Ultra-low power deep sustained optogenetic excitation of human ventricular cardiomyocytes with red-shifted opsins: a...

The main challenge in cardiac optogenetics is to have low-power, high-fidelity deep excitation of cells with minimal invasiveness and heating. We present a detailed computational study of optogenetic excitation of human ventricular cardiomyocytes (HVCMs) with new ChRmine, bReaChES and CsChrimson red-shifted opsins to overcome the challenge. Action potentials (APs) in ChRmine-expressing HVCMs can be triggered at 6 µW mm^-2 (10 ms pulse) and 0.7 µW mm^-2 (100 ms pulse) at 585 nm, which is two orders of magnitude lower than ChR2(H134R). This enables safe sustained excitation of deeply situated cardiac cells with ChRmine (7.46 mm) and with bReaChES (6.21 mm) with the light source at the pericardium surface. Deeper excitation up to 10.2 mm can be achieved with ChRmine by illuminating at 650 nm. Photostimulation conditions for minimum charge transfer during APs have been determined, which is important for tissue health under sustained excitation. The AP duration for all the opsins is constant up to 100 ms pulse width but increases thereafter. Interestingly, the AP frequency increases with irradiance under continuous illumination, but APs are suppressed at higher irradiances. The optimal range of irradiance for each opsin to excite HVCMs has been determined. Under optimal photostimulation conditions, each opsin can precisely excite APs up to 2.5 Hz, while latency and power of light pulse for each AP in a sequence remain most stable and an order of magnitude lower, respectively, in ChRmine-expressing HVCMs. The study highlights the importance of ChRmine and bReaChES for resynchronization, termination of ventricular tachycardia and designing optogenetic cardiac pacemakers with enhanced battery life. KEY POINTS: This work is the formulation of accurate theoretical models of optogenetic control of human ventricular cardiomyocytes (HVCMs) expressed with newly discovered opsins (ChRmine, bReaChES and CsChrimson). Under continuous illumination, action potentials in each opsin-expressing HVCMs can only be evoked in a certain range of irradiances. Action potentials in ChRmine-expressing HVCMs can be triggered at ultra-low power (6 µW mm^-2 at 10 ms pulse or 0.7 µW mm^-2 at 100 ms pulse at 585 nm), which is two to three orders of magnitude lower than reported results. Ongoing action potentials in ChRmine-expressing HVCMs can be suppressed by continuous illumination of 585 nm light at 2 µW mm^-2 . ChRmine enables sustained excitation due to its faster recovery from the desensitized state. Optogenetic excitation of deeply situated cardiac cells is possible up to ∼7.46 and 10.2 mm with ChRmine on illuminating the outer surface of pericardium at safe irradiance at 585 nm and 650 nm, respectively. The study opens up prospects for designing energy-efficient light-induced pacemakers, resynchronization and termination of ventricular tachycardia.