rdiology 31 (2015) 860e872

Canadian Journal of Ca

Review

Consequences of Circadian and Sleep Disturbances forthe Cardiovascular System

Faisal J. Alibhai, BSc, Elena V. Tsimakouridze, BSc, Cristine J. Reitz, BSc, W. Glen Pyle, PhD,

and Tami A. Martino, PhDCardiovascular Research Group, Department of Biomedical Sciences, University of Guelph, Ontario, Canada

ABSTRACTCircadian rhythms play a crucial role in our cardiovascular system.Importantly, there has been a recent flurry of clinical and experimentalstudies revealing the profound adverse consequences of disturbingthese rhythms on the cardiovascular system. For example, circadiandisturbance worsens outcome after myocardial infarction with impli-cations for patients in acute care settings. Moreover, disturbingrhythms exacerbates cardiac remodelling in heart disease models.Also, circadian dyssynchrony is a causal factor in the pathogenesis ofheart disease. These discoveries have profound implications for thecardiovascular health of shift workers, individuals with circadian andsleep disorders, or anyone subjected to the 24/7 demands of society.Moreover, these studies give rise to 2 new frontiers for translationalresearch: (1) circadian rhythms and the cardiac sarcomere, whichsheds new light on our understanding of myofilament structure, sig-nalling, and electrophysiology; and (2) knowledge translation, whichincludes biomarker discovery (chronobiomarkers), timing of therapies(chronotherapy), and other new promising approaches to improve themanagement and treatment of cardiovascular disease. Reconsideringcircadian rhythms in the clinical setting benefits repair mechanisms,and offers new promise for patients.

Received for publication October 31, 2014. Accepted January 8, 2015.

Corresponding author: Dr Tami A. Martino, Cardiovascular ResearchGroup, Biomedical Sciences/OVC Room 1646B, University of Guelph,Ontario N1G2W1, Canada. Tel.: þ1-519-824-4120 �54910; fax: þ1-519-767-1450.

E-mail: tmartino@uoguelph.caSee page 867 for disclosure information.

http://dx.doi.org/10.1016/j.cjca.2015.01.0150828-282X/� 2015 Canadian Cardiovascular Society. Published by Elsevier Inc. A

R�ESUM�ELes rythmes circadiens jouent un rôle crucial dans notre systèmecardiovasculaire. Notamment, une r�ecente avalanche d’�etudescliniques et exp�erimentales r�ev�elant les cons�equences ind�esirablesprofondes de la perturbation de ces rythmes sur le système car-diovasculaire ont �et�e r�ealis�ees. Par exemple, la perturbation desrythmes circadiens d�et�eriore les r�esultats cliniques après l’infarctus dumyocarde des patients en soins de phase aiguë. De plus, la pertur-bation des rythmes exacerbe le processus de remodelage cardiaquedes modèles de cardiopathie. Aussi, la dyssynchronie circadienne estun facteur causal dans la pathogenèse de la cardiopathie. Cesd�ecouvertes ont de profondes cons�equences sur la sant�e car-diovasculaire des travailleurs de quart, des individus ayant des trou-bles du rythme circadien et du sommeil, ou de tout individu soumistous les jours 24 heures sur 24 aux exigences de la soci�et�e. En outre,ces �etudes mettent en exergue 2 nouvelles frontières de la recherchetranslationnelle : 1) les rythmes circadiens et le sarcomère cardiaque,lequel jette un nouvel �eclairage sur notre compr�ehension de la struc-ture des myofilaments, de la signalisation et de l’�electrophysiologie; 2)l’application des connaissances, qui comprend la d�ecouverte des bio-marqueurs (chronobiomarqueurs), le meilleur moment des th�erapies(chronoth�erapie) et d’autres nouvelles approches prometteuses pouram�eliorer la prise en charge et le traitement de la maladie car-diovasculaire. La reconsid�eration des rythmes circadiens en milieuclinique favorise les m�ecanismes de r�eparation et s’avère prometteusepour les patients.

