A Singular Role of IK1 Promoting the Development of Cardiac Automaticity during Cardiomyocyte Differentiation by IK1 –Induced Activation of Pacemaker Current

Abstract

The inward rectifier potassium current (IK1) is generally thought to suppress cardiac automaticity by hyperpolarizing membrane potential (MP). We recently observed that IK1 could promote the spontaneously-firing automaticity induced by upregulation of pacemaker funny current (If) in adult ventricular cardiomyocytes (CMs). However, the intriguing ability of IK1 to activate If and thereby promote automaticity has not been explored. In this study, we combined mathematical and experimental assays and found that only IK1 and If, at a proper-ratio of densities, were sufficient to generate rhythmic MP-oscillations even in unexcitable cells (i.e. HEK293T cells and undifferentiated mouse embryonic stem cells [ESCs]). We termed this effect IK1-induced If activation. Consistent with previous findings, our electrophysiological recordings observed that around 50% of mouse (m) and human (h) ESC-differentiated CMs could spontaneously fire action potentials (APs). We found that spontaneously-firing ESC-CMs displayed more hyperpolarized maximum diastolic potential and more outward IK1 current than quiescent-yet-excitable m/hESC-CMs. Rather than classical depolarization pacing, quiescent mESC-CMs were able to fire APs spontaneously with an electrode-injected small outward-current that hyperpolarizes MP. The automaticity to spontaneously fire APs was also promoted in quiescent hESC-CMs by an IK1-specific agonist zacopride. In addition, we found that the number of spontaneously-firing m/hESC-CMs was significantly decreased when If was acutely upregulated by Ad-CGI-HCN infection. Our study reveals a novel role of IK1 promoting the development of cardiac automaticity in m/hESC-CMs through a mechanism of IK1-induced If activation and demonstrates a synergistic interaction between IK1 and If that regulates cardiac automaticity.

Combinatorial Treatment of Human Cardiac Engineered Tissues With Biomimetic Cues Induces Functional Maturation as Revealed by Optical Mapping of Action Potentials and Calcium Transients

“…Similar results were observed in the 1Hz paced condition (Supplementary Figure 2). Given its importance in automaticity (Azene et al, 2005;Tse et al, 2006;Xue et al, 2007;Lieu et al, 2008;Sun et al, 2017), we next measured the funny current (I f ) in hESC-VCMs isolated from hvCAS. Representative tracings were shown in Figure 3A.…”

Reciprocal interaction between IK1and Ifin biological pacemakers: A simulation study

“…HEK293 cell (44) and Kir2.1/HCN4 (42) showed that I K1 may actually recruit more I f by activating current at more negative membrane voltages, because I K1 was the only negative current in these experiments. Our simulation, however, did not yield such a result because the interaction of other positive currents (such as I NaK , I Kr and I Ks ) contributed to the hyperpolarization of membrane potential and helped the activation of I f .…”

“…I f has been shown to play an important role in generating pacemaking APs in both native (13,21,27,28,35,36) and engineered pacemakers (42). Experimentally it has been shown that high expression of HCN2 can initiate spontaneous beats in neonatal rat VMs (21,35) and improve spontaneous beats in rabbit CMs (13).…”

“…It has been suggested that a combined manipulation of I K1 and I f may be a better alternative for creating a bio-pacemaker (42). It has been demonstrated that the expression of transcriptional regulator TBX18, which influenced both I f and I K1 expression, generated appropriate autonomic responses in non-pacemaking CMs (10,31,43).…”

“…Computational modelling offers a means to investigate different approaches to generating stable pacemaking activity. It has been used to study possible roles of down-regulating I K1 in VMs (45)(46)(47) and combined action of overexpression of I f with down-regulation of I K1 in inducing spontaneous pacemaking activity in SAN (42). Bifurcation analysis has also been used to explore the effect of changes in some individual ion channel current on the pacemaking activities of SAN cells (48) and genesis of automaticity in VMs (46,49), showing the role of I K1 , I f , I st and Na + /Ca 2+ exchange current (I NaCa ) in modulating the initiation and stability of pacemaking activity (49).…”