1
|
Cressman A, Morales D, Zhang Z, Le B, Foley J, Murray-Stewart T, Genetos DC, Fierro FA. Effects of Spermine Synthase Deficiency in Mesenchymal Stromal Cells Are Rescued by Upstream Inhibition of Ornithine Decarboxylase. Int J Mol Sci 2024; 25:2463. [PMID: 38473716 PMCID: PMC10931026 DOI: 10.3390/ijms25052463] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2024] [Revised: 02/13/2024] [Accepted: 02/18/2024] [Indexed: 03/14/2024] Open
Abstract
Despite the well-known relevance of polyamines to many forms of life, little is known about how polyamines regulate osteogenesis and skeletal homeostasis. Here, we report a series of in vitro studies conducted with human-bone-marrow-derived pluripotent stromal cells (MSCs). First, we show that during osteogenic differentiation, mRNA levels of most polyamine-associated enzymes are relatively constant, except for the catabolic enzyme spermidine/spermine N1-acetyltransferase 1 (SAT1), which is strongly increased at both mRNA and protein levels. As a result, the intracellular spermidine to spermine ratio is significantly reduced during the early stages of osteoblastogenesis. Supplementation of cells with exogenous spermidine or spermine decreases matrix mineralization in a dose-dependent manner. Employing N-cyclohexyl-1,3-propanediamine (CDAP) to chemically inhibit spermine synthase (SMS), the enzyme catalyzing conversion of spermidine into spermine, also suppresses mineralization. Intriguingly, this reduced mineralization is rescued with DFMO, an inhibitor of the upstream polyamine enzyme ornithine decarboxylase (ODC1). Similarly, high concentrations of CDAP cause cytoplasmic vacuolization and alter mitochondrial function, which are also reversible with the addition of DFMO. Altogether, these studies suggest that excess polyamines, especially spermidine, negatively affect hydroxyapatite synthesis of primary MSCs, whereas inhibition of polyamine synthesis with DFMO rescues most, but not all of these defects. These findings are relevant for patients with Snyder-Robinson syndrome (SRS), as the presenting skeletal defects-associated with SMS deficiency-could potentially be ameliorated by treatment with DFMO.
Collapse
Affiliation(s)
- Amin Cressman
- Institute for Regenerative Cures, University of California Davis, Sacramento, CA 95817, USA; (A.C.); (D.M.); (Z.Z.); (B.L.)
| | - David Morales
- Institute for Regenerative Cures, University of California Davis, Sacramento, CA 95817, USA; (A.C.); (D.M.); (Z.Z.); (B.L.)
| | - Zhenyang Zhang
- Institute for Regenerative Cures, University of California Davis, Sacramento, CA 95817, USA; (A.C.); (D.M.); (Z.Z.); (B.L.)
| | - Bryan Le
- Institute for Regenerative Cures, University of California Davis, Sacramento, CA 95817, USA; (A.C.); (D.M.); (Z.Z.); (B.L.)
| | - Jackson Foley
- Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University, Baltimore, MD 21287, USA; (J.F.); (T.M.-S.)
| | - Tracy Murray-Stewart
- Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University, Baltimore, MD 21287, USA; (J.F.); (T.M.-S.)
| | - Damian C. Genetos
- Department of Anatomy, Physiology, and Cell Biology, School of Veterinary Medicine, University of California Davis, Davis, CA 95616, USA;
| | - Fernando A. Fierro
- Institute for Regenerative Cures, University of California Davis, Sacramento, CA 95817, USA; (A.C.); (D.M.); (Z.Z.); (B.L.)
- Department of Cell Biology and Human Anatomy, University of California Davis, Sacramento, CA 95817, USA
| |
Collapse
|
2
|
Boonamnaj P, Pandey RB, Sompornpisut P. Exploring polyamine interactions and binding pockets in SARS-CoV-2 ORF3a. J Mol Graph Model 2023; 122:108487. [PMID: 37086515 PMCID: PMC10083643 DOI: 10.1016/j.jmgm.2023.108487] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2023] [Revised: 04/03/2023] [Accepted: 04/05/2023] [Indexed: 04/24/2023]
Abstract
Ongoing global pandemic caused by coronavirus (COVID-19) requires urgent development of vaccines, treatments, and diagnostic tools. Open reading frame 3a (ORF3a) from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is considered to be a potential drug target for COVID-19 treatment. ORF3a is an accessory protein that plays a significant role in virus-host interactions and in facilitating host immune responses. Using putrescine, spermidine and spermine, an aliphatic polyamine for the activity suppression of ORF3a appears to be a promising approach in finding new targets for drug design. In this study, we explored the possible binding poses of polyamines to the ORF3a protein using a combination of various computational approaches i.e. pocket prediction, blind and site-specific molecular docking, molecular dynamics and ligand flooding simulations. The results showed that the tip of cytoplasmic domain and the upper tunnel of transmembrane domain of ORF3a provide a suitable binding site specific for the polyamines. MD simulations revealed the stability of spermidine binding in the upper tunnel pocket of ORF3a through salt bridge and hydrogen bond interactions between the amine groups of the ligand and negatively charged residues of ORF3a. These findings can be helpful in designing new therapeutic drugs.
Collapse
Affiliation(s)
- Panisak Boonamnaj
- The Center of Excellence in Computational Chemistry, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand
| | - R B Pandey
- School of Mathematics and Natural Sciences, University of Southern Mississippi, Hattiesburg, MS, 39406, USA
| | - Pornthep Sompornpisut
- The Center of Excellence in Computational Chemistry, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand.
| |
Collapse
|
3
|
Suma A, Granata D, Thomson AS, Carnevale V, Rothberg BS. Polyamine blockade and binding energetics in the MthK potassium channel. J Gen Physiol 2021; 152:151703. [PMID: 32342093 PMCID: PMC7335011 DOI: 10.1085/jgp.201912527] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2019] [Accepted: 03/24/2020] [Indexed: 11/20/2022] Open
Abstract
Polyamines such as spermidine and spermine are found in nearly all cells, at concentrations ranging up to 0.5 mM. These cations are endogenous regulators of cellular K+ efflux, binding tightly in the pores of inwardly rectifying K+ (Kir) channels in a voltage-dependent manner. Although the voltage dependence of Kir channel polyamine blockade is thought to arise at least partially from the energetically coupled movements of polyamine and K+ ions through the pore, the nature of physical interactions between these molecules is unclear. Here we analyze the polyamine-blocking mechanism in the model K+ channel MthK, using a combination of electrophysiology and computation. Spermidine (SPD3+) and spermine (SPM4+) each blocked current through MthK channels in a voltage-dependent manner, and blockade by these polyamines was described by a three-state kinetic scheme over a wide range of polyamine concentrations. In the context of the scheme, both SPD3+ and SPM4+ access a blocking site with similar effective gating valences (0.84 ± 0.03 e0 for SPD3+ and 0.99 ± 0.04 e0 for SPM4+), whereas SPM4+ binds in the blocked state with an ∼20-fold higher affinity than SPD3+ (Kd = 28.1 ± 3.1 µM for SPD3+ and 1.28 ± 0.20 µM for SPM4+), consistent with a free energy difference of 1.8 kcal/mol. Molecular simulations of the MthK pore in complex with either SPD3+ or SPM4+ are consistent with the leading amine interacting with the hydroxyl groups of T59, at the selectivity filter threshold, with access to this site governed by outward movement of K+ ions. These coupled movements can account for a large fraction of the voltage dependence of blockade. In contrast, differences in binding energetics between SPD3+ and SPM4+ may arise from distinct electrostatic interactions between the polyamines and carboxylate oxygens on the side chains of E92 and E96, located in the pore-lining helix.