Jürgen Aschoff was a physician, scientist, and cofounder of thefield of circadian biology. He introduced the notion that dayand night (diurnal) rhythms in physiology are a fundamentalfeature in most living organisms including humans.1 We nowknow that rhythms are regulated by the circadian system, byorchestration in the hypothalamic suprachiasmatic nucleus,

and through neural and hormonal outputs to all organsincluding the heart (Fig. 1A).2-7 Mechanistically, an exquisiteunderlying cellular clock mechanism has been identified, whichcontrols molecular rhythms in virtually all cells including car-diomyocytes (Fig. 1B).8,9 Over the past 5 decades numerousclinical and experimental studies have revealed a crucial role forthe circadian system in regulating healthy physiology. Mostrecently, studies by our group and others have shed light on theprofound adverse health consequences of circadian distur-bances, underlying cardiovascular, cancer, metabolic, respira-tory, psychiatric, and other disorders.6,10-15 In this review wefocus on the consequences of circadian disturbances on thepathogenesis and pathophysiology of heart disease.

ll rights reserved.

Figure 1. Circadian rhythms. (A) Schematic representation of the circadian hierarchical oscillator model and the outputs that influence cardio-vascular processes. The suprachiasmatic nucleus in the hypothalamus synchronizes peripheral organ clocks, including the heart, to entrain to the24-hour light-dark environment. (B) The molecular clock mechanism keeps 24-hour time via transcription-translation feedback loops. The primarynegative feedback loop consists of response element (RRE) driven Brain and Muscle ARNT Like 1: Circadian Locomotor Cycles Kaput (BMAL1:-CLOCK), which leads to Period (PER) and Cryptochrome (CRY) production, PER:CRY phosphorylation by Casein Kinase 1 d/ε (CKd/ε), and PER:CRYself-repression. BMAL1:CLOCK also drives Nuclear Receptor Subfamily 1, Group D, Member 1 (REV-ERBa) expression, which represses BMAL1.This mechanism regulates expression of clock-controlled genes (ccg) which regulate rhythmic biological processes.

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Circadian Biology and Normal CardiovascularPhysiology

Circadian rhythms in heart rate and blood pressure

Circadian rhythms are important for daily cyclic variationin mammalian physiology, for example our heart rate (HR)is highest during wake and lowest during sleep.16 Bloodpressure (BP) also displays a daily rhythm, indeed, Floras andcolleagues documented for the first time the remarkable BPsurge that occurs at the time of waking and morning activ-ity.17 BP is highest in the morning and decreases progres-sively (approximately 10%) to reach a nadir during sleep.18

These rhythms are consistent with the diurnal biases ofour autonomic nervous system, which oscillate over 24-hourperiods.19 Thus our cardiovascular physiology exhibitsstriking time of day-dependent oscillations. Remarkably,diurnal variations in HR and cardiac contractility observedin vivo still persist ex vivo, for example, when rodent heartsare perfused on a Langendorff apparatus.13,20 Thus it is notjust the neurohormonal axes that contribute to time of daycardiac physiology, but also the intrinsic circadian propertiesof the heart. Over the past decade this has given rise tonumerous experimental studies that show a direct role forthe circadian mechanism on our daily cardiovascularphysiology, as is demonstrated experimentally in rats21-24

and mice.13,25,26

Daily timing of onset of adverse cardiovascular events

Adverse cardiovascular events are a leading cause of deathworldwide and do not occur at random times throughout theday. An early morning peak of acute myocardial infarction(MI) was first reported in 1985,27 and in the decades since,despite changes in lifestyle and advances in medicine, thepattern of morning excess of MI persists.28-33 A diurnalpattern also occurs for infarct size,34-36 stroke,37 angina,38

ventricular tachyarrhythmia,39-41 defibrillation energy re-quirements,42 ventricular refractoriness,43 and sudden cardiacdeath.44-46 Physiologically, sympathovagal balance is thought

to play a causal role, because patients with autonomic nervoussystem dysfunction do not exhibit the early morning peak intiming of onset of MI.47-49 Several studies have highlightedadditional factors, many under circadian control, which in-crease the risk of adverse cardiovascular events, includingdiurnal rhythms in platelet activity,50-53 thrombosis orthrombolysis,54-56 and endothelial function.57 Mechanisti-cally, a direct role for the circadian mechanism underlyingtime of day dependence has been demonstrated inhumans52,58 and animal models.34,59-61

Diurnal variations in cardiomyocyte metabolism

As described, cardiac function displays a time of dayeffect, driven in part by the sympathetic environment andthe neurohormonal milieu. However, at the myocyte level,there is considerable new evidence that intrinsic cellularproperties also occur in a circadian manner, as has beenexquisitely detailed in studies by Young and colleagues. Tosummarize, this is especially evident from the ex vivoperfused rat or mouse heart models, which demonstratepersistence of time of day variations in cardiac metabolism,even in the absence of neural and hormonal cues.20,62,63

