Characteristic interactions with phosphatidylinositol 4,5-bisphosphate determine regulation of kir channels by diverse modulators

PIP2 is a necessary cofactor for ion channel function: how and why?

Phosphatidylinositol 4,5-bisphosphate (PIP2) is a minority phospholipid of the inner leaflet of plasma membranes. Many plasma membrane ion channels and ion transporters require PIP2 to function and can be turned off by signaling pathways that deplete PIP2. This review discusses the dependence of ion channels on phosphoinositides and considers possible mechanisms by which PIP2 and analogues regulate ion channel activity.

Electrostatic interaction of internal Mg2+ with membrane PIP2 Seen with KCNQ K+ channels

Activity of KCNQ (Kv7) channels requires binding of phosphatidylinositol 4,5-bisphosphate (PIP₂) from the plasma membrane. We give evidence that Mg²⁺ and polyamines weaken the KCNQ channel–phospholipid interaction. Lowering internal Mg²⁺ augmented inward and outward KCNQ currents symmetrically, and raising Mg²⁺ reduced currents symmetrically. Polyvalent organic cations added to the pipette solution had similar effects. Their potency sequence followed the number of positive charges: putrescine (+2) < spermidine (+3) < spermine (+4) < neomycin (+6) < polylysine (≫+6). The inhibitory effects of Mg²⁺ were reversible with sequential whole-cell patching. Internal tetraethylammonium ion (TEA) gave classical voltage-dependent block of the pore with changes of the time course of K⁺ currents. The effect of polyvalent cations was simpler, symmetric, and without changes of current time course. Overexpression of phosphatidylinositol 4-phosphate 5-kinase Iγ to accelerate synthesis of PIP₂ attenuated the sensitivity to polyvalent cations. We suggest that Mg²⁺ and other polycations reduce the currents by electrostatic binding to the negative charges of PIP₂, competitively reducing the amount of free PIP₂ available for interaction with channels. The dose–response curves could be modeled by a competition model that reduces the pool of free PIP₂. This mechanism is likely to modulate many other PIP₂-dependent ion channels and cellular processes.

Molecular determinants of PI(4,5)P2 and PI(3,4,5)P3 regulation of the epithelial Na+ channel

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) are physiologically important second messengers. These molecules bind effector proteins to modulate activity. Several types of ion channels, including the epithelial Na⁺ channel (ENaC), are phosphoinositide effectors capable of directly interacting with these signaling molecules. Little, however, is known of the regions within ENaC and other ion channels important to phosphoinositide binding and modulation. Moreover, the molecular mechanism of this regulation, in many instances, remains obscure. Here, we investigate modulation of ENaC by PI(3,4,5)P₃ and PI(4,5)P₂ to begin identifying the molecular determinants of this regulation. We identify intracellular regions near the inner membrane interface just following the second transmembrane domains in β- and γ- but not α-ENaC as necessary for PI(3,4,5)P₂ but not PI(4,5)P₂ modulation. Charge neutralization of conserved basic amino acids within these regions demonstrated that these polar residues are critical to phosphoinositide regulation. Single channel analysis, moreover, reveals that the regions just following the second transmembrane domains in β- and γ-ENaC are critical to PI(3,4,5)P₃ augmentation of ENaC open probability, thus, defining mechanism. Unexpectedly, intracellular domains within the extreme N terminus of β- and γ-ENaC were identified as being critical to down-regulation of ENaC activity and P_o in response to depletion of membrane PI(4,5)P₂. These regions of the channel played no identifiable role in a PI(3,4,5)P₃ response. Again, conserved positive-charged residues within these domains were particularly important, being necessary for exogenous PI(4,5)P₂ to increase open probability. We conclude that β and γ subunits bestow phosphoinositide sensitivity to ENaC with distinct regions of the channel being critical to regulation by PI(3,4,5)P₃ and PI(4,5)P₂. This argues that these phosphoinositides occupy distinct ligand-binding sites within ENaC to modulate open probability.

PIP2--the master key

The function of inwardly rectifying K+ (Kir) channels is highly diverse and therefore is tightly regulated by various environmental factors. In their article in this issue of Neuron, Rapedius et al. recognize a conserved structural mechanism for Kir channels gating by both pH and PIP2. In light of these findings and accumulated knowledge, PIP2 is suggested to have a common coregulatory role in the gating of Kir channels by all their soluble modulators.

Estrogen regulation of genes important for K+ channel signaling in the arcuate nucleus

Estrogen affects the electrophysiological properties of a number of hypothalamic neurons by modulating K(+) channels via rapid membrane actions and/or changes in gene expression. The interaction between these pathways (membrane vs. transcription) ultimately determines the effects of estrogen on hypothalamic functions. Using suppression subtractive hybridization, we produced a cDNA library of estrogen-regulated, brain-specific guinea pig genes, which included subunits from three prominent K+ channels (KCNQ5, Kir2.4, Kv4.1, and Kvbeta(1)) and signaling molecules that impact channel function including phosphatidylinositol 3-kinase (PI3K), protein kinase Cepsilon (PKCepsilon), cAMP-dependent protein kinase (PKA), A-kinase anchor protein (AKAP), phospholipase C (PLC), and calmodulin. Based on these findings, we dissected the arcuate nucleus from ovariectomized guinea pigs treated with estradiol benzoate (EB) or vehicle and analyzed mRNA expression using quantitative real-time PCR. We found that EB significantly increased the expression of KCNQ5 and Kv4.1 and decreased expression of KCNQ3 and AKAP in the rostral arcuate. In the caudal arcuate, EB increased KCNQ5, Kir2.4, Kv4.1, calmodulin, PKCepsilon, PLCbeta(4), and PI3Kp55gamma expression and decreased Kvbeta(1). The effects of estrogen could be mediated by estrogen receptor-alpha, which we found to be highly expressed in the guinea pig arcuate nucleus and, in particular, proopiomelanocortin neurons. In addition, single-cell RT-PCR analysis revealed that about 50% of proopiomelanocortin and neuropeptide Y neurons expressed KCNQ5, about 40% expressed Kir2.4, and about 60% expressed Kv4.1. Therefore, it is evident that the diverse effects of estrogen on arcuate neurons are mediated in part by regulation of K(+) channel expression, which has the potential to affect profoundly neuronal excitability and homeostatic functions, especially when coupled with the rapid effects of estrogen on K(+) channel function.

Cholinergic modulation of Kir2 channels selectively elevates dendritic excitability in striatopallidal neurons

Dopamine-depleting lesions of the striatum that mimic Parkinson's disease induce a profound pruning of spines and glutamatergic synapses in striatopallidal medium spiny neurons, leaving striatonigral medium spiny neurons intact. The mechanisms that underlie this cell type-specific loss of connectivity are poorly understood. The Kir2 K(+) channel is an important determinant of dendritic excitability in these cells. Here we show that opening of these channels is potently reduced by signaling through M1 muscarinic receptors in striatopallidal neurons, but not in striatonigral neurons. This asymmetry could be attributed to differences in the subunit composition of Kir2 channels. Dopamine depletion alters the subunit composition further, rendering Kir2 channels in striatopallidal neurons even more susceptible to modulation. Reduced opening of Kir2 channels enhances dendritic excitability and synaptic integration. This cell type-specific enhancement of dendritic excitability is an essential trigger for synaptic pruning after dopamine depletion, as pruning was prevented by genetic deletion of M1 muscarinic receptors.

