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Nguyen H, Glaaser IW, Slesinger PA. Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels. Front Physiol 2024; 15:1386645. [PMID: 38903913 PMCID: PMC11187414 DOI: 10.3389/fphys.2024.1386645] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2024] [Accepted: 04/08/2024] [Indexed: 06/22/2024] Open
Abstract
Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.
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Affiliation(s)
| | | | - Paul A. Slesinger
- Nash Family Department of Neuroscience, Icahn School of Medicine at Mount Sinai, New York, NY, United States
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2
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Lefevre TJ, Wei W, Mukhaleva E, Venkata SPM, Chandan NR, Abraham S, Li Y, Dessauer CW, Vaidehi N, Smrcka AV. Stabilization of Interdomain Interactions in G protein α i Subunits Determines Gα i Subtype Signaling Specificity. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.03.10.532072. [PMID: 37066214 PMCID: PMC10103935 DOI: 10.1101/2023.03.10.532072] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Highly homologous members of the Gαi family, Gαi1-3, have distinct tissue distributions and physiological functions, yet the functional properties of these proteins with respect to GDP/GTP binding and regulation of adenylate cyclase are very similar. We recently identified PDZ-RhoGEF (PRG) as a novel Gαi1 effector, however, it is poorly activated by Gαi2. Here, in a proteomic proximity labeling screen we observed a strong preference for Gαi1 relative to Gαi2 with respect to engagement of a broad range of potential targets. We investigated the mechanistic basis for this selectivity using PRG as a representative target. Substitution of either the helical domain (HD) from Gαi1 into Gαi2 or substitution of a single amino acid, A230 in Gαi2 to the corresponding D in Gαi1, largely rescues PRG activation and interactions with other Gαi targets. Molecular dynamics simulations combined with Bayesian network models revealed that in the GTP bound state, dynamic separation at the HD-Ras-like domain (RLD) interface is prevalent in Gαi2 relative to Gαi1 and that mutation of A230s4h3.3 to D in Gαi2 stabilizes HD-RLD interactions through formation of an ionic interaction with R145HD.11 in the HD. These interactions in turn modify the conformation of Switch III. These data support a model where D229s4h3.3 in Gαi1 interacts with R144HD.11 stabilizes a network of interactions between HD and RLD to promote protein target recognition. The corresponding A230 in Gαi2 is unable to form the "ionic lock" to stabilize this network leading to an overall lower efficacy with respect to target interactions. This study reveals distinct mechanistic properties that could underly differential biological and physiological consequences of activation of Gαi1 or Gαi2 by GPCRs.
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Affiliation(s)
- Tyler J. Lefevre
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI
- Program in Chemical Biology, University of Michigan, Ann Arbor, MI
| | - Wenyuan Wei
- Department of Integrative Biology and Pharmacology McGovern Medical School, UTHealth, Houston, TX
| | - Elizaveta Mukhaleva
- Department of Integrative Biology and Pharmacology McGovern Medical School, UTHealth, Houston, TX
| | | | - Naincy R. Chandan
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI
- Genentech, South San Francisco, CA
| | - Saji Abraham
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI
| | - Yong Li
- Department of Integrative Biology and Pharmacology McGovern Medical School, UTHealth, Houston, TX
| | - Carmen W. Dessauer
- Department of Integrative Biology and Pharmacology McGovern Medical School, UTHealth, Houston, TX
| | - Nagarajan Vaidehi
- Department of Computational and Quantitative Medicine, Beckman Research Institute of the City of Hope, Duarte, CA
| | - Alan V. Smrcka
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI
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3
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Luo H, Marron Fernandez de Velasco E, Wickman K. Neuronal G protein-gated K + channels. Am J Physiol Cell Physiol 2022; 323:C439-C460. [PMID: 35704701 PMCID: PMC9362898 DOI: 10.1152/ajpcell.00102.2022] [Citation(s) in RCA: 16] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP2, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na+ and Ca2+, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.
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Affiliation(s)
- Haichang Luo
- Department of Pharmacology, University of Minnesota, Minneapolis, MN, United States
| | | | - Kevin Wickman
- Department of Pharmacology, University of Minnesota, Minneapolis, MN, United States
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4
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Friesacher T, Reddy HP, Bernsteiner H, Carlo Combista J, Shalomov B, Bera AK, Zangerl-Plessl EM, Dascal N, Stary-Weinzinger A. A selectivity filter mutation provides insights into gating regulation of a K + channel. Commun Biol 2022; 5:345. [PMID: 35411015 PMCID: PMC9001731 DOI: 10.1038/s42003-022-03303-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2021] [Accepted: 03/22/2022] [Indexed: 11/13/2022] Open
Abstract
G-protein coupled inwardly rectifying potassium (GIRK) channels are key players in inhibitory neurotransmission in heart and brain. We conducted molecular dynamics simulations to investigate the effect of a selectivity filter (SF) mutation, G154S, on GIRK2 structure and function. We observe mutation-induced loss of selectivity, changes in ion occupancy and altered filter geometry. Unexpectedly, we reveal aberrant SF dynamics in the mutant to be correlated with motions in the binding site of the channel activator Gβγ. This coupling is corroborated by electrophysiological experiments, revealing that GIRK2wt activation by Gβγ reduces the affinity of Ba2+ block. We further present a functional characterization of the human GIRK2G154S mutant validating our computational findings. This study identifies an allosteric connection between the SF and a crucial activator binding site. This allosteric gating mechanism may also apply to other potassium channels that are modulated by accessory proteins. Gly selectivity filter (TIGYGYR) mutant of the GIRK2 channel causes rare but severe neurological disorder called the Keppen-Lubinsky syndrome. Here, the authors explore the molecular mechanism of action of this glycine to serine mutant causing disease and identify an allosteric connection between the selectivity filter and a crucial activator binding site.
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Affiliation(s)
- Theres Friesacher
- Department of Pharmaceutical Sciences, Division of Pharmacology and Toxicology, University of Vienna, Josef-Holaubek-Platz 2, 1090, Vienna, Austria
| | - Haritha P Reddy
- Department of Physiology and Pharmacology, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel.,Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras, Chennai, 600036, India
| | - Harald Bernsteiner
- Department of Pharmaceutical Sciences, Division of Pharmacology and Toxicology, University of Vienna, Josef-Holaubek-Platz 2, 1090, Vienna, Austria
| | - J Carlo Combista
- Department of Physiology and Pharmacology, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Boris Shalomov
- Department of Physiology and Pharmacology, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Amal K Bera
- Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras, Chennai, 600036, India
| | - Eva-Maria Zangerl-Plessl
- Department of Pharmaceutical Sciences, Division of Pharmacology and Toxicology, University of Vienna, Josef-Holaubek-Platz 2, 1090, Vienna, Austria
| | - Nathan Dascal
- Department of Physiology and Pharmacology, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel. .,Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel.
| | - Anna Stary-Weinzinger
- Department of Pharmaceutical Sciences, Division of Pharmacology and Toxicology, University of Vienna, Josef-Holaubek-Platz 2, 1090, Vienna, Austria.
