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

Direct observation of individual KCNQ1 potassium channels reveals their distinctive diffusive behavior

We have directly observed the trafficking and fusion of ion channel containing vesicles and monitored the release of individual ion channels at the plasma membrane of live mammalian cells using total internal reflection fluorescence microscopy. Proteins were fused in-frame with green or red fluorescent proteins and expressed at low level in HL-1 and HEK293 cells. Dual color imaging revealed that vesicle trafficking involved motorized movement along microtubules followed by stalling, fusion, and subsequent release of individual ion channels at the plasma membrane. We found that KCNQ1-KCNE1 complexes were released in batches of about 5 molecules per vesicle. To elucidate the properties of ion channel complexes at the cell membrane we tracked the movement of individual molecules and compared the diffusive behavior of two types of potassium channel complex (KCNQ1-KCNE1 and Kir6.2-SUR2A) to that of a G-protein coupled receptor, the A1 adenosine receptor. Plots of mean squared displacement against time intervals showed that mobility depended on channel type, cell type, and temperature. Analysis of the mobility of wild type KCNQ1-KCNE1 complexes showed the existence of a significant immobile subpopulation and also a significant number of molecules that demonstrated periodic stalling of diffusive movements. This behavior was enhanced in cells treated with jasplakinolide and was abrogated in a C-terminal truncated form (KCNQ1(R518X)-KCNE1) of the protein. This mutant has been identified in patients with the long QT syndrome. We propose that KCNQ1-KCNE1 complexes interact intermittently with the actin cytoskeleton via the C-terminal region and this interaction may have a functional role.

Sar1-GTPase-dependent ER exit of KATP channels revealed by a mutation causing congenital hyperinsulinism

The ATP-sensitive potassium (K(ATP)) channel controls insulin secretion by coupling glucose metabolism to excitability of the pancreatic beta-cell membrane. The channel comprises four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1), encoded by KCNJ11 and ABCC8, respectively. Mutations in these genes that result in reduced activity or expression of K(ATP) channels lead to enhanced beta-cell excitability, insulin hypersecretion and hypoglycaemia, and in humans lead to the clinical condition congenital hyperinsulinism (CHI). Here we have investigated the molecular basis of the focal form of CHI caused by one such mutation in Kir6.2, E282K. The study led to the discovery that Kir6.2 contains a di-acidic ER exit signal, (280)DLE(282), which promotes concentration of the channel into COPII-enriched ER exit sites prior to ER export via a process that requires Sar1-GTPase. The E282K mutation abrogates the exit signal, and thereby prevents the ER export and surface expression of the channel. When co-expressed, the mutant subunit was able to associate with the wild-type Kir6.2 and form functional channels. Thus unlike most mutations, the E282K mutation does not cause protein mis-folding. Since in focal CHI, maternal chromosome containing the K(ATP) channel genes is lost, beta-cells of the patient would lack wild-type Kir6.2 to rescue the mutant Kir6.2 subunit expressed from the paternal chromosome. The resultant absence of functional K(ATP) channels leads to insulin hypersecretion. Taken together, we conclude that surface expression of K(ATP) channels is critically dependent on the Sar1-GTPase-dependent ER exit mechanism and abrogation of the di-acidic ER exit signal leads to CHI.

Diabetic hyperglycemia aggravates seizures and status epilepticus-induced hippocampal damage

Epileptic seizures in diabetic hyperglycemia (DH) are not uncommon. This study aimed to determine the acute behavioral, pathological, and electrophysiological effects of status epilepticus (SE) on diabetic animals. Adult male Sprague-Dawley rats were first divided into groups with and without streptozotocin (STZ)-induced diabetes, and then into treatment groups given a normal saline (NS) (STZ-only and NS-only) or a lithium-pilocarpine injection to induce status epilepticus (STZ + SE and NS + SE). Seizure susceptibility, severity, and mortality were evaluated. Serial Morris water maze test and hippocampal histopathology results were examined before and 24 h after SE. Tetanic stimulation-induced long-term potentiation (LTP) in a hippocampal slice was recorded in a multi-electrode dish system. We also used a simulation model to evaluate intracellular adenosine triphosphate (ATP) and neuroexcitability. The STZ + SE group had a significantly higher percentage of severe seizures and SE-related death and worse learning and memory performances than the other three groups 24 h after SE. The STZ + SE group, and then the NS + SE group, showed the most severe neuronal loss and mossy fiber sprouting in the hippocampal CA3 area. In addition, LTP was markedly attenuated in the STZ + SE group, and then the NS + SE group. In the simulation, increased intracellular ATP concentration promoted action potential firing. This finding that rats with DH had more brain damage after SE than rats without diabetes suggests the importance of intensively treating hyperglycemia and seizures in diabetic patients with epilepsy.

Regulation of ATP-sensitive K+ channels by caveolin-enriched microdomains in cardiac myocytes

AIMS

ATP-sensitive potassium (K(ATP)) channels in the heart are critical regulators of cellular excitability and action potentials during ischaemia. However, little is known about subcellular localization of these channels and their regulation. The present study was designed to explore the potential role of caveolae in the regulation of K(ATP) channels in cardiac ventricular myocytes.

METHODS AND RESULTS

Both adult and neonatal rat cardiomyocytes were used. Subcellular fractionation by density gradient centrifugation, western blotting, co-immunoprecipitation, and immunofluorescence confocal microscopy were employed in combination with whole-cell voltage clamp recordings and siRNA gene silencing. We detected that the majority of K(ATP) channels on the plasma membrane of cardiac myocytes were localized in caveolin-3-enriched microdomains by cell fractionation and ultracentrifugation followed by western blotting. Immunofluorescence confocal microscopy revealed extensive colocalization of K(ATP) channel pore-forming subunit Kir6.2 and caveolin-3 along the plasma membrane. Co-immunoprecipitation of cardiac myocytes showed significant association of Kir6.2, adenosine A(1) receptors, and caveolin-3. Furthermore, whole-cell voltage clamp studies suggested that adenosine A(1) receptor-mediated activation of K(ATP) channels was largely eliminated by disrupting caveolae with methyl-beta-cyclodextrin or by small interfering RNA, whereas pinacidil-induced K(ATP) activation was not altered.

CONCLUSION

We demonstrate that K(ATP) channels are localized to caveolin-enriched microdomains. This microdomain association is essential for adenosine receptor-mediated regulation of K(ATP) channels in cardiac myocytes.

Neuronal activity regulates phosphorylation-dependent surface delivery of G protein-activated inwardly rectifying potassium channels

G protein-activated inwardly rectifying K(+) (GIRK) channels regulate neuronal excitability by mediating inhibitory effects of G protein-coupled receptors for neurotransmitters and neuromodulators. Notwithstanding many studies reporting modulation of GIRK channel function, whether neuronal activity regulates GIRK channel trafficking remains an open question. Here we report that NMDA receptor activation in cultured dissociated hippocampal neurons elevates surface expression of the GIRK channel subunits GIRK1 and GIRK2 in the soma, dendrites, and dendritic spines within 15 min. This activity-induced increase in GIRK surface expression requires protein phosphatase-1-mediated dephosphorylation of a serine residue (Ser-9) preceding the GIRK2 Val-13/Leu-14 (VL) internalization motif, thereby promoting channel recycling. Because activation of GIRK channels hyperpolarizes neuronal membranes, the NMDA receptor-induced regulation of GIRK channel trafficking may represent a dynamic adjustment of neuronal excitability in response to inhibitory neurotransmitters and/or neuromodulators.

