Evidence suggesting that the cardiomyocyte circadian clock modulates responsiveness of the heart to hypertrophic stimuli in mice.

Circadian dyssynchrony of an organism (at the whole-body level) with its environment, either through light-dark (LD) cycle or genetic manipulation of clock genes, augments various cardiometabolic diseases. The cardiomyocyte circadian clock has recently been shown to influence multiple myocardial processes, ranging from transcriptional regulation and energy metabolism to contractile function. The authors, therefore, reasoned that chronic dyssychrony of the cardiomyocyte circadian clock with its environment would precipitate myocardial maladaptation to a circadian challenge (simulated shiftwork; SSW). To test this hypothesis, 2- and 20-month-old wild-type and CCM (Cardiomyocyte Clock Mutant; a model with genetic temporal suspension of the cardiomyocyte circadian clock at the active-to-sleep phase transition) mice were subjected to chronic (16-wks) biweekly 12-h phase shifts in the LD cycle (i.e., SSW). Assessment of adaptation/maladaptation at whole-body homeostatic, gravimetric, humoral, histological, transcriptional, and cardiac contractile function levels revealed essentially identical responses between wild-type and CCM littermates. However, CCM hearts exhibited increased biventricular weight, cardiomyocyte size, and molecular markers of hypertrophy (anf, mcip1), independent of aging and/or SSW. Similarly, a second genetic model of selective temporal suspension of the cardiomyocyte circadian clock (Cardiomyocyte-specific BMAL1 Knockout [CBK] mice) exhibits increased biventricular weight and mcip1 expression. Wild-type mice exhibit 5-fold greater cardiac hypertrophic growth (and 6-fold greater anf mRNA induction) when challenged with the hypertrophic agonist isoproterenol at the active-to-sleep phase transition, relative to isoproterenol administration at the sleep-to-active phase transition. This diurnal variation was absent in CCM mice. Collectively, these data suggest that the cardiomyocyte circadian clock likely influences responsiveness of the heart to hypertrophic stimuli.

Short communication: ischemia/reperfusion tolerance is time-of-day-dependent: mediation by the cardiomyocyte circadian clock.

RATIONALE: Cardiovascular physiology and pathophysiology vary dramatically over the course of the day. For example, myocardial infarction onset occurs with greater incidence during the early morning hours in humans. However, whether myocardial infarction tolerance exhibits a time-of-day dependence is unknown. OBJECTIVE: To investigate whether time of day of an ischemic insult influences clinically relevant outcomes in mice. METHODS AND RESULTS: Wild-type mice were subjected to ischemia/reperfusion (I/R) (45 minutes of ischemia followed by 1 day or 1 month of reperfusion) at distinct times of the day, using the closed-chest left anterior descending coronary artery occlusion model. Following 1 day of reperfusion, hearts subjected to ischemia at the sleep-to-wake transition (zeitgeber time [ZT]12) resulted in 3.5-fold increases in infarct size compared to hearts subjected to ischemia at the wake-to-sleep transition (ZT0). Following 1 month of reperfusion, prior ischemic event at ZT12 versus ZT0 resulted in significantly greater infarct volume, fibrosis, and adverse remodeling, as well as greater depression of contractile function. Genetic ablation of the cardiomyocyte circadian clock (termed cardiomyocyte-specific circadian clock mutant [CCM] mice) attenuated/abolished time-of-day variations in I/R outcomes observed in wild-type hearts. Investigation of Akt and glycogen synthase kinase-3beta in wild-type and CCM hearts identified these kinases as potential mechanistic ties between the cardiomyocyte circadian clock and I/R tolerance. CONCLUSIONS: We expose a profound time-of-day dependence for I/R tolerance, which is mediated by the cardiomyocyte circadian clock. Further understanding of I/R tolerance rhythms will potentially provide novel insight regarding the etiology and treatment of ischemia-induced cardiac dysfunction.

Direct regulation of myocardial triglyceride metabolism by the cardiomyocyte circadian clock.

Maintenance of circadian alignment between an organism and its environment is essential to ensure metabolic homeostasis. Synchrony is achieved by cell autonomous circadian clocks. Despite a growing appreciation of the integral relation between clocks and metabolism, little is known regarding the direct influence of a peripheral clock on cellular responses to fatty acids. To address this important issue, we utilized a genetic model of disrupted clock function specifically in cardiomyocytes in vivo (termed cardiomyocyte clock mutant (CCM)). CCM mice exhibited altered myocardial response to chronic high fat feeding at the levels of the transcriptome and lipidome as well as metabolic fluxes, providing evidence that the cardiomyocyte clock regulates myocardial triglyceride metabolism. Time-of-day-dependent oscillations in myocardial triglyceride levels, net triglyceride synthesis, and lipolysis were markedly attenuated in CCM hearts. Analysis of key proteins influencing triglyceride turnover suggest that the cardiomyocyte clock inactivates hormone-sensitive lipase during the active/awake phase both at transcriptional and post-translational (via AMP-activated protein kinase) levels. Consistent with increased net triglyceride synthesis during the end of the active/awake phase, high fat feeding at this time resulted in marked cardiac steatosis. These data provide evidence for direct regulation of triglyceride turnover by a peripheral clock and reveal a potential mechanistic explanation for accelerated metabolic pathologies after prevalent circadian misalignment in Western society.