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Shalomov B, Handklo-Jamal R, Reddy HP, Theodor N, Bera AK, Dascal N. A revised mechanism of action of hyperaldosteronism-linked mutations in cytosolic domains of GIRK4 (KCNJ5). J Physiol 2021; 600:1419-1437. [PMID: 34957562 DOI: 10.1113/jp282690] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2021] [Accepted: 12/21/2021] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS Mutations in GIRK4 (KCNJ5) G-protein gated channels cause primary aldosteronism, a major cause of secondary hypertension. The primary mechanism is believed to be loss of K+ selectivity. R52H and E246K, aldosteronism-causing mutations in cytosolic N- and C- termini of GIRK4, were reported to cause loss of K+ selectivity. We show that R52H, E246K and G247R mutations render homotetrameric GIRK channels non-functional. In heterotetrameric context with GIRK1, these mutations impair membrane expression, interaction with Gβγ and open probability, but do not alter K+ selectivity or inward rectification. In human aldosterone-secreting cell line, a GIRK4 opener and overexpression of heterotetrameric GIRK1/4WT , but not over-expression of GIRK1/4 mutants, reduced aldosterone secretion. Aldosteronism-causing mutations in cytosolic domain of GIRK4 are loss-of-function mutations rather than gain-of-function, selectivity-loss mutations. Deciphering of exact biophysical mechanism that impairs the channel is crucial for setting the course of treatment. ABSTRACT G-protein gated, inwardly rectifying potassium channels (GIRK) mediate inhibitory transmission in brain and heart, and are present in adrenal cortex. GIRK4 (KCNJ5) subunits are abundant in the heart and adrenal cortex. Multiple mutations of KCNJ5 cause primary aldosteronism (PA). Mutations in the pore region of GIRK4 cause loss of K+ selectivity, Na+ influx, and depolarization of zona glomerulosa cells followed by hypersecretion of aldosterone. The concept of selectivity loss has been extended to mutations in cytosolic domains of GIRK4 channels, remote from the pore. We expressed aldosteronism-linked GIRK4R52H , GIRK4E246K , and GIRK4G247R mutants in Xenopus oocytes. Whole-cell currents of heterotetrameric GIRK1/4R52H and GIRK1/4E246K channels were greatly reduced compared to GIRK1/4WT . Nevertheless, all heterotetrameric mutants retained full K+ selectivity and inward rectification. When expressed as homotetramers, only GIRK4WT , but none of the mutants, produced whole-cell currents. Confocal imaging, single channel and Förster Resonance Energy Transfer (FRET) analyses showed: 1) reduction of membrane abundance of all mutated channels, especially as homotetramers, 2) impaired interaction with Gβγ subunits, and 3) reduced open probability of GIRK1/4R52H . VU0529331, a GIRK4 opener, activated homotetrameric GIRK4G247R channels, but not GIRK4R52H and GIRK4E246K . In human adrenocortical carcinoma cell line (HAC15), VU0529331 and over-expression of heterotetrameric GIRK1/4WT , but not over-expression of GIRK1/4 mutants, reduced aldosterone secretion. Our results suggest that, contrary to pore mutants of GIRK4, non-pore mutants R52H and E246K mutants are loss-of-function rather than gain-of-function/selectivity-loss mutants. Hence, GIRK4 openers may be a potential course of treatment for patients with cytosolic N- and C-terminal mutations. Abstract Figure: There are two mutations types in KCNJ5 (GIRK4) that can cause excessive secretion of aldosterone, leading to primary aldosteronism. Mutations of the first type render the channel non-selective to monovalent cations and often constitutively active, thus depolarizing the zona granulosa cells. This previously described mechanism underlies the disease-causing effects of mutations of amino acid residues located in the pore region (red color). Blockers of the channel may be useful as potential treatment to reduce aldosterone secretion. Here we show that mutations of the second type, located in the cytosolic domain remote from the pore, act by a different mechanism. They do not alter channel's ion selectivity or rectification but cause poor expression or poor activation by Gβγ, resulting in a reduction in cell's K+ conductance and depolarization. In this case, GIRK4 openers can potentially be useful to prevent the excessive aldosterone secretion. This article is protected by copyright. All rights reserved.
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Affiliation(s)
- Boris Shalomov
- Department of Physiology and Pharmacology, School of Medicine, and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Reem Handklo-Jamal
- Department of Physiology and Pharmacology, School of Medicine, and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Haritha P Reddy
- Department of Physiology and Pharmacology, School of Medicine, and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel.,Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras, Chennai, 600036, India
| | - Neta Theodor
- Department of Physiology and Pharmacology, School of Medicine, and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Amal K Bera
- Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras, Chennai, 600036, India
| | - Nathan Dascal
- Department of Physiology and Pharmacology, School of Medicine, and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, 69978, Israel
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6
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Encephalopathy-causing mutations in Gβ 1 ( GNB1) alter regulation of neuronal GIRK channels. iScience 2021; 24:103018. [PMID: 34522861 PMCID: PMC8426278 DOI: 10.1016/j.isci.2021.103018] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2021] [Revised: 08/04/2021] [Accepted: 08/18/2021] [Indexed: 11/24/2022] Open
Abstract
Mutations in the GNB1 gene, encoding the Gβ1 subunit of heterotrimeric G proteins, cause GNB1 Encephalopathy. Patients experience seizures, pointing to abnormal activity of ion channels or neurotransmitter receptors. We studied three Gβ1 mutations (K78R, I80N and I80T) using computational and functional approaches. In heterologous expression models, these mutations did not alter the coupling between G protein-coupled receptors to Gi/o, or the Gβγ regulation of the neuronal voltage-gated Ca2+ channel CaV2.2. However, the mutations profoundly affected the Gβγ regulation of the G protein-gated inwardly rectifying potassium channels (GIRK, or Kir3). Changes were observed in Gβ1 protein expression levels, Gβγ binding to cytosolic segments of GIRK subunits, and in Gβγ function, and included gain-of-function for K78R or loss-of-function for I80T/N, which were GIRK subunit-specific. Our findings offer new insights into subunit-dependent gating of GIRKs by Gβγ, and indicate diverse etiology of GNB1 Encephalopathy cases, bearing a potential for personalized treatment. GIRK channels are key players affected by GNB1 mutations under study (K78R and I80N/T) Effects of mutations (LoF or GoF) are channel subunit composition-specific The findings help to understand the GNB1 encephalopathy and to devise treatments The results yield new insights into mechanisms of Gβγ regulation of GIRKs
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Anderson A, Vo BN, de Velasco EMF, Hopkins CR, Weaver CD, Wickman K. Characterization of VU0468554, a New Selective Inhibitor of Cardiac G Protein-Gated Inwardly Rectifying K + Channels. Mol Pharmacol 2021; 100:540-547. [PMID: 34503975 DOI: 10.1124/molpharm.121.000311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2021] [Accepted: 09/01/2021] [Indexed: 11/22/2022] Open
Abstract
G protein-gated inwardly rectifying K+ (GIRK) channels are critical mediators of excitability in the heart and brain. Enhanced GIRK-channel activity has been implicated in the pathogenesis of supraventricular arrhythmias, including atrial fibrillation. The lack of selective pharmacological tools has impeded efforts to investigate the therapeutic potential of cardiac GIRK-channel interventions in arrhythmias. Here, we characterize a recently identified GIRK-channel inhibitor, VU0468554. Using whole-cell electrophysiological approaches and primary cultures of sinoatrial nodal cells and hippocampal neurons, we show that VU0468554 more effectively inhibits the cardiac GIRK channel than the neuronal GIRK channel. Concentration-response experiments suggest that VU0468554 inhibits Gβγ-activated GIRK channels in noncompetitive and potentially uncompetitive fashion. In contrast, VU0468554 competitively inhibits GIRK-channel activation by ML297, a GIRK-channel activator containing the same chemical scaffold as VU0468554. In the isolated heart model, VU0468554 partially reversed carbachol-induced bradycardia in hearts from wild-type mice but not Girk4-/- mice. Collectively, these data suggest that VU0468554 represents a promising new pharmacological tool for targeting cardiac GIRK channels with therapeutic implications for relevant cardiac arrhythmias. SIGNIFICANCE STATEMENT: Although cardiac GIRK-channel inhibition shows promise for the treatment of supraventricular arrhythmias, the absence of subtype-selective channel inhibitors has hindered exploration into this therapeutic strategy. This study utilizes whole-cell patch-clamp electrophysiology to characterize the new GIRK-channel inhibitor VU0468554 in human embryonic kidney 293T cells and primary cultures. We report that VU0468554 exhibits a favorable pharmacodynamic profile for cardiac over neuronal GIRK channels and partially reverses GIRK-mediated bradycardia in the isolated mouse heart model.
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Affiliation(s)
- Allison Anderson
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
| | - Baovi N Vo
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
| | - Ezequiel Marron Fernandez de Velasco
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
| | - Corey R Hopkins
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
| | - C David Weaver
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
| | - Kevin Wickman
- Graduate Program in Pharmacology (A.A., B.N.V.) and Department of Pharmacology (E.M.F.d.V., K.W.), University of Minnesota, Minneapolis, Minnesota; Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska (C.R.H.); and Departments of Pharmacology and Chemistry and Institute of Chemical Biology, Vanderbilt University, Nashville, Tennessee (C.D.W.)
