ATP-sensitive potassium channel traffic regulation by adenosine and protein kinase C

Rab35 GTPase positively regulates endocytic recycling of cardiac KATP channels

ATP-sensitive K⁺ (K_ATP) channel couples membrane excitability to intracellular energy metabolism. Maintaining K_ATP channel surface expression is key to normal insulin secretion, blood pressure and cardioprotection. However, the molecular mechanisms regulating K_ATP channel internalization and endocytic recycling, which directly affect the surface expression of K_ATP channels, are poorly understood. Here we used the cardiac K_ATP channel subtype, Kir6.2/SUR2A, and characterized Rab35 GTPase as a key regulator of K_ATP channel endocytic recycling. Electrophysiological recordings and surface biotinylation assays showed decreased K_ATP channel surface density with co-expression of a dominant negative Rab35 mutant (Rab35-DN), but not other recycling-related Rab GTPases, including Rab4, Rab11a and Rab11b. Immunofluorescence images revealed strong colocalization of Rab35-DN with recycling Kir6.2. Rab35-DN minimized the recycling rate of K_ATP channels. Rab35 also regulated K_ATP channel current amplitude in isolated adult cardiomyocytes by affecting its surface expression but not channel properties, which validated its physiologic relevance and the potential of pharmacologic target for treating the diseases with K_ATP channel trafficking defects.

Subcellular trafficking and endocytic recycling of KATP channels

Sarcolemmal/plasmalemmal ATP-sensitive K+ (KATP) channels have key roles in many cell types and tissues. Hundreds of studies have described how the KATP channel activity and ATP sensitivity can be regulated by changes in the cellular metabolic state, by receptor signaling pathways and by pharmacological interventions. These alterations in channel activity directly translate to alterations in cell or tissue function, that can range from modulating secretory responses, such as insulin release from pancreatic β-cells or neurotransmitters from neurons, to modulating contractile behavior of smooth muscle or cardiac cells to elicit alterations in blood flow or cardiac contractility. It is increasingly becoming apparent, however, that KATP channels are regulated beyond changes in their activity. Recent studies have highlighted that KATP channel surface expression is a tightly regulated process with similar implications in health and disease. The surface expression of KATP channels is finely balanced by several trafficking steps including synthesis, assembly, anterograde trafficking, membrane anchoring, endocytosis, endocytic recycling, and degradation. This review aims to summarize the physiological and pathophysiological implications of KATP channel trafficking and mechanisms that regulate KATP channel trafficking. A better understanding of this topic has potential to identify new approaches to develop therapeutically useful drugs to treat KATP channel-related diseases.

TASK-1 regulates mitochondrial function under hypoxia

TASK-1, TWIK-related acid-sensitive potassium channel 1, is a member of the two-pore- domain potassium channel family. It is constitutively active at resting potentials and strongly expressed in the heart. However, little is known about the role of TASK-1 channels in hypoxia. A cellular model of hypoxia and reoxygenation from rat heart-derived H9c2 cells or TASK-1 deficient HEK293T cells was employed to explore the role of TASK-1 channels in cytoprotection against hypoxia. The cell viability assay revealed that TASK-1 expression increased the number of viable cells subjected to 2 h of hypoxia followed by 2 h of reoxygenation (H/R). To dissect the protective role of TASK-1 on mitochondrial function, mitochondrial membrane potential (MMP) was assessed by tetramethylrhodamine fluorescence. It was demonstrated that MMP was significantly decreased by H/R, but it was maintained by TASK-1 expression or pretreatment with cyclosporin A, an inhibitor of mitochondrial permeability transition pore (mPTP). The effect of cyclosporin A on MMP was not further altered by TASK-1 expression. Moreover, TASK-1 expression significantly blocked cytochrome c release induced by H/R. While a small fraction of endogenous TASK-1 was found to colocalize with the mitochondrial marker MitoTracker in H9c2 cells, H/R did not alter the extent of colocalization of TASK-1 with MitoTracker. The total TASK-1 protein level was not significantly affected by H/R. In summary, we provided the evidence that TASK-1 channels confer cytoprotection against hypoxia-reoxygenation injury, possibly by their capacity of maintaining the mitochondrial membrane potential via inhibiting MPTP opening.

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.

Endocytosis: A Turnover Mechanism Controlling Ion Channel Function

Ion channels (IChs) are transmembrane proteins that selectively drive ions across membranes. The function of IChs partially relies on their abundance and proper location in the cell, fine-tuned by the delicate balance between secretory, endocytic, and degradative pathways. The disruption of this balance is associated with several diseases, such as Liddle's and long QT syndromes. Because of the vital role of these proteins in human health and disease, knowledge of ICh turnover is essential. Clathrin-dependent and -independent mechanisms have been the primary mechanisms identified with ICh endocytosis and degradation. Several molecular determinants recognized by the cellular internalization machinery have been discovered. Moreover, specific conditions can trigger the endocytosis of many IChs, such as the activation of certain receptors, hypokalemia, and some drugs. Ligand-dependent receptor activation primarily results in the posttranslational modification of IChs and the recruitment of important mediators, such as β-arrestins and ubiquitin ligases. However, endocytosis is not a final fate. Once internalized into endosomes, IChs are either sorted to lysosomes for degradation or recycled back to the plasma membrane. Rab proteins are crucial participants during these turnover steps. In this review, we describe the major ICh endocytic pathways, the signaling inputs triggering ICh internalization, and the key mediators of this essential cellular process.

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.

