Merging functional studies with structures of inward-rectifier K+ channels

Novel KCNJ10 Gene Variations Compromise Function of Inwardly Rectifying Potassium Channel 4.1

TheKCNJ10gene encoding Kir4.1 contains numerous SNPs whose molecular effects remain unknown. We investigated the functional consequences of uncharacterized SNPs (Q212R, L166Q, and G83V) on homomeric (Kir4.1) and heteromeric (Kir4.1-Kir5.1) channel function. We compared these with previously characterized EAST/SeSAME mutants (G77R and A167V) in kidney-derived tsA201 cells and in glial cell-derived C6 glioma cells. The membrane potentials of tsA201 cells expressing G77R and G83V were significantly depolarized as compared with WTKir4.1, whereas cells expressing Q212R, L166Q, and A167V were less affected. Furthermore, macroscopic currents from cells expressing WTKir4.1 and Q212R channels did not differ, whereas currents from cells expressing L166Q, G83V, G77R, and A167V were reduced. Unexpectedly, L166Q current responses were rescued when co-expressed with Kir5.1. In addition, we observed notable differences in channel activity between C6 glioma cells and tsA201 cells expressing L166Q and A167V, suggesting that there are underlying differences between cell lines in terms of Kir4.1 protein synthesis, stability, or expression at the surface. Finally, we determined spermine (SPM) sensitivity of these uncharacterized SNPs and found that Q212R-containing channels displayed reduced block by 1 μmSPM. At 100 μmSPM, the block was equal to or greater than WT, suggesting that the greater driving force of SPM allowed achievement of steady state. In contrast, L166Q-Kir5.1 channels achieved a higher block than WT, suggesting a more stable interaction of SPM in the deep pore cavity. Overall, our data suggest that G83V, L166Q, and Q212R residues play a pivotal role in controlling Kir4.1 channel function.

Optical Control of a Neuronal Protein Using a Genetically Encoded Unnatural Amino Acid in Neurons

Photostimulation is a noninvasive way to control biological events with excellent spatial and temporal resolution. New methods are desired to photo-regulate endogenous proteins expressed in their native environment. Here, we present an approach to optically control the function of a neuronal protein directly in neurons using a genetically encoded unnatural amino acid (Uaa). By using an orthogonal tRNA/aminoacyl-tRNA synthetase pair to suppress the amber codon, a photo-reactive Uaa 4,5-dimethoxy-2-nitrobenzyl-cysteine (Cmn) is site-specifically incorporated in the pore of a neuronal protein Kir2.1, an inwardly rectifying potassium channel. The bulky Cmn physically blocks the channel pore, rendering Kir2.1 non-conducting. Light illumination instantaneously converts Cmn into a smaller natural amino acid Cys, activating Kir2.1 channel function. We express these photo-inducible inwardly rectifying potassium (PIRK) channels in rat hippocampal primary neurons, and demonstrate that light-activation of PIRK ceases the neuronal firing due to the outflux of K(+) current through the activated Kir2.1 channels. Using in utero electroporation, we also express PIRK in the embryonic mouse neocortex in vivo, showing the light-activation of PIRK in neocortical neurons. Genetically encoding Uaa imposes no restrictions on target protein type or cellular location, and a family of photoreactive Uaas is available for modulating different natural amino acid residues. This technique thus has the potential to be generally applied to many neuronal proteins to achieve optical regulation of different processes in brains. The current protocol presents an accessible procedure for intricate Uaa incorporation in neurons in vitro and in vivo to achieve photo control of neuronal protein activity on the molecular level.

A synergistic blocking effect of Mg²⁺ and spermine on the inward rectifier K⁺ (Kir2.1) channel pore

Inward rectifier K⁺ channels (Kir2.1) exhibit an extraordinary rectifying feature in the current–voltage relationship. We have previously showed that the bundle–crossing region of the transmembrane domain constitutes the crucial segment responsible for the polyamine block. In this study, we demonstrated that the major blocking effect of intracellular Mg²⁺ on Kir2.1 channels is also closely correlated with K⁺ current flow, and the coupled movements of Mg²⁺ and K⁺ seem to happen in the same flux–coupling segment of the pore as polyamines. With a preponderant outward K⁺ flow, intracellular Mg²⁺ would also be pushed to and thus stay at the outermost site of a flux–coupling segment in the bundle–crossing region of Kir2.1 channels to block the pore, although with a much lower apparent affinity than spermine (SPM). However, in contrast to the evident possibilities of outward exit of SPM through the channel pore especially during strong membrane depolarization, intracellular Mg²⁺ does not seem to traverse the Kir2.1 channel pore in any case. Intracellular Mg²⁺ and SPM therefore may have a synergistic action on the pore–blocking effect, presumably via prohibition of the outward exit of the higher–affinity blocking SPM by the lower–affinity Mg²⁺.

Modular Design of the Selectivity Filter Pore Loop in a Novel Family of Prokaryotic 'Inward Rectifier' (NirBac) channels

Potassium channels exhibit a modular design with distinct structural and functional domains; in particular, a highly conserved pore-loop sequence that determines their ionic selectivity. We now report the functional characterisation of a novel group of functionally non-selective members of the prokaryotic 'inward rectifier' subfamily of K(+) channels. These channels share all the key structural domains of eukaryotic and prokaryotic Kir/KirBac channels, but instead possess unique pore-loop selectivity filter sequences unrelated to any other known ionic selectivity filter. The strikingly unusual architecture of these 'NirBac' channels defines a new family of functionally non-selective ion channels, and also provides important insights into the modular design of ion channels, as well as the evolution of ionic selectivity within this superfamily of tetrameric cation channels.

Neuronal and Cardiovascular Potassium Channels as Therapeutic Drug Targets: Promise and Pitfalls

Potassium (K(+)) channels, with their diversity, often tissue-defined distribution, and critical role in controlling cellular excitability, have long held promise of being important drug targets for the treatment of dysrhythmias in the heart and abnormal neuronal activity within the brain. With the exception of drugs that target one particular class, ATP-sensitive K(+) (KATP) channels, very few selective K(+) channel activators or inhibitors are currently licensed for clinical use in cardiovascular and neurological disease. Here we review what a range of human genetic disorders have told us about the role of specific K(+) channel subunits, explore the potential of activators and inhibitors of specific channel populations as a therapeutic strategy, and discuss possible reasons for the difficulty in designing clinically relevant K(+) channel modulators.

