The alpha subunit of Gq contributes to muscarinic inhibition of the M-type potassium current in sympathetic neurons

Kv7 Channels in Lung Diseases

Lung diseases constitute a global health concern causing disability. According to WHO in 2016, respiratory diseases accounted for 24% of world population mortality, the second cause of death after cardiovascular diseases. The Kv7 channels family is a group of voltage-dependent K⁺ channels (Kv) encoded by KCNQ genes that are involved in various physiological functions in numerous cell types, especially, cardiac myocytes, smooth muscle cells, neurons, and epithelial cells. Kv7 channel α-subunits are regulated by KCNE1–5 ancillary β-subunits, which modulate several characteristics of Kv7 channels such as biophysical properties, cell-location, channel trafficking, and pharmacological sensitivity. Kv7 channels are mainly expressed in two large groups of lung tissues: pulmonary arteries (PAs) and bronchial tubes. In PA, Kv7 channels are expressed in pulmonary artery smooth muscle cells (PASMCs); while in the airway (trachea, bronchus, and bronchioles), Kv7 channels are expressed in airway smooth muscle cells (ASMCs), airway epithelial cells (AEPs), and vagal airway C-fibers (VACFs). The functional role of Kv7 channels may vary depending on the cell type. Several studies have demonstrated that the impairment of Kv7 channel has a strong impact on pulmonary physiology contributing to the pathophysiology of different respiratory diseases such as cystic fibrosis, asthma, chronic obstructive pulmonary disease, chronic coughing, lung cancer, and pulmonary hypertension. Kv7 channels are now recognized as playing relevant physiological roles in many tissues, which have encouraged the search for Kv7 channel modulators with potential therapeutic use in many diseases including those affecting the lung. Modulation of Kv7 channels has been proposed to provide beneficial effects in a number of lung conditions. Therefore, Kv7 channel openers/enhancers or drugs acting partly through these channels have been proposed as bronchodilators, expectorants, antitussives, chemotherapeutics and pulmonary vasodilators.

Pharmacological Manipulation of K v 7 Channels as a New Therapeutic Tool for Multiple Brain Disorders

K _v 7 ("M-type," KCNQ) K⁺ currents, play dominant roles in controlling neuronal excitability. They act as a "brake" against hyperexcitable states in the central and peripheral nervous systems. Pharmacological augmentation of M current has been developed for controlling epileptic seizures, although current pharmacological tools are uneven in practical usefulness. Lately, however, M-current "opener" compounds have been suggested to be efficacious in preventing brain damage after multiple types of insults/diseases, such as stroke, traumatic brain injury, drug addiction and mood disorders. In this review, we will discuss what is known to date on these efforts and identify gaps in our knowledge regarding the link between M current and therapeutic potential for these disorders. We will outline the preclinical experiments that are yet to be performed to demonstrate the likelihood of success of this approach in human trials. Finally, we also address multiple pharmacological tools available to manipulate different K _v 7 subunits and the relevant evidence for translational application in the clinical use for disorders of the central nervous system and multiple types of brain insults. We feel there to be great potential for manipulation of K _v 7 channels as a novel therapeutic mode of intervention in the clinic, and that the paucity of existing therapies obligates us to perform further research, so that patients can soon benefit from such therapeutic approaches.

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

One drug-sensitive subunit is sufficient for a near-maximal retigabine effect in KCNQ channels

Retigabine is a widely studied potassium channel activator that is thought to interact with a conserved Trp side chain in the pore domain of Kv7 subunits. Yau et al. demonstrate that drug sensitivity in just one of the four subunits is sufficient for a near-maximal response to retigabine.

Retigabine is an antiepileptic drug and the first voltage-gated potassium (Kv) channel opener to be approved for human therapeutic use. Retigabine is thought to interact with a conserved Trp side chain in the pore of KCNQ2–5 (Kv7.2–7.5) channels, causing a pronounced hyperpolarizing shift in the voltage dependence of activation. In this study, we investigate the functional stoichiometry of retigabine actions by manipulating the number of retigabine-sensitive subunits in concatenated KCNQ3 channel tetramers. We demonstrate that intermediate retigabine concentrations cause channels to exhibit biphasic conductance–voltage relationships rather than progressive concentration-dependent shifts. This suggests that retigabine can exert its effects in a nearly “all-or-none” manner, such that channels exhibit either fully shifted or unshifted behavior. Supporting this notion, concatenated channels containing only a single retigabine-sensitive subunit exhibit a nearly maximal retigabine effect. Also, rapid solution exchange experiments reveal delayed kinetics during channel closure, as retigabine dissociates from channels with multiple drug-sensitive subunits. Collectively, these data suggest that a single retigabine-sensitive subunit can generate a large shift of the KCNQ3 conductance–voltage relationship. In a companion study (Wang et al. 2018. J. Gen. Physiol.https://doi.org/10.1085/jgp.201812014), we contrast these findings with the stoichiometry of a voltage sensor-targeted KCNQ channel opener (ICA-069673), which requires four drug-sensitive subunits for maximal effect.

Do TRPC channels support working memory? Comparing modulations of TRPC channels and working memory through G-protein coupled receptors and neuromodulators

Working memory is a crucial ability we use in daily life. However, the cellular mechanisms supporting working memory still remain largely unclear. A key component of working memory is persistent neural firing which is believed to serve short-term (hundreds of milliseconds up to tens of seconds) maintenance of necessary information. In this review, we will focus on the role of transient receptor potential canonical (TRPC) channels as a mechanism underlying persistent firing. Many years of in vitro work have been suggesting a crucial role of TRPC channels in working memory and temporal association tasks. If TRPC channels are indeed a central mechanism for working memory, manipulations which impair or facilitate working memory should have a similar effect on TRPC channel modulation. However, modulations of working memory and TRPC channels were never systematically compared, and it remains unanswered whether TRPC channels indeed contribute to working memory in vivo or not. In this article, we review the effects of G-protein coupled receptors (GPCR) and neuromodulators, including acetylcholine, noradrenalin, serotonin and dopamine, on working memory and TRPC channels. Based on comparisons, we argue that GPCR and downstream signaling pathways that activate TRPC, generally support working memory, while those that suppress TRPC channels impair it. However, depending on the channel types, areas, and systems tested, this is not the case in all studies. Further work to clarify involvement of specific TRPC channels in working memory tasks and how they are affected by neuromodulators is still necessary in the future.

Gq rather than G11 preferentially mediates nociceptor sensitization

Background

The G_q/11-protein signaling mechanism is essential throughout the nervous system, but little is known about the contribution of the individual G-protein GPCR signaling branches towards nociceptor activation and their specific role on nociceptor sensitization. We aimed to unravel the contribution of the G_q/11-signaling pathway towards nociceptor activation via a variety of classical inflammatory mediators signalling via different G-protein GPCRs and investigated the specific contribution of the individual G_q and G₁₁ G-Proteins in nociceptors.