The trafficking protein, EHD2, positively regulates cardiac sarcolemmal KATP channel surface expression: role in cardioprotection

ATP-sensitive K⁺ (K_ATP) channels uniquely link cellular energy metabolism to membrane excitability and are expressed in diverse cell types that range from the endocrine pancreas to neurons and smooth, skeletal, and cardiac muscle. A decrease in the surface expression of K_ATP channels has been linked to various disorders, including dysregulated insulin secretion, abnormal blood pressure, and impaired resistance to cardiac injury. In contrast, up-regulation of K_ATP channel surface expression may be protective, for example, by mediating the beneficial effect of ischemic preconditioning. Molecular mechanisms that regulate K_ATP channel trafficking are poorly understood. Here, we used cellular assays with immunofluorescence, surface biotinylation, and patch clamping to demonstrate that Eps15 homology domain-containing protein 2 (EHD2) is a novel positive regulator of K_ATP channel trafficking to increase surface K_ATP channel density. EHD2 had no effect on cardiac Na⁺ channels (Nav1.5). The effect is specific to EHD2 as other members of the EHD family-EHD1, EHD3, and EHD4-had no effect on K_ATP channel surface expression. EHD2 did not directly affect K_ATP channel properties as unitary conductance and ATP sensitivity were unchanged. Instead, we observed that the mechanism by which EHD2 increases surface expression is by stabilizing K_ATP channel-containing caveolar structures, which results in a reduced rate of endocytosis. EHD2 also regulated K_ATP channel trafficking in isolated cardiomyocytes, which validated the physiologic relevance of these observations. Pathophysiologically, EHD2 may be cardioprotective as a dominant-negative EHD2 mutant sensitized cardiomyocytes to ischemic damage. Our findings highlight EHD2 as a potential pharmacologic target in the treatment of diseases with K_ATP channel trafficking defects.-Yang, H. Q., Jana, K., Rindler, M. J., Coetzee, W. A. The trafficking protein, EHD2, positively regulates cardiac sarcolemmal K_ATP channel surface expression: role in cardioprotection.

Differential actions of AMP kinase on ATP-sensitive K+ currents in ventral tegmental area and substantia nigra zona compacta neurons

ATP-sensitive K⁺ (K-ATP) channels play significant roles in regulating the excitability of dopamine neurons in the substantia nigra zona compacta (SNC). We showed previously that K-ATP channel function is up-regulated by AMP-activated protein kinase (AMPK). This study extended these studies to the neurons adjacent to the SNC in the ventral tegmental area (VTA). Using patch pipettes to record whole-cell currents in slices of rat midbrain, we found that the AMPK activator A769662 increased the amplitude of currents evoked by the K-ATP channel opener diazoxide in presumed dopamine-containing VTA neurons. However, current evoked by diazoxide with A769662 was significantly smaller in VTA neurons compared to SNC neurons. Moreover, a significantly lower proportion of VTA neurons responded to diazoxide with outward current. However, A769662 was able to increase the incidence of diazoxide-responsive neurons in the VTA. In contrast, A769662 did not potentiate diazoxide-evoked currents in presumed non-dopamine VTA neurons. These results show that AMPK activation augments K-ATP currents in presumed dopamine neurons in the VTA and SNC, although diazoxide-evoked currents remain less robust in the VTA. We conclude that K-ATP channels may play important physiological roles in VTA and SNC dopamine neurons.

Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels

K_ATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P₂ and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing K_ATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

High glucose stimulates cell proliferation and Collagen IV production in rat mesangial cells through inhibiting AMPK-KATP signaling

PURPOSE

The present study investigated the putative mechanisms underlying effects of K_ATP channel on high glucose (HG)-induced mesangial cell proliferation and tissue inhibitors of metalloproteinases (TIMP)-2 and Collagen IV production.

METHODS

Rat mesangial cells were subjected to whole cell patch clamp to record the K_ATP channel currents under high glucose (HG, 30 mM) condition. Cell proliferation was measured using a CCK-8 assay. The production of TIMP-2 and Collagen IV and AMP-activated protein kinase (AMPK)-signaling pathway activity was assessed by ELISA and Western blotting, respectively. AMPK agonist (AICAR) was used to analyze the role of this kinase. The expression of K_ATP subunit (Kir6.1, Kir6.2, SUR1, SUR2A and SUR2B) was examined using quantitative real-time PCR (RT-PCR).

RESULTS

We found that HG was significant decreases in the expression of Kir6.1, SUB2A and SUB2B, three subunits of K_ATP, TIMP-2 production, K_ATP channel activity and AMPK activity, while it promoted the cell proliferation and Collagen IV production in rat mesangial cells. Pretreatment with K_ATP selective opener (diazoxide, DZX) significantly inhibited HG-induced mesangial cell proliferation, Collagen IV production and decrease in K_ATP channel activity in rat mesangial cells, which were reversed by pretreatment of 5-hydroxydecanoate, a selective inhibitor of K_ATP. Moreover, AICAR pretreatment inhibited HG-induced decrease in K_ATP channel activity.

CONCLUSIONS

Taken together, activating AMPK-K_ATP signaling may protect against HG-induced mesangial cell proliferation and Collagen IV production, and, thereby, provides new insights into the molecular mechanisms underlying early diabetic nephropathy (DN).

KATP channels in high glucose-induced rat mesangial cell proliferation and release of MMP-2 and fibronectin

ATP-sensitive potassium (KATP) channels are well characterized in cardiac, pancreatic and many other muscle cells. The purpose of this study was to determine if KATP channels play a role in diabetic nephropathy (DN). In the present study, functional expression of the KATP channel was examined in rat mesangial cells with or without high glucose (HG) stimulation. The mesangial cell proliferation and the release of matrix metalloproteinase (MMP)-2 and fibronectin in response to high glucose with a selective opener of KATP (diazoxide, DZX), or with a selective inhibitor of KATP (5-hydroxydecanoate, 5-HD) were also measured. The cell proliferation was observed using Cell Counting Kit-8 assay, and the mRNA expressions of KATP subunit, including Kir6.1, Kir6.2, sulfonylurea receptor 1 (SUR1), SUR2A and SUR2B, were assessed using quantitative real-time PCR. MMP-2 and fibronectin release was measured by ELISA. The present study clarified expression of SUR subunit of KATP in plasma. HG treatment could cause increased cell proliferation and release of MMP-2 and fibronectin in a dose-dependent manner. HG also significantly decreased the expression of Kir6.1, SUR2A and SUR2B. Pretreatment of DZX markedly decreased the expression of SUR1, SUR2A and SUR2B, but had no effect on Kir6.1 expression compared with HG alone, while these changes were inhibited by 5-HD pretreatment. Moreover, DZX also inhibited cell proliferation and release of MMP-2 and fibronectin in HG-induced rat mesangial cells, and that was corrected by 5-HD. These data suggest that HG stimulates mesangial cell proliferation and cellular matrix release via inhibiting KATP channel activity, leading us to propose that KATP channel dysfunction may be involved in the development of DN.

