**2. Voltage-gated sodium channels**

VGSCs play a critical role in the generation and propagation of APs in neurons, genetic alterations in VGSC genes are considered to be associated with epileptogenesis. Mammalian VGSC is composed of a large pseudotetrameric pore-forming α subunit with a molecular weight of 260 KDa, and one or more auxiliary β subunits (30–40 KDa) [3–5] (**Figure 2**). Nine subtypes of VGSC α subunits have been found in humans, including Nav1.1-Nav1.9, encoded by the genes SCN1A-SCN5A, SCN8A-SCN11A, respectively.

### **2.1 Nav1.1**

Nav1.1 is mainly distributed in the inhibitory GABAergic neurons of cerebellum and hippocampus. The Nav1.1 gene SCN1A is the clinically most relevant SCN gene for epilepsy. More than 1200 mutants have been identified to be associated with epilepsy; most of them are febrile seizures [6]. M145T mutation, a wellconserved amino acid in the first transmembrane segment of domain I of the

**173**

**Figure 2.**

*residues that form the ion selectivity filter.*

*Ion Channels in Epilepsy: Blasting Fuse for Neuronal Hyperexcitability*

Nav1.1 α-subunit, caused a reduction in peak sodium currents and a positive shift in the voltage dependence of activation [7], which provided the first evidence that the mild loss-of-function mutations in Nav1.1 may cause a significant portion of febrile seizures. Complete loss-of-function mutations in Nav1.1 cause severe myoclonic epilepsy of infancy (SMEI or Dravet's syndrome), which includes severe, intractable epilepsy and comorbidities of ataxia and cognitive impairment. Besides, homozygous null Nav1.1<sup>−</sup>/<sup>−</sup> mice developed ataxia and died on half a month of postnatal and did not change the voltage-dependent activity of VGSCs in hippocampal neurons. However, heterozygous Nav1.1+/<sup>−</sup> mice exhibited spontaneous seizures and sporadic deaths after 3 weeks, and the sodium current density was substantially reduced in inhibitory interneurons, except in excitatory pyramidal neurons [8]. So loss-of-function mutations in Nav1.1 can severely impair sodium currents and AP firing in hippocampal GABAergic inhibitory neurons. The functional downregulation in inhibitory neurons might cause the hyperexcitability of dentate granule or pyramidal neurons, which could lead to epilepsy in patients with SMEI. Experiments in mice have demonstrated that haploinsufficiency of Nav1.1 channels is sufficient to allow induction of seizures by elevated body temperature, supporting that haploinsufficiency of SCN1A is pathogenic in human SMEI which has striking temperature and age dependence of onset and progression of epilepsy [9]. What is more, SCN1A mutations were mostly missense mutations in GEFS+ patients, which are typically well controlled by treatment with antiepileptic drugs and no cognitive impairment is observed. The R1648H channels showed the reduced function in both excitatory and inhibitory neurons although the biophysical mechanisms were different, reducing peak sodium currents and enhancing slow inactivation in inhibitory neurons versus negatively shifted voltage dependence of fast inactivation in excitatory neurons [10]. The similar conclusion had been drawn when the R1648H mutation has been inserted into the mouse genome under the native promoter [11]. In light of these results, GEFS+ and SMEI may be caused by a continuum of mutational effects that selectively impair firing of GABAergic inhibitory neurons, which lead to increase in the excitability of the neural network [12].

*Structure of voltage-gated sodium channels. Schematic representation of VGSC subunits. The α subunit of the VGSC is illustrated together with β1 and β2 subunits; extracellular domains of the β subunits are shown as immunoglobulin-like folds, interacting with the loops in α subunits. Roman numerals indicate the domains of the α subunit; segments 5 and 6 (shown in green) are the pore-lining segments, and S4 helices (red) make up the voltage sensors. The red circle in the intracellular loop of domains III and IV indicates the inactivation gate IFM motif; Ψ, probable N-linked glycosylation site. The circles in reentrant loops in each domain represent the* 

*DOI: http://dx.doi.org/10.5772/intechopen.83698*

### *Ion Channels in Epilepsy: Blasting Fuse for Neuronal Hyperexcitability DOI: http://dx.doi.org/10.5772/intechopen.83698*

