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Chan TYC, Hung LY, Lam TYL, Sheng B, Leung FYK, Lee HHC. SCN4A-related congenital myopathy in a Han Chinese patient: A case report and literature review. Heliyon 2024; 10:e23663. [PMID: 38187266 PMCID: PMC10770507 DOI: 10.1016/j.heliyon.2023.e23663] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2023] [Revised: 11/23/2023] [Accepted: 12/09/2023] [Indexed: 01/09/2024] Open
Abstract
SCN4A mutations have been shown to be associated with myotonia, paramyotonia congenita, and periodic paralyses. More recently, loss-of-function variants in the SCN4A gene were also noted to be associated with rarer, autosomal recessive forms of congenital myasthenic syndrome and congenital myopathy. Diagnosis is challenging as the initial clinical presentation and histological features on muscle biopsies are non-specific. We report a Han Chinese patient presented with congenital myopathy with two missense SCN4A variants. The patient had an antenatal history of reduced fetal movements, polyhydramnios and a very preterm birth. At birth, she was noted to have low Apgar score, respiratory distress syndrome and hypotonia. Delayed motor development was noted in early childhood. Dysmorphic features such as an elongated face, dolichocephaly and high arched palate were present. At 16 years of age, the patient developed progressive muscle weakness and was wheelchair-bound by age 20. Muscle biopsy revealed non-specific changes only. Targeted hereditary myopathy panel testing by next generation sequencing revealed two previously unreported missense variants c.1841A > T p.(Asn614Ile) and c.4420G > A p.(Ala1474Thr) in the SCN4A gene. The clinical features of SCN4A-related congenital myopathy and myasthenic syndrome were reviewed. This case exemplifies the utility of next generation sequencing in the diagnosis of undifferentiated muscle disease.
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Affiliation(s)
- Tina Yee-Ching Chan
- Kowloon West Cluster Laboratory Genetic Service, Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong Special Administrative Region
| | - Ling-Yin Hung
- Kowloon West Cluster Laboratory Genetic Service, Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong Special Administrative Region
| | - Tiffany Yan-Lok Lam
- Department of Medicine and Geriatrics, Princess Margaret Hospital, Hong Kong Special Administrative Region
| | - Bun Sheng
- Department of Medicine and Geriatrics, Princess Margaret Hospital, Hong Kong Special Administrative Region
| | - Frank Ying-Kit Leung
- Kowloon West Cluster Laboratory Genetic Service, Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong Special Administrative Region
- Department of Pathology, Yan Chai Hospital, Hong Kong Special Administrative Region
| | - Hencher Han-Chih Lee
- Kowloon West Cluster Laboratory Genetic Service, Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong Special Administrative Region
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2
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Shandell MA, Quejada JR, Yazawa M, Cornish VW, Kass RS. Detection of Na v1.5 Conformational Change in Mammalian Cells Using the Noncanonical Amino Acid ANAP. Biophys J 2019; 117:1352-1363. [PMID: 31521331 PMCID: PMC6818161 DOI: 10.1016/j.bpj.2019.08.028] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2019] [Revised: 07/29/2019] [Accepted: 08/19/2019] [Indexed: 12/27/2022] Open
Abstract
Nav1.5 inactivation is necessary for healthy conduction of the cardiac action potential. Genetic mutations of Nav1.5 perturb inactivation and cause potentially fatal arrhythmias associated with long QT syndrome type 3. The exact structural dynamics of the inactivation complex is unknown. To sense inactivation gate conformational change in live mammalian cells, we incorporated the solvatochromic fluorescent noncanonical amino acid 3-((6-acetylnaphthalen-2-yl)amino)-2-aminopropanoic acid (ANAP) into single sites in the Nav1.5 inactivation gate. ANAP was incorporated in full-length and C-terminally truncated Nav1.5 channels using mammalian cell synthetase-tRNA technology. ANAP-incorporated channels were expressed in mammalian cells, and they exhibited pathophysiological function. A spectral imaging potassium depolarization assay was designed to detect ANAP emission shifts associated with Nav1.5 conformational change. Site-specific intracellular ANAP incorporation affords live-cell imaging and detection of Nav1.5 inactivation gate conformational change in mammalian cells.
