Regulation of Myocardial Metabolism by the Cardiomyocyte Circadian Clock

Abstract

On a daily basis, the heart is subjected to dramatic fluctuations in energetic demand and neurohumoral influences, many of which occur in a temporally predictable manner. In order to preserve cardiac performance, the heart must therefore maintain metabolic flexibility, even within the confines of a single day. Recent studies have established mechanistic links between time-of-day-dependent oscillations in myocardial metabolism and the cardiomyocyte circadian clock. More specifically, evidence suggests that this cell autonomous molecular mechanism regulates myocardial glucose uptake, flux through both glycolysis and the hexosamine biosynthetic pathway, and pyruvate oxidation, as well as glycogen, triglyceride, and protein turnover. These observations have led to the hypothesis that the cardiomyocyte circadian clock confers the selective advantage of anticipation of increased energetic demand during the awake period. Here, we review the accumulative evidence in support of this hypothesis thus far, and discuss the possibility that attenuation of these metabolic rhythms, through disruption of the cardiomyocyte circadian clock, contributes towards the etiology of cardiac dysfunction in various disease states.

Introduction

Anticipation is a selective advantage; intuitively, preparing for a specific stimulus or stressor before it occurs will facilitate an optimal response (in terms of both magnitude and timing of the response) [1]. A critical aspect of anticipation involves knowing when a distinct stimulus/stressor will present itself (i.e., prior to its onset) and in mammals this is conferred by circadian clocks [1, 2]. Circadian clocks are cell autonomous molecular mechanisms that reside within essentially all mammalian cells, including cardiomyocytes [2, 3]. These molecular mechanisms are comprised of a distinct set of transcription factors that form a series of positive and negative feedback loops, which oscillate with a periodicity of approximately 24 hours (Figure 1) [1, 2]. It has been estimated that circadian clocks modulate expression of approximately 10–15% of the transcriptome in a time-of-day-dependent manner [4–6]. Clock-regulated genes include critical modulators of transcription, translation, signal transduction, ion homeostasis, and metabolism [4–6]. Recent studies have highlighted a complex interaction between circadian clocks and metabolism, wherein clocks modulate metabolism over the course of the day, and metabolism fine tunes the clock [7, 8]. This interaction has been revealed both at the whole body and organ-specific level. [7, 8] The purpose of this review is to highlight recent advances regarding the influence that the cardiomyocyte circadian clock exerts over myocardial metabolism. We will initially provide a general overview on myocardial metabolism (Figure 2), followed by a summary regarding how the intrinsic cardiomyocyte clock modulates this important aspect of cardiac biology.

Figure 1. The cardiomyocyte circadian clock.

− represents a negative feedback loop. + represents a positive feedback loop. Clock controlled genes (also referred to as output genes) do not directly feedback on the circadian clock mechanism, and instead mediate time-of-day-dependent fluctuations in cardiac function.

Figure 2.

General overview of myocardial metabolic pathways.

Myocardial Metabolism – Energetics and Beyond

The ability of the heart to maintain contractile function and to rapidly adapt to acute changes in demand is predicated not only on the availability of carbon substrates for oxidative energy production but perhaps more importantly the metabolic flexibility needed to be able to utilize a continuously variable substrate milieu. The foundation of our understanding of metabolic regulation was established by the pioneering work of Randle and colleagues, who proposed in 1963 the classical glucose-fatty acid cycle in which increased fatty acid availability suppressed glucose oxidation [9]. This was followed over the next 2 decades or so, by increasingly sophisticated approaches to understand the molecular mechanisms underlying the reciprocal relationship between glucose and fatty acid oxidation. The result was an emphasis on critical metabolism regulators, with increased CPT1 activity and fatty acid oxidation leading to decreased PDH activity and glucose oxidation. However, in addition to acute responses to substrate availability, in the setting of sustained metabolic dysfunction (e.g., during diabetes) the heart develops a metabolic inflexibility (characterized by increased fatty acid oxidation and decreased glucose oxidation in the case of diabetes). These observations have led to speculation that metabolic inflexibility in states such as diabetes and hypertrophy significantly contributes towards the etiology of contractile dysfunction [10, 11]. For example, it has become widely accepted that increased dependence on fatty acids for energy production impairs cardiomyocyte function because they are less efficient fuels (i.e., greater oxygen requirement per ATP generated, compared to glucose) and they cause “excess” proton production due to an “uncoupling” between glucose oxidation and glycolysis [12]. Increased cycling of fatty acyl groups through the triglyceride pool will also decrease cardiac efficiency [13]. Recent research efforts have also focused on the mechanisms by which excess fatty acid availability (during states such as diabetes) leads to cardiac dysfunction through imbalances in cellular signaling cascades [14].

