Low-Power Optogenetic Excitation and Suppression of Human Ventricular Cardiomyocytes Expressed with ChRmine and HcKCR1

Low-power, high-fidelity optical excitation and the effective suppression of uncoordinated heartbeats are major challenges in cardiac optogenetics. Recently, a red-shifted, cation-conducting channelrhodopsin known as ChRmine, derived from the Rhodomonas lens (also attributed to the marine ciliate Tiarina fusus), has been discovered. ChRmine exhibits high light sensitivity and generates a large photocurrent for optical excitation. In addition, a potassium (K⁺)-conducting channelrhodopsin from Hyphochytrium catenoids (HcKCR1) has also been discovered. HcKCR1 exhibits a reversal potential close to the resting membrane potential of the targeted cardiac cells, making it suitable for optical suppression. In this paper, we present a theoretical model and a detailed analysis of optogenetic excitation and the suppression of cardiac activity using ChRmine and HcKCR1 in human ventricular cardiomyocytes (HVCMs). The present study shows that in ChRmine-expressing HVCMs, action potential (AP) can be triggered at 6 μW/mm² on illuminatation with 10 ms light pulse, which enables deeper excitation up to ~ 8 mm from the pericardial surface at safe light irradiances. High-fidelity optical pacing with ChRmine-expressing HVCMs is achievable up to 2.5 Hz at 0.7 ms light pulse and 0.58 mW/mm², which is an order of magnitude lower than the previously used opsins. HcKCR1 effectively suppresses action potentials by shunting the cell membrane potential to the resting state at a light irradiance of 1 mW/mm². Furthermore, HcKCR1 enables precise shortening of the AP duration at a very low irradiance of 1 µW/mm² on illuminating the cell during the repolarization phase. The present study highlights the advantages of the newly discovered ChRmine and HcKCR1 opsins for low-intensity optogenetic pacing and complete suppression of uncoordinated heartbeats. The results are useful for designing energy-efficient, light-induced cardiac pacemakers and for the effective treatment of cardioversion and tachycardia, as well as for extending the scale of cardiac optogenetics.