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Noebels JL, Avoli M, Rogawski MA, et al., editors. Jasper's Basic Mechanisms of the Epilepsies. 5th edition. New York: Oxford University Press; 2024. doi: 10.1093/med/9780197549469.003.0045
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.
Over the last decade, the epilepsy field has seen exponential growth in the discovery of new genes and pathogenic variants that cause epilepsy and developmental and epileptic encephalopathy (DEE) (Symonds et al., 2017; Hebbar and Mefford, 2020; Wang et al., 2020; Cornet and Cilio, 2019; El Kosseifi et al., 2019). This new era of discovery has been fueled by the availability of new sequencing and bioinformatics tools and by accelerated growth of the field of neurogenetics. Potassium channels have benefitted by this flourish of activity. Genes previously associated with epilepsy disorders have been reaffirmed and our understanding of their roles in genetic epilepsy has expanded (Fig. 45–1). Similarly, research has uncovered new genes that are associated with different types of epilepsy disorders (Fig. 45–1), shaping a landscape that not only informs on the possible causes of genetic disorders but also points toward potential new therapeutic approaches.
The potassium channel gene family and epilepsy channelopathies. Families are classified based on the number of transmembrane domains. The illustration shows the family grouping and a rendering of a representative atomic structure for each group. Genes (more...)
Our current understanding of potassium channel genes and their associated variants in epilepsy disorders has reached a level that could not fully be covered by one chapter. Indeed, over the last 10 years, several reviews have been published that provide a wealth of information about the different potassium channel families and epilepsy variants (Allen et al., 2020; Kohling and Wolfart, 2016; Cornet and Cilio, 2019; Brenner and Wilcox, 2012; D’Adamo et al., 2013; Oyrer et al., 2018; Villa and Combi, 2016; Cooper, 2012). Additionally, online resources like ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/), OMIM (Online Mendelian Inheritance in Man; https://www.omim.org/), and MARRVEL (Model organism Aggregated Resources for Rare Variant ExpLoration; http://marrvel.org/) provide further outlets to explore the impact of genetic variants in humans and model systems. To avoid simply replicating previous reviews and resources, this chapter takes a more functional perspective in discussing potassium channel genes and the mechanisms by which they can lead to epilepsy.
Highly systematic efforts have aimed to identify the potassium channels residing in neurons. Major advancements toward this goal include (1) the establishment by Hodgkin and Huxley in 1952 that a delayed potassium rectifier is responsible for the repolarization of the action potential (AP) (Hodgkin and Huxley, 1952a, 1952b); (2) the molecular identification of potassium channels in 1987, followed by the subsequent identification of the remaining potassium channel genes in the ensuing two decades (Gonzalez et al., 2012; Jegla et al., 2009); (3) the identification of the first potassium channels, KCNQ2 and KCNQ3, to underlie an epilepsy disorder in 1998 (Jentsch, 2000), ushering in a new era of potassium channelopathies in epilepsy; and (4) the development of pharmacological tools and potassium channel transgenic model systems in the last two decades that led to the identification of the functional roles of potassium channels in neurons. Thus, in this review, we try to explain the physiological and pathophysiological significance of the different potassium channel variants from the perspective of the known functional potassium channel cohorts in neurons discovered over the last 70 years. We primarily ground our discussion on forebrain neurons, as the neocortex and hippocampus play prominent roles in seizures and epilepsy. Similar discussions could occur for neurons in other regions such as the thalamus or cerebellum. Here, we first provide a brief introduction to potassium channels and then focus on the possible impact the different pathogenic potassium channel variants might have on neuronal physiology, synaptic transmission, and network excitability. We also describe new approaches to study potassium channel dysfunction and highlight that a full understanding of potassium channel disorders requires the integration of multiple approaches.
It is now well accepted that there are over 60 potassium channels in mammals (Jegla et al., 2009). As with most ion channels, there are two different ways that scientists typically refer to potassium channels. One is using the approved gene name given by the Human Genome Organization Gene Nomenclature Committee. In this system, all potassium channels and their auxiliary subunits start with the prefix “KCN” (Potassium Channel) followed by the family name designated with letters A to V. The second nomenclature approach is to classify potassium channels using guidelines by the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification, where potassium channels are designated as “K” followed by their subfamily name (“v” for voltage-gated, “Ca” for calcium-gated, “Na” for sodium-gated, “ir” for inward rectifiers, and “2P” for leak potassium channels) and ending with a number (1.1, 1.2, 2.1, etc.). In this review, we use both names interchangeably.
Potassium channels are classified into several families based on the number of transmembrane domains that they have (Gonzalez et al., 2012; Brenner and Wilcox, 2012). The inward rectifier family (2TM, Kir) has two transmembrane domains (2TMs) separated by a linker loop known as the P-loop (P stands for pore). The P-loop is a reentrant domain that only transverses part of the lipid bilayer housing the potassium channel selectivity filter. The selectivity filter is what allows potassium channels to discriminate potassium ions from sodium, calcium, and chloride ions (Gonzalez et al., 2012). The P-loop and its two associated transmembrane domains are also known as the pore domain. The next family is the four transmembrane (4TM) family that is made of 2TM domains linked together (also known as K2P channels). Members of this family have two P-loop domains and typically form what are referred to as potassium background “leak” channels. This occurs because 4TM potassium channels are open constitutively across the physiological membrane potential range. Another unique feature of 4TM channels is their structure that includes a “CAP” domain over the outer vestibule (Fig. 45–1) (Natale et al., 2021). Lastly, some members of this family lack a canonical intracellular gate and, therefore, gating instead occurs at the selectivity filter (Natale et al., 2021).
The largest family is the six-transmembrane (6TM) domain family that contains the voltage- gated potassium channels as well as the calcium- and sodium-activated potassium channels. With the exception of the KCNMA1 channels that have seven transmembrane domains, members of the 6TM family have one P-loop module linked to a 4TM domain that houses the voltage sensor. We note that the presence of a 4TM domain module does not always translate to a voltage-gated channel. For instance, small conductance calcium-activated potassium channels are not voltage-activated despite the presence of this domain, as the arginine-rich transmembrane S4 domain that confers voltage sensitivity to the potassium channels is uncharged in SK channels (Gonzalez et al., 2012). Irrespective of the family, a functional potassium channel requires the presence of four P-loop domains to come together. Thus, for the 2TM and 6TM domain family members, four independent potassium channels must join to form a tetrameric potassium channel. For the 4TM domain family members, only two channels must associate to form a multimer (Brenner and Wilcox, 2012; Gonzalez et al., 2012).
Over the last 10 years, significant progress has been made in determining the structures of potassium channels at the atomic level. Both X-ray crystallography and cryo-electron microscopy efforts have led to structures for multiple potassium channel members (Fig. 45–1). These snapshots of potassium channels, along with several decades of functional and mutagenesis studies, have revealed common principles but also unique differences among potassium channels.
The majority of potassium channels have the sequence TXGYG in their selectivity filter (D’Adamo et al., 2013; Natale et al., 2021; Black et al., 2021). Exceptions to this are the two-pore channels, where the tyrosine is replaced with additional amino acids in one of the pore domains, typically the second. The selectivity sequence allows potassium channels to conduct potassium in a diffusion-limited manner (Black et al., 2021). This is because the carboxyl groups of the selectivity filter amino acids and the side chain of the threonine amino acid mimic the water cage that potassium ions have when they are in aqueous solutions, allowing for an energetically favorable transition of potassium ions from the inner vestibule to the pore (Black et al., 2021). Although potassium channels discriminate between potassium and sodium ions, in the absence of potassium, potassium channels can conduct sodium (Korn and Ikeda, 1995). Thus, it is not surprising that selectivity filter variants can lead to potassium channels that have increased sodium permeability, leading to mixed conduction of potassium and sodium ions (Torkamani et al., 2014). Potassium conduction also depends on the selectivity filter structure and changes to its flexibility might alter conduction (Black et al., 2021).
In voltage-gated potassium channels, the first four transmembrane domains, known as S1–S4 or the voltage-sensor module, contain the voltage sensor. This module forms an outer and inner water vestibule separated by a hydrophobic plug within the lipid bilayer (Gonzalez et al., 2012). This allows the fourth transmembrane domain (S4), which contains a series of positively charged amino acids (arginines or lysines), to seemingly translate upward and rotate upon depolarization (via the helical screw model) (Gonzalez et al., 2012). Coupling between the voltage-sensor domain and the pore gate is mediated by residues in the S4-S5 linker, which interact with the residues in S6 that form the intracellular gate. In eukaryotic voltage- gated potassium channels, opening of the gate causes S6 to bend at a site known as PXP (proline-any amino acid-proline) (del Camino et al., 2005). This bend of the intracellular gate allows a hydrated potassium ion to enter the inner vestibule of the potassium channels before it sheds its water as it transverses the selectivity filter. Unlike all voltage-gated potassium channels, members of the KCNQ channel family do not have the PXP sequence in their S6; instead, they have a PAG sequence (Sun and MacKinnon, 2020), a sequence also found in silent nonexpressing voltage-gated potassium channels (KvS) (Bocksteins and Snyders, 2012). The PAG sequence prevents a complete bend of the intracellular gate, requiring potassium ions to squeeze through the gate (Sun and MacKinnon, 2020). Although voltage-gated potassium channels require movement of the voltage sensor to open, sodium- and calcium-activated channels require the presence of a ligand to fully open (Gonzalez et al., 2012). The ligand-binding site is found intracellularly and couples the binding of calcium and sodium to S6, causing the channel to open. Thus, these potassium channels open in response to an increase in intracellular sodium or calcium concentration (Gonzalez et al., 2012). By contrast, other potassium channels such as the KATP (i.e., KCNJ11) or GIRK (i.e., KCNJ6, KCNJ3) channels open due to changes to ATP levels or by G protein activation (Gonzalez et al., 2012).
