mechanisms. Perforated patch clamp recordings distinguished 11 currents in these ORNs. Derivatives of cAMP and/or cGMP antagonistically affected three of five K⁺ currents and two non-specific cation currents. The Ca²⁺ -dependent K⁺ current I_K(Ca ²⁺ ₎ and the sensitive pheromone-dependent K⁺ current I_K(cGMP-) , which both express fast kinetics, were inhibited by 8bcGMP, while a slow K⁺ current, I_K(cGMP+) , was activated by 8bcGMP. Furthermore, application of 8bcAMP blocked slowly activating, zero mV-reversing, non-specific cation currents, I_LL and I_cat(PKC?) , which remained activated in the presence of 8bcGMP. Their activations pull the membrane potential towards their 0-mV reversal potentials, in addition to increasing intracellular Ca²⁺ levels voltage- and I_LL -dependently. Twenty minutes after application, 8bcGMP blocked a TEA-independent K⁺ current, I_K(noTEA) , and a fast cation current, I_cat(nRP) , which both shift the membrane potential to negative values. We conclude that conditions of sensitization are maintained at high levels of cAMP, via specific opening/closure of ion channels that allow for fast kinetics, hyperpolarized membrane potentials, and low intracellular Ca²⁺ levels. In contrast, adaptation is supported via cGMP, which antagonizes cAMP, opening Ca²⁺ -permeable channels with slow kinetics that stabilize depolarized resting potentials. The antagonistic modulation of peripheral sensory neurons by cAMP or cGMP is reminiscent of pull-push mechanisms of neuromodulation at central synapses underlying metaplasticity.

Review of the mechanism underlying mefloquine-induced neurotoxicity

Mefloquine, a potent blood schizontocide, is effective against drug-resistant Plasmodium falciparum. This property, along with its unique pharmacokinetic profile, makes mefloquine a widely prescribed antimalarial drug. However, several epidemiological studies have raised concerns on the safety of mefloquine as prophylaxis for malaria. Well-documented side-effects of mefloquine include abnormal dreams, insomnia, anxiety, and depressed mood, as well as nausea and dizziness (the last two most frequent effects). The mechanisms that underlie the neurological/psychiatric complications of mefloquine are poorly understood. The aim of this study was to review the literature on the neurotoxic mechanisms of action of mefloquine to better understand its potential toxicity in the central nervous system, highlighting the mechanisms that lead to its psychiatric disorders. Experimental studies on the neurotoxic effects of mefloquine discussed herein include brain transporters of mefloquine, alteration in neurotransmitters, disruption on calcium (Ca²⁺) homeostasis and neuroinflammation, generation of oxidative stress response in neurons (involving glutathione, increased F2-isoprostanes, accumulation of cytosolic lipid globules), and alteration of voltage-dependent channels, as well as gap junction intercellular communications. Although several hypotheses have been proposed for the mechanisms that mediate mefloquine-induced brain damage, they are not fully understood, necessitating additional studies in the future.

Gs/Gq signaling switch in β cells defines incretin effectiveness in diabetes

By restoring glucose-regulated insulin secretion, glucagon-like peptide-1-based (GLP-1-based) therapies are becoming increasingly important in diabetes care. Normally, the incretins GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) jointly maintain normal blood glucose levels by stimulation of insulin secretion in pancreatic β cells. However, the reason why only GLP-1-based drugs are effective in improving insulin secretion after presentation of diabetes has not been resolved. ATP-sensitive K+ (KATP) channels play a crucial role in coupling the systemic metabolic status to β cell electrical activity for insulin secretion. Here, we have shown that persistent membrane depolarization of β cells due to genetic (β cell-specific Kcnj11-/- mice) or pharmacological (long-term exposure to sulfonylureas) inhibition of the KATP channel led to a switch from Gs to Gq in a major amplifying pathway of insulin secretion. The switch determined the relative insulinotropic effectiveness of GLP-1 and GIP, as GLP-1 can activate both Gq and Gs, while GIP only activates Gs. The findings were corroborated in other models of persistent depolarization: a spontaneous diabetic KK-Ay mouse and nondiabetic human and mouse β cells of pancreatic islets chronically treated with high glucose. Thus, a Gs/Gq signaling switch in β cells exposed to chronic hyperglycemia underlies the differential insulinotropic potential of incretins in diabetes.

KATP and TRPM2-like channels couple metabolic status to resting membrane potential of octopus neurons in the mouse ventral cochlear nucleus

ATP-sensitive potassium (K_ATP) channels and transient receptor potential melastatin 2 (TRPM2) channels are commonly expressed both pre- and postsynaptically in the central nervous system (CNS). We hypothesized that K_ATP and TRPM2 may couple metabolic status to the resting membrane potential of octopus neurons of the mouse ventral cochlear nucleus (VCN). Therefore, we studied the expression of K_ATP channels and TRPM2 channels in octopus cells by immunohistochemical techniques and their contribution to neuronal electrical properties by the electrophysiological patch clamp technique. In immunohistochemical staining of octopus cells, labelling with Kir6.2 and SUR1 antibodies was strong, and labelling with the SUR2 antibody was moderate, but labelling with Kir6.1 was very weak. Octopus cells had intense staining with TRPM2 antibodies. In patch clamp recordings, bath application of K_ATP channel agonists H₂O₂ (880 μM), ATZ (1 mM), cromakalim (50 μM), diazoxide (200 μM), NNC 55-0118 and NN 414 separately resulted in hyperpolarizations of resting potential to different extents. Application of 8-Bro-cADPR (50 μM), a specific antagonist of TRPM2 channels, in the presence of H₂O₂ (880 μM) resulted in further hyperpolarization by approximately 1 mV. The amplitudes of H₂O₂-induced outward K_ATP currents and ADPR-induced inward currents were 206.1 ± 31.5 pA (n = 4) and 136.8 ± 22.4 pA, respectively, at rest. Their respective reversal potentials were -77 ± 2.6 mV (n = 3) and -6.3 ± 2.9 (n = 3) and -6.3 ± 2.9 (n = 3). In conclusion, octopus cells appear to possess both K_ATP channels and TRPM2-like channels. K_ATP might largely be constituted by SUR1-Kir6.2 subunits and SUR2-Kir6.2 subunits. Both K_ATP and TRPM2-like channels might have a modulatory action in setting the membrane potential.

