The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function

Mammalian Per-Arnt-Sim proteins in environmental adaptation

The Per-Arnt-Sim (PAS) domain is conserved across the kingdoms of life and found in an ever-growing list of proteins. This domain can bind to and sense endogenous or xenobiotic small molecules such as molecular oxygen, cellular metabolites, or polyaromatic hydrocarbons. Members of this family are often found in pathways that regulate responses to environmental change; in mammals these include the hypoxia, circadian, and dioxin response pathways. These pathways function in development and throughout life to regulate cellular, organ, and whole-organism adaptive responses. Remarkably, in the case of the clock, this adaptation includes anticipation of environmental change. In this review, we summarize the roles of PAS domain-containing proteins in mammals. We provide structural evidence that functionally classifies both known and unknown biological roles. Finally, we discuss the role of PAS proteins in anticipation of and adaptation to environmental change.

Genetic background affects function and intracellular calcium regulation of mouse hearts

AIMS

The genetic background is currently under close scrutiny when determining cardiovascular disease progression and response to therapy. However, this factor is rarely considered in physiological studies, where it could influence the normal behaviour and adaptive responses of the heart. We aim to test the hypothesis that genetic strain variability is associated with differences in excitation-contraction coupling mechanisms, in particular those involved in cytoplasmic Ca(2+) regulation, and that they are concomitant to differences in whole-heart function and cell morphology.

METHODS AND RESULTS

We studied 8- to 10-week-old male C57BL/6, BALB/C, FVB, and SV129 mice. Echocardiography and radiotelemetry were used to assess cardiac function in vivo. FVB mice had increased left ventricular ejection fraction and fractional shortening with significantly faster heart rate (HR) and lack of diurnal variation of HR. Confocal microscopy, sarcomere length tracking, and epifluorescence were used to investigate cell volume, t-tubule density, contractility, and Ca(2+) handling in isolated ventricular myocytes. Sarcomere relaxation and time-to-peak of the Ca(2+) transient were prolonged in BALB/C myocytes, with more frequent Ca(2+) sparks and significantly higher sarcoplasmic reticulum (SR) Ca(2+) leak. There were no strain differences in the contribution of different Ca(2+) extrusion mechanisms. SV129 had reduced SR Ca(2+) leak with elevated SR Ca(2+) content and smaller cell volume and t-tubule density compared with myocytes from other strains.

CONCLUSION

These results demonstrate that a different genetic background is associated with physiological differences in cardiac function in vivo and differences in morphology, contractility, and Ca(2+) handling at the cellular level.

The cardiomyocyte circadian clock: emerging roles in health and disease

Circadian misalignment has been implicated in the development of obesity, diabetes mellitus, and cardiovascular disease. Time-of-day-dependent synchronization of organisms with their environment is mediated by circadian clocks. This cell autonomous mechanism has been identified within all cardiovascular-relevant cell types, including cardiomyocytes. Recent molecular- and genetic-based studies suggest that the cardiomyocyte circadian clock influences multiple myocardial processes, including transcription, signaling, growth, metabolism, and contractile function. Following an appreciation of its physiological roles, the cardiomyocyte circadian clock has recently been linked to the pathogenesis of heart disease in response to adverse stresses, such as ischemia/reperfusion, in animal models. The purpose of this review is therefore to highlight recent advances regarding the roles of the cardiomyocyte circadian clock in both myocardial physiology and pathophysiology (ie, health and disease).

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.

Circadian rhythms and metabolic syndrome: from experimental genetics to human disease

The incidence of the metabolic syndrome represents a spectrum of disorders that continue to increase across the industrialized world. Both genetic and environmental factors contribute to metabolic syndrome and recent evidence has emerged to suggest that alterations in circadian systems and sleep participate in the pathogenesis of the disease. In this review, we highlight studies at the intersection of clinical medicine and experimental genetics that pinpoint how perturbations of the internal clock system, and sleep, constitute risk factors for disorders including obesity, diabetes mellitus, cardiovascular disease, thrombosis and even inflammation. An exciting aspect of the field has been the integration of behavioral and physiological approaches, and the emerging insight into both neural and peripheral tissues in disease pathogenesis. Consideration of the cell and molecular links between disorders of circadian rhythms and sleep with metabolic syndrome has begun to open new opportunities for mechanism-based therapeutics.

Inotropic response of cardiac ventricular myocytes to beta-adrenergic stimulation with isoproterenol exhibits diurnal variation: involvement of nitric oxide

RATIONALE

Although >10% of cardiac gene expression displays diurnal variations, little is known of their impact on excitation-contraction coupling.

OBJECTIVE

To determine whether the time of day affects excitation-contraction coupling in rat ventricles.

METHODS AND RESULTS

Left ventricular myocytes were isolated from rat hearts at 2 opposing time points, corresponding to the animals resting or active periods. Basal contraction and [Ca(2+)](i) was significantly greater in myocytes isolated during the resting versus active periods (cell shortening 12.4+/-0.3 versus 11.0+/-0.2%; P<0.05 and systolic [Ca(2+)](i) 422+/-12 versus 341+/-9 nmol/L; P<0.01. This corresponded to a greater sarcoplasmic reticulum (SR) Ca(2+) load (672+/-20 versus 551+/-13 nmol/L P<0.001). The increase in systolic [Ca(2+)](i) in response to isoproterenol (>3 nmol/L) was also significantly greater in resting versus active period myocytes, reflecting a greater SR Ca(2+) load at this time. This diurnal variation in response of Ca(2+)-homeostasis to isoproterenol translated to a greater incidence of arrhythmic activity in resting period myocytes. Inhibition of neuronal NO synthase during stimulation with isoproterenol, further increased systolic [Ca(2+)](i) and the percentage of arrhythmic myocytes, but this effect was significantly greater in active period versus resting period myocytes. Quantitative RT-PCR analysis revealed a 2.65-fold increase in neuronal NO synthase mRNA levels in active over resting period myocytes (P<0.05).

CONCLUSIONS

The threshold for the development of arrhythmic activity in response to isoproterenol is higher during the active period of the rat. We suggest this reflects a reduction in SR Ca(2+) loading and a diurnal variation in neuronal NO synthase signaling.