Plasticity of sarcolemmal KATP channel surface expression: relevance during ischemia and ischemic preconditioning

Myocardial ischemia remains the primary cause of morbidity and mortality in the United States. Ischemic preconditioning (IPC) is a powerful form of endogenous protection against myocardial infarction. We studied alterations in KATP channels surface density as a potential mechanism of the protection of IPC. Using cardiac-specific knockout of Kir6.2 subunits, we demonstrated an essential role for sarcolemmal KATP channels in the infarct-limiting effect of IPC in the mouse heart. With biochemical membrane fractionation, we demonstrated that sarcolemmal KATP channel subunits are distributed both to the sarcolemma and intracellular endosomal compartments. Global ischemia causes a loss of sarcolemmal KATP channel subunit distribution and internalization to endosomal compartments. Ischemia-induced internalization of KATP channels was prevented by CaMKII inhibition. KATP channel subcellular redistribution was also observed with immunohistochemistry. Ischemic preconditioning before the index ischemia reduced not only the infarct size but also prevented KATP channel internalization. Furthermore, not only did adenosine mimic IPC by preventing infarct size, but it also prevented ischemia-induced KATP channel internalization via a PKC-mediated pathway. We show that preventing endocytosis with dynasore reduced both KATP channel internalization and strongly mitigated infarct development. Our data demonstrate that plasticity of KATP channel surface expression must be considered as a potentially important mechanism of the protective effects of IPC and adenosine.

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.

Isosteviol Sensitizes sarcKATP Channels towards Pinacidil and Potentiates Mitochondrial Uncoupling of Diazoxide in Guinea Pig Ventricular Myocytes

K_ATP channel is an important mediator or factor in physiological and pathological metabolic pathway. Activation of K_ATP channel has been identified to be a critical step in the cardioprotective mechanism against IR injury. On the other hand, desensitization of the channel to its opener or the metabolic ligand ATP in pathological conditions, like cardiac hypertrophy, would decrease the adaption of myocardium to metabolic stress and is a disadvantage for drug therapy. Isosteviol, obtained by acid hydrolysis of stevioside, has been demonstrated to play a cardioprotective role against diseases of cardiovascular system, like anti-IR injury, antihypertension, antihyperglycemia, and so forth. The present study investigated the effect of isosteviol (STV) on sarcK_ATP channel current induced by pinacidil and mitochondrial flavoprotein oxidation induced by diazoxide. Our results showed that preincubating cells with STV not only increased the current amplitude and activating rate of sarcK_ATP channels induced by pinacidil but also potentiated diazoxide-elicited oxidation of flavoprotein in mitochondria.

Angiotensin II stimulates internalization and degradation of arterial myocyte plasma membrane BK channels to induce vasoconstriction

Arterial smooth muscle cells (myocytes) express large-conductance Ca(2+)-activated K(+) (BK) channel α and auxiliary β1 subunits that modulate arterial contractility. In arterial myocytes, β1 subunits are stored within highly mobile rab11A-positive recycling endosomes. In contrast, BKα subunits are primarily plasma membrane-localized. Trafficking pathways for BKα and whether physiological stimuli that regulate arterial contractility alter BKα localization in arterial myocytes are unclear. Here, using biotinylation, immunofluorescence resonance energy transfer (immunoFRET) microscopy, and RNAi-mediated knockdown, we demonstrate that rab4A-positive early endosomes traffic BKα to the plasma membrane in myocytes of resistance-size cerebral arteries. Angiotensin II (ANG II), a vasoconstrictor, reduced both surface and total BKα, an effect blocked by bisindolylmaleimide-II, concanavalin A, and dynasore, protein kinase C (PKC), internalization, and endocytosis inhibitors, respectively. In contrast, ANG II did not reduce BKα mRNA, and sodium nitroprusside, a nitric oxide donor, did not alter surface BKα protein over the same time course. MG132 and bafilomycin A, proteasomal and lysosomal inhibitors, respectively, also inhibited the ANG II-induced reduction in surface and total BKα, resulting in intracellular BKα accumulation. ANG II-mediated BK channel degradation reduced BK currents in isolated myocytes and functional responses to iberiotoxin, a BK channel blocker, and NS1619, a BK activator, in pressurized (60 mmHg) cerebral arteries. These data indicate that rab4A-positive early endosomes traffic BKα to the plasma membrane in arterial myocytes. We also show that ANG II stimulates PKC-dependent BKα internalization and degradation. These data describe a unique mechanism by which ANG II inhibits arterial myocyte BK currents, by reducing surface channel number, to induce vasoconstriction.

Acute intermittent optogenetic stimulation of nucleus tractus solitarius neurons induces sympathetic long-term facilitation

Acute intermittent hypoxia (AIH) induces sympathetic and phrenic long-term facilitation (LTF), defined as a sustained increase in nerve discharge. We investigated the effects of AIH and acute intermittent optogenetic (AIO) stimulation of neurons labeled with AAV-CaMKIIa, hChR2(H134R), and mCherry in the nucleus of the solitary tract (NTS) of anesthetized, vagotomized, and mechanically ventilated rats. We measured renal sympathetic nerve activity (RSNA), phrenic nerve activity (PNA), power spectral density, and coherence, and we made cross-correlation measurements to determine how AIO stimulation and AIH affected synchronization between PNA and RSNA. Sixty minutes after AIH produced by ventilation with 10% oxygen in balanced nitrogen, RSNA and PNA amplitude increased by 80% and by 130%, respectively (P < 0.01). Sixty minutes after AIO stimulation, RSNA and PNA amplitude increased by 60% and 100%, respectively, (P < 0.01). These results suggest that acute intermittent stimulation of NTS neurons can induce renal sympathetic and phrenic LTF in the absence of hypoxia or chemoreceptor afferent activation. We also found that while acute intermittent optogenetic and hypoxic stimulations increased respiration-related RSNA modulation (P < 0.01), they did not increase synchronization between central respiratory drive and RSNA. We conclude that mechanisms that induce LTF originate within the caudal NTS and extend to other interconnecting neuronal elements of the central nervous cardiorespiratory network.

