Environmental cues signal through circadian control of sarcomeric gene expression during cardiomyocyte hypertrophy

Environmental disruption alters circadian clock gene expression, increasing the risk of adverse cardiac events and suggesting cardiomyocyte-specific circadian responses to external stimuli. Analyses of previously reported transcriptomic data revealed increased expression of Titin-cap (Tcap) in adult compared to embryonic myocytes and identified myosin light chain 2 (Myl2) as clock-controlled. Given cardiac sarcomeric roles of the encoded proteins, we hypothesized that extracellular cues driving postnatal cardiac maturation and hypertrophy influence time-of-day Tcap and Myl2 expression. Tcap induction was concomitant with neonatal myocyte binucleation, fetal gene suppression, and increased heart weight during the early phase of cardiac growth. Since norepinephrine stimulates β-adrenergic and α-adrenergic receptors, the latter driving clock-controlled transcriptional remodeling, phase‑response curves of the β‑adrenergic agonist isoproterenol (ISO) following α-adrenergic stimulation with phenylephrine (PE) were performed on neonatal rat ventricular myocytes (NRVM), revealing periodic myocyte hypertrophy and TCAP protein expression. PE entrenched ISO-mediated Tcap suppression, to which oscillatory Per2 and Myl2 transcription was impervious. Differential NRVM culture density revealed biomass-dependent changes in Per2 and Tcap transcription and hypertrophy timing. Hypoxia initiated myocyte atrophy and Bmal1-dependent Tcap transcription, reflected in decreased heart weight and increased Tcap expression in hypoxic neonatal rat hearts. Tcap depletion impaired Bmal1 and fetal hypertrophic gene expression, compromised hypoxia-mediated Myl2 transcriptional suppression, and aggravated hypoxia-induced atrophy. In summary, Tcap and Myl2 are circadian genes differentially influenced by environmental factors, including adrenergic stimulation, paracrine signaling, and O₂ tension during postnatal cardiac maturation. These findings have implications for the distinct regulation of myocyte growth and maturation, by external cues during the postnatal period.