Neuronal Voltage-gated Potassium Channels Research Articles

PO27-TH-15 Frequency of false positive neuronal voltage-gated potassium channel antibody detection in serological evaluation for neurological autoimmunity

PO27-TH-15 Frequency of false positive neuronal voltage-gated potassium channel antibody detection in serological evaluation for neurological autoimmunity

Atypical sialylated N-glycan structures are attached to neuronal voltage-gated potassium channels

Mammalian brains contain relatively high amounts of common and uncommon sialylated N-glycan structures. Sialic acid linkages were identified for voltage-gated potassium channels, Kv3.1, 3.3, 3.4, 1.1, 1.2 and 1.4, by evaluating their electrophoretic migration patterns in adult rat brain membranes digested with various glycosidases. Additionally, their electrophoretic migration patterns were compared with those of NCAM (neural cell adhesion molecule), transferrin and the Kv3.1 protein heterologously expressed in B35 neuroblastoma cells. Metabolic labelling of the carbohydrates combined with glycosidase digestion reactions were utilized to show that the N-glycan of recombinant Kv3.1 protein was capped with an oligo/poly-sialyl unit. All three brain Kv3 glycoproteins, like NCAM, were terminated with alpha2,3-linked sialyl residues, as well as atypical alpha2,8-linked sialyl residues. Additionally, at least one of their antennae was terminated with an oligo/poly-sialyl unit, similar to recombinant Kv3.1 and NCAM. In contrast, brain Kv1 glycoproteins consisted of sialyl residues with alpha2,8-linkage, as well as sialyl residues linked to internal carbohydrate residues of the carbohydrate chains of the N-glycans. This type of linkage was also supported for Kv3 glycoproteins. To date, such a sialyl linkage has only been identified in gangliosides, not N-linked glycoproteins. We conclude that all six Kv channels (voltage-gated K+ channels) contribute to the alpha2,8-linked sialylated N-glycan pool in mammalian brain and furthermore that their N-glycan structures contain branched sialyl residues. Identification of these novel and unique sialylated N-glycan structures implicate a connection between potassium channel activity and atypical sialylated N-glycans in modulating and fine-tuning the excitable properties of neurons in the nervous system.

Voltage-gated potassium channel autoimmunity mimicking creutzfeldt-jakob disease.

Rapidly progressive dementia has a variety of causes, including Creutzfeldt-Jakob disease (CJD) and neuronal voltage-gated potassium channel (VGKC) autoantibody-associated encephalopathy. To describe patients thought initially to have CJD but found subsequently to have immunotherapy-responsive VGKC autoimmunity. Observational, prospective case series. Department of Neurology, Mayo Clinic, and the Memory and Aging Center, University of California, San Francisco. Patients A clinical serologic cohort of 15 patients referred for paraneoplastic autoantibody evaluation. Seven patients were evaluated clinically by at least one of us. Clinical information for the remaining patients was obtained by physician interview or medical record review. Clinical features, magnetic resonance imaging abnormalities, electroencephalographic patterns, cerebrospinal fluid analyses, and responses to immunomodulatory therapy. All the patients presented subacutely with neurologic manifestations, including rapidly progressive dementia, myoclonus, extrapyramidal dysfunction, visual hallucinations, psychiatric disturbance, and seizures; most (60%) satisfied World Health Organization diagnostic criteria for CJD. Magnetic resonance imaging abnormalities included cerebral cortical diffusion-weighted imaging hyperintensities. Electroencephalographic abnormalities included diffuse slowing, frontal intermittent rhythmic delta activity, and focal epileptogenic activity but not periodic sharp wave complexes. Cerebrospinal fluid 14-3-3 protein or neuron-specific enolase levels were elevated in 5 of 8 patients. Hyponatremia was common (60%). Neoplasia was confirmed histologically in 5 patients (33%) and was suspected in another 5. Most patients' conditions (92%) improved after immunomodulatory therapy. Clinical, radiologic, electrophysiologic, and laboratory findings in VGKC autoantibody-associated encephalopathy may be confused with those of CJD. Serologic evaluation for markers of neurologic autoimmunity, including VGKC autoantibodies, may be warranted in suspected CJD cases.

