Probing the regulation of M (Kv7) potassium channels in intact neurons with membrane-targeted peptides

PIP2 modulates TRPC3 activity via TRP helix and S4-S5 linker

The transient receptor potential canonical type 3 (TRPC3) channel plays a pivotal role in regulating neuronal excitability in the brain via its constitutive activity. The channel is intricately regulated by lipids and has previously been demonstrated to be positively modulated by PIP2. Using molecular dynamics simulations and patch clamp techniques, we reveal that PIP2 predominantly interacts with TRPC3 at the L3 lipid binding site, located at the intersection of pre-S1 and S1 helices. We demonstrate that PIP2 sensing involves a multistep mechanism that propagates from L3 to the pore domain via a salt bridge between the TRP helix and S4-S5 linker. Notably, we find that both stimulated and constitutive TRPC3 activity require PIP2. These structural insights into the function of TRPC3 are invaluable for understanding the role of the TRPC subfamily in health and disease, in particular for cardiovascular diseases, in which TRPC3 channels play a major role.

Bidirectional Allosteric Coupling between PIP2 Binding and the Pore of the Oncochannel TRPV6

The epithelial ion channel TRPV6 plays a pivotal role in calcium homeostasis. Channel function is intricately regulated at different stages, involving the lipid phosphatidylinositol-4,5-bisphosphate (PIP2). Given that dysregulation of TRPV6 is associated with various diseases, including different types of cancer, there is a compelling need for its pharmacological targeting. Structural studies provide insights on how TRPV6 is affected by different inhibitors, with some binding to sites else occupied by lipids. These include the small molecule cis-22a, which, however, also binds to and thereby blocks the pore. By combining calcium imaging, electrophysiology and optogenetics, we identified residues within the pore and the lipid binding site that are relevant for regulation by cis-22a and PIP2 in a bidirectional manner. Yet, mutation of the cytosolic pore exit reduced inhibition by cis-22a but preserved sensitivity to PIP2 depletion. Our data underscore allosteric communication between the lipid binding site and the pore and vice versa for most sites along the pore.

Involvement of Ca2+ in Signaling Mechanisms Mediating Muscarinic Inhibition of M Currents in Sympathetic Neurons

Acetylcholine can excite neurons by suppressing M-type (KCNQ) potassium channels. This effect is mediated by M1 muscarinic receptors coupled to the Gq protein. Although PIP2 depletion and PKC activation have been strongly suggested to contribute to muscarinic inhibition of M currents (IM), direct evidence is lacking. We investigated the mechanism involved in muscarinic inhibition of IM with Ca2+ measurement and electrophysiological studies in both neuronal (rat sympathetic neurons) and heterologous (HEK cells expressing KCNQ2/KCNQ3) preparations. We found that muscarinic inhibition of IM was not blocked either by PIP2 or by calphostin C, a PKC inhibitor. We then examined whether muscarinic inhibition of IM uses multiple signaling pathways by blocking both PIP2 depletion and PKC activation. This maneuver, however, did not block muscarinic inhibition of IM. Additionally, muscarinic inhibition of IM was not prevented either by sequestering of G-protein βγ subunits from Gα-transducin or anti-Gβγ antibody or by preventing intracellular trafficking of channel proteins with blebbistatin, a class-II myosin inhibitor. Finally, we re-examined the role of Ca2+ signals in muscarinic inhibition of IM. Ca2+ measurements showed that muscarinic stimulation increased intracellular Ca2+ and was comparable to the Ca2+ mobilizing effect of bradykinin. Accordingly, 20-mM of BAPTA significantly suppressed muscarinic inhibition of IM. In contrast, muscarinic inhibition of IM was completely insensitive to 20-mM EGTA. Taken together, these data suggest a role of Ca2+ signaling in muscarinic modulation of IM. The differential effects of EGTA and BAPTA imply that Ca2+ microdomains or spatially local Ca2+ signals contribute to inhibition of IM.

