1
|
Alkassar M, Tudó À, Rambla-Alegre M, Ferreres L, Diogène J, Sureda FX, Campàs M. First record of paralytic shellfish toxins in marine pufferfish from the Spanish Mediterranean coast using cell-based assay, automated patch clamp and HPLC-FLD. CHEMOSPHERE 2024; 364:143053. [PMID: 39121960 DOI: 10.1016/j.chemosphere.2024.143053] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/17/2024] [Revised: 08/06/2024] [Accepted: 08/06/2024] [Indexed: 08/12/2024]
Abstract
Pufferfish is one of the most poisonous marine organisms, responsible for numerous poisoning incidents and some human fatalities due to its capability to accumulate potent neurotoxins such as tetrodotoxins (TTXs) and paralytic shellfish toxins (PSTs). In this study, tissue extracts (muscle, skin, liver, intestinal tract and gonads) obtained from sixteen pufferfish specimens of the Lagocephalus lagocephalus and Sphoeroides pachygaster species, collected along the Spanish Mediterranean coast, were analysed for the presence of voltage-gated sodium channel (also known as Nav channel) blockers using cell-based assay (CBA) and automated patch clamp (APC). No toxicity was observed in any of the S. pachygaster specimens, but toxicity was detected in the liver of most L. lagocephalus specimens. Instrumental analysis of these specimens, as well as in one Lagocephalus sceleratus specimen, by high-performance liquid chromatography coupled to fluorescence detection (HPLC-FLD) was performed, which confirmed the presence of PSTs only in L. lagocephalus specimens. This analysis reported the presence of saxitoxin (STX) and decarbamoylsaxitoxin (dcSTX) in all positive samples, being dcSTX the major analogue. These results demonstrate the ability of this species to accumulate PSTs, being the first report of the presence of PSTs in Mediterranean L.lagocephalus specimens. Furthermore, the presence of high PSTs contents in all five tested tissues of one L. lagocephalus specimen pointed the risk that the presence of this toxic fish in the Mediterranean Sea may represent for seafood safety and human health in case of accidental consumption.
Collapse
Affiliation(s)
- Mounira Alkassar
- IRTA, Ctra. Poble Nou Km 5.5, 43540, La Ràpita, Spain; Universitat Rovira I Virgili, Av. Països Catalans 26, 43007, Tarragona, Spain
| | - Àngels Tudó
- Universitat Rovira I Virgili, Av. Països Catalans 26, 43007, Tarragona, Spain
| | | | | | - Jorge Diogène
- IRTA, Ctra. Poble Nou Km 5.5, 43540, La Ràpita, Spain
| | - Francesc X Sureda
- Universitat Rovira I Virgili, Av. Països Catalans 26, 43007, Tarragona, Spain
| | - Mònica Campàs
- IRTA, Ctra. Poble Nou Km 5.5, 43540, La Ràpita, Spain.
| |
Collapse
|
2
|
Aballay-González A, Gallardo-Rodriguez JJ, Silva-Higuera M, Rivera A, Ulloa V, Delgado-Rivera L, Rivera-Belmar A, Astuya A. Neuro-2a cell-based assay for toxicity equivalency factor - proposal and evaluation in Chilean contaminated shellfish samples. Food Addit Contam Part A Chem Anal Control Expo Risk Assess 2019; 37:162-173. [DOI: 10.1080/19440049.2019.1676919] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Affiliation(s)
- Ambbar Aballay-González
- Laboratorio de Biotoxinas UdeC, Departamento de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile
- Centro de Investigación Oceanográfica COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile
| | | | - Macarena Silva-Higuera
- Laboratorio de Biotoxinas UdeC, Departamento de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile
- Centro de Investigación Oceanográfica COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile
| | - Alejandra Rivera
- Laboratorio de Biotoxinas UdeC, Departamento de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile
- Centro de Investigación Oceanográfica COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile
| | - Viviana Ulloa
- Laboratorio de Biotoxinas UdeC, Departamento de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile
| | - Lorena Delgado-Rivera
- Laboratorio de Toxinas Marinas y Micotoxinas, Sección de Química de Alimentos, Departamento de Salud Ambiental, Instituto de Salud Pública de Chile, Ñuñoa, Chile
| | - Andrea Rivera-Belmar
- Departamento de Alimentación y Nutrición, División de Salud y Política Pública, Subsecretaría de Salud Pública, Ministerio de Salud, Santiago, Chile
| | - Allisson Astuya
- Laboratorio de Biotoxinas UdeC, Departamento de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile
- Centro de Investigación Oceanográfica COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile
| |
Collapse
|
3
|
Aballay-Gonzalez A, Ulloa V, Rivera A, Hernández V, Silva M, Caprile T, Delgado-Rivera L, Astuya A. Matrix effects on a cell-based assay used for the detection of paralytic shellfish toxins in bivalve shellfish samples. Food Addit Contam Part A Chem Anal Control Expo Risk Assess 2016; 33:869-75. [DOI: 10.1080/19440049.2016.1166741] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Affiliation(s)
- Ambbar Aballay-Gonzalez
- Laboratory of Cell Culture and Marine Genomics, Marine Biotechnology Unit, Faculty of Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
| | - Viviana Ulloa
- Laboratory of Cell Culture and Marine Genomics, Marine Biotechnology Unit, Faculty of Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
| | - Alejandra Rivera
- Laboratory of Cell Culture and Marine Genomics, Marine Biotechnology Unit, Faculty of Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
- Sur-Austral COPAS Program, University of Concepcion, Concepción, Chile
| | - Víctor Hernández
- Natural Products Chemistry Laboratory, Botanic Department, Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
| | - Macarena Silva
- Laboratory of Cell Culture and Marine Genomics, Marine Biotechnology Unit, Faculty of Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
| | - Teresa Caprile
- Axon Guidance Laboratory, Department of Cell Biology, Faculty of Biological Sciences, University of Concepcion, Concepción, Chile
| | - Lorena Delgado-Rivera
- Laboratory of Marine Toxins and Mycotoxins, Food Chemistry Section, Environmental Health Department, Institute of Public Health Chile, Ñuñoa, Chile
| | - Allisson Astuya
- Laboratory of Cell Culture and Marine Genomics, Marine Biotechnology Unit, Faculty of Natural and Oceanographic Sciences, University of Concepcion, Concepción, Chile
- Sur-Austral COPAS Program, University of Concepcion, Concepción, Chile
| |
Collapse
|
4
|
Liu XP, Wooltorton JRA, Gaboyard-Niay S, Yang FC, Lysakowski A, Eatock RA. Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents. J Neurophysiol 2016; 115:2536-55. [PMID: 26936982 DOI: 10.1152/jn.00902.2015] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2015] [Accepted: 02/02/2016] [Indexed: 01/02/2023] Open
Abstract
Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.
Collapse
Affiliation(s)
- Xiao-Ping Liu
- Speech and Hearing Bioscience and Technology Program, Harvard-Massachusetts Institute of Technology Health Sciences and Technology Program, Cambridge, Massachusetts; Eaton-Peabody Laboratories, Massachusetts Eye and Ear, and Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts
| | | | - Sophie Gaboyard-Niay
- Department of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago, Illinois
| | - Fu-Chia Yang
- Dana-Farber Cancer Institute, Boston, Massachusetts; Department of Neurobiology, Harvard Medical School, Boston, Massachusetts; and
| | - Anna Lysakowski
- Department of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago, Illinois; Department of Otolaryngology-Head and Neck Surgery, University of Illinois at Chicago, Chicago, Illinois
| | - Ruth Anne Eatock
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, and Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts; Department of Neurobiology, Harvard Medical School, Boston, Massachusetts; and Department of Otolaryngology-Head and Neck Surgery, University of Illinois at Chicago, Chicago, Illinois
| |
Collapse
|
5
|
Goldschen-Ohm MP, Chanda B. Probing gating mechanisms of sodium channels using pore blockers. Handb Exp Pharmacol 2014; 221:183-201. [PMID: 24737237 DOI: 10.1007/978-3-642-41588-3_9] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Several classes of small molecules and peptides bind at the central pore of voltage-gated sodium channels either from the extracellular or intracellular side of the membrane and block ion conduction through the pore. Biophysical studies that shed light on the chemical nature, accessibility, and kinetics of binding of these naturally occurring and synthetic compounds reveal a wealth of information about how these channels gate. Here, we discuss insights into the structural underpinnings of gating of the channel pore and its coupling to the voltage sensors obtained from pore blockers including site 1 neurotoxins and local anesthetics.
Collapse
|
6
|
Huang CJ, Schild L, Moczydlowski EG. Use-dependent block of the voltage-gated Na(+) channel by tetrodotoxin and saxitoxin: effect of pore mutations that change ionic selectivity. ACTA ACUST UNITED AC 2013; 140:435-54. [PMID: 23008436 PMCID: PMC3457692 DOI: 10.1085/jgp.201210853] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Voltage-gated Na(+) channels (NaV channels) are specifically blocked by guanidinium toxins such as tetrodotoxin (TTX) and saxitoxin (STX) with nanomolar to micromolar affinity depending on key amino acid substitutions in the outer vestibule of the channel that vary with NaV gene isoforms. All NaV channels that have been studied exhibit a use-dependent enhancement of TTX/STX affinity when the channel is stimulated with brief repetitive voltage depolarizations from a hyperpolarized starting voltage. Two models have been proposed to explain the mechanism of TTX/STX use dependence: a conformational mechanism and a trapped ion mechanism. In this study, we used selectivity filter mutations (K1237R, K1237A, and K1237H) of the rat muscle NaV1.4 channel that are known to alter ionic selectivity and Ca(2+) permeability to test the trapped ion mechanism, which attributes use-dependent enhancement of toxin affinity to electrostatic repulsion between the bound toxin and Ca(2+) or Na(+) ions trapped inside the channel vestibule in the closed state. Our results indicate that TTX/STX use dependence is not relieved by mutations that enhance Ca(2+) permeability, suggesting that ion-toxin repulsion is not the primary factor that determines use dependence. Evidence now favors the idea that TTX/STX use dependence arises from conformational coupling of the voltage sensor domain or domains with residues in the toxin-binding site that are also involved in slow inactivation.
