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

Cardiac-specific mutation of Clock alters the quantitative measurements of physical activities without changing behavioral circadian rhythms

Even though peripheral circadian oscillators in the cardiovascular system are known to exist, the daily rhythms of the cardiovascular system are mainly attributed to autonomic or hormonal inputs under the control of the central oscillator, the suprachiasmatic nucleus (SCN). In order to examine the role of peripheral oscillators in the cardiovascular system, we used a transgenic mouse where the Clock gene is specifically disrupted in cardiomyocytes. In this cardiomyocyte-specific CLOCK mutant (CCM) mouse model, the circadian input from the SCN remains intact. Both CCM and wild-type (WT) littermates displayed circadian rhythms in wheel-running behavior. However, the overall wheel-running activities were significantly lower in CCM mice compared to WT over the course of 5 weeks, indicating that CCM mice either have lower baseline physical activities or they have lower physical adaptation abilities because daily wheel running, like routine exercise, induces physical adaptation over a period of time. Upon further biochemical analysis, it was revealed that the diurnal oscillations of phosphorylation states of several kinases and protein expression of the L-type voltage-gated calcium channel (L-VGCC) α1D subunit found in WT hearts were abolished in CCM hearts, indicating that in mammalian hearts, the daily oscillations of the activities of these kinases and L-VGCCs were downstream elements of the cardiac core oscillators. However, the phosphorylation of p38 MAPK exhibited robust diurnal rhythms in both WT and CCM hearts, indicating that cardiac p38 could be under the influence of the central clock through neurohormonal signals or be part of the circadian input pathway in cardiomyocytes. Taken together, these results indicate that the cardiac core oscillators have an impact in regulating circadian rhythmicities and cardiac function.

Rhythmic leptin is required for weight gain from circadian desynchronized feeding in the mouse

The neuroendocrine and metabolic effects of leptin have been extensively researched since the discovery, and the later identification, of the leptin gene mutated within the ob/ob mouse. Leptin is required for optimal health in a number of physiological systems (e.g. fertility, bone density, body weight regulation). Despite the extensive leptin literature and many observations of leptin's cyclical pattern over the 24-hour day, few studies have specifically examined how the circadian rhythm of leptin may be essential to leptin signaling and health. Here we present data indicating that a rhythmic leptin profile (e.g. 1 peak every 24 hours) leads to excessive weight gain during desynchronized feeding whereas non-rhythmic leptin provided in a continuous manner does not lead to excessive body weight gain under similar feeding conditions. This study suggests that feeding time can interact with leptin's endogenous rhythm to influence metabolic signals, specifically leading to excessive body weight gains during 'wrongly' timed feeding.

Pediatric obesity: a review for the child psychiatrist

Child and adolescent psychiatrists frequently encounter children who are obese in their practices and may be asked to work alongside primary care physicians and other specialists who treat youngsters with obesity. To offer expert consultation, they must understand all aspects of the pediatric obesity epidemic. By summarizing the relevant endocrinology, cardiology, nutrition, exercise science, and public health literature, this review of pediatric obesity assesses the epidemic's background, delineates the challenges of clinical care, and appraises the therapeutic recommendations for this population of patients and their families.