A network of phosphatidylinositol (4,5)-bisphosphate (PIP2) binding sites on the dopamine transporter regulates amphetamine behavior in Drosophila Melanogaster

Reward modulates the saliency of a specific drug exposure and is essential for the transition to addiction. Numerous human PET-fMRI studies establish a link between midbrain dopamine (DA) release, DA transporter (DAT) availability, and reward responses. However, how and whether DAT function and regulation directly participate in reward processes remains elusive. Here, we developed a novel experimental paradigm in Drosophila melanogaster to study the mechanisms underlying the psychomotor and rewarding properties of amphetamine (AMPH). AMPH principally mediates its pharmacological and behavioral effects by increasing DA availability through the reversal of DAT function (DA efflux). We have previously shown that the phospholipid, phosphatidylinositol (4, 5)-bisphosphate (PIP₂), directly interacts with the DAT N-terminus to support DA efflux in response to AMPH. In this study, we demonstrate that the interaction of PIP₂ with the DAT N-terminus is critical for AMPH-induced DAT phosphorylation, a process required for DA efflux. We showed that PIP₂ also interacts with intracellular loop 4 at R443. Further, we identified that R443 electrostatically regulates DA efflux as part of a coordinated interaction with the phosphorylated N-terminus. In Drosophila, we determined that a neutralizing substitution at R443 inhibited the psychomotor actions of AMPH. We associated this inhibition with a decrease in AMPH-induced DA efflux in isolated fly brains. Notably, we showed that the electrostatic interactions of R443 specifically regulate the rewarding properties of AMPH without affecting AMPH aversion. We present the first evidence linking PIP₂, DAT, DA efflux, and phosphorylation processes with AMPH reward.

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.

Identification of N-arachidonoyl dopamine as a highly biased ligand at cannabinoid CB1 receptors

BACKGROUND AND PURPOSE

N-arachidonyl dopamine (NADA) has been identified as a putative endocannabinoid, but there is little information about which signalling pathways it activates. The purpose of this study was to identify the signalling pathways activated by NADA in vitro.

EXPERIMENTAL APPROACH

Human or rat cannabinoid CB1 receptors were expressed in AtT20, CHO or HEK 293 cells. NADA displacement of radiolabelled cannabinoids, and CB1 receptor mediated activation of K channels or ERK phosphorylation, release of intracellular calcium ([Ca]i ) and modulation of adenylyl cyclase were measured in addition to NADA effects on CB1 receptor trafficking.

KEY RESULTS

At concentrations up to 30 μM, NADA failed to activate any signalling pathways via CB1 receptors, with the exception of mobilization of [Ca]i . The elevations of [Ca]i were insensitive to pertussis toxin, and reduced or abolished by blockers of Gq /11 -dependent processes including U73122, thapsigargin and a peptide antagonist of Gq /11 activation. Prolonged NADA incubation produced modest loss of cell surface CB1 receptors. The prototypical cannabinoid agonist CP55940 signalled as expected in all assays.

CONCLUSIONS AND IMPLICATIONS

NADA is an ineffective agonist at most canonical cannabinoid receptor signalling pathways, but did promote mobilization of [Ca]i via Gq -dependent processes and some CB1 receptor trafficking. This signalling profile is distinct from that of any known cannabinoid, and suggests that NADA may have a unique spectrum of effects in vivo. Our results also indicate that it may be possible to identify highly biased CB1 receptor ligands displaying a subset of the pharmacological or therapeutic effects usually attributed to CB1 ligands.

Phosphoinositide Modulation of Heteromeric Kv1 Channels Adjusts Output of Spiral Ganglion Neurons from Hearing Mice

Spiral ganglion neurons (SGNs) relay acoustic code from cochlear hair cells to the brainstem, and their stimulation enables electrical hearing via cochlear implants. Rapid adaptation, a mechanism that preserves temporal precision, and a prominent feature of auditory neurons, is regulated via dendrotoxin-sensitive low-threshold voltage-activated (LVA) K(+) channels. Here, we investigated the molecular physiology of LVA currents in SGNs cultured from mice following the onset of hearing (postnatal days 12-21). Kv1.1- and Kv1.2-specific toxins blocked the LVA currents in a comparable manner, suggesting that both subunits contribute to functional heteromeric channels. Confocal immunofluorescence in fixed cochlear sections localized both Kv1.1 and Kv1.2 subunits to specific neuronal microdomains, including the somatic membrane, juxtaparanodes, and the first heminode, which forms the spike initiation site of the auditory nerve. The spatial distribution of Kv1 immunofluorescence appeared mutually exclusive to that of Kv3.1b subunits, which mediate high-threshold voltage-activated currents. As Kv1.2-containing channels are positively modulated by membrane phosphoinositides, we investigated the influence of phosphatidylinositol-4,5-bisphosphate (PIP2) availability on SGN electrophysiology. Reducing PIP2 production using wortmannin, or sequestration of PIP2 using a palmitoylated peptide (PIP2-PP), slowed adaptation rate in SGN populations. PIP2-PP specifically inhibited the LVA current in SGNs, an effect reduced by intracellular dialysis of a nonhydrolysable analog of PIP2. PIP2-PP also inhibited heterologously expressed Kv1.1/Kv1.2 channels, recapitulating its effect in SGNs. Collectively, the data identify Kv1.1/Kv1.2 heteromeric channels as key regulators of action potential initiation and propagation in the auditory nerve, and suggest that modulation of these channels by endogenous phosphoinositides provides local control of membrane excitability.

