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Rosenthal JJC, Eisenberg E. Extensive Recoding of the Neural Proteome in Cephalopods by RNA Editing. Annu Rev Anim Biosci 2023; 11:57-75. [PMID: 36790891 DOI: 10.1146/annurev-animal-060322-114534] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/16/2023]
Abstract
The coleoid cephalopods have the largest brains, and display the most complex behaviors, of all invertebrates. The molecular and cellular mechanisms that underlie these remarkable advancements remain largely unexplored. Early molecular cloning studies of squid ion channel transcripts uncovered an unusually large number of A→I RNA editing sites that recoded codons. Further cloning of other neural transcripts showed a similar pattern. The advent of deep-sequencing technologies and the associated bioinformatics allowed the mapping of RNA editing events across the entire neural transcriptomes of various cephalopods. The results were remarkable: They contained orders of magnitude more recoding editing sites than any other taxon. Although RNA editing sites are abundant in most multicellular metazoans, they rarely recode. In cephalopods, the majority of neural transcripts are recoded. Recent studies have focused on whether these events are adaptive, as well as other noncanonical aspects of cephalopod RNA editing.
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Affiliation(s)
- Joshua J C Rosenthal
- The Eugene Bell Center, The Marine Biological Laboratory, Woods Hole, Massachusetts, USA;
| | - Eli Eisenberg
- Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
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2
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Ojala KS, Ginebaugh SP, Wu M, Miller EW, Ortiz G, Covarrubias M, Meriney SD. A high-affinity, partial antagonist effect of 3,4-diaminopyridine mediates action potential broadening and enhancement of transmitter release at NMJs. J Biol Chem 2021; 296:100302. [PMID: 33465376 PMCID: PMC7949096 DOI: 10.1016/j.jbc.2021.100302] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2020] [Revised: 01/12/2021] [Accepted: 01/14/2021] [Indexed: 12/03/2022] Open
Abstract
3,4-Diaminopyridine (3,4-DAP) increases transmitter release from neuromuscular junctions (NMJs), and low doses of 3,4-DAP (estimated to reach ∼1 μM in serum) are the Food and Drug Administration (FDA)-approved treatment for neuromuscular weakness caused by Lambert–Eaton myasthenic syndrome. Canonically, 3,4-DAP is thought to block voltage-gated potassium (Kv) channels, resulting in prolongation of the presynaptic action potential (AP). However, recent reports have shown that low millimolar concentrations of 3,4-DAP have an off-target agonist effect on the Cav1 subtype (“L-type”) of voltage-gated calcium (Cav) channels and have speculated that this agonist effect might contribute to 3,4-DAP effects on transmitter release at the NMJ. To address 3,4-DAP’s mechanism(s) of action, we first used the patch-clamp electrophysiology to characterize the concentration-dependent block of 3,4-DAP on the predominant presynaptic Kv channel subtypes found at the mammalian NMJ (Kv3.3 and Kv3.4). We identified a previously unreported high-affinity (1–10 μM) partial antagonist effect of 3,4-DAP in addition to the well-known low-affinity (0.1–1 mM) antagonist activity. We also showed that 1.5-μM DAP had no effects on Cav1.2 or Cav2.1 current. Next, we used voltage imaging to show that 1.5- or 100-μM 3,4-DAP broadened the AP waveform in a dose-dependent manner, independent of Cav1 calcium channels. Finally, we demonstrated that 1.5- or 100-μM 3,4-DAP augmented transmitter release in a dose-dependent manner and this effect was also independent of Cav1 channels. From these results, we conclude that low micromolar concentrations of 3,4-DAP act solely on Kv channels to mediate AP broadening and enhance transmitter release at the NMJ.
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Affiliation(s)
- Kristine S Ojala
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Scott P Ginebaugh
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Man Wu
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Evan W Miller
- Departments of Chemistry and Molecular & Cell Biology, University of California, Berkeley, Berkeley, California, USA
| | - Gloria Ortiz
- Departments of Chemistry and Molecular & Cell Biology, University of California, Berkeley, Berkeley, California, USA
| | - Manuel Covarrubias
- Department of Neuroscience, Thomas Jefferson University, Philadelphia, Pennsylvania, USA
| | - Stephen D Meriney
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
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Vallecillo-Viejo IC, Liscovitch-Brauer N, Diaz Quiroz JF, Montiel-Gonzalez MF, Nemes SE, Rangan KJ, Levinson SR, Eisenberg E, Rosenthal JJC. Spatially regulated editing of genetic information within a neuron. Nucleic Acids Res 2020; 48:3999-4012. [PMID: 32201888 PMCID: PMC7192619 DOI: 10.1093/nar/gkaa172] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2019] [Revised: 02/14/2020] [Accepted: 03/11/2020] [Indexed: 12/11/2022] Open
Abstract
In eukaryotic cells, with the exception of the specialized genomes of mitochondria and plastids, all genetic information is sequestered within the nucleus. This arrangement imposes constraints on how the information can be tailored for different cellular regions, particularly in cells with complex morphologies like neurons. Although messenger RNAs (mRNAs), and the proteins that they encode, can be differentially sorted between cellular regions, the information itself does not change. RNA editing by adenosine deamination can alter the genome's blueprint by recoding mRNAs; however, this process too is thought to be restricted to the nucleus. In this work, we show that ADAR2 (adenosine deaminase that acts on RNA), an RNA editing enzyme, is expressed outside of the nucleus in squid neurons. Furthermore, purified axoplasm exhibits adenosine-to-inosine activity and can specifically edit adenosines in a known substrate. Finally, a transcriptome-wide analysis of RNA editing reveals that tens of thousands of editing sites (>70% of all sites) are edited more extensively in the squid giant axon than in its cell bodies. These results indicate that within a neuron RNA editing can recode genetic information in a region-specific manner.