Findings

Using different transgenic mouse lines, lacking Gα_q, Gα₁₁ or both α-subunit of the G-proteins in primary nociceptive neurons, we analyzed the mechanical- and heat-sensitivity upon application of different GPCR-agonists that are known to play an important role under inflammatory conditions (e.g. ATP, Glutamate, Serotonin etc.). We found that the G_q/11-GPCR signaling branch constitutes a primary role in the manifestation of mechanical allodynia and a minor role in the development of thermal hyperalgesia. Moreover, with respect to the mediators used here, the G_q-protein is the principle G-protein among the G_q/11-protein family in nociceptive neurons leading to nociceptor sensitization.

Conclusions

Our results demonstrate that the G_q/11 signaling branch plays a primary role in nociceptor sensitization upon stimulation with classical GPCR ligands, contributing primarily towards the development of mechanically allodynia. Moreover, the deletion of the individual G-proteins led to the finding that the G_q-protein dominates the signalling machinery of the G_q/11 family of G-proteins in nociceptive neurons.

The actions of Pasteurella multocida toxin on neuronal cells

Pasteurella multocida toxin (PMT) activates the G-proteins Gα_i(1-3), Gα_q, Gα_11, Gα₁₂ and Gα₁₃ by deamidation of specific glutamine residues. A number of these alpha subunits have signalling roles in neurones. Hence we studied the action of this toxin on rat superior cervical ganglion (SCG) neurones and NG108-15 neuronal cells. Both Gα_q and Gα₁₁ could be identified in SCGs with immunocytochemistry. PMT had no direct action on Kv7 or Cav2 channels in SCGs. However PMT treatment enhanced muscarinic receptor mediated inhibition of M-current (Kv7.2 + 7. 3) as measured by a 19-fold leftward shift in the oxotremorine-M concentration–inhibition curve. Agonists of other receptors, such as bradykinin or angiotensin, that inhibit M-current did not produce this effect. However the amount of PIP₂ hydrolysis could be enhanced by PMT for all three agonists. In a transduction system in SCGs that is unlikely to be affected by PMT, Go mediated inhibition of calcium current, PMT was ineffective whereas the response was blocked by pertussis toxin as expected.

M1 muscarinic receptor evoked calcium mobilisation in transformed NG108-15 cells was enhanced by PMT. The calcium rises evoked by uridine triphosphate acting on endogenous P2Y₂ receptors in NG108-15 cells were enhanced by PMT. The time and concentration dependence of the PMT effect was different for the resting calcium compared to the calcium rise produced by activation of P2Y₂ receptors. PMT's action on these neuronal cells would suggest that if it got into the brain, symptoms of a hyperexcitable nature would be seen, such as seizures.

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Pasteurella multocida toxin (PMT) activates a range of G-protein alpha subunits.

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PMT increased muscarinic receptor mediated suppression of Kv7 potassium current in sympathetic neurones.

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PMT enhances both muscarinic and purinergic receptor mediated calcium mobilisation in NG108-15 cells.

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Both these events are mediated by the G-proteins Gq or G11.

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We would predict that the symptoms of central nervous system PMT toxicity would be hyperexcitable events such as seizures.

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.

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.

Differential PKC-dependent and -independent PKD activation by G protein α subunits of the Gq family: selective stimulation of PKD Ser⁷⁴⁸ autophosphorylation by Gαq

Protein kinase D (PKD) is activated within cells by stimulation of multiple G protein coupled receptors (GPCR). Earlier studies demonstrated a role for PKC to mediate rapid activation loop phosphorylation-dependent PKD activation. Subsequently, a novel PKC-independent pathway in response to Gαq-coupled GPCR stimulation was identified. Here, we examined further the specificity and PKC-dependence of PKD activation using COS-7 cells cotransfected with different Gq-family Gα and stimulated with aluminum fluoride (AlF4⁻). PKD activation was measured by kinase assays, and Western blot analysis of activation loop sites Ser⁷⁴⁴, a prominent and rapid PKC transphosphorylation site, and Ser⁷⁴⁸, a site autophosphorylated in the absence of PKC signaling. Treatment with AlF4⁻ potently induced PKD activation and Ser⁷⁴⁴ and Ser⁷⁴⁸ phosphorylation, in the presence of cotransfected Gαq, Gα11, Gα14 or Gα15. These treatments achieved PKD activation loop phosphorylation similar to the maximal levels obtained by stimulation with the phorbol ester, PDBu. Preincubation with the PKC inhibitor GF1 potently blocked Gα11-, Gα14-, and Gα15-mediated enhancement of Ser⁷⁴⁸ phosphorylation induced by AlF4⁻, and largely abolished Ser⁷⁴⁴ phosphorylation. In contrast, Ser⁷⁴⁸ phosphorylation was almost completely intact, and Ser⁷⁴⁴ phosphorylation was significantly activated in cells cotransfected with Gαq. Importantly, the differential Ser⁷⁴⁸ phosphorylation was also promoted by treatment of Swiss 3T3 cells with Pasteurella multocida toxin, a selective activator of Gαq but not Gα11. Taken together, our results suggest that Gαq, but not the closely related Gα11, promotes PKD activation in response to GPCR ligands in a unique manner leading to PKD autophosphorylation at Ser⁷⁴⁸.

Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or γ-aminobutyric acid.

Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons

Presynaptic imidazoline receptors (R(i-pre)) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of R(i-pre)-induced inhibition of NA release is unknown. We, therefore, investigated the effect of R(i-pre) activation on voltage-dependent Ca(2+) channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 μM), an R(i-pre) agonist as well as an α-adrenoceptor (R(α)) agonist, decreased Ca(2+) currents (I(Ca)) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 μM), which blocks the α(2)-adrenoceptor (R(α2)), cirazoline still inhibited I(Ca) by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 μM), which blocks R(i-pre) as well as R(α2). In addition, pretreatment with LY320135 (10 μM), another R(i-pre) antagonist, in combination with low-dose rauwolscine (3 μM), also blocked the R(α2)-resistant effect of cirazoline. Addition of guanosine-5-O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml(1)) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 μM) suppressed M current in SCG neurons cultured overnight. Finally, omega-conotoxin (omega-CgTx) GVIA (1 μM) obstructed cirazoline-induced current inhibition, and cirazoline (30 μM) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of omega-CgTx. Taken together, our results suggest that activation of R(i-pre) in SCG neurons reduced N-type I(Ca) in a pertussis toxin- and voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Regulation of voltage-gated Ca2+ channels by lipids

Great skepticism has surrounded the question of whether modulation of voltage-gated Ca(2+) channels (VGCCs) by the polyunsaturated free fatty acid arachidonic acid (AA) has any physiological basis. Here we synthesize findings from studies of both native and recombinant channels where micromolar concentrations of AA consistently inhibit both native and recombinant activity by stabilizing VGCCs in one or more closed states. Structural requirements for these inhibitory actions include a chain length of at least 18 carbons and multiple double bonds located near the fatty acid's carboxy terminus. Acting at a second site, AA increases the rate of VGCC activation kinetics, and in Ca(V)2.2 channels, increases current amplitude. We present evidence that phosphatidylinositol 4,5-bisphosphate (PIP(2)), a palmitoylated accessory subunit (beta(2a)) of VGCCs and AA appear to have overlapping sites of action giving rise to complex channel behavior. Their actions converge in a physiologically relevant manner during muscarinic modulation of VGCCs. We speculate that M(1) muscarinic receptors may stimulate multiple lipases to break down the PIP(2) associated with VGCCs and leave PIP(2)'s freed fatty acid tails bound to the channels to confer modulation. This unexpectedly simple scheme gives rise to unanticipated predictions and redirects thinking about lipid regulation of VGCCs.

