© 2000 American Heart Association, Inc.
Editorials |
Anchors Aweigh!
Ion Channels, Cytoskeletal Proteins, and Cellular Excitability
From Merck Research Laboratories, West Point, Pa.
Correspondence to Paul Bennett, PhD, Senior Director Ion Channel Pharmacology, WP 26-265, Merck Research Laboratories, 770 Sumneytown Pike, West Point, PA 19486. E-mail paul_bennett{at}merck.com
Key Words: Na+ channel • long-QT syndrome • antiarrhythmic agent • molecular biology • genetics
 |
Introduction |
|---|
 Top Introduction Ion Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
Genomics, proteomics, transgenics, molecular medicine: these are some of the scientific catch phrases of the 1990s. The hard work and high expectations of the past decade are beginning to influence reality. Results have accrued to the point that we can begin applying molecular knowledge to therapeutics. Although still in the formative stages, one cannot help but see the vast potential of the exponentially growing molecular knowledge base for understanding physiology and pathophysiology.
|
Ion Channel Disorders |
|---|
|
TopIntroductionIon Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
In recent years, increasing numbers of ion channelopathies—disorders involving mutations in ion channel genes—have been recognized. These disorders include periodic paralyzes, migraine, ataxias, epilepsy, and cardiac arrhythmias to name a few. Research efforts have been directed toward identifying the candidate ion channel genes and their mutations and understanding the functional consequences of these mutations. In many cases, the results have been highly rewarding with a biophysical phenotype that easily correlates with the ultimate clinical phenotype.1 There are also cases where mutations in channel proteins, all of which are known to lead to a clinical disorder, do not have, as yet, a phenotype that is consistent with an interpretable hypothesis. For example, in familial hemiplegic migraine, individual mutations in a Ca2+ channel
subunit gene can apparently
cause either loss or gain of function, depending on the
mutation.2 3 4 5 6 Perhaps different mutant channels behave
differently in their native environment and when interacting with
auxiliary proteins. Perhaps additional analysis will reveal a
mechanism. Yet at present, it is challenging to reconcile this
disparate behavior with the common clinical phenotype of
migraine. Presumably, this results from our as-yet limited knowledge of
the workings of this system and the role of Ca2+
channels in migraine. There are other examples of excitability disorders where there is no candidate gene. Five clinical variants of the cardiac long-QT syndrome (LQTS) have been identified, four of which result from mutations in voltage-gated ion channels.7 8 9 In most cases, the biophysical phenotype of the mutated channels correlates with the resulting clinical phenotype of prolonged QT intervals. LQT4 stands alone, and there are no known ion channel genes or mutations that correlate with this clinical variant. Furthermore, there are no known candidate ion channel genes in the segment of chromosome 4 linked to LQT4.
|
LQTS and Ankyrin |
|---|
|
TopIntroductionIon Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
In this issue of Circulation Research, Chauhan et al10 provide new data that may help address this difficult problem. Their article emphasizes the need to look beyond ion channels per se for mutations in modifier genes. Their study further focuses on the importance of an ion channel’s environment and the proteins with which it interacts. Chauhan et al10 have used state-of-the-art molecular and biophysical techniques to elucidate molecular events related to this clinical disorder by investigating the role of ankyrinB in Na+ channel behavior in mouse heart. Although Chauhan et al10 do not address specifically whether mutations in ankyrinB cause LQT4, the authors note the interesting coincidence that ankyrinB gene is located in the same human chromosomal region as LQT4. They demonstrate that ankyrinB knockout mouse Na+ channels have behavior consistent with LQT4 and therefore conclude that this might be a mechanism. This is a compelling, although not yet fully proven, idea.
Very little is known about this LQTS variant. Schott et al11 studied a 65-member family in which LQTS was associated with marked sinus bradycardia. Linkages to chromosomes 3 (LQT3), 7 (LQT2), and 11 (LQT1) were excluded. Genetic linkage was observed for a region of chromosome 4 located in the interval 4q25-q27. On the basis of current information, this region of human chromosome 4 contains few possible LQTS candidate genes. Genes that could conceivably play a role include phosphodiesterase 5A (4q26, PDE5A, OMIM No. 603310) and ANK2 (4q25-q27, ankyrin-2, ankyrinB OMIM No. 106410). The fibroblast growth factor-2 gene is in this region (4q25-q27 FGF2, FGFB OMIM No. 134920), but it is difficult to speculate how it might have a role.
|
Ion Channels and the Cytoskeleton |
|---|
|
TopIntroductionIon Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
Increasingly, it is recognized that ion channels are not isolated, functionally discrete islands in the vast sea of membrane lipids. Rather, they exist within highly coordinated mechanistic assemblies that include associated proteins. The roles of these associated proteins include feedback ion sensing, enzymatic activity, as well as helping to coordinate, localize, and target the assembly. A number of studies have associated ankyrins with ion channels in the nervous system, and ankyrin domains have been identified in ion channels,12 13 14 15 16 providing a conceivable link between ankyrins and LQT4, although other ankyrin-associated proteins could be implicated.
