Circulation Research. 2005;97:1213-1215
doi: 10.1161/01.RES.0000196744.62327.36
(Circulation Research. 2005;97:1213.)
© 2005 American Heart Association, Inc.
The Fire From Within
The Biggest Ca2+ Channel Erupts and Dribbles
Mark E. Anderson
From the University of Iowa, Carver College of Medicine, Iowa City.
Correspondence to Mark E. Anderson, MD, PhD, University of Iowa, Carver College of Medicine, 200 Hawkins Drive, Room E 315 GH, Iowa City, IA 53342-1081. E-mail mark-e-anderson{at}uiowa.edu
See related article, pages 1314–1322
Key Words: calmodulin kinase II • ryanodine receptor • heart failure • arrhythmias
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CaMKII Is a Pluripotent Signaling Molecule in Heart
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The multifunctional Ca
2+ and calmodulin (CaM)-dependent protein
kinase II (CaMKII) is a serine threonine kinase that is abundant
in heart where it phosphorylates Ca
2+i homeostatic proteins.
It seems likely that CaMKII plays an important role in cardiac
physiology because these target proteins significantly overlap
with the more extensively studied serine threonine kinase, protein
kinase A (PKA), which is a key arbiter of catecholamine responses
in heart. However, the physiological functions of CaMKII remain
poorly understood, whereas the potential role of CaMKII in signaling
myocardial dysfunction and arrhythmias has become an area of
intense focus. CaMKII activity and expression are upregulated
in failing human hearts and in many animal models of structural
heart disease.
1 CaMKII inhibitory drugs can prevent cardiac
arrhythmias
2,3 and suppress afterdepolarizations
4 that are a
probable proximate focal cause of arrhythmias in heart failure.
CaMKII inhibition in mice reduces left ventricular dilation
and prevents disordered intracellular Ca
2+ (Ca
2+i) homeostasis
after myocardial infarction.
5 CaMKII overexpression in mouse
heart causes severe cardiac hypertrophy, dysfunction, and sudden
death that is heralded by increased SR Ca
2+ leak
6; these findings
go a long way to making a case for CaMKII as a causative signal
in heart disease and arrhythmias but do not identify critical
molecular targets or test the potential role of CaMKII in a
large non-rodent animal model. The work by Ai et al in this
issue of
Circulation Research makes an important contribution
by demonstrating CaMKII upregulation causes increased Ca
2+ leak
from ryanodine receptor (RyR) Ca
2+ release channels in a clinically-relevant
model of structural heart disease.
7
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Ryanodine Receptors Are Central
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Ca
2+i release controls cardiac contraction, and most of the
Ca
2+i for contraction is released from the intracellular sarcoplasmic
reticulum (SR) through ryanodine receptors (RyR). RyRs are huge
proteins (565 kDa) that assemble with a fourfold symmetry to
form a functional Ca
2+ release channel. Approximately 90% of
the RyR is not directly required to form the pore but instead
protrudes into the cytoplasm where it binds numerous proteins,
including PKA, CaMKII, CaM, and FK12.6 (calstabin). Cardiac
contraction is initiated when Ca
2+ current (
ICa), through sarcolemmal
L-type Ca
2+ channels (LTCC), triggers RyR opening by a Ca
2+-induced
Ca
2+ release (CICR) mechanism. LTCCs "face off" with RyRs across
a highly ordered cytoplasmic cleft that delineates a kind of
Ca
2+ furnace during each CICR-initiated heart beat (
Figure).
CICR has an obvious need to function reliably, so it is astounding
to consider how this feed forward process is intrinsically unstable.
The increased instability of CICR in heart failure is directly
relevant to arrhythmias initiated by afterdepolarizations. RyRs
partly rely on a collaboration of Ca
2+-sensing proteins in the
SR lumen to grade their opening probability and the amount of
SR Ca
2+ release to a given
ICa stimulus. Thus the SR Ca
2+ content
is an important parameter for setting the inotropic state, and
heart failure is generally a condition of reduced SR Ca
2+ content
and diminished myocardial contraction.