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8
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van Aalst E, Wylie BJ. Cholesterol Is a Dose-Dependent Positive Allosteric Modulator of CCR3 Ligand Affinity and G Protein Coupling. Front Mol Biosci 2021; 8:724603. [PMID: 34490352 PMCID: PMC8417553 DOI: 10.3389/fmolb.2021.724603] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2021] [Accepted: 07/14/2021] [Indexed: 01/14/2023] Open
Abstract
Cholesterol as an allosteric modulator of G protein-coupled receptor (GPCR) function is well documented. This quintessential mammalian lipid facilitates receptor–ligand interactions and multimerization states. Functionally, this introduces a complicated mechanism for the homeostatic modulation of GPCR signaling. Chemokine receptors are Class A GPCRs responsible for immune cell trafficking through the binding of endogenous peptide ligands. CCR3 is a CC motif chemokine receptor expressed by eosinophils and basophils. It traffics these cells by transducing the signal stimulated by the CC motif chemokine primary messengers 11, 24, and 26. These behaviors are close to the human immunoresponse. Thus, CCR3 is implicated in cancer metastasis and inflammatory conditions. However, there is a paucity of experimental evidence linking the functional states of CCR3 to the molecular mechanisms of cholesterol–receptor cooperativity. In this vein, we present a means to combine codon harmonization and a maltose-binding protein fusion tag to produce CCR3 from E. coli. This technique yields ∼2.6 mg of functional GPCR per liter of minimal media. We leveraged this protein production capability to investigate the effects of cholesterol on CCR3 function in vitro. We found that affinity for the endogenous ligand CCL11 increases in a dose-dependent manner with cholesterol concentration in both styrene:maleic acid lipid particles (SMALPs) and proteoliposomes. This heightened receptor activation directly translates to increased signal transduction as measured by the GTPase activity of the bound G-protein α inhibitory subunit 3 (Gαi3). This work represents a critical step forward in understanding the role of cholesterol-GPCR allostery in regulation of signal transduction.
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Affiliation(s)
- Evan van Aalst
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, United States
| | - Benjamin J Wylie
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, United States
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9
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Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system. Pharmacol Ther 2021; 223:107808. [PMID: 33476640 DOI: 10.1016/j.pharmthera.2021.107808] [Citation(s) in RCA: 44] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2020] [Accepted: 01/05/2021] [Indexed: 12/15/2022]
Abstract
G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.
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10
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Berlin S, Artzy E, Handklo-Jamal R, Kahanovitch U, Parnas H, Dascal N, Yakubovich D. A Collision Coupling Model Governs the Activation of Neuronal GIRK1/2 Channels by Muscarinic-2 Receptors. Front Pharmacol 2020; 11:1216. [PMID: 32903404 PMCID: PMC7435011 DOI: 10.3389/fphar.2020.01216] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2020] [Accepted: 07/24/2020] [Indexed: 01/14/2023] Open
Abstract
The G protein-activated Inwardly Rectifying K+-channel (GIRK) modulates heart rate and neuronal excitability. Following G-Protein Coupled Receptor (GPCR)-mediated activation of heterotrimeric G proteins (Gαβγ), opening of the channel is obtained by direct binding of Gβγ subunits. Interestingly, GIRKs are solely activated by Gβγ subunits released from Gαi/o-coupled GPCRs, despite the fact that all receptor types, for instance Gαq-coupled, are also able to provide Gβγ subunits. It is proposed that this specificity and fast kinetics of activation stem from pre-coupling (or pre-assembly) of proteins within this signaling cascade. However, many studies, including our own, point towards a diffusion-limited mechanism, namely collision coupling. Here, we set out to address this long-standing question by combining electrophysiology, imaging, and mathematical modeling. Muscarinic-2 receptors (M2R) and neuronal GIRK1/2 channels were coexpressed in Xenopus laevis oocytes, where we monitored protein surface expression, current amplitude, and activation kinetics. Densities of expressed M2R were assessed using a fluorescently labeled GIRK channel as a molecular ruler. We then incorporated our results, along with available kinetic data reported for the G-protein cycle and for GIRK1/2 activation, to generate a comprehensive mathematical model for the M2R-G-protein-GIRK1/2 signaling cascade. We find that, without assuming any irreversible interactions, our collision coupling kinetic model faithfully reproduces the rate of channel activation, the changes in agonist-evoked currents and the acceleration of channel activation by increased receptor densities.
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Affiliation(s)
- Shai Berlin
- Department of Neuroscience, Rappaport Faculty of Medicine, Technion, Haifa, Israel
| | - Etay Artzy
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel
| | - Reem Handklo-Jamal
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel
| | - Uri Kahanovitch
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel
| | - Hanna Parnas
- Silberman Institute of Life Sciences, Hebrew University, Jerusalem, Israel
| | - Nathan Dascal
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel
| | - Daniel Yakubovich
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel Aviv-Yafo, Israel.,Department of Neonatology, Schneider Children's Hospital, Petah Tikva, Israel
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Evidence for an effect of receptor density on ligand occupancy and agonist EC 50. Sci Rep 2019; 9:19111. [PMID: 31836743 PMCID: PMC6910986 DOI: 10.1038/s41598-019-55361-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2019] [Accepted: 11/21/2019] [Indexed: 01/16/2023] Open
Abstract
Drug-receptor interaction theory predicts that proportional receptor occupancy is a function of ligand concentration as defined by a ligand-receptor affinity constant, and is independent of receptor density. However, we previously observed that the EC50 of 5-HT reduced as the density of 5-HT3 receptors increased, suggesting an effect of receptor density on occupancy. The current study was designed to maximise variability in experimentally observed currents and confirm this apparent contradiction prospectively. Xenopus oocytes were injected with RNA encoding 5-HT3A receptors under conditions designed to achieve varying receptor expression levels and 5-HT-evoked currents measured using two electrode voltage clamp. Results from 99 oocytes showed that as the maximal peak current increased from 0.05 µA to 12.1 µA there was a 3.7-fold reduction in EC50. Since occupancy and conductance are directly related in this system, this indicates that for a given concentration of 5-HT, proportional occupancy increases with increased receptor density. We conclude that normalising data masks this correlation, and can result in reduced accuracy of pharmacological measurements. We propose a mechanistic explanation for our observations.
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12
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Mutual action by Gγ and Gβ for optimal activation of GIRK channels in a channel subunit-specific manner. Sci Rep 2019; 9:508. [PMID: 30679535 PMCID: PMC6346094 DOI: 10.1038/s41598-018-36833-y] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2018] [Accepted: 11/29/2018] [Indexed: 01/06/2023] Open
Abstract
The tetrameric G protein-gated K+ channels (GIRKs) mediate inhibitory effects of neurotransmitters that activate Gi/o-coupled receptors. GIRKs are activated by binding of the Gβγ dimer, via contacts with Gβ. Gγ underlies membrane targeting of Gβγ, but has not been implicated in channel gating. We observed that, in Xenopus oocytes, expression of Gγ alone activated homotetrameric GIRK1* and heterotetrameric GIRK1/3 channels, without affecting the surface expression of GIRK or Gβ. Gγ and Gβ acted interdependently: the effect of Gγ required the presence of ambient Gβ and was enhanced by low doses of coexpressed Gβ, whereas excess of either Gβ or Gγ imparted suboptimal activation, possibly by sequestering the other subunit “away” from the channel. The unique distal C-terminus of GIRK1, G1-dCT, was important but insufficient for Gγ action. Notably, GIRK2 and GIRK1/2 were not activated by Gγ. Our results suggest that Gγ regulates GIRK1* and GIRK1/3 channel’s gating, aiding Gβ to trigger the channel’s opening. We hypothesize that Gγ helps to relax the inhibitory effect of a gating element (“lock”) encompassed, in part, by the G1-dCT; GIRK2 acts to occlude the effect of Gγ, either by setting in motion the same mechanism as Gγ, or by triggering an opposing gating effect.
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Yakubovich D, Berlin S, Kahanovitch U, Rubinstein M, Farhy-Tselnicker I, Styr B, Keren-Raifman T, Dessauer CW, Dascal N. A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ. PLoS Comput Biol 2015; 11:e1004598. [PMID: 26544551 PMCID: PMC4636287 DOI: 10.1371/journal.pcbi.1004598] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2015] [Accepted: 10/13/2015] [Indexed: 12/02/2022] Open
Abstract
G protein-gated K+ channels (GIRK; Kir3), activated by Gβγ subunits derived from Gi/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (Ievoked) and neurotransmitter-independent basal (Ibasal) GIRK activities are physiologically important, but mechanisms of Ibasal and its relation to Ievoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that Ibasal and Ievoked are interrelated: the extent of activation by neurotransmitter (activation index, Ra) is inversely related to Ibasal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates Ibasal and Ievoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse Ibasal-Ra correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gαi/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gαi/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα. Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K+ channel, GIRK (G protein-gated Inwardly Rectifying K+ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.
