Voltage-dependent metabolic regulation of Kv2.1 channels in pancreatic beta-cells

Kv2 channel-AMIGO β-subunit assembly modulates both channel function and cell adhesion molecule surface trafficking

The Kv2 channels encode delayed rectifier currents that regulate membrane potential in many tissues. They also have a non-conducting function to form stable junctions between the endoplasmic reticulum and plasma membranes, creating membrane contact sites that mediate functions distinct from membrane excitability. Therefore, proteins that interact with Kv2.1 and Kv2.2 channels can alter conducting and/or non-conducting channel properties. One member of the AMIGO family of proteins is an auxiliary β-subunit for Kv2 channels and modulates Kv2.1 electrical activity. However, the AMIGO family has two additional members of ∼50% similarity that have not yet been characterized as Kv2 β-subunits. In this work, we show that the surface trafficking and localization of all three AMIGOs are controlled by their assembly with both Kv2 channels. Additionally, assembly of each AMIGO with either Kv2.1 or Kv2.2 hyperpolarizes the channel activation midpoint by -10 mV. However, only AMIGO2 significantly slows inactivation and deactivation, leading to a prolonged open state of Kv2 channels. The co-regulatory effects of Kv2s and AMIGOs likely fine-tune both the electrical and non-electrical properties of the cells in which they are expressed.

The application of the Escherichia coli giant spheroplast for drug screening with automated planar patch clamp system

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Human Kv2.1 was expressed in the inner membrane of E. coli using prokaryotic codon.

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Bacterial spheroplasts large enough for the automated patch clamp were prepared by microfluidic chips.

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Kv2.1 current was recorded from the giant spheroplast by the automated patch clamp.

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E. coli spheroplasts were used for dose–response assay of potassium channel inhibitors.

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Our system will become the simple and sensitive drug assay method anyone can use.

Kv2.1, the voltage-gated ion channel, is ubiquitously expressed in variety of tissues and dysfunction of this ion channel is responsible for multiple diseases. Electrophysiological properties of ion channels are so far characterized with eukaryotic cells using the manual patch clamp which requires skilful operators and expensive equipments. In this research, we created a simple and sensitive drug screen method using bacterial giant spheroplasts and the automated patch clamp which does not require special skills. We expressed a eukaryotic voltage-gated ion channel Kv2.1 in Escherichia coli using prokaryotic codon, and prepared giant spheroplasts large enough for the patch clamp. Human Kv2.1 currents were successfully recorded from giant spheroplasts with the automated system, and Kv2.1-expressed E. coli spheroplasts could steadily reacted to the dose–response assay with TEA and 4-AP. Collectively, our results indicate for the first time that the bacterial giant spheroplast can be applied for practical pharmaceutical assay using the automated patch clamp.

Electrophysiological characterization of Ts6 and Ts7, K⁺ channel toxins isolated through an improved Tityus serrulatus venom purification procedure

In Brazil, Tityus serrulatus (Ts) is the species responsible for most of the scorpion related accidents. Among the Ts toxins, the neurotoxins with action on potassium channels (α-KTx) present high interest, due to their effect in the envenoming process and the ion channel specificity they display. The α-KTx toxins family is the most relevant because its toxins can be used as therapeutic tools for specific target cells. The improved isolation method provided toxins with high resolution, obtaining pure Ts6 and Ts7 in two chromatographic steps. The effects of Ts6 and Ts7 toxins were evaluated in 14 different types of potassium channels using the voltage-clamp technique with two-microelectrodes. Ts6 toxin shows high affinity for Kv1.2, Kv1.3 and Shaker IR, blocking these channels in low concentrations. Moreover, Ts6 blocks the Kv1.3 channel in picomolar concentrations with an IC₅₀ of 0.55 nM and therefore could be of valuable assistance to further designing immunosuppressive therapeutics. Ts7 toxin blocks multiple subtypes channels, showing low selectivity among the channels analyzed. This work also stands out in its attempt to elucidate the residues important for interacting with each channel and, in the near future, to model a desired drug.

