**3.2 Voltage-gated potassium channel subfamily KQT (KCNQ )**

Kv7 is its seventh member of Kv channel family (Kv1–Kv12). The Kv7.1 mutation mediates type 1 long QT syndrome (long-QT syndrome type 1, LQT1) and is therefore named KCNQ1 (K, potassium; CN, channel; Q, LQT). KCNQ has five subtypes of KCNQ1–KCNQ5, which play crucial roles in physiological functions. Dysfunction of KCNQ is associated with many diseases.

KCNQ1 is mainly distributed in the heart, which mediates cardiac delayedrectifier K+ current and maintains the normal repolarization process of cardiomyocytes [55]. KCNQ2–KCNQ5 are mainly distributed in central and peripheral neuronal tissues, of which KCNQ2 and KCNQ3 are distributed in brain regions [56]. KCNQ2 and KCNQ3 form functional heterotetramers, which are the main molecular bases for the formation of M currents that can be inhibited by acetylcholine M1 receptor activation [57]. Abundant KCNQ2 and KCNQ3 mutations could induce abnormal M currents, causing similarities in neonatal seizures and other nervous system diseases.

Benign familial neonatal seizure (BFNS) is an autosomal dominant idiopathic epilepsy syndrome that occurs on the 2nd to 8th day after birth and stops spontaneously after a few weeks. Whereas 15% of patients in later life may have recurrence of epilepsy [58]. With the study of pathogenic genes in epilepsy, 60–70% of patients with BFNS were found to be associated with KCNQ2 and KCNQ3 mutations. More than 80 different mutations have been reported on KCNQ2, and multiple mutations on KCNQ3 are associated with BFNS. Soldovieri et al. [58] studied the genes of 17 BFNS clinical patients. Sixteen different heterozygous mutations were found in KCNQ2, including 10 substitutions, 3 insertions/deletions, and 3 large deletions. One substitution was found in KCNQ3. Most of these mutations were novel, except for four KCNQ2 substitutions that were shown to be recurrent. Electrophysiological studies in mammalian cells revealed that homomeric or heteromeric KCNQ2 and/or KCNQ3 channels carrying mutant subunits with newly found substitutions displayed reduced current densities. Borgatti studied a BFNS family with four affected members: two of them exhibit BFNS only, while the other two, in addition to BFNS, present either with a severe epileptic encephalopathy or with focal seizures and mental retardation. All affected members of this family carry a novel missense mutation in the KCNQ2 gene (K526N), disrupting the tridimensional conformation of a C-terminal region of the channel subunit involved in accessory protein binding. When heterologously expressed in CHO cells, potassium channels containing mutant subunits in homomeric or heteromeric configuration with wild-type KCNQ2 and KCNQ3 subunits

exhibit an altered voltage-dependence of activation, without changes in intracellular trafficking and plasma membrane expression. The KCNQ2 K526N mutation might affect M-channel function by disrupting the complex biochemical signaling involving KCNQ2 C-terminus [59, 60]. KCNQ2 or KCNQ3 mutations cause M current to be downregulated, and the frequency of neuronal firing increases, leading to epilepsy.

### **3.3 G protein-coupled Kir channel**

Inward-rectifier potassium channels (Kir, IRK) are a specific subset of potassium channels. To date, seven subfamilies have been identified, which are associated with a variety of diseases [61]. The G-protein-coupled Kir (GIRK) channels belong to the subfamily of Kir3 (GIRKs) which are activated by ligand-stimulated G protein-coupled receptors (GPCRs). GPCRs, interacting with GIRK channels, facilitate their activation, resulting in hyperpolarization of the cell membrane [61].

GIRK channels have four identified subunits (GIRK1–4, encoded by KCNJ3, KCNJ6, KCNJ9, and KCNJ5, respectively) in mammals, existing in vivo both as homotetramers and heterotetramers with unique biophysical properties, regulation, and distribution [61, 62]. GIRK 1, 2, 3, and 4 subunits are expressed in the brain, localized in certain axons, postsynaptic, and presynaptic regions [63]. GIRK channels may be involved not only in slow inhibitory postsynaptic potentials but also in the presynaptic modulation of neuronal activity [61].

GIRK in the CNS is a heterotetramer composed of GIRK1 and GIRK2 subunits [63], which is responsible for maintaining the resting membrane potential and excitability of the neuron [64]. GIRK1 and GIRK2 subunits are found in the dendritic areas of neurons highly [63] correlate with the large concentration of GABAB receptors. Once the GABAB receptors are activated by their ligands, they can in turn activate IRK, mediating a significant part of the GABA postsynaptic inhibition [63].

Alterations in GIRK function have been associated with pathophysiology of severe brain disorders, including epilepsy. In this regard, a GIRK2 knockout mouse model resulted to be more susceptible to develop both spontaneous and induced seizures in respect to wild-type mice [65]. In particular, mice carrying a p Gly156Ser mutation displayed an epileptic phenotype [66]. Indeed, this mutation has been found to alter the putative ion-permeable, pore-forming domain of the channel, inducing Ca2+ overload in cells and reducing channel availability, leading thus to neurodegeneration and seizure susceptibility [67].

An increased expression of GIRK was observed in rat brain after an electroconvulsive shock, probably altering the excitability of granule cells and the functions of neurotransmitter receptors which are coupled to these channels [68]. Another evidence in support of a role of GIRK in epilepsy was provided by the demonstration that ML297, a potent and selective activator of GIRK, showed epileptogenic properties in mice [69]. On the other hand, the inhibition of GIRK activity by drugs causes seizures [70]. All these considerations imply that changes in Kir3 channel activity may alter the susceptibility to seizures.