SIGNIFICANCE STATEMENT

Rapid spike adaptation is an important feature of auditory neurons that preserves temporal precision. In spiral ganglion neurons, the primary afferents in the cochlea, adaptation is regulated by heteromeric ion channels composed of Kv1.1 and Kv1.2 subunits. These subunits colocalize to common functional microdomains, such as juxtaparanodes and the somatic membrane. Activity of the heteromeric channels is controlled by cellular availability of PIP2, a membrane phospholipid. This mechanism provides an intrinsic regulation of output from the auditory nerve, which could be targeted for therapeutic adjustment of hearing sensitivity.

Pepducins and Other Lipidated Peptides as Mechanistic Probes and Therapeutics

Lipopeptides based on the intracellular loops of cell-surface receptors, known as "Pepducins," represent a promising new class of compounds used for the study of membrane proteins and as potential therapeutics in a variety of diseases. Detailed knowledge of the three-dimensional structure of G-protein-coupled receptors (GPCRs) and delineation of the mechanisms of pepducin activation and biased G-protein signaling has facilitated the development of even more potent pepducin allosteric modulators.

Phosphoinositide control of membrane protein function: a frontier led by studies on ion channels

Anionic phospholipids are critical constituents of the inner leaflet of the plasma membrane, ensuring appropriate membrane topology of transmembrane proteins. Additionally, in eukaryotes, the negatively charged phosphoinositides serve as key signals not only through their hydrolysis products but also through direct control of transmembrane protein function. Direct phosphoinositide control of the activity of ion channels and transporters has been the most convincing case of the critical importance of phospholipid-protein interactions in the functional control of membrane proteins. Furthermore, second messengers, such as [Ca(2+)]i, or posttranslational modifications, such as phosphorylation, can directly or allosterically fine-tune phospholipid-protein interactions and modulate activity. Recent advances in structure determination of membrane proteins have allowed investigators to obtain complexes of ion channels with phosphoinositides and to use computational and experimental approaches to probe the dynamic mechanisms by which lipid-protein interactions control active and inactive protein states.

PIP2 regulates psychostimulant behaviors through its interaction with a membrane protein

Phosphatidylinositol (4,5)-bisphosphate (PIP₂) regulates the function of ion channels and transporters. Here, we demonstrate that PIP₂ directly binds the human dopamine (DA) transporter (hDAT), a key regulator of DA homeostasis and a target of the psychostimulant amphetamine (AMPH). This binding occurs through electrostatic interactions with positively charged hDAT N-terminal residues and is shown to facilitate AMPH-induced, DAT-mediated DA efflux and the psychomotor properties of AMPH. Substitution of these residues with uncharged amino acids reduces hDAT-PIP₂ interactions and AMPH-induced DA efflux, without altering the hDAT physiological function of DA uptake. We evaluated, for the first time, the significance of this interaction in vivo using locomotion as a behavioral assay in Drosophila melanogaster. Expression of mutated hDAT with reduced PIP₂ interaction in Drosophila DA neurons impairs AMPH-induced locomotion without altering basal locomotion. We present the first demonstration of how PIP₂ interactions with a membrane protein can regulate the behaviors of complex organisms.

