Elimination of potassium channel expression by antisense oligonucleotides in a pituitary cell line

Models of electrical activity: calibration and prediction testing on the same cell

Mathematical models are increasingly important in biology, and testability is becoming a critical issue. One limitation is that one model simulation tests a parameter set representing one instance of the biological counterpart, whereas biological systems are heterogeneous in their properties and behavior, and a model often is fitted to represent an ideal average. This is also true for models of a cell's electrical activity; even within a narrowly defined population there can be considerable variation in electrophysiological phenotype. Here, we describe a computational experimental approach for parameterizing a model of the electrical activity of a cell in real time. We combine the inexpensive parallel computational power of a programmable graphics processing unit with the flexibility of the dynamic clamp method. The approach involves 1), recording a cell's electrical activity, 2), parameterizing a model to the recording, 3), generating predictions, and 4), testing the predictions on the same cell used for the calibration. We demonstrate the experimental feasibility of our approach using a cell line (GH4C1). These cells are electrically active, and they display tonic spiking or bursting. We use our approach to predict parameter changes that can convert one pattern to the other.

Motor neuron inhibition-based gene therapy for spasticity

Spasticity is a condition resulting from excess motor neuron excitation, leading to involuntary muscle contraction in response to increased velocity of movement, for which there is currently no cure. Existing symptomatic therapies face a variety of limitations. The extent of relief that can be delivered by ablative techniques such as rhizotomy is limited by the potential for sensory denervation. Pharmacological approaches, including intrathecal baclofen, can be undermined by tolerance. One potential new approach to the treatment of spasticity is the control of neuromuscular overactivity through the delivery of genes capable of inducing synaptic inhibition. A variety of experiments in cell culture and animal models have demonstrated the ability of neural gene transfer to inhibit neuronal activity and suppress transmission. Similarly, enthusiasm for the application of gene therapy to neurodegenerative diseases of motor neurons has led to the development of a variety of strategies for motor neuron gene delivery. In this review, we discuss the limitations of existing spasticity therapies, the feasibility of motor neuron inhibition as a gene-based treatment for spasticity, potential inhibitory transgene candidates, strategies for control of transgene expression, and applicable motor neuron gene targeting strategies. Finally, we discuss future directions and the potential for gene-based motor neuron inhibition in therapeutic clinical trials to serve as an effective treatment modality for spasticity, either in conjunction with or as a replacement for presently available therapies.

Innovative approaches to anti-arrhythmic drug therapy

Normal cardiac function requires an appropriate and regular beating rate (cardiac rhythm). When the heart rhythm is too fast or too slow, cardiac function can be impaired, with derangements that vary from mild symptoms to life-threatening complications. Irregularities, particularly those involving excessively fast or slow rates, constitute cardiac 'arrhythmias'. In the past, drug treatment of cardiac arrhythmias has proven difficult, both because of inadequate effectiveness and a risk of serious complications. However, a variety of recent advances have opened up exciting possibilities for the development of novel and superior approaches to arrhythmia therapy. This article will review recent progress and future prospects for treating two particularly important cardiac arrhythmias: atrial fibrillation and ventricular fibrillation.

Contributions of Kv1.2, Kv1.5 and Kv2.1 subunits to the native delayed rectifier K(+) current in rat mesenteric artery smooth muscle cells

A large array of voltage-gated K(+) channel (Kv) genes has been identified in vascular smooth muscle tissues. This molecular diversity underlies the vast repertoire of native Kv channels that regulate the excitability of vascular smooth muscle tissues. The contributions of different Kv subunit gene products to the native Kv currents are poorly understood in vascular smooth muscle cells (SMCs). In the present study, Kv subunit-specific antibodies were applied intracellularly to selectively block various Kv channel subunits and the whole-cell outward Kv currents were recorded using the patch-clamp technique in rat mesenteric artery SMCs. Anti-Kv1.2 antibody (8 microg/ml) inhibited the Kv currents by 29.2 +/- 5.9% (n = 6, P < 0.05), and anti-Kv1.5 antibody (6 microg/ml) by 24.5 +/- 2.6% (n = 7, P < 0.05). Anti-Kv2.1 antibody inhibited the Kv currents in a concentration-dependent fashion (4-20 microg/ml). Co-application of antibodies against Kv1.2 and Kv2.1 (8 microg/ml each) induced an additive inhibition of Kv currents by 42.3 +/- 3.1% (n = 7, P < 0.05). In contrast, anti-Kv1.3 antibody (6 microg/ml) did not have any effect on the native Kv current (n = 6, P > 0.05). A control antibody (anti-GIRK1) also had no effect on the native Kv currents. This study demonstrates that Kv1.2, Kv1.5, and Kv2.1 subunit genes all contribute to the formation of the native Kv channels in rat mesenteric artery SMCs.