Ankyrins are a family of membrane-associated proteins that can be
divided into distinct groups that include erythrocyte-related isoforms
(ankyrinR) that show polarized distributions in
particular cells and so-called brain-related forms
(ankyrinB, ANK2) that display a broader
distribution. Tse et al17 isolated a human gene for
nonerythroid ankyrin, designated ANK2 on chromosome 4q25-q27. Ankyrins
are spectrin binding proteins that serve as adapters to coordinate
localization and assembly of interacting signaling
proteins.18 Ankyrins link ion channels and cell adhesion
molecules to the spectrin-based cytoskeleton and localize them to
specialized membrane domains. Spectrin is a rod-shaped protein
consisting of homologous and ß subunits. The ß subunit has an
actin binding domain and an ankyrin binding domain. Thus, ankyrin and
spectrin participate in cellular structural integrity as well as
facilitating the localization and interactions of communities of
signaling molecules including clustering of ion channels in excitable
membranes.12 16 The manipulation of the cytoskeleton of
cells, including cardiac cells, is known to modulate channel
function.12 14 15 16 19 20 With this emerging knowledge, it
is tantalizing to speculate that changes in any of the proteins within
such a complex may alter cardiac myocyte excitability through
modification of channel behavior.
|
Ankyrin Disruption Affects Na+ Cardiac Channels |
|---|
|
TopIntroductionIon Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
The study by Chauhan et al10 describes the properties of the cardiac Na+ channel in neonatal mouse heart cells lacking ankyrinB ]ankyrinB (-/-) mice[. AnkyrinB (-/-) mice have lower Na+ channel current density than wild-type mice, possibly indicating fewer cell surface Na+ channels. The voltage dependence of channel gating was altered as well, and recovery of Na+ channel inactivation was slower in the ankyrinB (-/-) mouse cells. Na+ channel open times were longer, and the channels exhibited uncharacteristic late openings at some membrane potentials. These behaviors are distinct from those of channels in wild-type mouse cells. Finally, the time to complete repolarization during the cardiac action potential was increased, possibly as a result of inappropriate opening of Na+ channels. Thus, in some ways, the functional consequences of disruption of interactions between the channel and ankyrin resemble the actions of a sodium channel blocker (slowed recovery from inactivation and reduced Na+ current density). Yet in other ways, this disruption causes the channels to behave like other LQT3 mutant Na+ channels.1 This "double hit" is predicted to be arrhythmogenic by virtue of both a prolonged QT interval and slowed conduction.
Some caution must be exercised when interpreting these results, however. It should be noted that these mice [ankyrinB (-/-)] are severely deranged with musculoskeletal defects, myopathy, and disruption of proteins involved in Ca2+ homeostasis.21 The mice show defects in the immune and nervous systems. Approximately 80% die by postnatal day 1, and 100% die within 21 days. Thus, there are numerous altered processes that could affect Na+ channel behavior. We can assume that if ankyrin is mutated in LQT4, the effects of the mutations must be much more subtle than those in the knockout mouse. Nevertheless, these results are exciting, and they demonstrate an important interaction between ankyrinB and the cardiac Na+ channel. They further emphasize the role that other proteins play in channel behavior and indicate that other candidate genes must always be considered as possible modifier genes.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
|
TopIntroductionIon Channel DisordersLQTS and AnkyrinIon Channels and the...Ankyrin Disruption Affects Na+...References |
|---|
1. Bennett PB, Yazawa K, Makita N, George AL Jr. Molecular mechanism for an inherited cardiac arrhythmia. Nature. 1995;376:683–685.[Medline] [Order article via Infotrieve]
2. Jen J. Calcium channelopathies in the central nervous system. Curr Opin Neurobiol. 1999;9:274–280.[Medline] [Order article via Infotrieve]
3.
Kraus RL, Sinnegger MJ, Glossmann H, Hering S,
Striessnig J. Familial hemiplegic migraine mutations change
1A Ca2+ channel
kinetics. J Biol Chem. 1998;273:5586–5590.
4.
Nyholt DR, Lea RA, Goadsby PJ, Brimage PJ, Griffiths
LR. Familial typical migraine: linkage to chromosome 19p13 and evidence
for genetic heterogeneity. Neurology. 1998;50:1428–1432.
5.
Hans M, Luvisetto S, Williams ME, Spagnolo M, Urrutia
A, Tottene A, Brust PF, Johnson EC, Harpold MM, Stauderman KA,
Pietrobon D. Functional consequences of mutations in the human
1A calcium channel subunit linked to familial
hemiplegic migraine. J Neurosci. 1999;19:1610–1619.
