Phosphatidylinositol 4,5-bisphosphate (PIP2) modulation of ATP and pH sensitivity in Kir channels. A tale of an active and a silent PIP2 site in the N terminus

From Crosstalk to Synergism: The Combined Effect of Cholesterol and PI(4,5)P2 on Inwardly Rectifying Potassium Channels

Inwardly rectifying potassium (Kir) channels are integral membrane proteins that control the flux of potassium ions across cell membranes and regulate membrane permeability. All eukaryotic Kir channels require the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for activation. In recent years, it has become evident that the function of many members of this family of channels is also mediated by another essential lipid-cholesterol. Here, we focus on members of the Kir2 and Kir3 subfamilies and their modulation by these two key lipids. We discuss how PI(4,5)P₂ and cholesterol bind to Kir2 and Kir3 channels and how they affect channel activity. We also discuss the accumulating evidence indicating that there is interplay between PI(4,5)P₂ and cholesterol in the modulation of Kir2 and Kir3 channels. In particular, we review the crosstalk between PI(4,5)P₂ and cholesterol in the modulation of the ubiquitously expressed Kir2.1 channel and the synergy between these two lipids in the modulation of the Kir3.4 channel, which is primarily expressed in the heart. Additionally, we demonstrate that there is also synergy in the modulation of Kir3.2 channels, which are expressed in the brain. These observations suggest that alterations in the relative levels PI(4,5)P₂ and cholesterol may fine-tune Kir channel activity.

Phosphatidylinositol 4,5-bisphosphate (PIP2) facilitates norepinephrine transporter dimerization and modulates substrate efflux

The plasmalemmal norepinephrine transporter (NET) regulates cardiovascular sympathetic activity by clearing extracellular norepinephrine in the synaptic cleft. Here, we investigate the subunit stoichiometry and function of NET using single-molecule fluorescence microscopy and flux assays. In particular, we show the effect of phosphatidylinositol 4,5-bisphosphate (PIP2) on NET oligomerization and efflux. NET forms monomers (~60%) and dimers (~40%) at the plasma membrane. PIP2 depletion results in a decrease in the average oligomeric state and decreases NET-mediated substrate efflux while not affecting substrate uptake. Mutation of the putative PIP2 binding residues R121, K334, and R440 to alanines does not affect NET dimerization but results in decreased substrate efflux that is not altered upon PIP2 depletion; this indicates that PIP2 interactions with these residues affect NET-mediated efflux. A dysregulation of norepinephrine and PIP2 signaling have both been implicated in neuropsychiatric and cardiovascular diseases. This study provides evidence that PIP2 directly regulates NET organization and function.

EAST/SeSAME Syndrome and Beyond: The Spectrum of Kir4.1- and Kir5.1-Associated Channelopathies

In 2009, two groups independently linked human mutations in the inwardly rectifying K+ channel Kir4.1 (gene name KCNJ10) to a syndrome affecting the central nervous system (CNS), hearing, and renal tubular salt reabsorption. The autosomal recessive syndrome has been named EAST (epilepsy, ataxia, sensorineural deafness, and renal tubulopathy) or SeSAME syndrome (seizures, sensorineural deafness, ataxia, intellectual disability, and electrolyte imbalance), accordingly. Renal dysfunction in EAST/SeSAME patients results in loss of Na+, K+, and Mg2+ with urine, activation of the renin-angiotensin-aldosterone system, and hypokalemic metabolic alkalosis. Kir4.1 is highly expressed in affected organs: the CNS, inner ear, and kidney. In the kidney, it mostly forms heteromeric channels with Kir5.1 (KCNJ16). Biallelic loss-of-function mutations of Kir5.1 can also have disease significance, but the clinical symptoms differ substantially from those of EAST/SeSAME syndrome: although sensorineural hearing loss and hypokalemia are replicated, there is no alkalosis, but rather acidosis of variable severity; in contrast to EAST/SeSAME syndrome, the CNS is unaffected. This review provides a framework for understanding some of these differences and will guide the reader through the growing literature on Kir4.1 and Kir5.1, discussing the complex disease mechanisms and the variable expression of disease symptoms from a molecular and systems physiology perspective. Knowledge of the pathophysiology of these diseases and their multifaceted clinical spectrum is an important prerequisite for making the correct diagnosis and forms the basis for personalized therapies.

From Bench to Biomolecular Simulation: Phospholipid Modulation of Potassium Channels

Potassium (K+) ion channels are crucial in numerous cellular processes as they hyperpolarise a cell through K+ conductance, returning a cell to its resting potential. K+ channel mutations result in multiple clinical complications such as arrhythmia, neonatal diabetes and migraines. Since 1995, the regulation of K+ channels by phospholipids has been heavily studied using a range of interdisciplinary methods such as cellular electrophysiology, structural biology and computational modelling. As a result, K+ channels are model proteins for the analysis of protein-lipid interactions. In this review, we will focus on the roles of lipids in the regulation of K+ channels, and how atomic-level structures, along with experimental techniques and molecular simulations, have helped guide our understanding of the importance of phospholipid interactions.

Simulating PIP2-Induced Gating Transitions in Kir6.2 Channels

ATP-sensitive potassium (KATP) channels consist of an inwardly rectifying K+ channel (Kir6.2) pore, to which four ATP-sensitive sulfonylurea receptor (SUR) domains are attached, thereby coupling K+ permeation directly to the metabolic state of the cell. Dysfunction is linked to neonatal diabetes and other diseases. K+ flux through these channels is controlled by conformational changes in the helix bundle region, which acts as a physical barrier for K+ permeation. In addition, the G-loop, located in the cytoplasmic domain, and the selectivity filter might contribute to gating, as suggested by different disease-causing mutations. Gating of Kir channels is regulated by different ligands, like Gβγ, H+, Na+, adenosine nucleotides, and the signaling lipid phosphatidyl-inositol 4,5-bisphosphate (PIP2), which is an essential activator for all eukaryotic Kir family members. Although molecular determinants of PIP2 activation of KATP channels have been investigated in functional studies, structural information of the binding site is still lacking as PIP2 could not be resolved in Kir6.2 cryo-EM structures. In this study, we used Molecular Dynamics (MD) simulations to examine the dynamics of residues associated with gating in Kir6.2. By combining this structural information with functional data, we investigated the mechanism underlying Kir6.2 channel regulation by PIP2.

SLC6 transporter oligomerization

Transporters of the solute carrier 6 (SLC6) family mediate the reuptake of neurotransmitters such as dopamine, norepinephrine, serotonin, GABA, and glycine. SLC6 family members are 12 transmembrane helix‐spanning proteins that operate using the transmembrane sodium gradient for transport. These transporters assume various quaternary arrangements ranging from monomers to complex stoichiometries with multiple subunits. Dopamine and serotonin transporter oligomerization has been implicated in trafficking of newly formed proteins from the endoplasmic reticulum to the plasma membrane with a pre‐fixed assembly. Once at the plasma membrane, oligomers are kept fixed in their quaternary assembly by interaction with phosphoinositides. While it remains unclear how oligomer formation precisely affects physiological transporter function, it has been shown that oligomerization supports the activity of release‐type psychostimulants. Most recently, single molecule microscopy experiments unveiled that the stoichiometry differs between individual members of the SLC6 family. The present overview summarizes our understanding of the influence of plasma membrane constituents on transporter oligomerization, describes the known interfaces between protomers and discusses open questions.