Inward rectifier K(+) current in human bronchial smooth muscle cells: inhibition with antisense oligonucleotides targeted to Kir2.1 mRNA

Inward rectifier K(+) (Kir) channels play an important role in forming membrane potential and then modulating muscle tone in certain types of smooth muscles. In cultured human bronchial smooth muscle cells (hBSMCs), Kir current was identified using whole-cell voltage clamp techniques and explored by using RT-PCR analysis of mRNA, Western blotting, and antisense oligonucleotide methods to block the synthesis of Kir channel protein. The K(+) current with strong inward rectification and high K(+) ion selectivity was observed. The current was unaffected by 4-aminopyridine, glibenclamide, and charybdotoxin, and hardly inhibited by tetraethylammonium, but was potently inhibited by extracellular Ba(2+). The IC(50) value of external Ba(2+) was approximately 1.3 microm. RT-PCR analysis of mRNA showed transcripts for Kir2.1, but not Kir1.1, Kir2.2, or Kir2.3. Treatment of cells with antisense oligonucleotides targeted to Kir2.1 resulted in a decrease in the current density of the Kir current and Kir protein expression, as compared with the mismatch-treated cells, whereas the current density of 4-AP-sensitive K(+) currents (K(V)) remained unaffected. The application of Ba(2+) markedly depolarized the membrane. These results demonstrate that Kir channel is present in human bronchial smooth muscle cells, and the Kir2.1 gene encodes the Kir channel protein in these cells.

Prostaglandin receptor subtypes, EP3C and EP4, mediate the prostaglandin E2-induced cAMP production and sensitization of sensory neurons

Although a number of prostaglandin E(2) (PGE(2)) receptor subtypes have been cloned, limited studies have been performed to elucidate subtypes that subserve specific actions of this eicosanoid, in part because of a paucity of selective receptor antagonists. Using reverse transcription-polymerase chain reaction (PCR) and antisense oligonucleotides, we examined which prostaglandin E(2) receptor (EP receptor) subtypes are expressed in sensory neurons and which mediate the PGE(2)-induced increase in cAMP production and augmentation of peptide release. Reverse transcription-PCR of cDNA isolated from rat sensory neurons grown in culture revealed PCR products for the EP1, EP2, EP3C, and EP4 receptor subtypes but not the EP3A or EP3B. Preexposing neuronal cultures for 48 h to antisense oligonucleotides of EP3C and EP4 mRNA diminished expression of the respective receptors by approximately 80%, abolished the PGE(2)-stimulated production of cAMP, and blocked the ability of PGE(2) to augment release of immunoreactive substance P and calcitonin gene-related peptide. Pretreating with individual antisense against the EP2, EP3C, or EP4 receptors or combinations of missense oligonucleotides had no effect on PGE(2)-induced activity. Treatment with antisense to EP3C and EP4 receptor subtypes did not alter the ability of forskolin to increase cAMP or enhance peptide release. These results demonstrate that sensory neurons are capable of expressing multiple EP receptor subtypes but that only the EP3C and EP4 receptors mediate PGE(2)-induced sensitization of sensory neurons.

Beta-amyloid peptide induces the expression of voltage dependent outward rectifying K+ channels in rat microglia

Upregulation of voltage-dependent outward rectifying K+ (Kv) channels has been reported in activated microglia. Since beta-amyloid peptide (A beta) is known to activate microglia, we tested whether the exposure of cultured rat microglia to A beta fragment 25-35 (A beta 25-35) induced the Kv current. A beta 25-35 in 5-200 nM concentration range significantly increased Kv current density, while there was small change in inward rectifying K+ current density. The full length A beta peptide (A beta 1-42) also increased Kv current. However, the control peptide, A beta 35-25, did not induce Kv current. Most of the Kv current induced by A beta was specifically blocked by the presence of antisense deoxyoligonucleotides against Kv1.3, and Kv1.5. Thus, it is concluded that we have identified Kv1.3 and Kv1.5 as the channel types expressed in A beta-treated microglia.

