Structural Basis for the Modulation of Human KCNQ4 by Small-Molecule Drugs

Fenestropathy of Voltage-Gated Sodium Channels

Voltage-gated sodium channels (Nav) are responsible for the initiation and propagation of action potentials in excitable cells. From pain to heartbeat, these integral membrane proteins are the ignition stations for every sensation and action in human bodies. They are large (>200 kDa, 24 transmembrane helices) multi-domain proteins that couple changes in membrane voltage to the gating cycle of the sodium-selective pore. Nav mutations lead to a multitude of diseases - including chronic pain, cardiac arrhythmia, muscle illnesses, and seizure disorders - and a wide variety of currently used therapeutics block Nav. Despite this, the mechanisms of action of Nav blocking drugs are only modestly understood at this time and many questions remain to be answered regarding their state- and voltage-dependence, as well as the role of the hydrophobic membrane access pathways, or fenestrations, in drug ingress or egress. Nav fenestrations, which are pathways that connect the plasma membrane to the central cavity in the pore domain, were discovered through functional studies more than 40 years ago and once thought to be simple pathways. A variety of recent genetic, structural, and pharmacological data, however, shows that these fenestrations are actually key functional regions of Nav that modulate drug binding, lipid binding, and influence gating behaviors. We discovered that some of the disease mutations that cause arrhythmias alter amino acid residues that line the fenestrations of Nav1.5. This indicates that fenestrations may play a critical role in channel's gating, and that individual genetic variation may also influence drug access through the fenestrations for resting/inactivated state block. In this review, we will discuss the channelopathies associated with these fenestrations, which we collectively name "Fenestropathy," and how changes in the fenestrations associated with the opening of the intracellular gate could modulate the state-dependent ingress and egress of drugs binding in the central cavity of voltage gated sodium channels.

Kv7.4 channel is a key regulator of vascular inflammation and remodeling in neointimal hyperplasia and abdominal aortic aneurysms

Inflammation has recently emerged as an important contributor for cardiovascular disease development and participates pivotally in the development of neointimal hyperplasia and abdominal aortic aneurysms (AAA) formation. Kv7.4/KCNQ4, a K⁺ channel, is one of the important regulators of vascular function but its role in vascular inflammation is unexplored. Here, we showed that the expression of Kv7.4 channel was elevated in the neointima and AAA tissues from mice and humans. Genetic deletion or pharmacological inhibition of Kv7.4 channel in mice alleviated neointimal hyperplasia and AAA formation via downregulation of a set of vascular inflammation-related genes, matrix metalloproteinases (MMP) 2/9, and intercellular adhesion molecule (ICAM-1). Furthermore, genetic deletion or inhibition of Kv7.4 channel suppressed the activation of tumor necrosis factor receptor 1 (TNFR1)-nuclear factor (NF)-κB signaling pathway via blockade of interaction between TNFR1 and TNFR1-associated death domain protein (TRADD) in vascular smooth muscle cells (VSMCs). Knockdown of Kv7.4 in vivo identified VSMC-expressed Kv7.4 as a major factor in vascular inflammation. Collectively, our findings suggest that Kv7.4 channel aggravates vascular inflammatory response, which promotes the neointimal hyperplasia and AAA formation. Inhibition of Kv7.4 channel may be a novel therapeutic strategy for vascular inflammatory diseases.

Structural insights into the lipid and ligand regulation of a human neuronal KCNQ channel

The KCNQ family (KCNQ1-KCNQ5) of voltage-gated potassium channels plays critical roles in many physiological and pathological processes. It is known that the channel opening of all KCNQs relies on the signaling lipid molecule phosphatidylinositol 4,5-bisphosphate (PIP2). However, the molecular mechanism of PIP2 in modulating the opening of the four neuronal KCNQ channels (KCNQ2-KCNQ5), which are essential for regulating neuronal excitability, remains largely elusive. Here, we report the cryoelectron microscopy (cryo-EM) structures of human KCNQ4 determined in complex with the activator ML213 in the absence or presence of PIP2. Two PIP2 molecules are identified in the open-state structure of KCNQ4, which act as a bridge to couple the voltage-sensing domain (VSD) and pore domain (PD) of KCNQ4 leading to the channel opening. Our findings reveal the binding sites and activation mechanisms of ML213 and PIP2 for neuronal KCNQ channels, providing a framework for therapeutic intervention targeting on these important channels.