L- and N-current but not M-current inhibition by M1 muscarinic receptors requires DAG lipase activity

Stimulation of postsynaptic M(1) muscarinic receptors (M(1)Rs) increases firing rates of both sympathetic and central neurons that underlie increases in vasomotor tone, heart rate, and cognitive memory functioning. At the cellular level, M(1)R stimulation modulates currents through various voltage-gated ion channels, including KCNQ K+ channels (M-current) and both L- and N-type Ca2+ channels (L- and N-current) by a pertussis toxin-insensitive, slow signaling pathway. Depletion of phosphatidylinositol-4,5-bisphosphate (PIP2) during M(1)R stimulation suffices to inhibit M-current. We found previously that following PIP2 hydrolysis by phospholipase C, activation of phospholipase A2 and liberation of a lipid metabolite, most likely arachidonic acid (AA) are necessary for L- and N-current modulation. Here we examined the involvement of a third lipase, diacylglycerol lipase (DAGL), in the slow pathway. We documented the presence of DAGL in superior cervical ganglion neurons, and then tested the highly selective DAGL inhibitor, RHC-80267, for its capacity to antagonize M(1)R-mediated modulation of whole-cell Ca2+ currents. RHC-80267 significantly reduced L- and N-current inhibition by the muscarinic agonist oxotremorine-M (Oxo-M) but did not affect their inhibition by exogenous AA. Moreover, voltage-dependent inhibition of N-current by Oxo-M remained in the presence of RHC-80267, indicating selective action on the slow pathway. RHC also blocked inhibition of recombinant N-current. In contrast, RHC-80267 had no effect on native M-current inhibition. These data are consistent with a role for DAGL in mediating L- and N-current inhibition. These results extend our previous findings that the signaling pathway mediating L- and N-current inhibition diverges from the pathway initiating M-current inhibition.

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).

Dual control of cardiac Na+ Ca2+ exchange by PIP(2): electrophysiological analysis of direct and indirect mechanisms

Cardiac Na(+)-Ca(2+) exchange (NCX1) inactivates in excised membrane patches when cytoplasmic Ca(2+) is removed or cytoplasmic Na(+) is increased. Exogenous phosphatidylinositol-4,5-bis-phosphate (PIP(2)) can ablate both inactivation mechanisms, while it has no effect on inward exchange current in the absence of cytoplasmic Na(+). To probe PIP(2) effects in intact cells, we manipulated PIP(2) metabolism by several means. First, we used cell lines with M1 (muscarinic) receptors that couple to phospholipase C's (PLCs). As expected, outward NCX1 current (i.e. Ca(2+) influx) can be strongly inhibited when M1 agonists induce PIP(2) depletion. However, inward currents (i.e. Ca(2+) extrusion) without cytoplasmic Na(+) can be increased markedly in parallel with an increase of cell capacitance (i.e. membrane area). Similar effects are incurred by cytoplasmic perfusion of GTPgammaS or the actin cytoskeleton disruptor latrunculin, even in the presence of non-hydrolysable ATP (AMP-PNP). Thus, G-protein signalling may increase NCX1 currents by destabilizing membrane cytoskeleton-PIP(2) interactions. Second, to increase PIP(2) we directly perfused PIP(2) into cells. Outward NCX1 currents increase as expected. But over minutes currents decline substantially, and cell capacitance usually decreases in parallel. Third, using BHK cells with stable NCX1 expression, we increased PIP(2) by transient expression of a phosphatidylinositol-4-phosphate-5-kinase (hPIP5KIbeta) and a PI4-kinase (PI4KIIalpha). NCX1 current densities were decreased by > 80 and 40%, respectively. Fourth, we generated transgenic mice with 10-fold cardiac-specific overexpression of PI4KIIalpha. This wortmannin-insensitive PI4KIIalpha was chosen because basal cardiac phosphoinositides are nearly insensitive to wortmannin, and surface membrane PI4-kinase activity, defined functionally in excised patches, is not blocked by wortmannin. Both phosphatidylinositol-4-phosphate (PIP) and PIP(2) were increased significantly, while NCX1 current densities were decreased by 78% with no loss of NCX1 expression. Most mice developed cardiac hypertrophy, and immunohistochemical analysis suggests that NCX1 is redistributed away from the outer sarcolemma. Cholera toxin uptake was increased 3-fold, suggesting that clathrin-independent endocytosis is enhanced. We conclude that direct effects of PIP(2) to activate NCX1 can be strongly modulated by opposing mechanisms in intact cells that probably involve membrane cytoskeleton remodelling and membrane trafficking.

PIP(2)-dependent inhibition of M-type (Kv7.2/7.3) potassium channels: direct on-line assessment of PIP(2) depletion by Gq-coupled receptors in single living neurons

The open state of M(Kv7.2/7.3) potassium channels is maintained by membrane phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)). They can be closed on stimulating receptors that induce PI(4,5)P(2) hydrolysis. In sympathetic neurons, closure induced by stimulating M1-muscarinic acetylcholine receptors (mAChRs) has been attributed to depletion of PI(4,5)P(2), whereas closure by bradykinin B(2)-receptors (B2-BKRs) appears to result from formation of IP(3) and release of Ca(2+), implying that BKR stimulation does not deplete PI(4,5)P(2). We have used a fluorescently tagged PI(4,5)P(2)-binding construct, the C-domain of the protein tubby, mutated to increase sensitivity to PI(4,5)P(2) changes (tubby-R332H-cYFP), to provide an on-line read-out of PI(4,5)P(2) changes in single living sympathetic neurons after receptor stimulation. We find that the mAChR agonist, oxotremorine-M (oxo-M), produces a near-complete translocation of tubby-R332H-cYFP into the cytoplasm, whereas bradykinin (BK) produced about one third as much translocation. However, translocation by BK was increased to equal that produced by oxo-M when synthesis of PI(4,5)P(2) was inhibited by wortmannin. Further, wortmannin 'rescued' M-current inhibition by BK after Ca(2+)-dependent inhibition was reduced by thapsigargin. These results provide the first direct support for the view that BK accelerates PI(4,5)P(2) synthesis in these neurons, and show that the mechanism of BKR-induced inhibition can be switched from Ca(2+) dependent to PI(4,5)P(2) dependent when PI(4,5)P(2) synthesis is inhibited.