The role of protein-protein interactions in the intracellular traffic of the potassium channels TASK-1 and TASK-3

The intracellular transport of membrane proteins is controlled by trafficking signals: Short peptide motifs that mediate the contact with COPI, COPII or various clathrin-associated coat proteins. In addition, many membrane proteins interact with accessory proteins that are involved in the sorting of these proteins to different intracellular compartments. In the K2P channels, TASK-1 and TASK-3, the influence of protein-protein interactions on sorting decisions has been studied in some detail. Both TASK paralogues interact with the adaptor protein 14-3-3; TASK-1 interacts, in addition, with the adaptor protein p11 (S100A10) and the endosomal SNARE protein syntaxin-8. The role of these interacting proteins in controlling the intracellular traffic of the channels and the underlying molecular mechanisms are summarised in this review. In the case of 14-3-3, the interacting protein masks a retention signal in the C-terminus of the channel; in the case of p11, the interacting protein carries a retention signal that localises the channel to the endoplasmic reticulum; and in the case of syntaxin-8, the interacting protein carries an endocytosis signal that complements an endocytosis signal of the channel. These examples illustrate some of the mechanisms by which interacting proteins may determine the itinerary of a membrane protein within a cell and suggest that the intracellular traffic of membrane proteins may be adapted to the specific functions of that protein by multiple protein-protein interactions.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

The role of ATP-sensitive potassium channels in cellular function and protection in the cardiovascular system

ATP-sensitive potassium channels (K(ATP)) are widely distributed and present in a number of tissues including muscle, pancreatic beta cells and the brain. Their activity is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels. Thus, they link cellular metabolism with membrane excitability. Recent studies using genetically modified mice and genomic studies in patients have implicated K(ATP) channels in a number of physiological and pathological processes. In this review, we focus on their role in cellular function and protection particularly in the cardiovascular system.

Optogenetic stimulation effectively enhances intrinsically generated network synchrony

Synchronized bursting is found in many brain areas and has also been implicated in the pathophysiology of neuropsychiatric disorders such as epilepsy, Parkinson’s disease, and schizophrenia. Despite extensive studies of network burst synchronization, it is insufficiently understood how this type of network wide synchronization can be strengthened, reduced, or even abolished. We combined electrical recording using multi-electrode array with optical stimulation of cultured channelrhodopsin-2 transducted hippocampal neurons to study and manipulate network burst synchronization. We found low frequency photo-stimulation protocols that are sufficient to induce potentiation of network bursting, modifying bursting dynamics, and increasing interneuronal synchronization. Surprisingly, slowly fading-in light stimulation, which substantially delayed and reduced light-driven spiking, was at least as effective in reorganizing network dynamics as much stronger pulsed light stimulation. Our study shows that mild stimulation protocols that do not enforce particular activity patterns onto the network can be highly effective inducers of network-level plasticity.

Convergence of nicotine-induced and auditory-evoked neural activity activates ERK in auditory cortex

Enhancement of sound-evoked responses in auditory cortex (ACx) following administration of systemic nicotine is known to depend on activation of extracellular-signaling regulated kinase (ERK), but the nature of this enhancement is not clear. Here, we show that systemic nicotine increases the density of cells immunolabeled for phosphorylated (activated) ERK (P-ERK) in mouse primary ACx (A1). Cortical injection of dihydro-β-erythroidine reduced nicotine-induced P-ERK immunolabel, suggesting a role for nicotinic acetylcholine receptors located in A1 and containing α4 and β2 subunits. P-ERK expressing cells were distributed mainly in layers 2/3 and more sparsely in lower layers, with many cells exhibiting immunolabel within pyramidal-shaped somata and proximal apical dendrites. About one-third of P-ERK positive cells also expressed calbindin. In the thalamus, P-ERK immunopositive cells were found in the nonlemniscal medial geniculate (MG) and adjacent nuclei, but were absent in the lemniscal MG. Pairing broad spectrum acoustic stimulation (white noise) with systemic nicotine increased P-ERK immunopositive cell density in ACx as well as the total amount of P-ERK protein, particularly the phosphorylated form of ERK2. However, narrow spectrum (tone) stimulation paired with nicotine increased P-ERK immunolabel preferentially at a site within A1 where the paired frequency was characteristic frequency (CF), relative to a second site with a spectrally distant CF (two octaves above or below the paired frequency). Together, these results suggest that ERK is activated optimally where nicotinic signaling and sound-evoked neural activity converge.

Regulation of cardiac ATP-sensitive potassium channel surface expression by calcium/calmodulin-dependent protein kinase II

Cardiac ATP-sensitive potassium (K(ATP)) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K(ATP) channels. We used real-time monitoring of K(ATP) channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K(ATP) channel subunits to track the dynamics of K(ATP) channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K(ATP) channels. This process required phosphorylation of threonine at 180 and 224 and an intact (330)YSKF(333) endocytosis motif of the K(ATP) channel Kir6.2 pore-forming subunit. A molecular model of the μ2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that μ2 docks by interaction with (330)YSKF(333) and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine- and lysine-rich loops on μ2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K(ATP) channel subunits. This mechanism couples the surface expression of cardiac K(ATP) channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.

Group I mGluR-mediated inhibition of Kir channels contributes to retinal Müller cell gliosis in a rat chronic ocular hypertension model

Müller cell gliosis, which is characterized by upregulated expression of glial fibrillary acidic protein (GFAP), is a universal response in many retinal pathological conditions. Whether down-regulation of inward rectifying K+ (Kir) channels, which commonly accompanies the enhanced GFAP expression, could contribute to Müller cell gliosis is poorly understood. We investigated changes of Kir currents, GFAP and Kir4.1 protein expression in Müller cells in a rat chronic ocular hypertension (COH) model, and explored the mechanisms underlying Müller cell gliosis. We show that Kir currents and Kir4.1 protein expression in Müller cells were reduced significantly, while GFAP expression was increased in COH rats, and these changes were eliminated by MPEP, a group I metabotropic glutamate receptors (mGluR I) subtype mGluR5 antagonist. In normal isolated Müller cells, the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) suppressed the Kir currents and the suppression was blocked by MPEP. The DHPG effect was mediated by the intracellular Ca2+ -dependent PLC/IP3-ryanodine/PKC signaling pathway, but the cAMP-PKA pathway was not involved. Moreover, intravitreal injection of DHPG in normal rats induced changes in Müller cells, similar to those observed in COH rats. The DHPG-induced increase of GFAP expression in Müller cells was obstructed by Ba2+, suggesting the involvement of Kir channels. We conclude that overactivation of mGluR5 by excessive extracellular glutamate in COH rats could contribute to Müller cell gliosis by suppressing Kir channels.

