Ligand activation mechanisms of human KCNQ2 channel

The human voltage-gated potassium channel KCNQ2/KCNQ3 carries the neuronal M-current, which helps to stabilize the membrane potential. KCNQ2 can be activated by analgesics and antiepileptic drugs but their activation mechanisms remain unclear. Here we report cryo-electron microscopy (cryo-EM) structures of human KCNQ2-CaM in complex with three activators, namely the antiepileptic drug cannabidiol (CBD), the lipid phosphatidylinositol 4,5-bisphosphate (PIP2), and HN37 (pynegabine), an antiepileptic drug in the clinical trial, in an either closed or open conformation. The activator-bound structures, along with electrophysiology analyses, reveal the binding modes of two CBD, one PIP2, and two HN37 molecules in each KCNQ2 subunit, and elucidate their activation mechanisms on the KCNQ2 channel. These structures may guide the development of antiepileptic drugs and analgesics that target KCNQ2.

Ancient medicinal plant rosemary contains a highly efficacious and isoform-selective KCNQ potassium channel opener

Voltage-gated potassium (Kv) channels in the KCNQ subfamily serve essential roles in the nervous system, heart, muscle and epithelia. Different heteromeric KCNQ complexes likely serve distinct functions in the brain but heteromer subtype-specific small molecules for research or therapy are lacking. Rosemary (Salvia rosmarinus) is an evergreen plant used medicinally for millennia for neurological and other disorders. Here, we report that rosemary extract is a highly efficacious opener of heteromeric KCNQ3/5 channels, with weak effects on KCNQ2/3. Using functional screening we find that carnosic acid, a phenolic diterpene from rosemary, is a potent, highly efficacious, PIP2 depletion-resistant KCNQ3 opener with lesser effects on KCNQ5 and none on KCNQ1 or KCNQ2. Carnosic acid is also highly selective for KCNQ3/5 over KCNQ2/3 heteromers. Medicinal chemistry, in silico docking, and mutagenesis reveal that carboxylate-guanidinium ionic bonding with an S4-5 linker arginine underlies the KCNQ3 opening proficiency of carnosic acid, the effects of which on KCNQ3/5 suggest unique therapeutic potential and a molecular basis for ancient neurotherapeutic use of rosemary.

Molecular basis for ligand activation of the human KCNQ2 channel

The voltage-gated potassium channel KCNQ2 is responsible for M-current in neurons and is an important drug target to treat epilepsy, pain and several other diseases related to neuronal hyper-excitability. A list of synthetic compounds have been developed to directly activate KCNQ2, yet our knowledge of their activation mechanism is limited, due to lack of high-resolution structures. Here, we report cryo-electron microscopy (cryo-EM) structures of the human KCNQ2 determined in apo state and in complex with two activators, ztz240 or retigabine, which activate KCNQ2 through different mechanisms. The activator-bound structures, along with electrophysiology analysis, reveal that ztz240 binds at the voltage-sensing domain and directly stabilizes it at the activated state, whereas retigabine binds at the pore domain and activates the channel by an allosteric modulation. By accurately defining ligand-binding sites, these KCNQ2 structures not only reveal different ligand recognition and activation mechanisms, but also provide a structural basis for drug optimization and design.

In silico re-engineering of a neurotransmitter to activate KCNQ potassium channels in an isoform-specific manner

Voltage-gated potassium (Kv) channel dysfunction causes a variety of inherited disorders, but developing small molecules that activate Kv channels has proven challenging. We recently discovered that the inhibitory neurotransmitter γ-aminobutyric acid (GABA) directly activates Kv channels KCNQ3 and KCNQ5. Here, finding that inhibitory neurotransmitter glycine does not activate KCNQs, we re-engineered it in silico to introduce predicted KCNQ-opening properties, screened by in silico docking, then validated the hits in vitro. Attaching a fluorophenyl ring to glycine optimized its electrostatic potential, converting it to a low-nM affinity KCNQ channel activator. Repositioning the phenyl ring fluorine and/or adding a methylsulfonyl group increased the efficacy of the re-engineered glycines and switched their target KCNQs. Combining KCNQ2- and KCNQ3-specific glycine derivatives synergistically potentiated KCNQ2/3 activation by exploiting heteromeric channel composition. Thus, in silico optimization and docking, combined with functional screening of only three compounds, facilitated re-engineering of glycine to develop several potent KCNQ activators.