Glucose-stimulated insulin secretion: A newer perspective

Existing concepts and models for glucose-stimulated insulin secretion (GSIS) are overviewed and a newer perspective has been formulated toward the physiological understanding of GSIS. A conventional model has been created on the basis of in vitro data on application of a square wave high glucose in the absence of any other stimulatory inputs. Glucose elicits rapid insulin release through an adenosine triphosphate-sensitive K(+) channel (KATP channel)-dependent mechanism, which is gradually augmented in a KATP channel-independent manner. Biphasic GSIS thus occurs. In the body, the β-cells are constantly exposed to stimulatory signals, such as glucagon-like peptide 1 (GLP-1), parasympathetic inputs, free fatty acid (FFA), amino acids and slightly suprathreshold levels of glucose, even at fasting. GLP-1 increases cellular cyclic adenosine monophosphate, parasympathetic stimulation activates protein kinase C, and FFA, amino acids and glucose generate metabolic amplification factors. Plasma glucose concentration gradually rises postprandially under such tonic stimulation. We hypothesize that these stimulatory inputs together make the β-cells responsive to glucose independently from its action on KATP channels. Robust GSIS in patients with a loss of function mutation of the sulfonylurea receptor, a subunit of KATP channels, is compatible with this hypothesis. Furthermore, pre-exposure of the islets to an activator of protein kinase A and/or C makes β-cells responsive to glucose in a KATP channel- and Ca(2+)-independent manner. We hypothesize that GSIS occurs in islet β-cells without glucose regulation of KATP channels in vivo, for which priming with cyclic adenosine monophosphate, protein kinase C and non-glucose nutrients are required. To understand the physiology of GSIS, comprehensive integration of accumulated knowledge is required.

α-Lipoic acid protects against arsenic trioxide-induced acute QT prolongation in anesthetized guinea pigs

Clinical use of arsenic trioxide (As₂O₃), which can induce the remission of relapsed or refractory acute promyelocytic leukemia, is often limited because of its cardiotoxicity. Symptoms of cardiotoxicity include acute cardiac conduction disturbances, such as QT prolongation. The present study was undertaken to evaluate the effects of α-lipoic acid (LA) on acute As₂O₃-induced ECG abnormalities (QTc interval prolongation) in anesthetized guinea pigs. Intravenous injection of As₂O₃ in guinea pigs caused QTc interval prolongation, which was significantly attenuated by co-treatment with LA (0.35, 3.5 and 35 mg/kg) in a dose-dependent manner. In isolated guinea pig cardiomyocytes, the decrease in IKs current induced by As₂O3 (1 μM) was rapidly restored to the basal level by the addition of LA (10 μM). Consistent with this finding, the As₂O₃-induced QTc interval prolongation was also improved rapidly by post-treatment with LA in guinea pigs. Electrospray ionization time-of-flight mass spectrometry analysis detected an expected peak of arsenic-LA complex in vitro, indicating that LA and As₂O3 form a new compound in vivo. In addition, pre-treatment with a chelating agent, British anti-Lewisite (BAL, 3.5 or 35 mg/kg), also attenuated the As₂O₃-induced QTc interval prolongation. In this study, co- and post-treatments with LA and pre-treatment with BAL ameliorated As₂O₃-induced acute QT prolongation in anesthetized guinea pigs. Because LA and probably BAL may bind to As₂O₃, these agents may exert protective effects through their chelating activity. Further studies are needed to determine whether LA is beneficial as a prophylactic or rescue agent for acute promyelocytic leukemia patients treated with As₂O₃.

Inhibition of insulin secretion from rat pancreatic islets by dexmedetomidine and medetomidine, two sedatives frequently used in clinical settings

The aim of this study was to determine whether dexmedetomidine (DEX) and medetomidine (MED), α2-adrenergic agonists clinically used as sedatives, influence insulin secretion from rat pancreatic islets. Islets were isolated from adult male Wistar rats after collagenase digestion. Static incubation was used to determine effects of DEX or MED on insulin secretion and ionic-channel currents of β-cells. Results indicate that both drugs dose-dependently inhibit insulin secretion, DEX more potently than MED. The inhibitory effects were attenuated by addition of yohimbine or by pretreatment of rats with pertussis toxin (PTX). 10 nM DEX decreased the current amplitude of voltage-dependent Ca2+ channels, but this did not occur when the N-type Ca2+ channel blocker ω-conotoxin was added. In the presence of tetraethylammonium, a classical voltage-gated K+ channel (Kv channel) blocker, the magnitude of inhibition of insulin secretion by MED was reduced. However, when tolbutamide, a specific blocker of the ATP-sensitive K+ channel (KATP channel), was present, the magnitude of MED inhibition of insulin secretion was not influenced, suggesting that Kv-channel activity alteration, but not that of KATP channels, is involved in MED-associated insulin secretory inhibition. The Kv-channel currents were increased during 1 nM MED exposure at membrane potentials ranging from -30 mV to -10 mV, where action potentials were generated in response to glucose stimulation. These results indicate that DEX and MED inhibit insulin secretion through an α2-adrenoceptor and PTX-sensitive GTP-binding protein pathway that eventually involves Kv channel activation and Ca2+ channel inhibition.

