Voltage-gated potassium channels and the diversity of electrical signalling

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

Biochemical and physiological properties of K+ channel-associated AKR6A (Kvβ) proteins

Voltage-gated potassium (Kv) channels play an essential role in the regulation of membrane excitability and thereby control physiological processes such as cardiac excitability, neural communication, muscle contraction, and hormone secretion. Members of the Kv1 and Kv4 families are known to associate with auxiliary intracellular Kvβ subunits, which belong to the aldo-keto reductase superfamily. Electrophysiological studies have shown that these proteins regulate the gating properties of Kv channels. Although the three gene products encoding Kvβ proteins are functional enzymes in that they catalyze the nicotinamide adenine dinucleotide phosphate (NAD[P]H)-dependent reduction of a wide range of aldehyde and ketone substrates, the physiological role for these proteins and how each subtype may perform unique roles in coupling membrane excitability with cellular metabolic processes remains unclear. Here, we discuss current knowledge of the enzymatic properties of Kvβ proteins from biochemical studies with their described and purported physiological and pathophysiological influences.

Involvement of sex hormonal regulation of K+ channels in electrophysiological and contractile functions of muscle tissues

Sex hormones, such as testosterone, progesterone, and 17β-estradiol, control various physiological functions. This review focuses on the sex hormonal regulation of K⁺ channels and the effects of such regulation on electrophysiological and contractile functions of muscles. In the cardiac tissue, testosterone and progesterone shorten action potential, and estrogen lengthens QT interval, a marker of increased risk of ventricular tachyarrhythmias. We have shown that testosterone and progesterone in physiological concentration activate KCNQ1 channels via membrane-delimited sex hormone receptor/eNOS pathways to shorten the action potential duration. Mitochondrial K⁺ channels are also involved in the protection of cardiac muscle. Testosterone and 17β-estradiol directly activate mitochondrial inner membrane K⁺ channels (Ca²⁺ activated K⁺ channel (K_Ca channel) and ATP-sensitive K⁺ channel (K_ATP channel)) that are involved in ischemic preconditioning and cardiac protection. During pregnancy, uterine blood flow increases to support fetal growth and development. It has been reported that 17β-estradiol directly activates large-conductance Ca²⁺-activated K⁺ channel (BK_Ca channel) attenuating arterial contraction. Furthermore, 17β-estradiol increases expression of BK_Ca channel β1 subunit which enhances BK_Ca channel activity by DNA demethylation. These findings are useful for understanding the mechanisms of sex or generation-dependent differences in the physiological and pathological functions of muscles, and the mechanisms of drug actions.

Genetically Encoded Voltage Indicators Are Illuminating Subcellular Physiology of the Axon

Everything we see and do is regulated by electrical signals in our nerves and muscle. Ion channels are crucial for sensing and generating electrical signals. Two voltage-dependent conductances, Na+ and K+, form the bedrock of the electrical impulse in the brain known as the action potential. Several classes of mammalian neurons express combinations of nearly 100 different varieties of these two voltage-dependent channels and their subunits. Not surprisingly, this variability orchestrates a diversity of action potential shapes and firing patterns that have been studied in detail at neural somata. A remarkably understudied phenomena exists in subcellular compartments of the axon, where action potentials initiate synaptic transmission. Ion channel research was catalyzed by the invention of glass electrodes to measure electrical signals in cell membranes, however, progress in the field of neurobiology has been stymied by the fact that most axons in the mammalian CNS are far too small and delicate for measuring ion channel function with electrodes. These quantitative measurements of membrane voltage can be achieved within the axon using light. A revolution of optical voltage sensors has enabled exploring important questions of how ion channels regulate axon physiology and synaptic transmission. In this review we will consider advantages and disadvantages of different fluorescent voltage indicators and discuss particularly relevant questions that these indicators can elucidate for understanding the crucial relationship between action potentials and synaptic transmission.

Genetic ablation or pharmacological inhibition of Kv1.1 potassium channel subunits impairs atrial repolarization in mice

Voltage-gated Kv1.1 potassium channel α-subunits, encoded by the Kcna1 gene, have traditionally been regarded as neural-specific with no expression or function in the heart. However, recent data revealed that Kv1.1 subunits are expressed in atria where they may have an overlooked role in controlling repolarization and arrhythmia susceptibility independent of the nervous system. To explore this concept in more detail and to identify functional and molecular effects of Kv1.1 channel impairment in the heart, atrial cardiomyocyte patch-clamp electrophysiology and gene expression analyses were performed using Kcna1 knockout ( Kcna1-/-) mice. Specifically, we hypothesized that Kv1.1 subunits contribute to outward repolarizing K+ currents in mouse atria and that their absence prolongs cardiac action potentials. In voltage-clamp experiments, dendrotoxin-K (DTX-K), a Kv1.1-specific inhibitor, significantly reduced peak outward K+ currents in wild-type (WT) atrial cells but not Kcna1-/- cells, demonstrating an important contribution by Kv1.1-containing channels to mouse atrial repolarizing currents. In current-clamp recordings, Kcna1-/- atrial myocytes exhibited significant action potential prolongation which was exacerbated in right atria, effects that were partially recapitulated in WT cells by application of DTX-K. Quantitative RT-PCR measurements showed mRNA expression remodeling in Kcna1-/- atria for several ion channel genes that contribute to the atrial action potential including the Kcna5, Kcnh2, and Kcnj2 potassium channel genes and the Scn5a sodium channel gene. This study demonstrates a previously undescribed heart-intrinsic role for Kv1.1 subunits in mediating atrial repolarization, thereby adding a new member to the already diverse collection of known K+ channels in the heart.

Gq-Coupled Muscarinic Receptor Enhancement of KCNQ2/3 Channels and Activation of TRPC Channels in Multimodal Control of Excitability in Dentate Gyrus Granule Cells

KCNQ (Kv7, "M-type") K⁺ channels and TRPC (transient receptor potential, "canonical") cation channels are coupled to neuronal discharge properties and are regulated via G_q/11-protein-mediated signals. Stimulation of G_q/11-coupled receptors both consumes phosphatidylinositol 4,5-bisphosphate (PIP₂) via phosphalipase Cβ hydrolysis and stimulates PIP₂ synthesis via rises in Ca²⁺ _i and other signals. Using brain-slice electrophysiology and Ca²⁺ imaging from male and female mice, we characterized threshold K⁺ currents in dentate gyrus granule cells (DGGCs) and CA1 pyramidal cells, the effects of G_q/11-coupled muscarinic M₁ acetylcholine (M₁R) stimulation on M current and on neuronal discharge properties, and elucidated the intracellular signaling mechanisms involved. We observed disparate signaling cascades between DGGCs and CA1 neurons. DGGCs displayed M₁R enhancement of M-current, rather than suppression, due to stimulation of PIP₂ synthesis, which was paralleled by increased PIP₂-gated G-protein coupled inwardly rectifying K⁺ currents as well. Deficiency of KCNQ2-containing M-channels ablated the M₁R-induced enhancement of M-current in DGGCs. Simultaneously, M₁R stimulation in DGGCs induced robust increases in [Ca²⁺]_i, mostly due to TRPC currents, consistent with, and contributing to, neuronal depolarization and hyperexcitability. CA1 neurons did not display such multimodal signaling, but rather M current was suppressed by M₁R stimulation in these cells, similar to the previously described actions of M₁R stimulation on M-current in peripheral ganglia that mostly involves PIP₂ depletion. Therefore, these results point to a pleiotropic network of cholinergic signals that direct cell-type-specific, precise control of hippocampal function with strong implications for hyperexcitability and epilepsy.SIGNIFICANCE STATEMENT At the neuronal membrane, protein signaling cascades consisting of ion channels and metabotropic receptors govern the electrical properties and neurotransmission of neuronal networks. Muscarinic acetylcholine receptors are G-protein-coupled metabotropic receptors that control the excitability of neurons through regulating ion channels, intracellular Ca²⁺ signals, and other second-messenger cascades. We have illuminated previously unknown actions of muscarinic stimulation on the excitability of hippocampal principal neurons that include M channels, TRPC (transient receptor potential, "canonical") cation channels, and powerful regulation of lipid metabolism. Our results show that these signaling pathways, and mechanisms of excitability, are starkly distinct between peripheral ganglia and brain, and even between different principal neurons in the hippocampus.

