KCNQ2 potassium channel variants are linked to developmental and epileptic encephalopathy (DEE). However, the mechanisms by which pathogenic variants, especially those outside known hotspots, such as the S4-S5 linker, lead to disease remain unknown. Here, we examined the H228R variant, a pathogenic mutation in the S4-S5 linker associated with DEE. We tested whether H228R induces KCNQ2 channel mistargeting in addition to its biophysical effects, given recent evidence of impaired trafficking in KCNQ2 DEE variants. We confirmed the H228R variant as a loss-of-function (LOF) when expressed as a homomer and as a dominant-negative when coexpressed with wild-type (WT) KCNQ3. Surprisingly, it exhibited some gain-of-function effects when coexpressed with WT KCNQ2. To determine its cellular localization in vivo, we used male and female heterozygous Kcnq2H228R knock-in mice, some of which die prematurely despite lack of increases in hippocampal excitatory neuron intrinsic excitability. We validated two different KCNQ2 antibodies in hippocampus via immunohistochemistry. These antibodies detected KCNQ2 in axons, with signal loss observed in Kcnq2 knock-out mice. Using these antibodies, we found that the H228R variant caused KCNQ2 channels to concentrate in the soma, strongly reducing their presence in axons. Further, analysis of heterozygous mice expressing both a FLAG-tagged WT KCNQ2 and H228R revealed that the FLAG-WT KCNQ2 could still traffic to axons, indicating that some KCNQ2 channels are correctly targeted within neurons. In summary, our results demonstrate that the LOF H228R variant disrupts the localization of variant KCNQ2 channels, suggesting mislocalization as a general endophenotype of KCNQ2 encephalopathy.

Homeostatic plasticity allows neurons to maintain their activity around a set point, contributing to neuronal network stability over time. Increasing or decreasing neuronal activity not only regulates the levels of ion channels in neurons, but also leads to structural plasticity of the axon initial segment (AIS), the site of action potential initiation. Although previous studies have shown that activity could change the length and position of the AIS along the axon to compensate for changes in neuronal activity, it is currently unknown whether homeostatic plasticity also regulates the periodicity of the AIS actin cytoskeleton. Actin filaments, which are components of the axon plasma membrane skeleton, form periodic rings with a 190-nm spacing. Here, we examined whether the integrity of the actin ring periodicity at the AIS is plastic, depending on prolonged changes in neuronal activity. To induce AIS homeostatic plasticity, we either increased neuronal activity chronically by blocking the AIS enriched Kv7 voltage-gated potassium channels with XE991 or chronically decreased it by using the pan-Kv7 channel opener retigabine (RTG), or alternatively, the voltage-gated sodium channel blocker TTX at different neuronal maturation stages (DIV10 and DIV16). We found that prolonged exposure to XE991 relocated AIS away from soma and disrupted the periodicity of actin rings at DIV10. However, the actin ring periodicity remained unaffected at DIV16. In comparison, long-term treatment with RTG or TTX induced relocation of AIS towards the soma and reduction in AIS length, respectively. In parallel, RTG and TTX disrupted the actin ring periodicity at both developmental stages. These observations suggest that homeostatic plasticity not only changes the AIS position and length, but it could also modulate the periodicity of the AIS actin membrane skeleton, raising the possibility that the mechanical properties of the AIS are also plastic. Statement of significance Homeostatic plasticity allows neurons to maintain their activity around a set point, contributing to neuronal network stability over time. Although previous studies have shown that activity could change the length and position of the AIS along the axon to compensate for changes in neuronal activity, it is currently unknown whether homeostatic plasticity also regulates the periodicity of the AIS actin cytoskeleton. Here, using super-resolution microscopy, we show that either blocking or activating Kv7 channels could disrupt the periodicity of actin depending on the maturation stage of cultured neurons. In comparison, blocking voltage-gated sodium channels disrupted the actin ring periodicity independent of the age of neurons. These observations suggest that homeostatic plasticity also modulates the periodicity of the AIS actin cytoskeleton. ### Competing Interest Statement The authors have declared no competing interest. U.S. National Science Foundation, 2210535 National Institutes of Health, https://ror.org/01cwqze88, S10OD023618