Metabolism and circadian rhythms--implications for obesity

Obesity has become a serious public health problem and a major risk factor for the development of illnesses, such as insulin resistance and hypertension. Human homeostatic systems have adapted to daily changes in light and dark in a way that the body anticipates the sleep and activity periods. Mammals have developed an endogenous circadian clock located in the suprachiasmatic nuclei of the anterior hypothalamus that responds to the environmental light-dark cycle. Similar clocks have been found in peripheral tissues, such as the liver, intestine, and adipose tissue, regulating cellular and physiological functions. The circadian clock has been reported to regulate metabolism and energy homeostasis in the liver and other peripheral tissues. This is achieved by mediating the expression and/or activity of certain metabolic enzymes and transport systems. In return, key metabolic enzymes and transcription activators interact with and affect the core clock mechanism. In addition, the core clock mechanism has been shown to be linked with lipogenic and adipogenic pathways. Animals with mutations in clock genes that disrupt cellular rhythmicity have provided evidence for the relationship between the circadian clock and metabolic homeostasis. In addition, clinical studies in shift workers and obese patients accentuate the link between the circadian clock and metabolism. This review will focus on the interconnection between the circadian clock and metabolism, with implications for obesity and how the circadian clock is influenced by hormones, nutrients, and timed meals.

Circadian proteins and genotoxic stress response

The circadian clock is an evolutionarily conserved time-keeping system that coordinates the physiology of the organism with daily changes in the environment. A growing body of evidence gradually leads to the conception that virtually all aspects of the biochemical, physiological, and behavioral functions of the animal are linked to circadian regulation. Moreover, proper synchronization of various processes through the activity of circadian components is important for the well-being of many organisms, including humans. The focus of this review is the circadian control of an organism's response to genotoxic stress, which is a major contributor to life-threatening human pathologies such as cancer and cardiovascular disease.

Attenuation of myocardial injury in mice with functional deletion of the circadian rhythm gene mPer2

Variations in circadian rhythms are evident in the incidence of cardiovascular disease, and the risk of cardiovascular events increases when rhythms are disrupted. The suprachiasmatic nucleus is the central circadian pacemaker that regulates the daily rhythm of peripheral organs. Diurnal rhythms have more recently been shown to exist in myocardial tissue and are involved in metabolism and contractile function. Thus we sought to determine whether the functional deletion of the circadian rhythm mouse periodic gene 2 (mPer2) would protect the heart against ischemic injury. Nonreperfused myocardial infarction was induced in anesthetized, ventilated C57 (n = 17) and mPer2 mutant (mPer2-M; n = 15) mice via permanent ligation of the left anterior descending coronary artery. At 4 days post-myocardial infarction, we observed a 43% reduction of infarct area in mPer2-M mice compared with wild-type mice. This is coincident with 25% less macrophage infiltration, 43% higher capillary density, 17% increase in hypertrophy, and 15% less cardiomyocyte apoptosis in the infarct zone. Also, matrix metalloproteinase-9 was expressed in inflammatory cells in both groups, but total protein was 40% higher in wild-type mice, whereas it was not elevated in mPer2-M mice in response to injury. The functional deletion of the mPer2 gene reduces the severity of myocardial infarct injury by limiting the inflammatory response, reducing apoptosis, and inducing cardiomyocyte hypertrophy, thus preserving cardiac function. These findings collectively imply that the disruption of the circadian clock gene mPer2 is protective. Understanding the interactions between circadian rhythm genes and cardiovascular disease may provide insights into potential preventative and therapeutic strategies for susceptible populations.

Weaving hypothesis of cardiomyocyte sarcomeres: discovery of periodic broadening and narrowing of intercalated disk during volume-load change

To investigate how cardiomyocytes change their length, echocardiographic and morphological studies were performed on rabbit hearts that were subjected to volume overload, overload removal, and repeated cycles of overload and overload removal. These conditions were created by arterio-venous fistula between the carotid artery and jugular vein, closure of the fistula, and cycles of repeatedly forming and closing fistula, respectively. After overload, hearts dilated and myocytes elongated. Intercalated disks repeatedly broadened and narrowed with a 2-day cycle, which continued for 8 weeks in many animals. The cycle consisted of shifts between five modes characterized by two interdigitation elongation-and-shortenings as follows: (I) flat with short ( approximately 1/4 to approximately 1/3 sarcomere long) interdigitations; (II) flat with long (one sarcomere long) interdigitations; (III) grooved with short interdigitations; (IV) grooved with long interdigitations; (V) flat with short interdigitations intermingled by sporadic long interdigitations; and return to (I). After overload removal, hearts contracted and myocytes shortened with similar 2-day broadening and narrowing cycle of intercalated disks, in which the five modes were reversed. Repeated overload and overload removal resulted in the repetition of myocyte elongation and shortening. We hypothesize that a single elongation-and-shortening event creates or disposes one sarcomere layer, and the two consecutive elongation-and-shortenings occur complementarily to each other so that the disks return to their original state after each cycle. Our hypothesis predicts that intercalated disks weave and unravel one sarcomere per myocyte per day.

Molecular time: an often overlooked dimension to cardiovascular disease

Diurnal rhythms influence cardiovascular physiology such as heart rate and blood pressure and the incidence of adverse cardiac events such as heart attack and stroke. For example, shift workers and patients with sleep disturbances, such as obstructive sleep apnea, have an increased risk of heart attack, stroke, and sudden death. Diurnal variation is also evident at the molecular level, as gene expression in the heart and blood vessels is remarkably different in the day as compared to the night. Much of the evidence presented here indicates that growth and renewal (structural remodeling) are highly dependent on processes that occur during the subjective night. Myocardial metabolism is also dynamic with substrate preference also differing day from night. The risk/benefit ratio of some therapeutic strategies and the appearance of biomarkers also vary across the 24-hour diurnal cycle. Synchrony between external and internal diurnal rhythms and harmony among the molecular rhythms within the cell is essential for normal organ biology. Cell physiology is 4 dimensional; the substrate and enzymatic components of a given metabolic pathway must be present not only in the right compartmental space within the cell but also at the right time. As a corollary, we show disrupting this integral relationship has devastating effects on cardiovascular, renal and possibly other organ systems. Harmony between our biology and our environment is vital to good health.