Mitochondria: A mirror into cellular dysfunction in heart disease

Cardiovascular (CV) disease is the single most significant cause of morbidity and mortality worldwide. The emerging global impact of CV disease means that the goals of early diagnosis and a wider range of treatment options are now increasingly pertinent. As such, there is a greater need to understand the molecular mechanisms involved and potential targets for intervention. Mitochondrial function is important for physiological maintenance of the cell, and when this function is altered, the cell can begin to suffer. Given the broad range and significant impacts of the cellular processes regulated by the mitochondria, it becomes important to understand the roles of the proteins associated with this organelle. Proteomic investigations of the mitochondria are hampered by the intrinsic properties of the organelle, including hydrophobic mitochondrial membranes; high proportion of basic proteins (pI greater than 8.0); and the relative dynamic range issues of the mitochondria. For these reasons, many proteomic studies investigate the mitochondria as a discrete subproteome. Once this has been achieved, the alterations that result in functional changes with CV disease can be observed. Those alterations that lead to changes in mitochondrial function, signaling and morphology, which have significant implications for the cardiomyocyte in the development of CV disease, are discussed.

Activation of protein kinase C alters the intracellular distribution and mobility of cardiac Na+ channels

Na(+) current derived from expression of the cardiac isoform SCN5A is reduced by receptor-mediated or direct activation of protein kinase C (PKC). Previous work has suggested a possible role for loss of Na(+) channels at the plasma membrane in this effect, but the results are controversial. In this study, we tested the hypothesis that PKC activation acutely modulates the intracellular distribution of SCN5A channels and that this effect can be visualized in living cells. In human embryonic kidney cells that stably expressed SCN5A with green fluorescent protein (GFP) fused to the channel COOH-terminus (SCN5A-GFP), Na(+) currents were suppressed by an exposure to PKC activation. Using confocal microscopy, colocalization of SCN5A-GFP channels with the plasma membrane under control and stimulated conditions was quantified. A separate population of SCN5A channels containing an extracellular epitope was immunolabeled to permit temporally stable labeling of the plasma membrane. Our results demonstrated that Na(+) channels were preferentially trafficked away from the plasma membrane by PKC activation, with a major contribution by Ca(2+)-sensitive or conventional PKC isoforms, whereas stimulation of protein kinase A (PKA) had the opposite effect. Removal of the conserved PKC site Ser(1503) or exposure to the NADPH oxidase inhibitor apocynin eliminated the PKC-mediated effect to alter channel trafficking, indicating that both channel phosphorylation and ROS were required. Experiments using fluorescence recovery after photobleaching demonstrated that both PKC and PKA also modified channel mobility in a manner consistent with the dynamics of channel distribution. These results demonstrate that the activation of protein kinases can acutely regulate the intracellular distribution and molecular mobility of cardiac Na(+) channels in living cells.

Measuring and evaluating the role of ATP-sensitive K+ channels in cardiac muscle

Since ion channels move electrical charge during their activity, they have traditionally been studied using electrophysiological approaches. This was sometimes combined with mathematical models, for example with the description of the ionic mechanisms underlying the initiation and propagation of action potentials in the squid giant axon by Hodgkin and Huxley. The methods for studying ion channels also have strong roots in protein chemistry (limited proteolysis, the use of antibodies, etc.). The advent of the molecular cloning and the identification of genes coding for specific ion channel subunits in the late 1980s introduced a multitude of new techniques with which to study ion channels and the field has been rapidly expanding ever since (e.g. antibody development against specific peptide sequences, mutagenesis, the use of gene targeting in animal models, determination of their protein structures) and new methods are still in development. This review focuses on techniques commonly employed to examine ion channel function in an electrophysiological laboratory. The focus is on the K(ATP) channel, but many of the techniques described are also used to study other ion channels.

Regulation of the ATP-sensitive potassium channel subunit, Kir6.2, by a Ca2+-dependent protein kinase C

The activity of ATP-sensitive potassium (K(ATP)) channels is governed by the concentration of intracellular ATP and ADP and is thus responsive to the metabolic status of the cell. Phosphorylation of K(ATP) channels by protein kinase A (PKA) or protein kinase C (PKC) results in the modulation of channel activity and is particularly important in regulating smooth muscle tone. At the molecular level the smooth muscle channel is composed of a sulfonylurea subunit (SUR2B) and a pore-forming subunit Kir6.1 and/or Kir6.2. Previously, Kir6.1/SUR2B channels have been shown to be inhibited by PKC, and Kir6.2/SUR2B channels have been shown to be activated or have no response to PKC. In this study we have examined the modulation of channel complexes formed of the inward rectifier subunit, Kir6.2, and the sulfonylurea subunit, SUR2B. Using a combination of biochemical and electrophysiological techniques we show that this complex can be inhibited by protein kinase C in a Ca(2+)-dependent manner and that this inhibition is likely to be as a result of internalization. We identify a residue in the distal C terminus of Kir6.2 (Ser-372) whose phosphorylation leads to down-regulation of the channel complex. This inhibitory effect is distinct from activation which is seen with low levels of channel activity.

A role of the sulfonylurea receptor 1 in endocytic trafficking of ATP-sensitive potassium channels

The ATP-sensitive potassium (K(ATP) ) channel consisting of sulfonylurea receptor 1 (SUR1) and inward-rectifier potassium channel 6.2 (Kir6.2) has a well-established role in insulin secretion. Mutations in either subunit can lead to disease due to aberrant channel gating, altered channel density at the cell surface or a combination of both. Endocytic trafficking of channels at the plasma membrane is one way to influence surface channel numbers. It has been previously reported that channel endocytosis is dependent on a tyrosine-based motif in Kir6.2, while SUR1 alone is unable to internalize. In this study, we followed endocytic trafficking of surface channels in real time by live-cell imaging of channel subunits tagged with an extracellular minimal α-bungarotoxin-binding peptide labeled with a fluorescent dye. We show that SUR1 undergoes endocytosis independent of Kir6.2. Moreover, mutations in the putative endocytosis motif of Kir6.2, Y330C, Y330A and F333I are unable to prevent channel endocytosis. These findings challenge the notion that Kir6.2 bears the sole endocytic signal for K(ATP) channels and support a role of SUR1 in this trafficking process.