KCNQ2 variants in children with neurodevelopmental impairment are difficult to assess due to their heterogeneity and unclear pathogenic mechanisms. We describe a child with neonatal-onset epilepsy, developmental impairment of intermediate severity, and KCNQ2 G256W heterozygosity. Analyzing prior KCNQ2 channel cryoelectron microscopy models revealed G256 as a node of an arch-shaped non-covalent bond network linking S5, the pore turret, and the ion path. Co-expression with G256W dominantly suppressed conduction by wild-type subunits in heterologous cells. Ezogabine partly reversed this suppression. Kcnq2G256W/+ mice have epilepsy leading to premature deaths. Hippocampal CA1 pyramidal cells from G256W/+ brain slices showed hyperexcitability. G256W/+ pyramidal cell KCNQ2 and KCNQ3 immunolabeling was significantly shifted from axon initial segments to neuronal somata. Despite normal mRNA levels, G256W/+ mouse KCNQ2 protein levels were reduced by about 50%. Our findings indicate that G256W pathogenicity results from multiplicative effects, including reductions in intrinsic conduction, subcellular targeting, and protein stability. These studies provide evidence for an unexpected and novel role for the KCNQ2 pore turret and introduce a valid animal model of KCNQ2 encephalopathy. Our results, spanning structure to behavior, may be broadly applicable because the majority of KCNQ2 encephalopathy patients share variants near the selectivity filter.

Abstract The past 70 years have provided a foundation for understanding potassium channel function at the single-channel, cellular, and circuit levels. Basic science of epilepsy has implicated potassium channel dysfunction in the pathogenesis of acquired epilepsy, and, more recently, genetic studies have revealed a large number of variants in potassium channel–encoding genes that cause epilepsy in humans. Identification and functional assessment of these variants have demonstrated they alter protein function, generating several hypotheses for how potassium channel dysfunction leads to the hypersynchronous neural activity characteristic of seizures. However, appreciation of the developmental, cellular, and synaptic complexity of the brain has increased in parallel to our knowledge of potassium channels, complicating the most straightforward idea that potassium channel dysfunction merely leads to depolarization of principal neurons, which causes seizures. The fact that many potassium channel–related epilepsies remain treatment refractory underscores the lack of a mechanistic understanding of this process. This chapter reviews our current understanding of the genetic and functional changes in potassium channels that may lead to epilepsy, within the framework of features of the excitable neuronal membrane known to be controlled by potassium currents. From there, the chapter expands into cell-type-specific roles of potassium channels, their effects on circuit development, and novel molecular interactions, and the evidence that these factors may play a role in epileptogenesis. Finally, the chapter highlights newer experimental approaches that have the potential to clarify the mechanisms through which potassium channel dysfunction leads to epilepsy and pave the way for more effective treatments.

A recent discovery established that axonal membrane is reinforced by the periodic axon membrane skeleton (PAMS) consisting of azimuthal actin rings connected longitudinally by spectrin tetramers. PAMS bestows the axon with distinct biomechanical properties. However, whether PAMS can regulate mobility of axonal channels non-associated to membrane skeleton is an open question. One such channel is the calcium-activated small conductance potassium (SK) channel, whose activity changes correlate with sleep, learning and memory, and diseases such as epilepsy, and schizophrenia.

