Reconstitution of muscarinic modulation of the KCNQ2/KCNQ3 K(+) channels that underlie the neuronal M current

Redox and nitric oxide-mediated regulation of sensory neuron ion channel function

SIGNIFICANCE

Reactive oxygen and nitrogen species (ROS and RNS, respectively) can intimately control neuronal excitability and synaptic strength by regulating the function of many ion channels. In peripheral sensory neurons, such regulation contributes towards the control of somatosensory processing; therefore, understanding the mechanisms of such regulation is necessary for the development of new therapeutic strategies and for the treatment of sensory dysfunctions, such as chronic pain.

RECENT ADVANCES

Tremendous progress in deciphering nitric oxide (NO) and ROS signaling in the nervous system has been made in recent decades. This includes the recognition of these molecules as important second messengers and the elucidation of their metabolic pathways and cellular targets. Mounting evidence suggests that these targets include many ion channels which can be directly or indirectly modulated by ROS and NO. However, the mechanisms specific to sensory neurons are still poorly understood. This review will therefore summarize recent findings that highlight the complex nature of the signaling pathways involved in redox/NO regulation of sensory neuron ion channels and excitability; references to redox mechanisms described in other neuron types will be made where necessary.

CRITICAL ISSUES

The complexity and interplay within the redox, NO, and other gasotransmitter modulation of protein function are still largely unresolved. Issues of specificity and intracellular localization of these signaling cascades will also be addressed.

FUTURE DIRECTIONS

Since our understanding of ROS and RNS signaling in sensory neurons is limited, there is a multitude of future directions; one of the most important issues for further study is the establishment of the exact roles that these signaling pathways play in pain processing and the translation of this understanding into new therapeutics.

Molecular underpinnings of ventral surface chemoreceptor function: focus on KCNQ channels

Central chemoreception is the mechanism by which CO₂/H(+) -sensitive neurons (i.e. chemoreceptors) regulate breathing in response to changes in tissue CO₂/H(+) . Neurons in the retrotrapezoid nucleus (RTN) directly regulate breathing in response to changes in tissue CO₂/H(+) and function as a key locus of respiratory control by integrating information from several respiratory centres, including the medullary raphe. Therefore, chemosensitive RTN neurons appear to be critically important for maintaining breathing, thus understanding molecular mechanisms that regulate RTN chemoreceptor function may identify therapeutic targets for the treatment of respiratory control disorders. We have recently shown that KCNQ (Kv7) channels in the RTN are essential determinants of spontaneous activity ex vivo, and downstream effectors for serotonergic modulation of breathing. Considering that loss of function mutations in KCNQ channels can cause certain types of epilepsy including those associated with sudden unexplained death in epilepsy (SUDEP), we propose that dysfunctions of KCNQ channels may be one cause for epilepsy and respiratory problems associated with SUDEP. In this review, we will summarize the role of KCNQ channels in the regulation of RTN chemoreceptor function, and suggest that these channels represent useful therapeutic targets for the treatment of respiratory control disorders.

Ability of naringenin, a bioflavonoid, to activate M-type potassium current in motor neuron-like cells and to increase BKCa-channel activity in HEK293T cells transfected with α-hSlo subunit

Background

Naringenin (NGEN) is a citrus bioflavonoid known to have beneficial health properties; however, the ionic mechanism of its actions remains largely unclear. In this study, we attempted to evaluate the possible effects of NGEN on K⁺ currents in NSC-34 neuronal cells and in HEK293T cells expressing α-hSlo.

Results

NGEN increased M-type K⁺ current (I_K(M)) in a concentration-dependent manner with an EC₅₀ value of 9.8 μM in NSC-34 cells. NGEN shifted the activation curve of I_K(M) conductance to the more negative potentials. In cell-attached recordings, NGEN or flupirtine enhanced the activity of M-type K⁺ (K_M) channels with no changes in single-channel amplitude. NGEN (10 μM) had minimal effect on erg-mediated K⁺ currents. Under cell-attached voltage-clamp recordings, NGEN decreased the frequency of spontaneous action currents and further application of linopirdine can reverse NGEN-induced inhibition of firing. In HEK293T cells expressing α-hSlo, this compound increased the amplitude of Ca²⁺-activated K⁺ current (I_K(Ca)). Under inside-out recordings, NGEN applied to the intracellular side of the detached patch enhanced the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. Moreover, from the study of a modeled neuron, burst firing of simulated action potentials (APs) was reduced in the presence of the increased conductances of both K_M and K_Ca channels. Fast-slow analysis of AP bursting from this model also revealed that as the conductances of both K_M and BK_Ca channels were increased by two-fold, the voltage nullcline was shifted in an upward direction accompanied by the compression of burst trajectory.

Conclusions

The present results demonstrate that activation of both K_M and BK_Ca channels caused by NGEN might combine to influence neuronal activity if similar channels were functionally co-expressed in central neurons in vivo.

PIP2 regulation of KCNQ channels: biophysical and molecular mechanisms for lipid modulation of voltage-dependent gating

Voltage-gated potassium (Kv) channels contain voltage-sensing (VSD) and pore-gate (PGD) structural domains. During voltage-dependent gating, conformational changes in the two domains are coupled giving rise to voltage-dependent opening of the channel. In addition to membrane voltage, KCNQ (Kv7) channel opening requires the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂). Recent studies suggest that PIP₂ serves as a cofactor to mediate VSD-PGD coupling in KCNQ1 channels. In this review, we put these findings in the context of the current understanding of voltage-dependent gating, lipid modulation of Kv channel activation, and PIP₂-regulation of KCNQ channels. We suggest that lipid-mediated coupling of functional domains is a common mechanism among KCNQ channels that may be applicable to other Kv channels and membrane proteins.

Kcnq1-5 (Kv7.1-5) potassium channel expression in the adult zebrafish

BACKGROUND

KCNQx genes encode slowly activating-inactivating K+ channels, are linked to physiological signal transduction pathways, and mutations in them underlie diseases such as long QT syndrome (KCNQ1), epilepsy in adults (KCNQ2/3), benign familial neonatal convulsions in children (KCNQ3), and hearing loss or tinnitus in humans (KCNQ4, but not KCNQ5). Identification of kcnqx potassium channel transcripts in zebrafish (Danio rerio) remains to be fully characterized although some genes have been mapped to the genome. Using zebrafish genome resources as the source of putative kcnq sequences, we investigated the expression of kcnq1-5 in heart, brain and ear tissues.

RESULTS

Overall expression of the kcnqx channel transcripts is similar to that found in mammals. We found that kcnq1 expression was highest in the heart, and also present in the ear and brain. kcnq2 was lowest in the heart, while kcnq3 was highly expressed in the brain, heart and ear. kcnq5 expression was highest in the ear. We analyzed zebrafish genomic clones containing putative kcnq4 sequences to identify transcripts and protein for this highly conserved member of the Kcnq channel family. The zebrafish appears to have two kcnq4 genes that produce distinct mRNA species in brain, ear, and heart tissues.

CONCLUSIONS

We conclude that the zebrafish is an attractive model for the study of the KCNQ (Kv7) superfamily of genes, and are important to processes involved in neuronal excitability, cardiac anomalies, epileptic seizures, and hearing loss or tinnitus.

Dynamic metabolic control of an ion channel

G-protein-coupled receptors mediate responses to external stimuli in various cell types. We are interested in the modulation of KCNQ2/3 potassium channels by the Gq-coupled M1 muscarinic (acetylcholine) receptor (M1R). Here, we describe development of a mathematical model that incorporates all known steps along the M1R signaling cascade and accurately reproduces the macroscopic behavior we observe when KCNQ2/3 currents are inhibited following M1R activation. Gq protein-coupled receptors of the plasma membrane activate phospholipase C (PLC) which cleaves the minor plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) into the second messengers diacylgycerol and inositol 1,4,5-trisphosphate, leading to calcium release, protein kinase C (PKC) activation, and PI(4,5)P2 depletion. Combining optical and electrical techniques with knowledge of relative abundance of each signaling component has allowed us to develop a kinetic model and determine that (i) M1R activation and M1R/Gβ interaction are fast; (ii) Gαq/Gβ separation and Gαq/PLC interaction have intermediate time constants; (iii) the amount of activated PLC limits the rate of KCNQ2/3 suppression; (iv) weak PLC activation can elicit robust calcium signals without net PI(4,5)P2 depletion or KCNQ2/3 channel inhibition; and (v) depletion of PI(4,5)P2, and not calcium/CaM or PKC-mediated phosphorylation, closes KCNQ2/3 potassium channels, thereby increasing neuronal excitability.