Collapse
Affiliation(s)
- Antonio Suma
- Institute for Computational Molecular Science, Temple University, Philadelphia, PA
| | - Daniele Granata
- Institute for Computational Molecular Science, Temple University, Philadelphia, PA
| | - Andrew S Thomson
- Department of Medical Genetics and Molecular Biochemistry, Temple University Lewis Katz School of Medicine, Philadelphia, PA
| | - Vincenzo Carnevale
- Institute for Computational Molecular Science, Temple University, Philadelphia, PA
| | - Brad S Rothberg
- Department of Medical Genetics and Molecular Biochemistry, Temple University Lewis Katz School of Medicine, Philadelphia, PA
| |
Collapse
|
4
|
Lysenin Channels as Sensors for Ions and Molecules. SENSORS 2020; 20:s20216099. [PMID: 33120957 PMCID: PMC7663491 DOI: 10.3390/s20216099] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/30/2020] [Revised: 10/19/2020] [Accepted: 10/23/2020] [Indexed: 12/18/2022]
Abstract
Lysenin is a pore-forming protein extracted from the earthworm Eisenia fetida, which inserts large conductance pores in artificial and natural lipid membranes containing sphingomyelin. Its cytolytic and hemolytic activity is rather indicative of a pore-forming toxin; however, lysenin channels present intricate regulatory features manifested as a reduction in conductance upon exposure to multivalent ions. Lysenin pores also present a large unobstructed channel, which enables the translocation of analytes, such as short DNA and peptide molecules, driven by electrochemical gradients. These important features of lysenin channels provide opportunities for using them as sensors for a large variety of applications. In this respect, this literature review is focused on investigations aimed at the potential use of lysenin channels as analytical tools. The described explorations include interactions with multivalent inorganic and organic cations, analyses on the reversibility of such interactions, insights into the regulation mechanisms of lysenin channels, interactions with purines, stochastic sensing of peptides and DNA molecules, and evidence of molecular translocation. Lysenin channels present themselves as versatile sensing platforms that exploit either intrinsic regulatory features or the changes in ionic currents elicited when molecules thread the conducting pathway, which may be further developed into analytical tools of high specificity and sensitivity or exploited for other scientific biotechnological applications.
Collapse
|
5
|
Nimigean CM. Polyamine block of MthK potassium channels. J Gen Physiol 2020; 152:151820. [PMID: 32459330 PMCID: PMC7335008 DOI: 10.1085/jgp.202012614] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Affiliation(s)
- Crina M. Nimigean
- Departments of Anesthesiology, and Physiology and Biophysics, Weill Cornell Medical College, New York, NY
| |
Collapse
|
6
|
Cheung TP, Choe JY, Richmond JE, Kim H. BK channel density is regulated by endoplasmic reticulum associated degradation and influenced by the SKN-1A/NRF1 transcription factor. PLoS Genet 2020; 16:e1008829. [PMID: 32502151 PMCID: PMC7299407 DOI: 10.1371/journal.pgen.1008829] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2020] [Revised: 06/17/2020] [Accepted: 05/05/2020] [Indexed: 12/19/2022] Open
Abstract
Ion channels are present at specific levels within subcellular compartments of excitable cells. The regulation of ion channel trafficking and targeting is an effective way to control cell excitability. The BK channel is a calcium-activated potassium channel that serves as a negative feedback mechanism at presynaptic axon terminals and sites of muscle excitation. The C. elegans BK channel ortholog, SLO-1, requires an endoplasmic reticulum (ER) membrane protein for efficient anterograde transport to these locations. Here, we found that, in the absence of this ER membrane protein, SLO-1 channels that are seemingly normally folded and expressed at physiological levels undergo SEL-11/HRD1-mediated ER-associated degradation (ERAD). This SLO-1 degradation is also indirectly regulated by a SKN-1A/NRF1-mediated transcriptional mechanism that controls proteasome levels. Therefore, our data indicate that SLO-1 channel density is regulated by the competitive balance between the efficiency of ER trafficking machinery and the capacity of ERAD. Excitable cells, such as neurons and muscles, are essential for the movement and behavior of animals. These cells express a set of specific types of ion channels that allow the selective passage of ions across the plasma membrane. The alteration in the levels of these ion channels influences cell excitability and the function of excitable cells. The regulation of ion channel trafficking and targeting is an effective way to control the function of excitable cells. The BK SLO-1 channel is a calcium-activated potassium channel that reduces excitability at presynaptic axon terminals and sites of muscle excitation. In a C. elegans genetic study, authors found that the delayed exit of SLO-1 channels from the ER causes their degradation by a mechanism called ER-associated degradation (ERAD). Interestingly, the same components that directly mediate SLO-1 ERAD also process a key transcriptional factor that maintains proteasome levels, thus indirectly influencing SLO-1 degradation. These data show that the levels of SLO-1 channels are regulated by the competitive balance between the efficiency of ER trafficking machinery and the capacity of ERAD.
Collapse
Affiliation(s)
- Timothy P. Cheung
- Center for Cancer Cell Biology, Immunology, and Infection, Department of Cell Biology and Anatomy, Chicago Medical School, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois, United States of America
- School of Graduate & Postdoctoral Studies, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois, United States of America
| | - Jun-Yong Choe
- School of Graduate & Postdoctoral Studies, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois, United States of America
- Department of Biochemistry and Molecular Biology, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois United States of America
| | - Janet E. Richmond
- Department of Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, United States of America
| | - Hongkyun Kim
- Center for Cancer Cell Biology, Immunology, and Infection, Department of Cell Biology and Anatomy, Chicago Medical School, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois, United States of America
- School of Graduate & Postdoctoral Studies, Rosalind Franklin University of Medicine & Science, North Chicago, Illinois, United States of America
- * E-mail:
| |
Collapse
|
7
|
Large-conductance Ca 2+- and voltage-gated K + channels form and break interactions with membrane lipids during each gating cycle. Proc Natl Acad Sci U S A 2019; 116:8591-8596. [PMID: 30967508 DOI: 10.1073/pnas.1901381116] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Membrane depolarization and intracellular Ca2+ promote activation of the large-conductance Ca2+- and voltage-gated (Slo1) big potassium (BK) channel. We examined the physical interactions that stabilize the closed and open conformations of the ion conduction gate of the human Slo1 channel using electrophysiological and computational approaches. The results show that the closed conformation is stabilized by intersubunit ion-ion interactions involving negative residues (E321 and E324) and positive residues (329RKK331) at the cytoplasmic ends of the transmembrane S6 segments ("RKK ring"). When the channel gate is open, the RKK ring is broken and the positive residues instead make electrostatic interactions with nearby membrane lipid oxygen atoms. E321 and E324 are stabilized by water. When the 329RKK331 residues are mutated to hydrophobic amino acids, these residues form even stronger hydrophobic interactions with the lipid tails to promote the open conformation, shifting the voltage dependence of activation to the negative direction by up to 400 mV and stabilizing the selectivity filter region. Thus, the RKK segment forms electrostatic interactions with oxygen atoms from two sources, other amino acid residues (E321/E324), and membrane lipids, depending on the gate status. Each time the channel opens and closes, the aforementioned interactions are formed and broken. This lipid-dependent Slo1 gating may explain how amphipathic signaling molecules and pharmacologically active agents influence the channel activity, and a similar mechanism may be operative in other ion channels.
