Ventricular arrhythmias are among the most severe complications of heart disease and can result in sudden cardiac death. Patients at risk currently receive implantable defibrillators that deliver electrical shocks to terminate arrhythmias on demand. However, strong electrical shocks can damage the heart and cause severe pain. Therefore, we have tested optogenetic defibrillation using expression of the light-sensitive channel channelrhodopsin-2 (ChR2) in cardiac tissue. Epicardial illumination effectively terminated ventricular arrhythmias in hearts from transgenic mice and from WT mice after adeno-associated virus–based gene transfer of ChR2. We also explored optogenetic defibrillation for human hearts, taking advantage of a recently developed, clinically validated in silico approach for simulating infarct-related ventricular tachycardia (VT). Our analysis revealed that illumination with red light effectively terminates VT in diseased, ChR2-expressing human hearts. Mechanistically, we determined that the observed VT termination is due to ChR2-mediated transmural depolarization of the myocardium, which causes a block of voltage-dependent Na⁺ channels throughout the myocardial wall and interrupts wavefront propagation into illuminated tissue. Thus, our results demonstrate that optogenetic defibrillation is highly effective in the mouse heart and could potentially be translated into humans to achieve nondamaging and pain-free termination of ventricular arrhythmia.

Optogenetic stimulation allows activation of cells with high spatial and temporal precision. Here we show direct optogenetic stimulation of skeletal muscle from transgenic mice expressing the light-sensitive channel Channelrhodopsin-2 (ChR2). Largest tetanic contractions are observed with 5-ms light pulses at 30 Hz, resulting in 84% of the maximal force induced by electrical stimulation. We demonstrate the utility of this approach by selectively stimulating with a light guide individual intralaryngeal muscles in explanted larynges from ChR2-transgenic mice, which enables selective opening and closing of the vocal cords. Furthermore, systemic injection of adeno-associated virus into wild-type mice provides sufficient ChR2 expression for optogenetic opening of the vocal cords. Thus, direct optogenetic stimulation of skeletal muscle generates large force and provides the distinct advantage of localized and cell-type-specific activation. This technology could be useful for therapeutic purposes, such as restoring the mobility of the vocal cords in patients suffering from laryngeal paralysis.

Aims Optogenetic pacing of the heart has been demonstrated in transgenic animals expressing Channelrhodopsin–2 (ChR2). However for the clinical use of optogenetics to treat cardiac arrhythmias, gene transfer to non–transgenic hearts is required. The aim of this study was to describe a reliable method for gene transfer of ChR2 into a sufficient percentage of cardiomyocytes to overcome the electrical sink of all the coupled non–expressing cardiomyocytes during optical pacing of the whole heart in vivo . Methods and Results Adeno–associated virus with cardiac tropism for expression of ChR2 in fusion to mCherry was systemically injected into wild type mouse hearts. Bright mCherry fluorescence was detected in the whole heart 4–10 weeks later. Single cell dissociation revealed that on average 58% cardiomyocytes were mCherry positive. These showed light–induced inward currents, action potentials and contractions. Pulsed illumination of the left ventricle induced ventricular pacing in vivo in 74% of mice and higher light intensities were required for reduced pulse duration or size of illumination. Non–responding hearts showed low AAV expression and the threshold for optical pacing was estimated to be 35–40% ChR2 expressing cardiomyocytes. Optical pacing in vivo was stable over extended periods without negative effects on normal sinus rhythm and ECG parameters after termination of stimulation indicating sufficient cardiac output during pacing. Conclusions Gene transfer generates sufficient ChR2 photocurrent for reliable optogenetic pacing in vivo and lays out the basis for future optogenetic pacemaker and pain–free defibrillation therapies.

Investigation of G_q signalling with pharmacological agonists of G_q-coupled receptors lacks spatio-temporal precision. The aim of this study was to establish melanopsin, a light-sensitive G_q-coupled receptor, as a new tool for the investigation of spatial and temporal effects of G_q stimulation on pacemaking in cardiomyocytes at an early developmental stage. A vector for ubiquitous expression of melanopsin was tested in HEK293FT cells, which showed light-induced production of inositol-1,4,5-trisphosphate and elevation of intracellular Ca²⁺ concentration. Mouse embryonic stem cells were stably transfected with this plasmid and differentiated into spontaneously beating embryoid bodies (EBs). Cardiomyocytes within EBs showed melanopsin expression and illumination (60 s, 308.5 nW/mm², 470 nm) of EBs increased beating rate within 10.2 ± 1.7 s to 317.1 ± 16.3% of baseline frequency. Illumination as short as 5 s was sufficient for generating the maximal frequency response. After termination of illumination, baseline frequency was reached with a decay constant of 27.1 ± 2.5 s. The light-induced acceleration of beating frequency showed a sigmoid dependence on light intensity with a half maximal effective light intensity of 41.7 nW/mm². Interestingly, EBs showed a high rate of irregular contractions after termination of high-intensity illumination. Local G_q activation by illumination of a small region in a functional syncytium of cardiomyocytes led to pacemaker activity within the illuminated area. Light-induced G_q activation in melanopsin-expressing cardiomyocytes increases beating rate and generates local pacemaker activity. We propose that melanopsin is a powerful optogenetic tool for the investigation of spatial and temporal aspects of G_q signalling in cardiovascular research.

Side effects on cardiac ion channels are one major reason for new drugs to fail during preclinical evaluation. Herein we propose a simple optogenetic screening tool measuring extracellular field potentials (FP) from paced cardiomyocytes to identify drug effects over the whole physiological heart range, which is essential given the rate-dependency of ion channel function and drug action. Human induced pluripotent stem cell-derived cardiomyocytes were transduced with an adeno-associated virus to express Channelrhodopsin2 and plated on micro-electrode arrays. Global pulsed illumination (470 nm, 1 ms, 0.9 mW/mm²) was applied at frequencies from 1 to 2.5 Hz, which evoked FP simultaneously in all cardiomyocytes. This synchronized activation allowed averaging of FP from all electrodes resulting in one robust FP signal for analysis. Field potential duration (FPD) was ~25% shorter at 2.5 Hz compared to 1 Hz. Inhibition of hERG channels prolonged FPD only at low heart rates whereas Ca²⁺ channel block shortened FPD at all heart rates. Optogenetic pacing also allowed analysis of the maximum downstroke velocity of the FP to detect drug effects on Na⁺ channel availability. In principle, the presented method is well scalable for high content cardiac toxicity screening or personalized medicine for inherited cardiac channelopathies.