Sensitization of TRPA1 by PAR2 contributes to the sensation of inflammatory pain

Proinflammatory agents trypsin and mast cell tryptase cleave and activate PAR2, which is expressed on sensory nerves to cause neurogenic inflammation. Transient receptor potential A1 (TRPA1) is an excitatory ion channel on primary sensory nerves of pain pathway. Here, we show that a functional interaction of PAR2 and TRPA1 in dorsal root ganglion (DRG) neurons could contribute to the sensation of inflammatory pain. Frequent colocalization of TRPA1 with PAR2 was found in rat DRG neurons. PAR2 activation increased the TRPA1 currents evoked by its agonists in HEK293 cells transfected with TRPA1, as well as DRG neurons. Application of phospholipase C (PLC) inhibitors or phosphatidylinositol-4,5-bisphosphate (PIP(2)) suppressed this potentiation. Decrease of plasma membrane PIP(2) levels through antibody sequestration or PLC-mediated hydrolysis mimicked the potentiating effects of PAR2 activation at the cellular level. Thus, the increased TRPA1 sensitivity may have been due to activation of PLC, which releases the inhibition of TRPA1 from plasma membrane PIP(2). These results identify for the first time to our knowledge a sensitization mechanism of TRPA1 and a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by TRPA1 activation.

Protein kinase C dependent inhibition of the heteromeric Kir4.1-Kir5.1 channel

Heteromultimerization of Kir4.1 and Kir5.1 leads to a channel with distinct functional properties. The heteromeric Kir4.1-Kir5.1 channel is expressed in the eye, kidney and brainstem and has CO(2)/pH sensitivity in the physiological range, suggesting a candidate molecule for the regulation of K(+) homeostasis and central CO(2) chemoreception. It is known that K(+) transport in renal epithelium and brainstem CO(2) chemosensitivity are subject to modulation by hormones and neurotransmitters that activate distinct intracellular signaling pathways. If the Kir4.1-Kir5.1 channel is involved in pH-dependent regulation of cellular functions, it may also be regulated by some of the intracellular signaling systems. Therefore, we undertook studies to determine whether PKC modulates the heteromeric Kir4.1-Kir5.1 channel. The channel expressed using a Kir4.1-Kir5.1 tandem dimer construct was inhibited by the PKC activator PMA in a dose-dependent manner. The channel inhibition was produced via reduction of the P(open). The effect of PMA was abolished by specific PKC inhibitors. In contrast, exposure of oocytes to forskolin (a PKA activator) had no significant effect on Kir4.1-Kir5.1 currents. The channel inhibition appeared to be independent of PIP(2) depletion and PKC-dependent internalization. Several consensus sequences of potential PKC phosphorylation sites were identified in the Kir4.1 and Kir5.1 subunits by sequence scan. Although the C-terminal peptides of both Kir4.1 and Kir5.1 were phosphorylated in vitro, site-directed mutagenesis of individual residues failed to reveal the PKC phosphorylation sites suggesting that the channel may have multiple phosphorylation sites. Taken together, these results suggest that the Kir4.1-Kir5.1 but not the homomeric Kir4.1 channel is strongly inhibited by PKC activation.

Crystal structure of a Kir3.1-prokaryotic Kir channel chimera

The Kir3.1 K(+) channel participates in heart rate control and neuronal excitability through G-protein and lipid signaling pathways. Expression in Escherichia coli has been achieved by replacing three fourths of the transmembrane pore with the pore of a prokaryotic Kir channel, leaving the cytoplasmic pore and membrane interfacial regions of Kir3.1 origin. Two structures were determined at 2.2 A. The selectivity filter is identical to the Streptomyces lividans K(+) channel within error of measurement (r.m.s.d.<0.2 A), suggesting that K(+) selectivity requires extreme conservation of three-dimensional structure. Multiple K(+) ions reside within the pore and help to explain voltage-dependent Mg(2+) and polyamine blockade and strong rectification. Two constrictions, at the inner helix bundle and at the apex of the cytoplasmic pore, may function as gates: in one structure the apex is open and in the other, it is closed. Gating of the apex is mediated by rigid-body movements of the cytoplasmic pore subunits. Phosphatidylinositol 4,5-biphosphate-interacting residues suggest a possible mechanism by which the signaling lipid regulates the cytoplasmic pore.

Diverse Kir modulators act in close proximity to residues implicated in phosphoinositide binding

Inwardly rectifying potassium (Kir) channels were the first shown to be directly activated by phosphoinositides in general and phosphatidylinositol bisphosphate (PIP(2)) in particular. Atomic resolution structures have been determined for several mammalian and bacterial Kir channels. Basic residues, identified through mutagenesis studies to contribute to the sensitivity of the channel to PIP(2), have been mapped onto the three dimensional channel structure and their localization has given rise to a plausible model that can explain channel activation by PIP(2). Moreover, mapping onto the three-dimensional channel structure sites involved in the modulation of Kir channel activity by a diverse group of regulatory molecules, revealed a striking proximity to residues implicated in phosphoinositide binding. These observations support the hypothesis that the observed dependence of diverse modulators on channel-PIP(2) interactions stems from their localization within distances that can affect PIP(2)-interacting residues.

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.

Phosphatidylinositol 4,5-bisphosphate interactions with the HERG K(+) channel

Regulation of ion channel activity plays a central role in controlling heart rate, rhythm, and contractility responses to cardiovascular demands. Dynamic beat-to-beat regulation of ion channels is precisely adjusted by autonomic stimulation of cardiac G protein-coupled receptors. The rapidly activating delayed rectifier K(+) current (I (Kr)) is produced by the channel that is encoded by human ether-a-gogo-related gene (HERG) and is essential for the proper repolarization of the cardiac myocyte at the end of each action potential. Reduction of I (Kr) via HERG mutations or drug block can lead to lethal cardiac tachyarrhythmias. Autonomic regulation of HERG channels is an area of active investigation with the emerging picture of a complex interplay of signal transduction events, including kinases, second messengers, and protein-protein interactions. A recently described pathway for regulation of HERG is through channel interaction with the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). Changes in cellular PIP2 concentrations may occur with Gq-coupled receptor activation. Here, we review the evidence for PIP2-HERG interactions, its potential biological significance, and unfilled gaps in our understanding of this regulatory mechanism.

Activation of inwardly rectifying potassium (Kir) channels by phosphatidylinosital-4,5-bisphosphate (PIP2): Interaction with other regulatory ligands

All members of the inwardly rectifying potassium channels (Kir1-7) are regulated by the membrane phospholipid, phosphatidylinosital-4,5-bisphosphate (PIP(2)). Some are also modulated by other regulatory factors or ligands such as ATP and G-proteins, which give them their common names, such as the ATP sensitive potassium (K(ATP)) channel and the G-protein gated potassium channel. Other more non-specific regulators include polyamines, kinases, pH and Na(+) ions. Recent studies have demonstrated that PIP(2) acts cooperatively with other regulatory factors to modulate Kir channels. Here we review how PIP(2) and co-factors modulate channel activities in each subfamily of the Kir channels.