α-Synuclein Aggregates Increase the Conductance of Substantia Nigra Dopamine Neurons, an Effect Partly Reversed by the KATP Channel Inhibitor Glibenclamide

Dopaminergic neurons (DNs) in the substantia nigra pars compacta (SNpc) form an important part of the basal ganglia circuitry, playing key roles in movement initiation and coordination. A hallmark of Parkinson’s disease (PD) is the degeneration of these SNpc DNs leading to akinesia, bradykinesia and tremor. There is gathering evidence that oligomeric α-synuclein (α-syn) is one of the major pathologic species in PD, with its deposition in Lewy bodies (LBs) closely correlated with disease progression. However, the precise mechanisms underlying the effects of oligomeric α-syn on DN function have yet to be fully defined. Here, we have combined electrophysiological recording and detailed analysis to characterize the time-dependent effects of α-syn aggregates (consisting of oligomers and possibly small fibrils) on the properties of SNpc DNs. The introduction of α-syn aggregates into single DNs via the patch electrode significantly reduced both the input resistance and the firing rate without changing the membrane potential. These effects occurred after 8–16 min of dialysis but did not occur with the monomeric form of α-syn. The effects of α-syn aggregates could be significantly reduced by preincubation with the ATP-sensitive K⁺ channel (KATP) inhibitor glibenclamide. These data suggest that accumulation of α-syn aggregates in DNs may chronically activate KATP channels leading to a significant loss of excitability and dopamine release.

Personalized Nutrition and -Omics

With change in global concern toward food quality over food quantity, consumer concern and choice of healthy food has become a matter of prime importance. It gave rise to concept of “personalized or precision nutrition”. The theory behind personalization of nutrition is supported by multiple factors including advances in food analytics, nutrition based diseases and public health programs, increasing use of information technology in nutrition science, concept of gene-diet interaction and growing consumer capacity or concern by better and healthy foods. The advances in “omics” tools and related analytical techniques have resulted into tremendous scope of their application in nutrition science. As a consequence, a better understanding of underlying interaction between diet and individual is expected with addressing of key challenges for successful implementation of this science. In this chapter, the above aspects are discussed to get an insight into driving factors for increasing concern in personalized nutrition.

Modulation of mitochondrial respiration rate and calcium-induced swelling by new cromakalim analogues

A cellular model of cardiomyocytes (H9c2 cell line) and mitochondria isolated from mouse liver were used to understand the drug action of BPDZ490 and BPDZ711, two benzopyran analogues of the reference potassium channel opener cromakalim, on mitochondrial respiratory parameters and swelling, by comparing their effects with those of the parent compound cromakalim. For these three compounds, the oxygen consumption rate (OCR) was determined by high-resolution respirometry (HRR) and their impact on adenosine triphosphate (ATP) production and calcium-induced mitochondrial swelling was investigated. Cromakalim did not modify neither the OCR of H9c2 cells and the ATP production nor the Ca-induced swelling. By contrast, the cromakalim analogue BPDZ490 (1) induced a strong increase of OCR, while the other benzopyran analogue BPDZ711 (2) caused a marked slowdown. For both compounds, 1 displayed a biphasic behavior while 2 still showed an inhibitory effect. Both compounds 1 and 2 were also found to decrease the ATP synthesis, with pronounced effect for 2, while cromakalim remained without effect. Overall, these results indicate that cromakalim, as parent molecule, does not induce per se any direct effect on mitochondrial respiratory function neither on whole cells nor on isolated mitochondria whereas both benzopyran analogues 1 and 2 display totally opposite behavior profiles, suggesting that compound 1, by increasing the maximal respiration capacity, might behave as a mild uncoupling agent and compound 2 is taken as an inhibitor of the mitochondrial electron-transfer chain.

'Resistance is futile?' - paradoxical inhibitory effects of KATP channel closure in glucagon-secreting α-cells

By secreting insulin and glucagon, the β- and α-cells of the pancreatic islets play a central role in the regulation of systemic metabolism. Both cells are equipped with ATP-regulated potassium (K_ATP ) channels that are regulated by the intracellular ATP/ADP ratio. In β-cells, K_ATP channels are active at low (non-insulin-releasing) glucose concentrations. An increase in glucose leads to K_ATP channel closure, membrane depolarization and electrical activity that culminates in elevation of [Ca²⁺ ]_i and initiation of exocytosis of the insulin-containing secretory granules. The α-cells are also equipped with K_ATP channels but they are under strong tonic inhibition at low glucose, explaining why α-cells are electrically active under hypoglycaemic conditions and generate large Na⁺ - and Ca²⁺ -dependent action potentials. Closure of residual K_ATP channel activity leads to membrane depolarization and an increase in action potential firing but this stimulation of electrical activity is associated with inhibition rather than acceleration of glucagon secretion. This paradox arises because membrane depolarization reduces the amplitude of the action potentials by voltage-dependent inactivation of the Na⁺ channels involved in action potential generation. Exocytosis in α-cells is tightly linked to the opening of voltage-gated P/Q-type Ca²⁺ channels, the activation of which is steeply voltage-dependent. Accordingly, the inhibitory effect of the reduced action potential amplitude exceeds the stimulatory effect resulting from the increased action potential frequency. These observations highlight a previously unrecognised role of the action potential amplitude as a key regulator of pancreatic islet hormone secretion.