Retigabine, a Kv7.2/Kv7.3-Channel Opener, Attenuates Drug-Induced Seizures in Knock-In Mice Harboring Kcnq2 Mutations

The hetero-tetrameric voltage-gated potassium channel K_v7.2/K_v7.3, which is encoded by KCNQ2 and KCNQ3, plays an important role in limiting network excitability in the neonatal brain. K_v7.2/K_v7.3 dysfunction resulting from KCNQ2 mutations predominantly causes self-limited or benign epilepsy in neonates, but also causes early onset epileptic encephalopathy. Retigabine (RTG), a K_v7.2/ K_v7.3-channel opener, seems to be a rational antiepileptic drug for epilepsies caused by KCNQ2 mutations. We therefore evaluated the effects of RTG on seizures in two strains of knock-in mice harboring different Kcnq2 mutations, in comparison to the effects of phenobarbital (PB), which is the first-line antiepileptic drug for seizures in neonates. The subjects were heterozygous knock-in mice (Kcnq2^Y284C/+ and Kcnq2^A306T/+) bearing the Y284C or A306T Kcnq2 mutation, respectively, and their wild-type (WT) littermates, at 63–100 days of age. Seizures induced by intraperitoneal injection of kainic acid (KA, 12mg/kg) were recorded using a video-electroencephalography (EEG) monitoring system. Effects of RTG on KA-induced seizures of both strains of knock-in mice were assessed using seizure scores from a modified Racine’s scale and compared with those of PB. The number and total duration of spike bursts on EEG and behaviors monitored by video recording were also used to evaluate the effects of RTG and PB. Both Kcnq2^Y284C/+ and Kcnq2^A306T/+ mice showed significantly more KA-induced seizures than WT mice. RTG significantly attenuated KA-induced seizure activities in both Kcnq2^Y284C/+ and Kcnq2^A306T/+ mice, and more markedly than PB. This is the first reported evidence of RTG ameliorating KA-induced seizures in knock-in mice bearing mutations of Kcnq2, with more marked effects than those observed with PB. RTG or other K_v7.2-channel openers may be considered as first-line antiepileptic treatments for epilepsies resulting from KCNQ2 mutations.

The Role of the Carboxyl Terminus Helix C-D Linker in Regulating KCNQ3 K+ Current Amplitudes by Controlling Channel Trafficking

In the central and peripheral nervous system, the assembly of KCNQ3 with KCNQ2 as mostly heteromers, but also homomers, underlies “M-type” currents, a slowly-activating voltage-gated K⁺ current that plays a dominant role in neuronal excitability. KCNQ3 homomers yield much smaller currents compared to KCNQ2 or KCNQ4 homomers and KCNQ2/3 heteromers. This smaller current has been suggested to result either from divergent channel surface expression or from a pore that is more unstable in KCNQ3. Channel surface expression has been shown to be governed by the distal part of the C-terminus in which helices C and D are critical for channel trafficking and assembly. A sequence alignment of this region in KCNQ channels shows that KCNQ3 possesses a longer linker between helix C and D compared to the other KCNQ subunits. Here, we investigate the role of the extra residues of this linker on KCNQ channel expression. Deletion of these residues increased KCNQ3 current amplitudes. Total internal reflection fluorescence imaging and plasma membrane protein assays suggest that the increase in current is due to a higher surface expression of the channels. Conversely, introduction of the extra residues into the linker between helices C and D of KCNQ4 reduced current amplitudes by decreasing the number of KCNQ4 channels at the plasma membrane. Confocal imaging suggests a higher fraction of channels, which possess the extra residues of helix C-D linker, were retained within the endoplasmic reticulum. Such retention does not appear to lead to protein accumulation and activation of the unfolded protein response that regulates protein folding and maintains endoplasmic reticulum homeostasis. Taken together, we conclude that extra helix C-D linker residues play a role in KCNQ3 current amplitudes by controlling the exit of the channel from the endoplasmic reticulum.