An M2-like muscarinic receptor enhances a delayed rectifier K+ current in rat sympathetic neurones

BACKGROUND AND PURPOSE

Resting superior cervical ganglion (SCG) neurones are phasic cells that switch to a tonic mode of firing upon muscarinic receptor stimulation. This effect is partially due to the muscarinic inhibition of the M-current. Because delayed rectifier K+ channels are essential to sustain tonic firing in central neurones, we asked whether the delayed rectifier current IKV in SCG neurones was modulated by the muscarinic receptors expressed in these cells.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp records of M-current and IKV were done in cultured or acutely dissociated rat SCG neurones. To characterize the receptor that regulates IKV, cells were bathed with muscarinic agonists and antagonists, relatively specific for receptor subtypes.

KEY RESULTS

The muscarinic agonist oxotremorine-M (Oxo-M) enhanced IKV by approximately 46% relative to its basal value. This effect remained unaltered when M-current was suppressed by linopirdine or Ba2+. Enhancement of IKV was insensitive to the M1-antagonist pirenzepine, whereas it was inhibited (approximately 60%) by the M2/4-antagonist himbacine. Further, the relatively specific M2-agonist bethanechol was as potent as Oxo-M in enhancing IKV. The modulation of IKV was insensitive to pertussis toxin (PTX), but was severely attenuated when internal ATP was replaced by its non-hydrolysable analogue AMP-PNP.

CONCLUSIONS AND IMPLICATIONS

These results suggest that an M2-like muscarinic receptor couples to a PTX-insensitive G-protein and to an ATP-dependent pathway to enhance IKV. Modulation of IKV must be taken into consideration in order to understand more precisely how muscarinic receptors acting on different ion channels regulate sympathetic excitability.

Angiotensin II regulates neuronal excitability via phosphatidylinositol 4,5-bisphosphate-dependent modulation of Kv7 (M-type) K+ channels

Voltage-gated Kv7 (KCNQ) channels underlie important K+ currents in many different types of cells, including the neuronal M current, which is thought to be modulated by muscarinic stimulation via depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP2). We studied the role of modulation by angiotensin II (angioII) of M current in controlling discharge properties of superior cervical ganglion (SCG) sympathetic neurons and the mechanism of action of angioII on cloned Kv7 channels in a heterologous expression system. In SCG neurons, which endogenously express angioII AT1 receptors, application of angioII for 2 min produced an increase in neuronal excitability and a decrease in spike-frequency adaptation that partially returned to control values after 10 min of angioII exposure. The increase in excitability could be simulated in a computational model by varying only the amount of M current. Using Chinese hamster ovary (CHO) cells expressing cloned Kv7.2 + 7.3 heteromultimers and AT1 receptors studied under perforated patch clamp, angioII induced a strong suppression of the Kv7.2/7.3 current that returned to near baseline within 10 min of stimulation. The suppression was blocked by the phospholipase C inhibitor edelfosine. Under whole-cell clamp, angioII moderately suppressed the Kv7.2/7.3 current whether or not intracellular Ca2+ was clamped or Ca2+ stores depleted. Co-expression of PI(4)5-kinase in these cells sharply reduced angioII inhibition, but did not augment current amplitudes, whereas co-expression of a PIP2 5'-phosphatase sharply reduced current amplitudes, and also blunted the inhibition. The rebound of the current seen in perforated-patch recordings was blocked by the PI4-kinase inhibitor, wortmannin (50 microM), suggesting that PIP2 re-synthesis is required for current recovery. High-performance liquid chromatographic analysis of anionic phospholipids in CHO cells stably expressing AT1 receptors revealed that PIP2 and phosphatidylinositol 4-phosphate levels are to be strongly depleted after 2 min of stimulation with angioII, with a partial rebound after 10 min. The results of this study establish how angioII modulates M channels, which in turn affects the integrative properties of SCG neurons.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

Pathways modulating neural KCNQ/M (Kv7) potassium channels

K(+) channels play a crucial role in regulating the excitability of neurons. Many K(+) channels are, in turn, regulated by neurotransmitters. One of the first neurotransmitter-regulated channels to be identified, some 25 years ago, was the M channel. This was categorized as such because its activity was inhibited through stimulation of muscarinic acetylcholine receptors. M channels are now known to be composed of subunits of the Kv7 (KCNQ) K(+) channel family. However, until recently, the link between the receptors and the channels has remained elusive. Here, we summarize recent developments that have begun to clarify this link and discuss their implications for physiology and medicine.

Altered expression of G(q/11alpha) protein shapes mGlu1 and mGlu5 receptor-mediated single cell inositol 1,4,5-trisphosphate and Ca(2+) signaling

The metabotropic glutamate (mGlu) receptors mGlu1 and mGlu5 mediate distinct inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) signaling patterns, governed in part by differential mechanisms of feedback regulation after activation. Single cell imaging has shown that mGlu1 receptors initiate sustained elevations in IP(3) and Ca(2+), which are sensitive to agonist concentration. In contrast, mGlu5 receptors are subject to cyclical PKC-dependent uncoupling and consequently mediate coincident IP(3) and Ca(2+) oscillations that are largely independent of agonist concentration. In this study, we investigated the contribution of G(q/11)alpha protein expression levels in shaping mGlu1/5 receptor-mediated IP(3) and Ca(2+) signals, using RNA interference (RNAi). RNAi-mediated knockdown of G(q/11)alpha almost abolished the single-cell increase in IP(3) caused by mGlu1 and mGlu5 receptor activation. For the mGlu1 receptor, this unmasked baseline Ca(2+) oscillations that persisted even at maximal agonist concentrations. mGlu5 receptor-activated Ca(2+) oscillations were still observed but were only initiated at high agonist concentrations. Recombinant overexpression of G(q)alpha enhanced IP(3) signals after mGlu1 and mGlu5 receptor activation. It is noteworthy that although mGlu5 receptor-mediated IP(3) and Ca(2+) oscillations in control cells were largely insensitive to agonist concentration, increasing G(q)alpha expression converted these oscillatory signatures to sustained plateau responses in a high proportion of cells. In addition to modulating temporal Ca(2+) signals, up- or down-regulation of G(q/11)alpha expression alters the threshold for the concentration of glutamate at which a measurable Ca(2+) signal could be detected. These experiments indicate that altering G(q/11)alpha expression levels differentially affects spatiotemporal aspects of IP(3) and Ca(2+) signaling mediated by the mGlu1 and mGlu5 receptors.