Focus on Kir7.1: physiology and channelopathy

Genetic studies have linked alterations in Kir7.1 channel to diverse pathologies. We summarize functional relevance of Kir7.1 channel in retinal pigment epithelium (RPE), regulation of channel function by various cytoplasmic metabolites, and mutations that cause channelopathies. At the apical membrane of RPE, K(+) channels contribute to subretinal K(+) homeostasis and support Na(+)/K(+) pump and Na(+)-K(+)-2Cl(-) cotransporter function by providing a pathway for K(+) secretion. Electrophysiological studies have established that barium- and cesium-sensitive inwardly rectifying K(+) (Kir) channels make up a major component of the RPE apical membrane K(+) conductance. Native human RPE expresses transcripts for Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2, and Kir6.1, albeit at levels at least 50-fold lower than Kir7.1. Kir7.1 is structurally similar to other Kir channels, consisting of 2 trans-membrane domains, a pore-forming loop that contains the selectivity filter, and 2 cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites and control the opening and closing of the channel. Recent evidence indicated that intrinsic sequence motifs present in Kir7.1 control surface expression. Mutant Kir7.1 channels are associated with inherited eye pathologies such as Snowflake Vitreoretinal Degeneration (SVD) and Lebers Congenital Amaurosis (LCA16). Based on the current evidence, mutations implicated in channelopathies have the potential to be used for genetic testing to diagnose blindness due to Kir7.1.

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.

Molecular Dynamics Simulations of KirBac1.1 Mutants Reveal Global Gating Changes of Kir Channels

Prokaryotic inwardly rectifying (KirBac) potassium channels are homologous to mammalian Kir channels. Their activity is controlled by dynamical conformational changes that regulate ion flow through a central pore. Understanding the dynamical rearrangements of Kir channels during gating requires high-resolution structure information from channels crystallized in different conformations and insight into the transition steps, which are difficult to access experimentally. In this study, we use MD simulations on wild type KirBac1.1 and an activatory mutant to investigate activation gating of KirBac channels. Full atomistic MD simulations revealed that introducing glutamate in position 143 causes significant widening at the helix bundle crossing gate, enabling water flux into the cavity. Further, global rearrangements including a twisting motion as well as local rearrangements at the subunit interface in the cytoplasmic domain were observed. These structural rearrangements are similar to recently reported KirBac3.1 crystal structures in closed and open conformation, suggesting that our simulations capture major conformational changes during KirBac1.1 opening. In addition, an important role of protein-lipid interactions during gating was observed. Slide-helix and C-linker interactions with lipids were strengthened during activation gating.

Much more than a leak: structure and function of K₂p-channels

Over the last decade, we have seen an enormous increase in the number of experimental studies on two-pore-domain potassium channels (K2P-channels). The collection of reviews and original articles compiled for this special issue of Pflügers Archiv aims to give an up-to-date summary of what is known about the physiology and pathophysiology of K2P-channels. This introductory overview briefly describes the structure of K2P-channels and their function in different organs. Its main aim is to provide some background information for the 19 reviews and original articles of this special issue of Pflügers Archiv. It is not intended to be a comprehensive review; instead, this introductory overview focuses on some unresolved questions and controversial issues, such as: Do K2P-channels display voltage-dependent gating? Do K2P-channels contribute to the generation of action potentials? What is the functional role of alternative translation initiation? Do K2P-channels have one or two or more gates? We come to the conclusion that we are just beginning to understand the extremely complex regulation of these fascinating channels, which are often inadequately described as 'leak channels'.

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.

Cocaine self-administration enhances excitatory responses of pyramidal neurons in the rat medial prefrontal cortex to human immunodeficiency virus-1 Tat

The medial prefrontal cortex (mPFC) plays a critical role in reward-motivated behaviors. Repeated cocaine exposure dysregulates the dorsal mPFC, and this is thought to contribute to cocaine-seeking and relapse of abstinent abusers. Neuropathology of the mPFC also occurs in human immunodeficiency virus (HIV)-positive individuals, and this is exaggerated in those who also abuse cocaine. The impact of the comorbid condition on mPFC neuronal function is unknown. To fill this knowledge gap, we performed a behavioral and electrophysiological study utilising adult male rats that self-administered cocaine by pressing a lever for 14 once-daily operant sessions. Saline-yoked (SAL-yoked) rats served as controls. Cue reactivity (CR) was used to indicate drug-seeking, assessed by re-exposing the rats to cocaine-paired cues wherein non-reinforced lever pressing was quantified 1 day (CR1) and 18-21 days (CR2) after the 14th operant session. Only cocaine self-administration (COC-SA) rats showed CR. One day after CR2, brain slices were prepared for electrophysiological assessment. Whole-cell patch-clamp recordings of dorsal (prelimbic) mPFC pyramidal neurons from COC-SA rats showed a significant increase in firing evoked by depolarizing currents as compared with those from SAL-yoked control rats. Bath application of the toxic HIV-1 protein transactivator of transcription (Tat) also depolarized neuronal membranes and increased evoked firing. The Tat-induced excitation was greater in the neurons from withdrawn COC-SA rats than in controls. Tat also reduced spike amplitude, and this co-varied with cocaine-seeking during CR2. Taken together, these novel findings provide support at the neuronal level for the concept that the increased excitability of mPFC pyramidal neurons following cocaine self-administration drives drug-seeking and augments the neuropathophysiology caused by HIV-1 Tat.

Molecular insights into the possible role of Kir4.1 and Kir5.1 in thyroid hormone biosynthesis

INTRODUCTION

Thyroid morphogenesis is a complex process. Inwardly rectifying potassium (Kir) genes play a role in hormone release, cell excitability, pH and K(+) homeostasis in many tissues.

OBJECTIVES

To investigate the thyroid developmental expression of three members, Kir4.1, Kir4.2 and Kir5.1, in mice. To postulate the K(+) channel role in thyroid hormone secretion.

MATERIAL AND METHODS

Quantitative RT-PCR analysis of Kir4.1, Kir4.2 and Kir5.1 in mice of different stages (E13.5-E18.5).