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Affiliation(s)
- Daniel Yakubovich
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Shai Berlin
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Uri Kahanovitch
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Moran Rubinstein
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Isabella Farhy-Tselnicker
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Boaz Styr
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Tal Keren-Raifman
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Carmen W. Dessauer
- Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, Texas, United States of America
| | - Nathan Dascal
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
- * E-mail:
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14
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Glaaser IW, Slesinger PA. Structural Insights into GIRK Channel Function. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2015; 123:117-60. [PMID: 26422984 DOI: 10.1016/bs.irn.2015.05.014] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
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.
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Affiliation(s)
- Ian W Glaaser
- Department of Neuroscience, Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA
| | - Paul A Slesinger
- Department of Neuroscience, Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, New York, USA.
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15
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Dascal N, Kahanovitch U. The Roles of Gβγ and Gα in Gating and Regulation of GIRK Channels. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2015; 123:27-85. [DOI: 10.1016/bs.irn.2015.06.001] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
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16
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de Velasco EMF, McCall N, Wickman K. GIRK Channel Plasticity and Implications for Drug Addiction. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2015; 123:201-38. [DOI: 10.1016/bs.irn.2015.05.011] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
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17
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Kahanovitch U, Tsemakhovich V, Berlin S, Rubinstein M, Styr B, Castel R, Peleg S, Tabak G, Dessauer CW, Ivanina T, Dascal N. Recruitment of Gβγ controls the basal activity of G-protein coupled inwardly rectifying potassium (GIRK) channels: crucial role of distal C terminus of GIRK1. J Physiol 2014; 592:5373-90. [PMID: 25384780 DOI: 10.1113/jphysiol.2014.283218] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The G-protein coupled inwardly rectifying potassium (GIRK, or Kir3) channels are important mediators of inhibitory neurotransmission via activation of G-protein coupled receptors (GPCRs). GIRK channels are tetramers comprising combinations of subunits (GIRK1-4), activated by direct binding of the Gβγ subunit of Gi/o proteins. Heterologously expressed GIRK1/2 exhibit high, Gβγ-dependent basal currents (Ibasal) and a modest activation by GPCR or coexpressed Gβγ. Inversely, the GIRK2 homotetramers exhibit low Ibasal and strong activation by Gβγ. The high Ibasal of GIRK1 seems to be associated with its unique distal C terminus (G1-dCT), which is not present in the other subunits. We investigated the role of G1-dCT using electrophysiological and fluorescence assays in Xenopus laevis oocytes and protein interaction assays. We show that expression of GIRK1/2 increases the plasma membrane level of coexpressed Gβγ (a phenomenon we term 'Gβγ recruitment') but not of coexpressed Gαi3. All GIRK1-containing channels, but not GIRK2 homomers, recruited Gβγ to the plasma membrane. In biochemical assays, truncation of G1-dCT reduces the binding between the cytosolic parts of GIRK1 and Gβγ, but not Gαi3. Nevertheless, the truncation of G1-dCT does not impair activation by Gβγ. In fluorescently labelled homotetrameric GIRK1 channels and in the heterotetrameric GIRK1/2 channel, the truncation of G1-dCT abolishes Gβγ recruitment and decreases Ibasal. Thus, we conclude that G1-dCT carries an essential role in Gβγ recruitment by GIRK1 and, consequently, in determining its high basal activity. Our results indicate that G1-dCT is a crucial part of a Gβγ anchoring site of GIRK1-containing channels, spatially and functionally distinct from the site of channel activation by Gβγ.
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Affiliation(s)
- Uri Kahanovitch
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Vladimir Tsemakhovich
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Shai Berlin
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Moran Rubinstein
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Boaz Styr
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Ruth Castel
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Sagit Peleg
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Galit Tabak
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Carmen W Dessauer
- Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, TX, 77030, USA
| | - Tatiana Ivanina
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
| | - Nathan Dascal
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv 69978, Israel
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18
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Wang W, Whorton MR, MacKinnon R. Quantitative analysis of mammalian GIRK2 channel regulation by G proteins, the signaling lipid PIP2 and Na+ in a reconstituted system. eLife 2014; 3:e03671. [PMID: 25049222 PMCID: PMC4135351 DOI: 10.7554/elife.03671] [Citation(s) in RCA: 66] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
GIRK channels control spike frequency in atrial pacemaker cells and inhibitory potentials in neurons. By directly responding to G proteins, PIP2 and Na+, GIRK is under the control of multiple signaling pathways. In this study, the mammalian GIRK2 channel has been purified and reconstituted in planar lipid membranes and effects of Gα, Gβγ, PIP2 and Na+ analyzed. Gβγ and PIP2 must be present simultaneously to activate GIRK2. Na+ is not essential but modulates the effect of Gβγ and PIP2 over physiological concentrations. Gαi1(GTPγS) has no effect, whereas Gαi1(GDP) closes the channel through removal of Gβγ. In the presence of Gβγ, GIRK2 opens as a function of PIP2 mole fraction with Hill coefficient 2.5 and an affinity that poises GIRK2 to respond to natural variations of PIP2 concentration. The dual requirement for Gβγ and PIP2 can help to explain why GIRK2 is activated by Gi/o, but not Gq coupled GPCRs. DOI:http://dx.doi.org/10.7554/eLife.03671.001 Though every cell in the body is surrounded by a membrane, there are a number of ways that molecules can pass through this membrane to either enter or leave the cell. Proteins from the GIRK family form channels in the membranes of mammalian cells, and when open these channels allow potassium ions to flow through the membrane to control the membrane's voltage. GIRK channels are found in the heart and in the central nervous system, and can be activated in a variety of ways. Sodium ions and molecules called ‘signaling lipids’ can regulate the activation of GIRK channels. These channels can also be caused to open by G proteins: proteins that are found inside cells and that help to transmit signals from the outside of a cell to the inside. Three G proteins—called Gα, Gβ, and Gγ—work together in a complex that functions a bit like a switch. When switched on, the Gα subunit is separated from the other two subunits (called Gβγ); and both parts can then activate different signaling pathways inside the cell. The Gβγ subunits and a signaling lipid have been known to regulate the opening of GIRK channels for a number of years, but these events have only been studied in the context of living cells. The specific role of each molecule, and whether the Gα subunit can also regulate the GIRK channels, remains unknown. Now Wang et al. have produced one type of mouse GIRK channel, called GIRK2, in yeast cells, purified this protein, and added it into an artificial membrane. This ‘reconstituted system’ allowed the regulation of a GIRK channel to be investigated under more controlled conditions than in previous experiments. Wang et al. found that the Gβγ subunits and the signaling lipid both need to be present to activate the GIRK2 channel. Sodium ions were not essential, but promoted further opening when Gβγ and the signaling lipid were already present. When locked in its ‘on’ state, the Gα subunit had no effect on GIRK2, but adding Gα locked in the ‘off’ state closed these channels by removing the Gβγ proteins. The findings of Wang et al. suggest that it should be possible to use a similar reconstituted system to investigate what allows different G proteins to activate specific signaling pathways. DOI:http://dx.doi.org/10.7554/eLife.03671.002
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Affiliation(s)
- Weiwei Wang
- Laboratory of Molecular Neurobiology and Biophysics, Howard Hughes Medical Institute, The Rockefeller University, New York, United States
| | - Matthew R Whorton
- Laboratory of Molecular Neurobiology and Biophysics, Howard Hughes Medical Institute, The Rockefeller University, New York, United States
| | - Roderick MacKinnon
- Laboratory of Molecular Neurobiology and Biophysics, Howard Hughes Medical Institute, The Rockefeller University, New York, United States
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19
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Dual regulation of G proteins and the G-protein-activated K+ channels by lithium. Proc Natl Acad Sci U S A 2014; 111:5018-23. [PMID: 24639496 DOI: 10.1073/pnas.1316425111] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Lithium (Li(+)) is widely used to treat bipolar disorder (BPD). Cellular targets of Li(+), such as glycogen synthase kinase 3β (GSK3β) and G proteins, have long been implicated in BPD etiology; however, recent genetic studies link BPD to other proteins, particularly ion channels. Li(+) affects neuronal excitability, but the underlying mechanisms and the relevance to putative BPD targets are unknown. We discovered a dual regulation of G protein-gated K(+) (GIRK) channels by Li(+), and identified the underlying molecular mechanisms. In hippocampal neurons, therapeutic doses of Li(+) (1-2 mM) increased GIRK basal current (Ibasal) but attenuated neurotransmitter-evoked GIRK currents (Ievoked) mediated by Gi/o-coupled G-protein-coupled receptors (GPCRs). Molecular mechanisms of these regulations were studied with heterologously expressed GIRK1/2. In excised membrane patches, Li(+) increased Ibasal but reduced GPCR-induced GIRK currents. Both regulations were membrane-delimited and G protein-dependent, requiring both Gα and Gβγ subunits. Li(+) did not impair direct activation of GIRK channels by Gβγ, suggesting that inhibition of Ievoked results from an action of Li(+) on Gα, probably through inhibition of GTP-GDP exchange. In direct binding studies, Li(+) promoted GPCR-independent dissociation of Gαi(GDP) from Gβγ by a Mg(2+)-independent mechanism. This previously unknown Li(+) action on G proteins explains the second effect of Li(+), the enhancement of GIRK's Ibasal. The dual effect of Li(+) on GIRK may profoundly regulate the inhibitory effects of neurotransmitters acting via GIRK channels. Our findings link between Li(+), neuronal excitability, and both cellular and genetic targets of BPD: GPCRs, G proteins, and ion channels.
