Midbody Positioning and Distance Between Daughter Nuclei Enable Unequivocal Identification of Cardiomyocyte Cell Division in Mice

Rationale: New strategies in the field of cardiac regeneration are directed at identifying proliferation-inducing substances to induce regrowth of myocardium. Current screening assays utilize neonatal cardiomyocytes and markers for cytokinesis, such as Aurora B-kinase. However, detection of cardiomyocyte division is complicated because of cell cycle variants, in particular, binucleation. Objective: To analyze the process of cardiomyocyte binucleation to identify definitive discriminators for cell cycle variants and authentic cardiomyocyte division. Methods and Results: Herein, we demonstrate by direct visualization of the contractile ring and midbody in Myh6 (myosin, heavy chain 6)-eGFP (enhanced green fluorescent protein)-anillin transgenic mice that cardiomyocyte binucleation starts by formation of a contractile ring. This is followed by irregular positioning of the midbody and movement of the 2 nuclei into close proximity to each other. In addition, the widespread used marker Aurora B-kinase was found to also label binucleating cardiomyocytes, complicating the interpretation of existing screening assays. Instead, atypical midbody positioning and the distance of daughter nuclei on karyokinesis are bona fide markers for cardiomyocyte binucleation enabling to unequivocally discern such events from cardiomyocyte division in vitro and in vivo. Conclusions: The 2 criteria provide a new method for identifying cardiomyocyte division and should be considered in future studies investigating cardiomyocyte turnover and regeneration after injury, in particular in the postnatal heart to prevent the assignment of false positive proliferation events.