Antisense suppression of potassium channel expression demonstrates its role in maturation of the action potential

A developmental increase in delayed rectifier potassium current (I(Kv)) in embryonic Xenopus spinal neurons is critical for the maturation of excitability and action potential waveform. Identifying potassium channel genes that generate I(Kv) is essential to understanding the mechanisms by which they are controlled. Several Kv genes are upregulated during embryogenesis in parallel with increases in I(Kv) and produce delayed rectifier current when heterologously expressed, indicating that they could encode channels underlying this current. We used antisense (AS) cRNA to test the contribution of xKv3.1 to the maturation of I(Kv), because xKv3.1 AS appears to suppress specifically heterologous expression of potassium current by xKv3.1 mRNA. The injection of xKv3.1 AS into embryos reduces endogenous levels of xKv3.1 mRNA in the developing spinal cord and reduces the amplitude and rate of activation of I(Kv) in 40% of cultured neurons, similar to the percentage of neurons in which endogenous xKv3.1 transcripts are detected. The current in these mature neurons resembles that at an earlier stage of differentiation before the appearance of xKv3.1 mRNA. Furthermore, AS expression increases the duration of the action potential in 40% of the neurons. No change in voltage-dependent calcium current is observed, suggesting that the decrease in I(Kv) is sufficient to account for lengthening of the action potential. Computer-simulated action potentials incorporating observed reductions in amplitude and rate of activation of I(Kv) exhibit an increase in duration similar to that observed experimentally. Thus xKv3.1 contributes to the maturation of I(Kv) in a substantial percentage of these developing spinal neurons.

Antisense suppression of potassium channel expression demonstrates its role in maturation of the action potential

A developmental increase in delayed rectifier potassium current (I(Kv)) in embryonic Xenopus spinal neurons is critical for the maturation of excitability and action potential waveform. Identifying potassium channel genes that generate I(Kv) is essential to understanding the mechanisms by which they are controlled. Several Kv genes are upregulated during embryogenesis in parallel with increases in I(Kv) and produce delayed rectifier current when heterologously expressed, indicating that they could encode channels underlying this current. We used antisense (AS) cRNA to test the contribution of xKv3.1 to the maturation of I(Kv), because xKv3.1 AS appears to suppress specifically heterologous expression of potassium current by xKv3.1 mRNA. The injection of xKv3.1 AS into embryos reduces endogenous levels of xKv3.1 mRNA in the developing spinal cord and reduces the amplitude and rate of activation of I(Kv) in 40% of cultured neurons, similar to the percentage of neurons in which endogenous xKv3.1 transcripts are detected. The current in these mature neurons resembles that at an earlier stage of differentiation before the appearance of xKv3.1 mRNA. Furthermore, AS expression increases the duration of the action potential in 40% of the neurons. No change in voltage-dependent calcium current is observed, suggesting that the decrease in I(Kv) is sufficient to account for lengthening of the action potential. Computer-simulated action potentials incorporating observed reductions in amplitude and rate of activation of I(Kv) exhibit an increase in duration similar to that observed experimentally. Thus xKv3.1 contributes to the maturation of I(Kv) in a substantial percentage of these developing spinal neurons.

Expression of mRNA for voltage-dependent and inward-rectifying K channels in GH3/B6 cells and rat pituitary

The expression of mRNA for voltage-dependent (Kv) and inward-rectifying K channels (Kir) was studied in clonal rat somato-mammotroph cells (GH3/B6 cells) and rat pituitary using reverse transcription-polymerase chain reaction (RT-PCR). In GH3/B6 cells transcripts for 16 different Kv channel alpha-subunits (seven Shaker-related: Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv3.2, Kv4.1, Kv5.1; six EAG: eag1, erg1, erg2, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and for five different Kir channel alpha-subunits (Kir1.1, Kir2.3, Kir3.2, Kir3.3, Kir6.2) were found. In addition, transcripts for a short isoform of Kvbeta2 and transcripts for Kvbeta3 subunits were present. In rat pituitary transcripts for 21 different Kv channel alpha-subunits (11 Shaker-related: Kv1.3, Kv1.4, Kv1.6, Kv2.1, Kv2.2, Kv3.2, Kv3.4, Kv4.1, Kv4.2, Kv4.3, Kv6.1; seven EAG: eag1, erg1-erg3, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and nine Kir channel alpha-subunits (Kir1.1, Kir2.2, Kir3.1-Kir3.4, Kir4.1, Kir6.1, Kir6. 2) were found. In addition, all tested auxiliary subunits (Kvbeta1-Kvbeta3, minK, SUR1, SUR2) are expressed in the pituitary. The results indicate that the macroscopic K currents in GH3/B6 and pituitary cells are presumably mediated by K channels constructed by a larger number of K channel alpha-subunits and auxiliary beta-subunits than previously distinguished electrophysiologically and pharmacologically.