Molecular characteristics of phosphoinositide binding

Phosphoinositides in general and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2) or PIP(2)) in particular have been recently found to function as important regulators of ion channels. Yet, while specific residues have been identified that affect channel-PIP(2) interactions, the precise binding site of PIP(2) has not been determined in any case. In addition to binding ion channels, however, phosphoinositides interact with a plethora of other proteins, and in a number of cases, the crystallographic structures of the complexes have been determined. Based on a database of 25 complexed crystallographic structures, we have addressed the molecular characteristics of phosphoinositide binding to proteins. Implications to phosphoinositide binding to ion channels are also discussed.

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.

Cell content of phosphatidylinositol (4,5)bisphosphate in Ehrlich mouse ascites tumour cells in response to cell volume perturbations in anisotonic and in isosmotic media

The labelling pattern of cellular phosphoinositides (PtdInsP(n)) was studied in Ehrlich ascites cells labelled in vivo for 24 h with myo-[2-(3)H]- or l-myo-[1-(3)H]inositol and exposed to anisotonic or isosmotic volume perturbations. In parallel experiments the cell volume ([(14)C]3-OMG space) was monitored. In hypotonic media the cells initially swelled osmotically and subsequently as expected showed a regulatory volume decrease (RVD) response. Concurrently, the cell content of PtdInsP(2) showed a marked, transient decrease and the content of PtdInsP a small, transient increase. The changes in PtdInsP(2) and PtdInsP content increased progressively with the extent of hypotonicity (in the range 1.00-0.50 relative osmolarity). No evidence was found for either hydrolysis of PtdInsP(2) or formation of PtdInsP(3). In hypertonic medium (relative osmolarity 1.50), cells initially shrank osmotically and subsequently as expected showed a small regulatory volume increase (RVI) response. Concurrently, the cell content of PtdInsP(2) showed a marked increase and the content of PtdInsP a small decrease, i.e. changes in the opposite direction of those seen in hypotonic media. In isosmotic media with high (100 mm) or low (0.8 mm) K(+) concentration, cells slowly swelled or shrank due to uptake or loss of isosmotic KCl. Under these conditions, with largely unchanged intracellular ionic strength, the cell content of PtdInsP(2) and PtdInsP remained constant. Our results show that PtdInsP(2) is not volume sensitive per se, and moreover that the regulatory volume adjustments in Ehrlich ascites cells are not mediated by PtdInsP(2) hydrolysis and its subsequent production of second messengers. The simplest interpretation of the observed effects would be that PtdInsP(2) is controlled by ionic strength, probably via activation/inhibition of phosphoinositide-specific phosphatases/kinases. In Ehrlich ascites cells, as shown previously, the opposing ion channels and transporters activated during RVD and RVI, respectively, are controlled with tight negative coordination by a common cell volume 'set-point' that is shifted in anisotonic media, but unchanged during cell swelling in isosmotic high K(+) medium. We hypothesize that PtdInsP(2) might orchestrate this 'set-point' shift.

Phosphatidylinositol-4,5-bisphosphate regulates NMDA receptor activity through alpha-actinin

Phosphatidylinositol-4,5-bisphosphate (PIP2) has been shown to regulate many ion channels, transporters, and other signaling proteins, but it is not known whether it also regulates neurotransmitter-gated channels. The NMDA receptors (NMDARs) are gated by glutamate and serve as a critical control point in synaptic function. Here we demonstrate that PIP2 supports NMDAR activity. In Xenopus oocytes, overexpression of phospholipase Cgamma (PLCgamma) or preincubation with 10 microm wortmannin markedly reduced NMDA currents. Stimulation of the epidermal growth factor receptor (EGFR) promoted the formation of an immunocomplex between PLCgamma and NMDAR subunits. Stimulation of EGFR or the PLCbeta-coupled M1 acetylcholine receptor produced a robust transient inhibition of NMDA currents. Wortmannin application blocked the recovery of NMDA currents from the inhibition. Using mutagenesis, we identified the structural elements on NMDAR intracellular tails that transduce the receptor-mediated inhibition, which pinpoint to the binding site for the cytoskeletal protein alpha-actinin. Mutation of the PIP2-binding residues of alpha-actinin dramatically reduced NMDA currents and occluded the effect of EGF. Interestingly, EGF or wortmannin affected the interaction between NMDAR subunits and alpha-actinin, suggesting that this protein mediates the effect of PIP2 on NMDARs. In mature hippocampal neurons, expression of the mutant alpha-actinin reduced NMDA currents and accelerated inactivation. We propose a model in which alpha-actinin supports NMDAR activity via tethering their intracellular tails to plasma membrane PIP2. Thus, our results extend the influence of PIP2 to the NMDA ionotropic glutamate receptors and introduce a novel mechanism of "indirect" regulation of transmembrane protein activity by PIP2.

The lipid connection-regulation of voltage-gated Ca(2+) channels by phosphoinositides

Recent findings have revealed a pivotal role for phospholipids phosphatidylinositol -4,5-biphosphate (PIP(2)) and phosphatidylinositol -3,4,5-trisphosphate (PIP(3)) in the regulation of high voltage-activated (HVA) Ca(2+) channels. PIP(2) exerts two opposing actions on HVA Ca(2+) channels: It stabilizes their activity but also produces a voltage-dependent inhibition that can be antagonized by protein kinase A (PKA) phosphorylation. PIP(2) depletion and arachidonic acid together mediate the slow, voltage-independent inhibition of HVA Ca(2+) channels by G( q/11 )-coupled receptors in neurons. A sufficient level of plasma membrane PIP(2) also appears to be necessary for G( betagamma )-mediated inhibition. On the other hand, increased production of PIP(3) by PI-3 kinases promotes trafficking of HVA Ca(2+) channels to the plasma membrane. This review discusses these findings and their implications.

Phosphoinositide-mediated gating of inwardly rectifying K(+) channels

Phosphoinositides, such as phosphatidylinositol-bisphosphate (PIP(2)), control the activity of many ion channels in yet undefined ways. Inwardly, rectifying potassium (Kir) channels were the first shown to be dependent on direct interactions with phosphoinositides. Alterations in channel-PIP(2) interactions affect Kir single-channel gating behavior. Aberrations in channel-PIP(2) interactions can lead to human disease. As the activity of all Kir channels depends on their interactions with phosphoinositides, future research will aim to understand the molecular events that occur from phosphoinositide binding to channel gating. The determination of atomic resolution structures for several mammalian and bacterial Kir channels provides great promise towards this goal. We have mapped onto the three-dimensional channel structure the position of basic residues identified through mutagenesis studies that contribute to the sensitivity of a Kir channel to PIP(2). The localization of these putative PIP(2)-interacting residues relative to the channel's permeation pathway has given rise to a testable model, which could account for channel activation by PIP(2).