Tamoxifen inhibition of kv7.2/kv7.3 channels

KCNQ genes encode five Kv7 K⁺ channel subunits (Kv7.1–Kv7.5). Four of these (Kv7.2–Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which regulates neuronal excitability. In this study, we demonstrate that tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits Kv7.2/Kv7.3 currents heterologously expressed in human embryonic kidney HEK-293 cells. Current inhibition by tamoxifen was voltage independent but concentration-dependent. The IC₅₀ for current inhibition was 1.68 ± 0.44 µM. The voltage-dependent activation of the channel was not modified. Tamoxifen inhibited Kv7.2 homomeric channels with a higher potency (IC₅₀ = 0.74 ± 0.16 µM). The mutation Kv7.2 R463E increases phosphatidylinositol- 4,5-bisphosphate (PIP₂) - channel interaction and diminished dramatically the inhibitory effect of tamoxifen compared with that for wild type Kv7.2. Conversely, the mutation Kv7.2 R463Q, which decreases PIP₂ -channel interaction, increased tamoxifen potency. Similar results were obtained on the heteromeric Kv7.2 R463Q/Kv7.3 and Kv7.2 R463E/Kv7.3 channels, compared to Kv7.2/Kv7.3 WT. Overexpression of type 2A PI(4)P5-kinase (PIP5K 2A) significantly reduced tamoxifen inhibition of Kv7.2/Kv7.3 and Kv7.2 R463Q channels. Our results suggest that tamoxifen inhibited Kv7.2/Kv7.3 channels by interfering with PIP₂-channel interaction because of its documented interaction with PIP₂ and the similar effect of tamoxifen on various PIP2 sensitive channels.

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.

Amphetamine actions at the serotonin transporter rely on the availability of phosphatidylinositol-4,5-bisphosphate

Nerve functions require phosphatidylinositol-4,5-bisphosphate (PIP2) that binds to ion channels, thereby controlling their gating. Channel properties are also attributed to serotonin transporters (SERTs); however, SERT regulation by PIP2 has not been reported. SERTs control neurotransmission by removing serotonin from the extracellular space. An increase in extracellular serotonin results from transporter-mediated efflux triggered by amphetamine-like psychostimulants. Herein, we altered the abundance of PIP2 by activating phospholipase-C (PLC), using a scavenging peptide, and inhibiting PIP2-synthesis. We tested the effects of the verified scarcity of PIP2 on amphetamine-triggered SERT functions in human cells. We observed an interaction between SERT and PIP2 in pull-down assays. On decreased PIP2 availability, amphetamine-evoked currents were markedly reduced compared with controls, as was amphetamine-induced efflux. Signaling downstream of PLC was excluded as a cause for these effects. A reduction of substrate efflux due to PLC activation was also found with recombinant noradrenaline transporters and in rat hippocampal slices. Transmitter uptake was not affected by PIP2 reduction. Moreover, SERT was revealed to have a positively charged binding site for PIP2. Mutation of the latter resulted in a loss of amphetamine-induced SERT-mediated efflux and currents, as well as a lack of PIP2-dependent effects. Substrate uptake and surface expression were comparable between mutant and WT SERTs. These findings demonstrate that PIP2 binding to monoamine transporters is a prerequisite for amphetamine actions without being a requirement for neurotransmitter uptake. These results open the way to target amphetamine-induced SERT-dependent actions independently of normal SERT function and thus to treat psychostimulant addiction.

Voltage-dependent open-channel block of G protein-gated inward-rectifying K(+) (GIRK) current in rat atrial myocytes by tamoxifen

Tamoxifen (Tmx) is a nonsteroidal selective estrogen receptor antagonist and is frequently used in the treatment and prevention of breast cancer. The compound and its metabolites have been reported to inhibit functions of different classes of membrane proteins, including various ion channels. For members of the inward-rectifying K(+) (Kir) channel family, interference of Tmx with binding of phosphatidylinositol 4,5-bisphosphate (PIP(2)) has been suggested as the mechanism underlying such inhibition. We have studied the inhibition of G protein-activated K(+) (GIRK) current by Tmx in isolated myocytes from hearts of adult rats using whole-cell voltage clamp and experimental conditions for measuring K(+) currents as inward currents (E (K) -50 mV; holding potential -90 mV). Extracellular Tmx reversibly inhibited GIRK current activated by acetylcholine (I (K(ACh))) with an EC(50) of 7.4 × 10(-7) M. This inhibition was composed of two components, a basal reduction in peak current and a block that required opening of channels by ACh. The open-channel block was partially relieved by depolarizing voltage steps in a voltage- and time-dependent fashion. A voltage-dependent open-channel block was not observed when I (K(ACh)) was measured as outward current (E (K) -90 mV; holding potential -40 mV). Intracellular application of Tmx via the patch clamp pipette at a concentration (7 × 10(-6) M) that caused a rapid inhibition of I (K(ACh)) upon extracellular application did not affect the current. Intracellular application of the H(2)O-soluble PIP(2) analog diC(8)-PIP(2) reduced the voltage-independent component of inhibition but had no effect on voltage-dependent open-channel block. The effects of 4-hydroxy-Tmx, a major active metabolite, tested at 2 × 10(-6) M, had effects on I (K(ACh)) analogous to those of Tmx. Inhibition of constitutive inward-rectifying K(+) current (I (K1)) in ventricular myocytes, carried by Kir2 complexes, by Tmx was devoid of a voltage-dependent component. This study suggests the voltage-dependent open-channel block of GIRK inward current as a novel mechanism of Tmx action.