Day/night variations in cardiac metabolism allow theheart to switch between different substrates (mainly glucoseand fatty acids) for energy sources depending on availabilityover the course of 24-hour periods.64,65 These intrinsiccardiomyocyte processes are facilitated in part by thecardiomyocyte-specific circadian clock mechanism.13,66

This circadian mechanism regulates messenger RNA(mRNA) and protein expression of key metabolic factors incardiomyocytes as described herein, thus controlling cellularprocesses in a time of day-dependent manner.

Circadian genomics

The heart is genetically a different organ in the day vs thenight. The first large-scale microarray study to demonstratethis revealed that approximately 8% of genes are rhythmic inmurine hearts under circadian (constant dark) conditions.67

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However, because humans (and medicine) exist in a diurnaland not circadian environment, we showed that approxi-mately 13% of genes in murine hearts are rhythmic undernormal day and night conditions.68 These genomic experi-ments recapitulate earlier findings of individual clock genecycling in polymerase chain reaction analyses,69 and more-over, identify many new rhythmic genes important for cardiacgrowth, renewal, transcription, translation, signalling path-ways, and metabolism.67,68 Our circadian heart microarraydata (and that of other researchers) are available through anopen access Web site created by the Hogenesch laboratory(Circadian Expression Profiles Data Base, Circa DB, http://bioinf.itmat.upenn.edu/circa/).70-73 Rhythmic cycling of thecore circadian mechanism genes has also been recentlydemonstrated in human heart tissue, in polymerase chainreaction analyses.74 Consistent with these observations is thepersistent 24-hour rhythmic oscillations in bioluminescence inheart and vascular tissue explants from transgenic rats inwhich luciferase expression is driven by the circadian mech-anism factor, Period 1 (Per1).75 Moreover, rhythmicity hasalso been shown to occur at the cellular level, in car-diomyocytes, vascular smooth muscle cells, endothelial cells,and fibroblasts in culture.76-80 It is worth noting that re-searchers generally perform rodent studies during the lightperiod, during rodent sleep time, and results are frequentlycompared with human wake time. These molecular studiesclearly demonstrate that timing of functional assessments ishighly critical in interpreting experimental findings. In sum-mary, the cell types in the heart possess circadian clockmechanisms that regulate a variety of clock-controlled outputgenes, which in turn influence the time of day processesessential for heart structure and function.

Circadian proteomics

Because it is the proteins and not directly the mRNA thatunderlies fundamental biochemical processes in cells, studieshave recently begun to explore circadian variation in thecellular proteome. We found that approximately 8% of thesoluble cardiac proteome exhibits diurnal variation in murineheart, using a large-scale 2-dimensional difference in gelelectrophoresis and mass spectrometry approach.81 Thecircadian mechanism plays a role in protein abundance in theheart; cardiomyocyte-specific Clock mutant mice have altereddiurnal abundance of proteins, including many rate-limitingenzymes crucial for vital metabolic pathways in the heart.81

In terms of how time of day protein abundance iscontrolled, we reported that approximately half of the diurnalvariation in the cardiac proteome can be accounted for byunderlying rhythmic changes at the mRNA level.81 Ourfindings are consistent with a previous report by Reddy et al.,who similarly observed approximately 50% of mRNA fluc-tuations underlying the hepatic proteome.82 Most recently,Luck et al. developed a statistical model that showed thatrhythmic production and degradation (half-life) at the mRNAand protein levels underlies daily protein abundance.83

Additional mechanisms that regulate diurnal molecularrhythms are glucocorticoid control of transcription,84,85 andtime of day variations in chromatin architecture.86 Thus, thereis a dynamic nature to the cardiac proteome, which underliesdiurnal variations in heart function.

Experimental Models of Circadian Disruptionand Consequences for the CardiovascularSystem

As highlighted in the preceding section, circadian rhythmsare fundamentally important for cardiovascular health. Theheart displays clear differences in time of day organ, cellular,and molecular physiology. Questions arise, therefore, as towhat happens when these rhythms are disturbed. Circadianrhythm disturbances can have multiple deleterious effects onorgan physiology, with profound clinical implications forcardiovascular and other diseases.6,10,87-89 In this section wefocus on experimental studies investigating the profoundadverse effects of circadian rhythm disturbance on the car-diovascular system.