Protein kinase C shifts the voltage dependence of KCNQ/M channels expressed in Xenopus oocytes

It is well established that stimulation of G(q)-coupled receptors such as the M1 muscarinic acetylcholine receptor inhibits KCNQ/M currents. While it is generally accepted that this muscarinic inhibition is mainly caused by the breakdown of PIP(2), the role of the subsequent activation of protein kinase C (PKC) is not well understood. By reconstituting M currents in Xenopus oocytes, we observed that stimulation of coexpressed M1 receptors with 10 microm oxotremorine M (oxo-M) induces a positive shift (4-30 mV, depending on which KCNQ channels are expressed) in the conductance-voltage relationship (G-V) of KCNQ channels. When we applied phorbol 12-myristate 13-acetate (PMA), a potent PKC activator, we observed a large positive shift (17.8 +/- 1.6 mV) in the G-V curve for KCNQ2, while chelerythrine, a PKC inhibitor, attenuated the shift caused by the stimulation of M1 receptors. By contrast, reducing PIP(2) had little effect on the G-V curve for KCNQ2 channels; although pretreating cells with 10 mum wortmannin for 30 min reduced KCNQ2 current amplitude by 80%, the G-V curve was shifted only slightly (5 mV). Apparently, the shift induced by muscarinic stimulation in Xenopus oocytes was mainly caused by PKC activation. When KCNQ2/3 channels were expressed in HEK 293T cells, the G-V curve seemed already to be shifted in a positive direction, even before activation of PKC, and PMA failed to shift the curve any further. That alkaline phosphatase in the patch pipette shifted the G-V curve in a negative direction suggests KCNQ2/3 channels are constitutively phosphorylated in HEK 293T cells.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

The use of Chinese hamster ovary (CHO) cells in the study of ion channels

The line of epithelial-like Chinese hamster ovary (CHO) cells was initiated by T.T. Puck in 1957. Since then, CHO cells have become a widely used mammalian expression system in industry and science. This paper discusses the different features of CHO cell physiology as well as the specific aspects of using these cells for ion channel studies; among the discussed features are the culturing and transfection of CHO cells, details of electrophysiological recordings from them and applications for the study of ion channel physiology and pharmacology. Examples of successful reconstitution of mammalian ion channels in CHO cells discussed in the paper include reconstitution of KCNQ channel regulation by muscarinic acetylcholine receptors and the study of the amiloride-sensitivity of epithelial sodium channels (ENaC).

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.

Protein kinase C bound with A-kinase anchoring protein is involved in muscarinic receptor-activated modulation of M-type KCNQ potassium channels

The second messenger for closure of M/KCNQ potassium channels in post-ganglionic neurons and central neurons had remained as a 'mystery in the neuroscience field' for over 25 years. However, recently the details of the pathway leading from muscarinic acetylcholine receptor (mAChR)-stimulation to suppression of the M/KCNQ-current were discovered. A key molecule is A-kinase anchoring protein (AKAP; AKAP79 in human, or its rat homolog, AKAP150) which forms a trimeric complex with protein kinase C (PKC) and KCNQ channels. AKAP79 or 150 serves as an adapter that brings the anchored C-kinase to the substrate KCNQ channel to permit the rapid and 'definitive' phosphorylation of serine residues, resulting in avoidance of signal dispersion. Thus, these findings suggest that mAChR-induced short-term modulation (or memory) does occur within the already well-integrated molecular complex, without accompanying Hebbian synapse plasticity. However, before this identity is confirmed, many other modulators which affect M-currents remain to be addressed as intriguing issues.

Muscarinic modulation of erg potassium current

We studied modulation of current in human embryonic kidney tsA-201 cells coexpressing rat erg1 channels with M(1) muscarinic receptors. Maximal current was inhibited 30% during muscarinic receptor stimulation, with a small positive shift of the midpoint of activation. Inhibition was attenuated by coexpression of the regulator of G-protein signalling RGS2 or of a dominant-negative protein, G(q), but not by N-ethylmaleimide or C3 toxin. Overexpression of a constitutively active form of G(q) (but not of G(13) or of G(s)) abolished the erg current. Hence it is likely that G(q/11), and not G(i/o) or G(13), mediates muscarinic inhibition. Muscarinic suppression of erg was attenuated by chelating intracellular Ca(2+) to < 1 nm free Ca(2+) with 20 mm BAPTA in the pipette, but suppression was normal if internal Ca(2+) was strongly clamped to a 129 nm free Ca(2+) level with a BAPTA buffer and this was combined with numerous other measures to prevent intracellular Ca(2+) transients (pentosan polysulphate, preincubation with thapsigargin, and removal of extracellular Ca(2+)). Hence a minimum amount of Ca(2+) was necessary for the inhibition, but a Ca(2+) elevation was not. The ATP analogue AMP-PCP did not prevent inhibition. The protein kinase C (PKC) blockers staurosporine and bisindolylmaleimide I did not prevent inhibition, and the PKC-activating phorbol ester PMA did not mimic it. Neither the tyrosine kinase inhibitor genistein nor the tyrosine phosphatase inhibitor dephostatin prevented inhibition by oxotremorine-M. Hence protein kinases are not needed. Experiments with a high concentration of wortmannin were consistent with recovery being partially dependent on PIP(2) resynthesis. Wortmannin did not prevent muscarinic inhibition. Our studies of muscarinic inhibition of erg current suggest a role for phospholipase C, but not the classical downstream messengers, such as PKC or a calcium transient.

Activation of M1 muscarinic receptors triggers transmitter release from rat sympathetic neurons through an inhibition of M-type K+ channels

Acetylcholine has long been known to excite sympathetic neurons via M1 muscarinic receptors through an inhibition of M-currents. Nevertheless, it remained controversial whether activation of muscarinic receptors is also sufficient to trigger noradrenaline release from sympathetic neurons. In primary cultures of rat superior cervical ganglia, the muscarinic agonist oxotremorine M inhibited M-currents with half-maximal effects at 1 microM and induced the release of previously incorporated [3H]noradrenaline with half-maximal effects at 10 microM. This latter action was not affected by the nicotinic antagonist mecamylamine which, however, abolished currents through nicotinic receptors elicited by high oxotremorine M concentrations. Ablation of the signalling cascades linked to inhibitory G proteins by pertussis toxin potentiated the release stimulating effect of oxotremorine M, and the half-maximal concentration required to stimulate noradrenaline release was decreased to 3 microM. Pirenzepine antagonized the inhibition of M-currents and the induction of release by oxotremorine M with identical apparent affinity, and both effects were abolished by the muscarinic toxin 7. These results indicate that one muscarinic receptor subtype, namely M1, mediates these two effects. Retigabine, which enhances M-currents, abolished the release induced by oxotremorine M, but left electrically induced release unaltered. Moreover, retigabine shifted the voltage-dependent activation of M-currents by about 20 mV to more negative potentials and caused 20 mV hyperpolarisations of the membrane potential. In the absence of retigabine, oxotremorine M depolarised the neurons and elicited action potential discharges in 8 of 23 neurons; in its presence, oxotremorine M still caused equal depolarisations, but always failed to trigger action potentials. Action potential waveforms caused by current injection were not affected by retigabine. These results indicate that the inhibition of M-currents is the basis for the stimulation of transmitter release from sympathetic neurons via M1 muscarinic receptors.