Collapse
|
7
|
Moczydlowski EG. The molecular mystique of tetrodotoxin. Toxicon 2012; 63:165-83. [PMID: 23261990 DOI: 10.1016/j.toxicon.2012.11.026] [Citation(s) in RCA: 78] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2012] [Accepted: 11/30/2012] [Indexed: 01/06/2023]
Abstract
In many respects tetrodotoxin (TTX) is the quintessential natural toxin. It is unequivocally toxic to mammals with LD(50) values for mice in the range of 10 μg/kg (intraperitoneal), 16 μg/kg (subcutaneous), and 332 μg/kg (oral) (Kao, 1966). Its biothreat status is recognized by its listing as a "Select Agent" by the US Department of Health and Human Services which includes regulated agents "determined to have the potential to pose a severe threat to both human and animal health" (http://www.selectagents.gov/). It has a well-defined cellular target (i.e., NaV channels) and pharmacological mode of action (i.e., block of nerve and muscle action potentials), and it is an indispensable chemical tool in neuroscience. It is widely distributed in marine and terrestrial ecosystems where it plays a role in the chemical ecology of predator-prey relationships and drives evolutionary selection of TTX-resistance (Hanifin, 2010; Williams, 2010; Zimmer and Ferrer, 2007). Lastly, TTX has acquired a certain mystique in scientific lore attributable to many fascinating aspects of its natural history and molecular interactions as presented in selected summary below. Additional information may be found in other excellent reviews (Fozzard and Lipkind, 2010; Kao, 1966; Lee and Ruben, 2008; Narahashi, 2001, 2008).
Collapse
Affiliation(s)
- Edward G Moczydlowski
- Nanobiology, Sandia National Laboratories, P.O. Box 5800, MS1413, Albuquerque, NM 87185-1413, USA
| |
Collapse
|
8
|
Investigations into matrix components affecting the performance of the official bioassay reference method for quantitation of paralytic shellfish poisoning toxins in oysters. Toxicon 2012; 59:215-30. [DOI: 10.1016/j.toxicon.2011.11.013] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2011] [Revised: 11/16/2011] [Accepted: 11/17/2011] [Indexed: 11/20/2022]
|
9
|
The tetrodotoxin binding site is within the outer vestibule of the sodium channel. Mar Drugs 2010; 8:219-34. [PMID: 20390102 PMCID: PMC2852835 DOI: 10.3390/md8020219] [Citation(s) in RCA: 91] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2009] [Revised: 01/10/2010] [Accepted: 01/28/2010] [Indexed: 12/21/2022] Open
Abstract
Tetrodotoxin and saxitoxin are small, compact asymmetrical marine toxins that block voltage-gated Na channels with high affinity and specificity. They enter the channel pore’s outer vestibule and bind to multiple residues that control permeation. Radiolabeled toxins were key contributors to channel protein purification and subsequent cloning. They also helped identify critical structural elements called P loops. Spacial organization of their mutation-identified interaction sites in molecular models has generated a molecular image of the TTX binding site in the outer vestibule and the critical permeation and selectivity features of this region. One site in the channel’s domain I P loop determines affinity differences in mammalian isoforms.
Collapse
|
10
|
Kaufmann SG, Westenbroek RE, Zechner C, Maass AH, Bischoff S, Muck J, Wischmeyer E, Scheuer T, Maier SKG. Functional protein expression of multiple sodium channel alpha- and beta-subunit isoforms in neonatal cardiomyocytes. J Mol Cell Cardiol 2009; 48:261-9. [PMID: 19426735 DOI: 10.1016/j.yjmcc.2009.04.017] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/27/2009] [Revised: 04/14/2009] [Accepted: 04/29/2009] [Indexed: 11/29/2022]
Abstract
Voltage-gated sodium channels are composed of pore-forming alpha- and auxiliary beta-subunits and are responsible for the rapid depolarization of cardiac action potentials. Recent evidence indicates that neuronal tetrodotoxin (TTX) sensitive sodium channel alpha-subunits are expressed in the heart in addition to the predominant cardiac TTX-resistant Na(v)1.5 sodium channel alpha-subunit. These TTX-sensitive isoforms are preferentially localized in the transverse tubules of rodents. Since neonatal cardiomyocytes have yet to develop transverse tubules, we determined the complement of sodium channel subunits expressed in these cells. Neonatal rat ventricular cardiomyocytes were stained with antibodies specific for individual isoforms of sodium channel alpha- and beta-subunits. alpha-actinin, a component of the z-line, was used as an intracellular marker of sarcomere boundaries. TTX-sensitive sodium channel alpha-subunit isoforms Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.4 and Na(v)1.6 were detected in neonatal rat heart but at levels reduced compared to the predominant cardiac alpha-subunit isoform, Na(v)1.5. Each of the beta-subunit isoforms (beta1-beta4) was also expressed in neonatal cardiac cells. In contrast to adult cardiomyocytes, the alpha-subunits are distributed in punctate clusters across the membrane surface of neonatal cardiomyocytes; no isoform-specific subcellular localization is observed. Voltage clamp recordings in the absence and presence of 20 nM TTX provided functional evidence for the presence of TTX-sensitive sodium current in neonatal ventricular myocardium which represents between 20 and 30% of the current, depending on membrane potential and experimental conditions. Thus, as in the adult heart, a range of sodium channel alpha-subunits are expressed in neonatal myocytes in addition to the predominant TTX-resistant Na(v)1.5 alpha-subunit and they contribute to the total sodium current.
Collapse
Affiliation(s)
- Susann G Kaufmann
- Medizinische Klinik und Poliklinik I, Universität Würzburg, Josef-Schneider-Strabetae 2, 97080 Würzburg, Germany
| | | | | | | | | | | | | | | | | |
Collapse
|
11
|
Llewellyn LE. Sodium channel inhibiting marine toxins. PROGRESS IN MOLECULAR AND SUBCELLULAR BIOLOGY 2009; 46:67-97. [PMID: 19184585 DOI: 10.1007/978-3-540-87895-7_3] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Saxitoxin (STX), tetrodotoxin (TTX) and their many chemical relatives are part of our daily lives. From killing people who eat seafood containing these toxins, to being valuable research tools unveiling the invisible structures of their pharmacological receptor, their global impact is beyond measure. The pharmacological receptor for these toxins is the voltage-gated sodium channel which transports Na ions between the exterior to the interior of cells. The two structurally divergent families of STX and TTX analogues bind at the same location on these Na channels to stop the flow of ions. This can affect nerves, muscles and biological senses of most animals. It is through these and other toxins that we have developed much of our fundamental understanding of the Na channel and its part in generating action potentials in excitable cells.
Collapse
Affiliation(s)
- Lyndon E Llewellyn
- Australian Institute of Marine Science, Townsville MC, QLD 4810, Australia.
| |
Collapse
|
12
|
Sun H, Varela D, Chartier D, Ruben PC, Nattel S, Zamponi GW, Leblanc N. Differential interactions of Na+ channel toxins with T-type Ca2+ channels. ACTA ACUST UNITED AC 2008; 132:101-13. [PMID: 18591418 PMCID: PMC2442173 DOI: 10.1085/jgp.200709883] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Two types of voltage-dependent Ca2+ channels have been identified in heart: high (ICaL) and low (ICaT) voltage-activated Ca2+ channels. In guinea pig ventricular myocytes, low voltage–activated inward current consists of ICaT and a tetrodotoxin (TTX)-sensitive ICa component (ICa(TTX)). In this study, we reexamined the nature of low-threshold ICa in dog atrium, as well as whether it is affected by Na+ channel toxins. Ca2+ currents were recorded using the whole-cell patch clamp technique. In the absence of external Na+, a transient inward current activated near −50 mV, peaked at −30 mV, and reversed around +40 mV (HP = −90 mV). It was unaffected by 30 μM TTX or micromolar concentrations of external Na+, but was inhibited by 50 μM Ni2+ (by ∼90%) or 5 μM mibefradil (by ∼50%), consistent with the reported properties of ICaT. Addition of 30 μM TTX in the presence of Ni2+ increased the current approximately fourfold (41% of control), and shifted the dose–response curve of Ni2+ block to the right (IC50 from 7.6 to 30 μM). Saxitoxin (STX) at 1 μM abolished the current left in 50 μM Ni2+. In the absence of Ni2+, STX potently blocked ICaT (EC50 = 185 nM) and modestly reduced ICaL (EC50 = 1.6 μM). While TTX produced no direct effect on ICaT elicited by expression of hCaV3.1 and hCaV3.2 in HEK-293 cells, it significantly attenuated the block of this current by Ni2+ (IC50 increased to 550 μM Ni2+ for CaV3.1 and 15 μM Ni2+ for CaV3.2); in contrast, 30 μM TTX directly inhibited hCaV3.3-induced ICaT and the addition of 750 μM Ni2+ to the TTX-containing medium led to greater block of the current that was not significantly different than that produced by Ni2+ alone. 1 μM STX directly inhibited CaV3.1-, CaV3.2-, and CaV3.3-mediated ICaT but did not enhance the ability of Ni2+ to block these currents. These findings provide important new implications for our understanding of structure–function relationships of ICaT in heart, and further extend the hypothesis of a parallel evolution of Na+ and Ca2+ channels from an ancestor with common structural motifs.
Collapse
Affiliation(s)
- Hui Sun
- Excigen, Inc., Baltimore, MD 21224, USA
| | | | | | | | | | | | | |
Collapse
|
13
|
Brette F, Orchard CH. No Apparent Requirement for Neuronal Sodium Channels in Excitation-Contraction Coupling in Rat Ventricular Myocytes. Circ Res 2006; 98:667-74. [PMID: 16484618 DOI: 10.1161/01.res.0000209963.02720.70] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The majority of Na channels in the heart are composed of the tetrodotoxin (TTX)-resistant (K
D
, 2 to 6 μmol/L) “cardiac” Na
V
1.5 isoform; however, TTX-sensitive (K
D
, 1 to 25 nmol/L) “neuronal” Na channel isoforms have recently been detected in several cardiac preparations. In the present study, we determined the functional subcellular localization of Na channel isoforms (according to their TTX sensitivity) in rat ventricular myocytes by recording
I
Na
in control and detubulated myocytes. We found that TTX-sensitive
I
Na
(K
D
, &8.8 nmol/L) makes up 14±3% of total
I
Na
in control and ≤4% in detubulated myocytes and calculated that &80% of TTX-sensitive
I
Na
is located in the t-tubules, where it generates &1/3 of t-tubular
I
Na
. In contrast, TTX-resistant
I
Na
is located predominantly (&78%) at the surface membrane. We also investigated the possible contribution of TTX-sensitive
I
Na
to excitation-contraction coupling, using 200 nmol/L TTX to selectively block TTX-sensitive
I
Na
. TTX decreased the rate of depolarization of the action potential by 10% but did not delay the rise of systolic Ca
2+
in the center of the cell (transverse confocal line scan), suggesting that TTX-sensitive
I
Na
does not play a role in synchronizing Ca
2+
release at the t-tubules; the amplitude of the Ca
2+
transient and contraction were also unchanged by 200 nmol/L TTX. The quantity of charge entering via
I
Ca
elicited by control or TTX action potential waveforms was similar, suggesting that the trigger for Ca
2+
release is not altered by blocking TTX-sensitive
I
Na
. We conclude that neuronal
I
Na
is concentrated at the t-tubules, but there is no evidence of a requirement for these channels in normal excitation-contraction coupling in ventricular myocytes.