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Affiliation(s)
| | - Noa Liscovitch-Brauer
- Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
| | - Juan F Diaz Quiroz
- The Eugene Bell Center, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02540, USA
| | | | - Sonya E Nemes
- The Eugene Bell Center, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02540, USA
| | - Kavita J Rangan
- The Eugene Bell Center, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02540, USA
| | - Simon R Levinson
- Department of Physiology and Biophysics, University of Colorado at Denver, Anschutz Medical Campus, Aurora, CO 80045, USA
| | - Eli Eisenberg
- Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
| | - Joshua J C Rosenthal
- The Eugene Bell Center, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02540, USA
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4
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Pisupati A, Mickolajczyk KJ, Horton W, van Rossum DB, Anishkin A, Chintapalli SV, Li X, Chu-Luo J, Busey G, Hancock WO, Jegla T. The S6 gate in regulatory Kv6 subunits restricts heteromeric K + channel stoichiometry. J Gen Physiol 2018; 150:1702-1721. [PMID: 30322883 PMCID: PMC6279357 DOI: 10.1085/jgp.201812121] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Revised: 07/03/2018] [Accepted: 09/26/2018] [Indexed: 11/24/2022] Open
Abstract
Atypical substitutions in the S6 activation gate sequence distinguish “regulatory” Kv subunits, which cannot homotetramerize due to T1 self-incompatibility. Pisupati et al. show that such substitutions in Kv6 work together with self-incompatibility to restrict Kv2:Kv6 heteromeric stoichiometry to 3:1. The Shaker-like family of voltage-gated K+ channels comprises four functionally independent gene subfamilies, Shaker (Kv1), Shab (Kv2), Shaw (Kv3), and Shal (Kv4), each of which regulates distinct aspects of neuronal excitability. Subfamily-specific assembly of tetrameric channels is mediated by the N-terminal T1 domain and segregates Kv1–4, allowing multiple channel types to function independently in the same cell. Typical Shaker-like Kv subunits can form functional channels as homotetramers, but a group of mammalian Kv2-related genes (Kv5.1, Kv6s, Kv8s, and Kv9s) encodes subunits that have a “silent” or “regulatory” phenotype characterized by T1 self-incompatibility. These channels are unable to form homotetramers, but instead heteromerize with Kv2.1 or Kv2.2 to diversify the functional properties of these delayed rectifiers. While T1 self-incompatibility predicts that these heterotetramers could contain up to two regulatory (R) subunits, experiments show a predominance of 3:1R stoichiometry in which heteromeric channels contain a single regulatory subunit. Substitution of the self-compatible Kv2.1 T1 domain into the regulatory subunit Kv6.4 does not alter the stoichiometry of Kv2.1:Kv6.4 heteromers. Here, to identify other channel structures that might be responsible for favoring the 3:1R stoichiometry, we compare the sequences of mammalian regulatory subunits to independently evolved regulatory subunits from cnidarians. The most widespread feature of regulatory subunits is the presence of atypical substitutions in the highly conserved consensus sequence of the intracellular S6 activation gate of the pore. We show that two amino acid substitutions in the S6 gate of the regulatory subunit Kv6.4 restrict the functional stoichiometry of Kv2.1:Kv6.4 to 3:1R by limiting the formation and function of 2:2R heteromers. We propose a two-step model for the evolution of the asymmetric 3:1R stoichiometry, which begins with evolution of self-incompatibility to establish the regulatory phenotype, followed by drift of the activation gate consensus sequence under relaxed selection to limit stoichiometry to 3:1R.
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Affiliation(s)
- Aditya Pisupati
- Department of Biology, Pennsylvania State University, University Park, PA.,Medical Scientist Training Program, College of Medicine, Pennsylvania State University, Hershey, PA
| | - Keith J Mickolajczyk
- Department of Biomedical Engineering, Pennsylvania State University, University Park, PA
| | - William Horton
- Department of Animal Science, Pennsylvania State University, University Park, PA
| | - Damian B van Rossum
- The Jake Gittlen Laboratories for Cancer Research, College of Medicine, Pennsylvania State University, Hershey, PA.,Division of Experimental Pathology, Department of Pathology, College of Medicine, Pennsylvania State University, Hershey, PA
| | - Andriy Anishkin
- Department of Biology, University of Maryland, College Park, MD
| | - Sree V Chintapalli
- Arkansas Children's Nutrition Center and Department of Pediatrics, University of Arkansas for Medical Sciences, Little Rock, AR
| | - Xiaofan Li
- Department of Biology, Pennsylvania State University, University Park, PA
| | - Jose Chu-Luo
- Department of Biology, Pennsylvania State University, University Park, PA
| | - Gregory Busey
- Department of Biology, Pennsylvania State University, University Park, PA
| | - William O Hancock
- Department of Biomedical Engineering, Pennsylvania State University, University Park, PA
| | - Timothy Jegla
- Department of Biology, Pennsylvania State University, University Park, PA .,Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, PA
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Kehl SJ. A Model of the Block of Voltage-Gated Potassium Kv4.2 Ionic Currents by 4-Aminopyridine. J Pharmacol Exp Ther 2017; 363:184-195. [PMID: 28864468 DOI: 10.1124/jpet.117.243097] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2017] [Accepted: 08/28/2017] [Indexed: 11/22/2022] Open
Abstract
Voltage clamp recordings of macroscopic currents were made from rat potassium-gated potassium 4.2(Kv4.2) channels expressed in human embryonic kidney (HEK293) cells with the main goals of quantifying the concentration, time, and voltage dependence of the block and to generate a state model that replicates the features of the block. When applied either externally or internally, the block of Kv4.2 currents by 4-aminopyridine (4AP) occurs at the holding potential (-80 mV), is affected by the stimulus frequency, and is relieved by membrane depolarization. The Kd for the tonic block at -80 mV was 0.9 ± 0.07 mM and was consistent with 1:1 binding. Relief of block during a step to 50 mV was well fitted by a single exponential with a time constant of ∼40 milliseconds. At -80 mV, the association rate constant was 0.08 mM-1 s-1, and the off-rate was 0.08 s-1 The state model replicates the features of the experimental data reasonably well by assuming that 4AP binds only to closed states, that 4AP binding and inactivation are mutually exclusive processes, and that the activation of closed-bound channels is the same as for closed channels. Since the open channel has a very low or no affinity for 4AP, channel opening promotes the unbinding of 4AP, which accounts for the reverse use dependence of the block.
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Affiliation(s)
- Steven J Kehl
- Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver, British Columbia, Canada
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6
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The Cole-Moore Effect: Still Unexplained? Biophys J 2016; 109:1312-6. [PMID: 26445430 DOI: 10.1016/j.bpj.2015.07.052] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2015] [Revised: 06/29/2015] [Accepted: 07/13/2015] [Indexed: 11/23/2022] Open
Abstract
In the first issue, on the first page of the Biophysical Journal in 1960, Cole and Moore provided the first confirmation of the Hodgkin and Huxley formulation of the sodium and potassium conductances that underlie the action potential. In addition, working with the squid giant axon, Cole and Moore noted that strong hyperpolarization preceding a depolarizing voltage-clamp pulse delayed the rise of the potassium conductance: once started, the time course of the rise was always the same but after significant hyperpolarization there was a long lag before the rise began. This phenomenon has come to be known as the Cole-Moore effect. Their article examines and disproves the hypothesis that the lag reflects the time required to refill the membrane with potassium ions after the ions are swept out of the membrane into the axoplasm by hyperpolarization. The work by Cole and Moore indirectly supports the idea of a membrane channel for potassium conductance. However, the mechanism of the Cole-Moore effect remains a mystery even now, buried in the structure of the potassium channel, which was completely unknown at the time.