Metabolic environment in substantia nigra reticulata is critical for the expression and control of hypoglycemia-induced seizures

Seizures represent a common and serious complication of hypoglycemia. Here we studied mechanisms of control of hypoglycemic seizures induced by insulin injection in fasted and nonfasted rats. We demonstrate that fasting predisposes rats to more rapid and consistent development of hypoglycemic seizures. However, the fasting-induced decrease in baseline blood glucose concentration cannot account for the earlier onset of seizures in fasted versus nonfasted rats. Data obtained with c-Fos immunohistochemistry and [14C]2-deoxyglucose uptake implicate a prominent involvement of the substantia nigra reticulata (SNR) among other structures in the hypoglycemic seizure control. This is supported by data showing that fasting decreases the SNR expression of K(ATP) channels, which link metabolism with activity, and is further confirmed with microinfusions of K(ATP) channel agonist and antagonist. Data obtained with whole-cell and perforated patch recordings from SNR neurons in slices in vitro demonstrate that both presynaptic and postsynaptic K(ATP) channels participate in the failure of the SNR to control hypoglycemic seizures. The results suggest that fasting and insulin-induced hypoglycemia can lead to impairment in the function of the SNR, leading thus to hypoglycemic seizures.

Chronic sustained and intermittent hypoxia reduce function of ATP-sensitive potassium channels in nucleus of the solitary tract

Activation of neuronal ATP-sensitive potassium (K(ATP)) channels is an important mechanism that protects neurons and conserves neural function during hypoxia. We investigated hypoxia (bath gassed with 95% N(2)-5% CO(2) vs. 95% O(2)-5% CO(2) in control)-induced changes in K(ATP) current in second-order neurons of peripheral chemoreceptors in the nucleus of the solitary tract (NTS). Hypoxia-induced K(ATP) currents were compared between normoxic (Norm) rats and rats exposed to 1 wk of either chronic sustained hypoxia (CSH) or chronic intermittent hypoxia (CIH). Whole cell recordings of NTS second-order neurons identified after 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) labeling of the carotid bodies were obtained in a brain stem slice. In Norm cells (n = 9), hypoxia (3 min) induced an outward current of 12.7 +/- 1.1 pA with a reversal potential of -73 +/- 2 mV. This current was completely blocked by the K(ATP) channel blocker tolbutamide (100 muM). Bath application of the K(ATP) channel opener diazoxide (200 muM, 3 min) evoked an outward current of 21.8 +/- 5.8 pA (n = 6). Hypoxia elicited a significantly smaller outward current in both CSH (5.9 +/- 1.4 pA, n = 11; P < 0.01) and CIH (6.8 +/- 1.7 pA, n = 6; P < 0.05) neurons. Diazoxide elicited a significantly smaller outward current in CSH (3.9 +/- 1.0 pA, n = 5; P < 0.05) and CIH (2.9 +/- 0.9 pA, n = 3; P < 0.05) neurons. Western blot analysis showed reduced levels of K(ATP) potassium channel subunits Kir6.1 and Kir6.2 in the NTS from CSH and CIH rats. These results suggest that hypoxia activates K(ATP) channels in NTS neurons receiving monosynaptic chemoreceptor afferent inputs. Chronic exposure to either sustained or intermittent hypoxia reduces K(ATP) channel function in NTS neurons. This may represent a neuronal adaptation that preserves neuronal excitability in crucial relay neurons in peripheral chemoreflex pathways.

Protein kinase C-epsilon induces caveolin-dependent internalization of vascular adenosine 5'-triphosphate-sensitive K+ channels

Vascular ATP-sensitive K(+) (K(ATP)) channels are critical regulators of arterial tone and, thus, blood flow in response to local metabolic needs. They are important targets for clinically used drugs to treat hypertensive emergency and angina. It is known that protein kinase C (PKC) activation inhibits K(ATP) channels in vascular smooth muscles. However, the mechanism by which PKC inhibits the channel remains unknown. Here we report that caveolin-dependent internalization is involved in PKC-epsilon-mediated inhibition of vascular K(ATP) channels (Kir6.1 and SUR2B) by phorbol 12-myristate 13-acetate or angiotensin II in human embryonic kidney 293 cells and human dermal vascular smooth muscle cells. We showed that Kir6.1 substantially overlapped with caveolin-1 at the cell surface. Cholesterol depletion with methyl-beta-cyclodextrin significantly reduced, whereas overexpression of caveolin-1 largely enhanced, PKC-induced inhibition of Kir6.1/SUR2B currents. Importantly, we demonstrated that activation of PKC-epsilon caused internalization of K(ATP) channels, the effect that was blocked by depletion of cholesterol with methyl-beta-cyclodextrin, expression of dominant-negative dynamin mutant K44E, or knockdown of caveolin-1 with small interfering RNA. Moreover, patch-clamp studies revealed that PKC-epsilon-mediated inhibition of the K(ATP) current induced by PMA or angiotensin II was reduced by a dynamin mutant, as well as small interfering RNA targeting caveolin-1. The reduction in the number of plasma membrane K(ATP) channels by PKC activation was further confirmed by cell surface biotinylation. These studies identify a novel mechanism by which the levels of vascular K(ATP) channels could be rapidly downregulated by internalization. This finding provides a novel mechanistic insight into how K(ATP) channels are regulated in vascular smooth muscle cells.

Investigation of K(ATP) channel endocytosis by immunofluorescence

Macroscopic ion channel currents (1) are a product of the channel open probability (P(o)), the single channel current (i) and the number of channels present on the cell surface (N) at any given time (I = P(o)iN). Intra-cellular trafficking pathways are proving to be of vital importance in regulating ion channel function since endocytosis, recycling and degradation all work in concert to maintain appropriate channel numbers on the cell surface. Immunofluorescence-based techniques provide a convenient and rapid method for the examination of these processes and have been used to investigate the intracellular trafficking of pancreatic ATP-sensitive potassium (K(ATP)) channels.

Investigation of K(ATP) channel endocytosis and cell surface density by Biotinylation and Western blotting

Macroscopic ion channel currents (I) are a product of the channel open probability (P(o)), the single channel current (i) and the number of channels present on the cell surface (N) at any given time (I = P(o)iN). Endocytosis has been shown to be one of the key determinants of cell surface channel density and defects of this process have been linked to diseases relating to ion channel dysfunction. Biotinylation allows the selective labelling and isolation of surface exposed proteins which can then be identified by Western blotting.

Chemiluminescence assays to investigate membrane expression and clathrin-mediated endocytosis of K(ATP) channels

Macroscopic ion channel currents (I) are a product of the channel open probability (P (o)), the single channel current (i) and the number of channels present on the cell surface (N) at any given time (I = P (o) iN). Endocytosis has been shown to be one of the key determinants of cell surface channel density and the defects of this process have been linked to diseases relating to ion channel dysfunction. Chemiluminescence-based techniques provide a rapid method for the examination of the rates of endocytosis and steady-state cell surface density of ion channels and have previously been used to investigate the endocytosis of pancreatic ATP-sensitive potassium (K(ATP)) channels.