Many studies of cardiac metabolism focus primarily on glucose and fatty acid utilization. Although under normal physiological conditions alternative fuels such as pyruvate and ketone bodies circulate at very low concentrations (i.e., <0.2mM), several studies suggest that they contribute significantly to oxidative energy production [15, 16]. Reliance on these alternative fuels likely increases further during physiologic states, such as increased physical activity. For example, during acute exercise bouts, circulating ketone bodies increase markedly [17]. Similarly, lactate, often considered a byproduct of glycolytic metabolism, is readily oxidized by the heart and circulating levels can be appreciable, easily reaching 3–5mM in response to moderate exercise [18]. The relative importance of considering these alternative fuels has been highlighted by us and others: when physiologically relevant mixtures of lactate and pyruvate are present in addition to glucose and fatty acids, increasing availability of exogenous fatty acids has little or no effect on glucose oxidation, but rather decreases lactate and pyruvate oxidation [19]. Similar observations were also seen in the diabetic heart, where decreased total carbohydrate oxidation is due to a reduction in lactate and pyruvate oxidation, with no change in glucose oxidation [19, 20].

While there is little doubt that acute augmentation of glucose uptake is cardioprotective and conversely prolonged exposure to high glucose impairs cardiomyocyte function, various studies (including some discussed above) suggest that these effects may extend beyond changes in oxidative energy metabolism [21, 22]. The hexosamine biosynthesis pathway (HBP) has long been recognized as a glucose or nutrient sensing pathway, and activation of this pathway has been implicated in the adverse effects associated with sustained hyperglycemia or diabetes [23, 24]. Glucose entry into the HBP is principally regulated by L-glutamine-D-fructose 6-phosphate amidotransferase (GFAT), which catalyzes the conversion of fructose-6-phosphate to glucosamine-6 phosphate, ultimately leading to the synthesis of UDP-GlcNAc (Figure 2). UDP-GlcNAc is the substrate not only for traditional protein glycosylation but also for O-GlcNAc transferase (OGT: uridine diphospho-N-acetylglucosamine: polypeptide β-N-acetylglucosaminyltransferase) a unique glycosyl transferase, which catalyzes the attachment of a single O-GlcNAc to specific Ser/Thr residues of nuclear and cytoplasmic proteins. This reversible post-translational modification of proteins draws parallels to phosphorylation, including modulation of protein activity, stability, and sub-cellular localization. OGT activity is very sensitive to changes in UDP-GlcNAc levels and thus the overall rate of O-GlcNAc synthesis appears to be tightly regulated by HBP flux; consequently, it has become fairly widely accepted that changes in the levels of O-GlcNAcylation represent a primary nutrient sensing mechanism in mammals. However, it should also be noted in addition to being regulated by OGT, protein O-GlcNAcylation is also regulated by O-GlcNAcase (OGA: β-N-acetylglucosaminidase), which catalyzes the removal of O-GlcNAc from proteins [23, 24].