### **Figure 2.**

*Epilepsy - Advances in Diagnosis and Therapy*

In this chapter, we summarize the epilepsy-associated VGIC genes, the mutations, corresponding phenotypes, and functional changes, aiming to provide clues

VGSCs play a critical role in the generation and propagation of APs in neurons, genetic alterations in VGSC genes are considered to be associated with epileptogenesis. Mammalian VGSC is composed of a large pseudotetrameric pore-forming α subunit with a molecular weight of 260 KDa, and one or more auxiliary β subunits (30–40 KDa) [3–5] (**Figure 2**). Nine subtypes of VGSC α subunits have been found in humans, including Nav1.1-Nav1.9, encoded by the genes SCN1A-SCN5A, SCN8A-

Nav1.1 is mainly distributed in the inhibitory GABAergic neurons of cerebellum and hippocampus. The Nav1.1 gene SCN1A is the clinically most relevant SCN gene for epilepsy. More than 1200 mutants have been identified to be associated with epilepsy; most of them are febrile seizures [6]. M145T mutation, a wellconserved amino acid in the first transmembrane segment of domain I of the

for evaluating the relationship between VGIC genes and epileptogenesis.

*VGICs-induced human epilepsy. LOF represents the loss-of-function mutation of VGICs.*

*Neuronal localization of some relevant voltage-gated ion channels. A schematic view of an excitatory pyramidal (orange), an inhibitory (green) neuron, and their synaptic connections is shown. Distinctive intracellular compartments are targeted by different populations of VGICs. Examples of which as mentioned in this chapter are shown here: in the somatodendritic compartment, Nav, Cav (L- and T-type), TRP, BK, and Kv channels; at axon initial segments (AIS) and nodes of Ranvier in pyramidal neurons, Nav1.2, Kv7 channels; at AIS of inhibitory neurons, Nav1.1; in the somatodendritic compartment of inhibitory neurons, BK and Nav1.6; in the presynaptic terminals, Cav P/Q type. GOF represents the gain-of-function mutation of* 

**2. Voltage-gated sodium channels**

SCN11A, respectively.

**2.1 Nav1.1**

**Figure 1.**

**172**

*Structure of voltage-gated sodium channels. Schematic representation of VGSC subunits. The α subunit of the VGSC is illustrated together with β1 and β2 subunits; extracellular domains of the β subunits are shown as immunoglobulin-like folds, interacting with the loops in α subunits. Roman numerals indicate the domains of the α subunit; segments 5 and 6 (shown in green) are the pore-lining segments, and S4 helices (red) make up the voltage sensors. The red circle in the intracellular loop of domains III and IV indicates the inactivation gate IFM motif; Ψ, probable N-linked glycosylation site. The circles in reentrant loops in each domain represent the residues that form the ion selectivity filter.*

Nav1.1 α-subunit, caused a reduction in peak sodium currents and a positive shift in the voltage dependence of activation [7], which provided the first evidence that the mild loss-of-function mutations in Nav1.1 may cause a significant portion of febrile seizures. Complete loss-of-function mutations in Nav1.1 cause severe myoclonic epilepsy of infancy (SMEI or Dravet's syndrome), which includes severe, intractable epilepsy and comorbidities of ataxia and cognitive impairment. Besides, homozygous null Nav1.1<sup>−</sup>/<sup>−</sup> mice developed ataxia and died on half a month of postnatal and did not change the voltage-dependent activity of VGSCs in hippocampal neurons. However, heterozygous Nav1.1+/<sup>−</sup> mice exhibited spontaneous seizures and sporadic deaths after 3 weeks, and the sodium current density was substantially reduced in inhibitory interneurons, except in excitatory pyramidal neurons [8]. So loss-of-function mutations in Nav1.1 can severely impair sodium currents and AP firing in hippocampal GABAergic inhibitory neurons. The functional downregulation in inhibitory neurons might cause the hyperexcitability of dentate granule or pyramidal neurons, which could lead to epilepsy in patients with SMEI. Experiments in mice have demonstrated that haploinsufficiency of Nav1.1 channels is sufficient to allow induction of seizures by elevated body temperature, supporting that haploinsufficiency of SCN1A is pathogenic in human SMEI which has striking temperature and age dependence of onset and progression of epilepsy [9]. What is more, SCN1A mutations were mostly missense mutations in GEFS+ patients, which are typically well controlled by treatment with antiepileptic drugs and no cognitive impairment is observed. The R1648H channels showed the reduced function in both excitatory and inhibitory neurons although the biophysical mechanisms were different, reducing peak sodium currents and enhancing slow inactivation in inhibitory neurons versus negatively shifted voltage dependence of fast inactivation in excitatory neurons [10]. The similar conclusion had been drawn when the R1648H mutation has been inserted into the mouse genome under the native promoter [11]. In light of these results, GEFS+ and SMEI may be caused by a continuum of mutational effects that selectively impair firing of GABAergic inhibitory neurons, which lead to increase in the excitability of the neural network [12].

