This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by London, B. Search for Related Content PubMed PubMed Citation Articles by London, B. Related Collections Related Article

(Circulation Research. 2006;99:1027.)
© 2006 American Heart Association, Inc.

Editorials

CaM Kinase Inhibition

Apply Directly to the Heart?

Barry London

From the Cardiovascular Institute, University of Pittsburgh, Pa.

Correspondence to Barry London, MD, PhD, Cardiovascular Institute, University of Pittsburgh, Scaife S572, 200 Lothrop Street, Pittsburgh, PA 15213-2582. Email londonb{at}upmc.edu

See related article, pages 1092–1099

Key Words: calcium • calmodulin • CaM kinase • arrhythmias • heart

Calcium/calmodulin-dependent protein kinases (CaM kinases) encompass a family of serine/threonine kinases (CaMKI-CaMKIV) that are regulated by calcium bound to calmodulin.^1,2 Four separate genes encode the CaMKII subunits (, ß, , ), and 6 to 12 subunits homo- or heteromultimerize to form the active enzyme. Each subunit contains an N-terminal catalytic domain which binds ATP and substrate, a regulatory domain which includes an autoinhibitory domain and a Ca/Calmodulin binding domain, and a C-terminal association (multimerization) domain. In the heart, a number of splice variants of CaMKII are expressed, including CaMKII₂ (CaMKII_C) and CaMKII₃ (CaMKII_B) which differ from each other by the presence of a nuclear localization signal between the regulatory domain and the association domain in CaMKII₃.³

CaMKII functions as a local calcium sensor in the heart. At baseline, the autoinhibitory domain prevents substrate binding to the enzyme.^1,2 In cardiac myocytes, intracellular Ca²⁺ [Ca²⁺]_i rises because of transmembrane influx through L-type Ca²⁺ channels (I_Ca) or the Na/Ca exchanger and release from internal stores such as the sarcoplasmic reticulum (SR). When intracellular Ca²⁺ rises, it binds to the EF-hand motifs at the N- and C-terminals calmodulin, an 150 amino acid ubiquitous intracellular protein. The Ca/CaM complex then binds to the regulatory domain of CaMKII and removes the inhibition of the autoinhibitory domain. Of note, activated CaMKII can be autophosphorylated at Thr287, which allows the enzyme to remain in the active state in the absence of an elevated intracellular Ca²⁺. Ultimately, dephosphorylation by intracellular phosphatases returns the enzyme to the inactive state.

A number of cellular targets for CaMKII have been identified using in vitro expression systems, CaMKII inhibitors in vivo, and transgenic mice. Nuclear CaMKII₃ activates a hypertrophic program via transcription factors such as CREB directly by phosphorylation and MEF2 indirectly by phosphyorylating class II histone deacetylases to release its inhibition.^4,5 Cytoplasmic CaMKII₂ associates with the L-type Ca²⁺ channel to allow Ca²⁺-dependent facilitation (an increase in I_Ca at increased stimulation frequencies) and phosphorylates the ryanodine receptor to increase SR leak.^6,7 Thus, CaMKII activation can affect intracellular Ca²⁺ and alter its own activity.

Transgenic mice overexpressing of either CaMKII₂ or CaMKII₃ in the heart develop a dilated cardiomyopathy, with the phenotype being more severe for the cytoplasmic isoform.^8,9 Transgenic overexpression of the CaMKII inhibitory peptide AC3-I, on the other hand, improves cardiac function and Ca²⁺ handling following myocardial infarction, protects against isoproterenol-induced hypertrophy and LV dysfunction, decreases phospholamban (PLN) phosphorylation at Thr17 and SR Ca²⁺ load, increases basal I_Ca, and eliminates Ca²⁺-dependent facilitation.¹⁰ Thus, CaMKII lies downstream from and parallels activation by the ß-adrenergic nervous system, suggesting that CaMKII blockade may have beneficial effects similar to ß-adrenergic blockers. In addition, CaMKII overexpression alters Ca²⁺ handling and ion channels in a way that would promote afterdepolarizations and arrhythmias. In a calcineurin overexpression mouse heart failure model, CaMKII suppression by KN93 reduces spontaneous arrhythmias, whereas CaMKII suppression by overexpression of AC3-I improves LV function and suppresses arrhythmias in Langendorff-perfused hearts.¹¹