A short motif in Kir6.1 consisting of four phosphorylation repeats underlies the vascular KATP channel inhibition by protein kinase C

Vascular ATP-sensitive K(+) channels are inhibited by multiple vasoconstricting hormones via the protein kinase C (PKC) pathway. However, the molecular substrates for PKC phosphorylation remain unknown. To identify the PKC sites, Kir6.1/SUR2B and Kir6.2/SUR2B were expressed in HEK293 cells. Following channel activation by pinacidil, the catalytic fragment of PKC inhibited the Kir6.1/SUR2B currents but not the Kir6.2/SUR2B currents. Phorbol 12-myristate 13-acetate (a PKC activator) had similar effects. Using Kir6.1-Kir6.2 chimeras, two critical protein domains for the PKC-dependent channel inhibition were identified. The proximal N terminus of Kir6.1 was necessary for channel inhibition. Because there was no PKC phosphorylation site in the N-terminal region, our results suggest its potential involvement in channel gating. The distal C terminus of Kir6.1 was crucial where there are several consensus PKC sites. Mutation of Ser-354, Ser-379, Ser-385, Ser-391, or Ser-397 to nonphosphorylatable alanine reduced PKC inhibition moderately but significantly. Combined mutations of these residues had greater effects. The channel inhibition was almost completely abolished when 5 of them were jointly mutated. In vitro phosphorylation assay showed that 4 of the serine residues were necessary for the PKC-dependent (32)P incorporation into the distal C-terminal peptides. Thus, a motif containing four phosphorylation repeats is identified in the Kir6.1 subunit underlying the PKC-dependent inhibition of the Kir6.1/SUR2B channel. The presence of the phosphorylation motif in Kir6.1, but not in its close relative Kir6.2, suggests that the vascular K(ATP) channel may have undergone evolutionary optimization, allowing it to be regulated by a variety of vasoconstricting hormones and neurotransmitters.

Brief ischemia causes long-term depression in midbrain dopamine neurons

Degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) plays an important role in the pathophysiology of neurodegenerative diseases like Parkinsonism and vascular dementia. SNc dopamine neurons both in vitro and in vivo show sensitivity to hypoxic/ischemic conditions and undergo degeneration. In acute brain slices, these dopamine neurons undergo hyperpolarization during hypoxia and hypoglycemia, which results in silencing of the neurons. However, the role that SNc excitatory synapses play in this process is poorly understood. Here we examined the effect of oxygen/glucose deprivation (OGD) on glutamatergic synaptic transmission in the SNc in a rat midbrain slice preparation. OGD for 5 min caused pre-synaptic ischemic long-term depression (iLTD) of glutamate transmission, as both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid- and N-methyl-D-aspartate receptor-mediated synaptic currents in SNc dopamine neurons were depressed to a similar extent. This depression began immediately after exposure to OGD and was not recovered upon washout of OGD. Pharmacological studies revealed that the iLTD was triggered by a rise in post-synaptic intracellular calcium and mediated by activation of pre-synaptic adenosine A(1) receptors, which reduced glutamate-dependent synaptic transmission by activating ATP-dependent potassium channels. Furthermore, we observed that iLTD did not occlude tetanic long-term depression (LTD) at the SNc excitatory synapses, suggesting that these two forms of LTD involve different pathways. Taken together, our results showed that brief exposure to hypoxia and hypoglycemia results in LTD of synaptic activity at glutamatergic synapses onto SNc neurons and this phenomenon could represent a protective mechanism by reducing ischemia-induced excitotoxic injury to dopamine neurons.

Trafficking of ATP-sensitive potassium channels in health and disease

K(ATP) channels (ATP-sensitive potassium channels), comprising four subunits each of Kir6.2 (inwardly rectifying potassium channel 6.2) and the SUR1 (sulfonylurea receptor 1), play a central role in glucose-stimulated insulin secretion by the pancreatic beta-cell. Changes in the number of channels at the cell surface are associated with genetic diseases of aberrant insulin secretion, including CHI (congenital hyperinsulinism) and NDM (neonatal diabetes mellitus). The present review summarizes advances in our understanding of the vesicular trafficking of normal K(ATP) channels and how genetic mutations in Kir6.2 interfere with such trafficking. A mutation, E282K, causing CHI, was found to disrupt a DXE [di-acidic ER (endoplasmic reticulum)-exit signal], thereby preventing its assembly into COPII (coatamer protein II)-coated vesicles and subsequent ER exit. The resultant decrease in the cell-surface density of the channel could explain the disease phenotype. Two mutations, Y330C and F333I, reported in patients with NDM, disrupted an endocytic traffic signal, thereby impairing CCV (clathrin-coated vesicle) formation and endocytosis. The consequent increase in the density of K(ATP) channels, together with an attenuated sensitivity to ATP reported previously, may account for the severe form of NDM.

Adenosine A(1) receptor: Functional receptor-receptor interactions in the brain

Over the past decade, many lines of investigation have shown that receptor-mediated signaling exhibits greater diversity than previously appreciated. Signal diversity arises from numerous factors, which include the formation of receptor dimers and interplay between different receptors. Using adenosine A₁ receptors as a paradigm of G protein-coupled receptors, this review focuses on how receptor-receptor interactions may contribute to regulation of the synaptic transmission within the central nervous system. The interactions with metabotropic dopamine, adenosine A_2A, A₃, neuropeptide Y, and purinergic P2Y₁ receptors will be described in the first part. The second part deals with interactions between A₁Rs and ionotropic receptors, especially GABA_A, NMDA, and P2X receptors as well as ATP-sensitive K⁺ channels. Finally, the review will discuss new approaches towards treating neurological disorders.

Glucose recruits K(ATP) channels via non-insulin-containing dense-core granules

beta cells rely on adenosine triphosphate-sensitive potassium (K(ATP)) channels to initiate and end glucose-stimulated insulin secretion through changes in membrane potential. These channels may also act as a constituent of the exocytotic machinery to mediate insulin release independent of their electrical function. However, the molecular mechanisms whereby the beta cell plasma membrane maintains an appropriate number of K(ATP) channels are not known. We now show that glucose increases K(ATP) current amplitude by increasing the number of K(ATP) channels in the beta cell plasma membrane. The effect was blocked by inhibition of protein kinase A (PKA) as well as by depletion of extracellular or intracellular Ca(2+). Furthermore, glucose promoted recruitment of the potassium inward rectifier 6.2 to the plasma membrane, and intracellular K(ATP) channels localized in chromogranin-positive/insulin-negative dense-core granules. Our data suggest that glucose can recruit K(ATP) channels to the beta cell plasma membrane via non-insulin-containing dense-core granules in a Ca(2+)- and PKA-dependent manner.

Molecular cell biology of KATP channels: implications for neonatal diabetes

ATP-sensitive potassium (KATP) channels play a key role in the regulation of insulin secretion by coupling glucose metabolism to the electrical activity of pancreatic beta-cells. To generate an electric signal of suitable magnitude, the plasma membrane of the beta-cell must contain an appropriate number of channels. An inadequate number of channels can lead to congenital hyperinsulinism, whereas an excess of channels can result in the opposite condition, neonatal diabetes. KATP channels are made up of four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1), encoded by the genes KCNJ11 and ABCC8, respectively. Following synthesis, the subunits must assemble into an octameric complex to be able to exit the endoplasmic reticulum and reach the plasma membrane. While this biosynthetic pathway ensures supply of channels to the cell surface, an opposite pathway, involving clathrin-mediated endocytosis, removes channels back into the cell. The balance between these two processes, perhaps in conjunction with endocytic recycling, would dictate the channel density at the cell membrane. In this review, we discuss the molecular signals that contribute to this balance, and how an imbalance could lead to a disease state such as neonatal diabetes.