Simplified molecular input line entry system-based optimal descriptors: QSAR modelling for voltage-gated potassium channel subunit Kv7.2

CORAL software has been used to build quantitative structure-activity relationships (QSARs) for the prediction of binding affinities (pEC50, i.e., minus decimal logarithm of the 50% effective concentration) of 35 potent inhibitors towards the voltage-gated potassium channel subunit Kv7.2. The pEC50 has been modelled using eight random splits, with the following representations of the molecular structure: (i) hydrogen-suppressed graph (HSG); (ii) simplified molecular input line entry system (SMILES); (iii) graph atomic orbitals (GAOs) and (iv) hybrid representation, which is HSG together with SMILES. These models have been examined using three methods, the classic scheme, balance correlation, and balance correlation with ideal slope. The QSAR model based on single optimal descriptors using SMILES provided the best accuracy for the prediction of the pEC50. The robustness of these models has been checked using parameters such as rm(2), r(*)m(2), [Formula: see text], and using a randomization technique. The best QSAR model based on single optimal descriptors has been applied to study the in vitro structure-activity relationships of pyrazolo[1,5-a]pyrimidin-7(4H)-one derivatives as Kv7.2 modulators. The pEC50 is found to be significantly increased by the incorporation of -OH, -NO2 or -Br groups in place of one -F, whereas -NH2 has a negative effect on the pEC50 values.

Pivoting between calmodulin lobes triggered by calcium in the Kv7.2/calmodulin complex

Kv7.2 (KCNQ2) is the principal molecular component of the slow voltage gated M-channel, which strongly influences neuronal excitability. Calmodulin (CaM) binds to two intracellular C-terminal segments of Kv7.2 channels, helices A and B, and it is required for exit from the endoplasmic reticulum. However, the molecular mechanisms by which CaM controls channel trafficking are currently unknown. Here we used two complementary approaches to explore the molecular events underlying the association between CaM and Kv7.2 and their regulation by Ca²⁺. First, we performed a fluorometric assay using dansylated calmodulin (D-CaM) to characterize the interaction of its individual lobes to the Kv7.2 CaM binding site (Q2AB). Second, we explored the association of Q2AB with CaM by NMR spectroscopy, using ¹⁵N-labeled CaM as a reporter. The combined data highlight the interdependency of the N- and C-lobes of CaM in the interaction with Q2AB, suggesting that when CaM binds Ca²⁺ the binding interface pivots between the N-lobe whose interactions are dominated by helix B and the C-lobe where the predominant interaction is with helix A. In addition, Ca²⁺ makes CaM binding to Q2AB more difficult and, reciprocally, the channel weakens the association of CaM with Ca²⁺.

Discovery of (S,E)-3-(2-fluorophenyl)-N-(1-(3-(pyridin-3-yloxy)phenyl)ethyl)-acrylamide as a potent and efficacious KCNQ2 (Kv7.2) opener for the treatment of neuropathic pain

Acrylamide (S)-6, a potent and efficacious KCNQ2 (Kv7.2) opener, demonstrated significant activity in two models of neuropathic pain and in the formalin test, suggesting that KCNQ2 openers may be useful in the treatment of neuropathic pain including diabetic neuropathy.

Surface expression and subunit specific control of steady protein levels by the Kv7.2 helix A-B linker

Kv7.2 and Kv7.3 are the main components of the neuronal voltage-dependent M-current, which is a subthreshold potassium conductance that exerts an important control on neuronal excitability. Despite their predominantly intracellular distribution, these channels must reach the plasma membrane in order to control neuronal activity. Thus, we analyzed the amino acid sequence of Kv7.2 to identify intrinsic signals that may control its surface expression. Removal of the interlinker connecting helix A and helix B of the intracellular C-terminus produces a large increase in the number of functional channels at the plasma membrane. Moreover, elimination of this linker increased the steady-state amount of protein, which was not associated with a decrease of protein degradation. The magnitude of this increase was inversely correlated with the number of helix A – helix B linkers present in the tetrameric channel assemblies. In contrast to the remarkable effect on the amount of Kv7.2 protein, removal of the Kv7.2 linker had no detectable impact on the steady-state levels of Kv7.3 protein.

