Circulation Research. 2000;86:367-368
(Circulation Research. 2000;86:367.)
© 2000 American Heart Association, Inc.
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
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Introduction
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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.
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Ion Channel Disorders
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In recent years, increasing numbers of ion
channelopathiesdisorders
involving mutations in ion channel
geneshave 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
Ca
2+ 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 Ca
2+
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.
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LQTS and Ankyrin
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In this issue of
Circulation Research, Chauhan
et al
10 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 channels environment and
the
proteins with which it interacts. Chauhan et al
10 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 al
10
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,
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.
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Ion Channels and the Cytoskeleton
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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.
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Ankyrin Disruption Affects Na+ Cardiac
Channels
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The study by Chauhan et al
10 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
[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.
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Footnotes
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The opinions expressed in this editorial are not necessarily
those of the editors or of the American Heart Association.
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