Collapse
|
8
|
Naranjo D, Moldenhauer H, Pincuntureo M, Díaz-Franulic I. Pore size matters for potassium channel conductance. J Gen Physiol 2016; 148:277-91. [PMID: 27619418 PMCID: PMC5037345 DOI: 10.1085/jgp.201611625] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2016] [Accepted: 08/10/2016] [Indexed: 01/31/2023] Open
Abstract
Ion channels are membrane proteins that mediate efficient ion transport across the hydrophobic core of cell membranes, an unlikely process in their absence. K+ channels discriminate K+ over cations with similar radii with extraordinary selectivity and display a wide diversity of ion transport rates, covering differences of two orders of magnitude in unitary conductance. The pore domains of large- and small-conductance K+ channels share a general architectural design comprising a conserved narrow selectivity filter, which forms intimate interactions with permeant ions, flanked by two wider vestibules toward the internal and external openings. In large-conductance K+ channels, the inner vestibule is wide, whereas in small-conductance channels it is narrow. Here we raise the idea that the physical dimensions of the hydrophobic internal vestibule limit ion transport in K+ channels, accounting for their diversity in unitary conductance.
Collapse
Affiliation(s)
- David Naranjo
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso 2360103, Chile
| | - Hans Moldenhauer
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso 2360103, Chile
| | - Matías Pincuntureo
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso 2360103, Chile Programa de Doctorado en Ciencias, mención Biofísica y Biología Computacional, Universidad de Valparaíso, Valparaíso 2360103, Chile
| | - Ignacio Díaz-Franulic
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Playa Ancha, Valparaíso 2360103, Chile Center for Bioinformatics and Integrative Biology, Universidad Andrés Bello, Santiago 8370146, Chile Fraunhofer Chile Research, Las Condes 7550296, Chile
| |
Collapse
|
9
|
Hoshi T, Heinemann SH. Modulation of BK Channels by Small Endogenous Molecules and Pharmaceutical Channel Openers. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2016; 128:193-237. [PMID: 27238265 DOI: 10.1016/bs.irn.2016.03.020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Voltage- and Ca(2+)-activated K(+) channels of big conductance (BK channels) are abundantly found in various organs and their relevance for smooth muscle tone and neuronal signaling is well documented. Dysfunction of BK channels is implicated in an array of human diseases involving many organs including the nervous, pulmonary, cardiovascular, renal, and urinary systems. In humans a single gene (KCNMA1) encodes the pore-forming α subunit (Slo1) of BK channels, but the channel properties are variable because of alternative splicing, tissue- and subcellular-specific auxiliary subunits (β, γ), posttranslational modifications, and a multitude of endogenous signaling molecules directly affecting the channel function. Initiatives to develop drugs capable of activating BK channels (channel openers) therefore need to consider the tissue-specific variability of BK channel structure and the potential interference with endogenously produced regulatory factors. The atomic structural basis of BK channel function is only beginning to be revealed. However, building on detailed knowledge of BK channel function, including its single-channel characteristics, voltage- and Ca(2+) dependence of channel gating, and modulation by diffusible messengers, a multi-tier allosteric model of BK channel gating (Horrigan and Aldrich (HA) model) has become a valuable tool in studying modulation of the channel. Using the conceptual framework of the HA model, we here review the functional impact of endogenous modulatory factors and select small synthetic compounds that regulate BK channel activity. Furthermore, we devise experimental approaches for studying BK channel-drug interactions with the aim to classify BK-modulating substances according to their molecular mode of action.
Collapse
Affiliation(s)
- T Hoshi
- University of Pennsylvania, Philadelphia, PA, United States.
| | - S H Heinemann
- Friedrich Schiller University Jena & Jena University Hospital, Jena, Germany
| |
Collapse
|
10
|
Mori MX, Inoue R. New experimental trends for phosphoinositides research on ion transporter/channel regulation. J Pharmacol Sci 2014; 126:186-97. [PMID: 25367262 DOI: 10.1254/jphs.14r14cp] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022] Open
Abstract
Phosphoinositides(4,5)-bisphosphates [PI(4,5)P2] critically controls membrane excitability, the disruption of which leads to pathophysiological states. PI(4,5)P2 plays a primary role in regulating the conduction and gating properties of ion channels/transporters, through electrostatic and hydrophobic interactions that allow direct associations. In recent years, the development of many molecular tools have brought deep insights into the mechanisms underlying PI(4,5)P2-mediated regulation. This review summarizes the methods currently available to manipulate the cell membrane PI(4,5)P2 level including pharmacological interventions as well as newly designed molecular tools. We concisely introduce materials and experimental designs suitable for the study of PI(4,5)P2-mediated regulation of ion-conducting molecules, in order to assist researchers who are interested in this area. It is our further hope that the knowledge introduced in this review will help to promote our understanding about the pathology of diseases such as cardiac arrhythmias, bipolar disorders, and Alzheimer's disease which are somehow associated with a disruption of PI(4,5)P2 metabolism.
Collapse
Affiliation(s)
- Masayuki X Mori
- Department of Synthetic Chemistry and Biological Chemistry, School of Engineering, Kyoto University, Japan
| | | |
Collapse
|
11
|
Properties of Slo1 K+ channels with and without the gating ring. Proc Natl Acad Sci U S A 2013; 110:16657-62. [PMID: 24067659 DOI: 10.1073/pnas.1313433110] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
High-conductance Ca(2+)- and voltage-activated K(+) (Slo1 or BK) channels (KCNMA1) play key roles in many physiological processes. The structure of the Slo1 channel has two functional domains, a core consisting of four voltage sensors controlling an ion-conducting pore, and a larger tail that forms an intracellular gating ring thought to confer Ca(2+) and Mg(2+) sensitivity as well as sensitivity to a host of other intracellular factors. Although the modular structure of the Slo1 channel is known, the functional properties of the core and the allosteric interactions between core and tail are poorly understood because it has not been possible to study the core in the absence of the gating ring. To address these questions, we developed constructs that allow functional cores of Slo1 channels to be expressed by replacing the 827-amino acid gating ring with short tails of either 74 or 11 amino acids. Recorded currents from these constructs reveals that the gating ring is not required for either expression or gating of the core. Voltage activation is retained after the gating ring is replaced, but all Ca(2+)- and Mg(2+)-dependent gating is lost. Replacing the gating ring also right-shifts the conductance-voltage relation, decreases mean open-channel and burst duration by about sixfold, and reduces apparent mean single-channel conductance by about 30%. These results show that the gating ring is not required for voltage activation but is required for Ca(2+) and Mg(2+) activation. They also suggest possible actions of the unliganded (passive) gating ring or added short tails on the core.
Collapse
|
12
|
Schroeder I, Thiel G, Hansen UP. Ca2+ block and flickering both contribute to the negative slope of the IV curve in BK channels. ACTA ACUST UNITED AC 2013; 141:499-505. [PMID: 23530139 PMCID: PMC3607826 DOI: 10.1085/jgp.201210956] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Single-channel current–voltage (IV) curves of human large-conductance, voltage- and Ca2+-activated K+ (BK) channels are quite linear in 150 mM KCl. In the presence of Ca2+ and/or Mg2+, they show a negative slope conductance at high positive potentials. This is generally explained by a Ca2+/Mg2+ block as by Geng et al. (2013. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.201210955) in this issue. Here, we basically support this finding but add a refinement: the analysis of the open-channel noise by means of β distributions reveals what would be found if measurements were done with an amplifier of sufficient temporal resolution (10 MHz), namely that the block by 2.5 mM Ca2+ and 2.5 mM Mg2+ per se would only cause a saturating curve up to +160 mV. Further bending down requires the involvement of a second process related to flickering in the microsecond range. This flickering is hardly affected by the presence or absence of Ca2+/Mg2+. In contrast to the experiments reported here, previous experiments in BK channels (Schroeder and Hansen. 2007. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.200709802) showed saturating IV curves already in the absence of Ca2+/Mg2+. The reason for this discrepancy could not be identified so far. However, the flickering component was very similar in the old and new experiments, regardless of the occurrence of noncanonical IV curves.