RESULTS

mRNA for Kir4.1, Kir4.2 and Kir5.1 were identified and increased with age in mice. Both Kir4.1 and Kir4.2 genes are better expressed after E16.5. Kir4.2 greatly increases from E13.5 to E16.5 (p ≤ 0.05).

CONCLUSION

Quantitative PCR shows that the mouse thyroid presents increased expression for Kir channels during development. The role of Kir in thyroid morphogenesis and differentiation might be understood in future studies. We speculate that thyroglobulin trafficking might be modulated by Kir4.1/5.1.

Lovastatin-Induced Phosphatidylinositol-4-Phosphate 5-Kinase Diffusion from Microvilli Stimulates ROMK Channels

We recently showed that lovastatin attenuates cyclosporin A (CsA)-induced damage of cortical collecting duct (CCD) principal cells by reducing intracellular cholesterol. Previous studies showed that, in cell expression models or artificial membranes, exogenous cholesterol directly inhibits inward rectifier potassium channels, including Kir1.1 (Kcnj1; the gene locus for renal outer medullary K(+) [ROMK1] channels). Therefore, we hypothesized that lovastatin might stimulate ROMK1 by reducing cholesterol in CCD cells. Western blots showed that mpkCCDc14 cells express ROMK1 channels with molecular masses that approximate the molecular masses of ROMK1 in renal tubules detected before and after treatment with DTT. Confocal microscopy showed that ROMK1 channels were not in the microvilli, where cholesterol-rich lipid rafts are located, but rather, the planar regions of the apical membrane of mpkCCDc14 cells. Furthermore, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], an activator of ROMK channels, was detected mainly in the microvilli under resting conditions along with the kinase responsible for PI(4,5)P2 synthesis, phosphatidylinositol-4-phosphate 5-kinase, type I γ [PI(4)P5K I γ], which may explain the low basal open probability and increased sensitivity to tetraethylammonium observed here for this channel. Notably, lovastatin induced PI(4)P5K I γ diffusion into planar regions and elevated PI(4,5)P2 and ROMK1 open probability in these regions through a cholesterol-associated mechanism. However, exogenous cholesterol alone did not induce these effects. These results suggest that lovastatin stimulates ROMK1 channels, at least in part, by inducing PI(4,5)P2 synthesis in planar regions of the renal CCD cell apical membrane, suggesting that lovastatin could reduce cyclosporin-induced nephropathy and associated hyperkalemia.

Silencing of Kir2 channels by caveolin-1: cross-talk with cholesterol

A growing number of studies show that different types of ion channels localize in caveolae and are regulated by the level of membrane cholesterol. Furthermore, it has been proposed that cholesterol-induced regulation of ion channels might be attributed to partitioning into caveolae and association with caveolin-1 (Cav-1). We tested, therefore, whether Cav-1 regulates the function of inwardly rectifying potassium channels Kir2.1 that play major roles in the regulation of membrane potentials of numerous mammalian cells. Our earlier studies demonstrated that Kir2.1 channels are cholesterol sensitive. In this study, we show that Kir2.1 channels co-immunoprecipitate with Cav-1 and that co-expression of Kir2.1 channels with Cav-1 in HEK293 cells results in suppression of Kir2 current indicating that Cav-1 is a negative regulator of Kir2 function. These observations are confirmed by comparing Kir currents in bone marrow-derived macrophages isolated from Cav-1(-/-) and wild-type animals. We also show, however, that Kir2 channels maintain their sensitivity to cholesterol in HEK293 cells that have very low levels of endogenous Cav-1 and in bone marrow-derived macrophages isolated from Cav-1(-/-) knockout mice. Thus, these studies indicate that Cav-1 and/or intact caveolae are not required for cholesterol sensitivity of Kir channels. Moreover, a single point mutation of Kir2.1, L222I that abrogates the sensitivity of the channels to cholesterol also abolishes their sensitivity to Cav-1 suggesting that the two modulators regulate Kir2 channels via a common mechanism.

mGluR1-mediated excitation of cerebellar GABAergic interneurons requires both G protein-dependent and Src-ERK1/2-dependent signaling pathways

Stimulation of type I metabotropic glutamate receptors (mGluR1/5) in several neuronal types induces slow excitatory responses through activation of transient receptor potential canonical (TRPC) channels. GABAergic cerebellar molecular layer interneurons (MLIs) modulate firing patterns of Purkinje cells (PCs), which play a key role in cerebellar information processing. MLIs express mGluR1, and activation of mGluR1 induces an inward current, but its precise intracellular signaling pathways are unknown. We found that mGluR1 activation facilitated spontaneous firing of mouse cerebellar MLIs through an inward current mediated by TRPC1 channels. This mGluR1-mediated inward current depends on both G protein-dependent and -independent pathways. The nonselective protein tyrosine kinase inhibitors genistein and AG490 as well as the selective extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitors PD98059 and SL327 suppressed the mGluR1-mediated current responses. Following G protein blockade, the residual mGluR1-mediated inward current was significantly reduced by the selective Src tyrosine kinase inhibitor PP2. In contrast to cerebellar PCs, GABA_B receptor activation in MLIs did not alter the mGluR1-mediated inward current, suggesting that there is no cross-talk between mGluR1 and GABA_B receptors in MLIs. Thus, activation of mGluR1 facilitates firing of MLIs through the TRPC1-mediated inward current, which depends on not only G protein-dependent but also Src–ERK1/2-dependent signaling pathways, and consequently depresses the excitability of cerebellar PCs.

Cholesterol binding to ion channels

Numerous studies demonstrated that membrane cholesterol is a major regulator of ion channel function. The goal of this review is to discuss significant advances that have been recently achieved in elucidating the mechanisms responsible for cholesterol regulation of ion channels. The first major insight that comes from growing number of studies that based on the sterol specificity of cholesterol effects, show that several types of ion channels (nAChR, Kir, BK, TRPV) are regulated by specific sterol-protein interactions. This conclusion is supported by demonstrating direct saturable binding of cholesterol to a bacterial Kir channel. The second major advance in the field is the identification of putative cholesterol binding sites in several types of ion channels. These include sites at locations associated with the well-known cholesterol binding motif CRAC and its reversed form CARC in nAChR, BK, and TRPV, as well as novel cholesterol binding regions in Kir channels. Notably, in the majority of these channels, cholesterol is suggested to interact mainly with hydrophobic residues in non-annular regions of the channels being embedded in between transmembrane protein helices. We also discuss how identification of putative cholesterol binding sites is an essential step to understand the mechanistic basis of cholesterol-induced channel regulation. Clearly, however, these are only the first few steps in obtaining a general understanding of cholesterol-ion channels interactions and their roles in cellular and organ functions.