Effects of circadian disruption on the cardiometabolic system

The presence of day-night variations in cardiovascular and metabolic functioning is well known. However, only recently it has been shown that cardiovascular and metabolic processes are not only affected by the behavioral sleep/wake cycle but are partly under direct control of the master circadian pacemaker located in the suprachiasmatic nucleus (SCN). Heart rate, cardiac autonomic activity, glucose metabolism and leptin-involved in appetite control-all show circadian variation (i.e., under constant behavioral and environmental conditions). This knowledge of behavioral vs. circadian modulation of cardiometabolic function is of clinical relevance given the morning peak in adverse cardiovascular incidents observed in epidemiological studies and given the increased risk for the development of diabetes, obesity, and cardiovascular disease in shift workers. We will review the evidence for circadian control of cardiometabolic functioning, as well its sensitivity to light and melatonin, and discuss potential implication for therapy.

Circadian biology and sleep: missing links in obesity and metabolism?

This supplement highlights key talks presented at the Pennington Symposium. The collected papers provide a state of the art review of circadian biology at the basic and clinical levels in the context of nutrition, obesity and sleep medicine. Investigators from multiple disciplines attempted to translate new information concerning molecular mechanisms into practical clinical applications, as well as foster new research hypotheses and directions to this exciting field of science and medicine. Furthermore, we hope to spark the interest and attention of the next generation of scientists who will tackle the questions presented by the changing interface between technology, lifestyle and biological rhythms.

Influence of intermittent hypoxia on the signal transduction pathways to inflammatory response and circadian clock regulation

AIMS

Obstructive sleep apnea syndrome (OSAS), characterized by intermittent hypoxia/reoxygenation (IHR), is often associated with changing levels of circulating inflammatory cytokines and causes excessive daytime sleepiness, mood disturbances, and cardiovascular disease. An abnormal rhythm in the expression of circadian clock genes is observed in OSAS patients, and is also implicated in OSAS-related clinical symptoms. IHR-induced signal transduction is thought to underlie OSAS-associated complications. The aim of this study is to elucidate the influence of IHR on signal transduction pathways to inflammatory response and circadian clock regulation.

MAIN METHODS

To evaluate the direct action of IHR on intracellular signaling, we used a cell culture model to explore the underlying transcriptional events initiated by IHR.

KEY FINDINGS

Treatment of cultured human lung adenocarcinoma epithelial cells (A549) with IHR resulted in the elevation of mRNA levels of an inflammation cytokine interleukin-6 (IL-6), due to activation of the signaling pathway of nuclear factor-kappaB, a potent transcriptional activator of IL-6. On the other hand, the treatment of cells with IHR had little effect on clock gene response element-driven transcription. As a consequence, there was no significant change in mRNA levels of clock genes in IHR-treated cells.

SIGNIFICANCE

These results suggest that IHR can activate signal transduction to an inflammatory response, but not to circadian clock regulation. The abnormal rhythm in the expression of clock genes in OSAS patients is attributable to the changed levels of circulating factors that have the ability to modulate clock gene expression.

Anticipating anticipation: pursuing identification of cardiomyocyte circadian clock function

Diurnal rhythms in myocardial physiology (e.g., metabolism, contractile function) and pathophyiology (e.g., sudden cardiac death) are well establish and have classically been ascribed to time-of-day-dependent alterations in the neurohumoral milieu. Existence of an intramyocellular circadian clock has recently been exposed. Circadian clocks enable the cell to anticipate environmental stimuli, facilitating a timely and appropriate response. Generation of genetically modified mice with a targeted disruption of the cardiomyocyte circadian clock has provided an initial means for deciphering the functions of this transcriptionally based mechanism and allowed predictions regarding which environmental stimuli the heart anticipates (i.e., "anticipating anticipation"). Recent studies show that the cardiomyocyte circadian clock influences myocardial gene expression, beta-adrenergic signaling, transcriptional responsiveness to fatty acids, triglyceride metabolism, heart rate, and cardiac output, as well as ischemia-reperfusion tolerance. In addition to reviewing current knowledge regarding the roles of the cardiomyocyte circadian clock, this article highlights putative frontiers in this field. The latter includes establishing molecular links between the cardiomyocyte circadian clock with identified functions, understanding the pathophysiological consequences of disruption of this mechanism, targeting resynchronization of the cardiomyocyte circadian clock for prevention/treatment of cardiovascular disease, linking the circadian clock with the cardiobeneficial effects of caloric restriction, and determining whether circadian clock genes are subject to epigenetic regulation. Information gained from studies investigating the cardiomyocyte circadian clock will likely translate to extracardiac tissues, such as skeletal muscle, liver, and adipose tissue.

Diurnal physiology: core principles with application to the pathogenesis, diagnosis, prevention, and treatment of myocardial hypertrophy and failure

The circadian system has been shown to be fundamentally important in human health and disease. Recently, there have been major advances in our understanding of daily rhythmicity, and its relevance to human physiology, and to the pathogenesis and treatment of cardiac hypertrophy and heart failure. Cardiovascular tissues, such as heart and blood vessels, show remarkable daily variation in gene expression, metabolism, growth, and remodeling. Moreover, synchrony of daily molecular and physiological rhythms is integral to healthy organ growth and renewal. Disruption of these rhythms adversely affects normal growth, also the remodeling mechanisms in disease, leading to gross abnormalities in heart and vessels. These observations provide new insights into the pathogenesis, diagnosis, treatment, and prevention of heart disease. In this review, we focus on the recent advances in circadian biology and cardiovascular function, with particular emphasis on how this applies to human myocardial hypertrophy and heart failure, and the implications and importance for translational medicine.

PET detection of the impact of dobutamine on myocardial glucose metabolism in women with type 1 diabetes mellitus

BACKGROUND

Our objective was to determine, in the hearts of women with type 1 diabetes mellitus (T1DM), whether the fate of extracted glucose is altered and, if so, what the impact of dobutamine is on myocardial substrate metabolism. In experimental models of T1DM, myocardial glycolysis and glucose oxidation are reduced with the impairment becoming more pronounced with dobutamine. Whether similar changes occur in humans with T1DM is unclear.