Calmodulin orchestrates the heteromeric assembly and the trafficking of KCNQ2/3 (Kv7.2/3) channels in neurons

Mutations in KCNQ2 and KCNQ3 genes are responsible for benign familial neonatal seizures and epileptic encephalopathies. Some of these mutations have been shown to alter the binding of calmodulin (CaM) to specific C-terminal motifs of KCNQ subunits, known as the A and B helices. Here, we show that the mutation I342A in the A helix of KCNQ3 abolishes CaM interaction and strongly decreases the heteromeric association with KCNQ2. The assembly of KCNQ2 with KCNQ3 is essential for their expression at the axon initial segment (AIS). We find that the I342A mutation alters the targeting of KCNQ2/3 subunits at the AIS. However, the traffic of the mutant channels was rescued by provision of exogenous CaM. We show that CaM enhances the heteromeric association of KCNQ2/KCNQ3-I342A subunits by binding to their B helices in a calcium-dependent manner. To further assert the implication of CaM in channel assembly, we inserted a mutation in the second coil-coil domain of KCNQ2 (KCNQ2-L638P) to prevent its heteromerization with KCNQ3. We observe that the expression of a Ca(2+)-insensitive form of CaM favours the assembly of KCNQ3 with KCNQ2-L638P. Our data thus indicate that both apoCaM and Ca(2+)/CaM bind to the C-terminal domains of KCNQ2 and KCNQ3 subunits, and regulate their heteromeric assembly. Hence, CaM may control the composition and distribution of KCNQ channels in neurons.

K+ channel modulation causes genioglossus inhibition in REM sleep and is a strategy for reactivation

Rapid eye movement (REM) sleep is accompanied by periods of upper airway motor suppression that cause hypoventilation and obstructive apneas in susceptible individuals. A common idea has been that upper airway motor suppression in REM sleep is caused by the neurotransmitters glycine and γ-amino butyric acid (GABA) acting at pharyngeal motor pools to inhibit motoneuron activity. Data refute this as a workable explanation because blockade of this putative glycine/GABAergic mechanism releases pharyngeal motor activity in all states, and least of all in REM sleep. Here we summarize a novel motor-inhibitory mechanism that suppresses hypoglossal motor activity largely in REM sleep, this being a muscarinic receptor mechanism linked to G-protein-coupled inwardly rectifying potassium (GIRK) channels. We then outline how this discovery informs efforts to pursue therapeutic targets to reactivate hypoglossal motor activity throughout sleep via potassium channel modulation. One such target is the inwardly rectifying potassium channel Kir2.4 whose expression in the brain is almost exclusive to cranial motor nuclei.

Kv7/M-type potassium channels in rat skin keratinocytes

Skin keratinocytes fulfil important signalling and protective functions. Immunocytochemical experiments revealed the unexpected presence of immunoreactivity for the M-type potassium channel subunit Kv7.2 in the keratinocyte layer of intact rat paw skin and in keratinocytes isolated from the skin of 1-day-old rats and cultured in vitro for 3–10 days. Application of the M-channel enhancer retigabine (3–10 μM) to isolated cultured rat keratinocytes: (a) increased outward membrane currents recorded under voltage clamp, (b) produced ~3 mV hyperpolarization at rest, (c) enhanced ~3-fold the release of ATP induced by the TRPV3 agonist carvacrol (1 mM) and (d) increased the amplitude of the carvacrol-induced intracellular Ca²⁺ transient measured with Fura-2. The effect of retigabine on ATP release was prevented by the M-channel blocking agent XE991. We conclude that rat skin keratinocytes possess M-channels that, when activated, can modify their physiological properties, with potential significance for their sensory and other biological functions.

Intracellular calcium level is an important factor influencing ion channel modulations by PLC-coupled metabotropic receptors in hippocampal neurons

Signaling pathways involving phospholipase C (PLC) are involved in various neural functions. Understanding how these pathways are regulated will lead to a better understanding of their roles in neural functions. Previous studies demonstrated that receptor-driven PLCβ activation depends on intracellular Ca(2+) concentration ([Ca(2+)]i), suggesting the possibility that PLCβ-dependent cellular responses are basically Ca(2+) dependent. To test this possibility, we examined whether modulations of ion channels driven by PLC-coupled metabotropic receptors are sensitive to [Ca(2+)]i using cultured hippocampal neurons. Muscarinic activation triggered an inward current at -100 mV (the equilibrium potential for K(+)) in a subpopulation of neurons. This current response was suppressed by pirenzepine (an M1-preferring antagonist), PLC inhibitor, non-selective cation channel blocker, and lowering [Ca(2+)]i. Using the neurons showing no response at -100 mV, effects of muscarinic activation on K(+) channels were examined at -40 mV. Muscarinic activation induced a transient decrease of the holding outward current. This current response was mimicked and occluded by XE991, an M-current K(+) channel blocker, suppressed by pirenzepine, PLC inhibitor and lowering [Ca(2+)]i, and enhanced by elevating [Ca(2+)]i. Similar results were obtained when group I metabotropic glutamate receptors were activated instead of muscarinic receptors. These results clearly show that ion channel modulations driven by PLC-coupled metabotropic receptors are dependent on [Ca(2+)]i, supporting the hypothesis that cellular responses induced by receptor-driven PLCβ activation are basically Ca(2+) dependent.

Muscarinic depolarization of layer II neurons of the parasubiculum

The parasubiculum (PaS) is a component of the hippocampal formation that sends its major output to layer II of the entorhinal cortex. The PaS receives strong cholinergic innervation from the basal forebrain that is likely to modulate neuronal excitability and contribute to theta-frequency network activity. The present study used whole cell current- and voltage-clamp recordings to determine the effects of cholinergic receptor activation on layer II PaS neurons. Bath application of carbachol (CCh; 10–50 µM) resulted in a dose-dependent depolarization of morphologically-identified layer II stellate and pyramidal cells that was not prevented by blockade of excitatory and inhibitory synaptic inputs. Bath application of the M₁ receptor antagonist pirenzepine (1 µM), but not the M₂-preferring antagonist methoctramine (1 µM), blocked the depolarization, suggesting that it is dependent on M₁ receptors. Voltage-clamp experiments using ramped voltage commands showed that CCh resulted in the gradual development of an inward current that was partially blocked by concurrent application of the selective Kv7.2/3 channel antagonist XE-991, which inhibits the muscarine-dependent K⁺ current I_M. The remaining inward current also reversed near E_K and was inhibited by the K⁺ channel blocker Ba²⁺, suggesting that M₁ receptor activation attenuates both I_M as well as an additional K⁺ current. The additional K⁺ current showed rectification at depolarized voltages, similar to K⁺ conductances mediated by Kir 2.3 channels. The cholinergic depolarization of layer II PaS neurons therefore appears to occur through M₁-mediated effects on I_M as well as an additional K⁺ conductance.

Regulation of voltage-gated potassium channels by PI(4,5)P2

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) regulates activities of numerous ion channels including inwardly rectifying potassium (K_ir) channels, KCNQ, TRP, and voltage-gated calcium channels. Several studies suggest that voltage-gated potassium (K_V) channels might be regulated by PI(4,5)P₂. Wide expression of K_V channels in different cells suggests that such regulation could have broad physiological consequences. To study regulation of K_V channels by PI(4,5)P₂, we have coexpressed several of them in tsA-201 cells with a G protein–coupled receptor (M₁R), a voltage-sensitive lipid 5-phosphatase (Dr-VSP), or an engineered fusion protein carrying both lipid 4-phosphatase and 5-phosphatase activity (pseudojanin). These tools deplete PI(4,5)P₂ with application of muscarinic agonists, depolarization, or rapamycin, respectively. PI(4,5)P₂ at the plasma membrane was monitored by Förster resonance energy transfer (FRET) from PH probes of PLCδ1 simultaneously with whole-cell recordings. Activation of Dr-VSP or recruitment of pseudojanin inhibited K_V7.1, K_V7.2/7.3, and K_ir2.1 channel current by 90–95%. Activation of M₁R inhibited K_V7.2/7.3 current similarly. With these tools, we tested for potential PI(4,5)P₂ regulation of activity of K_V1.1/K_Vβ1.1, K_V1.3, K_V1.4, and K_V1.5/K_Vβ1.3, K_V2.1, K_V3.4, K_V4.2, K_V4.3 (with different KChIPs and DPP6-s), and hERG/KCNE2. Interestingly, we found a substantial removal of inactivation for K_V1.1/K_Vβ1.1 and K_V3.4, resulting in up-regulation of current density upon activation of M₁R but no changes in activity upon activating only VSP or pseudojanin. The other channels tested except possibly hERG showed no alteration in activity in any of the assays we used. In conclusion, a depletion of PI(4,5)P₂ at the plasma membrane by enzymes does not seem to influence activity of most tested K_V channels, whereas it does strongly inhibit members of the K_V7 and K_ir families.

Restoration of ion channel function in deafness-causing KCNQ4 mutants by synthetic channel openers

BACKGROUND AND PURPOSE

DFNA2 is a frequent hereditary hearing disorder caused by loss-of-function mutations in the voltage-gated potassium channel KCNQ4 (Kv7.4). KCNQ4 mediates the predominant K(+) conductance, I(K,n) , of auditory outer hair cells (OHCs), and loss of KCNQ4 function leads to degeneration of OHCs resulting in progressive hearing loss. Here we explore the possible recovery of channel activity of mutant KCNQ4 induced by synthetic KCNQ channel openers.

EXPERIMENTAL APPROACH

Whole cell patch clamp recordings were performed on CHO cells transiently expressing KCNQ4 wild-type (wt) and DFNA2-relevant mutants, and from acutely isolated OHCs.