Diurnal disturbance worsens outcome after MI

MI triggers a temporally orchestrated inflammatoryresponse designed to clear the infarct of dead cells and debris,and at the same time activates reparative pathways for fibrosisand scar formation.90-92 We postulated that the circadianmechanism plays a key role in immune cell recruitment andinfarct healing after MI. That there is daily cycling of pe-ripheral blood cells,93-96 diurnal reactivity of immuno-cytes,95,97 and a circadian mechanism in immune cells98-102

provides indirect support for this hypothesis. Our recentstudy directly recognized, to our knowledge for the first time,that the circadian mechanism plays a causal role in immu-nocyte recruitment to infarcted myocardium, and that dis-turbing this mechanism worsens outcome after MI.103 Micewere infarcted using left anterior descending coronary arteryligation (MI model), and randomized to either a normaldiurnal or disrupted light:dark environment for 5 days, andthen maintained in normal diurnal conditions. Short-termdiurnal disruption altered innate immune responses crucialfor scar formation, leading to maladaptive cardiac remodelling(Fig. 2). One of the most intriguing findings is a role for thecircadian mechanism factor CLOCK in recruiting innateimmune cells after MI. This study has clinical relevance,because circadian and sleep disruptions can impair recovery inmodern intensive care units.104-107 Reconsidering how wepractice medicine, by maintaining the diurnal environmentand patient biological rhythms, provides a promising non-pharmaceutical approach to improve patient outcomes inacute clinical care settings.108

Diurnal disturbance worsens cardiac remodelling

The adverse consequences of diurnal disturbance on pre-existing heart disease was first demonstrated by Turek andcolleagues, who revealed the deleterious effects on survival incardiomyopathic hamsters subjected to chronic shifts in thelight and dark cycle.109 We showed that diurnal rhythmdisturbance exacerbates cardiac remodelling in mice withpressure overload-induced cardiac hypertrophy; and impor-tantly, that the adverse remodelling only ceased when theanimals were returned to their normal diurnal cycle.110 Thus,altering the photoperiod to one in which animals cannotentrain alters the relationship between the light and dark cy-cle, endogenous physiology, and cardiac gene and proteinlevels, such that individuals are “out of sync” with theirenvironment. This has direct consequences on cardiac

Figure 2. Short-term diurnal rhythm disturbance in the first few days after myocardial infarction (MI) significantly worsens remodelling and outcome.Mice (C57Bl/6) were subjected to left anterior descending coronary artery ligation (MI model) then randomized to either a normal 12-hour light-dark(L:D) environment (normal diurnal environment) or a disturbed L:D environment for 5 days (short-term diurnal rhythm and sleep disturbance). Afteronly 5 days, all animals were placed in normal 12-hour light-dark L:D environment for the duration of the experiment (8 weeks; 8w post-MI).Histologic analyses revealed that the disrupted-MI mice had greater left ventricular dilation and infarct expansion by 8 weeks after MI comparedwith the normal-MI mice (Masson’s Trichrome). Overall, we showed that short-term disturbance of circadian rhythms after MI alters immunocyterecruitment to infarcted myocardium and impairs healing; conversely, maintaining rhythms is crucial for benefiting wound healing and outcome.103

These preclinical findings offer promise for clinical translation, because maintaining diurnal rhythms in intensive and acute care units in the first fewdays after injury can benefit healing and outcome.

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physiology and pathophysiology, exacerbating underlyingheart disease.

Circadian dyssynchrony causes heart disease

The important consequences of circadian dyssynchrony onthe heart are perhaps best exemplified by the þ/tau hamsterstudies. These animals carry a mutation in the circadian clockmechanism gene, casein kinase 1 epsilon, such that theyexhibit an internal circadian period of approximately 22hours, even though they live in a 24-hour world.111 Theþ/tau heterozygotes exhibit abnormal entrainment to the 24-hour diurnal environment (earlier onset and a more frag-mented activity profile),112 and reduced longevity, comparedwith their wild type littermates.113 In terms of cardiovascularconsequences, the Sole and Ralph laboratory demonstratedthat circadian disorganization in the þ/tau hamsters is a causalfactor in the pathogenesis of dilated cardiomyopathy.114

Conversely, if the animals are housed in diurnal cyclesappropriate for their genotype (22 hours), then rhythmsnormalize and the heart disease is prevented.114 Notably, thetau/tau homozygotes have a 20-hour period and do not

entrain to a 24-hour light and dark cycle and thus they do notdevelop heart disease.114 Thus, these studies demonstrate theimportance of synchrony between the intrinsic circadianperiod and the external light-dark environment, highlightingthat abnormal entrainment by circadian dyssynchrony playsan aetiologic role in the pathogenesis of heart disease.