Growth of large and highly ordered 2D crystals of a K⁺ channel, structural role of lipidic environment

2D crystallography has proven to be an excellent technique to determine the 3D structure of membrane proteins. Compared to 3D crystallography, it has the advantage of visualizing the protein in an environment closer to the native one. However, producing good 2D crystals is still a challenge and little statistical knowledge can be gained from literature. Here, we present a thorough screening of 2D crystallization conditions for a prokaryotic inwardly rectifying potassium channel (>130 different conditions). Key parameters leading to very large and well-organized 2D crystals are discussed. In addition, the problem of formation of multilayers during the growth of 2D crystals is also addressed. An intermediate resolution projection map of KirBac3.1 at 6 Å is presented, which sheds (to our knowledge) new light on the structure of this channel in a lipid environment.

In vivo expression of a light-activatable potassium channel using unnatural amino acids

Optical control of protein function provides excellent spatial-temporal resolution for studying proteins in situ. Although light-sensitive exogenous proteins and ligands have been used to manipulate neuronal activity, a method for optical control of neuronal proteins using unnatural amino acids (Uaa) in vivo is lacking. Here, we describe the genetic incorporation of a photoreactive Uaa into the pore of an inwardly rectifying potassium channel Kir2.1. The Uaa occluded the pore, rendering the channel nonconducting, and, on brief light illumination, was released to permit outward K(+) current. Expression of this photoinducible inwardly rectifying potassium (PIRK) channel in rat hippocampal neurons created a light-activatable PIRK switch for suppressing neuronal firing. We also expanded the genetic code of mammals to express PIRK channels in embryonic mouse neocortex in vivo and demonstrated a light-activated PIRK current in cortical neurons. These principles could be generally expanded to other proteins expressed in the brain to enable optical regulation.

Silencing of the tandem pore domain halothane-inhibited K+ channel 2 (THIK2) relies on combined intracellular retention and low intrinsic activity at the plasma membrane

The tandem pore domain halothane-inhibited K(+) channel 1 (THIK1) produces background K(+) currents. Despite 62% amino acid identity with THIK1, THIK2 is not active upon heterologous expression. Here, we show that this apparent lack of activity is due to a unique combination of retention in the endoplasmic reticulum and low intrinsic channel activity at the plasma membrane. A THIK2 mutant containing a proline residue (THIK2-A155P) in its second inner helix (M2) produces K(+)-selective currents with properties similar to THIK1, including inhibition by halothane and insensitivity to extracellular pH variations. Another mutation in the M2 helix (I158D) further increases channel activity and affects current kinetics. We also show that the cytoplasmic amino-terminal region of THIK2 (Nt-THIK2) contains an arginine-rich motif (RRSRRR) that acts as a retention/retrieval signal. Mutation of this motif in THIK2 induces a relocation of the channel to the plasma membrane, resulting in measurable currents, even in the absence of mutations in the M2 helix. Cell surface delivery of a Nt-THIK2-CD161 chimera is increased by mutating the arginines of the retention motif but also by converting the serine embedded in this motif to aspartate, suggesting a phosphorylation-dependent regulation of THIK2 trafficking.

Identification of novel cholesterol-binding regions in Kir2 channels

Inwardly rectifying potassium (Kir) channels play an important role in setting the resting membrane potential and modulating membrane excitability. We have recently shown that cholesterol regulates representative members of the Kir family and that in the majority of the cases, cholesterol suppresses channel function. Furthermore, recent data indicate that cholesterol regulates Kir channels by specific sterol-protein interactions, yet the location of the cholesterol binding site in Kir channels is unknown. Using a combined computational-experimental approach, we show that cholesterol may bind to two nonanular hydrophobic regions in the transmembrane domain of Kir2.1 located between adjacent subunits of the channel. The location of the binding regions suggests that cholesterol modulates channel function by affecting the hinging motion at the center of the pore-lining transmembrane helix that underlies channel gating either directly or through the interface between the N and C termini of the channel.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

The bundle crossing region is responsible for the inwardly rectifying internal spermine block of the Kir2.1 channel

Inward rectifier potassium channels conduct K(+) across the cell membrane more efficiently in the inward than outward direction in physiological conditions. Voltage-dependent and flow-dependent blocks of outward K(+) currents by intracellular polyamines (e.g., spermine (SPM)) have been proposed as the major mechanisms underlying inward rectification. In this study, we show that the SPM blocking affinity curve is shifted according to the shift in K(+) reversal potential. Moreover, the kinetics of SPM entry to and exit from the binding site are correlatively slowed by specific E224 and E299 mutations, which always also disrupt the flux coupling feature of SPM block. The entry rates carry little voltage dependence, whereas the exit rates are e-fold decelerated per ∼15 mV depolarization. Interestingly, the voltage dependence remains rather constant among WT and quite a few different mutant channels. This voltage dependence offers an unprecedented chance of mapping the location (electrical distance) of the SPM site in the pore because these kinetic data were obtained along the preponderant direction of K(+) current flow (outward currents for the entry rate and inward currents for the exit rate) and thus contamination from flow dependence should be negligible. Moreover, double mutations involving E224 and A178 or M183 seem to alter the height of the same asymmetrical barrier between the SPM binding site and the intracellular milieu. We conclude that the SPM site responsible for the inward rectifying block is located at an electrical distance of ∼0.5 from the inside and is involved in a flux coupling segment in the bundle crossing region of the pore. With preponderant outward K(+) flow, SPM is "pushed" to the outmost site of this segment (∼D172). On the other hand, the blocking SPM would be pushed to the inner end of this segment (∼M183-A184) with preponderant inward K(+) flow. Moreover, E224 and E299 very likely electrostatically interact with the other residues (e.g., R228, R260) in the cytoplasmic domain and then allosterically keep the bundle crossing region in an open conformation appropriate for the flux coupling block of SPM.