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Major diversification of voltage-gated K+ channels occurred in ancestral parahoxozoans. Proc Natl Acad Sci U S A 2015; 112:E1010-9. [PMID: 25691740 DOI: 10.1073/pnas.1422941112] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
We examined the origins and functional evolution of the Shaker and KCNQ families of voltage-gated K(+) channels to better understand how neuronal excitability evolved. In bilaterians, the Shaker family consists of four functionally distinct gene families (Shaker, Shab, Shal, and Shaw) that share a subunit structure consisting of a voltage-gated K(+) channel motif coupled to a cytoplasmic domain that mediates subfamily-exclusive assembly (T1). We traced the origin of this unique Shaker subunit structure to a common ancestor of ctenophores and parahoxozoans (cnidarians, bilaterians, and placozoans). Thus, the Shaker family is metazoan specific but is likely to have evolved in a basal metazoan. Phylogenetic analysis suggested that the Shaker subfamily could predate the divergence of ctenophores and parahoxozoans, but that the Shab, Shal, and Shaw subfamilies are parahoxozoan specific. In support of this, putative ctenophore Shaker subfamily channel subunits coassembled with cnidarian and mouse Shaker subunits, but not with cnidarian Shab, Shal, or Shaw subunits. The KCNQ family, which has a distinct subunit structure, also appears solely within the parahoxozoan lineage. Functional analysis indicated that the characteristic properties of Shaker, Shab, Shal, Shaw, and KCNQ currents evolved before the divergence of cnidarians and bilaterians. These results show that a major diversification of voltage-gated K(+) channels occurred in ancestral parahoxozoans and imply that many fundamental mechanisms for the regulation of action potential propagation evolved at this time. Our results further suggest that there are likely to be substantial differences in the regulation of neuronal excitability between ctenophores and parahoxozoans.
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8
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Holmgren M, Rosenthal JJ. Regulation of Ion Channel and Transporter Function Through RNA Editing. Curr Issues Mol Biol 2014; 17:23-36. [PMID: 25347917 PMCID: PMC5248560] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/04/2023] Open
Abstract
A large proportion of the recoding events mediated by RNA editing are in mRNAs that encode ion channels and transporters. The effects of these events on protein function have been characterized in only a few cases. In even fewer instances are the mechanistic underpinnings of these effects understood. This review focuses on how RNA editing affects protein function and higher order physiology. In mammals, particular attention is given to the GluA2, an ionotropic glutamate receptor subunit, and K(v) 1.1, a voltage-dependent K+ channel, because they are particularly well understood. In K(v) addition, work on cephalopod K+ channels and Na+/K+-ATPases has also provided important clues on the rules used by RNA editing to regulate excitability. Finally, we discuss some of the emerging targets for editing and how this process may be used to regulate nervous function in response to a variable environment.
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Affiliation(s)
- Miguel Holmgren
- Molecular Neurophysiology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA
| | - Joshua J.C. Rosenthal
- Institute of Neurobiology and Department of Biochemistry, University of Puerto Rico Medical Sciences Campus, San Juan, Puerto Rico
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9
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Rosenthal JJC, Seeburg PH. A-to-I RNA editing: effects on proteins key to neural excitability. Neuron 2012; 74:432-9. [PMID: 22578495 DOI: 10.1016/j.neuron.2012.04.010] [Citation(s) in RCA: 109] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/11/2012] [Indexed: 10/28/2022]
Abstract
RNA editing by adenosine deamination is a process used to diversify the proteome. The expression of ADARs, the editing enzymes, is ubiquitous among true metazoans, and so adenosine deamination is thought to be universal. By changing codons at the level of mRNA, protein function can be altered, perhaps in response to physiological demand. Although the number of editing sites identified in recent years has been rising exponentially, their effects on protein function, in general, are less well understood. This review assesses the state of the field and highlights particular cases where the biophysical alterations and functional effects caused by RNA editing have been studied in detail.
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Affiliation(s)
- Joshua J C Rosenthal
- Institute of Neurobiology and Department of Biochemistry, University of Puerto Rico Medical Sciences Campus, San Juan, Puerto Rico 00901, USA
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10
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Abstract
To operate in the extreme cold, ion channels from psychrophiles must have evolved structural changes to compensate for their thermal environment. A reasonable assumption would be that the underlying adaptations lie within the encoding genes. Here, we show that delayed rectifier K(+) channel genes from an Antarctic and a tropical octopus encode channels that differ at only four positions and display very similar behavior when expressed in Xenopus oocytes. However, the transcribed messenger RNAs are extensively edited, creating functional diversity. One editing site, which recodes an isoleucine to a valine in the channel's pore, greatly accelerates gating kinetics by destabilizing the open state. This site is extensively edited in both Antarctic and Arctic species, but mostly unedited in tropical species. Thus adenosine-to-inosine RNA editing can respond to the physical environment.
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Affiliation(s)
- Sandra Garrett
- Institute of Neurobiology, University of Puerto Rico–Medical Sciences Campus, San Juan 00901, Puerto Rico
| | - Joshua J.C. Rosenthal
- Institute of Neurobiology, University of Puerto Rico–Medical Sciences Campus, San Juan 00901, Puerto Rico
- Department of Biochemistry, University of Puerto Rico–Medical Sciences Campus, San Juan 00936, Puerto Rico
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K+ Channels of Squid Giant Axons Open by an Osmotic Stress in Hypertonic Solutions Containing Nonelectrolytes. J Membr Biol 2011; 242:119-35. [DOI: 10.1007/s00232-011-9383-5] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2010] [Accepted: 06/27/2011] [Indexed: 10/18/2022]
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12
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Ion channels in key marine invertebrates; their diversity and potential for applications in biotechnology. Biotechnol Adv 2011; 29:457-67. [PMID: 21620946 DOI: 10.1016/j.biotechadv.2011.05.007] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2011] [Revised: 05/10/2011] [Accepted: 05/10/2011] [Indexed: 12/31/2022]
Abstract
Of the intra-membrane proteins, the class that comprises voltage and ligand-gated ion channels represents the major substrate whereby signals pass between and within cells in all organisms. It has been presumed that vertebrate and particularly mammalian ion channels represent the apex of evolutionary complexity and diversity and much effort has been focused on understanding their function. However, the recent availability of cheap high throughput genome sequencing has massively broadened and deepened the quality of information across phylogeny and is radically changing this view. Here we review current knowledge on such channels in key marine invertebrates where physiological evidence is backed up by molecular sequences and expression/functional studies. As marine invertebrates represent a much greater range of phyla than terrestrial vertebrates and invertebrates together, we argue that these animals represent a highly divergent, though relatively underused source of channel novelty. As ion channels are exquisitely selective sensors for voltage and ligands, their potential and actual applications in biotechnology are manifold.
