Visualization of Cell Cycle Variations and Determination of Nucleation in Postnatal Cardiomyocytes

Generation and Characterization of an Inducible Cx43 Overexpression System in Mouse Embryonic Stem Cells

Connexins (Cx) are a large family of membrane proteins that can form intercellular connections, so-called gap junctions between adjacent cells. Cx43 is widely expressed in mammals and has a variety of different functions, such as the propagation of electrical conduction in the cardiac ventricle. Despite Cx43 knockout models, many questions regarding the biology of Cx43 in health and disease remain unanswered. Herein we report the establishment of a Cre-inducible Cx43 overexpression system in murine embryonic stem (ES) cells. This enables the investigation of the impact of Cx43 overexpression in somatic cells. We utilized a double reporter system to label Cx43-overexpressing cells via mCherry fluorescence and exogenous Cx43 via fusion with P2A peptide to visualize its distribution pattern. We proved the functionality of our systems in ES cells, HeLa cells, and 3T3-fibroblasts and demonstrated the formation of functional gap junctions based on dye diffusion and FRAP experiments. In addition, Cx43-overexpressing ES cells could be differentiated into viable cardiomyocytes, as shown by the formation of cross striation and spontaneous beating. Analysis revealed faster and more rhythmic beating of Cx43-overexpressing cell clusters. Thus, our Cx43 overexpression systems enable the investigation of Cx43 biology and function in cardiomyocytes and other somatic cells.

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.

Solving the Polyploid Mystery in Health and Disease

Polyploidy (the more than doubling of a cell's genome) frequently arises during organogenesis, tissue repair, and age-associated diseases. Despite its prevalence, major gaps exist in how polyploid cells emerge and affect tissue function. Studies have begun to elucidate the signals required for polyploid cell growth as well as the advantages and disadvantages of polyploidy in health and disease. This review highlights the recent advances on the role and regulation of polyploidy in Drosophila and vertebrate models. The newly discovered versatility of polyploid cells has the potential to provide alternative strategies to promote tissue growth and repair, while limiting disease and dysfunction.