Maintenance of CaV2.2 channel-current by PIP2 unveiled by neomycin in sympathetic neurons of the rat

Membrane lipids are key determinants in the regulation of voltage-gated ion channels. Phosphatidylinositol 4,5-bisphosphate (PIP₂), a native membrane phospholipid, has been involved in the maintenance of the current amplitude and in the voltage-independent regulation of voltage-gated calcium channels (VGCC). However, the nature of the PIP₂ regulation on VGCC has not been fully elucidated. This work aimed to investigate whether the interacting PIP₂ electrostatic charges may account for maintaining the current amplitude of Ca_V2.2 channels. Furthermore, we tested whether charge shielding of PIP₂ mimics the voltage-independent inhibition induced by M₁ muscarinic acetylcholine receptor (M₁R) activation. Therefore, neomycin, a polycation that has been shown to block electrostatic interactions of PIP_2, was intracellularly dialyzed in superior cervical ganglion (SCG) neurons of the rat. Consistently, neomycin time-dependently diminished the calcium current amplitude letting the channel exhibit the hallmarks of the voltage-independent regulation. These results support that interacting PIP₂ charges not only underly the maintenance of the channel-current but also that charge screening of PIP₂ by itself unveils the voltage-independent features of Ca_V2.2 channels in SCG neurons.

G protein βγ subunits play a critical role in the actions of amphetamine

Abnormal levels of dopamine (DA) are thought to contribute to several neurological and psychiatric disorders including drug addiction. Extracellular DA levels are regulated primarily via reuptake by the DA transporter (DAT). Amphetamine, a potent psychostimulant, increases extracellular DA by inducing efflux through DAT. Recently, we discovered that G protein βγ subunits (Gβγ) interact with DAT, and that in vitro activation of Gβγ promotes DAT-mediated efflux. Here, we investigated the role of Gβγ in the actions of amphetamine in DA neurons in culture, ex vivo nucleus accumbens (NAc), and freely moving rats. Activation of Gβγ with the peptide myr-Ser-Ile-Arg-Lys-Ala-Leu-Asn-Ile-Leu-Gly-Tyr-Pro-Asp-Tyr-Asp (mSIRK) in the NAc potentiated amphetamine-induced hyperlocomotion, but not cocaine-induced hyperlocomotion, and systemic or intra-accumbal administration of the Gβγ inhibitor gallein attenuated amphetamine-induced, but not cocaine-induced hyperlocomotion. Infusion into the NAc of a TAT-fused peptide that targets the Gβγ-binding site on DAT (TAT-DATct1) also attenuated amphetamine-induced but not cocaine-induced hyperlocomotion. In DA neurons in culture, inhibition of Gβγ with gallein or blockade of the Gβγ-DAT interaction with the TAT-DATct1 peptide decreased amphetamine-induced DA efflux. Furthermore, activation of Gβγ with mSIRK potentiated and inhibition of Gβγ with gallein reduced amphetamine-induced increases of extracellular DA in the NAc in vitro and in freely moving rats. Finally, systemic or intra-accumbal inhibition of Gβγ with gallein blocked the development of amphetamine-induced, but not cocaine-induced place preference. Collectively, these results suggest that interaction between Gβγ and DAT plays a critical role in the actions of amphetamine and presents a novel target for modulating the actions of amphetamine in vivo.

Functional mapping of the N-terminal arginine cluster and C-terminal acidic residues of Kir6.2 channel fused to a G protein-coupled receptor

Ion channel-coupled receptors (ICCRs) are original man-made ligand-gated ion channels created by fusion of G protein-coupled receptors (GPCRs) to the inward-rectifier potassium channel Kir6.2. GPCR conformational changes induced by ligand binding are transduced into electrical current by the ion channel. This functional coupling is closely related to the length of the linker region formed by the GPCR C-terminus (C-ter) and Kir6.2N-terminus (N-ter). Manipulating the GPCR C-ter length allows to finely tune the channel regulation, both in amplitude and sign (opening or closing Kir6.2). In this work, we demonstrate that the primary sequence of the channel N-terminal domain is an additional parameter for the functional coupling with GPCRs. As for all Kir channels, a cluster of basic residues is present in the N-terminal domain of Kir6.2 and is composed of 5 arginines which are proximal to the GPCR C-ter in the fusion proteins. Using a functional mapping approach, we demonstrate the role of specific arginines (R27 and R32) for the function of ICCRs, indicating that the position and not the cluster of positively-charged arginines is critical for the channel regulation by the GPCR. Following observations provided by molecular dynamics simulation, we explore the hypothesis of interaction of these arginines with acidic residues, and using site-directed mutagenesis, we identified aspartate D307 and glutamate E308 residues as critical for the function of ICCRs. These results demonstrate the critical role of the N-terminal and C-terminal charged residues of Kir6.2 for its allosteric regulation by the fused GPCR.

Protein arginine methylation facilitates KCNQ channel-PIP2 interaction leading to seizure suppression

KCNQ channels are critical determinants of neuronal excitability, thus emerging as a novel target of anti-epileptic drugs. To date, the mechanisms of KCNQ channel modulation have been mostly characterized to be inhibitory via Gq-coupled receptors, Ca²⁺/CaM, and protein kinase C. Here we demonstrate that methylation of KCNQ by protein arginine methyltransferase 1 (Prmt1) positively regulates KCNQ channel activity, thereby preventing neuronal hyperexcitability. Prmt1+/- mice exhibit epileptic seizures. Methylation of KCNQ2 channels at 4 arginine residues by Prmt1 enhances PIP₂ binding, and Prmt1 depletion lowers PIP₂ affinity of KCNQ2 channels and thereby the channel activities. Consistently, exogenous PIP₂ addition to Prmt1+/- neurons restores KCNQ currents and neuronal excitability to the WT level. Collectively, we propose that Prmt1-dependent facilitation of KCNQ-PIP₂ interaction underlies the positive regulation of KCNQ activity by arginine methylation, which may serve as a key target for prevention of neuronal hyperexcitability and seizures.

DOI:http://dx.doi.org/10.7554/eLife.17159.001

In the brain, cells called neurons transmit information along their length in the form of electrical signals. To generate electrical signals, ions move into and out of neurons through ion channel proteins – such as the KCNQ channel – in the surface of these cells, which open and close to control the electrical response of the neuron.

Abnormally intense bursts of electrical activity from many neurons at once can cause seizures such as those experienced by people with epilepsy. A significant proportion of patients do not respond to current anti-seizure medications. Openers of KCNQ channels have emerged as a potential new class of anti-epileptic drugs. A better understanding of how KCNQ channels work, and how their opening by PIP₂lipid signals is regulated, could help to develop more effective therapies for epilepsy.