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Palavicini JP, O'Connell MA, Rosenthal JJC. An extra double-stranded RNA binding domain confers high activity to a squid RNA editing enzyme. RNA (NEW YORK, N.Y.) 2009; 15:1208-1218. [PMID: 19390115 PMCID: PMC2685509 DOI: 10.1261/rna.1471209] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/20/2008] [Accepted: 03/04/2009] [Indexed: 05/27/2023]
Abstract
RNA editing by adenosine deamination is particularly prevalent in the squid nervous system. We hypothesized that the squid editing enzyme might contain structural differences that help explain this phenomenon. As a first step, a squid adenosine deaminase that acts on RNA (sqADAR2a) cDNA and the gene that encodes it were cloned from the giant axon system. PCR and RNase protection assays showed that a splice variant of this clone (sqADAR2b) was also expressed in this tissue. Both versions are homologous to the vertebrate ADAR2 family. sqADAR2b encodes a conventional ADAR2 family member with an evolutionarily conserved deaminase domain and two double-stranded RNA binding domains (dsRBD). sqADAR2a differs from sqADAR2b by containing an optional exon that encodes an "extra" dsRBD. Both splice variants are expressed at comparable levels and are extensively edited, each in a unique pattern. Recombinant sqADAR2a and sqADAR2b, produced in Pichia pastoris, are both active on duplex RNA. Using a standard 48-h protein induction, both sqADAR2a and sqADAR2b exhibit promiscuous self-editing; however, this activity is particularly robust for sqADAR2a. By decreasing the induction time to 16 h, self-editing was mostly eliminated. We next tested the ability of sqADAR2a and sqADAR2b to edit two K+ channel mRNAs in vitro. Both substrates are known to be edited in squid. For each mRNA, sqADAR2a edited many more sites than sqADAR2b. These data suggest that the "extra" dsRBD confers high activity on sqADAR2a.
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Affiliation(s)
- Juan Pablo Palavicini
- Institute of Neurobiology, University of Puerto Rico Medical Sciences Campus, San Juan, Puerto Rico 00901
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14
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Colina C, Rosenthal JJC, DeGiorgis JA, Srikumar D, Iruku N, Holmgren M. Structural basis of Na(+)/K(+)-ATPase adaptation to marine environments. Nat Struct Mol Biol 2007; 14:427-31. [PMID: 17460695 DOI: 10.1038/nsmb1237] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2006] [Accepted: 03/22/2007] [Indexed: 02/07/2023]
Abstract
Throughout evolution, enzymes have adapted to perform in different environments. The Na(+)/K(+) pump, an enzyme crucial for maintaining ionic gradients across cell membranes, is strongly influenced by the ionic environment. In vertebrates, the pump sees much less external Na(+) (100-160 mM) than it does in osmoconformers such as squid (450 mM), which live in seawater. If the extracellular architecture of the squid pump were identical to that of vertebrates, then at the resting potential, the pump's function would be severely compromised because the negative voltage would drive Na(+) ions back to their binding sites, practically abolishing forward transport. Here we show that four amino acids that ring the external mouth of the ion translocation pathway are more positive in squid, thereby reducing the pump's sensitivity to external Na(+) and explaining how it can perform optimally in the marine environment.
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Affiliation(s)
- Claudia Colina
- Institute of Neurobiology, University of Puerto Rico-Medical Sciences Campus, San Juan, Puerto Rico 00901
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15
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Vervaeke K, Gu N, Agdestein C, Hu H, Storm JF. Kv7/KCNQ/M-channels in rat glutamatergic hippocampal axons and their role in regulation of excitability and transmitter release. J Physiol 2006; 576:235-56. [PMID: 16840518 PMCID: PMC1995637 DOI: 10.1113/jphysiol.2006.111336] [Citation(s) in RCA: 100] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
M-current (I(M)) plays a key role in regulating neuronal excitability. Mutations in Kv7/KCNQ subunits, the molecular correlates of I(M), are associated with a familial human epilepsy syndrome. Kv7/KCNQ subunits are widely expressed, and I(M) has been recorded in somata of several types of neurons, but the subcellular distribution of M-channels remains elusive. By combining field-potential, whole-cell and intracellular recordings from area CA1 in rat hippocampal slices, and computational modelling, we provide evidence for functional M-channels in unmyelinated axons in the brain. Our data indicate that presynaptic M-channels can regulate axonal excitability and synaptic transmission, provided the axons are depolarized into the I(M) activation range (beyond approximately -65 mV). Here, such depolarization was achieved by increasing the extracellular K(+) concentration ([K(+)](o)). Extracellular recordings in the presence of moderately elevated [K(+)](o) (7-11 mm), showed that the specific M-channel blocker XE991 reduced the amplitude of the presynaptic fibre volley and the field EPSP in a [K(+)](o)-dependent manner, both in stratum radiatum and in stratum lacknosum moleculare. The M-channel opener, retigabine, had opposite effects. The higher the [K(+)](o), the greater the effects of XE991 and retigabine. Similar pharmacological modulation of EPSPs recorded intracellularly from CA1 pyramidal neurons, while blocking postsynaptic K(+) channels with intracellular Cs(+), confirmed that active M-channels are located presynaptically. Computational analysis with an axon model showed that presynaptic I(M) can control Na(+) channel inactivation and thereby affect the presynaptic action potential amplitude and Ca(2+) influx, provided the axonal membrane potential is sufficiently depolarized. Finally, we compared the effects of blocking I(M) on the spike after-depolarization and bursting in CA3 pyramidal neuron somata versus their axons. In standard [K(+)](o) (2.5 mm), XE991 increased the ADP and promoted burst firing at the soma, but not in the axons. However, I(M) contributed to the refractory period in the axons when spikes were broadened by a low dose 4-aminopyridine (200 microm). Our results indicate that functional Kv7/KCNQ/M-channels are present in unmyelinated axons in the brain, and that these channels may have contrasting effects on excitability depending on their subcellular localization.
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Affiliation(s)
- K Vervaeke
- Department of Physiology at IMB and Centre for Molecular Biology and Neuroscience, CMBN, University of Oslo, PB 1103 Blindern, N-0317 Oslo, Norway
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16
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Kelley WP, Wolters AM, Sack JT, Jockusch RA, Jurchen JC, Williams ER, Sweedler JV, Gilly WF. Characterization of a novel gastropod toxin (6-bromo-2-mercaptotryptamine) that inhibits shaker K channel activity. J Biol Chem 2003; 278:34934-42. [PMID: 12815055 DOI: 10.1074/jbc.m301271200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A novel potassium channel antagonist has been purified from the defensive mucus secreted by Calliostoma canaliculatum, a marine snail found in the temperate coastal waters of the western Pacific. The toxin is expelled from the hypobranchial gland as part of a defensive response and is contained within a viscous matrix that minimizes dilution and degradation. The active compound was isolated by multistage microbore HPLC separations followed by bioactivity assays. Nuclear magnetic resonance, combined with electrospray ionization Fourier-transform ion cyclotron resonance and electrospray ionization ion trap mass spectrometry indicate that the active component is a heretofore unknown indole-derivative, a disulfide-linked dimer of 6-bromo-2-mercaptotryptamine (BrMT). Exudates from the hypobranchial glands of various marine mollusks have been sources for dye compounds such as 6-6 dibromoindigo, the ancient dye Tyrian purple. BrMT represents the first correlation of a hypobranchial gland exudate with a molecular response. Voltage clamp experiments with a number of K channel subtypes indicate that BrMT inhibits certain voltage-gated K channels of the Kv1 subfamily.