Pleiotropic effects of anti-diabetic drugs: A comprehensive review

Diabetes mellitus characterized by hyperglycaemia presents an array of comorbidities such as cardiovascular and renal failure, dyslipidemia, and cognitive impairments. Populations above the age of 60 are in an urgent need of effective therapies to deal with the complications associated with diabetes mellitus. Widely used anti-diabetic drugs have good safety profiles and multiple reports indicate their pleiotropic effects in diabetic patients or models. This review has been written with the objective of identifying the widely-marketed anti-diabetic drugs which can be efficiently repurposed for the treatment of other diseases or disorders. It is an updated, comprehensive review, describing the protective role of various classes of anti-diabetic drugs in mitigating the macro and micro vascular complications of diabetes mellitus, and differentiating these drugs on the basis of their mode of action. Notably, metformin, the anti-diabetic drug most commonly explored for cancer therapy, has also exhibited some antimicrobial effects. Unlike class specific effects, few instances of drug specific effects in managing cardiovascular complications have also been reported. A major drawback is that the pleiotropic effects of anti-diabetic drugs have been mostly investigated only in diabetic patients. Thus, for effective repurposing, more clinical trials devoted to analyse the effects of anti-diabetic drugs in patients irrespective of their diabetic condition, are required.

Mechanisms and Characteristics of Sulfonylureas and Glinides

BACKGROUND

Type 2 diabetes mellitus is a complex progressive endocrine disease characterized by hyperglycemia and life-threatening complications. It is the most common disorder of pancreatic cell function that causes insulin deficiency. Sulfonylurea is a class of oral hypoglycemic drugs. Over the past half century, these drugs, together with the subsequent non-sulfonylureas (glinides), have been the main oral drugs for insulin secretion.

OBJECTIVE

Through in-depth study, the medical profession considers it as an important drug for improving blood sugar control.

METHODS

The mechanism, characteristics, efficacy and side effects of sulfonylureas and glinides were reviewed in detail.

RESULTS

Sulfonylureas and glinides not only stimulated the release of insulin from pancreatic cells, but also had many extrapanular hypoglycemic effect, such as reducing the clearance rate of insulin in liver, reducing the secretion of glucagon, and enhancing the sensitivity of peripheral tissues to insulin in type 2 diabetes mellitus.

CONCLUSION

Sulfonylureas and glinides are effective first-line drugs for the treatment of diabetes mellitus. Although they have the risk of hypoglycemia, weight gain and cardiovascular disease, their clinical practicability and safety can be guaranteed as long as they are reasonably used.

Pharmacological chaperones of ATP-sensitive potassium channels: Mechanistic insight from cryoEM structures

ATP-sensitive potassium (K_ATP) channels are uniquely evolved protein complexes that couple cell energy levels to cell excitability. They govern a wide range of physiological processes including hormone secretion, neuronal transmission, vascular dilation, and cardiac and neuronal preconditioning against ischemic injuries. In pancreatic β-cells, K_ATP channels composed of Kir6.2 and SUR1, encoded by KCNJ11 and ABCC8, respectively, play a key role in coupling blood glucose concentration to insulin secretion. Mutations in ABCC8 or KCNJ11 that diminish channel function result in congenital hyperinsulinism. Many of these mutations principally hamper channel biogenesis and hence trafficking to the cell surface. Several small molecules have been shown to correct channel biogenesis and trafficking defects. Here, we review studies aimed at understanding how mutations impair channel biogenesis and trafficking and how pharmacological ligands overcome channel trafficking defects, particularly highlighting recent cryo-EM structural studies which have shed light on the mechanisms of channel assembly and pharmacological chaperones.

2,2-Dimethyl-3,4-dihydro-2H-1,4-benzoxazines as isosteres of 2,2-dimethylchromans acting as inhibitors of insulin release and vascular smooth muscle relaxants

The present study describes the synthesis and biological evaluation of 4-phenylureido/thioureido-substituted 2,2-dimethyl-3,4-dihydro-2H-1,4-benzoxazines as isosteres of corresponding 2,2-dimethylchromans reported to be pancreatic β-cell K_ATP channel openers. The benzoxazines were found to be less active as inhibitors of the glucose-induced insulin release than their corresponding chromans, while the myorelaxant activity of some 4-arylureido-substituted benzoxazines was more pronounced than that exhibited by their chroman counterparts. The myorelaxant activity of the most potent benzoxazine 8e was further characterized on rat aortic rings precontracted by 30 mM KCl in the presence of glibenclamide (10 μM) or precontracted by 80 mM extracellular KCl. Our findings indicate that, on vascular smooth muscle cells, the benzoxazine 8e mainly behaved as a calcium entry blocker.