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

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

Dual control of cardiac Na+ Ca2+ exchange by PIP(2): analysis of the surface membrane fraction by extracellular cysteine PEGylation

We describe a new assay to determine the fraction of cardiac Na(+)-Ca(2+) exchangers (NCX1) in the surface membrane of cells (F(surf)). An extracellular NCX1 disulphide bond is rapidly reduced by tris(2-carboxyethyl)phosphine hydrochloride (TCEP), cysteines are 'PEGylated' by alkylation with an impermeable conjugate of maleimide and a 5000 MW polyethylene glycol (MPEG), and F(surf) is quantified from Western blots as the fraction of NCX1 that migrates at a higher molecular weight. F(surf) remains less than 0.1 when NCX1 is expressed via transient transfections. Values of 0.15-0.4 are obtained for cell lines with stable NCX1 expression, 0.3 for neonatal myocytes and 0.6-0.8 for adult hearts. To validate the assay, we analysed an intervention that promotes clathrin-independent endocytosis in fibroblasts. Using BHK cells, removal of extracellular potassium (K(+)) caused yellow fluorescent protein (YFP)-tagged NCX1 to redistribute diffusely into the cytoplasm within 30 min, F(surf) decreased by 35%, and whole-cell exchange currents decreased by > 50%. In both HEK 293 and BHK cell lines, expression of human hPIP5Ibeta kinase significantly decreases F(surf). In BHK cells expressing M1 receptors, a muscarinic agonist (carbachol) causes a 40% decrease of F(surf) in normal media. This decrease is blocked by a high wortmannin concentration (3 mum), suggesting that type III phosphatidylinositol-4-kinase (PI4K) activity is required. As predicted from functional studies, carbachol increases F(surf) when cytoplasmic Ca(2) is increased by removing extracellular Na(+). Phorbol esters are without effect in BHK cells. In intact hearts, interventions that change contractility have no effect within 15 min, but we have identified two long-term changes. First, we analysed the diurnal dependence of F(surf) because messages for cardiac phosphatidylinositol-4-phosphate (PIP) 5-kinases increase during the light phase in entrained mice (i.e. during sleep). Cardiac phosphatidylinositol-(4,5)-bis-phosphate (PIP(2)) levels increase during the light phase and F(surf) decreases in parallel. Second, we analysed effects of aortic banding because NCX1 currents do not mirror the increases of NCX1 message and protein that occur in this model. F(surf) decreases significantly within 10 days, and cardiac PIP and PIP(2) levels are significantly increased. In summary, multiple experimental approaches suggest that PIP(2) synthesis favours NCX1 internalization, that NCX1 internalization is probably clathrin-independent, and that significant changes of NCX1 surface expression occur physiologically and pathologically in intact myocardium.

The inhibition of different types of potassium channels underlies the antidepressant-like effect of adenosine in the mouse forced swimming test

It was previously shown that the acute administration of adenosine elicits an antidepressant-like effect in the mouse forced swimming test (FST) by a mechanism dependent on the inhibition of the L-arginine-nitric oxide (NO)-guanylate cyclase pathway. Taken into account that the stimulation of this pathway is associated with the activation of K(+) channels, this study investigated the involvement of different types of K(+) channels in the effect of adenosine in the FST. Intracerebroventricular treatment of mice with tetraethylammonium (TEA, a non-specific inhibitor of K(+) channels, 25 pg/site), glibenclamide (an ATP-sensitive K(+) channel inhibitor, 0.5 pg/site), charybdotoxin (a large- and intermediate-conductance calcium-activated K(+) channel inhibitor, 25 pg/site) or apamin (a small-conductance calcium-activated K(+) channel inhibitor, 10 pg/site) was able to potentiate the action of subeffective doses of adenosine (1 mg/kg, i.p.) and fluoxetine (a serotonin reuptake inhibitor, 10 mg/kg, i.p.). Furthermore, the administration of adenosine or fluoxetine and the K(+) channel inhibitors, alone or in combination, did not affect the ambulatory behavior. Moreover, the reduction in the immobility time elicited by active doses of adenosine (10 mg/kg, i.p.) or fluoxetine (32 mg/kg, i.p.) in the FST was prevented by the pretreatment of mice with cromakalim (a K(+) channel opener, 10 microg/site, i.c.v.), without affecting the locomotion in an open-field. Together these results indicate that the modulatory effects of adenosine and fluoxetine on neuronal excitability, via inhibition of K(+) channels, may represent the final pathway of their antidepressant-like effects in the FST.

Arginine vasopressin inhibits Kir6.1/SUR2B channel and constricts the mesenteric artery via V1a receptor and protein kinase C

Kir6.1/SUR2B channel is the major isoform of K(ATP) channels in the vascular smooth muscle. Genetic disruption of either subunit leads to dysregulation of vascular tone and regional blood flows. To test the hypothesis that the Kir6.1/SUR2B channel is a target molecule of arginine vasopressin (AVP), we performed studies on the cloned Kir6.1/SUR2B channel and cell-endogenous K(ATP) channel in rat mesenteric arteries. The Kir6.1/SUR2B channel was expressed together with V1a receptor in the HEK-293 cell line. Whole cell currents of the transfected HEK cells were activated by K(ATP) channel opener pinacidil and inhibited by K(ATP) channel inhibitor glibenclamide. AVP produced a concentration-dependent inhibition of the pinacidil-activated currents with IC(50) 2.0 nM. The current inhibition was mediated by a suppression of the open-state probability without effect on single-channel conductance. An exposure to 100 nM PMA, a potent PKC activator, inhibited the pinacidil-activated currents, and abolished the channel inhibition by AVP. Such an effect was not seen with inactive phorbol ester. A pretreatment of the cells with selective PKC blocker significantly diminished the inhibitory effect of AVP. In acutely dissociated vascular smooth myocytes, AVP strongly inhibited the cell-endogenous K(ATP) channel. In isolated mesenteric artery rings, AVP produced concentration-dependent vasoconstrictions with EC(50) 6.5 nM. At the maximum effect, pinacidil completely relaxed vasoconstriction in the continuing exposure to AVP. The magnitude of the AVP-induced vasoconstriction was significantly reduced by calphostin-C. These results therefore indicate that the Kir6.1/SUR2B channel is a target molecule of AVP, and the channel inhibition involves G(q)-coupled V1a receptor and PKC.

Protein kinase C isoform-dependent modulation of ATP-sensitive K+ channels in mitochondrial inner membrane

The ATP-sensitive K(+) (K(ATP)) channels in both sarcolemmal (sarcK(ATP)) and mitochondrial inner membrane (mitoK(ATP)) are the critical mediators in cellular protection of ischemic preconditioning (IPC). Whereas cardiac sarcK(ATP) contains Kir6.2 and sulfonylurea receptor (SUR)2A, the molecular identity of mitoK(ATP) remains elusive. In the present study, we tested the hypothesis that protein kinase C (PKC) may promote import of Kir6.2-containing K(ATP) into mitochondria. Fluorescence imaging of isolated mitochondria from both rat adult cardiomyocytes and COS-7 cells expressing recombinant Kir6.2/SUR2A showed that Kir6.2-containing K(ATP) channels were localized in mitochondria and this mitochondrial localization was significantly increased by PKC activation with phorbol 12-myristate 13-acetate (PMA). Fluorescence resonance energy transfer microscopy further revealed that a significant number of Kir6.2-containing K(ATP) channels were localized in mitochondrial inner membrane after PKC activation. These results were supported by Western blotting showing that the Kir6.2 protein level in mitochondria from COS-7 cells transfected with Kir6.2/SUR2A was enhanced after PMA treatment and this increase was inhibited by the selective PKC inhibitor chelerythrine. Furthermore, functional analysis indicated that the number of functional K(ATP) channels in mitochondria was significantly increased by PMA, as shown by K(ATP)-dependent decrease in mitochondrial membrane potential in COS-7 cells transfected with Kir6.2/SUR2A but not empty vector. Importantly, PKC-mediated increase in mitochondrial Kir6.2-containing K(ATP) channels was blocked by a selective PKCepsilon inhibitor peptide in both COS-7 cells and cardiomyocytes. We conclude that the K(ATP) channel pore-forming subunit Kir6.2 is indeed localized in mitochondria and that the Kir6.2 content in mitochondria is increased by activation of PKCepsilon. PKC isoform-regulated mitochondrial import of K(ATP) channels may have significant implication in cardioprotection of IPC.

