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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Circ Res. Author manuscript; available in PMC 2016 May 8.
Published in final edited form as:
PMCID: PMC4428686
NIHMSID: NIHMS679161

A Novel Mechanism of Transient Outward Potassium Channel Current Regulation in the Heart: Implications for Cardiac Electrophysiology in Health and Disease

Michael S Bohnen, BA,* Vivek Iyer, MD MSE,** Kevin J Sampson, PhD,* and Robert S Kass, PhD*

The cardiac action potential (AP) results from the summation of ion channel activity that depolarizes and then repolarizes the plasma membrane, allowing for contraction and relaxation of the atrial and ventricular chambers. After the initial depolarization, a brief repolarization event occurs in human hearts that sets the critically important plateau phase of the AP during which calcium enters the cytosol leading to contraction; alteration of the plateau predisposes to arrhythmia. The brief repolarization prior to the plateau phase results from rapid activation of voltage gated potassium channels known collectively as transient-outward potassium channels. The K+ efflux through these channels causes a transient outward current known as Ito 1.

Ito may be further subdivided into a fast component Ito,f and a slow component Ito,s 2. Both Ito subtypes activate rapidly at membrane voltages positive to −30mV; the primary difference between them is the time to inactivation, as Ito,f inactivates over tens of milliseconds, while Ito,s inactivates over hundreds of milliseconds 3. Ito,f is comprised of pore forming alpha subunits Kv4.2 and/or Kv4.3, encoded by the KCND2 and KCND3 genes, respectively. The alpha subunit co-assembles with accessory subunits KChIP2 and/or DPP6, to form the functional channel 4.

Ito,f is abundantly expressed throughout the human heart, with a greater Ito,f current density present in the atria and Purkinje fibers than in the ventricular myocardium 2. The regional differential expression of Ito,f within the human heart contributes to the observed differences in the morphology of action potential waveforms throughout different cells in the heart. For example, atrial myocytes, where Ito,f is large, have a more triangular AP shape and repolarize faster than ventricular myocytes. The differences in AP morphology and duration serve physiological roles (eg. the longer plateau phase duration in the ventricles allows for a more prolonged and greater force of contraction), and alterations to them can lead to a wide variety of cardiac disease phenotypes. For Ito alone, studies demonstrate a correlation between increased Ito and early-onset lone atrial fibrillation, Brugada syndrome, and idiopathic ventricular fibrillation, while decreases in Ito have been demonstrated in heart failure2, 5, 6. The clinical phenotypes can be tied to the regulation of ion channel subunit expression. Gain-of-function mutations in KCND3 give rise to early-onset lone atrial fibrillation, while there is a relatively consistent decrease in Ito due to reduction in Kv4.3 expression in the setting of heart failure and concomitant ventricular remodeling 7. Implicit in this discussion is the understanding that regulation of Ito subunit expression has functional consequences on the action potential waveforms of different regions of the heart.

The continued improvements in our understanding of the molecular components of cardiac ion channels and post-transcriptional regulation of these components deepen our understanding of the pathophysiology of cardiac arrhythmia. While specific mutations in the pore forming subunits of Ito have been studied in patients with cardiac disease, a growing interest in the role of post-transcriptional and post-translational modifications of ion channels has evolved. Recent Ito studies have explored the impact of microRNAs on protein expression, and channel phosphorylation on Ito current density, which together highlight the importance of regulation of the Ito channel in cardiac myocytes 2. In the study by Li et al. in this issue of Circulation Research, the authors illustrate, for the first time, regulation of Ito by a cold-inducible RNA-binding protein, CIRP8.

RNA-binding proteins act as important regulators of gene expression 9. CIRP, which was first identified in murine germ cells exposed to low temperatures, is constitutively expressed in many bodily tissues, including the brain, testis, lung, and heart 10. CIRP acts as an RNA chaperone and regulatory molecule, controlling cellular processes such as RNA splicing, initiation of translation, and aiding the assembly, disassembly, and transport of proteins 11. In the setting of cell stressors, such as cold and hypoxia, CIRP expression is up-regulated 12. In the setting of hemorrhagic shock and sepsis, over-activity of CIRP causes overproduction of inflammatory cytokines, furthering hemodynamic instability. Before the work by Li et al., the role of CIRP in the heart was unclear.

Using CIRP-knockout rats, Li et al. reveal that the absence of CIRP results in a shortened QTc interval on electrocardiogram and decreased action potential duration (APD), tightly linked to an increase in Ito (Figure 1, panel A). Moreover, the CIRP-knockout rats did not have altered transcription of KCND2 or KCND3, but rather, had increased expression of Kv4.2 and Kv4.3 subunits, resulting in an approximate doubling of Ito density in ventricular myocytes. KChiP2 expression was unaffected, and expression of other key ion channels was not altered in the CIRP-knockout rats. These data suggest that CIRP selectively regulates KCND2 and KCND3 gene expression in rat heart by preventing excessive protein expression of the corresponding Kv4.2 and Kv4.3 subunits.

Figure 1
Computational modeling of impact of CIRP Ito downregulation in rat and human cardiomyocytes

This new finding adds to our understanding of the regulation of cardiac ion channels and action potential characteristics. Extrapolating the result from the rat model to larger mammals will prove interesting as alterations in Ito produce differing effects on APs depending on the morphology of the AP 1. Computational modeling, for example, predicts that in human ventricle, small decreases in Ito increase APD slightly, while large increases can shunt the AP and cause rapid repolarization (Figure 1, panel B). Modeling also predicts that this effect is altered in the atria and conducting system where a less pronounced “spike-and-dome” AP is the baseline. This diversity of effects of altering Ito in different regions of the heart likely explains the variety of cardiac disease phenotypes attributable to alterations in Ito.

As Ito plays a significant role in generating the normal cardiac action potential, pharmacological modulation, in addition to naturally occurring modulation, are active topics of research. One example, the experimental drug NS5806, increases peak Ito currents and slows channel inactivation in canine ventricular myocytes, and can recapitulate the Brugada Syndrome phenotype 13. Furthermore, in failing hearts in which a decrease in Ito expression has been shown to occur and to contribute to failure-induced action potential prolongation, NS5806 has been shown to rescue, at least in part, Ito expression 14, 15, suggesting that activation of Ito may serve in the treatment of heart failure. This proof-of-concept that Ito may be pharmacologically manipulated for therapeutic benefit potentially extends to other cardiac disease conditions wherein Ito imbalance occurs.

The discovery that CIRP regulates Ito expression brings CIRP to the forefront of gene regulation and pharmacology in Ito - dependent cardiovascular disease. CIRP may be actively released from cells, and a prior study developed neutralizing antisera containing IgG directed against CIRP, which successfully blocked CIRP and improved survival outcomes of hemorrhagic and septic animals 16. This study suggests that CIRP represents a potential novel therapeutic target, and opens the door for important questions to address: how does CIRP regulation impact the human heart? Does CIRP activity change in specific pathophysiologic settings, such as ischemic heart disease or heart failure? As for the interaction of CIRP with Ito channel components, does CIRP normally interfere with translation initiation of the alpha subunits Kv4.2 and Kv4.3, or another post-transcriptional process? Does CIRP affect normal folding of Kv4.2/Kv4.3, or does CIRP increase disassembly rates of Kv4.2/Kv4.3? These important questions, brought to the forefront by the paper by Li, et al., remain unanswered, with the field open to uncovering how exactly CIRP regulates Ito in health and in disease, and the therapeutic potential for CIRP regulation in the human heart.

Acknowledgments

Sources of Funding:

Supported by NIH 5R01 GM109762-02

Footnotes

Disclosures:

None.

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