Serologic Profiles Aiding the Diagnosis of Autoimmune Gastrointestinal Dysmotility

Autoimmune gastrointestinal dysmotility is a limited autoimmune dysautonomia occurring idiopathically or in the context of an anatomically remote neoplasm, previously documented or unsuspected. Here we report 24 Mayo Clinic patients in whom the profile of serum autoantibodies aided this diagnosis. All patients were ascertained serologically in the course of service evaluation for autoantibodies consistent with neurologic autoimmunity. Review of their histories, motility studies, and laboratory findings revealed that all had presented with subacute gastrointestinal dysmotility. Recorded motility abnormalities included esophageal dysmotility 8 (6 had achalasia), delayed gastric emptying 12, slow small intestinal transit 7, slow colonic transit 4, and pelvic floor dyssynergia 4. Four patients underwent abdominal surgery; 2 commenced total parenteral nutrition. Plasma membrane cation channel autoantibodies were detected in 23 patients: neuronal voltage-gated calcium channel (5 N-type and 1 P/Q-type), acetylcholine receptor (11 ganglionic-type and 4 muscle-type), and neuronal voltage-gated potassium channel autoantibodies (4). Two patients had antineuronal nuclear autoantibodies, type 1. Approximately half of the patients had neural autoantibodies (including skeletal muscle striational and glutamic acid decarboxylase, 65kd isoform) or other antibody markers of organ-specific autoimmunity (thyroid or gastric parietal cell specificities). Neoplasia was diagnosed in 11 patients (9 recent, 2 remote): lung, breast and endometrial, gastrointestinal and thymoma. Moderate to dramatic improvement in gastrointestinal symptoms was reported after immunotherapy in 4 of 4 patients treated and after pyridostigmine treatment in 2 of 2 patients treated. Autoimmune serology aids the diagnosis of autoimmune gastrointestinal dysmotility, both paraneoplastic and idiopathic, and might guide management.

Surface expression and distribution of voltage-gated potassium channels in neurons (Review)

The last decade has witnessed an exponential increase in interest in one of the great mysteries of nerve cell biology: Specifically, how do neurons know where to place the ion channels that control their excitability? Many of the most important insights have been gleaned from studies on the voltage-gated potassium channels (Kvs) which underlie the shape, duration and frequency of action potentials. In this review, we gather recent evidence on the expression, trafficking and maintenance mechanisms which control the surface density of Kvs in different subcellular compartments of neurons and how these may be regulated to control cell excitability.

Potassium Channel Antibody–Associated Encephalopathy Presenting With a Frontotemporal Dementia–like Syndrome

To describe a patient who presented with features suggestive of frontotemporal dementia (FTD) but with some atypical findings and antibodies to neuronal voltage-gated potassium channels (VGKC-Abs). Case report. Mater Misericordiae University Hospital, Dublin, Ireland. An 82-year-old man presented with progressive changes in personality, social conduct, and executive function with preservation of memory, deteriorating from baseline to requiring acute hospitalization within 6 months. Transient deterioration (episodic speech arrest) with spontaneous recovery, atypical for frontotemporal dementia, was observed. The patient had an elevated VGKC-Ab titer (2624 pM [normal range, < 100 pM]), elevated protein levels in cerebrospinal fluid, and a negative evaluation for malignancy. Magnetic resonance imaging of brain was normal but [(18)F]-fluorodeoxyglucose positron emission tomographic imaging revealed bifrontal hypometabolism. A marked and sustained improvement with steroid therapy was observed. Workup for a potentially reversible autoimmune-mediated encephalopathy, including a VGKC-Ab titer, should be considered in patients presenting with rapidly progressive behavioral and cognitive decline.

Whole-Body Tremulousness

Acquired generalized repetitive myoclonus may be mistaken for tremor. Distinguishing myoclonus has etiologic and therapeutic implications. To describe isolated generalized polymyoclonus and the outcomes of etiologic evaluations at the time of diagnosis. Computer search of the Mayo Movement Neurophysiology Laboratory database and medical records linkage system. Department of Neurology, Mayo Clinic. Nineteen adults with generalized repetitive myoclonus confirmed using surface electromyography (burst duration <50 milliseconds), and other neurologic features minimal or absent. Treatment of myoclonus and underlying causes. Clinical presentation and underlying etiologies. We identified 19 patients with isolated generalized polymyoclonus resembling whole-body tremor. Onset was most often subacute (12 patients), mean symptom duration was 1.8 years, and mean age at onset was 55 years. Referral diagnoses or patient complaints were tremor, tremulousness, or shaking in all but 5 patients. All the patients had repetitive myoclonus of all limbs, impairing gait in 14 patients. Surface electromyography confirmed nonperiodic muscle burst durations of less than 50 milliseconds, typical of myoclonus. Clinical and serologic screening for cancer and autoimmunity revealed metastatic breast cancer in 2 patients (1 positive for ganglionic acetylcholine receptor antibody) and antibody profiles implicating neurologic autoimmunity in 3 patients (CRMP-5 IgG or neuronal voltage-gated potassium channel antibodies). Medications known to occasionally trigger myoclonus (opioids, selective serotonin reuptake inhibitors, and a serotonin-norepinephrine reuptake inhibitor) were being taken by 7 patients. Myoclonus resolved after discontinuation of selective serotonin reuptake inhibitor therapy in 1 patient; drug discontinuation was declined and follow-up was inadequate in the other 6. Isolated whole-body tremulousness should raise the suspicion of generalized polymyoclonus, confirmed using routine surface electromyography. Recognition is important because the differential diagnosis includes autoimmunity and drug-induced myoclonus.

