Despite recent efforts to stimulate endogenous cardiomyocyte proliferation for cardiac regeneration, the lack of reliable in vivo methods for monitoring cardiomyocyte replication has hindered our understanding of its mechanisms. Thymidine analogs, used to label proliferating cells, are unsuitable for long-term cardiac regeneration studies as their DNA incorporation elicits a damage response, leading to their elimination. Here we present CycleTrack, a genetic strategy based on the transcriptional activation of Cre recombinase from a temporally regulated cyclin B2 promoter segment, for permanent labeling of cardiomyocytes passing through the G2/M phase. Using CycleTrack, we visualized cardiomyocyte turnover in neonatal and adult mice under various conditions, including pregnancy, increased ventricular afterload, and myocardial infarction. CycleTrack also provided visual and quantitative evidence of ventricular remuscularization following treatment with pro-regenerative microRNAs. We identify the subendocardium as a key site of mitotic activity and provide a mode of cardiomyocyte division along their short axis. CycleTrack is a powerful tool to monitor cardiomyocyte renewal during regenerative interventions.
