The heart is thought to be a postmitotic organ with terminally differentiated cardiomyocytes. Recently it has been demonstrated that there is evidence for cardiac turnover in the heart (Bergmann et al., 2009). However, the source of this cardiomyocyte renewal, which could be either existing cardiomyocytes or cardiac progenitor cells, remains elusive.

Our goal is to characterize the cell cycle status and DNA content of cardiomyocytes during postnatal development and after cardiac lesion in the mouse.

We have developed a new transgenic mouse model in which all cardiomyocyte nuclei are labelled by the fluorescence protein mCherry (αMHC-H2B-mCherry). This system allows the proper identification of cardiomyocyte nuclei in tissue sections and, after heart dissociation, the determination of the portion of cardiomyocytes and their degree of binucleation at different developmental stages.

To monitor cell cycle progression of cardiomyocytes in vivo and to visualize the onset of cell cycle variations like endoreduplication (DNA replication without karyokinesis or cytokinesis) and acytokinetic mitosis (karyokinesis without cytokinesis) postnatally, we used the eGFP-anillin system (Hesse et al., 2012). The eGFP-anillin fusion protein visualizes cytokinesis and midbody formation as hallmarks of cell division. The analysis of eGFP-anillin fluorescence in cardiomyocytes post-injury indicated that only a few border zone cardiomyocytes of the adult mouse heart are able to re-enter the cell cycle, but undergo endoreduplication and not cell division. Using high resolution microscopy of postnatal acute heart slices, we now try to live monitor the processes of endoreduplication and acytokinetic mitosis. In addition, this technique offers a novel, more physiological readout for the evaluation of putative proliferation inducing substances.

By combining the above mentioned transgenic models, we will be empowered to examine the plasticity of mono- and binuclear cardiomyocytes and to understand the cell biological mechanisms leading to terminal differentiation of cardiomyocytes.