### **2.2 Nav1.2**

The mutation of the Nav1.2 gene SCN2A is associated with various epilepsies, such as benign familial neonatal seizures (BFNIS), hereditary epilepsy with febrile seizures plus (GEFS+), Dravet's syndrome (DS), and other stubborn childhood epilepsy encephalopathy. Nav1.2 subunit is mainly distributed in the axon-initiating segment (AIS) and node of Ranvier. SCN2A mutations cause changes in VGSC function and expression and result in abnormal neuronal discharge. Because Nav1.2 plays an important role in the AIS area during the development, it is more common for infants to show SCN2A mutant-induced epilepsy encephalopathy [13]. BFNIS is the most common phenotype caused by gain-of-function missense mutations in SCN2A [14]. Up to now, at least 10 SCN2A mutations associated with BFNIS have been identified. SCN2A mutations are also found to result in the reduced expression of Nav1.2 on the surface of neurons [15]. Therefore, SCN2A mutants will lead to the decrease of sodium current density at node of Ranvier and AIS, seriously affecting the excitability of neurons [16]. For missense mutation of SCN2A, p.Tyr1589Cys causes a depolarizing shift of steady-state inactivation, increased persistent Na+ current, a slowing of fast inactivation, and an acceleration of its recovery, which contribute to neuronal hyperexcitability and familial epilepsy [17]. Due to the SCN2A mutation, early infantile epileptic encephalopathy (EIEE) patients with burst suppression and tonic-clonic migrating partial seizures showed a specific dose-dependent efficacy of VGSC blockers [18]. It is mainly caused by the dysfunction of VGSC [19]. By replacing neonatal Nav1.2 with adult Nav1.2 in mice, it has been suggested that neonatal Nav1.2 reduced neuronal excitability and had a significant impact on seizure susceptibility and behavior.

### **2.3 Nav1.3**

The SCN3A gene, clustered on human chromosome 2q24, encodes the Nav1.3 subtype [20], which is usually located in the soma of neurons. It is important in the integration of synaptic signals, determination of the depolarization threshold, and AP transmission [21]. In contrast to the rodent gene which is transiently expressed during development, human SCN3A is widely expressed in adult brain [22]. The first epilepsy-associated mutation (K354Q ) in SCN3A was found in 2008. K354Q mutation decreased inactivation rate and increased INaP [23]. The mutation is not sensitive to antiepilepsy drug carbamazepine or oxcarbazepine. K354Q mutation causes neuronal abnormal spontaneous discharge and membrane potential paroxysmal depolarization [24]. In 2014, four more missense variants were identified in SCN3A, which are R357Q, D766N, E1111K, and M1323V [25]. Compared to wildtype channels, R357Q caused smaller currents, slower activation, and depolarized voltage dependences of activation and inactivation. The E1111K mutation evoked a significantly greater level of persistent sodium current. All four mutants increase current activation in response to depolarizing voltage ramps. These findings support for a contribution of Nav 1.3 to childhood epilepsy. Recently, a novel SCN3A variant (L247P) was identified by whole exome sequencing of a child with focal epilepsy, developmental delay, and autonomic nervous system dysfunction. Voltage clamp analysis showed no detectable sodium currents in a heterologous expression system. To further test the possible clinical consequences of reduced SCN3A activity, they investigated the effect of a hypomorphic Scn3a allele (Scn3a Hyp) on seizure susceptibility and behavior using a gene trap mouse line. Heterozygous SCN3A mutant mice (SCN3A+/Hyp) neither exhibit spontaneous seizures nor hyperthermia-induced seizures, but they displayed increased susceptibility to electroconvulsive- and chemiconvulsive-induced seizures, which provide evidence that loss-of-function of SCN3A may contribute to increased seizure susceptibility [26].