Regulation of transient receptor potential (TRP) channels by phosphoinositides

This review summarizes the modulation of transient receptor potential (TRP) channels, by phosphoinositides. TRP channels are characterized by polymodal activation and a surprising complexity of regulation mechanisms. Possibly, most if not all TRP channels are modulated by phosphoinositides. Modulation by phosphatidylinositol 4,5-biphosphate (PIP(2)) has been shown in detail for TRP vanilloid (TRPV) 1, TRPV5, TRP melastatin (TRPM) 4, TRPM5, TRPM7, TRPM8, TRP polycystin 2, and the Drosophila TPR-like (TRPL) channels. This review describes mechanisms of modulation of TRP channels mainly by PIP(2) and discusses some future challenges of this fascinating topic.

M1 Muscarinic Receptor Modulation of Kir2 Channels Enhances Temporal Summation of Excitatory Synaptic Potentials in Prefrontal Cortex Pyramidal Neurons

The cholinergic innervation of the prefrontal cortex (PFC) plays a pivotal role in regulating executive functions. Muscarinic receptors activated by acetylcholine depolarize pyramidal neurons in the rodent PFC homologue, but the mechanisms mediating this modulation are controversial. To address this question, we studied the responses of layer V rat pre- and infralimbic cortex pyramidal neurons to muscarinic receptor stimulation. Consistent with previous findings, M1 receptor stimulation produced a strong depolarization, leading to tonic firing. Voltage-clamp analysis revealed that M1 activation reduced constitutively active inwardly rectifying (Kir2) K+ channel currents. Blocking protein kinase C activation or depleting intracellular Ca2+ stores did not affect the modulation. However, reversal of the modulation was prevented by the phosphoinositide kinase inhibitor, wortmanin, suggesting the modulation was mediated by depletions of membrane phosphatidylinositol-4,5-bisphosphate (PIP2). Reduction of Kir2 channel currents by M1 receptor stimulation significantly increased the temporal summation of excitatory synaptic potentials (EPSPs) evoked by repetitive stimulation of layer I. This action was complimented by M2/4 receptor mediated presynaptic inhibition of the same terminals. As a consequence of this dual modulation, the responses to a single, isolated afferent volley was reduced, but the response to a high-frequency afferent burst was potentiated.

Activation of epidermal growth factor receptor inhibits KCNQ2/3 current through two distinct pathways: membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation

KCNQ2/3 currents are the molecular basis of the neuronal M currents that play a critical role in neuron excitability. Many neurotransmitters modulate M/KCNQ currents through their G-protein-coupled receptors. Membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation are two mechanisms that have been proposed for modulation of KCNQ2/3 currents. In this study, we studied regulation of KCNQ2/3 currents by the epidermal growth factor (EGF) receptor, a member of another family of membrane receptors, receptor tyrosine kinases. We demonstrate here that EGF induces biphasic inhibition of KCNQ2/3 currents in human embryonic kidney 293 cells and in rat superior cervical ganglia neurons, an initial fast inhibition and a later slow inhibition. Additional studies indicate that the early and late inhibitions resulted from PtdIns(4,5)P2 hydrolysis and tyrosine phosphorylation, respectively. We further demonstrate that these two processes are mutually dependent. This study indicates that EGF is a potent modulator of M/KCNQ currents and provides a new dimension to the understanding of the modulation of these channels.

Selective inhibition of Kir currents by antihistamines

In the present study, the effects of antihistamines on inwardly rectifying potassium (Kir) channels expressed in Xenopus oocyte were investigated using two-electrode voltage clamp technique. Firstly, effects of antihistamines on two members of Kir2.0 sub-family, Kir2.1 and Kir2.3 were compared. For antihistamines that selectively block histamine H(1) receptor, the first-generation antihistamines mepyramine and diphenhydramine inhibited Kir2.3 current by 25.0+/-2.9% and 17.3+/-0.7% at concentrations of 100 microM, respectively. In contrast, the second- and third-generation antihistamines astemizole and desloratadine were completely devoid of any inhibitory effect on Kir2.3 current. Histamine H(2) receptor antagonist cimetidine, at 100 microM, failed to inhibit Kir2.3 current. On the other hand, Kir2.1 current was not sensitive to any of these drugs. The mepyramine-induced inhibition of Kir2.3 current was significantly reduced by a single point mutation in Kir2.3 (Kir2.3(I213L)), which enhances Kir2.3-PIP(2) interaction. Secondly, the effect of mepyramine was also tested on Kir3.4*, another member of Kir family. 100 microM mepyramine produced a 30.3+/-4.6% inhibition on Kir3.4* current. These results suggest that the first-generation histamine H(1) receptor antagonists selectively inhibit Kir currents. The inhibitory effect of antihistamines on Kir currents may be involved in their neuronal and cardiac toxic effects caused by drug overdosing.

Interaction of the Ca2+-sensing receptor with the inwardly rectifying potassium channels Kir4.1 and Kir4.2 results in inhibition of channel function

The Ca(2+)-sensing receptor (CaR), a G protein-coupled receptor, is expressed in many epithelial tissues including the parathyroid glands, kidney, and GI tract. Although its role in regulating PTH levels and Ca(2+) metabolism are best characterized, it may also regulate salt and water transport in the kidney as demonstrated by recent reports showing association of potent gain-of-function mutations in the CaR with a Bartter-like, salt-wasting phenotype. To determine whether this receptor interacts with novel proteins that control ion transport, we screened a human adult kidney cDNA library with the COOH-terminal 219 amino acid cytoplasmic tail of the CaR as bait using the yeast two-hybrid system. We identified two independent clones coding for approximately 125 aa from the COOH terminus of the inwardly rectifying K(+) channel, Kir4.2. The CaR and Kir4.2 as well as Kir4.1 (another member of Kir4 subfamily) were reciprocally coimmunoprecipitated from HEK-293 cells in which they were expressed, but the receptor did not coimmunoprecipitate with Kir5.1 or Kir1.1. Both Kir4.1 and Kir4.2 were immunoprecipitated from rat kidney extracts with the CaR. In Xenopus laevis oocytes, expression of the CaR with either Kir4.1 or Kir4.2 channels resulted in inactivation of whole cell current as measured by two-electrode voltage clamp, but the nonfunctional CaR mutant CaR(R796W), and that does not coimmunoprecipitate with the channels, had no effect. Kir4.1 and the CaR were colocalized in the basolateral membrane of the distal nephron. The CaR interacts directly with Kir4.1 and Kir4.2 and can decrease their currents, which in turn could reduce recycling of K(+) for the basolateral Na(+)-K(+)-ATPase and thereby contribute to inhibition of Na(+) reabsorption.