Collapse
Affiliation(s)
- Fabien Brette
- Department of Physiology, University of Bristol, United Kingdom.
| | | |
Collapse
|
14
|
Scheib H, McLay I, Guex N, Clare JJ, Blaney FE, Dale TJ, Tate SN, Robertson GM. Modeling the pore structure of voltage-gated sodium channels in closed, open, and fast-inactivated conformation reveals details of site 1 toxin and local anesthetic binding. J Mol Model 2006; 12:813-22. [PMID: 16508760 DOI: 10.1007/s00894-005-0066-y] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2005] [Accepted: 09/15/2005] [Indexed: 10/25/2022]
Abstract
In this work molecular modeling was applied to generate homology models of the pore region of the Na(v)1.2 and Na(v)1.8 isoforms of human voltage-gated sodium channels. The models represent the channels in the resting, open, and fast-inactivated states. The transmembrane portions of the channels were based on the equivalent domains of the closed and open conformation potassium channels KcsA and MthK, respectively. The critical selectivity loops were modeled using a structural template identified by a novel 3D-search technique and subsequently merged with the transmembrane portions. The resulting draft models were used to study the differences of tetrodotoxin binding to the tetrodotoxin-sensitive Na(v)1.2 (EC50: 0.012 microM) and -insensitive Na(v)1.8 (EC50: 60 microM) isoforms, respectively. Furthermore, we investigated binding of the local anesthetic tetracaine to Na(v)1.8 (EC50: 12.5 microM) in resting, conducting, and fast-inactivated state. In accordance with experimental mutagenesis studies, computational docking of tetrodotoxin and tetracaine provided (1) a description of site 1 toxin and local anesthetic binding sites in voltage-gated sodium channels. (2) A rationale for site 1 toxin-sensitivity versus -insensitivity in atomic detail involving interactions of the Na(v)1.2 residues F385-I and W943-II. (3) A working hypothesis of interactions between Na(v)1.8 in different conformational states and the local anesthetic tetracaine.
Collapse
Affiliation(s)
- Holger Scheib
- SBC Lab AG, Seebüelstrasse 26, 8185, Winkel, Switzerland.
| | | | | | | | | | | | | | | |
Collapse
|
15
|
Llewellyn LE. Saxitoxin, a toxic marine natural product that targets a multitude of receptors. Nat Prod Rep 2006; 23:200-22. [PMID: 16572228 DOI: 10.1039/b501296c] [Citation(s) in RCA: 243] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Saxitoxin (STX) was discovered early last century and can contaminate seafood and drinking water, and over time has become an invaluable research tool and an internationally regulated chemical weapon. Among natural products, toxins obtain a unique reputation from their high affinity and selectivity for their target pharmacological receptor, which for STX has long been considered to only be the voltage gated sodium channel. In recent times however, STX has been discovered to also bind to calcium and potassium channels, neuronal nitric oxide synthase, STX metabolizing enzymes and two circulatory fluid proteins, namely a transferrin-like family of proteins and a unique protein found in the blood of pufferfish.
Collapse
|
16
|
Abstract
Many substances, organic and manufactured, may induce peripheral nerve damage when exposed to them. The expected clinical phenotype is of a distal, sensory or sensorimotor polyneuropathy, often painful, with axonal characteristics on electrodiagnostic and histopathologic analysis. Treatment is limited; often, the only effective management is supportive care and avoidance from or removal of the offending toxin. Fortunately, the majority of toxic neuropathies are self-limited and improves gradually after toxin elimination.
Collapse
Affiliation(s)
- Patrick M Grogan
- Department of Neurology, Wilford Hall Medical Center, San Antonio, TX 78236, USA.
| | | |
Collapse
|
17
|
Llewellyn L, Negri A, Quilliam M. High affinity for the rat brain sodium channel of newly discovered hydroxybenzoate saxitoxin analogues from the dinoflagellate Gymnodinium catenatum. Toxicon 2004; 43:101-4. [PMID: 15037035 DOI: 10.1016/j.toxicon.2003.10.016] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2003] [Revised: 09/03/2003] [Accepted: 10/10/2003] [Indexed: 11/22/2022]
Abstract
The paralytic shellfish poison family has been recently extended by the discovery of several analogues possessing a hydoxybenzoate moiety instead of the carbamoyl group one finds in saxitoxin, the parent molecule of this toxin family. We have investigated the potency of these new analogues on a representative isoform of the pharmacological target of these toxins, the voltage gated sodium channel. These toxins were found to have K1's in the low nanomolar range, only slightly less potent than saxitoxin. The hydroxybenzoate group may increase the lipophilicity of these toxins and improve their ability to pass through epithelia and therefore its uptake and elimination in both intoxication victims and animals that bioaccumulate paralytic shellfish toxins.
Collapse
Affiliation(s)
- Lyndon Llewellyn
- Australian Institute of Marine Science, Marine Biotechnology, PMB 3, Townsville, MC 4810, Australia.
| | | | | |
Collapse
|
18
|
Morgan TP, Grosell M, Playle RC, Wood CM. The time course of silver accumulation in rainbow trout during static exposure to silver nitrate: physiological regulation or an artifact of the exposure conditions? AQUATIC TOXICOLOGY (AMSTERDAM, NETHERLANDS) 2004; 66:55-72. [PMID: 14687979 DOI: 10.1016/j.aquatox.2003.07.003] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The pattern of gill silver accumulation in rainbow trout during waterborne silver exposure has been reported to be unusual, reaching a peak in the first few hours of silver exposure followed by a marked decline with continued exposure. The potential causes of the pattern were investigated. Rainbow trout (1-5g) were exposed in a static system to 110mAg labeled AgNO(3) at a total concentration of 1.92microg Agl(-1) for 24h in synthetic soft water. Periodically throughout the exposure, gill and body 110mAg accumulation, gill and body 24Na uptake (from which whole body Na(+) uptake was calculated), gill Na(+)K(+)-ATPase activity, plus water silver (total and dissolved), Cl(-) and total organic carbon (TOC) concentrations were measured. Gill silver levels rapidly increased, peaked at 3h of exposure and then decreased until a plateau was reached at 12h of exposure. Body (minus gills) silver levels increased steadily over the exposure period until 18h of exposure. Whole body Na(+) uptake decreased, was maximally inhibited by 3h of exposure but recovered by 12h despite continued silver exposure. Gill Na(+)K(+)-ATPase activity was not inhibited until 5h of exposure. The water dissolved silver concentration declined by approximately 70% over the 24h exposure period and the TOC content of the water increased over three-fold during the first 2h of exposure. There was a decrease in the calculated contribution of Ag(+) (from 20.9 to 2.5%) and an increase in the calculated contribution of Ag-TOC complexes (from 77 to 97.3%) to the total water silver concentration over the first 2h of exposure. Apical silver uptake into the gills decreased over the initial 2.5h of exposure while basolateral silver export out of the gills to the body remained constant throughout the exposure. The results of this study suggest that: (1) physiological regulation of silver movement may explain the pattern of gill silver accumulation observed in rainbow trout, although not by a mechanism coupled to Na(+)K(+)-ATPase inhibition as originally proposed; (2) alternatively or additionally, a decreased bioavailability of silver, due to the static exposure conditions, may explain the pattern of gill accumulation; (3) the early inhibition of whole body Na(+) uptake observed during silver exposure occurs via a mechanism other than Na(+)K(+)-ATPase inhibition; and (4) gill silver accumulation may be an appropriate endpoint for biotic ligand modeling.
Collapse
Affiliation(s)
- Tammie P Morgan
- Department of Biology, McMaster University, Hamilton, Ont., Canada.
| | | | | | | |
Collapse
|
19
|
Floresca CZ. A comparison of the mu-conotoxins by [3H]saxitoxin binding assays in neuronal and skeletal muscle sodium channel. Toxicol Appl Pharmacol 2003; 190:95-101. [PMID: 12878039 DOI: 10.1016/s0041-008x(03)00153-4] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Sodium channels from rat brain, rat skeletal muscle, chick brain, and eel electroplax were compared by using the mu-conotoxins GIIIA, PIIIA, and StIII and [3H]saxitoxin. Rat skeletal muscle and eel electroplax sodium channels are equally sensitive to GIIIA, PIIIA, and StIII, displacing >90% of the [3H]saxitoxin binding sites in rat skeletal muscle and eel electroplax membranes and exhibiting inhibitory concentrations at half-maximal percentage specific binding (IC50) of 0.97 nM for GIIIA, 1.3 nM for PIIIA in rat skeletal muscle, and concentrations of 3.5 nM for GIIIA and 2.8 nM for PIIIA in eel electroplax. PIIIA and GIIIA at all concentrations inhibit only up to 10% of [3H]saxitoxin binding sites in chick brain membranes. Mu-conotoxin StIII at all concentrations inhibits only up to 10% of [3H]saxitoxin binding sites in rat brain membranes and displays two-site binding inhibition of the [3H]saxitoxin binding sites in rat skeletal muscle. PIIIA displaces >60% of the [3H]saxitoxin binding sites (IC50 of 44 nM) while GIIIA blocks out only 30% of the binding sites in rat brain sodium channels (IC50 of 69 nM). Thus, sodium channel subtypes can be classified into two categories: mu-conotoxin sensitive (i.e., subtypes predominantly found in rat skeletal muscle and eel electroplax) and insensitive (i.e., subtypes predominantly found in rat and chick brain).