Frequency-dependent regulation of rat hippocampal somato-dendritic excitability by the K+ channel subunit Kv2.1

The voltage-dependent potassium channel subunit Kv2.1 is widely expressed throughout the mammalian CNS and is clustered primarily on the somata and proximal dendrites, but not axons, of both principal neurones and inhibitory interneurones of the cortex and hippocampus. This expression pattern suggests that Kv2.1-containing channels may play a role in the regulation of pyramidal neurone excitability. To test this hypothesis and to determine the functional role of Kv2. 1-containing channels, cultured hippocampal slices were incubated with antisense oligonucleotides directed against Kv2.1 mRNA. Western blot analysis demonstrated that Kv2.1 protein content of cultured slices decreased > 90 % following 2 weeks of treatment with antisense oligonucleotides, when compared with either control missense-treated or untreated cultures. Similarly, Kv2.1 immunostaining was selectively decreased in antisense-treated cultures. Sustained outward potassium currents, recorded in both whole-cell and outside-out patch configurations, demonstrated a selective reduction of amplitude only in antisense-treated CA1 pyramidal neurones. Under current-clamp conditions, action potential durations were identical in antisense-treated, control missense-treated and untreated slices when initiated by low frequency stimulation (0.2 Hz). In contrast, spike repolarization was progressively prolonged during higher frequencies of stimulation (1 Hz) only in cells from antisense-treated slices. Similarly, action potentials recorded during electrographic interictal activity in the 'high [K+]o' model of epilepsy demonstrated pronounced broadening of their late phase only in cells from antisense-treated slices. Consistent with the frequency-dependent spike broadening, calcium imaging experiments from single CA1 pyramidal neurones revealed that high frequency Schaffer collateral stimulation resulted in a prolonged elevation of dendritic [Ca2+]i transients only in antisense-treated neurones. These studies demonstrate that channels containing Kv2.1 play a role in regulating pyramidal neurone somato-dendritic excitability primarily during episodes of high frequency synaptic transmission.

Dynamic regulation of K+ channel gene expression in differentiated cells

Recent studies have determined that K+ channel gene expression is dynamically controlled in endocrine, cardiac, and neuronal cells. This regulation is induced by physiological stimuli (e.g., hormones, transmitters, depolarization), drugs (e.g., opiates) and with pathophysiological conditions (e.g., seizures, hypertension). In many cases, alterations in subunit expression are driven by transcriptional changes. Furthermore, resultant changes in excitability can be produced within hours because of the rapid turnover of Kv-channel proteins. Finally, the consequences of altering K+-channel subunit are complex because a single gene product can participate in forming functionally distinct homomeric and heteromeric channels in the same cell. Thus, regulating K+-channel genes constitutes a novel mechanism for producing intricate long-term changes in excitability.

Modulation of cardiac Na+ current phenotype by beta1-subunit expression

Na+ current (INa) is smaller, activates and inactivates more slowly, and displays less negative voltage dependence of inactivation in the neonatal rat than in the adult rat. We have observed very similar changes when INa is recorded as a function of time in culture in mouse atrial tumor (AT-1) cells. The differences between mature and immature INa are reminiscent of those observed when skeletal muscle Na+ channel alpha subunits are expressed alone (immature) or with the beta1 subunit (mature). In the present experiments, we tested the hypothesis that suppression of beta1-subunit expression by antisense oligonucleotides would prevent the development of a mature INa. The mouse beta1 subunit was cloned from an AT-1 cDNA library and found to be identical to that in the rat at 216/218 amino acids. AT-1 cells exposed to anti-beta1 antisense oligonucleotides displayed an immature INa at day 8 in culture, whereas untreated cells or cells exposed to sense oligonucleotides displayed a mature INa. This result was observed with 2 different oligonucleotides, and neither affected the rapidly activating component of the delayed rectifier K+ current, another current recorded in AT-1 cells. These findings indicate that in these cells, the gating of INa is modulated by beta1 expression and that alpha-beta1 coexpression is required for the development of a mature cardiac INa phenotype.