Cell death-inducing cytotoxicity in truncated KCNQ4 variants associated with DFNA2 hearing loss

KCNQ4 encodes the homotetrameric voltage-dependent potassium ion channel Kv7.4, and is the causative gene for autosomal dominant nonsyndromic sensorineural hearing loss, DFNA2. Dominant-negative inhibition accounts for the observed dominant inheritance of many DFNA2-associated KCNQ4 variants. In addition, haploinsufficiency has been presumed as the pathological mechanism for truncated Kv7.4 variants lacking the C-terminal tetramerization region, as they are unlikely to exert a dominant-negative inhibitory effect. Such truncated Kv7.4 variants should result in relatively mild hearing loss when heterozygous; however, this is not always the case. In this study, we characterized Kv7.4^Q71fs (c.211delC), Kv7.4^W242X (c.725G>A) and Kv7.4^A349fs (c.1044_1051del8) in heterologous expression systems and found that expression of these truncated Kv7.4 variants induced cell death. We also found similar cell death-inducing cytotoxic effects in truncated Kv7.1 (KCNQ1) variants, suggesting that the generality of our findings could account for the dominant inheritance of many, if not most, truncated Kv7 variants. Moreover, we found that the application of autophagy inducers can ameliorate the cytotoxicity, providing a novel insight for the development of alternative therapeutic strategies for Kv7.4 variants.

Summary: Expression of truncated KCNQ4 variants lacking the C-terminal tetramerization domain results in cell-death inducing cytotoxicity, providing novel insight into the development of alternative therapeutic strategies for DFNA2 hearing loss.

The SUMO-specific protease SENP2 plays an essential role in the regulation of Kv7.2 and Kv7.3 potassium channels

Sentrin/small ubiquitin-like modifier (SUMO)-specific protease 2 (SENP2)-deficient mice develop spontaneous seizures in early life because of a marked reduction in M currents, which regulate neuronal membrane excitability. We have previously shown that hyper-SUMOylation of the Kv7.2 and Kv7.3 channels is critically involved in the regulation of the M currents conducted by these potassium voltage-gated channels. Here, we show that hyper-SUMOylation of the Kv7.2 and Kv7.3 proteins reduced binding to the lipid secondary messenger PIP₂. CaM1 has been shown to be tethered to the Kv7 subunits via hydrophobic motifs in its C termini and implicated in the channel assembly. Mutation of the SUMOylation sites on Kv7.2 and Kv7.3 specifically resulted in decreased binding to CaM1 and enhanced CaM1-mediated assembly of Kv7.2 and Kv7.3, whereas hyper-SUMOylation of Kv7.2 and Kv7.3 inhibited channel assembly. SENP2-deficient mice exhibited increased acetylcholine levels in the brain and the heart tissue because of increases in the vagal tone induced by recurrent seizures. The SENP2-deficient mice develop seizures followed by a period of sinus pauses or atrioventricular conduction blocks. Chronic administration of the parasympathetic blocker atropine or unilateral vagotomy significantly prolonged the life of the SENP2-deficient mice. Furthermore, we showed that retigabine, an M-current opener, reduced the transcription of SUMO-activating enzyme SAE1 and inhibited SUMOylation of the Kv7.2 and Kv7.3 channels, and also prolonged the life of SENP2-deficient mice. Taken together, the previously demonstrated roles of PIP2, CaM1, and retigabine on the regulation of Kv7.2 and Kv7.3 channel function can be explained by their roles in regulating SUMOylation of this critical potassium channel.

Drug treatment of epilepsy: From serendipitous discovery to evolutionary mechanisms

Epilepsy is a chronic brain disorder caused by abnormal firing of neurons. Up to now, using antiepileptic drugs is the main method of epilepsy treatment. The development of antiepileptic drugs lasted for centuries. In general, most agents entering clinical practice act on the balance mechanisms of brain "excitability-inhibition". More specifically, they target voltage-gated ion channels, GABAergic transmission and glutamatergic transmission. In recent years, some novel drugs representing new mechanisms of action have been discovered. Although there are about 30 available drugs in the market, it is still in urgent need of discovering more effective and safer drugs. The development of new antiepileptic drugs is into a new era: from serendipitous discovery to evolutionary mechanism-based design. This article presents an overview of drug treatment of epilepsy, including a series of traditional and novel drugs.