Calmodulin mediates Ca2+-dependent modulation of M-type K+ channels

To quantify the modulation of KCNQ2/3 current by [Ca2+]i and to test if calmodulin (CaM) mediates this action, simultaneous whole-cell recording and Ca2+ imaging was performed on CHO cells expressing KCNQ2/3 channels, either alone, or together with wild-type (wt) CaM, or dominant-negative (DN) CaM. We varied [Ca2+]i from <10 to >400 nM with ionomycin (5 microM) added to either a 2 mM Ca2+, or EGTA-buffered Ca2+-free, solution. Coexpression of wt CaM made KCNQ2/3 currents highly sensitive to [Ca2+]i (IC50 70 +/- 20 nM, max inhibition 73%, n = 10). However, coexpression of DN CaM rendered KCNQ2/3 currents largely [Ca2+]i insensitive (max inhibition 8 +/- 3%, n = 10). In cells without cotransfected CaM, the Ca2+ sensitivity was variable but generally weak. [Ca2+]i modulation of M current in superior cervical ganglion (SCG) neurons followed the same pattern as in CHO cells expressed with KCNQ2/3 and wt CaM, suggesting that endogenous M current is also highly sensitive to [Ca2+]i. Coimmunoprecipitations showed binding of CaM to KCNQ2-5 that was similar in the presence of 5 mM Ca2+ or 5 mM EGTA. Gel-shift analyses suggested Ca2+-dependent CaM binding to an "IQ-like" motif present in the carboxy terminus of KCNQ2-5. We tested whether bradykinin modulation of M current in SCG neurons uses CaM. Wt or DN CaM was exogenously expressed in SCG cells using pseudovirions or the biolistic "gene gun." Using both methods, expression of both wt CaM and DN CaM strongly reduced bradykinin inhibition of M current, but for all groups muscarinic inhibition was unaffected. Cells expressed with wt CaM had strongly reduced tonic current amplitudes as well. We observed similar [Ca2+]i rises by bradykinin in all the groups of cells, indicating that CaM did not affect Ca2+ release from stores. We conclude that M-type currents are highly sensitive to [Ca2+]i and that calmodulin acts as their Ca2+ sensor.

Experiments to test the role of phosphatidylinositol 4,5-bisphosphate in neurotransmitter-induced M-channel closure in bullfrog sympathetic neurons

Various neurotransmitters excite neurons by suppressing a ubiquitous, voltage-dependent, noninactivating K+ conductance called the M-conductance (gM). In bullfrog sympathetic ganglion neurons the suppression of gM by the P2Y agonist ATP involves phospholipase C (PLC). The present results are consistent with the involvement of the lipid and inositol phosphate cycles in the effects of both P2Y and muscarinic cholinergic agonists on gM. Impairment of resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP2) with the phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) slowed or blocked the recovery of agonist-induced gM suppression. This effect could not be attributed to an action of wortmannin on myosin light chain kinase or on phosphatidylinositol 3-kinase. Inhibition of PIP2 synthesis at an earlier point in the lipid cycle by the use of R59022 (40 microm) to inhibit diacylglycerol kinase also slowed the rate of recovery of successive ATP responses. This effect required several applications of agonist to deplete levels of various phospholipid intermediates in the lipid cycle. PIP2 antibodies attenuated the suppression of gM by agonists. Intracellular application of 20 microm PIP2 slowed the rundown of KCNQ2/3 currents expressed in COS-1 or tsA-201 cells, and 100 microm PIP2 produced a small potentiation of native M-current bullfrog sympathetic neurons. These are the results that might be expected if agonist-induced activation of PLC and the concomitant depletion of PIP2 contribute to the excitatory action of neurotransmitters that suppress gM.

AKAP150 signaling complex promotes suppression of the M-current by muscarinic agonists

M-type (KCNQ2/3) potassium channels are suppressed by activation of G(q/11)-coupled receptors, thereby increasing neuronal excitability. We show here that rat KCNQ2 can bind directly to the multivalent A-kinase-anchoring protein AKAP150. Peptides that block AKAP150 binding to the KCNQ2 channel complex antagonize the muscarinic inhibition of the currents. A mutant form of AKAP150, AKAP(DeltaA), which is unable to bind protein kinase C (PKC), also attenuates the agonist-induced current suppression. Analysis of recombinant KCNQ2 channels suggests that targeting of PKC through association with AKAP150 is important for the inhibition. Phosphorylation of KCNQ2 channels was increased by muscarinic stimulation; this was prevented either by coexpression with AKAP(DeltaA) or pretreatment with PKC inhibitors that compete with diacylglycerol. These inhibitors also reduced muscarinic inhibition of M-current. Our data indicate that AKAP150-bound PKC participates in receptor-induced inhibition of the M-current.

Activation of muscarinic m5 receptors inhibits recombinant KCNQ2/KCNQ3 K+ channels expressed in HEK293T cells

A variety of G-protein-coupled receptors regulate membrane excitability via M-type K(+) current (M-current) modulation. Muscarinic m1 and m3 acetylcholine receptors have both been implicated in the modulation of M-current. The muscarinic m5 receptor, like muscarinic m1 and m3 receptors, couples to phospholipase C via a pertussis toxin-insensitive G protein. Since a number of other receptors which activate phospholipase C also modulate M-current, we investigated if muscarinic m5 receptors could modulate recombinant M-type (KCNQ2/KCNQ3) K(+) channels after heterologous expression in human embryonic kidney (HEK) 293T cells. Application of Oxo-tremorine M to HEK293T cells expressing muscarinic m1, m3, or m5 receptors produced a similar robust inhibition of M-current, whereas muscarinic m2 and m4 receptor stimulation was without effect. Muscarinic m1, m3, or m5 receptor stimulation decreased the deactivation time constants of M-current at -50 mV. The inhibition of M-current by stimulation of muscarinic m1, m3, or m5 receptors was insensitive to overnight treatment with pertussis toxin or cholera toxin, which interfere with G(i/o) and G(s) G-protein signaling. These data suggest that muscarinic m1, m3, and m5 receptors inhibit M-channels via the activation of a common G protein.

Coupling of the nucleotide P2Y4 receptor to neuronal ion channels

1. G protein-linked P2Y nucleotide receptors are known commonly to stimulate the phosphoinositide signalling pathway. However, we have previously demonstrated that the cloned P2Y(2), P2Y(6) and P2Y(1) receptors couple to neuronal N-type Ca(2+) channels and to M-type K(+) channels. Here we investigate the coupling of recombinant, neuronally expressed rat- and human P2Y(4) receptors (rP2Y(4), hP2Y(4)) to those channels. 2. Rat sympathetic neurones were nuclear-injected with a P2Y(4) cDNA plasmid. A subsequent activation of rP2Y(4) or hP2Y(4) by UTP (100 micro M) in whole-cell (ruptured-patch) mode produced only about 12% inhibition of the N-type Ca(2+) current (I(Ca(N))). Surprisingly, in perforated patch mode, UTP produced much more inhibition of I(Ca(N)) (maximally 51%), with an IC(50) value of 273 nM. This inhibition was voltage-dependent and was blocked by co-expression of the betagamma-binding transducin Galpha-subunit. Pertussis toxin (PTX) pretreatment also suppressed I(Ca(N)) inhibition. 3. UTP inhibited the M-current, recorded in perforated patch mode, by (maximally) 52%, with IC(50) values of 21 nM for rP2Y(4) and 28 nM for hP2Y(4). This inhibition was not affected by PTX pretreatment. 4. With rP2Y(4), ATP inhibited the M-current (IC(50) 524 nM, 26 times weaker than UTP), whereas ATP had no agonist activity at hP2Y(4). This suggests a difference in agonist binding site between rP2Y(4) and hP2Y(4). 5. We conclude that, in contrast to other nucleotide receptors studied, the P2Y(4) receptor couples much more effectively to M-type K(+) channels than to Ca(2+) channels. Coupling to the Ca(2+) channels involves the betagamma-subunits of G(i/o)-proteins and requires a diffusible intracellular component that is lost in ruptured-patch recording.