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Affiliation(s)
- Wayne P Kelley
- Department of Chemistry and the Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61820, USA
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17
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Jerng HH, Gilly WF. Inactivation and pharmacological properties of sqKv1A homotetramers in Xenopus oocytes cannot account for behavior of the squid "delayed rectifier" K(+) conductance. Biophys J 2002; 82:3022-36. [PMID: 12023225 PMCID: PMC1302090 DOI: 10.1016/s0006-3495(02)75643-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
Considerable published evidence suggests that alpha-subunits of the cloned channel sqKv1A compose the "delayed rectifier" in the squid giant axon system, but discrepancies regarding inactivation properties of cloned versus native channels exist. In this paper we define the mechanism of inactivation for sqKv1A channels in Xenopus oocytes to investigate these and other discrepancies. Inactivation of sqKv1A in Xenopus oocytes was found to be unaffected by genetic truncation of the N-terminus, but highly sensitive to certain amino acid substitutions around the external mouth of the pore. External TEA and K(+) ions slowed inactivation of sqKv1A channels in oocytes, and chloramine T (Chl-T) accelerated inactivation. These features are all consistent with a C-type inactivation mechanism as defined for Shaker B channels. Treatment of native channels in giant fiber lobe neurons with TEA or high K(+) does not slow inactivation, nor does Chl-T accelerate it. Pharmacological differences between the two channel types were also found for 4-aminopyridine (4AP). SqKv1A's affinity for 4AP was poor at rest and increased after activation, whereas 4AP block occurred much more readily at rest with native channels than when they were activated. These results suggest that important structural differences between sqKv1A homotetramers and native squid channels are likely to exist around the external and internal mouths of the pore.
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Affiliation(s)
- Henry H Jerng
- Hopkins Marine Station, Pacific Grove, California 93950, USA
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18
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Rosenthal JJC, Bezanilla F. Extensive editing of mRNAs for the squid delayed rectifier K+ channel regulates subunit tetramerization. Neuron 2002; 34:743-57. [PMID: 12062021 DOI: 10.1016/s0896-6273(02)00701-8] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
We report the extensive editing of mRNAs that encode the classical delayed rectifier K+ channel (SqK(v)1.1A) in the squid giant axon. Using a quantitative RNA editing assay, 14 adenosine to guanine transitions were identified, and editing efficiency varied tremendously between positions. Interestingly, half of the sites are targeted to the T1 domain, important for subunit assembly. Other sites occur in the channel's transmembrane spans. The effects of editing on K+ channel function are elaborate. Edited codons affect channel gating, and several T1 sites regulate functional expression as well. In particular, the edit R87G, a phylogenetically conserved position, reduces expression close to 50-fold by regulating the channel's ability to form tetramers. These data suggest that RNA editing plays a dynamic role in regulating action potential repolarization in the giant axon.
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19
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Abstract
Voltage-gated potassium ion channels in axons underlie the repolarization phase of the membrane action potential and help to set the resting potential. In addition to being present in the axolemma, they are also found in axoplasm in small vesicles, 30-50 nm in diameter, which may serve as a reserve pool of K+ channel protein (Clay and Kuzirian [2000] J Neurobiol 45:172-184). We have developed a novel technique for extracting these vesicles from axoplasm, which relies on the ability of Texas red to bind to them, thereby reducing their buoyancy so that they are amenable to pelleting by ultracentrifugation (Clay and Kuzirian [2000] J Neurobiol 45:172-184). The mechanism underlying this process may be binding of Texas red to Hsc70, which is primarily a cytosolic protein. However, a small portion of it is located on the surface of vesicles. Kinesin is also on the vesicle surface. This protein is membrane bound in our in vitro vesicle preparation when solutions that do not contain MgATP are added to extruded axoplasm. The addition of MgATP to the solution appears to release a significant amount of kinesin from the vesicles, possibly by the Hsc70-MgATP catalysis mechanism recently proposed by Tsai et al.
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Affiliation(s)
- John R Clay
- Ion Channel Biophysics Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
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20
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Brock MW, Mathes C, Gilly WF. Selective open-channel block of Shaker (Kv1) potassium channels by s-nitrosodithiothreitol (SNDTT). J Gen Physiol 2001; 118:113-34. [PMID: 11429448 PMCID: PMC2233744 DOI: 10.1085/jgp.118.1.113] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2000] [Accepted: 05/22/2001] [Indexed: 11/22/2022] Open
Abstract
Large quaternary ammonium (QA) ions block voltage-gated K(+) (Kv) channels by binding with a 1:1 stoichiometry in an aqueous cavity that is exposed to the cytoplasm only when channels are open. S-nitrosodithiothreitol (SNDTT; ONSCH(2)CH(OH)CH(OH)CH(2)SNO) produces qualitatively similar "open-channel block" in Kv channels despite a radically different structure. SNDTT is small, electrically neutral, and not very hydrophobic. In whole-cell voltage-clamped squid giant fiber lobe neurons, bath-applied SNDTT causes reversible time-dependent block of Kv channels, but not Na(+) or Ca(2)+ channels. Inactivation-removed ShakerB (ShBDelta) Kv1 channels expressed in HEK 293 cells are similarly blocked and were used to study further the action of SNDTT. Dose-response data are consistent with a scheme in which two SNDTT molecules bind sequentially to a single channel, with binding of the first being sufficient to produce block. The dissociation constant for the binding of the second SNDTT molecule (K(d2) = 0.14 mM) is lower than that of the first molecule (K(d1) = 0.67 mM), indicating cooperativity. The half-blocking concentration (K(1/2)) is approximately 0.2 mM. Steady-state block by this electrically neutral compound has a voltage dependence (about -0.3 e(0)) similar in magnitude but opposite in directionality to that reported for QA ions. Both nitrosyl groups on SNDTT (one on each sulfur atom) are required for block, but transfer of these reactive groups to channel cysteine residues is not involved. SNDTT undergoes a slow intramolecular reaction (tau approximately 770 s) in which these NO groups are liberated, leading to spontaneous reversal of the SNDTT effect. Competition with internal tetraethylammonium indicates that bath-applied SNDTT crosses the cell membrane to act at an internal site, most likely within the channel cavity. Finally, SNDTT is remarkably selective for Kv1 channels. When individually expressed in HEK 293 cells, rat Kv1.1-1.6 display profound time-dependent block by SNDTT, an effect not seen for Kv2.1, 3.1b, or 4.2.