Collapse
Affiliation(s)
- Indra Schroeder
- Plant Membrane Biophysics, Technical University of Darmstadt, 64287 Darmstadt, Germany.
| | | | | |
Collapse
|
13
|
Geng Y, Wang X, Magleby KL. Lack of negative slope in I-V plots for BK channels at positive potentials in the absence of intracellular blockers. ACTA ACUST UNITED AC 2013; 141:493-7. [PMID: 23530138 PMCID: PMC3607821 DOI: 10.1085/jgp.201210955] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Large-conductance, voltage- and Ca(2+)-activated K(+) (BK) channels display near linear current-voltage (I-V) plots for voltages between -100 and +100 mV, with an increasing sublinearity for more positive potentials. As is the case for many types of channels, BK channels are blocked at positive potentials by intracellular Ca(2+) and Mg(2+). This fast block progressively reduces single-channel conductance with increasing voltage, giving rise to a negative slope in the I-V plots beyond about +120 mV, depending on the concentration of the blockers. In contrast to these observations of pronounced differences in the magnitudes and shapes of I-V plots in the absence and presence of intracellular blockers, Schroeder and Hansen (2007. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.200709802) have reported identical I-V plots in the absence and presence of blockers for BK channels, with both plots having reduced conductance and negative slopes, as expected for blockers. Schroeder and Hansen included both Ca(2+) and Mg(2+) in the intracellular solution rather than a single blocker, and they also studied BK channels expressed from α plus β1 subunits, whereas most previous studies used only α subunits. Although it seems unlikely that these experimental differences would account for the differences in findings between previous studies and those of Schroeder and Hansen, we repeated the experiments using BK channels comprised of α plus β1 subunits with joint application of 2.5 mM Ca(2+) plus 2.5 mM Mg(2+), as Schroeder and Hansen did. In contrast to the findings of Schroeder and Hansen of identical I-V plots, we found marked differences in the single-channel I-V plots in the absence and presence of blockers. Consistent with previous studies, we found near linear I-V plots in the absence of blockers and greatly reduced currents and negative slopes in the presence of blockers. Hence, studies of conductance mechanisms for BK channels should exclude intracellular Ca(2+)/Mg(2+), as they can reduce conductance and induce negative slopes.
Collapse
Affiliation(s)
- Yanyan Geng
- Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136, USA.
| | | | | |
Collapse
|
14
|
Weiger TM, Hermann A. Cell proliferation, potassium channels, polyamines and their interactions: a mini review. Amino Acids 2013; 46:681-8. [PMID: 23820618 DOI: 10.1007/s00726-013-1536-7] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2013] [Accepted: 06/13/2013] [Indexed: 01/18/2023]
Abstract
Polyamines, which are obligatory molecules involved in cell cycling and proliferation, are subject to a change in their free intracellular concentrations during the cell cycle. Potassium (K(+)) channels are also considered, but less well recognized, to be necessary for cell proliferation by either hyperpolarizing or depolarizing cells during the cell cycle. A block of polyamine synthesis as well as block or knockout of K(+) channels can halt cell proliferation. K(+) channels like BK (maxi calcium (Ca(2+))-activated K(+)), Kir (inward rectifier), M-type K(+)-and TASK (two-pore domain K(+)) channels or the delayed rectifier K(+) channels are modulated in their electrical properties by polyamines. Polyamines are most effective in blocking these channels when applied to the intracellular face of these channels except for TASK channels where they act only from the extracellular side. Quinidine, a general K(+) channel blocker, was found to reduce putrescine concentrations, to block the ornithine decarboxylase and halt cell proliferation. From these results, the question arises if there is an interaction between polyamines, K(+) channels and proliferation. It might be speculated that a decrease of intracellular polyamines allows more K(+) channels to be active, thus inducing hyperpolarization, while an increase of the polyamine concentration may block K(+) channel activity leading to depolarization of the membrane potential. On the other hand, a block or a deletion of K(+) channels may cause a decrease of the polyamine concentration in cells. More research is needed to test these hypotheses.
Collapse
Affiliation(s)
- Thomas M Weiger
- Division of Cellular and Molecular Neurobiology, Department of Cell Biology, University of Salzburg, Hellbrunnerstrasse 34, 5020, Salzburg, Austria,
| | | |
Collapse
|
15
|
Moscoso C, Vergara-Jaque A, Márquez-Miranda V, Sepúlveda RV, Valencia I, Díaz-Franulic I, González-Nilo F, Naranjo D. K⁺ conduction and Mg²⁺ blockade in a shaker Kv-channel single point mutant with an unusually high conductance. Biophys J 2013; 103:1198-207. [PMID: 22995492 DOI: 10.1016/j.bpj.2012.08.015] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2012] [Revised: 07/30/2012] [Accepted: 08/01/2012] [Indexed: 12/27/2022] Open
Abstract
Potassium channels exhibit a large diversity of single-channel conductances. Shaker is a low-conductance K-channel in which Pro475→Asp, a single-point mutation near the internal pore entrance, promotes 6- to 8-fold higher unitary current. To assess the mechanism for this higher conductance, we measured Shaker-P475D single-channel current in a wide range of symmetrical K(+) concentrations and voltages. Below 300 mM K(+), the current-to-voltage relations (i-V) showed inward rectification that disappeared at 1000 mM K(+). Single-channel conductance reached a maximum of ∼190 pS at saturating [K(+)], a value 4- to 5-fold larger than that estimated for the native channel. Intracellular Mg(2+) blocked this variant with ∼100-fold higher affinity. Near zero voltage, blockade was competitively antagonized by K(+); however, at voltages >100 mV, it was enhanced by K(+). This result is consistent with a lock-in effect in a single-file diffusion regime of Mg(2+) and K(+) along the pore. Molecular-dynamics simulations revealed higher K(+) density in the pore, especially near the Asp-475 side chains, as in the high-conductance MthK bacterial channel. The molecular dynamics also showed that K(+) ions bound distally can coexist with other K(+) or Mg(2+) in the cavity, supporting a lock-in mechanism. The maximal K(+) transport rate and higher occupancy could be due to a decrease in the electrostatic energy profile for K(+) throughout the pore, reducing the energy wells and barriers differentially by ∼0.7 and ∼2 kT, respectively.
Collapse
Affiliation(s)
- Cristian Moscoso
- Centro Interdisciplinario de Neurociencias de Valparaíso, Universidad de Valparaíso, Valparaíso, Chile
| | | | | | | | | | | | | | | |
Collapse
|
16
|
Smith FJ, Pau VPT, Cingolani G, Rothberg BS. Crystal structure of a Ba(2+)-bound gating ring reveals elementary steps in RCK domain activation. Structure 2012; 20:2038-47. [PMID: 23085076 DOI: 10.1016/j.str.2012.09.014] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2012] [Revised: 09/20/2012] [Accepted: 09/25/2012] [Indexed: 12/15/2022]
Abstract
RCK domains control activity of a variety of K(+) channels and transporters through binding of cytoplasmic ligands. To gain insight toward mechanisms of RCK domain activation, we solved the structure of the RCK domain from the Ca(2+)-gated K(+) channel, MthK, bound with Ba(2+), at 3.1 Å resolution. The Ba(2+)-bound RCK domain was assembled as an octameric gating ring, as observed in structures of the full-length MthK channel, and shows Ba(2+) bound at several positions. One of the Ba(2+) sites, termed C1, overlaps with a known Ca(2+)-activation site, determined by residues D184 and E210. Functionally, Ba(2+) can activate reconstituted MthK channels as observed in electrophysiological recordings, whereas Mg(2+) (up to 100 mM) was ineffective. Ba(2+) activation was abolished by the mutation D184N, suggesting that Ba(2+) activates primarily through the C1 site. Our results suggest a working hypothesis for a sequence of ligand-dependent conformational changes that may underlie RCK domain activation and channel gating.