Over the last decade KCNQ2 channels have arisen as fundamental and indispensable regulators of neonatal brain excitability, with KCNQ2 loss-of-function pathogenic variants being increasingly identified in patients with developmental and epileptic encephalopathy. However, the mechanisms by which KCNQ2 loss-of-function variants lead to network dysfunction are not fully known. An important remaining knowledge gap is whether loss of KCNQ2 function alters GABAergic interneuron activity early in development. To address this question, we applied mesoscale calcium imaging ex vivo in postnatal day 4-7 mice lacking KCNQ2 channels in interneurons (Vgat-ires-cre;Kcnq2f/f;GCamp5). In the presence of elevated extracellular potassium concentrations, ablation of KCNQ2 channels from GABAergic cells increased the interneuron population activity in the hippocampal formation and regions of the neocortex. We found that this increased population activity depends on fast synaptic transmission, with excitatory transmission promoting the activity and GABAergic transmission curtailing it. Together, our data show that loss of function of KCNQ2 channels from interneurons increases the network excitability of the immature GABAergic circuits, revealing a new function of KCNQ2 channels in interneuron physiology in the developing brain.

Small-conductance calcium-activated potassium (SK) channels show a ubiquitous distribution on neurons, in both somatodendritic and axonal regions. SK channels are associated with neuronal activity regulating action potential frequency, dendritic excitability, and synaptic plasticity. Although the physiology of SK channels and the mechanisms that control their surface expression levels have been investigated extensively, little is known about what controls SK channel diffusion in the neuronal plasma membrane. This aspect is important, as the diffusion of SK channels at the surface may control their localization and proximity to calcium channels, hence increasing the likelihood of SK channel activation by calcium. In this study, we successfully investigated the diffusion of SK channels labeled with quantum dots on human embryonic kidney cells and dissociated hippocampal neurons by combining a single-particle tracking method with total internal reflection fluorescence microscopy. We observed that actin filaments interfere with SK mobility, decreasing their diffusion coefficient. We also found that during neuronal maturation, SK channel diffusion was gradually inhibited in somatodendritic compartments. Importantly, we observed that axon barriers formed at approximately days in vitro 6 and restricted the diffusion of SK channels on the axon initial segment (AIS). However, after neuron maturation, SK channels on the AIS were strongly immobilized, even after disruption of the actin network, suggesting that crowding may cause this effect. Altogether, our work provides insight into how SK channels diffuse on the neuronal plasma membrane and how actin and membrane crowding impacts SK channel diffusion.

Gain-of-function (GOF) pathogenic variants in the potassium channels KCNQ2 and KCNQ3 lead to hyperexcitability disorders such as epilepsy and autism spectrum disorders. However, the underlying cellular mechanisms of how these variants impair forebrain function are unclear. Here, we show that the R201C variant in KCNQ2 has opposite effects on the excitability of two types of mouse pyramidal neurons of either sex, causing hyperexcitability in layer 2/3 (L2/3) pyramidal neurons and hypoexcitability in CA1 pyramidal neurons. Similarly, the homologous R231C variant in KCNQ3 leads to hyperexcitability in L2/3 pyramidal neurons and hypoexcitability in CA1 pyramidal neurons. However, the effects of KCNQ3 gain-of-function on excitability are specific to superficial CA1 pyramidal neurons. These findings reveal a new level of complexity in the function of KCNQ2 and KCNQ3 channels in the forebrain and provide a framework for understanding the effects of gain-of-function variants and potassium channels in the brain.SIGNIFICANCE STATEMENTKCNQ2/3 gain-of-function (GOF) variants lead to severe forms of neurodevelopmental disorders, but the mechanisms by which these channels affect neuronal activity are poorly understood. In this study, using a series of transgenic mice we demonstrate that the same KCNQ2/3 GOF variants can lead to either hyperexcitability or hypoexcitability in different types of pyramidal neurons [CA1 vs layer (L)2/3]. Additionally, we show that expression of the recurrent KCNQ2 GOF variant R201C in forebrain pyramidal neurons could lead to seizures and SUDEP. Our data suggest that the effects of KCNQ2/3 GOF variants depend on specific cell types and brain regions, possibly accounting for the diverse range of phenotypes observed in individuals with KCNQ2/3 GOF variants.