Fatty Acid and Related Potassium Kv2 Channel Blockers: Toxicity and Physiological Actions on Mosquitoes

Potassium channels constitute a very diverse group involved in neural signaling, neuronal activity, membrane potential maintenance, and action potential generation. Here, we tested the mammalian potassium channel blockers TRAM-34 and 5-hydroxydecanoate (5-HDC), as well as certain fatty acids (FA) that might fit in the lumen of the pore and block channel activity by obstructing K⁺ ion passage. Kv channel blockers could be leads for a novel pesticide type. Insecticidal activity was assessed by topical application to Anopheles gambiae adult mosquitoes, paralysis in a headless larval assay, at the cellular level with patch-clamp recordings of engineered HEK cells expressing AgKv2.1 channels, as well as central nervous system recordings from larval Drosophila melanogaster. With only one hydroxyl group difference, decanoic acid had a consistently greater effect than 5-HDC in blocking Kv channels, paralyzing larvae, and killing mosquitoes. The 11-dansylamino undecanoic acid (DAUDA) blockage of eukaryotic Kv channels is demonstrated for the first time, but it failed to kill adult mosquitoes. We synthesized alkyl esters from DAUDA and decanoic acid in an effort to improve cuticular penetration, but it had little impact upon adult toxicity. TRAM-34 and rolipram did not show activity on Kv channels nor potent insecticidal effect on adult mosquitoes. Furthermore, co-application of test compounds with permethrin did not increase mortality in adults. In conclusion, the compounds tested had modest insecticidal and synergistic activity.

A Xenopus oocyte model system to study action potentials

Voltage-gated Na⁺ and K⁺ channels are known to underlie the temporal characteristics of action potentials. Corbin-Leftwich et al. establish reliable action potential recordings from Xenopus oocytes coexpressing these channels and show how different K⁺ channel subtypes can modulate excitability.

Action potentials (APs) are the functional units of fast electrical signaling in excitable cells. The upstroke and downstroke of an AP is generated by the competing and asynchronous action of Na⁺- and K⁺-selective voltage-gated conductances. Although a mixture of voltage-gated channels has been long recognized to contribute to the generation and temporal characteristics of the AP, understanding how each of these proteins function and are regulated during electrical signaling remains the subject of intense research. AP properties vary among different cellular types because of the expression diversity, subcellular location, and modulation of ion channels. These complexities, in addition to the functional coupling of these proteins by membrane potential, make it challenging to understand the roles of different channels in initiating and “temporally shaping” the AP. Here, to address this problem, we focus our efforts on finding conditions that allow reliable AP recordings from Xenopus laevis oocytes coexpressing Na⁺ and K⁺ channels. As a proof of principle, we show how the expression of a variety of K⁺ channel subtypes can modulate excitability in this minimal model system. This approach raises the prospect of studies on the modulation of APs by pharmacological or biological means with a controlled background of Na⁺ and K⁺ channel expression.

Single-particle cryo-EM structure of a voltage-activated potassium channel in lipid nanodiscs

Voltage-activated potassium (Kv) channels open to conduct K⁺ ions in response to membrane depolarization, and subsequently enter non-conducting states through distinct mechanisms of inactivation. X-ray structures of detergent-solubilized Kv channels appear to have captured an open state even though a non-conducting C-type inactivated state would predominate in membranes in the absence of a transmembrane voltage. However, structures for a voltage-activated ion channel in a lipid bilayer environment have not yet been reported. Here we report the structure of the Kv1.2-2.1 paddle chimera channel reconstituted into lipid nanodiscs using single-particle cryo-electron microscopy. At a resolution of ~3 Å for the cytosolic domain and ~4 Å for the transmembrane domain, the structure determined in nanodiscs is similar to the previously determined X-ray structure. Our findings show that large differences in structure between detergent and lipid bilayer environments are unlikely, and enable us to propose possible structural mechanisms for C-type inactivation.

Potassium Channel Gain of Function in Epilepsy: An Unresolved Paradox

Exome and targeted sequencing have revolutionized clinical diagnosis. This has been particularly striking in epilepsy and neurodevelopmental disorders, for which new genes or new variants of preexisting candidate genes are being continuously identified at increasing rates every year. A surprising finding of these efforts is the recognition that gain of function potassium channel variants are actually associated with certain types of epilepsy, such as malignant migrating partial seizures of infancy or early-onset epileptic encephalopathy. This development has been difficult to understand as traditionally potassium channel loss-of-function, not gain-of-function, has been associated with hyperexcitability disorders. In this article, we describe the current state of the field regarding the gain-of-function potassium channel variants associated with epilepsy (KCNA2, KCNB1, KCND2, KCNH1, KCNH5, KCNJ10, KCNMA1, KCNQ2, KCNQ3, and KCNT1) and speculate on the possible cellular mechanisms behind the development of seizures and epilepsy in these patients. Understanding how potassium channel gain-of-function leads to epilepsy will provide new insights into the inner working of neural circuits and aid in developing new therapies.

Identification of VAPA and VAPB as Kv2 Channel-Interacting Proteins Defining Endoplasmic Reticulum-Plasma Membrane Junctions in Mammalian Brain Neurons

Membrane contacts between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are ubiquitous in eukaryotic cells and are platforms for lipid and calcium signaling and homeostasis. Recent studies have revealed proteins crucial to the formation and function of ER-PM junctions in non-neuronal cells, but little is known of the ER-PM junctions prominent in aspiny regions of mammalian brain neurons. The Kv2.1 voltage-gated potassium channel is abundantly clustered at ER-PM junctions in brain neurons and is the first PM protein that functions to organize ER-PM junctions. However, the molecular mechanism whereby Kv2.1 localizes to and remodels these junctions is unknown. We used affinity immunopurification and mass spectrometry-based proteomics on brain samples from male and female WT and Kv2.1 KO mice and identified the resident ER vesicle-associated membrane protein-associated proteins isoforms A and B (VAPA and VAPB) as prominent Kv2.1-associated proteins. Coexpression with Kv2.1 or its paralog Kv2.2 was sufficient to recruit VAPs to ER-PM junctions. Multiplex immunolabeling revealed colocalization of Kv2.1 and Kv2.2 with endogenous VAPs at ER-PM junctions in brain neurons from male and female mice in situ and in cultured rat hippocampal neurons, and KO of VAPA in mammalian cells reduces Kv2.1 clustering. The association of VAPA with Kv2.1 relies on a "two phenylalanines in an acidic tract" (FFAT) binding domain on VAPA and a noncanonical phosphorylation-dependent FFAT motif comprising the Kv2-specific clustering or PRC motif. These results suggest that Kv2.1 localizes to and organizes neuronal ER-PM junctions through an interaction with VAPs.SIGNIFICANCE STATEMENT Our study identified the endoplasmic reticulum (ER) proteins vesicle-associated membrane protein-associated proteins isoforms A and B (VAPA and VAPB) as proteins copurifying with the plasma membrane (PM) Kv2.1 ion channel. We found that expression of Kv2.1 recruits VAPs to ER-PM junctions, specialized membrane contact sites crucial to distinct aspects of cell function. We found endogenous VAPs at Kv2.1-mediated ER-PM junctions in brain neurons and other mammalian cells and that knocking out VAPA expression disrupts Kv2.1 clustering. We identified domains of VAPs and Kv2.1 necessary and sufficient for their association at ER-PM junctions. Our study suggests that Kv2.1 expression in the PM can affect ER-PM junctions via its phosphorylation-dependent association to ER-localized VAPA and VAPB.

