Age-dependent increase in c-Jun N-terminal kinase-2 activity: does this help to understand Ca2+-calmodulin-dependent protein-kinase II-mediated atrial arrhythmogenesis in human atrial fibrillation?

This editorial refers to ‘Transcriptional regulation of stress kinase JNK2 in pro-arrhythmic CaMKIIδ expression in the aged atrium’ by X. Gao et al., pp. 737–746. Atrial fibrillation (AF) is the most common arrhythmia, which shows a strong increase in prevalence with age and is associated with substantial morbidity and mortality, with worsening heart failure and thromboembolic events being the most prominent complications. Despite the enormous progress in our understanding of the underlying molecular mechanisms promoting AF maintenance and progression, current treatment options with drugs are only moderately effective and have profound side effects including proarrhythmia. Thus, there is a clear unmet need for novel drug options with increased effectivity and improved safety profiles. A better understanding of AF-pathophysiology is expected to foster this development and accumulating evidence has highlighted a central role for abnormal Ca2+-handling and increased Ca2+-calmodulin-dependent protein-kinase II (CaMKII) activity in AF-pathophysiology.1 CaMKII regulates multiple ion channels, Ca2+-handling and myofilament proteins and is overexpressed (hyperactive) in patients with persistent (chronic) AF (cAF)2,3 as well as in experimental AF.4–6 CaMKII upregulation promotes atrial arrhythmogenesis through modulation of multiple elements of electrical, Ca2+-handling and structural remodeling,1 positioning abnormal CaMKII as a nodal point of AF-promoting atrial remodeling. Despite the critical importance of increased CaMKII expression for AF pathophysiology, the precise molecular mechanisms of upregulated CaMKII in AF are poorly understood. In this issue of Cardiovascular Research, Gao et al.7 provide compelling evidence for a potential novel mechanism of increased CaMKII activity in AF. They show that in the human atrium phosphorylation (activity) of both c-Jun N-terminal kinase-2 (JNK2) and CaMKII increase in parallel with age and that constitutive JNK2-activation in mice and application of JNK2-agonist anisomycin increase CaMKIIδ-mRNA and -protein, whereas overexpression of dominant-negative JNK2 and specific JNK2-inhibition have the opposite effect. The JNK2-mediated increase in CaMKIIδ-mRNA was associated with phosphorylation (activation) of transcription factor c-jun (but not activating transcription-factor-2, ATF2) and resulted in a stronger c-jun binding to CaMKIIδ promoter along with an increase in transcriptional CaMKIIδ-activity, which was reversed by either JNK2 or c-jun siRNA knockdown. These data establish JNK2 as a novel transcriptional CaMKIIδ-regulator with potential implications for AF-pathophysiology. The results of Gao et al.7 are a logical extension of previous work showing that constitutive JNK2-activation in mice predisposes to inducible AF, likely via abnormal sarcoplasmic reticulum (SR) Ca2+-leak due to upregulated CaMKIIδ.8 Although CaMKIIδ-independent effects might also contribute JNK2 proarrhythmia (e.g. on connexin43),9 mice with genetic CaMKIIδ-inhibition do not develop pacing-induced AF after JNK2-activation with anisomycin, suggesting that JNK2 might induce AF exclusively via CaMKIIδ-activation, at least in mice. Besides increasing CaMKIIδ transcription,7,in vitro JNK2 directly phosphorylates the classical Thr287-autophosphorylation site of CaMKIIδ,8 although it remains to be demonstrated that this occurs in the intact heart. These novel findings raise several important questions. The relationship between JNK2 and CaMKIIδ is only correlative in human atria; whether JNK2 causally contributes to the higher CaMKIIδ-activity in AF patients requires direct demonstration. Patients with cAF exhibit enhanced CaMKIIδ protein levels with a proportional increase in stimulatory Thr287-autophosphorylation, along with reduced inhibitory Ser306-phosphorylation.3 Hyperoxidation at Met281/282 also contributes to the higher CaMKIIδ-activity in cAF patients.10 However, CaMKIIδ mRNA was unchanged in cAF patients, pointing to involvement of translational modifications (e.g. altered microRNA)11 instead of altered transcriptional activity. Although patients with paroxysmal AF also show increased SR Ca2+-leak, CaMKIIδ-activity was unchanged, likely because of the comparable patient′s age.12,13 These data suggest that the AF-related increase in CaMKIIδ-activity might be a consequence of the high atrial rate3 and the CaMKIIδ increases in experimental AF supports this hypothesis.4,5 Overall, the consequences of increased JNK2 for the human atrium remain unclear and require extensive further experimentation. One critical question is whether in vivo JNK2 phosphorylates CaMKIIδ at sites other than Thr287 (e.g. Ser232; Figure 1), reducing nuclear CaMKIIδ localization.14 The data in figure 2c of Gao et al.7 suggest a proportional increase in the cytosolic (CaMKIIδC) and nuclear (CaMKIIδB) splice CaMKIIδ variants. Since the splice CaMKIIδ variants form heterohexamers within the holoenzyme, JNK2-induced shifts in the composition of CaMKIIδ-heteromultimers might alter the microdomain-restricted splice CaMKIIδ variant-selective regulation of Ca2+-handling proteins and transcription factors, particularly in the aged atrium. Such shifts in CaMKIIδ-distribution might have important consequences for atrial remodeling. In addition, besides CaMKIIδ the heart expresses CaMKIIγ, which also contributes to adverse remodeling.15 Subsequent work should study whether JNK2 regulates the transcription of CaMKIIγ. Finally, the precise mechanisms of phosphorylation-mediated JNK2 activation in the atrium also need delineation. Known (black) and novel (red) molecular mechanisms of CaMKIIδ-activation. Age-related factors, genetic predisposition, risk factors and cardiovascular diseases increase expression and/or cause post-translational modifications (phosphorylation, oxidation and glycosylation) of various residues in the regulatory domain of CaMKIIδ via JNK2-dependent and JNK2-independent mechanisms. AP-1 = Activator protein-1 transcription factor; ATF2 = Activating transcription-factor-2; CaM = calmodulin. See text for further details. In conclusion, although CaMKII inhibition has been considered as a promising strategy to treat AF, CaMKII inhibitors have harmful effects on fertility and memory which limit their clinical applicability.1 The present paper establishes a molecular mechanism of transcriptional CaMKII regulation by JNK2 in heart. Targeting expression of CaMKII in heart may foster the development of novel therapeutic approaches for AF. Conflict of interest: Dobrev is on the Scientific Advisory Board of OMEICOS and ACESION and received speaker’s fees for educational lectures from Boston Scientific, Daiichi Sankyo and Servier. His laboratory executed a research contract for Omeicos. Lorenz has no conflicts of interest to disclose. The authors’work is supported by the NIH (R01-HL131517 and R01-136389 to D.D.), the DZHK (German Center for Cardiovascular Research, grant nos. 81X2800108, 81X2800161 81X2800136 to D.D.), the German Research Foundation (DFG, Do 769/4-1 to D.D., SFB688/TPA17 to K.L., Fonds 881067), Comprehensive Heart Failure Center Würzburg (project MY1 to K.L.) and the Ministry for Innovation, Science and Research of the Federal State of North Rhine-Westphalia (to K.L.).