Endocrine Abstracts

In neonates, cardiomyocytes exit the cell cycle thus establishing cardiomyocyte number for life. Further growth is through hypertrophy. Factors that advance the timing of the switch from hyperplastic to hypertrophic growth may increase risk of cardiac disease in adulthood. Early life administration of glucocorticoids is known to increase risk of cardiovascular disease. We hypothesized that dexamethasone, a synthetic glucocorticoid, causes precocious cell cycle exit of neonatal mouse cardiomyocytes. The Fluorescent Ubiquination-based Cell Cycle Indicator (Fucci2a) system reports on cell cycle in vivo: mCherry-hCdt1 (red) and mVenus-hGem (green) are differentially degraded through the cell cycle, labelling cells in the G1/G0 and S/G2/M phases, respectively. R26Fucci2aRTg/Tg dams were crossed with rat cardiac troponin T promoter (Tnnt2)-Cre+ males to drive Cre expression in cardiomyocytes of TNT.R26Fucci2a fetuses. Neonates were treated with dexamethasone (500ug/kg, i.p.) at postnatal day (P)1, P3 or P6. After 24 hours, neonatal hearts were collagenase digested and the number of mCherry vs mVenus positive cells was quantified by flow cytometry. Nucleation was assessed by Draq5 staining. Data are mean±SD. The proportion of mCherry+ cardiomyocytes (G1/G0) was high at all ages in vehicle treated mice and was further elevated in dexamethasone-treated neonates compared to vehicle (87.2±3.3% vs 94.1±1.3% P2, 62.4±6.1% vs 84.4±9%.0 P4, 77.0±4.4% vs 88.3±4.6% P7). The proportion of mVenus+ cardiomyocytes (S/G2/M) in vehicle-treated mice was highest at P4 (5.4±2.9%) treated mice compared to P2 (1.8±0.99%) and P7 (2.1±0.55%). Dexamethasone reduced the proportion of mVenus+ cardiomyocyte population to negligible levels at all ages (0.12±0.11% P2, 0.18±0.11% P4, 0.006±0.011% P7) and reduced the proportion of binucleated cardiomyocytes at P4 and P7. These data suggest that dexamethasone treatment in early life may cause early cell cycle exit in cardiomyocytes, with a lifelong associated reduction in cardiomyocyte number.