Cloning and expression of cardiac Kir2.1 and Kir2.2 channels in thermally acclimated rainbow trout

Potassium currents are plastic entities that modify electrical activity of the heart in various physiological conditions including chronic thermal stress. We examined the molecular basis of the inward rectifier K+ current (IK1) in rainbow trout acclimated to cold (4 degrees C, CA) and warm (18 degrees C, WA) temperature. Inward rectifier K+ channel (Kir)2.1 and Kir2.2 transcripts were expressed in atrium and ventricle of the trout heart, K(ir)2.1 being the major component in both cardiac chambers. The relative expression of Kir2.2 was, however, higher (P < 0.05) in atrium than ventricle. The density of ventricular IK1 was approximately 25% larger (P < 0.05) in WA than CA trout. Furthermore, the IK1 of the WA trout was 10 times more sensitive to Ba2+ (IC50 0.18 +/- 0.42 microM) than the IK1 of the CA trout (1.17 +/- 0.44 microM) (P < 0.05), and opening kinetics of single Kir2 channels was slower in WA than CA trout (P < 0.05). When expressed in COS-1 cells, the homomeric Kir2.2 channels demonstrated higher Ba2+ sensitivity (2.88 +/- 0.42 microM) than Kir2.1 channels (24.99 +/- 7.40 microM) (P < 0.05). In light of the different Ba2+ sensitivities of rainbow trout (om)Kir2.1 and omKir2.2 channels, it is concluded that warm acclimation increases either number or activity of the omK(ir)2.2 channels in trout ventricular myocytes. The functional changes in I(K1) are independent of omKir2 transcript levels, which remained unaltered by thermal acclimation. Collectively, these findings suggest that thermal acclimation modifies functional properties and subunit composition of the trout Kir2 channels, which may be needed for regulation of cardiac excitability at variable temperatures.

Regulation of TRP channels: a voltage–lipid connection

TRP (transient receptor potential) channels respond to a plethora of stimuli in a fine-tuned manner. We show here that both membrane potential and the level of PI (phosphatidylinositol) phosphates are efficient regulators of TRP channel gating. Recent work has shown that this regulation applies to several members of the TRPV (TRP vanilloid) subfamily (TRPV1 and TRPV5) and the TRPM (TRP melastatin) subfamily (TRPM4/TRPM5/TRPM7/TRPM8), whereas regulation of members of the TRPC subfamily is still disputed. The mechanism whereby PIP2 (PI 4,5-bisphosphate) acts on TRPM4, a Ca2+- and voltage-activated channel, is shown in detail in this paper: (i) PIP2 may bind directly to the channel, (ii) PIP2 induces sensitization to activation by Ca2+, and (iii) PIP2 shifts the voltage dependence towards negative and physiologically more meaningful potentials. A PIP2-binding pocket seems to comprise a part of the TRP domain and especially pleckstrin homology domains in the C-terminus.

An andersen-Tawil syndrome mutation in Kir2.1 (V302M) alters the G-loop cytoplasmic K+ conduction pathway

Loss-of-function mutations in the inward rectifier potassium channel, Kir2.1, cause Andersen-Tawil syndrome (ATS-1), an inherited disorder of periodic paralysis and ventricular arrhythmias. Here, we explore the mechanism by which a specific ATS-1 mutation (V302M) alters channel function. Val-302 is located in the G-loop, a structure that is believed to form a flexible barrier for potassium permeation at the apex of the cytoplasmic pore. Consistent with a role in stabilizing the G-loop in an open conformation, we found the V302M mutation specifically renders the channel unable to conduct potassium without altering subunit assembly or attenuating cell surface expression. As predicted by the position of the Val-302 side chain in the crystal structure, amino acid substitution analysis revealed that channel activity and phosphatidylinositol 4,5-bisphosphate (PIP2) sensitivity are profoundly sensitive to alterations in the size, shape, and hydrophobicity of side chains at the Val-302 position. The observations establish that the Val-302 side chain is a critical determinant of potassium conduction through the G-loop. Based on our functional studies and the cytoplasmic domain crystal structure, we suggest that Val-302 may influence PIP2 gating indirectly by translating PIP2 binding to conformational changes in the G-loop pore.

Modulation of Kir4.2 rectification properties and pHi-sensitive run-down by association with Kir5.1

Inwardly rectifying K+ channels (Kir) comprise seven subfamilies that can be subdivided further on the basis of cytosolic pH (pHi) sensitivity, rectification strength and kinetics, and resistance to run-down. Although distinct residues within each channel subunit define these properties, heteromeric association with other Kir subunits can modulate them. We identified such an effect in the wild-type forms of Kir4.2 and Kir5.1 and used this to further understand how the functional properties of Kir channels relate to their structures. Kir4.2 and a Kir4.2-Kir5.1 fusion protein were expressed in HEK293 cells. Inward currents from Kir4.2 were stable over 10 min and pHi-insensitive (pH 6 to 8). Conversely, currents from Kir4.2-Kir5.1 exhibited a pHi-sensitive run-down at slightly acidic pHi. At pHi 7.2, currents in response to voltage steps positive to EK were essentially time independent for Kir4.2 indicating rapid block by Mg2+. Coexpression with Kir5.1 significantly increased the blocking time constant, and increased steady-state outward current characteristic of weak rectifiers. Recovery from blockade at negative potentials was voltage dependent and 2 to 10 times slower in the homomeric channel. These results show that Kir5.1 converts Kir4.2 from a strong to a weak rectifier, rendering it sensitive to pHi, and suggesting that Kir5.1 plays a role in fine-tuning Kir4.2 activity.

Regulation of TRP channels by PIP(2)

Transient receptor potential (TRP) channels are regulated by a wide variety of physical and chemical factors. Recently, several members of the TRP channel family were reported to be regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2), PIP(2)). This review will summarize the current knowledge on PIP(2) regulation of TRP channels and discuss the possibility that PIP(2) is a common regulator of mammalian TRP channels.

Supervised membrane swimming: small G-protein lifeguards regulate PIPK signalling and monitor intracellular PtdIns(4,5)P2 pools

Regulation of PIPK (phosphatidylinositol phosphate kinase) and PtdIns(4,5)P2 signalling by small G-proteins and their effectors is key to many biological functions. Through selective recruitment and activation of different PIPK isoforms, small G-proteins such as Rho, Rac and Cdc42 modulate actin dynamics and cytoskeleton-dependent cellular events in response to extracellular signalling. These activities affect a number of processes, including endocytosis, bacterial penetration into host cells and cytolytic granule-mediated targeted cell killing. Small G-proteins and their modulators are also regulated by phosphoinositides through translocation and conformational changes. Arf family small G-proteins act at multiple sites as regulators of membrane trafficking and actin cytoskeletal remodelling, and regulate a feedback loop comprising phospholipase D, phosphatidic acid, PIPKs and PtdIns(4,5)P2, contributing to enhancement of PtdIns(4,5)P2-mediated cellular events and receptor signalling. Na+, Kir (inwardly rectifying K+), Ca2+ and TRP (transient receptor potential) ion channels are regulated by small G-proteins and membrane pools of PtdIns(4,5)P2. Yeast phosphatidylinositol 4-phosphate 5-kinases Mss4 and Its3 are involved in resistance against disturbance of sphingolipid biosynthesis and maintenance of cell integrity through the synthesis of PtdIns(4,5)P2 and downstream signalling through the Rom2/Rho2 and Rgf1/Rho pathways. Here, we review models for regulated intracellular targeting of PIPKs by small G-proteins and other modulators in response to extracellular signalling. We also describe the spatial and temporal cross-regulation of PIPKs and small G-proteins that is critical for a number of cellular functions.