Collapse
Affiliation(s)
- Frank J Smith
- Department of Biochemistry, Temple University School of Medicine, 3400 N. Broad Street, Philadelphia, PA 19140, USA
| | | | | | | |
Collapse
|
17
|
Chen RS, Geng Y, Magleby KL. Mg(2+) binding to open and closed states can activate BK channels provided that the voltage sensors are elevated. ACTA ACUST UNITED AC 2012; 138:593-607. [PMID: 22124117 PMCID: PMC3226971 DOI: 10.1085/jgp.201110707] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
BK channels are activated by intracellular Ca2+ and Mg2+ as well as by depolarization. Such activation is possible because each of the four subunits has two high-affinity Ca2+ sites, one low-affinity Mg2+ site, and a voltage sensor. This study further investigates the mechanism of Mg2+ activation by using single-channel recording to determine separately the action of Mg2+ on the open and closed states of the channel. To limit Mg2+ action to the Mg2+ sites, the two high-affinity Ca2+ sites are disabled by mutation. When the voltage is stepped from negative holding potentials to +100 mV, we find that 10 mM Mg2+ decreases the mean closed latency to the first channel opening 2.1-fold, decreases the mean closed interval duration 8.7-fold, increases mean burst duration 10.1-fold, increases the number of openings per burst 4.4-fold, and increases mean open interval duration 2.3-fold. Hence, Mg2+ can bind to closed BK channels, increasing their opening rates, and to open BK channels, decreasing their closing rates. To explore the relationship between Mg2+ action and voltage sensor activation, we record single-channel activity in macropatches containing hundreds of channels. Open probability (Po) is dramatically increased by 10 mM Mg2+ when voltage sensors are activated with either depolarization or the mutation R210C. The increased Po arises from large decreases in mean closed interval durations and moderate increases in mean open interval durations. In contrast, 10 mM Mg2+ has no detectable effects on Po or interval durations when voltage sensors are deactivated with very negative potentials or the mutation R167E. These observations are consistent with a model in which Mg2+ can bind to and alter the gating of both closed and open states to increase Po, provided that one or more voltage sensors are activated.
Collapse
Affiliation(s)
- Ren-Shiang Chen
- Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136, USA.
| | | | | |
Collapse
|
18
|
Geng Y, Niu X, Magleby KL. Low resistance, large dimension entrance to the inner cavity of BK channels determined by changing side-chain volume. ACTA ACUST UNITED AC 2011; 137:533-48. [PMID: 21576375 PMCID: PMC3105516 DOI: 10.1085/jgp.201110616] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Large-conductance Ca2+- and voltage-activated K+ (BK) channels have the largest conductance (250–300 pS) of all K+-selective channels. Yet, the contributions of the various parts of the ion conduction pathway to the conductance are not known. Here, we examine the contribution of the entrance to the inner cavity to the large conductance. Residues at E321/E324 on each of the four α subunits encircle the entrance to the inner cavity. To determine if 321/324 is accessible from the inner conduction pathway, we measured single-channel current amplitudes before and after exposure and wash of thiol reagents to the intracellular side of E321C and E324C channels. MPA− increased currents and MTSET+ decreased currents, with no difference between positions 321 and 324, indicating that side chains at 321/324 are accessible from the inner conduction pathway and have equivalent effects on conductance. For neutral amino acids, decreasing the size of the entrance to the inner cavity by substituting large side-chain amino acids at 321/324 decreased outward single-channel conductance, whereas increasing the size of the entrance with smaller side-chain substitutions had little effect. Reductions in outward conductance were negated by high [K+]i. Substitutions had little effect on inward conductance. Fitting plots of conductance versus side-chain volume with a model consisting of one variable and one fixed resistor in series indicated an effective diameter and length of the entrance to the inner cavity for wild-type channels of 17.7 and 5.6 Å, respectively, with the resistance of the entrance ∼7% of the total resistance of the conduction pathway. The estimated dimensions are consistent with the structure of MthK, an archaeal homologue to BK channels. Our observations suggest that BK channels have a low resistance, large entrance to the inner cavity, with the entrance being as large as necessary to not limit current, but not much larger.
Collapse
Affiliation(s)
- Yanyan Geng
- Department of Physiology and Biophysics, University of Miami Miller School of Medicine, FL 33136, USA. ygeng@-med.miami.edu
| | | | | |
Collapse
|
19
|
Wang W, Huang H, Hou D, Liu P, Wei H, Fu X, Niu W. Mechanosensitivity of STREX-lacking BKCa channels in the colonic smooth muscle of the mouse. Am J Physiol Gastrointest Liver Physiol 2010; 299:G1231-40. [PMID: 20864656 DOI: 10.1152/ajpgi.00268.2010] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Stretch sensitivity of Ca²(+)-activated large-conductance K(+) channels (BK(Ca)) has been observed in a variety of cell types and considered to be a potential mechanism in mechanoelectric transduction (MET). Mechanical stress is a major stimulator for the smooth muscle in the gastrointestinal (GI) tract. However, much about the role and mechanism of MET in GI smooth muscles remains unknown. The BK(Ca) shows a functional diversity due to intensive Slo I alternative splicing and different α/β-subunit assembly in various cells. The stress-regulated exon (STREX) insert is suggested to be an indispensable domain for the mechanosensitivity of BK(Ca). The purpose of this study was to determine whether the BK(Ca) in colonic myocytes of the adult mouse is sensitive to mechanical stimulation and whether the STREX insert is a crucial segment for the BK(Ca) mechanosensitivity. The α- and β1-subunit mRNAs and the α-subunit protein of the BK(Ca) channels were detected in the colonic muscularis. We found that the BK(Ca) STREX-lacking variant was abundantly expressed in the smooth muscle, whereas the STREX variant was not detectable. We demonstrated that the STREX-lacking BK(Ca) channels were also sensitive to membrane stretch. We suggest that in addition to the STREX domain, there are other additional structures in the channel responsible for mechanically coupling with the cell membrane.
Collapse
Affiliation(s)
- Wei Wang
- Dept. of Physiology, Capital Medical Univ., Beijing, PR China
| | | | | | | | | | | | | |
Collapse
|
20
|
Fologea D, Krueger E, Al Faori R, Lee R, Mazur YI, Henry R, Arnold M, Salamo GJ. Multivalent ions control the transport through lysenin channels. Biophys Chem 2010; 152:40-5. [PMID: 20724059 DOI: 10.1016/j.bpc.2010.07.004] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2010] [Revised: 07/27/2010] [Accepted: 07/28/2010] [Indexed: 11/30/2022]
Abstract
We report the effect of different ions on the conducting properties of lysenin channels inserted into planar lipid bilayer membranes. Our observations indicated that multivalent ions inhibited the lysenin channels conductance in a concentration dependent manner. The analysis performed on single channels revealed that multivalent ions induced reversible sub-conducting or closed states depending on the ionic charge and size. Good agreement is reported between experimental results and a theoretical model that is proposed to describe the interaction between divalent ions and lysenin channels as a simple isothermal absorption process.