METHODS AND RESULTS

Myocardial perfusion, oxygen consumption, and glucose and fatty acid metabolism were measured with positron emission tomography in 19 women, 7 normal volunteers (NVs) and 12 with T1DM. The NVs and 6 T1DM (DM1) patients were studied under baseline metabolic conditions and 6 T1DM patients were studied during hyperinsulinemic-euglycemic clamp (DM1-C), both at rest and during dobutamine. At rest, myocardial glucose uptake, glycolysis, glycogen storage, and oxidation were reduced by similar levels in DM1 patients compared with NVs (P < .05). During dobutamine, although myocardial glucose uptake was not different from DM1 patients at rest, fractional glycolysis was lower compared with NVs or DM1-C patients and reflected a lower glucose oxidation rate (P < .001). Measurements of myocardial glucose metabolism at rest and during dobutamine were comparable between NVs and DM1-C patients. During dobutamine, myocardial fatty acid uptake and oxidation increased in all 3 groups.

CONCLUSIONS

In women with T1DM, (1) myocardial glucose metabolism is impaired downstream from initial uptake, (2) these abnormalities become more pronounced with dobutamine and are paralleled by an increase in myocardial fatty acid metabolism, and (3) insulin restores glucose metabolism to levels observed in normal control subjects.

Effect of intermittent fasting on circadian rhythms in mice depends on feeding time

Calorie restriction (CR) resets circadian rhythms and extends life span. Intermittent fasting (IF) also extends life span, but its affect on circadian rhythms has not been studied. To study the effect of IF alongside CR, we imposed IF in FVB/N mice or IF combined with CR using the transgenic FVB/N alphaMUPA mice that, when fed ad libitum, exhibit spontaneously reduced eating and extended life span. Our results show that when food was introduced during the light period, body temperature peak was not disrupted. In contrast, IF caused almost arrhythmicity in clock gene expression in the liver and advanced mPer2 and mClock expression. However, IF restored the amplitudes of clock gene expression under disruptive light condition regardless whether the animals were calorically restricted or not. Unlike daytime feeding, nighttime feeding yielded rhythms similar to those generated during ad libitum feeding. Taken together, our results show that IF can affect circadian rhythms differently depending on the timing of food availability, and suggest that this regimen induces a metabolic state that affects the suprachiasmatic nuclei (SCN) clock.

Obstructive sleep apnoea and its cardiovascular consequences

Obstructive sleep apnoea (OSA) is a common disorder in which repetitive apnoeas expose the cardiovascular system to cycles of hypoxia, exaggerated negative intrathoracic pressure, and arousals. These noxious stimuli can, in turn, depress myocardial contractility, activate the sympathetic nervous system, raise blood pressure, heart rate, and myocardial wall stress, depress parasympathetic activity, provoke oxidative stress and systemic inflammation, activate platelets, and impair vascular endothelial function. Epidemiological studies have shown significant independent associations between OSA and hypertension, coronary artery disease, arrhythmias, heart failure, and stroke. In randomised trials, treating OSA with continuous positive airway pressure lowered blood pressure, attenuated signs of early atherosclerosis, and, in patients with heart failure, improved cardiac function. Current data therefore suggest that OSA increases the risk of developing cardiovascular diseases, and that its treatment has the potential to diminish such risk. However, large-scale randomised trials are needed to determine, definitively, whether treating OSA improves cardiovascular outcomes.

Regulation of feeding and metabolism by neuronal and peripheral clocks in Drosophila

Studies in mammals have indicated a connection between circadian clocks and feeding behavior, but the nature of the interaction and its relationship to nutrient metabolism are not understood. In Drosophila, clock proteins are expressed in many metabolically important tissues but have not been linked to metabolic processes. Here we demonstrate that Drosophila feeding behavior displays a 24 hr circadian rhythm that is regulated by clocks in digestive/metabolic tissues. Flies lacking clocks in these tissues, in particular in the fat body, also display increased food consumption but have decreased levels of glycogen and a higher sensitivity to starvation. Interestingly, glycogen levels and starvation sensitivity are also affected by clocks in neuronal cells, but the effects of neuronal clocks generally oppose those of the fat body. We propose that the input of neuronal clocks and clocks in metabolic tissues is coordinated to provide effective energy homeostasis.

The meter of metabolism

The circadian system orchestrates the temporal organization of many aspects of physiology, including metabolism, in synchrony with the 24 hr rotation of the Earth. Like the metabolic system, the circadian system is a complex feedback network that involves interactions between the central nervous system and peripheral tissues. Emerging evidence suggests that circadian regulation is intimately linked to metabolic homeostasis and that dysregulation of circadian rhythms can contribute to disease. Conversely, metabolic signals also feed back into the circadian system, modulating circadian gene expression and behavior. Here, we review the relationship between the circadian and metabolic systems and the implications for cardiovascular disease, obesity, and diabetes.

Linking the cardiomyocyte circadian clock to myocardial metabolism

INTRODUCTION

The energetic demands imposed upon the heart vary dramatically over the course of the day. In the face of equally commanding oscillations in the neurohumoral mileu, the heart must respond both rapidly and appropriately to its diurnal environment, for the survival of the organism. A major response of the heart to alterations in workload, nutrients, and various neurohumoral stimuli is at the level of metabolism. Failure of the heart to achieve adequate metabolic adaptation results in contractile dysfunction.

DISCUSSION

Substantial evidence is accumulating which suggests that a transcriptionally based timekeeping mechanism known as the circadian clock plays a role in mediating myocardial metabolic rhythms. Here, we provide an overview of our current knowledge regarding the interplay between the circadian clock within the cardiomyocyte and myocardial metabolism. This includes a particular focus on circadian clock mediated regulation of endogenous energy stores, as well as those mechanisms orchestrating circadian rhythms in metabolic gene expression.

CONCLUSION

An essential need to elucidate fully the functions of this molecular mechanism in the heart remains.

