Abstract 14091: Systemic Hypoxemia Induces Cardiomyocyte Cell Cycle Re-Entry but Few Myocytes Appear to Truly Divide

Abstract

Rationale: The adult mammalian heart has a very limited capacity to regenerate cardiomyocytes that die from pathological damage. Several studies suggest that those few new myocytes found after injury are derived from pre-existing cardiomyocytes. A recent study suggests that systemic hypoxemia in adult mice can induce their cardiac myocytes to reenter the cell cycle and form two daughter cells. The goal of the present experiments was to determine if hypoxemia can induce adult murine cardiomyocytes to reenter the cell cycle and divide. Hypothesis: Hypoxia induces adult cardiomyocytes to complete the cell cycle and form new cardiac myocytes. Methods and Results: EdU mini pumps were implanted in 2-month-old C57BL/6 mice. Mice were then placed in a hypoxia chamber and the oxygen was lowered by 1% every day for 14 days to reach a final concentration of 7% oxygen. The animals remained in hypoxia (7% Oxygen) for 2 weeks and then were euthanized. Hypoxic mice had an increase in heart weight and echocardiography documented an increase in cardiac pump function and LV wall thickness when compared to control mice. Analysis of isolated cardiac myocytes in C57BL/6 animals showed an increase in EdU+ cardiomyocytes in hypoxemia and DAPI fluorescence intensity was also increased in these EdU+ myocytes which may indicate DNA synthesis. In fixed heart tissue sections, hypoxic mice contained more EdU+ myocytes and had an increase in the total amount of EdU+ cells in the heart. To further evaluate cardiomyocyte proliferation, we employed a mosaic mouse model with double markers (MADM) in which myocytes entering the cell cycle express both red fluorescent protein (RFP) and green fluorescent protein (GFP) and therefore are yellow in color. When cells complete the cell cycle and divide they express only RFP or GFP. MADM mice were exposed to hypoxia at 7% Oxygen as described above. MADM mice subjected to hypoxemia had increased numbers of yellow myocytes versus controls. There was a much smaller number of hypoxic myocytes with just RFP or GFP. Conclusion: Systemic hypoxia induces cardiac myocyte cell cycle re-entry and DNA synthesis, but it seems that very few of these myocytes complete the cell cycle and divide into two daughter cells.