Influence of the Cardiomyocyte Circadian Clock on Autophagy in the Heart

Abstract

BackgroundIn addition to its roles during physiologic (e.g., fasting) and pathologic (e.g., ischemia) states, autophagy is critical for cellular ‘housekeeping’, playing an important role in the normal turnover of damaged proteins and organelles (e.g., mitochondria). Over the course of the day, cells/organs are subjected to profound fluctuations in energy supply/demand, as well as exposure to environmental stresses known to induce protein/organelle dysfunction. This is particularly true for the heart, whose workload and oxidative metabolism dramatically increase during the awake (versus sleep) period. Recent studies have shown that the cardiomyocyte circadian clock directly modulates myocardial processes in a time‐of‐day‐dependent fashion, including metabolism and contractile function. The importance of the cardiomyocyte circadian clock is highlighted by observations that genetic disruption of this mechanism in mice (i.e., cardiomyocyte‐specific BMAL1 knockout; CBK) results in age‐onset cardiomyopathy and premature death.HypothesisWe hypothesize that the cardiomyocyte circadian clock directly regulates autophagy in the heart (which might be important for the normal turnover of proteins/organelles).Methods and ResultsConsistent with time‐of‐day‐dependent oscillations in autophagy, the LC3II/I ratio and p62 protein levels both oscillate in the mouse heart over the course of the day; these oscillations occur in an antiphase manner (i.e., LC3II/I peaks during the light/sleep phase, whereas p62 peaks during the dark/awake phase). Somewhat surprisingly, time‐of‐day‐dependent oscillations in LC3II/I and p62 persist in CBK hearts. However, p62 protein levels are chronically elevated, whereas p62 mRNA are chronically decreased, in CBK hearts, suggesting a potential negative impact of circadian clock disruption on autophagic flux. Fasting‐induced stimulation of autophagy was therefore next investigated in control and CBK hearts. Despite persistent food withdrawal, autophagy markers exhibited time‐of‐day‐dependent oscillations in the fasted state; in the case of LC3II/I, oscillations in this marker were antiphase in hearts of fed versus fasted control mice. Importantly, fasting‐induced alterations in LC3II/I were significantly attenuated in CBK hearts. Key regulators of autophagy were subsequently investigated, revealing that phosphorylation of mTOR (marker of activation) was chronically elevated in CBK hearts, relative to littermate controls.SummaryThe cardiomyocyte circadian clock appears to be an important regulator of cardiac autophagy. We speculate that chronic impairment of autophagy in the heart following disruption of the cardiomyocyte circadian clock contributes to cardiomyopathy in CBK mice.Support or Funding InformationUAB School of Medicine Multi‐Investigator Pilot Grant;NIH R01 HL123574;NIH R01 HL122975