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Bodhinathan K, Slesinger PA. Alcohol modulation of G-protein-gated inwardly rectifying potassium channels: from binding to therapeutics. Front Physiol 2014; 5:76. [PMID: 24611054 PMCID: PMC3933770 DOI: 10.3389/fphys.2014.00076] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2013] [Accepted: 02/07/2014] [Indexed: 12/27/2022] Open
Abstract
Alcohol (ethanol)-induced behaviors may arise from direct interaction of alcohol with discrete protein cavities within brain proteins. Recent structural and biochemical studies have provided new insights into the mechanism of alcohol-dependent activation of G protein-gated inwardly rectifying potassium (GIRK) channels, which regulate neuronal responses in the brain reward circuit. GIRK channels contain an alcohol binding pocket formed at the interface of two adjacent channel subunits. Here, we discuss the physiochemical properties of the alcohol pocket and the roles of G protein βγ subunits and membrane phospholipid PIP2 in regulating the alcohol response of GIRK channels. Some of the features of alcohol modulation of GIRK channels may be common to other alcohol-sensitive brain proteins. We discuss the possibility of alcohol-selective therapeutics that block alcohol access to the pocket. Understanding alcohol recognition and modulation of brain proteins is essential for development of therapeutics for alcohol abuse and addiction.
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Affiliation(s)
- Karthik Bodhinathan
- Structural Biology and Peptide Biology Laboratories, The Salk Institute for Biological Studies La Jolla, CA, USA
| | - Paul A Slesinger
- Structural Biology and Peptide Biology Laboratories, The Salk Institute for Biological Studies La Jolla, CA, USA ; Department of Neuroscience, Icahn School of Medicine at Mount Sinai New York, NY, USA
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21
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Kienitz MC, Mintert-Jancke E, Hertel F, Pott L. Differential effects of genetically-encoded Gβγ scavengers on receptor-activated and basal Kir3.1/Kir3.4 channel current in rat atrial myocytes. Cell Signal 2014; 26:1182-92. [PMID: 24576551 DOI: 10.1016/j.cellsig.2014.02.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2013] [Revised: 01/23/2014] [Accepted: 02/13/2014] [Indexed: 12/23/2022]
Abstract
Opening of G-protein-activated inward-rectifying K(+) (GIRK, Kir3) channels is regulated by interaction with βγ-subunits of Pertussis-toxin-sensitive G proteins upon activation of appropriate GPCRs. In atrial and neuronal cells agonist-independent activity (I(basal)) contributes to the background K(+) conductance, important for stabilizing resting potential. Data obtained from the Kir3 signaling pathway reconstituted in Xenopus oocytes suggest that I(basal) requires free G(βγ). In cells with intrinsic expression of Kir3 channels this issue has been scarcely addressed experimentally. Two G(βγ)-binding proteins (myristoylated phosducin - mPhos - and G(αi1)) were expressed in atrial myocytes using adenoviral gene transfer, to interrupt G(βγ)-signaling. Agonist-induced and basal currents were recorded using whole cell voltage-clamp. Expression of mPhos and G(αi1) reduced activation of Kir3 current via muscarinic M(2) receptors (IK(ACh)). Inhibition of IK(ACh) by mPhos consisted of an irreversible component and an agonist-dependent reversible component. Reduction in density of IK(ACh) by overexpressed Gαi1, in contrast to mPhos, was paralleled by substantial slowing of activation, suggesting a reduction in density of functional M2 receptors, rather than G(βγ)-scavenging as underlying mechanism. In line with this notion, current density and activation kinetics were rescued by fusing the αi1-subunit to an Adenosine A(1) receptor. Neither mPhos nor G(αi1) had a significant effect on I(basal), defined by the inhibitory peptide tertiapin-Q. These data demonstrate that basal Kir3 current in a native environment is unrelated to G-protein signaling or agonist-independent free G(βγ). Moreover, our results illustrate the importance of physiological expression levels of the signaling components in shaping key parameters of the response to an agonist.
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Affiliation(s)
| | | | - Fabian Hertel
- Institute of Physiology, Ruhr-University Bochum, D-44780 Bochum, Germany
| | - Lutz Pott
- Institute of Physiology, Ruhr-University Bochum, D-44780 Bochum, Germany
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Constitutive Activity of the Acetylcholine-Activated Potassium Current IK,ACh in Cardiomyocytes. ADVANCES IN PHARMACOLOGY 2014; 70:393-409. [DOI: 10.1016/b978-0-12-417197-8.00013-4] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
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23
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Luján R, Marron Fernandez de Velasco E, Aguado C, Wickman K. New insights into the therapeutic potential of Girk channels. Trends Neurosci 2013; 37:20-9. [PMID: 24268819 DOI: 10.1016/j.tins.2013.10.006] [Citation(s) in RCA: 95] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2013] [Revised: 10/24/2013] [Accepted: 10/25/2013] [Indexed: 01/01/2023]
Abstract
G protein-dependent signaling pathways control the activity of excitable cells of the nervous system and heart, and are the targets of neurotransmitters, clinically relevant drugs, and drugs of abuse. G protein-gated inwardly rectifying potassium (K(+)) (Girk/Kir3) channels are a key effector in inhibitory signaling pathways. Girk-dependent signaling contributes to nociception and analgesia, reward-related behavior, mood, cognition, and heart-rate regulation, and has been linked to epilepsy, Down syndrome, addiction, and arrhythmias. We discuss recent advances in our understanding of Girk channel structure, organization in signaling complexes, and plasticity, as well as progress on the development of subunit-selective Girk modulators. These findings offer new hope for the selective manipulation of Girk channels to treat a variety of debilitating afflictions.
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Affiliation(s)
- Rafael Luján
- Instituto de Investigación en Discapacidades Neurológicas (IDINE), Departamento de Ciencias Médicas, Facultad de Medicina, Universidad Castilla-La Mancha, Campus Biosanitario, C/Almansa 14, 02008 Albacete, Spain.
| | | | - Carolina Aguado
- Instituto de Investigación en Discapacidades Neurológicas (IDINE), Departamento de Ciencias Médicas, Facultad de Medicina, Universidad Castilla-La Mancha, Campus Biosanitario, C/Almansa 14, 02008 Albacete, Spain
| | - Kevin Wickman
- Department of Pharmacology, University of Minnesota, 321 Church Street South East, Minneapolis, MN 55455, USA.
