M-current suppression in PC12 cells by bradykinin is mediated by a pertussis toxin-insensitive G-protein and modulated by intracellular calcium

Suppression of KV7/KCNQ potassium channel enhances neuronal differentiation of PC12 cells

Membrane potential shift driven by electrical activity is critical in determining the cell fate of proliferation or differentiation. As such, the ion channels that underlie the membrane electrical activity play an important role in cell proliferation/differentiation. KV7/KCNQ potassium channels are critical in determining the resting membrane potentials in many neuronal cells. However, the role of these channels in cell differentiation is not well studied. In the present study, we used PC12 cells as well as primary cultured rat cortical neurons to study the role and mechanism of KV7/KCNQ in neuronal differentiation. NGF induced PC12 cell differentiation into neuron-like cells with growth of neurites showing typical growth cone-like extensions. The Kv7/KCNQ blocker XE991 promoted NGF-induced neurite outgrowth, whereas Kv7/KCNQ opener retigabine (RTG) inhibited outgrowth. M-type Kv7 channels are likely involved in regulating neurite growth because overexpression of KCNQ2/Q3 inhibited neurite growth whereas suppression of KCNQ2/Q3 with shRNA promoted neurite growth. Membrane depolarization possibly underpins enhanced neurite growth induced by the suppression of Kv7/KCNQ. Additionally, high extracellular K(+) likely induced membrane depolarization and also promoted neurite growth. Finally, T-type Ca(2+) channels may be involved in membrane-depolarization-induced neurite growth. This study provides a new perspective for understanding neuronal differentiation as well as KV7/KCNQ channel function.

Sensitization of neonatal rat lumbar motoneuron by the inflammatory pain mediator bradykinin

Bradykinin (Bk) is a potent inflammatory mediator that causes hyperalgesia. The action of Bk on the sensory system is well documented but its effects on motoneurons, the final pathway of the motor system, are unknown. By a combination of patch-clamp recordings and two-photon calcium imaging, we found that Bk strongly sensitizes spinal motoneurons. Sensitization was characterized by an increased ability to generate self-sustained spiking in response to excitatory inputs. Our pharmacological study described a dual ionic mechanism to sensitize motoneurons, including inhibition of a barium-sensitive resting K⁺ conductance and activation of a nonselective cationic conductance primarily mediated by Na⁺. Examination of the upstream signaling pathways provided evidence for postsynaptic activation of B₂ receptors, G protein activation of phospholipase C, InsP3 synthesis, and calmodulin activation. This study questions the influence of motoneurons in the assessment of hyperalgesia since the withdrawal motor reflex is commonly used as a surrogate pain model.

DOI:http://dx.doi.org/10.7554/eLife.06195.001

When we accidentally place our hand on a hot stove, we normally experience a painful sensation that starts with the sensory nerves under our skin. These nerves respond by transmitting electrical impulses to our brain, where the painful sensation is then processed. At the same time, these impulses are also transmitted to the motor nerves that control the muscles in our hand to trigger an immediate reflex to withdraw the hand from the hot stove. Pain therefore has a useful role as it can reduce how bad an injury is.

People with a condition called hyperalgesia have an increased sensitivity to pain. This condition can result from a chemical called bradykinin ‘sensitizing’ the sensory nerves, causing them to transmit more electrical impulses in response to pain than normal. This makes the injury feel much more painful, and can make the pain last for longer than is beneficial.

It was less clear whether bradykinin also affects motor nerves and so triggers a withdrawal reflex. By recording the electrical activity of motor nerve cells taken from the spinal cords of newborn rats, Bouhadfane et al. now show that these motor nerves become more active when exposed to bradykinin.

Nerve cells generate electrical signals when ions—such as potassium, sodium, and calcium ions—move through channels in the membranes of the cell. Therefore, to investigate how bradykinin influences the electrical activity of motor nerves, Bouhadfane et al. exposed the cells to drugs that inhibit particular ion channels. This revealed that bradykinin sensitizes the motor nerves by blocking a type of potassium ion channel and activating another ion channel that mainly transports sodium ions. Furthermore, Bouhadfane et al. were able to identify the signaling pathways that allow bradykinin to affect the motor nerve cells.