As described above, potassium channels allow for potassium flux. This makes it possible for scientists to investigate the channel properties using electrophysiology. Typically, potassium channel variants are studied either in Xenopus laevis oocytes or in mammalian cells such as CHO or HEK293T cells. Oocytes have the advantage that they are large, allowing for considerable expression of ion channels on their surface. Additionally, their size makes it possible to perform two-electrode voltage clamp, allowing for detailed analysis of the potassium channel kinetics and biophysical properties. The use of oocytes is compatible with voltage-clamp fluorometry, allowing for simultaneous monitoring of potassium channel current activity and channel conformational changes such as voltage- sensor movement (Cowgill and Chanda, 2019). HEK293 and CHO cells are the preferred mammalian cell types for recordings as they have low native expression of potassium channels (CHO cells having less native expression than HEK293T cells) (Yu and Kerchner, 1998) and are amiable to whole-cell patch-clamp as well as high-throughput screening approaches testing hundreds of cells in parallel (Kang et al., 2019).
Following expression of a potassium channel variant in a cell type of interest, its biophysical properties are measured and it is typically classified as a loss-of-function (LOF) or gain-of-function (GOF) variant. As the name implies, LOF denotes reduced potassium current activity, whereas a GOF variant suggests an increase in potassium channel activity. Unfortunately, this binary classification, although easy to understand, does not fully capture the full spectrum of the identified variants. For instance, some variants change the selectivity of a potassium channel, allowing it to conduct both potassium and sodium ions. Such variants would be classified as LOF from the perspective of potassium ion flux, but GOF from the perspective of the potassium channel gaining a new property. For the purpose of this review, we define LOF and GOF from a functional potassium flux perspective. That is, any variant that leads to a decrease in potassium currents would be considered LOF, whereas any variant that increases the activity of a potassium channel would be considered GOF (Niday and Tzingounis, 2018). We note that defining a variant as LOF or GOF in heterologous cells might not necessarily translate to neurons. For instance, a variant that is a GOF in an expression system might not traffic in neurons, leading to an LOF. Therefore, the classification of LOF and GOF only defines the main effect of the variant on the channel properties in a heterologous system.
Although over 60 potassium channels have been molecularly identified, far fewer have been functionally confirmed in neurons (Storm, 1990; Rekling et al., 2000). Figure 45–2 shows a voltage response from a pyramidal neuron. In these traces, we have also indicated the potassium currents that are more likely to contribute to the different phases of the AP trajectory as well as to repetitive firing. Each potassium current has distinct biophysical, kinetic, and pharmacological properties, contributing to different phases of the voltage responses. As Figure 45–2 shows, potassium currents in neurons are classified as belonging to any of the following functional groups: Ileak, IKir, IA, ID, IK (also known as IKdr), IM, IfAHP (fast afterhyperpolarization, lasts < 100 ms), ImAHP (medium afterhyperpolarization, lasts 100–200 ms), and IsAHP (slow afterhyperpolarization, lasts >1 s). The different functional groups activate at different times and contribute to different processes (subthreshold potentials, spike repolarization, or various afterhyperpolarization components). Over the last 30 years, there has been some agreement as to which molecularly identified channels contribute to the different functional groups (Coetzee et al., 1999). Consequently, we group and then discuss the different epilepsy-associated genes based on the functional potassium current grouping to which they might belong in neurons.
Potassium currents in neurons. A. Potassium currents activated during an action potential waveform and repetitive firing. Left panels show the different potassium currents identified previously, whereas the right panels show the potassium channel genes (more...)
The IA potassium current, named by Connors and Stevens (1971), is a transient subthreshold activated potassium current that delays the onset of APs, contributes to AP repolarization, and controls the frequency of repetitive firing (Connor and Stevens, 1971). It is currently well established that members of the KCND (Kv4) family underlie the IA current (Coetzee et al., 1999; Jerng et al., 2004; Zemel et al., 2018; Carrasquillo and Nerbonne, 2014). In particular, a series of pharmacological, molecular, and genetic studies have demonstrated that KCND2 (Kv4.2) is the major somatodendritic IA potassium current in neurons. In axons and synaptic terminals, Kv1.4 channels likely contribute to the IA (Cooper et al., 1998). As such, we focus our discussion on members of the KCND family as they are primarily associated with epilepsy (Allen et al., 2020; Villa and Combi, 2016).
Although there have not been many reports of KCND2 variants, a 2014 study identified a de novo heterozygous variant in a pair of twins with infantile-onset refractory epilepsy and autism (Lee et al., 2014). This variant (V404M) was located in the S6 transmembrane domain, which is the domain that contributes to channel gating. Detailed biophysical analyses have demonstrated that this variant has three main effects: (1) increased inactivation of closed channels, that is, enhanced closed-state inactivation, (2) slowed recovery from inactivation, and (3) decreased closure of channels following activation (Lin et al., 2018). KCND3 variants have also been associated with epilepsy (Allen et al., 2020). KCND3 variants can be either LOF or GOF. LOF variants lead to either a decrease in surface trafficking or a right-shift of the activation threshold, whereas GOF variants lead to an increase in current density and slower inactivation.
KCND2 belongs to the voltage-gated potassium channel superfamily. It is primarily found in excitatory cells and has a distinct somatodendritic distribution, with increasing expression at distal ends of the dendrites (Trimmer, 2015). Because of this distribution and its subthreshold activation, KCND2 channels limit the invasion of backpropagating APs to distal dendrites (Kim et al., 2005; Tang and Thompson, 2012). The IA channel has several unique properties that KCND2 channels qualitatively recapitulate. Key aspects of IA and KCND2 channels are their activation at subthreshold membrane potentials, their rapid open and closed state inactivation, and their fast recovery from inactivation upon hyperpolarization (Jerng et al., 2004; Carrasquillo and Nerbonne, 2014). Although KCND2 is the pore-forming potassium channel that mediates the IA current in neurons, native IA is a multimeric complex that includes KCND2 and its auxiliary subunits K+ channel-interacting protein (KChiP) and dipeptidyl peptidase-like proteins (Carrasquillo and Nerbonne, 2014; Marionneau et al., 2011).
To understand the impact of this variant and possible mechanism of disease, we should consider the dual action of IA channels in neurons. A signature property of IA channels is their closed-state inactivation, which substantially occurs at resting membrane potentials (~–60 to –70 mV) (Jerng et al., 2004; Carrasquillo and Nerbonne, 2014). As the name implies, closed-state inactivation refers to the property of the IA channels to inactivate before they open. Consequently, the extent of closed-state inactivation determines the number of IA channels that are available to delay activation of AP generation following an incoming stimulus. Therefore, as the V404M mutation enhances closed-state inactivation, fewer IA channels would be available to prevent excitatory currents from increasing neuronal firing and to prevent backpropagating APs from depolarizing distal dendrites.
As mentioned, IA currents also contribute to the repolarization of the AP. As neurons reach the threshold of an AP, IA channels that have not yet been inactivated would activate. Based on their gating kinetics, IA channels activate soon after the peak of the AP. Previous work has indicated that less than 50% of the available IA channels activate during an AP and very few of these channels inactivate during the AP waveform (Mitterdorfer and Bean, 2002). Therefore, the IA deactivation kinetics would determine the speed of IA closure during the falling phase of the AP. As the V404M variant decreases the channel closure rate, the IA channel would remain open for a longer period of time, possibly speeding up the time course of an AP. Depending on the neuron type, such shortening of the AP time course might lead to bursting.
Although there have been few cases of patients with KCND2 variants in epilepsy, several studies have found that seizures and increased excitability can reduce the levels of the IA current in neurons (Tang and Thompson, 2012). Such a decrease in the IA current typically leads to decreased latency to the first AP following current injections from hyperpolarized membrane potentials.
KCND3 (Kv4.3) also contributes to the IA current (Allen et al., 2020). Previous studies using knockout mice have shown that KCND2 and KCND3 play distinct but complementary functions in neurons (Carrasquillo et al., 2012; Carrasquillo and Nerbonne, 2014; Norris and Nerbonne, 2010). For instance, deletion of Kcnd3 affects the firing properties of neurons following strong depolarizing current injections, whereas deletion of Kcnd2 leads to increased firing following small depolarizing inputs. A distinguishing feature of KCND3 over KCND2 channels is their recovery from inactivation, with KCND3 channels recovering faster than KCND2 channels. Therefore, KCND3 LOF variants would approximate the effects seen in Kcnd3 knockout mice, that is, an increase in repetitive firing following robust activity. In contrast, KCND3 GOF variants, which slow inactivation and have greater current density, would reduce repetitive firing, as recovery from inactivation between spikes would be prolonged.