Specific antibodies to the external vestibule of voltage-gated potassium channels block current

Using delayed-rectifier potassium channels as examples, we have designed two specific blockers by generating specific antipeptide antibodies to epitopes in the external vestibules of two channel proteins, Kv1.2 and Kv3.1. These antibodies reduced whole-cell Kv1.2 or Kv3.1 currents in transfected cells and the effect was blocked by the corresponding peptide antigen, but not by control peptides. A control antibody had little effect on Kv1.2 currents and the Kv1.2 blocker antibody had limited effect on other related potassium currents. Furthermore, the Kv1.2 blocking antibody inhibited dendrotoxin binding to Kv1.2 channel proteins in transfected cells. Moreover, using the Kv1.2 blocker antibody, we determined the presence and relative contribution of endogenous Kv1.2 to the overall endogenous K+ currents in NG108 neuronal cells. This guided design of specific channel blockers will facilitate future physiological studies on ion channel functions.

Long QT and ventricular arrhythmias in transgenic mice expressing the N terminus and first transmembrane segment of a voltage-gated potassium channel

Voltage-gated potassium channels control cardiac repolarization, and mutations of K+ channel genes recently have been shown to cause arrhythmias and sudden death in families with the congenital long QT syndrome. The precise mechanism by which the mutations lead to QT prolongation and arrhythmias is uncertain, however. We have shown previously that an N-terminal fragment including the first transmembrane segment of the rat delayed rectifier K+ channel Kv1.1 (Kv1.1N206Tag) coassembles with other K+ channels of the Kv1 subfamily in vitro, inhibits the currents encoded by Kv1.5 in a dominant-negative manner when coexpressed in Xenopus oocytes, and traps Kv1.5 polypeptide in the endoplasmic reticulum of GH3 cells. Here we report that transgenic mice overexpressing Kv1.1N206Tag in the heart have a prolonged QT interval and ventricular tachycardia. Cardiac myocytes from these mice have action potential prolongation caused by a significant reduction in the density of a rapidly activating, slowly inactivating, 4-aminopyridine sensitive outward K+ current. These changes correlate with a marked decrease in the level of Kv1.5 polypeptide. Thus, overexpression of a truncated K+ channel in the heart alters native K+ channel expression and has profound effects on cardiac excitability.

Inhibition of rat ventricular IK1 with antisense oligonucleotides targeted to Kir2.1 mRNA

The cardiac inward rectifying K+ current (IK1) is important in maintaining the maximum diastolic potential. We used antisense oligonucleotides to determine the role of Kir2.1 channel proteins in the genesis of native rat ventricular IK1. A combination of two antisense phosphorothioate oligonucleotides inhibited heterologously expressed Kir2.1 currents in Xenopus oocytes, either when coinjected with Kir2.1 cRNA or when applied in the incubation medium. Specificity was demonstrated by the lack of inhibition of Kir2.2 and Kir2.3 currents in oocytes. In rat ventricular myocytes (4-5 days culture), these oligonucleotides caused a significant reduction of whole cell IK1 (without reducing the transient outward K+ current or the L-type Ca2+ current). Cell-attached patches demonstrated the occurrence of multiple channel events in control myocytes (8, 14, 21, 35, 43, and 80 pS). The 21-pS channel was specifically knocked down in antisense-treated myocytes (fewer patches contained this channel, and its open frequency was reduced). These results demonstrate that the Kir2.1 gene encodes a specific native 21-pS K(+)-channel protein and that this channel has an essential role in the genesis of cardiac IK1.

Suppression of neuronal and cardiac transient outward currents by viral gene transfer of dominant-negative Kv4.2 constructs

To probe the molecular identity of transient outward (A-type) potassium currents, we expressed a truncated version of Kv4.2 in heart cells and neurons. The rat Kv4.2-coding sequence was truncated at a position just past the first transmembrane segment and subcloned into an adenoviral shuttle vector downstream of a cytomegalovirus promoter (pE1Kv4.2ST). We hypothesized that this construct would act as a dominant-negative suppressor of currents encoded by the Kv4 family by analogy to Kv1 channels. Cotransfection of wild-type Kv4.2 with a beta-galactosidase expression vector in Chinese hamster ovary (CHO)-K1 cells produced robust transient outward currents (Ito) after two days (14.0 pA/pF at 50 mV, n = 5). Cotransfection with pE1Kv4.2ST markedly suppressed the Kv4.2 currents (0.8 pA/pF, n = 6, p < 0.02; cDNA ratio of 2:1 Kv4.2ST:wild type), but in parallel experiments, it did not alter the current density of coexpressed Kv1.4 or Kv1.5 channels. Kv4.2ST also effectively suppressed rat Kv4.3 current when coexpressed in CHO-K1 cells. We then engineered a recombinant adenovirus (AdKv4.2ST) designed to overexpress Kv4.2ST in infected cells. A-type currents in rat cerebellar granule cells were decreased two days after AdKv4. 2ST infection as compared with those infected by a beta-galactosidase reporter virus (116.0 pA/pF versus 281.4 pA/pF in Ad beta-galactosidase cells, n = 8 each group, p < 0.001). Likewise, Ito in adult rat ventricular myocytes was suppressed by AdKv4.2ST but not by Adbeta-galactosidase (8.8 pA/pF versus 21.4 pA/pF in beta-galactosidase cells, n = 6 each group, p < 0.05). Expression of a GFP-Kv4.2ST fusion construct enabled imaging of subcellular protein localization by confocal microscopy. The protein was distributed throughout the surface membrane and intracellular membrane systems. We conclude that genes from the Kv4 family are the predominant contributors to the A-type currents in cerebellar granule cells and Ito in rat ventricle. Overexpression of dominant-negative constructs may be of general utility in dissecting the contributions of various ion channel genes to excitability.