KCNQ2 abnormality in BECTS: benign childhood epilepsy with centrotemporal spikes following benign neonatal seizures resulting from a mutation of KCNQ2

The molecular pathogenesis of benign childhood epilepsy with centrotemporal spikes (BECTS) remains unclear whereas mutations of the KCNQ2 and KCNQ3 genes have been identified as causes of benign familial neonatal convulsions. We report here a girl with benign neonatal convulsions followed by BECTS, for whom a mutation of KCNQ2 was identified. This case may provide the clue to the understanding of the molecular pathogenesis of BECTS.

Kv7 channels can function without constitutive calmodulin tethering

M-channels are voltage-gated potassium channels composed of Kv7.2-7.5 subunits that serve as important regulators of neuronal excitability. Calmodulin binding is required for Kv7 channel function and mutations in Kv7.2 that disrupt calmodulin binding cause Benign Familial Neonatal Convulsions (BFNC), a dominantly inherited human epilepsy. On the basis that Kv7.2 mutants deficient in calmodulin binding are not functional, calmodulin has been defined as an auxiliary subunit of Kv7 channels. However, we have identified a presumably phosphomimetic mutation S511D that permits calmodulin-independent function. Thus, our data reveal that constitutive tethering of calmodulin is not required for Kv7 channel function.

A novel missense mutation (N258S) in the KCNQ2 gene in a Turkish family afflicted with benign familial neonatal convulsions (BFNC)

Benign familial neonatal convulsions (BFNC) is a rare monogenic subtype of idiopathic epilepsy exhibiting autosomal dominant mode of inheritance. The disease is caused by mutations in the two homologous genes KCNQ2 and KCNQ3 that encode the subunits of the voltage-gated potassium channel. Most KCNQ2 mutations are found in the pore region and the cytoplasmic C domain. These mutations are either deletions/insertions that result in frameshift or truncation of the protein product, splice-site variants or missense mutations. This study reveals a novel missense mutation (N258S) in the KCNQ2 gene between the S5 domain and the pore of the potassium channel in two BFNC patients in a Turkish family. The absence of the mutation both in the healthy members of the family and in a control group, and the lack of any other change in the KCNQ2 gene of the patients indicate that N258S substitution is a pathogenic mutation leading to epileptic seizures in this family.

Deletions or duplications in KCNQ2 can cause benign familial neonatal seizures

BACKGROUND

Benign familial neonatal seizures are most often caused by mutations in the voltage-gated potassium channel subunit gene KCNQ2. More than 60 mutations have been described in BFNS families, approximately half of which lead to protein truncation. The hypothesis of this study was that deletion or duplication of >or=1 exons of KCNQ2 could cause BFNS in cases without coding or splicing mutations.

METHODS

Multiplex ligation-dependent probe amplification (MLPA) was used to test a group of 21 unrelated patients with clinical features consistent with either BFNS, benign familial neonatal-infantile seizures or sporadic neonatal seizures, for exonic deletions and duplications.

RESULTS

Three deletions and one duplication mutation were identified in four familial cases and cascade testing of their available family members showed that the mutations segregated with the phenotype in each family. The junction fragment for one of the deletions was amplified by PCR and sequenced to characterise the breakpoint and verify that a deletion had occurred.

CONCLUSIONS

Submicroscopic deletions or duplications of KCNQ2 are seen in a significant proportion of BFNS families: four of nine (44%) cases previously testing negative for coding or splice site mutation by sequencing KCNQ2 and KCNQ3. MLPA is an efficient second-tier testing strategy for KCNQ2 to identify pathogenic intragenic mutations not detectable by conventional DNA sequencing methods.