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

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

Circadian rhythms and cardiovascular health

The functional organization of the cardiovascular system shows clear circadian rhythmicity. These and other circadian rhythms at all levels of organization are orchestrated by a central biological clock, the suprachiasmatic nuclei of the hypothalamus. Preservation of the normal circadian time structure from the level of the cardiomyocyte to the organ system appears to be essential for cardiovascular health and cardiovascular disease prevention. Myocardial ischemia, acute myocardial infarct, and sudden cardiac death are much greater in incidence than expected in the morning. Moreover, supraventricular and ventricular cardiac arrhythmias of various types show specific day-night patterns, with atrial arrhythmias--premature beats, tachycardias, atrial fibrillation, and flutter - generally being of higher frequency during the day than night--and ventricular fibrillation and ventricular premature beats more common, respectively, in the morning and during the daytime activity than sleep span. Furthermore, different circadian patterns of blood pressure are found in arterial hypertension, in relation to different cardiovascular morbidity and mortality risk. Such temporal patterns result from circadian periodicity in pathophysiological mechanisms that give rise to predictable-in-time differences in susceptibility-resistance to cyclic environmental stressors that trigger these clinical events. Circadian rhythms also may affect the pharmacokinetics and pharmacodynamics of cardiovascular and other medications. Knowledge of 24-h patterns in the risk of cardiac arrhythmias and cardiovascular disease morbidity and mortality plus circadian rhythm-dependencies of underlying pathophysiologic mechanisms suggests the requirement for preventive and therapeutic interventions is not the same throughout the day and night, and should be tailored accordingly to improve outcomes.

Circadian profiles in the embryonic chick heart: L-type voltage-gated calcium channels and signaling pathways

Circadian clocks exist in the heart tissue and modulate multiple physiological events, from cardiac metabolism to contractile function and expression of circadian oscillator and metabolic-related genes. Ample evidence has demonstrated that there are endogenous circadian oscillators in adult mammalian cardiomyocytes. However, mammalian embryos cannot be entrained independently to light-dark (LD) cycles in vivo without any maternal influence, but circadian genes are well expressed and able to oscillate in embryonic stages. The authors took advantage of using chick embryos that are independent of maternal influences to investigate whether embryonic hearts could be entrained under LD cycles in ovo. The authors found circadian regulation of L-type voltage-gated calcium channels (L-VGCCs), the ion channels responsible for the production of cardiac muscle contraction in embryonic chick hearts. The mRNA levels and protein expression of VGCCα1C and VGCCα1D are under circadian control, and the average L-VGCC current density is significantly larger when cardiomyocytes are recorded during the night than day. The phosphorylation states of several kinases involved in insulin signaling and cardiac metabolism, including extracellular signal-regulated kinase (Erk), stress-activated protein kinase (p38), protein kinase B (Akt), and glycogen synthase kinase-3β (GSK-3β), are also under circadian control. Both Erk and p38 have been implicated in regulating cardiac contractility and in the development of various pathological states, such as cardiac hypertrophy and heart failure. Even though both Erk and phosphoinositide 3-kinase (PI3K)-Akt signaling pathways participate in complex cellular processes regarding physiological or pathological states of cardiomyocytes, the circadian oscillators in the heart regulate these pathways independently, and both pathways contribute to the circadian regulation of L-VGCCs.

Excessive sleepiness: under-recognized and essential marker for sleep/wake disorder management

BACKGROUND

Research during the past few decades has provided substantial evidence indicating that excessive sleepiness (ES) and associated sleep/wake disorders can result in significant morbidity and mortality. However, symptomatology (e.g., ES) and the relationships among common morbidities (e.g., cardiovascular disease, metabolic disorders, mood impairment) and sleep/wake disorders remain under-recognized in clinical practice, particularly in primary care. Yet assessment of sleep/wakefulness and associated symptoms can often be easily conducted in the primary care setting, providing valuable information to facilitate the diagnosis and management of sleep/wake disorders.

OBJECTIVE

To provide a conceptual and educational framework that helps primary care physicians comprehensively assess, differentially diagnosis, and appropriately manage patients presenting with ES or ES-related sleep/wake disorders.

METHODS

Comprised of six sleep specialists and six primary care physicians, the Sleep/Wake Disorders Working Group (SWG) used a modified, two-round Delphi approach to create and harmonize consensus recommendations for the assessment, diagnosis, treatment, and ongoing management of patients with common sleep/wake disorders related to ES.

RESULTS

After a review of the relevant literature, the SWG arrived at consensus on a number of clinical recommendations for the assessment and management ES and some of the most commonly associated sleep/wake disorders. Ten consensus statements – five each for assessment/diagnosis and treatment/ongoing care – were created for ES, insomnia, obstructive sleep apnea, circadian rhythm disorders, restless legs syndrome, and narcolepsy.