A process called methylation controls many biological tasks by changing the structure of key proteins inside cells. Although methylation occurs throughout the brain, its role in controlling how easily neurons are activated (a property known as “excitability”) remains unclear.

Kim, Jeong, Kim, Jung et al. now show that a protein called Prmt1 methylates the KCNQ channels in mice, and that this methylation is essential for suppressing seizures. Mice born without the Prmt1 protein developed epileptic seizures and the KCNQ channels in their neurons featured a reduced level of methylation. However, increasing the amount of PIP₂ in these neurons restored their excitability back to normal levels. The methylation of KCNQ channel proteins increases their affinity for PIP2, which is critical to open KCNQ channels. Kim et al. propose that these “opening” controllers balance the action of known “closers” of KCNQ channels to maintain neurons in a healthy condition.

In future, Kim et al. plan to investigate whether methylation affects the activity of other ion channels controlled by PIP₂. Such experiments will complement a more widespread investigation into other ways in which the Prtmt1 protein may control the activity of neurons.

DOI:http://dx.doi.org/10.7554/eLife.17159.002

Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells

Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly regulated. In this review, we focus on the principal ionic channels (KATP, Nav, and Cav channels) involved in GSIS and how classic and new proteins, hormones, and drugs regulate it. Moreover, we also discuss advances on how metabolic disorders such as metabolic syndrome and diabetes mellitus change channel activity leading to changes in insulin secretion.

Structural Insights into GIRK Channel Function

G protein-gated inwardly rectifying potassium (GIRK; Kir3) channels, which are members of the large family of inwardly rectifying potassium channels (Kir1-Kir7), regulate excitability in the heart and brain. GIRK channels are activated following stimulation of G protein-coupled receptors that couple to the G(i/o) (pertussis toxin-sensitive) G proteins. GIRK channels, like all other Kir channels, possess an extrinsic mechanism of inward rectification involving intracellular Mg(2+) and polyamines that occlude the conduction pathway at membrane potentials positive to E(K). In the past 17 years, more than 20 high-resolution atomic structures containing GIRK channel cytoplasmic domains and transmembrane domains have been solved. These structures have provided valuable insights into the structural determinants of many of the properties common to all inward rectifiers, such as permeation and rectification, as well as revealing the structural bases for GIRK channel gating. In this chapter, we describe advances in our understanding of GIRK channel function based on recent high-resolution atomic structures of inwardly rectifying K(+) channels discussed in the context of classical structure-function experiments.

Mutations in Nature Conferred a High Affinity Phosphatidylinositol 4,5-Bisphosphate-binding Site in Vertebrate Inwardly Rectifying Potassium Channels

All vertebrate inwardly rectifying potassium (Kir) channels are activated by phosphatidylinositol 4,5-bisphosphate (PIP2) (Logothetis, D. E., Petrou, V. I., Zhang, M., Mahajan, R., Meng, X. Y., Adney, S. K., Cui, M., and Baki, L. (2015) Annu. Rev. Physiol. 77, 81-104; Fürst, O., Mondou, B., and D'Avanzo, N. (2014) Front. Physiol. 4, 404-404). Structural components of a PIP2-binding site are conserved in vertebrate Kir channels but not in distantly related animals such as sponges and sea anemones. To expand our understanding of the structure-function relationships of PIP2 regulation of Kir channels, we studied AqKir, which was cloned from the marine sponge Amphimedon queenslandica, an animal that represents the phylogenetically oldest metazoans. A requirement for PIP2 in the maintenance of AqKir activity was examined in intact oocytes by activation of a co-expressed voltage-sensing phosphatase, application of wortmannin (at micromolar concentrations), and activation of a co-expressed muscarinic acetylcholine receptor. All three mechanisms to reduce the availability of PIP2 resulted in inhibition of AqKir current. However, time-dependent rundown of AqKir currents in inside-out patches could not be re-activated by direct application to the inside membrane surface of water-soluble dioctanoyl PIP2, and the current was incompletely re-activated by the more hydrophobic arachidonyl stearyl PIP2. When we introduced mutations to AqKir to restore two positive charges within the vertebrate PIP2-binding site, both forms of PIP2 strongly re-activated the mutant sponge channels in inside-out patches. Molecular dynamics simulations validate the additional hydrogen bonding potential of the sponge channel mutants. Thus, nature's mutations conferred a high affinity activation of vertebrate Kir channels by PIP2, and this is a more recent evolutionary development than the structures that explain ion channel selectivity and inward rectification.

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

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.

Phosphoinositides alter lipid bilayer properties

Phosphatidylinositol-4,5-bisphosphate (PIP₂), which constitutes ∼1% of the plasma membrane phospholipid, plays a key role in membrane-delimited signaling. PIP₂ regulates structurally and functionally diverse membrane proteins, including voltage- and ligand-gated ion channels, inwardly rectifying ion channels, transporters, and receptors. In some cases, the regulation is known to involve specific lipid–protein interactions, but the mechanisms by which PIP₂ regulates many of its various targets remain to be fully elucidated. Because many PIP₂ targets are membrane-spanning proteins, we explored whether the phosphoinositides might alter bilayer physical properties such as curvature and elasticity, which would alter the equilibrium between membrane protein conformational states—and thereby protein function. Taking advantage of the gramicidin A (gA) channels’ sensitivity to changes in lipid bilayer properties, we used gA-based fluorescence quenching and single-channel assays to examine the effects of long-chain PIP₂s (brain PIP₂, which is predominantly 1-stearyl-2-arachidonyl-PIP₂, and dioleoyl-PIP₂) on bilayer properties. When premixed with dioleoyl-phosphocholine at 2 mol %, both long-chain PIP₂s produced similar changes in gA channel function (bilayer properties); when applied through the aqueous solution, however, brain PIP₂ was a more potent modifier than dioleoyl-PIP₂. Given the widespread use of short-chain dioctanoyl-phosphoinositides, we also examined the effects of diC₈-phosphoinositol (PI), PI(4,5)P₂, PI(3,5)P₂, PI(3,4)P₂, and PI(3,4,5)P₃. The diC₈ phosphoinositides, except for PI(3,5)P₂, altered bilayer properties with potencies that decreased with increasing head group charge. Nonphosphoinositide diC₈ phospholipids generally were more potent bilayer modifiers than the polyphosphoinositides. These results show that physiological increases or decreases in plasma membrane PIP₂ levels, as a result of activation of PI kinases or phosphatases, are likely to alter lipid bilayer properties, in addition to any other effects they may have. The results further show that exogenous PIP₂, as well as structural analogues that differ in acyl chain length or phosphorylation state, alters lipid bilayer properties at the concentrations used in many cell physiological experiments.