Development-related aberrations in Kv1.1 α-subunit exert disruptive effects on bioelectrical activities of neurons in a mouse model of fragile X syndrome

Kv1.1, a Shaker homologue potassium channel, plays a critical role in homeostatic regulation of neuronal excitability. Aberrations in the functional properties of Kv1.1 have been implicated in several neurological disorders featured by neuronal hyperexcitability. Fragile X syndrome (FXS), the most common form of inherited mental retardation, is characterized by hyperexcitability in neural network and intrinsic membrane properties. The Kv1.1 channel provides an intriguing mechanistic candidate for FXS. We investigated the development-related expression pattern of the Kv1.1 α-subunit by using a Fmr1 knockout (KO) mouse model of FXS. Markedly decreased protein expression of Kv1.1 was found in neonatal and adult stages when compared to age-matched wild-type (WT) mice. Immunohistochemical investigations supported the delayed development-related increases in Kv1.1 expression, especially in CA3 pyramidal neurons. By applying a Kv1.1-specific blocker, dendrotoxin-κ (DTX-κ), we isolated the Kv1.1-mediated currents in the CA3 pyramidal neurons. The isolated DTX-κ-sensitive current of neurons from KO mice exhibited decreased amplitude, lower threshold of activation, and faster recovery from inactivation. The equivalent reduction in potassium current in the WT neurons following application of the appropriate amount of DTX-κ reproduced the enhanced firing abilities of KO neurons, suggesting the Kv1.1 channel as a critical contributor to the hyperexcitability of KO neurons. The role of Kv1.1 in controlling neuronal discharges was further supported by the parallel developmental trajectories of Kv1.1 expression, current amplitude, and discharge impacts, with a significant correlation between the amplitude of Kv1.1-mediated currents and Kv1.1-blocking-induced firing enhancement. These data suggest that the expression of the Kv1.1 α-subunit has a profound pathological relevance to hyperexcitability in FXS, as well as implications for normal development, maintenance, and control of neuronal activities.

Replacing voltage sensor arginines with citrulline provides mechanistic insight into charge versus shape

Activation of voltage-gated channels results from the outward movement of arginine residues on the S4 segments. Infield et al. use in vivo nonsense suppression to replace Shaker's S4 arginine residues with citrulline and reveal that a positive charge is required on R4 for voltage-dependent deactivation.

Voltage-dependent activation of voltage-gated cation channels results from the outward movement of arginine-bearing helices within proteinaceous voltage sensors. The voltage-sensing residues in potassium channels have been extensively characterized, but current functional approaches do not allow a distinction between the electrostatic and steric contributions of the arginine side chain. Here we use chemical misacylation and in vivo nonsense suppression to encode citrulline, a neutral and nearly isosteric analogue of arginine, into the voltage sensor of the Shaker potassium channel. We functionally characterize the engineered channels and compare them with those bearing conventional mutations at the same positions. We observe effects on both voltage sensitivity and gating kinetics, enabling dissection of the roles of residue structure versus positive charge in channel function. In some positions, substitution with citrulline causes mild effects on channel activation compared with natural mutations. In contrast, substitution of the fourth S4 arginine with citrulline causes substantial changes in the conductance–voltage relationship and the kinetics of the channel, which suggests that a positive charge is required at this position for efficient voltage sensor deactivation and channel closure. The encoding of citrulline is expected to enable enhanced precision for the study of arginine residues located in crowded transmembrane environments in other membrane proteins. In addition, the method may facilitate the study of citrullination in vivo.

Design, synthesis and evaluation of novel N-phenylbutanamide derivatives as KCNQ openers for the treatment of epilepsy

KCNQ (Kv7) has emerged as a validated target for the development of novel anti-epileptic drugs. In this paper, a series of novel N-phenylbutanamide derivatives were designed, synthesized and evaluated as KCNQ openers for the treatment of epilepsy. These compounds were evaluated for their KCNQ opening activity in vitro and in vivo. Several compounds were found to be potent KCNQ openers. Compound 1 with favorable in vitro activity was submitted to evaluation in vivo. Results showed that compound 1 owned significant anti-convulsant activity with no adverse effects. It was also found to posses favorable pharmacokinetic profiles in rat. This research may provide novel potent compounds for the discovery of KCNQ openers in treating epilepsy.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Shaker-related voltage-gated potassium channels Kv1 in human hippocampus

In this study, we investigated the tissue expression levels, alpha subunit composition and distribution of Shaker-related voltage-dependent potassium Kv1 channels in human hippocampus by combining western blotting experiments, toxin autoradiography, in vivo radioligand binding studies, immunoprecipitation and immunohistochemistry. Tissue expression of Kv1.1 and Kv1.2 α-subunits in human post-mortem brain tissue was confirmed in immunoblot analysis using a panel of specific monoclonal and polyclonal antibodies. Immunoprecipitation experiments using toxin-prelabeled Kv1 channels revealed that all toxin-sensitive Kv1 channels in human hippocampus contained either a Kv1.1 or Kv1.2 α-subunit with the majority being composed of Kv1.1/Kv1.2 heterotetramers. Receptor autoradiography suggested Kv1.1/Kv1.2 channel expression in the molecular layer of dentate gyrus. In accordance, immunohistochemical experiments also observed Kv1.1 and Kv1.2 α-subunits in the molecular layer of the dentate gyrus, in addition to the CA3 stratum lucidum and the CA1 stratum oriens. These findings indicate expression in axons and terminals of hippocampal pathways, namely the perforant path, the mossy fiber pathway and the Schaffer collaterals. Herein we present the first direct demonstration that Kv1.1 and Kv1.2 channel proteins are targeted to distinct compartments of the human hippocampal formation and that this expression pattern largely reflects their distribution profile in murine brain.

Increase of hyperpolarization-activated cyclic nucleotide-gated current in the aberrant excitability of spinal muscular atrophy

OBJECTIVE

The pathophysiology of spinal muscular atrophy (SMA) is still unclear.

METHODS

The nerve excitability test in SMA patients and a mouse model of SMA was carried out to explore the pathophysiology of nodal and internodal currents, and quantitative PCR, western blotting, and whole-cell patch-clamp recording were used for the identified hypothesis.

RESULTS

The nerve excitability test in SMA patients showed increased inward rectification in the current-threshold relationship and increased overshoot after hyperpolarizing threshold electrotonus, which indicates increased hyperpolarization-activated cyclic nucleotide-gated (HCN) current; these findings correlated with disease severity. Increased inward rectification in the current-threshold relationship was reproducible in a mouse model of mild SMA, and the abnormality preceded the decline of compound motor action potential amplitudes. Furthermore, quantitative PCR of spinal cord tissues and western blotting of the spinal cord and sciatic nerves showed increased HCN1 and HCN2 expression in SMA mice, and voltage-clamp recording in dissociated spinal motor neurons from SMA mice also showed increased HCN current density. Treatment with ZD7288, an HCN channel blocker, also reduced early mortality, improved motor function, and restored neuromuscular junction architecture in a mouse model of severe SMA.

INTERPRETATION

This study shows that increased HCN current underlies the pathophysiology of SMA and can be a novel non-SMN target for SMA therapy. Ann Neurol 2018;83:494-507.