Researchers now agree that members of the KCNA (Kv1) channel family underlie the neuronal ID, a voltage-activated potassium current. In particular, KCNA1, KCNA2, and possibly KCNA6 channels contribute to the ID current (Coetzee et al., 1999). For the purpose of this review we focus on KCNA1 and KCNA2, as an increasing number of variants in these channels have been identified in patients with epilepsy (Villa and Combi, 2016; Allen et al., 2020). As with other potassium channels, KCNA1 and KCNA2 can form either homomers or heteromers with each other. Additionally, KCNA1 and KCNA2 can associate with beta auxiliary subunits that can change their properties by shifting them from D-like currents to A-type currents (Gonzalez et al., 2012). A hallmark of the ID current is its sensitivity to low concentrations of 4-AP and α-dendrotoxin (Storm, 1988; Robertson et al., 1996), both of which are known chemoconvulsants.
Until recently, most variants associated with members of the KCNA family were associated with KCNA1 episodic ataxia 1 (EA1), comorbid with epilepsy (Brenner and Wilcox, 2012). Considering the high frequency of seizure occurrence in patients with EA1 and the finding that animal models lacking Kcna1 have seizures and SUDEP (i.e., Sudden Unexpected Death in Epilepsy) (Robbins and Tempel, 2012; Smart et al., 1998; Glasscock et al., 2010; Trosclair et al., 2020), we expect that KCNA1 variants can lead to seizures independent of ataxia. Consistent with this notion, KCNA1 variants in regions controlling KCNA1 gating lead to DEE (Allen et al., 2020). More recent work has identified a series of de novo variants in KCNA2 as well. These variants are LOF, GOF, or mixed (both LOF and GOF) variants leading to DEE (Masnada et al., 2017; Allen et al., 2020). Importantly, the most severe cases are in GOF variants that alter the activation and inactivation properties of the KCNA2 channels (Masnada et al., 2017; Niday and Tzingounis, 2018).
Similar to IA, ID potassium currents have dual roles in neuronal excitability, as they contribute to the subthreshold potassium currents as well as to the repolarization of the AP (Mitterdorfer and Bean, 2002; Storm, 1988, 1990; Cudmore et al., 2010). The ID current is a voltage-gated potassium current that activates rapidly but inactivates much slower than IA currents (seconds rather than milliseconds). Inactivation is voltage dependent such that at typical resting membrane potentials (–60 to –65 mV) a substantial amount of ID currents is inactivated. However, in neurons with much more hyperpolarized resting membrane potentials (–75 to –80 mV), ID currents are available. As the ID current inactivates and recovers from inactivation slowly, its main function is to delay the onset of the first AP and limit repetitive spiking. Earlier work has also suggested that ID currents control spike and network synchronization (Cudmore et al., 2010).
ID currents are very prominent in the axon initial segment, unmyelinated axons, and juxtaparanodal regions of myelinated axons (Rama et al., 2018; Trimmer, 2015). Due to their fast activation kinetics, ID currents activate shortly after the peak of the AP and are one of main contributors to AP repolarization in axons (Mitterdorfer and Bean, 2002; Rama et al., 2018). Indeed, the application of ID blockers in axons leads to a large prolongation of the axonal (but not somatic) AP time course, leading to a large calcium influx in axon terminals and subsequent neurotransmitter release (Rama et al., 2018). We note that the axon resting membrane potential is much more hyperpolarized than the resting membrane potential of the soma (Battefeld et al., 2014; Hu and Bean, 2018), increasing the ID currents’ availability and role during axonal APs.
Considering that the majority of KCNA1 variants lead to LOF, KCNA1 variants likely lead to seizures by reducing the levels of the ID currents in neurons that highly express the channel (Fig. 45–2). This would lead to an accelerated latency to the AP and prolongation of the axonal AP. Similar mechanisms are in play for LOF variants in KCNA2. This begs the question: how do GOF or mixed-effect variants alter excitability? GOF is attributed to a leftward shift in the voltage dependence of activation and LOF to a decreased maximum current density or increased likelihood of inactivation. To understand the mechanism of these diseases, one would have to model them in mice or study neurons derived from patients. Although it is currently unknown how KCNA2 GOF can lead to epilepsy (Niday and Tzingounis, 2018), we can speculate based on the known properties of the ID current in neurons. One possible explanation is that the GOF and mixed-effect variants only impact one aspect of the ID currents. For instance, a GOF variant might be more likely to delay activation of AP generation following a step depolarization or a ramp depolarization, but not necessarily impact the ability of the ID current to repolarize the AP. However, a large shift in the activation of the ID current would also promote inactivation, leading to decreased activity of the currents. Such functional neuronal LOF would increase excitability and might also lead to wider APs, allowing greater synaptic release of glutamate and network synchronization. Indeed, expression of epilepsy-associated KCNA1 variants in cultured hippocampal neurons broadens the AP, which leads to increased glutamate release in response to single APs and AP trains (Heeroma et al., 2009). At the network level, loss of Kcna1 increases fiber volley amplitudes, which is consistent with increased presynaptic depolarization and increases several measures of hippocampal network excitability (Simeone et al., 2013). Similarly, a mixed-effect variant might decrease the total amount of maximum ID current, thus only regulating the AP waveform but not the ability of the ID current to control the latency of APs. Further work using a combination of animal models and network modeling is necessary to understand the full spectrum of effects of KCNA2 mutations in epilepsy.
KCNB1, also known as Kv2.1, underlies the canonical delayed potassium channel rectifier (IK) found in the central nervous system; that is, it activates following the peak of the AP, and it does not inactivate for multiple seconds. Several studies using genetic and pharmacological approaches have established the involvement of Kv2 (KCNB) channels in the IK current (Murakoshi and Trimmer, 1999; Liu and Bean, 2014; Malin and Nerbonne, 2002). In addition to KCNB1, KCNB2 (Kv2.2) channels are expressed in neurons possibly as homomers or heteromers with KCNB1 (Malin and Nerbonne, 2002). As the majority of pathogenic variants related to epilepsy are found in KCNB1, we primarily focus our discussion on KCNB1 channels (Allen et al., 2020; Oyrer et al., 2018). A unique feature of KCNB1 channels is their association with silent potassium channels such as KCNF (Kv5), KCNG (Kv6), KCNV (Kv8), and KCNS (Kv9) (Bocksteins, 2016; Bocksteins and Snyders, 2012). Silent channels have the same topology as regular voltage-gated potassium channels; however, they cannot form functional homomeric channels because they do not traffic to the surface. Instead, they associate with members of the KCNB family to form heteromers. The precise subunit stoichiometry between KCNB and KvS channels is not fully established, with some earlier work suggesting a 3:1 KCNB:KvS ratio and more recent studies suggesting a 2:2 KCNB:KvS ratio (Moller et al., 2020). Independent of the subunit composition, the association of KvS with a Kv2 channel alters its biophysical properties. Indeed, studies have identified KCNV2 (Kv8.2) variants that lead to DEE likely due to the altered activity of KCNB1 channels (Jorge et al., 2011).
IK channels have several hallmarks that distinguish them from ID and IA channels (Storm, 1990). (1) They activate following the generation of the AP. Unlike ID and IA channels, they do not activate at subthreshold membrane potentials. (2) KCNB channels are found in proximal dendrites, the soma, and the axon initial segment (Trimmer, 2015). As a result, IK and Kv2 channels can influence dendritic, somatic, and axonal computations. (3) KCNB channels associate with vesicle-associated membrane protein-associated proteins to organize a complex between the plasma membrane and the endoplasmic reticulum (Kirmiz et al., 2018). This allows the formation of calcium domains, as this complex also includes voltage-gated calcium channels and ryanodine receptors (Vierra et al., 2019).
Over the last few years several KCNB1 variants were identified in patients with epileptic encephalopathy (Torkamani et al., 2014; de Kovel et al., 2017). Subsequently, additional patients with DEE were identified carrying KCNB1 variants (Allen et al., 2020). The variants were typically de novo, and in all cases the patients were heterozygous for the pathogenic variants. The DEE variants are distributed across all KCNB1 regions, with the majority of variants found in the transmembrane domains (Kang et al., 2019). Detailed functional characterization of a series of KCNB1 variants revealed that KCNB1 pathogenic variants are primarily LOF. However, the LOF mechanism varied, with some variants leading to decreased current density due to reduced surface expression, shift in voltage-activation toward more depolarized membrane potentials, or increased inactivation. A functional characterization also showed that the variants act as dominant negatives; thus, one copy of the channel is sufficient to lead to a dysfunctional tetrameric channel (Kang et al., 2019).