Energetics and location of phosphoinositide binding in human Kir2.1 channels

Kir2.1 channels are uniquely activated by phosphoinositide 4,5-bisphosphate (PI(4,5)P2) and can be inhibited by other phosphoinositides (PIPs). Using biochemical and computational approaches, we assess PIP-channel interactions and distinguish residues that are energetically critical for binding from those that alter PIP sensitivity by shifting the open-closed equilibrium. Intriguingly, binding of each PIP is disrupted by a different subset of mutations. In silico ligand docking indicates that PIPs bind to two sites. The second minor site may correspond to the secondary anionic phospholipid site required for channel activation. However, 96-99% of PIP binding localizes to the first cluster, which corresponds to the general PI(4,5)P2 binding location in recent Kir crystal structures. PIPs can encompass multiple orientations; each di- and triphosphorylated species binds with comparable energies and is favored over monophosphorylated PIPs. The data suggest that selective activation by PI(4,5)P2 involves orientational specificity and that other PIPs inhibit this activation through direct competition.

A Kir6.2 pore mutation causes inactivation of ATP-sensitive potassium channels by disrupting PIP2-dependent gating

In the absence of intracellular nucleotides, ATP-sensitive potassium (KATP) channels exhibit spontaneous activity via a phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent gating process. Previous studies show that stability of this activity requires subunit-subunit interactions in the cytoplasmic domain of Kir6.2; selective mutagenesis and disease mutations at the subunit interface result in time-dependent channel inactivation. Here, we report that mutation of the central glycine in the pore-lining second transmembrane segment (TM2) to proline in Kir6.2 causes KATP channel inactivation. Unlike C-type inactivation, a consequence of selectivity filter closure, in many K(+) channels, the rate of inactivation in G156P channels was insensitive to changes in extracellular ion concentrations or ion species fluxing through the pore. Instead, the rate of G156P inactivation decreased with exogenous application of PIP2 and increased when PIP2-channel interaction was inhibited with neomycin or poly-L-lysine. These findings indicate the G156P mutation reduces the ability of PIP2 to stabilize the open state of KATP channels, similar to mutations in the cytoplasmic domain that produce inactivation. Consistent with this notion, when PIP2-dependent open state stability was substantially increased by addition of a second gain-of-function mutation, G156P inactivation was abolished. Importantly, bath application and removal of Mg(2+)-free ATP or a nonhydrolyzable analog of ATP, which binds to the cytoplasmic domain of Kir6.2 and causes channel closure, recover G156P channel from inactivation, indicating crosstalk between cytoplasmic and transmembrane domains. The G156P mutation provides mechanistic insight into the structural and functional interactions between the pore and cytoplasmic domains of Kir6.2 during gating.

Molecular mechanisms of chloroquine inhibition of heterologously expressed Kir6.2/SUR2A channels

Chloroquine and related compounds can inhibit inwardly rectifying potassium channels by multiple potential mechanisms, including pore block and allosteric effects on channel gating. Motivated by reports that chloroquine inhibition of cardiac ATP-sensitive inward rectifier K(+) current (I(KATP)) is antifibrillatory in rabbit ventricle, we investigated the mechanism of chloroquine inhibition of ATP-sensitive potassium (K(ATP)) channels (Kir6.2/SUR2A) expressed in human embryonic kidney 293 cells, using inside-out patch-clamp recordings. We found that chloroquine inhibits the Kir6.2/SUR2A channel by interacting with at least two different sites and by two mechanisms of action. A fast-onset effect is observed at depolarized membrane voltages and enhanced by the N160D mutation in the central cavity, probably reflecting direct channel block resulting from the drug entering the channel pore from the cytoplasmic side. Conversely, a slow-onset, voltage-independent inhibition of I(KATP) is regulated by chloroquine interaction with a different site and probably involves disruption of interactions between Kir6.2/SUR2A and phosphatidylinositol 4,5-bisphosphate. Our findings reveal multiple mechanisms of K(ATP) channel inhibition by chloroquine, highlighting the numerous convergent regulatory mechanisms of these ligand-dependent ion channels.

Engineered interaction between SUR1 and Kir6.2 that enhances ATP sensitivity in KATP channels

The ATP-sensitive potassium (K_ATP) channel consisting of the inward rectifier Kir6.2 and SUR1 (sulfonylurea receptor 1) couples cell metabolism to membrane excitability and regulates insulin secretion. Inhibition by intracellular ATP is a hallmark feature of the channel. ATP sensitivity is conferred by Kir6.2 but enhanced by SUR1. The mechanism by which SUR1 increases channel ATP sensitivity is not understood. In this study, we report molecular interactions between SUR1 and Kir6.2 that markedly alter channel ATP sensitivity. Channels bearing an E203K mutation in SUR1 and a Q52E in Kir6.2 exhibit ATP sensitivity ∼100-fold higher than wild-type channels. Cross-linking of E203C in SUR1 and Q52C in Kir6.2 locks the channel in a closed state and is reversible by reducing agents, demonstrating close proximity of the two residues. Our results reveal that ATP sensitivity in K_ATP channels is a dynamic parameter dictated by interactions between SUR1 and Kir6.2.

Structural rearrangements underlying ligand-gating in Kir channels

Inward rectifier potassium (Kir) channels are physiologically regulated by a wide range of ligands that all act on a common gate, although structural details of gating are unclear. Here we show, using small molecule fluorescent probes attached to introduced cysteines, the molecular motions associated with gating of KirBac1.1 channels. The accessibility of the probes indicates a major barrier to fluorophore entry to the inner cavity. Changes in FRET between fluorophores attached to KirBac1.1 tetramers show that PIP₂-induced closure involves tilting and rotational motions of secondary structural elements of the cytoplasmic domain that couple ligand binding to a narrowing of the cytoplasmic vestibule. The observed ligand-dependent conformational changes in KirBac1.1 provide a general model for ligand-induced Kir channel gating at the molecular level.

ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 and Kir6.2 with diminished PIP2 sensitivity

ATP-sensitive potassium (K(ATP)) channels are inhibited by ATP and activated by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Both channel subunits Kir6.2 and sulfonylurea receptor 1 (SUR1) contribute to gating: while Kir6.2 interacts with ATP and PIP(2), SUR1 enhances sensitivity to both ligands. Recently, we showed that a mutation, E128K, in the N-terminal transmembrane domain of SUR1 disrupts functional coupling between SUR1 and Kir6.2, leading to reduced ATP and PIP(2) sensitivities resembling channels formed by Kir6.2 alone. We show here that when E128K SUR1 was co-expressed with Kir6.2 mutants known to disrupt PIP(2) gating, the resulting channels were surprisingly stimulated rather than inhibited by ATP. To explain this paradoxical gating behavior, we propose a model in which the open state of doubly mutant channels is highly unstable; ATP binding induces a conformational change in ATP-unbound closed channels that is conducive to brief opening when ATP unbinds, giving rise to the appearance of ATP-induced stimulation.

