Effect of Optogenetic Defibrillation on Cardiac Electrophysiology in Health and Ischemia

Introduction: Optogenetic defibrillation presents a promising approach for terminating cardiac arrhythmias through the activation of light-gated ion channels, named channelrhodopsins (ChR). Previous studies have examined cation non-selective ChR, such as Chlamydomonas ChR-2 (ChR2), and anion non-selective ChR, including Guillardia theta Anion ChR-1 (GtACR1).^[1] However, these variants depolarize resting cardiomyocytes (CM), potentially causing intracellular Ca²⁺ and Na⁺ overload. This could aggravate arrhythmogenesis, particularly in pathological conditions, such as acute ischemia. In contrast, ChR that show preferential permeability for K⁺, like Wobblia inhibitory ChR (WiChR),^[2] may be favourable for cardiac defibrillation, as they stabilise CM close to their natural resting membrane potential (RMP).

Methods: We developed a first computational model of WiChR, based on data obtained by single-cell patch-clamp recordings and fluorescent measurements of [K⁺]_i in ND7/23 cells. To simulate CM behaviour, we used the O’Hara model of human ventricular CM,^[3] and integrated either our WiChR model, or established models of ChR2^[4] or GtACR1.^[1] We simulated both healthy conditions as well as acute ischemia. To represent 7.5 min of ischemia, we raised extracellular K⁺ from 5.4 mM to 11 mM,^[5] included the K_ATP current with conductance 0.005 mS/µF,^[6] inhibited I_CaL and I_Na by 25%,^[6] and reduced I_NaK, I_pCa, and J_up by factors corresponding to 5.5 mM [ATP]_i and 78.8 mM [ADP]_i^[5] (compared to 10 mM and 15 mM for healthy conditions, respectively). For 15 min of ischemia, we extrapolated these values linearly. To evaluate CM behaviour without optogenetic intervention, we paused electrical pacing instead of illumination as reference.

Results: In all simulations, we applied illumination for 1.5 s. Under healthy conditions, CM RMP approached –79 mV, –43 mV, and –17 mV, for WiChR, GtACR1 and ChR2, respectively. Diastolic [Ca²⁺]_i during illumination scales with RMP (0.08 µM, 0.11 µM, and 0.19 µM for WiChR, GtACR1, and ChR2, respectively). Moreover, activation of ChR2 increases [Na⁺]_i by 0.2 mM, while WiChR leads to increased Ca²⁺ transient amplitude by 0.4 µM. Under ischemic conditions, changes due to ChR2 are more pronounced, while WiChR and GtACR1 temporarily decrease diastolic [Ca²⁺]_i (from 0.13 µM to 0.10 µM and 0.11 µM, respectively). Our results suggest that ChR that avoid pronounced depolarisation of CM RMP may be safer for defibrillation.

Outlook: We are currently performing simulations of human left ventricles reconstructed from patients with ischemic cardiomyopathy to explore how the ion selectivity and kinetics of different ChR variants affect the efficacy and safety of optogenetic defibrillation in tissue.

References:
[1]    Ochs AR et al. Front Physiol 2021/12:718622
[2]    Vierock J et al. Sci Adv 2022/8:eadd7729
[3]    O’Hara T et al. PLoS Comput Biol 2011/7:e1002061
[4]    Wülfers E et al. 2018 CinC 2018/45:1-4
[5]    Carpio E et al. Comput Biol Med 2022/141:105038
[6]    Dutta S et al. Prog Biopys Mol 2017/129:40-52


Fig. 1: Simulated effects of ChR2 (A), GtACR1 (B), and WiChR (C) on membrane voltage U (left), [Na⁺]_i (middle), and [Ca²⁺]_i (right).