KEY RESULTS

Various known KCNQ channel openers robustly enhanced KCNQ4 currents. The strongest potentiation was observed with a combination of zinc pyrithione plus retigabine. A similar albeit less pronounced current enhancement was observed with native I(K,n) currents in rat OHCs. DFNA2 mutations located in the channel's pore region abolished channel function and these mutant channels were completely unresponsive to channel openers. However, the function of a DFNA2 mutation located in the proximal C-terminus was restored by the combined application of both openers. Co-expression of wt and KCNQ4 pore mutants suppressed currents to barely detectable levels. In this dominant-negative situation, channel openers essentially restored currents back to wt levels, most probably through strong activation of only the small fraction of homomeric wt channels.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that by stabilizing the KCNQ4-mediated conductance in OHCs, chemical channel openers can protect against OHC degeneration and progression of hearing loss in DFNA2.

Distinct subunit contributions to the activation of M-type potassium channels by PI(4,5)P2

Low-threshold voltage-gated M-type potassium channels (M channels) are tetraheteromers, commonly of two Kv7.2 and two Kv7.3 subunits. Though gated by voltage, the channels have an absolute requirement for binding of the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)) to open. We have investigated the quantitative relation between the concentration of a water-soluble PI(4,5)P(2) analog, dioctanoyl-PI(4,5)P(2) (DiC(8)-PI(4,5)P(2)), and channel open probability (P(open)) by fast application of increasing concentrations of DiC(8)-PI(4,5)P(2) to the inside face of membrane patches excised from Chinese hamster ovary cells expressing M channels as heteromeric Kv7.2/7.3 subunits. The rationale for the experiments is that this will mimic the effect of changes in membrane PI(4,5)P(2) concentration. Single-channel conductances from channel current-voltage relations in cell-attached mode were 9.2 ± 0.1 pS with a 2.5-mM pipette [K(+)]. Plots of P(open) against DiC(8)-PI(4,5)P(2) concentration were best fitted using a two-component concentration-P(open) relationship with high and low affinity, half-maximal effective concentration (EC(50)) values of 1.3 ± 0.14 and 75.5 ± 2.5 µM, respectively, and Hill slopes of 1.4 ± 0.06. In contrast, homomeric channels from cells expressing only Kv7.2 or Kv7.3 constructs yielded single-component curves with EC(50) values of 76.2 ± 19.9 or 3.6 ± 1.0 µM, respectively. When wild-type (WT) Kv7.2 was coexpressed with a mutated Kv7.3 subunit with >100-fold reduced sensitivity to PI(4,5)P(2), the high-affinity component of the activation curve was lost. Fitting the data for WT and mutant channels to an activation mechanism with independent PI(4,5)P(2) binding to two Kv7.2 and two Kv7.3 subunits suggests that the two components of the M-channel activation curve correspond to the interaction of PI(4,5)P(2) with the Kv7.3 and Kv7.2 subunits, respectively, that channels can open when only the two Kv7.3 subunits have bound DiC(8)-PI(4,5)P(2), and that maximum channel opening requires binding to all four subunits.

Pore determinants of KCNQ3 K+ current expression

KCNQ3 homomeric channels yield very small macroscopic currents compared with other KCNQ channels or KCNQ2/3 heteromers. Two disparate regions of the channels--the C-terminus and the pore region--have been implicated in governing KCNQ current amplitudes. We previously showed that the C-terminus plays a secondary role compared with the pore region. Here, we confirm the critical role of the pore region in determining KCNQ3 currents. We find that mutations at the 312 position in the pore helix of KCNQ3 (I312E, I312K, and I312R) dramatically decreased KCNQ3 homomeric currents as well as heteromeric KCNQ2/3 currents. Evidence that these mutants were expressed in the heteromers includes shifted TEA sensitivity compared with KCNQ2 homomers. To test for differential membrane protein expression, we performed total internal reflection fluorescence imaging, which revealed only small differences that do not underlie the differences in macroscopic currents. To determine whether this mechanism generalizes to other KCNQ channels, we tested the effects of analogous mutations at the conserved I273 position in KCNQ2, with similar results. Finally, we performed homology modeling of the pore region of wild-type and mutant KCNQ3 channels to investigate the putative structural mechanism mediating these results. The modeling suggests that the lack of current in I312E, I312K, and I312R KCNQ3 channels is due to pore helix-selectivity filter interactions that lock the selectivity filter in a nonconductive conformation.

Comparison of the effects of DC031050, a class III antiarrhythmic agent, on hERG channel and three neuronal potassium channels

AIM

This study was conducted to test the selectivity of DC031050 on cardiac and neuronal potassium channels.

METHODS

Human ether-à-go-go related gene (hERG), KCNQ and Kv1.2 channels were expressed in CHO cells. The delayed rectifier potassium current (I(K)) was recorded from dissociated hippocampal pyramidal neurons of neonatal rats. Whole-cell voltage patch clamp was used to record the voltage-activated potassium currents. Drug-containing solution was delivered using a RSC-100 Rapid Solution Changer.

RESULTS

Both DC031050 and dofetilide potently inhibited hERG currents with IC(50) values of 2.3 ± 1.0 and 17.9 ± 1.2 nmol/L, respectively. DC031050 inhibited the I(K) current with an IC(50) value of 2.7 ± 1.5 μmol/L, which was >1000 times the concentration required to inhibit hERG current. DC031050 at 3 μmol/L did not significantly affect the voltage-dependence of the steady activation, steady inactivation of I(K), or the rate of I(K) from inactivation. Intracellular application of DC031050 (5 μmol/L) was insufficient to inhibit I(K). DC031050 up to 10 μmol/L had no effects on KCNQ2 and Kv1.2 channel currents.

CONCLUSION

DC031050 is a highly selective hERG potassium channel blocker with a substantial safety margin of activity over neuronal potassium channels, thus holds significant potential for therapeutic application as a class III antiarrhythmic agent.

Coordinated signal integration at the M-type potassium channel upon muscarinic stimulation

Several neurotransmitters, including acetylcholine, regulate neuronal tone by suppressing a non-inactivating low-threshold voltage-gated potassium current generated by the M-channel. Agonist dependent control of the M-channel is mediated by calmodulin, activation of anchored protein kinase C (PKC), and depletion of the phospholipid messenger phosphatidylinositol 4,5-bisphosphate (PIP2). In this report, we show how this trio of second messenger responsive events acts synergistically and in a stepwise manner to suppress activity of the M-current. PKC phosphorylation of the KCNQ2 channel subunit induces dissociation of calmodulin from the M-channel complex. The calmodulin-deficient channel has a reduced affinity towards PIP2. This pathway enhances the effect of concomitant reduction of PIP2, which leads to disruption of the M-channel function. These findings clarify how a common lipid cofactor, such as PIP2, can selectively regulate ion channels.

Facilitation of sympathetic neurotransmission by phosphatidylinositol-4,5-bisphosphate-dependent regulation of KCNQ channels in rat mesenteric arteries

Sympathetic nerves regulate vascular tone by releasing neurotransmitters into the vasculature. We previously demonstrated that bradykinin facilitates sympathetic neurotransmission in rat mesenteric arteries. Although little is known about the intracellular mechanism modulating this neurotransmission, recent cell line experiments have shown that the KCNQ channel, which is inhibited by the depletion of membrane phosphatidylinositol-4,5-bisphosphate (PIP₂), participates in the control of neurotransmission by bradykinin. In the present study, we examined the mechanism regulating neurotransmitter release from rat perivascular sympathetic nerves. Excitatory junction potentials (EJPs) elicited by repetitive nerve stimulation (1 Hz, 11 pulses, 20 μs, 20-50 V), a measure of sympathetic purinergic neurotransmission, were recorded with a conventional microelectrode technique in rat mesenteric arteries. Bradykinin (10⁻⁷  mol l⁻¹) significantly enhanced the amplitude of EJPs (n=22, P<0.05). This enhancing effect was abolished by N-type calcium-channel inhibition with ω-conotoxin GVIA (2 × 10⁻⁹ mol ⁻¹l, n=8). The blockade of phospholipase C with U-73122 (10(-6) mol l⁻¹, n=17) also eliminated the facilitatory effect of bradykinin. In addition, the effects of bradykinin were diminished by the prevention of PIP₂ resynthesis with wortmannin (10⁻⁵ mol l⁻¹ n=7) or KCNQ channel inhibition with XE-991 (10⁻⁵ mol l⁻¹, n=7). On the other hand, depletion of intracellular calcium stores with cyclopiazonic acid (3 × 10⁻⁶ mol l⁻¹, n=6) or the inhibition of protein kinase C with bisindolylmaleimide-I (10⁻⁶ mol l⁻¹, n=9) did not alter the action of bradykinin. These data demonstrate that the hydrolysis of PIP₂ by phospholipase C, which is activated by G(q/11)-coupled receptors, and subsequent KCNQ channel inhibition enhance sympathetic purinergic neurotransmission presumably via the activation of N-type calcium channels in rat mesenteric arteries.