Collapse
Affiliation(s)
- Daniel Fologea
- Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA.
| | | | | | | | | | | | | | | |
Collapse
|
21
|
Zhou JJ, Li MS, Qi J, Linsdell P. Regulation of conductance by the number of fixed positive charges in the intracellular vestibule of the CFTR chloride channel pore. ACTA ACUST UNITED AC 2010; 135:229-45. [PMID: 20142516 PMCID: PMC2828907 DOI: 10.1085/jgp.200910327] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Rapid chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is dependent on the presence of fixed positive charges in the permeation pathway. Here, we use site-directed mutagenesis and patch clamp recording to show that the functional role played by one such positive charge (K95) in the inner vestibule of the pore can be "transplanted" to a residue in a different transmembrane (TM) region (S1141). Thus, the mutant channel K95S/S1141K showed Cl(-) conductance and open-channel blocker interactions similar to those of wild-type CFTR, thereby "rescuing" the effects of the charge-neutralizing K95S mutation. Furthermore, the function of K95C/S1141C, but not K95C or S1141C, was inhibited by the oxidizing agent copper(II)-o-phenanthroline, and this inhibition was reversed by the reducing agent dithiothreitol, suggesting disulfide bond formation between these two introduced cysteine side chains. These results suggest that the amino acid side chains of K95 (in TM1) and S1141 (in TM12) are functionally interchangeable and located closely together in the inner vestibule of the pore. This allowed us to investigate the functional effects of increasing the number of fixed positive charges in this vestibule from one (in wild type) to two (in the S1141K mutant). The S1141K mutant had similar Cl(-) conductance as wild type, but increased susceptibility to channel block by cytoplasmic anions including adenosine triphosphate, pyrophosphate, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and Pt(NO(2))(4)(2-) in inside-out membrane patches. Furthermore, in cell-attached patch recordings, apparent voltage-dependent channel block by cytosolic anions was strengthened by the S1141K mutation. Thus, the Cl(-) channel function of CFTR is maximal with a single fixed positive charge in this part of the inner vestibule of the pore, and increasing the number of such charges to two causes a net decrease in overall Cl(-) transport through a combination of failure to increase Cl(-) conductance and increased susceptibility to channel block by cytosolic substances.
Collapse
Affiliation(s)
- Jing-Jun Zhou
- Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada
| | | | | | | |
Collapse
|
22
|
Tan K, Sather A, Robertson JL, Moy S, Roux B, Joachimiak A. Structure and electrostatic property of cytoplasmic domain of ZntB transporter. Protein Sci 2009; 18:2043-52. [PMID: 19653298 DOI: 10.1002/pro.215] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
ZntB is the distant homolog of CorA Mg(2+) transporter within the metal ion transporter superfamily. It was early reported that the ZntB from Salmonella typhimurium facilitated efflux of Zn(2+) and Cd(2+), but not Mg(2+). Here, we report the 1.90 A crystal structure of the intracellular domain of ZntB from Vibrio parahemolyticus. The domain forms a funnel-shaped homopentamer that is similar to the full-length CorA from Thermatoga maritima, but differs from two previously reported dimeric structures of truncated CorA intracellular domains. However, no Zn(2+) or Cd(2+) binding sites were identified in the high-resolution structure. Instead, 25 well-defined Cl(-) ions were observed and some of these binding sites are highly conserved within the ZntB family. Continuum electrostatics calculations suggest that the central pore of the funnel is highly attractive for cations, especially divalents. The presence of the bound Cl(-) ions increases the stability of cations along the pore suggesting they could be important in enhancing cation transport.
Collapse
Affiliation(s)
- Kemin Tan
- Midwest Center for Structural Genomics and Structural Biology Center, Argonne, Illinois 60439, USA
| | | | | | | | | | | |
Collapse
|
23
|
Mead-Savery FC, Wang R, Tanna-Topan B, Chen SRW, Welch W, Williams AJ. Changes in negative charge at the luminal mouth of the pore alter ion handling and gating in the cardiac ryanodine-receptor. Biophys J 2009; 96:1374-87. [PMID: 19217855 DOI: 10.1016/j.bpj.2008.10.054] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2007] [Accepted: 10/28/2008] [Indexed: 12/20/2022] Open
Abstract
We have tested the hypothesis that a high density of negative charge at the luminal mouth of the RyR2 pore plays a pivotal role in the high cation conductance and limited selectivity observed in this channel by introducing into each monomer a double point mutation to neutralize acidic residues in this region of the mouse RyR2 channel. The resultant channel, ED4832AA, is capable of functioning as a calcium-release channel in situ. Consistent with our hypothesis, the ED4832AA mutation altered the ion handling characteristics of single RyR2 channels. The mutant channel retains the ability to discriminate between cations and anions but cation conductance is altered significantly. Unitary K+ conductance is reduced at low levels of activity but increases dramatically as activity is raised and shows little sign of saturation. ED4832AA no longer discriminates between divalent and monovalent cations. In addition, the gating characteristics of single RyR2 channels are altered markedly by residue neutralization. Open probability in the ED4832AA channel is substantially higher than that of the wild-type channel. Moreover, at holding potentials in excess of +/-50 mV several subconductance states become apparent in ED4832AA and are more prevalent at very high holding potentials. These observations are discussed within the structural framework provided by a previously developed model of the RyR2 pore. Our data indicates that neutralization of acidic residues in the luminal mouth of the pore produces wide-ranging changes in the electric field in the pore, the interaction energies of permeant ions in the pore and the stability of the selectivity filter region of the pore, which together contribute to the observed changes ion handling and gating.
Collapse
Affiliation(s)
- Fiona C Mead-Savery
- Cardiac Medicine, National Heart and Lung Institute, Faculty of Medicine, Imperial College London, London SW3 6LY, United Kingdom
| | | | | | | | | | | |
Collapse
|
24
|
Integration of an electric-metal sensory experience in the Slo1 BK channel. Nat Struct Mol Biol 2008; 15:1130-2. [PMID: 18985066 DOI: 10.1038/nsmb1108-1130] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
|
25
|
Charpentier M, Bredemeier R, Wanner G, Takeda N, Schleiff E, Parniske M. Lotus japonicus CASTOR and POLLUX are ion channels essential for perinuclear calcium spiking in legume root endosymbiosis. THE PLANT CELL 2008; 20:3467-79. [PMID: 19106374 PMCID: PMC2630432 DOI: 10.1105/tpc.108.063255] [Citation(s) in RCA: 143] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2008] [Revised: 11/23/2008] [Accepted: 12/05/2008] [Indexed: 05/18/2023]
Abstract
The mechanism underlying perinuclear calcium spiking induced during legume root endosymbioses is largely unknown. Lotus japonicus symbiosis-defective castor and pollux mutants are impaired in perinuclear calcium spiking. Homology modeling suggested that the related proteins CASTOR and POLLUX might be ion channels. Here, we show that CASTOR and POLLUX form two independent homocomplexes in planta. CASTOR reconstituted in planar lipid bilayers exhibited ion channel activity, and the channel characteristics were altered in a symbiosis-defective mutant carrying an amino acid replacement close to the selectivity filter. Permeability ratio determination and competition experiments reveled a weak preference of CASTOR for cations such as potassium over anions. POLLUX has an identical selectivity filter region and complemented a potassium transport-deficient yeast mutant, suggesting that POLLUX is also a potassium-permeable channel. Immunogold labeling localized the endogenous CASTOR protein to the nuclear envelope of Lotus root cells. Our data are consistent with a role of CASTOR and POLLUX in modulating the nuclear envelope membrane potential. They could either trigger the opening of calcium release channels or compensate the charge release during the calcium efflux as counter ion channels.