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Ca2+-induced Ca2+ release (CICR) in health and disease. Each heart beat is initiated by cell membrane depolarization that opens Ca2+ channels. The Ca2+ current (ICa) induces ryanodine receptor (RyR) opening that allows release of myofilament activating Ca2+ for contraction. In healthy CICR, RyRs close during diastole while Ca2+ is removed from the cytoplasm by uptake into the sarcoplasmic reticulum (SR). In heart failure the SR has reduced Ca2+ content so that the amount of Ca2+ released to the myofilaments is smaller than in health. RyR hyperphosphorylation by CaMKII promotes repetitive RyR openings leading to a Ca2+ leak in diastole. This leak contributes to the reduction in SR Ca2+ content and can engage the electrogenic Na+-Ca2+ exchanger to trigger afterdepolarizations and arrhythmias.
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Kinases Facilitate Communication Between LTCCs and RyRs
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LTCCs and RyRs form the protein machinery for initiating contraction
in cardiac and skeletal muscle, but in cardiac muscle communication
between these proteins occurs without a requirement for physical
contact. PKA is preassociated with LTCCs and RyRs, and PKA-dependent
phosphorylation increases LTCC
8 and RyR
9opening. The resultant
increase in Ca
2+i is an important reason for the positive inotropic
response to cathecholamines. The multifunctional Ca
2+/calmodulin-dependent
protein kinase II (CaMKII) is activated by increased Ca
2+I,
and so catecholamine stimulation activates CaMKII in addition
to PKA.
5 In contrast to PKA, which is tightly linked to inotropy,
CaMKII inhibition does not cause a reduction in fractional shortening
during acute catecholamine stimulation in mice.
5 Prolonged catecholamine
exposure does reduce contractile function by uncertain mechanisms
that require CaMKII.
10 CaMKII colocalizes with LTCCs
11 and RyRs,
12 and CaMKII can also increase LTCC
13 and RyR
12 opening probability
in cardiac myocytes. The ultrastructural environment of LTCCs
and RyRs is well-suited for a Ca
2+i-responsive kinase to serve
as a coordinating signal between LTCCs and RyRs during CICR.
The recently identified role of CaMKII in heart failure suggests
the possibility that excessive CaMKII activity could cause or
contribute to CICR defects present in heart failure.
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Heart Failure Is a Disease of Disordered Ca2+i Homeostasis
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The key clinical phenotypes of contractile dysfunction and electrical
instability in heart failure involve problems with Ca
2+i homeostasis.
Broad changes in Ca
2+I-handling proteins can occur in various
heart failure models, but in general heart failure is marked
by a reduction in the capacity for SR Ca
2+ uptake, enhanced
activity of the sarcolemmal Na
+-Ca
2+ exchanger, and reduction
in CICR-coordinated SR Ca
2+ release. On the other hand, the
opening probability of individual LTCCs is increased in human
heart failure,
14 suggesting that posttranslational modifications
may also be mechanistically important for understanding these
Ca
2+i disturbances at Ca
2+ homeostatic proteins.
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Is Heart Failure a Disease of Enzymatic Over-Activity?
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Heart failure is marked by hyper-adrenergic tone, and beta adrenergic
receptor antagonist drugs (beta blockers) are a mainstay of
therapy for reducing mortality in heart failure patients. The
Marks group pioneered the concept that RyRs are hyperphosphorylated
by PKA in patients with heart failure and showed that successful
therapies, ranging from beta blockers to left ventricular assist
devices, reduce RyR phosphorylation in step with improved mechanical
function. They have developed a large body of evidence in patients
and in animal models that PKA phosphorylation of Ser2809 on
cardiac RyRs destabilizes binding of FK12.6 to RyRs and promotes
increased RyR opening that causes an insidious Ca
2+ leak. This
leak is potentially problematic because it can reduce SR Ca
2+ content (to depress inotropy), engage pathological Ca
2+-dependent
transcriptional programs (to promote myocyte hypertrophy), and
activate arrhythmia-initiating afterdepolarizations (to cause
sudden death). Indeed, RyR hyperphosphorylation can produce
arrhythmias as well as mechanical dysfunction, whereas a drug
that prevents FK12.6 dissociation from RyR also reduces or prevents
arrhythmias.
15 Taken together these findings make a strong case
that RyR hyperphosphorylation (a result of net excess kinase
activity) is a central event in heart failure and sudden death.
Not all findings point to hyperphosphorylation of RyR by PKA and subsequent FK12.6 dissociation as critical determinants of heart failure16 and arrhythmias.17 For example, studies in isolated and permeabilized ventricular myocytes failed to show an increase in RyR openings, called sparks, which are monitored by photoemission of a Ca2+-sensitive fluorescent dye.18 FKBP12.6 dissociation is not universally reported to follow RyR phosphorylation by PKA.19 Furthermore, FKBP12.6 binding to RyR is not affected during catecholamine stimulation that results in arrhythmias in a mouse model of catecholamine-induced ventricular tachycardia,20,21 a genetic disorder of hypersensitive RyR Ca2+ release. These findings challenge the PKA hypothesis and make room, conceptually, to consider the role of additional signals for modulating RyR activity in heart disease.