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Affiliation(s)
- Mathew W. Brock
- Hopkins Marine Station, Department of Biological Sciences
- Neurosciences Program, Stanford University, Pacific Grove, CA 93950
| | - Chris Mathes
- Hopkins Marine Station, Department of Biological Sciences
| | - William F. Gilly
- Hopkins Marine Station, Department of Biological Sciences
- Neurosciences Program, Stanford University, Pacific Grove, CA 93950
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21
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Ellis KC, Tenenholz TC, Jerng H, Hayhurst M, Dudlak CS, Gilly WF, Blaustein MP, Weber DJ. Interaction of a Toxin from the Scorpion Tityus serrulatus with a Cloned K+ Channel from Squid (sqKv1A). Biochemistry 2001; 40:5942-53. [PMID: 11352729 DOI: 10.1021/bi010173g] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A toxin from the scorpion Tityus serrulatus (TsTX-Kalpha) blocks native squid K(+) channels and their cloned counterpart, sqKv1A, at pH 8 ((native)K(d) approximately 20 nM; (sqKv1A)K(d) approximately 10 nM). In both cases, decreasing the pH below 7.0 significantly diminishes the TsTX-Kalpha effect (pK = 6.6). In the cloned squid channel, the pH dependence of the block is abolished by a single point mutation (H351G), and no change in toxin affinity was observed at higher pH values (pH > or =8.0). To further investigate the TsTX-Kalpha-sqKv1A interaction, the three-dimensional structure of TsTX-Kalpha was determined in solution by NMR spectroscopy, and a model of the TsTX-Kalpha-sqKv1A complex was generated. As found for other alpha-K toxins such as charybdotoxin (CTX), site-directed mutagenesis at toxin residue K27 (K27A, K27R, and K27E) significantly reduced the toxin's affinity for sqKv1A channels. This is consistent with the TsTX-Kalpha-sqKv1A model reported here, which has K27 of the toxin inserted into the ion conduction pathway of the K(+) channel. This toxin-channel model also illustrates a possible mechanism for the pH-dependent block whereby lysine residues from TsTX-Kalpha (K6 and K23) are repelled by protonated H351 on sqKv1A at low pH.
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MESH Headings
- Amino Acid Sequence
- Animals
- Cloning, Molecular
- Decapodiformes
- Delayed Rectifier Potassium Channels
- Ganglia, Invertebrate/chemistry
- Ganglia, Invertebrate/drug effects
- Ganglia, Invertebrate/physiology
- Models, Molecular
- Molecular Sequence Data
- Mutagenesis, Site-Directed
- Neurotoxins/chemistry
- Neurotoxins/genetics
- Neurotoxins/metabolism
- Neurotoxins/pharmacology
- Nuclear Magnetic Resonance, Biomolecular
- Oocytes
- Potassium Channel Blockers
- Potassium Channels/chemistry
- Potassium Channels/genetics
- Potassium Channels, Voltage-Gated
- Protein Structure, Secondary
- Recombinant Proteins/antagonists & inhibitors
- Recombinant Proteins/chemistry
- Recombinant Proteins/metabolism
- Scorpion Venoms/chemistry
- Scorpion Venoms/genetics
- Scorpion Venoms/pharmacology
- Sequence Homology, Amino Acid
- Xenopus laevis
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Affiliation(s)
- K C Ellis
- Department of Biochemistry, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
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22
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Abstract
A non inactivating potassium current known as the delayed rectifier plays a major role in membrane repolarization during an action potential. Whereas several candidate genes exist that code for potassium current, the identities of the molecular isotypes that are responsible in situ for membrane repolarization remain unidentified. We report that Kv2 channels play a major role in action potential repolarization. Kv2 channel elimination resulted in a reduction of the density of noninactivating potassium current and a prolonged impulse duration. In contrast, suppression of noninactivating current carried by Kv1 channels was much less effective in increasing action potential durations. Thus, whereas different potassium channels encode sustained potassium current, their contributions to action potential repolarization vary and require direct examination in situ. Our results indicate that Kv2 subunits function as classic delayed-rectifier channels in vertebrate neurons.
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23
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Blaine JT, Ribera AB. Kv2 channels form delayed-rectifier potassium channels in situ. J Neurosci 2001; 21:1473-80. [PMID: 11222637 PMCID: PMC6762936] [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: 02/19/2023] Open
Abstract
A non inactivating potassium current known as the delayed rectifier plays a major role in membrane repolarization during an action potential. Whereas several candidate genes exist that code for potassium current, the identities of the molecular isotypes that are responsible in situ for membrane repolarization remain unidentified. We report that Kv2 channels play a major role in action potential repolarization. Kv2 channel elimination resulted in a reduction of the density of noninactivating potassium current and a prolonged impulse duration. In contrast, suppression of noninactivating current carried by Kv1 channels was much less effective in increasing action potential durations. Thus, whereas different potassium channels encode sustained potassium current, their contributions to action potential repolarization vary and require direct examination in situ. Our results indicate that Kv2 subunits function as classic delayed-rectifier channels in vertebrate neurons.
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Affiliation(s)
- J T Blaine
- Department of Physiology and Biophysics, Medical Scientist Training Program, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA
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24
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Liu TI, Lebaric ZN, Rosenthal JJ, Gilly WF. Natural substitutions at highly conserved T1-domain residues perturb processing and functional expression of squid Kv1 channels. J Neurophysiol 2001; 85:61-71. [PMID: 11152706 DOI: 10.1152/jn.2001.85.1.61] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Shaker-type K-channel alpha-subunits (SqKv1A, B, D) expressed in neurons of the squid stellate ganglion differ in the length of their N-termini and in the species of amino acid present at several points in the T1 domain, an intracellular region involved in the tetramerization process during channel assembly. Heterologous expression of wild-type SqKv1A, B, and D in Xenopus oocytes reveals large differences in the level of both functional channels (assayed by whole-oocyte voltage clamp) and total channel protein (assayed by immunoblotting). Functional expression is poorest with SqKv1A and by far the best with SqKv1D. Biophysical properties of the three SqKv1 channels are essentially identical (assayed by cell-attached patch clamp). Site-directed mutagenesis was used to determine whether the observed differences in expression level are impacted by two residues in the T1 domain at which SqKv1A and B (but not D) differ from the consensus sequences found in many other taxa. In SqKv1A, glycine is substituted for arginine in an otherwise universally conserved sequence (FFDR in the T1(B) subdomain). In SqKv1B, glycine replaces serine in a sequence that is conserved within the Kv1 subfamily (SGLR in the T1(A) subdomain). Restoration of the consensus amino acid at these positions largely accounts for the observed differences in expression level. Analysis of the glycosylation state of aberrant versus restored alpha-subunits suggests that the anomalous amino acids in SqKv1A and B exert their influence during early steps in channel processing and assembly which take place in the endoplasmic reticulum (ER).
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Affiliation(s)
- T I Liu
- Neurosciences Program, Hopkins Marine Station, Stanford University, Pacific Grove, California 93950, USA
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25
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Abstract
We have localized the classical voltage-gated K(+) channel within squid giant axons by immunocytochemistry using the Kv1 antibody of Rosenthal et al. (1996). Widely dispersed patches of intense immunofluorescence were observed in the axonal membrane. Punctate immunofluorescence was also observed in the axoplasm and was localized to approximately 25-50-microm-wide column down the length of the nerve (axon diameter approximately 500 microm). Immunoelectronmicroscopy of the axoplasm revealed a K(+) channel containing vesicles, 30-50 nm in diameter, within this column. These and other vesicles of similar size were isolated from axoplasm using a novel combination of high-speed ultracentrifugation and controlled-pore size, glass bead separation column techniques. Approximately 1% of all isolated vesicles were labeled by K(+) channel immunogold reacted antibody. Incorporation of isolated vesicle fractions within an artificial lipid bilayer revealed K(+) channel electrical activity similar to that recorded directly from the axonal membrane by Llano et al. (1988). These K(+) channel-containing vesicles may be involved in cycling of K(+) channel protein into the axonal membrane. We have also isolated an axoplasmic fraction containing approximately 150-nm-diameter vesicles that may transport K(+) channels back to the cell body.