Turning receptors on and off with intracellular pepducins: new insights into G-protein-coupled receptor drug development

G-protein-coupled receptors (GPCRs) are a large family of remarkably versatile membrane proteins that are attractive therapeutic targets because of their involvement in a vast range of normal physiological processes and pathological diseases. Upon activation, intracellular domains of GPCRs mediate signaling to G-proteins, but these domains have yet to be effectively exploited as drug targets. Cell-penetrating lipidated peptides called pepducins target specific intracellular loops of GPCRs and have recently emerged as effective allosteric modulators of GPCR activity. The lipid moiety facilitates translocation across the plasma membrane, where pepducins then specifically modulate signaling of their cognate receptor. To date, pepducins and related lipopeptides have been shown to specifically modulate the activity of diverse GPCRs and other membrane proteins, including protease-activated receptors (PAR1, PAR2, and PAR4), chemokine receptors (CXCR1, CXCR2, and CXCR4), sphingosine 1-phosphate receptor-3 (S1P3), the melanocortin-4 receptor, the Smoothened receptor, formyl peptide receptor-2 (FPR2), the relaxin receptor (LGR7), G-proteins (Gα(q/11/o/13)), muscarinic acetylcholine receptor and vanilloid (TRPV1) channels, and the GPIIb integrin. This minireview describes recent advances made using pepducin technology in targeting diverse GPCRs and the use of pepducins in identifying potential novel drug targets.

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.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Cholesterol inhibits M-type K+ channels via protein kinase C-dependent phosphorylation in sympathetic neurons

M-type (KCNQ) potassium channels play an important role in regulating the action potential firing in neurons. Here, we investigated the effect of cholesterol on M current in superior cervical ganglion (SCG) sympathetic neurons, using the patch clamp technique. M current was inhibited in a dose-dependent manner by cholesterol loading with a methyl-beta-cyclodextrin-cholesterol complex. This effect was prevented when membrane cholesterol level was restored by including empty methyl-beta-cyclodextrin in the pipette solution. Dialysis of cells with AMP-PNP instead of ATP prevented cholesterol action on M currents. Protein kinase C (PKC) inhibitor, calphostin C, abolished cholesterol-induced inhibition whereas the PKC activator, PDBu, mimicked the inhibition of M currents by cholesterol. The in vitro kinase assay showed that KCNQ2 subunits of M channel can be phosphorylated by PKC. A KCNQ2 mutant that is defective in phosphorylation by PKC failed to show current inhibition not only by PDBu but also by cholesterol. These results indicate that cholesterol-induced inhibition of M currents is mediated by PKC phosphorylation. The inhibition of M currents by PDBu and cholesterol was completely blocked by PIP(2) loading, indicating that the decrease in PIP(2)-channel interaction underlies M channel inhibition by PKC-mediated phosphorylation. We conclude that cholesterol specifically regulates M currents in SCG neurons via PKC activation.

