doi: 10.1161/CIRCRESAHA.108.185090
© 2008 American Heart Association, Inc.
Editorials |
NFAT-Dependent Excitation–Transcription Coupling in Heart
From the Department of Physiology & Biophysics, University of Washington, Seattle.
Correspondence to Luis F. Santana, Department of Physiology & Biophysics, University of Washington, Box 357290, Seattle, WA 98195. E-mail santana{at}u.washington.edu
See related article, pages 733–742
Key Words: arrhythmias • calcium • L-type Ca2+ channels • Ito
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Introduction |
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 Top Introduction Step 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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Two articles from the laboratory of Stanley Nattel,1,2 one article in this issue of Circulation Research and an article in an upcoming issue of the journal, address an important, long-standing question in cardiac physiology: what are the molecular mechanisms underlying rate-dependent changes in the function of voltage-gated K+ and L-type Ca2+ channels in ventricular and atrial myocytes? Through a series of elegant experiments, they provide an interesting and unexpected answer to this difficult conundrum. In many ways, these 2 studies are excellent examples of how implementation of the problem-solving approach of Prof George Polya3 remains an invaluable tool to resolve complex problems in biology. Accordingly, I use a "Polyaesque" framework below to illustrate the importance and broad implications of the work by Xiao et al1 and Qi et al.2
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Step 1: Defining and Understanding the Problem |
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TopIntroductionStep 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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Multiples studies indicate that chronic increases in heart rate are associated with changes in the waveform of the action potential (AP) of atrial and ventricular myocytes. In atria, sustained, high-frequency electric activation rates are associated with shortening of the AP of atrial myocytes.4,5 This results in a decrease in the refractory period of the AP of atrial myocytes, which could increase the probability of atrial fibrillation, the most common cardiac arrhythmia. Interestingly, a decrease in depolarizing L-type Ca2+ channel current (ICa) function has been linked to decreased atrial AP duration during tachycardia.6,7 Downregulation of the transcript and protein levels of the pore-forming
subunit of L-type Ca2+ channels (Cav1.2) underlie decreased ICa function during atrial tachycardia and fibrillation. Like atrial myocytes, long-term tachycardia could also alter the waveform of the AP of ventricular myocytes and has been found to cause heart failure in canine.8 Chronic tachycardia decreases the amplitude of the transient outward K+ current (Ito), which alters phase 1 of the ventricular AP. This has the potential of altering excitation–contraction coupling9,10 and increasing arrhythmogenesis11 during heart failure. Rate-dependent decreases in Ito are caused, at least in part, by downregulation of transcript and protein levels of Kv4.312 channel subunits in human and canine ventricular myocytes.
As noted above, Xiao et al1 and Qi et al2 addressed a fundamental issue brought up by these findings: what are the signaling mechanisms that translate increased heart rate into downregulation of Cav1.2 and Kv4.3 genes in atrial and ventricular myocytes? Multiple lines of evidence suggest an answer to this important question. First, increasing AP firing rate increases [Ca2+]i in cardiac myocytes.13 Second, there are putative NFAT-binding sites in the promoter region of Cav1.2 and Kv4.3 genes.14,15 Third, activation of NFATc3 downregulates Ito in mouse ventricular myocytes.14,15 On the basis of these findings, Xiao et al1 and Qi et al2 hypothesized that rate-dependent changes in [Ca2+]i, as well as the activity of the Ca2+-dependent phosphatase calcineurin and the transcription factor NFATc3, could form part of a signaling cascade that downregulates Cav1.2 and Kv4.3 expression in atrial and ventricular myocytes during tachycardia.
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Step 2: Devising a Plan |
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TopIntroductionStep 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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To test their hypothesis, Xiao et al1 and Qi et al2 implemented a multidisciplinary, integrative approach. They used an in vitro (ie, cell culture) approach to submit atrial and ventricular myocytes to vary stimulation rates (0, 1, or 3 Hz) for 24 hours. Experiments involved the recording of [Ca2+]i, APs, ICa, and Ito and measurements of calcineurin activity and NFAT translocation in atrial and ventricular myocytes. Pharmacological tools were used to determine the role of Ca2+/calmodulin-dependent kinase (CaMK)II, calcineurin, NFATc3, and NFATc4 in Cav1.2 and Kv4.3 downregulation.