CONCLUSION

ES and ES-related sleep/wake disorders are commonly encountered in the primary care setting. By providing an educational framework for primary care physicians, the SWG hopes to improve patient outcomes by emphasizing recognition, prompt diagnosis, and appropriate ongoing management of ES and associated sleep/wake disorders.

Circadian disruption and metabolic disease: findings from animal models

Social opportunities and work demands have caused humans to become increasingly active during the late evening hours, leading to a shift from the predominantly diurnal lifestyle of our ancestors to a more nocturnal one. This voluntarily decision to stay awake long into the evening hours leads to circadian disruption at the system, tissue, and cellular levels. These derangements are in turn associated with clinical impairments in metabolic processes and physiology. The use of animal models for circadian disruption provides an important opportunity to determine mechanisms by which disorganization in the circadian system can lead to metabolic dysfunction in response to genetic, environmental, and behavioral perturbations. Here we review recent key animal studies involving circadian disruption and discuss the possible translational implications of these studies for human health and particularly for the development of metabolic disease.

Cardiac hypertrophy, low blood pressure, and low aldosterone levels in mice devoid of the three circadian PAR bZip transcription factors DBP, HLF, and TEF

The cardiovascular system is under the control of the circadian clock, and disturbed circadian rhythms can induce cardiovascular pathologies. This cyclic regulation is probably brought about by the circadian expression of genes encoding enzymes and regulators involved in cardiovascular functions. We have previously shown that the rhythmic transcription of output genes is, in part, regulated by the clock-controlled PAR bZip transcription factors DBP (albumin D-site binding protein), HLF (hepatic leukemia factor), and TEF (thyrotroph embryonic factor). The simultaneous deletion of all three PAR bZip transcription factors leads to increased morbidity and shortened life span. In the present study, we demonstrate that Dbp/ Tef/ Hlf triple knockout mice develop cardiac hypertrophy and left ventricular dysfunction associated with a low blood pressure. These dysfunctions are exacerbated by an abnormal response to this low blood pressure characterized by low aldosterone levels. The phenotype of PAR bZip knockout mice highlights the importance of circadian regulators in the modulation of cardiovascular functions.

Ecological measurements of light exposure, activity, and circadian disruption

Circadian rhythms are biological rhythms that repeat at approximately 24 hours. In humans, circadian rhythms have an average period of 24.2 hours. The 24-hour patterns of light and dark on the retina synchronize circadian rhythms to the local time on earth. Lighting characteristics affecting circadian rhythms are very different than those affecting visual responses. Lack of synchronization between the endogenous clock and the local time has been associated with a host of maladies. Therefore, it is important to measure circadian light exposures over the course of the 24-hour day and to be able to assess circadian entrainment and disruption in actual living environments. Presented is an overview of the recently developed Daysimeter, a personal measurement device for recording activity and circadian light-exposure. When the Daysimeter is worn on the head, two light sensors near the eye are used to estimate circadian light (CL_A) exposures over extended periods of time. Phasor analysis combines the measured periodic activity-rest patterns with the measured periodic light-dark patterns to assess behavioural circadian entrainment/disruption. As shown, day-shift and rotating-shift nurses exhibit remarkably different levels of behavioural circadian entrainment/disruption. These new ecological measurement and analysis techniques may provide important insights into the relationship between circadian disruption and well-being.