Collapse
Affiliation(s)
- Christina Z Floresca
- Marine Science Institute, University of the Philippines, Velasquez St., Diliman, Quezon City, Philippines 1101.
| |
Collapse
|
20
|
Doyle J, Brinkworth CS, Wegener KL, Carver JA, Llewellyn LE, Olver IN, Bowie JH, Wabnitz PA, Tyler MJ. nNOS inhibition, antimicrobial and anticancer activity of the amphibian skin peptide, citropin 1.1 and synthetic modifications. The solution structure of a modified citropin 1.1. EUROPEAN JOURNAL OF BIOCHEMISTRY 2003; 270:1141-53. [PMID: 12631273 DOI: 10.1046/j.1432-1033.2003.03462.x] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
A large number of bioactive peptides have been isolated from amphibian skin secretions. These peptides have a variety of actions including antibiotic and anticancer activities and the inhibition of neuronal nitric oxide synthase. We have investigated the structure-activity relationship of citropin 1.1, a broad-spectrum antibiotic and anticancer agent that also causes inhibition of neuronal nitric oxide synthase, by making a number of synthetically modified analogues. Citropin 1.1 has been shown previously to form an amphipathic alpha-helix in aqueous trifluoroethanol. The results of the structure-activity studies indicate the terminal residues are important for bacterial activity and increasing the overall positive charge, while maintaining an amphipathic distribution of residues, increases activity against Gram-negative organisms. Anticancer activity generally mirrors antibiotic activity suggesting a common mechanism of action. The N-terminal residues are important for inhibition of neuronal nitric oxide synthase, as is an overall positive charge greater than three. The structure of one of the more active synthetic modifications (A4K14-citropin 1.1) was determined in aqueous trifluoroethanol, showing that this peptide also forms an amphipathic alpha-helix.
Collapse
Affiliation(s)
- Jason Doyle
- Australian Institute for Marine Science, Townsville MC, Queensland, Australia
| | | | | | | | | | | | | | | | | |
Collapse
|
21
|
Renganathan M, Dib-Hajj S, Waxman SG. Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons. BRAIN RESEARCH. MOLECULAR BRAIN RESEARCH 2002; 106:70-82. [PMID: 12393266 DOI: 10.1016/s0169-328x(02)00411-4] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.
Collapse
Affiliation(s)
- M Renganathan
- Department of Neurology, Yale Medical School, New Haven, CT 06510, USA
| | | | | |
Collapse
|
22
|
Yatsuhashi T, Hisatome I, Kurata Y, Sasaki N, Ogura K, Kato M, Kinugasa R, Matsubara K, Yamawaki M, Yamamoto Y, Tanaka Y, Ogino K, Igawa O, Makita N, Shigemasa C. L-cysteine prevents oxidation-induced block of the cardiac Na+ channel via interaction with heart-specific cysteinyl residues in the P-loop region. Circ J 2002; 66:846-50. [PMID: 12224824 DOI: 10.1253/circj.66.846] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The present study investigated the protective effects of L-cysteine on the oxidation-induced blockade of Na+ channel a-subunits, hH1 (cardiac) and hSkM1 (skeletal), expressed in COS7 cells. Na+ currents were recorded by the whole-cell patch clamp technique (n = 3-7). L-cysteine alone blocked hH1 and hSkM1 in a dose-dependent manner, with saturating L-cysteine block at 3,000 micromol/L. Hg2+, a potent sulfhydryl oxidizing agent, blocked hH1 with a time to 50% inhibition (Time50%) of 20s. Preperfusion of COS7 cells with 100 micromol/L L-cysteine significantly slowed the Hg2+ block of hH1 (Time50% = 179 s). L-cysteine did not prevent Hg2+ block of hSkM1 (Time50% = 37s) or the C373Y hH1 mutant (Time50% = 43s). As for other sulfo-amino acids, homocysteine prevented the Hg2+ block of hH1, with the Time50% (70s) being significantly smaller than that of L-cysteine, whereas methionine did not prevent the Hg2+ block of hH1. L-cysteine did not prevent the Cd2+ block of hH1. These results indicate that L-cysteine selectively acts on heart-specific Cys373 in the P-loop region of hH1 to prevent Cys373 from the oxidation-induced sulfur-Hg-sulfur bridge formation.
Collapse
Affiliation(s)
- Toru Yatsuhashi
- Department of Medicine, Faculty of Medicine, Tottori University, Yonago, Japan
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
23
|
Chen Z, Alcayaga C, Suarez-Isla BA, O'Rourke B, Tomaselli G, Marban E. A "minimal" sodium channel construct consisting of ligated S5-P-S6 segments forms a toxin-activatable ionophore. J Biol Chem 2002; 277:24653-8. [PMID: 11973330 DOI: 10.1074/jbc.m111862200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (micro1) with a polyhistidine tag on the C terminus (muHis) and pore-only micro1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wild-type micro1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [3H]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (deltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 microm tetrodotoxin, deltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of micro1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.
Collapse
Affiliation(s)
- Zhenhui Chen
- Institute of Molecular Cardiobiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, 21205, USA
| | | | | | | | | | | |
Collapse
|
24
|
Maier SKG, Westenbroek RE, Schenkman KA, Feigl EO, Scheuer T, Catterall WA. An unexpected role for brain-type sodium channels in coupling of cell surface depolarization to contraction in the heart. Proc Natl Acad Sci U S A 2002; 99:4073-8. [PMID: 11891345 PMCID: PMC122650 DOI: 10.1073/pnas.261705699] [Citation(s) in RCA: 207] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Voltage-gated sodium channels composed of pore-forming alpha and auxiliary beta subunits are responsible for the rising phase of the action potential in cardiac muscle, but the functional roles of distinct sodium channel subtypes have not been clearly defined. Immunocytochemical studies show that the principal cardiac pore-forming alpha subunit isoform Na(v)1.5 is preferentially localized in intercalated disks, whereas the brain alpha subunit isoforms Na(v)1.1, Na(v)1.3, and Na(v)1.6 are localized in the transverse tubules. Sodium currents due to the highly tetrodotoxin (TTX)-sensitive brain isoforms in the transverse tubules are small and are detectable only after activation with beta scorpion toxin. Nevertheless, they play an important role in coupling depolarization of the cell surface membrane to contraction, because low TTX concentrations reduce left ventricular function. Our results suggest that the principal cardiac isoform in the intercalated disks is primarily responsible for action potential conduction between cells and reveal an unexpected role for brain sodium channel isoforms in the transverse tubules in coupling electrical excitation to contraction in cardiac muscle.
Collapse
Affiliation(s)
- Sebastian K G Maier
- Department of Pharmacology, University of Washington, Seattle, WA 98195, USA
| | | | | | | | | | | |
Collapse
|
25
|
Doyle J, Llewellyn LE, Brinkworth CS, Bowie JH, Wegener KL, Rozek T, Wabnitz PA, Wallace JC, Tyler MJ. Amphibian peptides that inhibit neuronal nitric oxide synthase. Isolation of lesuerin from the skin secretion of the Australian Stony Creek frog Litoria lesueuri. EUROPEAN JOURNAL OF BIOCHEMISTRY 2002; 269:100-9. [PMID: 11784303 DOI: 10.1046/j.0014-2956.2002.02630.x] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Two neuropeptides have been isolated and identified from the secretions of the skin glands of the Stony Creek Frog Litoria lesueuri. The first of these, the known neuropeptide caerulein 1.1, is a common constituent of anuran skin secretions, and has the sequence pEQY(SO3)TGWMDF-NH2. This neuropeptide is smooth muscle active, an analgaesic more potent than morphine and is also thought to be a hormone. The second neuropeptide, a new peptide, has been named lesueurin and has the primary structure GLLDILKKVGKVA-NH2. Lesueurin shows no significant antibiotic or anticancer activity, but inhibits the formation of the ubiquitous chemical messenger nitric oxide from neuronal nitric oxide synthase (nNOS) at IC(50) (16.2 microm), and is the first amphibian peptide reported to show inhibition of nNOS. As a consequence of this activity, we have tested other peptides previously isolated from Australian amphibians for nNOS inhibition. There are three groups of peptides that inhibit nNOS (IC(50) at microm concentrations): these are (a) the citropin/aurein type peptides (of which lesueurin is a member), e.g. citropin 1.1 (GLFDVIKKVASVIGGL-NH(2)) (8.2 microm); (b) the frenatin type peptides, e.g. frenatin 3 (GLMSVLGHAVGNVLG GLFKPK-OH) (6.8 microm); and (c) the caerin 1 peptides, e.g. caerin 1.8 (GLFGVLGSIAKHLLPHVVPVIAEKL-NH(2)) (1.7 microm). From Lineweaver-Burk plots, the mechanism of inhibition is revealed as noncompetitive with respect to the nNOS substrate arginine. When the nNOS inhibition tests with the three peptides outlined above were carried out in the presence of increasing concentrations of Ca(2+) calmodulin, the inhibition dropped by approximately 50% in each case. In addition, these peptides also inhibit the activity of calcineurin, another enzyme that requires the presence of the regulatory protein Ca(2+) calmodulin. It is proposed that the amphibian peptides inhibit nNOS by interacting with Ca(2+)calmodulin, and as a consequence, blocks the attachment of this protein to the calmodulin domain of nNOS.
Collapse
Affiliation(s)
- Jason Doyle
- Australian Institute of Marine Science, Townsville MC, Queensland, Australia
| | | | | | | | | | | | | | | | | |
Collapse
|
26
|
Vélez P, Sierralta J, Alcayaga C, Fonseca M, Loyola H, Johns DC, Tomaselli GF, Marbán E, Suárez-Isla BA. A functional assay for paralytic shellfish toxins that uses recombinant sodium channels. Toxicon 2001; 39:929-35. [PMID: 11223080 DOI: 10.1016/s0041-0101(00)00230-0] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Saxitoxin (STX) and its derivatives are highly toxic natural compounds produced by dinoflagellates commonly present in marine phytoplankton. During algal blooms ("red tides"), shellfish accumulate saxitoxins leading to paralytic shellfish poisoning (PSP) in human consumers. PSP is a consequence of the high-affinity block of voltage-dependent Na channels in neuronal and muscle cells. PSP poses a significant public health threat and an enormous economic challenge to the shellfish industry worldwide. The standard screening method for marine toxins is the mouse mortality bioassay that is ethically problematic, costly and time-consuming. We report here an alternative, functional assay based on electrical recordings in cultured cells stably expressing a PSP target molecule, the STX-sensitive skeletal muscle Na channel. STX-equivalent concentration in the extracts was calibrated by comparison with purified STX, yielding a highly significant correlation (R=0.95; N=30) between electrophysiological determinations and the values obtained by conventional methods. This simple, economical, and reproducible assay obviates the need to sacrifice millions of animals in mandatory paralytic shellfish toxin screening programs.