Adrenergic receptor stimulation suppresses oxidative metabolism in isolated rat islets and Min6 cells

Insulin secretion is stimulated by glucose metabolism and inhibited by catecholamines through adrenergic receptor stimulation. We determined whether catecholamines suppress oxidative metabolism in β-cells through adrenergic receptors. In Min6 cells and isolated rat islets, epinephrine decreased oxygen consumption rates compared to vehicle control or co-administration of epinephrine with α2-adrenergic receptor antagonist yohimbine. Epinephrine also decreased forskolin-stimulated oxygen consumption rates, indicating cAMP dependent and independent actions. Furthermore, glucose oxidation rates were decreased with epinephrine, independent of the exocytosis of insulin, which was blocked with yohimbine. We evaluated metabolic targets through proteomic analysis after 4 h epinephrine exposure that revealed 466 differentially expressed proteins that were significantly enriched for processes including oxidative metabolism, protein turnover, exocytosis, and cell proliferation. These results demonstrate that acute α2-adrenergic stimulation suppresses glucose oxidation in β-cells independent of nutrient availability and insulin exocytosis, while cAMP concentrations are elevated. Proteomics and immunoblots revealed changes in electron transport chain proteins that were correlated with lower metabolic reducing equivalents, intracellular ATP concentrations, and altered mitochondrial membrane potential implicating a new role for adrenergic control of mitochondrial function and ultimately insulin secretion.

Memantine has no effect on KATP channels in pancreatic β cells

Objective

Memantine, a drug for Alzheimer’s disease, is considered to suppress excessive stimulation of N-methyl-d-aspartic acid receptors and to prevent neuronal death. However, a recent report indicated that the neuronal K_ATP channel also can become a target of memantine. The K_ATP channel is a key regulator of insulin secretion in pancreatic β cells. Therefore, if memantine could inhibit the K_ATP channel in pancreatic β cells, it would be an effective drug for both Alzheimer’s disease and diabetes. However, there is no report on the effect of memantine on the K_ATP channel in pancreatic β cells. Therefore, we investigated whether memantine affect the blood glucose level, insulin secretion and K_ATP channel activity in pancreatic β cells.

Results

An intraperitoneal glucose tolerance test was performed with or without memantine (1 mg/kg) injection in intact mice. Insulin secretion from isolated islets was measured under low (2 mM) and high (20 mM) glucose concentrations with or without memantine (1 μM). The effect of memantine (1 μM) on K_ATP channel currents in isolated pancreatic β cells was recorded using the whole-cell patch-clamp technique. Memantine had no effect on the blood glucose level, insulin secretion from isolated islets or K_ATP channel current in pancreatic β cells.

Possible involvement of monoamine neurons in the emotional abnormality in Kir6.2-deficient mice

ATP-sensitive potassium (K_ATP) channels consist of two structurally different subunits: a pore-forming subunit of the Kir6.0-family (Kir6.1 or Kir6.2) and a regulatory sulfonylurea receptor subunit (SUR1, SUR2A or SUR2B). Although Kir6.2 is widely distributed in the brain, the mechanisms that underlie the impact of Kir6.2 on emotional behavior are not yet fully understood. To clarify the role of Kir6.2 in emotional behavior, in the present study, we investigated the behavioral characteristics of Kir6.2-knockout (Kir6.2^-/-) mice. Kir6.2^-/- mice showed impaired general behavior in a locomotor activity test and open field test. In addition, anxiety-like behavior was observed in the open field test, elevated plus-maze test and light-dark test. In particular, excessive anxiety-like behavior was observed in female Kir6.2^-/- mice. Moreover, we investigated whether Kir6.2 is expressed on monoamine neurons in the brain. Immunohistochemical studies showed that Kir6.2 was co-localized with tryptophan hydroxylase (TPH), a marker of serotonergic neurons, in dorsal raphe nuclei. Kir6.2 was also co-localized with tyrosine hydroxylase (TH), a marker of dopaminergic/noradrenergic neurons, in the ventral tegmental area and locus coeruleus. Next, we checked the protein levels of TH and TPH in the midbrain. Interestingly, TPH expression was significantly elevated in female Kir6.2^-/- mice. These results suggest that Kir6.2 in monoamine neurons, especially serotonergic neurons, could play a key role in emotional behavior.

β-Cell signalling and insulin secretagogues: A path for improved diabetes therapy

Insulin secretagogues including sulfonylureas, glinides and incretin-related drugs such as dipeptidyl peptidase 4 (DPP-4) inhibitors and glucagon-like peptide-1 receptor agonists are widely used for treatment of type 2 diabetes. In addition, glucokinase activators and G-protein-coupled receptor 40 (GPR40) agonists also have been developed, although the drugs are not clinically usable. These different drugs exert their effects on insulin secretion by different mechanisms. Recent advances in β-cell signalling studies have not only deepened our understanding of insulin secretion but also revealed novel mechanisms of insulin secretagogues. Clarification of the signalling mechanisms of the insulin secretagogues will contribute to improved drug therapy for diabetes.

ZD0947, a sulphonylurea receptor modulator, detects functional sulphonylurea receptor subunits in murine vascular smooth muscle ATP-sensitive K+ channels

In order to identify functional sulphonylurea receptor (SUR.x) subunits of native ATP-sensitive K⁺ channels (K_ATP channels) in mouse portal vein, the effects of ZD0947, a SUR.x modulator, were investigated on spontaneous portal vein contractions, macroscopic membrane currents and unitary currents recorded (using patch-clamp techniques) in freshly dispersed mouse portal vein myocytes. Spontaneous contractions in mouse portal vein were reversibly reduced by ZD0947 in a concentration-dependent manner (K_i =293nM). The relaxation elicited by 3µM ZD0947 was antagonized by the additional application of glibenclamide (300nM), but not gliclazide (100-300nM). In the conventional whole-cell configuration, 100µM ZD0947 elicited inward glibenclamide-sensitive currents at a holding potential of -60mV that demonstrated selectivity for K⁺(i.e. K_ATP currents). The peak amplitude of the membrane current elicited by 30µM or 100µM ZD0947 was smaller than that elicited by 100µM pinacidil at -60mV. In the cell-attached mode, 100µM ZD0947 activated glibenclamide-sensitive K⁺ channels with a conductance (35 pS) similar to that of recombinant Kir6.1/SUR2B channels that were expressed in HEK293 cells and activated by 100µM ZD0947. These results demonstrate that ZD0947 caused a significant vascular relaxation through the activation of K_ATP channels and that SUR2B may be the major functional subunit of SUR.x in mouse portal vein K_ATP channels, based on its pharmacological selectivity.