Characteristics and molecular basis of celecoxib modulation on K(v)7 potassium channels

BACKGROUND AND PURPOSE

Celecoxib is a selective cyclooxygenase-2 (COX-2) inhibitor used for the treatment of pain and inflammation. Emerging and accumulating evidence suggests that celecoxib can affect cellular targets other than COX, such as ion channels. In this study, we characterized the effects of celecoxib on K(v)7 K(+) channels and compared its effects with the well-established K(v)7 channel opener retigabine.

EXPERIMENTAL APPROACH

A perforated whole-cell patch technique was used to record K(v)7currents expressed in HEK 293 cells and M-type currents from rat superior cervical ganglion neurons.

KEY RESULTS

Celecoxib enhanced K(v)7.2-7.4, K(v)7.2/7.3 and K(v)7.3/7.5 currents but inhibited K(v)7.1 and K(v)7.1/KCNE1 currents and these effects were concentration dependent. The IC(50) value for inhibition of K(v)7.1 channels was approximately 4 µM and the EC(50) values for activation of K(v)7.2-7.4, K(v)7.2/K(v)7.3 and K(v)7.3/K(v)7.5 channels were approximately 2-5 µM. The effects of celecoxib were manifested by increasing current amplitudes, shifting the voltage-dependent activation curve in a more negative direction and slowing the deactivation of K(v)7 currents. 2,5-Dimethyl-celecoxib, a celecoxib analogue devoid of COX inhibition activity, has similar but greater effects on K(v)7currents. K(v)7.2(A235T) and K(v) 7.2(W236L) mutant channels, which have greatly attenuated responses to retigabine, showed a reversed response to celecoxib, from activation to inhibition.

CONCLUSIONS AND IMPLICATIONS

These results suggest that K(v)7 channels are targets of celecoxib action and provide new mechanistic evidence for understanding the effects of celecoxib. They also provide a new approach to developing K(v)7 modulators and for studying the structure-function relationship of K(v)7 channels.

The Kv7.2/Kv7.3 heterotetramer assembles with a random subunit arrangement

Voltage-gated K(+) channels composed of Kv7.2 and Kv7.3 are the predominant contributors to the M-current, which plays a key role in controlling neuronal activity. Various lines of evidence have indicated that Kv7.2 and Kv7.3 form a heteromeric channel. However, the subunit stoichiometry and arrangement within this putative heteromer are so far unknown. Here, we have addressed this question using atomic force microscopy imaging of complexes between isolated Kv7.2/Kv7.3 channels and antibodies to epitope tags on the two subunits, Myc on Kv7.2 and HA on Kv7.3. Initially, tsA 201 cells were transiently transfected with equal amounts of cDNA for the two subunits. The heteromer was isolated through binding of either tag to immunoaffinity beads and then decorated with antibodies to the other tag. In both cases, the distribution of angles between pairs of bound antibodies had two peaks, at around 90° and around 180°, and in both cases the 90° peak was about double the size of the 180° peak. These results indicate that the Kv7.2/Kv7.3 heteromer generated by cells expressing approximately equal amounts of the two subunits assembles as a tetramer with a predominantly 2:2 subunit stoichiometry and with a random subunit arrangement. When the DNA ratio for the two subunits was varied, copurification experiments indicated that the subunit stoichiometry was variable and not fixed at 2:2. Hence, there are no constraints on either the subunit stoichiometry or the subunit arrangement.

Phosphoinositide sensitivity of ion channels, a functional perspective

Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] are required for the activity of many different ion channels. This chapter will highlight various aspects of this paradigm, by discussing current knowledge on four different ion channel families: inwardly rectifying K(+) (Kir) channels, KCNQ voltage gated K(+) channels, voltage gated Ca(2+) (VGCC) channels and Transient Receptor Potential (TRP) channels. Our main focus is to discuss functional aspects of this regulation, i.e. how changes in the concentration of PtdIns(4,5)P(2) in the plasma membrane upon phospholipase C activation may modulate the activity of ion channels, and what are the major determinants of this regulation. We also discuss how channels act as coincidence detectors sensing phosphoinositide levels and other signalling molecules. We also briefly discuss the available methods to study phosphoinositide regulation of ion channels, and structural aspects of interaction of ion channel proteins with these phospholipids. Finally, in several cases the effect of PtdIns(4,5)P(2) is more complex than a simple dependence of ion channel activity on the lipid, and we will discuss some these complexities.

After-hyperpolarization currents and acetylcholine control sigmoid transfer functions in a spiking cortical model

Recurrent networks are ubiquitous in the brain, where they enable a diverse set of transformations during perception, cognition, emotion, and action. It has been known since the 1970's how, in rate-based recurrent on-center off-surround networks, the choice of feedback signal function can control the transformation of input patterns into activity patterns that are stored in short term memory. A sigmoid signal function may, in particular, control a quenching threshold below which inputs are suppressed as noise and above which they may be contrast enhanced before the resulting activity pattern is stored. The threshold and slope of the sigmoid signal function determine the degree of noise suppression and of contrast enhancement. This article analyses how sigmoid signal functions and their shape may be determined in biophysically realistic spiking neurons. Combinations of fast, medium, and slow after-hyperpolarization (AHP) currents, and their modulation by acetylcholine (ACh), can control sigmoid signal threshold and slope. Instead of a simple gain in excitability that was previously attributed to ACh, cholinergic modulation may cause translation of the sigmoid threshold. This property clarifies how activation of ACh by basal forebrain circuits, notably the nucleus basalis of Meynert, may alter the vigilance of category learning circuits, and thus their sensitivity to predictive mismatches, thereby controlling whether learned categories code concrete or abstract information, as predicted by Adaptive Resonance Theory.

K(V)7/KCNQ channels are functionally expressed in oligodendrocyte progenitor cells

Background

K_V7/KCNQ channels are widely expressed in neurons and they have multiple important functions, including control of excitability, spike afterpotentials, adaptation, and theta resonance. Mutations in KCNQ genes have been demonstrated to associate with human neurological pathologies. However, little is known about whether K_V7/KCNQ channels are expressed in oligodendrocyte lineage cells (OLCs) and what their functions in OLCs.

Methods and Findings

In this study, we characterized K_V7/KCNQ channels expression in rat primary cultured OLCs by RT-PCR, immunostaining and electrophysiology. KCNQ2-5 mRNAs existed in all three developmental stages of rat primary cultured OLCs. K_V7/KCNQ proteins were also detected in oligodendrocyte progenitor cells (OPCs, early developmental stages of OLCs) of rat primary cultures and cortex slices. Voltage-clamp recording revealed that the I_M antagonist XE991 significantly reduced K_V7/KCNQ channel current (I_K(Q)) in OPCs but not in differentiated oligodendrocytes. In addition, inhibition of K_V7/KCNQ channels promoted OPCs motility in vitro.

Conclusions

These findings showed that K_V7/KCNQ channels were functionally expressed in rat primary cultured OLCs and might play an important role in OPCs functioning in physiological or pathological conditions.

Contributions of Kv7-mediated potassium current to sub- and suprathreshold responses of rat layer II/III neocortical pyramidal neurons

After block of Kv1- and Kv2-mediated K(+) currents in acutely dissociated neocortical pyramidal neurons from layers II/III of rat somatosensory and motor cortex, the remaining current is slowly activating and persistent. We used whole cell voltage clamp to show that the Kv7 blockers linopirdine and XE-991 blocked a current with similar kinetics to the current remaining after combined block of Kv1 and Kv2 channels. This current was sensitive to low doses of linopirdine and activated more slowly and at more negative potentials than Kv1- or Kv2-mediated current. The Kv7-mediated current decreased in amplitude with time in whole cell recordings, but in most cells the current was stable for several minutes. Current in response to a traditional M-current protocol was blocked by muscarine, linopirdine, and XE-991. Whole cell slice recordings revealed that the Q₁₀ for channel deactivation was ∼2.5. Sharp electrode current-clamp recordings from adult pyramidal cells demonstrated that block of Kv7-mediated current with XE-991 reduced rheobase, shortened the latency to firing to near rheobase current, induced more regular firing at low current intensity, and increased the rate of firing to a given current injection. XE-991 did not affect single action potentials or spike frequency adaptation. Application of XE-991 also eliminated subthreshold voltage oscillations and increased gain for low-frequency inputs (<10 Hz) without affecting gain for higher frequency inputs. These data suggest important roles for Kv7 channels in subthreshold regulation of excitability, generation of theta-frequency subthreshold oscillations, regulation of interspike intervals, and biasing selectivity toward higher frequency inputs.

KCNQ5 channels control resting properties and release probability of a synapse

Little is known about which ion channels determine the resting electrical properties of presynaptic membranes. In recordings made from the rat calyx of Held, a giant mammalian terminal, we found resting potential to be controlled by KCNQ (Kv7) K(+) channels, most probably KCNQ5 (Kv7.5) homomers. Unlike most KCNQ channels, which are activated only by depolarizing stimuli, the presynaptic channels began to activate just below the resting potential. As a result, blockers and activators of KCNQ5 depolarized or hyperpolarized nerve terminals, respectively, markedly altering resting conductance. Moreover, the background conductance set by KCNQ5 channels, together with Na(+) and hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, determined the size and time course of the response to subthreshold stimuli. Signaling pathways known to directly affect exocytic machinery also regulated KCNQ5 channels, and increase or decrease of KCNQ5 channel activity controlled release probability through alterations in resting potential. Thus, ion channel determinants of presynaptic resting potential also control synaptic strength.

