Ohio State University
Publication Date: January 1, 2015
Cardiac arrhythmias are a hallmark feature of heart failure, and often times result in sudden cardiac death, claiming another life every 90 seconds. Current anti-arrhythmic agents are ineffective in that they are responsible for reflex tachyarrhythmias, and are involved with serious modulations of other voltage gated ion channels.
A key aspect of healthy contractile function and excitation-contraction (EC) coupling is Ca2+ homeostasis. Defective Ca2+ cycling in the heart via integral membrane proteins, voltage gated calcium channels, and other modulators results in heart failure.
Recent discoveries in the calcium-sensing gate mechanism of voltage gated calcium channels, widely involved in the calcium-induced calcium release (CICR) electrophysiological phenomena, provide new insights into the use of CaMKII as an increasingly profound therapeutic target for arrhythmogenesis.
The current review seeks to elucidate CaMKII as a key molecular mechanism underlying various forms of arrhythmogenesis, and how the protein kinase acts as a promising pharmaceutical target for improving cardiac health.
1.1. CaMKII as a Mechanism of Arrhythmogenesis
Ca2+/ Calmodulin-Dependent Protein Kinase II (CaMKII) is a multifunctional Ser/Thr protein kinase involved in the mediation of cardiac excitation-contraction coupling. CaMKII is widely involved in gene expression in the central nervous system and heart, and excessive biochemical activation of CaMKII has been linked to cardiac hypertrophy and various forms of sudden cardiac death. On the other hand, lowered CaMKII activation has been linked to sinoatrial node dysfunction (SND), which has also shown links to sudden cardiac death and arrhythmogenesis. Recent data show that CaMKII is also responsible for the phosphorylation of many key ion channels, particularly Nav1.5, a voltage gated sodium channel associated with the depolarization phase of the cardiac action potential. Through decades of research, dysfunction in ion channels such as Nav1.5, have been shown to have linkage with arrhythmogenesis, epilepsy and even sudden cardiac death. However, despite all of these efforts, little progress has been made in identifying the fundamental processes involved in active regulation of such ion channels.
1.2. Calcium-regulating proteins in EC-coupling
Calmodulin (CaM) is a relatively small, highly conserved protein only about 17 kDa large. Calmodulin serves as an intermediate messenger, binding calcium ions and resulting in interactions with different proteins, such as CaMKII. CaM is highly involved in intracellular calcium signaling, enabling Ca2+ overload based heart failure when at an un-optimal level of binding affinity. Calmodulin is a part of the EF hand protein family, and acts as a vital calcium sensor, binding calcium to proteins that are unable to bind the ions themselves. Steady state fluorescence experiments reveal calcium sensitivities, and help quantify association/disassociation rates, which are biophysically significant.
Large proteins like RyR2 are difficult to work with in regards to gene therapy, but with a small protein like calmodulin, and the effective soybean calmodulins (SCaM1 and SCaM 4), gene therapy of calmodulin to prevent heart failure and sudden cardiac death is biophysically viable. In order for this to be done successful, it is essential to understand the different binding activities of the different forms of soybean calmodulin, as well as to understand how the different terminals (N and C) bind calcium. This will allow for the engineering of certain transcripts of soybean calmodulin, ready to be implemented within a model for analysis.
1.3. CaMKII Signaling Complexes in Cardiac Excitability
Empirically, a βIV-spectrin/CaMKII signaling complex has been found to be imperative for membrane excitability of cardiomyocytes. The complex centers on the co-localization of ankyrin-G and Nav1.5, both of which are highly expressed in ventricular cardiomyocytes, at the intercalated disk. Ankyrin-G is shown to bind to the DII-DIII loop of the Nav1.5 channel alpha subunit, and this binding allows for proper localization of Nav1.5 to the intercalated disk. In addition, ankyrin-G has been shown to bind to βIV-spectrin, a cytoskeletal protein that in turn, binds to CaMKII near its carboxyl terminus.
This assures proper CaMKII targeting; in turn, proper localization of CaMKII to the intercalated disk ensures proper phosphorylation, and more importantly, normal function, of Nav1.5. However, certain mutant mice have been discovered to have erratic Nav1.5 channel activity as per disrupted CaMKII targeting (qv3J). These mice have a mutant form of βIV-spectrin, thus disrupting the proper targeting of CaMKII, and furthermore, minimizing optimal activity and phosphorylation of Nav1.5.
In heart, ankyrin associated protein complexes organize specialized membrane-domains with distinct electrical and structural properties in cardiac sinus node, atrial and ventricular cardiomyocytes . Disrupted ankyrin-G targeting has been associated with excitable cell disease; specifically in the βIV-spectrin/CaMKII signaling complex, defects in ankyrin-G targeting result in disruptive ankyrin-G/Nav1.5 interactions, resulting in Brugada Syndrome (BrS), a fatal cardiac arrhythmia characterized by ventricular fibrillation.
1.4. Conclusion and Future Work
CaMKII is multifunctional, and is responsible for imperative regulation of ion channels via phosphorylation, autophosphorylation, targeting and even coupling as seen with the dynamic Kv4.3-CaMKII coupling association and the βIV-spectrin/CaMKII signaling complex. Disrupted targeting of CaMKII and/or changes in autophosphorylation properties have been linked to cardiac hypertrophy and even sudden cardiac death through ventricular fibrillation. Understanding the regulation of CaMKII as a multifunctional kinase is essential in understanding the mediation of cardiac excitation-contraction coupling, and moreover, how optimum CaMKII activity in the heart is imperative for myocardial function.
1) Sakima Smith, Jerry Curran, Thomas J. Hund and Peter J. Mohler. “Defects in Cytoskeletal Signaling Pathways, Arrhythmia, and Sudden Cardiac Death.” Frontiers In Physiology. PubMed, 4 May 2012. Web. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3343379/pdf/fphys-03-00122.pdf>.
2) The ryanodine receptor store-sensing gate controls Ca2+ waves and Ca2+-triggered arrhythmias; Wenqian Chen, Ruiwu Wang, Biyi Chen, Xiaowei Zhong, Huihui Kong, Yunlong Bai, Qiang Zhou, and others; Nature Medicine, online 19 January 2014; DOI:10.1038/nm.3440; Abstract.