Cardiomyocyte heterogeneity in zebrafish development and regeneration

Contrary to adult mammals, zebrafish are able to regenerate their heart after cardiac injury. This regenerative response relies, in part, on the endogenous ability of cardiomyocytes (CMs) to dedifferentiate and proliferate to replenish the lost muscle. However, CM heterogeneity and population dynamics during development and regeneration remain poorly understood. Through comparative transcriptomic analyses of the developing and adult zebrafish heart, we identified tnnc2 and tnni4b.3 expression as markers for CMs at early and late developmental stages, respectively. Using newly developed reporter lines for these genes, we investigated their expression dynamics during development and regeneration, and observed interesting expression patterns. tnnc2 reporter lines label most CMs at embryonic stages, and this labeling declines rapidly during larval stages; in adult hearts expression is only detectable in a subset of CMs. Conversely, expression of a tnni4b.3 reporter is initially visible in outer curvature CMs at larval stages, and it is subsequently present in a vast majority of the CMs in adult hearts. Interestingly, we find that the adult CMs labeled by the embryonic reporter display higher levels of Tg(tp1:EGFP) expression, which indicates active Notch signaling. To further characterize this CM population, we performed transcriptomic analysis and found that it expresses genes encoding components of the Notch signaling pathway as well as markers of immature CMs. Moreover, during heart regeneration, proliferating CMs in the injured area activate the embryonic CM reporter. Overall, our findings provide further evidence of cardiomyocyte heterogeneity in the adult zebrafish heart.