Potential role of KCNQ/M-channels in regulating neuronal differentiation in mouse hippocampal and embryonic stem cell-derived neuronal cultures

Voltage-gated K(+) channels are key regulators of neuronal excitability, playing major roles in setting resting membrane potential, repolarizing the cell membrane after action potentials and affecting transmitter release. The M-type channel or M-channel is a unique voltage- and ligand-regulated K(+) channel. It is composed of the molecular counterparts KCNQ2 and KCNQ3 (also named Kv7.2 and Kv7.3) channels and expressed in the soma and dendrites of neurons. The present investigation examined the hypothesis that KCNQ2/3 channels played a regulatory role in neuronal differentiation and maturation. In cultured mouse embryonic stem (ES) cells undergoing neuronal differentiation and primary embryonic (E15-17) hippocampal cultures, KCNQ2 and KCNQ3 channels and underlying M-currents were identified. Blocking of KCNQ channels in these cells for 5 days using the specific channel blocker XE991 (10 μM) or linopirdine (30 μM) significantly decreased synaptophysin and syntaxin expression without affecting cell viability. Chronic KCNQ2/3 channel block reduced the expression of vesicular GABA transporter (v-GAT), but not vesicular glutamate transporter (v-GluT). Enhanced ERK1/2 phosphorylation was observed in XE991- and linopirdine-treated neural progenitor cells. In electrophysiological recordings, cells undergoing chronic block of KCNQ2/3 channels showed normal amplitude of mPSCs while the frequency of mPSCs was reduced. On the other hand, KCNQ channel opener N-Ethylmaleimide (NEM, 2 μM) increased mPSC frequency. Fluorescent imaging using fluorescent styryl-dye FM4-64 revealed that chronic blockade of KCNQ2/3 channels decreased endocytosis but facilitated exocytosis. These data indicate that KCNQ2/3 channels participate in the regulation of neuronal differentiation and show a tonic regulation on pre-synaptic transmitter release and recycling in developing neuronal cells.

Inhibition of SK and M channel-mediated currents by 5-HT enables parallel processing by bursts and isolated spikes

Although serotonergic innervation of sensory brain areas is ubiquitous, its effects on sensory information processing remain poorly understood. We investigated these effects in pyramidal neurons within the electrosensory lateral line lobe (ELL) of weakly electric fish. Surprisingly, we found that 5-HT is present at different levels across the different ELL maps; the presence of 5-HT fibers was highest in the map that processes intraspecies communication signals. Electrophysiological recordings revealed that 5-HT increased excitability and burst firing through a decreased medium afterhyperpolarization resulting from reduced small-conductance calcium-activated (SK) currents as well as currents mediated by an M-type potassium channel. We next investigated how 5-HT alters responses to sensory input. 5-HT application decreased the rheobase current, increased the gain, and decreased first spike latency. Moreover, it reduced discriminability between different stimuli, as quantified by the mutual information rate. We hypothesized that 5-HT shifts pyramidal neurons into a burst-firing mode where bursts, when considered as events, can detect the presence of particular stimulus features. We verified this hypothesis using signal detection theory. Our results indeed show that serotonin-induced bursts of action potentials, when considered as events, could detect specific stimulus features that were distinct from those detected by isolated spikes. Moreover, we show the novel result that isolated spikes transmit more information after 5-HT application. Our results suggest a novel function for 5-HT in that it enables differential processing by action potential patterns in response to current injection.

Combined phosphoinositide and Ca2+ signals mediating receptor specificity toward neuronal Ca2+ channels

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulates Ca(2+) (I(Ca)) and M-type K(+) currents in superior cervical ganglion sympathetic neurons. In those cells, M(1) muscarinic and AT(1) angiotensin types do not elicit Ca(2+)(i) signals and suppress both currents via depletion of PIP(2), whereas the B(2) bradykinin and P2Y purinergic types elicit robust IP(3)-mediated [Ca(2+)](i) rises and neither deplete PIP(2) nor inhibit I(Ca). We have suggested that this specificity arises from differential Ca(2+)(i) signals underlying receptor-specific stimulation of PIP(2) synthesis by phosphatidylinositol (PI) 4-kinase. Here, we investigate which PI 4-kinase isoform underlies this signal, whether stimulation of PI 4-phosphate 5-kinase is also required, and the origin of receptor-specific Ca(2+)(i) signals. Recordings of I(Ca) were used as a PIP(2) "biosensor." In control, stimulation of M(1), but not B(2) or P2Y, receptors robustly suppressed I(Ca). However, when PI 4-kinase IIIβ, diacylglycerol kinase, Rho, or Rho-kinase was blocked, agonists of all three receptors robustly suppressed I(Ca). Overexpression of exogenous M(1) receptors yielded large [Ca(2+)](i) rises by muscarinic agonist, and transfection of wild-type IRBIT decreased Ca(2+)(i) signals, whereas dominant negative IRBIT-S68A had little effect on B(2) or P2Y responses but greatly increased muscarinic responses. We conclude that overlaid on microdomain organization is IRBIT, setting a "threshold" for [IP(3)], assisting in fidelity of receptor specificity.

Characterization of a binding site for anionic phospholipids on KCNQ1

The KCNQ family of potassium channels underlie a repolarizing K(+) current in the heart and the M-current in neurones. The assembly of KCNQ1 with KCNE1 generates the delayed rectifier current I(Ks) in the heart. Characteristically these channels are regulated via G(q/11)-coupled receptors and the inhibition seen after phospholipase C activation is now thought to occur from membrane phosphatidylinositol (4,5)-bisphosphate (PIP(2)) depletion. It is not clear how KCNQ1 recognizes PIP(2) and specifically which residues in the channel complex are important. Using biochemical techniques we identify a cluster of basic residues namely, Lys-354, Lys-358, Arg-360, and Lys-362, in the proximal C terminus as being involved in binding anionic phospholipids. The mutation of specific residues in combination, to alanine leads to the loss of binding to phosphoinositides. Functionally, the introduction of these mutations into KCNQ1 leads to shifts in the voltage dependence of channel activation toward depolarized potentials and reductions in current density. Additionally, the biophysical effects of the charge neutralizing mutations, which disrupt phosphoinositide binding, mirror the effects we see on channel function when we deplete cellular PIP(2) levels through activation of a G(q/11)-coupled receptor. Conversely, the addition of diC8-PIP(2) to the wild-type channel, but not a PIP(2) binding-deficient mutant, acts to shift the voltage dependence of channel activation toward hyperpolarized potentials and increase current density. In conclusion, we use a combined biochemical and functional approach to identify a cluster of basic residues important for the binding and action of anionic phospholipids on the KCNQ1/KCNE1 complex.

The specific slow afterhyperpolarization inhibitor UCL2077 is a subtype-selective blocker of the epilepsy associated KCNQ channels

Mutations in members of the KCNQ channel family underlie multiple diseases affecting the nervous and cardiovascular systems. Despite their clinical relevance, research into these channels is limited by the lack of subtype-selective inhibitors, making it difficult to differentiate the physiological function of each family member in vivo. We have proposed that KCNQ channels might partially underlie the calcium-activated slow afterhyperpolarization (sAHP), a neuronal conductance whose molecular components are uncertain. Here, we investigated whether 3-(triphenylmethylaminomethyl)pyridine (UCL2077), identified previously as an inhibitor of the sAHP in neurons, acts on members of the KCNQ family expressed in heterologous cells. We found that 3 μM UCL2077 strongly inhibits KCNQ1 and KCNQ2 channels and weakly blocks KCNQ4 channels in a voltage-independent manner. In contrast, UCL2077 potentiates KCNQ5 channels at more positive membrane potentials, with little effect at negative membrane potentials. We found that the effect of UCL2077 on KCNQ3 is bimodal: currents are enhanced at negative membrane potentials and inhibited at positive potentials. We found that UCL2077 facilitates KCNQ3 currents by inducing a leftward shift in the KCNQ3 voltage-dependence, a shift dependent on tryptophan 265. Finally, we show that UCL2077 has intermediate effects on KCNQ2/3 heteromeric channels compared with KCNQ2 and KCNQ3 homomers. Together, our data demonstrate that UCL2077 acts on KCNQ channels in a subtype-selective manner. This feature should make UCL2077 a useful tool for distinguishing KCNQ1 and KCNQ2 from less-sensitive KCNQ family members in neurons and cardiac cells in future studies.

