doi: 10.1161/01.RES.0000208057.36708.48
© 2006 American Heart Association, Inc.
Editorials |
MAPK = Mitogen-Activated Protein KChIP2?
Unraveling Signaling Pathways Controlling Cardiac Ito Expression
From the Department of Medicine I (A.S.B., S.K.), University Hospital Grosshadern, Ludwig-Maximilians-University, Munich, Germany; and the Department of Cardiology (A.S.B.), Johns Hopkins University, Baltimore, Md.
Correspondence to Stefan Kääb, MD, Department of Medicine I, University Hospital Grosshadern, Ludwig-Maximilians-University, 81366 Munich, Germany. E-mail Stefan.Kaab{at}med.uni-muenchen.de
See related article, pages 386–393
Key Words: MAP Kinases • signal transduction • transient outward current • auxiliary subunit
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Introduction |
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Reduction in density of the cardiac calcium-independent transient outward potassium current (Ito1) is one of the most consistent findings in electrophysiological remodeling observed in animal models of left ventricular hypertrophy and failure as well as in the human heart under pathological conditions.1 Initially attributed to downregulation of the mRNA of pore-forming Kv4 subunits,2 a more complex picture emerged over time as several auxiliary subunits were identified, offering new potential regulatory elements for controlling Ito1 density. KChIP2 is a dazzling member of this group of beta-subunits which assembles with pore-forming Kv4 subunits in 4:4 complexes to produce native Ito1 channels.3 Several different KChIP2 splice variants are expressed in the human heart and the major KChIP2 isoform (KChIP2c) has been found to boost Ito1 current density and to speed up recovery from inactivation, thereby recapitulating several, albeit not all, features of native Ito1 channels.4 Importantly, myocytes isolated from KChIP2 knock-out mice exhibited a complete, selective loss of Ito1, whereas an
50% reduction was observed in cardiomyocytes from heterozygous animals, indicating that KChIP2 can quantitatively regulate Ito1.5 In line with this finding, KChIP2 mRNA typically distributes across the myocardial wall in a fashion paralleling the gradient of the transient outward current with fast recovery from inactivation encoded by Kv4 channels, with largest concentrations in subepicardium (EPI) and smallest in subendocardium (ENDO) in human and canine hearts.6 This suggests that KChIP2 is the limiting factor in cell surface expression of Ito1 channels, rather than the pore-forming Kv4 subunits which are expressed at equal levels across the ventricular wall in these species. In addition, KChIP2 is also greatly reduced in pathologic conditions with diminished Ito1 current density like heart failure and atrial fibrillation, further suggesting that KChIP2 is involved in controlling Kv4-mediated current levels.7 However, even though the discovery of this auxiliary subunit advanced our understanding of the regional gradients of Ito1 in health and disease, little was known about the regulatory elements upstream of KChIP2. |
Regulatory Pathways of Cardiac KChIP2 Expression |
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In this issue of Circulation Research, Jia and Takimoto try to elucidate the regulatory network controlling expression of Ito1 under pathological conditions.8 Using an in vivo and in vitro approach they demonstrate that KChIP2 protein and mRNA expression are reduced in aortic banded rat ventricles as well as in cultured neonatal rat ventricular myocytes after treatment with the alpha-adrenergic agent phenylephrine (PE) and the PKC activator, phorbol 12-myristate 13-acetate (PMA). Interestingly, in the rat heart, the reduction in KChIP2 mRNA and protein levels parallels the reduction of Ito1 current density, whereas there is only a modest decrease in Kv4.2 and Kv4.3 mRNA, supporting the notion of KChIP2 being the limiting factor in cell surface expression of functional Ito1 channels. Using pharmacological inhibitors of a wide variety of signaling pathways as well as adenoviral dominant-negative and constitutively active MEK and JNK constructs, the authors dissect the complex signaling pathways regulating KChIP2 expression. They show that the PE-induced reductions in KChIP2 mRNA levels are prevented by blocking JNK-signaling whereas PKC-activation, independent of PE, leads to reduction of KChIP2 mRNA levels via activation of the MEK/ERK branch of the MAPK. Perhaps most intriguing is the demonstration that inhibition of MEK/ERK activity caused an increase in Ito1 density, suggesting that this branch of the MAPK pathway might also influence basal expression of Ito1. Yet, there seem to be some methodological concerns with discrepancies between pharmacological and adenoviral-mediated inhibition of KChIP2 mRNA in the context of PE treatment, as pharmacological inhibition of MEK with U0126 was able to prevent the PE-induced reduction in KChIP2 mRNA, whereas dominant-negative MEK adenovirus had no apparent effect.8 Clearly, further experiments are necessary to explore the cross-talk between the different MAPK branches in more detail.