Ca2+/calmodulin disrupts AKAP79/150 interactions with KCNQ (M-Type) K+ channels

M-type channels are localized to neuronal, cardiovascular, and epithelial tissues, where they play critical roles in control of excitability and K(+) transport, and are regulated by numerous receptors via G(q/11)-mediated signals. One pathway shown for KCNQ2 and muscarinic receptors uses PKC, recruited to the channels by A-kinase anchoring protein (AKAP)79/150. As M-type channels can be variously composed of KCNQ1-5 subunits, and M current is known to be regulated by Ca(2+)/calmodulin (CaM) and PIP(2), we probed the generality of AKAP79/150 actions among KCNQ1-5 channels, and the influence of Ca(2+)/CaM and PIP(2) on AKAP79/150 actions. We first examined which KCNQ subunits are targeted by AKAP79 in Chinese hamster ovary (CHO) cells heterologously expressing KCNQ1-5 subunits and AKAP79, using fluorescence resonance energy transfer (FRET) under total internal reflection fluorescence (TIRF) microscopy, and patch-clamp analysis. Donor-dequenching FRET between CFP-tagged KCNQ1-5 and YFP-tagged AKAP79 revealed association of KCNQ2-5, but not KCNQ1, with AKAP79. In parallel with these results, CHO cells stably expressing M(1) receptors studied under perforated patch-clamp showed cotransfection of AKAP79 to "sensitize" KCNQ2/3 heteromers and KCNQ2-5, but not KCNQ1, homomers to muscarinic inhibition, manifested by shifts in the dose-response relations to lower concentrations. The effect on KCNQ4 was abolished by the T553A mutation of the putative PKC phosphorylation site. We then probed the role of CaM and PIP(2) in these AKAP79 actions. TIRF/FRET experiments revealed cotransfection of wild-type, but not dominant-negative (DN), CaM that cannot bind Ca(2+), to disrupt the interaction of YFP-tagged AKAP79(1-153) with CFP-tagged KCNQ2-5. Tonic depletion of PIP(2) by cotransfection of a PIP(2) phosphatase had no effect, and sudden depletion of PIP(2) did not delocalize GFP-tagged AKAP79 from the membrane. Finally, patch-clamp experiments showed cotransfection of wild-type, but not DN, CaM to prevent the AKAP79-mediated sensitization of KCNQ2/3 heteromers to muscarinic inhibition. Thus, AKAP79 acts on KCNQ2-5, but not KCNQ1-containing channels, with effects disrupted by calcified CaM, but not by PIP(2) depletion.

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

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

Differential contribution of SNARE-dependent exocytosis to inflammatory potentiation of TRPV1 in nociceptors

Potentiation of the pain-integrator ion channel transient receptor potential vanilloid type 1 (TRPV1) underlies thermal hyperalgesia mediated by a variety of proinflammatory factors. Two complementary mechanisms of TRPV1 inflammatory sensitization have been proposed, namely a decrease of its activation threshold and an increment of its surface expression in nociceptors. Here we investigated the involvement of regulated exocytosis to the inflammatory sensitization of TRPV1 in rat neonatal dorsal root ganglion neurons by proalgesic agents. The contribution of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent exocytosis was evaluated using a small peptide patterned after the synaptosomal-associated protein of 25 kDa (SNAP-25) protein that acts as a specific and potent inhibitor of neuronal exocytosis. We found that TRPV1 sensitization mediated by nerve growth factor, ATP, and IGF-I was accompanied by a higher channel expression in the neuronal plasma membrane, which was prevented by blockade of regulated exocytosis. In contrast, TRPV1 sensitization caused by bradykinin, IL-1beta, and artemin was insensitive to inhibition of SNARE-dependent vesicular fusion and was not due to an increase in TRPV1 surface expression. Therefore, it appears that some, but not all, proinflammatory agents sensitize rat nociceptors by promoting the recruitment of TRPV1 channels to the neuronal surface. These findings support the tenet that SNARE complex-mediated exocytosis of TRPV1 may be a valid therapeutic target to treat inflammatory pain.

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.

Neural KCNQ (Kv7) channels

KCNQ genes encode five Kv7 K(+) channel subunits (Kv7.1-Kv7.5). Four of these (Kv7.2-Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which widely regulates neuronal excitability, although other subunits may contribute to M-like currents in some locations. M-channels are closed by receptors coupled to Gq such as M1 and M3 muscarinic receptors; this increases neuronal excitability and underlies some forms of cholinergic excitation. Muscarinic closure results from activation of phospholipase C and consequent hydrolysis and depletion of membrane phosphatidylinositol-4,5-bisphosphate, which is required for channel opening. Some effects of M-channel closure, determined from transmitter action, selective blocking drugs (linopirdine and XE991) and KCNQ2 gene disruption or manipulation, are as follows: (i) in sympathetic neurons: facilitation of repetitive discharges and conversion from phasic to tonic firing; (ii) in sensory nociceptive systems: facilitation of A-delta peripheral sensory fibre responses to noxious heat; and (iii) in hippocampal pyramidal neurons: facilitation of repetitive discharges, enhanced after-depolarization and burst-firing, and induction of spontaneous firing through a reduction of action potential threshold at the axon initial segment. Several drugs including flupirtine and retigabine enhance neural Kv7/M-channel activity, principally through a hyperpolarizing shift in their voltage gating. In consequence they reduce neural excitability and can inhibit nociceptive stimulation and transmission. Flupirtine is in use as a central analgesic; retigabine is under clinical trial as a broad-spectrum anticonvulsant and is an effective analgesic in animal models of chronic inflammatory and neuropathic pain.