Melatonin and circadian biology in human cardiovascular disease

Diurnal rhythms influence cardiovascular physiology, i.e. heart rate and blood pressure, and they appear to also modulate the incidence of serious adverse cardiac events. Diurnal variations occur also at the molecular level including changes in gene expression in the heart and blood vessels. Moreover, the risk/benefit ratio of some therapeutic strategies and the concentration of circulating cardiovascular system biomarkers may also vary across the 24-hr light/dark cycle. Synchrony between external and internal diurnal rhythms and harmony among molecular rhythms within the cell are essential for normal organ biology. Diurnal variations in the responsiveness of the cardiovascular system to environmental stimuli are mediated by a complex interplay between extracellular (i.e. neurohumoral factors) and intracellular (i.e. specific genes that are differentially light/dark regulated) mechanisms. Neurohormones, which are particularly relevant to the cardiovascular system, such as melatonin, exhibit a diurnal variation and may play a role in the synchronization of molecular circadian clocks in the peripheral tissue and the suprachiasmatic nucleus. Moreover, mounting evidence reveals that the blood melatonin rhythm has a crucial role in several cardiovascular functions, including daily variations in blood pressure. Melatonin has antioxidant, anti-inflammatory, chronobiotic and, possibly, epigenetic regulatory functions. This article reviews current knowledge related to the biological role of melatonin and its circadian rhythm in cardiovascular disease.

Pediatric obesity: a review for the child psychiatrist

Child and adolescent psychiatrists frequently encounter children who are obese in their practices and may be asked to work alongside primary care physicians and other specialists who treat youngsters with obesity. To offer expert consultation, they must understand all aspects of the pediatric obesity epidemic. By summarizing the relevant endocrinology, cardiology, nutrition, exercise science, and public health literature, this review of pediatric obesity assesses the epidemic's background, delineates the challenges of clinical care, and appraises the therapeutic recommendations for this population of patients and their families.

Circadian light

The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.

CLOCK genetic variation and metabolic syndrome risk: modulation by monounsaturated fatty acids

BACKGROUND

Disruption of the circadian system may be causal for manifestations of the metabolic syndrome (MetS).

OBJECTIVE

The objective was to study the associations of 5 CLOCK polymorphisms with MetS features by analyzing fatty acid (FA) composition from dietary and red blood cell (RBC) membrane sources.

DESIGN

Participants (n = 1100) in the Genetics of Lipid Lowering Drugs and Diet Network (GOLDN) study were included. Dietary intake was estimated with a validated questionnaire. Anthropometric and biochemical measurements and genotypes were determined. Postprandial lipids and the FA composition of RBC membranes were analyzed.

RESULTS

CLOCK single nucleotide polymorphisms were significantly associated with obesity and individual components of MetS. For single nucleotide polymorphism rs4580704, minor allele carriers had a 46% lower risk of hypertension than did noncarriers. The monounsaturated fatty acid (MUFA) content of RBC membranes, particularly oleic acid, changed according to CLOCK genetic variants (P < 0.05). We identified significant gene-diet interactions associated with MetS at the CLOCK locus. By dichotomizing MUFA intake, we found different effects across rs4580704 genotypes for glucose (P = 0.020) and insulin resistance (P = 0.026). The protective effect of the minor allele on insulin sensitivity was only present when MUFA intake was >13.2% of energy. We also found different effects across CLOCK 3111T-->C genotypes for saturated fatty acid intake (% of energy) (P = 0.017). The deleterious effect of gene variants on waist circumference was only found with high saturated fatty acid intakes (>11.8%).

CONCLUSIONS

CLOCK polymorphisms interact with FAs to modulate MetS traits. The dietary source and membrane content of MUFAs are implicated in the relations between alterations in the circadian system and MetS.

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.

Direct regulation of myocardial triglyceride metabolism by the cardiomyocyte circadian clock

