Cardiac optogenetics: using light to monitor cardiac physiology

Abstract

Our current understanding of cardiac excitation and its coupling to contraction is largely based on ex vivo studies utilising fluorescent organic dyes to assess cardiac action potentials and signal transduction. Recent advances in optogenetic sensors open exciting new possibilities for cardiac research and allow us to answer research questions that cannot be addressed using the classic organic dyes. Especially thrilling is the possibility to use optogenetic sensors to record parameters of cardiac excitation and contraction in vivo. In addition, optogenetics provide a high spatial resolution, as sensors can be coupled to motifs and targeted to specific cell types and subcellular domains of the heart. In this review, we will give a comprehensive overview of relevant optogenetic sensors, how they can be utilised in cardiac research and how they have been applied in cardiac research up to now.

Keywords: Calcium cycling/excitation–contraction coupling; Cell signalling/signal transduction; Ion channels/membrane transport; Physiology.

Fig. 1

Motifs to target specific cardiac cells or cell organelles. a Overview of genes that are higher expressed in subareas of the heart and can be used to target specific cells [73]. The red region indicates atrial cells, the blue region ventricular cells. Genes from cardiac progenitor cells are between brackets, since it is unclear if they are indeed progenitor cell specific. Gene names can differ between species and gene expression may be dependent on developmental stage and specie. *Ly6a is only found in the mouse. b Overview of motifs that can be used to target specific locations within the cell

Fig. 2

Overview of cardiac optogenetic sensor designs. a–c each give an overview of a group of sensors that are relevant for cardiac research. For each sensor its mode of action is schematically visualised. Fluorescent proteins are depicted as coloured barrels, proteins or protein domains as white barrels. Coloured arrows indicate excitation/emission wavelengths. a Overview of optogenetic ion sensors. These sensors are based on proteins that can sense and bind the ion of interest with a high affinity. Upon binding, a conformational change occurs within the sensor, inducing or diminishing fluorescence or FRET [, –29, 35, 36, 40, 43, 44, 49, 50, 60, 72]. b Overview of optogenetic signal transduction sensors. These sensors consist of proteins or substrates that can bind the signalling molecule of interest. In case of a substrate, binding will result in activation or deactivation of the substrate. Ultimately, a conformational change of the sensor will induce or diminish fluorescence or FRET [6, 41, 45, 52, 53, 61, 63]. c Overview of optogenetic voltage sensors. VSD-based sensors are composed of a voltage-sensing transmembrane protein linked to either a single fluorescent protein or to a FRET fluorescent protein pair. When the membrane charges, the VSD displaces, giving rise to a fluorescent response [32, 42, 48, 62]. Opsin-based voltage sensors are based on microbial rhodopsin proton pumps and fluorescence is induced via a voltage-dependent shift in the acid–base equilibrium of the retinal Schiff base located in the proton pump [24, 25, 79]

Fig. 3

Solutions for cardiac movement artefacts. a Optical fibres are flexible and small, thus allowing the local recording of fluorescence even when movement occurs. The fibre ending is directly positioned against the cardiac tissue. b When studies are not limited by opening of the thorax, physical immobilisation of cardiac tissue can be achieved by attaching a rigid ring and applying a gentle vacuum. c Ratiometric imaging allows compensation of contraction related increases in fluorescence intensity, since it affects the intensities of both the donor and acceptor fluorophore the same. d Photoconversion can be used to convert specific cardiac regions and track these during contraction