Layer 5 Callosal Parvalbumin-Expressing Neurons: A Distinct Functional Group of GABAergic Neurons

Previous studies have shown that parvalbumin-expressing neurons (CC-Parv neurons) connect the two hemispheres of motor and sensory areas via the corpus callosum, and are a functional part of the cortical circuit. Here we test the hypothesis that layer 5 CC-Parv neurons possess anatomical and molecular mechanisms which dampen excitability and modulate the gating of interhemispheric inhibition. In order to investigate this hypothesis we use viral tracing to determine the anatomical and electrophysiological properties of layer 5 CC-Parv and parvalbumin-expressing (Parv) neurons of the mouse auditory cortex (AC). Here we show that layer 5 CC-Parv neurons had larger dendritic fields characterized by longer dendrites that branched farther from the soma, whereas layer 5 Parv neurons had smaller dendritic fields characterized by shorter dendrites that branched nearer to the soma. The layer 5 CC-Parv neurons are characterized by delayed action potential (AP) responses to threshold currents, lower firing rates, and lower instantaneous frequencies compared to the layer 5 Parv neurons. Kv1.1 containing K⁺ channels are the main source of the AP repolarization of the layer 5 CC-Parv and have a major role in determining both the spike delayed response, firing rate and instantaneous frequency of these neurons.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Ion Channels in Drug Discovery and Safety Pharmacology

Ion channels are membrane proteins involved in almost all physiological processes, including neurotransmission, muscle contraction, pace-making activity, secretion, electrolyte and water balance, immune response, and cell proliferation. Due to their broad distribution in human body and physiological roles, ion channels are attractive targets for drug discovery and safety pharmacology. Over the years ion channels have been associated to many genetic diseases ("channelopathies"). For most of these diseases the therapy is mainly empirical and symptomatic, often limited by lack of efficacy and tolerability for a number of patients. The search for the development of new and more specific therapeutic approaches is therefore strongly pursued. At the same time acquired channelopathies or dangerous side effects (such as proarrhythmic risk) can develop as a consequence of drugs unexpectedly targeting ion channels. Several noncardiovascular drugs are known to block cardiac ion channels, leading to potentially fatal delayed ventricular repolarization. Thus, the search of reliable preclinical cardiac safety testing in early stage of drug discovery is mandatory. To fulfill these needs, both ion channels drug discovery and toxicology strategies are evolving toward comprehensive research approaches integrating ad hoc designed in silico predictions and experimental studies for a more reliable and quick translation of results to the clinic side.Here we discuss two examples of how the combination of in silico methods and patch clamp experiments can help addressing drug discovery and safety issues regarding ion channels.

Clinical spectrum and genotype-phenotype associations of KCNA2-related encephalopathies

Recently, de novo mutations in the gene KCNA2, causing either a dominant-negative loss-of-function or a gain-of-function of the voltage-gated K+ channel Kv1.2, were described to cause a new molecular entity within the epileptic encephalopathies. Here, we report a cohort of 23 patients (eight previously described) with epileptic encephalopathy carrying either novel or known KCNA2 mutations, with the aim to detail the clinical phenotype associated with each of them, to characterize the functional effects of the newly identified mutations, and to assess genotype-phenotype associations. We identified five novel and confirmed six known mutations, three of which recurred in three, five and seven patients, respectively. Ten mutations were missense and one was a truncation mutation; de novo occurrence could be shown in 20 patients. Functional studies using a Xenopus oocyte two-microelectrode voltage clamp system revealed mutations with only loss-of-function effects (mostly dominant-negative current amplitude reduction) in eight patients or only gain-of-function effects (hyperpolarizing shift of voltage-dependent activation, increased amplitude) in nine patients. In six patients, the gain-of-function was diminished by an additional loss-of-function (gain-and loss-of-function) due to a hyperpolarizing shift of voltage-dependent activation combined with either decreased amplitudes or an additional hyperpolarizing shift of the inactivation curve. These electrophysiological findings correlated with distinct phenotypic features. The main differences were (i) predominant focal (loss-of-function) versus generalized (gain-of-function) seizures and corresponding epileptic discharges with prominent sleep activation in most cases with loss-of-function mutations; (ii) more severe epilepsy, developmental problems and ataxia, and atrophy of the cerebellum or even the whole brain in about half of the patients with gain-of-function mutations; and (iii) most severe early-onset phenotypes, occasionally with neonatal onset epilepsy and developmental impairment, as well as generalized and focal seizures and EEG abnormalities for patients with gain- and loss-of-function mutations. Our study thus indicates well represented genotype-phenotype associations between three subgroups of patients with KCNA2 encephalopathy according to the electrophysiological features of the mutations.

Scn2a deletion improves survival and brain-heart dynamics in the Kcna1-null mouse model of sudden unexpected death in epilepsy (SUDEP)

People with epilepsy have greatly increased probability of premature mortality due to sudden unexpected death in epilepsy (SUDEP). Identifying which patients are most at risk of SUDEP is hindered by a complex genetic etiology, incomplete understanding of the underlying pathophysiology and lack of prognostic biomarkers. Here we evaluated heterozygous Scn2a gene deletion (Scn2a+/-) as a protective genetic modifier in the Kcna1 knockout mouse (Kcna1-/-) model of SUDEP, while searching for biomarkers of SUDEP risk embedded in electroencephalography (EEG) and electrocardiography (ECG) recordings. The human epilepsy gene Kcna1 encodes voltage-gated Kv1.1 potassium channels that act to dampen neuronal excitability whereas Scn2a encodes voltage-gated Nav1.2 sodium channels important for action potential initiation and conduction. SUDEP-prone Kcna1-/- mice with partial genetic ablation of Nav1.2 channels (i.e. Scn2a+/-; Kcna1-/-) exhibited a two-fold increase in survival. Classical analysis of EEG and ECG recordings separately showed significantly decreased seizure durations in Scn2a+/-; Kcna1-/- mice compared with Kcna1-/- mice, without substantial modification of cardiac abnormalities. Novel analysis of the EEG and ECG together revealed a significant reduction in EEG-ECG association in Kcna1-/- mice compared with wild types, which was partially restored in Scn2a+/-; Kcna1-/- mice. The degree of EEG-ECG association was also proportional to the survival rate of mice across genotypes. These results show that Scn2a gene deletion acts as protective genetic modifier of SUDEP and suggest measures of brain-heart association as potential indices of SUDEP susceptibility.

Chansporter complexes in cell signaling

Ion channels facilitate diffusion of ions across cell membranes for such diverse purposes as neuronal signaling, muscular contraction, and fluid homeostasis. Solute transporters often utilize ionic gradients to move aqueous solutes up their concentration gradient, also fulfilling a wide variety of tasks. Recently, an increasing number of ion channel-transporter ('chansporter') complexes have been discovered. Chansporter complex formation may overcome what could otherwise be considerable spatial barriers to rapid signal integration and feedback between channels and transporters, the ions and other substrates they transport, and environmental factors to which they must respond. Here, current knowledge in this field is summarized, covering both heterologous expression structure/function findings and potential mechanisms by which chansporter complexes fulfill contrasting roles in cell signaling in vivo.