In the current issue of Circulation Research, Li et al study in detail the electrical remodeling in transgenic mice overexpressing the CaMKII inhibitory peptide AC3-I.¹² AC3-I transgenic mice have shorter action potential durations (APDs) and QT intervals despite increased I_Ca because of upregulation of the inward rectifier current I_K1 and the transient outward current I_to,f, with no significant changes in channel RNA or protein expression. These electrophysiological changes result from chronic AC3-I overexpression and are not reversed by modification of PKA activity nor mimicked by transient treatment of wild-type myocytes with AC3-I. Of note, the remodeling is eliminated when the AC3-I mice are crossed with PLN knockout mice, demonstrating that CaMKII-mediated alterations in PLN and, presumably, SR Ca²⁺ uptake/release are required for the electrical remodeling.

These findings extend our understanding of electrical remodeling in the heart. Most animal models and human studies of heart failure show APD prolongation because of decreases in outward K⁺ currents (usually I_to and I_K1) or changes in I_Ca and its inactivation.¹³ APD prolongation could prolong I_Ca and preserve contractile force, although it increases the risk of triggered arrhythmias and could contribute to Ca²⁺ overload. The study by Li et al demonstrates that CaMKII inhibition at physiologically relevant levels can upregulate repolarizing K⁺ currents. This raises the possibility that manipulations that inhibit CaMKII could both improve ventricular function and reverse maladaptive electrical remodeling in conditions such as heart failure. In other words, CaMKII inhibition may be antiarrhythmic in a way analogous to ß-adrenergic blockade.

The mechanism by which CaMKII inhibition leads to increased K⁺ current expression is unknown. The absence of any change in RNA or protein levels suggests either an increase in surface expression or a posttranslational modification that increases current. CaMKII inhibition and changes in SR Ca²⁺ dynamics could modify a previously unrecognized ion channel, ion channel ß subunit, binding protein or a chaperone. Alternately, changes in CaMKII and SR Ca²⁺ handling could lead to posttranslational modifications in the ion channels themselves directly via mechanisms such as phosphorylation or indirectly via changes in adrenergic activity. Whereas Li et al showed that acute manipulations of PKA do not affect repolarizing current levels, they did not rule out the possibility that more chronic changes may have an impact. It is also possible that an unrecognized effect of the transgene could lead to the phenotype by a circuitous route, and it will be important to document similar findings in another model system.

A number of other questions also remain unanswered. It is not clear which CaMKII isoform is responsible for the major physiological effects. Similarly, the source of Ca²⁺ responsible for enzyme activation, the role of local changes in intracellular [Ca²⁺], and the importance of rapid versus slow changes in intracellular [Ca²⁺] are unknown. The applicability of finding in the mouse to those in larger animals and humans is uncertain. Direct relevance to prevention of arrhythmias and sudden death in a clinically relevant model needs to be shown.

In summary, Li et al have identified an unexpected feedback regulation involving CaMKII, PLN, and sarcolemmal K⁺ channels in the mouse. The mechanisms underlying this regulation will be important to discern. Time will tell, however, whether the findings can be applied to the arrhythmogenic human heart.

	Acknowledgments

 
Sources of Funding

Funding from NIH-NHLBI HL062300 and HL077398, and an Established Investigator Award from the American Heart Association

Disclosures

None.

