Voltage-gated potassium channels KCNQs: Structures, mechanisms, and modulations

Phosphatidylinositol 4,5-bisphosphate activation mechanism of human KCNQ5

The human voltage-gated potassium channels KCNQ2, KCNQ3, and KCNQ5 can form homo- and heterotetrameric channels that are responsible for generating the neuronal M current and maintaining the membrane potential stable. Activation of KCNQ channels requires both the depolarization of membrane potential and phosphatidylinositol 4,5-bisphosphate (PIP2). Here, we report cryoelectron microscopy structures of the human KCNQ5-calmodulin (CaM) complex in the apo, PIP2-bound, and both PIP2- and the activator HN37-bound states in either a closed or an open conformation. In the closed conformation, a PIP2 molecule binds in the middle of the groove between two adjacent voltage-sensing domains (VSDs), whereas in the open conformation, one additional PIP2 binds to the interface of VSD and the pore domain, accompanying structural rearrangement of the cytosolic domain of KCNQ and CaM. The structures, along with electrophysiology analyses, reveal the two different binding modes of PIP2 and elucidate the PIP2 activation mechanism of KCNQ5.

Activation of the Voltage-Gated Potassium Channel by Amphiphilic Glycopeptides

Voltage-gated ion channels (VGICs) are allosterically modulated by glycosaminoglycan proteoglycans and sialic acid glycans. However, the structural diversity and heterogeneity of these biomolecules pose significant challenges to precisely delineate their underlying structure-activity relationships. Herein, we demonstrate how heparan sulfate (HS) and sialic acid synthetic glycans appended on amphiphilic glycopeptide backbone influence cell membrane persistence and modulate the gating of the Kv2.1 channel. Utilizing a panel of amphiphilic glycopeptides comprising HS disaccharides and sialic acid trisaccharide glycans, we observed that sulfation of HS and flexible α(2-6) sialylation result in prolonged persistence of glycopeptides on the cell membrane compared to non-sulfated HS and α(2-3) sialylation respective. This variation in glycocalyx composition was associated with a noticeable difference in the effects of these compounds on the activation and deactivation properties of the voltage-gated Kv2.1 channel with our strongest membrane associating compound demonstrating the most potent channel-activation propensity. Our findings demonstrate that sulfation charges on glycopeptide play a critical role in their membrane association propensities and endow them with VGIC activation properties. These results provide a valuable insight into the role of cell surface glycans in VGIC activities.

Targeting Kv7 Potassium Channels for Epilepsy

Voltage-gated Kv7 potassium channels, particularly Kv7.2 and Kv.7.3 channels, play a critical role in modulating susceptibility to seizures, and mutations in genes that encode these channels cause heterogeneous epilepsy phenotypes. On the basis of this evidence, activation of Kv7.2 and Kv.7.3 channels has long been considered an attractive target in the search for novel antiseizure medications. Ezogabine (retigabine), the first Kv7.2/3 activator introduced in 2011 for the treatment of focal seizures, was withdrawn from the market in 2017 due to declining use after discovery of its association with pigmentation changes in the retina, skin, and mucosae. A novel formulation of ezogabine for pediatric use (XEN496) has been recently investigated in children with KCNQ2-related developmental and epileptic encephalopathy, but the trial was terminated prematurely for reasons unrelated to safety. Among novel Kv7.2/3 openers in clinical development, azetukalner has shown dose-dependent efficacy against drug-resistant focal seizures with a good tolerability profile and no evidence of pigmentation-related adverse effects in early clinical studies, and it is now under investigation in phase III trials for the treatment of focal seizures, generalized tonic-clonic seizures, and major depressive disorder. Another Kv7.2/3 activator, BHV-7000, has completed phase I studies in healthy subjects, with excellent tolerability at plasma drug concentrations that exceed the median effective concentrations in a preclinical model of anticonvulsant activity, but no efficacy data in patients with epilepsy are available to date. Among other Kv7.2/3 activators in clinical development as potential antiseizure medications, pynegabine and CB-003 have completed phase I safety and pharmacokinetic studies, but results have not been yet reported. Overall, interest in targeting Kv7 channels for the treatment of epilepsy and for other indications remains strong. Future breakthroughs in this area could come from exploitation of mechanistic differences in the action of Kv7 activators, and from the development of molecules that combine Kv7 activation with other mechanisms of action.