Genome-wide DNA hydroxymethylation identifies potassium channels in the nucleus accumbens as discriminators of methamphetamine addiction and abstinence

Epigenetic consequences of exposure to psychostimulants are substantial but the relationship of these changes to compulsive drug taking and abstinence is not clear. Here, we used a paradigm that helped to segregate rats that reduce or stop their methamphetamine (METH) intake (nonaddicted) from those that continue to take the drug compulsively (addicted) in the presence of footshocks. We used that model to investigate potential alterations in global DNA hydroxymethylation in the nucleus accumbens (NAc) because neuroplastic changes in the NAc may participate in the development and maintenance of drug-taking behaviors. We found that METH-addicted rats did indeed show differential DNA hydroxymethylation in comparison with both control and nonaddicted rats. Nonaddicted rats also showed differences from control rats. Differential DNA hydroxymethylation observed in addicted rats occurred mostly at intergenic sites located on long and short interspersed elements. Interestingly, differentially hydroxymethylated regions in genes encoding voltage (Kv1.1, Kv1.2, Kvb1 and Kv2.2)- and calcium (Kcnma1, Kcnn1 and Kcnn2)-gated potassium channels observed in the NAc of nonaddicted rats were accompanied by increased mRNA levels of these potassium channels when compared with mRNA expression in METH-addicted rats. These observations indicate that changes in differentially hydroxymethylated regions and increased expression of specific potassium channels in the NAc may promote abstinence from drug-taking behaviors. Thus, activation of specific subclasses of voltage- and/or calcium-gated potassium channels may provide an important approach to the beneficial treatment for METH addiction.

mTOR referees memory and disease through mRNA repression and competition

Mammalian target of rapamycin (mTOR) activity is required for memory and is dysregulated in disease. Activation of mTOR promotes protein synthesis; however, new studies are demonstrating that mTOR activity also represses the translation of mRNAs. Almost three decades ago, Kandel and colleagues hypothesised that memory was due to the induction of positive regulators and removal of negative constraints. Are these negative constraints repressed mRNAs that code for proteins that block memory formation? Herein, we will discuss the mRNAs coded by putative memory suppressors, how activation/inactivation of mTOR repress protein expression at the synapse, how mTOR activity regulates RNA binding proteins, mRNA stability, and translation, and what the possible implications of mRNA repression are to memory and neurodegenerative disorders.

The chemical basis for electrical signaling

Electrical signals generated by minute currents of ions moving across cell membranes are central to all rapid processes in biology. Initiation and propagation of electrical signals requires voltage-gated sodium (Na_V) and calcium (Ca_V) channels. These channels contain a tetramer of membrane-bound subunits or domains comprising a voltage sensor and a pore module. Voltage-dependent activation occurs as membrane depolarization drives outward movements of positive gating changes in the voltage sensor via a sliding-helix mechanism, which leads to a conformational change in the pore module that opens its intracellular activation gate. A unique negatively charged site in the selectivity filter conducts hydrated Na⁺ or Ca²⁺ rapidly and selectively. Ion conductance is terminated by voltage-dependent inactivation, which causes asymmetric pore collapse. This Review focuses on recent advances in structure and function of Na_V and Ca_V channels that expand our current understanding of the chemical basis for electrical signaling mechanisms conserved from bacteria to humans.

Molecular mechanisms underlying pimaric acid-induced modulation of voltage-gated K+ channels

Voltage-gated K⁺ (K_V) channels, which control firing and shape of action potentials in excitable cells, are supposed to be potential therapeutic targets in many types of diseases. Pimaric acid (PiMA) is a unique opener of large conductance Ca²⁺-activated K⁺ channel. Here, we report that PiMA modulates recombinant rodent K_V channel activity. The enhancement was significant at low potentials (<0 mV) but not at more positive potentials. Application of PiMA significantly shifted the voltage-activation relationships (V_1/2) of rodent K_V1.1, 1.2, 1.3, 1.4, 1.6 and 2.1 channels (K_V1.1-K_V2.1) but K_V4.3 to lower potentials and prolonged their half-decay times of the deactivation (T_1/2D). The amino acid sequence which is responsible for the difference in response to PiMA was examined between K_V1.1-K_V2.1 and K_V4.3 by site-directed mutagenesis of residues in S5 and S6 segments of Kv1.1. The point mutation of Phe³³² into Tyr mimics the effects of PiMA on V_1/2 and T_1/2D and also abolished the further change by addition of PiMA. The results indicate that PiMA enhances voltage sensitivity of K_V1.1-K_V2.1 channels and suggest that the lipophilic residues including Phe³³² in S5 of K_V1.1-K_V2.1 channels may be critical for the effects of PiMA, providing beneficial information for drug development of K_V channel openers.

Inhibitory effects of cholinesterase inhibitor donepezil on the Kv1.5 potassium channel

Kv1.5 channels carry ultra-rapid delayed rectifier K⁺ currents in excitable cells, including neurons and cardiac myocytes. In the current study, the effects of cholinesterase inhibitor donepezil on cloned Kv1.5 channels expressed in HEK29 cells were explored using whole-cell recording technique. Exposure to donepezil resulted in a rapid and reversible block of Kv1.5 currents, with an IC₅₀ value of 72.5 μM. The mutant R476V significantly reduced the binding affinity of donepezil to Kv1.5 channels, showing the target site in the outer mouth region. Donepezil produced a significant delay in the duration of activation and deactivation, and mutant R476V potentiated these effects without altering activation curves. In response to slowed deactivation time course, a typical crossover of Kv1.5 tail currents was clearly evident after bath application of donepezil. In addition, both this chemical and mutant R476V accelerated current decay during channel inactivation in a voltage-dependent way, but barely changed the inactivation and recovery curves. The presence of donepezil exhibited the use-dependent block of Kv1.5 currents in response to a series of depolarizing pulses. Our data indicate that donepezil can directly block Kv1.5 channels in its open and closed states.

Loss of Function of KCNC1 is associated with intellectual disability without seizures

p.(Arg320His) mutation in the KCNC1 gene in human 11p15.1 has recently been identified in patients with progressive myoclonus epilepsies, a group of rare inherited disorders manifesting with action myoclonus, myoclonic epilepsy, and ataxia. This KCNC1 variant causes a dominant-negative effect. Here we describe three patients from the same family with intellectual disability and dysmorphic features. The three affected individuals carry a c.1015C>T (p.(Arg339*)) nonsense variant in KCNC1 gene. As previously observed in the mutant mouse carrying a disrupted KCNC1 gene, these findings reveal that individuals with a KCNC1 loss-of-function variant can present intellectual disability without seizure and epilepsy.

Cholinergic Interneurons Underlie Spontaneous Dopamine Release in Nucleus Accumbens

The release of dopamine from terminals in the NAc is regulated by a number of factors, including voltage-gated ion channels, D2-autoreceptors, and nAChRs. Cholinergic interneurons (CINs) drive dopamine release through activation of nAChRs on dopamine terminals. Using cyclic voltammetry in mouse brain slices, nAChR-dependent spontaneous dopamine transients and the mechanisms underlying the origin were examined in the NAc. Spontaneous events were infrequent (0.3 per minute), but the rate and amplitude were increased after blocking Kv channels with 4-aminopyridine. Although the firing frequency of CINs was increased by blocking glutamate reuptake with TBOA and the Sk blocker apamin, only 4-aminopyridine increased the frequency of dopamine transients. In contrast, inhibition of CIN firing with the μ/δ selective opioid [Met⁵]enkephalin (1 μm) decreased spontaneous dopamine transients. Cocaine increased the rate and amplitude of dopamine transients, suggesting that the activity of the dopamine transporter limits the detection of these events. In the presence of cocaine, the rate of spontaneous dopamine transients was further increased after blocking D2-autoreceptors. Blockade of muscarinic receptors had no effect on evoked dopamine release, suggesting that feedback inhibition of acetylcholine release was not involved. Thus, although spontaneous dopamine transients are reliant on nAChRs, the frequency was not strictly governed by the activity of CINs. The increase in frequency of spontaneous dopamine transients induced by cocaine was not due to an increase in cholinergic tone and is likely a product of an increase in detection resulting from decreased dopamine reuptake.SIGNIFICANCE STATEMENT The actions of dopamine in the NAc are thought to be responsible for endogenous reward and the reinforcing properties of drugs of abuse, such as psychostimulants. The present work examines the mechanisms underlying nAChR-induced spontaneous dopamine release. This study demonstrates that spontaneous dopamine release is (1) dependent of the activation of nicotinic receptors, (2) independent on the spontaneous activity of cholinergic interneurons, and (3) that cocaine increased the detection of dopamine transients by prolonging the presence and increasing the diffusion of dopamine in the extracellular space. The release of acetylcholine is therefore responsible for spontaneous dopamine transients, and cocaine augments dopamine tone without altering activity of cholinergic interneurons.