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

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

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.

Functional significance of axonal Kv7 channels in hippocampal pyramidal neurons

Members of the Kv7 family (Kv7.2-Kv7.5) generate a subthreshold K(+) current, the M- current. This regulates the excitability of many peripheral and central neurons. Recent evidence shows that Kv7.2 and Kv7.3 subunits are targeted to the axon initial segment of hippocampal neurons by association with ankyrin G. Further, spontaneous mutations in these subunits that impair axonal targeting cause human neonatal epilepsy. However, the precise functional significance of their axonal location is unknown. Using electrophysiological techniques together with a peptide that selectively disrupts axonal Kv7 targeting (ankyrin G-binding peptide, or ABP) and other pharmacological tools, we show that axonal Kv7 channels are critically and uniquely required for determining the inherent spontaneous firing of hippocampal CA1 pyramids, independently of alterations in synaptic activity. This action was primarily because of modulation of action potential threshold and resting membrane potential (RMP), amplified by control of intrinsic axosomatic membrane properties. Computer simulations verified these data when the axonal Kv7 density was three to five times that at the soma. The increased firing caused by axosomatic Kv7 channel block backpropagated into distal dendrites affecting their activity, despite these structures having fewer functional Kv7 channels. These results indicate that axonal Kv7 channels, by controlling axonal RMP and action potential threshold, are fundamental for regulating the inherent firing properties of CA1 hippocampal neurons.

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.

The C-terminus of Kv7 channels: a multifunctional module

Kv7 channels (KCNQ) represent a family of voltage-gated K(+) channels which plays a prominent role in brain and cardiac excitability. Their physiological importance is underscored by the existence of mutations in human Kv7 genes, leading to severe cardiovascular and neurological disorders such as the cardiac long QT syndrome and neonatal epilepsy. Kv7 channels exhibit some structural and functional features that are distinct from other Kv channels. Notably, the Kv7 C-terminus is long compared to other K(+) channels and is endowed with characteristic structural domains, including coiled-coils, amphipatic alpha helices containing calmodulin-binding motifs and basic amino acid clusters. Here we provide a brief overview of current insights and as yet unsettled issues about the structural and functional attributes of the C-terminus of Kv7 channels. Recent data indicate that the proximal half of the Kv7 C-terminus associates with one calmodulin constitutively bound to each subunit. Epilepsy and long QT mutations located in this proximal region impair calmodulin binding and can affect channel gating, folding and trafficking. The distal half of the Kv7 C-terminus directs tetramerization, employing tandem coiled-coils. Together, the data indicate that the Kv7 C-terminal domain is a multimodular structure playing a crucial role in channel gating, assembly and trafficking as well as in scaffolding the channel complex with signalling proteins.

Somatodendritic release of glutamate regulates synaptic inhibition in cerebellar Purkinje cells via autocrine mGluR1 activation

In the cerebellum, the process of retrograde signaling via presynaptic receptors is important for the induction of short- and long-term changes in inhibitory synaptic transmission at interneuron-Purkinje cell (PC) synapses. Endocannabinoids, by activating presynaptic CB1 receptors, mediate a short-term decrease in inhibitory synaptic efficacy, whereas glutamate, acting on presynaptic NMDA receptors, induces a longer-latency sustained increase in GABA release. We now demonstrate that either low-frequency climbing fiber stimulation or direct somatic depolarization of Purkinje cells results in SNARE-dependent vesicular release of glutamate from the soma and dendrites of PCs. The activity-dependent release of glutamate caused the activation of postsynaptic metabotropic glutamate receptor 1 (mGluR1) on PC somatodendritic membranes, resulting in the cooperative release of endocannabinoids and an mGluR1-mediated slow membrane conductance. The activity of excitatory amino acid transporters regulated the spatial spread of glutamate and thus the extent of PC mGluR1 activation. We propose that activity-dependent somatodendritic glutamate release and autocrine activation of mGluR1 on PCs provides a powerful homeostatic mechanism to dynamically regulate inhibitory synaptic transmission in the cerebellar cortex.