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Affiliation(s)
- Mia A Shandell
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York.
| | - Jose R Quejada
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York; Department of Rehabilitation and Regenerative Medicine, Columbia Stem Cell Initiative, Columbia University, New York, New York
| | - Masayuki Yazawa
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York; Department of Rehabilitation and Regenerative Medicine, Columbia Stem Cell Initiative, Columbia University, New York, New York
| | - Virginia W Cornish
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York; Department of Chemistry, Columbia University, New York, New York.
| | - Robert S Kass
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York.
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3
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Characterization of a novel LQT3 variant with a selective efficacy of mexiletine treatment. Sci Rep 2019; 9:12997. [PMID: 31506521 PMCID: PMC6736863 DOI: 10.1038/s41598-019-49450-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2019] [Accepted: 08/23/2019] [Indexed: 12/19/2022] Open
Abstract
Pathogenic variants in the human SCN5A gene encoding the a-subunit of the principle Na+ channel (Nav1.5) are associated with long QT syndrome (LQTS) 3. LQT3 patients display variable responses to Na+ channel blockers demanding for the development of variant-specific therapeutic strategies. Here we performed a combined electrophysiological analysis with in silico simulation of variant channel to elucidate mechanisms of therapeutic responsiveness. We identified a novel SCN5A variant (A1656D) in a LQTS patient with a distinct response to mexiletine resulting in suppression of non-sustained ventricular tachycardia and manifestation of premature atrial contraction. Patch clamp analysis revealed that A1656D variant exerted gain-of-function effects including hyperpolarizing shift of the voltage-dependence of activation, depolarizing shift in the voltage-dependence of inactivation, and slowing of fast inactivation. Among ranolazine, flecainide, and mexiletine, only mexiletine restored inactivation kinetics of A1656D currents. In silico simulation to assess the effect of A1656D variant on ventricular cardiac cell excitation predicted a prolonged action potential which is consistent with the prolonged QT and non-sustained ventricular tachycardia of the patient. It also predicted that only mexiletine suppressed the prolonged action potential of human ventricular myocytes expressing A1656D. These data elucidate the underlying mechanism of the distinct response to mexiletine in this patient.
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Gawali V, Todt H. Mechanism of Inactivation in Voltage-Gated Na+ Channels. CURRENT TOPICS IN MEMBRANES 2016; 78:409-50. [DOI: 10.1016/bs.ctm.2016.07.004] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
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NaV1.7 gain-of-function mutations as a continuum: A1632E displays physiological changes associated with erythromelalgia and paroxysmal extreme pain disorder mutations and produces symptoms of both disorders. J Neurosci 2008; 28:11079-88. [PMID: 18945915 DOI: 10.1523/jneurosci.3443-08.2008] [Citation(s) in RCA: 136] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Gain-of-function mutations of Na(V)1.7 have been shown to produce two distinct disorders: Na(V)1.7 mutations that enhance activation produce inherited erythromelalgia (IEM), characterized by burning pain in the extremities; Na(V)1.7 mutations that impair inactivation produce a different, nonoverlapping syndrome, paroxysmal extreme pain disorder (PEPD), characterized by rectal, periocular, and perimandibular pain. Here we report a novel Na(V)1.7 mutation associated with a mixed clinical phenotype with characteristics of IEM and PEPD, with an alanine 1632 substitution by glutamate (A1632E) in domain IV S4-S5 linker. Patch-clamp analysis shows that A1632E produces changes in channel function seen in both IEM and PEPD mutations: A1632E hyperpolarizes (-7 mV) the voltage dependence of activation, slows deactivation, and enhances ramp responses, as observed in Na(V)1.7 mutations that produce IEM. A1632E depolarizes (+17mV) the voltage dependence of fast inactivation, slows fast inactivation, and prevents full inactivation, resulting in persistent inward currents similar to PEPD mutations. Using current clamp, we show that A1632E renders dorsal root ganglion (DRG) and trigeminal ganglion neurons hyperexcitable. These results demonstrate a Na(V)1.7 mutant with biophysical characteristics common to PEPD (impaired fast inactivation) and IEM (hyperpolarized activation, slow deactivation, and enhanced ramp currents) associated with a clinical phenotype with characteristics of both IEM and PEPD and show that this mutation renders DRG and trigeminal ganglion neurons hyperexcitable. These observations indicate that IEM and PEPD mutants are part of a physiological continuum that can produce a continuum of clinical phenotypes.