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

Kv7.1 surface expression is regulated by epithelial cell polarization

The potassium channel KV7.1 is expressed in the heart where it contributes to the repolarization of the cardiac action potential. In addition, KV7.1 is expressed in epithelial tissues where it plays a role in salt and water transport. Mutations in the kcnq1 gene can lead to long QT syndrome and deafness, and several mutations have been described as trafficking mutations. To learn more about the basic mechanisms that regulate KV7.1 surface expression, we have investigated the trafficking of KV7.1 during the polarization process of the epithelial cell line Madin-Darby Canine Kidney (MDCK) using a modified version of the classical calcium switch. We discovered that KV7.1 exhibits a very dynamic localization pattern during the calcium switch. When MDCK cells are kept in low calcium medium, KV7.1 is mainly observed at the plasma membrane. During the first hours of the switch, KV7.1 is removed from the plasma membrane and an intracellular accumulation in the endoplasmic reticulum (ER) is observed. The channel is retained in the ER until the establishment of the lateral membranes at which point KV7.1 is released from the ER and moves to the plasma membrane. Our data furthermore suggest that while the removal of KV7.1 from the cell surface and its accumulation in the ER could involve activation of protein kinase C, the subsequent release of KV7.1 from the ER depends on phosphoinositide 3-kinase (PI3K) activation. In conclusion, our results demonstrate that KV7.1 surface expression is regulated by signaling mechanisms involved in epithelial cell polarization in particular signaling cascades involving protein kinase C and PI3K.

Molecular mechanisms in exercise-induced cardioprotection

Physical inactivity is increasingly recognized as modifiable behavioral risk factor for cardiovascular diseases. A partial list of proposed mechanisms for exercise-induced cardioprotection include induction of heat shock proteins, increase in cardiac antioxidant capacity, expression of endoplasmic reticulum stress proteins, anatomical and physiological changes in the coronary arteries, changes in nitric oxide production, adaptational changes in cardiac mitochondria, increased autophagy, and improved function of sarcolemmal and/or mitochondrial ATP-sensitive potassium channels. It is currently unclear which of these protective mechanisms are essential for exercise-induced cardioprotection. However, most investigations focus on sarcolemmal KATP channels, NO production, and mitochondrial changes although it is very likely that other mechanisms may also exist. This paper discusses current information about these aforementioned topics and does not consider potentially important adaptations within blood or the autonomic nervous system. A better understanding of the molecular basis of exercise-induced cardioprotection will help to develop better therapeutic strategies.

Syntaxin 1A regulates surface expression of beta-cell ATP-sensitive potassium channels

The pancreatic ATP-sensitive potassium (K(ATP)) channel consisting of four inwardly rectifying potassium channel 6.2 (Kir6.2) and four sulfonylurea receptor SUR1 subunits plays a key role in insulin secretion by linking glucose metabolism to membrane excitability. Syntaxin 1A (Syn-1A) is a plasma membrane protein important for membrane fusion during exocytosis of insulin granules. Here, we show that Syn-1A and K(ATP) channels endogenously expressed in the insulin-secreting cell INS-1 interact. Upregulation of Syn-1A by overexpression in INS-1 leads to a decrease, whereas downregulation of Syn-1A by small interfering RNA (siRNA) leads to an increase, in surface expression of K(ATP) channels. Using COSm6 cells as a heterologous expression system for mechanistic investigation, we found that Syn-1A interacts with SUR1 but not Kir6.2. Furthermore, Syn-1A decreases surface expression of K(ATP) channels via two mechanisms. One mechanism involves accelerated endocytosis of surface channels. The other involves decreased biogenesis and processing of channels in the early secretory pathway. This regulation is K(ATP) channel specific as Syn-1A has no effect on another inward rectifier potassium channel Kir3.1/3.4. Our results demonstrate that in addition to a previously documented role in modulating K(ATP) channel gating, Syn-1A also regulates K(ATP) channel expression in β-cells. We propose that physiological or pathological changes in Syn-1A expression may modulate insulin secretion by altering glucose-secretion coupling via changes in K(ATP) channel expression.

Endosomal KATP channels as a reservoir after myocardial ischemia: a role for SUR2 subunits

ATP-sensitive K(+) (K(ATP)) channels, composed of inward rectifier K(+) (Kir)6.x and sulfonylurea receptor (SUR)x subunits, are expressed on cellular plasma membranes. We demonstrate an essential role for SUR2 subunits in trafficking K(ATP) channels to an intracellular vesicular compartment. Transfection of Kir6.x/SUR2 subunits into a variety of cell lines (including h9c2 cardiac cells and human coronary artery smooth muscle cells) resulted in trafficking to endosomal/lysosomal compartments, as assessed by immunofluorescence microscopy. By contrast, SUR1/Kir6.x channels efficiently localized to the plasmalemma. The channel turnover rate was similar with SUR1 or SUR2, suggesting that the expression of Kir6/SUR2 proteins in lysosomes is not associated with increased degradation. Surface labeling of hemagglutinin-tagged channels demonstrated that SUR2-containing channels dynamically cycle between endosomal and plasmalemmal compartments. In addition, Kir6.2 and SUR2 subunits were found in both endosomal and sarcolemmal membrane fractions isolated from rat hearts. The balance of these K(ATP) channel subunits shifted to the sarcolemmal membrane fraction after the induction of ischemia. The K(ATP) channel current density was also increased in rat ventricular myocytes isolated from hearts rendered ischemic before cell isolation without corresponding changes in subunit mRNA expression. We conclude that an intracellular pool of SUR2-containing K(ATP) channels exists that is derived by endocytosis from the plasma membrane. In cardiac myocytes, this pool can potentially play a cardioprotective role by serving as a reservoir for modulating surface K(ATP) channel density under stress conditions, such as myocardial ischemia.