Dopamine D(2) receptor modulation of K(+) channel activity regulates excitability of nucleus accumbens neurons at different membrane potentials

The nucleus accumbens (NAc) is a forebrain area in the mesocorticolimbic dopamine (DA) system that regulates many aspects of drug addiction. Neuronal activity in the NAc is modulated by different subtypes of DA receptors. Although DA signaling has received considerable attention, the mechanisms underlying D(2)-class receptor (D(2)R) modulation of firing in medium spiny neurons (MSNs) localized within the NAc remain ambiguous. In the present study, we performed whole cell current-clamp recordings in rat brain slices to determine whether and how D(2)R modulation of K(+) channel activity regulates the intrinsic excitability of NAc neurons in the core region. D(2)R stimulation by quinpirole or DA significantly and dose-dependently decreased evoked Na(+) spikes. This D(2)R effect on inhibiting evoked firing was abolished by antagonism of D(2)Rs, reversed by blockade of voltage-sensitive, slowly inactivating A-type K(+) currents (I(As)), or eliminated by holding membrane potentials at levels in which I(As) was inactivated. It was also mimicked by inhibition of cAMP-dependent protein kinase (PKA) activity, but not phosphatidylinositol-specific phospholipase C (PI-PLC) activity. Moreover, D(2)R stimulation also reduced the inward rectification and depolarized the resting membrane potentials (RMPs) by decreasing "leak" K(+) currents. However, the D(2)R effects on inward rectification and RMP were blocked by inhibition of PI-PLC, but not PKA activity. These findings indicate that, with facilitated intracellular Ca(2+) release and activation of the D(2)R/G(q)/PLC/PIP(2) pathway, the D(2)R-modulated changes in the NAc excitability are dynamically regulated and integrated by multiple K(+) currents, including but are not limited to I(As), inwardly rectifying K(+) currents (I(Kir)), and "leak" currents (I(K-2P)).

Mechanism of inhibition of TREK-2 (K2P10.1) by the Gq-coupled M3 muscarinic receptor

TREK-2 is a member of the two-pore domain K(+) channel family and provides part of the background K(+) current in many types of cells. Neurotransmitters that act on receptors coupled to G(q) strongly inhibit TREK-2 and thus enhance cell excitability. The molecular basis for the inhibition of TREK-2 was studied. In COS-7 cells expressing TREK-2 and M(3) receptor, acetylcholine (ACh) applied to the bath solution strongly inhibited the whole cell current, and this was markedly reduced in the presence of U-73122, an inhibitor of PLC. The inhibition was also observed in cell-attached patches when ACh was applied to the bath solution. In inside-out patches, direct application of guanosine 5'-O-(3-thiotriphosphate) (10 microM), Ca(2+) (5 microM), or diacylglycerol (DAG; 10 microM) produced no inhibition of TREK-2 in >75% of patches tested. Phosphatidic acid, a product of DAG kinase, had no effect on TREK-2. Pretreatment of cells with 20 microM wortmannin, an inhibitor of phosphatidylinositol kinases, did not affect the inhibition or the recovery from inhibition of TREK-2, suggesting that phosphatidylinositol 4,5-bisphosphate depletion did not mediate the inhibition. Pretreatment of cells with a protein kinase C inhibitor (bisindolylmaleimide, 10 microM) markedly inhibited ACh-induced inhibition of TREK-2. Mutation of two putative PKC sites (S326A, S359C) abolished inhibition by ACh. Mutation of these amino acids to aspartate to mimic the phosphorylated state resulted in diminished TREK-2 current and no inhibition by ACh. These results suggest that the agonist-induced inhibition of TREK-2 via M(3) receptor occurs primarily via PKC-mediated phosphorylation.

Dendritic excitability of mouse frontal cortex pyramidal neurons is shaped by the interaction among HCN, Kir2, and Kleak channels

Dendritically placed, voltage-sensitive ion channels are key regulators of neuronal synaptic integration. In several cell types, hyperpolarization/cyclic nucleotide gated (HCN) cation channels figure prominently in dendritic mechanisms controlling the temporal summation of excitatory synaptic events. In prefrontal cortex, the sustained activity of pyramidal neurons in working memory tasks is thought to depend on the temporal summation of dendritic excitatory inputs. Yet we know little about how this is accomplished in these neurons and whether HCN channels play a role. To gain a better understanding of this process, layer V-VI pyramidal neurons in slices of mouse prelimbic and infralimbic cortex were studied. Somatic voltage-clamp experiments revealed the presence of rapidly activating and deactivating cationic currents attributable to HCN1/HCN2 channels. These channels were open at the resting membrane potential and had an apparent half-activation voltage near -90 mV. In the same voltage range, K+ currents attributable to Kir2.2/2.3 and K+-selective leak (Kleak) channels were prominent. Computer simulations grounded in the biophysical measurements suggested a dynamic interaction among Kir2, Kleak, and HCN channel currents in shaping membrane potential and the temporal integration of synaptic potentials. This inference was corroborated by experiment. Blockade of Kir2/Kleak channels caused neurons to depolarize, leading to the deactivation of HCN channels, the initiation of regular spiking (4-5 Hz), and enhanced temporal summation of EPSPs. These studies show that HCN channels are key regulators of synaptic integration in prefrontal pyramidal neurons but that their functional contribution is dependent on a partnership with Kir2 and Kleak channels.

Regulation of a family of inwardly rectifying potassium channels (Kir2) by the m1 muscarinic receptor and the small GTPase Rho

Inwardly rectifying potassium channels Kir2.1-Kir2.3 are important regulators of membrane potential and, thus, control cellular excitability. However, little is known about the regulation of these channels. Therefore, we studied the mechanisms mediating the regulation of Kir2.1-Kir2.3 by the G-protein-coupled m1 muscarinic receptor using the whole-cell patch-clamp technique and recombinant expression in the tsA201 mammalian cell line. Stimulation of the m1 muscarinic receptor inhibited all subtypes of inward rectifier tested, Kir2.1-Kir2.3. The inhibition of each channel subtype was reversible and was attenuated by the muscarinic receptor antagonist, atropine. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) mimicked the effects of m1 receptor activation by inhibiting Kir2.1 currents. However, PMA had no effect on Kir2.2 or Kir2.3. Inclusion of 200-microM guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) in the patch pipette solution prevented the effects of m1 muscarinic receptor stimulation on all three of the channel subtypes tested, confirming the mediation of the responses by G-proteins. Cotransfection with the activated mutant of the small GTPase Rho reduced current density, while C3 exoenzyme, a selective inhibitor of Rho, attenuated the m1 muscarinic receptor-induced inhibition of Kir2.1-Kir2.3. Also, buffering the intracellular calcium concentration with a high concentration of EGTA abolished the m1 receptor-induced inhibition of Kir2.1-Kir2.3, implicating a role for calcium in these responses. These results indicate that all three of the Kir2 channels are similarly inhibited by m1 muscarinic receptor stimulation through calcium-dependent activation of the small GTPase Rho.