Glibenclamide–sulfonylurea receptor 1 antagonist alleviates LPS-induced BV2 cell activation through the p38/MAPK pathway

We investigated the anti-neuroinflammatory activity and mechanism of glibenclamide, sulfonylurea receptor 1 (Sur1) antagonist, against LPS-induced microglial activationin vitro.

The ATP-sensitive K channel is seizure protective and required for effective dietary therapy in a model of mitochondrial encephalomyopathy

Effective therapies are lacking for mitochondrial encephalomyopathies (MEs). MEs are devastating diseases that predominantly affect the energy-demanding tissues of the nervous system and muscle, causing symptoms such as seizures, cardiomyopathy, and neuro- and muscular degeneration. Even common anti-epileptic drugs which are frequently successful in ameliorating seizures in other diseases tend to have a lower success rate in ME, highlighting the need for novel drug targets, especially those that may couple metabolic sensitivity to neuronal excitability. Furthermore, alternative epilepsy therapies such as dietary modification are gaining in clinical popularity but have not been thoroughly studied in ME. Using the Drosophila ATP6¹ model of ME, we have studied dietary therapy throughout disease progression and found that it is highly effective against the seizures of ME, especially a high fat/ketogenic diet, and that the benefits are dependent upon a functional K_ATP channel complex. Further experiments with K_ATP show that it is seizure-protective in this model, and that pharmacological promotion of its open state also ameliorates seizures. These studies represent important steps forward in the development of novel therapies for a class of diseases that is notoriously difficult to treat, and lay the foundation for mechanistic studies of currently existing therapies in the context of metabolic disease.

A Novel Diphenylthiosemicarbazide Is a Potential Insulin Secretagogue for Anti-Diabetic Agen

Insulin secretagogues are used for treatment of type 2 diabetes. We attempted to discover novel small molecules to stimulate insulin secretion by using in silico similarity search using sulfonylureas as query, followed by measurement of insulin secretion. Among 38 compounds selected by in silico similarity search, we found three diphenylsemicarbazides and one quinolone that stimulate insulin secretion. We focused on compound 8 (C8), which had the strongest insulin-secreting effect. Based on the structure-activity relationship of C8-derivatives, we identified diphenylthiosemicarbazide (DSC) 108 as the most potent secretagogue. DSC108 increased the intracellular Ca²⁺ level in MIN6-K8 cells. Competitive inhibition experiment and electrophysiological analysis revealed sulfonylurea receptor 1 (SUR1) to be the target of DSC108 and that this diphenylthiosemicarbazide directly inhibits ATP-sensitive K⁺ (K_ATP) channels. Pharmacokinetic analysis showed that DSC108 has a short half-life in vivo. Oral administration of DSC108 significantly suppressed the rises in blood glucose levels after glucose load in wild-type mice and improved glucose tolerance in the Goto-Kakizaki (GK) rat, a model of type 2 diabetes with impaired insulin secretion. Our data indicate that DSC108 is a novel insulin secretagogue, and is a lead compound for development of a new anti-diabetic agent.

Cerebrovascular Safety of Sulfonylureas: The Role of KATP Channels in Neuroprotection and the Risk of Stroke in Patients With Type 2 Diabetes

Sulfonylureas are ATP-sensitive potassium (KATP) channel blockers commonly used in the treatment of type 2 diabetes mellitus (T2DM). Activation of KATP channels plays a neuroprotective role in ischemia; thus, whether sulfonylureas affect the outcomes of stroke in patients with T2DM needs to be further studied. In our study, streptozotocin (STZ)-induced diabetic mice subjected to transient middle cerebral artery occlusion (MCAO) showed larger areas of brain damage and poorer behavioral outcomes. Blocking the KATP channel by tolbutamide increased neuronal injury induced by oxygen-glucose deprivation (OGD) in vitro and permanent MCAO (pMCAO) in vivo. Activating the KATP channel by diazoxide reduced the effects of both the OGD and pMCAO. Western blot analysis in STZ mouse brains indicated an early increase in protein levels of N-methyl-d-aspartate receptor 2B and postsynaptic density protein-95, followed by a decrease in phosphorylation of glycogen synthase kinase 3β. Our systematic meta-analysis indicated that patients with T2DM treated with sulfonylureas had a higher odds ratio for stroke morbidity than those who received comparator drugs. Taken together, these results suggest that sulfonylurea treatment in patients with T2DM may inhibit the neuroprotective effects of KATP channels and increase the risk of stroke.

Review: Insulin secretion: function and therapy of pancreatic beta-cells in diabetes

nsulin is secreted from the beta-cells of the pancreatic islets in response to an elevation of blood glucose concentration. This review describes a current view of the metabolic control of insulin secretion and the molecular mechanisms involved, including the role played by the beta-cell to ensure correct release of insulin as a result of electrical signals. It then considers what goes wrong in type 2 diabetes, a disease resulting from insufficient insulin secretion. It focuses on the influence of genetics exploring the theory of a genetic predisposition to type 2 diabetes, as well as the roles played by age and obesity. Finally, the mode of action of the hypoglycaemic sulphonylureas is discussed and the potential implications for the beta-cell associated with a sulphonylurea-based therapy.