The observations of Jia and Takimoto raise as many questions as they answer: What are signaling molecules upstream of MAPK (eg, MEKK1, MEK4/7, MEK1/2, small G proteins)? What are the downstream targets of MAPKs mediating transcriptional downregulation of KChIP2? In this respect, it is interesting to note that the transcription factor CREB, which is a target of ERK phosphorylation, has been shown to bind to the KChIP2 promoter region.9 Moreover, ERK has recently also been recognized to regulate Ito1 by direct phosphorylation of the pore-forming Kv4.2 subunit.10 Additional mechanisms, including phosphorylation by tyrosine kinases11 and mRNA degradation of Kv4.3 mRNA by angiotensin II and phenylephrine12 have also been shown to affect Ito1 density. To sort out the relative contribution of all these regulatory mechanisms affecting alpha- and beta-subunits of Ito1 channels will be a challenging task.
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Is MAPK Also Controlling Ito1 Expression in the Human Heart? |
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Perhaps the most relevant question is whether the findings obtained in a rat model are also applicable to the human heart, as there are marked differences in the molecular mechanisms regulating Ito1 expression in the hearts of large and small mammals. Several lines of evidence suggest that extrapolation from the conditions in rat to those in humans can only be done with appropriate caution. First, heart rate, action potential shape, and contractile properties are known to be species dependent: Although Ito1 plays a critical role in repolarization of the short action potential of rat myocardium, the functional significance of Ito1 in species with longer action potentials characterized by a prolonged positive plateau is less well defined. Because of the critical role of Ito1 in cardiac repolarization in rat and mouse, one can speculate that expression levels of Ito1 need to be more tightly regulated in small versus large mammals. Second, Kv4.2 is the major Ito1 pore-forming subunit in rat and mouse. In humans and dogs, Kv4.3 is the predominant isoform, whereas Kv4.2 is absent.13 Third, the gradient of Ito1 current density across the left ventricular free wall parallels transmural KChIP2 expression in human and canine myocardium.6 Conversely, in the rat heart, the transmural gradient in Ito1 expression is a product of a gradient in Kv4.2 gene expression rather than KChIP2 mRNA.6 This suggests that different regulatory pathways may be operative to control region- and disease-specific Ito1 expression patterns in small and large mammals.
So far, there is no experimental data to show that MAPKs are involved in the regulation of Ito1 channels in the human heart. However, there is circumstantial evidence to suggest that activation of MAPK pathways may be implicated in the generation of the transmural gradient of Ito1, as ERK activity is significantly greater in human ENDO compared with EPI.14 Therefore, one could hypothesize that high ENDO levels of activated ERK inhibit Kv4-mediated current in ENDO, whereas substantially less inhibition of steady-state Ito1 levels occurs in EPI. Additionally, regulation of Ito1 density via MAPK pathways could also play a significant role in heart failure. Even though the activation status of JNK in end-stage human heart failure is still a matter of debate,15 there is accumulating evidence that ERK1 is upregulated by mechanical stress, as this is the case in end-stage human heart failure.16 Activation of stress-activated MAP kinases could therefore contribute to diminished Ito1 by transcriptional downregulation of KChIP2. Given that activation of stress-activated MAP kinases has been observed to be a part of important cellular signaling events mediated through adrenergic receptors,17 one could speculate that the reduction of Ito1 in heart failure is a result of the altered neurohumoral environment in the failing heart.
What can we expect in the future? Given the plethora of possible regulatory mechanisms in macromolecular ion channel complexes, almost certainly, greater complexity is yet to come. Fortunately, understanding this complexity will generate a more detailed knowledge of homeostatic mechanisms of ion channel regulation and hence, cardiac electrophysiology. In this sense, the article of Jia and Takimoto8 can be viewed an important contribution toward achieving this goal.
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Acknowledgments |
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Support for this study is provided by DFG grant BA 3342/1-1 (to A.S.B.) and BMBF grants 01GS0499 and 01GI0204 (to S.K.).
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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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Related Article:
Circ. Res. 2006 98: 386-393.
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