Modulation of high-voltage activated Ca(2+) channels by membrane phosphatidylinositol 4,5-bisphosphate

Modulation of voltage-gated Ca(2+) channels controls activities of excitable cells. We show that high-voltage activated Ca(2+) channels are regulated by membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) with different sensitivities. Plasma membrane PIP(2) depletion by rapamycin-induced translocation of an inositol lipid 5-phosphatase or by a voltage-sensitive 5-phosphatase (VSP) suppresses Ca(V)1.2 and Ca(V)1.3 channel currents by approximately 35% and Ca(V)2.1 and Ca(V)2.2 currents by 29% and 55%, respectively. Other Ca(V) channels are less sensitive. Inhibition is not relieved by strong depolarizing prepulses. It changes the voltage dependence of channel gating little. Recovery of currents from inhibition needs intracellular hydrolysable ATP, presumably for PIP(2) resynthesis. When PIP(2) is increased by overexpressing PIP 5-kinase, activation and inactivation of Ca(V)2.2 current slow and voltage-dependent gating shifts to slightly higher voltages. Thus, endogenous membrane PIP(2) supports high-voltage activated L-, N-, and P/Q-type Ca(2+) channels, and stimuli that activate phospholipase C deplete PIP(2) and reduce those Ca(2+) channel currents.

Isoform-specific prolongation of Kv7 (KCNQ) potassium channel opening mediated by new molecular determinants for drug-channel interactions

Kv7 channels, especially Kv7.2 (KCNQ2) and Kv7.3 (KCNQ3), are key determinants for membrane excitability in the brain. Some chemical modulators of KCNQ channels are in development for use as anti-epileptic drugs, such as retigabine (D-23129, N-(2-amino-4-(4-fluorobenzylamino)-phenyl)), which was recently approved for clinical use. In addition, several other compounds were also reported to potentiate activity of the Kv7 channels. It is therefore of interest to investigate compound-channel interactions, so that more insights may be gained to aid future development of therapeutics. We have conducted a screen of 20,000 compounds for KCNQ2 potentiators using rubidium flux combined with atomic absorption spectrometry. Here, we report the characterization of a series of new structures that display isoform specificity and induce a marked reduction of deactivation distinct from that of retigabine. Furthermore, KCNQ2(W236L), a previously reported mutation that abolishes sensitivity to retigabine, remains fully sensitive to these compounds. This result, together with mutagenesis and other studies, suggests that the reported compounds confer a unique mode of action and involve new molecular determinants on the channel protein, consistent with the idea of recognizing a new site on channel protein.

Central cholinesterase inhibition enhances glutamatergic synaptic transmission

Central cholinergic overstimulation results in prolonged seizures of status epilepticus in humans and experimental animals. Cellular mechanisms of underlying seizures caused by cholinergic stimulation remain uncertain, but enhanced glutamatergic transmission is a potential mechanism. Paraoxon, an organophosphate cholinesterase inhibitor, enhanced glutamatergic transmission on hippocampal granule cells synapses by increasing the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in a concentration-dependent fashion. The amplitude of mEPSCs was not increased, which suggested the possibility of enhanced action potential-dependent release. Analysis of EPSCs evoked by minimal stimulation revealed reduced failures and increased amplitude of evoked responses. The ratio of amplitudes of EPSCs evoked by paired stimuli was also altered. The effect of paraoxon on glutamatergic transmission was blocked by the muscarinic antagonist atropine and partially mimicked by carbachol. The nicotinic receptor antagonist α -bungarotoxin did not block the effects of paraoxon; however, nicotine enhanced glutamatergic transmission. These studies suggested that cholinergic overstimulation enhances glutamatergic transmission by enhancing neurotransmitter release from presynaptic terminals.

Affinity for phosphatidylinositol 4,5-bisphosphate determines muscarinic agonist sensitivity of Kv7 K+ channels

Kv7 K⁺-channel subunits differ in their apparent affinity for PIP₂ and are differentially expressed in nerve, muscle, and epithelia in accord with their physiological roles in those tissues. To investigate how PIP₂ affinity affects the response to physiological stimuli such as receptor stimulation, we exposed homomeric and heteromeric Kv7.2, 7.3, and 7.4 channels to a range of concentrations of the muscarinic receptor agonist oxotremorine-M (oxo-M) in a heterologous expression system. Activation of M₁ receptors by oxo-M leads to PIP₂ depletion through G_q and phospholipase C (PLC). Chinese hamster ovary cells were transiently transfected with Kv7 subunits and M₁ receptors and studied under perforated-patch voltage clamp. For Kv7.2/7.3 heteromers, the EC₅₀ for current suppression was 0.44 ± 0.08 µM, and the maximal inhibition (Inhib_max) was 74 ± 3% (n = 5–7). When tonic PIP₂ abundance was increased by overexpression of PIP 5-kinase, the EC₅₀ was shifted threefold to the right (1.2 ± 0.1 µM), but without a significant change in Inhib_max (73 ± 4%, n = 5). To investigate the muscarinic sensitivity of Kv7.3 homomers, we used the A315T pore mutant (Kv7.3^T) that increases whole-cell currents by 30-fold without any change in apparent PIP₂ affinity. Kv7.3^T currents had a slightly right-shifted EC₅₀ as compared with Kv7.2/7.3 heteromers (1.0 ± 0.8 µM) and a strongly reduced Inhib_max (39 ± 3%). In contrast, the dose–response curve of homomeric Kv7.4 channels was shifted considerably to the left (66 ± 8 nM), and Inhib_max was slightly increased (81 ± 6%, n = 3–4). We then studied several Kv7.2 mutants with altered apparent affinities for PIP₂ by coexpressing them with Kv7.3^T subunits to boost current amplitudes. For the lower affinity (Kv7.2 (R463Q)/Kv7.3^T) or higher affinity (Kv7.2 (R463E)/Kv7.3^T) channels, the EC₅₀ and Inhib_max were similar to Kv7.4 or Kv7.3^T homomers (0.12 ± 0.08 µM and 79 ± 6% [n = 3–4] and 0.58 ± 0.07 µM and 27 ± 3% [n = 3–4], respectively). The very low-affinity Kv7.2 (R452E, R459E, and R461E) triple mutant was also coexpressed with Kv7.3^T. The resulting heteromer displayed a very low EC₅₀ for inhibition (32 ± 8 nM) and a slightly increased Inhib_max (83 ± 3%, n = 3–4). We then constructed a cellular model that incorporates PLC activation by oxo-M, PIP₂ hydrolysis, PIP₂ binding to Kv7-channel subunits, and K⁺ current through Kv7 tetramers. We were able to fully reproduce our data and extract a consistent set of PIP₂ affinities.

Phospholipase C-mediated inhibition of the M-potassium current by muscarinic-receptor activation in the vestibular primary-afferent neurons of the rat

The activation of the efferent vestibular system modifies the basal discharge and the dynamic response of primary-afferent neurons to head motion and gravitational stimuli. The efferent input to afferent neurons is mediated primarily by cholinergic synapses that activate both muscarinic and nicotinic receptors. Previously we had shown that the muscarinic-acetylcholine-receptor (mAChR) activation modulates the low-voltage-activated M-type potassium current (I(K,M)) in the vestibular-afferent neurons. In this work we studied the second-messenger system mediating the inhibition of I(K,M) after mAChR activation. For this, voltage and current-clamp recordings were obtained in the cultured vestibular-afferent neurons of the rat. The I(K,M) was measured during its deactivation. Response to current-pulse injection was also studied. The use of the mAChR agonist oxotremorine-M significantly reduced the amplitude of the I(K,M) and modified the discharge response to current pulses from single spike to multiple spiking, reducing the adaptation of the electrical discharge. The intracellular perfusion of the phospholipase C (PLC) inhibitor U73122 significantly attenuated the inhibitory action of the mAChR receptor agonist oxotremorine-M. Its inactive analog U73343 produced no significant action. The use of the phosphatidylinositol 4,5 bis-phosphate (PIP(2)) scavenger poly-l-lysine also led to a significant reduction of the I(K,M). Our results show that the mAChR mediated activation of PLC and subsequent PIP(2) depletion (caused by its hydrolysis), modulates the I(K,M) in the vestibular-afferent neurons, modifying their discharge response dynamics to current-pulse injection.

The muscarinic inhibition of the potassium M-current modulates the action-potential discharge in the vestibular primary-afferent neurons of the rat

There is consensus that muscarinic and nicotinic receptors expressed in vestibular hair cells and afferent neurons are involved in the efferent modulation of the electrical activity of the afferent neurons. However the underlying mechanisms of postsynaptic control in neurons are not well understood. In our work we show that the activation of muscarinic receptors in the vestibular neurons modulates the potassium M-current modifying the activity of afferent neurons. Whole-cell patch-clamp recordings were made on vestibular-afferent neurons isolated from Wistar rats (postnatal days 7-10) and held in primary culture (18-24 h). The M-current was studied during its deactivation after depolarizing voltage-clamp pulses. In 68% of the cells studied, those of larger capacitance, the M-current antagonists linopirdine and XE-991 reduced the amplitude of the M-current by 54%+/-7% and 50%+/-3%. The muscarinic-receptor agonist oxotremorine-M also significantly reduced the M-current by 58%+/-12% in the cells. The action of oxotremorine-M was blocked by atropine, thus indicating its cholinergic nature. The erg-channel blocker E-4031 did not significantly modify the M-current amplitude. In current-clamp experiments, linopirdine, XE-991, and oxotremorine-M modified the discharge response to current pulses from single spike to multiple spiking, reducing the adaptation of the electrical discharge. Our results indicate that large soma-size cultured vestibular-afferent neurons (most probably calyx-bearing neurons) express the M-current and that the modulation of this current by activation of muscarinic-receptor reduces its spike-frequency adaptation.