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Inanobe A, Nakagawa A, Kurachi Y. Conformational changes underlying pore dilation in the cytoplasmic domain of mammalian inward rectifier K+ channels. PLoS One 2013; 8:e79844. [PMID: 24244570 PMCID: PMC3823594 DOI: 10.1371/journal.pone.0079844] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2013] [Accepted: 09/25/2013] [Indexed: 11/18/2022] Open
Abstract
The cytoplasmic domain of inward rectifier K+ (Kir) channels associates with cytoplasmic ligands and undergoes conformational change to control the gate present in its transmembrane domain. Ligand-operated activation appears to cause dilation of the pore at the cytoplasmic domain. However, it is still unclear how the cytoplasmic domain supports pore dilation and how alterations to this domain affect channel activity. In the present study, we focused on 2 spatially adjacent residues, i.e., Glu236 and Met313, of the G protein-gated Kir channel subunit Kir3.2. In the closed state, these pore-facing residues are present on adjacent βD and βH strands, respectively. We mutated both residues, expressed them with the m2-muscarinic receptor in Xenopus oocytes, and measured the acetylcholine-dependent K+ currents. The dose-response curves of the Glu236 mutants tended to be shifted to the right. In comparison, the slopes of the concentration-dependent curves were reduced and the single-channel properties were altered in the Met313 mutants. The introduction of arginine at position 236 conferred constitutive activity and caused a leftward shift in the conductance-voltage relationship. The crystal structure of the cytoplasmic domain of the mutant showed that the arginine contacts the main chains of the βH and βI strands of the adjacent subunit. Because the βH strand forms a β sheet with the βI and βD strands, the immobilization of the pore-forming β sheet appears to confer unique properties to the mutant. These results suggest that the G protein association triggers pore dilation at the cytoplasmic domain in functional channels, and the pore-constituting structural elements contribute differently to these conformational changes.
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Affiliation(s)
- Atsushi Inanobe
- Department of Pharmacology, Graduate School of Medicine, Osaka University, Suita, Osaka, Japan
- Center for Advanced Medical Engineering and Informatics, Osaka University, Suita, Osaka, Japan
- * E-mail: (AI); (YK)
| | - Atsushi Nakagawa
- Laboratory of Supramolecular Crystallography, Institute for Protein Research, Osaka University, Suita, Osaka, Japan
| | - Yoshihisa Kurachi
- Department of Pharmacology, Graduate School of Medicine, Osaka University, Suita, Osaka, Japan
- Center for Advanced Medical Engineering and Informatics, Osaka University, Suita, Osaka, Japan
- * E-mail: (AI); (YK)
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25
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Zylbergold P, Sleno R, Hébert TE. A novel, radiolabel-free pulse chase strategy to study Kir3 channel ontogeny. J Recept Signal Transduct Res 2013; 33:144-52. [DOI: 10.3109/10799893.2013.764898] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
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26
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Sulaiman P, Xu Y, Fina ME, Tummala SR, Ramakrishnan H, Dhingra A, Vardi N. Kir2.4 surface expression and basal current are affected by heterotrimeric G-proteins. J Biol Chem 2013; 288:7420-9. [PMID: 23339194 DOI: 10.1074/jbc.m112.412791] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
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.
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Affiliation(s)
- Pyroja Sulaiman
- Department of Neuroscience, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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27
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Molecular basis of the facilitation of the heterooligomeric GIRK1/GIRK4 complex by cAMP dependent protein kinase. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1828:1214-21. [PMID: 23305758 PMCID: PMC3787752 DOI: 10.1016/j.bbamem.2012.12.016] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/25/2012] [Revised: 12/04/2012] [Accepted: 12/26/2012] [Indexed: 01/01/2023]
Abstract
G-protein activated inwardly rectifying K+ channels (GIRKs) of the heterotetrameric GIRK1/GIRK4 composition mediate IK + ACh in atrium and are regulated by cAMP dependent protein kinase (PKA). Phosphorylation of GIRK1/GIRK4 complexes promotes the activation of the channel by the G-protein Gβγ-dimer (“heterologous facilitation”). Previously we reported that 3 serines/threonines (S/Ts) within the GIRK1 subunit are phosphorylated by the catalytic subunit of PKA (PKA-cs) in-vitro and are responsible for the acute functional effects exerted by PKA on the homooligomeric GIRK1F137S (GIRK1⁎) channel. Here we report that homooligomeric GIRK4WT and GIRK4S143T (GIRK4⁎) channels are clearly regulated by PKA phosphorylation. Heterooligomeric channels of the GIRK1S385CS401CT407C/GIRK4WT composition, where the GIRK1 subunit is devoid of PKA mediated phosphorylation, exhibited reduced but still significant acute effects (reduction during agonist application was ≈ 49% compared to GIRK1WT/GIRK4WT). Site directed mutagenesis of truncated cytosolic regions of GIRK4 revealed four serines/threonines (S/Ts) that were heavily phosphorylated by PKA-cs in vitro. Two of them were found to be responsible for the acute effects exerted by PKA in vivo, since the effect of cAMP injection was reduced by ≈ 99% in homooligomeric GIRK4⁎T199CS412C channels. Coexpression of GIRK1WT/GIRK4T199CS412C reduced the acute effect by ≈ 65%. Only channels of the GIRK1S385CS401CT407C/GIRK4T199CS412C composition were practically devoid of PKA mediated effects (reduction by ≈ 97%), indicating that both subunits contribute to the heterologous facilitation of IK + ACh.
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28
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Wydeven N, Young D, Mirkovic K, Wickman K. Structural elements in the Girk1 subunit that potentiate G protein-gated potassium channel activity. Proc Natl Acad Sci U S A 2012; 109:21492-7. [PMID: 23236146 PMCID: PMC3535602 DOI: 10.1073/pnas.1212019110] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
G protein-gated inwardly rectifying K(+) (Girk/K(IR)3) channels mediate the inhibitory effect of many neurotransmitters on excitable cells. Girk channels are tetramers consisting of various combinations of four mammalian Girk subunits (Girk1 to -4). Although Girk1 is unable to form functional homomeric channels, its presence in cardiac and neuronal channel complexes correlates with robust channel activity. This study sought to better understand the potentiating influence of Girk1, using the GABA(B) receptor and Girk1/Girk2 heteromer as a model system. Girk1 did not increase the protein levels or alter the trafficking of Girk2-containing channels to the cell surface in transfected cells or hippocampal neurons, indicating that its potentiating influence involves enhancement of channel activity. Structural elements in both the distal carboxyl-terminal domain and channel core were identified as key determinants of robust channel activity. In the distal carboxyl-terminal domain, residue Q404 was identified as a key determinant of receptor-induced channel activity. In the Girk1 core, three unique residues in the pore (P) loop (F137, A142, Y150) were identified as a collective potentiating influence on both receptor-dependent and receptor-independent channel activity, exerting their influence, at least in part, by enhancing mean open time and single-channel conductance. Interestingly, the potentiating influence of the Girk1 P-loop is tempered by residue F162 in the second membrane-spanning domain. Thus, discontinuous and sometime opposing elements in Girk1 underlie the Girk1-dependent potentiation of receptor-dependent and receptor-independent heteromeric channel activity.
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Affiliation(s)
- Nicole Wydeven
- Department of Pharmacology, University of Minnesota, Minneapolis, MN 55455
| | - Daniele Young
- Department of Pharmacology, University of Minnesota, Minneapolis, MN 55455
| | - Kelsey Mirkovic
- Department of Pharmacology, University of Minnesota, Minneapolis, MN 55455
| | - Kevin Wickman
- Department of Pharmacology, University of Minnesota, Minneapolis, MN 55455
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29
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Mase Y, Yokogawa M, Osawa M, Shimada I. 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). J Biol Chem 2012; 287:19537-49. [PMID: 22511772 DOI: 10.1074/jbc.m112.353888] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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).
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Affiliation(s)
- Yoko Mase
- Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
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30
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Makary S, Voigt N, Maguy A, Wakili R, Nishida K, Harada M, Dobrev D, Nattel S. Differential Protein Kinase C Isoform Regulation and Increased Constitutive Activity of Acetylcholine-Regulated Potassium Channels in Atrial Remodeling. Circ Res 2011; 109:1031-43. [DOI: 10.1161/circresaha.111.253120] [Citation(s) in RCA: 81] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Rationale:
Atrial fibrillation (AF) causes atrial-tachycardia remodeling (ATR), with enhanced constitutive acetylcholine-regulated K
+
current (I
KAChC
) contributing to action potential duration shortening and AF promotion. The underlying mechanisms are unknown.
Objective:
To evaluate the role of protein-kinase C (PKC) isoforms in ATR-induced I
KAChC
activation.