The foundation for much of our understanding of the role of O-GlcNAc in regulating cellular function has been in the context of metabolic disease and nutrient excess [25]. Indeed many studies that have linked the adverse effects of hyperglycemia on various cells and tissue, including cardiomyocytes, to increased flux through the HBP and subsequent increases in O-GlcNAc levels [23]. However, it is now well established that protein O-GlcNAcylation goes well beyond simply mediating the adverse effects of hyperglycemia, and plays a critical role in regulating a diverse array of cellular processes, including cell survival and apoptosis, translation and transcription, signal transduction and proteasome function. Our knowledge of the impact of O-GlcNAcylation in heart has grown rapidly over the past decade or so; in addition to the detrimental effects of diabetes, acute activation of O-GlcNAc synthesis has been shown to be remarkably cardioprotective in the setting of ischemia/reperfusion and it has recently been reported that OGT and O-GlcNAc synthesis may be essential for the early activation of cardiomyocyte hypertrophic transcription pathways [26]. A number of key proteins involved in regulating metabolism are also known targets for O-GlcNAcylation, such as Akt, IRS1/2, GLUT4, glycogen synthase and most recently CD36/FAT (which contributes to transport of long chain fatty acids into the cell) [24, 27]. A number of transcription factors involved in metabolic regulation, including PPARα, FOXO1, CREB, ChREPB and LXRα have also been shown to be targets for O-GlcNAcylation [24, 25]. The potential importance of protein O-GlcNAcylation in acute physiologic regulation of cardiac function was also recently underscored by observations that total levels oscillate in the heart in a time-of-day-dependent manner [28].

Time-of-Day-Dependent Changes in Myocardial Metabolism

As highlighted in the preceding section, in order to maintain adequate cardiac output, the myocardium modifies metabolism in response to a host of environmental influences. Many of these environmental stimuli/stressors, such as workload, neural (e.g., sympathetic tone) and humoral (e.g., insulin) factors, fluctuate in a time-of-day-dependent manner [29–33]. For example, relative to the sleep phase the awake state is associated with an elevation in blood pressure and catecholamines, as well as other signals arising as a consequence of the transition from the fasting to fed state [34, 35]. Consequently, it is not surprising that myocardial metabolism exhibits a marked time-of-day-dependence at multiple levels with rhythms in carbohydrate, lipid, and protein turnover described for the heart both in vivo and ex vivo (as highlighted below).

During periods of increased energetic demand, the myocardium increases reliance on carbohydrate (glucose, glycogen and lactate) as a fuel [18, 36, 37]. Similarly, post-prandial elevations in circulating factors such as insulin are anticipated to promote glucose uptake and utilization by the heart, resulting in increased rates of glycolysis and glucose oxidation during the awake period (relative to the sleep phase), which persist in the ex vivo setting [28, 38]. A dichotomy exists for glycogen turnover, wherein increased cardiac work promotes glycogenolysis, while insulin stimulates glycogen synthesis [36, 39, 40]. However, it has been reported that in the heart net glycogen synthesis occurs during the active period (when cardiac work is increased), while net glycogenolysis occurs during the sleep phase [38]. Such observations appear to indicate that rhythms in feeding status exert dominance over glycogen metabolism; however, 24-hour rhythms in glycogen turnover observed in the ad libitum fed rodent persist even during prolonged fasting, suggesting that this is independent of feeding status [41]. With regards to glucose-mediated signaling in the heart, recent studies report that total protein O-GlcNAcylation is elevated in the heart during the active period, consistent with both increased substrate availability for the hexosamine biosynthetic pathway, as well as OGT expression, at this time [28].

In contrast to reliance on carbohydrate metabolism, acute alterations in workload have minimal effects on rates of myocardial fatty acid oxidation [36]. Non-oxidative fatty acid metabolism (e.g., triglyceride turnover) is exquisitely sensitive to a host of neurohumoral factors know to oscillate in a time-of-day-dependent manner (e.g., adrenergic and insulin stimulation) [13, 42, 43]. Taken together, it is therefore not surprising that triglyceride turnover, but not fatty acid oxidation, has been shown to exhibit diurnal oscillations in the heart [38, 44]. Analogous to glycogen turnover, net triglyceride synthesis is increased in the heart during the active period, while net lipolysis is elevated during the sleep phase, as evidenced in both in vivo and ex vivo models [44]. Regarding lipid-derived signaling in the heart, net phospholipid (and cholesterol ester) synthesis appears to be highest during the sleep phase [38]. The potential functional consequence of this was recently highlighted when hearts were challenged with an acute elevation in fatty acid availability; greatest fatty acid-induced depression of contractile dysfunction is observed during the sleep phase [38].