Circadian genes are expressed during early development in Xenopus laevis

BACKGROUND

Circadian oscillators are endogenous time-keeping mechanisms that drive twenty four hour rhythmic changes in gene expression, metabolism, hormone levels, and physical activity. We have examined the developmental expression of genes known to regulate circadian rhythms in order to better understand the ontogeny of the circadian clock in a vertebrate.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, genes known to function together in part of the core circadian oscillator mechanism (xPeriod1, xPeriod2, and xBmal1) as well as a rhythmic, clock-controlled gene (xNocturnin) were analyzed using in situ hybridization in embryos from neurula to late tailbud stages. Each transcript was present in the developing nervous system in the brain, eye, olfactory pit, otic vesicle and at lower levels in the spinal cord. These genes were also expressed in the developing somites and heart, but at different developmental times in peripheral tissues (pronephros, cement gland, and posterior mesoderm). No difference was observed in transcript levels or localization when similarly staged embryos maintained in cyclic light were compared at two times of day (dawn and dusk) by in situ hybridization. Quantitation of xBmal1 expression in embryonic eyes was also performed using qRT-PCR. Eyes were isolated at dawn, midday, dusk, and midnight (cylic light). No difference in expression level between time-points was found in stage 31 eyes (p = 0.176) but stage 40 eyes showed significantly increased levels of xBmal1 expression at midnight (RQ = 1.98+/-0.094) when compared to dawn (RQ = 1+/-0.133; p = 0.0004).

CONCLUSIONS/SIGNIFICANCE

We hypothesize that when circadian genes are not co-expressed in the same tissue during development that it may indicate pleiotropic functions of these genes that are separate from the timing of circadian rhythm. Our results show that all circadian genes analyzed thus far are present during early brain and eye development, but rhythmic gene expression in the eye is not observed until after stage 31 of development.

The PPAR trio: regulators of myocardial energy metabolism in health and disease

Common causes of heart failure are associated with derangements in myocardial fuel utilization. Evidence is emerging that metabolic abnormalities may contribute to the development and progression of myocardial disease. The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptor transcription factors has been shown to regulate cardiac fuel metabolism at the gene expression level. The three PPAR family members (alpha, beta/delta and gamma) are uniquely suited to serve as transducers of developmental, physiological, and dietary cues that influence cardiac fatty acid and glucose metabolism. This review describes murine PPAR loss- and gain-of-function models that have shed light on the roles of these receptors in regulating myocardial metabolic pathways and have defined key links to disease states including the hypertensive and diabetic heart.

Diurnal variations in myocardial metabolism

The heart is challenged by a plethora of extracellular stimuli over the course of a normal day, each of which distinctly influences myocardial contractile function. It is therefore not surprising that myocardial metabolism also oscillates in a time-of-day dependent manner. What is becoming increasingly apparent is that the heart exhibits diurnal variations in its intrinsic properties, including responsiveness to extracellular stimuli. This article summarizes our current knowledge regarding the mechanism(s) mediating diurnal variations in myocardial metabolism. Particular attention is focused towards the intramyocardial circadian clock, a cell autonomous molecular mechanism that appears to regulate myocardial metabolism both directly (e.g. triglyceride and glycogen metabolism) and indirectly (through modulation of the responsiveness of the myocardium to workload, insulin, and fatty acids). In doing so, the circadian clock within the cardiomyocyte allows the heart to anticipate environmental stimuli (such as changes in workload, feeding status) prior to their onset. This synchronization between the myocardium and its environment is enhanced by regular feeding schedules. Conversely, loss of synchronization may occur through disruption of the circadian clock and/or diurnal variations in neurohumoral factors (as observed during diabetes mellitus). Here, we discuss the possibility that loss of synchronization between the heart and its environment predisposes the heart to metabolic maladaptation and subsequent myocardial contractile dysfunction.

Compensated cardiac hypertrophy is characterised by a decline in palmitate oxidation

Cardiac hypertrophy is an independent risk factor in the development of heart failure. However, the cellular mechanisms underlying the transition from compensated hypertrophy to heart failure are incompletely understood. The aim of this study was to investigate changes in myocardial substrate utilisation and function in pressure-overload hypertrophy (using 13C NMR spectroscopy) in parallel with alterations in the expression pattern of genes involved in cardiac fatty acid and glucose uptake and oxidation. Left ventricular hypertrophy was induced surgically in Sprague-Dawley rats by inter-renal aortic constriction. Nine weeks later, hearts were perfused in the isovolumic mode with a physiological mixture of substrates including 5 mM 1-13C glucose, 1 mM 3-13C lactate, 0.1 mM U-13C pyruvate and 0.3 mM U-13C palmitate and cardiac function monitored simultaneously. Real-time PCR was used to determine mRNA levels of PPARalpha and PPARalpha-regulated metabolic enzymes. Results showed that at the stage of compensated hypertrophy, fatty acid oxidation (FAO) and expression of genes involved in FAO were markedly reduced, whilst pyruvate oxidation was enhanced, highlighting the fact that metabolic remodelling is an early event in the development of cardiac hypertrophy.

Thrombomodulin is a clock-controlled gene in vascular endothelial cells

Cardiovascular diseases are closely related to circadian rhythm, which is under the control of an internal biological clock mechanism. Although a biological clock exists not only in the hypothalamus but also in each peripheral tissue, the biological relevance of the peripheral clock remains to be elucidated. In this study we searched for clock-controlled genes in vascular endothelial cells using microarray technology. The expression of a total of 229 genes was up-regulated by CLOCK/BMAL2. Among the genes that we identified, we examined the thrombomodulin (TM) gene further, because TM is an integral membrane glycoprotein that is expressed primarily in vascular endothelial cells and plays a major role in the regulation of intravascular coagulation. TM mRNA and protein expression showed a clear circadian oscillation in the mouse lung and heart. Reporter analyses, gel shift assays, and chromatin immunoprecipitation analyses using the TM promoter revealed that a heterodimer of CLOCK and BMAL2 binds directly to the E-box of the TM promoter, resulting in TM promoter transactivation. Indeed, the oscillation of TM gene expression was abolished in clock mutant mice, suggesting that TM expression is regulated by the clock gene in vivo. Finally, the phase of circadian oscillation of TM mRNA expression was altered by temporal feeding restriction, suggesting TM gene expression is regulated by the peripheral clock system. In conclusion, these data suggest that the peripheral clock in vascular endothelial cells regulates TM gene expression and that the oscillation of TM expression may contribute to the circadian variation of cardiovascular events.