Circadian gene mutations and heart disease

Additional lines of evidence further support the notion thatcircadian mechanism gene factors underlie cardiovascularpathophysiology; several recent studies are summarized herein.(1) Whole body brain and muscle ARNT-like 1 (BMAL1)-/-

knockout mice exhibit a loss of diurnal variation in HR andBP,26 and develop dilated cardiomyopathy.115 (2)Cardiomyocyte-specific Bmal1 knockout mice also developdilated cardiomyopathy even though the disturbance is only inthe heart.66,116 (3) In gene mapping studies it was found thatBmal1 associates with hypertension loci in spontaneouslyhypertensive rats.117 (4) In a similar light, BMAL1 (and alsoneuronal PAS domain protein 2 [Npas2]) polymorphisms areassociated with hypertension in humans.117,118 (5) Mice with

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simultaneous loss of the circadian output genes develop car-diac hypertrophy and left ventricular dysfunction.119 (6)Cardiomyocyte-specific circadian mutant mice exhibit blunt-ed diurnal HR variation, reduced responsiveness of the heartto workload changes, and altered cardiac metabolism.13,65,120

(7) Period 2 (Per2) mice have attenuated injury in the coro-nary artery ligation model.121 (8) Adenosine receptor A2b-mediated stabilization of Per2 is protective against ischemicheart disease in mice.122 Collectively, these studies are crucialfor demonstrating relevant relationships between the circadianmechanism and cardiovascular disease genesis and pathology.

Translational Consequences of Circadian andSleep Disruption on the Cardiovascular Systemin Humans

Many of the mechanistic studies described herein thatinvestigated the effects of circadian and sleep disturbances onthe cardiovascular system, were performed in rodent modelsand cell culture. However, these have important translationalapplications for human health and for clinical cardiology, asdescribed herein.

Shift work and heart disease

Circadian rhythm disturbances are relevant to humans. Forexample, the World Health Organization describes shift workas an occupational disturbance of circadian rhythms,123 andclinical studies support an association between shift work andheart disease. Mechanistically, shift work alters sympatheticand vagal autonomic activity,124 and diurnal BP profiles,125

which might contribute to cardiovascular disease. Furthersupporting this notion, Scheer et al. demonstrated experi-mentally that the adverse effects of circadian misalignment inhealthy humans can affect endocrine, metabolic, and auto-nomic predictors of cardiovascular (and other) risk.126

Epidemiologic studies further support a link betweenshift work and heart disease. For example, an increased riskof ischemic heart disease was demonstrated in a longitudinalstudy of 504 paper mill shift workers in Sweden.127

Moreover, an increased risk of coronary heart disease wasreported for US female shiftwork nurses.128 Also, anincreased risk for MI was shown in a meta-analysis of 34studies pertaining to shift workers and cardiovascular dis-ease.129 Furthermore, a greater incidence of atherosclerosiswas demonstrated in a cross-sectional examination of 2510shift workers in Germany.130 Last, severe hypertension wasdemonstrated in a longitudinal study of 6495 Japanese maleshift workers.131 Because millions of North American andother individuals engage in shift work or irregular workschedules at some point in their careers,132 it is importantto determine how atypical work schedules affect perfor-mance efficiency,133 and minimize the potential for adversehealth consequences.