Vasoconstriction resulting from dynamic membrane trafficking of TRPM4 in vascular smooth muscle cells

The melastatin (M) transient receptor potential (TRP) channel TRPM4 mediates pressure and protein kinase C (PKC)-induced smooth muscle cell depolarization and vasoconstriction of cerebral arteries. We hypothesized that PKC causes vasoconstriction by stimulating translocation of TRPM4 to the plasma membrane. Live-cell confocal imaging and fluorescence recovery after photobleaching (FRAP) analysis was performed using a green fluorescent protein (GFP)-tagged TRPM4 (TRPM4-GFP) construct expressed in A7r5 cells. The surface channel was mobile, demonstrating a FRAP time constant of 168 +/- 19 s. In addition, mobile intracellular trafficking vesicles were readily detected. Using a cell surface biotinylation assay, we showed that PKC activation with phorbol 12-myristate 13-acetate (PMA) increased (approximately 3-fold) cell surface levels of TRPM4-GFP protein in <10 min. Similarly, total internal reflection fluorescence microscopy demonstrated that stimulation of PKC activity increased (approximately 3-fold) the surface fluorescence of TRPM4-GFP in A7r5 cells and primary cerebral artery smooth muscle cells. PMA also caused an elevation of cell surface TRPM4 protein levels in intact arteries. PMA-induced translocation of TRPM4 to the plasma membrane was independent of PKCalpha and PKCbeta activity but was inhibited by blockade of PKCdelta with rottlerin. Pressure-myograph studies of intact, small interfering RNA (siRNA)-treated cerebral arteries demonstrate that PKC-induced constriction of cerebral arteries requires expression of both TRPM4 and PKCdelta. In addition, pressure-induced arterial myocyte depolarization and vasoconstriction was attenuated in arteries treated with siRNA against PKCdelta. We conclude that PKCdelta activity causes smooth muscle depolarization and vasoconstriction by increasing the number of TRPM4 channels in the sarcolemma.

Expression of stretch-activated two-pore potassium channels in human myometrium in pregnancy and labor

BACKGROUND

We tested the hypothesis that the stretch-activated, four-transmembrane domain, two pore potassium channels (K2P), TREK-1 and TRAAK are gestationally-regulated in human myometrium and contribute to uterine relaxation during pregnancy until labor.

METHODOLOGY

We determined the gene and protein expression of K2P channels in non-pregnant, pregnant term and preterm laboring myometrium. We employed both molecular biological and functional studies of K2P channels in myometrial samples taken from women undergoing cesarean delivery of a fetus.

PRINCIPAL FINDINGS

TREK-1, but not TREK-2, channels are expressed in human myometrium and significantly up-regulated during pregnancy. Down-regulation of TREK-1 message was seen by Q-PCR in laboring tissues consistent with a role for TREK-1 in maintaining uterine quiescence prior to labor. The TRAAK channel was unregulated in the same women. Blockade of stretch-activated channels with a channel non-specific tarantula toxin (GsMTx-4) or the more specific TREK-1 antagonist L-methionine ethyl ester altered contractile frequency in a dose-dependent manner in pregnant myometrium. Arachidonic acid treatment lowered contractile tension an effect blocked by fluphenazine. Functional studies are consistent with a role for TREK-1 in uterine quiescence.

CONCLUSIONS

We provide evidence supporting a role for TREK-1 in contributing to uterine quiescence during gestation and hypothesize that dysregulation of this mechanism may underlie certain cases of spontaneous pre-term birth.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

HL-1 cells express an inwardly rectifying K+ current activated via muscarinic receptors comparable to that in mouse atrial myocytes

An inwardly rectifying K(+) current is present in atrial cardiac myocytes that is activated by acetylcholine (I(KACh)). Physiologically, activation of the current in the SA node is important in slowing the heart rate with increased parasympathetic tone. It is a paradigm for the direct regulation of signaling effectors by the Gbetagamma G-protein subunit. Many questions have been addressed in heterologous expression systems with less focus on the behaviour in native myocytes partly because of the technical difficulties in undertaking comparable studies in native cells. In this study, we characterise a potassium current in the atrial-derived cell line HL-1. Using an electrophysiological approach, we compare the characteristics of the potassium current with those in native atrial cells and in a HEK cell line expressing the cloned Kir3.1/3.4 channel. The potassium current recorded in HL-1 is inwardly rectifying and activated by the muscarinic agonist carbachol. Carbachol-activated currents were inhibited by pertussis toxin and tertiapin-Q. The basal current was time-dependently increased when GTP was substituted in the patch-clamp pipette by the non-hydrolysable analogue GTPgammaS. We compared the kinetics of current modulation in HL-1 with those of freshly isolated atrial mouse cardiomyocytes. The current activation and deactivation kinetics in HL-1 cells are comparable to those measured in atrial cardiomyocytes. Using immunofluorescence, we found GIRK4 at the membrane in HL-1 cells. Real-time RT-PCR confirms the presence of mRNA for the main G-protein subunits, as well as for M2 muscarinic and A1 adenosine receptors. The data suggest HL-1 cells are a good model to study IKAch.

Constitutive endocytic recycling and protein kinase C-mediated lysosomal degradation control K(ATP) channel surface density

Pancreatic ATP-sensitive potassium (K(ATP)) channels control insulin secretion by coupling the excitability of the pancreatic beta-cell to glucose metabolism. Little is currently known about how the plasma membrane density of these channels is regulated. We therefore set out to examine in detail the endocytosis and recycling of these channels and how these processes are regulated. To achieve this goal, we expressed K(ATP) channels bearing an extracellular hemagglutinin epitope in human embryonic kidney cells and followed their fate along the endocytic pathway. Our results show that K(ATP) channels undergo multiple rounds of endocytosis and recycling. Further, activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate significantly decreases K(ATP) channel surface density by reducing channel recycling and diverting the channel to lysosomal degradation. These findings were recapitulated in the model pancreatic beta-cell line INS1e, where activation of PKC leads to a decrease in the surface density of native K(ATP) channels. Because sorting of internalized channels between lysosomal and recycling pathways could have opposite effects on the excitability of pancreatic beta-cells, we propose that PKC-regulated K(ATP) channel trafficking may play a role in the regulation of insulin secretion.