Inherited neuromyotonia: A clinical and genetic study of a family

Neuromyotonia is a disorder of peripheral nerve hyperexcitability characterized by myokymia, muscle cramps and stiffness, delayed muscle relaxation after contraction (pseudomyotonia), and hyperhidrosis, associated with well described spontaneous electromyographic features. It is usually an acquired disorder associated with autoantibodies against neuronal voltage-gated potassium channels. However, mutations of KCNA1, encoding the K + channel subunit hKv1.1, have been reported in rare families with neuromyotonia, and mutations in KCNQ2, encoding voltage-gated potassium M channel subunit, in families with benign neonatal seizures and myokymia. We report a three-generation family with inherited neuromyotonia without evidence of immunological involvement. Genetic study excluded mutations in KCNA1, KCNA2, KCNA6 and KCNQ2 genes. Our study does not completely exclude the involvement of other genes encoding ion channels subunits in the pathogenesis of this disorder. Further studies of familial cases will shed light on the molecular basis of inherited neuromyotonia.

Voltage-gated potassium channel antibody-associated encephalitis with basal ganglia lesions

Antibodies reactive with neuronal voltage-gated potassium channels (VGKCs) have been reported in patients with limbic encephalitis, which affects the medial temporal lobes and is characterized by subacute onset of temporal lobe seizures, memory impairment, and personality changes.1–5 MRI of those patients showed only abnormal signal intensity located in the medial temporal lobes.1–5 We describe a patient with encephalitis associated with anti-VGKC antibody who had MRI abnormalities localized in the basal ganglia as well as in the medial temporal lobes that improved dramatically on corticosteroid treatment. A previously healthy, 23-year-old woman presented with a generalized seizure and was admitted to a nearby hospital, 2 months after onset of repeated short-term fevers with cervical lymphadenopathy. She showed disorientation and memory loss. Brain CT was negative. A CSF examination showed pleocytosis (12 lymphocytes/mm3) and a normal protein level. Viral encephalitis was diagnosed, and she was prescribed IV acyclovir. She continued to show severe memory loss and childishness. Brain MRI done 17 days after admission showed symmetric abnormal signal intensities localized in the bilateral …

Retigabine

Retigabine has anticonvulsant properties that appear to be primarily mediated by opening neuronal voltage-gated potassium channels. This action has been shown in neuronal KCNQ2/3 and KCNQ3/5 potassium channels. In addition to this unique action, retigabine also potentiates GABA-evoked currents in cortical neurons at high concentrations. When used as adjunctive therapy in patients with partial seizures, retigabine 600-1200 mg/day (200-400 mg three times daily) was associated with significant linear dose-dependent reductions in monthly seizure frequency compared with placebo in a large 16-week randomised phase II trial. Median monthly seizure frequency decreased from baseline by up to 35% among patients in the retigabine treatment arms compared with 13% in the placebo group. Retigabine 1200 mg/day was also significantly more effective than retigabine 600 mg/day. Responder rates, defined as the proportion of patients with > or = 50% reduction in seizure frequency, were significantly higher among patients in the retigabine 900 and 1200 mg/day groups than in those who received placebo. CNS-related adverse events were the most commonly reported treatment-emergent adverse events associated with retigabine in clinical trials. Across all three retigabine groups in the large phase II trial, somnolence (20.3%), dizziness (14.6%), confusion (12.3%) and speech disorder (11.3%) were the most frequent CNS-related adverse events.

No evidence for association between the KCNQ3 gene and susceptibility to idiopathic generalized epilepsy

Idiopathic generalized epilepsy (IGE) comprises a heterogeneous group of disorders, in which a high genetic predisposition and a complex mode of inheritance have been suggested. Recent identification of ion channel gene mutations in Mendelian epileptic disorders suggests genetically driven neuronal hyperexcitability as one important factor in epileptogenesis. Mutations in two neuronal voltage-gated potassium channel genes ( KCNQ2 and KCNQ3) have already been shown to cause epilepsy (BFNC), and we now tested the hypothesis that genetic variation in the KCNQ3 gene confers liability to common IGE subtypes. Length variation of two intragenic polymorphic markers ( D8S558 and D8S1835) were therefore assessed in 71 nuclear families ascertained for an affected child. However, the transmission-disequilibrium-test did not show significant differences between the transmitted and non-transmitted parental alleles. Thus, our findings do not provide evidence that genetic variation in the KCNQ3 gene exerts a relevant effect in the etiology of common IGE subtypes.