Circadian rhythms and insufficient sleep

Circadian rhythm disturbances are also relevant throughtheir intersection with sleep because sleep is under bothcircadian and homeostatic control. Indeed, a role for circa-dian mechanism genetic factors on sleep has been recentlydiscovered in humans. For example, familial advanced phase

sleep syndrome is associated with a missense mutation in theclock component PER2, which alters the circadian period.134

These individuals go to sleep very early in the night time,and routinely wake up in the extreme early morning hours.Although a direct correlation between circadian factors thatinfluence sleep and heart disease in humans has not yet beendemonstrated, factors affecting sleep duration clearly have adirect effect on cardiovascular health. For example, in today’ssociety, 40% of North Americans do not achieve the rec-ommended 7 hours of sleep,135 and short (<5 hours) or long(>9 hours) sleep durations are associated with an increasedrisk of heart disease.136 These findings are consistent withearlier reports of increased all-cause mortality risk (includingischemic heart disease) in individuals with sleeping patternsother than 7-8 hours per night.137,138 In addition to circa-dian mutations that cause sleep changes, circadian-coupledfactors disturbed by inadequate sleep might mechanisticallycontribute to heart disease pathophysiology.139-141 Intrigu-ingly, there are molecular consequences on diurnal geneexpression in the heart with as little as one night of sleepdeprivation. This was demonstrated in mice after 3, 6, 9, and12 hours of sleep deprivation, which altered expression of2470 genes in the heart in microarray analyses.142

Chronotypes, social clocks, and light at night

Circadian rhythms in some physiologic functions varybetween humans (eg, core body temperature, melatoninphase), leading to classifications of early/morning or late/evening chronotypes.143 Intriguingly, chronotype might in-fluence cardiovascular disease, because evening types appearto be more at risk for cardiovascular changes (hypertension,HR, BP) compared with morning types.144 A later chro-notype is also associated with social jet lagdthe misalign-ment of biological and social clocks145dand prevalence ofsocial jet lag in apparently healthy individuals mightcontribute to cardiovascular risk.146 Light at night includingthat from light-emitting diodes in television and computerscreens has broad consequences for our health, and is thesubject several interesting recent reports.147,148 These studiesare relevant to contemporary society and shed new light onhow our circadian phenotypes, lifestyle choices, and thediurnal environment can affect cardiovascular health anddisease.

New Frontiers: Circadian Rhythms and theCardiac Sarcomere

In addition to the mentioned experimental and clinicaldiscoveries, there are new frontiers for circadian rhythmsresearch important to our understanding of the cardiovascularsystem. The first involves circadian rhythms and the cardiacsarcomere, which is the main contractile apparatus in theheart. These studies give rise to new understanding ofmyofilament structure, signalling, and electrophysiology, asdetailed herein.

Circadian rhythms and myofilament structure

Cardiac myofilaments mediate heart performance(Fig. 3),149-152 and recent studies indicate a role for thecircadian mechanism in affecting myofilament and thus

Figure 3. The diurnal cardiac sarcomere. (A) The cardiac sarcomere, delineated by 2 Z-discs, sits at the functional centre of myocytes. (B) Car-diomyocyte contraction is a calcium-dependent interaction, with steric modulation by the troponin-tropomyosin complex, leading to myosin and actincross-bridges. ATP cleavage by myosin heads provides energy for the power stroke that pulls actin toward the centre of the cell and produces cellshortening. Cross-bridge cycling exhibits a circadian rhythm and disruption of CLOCK abolishes the diurnal cycling of cardiac myofilament activa-tion.81 (C) Key elements of the cardiac sarcomere.

Alibhai et al. 865Circadian Rhythm Disturbance and Heart Disease

cardiac function. For example, ex vivo rat hearts exhibit thegreatest contractile performance during the animal wake vssleep time, in Langendorff analysis.20 Conversely, this vari-ation in performance is ablated in hearts of diurnal-disruptedmice.81 At a molecular level, day/night rhythms in cardiacmyofilament activity are lost in cardiomyocyte-specific Clockmutant mice compared with wild type mice, thus furthersupporting the notion that cardiac contractile performancelinks to the circadian mechanism.81 That there are signifi-cant disruptions in sarcomere architecture in Bmal1-/- miceprovides further support for this notion.115 Our group alsodemonstrated a functional correlation between myofilamentactivity and protein phosphorylation as one mechanism forregulating diurnal myofilament function in the murineheart.81 Most recently we demonstrated that the expressionof murine cardiac Titin-cap (Tcap, telethonin), a key regu-lator of sarcomere structure and function, is regulated by thecircadian mechanism transcription factors CLOCK andBMAL1.153 In a similar light, Andrews and colleagues haveshown links between skeletal muscles and the circadian clockmechanism.154 Thus collectively, these studies indicate thatthe circadian mechanism plays an important and previouslyunrecognized role in contractile muscle composition andperformance.