Specificity of metabotropic glutamate receptor 2 coupling to G proteins

Metabotropic glutamate receptor 2 (mGluR2) is a class 3 G protein-coupled receptor and an important mediator of synaptic activity in the central nervous system. Previous work demonstrated that mGluR2 couples to pertussis toxin (PTX)-sensitive G proteins. However, the specificity of mGluR2 coupling to individual members of the G(i/o) family is not known. Using heterologously expressed mGluR2 in rat sympathetic neurons from the superior cervical ganglion (SCG), the mGluR2/G protein coupling profile was characterized by reconstituting coupling in PTX-treated cells expressing PTX-insensitive mutant Galpha proteins and Gbetagamma. By employing this method, it was demonstrated that mGluR2 coupled strongly with Galphaob, Galphai1, Galphai2, and Galphai3, although coupling to Galphaoa was less efficient. In addition, mGluR2 did not seem to couple to the most divergent member of the G(i/o) family, Galphaz, although Galphaz coupled strongly to the endogenous alpha2 adrenergic receptor. To determine which Galpha proteins may be natively expressed in SCG neurons, the presence of mRNA for various Galpha proteins was tested using reverse transcription-polymerase chain reaction. Strong bands were detected for all members of the G(i/o) family (Galphao, Galphai1, Galphai2, Galphai3, Galphaz) as well as for Galpha11 and Galphas. A weak signal was detected for Galphaq and no Galpha15 mRNA was detected.

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.

Antibodies and a cysteine-modifying reagent show correspondence of M current in neurons to KCNQ2 and KCNQ3 K+ channels

1. KCNQ K(+) channels are thought to underlie the M current of neurons. To probe if the KCNQ2 and KCNQ3 subtypes underlie the M current of rat superior cervical ganglia (SCG) neurons and of hippocampus, we raised specific antibodies against them and also used the cysteine-alkylating agent N-ethylmaleimide (NEM) as an additional probe of subunit composition. 2. Tested on tsA-201 (tsA) cells transfected with cloned KCNQ1-5 subunits, our antibodies showed high affinity and selectivity for the appropriate subtype. The antibodies immunostained SCG neurons and hippocampal sections at levels similar to those for channels expressed in tsA cells, indicating that KCNQ2 and KCNQ3 are present in SCG and hippocampal neurons. Some hippocampal regions contained only KCNQ2 or KCNQ3 subunits, suggesting the presence of M currents produced by channels other than KCNQ2/3 heteromultimers. 3. We found that NEM augmented M currents in SCG neurons and KCNQ2/3 currents in tsA cells via strong voltage-independent and modest voltage-dependent actions. Expression of individual KCNQ subunits in tsA cells revealed voltage-independent augmentation of KCNQ2, but not KCNQ1 nor KCNQ3, currents by NEM indicating that this action on SCG M currents likely localizes to KCNQ2. Much of the voltage-independent action is lost after the C242T mutation in KCNQ2. 4. The correspondence of NEM effects on expressed KCNQ2/3 and SCG M currents, along with the antibody labelling, provide further evidence that KCNQ2 and KCNQ3 subunits strongly contribute to the M current of neurons. The site of NEM action may be important for treatment of diseases caused by under-expression of these channels.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

ATP-inhibition of M current in frog sympathetic neurons involves phospholipase C but not Ins P(3), Ca(2+), PKC, or Ras

Suppression of the voltage-activated, noninactivating K(+) conductance (M conductance; g(M)) by muscarinic agonists, P(2Y) agonists or bradykinin increases neuronal excitability. All agonist effects are mediated, at least in part, via the Gq/(11) class of G protein. We found, using whole cell or perforated patch recording from bullfrog sympathetic B neurons that ATP-induced suppression of g(M) was attenuated by the phospholipase C (PLC) inhibitor, U73122 (IC(50) approximately 0.14 microM) but not by the inactive isomer, U73343. The ability of extracellularly applied U73122 to inhibit PLC was confirmed by its antagonism of ATP-induced elevation of intracellular Ca(2+) as measured by fura-2 photometry. ATP-induced g(M) suppression was not antagonized by the protein kinase C (PKC) inhibitor, chelerythrine (5 microM extracellular +10 microM intracellular), by the Ca(2+)-ATPase inhibitor, thapsigargin (5 microM), or by inositol trisphosphate (InsP(3)) receptor antagonists, heparin (approximaterly 300 microM) or xestospongin C (1.8 microM). The effect of ATP on g(M) was thus dependent on PLC yet independent of PKC and of InsP(3)-induced release of intracellular Ca(2+). We therefore tested the involvement of a PKC-independent action of diacylglycerol (DAG) that could occur via activation of Ras. This low-molecular-weight G protein is activated following DAG binding to Ras-GRP, a neuronal Ras-GTP exchange factor. However, impairment of Ras function by culturing neurons with isoprenylation inhibitors (perillic acid, 0.1 mM, or alpha-hydroxyfarnesyl-phosphonic acid, 10 microM) failed to affect ATP-induced g(M) suppression. Inhibition of MEK (mitogen-activated protein kinase), a downstream target of Ras, by using PD 98059 (10 microM) was also ineffective. The transduction mechanism used by ATP to suppress g(M) in frog sympathetic neurons therefore differs from the PLC-independent mechanism used by muscarine and from the PLC and Ca(2+)-dependent mechanism used by bradykinin and UTP in mammalian ganglia. The possibility remains that "lipid-signaling" mechanisms, perhaps involving PLC-induced depletion of phosphatidylinositol bisphosphate, are involved in PLC-mediated inhibition of g(M) by ATP in amphibian sympathetic neurons.