Loss- and gain-of-function variants in the gene encoding KCNQ2 channels are a common cause of developmental and epileptic encephalopathy, a condition characterized by seizures, developmental delays, breathing problems, and early mortality. To understand how KCNQ2 dysfunction impacts behavior in a mouse model, we focus on the control of breathing by neurons expressing the transcription factor Phox2b which includes respiratory neurons in the ventral parafacial region. We find Phox2b-expressing ventral parafacial neurons express Kcnq2 in the absence of other Kcnq isoforms, thus clarifying why disruption of Kcnq2 but not other channel isoforms results in breathing problems. We also find that Kcnq2 deletion or expression of a recurrent gain-of-function variant R201C in Phox2b-expressing neurons increases baseline breathing or decreases the central chemoreflex, respectively, in mice during the light/inactive state. These results uncover mechanisms underlying breathing abnormalities in KCNQ2 encephalopathy and highlight an unappreciated vulnerability of Phox2b-expressing ventral parafacial neurons to KCNQ2 pathogenic variants.

Brain-implanted stimulators are revolutionizing treatment of many neurological and psychiatric diseases, but still rely on temporary batteries for power, which require periodic replacements. Battery-free, self-sustainable deep brain stimulation (DBS) devices remain an unsolved challenge. Herein, we report a self-sustainable, battery-free, intermittent DBS system. This device is enabled by interfacing a high-performance bio-triboelectric nanogenerator (Bio-TENG) as an energy harvester with bio-supercapacitors as fast-charging energy storage units to intermittently drive a DBS pulse generator. The Bio-TENG acts as a smart breathing sensor and biomechanical energy harvester using the inhalation and exhalation motions of an inflated swine lung to charge bio-supercapacitors, which drive the pulse generator to stimulate neurons in mouse hippocampus tissues ex vivo. An intra-electrode triboelectrification strategy is used to boost the Bio-TENG power output. This sustainable intermittent power approach may provide a strategy for powering brain-machine bioelectronics.

Okur-Chung Neurodevelopmental Syndrome (OCNDS) is caused by heterozygous mutations to the CSNK2A1 gene, which encodes the alpha subunit of protein kinase CK2. The most frequently occurring mutation is lysine 198 to arginine (K198R). To investigate the impact of this mutation, we first generated a high-resolution phosphorylation motif of CK2WT, including the first characterization of specificity for tyrosine phosphorylation activity. A second high resolution motif representing CK2K198R substrate specificity was also generated. Here we report the impact of the OCNDS associated CK2K198R mutation. Contrary to prior speculation, the mutation does not result in a complete loss of function, but rather shifts the substrate specificity of the kinase. Broadly speaking the mutation leads to 1) a decreased preference for acidic residues in the +1 position, 2) a decreased preference for threonine phosphorylation, 3) an increased preference for tyrosine phosphorylation, and 4) an alteration of the tyrosine phosphorylation specificity motif. To further investigate the result of this mutation we have developed a probability-based scoring method, allowing us to predict shifts in phosphorylation in the K198R mutant relative to the wild type kinase. As an initial step we have applied the methodology to the set of axonally localized ion channels in an effort to uncover potential alterations of the phosphoproteome associated with the OCNDS disease condition.

KCNQ2 and KCNQ3 channels are associated with multiple neurodevelopmental disorders and are also therapeutic targets for neurological and neuropsychiatric diseases. For more than two decades, it has been thought that most KCNQ channels in the brain are either KCNQ2/3 or KCNQ3/5 heteromers. Here, we investigated the potential heteromeric compositions of KCNQ2-containing channels. We applied split-intein protein trans-splicing to form KCNQ2/5 tandems and coexpressed these with and without KCNQ3. Unexpectedly, we found that KCNQ2/5 tandems form functional channels independent of KCNQ3 in heterologous cells. Using mass spectrometry, we went on to demonstrate that KCNQ2 associates with KCNQ5 in native channels in the brain, even in the absence of KCNQ3. Additionally, our functional heterologous expression data are consistent with the formation of KCNQ2/3/5 heteromers. Thus, the composition of KCNQ channels is more diverse than has been previously recognized, necessitating a re-examination of the genotype/phenotype relationship of KCNQ2 pathogenic variants.