A cellular model for long QT syndrome. Trapping of heteromultimeric complexes consisting of truncated Kv1.1 potassium channel polypeptides and native Kv1.4 and Kv1.5 channels in the endoplasmic reticulum

We demonstrated that overexpression of a cRNA encoding a truncated potassium channel polypeptide that contains the NH2 terminus and the first transmembrane segment (Kv1.1N206Tag) abolished the expression of Kv1.1 and Kv1.5 outward currents in Xenopus oocytes (Babila, T., Moscucci, A., Wang, H., Weaver, F. E. & Koren, G. (1994) Neuron 12, 615-626). Recently, we showed that expression of Kv1.1N206Tag in the heart of transgenic mice resulted in the creation of mice with prolongation of the surface electrocardiogram's QT interval (London, B., Han, X., Folco, E. & Koren, G. (1996) Biophys. J. 70, A2601). To study the dominant negative mechanism of Kv1.1N206Tag, we overexpressed it in GH3 cells, a pituitary cell line expressing Kv1. 5 and Kv1.4. RNase protection analysis comparing the steady-state levels of native Kv1.5 and Kv1.1N206Tag transcripts revealed an excess of Kv1.1N206Tag transcript. Immunoprecipitation analysis using 12CA5 monoclonal antibody detected a 25-kDa polypeptide in the transfected cells. The half-life of Kv1.1N206Tag was 2.6 h. Subcellular fractionation of cell lysates labeled with [35S]methionine revealed that Kv1.1N206Tag polypeptide is detectable in the particulate (membranous) fraction, but not in the soluble (cytosol) fraction. A series of double immunoprecipitations with 12CA5 and polyclonal antibodies against Kv1.5 and Kv1.4 revealed that Kv1.1N206Tag forms heteromultimeric complexes with the native Kv1.4 and Kv1.5 polypeptides. The steady-state levels of Kv1.5 were not affected by the overexpression of Kv1.1N206Tag. Immunofluorescence colocalization and confocal microscopy analyses revealed that Kv1.1N206TagFlag did not reach the plasma membrane, and its distribution pattern was characteristic to that of a resident endoplasmic reticulum polypeptide. Our observations establish that the negative effect of Kv1.1N206Tag is mediated by the formation of heteromultimeric complexes with the native channels and by the retention of these complexes in the endoplasmic reticulum.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Blockade of clomipramine and amitriptyline analgesia by an antisense oligonucleotide to mKv1.1, a mouse Shaker-like K+ channel

The effect of an antisense oligonucleotide to the K+ channel coding mKv1.1 mRNA on antinociception induced by the tricyclic antidepressants, clomipramine (20-35 mg kg(-1) s.c.) and amitriptyline (10-25 mg kg(-1) s.c.), was investigated in the mouse hot-plate test. Antisense oligonucleotide (0.5-1.0-2.0-3.0 nmol per i.c.v. injection) produced a dose-dependent inhibition of clomipramine and amitriptyline antinociception 72 h after the last i.c.v. injection. The sensitivity to both analgesic drugs returned 7 days after antisense oligonucleotide injection, indicating the absence of irreversible damage or toxicity. Treatment with a degenerated oligonucleotide did not modify the clomipramine- and amitriptyline-induced antinociception in comparison with that in naive (unpretreated controls), vector and saline i.c.v.-injected mice. A quantitative reverse transcription-polymerase chain reaction (RT-PCR) study demonstrated a reduction in mRNA levels only in the antisense oligonucleotide treated group. Antisense oligonucleotide, degenerated oligonucleotide or vector pretreatment, in the range of doses used, did not produce any behavioural impairment as revealed by the mouse rotarod and hole-board tests. The present results indicate that modulation of the mKv1.1 K+ channel plays an important role in the central analgesia induced by the tricyclic antidepressants, clomipramine and amitriptyline.