6.
Battistini S, Stenirri S, Piatti M, Gelfi C, Righetti
PG, Rocchi R, Giannini F, Battistini N, Guazzi GC, Ferrari M, Carrera
P. A new CACNA1A gene mutation in acetazolamide-responsive
familial hemiplegic migraine and ataxia. Neurology. 1999;53:38–43.
7. Keating MT, Sanguinetti MC. Molecular genetic insights into cardiovascular disease. Science. 1996;272:681–685.[Abstract]
8. Vincent GM. The molecular genetics of the long QT syndrome: genes causing fainting and sudden death. Annu Rev Med. 1998;49:263–274.[Medline] [Order article via Infotrieve]
9. Roden DM, George AL Jr, Bennett PB. Recent advances in understanding the molecular mechanisms of the long QT syndrome. J Cardiovasc Electrophysiol. 1995;6:1023–1031.[Medline] [Order article via Infotrieve]
10.
Chauhan VS, Tuvia S, Buhusi M, Bennett V, Grant AO.
Abnormal cardiac Na+ channel properties and QT
heart rate adaptation in neonatal ankyrinB
knockout mice. Circ Res. 2000;86:441–447.
11. Schott JJ, Charpentier F, Peltier S, Foley P, Drouin E, Bouhour JB, Donnelly P, Vergnaud G, Bachner L, Moisan JP. Mapping of a gene for long QT syndrome to chromosome 4q25–27. Am J Hum Genet. 1995;57:1114–1122.[Medline] [Order article via Infotrieve]
12. Srinivasan Y, Elmer L, Davis J, Bennett V, Angelides K. Ankyrin and spectrin associate with voltage-dependent sodium channels in brain. Nature. 1988;333:177–180.[Medline] [Order article via Infotrieve]
13.
Zhou D, Lambert S, Malen PL, Carpenter S, Boland LM,
Bennett V. AnkyrinG is required for clustering of
voltage-gated Na channels at axon initial segments and for normal
action potential firing. J Cell Biol. 1998;143:1295–1304.
14.
Wood SJ, Slater CR. ß-Spectrin is colocalized
with both voltage-gated sodium channels and
ankyrinG at the adult rat neuromuscular junction.
J Cell Biol. 1998;140:675–684.
15.
Undrovinas AI, Shander GS, Makielski JC. Cytoskeleton
modulates gating of voltage-dependent sodium channel in heart.
Am J Physiol. 1995;269:H203–H214.
16.
Srinivasan Y, Lewallen M, Angelides KJ. Mapping the
binding site on ankyrin for the voltage-dependent sodium channel from
brain. J Biol Chem. 1992;267:7483–7489.
17. Tse WT, Menninger JC, Yang-Feng TL, Francke U, Sahr KE, Lux SE, Ward DC, Forget BG. Isolation and chromosomal localization of a novel nonerythroid ankyrin gene. Genomics. 1991;10:858–866.[Medline] [Order article via Infotrieve]
18. Lambert S, Bennett V. From anemia to cerebellar dysfunction. A review of the ankyrin gene family. Eur J Biochem. 1993;211:1–6.[Medline] [Order article via Infotrieve]
19.
Maltsev VA, Undrovinas AI. Cytoskeleton modulates
coupling between availability and activation of cardiac sodium channel.
Am J Physiol. 1997;273:H1832–H1840.
20.
Rasband MN, Peles E, Trimmer JS, Levinson SR, Lux SE,
Shrager P. Dependence of nodal sodium channel clustering on paranodal
axoglial contact in the developing CNS. J Neurosci. 1999;19:7516–7528.
21.
Scotland P, Zhou D, Benveniste H, Bennett V. Nervous
system defects of ankyrinB (-/-) mice suggest
functional overlap between the cell adhesion molecule L1 and 440-kD
ankyrinB in premyelinated axons.
J Cell Biol. 1998;143:1305–1315.
This article has been cited by other articles:
 |
|
 |
|
  J. M. Nerbonne and R. S. Kass Molecular Physiology of Cardiac Repolarization Physiol Rev, October 1, 2005; 85(4): 1205 - 1253. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Brette, K. Komukai, and C. H. Orchard Validation of formamide as a detubulation agent in isolated rat cardiac cells Am J Physiol Heart Circ Physiol, October 1, 2002; 283(4): H1720 - H1728. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 X. Yang, P. J I Salas, T. V Pham, B. J Wasserlauf, M. J D Smets, R. J Myerburg, H. Gelband, B. F Hoffman, and A. L Bassett Cytoskeletal actin microfilaments and the transient outward potassium current in hypertrophied rat ventriculocytes J. Physiol., June 1, 2002; 541(2): 411 - 421. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||