Margatoxin-bound quantum dots as a novel inhibitor of the voltage-gated ion channel Kv1.3

Venom-derived ion channel inhibitors have strong channel selectivity, potency, and stability; however, tracking delivery to their target can be challenging. Herein, we utilized luminescent quantum dots (QDs) conjugated to margatoxin (MgTx) as a traceable vehicle to target a voltage-dependent potassium channel, Kv1.3, which has a select distribution and well-characterized role in immunity, glucose metabolism, and sensory ability. We screened both unconjugated (MgTx) and conjugated MgTx (QD-MgTx) for their ability to inhibit Shaker channels Kv1.1 to Kv1.7 using patch-clamp electrophysiology in HEK293 cells. Our data indicate that MgTx inhibits 79% of the outward current in Kv1.3-transfected cells and that the QD-MgTx conjugate is able to achieve a similar level of block, albeit a slightly reduced efficacy (66%) and at a slower time course (50% block by 10.9 ± 1.1 min, MgTx; vs. 15.3 ± 1.2 min, QD-MgTx). Like the unbound peptide, the QD-MgTx conjugate inhibits both Kv1.3 and Kv1.2 at a 1 nM concentration, whereas it does not inhibit other screened Shaker channels. We tested the ability of QD-MgTx to inhibit native Kv1.3 expressed in the mouse olfactory bulb (OB). In brain slices of the OB, the conjugate acted similarly to MgTx to inhibit Kv1.3, causing an increased action potential firing frequency attributed to decreased intraburst duration rather than interspike interval. Our data demonstrate a retention of known biophysical properties associated with block of the vestibule of Kv1.3 by QD-MgTx conjugate compared to that of MgTx, inferring QDs could provide a useful tool to deliver ion channel inhibitors to targeted tissues in vivo.

Paternal age effects on sperm FOXK1 and KCNA7 methylation and transmission into the next generation

Children of older fathers carry an increased risk for developing autism and other disorders. To elucidate the underlying mechanisms, we investigated the correlation of sperm DNA methylation with paternal age and its impact on the epigenome of the offspring. Methylation levels of nine candidate genes and LINE-1 repeats were quantified by bisulfite pyrosequencing in sperm DNA of 162 donors and 191 cord blood samples of resulting children (conceived by IVF/ICSI with the same sperm samples). Four genes showed a significant negative correlation between sperm methylation and paternal age. For FOXK1 and KCNA7, the age effect on the sperm epigenome was replicated in an independent cohort of 188 sperm samples. For FOXK1, paternal age also significantly correlated with foetal cord blood (FCB) methylation. Deep bisulfite sequencing and allele-specific pyrosequencing allowed us to distinguish between maternal and paternal alleles in FCB samples with an informative SNP. FCB methylation of the paternal FOXK1 allele was negatively correlated with paternal age, whereas maternal allele was unaffected by maternal age. Since FOXK1 duplication has been associated with autism, we studied blood FOXK1 methylation in 74 children with autism and 41 age-matched controls. The FOXK1 promoter showed a trend for accelerated demethylation in the autism group. Dual luciferase reporter assay revealed that FOXK1 methylation influences gene expression. Collectively, our study demonstrates that age-related DNA methylation changes in sperm can be transmitted to the next generation and may contribute to the increased disease risk in offspring of older fathers.

Dominant KCNA2 mutation causes episodic ataxia and pharmacoresponsive epilepsy

OBJECTIVE

To identify the genetic basis of a family segregating episodic ataxia, infantile seizures, and heterogeneous epilepsies and to study the phenotypic spectrum of KCNA2 mutations.

METHODS

A family with 7 affected individuals over 3 generations underwent detailed phenotyping. Whole genome sequencing was performed on a mildly affected grandmother and her grandson with epileptic encephalopathy (EE). Segregating variants were filtered and prioritized based on functional annotations. The effects of the mutation on channel function were analyzed in vitro by voltage clamp assay and in silico by molecular modeling. KCNA2 was sequenced in 35 probands with heterogeneous phenotypes.

RESULTS

The 7 family members had episodic ataxia (5), self-limited infantile seizures (5), evolving to genetic generalized epilepsy (4), focal seizures (2), and EE (1). They had a segregating novel mutation in the shaker type voltage-gated potassium channel KCNA2 (CCDS_827.1: c.765_773del; p.255_257del). A rare missense SCN2A (rs200884216) variant was also found in 2 affected siblings and their unaffected mother. The p.255_257del mutation caused dominant negative loss of channel function. Molecular modeling predicted repositioning of critical arginine residues in the voltage-sensing domain. KCNA2 sequencing revealed 1 de novo mutation (CCDS_827.1: c.890G>A; p.Arg297Gln) in a girl with EE, ataxia, and tremor.

CONCLUSIONS

A KCNA2 mutation caused dominantly inherited episodic ataxia, mild infantile-onset seizures, and later generalized and focal epilepsies in the setting of normal intellect. This observation expands the KCNA2 phenotypic spectrum from EE often associated with chronic ataxia, reflecting the marked variation in severity observed in many ion channel disorders.

Pharmacological Conversion of a Cardiac Inward Rectifier into an Outward Rectifier Potassium Channel

Potassium (K(+)) channels are crucial for determining the shape, duration, and frequency of action-potential firing in excitable cells. Broadly speaking, K(+) channels can be classified based on whether their macroscopic current outwardly or inwardly rectifies, whereby rectification refers to a change in conductance with voltage. Outwardly rectifying K(+) channels conduct greater current at depolarized membrane potentials, whereas inward rectifier channels conduct greater current at hyperpolarized membrane potentials. Under most circumstances, outward currents through inwardly rectifying K(+) channels are reduced at more depolarized potentials. However, the acetylcholine-gated K(+) channel (KACh) conducts current that inwardly rectifies when activated by some ligands (such as acetylcholine), and yet conducts current that outwardly rectifies when activated by other ligands (for example, pilocarpine and choline). The perplexing and paradoxical behavior of KACh channels is due to the intrinsic voltage sensitivity of the receptor that activates KACh channels, the M2 muscarinic receptor (M2R). Emerging evidence reveals that the affinity of M2R for distinct ligands varies in a voltage-dependent and ligand-specific manner. These intrinsic receptor properties determine whether current conducted by KACh channels inwardly or outwardly rectifies. This review summarizes the most recent concepts regarding the intrinsic voltage sensitivity of muscarinic receptors and the consequences of this intriguing behavior on cardiac physiology and pharmacology of KACh channels.

Automated evolutionary optimization of ion channel conductances and kinetics in models of young and aged rhesus monkey pyramidal neurons

Conductance-based compartment modeling requires tuning of many parameters to fit the neuron model to target electrophysiological data. Automated parameter optimization via evolutionary algorithms (EAs) is a common approach to accomplish this task, using error functions to quantify differences between model and target. We present a three-stage EA optimization protocol for tuning ion channel conductances and kinetics in a generic neuron model with minimal manual intervention. We use the technique of Latin hypercube sampling in a new way, to choose weights for error functions automatically so that each function influences the parameter search to a similar degree. This protocol requires no specialized physiological data collection and is applicable to commonly-collected current clamp data and either single- or multi-objective optimization. We applied the protocol to two representative pyramidal neurons from layer 3 of the prefrontal cortex of rhesus monkeys, in which action potential firing rates are significantly higher in aged compared to young animals. Using an idealized dendritic topology and models with either 4 or 8 ion channels (10 or 23 free parameters respectively), we produced populations of parameter combinations fitting the target datasets in less than 80 hours of optimization each. Passive parameter differences between young and aged models were consistent with our prior results using simpler models and hand tuning. We analyzed parameter values among fits to a single neuron to facilitate refinement of the underlying model, and across fits to multiple neurons to show how our protocol will lead to predictions of parameter differences with aging in these neurons.