Identification and characterization of a new class of trafficking motifs for controlling clathrin-independent internalization and recycling

Plasma membrane proteins such as receptors and ion channels allow a cell to communicate with its environment and regulate many intracellular activities. Thus, the proper control of the surface number of these proteins is essential for maintaining the structural and functional homeostasis of a cell. Internalization and recycling plays a key role in determining the surface density of receptors and channels. Whereas the clathrin-mediated internalization and its associated recycling have been the focus of research in this field, recent studies have revealed that an increasing number of receptors and channels enter a cell via clathrin-independent pathways. However, little is known about the trafficking motifs involved in controlling clathrin-independent internalization and various associated recycling pathways. By using a potassium channel as a model system, we identified a class of trafficking motifs that function along a clathrin-independent pathway to increase the surface density of a membrane protein by preventing its rapid internalization and/or facilitating its recycling via the ADP-ribosylation factor 6-dependent recycling pathway. Moreover our data suggest that these motifs may enhance the association of membrane proteins with the EFA6 family of guanine nucleotide exchange factors for ADP-ribosylation factor 6.

Verrucotoxin inhibits KATP channels in cardiac myocytes through a muscarinic M3 receptor-PKC pathway

Verrucotoxin is the major component of venom from the stonefish (Synanceia verrucosa). Stings from the dorsal spines of the stonefish produce intensive pain, convulsions, hypotension, paralysis, respiratory weakness and collapse of the cardiovascular system, occasionally leading to death. It has been reported that verrucotoxin might modulate ATP-sensitive K+ (KATP) current in frog atrial fibers. However, the mechanism by which verrucotoxin acts on KATP current remains unclear. In this study, we examined whether verrucotoxin inhibited KATP current in guinea pig ventricular myocytes, using the patch clamp method. Verrucotoxin suppressed KATP current induced by pinacidil (KATP channel opener) in a concentration-dependent manner, with a half maximum concentration of 16.3 microg/ml. The effect of verrucotoxin on KATP current was suppressed by atropine (1 microM), a muscarinic receptor antagonist, or by 4-diphenylacetoxy-N-methylpiperidine (100 nM), a muscarinic M3 receptor antagonist. Furthermore, the effect of verrucotoxin on KATP current was attenuated by the protein kinase C (PKC) inhibitor chelerythrine (10 microM) and calphostin C (10 microM), yet not by the cAMP-dependent protein kinase (PKA) inhibitor H-89 (0.5 microM). These results suggest that verrucotoxin inhibits KATP current through the muscarinic M3 receptor-PKC pathway. These findings enhance our understanding of the toxic effects of verrucotoxin from the stonefish.

Microtubule plus-end-tracking proteins target gap junctions directly from the cell interior to adherens junctions

Gap junctions are intercellular channels that connect the cytoplasms of adjacent cells. For gap junctions to properly control organ formation and electrical synchronization in the heart and the brain, connexin-based hemichannels must be correctly targeted to cell-cell borders. While it is generally accepted that gap junctions form via lateral diffusion of hemichannels following microtubule-mediated delivery to the plasma membrane, we provide evidence for direct targeting of hemichannels to cell-cell junctions through a pathway that is dependent on microtubules; through the adherens-junction proteins N-cadherin and beta-catenin; through the microtubule plus-end-tracking protein (+TIP) EB1; and through its interacting protein p150(Glued). Based on live cell microscopy that includes fluorescence recovery after photobleaching (FRAP), total internal reflection fluorescence (TIRF), deconvolution, and siRNA knockdown, we propose that preferential tethering of microtubule plus ends at the adherens junction promotes delivery of connexin hemichannels directly to the cell-cell border. These findings support an unanticipated mechanism for protein delivery to points of cell-cell contact.

AMP-activated protein kinase mediates preconditioning in cardiomyocytes by regulating activity and trafficking of sarcolemmal ATP-sensitive K(+) channels

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that is complex and yet to be fully defined. AMP-activated kinase (AMPK) is activated in cells under conditions associated with ATP depletion and increased AMP/ATP ratio. In the present study, we have taken advantage of a cardiac phenotype overexpressing a dominant negative form of the alpha2 subunit of AMPK to analyze the role, if any, that AMPK plays in preconditioning the heart. We have found that myocardial preconditioning activates AMPK in wild type, but not transgenic mice. Cardiac cells from transgenic mice could not be preconditioned, as opposed to cells from the wild type. The cytoprotective effect of AMPK was not related to the effect that preconditioning has on mitochondrial membrane potential as revealed by JC-1, a mitochondrial membrane potential-sensitive dye, and laser confocal microscopy. In contrast, experiments with di-8-ANEPPS, a sarcolemmal-potential sensitive dye, has demonstrated that intact AMPK activity is required for preconditioning-induced shortening of the action membrane potential. The preconditioning-induced activation of sarcolemmal K(ATP) channels was observed in wild type, but not in transgenic mice. HMR 1098, a selective inhibitor of sarcolemmal K(ATP) channels opening, inhibited preconditioning-induced shortening of action membrane potential as well as cardioprotection afforded by AMPK. Immunoprecipitation followed by Western blotting has shown that AMPK is essential for preconditioning-induced recruitment of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that AMPK mediates preconditioning in cardiac cells by regulating the activity and recruitment of sarcolemmal K(ATP) channels without being a part of signaling pathway that regulates mitochondrial membrane potential.

A1 adenosine receptor-mediated modulation of neuronal ATP-sensitive K channels in rat substantia nigra

ATP-sensitive K (K(ATP)) channels, widely expressed in cytoplasmic membranes of neurons, couple cell metabolism to excitability. They are considered to play important roles in controlling seizure activity during hypoxia and in neuroprotection against cell damage during hypoxia, ischemia and excitotoxicity. It is known that adenosine augments the opening of cardiac surface K(ATP) channels by reducing the sensitivity of these channels to ATP blockade. We investigated whether a similar modulation occurs in neuronal channels. Whole cell voltage-clamp recordings were made using rat midbrain slices to record the membrane current and conductance in principal neurons of the substantia nigra pars compacta (SNc). When the pipette solution contained 1 mM ATP, the membrane current at -60 mV and cellular conductance remained stable for at least 15 min. When slices were treated with (-)-N(6)-2-phenylisopropyl adenosine (R-PIA), a selective agonist for A(1) adenosine receptors, in the same condition, the outward current developed slowly to the amplitude of 109.9+/-26.6 pA, and conductance increased to 229+/-50% of the baseline. These changes were strongly inhibited by 200 microM tolbutamide, a K(ATP) channel blocker, suggesting that opening of K(ATP) channels mediated these changes. Pretreatment with 8-cyclopentyltheophylline (CPT), a selective A(1) adenosine receptor antagonist, abolished the outward current and conductance increases. Treatment of adenosine resulted in the similar changes sensitive to tolbutamide. These changes were abolished by CPT. These results suggest that activation of A(1) adenosine receptors promotes the opening of K(ATP) channels in principal neurons of the SNc by removing the blockade by ATP.