The study implies that the neuronal circuitry for pain does not rely exclusively on sensory nerve cells but should also integrate motor nerve cells. A future challenge remains in developing a protocol to resolve the contribution of motor nerve cells to hyperalgesia assessed by reflex withdrawal.

DOI:http://dx.doi.org/10.7554/eLife.06195.002

Activity-dependent transcriptional regulation of M-Type (Kv7) K(+) channels by AKAP79/150-mediated NFAT actions

M-type K(+) channels, encoded by KCNQ2-KCNQ5 genes, play key roles in regulation of neuronal excitability; however, less is known about the mechanisms controlling their transcriptional expression. Here, we discovered a mechanism regulating KCNQ2/3 transcriptional expression by neuronal activity in rodent neurons, involving activation of calcineurin and nuclear factor of activated T cell (NFAT) transcription factors, orchestrated by A kinase-anchoring protein (AKAP)79/150. The signal requires Ca(2+) influx through L-type Ca(2+) channels and both local and global Ca(2+) elevations. We postulate increased M-channel expression to act as a negative feedback to suppress neuronal hyperexcitability, demonstrated by profoundly upregulated KCNQ2/3 transcription in hippocampi from wild-type, but not AKAP150(-/-), mice after drug-induced seizures. Thus, we suggest a distinct role of AKAP79/150 and the complex it organizes in activity-dependent M-channel transcription, which may potentially serve throughout the nervous system to limit overexcitability associated with disease states such as epilepsy.

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.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Activation of a PTX-insensitive G protein is involved in histamine-induced recombinant M-channel modulation

The M-type potassium current (I(M)) plays a dominant role in regulating membrane excitability and is modulated by many neurotransmitters. However, except in the case of bradykinin, the signal transduction pathways involved in M-channel modulation have not been fully elucidated. The channels underlying I(M) are produced by the coassembly of KCNQ2 and KCNQ3 channel subunits and can be expressed in heterologous systems where they can be modulated by several neurotransmitter receptors including histamine H(1) receptors. In HEK293T cells, histamine acting via transiently expressed H(1)R produced a strong inhibition of recombinant M-channels but had no overt effects on the voltage dependence or voltage range of I(M) activation. In addition, the modulation of I(M) by histamine was not voltage sensitive, whereas channel gating, particularly deactivation, was accelerated by histamine. Non-hydrolysable guanine nucleotide analogues (GDP-beta-S and GTP-gamma-S) and pertussis toxin (PTX) treatment demonstrated the involvement of a PTX-insensitive G protein in the signal transduction pathway mediating histamine-induced I(M) modulation. Abrogation of the histamine-induced modulation of I(M) by expression of a C-terminal construct of phospholipase C (PLC-beta1-ct), which buffers activated Galpha(q/11) subunits, implicates this G protein alpha subunit in the modulatory pathway. On the other hand, abrogation of the histamine-induced modulation of I(M) by expression of two constructs which buffer free betagamma subunits, transducin (Galphat) and a C-terminal construct of a G protein receptor kinase (MAS-GRK2-ct), implicates betagamma dimers in the modulatory pathway. These findings demonstrate that histamine modulates recombinant M-channels in HEK293T cells via a PTX-insensitive G protein, probably Galpha(q/11), in a similar manner to a number of other G protein-coupled receptors. However, histamine-induced I(M) modulation in HEK293T cells is novel in that betagamma subunits in addition to Galpha(q/11) subunits appear to be involved in the modulation of KCNQ2/3 channel currents.