Inhibition of hKv2.1, a major human neuronal voltage-gated K + channel, by meclofenamic acid

Using the standard patch clamp whole cell recording method, we assessed the pharmacological activity of four fenamate nonsteroidal anti-inflammatory drugs, meclofenamic acid, flufenamic acid, mefenamic acid and niflumic acid, on hKv2.1, a major human neuronal voltage-gated potassium channel stably expressed heterologously in Chinese hamster ovary cells. Meclofenamic acid inhibited hKv2.1 in a concentration-dependent manner whereas the other three fenamates had weaker or no effect on these channels at a concentration of 100 μM. The estimated IC 50 of meclofenamic acid was 56.0 μM for hKv2.1 compared an IC 50 of 155.9 μM for another human neuronal K channel (hKv1.1). Meclofenamic acid reached its maximum inhibition within 5 min of bath application and its effect was readily reversed upon wash. Kinetic analysis revealed that this drug did not alter the channel activation or deactivation time courses. Moreover, the effect of meclofenamic acid on hKv2.1 channels was not voltage-dependent. Indomethacin, another inhibitor of the cyclooxygenase that catalyses the synthesis of prostaglandin from arachidonic acid, had no effect on either hKv2.1 or hKv1.1. These results indicate that meclofenamic acid inhibits hKv2.1 more potently than hKv1.1 and it is likely that this compound acts directly on the channel proteins.

Regulation of a human neuronal voltage-gated potassium channel (hKv1.1) by protein tyrosine phosphorylation and dephosphorylation.

Regulation of a human neuronal voltage-gated potassium channel (hKv1.1) by protein tyrosine phosphorylation and dephosphorylation.

Mouse brain potassium channel ?1 subunit mRNA: Cloning and distribution during development

The pore-forming alpha subunits of voltage-gated potassium channels in neurons and other excitable cells are expressed in association with accessory beta subunits. These subunits both promote insertion of channel complexes into surface membranes and influence their electrophysiological properties. As part of an effort to understand the regulation of voltage-gated potassium channels during development, we cloned the mouse homolog of the rat Kvbeta1 potassium channel subunit. Kvbeta1 subunits are known to associate preferentially with Shaker (Kv1)-related alpha subunits. We then used a digoxigenin-tagged cRNA probe and in situ hybridization techniques to visualize the appearance of Kvbeta1 mRNA transcripts during late embryonic and early neonatal development of the mouse brain. We detected Kvbeta1-specific labeling of cells in hippocampus, cerebral cortex, caudate putamen, colliculus, and cerebellum. In hippocampus, we observed Kvbeta1 mRNA in CA3 pyramidal neurons at the earliest time examined, embryonic day 16 (E16). Between E16 and postnatal day 7 (P7), cell labeling increased uniformly across the pyramidal neurons of Ammon's horn (CA1, CA2, and CA3). Subsequently, between P7 and P22, regional differences characteristic of mature hippocampus appeared-intense labeling of neurons in CA3 and CA1, and less in CA2. In cortex, labeling of cells in the subplate and cortical plate layers was observed at E16. During development, the intensity of this labeling increased, and labeled cells persisted into the adult stage in the deep cortical layer (VIb) formed from subplate neurons. Additional labeling of scattered solitary cells in cortical layers II-VIa emerged between P3 and P7 and was prominent in mature cortex. In caudate putamen, Kvbeta1-labeled cells were observed at P1 and were restricted to the lateral and rostral half of the caudate. During development, labeling expanded caudally and medially and eventually filled the mature caudate putamen. In colliculus, a small population of inferior colliculus cells showed labeling at P7, and additional labeling of scattered cells appeared during development. In superior colliculus, labeling was observed only in the adult deep gray layer. In cerebellum, intense labeling was observed in Purkinje cells at all stages between P1 and adult. Labeling was also seen in granule neurons in the external granule layer at early postnatal stages and in the inner granule layer beginning at P7.

Modulation of cloned human neuronal voltage-gated potassium channels (hKv1.1 and hKv2.1) by neurosteroids.