Circadian rhythms and myofilament signalling

Cyclic adenosine monophosphate-dependent protein ki-nase (PKA) targets the myofilament complex (troponin I,myosin binding protein C, titin, and others) and thus has a

powerful ability to affect myofilament function. PKA geneexpression is regulated by the cardiomyocyte clock,13 whichmight help explain diurnal variations in myocardial contrac-tility. Additionally, there is diurnal variation in b-adrenergicreceptor activation, a transmembrane receptor coupled withPKA activation in cardiac myocytes.155,156 Diurnal disruptiondecreases myofilament protein phosphorylation by PKA afterMI in mice.103 Together, these findings show the importanceof circadian variations in the b-adrenergic-PKA cascade, formyofilament regulation partly under control of the car-diomyocyte clock mechanism, and a pathway by whichchanges in myofilament function can influence the ability ofthe heart to respond to changes in driving factors.

Circadian mechanism and cardiac electrophysiology

As alluded to herein, the circadian mechanism plays a rolein cardiac contractility via time of day fluctuations in con-tractile proteins. Mechanistically, this might also be regulatedintrinsically by ion channels in the heart that influencemyofilament activation,156,157 some of which exhibit diurnalvariation. Time of day-dependent oscillations have been re-ported in the heart for the voltage-dependent potassiumchannels Kv1.5 and Kv4.2,

158 and the L-type voltage-gatedcalcium channel subunit VGCCa1D.159 Also, L-type cal-cium channel current densities increase during the activeperiod, which is consistent with the nocturnal elevation inmurine HR.155,157 Moreover, circadian variations in potas-sium channel interacting protein-2 affect potassium channelopening in a manner that is consistent with changes in action

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potential duration and circadian variations in HR. In vivo, thekruppel-like factor 15 (circadian output gene) knockout micedisplay increased susceptibility to ventricular arrhythmias, andreduced expression of potassium channel interacting protein2.160 Finally, there is loss of the rhythmic sodium channelvoltage-gated type V a subunit, and increased susceptibility toarrhythmias, in inducible cardiomyocyte-specific Bmal1-/-

mice.161 These studies provide new insights for understandingthe diurnal nature of cardiac electrophysiology, HR regula-tion, and arrhythmias.

New Frontiers: Translational ChronocardiologyAnother exciting new frontier is knowledge translation.

This is important for rhythms research to have a beneficialeffect on the quality of life for people with heart disease. Onearea of application is chronobiomarkers, in which temporalgene (microarray) and protein (proteomic) metadata are usedto identify new biomarkers of heart disease. A second area ofapplication is chronotherapy, which involves timing therapiesto improve the management of cardiovascular (and other)diseases. These approaches and relevance to clinical cardiologyare described in more detail herein.

Chronobiomarkers

Applying circadian concepts to clinical cardiology offersnew opportunities for discovering biomarkers, with a varietyof different technical approaches. For example, Ueda et al.estimate tissue time of day in mice by measuring expression ofapproximately 100 genes across a molecular time table inmicroarray analyses.162 Body time in mice163 and humans164

can also be by estimated using metabolomic studies. Wediscovered time of day biomarkers in murine cardiac hyper-trophy using microarray profiling and a novel time seriesalgorithm termed DeltaGene,165 and de novo chrono-biomarkers in murine plasma samples over 24-hour pe-riods.166 Collectively, these preclinical proof of conceptstudies are important for designing new approaches forbiomarker discovery, by taking advantage of the body’s nat-ural 24-hour molecular rhythms. Moreover, they have prac-tical application for chronotherapy (described herein), whichrequires easily accessible markers of body time to optimize thetiming of drug treatments.

Chronotherapy benefits (reverse) cardiac remodelling

Chronotherapy improves therapeutic efficacy (and possiblyreduces toxicity) by timing treatments with our daily physi-ology. For example, in a preclinical study on heart disease, weshowed that sleep-time administration of the short-actingangiotensin-converting enzyme inhibitor captopril markedlyreduced cardiac remodelling in mice with pressure overload-induced cardiac hypertrophy; conversely, administration atwake time had no measureable benefit on heart damage.167

Mechanistically, the sleep-time benefit is associated withdiurnal rhythms in the renin-angiotensin-aldosterone systemand cardiac genes and proteins, which peak during the rodentsleep time. Indeed, there is significant potential for chrono-therapy, because it has been recently reported that many best-selling drugs and the World Health Organization essential

medicines have short half-lives for directly targeting genepathways at specific times of day or night.168