Sympathoexcitation by bradykinin involves Ca2+-independent protein kinase C

Bradykinin has long been known to excite sympathetic neurons via B(2) receptors, and this action is believed to be mediated by an inhibition of M-currents via phospholipase C and inositol trisphosphate-dependent increases in intracellular Ca(2+). In primary cultures of rat superior cervical ganglion neurons, bradykinin caused an accumulation of inositol trisphosphate, an inhibition of M-currents, and a stimulation of action potential-mediated transmitter release. Blockade of inositol trisphosphate-dependent signaling cascades failed to affect the bradykinin-induced release of noradrenaline, but prevented the peptide-induced inhibition of M-currents. In contrast, inhibition or downregulation of protein kinase C reduced the stimulation of transmitter release, but not the inhibition of M-currents, by bradykinin. In cultures of superior cervical ganglia, classical (alpha, betaI, betaII), novel (delta, epsilon), and atypical (zeta) protein kinase C isozymes were detected by immunoblotting. Bradykinin induced a translocation of Ca(2+)-independent protein kinase C isoforms (delta and epsilon) from the cytosol to the membrane of the neurons, but left the cellular distribution of other isoforms unchanged. This activation of Ca(2+)-independent protein kinase C enzymes was prevented by a phospholipase C inhibitor. The bradykinin-dependent stimulation of noradrenaline release was reduced by inhibitors of classical and novel protein kinase C isozymes, but not by an inhibitor selective for Ca(2+)-dependent isoforms. These results demonstrate that bradykinin B(2) receptors are linked to phospholipase C to simultaneously activate two signaling pathways: one mediates an inositol trisphosphate- and Ca(2+)-dependent inhibition of M-currents, the other one leads to an excitation of sympathetic neurons independently of changes in M-currents through an activation of Ca(2+)-insensitive protein kinase C.

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated-patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 +/- 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca(2+) stores with thapsigargin (100 nM), and was reduced by 40 % with apamin (100 nM). Membrane resistance increased by about 60 % during the histamine-induced depolarization suggesting inhibition of a K(+) channel. An inward current relaxation, typical of an M-current, was observed in response to negative voltage steps from a holding potential of -30 mV. This current reversed at -81.6 +/- 1.8 mV and was abolished by the M-channel inhibitor linopirdine (100 microM). During application of histamine, the amplitude of M-currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range -150 to -70 mV, and glibenclamide (10 microM) had no effect on either resting membrane potential or the response to histamine. The results show that an M-current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage-operated Ca(2+) channels in these cells.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Retigabine, a novel anti-convulsant, enhances activation of KCNQ2/Q3 potassium channels

Retigabine [N-(2-amino-4-[fluorobenzylamino]-phenyl) carbamic acid; D-23129] is a novel anticonvulsant, unrelated to currently available antiepileptic agents, with activity in a broad range of seizure models. In the present study, we sought to determine whether retigabine could enhance current through M-like currents in PC12 cells and KCNQ2/Q3 K(+) channels expressed in Chinese hamster ovary cells (CHO-KCNQ2/Q3). In differentiated PC12 cells, retigabine enhanced a linopirdine-sensitive current. The effect of retigabine was associated with a slowing of M-like tail current deactivation in these cells. Retigabine (0.1 to 10 microM) induced a potassium current and hyperpolarized CHO cells expressing KCNQ2/Q3 cells but not in wild-type cells. Retigabine-induced currents in CHO-KCNQ2/Q3 cells were inhibited by 60.6 +/- 11% (n = 4) by the KCNQ2/Q3 blocker, linopirdine (10 microM), and 82.7 +/- 5.4% (n = 4) by BaCl(2) (10 mM). The mechanism by which retigabine enhanced KCNQ2/Q3 currents involved large, drug-induced, leftward shifts in the voltage dependence of channel activation (-33.1 +/- 2.6 mV, n = 4, by 10 microM retigabine). Retigabine shifted the voltage dependence of channel activation with an EC(50) value of 1.6 +/- 0.3 microM (slope factor was 1.2 +/- 0.1, n = 4 to 5 cells per concentration). Retigabine (0.1 to 10 microM) also slowed the rate of channel deactivation, predominantly by increasing the contribution of a slowly deactivating tail current component. Our findings identify KCNQ2/Q3 channels as a molecular target for retigabine and suggest that activation of KCNQ2/Q3 channels may be responsible for at least some of the anticonvulsant activity of this agent.