Anti-seizure drug (ASD) targets are widely expressed in both excitatory and inhibitory neurons. It remains unknown if the action of an ASD upon inhibitory neurons could counteract its beneficial effects on excitatory neurons (or vice versa), thereby reducing the efficacy of the ASD. Here, we examine whether the efficacy of the ASD retigabine (RTG) is altered after removal of the Kv7 potassium channel subunit KCNQ2, one of its drug targets, from parvalbumin-expressing interneurons (PV-INs). Parvalbumin-Cre (PV-Cre) mice were crossed with Kcnq2 -floxed ( Kcnq2 fl/fl ) mice to conditionally delete Kcnq2 from PV-INs. In these conditional knockout mice (cKO, PV- Kcnq2 fl/fl ), RTG (10 mg/kg, i.p.) significantly delayed the onset of either picrotoxin (PTX, 10 mg/kg, i.p)- or kainic acid (KA, 30mg/kg, i.p.)-induced convulsive seizures compared to vehicle, while RTG was not effective in wild-type littermates (WT). Immunostaining for KCNQ2 and KCNQ3 revealed that both subunits were enriched at axon initial segments (AISs) of hippocampal CA1 PV-INs, and their specific expression was selectively abolished in cKO mice. Accordingly, the M-currents recorded from CA1 PV-INs and their sensitivity to RTG were significantly reduced in cKO mice. While the ability of RTG to suppress CA1 excitatory neurons in hippocampal slices was unchanged in cKO mice, its suppressive effect on the spike activity of CA1 PV-INs was significantly reduced compared with WT mice. In addition, the RTG-induced suppression on intrinsic membrane excitability of PV-INs in WT mice was significantly reduced in cKO mice. These findings suggest that preventing RTG from suppressing PV-INs improves its anticonvulsant effect. Significance Statement One out of three patients with epilepsy remain resistant to anti-seizure drugs (ASD). Therefore, strategies that can enhance the efficacy of current ASDs or aid in the development of novel ASDs are in high demand. Here we show that by sparing a subset of inhibitory neurons, the ASD retigabine becomes more efficacious. These findings indicate that developing the ASDs that have relatively greater effects on excitatory over inhibitory neurons may be an effective strategy to improve the efficacy of ASDs.

Since its discovery in the mid-20th century, the Hodgkin-Huxley biophysical model of the squid giant axon's (SGA's) neurophysiology has traditionally served as the basis for the teaching of action potential (AP) dynamics in the physiology classroom. This model teaches that leak conductances set membrane resting potential; that fast, inactivating, voltage-gated sodium channels effect the SGA AP upstroke; and that delayed, rectifying, noninactivating voltage-gated potassium channels carry AP repolarization and the early part of the afterhyperpolarization (AHP). This model serves well to introduce students to the fundamental ideas of resting potential establishment and maintenance, as well as basic principles of AP generation and propagation. Furthermore, the Hodgkin-Huxley SGA model represents an excellent and accessible starting point for discussion of the concept of AP threshold and the role of passive electrical properties of the neuron. Additionally, the introduction of the Hodgkin-Huxley model of the SGA AP permits the integration of physiological principles, as instructors ask students to apply previously studied principles of transporter and channel biophysics to the essential physiological phenomenon of electrical signal conduction. However, both some early observations as well as more recent evidence strongly suggest that this seminal invertebrate model of AP dynamics does not appropriately capture the full story for mammalian axons. We review recent evidence that mammalian axonal nodes of Ranvier repolarize largely (though not exclusively) through the activity of leak potassium-ion (K+) conductances carried through two-pore domain (K-2P) channels. We call for changes to physiology textbooks and curricula to highlight this remarkable difference in invertebrate and mammalian AP repolarization mechanisms. NEW & NOTEWORTHY Historically, physiology courses have typically taught that action potential repolarization occurs exclusively due to the activation of delayed-rectifier voltage-gated potassium channels. Here, we review and highlight recent evidence that leak potassium channels of the two-pore domain (K-2P) class may largely serve this repolarization role at mammalian nodes of Ranvier. We call for the inclusion of these ideas in physiology curricula at all levels, from high school to graduate school.