Two voltage-gated potassium channels, hKv1.1 and hKv2.1, were stably expressed in Chinese hamster ovary cells. The effects of neurosteroids, pregnenolone sulfate (PS), 5alpha-pregnan-3alpha-ol-20-one sulfate (5alpha3alphaP), and 5beta-pregnan-3alpha-ol-20-one sulfate (5beta3alphaP), on hKv1.1 and hKv2.1 were studied using the patch-clamp method. PS, 5alpha3alphaP, and 5beta3alphaP shifted current/voltage (I/V) relationship of both channels to the left by about 10 mV. The neurosteroids also increased the current amplitude voltage dependently. The activation time constants (taua) for hKv1.1 and hKv2.1 were voltage dependent and decreased with membrane depolarization. At -20 mV, PS reduced taua for hKv1.1 by 8 ms and 15 ms at 10 and 100 microM respectively. PS also significantly decreased the taua of hKv2.1 by 30 ms (10 microM) and 38 ms (100 microM) at +10 mV. However, only at 100 microM did 5alpha3alphaP or 5beta3alphaP significantly alter the taua of hKv1.1. Neither 5alpha3alphaP nor 5beta3alphaP affected the taua of hKv2.1. Deactivation time constants (taud) were also voltage dependent and, in contrast to taua, taud was prolonged with membrane depolarization. At 100 microM, PS, 5alpha3alphaP, and 5beta3alphaP significantly increased the taud for hKv1.1 but did not affect the taud for hKv2.1.

Tyrosine phosphorylation modulates current amplitude and kinetics of a neuronal voltage-gated potassium channel.

The modulation of the Kv1.3 potassium channel by tyrosine phosphorylation was studied. Kv1.3 was expressed in human embryonic kidney (HEK 293) cells, and its activity was measured by cell-attached patch recording. The amplitude of the characteristic C-type inactivating Kv1.3 current is reduced by >95%, in all cells tested, when the channel is co-expressed with the constitutively active nonreceptor tyrosine kinase, v-Src. This v-Src-induced suppression of current is accompanied by a robust tyrosine phosphorylation of the channel protein. No suppression of current or tyrosine phosphorylation of Kv1.3 protein is observed when the channel is co-expressed with R385A v-Src, a mutant with severely impaired tyrosine kinase activity. v-Src-induced suppression of Kv1.3 current is relieved by pretreatment of the HEK 293 cells with two structurally different tyrosine kinase inhibitors, herbimycin A and genistein. Furthermore, Kv1.3 channel protein is processed properly and targeted to the plasma membrane in v-Src cotransfected cells, as demonstrated by confocal microscopy using an antibody directed against an extracellular epitope on the channel. Thus v-Src-induced suppression of Kv1.3 current is not mediated through decreased channel protein expression or interference with its targeting to the plasma membrane. v-Src co-expression also slows the C-type inactivation and speeds the deactivation of the residual Kv1.3 current. Mutational analysis demonstrates that each of these modulatory changes, in current amplitude and kinetics, requires the phosphorylation of Kv1.3 at multiple tyrosine residues. Furthermore, a different combination of tyrosine residues is involved in each of the modulatory changes. These results emphasize the complexity of signal integration at the level of a single ion channel.

The effects of piracetam and its novel peptide analogue GVS-111 on neuronal voltage-gated calcium and potassium channels

1. 1. With the use of the two-microelectrode voltage-clamp method, three types of voltage-activated ionic currents were examined in isolated neurons of the snail Helix pomatia: high-threshold Ca 2+ current ( I ca ), high-threshold Ca 2+-dependent K + current ( I K( Ca) ) and high-threshold K + current independent of Ca 2+ ( I K( v) ). 2. 2. The effect of bath application of the nootropics piracetam and a novel piracetam peptide analog, ethyl ester of N-phenyl-acetyl-l-prolyl-glycine (GVS-111), on these three types of voltage-activated ionic currents was studied. 3. 3. In more than half of the tested cells, I ca was resistant to both piracetam and GVS-111. In the rest of the cells, I Ca decreased 19±7% with 2 mM of piracetam and 39±14% with 2 μM of GVS-111. 4. 4. I K( V) in almost all cells tested was resistant to piracetam at concentrations up to 2 μM. However, I K( V) in two-thirds of the cells was sensitive to GVS-111, being supressed 49±18% with 1 μM GVS-111. 5. 5. I K( ca) appeared to be the most sensitive current of those studied to both piracetam and GVS-111. Piracetam at 1 mM and GVS-111 at 0.1 μM decreased the amplitude of I K( Ca) in most of the cells examined by 49±19% and 69±24%, respectively. 6. 6. The results suggest that piracetam and GVS-111 suppression of voltage-activated calcium and potassium currents of the neuronal membrane may regulate (both up and down) Ca 2+influx into neurons.