Action potential broadening in a presynaptic channelopathy

Brain development and interictal function are unaffected in many paroxysmal neurological channelopathies, possibly explained by homoeostatic plasticity of synaptic transmission. Episodic ataxia type 1 is caused by missense mutations of the potassium channel Kv1.1, which is abundantly expressed in the terminals of cerebellar basket cells. Presynaptic action potentials of small inhibitory terminals have not been characterized, and it is not known whether developmental plasticity compensates for the effects of Kv1.1 dysfunction. Here we use visually targeted patch-clamp recordings from basket cell terminals of mice harbouring an ataxia-associated mutation and their wild-type littermates. Presynaptic spikes are followed by a pronounced afterdepolarization, and are broadened by pharmacological blockade of Kv1.1 or by a dominant ataxia-associated mutation. Somatic recordings fail to detect such changes. Spike broadening leads to increased Ca²⁺ influx and GABA release, and decreased spontaneous Purkinje cell firing. We find no evidence for developmental compensation for inherited Kv1.1 dysfunction.

Episodic ataxia type 1 is caused by mutations in the potassium channel Kv1.1, which is found in cerebellar basket cells. Here, the authors use electrophysiology techniques to characterize these mutant channels, and observe that the changes result in decreased spontaneous Purkinje cell firing with no evidence for developmental compensation.

Therapeutic Approaches to Genetic Ion Channelopathies and Perspectives in Drug Discovery

In the human genome more than 400 genes encode ion channels, which are transmembrane proteins mediating ion fluxes across membranes. Being expressed in all cell types, they are involved in almost all physiological processes, including sense perception, neurotransmission, muscle contraction, secretion, immune response, cell proliferation, and differentiation. Due to the widespread tissue distribution of ion channels and their physiological functions, mutations in genes encoding ion channel subunits, or their interacting proteins, are responsible for inherited ion channelopathies. These diseases can range from common to very rare disorders and their severity can be mild, disabling, or life-threatening. In spite of this, ion channels are the primary target of only about 5% of the marketed drugs suggesting their potential in drug discovery. The current review summarizes the therapeutic management of the principal ion channelopathies of central and peripheral nervous system, heart, kidney, bone, skeletal muscle and pancreas, resulting from mutations in calcium, sodium, potassium, and chloride ion channels. For most channelopathies the therapy is mainly empirical and symptomatic, often limited by lack of efficacy and tolerability for a significant number of patients. Other channelopathies can exploit ion channel targeted drugs, such as marketed sodium channel blockers. Developing new and more specific therapeutic approaches is therefore required. To this aim, a major advancement in the pharmacotherapy of channelopathies has been the discovery that ion channel mutations lead to change in biophysics that can in turn specifically modify the sensitivity to drugs: this opens the way to a pharmacogenetics strategy, allowing the development of a personalized therapy with increased efficacy and reduced side effects. In addition, the identification of disease modifiers in ion channelopathies appears an alternative strategy to discover novel druggable targets.

Altered potassium channel distribution and composition in myelinated axons suppresses hyperexcitability following injury

Neuropathic pain following peripheral nerve injury is associated with hyperexcitability in damaged myelinated sensory axons, which begins to normalise over time. We investigated the composition and distribution of shaker-type-potassium channels (Kv1 channels) within the nodal complex of myelinated axons following injury. At the neuroma that forms after damage, expression of Kv1.1 and 1.2 (normally localised to the juxtaparanode) was markedly decreased. In contrast Kv1.4 and 1.6, which were hardly detectable in the naïve state, showed increased expression within juxtaparanodes and paranodes following injury, both in rats and humans. Within the dorsal root (a site remote from injury) we noted a redistribution of Kv1-channels towards the paranode. Blockade of Kv1 channels with α-DTX after injury reinstated hyperexcitability of A-fibre axons and enhanced mechanosensitivity. Changes in the molecular composition and distribution of axonal Kv1 channels, therefore represents a protective mechanism to suppress the hyperexcitability of myelinated sensory axons that follows nerve injury.

Modeling of the Binding of Peptide Blockers to Voltage-Gated Potassium Channels: Approaches and Evidence

Modeling of the structure of voltage-gated potassium (KV) channels bound to peptide blockers aims to identify the key amino acid residues dictating affinity and provide insights into the toxin-channel interface. Computational approaches open up possibilities for in silico rational design of selective blockers, new molecular tools to study the cellular distribution and functional roles of potassium channels. It is anticipated that optimized blockers will advance the development of drugs that reduce over activation of potassium channels and attenuate the associated malfunction. Starting with an overview of the recent advances in computational simulation strategies to predict the bound state orientations of peptide pore blockers relative to KV-channels, we go on to review algorithms for the analysis of intermolecular interactions, and then take a look at the results of their application.

Phosphorylation of K+ channels at single residues regulates memory formation

Phosphorylation is a ubiquitous post-translational modification of proteins, and a known physiological regulator of K⁺ channel function. Phosphorylation of K⁺ channels by kinases has long been presumed to regulate neuronal processing and behavior. Although circumstantial evidence has accumulated from behavioral studies of vertebrates and invertebrates, the contribution to memory of single phosphorylation sites on K⁺ channels has never been reported. We have used gene targeting in mice to inactivate protein kinase A substrate residues in the fast-inactivating subunit K_v4.2 (T38A mutants), and in the small-conductance Ca²⁺-activated subunit SK1 (S105A mutants). Both manipulations perturbed a specific form of memory, leaving others intact. T38A mutants had enhanced spatial memory for at least 4 wk after training, whereas performance in three tests of fear memory was unaffected. S105A mutants were impaired in passive avoidance memory, sparing fear, and spatial memory. Together with recent findings that excitability governs the participation of neurons in a memory circuit, this result suggests that the memory type supported by neurons may depend critically on the phosphorylation of specific K⁺ channels at single residues.

Normal human CD4(+) helper T cells express Kv1.1 voltage-gated K(+) channels, and selective Kv1.1 block in T cells induces by itself robust TNFα production and secretion and activation of the NFκB non-canonical pathway

TNFα is a very potent and pleiotropic pro-inflammatory cytokine, essential to the immune system for eradicating cancer and microorganisms, and to the nervous system, for brain development and ongoing function. Yet, excess and/or chronic TNFα secretion causes massive tissue damage in autoimmune, inflammatory and neurological diseases and injuries. Therefore, many patients with autoimmune/inflammatory diseases receive anti-TNFα medications. TNFα is secreted primarily by CD4(+) T cells, macrophages, monocytes, neutrophils and NK cells, mainly after immune stimulation. Yet, the cause for the pathologically high and chronic TNFα secretion is unknown. Can blocking of a particular ion channel in T cells induce by itself TNFα secretion? Such phenomenon was never revealed or even hypothesized. In this interdisciplinary study we discovered that: (1) normal human T cells express Kv1.1 voltage-gated potassium channel mRNA, and the Kv1.1 membrane-anchored protein channel; (2) Kv1.1 is expressed in most CD4(+)CD3(+) helper T cells (mean CD4(+)CD3(+)Kv1.1(+) T cells of 7 healthy subjects: 53.09 ± 22.17 %), but not in CD8(+)CD3(+) cytotoxic T cells (mean CD8(+)CD3(+)Kv1.1(+) T cells: 4.12 ± 3.04 %); (3) electrophysiological whole-cell recordings in normal human T cells revealed Kv currents; (4) Dendrotoxin-K (DTX-K), a highly selective Kv1.1 blocker derived from snake toxin, increases the rate of rise and decay of Kv currents in both resting and activated T cells, without affecting the peak current; (5) DTX-K by itself induces robust TNFα production and secretion by normal human T cells, without elevating IFNγ, IL-4 and IL-10; (6) intact Ca(2+) channels are required for DTX-induced TNFα secretion; (7) selective anti-Kv1.1 antibodies also induce by themselves TNFα secretion; (8) DTX-K activates NFκB in normal human T cells via the unique non-canonical-pathway; (9) injection of Kv1.1-blocked human T cells to SCID mice, causes recruitment of resident mouse cells into the liver, alike reported after TNFα injection into the brain. Based on our discoveries we speculate that abnormally blocked Kv1.1 in T cells (and other immune cells?), due to either anti-Kv1.1 autoimmune antibodies, or Kv1.1-blocking toxins alike DTX-K, or Kv1.1-blocking genetic mutations, may be responsible for the chronic/excessive TNFα in autoimmune/inflammatory diseases. Independently, we also hypothesize that selective block of Kv1.1 in CD4(+) T cells of patients with cancer or chronic infectious diseases could be therapeutic, since it may: a. augment beneficial secretion and delivery of TNFα to the disease-affected sites; b. induce recruitment and extravasation of curative immune cells and factors; c. improve accessibility of drugs to the brain and few peripheral organs thanks to TNFα-induced increased permeability of organ's barriers.