A carboxy-terminal inter-helix linker as the site of phosphatidylinositol 4,5-bisphosphate action on Kv7 (M-type) K+ channels

The regulation of M-type (KCNQ [Kv7]) K⁺ channels by phosphatidylinositol 4,5-bisphosphate (PIP₂) has perhaps the best correspondence to physiological signaling, but the site of action and structural motif of PIP₂ on these channels have not been established. Using single-channel recordings of chimeras of Kv7.3 and 7.4 channels with highly differential PIP₂ sensitivities, we localized a carboxy-terminal inter-helix linker as the primary site of PIP₂ action. Point mutants within this linker in Kv7.2 and Kv7.3 identified a conserved cluster of basic residues that interact with the lipid using electrostatic and hydrogen bonds. Homology modeling of this putative PIP₂-binding linker in Kv7.2 and Kv7.3 using the solved structure of Kir2.1 and Kir3.1 channels as templates predicts a structure of Kv7.2 and 7.3 very similar to the Kir channels, and to the seven-β-sheet barrel motif common to other PIP₂-binding domains. Phosphoinositide-docking simulations predict affinities and interaction energies in accord with the experimental data, and furthermore indicate that the precise identity of residues in the interacting pocket alter channel–PIP₂ interactions not only by altering electrostatic energies, but also by allosterically shifting the structure of the lipid-binding surface. The results are likely to shed light on the general structural mechanisms of phosphoinositide regulation of ion channels.

Homomeric and heteromeric assembly of KCNQ (Kv7) K+ channels assayed by total internal reflection fluorescence/fluorescence resonance energy transfer and patch clamp analysis

M-type K(+) channels, consisting of KCNQ1-5 (Kv7.1-7.5) subunits, form a variety of homomeric and heteromeric channels. Whereas all the subunits can assemble into homomeric channels, the ability of the subunits to assemble into heteromultimers is highly variable. KCNQ3 is widely thought to co-assemble with several other KCNQ subtypes, whereas KCNQ1 and KCNQ2 do not. However, the existence of other subunit assemblies is not well studied. To systematically explore the heteromeric assembly of KCNQ channels in individual living cells, we performed fluorescence resonance energy transfer (FRET) between cyan fluorescent protein- and yellow fluorescent protein-tagged KCNQ subunits expressed in Chinese hamster ovary cells under total internal reflection fluorescence microscopy in which excitation light only penetrates several hundred nanometers into the cell, thus isolating membrane events. We found significant FRET between homomeric subunits as expected from their functional expression in heterologous expression systems. Also as expected from previous work, robust FRET was observed between KCNQ2 and KCNQ3. KCNQ3 and KCNQ4 also showed substantial FRET as did KCNQ4 and KCNQ5. To determine functional assembly of KCNQ4/KCNQ5 heteromers, we performed two types of experiments. In the first, we constructed a mutant tetraethylammonium ion-sensitive KCNQ4 subunit and tested its assembly with KCNQ5 by patch clamp analysis of the tetraethylammonium ion sensitivity of the resulting current; however, those data were not conclusive. In the second, we co-expressed a KCNQ4 (G285S) pore mutant with KCNQ5 and found the former to act as a dominant negative, suggesting co-assembly of the two types of subunits. These data confirm that among the allowed assembly conformations are KCNQ3/4 and KCNQ4/5 heteromers.

Low expression of Kv7/M channels facilitates intrinsic and network bursting in the developing rat hippocampus

Early in development, network activity in the hippocampus is characterized by recurrent synchronous bursts, whose cellular correlates are giant depolarizing potentials (GDPs). The propensity for generating GDPs is attributed to GABAergic synaptic transmission being depolarizing and excitatory in neonatal neurons. However, developmental regulation of intrinsic conductances may also influence GDPs generation. A likely candidate is the non-inactivating, low-threshold, muscarinic-sensitive K(+) current (M current; I(m)), which down-regulates intrinsic bursting activity in adult hippocampal pyramidal neurons. Western blot analysis of homogenates of the CA3 hippocampal region showed that expression of the Kv7.2 subunit, one of the constituents of neuronal M channels, is weak in neonatal neurons, and markedly increases after the first postnatal week. Likewise, the density of I(m) was very low in neonatal CA3 pyramidal cells and increased later on. Spontaneously occurring intrinsic bursts in neonatal neurons were longer and more robust, and recurred more regularly, than in juvenile neurons. The I(m) blocker linopirdine only mildly affected intrinsic bursting in neonatal neurons, but strongly facilitated and regularized it in juvenile neurons. We conclude that the low expression of Kv7/M channels and the depolarizing action of GABA early after birth enhance intrinsic bursting and neuronal synchronization leading to generation of GDPs within the hippocampal network.

The therapeutic potential of neuronal K V 7 (KCNQ) channel modulators: an update

BACKGROUND

Neuronal KCNQ channels (K(V)7.2-5) represent attractive targets for the development of therapeutics for chronic and neuropathic pain, migraine, epilepsy and other neuronal hyperexcitability disorders, although there has been only modest progress in translating this potential into useful therapeutics.

OBJECTIVE

Compelling evidence of the importance of K(V)7 channels as neuronal regulatory elements, readily amenable to pharmacological modulation, has sustained widespread interest in these channels as drug targets. This review will update readers on key aspects of the characterization of these important ion channel targets, and will discuss possible current barriers to their exploitation for CNS therapeutics.

METHODS

This article is based on a review of recent literature, with a focus on data pertaining to the roles of these channels in neurophysiology. In addition, I review some of the regulatory elements that influence the channels and how these may relate to channel pharmacology, and present a review of recent advances in neuronal K(V)7 channel pharmacology.

CONCLUSIONS

These channels continue to be valid and approachable targets for CNS therapeutics. However, we may need to understand more about the roles of neuronal K(V)7 channels during the development of disease states, as well as to pay more attention to a detailed analysis of the molecular pharmacology of the different channel subfamily members and the modes of interaction of individual modulators, in order to successfully target these channels for therapeutic development.

NGF inhibits M/KCNQ currents and selectively alters neuronal excitability in subsets of sympathetic neurons depending on their M/KCNQ current background

M/KCNQ currents play a critical role in the determination of neuronal excitability. Many neurotransmitters and peptides modulate M/KCNQ current and neuronal excitability through their G protein-coupled receptors. Nerve growth factor (NGF) activates its receptor, a member of receptor tyrosine kinase (RTK) superfamily, and crucially modulates neuronal cell survival, proliferation, and differentiation. In this study, we studied the effect of NGF on the neuronal (rat superior cervical ganglion, SCG) M/KCNQ currents and excitability. As reported before, subpopulation SCG neurons with distinct firing properties could be classified into tonic, phasic-1, and phasic-2 neurons. NGF inhibited M/KCNQ currents by similar proportion in all three classes of SCG neurons but increased the excitability only significantly in tonic SCG neurons. The effect of NGF on excitability correlated with a smaller M-current density in tonic neurons. The present study indicates that NGF is an M/KCNQ channel modulator and the characteristic modulation of the neuronal excitability by NGF may have important physiological implications.

Developmental changes in KCNQ2 and KCNQ3 expression in human brain: possible contribution to the age-dependent etiology of benign familial neonatal convulsions

Several mutations of KCNQ2 and KCNQ3 are considered to be associated with benign familial neonatal convulsions (BFNC). BFNC is characterized by seizures starting within several days of life and spontaneous remission within weeks to months. KCNQ channel is a heteromeric voltage-dependent potassium channel consisting of KCNQ2 and KCNQ3 subunits. To clarify the age-dependent etiology of BFNC, we examined the developmental changes in KCNQ2 and KCNQ3 expression in human hippocampus, temporal lobe, cerebellum and medulla oblongata obtained from 23 subjects who died at 22 gestation weeks to adulthood. Formalin-fixed and paraffin-embedded specimens were used for immunohistochemistry. Unique developmental changes in KCNQ2 and KCNQ3 were found in each region. A high expression of KCNQ2 was identified in the hippocampus, temporal cortex, cerebellar cortex and medulla oblongata in fetal life, but such expression decreased after birth. The expression of KCNQ3 increased in late fetal life to infancy. Simultaneous and high expressions of KCNQ2 and KCNQ3 were observed in each region from late fetal life to early infancy, coinciding with the time when BFNC occurs. Such coexpression may contribute to the pathogenesis of BFNC.