Methods and Results:
Cells from ATR-dogs (400-bpm atrial pacing for 1 week) were compared to control dog cells. In vitro tachypaced (TP; 3 Hz) canine atrial cardiomyocytes were compared to parallel 1-Hz paced cells. I
KAChC
single-channel activity was assessed in cell-attached and cell-free (inside-out) patches. Protein expression was assessed by immunoblot. In vitro TP activated I
KAChC
, mimicking effects of in vivo ATR. Discrepant effects of PKC activation and inhibition between control and ATR cells suggested isoform-selective effects and altered PKC isoform distribution. Conventional PKC isoforms (cPKC; including PKCα) inhibited, whereas novel isoforms (including PKCε) enhanced, acetylcholine-regulated K
+
current (I
KACh
) in inside-out patches. TP and ATR downregulated PKCα (by 33% and 37%, respectively) and caused membrane translocation of PKCε, switching PKC predominance to the stimulatory novel isoform. TP increased [Ca
2+
]
i
at 2 hours by 30%, with return to baseline at 24 hours. Buffering [Ca
2+
]
i
during TP with the cell-permeable Ca
2+
chelator BAPTA-AM (1 μmol/L) or inhibiting the Ca
2+
-dependent protease calpain with PD150606 (20 μmol/L) prevented PKCα downregulation and TP enhancement of I
KAChC
. PKCε inhibition with a cell-permeable peptide inhibitor suppressed TP/ATR-induced I
KAChC
activation, whereas cPKC inhibition enhanced I
KAChC
activity in 1-Hz cells.
Conclusions:
PKC isoforms differentially modulate I
KACh
, with conventional Ca
2+
-dependent isoforms inhibiting and novel isoforms enhancing activity. ATR causes a rate-dependent PKC isoform switch, with Ca
2+
/calpain-dependent downregulation of inhibitory PKCα and membrane translocation of stimulatory PKCε, enhancing I
KAChC
. These findings provide novel insights into mechanisms underlying I
KAChC
dysregulation in AF.
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Affiliation(s)
- Samy Makary
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Niels Voigt
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Ange Maguy
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Reza Wakili
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Kunihiro Nishida
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Masahide Harada
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Dobromir Dobrev
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
| | - Stanley Nattel
- From the Department of Medicine and Research Center (S.M., A.M., R.W., K.N., M.H., S.N.), Montreal Heart Institute and Université de Montréal, Montreal, Quebec, Canada; Physiology Department, Faculty of Medicine, Suez Canal University, Egypt (S.M.); Department of Pharmacology and Therapeutics (S.N.), McGill University, Montreal, Quebec, Canada; Division of Experimental Cardiology (S.M., N.V., D.D.), Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany; Department of Medicine I (R.W
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31
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Berlin S, Tsemakhovich VA, Castel R, Ivanina T, Dessauer CW, Keren-Raifman T, Dascal N. Two distinct aspects of coupling between Gα(i) protein and G protein-activated K+ channel (GIRK) revealed by fluorescently labeled Gα(i3) protein subunits. J Biol Chem 2011; 286:33223-35. [PMID: 21795707 DOI: 10.1074/jbc.m111.271056] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
G protein-activated K(+) channels (Kir3 or GIRK) are activated by direct interaction with Gβγ. Gα is essential for specific signaling and regulates basal activity of GIRK (I(basal)) and kinetics of the response elicited by activation by G protein-coupled receptors (I(evoked)). These regulations are believed to occur within a GIRK-Gα-Gβγ signaling complex. Fluorescent energy resonance transfer (FRET) studies showed strong GIRK-Gβγ interactions but yielded controversial results regarding the GIRK-Gα(i/o) interaction. We investigated the mechanisms of regulation of GIRK by Gα(i/o) using wild-type Gα(i3) (Gα(i3)WT) and Gα(i3) labeled at three different positions with fluorescent proteins, CFP or YFP (xFP). Gα(i3)xFP proteins bound the cytosolic domain of GIRK1 and interacted with Gβγ in a guanine nucleotide-dependent manner. However, only an N-terminally labeled, myristoylated Gα(i3)xFP (Gα(i3)NT) closely mimicked all aspects of Gα(i3)WT regulation except for a weaker regulation of I(basal). Gα(i3) labeled with YFP within the Gα helical domain preserved regulation of I(basal) but failed to restore fast I(evoked). Titrated expression of Gα(i3)NT and Gα(i3)WT confirmed that regulation of I(basal) and of the kinetics of I(evoked) of GIRK1/2 are independent functions of Gα(i). FRET and direct biochemical measurements indicated much stronger interaction between GIRK1 and Gβγ than between GIRK1 and Gα(i3). Thus, Gα(i/o)βγ heterotrimer may be attached to GIRK primarily via Gβγ within the signaling complex. Our findings support the notion that Gα(i/o) actively regulates GIRK. Although regulation of I(basal) is a function of Gα(i)(GDP), our new findings indicate that regulation of kinetics of I(evoked) is mediated by Gα(i)(GTP).
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Affiliation(s)
- Shai Berlin
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
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32
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Leal-Pinto E, Gómez-Llorente Y, Sundaram S, Tang QY, Ivanova-Nikolova T, Mahajan R, Baki L, Zhang Z, Chavez J, Ubarretxena-Belandia I, Logothetis DE. Gating of a G protein-sensitive mammalian Kir3.1 prokaryotic Kir channel chimera in planar lipid bilayers. J Biol Chem 2010; 285:39790-800. [PMID: 20937804 DOI: 10.1074/jbc.m110.151373] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Kir3 channels control heart rate and neuronal excitability through GTP-binding (G) protein and phosphoinositide signaling pathways. These channels were the first characterized effectors of the βγ subunits of G proteins. Because we currently lack structures of complexes between G proteins and Kir3 channels, their interactions leading to modulation of channel function are not well understood. The recent crystal structure of a chimera between the cytosolic domain of a mammalian Kir3.1 and the transmembrane region of a prokaryotic KirBac1.3 (Kir3.1 chimera) has provided invaluable structural insight. However, it was not known whether this chimera could form functional K(+) channels. Here, we achieved the functional reconstitution of purified Kir3.1 chimera in planar lipid bilayers. The chimera behaved like a bona fide Kir channel displaying an absolute requirement for PIP(2) and Mg(2+)-dependent inward rectification. The channel could also be blocked by external tertiapin Q. The three-dimensional reconstruction of the chimera by single particle electron microscopy revealed a structure consistent with the crystal structure. Channel activity could be stimulated by ethanol and activated G proteins. Remarkably, the presence of both activated Gα and Gβγ subunits was required for gating of the channel. These results confirm the Kir3.1 chimera as a valid structural and functional model of Kir3 channels.
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Affiliation(s)
- Edgar Leal-Pinto
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, Virginia 23298, USA
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33
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Zylbergold P, Ramakrishnan N, Hebert T. The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels. Channels (Austin) 2010; 4:411-21. [PMID: 20855978 DOI: 10.4161/chan.4.5.13327] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.
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Affiliation(s)
- Peter Zylbergold
- Department of Pharmacology and Therapeutics, McGill University, Québec, Canada
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34
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Tselnicker I, Tsemakhovich VA, Dessauer CW, Dascal N. Stargazin modulates neuronal voltage-dependent Ca(2+) channel Ca(v)2.2 by a Gbetagamma-dependent mechanism. J Biol Chem 2010; 285:20462-71. [PMID: 20435886 DOI: 10.1074/jbc.m110.121277] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Loss of neuronal protein stargazin (gamma(2)) is associated with recurrent epileptic seizures and ataxia in mice. Initially, due to homology to the skeletal muscle calcium channel gamma(1) subunit, stargazin and other family members (gamma(3-8)) were classified as gamma subunits of neuronal voltage-gated calcium channels (such as Ca(V)2.1-Ca(V)2.3). Here, we report that stargazin interferes with G protein modulation of Ca(V)2.2 (N-type) channels expressed in Xenopus oocytes. Stargazin counteracted the Gbetagamma-induced inhibition of Ca(V)2.2 channel currents, caused either by coexpression of the Gbetagamma dimer or by activation of a G protein-coupled receptor. Expression of high doses of Gbetagamma overcame the effects of stargazin. High affinity Gbetagamma scavenger proteins m-cbetaARK and m-phosducin produced effects similar to stargazin. The effects of stargazin and m-cbetaARK were not additive, suggesting a common mechanism of action, and generally independent of the presence of the Ca(V)beta(3) subunit. However, in some cases, coexpression of Ca(V)beta(3) blunted the modulation by stargazin. Finally, the Gbetagamma-opposing action of stargazin was not unique to Ca(V)2.2, as stargazin also inhibited the Gbetagamma-mediated activation of the G protein-activated K(+) channel. Purified cytosolic C-terminal part of stargazin bound Gbetagamma in vitro. Our results suggest that the regulation by stargazin of biophysical properties of Ca(V)2.2 are not exerted by direct modulation of the channel but via a Gbetagamma-dependent mechanism.