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SCN5A channelopathies--an update on mutations and mechanisms. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2008; 98:120-36. [PMID: 19027780 DOI: 10.1016/j.pbiomolbio.2008.10.005] [Citation(s) in RCA: 124] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Voltage-gated Na+ channels mediate the rapid upstroke of the action potential in excitable tissues. Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. Mutations in SCN5A are associated with distinct cardiac excitation disorders often resulting in life-threatening arrhythmias. This review outlines the currently known SCN5A mutations linked to three distinct cardiac rhythm disorders: long QT syndrome subtype 3 (LQT3), Brugada syndrome (BS), and cardiac conduction disease (CCD). Electrophysiological properties of the mutant channels are summarized and discussed in terms of Na+ channel structure-function relationships and regarding molecular mechanisms underlying the respective cardiac dysfunction. Possible reasons for less convincing genotype-phenotype correlations are suggested.
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Fraceto LF, Oyama S, Nakaie CR, Spisni A, de Paula E, Pertinhez TA. Interaction of local anesthetics with a peptide encompassing the IV/S4–S5 linker of the Na+ channel. Biophys Chem 2006; 123:29-39. [PMID: 16687202 DOI: 10.1016/j.bpc.2006.03.010] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2006] [Revised: 03/17/2006] [Accepted: 03/19/2006] [Indexed: 11/16/2022]
Abstract
The peptide pIV/S4-S5 encompasses the cytoplasmic linker between helices S4-S5 in domain IV of the voltage-gated Na+ channel, residues 1644-1664. The interaction of two local anesthetics (LA), lidocaine and benzocaine, with pIV/S4-S5 has been studied by DOSY, heteronuclear NMR 1H-15N-HSQC spectroscopy and computational methods. DOSY indicates that benzocaine, a neutral ester, exhibits stronger interaction with pIV/S4-S5 than lidocaine, a charged amine-amide. Weighted average chemical shifts, Deltadelta(1H-15N), show that benzocaine affects residues L1653, M1655 and S1656 while lidocaine slightly perturbs residues I1646, L1649 and A1659, L1660, near the N- and C-terminus, respectively. Computational methods confirmed the stability of the benzocaine binding and the existence of two binding sites for lidocaine. Even considering that the approach of studying the peptide in the presence of a co-solvent (TFE/H2O, 30%/70% v/v) has an inherently limited implication, our data strongly support the existence of multiple LA binding sites in the IV/S4-S5 linker, as suggested in the literature. In addition, we consider that LA can bind to the S4-S5 linker with diverse binding modes and strength since this linker is part of the receptor for the "inactivation gate particle". Conditions for devising new functional studies, aiming to better understand Na+ channel functionality as well as the various facets of LA pharmacological activity are proposed in this work.