Phosphatidylinositol 4,5-bisphosphate hydrolysis mediates histamine-induced KCNQ/M current inhibition

The M-type potassium channel, of which its molecular basis is constituted by KCNQ2-5 homo- or heteromultimers, plays a key role in regulating neuronal excitability and is modulated by many G protein-coupled receptors. In this study, we demonstrate that histamine inhibits KCNQ2/Q3 currents in human embryonic kidney (HEK)293 cells via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis mediated by stimulation of H(1) receptor and phospholipase C (PLC). Histamine inhibited KCNQ2/Q3 currents in HEK293 cells coexpressing H(1) receptor, and this effect was totally abolished by H(1) receptor antagonist mepyramine but not altered by H(2) receptor antagonist cimetidine. The inhibition of KCNQ currents was significantly attenuated by a PLC inhibitor U-73122 but not affected by depletion of internal Ca(2+) stores or intracellular Ca(2+) concentration ([Ca(2+)](i)) buffering via pipette dialyzing BAPTA. Moreover, histamine also concentration dependently inhibited M current in rat superior cervical ganglion (SCG) neurons by a similar mechanism. The inhibitory effect of histamine on KCNQ2/Q3 currents was entirely reversible but became irreversible when the resynthesis of PIP(2) was impaired with phosphatidylinsitol-4-kinase inhibitors. Histamine was capable of producing a reversible translocation of the PIP(2) fluorescence probe PLC(delta1)-PH-GFP from membrane to cytosol in HEK293 cells by activation of H(1) receptor and PLC. We concluded that the inhibition of KCNQ/M currents by histamine in HEK293 cells and SCG neurons is due to the consumption of membrane PIP(2) by PLC.

Calmodulin binding to M-type K+ channels assayed by TIRF/FRET in living cells

Calmodulin (CaM) binds to KCNQ2-4 channels within their carboxy termini, where it regulates channel function. The existing data have not resolved the Ca2+ dependence of the interaction between the channels and CaM. We performed glutathione S-transferase (GST)-pull-down assays between purified KCNQ2-4 carboxy termini and CaM proteins to determine the Ca2+ dependence of the interaction in vitro. The assays showed substantial Ca2+ dependence of the interaction of the channels with wild-type (WT) CaM, but not with dominant-negative (DN) CaM. To demonstrate CaM-channel interactions in individual living cells, we performed fluorescence resonance energy transfer (FRET) between ECFP-tagged KCNQ2-4 channels and EYFP-tagged CaM expressed in CHO cells, performed under total internal reflection fluorescence (TIRF) microscopy, in which excitation light only penetrates several hundred nanometres into the cell, thus isolating membrane events. FRET was assayed between the channels and either WT or DN CaM, performed under conditions of normal [Ca2+]i, low [Ca2+]i or high [Ca2+]i induced by empirically optimized bathing solutions. The FRET data suggest a strong Ca2+ dependence for the interaction between WT CaM and KCNQ2, but less so for KCNQ3 and KCNQ4. FRET between all KCNQ2-4 channels and DN CaM was robust, and not significantly Ca2+ dependent. These data show interactions between CaM and KCNQ channels in living cells, and suggest that the interactions between KCNQ2-4 channels and CaM are likely to have Ca2+-dependent and Ca2+-independent components.

Combinatorial augmentation of voltage-gated KCNQ potassium channels by chemical openers

Noninactivating potassium current formed by KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) subunits resembles neuronal M-currents which are activated by voltage and play a critical role in controlling membrane excitability. Activation of voltage-gated potassium channels by a chemical opener is uncommon. Therefore, the mechanisms of action are worthy further investigation. Retigabine and zinc pyrithione are two activators for KCNQ channels but their molecular interactions with KCNQ channel remain largely elusive. Here we report that retigabine and zinc pyrithione recognize two different sites of KCNQ2 channels. Their agonistic actions are noncompetitive and allow for simultaneous binding of two different activators on the same channel complex, hence giving rise to combinatorial potentiation with characteristic properties of both openers. Examining their effects on mutant channels, we showed zinc pyrithione is capable of opening nonconductive channels and coapplication of zinc pyrithione and retigabine could restore a disease mutant channel similar to wild type. Our results indicate two independent activator binding sites present in KCNQ channels. The resultant combinatorial potentiation by multiple synthetic chemical openers indicates that KCNQ channels are accessible to various types and combinations of pharmacological regulation.

GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current

The CB(1) cannabinoid receptor mediates many of the psychoactive effects of Delta(9)THC, the principal active component of cannabis. However, ample evidence suggests that additional non-CB(1)/CB(2) receptors may contribute to the behavioral, vascular, and immunological actions of Delta(9)THC and endogenous cannabinoids. Here, we provide further evidence that GPR55, a G protein-coupled receptor, is a cannabinoid receptor. GPR55 is highly expressed in large dorsal root ganglion neurons and, upon activation by various cannabinoids (Delta(9)THC, the anandamide analog methanandamide, and JWH015) increases intracellular calcium in these neurons. Examination of its signaling pathway in HEK293 cells transiently expressing GPR55 found the calcium increase to involve G(q), G(12), RhoA, actin, phospholipase C, and calcium release from IP(3)R-gated stores. GPR55 activation also inhibits M current. These results establish GPR55 as a cannabinoid receptor with signaling distinct from CB(1) and CB(2).

K+ M-current regulates the transition to seizures in immature and adult hippocampus

PURPOSE

Loss-of-function mutations in Kv7.2 or Kv7.3 K(+) channel subunits underlies the neonatal epilepsy benign familial neonatal convulsions (BFNC). These two subunits interact to form a functional K(+) channel that underlies the M-current (I(M)), a voltage-dependent noninactivating K(+) current. In BFNC, seizures begin shortly after birth, and spontaneously remit in the first few months of life. The nature of this window of vulnerability is unclear. We address this issue using a hippocampal slice model, to study the effects of I(M) blockade or augmentation on epileptiform activity.

METHODS

We used the Mg(+)(+)-free seizure model in adult and immature (P8-P15) acute rat hippocampal slices. We recorded from both CA1 and CA3 regions using extracellular and intracellular methods.

RESULTS

When M-channels are blocked pharmacologically, the transition from interictal to ictal bursting becomes much more likely, especially in immature brain. We also show augmentation of I(M) is effective in stopping ictal events in immature brain, at the developmental age that approximates a human newborn in cortical development. I(M) appears to counter the sustained N-methyl-D-aspartate (NMDA) receptor-mediated depolarizations needed to trigger an ictal event. The increased likelihood of ictal bursting by I(M) blockade is not shared by other selective K(+) channel blockers that increase hippocampal excitability.

CONCLUSIONS

Voltage-dependent M-channels are activated during interictal bursts and contribute to burst termination. When these channels are compromised, interictal burst duration becomes sufficient to trigger the sustained depolarizations that underlie ictal bursts. This transition to ictal bursts upon I(M) blockade is especially likely to occur in immature hippocampus. This selective function of M-channels likely contributes to the transient window of vulnerability to seizures that occurs with BFNC.

Mechano- and chemosensitivity of rat nodose neurones--selective excitatory effects of prostacyclin

Nodose ganglion sensory neurones exert a significant reflex autonomic influence. We contrasted their mechanosensitivity, excitability and chemosensitivity in response to the stable prostacyclin (PGI2) analogue carbacyclin (cPGI) in culture. Under current clamp conditions we measured changes in membrane potential (DeltamV) and action potential (AP) responses to mechanically induced depolarizations and depolarizing current injections before and after superfusion of cPGI (1 microM and 10 microM). Chemosensitivity was indicated by augmentation of AP firing frequency and increased maximum gain of AP frequency (max. dAP/dDeltamV), during superfusion with cPGI. Results indicate that two groups of neurones, A and B, are mechanosensitive (MS) and one group, C, is mechanoinsensitive (MI). Group A shows modest depolarization without AP generation during mechanical stimulation, and no increase in max. dAP/dDeltamV, despite a marked increase in electrical depolarization with cPGI. Group B shows pronounced mechanical depolarization accompanied by enhanced AP discharge with cPGI, and an increase in max. dAP/dDeltamV. Group C remains MI after cPGI but is more excitable and markedly chemosensitive (CS) with a pronounced enhancement of max. dAP/dDeltamV with cPGI. The effect of cPGI on ionic conductances indicates that it does not sensitize the mechanically gated depolarizing degenerin/epithelial Na+ channels (DEG/ENaC), but it inhibits two voltage-gated K+ currents, Maxi-K and M-current, causing enhanced AP firing frequency and depolarization, respectively. We conclude that MS nodose neurones may be unimodal MS or bimodal MS/CS, and that MI neurones are unimodal CS, and much more CS to cPGI than MS/CS neurones. We suggest that the known excitatory effect of PGI2 on baroreceptor and vagal afferent fibres is mediated by inhibition of voltage-gated K+ channels (Maxi-K and M-current) and not by an effect on mechanically gated DEG/ENaC channels.

Modulation of ERG Channels by XE991

In neuronal tissue, KCNQ2-5 channels conduct the physiologically important M-current. In some neurones, the M-current may in addition be conducted partly by ERG potassium channels, which have widely overlapping expression with the KCNQ channel subunits. XE991 and linopiridine are known to be standard KCNQ potassium channel blockers. These compounds have been used in many different tissues as specific pharmacological tools to discern native currents conducted by KCNQ channels from other potassium currents. In this article, we demonstrate that ERG1-2 channels are also reversibly inhibited by XE991 in the micromolar range (EC(50) 107 microM for ERG1). The effect has been characterized in Xenopus laevis oocytes expressing ERG1-2 and in the mammalian HEK293 cell line stably expressing ERG1 channels. The IC(50) values for block of KCNQ channels by XE991 range 1-65 microM. In conclusion, great care should be taken when choosing the concentration of XE991 to use for experiments on native potassium channels or animal studies in order to be able to conclude on selective KCNQ channel-mediated effects.