Researchers have extensively studied KCNB-mediated IK currents over the last two decades with multiple studies showing that KCNB channels control the firing properties of various types of neurons (i.e., pyramidal neurons of the hippocampus, entorhinal cortex and neocortex, and superior cervical ganglion cells) (Liu and Bean, 2014; Malin and Nerbonne, 2002; Guan et al., 2013; Kimm et al., 2015; Honigsperger et al., 2017). KCNB channels contribute to the later phase of the AP repolarization as they have slower activation kinetics than IA and ID channels (Mitterdorfer and Bean, 2002). Therefore, the IK current operates when the membrane potential is closer to the potassium equilibrium potential (EK). This might explain why neurons have very large IK currents, as a high channel density would overcome otherwise small currents caused by a low driving force (Vm-EK). Additionally, as IK and KCNB channels have relative slow deactivation kinetics, they could also contribute to the fast afterhyperpolarization (fAHP), which helps relieve the inactivation of sodium channels. Consequently, blocking Kv2 channels using the dominant negative variants leads to greater susceptibility to depolarization block due to cumulative sodium channel inactivation (Guan et al., 2013). This cumulative sodium channel inactivation results in smaller APs and eventually AP failure. Although most variants lead to reduced KCNB1 current levels, some variants such as KCNB1G279R result in loss of potassium selectivity (Torkamani et al., 2014). This variant also leads to voltage-independent cationic leak currents, such that KCNB1 channels allow sodium flux at resting membrane potentials. This additional sodium conductance at subthreshold potentials would likely add to the subthreshold persistent sodium current present in pyramidal neurons, leading to increased neuronal excitability and greater excitatory postsynaptic potential amplification (Carter et al., 2012). Consistent with this, expression of KCNB1 variants in cultured neurons lead to decreased input resistance and repetitive firing (Saitsu et al., 2015). Thus, these effects would be more severe compared to a variant that simply reduces KCNB1 current density. Consistent with this prediction, Kcnb1G279R mice exhibit spontaneous seizures (Hawkins et al., 2021), a phenotype more severe that the one previously described for Kcnb1 knockout mice—mice that have reduced thresholds for seizures but do not exhibit spontaneous seizures (Speca et al., 2014).
KCNC1, also known as Kv3.1, is a delayed potassium channel rectifier with a high threshold for activation (V0.5 = –10 mV) and no fast inactivation. Kv3.1 are known for their fast activation, their very fast closing kinetics, and, more recently, their resurgent current during AP repolarization (Kaczmarek and Zhang, 2017; Labro et al., 2015). Therefore, the biophysical properties of Kv3 channels allow them to shorten AP duration, causing neurons to very rapidly repolarize and minimizing the refractory period between APs. Consequently, KCNC1 expression permits neurons such as fast-spiking GABAergic neurons or neurons of the auditory system to fire at high frequency. The faster AP repolarization also limits the spike-evoked calcium flux in terminals, thus also regulating synaptic transmission properties.
Currently, only a few cases of epilepsy patients associated with KCNC1 variants have been identified (Allen et al., 2020). In particular, a group of patients with childhood-onset progressive myoclonic epilepsy carried a recurrent de novo heterozygous KCNC1 variant (Muona et al., 2015; Oliver et al., 2017). The variant (R320H) is within the KCNC1 voltage sensor. In oocytes, this variant led to strongly reduced currents even when coexpressed with wild-type channels (80% current reduction). Thus, the variant acts as a dominant negative. Moreover, the variant alters the KCNC1 gating, shifting activation toward hyperpolarized membrane potentials. Additional variants that might not act as dominant negatives have also been recently identified (Cameron et al., 2019).
Over the years, multiple groups have studied the role of KCNC1 channels in controlling fast-spiking activity. Although Kcnc1 knockout mice have subtle behavioral phenotypes, double Kcnc1/Kcnc3 knockout mice have severe ataxia and myoclonus (Espinosa et al., 2001). Considering that KCNC1, KCNC2, and KCNC3 co-localize in multiple neuronal types, the effects of KCNC1 R230H might reflect LOF activity in KCNC1 heteromeric rather than homomeric channels. The reduction of the KCNC1 current would likely transform a rapidly firing neuron to more regular adapting neuron (Lien and Jonas, 2003). This would decrease the AP firing frequency, leading to altered synaptic transmission during repetitive stimulations. As KCNC1 channels are highly expressed in fast-spiking GABAergic interneurons, their decreased firing frequency might lead to reduced feed-forward inhibition. KCNC1 also plays a role in limiting and sharpening the presynaptic waveform, presynaptic calcium influx, and neurotransmitter release, especially at synapses made by fast-spiking GABAergic neurons (Hoppa et al., 2014; Goldberg et al., 2005). Thus, KCNC1 dysfunction may also increase synaptic depression during high-frequency trains at FS GABAergic synapses.
Members of the KCNQ channel family (also referred to as Kv7) mediate the M-current and the medium afterhyperpolarization current (mAHP) (as well as the slow AHP in some neurons) (Wang et al., 1998; Peters et al., 2005; Soh et al., 2014; Laker et al., 2021). The KCNQ subfamily belongs to the 6TM family; thus, it has a P-loop domain and a voltage-sensing module. The KCNQ subfamily consists of five members, named KCNQ1–KCNQ5 (Jentsch, 2000; Greene and Hoshi, 2017). In the context of potassium channels and epilepsy, we primarily focus on two members of this family, KCNQ2 and KCNQ3, as they are the KCNQ members most commonly associated with epilepsy (Allen et al., 2020; Villa and Combi, 2016; Oyrer et al., 2018). Researchers have recently identified KCNQ5 channels in some patients with epilepsy, but the current knowledge about KCNQ5 variants is very limited (Lehman et al., 2017) and few studies have focused on KCNQ1 variants and epilepsy (Goldman et al., 2009).
In 1998, positional cloning of patients with rare self-limiting pediatric epilepsy disorders led to the discovery of the KCNQ2 and KCNQ3 channels (Jentsch, 2000). Soon after their discovery, several groups showed that KCNQ2 and KCNQ3 channels form heteromers leading to the M-current (Wang et al., 1998), a slowly activating noninactivating potassium current (Brown and Adams, 1980). The M-current starts activating before the AP threshold is reached, acting as a brake to incoming activity (Cooper and Jan, 2003). As KCNQ2 and KCNQ3 channels are expressed in myelinated and nonmyelinated axons, these channels also set the resting membrane potentials of axons (Hu and Bean, 2018; Battefeld et al., 2014). KCNQ2 and KCNQ3 channels exhibit slow activation kinetics, preventing them from being substantially activated during an AP. However, these channels can activate after a brief bout of activity or in neurons with prominent afterdepolarization (Yue and Yaari, 2004). The activation of KCNQ2/3 channels following an afterdepolarization or brief bout of activity leads to an mAHP that lasts no more than 50–100 ms (Bean, 2007).
Numerous excellent resources have described the role of KCNQ2 and KCNQ3 channels in physiology and in neurological disorders (Nappi et al., 2020; Cooper, 2012; El Kosseifi et al., 2019; Cornet and Cilio, 2019). Here, we summarize some of the key findings that have emerged in relation to KCNQ2 and KCNQ3 variants. Initially, KCNQ2 and KCNQ3 LOF variants were identified in patients with benign familial neonatal epilepsy (BFNE), a self-limiting pediatric epilepsy disorder (Cooper, 2012); currently known as self-limited (familial) neonatal epilepsy (SLFNE). In 2012, however, research demonstrated that LOF KCNQ2 variants can also lead to severe epilepsy disorders such as Ohtahara syndrome (Weckhuysen et al., 2012). Following this finding, many new variants were identified in patients with neonatal epilepsy. It is now well established that KCNQ2 variants most frequently arise in neonates that present with seizures within a few hours or days after birth, eventually leading to DEE (El Kosseifi et al., 2019). For self-limiting neonatal epilepsy, KCNQ2 variants can occur across the protein, although the S2-S3 linker could be considered a hotspot (Goto et al., 2019). However, for patients with DEE, most variants concentrate into three areas: the pore region, the voltage-sensor domain, and a C-terminus domain required for binding of the KCNQ2 auxiliary subunit calmodulin (Zhang et al., 2020; Goto et al., 2019). Variants in the voltage-sensor domain can be either LOF or GOF variants. Patients with GOF variants present with symptoms distinct from patients with LOF (El Kosseifi et al., 2019). In particular, these patients do not have seizures as neonates; rather, they exhibit myoclonic spasms and severe respiratory dysfunction. KCNQ2 GOF can lead to premature lethality, likely due to failure to breathe and to regulate chemoreflexes (Mulkey et al., 2017).
Studies have shown that KCNQ3 LOF variants also lead to self-limiting neonatal epilepsy and, more recently, to severe forms of epilepsy (Lauritano et al., 2019; Nappi et al., 2020); however, this requires the loss of both functional copies of KCNQ3 channels (Lauritano et al., 2019). In contrast, one dysfunctional copy of KCNQ2 can lead to DEE. KCNQ3 GOF variants have also been identified. Such variants, also identified in the voltage-sensor domain, lead to autism spectrum disorders and electrical status epilepticus in sleep (ESES) (Sands et al., 2019).
Unlike many potassium channels, our understanding of the function of KCNQ2 and KCNQ3 channels in neurons is well established. However, the mechanism by which KCNQ2 and KCNQ3 variants lead to a spectrum of symptoms is not known. As described previously, KCNQ2 and KCNQ3 channels mediate the M-current and the mAHP. This regulation has been established through multiple studies using both pharmacological agents and a series of transgenic mice (Nappi et al., 2020; Greene and Hoshi, 2017; Springer et al., 2021). KCNQ2 and KCNQ3 channels are unique in that their expression occurs early in development, well before neurons have matured, followed by high expression in the axons (Springer et al., 2021; Dirkx et al., 2020). As a result, changes in KCNQ2 and KCNQ3 channel activity can alter the electrophysiological properties of developing neurons and, later, developing axons. Typically, KCNQ2 and KCNQ3 channels counteract the depolarizing activity of the persistent sodium current that resides in most, if not all, neurons (Golomb et al., 2006; Verneuil et al., 2020). LOF KCNQ2 and KCNQ3 variants increase excitability as the persistent sodium current proceeds unabated. Depending on the neuron activity, the undiminished persistent sodium current can lead to increased neuronal firing activity and sustained depolarization. Indeed, recent work using conditional Kcnq2 knockout mice has shown that loss of KCNQ2 channels leads to widespread neuronal excitability and, importantly, seizures immediately followed by spreading depression (Aiba and Noebels, 2021). Considering that KCNQ2 and KCNQ3 channels occur as heteromers, it is unclear why KCNQ2 but not KCNQ3 de novo variants lead to DEE. One possible explanation is that KCNQ2 channels are expressed earlier than KCNQ3 channels (Dirkx et al., 2020; Springer et al., 2021). Consequently, in early life, the majority of M-current channels would contain either one or two KCNQ2 pathogenic variant subunits per tetramer. As neurons mature, KCNQ3 levels would steadily increase to a level similar to that of KCNQ2 channels, leading to channels with one KCNQ2 variant per tetramer. Thus, the difference between KCNQ2- and KCNQ3-related epilepsy might result from the developmental trajectory of KCNQ2 and KCNQ3 channels.