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Affiliation(s)
- Leonardo F Fraceto
- Center of Molecular and Structural Biology, LNLS, Campinas, Brazil; Department of Biochemistry, Institute of Biology, State University of Campinas, Campinas, Brazil
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Popa MO, Alekov AK, Bail S, Lehmann-Horn F, Lerche H. Cooperative effect of S4-S5 loops in domains D3 and D4 on fast inactivation of the Na+ channel. J Physiol 2004; 561:39-51. [PMID: 15459238 PMCID: PMC1665345 DOI: 10.1113/jphysiol.2004.065912] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Cytoplasmic S4-S5 loops have been shown to be involved in fast inactivation of voltage-gated ion channels. We studied mutations in these loops and their potential cooperative effects in domains D3 (N1151C, A1152C, I1160C/A) and D4 (F1473C, L1482C/A) of the human skeletal muscle Na(+) channel alpha-subunit (hNa(v)1.4) using expression in tsA201 cells and the whole cell patch-clamp technique. All cysteine mutations were accessible to intracellularly applied sulfhydryl reagents which considerably destabilized fast inactivation. For different combinations of corresponding D3/D4 double mutations, fast inactivation could be almost completely removed. Thermodynamic cycle analysis indicated an additive effect for N1151C/F1473C and a significant cooperative effect for I1160/L1482 double mutations. Application of oxidizing reagents such as Cu-phenanthroline to link two cysteines via a disulfide bridge did not reveal evidence for a direct physical interaction of cysteines in D3 and D4. In addition to the pronounced alterations of fast inactivation, mutations of I1160 shifted steady-state activation in the hyperpolarizing direction and slowed the kinetics of both activation and deactivation. Sulfhydryl reagents had charge-dependent effects on I1160C suggesting interaction with negative charges in another protein region. We conclude that fast inactivation of the Na(+) channel involves both S4-S5 loops in D3 and D4 in a cooperative manner. D3/S4-S5 also plays an important role in activation and deactivation.
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Affiliation(s)
- M Oana Popa
- Department of Applied Physiology, University of Ulm, D-89069 Ulm, Germany
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Yarov-Yarovoy V, McPhee JC, Idsvoog D, Pate C, Scheuer T, Catterall WA. Role of amino acid residues in transmembrane segments IS6 and IIS6 of the Na+ channel alpha subunit in voltage-dependent gating and drug block. J Biol Chem 2002; 277:35393-401. [PMID: 12130650 DOI: 10.1074/jbc.m206126200] [Citation(s) in RCA: 190] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Alanine-scanning mutagenesis of transmembrane segments IS6 and IIS6 of the rat brain Na(v)1.2 channel alpha subunit identified mutations N418A in IS6 and L975A in IIS6 as causing strong positive shifts in the voltage dependence of activation. In contrast, mutations V424A in IS6 and L983A in IIS6 caused strong negative shifts. Most IS6 mutations opposed inactivation from closed states, but most IIS6 mutations favored such inactivation. Mutations L421C and L983A near the intracellular ends of IS6 and IIS6, respectively, exhibited significant sustained Na(+) currents at the end of 30-ms depolarizations, indicating a role for these residues in Na(+) channel fast inactivation. These residues, in combination with residues at the intracellular end of IVS6, are well situated to form an inactivation gate receptor. Mutation I409A in IS6 reduced the affinity of the local anesthetic etidocaine for the inactivated state by 6-fold, and mutations I409A and N418A reduced use-dependent block by etidocaine. No IS6 or IIS6 mutations studied affected inactivated-state affinity or use-dependent block by the neuroprotective drug sipatrigine (compound 619C89). These results suggest that the local anesthetic receptor site is formed primarily by residues in segments IIIS6 and IVS6 with the contribution of a single amino acid in segment IS6.
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Affiliation(s)
- Vladimir Yarov-Yarovoy
- Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280, USA
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10
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Abstract
Cardiac sodium (Na) channels are dynamic molecules that undergo rapid structural changes in response to the changing electrical field in the myocardium. Inherited mutations in SCN5A, the gene encoding the cardiac Na channel, provoke life-threatening cardiac arrhythmias, often by modifying these voltage-dependent conformational changes. These disorders (i.e. the long QT syndrome and Brugada syndrome) may serve as valuable models for understanding the mechanistic linkages between Na channel dysfunction and cardiac arrhythmias in more common, acquired conditions such as cardiac ischemia. In addition, the balance between therapeutic and adverse effects from Na channel blockade by antiarrhythmic compounds may be shifted by subtle alterations in Na channel function. This review examines recent studies that tie key loci in the Na channel primary sequence to its dynamic function, while examining the emerging themes linking Na channel structure, function, and pharmacology to inherited and acquired disorders of cardiac excitability.