Hypertension chronotherapy

Chronotherapy can also be effective in the treatment ofhypertension. Most people have a diurnal BP profile thatexhibits a dip of approximately 10% at night.18 Conversely,hypertensive nondippers169 or normotensive individuals withattenuated nocturnal dips in BP170 have an increased risk ofheart disease. Ambulatory BP monitoring, and especiallycovering the night time, is a useful prognostic indicator forheart disease,171-173 and is recommended by many interna-tional societies.174 Chronotherapy to treat hypertension atnight helps to restore the day/night BP rhythms and reducesthe risk of cardiovascular disease.175-178 For example, dayand night drug administration manage daytime hyperten-sion, however, only nighttime administration of ramipril,179

or telmisartan,180 or valsartan and amlodipine181 restores thenocturnal BP dip. Furthermore, the clinical trial termedAmbulatory Blood Pressure Monitoring in the Prediction ofCardiovascular Events and Effects of Chronotherapy(MAPEC) demonstrated prospectively that bedtime chro-notherapy but not morning administration of antihyper-tensive medication improves BP control and reducescardiovascular disease risk.182

Chronotherapy and other cardiovascular conditions:obstructive sleep apnea, hemodialysis, aspirin

Chronotherapy also holds significant clinical applicabilityin other conditions associated with heart disease. For example,obstructive sleep apnea (OSA) is a common respiratory dis-order of sleep, with underlying circadian-coupled mechanismsthat if left untreated can lead to the development and exac-erbation of heart disease and heart failure.183-188 Conversely,nocturnal continuous positive airway pressure (CPAP) therapybenefits patients with OSA and heart failure, reduces sym-pathetic activity,189,190 decreases BP during sleep, and im-proves left ventricular function.191-194 At a molecularcircadian level OSA appears to cause circadian clock genedysfunction, and nocturnal CPAP therapy helps to improve it.This was demonstrated by comparing Per1 mRNA expressionin peripheral blood cells of healthy subjects, OSA patients,and OSA patients treated with CPAP.195

Several additional lines of evidence further support thenotion that timing of therapy benefits the cardiovascularsystem. For example, nocturnal hemodialysis of patients withcardio-renal disease reduces left ventricular hypertrophy196,197

and improves myocardial function,198 compared with patientsgiven conventional daytime hemodialysis. In another study,low-dose aspirin taken at night reduced the circadian rhythmof platelet reactivity in healthy individuals; it is postulated thatthis might also reduce the risk of acute cardiovascular eventsduring the peak morning hours.199 Thus, these time-dependent effects of therapies have significant potential forbenefitting many patients in clinical settings.

Conclusions and Final DirectionsClinical and experimental studies demonstrate the impor-

tant role of circadian rhythms for healthy cardiovascular

Alibhai et al. 867Circadian Rhythm Disturbance and Heart Disease

physiology. Disturbing of rhythms has direct adverse conse-quences on cardiac physiology and pathophysiology. Futurestudies will undoubtedly provide new insights into themechanisms and molecular pathways that drive these pathol-ogies. In terms of translation, significant opportunities exist toincrease our understanding of circadian rhythms on cardio-vascular disease. For example, experiments investigatingcircadian regulation of cerebrovascular conditions and strokeare virtually unknown despite this being the third leadingcause of death in Canada with an estimated 50,000 strokes peryear.200 Also, metabolic disorders are considered major riskfactors for cardiovascular disease, and thus circadian rhythmsresearch applied to prevalent clinical conditions such as dia-betes and obesity could be fruitful areas of investigation.Determining the relationship between the circadian mecha-nism and cardiopulmonary conditions (eg, OSA and others)provides new opportunities to improve the diagnosis andtreatment of heart and lung disease. Upcoming areas forknowledge translation include development of databases forpatients with heart disease concurrent with circadian and/orsleep disorders, collecting circadian biometric data (eg, 24-hour HR and BP profiles, gene and protein biomarkers) inchronobiology and sleep clinics, and chronotherapeutic stra-tegies to improve the effectiveness of existing treatments.Thus, circadian rhythms research advances our understandingof cardiovascular health and disease, and leads to novel stra-tegies for management and treatment of heart disease inclinical settings.

Funding SourcesCanadian Institutes of Health Research grant to T.A.

Martino.

DisclosuresThe authors have no conflicts of interest to disclose.

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