Anionic phospholipid interactions with the potassium channel KcsA: simulation studies

Molecular dynamics (MD) simulations have been used to unmask details of specific interactions of anionic phospholipids with intersubunit binding sites on the surface of the bacterial potassium channel KcsA. Crystallographic data on a diacyl glycerol fragment at this site were used to model phosphatidylethanolamine (PE), or phosphatidylglycerol (PG), or phosphatidic acid (PA) at the intersubunit binding sites. Each of these models of a KcsA-lipid complex was embedded in phosphatidyl choline bilayer and explored in a 20 ns MD simulation. H-bond analysis revealed that in terms of lipid-protein interactions PA > PG >> PE and revealed how anionic lipids (PG and PA) bind to a site provided by two key arginine residues (R(64) and R(89)) at the interface between adjacent subunits. A 27 ns simulation was performed in which KcsA (without any lipids initially modeled at the R(64)/R(89) sites) was embedded in a PE/PG bilayer. There was a progressive specific increase over the course of the simulation in the number of H-bonds of PG with KcsA. Furthermore, two specific PG binding events at R(64)/R(89) sites were observed. The phosphate oxygen atoms of bound PG formed H-bonds to the guanidinium group of R(89), whereas the terminal glycerol H-bonded to R(64). Overall, this study suggests that simulations can help identify and characterize sites for specific lipid interactions on a membrane protein surface.

Ligand-dependent linkage of the ATP site to inhibition gate closure in the KATP channel

Major advances have been made on the inhibition gate and ATP site of the K(ir)6.2 subunit of the K(ATP) channel, but little is known about conformational coupling between the two. ATP site mutations dramatically disrupt ATP-dependent gating without effect on ligand-independent gating, observed as interconversions between active burst and inactive interburst conformations in the absence of ATP. This suggests that linkage between site and gate is conditionally dependent on ATP occupancy. We studied all substitutions at position 334 of the ATP site in K(ir)6.2deltaC26 that express in Xenopus oocytes. All substitutions disrupted ATP-dependent gating by 10-fold or more. Only positive-charged arginine or lysine at 334, however, slowed ligand-independent gating from the burst, and this was in some but not all patches. Moreover, the polycationic peptide protamine reversed the slowed gating from the burst of 334R mutant channels, and speeded the slow gating from the burst of wild-type SUR1/K(ir)6.2 in the absence of ATP. Our results support a two-step ligand-dependent linkage mechanism for K(ir)6.2 channels in which ATP-occupied sites function to electrostatically dissociate COOH-terminal domains from the membrane, then as in all K(ir) channels, free COOH-terminal domains and inner M2 helices transit to a lower energy state for gate closure.

Functional expression of Kir2.x in human aortic endothelial cells: the dominant role of Kir2.2

Inward rectifier K+channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K+current ( IK) demonstrated that 1) IKcurrent sensitivity to Ba2+and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of ∼4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced ∼50% by dnKir2.1 and ∼85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K+conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells.

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.

Molecular simulations and lipid–protein interactions: potassium channels and other membrane proteins

Molecular dynamics simulations may be used to probe the interactions of membrane proteins with lipids and with detergents at atomic resolution. Examples of such simulations for ion channels and for bacterial outer membrane proteins are described. Comparison of simulations of KcsA (an α-helical bundle) and OmpA (a β-barrel) reveals the importance of two classes of side chains in stabilizing interactions with the head groups of lipid molecules: (i) tryptophan and tyrosine; and (ii) arginine and lysine. Arginine residues interacting with lipid phosphate groups play an important role in stabilizing the voltage-sensor domain of the KvAP channel within a bilayer. Simulations of the bacterial potassium channel KcsA reveal specific interactions of phosphatidylglycerol with an acidic lipid-binding site at the interface between adjacent protein monomers. A combination of molecular modelling and simulation reveals a potential phosphatidylinositol 4,5-bisphosphate-binding site on the surface of Kir6.2.

Identification of a functional phosphatidylinositol 3,4,5-trisphosphate binding site in the epithelial Na+ channel

Membrane phospholipids, such as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), are signaling molecules that can directly modulate the activity of ion channels, including the epithelial Na(+) channel (ENaC). Whereas PI(3,4,5)P(3) directly activates ENaC, its binding site within the channel has not been identified. We identify here a region of gamma-mENaC just following the second trans-membrane domain (residues 569-583) important to PI(3,4,5)P(3) binding and regulation. Deletion of this track decreases activity of ENaC heterologously expressed in Chinese hamster ovary cells. K-Ras and its first effector phosphoinositide 3-OH kinase (PI3-K), as well as RhoA and its effector phosphatidylinositol 4-phosphate 5-kinase increase ENaC activity. Whereas the former, via generation of PI(3,4,5)P(3), increases ENaC open probability, the latter increases activity by increasing membrane levels of the channel. Deletion of the region just distal to the second trans-membrane domain disrupted regulation by K-Ras and PI3-K but not RhoA and phosphatidylinositol 4-phosphate 5-kinase. Moreover, PI(3,4,5)P(3) binds ENaC with deletion of the region following the second transmembrane domain disrupting this interaction and disrupting direct activation of the channel by PI(3,4,5)P(3). Mutation analysis revealed the importance of conserved positive and negative charged residues as well as bulky amino acids within this region to modulation of ENaC by PI3-K. The current results identify the region just distal to the second trans-membrane domain within gamma-mENaC as being part of a functional PI(3,4,5)P(3) binding site that directly impacts ENaC activity. Phospholipid binding to this site is probably mediated by the positively charged amino acids within this track, with negatively charged and bulky residues also influencing specificity of interactions.

Molecular dynamics simulation approaches to K channels: conformational flexibility and physiological function

Molecular modeling and simulations enable extrapolation for the structure of bacterial potassium channels to the function of their mammalian homologues. Molecular dynamics simulations have revealed the concerted single-file motion of potassium ions and water molecules through the selectivity filter of K channels and the role of filter flexibility in ion permeation and in "fast gating." Principal components analysis of extended K channel simulations suggests that hinge-bending of pore-lining M2 (or S6) helices plays a key role in K channel gating. Based on these and other simulations, a molecular model for gating of inward rectifier K channel gating is presented.

Sequential phosphorylation mediates receptor- and kinase-induced inhibition of TREK-1 background potassium channels

Background potassium channels determine membrane potential and input resistance and serve as prominent effectors for modulatory regulation of cellular excitability. TREK-1 is a two-pore domain background K+ channel (KCNK2, K2P2.1) that is sensitive to a variety of physicochemical and humoral factors. In this work, we used a recombinant expression system to show that activation of G alpha(q)-coupled receptors leads to inhibition of TREK-1 channels via protein kinase C (PKC), and we identified a critical phosphorylation site in a key regulatory domain that mediates inhibition of the channel. In HEK 293 cells co-expressing TREK-1 and either the thyrotropin-releasing hormone receptor (TRHR1) or the Orexin receptor (Orx1R), agonist stimulation induced robust channel inhibition that was suppressed by a bisindolylmaleimide PKC inhibitor but not by a protein kinase A blocker ((R(p))-cAMP-S). Channel inhibition by agonists or by direct activators of PKC (phorbol dibutyrate) and PKA (forskolin) was disrupted not only by alanine or aspartate mutations at an identified PKA site (Ser-333) in the C terminus, but also at a more proximal regulatory site in the cytoplasmic C terminus (Ser-300); S333A and S300A mutations enhanced basal TREK-1 current, whereas S333D and S300D substitutions mimicked phosphorylation and strongly diminished currents. When studied in combination, TREK-1 current density was enhanced in S300A/S333D but reduced in S300D/S333A mutant channels. Channel mutants were expressed and appropriately targeted to cell membranes. Together, these data support a sequential phosphorylation model in which receptor-induced kinase activation drives modification at Ser-333 that enables subsequent phosphorylation at Ser-300 to inhibit TREK-1 channel activity.