Mitochondria and heart failure

PURPOSE OF REVIEW

Energetic abnormalities in cardiac and skeletal muscle occur in heart failure and correlate with clinical symptoms and mortality. It is likely that the cellular mechanism leading to energetic failure involves mitochondrial dysfunction. Therefore, it is crucial to elucidate the causes of mitochondrial myopathy, in order to improve cardiac and skeletal muscle function, and hence quality of life, in heart failure patients.

RECENT FINDINGS

Recent studies identified several potential stresses that lead to mitochondrial dysfunction in heart failure. Chronically elevated plasma free fatty acid levels in heart failure are associated with decreased metabolic efficiency and cellular insulin resistance. Tissue hypoxia, resulting from low cardiac output and endothelial impairment, can lead to oxidative stress and mitochondrial DNA damage, which in turn causes dysfunction and loss of mitochondrial mass. Therapies aimed at protecting mitochondrial function have shown promise in patients and animal models with heart failure.

SUMMARY

Despite current therapies, which provide substantial benefit to patients, heart failure remains a relentlessly progressive disease, and new approaches to treatment are necessary. Novel pharmacological agents are needed that optimize substrate metabolism and maintain mitochondrial integrity, improve oxidative capacity in heart and skeletal muscle, and alleviate many of the clinical symptoms associated with heart failure.

Daytime Cheyne-Stokes Respiration in Ambulatory Patients With Severe Congestive Heart Failure Is Associated With Increased Mortality

BACKGROUND

Cheyne-Stokes respiration (CSR) frequently occurs in patients with severe heart failure during sleep and may increase mortality. Daytime CSR supposedly poses an even greater risk, but its prevalence and prognostic importance remain elusive. Therefore, we investigated the circadian prevalence of CSR and its influence on survival in patients with heart failure.

METHODS

In 60 consecutive ambulatory patients (mean age+/-SE, 58.0+/-1.5 years; 6 women) with stable severe heart failure (left ventricular ejection fraction, 26+/-1%; New York Heart Association [NYHA] class, 2.6+/-0.1), the breathing pattern was unobtrusively monitored during 24 h of usual activities with a portable respiratory inductive plethysmograph.

RESULTS

During nights, 62% of patients had >or=15 periodic breathing cycles per hour; during days, the corresponding prevalence was 16%. CSR prevailed in 32+/-3% of the night and in 10+/-2% of the day, with peaks at 4:00 am, 2:00 pm, and 6:00 pm. Eighteen patients with CSR during >or=10% of the daytime lived shorter without heart transplantation than 42 patients with <10% of daytime CSR (p<0.05) during 836+/-27 days of follow-up. CSR during >or=10% of the daytime was an independent predictor of mortality (hazard ratio, 3.8; 95% confidence interval, 1.1 to 12.7; p<0.05) when controlling for age, sex, brain natriuretic peptide, left ventricular ejection fraction, and NYHA class.

CONCLUSIONS

CSR occurs in 62% of patients with severe heart failure at night and in 16% during the day. Since daytime CSR is associated with reduced survival, solely performing sleep studies may not allow to adequately assess prognosis and tailor treatment in patients with severe heart failure.

Assembly and maintenance of the sarcomere night and day

The assembly of sarcomeric proteins into the highly organized structure of the sarcomere is an ordered and complex process involving an array of structural and associated proteins. The sarcomere has shown itself to be considerably more complex than ever envisaged and may be considered one of the most complex macromolecular assemblies in biology. Studies over the last decade have helped to put a new face on the sarcomere, and, as such, the sarcomere is being redefined as a dynamic network of proteins capable of generating force and signalling with other cellular compartments and metabolic enzymes capable of controlling many facets of striated myocyte biology.

Rapid attenuation of circadian clock gene oscillations in the rat heart following ischemia-reperfusion

The intracellular circadian clock consists of a series of transcriptional modulators that together allow the cell to perceive the time of day. Circadian clocks have been identified within various components of the cardiovascular system (e.g. cardiomyocytes, vascular smooth muscle cells) and possess the potential to regulate numerous aspects of cardiovascular physiology and pathophysiology. The present study tested the hypothesis that ischemia/reperfusion (I/R; 30 min occlusion of the rat left main coronary artery in vivo) alters the circadian clock within the ischemic, versus non-ischemic, region of the heart. Left ventricular anterior (ischemic) and posterior (non-ischemic) regions were isolated from I/R, sham-operated, and naïve rats over a 24-h period, after which mRNAs encoding for both circadian clock components and known clock-controlled genes were quantified. Circadian clock gene oscillations (i.e. peak-to-trough fold differences) were rapidly attenuated in the I/R, versus the non-ischemic, region. Consistent with decreased circadian clock output, we observe a rapid induction of E4BP4 in the ischemic region of the heart at both the mRNA and protein levels. In contrast with I/R, chronic (1 week) hypobaric chamber-induced hypoxia did not attenuate oscillations in circadian clock genes in either the left or right ventricle of the rat heart. In conclusion, these data show that in a rodent model of myocardial I/R, circadian clocks within the ischemic region become rapidly impaired, through a mechanism that appears to be independent of hypoxia.

Aryl hydrocarbon receptor nuclear translocator-like (BMAL1) is associated with susceptibility to hypertension and type 2 diabetes

Many aspects of physiology and behavior follow a circadian rhythm. Brain and muscle Arnt-like protein-1 (BMAL1) is a key component of the mammalian molecular clock, which controls circadian oscillations. In the rat, the gene encoding Bmal1 is located within hypertension susceptibility loci. We analyzed the SNP distribution pattern in a congenic interval associated with hypertension in the spontaneously hypertensive rat (SHR), and we show that Bmal1 maps close to a region genetically divergent between SHR and its normotensive (Wistar-Kyoto) counterpart. Bmal1 sequencing in rat strains identified 19 polymorphisms, including an SHR promoter variant that significantly affects Gata-4 activation of transcription in transient transfection experiments. A genetic association study designed to test the relevance of these findings in 1,304 individuals from 424 families primarily selected for type 2 diabetes showed that two BMAL1 haplotypes are associated with type 2 diabetes and hypertension. This comparative genetics finding translated from mouse and rat models to human provides evidence of a causative role of Bmal1 variants in pathological components of the metabolic syndrome.