Neuronal and Cardiovascular Potassium Channels as Therapeutic Drug Targets: Promise and Pitfalls

Potassium (K(+)) channels, with their diversity, often tissue-defined distribution, and critical role in controlling cellular excitability, have long held promise of being important drug targets for the treatment of dysrhythmias in the heart and abnormal neuronal activity within the brain. With the exception of drugs that target one particular class, ATP-sensitive K(+) (KATP) channels, very few selective K(+) channel activators or inhibitors are currently licensed for clinical use in cardiovascular and neurological disease. Here we review what a range of human genetic disorders have told us about the role of specific K(+) channel subunits, explore the potential of activators and inhibitors of specific channel populations as a therapeutic strategy, and discuss possible reasons for the difficulty in designing clinically relevant K(+) channel modulators.

Ion channel degeneracy enables robust and tunable neuronal firing rates

Firing rate is an important means of encoding information in the nervous system. To reliably encode a wide range of signals, neurons need to achieve a broad range of firing frequencies and to move smoothly between low and high firing rates. This can be achieved with specific ionic currents, such as A-type potassium currents, which can linearize the frequency-input current curve. By applying recently developed mathematical tools to a number of biophysical neuron models, we show how currents that are classically thought to permit low firing rates can paradoxically cause a jump to a high minimum firing rate when expressed at higher levels. Consequently, achieving and maintaining a low firing rate is surprisingly difficult and fragile in a biological context. This difficulty can be overcome via interactions between multiple currents, implying a need for ion channel degeneracy in the tuning of neuronal properties.

Spontaneous seizures in Kcna1-null mice lacking voltage-gated Kv1.1 channels activate Fos expression in select limbic circuits

Mice lacking voltage-gated Kv1.1 channels as a result of deletion of the Kcna1 gene are an extensively utilized genetic model of human epilepsy and sudden unexpected death in epilepsy because of their frequent seizures and genotypic-phenotypic similarity to the human condition. Ictal behaviors, electrophysiological recordings, and gene expression studies suggest limbic circuits are critical for epilepsy in Kcna1-null mice, but the exact brain networks recruited by seizures remain unknown. In this study, Fos protein expression patterns were used to map limbic brain regions with increased neuronal activity at baseline and during spontaneous seizures in Kcna1-null mice by comparing seizing and non-seizing knockouts and wild-type controls. Basal Fos levels were unchanged in non-seizing knockout mice compared to wild types for all brain regions examined except the dentate gyrus granule cell layer which exhibited a significant decrease in Fos-positive cells. Following seizures, Kcna1-null brains exhibited significantly increased Fos labeling in the basolateral amygdala and the dentate hilus region, but not in other principal cell layers of the hippocampal formation. The selective Fos activation in the amygdala following seizures suggests that extra hippocampal limbic circuits may be critically involved with seizure generation or spread in Kcna1-null mice. Fos protein expression patterns were analyzed using immunohistochemistry to provide the first map of brain regions recruited by spontaneous seizures in mice lacking Kv1.1 channels, an extensively used genetic model of epilepsy. Seizures significantly increased Fos expression in the amygdala and hilus by about fourfold, suggesting an important contribution by extrahippocampal networks to epilepsy in this model.

Kv1 channels and neural processing in vestibular calyx afferents

Potassium-selective ion channels are important for accurate transmission of signals from auditory and vestibular sensory end organs to their targets in the central nervous system. During different gravity conditions, astronauts experience altered input signals from the peripheral vestibular system resulting in sensorimotor dysfunction. Adaptation to altered sensory input occurs, but it is not explicitly known whether this involves synaptic modifications within the vestibular epithelia. Future investigations of such potential plasticity require a better understanding of the electrophysiological mechanisms underlying the known heterogeneity of afferent discharge under normal conditions. This study advances this understanding by examining the role of the Kv1 potassium channel family in mediating action potentials in specialized vestibular afferent calyx endings in the gerbil crista and utricle. Pharmacological agents selective for different sub-types of Kv1 channels were tested on membrane responses in whole cell recordings in the crista. Kv1 channels sensitive to α-dendrotoxin and dendrotoxin-K were found to prevail in the central regions, whereas K(+) channels sensitive to margatoxin, which blocks Kv1.3 and 1.6 channels, were more prominent in peripheral regions. Margatoxin-sensitive currents showed voltage-dependent inactivation. Dendrotoxin-sensitive currents showed no inactivation and dampened excitability in calyces in central neuroepithelial regions. The differential distribution of Kv1 potassium channels in vestibular afferents supports their importance in accurately relaying gravitational and head movement signals through specialized lines to the central nervous system. Pharmacological modulation of specific groups of K(+) channels could help alleviate vestibular dysfunction on earth and in space.

Selectivity filters and cysteine-rich extracellular loops in voltage-gated sodium, calcium, and NALCN channels

How nature discriminates sodium from calcium ions in eukaryotic channels has been difficult to resolve because they contain four homologous, but markedly different repeat domains. We glean clues from analyzing the changing pore region in sodium, calcium and NALCN channels, from single-cell eukaryotes to mammals. Alternative splicing in invertebrate homologs provides insights into different structural features underlying calcium and sodium selectivity. NALCN generates alternative ion selectivity with splicing that changes the high field strength (HFS) site at the narrowest level of the hourglass shaped pore where the selectivity filter is located. Alternative splicing creates NALCN isoforms, in which the HFS site has a ring of glutamates contributed by all four repeat domains (EEEE), or three glutamates and a lysine residue in the third (EEKE) or second (EKEE) position. Alternative splicing provides sodium and/or calcium selectivity in T-type channels with extracellular loops between S5 and P-helices (S5P) of different lengths that contain three or five cysteines. All eukaryotic channels have a set of eight core cysteines in extracellular regions, but the T-type channels have an infusion of 4–12 extra cysteines in extracellular regions. The pattern of conservation suggests a possible pairing of long loops in Domains I and III, which are bridged with core cysteines in NALCN, Cav, and Nav channels, and pairing of shorter loops in Domains II and IV in T-type channel through disulfide bonds involving T-type specific cysteines. Extracellular turrets of increasing lengths in potassium channels (Kir2.2, hERG, and K2P1) contribute to a changing landscape above the pore selectivity filter that can limit drug access and serve as an ion pre-filter before ions reach the pore selectivity filter below. Pairing of extended loops likely contributes to the large extracellular appendage as seen in single particle electron cryo-microscopy images of the eel Na_v1 channel.