Collapse
Affiliation(s)
- Myriam Charpentier
- Ludwig-Maximilians-Universität München, Faculty of Biology, Genetics, 80638 München, Germany
| | | | | | | | | | | |
Collapse
|
26
|
Carvacho I, Gonzalez W, Torres YP, Brauchi S, Alvarez O, Gonzalez-Nilo FD, Latorre R. Intrinsic electrostatic potential in the BK channel pore: role in determining single channel conductance and block. ACTA ACUST UNITED AC 2008; 131:147-61. [PMID: 18227273 PMCID: PMC2213566 DOI: 10.1085/jgp.200709862] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
The internal vestibule of large-conductance Ca2+ voltage-activated K+ (BK) channels contains a ring of eight negative charges not present in K+ channels of lower conductance (Glu386 and Glu389 in hSlo) that modulates channel conductance through an electrostatic mechanism (Brelidze, T.I., X. Niu, and K.L. Magleby. 2003. Proc. Natl. Acad. Sci. USA. 100:9017–9022). In BK channels there are also two acidic amino acid residues in an extracellular loop (Asp326 and Glu329 in hSlo). To determine the electrostatic influence of these charges on channel conductance, we expressed wild-type BK channels and mutants E386N/E389N, D326N, E329Q, and D326N/E329Q channels on Xenopus laevis oocytes, and measured the expressed currents under patch clamp. Contribution of E329 to the conductance is negligible and single channel conductance of D326N/E329Q channels measured at 0 mV in symmetrical 110 mM K+ was 18% lower than the control. Current–voltage curves displayed weak outward rectification for D326N and the double mutant. The conductance differences between the mutants and wild-type BK were caused by an electrostatic effect since they were enhanced at low K+ (30 mM) and vanished at high K+ (1 M K+). We determine the electrostatic potential change, Δφ, caused by the charge neutralization using TEA+ block for the extracellular charges and Ba2+ for intracellular charges. We measured 13 ± 2 mV for Δφ at the TEA+ site when turning off the extracellular charges, and 17 ± 2 mV for the Δφ at the Ba2+ site when the intracellular charges were turned off. To understand the electrostatic effect of charge neutralizations, we determined Δφ using a BK channel molecular model embedded in a lipid bilayer and solving the Poisson-Boltzmann equation. The model explains the experimental results adequately and, in particular, gives an economical explanation to the differential effect on the conductance of the neutralization of charges D326 and E329.
Collapse
|
27
|
Abstract
We suggest that bubbles are the bistable hydrophobic gates responsible for the on-off transitions of single channel currents. In this view, many types of channels gate by the same physical mechanism-dewetting by capillary evaporation-but different types of channels use different sensors to modulate hydrophobic properties of the channel wall and thereby trigger and control bubbles and gating. Spontaneous emptying of channels has been seen in many simulations. Because of the physics involved, such phase transitions are inherently sensitive, unstable threshold phenomena that are difficult to simulate reproducibly and thus convincingly. We present a thermodynamic analysis of a bubble gate using morphometric density functional theory of classical (not quantum) mechanics. Thermodynamic analysis of phase transitions is generally more reproducible and less sensitive to details than simulations. Anesthetic actions of inert gases-and their interactions with hydrostatic pressure (e.g., nitrogen narcosis)-can be easily understood by actions on bubbles. A general theory of gas anesthesia may involve bubbles in channels. Only experiments can show whether, or when, or which channels actually use bubbles as hydrophobic gates: direct observation of bubbles in channels is needed. Existing experiments show thin gas layers on hydrophobic surfaces in water and suggest that bubbles nearly exist in bulk water.
Collapse
|
28
|
Suh BC, Hille B. Electrostatic interaction of internal Mg2+ with membrane PIP2 Seen with KCNQ K+ channels. ACTA ACUST UNITED AC 2007; 130:241-56. [PMID: 17724161 PMCID: PMC2151647 DOI: 10.1085/jgp.200709821] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Activity of KCNQ (Kv7) channels requires binding of phosphatidylinositol 4,5-bisphosphate (PIP2) from the plasma membrane. We give evidence that Mg2+ and polyamines weaken the KCNQ channel–phospholipid interaction. Lowering internal Mg2+ augmented inward and outward KCNQ currents symmetrically, and raising Mg2+ reduced currents symmetrically. Polyvalent organic cations added to the pipette solution had similar effects. Their potency sequence followed the number of positive charges: putrescine (+2) < spermidine (+3) < spermine (+4) < neomycin (+6) < polylysine (≫+6). The inhibitory effects of Mg2+ were reversible with sequential whole-cell patching. Internal tetraethylammonium ion (TEA) gave classical voltage-dependent block of the pore with changes of the time course of K+ currents. The effect of polyvalent cations was simpler, symmetric, and without changes of current time course. Overexpression of phosphatidylinositol 4-phosphate 5-kinase Iγ to accelerate synthesis of PIP2 attenuated the sensitivity to polyvalent cations. We suggest that Mg2+ and other polycations reduce the currents by electrostatic binding to the negative charges of PIP2, competitively reducing the amount of free PIP2 available for interaction with channels. The dose–response curves could be modeled by a competition model that reduces the pool of free PIP2. This mechanism is likely to modulate many other PIP2-dependent ion channels and cellular processes.
Collapse
Affiliation(s)
- Byung-Chang Suh
- Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, WA 98195, USA
| | | |
Collapse
|
29
|
Schroeder I, Hansen UP. Saturation and microsecond gating of current indicate depletion-induced instability of the MaxiK selectivity filter. ACTA ACUST UNITED AC 2007; 130:83-97. [PMID: 17591987 PMCID: PMC2154363 DOI: 10.1085/jgp.200709802] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed with a high temporal resolution (50-kHz filter) in symmetrical solutions with 50, 150, or 400 mM KCl and 2.5 mM CaCl(2) and 2.5 mM MgCl(2). At membrane potentials >+100 mV, the single-channel current showed a negative slope resistance, concomitantly with a flickery block, which was not influenced by Ca(2+) or Mg(2+). The analysis of the amplitude histograms by beta distributions revealed that current in this voltage range was reduced by two effects: rate limitation at the cytosolic side of the pore and gating with rate constants 10-20-fold higher than the cutoff frequency of the filter (i.e., dwell times in the microsecond range). The data were analyzed in terms of a model that postulates a coupling between both effects; if the voltage over the selectivity filter withdraws ions from the cavity at a higher rate than that of refilling from the cytosol, the selectivity filter becomes instable because of ion depletion, and current is interrupted by the resulting flickering. The fit of the IV curves revealed a characteristic voltage of 35 mV. In contrast, the voltage dependence of the gating factor R, i.e., the ratio between true and apparent single-channel current, could be fitted by exponentials with a characteristic voltage of 60 mV, suggesting that only part of the transmembrane potential is felt by the flux through the selectivity filter.