In contrast to LOF variants, GOF variants, in principle, might lead to hyperpolarization of the membrane potential (Niday and Tzingounis, 2018). Thus, the question arises: how do KCNQ2 and KCNQ3 GOF channels lead to hyperexcitability in infants? One possibility might be cell-type localization. KCNQ2 and KCNQ3 channels are also expressed in interneurons (Fig. 45–2) (Cooper et al., 2001), and their deletion leads to elevated interneuron excitability (Soh et al., 2018). Thus, KCNQ2 and KCNQ3 variants might decrease interneuron firing activity, preferentially leading to network disinhibition. Another unexplored possibility is homeostatic adaptation. Earlier work has shown that hippocampal inactivity (induced by application of the sodium channel blocker tetrodotoxin) leads to seizures later in life (Galvan et al., 2000); thus, the presence of increased KCNQ2/3 activity early in development might lead to a later homeostatic rebound of activity, resulting in seizures. A future challenge is to define the mechanisms not only of KCNQ2 and KCNQ3 LOF variants but also GOF variants using a combination of conditional knock-in models and patient-derived pluripotent cells.
For the purpose of this section, we discuss the fAHP mediated by calcium- and voltage-activated potassium currents (also known as either big K channels or maxi K channels due to their large potassium conductance, which exceeds 100 pS; in comparison KCNQ2 has a 5–10 pS conductance) (Gonzalez et al., 2012; Bailey et al., 2019; Contet et al., 2016). As mentioned previously, KCNB (Kv2) channels can also contribute to the fAHP due to their slow channel deactivation kinetics (see IK above).
KCNMA1 channels are tetramers of the alpha subunit KCNMA1. Although KCNMA1 channels belong to the 6TM potassium channel family, they actually have seven transmembrane domains leading to an extracellular N-terminus (Gonzalez et al., 2012). The additional transmembrane domain known as S0 allows KCNMA1 channels to interact with auxiliary subunits (beta and gamma), which regulate both its biophysical and pharmacological properties (Gonzalez-Perez and Lingle, 2019). Unlike most 6TM channels, KCNMA1 channels have an extensive C-terminus that houses two calcium-binding domains, known as regulators of K+ conductance (RCK) domains (RCK1, RCK2). Although KCNMA1 channels have an S4 region and respond to depolarization, their opening does not occur within the physiological range. Rather, calcium binding to KCNMA1 drives their voltage activation to hyperpolarized membrane potentials, allowing KCNMA1 channels to open during an AP (Contet et al., 2016). In particular, upon depolarization and calcium entry during AP repolarization, KCNMA1 channels open rapidly (Niday and Bean, 2021), limiting the duration of the AP. This tight control between calcium, AP, and KCNMA1 channel activation is possible because KCNMA1 channels are located in a nano-domain complex with calcium channels (Fakler and Adelman, 2008). A unique feature of KCNMA1 channels is their extensive alternative splicing, which allows different cell types to tune the KCNMA1 channel calcium and voltage sensitivity (Bailey et al., 2019).
KCNMA1 channels are well known as the first potassium channels to exhibit GOF variants associated with epilepsy (Brenner and Wilcox, 2012). The first variant (D434G) increases the open probability of KCNMA1 channels, boosting their calcium sensitivity and leading to faster activation and slower deactivation (Brenner and Wilcox, 2012). Since that initial discovery, multiple additional variants have been identified. However, currently, the majority of known variants are LOF rather than GOF (Bailey et al., 2019). As in most cases, most variants are de novo, heterozygous, and concentrated in either the intracellular C-terminus, the selectivity filter, or the linker between the C- terminus and the S6 domain (Bailey et al., 2019). Although it is unknown whether the epilepsy-associated KCNMA1 channel variants act as dominant negatives, a recent characterization of a variant identified in children with congenital and progressive cerebellar ataxia showed that this variant acted as a dominant negative (Du et al., 2020), raising this possibility for KCNMA1 channels in general. Currently, genotype–phenotype relationships between patients with LOF and GOF variants have not revealed any differences (Bailey et al., 2019). Consequently, patients with either type of variant can exhibit generalized tonic, absence, or myoclonic seizures.
Researchers have extensively studied the role of KCNMA1 channels in neuronal physiology (Contet et al., 2016). This focus is partly due to the availability of specific blockers for KCNMA1 channels, starting in the 1980s. In many neurons, inhibiting KCNMA1 channels increases the firing frequency of the recorded neurons. However, this property is not general. For instance, blocking KCNMA1 channels in hippocampal pyramidal neurons leads to a decrease in the firing rate (Gu et al., 2007). The net effect of KCNMA1 inhibition on neuronal excitability is complex and heavily depends on firing frequency as well as the molecular context: auxiliary subunits (Gonzalez-Perez and Lingle, 2019), calcium channels (Berkefeld and Fakler, 2008), and the identity of other potassium channels resident in neurons (Kimm et al., 2015; Gu et al., 2007).
The primary effect of KCNMA1 channels is to rapidly respond to calcium as it enters neurons during AP repolarization. As a result, KCNMA1 channel activation contributes to the late falling phase of the AP, a time point that overlaps with the activation of IK KCNB channels. Their large single-ion conductance allows KCNMA1 channels to operate at the foot of the AP repolarization. Blocking KCNMA1 channels changes the time course of the AP, leading to its prolongation. Such widening may allow greater recruitment of KCNB1 channels, as they have slow activation kinetics (Gu et al., 2007). This increased recruitment would lead to a longer-lasting KCNB1-mediated AHP, leading to a lower rate of neuronal firing. Thus, variants that lead to KCNMA1 channel LOF might reduce firing but allow a greater calcium influx as the AP time course will be longer. Such an AP prolongation in synaptic terminals would lead to a greater neurotransmission release, and possibly to a greater afterdepolarization and bursting in the soma. In contrast, KCNMA1 GOF variants decrease the width of the AP, limiting the recruitment of KCNB1 channels and allowing for a briefer fAHP, depending on the clearance of calcium from the plasma membrane. In turn, this situation would allow both faster recovery of sodium channels from inactivation and a briefer refractory period between APs (Brenner and Wilcox, 2012). Thus, GOF variants that activate more rapidly than wild-type channels would lead to a larger fAHP and less recruitment of slow-activating KCNB1 potassium channels (Kimm et al., 2015). Therefore, the effect of KCNMA1 channel inhibition or activation on firing depends on the interplay between KCNMA1 channels and the potassium currents in different cell types.
AP trains or sustained depolarizations can activate slowly decaying (seconds) hyperpolarizing currents that can be grouped together as IsAHP. These currents induce spike frequency adaptation or increase inter-burst intervals. Researchers have proposed many different molecular substrates that underlie these currents, including KCNQ channels (Tzingounis and Nicoll, 2008; Laker et al., 2021), the Na+-K+ ATPase (Gulledge et al., 2013), KATP channels (KCNJ11) (Tanner et al., 2011; Laker et al., 2021), and Na+-activated K+ channels (KNa, KCNT1-2) (Wallén et al., 2007). KCNJ11 variants cause a neurological syndrome termed developmental delay, epilepsy, and neonatal diabetes (DEND) (see section on IKir below) (Gloyn et al., 2006); however, KCNQ and KCNT1 variants are most strongly associated with epilepsy and DEE. As KCNQ channels were covered above, we focus this section on KCNT1-2.
KCNT1 and KCNT2 encode 6TM subunits that can form homomeric or heteromeric channels that are potassium selective, have a high single-channel conductance (≈ 100 pS), and show little inactivation (Joiner et al., 1998; Bhattacharjee et al., 2003). Unlike their voltage-gated relatives, these channels lack the canonical voltage sensor. Instead, they are largely gated by sodium, as their open probability increases as a function of sodium concentration available to the C-terminal (intracellular) region, where the sodium binding sites are located (Zhang et al., 2010; Hite et al., 2015). The concentration of sodium required to activate the channels, however, is quite high (EC50 ≈ 30–50 mM) (Bhattacharjee et al., 2003), leading to the hypothesis that they are only activated after very strong depolarizations that lead to the accumulation of intracellular sodium beyond normal levels (≈ 5 mM). In line with this concept, several studies have reported that an IKNa activated by repetitive stimulation contributes to the sAHP (Kim and Mccormick, 1998; Schwindt et al., 1989; Franceschetti et al., 2003). Other recent studies, however, have suggested that IKNa can be activated at subthreshold potentials via a subthreshold persistent sodium current (Budelli et al., 2009; Hage and Salkoff, 2012), and they can modulate membrane excitability either before or after a single AP (Martinez-Espinosa et al., 2015). Thus, although IKNa and the KCNT1-mediated current have traditionally been considered components of the sAHP, their role in some types of neurons may approach those of voltage-gated or even leak potassium channels.