Depolarization activates the phosphoinositide phosphatase Ci-VSP, as detected in Xenopus oocytes coexpressing sensors of PIP2

Voltage-evoked signals play critical roles in neural activities, muscle contraction and exocytosis. Ciona voltage-sensor containing phosphatase (Ci-VSP) consists of the transmembrane voltage sensor domain (VSD) and a cytoplasmic domain of phosphoinositide phosphatase, homologous to phosphatase and tensin homologue deleted on chromosome 10 (PTEN). Previous experiments utilizing potassium channels as the sensor for phosphoinositides have demonstrated that phosphatase activities of Ci-VSP are voltage dependent. However, it still remained unclear whether enzyme activity is activated by depolarization or hyperpolarization. Further, a large gap in voltage dependency was found between the charge movement of the VSD and potassium channel-reporting phosphatase activities. In this study, voltage-dependent dynamics of phosphoinositides mediated by Ci-VSP were examined by confocal imaging and electrical measurements in Xenopus oocytes. Imaging of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) using green fluorescent protein (GFP)-tagged pleckstrin homology (PH) domains from phospholipase C delta subunit (PLC-delta) showed that PtdIns(4,5)P(2) concentration is reduced during depolarization. In the presence of Ci-VSP, IRK1 channels with higher sensitivity to phosphoinositide than GIRK2 channels decreased their magnitude during depolarization over 0 mV, indicating that the PtdIns(4,5)P(2) level is reduced upon depolarization. KCNQ2/3 channels coexpressed with Ci-VSP exhibited voltage-dependent decay of the outward current that became sharper with higher depolarization in a voltage range up to 100 mV. These results indicate that Ci-VSP has an activity that depletes PtdIns(4,5)P(2) unlike PTEN and that depolarization-activated voltage sensor movement is translated into activation of phosphatase activity.

Self-assembly of the isolated KCNQ2 subunit interaction domain

Mutations in the KCNQ2 gene cause myokymia and neonatal epilepsy, indicating that this K(+) channel regulates the excitability of lower motoneurons and CNS neurons. Little is known about the parameters that direct the assembly of this multimeric molecule and other KCNQ subunits. Here, we show that the carboxy-terminal subunit interaction domain of KCNQ2 autonomously folds and assembles into tetramers. This domain contains a bipartite coiled-coil motif. Whereas structural integrity of the second coiled-coil motif is crucial for tetramer formation, that of the first motif is less important. These data suggest a crucial role of coiled-coil motifs in tetrameric KCNQ channel assembly.

Polarized axonal surface expression of neuronal KCNQ channels is mediated by multiple signals in the KCNQ2 and KCNQ3 C-terminal domains

The M channels, important regulators of neuronal excitability, are voltage-gated potassium channels composed of KCNQ2-5 subunits. Mutations in KCNQ2 and KCNQ3 cause benign familial neonatal convulsions (BFNC), dominantly inherited epilepsy and myokymia. Crucial for their functions in controlling neuronal excitability, the M channels must be placed at specific regions of the neuronal membrane. However, the precise distribution of surface KCNQ channels is not known. Here, we show that KCNQ2/KCNQ3 channels are preferentially localized to the surface of axons both at the axonal initial segment and more distally. Whereas axonal initial segment targeting of surface KCNQ channels is mediated by ankyrin-G binding motifs of KCNQ2 and KCNQ3, sequences mediating targeting to more distal portion of the axon reside in the membrane proximal and A domains of the KCNQ2 C-terminal tail. We further show that several BFNC mutations of KCNQ2 and KCNQ3 disrupt surface expression or polarized surface distribution of KCNQ channels, thereby revealing impaired targeting of KCNQ channels to axonal surfaces as a BFNC etiology.