Regulation of M(Kv7.2/7.3) channels in neurons by PIP(2) and products of PIP(2) hydrolysis: significance for receptor-mediated inhibition

M-channels are voltage-gated K+ channels that regulate the excitability of many neurons. They are composed of Kv7 (KCNQ) family subunits, usually Kv7.2 + Kv7.3. Native M-channels and expressed Kv7.2 + 7.3 channels are inhibited by stimulating G(q/11)-coupled receptors - prototypically the M1 muscarinic acetylcholine receptor (M1-mAChR). The channels require membrane phosphatidylinositol-4,5-bisphosphate (PIP(2)) to open and the effects of mAChR stimulation result primarily from the reduction in membrane PIP(2) levels following G(q)/phospholipase C-catalysed PIP(2) hydrolysis. However, in sympathetic neurons, M-current inhibition by bradykinin appears to be mediated through the release and action of intracellular Ca(2)+ by inositol-1,4,5-trisphosphate (IP(3)), a product of PIP(2) hydrolysis, rather than by PIP(2) depletion. We have therefore compared the effects of bradykinin and oxotremorine-M (a muscarinic agonist) on membrane PIP(2) in sympathetic neurons using a fluorescently tagged mutated C-domain of the PIP(2) binding probe, 'tubby'. In concentrations producing equal M-current inhibition, bradykinin produced about one-quarter of the reduction in PIP(2) produced by oxotremorine-M, but equal reduction when PIP(2) synthesis was blocked with wortmannin. Likewise, wortmannin restored bradykinin-induced M-current inhibition when Ca(2)+ release was prevented with thapsigargin. Thus, inhibition by bradykinin can use product (IP(3)/Ca(2)+)-dependent or substrate (PIP(2)) dependent mechanisms, depending on Ca(2)+ availability and PIP(2) synthesis rates.

Decrease in PIP(2) channel interactions is the final common mechanism involved in PKC- and arachidonic acid-mediated inhibitions of GABA(B)-activated K+ current

We showed in our previous study that in hippocampal CA1 neurons the stimulation of muscarinic receptors inhibited the GIRK current (I(GIRK)) via a PLC/PKC pathway, whereas group I metabotropic glutamate receptors (mGluR) inhibited I(GIRK) via a PLA(2)/arachidonic acid pathway. In this study, we present evidence that receptor-mediated signalling pathways activated by the two G(q)-coupled receptors (G(q)PCRs) converge on the inhibition of GIRK channel-PIP(2) interaction. I(GIRK) was activated in acutely isolated hippocampal CA1 neurons by repetitive application of baclofen, a GABA(B) receptor agonist, with a 2-3 min interval. When both CCh and DHPG were pretreated before the second I(GIRK) activation, the magnitude of the second I(GIRK) was 52.2 +/- 2.5% of the first I(GIRK), which was not significantly different from the magnitude of inhibition by CCh or DHPG alone. This result shows that the effects of muscarinic receptor and group I mGluR stimulation on I(GIRK) are not additive but occlusive, suggesting that each pathway may converge to a common mechanism that finally regulates I(GIRK). To test the involvement of PIP(2) in this mechanism, the effect of CCh and DHPG on I(GIRK) was tested in cells loaded with exogenous PIP(2). The inhibition of I(GIRK) by CCh or DHPG was almost completely abolished in PIP(2)-loaded cells. We confirmed that the inhibition of I(GIRK) by direct application of phorbol ester or arachidonic acid was also completely reversed in PIP(2)-loaded cells. These results indicate that the decrease in PIP(2)-channel interactions is the final common mechanism responsible for G(q)PCR-induced inhibitions of I(GIRK) mediated by PKC and arachidonic acid.

SNAP-23 and syntaxin-2 localize to the extracellular surface of the platelet plasma membrane

SNARE proteins direct membrane fusion events required for platelet granule secretion. These proteins are oriented in cell membranes such that most of the protein resides in a cytosolic compartment. Evaluation of SNARE protein localization in activated platelets using immunonanogold staining and electron microscopy, however, demonstrated expression of SNAP-23 and syntaxin-2 on the extracellular surface of the platelet plasma membrane. Flow cytometry of intact platelets confirmed trypsin-sensitive SNAP-23 and syntaxin-2 localization to the extracellular surface of the plasma membrane. Acyl-protein thioesterase 1 and botulinum toxin C light chain released SNAP-23 and syntaxin-2, respectively, from the surface of intact platelets. When resting platelets were incubated with both acyl-protein thioesterase 1 and botulinum toxin C light chain, a complex that included both SNAP-23 and syntaxin-2 was detected in supernatants, indicating that extracellular SNARE proteins retain their ability to bind one another. These observations represent the first description of SNARE proteins on the extracellular surface of a cell.

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.