The best thing since sliced bread? Optical mapping of transverse cardiac slices in the mouse heart

Mice are a mainstay in cardiovascular and cardiac electrophysiology research. Although novel technologies have emerged that allow for genetic manipulation in larger species, due to the vast number of existing genetically modified murine models, combined with small size, low cost, and numerous established surgical models, the mouse will likely remain a pillar of mechanistic cardiovascular research for decades to come. A detailed and comprehensive understanding of similarities and key differences between mice and larger species is therefore of paramount importance for meaningful translation of findings from the mouse heart to human patients. The cardiac electrophysiology community is acutely aware of differences between the murine and human heart, especially with regard to ion channels and corresponding currents that shape the action potential (Bartos et al. 2015). Although there is an abundance of data on murine ionic currents, as well as intracellular Ca2+ handling and excitation-contraction coupling, there is a sparsity of information on regional heterogeneity of these properties throughout the mouse heart (i.e. right versus left ventricle, apico-basal, transmural). This knowledge gap probably exists due to the small size of the heart, as it is technically challenging to isolate myocytes from precise locations or to dissect and perfuse ventricular wedge or other tissue preparations, as is common in larger species. However, knowledge of these regional heterogeneities is absolutely necessary for understanding drug effects, pathological remodelling during disease, and ultimately, mechanisms of arrhythmogenesis. In this issue of The Journal of Physiology, Wen and colleagues (2018) present a novel optical mapping approach using short-axis tissue slices from the mouse heart that provides for unprecedented, and potentially high-throughput, characterization of action potentials (APs) and intracellular Ca2+ transients (CaTs) everywhere in the mouse ventricles. Following dye loading via traditional Langendorff perfusion, the ventricles were embedded in low-melt agarose and thin (300 μm) short-axis slices were cut from apex to base, resulting in approximately 20 slices comprising the entire mouse ventricles (akin to a loaf of sliced bread). Optical mapping was then performed on each slice, allowing for complete characterization of APs and CaTs throughout the entire heart with a high degree of regional precision, since the apico-basal location of each slice is known, and every slice is fully transmural. This rich data set allowed for characterization of regional AP and CaT properties, including AP and CaT durations, alternans thresholds, and transmural activation and conduction velocities. Importantly, the values of epicardial parameters and conduction velocity are in agreement with published optical mapping data from intact murine hearts, suggesting the slice approach is not dramatically damaging or altering tissue properties. Using this elegant approach, the authors found significant transmural gradients in AP and CaT durations that are similar to those observed in larger species, with longer durations on the endocardium versus epicardium (Laurita et al. 2003). Longer CaT durations are indicative of slower sarcoplasmic reticulum Ca2+ reuptake, which has been linked to arrhythmogenic Ca2+ alternans. Indeed, just as in the canine heart, transmural imaging of the mouse ventricles revealed more severe Ca2+ alternans on the endocardium where CaT duration was prolonged (Laurita et al. 2003). This strong agreement in transmural properties between mice and larger species suggests that the mouse may reliably reproduce some aspects of ventricular transmural heterogeneity. No significant apico-basal gradients were observed in the mouse heart, but AP and CaT durations were found to be shorter in the right versus left ventricle, mimicking right-left ventricular gradients in larger species (Volders et al. 1999). Of course, this approach is not limited to detailed characterization of healthy mouse hearts. This technique may prove extremely valuable for characterizing regional arrhythmogenic changes in genetic murine models of cardiovascular disease (e.g. catecholaminergic polymorphic ventricular tachycardia, hypertrophic cardiomyopathy), as well as comprehensive regional characterization of remodelling in acquired pathologies (myocardial infarction, pressure-overload heart failure) that may not be possible in larger species. The slice approach may also prove useful for higher throughput studies. Depending on the optics, camera resolution, and slice size and position, it may be possible to image several or all slices from a single mouse heart simultaneously rather than sequentially. Wen and colleagues are not the first to optically map cardiac tissue slices, as there are previous reports of mapping of heart slices in larger species, including human hearts where viral transfection was also achieved by maintaining the slices in culture (Kang et al. 2016). Whether such tissue culture and transfection approaches are possible in the mouse slices remains to be determined. But any way you slice it, cardiac tissue slices and corresponding technologies to study them are emerging as valuable experimental techniques that will likely lead to novel insights for decades to come. None declared. C. M. Ripplinger is supported in part by AHA 16GRNT30960054 and NIH R01HL111600.