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

Neurobiology of circadian systems

Time is a dimension tightly associated with the biology of living species. There are cycles of varied lengths in biological activities, from very short (ultradian) rhythms to rhythms with a period of approximately one day (circadian) and rhythms with longer cycles, of a week, a month, a season, or even longer. These rhythms are generated by endogenous biological clocks, i.e. time-keeping structures, rather than being passive reactions to external fluctuations. In mammals, the suprachiasmatic nucleus (SCN) is the major pacemaker. The pineal gland, which secretes melatonin, is the major pacemaker in other phyla. There also exist biological clocks generating circadian rhythms in peripheral tissues, for example the liver. A series of clock genes generates the rhythm through positive and negative feedback effect of proteins on their own synthesis, and this system oscillates with a circadian period. External factors serve as indicators of the astronomical (solar) time and are called zeitgebers, literally time-givers. Light is the major zeitgeber, which resets daily the SCN circadian clock. In the absence of zeitgebers, the circadian rhythm is said to be free running; it has a period that differs from 24 hours. The SCN, together with peripheral clocks, enables a time-related homeostasis, which can become disorganized in its regulation by external factors (light, social activities, food intake), in the coordination and relative phase position of rhythms, or in other ways. Disturbances of rhythms are found in everyday life (jet lag, shift work), in sleep disorders, and in several psychiatric disorders including affective disorders. As almost all physiological and behavioural functions in humans occur on a rhythmic basis, the possibility that advances, delays or desynchronization of circadian rhythms might participate in neurological and psychiatric disorders has been a theme of research. In affective disorders, a decreased circadian amplitude of several rhythms as well as a phase advance or delay have been described, leading to hypotheses about changes in biological clocks themselves or in their sensitivity to environmental factors, such as light or social cues. Molecular genetics studies have suggested the involvement of circadian clock genes, but no tight association has yet been found. Agomelatine is an antidepressant, agonist at melatonergic MT(1), MT(2) receptors and antagonist at 5-HT(2C) receptors, and is able to phase advance circadian rhythms in humans. The fact that non-pharmacological (light therapy, sleep deprivation, rhythm therapy) and pharmacological (lithium, antidepressants, agomelatine) therapies of affective disorders influence circadian rhythms indicates that biological clocks play a role in the pathophysiology of these disorders.

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.

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.

Energy-responsive timekeeping

An essential component of energy homeostasis lies in an organism's ability to coordinate daily patterns in activity, feeding, energy utilization and energy storage across the daily 24-h cycle. Most tissues of the body contain the molecular clock machinery required for circadian oscillation and rhythmic gene expression. Under normal circumstances, behavioural and physiological rhythms are orchestrated and synchronized by the suprachiasmatic nucleus (SCN) of the hypothalamus, considered to be the master circadian clock. However, metabolic processes are easily decoupled from the primarily light-driven SCN when food intake is desynchronized from normal diurnal patterns of activity. This dissociation from SCN based timing demonstrates that the circadian system is responsive to changes in energy supply and metabolic status. There has long been evidence for the existence of an anatomically distinct and autonomous food-entrainable oscillator (FEO) that can govern behavioural rhythms, when feeding becomes the dominant entraining stimulus. But now rapidly growing evidence suggests that core circadian clock genes are involved in reciprocal transcriptional feedback with genetic regulators of metabolism, and are directly responsive to cellular energy supply. This close interaction is likely to be critical for normal circadian regulation of metabolism, and may also underlie the disruption of proper metabolic rhythms observed in metabolic disorders, such as obesity and type-II diabetes.

Chronobiology, genetics and metabolic syndrome

PURPOSE OF REVIEW

Circadian rhythms are such an innate part of our lives that we rarely pause to speculate why they even exist. Recently, some studies have suggested that the disruption of the circadian system may be causal for the manifestations of metabolic syndrome (MetS). This review summarizes the latest evidence of the existing interaction among chronobiology, genetics and MetS.

RECENT FINDINGS

Shift work, sleep deprivation and bright light exposure at night are related to increased adiposity and prevalence of MetS. Animal models have revealed that mice with circadian locomotor output cycles kaput (clock) gene disruption are prone to develop a phenotype resembling MetS. Moreover, studies in humans have shown that clock genes are expressed in adipose tissue, and that both their levels of expression and their genetic variants correlate with different components of the MetS. Current studies are illustrating the particular role of different clock gene variants and their predicted haplotypes in MetS.