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

Prostaglandin E2 promotes Na1.8 trafficking via its intracellular RRR motif through the protein kinase A pathway

Voltage-gated sodium channels (Na(v)) are essential for the initiation and propagation of action potentials in neurons. Na(v)1.8 activity is regulated by prostaglandin E(2) (PGE(2)). There is, however, no direct evidence showing the regulated trafficking of Na(v)1.8, and the molecular and cellular mechanism of PGE(2)-induced sodium channel trafficking is not clear. Here, we report that PGE(2) regulates the trafficking of Na(v)1.8 through the protein kinase A (PKA) signaling pathway, and an RRR motif in the first intracellular loop of Na(v)1.8 mediates this effect. In rat dorsal root ganglion (DRG) neurons, prolonged PGE(2) treatment enhanced Na(v)1.8 currents by increasing the channel density on the cell surface. Activation of PKA by forskolin had the same effect on DRG neurons and human embryonic kidney 293T cells expressing Na(v)1.8. Inhibition of PKA completely blocked the PGE(2)-promoted effect on Na(v)1.8. Mutation of five PKA phosphorylation sites or the RRR motif in the first intracellular loop of Na(v)1.8 abolished the PKA-promoted Na(v)1.8 surface expression. Furthermore, a membrane-tethered peptide containing the intracellular RRR motif disrupted the PGE(2)-induced promotion of the Na(v)1.8 current in DRG neurons. Our data indicate that PGE(2) promotes the surface expression of Na(v)1.8 via an intracellular RRR motif, and provide a novel mechanism for functional modulation of Na(v)1.8 by hyperalgesic agents.

PKC-permitted elevation of sarcolemmal KATP concentration may explain female-specific resistance to myocardial infarction

The female myocardium, relative to that of the male, exhibits sustained resistance to ischaemic tissue injury, a phenomenon termed sex-specific cardioprotection (SSC). SSC is dependent upon the sarcolemmal K(ATP) channel (sarcK(ATP)), and protein kinase C (PKC). Here we investigate whether PKC-mediated regulation of sarcK(ATP) concentration can explain this endogenous form of protection. Hearts from male (M) and female (F) rats were Langendorff-perfused for 30 min prior to either regional ischaemia-reperfusion (I/R), or global ischaemia (GISC). For both protocols, pre-ischaemic blockade of PKC was achieved by chelerythrine (Chel) in male (M + C) and female (F + C) hearts. Additional female hearts underwent sarcK(ATP) antagonism during I/R by HMR-1098 (HMR), either alone or in combination with Chel (HMR + Chel). GISC hearts were fractionated to assess cellular distribution of PKC and sarcK(ATP). Sex-specific infarct resistance was apparent under control I/R (F, 23 +/- 3% vs. M, 36 +/- 4%, P < 0.05) and abolished by Chel (F + C, 36 +/- 3%). Female infarct resistance was susceptible to sarcK(ATP) blockade (Control, 16 +/- 2% vs. HMR, 27 +/- 3%), and PKC blockade had no additional effect (HMR + Chel, 26 +/- 2%). The prevalence of Kir6.2 and SUR2 was higher in the sarcolemmal fractions of females (Kir6.2: F, 1.24 +/- 0.07 vs. M, 1.02 +/- 0.06; SUR2: F, 3.16 +/- 0.22 vs. M, 2.45 +/- 0.09; ratio units), but normalized by Chel (Kir6.2: F, 1.06 +/- 0.07 vs. M, 0.99 +/- 0.06; SUR2: F, 2.99 +/- 0.09 vs. M, 2.82 +/- 0.22, M; ratio units). Phosphorylation of sarcolemmal PKC was reduced by Chel (p-PKC/PKC: control, 0.43 +/- 0.02; Chel, 0.29 +/- 0.01; P < 0.01). We conclude that PKC-mediated regulation of sarcK(ATP) may account for the physiologically sustainable dependence of SSC upon both PKC and sarcK(ATP), and that this regulation involves PKC-permitted enrichment of the female sarcolemma with sarcK(ATP). As such, the PKC-sarcK(ATP) axis may represent a target for sustainable prophylactic induction of cardioprotection.

Glucose deprivation regulates KATP channel trafficking via AMP-activated protein kinase in pancreatic beta-cells

OBJECTIVE

AMP-activated protein kinase (AMPK) and the ATP-sensitive K(+) (K(ATP)) channel are metabolic sensors that become activated during metabolic stress. AMPK is an important regulator of metabolism, whereas the K(ATP) channel is a regulator of cellular excitability. Cross talk between these systems is poorly understood.

RESEARCH DESIGN AND METHODS

Rat pancreatic beta-cells or INS-1 cells were pretreated for 2 h at various concentrations of glucose. Maximum K(ATP) conductance (G(max)) was monitored by whole-cell measurements after intracellular ATP washout using ATP-free internal solutions. K(ATP) channel activity (NPo) was monitored by inside-out patch recordings in the presence of diazoxide. Distributions of K(ATP) channel proteins (Kir6.2 and SUR1) were examined using immunofluorescence imaging and surface biotinylation studies. Insulin secretion from rat pancreatic islets was measured using an enzyme immunoassay.

RESULTS

G(max) and NPo in cells pretreated with glucose-free or 3 mmol/l glucose solutions were significantly higher than in cells pretreated in 11.1 mmol/l glucose solutions. Immunofluorescence imaging and biotinylation studies revealed that glucose deprivation induced an increase in the surface level of Kir6.2 without affecting the total cellular amount. Increases in G(max) and the surface level of Kir6.2 were inhibited by compound C, an AMPK inhibitor, and siAMPK transfection. The effects of glucose deprivation on K(ATP) channels were mimicked by an AMPK activator. Glucose deprivation reduced insulin secretion, but this response was attenuated by compound C.

CONCLUSIONS

K(ATP) channel trafficking is regulated by energy status via AMPK, and this mechanism may play a key role in inhibiting insulin secretion under low energy status.