Cholesterol sensitivity of KIR2.1 depends on functional inter-links between the N and C termini

In recent years, cholesterol has been emerging as a major regulator of ion channel function. We have previously shown that cholesterol suppresses Kir2 channels, a subfamily of constitutively active strongly rectifying K (+) channels. Furthermore, our earlier studies have shown that cholesterol sensitivity of Kir2 channels depends on a group of residues that form a belt-like structure around the cytosolic pore of the channel in proximity to the transmembrane domain. In this study, we focus on the contributions of different structural domains of Kir2 channels in the regulation of their cholesterol sensitivity. Focusing on the mildest mutation in the sensitivity belt, L222I, we show that the sensitivity of the channel to cholesterol can be restored by crosstalk between three distinct cytosolic regions: the C-terminal CD loop, the EF and GA loops of the C-terminus, and the βA sheet of the N-terminus. Thus, in addition to the importance of residues that affect the cytosolic G-loop gate in the sensitivity of Kir2 channels to cholesterol, our data suggest an important role to the interactions at the interface between the channel's N- and C- termini that couple the intracellular domains of its four subunits during gating.

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.

Influence of lipids on protein-mediated transmembrane transport

Transmembrane proteins are responsible for transporting ions and small molecules across the hydrophobic region of the cell membrane. We are reviewing the evidence for regulation of these transport processes by interactions with the lipids of the membrane. We focus on ion channels, including potassium channels, mechanosensitive and pentameric ligand gated ion channels, and active transporters, including pumps, sodium or proton driven secondary transporters and ABC transporters. For ion channels it has been convincingly shown that specific lipid-protein interactions can directly affect their function. In some cases, a combined approach of molecular and structural biology together with computer simulations has revealed the molecular mechanisms. There are also many transporters whose activity depends on lipids but understanding of the molecular mechanisms is only beginning.

KCNJ2 mutation in short QT syndrome 3 results in atrial fibrillation and ventricular proarrhythmia

We describe a mutation (E299V) in KCNJ2, the gene that encodes the strong inward rectifier K(+) channel protein (Kir2.1), in an 11-y-old boy. The unique short QT syndrome type-3 phenotype is associated with an extremely abbreviated QT interval (200 ms) on ECG and paroxysmal atrial fibrillation. Genetic screening identified an A896T substitution in a highly conserved region of KCNJ2 that resulted in a de novo mutation E299V. Whole-cell patch-clamp experiments showed that E299V presents an abnormally large outward IK1 at potentials above -55 mV (P < 0.001 versus wild type) due to a lack of inward rectification. Coexpression of wild-type and mutant channels to mimic the heterozygous condition still resulted in a large outward current. Coimmunoprecipitation and kinetic analysis showed that E299V and wild-type isoforms may heteromerize and that their interaction impairs function. The homomeric assembly of E299V mutant proteins actually results in gain of function. Computer simulations of ventricular excitation and propagation using both the homozygous and heterozygous conditions at three different levels of integration (single cell, 2D, and 3D) accurately reproduced the electrocardiographic phenotype of the proband, including an exceedingly short QT interval with merging of the QRS and the T wave, absence of ST segment, and peaked T waves. Numerical experiments predict that, in addition to the short QT interval, absence of inward rectification in the E299V mutation should result in atrial fibrillation. In addition, as predicted by simulations using a geometrically accurate three-dimensional ventricular model that included the His-Purkinje network, a slight reduction in ventricular excitability via 20% reduction of the sodium current should increase vulnerability to life-threatening ventricular tachyarrhythmia.

Kir2.4 surface expression and basal current are affected by heterotrimeric G-proteins

Kir2.4, a strongly rectifying potassium channel that is localized to neurons and is especially abundant in retina, was fished with yeast two-hybrid screen using a constitutively active Gαo1. Here, we wished to determine whether and how Gαo affects this channel. Using transfected HEK 293 cells and retinal tissue, we showed that Kir2.4 interacts with Gαo, and this interaction is stronger with the GDP-bound form of Gαo. Using two-electrode voltage clamp, we recorded from oocytes that were injected with Kir2.4 mRNA and a combination of G-protein subunit mRNAs. We found that the wild type and the inactive mutant of Gαo reduce the Kir2.4 basal current, whereas the active mutant has little effect. Other pertussis-sensitive Gα subunits also reduce this current, whereas Gαs increases it. Gβγ increases the current, whereas m-phosducin, which binds Gβγ without affecting the state of Gα, reduces it. We then tested the effect of G-protein subunits on the surface expression of the channel fused to cerulean by imaging the plasma membranes of the oocytes. We found that the surface expression is affected, with effects paralleling those seen with the basal current. This suggests that the observed effects on the current are mainly indirect and are due to surface expression. Similar results were obtained in transfected HEK cells. Moreover, we show that in retinal ON bipolar cells lacking Gβ3, localization of Kir2.4 in the dendritic tips is reduced. We conclude that Gβγ targets Kir2.4 to the plasma membrane, and Gαo slows this down by binding Gβγ.

An inwardly rectifying K+ channel is required for patterning

Mutations that disrupt function of the human inwardly rectifying potassium channel KIR2.1 are associated with the craniofacial and digital defects of Andersen-Tawil Syndrome, but the contribution of Kir channels to development is undefined. Deletion of mouse Kir2.1 also causes cleft palate and digital defects. These defects are strikingly similar to phenotypes that result from disrupted TGFβ/BMP signaling. We use Drosophila melanogaster to show that a Kir2.1 homolog, Irk2, affects development by disrupting BMP signaling. Phenotypes of irk2 deficient lines, a mutant irk2 allele, irk2 siRNA and expression of a dominant-negative Irk2 subunit (Irk2DN) all demonstrate that Irk2 function is necessary for development of the adult wing. Compromised Irk2 function causes wing-patterning defects similar to those found when signaling through a Drosophila BMP homolog, Decapentaplegic (Dpp), is disrupted. To determine whether Irk2 plays a role in the Dpp pathway, we generated flies in which both Irk2 and Dpp functions are reduced. Irk2DN phenotypes are enhanced by decreased Dpp signaling. In wild-type flies, Dpp signaling can be detected in stripes along the anterior/posterior boundary of the larval imaginal wing disc. Reducing function of Irk2 with siRNA, an irk2 deletion, or expression of Irk2DN reduces the Dpp signal in the wing disc. As Irk channels contribute to Dpp signaling in flies, a similar role for Kir2.1 in BMP signaling may explain the morphological defects of Andersen-Tawil Syndrome and the Kir2.1 knockout mouse.