A conserved tryptophan at the membrane-water interface acts as a gatekeeper for Kir6.2/SUR1 channels and causes neonatal diabetes when mutated

We identified a novel heterozygous mutation, W68R, in the Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel, in a patient with transient neonatal diabetes. This tryptophan is absolutely conserved in mammalian Kir channels. The functional effects of mutations at residue 68 of Kir6.2 were studied by heterologous expression in Xenopus oocytes, and by homology modelling. We found the Kir6.2-W68R mutation causes a small reduction in ATP inhibition in the heterozygous state and an increase in the whole-cell KATP current. This can explain the clinical phenotype of the patient. The effect of the mutation was not charge or size dependent, the order of potency for ATP inhibition being W Collapse

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MESH Headings

• ATP-Binding Cassette Transporters/genetics
• ATP-Binding Cassette Transporters/metabolism
• Amino Acid Sequence
• Animals
• Cell Membrane/genetics
• Cell Membrane/metabolism
• Child
• Conserved Sequence/genetics
• Diabetes Mellitus/genetics
• Diabetes Mellitus/metabolism
• Female
• Genetic Carrier Screening
• Humans
• Infant, Newborn
• Infant, Newborn, Diseases/genetics
• Infant, Newborn, Diseases/metabolism
• Molecular Sequence Data
• Mutation/genetics
• Oocytes/metabolism
• Phenotype
• Potassium Channels, Inwardly Rectifying/genetics
• Potassium Channels, Inwardly Rectifying/metabolism
• Rats
• Receptors, Drug/genetics
• Receptors, Drug/metabolism
• Sulfonylurea Receptors
• Tryptophan/genetics
• Xenopus laevis

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Grants

• Wellcome Trust
• 084655 Wellcome Trust
• 089795 Wellcome Trust
• 076436 Wellcome Trust

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Affiliation(s)

Roope Männikkö Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
Phillip J Stansfeld
Alexandra S Ashcroft
Andrew T Hattersley
Mark S P Sansom
Sian Ellard
Frances M Ashcroft

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Rapid signaling responses in Sertoli cell membranes induced by follicle stimulating hormone and testosterone: calcium inflow and electrophysiological changes

This minireview describes the rapid signaling actions of follicle stimulating hormone (FSH) and testosterone in immature Sertoli cells mainly related to Ca(2+) inflow and the electrophysiological changes produced by hormones. The rapid membrane actions of FSH occur in a time frame of seconds to minutes, which include membrane depolarization and the stimulation of (45)Ca(2+) uptake. These effects can be prevented by pertussis toxin (PTX), suggesting that they are likely mediated by Gi-protein coupled receptor activation. Furthermore, these effects were inhibited by verapamil, a blocker of the L-type voltage-dependent Ca(2+) channel (VDCC). Finally, FSH stimulation of (45)Ca(2+) uptake was inhibited by the (phosphoinositide 3-kinase) PI3K inhibitor wortmannin. These results suggest that the rapid action of FSH on L-type Ca(2+) channel activity in Sertoli cells from pre-pubertal rats is mediated by the Gi/Gβγ/PI3Kγ pathway, independent of its effects on insulin-like growth factor type I (IGF-I). Testosterone depolarizes the membrane potential and increases the resistance and the (45)Ca(2+) uptake in Sertoli cells of the seminiferous tubules of immature rats. These actions were nullified by diazoxide (K(+)(ATP) channel opener). Testosterone actions were blocked by both PTX and the phospholipase C (PLC) inhibitor U73122, suggesting the involvement of PLC - phosphatidylinositol 4-5 bisphosphate (PIP2) hydrolysis via the Gq protein in the testosterone-mediated pathway. These results indicate that testosterone acts on the Sertoli cell membrane through the K(+)(ATP) channels and PLC-PIP2 hydrolysis, which closes the channel, depolarizes the membrane and stimulates (45)Ca(2+) uptake. These results demonstrate the existence of rapid non-classical pathways in immature Sertoli cells regulated by FSH and testosterone.

Carvedilol inhibits Kir2.3 channels by interference with PIP₂-channel interaction

Carvedilol, a β- and α-adrenoceptor blocker, is used to treat congestive heart failure, mild to moderate hypertension, and myocardial infarction. It has been proposed to block K(ATP) channels by binding to the bundle crossing region at a domain including cysteine at position 166, and thereby plugging the pore region. However, carvedilol was reported not to affect Kir2.1 channels, which lack 166 Cys. Here, we demonstrate that carvedilol inhibits Kir2.3 carried current by an alternative mechanism. Carvedilol inhibited Kir2.3 channels with at least 100 fold higher potency (IC(50)=0.49 μM) compared to that for Kir2.1 (IC(50)>50 μM). Kir2.3 channel inhibition was concentration-dependent and voltage-independent. Increasing Kir2.3 channel affinity for PIP(2), by a I213L point mutation, decreased the inhibitory effect of carvedilol more than twentyfold (IC(50)=11.1 μM). In the presence of exogenous PIP(2), Kir2.3 channel inhibition by carvedilol was strongly reduced (80 vs. 2% current inhibition). These results suggest that carvedilol, as other cationic amphiphilic drugs, inhibits Kir2.3 channels by interfering with the PIP(2)-channel interaction.

Random mutagenesis screening indicates the absence of a separate H(+)-sensor in the pH-sensitive Kir channels

Several inwardly-rectifying (Kir) potassium channels (Kir1.1, Kir4.1 and Kir4.2) are characterised by their sensitivity to inhibition by intracellular H(+) within the physiological range. The mechanism by which these channels are regulated by intracellular pH has been the subject of intense scrutiny for over a decade, yet the molecular identity of the titratable pH-sensor remains elusive. In this study we have taken advantage of the acidic intracellular environment of S. cerevisiae and used a K(+) -auxotrophic strain to screen for mutants of Kir1.1 with impaired pH-sensitivity. In addition to the previously identified K80M mutation, this unbiased screening approach identified a novel mutation (S172T) in the second transmembrane domain (TM2) that also produces a marked reduction in pH-sensitivity through destabilization of the closed-state. However, despite this extensive mutagenic approach, no mutations could be identified which removed channel pH-sensitivity or which were likely to act as a separate H(+) -sensor unique to the pH-sensitive Kir channels. In order to explain these results we propose a model in which the pH-sensing mechanism is part of an intrinsic gating mechanism common to all Kir channels, not just the pH-sensitive Kir channels. In this model, mutations which disrupt this pH-sensor would result in an increase, not reduction, in pH-sensitivity. This has major implications for any future studies of Kir channel pH-sensitivity and explains why formal identification of these pH-sensing residues still represents a major challenge.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

KCNJ10 gene mutations causing EAST syndrome (epilepsy, ataxia, sensorineural deafness, and tubulopathy) disrupt channel function

Mutations of the KCNJ10 (Kir4.1) K(+) channel underlie autosomal recessive epilepsy, ataxia, sensorineural deafness, and (a salt-wasting) renal tubulopathy (EAST) syndrome. We investigated the localization of KCNJ10 and the homologous KCNJ16 in kidney and the functional consequences of KCNJ10 mutations found in our patients with EAST syndrome. Kcnj10 and Kcnj16 were found in the basolateral membrane of mouse distal convoluted tubules, connecting tubules, and cortical collecting ducts. In the human kidney, KCNJ10 staining was additionally observed in the basolateral membrane of the cortical thick ascending limb of Henle's loop. EM of distal tubular cells of a patient with EAST syndrome showed reduced basal infoldings in this nephron segment, which likely reflects the morphological consequences of the impaired salt reabsorption capacity. When expressed in CHO and HEK293 cells, the KCNJ10 mutations R65P, G77R, and R175Q caused a marked impairment of channel function. R199X showed complete loss of function. Single-channel analysis revealed a strongly reduced mean open time. Qualitatively similar results were obtained with coexpression of KCNJ10/KCNJ16, suggesting a dominance of KCNJ10 function in native renal KCNJ10/KCNJ16 heteromers. The decrease in the current of R65P and R175Q was mainly caused by a remarkable shift of pH sensitivity to the alkaline range. In summary, EAST mutations of KCNJ10 lead to impaired channel function and structural changes in distal convoluted tubules. Intriguingly, the metabolic alkalosis present in patients carrying the R65P mutation possibly improves residual function of KCNJ10, which shows higher activity at alkaline pH.