The relationship between nutrition and circadian rhythms in mammals

The master clock located in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus regulates circadian rhythms in mammals. The clock is an intracellular, transcriptional mechanism sharing the same molecular components in SCN neurons and in peripheral cells, such as the liver, intestine, and retina. The circadian clock controls food processing and energy homeostasis by regulating the expression and/or activity of enzymes involved in cholesterol, amino acid, lipid, glycogen, and glucose metabolism. In addition, many hormones involved in metabolism, such as insulin, glucagon, adiponectin, corticosterone, leptin, and ghrelin, exhibit circadian oscillation. Furthermore, disruption of circadian rhythms is involved in the development of cancer, metabolic syndrome, and obesity. Metabolism and food intake also feed back to influence the biological clock. Calorie restriction (CR) entrains the SCN clock, whereas timed meals entrain peripheral oscillators. Furthermore, the cellular redox state, dictated by food metabolism, and several nutrients, such as glucose, ethanol, adenosine, caffeine, thiamine, and retinoic acid, can phase-shift circadian rhythms. In conclusion, there is a large body of evidence that links feeding regimens, food components, and the biological clock.

Robust circadian rhythm in heart rate and its variability: influence of exogenous melatonin and photoperiod

Heart rate (HR) and heart rate variability (HRV) undergo marked fluctuations over the 24-h day. Although controversial, this 24-h rhythm is thought to be driven by the sleep-wake/rest-activity cycle as well as by endogenous circadian rhythmicity. We quantified the endogenous circadian rhythm of HR and HRV and investigated whether this rhythm can be shifted by repeated melatonin administration while exposed to an altered photoperiod. Eight healthy males (age 24.4 +/- 4.4 years) participated in a double-blind cross-over design study. In both conditions, volunteers were scheduled to 16 h-8 h rest : wake and dark : light cycles for nine consecutive days preceded and followed by 29-h constant routines (CR) for assessment of endogenous circadian rhythmicity. Melatonin (1.5 mg) or placebo was administered at the beginning of the extended sleep opportunities. For all polysomnographically verified wakefulness periods of the CR, we calculated the high- (HF) and low- (LF) frequency bands of the power spectrum of the R-R interval, the standard deviation of the normal-to-normal (NN) intervals (SDNN) and the square root of the mean-squared difference of successive NN intervals (rMSSD). HR and HRV variables revealed robust endogenous circadian rhythms with fitted maxima, respectively, in the afternoon (16:36 hours) and in the early morning (between 05:00 and 06:59 hours). Melatonin treatment phase-advanced HR, HF, SDNN and rMSSD, and these shifts were significantly greater than after placebo treatment. We conclude that endogenous circadian rhythmicity influences autonomic control of HR and that the timing of these endogenous rhythms can be altered by extended sleep/rest episodes and associated changes in photoperiod as well as by melatonin treatment.

Treatment with beta2-adrenoceptor agonist in vivo induces human clock gene, Per1, mRNA expression in peripheral blood

This study examined whether in vivo exposure to a beta2-adrenoceptor agonist, tulobuterol, induces human Period1 (hPer1) mRNA expression in cells from peripheral whole blood. In one experiment, oral tulobuterol was administered to five healthy volunteers at 22:00 h, while in another, a transdermally tulobuterol patch was applied to the same five subjects at 20:00 h. In each experiment, serum tulobuterol concentrations were measured at four time points, and total RNA was isolated from peripheral blood cells for determinations of hPer1 mRNA expression by real-time polymerase chain reaction. Both the tulobuterol tablet and the transdermal patch increased hPer1 mRNA expression, suggesting that analyses of human peripheral blood cells could reliably represent peripheral clock gene mRNA expression in vivo.

Potential role for peripheral circadian clock dyssynchrony in the pathogenesis of cardiovascular dysfunction

Circadian clocks are intracellular molecular mechanisms designed to allow the cell, organ, and organism to prepare for an anticipated stimulus prior to its onset. In order for circadian clocks to maintain their selective advantage, they must be entrained to the environment. Light, sound, temperature, physical activity (including sleep/wake transitions), and food intake are among the strongest environmental factors influencing mammalian circadian clocks. Normal circadian rhythmicities in these environmental factors have become severely disrupted in our modern day society, concomitant with increased incidence of type 2 diabetes mellitus, obesity, and cardiovascular disease. Here, we review our current knowledge regarding the roles of peripheral circadian clocks, concentrating on those found within tissues directly involved in metabolic homeostasis and cardiovascular function. We propose that both inter- and intra-organ dyssynchronization, through alteration/impairment of peripheral circadian clocks, accelerates the development of cardiovascular disease risk factors associated with cardiometabolic syndrome.

Circadian rhythms in the CNS and peripheral clock disorders: function of clock genes: influence of medication for bronchial asthma on circadian gene

Bronchial asthma is a chronic inflammatory disorder of the airways, in which inflammation causes bronchial hyper-responsiveness and flow limitation in the presence of various stimuli. Pulmonary function in asthmatic patients frequently deteriorates between midnight and early morning, which has suggested a role for chronotherapy. Although relationships between bronchial asthma and the function of clock genes remain unclear, some medications given for asthma such as glucocorticoids or beta(2)-adrenoceptor agonists may influence clock genes in vivo. In our studies of clock gene mRNA expressions in human bronchial epithelial cells in vitro and peripheral blood cells in vivo, we demonstrated that glucocorticoid or beta(2)-adrenoceptor agonist treatment strongly induced human Per1 mRNA expression both in vitro and in vivo. Human peripheral blood cells provide a useful indication of peripheral clock gene mRNA expression in vivo.

