Endocrine Abstracts

During early life, the majority of cardiomyocytes exit the cell cycle and undergo terminal differentiation, becoming binucleated. This establishes the number of cardiomyocytes for the remainder of the lifetime, with subsequent consequences for cardiac resilience in adulthood. Activation of the glucocorticoid receptor (GR) is important for heart maturation: fetal mice lacking GR in cardiomyocytes (SMGRKO) show structural and functional cardiac immaturity. Young adult male and female SMGRKO mice have heavier hearts but similar cardiomyocyte size compared to littermate controls suggesting that SMGRKO mice have more cardiomyocytes. We hypothesized that cessation of cardiomyocyte proliferation and subsequent binucleation will be delayed in neonatal SMGRKO mice, contributing to increased cardiomyocyte endowment. DNA replication, a marker of both proliferation and binucleation, was measured by incorporation of EdU, a thymidine analogue. Neonatal SMGRKO mice and littermate controls were injected with EdU (50mg/g bw) 4h prior to euthanasia at postnatal (P) days 2, 4, 7 or 10. Hearts were fixed, embedded, sectioned and immuno-stained for cardiac troponin and EdU. Nuclei were counter-stained with DAPI. The percentage of EdU-positive cardiomyocytes was measured. SMGRKO mice exhibited a different profile of DNA replication compared to controls. The percentage of EdU-positive cardiomyocytes was similar in both genotypes at P2, lower in SMGRKO mice at P4 (14.0±1.9 versus 11.4±1.6%, mean±SD, n=8) and higher at P7 (3.7±0.7% v 6.0±1.1%, n=7–11) with a trend towards greater DNA replication at P10. Mice lacking GR in cardiomyocytes show an altered pattern of DNA replication in the neonatal period, with elevated DNA replication during the later neonatal period. This suggests altered cardiomyocyte proliferation and/or binucleation, with delayed terminal differentiation of cardiomyocytes in the absence of GR. Future studies will compare the time course of binucleation in SMGRKO and control animals.