Stabilization of Kv4 protein by the accessory K⁺ channel interacting protein 2 (KChIP2) subunit is required for the generation of native myocardial fast transient outward K⁺ currents

Abstract

Key points• The cytosolic K + channel accessory subunit, K + channel interacting protein 2 (KChIP2), was previously suggested to be critical in the generation of cardiac fast transient outward current (I to,f ) channels.• The experiments presented here revealed the novel finding that targeted deletion of KChIP2 results in the complete loss of the Kv4.2 protein, although Kcnd2 (Kv4.2) transcript expression is not decreased in KChIP2 −/− ventricles.• In contrast, the slow transient outward current, I to,s , is increased in KChIP2 −/− left ventricular apex myocytes and ventricular action potential waveforms in KChIP2 −/− and WT mice are not significantly different.• These results demonstrate the critical role of KChIP2 in the stabilization of native Kv4 proteins and that the loss of the Kv4.2 protein underlies the elimination of I to,f in KChIP2 −/− myocytes.• Taken together, the results here demonstrate that electrical remodelling compensates for the elimination of I to,f , maintaining physiological action potential repolarization in mouse myocardium.Abstract The fast transient outward K + current (I to,f ) underlies the early phase of myocardial action potential repolarization, contributing importantly to the coordinated propagation of activity in the heart and to the generation of normal cardiac rhythms. Native I to,f channels reflect the tetrameric assembly of Kv4 pore-forming (α) subunits, and previous studies suggest roles for accessory and regulatory proteins in controlling the cell surface expression and the biophysical properties of Kv4-encoded I to,f channels. Here, we demonstrate that the targeted deletion of the cytosolic accessory subunit, K + channel interacting protein 2 (KChIP2), results in the complete loss of the Kv4.2 protein, the α subunit critical for the generation of mouse ventricular I to,f . Expression of the Kcnd2 (Kv4.2) transcript in KChIP2 −/− ventricles, however, is unaffected. The loss of the Kv4.2 protein results in the elimination of I to,f in KChIP2 −/− ventricular myocytes. In parallel with the elimination of I to,f , the slow transient outward K + current (I to,s ) is upregulated and voltage-gated Ca 2+ currents (I Ca,L ) are decreased. In addition, surface electrocardiograms and ventricular action potential waveforms in KChIP2 −/− and wild-type mice are not significantly different, suggesting that the upregulation of I to,s and the reduction in I Ca,L compensate for the loss of I to,f . Additional experiments revealed that I to,f is not 'rescued' by adenovirus-mediated expression of KChIP2 in KChIP2 −/− myocytes, although I Ca,L densities are increased. Taken together, these results demonstrate that association with KChIP2 early in the biosynthetic pathway

Reductions in the Cardiac Transient Outward K+ Current Ito Caused by Chronic β-Adrenergic Receptor Stimulation Are Partly Rescued by Inhibition of Nuclear Factor κB

“…The involvement of NF-B in ␤-AR-mediated effects on I to,f is consistent with previous studies showing that KChIP2 facili- tates insertion of I to,f channels into cell membranes (30) while also limiting channel degradation (31). Importantly, NF-B inhibition had no measureable effects on I to,f densities in the absence of ␤-AR stimulation, despite causing elevations of KChIP2 mRNA levels under these conditions.…”

Notch-Mediated Epigenetic Regulation of Voltage-Gated Potassium Currents

“…Previous studies have shown that complete loss of KChIP2 in mouse ventricular myocardium results in elimination of I to,f 29 . In Notch GOF mice, KChIP2 protein levels are reduced by about 50% (Figure 3D).…”

“…In Notch GOF mice, KChIP2 protein levels are reduced by about 50% (Figure 3D). To explore the hypothesis that the reduction of KChIP2 in Notch GOF is sufficient to attenuate I to,f , whole-cell Kv current recordings were obtained from LV myocytes isolated from KChIP2 +/− mice lacking one copy of Kcnip2 29 . Analyses of the voltage-clamp data obtained in these experiments revealed that mean ± SEM I K,peak ( P = 0.018) and I to,f ( P = 0.0026) densities were significantly lower in KChIP2 +/− LV myocytes, than in control LV myocytes (Online Figure IV; Table 2), whereas I K,slow , I ss and I K1 densities in KChIP2 +/− and control LV myocytes were not significantly different (Online Figure IV; Table 2).…”

Transmural gradients in ion channel and auxiliary subunit expression

“…Conversely, adenoviral-mediated overexpression of KChIP2 was unable to restore I to,f density in cultured adult mouse KChIP2(−/−) myocytes (Foeger et al, 2013) or to increase it in cultured adult canine endocardial myocytes (Rosati and McKinnon, unpublished). The apparent failure of KChIP2 to rescue expression of I to,f in adult cultured myocytes may reflect a technical issue, since transcription of multiple ion channel subunit genes, including Kv4.2, can be markedly down-regulated in these cells when placed in culture (Hershman and Levitan, 1998), Rosati and McKinnon, unpublished).…”