CARNITINE PALMITOYL TRANSFERASE-I INHIBITION IS NOT ASSOCIATED WITH CARDIAC HYPERTROPHY IN RATS FED A HIGH-FAT DIET

1. Cardiac lipotoxicity is characterized by hypertrophy and contractile dysfunction and can be triggered by impaired mitochondrial fatty acid oxidation and lipid accumulation. The present study investigated the effect of dietary fatty acid intake alone and in combination with inhibition of mitochondrial fatty acid uptake with the carnitine palmitoyl transferase (CPT)-I inhibitor oxfenicine. Long-chain fatty acids activate peroxisome proliferator-activated receptors (PPAR), thus mRNA levels of PPAR target genes were measured. 2. Rats were untreated or given the CPT-I inhibitor oxfenicine (150 mg/kg per day) and were fed for 8 weeks with either: (i) standard low-fat chow (10% of energy from fat); (ii) a long-chain saturated fatty acid diet; (iii) a long-chain unsaturated fatty acid diet; or (iv) a medium-chain fatty acid diet (which bypasses CPT-I). High-fat diets contained 60% of energy from fat. 3. Cardiac triglyceride content was increased in the absence of oxfenicine in the saturated fat group compared with other diets. Oxfenicine treatment further increased cardiac triglyceride stores in the saturated fat group and caused a significant increase in the unsaturated fat group. Despite elevations in triglyceride stores, left ventricular mass, end diastolic volume and systolic function were unaffected. 4. The mRNA levels of PPAR-regulated genes were increased by the high saturated and unsaturated fat diets compared with standard chow or the medium chain fatty acid chow. Oxfenicine did not further upregulate PPARalpha target genes within each dietary treatment group. 5. Taken together, the data suggest that consuming a high-fat diet or inhibiting CPT-I do not result in cardiac hypertrophy or cardiac dysfunction in normal rats.

Circadian Rhythms in the CNS and Peripheral Clock Disorders: Role of the Biological Clock in Cardiovascular Diseases

The cardiovascular diseases are closely related to circadian rhythm, which is under the control of the biological clock. Clock genes show circadian oscillation not only in the suprachiasmatic nucleus but also in peripheral tissues, suggesting the existence of the peripheral clock. We previously demonstrated that plasminogen activator inhibitor-1 (PAI-1) might be an output gene of the peripheral clock. To further elucidate the functional relevance of the peripheral clock in the cardiovascular system, we screened target genes of the peripheral clock by cDNA microarray analysis. A total of 29 genes including transcription factor, growth factors, and membrane receptors were upregulated by CLOCK/BMAL and showed circadian oscillation. These results suggest that cardiovascular systems have their own peripheral clocks, and at least in part, they may regulate the circadian oscillation of cardiovascular function directly. These results potentially provide a molecular basis for the circadian variation of cardiovascular function and novel strategies for the prevention and treatment of cardiovascular diseases.

Chronobiological analysis of circadian patterns in transcription of seven key clock genes in six peripheral tissues in mice

The molecular clock machinery in mammals consists of a number of clock genes (CGs) and their resultant proteins that form interlocking transcription-translation feedback loops. These loops generate and maintain the 24 h mRNA and protein oscillations and consequential biological and physiological rhythms. To understand whether peripheral oscillators share similarly-timed clock machinery, the temporal expression patterns of the seven recognized key CGs (mPer1, mPer2, mCry1, mCry2, mRev-erb alpha, mClock, and mBmal1) were examined simultaneously in six peripheral tissues in mice every 4 h for 24 h in synchronized light-dark conditions using real time PCR assays. Time series were analyzed for time-effect by ANOVA and for rhythm characteristics by the single cosinor fitting procedure. The expression levels of most CGs were comparable in liver, kidney, and spleen, but mBmal1 and mCry1 were more abundant in the thymus, and mPer1, mCry1, and mCry2 were more abundant in the testis. In addition, mCry2 was dramatically lower in the kidney, spleen, and thymus; mPer2 was significantly lower in the spleen, testis, and thymus; and all of the genes tested were strikingly less abundant in peripheral blood. A significant 24 h rhythmic component was found for each CG in the liver and kidney and for some CGs in other tissues. Of note, a 12 h ultradian rhythmic component was also found in mRNA expression for some CGs in several of the tissues and was the only significant oscillation observed for CGs in the testis. Ultradian oscillations were also observed for mPer1 in the testis (8 h) and thymus (12 h and 8 h) in a second study where mice were sampled every 2 h. The present results suggest that the functioning of the molecular circadian clock may be modified to some extent between peripheral tissues, as denoted by differences in amplitude and phasing, and operates differently or is less developed in tissues containing differentiating cells (i.e., testis and thymus), as denoted by the presence of ultradian patterns resulting in two or more peaks within 24 h.

The circadian protein Clock localizes to the sarcomeric Z-disk and is a sensor of myofilament cross-bridge activity in cardiac myocytes

In the mammalian heart, the circadian protein Clock regulates glucose and fatty acid metabolism. In this study, we determined some of the factors that regulate Clock expression and subcellular distribution in myocytes. Using immunochemistry and biochemical subcellular fractionation, we have shown that Clock localizes to the Z-disk of the myofilaments. Increasing calcium and cross-bridge cycling with 10 microM phenylephrine for 48 h resulted in a threefold increase in Clock and a translocation of the protein to the nucleus. When myofilament cross-bridge cycling was inhibited with 10 microM verapamil or 7.5mM butanedione monoxime for 48 h, both significantly reduced the presence of Clock in the nucleus and cytoskeleton. These results suggest that the expression and subcellular distribution of Clock can be altered by changes in cross-bridge cycling, a major source of energy expenditure in myocytes. We suggest that the circadian Clock protein may help coordinate the sensing of energy expenditure with energy supply.

Shift work and pathological conditions

Shift work exerts major influences on the physiological functions of the human body. These are primarily mediated by the disruption of circadian rhythms since most body functions are circadian rhythmic. Next to the disturbances caused by changes in the circadian system, shift work has also been suggested to be related to a number of other health disorders. The present study summarizes recently published data on the potential relationship between disorders and shift working.