Regulation of ion channels by phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate is a signaling phospholipid of the plasma membrane that has a dynamically changing concentration. In addition to being the precursor of inositol trisphosphate and diacylglycerol, it complexes with and regulates many cytoplasmic and membrane proteins. Recent work has characterized the regulation of a wide range of ion channels by phosphatidylinositol 4,5-bisphosphate, helping to redefine the role of this lipid in cells and in neurobiology. In most cases, phosphatidylinositol 4,5-bisphosphate increases channel activity, and its hydrolysis by phospholipase C reduces channel activity.

PKC-delta sensitizes Kir3.1/3.2 channels to changes in membrane phospholipid levels after M3 receptor activation in HEK-293 cells

G protein-gated inward rectifier (Kir3) channels are inhibited by activation of G(q/11)-coupled receptors and this has been postulated to involve the signaling molecules protein kinase C (PKC) and/or phosphatidylinositol 4,5-bisphosphate (PIP(2)). Their precise roles in mediating the inhibition of this family of channels remain controversial. We examine here their relative roles in causing inhibition of Kir3.1/3.2 channels stably expressed in human embryonic kidney (HEK)-293 cells after muscarinic M(3) receptor activation. In perforated patch mode, staurosporine prevented the G(q/11)-mediated, M(3) receptor, inhibition of channel activity. Recovery from M(3)-mediated inhibition was wortmannin sensitive. Whole cell currents, where the patch pipette was supplemented with PIP(2), were still irreversibly inhibited by M(3) receptor stimulation. When adenosine A(1) receptors were co-expressed, inclusion of PIP(2) rescued the A(1)-mediated response. Recordings from inside-out patches showed that catalytically active PKC applied directly to the intracellular membrane face inhibited the channels: a reversible effect modulated by okadaic acid. Generation of mutant heteromeric channel Kir3.1S185A/Kir3.2C-S178A, still left the channel susceptible to receptor, pharmacological, and direct kinase-mediated inhibition. Biochemically, labeled phosphate is incorporated into the channel. We suggest that PKC-delta mediates channel inhibition because recombinant PKC-delta inhibited channel activity, M(3)-mediated inhibition of the channel, was counteracted by overexpression of two types of dominant negative PKC-delta constructs, and, by using confocal microscopy, we have demonstrated translocation of green fluorescent protein-tagged PKC-delta to the plasma membrane on M(3) receptor stimulation. Thus Kir3.1/3.2 channels are sensitive to changes in membrane phospholipid levels but this is contingent on the activity of PKC-delta after M(3) receptor activation in HEK-293 cells.

PI(4,5)P2 regulates the activation and desensitization of TRPM8 channels through the TRP domain

The subjective feeling of cold is mediated by the activation of TRPM8 channels in thermoreceptive neurons by cold or by cooling agents such as menthol. Here, we demonstrate a central role for phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in the activation of recombinant TRPM8 channels by both cold and menthol. Moreover, we show that Ca(2+) influx through these channels activates a Ca(2+)-sensitive phospholipase C and that the subsequent depletion of PI(4,5)P(2) limits channel activity, serving as a unique mechanism for desensitization of TRPM8 channels. Finally, we find that mutation of conserved positive residues in the highly conserved proximal C-terminal TRP domain of TRPM8 and two other family members, TRPM5 and TRPV5, reduces the sensitivity of the channels for PI(4,5)P(2) and increases inhibition by PI(4,5)P(2) depletion. These data suggest that the TRP domain of these channels may serve as a PI(4,5)P(2)-interacting site and that regulation by PI(4,5)P(2) is a common feature of members of the TRP channel family.

In vivo and in vitro functional characterization of Andersen's syndrome mutations

The inward rectifier K(+) channel Kir2.1 carries all Andersen's syndrome mutations identified to date. Patients exhibit symptoms of periodic paralysis, cardiac dysrhythmia and multiple dysmorphic features. Here, we report the clinical manifestations found in three families with Andersen's syndrome. Molecular genetics analysis identified two novel missense mutations in the KCNJ2 gene leading to amino acid changes C154F and T309I of the Kir2.1 open reading frame. Patch clamp experiments showed that the two mutations produced a loss of channel function. When co-expressed with Kir2.1 wild-type (WT) channels, both mutations exerted a dominant-negative effect leading to a loss of the inward rectifying K(+) current. Confocal microscopy imaging in HEK293 cells is consistent with a co-assembly of the EGFP-fused mutant proteins with WT channels and proper traffick to the plasma membrane to produce silent channels alone or as hetero-tetramers with WT. Functional expression in C2C12 muscle cell line of newly as well as previously reported Andersen's syndrome mutations confirmed that these mutations act through a dominant-negative effect by altering channel gating or trafficking. Finally, in vivo electromyographic evaluation showed a decrease in muscle excitability in Andersen's syndrome patients. We hypothesize that Andersen's syndrome-associated mutations and hypokalaemic periodic paralysis-associated calcium channel mutations may lead to muscle membrane hypoexcitability via a common mechanism.

Differential sensitivity of Kir2 inward-rectifier potassium channels to a mitochondrial uncoupler: identification of a regulatory site

The aim of this study was to gain insight into the mechanism by which members of the K(ir)2 subfamily are differentially sensitive to agents that inhibit mitochondrial function by identifying responsible site(s) in K(ir)2 proteins. K(ir)2 channels were expressed in Xenopus laevis oocytes and assayed by two-electrode voltage clamp and patch clamp. Incubation of oocytes in carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), a mitochondrial uncoupler, inhibited K(ir)2.2 and K(ir)2.3, but not K(ir)2.1. Replacement of the first 44 amino acids of K(ir)2.2 the or of first 19 K(ir)2.3 with the first 45 of K(ir)2.1 did not affect the sensitivity of the channels to FCCP. In contrast, a larger substitution of K(ir)2.1 N-terminal sequence (1-78) into K(ir)2.2 or K(ir)2.3 produced channels that were resistant to FCCP. Sequence alignment between residues 46 and 78 (K(ir)2.1 numbering) revealed four residues that are the same in K(ir)2.2 and K(ir)2.3 but different in K(ir)2.1. Each of these four residues in the resistant chimera was converted back to the K(ir)2.2/K(ir)2.3 amino acid. Three of the mutants (D51N, I59A, and G65S) were not sensitive to FCCP, but the H53Q mutant was sensitive. K(ir)2.1-H53A and K(ir)2.1-H53E were also sensitive. In contrast, K(ir)2.1-H53R and K(ir)2.1-H53K were recovered during resistant. K(ir)2.2 and K(ir)2.3 currents perfusion of inside-out patches from FCCP-treated oocytes. FCCP was without effect on K(ir)2.2 and K(ir)2.3 when applied directly to inside-out patches. Together, these results suggest inhibition of K(ir)2.2 and K(ir)2.3 by a ligand that bears a positive charge and is produced by an intracellular action of FCCP.