KCNT1 variants cause a range of childhood epilepsy syndromes, including DEE (Ohba et al., 2015), epilepsy of infancy with migratin focal seizures (EIMFS) (Barcia et al., 2012), and sleep-realted hypermotor epilepsy (SHE) (Heron et al., 2012; Moller et al., 2015). EIMFS is the more severe form of KCNT1-related disease, characterized by frequent seizures that appear before 6 months of age and by profound developmental delay. SHE is usually less severe than EIMFS, but it is characterized by childhood-onset seizures that often occur in clusters at night, accompanied by cognitive comorbidities and psychiatric and behavioral problems. Both forms of KCNT1-related epilepsy are often refractory to treatment (Gertler et al., 2018).
Interestingly, KCNT1 variants are almost exclusively missense. Electrophysiological assessment of these human KCNT1 variants in heterologous systems has shown that they exhibit increased peak potassium current magnitude and faster activation times, potentially due to increased channel cooperativity (Kim et al., 2014), enhanced Na+ sensitivity, and/or increased probability of channel opening (Tang et al., 2016; McTague et al., 2018). Although rare, a handful of KCNT2 variants that cause DEE have also been reported, which either increase sodium permeability of the channel or reduce current amplitude (Gururaj et al., 2017; Ambrosino et al., 2018; Mao et al., 2020).
Quinidine, a Food and Drug Administration (FDA)-approved drug that crosses the blood–brain barrier, has been previously shown to inhibit KNa currents (Yang et al., 2006). Researchers subsequently tested this drug on mutant KCNT1 channels and found that it decreased the activity of these channels to wild-type levels (McTague et al., 2018; Milligan et al., 2014). Based on these results, quinidine was tested as a precision therapy to treat epilepsy caused by KCNT1 variants. Although some early cases were promising (Bearden et al., 2014), further reports with larger cohorts in controlled trials showed limited therapeutic benefit (Mullen et al., 2018; Chong et al., 2016; Fitzgerald et al., 2019).
It is clear that KCNT1 variants greatly increase steady-state potassium currents in heterologous cells, by as much as 10-fold. Two recent studies also reported increased Na+-sensitive potassium currents in human and mouse neurons expressing KCNT1 variants, suggesting that this increase in current magnitude persists in neurons (Shore et al., 2020; Quraishi et al., 2019). Based on the proposed role of IKNa in the sAHP, one may have expected larger sAHPs in neurons expressing KCNT1 variants; however, neither study reported increased sAHPs. In contrast, both KCNT1 variants, one of which causes EIMFS and the other SHE, decreased AP width and increased the fAHP, suggesting that the KCNT1 variants activate on a time scale that is rapid enough to affect single APs. One of the studies, however, found an increased AP number in response to sustained current steps (Quraishi et al., 2019), while the other found decreased AP number in GABAergic neurons (Shore et al., 2020), suggesting that the effects of KCNT1 variants on cellular physiology are cell type dependent (Fig. 45–3). The changes in GABAergic neurons are consistent with the high expression of KCNT1 in interneurons (Figs. 45–2 and 45–3).
Wide-field calcium imaging to characterize and localize abnormal brain activity in a mouse model of human KCNT1-related epilepsy. A. Wide-field fluorescence image of GCaMP6s expressed in the brain of a mouse carrying the Y796H variant in KCNT1 that causes (more...)
Members of the KCNJ family belong to the inwardly rectifying potassium family (Gonzalez et al., 2012). As the name implies, these channels allow large inward currents when the membrane potential is negative compared to the EK, and much smaller currents when the resting membrane potential is positive compared to the EK. The smaller outward currents are due to intracellular polyamines (i.e., spermine) or Mg2+ blocking the channels as the membrane depolarizes. Typically, the main functions of these channels are to set the resting membrane potential and to rapidly buffer extracellular potassium following bouts of activity, as the buildup of extracellular potassium shifts the EK positive to the resting membrane potential, leading to inward potassium flux and membrane depolarization. KCNJ10 (Kir4.1) is highly expressed in astrocytes and oligodendrocytes, and its primary function is to facilitate and speed the clearance of potassium from the extracellular space along with sodium-potassium pumps (Beckner, 2020). KCNJ10 can be found either as homomeric channels or heteromeric channels with KCNJ16 (Kir5.1) (Ohno et al., 2018; Gonzalez et al., 2012).
KCNJ11, also known as a KATP channel, is an inward rectifier whose gating depends on intracellular ATP levels. Increased levels of ATP lead to its closure; thus, KATP channels sense the cell’s metabolic state as they follow the ebb and flow of the ATP concentration in neurons (Gonzalez et al., 2012). The canonical KATP channel is an octameric complex of KCNJ11 subunits and sulfonylurea receptor subunits with a 1:1 ratio, and it is expressed primarily in neurons. KCNJ2 has also been associated with epilepsy, but due to the limited number of examples, we will not discuss it any further (Villa and Combi, 2016).
Previous research has associated KCNJ10 LOF variants with autosomal recessive epilepsy, ataxia, sensorineural deafness, (salt-wasting) renal tubulopathy (EAST syndrome), as well as seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (SeSAME) syndrome. The majority of variants led to reduced currents, either as homomers or heteromers when coexpressed with KCNJ16 (Brenner and Wilcox, 2012; Villa and Combi, 2016; Ohno et al., 2018). More recent work has identified KCNJ10 GOF variants in patients with autism and epilepsy (Villa and Combi, 2016). Expression of the identified variants in Xenopus laevis oocytes led to greater inward currents, possibly due to an increase in KCNJ10 surface trafficking.
KCNJ11 variants have been previously associated with DEND syndrome (Villa and Combi, 2016; Brenner and Wilcox, 2012). Unlike KCNJ10 variants, which are primarily LOF variants, the KCNJ11 variants are most commonly GOF. In particular, DEND-associated KCNJ11 variants prevent ATP-mediated channel closure, increasing their likelihood to remain open.
Bouts of APs lead to elevated levels of extracellular potassium. This behavior is particularly evident following seizures, in which potassium levels can reach 10–12 millimolar (de Curtis et al., 2018). Even modest changes in extracellular potassium levels can depolarize neurons, which may alter neuronal excitability. As noted previously, clearance of extracellular potassium following activity is partly mediated by buffering (i.e., transport) of potassium by either astrocytes or oligodendrocytes. KCNJ10 channels are one of the key channels aiding in the clearance of extracellular potassium, as these channels are highly expressed in astrocytes as well as oligodendrocyte precursor cells (OPCs) and mature oligodendrocytes (Larson et al., 2018; Zhang et al., 2014; Djukic et al., 2007; Beckner, 2020). Consistent with the critical function of KCNJ10 in astrocytes and oligodendrocytes, conditional deletion of Kcnj10 from either cell type decreases their potassium- buffering capacity (Djukic et al., 2007; Larson et al., 2018). This result likely explains the finding that Kcnj10-deficient mice have spontaneous seizures. Therefore, LOF KCNJ10 variants may lead to seizures and epilepsy as either astrocytes or oligodendrocytes fail to rapidly clear extracellular potassium levels. GOF variants might have the opposite effect, that is, a higher clearance rate of extracellular potassium and a reduction of basal extracellular potassium, allowing neurons to fire even more rapidly (Niday and Tzingounis, 2018).
The primary function of KATP channels is likely neuronal. Two studies have suggested that KCNJ11 channels contribute to the slow AHP, a slow potassium conductance, which drives spike frequency adaptation in neurons (Tanner et al., 2011; Laker et al., 2021). Therefore, following a train of APs, sodium–potassium pump activity increases, resulting in an increased consumption of ATP. The decreased ATP levels would allow KCNJ11 channels to open, leading to a sustained hyperpolarization. Similarly, in some neurons, increased neuronal synaptic activity leads to NMDA receptor-dependent calcium influx. Calcium can then bind to plasma membrane calcium ATPase (PMCA), leading to decreased ATP levels and subsequent activation of dendritic KCNJ11 channels, which limits the activation of NMDA receptors (Knowlton et al., 2018). How then do KCNJ11 GOF variants lead to hyperexcitability? If pathogenic variants lead to either increased KCNJ11 basal activity or increased ATP affinity such that fewer KCNJ11 channels are available to respond to changes in the intracellular ATP concentration, then fewer KCNJ11 channels would be available to hyperpolarize the membrane following activity.
KCNK4 channels, more commonly known as TRAAK channels, are members of the 2TM (K2P) domain potassium channel family. These channels are constitutively open; thus, they are viewed as “leak” background channels (Natale et al., 2021; Gonzalez et al., 2012). However, this behavior does not imply that their opening is not regulated. Previous work has shown that the open probability of TRAAK channels is regulated by various stimuli, including chemicals such as polyunsaturated fatty acids (PUFAs). Both mechanical stimulation and increases in temperature further activate TRAAK channels. TRAAK channels are found across the nervous system, and recent work has shown that they are highly expressed in nodes of Ranvier. In particular, immunohistochemical experiments have shown that TRAAK channels are restricted to nodes (no expression in AIS) and can be found in 80% of some central and peripheral axons (Brohawn et al., 2019; Kanda et al., 2019).