Astaxanthin Inhibits Ferroptosis of Hippocampal Neurons in Kainic Acid-Induced Epileptic Mice by Activating the Nrf2/GPX4 Signaling Pathway

BACKGROUND

Epilepsy, a prevalent neurological disorder, is distinguished by episodic abnormal discharges of neurons within the brain, resulting in transient brain dysfunction. Prior research has identified a novel form of cell death termed ferroptosis, which is intricately linked to the initiation and progression of epilepsy. It has been demonstrated that astaxanthin (AST) can inhibit ferroptosis by enhancing the activity of nuclear factor erythroid 2-related factor 2 (Nrf2), thereby providing cytoprotection. Therefore, this study aims to investigate whether AST can alleviate neuronal ferroptosis in epilepsy by activating the Nrf2/GPX4 pathway, thereby exerting a neuroprotective effect.

METHODS

By constructing a kainic acid (KA)-induced epilepsy mouse model and a KA-induced HT22 cell model, we employed behavioral testing, Western blot analysis, quantitative real-time reverse transcription qRT-PCR, ferroptosis-related assay kits, immunofluorescence staining, and other methods. These methodologies were utilized to investigate the protective effects and underlying mechanisms of AST on ferroptosis in KA-induced epileptic mice and HT22 neurons.

RESULTS

Our results demonstrate that AST pretreatment alleviates KA-induced epileptic behaviors and cognitive impairments in mice and mitigates ferroptosis indicators such as lipid peroxidation and mitochondrial morphological alterations. This neuroprotective effect appears to be mediated by the activation of the Nrf2/GPX4 signaling axis. In vitro studies further revealed that AST confers neuroprotection against KA-induced HT22 neuronal cell death, an effect that is abrogated by an Nrf2 inhibitor. Hence, the neuroprotective properties of AST are significantly associated with the modulation of the Nrf2-mediated ferroptosis pathway, as corroborated by bioinformatics analyses.

CONCLUSION

The AST effectively inhibits neuronal ferroptosis in both in vivo and in vitro epilepsy models via the Nrf2/GPX4 pathway. This finding suggests that AST holds promise as a potential therapeutic agent for the treatment of epilepsy.

Potentiation of Nicotine-Induced Currents by QO58, a Kv7 Channel Opener, in Intracardiac Ganglion Neurons of Rats

QO58 (5-(2,6-dichloro-5-fluoropyridin-3-yl)-3-phenyl-2-(trifluoromethyl)-1H-[1,5-a] pyrimidin-7-one) is currently used as a specific activator of the Kv7 (KCNQ) family of K+ channels. Here, we report an unexpected potentiating effect of this drug on nicotinic acetylcholine receptors. We recorded the whole-cell responses to the rapid application of nicotine with the Cs+-based pipette solution in intracardiac ganglion neurons freshly dissociated from the rat heart. Nicotine-induced inward currents were concentration-dependently blocked by mecamylamine, but not by 1 μM atropine at a holding potential of -60 mV. While the application of QO58 per se evoked a persistent inward current at this holding potential, 10 μM QO58 potentiated the peak amplitude of the nicotine-induced current. The QO58-induced inward currents were inhibited by the Kv7 channel blockers XE991 and Ba2+, but not by mecamylamine. On the other hand, the nicotine-induced current potentiated by QO58 was fully inhibited by mecamylamine. The facilitatory action of QO58 on the nicotinic response was unaffected by Ba2+. QO58 did not affect the reversal potential of the nicotine-induced current. QO58 apparently shifted the concentration-response curve of nicotine to the left. The half-maximal effective concentrations for nicotine in the absence and presence of 10 μM QO58 were 10.2 and 4.3 μM, respectively. These results suggest that QO58 acts as a positive allosteric modulator of nicotinic acetylcholine receptors. Given the prevalence of nicotinic receptor signaling, the present observations should be considered in future studies on the roles of Kv7 channels in the function of neural circuits and diseases.

Alleviating the Effects of Short QT Syndrome Type 3 by Allele-Specific Suppression of the KCNJ2 Mutant Allele

Short QT syndrome type 3 (SQTS3 or SQT3), which is associated with life-threatening cardiac arrhythmias, is caused by heterozygous gain-of-function mutations in the KCNJ2 gene. This gene encodes the pore-forming α-subunit of the ion channel that carries the cardiac inward rectifier potassium current (IK1). These gain-of-function mutations either increase the amplitude of IK1 or attenuate its rectification. The aim of the present in silico study is to test to which extent allele-specific suppression of the KCNJ2 mutant allele can alleviate the effects of SQT3, as recently demonstrated in in vitro studies on specific heterozygous mutations associated with long QT syndrome type 1 and 2 and short QT syndrome type 1. To this end, simulations were carried out with the two most recent comprehensive models of a single human ventricular cardiomyocyte. These simulations showed that suppression of the mutant allele can, at least partially, counteract the effects of the mutation on IK1 and restore the action potential duration for each of the four SQT3 mutations that are known by now. We conclude that allele-specific suppression of the KCNJ2 mutant allele is a promising technique in the treatment of SQT3 that should be evaluated in in vitro and in vivo studies.