Compared to carbohydrate and lipid metabolism, substantially less is known regarding time-of-day-dependent oscillations in protein turnover and amino acid metabolism in the heart. Intuitively, one would predict that during the active period post prandial signals, such as insulin would promote net protein synthesis as well as amino acid uptake, while increased energetic demand at this time might exert an opposing action. However, previously published studies reveal that net protein synthesis is increased in the myocardium during the sleep phase in the in vivo setting, and that they persist during fasting, suggesting that post-prandial signals do not play a dominant role [45]. Indirect evidence also exists that amino acid metabolism exhibits a diurnal variation in the heart, including observations that steady state levels of distinct amino acids are altered in the heart in a time-of-day-dependent manner, and that microarray studies reveal that gene expression of a number of key enzymes involved in amino acid metabolism similarly exhibit diurnal variations in the heart [4–6, 46]. Clearly additional studies are required to address important questions related to various aspects of organelle and protein turnover, as well as the mechanisms underlying rhythms in these fundamental processes in the heart.

Circadian Clock Regulation of Myocardial Metabolism

While it is evident from the discussion above that carbohydrate, lipid, and protein metabolism exhibit time-of-day-dependent rhythms in the heart questions still remain as to how this regulation occurs and what the advantage of such rhythms might be. Insights into these fundamental questions are beginning to emerge, and are highlighted below.

Signals or stressors associated with active/sleep- and/or feeding/fasting-transitions could potentially mediate one or more aspects of diurnal variations in myocardial metabolism. These signals, namely workload, adrenergic stimulation, and/or insulin, fluctuate concomitantly with nutrient availability/intake. However, as alluded to above, a number of observations are inconsistent with this hypothesis, including the persistence of a number of metabolic oscillations both during prolonged fasting and under controlled ex vivo conditions. Furthermore, with regard to protein synthesis, oscillations in rates of this process are antiphase to the circulating levels of its major activator, insulin; in other words, rates of protein synthesis are at their highest when insulin levels are lowest [45, 47, 48]. Collectively, these observations suggest that time-of-day-dependent oscillations in myocardial metabolism cannot be mediated purely by factors extrinsic to the heart. One intrinsic mechanism in the heart, with the capability of modulating myocardial processes in a time-of-day-dependent manner, is the cardiomyocyte circadian clock [3, 29, 49].

Circadian clocks reside within essentially every mammalian cell, and influence cellular functions in a time-of-day-dependent manner [1, 2]. Accordingly, mouse models wherein circadian clock components have been genetically ablated in a ubiquitous manner exhibit varying degrees of circadian disruption at multiple levels, including behavioral, cellular, and molecular [50–52]. Two distinct mouse models have recently been generated in which the circadian clock mechanism has been disrupted in a cardiomyocyte-specific manner. These models each targeted one of two critical circadian clock components: CLOCK (disrupted in cardiomyocyte-specific CLOCK mutant [CCM] mice) and BMAL1 (disrupted in cardiomyocyte-specific BMAL1 knockout [CBK] mice) [6, 53]. In contrast to those animal models where the circadian clock was disrupted in the whole animal, these cardiac specific models exhibit normal behavioral and neurohumoral rhythms, enabling us to specifically explore the role of the cardiomyocyte circadian clock in the regulation of myocardial metabolism.

