Abstract 188: Sarcomere Protein Cardiac Myosin Binding Protein C Regulates Cardiomyocyte Proliferation

Abstract

Introduction: Wildtype neonatal murine cardiomyocytes become bi-nucleated, hypertrophy and terminally differentiate, and concurrently develop well-organized striation patterns of sarcomeres of myofibrils. Neonatal mice lacking cardiac myosin binding protein C (Mybpc t/t ) have subtle abnormalities in myofibrillar structure and ventricular dilation, but remain healthy life-long. Hypothesis: Deficits in MybpC impacts formation of bi-nucleated and cell proliferation of cardiomyocytes. Methods: We studied the cardiac development in Mybpc t/t mice and hearts depleted of cMyBP-C by adeno-associated virus (AAV)-mediated RNAi. Results: Mybpc t/t hearts presented significant increase in mono-nucleated cardiomyocytes (18.28 ± 1.68%) and decrease in bi-nucleated cardiomyocytes (74.87 ± 87%), compared to wildtype hearts (mono-nucleated: 5.85% ± 1.14%, p<0.001; bi-nucleated: 89.83 ± 2.04%, p<0.001, n=4 in each genotype). Similar distribution of mono-nucleated (25.25 ± 3.15%) and bi-nucleated cardiomyocytes (73.55 ± 3.8%) was also observed in cardiomyocytes depleted of cMyBP-C by AAV-mediated RNAi (n=4). Significant increase in EdU incorporation was observed in Mybpc t/t cardiomyocytes (47.02 ± 2.5%) compared to wildtype cardiomyocytes (29.35 ± 3.06%, p<0.005). Phosphorylated histone H3, a marker of mitosis, was approximately two-fold more positive in Mybpc t/t than wildtype cardiomyocytes at postnatal day 3, day 7 and day10 (p<0.01). In addition, we observed a 15-fold increase of Aurora B expression within the cytoplasmic bridge between cardiomyocytes undergoing cytokinesis in Mybpc t/t hearts compared to wildtype hearts at postnatal day 7 (p<0.001). The predicted consequence of protracted cardiomyocyte proliferation in neonatal Mybpc t/t mice, a significantly increased number of cardiomyocytes, was demonstrated. In comparison to wildtype, the number of cardiomyocytes in Mybpc t/t hearts was significantly increased by 40% (n=3 in each group, p<0.01). Conclusions: Loss of cMyBP-C causes neonatal cardiomyocytes to undergo extra round of cytokinesis and continued progression of the cell cycles. Changes in cell number, rather than cell size, may be responsible for morphologic abnormalities of some cardiomyopathies.