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

Top References

 

1. Zhang T, Brown JH. Role of Ca²⁺/calmodulin-dependent protein kinase II in cardiac hypertrophy and heart failure. Cardiovasc Res. 2004; 63: 476–486.[Abstract/Free Full Text]
2. Anderson ME. Calmodulin kinase signaling in heart: an intriguing candidate target for therapy of myocardial dysfunction and arrhythmias. Pharmacol Ther. 2005; 106: 39–55.[CrossRef][Medline] [Order article via Infotrieve]
3. Hoch B, Meyer R, Hetzer R, Krause E-G, Karczewski P. Identification and expression of -isoforms of the multifunctional Ca²⁺/calmodulin-depedent protein kinase in failing and nonfailing human myocardium. Circ Res. 1999; 84: 713–721.[Abstract/Free Full Text]
4. Sheng M, Thompson MA, Greenberg ME. CREB: a Ca²⁺-regulated transcription factor phosphorylated by calmodulin-dependent kinases. Science. 1991; 252: 1427–1430.[Abstract/Free Full Text]
5. Lu J, McKinsey TA, Nicol RL, Olson EN. Signal-dependent activation of the MEF2 transcription factor by dissociation from histone deacetylases. Proc Natl Acad Sci U S A. 2000; 97: 4070–4075.[Abstract/Free Full Text]
6. Anderson MD, Braun AP, Shulman H, Premack BA. Multifunctional Ca²⁺/calmodulin-dependent protein kinase mediates Ca²⁺-induced enhancement of the L-type Ca²⁺ current in rabbit ventricular myocytes. Circ Res. 1994; 75: 854–861.[Abstract/Free Full Text]
7. Wehrens XH, Lehnart SE, Reiken SR, Marks AR. Ca²⁺/calmodiulin-dependent protein kinase II phosphorylation regulates the cardiac ryanodine receptor. Circ Res. 2004; 94: E61–E70.[CrossRef][Medline] [Order article via Infotrieve]
8. Zhang T, Johnson EN, Gu Y, Morissette MR, Sah VP, Gigena MS, Belke DD, Dillmann WH, Rogers TB, Shulman H, Ross J Jr., Brown JH. The cardiac-specific nuclear _B isoform of Ca²⁺/calmodulin-dependent protein kinase II induces hypertrophy and dilated cardiomyopathy associated with increased protein phosphatase 2A activity. J Biol Chem. 2002; 277: 1261–1267.[Abstract/Free Full Text]
9. Zhang T, Maier LS, Dalton ND, Miyamoto S, Ross J Jr., Bers DM, Brown JH. The _C isoform of CaMKII is activated in cardiac hypertrophy and induces dilated cardiomyopathy and heart failure. Circ Res. 2003; 92: 912–919.[Abstract/Free Full Text]
10. Zhang R, Khoo MSC, Wu Y, Yang Y, Grueter CE, Ni G, Price EE Jr., Their W, Guatimosim S, Song L-SS, Madu EC, Shah AN, Vishnivetskaya TA, Atkinson JB, Gurevich VV, Salama G, Lederer WJ, Colbran RJ, Anderson ME. Calmodulin kinase II inhibition protects against structural heart disease. Nat Med. 2005; 11: 409–417.[CrossRef][Medline] [Order article via Infotrieve]
11. Khoo MSC, Li J, Singh MV, Yang Y, Kannankeril P, Wu Y, Grueter CE, Guan X, Oddis CV, Zhang R, Mendes L, Ni G, Madu EC, Yang J, Bass M, Gomez RJ, Wadzinski BE, Olson EN, Colbran RJ, Anderson ME. Death, cardiac dysfunction, and arrhythmias are increased by calmodulin kinase II in calcineurin cardiomyopathy. Circulation. 2006; 114: 1352–1359.[Abstract/Free Full Text]
12. Li J, Marionneau C, Zhang R, Shah V, Hell JW, Nerbonne JM, Anderson ME. Calmodulin kinase II inhibition shortens action potential duration by up-regulation of K⁺ currents. Circ Res. 2006; 99: 1092–1099.[Abstract/Free Full Text]
13. Tomaselli GF, Zipes DP. What causes sudden death in heart failure? Circ Res. 2004; 95: 754–763.[Abstract/Free Full Text]

Related Article:

Calmodulin Kinase II Inhibition Shortens Action Potential Duration by Upregulation of K⁺ Currents

Jingdong Li, Céline Marionneau, Rong Zhang, Vaibhavi Shah, Johannes W. Hell, Jeanne M. Nerbonne, and Mark E. Anderson
Circ. Res. 2006 99: 1092-1099. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by London, B. Search for Related Content PubMed PubMed Citation Articles by London, B. Related Collections Related Article