Thus far, there have been few reports of KCNK4 variants associated with epilepsy. In particular, researchers have identified a series of de novo GOF variants in patients with facial dysmorphism, hypertrichosis, epilepsy, intellectual/developmental delay, and gingival overgrowth syndrome (FHEIG) (Bauer et al., 2018). These variants occur in the transmembrane domains of KCNK4 channels, leading to higher basal TRAAK activity.
For many years, researchers have known that the mammalian nodes of Ranvier express high levels of leak background channels (Avalos Prado and Sandoz, 2019). Mammalian nodes of Ranvier are unique in that they express very few voltage-gated potassium channels. Instead, in rodents, most voltage-gated potassium channels are located in the paranodal regions or juxta-paranodal regions (members of the KCNQ channel family are an exception) (Trimmer, 2015). Consequently, the AP repolarization in mammalian nodes of Ranvier is driven by sodium channel inactivation and repolarization by leak channels. Indeed, a reduction in TRAAK currents using TRAAK-specific blockers or short hairpin RNA interference led to a depolarized membrane potential (~5 mV), reduced AP amplitude, and increased AP time course. The reduction in the AP amplitude was due to the decrease in available sodium channels (depolarization inactivates sodium channels) (Brohawn et al., 2019; Kanda et al., 2019). An advantage of AP repolarization by leak potassium channels is a reduced fAHP; thus, the refractory period between successive APs is brief and is directly dependent on the expression levels of TRAAK channels. Therefore, GOF variants that increase the leak potassium current density would lead to a faster AP repolarization and AP propagation. This behavior would allow neurons to fire at a higher rate. We note that although GOF variants would increase the basal levels of TRAAK currents, they would also prevent any additional positive modulation by modulators such as PUFAs (i.e., arachidonic acid).
Members of the KCNH family (KCNH1, KCNH2) have also been implicated in epilepsy (Aubert Mucca et al., 2021; Kessi et al., 2020; Miyamoto et al., 2019; Villa and Combi, 2016). Most notably, pathogenic variants of KCNH1 (also known as Kv10.1) have been implicated in Temple-Baraitser syndrome, which is characterized by symptoms including epilepsy. Importantly, all of the identified variants have been GOF. KCNH1 channels belong to the voltage-gated potassium channel family but also have a few unique features that distinguish them from the canonical members of the 6TM family. For instance, the voltage-sensor module is not domain swapped (Whicher and MacKinnon, 2016). In Kv channels, the voltage-sensor module of one subunit directly interacts with the pore components of another channel subunit, also known as a domain swap. This allows for cooperativity among subunits during activation. Consequently, KCNH1 can show voltage-dependent gating even when the S4-S5 linker is not covalently attached to the pore domain, unlike other voltage-gated potassium channels (Lorinczi et al., 2015). In typical voltage-gated potassium channels and unlike KCNH1, the S4-S5 linker partly acts as a mechanical lever during gating. In addition to the absence of domain swapping, KCNH1 channels have an eag domain (composed of Per-Arnt-Sim [PAS] and PAS-cap domains) in their N-terminus as well as a cyclic nucleotide-binding homology domain (CNBHD), which is connected to the pore through a C-linker domain. The eag and CNBHD domains interact together to control gating of KCNH channels (Whicher and MacKinnon, 2019). Currently, the function of KCNH1 in neurons is unclear, with one study suggesting control of calcium influx in presynaptic terminals during repetitive stimulation. Thus, GOF variants might alter presynaptic release properties; however, any proposed mechanism is premature at this time (Niday and Tzingounis, 2018).
Epilepsy arises from either LOF or GOF variants. GOF variants seem to cause very severe epilepsy phenotypes and occur frequently in DEE.
For 6TM voltage-gated family members, many GOF variants arise in voltage-sensor modules. LOF variants primarily associate with the pore, regions critical for trafficking, and ligand-binding sites.
The majority of epilepsy associated potassium channels are found in axons (KCNA1,2; KCNB1; KCNQ2,3; KCNT1,2; KCNK4).
Potassium channels enriched in either glutamatergic or GABAergic neurons lead to epilepsy and DEE.
Despite the substantial progress in clarifying the effects of epilepsy-associated potassium channel variants on the biophysical properties and function of channels, and in some cases their effects on neuronal physiology, we are still far from understanding how these changes ultimately lead to hyperexcitable, hypersynchronous neural networks, seizures, and epilepsy. Lying between these two points is an enormous cellular, developmental, and synaptic complexity, as well as a lack of animal models that recapitulate key features of potassium channel-associated epilepsies. Below, we highlight new concepts and approaches that promise to improve our ability to identify the changes most relevant to disease and translatability of animal and human cell-derived models, innovations that are necessary to drive the discovery of new therapies for these disorders.
The majority of our knowledge regarding potassium channel physiology has arisen from studies in juvenile and adult animals. As a result, the current interpretation of how potassium channel variants impact physiology is based on a limited developmental window. As shown in Figure 45–2, multiple potassium channels associated with epilepsy and DEE are expressed early in development, before neurons have fully matured (>P28 in rodents). Similar mRNA expression patterns can be seen in human transcriptome databases (Li et al., 2018; https://hbatlas.org/). Immature pyramidal neurons exhibit substantially different biophysical properties than mature neurons. For instance, immature neurons have a more depolarized resting membrane potential, a much higher input resistance, and produce action potentials with smaller amplitudes and longer durations.
Despite these differences, McCormick and Prince (1987) and later Foehring and colleagues (Guan et al., 2011) observed that the frequency-current relationship (input-output relationship) of immature to older (more mature) neurons remained similar across development; that is, neurons can fire repetitively soon after post-birth. Rather, the major difference between immature (first week postnatally) and mature neurons lies in the slope of the frequency-current relationship (gain) and the range of frequencies to which the neurons can respond. A similar effect occurs for fast-spiking interneurons, which can fire repetitively during the first week of life, yet quickly mature to exhibit fast spiking nonadapting properties by the second of week of life (Goldberg et al., 2011). For neurons to achieve this behavior, McCormick and Prince (1987) proposed that the ratios between different types of ion channels, such as potassium channels, are maintained across development. Consistent with this proposal, Guan et al. (2011) found that the contribution of Kv2 (KCNB) and Kv4 (KCND) voltage-gated potassium currents remained constant across development. Therefore, any potassium channel variant, LOF or GOF, would shift the ratio of ionic conductances away from preexisting optimal firing ratios, which might change the trajectory of synapse maturation and network excitability.
As an overarching concept, early activity in excitatory and inhibitory neurons regulates the subsequent development of synaptic networks. The prevalent notion of “fire together, wire together” highlights the importance of finely tuned, early-stage excitability in the developing brain for establishing appropriate synaptic networks. If an immature neuron has altered intrinsic excitability due to the presence of a pathogenic potassium channel variant, dysregulation will arise in the neuronal excitability as well as the transcriptional profile, cell fate trajectory, and proper integration into functional neural circuits. Thus, altered excitability during postnatal development, owing to pathogenic variants in potassium channels, might initiate activity-dependent changes in the transcriptional profile of multiple cell types in the developing brain distributed across multiple brain regions. Indeed, a recent study found that human neurons expressing the recurrent Kcnq2R581Q variant led to changes in the potassium channel transcriptome (Simkin et al., 2021). Similarly, overexpression of Aplysia shaker channels (corresponding to Kv1 channels) in mice led to hyperexcitability due to the downregulation of potassium channels (Sutherland et al., 1999; Williams and Sutherland, 2004), likely in an effort to overcome the hypoexcitability effects caused by a greater expression of potassium conductance in neurons (Sutherland et al., 1999). Recently, research demonstrated that the deletion of KCNQ2/3 from interneurons leads to greater excitatory and inhibitory synaptic transmission, resulting in a lack of change to the excitation/inhibition (E/I) ratio in pyramidal neurons. However, the cost of maintaining the E/I ratio at the same level led to greater susceptibility to seizures, because the excitatory drive was higher than that in wild-type neurons (Soh et al., 2018). Similarly, expression of a Kcnt1 variant led to increases in connection probability between homotypic neuron pairs in vitro, a change accompanied by increased network burst firing (Shore et al., 2020).
Thus, we propose that to understand the effect of potassium variants in epilepsy, we must understand not only how a potassium channel variant changes neuronal excitability but also how it impacts the development of neuronal properties. For this purpose, a greater number of in vivo animal models and generation of patient derived neurons and organoids are needed. For potassium channels in which variants predominantly cause loss of protein expression, knockout animals likely provide adequate models for disease. However, most of the severe human epilepsies associated with potassium channel variants are caused by variants that have complex effects on channel behavior, meaning that animal models should incorporate orthologous human variants. Recent genome editing technologies have made the generation of mice with pathogenic human potassium channel variants cheaper and faster. Studies on these new models should not only focus on adult mice but also on perinatal mice, as changes in neuronal excitability occurring early in development might alter neuronal cell fate, neurogenesis, and synapse maturation and connectivity.