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Affiliation(s)
- J R Balser
- Department of Anesthesiology, Vanderbilt University School of Medicine, Nashville, TN 37232-6602, USA.
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Kambouris NG, Nuss HB, Johns DC, Marbán E, Tomaselli GF, Balser JR. A revised view of cardiac sodium channel "blockade" in the long-QT syndrome. J Clin Invest 2000; 105:1133-40. [PMID: 10772658 PMCID: PMC300835 DOI: 10.1172/jci9212] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Mutations in SCN5A, encoding the cardiac sodium (Na) channel, are linked to a form of the congenital long-QT syndrome (LQT3) that provokes lethal ventricular arrhythmias. These autosomal dominant mutations disrupt Na channel function, inhibiting channel inactivation, thereby causing a sustained ionic current that delays cardiac repolarization. Sodium channel-blocking antiarrhythmics, such as lidocaine, potently inhibit this pathologic Na current (I(Na)) and are being evaluated in patients with LQT3. The mechanism underlying this effect is unknown, although high-affinity "block" of the open Na channel pore has been proposed. Here we report that a recently identified LQT3 mutation (R1623Q) imparts unusual lidocaine sensitivity to the Na channel that is attributable to its altered functional behavior. Studies of lidocaine on individual R1623Q single-channel openings indicate that the open-time distribution is not changed, indicating the drug does not block the open pore as proposed previously. Rather, the mutant channels have a propensity to inactivate without ever opening ("closed-state inactivation"), and lidocaine augments this gating behavior. An allosteric gating model incorporating closed-state inactivation recapitulates the effects of lidocaine on pathologic I(Na). These findings explain the unusual drug sensitivity of R1623Q and provide a general and unanticipated mechanism for understanding how Na channel-blocking agents may suppress the pathologic, sustained Na current induced by LQT3 mutations.
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Affiliation(s)
- N G Kambouris
- Department of Anesthesiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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Chen Z, Ong BH, Kambouris NG, Marbán E, Tomaselli GF, Balser JR. Lidocaine induces a slow inactivated state in rat skeletal muscle sodium channels. J Physiol 2000; 524 Pt 1:37-49. [PMID: 10747182 PMCID: PMC2269847 DOI: 10.1111/j.1469-7793.2000.t01-1-00037.x] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
1. Local anaesthetics such as lidocaine (lignocaine) interact with sodium channels in a manner that is exquisitely sensitive to the voltage-dependent conformational state of the ion channel. When depolarized in the presence of lidocaine, sodium channels assume a long-lived quiescent state. Although studies over the last decade have localized the lidocaine receptor to the inner aspect of the aqueous pore, the mechanistic basis of depolarization-induced 'use-dependent' lidocaine block remains uncertain. 2. Recent studies have shown that lowering the extracellular Na+ concentration ([Na+]o) and mutations in the sodium channel outer P-loop modulate occupancy of a quiescent 'slow' inactivated state with intermediate kinetics (termed IM) that involves structural rearrangements in the outer pore. 3. Site-directed mutagenesis and ion-replacement experiments were performed using voltage-clamped Xenopus oocytes and cultured (HEK-293) cells expressing wild-type and mutant rat skeletal muscle (mu1) sodium channels. 4. Our results show that lowering [Na+]o potentiates use-dependent lidocaine block. The effect of [Na+]o is maintained despite a III-IV linker mutation that partially disrupts fast inactivation (F1304Q). In contrast, the effect of lowering [Na+]o on lidocaine block is reduced by a P-loop mutation (W402A) that limits occupancy of IM. 5. Our findings are consistent with a simple allosteric model where lidocaine binding induces channels to occupy a native slow inactivated state that is inhibited by [Na+]o.
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Affiliation(s)
- Z Chen
- Departments of Anesthesiology and Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
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Catterall WA. From ionic currents to molecular mechanisms: the structure and function of voltage-gated sodium channels. Neuron 2000; 26:13-25. [PMID: 10798388 DOI: 10.1016/s0896-6273(00)81133-2] [Citation(s) in RCA: 1542] [Impact Index Per Article: 64.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Affiliation(s)
- W A Catterall
- Department of Pharmacology, University of Washington, Seattle 98195, USA.