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.

Regulation of Adenosine-activated GIRK Channels by Gq-coupled Receptors in Mouse Atrial Myocytes

Adenosine (Ado) is an important mediator of the endogenous defense against ischemia-induced injury in the heart. The action of Ado is mediated by activation of G protein-gated inwardly rectifying K(+) (GIRK) channels. In turn, GIRK channels are inhibited by reducing phosphatidylinositol 4,5-bisphosphate (PIP(2)) through Gq protein-coupled receptors (GqPCRs). We previously found that GIRK channels activated by acetylcholine, a muscarinic M2 acetylcholine receptor agonist, are inhibited by GqPCRs in a receptor-specific manner. However, it is not known whether GIRK channels activated by Ado signaling are also regulated by GqPCRs. Presently, this was investigated in mouse atrial myocytes using the patch clamp technique. GIRK channels were activated by 100 microM Ado. When Ado was repetitively applied at intervals of 5~6 min, the amplitude of second Ado-activated GIRK currents (I(K(Ado))) was 88.3+/-3.7% of the first I(K(Ado)) in the control. Pretreatment of atrial myocytes with phenylephrine, endothelin-1, or bradykinin prior to a second application of Ado reduced the amplitude of the second I(K(Ado)) to 25.5+/-11.6%, 30.5+/-5.6%, and 96.0+/-2.7%, respectively. The potency of I(K(Ado)) inhibition by GqPCRs was different with that observed in acetylcholine-activated GIRK currents (I(K(ACh))) (endothelin-1>phenylephrine>bradykinin). I(K(Ado)) was almost completely inhibited by 500 microM of the PIP(2) scavenger neomycin, suggesting low PIP(2) affinity of I(K(Ado)). Taken together, these results suggest that the crosstalk between GqPCRs and the Ado-induced signaling pathway is receptor-specific. The differential change in PIP(2) affinity of GIRK channels activated by Ado and ACh may underlie, at least in part, their differential responses to GqPCR agonists.

Phosphatidylinositol 4,5-bisphosphate activates Slo3 currents and its hydrolysis underlies the epidermal growth factor-induced current inhibition

The Slo3 gene encodes a high conductance potassium channel, which is activated by both voltage and intracellular alkalinization. Slo3 is specifically expressed in mammalian sperm cells, where it gives rise to pH-dependent outwardly rectifying K(+) currents. Sperm Slo3 is the main current responsible for the capacitation-induced hyperpolarization, which is required for the ensuing acrosome reaction, an exocytotic process essential for fertilization. Here we show that in intact spermatozoa and in a heterologous expression system, the activation of Slo3 currents is regulated by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Depletion of endogenous PIP(2) in inside-out macropatches from Xenopus oocytes inhibited heterologously expressed Slo3 currents. Whole-cell recordings of sperm Slo3 currents or of Slo3 channels co-expressed in Xenopus oocytes with epidermal growth factor receptor, demonstrated that stimulation by epidermal growth factor (EGF) could inhibit channel activity in a PIP(2)-dependent manner. High concentrations of PIP(2) in the patch pipette not only resulted in a strong increase in sperm Slo3 current density but also prevented the EGF-induced inhibition of this current. Mutation of positively charged residues involved in channel-PIP(2) interactions enhanced the EGF-induced inhibition of Slo3 currents. Overall, our results suggest that PIP(2) is an important regulator for Slo3 activation and that receptor-mediated hydrolysis of PIP(2) leads to inhibition of Slo3 currents both in native and heterologous expression systems.

Thiopental inhibits function of different inward rectifying potassium channel isoforms by a similar mechanism

Thiopental is a well-known intravenous barbiturate anesthetic with important cardiac side effects. The actions of thiopental on the transmembrane ionic currents that determine the resting potential and action potential duration in cardiomyocytes have been studied widely. We aimed at elucidating the characteristics and mechanism of inhibition by thiopental on members of the subfamily of inward rectifying Kir2.x (Kir2.1, 2.2 and 2.3), Kir1.1 and Kir6.2/SUR2A channels. These inward rectifier potassium channels were transfected in HEK-293 cells and macroscopic currents were recorded in the whole-cell and inside-out configurations of the patch-clamp technique. Thiopental inhibited Kir2.1, Kir2.2, Kir2.3, Kir1.1 and Kir6.2/SUR2A currents with similar potency; in whole-cell experiments 30 microM thiopental decreased Kir2.1, Kir2.2, Kir2.3 and Kir1.1 currents to 55+/-6, 39+/-8, 42+/-5 and 49+/-5% at -120 mV, respectively. Point mutations on Kir2.3 (I213L) or Kir2.1 (L222I) did not modify the potency of block. Thiopental inhibited all Kir channels in a concentration-dependent and voltage-independent manner. Also, the time course of thiopental inhibition was slow (T(1/2) approximately 4 min) and independent of external or internal drug application. However, in the presence of PIP(2), inhibition by thiopental on Kir2.1 was significantly decreased. Thiopental at clinically relevant concentrations significantly inhibited all Kir channels evaluated in this work. The reduction of thiopental effects during PIP(2) treatment suggests that thiopental inhibition on Kir2.1 channels is related to channel-PIP(2) interaction.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Crystal structure of the eukaryotic strong inward-rectifier K+ channel Kir2.2 at 3.1 A resolution

Inward-rectifier potassium (K+) channels conduct K+ ions most efficiently in one direction, into the cell. Kir2 channels control the resting membrane voltage in many electrically excitable cells, and heritable mutations cause periodic paralysis and cardiac arrhythmia. We present the crystal structure of Kir2.2 from chicken, which, excluding the unstructured amino and carboxyl termini, is 90% identical to human Kir2.2. Crystals containing rubidium (Rb+), strontium (Sr2+), and europium (Eu3+) reveal binding sites along the ion conduction pathway that are both conductive and inhibitory. The sites correlate with extensive electrophysiological data and provide a structural basis for understanding rectification. The channel's extracellular surface, with large structured turrets and an unusual selectivity filter entryway, might explain the relative insensitivity of eukaryotic inward rectifiers to toxins. These same surface features also suggest a possible approach to the development of inhibitory agents specific to each member of the inward-rectifier K+ channel family.