Shal-type channels contribute to the Ca2+-independent transient outward K+ current in rat ventricle

1. The hypothesis that Kv4.2 and Kv4.3 are two of the essential K+ channel isoforms underlying the Ca2+-independent transient outward K+ current (It) in rat ventricle has been tested using a combination of electrophysiological measurements and antisense technology in both native myocytes and a stably transfected mammalian cell line, mouse Ltk- cells (L-cells). 2. The transient outward currents generated by Kv4.2 channels in L-cells exhibit rapid activation and inactivation properties similar to those produced by It in rat ventricular cells. The current-voltage relationships and the voltage dependence of steady-state inactivation are also very similar in these two preparations. However, the recovery from inactivation of Kv4.2 is much slower (time constant, 378 ms) than that of It in rat ventricular cells (58 ms). 3. The K+ current due to Kv4.2 can be blocked by millimolar concentrations of 4-aminopyridine in L-cells; a similar pharmacological response has been observed in rat ventricular myocytes. 4. Quinidine inhibits Kv4.2 in L-cells and It in rat ventricular cells in a similar fashion. In L-cells quinidine reduced the amplitude of Kv4.2 and accelerated its time course of inactivation, suggesting that quinidine may act as an open channel blocker of Kv4.2, as has been described for It in rat ventricle. 5. To provide further independent evidence that Kv4.2 and Kv4.3 channel isoforms contribute to It in rat ventricular cells, the effects of 20-mer antisense phosphorothioate oligodeoxynucleotides directed against Kv4.2 and Kv4.3 mRNAs were examined in ventricular myocytes isolated from 14- and 20-day-old rats, and in L-cells. In both preparations, Kv4.2 antisense pretreatment significantly reduced the transient outward K+ current (by approximately 55-60%). Similar reduction of It was produced by the Kv4.3 antisense oligonucleotide on the 14-day-old rat myocytes. 6. In 14-day rat ventricular cells, combination of Kv4.2 and Kv4.3 antisense oligonucleotides did not produce a significantly larger reduction of It than that observed after pretreatment with either antisense oligonucleotide alone. 7. L-cells stably transfected with Kv4.2 were treated with Kv4.3 antisense oligonucleotide to evaluate the possibility of cross-reactivity between Kv4.3 antisense and Kv4.2 mRNA. This antisense treatment produced no change in It, verifying the lack of cross-reactivity. 8. These biophysical and pharmacological results together with the antisense data show that Kv4.2 and Kv4.3 are essential components of the Ca2+-independent transient outward K+ current, It, in rat ventricular myocytes.

Antisense oligodeoxynucleotides directed against Kv1.5 mRNA specifically inhibit ultrarapid delayed rectifier K+ current in cultured adult human atrial myocytes

Several cloned K+ channel subunits are candidates to underlie macroscopic currents in the human heart, but direct evidence bearing on their role is lacking. The Kv1.5 K+ channel subunit has been suggested to play a potential role in human cardiac ultrarapid delayed rectifier (IKur) and transient outward (Ito) currents. To evaluate the role of proteins encoded by the Kv1.5 gene, we incubated cultured human atrial myocytes for 48 hours in medium containing antisense phosphorothioate oligodeoxynucleotides directed against octodecameric segments of the Kv1.5 mRNA coding sequence, the same concentration of homologous oligodeoxynucleotides with four mismatch mutations, or vehicle (control group). Cells exposed to antisense showed a highly significant (approximately 50%) reduction in IKur whether measured by step current at the end of a 400-millisecond depolarizing pulse, tail current at -20 mV, or current sensitive to a concentration of 4-aminopyridine (50 mumol/L) that is highly selective for IKur compared with control cells or cells exposed to mismatch oligodeoxynucleotides. In contrast, Ito was not different among the three experimental groups. When cultured human ventricular myocytes were exposed to Kv1.5 antisense oligodeoxynucleotides with the same controls, no changes occurred in either Ito or the sustained current at the end of a depolarizing pulse. We conclude that Kv1.5 channel subunits are essential to the expression of IKur and do not play a role in Ito in cultured human atrial myocytes. These studies provide the first direct evidence with an antisense approach for the equivalence between a macroscopic cardiac K+ current and a cloned K+ channel subunit and offer insights into the molecular electrophysiology of the human heart.