Glucose-sensitivity of the afterhyperpolarization potential: role of SK1 channel in insulin-secreting cells

The role of the small-conductance, calcium-activated SK potassium channel in regulating pancreatic β cell function remains controversial with conflicting pharmacological results. In this study, we used current clamp recordings to further characterize the function of SK channels in INS-1 cell line. We compared afterhyperpolarization potential (AHP) responses of SK1-downregulated cells with those of control INS-1 cells. They were tested with and without the presence of glucose. We found that cells in which SK1 channel subunit expression had been downregulated exhibited AHPs in the presence of 20mM glucose while control INS-1 cells had AHPs only in the absence of glucose. Our findings show that the glucose-dependence of the AHP in the rat INS-1 cell line depends only on SK1 channel subunit expression.

Signal integration at the level of ion channel and exocytotic function in pancreatic β-cells

Whole body energy balance is ensured by the exquisite control of insulin secretion, the dysregulation of which has serious consequences. Although a great deal has been learned about the control of insulin secretion from pancreatic β-cells in the past 30 years, there remains much to be understood about the molecular mechanisms and interactions that underlie the precise control of this process. Numerous molecular interactions at the plasma membrane mediate the excitatory and amplifying events involved in insulin secretion; this includes interactions between ion channels, signal transduction machinery, and exocytotic proteins. The present Perspectives article considers evidence that key membrane and membrane-associated proteins essential to insulin secretion are regulated in concert as a functional unit, ensuring an integrated excitatory and exocytotic response to the signals that control insulin secretion.

Sialic acids attached to O-glycans modulate voltage-gated potassium channel gating

Neuronal, cardiac, and skeletal muscle action potentials are produced and conducted through the highly regulated activity of several types of voltage-gated ion channels. Voltage-gated potassium (K(v)) channels are responsible for action potential repolarization. Glycans can be attached to glycoproteins through N- and O-linkages. Previous reports described the impact of N-glycans on voltage-gated ion channel function. Here, we show that sialic acids attached through O-linkages modulate gating of K(v)2.1, K(v)4.2, and K(v)4.3. The conductance-voltage (G-V) relationships for each isoform were shifted uniquely by a depolarizing 8-16 mV under conditions of reduced sialylation. The data indicate that sialic acids modulate K(v) channel activation through apparent electrostatic mechanisms that promote channel activity. Voltage-dependent steady-state inactivation was unaffected by changes in sialylation. N-Linked sialic acids cannot be responsible for the G-V shifts because K(v)4.2 and K(v)4.3 cannot be N-glycosylated, and immunoblot analysis confirmed K(v)2.1 is not N-glycosylated. Glycosidase gel shift analysis suggested that K(v)2.1, K(v)4.2, and K(v)4.3 were O-glycosylated and sialylated. To confirm this, azide-modified sugar residues involved specifically in O-glycan and sialic acid biosynthesis were shown to incorporate into all three K(v) channel isoforms using Cu(I)-catalyzed cycloaddition chemistry. Together, the data indicate that sialic acids attached to O-glycans uniquely modulate gating of three K(v) channel isoforms that are not N-glycosylated. These data provide the first evidence that external O-glycans, with core structures distinct from N-glycans in type and number of sugar residues, can modulate K(v) channel function and thereby contribute to changes in electrical signaling that result from regulated ion channel expression and/or O-glycosylation.

Voltage-gated potassium channel Kv1.3 blocker as a potential treatment for rat anti-glomerular basement membrane glomerulonephritis

The voltage-gated potassium channel Kv1.3 has been recently identified as a molecular target that allows the selective pharmacological suppression of effector memory T cells (T(EM)) without affecting the function of naïve T cells (T(N)) and central memory T cells (T(CM)). We found that Kv1.3 was expressed on glomeruli and some tubules in rats with anti-glomerular basement membrane glomerulonephritis (anti-GBM GN). A flow cytometry analysis using kidney cells revealed that most of the CD4(+) T cells and some of the CD8(+) T cells had the T(EM) phenotype (CD45RC(-)CD62L(-)). Double immunofluorescence staining using mononuclear cell suspensions isolated from anti-GBM GN kidney showed that Kv1.3 was expressed on T cells and some macrophages. We therefore investigated whether the Kv1.3 blocker Psora-4 can be used to treat anti-GBM GN. Rats that had been given an injection of rabbit anti-rat GBM antibody were also injected with Psora-4 or the vehicle intraperitoneally. Rats given Psora-4 showed less proteinuria and fewer crescentic glomeruli than rats given the vehicle. These results suggest that T(EM) and some macrophages expressing Kv1.3 channels play a critical role in the pathogenesis of crescentic GN and that Psora-4 will be useful for the treatment of rapidly progressive glomerulonephritis.