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Affiliation(s)
- Isabella Tselnicker
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel.
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35
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Berlin S, Keren-Raifman T, Castel R, Rubinstein M, Dessauer CW, Ivanina T, Dascal N. G alpha(i) and G betagamma jointly regulate the conformations of a G betagamma effector, the neuronal G protein-activated K+ channel (GIRK). J Biol Chem 2009; 285:6179-85. [PMID: 20018875 DOI: 10.1074/jbc.m109.085944] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Stable complexes among G proteins and effectors are an emerging concept in cell signaling. The prototypical G betagamma effector G protein-activated K(+) channel (GIRK; Kir3) physically interacts with G betagamma but also with G alpha(i/o). Whether and how G alpha(i/o) subunits regulate GIRK in vivo is unclear. We studied triple interactions among GIRK subunits 1 and 2, G alpha(i3) and G betagamma. We used in vitro protein interaction assays and in vivo intramolecular Förster resonance energy transfer (i-FRET) between fluorophores attached to N and C termini of either GIRK1 or GIRK2 subunit. We demonstrate, for the first time, that G betagamma and G alpha(i3) distinctly and interdependently alter the conformational states of the heterotetrameric GIRK1/2 channel. Biochemical experiments show that G betagamma greatly enhances the binding of GIRK1 subunit to G alpha(i3)(GDP) and, unexpectedly, to G alpha(i3)(GTP). i-FRET showed that both G alpha(i3) and G betagamma induced distinct conformational changes in GIRK1 and GIRK2. Moreover, GIRK1 and GIRK2 subunits assumed unique, distinct conformations when coexpressed with a "constitutively active" G alpha(i3) mutant and G betagamma together. These conformations differ from those assumed by GIRK1 or GIRK2 after separate coexpression of either G alpha(i3) or G betagamma. Both biochemical and i-FRET data suggest that GIRK acts as the nucleator of the GIRK-G alpha-G betagamma signaling complex and mediates allosteric interactions between G alpha(i)(GTP) and G betagamma. Our findings imply that G alpha(i/o) and the G alpha(i) betagamma heterotrimer can regulate a G betagamma effector both before and after activation by neurotransmitters.
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Affiliation(s)
- Shai Berlin
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel.
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36
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Schreibmayer W. Emerging role(s) of G-protein alpha-subunits in the gating of GIRKs. J Physiol 2009; 587:4125-6. [PMID: 19720846 DOI: 10.1113/jphysiol.2009.179234] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Affiliation(s)
- Wolfgang Schreibmayer
- Molecular Physiology Laboratory, Institute of Biophysics, Medical University of Graz, Harrachgasse 21/4; A-8010 Graz, Austria.
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37
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Slepak VZ. Structure, function, and localization of Gβ5-RGS complexes. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2009; 86:157-203. [PMID: 20374716 DOI: 10.1016/s1877-1173(09)86006-7] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
Abstract
Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5-R7 complexes, (2) regional distribution of Gβ5-R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5-R7 complexes, and (4) novel binding partners of Gβ5-R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5-R7 function in vivo.
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Affiliation(s)
- Vladlen Z Slepak
- Department of Molecular and Cellular Pharmacology and the Neuroscience Program, University of Miami School of Medicine, Miami, Florida 33136, USA
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Müllner C, Steinecker B, Gorischek A, Schreibmayer W. Identification of the structural determinant responsible for the phosphorylation of G-protein activated potassium channel 1 by cAMP-dependent protein kinase. FEBS J 2009; 276:6218-26. [DOI: 10.1111/j.1742-4658.2009.07325.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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39
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Rubinstein M, Peleg S, Berlin S, Brass D, Keren-Raifman T, Dessauer CW, Ivanina T, Dascal N. Divergent regulation of GIRK1 and GIRK2 subunits of the neuronal G protein gated K+ channel by GalphaiGDP and Gbetagamma. J Physiol 2009; 587:3473-91. [PMID: 19470775 DOI: 10.1113/jphysiol.2009.173229] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
G protein activated K+ channels (GIRK, Kir3) are switched on by direct binding of Gbetagamma following activation of Gi/o proteins via G protein-coupled receptors (GPCRs). Although Galphai subunits do not activate GIRKs, they interact with the channels and regulate the gating pattern of the neuronal heterotetrameric GIRK1/2 channel (composed of GIRK1 and GIRK2 subunits) expressed in Xenopus oocytes. Coexpressed Galphai3 decreases the basal activity (Ibasal) and increases the extent of activation by purified or coexpressed Gbegagamma. Here we show that this regulation is exerted by the 'inactive' GDP-bound Galphai3GDP and involves the formation of Galphai3betagamma heterotrimers, by a mechanism distinct from mere sequestration of Gbetagamma 'away' from the channel. The regulation of basal and Gbetagamma-evoked current was produced by the 'constitutively inactive' mutant of Galphai3, Galphai3G203A, which strongly binds Gbetagamma, but not by the 'constitutively active' mutant, Galphai3Q204L, or by Gbetagamma-scavenging proteins. Furthermore, regulation by Galphai3G203A was unique to the GIRK1 subunit; it was not observed in homomeric GIRK2 channels. In vitro protein interaction experiments showed that purified Gbetagamma enhanced the binding of Galphai3GDP to the cytosolic domain of GIRK1, but not GIRK2. Homomeric GIRK2 channels behaved as a 'classical' Gbetagamma effector, showing low Ibasal and strong Gbetagamma-dependent activation. Expression of Galphai3G203A did not affect either Ibasal or Gbetagamma-induced activation. In contrast, homomeric GIRK1* (a pore mutant able to form functional homomeric channels) exhibited large Ibasal and was poorly activated by Gbegagamma. Expression of Galphai3GDP reduced Ibasal and restored the ability of Gbetagamma to activate GIRK1*, like in GIRK1/2. Transferring the unique distal segment of the C terminus of GIRK1 to GIRK2 rendered the latter functionally similar to GIRK1*. These results demonstrate that GIRK1 containing channels are regulated by both Galphai3GDP and Gbetagamma, while GIRK2 is a Gbetagamma-effector insensitive to Galphai3GDP.
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Affiliation(s)
- Moran Rubinstein
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel.
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Structural diversity in the RGS domain and its interaction with heterotrimeric G protein alpha-subunits. Proc Natl Acad Sci U S A 2008; 105:6457-62. [PMID: 18434541 DOI: 10.1073/pnas.0801508105] [Citation(s) in RCA: 146] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Regulator of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Galpha subunits and thus facilitate termination of signaling initiated by G protein-coupled receptors (GPCRs). RGS proteins hold great promise as disease intervention points, given their signature role as negative regulators of GPCRs-receptors to which the largest fraction of approved medications are currently directed. RGS proteins share a hallmark RGS domain that interacts most avidly with Galpha when in its transition state for GTP hydrolysis; by binding and stabilizing switch regions I and II of Galpha, RGS domain binding consequently accelerates Galpha-mediated GTP hydrolysis. The human genome encodes more than three dozen RGS domain-containing proteins with varied Galpha substrate specificities. To facilitate their exploitation as drug-discovery targets, we have taken a systematic structural biology approach toward cataloging the structural diversity present among RGS domains and identifying molecular determinants of their differential Galpha selectivities. Here, we determined 14 structures derived from NMR and x-ray crystallography of members of the R4, R7, R12, and RZ subfamilies of RGS proteins, including 10 uncomplexed RGS domains and 4 RGS domain/Galpha complexes. Heterogeneity observed in the structural architecture of the RGS domain, as well as in engagement of switch III and the all-helical domain of the Galpha substrate, suggests that unique structural determinants specific to particular RGS protein/Galpha pairings exist and could be used to achieve selective inhibition by small molecules.
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