Collapse
Affiliation(s)
- P Vélez
- Department of Physiology, Faculty of Sciences, University of Valparaíso, P.O. Box 5030, Valparaíso, Chile
| | | | | | | | | | | | | | | | | |
Collapse
|
27
|
Birinyi A, Parker D, Antal M, Shupliakov O. Zinc co-localizes with GABA and glycine in synapses in the lamprey spinal cord. J Comp Neurol 2001; 433:208-21. [PMID: 11283960 DOI: 10.1002/cne.1136] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The presence of zinc in synaptic terminals in the lamprey spinal cord was examined utilizing a modification of the Timm's sulfide silver method and with the fluorescent marker 6-methoxy-8-quinolyl-p-toluenesulfonamide (TSQ). Axons labeled with a Timm's staining method were predominantly located in the lateral region of the dorsal column. This correlated with a maximum of TSQ fluorescence in this region of the spinal cord. Single labeled terminals accumulating Timm reaction product were also found throughout the gray matter and fiber tracts. At the ultrastructural level, zinc was located in a population of synaptic terminals that co-localized gamma-aminobutyric acid (GABA) and glycine. Possible effects of Zn2+ on neuronal activity were examined. In spinobulbar interneurons, which receive GABAergic input in the dorsal column, zinc potentiated responses to GABA application, but it did not affect responses to GABA in motoneurons. Responses in motoneurons to pressure application of glycine were also not affected by Zn2+. Zinc, however, potentiated monosynaptic glycinergic inhibitory postsynaptic potentials (IPSPs) evoked in motoneurons by inhibitory locomotor network interneurons and increased frequency, but not amplitude of spontaneous miniature IPSPs recorded in the presence of tetrodotoxin (TTX), suggesting presynaptic effects. Glutamate responses and the amplitude of monosynaptic excitatory postsynaptic potentials (EPSPs) in motoneurons were reduced by zinc. These effects appeared to be mediated largely postsynaptically through an effect on the N-methyl-D-aspartate (NMDA) component of the glutamatergic input. Our results thus show that free zinc is present in inhibitory synaptic terminals in the lamprey spinal cord, and that it may function as a modulator of inhibitory synaptic transmission.
Collapse
Affiliation(s)
- A Birinyi
- Department of Anatomy, Histology, and Embryology, University Medical School of Debrecen, H-4012, Debrecen, Hungary
| | | | | | | |
Collapse
|
28
|
Hisatome I, Kurata Y, Sasaki N, Morisaki T, Morisaki H, Tanaka Y, Urashima T, Yatsuhashi T, Tsuboi M, Kitamura F, Miake J, Takeda SI, Taniguchi SI, Ogino K, Igawa O, Yoshida A, Sato R, Makita N, Shigemasa C. Block of sodium channels by divalent mercury: role of specific cysteinyl residues in the P-loop region. Biophys J 2000; 79:1336-45. [PMID: 10968996 PMCID: PMC1301028 DOI: 10.1016/s0006-3495(00)76386-7] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Divalent mercury (Hg(2+)) blocked human skeletal Na(+) channels (hSkM1) in a stable dose-dependent manner (K(d) = 0.96 microM) in the absence of reducing agent. Dithiothreitol (DTT) significantly prevented Hg(2+) block of hSkM1, and Hg(2+) block was also readily reversed by DTT. Both thimerosal and 2,2'-dithiodipyridine had little effect on hSkM1; however, pretreatment with thimerosal attenuated Hg(2+) block of hSkM1. Y401C+E758C rat skeletal muscle Na(+) channels (mu1) that form a disulfide bond spontaneously between two cysteines at the 401 and 758 positions showed a significantly lower sensitivity to Hg(2+) (K(d) = 18 microM). However, Y401C+E758C mu1 after reduction with DTT had a significantly higher sensitivity to Hg(2+) (K(d) = 0.36 microM) than wild-type hSkM1. Mutants C753Amu1 (K(d) = 8.47 microM) or C1521A mu1 (K(d) = 8.63 microM) exhibited significantly lower sensitivity to Hg(2+) than did wild-type hSkM1, suggesting that these two conserved cysteinyl residues of the P-loop region may play an important role in the Hg(2+) block of the hSkM1 isoform. The heart Na(+) channel (hH1) was significantly more sensitive to low-dose Hg(2+) (K(d) = 0.43 microM) than was hSkM1. The C373Y hH1 mutant exhibited higher resistance (K(d) = 1.12 microM) to Hg(2+) than did wild-type hH1. In summary, Hg(2+) probably inhibits the muscle Na(+) channels at more than one cysteinyl residue in the Na(+) channel P-loop region. Hg(2+) exhibits a lower K(d) value (<1. 23 microM) for inhibition by forming a sulfur-Hg-sulfur bridge, as compared to reaction at a single cysteinyl residue with a higher K(d) value (>8.47 microM) by forming sulfur-Hg(+) covalently. The heart Na(+) channel isoform with more than two cysteinyl residues in the P-loop region exhibits an extremely high sensitivity (K(d) < 0. 43 microM) to Hg(+), accounting for heart-specific high sensitivity to the divalent mercury.
Collapse
Affiliation(s)
- I Hisatome
- First Department of Internal Medicine, Tottori University Faculty of Medicine, Yonago 683, Japan.
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
29
|
Kurata Y, Sato R, Hisatome I, Imanishi S. Mechanisms of cation permeation in cardiac sodium channel: description by dynamic pore model. Biophys J 1999; 77:1885-904. [PMID: 10512810 PMCID: PMC1300471 DOI: 10.1016/s0006-3495(99)77031-1] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
The selective permeability to monovalent metal cations, as well as the relationship between cation permeation and gating kinetics, was investigated for native tetrodotoxin-insensitive Na-channels in guinea pig ventricular myocytes using the whole-cell patch clamp technique. By the measurement of inward unidirectional currents and biionic reversal potentials, we demonstrate that the cardiac Na-channel is substantially permeable to all of the group Ia and IIIa cations tested, with the selectivity sequence Na(+) >/= Li(+) > Tl(+) > K(+) > Rb(+) > Cs(+). Current kinetics was little affected by the permeant cation species and concentrations tested (</=160 mM), suggesting that the permeation process is independent of the gating process in the Na-channel. The permeability ratios determined from biionic reversal potentials were concentration and orientation dependent: the selectivity to Na(+) increased with increasing internal [K(+)] or external [Tl(+)]. The dynamic pore model describing the conformational transition of the Na-channel pore between different selectivity states could account for all the experimental data, whereas conventional static pore models failed to fit the concentration-dependent permeability ratio data. We conclude that the dynamic pore mechanism, independent of the gating machinery, may play an important physiological role in regulating the selective permeability of native Na-channels.
Collapse
Affiliation(s)
- Y Kurata
- Department of Physiology, Kanazawa Medical University, Ishikawa 920-0293, Japan
| | | | | | | |
Collapse
|
30
|
Chen DP, Xu L, Tripathy A, Meissner G, Eisenberg B. Selectivity and permeation in calcium release channel of cardiac muscle: alkali metal ions. Biophys J 1999; 76:1346-66. [PMID: 10049318 PMCID: PMC1300114 DOI: 10.1016/s0006-3495(99)77297-8] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Current was measured from single open channels of the calcium release channel (CRC) of cardiac sarcoplasmic reticulum (over the range +/-180 mV) in pure and mixed solutions (e.g., biionic conditions) of the alkali metal ions Li+, K+, Na+, Rb+, Cs+, ranging in concentration from 25 mM to 2 M. The current-voltage (I-V) relations were analyzed by an extension of the Poisson-Nernst-Planck (PNP) formulation of electrodiffusion, which includes local chemical interaction described by an offset in chemical potential, which likely reflects the difference in dehydration/solvation/rehydration energies in the entry/exit steps of permeation. The theory fits all of the data with few adjustable parameters: the diffusion coefficient of each ion species, the average effective charge distribution on the wall of the pore, and an offset in chemical potential for lithium and sodium ions. In particular, the theory explains the discrepancy between "selectivities" defined by conductance sequence and "selectivities" determined by the permeability ratios (i.e., reversal potentials) in biionic conditions. The extended PNP formulation seems to offer a successful combined treatment of selectivity and permeation. Conductance selectivity in this channel arises mostly from friction: different species of ions have different diffusion coefficients in the channel. Permeability selectivity of an ion is determined by its electrochemical potential gradient and local chemical interaction with the channel. Neither selectivity (in CRC) seems to involve different electrostatic interaction of different ions with the channel protein, even though the ions have widely varying diameters.
