Cardiac Optogenetics and Optical Mapping - Overcoming Spectral Congestion in All-Optical Cardiac Electrophysiology

Abstract

Optogenetic control of the heart is an emergent technology that offers unparalleled spatio-temporal control of cardiac dynamics via light-sensitive ion pumps and channels (opsins). This fast-evolving technique holds broad scope in both clinical and basic research setting. Combination of optogenetics with optical mapping of voltage or calcium fluorescent probes facilitates 'all-optical' electrophysiology, allowing precise optogenetic actuation of cardiac tissue with high spatio-temporal resolution imaging of action potential and calcium transient morphology and conduction patterns. In this review, we provide a synopsis of optogenetics and discuss in detail its use and compatibility with optical interrogation of cardiac electrophysiology. We briefly discuss the benefits of all-optical cardiac control and electrophysiological interrogation compared to traditional techniques, and describe mechanisms, unique features and limitations of optically induced cardiac control. In particular, we focus on state-of-the-art setup design, challenges in light delivery and filtering, and compatibility of opsins with fluorescent reporters used in optical mapping. The interaction of cardiac tissue with light, and physical and computational approaches to overcome the 'spectral congestion' that arises from the combination of optogenetics and optical mapping are discussed. Finally, we summarize recent preclinical work applications of combined cardiac optogenetics and optical mapping approach.

Keywords: action potential; arrhythmias; calcium; cardiac; conduction (action potential); fluorescence; optical mapping; optogenetic.

FIGURE 1

All-optical electrophysiology. Cardiac preparations treated with opsins and sensors allow simultaneous optically driven control of the heart (pacing or modulation of the action potential) and optical recording of action potential or calcium handling across the myocardium.

FIGURE 2

Optogenetic control of the cardiac action potential. (A) Opsins such as channelrhodopsins, by conducting cations such as H⁺_, Na⁺_, K⁺_, Ca²⁺ on light activation, can depolarize the cell membrane and hence initiate or prolong the cardiac action potential. (B) Hyperpolarizing opsins such as Halorhodopsins (Cl^- pumps), Bacteriorhodopsins (H⁺ pumps) and Anion Channelrhodopsins (I^-, Br^-, Cl^-_, F^- channels) can shorten or completely suppress the action potential.

FIGURE 3

Excitation spectra of the depolarizing opsin ChR2-H134 and voltage sensitive sensors di-4-ANEPPS and di-4-ANBDQBS. Significant overlap exists between the excitation spectra of ChR2 and di-4-ANEPPS, preventing imaging of di-4-ANEPPS without perturbing the membrane potential of ChR2 expressing cells. However, the red-shifted spectra of di-4-ANBDQBS does allow for excitation without simultaneous activation of ChR2.

FIGURE 4

Example effect of idealized optical filters on ‘white’ light. Arrows indicate path of light with indicated spectra pre and post filtering. (A) Bandpass filter. All wavelengths of light, other than those within the transmission window are absorbed. (B) Longpass filter. Light with wavelengths above the central wavelength (CWL) are transmitted, and all other wavelengths are absorbed. (C) Dichroic mirror. As with the longpass filter, light with wavelengths above the CWL are transmitted. Light with wavelengths below the CWL is reflected at an angle of incidence, commonly 45^°.