Effects of Bisphenol A on ion channels: Experimental evidence and molecular mechanisms

Bisphenol A (BPA) is an endocrine-disrupting chemical (EDC) produced in huge quantities in the manufacture of polycarbonate plastics and epoxy resins. It is present in most humans in developed countries, acting as a xenoestrogen and it is considered an environmental risk factor associated to several diseases. Among the whole array of identified mechanisms by which BPA can interfere with physiological processes in living organisms, changes on ion channel activity is one of the most poorly understood. There is still little evidence about BPA regulation of ion channel expression and function. However, this information is key to understand how BPA disrupts excitable and non-excitable cells, including neurons, endocrine cells and muscle cells. This report is the result of a comprehensive literature review on the effects of BPA on ion channels. We conclude that there is evidence to say that these important molecules may be key end-points for EDCs acting as xenoestrogens. However, more research on channel-mediated BPA effects is needed. Particularly, mechanistic studies to unravel the pathophysiological actions of BPA on ion channels at environmentally relevant doses.

Mechanism of Proarrhythmic Effects of Potassium Channel Blockers

Any disturbance of electrical impulse formation in the heart and of impulse conduction or action potential (AP) repolarization can lead to rhythm disorders. Potassium (K(+)) channels play a prominent role in the AP repolarization process. In this review we describe the causes and mechanisms of proarrhythmic effects that arise as a response to blockers of cardiac K(+) channels. The largest and chemically most diverse groups of compound targets are Kv11.1 (hERG) and Kv7.1 (KvLQT1) channels. Finally, the proarrhythmic propensity of atrial-selective K(+) blockers inhibiting Kv1.5, Kir3.1/3.4, SK, and K2P channels is discussed.

DNA methylation regulates hypothalamic gene expression linking parental diet during pregnancy to the offspring's risk of obesity in Psammomys obesus

BACKGROUND/OBJECTIVE

The rising incidence of obesity is a major public health issue worldwide. Recent human and animal studies suggest that parental diet can influence fetal development and is implicated with risk of obesity and type 2 diabetes in offspring. The hypothalamus is central to body energy homoeostasis and appetite by controlling endocrine signals. We hypothesise that offspring susceptibility to obesity is programmed in the hypothalamus in utero and mediated by changes to DNA methylation, which persist to adulthood. We investigated hypothalamic genome-wide DNA methylation in Psammomys obesus diet during pregnancy to the offspring's risk of obesity.

METHODS

Using methyl-CpG binding domain capture and deep sequencing (MBD-seq), we examined the hypothalamus of offspring exposed to a low-fat diet and standard chow diet during the gestation and lactation period.

RESULTS

Offspring exposed to a low-fat parental diet were more obese and had increased circulating insulin and glucose levels. Methylome profiling identified 1447 genomic regions of differential methylation between offspring of parents fed a low-fat diet compared with parents on standard chow diet. Pathway analysis shows novel DNA methylation changes of hypothalamic genes associated with neurological function, nutrient sensing, appetite and energy balance. Differential DNA methylation corresponded to changes in hypothalamic gene expression of Tas1r1 and Abcc8 in the offspring exposed to low-fat parental diet.

CONCLUSION

Subject to parental low-fat diet, we observe DNA methylation changes of genes associated with obesity in offspring.

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.

The antidiabetic agent glibenclamide protects airway hyperresponsiveness and inflammation in mice

Glibenclamide has a newly discovered role in inflammation regulation besides its antidiabetic effect. As an inhibitor of ATP-sensitive potassium (KATP) channel, glibenclamide antagonizes the relaxation of the tracheal smooth muscle. This indicates that glibenclamide might attenuate airway inflammation while aggravate airway hyperresponsiveness (AHR) in asthmatics. Clinically, many diabetics with asthma are prescribed with glibenclamide to control blood glucose. However, whether glibenclamide could exert any effects on asthmatic inflammation remains unknown. Using an ovalbumin (OVA)-induced mouse model of asthma, we evaluated the effects of glibenclamide on the AHR and inflammation. Interestingly, glibenclamide reduced all the cardinal features of asthma in OVA-challenged mice, including AHR, airway inflammation, and T-helper type 2 (Th2) cytokines. Glibenclamide also downregulated OVA-induced expressions of vascular cell adhesion molecule 1 (VCAM-1) and phosphorylated signal transducer and activator of transcription 6 (p-STAT6) in the lung. In addition, increased sulfonylurea receptor 1 (SUR1) expression in the lung was observed after the OVA challenge. These findings suggest that the classic sulfonylurea glibenclamide plays an important protective role in the development of asthma, which not only provides the evidence for the safety of prescribed glibenclamide in diabetics combined with asthma but also indicates a possible new therapeutic for asthma via targeting glibenclamide-related pathways.

Complex Membrane Channel Blockade: A Unifying Hypothesis for the Prodromal and Acute Neuropsychiatric Sequelae Resulting from Exposure to the Antimalarial Drug Mefloquine

The alkaloid toxin quinine and its derivative compounds have been used for many centuries as effective medications for the prevention and treatment of malaria. More recently, synthetic derivatives, such as the quinoline derivative mefloquine (bis(trifluoromethyl)-(2-piperidyl)-4-quinolinemethanol), have been widely used to combat disease caused by chloroquine-resistant strains of the malaria parasite, Plasmodium falciparum. However, the parent compound quinine, as well as its more recent counterparts, suffers from an incidence of adverse neuropsychiatric side effects ranging from mild mood disturbances and anxiety to hallucinations, seizures, and psychosis. This review considers how the pharmacology, cellular neurobiology, and membrane channel kinetics of mefloquine could lead to the significant and sometimes life-threatening neurotoxicity associated with mefloquine exposure. A key role for mefloquine blockade of ATP-sensitive potassium channels and connexins in the substantia nigra is considered as a unifying hypothesis for the pathogenesis of severe neuropsychiatric events after mefloquine exposure in humans.