A tandem Di-hydrophobic motif mediates clathrin-dependent endocytosis via direct binding to the AP-2 ασ2 subunits

Select plasma membrane proteins can be marked as cargo for inclusion into clathrin-coated pits by common internalization signals (e.g. YXXΦ, dileucine motifs, NPXY) that serve as universal recognition sites for the AP-2 adaptor complex or other clathrin-associated sorting proteins. However, some surface proteins, such as the Kir2.3 potassium channel, lack canonical signals but are still targeted for clathrin-dependent endocytosis. Here, we explore the mechanism. We found an unusual endocytic signal in Kir2.3 that is based on two consecutive pairs of hydrophobic residues. Characterized by the sequence ΦΦXΦΦ (a tandem di-hydrophobic (TDH) motif, where Φ is a hydrophobic amino acid), the signal shows no resemblance to other endocytic motifs, yet it directly interacts with AP-2 to target the Kir2.3 potassium channel into the endocytic pathway. We found that the tandem di-hydrophobic motif directly binds to the ασ2 subunits of AP-2, interacting within a large hydrophobic cleft that encompasses part of the docking site for di-Leu signals, but includes additional structures. These observations expand the repertoire of clathrin-dependent internalization signals and the ways in which AP-2 can coordinate endocytosis of cargo proteins.

Tonic inhibition of TRPV3 by Mg2+ in mouse epidermal keratinocytes

The transient receptor potential vanilloid 3 channel (TRPV3) is abundantly expressed in epidermal keratinocytes and plays important roles in sensory biology and skin health. Mg²⁺ deficiency causes skin disorders under certain pathological conditions such as type 2 diabetes mellitus. In this study, we investigated the effect of Mg²⁺ on TRPV3 in primary epidermal keratinocytes. Extracellular Mg²⁺ ([Mg²⁺]_o) inhibited TRPV3-mediated membrane current and calcium influx. TRPV3 activation induced a calcium signaling pathway culminating in activation of the cAMP response element binding (CREB). TRPV3 inhibition by [Mg²⁺]_o, the TRPV3 blocker ruthenium red or TRPV3 siRNA suppressed this response. In TRPV3-expressing Chinese hamster ovary (CHO) cells, both extracellular and intracellular Mg²⁺ inhibited TRPV3 single-channel conductance but not open probability. Neutralization of an aspartic acid residue (D641) in the extracellular pore loop or two acidic residues (E679, E682) in the inner pore region significantly attenuated the inhibitory effect of extracellular or intracellular Mg²⁺ on TRPV3-mediated signaling, respectively. Our findings suggest that epidermal TRPV3 is tonically inhibited by both extracellular and intracellular Mg²⁺, which act on both sides of the channel pore loop. Mg²⁺ deficiency may promote the function of TRPV3 and contribute to the pathogenesis of skin diseases.

Structural basis for modulation of gating property of G protein-gated inwardly rectifying potassium ion channel (GIRK) by i/o-family G protein α subunit (Gαi/o)

G protein-gated inwardly rectifying potassium channel (GIRK) plays a crucial role in regulating heart rate and neuronal excitability. The gating of GIRK is regulated by the association and dissociation of G protein βγ subunits (Gβγ), which are released from pertussis toxin-sensitive G protein α subunit (Gα(i/o)) upon GPCR activation in vivo. Several lines of evidence indicate that Gα(i/o) also interacts directly with GIRK, playing functional roles in the signaling efficiency and the modulation of the channel activity. However, the underlying mechanism for GIRK regulation by Gα(i/o) remains to be elucidated. Here, we performed NMR analyses of the interaction between the cytoplasmic region of GIRK1 and Gα(i3) in the GTP-bound state. The NMR spectral changes of Gα upon the addition of GIRK as well as the transferred cross-saturation (TCS) results indicated their direct binding mode, where the K(d) value was estimated as ∼1 mm. The TCS experiments identified the direct binding sites on Gα and GIRK as the α2/α3 helices on the GTPase domain of Gα and the αA helix of GIRK. In addition, the TCS and paramagnetic relaxation enhancement results suggested that the helical domain of Gα transiently interacts with the αA helix of GIRK. Based on these results, we built a docking model of Gα and GIRK, suggesting the molecular basis for efficient GIRK deactivation by Gα(i/o).

The function and molecular identity of inward rectifier channels in vestibular hair cells of the mouse inner ear

Inner ear hair cells respond to mechanical stimuli with graded receptor potentials. These graded responses are modulated by a host of voltage-dependent currents that flow across the basolateral membrane. Here, we examine the molecular identity and the function of a class of voltage-dependent ion channels that carries the potassium-selective inward rectifier current known as I(K1). I(K1) has been identified in vestibular hair cells of various species, but its molecular composition and functional contributions remain obscure. We used quantitative RT-PCR to show that the inward rectifier gene, Kir2.1, is highly expressed in mouse utricle between embryonic day 15 and adulthood. We confirmed Kir2.1 protein expression in hair cells by immunolocalization. To examine the molecular composition of I(K1), we recorded voltage-dependent currents from type II hair cells in response to 50-ms steps from -124 to -54 in 10-mV increments. Wild-type cells had rapidly activating inward currents with reversal potentials close to the K(+) equilibrium potential and a whole-cell conductance of 4.8 ± 1.5 nS (n = 46). In utricle hair cells from Kir2.1-deficient (Kir2.1(-/-)) mice, I(K1) was absent at all stages examined. To identify the functional contribution of Kir2.1, we recorded membrane responses in current-clamp mode. Hair cells from Kir2.1(-/-) mice had significantly (P < 0.001) more depolarized resting potentials and larger, slower membrane responses than those of wild-type cells. These data suggest that Kir2.1 is required for I(K1) in type II utricle hair cells and contributes to hyperpolarized resting potentials and fast, small amplitude receptor potentials in response to current inputs, such as those evoked by hair bundle deflections.