Collapse
Affiliation(s)
- Indra Schroeder
- Department of Structural Biology, University of Kiel, 24098 Kiel, Germany
| | | |
Collapse
|
30
|
Kurata HT, Cheng WW, Arrabit C, Slesinger PA, Nichols CG. The role of the cytoplasmic pore in inward rectification of Kir2.1 channels. ACTA ACUST UNITED AC 2007; 130:145-55. [PMID: 17635958 PMCID: PMC2151631 DOI: 10.1085/jgp.200709742] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Steeply voltage-dependent block by intracellular polyamines underlies the strong inward rectification properties of Kir2.1 and other Kir channels. Mutagenesis studies have identified several negatively charged pore-lining residues (D172, E224, and E299, in Kir2.1) in the inner cavity and cytoplasmic domain as determinants of the properties of spermine block. Recent crystallographic determination of the structure of the cytoplasmic domains of Kir2.1 identified additional negatively charged residues (D255 and D259) that influence inward rectification. In this study, we have characterized the kinetic and steady-state properties of spermine block in WT Kir2.1 and in mutations of the D255 residue (D255E, A, K, R). Despite minimal effects on steady-state blockade by spermine, D255 mutations have profound effects on the blocking kinetics, with D255A marginally, and D255R dramatically, slowing the rate of block. In addition, these mutations result in the appearance of a sustained current (in the presence of spermine) at depolarized voltages. These features are reproduced with a kinetic model consisting of a single open state, two sequentially linked blocked states, and a slow spermine permeation step, with residue D255 influencing the spermine affinity and rate of entry into the shallow blocked state. The data highlight a “long-pore” effect in Kir channels, and emphasize the importance of considering blocker permeation when assessing the effects of mutations on apparent blocker affinity.
Collapse
Affiliation(s)
- Harley T Kurata
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO 63110, USA
| | | | | | | | | |
Collapse
|
31
|
Lin X, Veenstra RD. Effect of transjunctional KCl gradients on the spermine inhibition of connexin40 gap junctions. Biophys J 2007; 93:483-95. [PMID: 17468172 PMCID: PMC1896261 DOI: 10.1529/biophysj.106.098517] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2006] [Accepted: 03/19/2007] [Indexed: 01/13/2023] Open
Abstract
Spermine inhibits rat connexin40 (Cx40) gap junctions. Glutamate residues at positions 9 and 13 and a basic amino acid (HKH) motif at positions 15-17 on the amino terminal domain are essential for this inhibitory activity. Questions remain as to whether spermine occludes the channel within the ion permeation pathway. To examine this question, cis or trans [KCl] was systematically lowered and the equilibrium dissociation constants (K(d)) and kinetics of unilateral spermine block on wild-type Cx40 gap junctions were determined. Asymmetric reductions in the trans [KCl] produced noticeable asymmetric shifts in the V(1/2) and G(min) values that progressively resembled G(j)-V(j) relationships observed in heterotypic connexin gap junction combinations. As cis or trans [KCl] was reduced by 25%, 50%, or 75% relative to the spermine-containing side, the transjunctional voltage (V(j))-dependent K(d) values increased or decreased, respectively. The spermine on-rates and off-rates, calculated from the junctional current decay and recovery time constants, were similarly affected. Hill coefficients for the spermine dose-response curves were approximately 0.58, indicative of negative cooperativity and possible multiple spermine inhibitory sites. The equivalent "electrical distance" (delta) ranged from 0.61 at 25% cis [KCl] to 1.4 at 25% trans [KCl], with a Hill coefficient of 1.0. Symmetrical reductions in [KCl] resulted in intermediate decreases in the spermine K(d)s, indicative of a minor electrostatic effect and a more significant effect of the transjunctional KCl electrodiffusion potential on the spermine association and dissociation rates. These data are consistent with a single spermine molecule being sufficient to occlude the Cx40 gap junction channel within the KCl permeation pathway.
Collapse
Affiliation(s)
- Xianming Lin
- Department of Pharmacology, The State University of New York Upstate Medical University, Syracuse, New York 13210, USA
| | | |
Collapse
|
32
|
Li Y, Berke I, Chen L, Jiang Y. Gating and inward rectifying properties of the MthK K+ channel with and without the gating ring. ACTA ACUST UNITED AC 2007; 129:109-20. [PMID: 17261840 PMCID: PMC2154359 DOI: 10.1085/jgp.200609655] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
In MthK, a Ca2+-gated K+ channel from Methanobacterium thermoautotrophicum, eight cytoplasmic RCK domains form an octameric gating ring that controls the intracellular gate of the ion conduction pore. The binding of Ca2+ ions to the RCK domains alters the conformation of the gating ring, thereby opening the gate. In the present study, we examined the Ca2+- and pH-regulated gating and the rectifying conduction properties of MthK at the single-channel level. The open probability (Po) of MthK exhibits a sigmoidal relationship with intracellular [Ca2+], and a Hill coefficient >1 is required to describe the dependence of Po on [Ca2+], suggesting cooperative Ca2+ activation of the channel. Additionally, intracellular Ca2+ also blocks the MthK pore in a voltage-dependent manner, rendering an apparently inwardly rectifying I-V relation. Intracellular pH has a dual effect on MthK gating. Below pH 7.5, the channel becomes insensitive to Ca2+. This occurs because the gating ring is structurally unstable at this pH and tends to disassemble (Ye, S., Y. Li, L. Chen, and Y. Jiang. 2006. Cell. 126:1161-1173). In contrast, above pH 7.5, a further increase in pH shifts the Po-[Ca2+] relation towards a lower Ca2+ concentration, augments Po at saturating [Ca2+], and activates the channel even in the absence of Ca2+. Channel activity is marked by bursts of rapid openings and closings separated by relatively longer interburst closings. The duration of interburst closing and the burst length are highly Ca2+ and pH dependent, whereas the kinetics of intraburst events is Ca2+ and pH independent. The rapid intraburst openings and closings are also observed with the isolated MthK pore lacking the attached intracellular gating ring. The fast kinetic events, independent of both Ca2+ and pH, therefore appear to be determined by processes occurring within the ion conduction pore, whereas the slow events reflect the gating process controlled by Ca2+ and pH through the gating ring.
Collapse
Affiliation(s)
- Yang Li
- Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | | | | | | |
Collapse
|
33
|
Grabe M, Bichet D, Qian X, Jan YN, Jan LY. K+ channel selectivity depends on kinetic as well as thermodynamic factors. Proc Natl Acad Sci U S A 2006; 103:14361-6. [PMID: 16983068 PMCID: PMC1570128 DOI: 10.1073/pnas.0606662103] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Potassium channels are necessary for a number of essential biological tasks such as the generation of action potentials and setting the resting membrane potential in cells, both of which require that these channels selectively permit the passage of potassium ions while suppressing the flow of other ions. Generally, this selectivity is attributed to a narrow stretch of the channel known as the selectivity filter. Over this stretch ions are dehydrated, and the backbone oxygen atoms of the protein mimic the ion's loss of coordination by water. However, channels are long pores with spatially distinct ion-binding sites that all must be traversed during ion permeation. We have shown that selectivity of mutant Kir3.2 (GIRK2) channels can be substantially amplified by introducing acidic residues into the cavity, a binding site below the selectivity filter. Here, we carry out electrostatic calculations on homology models to quantify the degree of stabilization that these mutations have on ions in the cavity. We then construct a multiion model of ion permeation to calculate the channel's permeability to potassium relative to sodium. This kinetic model uses rates derived from the electrostatic calculations and demonstrates that nonselective electrostatic stabilization of cations in the cavity can amplify channel selectivity independently of the selectivity filter. This nonintuitive result highlights the dependence of channel properties on the entire channel architecture and suggests that selectivity may not be fully understood by focusing solely on thermodynamic considerations of ion dehydration and the energetics of the selectivity filter.
Collapse
Affiliation(s)
- Michael Grabe
- Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143
| | - Delphine Bichet
- Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143
| | - Xiang Qian
- Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143
| | - Yuh Nung Jan
- Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143
| | - Lily Yeh Jan
- Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143
- To whom correspondence should be addressed. E-mail:
| |
Collapse
|