SUMMARY

The circadian system has an important impact on metabolic disturbances and vice versa. Although the precise mechanism linking the MetS to chronodisruption is not well known, hypotheses point to the internal desynchronization between different circadian rhythms. The novelty of this area of research is contributing to the development of new and intriguing studies, particularly those focused on the association between different clock genes polymorphisms and MetS traits.

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.

Shift work and its effects on the cardiovascular system

The practice of shift-work scheduling has long been part of normal work duties in emergency services such as health and security. It is only recently, in the wake of growing job opportunities and booming industries, where more employees are needed to keep services running over 24-hour periods that studies on the effects of shift work on workers' health have begun to delve deeper. The desynchronisation that occurs in circadian rhythms, with respect to sleep cycles, predisposes employees to coronary heart disease, gastrointestinal disturbances, increased risk of breast cancer and poor pregnancy outcomes. This literature review focuses on circadian rhythms, their molecular components, disturbances of these rhythms as a result of shift work and the adverse effects thereof on the cardiovascular system.

Molecular clocks, type 2 diabetes and cardiovascular disease

The westernised world is in the midst of an epidemic of type 2 diabetes and associated cardiovascular disease. These closely interlinked conditions have a common pathophysiological basis underpinned by insulin resistance and the metabolic syndrome. Contemporary changes in environmental factors on a background of genetic susceptibility are thought to account for the increases seen. Life on earth is governed by the 24-hour environment of light and darkness cycling with the rotation of the earth. Numerous metabolic and physiological pathways are coordinated to this 24-hour cycle by an endogenous clock. Recent epidemiological evidence and animal data suggest that disturbance of circadian rhythms through genetic and environmental influences on the molecular clock is pivotal in the pathogenesis of obesity, type 2 diabetes and cardiovascular disease. This review describes current knowledge on the topic.

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.

Disruption of the circadian clock within the cardiomyocyte influences myocardial contractile function, metabolism, and gene expression

Virtually every mammalian cell, including cardiomyocytes, possesses an intrinsic circadian clock. The role of this transcriptionally based molecular mechanism in cardiovascular biology is poorly understood. We hypothesized that the circadian clock within the cardiomyocyte influences diurnal variations in myocardial biology. We, therefore, generated a cardiomyocyte-specific circadian clock mutant (CCM) mouse to test this hypothesis. At 12 wk of age, CCM mice exhibit normal myocardial contractile function in vivo, as assessed by echocardiography. Radiotelemetry studies reveal attenuation of heart rate diurnal variations and bradycardia in CCM mice (in the absence of conduction system abnormalities). Reduced heart rate persisted in CCM hearts perfused ex vivo in the working mode, highlighting the intrinsic nature of this phenotype. Wild-type, but not CCM, hearts exhibited a marked diurnal variation in responsiveness to an elevation in workload (80 mmHg plus 1 μM epinephrine) ex vivo, with a greater increase in cardiac power and efficiency during the dark (active) phase vs. the light (inactive) phase. Moreover, myocardial oxygen consumption and fatty acid oxidation rates were increased, whereas cardiac efficiency was decreased, in CCM hearts. These observations were associated with no alterations in mitochondrial content or structure and modest mitochondrial dysfunction in CCM hearts. Gene expression microarray analysis identified 548 and 176 genes in atria and ventricles, respectively, whose normal diurnal expression patterns were altered in CCM mice. These studies suggest that the cardiomyocyte circadian clock influences myocardial contractile function, metabolism, and gene expression.

Sleep and circadian rhythms: key components in the regulation of energy metabolism

In this review, we present evidence from human and animal studies to evaluate the hypothesis that sleep and circadian rhythms have direct impacts on energy metabolism, and represent important mechanisms underlying the major health epidemics of obesity and diabetes. The first part of this review will focus on studies that support the idea that sleep loss and obesity are "interacting epidemics." The second part will discuss recent evidence that the circadian clock system plays a fundamental role in energy metabolism at both the behavioral and molecular levels. These lines of research must be seen as in their infancy, but nevertheless, have provided a conceptual and experimental framework that potentially has great importance for understanding metabolic health and disease.