Collapse
Affiliation(s)
- D P Chen
- Department of Molecular Biophysics and Physiology, Rush Medical College, Chicago, Illinois 60612, USA.
| | | | | | | | | |
Collapse
|
31
|
Gandhi CS, Shuck K, Lear JD, Dieckmann GR, DeGrado WF, Lamb RA, Pinto LH. Cu(II) inhibition of the proton translocation machinery of the influenza A virus M2 protein. J Biol Chem 1999; 274:5474-82. [PMID: 10026160 DOI: 10.1074/jbc.274.9.5474] [Citation(s) in RCA: 106] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The homotetrameric M2 integral membrane protein of influenza virus forms a proton-selective ion channel. An essential histidine residue (His-37) in the M2 transmembrane domain is believed to play an important role in the conduction mechanism of this channel. Also, this residue is believed to form hydrogen-bonded interactions with the ammonium group of the anti-viral compound, amantadine. A molecular model of this channel suggests that the imidazole side chains of His-37 from symmetry-related monomers of the homotetrameric pore converge to form a coordination site for transition metals. Thus, membrane currents of oocytes of Xenopus laevis expressing the M2 protein were recorded when the solution bathing the oocytes contained various transition metals. Membrane currents were strongly and reversibly inhibited by Cu2+ with biphasic reaction kinetics. The biphasic inhibition curves may be explained by a two-site model involving a fast-binding peripheral site with low specificity for divalent metal ions, as well as a high affinity site (Kdiss approximately 2 microM) that lies deep within the pore and shows rather slow-binding kinetics (kon = 18.6 +/- 0.9 M-1 s-1). The pH dependence of the interaction with the high affinity Cu2+-binding site parallels the pH dependence of inhibition by amantadine, which has previously been ascribed to protonation of His-37. The voltage dependence of the inhibition at the high affinity site indicates that the binding site lies within the transmembrane region of the pore. Furthermore, the inhibition by Cu2+ could be prevented by prior application of the reversible blocker of M2 channel activity, BL-1743, providing further support for the location of the site within the pore region of M2. Finally, substitutions of His-37 by alanine or glycine eliminated the high affinity site and resulted in membrane currents that were only partially inhibited at millimolar concentrations of Cu2+. Binding of Cu2+ to the high affinity site resulted in an approximately equal inhibition of both inward and outward currents. The wild-type protein showed very high specificity for Cu2+ and was only partially inhibited by 1 mM Ni2+, Pt2+, and Zn2+. These data are discussed in terms of the functional role of His-37 in the mechanism of proton translocation through the channel.
Collapse
Affiliation(s)
- C S Gandhi
- Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208-3520, USA
| | | | | | | | | | | | | |
Collapse
|
32
|
Kurata Y, Hisatome I, Tsuboi M, Uenishi H, Zhang G, Oyaizu M, Sato R, Imanishi S. Effect of sulfhydryl oxidoreduction on permeability of cardiac tetrodotoxin-insensitive sodium channel. Life Sci 1998; 63:1023-35. [PMID: 9749825 DOI: 10.1016/s0024-3205(98)00364-6] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Effects of sulfhydryl oxidizing and reducing agents on permeability of the tetrodotoxin (TTX)-insensitive Na-channel were investigated in guinea-pig ventricular myocytes using the whole-cell patch-clamp technique. Mercury chloride (HgCl2) at 1-100 microM irreversibly blocked Na+ currents with no significant changes in the gating kinetics. In contrast, the hydrophilic sulfhydryl oxidizing agent, thimerosal at 50-100 microM little affected Na+ permeation through the Na-channel. The Hg2+-induced block of Na+ current could be readily reversed by 1,4-dithiothreitol (DTT), an agent that reduces disulfide bonds. These results indicate that the formation of sulfur-Hg-sulfur bridge is essential for Hg2+ block. Pretreatment with DTT prevented the Hg2+ block of Na+ current, whereas Zn2+ and Cd2+ retained their abilities to block Na+ current after DTT treatment. An application of Zn2+ or Cd2+ resulted in the restoration of Hg2+ sensitivity of the DTT-treated channel. A conformational model for the Na-channel with multiple free sulfhydryl groups and native disulfide bonds could account for our experimental data regarding the effects of sulfhydryl modifying agents on the channel permeability. We conclude that the cardiac TTX-insensitive Na-channel contains functionally important free sulfhydryl groups and disulfide bonds which are accessible from the extracellular side by an aqueous pathway. These sulfhydryls would be capable of modulating the Na-channel permeability by affecting the conformation of channel pore region.
Collapse
Affiliation(s)
- Y Kurata
- Department of Physiology, Kanazawa Medical University, Ishikawa, Japan
| | | | | | | | | | | | | | | |
Collapse
|
33
|
Abstract
We report the characterization of a new sodium channel blocker, mu-conotoxin PIIIA(mu-PIIIA). The peptide has been synthesized chemically and its disulfide bridging pattern determined. The structure of the new peptide is: [sequence: see text] where Z = pyroglutamate and O = 4-trans-hydroxyproline. We demonstrate that Arginine-14 (Arg14) is a key residue; substitution by alanine significantly decreases affinity and results in a toxin unable to block channel conductance completely. Thus, like all toxins that block at Site I, mu-PIIIA has a critical guanidinium group. This peptide is of exceptional interest because, unlike the previously characterized mu-conotoxin GIIIA (mu-GIIIA), it irreversibly blocks amphibian muscle Na channels, providing a useful tool for synaptic electrophysiology. Furthermore, the discovery of mu-PIIIA permits the resolution of tetrodotoxin-sensitive sodium channels into three categories: (1) sensitive to mu-PIIIA and mu-conotoxin GIIIA, (2) sensitive to mu-PIIIA but not to mu-GIIIA, and (3) resistant to mu-PIIIA and mu-GIIIA (examples in each category are skeletal muscle, rat brain Type II, and many mammalian CNS subtypes, respectively). Thus, mu-conotoxin PIIIA provides a key for further discriminating pharmacologically among different sodium channel subtypes.
Collapse
|
34
|
Shon KJ, Olivera BM, Watkins M, Jacobsen RB, Gray WR, Floresca CZ, Cruz LJ, Hillyard DR, Brink A, Terlau H, Yoshikami D. mu-Conotoxin PIIIA, a new peptide for discriminating among tetrodotoxin-sensitive Na channel subtypes. J Neurosci 1998; 18:4473-81. [PMID: 9614224 PMCID: PMC6792697] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
We report the characterization of a new sodium channel blocker, mu-conotoxin PIIIA(mu-PIIIA). The peptide has been synthesized chemically and its disulfide bridging pattern determined. The structure of the new peptide is: [sequence: see text] where Z = pyroglutamate and O = 4-trans-hydroxyproline. We demonstrate that Arginine-14 (Arg14) is a key residue; substitution by alanine significantly decreases affinity and results in a toxin unable to block channel conductance completely. Thus, like all toxins that block at Site I, mu-PIIIA has a critical guanidinium group. This peptide is of exceptional interest because, unlike the previously characterized mu-conotoxin GIIIA (mu-GIIIA), it irreversibly blocks amphibian muscle Na channels, providing a useful tool for synaptic electrophysiology. Furthermore, the discovery of mu-PIIIA permits the resolution of tetrodotoxin-sensitive sodium channels into three categories: (1) sensitive to mu-PIIIA and mu-conotoxin GIIIA, (2) sensitive to mu-PIIIA but not to mu-GIIIA, and (3) resistant to mu-PIIIA and mu-GIIIA (examples in each category are skeletal muscle, rat brain Type II, and many mammalian CNS subtypes, respectively). Thus, mu-conotoxin PIIIA provides a key for further discriminating pharmacologically among different sodium channel subtypes.
Collapse
Affiliation(s)
- K J Shon
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, USA
| | | | | | | | | | | | | | | | | | | | | |
Collapse
|
35
|
Negri A, Llewellyn L. Comparative analyses by HPLC and the sodium channel and saxiphilin 3H-saxitoxin receptor assays for paralytic shellfish toxins in crustaceans and molluscs from tropical North West Australia. Toxicon 1998; 36:283-98. [PMID: 9620576 DOI: 10.1016/s0041-0101(97)00119-0] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The increased frequency and distribution of red tides requires the development of high-throughput detection methods for paralytic shellfish toxins (PST). Community ethics also requires that there be a reduced reliance upon the standard mouse bioassay. A biomolecular assay such as the sodium channel 3H-saxitoxin binding assay can satisfy both of these requirements but may be compromised by cross-reactivity with the structurally unrelated tetrodotoxins (TTX). This study utilised the sodium channel assay but also an alternative 3H-saxitoxin binding assay based upon a saxiphilin isoform from the centipede Ethmostigmus rubripes to screen for PSTs. Saxiphilin is a novel transferrin which binds saxitoxin (STX) but differs from the sodium channel in not having any measurable affinity for TTX. A detailed analysis of toxin composition was achieved by high performance liquid chromatography (HPLC). Various crustaceans and molluscs accumulate PSTs and TTX, thus proving useful biomarkers for these toxins in their immediate environment and an ideal challenge to the detection and analysis of PSTs in this presumptive screening program. Also, there has been little investigation of PSTs in invertebrates from the Indian Ocean so this region was selected to extend our knowledge of the distribution of these toxins. 190 crabs and shellfish encompassing 31 species were collected from reefs along the North-West Australian coast and tested for PSTs and TTX by sodium channel and saxiphilin bioassays as well as HPLC. PSTs were detected in 18 species of crabs and shellfish of the 31 species tested. Eight of these species have not been previously described as toxic, these being the crabs Euzanthus exsculptus, Lophozozymus octodentatus, Metopograpsus frontalis, Pilumnus pulcher, Platypodia pseudogranulosa and Portunus pelagicus, and the molluscs Tectus fenestratus and Trochus hanleyanus. By HPLC, only one or both of STX and decarbamoyl-STX was detected in any extract. Some extracts markedly inhibited 3H-saxitoxin binding by the sodium channel but not by saxiphilin. The close agreement between toxin quantification by the PST specific methods of HPLC and the saxiphilin bioassay is indicative that the additional toxicity detected by the sodium channel assay is TTX.