Throughout this review, we have considered the impact of potassium channel variants only from the perspective of potassium conduction. However, several reports have shown that potassium channels can also initiate signaling cascades independent of their function as potassium sieves. Recently, a study reported that KCNC3 channels help nucleate F-actin in the calyx of Held, a property independent of KCNC3’s potassium conductance (Wu et al., 2021). By organizing the F-actin cytoskeleton, KCNC3 regulates multiple aspects of synaptic transmission, including short-term plasticity. Similarly, earlier work on KCNB1 channels has shown that independent of their ion conduction, they can lead to downstream activation of Ras/Akt signaling in multiple cell types. Interestingly, some KCNB1 pathogenic variants (KCNB1R312H, KCNB1F416L) are not able to activate the Ras/Akt signaling pathway, suggesting that KCNB1 variants might have pleiotropic effects (Yu et al., 2019). KCNT1 channels have also been shown to regulate signaling independent of their ion channel properties. For instance, the KCNT1 C- terminus interacts with a variety of proteins such as fragile X mental retardation protein (FMRP), cytoplasmic FMRP interacting protein, and phosphatase and actin regulator protein 1 (Ali et al., 2020; Brown et al., 2010). These interactions regulate downstream signaling and the interactions between KCNT1 and the actin cytoskeleton. Thus, an emerging picture is that potassium channels also act as a hub and scaffold for signaling molecules, raising the possibility that some potassium channel variants might alter network excitability independent of their effects on potassium flux.
One of the most unexpected developments over the last 10 years has been the recognition that some potassium channels bind to solute carriers (transporters). In a series of studies, Abbott and colleagues demonstrated that KCNQ2 channels directly interact with the myo-inositol sodium contransporters SMIT1 and SMIT2 (Manville and Abbott, 2019). The formation of the SMIT-KCNQ complex not only allows for local buildup of myo-inositol within the KCNQ2 channel vicinity (myo-inositol is the precursor of PIP2, a phospholipid necessary for the function of KCNQ2 channels) but also regulates the pharmacological and ion conduction properties of KCNQ2 channels (Manville and Abbott, 2019). SMITs wedge between neighboring voltage sensors, gaining access to the pore module. Similar to KCNQ channels, KCNA1 and KCNA2 channels directly bind to the sodium-independent neutral amino acid transporter LAT1 (Slc7A5), which in turn modulates multiple biophysical facets of KCNA channels (Baronas et al., 2018). Importantly, the interaction of Slc7A5 with two KCNA2 GOF variants dramatically reduced their protein expression and increased their inactivation, transforming them from GOF variants to LOF variants. These examples highlight the need to identify the protein interactome of disease-associated potassium channels, as the interactions between potassium channels and their accessory/auxiliary partners might determine whether a variant is LOF or GOF. To achieve this, mass spectroscopy of brain complexes using more specific antibodies or epitope-tagged knock-in mice is necessary.
A fundamental problem in trying to link altered channel function and cellular physiology to network dysfunction is identifying the relevant brain regions, circuits, developmental time points, and cell types that mediate the pathological effects of gene variants, so that interrogations of cellular and channel function can be carried out in a context that is relevant to the disease phenotypes. In some cases, traditional analysis of protein expression patterns via immunostaining can constrain these parameters; however, many potassium channels show widespread expression in the central nervous system. Furthermore, as described above, the properties and activities of many potassium channels are dependent on interactions with other potassium channels, and other ion channels, receptors, and interacting molecules, meaning that their role in disease may be heavily dependent on their local environment. Fortunately, researchers are beginning to adopt new technologies in order to identify brain regions and cell types that show pathological or epileptiform activity and to perform more detailed analysis of potassium channel dysfunction. Here, we highlight a few of these technologies.
Recording the activity of single neurons via patch-clamp electrophysiology has formed the backbone of potassium channel research for decades. Large-scale imaging and electrophysiological recording approaches, however, can simultaneously monitor the activity of increasing numbers of neurons both in vivo and in vitro. Although it suffers from poor temporal precision, calcium imaging is a relatively straightforward way to simultaneously monitor neural activity in several brain areas or specific cell types. Wide-field calcium imaging, which has a large enough field of view to capture the entire dorsal cortex or whole brain slices in mice (Wekselblatt et al., 2016), has been employed to monitor the emergence and spread of epileptiform activity in numerous pharmacological models, including 4-AP-induced epileptiform activity and tumor-induced epilepsy (Daniel et al., 2015; Liou et al., 2018; Turrini et al., 2017; Rossi et al., 2017; Liu and Baraban, 2019; Hatcher et al., 2020). Recently, researchers have also used this method to investigate mouse genetic models of potassium channel dysfunction, including Kcnt1 (Shore et al., 2020) and Kcnq2/3 mice (Hou et al., 2021), and found that changes in the neural activity of these models are brain region specific. Subsequent studies then targeted these regions for more detailed interrogation with patch-clamp electrophysiology in order to better relate the cellular changes to the epileptiform activity. Wide-field imaging can also be used as a prelude to two-photon (2P) imaging (Rossi et al., 2018). Although current 2P approaches cannot survey the large sizes of tissue that wide-field can, it offers the ability to characterize changes in AP firing in specific cell types or localize changes to specific cortical layers in awake, behaving animals (Tran et al., 2020; Wenzel et al., 2017, 2019; Fig. 45–3). For deeper brain structures that are difficult to access with 2P, calcium imaging using fiber photometry can monitor activity with cell type specificity, albeit with limited spatial information (Zhang et al., 2019). Our current understanding of the role of potassium channels in epilepsy is largely based on studies in hippocampal and neocortical neurons. Contributions from thalamic, striatal, or basal forebrain circuits are also likely, however, as a study recently demonstrated for STXBP1- and SCN2A-related epilepsy in mice (Miyamoto et al., 2019).
In addition to imaging approaches, researchers are increasingly applying high-density probes and grids to localize epileptiform activity (Khodagholy et al., 2015; McCafferty et al., 2018). These electrical approaches sacrifice detailed spatial information and, usually, the ability to record from genetically defined populations. However, they have superior temporal resolution, which gives investigators the ability to characterize dynamic changes in AP firing patterns caused by potassium channel variants. Laminar probes in particular can be inserted into the brain at varying angles to sample multiple brain regions, and they can pick up both single-unit activity and local field potentials (Steinmetz et al., 2018). Thus, we expect that both large-scale imaging and electrophysiology approaches will be increasingly adopted, both to contextualize the relevant changes in brain activity and to study the effects of potassium channel dysfunction on firing patterns of specific cell types. Although we have a solid understanding of the effects of potassium channel variants on the firing patterns of neurons in vitro and ex vivo, it remains unclear how they affect the firing patterns of neurons in vivo, both interictally and preictally.
A second, complimentary, large-scale approach to identifying primary pathways that contribute to potassium channelopathies and epilepsy is the use of transcriptomics (Lein et al., 2017). Recent advances in RNA sequencing technology and the resulting studies have led to databases that can be searched to identify brain regions and cell types in which a potassium channel of interest is expressed, along with screening of known interacting proteins (Saunders et al., 2018; Tasic et al., 2016; Favuzzi et al., 2019). Some of these transcriptomic studies have been performed at different developmental time points (Li et al., 2018; Foldy et al., 2016), giving investigators snapshots not only of cell-type-specific expression of their favorite potassium channel subunit but also of how the expression evolves over time (Fig. 45–2). Mining databases for clues to pathogenic mechanisms offers an attractive way to generate novel hypotheses, which can then be tested with the use of cell-type-specific genetic approaches. An even more powerful way to use transcriptomics to advance our understanding of the pathogenic mechanisms of potassium channel dysfunction is to perform spatial transcriptomics in animal or human cell potassium channel disease models (Lein et al., 2017). With this approach, transcriptional network changes can be followed across multiple brain regions (neocortex, hippocampus, thalamus, hypothalamus) and developmental time points in parallel.
Over the last 10 years, we have seen an explosion in the identification of new potassium channel variants underlying multiple types of epilepsy-associated disorders and syndromes. The acceleration in identifying new potassium channelopathies highlights the need to better understand the function of potassium channels in neurons and to integrate preexisting and emergent experimental approaches. Combining multiple new approaches and bringing together teams of investigators with complementary expertise could help unravel the complexity of potassium channelopathies and epilepsy. To make substantial progress in our understanding of potassium channelopathies and epilepsy as well as to develop new therapeutic strategies, we must address a series of open questions: (1) Determine the impact of potassium channel variants on brain physiology and epilepsy-related behaviors across the lifespan of an organism. Potassium channel expression is dynamic across development; thus, we must understand their impact at different developmental ages. This would allow us to identify the best therapeutic windows for interventions. (2) Identify the cell types and circuits that drive the epilepsy phenotype and comorbidities. Although a potassium channel might exhibit broad expression, functionally it might have a more restrictive role. By canvassing and leveraging the new transcriptome databases, we can gain insight into which circuits we should target first. (3) Determine whether epilepsy is due to ongoing activity of potassium channel variants or due to remodeling of the nervous system circuits. This is a fundamental question, as it would inform whether targeting a potassium channel variant is the best therapeutic strategy for a particular disorder. (4) Define the role of potassium channels and their variants across the central, peripheral, and enteric nervous systems. Many potassium channels implicated in epilepsy are expressed across the nervous system, likely contributing to the various comorbidities associated with various potassium channelopathies and epilepsy. Addressing these questions over the next 10 years will fundamentally improve our understanding of potassium channelopathies in epilepsy and allow us to deliver the promise of precision medicine.
The authors declare no relevant conflicts.
This is an open access publication, available online and distributed under the terms of a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International licence (CC BY-NC-ND 4.0), a copy of which is available at https://creativecommons.org/licenses/by-nc-nd/4.0/. Subject to this license, all rights are reserved.
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