Glucose Induces Slow-Wave Sleep by Exciting the Sleep-Promoting Neurons in the Ventrolateral Preoptic Nucleus: A New Link between Sleep and Metabolism

Sleep-active neurons located in the ventrolateral preoptic nucleus (VLPO) play a crucial role in the induction and maintenance of slow-wave sleep (SWS). However, the cellular and molecular mechanisms responsible for their activation at sleep onset remain poorly understood. Here, we test the hypothesis that a rise in extracellular glucose concentration in the VLPO can promote sleep by increasing the activity of sleep-promoting VLPO neurons. We find that infusion of a glucose concentration into the VLPO of mice promotes SWS and increases the density of c-Fos-labeled neurons selectively in the VLPO. Moreover, we show in patch-clamp recordings from brain slices that VLPO neurons exhibiting properties of sleep-promoting neurons are selectively excited by glucose within physiological range. This glucose-induced excitation implies the catabolism of glucose, leading to a closure of ATP-sensitive potassium (KATP) channels. The extracellular glucose concentration monitors the gating of KATP channels of sleep-promoting neurons, highlighting that these neurons can adapt their excitability according to the extracellular energy status. Together, these results provide evidence that glucose may participate in the mechanisms of SWS promotion and/or consolidation.

SIGNIFICANCE STATEMENT

Although the brain circuitry underlying vigilance states is well described, the molecular mechanisms responsible for sleep onset remain largely unknown. Combining in vitro and in vivo experiments, we demonstrate that glucose likely contributes to sleep onset facilitation by increasing the excitability of sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO). We find here that these neurons integrate energetic signals such as ambient glucose directly to regulate vigilance states accordingly. Glucose-induced excitation of sleep-promoting VLPO neurons should therefore be involved in the drowsiness that one feels after a high-sugar meal. This novel mechanism regulating the activity of VLPO neurons reinforces the fundamental and intimate link between sleep and metabolism.

Molecular analysis of ATP-sensitive K(+) channel subunits expressed in mouse portal vein

BACKGROUND

Several combinations of inwardly rectifying K(+) channel 6.x family pore-forming (KIR6.x) subunits associated with sulphonylurea receptor (SUR.x) subunits have been detected among ATP-sensitive K(+) (KATP) channels. It remains to be established which of these is expressed in native vascular smooth muscle.

METHODS

Pharmacological and electrophysiological properties of KATP channels in mouse portal vein were investigated using tension measurements and patch-clamp techniques. Molecular biological analyses were also performed to investigate the structural properties of these channels.

RESULTS

Spontaneous contractions in mouse portal vein were reversibly reduced by pinacidil and MCC-134, and the pinacidil-induced relaxation was antagonized by glibenclamide and U-37883A. In cell-attached mode, pinacidil activated glibenclamide-sensitive K(+) channels with a conductance (35 pS) similar to that of KIR6.1. RT-PCR analysis revealed the expression of KIR6.1, KIR6.2 and SUR2B transcripts. Using real-time PCR methods, the quantitative expression of KIR6.1 was much greater than that of KIR6.2. Immunohistochemical studies indicated the presence of KIR6.1 and SUR2B proteins in the smooth muscle layers of mouse portal vein and in single smooth muscle cells dispersed from mouse portal vein.

CONCLUSIONS

The results indicate that native KATP channels in mouse portal vein are likely to be composed of a heterocomplex of KIR6.1 and SUR2B subunits.

Molecular determinants of ATP-sensitive potassium channel MgATPase activity: diabetes risk variants and diazoxide sensitivity

Molecular interactions between two residues in the sulfonylurea receptor (SUR) subunit of the ATP-sensitive potassium channel influence MgATPase activity. This interaction may provide a mechanism for the increased diabetes risk associated with a common channel variant and determines sensitivity to diazoxide.

ATP-sensitive K⁺ (K_ATP) channels play an important role in insulin secretion. K_ATP channels possess intrinsic MgATPase activity that is important in regulating channel activity in response to metabolic changes, although the precise structural determinants are not clearly understood. Furthermore, the sulfonylurea receptor 1 (SUR1) S1369A diabetes risk variant increases MgATPase activity, but the molecular mechanisms remain to be determined. Therefore, we hypothesized that residue–residue interactions between 1369 and 1372, predicted from in silico modelling, influence MgATPase activity, as well as sensitivity to the clinically used drug diazoxide that is known to increase MgATPase activity. We employed a point mutagenic approach with patch-clamp and direct biochemical assays to determine interaction between residues 1369 and 1372. Mutations in residues 1369 and 1372 predicted to decrease the residue interaction elicited a significant increase in MgATPase activity, whereas mutations predicted to possess similar residue interactions to wild-type (WT) channels elicited no alterations in MgATPase activity. In contrast, mutations that were predicted to increase residue interactions resulted in significant decreases in MgATPase activity. We also determined that a single S1369K substitution in SUR1 caused MgATPase activity and diazoxide pharmacological profiles to resemble those of channels containing the SUR2A subunit isoform. Our results provide evidence, at the single residue level, for a molecular mechanism that may underlie the association of the S1369A variant with type 2 diabetes. We also show a single amino acid difference can account for the markedly different diazoxide sensitivities between channels containing either the SUR1 or SUR2A subunit isoforms.