The time-of-day-dependent rhythms in glycolysis, glycogen synthesis, glucose oxidation, protein O-GlcNAcylation, and triglyceride synthesis which are all present in wild-type hearts are all abolished in CCM hearts, demonstrating their regulation by the intrinsic cardiomyocyte circadian clock [28, 44]. Although protein synthesis rates have yet to be determined in either of these models, indirect evidence suggests that the cardiomyocyte circadian clock may also influence this aspect of myocardial metabolism. For example, acute (1 week) isoproterenol-induced cardiac hypertrophy was shown to exhibit a time-of-day-dependence, with greatest hypertrophic growth during the sleep phase, which is consistent with greater protein synthesis rates during this time period: these time-of-day-dependent oscillations in hypertrophic growth are absent in CCM hearts [53]. As noted above, studies of cardiac metabolic regulation typically focus on glucose and fatty acid metabolism; however, Stowe et al have shown that the murine heart exhibits a relatively high reliance on ketone body utilization, even in the fed state [16]. More recently, in preliminary studies using both CCM and CBK mice we have found that myocardial ketone body utilization also appears to be under direct clock control (Young et al, unpublished observations).

These mouse models have also provided a valuable platform for identifying putative mediators linking the cardiomyocyte circadian clock with myocardial metabolism. A number of well-established key modulators of cardiac metabolism have been identified as being under the control of the cardiomyocyte circadian clock, including AMPK, GSK3β, Akt, OGT, GLUL, BDH1, DGAT2, and NAMPT [28, 44, 54]. Circadian regulation of NAMPT suggests influence of the cardiomyocyte clock on myocardial NAD metabolism, similar to that recently described studies in the liver [55, 56]. Indeed, NAD levels oscillate in wild-type hearts, and are chronically repressed in CCM hearts [44, 57]. The regulation of NAD levels in a diurnal manner by the cardiomyocyte circadian clock has the potential for impacting a host of NAD-dependent processes, particularly metabolism. It should be noted that myocardial metabolism in turn likely acts in a ‘feedback’ manner, to influence the cardiomyocyte circadian clock. For example, sirtuins (deacetylases) are sensitive to cellular NAD levels, and have been shown to deacetylate two established circadian clock components (BMAL1 and Per2) in a time-of-day-dependent manner [58–60].

As noted above O-GlcNAcylation is a nutrient driven post-translational modification of proteins, which has been linked to the regulation of a diverse array of cellular processes. Many of the proteins that have been shown to regulate cardiac metabolism and well as be modulated by the circadian clock, such as GSK3β, Akt and AMPK have all also been reported to be targets for O-GlcNAc modification [24, 25]. In the case of GSK3β and Akt, increased O-GlcNAcylation attenuates their activity, whereas O-GlcNAcylation of AMPK has been shown to increase its activity. It is also of note that GFAT, which regulates glucose entry into the HBP is phosphorylated by AMPK and this increases its activity thereby increasing glucose entry into the HBP. Given the substantial overlap between the circadian clock and O-GlcNAcylation in their regulatory roles we postulated that there may be an interrelationship between the two. We recently demonstrated not only that overall O-GlcNAc levels but also that OGT which catalyzes O-GlcNAc synthesis are both under circadian control [28]. Moreover we also found that that BMAL1 and Per1 are both O-GlcNAc modified, and furthermore that O-GlcNAcylation influences the timing of the circadian clock [28]. Overall this study demonstrated for the first time a close and intertwined relationship between cardiac metabolism, protein O-GlcNAcylation and the circadian clock. The fact that these observations may be more generally relevant was supported by subsequent studies in drosophila where altered OGT expression was shown to influence circadian rhythms and that dPER, the Drosophila PERIOD protein was an O-GlcNAc target [61].

While there is no doubt that the cardiomyocyte circadian clock and myocardial metabolism are intertwined, the question remains as to what advantage might arise from such a close relationship? The answer likely resides within the general function of circadian clocks: anticipation. What might the heart be anticipating? In general terms, the myocardium has two ‘major’ behavioral oscillations to contend with on a daily basis. These are awake/sleep and feeding/fasting rhythms. In terms of evolutionary selective pressures, it is important to note that not only are awake/sleep cycles more predictable on a daily basis but also that foraging for food, avoidance of predation, and reproduction, which occur during the awake period, are all energetically demanding, on both the organism as a whole, as well as the heart. Energetic demands during the active period remain high, even if the animal in the wild is not successful in its forage for food. As such, organisms that are able to anticipate this scenario (i.e., rhythms of physical activity/energetic demand independent of feeding status) would undoubtedly have an evolutionary selective advantage. We hypothesize that the cardiomyocyte circadian clock confers this selective advantage.