PIP(2)-binding site in Kir channels: definition by multiscale biomolecular simulations

Phosphatidylinositol bisphosphate (PIP(2)) is an activator of mammalian inwardly rectifying potassium (Kir) channels. Multiscale simulations, via a sequential combination of coarse-grained and atomistic molecular dynamics, enabled exploration of the interactions of PIP(2) molecules within the inner leaflet of a lipid bilayer membrane with possible binding sites on Kir channels. Three Kir channel structures were investigated: X-ray structures of KirBac1.1 and of a Kir3.1-KirBac1.3 chimera and a homology model of Kir6.2. Coarse-grained simulations of the Kir channels in PIP(2)-containing lipid bilayers identified the PIP(2)-binding site on each channel. These models of the PIP(2)-channel complexes were refined by conversion to an atomistic representation followed by molecular dynamics simulation in a lipid bilayer. All three channels were revealed to contain a conserved binding site at the N-terminal end of the slide (M0) helix, at the interface between adjacent subunits of the channel. This binding site agrees with mutagenesis data and is in the proximity of the site occupied by a detergent molecule in the Kir chimera channel crystal. Polar contacts in the coarse-grained simulations corresponded to long-lived electrostatic and H-bonding interactions between the channel and PIP(2) in the atomistic simulations, enabling identification of key side chains.

Subtype-specific regulation of P2X3 and P2X2/3 receptors by phosphoinositides in peripheral nociceptors

Background

P2X3 and P2X2/3 purinergic receptor-channels, expressed in primary sensory neurons that mediate nociception, have been implicated in neuropathic and inflammatory pain responses. The phospholipids phosphatidylinositol 4,5-bisphosphate (PIP₂) and phosphatidylinositol 3,4,5-trisphosphate (PIP₃) are involved in functional modulation of several types of ion channels. We report here evidence that these phospholipids are able to modulate the function of homomeric P2X3 and heteromeric P2X2/3 purinoceptors expressed in dorsal root ganglion (DRG) nociceptors and in heterologous expression systems.

Results

In dissociated rat DRG neurons, incubation with the PI3K/PI4K inhibitor wortmannin at 35 μM induced a dramatic decrease in the amplitude of ATP- or α,β-meATP-evoked P2X3 currents, while incubation with 100 nM wortmannin (selective PI3K inhibition) produced no significant effect. Intracellular application of PIP₂ was able to fully reverse the inhibition of P2X3 currents induced by wortmannin. In Xenopus oocytes and in HEK293 cells expressing recombinant P2X3, 35 μM wortmannin incubation induced a significant decrease in the rate of receptor recovery. Native and recombinant P2X2/3 receptor-mediated currents were inhibited by incubation with wortmannin both at 35 μM and 100 nM. The decrease of P2X2/3 current amplitude induced by wortmannin could be partially reversed by application of PIP₂ or PIP₃, indicating a sensitivity to both phosphoinositides in DRG neurons and Xenopus oocytes. Using a lipid binding assay, we demonstrate that the C-terminus of the P2X2 subunit binds directly to PIP₂, PIP₃ and other phosphoinositides. In contrast, no direct binding was detected between the C-terminus of P2X3 subunit and phosphoinositides.

Conclusion

Our findings indicate a functional regulation of homomeric P2X3 and heteromeric P2X2/3 ATP receptors by phosphoinositides in the plasma membrane of DRG nociceptors, based on subtype-specific mechanisms of direct and indirect lipid sensing.

Regulation of Kir channels in bovine retinal pigment epithelial cells by phosphatidylinositol 4,5-bisphosphate

The inwardly rectifying K+ (Kir) current in mammalian retinal pigment epithelial (RPE) cells, which is largely mediated by Kir7.1 channels, is stable in cells dialyzed with MgATP but runs down when intracellular ATP is depleted. A potential mechanism for this rundown is a decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) regeneration by ATP-dependent lipid kinases. Here, we used the whole cell voltage-clamp technique to investigate the membrane PIP2 dependence of Kir channels in isolated bovine RPE cells. When RPE cells were dialyzed with ATP-free solution containing PIP2 (25-50 microM), rundown persisted but was markedly reduced. Removal of Mg2+ from the pipette solution also slowed rundown, indicating that elevated intracellular Mg2+ concentration contributes to rundown. Cell dialysis with the PIP2 scavenger neomycin in MgATP solution diminished Kir current in a voltage-dependent manner, suggesting that it acted at least in part by blocking the Kir channel. Kir current in MgATP-loaded cells was partially inhibited by bath application of quercetin (100 microM), phenylarsine oxide (100 microM), or wortmannin (50 microM), inhibitors of phosphatidylinositol (PI) kinases, and was completely inhibited by cell dialysis with 2 mM adenosine, a PI4 kinase inhibitor. Both LY-294002 (100 microM), an inhibitor of PI3 kinases, and its inactive analog LY-303511 (100 microM) rapidly and reversibly inhibited Kir current, suggesting that these compounds act as direct channel blockers. We conclude that the activity of Kir channels in the RPE is critically dependent on the regeneration of membrane PIP2 by PI4 kinases and that this may explain the dependence of these channels on hydrolyzable ATP.

Analysis of two KCNJ11 neonatal diabetes mutations, V59G and V59A, and the analogous KCNJ8 I60G substitution: differences between the channel subtypes formed with SUR1

beta-Cell-type K(ATP) channels are octamers assembled from Kir6.2/KCNJ11 and SUR1/ABCC8. Adenine nucleotides play a major role in their regulation. Nucleotide binding to Kir6.2 inhibits channel activity, whereas ATP binding/hydrolysis on sulfonylurea receptor 1 (SUR1) opposes inhibition. Segments of the Kir6.2 N terminus are important for open-to-closed transitions, form part of the Kir ATP, sulfonylurea, and phosphoinositide binding sites, and interact with L0, an SUR cytoplasmic loop. Inputs from these elements link to the pore via the interfacial helix, which forms an elbow with the outer pore helix. Mutations that destabilize the interfacial helix increase channel activity, reduce sensitivity to inhibitory ATP and channel inhibitors, glibenclamide and repaglinide, and cause neonatal diabetes. We compared Kir6.x/SUR1 channels carrying the V59G substitution, a cause of the developmental delay, epilepsy, and neonatal diabetes syndrome, with a V59A substitution and the equivalent I60G mutation in the related Kir6.1 subunit from vascular smooth muscle. The substituted channels have increased P(O) values, decreased sensitivity to inhibitors, and impaired stimulation by phosphoinositides but retain sensitivity to Ba(2+)-block. The V59G and V59A channels are either not, or poorly, stimulated by phosphoinositides, respectively. Inhibition by sequestrating phosphatidylinositol 4,5-bisphosphate with neomycin and polylysine is reduced in V59A, and abolished in V59G channels. Stimulation by SUR1 is intact, and increasing the concentration of inhibitory ATP restores the sensitivity of Val-59-substituted channels to glibenclamide. The I60G channels, strongly dependent on SUR stimulation, remain sensitive to sulfonylureas. The results suggest the interfacial helix dynamically links inhibitory inputs from the Kir N terminus to the gate and that sulfonylureas stabilize an inhibitory configuration.