Collapse
Affiliation(s)
- A Negri
- Australian Institute of Marine Science, Dampier
| | | |
Collapse
|
36
|
Gessner BD, Middaugh JP, Doucette GJ. Paralytic shellfish poisoning in Kodiak, Alaska. West J Med 1997; 167:351-3. [PMID: 9392992 PMCID: PMC1304631] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- B D Gessner
- Alaska Division of Public Health, Anchorage 99501, USA
| | | | | |
Collapse
|
37
|
Li RA, Tsushima RG, Kallen RG, Backx PH. Pore residues critical for mu-CTX binding to rat skeletal muscle Na+ channels revealed by cysteine mutagenesis. Biophys J 1997; 73:1874-84. [PMID: 9336183 PMCID: PMC1181088 DOI: 10.1016/s0006-3495(97)78218-3] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
We have studied mu-conotoxin (mu-CTX) block of rat skeletal muscle sodium channel (rSkM1) currents in which single amino acids within the pore (P-loop) were substituted with cysteine. Among 17 cysteine mutants expressed in Xenopus oocytes, 7 showed significant alterations in sensitivity to mu-CTX compared to wild-type rSkM1 channel (IC50 = 17.5 +/- 2.8 nM). E758C and D1241C were less sensitive to mu-CTX block (IC50 = 220 +/- 39 nM and 112 +/- 24 nM, respectively), whereas the tryptophan mutants W402C, W1239C, and W1531C showed enhanced mu-CTX sensitivity (IC50 = 1.9 +/- 0.1, 4.9 +/- 0.9, and 5.5 +/- 0.4 nM, respectively). D400C and Y401C also showed statistically significant yet modest (approximately twofold) changes in sensitivity to mu-CTX block compared to WT (p < 0.05). Application of the negatively charged, sulfhydryl-reactive compound methanethiosulfonate-ethylsulfonate (MTSES) enhanced the toxin sensitivity of D1241C (IC50 = 46.3 +/- 12 nM) while having little effect on E758C mutant channels (IC50 = 199.8 +/- 21.8 nM). On the other hand, the positively charged methanethiosulfonate-ethylammonium (MTSEA) completely abolished the mu-CTX sensitivity of E758C (IC50 > 1 microM) and increased the IC50 of D1241C by about threefold. Applications of MTSEA, MTSES, and the neutral MTSBN (benzyl methanethiosulfonate) to the tryptophan-to-cysteine mutants partially or fully restored the wild-type mu-CTX sensitivity, suggesting that the bulkiness of the tryptophan's indole group is a determinant of toxin binding. In support of this suggestion, the blocking IC50 of W1531A (7.5 +/- 1.3 nM) was similar to W1531C, whereas W1531Y showed reduced toxin sensitivity (14.6 +/- 3.5 nM) similar to that of the wild-type channel. Our results demonstrate that charge at positions 758 and 1241 are important for mu-CTX toxin binding and further suggest that the tryptophan residues within the pore in domains I, III, and IV negatively influence toxin-channel interaction.
Collapse
Affiliation(s)
- R A Li
- Department of Medicine, University of Toronto, Ontario, Canada
| | | | | | | |
Collapse
|
38
|
|
39
|
Conti F, Gheri A, Pusch M, Moran O. Use dependence of tetrodotoxin block of sodium channels: a revival of the trapped-ion mechanism. Biophys J 1996; 71:1295-312. [PMID: 8874004 PMCID: PMC1233597 DOI: 10.1016/s0006-3495(96)79330-x] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The use-dependent block of sodium channels by tetrodotoxin (TTX) has been studied in cRNA-injected Xenopus oocytes expressing the alpha-subunit of rat brain IIA channels. The kinetics of stimulus-induced extra block are consistent with an underlying relaxation process involving only three states. Cumulative extra block induced by repetitive stimulations increases with hyperpolarization, with TTX concentration, and with extracellular Ca2+ concentration. We have developed a theoretical model based on the suggestion by Salgado et al. that TTX blocks the extracellular mouth of the ion pore less tightly when the latter has its external side occupied by a cation, and that channel opening favors a tighter binding by allowing the escape of the trapped ion. The model provides an excellent fit of the data, which are consistent with Ca2+ being more efficient than Na+ in weakening TTX binding and with bound Ca2+ stabilizing the closed state of the channel, as suggested by Armstrong and Cota. Reports arguing against the trapped-ion mechanism are critically discussed.
Collapse
Affiliation(s)
- F Conti
- Istituto di Cibernetica e Biofisica, CNR, Genoa, Italy.
| | | | | | | |
Collapse
|
40
|
Zeng D, Kyle JW, Martin RL, Ambler KS, Hanck DA. Cardiac sodium channels expressed in a peripheral neurotumor-derived cell line, RT4-B8. THE AMERICAN JOURNAL OF PHYSIOLOGY 1996; 270:C1522-31. [PMID: 8967455 DOI: 10.1152/ajpcell.1996.270.5.c1522] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
RT4-B is one of several cell lines derived from a multipotent stem cell line, RT4-AC, which originated from a rat peripheral neurotumor. Based on Northern blot and ribonuclease protection experiments, RT4-B8 cells have been proposed to express rat cardiac Na channel mRNA as the major isoform. We report here direct electrophysiological evidence that the expressed voltage-gated Na channels in the RT4-B8 cell line are of the cardiac phenotype with no evidence for subpopulations expressing other Na channel isoforms. Current activation half point (conductance) was -41 +/- 5 mV (n = 7) and the steady-state voltage-dependent availability half point was -89 +/- 1 mV. As expected for cardiac Na channels, the half concentration of block for tetrodotoxin block was 0.74 microM, for saxitoxin (STX) was 0.15 microM, and for the class 2B divalent cation Cd2+ was 67 microM. Block was well described by single-site dose-response relationships with no indication of a subpopulation with "neuronal" affinity. Single-channel conductance (140 mM Na+) was 10 pS and predicted the average number of channels open at peak Na current to be 3 channels/microns2. [3H]STX binding data were also consistent with a single population of low-affinity STX binding sites and predicted channel density to be 11 sites/microns2. No inwardly or outwardly rectifying K or Ca currents were detected electrophysiologically, although in some cells a small time-independent Cl current was detected. Reverse transcription-polymerase chain reaction of mRNA isolated from RT4-B8 cells demonstrated the presence of rat cardiac (rH1) and brain IIa alpha-subunit mRNA, as well as mRNA for the Na channel beta 1-subunit. Northern blot analysis confirmed the predominance of the rat cardiac Na mRNA compared with brain IIa. The beta 1-subunit mRNA levels were significantly lower than those detected in rat brain and heart mRNA but were comparable to the low level of beta 1-subunit mRNA detected in isolated rat ventricular myocytes.
Collapse
Affiliation(s)
- D Zeng
- Department of Medicine, University of Chicago, Illinois 60637, USA
| | | | | | | | | |
Collapse
|
41
|
Tomaselli GF, Chiamvimonvat N, Nuss HB, Balser JR, Pérez-García MT, Xu RH, Orias DW, Backx PH, Marban E. A mutation in the pore of the sodium channel alters gating. Biophys J 1995; 68:1814-27. [PMID: 7612823 PMCID: PMC1282084 DOI: 10.1016/s0006-3495(95)80358-9] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Ion permeation and channel gating are classically considered independent processes, but site-specific mutagenesis studies in K channels suggest that residues in or near the ion-selective pore of the channel can influence activation and inactivation. We describe a mutation in the pore of the skeletal muscle Na channel that alters gating. This mutation, I-W53C (residue 402 in the mu 1 sequence), decreases the sensitivity to block by tetrodotoxin and increases the sensitivity to block by externally applied Cd2+ relative to the wild-type channel, placing this residue within the pore near the external mouth. Based on contemporary models of the structure of the channel, this residue is remote from the regions of the channel known to be involved in gating, yet this mutation abbreviates the time to peak and accelerates the decay of the macroscopic Na current. At the single-channel level we observe a shortening of the latency to first opening and a reduction in the mean open time compared with the wild-type channel. The acceleration of macroscopic current kinetics in the mutant channels can be simulated by changing only the activation and deactivation rate constants while constraining the microscopic inactivation rate constants to the values used to fit the wild-type currents. We conclude that the tryptophan at position 53 in the domain IP-loop may act as a linchpin in the pore that limits the opening transition rate. This effect could reflect an interaction of I-W53 with the activation voltage sensors or a more global gating-induced change in pore structure.
Collapse
Affiliation(s)
- G F Tomaselli
- Department of Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA
| | | | | | | | | | | | | | | | | |
Collapse
|
42
|
Kirsch GE, Alam M, Hartmann HA. Differential effects of sulfhydryl reagents on saxitoxin and tetrodotoxin block of voltage-dependent Na channels. Biophys J 1994; 67:2305-15. [PMID: 7696471 PMCID: PMC1225615 DOI: 10.1016/s0006-3495(94)80716-7] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
We have probed a cysteine residue that confers resistance to tetrodotoxin (TTX) block in heart Na channels, with membrane-impermeant, cysteine-specific, methanethiosulfonate (MTS) analogs. Covalent addition of a positively charged group to the cysteinyl sulfhydryl reduced pore conductance by 87%. The effect was selectively prevented by treatment with TTX, but not saxitoxin (STX). Addition of a negatively charged group selectively inhibited STX block without affecting TTX block. These results agree with models that place an exposed cysteinyl sulfhydryl in the TTX site adjacent to the mouth of the pore, but do not support the contention that STX and TTX are interchangeable. The surprising differences between the two toxins are consistent with the hypothesis that the toxin-receptor complex can assume different conformations when STX or TTX bound.
Collapse
Affiliation(s)
- G E Kirsch
- Department of Anesthesiology, Baylor College of Medicine, Houston, Texas 77030
| | | | | |
Collapse
|
43
|
|
44
|
Llewellyn LE, Moczydlowski EG. Characterization of saxitoxin binding to saxiphilin, a relative of the transferrin family that displays pH-dependent ligand binding. Biochemistry 1994; 33:12312-22. [PMID: 7918453 DOI: 10.1021/bi00206a039] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Saxiphilin is a 91 kDa saxitoxin-binding protein that is homologous to members of the transferrin family of Fe(3+)-binding proteins noted for pH-dependent release of Fe3+. The mechanism of toxin binding to purified native saxiphilin from the bullfrog (Rana catesbeiana) was studied using [3H]saxitoxin. At pH 7.4 and 0 degrees C [3H]saxitoxin binds to a single site on saxiphilin with a KD of approximately 0.2 nM. The pH dependence of [3H]saxitoxin binding follows a one-site titration curve in the range of pH 9-4 with maximal binding from pH 9 to 7 and half-inhibition at pH 5.7. Inhibition of toxin binding at low pH is the combined result of a decrease in the rate of toxin association and an increase in the rate of toxin dissociation. The dependence of the apparent rate constants for [3H]saxitoxin association and dissociation on [H+] can be accounted for by a four-state model of allosteric interaction between the toxin-binding site and a single titratable residue of saxiphilin with a pKa of 7.2 in the toxin-free form and 4.3 in the toxin-bound form. From 0 to 25 degrees C, the temperature dependence of [3H]saxitoxin binding to saxiphilin is characterized by delta H degrees = -8.3 kcal mol-1, delta S degrees = 13.8 cal mol-1 K-1, and activation energies of 22.5 kcal mol-1 for dissociation and 11.1 kcal mol-1 for association. Binding of [3H]saxitoxin to saxiphilin is competitively inhibited with low affinity by a variety of divalent metal and lanthanide cations. Inhibition of toxin binding by the carboxyl-methylating reagent trimethyloxonium is prevented by pre-equilibration with [3H]saxitoxin, implicating the presence of one or more carboxyl groups in the binding site. Functional similarities suggest that the saxitoxin-binding site of saxiphilin is located in an interdomain cleft analogous to the location of one of the two homologous Fe(3+)-binding sites of transferrins. On the basis of residue substitutions between saxiphilin and transferrins, it is proposed that the saxitoxin-binding site is located in the carboxy terminal lobe of saxiphilin and that binding is modulated by protonation of a conserved histidine residue.