Based on evidence to date, we propose a hypothetic model in which the cardiomyocyte circadian clock allows the heart to anticipate daily rhythms in physical activity (particularly during periods of prolonged fasting), through time-of-day-dependent modulation of myocardial metabolism (Figure 3). During periods of increased energetic demand, the myocardium increases its reliance on carbohydrate as a fuel [36, 37]. Consistent with the latter, the cardiomyocyte circadian clock increases glucose uptake and utilization during the active phase, when myocardial AMPK activity peaks [28, 38, 44]. During fasting, inhibition of pyruvate dehydrogenase (i.e., Randal cycle) minimizes carbohydrate oxidation capacity [9]. However, circadian clock mediated increased glucose uptake during the active period would provide glycolytic and glycogenesis precursors, increased flux through the HBP, glycolysis, and glycogen storage (for subsequent utilization during the resting period), despite prolonged fasting. Indeed, as an example, in contrast to most tissues, myocardial glycogen stores increase during an overnight fast [62]. Similar to glycogen synthesis, the circadian clock mediates increased triglyceride synthesis during the active period [44]. Increased circulating fatty acids during prolonged fasting, coupled with circadian clock-mediated increased triglyceride synthesis capacity during the active period, promote triglyceride accumulation in the heart (for subsequent utilization during the resting period). Furthermore, circadian clock-dependent increased transcriptional responsiveness of the heart to fatty acids during the active period will reinforce promotion of fatty acid uptake and oxidation during prolonged fasting through changes in maximal capacity [63].

Figure 3. Hypothetical model for cardiomyocyte clock-mediated metabolic anticipation of increased energetic demand during the active period compared to the sleep period.

Thickness of line indicates predicted flux through a pathway. Dashed line represents positive regulatory effect. * represents cardiomyocyte circadian clock control.

During periods of sustained activity (e.g., >1 hour; which potentially occurs during foraging for food), circulating ketone bodies elevate markedly [17]. Cardiomyocyte circadian clock promotion of ketone body utilization during the active period would likely provide a significant fuel source for the contracting myocardium. Similarly, circulating lactate levels also become elevated during exercise [18]; however, whether the cardiomyocyte circadian clock promotes lactate utilization during the active period is currently unknown. During periods of increased physical activity, the likelihood of protein damage (e.g., through oxidative stress) is increased. Protein and organelle turnover is an energetically demanding process, which could potentially compete with contractile processes during the active period. Consistent with this rationale, the cardiomyocyte circadian clock likely increases net myocardial protein synthesis during the sleep/less active phase, as a means to replace damaged proteins/organelles at this time.

Implications for Health and Disease

It is clear from animal-based studies reported to date that cardiomyocyte circadian clock-mediated rhythms in myocardial metabolism exhibit close correlates with physical activity rhythms [28, 38, 64]. This includes increased glucose uptake, glycolytic flux, and oxidation during the active period (even when assessed ex vivo) [28, 38, 64]. As discussed above, increased glucose metabolism during the active period will undoubtedly act as an important source of ATP to meet increased energetic demands at this time. In doing so, the cardiomyocyte circadian clock therefore likely facilitates increased heart rate and cardiac output during the active period. Consistent with this hypothesis, CCM hearts exhibit decreased heart rate and cardiac output diurnal variations, due to an impairment in contractile function during the active period [6].