Piezo proteins are pore-forming subunits of mechanically activated channels

Mechanotransduction has an important role in physiology. Biological processes including sensing touch and sound waves require as-yet-unidentified cation channels that detect pressure. Mouse Piezo1 (MmPiezo1) and MmPiezo2 (also called Fam38a and Fam38b, respectively) induce mechanically activated cationic currents in cells; however, it is unknown whether Piezo proteins are pore-forming ion channels or modulate ion channels. Here we show that Drosophila melanogaster Piezo (DmPiezo, also called CG8486) also induces mechanically activated currents in cells, but through channels with remarkably distinct pore properties including sensitivity to the pore blocker ruthenium red and single channel conductances. MmPiezo1 assembles as a ∼1.2-million-dalton homo-oligomer, with no evidence of other proteins in this complex. Purified MmPiezo1 reconstituted into asymmetric lipid bilayers and liposomes forms ruthenium-red-sensitive ion channels. These data demonstrate that Piezo proteins are an evolutionarily conserved ion channel family involved in mechanotransduction.

Structure of a KirBac potassium channel with an open bundle crossing indicates a mechanism of channel gating

KirBac channels are prokaryotic homologs of mammalian inwardly rectifying (Kir) potassium channels, and recent crystal structures of both Kir and KirBac channels have provided major insight into their unique structural architecture. However, all of the available structures are closed at the helix bundle crossing, and therefore the structural mechanisms that control opening of their primary activation gate remain unknown. In this study, we engineered the inner pore-lining helix (TM2) of KirBac3.1 to trap the bundle crossing in an apparently open conformation and determined the crystal structure of this mutant channel to 3.05 Å resolution. Contrary to previous speculation, this new structure suggests a mechanistic model in which rotational 'twist' of the cytoplasmic domain is coupled to opening of the bundle-crossing gate through a network of inter- and intrasubunit interactions that involve the TM2 C-linker, slide helix, G-loop and the CD loop.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.

Interactions of cations with the cytoplasmic pores of inward rectifier K(+) channels in the closed state

Ion channels gate at membrane-embedded domains by changing their conformation along the ion conduction pathway. Inward rectifier K(+) (Kir) channels possess a unique extramembrane cytoplasmic domain that extends this pathway. However, the relevance and contribution of this domain to ion permeation remain unclear. By qualitative x-ray crystallographic analysis, we found that the pore in the cytoplasmic domain of Kir3.2 binds cations in a valency-dependent manner and does not allow the displacement of Mg(2+) by monovalent cations or spermine. Electrophysiological analyses revealed that the cytoplasmic pore of Kir3.2 selectively binds positively charged molecules and has a higher affinity for Mg(2+) when it has a low probability of being open. The selective blocking of chemical modification of the side chain of pore-facing residues by Mg(2+) indicates that the mode of binding of Mg(2+) is likely to be similar to that observed in the crystal structure. These results indicate that the Kir3.2 crystal structure has a closed conformation with a negative electrostatic field potential at the cytoplasmic pore, the potential of which may be controlled by conformational changes in the cytoplasmic domain to regulate ion diffusion along the pore.

R-Isovaline: a subtype-specific agonist at GABA(B)-receptors?

The R-enantiomer of isovaline, an analgesic amino acid, has a chemical structure similar to glycine and GABA. Although its actions on thalamic neurons are strychnine-resistant and independent of the Cl(-) gradient, R-isovaline increases membrane conductance for K(+). The purpose of this study was to determine if R-isovaline activated metabotropic GABA(B) receptors. We used whole-cell voltage-clamp recordings to characterize the effects of R-isovaline applied by bath perfusion and local ejection from a micropipette to thalamic neurons in 250 μm thick slices of rat brain. The immunocytochemical methods that we employed to visualize GABA(B1) and GABA(B2) receptor subunits showed extensive staining for both subunits in ventrobasal nuclei, which were the recording sites. Bath or local application of R-isovaline caused a slowly developing increase in conductance and outward rectification in 70% (54/77) of neurons, both effects reversing near the K(+) Nernst potential. As with the GABA(B) agonist baclofen, G proteins likely mediated the R-isovaline effects because they were susceptible to blockade by non-hydrolyzable substrates of guanosine triphosphate. The GABA(B) antagonists CGP35348 and CGP52432 prevented the conductance increase induced by R-isovaline, applied by bath or local ejection. The GABA(B) allosteric modulator CGP7930 enhanced the R-isovaline induced increase in conductance. At high doses, antagonists of GABA(A), GABA(C), glycine(A), μ-opioid, and nicotinic receptors did not block R-isovaline responses. The observations establish that R-isovaline increases the conductance of K(+) channels coupled to metabotropic GABA(B) receptors. Remarkably, not all neurons that were responsive to baclofen responded to R-isovaline. The R-isovaline-induced currents outlasted the fast baclofen responses and persisted for a 1-2-h period. Despite some similar actions, R-isovaline and baclofen do not act at identical GABA(B) receptor sites. The binding of R-isovaline and baclofen to the GABA(B) receptor may not induce the same conformational changes in receptor proteins or components of the intracellular signaling pathways.

Epidermal growth factor receptor tyrosine kinase regulates the human inward rectifier potassium K(IR)2.3 channel, stably expressed in HEK 293 cells

BACKGROUND AND PURPOSE

The detailed molecular modulation of inward rectifier potassium channels (including the K(IR) 2.3 channel) is not fully understood. The present study was designed to determine whether human K(IR) 2.3 (K(IR) 2.3) channels were regulated by protein tyrosine kinases (PTKs).

EXPERIMENTAL APPROACH

Whole-cell patch voltage-clamp, immunoprecipitation, Western blot analysis and site-directed mutagenesis were employed to determine the potential PTK phosphorylation of Kir2.3 current in HEK 293 cells stably expressing Kir2.3 gene.

KEY RESULTS

The broad-spectrum PTK inhibitor genistein (10 µM) and the selective epidermal growth factor (EGF) kinase inhibitor AG556 (10 µM) reversibly decreased K(IR) 2.3 current and the effect was reversed by the protein tyrosine phosphatase inhibitor, orthovanadate (1 mM). Although EGF (100 ng·mL(-1) ) and orthovanadate enhanced K(IR) 2.3 current, this effect was antagonized by AG556. However, the Src-family tyrosine kinase inhibitor PP2 (10 µM) did not inhibit K(IR) 2.3 current. Tyrosine phosphorylation of K(IR) 2.3 channels was decreased by genistein or AG556, and was increased by EGF or orthovanadate. The decrease of tyrosine phosphorylation of K(IR) 2.3 channels by genistein or AG556 was reversed by orthovanadate or EGF. Interestingly, the response of K(IR) 2.3 channels to EGF or AG556 was lost in the K(IR) 2.3 Y234A mutant channel.