Adenovirus-mediated G(alpha)(q)-protein antisense transfer in neurons replicates G(alpha)(q) gene knockout strategies

Antisense approaches are increasingly used to dissect signaling pathways linking cell surface receptors to intracellular effectors. Here we used a recombinant adenovirus to deliver G-protein alpha(q) antisense into rat superior cervical ganglion (SCG) neurons and neuronal cell lines to dissect G(alpha)(q)-mediated signaling pathways in these cells. This approach was compared with other G(alpha)(q) gene knockdown strategies, namely, antisense plasmid and knockout mice. Infection with adenovirus expressing G(alpha)(q) antisense (G(alpha)(q)AS AdV) selectively decreased immunoreactivity for the G(alpha)(q) protein. Expression of other G(alpha) protein subunits, such as G(alpha)(oA/B,) was unaltered. Consistent with this, modulation of Ca(2+) currents by the G(alpha)(q)-coupled M(1) muscarinic receptor was severely impaired in neurons infected with G(alpha)(q)AS AdV whereas modulation via the G(alpha)(oA)-coupled M(4) muscarinic receptor was unchanged. In agreement, activation of phospholipase C and consequent mobilization of intracellular Ca(2+) by UTP receptors was lost in NG108-15 cells infected with G(alpha)(q)AS AdV but not in cells infected with the control GFP-expressing adenovirus. Results obtained with this recombinant AdV strategy qualitatively and quantitatively replicated results obtained using SCG neurons microinjected with G(alpha)(q) antisense plasmids or SCG neurons from G(alpha)(q) knockout mice. This combined antisense/recombinant adenoviral approach can therefore be useful for dissecting signal transduction mechanisms in SCG and other neurons.

Signaling microdomains define the specificity of receptor-mediated InsP(3) pathways in neurons

M(1) muscarinic (M(1)AChRs) and B(2) bradykinin (B(2)Rs) receptors are two PLCbeta-coupled receptors that mobilize Ca(2+) in nonexcitable cells. In many neurons, however, B(2)Rs but not M(1)AChRs mobilize intracellular Ca(2+). We have studied the membrane organization and dynamics underlying this coupling specificity by using Trp channels as biosensors for real-time detection of PLCbeta products. We found that, in sympathetic neurons, although both receptors rapidly produced DAG and InsP(3) as messengers, only InsP(3) formed by B(2)Rs has the ability to activate IP(3)Rs. This exclusive coupling results from spatially restricted complexes linking B(2)Rs to IP(3)Rs, a missing partnership for M(1)AChRs. These complexes allow fast and localized rises of InsP(3), necessary to activate the low-affinity neuronal IP(3)R. Thus, these signaling microdomains are of critical importance for the induction of selective responses, discriminating proinflammatory information associated with B(2)Rs from cholinergic neurotransmission.

Constitutive activation of G-proteins by polycystin-1 is antagonized by polycystin-2

Polycystin-1 (PC1), a 4,303-amino acid integral membrane protein of unknown function, interacts with polycystin-2 (PC2), a 968-amino acid alpha-type channel subunit. Mutations in their respective genes cause autosomal dominant polycystic kidney disease. Using a novel heterologous expression system and Ca(2+) and K(+) channels as functional biosensors, we found that full-length PC1 functioned as a constitutive activator of G(i/o)-type but not G(q)-type G-proteins and modulated the activity of Ca(2+) and K(+) channels via the release of Gbetagamma subunits. PC1 lacking the N-terminal 1811 residues replicated the effects of full-length PC1. These effects were independent of regulators of G-protein signaling proteins and were lost in PC1 mutants lacking a putative G-protein binding site. Co-expression with full-length PC2, but not a C-terminal truncation mutant, abrogated the effects of PC1. Our data provide the first experimental evidence that full-length PC1 acts as an untraditional G-protein-coupled receptor, activity of which is physically regulated by PC2. Thus, our study strongly suggests that mutations in PC1 or PC2 that distort the polycystin complex would initiate abnormal G-protein signaling in autosomal dominant polycystic kidney disease.

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.

Multiple pertussis toxin-sensitive G-proteins can couple receptors to GIRK channels in rat sympathetic neurons when expressed heterologously, but only native G(i)-proteins do so in situ

Although many G-protein-coupled neurotransmitter receptors are potentially capable of modulating both voltage-dependent Ca(2+) channels (I(Ca)) and G-protein-gated K(+) channels (I(GIRK)), there is a substantial degree of selectivity in the coupling to one or other of these channels in neurons. Thus, in rat superior cervical ganglion (SCG) neurons, M(2) muscarinic acetylcholine receptors (mAChRs) selectively activate I(GIRK) whereas M(4) mAChRs selectively inhibit I(Ca). One source of selectivity might be that the two receptors couple preferentially to different G-proteins. Using antisense depletion methods, we found that M(2) mAChR-induced activation of I(GIRK) is mediated by G(i) whereas M(4) mAChR-induced inhibition of I(Ca) is mediated by G(oA). Experiments with the beta gamma-sequestering peptides alpha-transducin and beta ARK1(C-ter) indicate that, although both effects are mediated by G-protein beta gamma subunits, the endogenous subunits involved in I(GIRK) inhibition differ from those involved in I(Ca) inhibition. However, this pathway divergence does not result from any fundamental selectivity in receptor-G-protein-channel coupling because both I(GIRK) and I(Ca) modulation can be rescued by heterologously expressed G(i) or G(o) proteins after the endogenously coupled alpha-subunits have been inactivated with Pertussis toxin (PTX). We suggest instead that the divergence in the pathways activated by the endogenous mAChRs results from a differential topographical arrangement of receptor, G-protein and ion channel.

Neurotensin excitation of serotonergic neurons in the rat nucleus raphe magnus: ionic and molecular mechanisms

To understand the cellular and molecular mechanisms by which neurotensin (NT) induces an analgesic effect in the nucleus raphe magnus (NRM), whole-cell patch-clamp recordings were performed to investigate the electrophysiological effects of NT on acutely dissociated NRM neurons. Two subtypes of neurons, primary serotonergic and secondary non-serotonergic cells, were identified from acutely isolated NRM neurons. During current-clamp recordings, NT depolarized NRM serotonergic neurons and evoked action potentials. Voltage-clamp recordings showed that NT excited serotonergic neurons by enhancing a voltage-insensitive and non-selective cationic conductance. Both SR48692, a selective antagonist of subtype 1 neurotensin receptor (NTR-1), and SR 142948A, a non-selective antagonist of NTR-1 and subtype 2 neurotensin receptor (NTR-2), failed to prevent neurotensin from exciting NRM serotonergic neurons. NT-evoked cationic current was inhibited by the intracellular administration of GDP-beta-S. NT failed to induce cationic currents after dialyzing serotonergic neurons with the anti-G(alphaq/11) antibody. Cellular Ca(2+) imaging study using fura-2 showed that NT induced the calcium release from the intracellular store. NT-evoked current was blocked after the internal perfusion of heparin, an IP(3) receptor antagonist, or BAPTA, a fast Ca(2+) chelator. It is concluded that neurotensin enhancement of the cationic conductance of NRM serotonergic neurons is mediated by a novel subtype of neurotensin receptors. The coupling mechanism via G(alphaq/11) proteins is likely to involve the generation of IP(3), and subsequent IP(3)-evoked Ca(2+) release from intracellular stores results in activating the non-selective cationic conductance.