Both PKA and CaMKII may phosphorylate Ser2809, but recently CaMKII was found to exclusively phosphorylate Ser2815 and this phosphorylation caused increased RyR opening.12 However, the PKA and CaMKII responses may be mechanistically distinct because CaMKII evoked increased RyR opening in the absence of FK12.6 dissociation. These findings together with the fact that CaMKII activity is recruited under conditions of increased PKA activity suggest that CaMKII might also be important in regulating RyRs in heart failure.
The article by Ai et al shows that expression of a CaMKII splice variant that is resident in cytoplasm (CaMKII
c) was increased, and there was enhanced phosphorylation of the recently identified CaMKII site (Ser2815) on RyR. Both Ser2815 and the PKA site (Ser2809) were hyperphosphorylated in failing hearts, but phosphorylation of the CaMKII site was greater than the PKA site. Because both Ser2809 and Ser2815 can increase RyR openings, it seemed likely that PKA and CaMKII would work together to increase Ca2+ leak. Surprisingly, CaMKII inhibition but not PKA inhibition suppressed the leak. These experiments were performed with meticulous attention to matching SR Ca2+ load, a technically difficult accomplishment that is not performed by most groups evaluating SR Ca2+ release. Thus, differences in the SR intraluminal Ca2+ could not account for these findings. Although these experiments were carefully controlled, one potential limitation is that the experiments relied exclusively on CaMKII and PKA inhibitor drugs that are notorious for nonspecific actions at ion channel proteins. They also showed that the ratio of inositol tris phosphate receptors (IP3R) to RyRs was increased in failing left ventricular myocytes. IP3R are important for regulating Ca2+i in many cells types, including atrial myocytes, but their role in ventricle remains uncertain. The finding that the IP3R are increased at the expense of RyR suggests that Ca2+i release sites are fundamentally reordered in heart failure but leaves the impact of this change untested. IP3R are also a target for CaMKII, so interesting questions remain about the potential role for this channel and CaMKII in heart failure, at least in this model.
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What We Learned and What We Need to Know
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CaMKII activity seems to be part and parcel of the adrenergic
signaling seen in structural heart disease. This work shows
us that CaMKII can contribute directly to increased SR Ca
2+ leak in a clinically relevant model of heart failure that is
marked by arrhythmias and sudden death.
22 Acute experiments
with CaMKII inhibitory drugs strongly suggest that SR Ca
2+ leak
is principally linked to CaMKII rather than PKA activity. Excessive
SR Ca
2+ release can activate inward (forward mode) Na
+-Ca
2+ exchanger current to cause delayed afterdepolarizations and
arrhythmias and CaMKII inhibition can prevent these inward Na
+-Ca
2+ exchanger currents.
23 An important next step toward translating
these findings will be to evaluate the effects of chronic CaMKII
inhibition in this model to see whether it reverses cardiac
dysfunction, arrhythmias, and whether chronic CaMKII inhibitor
therapy can stop the RyR leak to refill the SR. It will be necessary
to have improved pharmacological agents with fewer nonspecific
effects to convincingly perform these experiments. These future
experiments will tell us whether CaMKII inhibition is a potentially
viable therapy for structural heart disease and arrhythmias
in a non-genetic non-mouse model. We need to know whether CaMKII
inhibition is really a highly-specific form of beta blockade
that can preserve inotropic responses to catecholamines while
preventing the adverse consequences of catecholamines in heart
failure.
5
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Acknowledgments
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This work was supported in part by grants from the National
Institutes of Health (HL070250, HL62494, and HL046681). Dr Anderson
is an Established Investigator of the American Heart Association.
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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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Related Article:
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Ca2+/Calmodulin–Dependent Protein Kinase Modulates Cardiac Ryanodine Receptor Phosphorylation and Sarcoplasmic Reticulum Ca2+ Leak in Heart Failure
- Xun Ai, Jerry W. Curran, Thomas R. Shannon, Donald M. Bers, and Steven M. Pogwizd
Circ. Res. 2005 97: 1314-1322.
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CaMKII Is a Pluripotent...
Ryanodine Receptors Are Central