Noradrenaline release from rat sympathetic neurones triggered by activation of B2 bradykinin receptors

1. The role of bradykinin receptors in the regulation of sympathetic transmitter release was investigated in primary cultures of neurones dissociated from superior cervical ganglia of neonatal rats. These cultures were loaded with [3H]-noradrenaline and the outflow of radioactivity was determined under continuous superfusion. 2. Bradykinin (100 nmol l[-1] applied for 10 min) caused a transient increase in tritium outflow that reached a peak within four minutes after the beginning of the application and then declined towards the baseline, despite the continuing presence of the peptide. ATP (100 micromol l[-1]) and nicotine (10 micromol l[-1]) caused elevations in 3H outflow with similar kinetics, whereas outflow remained elevated during a 10 min period of electrical field stimulation (0.5 ms, 50 mA, 50 V cm[-1], 1.0 Hz). 3. When bradykinin was applied for periods of 2 min, the evoked 3H overflow was half-maximal at 12 nmol l(-1) and reached a maximum of 2.3% of cellular radioactivity. The preferential B1 receptor agonist des-Arg9-bradykinin failed to alter 3H outflow. The B2 receptor antagonists, [D-Phe7]-bradykinin (1 micromol l[-1]) and Hoe 140 (10 nmol l[-1]), per se did not alter 3H outflow, but shifted the concentration-response curve for bradykinin-evoked 3H overflow to the right by a factor of 7.9 and 4.3, respectively. 4. Bradykinin-induced overflow was abolished in the absence of extracellular Ca2+ and in the presence of either 1 micromol l(-1) tetrodotoxin or 300 micromol l(-1) Cd2+, as was electrically-induced overflow. Activation of alpha2-adrenoceptors by 1 micromol l(-1) UK 14,304 reduced both bradykinin- and electrically-triggered overflow. The Ca2+-ATPase inhibitor thapsigargin (0.3 micromol l[-1]) failed to alter either type of stimulated overflow. Caffeine (10 mmol l[-1]) enhanced bradykinin-induced overflow, but reduced overflow triggered by electrical field stimulation. 5. Inclusion of Ba2+ (0.1 to 1 mmol l[-1]) in the superfusion medium enhanced electrically induced overflow by approximately 100% and potentiated bradykinin-triggered overflow by almost 400%. Application of 1 mmol l(-1) Ba2+ for periods of 2 min triggered 3H overflow, and this overflow was abolished by 1 micromol l(-1) tetrodotoxin and enhanced by 10 mmol l(-1) caffeine. In contrast, inclusion of tetraethylammonium (0.1 to 1 mmol l[-1]) in the superfusion buffer caused similar increases of bradykinin- and electrically evoked 3H overflow (by about 100%), and tetraethylammonium, when applied for 2 min, failed to alter 3H outflow. 6. Treatment of cultures with 100 ng ml(-1) pertussis toxin caused a significant increase in bradykinin-, but not in electrically-, evoked tritium overflow. Treatment with 100 ng ml(-1) cholera toxin reduced both types of stimulated 3H overflow. 7. These data reveal bradykinin as a potent stimulant of action potential-mediated and Ca2+-dependent transmitter release from rat sympathetic neurones in primary cell culture. This neurosecretory effect of bradykinin involves activation of B2-receptors, presumably linked to pertussis- and cholera toxin-insensitive G proteins, most likely members of the Gq family. Results obtained with inhibitors of muscarinic K+ (KM) channels, like caffeine and Ba2+, indicate that the secretagogue action of bradykinin probably involves inhibition of these K+ channels.

Nonstationary noise analysis of M currents simulated and recorded in PC12 cells

M current relaxations recorded in PC12 cells were subjected to nonstationary noise analysis (NSNA) to obtain estimates of single-channel current (i), channel number (N), and open probability (Po) for the channels responsible for M current. The analysis was constrained such that N and single-channel conductance were the same at two potentials. The relation between variance and current indicated that the fraction of channels open was 0.58 +/- 0.06 (mean +/- SD) and 0.05 +/- 0.04 (mean +/- SD: n = 9) at -33 and -63 mV, respectively. The single M channel conductance was 4.0 pS, and a density of 1 functional M channel per 4 microm2 was estimated. Monte Carlo simulations of a two-state model of M channels were used to obtain sets of simulated macroscopic M currents that were subjected to the same NSNA procedure so as to evaluate the accuracy of M channel parameters obtained with this method. The influence of current rundown and filter frequency on estimates of i, N, and Po were evaluated. The single-channel parameters estimated from the simulations differed by < 10% from actual values at any level of current rundown, N, or Po. The dispersion in the estimation of N and Po increased as Po decreased. Decreasing filter frequency caused an underestimation of i, paralleled by an overestimation of N. The estimation of Po was relatively immune to the filter frequency, especially for data simulated with Po = 0.77.