Collapse
Affiliation(s)
- L E Llewellyn
- Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06520-8066
| | | |
Collapse
|
45
|
Satin J, Limberis JT, Kyle JW, Rogart RB, Fozzard HA. The saxitoxin/tetrodotoxin binding site on cloned rat brain IIa Na channels is in the transmembrane electric field. Biophys J 1994; 67:1007-14. [PMID: 7811911 PMCID: PMC1225453 DOI: 10.1016/s0006-3495(94)80566-1] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
The rat brain IIa (BrIIa) Na channel alpha-subunit and the brain beta 1 subunit were coexpressed in Xenopus oocytes, and peak whole-oocyte Na current (INa) was measured at a test potential of -10 mV. Hyperpolarization of the holding potential resulted in an increased affinity of STX and TTX rested-state block of BrIIa Na channels. The apparent half-block concentration (ED50) for STX of BrIIa current decreased with hyperpolarizing holding potentials (Vhold). At Vhold of -100 mV, the ED50 was 2.1 +/- 0.4 nM, and the affinity increased to a ED50 of 1.2 +/- 0.2 nM with Vhold of -140 mV. In the absence of toxin, the peak current amplitude was the same for all potentials negative to -90 mV, demonstrating that all of the channels were in a closed conformation and maximally available to open in this range of holding potentials. The Woodhull model (1973) was used to describe the increase of the STX ED50 as a function of holding potential. The equivalent electrical distance of block (delta) by STX was 0.18 from the extracellular milieu when the valence of STX was fixed to +2. Analysis of the holding potential dependence of TTX block yielded a similar delta when the valence of TTX was fixed to +1. We conclude that the guanidinium toxin site is located partially within the transmembrane electric field. Previous site-directed mutagenesis studies demonstrated that an isoform-specific phenylalanine in the BrIIa channel is critical for high affinity toxin block. Therefore, we propose that amino acids at positions corresponding to this Phe in the BrIIa channel, which lie in the outer vestibule of the channel adjacent to the pore entrance,are partially in the transmembrane potential drop.
Collapse
Affiliation(s)
- J Satin
- Department of Medicine, University of Chicago, Illinois 60637
| | | | | | | | | |
Collapse
|
46
|
Satin J, Kyle JW, Fan Z, Rogart R, Fozzard HA, Makielski JC. Post-repolarization block of cloned sodium channels by saxitoxin: the contribution of pore-region amino acids. Biophys J 1994; 66:1353-63. [PMID: 8061191 PMCID: PMC1275856 DOI: 10.1016/s0006-3495(94)80926-9] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Sodium channels expressed in oocytes exhibited isoform differences in phasic block by saxitoxin (STX). Neuronal channels (rat IIa co-expressed with beta 1 subunit, Br2a + beta 1) had slower kinetics of phasic block for pulse trains than cardiac channels (RHI). After the membrane was repolarized from a single brief depolarizing step, a test pulse at increasing intervals showed first a decrease in current (post-repolarization block) then eventual recovery in the presence of STX. This block/unblock process for Br2a + beta 1 was 10-fold slower than that for RHI. A model accounting for these results predicts a faster toxin dissociation rate and a slower association rate for the cardiac isoform, and it also predicts a shorter dwell time in a putative high STX affinity conformation for the cardiac isoform. The RHI mutation (Cys374-->Phe), which was previously shown to be neuronal-like with respect to high affinity tonic toxin block, was also neuronal-like with respect to the kinetics of post-repolarization block, suggesting that this single amino acid is important for conferring isoform-specific transition rates determining post-repolarization block. Because the same mutation determines both sensitivity for tonic STX block and the kinetics of phasic STX block, the mechanisms accounting for tonic block and phasic block share the same toxin binding site. We conclude that the residue at position 374, in the putative pore-forming region, confers isoform-specific channel kinetics that underlie phasic toxin block.
Collapse
Affiliation(s)
- J Satin
- Cardiac Electrophysiology Laboratory, University of Chicago, Illinois 60637
| | | | | | | | | | | |
Collapse
|
47
|
Schild L, Moczydlowski E. Permeation of Na+ through open and Zn(2+)-occupied conductance states of cardiac sodium channels modified by batrachotoxin: exploring ion-ion interactions in a multi-ion channel. Biophys J 1994; 66:654-66. [PMID: 8011896 PMCID: PMC1275762 DOI: 10.1016/s0006-3495(94)80839-2] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Mammalian heart sodium channels inserted into planar bilayers exhibit a distinctive subconductance state when single batrachotoxin-modified channels are exposed to external Zn2+. The current-voltage behavior of the open state and the Zn(2+)-induced substate was characterized in the presence of symmetrical Na+ ranging from 2 to 3000 mM. The unitary conductance of the open state follows a biphasic dependence on [Na+] that can be accounted for by a 3-barrier-2-site model of Na+ permeation that includes double occupancy and Na(+)-Na+ repulsion. The unitary conductance of the Zn2+ substate follows a monophasic dependence on [Na+] that can be explained by a similar 3-barrier-2-site model with low affinity for Na+ and single occupancy due to repulsive interaction with a Zn2+ ion bound near the external entrance to the pore. The apparent association rate of Zn2+ derived from dwell-time analysis of flickering events is strongly reduced as [Na+] is raised from 50 to 500 mM. The apparent dissociation rate of Zn2+ is also enhanced as [Na+] is increased. While not excluding surface charge effects, such behavior is consistent with two types of ion-ion interactions: 1) A competitive binding interaction between Zn2+ and Na+ due to mutual competition for high affinity sites in close proximity. 2) A noncompetitive, destabilizing interaction resulting from simultaneous occupancy by Zn2+ and Na+. The repulsive influence of Zn2+ on Na+ binding in the cardiac Na+ channel is similar to that which has been proposed to occur between Ca2+ and Na+ in structurally related calcium channels. Based on recent mutagenesis data, a schematic model of functionally important residues in the external cation binding sites of calcium channels and cardiac sodium channels is proposed. In this model, the Zn(2+)-induced subconductance state results from Zn2+ binding to a site in the external vestibule that is close to the entrance of the pore but does not occlude it.
Collapse
Affiliation(s)
- L Schild
- Institut de Pharmacologie, de l'Universite de Lausanne, Switzerland
| | | |
Collapse
|
48
|
Lönnendonker U. Use dependence of guanidinium toxins in frog myelinated nerve: evidence for features of native voltage-gated sodium channels. Prog Neurobiol 1994; 42:359-74. [PMID: 8058967 DOI: 10.1016/0301-0082(94)90080-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- U Lönnendonker
- I. Physiologisches Institut, Universität des Saarlandes, Homburg, Germany
| |
Collapse
|
49
|
Lipkind GM, Fozzard HA. A structural model of the tetrodotoxin and saxitoxin binding site of the Na+ channel. Biophys J 1994; 66:1-13. [PMID: 8130328 PMCID: PMC1275657 DOI: 10.1016/s0006-3495(94)80746-5] [Citation(s) in RCA: 210] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Biophysical evidence has placed the binding site for the naturally occurring marine toxins tetrodotoxin (TTX) and saxitoxin (STX) in the external mouth of the Na+ channel ion permeation pathway. We developed a molecular model of the binding pocket for TTX and STX, composed of antiparallel beta-hairpins formed from peptide segments of the four S5-S6 loops of the voltage-gated Na+ channel. For TTX the guanidinium moiety formed salt bridges with three carboxyls, while two toxin hydroxyls (C9-OH and C10-OH) interacted with a fourth carboxyl on repeats I and II. This alignment also resulted in a hydrophobic interaction with an aromatic ring of phenylalanine or tyrosine residues for the brainII and skeletal Na+ channel isoforms, but not with the cysteine found in the cardiac isoform. In comparison to TTX, there was an additional interaction site for STX through its second guanidinium group with a carboxyl on repeat IV. This model satisfactorily reproduced the effects of mutations in the S5-S6 regions and the differences in affinity by various toxin analogs. However, this model differed in important ways from previously published models for the outer vestibule and the selectivity region of the Na+ channel pore. Removal of the toxins from the pocket formed by the four beta-hairpins revealed a structure resembling a funnel that terminated in a narrowed region suitable as a candidate for the selectivity filter of the channel. This region contained two carboxyls (Asp384 and Glu942) that substituted for molecules of water from the hydrated Na+ ion. Simulation of mutations in this region that have produced Ca2+ permeation of the Na+ channel created a site with three carboxyls (Asp384, Glu942, and Glu1714) in proximity.
Collapse
Affiliation(s)
- G M Lipkind
- Cardiac Electrophysiology Laboratories, University of Chicago, Illinois 60637
| | | |
Collapse
|
50
|
Abstract
Acutely dissociated neurons from the superficial layers of the medial entorhinal cortex of the rat were studied under voltage clamp using the whole-cell patch-clamp configuration. Neurons from the medial entorhinal cortex exhibit a tetrodotoxin (TTX)-resistant Na+ current (ITTX-R; IC50 approximately 146 nM), in addition to the normal TTX-sensitive Na+ current (ITTX-S; IC50 approximately 6 nM). ITTX-R was found in both putative stellate and putative pyramidal neurons from the medial entorhinal cortex. ITTX-R is kinetically indistinguishable from ITTX-S, but can be distinguished from ITTX-S based on its enhanced sensitivity to block by Cd2+, La3+, and Zn2+. ITTX-R is kinetically and pharmacologically similar to the TTX-resistant Na+ current found in cardiac muscle.
Collapse
Affiliation(s)
- J A White
- Department of Biological Sciences, University of Iowa, Iowa City 52242-1324
| | | | | |
Collapse
|