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Affiliation(s)
- J R Clay
- Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
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26
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Molecular underpinnings of motor pattern generation: differential targeting of shal and shaker in the pyloric motor system. J Neurosci 2000. [PMID: 10964967 DOI: 10.1523/jneurosci.20-17-06619.2000] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The patterned activity generated by the pyloric circuit in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, results not only from the synaptic connectivity between the 14 component neurons but also from differences in the intrinsic properties of the neurons. Presumably, differences in the complement and distribution of expressed ion channels endow these neurons with many of their distinct attributes. Each pyloric cell type possesses a unique, modulatable transient potassium current, or A-current (I(A)), that is instrumental in determining the output of the network. Two genes encode A-channels in this system, shaker and shal. We examined the hypothesis that cell-specific differences in shaker and shal channel distribution contribute to diversity among pyloric neurons. We found a stereotypic distribution of channels in the cells, such that each channel type could contribute to different aspects of the firing properties of a cell. Shal is predominantly found in the somatodendritic compartment in which it influences oscillatory behavior and spike frequency. Shaker channels are exclusively localized to the membranes of the distal axonal compartments and most likely affect distal spike propagation. Neither channel is detectably inserted into the preaxonal or proximal portions of the axonal membrane. Both channel types are targeted to synaptic contacts at the neuromuscular junction. We conclude that the differential targeting of shaker and shal to different compartments is conserved among all the pyloric neurons and that the channels most likely subserve different functions in the neuron.
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27
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Baro DJ, Ayali A, French L, Scholz NL, Labenia J, Lanning CC, Graubard K, Harris-Warrick RM. Molecular underpinnings of motor pattern generation: differential targeting of shal and shaker in the pyloric motor system. J Neurosci 2000; 20:6619-30. [PMID: 10964967 PMCID: PMC6772986] [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: 02/17/2023] Open
Abstract
The patterned activity generated by the pyloric circuit in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, results not only from the synaptic connectivity between the 14 component neurons but also from differences in the intrinsic properties of the neurons. Presumably, differences in the complement and distribution of expressed ion channels endow these neurons with many of their distinct attributes. Each pyloric cell type possesses a unique, modulatable transient potassium current, or A-current (I(A)), that is instrumental in determining the output of the network. Two genes encode A-channels in this system, shaker and shal. We examined the hypothesis that cell-specific differences in shaker and shal channel distribution contribute to diversity among pyloric neurons. We found a stereotypic distribution of channels in the cells, such that each channel type could contribute to different aspects of the firing properties of a cell. Shal is predominantly found in the somatodendritic compartment in which it influences oscillatory behavior and spike frequency. Shaker channels are exclusively localized to the membranes of the distal axonal compartments and most likely affect distal spike propagation. Neither channel is detectably inserted into the preaxonal or proximal portions of the axonal membrane. Both channel types are targeted to synaptic contacts at the neuromuscular junction. We conclude that the differential targeting of shaker and shal to different compartments is conserved among all the pyloric neurons and that the channels most likely subserve different functions in the neuron.
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Affiliation(s)
- D J Baro
- Institute of Neurobiology and Department of Biochemistry, Medical Sciences Campus, University of Puerto Rico, San Juan, Puerto Rico 00901, USA.
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28
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Nguyen TD, Rabe H, Terlau H, Jeserich G. Isolation and heterologous expression of two genomic clones encoding Shaker-related potassium channels of trout CNS. J Neurosci Res 2000; 60:174-83. [PMID: 10740222 DOI: 10.1002/(sici)1097-4547(20000415)60:2<174::aid-jnr6>3.0.co;2-i] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Two Shaker-related potassium channel genes (termed tsha1 and tsha2) expressed in the CNS of trout were cloned and sequenced. The coding regions of both genes were not interrupted by introns and exhibited a high overall sequence similarity to other members of the Shaker subfamily. By computer-assisted sequence alignments, tsha1 was identified as a fish homologue to the mammalian Kv1.2 subtype of potassium channels, whereas tsha2 did not show a preferential sequence homology but shared a uniform similarity to Kv1.1, Kv1.2, and Kv1.3. Upon heterologous expression in a mammalian glial cell line, both channels exhibited delayed rectifier current properties that differed from each other by their threshold potentials of activation and their pharamcological features: wheras the tsha1-mediated current was efficiently blocked by submicromolar concentrations of alpha-DTX but not by TEA, tsha2 was highly TEA-sensitive, correlating well with differences in the amino acid structure of the pore outer mouth region. As revealed by RT-PCR, Shaker-related potassium channels were sequentially expressed during trout brain development: tsha2 was found initially at stage 34, followed by tsha1 at stage 36, whereas two other members of the Shaker family (tsha3 and tsha4) were detectable much earlier (stage 30, hatching). In the mature brain tissue, no regional specialization of Shaker channel subtype expression was noted.
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Affiliation(s)
- T D Nguyen
- Abteilung Zoophysiologie, Universität Osnabrück, Osnabrück, Germany
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29
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Abstract
In the voltage-gated ion channels of every animal, whether they are selective for K+, Na+ or Ca2+, the voltage sensors are the S4 transmembrane segments carrying four to eight positive charges always separated by two uncharged residues. It is proposed that they move across the membrane in a screw-helical fashion in a series of three or more steps that each transfer a single electronic charge. The unit steps are stabilized by ion pairing between the mobile positive charges and fixed negative charges, of which there are invariably two located near the inner ends of segments S2 and S3 and a third near the outer end of either S2 or S3. Opening of the channel involves three such steps in each domain.
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Affiliation(s)
- R D Keynes
- Physiological Laboratory, University of Cambridge, UK.
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30
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Pascual JM, Shieh CC, Kirsch GE, Brown AM. Contribution of the NH2 terminus of Kv2.1 to channel activation. THE AMERICAN JOURNAL OF PHYSIOLOGY 1997; 273:C1849-58. [PMID: 9435489 DOI: 10.1152/ajpcell.1997.273.6.c1849] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Opening and closing of voltage-operated channels requires the interaction of diverse structural elements. One approach to the identification of channel domains that participate in gating is to locate the sites of action of modifiers. Covalent reaction of Kv2.1 channels with the neutral, sulfhydryl-specific methylmethanethiosulfonate (MMTS) caused a slowing of channel gating with a predominant effect on the kinetics of activation. These effects were also obtained after intracellular, but not extracellular, application of a charged MMTS analog. Single channel analysis revealed that MMTS acted primarily by prolonging the latency to first opening without substantially affecting gating transitions after the channel first opens and until it inactivates. To localize the channel cysteine(s) with which MMTS reacts, we generated NH2- and COOH-terminal deletion mutants and a construct in which all three cysteines in transmembrane regions were substituted. Only the NH2-terminal deletion construct gave rise to currents that activated slowly and displayed MMTS-insensitive kinetics. These results show that the NH2-terminal tail of Kv2.1 participates in transitions leading to activation through interactions involving reduced cysteine(s) that can be modulated from the cytoplasmic phase.