Protein kinase C-dependent enhancement of activity of rat brain NCKX2 heterologously expressed in HEK293 cells

Different members of the Na+/Ca2++K+ exchanger (NCKX) family are present in distinct brain regions, suggesting that they may have cell-specific functions. Many neuronal channels and transporters are regulated via phosphorylation. Regulation of the rat brain NCKXs by protein kinases, however, has not been described. Here, we report an increase in NCKX2 activity in response to protein kinase C (PKC) activation. Outward current of NCKX2 heterologously expressed in HEK293 cells was enhanced by beta-phorbol dibutyrate (PDBu), whereas PDBu had little effect on activity of NCKX3 or NCKX4. The PDBu-induced enhancement (PIE) of NCKX2 activity was abolished by PKC inhibitors and significantly reduced when the dominant negative mutant of PKCepsilon (K437R) was overexpressed. Moreover, PDBu accelerated the decay rate of the Ca2+ transient at the calyx of Held, where NCKX is the major Ca2+-clearance mechanism. Intracellular perfusion with alkaline phosphatase completely inhibited PIE. Consistently, beta-phorbol myristate acetate (PMA), but not 4alpha-PMA, induced a 3-fold stimulation of 32P incorporation into NCKX2 expressed in HEK293 cells. To investigate the sites involved, PIE of wild-type NCKX2 was compared with mutant NCKX2 in which the three putative PKC consensus sites were replaced with alanine, either individually or in combination. Double-site mutation involving Thr-476 (T166A/T476A and T476A/S504A) disrupted PIE, whereas single mutation of Thr-166, Thr-476, or Ser-504 or the double mutant T166A/S504A failed to completely prevent PIE. These findings suggest that PKC-mediated activation of NCKX2 is sensitive to mutation of multiple PKC consensus sites via a mechanism that may involve several phosphorylation events.

Increased ATP-sensitive K+ channel expression during acute glucose deprivation

ATP-sensitive potassium (KATP) channels play a central role in glucose-stimulated insulin secretion (GSIS) by pancreatic beta-cells. Activity of these channels is determined by their open probability (Po) and the number of channels present in a cell. Glucose is known to reduce Po, but whether it also affects the channel density is unknown. Using INS-1 model beta-cell line, we show that the expression of K(ATP) channel subunits, Kir6.2 and SUR1, is high at low glucose, but declines sharply when the ambient glucose concentration exceeds 5mM. In response to glucose deprivation, channel synthesis increases rapidly by up-regulating translation of existing mRNAs. The effects of glucose deprivation could be mimicked by pharmacological activation of 5'-AMP-activated protein kinase with 5-aminoimidazole-4-carboxamide ribonucleotide and metformin. Pancreatic beta-cells which have lost their ability for GSIS do not show such changes implicating a possible (patho-)physiological link between glucose-regulated KATP channel expression and the capacity for normal GSIS.

Kir6.2 mutations causing neonatal diabetes prevent endocytosis of ATP-sensitive potassium channels

ATP-sensitive potassium (KATP) channels couple the metabolic status of a cell to its membrane potential-a property that endows pancreatic beta-cells with the ability to regulate insulin secretion in accordance with changes in blood glucose. The channel comprises four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1). Here, we report that KATP channels undergo rapid internalisation from the plasma membrane by clathrin-mediated endocytosis. We present several lines of evidence to demonstrate that endocytosis is mediated by a tyrosine based signal (330YSKF333) located in the carboxy-terminus of Kir6.2 and that SUR1 has no direct role. We show that genetic mutations, Y330C and F333I, which cause permanent neonatal diabetes mellitus, disrupt this motif and abrogate endocytosis of reconstituted mutant channels. The resultant increase in the surface density of KATP channels would predispose beta-cells to hyperpolarise and may account for reduced insulin secretion in these patients. The data imply that endocytosis of KATP channels plays a crucial role in the (patho)-physiology of insulin secretion.

Phosphorylation regulates an inwardly rectifying ATP-sensitive K(+)- conductance in proximal tubule cells of frog kidney

K(+) channels in the renal proximal tubule play an important role in salt reabsorption. Cells of the frog proximal tubule demonstrate an inwardly rectifying, ATP-sensitive K(+) conductance that is inhibited by Ba(2+), G(Ba). In this paper we have investigated the importance of phosphorylation state on the activity of G(Ba) in whole-cell patches. In the absence of ATP, G(Ba) decreased over time; this fall in G(Ba) involved phosphorylation, as rundown was inhibited by alkaline phosphatase and was accelerated by the phosphatase inhibitor F(-)(10 mM: ). Activation of PKC using the phorbol ester PMA accelerated rundown via a mechanism that was dependent on phosphorylation. In contrast, the inactive phorbol ester PDC slowed rundown. Inclusion of the PKC inhibitor PKC-ps in the pipette inhibited rundown. These data indicate that PKC-mediated phosphorylation promotes channel rundown. Rundown was prevented by the inclusion of PIP-2 in the pipette. PIP-2 also abrogated the PMA-mediated increase in rundown, suggesting that regulation of G(Ba) by PIP-2 occurred downstream of PKC-mediated phosphorylation. G-protein activation inhibited G(Ba), with initial currents markedly reduced in the presence of GTPgammas. These properties are consistent with G(Ba) being a member of the ATP-sensitive K(+) channel family.

Hide and run. Arginine-based endoplasmic-reticulum-sorting motifs in the assembly of heteromultimeric membrane proteins

Arginine-based endoplasmic reticulum (ER)-localization signals are sorting motifs that are involved in the biosynthetic transport of multimeric membrane proteins. After their discovery in the invariant chain of the major histocompatibility complex class II, several hallmarks of these signals have emerged. They occur in polytopic membrane proteins that are subunits of membrane protein complexes; the presence of the signal maintains improperly assembled subunits in the ER by retention or retrieval until it is masked as a result of heteromultimeric assembly. A distinct consensus sequence and their position independence with respect to the distal termini of the protein distinguish them from other ER-sorting motifs. Recognition by the coatomer (COPI) vesicle coat explains ER retrieval. Often, di-leucine endocytic signals occur close to arginine-based signals. Recruitment of 14-3-3 family or PDZ-domain proteins can counteract ER-localization activity, as can phosphorylation. This, and the occurrence of arginine-based signals in alternatively spliced regions, implicates them in the regulated surface expression of multimeric membrane proteins in addition to their function in quality control.

The M3/M4 cytoplasmic loop of the α1 subunit restricts GABAARs lateral mobility: A study using fluorescence recovery after photobleaching

A crucial problem in neurobiology is how neurons are able to maintain neurotransmitter receptors at specific membrane domains. The large structural heterogeneity of gamma aminobutyric acid receptors (GABAARs) led to the hypothesis that there could be a link between GABAAR gene diversity and the targeting properties of the receptor complex. Previous studies using Fluorescence Recovery After Photobleaching (FRAP) have shown a restricted mobility in GABAARs containing the alpha1 subunit. The M3/M4 cytoplasmic loop is the region of the alpha1 subunit with the lowest sequence homology to other subunits. Therefore, we asked whether the M3/M4 loop is involved in cytoskeletal anchoring and GABAAR clustering. A series of alpha1 chimeric subunits was constructed: alpha1CH (control subunit), alpha1CD (Cytoplasmic loop deleted), alpha1CD2, and alpha1CD3 (alpha1 with the M3/M4 loop from the alpha2 and alpha3 subunits, respectively). Our results using FRAP indicate an involvement of the M3/M4 cytoplasmic loop of the alpha1 subunit in controlling receptor lateral mobility. On the other hand, inmunocytochemical approaches showed that this domain is not involved in subunit targeting to the cell surface, subunit-subunit assembly, or receptor aggregation.

The Role of Protein Kinase C in Cerebral Ischemic and Reperfusion Injury

Background and Purpose— Stroke is a leading cause of disability and death in the United States, yet limited therapeutic options exist. The need for novel neuroprotective agents has spurred efforts to understand the intracellular signaling pathways that mediate cellular response to stroke. Protein kinase C (PKC) plays a central role in mediating ischemic and reperfusion damage in multiple tissues, including the brain. However, because of conflicting reports, it remains unclear whether PKC is involved in cell survival signaling, or mediates detrimental processes.

Summary of Review— This review will examine the role of PKC activity in stroke. In particular, we will focus on more recent insights into the PKC isozyme-specific responses in neuronal preconditioning and in ischemia and reperfusion-induced stress.

Conclusion— Examination of PKC isozyme activities during stroke demonstrates the clinical promise of PKC isozyme-specific modulators for the treatment of cerebral ischemia.