CONCLUSION AND IMPLICATIONS

These results demonstrate that the EGF receptor tyrosine kinase up-regulates the K(IR) 2.3 channel via phosphorylation of the Y234 residue of the WT protein. This effect may be involved in the endogenous regulation of cellular electrical activity.

Cholesterol regulates prokaryotic Kir channel by direct binding to channel protein

Cholesterol is a major regulator of a variety of ion channels but the mechanisms underlying cholesterol sensitivity of ion channels are still poorly understood. The key question is whether cholesterol regulates ion channels by direct binding to the channel protein or by altering the physical environment of lipid bilayer. In this study, we provide the first direct evidence that cholesterol binds to prokaryotic Kir channels, KirBac1.1, and that cholesterol binding is essential for its regulatory effect. Specifically, we show that cholesterol is eluted together with the KirBac1.1 protein when separated on an affinity column and that the amount of bound cholesterol is proportional to the amount of the protein. We also show that cholesterol binding to KirBac1.1 is saturable with a K(D) of 390μM. Moreover, there is clear competition between radioactive and non-radioactive cholesterol for the binding site. There is no competition, however, between cholesterol and 5-Androsten 3β-17 β-diol, a sterol that we showed previously to have no effect on KirBac1.1 function. Finally, we show that cholesterol-KirBac1.1 binding is significantly inhibited by trifluoperazine, known to inhibit cholesterol binding to other proteins, and that inhibition of cholesterol-KirBac1.1 binding results in full recovery of the channel activity. Collectively, results from this study indicate that cholesterol-induced suppression of KirBac1.1 activity is mediated by direct interaction between cholesterol and the channel protein.

Integrin α9β1-mediated cell migration in glioblastoma via SSAT and Kir4.2 potassium channel pathway

The α9β1 integrin accelerates cell migration through binding of the α9 cytoplasmic domain to SSAT, which catalyzes the catabolism of higher order polyamines, spermidine and spermine, to the lower order polyamine, putrescine. SSAT levels were downregulated at both the mRNA and protein levels by shRNA-mediated simultaneous knockdown of MMP-9 and uPAR/cathepsin B. In addition, we noted a prominent reduction in the expression of SSAT with MMP-9 and uPAR/cathepsin B knockdown in the tumor regions of 5310 injected nude mice brains. Further, SSAT knockdown in glioma xenograft cells significantly reduced their migration potential. Interestingly, MMP-9, uPAR and cathepsin B overexpression in these xenograft cells significantly elevated SSAT mRNA and protein levels. The migratory potential of MMP-9/uPAR/cathepsin B-overexpressed 4910 and 5310 cells was not affected by either glybenclamide (Kir 6.x inhibitor) or tertiapin-Q (Kir 1.1 and 3.x inhibitor) but instead was significantly inhibited by either barium or Kir4.2 siRNA treatments. Co-localization of α9 integrin with Kir4.2 was observed in both 4910 and 5310 xenograft cells. However, MMP-9 and uPAR/cathepsin B knockdown in these cells prominently reduced the co-localization of α9 with Kir4.2. Taken together, our results clearly demonstrate that α9β1 integrin-mediated cell migration utilizes SSAT and the Kir4.2 potassium channel pathway, and inhibition of the migratory potential of these glioma xenograft cells by simultaneous knockdown of MMP-9 and uPAR/cathepsin B could be attributed to the reduced SSAT levels and co-localization of α9 integrin with Kir4.2 inward rectifier potassium channels.

Voltage-gated calcium channels

Voltage-gated calcium (Ca(2+)) channels are key transducers of membrane potential changes into intracellular Ca(2+) transients that initiate many physiological events. There are ten members of the voltage-gated Ca(2+) channel family in mammals, and they serve distinct roles in cellular signal transduction. The Ca(V)1 subfamily initiates contraction, secretion, regulation of gene expression, integration of synaptic input in neurons, and synaptic transmission at ribbon synapses in specialized sensory cells. The Ca(V)2 subfamily is primarily responsible for initiation of synaptic transmission at fast synapses. The Ca(V)3 subfamily is important for repetitive firing of action potentials in rhythmically firing cells such as cardiac myocytes and thalamic neurons. This article presents the molecular relationships and physiological functions of these Ca(2+) channel proteins and provides information on their molecular, genetic, physiological, and pharmacological properties.

Identification of a novel bacterial K(+) channel

In an attempt to explore unknown K(+) channels in mammalian cells, especially ATP-sensitive K(+) (K(ATP)) channels, we compared the sequence homology of Kir6.1 and Kir6.2, two pore-forming subunits of mammalian K(ATP) channel genes, with bacterial genes that code for selective proteins with confirmed or putative ion transport properties. BLAST analysis revealed that a prokaryotic gene (ydfJ) expressed in Escherichia coli K12 strain shared 8.6% homology with Kir6.1 and 8.3% with Kir6.2 genes. Subsequently, we cloned and sequenced ydfJ gene from E. coli K12 and heterologously expressed it in mammalian HEK-293 cells. The whole-cell patch-clamp technique was used to record ion channel currents generated by ydfJ-encoded protein. Heterologous expression of ydfJ gene in HEK-293 cells yielded a novel K(+) channel current that was inwardly rectified and had a reversal potential close to K(+) equilibrium potential. The expressed ydfJ channel was blocked reversibly by low concentration of barium in a dose-dependent fashion. Specific K(ATP) channel openers or blockers did not alter the K(+) current generated by ydfJ expression alone or ydfJ coexpressed with rvSUR1 or rvSUR2B subunits of K(ATP) channel complex. Furthermore, this coexpressed ydfJ/rvSUR1 channels were not inhibited by ATP dialysis. On the other hand, ydfJ K(+) currents were inhibited by protopine (a nonspecific K(+) channel blocker) but not by dofetilide (a HERG channel blocker). In summary, heterologously expressed prokaryotic ydfJ gene formed a novel functional K(+) channel in mammalian cells.