Recent work has established that axons have a periodic skeleton structure comprising of azimuthal actin rings connected via longitudinal spectrin tetramer filaments. This structure endows the axon with structural integrity and mechanical stability. Additionally, voltage-gated sodium channels follow the periodicity of the active-spectrin arrangement, spaced ∼190 nm segments apart. The impact of this periodic arrangement of sodium channels on the generation and propagation of action potentials is unknown. To address this question, we simulated an action potential using the Hodgkin-Huxley formalism in a cylindrical compartment, but instead of using a homogeneous distribution of voltage-gated sodium channels in the membrane, we applied the experimentally determined periodic arrangement. We found that the periodic distribution of voltage-gated sodium channels does not significantly affect the generation or propagation of action potentials but instead leads to large, localized sodium action currents caused by high-density sodium nanodomains. Additionally, our simulations show that the distance between periodic sodium channel strips could control axonal excitability, suggesting a previously underappreciated mechanism to regulate neuronal firing properties. Together, this work provides a critical new insight into the role of the periodic arrangement of sodium channels in axons, providing a foundation for future experimental studies.

Dysfunctional dopamine signaling is implicated in various neuropsychological disorders. Previously, we reported that dopamine increases D1 receptor (D1R)-expressing medium spiny neuron (MSN) excitability and firing rates in the nucleus accumbens (NAc) via the PKA/Rap1/ERK pathway to promote reward behavior. Here, the results show that the D1R agonist, SKF81297, inhibits KCNQ-mediated currents and increases D1R-MSN firing rates in murine NAc slices, which is abolished by ERK inhibition. In vitro ERK phosphorylates KCNQ2 at Ser414 and Ser476; in vivo, KCNQ2 is phosphorylated downstream of dopamine signaling in NAc slices. Conditional deletion of Kcnq2 in D1R-MSNs reduces the inhibitory effect of SKF81297 on KCNQ channel activity, while enhancing neuronal excitability and cocaine-induced reward behavior. These effects are restored by wild-type, but not phospho-deficient KCNQ2. Hence, D1R-ERK signaling controls MSN excitability via KCNQ2 phosphorylation to regulate reward behavior, making KCNQ2 a potential therapeutical target for psychiatric diseases with a dysfunctional reward circuit.

Retigabine is a first-in-class potassium channel opener approved for patients with epilepsy. Unfortunately, several side effects have limited its use in clinical practice, overshadowing its beneficial effects. Multiple studies have shown that retigabine acts by enhancing the activity of members of the voltage-gated KCNQ (Kv7) potassium channel family, particularly the neuronal KCNQ channels KCNQ2-KCNQ5. However, it is currently unknown whether retigabine's action in neurons is mediated by all KCNQ neuronal channels or by only a subset. This knowledge is necessary to elucidate retigabine's mechanism of action in the central nervous system and its adverse effects and to design more effective and selective retigabine analogs. In this study, we show that the action of retigabine in excitatory neurons strongly depends on the presence of KCNQ3 channels. Deletion of Kcnq3 severely limited the ability of retigabine to reduce neuronal excitability in mouse CA1 and subiculum excitatory neurons. In addition, we report that in the absence of KCNQ3 channels, retigabine can enhance CA1 pyramidal neuron activity, leading to a greater number of action potentials and reduced spike frequency adaptation; this finding further supports a key role of KCNQ3 channels in mediating the action of retigabine. Our work provides new insight into the action of retigabine in forebrain neurons, clarifying retigabine's action in the nervous system.NEW & NOTEWORTHY Retigabine has risen to prominence as a first-in-class potassium channel opener approved by the Food and Drug Administration, with potential for treating multiple neurological disorders. Here, we demonstrate that KCNQ3 channels are the primary target of retigabine in excitatory neurons, as deleting these channels greatly diminishes the effect of retigabine in pyramidal neurons. Our data provide the first indication that retigabine controls neuronal firing properties primarily through KCNQ3 channels.