Constitutive opening of the Kv7.2 pore activation gate causes KCNQ2-developmental encephalopathy

Pathogenic variants in KCNQ2 encoding Kv7.2 voltage-gated potassium channel subunits cause developmental encephalopathies (KCNQ2-encephalopathies), both with and without epilepsy. We herein describe the clinical, in vitro, and in silico features of two encephalopathy-causing variants (A317T, L318V) in Kv7.2 affecting two consecutive residues in the S6 activation gate that undergoes large structural rearrangements during pore opening; the disease-causing A356T variant in KCNQ3, paralogous to the A317T variant in KCNQ2, was also investigated. Currents through KCNQ2 mutant channels displayed increased density, hyperpolarizing shifts in activation gating, faster activation and slower deactivation kinetics, and resistance to changes in the cellular concentrations of phosphatidylinositol 4,5-bisphosphate (PIP2), a critical regulator of Kv7 channel function; all these features are consistent with a strong gain-of-function effect. An increase in the probability of single-channel opening, with no change in membrane abundance or single-channel conductance, was responsible for the observed gain-of-function effects. All-atom molecular dynamics simulations revealed that the mutations widened the inner pore gate and stabilized a constitutively open channel configuration in the closed state, with minimal effects on the open conformation. Thus, mutation-induced stabilization of the inner pore gate open configuration is a molecular pathogenetic mechanism for KCNQ2-related encephalopathies.

Lipid metabolism: Novel approaches for managing idiopathic epilepsy

Epilepsy is a common neurological condition characterized by abnormal neuronal activity, often leading to cellular damage and death. There is evidence to suggest that lipid imbalances resulting in cellular death play a key role in the development of epilepsy, including changes in triglycerides, cholesterol, sphingolipids, phospholipids, lipid droplets, and bile acids (BAs). Disrupted lipid metabolism acts as a crucial pathological mechanism in epilepsy, potentially linked to processes such as cellular ferroptosis, lipophagy, and immune modulation of gut microbiota (thus influencing the gut-brain axis). Understanding these mechanisms could open up new avenues for epilepsy treatment. This study investigates the association between disturbances in lipid metabolism and the onset of epilepsy.

Evolution of Bioelectric Membrane Potentials: Implications in Cancer Pathogenesis and Therapeutic Strategies

Electrophysiology typically deals with the electrical properties of excitable cells like neurons and muscles. However, all other cells (non-excitable) also possess bioelectric membrane potentials for intracellular and extracellular communications. These membrane potentials are generated by different ions present in fluids available in and outside the cell, playing a vital role in communication and coordination between the cell and its organelles. Bioelectric membrane potential variations disturb cellular ionic homeostasis and are characteristic of many diseases, including cancers. A rapidly increasing interest has emerged in sorting out the electrophysiology of cancer cells. Compared to healthy cells, the distinct electrical properties exhibited by cancer cells offer a unique way of understanding cancer development, migration, and progression. Decoding the altered bioelectric signals influenced by fluctuating electric fields benefits understanding cancer more closely. While cancer research has predominantly focussed on genetic and molecular traits, the delicate area of electrophysiological characteristics has increasingly gained prominence. This review explores the historical exploration of electrophysiology in the context of cancer cells, shedding light on how alterations in bioelectric membrane potentials, mediated by ion channels and gap junctions, contribute to the pathophysiology of cancer.

G protein βγ regulation of KCNQ-encoded voltage-dependent K channels

The KCNQ family is comprised of five genes and the expression products form voltage-gated potassium channels (Kv7.1-7.5) that have a major impact upon cellular physiology in many cell types. Each functional Kv7 channel forms as a tetramer that often associates with proteins encoded by the KCNE gene family (KCNE1-5) and is critically reliant upon binding of phosphatidylinositol bisphosphate (PIP2) and calmodulin. Other modulators like A-kinase anchoring proteins, ubiquitin ligases and Ca-calmodulin kinase II alter Kv7 channel function and trafficking in an isoform specific manner. It has now been identified that for Kv7.4, G protein βγ subunits (Gβγ) can be added to the list of key regulators and is paramount for channel activity. This article provides an overview of this nascent field of research, highlighting themes and directions for future study.