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Step 3: Carrying Out the Plan |
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TopIntroductionStep 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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Using these approaches, Xiao et al1 and Qi et al2 elegantly demonstrated that increasing the AP firing rate from 0 to 1 to 3 Hz increased [Ca2+]i, thereby activating calcineurin. In ventricular myocytes, but not atrial myocytes, calcineurin is activated through a CaMKII-dependent mechanism. Activation of calcineurin dephosphorylates NFATc3 and NFATc4, which allows these transcription factors to translocate into the nuclei of atrial (NFATc3 and NFATc4) and ventricular myocytes (NFATc3 only), in which they can modulate the expression of Cav1.2 (atrial myocytes) and the Kv4.3 (ventricular myocytes) genes. The data supporting this model are compelling. (1) Increasing AP firing rates from 0 to 1 to 3 Hz downregulated ICa and Ito function and Cav1.2 and Kv4.3 transcripts and proteins in atrial and ventricular myocytes. (2) It also increased diastolic and systolic [Ca2+]i in these cells, and (3) calcineurin activity was higher in cells stimulated at 3 Hz than at 1 Hz. In ventricular myocytes, but not atrial myocytes, rate-dependent changes in calcineurin activity required CaMKII function. (4) Finally, increases in AP frequency augmented NFATc3 (ventricular and atrial) and NFATc4 (atrial) translocation in atrial and ventricular myocytes.
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Step 4: Looking Back, Examination of the Results Obtained |
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TopIntroductionStep 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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The findings of Xiao et al1 and Qi et al2 are important because they establish, for the first time, a well-defined signaling pathway for the rate-dependent regulation of Cav1.2 and Kv4.3 expression in atrial and ventricular myocytes. However, the importance of these findings goes beyond these particular experimental conditions. Note that previous studies suggested a causal relationship between calcineurin/NFAT signal activation and the expression of specific voltage-gated ion channels in mouse ventricular myocytes.14,15 Yet, the role of this signaling pathway is not limited to cardiac myocytes. In smooth muscle, NFATc3 activation decreased expression of Kv2.115,16 and the β1 subunit of large conductance Ca2+-activated K+ channels17 during the development of hypertension. Together with the work of Xiao et al1 and Qi et al2 in canine atrial and ventricular myocytes, these studies support the provocative hypothesis that activation of NFATc3 may be a general mechanistic point of convergence among stimuli that regulate expression of voltage-gated ion channels in the cardiovascular system.
As with any good study, the impact of the work Xiao et al1 and Qi et al2 is not limited to the questions they answered, but the questions they raised. For example, whereas Xiao et al,1 as well as others,14,15 have linked NFAT activation to downregulation of Kv4.3 in adult ventricular myocytes, a recent study suggests that NFAT could upregulate Kv4 channels in neonatal ventricular myocytes.18 How could these seemingly contradictory findings be reconciled? One possibility is that NFAT associates with other transcription factors and binding partners, and whether NFAT upregulate or downregulate the expression of specific genes would depend on the molecular identity of the proteins this transcription factor associates with. Another question brought up by the work of Qi et al2 is why does NFATc3/c4 activation decreases Cav1.2 in atrial but not in ventricular myocytes? Future studies should address these important issues.
In addition to inducing Kv4.3 and Cav1.2 channel downregulation, activation of calcineurin/NFAT signaling causes hypertrophy.19 Inhibition of calcineurin decreases ventricular hypertrophy by up to 40% after myocardial infarction.20,21 In contrast, Xiao et al1 and Qi et al2 suggest that preventing calcineurin and NFAT activation completely prevents rate-dependent Ito and ICa remodeling. Given that calcineurin/NFAT inhibition only partially blocks the development of hypertrophy, whereas Ito and ICa downregulation is completely prevented, it is intriguing to speculate that there may be less redundancy in the pathways leading to Kv4.3 and Cav1.2 downregulation in atrial and ventricular myocytes.
Let us end by highlighting a central tenet in the problem-solving approach of Polya: that analyzing how others solved a particular problem could help answering new questions in the future. With this in mind, the work by Xiao et al1 and Qi et al2 forms part of the emerging field of excitation-transcription coupling.22 The approaches and concepts developed by Xiao et al1 and Qi et al2 represent an important contribution to this field and, if one is to follow the advice of Polya, should help to unravel and clearly define signaling pathways responsible for the regulation of the expression and function of voltage-gated ion channels in excitable cells under physiological and pathophysiological conditions.
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Acknowledgments |
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Sources of Funding
Supported by NIH grant HL085686. L.F.S. is an Established Investigator of the American Heart Association.
Disclosures
None.
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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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References |
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TopIntroductionStep 1: Defining and...Step 2: Devising a...Step 3: Carrying Out...Step 4: Looking Back,...References |
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Related Article:
Circ. Res. 2008 103: 733-742.
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