Downregulation of the HERG (KCNH2) K+ channel by ceramide: evidence for ubiquitin-mediated lysosomal degradation

The HERG (KCNH2) potassium channel underlies the rapid component of the delayed rectifier current (Ikr), a current contributing to the repolarisation of the cardiac action potential. Mutations in HERG can cause the hereditary forms of the short-QT and long-QT syndromes, predisposing to ventricular arrhythmias and sudden cardiac death. HERG is expressed mainly in the cell membrane of cardiac myocytes, but has also been identified in cell membranes of a range of other cells, including smooth muscle and neurones. The mechanisms regulating the surface expression have however not yet been elucidated. Here we show, using stable HERG-expressing HEK 293 cells, that ceramide evokes a time-dependent decrease in HERG current which was not attributable to a change in gating properties of the channel. Surface expression of the HERG channel protein was reduced by ceramide as shown by biotinylation of surface proteins, western blotting and immunocytochemistry. The rapid decline in HERG protein after ceramide stimulation was due to protein ubiquitylation and its association with lysosomes. The results demonstrate that the surface expression of HERG is strictly regulated, and that ceramide modifies HERG currents and targets the protein for lysosomal degradation.

Role of ubiquitin-proteasome degradation pathway in biogenesis efficiency of {beta}-cell ATP-sensitive potassium channels

ATP-sensitive potassium (K(ATP)) channels of pancreatic beta-cells mediate glucose-induced insulin secretion by linking glucose metabolism to membrane excitability. The number of plasma membrane K(ATP) channels determines the sensitivity of beta-cells to glucose stimulation. The K(ATP) channel is formed in the endoplasmic reticulum (ER) on coassembly of four inwardly rectifying potassium channel Kir6.2 subunits and four sulfonylurea receptor 1 (SUR1) subunits. Little is known about the cellular events that govern the channel's biogenesis efficiency and expression. Recent studies have implicated the ubiquitin-proteasome pathway in modulating surface expression of several ion channels. In this work, we investigated whether the ubiquitin-proteasome pathway plays a role in the biogenesis efficiency and surface expression of K(ATP) channels. We provide evidence that, when expressed in COS cells, both Kir6.2 and SUR1 undergo ER-associated degradation via the ubiquitin-proteasome system. Moreover, treatment of cells with proteasome inhibitors MG132 or lactacystin leads to increased surface expression of K(ATP) channels by increasing the efficiency of channel biogenesis. Importantly, inhibition of proteasome function in a pancreatic beta-cell line, INS-1, that express endogenous K(ATP) channels also results in increased channel number at the cell surface, as assessed by surface biotinylation and whole cell patch-clamp recordings. Our results support a role of the ubiquitin-proteasome pathway in the biogenesis efficiency and surface expression of beta-cell K(ATP) channels.

Proteasomal degradation of Kir6.2 channel protein and its inhibition by a Na+ channel blocker aprindine

ATP-sensitive K+ channels (K(ATP):SUR2A+Kir6.2) play a pivotal role in cardiac protection against ischemia and reperfusion injury. When expressed in COS cells, Kir6.2 was short-lived with a half-life time of 1.9 h. The half-life time of Kir6.2 was prolonged by proteasome inhibitors MG132, ALLN, proteasome inhibitor 1, and lactacystine, but not at all by a lysosomal inhibitor chloroquine. MG132 also increased the level of ubiquitinated Kir6.2 without affecting its localization in the endoplasmic reticulum and Golgi apparatus. In electrophysiological recordings, MG132 augmented nicorandil-activated K(ATP) currents in COS cells expressing SUR2A and Kir6.2 as well as the same currents in neonatal rat cardiomyocytes. Like MG132, a Na+ channel blocker aprindine prolonged the half-life time of Kir6.2 and augmented K(ATP). Finally, both aprindine and MG132 inhibited the 20S proteasome activity in vitro. These results suggest a novel activity of aprindine to enhance K(ATP) currents by inhibiting proteasomal degradation of Kir 6.2 channels, which may be beneficial in the setting of cardiac ischemia.

Trafficking of potassium channels

Recent progress in our understanding of the trafficking of potassium channels can be seen in particular when considering the Kv-type channels. To date, we have discovered that folding of the Kv1.3 T1 domain begins in the ribosomal exit tunnel, and that the cell surface expression of Kv4 channels is enhanced by the presence of two recently identified accessory subunits. Current advances are beginning to enable us to understand the Kv supermolecular complex containing these subunits in crystallographic detail. In addition, determinants that govern the dendritic or axonal targeting of Kv channels have also been identified. In terms of the bigger picture, the careful analysis of gene expression patterns in the brain paves the way for studying trafficking in a physiological context. Indeed, neuronal activity has recently been shown to fine-tune the localization of Kv2.1 channels in microdomains of the neuronal plasma membrane.

Increase of delayed rectifier potassium currents in large aspiny neurons in the neostriatum following transient forebrain ischemia

Large aspiny (LA) neurons in the neostriatum are resistant to cerebral ischemia whereas spiny neurons are highly vulnerable to the same insult. Excitotoxicity has been implicated as the major cause of neuronal damage after ischemia. Voltage-dependent potassium currents play important roles in controlling neuronal excitability and therefore influence the ischemic outcome. To reveal the ionic mechanisms underlying the ischemia-resistance, the delayed rectifier potassium currents (Ik) in LA neurons were studied before and at different intervals after transient forebrain ischemia using brain slices and acute dissociation preparations. The current density of Ik increased significantly 24 h after ischemia and returned to control levels 72 h following reperfusion. Among currents contributing to Ik, the margatoxin-sensitive currents increased 24 h after ischemia while the KCNQ/M current remained unchanged after ischemia. Activation of protein kinase A (PKA) down-regulated Ik in both control and ischemic LA neurons, whereas inhibition of PKA only up-regulated Ik and margatoxin-sensitive currents 72 h after ischemia, indicating an active PKA regulation on Ik at this time. Protein tyrosine kinases had a tonic inhibition on Ik to a similar extent before and after ischemia. Compared with that of control neurons, the spike width was significantly shortened 24 h after ischemia due to facilitated repolarization, which could be reversed by blocking margatoxin-sensitive currents. The increase of Ik in LA neurons might be one of the protective mechanisms against ischemic insult.

Organ arrest, protection and preservation: natural hibernation to cardiac surgery

Cardiac surgery continues to be limited by an inability to achieve complete myocardial protection from ischemia-reperfusion injury. This paper considers the following questions: (1) what lessons can be learned from mammalian hibernators to improve current methods of human myocardial arrest, protection and preservation? and (2) can the human heart be pharmacologically manipulated during acute global ischemia to act more like the heart of a hibernating mammal? After reviewing the major entropy-slowing strategies of hibernation, a major player identified in the armortarium is maintenance of the membrane potential. The resting membrane potential of the hibernator's heart appears to be maintained close to its pre-torpid state of around -85 mV. In open-heart surgery, 99% of all surgical heart arrest solutions (cardioplegia) employ high potassium (>16 mM) which depolarises the membrane voltage from -85 to around -50 mV. However, depolarising potassium cardioplegia has been increasingly linked to myocyte and microvascular damage leading to functional loss during reperfusion. Our recent work has been borrowed from hibernation biology and is focused on a very different arrest strategy which 'clamps' the membrane near its resting potential and depresses O2 consumption from baseline by about 90%. The new 'polarising' cardioplegia incorporates adenosine and lidocaine (AL) as the arresting combination, not high potassium. Studies in the isolated rat heart show that AL cardioplegia delivered at 37 degrees C can arrest the heart for up to 4 h with 70-80% recovery of the cardiac output, 85-100% recovery of heart rate, systolic pressure and rate-pressure product and 70-80% of baseline coronary flows. Only 14% of hearts arrested with crystalloid St. Thomas' solution No. 2 cardioplegia survived after 4 h. In conclusion, maintenance of the myocardial membrane potential near or close to its resting state appears to be an important feature of the hibernator's heart that may find great utility in surgical arrest and cellular preservation strategies. Identifying and safely turning 'off' and 'on' the entropy-slowing genes to down-regulate the hibernator's heart and applying this to human organs and tissues remains a major challenge for future genomics and proteomics.