Manipulation of the delayed rectifier Kv1.5 potassium channel in glial cells by antisense oligodeoxynucleotides

Glial cells have been shown to express several biophysically and pharmacology distinct potassium channel types. However, the molecular identity of most glial K+ channels is unknown. We have developed an antibody specific for the Shaker type potassium channel Kv1.5 protein, and demonstrate by immunohistochemistry the presence of this channel in glial cells of adult rat hippocampal and cerebellar slices, as well as in cultured spinal cord astrocytes. Immunoreactivity was particularly intense in the endfoot processes of astrocytes surrounding the microvasculature of the hippocampus. The specific contribution of this channel protein to the delayed rectifying K+ current of spinal cord astrocytes was determined by incubating these cells with antisense oligodeoxynucleotides complementary to the mRNA coding for Kv1.5 protein. Such treatment reduced delayed rectifier current density and shifted the potassium current steadystate inactivation, without altering current activation, cell capacitance, or cell resting potential. The tetraethylammonium acetate (TEA) sensitivity of astrocytic delayed rectifier current was enhanced following antisense oligodeoxynucleotide treatment, suggesting that Kv1.5 channel protein may provide a significant component of the TEA-insensitive current in this preparation. Our results suggest that Kv1.5 is widely expressed in glial cells of brain and spinal cord and that delayed rectifying K+ currents in astrocytes are largely mediated by Kv1.5 channel protein.

Temporal regulation of Shaker- and Shab-like potassium channel gene expression in single embryonic spinal neurons during K+ current development

A developmental increase in density of delayed rectifier potassium current (IKv) in embryonic Xenopus spinal neurons shortens action potential durations and limits calcium influx governing neuronal differentiation. Although previous work demonstrates that maturation of IKv depends on general mRNA synthesis, it is not known whether increases in K+ channel gene transcripts direct maturation of the current. Accordingly, the developmental appearance of specific Kv potassium channel genes was determined using single-cell reverse transcription-PCR techniques after whole-cell recording of IKv during the period of its development. Detection of a coexpressed housekeeping gene along with the potassium channel gene controlled for successful aspiration of cellular mRNA and allowed scoring of cells in which Kv gene transcripts were not detected. Diverse types of Xenopus spinal neurons exhibit homogeneous development of IKv both in vivo and in culture. In contrast, transcripts of two genes encoding delayed rectifier current, Kv1.1 (Shaker) and Kv2.2 (Shab), are expressed heterogeneously during the period in which the current develops. Kv1.1 mRNA achieves maximal appearance in approximately 30% of cells, while IKv is immature; Kv2.2 mRNA appears later in approximately 60% of mature neurons. Kv1.1 and 2.2 are thus candidates for generation of IKv, and spinal neurons are a heterogeneous population with respect to potassium channel gene expression. Moreover, correlation of gene expression with current properties shows that neurons lacking Kv2.2 have a characteristic voltage dependence of activation of IKv.

Temporal regulation of Shaker- and Shab-like potassium channel gene expression in single embryonic spinal neurons during K+ current development

A developmental increase in density of delayed rectifier potassium current (IKv) in embryonic Xenopus spinal neurons shortens action potential durations and limits calcium influx governing neuronal differentiation. Although previous work demonstrates that maturation of IKv depends on general mRNA synthesis, it is not known whether increases in K+ channel gene transcripts direct maturation of the current. Accordingly, the developmental appearance of specific Kv potassium channel genes was determined using single-cell reverse transcription-PCR techniques after whole-cell recording of IKv during the period of its development. Detection of a coexpressed housekeeping gene along with the potassium channel gene controlled for successful aspiration of cellular mRNA and allowed scoring of cells in which Kv gene transcripts were not detected. Diverse types of Xenopus spinal neurons exhibit homogeneous development of IKv both in vivo and in culture. In contrast, transcripts of two genes encoding delayed rectifier current, Kv1.1 (Shaker) and Kv2.2 (Shab), are expressed heterogeneously during the period in which the current develops. Kv1.1 mRNA achieves maximal appearance in approximately 30% of cells, while IKv is immature; Kv2.2 mRNA appears later in approximately 60% of mature neurons. Kv1.1 and 2.2 are thus candidates for generation of IKv, and spinal neurons are a heterogeneous population with respect to potassium channel gene expression. Moreover, correlation of gene expression with current properties shows that neurons lacking Kv2.2 have a characteristic voltage dependence of activation of IKv.