Sulfonylurea Receptor 1 in Humans with Post-Traumatic Brain Contusions

Post-traumatic brain contusions (PTBCs) are traditionally considered primary injuries and can increase in size, generate perilesional edema, cause mass effect, induce neurological deterioration, and cause death. Most patients experience a progressive increase in pericontusional edema, and nearly half, an increase in the hemorrhagic component itself. The underlying molecular pathophysiology of contusion-induced brain edema and hemorrhagic progression remains poorly understood. The aim of this study was to investigate sulfonylurea 1/transient receptor potential melastatin 4 (SUR1-TRPM4) ion channel SUR1 expression in various cell types (neurons, astrocytes, endothelial cells, microglia, macrophages, and neutrophils) of human brain contusions and whether SUR1 up-regulation was related to time postinjury. Double immunolabeling of SUR1 and cell-type- specific proteins was performed in 26 specimens from traumatic brain injury patients whose lesions were surgically evacuated. Three samples from limited brain resections performed for accessing extra-axial skull-base tumors or intraventricular lesions were controls. We found SUR1 was significantly overexpresed in all cell types and was especially prominent in neurons and endothelial cells (ECs). The temporal pattern depended on cell type: 1) In neurons, SUR1 increased within 48 h of injury and stabilized thereafter; 2) in ECs, there was no trend; 3) in glial cells and microglia/macrophages, a moderate increase was observed over time; and 4) in neutrophils, it decreased with time. Our results suggest that up-regulation of SUR1 in humans point to this channel as one of the important molecular players in the pathophysiology of PTBCs. Our findings reveal opportunities to act therapeutically on the mechanisms of growth of traumatic contusions and therefore reduce the number of patients with neurological deterioration and poor neurological outcomes.

Counterregulatory responses to hypoglycemia differ between glimepiride and glyburide in non diabetic individuals

OBJECTIVE

Reported rates of hypoglycemia in patients with type 2 diabetes mellitus are lower with glimepiride as compared to glyburide. The aim of this study was to determine whether physiologic differences in counterregulatory neuroendocrine and metabolic mechanisms during hypoglycemia provide a basis for the observed clinical differences between glimepiride and glyburide.

RESEARCH DESIGN AND METHODS

Non-diabetic volunteers (age 38±2years, BMI 26±1kg/m(2)) were studied in a single-blind fashion during separate 2day randomized protocols consisting of 2h hyperinsulinemic (9pmol/kg/min) euglycemic (4.9±0.1mmol) and hypoglycemic (2.9±0.1mmol/L) clamps. Individuals received biologically equivalent doses of glimepiride (4mg) or glyburide (10mg) 1h prior to each glucose clamp (n=11) as well as a control group of placebo studies. Glucose kinetics were calculated using D-Glucose-6-6d2.

RESULTS

Insulin and C-peptide levels were increased (p<0.05) during euglycemia in both sulfonylurea groups as compared to placebo. However, despite equivalent hypoglycemia, insulin and C-peptide levels were higher (p<0.05) only after glyburide. Glucagon responses and endogenous glucose production (EGP) were decreased (p<0.05) during hypoglycemia following glyburide administration as compared to glimepiride. Glyburide reduced (p<0.05) norepinephrine responses during euglycemic clamps. In addition combined epinephrine and norepinephrine responses during hypoglycemia were reduced (p<0.05) following glyburide as compared to placebo. Leptin levels fell by a greater amount (p<0.05) during hypoglycemia with both sulfonylureas as compared to placebo.

CONCLUSIONS

In summary, glimepiride and glyburide can both similarly increase insulin and C-peptide levels during hyperinsulinemic euglycemia. However, during moderate hyperinsulinemic hypoglycemia (2.9mmol/L) glyburide resulted in increased C-peptide and insulin, but blunted glucagon, sympathetic nervous system and EGP responses. We conclude that glyburide can acutely reduce key neuroendocrine and metabolic counterregulatory defenses during hypoglycemia in healthy individuals.

Role of Epac2A/Rap1 signaling in interplay between incretin and sulfonylurea in insulin secretion

Incretin-related drugs and sulfonylureas are currently used worldwide for the treatment of type 2 diabetes. We recently found that Epac2A, a cAMP binding protein having guanine nucleotide exchange activity toward Rap, is a target of both incretin and sulfonylurea. This suggests the possibility of interplay between incretin and sulfonylurea through Epac2A/Rap1 signaling in insulin secretion. In this study, we examined the combinatorial effects of incretin and various sulfonylureas on insulin secretion and activation of Epac2A/Rap1 signaling. A strong augmentation of insulin secretion by combination of GLP-1 and glibenclamide or glimepiride, which was found in Epac2A(+/+) mice, was markedly reduced in Epac2A(-/-) mice. In contrast, the combinatorial effect of GLP-1 and gliclazide was rather mild, and the effect was not altered by Epac2A ablation. Activation of Rap1 was enhanced by the combination of an Epac-selective cAMP analog with glibenclamide or glimepiride but not gliclazide. In diet-induced obese mice, ablation of Epac2A reduced the insulin secretory response to coadministration of the GLP-1 receptor agonist liraglutide and glimepiride. These findings clarify the critical role of Epac2A/Rap1 signaling in the augmenting effect of incretin and sulfonylurea on insulin secretion and provide the basis for the effects of combination therapies of incretin-related drugs and sulfonylureas.