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

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

Protection against ventricular arrhythmias and cardiac death using adenosine and lidocaine during regional ischemia in the in vivo rat

Despite decades of research, there are few effective ways to treat ventricular fibrillation (VF), ventricular tachycardia (VT), or cardiac ischemia that show a significant survival benefit. Our aim was to investigate the combined therapeutic effect of two common antiarrhythmic compounds, adenosine and lidocaine (AL), on mortality, arrhythmia frequency and duration, and infarct size in the rat model of regional ischemia. Sprague-Dawley rats (n = 49) were anesthetized with pentobarbital sodium (60 mg.ml(-1).kg(-1) i.p.) and instrumented for regional coronary occlusion (30 min) and reperfusion (120 min). Heart rate, blood pressure, and a lead II electrocardiogram were recorded. Intravenous pretreatment began 5 min before ischemia and extended throughout ischemia, terminating at the start of reperfusion. After 120 min, hearts were removed for infarct size measurement. Mortality occurred in 58% of saline controls (n = 12), 50% of adenosine only (305 microg.kg(-1).min(-1), n = 8), 0% in lidocaine only (608 microg.kg(-1).min(-1), n = 8), and 0% in AL at any dose (152, 305, or 407 microg.kg(-1).min(-1) adenosine plus 608 microg.kg(-1).min(-1) lidocaine, n = 7, 8, and 6). VT occurred in 100% of saline controls (18 +/- 9 episodes), 50% of adenosine-only (11 +/- 7 episodes), 83% of lidocaine-only (23 +/- 11 episodes), 60% of low-dose AL (2 +/- 1 episodes, P < 0.05), 57% of mid-dose AL (2 +/- 1 episodes, P < 0.05), and 67% of high-dose AL rats (6 +/- 3 episodes). VF occurred in 75% of saline controls (4 +/- 3 episodes), 100% of adenosine-only-treated rats (3 +/- 2 episodes), and 33% lidocaine-only-treated rats (2 +/- 1 episodes) of the rats tested. There was no deaths and no VF in the low- and mid-dose AL-treated rats during ischemia, and only one high-dose AL-treated rat experienced VF (25.5 sec). Infarct size was lower in all AL-treated rats but only reached significance with the mid-dose treatment (saline controls 61 +/- 5% vs. 38 +/- 6%, P < 0.05). We conclude that a constant infusion of a solution containing AL virtually abolished severe arrhythmias and prevented cardiac death in an in vivo rat model of acute myocardial ischemia and reperfusion. AL combinational therapy may provide a primary prevention therapeutic window in ischemic and nonischemic regions of the heart.

ATP-sensitive K(+) channels regulate the concentrative adenosine transporter CNT2 following activation by A(1) adenosine receptors

This study describes a novel mechanism of regulation of the high-affinity Na(+)-dependent adenosine transporter (CNT2) via the activation of A(1) adenosine receptors (A(1)R). This regulation is mediated by the activation of ATP-sensitive K(+) (K(ATP)) channels. The high-affinity Na(+)-dependent adenosine transporter CNT2 and A(1)R are coexpressed in the basolateral domain of the rat hepatocyte plasma membrane and are colocalized in the rat hepatoma cell line FAO. The transient increase in CNT2-mediated transport activity triggered by (-)-N(6)-(2-phenylisopropyl)adenosine is fully inhibited by K(ATP) channel blockers and mimicked by a K(ATP) channel opener. A(1)R agonist activation of CNT2 occurs in both hepatocytes and FAO cells, which express Kir6.1, Kir6.2, SUR1, SUR2A, and SUR2B mRNA channel subunits. With the available antibodies against Kir6.X, SUR2A, and SUR2B, it is shown that all of these proteins colocalize with CNT2 and A(1)R in defined plasma membrane domains of FAO cells. The extent of the purinergic modulation of CNT2 is affected by the glucose concentration, a finding which indicates that glycemia and glucose metabolism may affect this cross-regulation among A(1)R, CNT2, and K(ATP) channels. These results also suggest that the activation of K(ATP) channels under metabolic stress can be mediated by the activation of A(1)R. Cell protection under these circumstances may be achieved by potentiation of the uptake of adenosine and its further metabolization to ATP. Mediation of purinergic responses and a connection between the intracellular energy status and the need for an exogenous adenosine supply are novel roles for K(ATP) channels.

Hypoxia-induced preconditioning in adult stimulated cardiomyocytes is mediated by the opening and trafficking of sarcolemmal KATP channels

The opening of sarcolemmal and mitochondrial ATP-sensitive K(+) (KATP) channels in the heart is believed to mediate ischemic preconditioning, a phenomenon whereby brief periods of ischemia/reperfusion protect the heart against myocardial infarction. Here, we have applied digital epifluorescent microscopy, immunoprecipitation and Western blotting, perforated patch clamp electrophysiology, and immunofluorescence/laser confocal microscopy to examine the involvement of KATP channels in cardioprotection afforded by preconditioning. We have shown that adult, stimulated-to-beat, guinea-pig cardiomyocytes survived in sustained hypoxia for approximately 17 min. An episode of 5-min-long hypoxia/5-min-long reoxygenation before sustained hypoxia dramatically increased the duration of cellular survival. Experiments with different antagonists of KATP channels, applied at different times during the experimental protocol, suggested that the opening of sarcolemmal KATP channels at the beginning of sustained hypoxia mediate preconditioning. This conclusion was supported by perforated patch clamp experiments that revealed activation of sarcolemmal KATP channels by preconditioning. Immunoprecipitation and Western blotting as well as immunofluorescence and laser confocal microscopy showed that the preconditioning is associated with the increase in KATP channel proteins in sarcolemma. Inhibition of trafficking of KATP channel subunits prevented preconditioning without affecting sensitivity of cardiomyocytes to hypoxia in the absence of preconditioning. We conclude that the preconditioning is mediated by the activation and trafficking of sarcolemmal KATP channels.

Long-term plasticity of intrinsic excitability: learning rules and mechanisms

Spatio-temporal configurations of distributed activity in the brain is thought to contribute to the coding of neuronal information and synaptic contacts between nerve cells could play a central role in the formation of privileged pathways of activity. Synaptic plasticity is not the exclusive mode of regulation of information processing in the brain, and persistent regulations of ionic conductances in some specialized neuronal areas such as the dendrites, the cell body, and the axon could also modulate, in the long-term, the propagation of neuronal information. Persistent changes in intrinsic excitability have been reported in several brain areas in which activity is elevated during a classical conditioning. The role of synaptic activity seems to be a determinant in the induction, but the learning rules and the underlying mechanisms remain to be defined. We discuss here the role of synaptic activity in the induction of intrinsic plasticity in cortical, hippocampal, and cerebellar neurons. Activation of glutamate receptors initiates a long-term modification in neuronal excitability that may represent a parallel, synergistic substrate for learning and memory. Similar to synaptic plasticity, long-lasting intrinsic plasticity appears to be bidirectional and to express a certain level of input or cell specificity. These nonsynaptic forms of plasticity affect the signal propagation in the axon, the dendrites, and the soma. They not only share common learning rules and induction pathways with the better-known synaptic plasticity such as NMDA receptor dependent LTP and LTD, but also contribute in synergy with these synaptic changes to the formation of a coherent engram.