**175**

K+

ability. K+

activated K+

*Ion Channels in Epilepsy: Blasting Fuse for Neuronal Hyperexcitability*

Nav1.6, mainly distributed to the soma and synaptic origin, is important for APs generation and propagation [27]. In the development process, Nav1.2 is gradually replaced by Nav1.6 in the mature node of Ranvier [28]. The first heterozygous missense mutation (p.Asn1768Asp) in the Nav1.6 gene SCN8A was identified in 2012 by whole-genome sequencing (WGS) in a patient with severe epileptic encephalopathy who exhibited early-onset seizures, autistic features, intellectual disability, ataxia, and sudden unexpected death in epilepsy (SUDEP) [29]. Since this initial discovery, more than 100 pathogenic SCN8A variants have been identified in patients with epilepsy [30]. Most of the SCN8A variants have been detected

Different mutations in the SCN8A gene encoding Nav1.6 have different effects on epilepsy. For the missense mutation V929F, an evolutionarily conserved residue in the pore loop of domain II of Nav1.6, it was found that heterozygous mutations produced well-defined spike-wave discharges and are associated to absence epilepsy in mice [31]. However, missense mutations in Scn8amed−jo were able to improve the epilepsy symptoms of SCN1A+/<sup>−</sup> heterozygotes. The mechanism might be the decrease in Nav1.6 expression of excitatory neurons compensating for the loss of Nav1.1 in inhibitory neurons [32]. Recently, more and more de novo and inherited SCN8A epilepsy mutations were detected by gene panel analysis [33]. For example, loss-of-function mutants [34], underlying the complex seizure phenotype, were identified using specific mouse line. It was suggested that decreasing Scn8a expression in cortical excitatory neurons could reduce seizures. On the contrary, the decreasing expression of SCN8A in the thalamic reticular nucleus (RT) leads to absence seizures. Loss of Scn8a will impair tonic firing mode behavior and impair desynchronizing recurrent RT-RT synaptic inhibition in the thalamic reticular nucleus, which suggested that Scn8a-mediated hypofunction in cortical circuits, conferring convulsive seizure resistance, while hypofunction in the thalamus is sufficient to

The SCN9A gene encodes the Nav1.7 subtype, which was initially identified in the peripheral nervous system, sympathetic ganglion, and olfactory sensory neurons [35–38]. Nav1.7 is also found expressed in the central nervous system such as in the cerebral cortex and hippocampus [39]. A missense mutation of SCN9A (N641Y), at a conserved amino acid residue located at the intracellular loop between domain I and II, is associated with a family of febrile seizures (FS, N641Y). Mice carrying N641Y mutations were more susceptible to electrical stimulation-induced clonic and tonic seizures [40]. However, it is still unclear how SCN9A gene mutation

channels control the resting membrane potential and enable rapid repolariza-

b subunits. Kv channels are the largest ion channel group (Kv1–Kv12) that are expressed substantially in the CNS. Dysfunction of Kv channels including Ca2+-

channels, are associated with epilepsy [2].

channels are composed of four pore forming a subunits and modulatory

currents, thus limiting neuronal excit-

*DOI: http://dx.doi.org/10.5772/intechopen.83698*

**2.4 Nav1.6**

in individuals with EIEE.

generate absence seizures.

caused epilepsy in the CNS.

**3. Potassium channels**

tion of the AP by producing outward K+

**2.5 Nav1.7**

*Ion Channels in Epilepsy: Blasting Fuse for Neuronal Hyperexcitability DOI: http://dx.doi.org/10.5772/intechopen.83698*
