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(Circulation Research. 2000;86:367.)
© 2000 American Heart Association, Inc.

Editorials

Anchors Aweigh!

Ion Channels, Cytoskeletal Proteins, and Cellular Excitability

Paul B. Bennett

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 Disorders LQTS and Ankyrin Ion 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

Top Introduction Ion Channel Disorders LQTS and Ankyrin Ion 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

Top Introduction Ion Channel Disorders LQTS and Ankyrin Ion 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 ankyrin_B in Na⁺ channel behavior in mouse heart. Although Chauhan et al¹⁰ do not address specifically whether mutations in ankyrin_B cause LQT4, the authors note the interesting coincidence that ankyrin_B gene is located in the same human chromosomal region as LQT4. They demonstrate that ankyrin_B 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, ankyrin_B 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

Top Introduction Ion Channel Disorders LQTS and Ankyrin Ion 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 (ankyrin_R) that show polarized distributions in particular cells and so-called brain-related forms (ankyrin_B, 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

Top Introduction Ion Channel Disorders LQTS and Ankyrin Ion 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 ankyrin_B ]ankyrin_B (-/-) mice[. Ankyrin_B (-/-) 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 [ankyrin_B (-/-)] 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 ankyrin_B 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

Top Introduction Ion Channel Disorders LQTS and Ankyrin Ion Channels and the... Ankyrin Disruption Affects Na+... References

 

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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.[Abstract/Free Full Text]
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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]
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14. Wood SJ, Slater CR. ß-Spectrin is colocalized with both voltage-gated sodium channels and ankyrin_G at the adult rat neuromuscular junction. J Cell Biol. 1998;140:675–684.[Abstract/Free Full Text]
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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]
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