Novel KCNQ4 variants in different functional domains confer genotype- and mechanism-based therapeutics in patients with nonsyndromic hearing loss

Loss-of-function variant in the gene encoding the KCNQ4 potassium channel causes autosomal dominant nonsyndromic hearing loss (DFNA2), and no effective pharmacotherapeutics have been developed to reverse channel activity impairment. Phosphatidylinositol 4,5-bisphosphate (PIP₂), an obligatory phospholipid for maintaining KCNQ channel activity, confers differential pharmacological sensitivity of channels to KCNQ openers. Through whole-exome sequencing of DFNA2 families, we identified three novel KCNQ4 variants related to diverse auditory phenotypes in the proximal C-terminus (p.Arg331Gln), the C-terminus of the S6 segment (p.Gly319Asp), and the pore region (p.Ala271_Asp272del). Potassium currents in HEK293T cells expressing each KCNQ4 variant were recorded by patch-clamp, and functional recovery by PIP₂ expression or KCNQ openers was examined. In the homomeric expression setting, the three novel KCNQ4 mutant proteins lost conductance and were unresponsive to KCNQ openers or PIP₂ expression. Loss of p.Arg331Gln conductance was slightly restored by a tandem concatemer channel (WT-p.R331Q), and increased PIP₂ expression further increased the concatemer current to the level of the WT channel. Strikingly, an impaired homomeric p.Gly319Asp channel exhibited hyperactivity when a concatemer (WT-p.G319D), with a negative shift in the voltage dependence of activation. Correspondingly, a KCNQ inhibitor and chelation of PIP₂ effectively downregulated the hyperactive WT-p.G319D concatemer channel. Conversely, the pore-region variant (p.Ala271_Asp272del) was nonrescuable under any condition. Collectively, these novel KCNQ4 variants may constitute therapeutic targets that can be manipulated by the PIP₂ level and KCNQ-regulating drugs under the physiological context of heterozygous expression. Our research contributes to the establishment of a genotype/mechanism-based therapeutic portfolio for DFNA2.

Ligand modulation of KCNQ-encoded (KV7) potassium channels in the heart and nervous system

KCNQ-encoded (K_V7) potassium channels are diversely distributed in the human tissues, associated with many physiological processes and pathophysiological conditions. These channels are increasingly used as drug targets for treating diseases. More selective and potent molecules on various types of the K_V7 channels are desirable for appropriate therapies. The recent knowledge of the structure and function of human KCNQ-encoded channels makes it more feasible to achieve these goals. This review discusses the role and mechanism of action of many molecules in modulating the function of the KCNQ-encoded potassium channels in the heart and nervous system. The effects of these compounds on K_V7 channels help to understand their involvement in many diseases, and to search for more selective and potent ligands to be used in the treatment of many disorders such as various types of cardiac arrhythmias, epilepsy, and pain.

Chemical modulation of Kv7 potassium channels

The rising interest in Kv7 modulators originates from their ability to evoke fundamental electrophysiological perturbations in a tissue-specific manner. A large number of therapeutic applications are, in part, based on the clinical experience with two broad-spectrum Kv7 agonists, flupirtine and retigabine. Since precise molecular structures of human Kv7 channel subtypes in closed and open states have only very recently started to emerge, computational studies have traditionally been used to analyze binding modes and direct the development of more potent and selective Kv7 modulators with improved safety profiles. Herein, the synthetic and medicinal chemistry of small molecule modulators and the representative biological properties are summarized. Furthermore, new therapeutic applications supported by in vitro and in vivo assay data are suggested.