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Affiliation(s)
- J M Pascual
- Center for Molecular Recognition, Columbia University, New York, New York 10032, USA
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31
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Patton DE, Silva T, Bezanilla F. RNA editing generates a diverse array of transcripts encoding squid Kv2 K+ channels with altered functional properties. Neuron 1997; 19:711-22. [PMID: 9331360 DOI: 10.1016/s0896-6273(00)80383-9] [Citation(s) in RCA: 104] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
We have cloned a Kv2 potassium channel from squid optic lobe termed sqKv2. Multiple overlapping sqKv2 cDNA clones differed from one another at specific positions by purine transitions. To test whether the purine transitions were generated by RNA editing, we compared a 360 nucleotide genomic sequence with corresponding cDNA sequences (encoding S4-S6) isolated from individual animals and lying on a single gene and exon. cDNA sequences differed from genomic sequence at 17 positions, resulting in 28 unique sequences. There was invariantly an adenosine in the genomic sequence and a guanosine in the edited cDNA sequences. Two of the edits altered the rates of channel closure and slow inactivation. These results extend selective RNA editing to invertebrate taxa and represents a novel mechanism for the posttranscriptional modulation of voltage-gated ion channels.
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Affiliation(s)
- D E Patton
- Department of Physiology, University of California School of Medicine, Los Angeles 90095, USA
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32
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A family of delayed rectifier Kv1 cDNAs showing cell type-specific expression in the squid stellate ganglion/giant fiber lobe complex. J Neurosci 1997. [PMID: 9185544 DOI: 10.1523/jneurosci.17-13-05070.1997] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Squid giant axons are formed by giant fiber lobe (GFL) neurons of the stellate ganglion (SG). Other large motoneurons in the SG form a parallel system. A small family of cDNAs (SqKv1A-D) encoding Kv1 alpha-subunits was identified in a squid (Loligo opalescens) SG/GFL library. Members have distinct 5' untranslated regions (UTRs) and initial coding regions, but beyond a certain point (nucleotide 34 of SqKv1A) only nine differences exist. 3' UTRs are identical. Predicted alpha-subunits are nearly identical, and only the N termini differ significantly, primarily in length. RNase protection assays that use RNA isolated from specific SG regions show that SqKv1A mRNA is expressed prominently in the GFL but not in the SG proper. SqKv1B yields the opposite pattern. SqKv1D also is expressed only in the SG. SqKv1C expression was not detectable. In situ hybridizations confirm these results and reveal that SqKv1B mRNA is abundant in many large neurons of the SG, whereas SqKv1D expression is limited to small isolated clusters of neurons. SqKv1A and B are thus the predominant Kv1 mRNAs in the SG/GFL complex. Activation properties of SqKv1A and B channels expressed in oocytes are very similar to one another and compare favorably with properties of native delayed rectifier channels in GFL neurons and large SG neurons. The Kv1 complement in these squid neurons thus seems to be relatively simple. Several differences exist between cloned and native channels, however, and may reflect differences in the cellular environments of oocytes and neurons.
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33
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Rosenthal JJ, Liu TI, Gilly WF. A family of delayed rectifier Kv1 cDNAs showing cell type-specific expression in the squid stellate ganglion/giant fiber lobe complex. J Neurosci 1997; 17:5070-9. [PMID: 9185544 PMCID: PMC6573319] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/1997] [Revised: 04/16/1997] [Accepted: 04/21/1997] [Indexed: 02/04/2023] Open
Abstract
Squid giant axons are formed by giant fiber lobe (GFL) neurons of the stellate ganglion (SG). Other large motoneurons in the SG form a parallel system. A small family of cDNAs (SqKv1A-D) encoding Kv1 alpha-subunits was identified in a squid (Loligo opalescens) SG/GFL library. Members have distinct 5' untranslated regions (UTRs) and initial coding regions, but beyond a certain point (nucleotide 34 of SqKv1A) only nine differences exist. 3' UTRs are identical. Predicted alpha-subunits are nearly identical, and only the N termini differ significantly, primarily in length. RNase protection assays that use RNA isolated from specific SG regions show that SqKv1A mRNA is expressed prominently in the GFL but not in the SG proper. SqKv1B yields the opposite pattern. SqKv1D also is expressed only in the SG. SqKv1C expression was not detectable. In situ hybridizations confirm these results and reveal that SqKv1B mRNA is abundant in many large neurons of the SG, whereas SqKv1D expression is limited to small isolated clusters of neurons. SqKv1A and B are thus the predominant Kv1 mRNAs in the SG/GFL complex. Activation properties of SqKv1A and B channels expressed in oocytes are very similar to one another and compare favorably with properties of native delayed rectifier channels in GFL neurons and large SG neurons. The Kv1 complement in these squid neurons thus seems to be relatively simple. Several differences exist between cloned and native channels, however, and may reflect differences in the cellular environments of oocytes and neurons.
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Affiliation(s)
- J J Rosenthal
- Hopkins Marine Station, Department of Biological Sciences, Stanford University, Pacific Grove, California 93950, USA
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Mathes C, Rosenthal JJ, Armstrong GM, Gilly WF. Fast inactivation of delayed rectifier K conductance in squid giant axon and its cell bodies. J Gen Physiol 1997; 109:435-48. [PMID: 9101403 PMCID: PMC2219430 DOI: 10.1085/jgp.109.4.435] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/1996] [Accepted: 01/09/1997] [Indexed: 02/04/2023] Open
Abstract
Inactivation of delayed rectifier K conductance (gk) was studied in squid giant axons and in the somata of giant fiber lobe (GFL) neurons. Axon measurements were made with an axial wire voltage clamp by pulsing to VK (approximately -10 mV in 50-70 mM external K) for a variable time and then assaying available gK with a strong, brief test pulse. GFL cells were studied with whole-cell patch clamp using the same prepulse procedure as well as with long depolarizations. Under our experimental conditions (12-18 degrees C, 4 mM internal MgATP) a large fraction of gK inactivates within 250 ms at -10 mV in both cell bodies and axons, although inactivation tends to be more complete in cell bodies. Inactivation in both preparations shows two kinetic components. The faster component is more temperature-sensitive and becomes very prominent above 12 degrees C. Contribution of the fast component to inactivation shows a similar voltage dependence to that of gK, suggesting a strong coupling of this inactivation path to the open state. Omission of internal MgATP or application of internal protease reduces the amount of fast inactivation. High external K decreases the amount of rapidly inactivating IK but does not greatly alter inactivation kinetics. Neither external nor internal tetraethylammonium has a marked effect on inactivation kinetics. Squid delayed rectifier K channels in GFL cell bodies and giant axons thus share complex fast inactivation properties that do not closely resemble those associated with either C-type or N-type inactivation of cloned Kvl channels studied in heterologous expression systems.
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Affiliation(s)
- C Mathes
- Hopkins Marine Station, Department of Biological Sciences, Stanford University, Pacific Grove, California 93950, USA
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