Maintenance of circadian synchronization (i.e., appropriate alignment of intrinsic rhythms with rhythms in environmental stimuli/stressors) is essential for cardiovascular function. This concept was highlighted by elegant studies by Martino and colleagues [65]. Hamsters carrying one mutant allele for tau (casein kinase 1ε) possess an intrinsic circadian clock with a periodicity of approximately 22 hours (as opposed to 24 hours in wild-type hamsters). When heterozygous tau hamsters are housed within a normal 12h:12h light:dark cycle (i.e., dyssynchronization between endogenous and environmental rhythms), age-dependent cardiovascular disease (both cardiac and renal) develops. However, when these same mutant hamsters are housed in an 11h:11h light:dark cycle (i.e., synchronization between endogenous and environmental rhythms), age-dependent cardiovascular disease development is not observed [65]. Given that circadian clocks directly modulate metabolism in a time-of-day-dependent manner, the possibility arises that a mismatch between myocardial metabolism rhythms (intrinsically driven) and energetic demand (extrinsically driven) might contribute towards contractile dysfunction. Indeed, the heart clock has been shown to be modulated by a host of cardiovascular disease risk factors, including pressure overload, hypertension, diabetes mellitus, obesity, coronary artery ligation, simulated shift work, and aging [53, 63, 66–69]. Although a plethora of studies have reported altered myocardial metabolism in response to many of these stresses, we are unaware of studies that have directly assessed whether diurnal variations in myocardial metabolic fluxes are altered. Indirect evidence, including time-of-day-dependent oscillations in metabolic gene expression, suggests that this may be the case [66, 68–71]. Clearly additional studies are required to determine the impact of disrupted diurnal variations in myocardial metabolism on the etiology of cardiac contractile dysfunction.

Challenges and Future Directions

Figure 2 outlines an evidence-based hypothetical selective advantage for regulation of myocardial metabolism by the cardiomyocyte circadian clock (i.e., anticipation of daily activity rhythms, independent of feeding status). Future studies are required to provide further support for, or to refute against, the validity of this model. If true, one would predict that CCM and/or CBK mice would exhibit poor cardiac function and/or exercise performance during periods of prolonged fasting. Additional studies are also required in order to firmly establish mechanistic links between the cardiomyocyte circadian clock and myocardial metabolism. Examples of unanswered questions include “does cardiomyocyte circadian clock regulation of AMPK mediate time-of-day-dependent oscillations in myocardial glucose uptake and utilization?”, “does the cardiomyocyte circadian clock anticipate utilization of alternative fuels (e.g., ketone bodies, lactate) during the active period?”, and “what role does cardiomyocyte circadian clock regulation of protein O-GlcNAcylation play in cardiac biology?”. A third area of interest revolves around the concept that disruption of normal circadian clock function, and subsequent disruption of diurnal rhythms in myocardial metabolism, contributes to the pathogenesis of cardiovascular disease development. This is particularly prudent in light of the fact of circadian clocks are exquisitely sensitive to environmental cues, many of which are common cardiovascular disease risk factors. This includes nutrient intake: alterations in the quantity, quality, and timing of food intake modulate circadian clocks in tissue-specific manners [72, 73]. As such, ‘inappropriate’ timing of food intake can cause internal circadian dyssynchrony (e.g., sleep-phase restricted feeding causes a 8-hour phase shift in the liver clock, but only a 4-hour phase shift in the heart clock, resulting in liver-heart dyssychrony; Young, unpublished observations). Future studies are required to elucidate fully the pathologic consequences of environment-induced circadian dyssynchrony on cardiac function. As we move closer to a 24-hour society, with continuous access to light and food, disruption of circadian biology is becoming common place.

Summary

Both myocardial metabolism and contractile function oscillate as a function of time-of-day. During the active/awake period, increased energetic demand is associated with greater rates of myocardial carbohydrate utilization, in addition to storage of glucose and fatty acids as glycogen and triglyceride (respectively). The cardiomyocyte circadian clock appears to play a pivotal role in mediating daily rhythms in myocardial metabolism. This has given rise to the hypothesis that the cardiomyocyte circadian clock allows the heart to anticipate periods of increased energetic demand, independent of feeding status. Whether disruption of the cardiomyocyte circadian clock in response to environmental cues contributes to the pathogenesis of cardiac dysfunction through modulation of myocardial metabolism is an attractive hypothesis that requires further interrogation.

Highlights.

1. The cardiomyocyte circadian clock directly regulates myocardial metabolism in a time-of-day-dependent manner

2. The cardiomyocyte circadian clock allows the heart to anticipate environmental stresses/stimuli

3. Circadian dyssynchrony likely contributes towards the etiology of cardiovascular disease