DEND mutation in Kir6.2 (KCNJ11) reveals a flexible N-terminal region critical for ATP-sensing of the KATP channel

ATP-sensitive K(+)-channels link metabolism and excitability in neurons, myocytes, and pancreatic islets. Mutations in the pore-forming subunit (Kir6.2; KCNJ11) cause neonatal diabetes, developmental delay, and epilepsy by decreasing sensitivity to ATP inhibition and suppressing electrical activity. Mutations of residue G53 underlie both mild (G53R,S) and severe (G53D) forms of the disease. All examined substitutions (G53D,R,S,A,C,F) reduced ATP-sensitivity, indicating an intolerance of any amino acid other than glycine. Surprisingly, each mutation reduces ATP affinity, rather than intrinsic gating, although structural modeling places G53 at a significant distance from the ATP-binding pocket. We propose that glycine is required in this location for flexibility of the distal N-terminus, and for an induced fit of ATP at the binding site. Consistent with this hypothesis, glycine substitution of the adjacent residue (Q52G) partially rescues ATP affinity of reconstituted Q52G/G53D channels. The results reveal an important feature of the noncanonical ATP-sensing mechanism of K(ATP) channels.

Phosphatidylinositol-4,5-bisphosphate (PIP2) regulation of strong inward rectifier Kir2.1 channels: multilevel positive cooperativity

Inwardly rectifying potassium (Kir) channels are gated by the interaction of their cytoplasmic regions with membrane-bound phosphatidylinositol-4,5-bisphosphate (PIP(2)). In the present study, we examined how PIP(2) interaction regulates channel availability and channel openings to various subconductance levels (sublevels) as well as the fully open state in the strong inward rectifier Kir2.1 channel. Various Kir2.1 channel constructs were expressed in Xenopus oocytes and single channel or macroscopic currents were recorded from inside-out patches. The wild-type (WT) channel rarely visited the subconductance levels under control conditions. However, upon reducing Kir2.1 channel interaction with PIP(2) by a variety of interventions, including PIP(2) antibodies, screening PIP(2) with neomycin, or mutating PIP(2) binding sites (e.g. K188Q), visitation to the sublevels was markedly increased before channels were converted to an unavailable mode in which they did not open. No channel activity was detected in channels with the double mutation K188A/R189A, a mutant which exhibits extremely weak interaction with PIP(2). By linking subunits together in tandem dimers or tetramers containing mixtures of WT and K188A/R189A subunits, we demonstrate that one functional PIP(2)-interacting WT subunit is sufficient to convert channels from the unavailable to the available mode with a high open probability dominated by the fully open state, with similar kinetics as tetrameric WT channels. Occasional openings to sublevels become progressively less frequent as the number of WT subunits increases. Quantitative analysis reveals that the interaction of PIP(2) with WT subunits exerts strong positive cooperativity in both converting the channels from the unavailable to the available mode, and in promoting the fully open state over sublevels. We conclude that the interaction of PIP(2) with only one Kir2.1 subunit is sufficient for the channel to become available and to open to its full conductance state. Interaction with additional subunits exerts positive cooperativity at multiple levels to further enhance channel availability and promote the fully open state.

A short motif in Kir6.1 consisting of four phosphorylation repeats underlies the vascular KATP channel inhibition by protein kinase C

Vascular ATP-sensitive K(+) channels are inhibited by multiple vasoconstricting hormones via the protein kinase C (PKC) pathway. However, the molecular substrates for PKC phosphorylation remain unknown. To identify the PKC sites, Kir6.1/SUR2B and Kir6.2/SUR2B were expressed in HEK293 cells. Following channel activation by pinacidil, the catalytic fragment of PKC inhibited the Kir6.1/SUR2B currents but not the Kir6.2/SUR2B currents. Phorbol 12-myristate 13-acetate (a PKC activator) had similar effects. Using Kir6.1-Kir6.2 chimeras, two critical protein domains for the PKC-dependent channel inhibition were identified. The proximal N terminus of Kir6.1 was necessary for channel inhibition. Because there was no PKC phosphorylation site in the N-terminal region, our results suggest its potential involvement in channel gating. The distal C terminus of Kir6.1 was crucial where there are several consensus PKC sites. Mutation of Ser-354, Ser-379, Ser-385, Ser-391, or Ser-397 to nonphosphorylatable alanine reduced PKC inhibition moderately but significantly. Combined mutations of these residues had greater effects. The channel inhibition was almost completely abolished when 5 of them were jointly mutated. In vitro phosphorylation assay showed that 4 of the serine residues were necessary for the PKC-dependent (32)P incorporation into the distal C-terminal peptides. Thus, a motif containing four phosphorylation repeats is identified in the Kir6.1 subunit underlying the PKC-dependent inhibition of the Kir6.1/SUR2B channel. The presence of the phosphorylation motif in Kir6.1, but not in its close relative Kir6.2, suggests that the vascular K(ATP) channel may have undergone evolutionary optimization, allowing it to be regulated by a variety of vasoconstricting hormones and neurotransmitters.

Functional analysis of two Kir6.2 (KCNJ11) mutations, K170T and E322K, causing neonatal diabetes

Heterozygous activating mutations in Kir6.2 (KCNJ11), the pore-forming subunit of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel, are a common cause of neonatal diabetes (ND). We assessed the functional effects of two Kir6.2 mutations associated with ND: K170T and E322K. K(ATP) channels were expressed in Xenopus oocytes, and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of sulphonylurea receptor (SUR)). Both mutations reduced the sensitivity of the K(ATP) channel to inhibition by MgATP and enhanced whole-cell K(ATP) currents. In pancreatic beta cells, such an increase in the K(ATP) current is expected to reduce insulin secretion and thereby cause diabetes. The E322K mutation was without effect when Kir6.2 was expressed in the absence of SUR1, suggesting that this residue impairs coupling to SUR1. This is consistent with its predicted location on the outer surface of the tetrameric Kir6.2 pore. The kinetics of K170T channel opening and closing were altered by the mutation, which may contribute to the lower ATP sensitivity. Neither mutation affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting that patients carrying these mutations may respond to these drugs.

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.

Identification of the PIP2-binding site on Kir6.2 by molecular modelling and functional analysis

ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to electrical activity by regulating K(+) fluxes across the plasma membrane. Channel closure is facilitated by ATP, which binds to the pore-forming subunit (Kir6.2). Conversely, channel opening is potentiated by phosphoinositol bisphosphate (PIP(2)), which binds to Kir6.2 and reduces channel inhibition by ATP. Here, we use homology modelling and ligand docking to identify the PIP(2)-binding site on Kir6.2. The model is consistent with a large amount of functional data and was further tested by mutagenesis. The fatty acyl tails of PIP(2) lie within the membrane and the head group extends downwards to interact with residues in the N terminus (K39, N41, R54), transmembrane domains (K67) and C terminus (R176, R177, E179, R301) of Kir6.2. Our model suggests how PIP(2) increases channel opening and decreases ATP binding and channel inhibition. It is likely to be applicable to the PIP(2)-binding site of other Kir channels, as the residues identified are conserved and influence PIP(2) sensitivity in other Kir channel family members.