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Peter W, Mitrovic N, Schiebe M, Lehmann-Horn F, Lerche H. A human muscle Na+ channel mutation in the voltage sensor IV/S4 affects channel block by the pentapeptide KIFMK. J Physiol 1999; 518:13-22. [PMID: 10373685 PMCID: PMC2269396 DOI: 10.1111/j.1469-7793.1999.0013r.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
1. Whole cell patch clamping of transfected HEK293 cells was used to examine the effects of a pentapeptide (KIFMK) containing the proposed inactivation particle of the Na+ channel on two mutations causing myotonia. One mutation (R1448P) is located in the voltage sensor IV/S4, and the other one (G1306E) near the postulated inactivation gate within the III-IV linker. 2. In the absence of peptide, currents of wild-type (WT) and mutant human muscle Na+ channels decayed monoexponentially with inactivation time constants that were 5-fold (R1448P) and 3-fold (G1306E) larger for the mutants. Upon intracellular application of KIFMK (0.3-1 mM) the current decay became biexponential with an additional fast decaying component that increased in amplitude with depolarization. 3. Furthermore, the peptide induced large tail currents upon repolarization, indicating that KIFMK prevents inactivation by blocking open Na+ channels. The peak of this tail current decreased only slowly with depolarizations of increasing duration. The voltage dependence of this decline indicated that the dissociation rate of the charged peptide decreased with depolarization. Increased external [Na+] ([Na+]e) antagonized block by KIFMK, consistent with a pore-blocking mechanism. 4. The results are discussed with regard to a three-state model for one open, an absorbing inactivated and one blocked state with voltage-dependent on- and off-rates for peptide binding. The peptide had qualitatively similar effects on WT and both mutants, indicating that the freely diffusible peptide accelerates the current decay in all three clones. However, for the R1448P mutation the affinity for KFIMK was decreased and the voltage dependence of peptide block was changed in a similar way to the voltage dependence of inactivation. These data suggest that the mutation R1448P affects the voltage-dependent formation of a receptor site for both the inactivation particle and KIFMK.
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Affiliation(s)
- W Peter
- Department of Applied Physiology, University of Ulm, D-89069 Ulm, Germany
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Abstract
Computations on sodium channel gating were conducted using a closed-open-inactivated coupled kinetic scheme. The time constant of inactivation (tauh) derives a voltage dependency from coupling to voltage-dependent activation even when rate constants between inactivated and other states are strictly voltage independent. The derived voltage dependency does not require any physical, molecular link between the structures responsible for inactivation and the charges producing voltage-dependent activation. The only requirement is that the closed to inactivated rate constant (kCI) differs from the open to inactivated (kOI), consistent with experimental results. A number of mutations and other treatments uncouple sodium channel activation and inactivation in that the voltage dependency of tauh is substantially reduced while voltage-dependent activation persists. However, a clear basis for uncoupling has not been described. A variety of experimental results are accounted for just by changes in the difference between kOI and kCI. In wild type channels, kOI > kCI and inactivation develops with a delay whose time constant is just that for channel opening. Mutations that reduce the kOI - kCI difference reduce the amplitude of the delay process and the derived voltage dependency of tauh. If kOI = kCI, inactivation develops as a single exponential (no matter what the number of closed states), activation and inactivation become independent, parallel processes, and any voltage dependency of tauh is then entirely intrinsic to inactivation. If kOI < kCI, inactivation develops as the sum of exponentials, tauh at negative potentials speeds and then slows with more positive potentials. These predicted kOI < kCI effects have all been seen experimentally (O'Leary, M.E., L.-Q. Chen, R.G. Kallen, and R. Horn. 1995. J. Gen. Physiol. 106: 641-658). An open to closed rate constant of zero also removes the derived voltage dependency of tauh, but activation and inactivation are still coupled and the inactivation delay remains.
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Affiliation(s)
- L Goldman
- Department of Physiology, School of Medicine, University of Maryland, Baltimore, Maryland 21201, USA
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