Structural determinants of M-type KCNQ (Kv7) K+ channel assembly

The ability of KCNQ (Kv7) channels to form hetero-oligomers is of high physiological importance, because heteromers of KCNQ3 with KCNQ2 or KCNQ5 underlie the neuronal M-current, which modulates neuronal excitability. In KCNQ channels, we recently identified a C-terminal subunit interaction (si) domain that determines their subunit-specific assembly. Within this si domain, there are two motifs that comprise approximately 30 amino acid residues each and that exhibit a high probability for coiled-coil formation. Transfer of the first or the second coiled-coil (TCC) domain from KCNQ3 into the KCNQ1 scaffold resulted in chimeras KCNQ1(TCC1)Q3 and KCNQ1(TCC2)Q3, both of which coimmunoprecipitated with KCNQ2. However, only KCNQ1(TCC2)Q3 enhanced KCNQ2 currents and surface expression or exerted a strong dominant-negative effect on KCNQ2. Deletion of TCC2 within KCNQ2 yielded functional homomeric channels but prevented the current augmentation measured after coexpression of KCNQ2 and KCNQ3. In contrast, deleting TCC1 within KCNQ2 did not give functional homomeric KCNQ2 or heteromeric KCNQ2/KCNQ3 channels. Mutations that disrupted the predicted coiled-coil structure of TCC1 in KCNQ2 or KCNQ3 abolished channel activity after expressing these constructs singly or in combination, whereas helix-breaking mutations in TCC2 of KCNQ2 gave functional homomeric channels but prevented the heteromerization with KCNQ3. In contrast, KCNQ3 carrying a coiled-coil disrupting mutation in TCC2 hetero-oligomerized with KCNQ2. Our data suggest that the TCC1 domains of KCNQ2 and KCNQ3 are required to form functional homomeric as well as heteromeric channels, whereas both TCC2 domains facilitate an efficient transport of heteromeric KCNQ2/KCNQ3 channels to the plasma membrane.

Identification by mass spectrometry and functional characterization of two phosphorylation sites of KCNQ2/KCNQ3 channels

Neuronal potassium channel subunits of the KCNQ (Kv7) family underlie M-current (I(M)), and may also underlie the slow potassium current at the node of Ranvier, I(Ks). I(M) and I(Ks) are outwardly rectifying currents that regulate excitability of neurons and myelinated axons, respectively. Studies of native I(M) and heterologously expressed Kv7 subunits suggest that, in vivo, KCNQ channels exist within heterogeneous, multicomponent protein complexes. KCNQ channel properties are regulated by protein phosphorylation, protein-protein interactions, and protein-lipid interactions within such complexes. To better understand the regulation of neuronal KCNQ channels, we searched directly for posttranslational modifications on KCNQ2/KCNQ3 channels in vivo by using mass spectrometry. Here we describe two sites of phosphorylation. One site, specific for KCNQ3, appears functionally silent in electrophysiological assays but is located in a domain previously shown to be important for subunit tetramerization. Mutagenesis and electrophysiological studies of the second site, located in the S4-S5 intracellular loop of all KCNQ subunits, reveal a mechanism of channel inhibition.

Decreased subunit stability as a novel mechanism for potassium current impairment by a KCNQ2 C terminus mutation causing benign familial neonatal convulsions

KCNQ2 and KCNQ3 K+ channel subunits underlie the muscarinic-regulated K+ current (I(KM)), a widespread regulator of neuronal excitability. Mutations in KCNQ2- or KCNQ3-encoding genes cause benign familiar neonatal convulsions (BFNCs), a rare autosomal-dominant idiopathic epilepsy of the newborn. In the present study, we have investigated, by means of electrophysiological, biochemical, and immunocytochemical techniques in transiently transfected cells, the consequences prompted by a BFNC-causing 1-bp deletion (2043deltaT) in the KCNQ2 gene; this frameshift mutation caused the substitution of the last 163 amino acids of the KCNQ2 C terminus and the extension of the subunit by additional 56 residues. The 2043deltaT mutation abolished voltage-gated K+ currents produced upon homomeric expression of KCNQ2 subunits, dramatically reduced the steady-state cellular levels of KCNQ2 subunits, and prevented their delivery to the plasma membrane. Metabolic labeling experiments revealed that mutant KCNQ2 subunits underwent faster degradation; 10-h treatment with the proteasomal inhibitor MG132 (20 microm) at least partially reversed such enhanced degradation. Co-expression with KCNQ3 subunits reduced the degradation rate of mutant KCNQ2 subunits and led to their expression on the plasma membrane. Finally, co-expression of KCNQ2 2043deltaT together with KCNQ3 subunits generated functional voltage-gated K+ currents having pharmacological and biophysical properties of heteromeric channels. Collectively, the present results suggest that mutation-induced reduced stability of KCNQ2 subunits may cause epilepsy in neonates.