Activation of KCNQ4 as a Therapeutic Strategy to Treat Hearing Loss

Potassium voltage-gated channel subfamily q member 4 (KCNQ4) is a voltage-gated potassium channel that plays essential roles in maintaining ion homeostasis and regulating hair cell membrane potential. Reduction of the activity of the KCNQ4 channel owing to genetic mutations is responsible for nonsyndromic hearing loss, a typically late-onset, initially high-frequency loss progressing over time. In addition, variants of KCNQ4 have also been associated with noise-induced hearing loss and age-related hearing loss. Therefore, the discovery of small compounds activating or potentiating KCNQ4 is an important strategy for the curative treatment of hearing loss. In this review, we updated the current concept of the physiological role of KCNQ4 in the inner ear and the pathologic mechanism underlying the role of KCNQ4 variants with regard to hearing loss. Finally, we focused on currently developed KCNQ4 activators and their pros and cons, paving the way for the future development of specific KCNQ4 activators as a remedy for hearing loss.

Structural Mechanism of ω-Currents in a Mutated Kv7.2 Voltage Sensor Domain from Molecular Dynamics Simulations

Activation of voltage-gated ion channels is regulated by conformational changes of the voltage sensor domains (VSDs), four water- and ion-impermeable modules peripheral to the central, permeable pore domain. Anomalous currents, defined as ω-currents, have been recorded in response to mutations of residues on the VSD S4 helix and associated with ion fluxes through the VSDs. In humans, gene defects in the potassium channel Kv7.2 result in a broad range of epileptic disorders, from benign neonatal seizures to severe epileptic encephalopathies. Experimental evidence suggests that the R207Q mutation in S4, associated with peripheral nerve hyperexcitability, induces ω-currents at depolarized potentials, but the fine structural details are still elusive. In this work, we use atom-detailed molecular dynamics simulations and a refined model structure of the Kv7.2 VSD in the active conformation in a membrane/water environment to study the effect of R207Q and four additional mutations of proven clinical importance. Our results demonstrate that the R207Q mutant shows the most pronounced increase of hydration in the internal VSD cavity, a feature favoring the occurrence of ω-currents. Free energy and kinetics calculations of sodium permeation through the native and mutated VSD indicate as more favorable the formation of a cationic current in the latter. Overall, our simulations establish a mechanistic linkage between genetic variations and their physiological outcome, by providing a computational description that includes both thermodynamic and kinetic features of ion permeation associated with ω-currents.

Dynein regulates Kv7.4 channel trafficking from the cell membrane

The dynein motor protein transports proteins away from the cell membrane along the microtubule network. Recently, we found the microtubule network was important for regulating the membrane abundance of voltage-gated Kv7.4 potassium channels in vascular smooth muscle. Here, we aimed to investigate the influence of dynein on the microtubule-dependent internalization of the Kv7.4 channel. Patch-clamp recordings from HEK293B cells showed Kv7.4 currents were increased after inhibiting dynein function with ciliobrevin D or by coexpressing p50/dynamitin, which specifically interferes with dynein motor function. Mutation of a dynein-binding site in the Kv7.4 C terminus increased the Kv7.4 current and prevented p50 interference. Structured illumination microscopy, proximity ligation assays, and coimmunoprecipitation showed colocalization of Kv7.4 and dynein in mesenteric artery myocytes. Ciliobrevin D enhanced mesenteric artery relaxation to activators of Kv7.2–Kv7.5 channels and increased membrane abundance of Kv7.4 protein in isolated smooth muscle cells and HEK293B cells. Ciliobrevin D failed to enhance the negligible S-1–mediated relaxations after morpholino-mediated knockdown of Kv7.4. Mass spectrometry revealed an interaction of dynein with caveolin-1, confirmed using proximity ligation and coimmunoprecipitation assays, which also provided evidence for interaction of caveolin-1 with Kv7.4, confirming that Kv7.4 channels are localized to caveolae in mesenteric artery myocytes. Lastly, cholesterol depletion reduced the interaction of Kv7.4 with caveolin-1 and dynein while increasing the overall membrane expression of Kv7.4, although it attenuated the Kv7.4 current in oocytes and interfered with the action of ciliobrevin D and channel activators in arterial segments. Overall, this study shows that dynein can traffic Kv7.4 channels in vascular smooth muscle in a mechanism dependent on cholesterol-rich caveolae.